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 A MANUAL 
 
 OF 
 
 PHOTOGRAPHIC CHEMISTRY. 
 
A MANUAL 
 PHOTOGRAPHIC CHEMISTRY, 
 
 THEORETICAL AND PRACTICAL. 
 
 BY 
 
 T. EEEDEEICK HAEDWICH, 
 
 LATE DEMONSTRATOR OF CHEMISTRY AND LECTURER ON PHOTOGRAPHY 
 IS KING'S COLLEGE, LONDON. 
 
 Sebentfj (SHitton. 
 
 EDITED BY 
 GEORGE DAWSON, M.A., 
 
 LECTURER ON PHOTOGRAPHY IN KING'S COLLEGE, 
 AND 
 
 EDWARD HADOW, F.C.S., M.R.C.S., 
 
 DEMONSTRATOR OF CHEMISTRY IN KING'S COLLEGE. 
 
 LONDON : 
 JOHN CHURCHILL & SONS, NEW BURLINGTON STREET. 
 
 . 
 
 [The Author reserves to himself the right of translating this Edition. ~\ 
 
FEINTED BY 
 
 JOHN EDWAKD TATI.OB, LITTLE QUEEN STREET, 
 LINCOLN'S INN FIELDS. 
 
PREFACE TO THE SEVENTH EDITION. 
 
 IN this Edition a different arrangement of the Parts of 
 the Work has been adopted, " The Outlines of General 
 Chemistry " having been placed first. This alteration may 
 seem unimportant to those who consult the Work merely 
 for practical details, but it is hoped that the re-arrange- 
 ment will prove satisfactory to those readers who wish to 
 peruse it continuously, that they may obtain a sound theo- 
 retical foundation before proceeding to practice. Although 
 this portion of the Book has been in a great measure re- 
 written, old-fashioned definitions of Acids and Salts, and 
 modes of representing chemical changes, have beenretained, 
 since later and more exact views could hardly be intro- 
 duced without the frequent employment of new terms, 
 which seem out of place in a Manual treating only of what 
 is theoretical for practical ends. Those Atomic Weights 
 are given which accord with the ordinary chemical names 
 familiar to Photographers. The " Vocabulary of Chemi- 
 cals " has been revised and enlarged ; but, as in former 
 Editions, the processes given in this Part are such as suit 
 the amateur rather than the manufacturer. Some re- 
 marks on the Elements which enter into the composition 
 of photographic chemicals have also been added, with the 
 view of enabling the reader to be more independent of 
 other works on Chemistry. 
 
 In Part II., Mr. Hardwich's theories of the Action of 
 Light on Salts of Silver are left much as they stood, 
 
VI PEEFACE TO THE SEVENTH EDITION. 
 
 inasmuch as his views on these points accord generally 
 with those of the Editors, and have been further confirmed 
 by the researches of Monckhoven, and more recently by 
 those of Dr. "Vogel, of Berlin. The new Lenses which have 
 come prominently into notice during the last three years 
 have been described, and the theory of Binocular Vision 
 and of the Stereoscope has been more fully explained. 
 
 In discussing the theory of Collodion Negatives, care 
 has been taken to distinguish between the modes of treat- 
 ment best suited for Iodized and Bromo-iodized Collodions, 
 but in such a way as to interfere as little as possible with 
 the original arrangement of the Part. 
 
 In Part III., the same distinction between the two kinds 
 of Collodion has been maintained throughout, and separate 
 formula? and instructions for manipulation have been given 
 for each ; the object being to explain the details of the Art 
 as at present practised, so far as they have been thoroughly 
 tested. Some Sections have been re-written, and in others it 
 has been found necessary to condense and to curtail, so as to 
 admit more important matter within the prescribed limits. 
 
 In the Appendix some additions have been also made, 
 and the Index has been rendered more complete. 
 
 In conclusion, the Editors trust that their efforts will 
 not prove to have been unsuccessful in maintaining for this 
 Manual the position it has hitherto held as " The Photo- 
 grapher's Guide." Whether they have succeeded or not, 
 they have endeavoured conscientiously to carry out the 
 expressed wish of Mr. Hardwich, who has been truly called 
 "the Photographer's best friend," and who has only re- 
 linquished his favourite pursuit because he has found a 
 higher and more important field of labour. 
 
 GEOEGE DAWSON. 
 EDWAED A. HADOW. 
 
 King's College, London, 
 January, 1864. 
 
PREFACE TO THE SIXTH EDITION. 
 
 IN preparing a new Edition of his Work, the Author 
 finds it growing under his hands in a way not originally 
 anticipated. The only apology he can offer is, that he 
 hears the * Manual ' spoken of as being useful for refer- 
 ence rather than for continuous perusal, and if so, each 
 Part, and also each separate Chapter, ought to be as 
 nearly as possible complete in itself, even at the expense 
 of some repetition. In furtherance of this object, the em- 
 ployment of type of two sizes has been adopted, which 
 will enable the reader to distinguish the elementary parts 
 of the Work from those which, being more elaborate and 
 minute, may be consulted only on occasion*. 
 
 The Third Chapter, treating of latent images, has un- 
 dergone some alteration in this Edition, and the Author 
 having definitely adopted a particular hypothesis, has 
 given it that prominence which it appears to him to de- 
 serve. 
 
 With reference to Collodion, he believes that every 
 statement which he makes concerning it may be implicitly 
 received, since his opportunities of fixing correct data have 
 been unusually great. At the same time he wishes it to 
 be understood that he has now relinquished the com- 
 mercial manufacture of Collodion, and that for the future 
 no responsibility will attach to him. 
 
Vlll PBEFACE TO THE SIXTH EDITION. 
 
 Photographic printing has reached a point beyond which 
 any further advance will be difficult. The Chapters rela- 
 ting to this subject have been once more re-arranged, but 
 it is not anticipated that such a proceeding will again be 
 necessary unless our present modes should be superseded. 
 
 Dry Collodion photography is in this Edition placed in 
 the first or theoretical, as well as in the second or practical 
 part of the Work. Major Russell has communicated aii 
 improved method, of the utility of which high expectations 
 are entertained. 
 
 In conclusion, the Writer desires to thank those kind 
 friends who have assisted him in collecting materials for 
 this Edition, and to acknowledge his obligations to the 
 authors of papers published during the past two years. 
 His aim throughout has been to show strict impartiality 
 in recording the results of his fellow-labourers, and if he 
 has failed in doing so, the fault was not in the intention. 
 
 London, January 1st, 1861. 
 
CONTENTS. 
 
 Page 
 1 
 
 PART I. 
 OUTLINES OF GENERAL CHEMISTRY. 
 
 CHAPTER I. 
 
 THE CHEMICAL ELEMENTS AND THEIE COMBINATIONS. 
 
 The more important Elementary Bodies, with their Sym- 
 bols and Atomic Weights. Compounds and Mixtures. 
 Bases. Acids. Neutral Oxides. Salts. Laws of 
 combination by weight. The Atomic theory. Sym- 
 bols and formulae. Chemical nomenclature. Che- 
 mical changes. Chemistry of organic bodies ... 9 
 
 CHAPTER II. 
 
 DESCRIPTION OF PHOTOGEAPHIC CHEMICALS AND THEIE 
 
 ELEMENTS, ALPHABETICALLY AEBANGED .... 29 
 
X CONTENTS. 
 
 PART II. 
 
 THE THEORY OF PHOTOGRAPHIC PROCESSES. 
 CHAPTER I. 
 
 Page 
 HISTOEICAL SKETCH OF PHOTOGEAPHY 137 
 
 CHAPTER II. 
 
 THE SALTS OP SILYER EMPLOYED IN PHOTOGEAPHY. 
 
 Section I. The Chemistry of the Salts of Silver. The pre- 
 paration and properties of the Nitrate of Silver of the 
 Chloride, Bromide, and Iodide of Silver. The Chemis- 
 try of the Oxides of Silver 123 
 
 Section II. The Photographic Properties of the Salts of 
 Silver. The action of Light upon Nitrate of Silver 
 upon Chloride, Bromide, and Iodide of Silver upon 
 organic Salts of Silver. Simple experiments with sen- 
 sitive Photographic Paper. Nature of the chemical 
 changes produced by light vipon the Compounds of 
 Silver 129 
 
 CHAPTER III. 
 
 ON THE DEVELOPMENT OF AN INVISIBLE IMAGE. 
 
 Simple experiments illustrating the process 137 
 
 Section I. The Substances employed as Developers. The 
 chemistry of the principal reducing agents, Gallic Acid, 
 Pyrogallic Acid, and the Protosalts of Iron. Reduc- 
 tion of Salts of Silver by Developing Agents. Con- 
 ditions which retard or otherwise modify the process 
 
 of reduction 138 
 
 Section II. The Formation and Development of the Latent 
 Image. Hypothesis on the nature of the image. 
 The mode of its development. Causes which affect 
 
CONTENTS. XI 
 
 Page 
 
 the sensitiveness of the plates. Fogging and other 
 irregularities of development. Moser's researches. 
 Experiments of Norris, Busk, and others. The Che- 
 mical nature of developed images 145 
 
 CHAPTER IY. 
 
 ON FIXING THE PHOTOGRAPHIC IMAGE. 
 
 Chemistry of the various substances which may be em- 
 ployed as Fixing Agents. Ammonia, Alkaline Chlo- 
 rides, Bromides, and Iodides. Hyposulphite of Soda. 
 Cyanide of Potassium. Sulphocyanides.: Relative 
 affinities of Chlorine, Iodine, and Hyposulphurous 
 Acid for Silver 166 
 
 CHAPTER V. 
 
 ON THE NATURE AND PROPERTIES OP LIGHT. 
 
 Section I. The compound Nature of Light, with the laws 
 of refraction. Decomposition of Light into elemen- 
 tary coloured rays. Phenomena of simple refraction 
 by parallel and inclined surfaces. The various forms 
 of Lenses. The Foci of Lenses. Formation of a Lu- 
 minous Image by a Lens. Enlarged and reduced 
 images. Lenses of long and short focus 172 
 
 Section II. The Photographic Camera. Its simplest form. 
 Chromatic aberration : visual and actinic focus. 
 Spherical aberration. Curvature of the field. Varia- 
 tion of focus for near and distant objects. The single 
 Actinic View Lens. The Aplanatic Lens. The Or- 
 thoscopic or Orthographic Lens. The Triplet Lens. 
 Sutton's Panoramic Lens. The Globe Lens. The 
 Petzval Orthoscopic Lens. The double, or Portrait 
 combination Lens. On distortion in Lenses . . . 180 
 
 Section III. On Actinism. Diagram of Chemical Spec- 
 trum. Illustrative experiments. Superior sensibility 
 
Xll CONTENTS. 
 
 Page 
 
 of Bromide of Silver to coloured light. Mode in 
 which dark-coloured objects impress the film. Actinic 
 
 transparency 198 
 
 Section IV. On Binocular Vision and the Stereoscope. 
 Phenomena of Binocular Vision. Theory of the Ste- 
 reoscope. Wheatstone's reflecting Stereoscope. 
 Brewster's Stereoscope. Eules for taking Stereoscopic 
 pictures 205 
 
 CHAPTEE VI. 
 
 COLLODION AND THE NITEATE BATH. 
 
 Section I. Collodion. Pyroxyline its chemical composi- 
 tion. Parchmentizing action of Oil of Vitriol. The 
 solution of the Pyroxyline in Ether and Alcohol. 
 The changes which Iodized Collodion undergoes by 
 keeping. Observations on some points of importance 
 in the manufacture of Photographic Collodion . . . 216 
 
 Section II. The Chemistry of the Nitrate Bath. Its sol- 
 vent action on Iodide of Silver. Acidity and Alka- 
 linity of the Nitrate Bath. Formation of Acetate of 
 Silver in the Bath. Contamination by organic mat- 
 ters. Use of fused Nitrate of Silver for the Bath . . 247 
 
 CHAPTEE VII. 
 
 ON POSITIVE AND NEGATIVE COLLODION PHOTOGEAPHS. 
 
 Definition of the terms Positive and Negative. The same 
 
 Photograph often capable of representing both varieties 255 
 
 Section I. Collodion Positives. Process with attenuated 
 films. Improved mode with Bromide in the Collo- 
 dion. Proper condition of the Nitrate Bath. Deve- 
 lopers for Collodion Positives. Fixing Agents. A 
 process for whitening Grlass Positives by means of 
 Chloride of Mercury 257 
 
 Section II. Collodion Negatives. Conditions which affect 
 
CONTENTS. Xlii 
 
 Page 
 
 the formation and development of the Negative. 
 Iodized and Bromo-iodized Negative Collodion. The 
 Nitrate Bath and developers suitable for Iodized Col- 
 lodion. Ditto for Bromo-iodized Collodion. Fixing 
 liquids. Artificial modes of strengthening Negatives . 267 
 
 CHAPTEE YIIL 
 
 ON THE THEORY OP POSITIVE FEINTING. 
 
 General observations 287 
 
 Section I. Printing the Proof. The stages of its darkening 
 by light. The function of each constituent of the sen- 
 sitive paper. Observations on the various conditions 
 which affect the sensitiveness of the paper, and the 
 vigour* of the image 288 
 
 Section II. Toning the Proof. Toning by Sulphur. The 
 Salts of Gold as toning agents. The alkaline Gold 
 Toning Bath with the more important of the condi- 
 tions which affect its action - ... 305 
 
 Section III. Fixing the Proof. The properties of Hypo- 
 sulphite of Silver. Conditions of a proper Fixing. 
 Injurious states of the Fixing Bath 313 
 
 Section IV. The Fading of Photographic Prints. The 
 causes which produce it. The comparative perma- 
 nency of Prints. The mode of testing permanency . 319 
 
 CHAPTEE IX. 
 
 COLLODION PEESEEVATIVE AND DEY COLLODION PEOCESSES. 
 
 The Method of Messrs. Spiller and Crookes. Shadbolt's 
 ^HoneyJProcess. The Oxymel Process. Dry Process 
 of the Abbe Despratz. Norris'a Eesearches. Tau- 
 penot's Collodio-Albumen Process. FothergilTs Pro- 
 cess. EusselT s Tannin Process. Collodion for the Dry 
 Processes. General Observations 327 
 
XIV CONTENTS. 
 
 CHAPTER X. 
 
 ON THE DAGUEEEEOTYPE AND OTHER PEOCESSES. 
 
 Page 
 
 Section I. The Daguerreotype. The nature of the sensi- 
 tive film. Development of the Latent Image. The 
 strengthening of the Image by means of Hyposulphite 
 of Gold 352 
 
 Section II. The Processes of Talbot, etc. The Calotype. 
 The waxed Paper Process of Le Grey. The Albumen 
 Negative Process 356 
 
 Section III. Photographic Processes by Sir John Herschel 
 and others. The Cyanotype. The Chrysotype. Ura- 
 nium Printing Process. Processes with Bichromate of 
 Potash. Carbon Printing. Photo-lithography. 
 Photo-zincography. Photo-galvanography, and Pho- 
 to-glyphy 362 
 
 PART III. 
 PRACTICE OF PHOTOGRAPHY UPON COLLODION. 
 
 CHAPTER I. 
 
 PHOTOGEAPHIC CHEMICALS. 
 
 Section I. Preparation of plain Collodion. Mode of 
 preparing soluble Pyroxyline by the Nitre process. 
 Commercial process by the mixed Acids. Manipula- 
 tions and Precautions to be observed. Positive and 
 Negative Collodion 373 
 
 Section II. Formulae for Solutions for direct Positives. 
 The iodized Collodion. The Nitrate Bath. Develop- 
 ing fluids. Fixing liquid. Varnishes 391 
 
 Section III. Formula for Negative Solutions. Iodized 
 Collodion and Nitrate Bath. Bromo-iodized Collo- 
 dion and Nitrate Bath. Developing fluids. Fixing 
 liquid.^Yarnish 398 
 
CONTENTS. XV 
 
 CHAPTEE II. 
 
 MANIPULATIONS AND PEOCESSES. 
 
 Page 
 
 Section I. Manipulations of the Collodion process. 
 Cleaning the Plates. Coating with Collodion. Exci- 
 ting. Exposing. Developing. Fixing. Varnishing 407 
 
 Section II. Positive and Negative Portraiture. Con- 
 struction of Glass Houses. Working in the open Air. 
 Arrangement of the apparatus. Management of 
 Portrait Lenses. Manipulations in taking Glass Posi- 
 tives. Observations on the Collodion, etc., for Nega- 
 tive Portraits. Carte-de-Visite Portraits .... 427 
 
 Section III. Landscape Photography. The apparatus. 
 
 The Chemicals. The manipulations 444 
 
 Section IY. On copying Engravings and Maps. Hints in 
 selecting the Lenses. The difficulties to be antici- 
 pated. The proper state of the chemicals . . . . 455 
 
 Section Y. Rules for taking Stereoscopic and instantaneous 
 Photographs. Latimer Clark's arrangement. Instan- 
 taneous Photography 465 
 
 Section YI. Photographic delineation of Microscopic Ob- 
 jects. Arrangement of apparatus. Mode of finding 
 the chemical Focus. Experience of various operators 473 
 
 Section YII. Photography in hot climates. Remarks on 
 a stable Pyroxyline. Use of Alcohol to retard Evapo- 
 ration of Collodion. Apparatus and package . . . 478 
 
 CHAPTEE III. 
 
 THE PRACTICAL DETAILS OF PHOTOGRAPHIC PRINTING. 
 
 Section I. The ordinary process of Positive Printing. Se- 
 lection of the paper. Preparation of Albuminized 
 paperof plain paper of Ammonio-Nitrate paper. 
 Preparation of the Alkaline Gold Toning Bath. Pre- 
 paration of the Fixing Bath. Manipulatory details of 
 Photographic Printing. Mode of washing and mount- 
 ing the Proofs. Vignetting 486 
 
 Section II. Positive Printing by Development. A process 
 on Iodide and Bromide of Silver. rProcess on Chloride 
 and Citrate of Silver. On Printing Transparencies . 507 
 
XVI CONTENTS. 
 
 CHAPTER IV. 
 
 CLASSIFICATION OF CAUSES OF FAILTJEE IN THE COLLODION 
 
 PEOCESS. 
 
 Page 
 
 Section I. Imperfections in Negative and Positive Col- 
 lodion Photographs. Fogging. Transparent and 
 opaque spots. Markings of all kinds. Under- and 
 over-exposure, etc. Defects in Collodion Positives . 517 
 Section II. Imperfections in Paper Positives .... 536 
 
 CHAPTER V. 
 
 DEY COLLODION AND COLLODIO-ALBTJMEN PEOCESSES. 
 
 Practical details of Russell's Tannin Process of FothergilTs 
 Process of Taupenot's Collodio- Albumen Process. 
 Modification of the latter by Messrs. Petschler and 
 Mann . 540 
 
 APPENDIX. 
 
 To find the equivalent focus of a lens and the angle of view 
 included. Quantitative Testing of Nitrate Baths. 
 Table of Specific Gravity of Nitrate of Silver solutions. 
 Recovery of Silver from waste solutions and resi- 
 dues. Reduction of Chloride of Silver. Reduction of 
 Sulphide of Silver. Separation of Silver and Gold. 
 Removal of Iodide of Silver from Nitrate Baths. 
 Mode of taking the Specific Gravity of Liquids. On 
 Filtration. The use of Test-papers. The removal of 
 Silver stains from the hands, etc. Cleansing of Bot- 
 tles. Removal of Varnish from finished pictures. 
 Lacquer for blacking brass-work. Dr. Ure's tables 
 of the strength of Sulphuric and Nitric Acid of dif- 
 ferent densities. Proportion of Absolute Alcohol in 
 Spirit of different densities. Baume's Hydrometer 
 Tables for liquids heavier and lighter than water. 
 Fahrenheit and Centigrade Thermometer scales. 
 Table of Weights and Measures. Twaddell's Hydro- 
 meter . , 561 
 
A MANUAL 
 
 OF 
 
 PHOTOGRAPHIC CHEMISTRY. 
 
 INTRODUCTION. 
 
 IN attempting to impart knowledge on any subject, it is 
 not sufficient that the writer should himself be acquainted 
 with that which he professes to teach. Even supposing 
 such to be the case, yet much of the success of his effort 
 must depend upon the manner in which the information is 
 conveyed ; for as, on the one hand, a system of extreme 
 brevity always fails of its object, so, on the other, a mere 
 compilation of facts imperfectly explained tends only to 
 confuse the reader. 
 
 A middle course between these extremes is perhaps the 
 best to adopt ; that is, to make selection of certain funda- 
 mental points, and to explain them with some minuteness, 
 leaving others of less importance to be dealt with in a more 
 summary manner, or to be altogether omitted. 
 
 But independently of observations of this kind, applying 
 to educational instruction in general, it may be remarked, 
 that there are sometimes difficulties of a more formidable 
 description to be overcome. For instance, in treating of a 
 science, like Photography, which involves numerous and 
 complex reactions, there is great danger of attributing 
 effects to wrong causes. Perhaps none but he who has 
 
 B 
 
2 INTRODUCTION. 
 
 himself worked in the laboratory can estimate this point in 
 its proper light. In an experiment where the quantities of 
 material acted upon are infinitesimally small, and the che- 
 mical changes involved of a most refined and subtle de- 
 scription, it is soon discovered that the slightest variation 
 in the usual conditions suffices to alter the result. 
 
 Nevertheless Photography is truly a science, governed 
 by fixed laws ; and hence, as our knowledge increases, we 
 may fairly hope that uncertainty will cease, and the same 
 precision be attained as that with which chemical opera- 
 tions are usually performed. 
 
 The intention of the author in writing this work, is to 
 impart a thorough knowledge of what may be termed the 
 " First Principles of Photography," that the amateur may 
 arm himself with a theoretical acquaintance with the sub- 
 ject before proceeding to its practice. To assist this ob- 
 ject, care will be taken to avoid needless complexity in the 
 formulae, and all ingredients will be omitted which are not 
 believed to be useful. 
 
 The impurities of chemicals will be pointed out as far as 
 possible, and special directions given for their removal. 
 
 Amongst the great variety of Photographic processes 
 which have been devised, those only will be selected which 
 are correct on theoretical grounds, and are found in prac- 
 tice to succeed. 
 
 As the work is addressed to one supposed to be unac- 
 quainted both with Chemistry and Photography, pains will 
 be taken to avoid the employment of all technical terms 
 of which an explanation has not previously been given. 
 
 A SKETCH OF THE MAIN DIVISIONS TO BE ADOPTED, WITH 
 THE PRINCIPAL SUBJECT-MATTER OF EACH. 
 
 The title given to the Work is " A Manual of Photo- 
 graphic Chemistry," and it is proposed to include in it a 
 familiar explanation of the nature of the various chemical 
 agents employed in the Art of Photography, with the ra- 
 tionale of the manner in which they are thought to act. 
 
INTRODUCTION. 3 
 
 The division adopted is threefold. 
 
 PART I. contains a statement of the Laws of Chemical 
 combination, an explanation of Chemical terms, and of 
 Symbolic notation (or short-hand) ; and also a list of Photo- 
 graphic chemicals alphabetically arranged, including their 
 preparation and properties as far as required for their 
 employment in the Art. 
 
 Although Photography may certainly be practised suc- 
 cessfully without a knowledge of Chemistry, yet a slight 
 acquaintance with this science is so material an assistance 
 in enabling a Photographer to explain, avoid, and escape 
 from his difficulties, that it is recommended to the reader 
 to take the trouble to master the first principles contained 
 in Chapter I., and also to refer to the Vocabulary when a 
 chemical is employed for the first time. 
 
 PART II., or "the Science of Photography," includes a 
 full desci'iption of the chemical action of Light upon the 
 Salts of Silver, with its application to artistic purposes ; 
 all mention of manipulatory details, and of quantities of 
 ingredients, being, as a rule, omitted. 
 
 In this division of the Work will be found ten Chap- 
 ters, the contents of which are as follows : 
 
 Chapter I. is a sketch of the history of Photography, 
 intended to convey a general notion of the origin and pro- 
 gress of the Art, without dwelling on minute particulars. 
 
 Chapter II. describes the Chemistry of the Salts of 
 Silver employed by Photographers ; their preparation and 
 properties ; the phenomena of the action of Light upon 
 them, with experiments illustrating it. 
 
 Chapter III. leads us on to the formation of an invisible 
 image upon a sensitive surface, with the development or 
 bringing out to view of the same by means of chemical 
 reagents. This point, being of fundamental importance, 
 is described carefully ; the reduction of metallic oxides, 
 the properties of the bodies employed to reduce, and the 
 hypotheses which have been entertained on the nature of 
 the Light's action, are all minutely explained. 
 
 Chapter IV. treats of the fixing of Photographic im- 
 
4 ' INTKODUCTION. 
 
 pressions, in order to render them indestructible by dif- 
 fused light. 
 
 Chapter V. contains a sketch of the Optics of Photo- 
 graphy the decomposition of white Light into its elemen- 
 tary rays, the Photographic properties of the different co- 
 lours, the refraction of Light, and construction of Lenses. 
 In the last Section of this Chapter will be found a short 
 sketch of the history and use of the Stereoscope. 
 
 Chapter VI. explains the chemistry of Pyroxyline, with 
 its solution in Alcoholized Ether, or Collodion ; also of the 
 Nitrate Bath, in which the photographic plates are ren- 
 dered sensitive to light. 
 
 Chapter VII. describes the classification of Collodion 
 Photographs as Positives and Negatives, with the distinc- 
 tive peculiarities of each, and the most approved modes of 
 obtaining them. 
 
 Chapter VIII. contains the theory of the production of 
 Positive Photographs upon paper. In this Chapter will 
 be found an explanation of the complex chemical changes 
 involved in printing Positives, with the precautions which 
 are required to ensure the permanency of the finished 
 proofs. 
 
 Chapter IX. is an epitome of all that is known of the 
 dry Collodion and Collodion preservative processes ; their 
 history, and the scientific principles upon which they are 
 founded. 
 
 Chapter X. is supplementary to the others, and a brief 
 notice of it will suffice. It explains the theory of the Pho- 
 tographic processes of Daguerre and Talbot ; especially 
 noticing those points in which they may be contrasted with 
 Photography upon Collodion, but omitting all description 
 of manipulatory details, which if included would extend 
 the Work beyond its proposed limits. Photo-lithography, 
 lately improved and perfected, is also alluded to in this 
 Chapter. 
 
 The title of the third principal division of the Work, 
 viz. " The practice of Photography upon Collodion," ex- 
 plains itself. Attention however may be invited to the 
 
INTEODUCTION. 5 
 
 fourth Chapter, in which a classification is given of the 
 principal imperfections in Photographs, with short direc- 
 tions for their removal. 
 
 The reader will at once gather from this sketch of the 
 contents of the volume before him, that whilst the general 
 theory of every Photographic process is described, with 
 the preparation and properties of the chemicals employed, 
 minute directions in the minor points of manipulation are 
 restricted to Photography upon Collodion, that branch of 
 the Art being the one to which the time and attention of 
 the author have been especially directed. Collodion is al- 
 lowed by all to be a good vehicle for the sensitive Silver 
 Salts, and successful results can be obtained with a very 
 small expenditure of time and trouble, if the solutions em- 
 ployed in the process are prepared in a state of purity. 
 
PART I. 
 
 OUTLINES OF GENERAL CHEMISTRY. 
 
OUTLINES OF GENERAL CHEMISTRY. 
 
 CHAPTER I. 
 
 ALL substances with which we are acquainted, whether 
 solid, liquid, or gaseous, are either elements, or compounds 
 of elements, or mixtures of elements, or mixtures of com- 
 pounds. It is necessary, in the first place, to understand 
 these terms clearly. 
 
 ELEMENTS. 
 
 An Element is a substance which consists of one kind of 
 matter only, or which has never yet been decomposed or 
 separated into two or more kinds of matter. The number 
 of such simple substances at present known is 64 ; of these 
 however only the following 34 need be mentioned, as being 
 the most common, and as including all that are alluded to 
 in this Work. The meaning of the symbols and atomic 
 weights attached to each element will be explained after- 
 wards. 
 
 Symbol. Atomic Wt. 
 
 Oxygen O 8 
 
 Hydrogen H 1 
 
 Non-Metallic, Nitrogen N 14 
 
 Elements. 1 Chlorine 01 35'5 
 
 Iodine I 127 
 
 Bromine Br 80 
 
10 
 
 THE CHEMICAL ELEMENTS 
 
 Symbol. Atomic Wt. 
 
 Fluorine F 19 
 
 Non-Metallic I Carbon C 6 
 
 Elements ( Boron B 11 
 
 continued. Phosphorus P 31 
 
 I Sulphur S 16 
 
 Potassium K 39 
 
 Sodium ...... Na 23 
 
 Lithium L 7 
 
 Barium Ba 68'5 
 
 Calcium Ca 20 
 
 Strontium Sr 43'8 
 
 Magnesium Mg 12' 16 
 
 Aluminium Al 13*75 
 
 Chromium Cr 26'3 
 
 Iron Fe 28 
 
 Zinc Zn 32'7 
 
 Manganese Mn 27'5 
 
 Uranium U 60 
 
 Cadmium Cd 56 
 
 Copper Cu 31'7 
 
 Lead Pb 103'5 
 
 Tin Sn 59 
 
 Antimony Sb 122 
 
 Arsenic As 75 
 
 Gold Au 196-6 
 
 Mercury Hg 100 
 
 Silver Ag 108 
 
 Platinum . Pt 98 '6 
 
 Metals. 
 
 COMPOUNDS AND MIXTUEES. 
 
 These elementary substances are capable of uniting 
 with each other to form Compounds, which differ from 
 their elements in appearance and properties, the more com- 
 pletely that the latter were opposed to each other in their 
 properties. Thus the gaseous element Chlorine is capable 
 of combining with the metal Sodium, to form with it the 
 well-known solid, common Salt, in which neither a gas nor 
 
AND THEIE COMBINATIONS. 11 
 
 a metal can be detected by the senses. The black solid, 
 Iodine, unites with the metal Potassium, to form the white 
 compound Iodide of Potassium. The two gases Oxygen 
 and Hydrogen form by their union, the liquid, Water. 
 Chemical combination is often attended with heat and light, 
 the union of bodies with the Oxygen of the air, when ac- 
 companied with these phenomena, being termed combustion. 
 
 Mixture differs entirely from Chemical combination, 
 in that it leaves the properties of the ingredients compara- 
 tively unchanged and perceptible to the senses, it is not 
 attended with heat, excepting that which may be due to 
 the friction used, and very simple means usually serve to 
 separate the ingredients again. Thus gunpowder is an 
 example of the most perfect mixture of the three ingre- 
 dients, Charcoal, Nitrate of Potash, and Sulphur ; yet the 
 sense of taste will detect the Nitre, the Sulphur may be 
 discovered by the smell when rubbed, and the eye perceives 
 the colour of the Charcoal. Sulphur and finely-divided Iron 
 might be so intimately mixed, that the two ingredients 
 could not be separately distinguished by the eye, but the 
 Iron might still be separated by a magnet from the Sulphur; 
 if however true combination were brought about by the 
 application of a gentle heat, a great increase of heat is 
 suddenly perceived, and if the materials were in the right 
 proportions, the magnet would no longer discover any Iron 
 in the resulting compound. Hydrogen and Oxygen gases 
 may be mixed, and they will remain gases still, but when 
 combined the gaseous character is lost, and the liquid, water, 
 results. 
 
 Of the compounds produced by the union of two ele- 
 ments, the class of Oxides, formed by the combination of 
 Oxygen with another element, is the most important. 
 These Oxides form three important classes, namely Bases, 
 Acids, and neutral Oxides, each of which must be sepa- 
 rately considered. 
 
 The Basic Oxides are found only among the metallic 
 
12 THE CHEMICAL ELEMENTS 
 
 Oxides, although the whole of the metals do not furnish 
 basic Oxides. The most perfect examples of Bases are 
 the Oxides of the first seven metals given in the list of 
 elements ; the Oxides of the first three, Potassium, Sodium, 
 and Lithium, known as Potash, Soda, and Lithia, are dis- 
 tinguished from other bases by the term "Alkali;" the 
 Oxides of the other four are known as " Alkaline " Earths. 
 The Alkalies and Alkaline Earths exhibit all the true 
 characteristics of bases in perfection, viz. causticity and 
 corrosiveness, and the power of uniting with another class 
 of compounds, the Acids, equally caustic and corrosive, and 
 of producing compounds comparatively inactive and harm- 
 less, they possess also the property of changing the red of 
 Litmus to blue, and the yellow of Turmeric to brown. The 
 property of Litmus and Turmeric in changing colour with 
 bases and acids has caused them to be used for " test- 
 papers," which serve to detect an exceedingly minute 
 amount of one of the stronger bases, or acids in a free 
 state. The other metallic oxides termed bases exhibit the 
 basic character in very variable degrees, some, such as the 
 Oxides of Silver and Lead, " neutralize " the strongest acids 
 perfectly, so that the product no longer affects test-paper ; 
 others, such as the Oxides of Zinc and Iron, neutralize 
 acids, but imperfectly, so that the products of the combina- 
 tion always show an Acid reaction (see ACIDS) with test- 
 paper. (With regard to Ammonia, see the Vocabulary.) 
 
 ACIDS. 
 
 The most important of the Acid Oxides are those of non- 
 metallic elements ; only a few of the metals, as Arsenic, 
 Chromium, and Manganese, furnishing acid oxides. Some 
 metals, as Chromium and Manganese, have both basic and 
 acid oxides ; when this is the case, the acid oxide always 
 contains the most Oxygen. 
 
 Taking the Oxides of Nitrogen and Sulphur known as 
 Nitric and Sulphuric Acids as types, the characteristics of 
 acids may be stated to be, intense sourness and corro- 
 siveness, the power of " neutralizing" the strongest bases, 
 
AND THEIB COMBINATIONS. 13 
 
 so that the product no longer affects test-paper, and the 
 property of changing blue Litmus to red, even when ex- 
 tremely dilute. The acid oxides, like the bases, possess 
 these characters in very different degrees, the weaker bases 
 and acids approaching each other in character, and in the 
 same proportion, having little tendency to combine. 
 
 The stronger Oxygen Acids always retain, even in their 
 concentrated state, a certain amount of water, which is 
 essential to them. If deprived of this, they form what are 
 termed " Anhydrides," which are essentially different from 
 acids. 
 
 Hydrogen Acids. While speaking of acids, it may be 
 as well to mention that besides the acids above alluded 
 to, which are Oxides, there are others which contain no 
 Oxygen, but are compounds of Hydrogen with certain 
 elements, chiefly Chlorine, Bromine, Iodine, Fluorine, and 
 Sulphur, and with certain compounds which perform many 
 of the functions of these elements (see CYANOGEN, in Voca- 
 bulary). These acids are distinguished by the prefix 
 Hydro-, the rest of the name being derived from the other 
 element of the acid ; thus the compound of Chlorine and 
 Hydrogen is termed Hydrochloric Acid, that of Hydrogen 
 and Iodine, Hydriodic Acid. Most of these Hydrogen 
 Acids are gaseous bodies, and the liquids known under 
 their names are solutions of the same in water. 
 
 NEUTBAL OXIDES. 
 
 Besides forming Acid and Basic Oxides, Oxygen also 
 unites ki a few cases with elements to form neutral bodies 
 which have no tendency to unite with either acids or bases. 
 Such are two of the Oxides of Nitrogen, the Oxide of 
 Carbon, and the black Oxide of Manganese. 
 
 SALTS. 
 
 The combination of an Acid with a Base produces what 
 is termed a Salt. This class of compounds is a very large 
 and important one, and includes many substances that 
 would not ordinarily be recognized under that term. 
 
14 THE CHEMICAL ELEMENTS 
 
 When the affinities of the acid and base of the salt are 
 about equal, as is the case, for example, with Sulphuric Acid 
 and Potash in Sulphate of Potash, the salt has no action 
 on test-paper. In many cases however the reaction of the 
 acid or base preponderates ; thus Sulphate of Iron, a com- 
 pound of a powerful acid with the weak base Oxide of 
 Iron, always has an acid reaction : Carbonate of Soda, on 
 the other hand, consists of a strong base and a feeble acid, 
 the Carbonic, and therefore is always alkaline to test-paper. 
 Both of these salts, though far from neutral in reaction, are 
 yet neutral in composition. (See NOMENCLATURE, p. 22.) 
 Salts, derived from Oxygen Acids, always contain the 
 whole of the acid and base, but with Hydrogen Acids the 
 case is different ; in these, the Hydrogen of the acid unites 
 with the Oxygen of the base to form water, which sepa- 
 rates on evaporation to dryness, and the other element of 
 the acid unites with the metal of the base to form what 
 is termed a " Haloid Salt," a name derived from a\s, aXos 
 (Halos), Sea Salt, because common Salt, Chloride of Sodium, 
 is a familiar example. This compound may be formed by 
 the union of Hydrochloric Acid and Soda(Oxide of Sodium), 
 and would be termed Hydrochlorate of Soda if it retained 
 the acid and base entire, but as each is decomposed, one 
 giving up its Hydrogen, and the other its Oxygen to form 
 water, which separates, the Chlorine of the acid and So- 
 dium of the base alone remain, and the product of their 
 combination is termed Chloride of Sodium, which contains 
 no longer either acid or base, and yet is a true Salt. 
 Examples of such salts are Iodides, Bromides, Fluorides, 
 and Chlorides of the various metals. The Haloid salts 
 however, when decomposed, yield products similar to the 
 Oxyacid salts. For instance, if Iodide of Potassium be 
 dissolved in water, and dilute Sulphuric Acid added, this 
 acid, being powerful in its chemical affinities, tends to appro- 
 priate to itself the alkali ; it does not however remove Po- 
 tassium and liberate Iodine, but takes the Oxide of Potas- 
 sium and sets free Hydriodic Acid. In other words, as 
 an atom of water is produced during the formation of a 
 
AND THEIE COMBINATIONS. 15 
 
 Hydracid salt, so are the elements of water separated in 
 the decomposition of a Hydracid salt. The reaction of 
 dilute Sulphuric Acid upon Iodide of Potassium may be 
 stated thus : 
 
 Sulphuric Acid plus (Iodine Potassium) plus (Hydrogen Oxygen 
 equals (Sulphuric Acid, Oxygen Potassium) or Sulphate of Potash, 
 and (Hydrogen Iodine) or Hydriodic Acid. 
 
 All metallic salts with Oxygen acids contain the metal 
 in the state of Oxide, although in the name of the salt the 
 word Oxide is omitted ; thus Sulphate of Iron is strictly 
 Sulphate of Oxide of Iron, and Nitrate of Silver, Nitrate 
 of Oxide of Silver. The Oxides of the Alkaline and Earthy 
 Metals, being known long before their Elementary Metals 
 were discovered, have each a name more familiar than that 
 of the metals, hence in the salts with Oxygen acids the 
 former name is employed rather than the latter ; thus Ni- 
 trate of Lime is spoken of, rather than Nitrate of Oxide of 
 Calcium. In the haloid salts of these elements, the name 
 of the metal is necessarily made use of; thus Hydrochloric 
 Acid and Lime form Chloride of Calcium (and water). 
 
 Since Acids do not combine with Metals, but only with 
 their Oxides, every metal when dissolving in an acid 
 obtains Oxygen, either from the water or from the acid, 
 or, in the case of Hydracids, displaces the Hydrogen of the 
 acid, and combines with the remaining element of the 
 acid. Thus when Zinc or Iron dissolves in dilute Sul- 
 phuric Acid, water is decomposed, the Oxygen combining 
 with %e metals to form oxides, which unite with the 
 acid, and the other element of the water, Hydrogen, es- 
 capes with effervescence. When the same metals dissolve 
 in a Hydrogen acid, as Hydrochloric, Hydrogen is evolved 
 from the acid, and the remaining element, Chlorine, forms 
 with the Zinc or Iron, Chloride of Zinc or Iron. On the 
 other hand, when Silver dissolves in Nitric Acid, the Oxy- 
 gen required to form Oxide of Silver is derived from the 
 acid, which is thus reduced to a lower Oxide of Nitrogen, 
 which escapes with effervescence. 
 
16 THE CHEMICAL ELEMENTS 
 
 LAWS OF COMBINATION BY WEIGHT. 
 
 These are four in number, and they are of much import- 
 ance. 
 
 1. Law of Definite Proportion. " Every Chemical com- 
 pound has a perfectly definite composition, and the same 
 compound, from whatever source derived, always has the 
 same elements in the same proportions." Thus, water 
 from any part of the world, when purified, will be found 
 to contain 88'9 of Oxygen, and 11*1 of Hydrogen in 100 
 parts. 
 
 2. Law of Multiple Proportions. "If one body can 
 combine with another in more than one proportion, the 
 other proportions are either double, treble, etc., of the first, 
 or bear some nearly equally simple proportion." This may 
 be illustrated by the series of compounds of Oxygen and 
 Nitrogen. 
 
 Nitrogen. Oxygen. 
 
 Protoxide of Nitrogen . . JU] + 8 = (Tj 
 
 Deutoxide of Nitrogen . . |14J +16 = | 8J8"| 
 
 Nitrous Acid |14| + 24 = |1F~ 
 
 Peroxide of Nitrogen . . . 
 
 Nitric Acid ..'.... |14| +40= |8 8 8|8|8| 
 
 Here it is seen that Chemical combination of two bodies, 
 if in more than one proportion, is in successive steps ; thus, 
 14 of Nitrogen having taken up 8 of Oxygen, the next 
 compound contains another whole 8, and so on with each 
 in succession. An example of a little less simple series of 
 combinations is seen in the Oxides of Iron. 
 
 Iron. Oxygen. 
 
 Protoxide of Iron . . 28 +8 
 
 Sesquioxide .... 56 (=28x2) +24 (=8x3) 
 Ferric Acid .... 28 +24 (=8x3) 
 
 3. Law of Equivalent Proportions. " If certain bodies, 
 as A, B, and C, unite each separately with another, X, 
 the proportions in which they combine with X to form 
 their simplest compounds, are the same (or some simple 
 
AND THEIR COMBINATIONS. 17 
 
 proportion of the same) in which they combine with each 
 other to form their simplest compounds." 
 
 Thus 8 of Oxygen combines with 35*5 of Chlorine to 
 form Hypochlorous Acid, and with 1 of Hydrogen to form 
 water ; and Chlorine and Hydrogen themselves combine in 
 these same proportions, 35 '5 of Chlorine to 1 of Hydro- 
 gen (Hydrochloric Acid). 
 
 Again, 8 of Oxygen combines with 39 of Potassium, 39 
 of Potassium with" 127 of Iodine, 127 of Iodine with 108 
 of Silver, and 108 of Silver combines with 8 of Oxygen to 
 form Oxide of Silver. 
 
 From the above statements it is evident that every ele- 
 ment has a certain fixed proportion in which (or in mul- 
 tiples of which) it combines with other elements, and that 
 if a certain number be fixed on as the combining proper* 
 tion of one element, the combining proportions of all the 
 other elements may also be represented by fixed numbers. 
 Such numbers have been assigned to each element ; in 
 this country Hydrogen, having the smallest combining 
 number, has the combining number 1 assigned to it, and 
 the numbers of other elements are in proportion ; these 
 are the numbers attached to each element under the head 
 of " Atomic Weight," in the list (page 9). The reason for 
 calling these numbers " atomic weights " will be explained 
 shortly. 
 
 4. Law of Combining numbers of Compounds. The com- 
 bining number, or atomic weight, of a compound is the 
 sum of the combining numbers of its constituents. Thus 
 8 of Oxygen with 1 of Hydrogen forms water, of which the 
 atomic weight is therefore 8 + 1 = 9. Nitric Acid consists 
 of 14 of Nitrogen and 8 X 5 = 40 of Oxygen, it has there- 
 fore the atomic weight of 40 + 14= 54. 
 
 ATOMIC THEORY. 
 
 The remarkable facts stated in these laws have led to a 
 conjecture or theory, termed the Atomic Theory, that 
 however minutely divisible matter may be, it is notprac- 
 tically infinitely divisible, but that it consists of ultimate 
 
 c 
 
18 THE CHEMICAL ELEMENTS 
 
 particles which never are divided in all the changes which 
 matter undergoes, and which are therefore termed atoms, 
 from aropos (uncut). It is supposed that the simplest 
 combinations of two elements consist of 1 atom of one 
 united to 1 atom of the other, and as the numbers attached 
 to the elements represent the proportions of the simplest 
 combinations, they also represent the proportional weights 
 of the atoms of each of the elements, or their atomic 
 weights. Thus water consists of 8 of Oxygen and 1 of 
 Hydrogen, and if it be assumed that it is a compound of 
 an equal number of atoms of each element, then it fol- 
 lows that if an atom of Hydrogen weigh 1, an atom of 
 Oxygen weighs 8. 
 
 In describing the composition of a substance, it is com- 
 mon to speak of it as consisting of one atom of one element 
 combined with one atom (or more) of another element, 
 whatever the quantity of the substance may be, meaning 
 that the minutest particle or atom of it has this composition. 
 
 The indivisibility of the atoms explains the step-like 
 character of successive combinations of two elements, of 
 which examples were given in page 16. The atomic struc- 
 ture of compounds may be represented as in the following 
 diagram. 
 
 Fig. 1 is a compound atom of Sulphuric Acid, consisting 
 of an atom of Sulphur united with 3 atoms of Oxygen ; 
 Fig. 2 is an atom of Peroxide of Nitrogen, Nitrogen 1 atom, 
 Oxygen 4 atoms ; and Fig. 3, an atom of Nitric Acid, com- 
 posed of Nitrogen 1 atom, Oxygen 5 atoms. ~* 
 
 Fig. 1. Pig. 2. Fig. 3. 
 
 Practical application of the Laws of Combination. The 
 
AND THEIE COMBINATIONS. 19 
 
 utility of being acquainted with the law of combining pro- 
 portions is obvious when their nature is understood. As 
 bodies both unite with and replace each other in equiva- 
 lents, a simple calculation shows at once how much of each 
 element or compound will be required in a given reac- 
 tion. Thus, supposing it be desired to convert 100 grains 
 of Nitrate of Silver into Chloride of Silver, the weight of 
 Chloride of Sodium which will be necessary is deduced 
 thus : one equivalent, or 170 parts, of Nitrate of Silver 
 is decomposed by an equivalent, or 58*5 parts, of Chloride 
 of Sodium. Therefore as 170 I 58'5 : ; 100 ; 34'4 ; that is, 
 34'4 grains of salt will precipitate, in the state of Chloride, 
 the whole of the Silver contained in 100 grains of Nitrate. 
 
 So again, in order to form the Iodide of Silver, what are 
 the proportions in which the two salts should be mixed ? 
 The equivalent of Iodide of Potassium is 166, and that of 
 Nitrate of Silver is 170. These numbers so nearly corre- 
 spond, that it is common to direct that equal weights of the 
 two salts should be taken. 
 
 One more illustration will suffice. Supposing it be re- 
 quired to form 20 grains of Iodide of Silver, how much 
 Iodide of Potassium and Nitrate of Silver must be used ? 
 One equivalent, or 166 parts, of Iodide of Potassium, will 
 yield an equivalent, or 235 parts, of Iodide of Silver ; 
 therefore as 235 : 166 : : 20 ; 14" 1. Hence, if 14' 1 grains 
 of the Iodide of Potassium be dissolved in water, and an 
 equivalent quantity, viz. 14'5 grains, of the Nitrate of 
 Silver added, the yellow precipitate, when washed and 
 dried, will weigh precisely 20 grains. 
 
 SYMBOLS AND FOEMULJ3. 
 
 The understanding of the composition and changes of 
 chemical compounds is greatly facilitated by the use of 
 Symbols to represent the atoms of the elements in a com- 
 pound. An atom of an element is represented by a letter, 
 usually the first letter of its name, as C for an atom of 
 Carbon, O for an atom of Oxygen ; sometimes the symbol 
 is derived from the Latin name, as Pb for Lead (Plum- 
 
20 THE CHEMICAL ELEMENTS 
 
 bum), Cu for Copper (Cuprum). A number prefixed, as 
 2 O, or following, but below the line, as O 2 , signifies 2 
 atoms of Oxygen = 16. Two symbols placed side by side 
 represent combination, thus H Cl represents a compound 
 of 1 atom of Hydrogen with 1 atom of Chlorine. Sym- 
 bols of compounds are termed Formula. In such com- 
 pounds . the metal is always placed to the left, and the 
 Oxygen, Chlorine, or other non-metallic body, to the right ; 
 thus Fe O or Zn Cl instead of O Fe or Cl Zn. In like man- 
 ner, in the formulae of Salts the base is placed to the left, 
 and the acid to the right. Small figures placed below the 
 line relate only to the elements which they follow ; thus 
 Fe 2 O 3 represents 2 atoms of Iron combined with 3 atoms 
 of Oxygen, but large numbers placed before a compound 
 relate to all the elements until the next comma, or full- 
 stop, or sign of -f . Thus 3Ca,PO 5 represents 3 atoms 
 of Lime combined with 1 atom of Phosphoric Acid. When 
 large figures precede formulae enclosed in brackets, they 
 affect the whole, whether commas, full-stops, or signs of 
 -f be present or not. Thus 2 (Fe 2 O 3 , 3 S O 3 ) signifies 2 atoms 
 of the compound of 3 of Sulphuric Acid with 1 of Sesquioxide 
 of Iron. The sign + represents less intimate combination ; 
 thus FeO,SO 3 + 7HO represents crystallized Sulphate of 
 Iron, in which the 7 HO can be separated readily by heat, 
 leaving the Sulphate of Iron undecomposed. The same 
 sign + is also used in equations to represent addition or 
 mixture ; thus NaCl + Ag O, NO S = Ag Cl + Na O, NO 5 sig- 
 nifies a mixture of Chloride of Sodium with Nitrate of Sil- 
 ver, producing a mixture of Chloride of Silver and Nitrate 
 of Soda.- 
 
 NOMENCLATURE OF COMPOUNDS. 
 
 Compounds of two Elements. A compound of two ele- 
 ments is distinguished by the name of the first termina- 
 ting in -ide; thus FeO = Oxide of Iron, NaCl=Chlon'cfe 
 of Sodium, K 1= Iodide of Potassium. The same termi- 
 nation is also used with compound bodies which possess 
 neither basic nor acid characters when they unite with 
 
AND THEIR COMBINATIONS. 21 
 
 each other or with an element ; thus Cyanogen, C 2 N, a 
 compound neither acid nor basic, unites with Potassium, 
 forming Cyanide of Potassium, K, C 2 N or KCy. So also 
 the salts produced by the combination of Hydrogen acids 
 with bases, since they are compounds of only two elements, 
 in consequence of the separation of Hydrogen and Oxy- 
 gen as water, are named in the same way. Salts of this 
 kind may be produced either by the union of two elements, 
 or by the union of an acid and base. 
 
 K + I=ZI KO + HI=KI+HO. 
 
 Na + Cl=NaCl 
 
 The combinations of Sulphur, Carbon, and Phosphorus 
 with other elements used to be distinguished by the ter- 
 mination -uret, as FeS=Sulphwretf of Iron, but the ter- 
 mination -ide is now generally used with all binary com- 
 pounds, or compounds of two elements. Phosphide, or 
 Sulphide, or Carbide of Iron, are now the terms used in- 
 stead of Phosphuret, Sulphuret, and Carburet of Iron. 
 
 When one element combines with another in more than 
 one proportion, the compound which consists of an atom 
 of each is distinguished by the prefix proto-, as FeCl= < 
 .Profochloride of Iron, N O = Protoxide of Nitrogen ; the 
 compound of 3 atoms of the first-named* with 2 atoms of 
 the second element has the prefix sesqui-, as l?e 2 C] 3 = Ses- 
 quichloride of Iron ; the prefix hi- or bin- signifies 2 atoms 
 of the first to 1 of the second, and per-, the compound, 
 not acid, which contains the largest amount of the same 
 element; thus, MnO 2 is the Binoxide and also the Per- 
 oxide of Manganese, as it is the highest non-acid oxide of 
 that metal. 
 
 Compounds which contain more than one atom of the 
 second-named element to one of the first, are classified as 
 * w6-compounds ; the quantity is defined by other prefixes, 
 as di- and tri-, which are the reverse of bi-' and ter-, and 
 
 * First and second relates to the order of the elements in the names of 
 the compounds, and not to their order in the formulae. 
 
22 THE CHEMICAL ELEMENTS 
 
 signify one atom of the first-named to two and three re- 
 spectively of the other element, thus S 2 Cl=_D/chloride of 
 Sulphur. 
 
 Acids and Salts. If but one acid is known as resulting 
 from the union of two elements, its name ends in -ic; this 
 is always the case with the Hydrogen acids. In the case 
 of Oxygen acids, when there is a series of acids produced 
 by the combination of an element with Oxygen, that 
 which contains the largest amount of Oxygen terminates 
 in -ic, the next which contains less ends in -oiis, and if 
 there be a third with still less the prefix hypo- is used : 
 thus NO 5 = Nitric Acid, NO 3 = Nitron Acid; SO 3 = 
 Sulphuric Acid, SO 2 = Sulphurous Acid, and S 2 O 2 = Hypo- 
 sulphuroMs Acid. 
 
 Sometimes after distinguishing the highest known acid 
 of a series with the termination -ic, another acid is dis- 
 covered with still more Oxygen, in this case the new com- 
 pound has the prefix per- attached; thus, after C1O 5 = 
 Chloric Acid was named, another acid, C1O 7 , was discovered, 
 and named Perchloric Acid. 
 
 In the name of a salt the -ic of the acid is changed into 
 -ate, and -ous into -ite ; thus, Sulphuric Acid forms Sul- 
 phates, and Sulphurows Acid forms Sulphas ; the prefixes 
 of the acids, as Hypo- or Per-, being retained in the salt, 
 as .Hyposulphite of Soda, Permanganate of Potash. 
 
 A chemically neutral salt is one which has as many 
 atoms of acid as there are atoms of Oxygen in the base, 
 thus FeO,SO 3 and Fe 2 O 3 ,3SO 3 are each chemically neu- 
 tral (though from the weakness of their bases, quite acid to 
 test-paper) ; such salts are most correctly distinguished as 
 Sulphate of the Protoxide, and Sulphate of the Sesquioxide 
 of Iron, the name of the oxide indicating the number of 
 atoms of acid they must contain ; commonly, however, the 
 prefix which distinguishes the oxide is transferred to the 
 acid, and the above-named salts are thus called Proto- 
 sulphate and Sesquisulpli&te of Iron. Prefixes to the acid 
 in a salt ought properly to be confined to those cases in 
 which there are not the same number of atoms of acid as 
 
AND THEIE COMBINATIONS. 23 
 
 there are of Oxygen in the base, the prefixes sesqui-, bi-, 
 ter-, di-, and tri- having the same meanings as in the case 
 of compounds of two elements (page 21), the three first 
 being applied to chemically acid, and the two latter to 
 chemically basic salts; thus KO, 2 CrO 3 = -Bichromate of 
 Potash; EX), 2 SO 3 = ^sulphate of Potash (fused); and 
 2PbO,NO 5 = Z)mitrate of Lead, are rightly named. 
 
 In salts derived from Hydrogen acids, since they are 
 binary compounds, the prefix of the oxide is necessarily 
 transferred to the residue of the acid ; thus Hydrochloric 
 Acid forms with Sesquioxide of Iron, Sesquichloride of 
 Iron (and water). Fe 2 O 3 + 3HCl= Fe 2 Cl 3 + 3HO. Basic 
 salts of Hydrogen acids necessarily contain some oxide, 
 and are therefore distinguished by the prefix Oxy-, thus : 
 2ZnO + HCl = HO + (ZnCl,ZnO=O^chlorideofZinc). 
 
 Bibasic and Tribasic Acids. The rule that a neutral 
 salt contains one atom of acid for every one atom of base, 
 is true of the generality of acids, such being termed 
 Monobasic; acids however exist, one atom of which re- 
 quires two atoms of base to form a neutral salt; these acids 
 are termed Bibasic. Examples are seen in Pyrophosphoric 
 Acid, PO 5 , and Tartaric Acid, of which one atom is re- 
 presented by T. Neutral Pyrophosphate and Tartrate 
 of Soda have the composition : 
 
 Pyrosphate Soda . . . 2NaO,PO 5 . 
 Tartrate Soda .... 2NaO,T. 
 
 There are again other acids, one atom of which re- 
 quires three atoms of base to form a neutral salt. Ordi- 
 nary Phosphoric Acid and Citric Acid are examples, 
 their neutral salts of Lime having the formulae : 
 
 Phosphate of Lime . . . 3CaO,PO 5 . 
 Citrate of Lime . . . . 3CaO,Clt7 
 
 CHEMICAL CHANGES. 
 
 Chemical changes, whether they be the result of com- 
 bination or of decomposition, or of interchange of ele- 
 
24 THE CHEMICAL ELEMENTS 
 
 rnents between compounds, are often attended with striking 
 phenomena, such as the evolution of light and heat, or 
 with remarkable alterations of condition, as from gas to 
 liquid, or from liquid to solid, etc. ; but the changes most 
 familiar to the Photographer and the Analyst are those 
 of colour and of solubility. With regard to the latter, it 
 is well here to explain one or two terms. A body is said 
 to dissolve in a liquid, when it disappears in the liquid, 
 leaving the latter transparent, even though it may be 
 coloured. Two transparent solutions, when mixed, often 
 produce a precipitate, a term which is applied to inso- 
 luble matter suddenly appearing in a liquid previously 
 clear, and rendering it opaque and turbid. 
 
 Precipitates can be separated either by Decantation or 
 Filtration. The first method, Decantation, is, allowing 
 the precipitate to fall by its own weight to the bottom of 
 the liquid, and then pouring the latter off. Filtration is 
 effected by pouring the whole liquid and precipitate on a 
 piece of porous paper, folded into a conical form, termed 
 a Filter, and supported in a Funnel ; the liquid passes 
 through the paper in a transparent state, and the Pre- 
 cipitate remains upon it. But chemical changes are here 
 referred to chiefly for the purpose of pointing out the 
 varieties of internal change which bodies may undergo 
 when brought into contact with each other ; these in- 
 ternal changes manifesting themselves often in the visible 
 phenomena above .alluded to. Alterations of composition 
 may usually be referred to one of the following heads : 
 
 1. Simple and direct union of two elements. Thus 
 Sulphur combines with the Oxygen of the atmosphere 
 when heated to a certain point, the union being known as 
 "burning," and the product is Sulphurous Acid, SO 2 . 
 The element Iron "rusts" by combining with Oxygen. 
 
 2. Union of compounds with elements. This Sulphu- 
 rous Acid, S O 2 , by exposure to air can combine with more 
 Oxygen. S O 2 + O = S O 3 = Sulphuric Acid. Protoxide of 
 Iron, FeO, by exposure to air becomes Fe 2 O 3 . 
 
AND THEIE COMBINATIONS. 25 
 
 3. Union of compounds with each other, as in the for- 
 mation of salts with Oxygen Acids. 
 
 KO + SO 3 =KO,SO 3 Sulphate of Potash. 
 
 Two salts sometimes unite together to form a double 
 salt. Alum is an example, it is a double salt of Sulphate 
 of Alumina and Sulphate of Potash. 
 
 The double Chloride of G-old and Sodium is another 
 example, AuCJ 3 + NaCl + 4HO. 
 
 4. Displacement, or single decomposition, is where one 
 element or compound displaces another element or com- 
 pound from its state of combination. 
 
 Thus Chlorine acting on Iodide of Potassium, imme- 
 dately takes the Potassium, and sets Iodine free, a change 
 which is rendered evident from the black colour of the 
 Iodine. !KI+C1 = KC1+L 
 
 One metal often precipitates another from its solution 
 by displacing it ; thus Zinc placed in a solution of Ace- 
 tate of Lead combines with the Acetic Acid and Oxygen 
 of the Oxide of Lead, and is dissolved, while the latter is 
 thrown down in an arborescent form, known as the " Lead 
 tree." PbO, Acet + Zn=ZnO, Acet + Pb. 
 
 A piece of metallic Copper, placed in a solution of Ni- 
 trate of Silver, will speedily throw down metallic Silver, 
 and form Nitrate of Copper. 
 
 One Oxide is often used to separate another Oxide ; 
 thus Oxide of Silver is obtained by adding a solution of 
 Potash to a solution of E itrate of Silver. 
 
 5. Double decomposition is the mutual interchange of 
 the atoms of two compounds ; in the case of solutions, this 
 is often attended with precipitation ; thus, in the pre- 
 paration of Nitrate of Iron from Nitrate of Baryta and 
 Sulphate of Iron, there is a mutual interchange, resulting 
 in Nitrate of Iron and Sulphate of Baryta, which is pre- 
 cipitated. 
 
 FeO,SO 3 + BaO,NO.=:FeO,NO 5 + BaO,SO 3 . 
 
 In the sensitizing of Salted Paper with Nitrate of Silver, 
 
26 THE CHEMICAL ELEMENTS 
 
 Chloride of Silver is precipitated in the Paper, the change 
 being AgO,NO 6 + NaCl = NaO,NOj+AgCl. 
 
 6. Substitution. This is only a variety of displacement 
 or of double decomposition, but the term is used chiefly 
 in cases where one of the compounds that has undergone 
 the displacement, is little, if at all, altered in appearance, 
 as in the case of Cotton transformed into Gun-cotton. 
 Here a certain number of atoms of Hydrogen have been 
 displaced by NO 4 = Peroxide of Nitrogen, but the ap- 
 pearance of the Cotton is little, if at all, changed. 
 
 
 7. Decomposition, into simpler compounds or ele- 
 ments, as of Chloride of Silver by light, which resolves 
 it into Subchloride of Silver and Chlorine ; strong Nitric 
 Acid by light is decomposed into NO 4 and O. Chlorate 
 and lodate of Potash by heat are resolved into Oxygen, 
 and Chloride and Iodide of Potassium. 
 
 Several of the compounds of Gold, Silver, Platinum, 
 and Mercury are entirely decomposed into their elements 
 by heat. 
 
 ON THE CHEMISTRY OF ORGANIC SUBSTANCES. 
 
 The term organic is applied to substances produced 
 either by vegetables or animals, or which are obtained by 
 chemical processes from them ; thus Wood, and Acetic 
 Acid procured by its distillation, and Sugar, with the 
 Alcohol derived from it by fermentation, are organic sub- 
 stances. 
 
 All organic bodies are compounds of Carbon, and the 
 class embraces a great variety of products, which, like 
 inorganic bodies, include neutral, acid, basic, and saline 
 compounds. 
 
 The organic acids are numerous, such as Acetic Acid, 
 Tartaric, Citric, and a variety of others. 
 
 The neutral substances cannot easily be assimilated to 
 any class of inorganic compounds ; as examples, take 
 Starch, Sugar, Lignine, etc. 
 
AND THEIE COMBINATIONS. 27 
 
 The bases are also a large class. Morphia, obtained 
 from Opium ; Quinia, from Quinine ; Nicotine, from To- 
 bacco, are illustrations. 
 
 Besides these, there are large classes of compounds 
 known as Alcohols, Ethers, Aldehyds, etc., to which there 
 is nothing analogous in inorganic chemistry. 
 
 Alcohols are Carbon compounds, which by union with 
 acids form neutral bodies, termed Ether, Water being 
 separated. 
 
 Ethers are either Alcohol + Acid minus Water, or 2 Al- 
 cohol minus 2 Water. 
 
 Aldehyds are Alcohols minus Hydrogen, and by the 
 absorption of Oxygen they form Acids. 
 
 Composition of organic and inorganic bodies contrasted. 
 There are more than fifty elementary substances found 
 in the inorganic kingdom, but onljfour, commonly speak- 
 ing, in the organic : these four are Carbon, Hydrogen, 
 Nitrogen, and Oxygen. 
 
 Some organic bodies oil of turpentine, naphtha, etc. 
 contain only Carbon and Hydrogen ; many others, such as 
 sugar, gum, alcohols, fats, vegetable acids, Carbon, Hy- 
 drogen, and Oxygen. The Nitrogenous bodies, so called, 
 contain Nitrogen in addition to the other elements : such 
 are Albumen, Caseine, Gelatine, and all natural organic 
 bases ; Sulphur and Phosphorus are also present in many 
 of the Nitrogenous bodies, but only to a small extent. 
 
 Organic substances, although simple as regards the 
 number of elements involved in their formation, are often 
 highly complex in the arrangement of the atoms j this 
 may be illustrated by the following formulae : 
 
 Starch C 24 H 20 O 20 . 
 
 Cane Sugar .... C^H^O^. 
 Grape Sugar .... C 24 H 28 O 28 . 
 
 Inorganic bodies, as already shown, unite in pairs, 
 two elements join to form a binary compound ; two binary 
 compounds produce a salt ; two salts associated together 
 form a double salt. With organic bodies however the ar- 
 
28 CHEMICAL ELEMENTS AND THEIB COMBINATIONS. 
 
 rangement is often different, the elementary atoms are 
 all grouped equally in one compound atom, which is highly 
 complex in structure, and cannot be split up into binary 
 products. 
 
 Observe also, as characteristic of Organic Chemistry, 
 the apparent similarity in composition between bodies 
 which differ widely in properties. As examples take Lig- 
 nine, or cotton fibre, and Starch, each of which contains 
 the same percentage of Carbon, Hydrogen, and Oxygen. 
 
 Mode of distinguishing between Organic and Inorganic 
 matter. A simple means of doing this is as follows : 
 place the suspected substance upon a piece of Platinum- 
 foil, and heat it to redness with a spirit-lamp : if it first 
 blackens, and then burns completely away, it is probably 
 of organic origin. This test depends upon the fact, that 
 all organic bodies contain Carbon, and that their other 
 constituent elements are either themselves volatile, or ca- 
 pable of forming volatile combinations with Oxygen. In- 
 organic substances, on the other hand, are often unaffected 
 by heat, or, if volatile, are dissipated without previous 
 charring. 
 
 The action of heat upon organic matter may be illus- 
 trated by the combustion of coal or wood in an ordinary 
 furnace ; first, an escape of Carbon and Hydrogen, united 
 in the form of volatile gaseous matter, takes place, leaving 
 behind a black cinder, which consists of Carbon and inor- 
 ganic matter combined; afterwards this Carbon burns 
 away into Carbonic Acid, and a grey ash is left, which is 
 composed of inorganic salts, and is indestructible by heat. 
 
 This test is of course not applicable to organic bodies 
 volatile without decomposition, as Alcohol, Ether, Cam- 
 phor, etc. ; but these also are generally not the varieties 
 of organic matter which trouble the Photographer. 
 
29 
 
 CHAPTER II. 
 
 VOCABULARY OP PHOTOGRAPHIC CHEMICALS.* 
 
 ACETIC ACID. 
 Formula of Glacial Acid, HO,C 4 H 3 O 3 =60.t 
 
 ACETIC Acid is a product of the oxidation of Alcohol, 
 C 4 H 6 O 2 . Alcohol, when perfectly pure, is not affected 
 by exposure to air ; but if it be diluted, and a portion 
 of yeast be added, it soon acts as a ferment, and causes 
 the spirit to unite with Oxygen derived from the atmo- 
 sphere, and so to become sour from formation of Acetic 
 Acid, or "vinegar," C 4 H 6 O 2 + O 4 =HO,C 4 H 3 O 3 +2HO. 
 
 Acetic Acid is also produced on a large scale by heating 
 wood in close vessels : a substance distils over which is 
 Acetic Acid contaminated with empyreumatic and tarry 
 matter ; it is termed Pyroligneous Acid, and is much used 
 in commerce. 
 
 The most concentrated Acetic Acid may be obtained by 
 neutralizing common vinegar with Carbonate of Soda, and 
 crystallizing out the Acetate of Soda so formed ; the Ace- 
 tate must then be deprived of its water of crystallization, 
 and fused at a gentle heat. After cooling, 82 parts 
 
 * N.B. The arrangement in this Vocabulary is made with reference to the 
 second constituent in compounds. Thus, Acetic Ether will be found under 
 the head of " Ether, Acetic," Nitrate of Silver as " Silver, Nitrate of," 
 Iodide of Potassium as " Potassium, Iodide of," etc. Acids will be found 
 under their special names. 
 
 t The number affixed to each Formula represents the Atomic Weight of 
 the compound obtained by adding up the weights of the atoms of the ele- 
 ments. 
 
30 VOCABULARY OF 
 
 of the salt are to be distilled with 98 of strong Sulphuric 
 Acid, which removes the Soda and liberates Acetic Acid : 
 the Acetic Acid, being volatile, distils over, and may be 
 condensed. 
 
 NaO,C 4 H 3 O s + HOSO 3 =NaO,SO 3 -fHO,C 4 H 3 O 3 . 
 
 Properties of Acetic Acid. The strongest acid contains 
 only a. single atom of water ; it is sold under the name 
 of " Glacial Acetic Acid," so called from its property of 
 solidifying at a moderate^ low temperature. At about 
 50 the crystals melt, and form a limpid liquid of pungent 
 odour and a density nearly corresponding to that of water; 
 the specific gravity of Acetic Acid however is no test of 
 its real strength, which can only be estimated by analysis. 
 
 The commercial Glacial Acetic Acid, so termed, is usually 
 diluted with water, and sometimes a trace of Sulphurous 
 Acid is introduced, as the Writer is informed, to confer 
 the property of solidifying in cold weather, and thus to 
 give an appearance of strength. It is however probably 
 due in most cases to the decomposition of the Sulphuric 
 Acid used to obtain it. Sulphurous and Hydrochloric 
 Acids are both injurious in Photographic Processes, from 
 their property of precipitating Nitrate of Silver. To de- 
 tect them, proceed as follows : dissolve a small crystal of 
 Nitrate of Silver in a few drops of water, and add to it 
 about half a drachm of the Glacial Acid; the mixture 
 should remain quite clear even when exposed to light. Hy- 
 drochloric and Sulphurous Acid produce a white deposit 
 of Chloride or Sulphite of Silver, distinguishable by 
 Nitric Acid, which dissolves the Sulphite, but leaves the 
 Chloride unchanged ; and if Aldehyde or volatile tarry 
 matter be present in the Acetic Acid, the mixture with Ni- 
 trate of Silver, although clear at first, becomes discoloured 
 by the action of light. 
 
 Glacial Acetic Acid sometimes has a smell of garlic. In 
 this case it probably contains an organic Sulphur Acid, 
 and is unfit for use. 
 
 Many employ a cheaper form of Acetic Acid, sold by 
 druggists as " Beaufoy's " acid ; it should be of the strength 
 
PHOTOGRAPHIC CHEMICALS. 31 
 
 of the Acetic Acid Fortiss. of the London Pharmacopoeia, 
 containing 30 per cent, real acid. It will be advisable 
 to test it for Sulphuric Acid (see Sulphuric Acid), and 
 other impurities, before use. When a certain quantity 
 of the Glacial Acid is advised in a formula, take three 
 times as much of the Beaufoy's Acid. The Writer fre- 
 quently uses it, and finds it more constant in its properties 
 than the " Glacial Acid " so called. 
 
 ALBUMEN. 
 
 Albumen is an organic principle found both in the 
 animal and vegetable kingdom. Its properties are best 
 studied in the white of egg, which is a very pure form of 
 Albumen. 
 
 Albumen is capable of existing in two states ; in one of 
 which it is soluble, in the other insoluble in water. The 
 aqueous solution of the soluble variety gives a slightly 
 alkaline reaction to test-paper ; it is somewhat thick and 
 glutinous, but becomes more fluid on the addition of a 
 small quantity of an alkali, such as Potash or Ammonia. 
 
 Soluble Albumen may be converted into the insoluble 
 form in the following ways : 
 
 1. Sy the application of heat. A moderately strong 
 solution of Albumen becomes opalescent and coagulates 
 on being heated to about 150 Fahrenheit, but a tempera- 
 ture of 212 is required if the liquid is very dilute. A 
 layer of dried Albumen is not rendered insoluble by dry 
 heat of 212. 
 
 2. By addition of strong acids. Nitric Acid coagulates 
 Albumen perfectly without the aid of heat. Acetic Acid 
 however acts differently, appearing to enter into combina- 
 tion with the Albumen, and forming a compound soluble 
 in warm water acidified by Acetic Acid. 
 
 3. By the action of metallic salts. Many of the salts 
 of the metals coagulate Albumen completely. Nitrate of 
 Silver does so ; also the Bichloride of Mercury. Ammo- 
 nio-Nitrate of Silver however does not coagulate Albumen. 
 
 The white precipitate formed on mixing Albumen with 
 
32 VOCABTJLABY OF 
 
 Nitrate of Silver is a chemical compound of tlie animal 
 matter with Protoxide of Silver, and has been termed Al- 
 buminate of Silver : its, properties will be described after- 
 wards. On heating in a current of Hydrogen gas, it as- 
 sumes a brick-red colour, being probably reduced to the 
 condition of a compound with Suboxide of Silver. It is 
 then almost insoluble in Ammonia, but enough dissolves to 
 tinge the liquid wine-red. The red coloration of solution 
 of Nitrate of Silver employed in sensitizing the Albumi- 
 nized Photographic paper is doubtless produced by the 
 same compound, although often referred to the presence of 
 Sulphide of Silver. 
 
 Albumen also combines with Lime and Baryta. When 
 Chloride of Barium is used with Albumen, a white precipi- 
 tate of this kind usually forms. 
 
 By long keeping, Albumen loses its alkaline reaction and 
 becomes sour and more limpid than at first. Mucous 
 threads like cobwebs form in it, which appear to be caused 
 by oxidation. Ammonia added to Albumen is said to pre- 
 serve it for a longer time, and a lump of camphor floated 
 in the liquid has also a good effect. Decomposed Albu- 
 men usually contains Sulphuretted Hydrogen. 
 
 Chemical composition of Albumen. Albumen belongs to 
 the Nitrogenous class of organic substances (see page 27). 
 It also contains small quantities of Sulphur and Phos- 
 phorus ; when burnt, it leaves behind an ash in which 
 Chloride and Phosphate can be detected. 
 
 ALCOHOL. 
 Formula, C 4 H 6 O 2 = 46. 
 
 Alcohol is obtained by the careful distillation of any 
 spirituous or fermented liquor. If wine or beer be placed 
 in a retort, and heat applied, the Alcohol, being more vo- 
 latile than water, rises first, and may be condensed in an 
 appropriate receiver; a portion of the vapour of water 
 however passes over with the Alcohol, and dilutes it to a 
 certain extent, forming what is termed " Spirits of Wine." 
 
 
PHOTOGRAPHIC CHEMICALS. 33 
 
 Much of this water may be removed by redistillation from 
 Carbonate of Potash ; but in order to render the Alcohol 
 thoroughly anhydrous, it is necessary to employ quick- 
 lime, which possesses a still greater attraction for water. 
 For this purpose strong Alcohol of '823 should be left 
 in contact with powdered quicklime for three or four days, 
 or until the latter has ceased to swell from absorption of 
 water, after which it is separated by distillation, the retort 
 being placed in a water-bath. 
 
 Properties of Alcohol. Pure anhydrous or absolute Al- 
 cohol is a limpid liquid, of an agreeable odour and pungent 
 taste ; sp. gr. at 60, 794. It absorbs vapour of water, 
 and becomes diluted by exposure to damp air ; boils at 173 
 Fahr. It has never been frozen. 
 
 Alcohol distilled from Carbonate of Potash has a specific 
 gravity of '815 to "823, and contains 90 to 93 per cent, of 
 real spirit. 
 
 The specific gravity of ordinary rectified Spirits of Wine 
 is about '836, and it. contains 80 to 83 per cent, of absolute 
 Alcohol. 
 
 Different Commercial qualities of Alcohol. Alcohol 
 really absolute is not often used in Photography ; the ex- 
 pense of making it is very great, and it could not be pre- 
 pared at a profit. A spirit with less than four per cent, 
 of water (sp. gr. *805) may be obtained by agitating com- 
 mercial Spirits of Wine first with Carbonate of Potash in 
 the manner presently to be advised, and then with a com- 
 mon quality of dry Chloride of Calcium. Put in about 
 three-quarters of a pound of the Chloride of Calcium to 
 half a gallon of Spirit of '815 ; the greater part dissolves 
 with perceptible rise of temperature. Draw over as muc-h 
 as possible in a steam bath ; and, to prevent the residue in 
 the retort from setting into a hard mass, it is well to add 
 a little water to it, after the distillation is completed. 
 In this way the commercial absolute Alcohol is, usually 
 prepared. 
 
 The next quality of Spirit is the strong Alcohol of ^815 
 to '823. This may be obtained by agitating Spirits of 
 
 D 
 
34 VOCABULARY OF 
 
 
 Wine, '836, with an excess of dry Carbonate of Potash. 
 The salt termed Carbonate of Potash is a deliquescent salt, 
 having a great attraction for water ; consequently when 
 Spirit of Wine is shaken with Carbonate of Potash, a 
 portion of water is removed, the salt dissolving in it and 
 forming a dense liquid, which refuses to mix with the Al- 
 cohol, and sinks to the bottom. At the expiration of two 
 or three days, if the bottle has been shaken frequently, the 
 action is complete, and the lower stratum of fluid may be 
 drawn off and rejected. 
 
 In order to obtain the greatest amount of concentration, 
 it will be necessary to have a series of vessels, each con- 
 taining about a pound of the Carbonate to a gallon of 
 Spirit. The Alcohol may be passed from one to the other, 
 and should not be distilled until the finely-powdered Car- 
 bonate can be shaken about in the liquid without being 
 wetted. The Carbonate which the Writer employs is 
 nearly pure, and costs from a shilling to eighteen-pence per 
 pound ; it must be dried on a hot metal plate before use. 
 A commoner variety would answer, but it has the disad- 
 vantage of clotting together at the bottom of the vessel, 
 and of not dissolving into a clear liquid. 
 
 A third quality of commercial Alcohol is the rectified 
 Spirit of Wine of '836 already referred to ; it is very suit- 
 able for adding to developing fluids, etc., but not suffici- 
 ently strong for good Collodion. . 
 
 Alcohol for Collodion Photography. For a long time 
 the Writer was in the habit of employing rectified Spirits 
 of Wine for the preparation of Collodion, increasing its 
 strength as far as necessary, by means of dry Carbonate of 
 Potash ; but having at length become dissatisfied with the 
 smell of certain samples of this rectified Spirit, he has 
 since used a strong Alcohol obtained by one distillation. 
 In rectifying Spirit, a liquid known as " faints " is some- 
 times mixed with the purer Spirit for the sake of economy, 
 and these faints are invariably contaminated with essential 
 Oils. It is most important in Photography to avoid essen- 
 tial Oils, and therefore the grain Spirit obtained by one dis- 
 
PHOTOGRAPHIC CHEMICALS. 35 
 
 tillation in a Coffey's still may be preferred. In taking the 
 specific gravity of this grain Spirit, which varies from '817 
 to *819 at 60 Fahr., we see at once the advantage likely 
 to accrue from its employment, since the fusel Oil, which 
 boils at a more elevated temperature, cannot rise so high 
 in the still, and is separated. The smell of the grain Spirit 
 is very sweet ; and although it is not quite so strong as is 
 required, yet by converting a portion of it into Alcohol of 
 805, by means of dry Chloride of Calcium, and mixing 
 this with the remainder, the correct specific gravity may 
 easily be obtained. 
 
 The reaction to test-paper of the pure grain Spirit should 
 be quite neutral ; but in some instances a trace of acid is 
 present, so that each half-gallon of Spirit requires about 
 one drop of solution of Ammonia of sp. gr. 0'93. The 
 Writer has never yet found in this or in any other Spirit 
 the alkaline reaction which is exhibited by Ether. 
 
 ALCOHOL (Methylated). 
 
 Spirit of Wine containing ten per cent, of Wood Naph- 
 tha is allowed by the excise laws to be sold free of duty, 
 under the name of " Methylated Spirit." The quality, 
 however, is often inferior, since residues containing fusel 
 oil and other volatile bodies are usually rectified for the 
 purpose of methylating. 
 
 ALCOHOL, METHYLIC. 
 Formula, C 2 H 4 O 2 == 32. 
 
 This liquid is one of the products of the destructive 
 distillation of wood, and constitutes a large portion of 
 the inflammable fluid called Wood Naphtha, or Pyro- 
 xylic Spirit. In its pure state it much resembles com- 
 mon Alcohol in its properties. As common Alcohol by 
 oxidation furnishes an Acid, the Acetic, so Methylic 
 Alcohol in like manner yields a similar Acid, called For- 
 mic Acid. 
 
36 YOCABULAEY OF 
 
 AMMONIA. 
 Formula, NH 3 =17. 
 
 The liquid known by this name is an aqueous solution 
 of the volatile gas Ammonia. Ammoniacal gas contains 
 one atom of Nitrogen combined with three of Hydrogen : 
 these two elementary bodies, when free, show no tendency 
 to combine, but they can be made to unite under certain 
 circumstances, and the result is Ammonia. 
 
 Properties of Ammonia. Ammoniacal gas is soluble in 
 water to a large extent : the solution possessing those pro- 
 perties which are termed alkaline (p. 12). Ammonia how- 
 ever differs from the other alkalies in one important par- 
 ticular it is volatile : hence the original colour of tur- 
 meric-paper affected by Ammonia is restored on the appli- 
 cation of heat. Solution of Ammonia absorbs Carbonic 
 Acid rapidly from the air, and is converted into Carbonate 
 of Ammonia ; it should therefore be preserved in stoppered 
 bottles. Besides Carbonate, commercial Ammonia often 
 contains Chloride of Ammonium, recognized by the white 
 precipitate given by Nitrate of Silver after acidifying with 
 pure Nitric Acid. 
 
 The strength of commercial Ammonia varies greatly ; 
 that sold for pharmaceutical purposes under the name of 
 Liquor Ammonia? contains about ten per cent, of real Am- 
 monia. The sp. gr. of aqueous Ammonia diminishes with 
 the portion of Ammonia present, the Liquor Ammonise 
 being usually about '936. It is a common error in Pho- 
 tographic operations to confound the diluted Ammonia 
 with the Liquor Ammoniae Fort., which is so strong that 
 a single drop will neutralize a large portion of acid, and 
 has sp. gr. of only '88. 
 
 Ammonia has no resemblance in composition to other 
 bases which are metallic oxides, and it differs from them 
 also, in that it never combines with Oxygen acids without 
 including water as an essential ingredient in the compound. 
 Thus Sulphate of Ammonia, dry, contains 
 + HO+SO 3 = NH 4 OSO 3 . 
 
 
PHOTOGEAPHIC CHEMICALS. 37 
 
 Yet the salts of Ammonia have a close resemblance in 
 properties to those of the alkalies Potash and Soda, and 
 this resemblance has led to the Ammonium Theory, which 
 developes a close resemblance in composition between the 
 salts of Ammonia and of the alkalies, and also explains 
 why water is essential to the salts with Oxygen acids. 
 
 Ammonium Theory. This theory assumes that in all the 
 salts of Ammonia, the Ammonia, NH 3 , does not exist as 
 such, but is united to an atom of Hydrogen, either of the 
 acid, as in the case of Hydrochloric Acid, or of the 
 water, as in the case of Oxygen acids like Sulphuric 
 Acid, to form a kind of compound metal, N H 4 , termed Am- 
 monium, and which in its salts acts a part similar to that 
 of the metals Potassium and Sodium, in the salts of 
 Potash and Soda ; and as Potash and Soda only exist in 
 the salts witli Oxygen acids, so water is only needed in 
 the salts of Ammonia with Oxygen acids, the Hydrogen 
 of the water forming the Ammonium, and the Oxygen of 
 the water converting it into an oxide, analogous to me- 
 tallic oxides ; and thus it would appear that NH 3 is not 
 the true base, but NH 3 +HO = (NH 4 )O, Oxide of Ammo- 
 nium. The following formulae will show the resemblance 
 between salts of Potash and Ammonia from this point of 
 view : 
 
 N H, + H Cl = (N H 4 )C1 = Chloride of Ammonium. 
 
 K,C1 Chloride of Potassium. 
 
 NH 3 +HO+SO 3 =(NH 4 )OSO 3 = Sulphate of Ammonia. 
 KO SO 3 = Sulphate of Potash. 
 
 As the names of the salts derived from Hydrogen acids 
 terminate in ide (see page 14), so by means of this theory 
 the corresponding salts of Ammonia may be so termed ; 
 thus, Hydrochlorate of Ammonia N H 3 , H Cl, is also a Chlor- 
 ide of Ammonium, NH 4 C1, resembling thus Chloride of 
 Potassium in name as well as properties. 
 
 AMMONIA, CARBONATE OF. 
 
 This occurs commercially in lumps of a considerable size, 
 obtained by sublimation. Its composition is rather vari- 
 
38 VOCABULARY OF 
 
 able. When first formed it has nearly the composition of 
 a Sesquicarbonate, but by exposure to the air neutral Car- 
 bonate of Ammonia escapes, and a white powder is left 
 which is a Bicarbonate. 
 
 AMMONIA, HYDEOSULPHATE OF. 
 Formula, N H 4 S, H S = 51. 
 
 The liquid known by this name, and formed by passing 
 Sulphuretted Hydrogen gas into Ammonia, is a double 
 Sulphide of Hydrogen and Ammonium. In the prepara- 
 tion, the passage of the gas is to be continued until the 
 solution gives no precipitate with Sulphate of Magnesia, 
 and smells strongly of Hydrosulphuric Acid. 
 
 Properties. Colourless at first, but afterwards changes 
 to yellow, from liberation and subsequent solution of Sul- 
 phur. Becomes milky on the addition of any acid. Pre- 
 cipitates, in the form of Sulphide, the last 14 metals of 
 the list of elements (page 10). Alumina and Chromium 
 are also thrown down, but as Oxides. An excess of Sul- 
 phide of Ammonium redissolves the Sulphides of Gold, 
 Platinum, Tin, Arsenic, and Antimony. 
 
 Hydrosulphate of Ammonia is employed in Photography 
 to darken the Collodion image, and also as a test for the 
 presence of Silver in certain solutions. 
 
 AMMONIA, NITEATE OF. 
 Formula, N H 4 O, NO 5 = 80. 
 
 This is a neutral combination of Nitric Acid and Am- 
 monia, which may be crystallized without difficulty. It is 
 gradually formed in the Photographic Nitrate Bath when 
 compounds of Ammonium are used in iodizing. 
 
 Nitrate of Ammonia is not of itself alkaline, but inas- 
 much as it is solvent of Oxide and also of Carbonate of 
 Silver, a Bath containing Nitrate of Ammonia will give a 
 strong Alkaline reaction on adding to it either Potash, 
 Ammonia, or Chalk. 
 
PHOTOGRAPHIC CHEMICALS. 39 
 
 AMMONIUM, BKOMIDE OF. 
 Formula, NH 4 Br= 98. 
 
 This is a crystallized salt, which may be prepared by 
 precipitating Bromide of Calcium by Carbonate of Am- 
 monia. It is very soluble in water, and is more easily 
 dissolved by Alcohol and Ether than the corresponding 
 Bromide of Potassium. It does not become coloured 011 
 keeping, like the Iodide of Ammonium. 
 
 Bromide of Ammonium is a better form of Bromide for 
 Collodion than the corresponding Salt of Potassium or 
 Cadmium, and it can usually be obtained in a pure state. 
 
 AMMONIUM, CHLOEIDE OF. 
 Formula, NH 4 C1 = 53'5. 
 
 This salt, also known as Muriate or Hydrochlorate of 
 Ammonia, occurs in commerce in the form of colourless 
 and translucent masses, which are procured by sublimation, 
 the dry salt being volatile when strongly heated. It dis- 
 solves in an equal weight of boiling, or in three parts of 
 cold water. It contains more Chlorine in proportion to the 
 weight used than Chloride of Sodium, the atomic weights 
 of the two being as 53'5 to 58'5. 
 
 Chloride of Ammonium is easily obtained in a pure state, 
 and is, on the whole, more suitable for salting paper than 
 either Chloride of Sodium or Chloride of Barium. 
 
 AMMONIUM, IODIDE OF. 
 Formula, NH 4 I = 145. 
 
 Properties. Usually sold in small crystals, which have 
 more or less of a yellow colour. An unstable substance 
 and prone to liberation of Iodine, but may be kept for an 
 unlimited length of time, if thoroughly dried and put away 
 in a dark place. Very much more soluble in Alcohol than 
 Iodide of Potassium, and not precipitated on adding Ether. 
 Also soluble to a large extent in water : Alkaline reaction 
 to litmus-paper. 
 
40 VOCABULARY OF 
 
 Preparation. In former editions the "Writer has de- 
 scribed a mode of preparing Iodide of Ammonium by dis- 
 solving Iodine in Hydrosulphate of Ammonia ; but a more 
 extended experience has shown him that traces of a Sul- 
 phur compound are often present in the Iodide made in 
 that way. In order therefore to obtain an Ammonium 
 compound equal to the Iodide of Potassium as regards sen- 
 sitiveness in Collodion, he recommends that an Iodide of 
 Calcium be first formed, and afterwards precipitated by 
 Carbonate of Ammonia. This process is more trouble- 
 some than the last, but gives a very good result. Dry the 
 crystals in vacuo over Sulphuric Acid. 
 
 Impurities. Sulphate of Ammonia is often present to a 
 large extent in commercial Iodide of Ammonium, and 
 sometimes Carbonate of Ammonia, introduced for the pur- 
 pose of preserving it in a colourless condition. Make a 
 concentrated aqueous solution, and add a few drops of 
 solution of Chloride of Barium. A white deposit indi- 
 cates either Sulphate or Carbonate ; but if the precipitate 
 is at once taken up on dropping in one or two drops of 
 Glacial Acetic Acid, it consists of Carbonate of Baryta. 
 Sulphate of Ammonia is mostly left behind on dissolving 
 the Iodide in Alcohol of '805, sold as absolute ; but the 
 Carbonate dissolves, and Collodion iodized with this solu- 
 tion, eventually renders the Bath alkaline by producing 
 Ammonio-Nitrate of Silver. 
 
 To decolorize Iodide of Ammonium which has been 
 decomposed by keeping, shake it up with a little Ether, 
 pure or methylated ; the Iodine will be dissolved out, and, 
 unless Alcohol be present, no great loss from solution will 
 result. 
 
 AQUA REGIA. See NITKO-HYDEOCHLOEIC ACID. 
 
 AESENIC, BEOMIDE OF. 
 Formula, AsBr 3 = 315. 
 
 Bromide of Arsenic has been used in Collodion for the 
 purpose of increasing the intensity of the developed image. 
 
PHOTOGEAPHIC CHEMICALS. 41 
 
 A solution suitable for that purpose may be prepared by 
 reducing Metallic Arsenic to a fine powder and placing it 
 in a dry bottle with Alcohol of sp. gr. '805. Bromine is 
 then to be dropped into the Alcohol, when immediate com- 
 bination will ensue : the Arsenic must always be in excess. 
 The addition of water decomposes Bromide of Arsenic 
 into Arsenious Acid and Hydrobromic Acid ; hence the 
 necessity for using Alcohol of considerable strength. 
 
 AUEO-CHLOEIDE OF SODIUM. 
 See SODIUM, AUEO-CHLOEIDE OF. 
 
 BAEIUM, CHLOEIDE OF. 
 
 Formula, BaCl+2Aq. = 122. 
 
 Barium is a metallic element very closely allied to Cal- 
 cium, the elementary basis of Lime. The Chloride of Ba- 
 rium is commonly employed as a test for Sulphuric Acid, 
 with which it forms an insoluble precipitate of Sulphate 
 of Baryta. It also slightly alters the colour of the Pho- 
 tographic image when used in preparing Positive paper, 
 which may be due, in some measure, to a chemical com- 
 bination of Baryta with Albumen : but it must be remem- 
 bered that this Chloride, from its high atomic weight, con- 
 tains less Chlorine than the alkaline Chlorides. 
 
 Properties of Chloride of Barium. Chloride of Barium 
 occurs in the form of white crystals, soluble in about two 
 parts of water, at common temperatures. These crystals 
 contain two atoms of water of crystallization, which are 
 expelled at 212, leaving the anhydrous Chloride. 
 
 BAEYTA, NITEATE OF. 
 Formula, BaO,NO 5 = 130'5. 
 
 Nitrate of Baryta forms octohedral crystals, which are 
 anhydrous. It is considerably less soluble than the Chlo- 
 ride of Barium, requiring for solution twelve parts of cold 
 and four of boiling water. It may be substituted for the 
 Nitrate of Lead in the preparation of Protonitrate of Iron. 
 
42 VOCABULABY OF 
 
 BENZOLE. 
 Formula, C 12 H 6 = 78. 
 
 A limpid liquid, obtained commercially by distilling off 
 the most volatile constituent of the substance known as 
 " Coal Naphtha." It does not mix with water, but is dis- 
 solved in any quantity by Alcohol or Ether. 
 
 Benzole is an excellent solvent of fats and oils, and may 
 be employed for removing grease-spots. It also dissolves 
 Gutta-percha and Caoutchouc. A rapidly drying varnish 
 may be made with Benzole, but it should first be purified 
 by redistillation, since ordinary Benzole sometimes leaves 
 a greasy residue on drying. 
 
 BITUMEN OF JUDEA. 
 
 A substance like Pitch in appearance and brittleness, 
 found abundantly in various parts of the world, as in 
 Judea, floating on the Dead Sea, and in Trinidad in the 
 Pitch Lake. It is also known as Asphalt. Soluble in 
 Coal Naphtha. 
 
 BEOMINE. 
 
 Symbol, Br = 80. 
 
 This elementary substance is obtained from the uncrys- 
 tallizable residuum of sea-water, termed bittern. It exists 
 in the water in very minute proportions, combined with 
 Magnesium in the form of a soluble Bromide of Magne- 
 sium. 
 
 Properties. Bromine is a deep reddish-brown liquid of 
 a disagreeable odour, which gives off ruddy vapours at 
 common temperatures ; sparingl} 7 soluble in water (1 part 
 in 23, Lowig), but more abundantly so in Alcohol, and 
 especially in Ether. It is very heavy, having a specific 
 gravity of 3'0. 
 
 Bromine is closely analogous to Chlorine and Iodine in 
 its chemical properties. It stands intermediately between 
 the two ; its affinities being stronger than those of Iodine, 
 but weaker than Chlorine (see CHLORINE). 
 
PHOTOGRAPHIC CHEMICALS. 43 
 
 It forms a large class of salts, of which the Bromides 
 of Potassium, Ammonium, Cadmium, and Silver are the 
 most familiar to Photographers. 
 
 CADMIUM. 
 Symbol, Cd = 56. 
 
 A white metal, resembling Tin in its physical properties. 
 Its Oxide is found associated with Zinc in certain of the 
 ores of the latter, but the two are easily separated. 
 
 The Iodide and Bromide of Cadmium are used in Pho- 
 tography, on account of their permanency and the facility 
 with which they dissolve in Ether and Alcohol. 
 
 Metallic Cadmium is sometimes employed to remove 
 free Iodine from Collodion. 
 
 CADMIUM, IODIDE OF. 
 
 Formula, Cdl = 183. 
 
 This salt is formed by heating filings of metallic Cad- 
 mium with Iodine, or by mixing the two with addition of 
 water. 
 
 Iodide of Cadmium is very soluble, both in Alcohol 
 and Water ; the solution yielding on evaporation large 
 six-sided tables of a pearly lustre, which are permanent in 
 the air. The commercial Iodide is sometimes contaminated 
 with Iodide of Zinc, the crystals being imperfectly formed 
 and slowly liberating Iodine when dissolved in Ether and 
 Alcohol. Pure Iodide of Cadmium remains nearly or 
 quite colourless in Collodion, if the fluid be kept in a cool 
 and dark place. 
 
 CALCIUM, CHLOEIDE OF. 
 Formula, CaCl = 55 "5. 
 
 This salt is found in sea-water ; but is usually prepared 
 artificially by dissolving Chalk in Hydrochloric Acid and 
 evaporating. When strongly dried, it occurs in lumps 
 which are hard and difficult to pulverize. 
 
 Chloride of Calcium has a great attraction for water, 
 
44 VOCABULARY OF 
 
 and is used for drying gases and other purposes. Exposed 
 to the air, it soon deliquesces from absorption of atmo- 
 spheric moisture. It is very soluble in Alcohol with evo- 
 lution of heat, and when the liquid is subjected to distilla- 
 tion, a highly concentrated spirit passes over, leaving a 
 viscid mass which crystallizes on cooling, and contains 
 nearly sixty per cent, of Alcohol in a state of loose chemical 
 combination with the Chloride. 
 
 Pure dry Chloride of Calcium is an expensive salt, but 
 a common quality, sufficiently good for the use above de- 
 scribed, may be obtained at a lower price. 
 
 CALCIUM, IODIDE OF. 
 Formula, Cal = 147. 
 
 This salt, useful for the preparation of some other 
 Iodides, may be obtained either by saturating Hydriodic 
 Acid with Carbonate of Lime, or by digesting Iodine with 
 metallic Iron and Water until the liquid is purely green, 
 then adding excess of Lime and filtering off the solution 
 from the precipitate and evaporating. 
 
 CAMPHOE. 
 
 Formula, C 20 H 16 O 2 = 152. 
 
 This substance is the produce of the Laurus Camphora 
 of Japan and China. It occurs when pure in sublimed 
 masses, which are tough and cannot be reduced to powder 
 without the aid of a few drops of spirit. It evaporates 
 slowly in the air at ordinary temperatures. 
 
 Camphor is soluble in Alcohol, but very sparingly in- 
 deed in water. Hence a piece of any size may be put into 
 the liquid without fear of adding an excess. For its use 
 in detecting grease, see page 111. 
 
 CAOUTCHOUC. 
 
 This substance, known also as India-rubber, is the in- 
 spissated milky juice of trees growing in South America 
 and the East Indies. It is insoluble in water and in Al- 
 cohol. 
 
PHOTOGEAPHIC CHEMICALS. 45 
 
 Chloroform is the most perfect solvent for Caoutchouc, 
 and leaves it unchanged on evaporation. Benzole also 
 acts upon it. Mineral Naphtha takes it up on applying 
 heat, but the residue after evaporation is sticky. 
 
 By combination with Sulphur, Caoutchouc undergoes a 
 change of properties, familiar to us in the article sold as 
 vulcanized India-rubber. 
 
 CARBON. 
 
 Symbol, C = 6. 
 
 Carbon is an element, which in a state nearly pure, is 
 familiar to all as Charcoal or Lamp-black. The diamond 
 is crystallized Carbon perfectly pure. By combustion in 
 the air it yields two Oxides, CO and CO 2 . The first or 
 Carbonic Oxide, C O, is produced when the supply of air 
 is small, and the other Oxide, Carbonic Acid, CO 2 , when 
 the combustion is perfect. 
 
 It is essentially tlie organic element, as every organic 
 body is a compound of Carbon. 
 
 CAKBON, BISULPHIDE OF. 
 
 Formula, CS 2 = 38. 
 
 Sulphur brought into contact with Charcoal at a bright 
 red-heat, at once combines with it to form the above com- 
 pound. It is a colourless, very volatile, and inflammable 
 liquid, producing great cold by evaporation. It is heavier 
 than water, in which it is insoluble, but it mixes readily 
 with Alcohol and Ether. 
 
 CA SEINE. fi 
 
 CELLULOSE. 
 
 Formula, C 36 H 30 O 30 = 486. 
 
 This term is applied to the pure form of vegetable fibre 
 existing in carefully bleached cotton, flax, etc. Cellulose 
 is the material which forms the cell-wall of the tissues of 
 plants, and Lignine is the deposit formed in the interior 
 
46 VOCABULARY OF 
 
 of the cell. Hence the former is most abundant in the 
 pith and newly-formed wood, whilst the latter is found 
 in parts of older growth which have become hard and 
 solid. 
 
 A new solvent for Cellulose has lately been discovered, 
 prepared by exposing copper-turnings to the air in con- 
 tact with Ammonia. The presence of a saline body like 
 Nitrate of Ammonia prevents the Cellulose from being 
 dissolved, and hence the ordinary blue liquid obtained by 
 adding Ammonia to a salt of Copper will not answer the 
 purpose. 
 
 For further particulars as the properties of Cellulose, 
 see Art. "Cotton." 
 
 CHAECOAL, ANIMAL. 
 
 Animal Charcoal is obtained by heating animal sub- 
 stances, such as bones, dried blood, horns, etc., to redness, 
 in close vessels, until all volatile empyreumatic matters 
 have been driven off, and a residue of Carbon remains. 
 When prepared from bones, it contains a large quantity 
 of inorganic matter in the shape of Carbonate and Phos- 
 phate of Lime. Animal Charcoal is freed from these 
 earthy salts by repeated digestion in Hydrochloric Acid ; 
 but unless very carefully washed it is apt to retain an acid 
 reaction, and so to liberate free Nitric Acid when added 
 to solution of Nitrate of Silver. 
 
 Properties. Animal Charcoal, when pure, consists solely 
 of Carbon, and burns away in the air without leaving any 
 residue : it is remarkable for its property of decolorizing 
 solutions; the organic colouring substance being separated, 
 but not actually destroyed, as it is by Chlorine employed 
 as a bleaching agent. This power of absorbing colouring 
 matter is not possessed in an equal degree by all varieties 
 of Charcoal, but is in great measure peculiar to those de- 
 rived from the animal kingdom. 
 
PHOTOGRAPHIC CHEMICALS. 47 
 
 CHLOKINE. 
 Symbol, Cl = 35'5. 
 
 Chlorine is an element, found abundantly in nature, 
 combined with metallic Sodium in the form of Chloride of 
 Sodium, or Sea- Salt. 
 
 Preparation. By the action of Hydrochloric Acid on 
 a natural product known as Binoxide of Manganese, 
 Mn O 2 . The reaction may be thus represented : 
 MnO 2 + 2HCl=MnCl + 2HO + Cl. 
 
 Properties. Chlorine is a greenish-yellow gas, of a 
 pungent and suffocating odour ; soluble to a considerable 
 extent in water, the solution possessing the odour and 
 colour of the gas. It is nearly 2| times as heavy as a cor- 
 responding bulk of atmospheric air. 
 
 Chemical properties. Chlorine belongs to a small na- 
 tural group of elements which contains also Bromine, 
 Iodine, and Fluorine. They are characterized by having 
 a strong affinity for Hydrogen, and also for the metals ; 
 but are comparatively indifferent to Oxygen. Many me- 
 tallic substances actually undergo combustion when pro- 
 jected into an atmosphere of Chlorine, the union between 
 the two taking place with extreme violence. The cha- 
 racteristic bleaching properties of Chlorine gas are ex- 
 plained in the same manner : Hydrogen is removed from 
 the organic substance, and in that way the structure is 
 broken up and the colour destroyed. 
 
 Chlorine is more powerful in its affinities than either 
 Bromine or Iodine. The salts formed by these three ele- 
 ments are closely analogous in composition and often in 
 properties. Those of the Alkalies, Alkaline Earths, and 
 many of the Metals, are soluble in water : but the Silver 
 salts are insoluble ; the Lead salts sparingly so. 
 
 The combinations of Chlorine, Bromine, Iodine, and 
 Fluorine, with Hydrogen, are acids, which by combina- 
 tion with bases form haloid salts and water (p. 14). 
 
 The test by which the presence of Chlorine is detected, 
 in its free state, or in its soluble metallic compounds, is 
 
48 VOCABULAEY OF 
 
 Nitrate of Silver ; it gives a white curdy precipitate 01 
 Chloride of Silver, insoluble in Nitric Acid, but soluble in 
 Ammonia. The solution of Nitrate of Silver employed 
 as the test must not contain Iodide of Silver, since this 
 compound is precipitated by dilution. 
 
 CHLOROFORM. 
 Formula, C 2 HC1 3 119'5. 
 
 This volatile liquid is obtained by the action of Chloride 
 of Lime upon dilute Alcohol. It does not mix with water, 
 but is very soluble in Spirit. 
 
 Chloroform is the best solvent known for Caoutchouc, 
 and it also dissolves Gutta-percha readily. Amber and 
 many other resins are more or less soluble in Chloroform ; 
 and this solvent is well suited for the preparation of Pho- 
 tographic Varnishes, from its volatility, and from its having 
 no solvent power on any of the varieties of Collodion films, 
 some of which would be dissolved by an Alcohol Varnish. 
 
 CHROMIUM. 
 Symbol, Cr = 26'3. 
 
 Chromium is a metallic element found in the state of 
 Oxide combined with Oxide of Iron in the mineral termed 
 Chrome Ironstone. Its most important Oxides are the 
 basic Oxide Cr 2 O 3 and Chromic Acid, CrO 3 . 
 
 Chromate of Potash, the source of all the preparations 
 of Chromium, is obtained from the ore by fusing it for 
 a long time with Nitrate of Potash; the latter imparts 
 Oxygen, which changes Cr 2 O 3 into two atoms of CrO 3 , 
 which with the Potash forms Chromate : this is afterwards 
 dissolved out and crystallized. 
 
 The Chromate acidified with Nitric Acid yields the Bi- 
 chromate of Potash, which is purified by crystallization. 
 
IHOTOdRAPHIC CHEMICALS. 4# 
 
 CITRIC ACID. 
 Formula, 3HO,C 12 H 5 O n +2HO = 210. 
 
 This acid is found abundantly in lemon-juice and in lime- 
 juice. It occurs in commerce in the form of large crystals, 
 which are soluble in less than their own weight of water 
 at 60. 
 
 Commercial Citric Acid is sometimes mixed with Tar- 
 taric Acid. The adulteration may be discovered by making 
 a concentrated solution of the acid and adding Acetate of 
 Potash : crystals of Bitartrate of Potash will separate if 
 Tartaric Acid be present. 
 
 Citric Acid is a tribasic acid (p. 23), that is, one atom of 
 it combines with three atoms of base to form a neutral 
 salt; thus dry Citrate of Soda is 3NaO,C 12 H 5 O n . 
 
 A single atom of Citric Acid decomposes three atoms 
 of Carbonate of Soda. One atom of Tartaric Acid how- 
 ever decomposes only two atoms of the Carbonate, so that 
 if Citric Acid be adulterated with Tartaric Acid the alkali 
 is in excess when the proportions given in the Photo- 
 graphic formulae for making Citrates are adopted. 
 
 Citric Acid retards the reduction of Silver Salts by a 
 developer, when the acid is present in a free state. But 
 Citric Acid previously neutralized by an Alkali acts as a 
 reducing agent, and precipitates metallic Gold from a so- 
 lution of Chloride of G-old. 
 
 COTTON. 
 
 Formula, C 36 H 30 O 30 = 486. 
 
 The Cotton of commerce has been obtained principally 
 from America, India, Egypt, and the South Sea Islands ; 
 the largest supply, up to 1862, came from America, but at 
 present the sources of supply are numerous and rather un- 
 certain. There is a considerable difference in the quality 
 of Cotton as regards length of fibre, etc., and hence the 
 market price of the raw material varies greatly. 
 
 Rough Cotton as it is imported is not in a state to be 
 used commercially ; it must first be carefully combed and 
 
50 VOCABULAEY OF 
 
 cleaned. Even the finest Cotton Wool, however, as it is pre- 
 pared for the jeweller and for surgical purposes, may with 
 advantage be further purified if required in making Pho- 
 tographic Pyroxyline. The fibre of the Cotton, as it ex- 
 ists in the pod, is encased by a film of oily or resinous 
 matter, which, when the Cotton is immersed in Nitro- Sul- 
 phuric Acid of the correct strength for making soluble 
 Pyroxyline, and at a temperature of 150 Fahr., resists for 
 a time the action of the Nitric Acid, and much squeezing 
 with the glass rod is required to wet the Cotton, and to 
 make it imbibe the fluid : bubbles of air are entangled at 
 first, and cannot easily be expelled, but almost immedi- 
 ately afterwards an evolution of red fumes takes place, and 
 the fibre is destroyed by oxidation. On the other hand, 
 with Cotton which has been previously treated with a 
 diluted alkali so as to saponify the resin and remove it in 
 a soluble form, there is no difficulty whatever in making it 
 absorb the acid ; it sucks up the liquid like a sponge, and 
 remains during the whole time of the digestion without 
 dissolving. From the finest qualities of Cotton Wool, 
 traces of soluble matter are extracted by Potash, sufficient 
 to impart a strong yellow colour to the alkaline liquid, 
 and which, if permitted to remain, deoxidize that portion 
 of the Nitric Acid immediately in contact with the fibre, 
 and so far weaken it as to ensure the immediate solution 
 of a part of the Cotton. 
 
 Fine Cotton is usually sent out in packets of one pound 
 each, which may be divided into quarters, and each quar- 
 ter boiled gently for two hours, in a solution of two ounces 
 of Potash (at two shillings and sixpence per pound) to a 
 gallon of water. The mass is lifted out, and well squeezed, 
 with repeated changes of water, for about twenty minutes, 
 after which it is spread out to dry. It is important to re- 
 move the whole of the Potash, and to disturb the fibre 
 of the Cotton as little as possible, since if it become 
 knotted and twisted, the action of the Nitric Acid will be 
 interfered with. 
 
 It may perhaps be suggested that the Potash is likely 
 
PHOTOGRAPHIC CHEMICALS. 51 
 
 to exercise a chemical or modifying action on the fibre of 
 the Cotton, but it does not appear to have any such effect 
 when used in dilute solution. A somewhat stronger Potash 
 has been stated to affect the cellulose by degrees, and to 
 produce more or less of the condition which we see in old 
 and rotten calico, as compared with the new material ; but 
 there need be no apprehension of this in the process above 
 described, because Cotton fabrics which have been weak- 
 ened by wear and repeated washings, become more easily 
 soluble in Nitro- Sulphuric Acid, whereas the Cotton- wool, 
 by boiling in weak alkali, is rendered less soluble in the 
 Acids. 
 
 It has not at present been absolutely proved, that all va- 
 rieties of Cotton correspond as regards the solvent action 
 of the weak Nitro- Sulphuric Acid, even when the cleans- 
 ing has been correctly performed : the removal of the re- 
 sinous impurity, however, is undoubtedly a point of im- 
 portance. 
 
 CYANOGEN. 
 
 Formula, C 2 N = 26. Symbol, Cy. 
 
 Cyanogen is a very remarkable compound which be- 
 haves in many respects like the elements, Chlorine, Bro- 
 mine, and Iodine. Like them it unites with Hydrogen to 
 form an Acid, Hydrocyanic or Prussic Acid ; and like 
 them it combines with metals to form Haloid Salts (page 
 14). Cyanide of Silver, AgC 2 N or AgCy, much resembles 
 Chloride of Silver in properties. 
 
 Cyanogen has a capacity for forming complex compounds 
 with certain metals, in which the metals can no longer be 
 discovered by the usual tests; thus Iron for example 
 enters into the composition of the Ferrocyanides, but no 
 ordinary test would discover it. 
 
 Cyanogen in the uncombined state is a gas ; it cannot be 
 formed directly from its elements, Carbon and Nitrogen, 
 but may easily be procured by heating one of its com- 
 pounds, Cyanide of Mercury, HgCy = Hg + Cy. 
 
52 VOCABULARY OF 
 
 ETHEE. 
 
 Formula, C s H 10 O a = 74. 
 
 Ether is obtained by distilling a mixture of Sulphuric 
 Acid and Alcohol. Although in the action of the Acid 
 upon the Alcohol there are some complicated changes, yet 
 the ultimate effect is quite simple, and consists in the re- 
 moval of two atoms of Water from two atoms of Alcohol, 
 and the Alcohol residues, united together, form Ether. 
 
 Alcohol. Ether. 
 
 2(C 4 H 6 2 )-2HO = C 8 H 10 2 . 
 
 This Ether is often termed Sulphuric Ether from its 
 mode of formation, but it contains no Sulphuric Acid, 
 whereas other Ethers, such as Acetic Ether and Nitrous 
 Ether, retain the acids, whose names they bear, as essen- 
 tial ingredients. 
 
 Properties of Ether. Ether is a limpid and colourless 
 fluid, with a peculiar and fragrant odour. Specific gravity, 
 at 60, about '720. Boils at 98 Fahrenheit. The vapour 
 is exceedingly dense, and may be seen passing off from 
 the liquid and falling to the ground : hence the danger 
 of pouring Ether from one bottle to another if a flame be 
 near at hand. 
 
 Ether does not mix with water in all proportions ; when 
 the two are shaken together, after a short time the former 
 rises and floats upon the surface. In this way a mixture 
 of Ether and Alcohol may be purified to some extent, as 
 in the common process of washing Ether. The water em- 
 ployed however always retains a certain portion of Ether 
 (about a tenth part of its bulk), and acquires a strong 
 ethereal odour ; washed Ether also contains water in small 
 quantity. 
 
 Bromine and Iodine are both soluble in Ether, and gra- 
 dually react upon and decompose it. The strong alkalies, 
 such as Potash and Soda, also decompose Ether slightly 
 after a time, but not immediately. Nitric Acid oxidizes 
 it with evolution of red fumes. Exposed to air and light, 
 Ether is ozonized and acquires a peculiar odour : after a 
 
PHOTOSEAPHIC CHEMICALS. 53 
 
 time Acetic Acid and Acetic Ether are formed by a further 
 action of Oxygen. 
 
 Ether dissolves fatty and resinous substances readily, 
 but inorganic salts are mostly insoluble in this fluid. 
 Hence Iodide of Potassium and other substances dissolved 
 in Alcohol are precipitated to a certain extent by the ad- 
 dition of Ether. 
 
 Ether for Photographic use. The preparation of pure 
 Ether is of the utmost importance in Collodion Photo- 
 graphy, since traces of foreign bodies, which analysis fails 
 to detect, may produce an injurious effect. In the distilla- 
 tion of Ether, the crude product first obtained is conta- 
 minated with Oil of Wine, and contains foul-smelling or- 
 ganic bodies, from which it is partially freed by digestion 
 with Lime or a caustic alkali ; the object being to saponify 
 the impurities and retain them in the body of the retort. 
 It sometimes happens in this process that minute traces 
 of the alkali are mechanically carried over by the Ether, 
 and these are injurious to the keeping qualities of Collo- 
 dion. 
 
 A quality of commercial Ether, having a sp. gr. of about 
 '750 and answering to the demands of the Pharmacopoeia, 
 is much used for making Collodion. This Ether however 
 often leaves an unpleasant smell when allowed to evapo- 
 rate in the hand, and hence the Writer for some time 
 employed in preference a stronger quality, sp. gr. '728, 
 obtained by agitating the above with caustic alkali and 
 drawing over the first portions only. 
 
 Even with these precautions however it was found diffi- 
 cult to obtain a really uniform substance, for although the 
 specific gravity was nearly the same, a perceptible differ- 
 ence in smell could be detected, and the keeping proper- 
 ties of the plain Collodion were found to vary. The fol- 
 lowing plan was therefore adopted and with manifest ad- 
 vantage, viz. to agitate the best Ether in a vessel contain- 
 ing water acidulated with Sulphuric Acid, and then to 
 draw over the first half in a retort ; in this way any traces 
 of alkali are retained, and the impurities if present accu- 
 
54 VOCABULARY OF 
 
 mulate in the last portions.* The product appears inva- 
 riably to restore the blue colour of reddened litmus-paper, 
 and therefore this alkaline reaction is probably peculiar to 
 Ether, although it has been stated in chemical works that 
 Ether is neither acid nor alkaline. 
 
 There are some points with regard to Ether not precisely 
 understood. The Author has seen a sample of Ether so 
 light that a test tube containing it nearly boiled when 
 held in the hand, and yet this same sample injured the 
 quality of all the Collodion to which it was added, chan- 
 ging the Negative picture into an image positive by trans- 
 mitted light. 
 
 Ether prepared from Methylated Spirit (see Alcohol) 
 is extensively employed in the manufacture of Collodion, 
 and especially of Positive Collodion. Great exertions have 
 been lately made to improve the quality of this material, 
 but at one time it was so variable that two samples could 
 scarcely be found to give corresponding results. The pre- 
 sence of the gaseous Methylic Ether introduces a diffi- 
 culty, inasmuch as this substance is more prone to absorb 
 Oxygen than pure Ether. 
 
 A test often applied to Ether is to agitate it with an 
 equal bulk of strong Alcohol containing Iodide of Potas- 
 sium dissolved in the proportion of 3^ grains to the ounce 
 of spirit. The mixture should remain nearly or quite 
 colourless for several days, when the thermometer is at 
 60 Fahr. It is possible to make any Ether stand the 
 test of Iodide of Potassium most perfectly, by agitating it 
 with a little dry Carbonate of Potash ; but the resulting 
 Collodion would be injured rather than improved by such 
 a process, since Carbonated Alkali decomposes Gun-Cot- 
 ton. When, however, we obtain a sample of Ether which 
 has been distilled from Sulphuric Acid, and yet find it to 
 remain colourless for a long time on adding Iodide of Po- 
 
 * The remaining half of the Ether may after distillation be employed 
 for Positive Collodion, or for Bromo-iodized Negative Collodion, in which 
 the quality of the Ether is not of so much importance as in simply iodized 
 Collodion. 
 

 PHOTOGBAPHIC CHEMICALS. 55 
 
 tassium, we may be assured of its perfect freedom from 
 the "ozonized" principle.* 
 
 The amount of Alcohol in a sample of Ether may be 
 ascertained by agitating it in a graduated measure-glass 
 with half its bulk of a saturated solution of Chloride of 
 Calcium ; this should be poured into the tube first, its 
 height noted, and the Ether poured on its surface, the tube 
 being closed by a stopper or the thumb, the two are agi- 
 tated together and then allowed to stand until they sepa- 
 rate, the increase in bulk of the Chloride of Calcium indi- 
 cates the amount of Alcohol present. 
 
 Water is detected in Ether by the turbidity which it 
 causes when the latter is dropped into Spirits of Turpen- 
 tine ; if no water is present, the two mix perfectly. 
 
 An ozonized condition of Ether is at once detected, on 
 agitating it with a little of a solution of Iodide of Potas- 
 sium in Water or Alcohol, by its colouring the latter more 
 or less yellow from the liberation of Iodine. 
 
 The difference between bad and good Ether is seen most 
 evidently after long keeping. Supposing white light to 
 be excluded, a pure sample of Ether may be placed in a 
 bottle, only half full, and at the expiration of two or three 
 months it will scarcely become coloured on the first addi- 
 tion of Iodide of Potassium. Ether only partially purified 
 will often stand the test of Iodide of Potassium when 
 freshly distilled, but it will soon acquire the property of 
 liberating Iodine when it is stowed away for keeping. 
 
 ETHER, ACETIC. 
 Formula, C 4 H 5 0,C 4 H 3 O 3 =88. 
 
 This compound Ether may be produced by distilling 
 Acetate of Soda with Sulphuric Acid and Alcohol. It is 
 a limpid liquid with a sweet and fragrant smell. It is also 
 spontaneously formed in small quantities in ordinary Ether, 
 
 * The condition of Ether known as "ozonized," is the same as that to 
 which the term " acid " is often applied ; but in testing samples of Ether, we 
 rarely find them acid to test-paper. 
 
56 . VOCABULAEY OF 
 
 or more especially in Ether containing dissolved Pyroxy- 
 line, its production being due to a slow process of oxida- 
 tion. In a developing fluid or a Nitrate Bath containing 
 both Acetic Acid and Alcohol, the peculiar smell of Acetic 
 Ether is soon to be detected. 
 
 Acetic Ether is a powerful solvent for Pyroxyline ; being 
 capable of dissolving the most explosive Gun-Cotton which 
 is quite insoluble in ordinary Ether and Alcohol, but it 
 leaves it on evaporation in the form of a white powder. 
 Its presence in Collodion is very injurious, since it de- 
 stroys the tenacity and transparency of the film. 
 
 ETHEE, METHYLIC. 
 
 Formula, C 4 H 6 O 2 = 42. 
 
 This Ether bears the same relation to Pyroxylic Alcohol 
 or Wood Spirit, that ordinary Ether bears to Alcohol from 
 wine. It is a gaseous substance, which dissolves to a cer- 
 tain extent in water, but more abundantly so in Alcohol 
 or Ether. 
 
 "Methylated Ether," the peculiarities of which have 
 been pointed out under the head of Ether, is made from 
 Spirit of Wine containing ten per cent, of wood spirit (see 
 ALCOHOL, METHYLATED). It must therefore contain a 
 portion of gaseous Methylic Ether dissolved in an excess 
 of common Ether. 
 
 FQEMIC ACID. 
 Formula (of concentrated Acid), HO,C 2 HO 3 46. 
 
 As one atom of common Alcohol by the action of four 
 atoms of Oxygen yields Acetic Acid, so one atom of Me- 
 thylic Alcohol by the action of four atoms of Oxygen 
 furnishes Formic Acid. 
 
 Com. Alcohol. Acetic Acid. 
 
 C 4 H 6 2 + 4 = HO,C 4 H 3 3 + 2HO. 
 
 Meth. Alcohol. Formic Acid. 
 
 C 2 H 4 2 + 4 = HO,C 2 H0 3 + 2HO. 
 
PHOTOGRAPHIC CHEMICALS. 57 
 
 Formic Acid is not however usually prepared in this 
 way, the common method is to obtain it by the oxidation 
 of Starch ; for this purpose, 1 of Starch, 4 of Binoxide of 
 Manganese, and 4 of Water are mixed in a large retort ; 
 4 of Sulphuric Acid is then added, effervescence takes 
 place, when this has ceased, distil ; an impure Acid comes 
 over, which is neutralized with Carbonate of Soda. The 
 Formiate of Soda purified by crystallization is distilled 
 with an equivalent of Sulphuric Acid, when the pure For- 
 mic Acid passes over into the receiver. 
 
 When required on the large scale, a better source is 
 Oxalic Acid. Equal weights of Oxalic Acid and Glyce- 
 rine, with one-eighth of Water, are heated in a retort to 
 220Fahr. Effervescence ensues andcontinues for some time, 
 during which some Formic Acid distils over. To obtain 
 the remainder, add one-third of Water and distil again ; 
 repeat addition of Water until about four parts have dis- 
 tilled over. The concentrated Acid is obtained from the 
 Soda Salt as before. The same Glycerine will serve for 
 fresh Oxalic Acid again and again. 
 
 Properties. When concentrated, this is a fuming and 
 corrosive Acid ; it always retains one atom of Water, from 
 which it cannot be deprived without becoming decomposed 
 into Carbonic Oxide and Water. 
 
 HO,C 2 HO 3 -HO = 2CO + HO. 
 
 It forms a white sparingly soluble salt with Oxide of 
 Silver, which, with the least elevation of temperature, yields 
 Metallic Silver. 
 
 AgO,C 2 HO 3 =:CO 2 + CO-f HO + Ag. 
 
 Its name is derived from Formica, an Ant, an insect 
 which stings with this Acid. 
 
 GALLIC ACID. 
 Formula, 3HO,C 14 H 3 O 7 + 2HO = 188. 
 
 Gallic Acid is obtained from the Tannic Acid by a 
 species of fermentation. The powdered galls mixed with 
 water, or their infusion, is left for some weeks, during 
 
58 . VOCABULARY OF 
 
 which it becomes mouldy, Oxygen is absorbed and Car- 
 bonic Acid is given off, and Gallic Acid is deposited in 
 abundance. The mouldy paste is squeezed, to get rid of 
 foreign matters, and then boiled in water, which on cool- 
 ing deposits the Gallic Acid. By digestion with Animal 
 Charcoal and recrystallization it is obtained pure. 
 
 Gallic Acid, like Tannic Acid, gives an intensely blue- 
 black colour with salts of Peroxide of Iron, but it differs 
 from the latter in giving no precipitate with Gelatine. 
 
 GELATINE. 
 
 This is a nitrogenized organic substance somewhat ana- 
 logous to Albumen, but differing from it in properties. 
 It is obtained by subjecting bones, hoofs, horns, calves' 
 feet, etc., to the action of boiling water. The jelly formed 
 on cooling is termed size, or, when dried and cut into 
 slices, glue. Gelatine, as it is sold in the shops, is a pure 
 form of Glue. Isinglass is gelatine prepared, chiefly in 
 Russia, from the air-bladders of certain species of stur- 
 geon. 
 
 Properties of Gelatine. Gelatine softens and swells up 
 in cold water, but does not dissolve until heated ; the hot 
 solution, on cooling, forms a tremulous jelly. One ounce 
 of cold water will retain about three grains of Isinglass 
 without gelatinizing ; but much depends upon the tempe- 
 rature, a few degrees greatly affecting the result. The 
 solution forms an insoluble precipitate with Tannic Acid, 
 which has the composition of Leather. 
 
 When long boiled in water, and especially in presence 
 of an acM, such as the Sulphuric, Gelatine undergoes a 
 peculiar modification, and the solution loses either partially 
 or entirely its property of solidifying to a jelly. 
 
 The reactions of Gelatine with Nitrate of Silver will be 
 described afterwards. 
 
 GLYCERINE. 
 
 Formula, C 6 H 8 O 6 = 92. 
 Fatty bodies are resolved by treatment with an alkali 
 
PHOTOGBAPHIC CHEMICALS. 59 
 
 into an acid, which combines with the alkali, forming a 
 soap, and Glycerine, which remains in solution. 
 
 Pure Glycerine, as obtained by Price's patent process of 
 distillation, is a sweet viscid liquid of sp. gr. about T23 ; 
 miscible in all proportions with water and Alcohol. It is 
 neutral to test-paper. It has little or no action upon Ni- 
 trate of Silver in the dark, and reduces it very slowly even 
 when exposed to light. 
 
 GLYCYEKHIZINE. 
 
 Glycyrrhizine, obtained from the fresh root of Liquorice, 
 is a substance intermediate in properties between a sugar 
 and a resin. Sparingly soluble in water, but very soluble 
 in Alcohol. It precipitates strong solution of Nitrate of 
 Silver white, but the deposit becomes reddened by expo- 
 sure to light. 
 
 A mode of preparing Glycyrrhizine has been described 
 in which Carbonate of Potash is used, but the Author 
 does not find it to succeed. The process given in Miller's 
 ' Elements of Chemistry ' may however be depended on. 
 Procure fresh Liquorice- root and slice it very fine trans- 
 versely ; then pound well in an iron mortar, and add as 
 much boiling water as will just cover it. Place the jug 
 in a warm situation for twelve hours^and afterwards press 
 out the juice as far as possible by means of an iron screw- 
 press. Evaporation by artificial heat is useless, and serves 
 only to decompose the Glycyrrhizne. 
 
 Next mix Oil of Vitriol with an equal bulk of water and 
 allow it to cool. Then add it to the strong infusion until 
 test-paper becomes immediately red when dipped in the 
 liquid, and a thick yellow deposit forms. A few drachms 
 more or less of the Sulphuric Acid make but little differ- 
 ence, but if too much be used, the trouble of washing out 
 the acid from the Glycyrrhizine will be increased. 
 
 Leave it for twelve hours, when the yellow deposit may 
 be collected on a cloth. Begin by draining it nearly dry, 
 and then wash it with a moderate quantity of water to 
 remove the excess of Sulphuric Acid. Now squeeze it in 
 
60 VOCABULARY OP 
 
 a powerful press until it is dry enough to powder in the 
 hand, and digest it without heat in Absolute Alcohol, about 
 four or five times its weight. The pure Glycyrrhizine dis- 
 solves, and a bulky mass of vegetable Albumen is left be- 
 hind. Lastly, evaporate at a temperature not higher than 
 120 Fahr. 
 
 The Only difficulty in the process is the washing of the 
 impure Glycyrrhizine with water to remove the Sulphuric 
 Acid, since the sugar is partially soluble in water, and even 
 with proper care much of it is lost in the process. For this 
 reason the Author thinks it better not to use too much 
 acid in precipitating the Glycyrrhizine, although the quan- 
 tity of deposit is greater when the acid is added freely. 
 
 The Glycyrrhizine exists in the root in combination with 
 bases, Lirne, etc., and is soluble in water, but insoluble in 
 Alcohol. The Sulphuric Acid removes the bases, and the 
 sugar-resin is then much more soluble in Alcohol than in 
 Water. Alkalies and Alkaline Carbonates gradually de- 
 compose Glycyrrhizine, with production of a strong yel- 
 low colour. 
 
 GOLD, CHLOEIDE OF. 
 Formula, And, = 303-1. 
 
 Formula of Crystals = AuC] 3 HC1 = 339'6. 
 
 This salt is formed by dissolving pure metallic Gold in 
 Nitro-Hydrochloric Acid, and evaporating at a gentle heat. 
 The solution affords deliquescent crystals of a deep orange 
 colour, which are a double Chloride of Gold and Hydrogen. 
 
 Chloride of Gold, in a state fit for Photographic use, 
 may easily be obtained by the following process : Place 
 a half-sovereign in any convenient vessel, and pour on it 
 half a drachm of Nitric Acid mixed with two and a half 
 drachms of Hydrochloric Acid and three drachms of water ; 
 digest by a gentle heat, but do not boil the acid, or much 
 of the Chlorine will be driven off in the form of gas. At 
 the expiration of a few hours add fresh JSTitro-Hydrochloric 
 Acid in quantity the same as at first, which will probably 
 
PHOTOGEAPHIC CHEMICALS. 61 
 
 T>e sufficient, but if not, repeat the process a third time.* 
 Next, dilute largely with distilled water, and add a filtered 
 aqueous solution of common Sulphate of Iron (6 parts to 
 1 of Gold) ; collect the precipitated Gold, which is now 
 free from Copper ; redissolve in Aqua Hegia as at first, 
 and evaporate to dryness on a water bath. 
 
 The weight of a half-sovereign is about 61 grains, of 
 which 56 grains are pure gold. This is equivalent to 86 
 grains of Chloride of Gold, which will be the quantity con- 
 tained in the solution. 
 
 Properties of Chloride of Gold. The crystals contain 
 Hydrochloric Acid, and are of a bright yellow collour ; but 
 when the H Cl has been expelled, AuCl 3 remains as a dark- 
 red amorphous mass (Leo ruler of the alchemists). Chlo- 
 ride of Gold is decomposed with precipitation of metallic 
 Gold by Charcoal, Sulphurous Acid, and many of the ve- 
 getable acids, such as Citric, Tartaric, etc. ; also by Pro- 
 tosulphate and Protonitrate of Iron. It tinges the cuticle 
 of an indelible purple tint. It is soluble in Alcohol and 
 in Ether. 
 
 The most convenient mode of keeping the Chloride is 
 in aqueous solution, one grain to the drachm of water. 
 Sometimes a partial reduction of Gold will take place upon 
 the sides of the bottle, but the quantity so deposited is very 
 small, and may be disregarded. 
 
 GOLD, FULMINATING. 
 
 This is a yellowish-brown substance, precipitated on add- 
 ing Ammonia to a strong solution of Chloride of Gold. 
 
 It may be dried carefully at 212, but explodes violently 
 on being heated suddenly to about 290. Friction also 
 causes it to explode when dry ; but the moist powder may 
 be rubbed or handled without danger. It is decomposed 
 by Sulphuretted Hydrogen. 
 
 Fulminating Gold is probably an Aurate of Ammonia, 
 
 * Observe that if any Silver be present in the Gold coin it will produce a 
 flocculent deposit of Chloride of Silver, thus preventing the solution from 
 being complete. 
 
62 VOCABULARY OF 
 
 containing two atoms of Ammonia to one atom of Peroxide 
 of Gold. 
 
 GOLD, HYPOSULPHITE OF. 
 Formula, AuO,S 2 O 2 . = 252'6. 
 
 Hyposulphite of Gold is produced by the reaction of 
 Chloride of Gold upon Hyposulphite of Soda. 
 
 The salt sold in commerce as Sel d'or is a double Hypo- 
 sulphite of Gold and Soda, having the composition 
 
 3(NaO,S 2 O 2 ) + AuO,S 2 O 2 +4HO = 525-6, 
 and containing 37'4 per cent, of metallic Gold when pure. 
 It is formed by adding one part of Chloride of Gold, in 
 solution, to three parts of Hyposulphite of Soda, and pre- 
 cipitating the resulting salt by Alcohol : the Chloride of 
 Gold must be added to the Hyposulphite of Soda, and not 
 the Soda Salt to the Gold. 
 
 Properties. Hyposulphite of Gold is unstable and can- 
 not exist in an isolated state, quickly passing into Sulphur, 
 Sulphuric Acid, and metallic Gold. When combined with 
 excess of Hyposulphite of Soda in the form of Sel d'or, it 
 is more permanent. 
 
 Sel d'or occurs crystallized in fine needles, which are 
 very soluble in water. The commercial article is often 
 impure, containing little else than Hyposulphite of Soda, 
 with a trace of Gold. It may be analysed by adding a few 
 drops of strong Nitric Acid (free from Chlorine), diluting 
 with water, and afterwards collecting and igniting the yel- 
 low powder, which is metallic Gold. 
 
 GEAPE SUGAE. 
 
 Formula, C 12 H 12 O 12 + 2HO = 198. 
 
 This modification of Sugar, often termed Granular Su- 
 gar, or Glucose, exists abundantly in the juice of grapes 
 and in many other varieties of fruit. It forms the saccha- 
 rine concretion found in honey, raisins, dried figs, etc. It 
 may be produced artificially, by the action of dilute mine- 
 ral acids upon Starch or Cane Sugar. 
 
 Properties. Grape Sugar crystallizes slowly and with 
 
PHOTOGRAPHIC CHEMICALS. 63 
 
 difficulty from a concentrated aqueous solution, in small 
 hemispherical nodules, which are hard, and feel gritty be- 
 tween the teeth. It is much less sweet to the taste than 
 Cane Sugar, and is not so soluble in water (1 part dissolves 
 in 1| of cold water). 
 
 Grape Sugar decomposes the salts of the noble metals, 
 reducing them by degrees to the metallic state, even with- 
 out the aid of light ; Cane Sugar does not possess these pro- 
 perties to an equal extent. A very delicate test for Grape 
 Sugar is founded upon its reducing power on Oxide of Cop- 
 per in presence of free Alkalies. The mode of applying 
 the test is to add to the solution of Sugar, a drop of solu- 
 tion of Sulphate of Copper, and then enough solution of 
 Potash to render the liquid strongly Alkaline ; no precipi- 
 tation of Oxide of Copper takes place, when either Cane 
 or Grape Sugar are present, but the blue colour of the so- 
 lution becomes much more intense. On now gently warm- 
 ing the mixture, if Grape Sugar is present, a yellow or 
 reddish precipitate will soon be seen at the top of the liquid, 
 and will gradually spread throughout the whole. If Cane 
 Sugar only be present, no such precipitate or change of 
 colour takes place, even if the liquid be boiled. 
 
 GUM AEABIC. 
 
 This substance is met with in the juices of many plants, 
 but is found in its purest form in the transparent tears 
 which exude from various species of Acacia. 
 
 Gum Arabic is now believed to consist of an organic 
 substance resembling mucilage of Tragacanth, combined 
 with a portion of Lime or other base. The mucilaginous 
 constituent in the pure state, when once dried, is not solu- 
 ble in water, but simply swells up when wetted, without 
 passing into solution. The addition of an alkali, or of an 
 alkaline earth such as Lime, renders it completely soluble 
 in water. 
 
 HONEY. 
 
 This substance contains two distinct kinds of Sugar, 
 
64 VOCABULARY OF 
 
 crystallizable Grape Sugar, and an uncrystallizable sub- 
 stance. The agreeable taste of Honey probably depends 
 upon the latter, but its reducing power on metallic oxides 
 is due to the former. Pure Grape Sugar can readily be 
 obtained from inspissated Honey, by treating it with Al- 
 cohol, which washes out the syrup, but leaves the crystal- 
 line portion. 
 
 For Photographic use the Virgin Honey should be 
 obtained direct from the comb, much of the commercial 
 article being adulterated with flour, which causes it to fer- 
 ment on the addition of water. 
 
 HYDEIODIC ACID. 
 
 Formula, HI= 128. 
 
 This is a gaseous compound of Hydrogen and Iodine, 
 corresponding in composition to the Hydrochloric Acid. 
 It cannot however, from its instability, be obtained in the 
 same manner, since, on distilling an Iodide with Sulphuric 
 Acid, the Hydriodic Acid first formed is subsequently 
 decomposed by the Sulphuric Acid. An aqueous solution 
 of Hydriodic Acid is easily prepared by passing Sulphu- 
 retted Hydrogen gas through water standing over powdered 
 Iodine, until the liquid, which about the middle of the 
 operation becomes very brown on shaking, from solution 
 of excess of Iodine, is just decolorized again. Sulphur 
 separates out abundantly, and can be removed either by 
 filtering or decantation. The solution ought not to smell 
 of Sulphuretted Hydrogen, or blacken paper dipped in 
 Acetate of Lead or Nitrate of Silver, held over it. Its 
 formation is as follows : H S + 1 HI + S. 
 
 Properties. Hydriodic Acid gas is very soluble in 
 water, yielding a strongly acid liquid. The solution, 
 colourless at first, soon becomes brown from liberation of, 
 free Iodine. It may be restored to its original condition 
 by adding solution of Sulphuretted Hydrogen and allow 
 ing Sulphur to subside. 
 
 
PHOTOGRAPHIC CHEMICALS. 65 
 
 HYDEOCHLOEIC ACID. 
 Formula, H 01 = 36-5. 
 
 Hydrochloric Acid is a volatile gas, which may be libe- 
 rated from most of the salts termed Chlorides by the action 
 of Sulphuric Acid. The acid, by its superior affinities, 
 decomposes these salts in the following manner : 
 NaCl + HO, SO 3 =NaO, SO 3 + HC1. 
 
 Properties. Abundantly soluble in water, forming the 
 liquid Hydrochloric or Muriatic Acid of Commerce. The 
 most concentrated solution of Hydrochloric Acid has a 
 sp. gr. 1-2, and contains about 40 per cent, of gas ; that 
 commonly sold is somewhat weaker, sp. gr. 1'14 to 1*16, 
 containing about 28 per cent, real acid. 
 
 Pure Hydrochloric Acid is colourless, and fumes in the 
 air. The yellow colour of the commercial acid depends 
 upon the presence of traces of Perchloride of Iron, or of 
 organic matter; commercial Muriatic Acid also often con- 
 tains free Chlorine and Sulphuric Acid. 
 
 HYDEOGEN. 
 Symbol, H = l. 
 
 Hydrogen is an element in the form of a gas, whose 
 Oxide is Water. It is obtained from Water by adding to 
 it metallic Zinc and a tenth of Sulphuric Acid ; the Oxy- 
 gen of the Water unites with the Zinc to form the base 
 Oxide of Zinc, which with the acid forms Sulphate of 
 Zinc. (See SALTS, Chap. I.) 
 
 It is very combustible, the product being water. United 
 with the elements, Chlorine, Iodine, Bromine, Fluorine, 
 and Sulphur, it forms acids (and, indeed, according to one 
 mode of viewing acids it is the acidifying principle, all acids 
 being thus regarded as salts of Hydrogen). 
 
 HYDEOSULPHUEIC ACID. 
 
 Formula, HS = 17. 
 This gas, also known as Sulphuretted Hydrogen, is a 
 
66 . VOCABULARY OF 
 
 compound of Sulphur and Hydrogen, analogous in com- 
 position to the Hydrochloric and Hydriodic Acids. It is 
 usually prepared by the action of dilute Sulphuric Acid 
 upon Sulphide of Iron, the decomposition being similar to 
 that involved in the preparation of the Hydrogen Acids 
 generally : FeS + HO, S O 3 = FeO, SO 3 + H S. 
 
 Properties. Cold water absorbs three times its bulk 
 of Hydrosulphuric Acid, and acquires the peculiar putrid 
 odour and poisonous qualities of the gas. The solution is 
 faintly acid to test-paper, and becomes opalescent on keep- 
 ing, from gradual separation of Sulphur. It is decom- 
 posed by Nitric Acid, and also by Chlorine and Iodine. 
 It precipitates the last ten metals in the list of elements 
 from their solutions in the state of Sulphides ; but Iron 
 and other metals of that class are not affected, in presence 
 of free acid. Hydrosulphuric Acid is constantly employed 
 in the chemical laboratory for these and other purposes. 
 
 ICELAND MOSS. 
 
 Cetraria Islandica. A species of Lichen found in Ice- 
 land and the mountainous parts of Europe ; when boiled 
 in water, it first swells up, and then yields a substance 
 which gelatinizes on cooling. 
 
 It contains Lichen Starch, a bitter crystallizable prin- 
 ciple soluble in Alcohol, termed " Cetrarine " and common 
 Starch ; traces of Gallic Acid and Bitartrate of Potash are 
 also present. 
 
 IODINE. 
 
 Symbol, I = 127. 
 
 Iodine is chiefly prepared at Glasgow, from Jcelp, which 
 is the fused ash obtained on burning seaweeds. The wa- 
 ters of the ocean contain minute quantities of the Iodides 
 of Sodium and Magnesium, which are separated and stored 
 up by the growing tissues of the marine plant. 
 
 In the preparation, the mother-liquor of kelp (which is 
 the liquid that remains after most of the salts, which con- 
 tain no Iodine, have been separated by crystallization) is 
 
PHOTOGRAPHIC CHEMICALS. 67 
 
 distilled at a gentle heat with a certain proportion of Sul- 
 phuric Acid and Binoxide of Manganese. The decompo- 
 sition is as follows : 
 
 Nal + MnO 2 + 2SO 3 = NaO,SO 3 + MnO,SO 3 + I. 
 The Iodine sublimes in purple vapours, which condense to 
 black crystals. 
 
 Properties. Iodine has a bluish-black colour and me- 
 tallic lustre : it stains the skin yellow, and has a pungent 
 smell, like diluted Chlorine. It is extremely volatile when 
 moist, boils at 347, and produces dense violet coloured 
 fumes, which condense in brilliant plates. Specific gravity, 
 4'946. Iodine is very sparingly soluble in water, 1 part 
 requiring 7000 parts for perfect solution : even this mi- 
 nute quantity however tinges the liquid of a brown colour. 
 Alcohol and Ether dissolve it more abundantly, forming 
 dark-brown solutions. Iodine also dissolves freely in solu- 
 tions of the alkaline Iodides, such as the Iodide of Potas- 
 sium, of Sodium, and of Ammonium. 
 
 Chemical Properties. Iodine belongs to the Chlorine 
 group of elements, characterized by forming acids with 
 Hydrogen, and combining extensively with the metals (see 
 Chlorine). They are however comparatively indifferent 
 to Oxygen, and also to each other. The Iodides of the 
 alkalies and alkaline earths are soluble in water ; also 
 those of Iron, Zinc, Cadmium, etc. The Iodides of Lead 
 and Mercury are nearly, and the Iodide of Silver is quite, 
 insoluble. ' 
 
 Iodine possesses the property of forming a compound 
 of a deep blue colour with Starch. In using this as a test, 
 it is necessary first to liberate the Iodine (if in combina- 
 tion) by means of Chlorine, avoiding an excess, or by 
 means of Nitric Acid saturated with Peroxide of Nitrogen. 
 The presence of Alcohol or Ether interferes to a certain 
 extent with the result. 
 
 IODOFORM. 
 Formula, C 2 HI 3 =: 394. 
 
 The composition of this substance is analogous to that 
 
68 . VOCABULARY OF 
 
 of Chloroform, Iodine being substituted for Chlorine. It- 
 is obtained on boiling together Iodine, Carbonate of Potash, 
 and Alcohol. 
 
 lodoform occurs in yellow nacreous crystals, which have 
 a saffron-like odour. It is insoluble in water, but soluble 
 in spirit. 
 
 IKON, PKOTACETATE OF. 
 Formula, FeO,C 4 H 3 O 3 . 
 
 There are two Acetates of Iron, a Protacetate, which is 
 nearly colourless, and a Peracetate, which is red ; the for- 
 mer only is used in Photography. 
 
 A solution of the Protacetate containing a slight excess 
 of the Sulphate, but sufficiently pure for Photographic pur- 
 poses, may be made by dissolving 12 grains of Protosulphate 
 of Iron and 12 grains of crystallized Acetate of Lead, each 
 in half an ounce of water, mixing, and filtering from the 
 white deposit, which is Sulphate of Lead. The solution is 
 very unstable, and soon deposits a reddish Subsalt, or if 
 free Acetic Acid be present, it assumes a red colour. 
 
 A mixture of Acetate of Soda with Protosulphate of 
 Iron, six grains of the former and twelve grains of the 
 latter to an ounce of water, acts in Photography very much 
 in the same manner as the pure solution of the Acetate. 
 
 IKON, AMMONIO-CITKATE OF. 
 
 This beautiful pharmaceutical preparation is a basic salt 
 containing Citric Acid in union both with Ammonia and 
 with Peroxide of Iron. It is met with in the form of thin 
 transparent-brown scales, produced by drying a syrupy 
 solution on warm plates. 
 
 The reaction of certain tests upon the Ammonio-Citrate 
 and also upon the Ammonio-Tartrate of Iron, is peculiar. 
 Free Ammonia, which usually throws down a red Sesqui- 
 oxide from the Persalts of Iron, produces no deposit with 
 these compounds ; and Ferrocyanide of Potassium, which 
 usually precipitates Prussian blue, simply produces a 
 
PHOTOGRAPHIC CHEMICALS. 
 
 purple colour ; the blue precipitate is, however, obtained on 
 acidifying the liquid. The presence of the vegetable acid 
 and the basic character due to Ammonia are the causes of 
 these anomalies. 
 
 PEECHLOEIDE OF. 
 Formula, Fe 2 Cl 3 = 162'5. 
 
 There are two Chlorides of Iron, corresponding in com- 
 position to the Protoxide and the Sesquioxide respectively. 
 The Protochloride is very soluble in water, forming a green 
 solution, which precipitates a dirty white Protoxide on the 
 addition of an alkali. The Perchloride, on the other hand, 
 is dark- brown, and gives with alkalies a reddish-brown 
 precipitate of Peroxide. 
 
 Properties. Perchloride of Iron may be obtained in a 
 state of purity by heating Iron wire in excess of Chlorine ; 
 it condenses in the shape of brilliant and iridescent brown 
 crystals, which are volatile, and dissolve in water, the so- 
 lution being acid to test-paper ; a more easy mode of pre- 
 paring the solution, however, is by digesting Hydrochloric 
 Acid with excess of Peroxide of Iron. It is soluble in 
 Alcohol, forming the Tinctura Ferri Sesquichloridi of the 
 Pharmacopoeia. Commercial Perchloride of Iron ordi- 
 narily contains an excess of Hydrochloric Acid. 
 
 IRON, IODIDE OF. 
 
 Formula, Fel = 155. 
 
 Iodide of Iron is prepared by digesting an excess of 
 Iron filings wdth pulverized Iodine and water. It is very 
 soluble in water and in Alcohol, but the solution rapidly 
 absorbs Oxygen, and deposits Peroxide of Iron ; hence the 
 importance of preserving it in contact with metallic Iron, 
 with which the separated Iodine may recombine. By very 
 careful evaporation, hydrated crystals of Proto-iodide may 
 be obtained, but the composition of the solid salt usually 
 sold under that name cannot be depended on. 
 
 A Periodide of Iron, corresponding to the Perchloride, 
 does not exist. 
 
70 . VOCABULARY OF 
 
 IKON, PEOTONITEATE OF. 
 Formula, FeO,NO 5 + 7Aq. = 153. 
 
 This salt, by careful evaporation in vacua over Sul- 
 phuric Acid, forms transparent crystals, of a light green 
 colour, and containing 7 atoms of water, like the Protosul- 
 phate. It is exceedingly unstable, and soon becomes red 
 from decomposition, unless preserved out of contact with 
 air. The preparation of a solution of Protonitrate of Iron 
 suitable for developing Collodion Positives, will be given 
 in Part III. 
 
 IEON, PEOTOSULPHATE OF. 
 Formula, FeO, S O 3 + 7 Aq. = 139. 
 
 This salt, commonly called Copperas, is much used in 
 the arts ; it is employed in Photography to develope the 
 latent image. It dissolves in rather more than an equal 
 weight of cold water, or in less of boiling water. 
 
 Aqueous solution of Sulphate of Iron becomes oxidized 
 by exposure to the air, it also is able to absorb the gas 
 Binoxide of Nitrogen, acquiring a deep olive-brown colour ; 
 as this gaseous Binoxide is itself a reducing agent, the 
 liquid so formed has been proposed as a more energetic 
 developer than the Sulphate of Iron alone. 
 
 KAOLIN, OE CHINA CLAY. 
 
 This is prepared, by careful levigation, from mouldering 
 granite and other disintegrated felspathic rocks. It is a 
 Silicate of Alumina, containing Silicic Acid or Flint, united 
 with the base Alumina. Kaolin is perfectly insoluble in 
 water and in acids, and produces no decomposition in solu- 
 tion of Nitrate of Silver. It is employed by Photographers 
 to decolorize solutions of Nitrate of Silver which have be- 
 come brown from the action of Albumen or other organic 
 matters. 
 
 Commercial Kaolin may contain chalk, in which state 
 it produces alkalinity in solution of Nitrate, of Silver when 
 
PHOTOGKAPHIC CHEMICALS. 71 
 
 Salts of Ammonia are present. The impurity, detected 
 by its effervescence with acids, is removed by washing the 
 Kaolin in diluted vinegar and subsequently in water. 
 
 LEAD, ACETATE OF. 
 Formula, PbO,C 4 H 3 O 3 + 3HO = 189'6. 
 
 The neutral Acetate of Lead is a very abundant sub- 
 stance in commerce, and is known as Sugar of Lead. It 
 is prepared by digesting Oxide of Lead in Pyroligneous 
 or Acetic Acid, and crystallizes in acicular masses. 
 
 Acetate of Lead is easily soluble in cold water, but the 
 solution is usually milky, either from the presence of a 
 little Carbonate mixed with the Acetate, or from Carbonic 
 Acid or Carbonate of Lime in the water used. The crys- 
 tals employed in Pharmacy and sold at a low price are 
 usually pure enough for Photographic purposes. 
 
 LEAD, NITEATE OF. 
 Formula, PbO,NO 5 = 165'6. 
 
 Nitrate of Lead is obtained by dissolving the metal, or 
 the Oxide of Lead, in excess of Nitric Acid, diluted with 
 2 parts of water. It crystallizes on evaporation in white 
 anhydrous tetrahedra and octohedra, which are hard, and 
 decrepitate on being heated ; they are soluble in .8 parts 
 of water at 60. 
 
 Nitrate of Lead forms with Sulphuric Acid, or with so- 
 luble Sulphates, a white precipitate, which is the insoluble 
 Sulphate of Lead. The Iodide of Lead is also very spa- 
 ringly soluble in water. 
 
 LIGNINE. See CELLULOSE. 
 
 LITHIUM, IODIDE OF. 
 
 Formula, L I -f 6 Aq. = 188. 
 
 Lithium is one of the rarer elements, found in the 
 mineral kingdom, and the basic reactions of its Oxide are so 
 strong that it takes its place with the alkalies Potash and 
 
72 VOCABULAEY OF 
 
 Soda. Metallic Lithium, though a metal, is the lightest 
 solid body known, its density being little more than one- 
 half of that of water. 
 
 Iodide of Lithium has been proposed for Photographic 
 use, being more easily soluble in Alcohol than Iodide of 
 Potassium. Its deliquescent nature however is an ob- 
 jection, .as also is the difficulty and expense of obtaining 
 it commercially in a pure state. A solution of the Iodide 
 may be prepared by mixing equivalent proportions of 
 crystallized Sulphate of Lithia and Iodide of Calcium in 
 concentrated aqueous solution, evaporating to dryness 
 over Sulphuric Acid in vacua, and exhausting the dry re- 
 sidue with Alcohol of *805. Iodide of Calcium is prefer- 
 able to Iodide of Barium, in consequence of the latter 
 being more frequently contaminated with an excess of 
 
 LITMUS. 
 
 Litmus is a vegetable substance prepared from various 
 lichens, which are principally collected on rocks adjoining 
 the sea. The blue colouring-matter is extracted by a 
 peculiar process, and is afterwards made up into a paste 
 with chalk, plaster of Paris, etc. 
 
 Litmus occurs in commerce in the form of small cubes 
 of a fine violet colour. In using it for the preparation of 
 test-papers, it is digested in hot water, the solution con- 
 centrated at a gentle heat, and sheets of porous paper 
 soaked in the blue liquid so formed. The red papers are 
 prepared at first in the same manner, but are afterwards 
 placed in water which has been rendered faintly acid with 
 Sulphuric or Hydrochloric Acid. Papers are prepared 
 also of a purplish tint, which becomes full blue with alka- 
 lies, and bright red with acids. 
 
 MAGNESIUM, IODIDE OF. 
 Formula, Mgl + Aq. ? 
 
 This Iodide has been recommended for Collodion in con- 
 sequence of its ready solubility in Alcohol. Commercial 
 
PHOTOGEAPHIC CHEMICALS. 73 
 
 Iodide of Magnesium usually contains free Iodine in some 
 quantity, but a colourless solution of the Iodide sufficiently 
 pure for Photographic use may easily be obtained as fol- 
 lows : Take crystallized Sulphate of Magnesia and heat 
 it in a porcelain capsule on a sand bath, until all the water 
 of crystallization has been expelled, and a dry white mass 
 remains. Pulverize twenty-five grains of this substance 
 very finely, and rub it up in a glass mortar with thirty- 
 two grains of pure Iodide of Potassium, likewise finely 
 powdered. Then introduce it into a dry bottle with two 
 fluid ounces of Alcohol of '805, sold as absolute, and agi- 
 tate for ten minutes or a quarter of an hour. Filter out 
 the insoluble Sulphate of Potash and the excess of Sul- 
 phate of Magnesia, when the solution will be fit for use ; 
 it must be added in the usual proportion, viz. two drachms 
 of the iodizing liquid to six of plain Collodion. 
 
 MEECUEY, CHLOKIDE OF 
 
 (formerly BICHLOEIDE). 
 
 Formula, HgCl= 135-5. 
 
 This salt, also called Corrosive Sublimate, and often Si- 
 chloride of Mercury (the atomic weight of Mercury being 
 doubled), may be formed by heating Mercury in excess of 
 Chlorine, or, more economically, by subliming a mixture 
 of Sulphate of Mercury and Chloride of Sodium. 
 
 Properties. A very corrosive and poisonous salt, usu- 
 ally sold in semitransparent, crystalline masses, or in the 
 state of powder. Soluble in 16 parts of cold, and in 3 
 of hot water ; more abundantly so in Alcohol, and also in 
 Ether. The solubility in water may be increased by the 
 addition of free Hydrochloric Acid, or of Chloride of Am- 
 monium. 
 
 The Subchloride of Mercury is an insoluble white pow- 
 der, commonly known under the name of Calomel. 
 
 MILK. 
 
 The Milk of herbivorous animals contains three prin- 
 
74 VOCABULAEY OF 
 
 cipal constituents Fatty matter, Caseine, and Sugar ; in 
 addition to these, small quantities of the Alkaline Chlo- 
 rides and Phosphates, and of Phosphates of Lime and 
 Magnesia, are present. 
 
 The fatty matter is contained in small cells, and forms 
 the greater part of the cream which rises to the surface of 
 the milk on standing ; hence skimmed milk is to be pre- 
 ferred for Photographic use. 
 
 The second constituent, Caseine, is an organic principle 
 somewhat analogous to Albumen in composition and pro- 
 perties. It differs from it, however, in not being coagu- 
 lated by boiling, but it is, on the other hand, precipitated 
 by Acetic Acid. The substance termed "rennet," which 
 is the dried stomach of the calf, possesses the property of 
 coagulating Caseine, but the exact mode of its action is 
 unknown. .Sherry wine is also commonly employed to 
 curdle milk ; but brandy and other spirituous liquids, 
 when free from acid and astringent matter, have no effect. 
 
 In all these cases, a portion of the Caseine usually re- 
 mains in a soluble form in the wliey ; but when the Milk 
 is coagulated by the addition of acids, the quantity so left 
 is very small, and hence the use of the rennet is to be pre- 
 ferred, since the presence of Caseine facilitates the reduc- 
 tion of the sensitive Silver salts by light. 
 
 Caseine combines with Oxide of Silver in the same man- 
 ner as Albumen, forming a white coagulum, which becomes 
 brick-red on exposure to light. 
 
 Sugar of Milk, the third principal constituent, differs 
 from both cane and grape sugar ; it may be obtained by 
 evaporating whey until crystallization begins to take place. 
 It is hard and gritty, and only slightly sweet ; slowly so- 
 luble, without forming a syrup, in about two and a half 
 parts of boiling, and six of cold water. It does not fer- 
 ment and form Alcohol on the addition of yeast, like grape 
 sugar, but by the action of decomposing animal matter is 
 converted into Lactic Acid. 
 
 When skimmed Milk is exposed to the air for some 
 hours, it gradually becomes sour, from Lactic Acid formed 
 
PHOTOCEAPHIC CHEMICALS. 75 
 
 in this way ; and if then heated to ebullition, the Caseine 
 coagulates very perfectly. 
 
 NAPHTHA. 
 
 This name is applied to several liquids sold in com- 
 merce. First, Wood Naphtha, often used for burning in 
 lamps, which is the same as Pyroxylic Spirit or Methylic 
 Alcohol, q. v. ; second, Coal Naphtha, a volatile liquid 
 which distils over in the process of manufacturing gas ; 
 and, thirdly, Mineral Naphtha, found in the soil of cer- 
 tain places in Europe and Asia, and recently very abund- 
 antly in Canada and Pennsylvania, in America. 
 
 The two latter substances, to which the name of Naphtha 
 ought to be restricted, possess the same leading charac- 
 ters, in that they are Hydrocarbons, burning with a bright, 
 smoky flame ; they do not mix with water, and they dis- 
 solve Caoutchouc, in all which respects they differ from 
 " Wood Naphtha." (See BENZOLE.) 
 
 NITRIC ACID. 
 Formula, NO 5 =54. 
 
 Nitric Acid, or Aqua-fortis, is prepared by adding Sul- 
 phuric Acid to Nitrate of Potash, and distilling the mix- 
 ture in a retort. Sulphate of Potash and free Nitric Acid 
 are formed, the latter of which, being volatile, distils over 
 in combination with one atom of water previously united 
 with the Sulphuric Acid. 
 
 Properties. Anhydrous Nitric Acid is a solid substance, 
 white and crystalline, but it cannot be prepared except by 
 an expensive and complicated process. 
 
 The strongest liquid Nitric Acid contains 1 atom of 
 water, and has a sp. gr. of about T5 ; if perfectly pure, it 
 is colourless, but usually it has a slight yellow tint, from 
 partial decomposition into Peroxide of Nitrogen : it fumes 
 strongly in the air. 
 
 The strength of commercial Nitric Acid is subject to 
 much variation. An acid of sp. gr. 1'42, containing about 
 
76 VOCABULARY OF 
 
 4 atoms of water, is often met with. If the specific gravity 
 is much lower than this (less than 1'36), it will scarcely be 
 adapted for the preparation of Pyroxyline. The yellow 
 Nitrous Acid, so called, is a strong Nitric Acid partially 
 saturated with the brown vapours of Peroxide of Nitrogen : 
 it has a high specific gravity, usually about T45, but this 
 is deceptive, being caused in part by the presence of the 
 Peroxide. On mixing with Sulphuric Acid, the colour 
 disappears, a compound being formed which has been 
 termed a Sulphate of Nitrous Acid. 
 
 In the Appendix a Table is given which exhibits the 
 quantity of real anhydrous Nitric Acid contained in samples 
 of different densities. 
 
 Nitric Acid is a powerful oxidizing agent ; it attacks all 
 the common metals, with the exception of Gold and Pla- 
 tinum. Metals dissolving in acids usually derive the 
 necessary amount of Oxygen (see SALTS, Chap. I.) from 
 the water, Hydrogen being given off; but when Nitric 
 Acid is the solvent, Oxygen is derived from it, and N O 2 
 or NO 3 or NO 4 escapes. Animal substances, such as the 
 cuticle, nails, etc., are tinged of a permanent yellow co- 
 lour, and are deeply corroded by a prolonged application. 
 Nitric Acid forms a numerous class of salts, all of which 
 are soluble in water; hence its presence cannot be deter- 
 mined by any precipitating reagent, in the same manner 
 as that of Hydrochloric and Sulphuric Acid. 
 
 The principal impurities in commercial Nitric Acid are 
 Chlorine and Sulphuric Acid; also Peroxide of Nitrogen, 
 which tinges the acid yellow, as already described. Chlo- 
 rine is detected by diluting the acid with an equal bulk of 
 distilled water, and adding a few drops of Nitrate of Silver, 
 a milkiness, which is Chloride of Silver in suspension, 
 indicates the presence of Chlorine. In testing for Sulphu- 
 ric Acid, dilute the Nitric Acid as before, and drop in a 
 single drop of solution of Chloride of Barium ; if Sulphu- 
 ric Acid be present, an insoluble precipitate of Sulphate 
 of Baryta will be formed. 
 
 Nitric Acid for Photographic use. A convenient form 
 
PHOTOGBAPHIC CHEMICALS. 77 
 
 of acid for preparing Pyroxyline is the Nitric Acid of 1'45. 
 The question has been asked, why so concentrated an acid 
 is recommended, seeing that it is afterwards to be diluted 
 with water P There are two reasons ; first, because this 
 acid is cheaper in the end, and perhaps more uniform 
 than a weaker acid ; and secondly, it is important that both 
 the Sulphuric and Nitric Acid should be as strong as 
 possible, in order to allow of the use of sufficient water 
 to raise the temperature of the resulting Nitro-Sulphuric 
 Acid at once to the proper point, and so to obviate the 
 necessity of employing artificial heat. 
 
 Only an apparent saving of expense is effected by using 
 the yellow acid sold in commerce as "Acid Nitros," in 
 place of the pure acid of equal strength. For some time 
 the Writer was of opinion that this might be done with 
 impunity; it subsequently appeared, however, that the 
 quantity of Chlorine in the " Nitros Acid" was more vari- 
 able than had been supposed, and that it must necessarily 
 remain so, inasmuch as the cargoes of Nitrate of Soda, 
 from which the acid is made, differ in quality, and no pre- 
 liminary process of purification is resorted to, with a view 
 of eliminating the Chloride. Chlorine, when present in 
 the Nitric Acid in more than a certain proportion, ap- 
 pears to decompose the Pyroxyline, and to cause its partial 
 solution in the Nitro- Sulphuric Acid : hence it becomes 
 necessary to work with a smaller quantity of water, and 
 consequently an inferior Pyroxyline is obtained. 
 
 NITROGEN. 
 Symbol, N=14 
 
 Nitrogen is an element, existing as a gas, in the free 
 state. Our atmosphere consists of a mixture of the two 
 elements Nitrogen and Oxygen, in the proportion of 4 
 volumes of the first with 1 volume of the second. 
 
 Nitrogen, when free, is remarkably devoid of sensible 
 properties, and can scarcely be made directly to unite with 
 any element. Its compounds are, however, numerous and 
 important, Nitric Acid, NO 5 , being one of its oxides; 
 
78 VOCABULARY OF 
 
 Ammonia, Itf H 3 , is its compound with Hydrogen, and Cy- 
 anogen, C 2 N, its compound with Carbon. 
 
 NITEOGEN, BINOXIDE OF. 
 
 Formula, NO 2 = 30. 
 
 Binoxide of Nitrogen is a colourless gas given off during 
 the solution of Copper and other metals, in dilute Nitric 
 Acid ; its mode of formation during the solution of Copper 
 is according to this equation : 
 
 3Cu+4NO 5 = 3(CuO,NO 5 )+NO 2 . 
 
 This gas, coming into contact with the Oxygen of the 
 air, is instantly reddened by combining with it, and form- 
 ing Nitrous Acid, NO 3 , and Peroxide of Nitrogen, NO 4 . 
 Hence if Copper is dissolved in dilute Nitric Acid in an 
 open vessel, red vapours are seen all the time ; but if in a 
 flask fitted with a cork and tube, the red vapours at first 
 formed will be expelled after a while, and the flask will 
 then be filled with the colourless NO 2 , which will redden 
 on removing the cork and tube. 
 
 NITEOGEN, PEEOXIDE OF. 
 Formula, NO 4 = 46. 
 
 The red fumes which appear during the solution of 
 metals in Nitric Acid, are chiefly vapours of Peroxide of 
 Nitrogen. In its pure state it forms a very volatile, orange 
 liquid. This compound, in contact with bases, forms a 
 mixture of Nitrite and Nitrate. 
 
 2O + 2NO 4 =KO,NO 3 +E:O,NO 5 . 
 
 The compounds NO 2 , NO 3 , and NO 4 dissolve in solu- 
 tion of Sulphate of Iron, colouring the latter of a deep 
 brownish-black. 
 
 NITEO-GLUCOSE. 
 
 When 3 fluid ounces of cold Nitro- Sulphuric Acid, con- 
 sisting of 2 ounces of Oil of Vitriol and 1 ounce of highly 
 concentrated Nitric Acid, are mixed with 1 ounce of finely 
 powdered Cane Sugar, there is formed at first a thin, trans- 
 
PHOTOGRAPHIC CHEMICALS. 79 
 
 parent, pasty mass. If it is stirred with a glass rod for a 
 few minutes without interruption, the paste coagulates as 
 it were, and separates from the liquid as a thick tenacious 
 mass, aggregating into lumps, which can easily be removed 
 from the acid mixture. 
 
 This substance is a substitution compound, and is de- 
 rived from Sugar as Gun-cotton is from Cotton, by the 
 displacement of a part of the H in Sugar by N O 4 . It has 
 a very acid and intensely bitter taste. Kneaded in warm 
 water until the latter no longer reddens litmus-paper, it 
 acquires a silvery colour and a beautiful silky lustre. 
 When dissolved in Collodion, it ozonizes the Ether on 
 keeping for some months, and hence there is a rapid libe- 
 ration of Iodine when the iodizer is added. Alkalies de- 
 compose Nitro-Glucose, evolving a smell of burnt sugar. 
 The product lessens the sensitiveness of Collodion, but 
 increases the intensity of the image. 
 
 NITEO-HYDEOCHLOEIC ACID. 
 
 This liquid is the Aqua Eegia of the old alchemists. It 
 is produced by mixing Nitric and Hydrochloric Acids ; the 
 Oxygen of the former combines with the Hydrogen of the 
 latter, forming water and liberating Chlorine. 
 
 The presence of free Chlorine confers on the mixture the 
 power of dissolving Gold and Platinum, which neither of 
 the two acids possesses separately. In preparing Aqua 
 Eegia it is usual to mix one part, by measure, of Nitric 
 Acid with four of Hydrochloric Acid, and to dilute with 
 an equal bulk of water. The application of a gentle heat 
 assists the solution of the metal ; but if the temperature 
 rises to the boiling-point, effervescence and loss of Chlo- 
 rine take place. 
 
 NITEOUS ACID. 
 Formula, NO 3 =38. 
 Nitrous Acid, in the free state, at ordinary tempera- 
 
80 VOCABULARY OF 
 
 tures is a reddish gas, which easily decomposes into two 
 other compounds, the red Peroxide of Nitrogen and the 
 colourless Deutoxide. 
 
 2N0 3 =N0 4 + N0 2 . 
 
 United with bases, however, it forms stable salts. The 
 Nitrite of Potash may be obtained by strongly fusing 
 Nitrate of Potash until 2 atoms of Oxygen have been ex- 
 pelled. Nitrite of Silver is a sparingly soluble salt, crys- 
 tallizing in long needles. Other Nitrites are usually 
 obtained from Nitrite of Potash by double decomposition. 
 (See SILVER, NITRITE or.) NO 3 is a weak acid, being 
 displaced even by Acetic Acid. 
 
 NITEO-SULPHUEIC ACID. 
 
 This name is applied to a mixture of Nitric and Sul- 
 phuric Acids, the properties and preparation of which will 
 be afterwards described. . 
 
 OEGANIC MATTEE. 
 
 This term is often loosely employed in this Work to 
 designate varieties of non-volatile vegetable and animal 
 substances of unknown composition, which are prone to 
 change by absorption of Oxygen. Bodies which have a 
 definite formula, however, are often included in the class 
 of "Organic matters" if they easily pass by oxidation 
 into ill-defined products. Thus, the Sugar of Liquorice, 
 which soon decomposes and becomes brown, may be re- 
 ferred to under that head : but a stable vegetable acid 
 like Acetic Acid, or a neutral substance not readily oxidi- 
 zable, such as Glycerine, although really organic in compo- 
 sition, would not usually be referred to by this term. 
 
 Organic matter of the kind above alluded to reacts upon 
 Nitrate of Silver in presence of light, reducing it more 
 or less perfectly to the metallic state, and becoming itself 
 oxidized. 
 
 
 
PHOTOGRAPHIC CHEMICALS. 81 
 
 OXYGEN. 
 
 Symbol, O = 8. 
 
 Oxygen gas is one of the elements ; it is very abun- 
 dantly distributed, both in the free state and in combina- 
 tion. In the former state it exists in our atmosphere, 
 but mixed with four times its bulk of the elementary gas 
 Nitrogen. Its compounds, as stated in Chapter I., are 
 either acids, bases, or neutral bodies. It can be ob- 
 tained in a state of almost perfect purity and in great 
 abundance by heating the salt Chlorate of Potash, 
 KO,C10 5 , to gentle redness in a flask or retort ; Chloride 
 of Potassium remains. KO,C1O 5 = E:C1 + 6O. This 
 decomposition is rendered more easy by first mixing 
 the powdered Chlorate with one-fourth of well-dried, and 
 powdered, Black Oxide of Manganese. 
 
 OXYMEL. 
 
 This syrup of Honey and Vinegar is prepared as fol- 
 lows. Take of 
 
 Honey 1 pound. 
 
 Acid, Acetic, fortiss. (Beaufoy's Acid) 11 drachms. 
 
 Water 13 drachms. 
 
 Stand the pot containing the Honey in boiling water 
 until a scum rises to the surface, which is to be removed 
 two or three times. Then add the Acetic Acid and the 
 water, and skim once more if required. Allow to cool, 
 and it will be fit for use. 
 
 OZONE. 
 
 Oxygen, in the state in which it usually exists in the 
 atmosphere, exhibits no very powerful chemical proper- 
 ties at ordinary temperatures ; thus, it has no action on 
 Iodide of Potassium, it cannot displace the Iodine to 
 combine with the Potassium. It appears, however, that 
 Oxygen, though an element, is capable of taking on a 
 
82 VOCABULAEY OF 
 
 more active form, in which condition it almost resembles 
 Chlorine in its tendency to combine with bodies. In this 
 state it instantly attacks Iodides of Potassium, Sodium, 
 etc., forming Potash, Soda, etc., and setting Iodine free. 
 To this modification of Oxygen the term Ozone has been 
 applied, a name derived from o&> (ozo, I smell), on account 
 of its having a remarkable odour. 
 
 Some organic bodies, as Ether and Spirits of Turpen- 
 tine, have the power of converting a portion of the Oxygen 
 of the air into Ozone, which they loosely retain. This 
 condition of Ether can easily be produced by thrusting a 
 red-hot wire* into the vapour in a bottle containing a 
 little Ether. On shaking the bottle afterwards, and test- 
 ing with a solution of Iodide of Potassium, Ozone will be 
 indicated by the liberation of Iodine. This condition of 
 Ether occurs spontaneously after a time ; it may be got 
 rid of by distillation from solid Potash. 
 
 PHOSPHOEUS. 
 Symbol, P= 31. 
 
 Phosphorus is an element, one of whose Oxides, Phos- 
 phoric Acid, PO 5 , combined with Lime and Magnesia, 
 forms the principal ingredient of the earthy matter of 
 the bones of animals, this being the source whence this 
 element is obtained. Burnt bones are treated with Sul- 
 phuric Acid, which unites with the Lime, setting a part of 
 the Phosphoric Acid free, which is afterwards decomposed 
 by strongly heating with Charcoal; the Charcoal com- 
 bines with the Oxygen, and the Phosphorus, set free, dis- 
 tils over from the retort in which the operation is con- 
 ducted. 
 
 A semitransparent, waxy-looking body, highly inflam- 
 mable, shines in the dark; this slow combustion very 
 easily passing into a vivid one with emission of great heat. 
 
 * No flame should ever be brought near an Ether bottle, for fear of an ex- 
 plosion. 
 
PHOTOGBAPHIC CHEMICALS. 83 
 
 It forms three compounds with Oxygen : 
 
 Hypophosphorous Acid . PO =39. 
 Phosphorous Acid . . . PO 3 = 55. 
 Phosphoric Acid . . . . PO 5 = 71. 
 
 PHOSPHOKIC ACID. 
 Formula (Hydrated), 3HO,PO 5 = 98. 
 
 This is the highest Oxide of Phosphorus, and may be 
 obtained by dissolving Phosphorus cautiously in Nitric 
 Acid. Phosphorus burnt in the air gives off abundance 
 of white fumes, which are anhydrous Phosphoric Acid. 
 These, treated with water, take up three equivalents, and 
 form the same acid as that obtained by the action of 
 Nitric Acid. All the salts of this acid contain three atoms 
 either of base or of base and water together, independent 
 of water of crystallization. Common Phosphate of Soda, 
 deprived of water of crystallization, is 2NaO,HO,PO 5 . 
 Such an acid, requiring three atoms of base, is termed 
 tribasic. The neutral salts of this tribasic acid give a 
 yellow precipitate with Nitrate of Silver, soluble in Nitric 
 Acid. 
 
 If this tribasic acid be deprived of one-third of its 
 essential water, an acid is obtained with different proper- 
 ties, 2HO,P,O 5 . It is bibasic, and its salts give a white 
 precipitate with Nitrate of Silver : it is termed Pyrophos- 
 phoric Acid. 
 
 If another atom of water be removed, a third acid, 
 termed Metaphosphoric Acid, is left, HO,PO 5 . It is mo- 
 nobasic, and its salts give, like the pyrophosphates, a 
 white precipitate with Nitrate of Silver. 
 
 POTASH. 
 Formula, KO,HO = 56. 
 
 Potash, the Oxide of Potassium, is obtained from Car- 
 bonate of Potash by separating the Carbonic Acid by 
 means of Caustic Lime. Lime is a more feeble base than 
 Potash, but the Carbonate of Lime, being insoluble in 
 
84 VOCABULABY OF 
 
 water, is at once formed on adding Milk of Lime to a 
 solution of Carbonate of Potash in not less than twelve 
 parts of water. 
 
 Properties. Usually met with in the form of solid 
 lumps, or in cylindrical sticks, which are formed by melt- 
 ing the Potash and running it into a mould. It always 
 contains one atom of water, which cannot be driven off by 
 the application of heat. 
 
 Potash is soluble almost to any extent in water, much 
 heat being evolved. The solution is powerfully alkaline, 
 and acts rapidly upon the skin; it dissolves fatty and 
 resinous bodies, converting them into soaps. Solution of 
 Potash absorbs Carbonic Acid quickly from the air, and 
 should therefore be preserved in stoppered bottles; the 
 glass stoppers must be wiped occasionally, in order to 
 prevent them from becoming immovably fixed by the 
 solvent action of the Potash upon the Silica of the glass. 
 
 The Liquor Potassae of the London Pharmacopoeia has 
 a sp. gr. of 1'063, and contains about 5 per cent, of real 
 Potash. It is usually contaminated with Carbonate of 
 Potash, which causes it to effervesce on the addition of 
 acids ; also, to a less extent, with Sulphate of Potash, 
 Chloride of Potassium, Silica, etc. 
 
 POTASH, BICHROMATE OF. 
 Formula, KO,2CrO 3 = 147'5. 
 
 This salt is largely manufactured at Glasgow for the 
 use of the calico printers, from a native compound of the 
 oxides of Chromium and Iron. It occurs in fine orange- 
 coloured crystals, which are soluble in about ten parts of 
 water at 60 Fahrenheit. 
 
 There are two Chromates of Potash, a neutral Chro- 
 mate, which is yellow and contains an atom of each consti- 
 tuent ; and a Bichromate, orange red, as before mentioned, 
 and having two atoms of acid to one of base. The Chromic 
 acid of Bichromate of Potash, in contact with organic 
 bodies such as Gelatine, is decomposed by light, yielding 
 
PHOTOGRAPHIC CHEMICALS. 85 
 
 up half its Oxygen to the' organic body, and being itself 
 reduced to a lower Oxide of Chromium. 
 2CrO 3 3O = Cr 2 O 3 . 
 
 The organic body thus oxidized, if previously soluble 
 in water, is often, as in the case of Gelatine, rendered in- 
 soluble in water, and upon this fact are founded various 
 Photographic processes. (See CHROMIUM.) 
 
 POTASH, CAEBONATE OF. 
 Formula, K O, C O 2 = 69. 
 
 The impure Carbonate of Potash, termed PearlasJi, is 
 obtained from the ashes of wood and vegetable matter, in 
 the same manner as Carbonate of Soda used to be pre- 
 pared from the ashes of seaweeds. Salts of Potash and of 
 Soda appear essential to vegetation, the former to land 
 and the latter to sea plants, and are absorbed and ap- 
 propriated by the living tissues of the plants. They exist 
 in the vegetable structure combined with organic acids in 
 the form of salts, Oxalate, Tartrate, etc., which, when 
 burned, are converted into Carbonate. 
 
 Properties. The Pearlash of commerce contains large 
 and variable quantities of Chloride of Potassium, Sulphate 
 of Potash, etc. A purer Carbonate is sold, which is free 
 from Sulphates, and with only a trace of Chlorides. Car- 
 bonate of Potash is a strongly alkaline salt, deliquescent, 
 and soluble in twice its weight of cold water ; insoluble in 
 Alcohol, and employed to deprive it of water. 
 
 POTASH, NITEATE OF. 
 Formula, KO,NO 5 =101. 
 
 This salt, also termed Nitre, or Saltpetre, is an abundant 
 natural product, found efflorescent upon the soil in certain 
 parts of the East Indies. It is also produced artificially 
 in what are called Nitre-beds. 
 
 There are different qualities of Nitre sold in commerce, 
 some of which contain much Chloride of Potassium, de- 
 
86 VOCABULARY OF 
 
 tected on dissolving the Nitre in distilled water, and add- 
 ing a drop or two of solution of Nitrate of Silver. This 
 impurity is injurious when the Nitre is employed for Pho- 
 tographic use : in the manufacture of Pyroxyline it de- 
 composes the Nitric Acid ; and in the case of positive de- 
 veloping solutions, the presence of Chloride in the Nitre 
 seem to produce a white cloudiness on the film. 
 
 A quality of Nitre which answers very well for making 
 Pyroxyline can be obtained at the operative chemists' at 
 one shilling per pound ; it is often sold as pure Nitre, but 
 usually contains sufficient Chloride to produce an opales- 
 cence with Nitrate of Silver ; if the impurity is in larger 
 quantity, and produces a decided precipitate with Nitrate 
 of Silver, the sample must be rejected. 
 
 POTASH AND SODA, TAETEATE OF. 
 Formula, KO,NaO,C 8 H 4 O 10 + 8Aq. = 282. 
 
 This substance is often sold in commerce under the 
 name of Hochelle Salt. It is prepared by adding Soda to 
 the Bitartrate of Potash, and forms large crystals, having 
 the appearance of six-sided prisms, perfect only on one 
 side. 
 
 POTASSIUM, BROMIDE OF. 
 Formula, KBr=119. 
 
 Bromide of Potassium is prepared by adding Bromine 
 to Caustic Potash, and heating the product, which is a 
 mixture of Bromide of Potassium and Bromate of Potash 
 to redness, in order to drive off the Oxygen from the 
 latter salt. It crystallizes in anhydrous cubes, like the 
 Chloride and Iodide of Potassium ; it is easily soluble in 
 water, but very sparingly so in Alcohol; it yields red 
 fumes of Bromine when acted upon by hot and strong 
 Sulphuric Acid. 
 
 This salt is useful in Photographic processes on paper, 
 but in Collodion it is liable to cause turbidity and spots. 
 
PHOTOGBAPHIC CHEMICALS. 87 
 
 POTASSIUM, CYANIDE OF. 
 Formula, Z,C 2 N, or KCy=65. 
 
 Cyanide of Potassium may be regarded either as a com- 
 pound of Cyanogen and Potassium (see CYANOGEN), or as 
 a salt, derived from Hydrocyanic, or " Prussic " Acid. 
 
 KO + HCy^KCy + HO. 
 
 It is obtained from Ferrocyanide of Potassium by mixing 
 eight parts of this salt, thoroughly dried, with three of 
 dry Carbonate of Potash, and fusing in a covered earthen 
 crucible. "When effervescence has ceased, and Iron, which 
 separates, has settled down, the clear liquid is poured off, 
 and on cooling solidifies into the white mass known as 
 Cyanide of Potassium. Thus obtained it is not quite pure, 
 and, as sold in commerce, it often only contains about half 
 its weight of true Cyanide ; but the impurities present, of 
 which Carbonate of Potash is the principal, do not pro- 
 duce any injurious effect beyond lessening the strength of 
 the salt. It is highly poisonous, and, if treated with an 
 acid, it gives off the very volatile and poisonous Hydro- 
 cyanic Acid, the vapour of which, incautiously inhaled, 
 might cause fainting, or even death. 
 
 POTASSIUM, FLUOEIDE OF. 
 Formula, KF = 58. 
 
 Preparation. Fluoride of Potassium is formed by sa- 
 turating Hydrofluoric Acid with Potash, and evaporating 
 to dryness in a platinum vessel. Hydrofluoric Acid con- 
 tains the element Fluorine combined with Hydrogen ; it 
 is a powerfully acid and corrosive liquid, formed by de- 
 composing Fluor Spar, which is a Fluoride of Calcium, 
 with strong Sulphuric Acid ; the action which takes place 
 being precisely analogous to that involved in the prepa- 
 ration of Hydrochloric Acid. 
 
 CaF + HO,SO 3 =CaO,SO 3 + HF. 
 
 Properties. A deliquescent salt, occurring in small and 
 imperfect crystals. Very soluble in water ; the solution 
 
88 VOCABULAKY OF 
 
 acting upon glass in the same manner as Hydrofluoric 
 Acid. 
 
 POTASSIUM, IODIDE OF. 
 Formula, K I =166. 
 
 This salt may be prepared by dissolving Iodine in Liquor 
 Potassse until the solution begins to acquire a brown co- 
 lour: a mixture of Iodide of Potassium and lodate of 
 Potash (KO,IO 5 ) is thus formed ; but by evaporation and 
 heating to redness, the latter salt parts with its Oxygen, 
 and is converted into Iodide of Potassium. 
 
 Properties. It forms cubic and prismatic crystals, which 
 are hard, and very slightly if at all deliquescent. Soluble 
 in less than an equal weight of water at 60 ; it is also 
 soluble in Alcohol, but not in Ether. The proportion of 
 Iodide of Potassium contained in a saturated alcoholic 
 solution, varies with the strength of the spirit, with com- 
 mon Spirits of Wine, sp. gr. *836, it would be about 8 
 grains to the drachm ; with Alcohol rectified from Carbo- 
 nate of Potash, sp. gr. '823, 4 or 5 grains ; with Alcohol 
 of '805, 2 grains. The solution of Iodide of Potassium is 
 instantly coloured brown by free Chlorine or Ozone ; also 
 very rapidly by Nitric Acid containing Peroxide of Nitro- 
 gen ; ordinary acids however act less quickly, Hydriodic 
 Acid being first formed, and subsequently decomposing 
 spontaneously. 
 
 Commercial Iodide of Potassium. A very pure Iodide 
 of Potassium has lately been prepared purposely for Pho- 
 tography, in crystals which have a slightly yellow tinge. 
 Much of the Iodide, however, employed for medical and 
 other purposes, is contaminated with foreign salts. 
 
 Carbonate of Potash is the most common impurity : 
 when it is present, the Iodide forms small and imperfect 
 crystals, which become moist on exposure to the air, from 
 the deliquescent nature of the Alkaline Carbonate. 
 
 Sulphate of Potash is also a common impurity ; it may 
 be detected by Chloride of Barium. Chloride of Potas- 
 
PHOTOGBAPHIC CHEMICALS. 89 
 
 slum, when present in the Iodide, may be detected as fol- 
 lows : Precipitate the salt by an equal weight of Nitrate 
 of Silver, and treat the yellow mass with solution of Am- 
 monia : if any Chloride of Silver be present, it dissolves in 
 the Ammonia, and, after nitration, is precipitated in white 
 curds by the addition of an excess of pure Nitric Acid. 
 If the Nitric Acid employed is not pure, but contains 
 traces of free Chlorine, the Iodide of Silver must be well 
 washed with distilled water before treating it with Am- 
 monia, or the excess of free Nitrate of Silver dissolving 
 in the Ammonia would, on neutralizing, produce Chloride 
 of Silver, and so cause an error. 
 
 lodate of Potash is a fourth impurity often found in 
 Iodide of Potassium : to detect it, add a drop of dilute Sul- 
 phuric Acid, or a crystal of Citric Acid, to the solution of 
 the Iodide ; if lodate be present, the liquid will immedi- 
 ately become yellow from liberation of free Iodine. The 
 rationale of this reaction is as follows: The Sulphuric 
 Acid unites with the base of the salt, and liberates Hy- 
 driodic Acid (HI), which is a colourless compound; but 
 if lodic Acid (I O 5 ) be also present, it decomposes the Hy- 
 driodic Acid first formed, oxidizing the Hydrogen into 
 Water (HO), and setting free the Iodine, or in formulae : 
 
 The immediate production of a yellow colour on adding a 
 weak acid to aqueous solution of Iodide of Potassium, is 
 therefore a proof of the presence of an lodate. 
 
 Tests for purity of Iodide. Solution of Chloride of Ba- 
 rium is commonly used to detect impurities in Iodide of 
 Potassium ; it forms a white precipitate if Carbonate, lo- 
 date, or Sulphate be present. In the two former instances 
 the precipitate dissolves on the addition of a single drop 
 of pure dilute Nitric Acid, but in the latter case it is in- 
 soluble. The commercial Iodide however is rarely so pure 
 as to remain quite clear on the addition of Chloride of 
 Barium. 
 
 A simple test for Carbonate of Potash, which is the 
 most objectionable impurity, is to dissolve the Iodide in 
 
90 VOCABULAEY OF 
 
 about three times its weight of Lime-water; a turbidity 
 will indicate the presence of Carbonate. 
 
 The mere fact of reddened Litmus-paper becoming blue 
 in solution of Iodide of Potassium is no proof of impurity, 
 since the finest crystals which can be obtained have an 
 alkaline reaction. But if an Alcoholic solution of the 
 Iodide remains quite colourless when exposed for several 
 days to a strong light, it is almost certain that an excess 
 of alkali is present : the chemically pure Iodide of Potas- 
 sium is gradually decomposed by light, and assumes a faint 
 straw-yellow tint, returning however to its colourless con- 
 dition on putting the bottle again in a dark place. 
 
 Iodide of Potassium may, when required, be purified by 
 recrystallizing it from Spirit, or by dissolving it in Alcohol 
 of '805, in which Carbonate, Sulphate, and lodate are 
 insoluble. 
 
 POTASSIUM, SULPHIDE OF. 
 
 There are many Sulphides of Potassium, but the one 
 commonly employed by Photographers, and sold in com- 
 merce as "Liver of Sulphur," is an impure Tersulphide. 
 It is prepared by heating Sulphur with Carbonate of 
 Potash, the result of which is that a portion of the Sulphur 
 is oxidized into Sulphuric Acid, and combines with Potash, 
 forming Sulphate of Potash, whilst another portion enters 
 into combination with Potassium, producing a Sulphide 
 of Potassium containing three atoms of Sulphur to one of 
 Potassium. 
 
 Sulphide of Potassium is used for the same purposes as 
 Hydrosulphate of Ammonia, q. v. 
 
 POTASSIUM, SULPHOCYANIDE OF. 
 
 See SULPHOCYANIDES. 
 
 PYEOGALLIC ACID. 
 
 Formula, C 12 H 6 O 6 =126. 
 This substance, obtained by the action of heat of 420 on 
 
PHOTOGRAPHIC CHEMICALS. 91 
 
 Gallic Acid, is now made in very large quantities, and the 
 desire of the purchaser to economize has acted injuriously 
 upon its manufacture, as upon that of other Photographic 
 Chemicals. The price at which Pyrogallic Acid is sold 
 offers no inducement to the expenditure of much pains in 
 its preparation, and hence some samples prove unsatis- 
 factory. Impurities of an organic nature are occasionally 
 found in Pyrogallic Acid, which have the effect of lessening 
 its reducing powers, and of adding to the opacity of the 
 deposit : hence, on applying it to the surface of the film, 
 the blacks of the Negative become very dense and smoky, 
 but the delicate shadows refuse to develope. 
 
 PYEOXYLIC SPIEIT. See ALCOHOL, METHYLIC. 
 
 PYROXYLINS. 
 
 General Formula, C 36 H (30) _ a; (NO 4 ) a .O 30 . 
 
 The name Pyroxyline is applied to a series of compounds, 
 obtained by the action of mixed Nitric and Sulphuric Acids 
 on vegetable fibres ; they are all more or less explosive, 
 the strongest being Gun-cotton ; the soluble varieties are 
 termed sometimes Collodion Wool. They are all substi- 
 tution compounds, being derived from the vegetable fibre 
 by the substitution of atoms of N O 4 for an equal number 
 of atoms of H in the fibre. 
 
 As the preparation of the varieties suitable to Photo- 
 graphy will be fully entered into hereafter, it is only ne- 
 cessary here to make a few general remarks. 
 
 When Cotton, C 36 H 30 O 30 , is immersed in a mixture of 
 Nitric and Sulphuric Acids, cold, four varieties of Pyroxy- 
 line may be obtained, depending upon the strength of the 
 acids used. 
 
 A.-C 36 H 21 (N0 4 ) 9 30 . 
 B.-C 36 H 22 (N0 4 ) S 30 . 
 C--C 36 H 23 (N0 4 ) 7 30 . 
 D.-C 36 H 24 (N0 4 ) 6 30 . 
 
 A. is true Gun-cotton, insoluble in Ether and Alcohol ; 
 soluble only in Acetic Ether. 
 
92 VOCABULAKT OF 
 
 B. soluble in Ether + Alcohol ; insoluble in glacial Acetic 
 Acid. 
 
 C. soluble in Ether + Alcohol j soluble in glacial Acetic 
 Acid. 
 
 D. disintegrated and clotted ; difficultly soluble in Ether 
 + Alcohol ; quite soluble in Acetic Acid. 
 
 None of these compounds prepared cold are, however, 
 suitable for Photography, as their solutions in Ether are 
 not sufficiently fluid. All Photographic Pyroxyline is pre- 
 pared with hot acids, heat being found remarkably to mo- 
 dify the products which, thus prepared, have not yet been 
 thoroughly analyzed. A mixture of acids which, if cold, 
 would give an insoluble cotton, will when hot often pro- 
 duce a perfectly soluble product, and another acid mix- 
 ture which would yield a pyroxyline giving a very viscid 
 Collodion with 4 grs. to the ounce, will, when used hot, 
 give a product soluble to the extent of 7 or 8 grs. to the 
 ounce, and yet the solution will be quite fluid. 
 
 When in the dry state, these compounds are rather harsh 
 to the touch, and produce a crackling sensation between 
 the fingers, like Starch. They are somewhat hygroscopic, 
 but when carefulty dried are excellent insulators of electri- 
 city, and become highly electric by friction. Strong Sul- 
 phuric Acid dissolves them in the cold without evolving 
 any gas, and the solution is colourless unless fibres of un- 
 changed cotton are present. Cold Nitric Acid of a certain 
 strength also dissolves them slowly, but dilution with water 
 throws them down again. Potash dissolves Gun-cotton 
 with decomposition and evolution of Ammonia, and the 
 solution contains Nitrate of Potash, Nitrite of Potash, and 
 an organic Salt of Potash : the properties of the latter are 
 those of a reducing agent, and when mixed with Ammo- 
 niacal Nitrate of Silver it may be used for silvering mir- 
 rors ; by absorption of Oxygen it passes gradually into the 
 state of Oxalate of Potash. 
 
 SEL D'OE. See GOLD, HYPOSULPHITE OF. 
 
PHOTOGRAPHIC CHEMICALS. 93 
 
 SILVEE. 
 Symbol, Ag = 108. 
 
 This metal, the Luna or Diana of the Alchemists, is 
 found native in Peru and Mexico ; but its principal ore is 
 the Sulphide. 
 
 When pure it has a sp. gr. of 1O5, and is very malleable 
 and ductile ; melts at a bright red heat. Silver does not 
 oxidize in the air, but when exposed to an impure atmo- 
 sphere containing traces of Sulphuretted Hydrogen, it is 
 slowly tarnished, from formation of Sulphide of Silver. 
 It dissolves in strong boiling Sulphuric Acid, but the best 
 solvent for it is Nitric Acid. 
 
 The standard coin of the realm is an alloy of Silver and 
 Copper, containing 92*5 per cent, of Silver. 
 
 To prepare pure Nitrate of Silver from it, dissolve in 
 Nitric Acid by aid of heat, and evaporate until crystals 
 are obtained. Then wash the crystals with a little dilute 
 Nitric Acid, redissolve them in water, and crystallize by 
 evaporation a second time. 
 
 The process is also occasionally conducted by boiling 
 down the impure acid solution of the Silver to dryness, 
 without any crystallization, and fusing the product pretty 
 strongly, until a portion taken out, dissolved in water, and 
 filtered from Oxide of Copper, ceases to give a blue colour 
 with Ammonia, showing that the Nitrate of Copper is quite 
 decomposed ; afterwards recrystallize as before. Perhaps 
 the easiest method is, to dissolve the alloy in Nitric Acid, 
 immerse a piece of metallic Copper until the Silver is 
 wholly precipitated, remove the Copper, wash the Silver 
 well with water, then with a little Nitrate of Silver to re- 
 move any adhering Copper, and lastly dissolve in Nitric 
 Acid and crystallize ; or else take the Silver as precipitated 
 by Copper, wash it and dissolve in Nitric Acid, avoiding all 
 excess (or if excess has been added, evaporate carefully to 
 dryness to expel it) ; and to neutralize and remove traces 
 of Copper, add Oxide of Silver to the boiling solution until 
 
94 VOCABULAEY OF 
 
 on filtering and testing a portion with Ammonia, no blue 
 colour is perceived, then filter the whole. This solution, 
 if of the right strength (see Appendix), might be used at 
 once for Photographic purposes. 
 
 SILYEE, ACETATE OF. 
 
 Formula, AgO,C 4 H 3 O 3 =167. 
 
 This is a difficultly soluble salt, deposited in white lamellar 
 crystals when an Acetate is added to a strong solution of 
 Nitrate of Silver. 
 
 It is sometimes used as an addition to the Negative Ni- 
 trate Bath, but as the commercial Acetate of Silver is 
 often impure and contaminated with Carbonate of Silver, 
 which would make the Bath alkaline, it is better to use 
 the Acetate of Soda, according to the directions to be given 
 afterwards. 
 
 SILYEE, ALBUMINATE OF. 
 
 This name has been given to the insoluble white sub- 
 stance precipitated on adding Nitrate of Silver to a solu- 
 tion of Albumen, and which analysis shows to contain 
 Oxide of Silver combined with the animal matter. It is 
 reducible to a red subcompound, both by white light and 
 by Hydrogen Gas. 
 
 SILYEE, AMMONIO-NITEATE OF. 
 
 Crystallized Nitrate of Silver absorbs Ammoniacal gas 
 rapidly, with production of heat sufficient to fuse the re- 
 sulting compound, which is white, and contains 22'5 per 
 cent, of Ammonia with 77'5 per cent, of Nitrate of Silver. 
 The solution however which Photographers employ, is pre- 
 pared by adding to the solution of Nitrate of Silver, quite 
 neutral, a pure solution of Ammonia until the precipitate 
 which first forms is nearly redissolved, and then filtering ; 
 the compound thus produced in solution has the composi- 
 tion NH 2 Ag + NH 4 ONO 5 , and contains 16'67 per cent, 
 of Ammonia. 
 
PHOTOGBAPHIC CHEMICALS. 95 
 
 In sensitizing salted paper by the Ammonio-Nitrate of 
 Silver, Ammonia is necessarily set free. Thus 
 
 SILVER, BROMIDE OF. 
 
 Formula, AgBr= 188. 
 
 Bromide of Silver is a salt of Silver which may be pro- 
 duced either by direct union of its elements, as in the 
 Daguerreotype, or by double decomposition between Ni- 
 trate of Silver and an alkaline Bromide, as in the wet 
 processes of Photography. 
 
 A g O,NO 5 + KBr=KO,NO 5 +AgBr. 
 
 On mixing the two solutions, the Bromide falls as a 
 curdy, yellowish-white precipitate, insoluble in water or 
 Nitric Acid, but soluble in alkaline Hyposulphites, Cya- 
 nides and Sulphocyanides, and in strong Ammonia. Turns 
 grey on exposure to light. 
 
 SILYEE, CARBONATE OF. 
 Formula, AgO,CO 2 =138. 
 
 This is a white or yellowish powder deposited on adding 
 any soluble Carbonate to solution of Nitrate of Silver. It 
 is only slightly soluble in water, or in solution of Nitrate 
 of Silver, but yet sufficiently so to produce an alkaline 
 reaction to Litmus. More soluble in water containing 
 Nitrate of Ammonia, and freely so in Ammonia itself. 
 Also dissolved by dilute Nitric or Acetic Acid, forming a 
 Nitrate or Acetate. 
 
 SILVER, CHLORIDE OF. 
 Formula, AgCl=143'5. 
 
 Chloride of Silver may be obtained by the direct union 
 of Silver and Chlorine, or, as in the preparation of papers 
 for Positive printing, by double decomposition of Nitrate 
 of Silver and an alkaline Chloride. 
 
 O 5 + NaCl=NaO,NO 5 
 
96 VOCABULARY OF 
 
 On mixing the solutions, the Chloride of Silver falls as 
 a curdy white precipitate, insoluble in water and Nitric 
 Acid, very soluble in alkaline Hyposulphites, Cyanides, 
 and Sulphocyanides, and in Ammonia even when dilute. 
 On exposure to light, it goes through various shades of 
 violet until it becomes finally black. 
 
 SILYEE, CITEATE OF. 
 Formula, 3AgO,C 12 H 5 O n =513. 
 
 A white salt precipitated on adding a soluble Citrate to 
 solution of Nitrate of Silver. Its property of being re- 
 duced to a coloured subsalt by the action of light, or by a 
 deoxidizing agent, renders it useful in Photography. 
 
 Citric Acid added to solution of Nitrate of Silver pro- 
 duces no precipitation. This acid is what is termed tri- 
 basic, i. e. an acid, one atom of which combines with three 
 atoms of base to form a neutral Salt. 
 
 SILYEE, FLUOEIDE OF. 
 
 Formula, AgF= 127. 
 
 This compound differs essentially from those last de- 
 scribed in being soluble in water. The dry salt fuses on 
 being heated, and is reduced by a higher temperature, or 
 by exposure to light. 
 
 SILYEE, HYPOSULPHITE OF. 
 
 Formula, Ag O, S 2 O 2 = 164. 
 
 This salt may be obtained by adding a dilute solution 
 of Nitrate of Silver to an excess of concentrated Hypo- 
 sulphite of Soda ; it is thrown down, mixed with a little 
 Sulphide of Silver, from which it is separated after wash- 
 ing by Ammonia, which dissolves the Hyposulphite of 
 Silver, and from which it is precipitated by exact neutrali- 
 zation with Nitric Acid. 
 
 White, nearly insoluble, tastes sweet ; it is very unstable, 
 easily decomposing into Sulphide of Silver and Sulphuric 
 Acid. AgO,S 2 O 2 =AgS + SO 3 . 
 
PHOTOGRAPHIC CHEMICALS. 97 
 
 It forms two double salts with Hyposulphite of Soda : 
 one having the formula, 
 
 2(NaO,S 2 2 ) + AgO,S 2 2 , 
 is very soluble in water ; the other salt, 
 
 NaO,S 2 O 2 + AgO,S 2 O 2 , 
 
 is sparingly soluble. These salts, when in solution, are 
 precipitated by Iodide of Potassium, but not by Chloride 
 of Sodium. They are far more stable than the simple Hy- 
 posulphite. Taste, exceedingly sweet. 
 
 SILVEE, IODIDE OF. 
 Formula, Agl=235. 
 
 Iodide of Silver in the Daguerreotype process is pro- 
 duced by the direct union of its elements, but in the wet 
 processes of Photography by double decomposition of Ni- 
 trate of Silver and an alkaline Iodide. 
 
 AgO,N0 5 + KI=0,N0 5 +AgI. 
 
 Obtained by mixing solutions of Nitrate of Silver and 
 an alkaline Iodide, it forms a yellow precipitate. 
 
 This compound differs from the Chloride and Bromide 
 of Silver in being insoluble in Ammonia, but it resembles 
 them in being soluble in alkaline Hyposulphites, Cyanides, 
 and Sulphocyanides. Heat changes it temporarily to deep 
 yellow. Light turns it brown. 
 
 SILVER, NITEATE OF. 
 Formula, Ag O, NO 5 = 170. 
 
 Preparation and Properties. The preparation of Ni- 
 trate of Silver has been sufficiently described under the 
 head of Silver, to which the reader is referred. There 
 are, however, some points of practical importance yet to 
 be considered. 
 
 Pure Nitrate of Silver may be made from alloys of 
 Silver and Copper, as described (art. SILVEE) ; but since 
 the heat which is necessary to decompose the Nitrate of 
 
 H 
 
98 . VOCABULARY OF 
 
 Copper often produces Nitrite of Silver, it is advised that 
 Silver nearly free from Copper should be chosen in pre- 
 ference. The consumption of Nitrate of Silver in Photo- 
 graphy has "bow become very large, and it fortunately 
 happens that the crystallized salt can be obtained in al- 
 most any quantity at a moderate price, being a bye-pro- 
 duct in the operations of parting Grold and Silver, which 
 are carried on in the refineries. The assay processes also 
 yield a portion of the Nitrate sold in commerce, but not by 
 any means the greater part. 
 
 This facility of obtaining commercial crystallized Nitrate 
 of Silver at a price which is very little above that of the 
 metal it contains, tas however acted injuriously as regards 
 the purity of the article, since it necessarily leaves its 
 manufacture in the hands of a few individuals, \vho are not 
 able to pay that attention to it which is needed. The crys- 
 tals are usually sent out simply dried off from the Nitric 
 Acid* as a refuse product on which no profit can be ex- 
 pected. Intentional adulteration, however, is not practised 
 by the large producers of Nitrate of Silver, as far as the 
 Author is aware. 
 
 The purity of Nitrate of Silver may easily be ascer- 
 tained by dissolving a portion in distilled water, and pre- 
 cipitating the solution entirely with pure Hydrochloric 
 Acid ; the liquid filtered from the precipitate should leave 
 no residue on evaporation to dry ness. 
 
 Nitrate of Potash and Nitrate of Copper have been 
 spoken of as occasional impurities in commercial Nitrate 
 of Silver, but the presence of these salts in small quantity 
 would have little effect except in reducing the strength of 
 the Baths. The peculiar Photographic action of bad Ni- 
 trate of Silver is probably to be referred to a different 
 cause, viz. to the presence of oxidized organic matter. In 
 the assay processes, fragments of charcoal are introduced 
 to prevent the acid from bumping as it dissolves the Silver ; 
 we have good reason for believing that during this process, 
 the Nitric Acid oxidizes the charcoal into a substance 
 which has an affinity for the Silver salt ; and the Author 
 
PHOTOGRAPHIC CHEMICALS. 99 
 
 has found that Nitrate of Silver so produced is altogether 
 unfit for Collodion Photography. 
 
 When old Nitrate Baths are precipitated by Zinc, organic 
 matter is carried down by the reduced Silver,' and the pro- 
 duct, if converted into Nitrate of Silver without previous 
 fusion, is useless for Photographic purposes. 
 
 The Author has also detected impurity of a similar kind, 
 but in less quantity, in cases where no charcoal had been 
 employed. Pieces of straw, etc., may perhaps fall into the 
 acid ; but, however this may be, the fact is certain, that 
 Nitrate of Silver prepared by dissolving Silver in Nitric 
 Acid, and evaporating to dryness without any crystalliza- 
 tion, cannot be depended on for Photography. 
 
 It has been suggested that the Nitrate of Silver for the 
 Bath should be made purposely by dissolving pure Silver 
 in pure Nitric Acid. Probably however no manufacturer 
 would care to compete with the refiners, who are in a 
 better position to supply the article at a reasonable price. 
 Recry stabilization seems therefore to be the proper remedy. 
 The first crop of crystals may be dried off from the acid, 
 and then crystallized a second time from distilled water ; 
 after w^hich the product will be in a pure state if charcoal 
 and substances of that kind have been excluded. This 
 second crystallization ought not, according to a competent 
 authority, to add more than fourpence, or at most sixpence, 
 to the price per ounce. 
 
 A saturated solution of the purified crystals slowly 
 restores the blue/colour of reddened litmus-paper, if the 
 Nitric Acid be expelled by heating to 240 previous to 
 the second crystallization. This proceeding, however, is 
 not actually necessary, inasmuch as a trace of adhering 
 Nitric Acid can always be removed by Carbonate of Soda 
 when making the Bath ; and it is well known that the pre- 
 sence of a little acid facilitates the crystallization of Nitrate 
 of Silver. 
 
100 VOCABULARY OF 
 
 SILVEE, NITEITE OF. 
 Formula, AgO,N0 3 = 154. 
 
 Nitrite of Silver is a compound of Nitrous Acid, or NO 3 , 
 with Oxide of Silver. It may be formed by heating Ni- 
 trate of Silver, so as to drive off a portion of its Oxygen, 
 or by mixing Nitrate of Silver and Nitrate of Potash in 
 equal parts, fusing strongly, and dissolving in a small 
 quantity of boiling water : on cooling, the Nitrite of Silver 
 crystallizes out, and may be purified by pressing in blot- 
 ting-paper ; but the best method of producing it is, to fuse 
 pure Nitrate of Potash (free from chloride) in a crucible 
 at a strong red-heat, until a portion removed, dissolved in 
 water and tested with Nitrate of Silver, is found to give a 
 brownish precipitate due to a portion of free Potash ; when 
 this is the case, it is allowed to cool, and 1 part of the pro- 
 duct is then dissolved in 20 of boiling water and mixed 
 with 2 parts of Nitrate of Silver, also dissolved in 20 of 
 boiling water, mix, filter rapidly through a hot filter ; on 
 cooling, the Nitrite of Silver crystallizes out abundantly 
 from the liquid. 
 
 Properties. Nitrite of Silver is soluble in 120 parts of 
 cold water : easily soluble in boiling water, and crystallizes 
 on cooling, in long slender needles. A small quantity 
 dissolved in the Negative Bath increases the sensitiveness 
 and intensity of the Collodion surface, but it has a ten- 
 dency to produce fogging. 
 
 SILVEE, OXIDE OF. 
 Formula, AgO = 116. 
 
 Oxide of Silver may be obtained by adding a solution 
 of Potash (pure or nearly free from Chloride) to one of 
 Nitrate of Silver ; a brown-black precipitate of Oxide of 
 Silver falls, which may be collected on a filter and washed, 
 or it may be allowed to settle down, the clear liquid poured 
 off, fresh water added, and the Oxide again allowed to 
 settle ; this being repeated until the Nitrate of Potash and 
 Potash have been removed, which may be ascertained 
 
 
PHOTOGRAPHIC CHEMICALS. 101 
 
 either by evaporating a little on a slip of glass, or, if ex- 
 cess of Potash was added in the first place, by testing with 
 red litmus-paper. It may be kept in a bottle in a moist 
 state, its use being to neutralize Nitrate of Silver Baths, 
 and to separate Oxide of Copper from Nitrate of Silver ; 
 the bottle should be kept in the dark. 
 
 SILVEE, SULPHIDE OF. 
 
 Formula, AgS = 124. 
 
 This compound, sometimes termed Sulphuret of Silver, 
 is formed by the action of Sulphur upon metallic Silver, 
 or of Sulphuretted Hydrogen or Hydrosulphate of Am- 
 monia upon the Silver salts : the decomposition of Hypo- 
 sulphite of Silver also furnishes the black Sulphide. 
 
 Sulphide of Silver is insoluble in water, and in those 
 substances which dissolve the Chloride, Bromide, and 
 Iodide, such as Ammonia, Hyposulphites, Cyanides, etc. ; 
 but it dissolves in Nitric Acid, being converted into soluble 
 Sulphate and Nitrate of Silver. 
 
 The colour of precipitated Sulphide of Silver varies with 
 the quantity present ; it is black when in mass, but yellow- 
 ish-brown in a state of fine division. 
 
 SODA, ACETATE OF. 
 Formula, NaO,C 4 H 3 O 3 + 6HO = 136. 
 
 An easily crystallizable salt, employed commercially as 
 a source of Acetic Acid. There are several qualities, some 
 of which contain brown empyreumatic matter, but the re- 
 crystallized salt can usually be obtained pure. It is readily 
 soluble in water and also in Alcohol. 
 
 This Acetate is undoubtedly the best form to employ in 
 Photography, since Acetate of Potash is deliquescent, and 
 cannot easily be kept dry. 
 
 SODA, CAEBONATE OF. 
 
 Formula, NaO,CO 2 + 10 Aq. = 143. 
 This salt was formerly obtained from the ashes of sea- 
 
102 . VOCABULAEY OF 
 
 weeds, but is now more economically manufactured on a 
 large scale from common Salt. The Chloride of Sodium is 
 converted into Sulphate of Soda, and the Sulphate into 
 Carbonate of Soda by roasting with Coal and Chalk. 
 
 Properties. The perfect crystals contain ten atoms of 
 water, which are driven off by the application of heat, 
 leaving a white powder the anhydrous Carbonate. Com- 
 mon Washing ' Soda is a Protocarbonate, contaminated to 
 a certain extent with Chloride of Sodium and Sulphate 
 of Soda. The Carbonate used for effervescing draughts is 
 either a Bicarbonate with 1 atom of water, or a Sesquicar- 
 bonate, containing about 40 per cent, of real alkali ; it is 
 therefore nearly double as strong as the washing Carbo- 
 nate, which contains about 22 per cent, of Soda. The 
 crystallized Carbonate of Soda is soluble in twice its weight 
 of water at 60, the solution being strongly alkaline to test 
 papers. 
 
 Bicarbonate or Sesquicarbonate of Soda is less soluble 
 than the ordinary Carbonate, requiring from ten to thirteen 
 parts of cold water to retain it in solution ; its composition 
 when pure is NaO,CO 2 
 
 SODA, CITRATE OF. 
 Formula, 3NaO,C 12 H 5 O 11 + 11 HO = 357. 
 
 This salt, the composition of which is analogous to the 
 Citrate of Silver, q. v., yields well-defined crystals, but as 
 it is sometimes difficult to obtain it in commerce, instruc- 
 tions are given in this Work for making it extemporane- 
 ously by neutralizing 56 grains of pure Citric Acid with 
 66 grains of dry Sesquicarbonate of Soda, quantities 
 which will yield 95 grains of Citrate of Soda. 
 
 The crystals are very soluble in water, and produce a 
 white flocculent deposit of Citrate of Silver in solution of 
 Nitrate of Silver. 
 
 SODA, HYPOSULPHITE OF. 
 
 Formula, JNaO, S 2 O 2 + 5 Aq. = 124. 
 The properties of Hyposulphurous Acid and the Hypo- 
 
PHOTOGRAPHIC CHEMICALS. 103 
 
 sulphite of Soda are described in Chap. IV.Partll. of the 
 present Work. The crystallized salt includes five atoms 
 of water of crystallization. 
 
 Hyposulphite of Soda is now made on an extensive scale, 
 and is sold at a low price. Some of the inferior qualities 
 have a yellow colour, and are said to contain a portion of 
 Sulphide or free Sulphur ; but most of the Salt sold in 
 commerce is sufficiently pure in the state in which it comes 
 from the manufacturers. The Salt when heated first fuses 
 in its water of crystallization, then dries up and lastly 
 burns with a bluish flame. 
 
 SODA, PHOSPHATE OF. 
 Formula, 2NaO,HO,PO 5 + 24 Aq. = 358. 
 
 There are several Phosphates of Soda, but the common 
 Phosphate has the above composition. It forms large 
 transparent crystals which effloresce, and become opaque 
 on the exterior ; soluble in four parts of cold water, the 
 solution possessing a faint alkaline reaction ; precipitates 
 Nitrate of Silver yellow. 
 
 Pyro-Phosphate of Soda, so called, is prepared by heat- 
 ing the ordinary Phosphate until the whole of the water 
 has been expelled ; when the residue is dissolved in water 
 it is found to have been completely altered in properties, 
 as it precipitates Nitrate of Silver white instead of yellow ; 
 it crystallizes in prisms. Formula, 2NaO,PO 5 . 
 
 SODIUM, AUBO-CHLOKIDE OF. 
 
 Formula, NaCl,AuCl 3 + 4Aq. = 397'6. 
 
 This salt is a double Chloride of Gold and Sodium, 
 with four atoms of water of crystallization. It has been 
 proposed as a convenient substitute for the ordinary Chlo- 
 ride of Gold, as it is not deliquescent, and forms long 
 orange-yellow, four-sided prisms. Another advantage is, 
 the absence of free acid in the double salt, whereas the 
 ordinary Chloride of Gold always contains Hydrochloric 
 Acid. The Auro- Chloride however will not in all pro- 
 
104 . VOCABULARY OF 
 
 bability come into general use in Photography, since it is 
 evidently open to adulteration, and although theoretically 
 neutral, yet the manufacturers themselves allow that it 
 will not be safe to calculate upon perfect neutrality, since 
 traces of Hydrochloric Acid are apt to remain in the in- 
 terstices of the crystals. 
 
 It is prepared by evaporating to dryness a solution of 
 4 of Gold in Aqua Eegia, dissolving the residue in water, 
 adding 1 part of common salt, evaporating the whole down 
 to 4 parts and leaving it to crystallize by cooling. 
 
 The pure salt contains 497 per cent, of Gold. The 
 amount of Gold may be ascertained by taking a given 
 weight of the compound, dissolving it in water, and boiling 
 it with half its weight of Oxalic Acid ; the whole of the 
 Gold separates in the metallic state, and may then be 
 washed, dried, and weighed. The Gold may also be sepa- 
 rated by the addition of fi ve*times the weight of Sulphate 
 of Iron, according to directions under GOLD, CHLOKIDE OF. 
 
 SODIUM, CHLOEIDE OF. 
 Formula, NaCl = 58'5. 
 
 Common Salt exists abundantly in nature, both in the 
 form of solid rock-salt, and dissolved in the waters of the 
 ocean. 
 
 Properties. Fusible without decomposition at low red- 
 ness, but sublimes at higher temperatures ; the melted 
 salt concretes into a hard white mass on cooling. Nearly 
 insoluble in absolute Alcohol, but dissolves in minute 
 quantity in rectified spirit. Soluble in three parts of water, 
 both hot and cold. Crystallizes in cubes, which are anhy- 
 drous. 
 
 Impurities of Common Salt. Table Salt usually con- 
 tains some Chloride of Magnesium, which, being deli- 
 quescent, produces a dampness by absorption of atmo- 
 spheric moisture : Sulphate of Soda is also commonly pre- 
 sent. The salt may be purified by repeated recrystalliza- 
 tion, but it is more simple to prepare the pure compound 
 directly, by neutralizing Hydrochloric Acid with Carbonate 
 of Soda. 
 
 
PHOTOGKAPHIC CHEMICALS. 105 
 
 SUGAB OF MILK. See MILK. 
 
 SULPHOCYANIDES. 
 
 General Formula, M,S 2 C 2 N, or M,S 2 Cy. 
 
 Sulphocyanides are Salts which differ from the Cyanides 
 in containing 2 atoms of Sulphur ; the alkaline Cyanides 
 fused with Sulphur combine with it readily and furnish 
 Sulphocyanides. K Cy + S 2 = K, S 2 Cy . 
 
 As Cyanides treated with an acid yield Hydrocyanic 
 Acid, so the Sulphocyanides in like manner yield Hydro- 
 sulphocyanic Acid, which differs from the former acid in 
 not being volatile or poisonous, and in giving an intensely 
 blood-red colour with salts of Peroxide of Iron. These 
 salts have been lately proposed as agents for dissolving 
 out the unchanged Salts of Silver ; their mode of action is 
 however different from that of the Cyanides and Hypo- 
 sulphites, in that their solvent power depends on their 
 state of concentration, which is not the case with the two 
 latter classes of salts. 
 
 SULPHOCYANIDE OF POTASSIUM. 
 
 Formula, K, S 2 Cy = 97. 
 
 Dried Ferrocyanide of Potassium 3 parts, Sulphur 2 
 parts, and dry Carbonate of Potash 1 part, are intimately 
 mixed and fused for some time in a covered crucible. The 
 melted mass, when cold, may be treated with boiling water 
 and filtered, and if the liquid be concentrated on cooling, 
 this salt will separate in prismatic crystals sufficiently 
 pure for Photographic purposes. 
 
 SULPHOCYANIDE OF AMMONIUM. 
 
 Formula, NH 4 ,S 2 Cy = 76. 
 
 Mr. Emerson Reynolds gives the following method for 
 the preparation of this Salt (' British Journal of Photo- 
 
106 VOCABULARY OF 
 
 graphy,' 1863). In a well-stoppered "Winchester quart 
 bottle, place : 
 
 Bisulphide of Carbon . 16 ounces by weight. 
 
 Methylated Spirit ... 12 by measure. 
 
 Liq. Ammonise (88) . . 36 ,, by measure. 
 
 Water ....... sufficient to fill the bottle. 
 
 These should be mixed in the above order, shaking between 
 each addition ; when the bottle is full, the stopper is in- 
 serted and securely tied down. The bottle is kept at 
 about 95 Fahr. for several days until the Bisulphide is all 
 dissolved. The deep orange liquid is then placed in a re- 
 tort, and distilled until Sulphur begins to separate. The 
 residue is now poured out into a basin and Sulphuric Acid 
 added till the liquid strongly reddens litmus-paper. It is 
 then allowed to stand for two hours, after which it is 
 filtered and neutralized with Ammonia, and gently eva- 
 porated to dryness. If the Sulphocyanide is required to 
 be quite pure it must be dissolved out by warm methylated 
 spirit. Its derivation from Bisulphide of Carbon, CS 2 , is 
 as follows : 2CS 2 + 4NH 3 =NH 4 ,C 2 NS 2 + 
 
 SFLPHUEETTED HYDBOGEN. 
 See HYDROSTJLPHUBIC ACID. 
 
 'SULPHUKIC ACID. 
 Formula, SO 3 = 40. 
 
 Sulphuric Acid is the highest Oxide of Sulphur ; the liquid 
 acid, when most concentrated, always has 1 atom of water, 
 HO,SO 3 . The commercial process for the manufacture 
 of Sulphuric Acid is exceedingly ingenious and beautiful, 
 but it involves many complicated reactions. The Sulphur 
 is first burnt into gaseous Sulphurous Acid, SO 2 , and 
 then, by the agency of Binoxide of Nitrogen gas, an addi- 
 tional atom of Oxygen is carried from the atmosphere to 
 the S O 9 , converting it to S O 3 , or Sulphuric Acid. 
 
 Properties. Anhydrous Sulphuric Acid is a white crys- 
 talline solid. The strongest liquid acid always contains 
 
PHOTOGRAPHIC CHEMICALS. 107 
 
 one atom of water, wliich is chemically combined with it, 
 and cannot be driven off by the application of heat. 
 
 This monoJiydrated Sulphuric Acid, represented by the 
 formula H O, S O 3 , is a dense fluid, having a specific gravity 
 of about 1'845 ; boils at 620, and distils without decom- 
 position. It is not volatile at common temperatures, and 
 therefore does not fume like Nitric or Hydrochloric Acid, 
 The concentrated acid may be cooled down even to zero 
 without solidifying ; but a weaker compound, containing 
 twice the quantity of water, and termed glacial Sulphuric 
 Acid, crystallizes at 40 Fahr. Sulphuric Acid is intensely 
 acid and caustic, but it does not destroy the skin or dis- 
 solve metals so readily as Nitric Acid. It has an energetic 
 attraction for water, and when the two are mixed, con- 
 densation ensues, and much heat is evolved ; four parts of 
 acid and one of water produce a temperature equal to that 
 of boiling water. Mixed with Nitric Acid and a certain 
 proportion of water, it forms the liquid called by Photo- 
 graphers Nitro-Sulphuric Acid. 
 
 Sulphuric Acid possesses intense chemical powers, and 
 displaces the greater number of ordinary acids from their 
 salts. It chars organic substances, by removing the ele- 
 ments of water, and converts Alcohol into Ether in a 
 similar manner. The strength of a given sample of Sul- 
 phuric Acid may be calculated, nearly, from its specific 
 gravity, and a Table is given by Dr. Ure for that purpose. 
 (See Appendix.) It is quite necessary however that the 
 temperature should be attended to in taking the specific 
 gravity, since a few degrees higher or lower than the point 
 mentioned cause a considerable difference. 
 
 Impurities of Commercial Sulphuric Acid. The liquid 
 acid sold as Oil of Vitriol is tolerably constant in compo- 
 sition, and seems to be as well adapted for Photographic 
 use as the pure Sulphuric Acid, which is far more ex- 
 pensive. The specific gravity is often about 1*836 at 60, 
 but an acid of T843 or 1-845 at 60 Fahr. can always be 
 obtained of the manufacturers, and is preferable for em- 
 ployment in Photography. If a drop, evaporated upon 
 
108 VOCABULARY OF 
 
 Platinum foil, gives a fixed residue, probably Bisulphate 
 of Potash is present. A milkiness, on dilution, indicates 
 Sulphate of Lead ; a little Sulphate of Lead however would 
 not interfere, and may be disregarded. Commercial Sul- 
 phuric Acid sometimes bleaches Sulphate of Indigo, on 
 boiling a diluted solution : this is due to traces of Nitric 
 Acid present in the Oil of Vitriol. 
 
 Test for Sulphuric Acid. If the presence of Sulphuric 
 Acid, or a soluble Sulphate, be suspected in any liquid, it 
 may be tested for by adding a few drops of dilute solution 
 of Chloride of Barium, or of Nitrate of Baryta. A white 
 precipitate, insoluble in Nitric Acid, indicates Sulphuric 
 Acid. If the liquid to be tested is very acid, from Nitric 
 or Hydrochloric Acid, it must be largely diluted before 
 testing, or a crystalline precipitate will form, caused by 
 the sparing solubility of the Chloride of Barium itself in 
 acid solutions. 
 
 TANNIN. 
 Formula, C 54 H 22 O 34 = 618. 
 
 Various organic substances, possessing an astringent 
 action, have been termed " Tannin ;" such, for instance, as 
 the extractive matters from bark, used in tanning hides, 
 the astringent principles of Tea and Coffee, etc. The most 
 important however is the Tannin of the Gall Nut, known 
 as " Tannic Acid " or " Gallo-Tannic Acid." 
 
 Gall Nuts contain as much as two-thirds of their weight 
 of Tannic Acid, which is extracted by reducing the nuts 
 to powder, and digesting them with washed Ether : the 
 decanted liquid separates on standing into two portions, 
 the lower being an aqueous solution of Tannic Acid, the 
 watery constituent of which is derived from the washed 
 Ether (see ETHEE). On evaporating the aqueous solu- 
 tion to dryness, a porous buff-coloured residue of Amor- 
 phous Tannic Acid is obtained. 
 
 Tannic Acid is freely soluble in water, but it rarely 
 yields a clear solution, on account of traces of remaining 
 resinous matter. The reaction to test-paper is slightly 
 
PHOTOGEAPHIC CHEMICALS. 109 
 
 acid, and on adding an alkali a " Tannate " is formed, but 
 the alkaline Tannates are very unstable, and tend to ab- 
 sorb Oxygen and become brown. Tannic Acid gives with, 
 the Persalts of Iron a violet-black precipitate, which is 
 the basis of common Writing Ink. Solutions of Gelatine 
 are precipitated by Tannic Acid in the form of Tanno-Ge- 
 latine, the material of Leather : Albumen is also coagulated 
 by Tannin. When Tannic Acid is heated to about 600 
 Fahr., it is decomposed, and yields Pyrogallic Acid. Ni- 
 trate of Silver is not precipitated by Tannic Acid, but suf- 
 fers a slow reduction to the state of Metallic Silver. 
 
 TAETAEIC ACID. 
 Formula, 2HO,C 8 H 4 O 10 = 150. 
 
 This Acid is derived from a substance called Tartar, 
 deposited from the juice of the Grape during fermentation. 
 This Tartar is an Acid Tartrate of Potash, and purified by 
 crystallization and deprived of its base it furnishes this 
 Acid. Tartaric Acid forms colourless, transparent, rhombic 
 prisms, very soluble in Water and Alcohol ; it is a bibasic 
 acid. The neutral salt of Potash, 2.KO,C 8 H 4 O 10 , is very 
 soluble in water. The acid salt, KO,HO,C S H 4 O 10 , known 
 commonly as Cream of Tartar when purified, is only spa- 
 ringly soluble, and forms the bulk of Tartar from wine. 
 
 For the mode of distinguishing this Acid from Citric 
 Acid, see the latter in the Vocabulary. 
 
 TETEATHIONIC ACID. 
 
 Formula, S 4 O 5 = 104. 
 
 This acid is one of the Oxides of Sulphur, belonging to 
 the interesting series designated by Berzelius the " Poly- 
 thionic Acids." The composition of this series may be re- 
 presented as follows : Formal*. 
 
 Dithionic Acid S 2 O 5 . 
 
 Trithionic Acid .... S 3 O 5 . 
 Tetrathionic Acid . . . S 4 O 5 . 
 Pentathionic Acid . . . S 5 O 5 . 
 
110 VOCABULARY OF 
 
 The amount of Oxygen in all is the same, that of the other 
 element increases progressively ; hence the highest mem- 
 ber of the series might by losing Sulphur descend gradually 
 until it reached the condition of the lowest. Such a tran- 
 sition is not only theoretically possible, but there is an 
 actual tendency to it, all the acids being unstable with 
 the exception of the Dithionic. The Alkaline Salts of 
 these acids are more unstable than the acids themselves ; 
 a solution of Tetrathionate of Soda becomes milky in the 
 course of a few days from deposition of Sulphur, and, if 
 tested, is then found to contain THthionate and eventually 
 Dithionate of Soda. 
 
 The presence of Hyposulphite of Soda increases the 
 instability of Tetrathionate of Soda. A solution of the 
 latter may be preserved for many hours unchanged, but 
 if a few crystals of Hyposulphite of Soda be dropped in, 
 it begins very shortly to deposit Sulphur, and continues 
 to do so for several days. At the same time the liquid 
 acquires an acid reaction to test-paper, and produces effer- 
 vescence on the addition of Carbonate of Lime. 
 
 Tetrathionate of Soda may be formed by acting on 
 Hyposulphite of Soda with Iodine ; a colourless solution 
 is obtained, which, in addition to the new salt, contains 
 Iodide of Sodium. Perchloride of Iron, Chloride of Cop- 
 per, and Chloride of Gold, decompose Hyposulphite of 
 Soda and form Tetrathionate, amongst other products. 
 Acids in a free state have often the same effect, since they 
 liberate Sulphurous Acid, which, in contact with Hyposul- 
 phite of Soda, forms Trithionate and Tetrathionate of Soda. 
 
 TURMERIC. 
 
 Turmeric is the rhizome of an Indian plant. It pos- 
 sesses a yellow colouring matter, soluble in water and al- 
 cohol. Paper brushed with a strong decoction of turmeric, 
 is used by the Chemist as a 'test for alkalinity ; free alka- 
 lies or alkaline bases 'change the colour from yellow to 
 brown. It is not so sensitive as reddened Litmus, but is 
 useful as indicating a strong alkaline reaction. 
 
PHOTOGRAPHIC CHEMICALS. Ill 
 
 URANIUM, NITBATE OF. 
 Formula, U 2 O 3 ,NO 5 + 6Aq. = 252. 
 
 This salt may be formed by dissolving either of the 
 Oxides of Uranium in Nitric Acid and evaporating over a 
 water bath. It forms crystals of a greenish-yellow colour, 
 which are efflorescent in warm dry air. Soluble in water, 
 Alcohol, and Ether. 
 
 Nitrate of Uranium is not an expensive salt, and has an 
 advantage in that respect over the Tartrate, which has 
 been recommended in preference. Its solution in Ether 
 exposed to the sun's rays, deposits a green Oxide, having 
 the composition U 3 O 4 . 
 
 WATEE. 
 Formula, HO = 9. 
 
 Water is an Oxide of Hydrogen, containing single atoms 
 of each of the gases. 
 
 Distilled water is water which has been vaporized and 
 again condensed ; by this means it is freed from earthy 
 and saline impurities, which, not being volatile, are left in 
 the body of the retort. Pure distilled water leaves no re- 
 sidue on evaporation, and should remain perfectly clear on 
 the addition of Nitrate of Silver, even when exposed to the 
 light ; it should also be neutral to test-paper. 
 
 The condensed water of steam-boilers, often sold in pro- 
 vincial towns and elsewhere as distilled water, is apt to be 
 contaminated with oily and empyreumatic matter, which 
 discolours Nitrate of Silver, and is therefore injurious. 
 Newly-made metal stills also fail at first in giving a pure 
 product, from the presence of grease or dust in the sol- 
 dered joints. Mr. Tomlinson* has shown that a very mi- 
 nute amount of grease on water may be detected by Cam- 
 phor, which, thrown on water free from grease, rapidly 
 
 * See ' Experimental Essays,' by Mr. C. Tomlinson, published by Virtue, 
 Brothers. 
 
 *4{''& ' 'JJ* 
 
112 VOCABULARY OF 
 
 spins round, until it has dissolved, or has evaporated, but 
 this motion is prevented by a trace of fatty matter. 
 
 Bain water, having undergone a natural process of dis- 
 tillation, is free from inorganic salts, but it usually contains 
 a minute portion of Ammonia, which gives it an alkaline 
 reaction to test-paper. It is very good for Photographic 
 purposes if collected in clean vessels, but when taken from 
 a common rain-water tank should always be examined, 
 and if much organic matter be present, tinging it of a 
 brown colour, and gradually reducing Nitrate of Silver in 
 presence of light, it must be rejected. 
 
 Spring or River water, commonly known as "hard 
 water," usually contains Sulphate of Lime, and Carbonate 
 of Lime dissolved in Carbonic Acid ; also Chloride of So- 
 dium in greater or less quantity. On boiling the water 
 for twenty minutes or half an hour, the Carbonic Acid gas 
 is evolved, and the greater part of the Carbonate of Lime 
 (if any be present) deposits, forming an incrustation, 
 which dissolves in Acetic Acid with effervescence. Spring- 
 water is less likely to contain brown organic matter than 
 rain-water. 
 
 In testing water for Carbonates, Sulphates, and Chlo- 
 rides, divide it into two parts, and add to the first a dilute 
 solution of Chloride of Barium, and to the second, Nitrate 
 of Silver,* a milkiness indicates the presence of either 
 Carbonate or Sulphate in the first case, or of Carbonate 
 or Chloride in the second. Next, acidify the two liquids 
 with a few drops of Grlacial Acetic Acid : if the opales- 
 cence disappears and the fluid becomes clear, Carbonates 
 are present ; but if, as is more frequently the case, the 
 cloudiness is only partially removed by the Acetic Acid, 
 then the Carbonate is mixed with Sulphate or Chloride as 
 the case may be. 
 
 Water for the Nitrate Bath. Common hard water can 
 often be used for making a Nitrate Bath when nothing 
 
 * The Photographic Nitrate Bath cannot be used as a test, since the Iodide 
 of Silver it contains is precipitated on dilution, giving a milkiness which 
 might be mistaken for Chloride of Silver. 
 
PHOTOGRAPHIC CHEMICALS. 113 
 
 better is at hand. The Chlorides it contains are preci- 
 pitated by the Nitrate of Silver, leaving small quantities 
 of soluble Nitrates in solution, which are not injurious. 
 Carbonate of Lime, if present, neutralizes acid, and may 
 render the Bath alkaline if Salts of Ammonia are present. 
 Sulphate of Lime causes no precipitation, and ought not 
 theoretically to produce any injurious effect. It has how- 
 ever been spoken against by some, but whether from prac- 
 tical experience or not the Writer is uninformed. Or- 
 ganic matter will almost certainly be injurious to the Bath, 
 and therefore, unless the purity of the rain-water can be 
 guaranteed, spring- water will be preferable. 
 
 Water for the developing solutions. Carbonate of Lime 
 in a water ought not to unfit it for the developing liquid in 
 presence of free Acetic Acid, because the chalk would be 
 decomposed under such circumstances, and converted into 
 Acetate of Lime, which would probably assist a little in 
 increasing the intensity. In the case of Sulphate of Iron, 
 chalky water throws down a small quantity of Oxide 
 of Iron, which produces turbidity and a rusty colour at 
 the edges of the pictures ; but any acid added to the de- 
 veloper dissolves it and renders the liquid again clear. 
 Chalk in water used for the Gallic Acid developer, makes 
 it discolour quickly with Nitrate of Silver, but a minim 
 of Acetic Acid to each two ounces is a remedy, as before 
 shown. 
 
 Soluble Chlorides are always injurious in water used for 
 developing, because they decompose the Nitrate of Silver 
 on the film, producing a curdy precipitate and lessening 
 the available strength. Acids have no effect in removing 
 Chlorides, but they may be separated by shaking up the 
 water with a graduated quantity of Nitrate of Silver, and 
 filtering it. In this process however there is always a diffi- 
 culty in knowing how much of the Nitrate will be needed, 
 since the least excess over the quantity required to de- 
 compose the salt, causes the water to blacken on adding 
 Pyrogallic Acid. This might be obviated however by keep- 
 ing the Pyrogallic Acid dissolved in Glacial Acetic Acid, 
 
114 VOCABULABY OF PHOTOGRAPHIC CHEMICALS. 
 
 and adding the water only when required. Bear in mind 
 also that it will not be necessary to remove every trace of 
 salt from the water, since it is quite possible to bring out 
 a perfect image with a developing solution which produces 
 a decided turbidity on first touching the film ; especially 
 so if a few drops of solution of Nitrate of Silver be added 
 to the developer immediately before use. 
 
 Water for washing preserved plates. A faint opales- 
 cence on adding a few drops of solution of Nitrate of Silver 
 may be disregarded, but if the water becomes decidedly 
 milky, try the following mode of purifying it. To each 
 pint add a fluid drachm of the Negative Nitrate Bath, and 
 set ijj aside for twelve hours to deposit. It may be fil- 
 tered at the time, but the precipitate is liable to choke the 
 filter and make it run slowly. The above proceeding will 
 probably leave one of the two precipitating salts in excess, 
 but the Writer gives the proportions which he found to 
 answer in the case of a hard water drawn from a well at 
 the seaside, containing a mixture of Chloride of Sodium 
 and Carbonate of Lime. Graduated quantities of the Ni- 
 trate Bath were dropped into this water at intervals of a 
 few hours, and it was found that all further precipitation 
 ceased when 60 minims had been added to each pint. 
 

 PART II. 
 
 THE THEORY OF PHOTOGRAPHIC 
 
 PROCESSES. 
 
117 
 
 A 
 
 THEORY OF PHOTOGRAPHIC PROCESSES. 
 
 CHAPTER I. 
 
 HISTOEICAL SKETCH OF PHOTOGRAPHY. 
 
 THE Art of Photography, which has now attained such 
 perfection, and has become so popular amongst all classes, 
 is one of comparatively recent introduction. 
 
 The word Photography means literally "writing by 
 means of Light ;" and it includes all processes by which 
 any kind of picture can be obtained by the chemical agency 
 of Light, without reference to the nature of the sensitive 
 surface upon which it acts. 
 
 The philosophers of antiquity, although chemical changes 
 due to the influence of Light were continually passing be- 
 fore their eyes, do not appear to have directed their atten- 
 tion to them. Some of the Alchemists indeed noticed the 
 fact that a substance termed by them " Horn Silver," 
 which was Chloride of Silver which had undergone fusion, 
 became blackened by exposure to Light ; but their ideas 
 on such subjects being of the most erroneous nature, no- 
 thing resulted from the discovery. 
 
 The first philosophical examination of the decomposing 
 action of Light upon compounds containing Silver was 
 made by the illustrious Scheele, no longer than three-quar- 
 ters of a century ago, viz. in 1777. It was also remarked 
 
118 THEORY OF PHOTOGEAPHT. 
 
 by him that some of the coloured rays of Light were pecu- 
 liarly active in promoting the change. 
 
 Earliest application of these facts to purposes of Art. 
 The first attempts to render the blackening of Silver Salts 
 by Light available for artistic purposes were made by 
 Wedgwood and Davy about A.D. 1802. A sheet of white 
 paper or of white leather was saturated with a solution of 
 Nitrate of Silver, and the shadow of the figure intended to 
 be copied projected upon it. Under these circumstances 
 the part on which the shadow fell remained white, whilst 
 the surrounding exposed parts gradually darkened under 
 the influence of the sun's rays. 
 
 Unfortunately these and similar experiments, which ap- 
 peared at the outset to promise well, were checked by the 
 experimentalists being unable to discover any means of 
 fixing the pictures, so as to render them indestructible by 
 diffused Light. The unchanged Silver Salt being per- 
 mitted to remain in the white portions of the paper, na- 
 turally caused the proofs to blacken in every part, unless 
 carefully preserved in the dark. 
 
 Introduction of the Camera Obscura, and other Improve- 
 ments in Photography. The " Camera Obscura," or dark- 
 ened chamber, by means of which a luminous image of an 
 object may be formed, was invented by Baptista Porta, of 
 Padua ; but the preparations employed by Wedgwood were 
 not sufficiently sensitive to be easily affected by the sub- 
 dued light of that instrument. 
 
 In the year 1814, however, twelve years subsequent 
 to the publication of Wedgwood's paper, M. Niepce, of 
 Chalons, having directed his attention to the subject, suc- 
 ceeded in perfecting a process in which the Camera could 
 be employed, although the sensibility was still so low that 
 an exposure of some hours was required to produce the 
 effect. 
 
 In the process of M. Niepce, which was termed " Helio- 
 graphy," or " sun-drawing," the use of the Silver Salts 
 was discarded, and a resinous substance, known as " Bitu- 
 men of Judsea," substituted. This resin was smeared on 
 
HISTOEICAL SKETCH. 119 
 
 the surface of a metal plate, and exposed to the luminous 
 image. The light in acting upon it so changed its pro- 
 perties, that it became insoluble in certain essential oils. 
 Hence, on subsequent treatment with the oleaginous sol- 
 vent, the shadows dissolved away, and the lights were repre- 
 sented by the unaltered resin remaining on the plate. 
 
 The Discoveries of Ml. Daguerre. MM. Niepce and 
 Daguerre appear at one time to have been associated as 
 partners, for the purpose of mutually prosecuting their re- 
 searches ; but it was not until after the death of the for- 
 mer, viz. in 1839, that the process named the Daguerreo- 
 type was given to the world. Daguerre was dissatisfied 
 with the slowness of action of the Bitumen sensitive sur- 
 face, and directed his attention mainly to the use of the 
 Salts of Silver, which are thus again brought before our 
 notice. 
 
 Even the earlier specimens of the Daguerreotype, al- 
 though far inferior to those subsequently produced, pos- 
 sessed a beauty which had not been attained by any Pho- 
 tographs prior to that time. 
 
 The sensitive plates of Daguerre were prepared by ex- 
 posing a silvered tablet to the action of the vapour of 
 Iodine, so as to form a layer of Iodide of Silver upon the 
 surface. By a short exposure in the Camera an effect was 
 produced, not visible to the eye, but appearing when the 
 plate was subjected to the vapour of Mercury. This fea- 
 ture, viz. the production of a latent image upon Iodide of 
 Silver, with its subsequent development by a chemical 
 reagent, is one of the first importance. Its discovery at 
 once reduced the time of taking a picture from hours to 
 minutes, and promoted the utility of the Art. 
 
 Daguerre also succeeded infixing his proofs, by removal 
 of the unaltered Iodide of Silver from the shadows. The 
 processes employed however were imperfect, and the mat- 
 ter was not set at rest until the publication of a paper by 
 Sir John Herschel, on the property possessed by " Hypo- 
 sulphites " of dissolving the Salts of Silver insoluble in 
 water. 
 
120 THEOET OF PHOTOGRAPHY. 
 
 On a means of Multiplying Photographic Impressions 
 and other Discoveries of Mr. Fox Talbot. The first com- 
 munication made to the Royal Society by Mr. Fox Talbot, 
 in January, 1839, related to the preparation of a more 
 sensitive paper than had been previously known, and also 
 to a method of fixing the pictures by common Salt. It 
 was directed that the paper should be dipped first in solu- 
 tion of Chloride of Sodium, and then in Nitrate of Silver. 
 By proceeding in this way a white substance termed Clilo- 
 ride of Silver is formed, more sensitive to light than the 
 Nitrate of Silver originally employed by Wedgwood and 
 Davy. The object is laid in contact with the prepared 
 paper, and being exposed to light, a copy is obtained, which 
 is Negative, id est, with the light and shade reversed. 
 The discovery of a way of fixing these Negatives was of 
 great importance, as it not only rendered them unalterable 
 by light, but it further allowed of their being used to ob- 
 tain copies having the light and shade correct, or Positive 
 copies, which was effected by laying the Negative over a 
 second sheet of prepared sensitive paper, so as to allow 
 the sun's light to pass through the transparent parts, 
 tinder these circumstances, when the Negative is raised, 
 a natural representation of the object is found below; 
 the tints having been again reversed by the second ope- 
 ration. 
 
 This production of a Negative Photograph, from which 
 any number of Positive copies may be obtained, is a car- 
 dinal point in Mr. Talbot's invention, and one of great im- 
 portance. 
 
 Another most important discovery was that of the exist- 
 ence of an invisible image impressed on the paper by a 
 much shorter exposure to light than was necessary to pro- 
 duce a visible one, and capable of being afterwards ren- 
 dered visible. Mr. Talbot took out a patent for a process 
 of this kind in 1841 . Mr. Brayley had, however, as early 
 as 1839, described in a lecture at the London Institution 
 a similar method communicated to him by Mr. Eeade. 
 Mr. Talbot's process was, however, a decided advance on 
 
HISTORICAL SKETCH. 121 
 
 the former, in consequence of his employing Acetic Acid 
 to check action not due to light. In this process, a sheet 
 of paper is first coated with Iodide of Silver by soaking it 
 alternately in Iodide of Potassium and Nitrate of Silver ; 
 it is then washed with solution of Gallic Acid, contain- 
 ing Nitrate of Silver and Acetic Acid (sometimes termed 
 Gallo- Nitrate of Silver), by which the sensibility to light 
 is greatly augmented. An exposure in the Camera of 
 some seconds or minutes, according to the brightness of 
 the light, impresses an invisible image, which is brought 
 out by treating the plate with a fresh portion of the mix- 
 ture of Gallic Acid, Nitrate of Silver, and Acetic Acid 
 employed in exciting. 
 
 On the use of Glass Plates to retain sensitive Films. 
 The principal defects in the Calotype process are attribu- 
 table to the coarse and irregular structure of the fibre of 
 paper, even when manufactured with the greatest care, and 
 expressly for Photographic purposes. In consequence of 
 this, the same amount of exquisite definition and sharpness 
 of outline as that resulting from the use of metal plates, 
 cannot be obtained. 
 
 We are indebted to Sir John Herschel for the first em- 
 ployment of glass plates to receive sensitive Photographic 
 films. 
 
 The Iodide of Silver may be retained upon the glass by 
 means of a layer of Albumen or white of egg, as proposed 
 by M. Niepce de Saint- Victor, nephew to the original dis- 
 coverer of the same name. 
 
 A more important improvement still was the employ- 
 ment of " Collodion " for a similar purpose. 
 
 Collodion is an ethereal solution of Pyroxyline, a weakly 
 explosive variety of Gun-cotton. On evaporation it leaves 
 a transparent layer, resembling goldbeater's skin, which 
 adheres to the glass with some tenacity. M. Le Grey, of 
 Paris, originally suggested that this substance might per- 
 haps be rendered available in Photography, but our own 
 countryman, the late Mr. Archer, was the first to carry 
 out the idea practically. In a communication to ' The 
 
122 THEOEY OF PEOTOGEAPHY. 
 
 Chemist ' in the autum of 1851, this gentleman gave a de- 
 scription of the Collodion process much as it now stands ; 
 at the same time proposing the substitution of P^ro-gallic 
 Acid for the Gallic Acid previously employed in develop- 
 ing the image. 
 
 At that period no idea could have been entertained of 
 the stimulus which this discovery would render to the pro- 
 gress of the Art ; but experience has now abundantly de- 
 monstrated, that, as far as all qualities most desirable in a 
 Photographic process are concerned, none at present known 
 can excel, or perhaps equal, the Collodion process. 
 
123 
 
 CHAPTER II. 
 
 THE SALTS OF SILYEB, EMPLOYED IN PHOTOGRAPHY. 
 
 BY the term of Salt of Silver \ve understand that the com- 
 pound in question contains Silver, but not in its elemen- 
 tary form ; the metal is in fact in a state of chemical union 
 with other elements which disguise its physical properties, 
 so that the Salt possesses none of the external characters 
 of the Silver from which it was produced. (See Part I. 
 page 13, on Salts.) 
 
 The contents of this Chapter may be arranged in three 
 Sections : the first describing the Chemistry of the Salts 
 of Silver; the second, the action of Light upon them; 
 the third, the preparation of a sensitive surface, with ex- 
 periments illustrating the formation of the Photographic 
 image. 
 
 SECTION I. 
 
 Chemistry of the Salts of Silver. 
 
 The principal Salts of Silver employed in the Pho- 
 tographic processes are four in number, viz. Nitrate of 
 Silver, Chloride of Silver, Iodide of Silver, and Bromide 
 of Silver. In addition to these, it will be necessary to de- 
 scribe the Oxides of Silver. 
 
 THE PREPARATION AND PROPERTIES OF THE NITRATE 
 OF SILVER. 
 
 Nitrate of Silver is prepared by dissolving metallic Silver 
 
124 THEOBY OF PHOTOGRAPHY. 
 
 in Nitric Acid. Nitric Acid is a powerfully acid and cor- 
 rosive substance, containing two elementary bodies united 
 in definite proportions : these are Nitrogen and Oxygen ; 
 the latter being present in greatest quantity. 
 
 Nitric Acid is a powerful solvent for the metallic bodies 
 generally. To illustrate its action in that particular, as 
 contrasted with other acids, place pieces of silver-foil in 
 two test-tubes, the one containing dilute Sulphuric, the 
 other dilute Nitric Acid ; on the application of heat a vio- 
 lent action soon commences in the latter, but the former 
 is unaffected. In order to understand this, it must be 
 borne in mind that when a metallic substance dissolves in 
 an acid, the nature of the solution is different from that 
 of an aqueous solution of salt or sugar. If salt water be 
 boiled down until the whole of the water has evaporated, 
 the salt is recovered with properties the same as at first ; 
 but if a similar experiment be made with a solution of 
 Silver in Nitric Acid, the result is different : in that case 
 we do not obtain metallic Silver on evaporation, but Silver 
 combined with Oxygen and Nitric Acid, both of which are 
 in a state of chemical combination with the metal. 
 
 The Oxygen required for the Oxide of Silver which is 
 united with the Nitric Acid, is derived from another part 
 of the same Acid, one atom of which by supplying Oxygen 
 to three atoms of metallic Silver is thereby reduced to 
 Deutoxide of Nitrogen which escapes with effervesence. 
 The following equation explains these changes : 
 3Ag + 4N0 5 = 3(AgO,N0 5 )+NO ? .* 
 
 It is this instability of Nitric Acid its readiness to part 
 with Oxygen which renders it superior to the Sulphuric 
 and to most acids in dissolving Silver and various other 
 substances, both organic and inorganic. 
 
 Properties of Nitrate of Silver. In preparing Nitrate 
 of Silver, when the metal is dissolved, the solution is boiled 
 down and set aside to crystallize. The salt however as so 
 obtained is still acid to test-paper, and requires either re- 
 
 * The preparation of Nitrate of Silver from alloys of Silver and Copper is 
 described in Part I., Art. " Silver." 
 
SALTS OF SILVEE. 125 
 
 tallization, or careful heating to about 300 Fahren- 
 heit. It is this retention of small quantities of Nitric Acid, 
 and other impiirities, which renders much of the commer- 
 cial Nitrate of Silver useless for Photography, until after 
 a second crystallization. 
 
 Pure Nitrate of Silver occurs in the form of white crys- 
 talline plates, which are very heavy and dissolve readily 
 in an equal weight of cold water. The solubility is much 
 lessened by the presence of free Nitric Acid, and in the 
 concentrated Nitric Acid the crystals are almost insoluble. 
 Boiling Alcohol takes up about one-fourth part of its 
 weight of the crystallized Nitrate, but deposits nearly the 
 whole on cooling. Nitrate of Silver has an intensely bitter 
 and nauseous taste ; acting as a caustic, and corroding the 
 skin by a prolonged application. Its aqueous solution does 
 not redden blue litmus-paper ; on the contrary, the pure 
 re-crystallized and dried Nitrate of Silver restores the blue 
 colour of paper previously reddened. 
 
 Heated in a crucible the salt melts, and when poured 
 into a mould and solidified, forms the white lunar caustic 
 of commerce. At a still higher temperature it is decom- 
 posed, and bubbles of Oxygen Gas are evolved : the melted 
 mass cooled and dissolved in water yields a solution, which 
 contains Nitrite in addition to Nitrate of Silver.* 
 
 THE CHEMISTRY OF THE CHLORIDES OF SILVER. 
 
 Preparation of Protochloride of Silver (AgCl). The 
 ordinary white Chloride of Silver may be prepared in two 
 ways, by the direct action of Chlorine upon metallic 
 Silver, and by double decomposition between two salts. 
 
 If a plate of polished silver be exposed to a current of 
 Chlorine Gas,t it becomes after a short time coated on 
 the surface with a superficial film of white powder. This 
 
 * Nitrite of Silver differs from the Nitrate in containing less Oxygen, and 
 is formed from it by the abstraction of two atoms of that element; it 13 
 described in the Vocabulary, Part I. 
 
 t *'or the properties of the element " Chlorine," see the Vocabulary. 
 
126 THEOEY OF PHOTOGEAPHY. 
 
 powder is Chloride of Silver, containing the two elements 
 Chlorine and Silver united in single equivalents. 
 
 Preparation of Chloride of Silver by double decomposi- 
 tion. l n order to illustrate this, take a solution in water 
 of Chloride of Sodium or " common salt," and mix it with 
 a solution containing Nitrate of Silver; immediately a 
 dense, curdy, white precipitate falls, which is the sub- 
 stance in question. 
 
 In this reaction the elements change places ; the Chlo- 
 rine leaves the Sodium with which it was previously com- 
 bined, and crosses over to the Silver; the Oxygen and 
 Nitric Acid are released from the Silver, and unite with 
 the Sodium ; thus 
 
 This interchange of elements is an example of double 
 decomposition ; further illustrations of it are given in the 
 first Chapter of Part I. 
 
 In preparing Chloride of Silver by double decomposition, 
 the white clotty masses which first form must be washed 
 repeatedly with water, in order to free them from soluble 
 Nitrate of Soda, the other product of the change. When 
 this is done, the salt is in a pure state, and may be dried, 
 etc., in the usual way. 
 
 Properties of Chloride of Silver. Chloride of Silver 
 forms a soft white powder resembling common chalk or 
 whiting. It is tasteless and insoluble in water ; unaffected 
 by boiling with the strongest Nitric Acid, but sparingly 
 dissolved by concentrated Hydrochloric Acid. 
 
 Ammonia dissolves Chloride of Silver freely, as do solu- 
 tions of Hyposulphite of Soda and Cyanide of Potassium. 
 Concentrated solutions of the Sulphocyanide, Chloride, 
 Iodide, and Bromide of Potassium, Sodium, and Am- 
 monium, are likewise solvents of Chloride of Silver, but to 
 a limited extent, as will be more fully shown in Chapter 
 IV., when treating of the modes of fixing the Photographic 
 proofs. 
 
 Dry Chloride of Silver carefully heated to redness fuses, 
 and concretes on cooling into a tough and semitranspareiit 
 
SALTS OF SILVER. 127 
 
 substance, which has been termed horn silver or luna 
 cornea. 
 
 Placed in contact with metallic Zinc or Iron acidified 
 with dilute Sulphuric Acid, Chloride of Silver is reduced 
 to the metallic state, the Chlorine passing to the other 
 metal under the decomposing influence of the galvanic 
 current which is established. Ag Cl -f Zn = Zn Cl + Ag. 
 
 Preparation and Properties of Subckloride of Silver. If 
 a plate of polished Silver be dipped into a solution of Per- 
 chloride of Iron, a black stainis produced, the Perchloride 
 sinking to the state of a .Profo-chloride of Iron, and losing 
 a portion of Chlorine, which passes to the Silver and con- 
 verts it superficially into a Subchloride of Silver. 
 
 This compound differs from the white Chloride of Silver 
 in containing less Chlorine. As it has not been obtained 
 sufficiently pure for analysis, no formula can be assigned 
 to it. The only facts certainly ascertained with regard to 
 the Subchloride of Silver are, that it is a pulverulent 
 substance of a bluish-black colour, not easily affected by 
 Citric Acid, but decomposed by fixing agents, such as 
 Ammonia, Hyposulphite of Soda, or Cyanide of Potassium, 
 into Chloride of Silver, which dissolves, and insoluble 
 metallic Silver. 
 
 THE CHEMISTRY OF IODIDE OF SILVER. 
 
 The properties of Iodine are described in the first Part 
 of the Work : they are analogous to those of Chlorine -and 
 Bromine, the Silver Salts formed by these elements bear- 
 ing also a strong resemblance to each other. 
 
 Preparation and Properties of Iodide of Silver (Agl). 
 Iodide of Silver may be formed in an analogous manner 
 to the Chloride, viz. by the direct action of the vapour of 
 Iodine upon metallic Silver, or by double decomposition 
 between solutions of Iodide of Potassium and Nitrate of 
 Silver. 
 
 When prepared by the latter mode it forms an impalp- 
 able powder, the colour of which varies with the manner 
 
128 THEORY OF PHOTOGRAPHY. 
 
 of precipitation. If the Iodide of Potassium be in excess, 
 the Iodide of Silver falls to the bottom of the vessel nearly 
 white ; but with an excess of Nitrate of Silver it is of a 
 straw-yellow tint. This point may be noticed, because the 
 yellow condition is the one adapted for Photographic use, 
 the other being insensible to the influence of Light. 
 
 Iodide of Silver is tasteless and inodorous : insoluble in 
 water and in dilute Nitric Acid. It is scarcely dissolved by 
 Ammonia, which serves to distinguish it from the Chlo- 
 ride of Silver, freely soluble in that liquid. Hyposulphite 
 of Soda and Cyanide of Potassium both dissolve Iodide 
 of Silver ; it is also soluble in solutions of Sulphocyanide, 
 Bromide, and Iodide, of Potassium, Sodium, and Ammo- 
 nium, as will be further explained in Chapter IV. 
 
 Iodide of Silver is reduced by Metallic Zinc in the same 
 manner as the Chloride of Silver, forming soluble Iodide 
 of Zinc and leaving a black powder of metallic Silver. 
 n = ZnI + Ag. 
 
 THE PREPARATION AND PROPERTIES OF BROMIDE 
 OF SILVER. 
 
 This substance so closely resembles the corresponding 
 salts containing Chlorine and Iodine, that a short notice 
 of it will suffice. 
 
 Bromide of Silver (AgBr) is prepared by exposing a 
 silvered plate to the vapour of Bromine, or by adding solu- 
 tion of Bromide of Potassium to Nitrate of Silver. It is 
 insoluble in water, slightly yellow in colour, and distin- 
 guished from Iodide of Silver by dissolving in strong Am- 
 monia and in Chloride of Ammonium. It is freely soluble 
 in Hyposulphite of Soda and in Cyanide of Potassium. 
 
 The properties of the element Bromine are described in 
 Part I. 
 
 CHEMISTRY OF THE OXIDES OF SILVER. 
 
 The Protoxide of Silver (AgO). If a little Potash or 
 Ammonia be added to solution of Nitrate of Silver, an 
 
SALTS OF SILVEE. 129 
 
 olive-brown substance is formed, which, on standing, col- 
 lects at the bottom of the vessel. This is Oxide of Silver, 
 displaced from its previous state of combination with Nitric 
 Acid by the stronger oxide, Potash. Oxide of Silver is 
 soluble to a very minute extent in pure water, the solution 
 possessing an alkaline reaction to Litmus ; it is easily dis- 
 solved by Nitric or Acetic Acid, forming a neutral Nitrate 
 or Acetate of Silver ; it also dissolves in Ammonia (Am- 
 monio-Nitrate of Silver), Nitrate of Ammonia, Hyposul- 
 phite of Soda, and Cyanide of Potassium. 
 
 The Suboxide of Silver (Ag 2 O ?). This compound is dis- 
 tinguished from the Protoxide by the action of Hydro- 
 chloric Acid, which with the latter produces the white 
 Chloride, but with the latter the dark Subchloride. It 
 has probably never been obtained pure, but a certain 
 amount is obtained, mixed however with Silver and Prot- 
 oxide, on boiling Arsenite of Silver, 3AgO,AsO 3 , with 
 a solution of Potash : a black powder results, which is the 
 above mixture. On treating this powder, after washing, 
 with Hydrochloric Acid, a mixture of Chloride, Subchlo- 
 ride, and metallic Silver is produced, from which, after 
 washing, the latter may be dissolved out by Nitric Acid, 
 leaving the dark mixture of Chlorides. 
 
 SECTION II. 
 
 On the Photographic Properties of the Salts of Silver. 
 
 In addition to the Salts of Silver described in the First 
 Section of this Chapter there are many others well known 
 to chemists, as the Acetate of Silver, the Sulphate, the Ci- 
 trate of Silver, etc. Some occur in crystals which are solu- 
 ble in water, whilst others are pulverulent and insoluble. 
 
 The Salts of Silver formed by colourless Acids are 
 white when first prepared, and remain so if kept in a dark 
 place; but they possess the remarkable peculiarity of being 
 darkened by exposure to Light, either alone or in contact 
 with organic substances. 
 
130 THEOEY OF PHOTOGEAPHY. 
 
 Action of Light upon the Nitrate of Silver. The Nitrate 
 of Silver is one of the most permanent of the Silver salts. 
 It may be preserved unchanged in the crystalline form, or 
 in solution in distilled water, for an indefinite length of 
 time, even when constantly exposed to the light of day. 
 
 Nitrate of Silver may, however, be rendered susceptible 
 to the influence of Light, by adding to its solution organic 
 matter, vegetable or animal. The phenomena produced in 
 this case are well illustrated by dipping a sheet of white 
 paper in solution of Nitrate of Silver, and exposing it to 
 the direct rays of the sun ; it slowly darkens, until it be- 
 comes nearly black. The stains upon the skin produced 
 by handling Nitrate of Silver are caused in the same way, 
 and are seen most evidently when the part has been ex- 
 posed to light. 
 
 The varieties of organic matter which especially facilitate 
 the blackening of Nitrate of Silver are such as tend to ab- 
 sorb Oxygen; hence pure vegetable fibre, free from Chlo- 
 rides, such, for instance, as the Swedish filtering-paper, is 
 not rendered very sensitive by being simply brushed with 
 solution of the Nitrate, but a little grape sugar added soon 
 determines the decomposition. 
 
 Decomposition of Chloride, Bromide, and Iodide of Silver 
 by Light. Pure Chloride of Silver prepared in the moist 
 way changes slowly from white to violet on exposure to 
 light. Bromide of Silver becomes of a grey colour, but is 
 less affected than the Chloride. Iodide of Silver (if free 
 from excess of Nitrate of Silver) does not alter in appear- 
 ance by exposure even to the sun's rays, but retains its 
 yellow tint unchanged. Of these three compounds there- 
 fore, Chloride of Silver is the most visibly acted on by light, 
 and papers prepared with this salt will become far darker 
 on exposure, than others coated with Bromide or Iodide of 
 Silver. 
 
 There are certain conditions which render the action of 
 light upon the Chloride of Silver more decided. These are, 
 first, an excess of a soluble salt of Silver such as the Nitrate, 
 and, second, the presence of organic matter. Pure Chloride 
 
SALTS OF SILVEE. 131 
 
 of Silver would be useless as a Photographic agent, but a 
 Chloride with excess of Nitrate takes a strong impression. 
 Even Iodide of Silver, ordinarily unaffected, is blackened 
 by light when moistened with a solution of the Nitrate of 
 Silver. 
 
 Organic matter combined with Chloride and Nitrate of 
 Silver gives a still higher degree of darkening in the solar 
 ray, and in this way the Photographic papers are prepared. 
 
 Action of Light upon organic compounds of Silver. On 
 adding diluted Albumen, or white of egg, to solution of 
 Nitrate of Silver, a flocculent deposit forms, which is a com- 
 pound of the animal matter with Oxide of Silver, and is 
 known as "Albuminate of Silver." This substance is at 
 first white, but on exposure to light it assumes a brick-red 
 colour. 
 
 Caseine, the animal principle of milk, is coagulated by 
 Nitrate of Silver, the product behaving in the same manner 
 as the Albuminate when exposed to light. Gelatine does 
 not precipitate Nitrate of Silver : but if a sheet of trans- 
 parent Gelatine be allowed to imbibe a solution of the 
 Nitrate, it combines with it, and the product becomes of a 
 clear ruby-red tint on exposure to light. 
 
 Many other organic compounds of Silver are darkened 
 by light. The white Citrate of Silver changes to a red 
 colour. Glycyrrhizin, the Sugar of Liquorice, also forms 
 a white compound with Oxide of Silver, which becomes 
 brown or red in the sun's rays. 
 
 The photographic use of the organic Salts of Silver, and 
 the extent to which they are affected by light, will be fur- 
 ther considered in the Eighth Chapter, when speaking of 
 the theory of Positive printing. 
 
 SIMPLE EXPERIMENTS ILLUSTRATING THE ACTION OF LIGHT 
 UPON A SENSITIVE LAYEB OF CHLOEIDE OF SILVEE ON 
 PAPEE. 
 
 In the performance of the most simple experiments on 
 the decomposition of Silver Salts by Light, the student 
 
132 THEOEY OF PHOTOGRAPHY. 
 
 may employ ordinary test-tubes, in which small quantities 
 of the two liquids required for the double decomposition 
 may be mixed together. 
 
 When however concentrated solutions are used in this 
 way, the insoluble Silver Salt falls in dense and clotted 
 masses, which, exposed to the sun's rays, quickly blacken 
 on the exterior, but the inside is protected, and remains 
 white. It is of importance therefore in Photography that 
 the sensitive material should exist in the form of a surface, 
 in order that the various particles of which it is composed 
 may each one individually be brought into relation with 
 the disturbing force. 
 
 Full directions for the preparation of sensitive Photo- 
 graphic paper are given in the third division of this work. 
 The following is the theory of the process : A sheet of 
 paper is treated with solution of Chloride of Sodium or 
 Ammonium, and subsequently with Nitrate of Silver; 
 hence results a formation of Chloride of Silver in a fine 
 state of division, with an excess of Nitrate of Silver, since 
 the Silver bath is applied after the salting solution, and is 
 made purposely of a greater strength. 
 
 Illustrative Experiment No. I. Place a square of sen- 
 sitive paper, prepared as above (for full directions see 
 Part III.), in the direct rays of the sun, and observe the 
 gradual process of darkening which takes place ; the sur- 
 face passes through a variety of changes in colour until 
 it becomes of a deep chocolate-brown. If the Light is to- 
 lerably intense, the brown shades are probably reached in 
 from three to five minutes ; but the sensibility of the paper, 
 and also the nature of the tints, will vary much with the 
 character of the organic matter present. 
 
 Experiment No. II. Lay a device cut from black paper 
 upon a sheet of sensitive paper, and compress the two to- 
 gether by means of a sheet of glass. After a proper length 
 of exposure the figure will be exactly copied with the tint 
 reversed : the black paper protecting the sensitive Chloride 
 beneath, produces a white figure upon a dark ground. 
 
 Experiment No. III. Repeat the last experiment, sub- 
 
SALTS OF SILVER. 133 
 
 stituting a piece of lace or gauze-wire for the paper device. 
 This is intended to show the minuteness with which objects 
 can be copied, since the smallest filament will be distinctly 
 represented. 
 
 Experiment No. TV. Take an engraving in which the 
 contrast of light and shade is tolerably well marked, and 
 having laid it closely in contact with the sensitive paper, 
 expose as before. This experiment shows that the surface 
 darkens in degrees proportionate to the intensity of the 
 light, so that the half shadows of the engraving are ac- 
 curately maintained, and a pleasing gradation of tone is 
 produced. 
 
 In the darkening of Photographic papers, the action of 
 the light is quite superficial, and although the black colour 
 may be intense, yet the amount of reduced Silver which 
 forms it is so small that it cannot be conveniently esti- 
 mated by chemical reagents. This is well shown by the re- 
 sults of an analysis performed by the Author, in which the 
 total weight of Silver obtained from the blackened sheet 
 measuring nearly 24 by 18 inches amounted to less than 
 half a grain. It becomes therefore of great importance in 
 preparing sensitive paper to attend to the condition of the 
 surface layer of particles, the action rarely extending to 
 those beneath. The use of Albumen, Gelatine, etc., which 
 will be explained in the Eighth Chapter, has reference to 
 this amongst other advantages, and secures a better and 
 more sharply defined print. 
 
 THE NATURE OF THE CHEMICAL CHANGES PEODUCED 
 BY LIGHT UPON COMPOUNDS OF SILVER. 
 
 Action of Light upon Chloride of Silver. This may be studied 
 by suspending pure Chloride of Silver in distilled water, and ex- 
 posing it to the sun's rays for several days. When the process 
 of darkening has proceeded to some extent, the supernatant 
 liquid will be found to contain free Chlorine. 
 
 The luminous rays appear to loosen the affinity of the elements 
 Chlorine and Silver for each other ; hence a portion of Chlorine 
 is separated, and the white Protochloride is converted into a violet 
 $Mi>chloride of Silver. If an atom of Nitrate of Silver be present, 
 
134 THEOEY OF PHOTOGRAPHY. 
 
 the liberated Chlorine unites with it, displacing Niti'ic Acid, and 
 forming again Chloride of Silver which is decomposed in its turn. 
 The excess of Nitrate of Silver thus assists the darkening of 
 Chloride of Silver, by rendering the chain of chemical affinities 
 more complete, and preventing an accumulation of Chlorine in 
 the liquid. 
 
 The violet-coloured compound, the product of the darkening 
 action of light, has been spoken of by some writers as a mixture 
 of Chloride of Silver and Metallic Silver, but the fact that white 
 Chloride of Silver will darken in the sun's rays, even when covered 
 with strong Nitric Acid, proves that the colour is not due to me- 
 tallic Silver, but to a Subchloride. 
 
 The properties of this Subchloride are as follows : It is very 
 little affected by Nitric Acid, but quickly acted on by those bodies 
 which Photographers employ as fixing agents, viz. Ammonia, 
 Hyposulphite of Soda, and Cyanide of Potassium; the greater 
 part being dissolved by the fixing agent, and a minute portion of 
 Metallic Silver remaining insoluble. 
 
 Chemical Changes on Organic Salts of Silver. White Al- 
 buminate of Silver is soluble in Ammonia, and also in Hyposul- 
 phite of Soda ; but after having been reddened by exposure to 
 light, very little effect is produced upon it by these fixing agents. 
 Grelatine saturated with Nitrate of Silver and exposed to light, 
 loses its characteristic property of dissolving in hot water, but 
 when treated with boiling solution of Potash, it is taken up, 
 forming a clear liquid of a blood-red colour. 
 
 From these facts and others not mentioned, we infer that in the 
 action of light upon Organic Salts of Silver, the Oxide of Silver 
 loses Oxygen and sinks to the state of a Suboxide, just as Chloride 
 of Silver parts with a portion of Chlorine when exposed to light 
 and becomes a Subchloride. The organic body is probably 
 oxidized, and the products are left in combination with each 
 other. The present state of our knowledge will not allow of 
 more than a general indication of the nature of these changes ; 
 no exact formula can be given. 
 
 Nature of the Blackening of Photographic Papers. In this 
 case both Chloride of Silver and Nitrate of Silver are present, 
 supported by an organic basis. To ascertain the nature of the 
 darkening action of light upon a surface of this kind, it is better to 
 spread the Chloride upon a glass plate, and to make two experi- 
 ments, adding organic matter in one, and omitting it in the other. 
 
CHANGES BY LIGHT. 135 
 
 Chloride of Silver does not readily adhere to glass unless the sur- 
 face of the glass be finely ground, and even then very careful mani- 
 pulation is required. A more simple plan is to employ Collodion 
 as a vehicle for the Chloride, and this we may do with impunity, 
 since it is known that Pyroxyline, the basis of Collodion, has com- 
 paratively little effect upon the Salts of Silver, and behaves to- 
 wards them almost in the manner of an inert substance, such as 
 glass or porcelain. Having therefore taken a sample of ordinary 
 Collodion, dissolve in each ounce seven grains of Chloride of Zinc ; 
 then coat two glass plates in the usual way, allow them to become 
 quite dry, and immerse in a Nitrate bath. Rear one of the plates 
 on end to drain, but wash the other with water, and pour over it a 
 little diluted Albumen, afterwards drying a second time, and re- 
 dipping in the bath. In the course of a few hours, expose both 
 to sun-light. It will be found that the Chloride of Silver upon 
 Collodion alone, darkens to a violet-blue tint ; but that on Albu- 
 men and Collodion mixed, changes to a deep olive-brown. Now 
 apply a drop of Nitric Acid to the two plates ; the violet sub- 
 chloride will be very little affected, but the brown image contain- 
 ing a suboxide will dissolve. Next treat the plates with Am- 
 monia ; this will dissolve the violet and produce no effect upon 
 the brown image. It is clear therefore, from the action of these 
 tests, that the presence of Albumen makes an important differ- 
 ence hi the nature and properties of the image. 
 
 But in order to investigate the subject thoroughly, the darkened 
 plates must be examined after, as well as before treatment with 
 a fixing agent, since a photographic picture cannot be said to be 
 completed until it has passed through the bath of Hyposulphite 
 of Soda. We therefore prepare a fresh series of films of Chloride 
 and Nitrate of Silver in the manner above described, viz. some on 
 dry Collodion, and others upon Albumen, and having, by repeated 
 treatment with fresh Nitrate of Silver, and a long exposure to the 
 sun's rays, obtained the maximum of darkening, we immerse them 
 in a solution of Hyposulphite of Soda, wash carefully, and dry. 
 Comparing the two sets of images after fixing, we find those on 
 the Collodion, grey by reflected light, and pale slaty-blue by 
 transmitted light. The images on the Albumen, on the other 
 hand, are of a dark olive-brown by reflection, and almost abso- 
 lutely opaque when examined by transmission. The following 
 tests are then applied : a. Mercury ; amalgamation with the 
 
136 THEOBY OF PHOTOGKAPHY. 
 
 Collodion image, but none with the Albumen. 5. Cyanide of 
 Potassium: no action on the grey and transparent image, but 
 gradual solution of the brown opaque image, c. Sulphuretted 
 Hydrogen water: darkens the image on Collodion, but gradually 
 bleaches that on Albumen, d. Permanganate of Potash, if in 
 dilute solution, scarcely acts on the slaty-blue image, but oxidizes 
 the brown image. 
 
 Putting these facts together, we conclude that photographs 
 formed on pure Chloride and Nitrate of Silver consist of a Sub- 
 chloride of Silver before fixing with Hyposulphite of Soda, and 
 of Metallic Silver after fixing. In practice, however, it is found 
 impossible to produce a picture on pure Chloride of Silver, the 
 image being altogether too faint, and lacking depth of colour 
 by reflected light. When Albumen, Gelatine, or analogous sub- 
 stances are present, the image contains Suboxide of Silver com- 
 bined with organic matter : and this combination is not entirely 
 ' destroyed in the process of fixing, for although both Subchlo- 
 ride and Suboxide of Silver are instantly decomposed by fixing 
 agents, yet the latter does not appear to be so when united with 
 Albumen. 
 
 The action of tests upon the organic image will vary with the 
 length of time the plates have been exposed to light. The more 
 intense the blackening, the less evident is the action of solvents, 
 such as the Cyanide of Potassium, etc. Hence the half-tones of 
 a Photographic picture often dissolve in fixing, whilst the full 
 shadows remain. Unstable Salts of Gold, on the other hand, 
 when applied to an image on paper, deposit the metal more readily 
 on those parts which have been completely blackened, and less so 
 upon the lighter shades. Again, if Photographic papers be pre- 
 pared with two different kinds of organic matter, as, for example, 
 with Iceland moss in one case and Albumen in another, the 
 images obtained will not exactly correspond in properties. And 
 if, in a third experiment, the paper be rendered sensitive upon a 
 solution of Ammonio-nitrate of Silver in place of Nitrate of Silver, 
 the reaction with tests will again be different. Therefore, al- 
 though the Photograph contains both organic matter and Silver, 
 no certain formula can be given for its composition. 
 
 The consideration of the effects of organic matter in modifying 
 the properties of the image will be resumed in the Chapter treat- 
 ing of photographic printing. 
 
137 
 
 CHAPTER III. 
 
 ON THE DEVELOPMENT OF AN INVISIBLE IMAGE BY 
 MEANS OF A KEDTTCING AGENT. 
 
 IT has been shown in the previous Chapter that the ma- 
 jority of the Salts of Silver, either alone or in contact 
 with organic matter, are darkened in colour on exposure 
 to light, and, by the loss of Oxygen, Chlorine, etc., be- 
 come reduced to the condition of Subsalta. 
 
 Many of the same compounds are also susceptible of a 
 change under the influence of light, which is even more 
 remarkable. This change takes place after a comparatively 
 short exposure, and does not affect the appearance of the 
 sensitive layer. The impression, however, although in- 
 visible at first, is brought out by treating the plate with 
 certain chemical agents which are without effect on the 
 original unchanged salt. 
 
 Experiments illustrating the Formation of an Invisible 
 Image. Take a sheet of sensitive paper, prepared with 
 Iodide of Silver by the method given in the third Chapter 
 of Part III., and having divided it into two parts, expose 
 one of them to the luminous rays for a few seconds. No 
 visible decomposition takes place, but on removing the 
 pieces to a room dimly illuminated, and brushing with a 
 solution of Gallic Acid, a manifest difference will be ob- 
 served ; the one being unaffected, whilst the other darkens 
 gradually until it becomes black. 
 
 Experiment II. A prepared sheet is shielded in certain 
 parts by an opaque substance, and then after the requisite 
 exposure, which is easily ascertained by a few trials, treated 
 
138 THEORY OP PHOTOGRAPHY. 
 
 with, the Gallic Acid as before ; in this case the protected 
 part remains white, whilst the other darkens to a greater 
 or less extent. In the same way, copies of leaves, engra- 
 vings, etc., may be made, very correct in the shading, and 
 much resembling those produced by the prolonged action 
 of light alone upon the Chloride of Silver. 
 
 'A great economy of time is effected by employing a sub- 
 stance like Gallic Acid to develope or bring out to view an 
 invisible image, in preference to forming the picture by 
 the direct action of light, unassisted by a developer. This 
 is well shown in the results of some experiments con- 
 ducted by M. Claudet in the Daguerreotype process : he 
 found that with a sensitive layer of Bromo-Iodide of Silver, 
 an intensity of light three thousand times greater was 
 required if the use of a developer was omitted, and the 
 exposure continued until the picture became visible upon 
 the plate. 
 
 To increase the sensitiveness of Photographic prepara- 
 tions is a point of great consequence ; and indeed, when 
 the Camera is used, from the low intensity of the luminous 
 image formed in that instrument, no other plan than the 
 one above described would be practicable. Hence the ad- 
 vancement, and indeed the very origin, of the Photographic 
 Art, may be dated from the first discovery of a process for 
 bringing out to view an invisible image by means of a re- 
 ducing agent. 
 
 The present Chapter is divided into two Sections : first, 
 the properties of the substances usually employed in redu- 
 cing the Salts of Silver, with their mode of action ; and se- 
 cond, the formation and development of the invisible image. 
 
 SECTION I. 
 
 The various Substances employed as Reducing Agents. 
 
 The most important of the developers are as follows : 
 Gallic Acid, Pyrogallic Acid, and the Protosalts of Iron. 
 
 GALLIC AND PYROGALLIC ACIDS. 
 
 a. Gallic Acid. Gallic Acid is obtained from Gall 
 
EEDUCING AGENTS. 139 
 
 Nuts, which, are peculiar excrescences formed upon the 
 branches and shoots of the Quercus infectoria by the 
 puncture of a species of insect. The best kind is im- 
 ported from Turkey, and sold in commerce as Aleppo 
 Galls. Gall Nuts do not contain Gallic Acid ready formed, 
 but an analogous chemical principle termed Tannic Acid, 
 well known for its astringent properties and employment 
 in the process of tanning raw hides. 
 
 Gallic Acid is produced by the decomposition and oxi- 
 dation of Tannic Acid^when powdered galls are exposed 
 for a long time in a moist state to the action of the air. 
 By boiling the mass with water and filtering whilst hot, 
 the acid is extracted, and, being sparingly soluble in cold 
 water, crystallizes on cooling. 
 
 Gallic Acid occurs in the form of long silky needles, 
 soluble in 100 parts of cold and 3 of boiling water ; they 
 are also readily soluble in Alcohol, but sparingly in Ether. 
 The aqueous solution becomes mouldy on keeping, to ob- 
 viate which the addition of a lump of Camphor or a drop 
 or two of Oil of Cloves is recommended. 
 
 Gallic Acid is a feeble acid, scarcely reddening litmus ; 
 it forms salts with metallic Oxides, but those of Silver and 
 Gold are reduced by it to the metallic state. 
 
 b. Pyrogallic Acid. The term pyro- prefixed to Gallic 
 Acid implies that the new substance is obtained by the 
 action of heat upon that body. At a temperature of about 
 410 Fahr., Gallic Acid is decomposed, and gives off a 
 white vapour, which condenses in lamellar crystals ; this 
 is Pyrogallic Acid. 
 
 Pyrogallic Acid is very soluble in cold water, and in Al- 
 cohol and Ether; the solution decomposes and becomes 
 brown by exposure to the air. It gives an indigo-blue 
 colour with Protosulphate of Iron, which changes to dark 
 green if any Persulphate be present. 
 
 Although termed an acid, this substance is strictly neu- 
 tral: it does not redden litmus-paper, and forms no salts. 
 It has an affinity for Oxygen, which is greatly augmented 
 by the presence of an alkali : a mixture of this kind 
 
140 THEORY OF PHOTOGBAPHY. 
 
 made by adding Liquor Potassae to Pyrogallic Acid, is 
 often employed for absorbing the Oxygen contained in 
 atmospheric air. 
 
 Commercial Pyrogallic Acid is often contaminated with 
 empyreumatic oil, and also with a black insoluble sub- 
 stance known as Metagallic Acid, formed when the heat 
 is raised above the proper temperature in the process of 
 manufacture. 
 
 THE SALTS OF IRON. 
 
 There are two Oxides of Iron which form Salts, viz. the 
 Protoxide of Iron, containing an atom of Oxygen to one 
 of metal ; and the Peroxide, with an atom and a half of 
 Oxygen to one of metal. As half atoms, however, are not 
 allowed in chemical language, it is usual to say that the 
 Peroxide of Iron contains three atoms of Oxygen to two 
 of metallic Iron. 
 
 Expressed in symbols, the composition is as follows : 
 Protoxide of Iron, Fe,O. 
 Peroxide of Iron, Fe 2 ,O 3 . 
 
 The Proto- and Persalts of Iron do not resemble each 
 other in their physical and chemical properties. The for- 
 mer are usually of an apple-green colour, and their aqueous 
 solutions are almost colourless, if not highly concentrated. 
 The latter, on the other hand, are dark, and give a yellow 
 or even blood-red solution. 
 
 The following experiment will serve to illustrate the 
 properties of both classes of salts : Take a crystal of Pro- 
 tosulphate of Iron, and, having reduced it to powder, pour 
 a little Nitric Acid upon it in a test-tube. On the appli- 
 cation of heat, abundance of fumes will be given off, and 
 a red solution obtained. The Nitric Acid in this reac- 
 tion imparts Oxygen, and converts the -ZVofosulphate en- 
 tirely into a Per salt of Iron. It is this feature, viz. the 
 tendency to absorb Oxygen, and to pass into the state 
 of Persalts, which makes the Protosalts of Iron useful in 
 Photography. 
 
 There are two Protosalts of Iron commonly employed 
 
BEDUCING AGENTS. 141 
 
 by Photographers : the Protosulphate and the Protonitrate 
 of Iron. 
 
 a. Protosulphate of Iron. This salt, often termed Cop- 
 peras or Green Vitriol, is an abundant substance, and is 
 used for a variety of purposes in the arts. Commercial 
 Sulphate of Iron, however, being prepared on a large scale, 
 requires recrystallization to render it sufficiently pure for 
 Photographic purposes. 
 
 Pure Sulphate of Iron occurs in the form of large trans- 
 parent, prismatic crystals, of a delicate green colour : by 
 exposure to the air they gradually absorb Oxygen and be- 
 come rusty on the surface. Solution of Sulphate of Iron, 
 colourless at first, afterwards changes to a red tint, and 
 deposits a brown powder ; this powder is a basic Persul- 
 phate of Iron, that is, a Persulphate containing an excess 
 of the oxide or base. By the addition of Sulphuric or 
 Acetic Acid to the solution, the formation of a deposit is 
 prevented, the brown powder being soluble in acid liquids. 
 
 The Crystals of Sulphate of Iron include a large quan- 
 tity of water of crystallization, a part of which they lose 
 by exposure to dry air. By a higher temperature, the 
 salt may be rendered perfectly anhydrous, in which state 
 it forms a white powder. 
 
 b. Protonitrate of Iron. This salt is prepared by double 
 decomposition between Nitrate of Baryta or Lead and 
 Protosulphate of Iron. It is an unstable substance, and 
 crystallizes with great difficulty ; its aqueous solution is 
 pale-green at first, but it is very prone to decomposition, 
 even more so than the corresponding Sulphate of Iron. 
 
 THE REDUCTION OF SALTS OF SILVER BY DEVELOPING 
 AGENTS. 
 
 These compounds, termed developers, have a tendency, 
 when mixed with soluble Salts of Silver, to set Silver free 
 either by combining with all the Elements with which the 
 Silver was united, or at least with the Oxygen of its Salts, 
 which being removed, the Acid has no longer an affinity 
 
142 THEORY OF PHOTOGEAPHY. 
 
 for the metal Silver, which when set free in either case, is 
 deposited on the parts of the sensitive plate that have been 
 touched by Light, causing them to darken or develope. 
 
 Salts of Iron develope by the first method, by uniting 
 with the Acid and Oxygen of the Silver Salt. The follow. 
 ing equation explains the action of Nitrate of Iron : 
 
 Sulphate of Iron acts in the same way, but the presence 
 of the two Acids gives the equation a more complex ap- 
 pearance. 
 
 3(AgO,N0 5 ) + 6(FeO,S0 3 )= 
 2(Fe 2 O 3 ,3SO 3 )+Fe 2 O 3 ,3NO 5 
 
 The binary Salts of Silver which contain neither Oxygen 
 nor Acid, such as Chloride, Iodide, etc., present no obstacle 
 to reduction from their composition; for although these 
 Silver salts are not generally reducible by developing 
 agents, yet Chloride of Gold, which has a precisely similar 
 composition, is readily reducible, the decomposition by 
 Sulphate of Iron being as follows : 
 AuCl 3 + 6(FeO,SO 3 ) = 2(Fe 2 O 3 .3SO 3 ) + Fe 2 Cl 3 +Au. 
 Organic developing agents, such as Gallic or Pyrogallic 
 Acid, appear to reduce Silver to the metallic state from 
 its Salts by the second method ; they remove Oxygen, the 
 tie which binds the Acid and Metal together, and the two 
 latter are thus set free from each other. 
 
 The following simple diagram may assist the compre- 
 hension of the change : 
 
 Compound Atom of Pyrogallic 
 Nitrate of Silver. Acid. 
 
 The Oxygen being removed by the Pyrogallic Acid, the 
 
BEDUCING AGENTS. 143 
 
 Nitric Acid, NO 5 is set free in the liquid, together with 
 metallic Silver, Ag, which is deposited on the latent image. 
 
 The decomposition of a salt containing no Oxygen may 
 yet be brought about by the affinity of a developer for 
 Oxygen. If water is present, the latter furnishes the Oxygen 
 for the developer, and the Hydrogen of the water unites 
 with the Element previously combined with the metal, 
 setting the latter free. 
 
 The following simple diagram, in which however the 
 number of the atoms concerned is omitted, may explain 
 the reduction of Chloride of Gold by Pyrogallic Acid : 
 
 Compound Atom of Compound Atom Pyrogallic 
 
 Chloride of Gold. of Water. Acid. 
 
 It is not here intended to be implied that all developed 
 images are necessarily metallic. The above remarks are 
 made chiefly with the view of explaining how developers 
 act when a metallic image is obtained ; but in certain cases 
 the reduction appears to stop short of this, resulting in the 
 deposition of a compound intermediate between Silver and 
 its original Salt. 
 
 In the above diagram the symbol Au represents Gold, 
 Cl Chlorine, H Hydrogen, and O Oxygen. An atom of 
 water is decomposed, the Hydrogen combining with the 
 Chlorine to form Hydrochloric Acid (HC1), and the Oxy- 
 gen passing to the reducing agent as before. 
 
 Hence there would be no theoretical difficulty in sup- 
 posing a reduction of Iodide of Silver, if we associated 
 with it an atom of water to furnish the Oxygen. We shall 
 presently see however that pure Iodide of Silver is unaf- 
 fected by a reducing agent, and that the compound which 
 
144 THEOBY OF PHOTOGRAPHY. 
 
 blackens on the application of the Pyrogallic Acid is an 
 Iodide with excess of Nitrate of Silver (p. 150). 
 
 Presence of Free Acids, etc., in Development. Acids 
 exercise a retarding effect upon the reduction of salts of 
 Silver by the developing agents, and especially acids, like 
 Nitric or Sulphuric Acid, which have strong affinities for 
 bases. Solution of Pyrogallic Acid mixed with Nitrate of 
 Silver produces a deposit immediately, and Salts of Iron 
 will also separate Silver before long ; but if a little Acetic 
 Acid be previously added, the precipitation is more gra- 
 dual, and when both solutions are strongly acidified with 
 Nitric Acid, it is for a time suspended. On the other hand, 
 alkaline liquids produce an opposite effect, and favour 
 quick reduction. 
 
 Comparative Strength of reducing Agents. Sulphate of 
 Iron acts with rapidity. Nitrate of Iron and Gallic Acid 
 are both feeble reducing agents. Pyrogallic Acid is 
 stronger, a smaller quantity sufficing to produce the ef- 
 fect. Oil of Cloves, Grape Sugar, Aldehyde, Honey, etc., 
 are capable of reducing Salts of Silver, but they act too 
 slowly to be employed with advantage in Photography. 
 
 Effect of Temperature. Reduction, of the Salts of Silver 
 proceeds more rapidly in proportion as the temperature 
 rises. In cold weather it will be found that the develop- 
 ment is slower than usual, and that greater strength of the 
 reducing agent and more free Nitrate of Silver is required 
 to produce the effect. The action of Sulphate of Iron, 
 however, is considered to be less affected by depression of 
 temperature than that of Pyrogallic Acid. 
 
 On the other hand, if the heat of the Atmosphere be 
 excessive, the tendency to rapid reduction will be greatly 
 increased, the solutions decomposing each other almost im- 
 mediately on mixing. In this case the remedy will be to 
 use Acetic Acid freely, or in place of it Citric Acid, which, 
 as a retarding agent, is at least twenty times stronger than 
 Acetic Acid. 
 
 Varieties of Colour in deposited Silver. The precipitate 
 of metallic Silver obtained by the action of reducing agents 
 
THE LATENT IMAGE. 145 
 
 upon the Nitrate, varies much in colour and in general ap- 
 pearance. If Gallic or Pyrogallic Acid be employed, it is 
 a black powder ; whilst the Salts of Iron, and especially 
 the same with free Nitric Acid added, produce a sparkling 
 precipitate, resembling what is termed frosted silver. 
 Grape Sugar and many of the essential oils, such as Oil of 
 Cloves, etc., separate the metal from Ammonio-Nitrate of 
 Silver in the form of a brilliant mirror film, and are often 
 employed in silvering glass. 
 
 In remarking upon these peculiarities in the molecular 
 condition of precipitated Silver, it should be observed that 
 the appearance of a metal whilst in mass is no indication of 
 its colour when in the state of fine powder. Platinum and 
 Iron, both bright metals, and susceptible of a high polish, 
 are dull and intensely black when in a fine sjtate of divi- 
 sion ; Gold is of a purple or yellowish-brown ; Mercury a 
 dirty grey. 
 
 SECTION II. 
 Tlie Formation and Development of the Latent Image. 
 
 It was shown in the Second Chapter that the continued 
 action of white light upon certain of the Salts of Silver re- 
 sulted in the separation of elements like Chlorine and Oxy- 
 gen and the partial reduction of the compound. We have 
 seen in this Chapter that bodies possessing affinity for 
 Oxygen, such as Sulphate of Iron and Pyrogallic Acid, 
 produce a similar effect ; acting in some cases with great 
 energy and precipitating metallic Silver in a pure state. 
 
 In forming an extemporaneous theory on the production 
 of the Photographic image by development, it would there- 
 fore be natural to suppose that the process consisted in 
 commencing a reducing action upon the sensitive surface 
 by means of the luminous image of the Camera, and after- 
 wards completing or carrying on the same by the applica- 
 tion of the developing solution. We may remark how- 
 ever that this hypothesis is not the one most strongly sup- 
 ported by facts, and that it will certainly lead us astray if 
 
 L 
 
146 THEORY OF PHOTOGEAPHY. 
 
 too far pressed. The idea of the luminous image in the 
 Camera originating a chemical change, and the reducing 
 agent carrying on that change to its completion, might 
 lead to the inference that the two agencies were exactly 
 similar in their mode of operation, and that the one could 
 be substituted for the other : that when, for instance, the 
 exposure in the Camera had been very brief, the defect 
 would be remedied by prolonging the development. Such 
 a notion is quite erroneous. A definite time is occupied in 
 the formation of the invisible image, which cannot be 
 shortened or extended beyond its proper limits with im- 
 punity. So far from the action of the developing solution 
 being facilitated when the plate has been left in the Camera 
 for an unusually long time, it is often retarded thereby ; 
 and on the other hand, unless the exposure be sufficient, it 
 will be in vain to expect to produce a picture by strengthen- 
 ing the reducing agent or keeping it upon the plate for a 
 longer time. Thus we see that " the invisible image " has 
 a real existence, and that its formation by the solar ray is 
 a phenomenon quite distinct from any after-process of de- 
 velopment. 
 
 HYPOTHESIS ON THE NATUEE OF THE INVISIBLE IMAGE. 
 
 When a sensitive film containing a Salt of Silver is ex- 
 posed in the Camera, and receives a latent impression ca- 
 pable of subsequent development, an inquiry arises as to 
 the exact condition of the film-particles on which the light 
 has acted. The most careful inspection, even with the aid 
 of a microscope, fails to detect any difference between the 
 exposed and non-exposed portions of the plate. Chemical 
 solvents likewise act in the same manner both before and 
 after the insolation, thus showing that there is no actual 
 separation of the elements of the film, analogous to the 
 elimination of Chlorine from Chloride of Silver by the pro- 
 longed action of the sun's rays. 
 
 The formation of the invisible image is supposed to be 
 a molecular change, unattended with any separation of 
 

 THE LATENT IMAGE. 147 
 
 elements such as occurs in the case of a visible image im- 
 pressed by light. The following diagrams will assist me- 
 chanically in conveying an idea of what we mean by a 
 molecular change. 
 
 Let fig. 1 represent the Iodide of Silver in its normal 
 state, and fig. 2 the same Iodide after its impression in 
 the Camera. The percentage composition has not been 
 changed, for supposing one of the circles to represent 
 Iodine and the other Silver, we have in both figures a 
 single atom of each. The arrangement of the constituent 
 atoms however has been modified in the second figure, 
 where they are so placed as to touch each other only at 
 the edges. Let it be borne in mind that this representation 
 is purely hypothetical. We observe certain differences in 
 the properties of the Iodide after its insolation, and sup- 
 pose them to indicate corresponding differences in mole- 
 cular arrangement ; but we cannot speak with certainty, 
 since the ultimate atoms of bodies are too minute to come 
 within the range of our observation. 
 
 We now proceed to examine further the tangible differ- 
 ences between the impressed and the non-impressed atom 
 of Iodide of Silver. If the two be exposed for a short 
 time over Mercury slightly warmed, the metallic vapour 
 will condense upon the former, but not upon the latter ; 
 and in this way the latent picture is made visible in the 
 Daguerreotype process. So, again, if Nitrate of Silver pre- 
 viously mixed with a reducing agent be applied to the 
 same two atoms, it will be seen that the impressed or mo- 
 dified Iodide hastens the decomposition of the mixture, 
 whereas the Iodide prepared in total darkness exercises no 
 
148 THEOEY OF PHOTOGEAPHY. 
 
 such effect. We have indeed learnt from the observations 
 in the last Section that reducing agents themselves decom- 
 pose Nitrate of Silver, and throw down the metal in a 
 pure form. This reduction however is usually slow and 
 gradual, and when a little free acid, such as Nitric or Acetic 
 Acid, is added, the mixture remains perfectly clear for a 
 period of five minutes or longer. Prepare therefore such 
 a mixture with the proportions correctly adjusted, and 
 drop into it two particles, one of the ordinary and the 
 other of the impressed Iodide of Silver ; when it will be 
 found that the first remains yellow, but that the second 
 immediately becomes black and is incrusted with Metallic 
 Silver. 
 
 There are other instances of the surfaces of bodies un- 
 dergoing a molecular change, not to be detected by the 
 eye, but yet sufficient to affect the affinities of compounds 
 brought into contact with such surfaces. 
 
 An instructive illustration of this may be conducted as 
 follows: Take a clean glass vessel, and having poured 
 into it a strong aqueous solution of Chloride of Potassium, 
 add a little Tartaric Acid, and stir briskly with a glass 
 rod. In a few moments the parts of the glass touched by 
 the rod will be marked by a line of minute crystals. In 
 this case there is a disposition in the liquid to cr} r stallize ; 
 but the rubbed state of the glass, where the rod has touched 
 it, facilitates the change, and determines the point at 
 which the crystallization shall commence. The deposit is 
 adherent and is not removed by emptying the capsule of 
 its contents and washing it out with alcohol : warm water, 
 however, dissolves the crystals of Bitartrate, and the sur- 
 face of the glass is then found intact ; the rubbed condi- 
 tion, as we have termed it, being temporary and quite 
 invisible. 
 
 One additional illustration of molecular in contradistinc- 
 tion to chemical change will suffice. When an engraving 
 which has been framed and suspended for a length of time 
 is taken down to be cleaned, it will sometimes happen that 
 the glass, however carefully polished by rubbing, will ex- 
 
THE LATENT IMAGE. 
 
 liibit the outlines of the picture when breathed on. The 
 breath settles in certain parts and not in others, in conse- 
 quence of variations in the surface condition. When the 
 glass is put aside, the invisible image gradually vanishes, 
 and the molecules return to their original state. 
 
 Granting, therefore, that a ray of white light in acting 
 on Iodide of Silver, so modifies it that it acquires the pro- 
 perty of accelerating chemical change in the mixture known 
 to Photographers as the developing agent, we observe in 
 the next place, that it is not necessary to suppose that the 
 invisible modification is permanent, or that the particles 
 of the Iodide, once disturbed, cannot afterwards be restored 
 to their normal state. On the contrary, there are facts 
 which prove that the invisible image may completely dis- 
 appear, leaving the plate in a condition to receive a second 
 impression in the Camera. 
 
 Mechanical diagrams may here be again employed ; and 
 their use will be free from objection, if it be understood 
 that they are meant only to assist in fixing facts upon 
 the mind of the reader, and like a system of artificial 
 memory, may be laid aside when this important object has 
 been accomplished. 
 
 Fig. 1. Fig. 2. 
 
 In the first figure we see two ovals, slightly different in 
 size, and turning on a common centre ; in (a) their long 
 diameters correspond, but in (b) they cross each other, 
 whilst in (c) they are made to coincide once more by a fur- 
 ther action of the same force. In fig. 2 we have the more 
 familiar illustration of waves : first an elevation, then a 
 corresponding depression. Now in the photographic pro- 
 cess we find that the invisible effect of the light rises gra- 
 dually to a certain pitch of intensity, but beyond that it 
 appears to descend as it were on the other side : hence 
 
150 THEOEY OF PHOTOGBAPHY. 
 
 the particle of Iodide of Silver first acquires tlie property 
 of blackening the developer, but loses it again if left too 
 long in the Camera. 
 
 THE DEVELOPMENT OF THE INVISIBLE IMAGE. 
 
 The amateur commencing the study of Photography 
 will probably form an incorrect idea of the real nature of 
 a developing agent. Being in the habit of applying to the 
 surface of the wet Collodion a solution of Pyrogallic Acid 
 or of Sulphate of Iron, he terms these compounds "the 
 developers," and forgets that a floating layer of solution of 
 Nitrate of Silver is also present, derived from the bath. 
 To impress upon the mind the fact that the developer is 
 not the simple reducing agent, but the same mixed with 
 Nitrate of Silver, let the following experiment be made. 
 Coat a plate with Iodized Collodion, pass it through the 
 Nitrate Bath, and wash it carefully back and front with 
 distilled water. Now expose for an instant to diffused 
 daylight, and apply the Pyrogallic Acid : the plate will 
 remain perfectly clear and transparent. The invisible im- 
 pression however is truly present, and the insolated Io- 
 dide is ready to play its part, but no blackening takes place, 
 because the reducing agent has nothing to act upon. Let 
 the Pyrogallic Acid, therefore, be poured back into a mea- 
 sure containing a few drops of solution of Nitrate of Silver 
 and applied a second time to the plate, when the film will 
 begin to darken and continue to do so until it is quite 
 opaque. 
 
 The Second Stage of the Development. This consists in 
 strengthening the image first formed, by an additional de- 
 posit of Silver. Take a sensitive Collodion plate, and 
 having impressed an invisible image upon it by a proper 
 exposure in the Camera, remove it to the dark room, and 
 pour over it solution of Pyrogallic Acid. When the pic- 
 ture has fully appeared, stop the action by washing the 
 plate with water. An examination of the image at this 
 stage will show that it is perfect in the details, but pale 
 and translucent. Now take the plate and treat it with 
 
DEVELOPMENT OF LATENT IMAGE. 151 
 
 Pyrogallic Acid to which fresh Nitrate of Silver has been 
 added ; immediately the picture will become much blacker, 
 and will continue to darken, even to complete opacity, if 
 the supply of Nitrate be kept up. The same result may 
 be obtained after the Iodide of Silver has been removed 
 from the plate by Hyposulphite of Soda or Cyanide of 
 Potassium ; and in such a case it is evident that the addi- 
 tional deposit upon the image must be produced from the 
 Nitrate of Silver and not from the Iodide of Silver. Ob- 
 serve, also, that this additional deposit forms only upon tlie 
 image, exhibiting no affinity for the unaltered Iodide upon 
 the part of the plate corresponding to the shadows of the 
 picture, but attaching itself in preference to those parts 
 already blackened by the developer. 
 
 The second stage of the development, in which a feeble 
 image is strengthened and rendered more opaque, is a pro- 
 cess bearing a close resemblance to the growth of a crystal 
 in a saturated liquid, by aggregation of fresh particles ; and 
 after the picture has reached its full density, a series of 
 elevations may often be seen upon the plate, corresponding 
 to the lines of the image. 
 
 OBJECTIONS TO THE FOREGOING VIEWS CONSIDERED. 
 
 The theory of the formation and development of the 
 latent image given in the preceding pages was not origi- 
 nated by the author of this work, nor can he at present 
 specify any one individual to whom it may be ascribed. 
 Objections have been urged against it, and other hypo- 
 theses adopted in preference, not only in this country but 
 also in France. It has been said, for instance, that the 
 process of photographic printing upon Chloride of Silver 
 involves a partial reduction of the Chloride by the action 
 of light, and hence it is reasonable to suppose that the 
 Iodide of Silver suffers reduction in the developing pro- 
 cess. We must bear in mind, however, that there is a great 
 difference in the intensity of the light to which the sensi- 
 tive surface is subjected in the two cases mentioned, and 
 not only so, but that the Iodide of Silver is more diffi- 
 
152 THEORY OF PHOTOGRAPHY. 
 
 cult to reduce than the Chloride. In fact, as we have 
 already shown, the solar ray has no effect in discolouring 
 pure Iodide of Silver, although it quickly blackens pure 
 Chloride of Silver. If the invisible image were simply 
 a disturbance of ultimate particles, rendering their after- 
 separation by the reducing agent more easy, we should ex- 
 pect to find Chloride of Silver receiving the image of the 
 Camera with greater rapidity than Iodide of Silver. The 
 contrary however is the case, and we are probably within 
 the mark in stating the sensitiveness of Iodide of Silver to 
 be ten times greater than that of Bromide and sixty times 
 greater than that of Chloride of Silver. 
 
 Another argument militating strongly against the view 
 often expressed that the latent image consists of Iodide of 
 Silver reduced in inappreciable quantities, or of Iodide 
 of Silver with an increased tendency to reduction, is the 
 fact before mentioned, that the rapidity of the develop- 
 ment is not in proportion to the length of the exposure in 
 the Camera. On the over-exposed sky of a landscape 
 the Iodide will sometimes remain yellow and unchanged 
 throughout the whole process, whilst the less exposed 
 parts of the plate darken under the influence of the deve- 
 loper. 
 
 The opponents of the views which we have given of 
 the purely molecular character of the disturbance in the 
 Camera base their objections chiefly on phenomena ob- 
 served in the practice of the dry Collodion processes. It 
 is not unusual in such methods to obtain visible indica- 
 tions of the picture upon the surface of the film before the 
 developer is applied ; and even where no such indications 
 present themselves, there yet often exists a remarkable 
 difference between the exposed and the non-exposed parts 
 to which as yet we have not directed the reader's atten- 
 tion. Mr. Young, of Manchester, was the first to notice 
 the phenomenon, and to show that if an exposed plate be 
 immersed in solution of Hyposulphite of Soda until the 
 yellow Iodide of Silver is dissolved, a picture will still 
 appear on applying the usual developing mixture. The 
 
THE LATENT IMAGE. 153 
 
 effect cannot be obtained on a wet Collodion plate and 
 only imperfectly on a plate preserved by honey, the best 
 substance to exhibit it being Albumen. The Author has 
 experimented with a view of accounting for a pheno- 
 menon so remarkable as the development of an invisible 
 image upon a plate previously cleared of its Iodide of Sil- 
 ver, and the explanation which he would suggest is as 
 follows : Pure Iodide of Silver is molecularly modified 
 by the Camera image, but its properties as regards the 
 action of Hyposulphite of Soda are unaffected thereby : 
 upon a plate so constituted no image could be developed 
 after fixing. The sensitive surface of the Collodion film 
 is never however an absolutely pure Iodide of Silver, and 
 sometimes very far from being so. It contains traces of 
 an organic compound of Silver in the wet Collodion pro- 
 cess, and more appreciable quantities of the same in the 
 dry processes. Now, with a compound film of this kind, 
 it may be supposed that whilst the Iodide of Silver under- 
 goes its peculiar change in the Camera, the organic com- 
 bination of Silver likewise changes, but in a different 
 manner. Without altering in appearance it gradually 
 loses its solubility in fixing agents, so that when the Hypo- 
 sulphite of Soda is applied its removal is not effected. 
 We do not however perceive any picture upon the film, 
 because the organic combinations of Silver are transparent 
 in thin layers, even when they contain Iodide of Silver. 
 An Albumino-Iodide of Silver, for instance, may be pre- 
 pared in clear and colourless lumps like the finest jelly, 
 and it is not difficult to conceive that such a substance may 
 remain in an invisible condition upon Mr. Young's plates 
 even after fixing. 
 
 If the above Hypothesis be correct, there exist two 
 varieties of the invisible image: first, that upon simple 
 Iodide of Silver, which enables it to condense Mercury or 
 to determine chemical change in the unstable developer ; 
 secondly, the image upon Iodide of Silver placed in con- 
 tact with an organic compound of Silver. In the latter 
 case the movement of the particles of the Iodide is sup- 
 
154 THEORY OF PHOTOGEAPHY. 
 
 posed to be propagated to the other body, and changes are 
 thus originated, visible or latent, according to the dura- 
 tion of the exposure, and the properties of the superadded 
 substance. An organic Salt of Silver contains in itself the 
 elements of its own reduction, and hence we are not sur- 
 prised to find a catalytic action exerted upon it by the 
 actinically excited Iodide of Silver. 
 
 CAUSES WHICH AFFECT THE SENSITIVENESS OF IODIDE 
 OF SILVEE. 
 
 By the term sensitiveness we understand a facility of 
 receiving an impression from a very feeble ray of light, or 
 of receiving it quickly from a bright ray. This impression, 
 however, need not be followed by a vigorous or intense 
 development, in order to constitute sensitiveness. On the 
 contrary, it often happens that the most sensitive film 
 yields a feeble picture, and when the Iodide is so prepared 
 as to produce an opaque picture, then it is less sensitive, 
 and requires to be exposed in the Camera for a longer time. 
 
 It has already been stated that although Iodide of Sil- 
 ver is less affected by direct sunlight than Bromide or 
 Chloride of Silver, yet that it is more sensitive to the re- 
 ception of the invisible image than either of those com- 
 pounds. Placing Iodide of Silver therefore at the head 
 of the list, we remark that, in the state in which it is used 
 upon Collodion, it possesses the highest degree of sensitive- 
 ness when there is an excess of Nitrate of Silver ; and if 
 the experiment be made of washing the plate carefully 
 with distilled water so as to remove the free Nitrate, a 
 longer exposure in the Camera will be required. The sen- 
 sitiveness does not, however, increase uniformly with the 
 amount of free Nitrate, and it has been found in the Col- 
 lodion process that no advantage can be gained by using 
 a solution of Nitrate of Silver stronger than that usually 
 recommended for the Bath. 
 
 Strong acids, like Nitric Acid, greatly diminish the sen- 
 sitiveness of Iodide of Silver. Even a weak vegetable acid, 
 such as the Acetic, has a similar though less decided effect, 
 
SENSITIVENESS. 155 
 
 and if the Collodion film be rendered very acid with 
 Acetic Acid before its exposure in the Camera, it will be 
 found impossible to take a picture rapidly, even after 
 strengthening the developer to the utmost limits. 
 
 It has long been remarked that the use of bodies like 
 Albumen, Gelatine, Caseiue, etc., which combine with Ox- 
 ides of Silver, retards the action of light upon Iodide of 
 Silver ; and one principal reason why the Collodion film 
 is so sensitive is believed to be that Pyroxylin e, the basis 
 of Collodion, is a substance peculiarly indifferent to the 
 Salts of Silver, and exhibits very little tendency to com- 
 bine with them. The Photographer often employs Albu- 
 men, but he does so with a view of increasing the opacity 
 of the image, and not for the purpose of adding to the 
 sensitiveness. Whilst discussing Mr. Young's experi- 
 ments, we have distinguished between simple Iodide of 
 Silver and Iodide with addition of organic compounds. 
 The latter combination gives the more vigorous picture, 
 but the former is superior in sensitiveness. 
 
 The great sensitiveness of Iodide of Silver on Collodion 
 may also be due in a measure to mechanical causes. The 
 loose state of coagulation in a Collodion film and the ex- 
 quisitely fine state of division in which the particles are 
 deposited must be favourable to molecular and chemical 
 change. It will be seen as we proceed that success in 
 this process depends very much upon correctly balancing 
 the different solutions, in order that the Iodide of Silver 
 may be thrown down in a state favourable to penetration 
 by the developer. 
 
 Before passing to the next division of our subject, it may 
 further be remarked, that in any Photographic process in 
 which an invisible image is produced, the time occupied 
 in forming that image will vary more or less with the 
 nature of the agent by which it is to be developed. Bodies 
 like the Protosulphate of Iron, possessing a strong affinity 
 for Oxygen, will throw down Metallic Silver upon a sur- 
 face of Iodide of Silver which has not undergone a suffi- 
 ciently decided modification in the Camera to be affected 
 
156 THEOEY OF PHOTOGRAPHY. 
 
 by a weaker developer like Gallic Acid. It is probable 
 that the future of instantaneous Photography lies in- the 
 discovery of a developer even more unstable than a mixture 
 of Sulphate of Iron and Nitrate of Silver; for the greater 
 the instability, the less the need of a previously disturbing 
 force to originate motion in the particles. 
 
 The controlling influence of the developer upon the sen- 
 sitiveness of Iodide of Silver, must be borne in mind in 
 instituting comparisons between the Daguerreotype and 
 the Collodion process. The employment of Bromine in 
 conjunction with Iodine increases the sensitiveness of the 
 Daguerreotype, but we have no reason to suppose that 
 such would be the case if the image were developed in 
 a different manner. The vapour of Mercury condenses 
 more readily upon the insolated J?romo-iodide than upon 
 the simple Iodide of Silver, but another vapour might 
 condense less readily, and experiment is the only safe 
 guide. 
 
 Whilst upon this subject, we also remark that the Iodide 
 of Silver formed in the manner described in the Tenth 
 Chapter on the Daguerreotype, ought not to be compared 
 photographically with Iodide of Silver thrown down from 
 aqueous solutions. In the case of the metal plate there 
 is a substratum of Metallic Silver, and it is not certain 
 that the composition of this Iodide corresponds to that 
 precipitated in the moist way. The action of light upon 
 it certainly differs, for whereas the humid Iodide remains 
 unaffected in the sun's rays, the Iodide of the Daguerreo- 
 typist gradually becomes insoluble in solution of Hypo- 
 sulphite of Soda. 
 
 ILLUSTRATIONS OP ABNORMAL DEVELOPMENT. 
 
 The characteristics of the proper development of a 
 latent image are that the action of the reducing agent 
 should cause a blackening in the parts touched by light, 
 but produce no effect upon those which have remained in 
 shadow. In operating both on Collodion and paper, how- 
 ever, we find a liability to failure in this respect ; the film 
 
ABNOEMAL DEVELOPMENT. 157 
 
 beginning, after the application of the developer, to change 
 in colour to a greater or less extent over the whole sur- 
 face. 
 
 Let us see how the above defect, known to the Photo- 
 grapher as " fogging," may be originated. Supposing 
 the invisible image to be duly formed, it is yet necessary 
 for its correct development that a proper balance should 
 be maintained between the constituents of the developer. 
 The reducing agent must not be too powerful, nor the quan- 
 tity of titrate of Silver too large, otherwise, although the 
 deposited Silver will fall principally upon the image, a por- 
 tion of it will be thrown down upon the shadows. It will 
 also be found that the quantity of Nitrate of Silver being 
 correctly balanced, fogging will certainly ensue if a little 
 of this Nitrate be changed into Oxide of Silver by the 
 addition of an alkali, since the tendency to reduction is 
 then so strong that it cannot be controlled. Even when 
 the Nitrate is accurately neutral, care and avoidance of all 
 disturbing causes will be required to prevent a deposition 
 of Silver upon the shadows of the image ; especially when 
 Nitrite of Silver or Acetate of Silver are present, both 
 of which salts are more easily reduced than the Nitrate 
 of Silver. 
 
 The use of Acid is the principal resource in obviating 
 cloudiness of the image. Acids lessen the facility of re- 
 duction of the salts of Silver by developing agents (p. 144), 
 and hence when they are present the metal is deposited 
 more slowly, and only on the parts where the action of the 
 light has so modified the particles of Iodide as to favour 
 the decomposition : whereas if acids be absent, or present 
 in insufficient quantity, the equilibrium of the mixture of 
 Nitrate of Silver and reducing agent which constitutes the 
 developer is so unstable, that any rough point or sharp 
 edge becomes a centre from which the chemical action, 
 once started, radiates to all parts of the plate. 
 
 Observe, however, that although we employ free acids 
 in the developer to regulate the process and prevent the 
 chemical actions from running riot, yet if we carry the 
 
158 THEORY OF PHOTOGEAPHY. 
 
 proportion of the acid too far, or employ too strong an 
 acid, we may originate the very defect we wish to avoid. 
 Excess of Nitric Acid is a cause of fogging, if not as potent 
 as excess of alkali, yet very decided. In this case the op- 
 position to reduction is so strong, that for a long time after 
 applying the developer to the plate no Silver is deposited : 
 eventually spangles of metal are seen, which adhere like 
 sparkling dust to the shadows. 
 
 A more or less evident amount of fogging must be anti- 
 cipated when the exposure in the Camera was too short. 
 The latent image being in such a case very weak and im- 
 perfectly formed, does not act. properly in attracting the 
 Metallic Silver from the developer. When Nitrate of 
 Silver and a reducing agent are present at the same time 
 upon a film, Metallic Silver must very shortly fall upon 
 some portion of the plate ; hence the best security against 
 fogging is a vigorous latent image, because in that case 
 the action is so rapid upon the exposed parts that the 
 energy of the developer is soon spent. Those films of 
 Iodide of Silver which are prepared with especial reference 
 to the production of vigorous images always give clean 
 pictures, whereas the more sensitive Iodide of Silver is 
 very liable to fog, since the latent image, although quickly 
 formed, is not very deep or decided. 
 
 Spots upon the plate are instances of abnormal develop- 
 ment, which may often be explained in a similar manner. 
 In such a case the latent image upon the Iodide of Silver 
 is not always sufficiently vigorous to exhaust with rapidity 
 the mixture of reducing agent and Nitrate of Silver which 
 constitutes the developer. Each spot exhibits, it is true, 
 a central nucleus of extraneous matter, but this does not 
 invalidate the argument. Such microscopic nuclei are 
 always present upon the sensitive film to a greater or less 
 extent, but if the invisible image be well formed, they are 
 not so liable to increase in size during development. Full 
 occupation must, so to speak, be provided for the developer, 
 otherwise it will search out these minute particles, and 
 surround them with deposited Silver. Filtering of solu- 
 
EESEABCHES OF MOSEE. 159 
 
 tions and avoidance of dust are useful remedies for spots, 
 but so likewisejs attention to the state of the sensitive film 
 as regards vigorous and intense development. If however 
 the energy of development be too great, the spots may 
 recur ; just as fogging of the film is sometimes due to defi- 
 ciency of acid, and sometimes to excess of acid. 
 
 EESEAECHES OF M. MOSEE AND OTHEES ON INVISIBLE 
 IMAGES. 
 
 The papers of M. Ludwig Moser ' On the Formation and De- 
 velopment of Invisible Images,' published in 1842, explain so 
 clearly many remarkable phenomena of occasional occurrence in 
 the Photographic processes, that no apology need be offered for 
 referring to them. 
 
 His first proposition may be stated thus : " If a polished sur- 
 face has been touched in particular parts by any body, it acquires 
 the property of precipitating certain vapours on these spots dif- 
 ferently to what it does on the other untouched parts." To illus- 
 trate this, take a thin plate of metal, having characters excised ; 
 warm it gently, and lay it upon the surface of a clean mirror glass 
 for a few minutes : then remove, allow to cool, and breathe upon 
 the glass, when the outlines of the device will be distinctly seen. 
 A plate of polished Silver may be substituted for the glass, and 
 in place of developing the image by the breath, it may be brought 
 out by Mercurial vapour. 
 
 The second proposition of M. Moser was as follows : " LigJit 
 acts on bodies, and its influence may be tested by vapours that 
 adhere to the substance." A plate of mirror glass is exposed in 
 the Camera to a bright and intense light ; it is then removed and 
 breathed upon, when an image before invisible will be developed, 
 the breath settling most strongly upon the parts where the light 
 has acted. A plate of polished Silver may be used as before in- 
 stead of glass, the vapour of Mercury or of water being employed 
 to develope the image. A Silver plate exposed to the vapour of 
 Iodine until Iodide of Silver has formed upon its surface, is still 
 more sensitive to the influence of light, and receives a very perfect 
 impression under the subsequent action of the Mercury. 
 
 The above and other experiments of M. Moser have a practical 
 significance, for it has since been found that the same condition 
 
160 THEORY OF PHOTOGRAPHY. 
 
 of surface which causes a vapour to settle in a peculiar manner 
 may also affect the behaviour of the Iodide of Silver when treated 
 with a mixture of Nitrate of Silver and a reducing agent. Thus, if 
 a clean glass plate be touched in certain spots by the warm finger, 
 the impression soon disappears, but is again seen on breathing 
 upon the glass ; and if this same plate be coated with a very deli- 
 cate layer of Iodized Collodion and passed through the Nitrate 
 bath, the solution of Pyrogallic Acid will often produce a well- 
 defined outline of the figure even before the plate has been ex- 
 posed in the Camera. This experiment is an instructive one, and 
 shows the necessity of cleaning the plates used in Photography 
 with care. If there be any irregularity in the manner in which 
 the breath settles upon the glass when it is breathed on, a con- 
 dition of surface exists at that point which will probably so 
 modify the layer of Iodide of Silver, that the action of the deve- 
 loping fluid will be in some way interfered with. 
 
 Glass plates with Collodion pictures on them should be cleaned 
 very carefully before being again used, or the old impression will 
 re-appear during development. Plates packed in sheets of news- 
 paper often show the letters in the same way when the Pyrogallic 
 Acid is applied : traces of organic matter in all probability are 
 present on the surface of the glass, and it is only by long soaking 
 in chemical solutions that these invisible images can be destroyed. 
 It may in every case be assumed that the existence of the invi- 
 sible image can be detected by breathing, and that a glass which 
 does not affect the breath is photographically clean. 
 
 M. Moser in the same series of papers calls attention to the 
 peculiar phenomena resulting from over-action of light. "If light," 
 he says, " acts on Iodide of Silver, it imparts to it the power of 
 condensing mercurial vapours, but if it acts beyond a certain 
 time, it then diminishes this power, and at length takes it away 
 altogether." This observation, like the last, is a practical one, 
 for in all photographic processes we have to contend against the 
 phenomenon known as solarization. Over-exposure in the Ca- 
 mera will invariably weaken the action of the developer more or 
 less, but nevertheless the tendency to solarize may be overcome 
 in a measure by associating other substances with the Iodide of 
 Silver, so as to impart greater stability to the invisible image. 
 Bromide of Silver appears to exercise such an effect, as also 
 does an acid condition of the film. The presence of organic 
 
THE LATENT IMAGE. 161 
 
 Salts of Silver in the film Hkewise influences the amount of 
 solarization. 
 
 Observations by Norris and others. When a sensitive Collodion 
 plate is washed in water before its exposure in the Camera, a 
 mixture of Pyrogallic Acid and Nitrate of Silver is required to 
 develope it. The mixture is usually made before the developer 
 is applied to the film, and this is of more importance than would 
 appear. If the Pyrogallic Acid solution be used alone without 
 any Nitrate of Silver, it will frequently obliterate the latent im- 
 pression, so that a second picture may be taken upon the same 
 plate. Such however is not invariably the case ; if a mere trace 
 of Nitrate of Silver remain in the pores of the Collodion, there will 
 be development instead of obliteration. A washed Collodion film 
 may have its free Nitrate of Silver restored to it by dipping in 
 the Bath after exposure, but it will, as a rule, develope with un- 
 usual energy after such treatment, and the picture will be taken 
 by a shorter .exposure than if the plate were simply flooded with 
 a mixture of Pyrogallic Acid and Nitrate of Silver. 
 
 Dilute acids gradually obliterate the latent image on washed 
 Colloclio-iodide of Silver. When an exposed plate is left for a 
 time over the vapour of Acetic Acid, the impression gradually 
 disappears and the film returns to a sensitive condition. On re- 
 peating this experiment, the author finds that the second latent 
 image requires a longer exposure in the Camera, and developes in 
 a somewhat different manner from the first. 
 
 Dry Iodide of Silver on Collodion, gradually loses its power of 
 receiving the latent image, when placed in contact with certain 
 other bodies. Thus the extreme edges of dry plates, where they 
 touch the grooves of the carrying-box, often remain transparent 
 on developing, and exhibit no indications of an impression. 
 Traces of organic matter have a similar effect on the Iodide in 
 the wet process, as may be proved by allowing a drop of saliva to 
 fall on a glass plate, and then carefully removing it with a silk 
 handkerchief. On Collodionizing the glass and passing it through 
 the Nitrate Bath, there will be a transparent mark of defective 
 development on that particular part, showing that the Iodide 
 had not undergone the usual change in the Camera. 
 
 Grove's Electrical Images. Mr. Grove has succeeded in pro- 
 ducing latent images by electricity. In the experiments described, 
 a plate of glass was electrized in certain parts, and then breathed 
 
162 THEORY OF PHOTOGEAPHY. 
 
 on or exposed to the fumes of Hydrofluoric Acid ; in either case 
 the vapour settled exclusively upon the wow-electrical part of the 
 glass. When the glass was first electrized, and afterwards coated 
 with Iodide of Silver and exposed to light, Pyrogallic Acid pro- 
 duced no reduction. 
 
 Experiments of Busk and others. Mr. Busk has stated in a 
 paper to the Blackheath Photographic Society, that ordinary 
 writing-paper sensitized with Nitrate of Silver undergoes an invi- 
 sible change when placed in contact with certain surfaces ; and 
 that in consequence of this change the paper loses its property of 
 being darkened by exposure to light. To exhibit the phenomenon 
 in perfection he recommends that the Nitrate Bath should be 
 rather strongly acidified with Tartaric or Acetic Acid. A few 
 hours' contact with an engraving will then produce the effect, 
 and those portions of the sensitive paper which have been touched 
 by the blacks of the engraving will remain white in the solar 
 ray, whilst the other untouched parts will darken in the usual 
 manner. 
 
 Mr. Malone observes with reference to the above, that much 
 depends upon the composition of the colouring-ink of the engra- 
 ving, and that one kind of ink will suspend the reducing process 
 in a sensitive paper, whilst another will fail in doing so. His 
 mode of procedure is as follows : The paper is rendered sensitive 
 upon a neutral Nitrate Bath, and is then placed between the 
 leaves of a book. Spontaneous darkening gradually ensues, but 
 the parts opposite to the black letters remain white, in conse- 
 quence of some unexplained influence exerted by the ink. 
 
 Invisible Images produced by the Agency of Ozone. M. The- 
 nard has shown that ordinary paper exposed to a current of 
 ozonized Oxygen, experiences a change not visible to the eye, but 
 becoming so when the paper is laid in contact with a sensitive 
 sheet containing Chloride and Nitrate of Silver : blackening of 
 the Nitrate takes place opposite to the ozonized surface. When 
 the ozonized paper is enclosed in a damp tube, a peculiar odour 
 is gradually developed, due in all probability to an oxidation of 
 the organic matter into a body resembling Aldehyde or Formic 
 Acid in its property of reducing Nitrate of Silver to the metallic 
 state. These experiments of M. Thenard were undertaken in 
 consequence of the publication of a series of observations by M. 
 Niepce de St. Victor, attributing similar effects to the agency of 
 
THE DEVELOPED IMAGE. 163 
 
 latent light. The ingenuity of M. Niepce is praiseworthy, but 
 the opinion entertained at the present time appears to be that the 
 phenomena he has described are susceptible of a simpler explana- 
 tion than that involved in the supposition of stored-up light. 
 
 CHEMICAL NATURE OF DEVELOPED IMAGES. 
 
 Photographic pictures obtained by developing an invisible 
 image with a reducing agent, contain usually a far larger quan- 
 tity of metallic Silver than those produced by the long-continued 
 action of light alone. Hence they are less easily injured by de- 
 structive agents, such as Cyanide of Potassium, Oxidizers, etc. 
 The plan which has been adopted to ascertain the actual quan- 
 tity of Silver present in an image, is first to convert the deposit 
 into Chloride of Silver, and afterwards into Sulphide of Silver, by 
 immersion in a solution of a soluble Sulphide. The more opaque 
 the image appears after this treatment, the greater the quantity 
 of real Silver. 
 
 Developed images vary, however, materially in their nature and 
 properties, according to the circumstances under which they were 
 produced. Photographic prints, for instance, taken on Chloride 
 of Silver, by development after a moderate amount of exposure, 
 resemble very closely images obtained by the direct action of light 
 alone, whilst images like Collodion Positives are essentially dif- 
 ferent, and do not react in the same manner with tests. The 
 former, indeed, may be compared to images formed by direct 
 sun -light upon Chloride of Silver combined with organic matter ; 
 but the latter to images upon pure Chloride of Silver, which, as 
 before shown, consist, after fixing, of metallic Silver only. The 
 following conditions may be mentioned as affecting the character 
 of the developed image. 
 
 a. The Surface used to sustain tTie Sensitive Layer. Albumen 
 and Gelatine are favourable to the production of a brown and 
 opaque image, which will bear magnifying without showing sepa- 
 rate particles. Cyanide of Potassium and Oxidizing agents dis- 
 solve this image, and it will not amalgamate with Mercury. It is 
 probably not Metallic, but partly organic in its nature. Collodion, 
 on the contrary, often gives a grey and slaty image, the ultimate 
 particles of which are seen on magnifying. It amalgamates with 
 hot Mercury, but is little affected by fixing agents and oxidizers. 
 This image is nearly or quite metallic. 
 
164 THEOET OF PHOTOGEAPHT. 
 
 Certain organic matters may be dissolved in the Collodion, and 
 the result will be to produce an image intermediate in properties 
 between the two last described. Glycyrrhizine and Grape Sugar 
 may be mentioned in illustration. 
 
 b. The Nature of the Sensitive Salt. Bromide of Silver added 
 to the Iodide in an Albumen or a Gelatine process, produces an 
 image with increased opacity, and one which from the action of 
 tests we conclude to be more organic in its nature. Bromide of 
 Silver in Collodion, however, diminishes the intensity, and renders 
 the image grey and metallic. In Collodion containing certain kinds 
 of organic matter purposely added, Bromide behaves in the same 
 manner as in Albumen, increasing the opacity of the developed 
 image. 
 
 In the case of Chloride of Silver employed to receive a latent 
 image, and supported by a surface of Gelatine or Albumen, the 
 impression is usually slightly visible before development ; and 
 when such is the case the resulting picture will be organic in its 
 nature, and behave with tests more in the manner of an ordinary 
 Photographic paper darkened by light. 
 
 c. The Developing Agent employed. This subject has already 
 been noticed. Organic developers, Gallic and Pyrogallic Acids, 
 tend to produce images of the opaque kind ; but the inorganic 
 protosalts of Iron, grey and metallic images. Acetate of Iron 
 resembles Pyrogallic Acid in its action more than it resembles 
 the Sulphate of Iron. 
 
 The addition of a strong acid like Nitric Acid to the developer, 
 produces a deposit, the particles of which are large and crystal- 
 line, with small opacity by transmitted light ; but an alkaline 
 condition of the solutions is favourable to a brown deposit, with 
 the particles in a state of fine division. 
 
 d. The stage of the Development. The red image first formed 
 on the application of the developer to a gelatinized or albuminized 
 surface of Iodide of Silver is less metallic, and more easily injured 
 by destructive tests, than the black image, which is the result of 
 prolonging the action. Developed Photographic prints, when 
 of a bright red colour after fixing, correspond in properties to 
 those obtained by the direct action of light on paper prepared 
 with Chloride of Silver, and less closely to Collodion, or even 
 to fully developed Talbotype Negatives. 
 
 e. The intensity of the light by which the Invisible Image was 
 
THE DEVELOPED IMAGE. 165 
 
 formed. A very strong light acting for a short time, is followed 
 by the development of the red image, the particles of which are 
 finely divided, and easily acted on by solvents and oxidizing 
 agents. A dull light, on the other hand, acting for a longer time, 
 is succeeded by an image of the metallic variety, which amalga- 
 mates with Mercury. The two varieties of deposit may often be 
 seen on the same picture, the former in the high lights, as the 
 sky, the latter in the deep shadows where the chemical rays are 
 feeble. 
 
166 
 
 CHAPTER IV. 
 
 ON FIXING THE PHOTOGEAPHIC IMAGE. 
 
 A SENSITIVE layer of Chloride or Iodide of Silver on which 
 an image may be formed, either with or without the aid 
 of a developing agent, must pass through further treat- 
 ment in order to render it indestructible by diffused light- 
 The image itself is sufficiently permanent, and cannot be 
 said, in correct language, to need fixing ; but the un- 
 changed Silver Salt which surrounds it, being still sensi- 
 tive to light, tends to be decomposed in its turn, and by 
 darkening, to obliterate the picture. 
 
 In order that any body may be employed with success 
 as a fixing agent, it is required not only that it should dis- 
 solve unchanged Chloride or Iodide of Silver, but that it 
 should produce no injurious effect upon the image. A 
 solvent action upon the image is most liable to happen 
 when the agency of light alone, without a developer, has 
 been employed : in that case the darkened surface, not 
 being reduced perfectly to the metallic state, remains solu- 
 ble to a certain extent in the fixing liquid. 
 
 AMMONIA AND OTHER FIXING AGENTS. 
 
 Ammonia. Ammonia dissolves Chloride of Silver rea- 
 dily, but not Iodide of Silver : hence its use is necessarily 
 confined to the direct sun proofs upon paper. Even these 
 however cannot advantageously be fixed in Ammonia un- 
 less a deposit of Gold has been previously produced upon 
 the surface by a process of "toning," presently to be ex- 
 
FIXING AGENTS. 167 
 
 plained : an unpleasant red tint is always caused by Am- 
 monia acting upon the darkened material of a sun picture 
 as it comes from the printing-frame. 
 
 The principal objections to the use of Ammonia as a 
 fixing agent are its pungent odour, and also the fact that 
 it is scarcely strong enough in its affinities to dissolve the 
 Oxide of Silver when associated with Albumen and simi- 
 lar bodies. 
 
 Chloride and Iodide as Fixing Agents. The Chlorides 
 of Potassium, Ammonium, and Sodium possess the pro- 
 perty of dissolving a small portion of Chloride of Silver. 
 In the act of solution a double salt is formed, a compound 
 of Chloride of Sodium with Chloride of Silver, which may 
 be crystallized out by allowing the liquid to evaporate 
 spontaneously. The earlier Photographers employed a 
 saturated solution of common Salt for fixing paper prints ; 
 but the fixing action of the Alkaline Chloride is very slow 
 and imperfect. 
 
 The Iodide of Potassium has been used as a fixing agent 
 for Iodide of Silver. It dissolves it by forming a double 
 salt in the manner before described. 
 
 It is important to remark in the solution of the insoluble 
 Silver Salts by Alkaline Chlorides, Iodides, etc., that the 
 amount dissolved is not in proportion to the quantity of the 
 solvent, but to the degree of concentration of its aqueous 
 solution. The reason is that the double salt formed is de- 
 composed by a large quantity of water. Hence it is only 
 a saturated solution of Chloride of Sodium which possesses 
 any considerable power of fixing ; and with the Iodide of 
 Potassium the same rule holds good, the stronger the solu- 
 tion the more Iodide of Silver will be taken up by a given 
 weight. The addition of water produces milkiness and a 
 deposit of the Silver Salt previously dissolved. 
 
 HYPOSULPHITE OF SODA AS A FIXING AGENT. 
 
 Hyposulphurous Acid, S 2 O 2 , is one of the Oxides of 
 Sulphur. It is, as its name implies, of an, acid nature, and 
 
168 THEOBY OF PHOTOGRAPHY. 
 
 the proportion of its oxygen places it upon the list imme- 
 diately below Sulphurous Acid (" upo," under). 
 
 The Hyposulphite of Soda employed by Photographers 
 is a neutral combination of Hyposulphurous Acid and the 
 alkali Soda : it is selected as being more economical in pre- 
 paration than other Hyposulphites adapted for fixing. It 
 occurs in the form of large translucent groups of crystals, 
 which include five atoms of water. These crystals are so- 
 luble in water almost to any extent, the solution being at- 
 tended with the production of cold ; they have a nauseous 
 and bitter taste. 
 
 Hyposulphite of Soda is now made on an extensive scale, 
 and sold at a low price. It is usually sufficiently pure in 
 the state in which it comes from the manufacturers. 
 
 In the solution of Silver compounds by Hyposulphite of 
 Soda a double decomposition always takes place ; thus : 
 
 Hyposulphite of Soda -f- Chloride of Silver 
 = Hyposulphite of Silver -f Chloride of Sodium. 
 
 The Hyposulphite of Silver combines with an excess of 
 Hyposulphite of Soda, and forms a soluble double salt, 
 which may be crystallized out by evaporating the solution. 
 This compound possesses an intensely sweet taste, and con- 
 tains one atom of Hyposulphite of Silver, with two of 
 Hyposulphite of Soda. (AgO, S 2 O 2 +2NaO, S 2 O 2 + 2HO.) 
 In addition to this there is a second double Salt, differing 
 from the first in being very sparingly soluble in water : it 
 contains single atoms of each constituent, and may be re- 
 presented by the formula AgO, S 2 O 2 + NaO, S 2 O 2 + H O. 
 
 The quantity of Chloride of Silver which Hyposulphite 
 of Soda will easily dissolve may be stated roughly as about 
 one-third of its weight. If the proportion of Chloride be in- 
 creased to more than one-half of the Hyposulphite, there 
 will usually be an abundant deposition of minute sparkling 
 crystals, since the second, or sparingly soluble double salt 
 of Hyposulphite of Silver and Soda will then be formed. 
 
 When Hyposulphite of Soda is employed as a fixing 
 agent it is a safe precaution to convert any Nitrate of 
 
FIXING AGENTS. 169 
 
 Silver which may be present into Chloride of Silver, other- 
 wise there will be a danger of the solution becoming dis- 
 coloured from a decomposition into Sulphide of Silver. 
 This change will be explained more particularly in the 
 Section on the fixing of paper proofs, in Chapter VIII. 
 
 Iodide of Silver is dissolved by Hyposulphite of Soda 
 more slowly than Chloride of Silver, and the amount even- 
 tually taken up is far less one part by weight of Iodide 
 of Silver requiring about twenty-four parts by weight of 
 Hyposulphite of Soda in a cold solution ; but if the solu- 
 tion be heated, a much larger quantity of Iodide of Silver 
 is dissolved. 
 
 CYANIDE OF POTASSIUM AS A FIXING AGENT. 
 
 For the chemistry of Cyanogen see the Vocabulary. 
 
 The Cyanide of Potassium is the salt most frequently 
 employed in fixing, but Cyanide of Sodium will answer 
 the purpose equally well. Cyanide of Potassium, the com- 
 mercial manufacture of which is described in the Vocabu- 
 lary, occurs in the form of fused lumps of considerable 
 size. In this state it is usually contaminated with a large 
 percentage of Carbonate of Potash, amounting in some 
 cases to more than half its weight. By boiling in proof 
 Spirit the Cyanide may be extracted and crystallized, but 
 this operation is scarcely required as far as its use in Pho- 
 tography is concerned. 
 
 Cyanide of Potassium absorbs moisture on exposure to 
 the air. It is very soluble in water, but the solution de- 
 composes on keeping ; changing in colour and evolving the 
 odour of Prussia Acid, which is a Cyanide of Hydrogen. 
 Cyanide of Potassium is highly poisonous, and must be 
 used with caution. 
 
 Solution oi Cyanide of Potassium^s a most energetic 
 agent in dissolving the insoluble Silver Salts ; far more so 
 than the Hyposulphite of Soda. The Salts are in all cases 
 converted into Cyanide, and exist in the solution in the 
 form of a soluble double Salt, which, unlike the double 
 Sulphocyanide, is not decomposed by dilution with water. 
 
170 THEOEY OF PHOTOGEAPHY. 
 
 Cyanide of Potassium is not adapted for fixing Positive 
 proofs upon Chloride of Silver; and even when a deve- 
 loper has been used, unless the solution of the Cyanide is. 
 tolerably dilute, it is apt to attack the image, converting 
 it superficially into Cyanide of Silver, and then dissolving 
 it in the form of a double Cyanide of Potassium and 
 Silver. 
 
 The solvent powers of Cyanide of Potassium on Metallic 
 Silver are much increased by the addition of a little Iodine 
 to its aqueous solution ; a colourless liquid is formed, which 
 has been termed " lodo-cyanide of Potassium." 
 
 SULPHOCYANIDES AS FIXING AGENTS. 
 
 Sulphocyanides of Potassium and Ammonium have been 
 lately proposed as fixing agents ; they resemble the alkaline 
 chlorides used for this purpose, inasmuch as their solvent 
 power depends on the degree of concentration of the solution, 
 but they greatly exceed the latter salts in their power of 
 dissolving the insoluble Salts of Silver, when strong solu- 
 tions are used. 
 
 Though inferior to Hyposulphites in this varying degree 
 of solvent power, they have the advantage of imparting no 
 Sulphur to the print, and they preserve the tones clear 
 and free from mealiness. 
 
 For chemical composition, etc., see the Vocabulary. 
 
 OBSEEVATIONS ON THE EELATIVE AFFINITIES OF CHLOEINE, 
 IODINE, AND HYPOSULPHUEOUS ACID FOE SILVEE. 
 
 Hyposulphurous Acid is usually viewed as a body possessing 
 extraordinary affinities for Silver, inasmuch as a solution of 
 Hyposulphite of Soda decomposes both Iodide and Chloride of 
 Silver. It may be shown, however, on the other hand, that a 
 solution of a Chloride or an Iodide will decompose Hyposulphite 
 of Silver under some circumstances ; and hence the relative affi- 
 nities are less evident than may appear. The use of these fixing 
 agents, indeed, depends upon their being present in large excess 
 over the Salt of Silver to be dissolved, in order that a true double 
 salt may be formed. When two different fixing agents are em- 
 
 
FIXING AGENTS. 171 
 
 ployed in a state of mixture, that particular one may prevail 
 which is present in largest quantity, even although its affinities 
 for Silver are weaker than those of the other. 
 
 Thus although Chloride of Silver dissolves easily in solution 
 of Hyposulphite of Soda, we find that Hyposulphite of Silver 
 is decomposed and converted into Chloride of Silver when treated 
 with a saturated solution of Chloride of Sodium. And the same 
 remark applies to Iodide of Silver ; it is decomposed by a large 
 excess of Hyposulphite of Soda, but not by a small excess. The 
 addition of Iodide of Potassium to a fixing bath of Hyposul- 
 phite nearly saturated with Silver Salt, will precipitate a portion 
 of the Silver in the form of the yellow Iodide of Silver. 
 
 Hyposulphite of Soda will not dissolve more Chloride of Silver 
 than is required to form the compound represented by the for- 
 mula AgO,S 2 Oo4-NaO,S 2 O 2 , or one equivalent of Chloride of 
 Silver requires two equivalents of Hyposulphite at least for its 
 solution, while one equivalent of Iodide of Silver requires about 
 forty -six equivalents of Hyposulphite at ordinary temperatures ; 
 but if the solution be warm, a much larger amount of Iodide 
 of Silver can be dissolved, which, however, separates again on 
 cooling. 
 
172 
 
 CHAPTER V. 
 
 ON THE NATURE AND PROPERTIES OF LIGHT. 
 
 THE present Chapter is devoted to a discussion of the more 
 remarkable properties of Light ; the object being to select 
 certain prominent points, and to state them as clearly as 
 possible, referring, for information of a more complete 
 kind, to acknowledged works on the subject of Optics. 
 
 The Chapter will be divided into four Sections : first, 
 the compound nature of Light, with the Laws of refraction 
 of Light ; second, Lenses and the Camera ; third, Actinism; 
 fourth, Binocular Vision and the Stereoscope. 
 
 SECTION I. 
 
 The Compound Nature of Light. 
 
 The ideas entertained on the subject of Light, before the 
 time of Sir Isaac Newton, were vague and unsatisfactor}^ 
 It was shown by that eminent philosopher, that a ray of 
 sunlight was not homogeneous, as had been supposed, but 
 consisted of several rays of vivid colours, united and inter- 
 mingled. 
 
 This fact may be demonstrated by throwing a pencil of 
 Sunlight obliquely upon one side of a prism, and receiving 
 the oblong image so formed, upon a white screen. 
 
 The space illuminated and coloured by a pencil of rays 
 analysed in this way is called "the Solar Spectrum." The 
 action of a prism in decomposing white light will be more 
 
THE SOLAE SPECTEUM. 
 
 173 
 
 fully explained at page 198. At present we notice only 
 that seven principal colours may be distinguished in the 
 Solar Spectrum, viz. red, orange, yellow, green, blue, 
 
 Violet. 
 
 indigo, and violet. Sir David Brewster has made observa- 
 tions which lead him to suppose that the primary colours 
 are in reality but three in number, viz. red, yellow, and 
 blue, and that the others are compound, being produced by 
 two or more of these overlapping each other ; thus the 
 red and yellow spaces intermingled constitute orange; the 
 yellow and blue spaces, green. 
 
 The composition of white light from the seven prismatic 
 colours may be roughly proved by painting them on the 
 face of a wheel, and causing it to rotate rapidly : this 
 blends them together, and a sort of greyish-white is the 
 result. The white is imperfect, because the colours em- 
 ployed cannot possibly be obtained of the proper tints or 
 laid on in the exact proportions. 
 
 The decomposition of light is effected in other ways be- 
 sides that already given : 
 
 First, by reflection from the surfaces bf coloured bodies. 
 All substances throw off rays of light which impinge upon 
 the retina of the eye and produce the phenomena of vision. 
 Colour is caused by a portion only, and not the whole, of 
 the elementary rays, being projected in this way. Sur- 
 faces termed white reflect all the rays ; coloured surfaces 
 absorb some and reflect others : thus red substances reflect 
 only red rays, yellow substances, yellow rays, etc., the ray 
 
174 THEORY OF PHOTOGRAPHY. 
 
 which is reflected in all cases deciding the colour of the 
 substance. 
 
 Secondly, light may be decomposed by transmission 
 through media which are transparent to certain rays, but 
 opaque to others. 
 
 Ordinary transparent glass allows all the rays constitu- 
 ting white light to pass ; but by the addition of certain 
 metallic oxides to it whilst in a state of fusion, its proper- 
 ties are modified, and it becomes coloured. Glass stained 
 by Oxide of Cobalt is permeable only to blue rays. Oxide 
 of Silver imparts a pure yellow tint ; Oxide of Gold or 
 Suboxide of Copper a ruby red, etc. 
 
 PEOPERTIES OF LIGHT. 
 
 The agency of Light produces a variety of distinct effects 
 upon the bodies which surround us. These may be classed 
 together as the properties of light. They are of three 
 kinds the phenomena of colour and vision, of heat, and 
 of chemical action. 
 
 By resolving white light into its constituent rays, we 
 find that each of these properties is associated more par- 
 ticularly with certain of the elementary colours. 
 
 The yellow is decidedly the most luminous ray. On ex- 
 amining the Solar Spectrum, it is seen that the brightest 
 part is that occupied by the yellow, and that the light di- 
 minishes rapidly on each side. So, again, rooms glazed 
 with yellow glass always appear abundantly illuminated, 
 whilst the effect of red or blue glass is dark and sombre. 
 The yellow colour therefore constitutes that portion of 
 white light by which surrounding objects are rendered 
 visible ; it is essentially the visual ray. 
 
 The heating properties of the sunlight reside principally 
 in the red ray, as is shown by the expansion of a mercu- 
 rial thermometer placed in that part of the spectrum. 
 
 The chemical action of light corresponds more to the 
 indigo and violet rays, and is wanting, as regards its in- 
 fluence upon Iodide of Silver, both in the red and yellow. 
 Strictly speaking, however, it cannot be localized in either 
 
REFRACTION OF LIGHT. 175 
 
 of the coloured spaces, as will be more fully shown in the 
 third Section of this Chapter, to which the reader is re- 
 ferred. 
 
 REFRACTION OF LIGHT, AND LENSES. 
 
 A ray of light, in its passage through any transparent 
 medium, travels in a straight line as long as the density 
 of the medium continues unchanged. But if the density 
 varies, becoming either greater or less, then the ray is re- 
 fracted, or bent out of the course which it originally pur- 
 sued. The degree to which the refraction or bending takes 
 place depends upon the nature of the new medium, and in 
 particular upon its density as compared with that of the 
 medium which the ray had previously traversed. Hence 
 Water refracts light more powerfully than Air, and Glass 
 more so than Water. 
 
 The following diagram illustrates the refraction of a ray 
 of light: 
 
 The dotted line is drawn perpendicularly to the surface, 
 and it is seen that the ray of light on entering is bent 
 towards this line. On emerging, on the other hand, it is 
 bent to an equal extent, away from the perpendicular, so 
 that it proceeds in a course parallel to, but not coincident 
 with its original direction. If we suppose the new me- 
 dium, in place of being more dense than the old, to be less 
 dense, then the conditions are exactly reversed,-M:he ray is 
 bent away from the perpendicular on entering, and towards 
 it on leaving. 
 
176 
 
 THEOSY OF PHOTOGRAPHY. 
 
 It must be observed that tlie laws of refraction apply 
 only to rays of light which fall npon the medium at an 
 angle : if they enter perpendicularly in the direction ot 
 the dotted lines in the last figure they pass straight 
 through without suffering refraction. 
 
 Notice also, that it is at the surface of bodies that the 
 deflecting power acts. The ray is bent on entering, and 
 bent again on leaving ; but whilst within the medium it 
 continues in a straight line. Hence it is evident that by 
 variously modifying the surfaces of refractive media the 
 rays of light may be diverted almost at pleasure. This 
 will be rendered clear by a few simple diagrams. 
 
 In the figures given below, the dotted lines represent 
 perpendiculars to the surface at the point where the ray 
 Fig. 1. Fig. 2. 
 
 A 
 
 falls, and it is seen that the usual law of bending towards 
 the perpendicular on entering, and away from it on leaving 
 the dense medium, is in each case correctly observed. 
 
LENSES. 
 
 177 
 
 Fig. 2, in the last page, termed a prism, bends the ray 
 permanently to one side ; fig. 3, consisting of two prisms 
 placed base to base, causes rays before parallel to meet in 
 a point ; and conversely, fig. 4, having prisms placed edge 
 to edge, diverts them further asunder. 
 
 The various forms of Lenses. The phenomena of the 
 refraction of light are seen in the case of curved surfaces 
 in the same manner as with those which are plane. 
 
 Glasses ground of a curvilinear form are termed Lenses. 
 The following are examples. 
 
 Fig. 1. Fig. 2. Fig. 3. 
 
 
 Fig. 1 is a biconvex lens ; fig. 2, a biconcave lens ; and 
 fig. 3, a meniscus lens. 
 
 As far as regards their refractive powers, such figures 
 may be represented, nearly, by others bounded by straight 
 lines, and thus it becomes evident that a biconvex lens 
 tends to condense rays of light to a point, and a biconcave 
 to scatter them. A meniscus combines both actions, but 
 the rays are eventually bent together, the convex curve of 
 a meniscus lens being always greater than the concave. 
 
 The Foci of Lenses. It has been shown that convex 
 lenses tend to condense rays of light and bring them to- 
 gether to a point. This point is termed "the focus" of 
 the Lens. 
 
 The following laws as regards the focus may be laid 
 down : That rays of light which are pursuing a parallel 
 course at the time they enter the Lens are brought to a 
 
 N 
 
178 THEOEY OF PHOTOGRAPHY. 
 
 focus at a point nearer to the Lens than diverging rays. 
 The rays proceeding from very distant objects are parallel ; 
 those from objects near at hand diverge. The sun's rays 
 are always parallel, arid the divergence of the others be- 
 comes greater as the distance from the Lens is less. 
 
 The focus of a Lens for parallel rays is termed the "prin- 
 cipal focus," and is not subject to variation ; this is the 
 point referred to when the focal length of a Lens is spoken 
 of. When the rays are not parallel, but diverge from a 
 point, that point is associated with the focus, and the two 
 are termed " conjugate foci." 
 
 In the above diagram A is the principal focus, and B 
 and C are conjugate foci. Any object placed at B has its 
 focus at C, and conversely when placed at C it is in focus 
 atB. 
 
 Therefore, although the principal focus of a Lens (as de- 
 termined by the degree of its convexity) is always the same, 
 yet the focus for objects near at hand varies, being longer 
 as they are brought closer to the Lens. 
 
 Formation of a Luminous Image by a Lens. As the 
 rays of light proceeding from a point are brought to a 
 focus by means of a Lens, so are they when they proceed 
 from an object, and in that case an image of the object is 
 the result. 
 
 In Order that the course pursued by pencils of rays pro- 
 ceeding from an object may be easily traced, the lines from 
 the barb of the arrow in the figure in the next page are 
 dotted. Observe that the object is necessarily inverted, 
 and also that those rays which traverse the central point 
 of the Lens, or the centre of the axis, as it is termed, are 
 not bent away, but pursue a course either coincident with, 
 
LENSES. 
 
 179 
 
 or parallel to the original, as in the case of refracting medh 
 with parallel surfaces. 
 
 Enlarged and reduced Images. When an object is 
 placed at some distance in front of a Lens an image is 
 formed, smaller than the object; but if the object be ad- 
 vanced nearer to the Lens the image increases in size until 
 it becomes, first equal to, and then larger than the original, 
 the focus at the same time receding to a greater distance 
 from the Lens. 
 
 Thus if a photographic portrait be approximated to a 
 Lens, an image of the life size may be thrown upon a 
 screen, just as the rays proceeding from the living model 
 were in the first instance employed to form the portrait. 
 The two planes, in fact, of the object and of the image are 
 strictly conjugate foci (p. 178), and it is immaterial from 
 which of the two, anterior or posterior, the rays of light 
 proceed. 
 
 Amplifying instruments, like Woodward's Patent Solar 
 Camera, are constructed upon the above principle, and 
 are found useful in enlarging photographs to three or four 
 times their original diameter. A loss of definition always 
 results in this process, which may conveniently be remedied 
 by the pencil of the artist. 
 
 Lenses of long and short focus. A Lens of short focus 
 placed at a given distance from an object forms a small 
 image, the rays being strongly refracted. If the Lens be 
 brought nearer to the object, the image becomes larger, as 
 
180 THEOEY OF PHOTOGRAPHY. 
 
 above shown ; but at a certain point the approximation 
 must be stopped, or the Lens will be strained, as the opti- 
 cians term it, and distortion will result. Therefore Lenses 
 are purposely constructed of a longer focus when a large 
 image is required. 
 
 Long-focus lenses for taking large pictures have usually 
 a considerable diameter, but it will be understood that the 
 size of a Lens has nothing whatever to do with the size of 
 the image. The focal length of the Lens, at a given distance, 
 determines the size of the image ; nevertheless, since the 
 luminous rays are diffused over a large space when the fo- 
 cus is long, the optician is compelled in such a case to in- 
 crease the aperture of the glass in order to transmit more 
 light. 
 
 The focal length of a single actinic combination is usually 
 found, by focussing a distant object, and then measuring 
 the distance from the back of the lens to the refracted 
 image on the screen. This distance represents the focal 
 length of the lens. It is not strictly accurate, but near 
 enough for all practical purposes. The focal length of a 
 double or triple combination of lenses cannot be thus esti- 
 mated, because the point from which it is measured is si- 
 tuated somewhere in front of the back combination; hence 
 a more complicated process of measurement must be adopt- 
 ed to determine what is called in this case the " equivalent 
 focus," or the distance from the screen, at which a single 
 lens must be placed in order to give the same-sized image. 
 (See Appendix.) 
 
 SECTION IL 
 
 The Photographic Camera. 
 
 The Photographic Camera, in its essential nature, is an 
 extremely simple instrument. It consists merely of a dark 
 chamber, having an aperture in front in which a Lens is 
 inserted. The accompanying figure shows the simplest 
 form of Camera. 
 
 : 
 
THE CAMERA. 181 
 
 The body is represented as consisting of two sliding por- 
 tions; but the same object of lengthening or shortening 
 
 the focal distance may be attained by making the Lens it- 
 self movable. A luminous image of any object placed in 
 front of the Camera is formed by means of the Lens, and 
 received upon a surface of ground glass at the back part of 
 the instrument. When the Camera is required for use, the 
 object isfocussed upon the ground glass, which is then re- 
 moved, and a slide containing the sensitive layer inserted 
 in its place. 
 
 The luminous image, as formed upon the ground glass, 
 is termed the " Field" of the Camera; it is spoken of as 
 being flat or curved, sharp or indistinct, etc. These and 
 other peculiarities depending upon the construction of the 
 Lens will now be explained. 
 
 Chromatic Aberration: Visual and Actinic Focus. 
 The outside of a biconvex lens resembles the sharp edge 
 of a prism, and necessarily produces decomposition in the 
 white light which passes through it. The luminous image 
 of an uncorrected Lens, is in consequence bounded by co- 
 loured fringes. 
 
 The action of a prism in separating white light into its 
 constituent rays may be simply explained. The indigo 
 and violet rays are more bent away than the yellow and 
 red, and consequently they are separated from them, and 
 occupy a higher position in the Spectrum. (See the dia- 
 gram at p. 173.) A little reflection will show that in con- 
 sequence of this unequal refrangibility of the coloured 
 rays, white light must invariably be decomposed on enter- 
 ing any dense medium obliquely. This is indeed the case ; 
 
182 THEOEY OF PHOTOGEAPHY. 
 
 but if the surfaces of the medium are parallel to each other 
 the effect is not seen, because the rays recombine on their 
 emergence, being bent to the same extent in the opposite 
 direction. Hence light is transmitted colourless through 
 an ordinary pane of glass, but yields the tints of the Spec- 
 trum in its passage through a prism or a lens, where the 
 two surfaces are inclined to each other at an acute angle. 
 
 The same causes which produce chromatic aberration in 
 a Lens, tend also to separate the chemical from the visual 
 
 focus. The violet and indigo rays are more strongly bent 
 in than the yellow, and still more than the red ; conse- 
 quently the focus for each of those colours is at a different 
 point. The above diagram shows this. 
 
 V represents the focus of the violet ray, Y of the yellow, 
 and E of the red. Hence, as the chemical action corre- 
 sponds more to the violet, the most marked actinic effect 
 would be produced nearly at V. The luminous portion of 
 the spectrum however is the yellow, consequently the visual 
 focus is at Y. 
 
 Photographers have long recognized this point; and 
 therefore, with ordinary Lenses, not corrected for actin- 
 ism, rules are laid down as to the exact distance which 
 the sensitive plate should be shifted away from the visual 
 focus in order to obtain the greatest amount of distinctness 
 of outline in the image impressed by chemical action. 
 
 Actinic aberration is corrected by combining two lenses 
 cut from varieties of glass which differ in their power of 
 separating the coloured rays. These are generally the 
 dense flint-glass containing Oxide of Lead, and the light 
 crown-glass. Of the two lenses, the one is biconvex, and 
 the other biconcave; so that when fitted together they 
 
SPHEBICAL ABERRATION. 
 
 183 
 
 produce a compound Actinic lens of a meniscus form, as 
 shown in the diagram. 
 
 The first Lens in this figure is the flint- and the second 
 the crown-glass. Of the two the biconvex is the most 
 powerful, so as to overcome the other, and produce a total 
 of refraction to the required extent. Each of the Lenses 
 is made from glass of different dispersive power to that 
 of its coadjutor ; and the effect of passing the rays through 
 both, is, by overlapping the coloured spaces, to unite the 
 complementary tints, and to re-form white light. 
 
 Spherical or Axial Aberration. Spherical aberration 
 is the property possessed by Lenses which are segments of 
 spheres, of refracting rays of light unequally at different 
 parts of their surfaces. The following diagram shows this 
 in an exaggerated degree : 
 
 
 Observe that the dotted lines which fall upon the cir- 
 cumference of the Lens are brought to a focus at a point 
 nearer to the Lens than those passing through the centre ; 
 in other words, the outside of the Lens refracts light more 
 powerfully. This causes a degree of confusion and indis- 
 tinctness in the image, from various rays crossing, and 
 interfering with each other. 
 
 In correcting for spherical aberration, the first point to 
 be attended to, is the curve of the Lens itself. A. bicon- 
 
184 THEORY OF PHOTOGRAPHY. 
 
 vex Lens, as shown in the above diagram, is the worst 
 form. The Meniscus Lens (p. 177) is preferable, and is 
 
 usually employed. But a Lens of the shape given above, 
 one surface of which is a section of an ellipse, and the 
 other of a circle struck from the furthest of the two foci of 
 that ellipse, is the most perfect, and produces no aberration 
 in the case of rays parallel with the axis. The difficulty 
 of grinding such a Lens, however, is very great. 
 
 But spherical aberration, even in a Lens which is badly 
 made as regards form, may be remedied in another way, 
 viz. by using a diaphragm or stop, so as to cut off a portion 
 of the light, and prevent the same pencil of rays from fall- 
 ing both on the centre and on the edge of the Lens. As 
 the action of a stop is more fully explained a few pages 
 in advance, we defer the further consideration of it for the 
 present. 
 
 Curvature of the Field. In using a Camera mounted 
 with a common Meniscus Lens, it will be observed that 
 when the centre of the field has been focussed, the outer 
 portions of the image appear indistinct, whilst if the ground 
 glass be pushed in a little, the outside becomes sharp, but 
 the centre is thrown out of focus. This defect is some- 
 times attributed to spherical aberration, but incorrectly 
 so, because indistinctness from that cause is seen alike at 
 every portion of the field. The want of definition which 
 can be remedied by shifting the position of the focussing- 
 screen we refer rather to curvature of the field, or to the 
 fact that the image of a flat surface, formed by a Lens, 
 
CUEVATTJBE OF THE FIELD. 185 
 
 produces not a plane, but a portion of a hollow sphere, as 
 shown by the inverted arrow, page 179. 
 
 In the annexed diagram the centre line running at 
 right angles to the general direction of the Lens is the 
 axis; an imaginary line, on which the Lens might be rotated 
 as a wheel is turued on its axle. The lines A A represent 
 rays of light falling parallel to the axis ; and the dotted lines, 
 others which have an oblique direction ; B and C show the 
 
 A- 
 
 A- 
 
 points at which the two foci are formed. Observe that 
 these points, although equidistant from the centre of the 
 Lens, do not fall in the same vertical plane, and there- 
 fore they cannot both be received simultaneously upon the 
 ground glass of the Camera, which would occupy the posi- 
 tion of the perpendicular double line in the diagram. 
 
 The defects due to curvature of the field, might be 
 avoided by bending inwards the glass or paper used to 
 sustain the sensitive Iodide, in such a way as to meet the 
 image, as is done in the apparatus for Sutton's Panoramic 
 Lens : but practically this plan, though possessing some 
 advantages, is in other respects objectionable. The 
 image itself therefore must be flattened out, and this can 
 be effected by means of the diaphragms before alluded to 
 in the paragraph on spherical aberration : the position and 
 use of which will be shown in the following pages. 
 
 Variation of Focus for Near and Distant Objects. 
 Lenses employed with the full aperture, do not render 
 near and remote objects sharp upon the ground glass of 
 the Camera at the same time. If the foreground is in 
 focus, the Lens must be thrown inwards to make the dis- 
 tance clear, and vice versa. This is a necessary conse- 
 
186 THEORY OF PHOTOGRAPHY. 
 
 quence of the focal plane of any object varying with the 
 position of the object in regard to the Lens (p. 178). 
 
 The action of a diaphragm or stop, already spoken of 
 under the head of " Spherical Aberration" and " Curvature 
 of the Field," is seen to still greater advantage in remedy- 
 ing the defect now complained of. 
 
 Fig. 1 represents rays of light radiating in all directions 
 from a luminous point and falling on the entire surface of 
 
 Fig. 1. Fig. 2. 
 
 a Lens ; fig. 2, the action of the diaphragm in intercept- 
 ing that portion of the rays which otherwise would im- 
 pinge upon the outside of the glass. The focus in each 
 case is at the point where the rays cross each other after 
 refraction. Now in fig. 1 suppose the greyed screen, re- 
 presented by the dotted line, to be advanced nearer to the 
 Lens, or to be withdrawn from it, even in the least degree, 
 the image would immediately be out of focus ; but in the 
 second figure, the rays run so nearly parallel, that the ef- 
 fect of a slight change of position would not be perceptible. 
 The refracted pencil is as it were sharpened and drawn 
 out, so that the focus has considerable depth, and is no 
 longer confined to a single plane. The first figure repre- 
 sents what is termed a large angular pencil of rays, and 
 fig. 2 a small angular pencil : the latter alone can be em- 
 ployed when near and distant objects are to be simultane- 
 ously depicted. 
 
 The focal variation for near and distant objects is much 
 more considerable when Lenses of very long focus are 
 employed, and (always supposing the distance from the 
 object to remain the same in the two cases) it becomes 
 less evident with Lenses of short focus. Hence the small 
 Lenses used for stereoscopic Photography have usually 
 some depth of focus with a comparatively large aperture. 
 
LENSES. 
 
 187 
 
 THE SINGLE ACTINIC VIEW LENS. 
 
 The following diagram will serve to explain the con- 
 struction of this Lens. 
 
 The glass is an Actinic meniscus, placed with its 
 concave side towards the object. In front of the Lens 
 
 and at a distance from it equal to a fifth part of its focal 
 length, are fixed a series of diaphragms, the largest being 
 employed for subjects nearly in one plane ; and the smaller 
 for those embracing both foreground and distance. 
 
 This form of lens is dependent entirely upon the stop 
 or diaphragm for correct action, and could not be used 
 with the full aperture. For in the first place the spherical 
 aberration is only partially overcome by the curves given 
 to the two glasses used in actinizing, and hence a resi- 
 dual error which must be corrected by the stop. Secondly, 
 the curvature of field is so considerable, that only a small 
 surface could be covered flat by the Lens of full aperture. 
 It is for the purpose of dealing with this latter error that 
 the diaphragms are fixed in so peculiar a position with re- 
 ference to the glass. By referring to the diagram it will 
 be seen that the effect of a stop placed at a distance in 
 front of the Lens is to direct the oblique rays, represented 
 in the diagram by dotted lines, through the outside por- 
 tions only of the glass ; upon which they fall at such an 
 angle that the focus is lengthened out, and consequently 
 the field instead of being concave is nearly flat. If the 
 diaphragm were pushed in until it came in contact with 
 the Lens, the effect would then be similar to using a small 
 
188 THEORY OF PHOTOGRAPHY. 
 
 Lens having the same diameter as the aperture of the stop; 
 and the field of the Camera would be indistinct at the 
 edges, from curvature of the image. Hence the View Lens 
 is not unnecessarily large and cumbrous, as might at first 
 appear ; and every portion of the glass is really employed 
 in forming the picture, the centre taking the middle part, 
 and the circumference of the Lens refracting the rays which 
 proceed from the sides of the object. 
 
 THE APLANATIC LENS. 
 
 The Aplanatic Lens, patented by Mr. Grubb, of Dublin, 
 resembles the ordinary Actinic View Lens in outward form, 
 but differs entirely in the mode in which it is actinized,* 
 the crown-glass taking the place of the flint, and vice versa. 
 It is constructed by cementing a crown-glass Meniscus 
 Lens of suitable focus to a concavo-convex of flint, the con- 
 cave side of the latter being ground to the same curve as 
 the convex side of the flint, so as to admit of perfect 
 contact. 
 
 The advantage claimed for this mode of actinizing a 
 lens, is a considerable reduction thereby in the amount of 
 spherical aberration as compared with the ordinary form of 
 View Lens ; hence follows also another advantage, in en- 
 abling the stop to be placed nearer the front of the Lens 
 than its usual position, which would to some extent dimi- 
 nish the barrel-shaped distortion common to all single 
 combinations. 
 
 THE ORTHOSCOPIC OR ORTHOGRAPHIC LENS. 
 
 This instrument, constructed from calculations by Pro- 
 fessor Petzval, was proposed to supersede the old form of 
 View Lens, as giving a more truthful representation of the 
 object. The name indicates the straightness of the lines 
 
 * Some apology may be required for coining a new verb, but the adoption 
 of some such expression as " to actinize " is necessary in order to distinguish 
 between an " achromatic" lens, that is, corrected for colour, and one cor- 
 rected for the chemical focus. These points lie in different parts of the 
 spectrum, and are caused by the different refrangibility of the different 
 coloured rays ; see p. 182. Hence an achromatic lens has not its visual ne- 
 cessarily coincident with its chemical or actinic focus. In the early days of 
 
LENSES. 189 
 
 produced by the Lena, but, as we shall presently show, it 
 is a misnomer. Nevertheless there are advantages in the 
 Orthographic Lens which will secure its continued use. 
 
 The above diagram is a sectional view of the Petzval 
 Orthoscopic Lens. It has two actinic combinations, the 
 posterior of which (consisting of a biconcave and a menis- 
 cus, touching at their edges) is a dispersing Lens (p. 177), 
 and lengthens out the focus of the rays converged by the 
 first. The use of this negative or dispersing Lens consti- 
 tutes the peculiarity of this instrument, and confers many 
 advantages. 
 
 In the first place, the Negative Lens enables the opti- 
 cian to overcome spherical aberration without employiog 
 a small diaphragm. This lens is not itself free from the 
 above-named defect, but the aberration is in an opposite 
 direction to that of the first lens, consequently the two 
 neutralize each other and the picture is sufficiently sharp. 
 
 More important still is the effect which the dispersing 
 Lens exerts in improving the oblique pencils, or those 
 which form the outside portions of the picture. They are 
 so far lengthened out as to lessen the concavity of the field, 
 and to render the image sufficiently flat to fall correctly 
 upon the focussing-screen. In this respect the Ortho- 
 graphic Lens is very perfect, and is universally allowed to 
 
 Photography, Professor Hunt strongly urged the adoption of some such 
 term, and proposed the word " di-actinic," that is, admitting of the passage 
 of the chemical rays. The word " actinic " has now come into general use 
 and is not likely to be set aside ; it has therefore been proposed to coin the 
 verb from the shorter adjective in preference to the longer, as being suffi- 
 ciently distinctive. 
 
190 THEORY OF PHOTOGRAPHY. 
 
 be surpassed by none in its power of covering a large ex- 
 tent of surface. 
 
 THE TRIPLET LENS. 
 
 The exact origin of the Triplet Lens for photographic 
 work is not easy to define, since there are conflicting claims 
 for the honour of first having introduced it. It would ap- 
 pear, that so early as the year 1853, the late Mr. F. Scott 
 Archer was in the habit of using a double combination 
 Portrait Lens for taking landscapes and interiors, and 
 when thus applied, a stop with a small aperture was placed 
 between the components of the combination, not in the 
 convenient manner subsequently introduced by Mr. 
 Waterhouse, but similarly situated. Mr. Archer, finding 
 that for interiors his lens was sometimes of inconveniently 
 long focus, introduced a slightly convex lens of small dia- 
 meter instead of the diaphragm, thus shortening the focus 
 and enabling him to include more of the subject on the 
 same-sized plate. Finding this arrangement work satis- 
 factorily, he then substituted for ordinary landscape pur- 
 poses a concave lens of similar dimensions in the place of 
 the convex, thereby lengthening the combined focus, so as 
 to enable him to use larger plates with the same lens. 
 From evidence still existing, these additions were not em- 
 pirical, but made with the definite objects in view. The 
 small convex and concave lenses were not at first actinic, 
 from motives of economy the requisite slight corrections 
 for variation of chemical focus being made after focussing. 
 Shortly after Mr. Archer introduced the third lens, M. 
 Chevalier constructed something of the same kind for the 
 late Mr. P. W. Fry, and subsequently M. Derogy adopted 
 the same device for obtaining a sort of multum in parvo 
 lens. 
 
 About four or five years ago, Mr. Thomas Sutton, of 
 Jersey, devised a form of lens which he called the " Sym- 
 metrical Triplet," which, as its name implies, consisted of 
 one concave and two compound convex combinations, the 
 latter being exactly alike and placed one at each end of 
 
LENSES. 191 
 
 the tube-mounting. The object aimed at was to correct 
 the deviation of the pencils of rays so that they should be 
 incident and emerge in directions parallel to one another. 
 Mr. G-oddard, of Isleworth, then constructed some triple 
 combinations, not adhering however to the symmetrical 
 form, and subsequently M. Dallmeyer adopted the same 
 principle of employing for landscape lenses three actinic 
 combinations ; these however were not mere copies of any 
 of the preceding, but the formulae for their construction 
 were worked out quite independently by M. Dallmeyer, 
 so that they only resembled the others in consisting of 
 three parts, the central one being a concave lens. 
 
 Somewhat later, Mr. Ross adopted a form of lens con- 
 sisting of three parts, arranged as were the others ; but 
 here again the formulae were calculated anew by Mr. Ross, 
 and any person who will take the trouble of examining 
 them, will perceive at once that they differ in construction 
 as much from those of M. Dallmeyer as the last-named 
 optician's did from those of his predecessors. 
 
 The above sectional diagram of the latest form of the 
 " Actinic Triplet " will show its construction. It consists 
 of two Positive or condensing plano-convex lenses, A (the 
 anterior) and C (the posterior), so arranged that without 
 the intervention of the central lens they would form a 
 combination of short focus somewhat similar to the ordinary 
 portrait lens, but possessing greater curvature of field. 
 
192 THEORY OF PHOTOGRAPHY. 
 
 The Negative or dispersing lens, B, while from its 
 smallness it acts to some extent as a stop, elongates the 
 pencils of rays over the entire field and is so constructed 
 as also to act more powerfully on the extreme than on the 
 central pencils, thereby flattening the field. For this 
 latter object it was first used by Petzval in his Orthoscopic 
 and subsequently by Mr. Button in his Triplet combination. 
 The axis of the emergent being parallel to that of the inci- 
 dent pencils renders the marginal lines straight, and gives 
 an entire freedom from distortion a defect which consti- 
 tutes the great drawback to the use of the Petzval and the 
 single combinations for architectural and copying purposes. 
 In common with these lenses, it also gives nearly equal 
 illumination over the whole field. Each lens is rendered 
 actinic, and the contact surfaces are cemented in order to 
 reduce the reflecting surfaces to a minimum. The stop 
 D, placed immediately in front of the Negative lens, 
 serves the purpose of still more flattening the field and 
 diminishing spherical aberration. 
 
 Comparing the Triplet and the Orthoscopic with the 
 single combination as regards rapidity of Photographic 
 action, the advantage belongs to the latter in the case of 
 equal apertures and equal focal lengths, since in the Ortho- 
 scopic and Triplet combinations a part of the actinic power 
 is lost in passing through so many glasses and also by sur- 
 face reflection. In the case of the Triplet where the stop is 
 placed behind the front combination, theoretical reasons 
 would lead to the conclusion that it is quicker in action 
 than the single lens, inasmuch as the rays before being 
 intercepted by the stop are already converging, and there- 
 fore more concentrated. Practically this gain does not 
 seem sufficient to counterbalance the loss by reflection and 
 absorption. 
 
 BUTTON'S PANORAMIC LENS. 
 
 This compound lens, the invention of Mr. Button, of 
 Jersey, is composed of two single, thick, concavo-convex 
 lenses, A and B, the curved surfaces of which are portions 
 
LENSES. 193 
 
 of concentric spheres. They are secured to a brass mount 
 (not shown in the diagram) in such a way that the interior 
 space C can be filled with distilled water, which serves to 
 
 X 
 
 render the Lens actinic. In this space and midway be- 
 tween the lenses a diaphragm D E is placed, with a central 
 aperture of an elliptical form, having placed in front of it, 
 towards the anterior Lens A, two upright, thin partitions, 
 which serve to equalize the light. 
 
 This lens includes with fine definition and equal illumi- 
 nation a far larger angle of view than any other Lens 
 hitherto introduced, viz. upwards of 100 on the base line. 
 A serious drawback, however, to its general use, is its 
 great curvature of field, which necessitates the employment 
 of curved plates to receive the image at its point of focus. 
 It is not constructed for architectural and copying pur- 
 poses, but solely for landscapes, where a large angle with 
 good definition is required. 
 
 THE GLOBE LENS. 
 
 The Globe Lens, patented by Messrs. Harrison and 
 Schnitzer, of New York, has recently been prominently 
 brought into notice in this country. It consists of two 
 actinic meniscus combinations A and B, placed with their 
 concave sides facing each other at such a distance that 
 their outer curved surfaces, if continued, would form a per- 
 fect sphere, as represented by the dotted lines. A portion 
 of the light from the object, refracted by the anterior Lens 
 
194 
 
 THEORY OF PHOTOGRAPHY. 
 
 A, passes through, the aperture E of the diaphragm CD, 
 placed midway between the Lenses, and is again refracted 
 
 by the posterior Lens B to a focus on the flat focussing 
 screen. 
 
 The advantages claimed for this Lens are absolute free- 
 dom from distortion flat field and a large angle of view. 
 These conditions it fulfils, but at the expense of a large 
 amount of spherical aberration of the positive character, 
 which interferes much with sharpness of definition, unless 
 the stop be reduced to an extremely small size. It is 
 therefore a slow-working Lens, and being also expensive, 
 although it possesses many excellent qualities, will not 
 probably come into general use in this country. 
 
 THE DOUBLE OR PORTRAIT COMBINATION OF ACTINIC 
 LENSES. 
 
 The Portrait combination of Lenses is constructed to 
 give a perfectly sharp image with an aperture of full size, 
 and thus to ensure rapidity of action by admitting a large 
 volume of light. The following diagram gives a sectional 
 view : 
 
LENSES. 195 
 
 In this combination the front Lens is an Actinic plano- 
 convex, with the convex side turned toward the object; 
 and the second, which takes up the rays and refracts them 
 further, is an actinic Biconvex Lens; there are therefore 
 in all, four distinct glasses concerned in forming the image, 
 which may appear at first to be an unnecessarily complex 
 arrangement. It is found however that by combining two 
 glasses of different curves, the aberrations of one correct 
 those of the other to a great extent, and the field is suffi- 
 ciently distinct without the employment of a diaphragm. 
 
 The manufacture of Portrait Lenses is a point of great 
 difficulty, care being required in grinding the glasses, in 
 order to avoid distortion of the image : hence the most 
 rapid Portrait Lenses, having large aperture and short 
 focus, are often useless unless purchased of a good maker. 
 
 The Portrait Lens however, although it produces a per- 
 fectly sharp image with the full aperture, cannot be made 
 to embrace objects at different distances from the Camera, 
 and the field is also more or less curved and consequently 
 indistinct at the edges. Stops therefore are occasionally 
 used, when length of exposure is not an object, in order to 
 sharpen out the refracted pencils arid give depth to the 
 focus. They may be required in taking groups in a good 
 light, or in using the Lens to copy pictures, where it is ne- 
 cessary to cover a plate of a full size quite up to the edge. 
 
 A question arises as to the proper place for a stop in a 
 Portrait Lens, whether it should be placed in front of or 
 between the glasses ? The former position is convenient, 
 but the latter is more correct in some instances, and by a 
 mechanical arrangement introduced by Mr. Waterhouse, a 
 diaphragm may be so placed, without removing the glasses. 
 A stop in front of the anterior Lens diminishes the avail- 
 able size of the field, but renders it comparatively flat, 
 whilst one placed between the glasses increases the size of 
 the field, although leaving a little of the original curvature. 
 , In both cases the brilliancy of the image is lessened and 
 the time of exposure prolonged, but not to so great an ex- 
 tent when the stop is central. 
 
196 
 
 THEORY OF PHOTOGRAPHY. 
 
 The inquiry is frequently made why the Portrait Lens 
 gives so concave a field, since it is well known that the op- 
 tician has it in his power to flatten the field very materially 
 by altering the curves of the constituent Lenses. The 
 answer is, that curvature of field is not esteemed a serious 
 objection in the case of portrait Lenses, because the sides 
 of the object are usually nearer to the Lens than the cen- 
 tre. And it has been found that in grinding the glasses 
 in such a way as to secure great flatness of field, the de- 
 fining power of the Lens is injured as regards the oblique 
 pencils, in consequence of the introduction of an error 
 known as " astigmation." 
 
 ON DISTORTION IN LENSES. 
 
 If the single Actinic or Aplanatic View Lens be em- 
 ployed to photograph a rectangular figure like the diagram 
 No. 1, the outside lines will be found on close inspection to 
 be slightly curved inwards at the extremities, thus pro- 
 ducing a barrel- shaped copy, as shown in No. 2. If in- 
 stead of these the Orthographic Lens be used, the result- 
 ing photograph will be of an hour-glass shape (No. 3), the 
 lines being bent outwards at the extremities. 
 
 No. l. 
 
 No. 3. 
 
 r-7 
 
 Observe that the central lines in each figure are per- 
 fectly straight, and that the distortion is most discernible 
 at the edges. The larger the copy in proportion to the 
 original, the greater the distortion. 
 
 Distortion by Lenses is easily understood if we bear in 
 mind the remarks made at page 183. The outside of the 
 Lens refracts light more powerfully than the centre ; and 
 hence, when the whole surface of the Lens is employed to 
 form the image, we have that confusion and overlapping 
 which constitutes spherical aberration. To remedy this, a 
 
LENSES. 
 
 197 
 
 stop is so placed that the centre and .the outside of the 
 Lens no longer co-operate, but work independently of each 
 other, and form separate portions of the image. The la- 
 teral parts of the picture are then confined to the edges of 
 the Lens, and as these edges refract more powerfully than 
 the centre, a bending of the rays inwards takes place, thus 
 constituting distortion. When the stop or diaphragm 
 however is placed at a distance behind the Lens instead 
 of in front of it, the rays cross each other after refrac- 
 tion, and thus the same bending which before produced 
 an inward distortion, now causes a distortion outwards.* 
 
 Fig. 1. 
 
 Fig. 2. 
 
 Observe in the first of the above figures, which has the 
 diaphragm in front, that the image of the barb of the 
 arrow is formed by the lower edge of the Lens. In the 
 second figure, the diaphragm being placed behind the Lens, 
 the barb is formed by the upper edge, and thus the dis- 
 tortion is in the opposite direction. In both figures those 
 rays only are inserted to which the diaphragm permits a 
 passage. 
 
 In the Orthographic combination, the back Lens being 
 smaller than the front acts as a diaphragm to it, and hence 
 the lines are bent outwards. The distortion of the Ortho- 
 graphic Lens however is not so great as that of the old 
 View Lens, because the diaphragm is nearer to the front 
 glass. The distortion of an ordinary View Lens would be 
 nearly removed if the diaphragm were pushed in until it 
 came in contact with the glass, but in that case the field 
 would be too much curved to give correct definition at the 
 margins of the picture. For this reason the Aplanatic 
 
 * Vide ' British Journal of Photography,' vol. vi. p. 222, observations by 
 the Editor. 
 
198 
 
 THEOEY OF PHOTOGEAPHT. 
 
 Lens, which admits of the stop being placed nearer the 
 front, is assumed to have less distortion than the single 
 Lens. 
 
 The ordinary compound Lens with a central stop gives a 
 near approach to a correct refraction of objects in their pre- 
 cise relative position, on the focussing screen. The front 
 Lens of such a combination having the diaphragm behind it 
 produces the hour-glass distortion, whilst the back Lens 
 with the diaphragm in front produces the barrel-shaped 
 distortion. The two distortions thus tend to neutralize 
 each other, and straighter lines are the result. But the 
 most perfect are unquestionably the Triplet and the Har- 
 rison Globe Lens, both which may be said to be mathema- 
 tically correct. 
 
 On Actinism. 
 
 The term " actinism " (Gr. CIKTIS, a ray or flash) has 
 been proposed as convenient to designate the property 
 possessed by light of producing chemical change ; the rays 
 to which the effect is especially due being known as actinic 
 rays. 
 
 If the pure Solar Spectrum formed by prismatic ana- 
 lysis in the manner represented at page 173 be allowed to 
 impinge upon a prepared sensitive surface of Iodide of 
 Silver, the latent image being subsequently developed by 
 a reducing agent, the effect produced will be something 
 similar to that represented in the following diagram : 
 
 Fig. 1. 
 
ACTINISM. 199 
 
 Fig. 1 shows tlie visible spectrum as it appears to the 
 eye ; the brightest part being in the yellow space, and the 
 light gradually shading off until it ceases to be seen. Fig. 2 
 represents the effect (visible after development) of throwing 
 the Spectrum upon Iodide of Silver. Observe that the 
 darkening characteristic of chemical action is most evident 
 in the upper spaces, where the light is feeble, and is alto- 
 gether absent at the point corresponding to the bright yel- 
 low spot of the visible spectrum. The actinic and luminous 
 spectra are therefore totally distinct from each other, and 
 the word " Photography," which signifies the process of 
 taking pictures by light, is in reality inaccurate. 
 
 To those who have not the opportunity of working with 
 the Solar Spectrum, the following experiments will be 
 useful in illustrating the photographic value of different 
 colours : 
 
 Experiment /.Take a sheet of sensitive paper prepared 
 with Chloride of Silver, and lay upon it strips of blue, 
 yellow, and red glass. On exposure to the sun's rays for 
 a few minutes, the part beneath the blue glass darkens 
 rapidly, whilst that covered by the red and yellow glass is 
 perfectly protected. This result is the more striking from 
 the extreme transparency of the yellow glass, giving the 
 idea that the Chloride would certainly be blackened first 
 at that point. On the other hand, the blue glass appears 
 very dark, and effectually conceals the tissue of the paper 
 from view. 
 
 Experiment II. Select a vase of flowers of different 
 shades of scarlet, blue, and yellow, and make a Photo- 
 graphic copy of them, by development, upon Iodide of 
 Silver. The blue tints will be found to act most violently 
 upon the sensitive compound, whilst the reds and yellows 
 will be scarcely visible ; were it not that it is difficult to 
 procure in nature pure and homogeneous tints, free from 
 admixture with other colours, they would make no impres- 
 sion whatever upon the plate. 
 
 In exemplifying further the importance of distinguish- 
 ing between visual and actinic rays of light, we may observe 
 
200 THEORY OF PHOTOGEAPHY. 
 
 that if the two were in all respects the same, Photography 
 must cease to exist as an Art. It would be impossible to 
 make use of the more sensitive chemical preparations, from 
 the difficulties which would attend the previous prepara- 
 tion and subsequent development of the plates. These 
 operations are now conducted in what is termed a dark 
 room ; but it is dark only in a Photographic sense ; being 
 illuminated by means of yellow light, which, whilst it en- 
 ables the operator easily to watch the progress of the work, 
 produces no injurious effect upon the sensitive surfaces. If 
 the windows of the room were glazed with Hue in place 
 of yellow glass, then it would be strictly a " dark room," 
 but one altogether unfitted for the purpose intended. 
 
 In selecting glass for the Photographic dark room, con- 
 siderable difference will be found in the actinic opacity of 
 the various shades of colour. A dark orange-yellow is 
 more impervious to chemical rays than a lighter canary- 
 yellow, whilst a blood-red colour possesses almost perfect 
 opacity, a Collodion sensitive plate being unaffected even 
 by the direct rays of the sun shining through the glass. 
 Eooms glazed with red glass, however, are extremely pain- 
 ful to the eyes, the retina3 being rendered after a time 
 insensitive to this colour, and all objects assuming the com- 
 plementary tint, green, on emerging into the daylight. 
 
 Another point connected with this subject of visual and 
 actinic rays is the extent to which the sensibility of the 
 Photographic compounds is influenced by atmospheric con- 
 ditions not visibly interfering with the brightness of the 
 light. It would be natural to suppose that those days on 
 which the sun's rays are very powerful would be the best 
 for rapid impression, but such is not by any means the 
 case. If the light is at all of a yellow cast, however bright 
 it may be, its actinic power will be small. 
 
 It will often be observed also in working towards the 
 evening, that a sudden . diminution of sensibility in the 
 plates begins to be perceptible at a time when but little 
 difference can be detected in the brilliancy of the light ; 
 the sun sinks behind a golden cloud, and all chemical action 
 is soon at an end. 
 
ACTINISM. 
 
 201 
 
 In the same manner is explained the difficulty of obtain- 
 ing Photographs in the glowing light of tropical climates ; 
 the superiority of the spring months to those of the mid- 
 summer ; of the morning sun to that of the afternoon, etc. 
 April and May are usually considered the best months for 
 rapid impression in this country ; but the light continues 
 good until the end of July. In August and September a 
 longer exposure of the plates will be required. 
 
 THE SUPEBIOE SENSIBILITY OF BROMIDE OF SILVER TO 
 COLOUEED LIGHT. 
 
 In copying the Solar Spectrum alternately upon a sur- 
 face of Iodide, and Bromide of Silver, we notice a difference 
 in the Photographic properties of these two salts. The 
 latter is affected more extensively, to a point lower in the 
 spectrum, than the former. In the case of the Iodide of 
 Silver, the action ceases in the Blue space ; but with the 
 Bromide it reaches to the Green. This is shown in the 
 following diagrams : 
 
 Fig. 1. Fig. 2. Fig 3. 
 
 Violet. 
 
 ludigo. 
 
202 THEORY OF PHOTOGRAPHY. 
 
 Fig. 1 represents the chemical spectrum on Bromide of 
 Silver ; fig. 2, the same upon Iodide of Silver ; and fig. 3, 
 the visible spectrum. 
 
 It might perhaps be supposed that the superior sensi- 
 bility of the Bromide of Silver to green rays of light would 
 render that salt useful to the Photographer in copying land- 
 scape scenery. It is doubtful however whether such is the 
 case, and for the following reasons : 
 
 A reference to the foregoing diagrams which are shaded 
 to represent nearly the relative intensity of the chemical 
 action exerted by the rays at different points of the spec- 
 trum will show that the maximum point of blackness is in 
 the indigo and violet space, the action being more feeble in 
 the blue space lower down ; there are also highly refran- 
 gible rays extending upwards beyond the visible colours, 
 and these invisible rays are actively concerned in the for- 
 mation of the image. Hence we see that the amount of 
 effect produced by a pure green, or even a light blue tint, 
 upon a surface of Bromide of Silver is very small as com- 
 pared with that of an indigo or violet ; and since radiations 
 of all kinds are present in copying natural objects, the 
 green tints have not time to act before the image is im- 
 pressed by the more refrangible rays. 
 
 Sir John Herschel proposed to render coloured light 
 available in Photography by separating the actinic rays 
 of high refrangibility, and working only with those which 
 correspond to the blue and green spaces in the spectrum. 
 This may be done by placing in front of the Camera a 
 vertical glass trough containing a solution of Sulphate of 
 Quinine. Professor Stokes has shown that this liquid 
 possesses remarkable properties. In transmitting rays of 
 light it modifies them so that they emerge of lower refran- 
 gibility, and incapable of producing the same actinic effect. 
 Solution of Sulphate of Quinine, although appearing nearly 
 colourless to the eye, is, if we may use the term, opaque to 
 all actinic rays higher than the blue-coloured space. The 
 proposition of Sir John Herschel above referred to was 
 to employ a bath of Sulphate of Quinine, and having elimi- 
 
ACTINISM. 203 
 
 nated the actinic rays of high refrangibility, to work upon 
 Bromide of Silver with those corresponding to the lower- 
 coloured spaces. In this way he conceived that a more na- 
 tural effect would be obtained. 
 
 If Photographic compounds should be discovered of 
 greater sensibility that any we at present possess, the use 
 of the Quinine Bath will perhaps be adopted ; but at pre- 
 sent we are compelled to trust to the superior intensity of 
 the invisible rays for the formation of the image. In work- 
 ing with artificial light, such as gas or camphine, the case 
 is different. Actinic rays of high refrangibility are com- 
 paratively wanting in gas-light, and the great bulk of the 
 Photographic rays lie within the limits of the visible spec- 
 trum : the action is therefore more energetic upon Bromide 
 than on Iodide of Silver. 
 
 The fact that the natural colours are not correctly repre- 
 sented in Photography, is often urged in depreciation of 
 the art ; " when lights are represented by shadows," it 
 is said, "how can a truthful picture be expected?" The 
 insensitiveness of Iodide of Silver to the green and yellow 
 colours occupying the lower portion of the spectrum would 
 indeed present an insuperable difficulty if the tints of Na- 
 ture were pure and homogeneous. Such however is not 
 the case. Even the most sombre colours are accompanied 
 by scattered rays of white light in quantity amply suffi- 
 cient to affect the sensitive film. 
 
 This is especially seen when the coloured body possesses 
 a good reflecting surface ; and hence some varieties of foli- 
 age, as for instance the Ivy, with its smooth and polished 
 leaf, are more easily photographed than others. So again 
 with regard to drapery in the department of portraiture 
 it is necessary to attend not only to the colour, but also 
 to the material of which it is composed. Silks and satins 
 are favourable, as reflecting much light, whilst velvets and 
 coarse stuffs of all kinds, if at all dark, produce very little 
 effect upon the sensitive film- 
 
204 THEOliY OF PHOTOGEAPHY. 
 
 THE ACTINIC PROPERTIES OF TRANSPARENT AND COLOUR- 
 LESS MEDIA. 
 
 The preceding observations with regard to the different 
 degrees of permeability of blue and yellow glass to the 
 Photographic or Actinic, and the visible rays of light, go 
 to prove how little we can judge, by the mere appearance 
 alone, of the capacity of a medium for transmitting che- 
 mical rays. But Dr. Miller has recently communicated to 
 the Royal Society a number of interesting researches, which 
 show that bodies absolutely colourless and transparent 
 differ greatly in their permeability to the Actinic rays, as 
 shown by the remarkable difference in the comparative 
 lengths of the spectra formed on Iodized Collodion, when 
 light from a certain fixed and uniform source was made 
 to pass, first through the colourless and transparent body 
 under examination, and then through a Quartz prism on 
 to the sensitive film. Quartz was employed for this pur- 
 pose, as it proved to be the true glass for chemical rays. 
 The following numbers express the comparative lengths 
 of the spectra obtained when the light had first passed 
 through the corresponding media : 
 
 Quartz 74 
 
 Ice and Water 74 
 
 Fluor Spar 74 
 
 Eock Salt 63 
 
 Oil of Turpentine 8 
 
 Phosphorus 
 
 Mint Glass 9 
 
 Window 16 
 
 Plate 15 
 
 Crown 10 
 
 These experiments go to show how much Photographic 
 power is lost by the transmission of light through the 
 glass of the Lenses, and how superior Quartz would be 
 if it could only be employed for such a purpose. Water, 
 which is used for the interior of Mr. Sutton's Lens, is seen 
 to be remarkably transparent to chemical rays ; but this 
 
THE STEREOSCOPE. 205 
 
 advantage is in great measure lost by the passage of the 
 light through the glass sides. 
 
 SECTION IV. 
 On Binocular Vision and the Stereoscope. 
 
 An object is said to be " stereoscopic " (errepeop, solid, 
 and 0-/C07T60), I see) when it stands out in relief, and gives 
 to the eye the impression of solidity. 
 
 This subject was first explained by Professor Wheat- 
 stone in a memoir on binocular vision, published in the 
 ' Philosophical Transactions ' for 1838 ; in which he shows 
 that solid bodies project different perspective figures upon 
 each retina, and that the illusion of solidity may be arti- 
 ficially produced by means of the " Stereoscope." 
 - The phenomena of binocular vision may be simply 
 stated as follows : If a cube, or a small box of an oblong 
 form, be placed at a short distance in front of the ob- 
 server, and viewed attentively with the right and left eye 
 separately and in succession, it will be found that the figure 
 perceived in the two cases is different ; that each eye sees 
 more of one side of the box, and less of the other ; and 
 that in neither instance is the effect exactly the same as 
 that given by the two eyes employed conjointly. 
 
 A silver pencil-case, or a pen-holder, may be used to 
 illustrate the same fact. It should be held at about six or 
 eight inches distant from the root of the nose, and quite 
 at right angles to the face, so that the length of the pencil 
 is concealed by the point. Then, whilst it remains fixed 
 in this position, the left and right eye are to be alternately 
 closed : in each case a portion of the opposite side of the 
 pencil will be rendered visible. 
 
 The diagrams on the next page exhibit the appearance 
 of a bust as seen by each eye successively. 
 
 Observe that the second figure, which represents the 
 impression received by the right eye, is more of a full face 
 than fig. 1, which, being viewed from a point removed a 
 little to the left, partakes of the character of a profile, 
 
206 THEOEY OF PHOTOGRAPHY, 
 
 The human eyes are placed about 2| inches, or from 
 that to 2f inches, asunder; hence it follows that, the points 
 of sight being separated, a dissimilar image of a solid 
 
 Fig. 1. Fig. 2. 
 
 object is formed on the retina of each eye. We do not 
 however see two images, but a single one, which is ste- 
 reoscopic. 
 
 In looking at a picture painted on a flat surface the case 
 is different ; each of the eyes, as before, receives an image, 
 but these images are in every respect similar; conse- 
 quently the impression of solidity is wanting. A single 
 picture, therefore, cannot be made to appear stereoscopic. 
 To convey the illusion, two pictures must be employed, the 
 one being a right and the other a left perspective projec- 
 tion of the object. The pictures must also be so arranged, 
 that each is presented to its own eye, and that the two ap- 
 pear to proceed from the same spot. 
 
 The reflecting stereoscope, employed to effect this, forms 
 luminous images of the binocular pictures, and throws these 
 images together, so that, on looking into the instrument, 
 only a single image is seen, in a central position. It 
 should, however, be understood, that no optical arrange- 
 ment of any kind is indispensably required, since it is quite 
 possible, with a little effort, to combine the two images by 
 
THE STEREOSCOPE. 207 
 
 the unaided organs of vision, Tlie following diagram will 
 make this obvious : 
 
 B 
 
 The circles A and B represent two wafers, which are 
 stuck on paper at a distance of about 1| inch from each 
 other. They are then viewed, either by squinting strongly, 
 until the right eye looks at the left wafer and the left eye 
 at the right wafer, or else by focussing the eyes for a dis- 
 tance beyond the wafers, which is easily effected by look- 
 ing at a point midway between, but giving the attention 
 to the indistinct wafers on each side, until the right eye 
 at length looks at the right and the left eye at the left 
 wafer ; in both cases the two wafers at first appear dou- 
 ble, four images being seen, of which two gradually ap- 
 proach until they coalesce, and the resulting image ap- 
 pears in the first case in front, and in the latter case behind 
 the paper. The mode in which coalescence is effected in 
 both cases may be thus explained. 
 
 The perception of a single object, while using two eyea 
 in ordinary vision, is due to the eyes being directed to the 
 object in such a manner, that the picture formed in each 
 eye falls on similar parts of the retina. If, while viewing 
 an object, one of the eyes be pressed by the finger the ob- 
 ject becomes double, from the position on the two retina 
 not corresponding. From the same cause, on first trying 
 to fix the two eyes on the two wafers, they appear double 
 until the optic axis of each eye (or a line of indefinite 
 length drawn through the centre of the pupil from the 
 central and most distinctly seeing part of the retina) is 
 directed fully to each wafer. When this is the case, a si- 
 milar picture falls on similar parts of each, and the two 
 objects are perceived by the mind as one. The other less 
 distinct images, one on each side of the principal one, are 
 due to each eye perceiving its neighbour's wafer, but on 
 points of the eyes that do not correspond. 
 
208 THEOEY OF PHOTOGRAPHY. 
 
 The cause of the combined image standing out in front 
 or behind the paper is due to the mind always referring 
 the place of an object to the point where the optic axes meet, 
 and when the two wafers are united by squinting, this point 
 is in front, and when by distant focussing, behind the paper. 
 This will be evident from the following diagrams : 
 
 Fig. 3. Fig. 4. 
 
 E and L represent the two eyes, and A and B the two wafers. 
 
 Fig. 3 represents union by squinting ; the optic axis of 
 each eye is directed to the opposite wafer, and the image 
 is referred by the mind to P in front, where the axes cross. 
 Fig. 4 shows the ordinary way of viewing with the Stereo- 
 scope, where each eye is directed to its own wafer. Here 
 the optic axes meet beyond the wafers at P, and the com- 
 pound image is perceived in the same place. 
 
 If, instead of two similar objects as wafers, the right 
 and left perspective views of an object, such as the follow- 
 ing of a hollow hexagonal cone, are viewed in a similar 
 
 Fig. 5. 
 
 manner, a sensation of depth and elevation is obtained as 
 
STEEEOSCOPIC VISION. 
 
 209 
 
 soon as the images coalesce, the different parts of the 
 cone appear at different distances from the observer, and, 
 what is more singular, the appearances are reversed by 
 the two modes of causing the pictures to coalesce. The 
 above diagram, viewed by focussing the eye for a distance, 
 or by the Stereoscope, represents a cone, with the small 
 hexagon distant ; but by squinting, the small end is made 
 to stand out towards the observer. As the former is al- 
 ways the way of viewing objects by the Stereoscope, it is 
 only necessary here to explain how the sensation of depth 
 and distance is produced by distant focussing, or viewing 
 each half with the eye of the same side, and for the sake 
 of simplicity only four points in each figure, A, C, D, B, 
 
 Pig. 6. 
 
210 THEORY OF PHOTOGRAPHY. 
 
 and a, c, d, b, need be taken for the purpose of illustrating 
 the mode in which the eye has to unite every correspond- 
 ing pair of points in turn. In the annexed diagram, these 
 letters represent the same points of the hexagons, as in 
 fig. 5. 
 
 The points A and a, which correspond in the two larger 
 hexagons, are united when the right optic axis R a is 
 directed to a, and the left LA to A ; but the resulting 
 single point appears to be at E, where these axes meet : 
 so, in like manner, B and b unite, and form one point, ap- 
 parently, at F. The points C c of the smaller hexagons 
 being more distant from each other than A from a, or B 
 from b, can only be united by a more distant prolongation 
 of the optic axes L c and E c to the point of union G, 
 which thus appears to be the position of these points of 
 the smaller hexagons, and so with regard to D and d', and 
 as these points are merely taken as samples of the whole, 
 the result is that the corresponding parts of the lesser 
 hexagons being more distant from each other than those 
 of the greater hexagons, the former, when united, appear 
 more distant than the latter, and the figure stands out 
 from the paper. 
 
 The Stereoscope is merely an instrument to cause a su- 
 perposition of the right and left views ; but this alone is 
 not sufficient to produce a stereoscopic effect, as the fol- 
 lowing diagram will show, in which the two halves of 
 
 Fig. 7. 
 
 fig. 5 are represented superposed, the two outer hexa- 
 gons coincide, but the similar parts of the two inner need 
 still to be brought into union in the manner represented 
 
THE STEEEOSCOPE. 211 
 
 n fig. 6, and the various distances at which the optic axes 
 meet appear to the mind to be the distances of the objects 
 represented. 
 
 In Mr. Wheatstone's reflecting Stereoscope, mirrors are 
 used. The principle of the instrument is as follows : 
 Objects placed in front of a mirror have their reflected 
 images apparently behind the mirror. By arranging two 
 mirrors at a certain inclination to each other, the images 
 of the double picture may be made to approach until they 
 coalesce, and the eye perceives a single one only. The 
 following diagram will explain this. 
 
 Fig. 8. 
 
 The rays proceeding from the star on either side pass 
 in the direction of the arrows, being thrown off from the 
 mirror (represented by the thick black line) and entering 
 the eyes at R and L. The reflected images appear to the 
 mind behind the mirror, uniting at the point A, where the 
 optic axes meet. 
 
 The reflecting Stereoscope is adapted principally for 
 viewing large pictures. It is a very perfect instrument, 
 and admits of a variety of adjustments, by which the ap- 
 parent size and distance of the Stereoscopic image may be 
 varied almost at pleasure. 
 
 The refracting Stereoscope is a more portable form of 
 apparatus. A sectional view of the common form is given 
 in the diagram (fig. 9). 
 
 The brass tubes to which the eyes of the observer are 
 
212 
 
 THEOEY OF PHOTOGRAPHY. 
 
 applied contain each a semi-lens, formed by dividing a 
 common lens through the centre and cutting each half into 
 
 Fig. 9. 
 
 a circular form (fig. 10). The half-lens viewed in section 
 (fig. 11) is therefore of a prismatic shape, and when placed 
 with its sharp edge as in the diagram (fig. 9), alters the 
 direction of the rays of light proceeding from the picture, 
 bending them outwards or away from the centre, so that 
 
 Fig. 10. 
 
 Fig. 11. 
 
 an accordance with the law previously mentioned, the mind 
 refers them to the direction of the prolonged optic axes, 
 represented for one pair of points by dotted lines (fig. 9), 
 
STEREOGRAPHY. 213 
 
 and the corresponding parts appear to be at the various 
 distances, at which the optic axes, directed to them, meet. 
 In the instrument as it is often sold, one of the lenses is 
 made movable, and by turning it round with the finger and 
 thumb it will be seen that the positions of the images may 
 be shifted at pleasure. 
 
 Stereoscopes are now often mounted with whole lenses 
 in place of the semi-lenses above described. In that case 
 the images coalesce by an involuntary action of the eyes 
 of the observer. If indeed the whole lenses were set wider 
 apart than 2| inches, it might be conjectured that vision 
 would take place principally through the inside edges, and 
 that the same effect as that of a semi-lens would be pro- 
 duced. A single experiment, however, will suffice to show 
 that even when the centres of the lenses are exactly op- 
 posite to the eyes, the two pictures combine without any 
 effort. 
 
 Rules for talcing Binocular Photographs. In viewing 
 very distant objects with the eyes, the images formed on 
 the retinae are not sufficiently dissimilar to produce a very 
 Stereoscopic effect ; hence it is often required, in taking 
 binocular pictures, to separate the Cameras more widely 
 than the two eyes are separated, in order to give a suffi- 
 cient appearance of relief. Mr. Wheatstone's original 
 directions were, to allow about one foot of separation for 
 each twenty-five feet of distance ; but considerable latitude 
 may be permitted. 
 
 If the Cameras are not placed far enough apart, the di- 
 mensions of the stereoscopic image from before backwards 
 will be too small, statues looking like bas-reliefs, and the 
 circular trunks of trees appearing oval, with the long dia- 
 meter transverse. On the other hand, when the separation 
 is too wide, the reverse obtains, objects for instance 
 which are square, assuming an oblong shape pointing 
 towards the observer. 
 
 To understand the cause of this, it is necessary to refer to 
 fig. 6, where it was shown that those corresponding parts in 
 the two halves of a stereoscopic picture which are most 
 
214 THEORY OF PHOTOGBAPHY. 
 
 widely separated, appear most distant when united in the 
 Stereoscope, and hence the greater the separation, the 
 greater is the apparent distance of the object ; but the 
 effect of separating the Cameras too widely is to produce 
 too great separation of the distant parts of an object, as a 
 little reflection on fig. 5 will show ; it is evident that if 
 the eyes, or the Cameras which represent them, were sepa- 
 rated further in viewing the hollow cone, more of the sides 
 D B and a c would be seen, and consequently the smaller 
 hexagons would be placed more apart, and so when the 
 two images were united in the Stereoscope, the cone would 
 appear longer. 
 
 This effect is most observable when the picture embraces 
 a variety of objects, situated in different planes. In the 
 case of views which are quite distant, no near objects being 
 admitted, the Cameras may be placed with especial refe- 
 rence to them, even as far as twelve feet apart, without 
 producing distortion. 
 
 It is sometimes observable, in looking at Stereoscopic 
 pictures, that they convey an erroneous impression of the 
 real size and distance of the object. For instance, in 
 using the large reflecting Stereoscope, if, when the adjust- 
 ments have been made and the images properly united, 
 the two pictures be moved slowly forward, the eyes re- 
 maining fixed upon the mirrors, the Stereoscopic image 
 will gradually change its character, the various objects it 
 embraces appearing to become diminished in size, and ap- 
 proaching near to the observer ; whilst if the pictures be 
 pushed backwards, the image will enlarge and recede to a 
 distance. So, again, if an ordinary slide for the lenticular 
 Stereoscope be divided in the centre, and looking into 
 the instrument until the images coalesce the two halves 
 be slowly separated from each other, the solid picture will 
 seem to become larger and to recede from the eye. 
 
 It is easy to understand the cause of this. When the 
 pictures in the reflecting Stereoscope are moved forwards, 
 the convergence of the optic axes is increased : the image 
 therefore appears nearer, in accordance with the last-men- 
 
STEBEOGEAPHY. 215 
 
 tioned law. But to convey the impression of proximity 
 to an object is equivalent to an apparent diminution in its 
 size, for we judge of the dimensions of a body very much 
 in relation to its supposed distance. Of two figures in 
 bronze or marble, for instance, appearing of the same 
 height, one known to be a hundred yards off might be 
 considered colossal, whilst the other, obviously near at 
 hand, would be viewed as a statuette. 
 
 These facts, with others not mentioned, are of great inter- 
 est and importance, but their further consideration does 
 not fall within the bounds originally prescribed for this 
 work. The practical details of Stereoscopic Photography 
 have been arranged in a distinct Section, and will be found 
 included in the Third Part.* 
 
 * For a full and detailed explanation of the Stereoscopic phenomena, see 
 ' Brewster on the Stereoscope,' published by Murray. 
 
216 
 
 HAPTER VI. 
 
 ON COLLODION AND THE NITBATE BATH. 
 
 IN the preceding part of this Work the physical and che- 
 mical properties of Chloride and Iodide of Sil\ r er have been 
 described, together with the changes which they experience 
 by the action of Light. Nothing however has been said of 
 the surface used to support the Iodide of Silver, and to ex- 
 pose it in a finely-divided state to the influence of the ac- 
 tinic radiation. This omission will now be supplied. 
 
 The sensibility of Iodide of Silver upon Collodion is 
 greatly superior to that of the same salt employed in con- 
 junction with any other vehicle at present known. The 
 causes of this superior sensitiveness, as far as ascertained, 
 may be referred to the very finely-divided state of the Io- 
 dide of Silver, and to the Collodion film exerting little or 
 no chemical action on Salts of Silver. It must however 
 be allowed that there are many points affecting the sensi- 
 tiveness of Iodide of Silver, both mechanical and chemical, 
 of the nature of which we are ignorant. 
 
 SECTION I. 
 
 Collodion. 
 
 Collodion (so named from the Greek word KoXAaw, to 
 stick) is a glutinous, transparent fluid, prepared by dis- 
 solving Pyroxyline in a mixture of Ether and Alcohol. 
 It was originally used for surgical purposes only, being 
 
PTEOXYLINE. 217 
 
 smeared over wounds and raw surfaces, to preserve them 
 from contact with the air by the tough film which it leaves 
 on evaporation. 
 
 The constituents of Photographic Collodion are three, 
 viz. the Pyroxyline ; the fluids used to dissolve it ; and the 
 iodizing compounds. Each of these will be treated in suc- 
 cession. 
 
 CHEMISTRY OF PYROXYLINE. 
 
 Pyroxyline is Cotton or Linen which has been altered in 
 composition and properties by treatment with certain acids. 
 
 Cotton and Linen, when examined by the microscope, 
 are seen to consist of fibres, and these fibres have a constant 
 composition, containing three elementary bodies, Carbon, 
 Hydrogen, and Oxygen, united in fixed proportions. To 
 the combination the term Lignine or Cellulose* has been 
 applied. 
 
 Cellullose is a definite chemical compound, in the same 
 sense as Starch or Sugar, and when treated with various 
 reagents, exhibits properties peculiar to itself. It is inso- 
 luble in most liquids, such as Water, Alcohol, Ether, etc., 
 and also in dilute acids ; but when acted upon by Nitric 
 Acid of a certain strength it dissolves. 
 
 Preparation of Pyroxyline. If, instead of treating Cot- 
 ton with Nitric Acid, a mixture of Nitric and Sulphuric 
 Acid in certain proportions be used, the effect is peculiar. 
 The fibres contract slightly, but undergo no other visible 
 change. On experimenting further, however, the proper- 
 ties of the cotton are found to have been affected. Its 
 weight has increased by more than one-half ; it has be- 
 come soluble in various liquids, such as Acetic Ether, 
 Ether and Alcohol, etc., and on contact with flame it no 
 longer burns quietly, but explodes with greater or less 
 violence. 
 
 To produce the peculiar change by which Cotton is con- 
 verted into Pyroxyline, both Nitric and Sulphuric Acid 
 
 * Lignine and Cellulose are not precisely the same. The latter is the ma- 
 terial composing the cell-wall; the former, the contained matter of the cell. 
 
218 THEOET OF PHOTOGRAPHY. 
 
 are employed ; but the former plays the most important 
 part. On analysing Pyroxyline, an Oxide of Nitrogen, 
 NO 4 , is detected in it, but no Sulphuric Acid. One prin- 
 cipal use of the latter substance is to prevent the Nitric 
 Acid from dissolving the Pyroxyline, and to produce a 
 higher degree of concentration by absorbing a portion of 
 the water. 
 
 The property possessed by Sulphuric Acid of removing 
 water from other bodies, may be illustrated by filling a 
 small vessel of any kind to about two-thirds with Oil of 
 Yitriol, and setting it aside for a few days ; at the end of 
 that time, and especially if the atmosphere be damp, it will 
 have absorbed sufficient moisture to cause it to flow over 
 the edge. 
 
 Now even the strongest reagents employed in chemistry 
 contain, almost invariably, water in greater or lesser quan- 
 tity. The strongest Nitric Acid always contains an atom 
 of water, but the commercial Acid often has much more. 
 The effect therefore of mixing strong Oil of Vitriol with 
 commercial Nitric Acid is to remove water in proportion 
 to the amount used, and to produce a liquid containing 
 Nitric Acid in a high state of concentration, and Sul- 
 phuric Acid more or less diluted. This liquid is the Nitro- 
 Sulphuric Acid employed in the preparation of Pyroxyline. 
 
 Commercial Nitric Acid is so variable in composition, 
 that it is often found desirable to substitute Nitrate of 
 Potash for it in making a Nitro- Sulphuric Acid of a de- 
 finite strength. When Strong Sulphuric Acid is poured 
 upon Nitrate of Potash in a state of fine powder, a solu- 
 tion is obtained containing Bisulphate of Potash and free 
 Nitric Acid. The reaction may be thus represented : 
 
 Composition of Pyroxyline. This substance, as was 
 before stated, is Cotton, or some other variety of Cellulose, 
 in which a portion of the Hydrogen has been replaced by 
 NO 4 , Peroxide of Nitrogen. In order to understand in 
 what state this body is combined with cotton fibre, it is 
 necessary to understand the nature of substitution. 
 
PYBOXYLINE. 219 
 
 By the careful study of the action of Chlorine, and of 
 Nitric Acid, upon various organic substances, a remark- 
 able series of compounds has been discovered, containing 
 a portion of Chlorine or of Peroxide of Nitrogen in the 
 place of Hydrogen. The peculiarity of these substances 
 is, that they strongly resemble the originals in their phy- 
 sical, and often in their chemical properties. It might 
 have been supposed that agents of such active chemical 
 affinities as Chlorine and Oxide of Nitrogen would, by 
 their mere presence in a body, produce a marked effect ; 
 yet it is not so in the case before us. The primitive type 
 or constitution of the substance modified remains the same, 
 even the crystalline form being often unaffected. It seems 
 as if the body by which the Hydrogen is displaced had 
 stepped in quietly and taken up its position in the frame- 
 work of the whole without disturbance. Many compounds 
 of this kind are known ; they are termed by chemists 
 " substitution compounds." The law invariably observed 
 is, that the substitution takes place in equal atoms : a single 
 atom of Chlorine, for instance, displaces one of Hydro- 
 gen ; two of Chlorine displace two of Hydrogen, and so 
 on, until, in some cases, the whole of the latter element is 
 separated. 
 
 In illustration of these remarks, take the following in- 
 stances : Acetic Acid contains Carbon, Hydrogen, and 
 Oxygen ; by the action of Chlorine the Hydrogen may be 
 removed in the form of Hydrochloric Acid and an equal 
 number of atoms of Chlorine be substituted. In this way 
 a new compound is formed, termed Chloracetic Acid, re- 
 sembling in many important particulars the Acetic Acid 
 itself. Notice particularly that the peculiar properties 
 characteristic of Chlorine are completely masked in the 
 substitution body, and no indication of its presence is ob- 
 tained by the usual tests. A soluble Chloride gives with 
 Nitrate of Silver a white precipitate of Chloride of Silver, 
 unaffected by Acids, but the Chloracetic Acid does not ; 
 hence it is plain that the Chlorine exists in a peculiar and 
 intimate state of combination different from what is usual. 
 
220 THEOEY OF PHOTOGBAPHY. 
 
 Pyroxyline affords another illustration of substitution. 
 Omitting, for the sake of simplicity, the number of atoms 
 concerned in the change, the action of concentrated Nitric 
 Acid upon ligneous fibre may be thus explained : 
 
 f Carbon 
 Cotton o, |^rogen +NitrieAc;d 
 
 I Oxygen 
 
 equals 
 f Carbon 
 
 Pyroxyline o, K&trogen + 
 
 L Oxygen 
 Or in symbols : 
 
 By a reference to the formula, it will be seen that the 
 fifth atom of Oxygen contained in the Nitric Acid takes 
 one of Hydrogen, and forms an atom of Water ; the N O 4 
 then steps in, to fill the gap which the atom of Hydrogen 
 has left. All this is done with so little disturbance that 
 even the fibrous structure of the Cotton remains as before. 
 
 Various forms of Pyroxyline. There are varieties of 
 Pyroxyline, depending upon the strength of the Nitro- 
 Sulphuric Acid employed in its preparation. The first of 
 these is the most explosive Gun Cotton. This kind of 
 Pyroxyline contains the largest amount of Peroxide of 
 Nitrogen ; it dissolves only in Acetic Ether, and is left on 
 evaporation as a white powder. It is produced by the 
 strongest Nitro- Sulphuric Acid which can be made. 
 
 The Pyroxyline used by Photographers is formed by 
 a Nitro- Sulphuric Acid slightly weaker than the last, and 
 contains a smaller percentage of Peroxide of Nitrogen. 
 It is less explosive than Gun Cotton, and dissolves not 
 only in Acetic Ether, but also in a mixture of Ether and 
 Alcohol. 
 
 A form of Pyroxyline, not used in Photography, re- 
 sembles Xyloidine, the substance produced by treating 
 starch with Nitric Acid. It is lower on the scale than 
 
PYROXYLINE. 221 
 
 the others, and contains less of Peroxide of Nitrogen. It 
 dissolves in Acetic Ether, in Ether and Alcohol, and in 
 Acetic Acid. The ethereal solution leaves, on evapora- 
 tion, an opaque film, which is highly combustible, but not 
 explosive. 
 
 By bearing in mind the properties of these compounds, 
 which will be found more carefully described in the Vo- 
 cabulary, Part I., Art. Pyroxyline, many of the anoma- 
 lies complained of in the manufacture of Pyroxyline dis- 
 appear. If the Nitro- Sulphuric Acid employed is too 
 strong, the product will be insoluble in Ether ; whilst if 
 it be too weak, the fibres will be gelatinized and partly dis- 
 solved by the Acids in the form of Xyloidine. 
 
 Parchmentizing action of Oil of Vitriol upon Cellu- 
 lose. Photographic Pyroxyline is found to vary in its phy- 
 sical properties in a manner which can only be explained 
 by studying a peculiar effect exercised by Sulphuric Acid 
 of a certain strength upon cotton fibre. 
 
 To demonstrate this action, ordinary blotting-paper is 
 floated for a few seconds upon Oil of Vitriol previously 
 diluted with half its bulk of water and allowed to cool : it 
 is then abundantly washed and hung up to dry. The 
 Sulphuric Acid must not be too strong, or it will char the 
 paper, nor too weak, lest it cause partial conversion into 
 dextrine and consequent solution. If it be of the right 
 strength (sp. gr. 1*68), the paper shrinks considerably, and 
 acquires the toughness and general appearance of parch- 
 ment. It will support a heavy weight without tearing, 
 and is so close in texture as to be almost impermeable by 
 liquids. 
 
 Its chemical properties are the same as those of ordinary 
 paper, and analysis fails to detect any difference in compo- 
 sition. When acted on by Nitric Acid this material is 
 converted into Pyroxyline in the usual way, the Collodion 
 film possessing the characteristic toughness and close tex- 
 ture of the artificial parchment itself. 
 
 With these facts before us we can readily understand 
 that a mixture like the Nitro- Sulphuric Acid used in mak- 
 
222 THEORY OF PHOTOGRAPHY. 
 
 ing Pyroxyline, at the same time that it forms a substitu- 
 tion compound containing Peroxide of Nitrogen, must also 
 tend to parchmentize the fibre of the cotton to a greater 
 or less extent. This is indeed the case, and by altering 
 the relative proportions of the two acids in the mixture, 
 and increasing or diminishing the quantity of Water, the 
 action can be regulated at will, and modifications produced 
 which are highly useful in the manufacture of Collodion. 
 We shall also find, as we proceed, that the temperature of 
 the Nitro- Sulphuric Acid at the time of immersing the 
 cotton affects the action of the acids, and consequently the 
 properties of the resulting Pyroxyline. 
 
 THE SOLUTION OF PYROXYLINE IN ETHER AND ALCOHOL, 
 OR "COLLODION." 
 
 Pyroxyline is freely soluble in Acetic Ether, but this 
 liquid is not adapted for the purpose required, inasmuch 
 as on evaporation it leaves the Pyroxyline in the form of 
 a white powder, and not as a transparent layer. 
 
 The rectified Ether of commerce has been found to 
 answer better than any other substance as a solvent for 
 Pyroxyline. 
 
 If the sp. gravity be about "750, Ether contains inva- 
 riably a small proportion of Alcohol, which appears to be 
 necessary ; the solution not taking place with absolutely 
 pure Ether. The Pyroxyline, if properly prepared, begins 
 almost immediately to gelatinize by the action of the Ether, 
 and is soon completely dissolved. In this state it forms a 
 slimy solution, which, when poured out on a glass plate, 
 dries up into a horny transparent layer. 
 
 In preparing Collodion for Photographic purposes, we 
 find that its physical properties are liable to considerable 
 variation. Sometimes it appears very thin and fluid, flow- 
 ing on the glass almost like water, whilst at others it is 
 thick and glutinous. These differences relate not only to 
 the quality of the Pyroxyline itself as shown above, but 
 also to the proportions of the Solvents employed. 
 
 When the Ether is in large excess, the film is inclined 
 
COLLODION. 223 
 
 to be strong and tough, so that it can often be raised by 
 one corner and lifted completely off the plate without tear- 
 ing. It is also very contractile, and a portion of the Col- 
 lodion poured on the hand draws together and puckers the 
 skin as it dries. 
 
 These properties, produced by Ether in large proportion, 
 disappear entirely on the addition of more Alcohol. The 
 transparent layer is now soft and easily torn, possessing 
 but little coherency ; and this is still more manifest when 
 the Alcohol contains water. 
 
 To appreciate the action of water in Collodion, add a 
 few drops purposely ; the Pyroxyline will be precipitated 
 in flakes to the bottom of the bottle. On agitation the 
 precipitate redissolves, but the Collodion is rendered glu- 
 tinous, and the film left on evaporation is no longer homo- 
 geneous and transparent, but semi-opaque, reticulated or 
 honeycombed, and so rotten, that a stream of water pro* 
 jected upon the plate, washes it away. 
 
 Commercial Spirit of Wine often contains water in 
 excess, and when such is the case, the proportion of 
 Ether in the Collodion must be very much increased. 
 Collodion made in this way is often highly fluid and struc- 
 tureless at first, adhering to the glass with some tenacity, 
 and having a short texture ; but it tends to become rotten 
 when used to coat many plates successively, the water on 
 account of its lesser volatility accumulating in injurious 
 quantity in the last portions. 
 
 CHANGES IN COLLODION AFTEE IODIZING. 
 
 Collodion iodized with the Iodide of Potassium, Am- 
 monium, Magnesium, or Zinc, soon assumes a yellow tint, 
 which in the course of a few days or weeks, deepens to a 
 full brown. This gradual coloration is due to a develop- 
 ment of free Iodine, by a peculiar action of the Ether, 
 which is a fluid very prone to decomposition. 
 
 Ether may, with proper precautions, be preserved for a 
 long time in a pure state, but on exposure to the joint 
 
224 THEORY OF PHOTOGEAPHY. 
 
 action of air and light it undergoes a slow process of oxi- 
 dation, attended with formation of Acetic Ether and ozone, 
 or Oxygen in an allotropic and active condition, which ap- 
 pears to be dissolved in the Ether. Iodide of Potassium 
 or Ammonium is decomposed by Ether in this state, the 
 ozone liberating Iodine, which dissolves and tinges the 
 liquid yellow. 
 
 Ether containing Pyroxyline dissolved, tends to assume 
 the above state more quickly than pure Ether, and espe- 
 cially so when the Pyroxyline has undergone decomposi- 
 tion, and set free a portion of the Peroxide of Nitrogen. 
 It is therefore of the utmost importance that plain Collo- 
 dion should be kept in a cool place, and especially that it 
 should never be exposed to light ; even ordinary daylight 
 will, in the course of ten days or a fortnight, confer this 
 property of striking an immediate colour on the addition 
 of the iodizer. 
 
 The straw-yellow colour which usually follows in the 
 course of an hour or so after iodizing Collodion, is due to 
 the ozonization of the Ether above described, but the 
 further increase of that colour on keeping the Collodion 
 for several weeks is more dependent upon the Pyroxyline; 
 for if the Pyroxyline be omitted from the solution, and 
 the Iodide added to the Ether and Alcohol simply, then 
 the development of free Iodine proceeds very slowly. The 
 rate of change under the decomposing effect of Pyroxyline 
 is mainly affected by the stability of the particular Iodide 
 which is employed. Iodide of Ammonium is the least 
 stable of the iodides, and Iodide of Cadmium the most so ; 
 Iodide of Potassium being intermediate. Collodion iodized 
 with Iodide of Cadmium usually remains nearly colourless 
 to the last drop, if made with pure Ether, and kept in a 
 cool and dark place. 
 
 As the presence of free Iodine in Collodion affects its 
 photographic properties, it may sometimes be necessary to 
 remove it. This is done by inserting a strip of Silver 
 foil, which decolorizes the liquid, by forming Iodide of 
 Silver, soluble in the excess of Alkaline Iodide. Metallic 
 
PYBOXYLINE. 225 
 
 Cadmium and Metallic Zinc have the same effect. Metallic 
 Mercury also decolorizes brown Collodion, but the Iodide 
 of Mercury formed has the effect of lessening the sensi- 
 tiveness of the film to light. 
 
 FUBTHEE OBSERVATIONS ON THE MANUFACTTJBE OF 
 PHOTOGBAPHIC COLLODION. 
 
 In order to conduct the manufacture of Collodion with suc- 
 cess, a number of minor points must be considered, which could 
 not conveniently be included in the general description given 
 above. These relate not only to the Pyroxyline, but also to the 
 Solvents, and the iodizing compounds : they will now be ex- 
 plained, type of a smaller size being employed for the sake of dis- 
 tinction. 
 
 The most important point is the composition of the Nitro- 
 Sulphuric Acid, in which the Pyroxyline is prepared ; for it will 
 be found that the slightest variation in the quantity of water ; 
 in the relative proportions of the two constituent acids ; or in 
 the temperature at the time of putting in the Cotton, will af- 
 fect the result. It will be advisable therefore to examine these 
 matters separately, and in addition to speak of the condition of 
 the fibre itself, which is important both chemically and mechani- 
 cally. 
 
 a. Effect of varying the proportion of Water in the Nitro- Sul- 
 phuric Acid. When many separate quantities of Pyroxyline are 
 made in the same Nitro-Sulphuric Acid, the mixture becomes 
 gradually weaker by the abstraction of the elements of water from 
 the fibre, and each successive portion of Pyroxyline is less in the 
 state of explosive Gun Cotton, and more in that of Xyloidine, 
 than the one which preceded it. 
 
 There is considerable difference in the physical properties of 
 Collodions made from strong and weak samples of Pyroxyline 
 prepared as above described. In the first instance, when the pro- 
 duct is only one step removed from Gun Cotton, the Collodion 
 pours slowly upon the glass, and tends to thicken at the edges. 
 The film is not very adherent, and will sometimes split away on 
 drying. In attempting to coat a large plate, a wavy appearance, 
 often known as woolliness of the film, is seen at the lower corner. 
 With Pyroxyline made in weak acids, on the other hand, the 
 Collodion is more limpid and free from structure, the film of 
 
 Q 
 
226 THEORY OF PHOTOGRAPHY. 
 
 Iodide when formed in the Bath presenting an even appearance 
 throughout. 
 
 When the amount of water in the Nitro- Sulphuric Acid is 
 carried still further, one of two things happens ; either the Cotton 
 instantly dissolves in the acid, or it is more or less disintegrated 
 without actually dissolving. The product in the latter case is not 
 entirely soluble in Ether and Alcohol, but leaves behind a re- 
 sidue consisting in part of unaltered Cotton fibre. The portion 
 which is taken up by the Ethereal Solvent, may be considered 
 as nearly corresponding to the lowest substitution compound, 
 D, page 91, and very commonly the solution of this compound 
 leaves behind an opaque film on evaporation. 
 
 Here it may be proper to explain the reason why a weak Ni- 
 tro-Sulphuric Acid sometimes dissolves the Cotton, and some- 
 times merely disintegrates it. Supposing the temperature, and 
 the Cotton used, to be the same, yet the solvent action of the 
 acids will vary with the relative proportions of Sulphuric and 
 Nitric Acid present, even although the amount of water be cor- 
 rectly adjusted to that of the Sulphuric Acid. Thus a warm diluted 
 Nitric Acid used alone acts upon the Cotton rapidly and dissolves 
 it ; but if a very small portion of diluted Sulphuric Acid be added, 
 the tendency to dissolve the Cotton is lessened, or if the Cotton 
 be already dissolved the diluted Sulphuric Acid throws it down 
 again. If however the proportion of the diluted Sulphuric Acid 
 be very considerably increased, until it reaches to as much as 
 three times the bulk of the diluted Nitric Acid, then the ten- 
 dency to dissolve the Cotton becomes greater than in the case of 
 a mixture containing equal volumes of the two diluted acids. 
 
 b. A peculiar action of the Oil of Vitriol in the Process. 
 To demonstrate this, take a sheet of ordinary bibulous paper, 
 and having cut it into separate pieces, float them ttpon Sulphuric 
 Acid diluted with half its bulk of water and cooled : allow five 
 seconds for the first, ten for the second, twenty for the third, and 
 so on, until the last piece is gelatinized and dissolved ; then re- 
 move the Sulphuric Acid carefully by washing, and convert them 
 into Pyroxyline. A marked difference will be perceptible between 
 the samples of Collodion so obtained. The first noticeable ef- 
 fect of the previous parchmentizing, will be an increased fluidity 
 and freedom from structural lines. The Collodion when poured 
 upon a glass sets very rapidly, and with such firmness that the 
 
PYEOXYLTNE. 227 
 
 finger may be rubbed backwards and forwards without disturbing 
 it. The film on lifting from the Bath soon becomes partially sur- 
 face-dry, and repels developers or fixing agents ; when washed 
 with water and dried, it forms a dense and highly- varnished surface 
 nearly impenetrable by liquids. The moist film after develop- 
 ment with Pyrogallic Acid and fixing, appears unusually tough, 
 and will bear pumping on without injury. It is also very con- 
 tractile, and tends to draw itself away from the edges of the glass. 
 When pushed aside it can be pulled back again like the finger of 
 a glove. The fixing agent never removes any portion of the 
 image from this Collodion because the Iodide of Silver is in the 
 film and not only upon its surface. 
 
 The above-mentioned properties, imparted by the preliminary 
 action of Oil of Vitriol, are not seen to an equal extent in all the 
 pieces of paper used in the experiments, but are more decidedly 
 evident in proportion to the time during which the acid was al- 
 lowed to act. If however the action of the acid be carried to 
 that point at which the paper begins to soften and become semi- 
 gelatinous, then the resulting Collodion will be entirely different, 
 the film being rotten and powdery. This we shall presently show 
 to be due to a disintegration by the Nitric Acid contained in 
 the Nitro-Sulphuric Acid ; which disintegration is exhibited more 
 strongly when the fibre is previously changed, nearly into dextrine, 
 by the prolonged action of Sulphuric Acid. 
 
 In addition to these effects produced by parchmentizing the 
 fibre in different degrees before converting it into Pyroxyline, 
 there are others which deserve notice. The solubility of the pro- 
 duct in Ether and Alcohol is much increased, so that the exact 
 amount of water in the Nitro-Sulphuric Acid becomes a point of 
 less importance as affecting solubility, and an acid strong enough 
 to make ordinary paper explosive and insoluble, answers perfectly 
 in the case of paper previously parchmentized. 
 
 To secure the parchmentizing action in the ordinary process of 
 making Pyroxyline, the amount of diluted Sulphuric Acid pre- 
 sent in the Nitro-Sulphuric Acid should be considerably greater 
 than that of the Nitric Acid. The Cotton then shrinks into a 
 small compass, and the resulting Collodion will be tough and 
 strong. The action of the Oil of Vitriol in the process evidently 
 precedes that of the Nitric Acid, since we find that cotton fibre 
 which has once been converted into Pyroxyline is no longer a 
 
228 THEOEY OF PHOTOGRAPHY. 
 
 fected by diluted Sulphuric Acid, even when immersed for several 
 hours. 
 
 A peculiar action of the Nitric Add in the process of making 
 Pyroxyline. The proper action of Nitric Acid in this process is, 
 as before shown, to communicate Peroxide of Nitrogen to the 
 fibre, and so to convert the Cellulose into Pyroxyline ; the stronger 
 the Nitric Acid, the greater the amount of peroxide imparted. 
 We notice, however, another action of Nitric Acid upon Pyroxy- 
 line, by which the properties of the latter are much altered. This 
 second or modifying action, if we may so term it, is exerted not so 
 much by a concentrated, as by a diluted Nitric Acid, and more by 
 a hot Nitric Acid than by the same acid employed cold. To ex- 
 hibit this secondary action of Nitric Acid, take ordinary Pyroxy- 
 line made by the formulae given in the Third Part of this Work, 
 and dip it for an instant in Nitric Acid of sp. gr. T45, mixed 
 with a third of its bulk of Sulphuric Acid (to prevent it from dis- 
 solving the cotton) and heated to about 140 or 150 Fahrenheit.* 
 The following are the main characteristics of Collodion pre- 
 pared from Pyroxyline so treated : It does not set rapidly 
 upon the glass like the parchmentized Collodion last described, 
 but remains liquid for a minute, or even longer, after which it 
 rubs under the finger in a soapy manner, instead of bearing 
 friction without injury. After passing through the bath the 
 film is creamy, retains its surface-moisture for an unusually long 
 time, and on washing with water and drying presents a porous 
 surface, quite lustreless, and without any varnished appearance. 
 This film, when wet, will not bear pumping on, but is so rotten 
 that a small stream of water allowed to impinge upon it makes 
 a round hole. The Collodion is non- contractile, and so far from 
 admitting of being pushed backwards and forwards after fixing 
 with the Hyposulphite, it at once breaks away in short pieces 
 under such treatment. Fixing agents will often remove the image 
 from a Collodion of this kind, because the Iodide of Silver is not 
 imprisoned by the Pyroxyline, but lies loosely upon its surface. 
 
 * In addition to this modified Pyroxyline produced by hot Nitric Acid 
 mixed with a little Sulphuric Acid, a remarkable change of properties may 
 be produced by the pure Nitric Acid of V45 employed cold, and without any 
 admixture of Sulphuric Acid. The Pyroxyline gradually becomes opaque, 
 and loses its solubility in Ether and Alcohol ; eventually it dissolves in the 
 cold Nitric Acid without any evolution of gas, and if water be then added, 
 opaque white flakes are thrown down, whu-h, when treated with Ether and 
 Alcohol, simply swell up without passing into solution. 
 
PYEOXTLINE. 229 
 
 Here we repeat the remark before made when speaking of the 
 action of Sulphuric Acid, viz. that the secondary or disintegrating 
 action of Nitric Acid is always greater when the parchmentizing 
 effect has been previously produced ; and we may also anticipate 
 an observation to be made a few pages in advance, by adding that 
 one kind of cellulose, such as flax, may be disintegrated by the 
 Nitric Acid more readily than another, viz. cotton. 
 
 To secure the full disintegrating action of weak Nitric Acid in 
 the ordinary process of preparing Pyroxyline, the bulk of Nitric 
 Acid present in the Nitro-Sulphuric A.cid should be at least equal 
 to that of the Sulphuric Acid, and may with advantage be many 
 times greater. The cotton will then assume an opaque appear- 
 ance on being dipped in the mixture, and the Collodion will lack 
 those properties of strength and toughness before referred to. 
 
 Composition, by volume, of Nitro-Sulphuric Acid for preparing 
 Photographic Pyroxyline. The following table 
 Oil of Vitriol, Pure Nitric Acid, 
 1-845 at 60 F. T45 at 60 F. Water. 
 
 No. 1 .... 3 .... 1 .... i 
 No. 2 .... 2 .... 1 .... | 
 No. 3 . . . . 1 . . . . 1 . . . . -& 
 No.4 . . . . 1 . . . . 2 . . . . -^ 
 No. 5 .... 1 .... 3 .... 
 exhibits the composition of five different mixtures of Sulphuric 
 and Nitric Acid, in which an attempt has been made to graduate 
 the proportion of water in such a way that the percentage of 
 Peroxide of Nitrogen imparted to immersed cotton fibre may be 
 nearly the same in each : the table was constructed by simple ex- 
 periment, taking care in each case to work with the maximum 
 quantity of water, and stopping the addition of water only when 
 it was found that the product left a thick sediment on being dis- 
 solved in Ether and Alcohol. 
 
 One hundred grains of purified and dried cotton immersed in 
 either of these five mixtures, with the precautions necessary to 
 prevent any loss from solution, ought to weigh one hundred and 
 sixty grains : the Pyroxyline in each case is soluble, to a great ex- 
 tent, in glacial Acetic Acid, and also in boiling absolute Alcohol ; 
 whilst the resulting Collodion does not produce an entirely opaque 
 film. The substitution body formed is therefore above compound 
 D, or Xyloidine, but not above compound C (see the Vocabulary, 
 
230 THEORY OF PHOTOGRAPHY. 
 
 art. PYKOXYLINE). A more careful examination seems to indi- 
 cate that if there be a difference in the strength of the mixtures 
 given in the above table, No. 1 is somewhat stronger than No. 5.* 
 The Collodion however from No. 1 is more fluid than that from 
 No. 5, thus showing that in addition to high temperature and 
 dilution of the mixture with water, an excess of Sulphuric Acid 
 has to do with flowing properties of Collodion. A greater amount 
 of fluidity than exists even in the Collodion from No. 1 may be 
 produced by dipping the Pyroxyline first in No. 1, to secure the 
 full action of the Sulphuric Acid, and afterwards in No. 5 ; the 
 weak Nitric Acid will then act more decidedly than it would have 
 done upon a product produced by one immersion. 
 
 The temperature employed for the above table of acids may be 
 150 Fahr. ; and, in making the Pyroxyline, we find that the 
 lower numbers give a product which has an opaque appearance ; 
 whereas the Pyroxyline made by Nos. 1 and 2 exhibits no opacity. 
 The five samples of Collodion differ very much in the rapidity 
 with which they set upon the glass, and also in their physical 
 structure : the first setting rapidly and producing a horny film ; 
 the last scarcely possessing any power of setting. The only way 
 of overcoming this, and putting them on something like a par, 
 is by varying the proportions of Ether and Alcohol in the sol- 
 vents, using more Alcohol in the former, and more Ether in the 
 latter. Observe, however, that although the lower members of 
 the series in the above table ought to yield a Collodion setting 
 less rapidly upon the glass than the upper members, yet that this 
 observation only applies when the full quantity of water is em- 
 ployed in the Nitro-Sulphuric Acid. For as before shown, the 
 setting power is injured by a weak Nitric Acid, but not so much 
 by a stronger acid : hence No. 5 would produce a Collodion with 
 
 * If any one should desire to examine the above acids as regards their 
 strength or power of imparting NO 4 to the fibre of Cotton, let it be borne 
 in mind that in the case of No. 1 the percentage amount of NO 4 in the Pyr- 
 oxyline will increase considerably if the Cotton be permitted to remain for 
 several hours in the cold acid. The proportion of Nitric Acid in this mix- 
 ture is so small, being little more than one-fifth of the total bulb, that the 
 Nitric Acid is in a measure exhausted in acting upon the fibre, and conse- 
 quently a rather low substitution body is formed : the Cotton imbibes the 
 liquid like a sponge, and for the first half-hour only those portions of the 
 acid which touch the fibre produce any effect ; afterwards, however, a pro- 
 cess of diffusion goes on in the liquid, and fresh acid coming into contact 
 with the fibre, more Peroxide of Nitrogen is communicated to it. 
 
PYROXYLINE. 231 
 
 sufficient power of setting if the amount of water in the Nitric 
 Acid were reduced ; but in this case the film would possess the 
 objectionable properties of Pyroxyline made in strong acids, being 
 glutinous and difficult to pour. 
 
 Effect of Raising the Temperature. Although we have spoken 
 of fluidity of Collodion, and tenacity of the film as affected by 
 the relative proportions of water, Sulphuric Acid, and Nitric 
 Acid present in the Nitro-Sulphuric Acid, yet such remarks must 
 be taken very much in connection with the temperature at the 
 time of putting in the Cotton, since the physical modifications 
 produced by the two constituent acids are seen in an exaggerated 
 degree when the Nitro-Sulphuric mixture is heated. An acid 
 which gives an insoluble and explosive product in the cold, will 
 yield Pyroxyline perfectly soluble at a higher temperature. Py- 
 roxyline prepared in acids barely warm, makes Collodion which 
 is glutinous and difficult to pour on large glasses, even although 
 containing as little as two or three grains of the soluble Cotton 
 to the ounce. The film soon becomes surface-dry, and repels the 
 developer so that it cannot be made to flow up to the edge : on 
 examination after fixing, it shows cellular spaces and structural 
 lines. Hot acids, on the other hand, yield Pyroxyline of the 
 kind which is easily soluble in Ether and Alcohol to the extent 
 of eight or ten grains to the ounce of solvents, and the resulting 
 Collodion is limpid and adherent to the glass. There is also an 
 absence of structural marking in this Collodion, the transparent 
 layer being nearly homogeneous, even when highly magnified. 
 
 The reader will perceive that the above physical effects of in- 
 creased temperature are almost identical with those before attri- 
 buted to a dilution of the Nitro-Sulphuric Acid with water. This 
 is quite natural, because heat and dilution co-operate in increasing 
 the action of the acids upon the fibre. What we have to do in 
 preparing a Pyroxyline for fluid and adhesive Collodion, is to 
 hit the exact point at which disintegration of the fibre commences, 
 and to add more water, and raise the temperature a few degrees, 
 if after an immersion of five or ten minutes the Pyroxyline ap- 
 pears strong and unyielding, like ordinary cotton. 
 
 Different forms of Cellulose. Cotton, Straw, Pith, Flax, etc., 
 with the manufactured fabrics produced from the same, may be 
 converted into soluble Pyroxyline, but the product will be more 
 or less different in each case. When a rather concentrated Nitro- 
 
232 THEORY OF PHOTOGRAPHY. 
 
 Sulphuric Acid is used, Cotton may give a glutinous Collodion, 
 and Calico a fluid Collodion. In another acid, weaker than the 
 last, the Cotton succeeds well, whilst Calico instantly dissolves. 
 The difference in the two cases appears to depend principally 
 upon the thickness of the fibre ; Calico produces Pyroxyline of 
 the fluid kind, and is partially dissolved, because the Nitric Acid, 
 in acting on the outside portions of the closely-twisted fibre, is 
 reduced in strength, and hence the interior of the fibre is left 
 more nearly in the condition of Xyloidine. It is possible, also, 
 that with this weakening of the acid there may be a corresponding 
 rise of temperature, which would assist in producing a powdery 
 Pyroxyline, or in causing solution. 
 
 Linen, even when selected of the same thickness, yields a more 
 limpid Collodion than Calico, and one of which the film is less 
 tenacious and contractile. These peculiarities cannot be satisfac- 
 torily explained, unless they depend upon a difference in compo- 
 sition ; which is not improbable, since it is known that Flax can 
 be distinguished from Cotton by chemical tests, being more easily 
 discoloured by the action of alkalies. 
 
 Both Linen and Calico undergo a change by constant use, which 
 is recognized on making the material into Pyroxyline. Old and 
 rotten rags are quickly disintegrated by the acid mixture, and 
 the Pyroxyline produces a highly-structureless Collodion, which 
 adheres with much tenacity to the glass. Paper has been at 
 different times much recommended for the preparation of Pyr- 
 oxyline, but it will easily be gathered from the above remarks 
 that it is an unfit material ; for not only do different samples of 
 paper vary greatly in thickness, which, as before shown, would 
 affect the action of the Nitric Acid, but, being made both from 
 Cotton and Linen rags, some of which rags are new, and others 
 old and rotten, they cannot be expected to produce uniformity. 
 The Swedish filtering paper imported into this country for che- 
 mical purposes, has been stated to be uniform, but the Writer 
 does not find the assertion correct ; the Pyroxyline being at one 
 time structureless, and at another comparatively glutinous, even 
 with the same Acids. 
 
 In addition to the materials mentioned above, others have been 
 tried, such as China Grass, the Pith of the Jerusalem Artichoke, 
 the Fibre of the Aloe, etc., but the result was only to confirm 
 the opinion above expressed, that on each material the Sulphuric 
 
PYKOXYLINE. 233 
 
 Acid, and also the Nitric Acid, produces a different effect. The 
 Fibre of the Grasses, including Flax, appears to be more easily 
 disintegrated and dissolved both by Acids and Alkalies than that 
 of Cotton, and to be convertible into sugar with greater facility. 
 
 A point of some importance, as regards the manufacture of 
 Pyroxyline, is the cleansing of the Fibre thoroughly from adhering 
 resinous matter, which, if allowed to remain, deoxidizes a portion 
 of the Nitric Acid, and so far weakens it as .to ensure the imme- 
 diate destruction of a portion of the cellulose at high temperatures. 
 A convenient substance to employ in cleansing is a diluted alkali, 
 which converts the resin into a soap more or less soluble in water. 
 It is probable that the differences which have been said to exist 
 between Cotton of various growths may depend in part upon the 
 presence or absence of this resinous matter. The Manufacturer 
 who wishes to work with great accuracy, and to employ the largest 
 quantity of water possible in the Nitro-Sulphuric Acid, should 
 also bear in mind that Cotton is a hygroscopic substance, and 
 requires to be artificially dried. A minute proportion of moisture 
 present upon the very surface of the Cotton, would produce a 
 greater effect in causing solution than the same quantity of water 
 added to the Nitro-Sulphuric Acid, since it would dilute only 
 that portion of the Acid which touches the Fibre, and thus would 
 cause a rise in the temperature. 
 
 Photographic Properties of Various Kinds of Pyroxyline. In 
 preparing Pyroxyline from the five different mixtm-es given in the 
 table of composition of Nitro-Sulphuric Acid at page 229, it is 
 found to differ in its Photographic properties as well as in its 
 physical properties. As regards intensity of the Negative image, 
 we observe that Pyroxyline made in the mixtures at the top of 
 the scale, which contain an excess of the diluted Oil of Vitriol, 
 tends to produce more intense images than Pyroxyline fronl No. 4 
 or No. 5, in which the diluted Nitric Acid is in excess. The 
 difference in the two cases is caused by the Oil of Vitriol, since 
 the same effect of increasing intensity can be obtained by parch- 
 mentizing the Fibre first, and converting it into Pyroxyline after- 
 wards ; and it is not improbable that this action of the Sulphuric 
 Acid in increasing intensity depends upon a conversion of the 
 Cellulose into a substance resembling dextrine in its photographic 
 action. 
 
 Irrespective of the proportions of the two Acids, the quantity 
 
234 THEORY OF PHOTOGRAPHY. 
 
 of water in any mixture of Nitro-Sulphurie Acid will affect the 
 photographic intensity of the resulting Pyroxyline and Collodion. 
 This effect is evidently due to the same action of weak Nitric Acid 
 as that which causes it to disintegrate the Fibre, for if we take a 
 sample of Pyroxyline previously parchmentized by Oil of Vitriol, 
 and capable of yielding great intensity in Collodion, we may 
 destroy its properties in that respect most completely by dipping 
 it for an instant in a, warm mixture containing an excess of very 
 weak Nitric Acid. When speaking of the Negative Nitrate Bath, 
 it will be shown that it loses its power of producing a dense 
 picture if a little organic matter oxidized by Nitric Acid be added 
 to it : it appears, therefore, that Nitric Acid is capable of pro- 
 ducing with organic bodies a substance of unknown composition, 
 which is injurious to the intensity of the photographic image. 
 This subject, however, is not at present clearly understood, and 
 we must, therefore, be satisfied with indicating the facts as they 
 stand. 
 
 The temperature of the Nitro-Sulphuric Acid at the time of 
 immersing the Cotton invariably affects the photographic pro- 
 perties of Pyroxyline. At high temperatures, a portion of the 
 Fibre is converted into a substance which has a bitter taste, 
 and turns brown when treated with Alkalies. This substance 
 is believed to be Nitro- Glucose, formed by the action of strong 
 Nitric Acid upon Grape Sugar : the Grape Sugar itself being 
 produced from the Cellulose by contact with the warm and diluted 
 Sulphuric Acid. In studying the effect which this bitter product 
 of decomposition is likely to produce, we may prepare Nitro- 
 Glucose, and add it to Collodion. Nitro-Glucose diminishes the 
 sensitiveness of the film to weak rays of light, but increases the 
 rapidity and intensity of the development in Negative pictures. 
 Vigorous images are produced even in a very dull light, but they 
 are always liable to be black and white without middle tints, or 
 to solarize, and become extremely red in the most exposed parts 
 of the film, where the light acts strongly. Collodion made at 
 very high temperatures, although possessed of great fluidity, ad- 
 hesiveness, and freedom from structural lines, with other physical 
 advantages before enumerated, is found less useful for ordinary 
 work than samples prepared at a lower temperature, and which 
 are not so good in physical properties, being somewhat ropy in 
 hot weather, and drying up more quickly after sensitizing. A 
 
PYEOXYLINE. 235 
 
 film of this latter kind is very sensitive, and every radiation makes 
 a distinct impression even after the shortest exposure. 
 
 The author of this Work is inclined to attribute the peculi- 
 arities of Collodion made from Linen, or from paper manufac- 
 tured out of partially decomposed rags, to the presence of Nitro- 
 Glucose in the Pyroxyline. He has found that unless the tem- 
 perature of the Nitre-Sulphuric Acid be kept low, there is a 
 peculiar disposition to form the bitter resin, not only in Linen 
 Fibre, and partially-decayed Cotton Fibre, but also in the pure 
 Cotton Fibre previously converted into vegetable parchment by 
 the action of diluted Sulphuric Acid. In each case the Collo- 
 dion is highly intense, and when shaken up with Carbonate of 
 Potash assumes an amber-yellow colour, whereas a Pyroxyline 
 nearly free from the bitter matter remains colourless for a time 
 on treating the Collodion with Carbonate of Potash. 
 
 It may be proper before leaving this part of our subject to say 
 a few words on an action which Chlorine appears to exert in the 
 manufacture of Pyroxyline. The yellow Nitric Acid of commerce 
 invariably contains a portion of Chlorine, and this is found to 
 exert a decomposing action upon the Fibre, the result of which 
 is to increase the fluidity of the resulting Collodion and also its 
 intensity, but somewhat to diminish its sensitiveness. It is ad- 
 visable therefore to employ a Nitric Acid from which the Chlo- 
 rine has been eliminated, since any amount of intensity of Collo- 
 dion may be obtained by sufficiently increasing the proportion of 
 diluted Oil of Vitriol in the Nitro-Sulphuric Acid, or by raising 
 the temperature, and avoiding the use of too much water in the 
 acids. 
 
 In examining commercial samples of Pyroxyline prepared from 
 Cotton-wool, the Writer has met with a variety which gives 
 great intensity of Collodion with an average amount of fluidity. 
 A quality of Pyroxyline similar to this may be prepared in a mix- 
 ture of Nitro-Sulphuric Acid containing about two measures of 
 Oil of Vitriol to one measure of Nitric Acid of T45 (contaminated 
 with a little Chlorine), and a quantity of water decidedly less than 
 that given in the table at page 229. The temperature of the acid 
 mixture must be sufficiently raised to disintegrate the fibre, and 
 so to produce a Pyroxyline which occupies a small space when dry, 
 has rather a yellow aspect, and is inclined to be dusty. The film 
 from the resulting Collodion is not very sensitive, since it contains 
 
236 THEOET OF PHOTOGEAPHY. 
 
 a notable portion of the bitter resin, but it answers remarkably 
 well for a negative Collodion prepared with a mixed Iodide and 
 Bromide, and its manufacture requires less nicety than the for- 
 mula which the Author adopts, since the quantity of water in the 
 acids is not so large. 
 
 Spontaneous Decomposition in Pyroxyline. The Author has 
 occasionally failed when using samples of Pyroxyline which have 
 been kept for many months after preparation. A partial libera- 
 tion of Oxides of Nitrogen appears to take place in some instances, 
 forming an atmosphere of red fumes within the bottle. Pyroxy- 
 line which has undergone much decomposition from the use of a 
 very high temperature in the process of manufacture may be ex- 
 pected to change in this way, and especially so if the acids are 
 not thoroughly removed by washing ; a little Sulphuric Acid left 
 in the ' Pyroxyline would keep it continually damp, and perfect 
 dryness is essential to the stability of Pyroxyline. From phe- 
 nomena observed in the decomposition of Nitro- Glucose, it is 
 probable that constant exposure to light favours the change. 
 The products of the spontaneous decomposition of Gun Cotton 
 appear to be Oxalic Acid and a neutral organic substance having 
 the composition of Gum.* 
 
 THE PLAIN COLLODION. 
 
 The relative proportions of Ether and Alcohol in Collodion 
 affect both the physical and Photographic properties of the so- 
 lution. This subject has been already alluded to at p. 223, but 
 there are some additional observations to be made. 
 
 The use of an excess of Alcohol is advantageous in preventing 
 the Ether from evaporating quickly in hot weather. At tempe- 
 ratures of 90 and 100 Ealir. it is almost impossible, with Col- 
 lodion containing but little Alcohol, to coat a large glass and 
 immerse it in the Bath before the upper part becomes dry ; 
 consequently an uneven coating of Iodide is produced, with a 
 blueness at the top edge. This happens especially when the 
 Spirits are very strong and almost free from water. Another 
 advantage of making Collodion with a large quantity of Alcohol 
 is, that it is more readily wetted by the Bath solution, and 
 does not throw the liquid into greasy lines upon the surface of 
 
 * For some further properties of Pyroxyline not mentioned in this Sec- 
 tion, see the Vocabulary, Part I. 
 
PLAIN COLLODION. 237 
 
 the film. There is however a practical limit to the use of Alco- 
 hol, inasmuch as not being a complete solvent of Pyroxyline it 
 alters the structure of the film, rendering it gelatinous. 
 
 The quality of the Pyroxyline is the first important point to 
 be considered in determining the relative proportions of Ether 
 and Alcohol. With a sample of Pyroxyline made at a low tem- 
 perature and in rather strong Nitro-Sulphuric Acid containing a 
 minimum of Sulphuric Acid, barely enough Alcohol should be 
 used in the plain Collodion to confer the requisite solubility, viz. 
 an eighth or a twelfth part, by bulk, of the Ether ; otherwise the 
 iodized* Collodion will be very tender and easily torn, glutinous 
 and difficult to pour, loosely adherent to the glass, and full of 
 crapy lines and structural markings. 
 
 Pyroxyline prepared from Cotton-wool by the formula with 
 a large excess of Oil of Vitriol will bear more Alcohol than 
 the last, and with some advantage, for the contractility which 
 makes it separate from the edges of the glass is lessened by the 
 addition of Alcohol. The tendency to set quickly arid produce 
 water-markings at the upper edge of the plate is also to a great 
 extent obviated, as is also the rapid surface-drying of the film 
 after taking it from the Bath, which causes it to repel the deve- 
 loper, as before shown. The film, being nearly structureless and 
 very tough, will bear a quantity of Spirits which in other cases 
 would produce crapy lines and tenderness. If the Pyroxyline 
 of this formula be made in acids containing the largest possible 
 quantity of water, the proportion of Alcohol in the plain Collodion 
 may be one-half of the Ether, which will give, after iodizing, 
 equal bulks of the two Solvents ; but if the Nitro-Sulphuric 
 Acid was made with less water, then the proportion of Alcohol 
 in the plain Collodion must not exceed one-third of the Ether, 
 or the film will be woolly at the lower corner of the plate. 
 
 It will be found that the solubility of this tough kind of Py- 
 roxyline is increased by employing the maximum quantity of 
 Alcohol, so that if the plain Collodion be diluted with Ether, a 
 precipitate will take place. With other kinds of Pyroxyline 
 differently prepared, the addition of Ether to the plain Collodion 
 produces no precipitate. 
 
 * This observation supposes the plain Collodion and the iodizing compound 
 to be kept in separate solutions ; two measures of the latter being added to 
 six measures of the former. 
 
238 THEOEY OF PHOTOGRAPHY. 
 
 In the case of Pyroxyline prepared in Nitro-Sulphuric Acid 
 containing equal bulks of Oil of Vitriol and Nitric Acid, with 
 the maximum of water, it is advisable to reduce the quantity of 
 Alcohol somewhat ; for if too much Alcohol be employed, the set- 
 ting of the Pyroxyline will be so greatly retarded that the upper 
 edge of the film will become dry before the lower part has soli- 
 dified sufficiently to take the Bath without precipitation of the 
 Pyroxyline. Such an effect could not happen in the case of the 
 first formula of page 229, containing Oil of Vitriol in excess, be- 
 cause it would be impossible to use such a mixture in a state suf- 
 ficiently weak to destroy the property of setting in the resulting 
 Pyroxyline : before that point was reached the cotton would dis- 
 solve in the acid. 
 
 The exact strength of the Alcohol used in Photography must 
 always be noted, since the effect of water when present in any 
 quantity is to produce viscidity of Collodion, and more rapid de- 
 composition under the influence of the iodizing compound. The 
 experience of Photographers is favourable to the employment of 
 even a stronger spirit than that usually recommended ; and when 
 it can be obtained, the Alcohol of '805 at 60 Fahr., sold in com- 
 merce as absolute, may be preferred, as a rule. 
 
 With the most horny kind of Pyroxyline, however, prepared by 
 the first formula of page 229, a little water appears necessary, to 
 open out the structure of the film and prevent it from assuming a 
 condition in which it resembles Gutta Percha in being impervious 
 to liquids ; but in the case of Pyroxyline from formula No. 3, of 
 page 229, such as is recommended for Positives, the film will be 
 sufficiently porous, even with the whole of the Alcohol in the 
 absolute state. With formula No. 1 the Alcohol of '805 may also 
 be employed, if the proportion be increased until it nearly doubles 
 that of the Ether. 
 
 Photographic Effects of Excess of Alcohol in Collodion. The 
 addition of Alcohol to Collodion lessens the contractility of the 
 film, and renders it soft and gelatinous. These conditions are fa- 
 vourable to sensitiveness, perhaps from the play of affinities being 
 promoted by the loose manner in which the particles of Iodide 
 are held together. The extra sensitiveness obtained by use of 
 Alcohol however does not increase after a certain point ; on th 
 contrary, it diminishes, for it appears to be necessary to extreme 
 sensitiveness that the film should coagulate within a certain time 
 
PLAIN COLLODION. . 239 
 
 after it has been coated, and therefore the addition of Alcohol 
 must be stopped when the film loses its ready setting qualities, 
 and is not coherent under the finger. Hence a porous Collodion 
 is soon injured in sensitiveness on adding too much Alcohol, but 
 a strong and tough Pyroxyline will bear equal bulks of the two 
 Solvents without loss in that respect. 
 
 The above observation as to the effect of excess of Alcohol in 
 diminishing the sensitiveness of the film to dark objects applies 
 particularly when the atmosphere is cold and damp, and evapo- 
 ration is retarded. At a very high temperature and in a dry air 
 it does not apply, since the presence of Alcohol is then useful in 
 preventing the film from becoming surface-dry, especially when 
 it is necessary to keep the sensitive plate for a long time between 
 exposure and development. 
 
 Intensity of Negative is much favoured by using a full quantity 
 of Alcohol,* and particularly when large glasses are coated, and 
 long focus lenses, which work slowly, are employed. The differ- 
 ence is doubtless due in part to the structure of the film being 
 opened out by the Alcohol, so as to assist the developer in pene- 
 trating, and partly to the rapidity of evaporation and consequent 
 surface-drying being diminished. Hence in hot weather the Al- 
 cohol acts very beneficially, and with Collodion made almost en- 
 tirely of Ether, the Negatives are unusually weak at such times. 
 Pyroxyline of the horny kind is especially liable to lose intensity 
 at high temperature unless the Alcohol be added freely so as to 
 prevent the film from becoming hard and impervious. 
 
 Decomposition of Plain Collodion by Keeping. Plain Collo- 
 dion tends to become more limpid by keeping, and often acquires 
 the property of eliminating Iodine rapidly from the iodizer ; but 
 the rapidity with which the change takes place, varies much with 
 the mode of preparing the Pyroxyline and with the quality of the 
 Ether. Supposing all the materials to be pure, the decomposi- 
 tion after keeping for several months in a cool and dark place is 
 very slight, and is generally considered to improve the quality of 
 the Negative rather than otherwise : it imparts a slightly red or 
 purple tone to the image, without much affecting the sensitiveness, 
 and hence many prefer to keep a stock of Collodion always on 
 hand, that it may settle down clear, and " ripen." This proceeding 
 
 * For full particulars as to the purity of Alcohol for Photographic pur- 
 poses see the Vocabulary, Part I. 
 

 
 240 THEORY OF PHOTOGEAPHY. 
 
 however would be very far from safe with an unstable Pyroxy- 
 line, or an inferior Ether, since the Oxides of Nitrogen would 
 then be set free from the Gun Cotton, and injury to the sensi- 
 tiveness would result from elimination of Iodine after iodizing. 
 
 When great stability is an object, as in exporting Collodion 
 to foreign climates, the Pyroxyline ought not to be made in such 
 a way as to produce much decomposition in the Mtro-Sulphuric 
 Acid. For instance, if we take pieces of old linen and immerse 
 them at a temperature of 150 Fahrenheit in a Mtro-Sulphuric 
 Acid made purposely very weak, the greater part will dissolve, 
 but a few fragments remain, which, when washed and applied to 
 the tongue, have a bitter taste. In this case the Pyroxyline is 
 partially decomposed in the Acid, and Collodion made from the 
 product might work well at first, but after keeping for twelve 
 months in the plain state would probably be as thin as water, 
 and produce a rotten and insensitive film. 
 
 The best kind of Pyroxyline for yielding a stable Collodion, 
 according to the Author's experience, is that made from Cotton- 
 wool, and at rather a low temperature. This preparation has 
 been proved to retain its original properties nearly unchanged for 
 nine months. The test for decomposition of Pyroxyline is agita- 
 tion of the Collodion with dry Carbonate of Potash. When so 
 treated it should remain colourless for a certain time : if it assume 
 a brown tint in less than two hours, traces of the compound above 
 named as resembling Nitro- Glucose are present. 
 
 Plain Collodion, made at a temperature of 170 to 180 Fahr., 
 will keep well for a few weeks, but when exported to a distant 
 climate, and subjected to an elevated temperature, it liberates Io- 
 dine from Iodide of Potassium somewhat quickly, and is useful 
 only for taking Positives, or Negative views when shortness of 
 exposure is not an object. Carbonate of Potash shaken up with 
 this kind of Collodion strikes a brown colour. 
 
 A point which should not be overlooked in preparing Collodion 
 for keeping, is the length of time during which the fibre of the 
 Cellulose remains in the Nitro-Sulphuric Acid. A short immer- 
 sion (five minutes) is the best, but if the material be left for 
 half an hour or longer, as may sometimes be done without solu- 
 tion taking place, the Collodion will often acquire by degrees the 
 property of striking a brown colour with Iodide of Potassium. 
 
 The quality of the Ether is very important in making a Collodion 
 

 PLAIN COLLODION. 241 
 
 of uniform properties. It is possible to obtain an Ether which 
 will remain for months without assuming the condition known as 
 " ozonized," in which it liberates Iodine from Iodide of Potassium; 
 but Ether containing Aldehyde or organic impurity will soon 
 change. 
 
 The Photographer will be guided partly by the quality of his 
 Ether in deciding as to the best formula for a Pyroxyline, because 
 an inferior Ether soon renders the Collodion rather limpid through 
 formation of Acetic Ether or some similar product, by which the 
 setting power of the Pyroxyline is diminished. It must also be 
 borne in mind that when this impure Ether liberates Iodine in 
 the iodized Collodion, a corresponding portion of the alkali Pot- 
 ash is set free, which, as we shall presently show, adds to the lim- 
 pidity, and lessens the setting powers. To meet this difficulty, 
 the Pyroxyline should be made slightly more tough and con- 
 tractile than is necessary, which may be effected by reducing the 
 temperature of the Acid a few degrees. Also, since impure Ether 
 soon becomes ozonized, a high temperature of Mtro-Sulphuric 
 Acid ought to be avoided, as likely to render the Pyroxyline 
 unstable. 
 
 Inferior Ether usually produces a more intense Collodion than 
 pure Ether, but the plain Collodion does not keep for any length 
 of time without change, soon losing its sensitiveness, and yielding 
 pictures which are black and white without middle tints. A rapid 
 elimination of Iodine takes place on adding the iodizer, and the 
 film is weak and rotten. In some cases, commercial Ether is 
 contaminated with organic Oils having a foul smell, or contains 
 traces of the Alkali employed in the processes of purification. 
 The latter impurity is particularly objectionable, because Alkalies 
 and Carbonated Alkalies decompose Collodion, rendering it lim- 
 pid, and destroying the setting properties. " Methylated Ether " 
 (see the Vocabulary, Part I.) is largely employed in the manu- 
 facture of Collodion, but where expense is not an object, the 
 Writer recommends the pure Ether in preference. (Read the 
 article ETHEE, in the Vocabulary, for further particulars.) 
 
 IODIZING SOLUTIONS. 
 
 Different Iodides vary in their effect upon plain Collodion. 
 Those which have an acid reaction, like the Iodide of Cadmium, 
 may be expected to increase the glutinosity. But Alkaline 
 
 E 
 
212 THEORY OF PHOTOGBAPHY. 
 
 Iodides, such as Iodide of Potassium or Ammonium, render it 
 limpid and structureless. Fixed Alkalies or Alkaline Carbonates 
 hare a more marked effect than Alkaline Iodides, as may be 
 shown by agitating plain Collodion with powdered Carbonate of 
 Potash : in the course of a few days it becomes as fluid as water. 
 The first effect of the Alkali, however, upon Pyroxyline, made by 
 the formula with excess of Oil of Yitriol, is to render the Collodion 
 fflairy, so that the bottle may often be completely inverted with- 
 out any immediate loss. The same observation applies to Alkaline 
 Iodides : they thicken the Collodion slightly in the first instance, 
 and afterwards render it more limpid. When Collodion gradually 
 becomes limpid under the action of an Alkaline Iodide, it loses 
 its power of setting upon the glass ; but there is a remarkable 
 difference between Collodions in the rapidity of this change, and 
 those which are tough and unyielding when newly made withstand 
 the action of the Alkali for a much longer time. 
 
 The reader will perceive that the effects now attributed to the 
 action of alkaline Iodides upon Collodion, viz. limpidity, and a 
 diminished power of setting, are the same as those spoken of 
 under the head of dilution of the Nitro-Sulphuric Acid with 
 water. There is, however, this important difference in the two 
 cases, that in the former an increase of photographic intensity 
 accompanies the porosity, but in the latter the intensity is di- 
 minished, as already shown at page 234. 
 
 Iodide of Cadmium. The stability of this compound is its 
 great recommendation : Ether, unless highly ozonized, has little 
 or no effect in. liberating Iodine from it. Another advantage of 
 the Iodide of Cadmium is, that it does not destroy the setting 
 properties of the Pyroxyline as the alkaline Iodides eventually do. 
 
 The Collodion should be prepared purposely when Iodide of 
 Cadmium is used as an iodizer. Collodion containing Pyroxyline 
 made at a low temperature and in rather strong acids, works 
 tolerably well after iodizing with the Potassium or Ammonium 
 compound, and keeping until liberation of Iodine and liquefaction 
 have taken place ; but with Iodide of Cadmium it would be in 
 every respect unsatisfactory, flowing in a slimy manner upon the 
 plate, exhibiting crapy lines, repelling the developer, and splitting 
 away from the glass on drying. When the Pyroxyline is made 
 in strong acids at a very high temperature, the Collodion is often 
 eufficiently fluid, but yet is not altogether adapted for iodizing 
 
IODIDES FOR COLLODION. 243 
 
 with the Cadmium Salt, from its tendency to produce " woolliness" 
 or unevenness of film at the lower edge of the plate. The proper 
 kind is that prepared in weak acids, and sufficiently parchmentized 
 by the Oil of Yitriol to give the requisite degree of intensity. 
 
 Iodide of Potassium. This Salt is well adapted for iodizing 
 Collodion not required to possess keeping properties, but its 
 sparing solubility in Alcohol and Ether is an objection. Collo- 
 dion containing 4J- parts of Ether of '725 to 3 of Alcohol '816, 
 will carry nearly three grains of Iodide of Potassium to the ounce. 
 With equal biilks of Ether and Alcohol the full quantity, yiz. 
 four grains, may be dissolved. Five parts of Ether with three of 
 Alcohol will not take more than 2 or 2j grains to the ounce. 
 The admixture of Iodide of Cadmium with Iodide of Potassium 
 increases the solubility of the latter Salt, by forming a double 
 Salt, the Iodide of Potassium and Cadmium. To produce this 
 compound, which is well fitted for iodizing Collodion containing 
 five parts Ether to three of Alcohol of '816, equal weights of the 
 two Iodides may be taken. These proportions are not strictly 
 correct, but they are sufficiently near for practical purposes. 
 
 It may also be remarked that the quantity of an alkaline Iodide 
 which any Collodion will carry, depends partly upon the Pyroxy- 
 line. With that particular kind of Pyroxyline which is recom- 
 mended for Negatives in this Work, no more than 3| grains per 
 ounce must be employed, otherwise the Iodide of Silver will 
 be precipitated upon the surface of the film at the lower edge, 
 and marks will result. By mixing the Iodide of Potassium 
 with Iodide of Cadmium, or by using Iodide of Cadmium alone, 
 a larger quantity of Iodide may be used without the appearance 
 of markings. 
 
 The effect of using an iodizing solution containing more Iodide 
 of Potassium than the Collodion will retain in solution, is not 
 always to produce an immediate precipitation, such as would 
 result if the Pyroxyline were omitted, and the iodizing solution 
 added to an equivalent quantity of Ether and Alcohol. The 
 presence of the Pyroxyline prevents a visible deposition, but the 
 Collodion will produce a spotted image, and after standing for 
 some days, crystals will form upon the sides of the bottle. - 
 
 When the whole of the Alcohol contained in the Collodion is 
 of the strength of '805, commercially known as " absolute," it 
 will scarcely be safe to iodize with Iodide of Potassium only, 
 
244 THEORY OF PHOTOGEAPHY. 
 
 unless the proportion of Alcohol is nearly double that of the 
 Ether : much, however, will depend upon the degree of dryness 
 of the Ether itself, since the commercial Ether often contains 
 Water dissolved in it, in sufficient quantity to prevent the pre- 
 cipitation of the Iodide of Potassium. The preparation of an 
 iodizing solution of Iodide of Potassium is a troublesome process 
 with Alcohol of '805, since it is necessary to pulverize the Iodide 
 very carefully, and to boil the spirit upon it ; in cold weather, 
 such an iodizer is apt to deposit cubical crystals upon the sides of 
 the bottle containing it. 
 
 Commercial Iodide of Potassium is often contaminated with 
 Carbonate of Potash. This Salt has an injurious action not only 
 in throwing a white deposit of Carbonate of Cadmium when the 
 mixed Iodides are used, but also in reacting upon the Collodion, 
 and producing rapid liquefaction, as before shown. Pure Iodide 
 of Potassium is now prepared purposely for Photography : the 
 crystals have a slightly yellow tint, and the Alcoholic solution 
 liberates Iodine slowly on exposure to Light. 
 
 Iodide of Ammonium is useful in iodizing Collodion, when it 
 is required to add also a portion of Bromide ; if any Iodide of 
 Potassium were present in such a case, a white deposit would 
 form, on account of the sparing solubility of Bromide of Potas- 
 sium in Spirits free from water. Collodion containing 44 drachms 
 of Ether to 3^ of Alcohol '816, will not carry more than a quarter 
 of a grain of Bromide of Potassium to the ounce, but it will easily 
 dissolve a much larger quantity of Bromide of Ammonium or 
 Bromide of Cadmium. The only objection to the use of Iodide 
 of Ammonium is its instability, and the difficulty of invariably 
 obtaining it in a pure state, the Writer having found that most 
 of the samples prepared by aid of Sulphuretted Hydrogen or 
 Hydrosulphate of Ammonia, contain traces of a Sulphur com- 
 pound, and are inferior to those made in the moist way, by pre- 
 cipitation. When Iodide of Ammonium becomes brown by keep- 
 ing, it may be decolorized by shaking it up in a bottle with a 
 little Ether, and drying upon blotting-paper. 
 
 Iodide of Sodium is intermediate in solubility between Iodide 
 of Potassium and Iodide of Ammonium, and would be a valuable 
 compound for iodizing if it could be obtained commercially in a 
 pure state. 
 
 Iodide of Iron was formerly used in Photography, but has now 
 
IODIDES IN COLLODION. 245 
 
 become obsolete. It produces a very sensitive Collodion at first, 
 but soon reacts upon the Pyroxyline, and the Collodion becomes 
 reduced to the condition of a jelly. The Nitrate Bath is also 
 thrown out of order, Protonitrate of Iron being formed, which 
 precipitates Metallic Silver on the sides of the trough. 
 
 Chemical and Photographic action of the various Iodides in 
 Collodion. With recently iodized Collodion the difference in 
 sensitiveness between the various Iodides is not very marked if 
 they are in a pure state. The presence of Carbonate of Potash, 
 lodate of Potash, or Chloride of Potassium in the commercial 
 Iodide of Potassium at once diminishes the sensitiveness ; and 
 any trace of a Sulphur compound in the Iodide of Ammonium 
 will have the same effect. 
 
 The intensity of Negative Collodion does not vary materially 
 with the Iodide, if the Collodion be tested soon after iodizing. 
 Nevertheless by close observation minute differences can be de- 
 tected, and Iodide of Potassium will be found to give a somewhat 
 more vigorous picture than Iodide of Ammonium. With any 
 Iodide also the intensity will be lessened by adding too large a 
 quantity of the Iodide, and especially when the setting power of 
 the Collodion is small. If from any cause, such as excess of 
 Iodide, deficiency of setting power in the Collodion, etc., the sen- 
 sitive film of Iodide be allowed to lie loosely upon the surface of 
 the Collodion, the picture will be very feeble, and will often fall 
 away when the fixing agent is applied. 
 
 After Collodion has been kept for a time in the iodized state, 
 both the sensitiveness and the intensity will vary with the Iodide 
 chosen, because, as before shown, an afiinity exists between the 
 Pyroxyline and the base of the Iodide. Alkaline Iodides are 
 soon decomposed by Collodion, and hence a loss of sensitiveness, 
 depending partly upon the retarding effect of free Iodine, and 
 partly upon the new compound formed by the liberated Alkali. 
 The liquefaction of the Collodion, and its diminished power of 
 setting upon the glass, must also be supposed to be injurious to 
 sensitiveness (p. 238) . The intensity however will increase in con- 
 sequence of these changes, since free Iodine, although it lessens 
 intensity in a dull light, tends to prevent feebleness of the image 
 from solarization in a strong light ; and the compound formed 
 by the trace of liberated Alkali, whatever be its nature, has a 
 direct action in adding to the intensity. The liquefaction of the 
 
246 THEOET OF PHOTOGRAPHY. 
 
 Collodion is also serviceable up to a certain point, by removing 
 the impermeability to the developer which the more dense and 
 horny kinds of Collodion exhibit. 
 
 Iodide of Cadmium is the most stable of all the Iodides, yet 
 with Iodide of Cadmium' as an iodizer, an amount of change suf- 
 ficient to lessen the sensitiveness of the Collodion to very dark 
 objects eventually takes place, and with an unstable Pyroxyline 
 or impure Ether, this change may be evident even in a few weeks. 
 Iodide of Cadmium also increases the intensity of Collodion after 
 a long keeping, especially with Pyroxyline made in rather strong 
 acids. In such a case the Collodion gradually becomes slightly 
 gelatinous, the film solarizing strongly in the parts most acted on 
 by the Light, and the image exhibiting a bright red colour when 
 viewed by transmission. This state of Collodion is well fitted for 
 photographing in a dull light. 
 
 The nature of the Pyroxyline must also be taken into account in 
 estimating the probable effect of iodizing with an Alkaline Iodide. 
 If it has undergone decomposition in the manufacture, the sensi- 
 tiveness of the Collodion will soon be lost ; but if prepared at 
 low temperatures, it remains for a longer time unchanged : of all 
 kinds of Pyroxyline the least stable after iodizing is that prepared 
 from Linen, in weak acids, and at a high temperature. 
 
 A point which affects the keeping qualities of a Collodion, or 
 its stability after iodizing, is the presence of Bromide combined 
 with the Iodide. Collodion so made assumes the usual yellow 
 tint when mixed with the iodizing solution ; but in the course of 
 some hours either the whole or a part of the free Iodine appears 
 to be re-absorbed. This change takes place most rapidly in the 
 case of methylated Ether, but it may be seen more or less even 
 with pure Ether and pure Alcohol. No satisfactory explanation 
 can be given. 
 
 To facilitate the comprehension of the decompositions which 
 take place in iodized Collodion, let the following experiments be 
 made. Take Nitro-Grlucose and add it in small quantity to or- 
 dinary Collodion iodized with the Potassium compound : the 
 elimination of Iodine will be somewhat more rapid than usual, 
 the Collodion at the same time losing sensitiveness and gaining 
 intensity. Next dissolve Nitro-Glucose in Spirits of Wine, and 
 boil it in a test-tube with powdered Carbonate of Potash : the 
 liquid becomes brown, and evolves a smell of burnt sugar : a 
 
THE NITRATE BATH. 247 
 
 few drops of it in iodized Collodion rapidly destroy the sensi- 
 tiveness, but add much to the blackness of the Negative. In a 
 third experiment introduce a portion of a reducing agent, such 
 as grape-sugar, into iodized Collodion : the result will be to 
 lessen the sensitiveness on keeping, and to increase the intensity. 
 The experiment last described shows that not only the nature of 
 the particular Iodide, and that of the Pyroxyline, but also the 
 presence of foreign organic substances may affect the keeping 
 properties of iodized Collodion ; hence the importance of using 
 Ether which has been carefully freed from traces of Aldehyde, 
 etc., since even the employment of an Iodide as stable as that of 
 Cadmium will not prevent decomposition if substances are pre- 
 sent in the Collodion which have an affinity for Oxygen. 
 
 Another experiment, illustrating the effect of. changes in Col- 
 lodion after iodizing, is the following : Take Nitrite of Soda, 
 and boil with it Alcohol of "805 until a saturated solution has 
 been obtained : a few minims of the liquid added to an ounce of 
 iodized Collodion, will increase the intensity. A nitrite is one of 
 the products of decomposition of Collodion by alkalies or al- 
 kaline Iodides, and may sometimes be seen in the form of well- 
 defined crystals at the bottom of the bottle. 
 
 SECTION II. 
 The Chemistry of the Nitrate Bath. 
 
 The solution of Nitrate of Silver in which the plate 
 coated with Iodized Collodion is dipped, to form the layer 
 of Iodide of Silver, is known technically as the Nitrate 
 Bath. The Chemistry of Nitrate of Silver has been ex- 
 plained at page 124, but there are some points relating to 
 the properties of its aqueous solution which require a fur- 
 ther notice. 
 
 Solubility of Iodide of Silver in the Nitrate Bath. 
 Aqueous solution of Nitrate of Silver may be mentioned 
 in the list of solvents of Iodide of Silver. The proportion 
 dissolved is in all cases small, but it increases with the 
 strength of the solution. If no attention were paid to this 
 point, and the precaution of previously saturating the Ni- 
 trate Bath with Iodide of Silver neglected, Iodide of Silver 
 
248 THEORY OF PHOTOGRAPHY 
 
 in the film would be dissolved when left too long in the 
 liquid. 
 
 This solvent power of Nitrate of Silver on the Iodide is 
 well shown by taking the excited Collodion plate out of the 
 Bath, and allowing it to dry spontaneously. The layer of 
 Nitrate on the surface, becoming concentrated by evapora- 
 tion, dissolves the Iodide and produces a transparent, 
 spotted appearance. 
 
 In the solution of Iodide of Silver by Nitrate of Silver 
 a double salt is formed, which corresponds in properties 
 to the double Iodide of Potassium and Silver in being 
 decomposed by the addition of water. Consequently, in 
 order to saturate a Bath with Iodide of Silver it is only 
 necessary to dissolve the total weight of Nitrate of Silver in 
 a small bulk of water, and to add to it a few grains of an 
 Iodide ; perfect solution takes place, and on subsequent dilu- 
 tion with the full amount of water, the excess of Iodide 
 of Silver is precipitated in the form of a milky deposit. 
 
 The above-named double salt of Iodide of Silver and 
 Nitrate of Silver has been termed by Schnauss the " lodo- 
 Nitrate of Silver;" and although in the Collodion Nitrate 
 Bath it is present only in small quantities, and in a state 
 of solution, it is quite possible to obtain it in well-defined 
 crystals. No corresponding compounds containing Bro- 
 mide and Chloride of Silver are known, and hence it will 
 not be necessary to saturate the Bath with those salts 
 when it is desired to use them in Collodion. 
 
 Although the addition of water to the Negative Bath 
 renders it milky by precipitation of Iodide of Silver, dilu- 
 tion with Alcohol has not the same effect. 
 
 Acid condition of Nitrate of Silver. A solution of Ni- 
 trate of Silver prepared from the commercial Nitrate has 
 usually an acid reaction ; the crystals having been imper- 
 fectly drained from the acid mother-liquor in which they 
 were formed. Hence, in making a new Bath it is advi- 
 sable not only to saturate it with Iodide of Silver, but to 
 neutralize the free Nitric Acid it contains. 
 The quantity of this Acid is very variable. If the 
 

 THE NITEATE BATH. 249 
 
 Nitrate has been carefully dried at 240 Fahrenheit, and 
 then crystallized a second time, no Nitric Acid can be de- 
 tected, the concentrated aqueous solution slowly restoring 
 the blue colour of reddened Litmus. This alkaline effect 
 upon reddened Litmus the Writer believes to be the pro- 
 per reaction of pure Nitrate of Silver, since he finds it to 
 exist in samples of Nitrate which have never undergone 
 fusion. On the other hand, Nitrate of Silver which has 
 been crystallized only once from the acid mother-liquor, 
 without any attempt at careful drying, is often so deci- 
 dedly acid that it cannot be employed in Photography 
 until neutralized. 
 
 The Nitrate Bath, although perfectly neutral when first 
 prepared, may become acid by continued use, if Collo- 
 dion containing much, free Iodine be constantly employed. 
 In that case a portion of Nitric Acid is liberated, and 
 lodate and Iodide of Silver are formed, thus : 
 
 If the Bath contains Acetate of Silver, free Iodine libe- 
 rates Acetic Acid in place of Nitric Acid ; and Nitric Acid 
 added to such a Bath neutralizes itself and displaces Acetic 
 Acid, thus : 
 
 AgO,Acet. + NO 5 = AgO,NO 5 + Acetic Acid. 
 
 The actual quantity of acid liberated by Collodion which 
 has become brown from decomposition, is very inconsider- 
 able, and it is quite a mistake to be continually neutralizing 
 the Nitrate Bath. When, however, the trough which holds 
 the Bath is narrow, and the plates large, a minute addition 
 of Alkali may occasionally be required, to prevent the 
 film from losing sensitiveness, and yielding weak metallic 
 Negatives. 
 
 Alkaline condition of the Bath. By "alkalinity" of the 
 Bath is meant a condition in which the blue tint is restored 
 to reddened litmus-paper. This change, when rapid, indi- 
 cates that a free Oxide is present in solution, which by 
 combining with the acid in the reddened paper neutralizes 
 it and removes the red colour. 
 
 If a small portion of caustic Potash or Ammonia be 
 
250 THEORY OF PHOTOGRAPHY. 
 
 added to a strong solution of Nitrate of Silver, it produces 
 a brown precipitate, which is Oxide of Silver. The solution, 
 however, from which the precipitate has separated, is not 
 left in a neutral state, but possesses a distinct alkaline 
 reaction, since Oxide of Silver is sparingly soluble in water, 
 and the solution restores the blue colour of reddened Lit- 
 mus. Both Oxide of Silver and Carbonate of Silver are 
 also abundantly soluble in water containing Nitrate of Am- 
 monia ; which salt is continually accumulating in the Bath 
 when compounds of Ammonium are used for iodizing. 
 
 An alkaline Bath is fatal to success in Photography, pro- 
 ducing that universal darkening of the film on applying 
 the developer to which the name of " fogging " has been 
 given. Hence care must be used in adding to the Bath 
 substances which tend to make it alkaline. Collodion con- 
 taining free Ammonia, often sold in the shops, gradually 
 does so. The use of Potash or Carbonate of Soda to neu- 
 tralize the Bath, or even of Chalk or Marble, if Salts of 
 Ammonia are present, has the same effect, when an excess 
 is employed ; and hence a trace of Acetic or Nitric Acid 
 must afterwards be added. 
 
 The mode of testing a Bath for alkalinity is as follows : 
 a strip of porous blue litmus-paper is taken and held to 
 the mouth of a bottle of glacial Acetic Acid until it be- 
 comes reddened ; it is then placed in the liquid to be ex- 
 amined and left for ten minutes or a quarter of an hour. 
 If free Oxide of Silver be present in solution, the origi- 
 nal blue colour of the paper will be gradually restored. 
 This experiment must not be made in a strong light, or the 
 litmus-paper will darken, and the blue colour be obscured. 
 Indeed it is always somewhat difficult to examine a Ni- 
 trate Bath by test-papers, since the pure Nitrate of Silver 
 has a slightly alkaline reaction ; and hence the Photogra- 
 phic effect of alkalinity, viz. cloudiness of the image, will 
 to the amateur afford a more certain guide. The Writer 
 believes that the use of Ammonia for the purpose of neu- 
 tralizing the Bath is the most common cause of failure 
 from alkalinity, few being aware that a single drop of 
 
THE NITRATE BATH. 251 
 
 strong Ammonia will neutralize a comparatively large 
 quantity of acid. 
 
 Acetate of Silver in the Nitrate Bath. In preparing a 
 new Bath, if the crystals of Nitrate of Silver are acid, it 
 is usual to add an alkali in small quantity. This removes 
 the Nitric Acid, but leaves the solution faintly alkaline 
 from Oxide of Silver. If Acetic or Nitric Acid is then 
 dropped in, it forms Acetate or Nitrate of Silver by com- 
 bination with the Oxide. 
 
 Acetate of Silver is not formed by the simple addition of 
 Acetic Acid to the Bath, because its production under such 
 circumstance would imply the liberation of Nitric Acid ; 
 but if an alkali be present to neutralize the Nitric Acid, 
 then the double decomposition takes place, thus- 
 Acetate of Soda -f- Nitrate of Silver 
 = Acetate of Silver + Nitrate of Soda. 
 
 Acetate of Silver is a white flaky salt, sparingly soluble 
 in water. It dissolves in the Bath only in small proportion, 
 but yet sufficiently to affect the Photographic properties of 
 the sensitive Collodion film. The observance of the follow- 
 ing simple rules will obviate its production in injurious 
 quantity : First, when it is required to remove free Nitric 
 Acid from a bath not containing Acetic Acid a solution of 
 Potash or Carbonate of Soda may be dropped in freely ; 
 but the liquid must be filtered before adding any Acetic 
 Acid, otherwise the brown deposit of Oxide of Silver will 
 be taken up by the Acetic Acid, and the Bath will be 
 charged with Acetate of Silver. Secondly, in dealing 
 with a Bath containing both Nitric and Acetic Acid, em- 
 ploy an alkali much diluted (Liquor Ammonise with 10 
 parts of Water), and add a single drop at a time, coating 
 and trying a plate between each addition ; the Nitric Acid 
 will neutralize itself before the Acetic, and with care there 
 will be no formation of Acetate of Silver in quantity. 
 
 The question is sometimes asked, how Acetic Acid may 
 be removed from the Nitrate Bath, and Nitric Acid sub- 
 stituted? This operation is somewhat difficult to effect. 
 Nitric Acid, when present in excess, can be neutralized 
 
252 THEOET OF PHOTOGKAPHY. 
 
 and converted into a Nitrate, which is nearly or quite inert 
 in Photography ; but to neutralize Acetic Acid is to form 
 an Acetate, which is not inert ; and the subsequent addi- 
 tion of Nitric Acid to such a solution again liberates Ace- 
 tic Acid. Evaporation to dryness with a little Nitric Acid 
 is the only means of effectually eliminating the Acetic 
 Acid from the Bath. 
 
 Organic Matter in the Nitrate Bath. Nitrate of Silver 
 has an affinity for certain kinds of organic matter, and 
 when such substances are present, the Photographic action 
 of the Bath is in some way interfered with. 
 
 Commercial crystallized Nitrate of Silver is frequently 
 contaminated with traces of an impurity, which is probably 
 produced by organic matters falling into the Nitric Acid 
 employed in dissolving the Silver. Eepeated recrystalliza- 
 tion is required to remove this substance. If allowed to 
 remain, it injures the sensitiveness of the film to feeble 
 radiations, makes the Negative weak and metallic, reverses 
 the action of the light, and produces either fogging or 
 markings of various kinds, the result of irregular reduction 
 of Silver. 
 
 Solutions of Nitrate of Silver often acquire organic con- 
 tamination by being kept in troughs of Gutta-pereha. 
 Pure Gutta-percha seems to have little action upon Nitrate 
 of Silver, but the commercial article is invariably impure. 
 Caoutchouc seems also to be without action, but Caout- 
 chouc vulcanized with Sulphur, such as is used for the 
 tops of watertight baths, will decompose the Nitrate of 
 Silver by degrees. Baths injured by impure Gutta-percha 
 produce fogging of the film, streaks of irregular develop- 
 ment, and quick discoloration of the solution of Pyrogallic 
 Acid, attended with variations in the density of the Nega- 
 tive picture. 
 
 Albumen and Gelatine soon decompose Nitrate of Silver, 
 and hence the dipping of a few dry albuminized plates, or 
 the floating of chloride-papers intended for the printing 
 process, upon a Collodion Bath, would effectually disorder 
 it, and almost certainly give rise to fogging. 
 
THE NITBATE BATH. 253 
 
 Alcohol and Ether react very slowly upon solution of 
 Nitrate of Silver, and Pyroxyline is almost without effect. 
 Henae when pure Collodion is employed, the bath may be 
 kept for many months without much appreciable change. 
 It should however be carefully excluded from light, or the 
 sides of the bottle will be covered with a delicate layer 
 of reduced Silver. Methylated Spirit of Wine is seldom 
 sufficiently free from volatile oil to remain long in the 
 bath without producing partial reduction. 
 
 Collodion containing common Eosin, has been recom- 
 mended for use in the dry Collodion process ; but the Eosin 
 finds its way into the bath, and spoils it for the wet process, 
 soon producing solarization of the most exposed parts of 
 the plate, and altering the appearance of the film of Iodide, 
 so that it becomes pale and blue, instead of being yellow 
 and creamy. 
 
 The effect produced by organic matters in the bath will 
 be more intelligible if we explain that not only Nitrate of 
 Silver, but also Iodide of Silver has an affinity for a cer- 
 tain class of these bodies. Hence when a Collodion film is 
 dipped in a bath of the kind described, a trace of organic 
 matter is carried down and retained in the film. This ap- 
 plies especially to Albumen, which has so decided an affinity 
 for Iodide of Silver, that its presence will entirely alter 
 the colour of the film, and render it blue and transparent. 
 A state of bath in which the Collodion film, instead of 
 being yellow and creamy, appears blue or opalescent, may 
 be produced by organic matter, as well as by a deficiency 
 of Nitrate of Silver. 
 
 To remove organic matter from the bath, a plan sug- 
 gested by Mr. Barber is often employed. The solution, 
 having been rendered slightly alkaline by Ammonia, is 
 exposed for two or three days to a bright sunlight in a trans- 
 parent bottle, when the greater part of the organic com- 
 pound separates in combination with the Oxide of Silver. 
 This mode is generally successful in restoring a Bromo- 
 iodized Collodion bath to a good working condition, but 
 often fails in rendering it useful for a simply Iodized Collo- 
 dion and Pyrogallic developer. 
 
254 THEOET OF PHOTOGRAPHY. 
 
 Use of Fused Nitrate of Silver for the Bath. Fusion 
 has been resorted to with a view of expelling traces of 
 Nitric Acid from the crystallized Nitrate of Silver and 
 thus lessening the trouble of preparing the bath. It must 
 be observed however that decomposition of the Nitrate is 
 liable to occur in melting, if the crystals are not chemically 
 pure. A .trace of any organic substance, such as a bit of 
 cork, or a fragment of gelatine adhering to the sides of the 
 porcelain dish, would at the melting-point of Nitrate of 
 Silver remove Oxygen, and produce Nitrite of Silver : a 
 safer plan is therefore to pulverize the Nitrate and dry it 
 in a hot-air bath, at about twenty degrees above the tem- 
 perature of boiling water. Supposing the Nitrate to be 
 absolutely free from organic matter, there would be no 
 danger of forming Nitrite in the process of fusion, since 
 the temperature at which Nitrate of Silver decomposes is 
 far above its melting-point. 
 
 Nitrate of Silver which has been much decomposed in 
 melting, produces a great peculiarity of development. In 
 the Positive glass process the whites are often solarized, 
 and appear blue by reflected light ; whilst in the Negative 
 process the image is highly intense, with over-action in 
 parts like the sky, and the film usually fogs slightly towards 
 the end of the development : on applying the developer the 
 whole picture starts out instantly, and the solution of Pyro- 
 gallic Acid becomes muddy instead of assuming by degrees 
 the colour of sherry wine. These effects are not unlike 
 those previously described as due to certain kinds of or- 
 ganic matter in the bath, the reduction of Silver by the 
 developer being in both cases facilitated. 
 
255 
 
 CHAPTER VII. 
 
 ON POSITIVE AND NEGATIVE COLLODION PHOTO- 
 GRAPHS. 
 
 THE terms "Positive" and "Negative " occur so frequently 
 in all works upon the subject of Photography, that it will 
 be impossible for the student to make progress without 
 thoroughly understanding their meaning. 
 
 A Positive may be defined to be a Photograph which 
 gives a natural representation of an object, as it appears 
 to the eye. 
 
 A Negative Photograph, on the other hand, has the 
 lights and shadows reversed, so that the appearance of the 
 object is changed or negatived. 
 
 In Photographs taken upon Chloride of Silver, either in 
 the Camera or by superposition, the effect must necessarily 
 be Negative; the Chloride being darkened by luminous 
 rays, the lights are represented by shadows. 
 
 The following simple diagrams will make this obvious. 
 
 Fig. 1 is an opaque image drawn upon a transparent 
 ground ; fig. 2 represents the effect produced by placing it 
 
 Fig. 1. Fig. 2. Fig. 3. 
 
256 
 
 THEORY OF PHOTOGRAPHY. 
 
 in contact with a layer of sensitive Chloride and exposing 
 to light ; and fig. 3 is the result of copying this negative 
 again on Chloride of Silver. 
 
 Fig. 3 therefore is a Positive copy of fig. 1, obtained by 
 means of a Negative. By the first operation the tints are 
 reversed ; by the second, being reversed again, they are 
 made to .correspond to the original : hence the possession 
 of a Negative enables us to obtain Positive copies of the 
 object, indefinite in number and all precisely similar in 
 appearance. This capability of multiplying impressions 
 is of the utmost importance, and has rendered the produc- 
 tion of good Negative Photographs of greater consequence 
 than any other branch of the Art. 
 
 The same Photograph may often be made to show either 
 as a Positive or as a Negative. For instance, supposing a 
 piece of silver-leaf to be cut into the shape of a cross and 
 pasted on a square of glass, the appearance presented by 
 it would vary under different circumstances. 
 
 Fig. 1 represents it placed on a layer of black velvet ; 
 
 Fig. 1. Fig. 2. 
 
 fig. 2 as held up to the light. If we term it Positive in 
 the first case, i. e. by reflected light, then it is Negative 
 in the second, that is, by transmitted light. The explana- 
 tion is obvious. 
 
 Therefore to carry our original definition of Positives 
 and Negatives a little further, we may say that the former 
 are usually viewed by reflected, and the latter by trans- 
 mitted light. 
 
 All Photographs however cannot be made to represent 
 
COLLODION POSITIVES. 257 
 
 both Positives and Negatives. In order to possess this 
 capability, it is necessar} 7 " that a part of the image should 
 be transparent, and the other opaque but with a bright 
 surface. These conditions are fulfilled when the Iodide of 
 Silver upon Collodion is employed, in conjunction with a 
 developing agent. 
 
 Every Collodion picture is to a certain extent both Nega- 
 tive and Positive, and hence the processes for obtaining 
 both varieties of Photographs are substantially the same. 
 Although however the general characters of a Positive and 
 a Negative are similar, there are some points of difference. 
 A surface which appears perfectly opaque when looked 
 down upon, becomes somewhat translucent on being held 
 up to the light ; hence, to give the same effect, the deposit 
 of metal in a Negative must be proportionally thicker 
 than in a Positive ; otherwise the minor details of the 
 image will be invisible, from not obstructing the light suf- 
 ficiently. 
 
 With these preliminary remarks, we are prepared to 
 investigate more closely the rationale of the processes 
 for obtaining Collodion Positives and Negatives. All that 
 refers to paper Positives upon Chloride of Silver will be 
 treated in a subsequent Chapter. 
 
 SECTION I. 
 
 On Collodion Positives. 
 
 Collodion Positives are sometimes termed direct, because 
 obtained by a single operation. The chloride of Silver 
 acted upon by light alone, is not adapted to yield direct 
 Positives, the reduced surface being dark and incapable of 
 representing the lights of a picture. Hence a developing 
 agent is necessarily employed, and the Iodide of Silver 
 substituted for the Chloride, as being a more sensitive pre- 
 paration. Collodion Positives are closely allied in their 
 nature to Daguerreotypes. The difference between the 
 two consists principally in the surface used to sustain the 
 
 s 
 
258 THEORY OP PHOTOGEAPHY. 
 
 sensitive layer, and the nature of the substance by which 
 the invisible image is developed. 
 
 In a Collodion Positive the lights are formed by a bright 
 surface of reduced Silver, and the shadows by a black 
 background showing through the transparent portions of 
 the plate. 
 
 Two main points are to be attended to in the production 
 of these Photographs. 
 
 First, to obtain an image distinct in every part, but of 
 comparatively small intensity. If the deposit of reduced 
 metal be too thick, the dark background is not seen to 
 a sufficient extent, and the picture in consequence is defi- 
 cient in shadow. 
 
 Secondly, to whiten the surface of the reduced metal 
 as much as possible, in order to produce a sufficient con- 
 trast of light and shade. Iodide of Silver developed in 
 the usual way presents a dull yellow appearance which is 
 sombre and unpleasing. 
 
 The Collodionfor Positives. Ordinary Collodion iodized 
 for Negatives and giving a thick yellow film, usually fails 
 in taking good Positives. If the exposure in the Camera 
 be sufficiently long to impress dark shadows, the lighter 
 parts of the image develope with such rapidity that the gra- 
 dation of tone is lost by excessive deposit of Silver. The 
 addition of Nitric Acid to the Bath is to a certain extent a 
 remedy, inasmuch as it lessens the intensity of the reduc- 
 tion and gives the Silver a sparkling appearance ; never- 
 theless Positives taken in this way are always unpleasing, 
 and cannot be compared with a good Daguerreotype in 
 softness and delicacy. 
 
 A better class of picture may often be obtained by dilu- 
 ting down a sample of Collodion with Ether and Alcohol 
 until it gives a pale-bluish film in the Bath. The pro- 
 portion of Iodide of Silver being in that case small, the 
 action of the high lights is less violent, and the shadows 
 are allowed more time to impress themselves. 
 
 The employment of a very thin film for Positives is not 
 invariably a successful process. The particles of the Iodide 
 
COLLODION POSITIVES. 259 
 
 of Silver being closely in contact with tlie glass, unusual 
 care is required in cleaning the plates in order to avoid 
 stains; and as the amount of free ISitrate of Silver retained 
 upon the surface of the film is small, circular patches of 
 imperfect development, causing blue and green stains, are 
 liable to occur, unless the reducing agent be scattered 
 evenly and perfectly over the surface. Also if free Iodine 
 or organic substances which have a retarding effect on the 
 action of light be present to a considerable extent, the 
 Collodion will not work well with 'a small proportion of 
 Iodide : in such a case a creamy and dense layer of the 
 sensitive Salt is required, to give a sufficient power of re- 
 sistance. 
 
 In attempting to dilute down the Nitrate Bath at the 
 same time with the Collodion, we gain in some respects an 
 advantage. Excess of development is obviated, and the 
 picture shows well on the surface of the film. The em- 
 ployment of a very weak Nitrate Bath (twenty grains of 
 Nitrate of Silver to the ounce of water) in the Positive 
 process is not however advisable on the whole. It becomes 
 necessary, on account of the small quantity of Silver pre- 
 sent, to exclude all free Nitric Acid, and even to avoid the 
 employment of a Collodion too highly tinted with Iodine. 
 On the other hand, with a strong Nitrate Bath, and a 
 tolerably dense film of Iodide of Silver, a better result is 
 often secured by the use of Nitric Acid, as will presently 
 be shown. 
 
 These and other like processes, in which films of Iodide 
 only are employed, have now been almost superseded by 
 others, founded on a peculiar property which Bromide 
 possesses when added in moderate proportion to an ordi- 
 nary iodized Collodion. It alters the molecular state of 
 the Silver reduced during development, rendering it me- 
 tallic : the image also is superficial, and sufficiently trans- 
 lucent to give a perfect gradation of tone. When Bromide 
 is employed, in conjunction with Sulphate of Iron as a de- 
 veloper, the quality of the Pyroxyline is not of the same 
 importance as in the case of pictures taken with a simply 
 iodized Collodion. 
 
260 THEOEY OF PHOTOGRAPHY. 
 
 To produce a very faint and superficial image, a weak 
 Nitro- Sulphuric Acid must be used for the Pyroxyline 
 (p. 233), but the objection is that after a certain point of 
 dilution with Water, a quality of Pyroxyline is obtained 
 which becomes opaque on drying. This is comparatively 
 immaterial when transparent varnish is applied to the 
 image, but it spoils the appearance of the picture when 
 mounted without varnish, as is sometimes preferred. 
 
 With regard to the quantity of Bromide which may be 
 introduced, the operator should be guided by the aspect 
 of the developed image. If, when exposed long enough to 
 form the shadows, the high lights appear too dense, more 
 Bromide must be added : whilst, on the other hand, if the 
 Positive is grey and feeble, and this defect is not due to 
 over-exposure, the proportion of Bromide may be reduced. 
 The American operators, who are very successful with 
 Collodion Positives, employ more Bromide than is recom- 
 mended in this Work, but their Pyroxyline may possibly 
 be made in a different manner. 
 
 One effect produced by Bromide in Positive Collodion, 
 is seen in keeping the lines of the picture sharp and clear 
 during development. When the Bath is strongly acidified 
 with JSTitric Acid and a simply iodized Collodion is em- 
 ployed, the image is often blurred by an irregular deposit 
 of Silver, giving an appearance like defective focussing. 
 With Collodion containing mixed Iodide and Bromide in 
 the proper proportions, this does not so frequently happen, 
 the image remaining clear and free from stains. 
 
 Does Bromide increase or diminish the sensitiveness of 
 the film to a weak light ? This is a question which can only 
 be answered by considering the chemical state of the Col- 
 lodion, Bath, and developer. With a weak solution of Pyro- 
 gallic Acid as a developer, and a dilute Bath, Bromide seems 
 to diminish the sensitiveness considerably, but this appear- 
 ance is delusive, since the latent image is really present, and 
 simply requires a stronger reducing agent and more Ni- 
 trate of Silver to bring it out. On increasing the strength 
 of the Bath therefore and using Sulphate of Iron as a 
 
COLLODION POSITIVES. 261 
 
 developer, the whole of the details will show themselves 
 after a minimum of exposure in the Camera. 
 
 There are states of Collodion also in which the use of 
 Bromide may increase the sensitiveness, viz. when decom- 
 position has taken place. Every practical operator learns 
 by experience that Positive Collodion containing Bromide 
 does not lose sensitiveness after iodizing in the same rapid 
 manner as Negative Collodion prepared without Bromide. 
 Doubtless the difference in the developing solutions Sul- 
 phate of Iron in the one case and Pyrogallic Acid in the 
 other has something to do with this result ; but a peculiar 
 action of the Bromide in rendering the Collodion indepen- 
 dent of organic changes due to keeping after iodizing must 
 be allowed. 
 
 A comparatively newly-iodized Collodion is seldom in 
 the best possible state for taking Positives ; for if it be 
 colourless or nearly so, the plates will show clouding after 
 development. By keeping the Collodion for several weeks, 
 a portion of the Iodine is liberated, which has a great 
 effect in giving brilliancy to the picture, by preserving the 
 shadows from the reducing action of the developer. 
 
 The Nitrate Bath. An ordinary neutral thirty-grain 
 solution of Nitrate of Silver, such as is used for Nega- 
 tives, may be employed also for Positives, and especially 
 so if the Collodion be purposely kept after iodizing until it 
 assumes a yellow tint, and liberates enough iodine to pre- 
 vent the clouding which might otherwise take place in the 
 absence of acid. Negative Baths, however, which contain 
 Acetate of Silver, or any kind of organic matter, or which 
 have been made from Nitrate of Silver previously decom- 
 posed by strong fusion, are not well adapted for Positives, 
 since these conditions promote solarization, a defect which 
 shows itself in Positives by discoloured whites on the most 
 exposed parts of the film. 
 
 When the Nitrate Bath is made purposely for direct 
 Positives, it is always better to acidify it with Nitric Acid 
 rather than with Acetic Acid, as used for Negatives. The 
 Nitric Acid diminishes the density of the deposited image, 
 
262 THEOEY OF PHOTOGBAPHY. 
 
 renders it whiter and more metallic, and assists in preserv- 
 ing the clearness of the plate. The mistake has sometimes 
 been made of using too much Acid in proportion to the 
 strength of the Bath, which not only lessens the sensitive- 
 ness of the film, but also, as before shown, promotes a 
 blurring of the image, and a fogging from irregular reduc- 
 tion. Even with a Collodion containing Bromide, there 
 will be a tendency to staining and imperfect development, 
 if the amount of Nitric Acid be carried as far as is some- 
 times recommended, in the case of a Bath of thirty grains 
 to the ounce. 
 
 Many successful operators use a Bath of forty grains to 
 the ounce for Collodion Positives, in order to shorten the 
 exposure in the Camera, and give brilliancy to the image. 
 This solution will bear a far larger addition of Nitric Acid 
 than the ordinary Bath of thirty grains ; and, always sup- 
 posing Bromide to be present in the Collodion, satisfactory 
 pictures may be obtained with a quantity of Nitric Acid 
 as large as two minims to the ounce. Neither is the sensi- 
 tiveness very much impaired by the excess of Acid ; thus 
 showing how many conditions have to be taken into ac- 
 count in considering the formation and development of the 
 Collodion picture. With a Bath very strongly acidified, 
 it would be impossible to get any result if the proportion 
 of Nitrate of Silver and Iodide of Silver in the Collodion 
 film were both reduced to one-half. The Acid and Nitrate 
 of Silver must, in fact, be balanced against each other, and 
 when the Collodion film is lessened in opacity, the deve- 
 loper must be considerably strengthened, otherwise the 
 latent image, although actually present, cannot be made 
 to appear in the half-shadows. 
 
 The Author's experience is not favourable to the em- 
 ployment of a forty-grain Bath in the Positive glass pro- 
 cess as far as beginners in the Art are concerned. He 
 finds that there are fewer failures when the Bath contains 
 thirty grains of Nitrate of Silver to the ounce, and is only 
 slightly acid. Stains of reduced Silver at the edges of the 
 plate are not easily avoided in the case of a strong and 
 
COLLODION POSITIVES. 263 
 
 acid Bath, and if the developer is too weak, there are me- 
 tallic spangles on the shadows. 
 
 A Positive Nitrate Bath, containing both Alcohol and 
 Nitric Acid, ought never to be exposed to the light ; the 
 writer has observed that when this precaution is neglected, 
 the solution soon produces clouded pictures. 
 
 Developers for Collodion Positives. In the case of a 
 Collodion which does not contain Bromide, Pyrogallic Acid 
 is occasionally employed as a developer. When mixed 
 with Acetic Acid, as is usual for Negative pictures, it 
 produces a surface which is dull and yellow, but this may 
 be obviated by substituting a small quantity of Nitric Acid 
 for the Acetic (Pyrogallic Acid, 2 grains ; Nitric Acid, 
 1 drop ; Water, 1 ounce). The surface produced by Pyro- 
 gallic Acid with Nitric Acid is lustreless, but very white, 
 if the solution be used of the proper strength. On at- 
 tempting to increase the amount of Nitric Acid the de- 
 posit becomes metallic, and the half-tones of the picture 
 are injured ; since Pyrogallic Acid, although an active de- 
 veloper, does not allow of the addition of mineral acid to 
 the same extent as the Salts of Iron : it requires also, 
 when combined with Nitric Acid, a fair proportion of 
 Nitrate of Silver on the film, or the development will be 
 imperfect in parts of the plate, producing green and blue 
 stains ; if such stains occur, a few drops of a solution of 
 Nitrate of Silver may be added to the developer before 
 use. 
 
 Sulphate of Iron. This salt is a more energetic redu- 
 cing agent than the last, and is better fitted for employ- 
 ment with a Collodion containing a portion of Bromide, 
 the action of Bromide being to retard the development. 
 To produce by means of Sulphate of Iron a dead white 
 tint with absence of metallic lustre, it may be used in con- 
 junction with Acetic Acid. 
 
 The addition of Nitric Acid to Sulphate of Iron modi- 
 fies the development, making it more slow and gradual, 
 and producing a bright sparkling surface of reduced Silver. 
 Too much of this acid however must not be used, or the 
 
264 THEORY OF PHOTOGRAPHY. 
 
 action will be irregular. The Nitrate Bath also should be 
 tolerably concentrated, in order to compensate for the 
 retarding effect of Nitric Acid upon the development. 
 The blue and transparent films of Iodide of Silver, are not 
 well adapted for Positives to be developed in this way. 
 Such films require the most vigorous developer possible ; 
 Acetic Acid should therefore be used instead of Nitric 
 Acid. 
 
 Protonitrate of Iron. This salt is remarkable as giving 
 a surface of brilliant metallic lustre without any addition 
 of free acid. Theoretically, its action may be considered 
 as closely corresponding to that of Sulphate of Iron with 
 Nitric Acid added. There are however slight differences 
 between them, which are in favour of the protonitrate, as 
 regards the colour of the image. 
 
 The Nitrate is, of all the Protosalts of Iron, by far the 
 most feeble developer, and is seldom used alone in Pho- 
 tography. In the case of Bromo-iodized Collodion very 
 little dependence must be placed upon the Protonitrate of 
 Iron as far as reducing Metallic Silver is concerned, but 
 it may be added to solution of Sulphate of Iron when the 
 peculiar metallic tone which it imparts, is desired. Begin- 
 ners often fail in using this developer, from not allowing a 
 sufficient excess of the Sulphate of Iron in its preparation, 
 and from supposing that the Protonitrate is equally as 
 strong a reducing agent as the Protosulphate, whereas it 
 is at least twenty times weaker. 
 
 The Tone and Colour of Positives. The same chemicals 
 yield such different results in the hands of various ope- 
 rators, that some have spoken of manipulation and prac- 
 tice as more essential than theory. Doubtless the exact 
 time of exposure in the Camera is important, because a 
 short exposure always produces an image of a whiter colour 
 and more transparent shadow than a long exposure. In 
 the case of Negatives, the image, viewed by transmitted 
 light, is often of a jet-black when under-exposed, but of a 
 ruby-red when over-exposed. So in Positives, it is always 
 brighter and more metallic when the action of the light 
 
COLLODION POSITIVES. 265 
 
 is stopped at the proper time. There are other causes of 
 variation in the colour of the Positive image, such as 
 changes in the actinic intensity of the light ; and the ope- 
 rator may expect to produce a better quality of picture 
 when the light is strong. 
 
 The gradation of tone in Collodion Positives will also be 
 affected by the mode in which the light falls upon the sub- 
 ject ; for when it possesses great actinic power, there is 
 always a tendency to an excessive reflection from the high 
 lights ; and if this be not counteracted by an arrangement 
 of curtains, to be described in the third part of the Work, 
 it will require the utmost attention to the state of the che- 
 micals, to prevent excess of intensity, and loss of good 
 shading. 
 
 The mode of developing the Positive image has an effect 
 upon the gradation, independently of the exposure, or of 
 peculiarities in the Collodion. The theory is as follows : 
 When the Nitrate of Silver is present upon the film in 
 large quantity relatively to the Sulphate of Iron, the ten- 
 dency is to give great contrast of image, and to produce a 
 bold picture, which appears to stand out from the glass. 
 The deposit of Silver in such a case falls more abundantly 
 upon the high lights, and less so upon the shadows, so that 
 the shading in the face and lighter parts is often lost. On 
 the other hand, when the quantity of Nitrate of Silver is 
 much reduced in relation to the Sulphate of Iron, the 
 image developes more slowly, and is soft and full of half- 
 tone : in extreme cases the high lights are not sufficiently 
 opaque, and the features of the sitter, after backing up with 
 the black varnish, are those of a Negro. This rule applies 
 to every kind of developed picture, and the practical infer- 
 ence from it is, that when the Positive is too vigorous, and 
 the details in the face are lost, a large quantity of develop- 
 ing solution should be scattered over the film so as to wash 
 off a portion of the free Nitrate of Silver. Experimental 
 trial will convince the operator how extremely minute is 
 the proportion of Nitrate of Silver actually required for 
 the formation of the image ; but no exact directions can be 
 
266 THEORY OF PHOTOGBAPHY. 
 
 given, since the quantity will vary with the density of the 
 sensitive film, the brightness of the light, the amount of 
 retarding acid, and the strength of the developer. 
 
 Fixing Agents for Positives. Hyposulphite of Soda is 
 not usually employed for fixing Collodion Positives, since 
 it contains Sulphur, and is somewhat unstable ; hence the 
 image is liable to be superficially darkened to an appreci- 
 able extent. Cyanide of Potassium is free from this ob- 
 jection, and always produces a whiter picture than Hypo- 
 sulphite. The strong solvent powers of Cyanide of Potas- 
 sium must be borne in mind, when using it as a fixing 
 agent ; for if the solution be too concentrated or allowed 
 to remain a long time upon the plate, the middle tints first 
 become whitened from conversion into Cyanide of Silver, 
 and immediately afterwards begin to dissolve. Neither 
 should the plate be exposed to a strong light whilst the 
 Cyanide is upon it, as the action of light favours the conver- 
 sion of the image into Cyanide of Silver. Sulphocyanides 
 of Potassium and Ammonium will probably be found use- 
 ful fixing agents for Positives, as they are far less likely 
 than the Cyanide to endanger the delicate half-tones of the 
 picture. 
 
 A Process for Whitening Positives by Corrosive Sub- 
 limate. In place of brightening the Positive image by 
 modifying the developer, it was proposed some time since 
 by Mr. Archer to effect the same object by the use of the 
 Chloride of Mercury, Hg Cl. 
 
 The image is first developed in the usual way, fixed, and 
 washed. It is then treated with a solution of the Chloride 
 (thirty grains dissolved in an ounce of hot water), the effect 
 of which is to produce almost immediately an interesting 
 series of changes in colour. The surface first darkens 
 considerably, until it becomes of an ash-grey, approaching 
 to black ; shortly it begins to get lighter, and assumes a 
 pure white tint, or a white slightly inclining to blue. 
 
 The rationale of this reaction appears to be, that the 
 Chlorine of the mercurial salt divides itself between the 
 Mercury and the Silver, producing a compound of Calomel 
 and Chloride of Silver, which is not affected by light. 
 
COLLODION NEGATIVES. 267 
 
 Positive Photographs whitened by Chloride of Mer- 
 cury have usually more or less of a bluish tone, but this 
 may be overcome by associating other chemicals with the 
 Corrosive Sublimate. A more serious objection is the in- 
 stability of the picture after whitening, since it has been 
 stated that these images are liable to fade unless special 
 precautions be taken. 
 
 SECTION II. 
 
 On, Collodion Negatives. 
 
 As in the case of a direct Positive we require an image 
 which is feeble though distinct, so, on the other hand, for 
 a Negative, it is necessary to obtain one of considerable 
 intensity. In the Chapter immediately following the pre- 
 sent, it will be shown that in using glass Negatives to pro- 
 duce Positive copies upon Chloride of Silver paper, a good 
 result cannot be secured unless the Negative is sufficiently 
 dark to obstruct light strongly. 
 
 There are various conditions affecting the formation and 
 development of a Photographic Negative, which ought 
 to be studied by one who aims at perfection in the Art. 
 Some of these are the quality of the light at the time of 
 taking the picture, the focal length and aperture of the 
 Lens, the state of the atmosphere, etc. To meet the dif- 
 ferent cases which may arise, the Collodion, the Nitrate 
 Bath, and the developing solution may each or all be con- 
 siderably modified. 
 
 With regard to the first of these conditions we may ob- 
 serve that the mode in which a Negative developes, and 
 its colour when the process is completed, vary much with 
 the actinic intensity of the light. Pictures taken by a 
 short exposure in a strong light, develope easily under the 
 Pyrogallic Acid. The first deposit of Silver is often of a 
 red colour when held against the light, and this influences 
 the subsequent precipitation (see p. 150), so that the Ne- 
 gative continues to darken until the whole of the Nitrate 
 of Silver in the film has been decomposed. When exa- 
 
268 THEORY OF PHOTOGRAPHY. 
 
 mined after fixing, it shows a plum-colour or yellow bloom 
 by reflected light, and is often brown by transmitted light. 
 In dull wintry weather, on the contrary, and especially when 
 the atmosphere is loaded with aqueous vapour, the Nega- 
 tive picture is slow in appearing under the action of the 
 developer, and the image first formed is rather of a violet- 
 blue than of a red colour. In consequence of this, the sub- 
 sequent deposit of Silver is likewise of a violet-blue, and 
 a much longer time is expended in bringing the develop- 
 ment up to the proper point. The particles of precipitated 
 Silver are always larger when the reducing process is slow, 
 and thus Negatives taken in a bad light often prove in 
 printing to be less intense than they appear, whereas those 
 taken in a bright light are usually more intense chemically 
 than visually. 
 
 When the strength of the light falls very low indeed, as, 
 for instance, in working in a room covered in with glass 
 of a greenish tint, fatal to quick Photographic action, the 
 darker parts of the picture fail to impress the film, and the 
 Negative shows only patches of black and-white ; or, with 
 a more sensitive Collodion, the whole picture appears, but is 
 extremely feeble and indistinct, a deposit of Silver falling 
 upon the shadows and giving a cloudy aspect. On the 
 other hand, in a light of extreme brilliancy, reddening 
 from solarization is to be appehrended, the actinic rays 
 beingreflected fromthehigh lights of the subject in agreater 
 proportion than from the shadows : this may happen es- 
 pecially with a sky free from fleecy clouds, which when 
 present have a great effect in illuminating the shadows 
 and diminishing excessive contrast. When solarization of 
 Negatives occurs from great intensity of light, the image 
 often starts out almost instantaneously on applying the 
 Pyrogallic Acid, and after fixing by the Hyposulphite of 
 Soda exhibits an intensely red appearance of the high 
 lights, with a steel-blue colour of the same parts viewed by 
 looking down upon them. 
 
 We notice in the second place the focal length and aper- 
 ture of the Lens, as a condition influencing the quality of 
 
 
COLLODION NEGATIVES. 269 
 
 the developed Negative. If, for instance, two Cameras 
 be pitched side by side, at the same view, one being a Ste- 
 reoscopic instrument with a single Lens of 4^-inch focus, 
 and the other fitted with a twenty-inch Lens, suitable for 
 glasses of twelve inches diameter, the plates, even when 
 properly exposed, will behave differently under the action 
 of the developer, the Stereoscopic picture showing by far 
 the greater intensity, contrast of tint, and tendency to 
 solarization. The relative power of the Lenses makes the 
 difference, the short-focus Lens producing a brilliant image 
 which impresses the sensitive film in less than a third of 
 the time required by the other. A second experiment, 
 equally instructive, may be made by surrounding a group 
 of statuary with a dark drapery, so as to produce a strong 
 contrast of light and shade. On copying such an object 
 with the full aperture of a short-focus Lens, it will be 
 found almost impossible, unless the composition of the Col- 
 lodion be modified in a manner presently to be explained, 
 to get the whole of the picture simultaneously. Either 
 the whites will be well rendered, and the blacks wanting, 
 or by a longer exposure the shadows will be well brought 
 out, but the lights solarized. Now take the same Lens 
 and cut down the effective aperture by a middle-sized 
 diaphragm, allowing proportionally longer time in the 
 Camera, when a perfect Negative will be produced. This 
 rule, then, may be stated as follows : A Camera image 
 of great actinic intensity often produces a Collodion Ne- 
 gative with exaggerated contrast of light and shade, and 
 conversely an image of low intensity tends to give a pic- 
 ture which is too uniform, and deficient in extreme tints. 
 Availing himself of this knowledge, the skilful operator 
 alters the size of his diaphragms according to the quality 
 of picture desired, and remedies solarization by diminish- 
 ing the supply of light. 
 
 The amateur, on commencing Photography, is often at 
 a loss to understand apparent discrepancies in the formulse 
 employed by various operators, and in their general mode 
 of working. In one instance a plate of twenty inches by 
 
270 THEOBY OF PHOTOGRAPHY. 
 
 seventeen is developed to the full intensity with a single 
 application of the reducing agent ; in another, a Negative 
 of half that size is found to require repeated treatment 
 with Pyrogallic Acid and Nitrate of Silver, and to occupy 
 ten or fifteen minutes in reaching the full opacity. Sup- 
 posing the Lens and chemicals to be the same in each case, 
 the difference may depend upon the character of the sub- 
 ject. When a distant landscape is Photographed, a large 
 number of rays of light are concentrated upon the film : 
 but if an object like an old picture or a faded drawing be 
 copied without any reduction in size, the light is very 
 feeble. No careful timing of the exposure will cause two 
 such Negatives to develope in the same way, because the 
 long-continued action of a weak light in the Camera does 
 not correspond to the shorter action of an intense light. 
 The molecules of Iodide of Silver are differently affected 
 in the two cases, and consequently they behave differ- 
 ently when treated with the mixture which constitutes the 
 developer. 
 
 The Collodion for Negatives. A perusal of the First 
 Section of the last Chapter (p. 234) will show that the pro- 
 perties of Negative Collodion vary much with the mode 
 of preparing the Pyroxyline. Upon this subject it is not 
 necessary to enlarge at present, since it will again be 
 alluded to in the Chapter on dry Collodion. Distinguish- 
 ing terms however are always useful, and hence we pro- 
 pose to speak of Pyroxyline with, and Pyroxyline without 
 organic reactions towards the Salts of Silver; the for- 
 mer being the variety which yields the intense Negative 
 images. 
 
 Pyroxyline without organic reactions may be expected 
 to give the greater sensitiveness of the two, and the better 
 keeping qualities after iodizing. It is however difficult to 
 use it in consequence of the increased liability to fogging 
 and spots of all kinds (p. 158). Also when the intensity 
 and contrast of the image are too small, as frequently hap- 
 pens with such Collodion, they cannot easily be increased, 
 whereas excessive intensity may readily be reduced. The 
 
COLLODION NEGATIVES. 271 
 
 Pyroxyline with organic reactions is therefore preferred 
 by the majority of operators. 
 
 For the sake of greater simplicity, the Author has given 
 only one formula for Negative Pyroxyline in the First Chap- 
 ter of the third or practical division of the Work. This 
 formula appears to answer well for a normal Collodion, 
 applicable to all purposes. Those however who wish to 
 prepare a Pyroxyline suitable for any one especial kind of 
 work, may read the section in Part III. on portraits, land- 
 scape photography, and copying, where they will find the 
 subject examined more fully. 
 
 Independently of the Pyroxyline, Negative Collodion 
 may be described as of two kinds, the simply iodized, and 
 the .Sromo-iodized Collodion. Each has its advantages 
 and likewise its defects. The first gives the greater sen- 
 sitiveness, and also the greater intensity, but it is more 
 liable to spots and markings, and the purity of the chemi- 
 cals is of greater importance than when Bromo-iodized 
 Collodion is employed. 
 
 Let us examine the above points more minutely. It must 
 be allowed that the addition of a Bromide to Negative 
 Collodion impairs the sensitiveness. Much indeed depends 
 upon the mode of developing, but even when the reducing 
 agent is strong, and the Nitrate of Silver abundant, the 
 Bromo-iodized Collodion is still inferior in that respect. 
 If such a Collodion be found on trial to yield a picture in 
 eight seconds, a pure, simply iodized Cadmium Collodion 
 will probably give the same picture in five seconds. Hence 
 in the dull winter months, many are necessitated to employ 
 Iodides alone, who at more favourable times use combina- 
 tions of Iodide and Bromide. 
 
 The excessive sensitiveness of simply iodized Collodion 
 however is not seen in perfection, unless all the materials 
 are pure, and hence some have stated that the addition of 
 a small portion of a Bromide increases the sensitiveness. 
 Experiments leading to such a conclusion were probably 
 made with a Collodion which from organic decomposition 
 of the Pyroxyline or other causes, had acquired the pro- 
 
272 THEOEY OF PHOTOGRAPHY. 
 
 perty of producing an intense Negative, and in this case the 
 action of theBromide, as already shown at page261,is pecu- 
 liar. A Pyroxyline without strong organic reactions is pro- 
 per for the most sensitive iodized Collodion, and the Ether 
 must also be very pure. When this extreme purity of the 
 Ether cannot be secured, Iodide of Cadmium should be 
 used in place of Iodide of Potassium, to preserve the fluid 
 in a neutral and colourless condition. The manufacturer 
 of Collodion who examines its properties shortly after its 
 
 eparation finds little or no difference between the Iodide 
 of Potassium and the Iodide of Cadmium. The purchaser 
 however usually esteems the latter to be superior as re- 
 gards sensitiveness, because the plain Collodion has often 
 been kept for a time before he obtains it, and has ac- 
 quired the property of displacing Iodine : hence with the 
 Potassium iodizer the colour changes quickly to orange- 
 yellow, and the action of the light is retarded. Iodide of 
 Cadmium, in consequence of its stability, would have super- 
 seded the other Iodides, had it not been for its glutinizing 
 action on Pyroxyline, a serious objection in covering large 
 surfaces of glass. 
 
 The effect of Bromide in diminishing the intensity of the 
 image has already been considered in the section on Posi- 
 tives. It is often used for the same purpose in Negative 
 Collodion, because when the light is strong and the Lens 
 powerful, the intensity and contrast may be in excess, and 
 if so, either a black and white picture without middle 
 tints, or a red picture defective from solarization will be 
 obtained. The Bromide effectually remedies both these 
 causes of failure ; frequently indeed it originates an error 
 in the opposite direction. On looking at a Negative taken 
 with a Bromo-iodized Collodion, we see that the peculiar 
 metallic effect given by the Bromide is most evident in 
 the half-shadows of the picture, where the light acts feebly. 
 Provided there be a sufficiency of light, enough intensity 
 will be obtainable in presence of Bromide, but when the 
 light is too much reduced, the image will be rendered 
 weak and translucent thereby. A Negative must possess 
 
NEGATIVE COLLODION. 273 
 
 a certain amount of intensity, and also a proper colour, 
 if it is to possess what the Photographer terms "good 
 printing qualities." These qualities cannot be secured 
 by merely pushing the development and piling up par- 
 ticles of metallic Silver : much will depend upon the co- 
 lour of the image in its early stage, which should be of a 
 soft red, and should appear nearly homogeneous when mag- 
 nified. The employment of Bromide should be suspended 
 when the particles of Silver become large and crystalline, 
 since this condition of image is too permeable to the chemi- 
 cal Light in the process of printing. 
 
 The use of Bromides in Negative Collodion must be 
 regulated by the nature of the Pyroxyline, as well as by 
 the brilliancy of the light. The more marked the organic 
 reactions of the Pyroxyline, the greater the proportion 
 of Bromide admissible. This rule is the same as that laid 
 down for Collodion Positives (p. 260), and the directions 
 are to examine the Negative, and if the contrast between 
 the high lights and the shadows is too marked, to increase 
 the quantity of Bromide. There are varieties of Pyro- 
 xyline which will not bear any addition of Bromide, the 
 contrast of the image being too small, even with simple 
 Iodide. 
 
 Apart from all considerations of density of Negative 
 depending either upon the light or the Pyroxyline, it must 
 be admitted that the employment of Bromide in Negative 
 Collodion is most desirable for other and distinct reasons. 
 The latent image produced in the Camera appears to be 
 of a more definite description upon a bromo-iodized than 
 upon a simply iodized Collodion. In the case of the latter, 
 disturbances of all kinds are apt to arise during develop- 
 ment ; the deposit is defective in one part of the plate, 
 and irregular in another, so that to secure a Negative free 
 from all blemish is a rare occurrence. Much indeed de- 
 pends upon the skill of the operator and the purity of his 
 chemicals, but this does not lessen the value of the Bro- 
 mide, which undoubtedly exercises a remarkable influence 
 in preventing, not only over-action of light, but likewise 
 
 T 
 
274 THEORY OF PHOTOGRAPHY. 
 
 transparent markings, vertical lines, and spots of all kinds. 
 Iodide of Silver, when associated with Bromide, receives 
 the molecular modification in the Camera somewhat tardily, 
 but when once impressed, it retains the image with greater 
 force, and is not equally liable to the reception of false 
 images from moisture or traces of organic matter upon the 
 glass (p. 160). 
 
 Although in anticipation of our subject, we may here 
 remark in addition, that those minor decompositions and 
 impurities in the Negative Nitrate Bath, which so fre- 
 quently disturb the image in the case of a simply iodized 
 Collodion, will not produce the same effect when the film 
 contains Iodide and Bromide conjoined. To prove the 
 truth of this assertion, the Writer on one occasion collected 
 several impure Nitrate Baths, none of which would yield 
 a perfect Negative with ordinary Collodion, and yet he 
 succeeded in every case in taking a good impression with 
 Bromo-iodized Collodion. Bromide therefore may be 
 viewed as a useful adjunct when retarding impurities of 
 various kinds are present, against which the unassisted 
 Iodide would be unable to contend. 
 
 The foregoing remarks on the comparative advantages of 
 Iodized and Bromo-iodized Collodions apply more particu- 
 larly to their use with a Pyrogallic developer. Experience 
 has proved that the former, except under the exceptional 
 conditions mentioned, is better adapted for a Pyrogallic, 
 and the latter for an Iron developer. In the practical 
 instructions of Part III. it has therefore been deemed 
 advisable to give separately the formulae and the mode of 
 manipulation which have been found most applicable to 
 each kind. 
 
 Changes in Negative Collodion after Iodizing. All Col- 
 lodion loses sensitiveness more or less after iodizing, and 
 especially so when the Pyroxyline is unstable and liable 
 to part with a portion of its Peroxide of Nitrogen ; also 
 when the Ether is impure and in an ozonized condition. 
 Practically we esteem a Collodion iodized with Iodide of 
 Cadmium as uniform in properties, since if the chemicals 
 
NEGATIVE COLLODION. 275 
 
 are of good quality it will retain its sensitiveness nearly 
 unchanged for twelve months after iodizing. Next in sta- 
 bility to the Cadmium Collodion stands the Bromo-iodized 
 Collodion, which remains unchanged for a far longer time 
 than a simply iodized Collodion, and will retain a fair share 
 of sensitiveness for many weeks. 
 
 Negative Collodion produces as a rule a more intense 
 image when kept for a time in the iodized state ; and this is 
 true not only of Collodion iodized with the alkaline Iodides, 
 but also of that containing Iodide of Cadmium. Bromo- 
 iodized Collodion also gains in intensity as it gradually 
 decomposes, and the quality of the Negatives is usually 
 better after a few weeks' keeping than at first; they have 
 more of a red or black tone, and exhibit greater contrast. 
 
 When alkaline Iodides are used, both the simple and 
 the Bromo-iodized Collodion become more limpid by keep- 
 ing. This is an advantage in coating full-sized plates, and 
 since the setting powers of the Collodion are lessened, it 
 becomes far easier to cover a large surface before gelatini- 
 zation ensues. Old Iodized Collodion of this kind is like- 
 wise more porous and permeable by the developer than 
 newly iodized, which affords another reason why it is 
 especially suitable for large plates. 
 
 Clearness of the transparent parts in Negatives, with 
 increased sharpness of outline, are both effects of keeping 
 Collodion after iodizing. These peculiarities, as well as 
 the comparative absence of spots and markings, are due 
 partly to the " organic reactions " developed in old Col- 
 lodion, and partly to the acid state of the film, when the 
 Collodion is brown from free Iodine. 
 
 On keeping simply iodized Negative Collodion for a 
 much longer time, the amount of free Iodine often becomes 
 so great that the colour deepens to a red, and a portion of 
 Nitric Acid is liberated on dipping the film into the Bath, 
 sufficiently large to lower the density of the Negative, and 
 make it grey and metallic. 
 
 The Negative Nitrate Bath, for Iodized Collodion. 
 In the Second Section of the last Chapter the Chemistry, 
 
276 THEORY OF PHOTOGRAPHY. 
 
 of the Nitrate Bath was explained, with the action of or- 
 ganic substances upon it, and the marked effect which 
 they produce upon the development of the Negative image. 
 
 Supposing the Nitrate of Silver to be chemically pure, 
 a question arises as to the proper state of the Negative 
 Bath as regards strength, acidity, presence of Acetate of 
 Silver, etc. On these points opinions are divided. Much 
 depends upon the quality of the Collodion, and therefore 
 the observations now to be made may be said to apply 
 principally to the use of a simply iodized Collodion pre- 
 pared by the formulae given in this Work, and not to a 
 Collodion containing both Bromide and Iodide. 
 
 Negative Baths have been used with as much as forty 
 grains of Nitrate of Silver to the ounce of water ; thirty 
 grains, however, is quite sufficient, and any proportion 
 beyond this would only add to the rapidity of develop- 
 ment and tendency to staining in hot weather. When a 
 Bath, originally made with thirty grains of Nitrate of 
 Silver to the ounce of water, has been much used, the 
 proportion of Nitrate will be found on analysis to have 
 been considerably reduced. Ordinary Collodion dipped in 
 such a Bath produces a less creamy film than usual, and 
 the sensitiveness will be found to be much diminished. 
 
 The reaction of the Bath to test-paper ought to be either 
 neutral or slightly acid, an alkaline reaction being always 
 injurious. A neutral Bath produces a more dense Nega- 
 tive in dull weather, but is apt to give solarization in 
 bright sunshine. The presence of Acetic Acid obviates 
 this in great measure. An impression is not uncommonly 
 entertained that any Acid in the Bath greatly diminishes the 
 sensitiveness. With the Collodion described in Part III., 
 the Writer does not find such to be the case, and therefore, 
 in hot weather, and with a neutral colourless Collodion, he 
 recommends that Acetic Acid should be used in the Bath 
 to prevent rapid discoloration on applying the developer. 
 A minim of the Glacial Acid may be dropped into each 
 four or five ounces, if the subject be well lighted. In using 
 an Acid Bath, however, the developer should be propor- 
 
NEGATIVE NITBATE BATH. 277 
 
 tionally strengthened if the temperature or the light after- 
 wards fall. 
 
 The use of Nitric Acid in the Negative Bath has been 
 usually condemned as interfering with the precipitation of 
 the Silver in the opaque form, but in this instance also 
 everything depends upon the Collodion and the light. 
 Pyroxyline with organic reactions tends so strongly to 
 produce a brown or red image in bright sunshine, that the 
 Collodion can be w r orked successfully in a Bath containing 
 a trace of Nitric Acid. In circumstances favourable to 
 rapid development and solarization, such as Stereoscopic 
 Photography with Lenses of a short focus, the presence of 
 a minute quantity of Nitric Acid is often a positive advan- 
 tage, and no marked effect in preventing the development 
 of details in the shadows will be perceived. 
 
 The employment of Acetate of Silver in the Bath met 
 with much favour in the early days of Photography, but 
 principally so because the preparation of pure Nitrate of 
 Silver and the chemistry of Collodion were not understood. 
 There is in fact an impurity common in commercial Nitrate 
 of Silver (see p. 252) which lowers the density of Negatives, 
 and since Acetate always increases the density, its use has 
 been found in such cases to be an improvement. So, again, 
 Pyroxyline made at low temperatures and in weak Acids 
 produces a violet image, and hence Acetate of Silver, 
 which changes the colour from blue to red, increases the 
 intensity. When we add to this the fact that the pre- 
 sence of Acetate frees the Bath effectually from Nitric 
 Acid, and thus enables a Collodion to be used which has 
 been kept a long time, and is very brown from excess 
 of free Iodine, it is not difficult to understand why it 
 has been so extensively employed. If pure recrystallized 
 Nitrate of Silver be selected in the first instance, and 
 proper attention paid to the state of the Collodion, both 
 as regards the length of time it has been kept after io- 
 dizing, and the quality of the Pyroxyline, full intensity of 
 Negative may be obtained without the use of Acetate, even 
 under somewhat unfavourable conditions. 
 
278 THEORY OF PHOTOGRAPHY. 
 
 The objections to the employment of Acetate of Silver 
 in the Bath are that it renders the solution more liable 
 to change by keeping, favours red solarization in sunny 
 weather, and at high temperatures increases the chance 
 of spots, markings, and discoloration of the developer on 
 touching the film. 
 
 The Negative Nitrate Bath for Bromo-iodized Collo- 
 dion. Experience has shown that a Nitrate Bath specially 
 prepared for an Iodized Collodion with Pyrogallic de- 
 veloper, is not the best suited for a Bromo-iodized Collo- 
 dion with Iron developer, and vice versd. In the former 
 case the aim is to obtain conditions of the Nitrate Bath 
 which will combine the greatest sensitiveness with suffi- 
 cient intensity of Negative at one development. When 
 the chemicals are pure such a combination is easily attain- 
 able. In the latter case density and sensitiveness combined 
 seem incompatible. But as intensity of image is quite 
 under control, by a subsequent re-development, it is con- 
 sidered more important to prepare the Nitrate Bath almost 
 entirely with the view of securing the highest sensitiveness. 
 Acetate of Silver and Acetic Acid, which in the Nitrate 
 Bath for Iodized Collodion often exercise a beneficial 
 effect in increasing both density and sensitiveness, here 
 act differently. They increase the intensity of image, 
 but tend to lengthen the exposure. A small proportion of 
 Nitric Acid, on the other hand, materially increases the 
 sensitiveness, and at the same time lowers the vigour of 
 the image ; but since the latter can be raised to any extent 
 by subsequent development, it is generally sacrificed in 
 the first instance in order to secure the former. Directions 
 for the preparation of this Bath will be found in Part III. 
 
 Developing Solutions for Collodion Negatives. Three 
 formulae are given in Part III., each of which has its ad- 
 vantages. The first contains Pyrogallic Acid with Acetic 
 Acid ; the second, Pyrogallic Acid with Citric Acid ; and 
 the third, Sulphate of Iron with Acetic Acid. 
 
 Developers for Iodized Collodion. Pyrogallic Acid with 
 Acetic Acid is the form most commonly employed, the use 
 
. NEGATIVE DEVELOPERS. 279 
 
 of the Acetic Acid being to moderate the violence of the 
 action, and to preserve those parts of the Iodide which 
 have not been touched by light. It has also the advan- 
 tage of causing the solution to flow easily upon the film, 
 thus forming a substitute for Alcohol, which would other- 
 wise be required to prevent greasiness and streaks. The 
 ordinary strength of the solution of Pyrogallic Acid is 
 one grain to the ounce of Water, but as regards the 
 quantity of Acetic Acid, the practice of operators varies. 
 The intensity is greatest when the minimum proportion of 
 about five minims to the ounce is used, but in this case a 
 portion of spirit will be required in covering large plates. 
 With a Negative Collodion giving abundance of intensity 
 it is better in every respect to employ the full strength of 
 acid, viz. from twenty to thirty minims to the ounce, by 
 which the reduction will be rendered more even, and stains 
 of irregular action prevented. 
 
 Attention should here be drawn to the decomposition 
 which commercial Pyrogallic Acid experiences by keeping. 
 In the course of a very few days in hot weather the solu- 
 tion becomes yellow, and not only loses, in some measure, 
 its power of developing the weakest radiations, but rapidly 
 discolours the Nitrate upon the film. Solution of Sulphate 
 of Iron also becomes yellow by keeping, but in this case no 
 injurious effect results except in the weakening of the de- 
 veloper. 
 
 Pyrogallic Acid with Citric Acid may be viewed as a 
 weaker reducing agent than the last, and one less likely to 
 develope the dark shadows after a short exposure. Citric 
 Acid, in fact, is an agent of far greater power in retarding 
 reduction of Silver, than Acetic Acid, and three-quarters of a 
 grain will be found more than equivalent to twenty minims 
 of the latter. This property of Citric Acid is an advantage 
 when working at high temperatures, as, for instance, in a 
 tent heated by the direct rays of the sun. The ordinary 
 developer then acts so quickly that it is impossible to cover 
 the plate before the reduction begins, and the discoloration 
 on touching the film is rapid, so that the pictures are al- 
 
280 THEORY OF PHOTOGEAPHY. 
 
 most invariably weak and red, with stains and spots. In 
 this state of things a stronger Acid will be found service- 
 able, and although the ordinary proportions are twice as 
 much of Pyrogallic Acid as of Citric Acid, yet in extreme 
 cases, the relative amount of Acid may be doubled, and a 
 grain of each constituent of the formula be dissolved in 
 an ounce of Water. The solution will flow evenly over 
 the film on adding Alcohol, and the image will not ap- 
 pear until after an interval of twenty or thirty seconds. 
 At so high a temperature the reducing power of even a 
 feeble developer will be quite enough to bring out the 
 shadows. 
 
 Citric Acid in the developing solution changes the colour 
 of the Negative from brown to blue, and in consequence 
 the real intensity of the image is somewhat less than it 
 appears. It also has a marked effect in preventing red 
 solarization in a brilliant light, and in preserving the sur- 
 face of the film from fogging. Hence it is particularly 
 adapted for distant landscape views including sky and 
 water, or for other subjects well lighted ; whilst, on the 
 other hand, it is not suited for working in a glass house in 
 a bad light and in cold weather, nor for copying works of 
 art with long-focus Lenses, nor for taking interiors. In all 
 such cases we may anticipate, when using a developer con- 
 taining Citric Acid, that the image in its early stage of 
 development will be weak and metallic, showing nothing 
 of that tone of red which is so essential to the proper con- 
 tinuance of the precipitation. There will also be a loss of 
 half-tint, from the reducing agent being too weak to bring 
 out the darker portions of the image. 
 
 Pyrogallic Acid with Citric Acid forms a good developer 
 for a Cadmium landscape Collodion. Free Iodine being 
 absent from this Collodion, the film on leaving the Bath 
 is nearly neutral, and no Nitric Acid is present. Con- 
 sequently the high lights over-act, and render the image 
 very feeble unless the developer contain a stronger acid 
 than Acetic Acid. 
 
 Sulphate of Iron is a developer of great power, and may 
 
NEGATIVE DEVELOPERS. 281 
 
 be employed with advantage when its action is correctly 
 understood. Being a substance belonging to the mineral 
 kingdom, it is not favourably constituted for producing that 
 opacity of image which is characteristic of Pyrogallic Acid ; 
 but on the other hand it is a much stronger reducing 
 agent, and will bring out a perfect picture, when from 
 some opposing conditions, the ordinary developer proves 
 ineffectual. 
 
 In order that an Iodized Collodion may be adapted for 
 developing with Sulphate of Iron, so as to give sufficient 
 intensity at one operation, it ought to be of that kind 
 which gives strong contrast of image. It must likewise 
 be a Collodion working clean and free from fogging, inas- 
 much as the Sulphate of Iron has a tendency to precipi- 
 tate the Silver in an irregular manner upon the film greater 
 than that of the Pyrogallic Acid. 
 
 The state of the Nitrate Bath must also be considered 
 in making choice of a developer. Newly -prepared Baths 
 which yield rather faint Negatives with Pyrogallic Acid, 
 seldom succeed with Sulphate of Iron : the image becomes 
 rapidly fogged and is useless. Baths made from fused 
 Nitrate of Silver ; Baths containing Acetate ; and old 
 Baths contaminated with organic matter, can on the other 
 hand be worked more successfully with Sulphate of Iron 
 than with Pyrogallic Acid. The rule appears to be that 
 if the solution is in a state for giving very red Negatives 
 with great intensity and freedom from fogging, the inor- 
 ganic developer will be the better of the two : for in that 
 case the organic reactions of the film are already at their 
 maximum, and do not further need an organic developer 
 like Pyrogallic Acid. 
 
 Depression of temperature is always an indication for 
 the use of Sulphate of Iron, and in such a case its superiority 
 is especially evident. When the thermometer sinks to 40 
 Fahrenheit, it will be found that the ordinary solution of 
 Pyrogallic Acid and Acetic Acid acts very slowly, and, in 
 the case of Collodion which has undergone a little organic 
 decomposition, it does not bring out the dark shades effec- 
 
282 THEOEY OF PHOTOGRAPHY. 
 
 tually, so that on examining the Negative after fixing, it 
 appears to have received an insufficient amount of exposure 
 in the Camera. An increase in the strength of the solu- 
 tion, using more of the Pyrogallic Acid and less Acetic 
 Acid, does not altogether remedy the defect, although it 
 adds to the opacity of the parts which have received most 
 light. The substitution of Citric Acid for the Acetic 
 would weaken the reducing power, and be a positive evil. 
 Nothing then remains but either to warm the Bath and 
 developing room by a stove, or to employ the Sulphate of 
 Iron. 
 
 The difficulty of securing a proper gradation of tone is 
 especially felt, not only in cold weather, but also when 
 the picture embraces a variety of objects which contrast 
 strongly in their power of reflecting light. Pyrogallic Acid 
 as a developer is apt either to destroy the definition in the 
 light parts by producing absolute opacity of the Negative, 
 or to exhibit all the effects of over-action of light and red 
 solarization. In this way the folds of white drapery suffer, 
 and the distance in landscapes is lost. Such subjects can 
 be photographed successfully, either by using a feeble 
 Negative Collodion destitute of organic reactions, or by 
 developing with Sulphate of Iron. Monuments of white 
 marble standing out against the sky, with cypress-trees in 
 the background, have been well copied by using Sulphate 
 of Iron ; and although equally good results may be ob- 
 tained with Pyrogallic Acid, by giving a long exposure 
 and working with a small aperture to the Lens yet this 
 can only be expected when the Bath and Collodion are in 
 the most perfect working order. 
 
 There are other causes of imperfect gradation of tone 
 which Sulphate of Iron is employed to remedy. When, 
 for instance, the atmosphere is yellow or murky, and no 
 clouds exist to throw back the light into the shadows, it 
 becomes very difficult to work a simply iodized Collodion 
 successfully with Pyrogallic Acid : the blacks of the Nega- 
 tive are too opaque, and the shadows too transparent. A 
 longer exposure in the Camera in this instance is only a 
 
NEGATIVE DEVELOPEES. 283 
 
 partial remedy, because it invariably flattens the picture, 
 destroying its rotundity and stereoscopic effect. The use 
 of Sulphate of Iron is exactly adapted to meet the case, 
 for it has great power in developing weak radiations, whilst 
 at the same time it precipitates the Silver in a compara- 
 tively metallic and translucent form. The resulting Ne- 
 gative is therefore soft, and free from violent contrasts of 
 light and shade. 
 
 It has been said that the employment of Sulphate of 
 Iron invariably shortens the necessary exposure in the 
 Camera, but this statement is incorrect. That it does so 
 in a bad light and at a low temperature is certain, but 
 probably if the experiment be made under opposite con- 
 ditions, the same result will not be obtained. In the case 
 of a sun-lit view, for instance, taken in the spring months, 
 the Writer finds Pyrogallic Acid abundantly strong enough 
 to bring out the weaker radiations with a minimum of 
 exposure, the Collodion being supposed to contain only 
 Iodides. 
 
 Acetate of Iron has been used in Photography : it pro- 
 duces nearly as much density as Pyrogallic Acid, and at 
 the same time is equal to the Sulphate in its power of deve- 
 loping the shadows. A similar effect may be obtained by 
 mixing the ordinary Sulphate of Iron with half its weight 
 of crystallized Acetate of Soda. 
 
 Effect of Varying the Mode of Development. The re- 
 marks made in the last Section, on the contrast in Collo- 
 dion Positives as affected by the mode of development, 
 apply also in the case of Negatives. We have already 
 seen that an ordinary Iodized Collodion may, when em- 
 ployed in the subdued light of a glass studio, produce a 
 soft Negative, and yet in a glare of sunshine it may yield 
 an intense Negative. If the intensity be greater than is 
 desirable, it is in the power of the operator to remedy it 
 in a measure by removing a portion of the free Nitrate of 
 Silver from the surface of the film. To effect this, it will 
 only be necessary to flood the plate with a large quantity 
 of a diluted solution of Pyrogallic Acid containing perhaps 
 
284 THEORY OF PHOTOGRAPHY. 
 
 half a grain of the reducing agent to the ounce of water : 
 or, more effectually still, to wash the plate with water, after 
 exposure, and then to develope it by the addition of a few 
 drops of the Bath solution to the Pyrogallic Acid. The 
 picture thus obtained will have less contrast and solarization 
 than before, and the developing action may be pushed 
 far enough to bring out the deepest shadows, without 
 adding too much to the intensity of the lights. When the 
 converse of the foregoing happens, and the image on a 
 simply Iodized Collodion is deficient in contrast, it is re- 
 commended to increase the re]ative proportion of Nitrate 
 of Silver, by making an addition of that substance to the 
 developer before applying it to the film. 
 
 Developer for Bromo-iodized Collodion. However much 
 opinions may be divided as to the relative merits of Pyro- 
 gallic Acid and Sulphate of Iron as developers for a sim- 
 ply Iodized Collodion, no doubt exists that the latter is 
 the more suitable for a Bromo-iodized Collodion. The 
 great reducing power of the Salts of Iron is precisely what 
 we require in the presence of Bromide of Silver, a salt 
 which considerably retards reduction ; and, as already 
 shown, the fine red or black tone which Pyrogallic Acid 
 imparts to the Negatives is not seen when the Collodion 
 contains Bromide. The tendency to produce fogging which 
 the Salts of Iron exhibit in the case of a simply iodized 
 Collodion, is nearly absent when the Collodion contains 
 Bromide. A subsequent development with Pyrogallic 
 Acid will almost in every instance be required, which ren- 
 ders the operation somewhat tedious, but it is amply com- 
 pensated for by the increased vigour and contrast thereby 
 obtained. 
 
 Fixing Agents for Negatives. Cyanide of Potassium 
 acts quickly in removing the Iodide of Silver, and the 
 plates do not require much subsequent washing ; the film 
 is also left in a favourable state for continuing the develop- 
 ment with mixed Pyrogallic Acid and Nitrate of Silver 
 when required. We recommend, however, Sulphocya- 
 nides or Hyposulphite in preference to the Cyanide of 
 
NEGATIVE STRENGTHENED. 285 
 
 Potassium, as safer in the hands of a beginner. Negative 
 images are more easily dissolved by fixing agents than 
 Collodion Positives, and therefore, unless much care be 
 exercised, the application of Cyanide lowers the intensity 
 materially, and whitens the surface of the picture by con- 
 verting it into Cyanide of Silver. This is especially the 
 case in working in the open air, and attempting to fix the 
 image whilst the sun is shining upon the plate. Collodion 
 Negatives developed with Sulphate of Iron are less soluble 
 in solution of Cyanide of Potassium than those in which 
 Pyrogallic Acid is used as the reducing agent ; and the 
 use of Bromide in the Collodion likewise diminishes the 
 solubility. 
 
 Modes of strengthening a finished Impression which is 
 too feeble to be used as a Negative. The ordinary plan 
 of pushing the development cannot be applied with advan- 
 tage after the picture has been washed and dried. In that 
 case, if it is found to be too feeble to print well, its inten- 
 sity may be increased by one of the following methods : 
 
 1. Treatment of the image with Sulphuretted Hydrogen 
 or Hydrosulphate of Ammonia. The object is to convert 
 the metallic Silver into Sulphide of Silver, and if this 
 could be done, it would be of service. The mere applica- 
 tion of an Alkaline Sulphide has however but little effect 
 upon the image, excepting to darken its surface and de- 
 stroy the Positive appearance by reflected light : the struc- 
 ture of the metallic deposit is too dense to admit of the 
 Sulphur reaching its interior. 
 
 This may be obviated by first converting the image into 
 the white Salt of Mercury and Silver by the application 
 of Corrosive Sublimate, and afterwards treating it with a 
 solution of Sulphuretted Hydrogen or Hydrosulphate of 
 Ammonia. Negatives produced in this way are of a brown- 
 yellow colour by transmitted light, and remarkably opaque 
 to chemical rays. 
 
 The employment of Corrosive Sublimate has one se- 
 rious drawback, viz. the injurious effect which even a 
 trace of a Salt of Mercury exerts upon the sensitiveness 
 of Iodide of Silver: hence if the glass plates are not 
 
286 THEORY OF PHOTOGRAPHY, 
 
 cleaned with extraordinary care, or if the slightest portion 
 of the Mercury Salts finds its way into the Bath, injurious 
 effects will follow. The Writer dispenses entirely with the 
 employment of the Sublimate, and acts on the image with 
 a solution of Iodine in Iodide of Potassium until it is con- 
 verted into Iodide of Silver, after which the Hydrosul- 
 phate is applied in the usual way. The Hydrosulphate is 
 in fact the principal agent in producing the intensity, and 
 no other chemical is actually required, excepting for the 
 purpose of rendering the image sufficiently porous to allow 
 of a proper penetration by the Sulphur. Practical direc- 
 tions for carrying out this intensifying process, will be 
 found in Part III. under the head of " Copying Maps and 
 Diagrams." 
 
 2. MM. Barreswil and Davannes Process. The image 
 is converted into Iodide of Silver by treating it with a 
 solution made by dissolving a grain of Iodine in an ounce 
 of water by the aid of a little Alcohol. It is then washed, 
 to remove the excess of Iodine, exposed to the light, 
 and a portion of the ordinary developing solution, mixed 
 with Nitrate of Silver, poured over it. The changes which 
 ensue are precisely the same as those already described 
 (p. 150) ; the whole object of the process being to bring the 
 metallic surface back again into the condition of Iodide of 
 Silver modified by light, that the developing action may 
 be commenced afresh, and more Silver deposited from the 
 Nitrate in the usual way. 
 
 In a former Edition a solution of Iodine in Iodide of 
 Potassium was recommended for the conversion of the 
 image into Iodide in this intensifying process. This method 
 however is liable to fail, for if the solution be kept upon 
 the plate until the whole image becomes yellow, the sen- 
 sitiveness to light is in great measure lost. At present 
 therefore the Writer is of opinion that a weak aqueous 
 solution of Iodine is preferable, and that the condition of 
 the film so produced is analogous to that of the Daguerreo- 
 type, the metallic Silver being acted on superficially, and 
 never entirely converted into Iodide. 
 
287 
 
 CHAPTER VIII. 
 
 ON THE THEOBY OF POSITIVE PEINTING. 
 
 THE subject of Collodion Negatives having been explained 
 in the previous Chapter, we proceed to show how they may 
 be made to yield an indefinite number of copies with the 
 lights and shadows correct as in nature. 
 
 Such copies are termed " Positives," or sometimes 
 " Positive prints," to distinguish them from direct Posi- 
 tives upon Collodion. 
 
 There are two distinct modes of obtaining photographic 
 prints ; first, by development, or, as it is termed, by the 
 Negative process, in which a layer of Iodide or Chloride of 
 Silver is employed, and the invisible image developed by 
 Gallic Acid ; and second, by the direct action of light upon 
 a surface of Chloride of Silver, no developer being used. 
 These processes, involving chemical changes of great deli- 
 cacy, require a careful explanation. 
 
 The action of light upon Chloride of Silver was described 
 in Chapter II. It was shown that a gradual process of 
 darkening took place, the compound being reduced to the 
 condition of a coloured subsalt ; also, that the perfection 
 of the change was increased by the presence of excess of 
 Nitrate of Silver, and of organic matters, such as Gelatine, 
 Albumen, etc. 
 
 We have now to suppose that a sensitive paper has 
 been prepared in this way, and that a Negative having 
 been laid in contact with it, the combination has been ex- 
 posed to the agency of light for a sufficient length of time. 
 
288 THEOEY OF PHOTOGRAPHY. 
 
 Upon removing the glass, a Positive representation of the 
 object will be found below, of great beauty and detail. 
 Now if this image were in its nature fixed and permanent, 
 or if there were means of making it so, without injury to 
 the tint, the production of Paper Positives would be a 
 simple department of the Photographic art; for it will 
 be found that with almost any Negative, and with sen- 
 sitive paper however prepared, the picture will look tole- 
 rably well on its first removal from the printing-frame. 
 Immersion in the bath of Hyposulphite of Soda however, 
 which is essentially necessary in order to fix the picture, 
 produces an unfavourable effect upon the tint, decom- 
 posing the violet- coloured Subchloride of Silver, and leav- 
 ing behind a red substance which appears to be united to 
 the fibre of the paper. 
 
 Other chemical operations are therefore required to re- 
 move the objectionable red colour of the print, and hence 
 the consideration of the subject is naturally divided into 
 two parts ; first, the means by which the paper is rendered 
 sensitive, and the image impressed upon it; and secondly, 
 the subsequent fixing and toning, as it may be termed, of 
 the proof. 
 
 The present Chapter will also include, in an additional 
 Section, a condensed account of the most important facts 
 relating to the properties and the mode of preservation of 
 photographic prints. 
 
 SECTION I. 
 
 Printing the Proof., 
 
 When a sheet of the photographically prepared paper 
 is exposed to the light, we observe it to assume various 
 colours, each one deeper and more intense than that which 
 preceded it. These shades of colour are not always the 
 same, but vary more or less with the mode of preparing 
 the paper, as will presently be shown. The sequence of 
 tints, in the case of a paper prepared simply with Chloride 
 .and Nitrate of Silver, is as follows: Pale violet, violet- 
 
POSITIVE FEINTING. 289 
 
 blue, slate-blue, bronze or copper-colour. When the 
 bronzed stage is readied, there is no further change. 
 
 On immersing the paper darkened as above described, 
 in the fixing bath of Hyposulphite, the violet tones are 
 destroyed, and the print assumes a red or brown colour, 
 which is most intense in the parts where the light has 
 acted longest. Therefore we see, that, to produce a good 
 photographic print, the Negative must possess consider- 
 able opacity in the dark parts ; for if it be pale and feeble, 
 the light passes rapidly through it and darkens the paper 
 universally, before the exposure has been sufficiently pro- 
 longed to ensure the requisite degree of reduction ; hence 
 the deepest shadows of the resulting Positive are not dark 
 enough, and there is a want of contrast which is fatal to 
 the effect. A good Negative should be so opaque as to 
 preserve the lights of the printed image beneath clear, un- 
 til the darkest shades are about to pass into the bronze or 
 coppery condition. If the amount of intensity be less than 
 this, the finest effect cannot be obtained. 
 
 Let us now pass on to consider more carefully the exact 
 function of each of the constituents of the sensitive sheet, 
 and to show how the effect may be varied by altering their 
 relative proportions, or by introducing substances not 
 usually employed. 
 
 The printing process in its most simple form may be 
 conducted as follows : Take pure Swedish filtering-paper, 
 free from size and other extraneous matters, and float it upon 
 a solution of Nitrate of Silver containing about one hun- 
 dred grains to the ounce of water : then dry and expose it 
 to a strong sunlight. The darkening action will take place, 
 but with such extreme slowness as at first to convey the 
 impression that the paper is quite insensitive to light ; by 
 perseverance, however, for three or four days, a pale-brown 
 tone will be obtained. One cause of this difficulty with 
 the simply Nitrated paper is that the Nitric Acid in the 
 Nitrate of Silver retards the reduction, and the pure fibre 
 of paper does not possess a sufficient affinity for Oxygen 
 to enable it to overcome the opposition. It is possible 
 
 u 
 
290 THEORY OF PHOTOGRAPHY. 
 
 however to counteract the Nitric Acid by adding Am- 
 monia so as to produce Ammonio-Nitrate of Silver : this 
 accelerates the change considerably, and a few hours' ex- 
 posure to strong sunlight will then give the requisite 
 opacity. 
 
 Swedish paper, however, although prepared with Am- 
 monio-Nitrate of Silver, darkens very slowly excepting in 
 strong sunlight, and the Photographer will find by experi- 
 ment that a minute quantity of Chloride of Silver in the 
 prepared paper will enable him to obtain the desired result 
 in minutes instead of hours of exposure. Other insoluble 
 Salts of Silver, such as the Phosphate and Citrate, render 
 the prepared paper more sensitive than when it has been 
 treated with a soluble Salt of Silver only. The part there- 
 fore which we assign to such insoluble Salts is that of 
 "accelerators" to the luminous agency, and of all accele- 
 rators the Chloride of Silver appears to be the most re- 
 markable. 
 
 The following experiment will prove instructive in fur- 
 ther exhibiting the function of the Chloride of Silver in 
 ordinary sensitive paper. Take a piece of ordinary bibu- 
 lous paper and float it for an instant upon the Nitrate of 
 Silver solution which- Photographers employ in printing ; 
 then blot it off, and immerse for five minutes in a solution 
 of common salt containing ten grains to the ounce. This 
 paper when freed from excess of salt by washing in dis- 
 tilled water, may be viewed as containing only Chloride of 
 Silver in contact with the Cellulose. On exposing it to the 
 light it will be found to change rather quickly to a pale- 
 violet tone. At that point, however, the reducing action 
 will be suspended, and when the fixing bath of Hyposul- 
 phite of Soda is brought to bear upon the image, it will 
 nearly disappear in consequence of decomposition and solu- 
 tion. Pictures printed upon Chloride of Silver only would 
 be altogether wanting in contrast, consisting only of half- 
 tints, without any depth of shadow. 
 
 In a second experiment, take several strips of paper pre- 
 pared as described for experiment No. 1, and apply washes 
 
POSITIVE FEINTING. 291 
 
 of Nitrate of Silver of various degrees of concentration, 
 such as 5, 10, 20, 40, 80, and 100 grains to the ounce of 
 water. On drying the strips, and exposing them succes- 
 sively beneath the same Negative matrix, it will be found 
 that the pictures become more and more vigorous in pro- 
 portion as the Nitrate of Silver solution increases in 
 strength, but that beyond a certain point a further increase 
 in the concentration of the Nitrate of Silver does not add 
 to the effect. 
 
 Having performed the above experiments, we are pre- 
 pared to conclude therefrom, that the office of the free 
 Nitrate of Silver is to furnish the material which composes 
 the metallic part of the image, and so to give intensity ; the 
 Chloride meanwhile accelerating the change and adding to 
 the sensitiveness of the prepared paper. 
 
 It remains now to consider the action of the organic sup- 
 porting basis, and this is perhaps equally important with 
 that of the other constituents, although in the present state 
 of our knowledge it cannot be defined with the same pre- 
 cision. If we take a sheet of Swedish filtering-paper and 
 immerse it in cold Nitro- Sulphuric Acid, in such a way as 
 to wet only one-half of the paper, the surface after wash- 
 ing and drying will consist in part of Cellulose and in part 
 of Pyroxyline. Now it has already been shown (p. 135) that 
 Pyroxyline is in a manner indifferent to the Salts of Silver, 
 and that Chloride of Silver, supported by Pyroxyline, be- 
 haves in the sun's ray much in the same manner as Chloride 
 of Silver supported upon a glass plate : hence we should 
 anticipate that if a sheet of paper were converted only par- 
 tially into Pyroxyline, and subsequently treated with Salt 
 and Nitrate of Silver, the two halves would behave diffe- 
 rently on exposure. This expectation is correct, and the 
 experiment will show not only that the darkening is more 
 decided upon the unaffected Cellulose, but that the image is 
 less dissolved by the fixing bath, and has a softer and more 
 velvety shade of colour. In the one case the fixed print 
 is of a warm red, and tones in solution of Chloride of Gold 
 to a fine purple-black j in the other, it is very faint and 
 
292 THEOBY OF PHOTOGRAPHY. 
 
 metallic, or, if examined after toning, cold and slaty in 
 aspect. 
 
 We may therefore add to what we have before said, 
 the following statement, viz. that whereas the function of 
 the Chloride is to impart sensitiveness in Photographic 
 printing, and that of the Nitrate of Silver to give inten- 
 sity, the Organic Matter acts by brightening the colour. 
 The artist requires an image which after simply fixing 
 shall be of a warm red tone, and thus be capable of yield* 
 ing a full brown or black on subsequent treatment with 
 the Gold solution ; he will find by experience that organic 
 compounds of Silver in the paper, produced by adding 
 Albumen or similar substances to the salting bath, will 
 afford him the means of obtaining these varied tones, and 
 that without them the picture will lack richness of effect. 
 . The chemist may perhaps be disposed to inquire more 
 particularly how the organic substance acts ; but we must 
 be guarded in answering this question, because it involves 
 the consideration of a class of actions which belong to an 
 obscure Chapter of Chemistry. It is known that many 
 oxides and subsalts of metals, attach themselves in a pe- 
 culiar way to animal and vegetable fibre, although the 
 precise nature of the union is uncertain. These same 
 Oxides commonly exhibit an affinity for the colouring mat- 
 ters used in dyeing, and are known as "Mordants," be- 
 cause they bind the colours on to the cloth and fix them so 
 that they resist the action of water. An ordinary " iron 
 mould " is a familiar instance of this kind of action, the 
 red stain upon the linen consisting of an Oxide or Subsalt 
 of Iron, adhering to the fibre. Organic substances satu- 
 rated with Bichromate of Potash and exposed to light, 
 furnish another example, for it has been shown that the 
 Bichromate becomes in such a case reduced to an Oxide of 
 Chromium, which is a true mordant, although a feeble one. 
 Now in the process of Photographic printing we suppose 
 that besides the formation of Subchloride of Silver, Pro- 
 toxide of Silver is reduced to the state of a lower or sub- 
 oxide, and that this suboxide combines with the Cellulose. 
 
POSITIVE FEINTING. 293 
 
 Further, the fact of the Cellulose or other organic matter 
 having an affinity for a suboxide facilitates the formation 
 of that substance, and enables it when formed to withstand 
 the action of bodies like Hyposulphite of Soda, which are 
 known to possess the property of decomposing Suboxide 
 of Silver when existing in an uncombined state. 
 
 If Photographic printing can be shown to bear any ana- 
 logy to the operations employed in the art of dyeing, it 
 would be anticipated that certain kinds of fibre would ex- 
 hibit the affinity for the mordant oxide more completely 
 than others. It is well known, for instance, that woollen 
 stuffs take certain dyes with more facility than materials 
 made of linen or cotton, so that if a cloth be woven partly 
 of wool and partly of cotton, a colour may be fast upon 
 the former but removable by washing from the latter. So 
 in Photography we find that cotton immersed in Nitrate 
 of Silver is less readily affected by the sun's rays than 
 wool or silk. In speaking of the use of animal substances 
 in printing we must not indeed lose sight of the fact that 
 these tissues invariably contain traces of Chloride and also 
 of Phosphate. The pure animal fibre, however, is believed 
 to play an important part in the process, quite indepen- 
 dently of any inorganic salts. This action of the animal 
 matters we now proceed to consider further. 
 
 Swedish paper, prepared with Chloride and Nitrate of 
 Silver, although sufficiently sensitive, could not be used in 
 Photography. The picture would exhibit a complete want 
 of definition, and would also appear to be sunk in the sub- 
 stance of the paper so as to be seen more distinctly by 
 transmitted than by reflected light. If the Salt were em- 
 ployed in anything like quantity, Chloride of Silver would 
 form in loose flakes upon the surface, and would burst 
 out and fall away into the Bath. The sizing of Photo- 
 graphic paper has undoubtedly a mechanical action, keep- 
 ing the chemicals upon the surface, and thus securing an 
 even layer of Chloride of Silver in a state of excessive 
 division. This however, as already shown, is not the sole 
 use of the sizing, for when it consists of Albumen or Gela- 
 
294 THEORY OF PHOTOGRAPHY. 
 
 tine it communicates a fine red tint to the image, and gives 
 TV hat artists term an aspect of transparency to the whole 
 picture. 
 
 If we examine the action of the animal sizing chemically, 
 we find that Albumen, Caseine, and G-elatine, all withdraw 
 from the Bath large quantities of Nitrate of Silver, so that 
 the solution becomes continually weaker. A sheet of trans- 
 parent Gelatine, allowed to swell up by imbibing a solution 
 of Nitrate of Silver, of the strength of twenty grains to 
 the ounce of Water, will appropriate nearly the whole of 
 the dissolved Nitrate, so that the liquid expelled from it 
 by squeezing will yield less than three grains of Silver to 
 the ounce. The Gelatine forms with the Nitrate of Silver 
 a compound which may be designated Gelatino-Nitrate of 
 Silver, and which is highly photographic and colorific. 
 Chloride of Silver thrown down in presence of this Gela- 
 tino-Nitrate, does not clot together in the same manner as 
 the pure Chloride of Silver, but exists in a state of exces- 
 sive division, and remains for a long time without subsiding 
 to the bottom of the liquid. Exposed to light the Gela- 
 tino-Nitrate of Silver darkens to a ruby-red colour ; and 
 Chloride of Silver precipitated from an aqueous solution of 
 the Gelatino-Nitrate never assumes, in the sun's rays, the 
 slate-blue colour characteristic of Subchloride of Silver, but 
 changes quickly to a chocolate-brown tone. In the case of 
 a paper sized with Albumen or Gelatine, and subsequently 
 salted and rendered sensitive, the action of the light is evi- 
 dently compound, for the Chloride decomposes at the same 
 time that the Oxide of Silver is reduced by the animal mat- 
 ter. The behaviour of the darkened paper always varies 
 with the proportion which the Chloride bears to the organic 
 substance : when the former is relatively large, the print 
 exhibits the violet shades on its removal from the frame, 
 and dissolves considerably in the fixing bath; but with 
 the organic matters in excess, the colour of the print is 
 brick-red from the very first, and the tones are less affected 
 by the solvent action of the Hyposulphite of Soda. 
 
POSITIVE FEINTING. 295 
 
 FUETHER OBSEEVATIONS ON THE FEINTING OF FHOTO- 
 GEAFHIC PEOOFS. 
 
 In considering the process of Positive printing more carefully, 
 we may divide it as follows : The Paper ; Salting Bath ; Sensi- 
 tizing Solution ; Various Kinds of Organic Matter. 
 
 The Paper. The quality of paper sold for Photographic pur- 
 poses is variable, and often inferior. A difference exists in the 
 length and thickness of the fibre of various kinds of Cellulose, 
 so that the resulting paper may be either coarse or smooth grained. 
 Its smoothness cannot be estimated until the print has passed 
 through all the processes of fixing and washing, because by hot- 
 pressing and other appliances, it is easy to get up a fictitious 
 glaze. 
 
 Supposing the paper to be properly made in the first instance, 
 yet much will depend upon the perfection of the process adopted 
 for sizing. The defects which occur in a paper badly sized are 
 of the following kind : First, portions of the finished pictures 
 are pale in colour, and have a spotty appearance, due to an in- 
 equality in the imbibition of liquid by the paper ; some parts of 
 which being comparatively impermeable by the Nitrate of Silver, 
 and others more porous, the surface is unequally sensitive, and 
 darkens in an irregular manner. Secondly, when the sizing is 
 very soft, the chemicals sink too deeply into the paper, and the 
 proofs are what is termed mealy, and ineffective. Albumen, even 
 when employed without any addition of water, gives very little 
 glaze upon a paper of this kind, and no surface vigour can be 
 obtained : the sheets are often very tender, so that they become 
 torn in the many washings to which the photographic proofs are 
 necessarily subjected ; and when Albumen is used, there is a 
 strong tendency to a superficial blistering in the fixing bath, or 
 in the washing waters, inasmuch as the Alkaline solutions used 
 in toning tend to lessen the tenacity of the size. Thirdly, paper 
 may be too strongly sized, and when such is the case, the amount 
 of gloss given by Albumen is considerable, but the prints are not 
 easily toned in the Gold solutions, and are fixed with difficulty. 
 
 There are two principal modes of sizing paper : first, with a 
 mixture of Starch and Resin partially saponified by an Alkali ; 
 and second, with Gelatine hardened by Alum. The first method 
 is principally practised on the Continent, and the latter in the 
 
296 THEOEY OF PHOTOGRAPHY. 
 
 papers of home manufacture. Papers sized with Starch and 
 saponified Eesin have necessarily an alkaline reaction, whilst the 
 Gelatine sized papers exhibit an acidity due to the Alum. In 
 each case the paper improves more or less by keeping, because 
 the size becomes gradually harder, and the sheets are, in conse- 
 quence, less easily torn in the washings. Opinions are divided as 
 to which mode of sizing is to be preferred, but the general im- 
 pression is, that the Starch offers more mechanical advantages 
 when Albumen is to be used in the salting solution, whereas the 
 Gelatine size gives a better surface layer of Chloride of Silver 
 in the case of plain salted paper prepared without Albumen. 
 Towgood's paper is held in much esteem for printing by the 
 Ammonio- Nitrate process, and probably answers better than any 
 other in the market, but it is not well adapted for Albuminizing. 
 Papier Rive, so called from the place where it is manufactured 
 in France, is a starch-sized paper, with a hard, smooth surface, 
 not easily permeable by liquids ; hence it takes a high gloss on 
 the Albumen. It is somewhat rotten in texture, and apt to tear 
 in the washing, but it assumes a fine, brilliant colour in the 
 toning-bath, and is therefore much used for carte-de-visite and 
 stereoscopic prints. The German paper, usually called Saxe, 
 also starch-sized, is stronger in texture than the Eive, and well 
 fitted for large photographs. From its more porous texture, 
 it does not assume so high a gloss on the Albumen as the 
 Eive. 
 
 The Photographic properties of the paper are much affected by 
 the mode of sizing adopted, even when Albumen is afterwards 
 used, for the picture is probably formed partly upon the Albumen, 
 and partly in the sizing. English papers tend to give red tones 
 which become brown or chocolate-coloured in the finished print. 
 This is due in part to the use of Gelatine, which, as before shown, 
 forms a compound with Nitrate of Silver, darkening in the sun 
 to a ruby -red colour ; but in part to the Alum employed to harden 
 the Gelatine, since Alum is an acid salt, and acids tend to impart 
 a foxy-red tone to the image. The foreign papers sized with 
 Starch or Eesin, produce tones which are of a sepia-brown after 
 fixing, and of a purple-black when treated with the Solution of 
 Gold ; the reason is partly because the Starch and Eesin do 
 not, like Gelatine, exert a very marked action in reddening the 
 picture, and partly because the sizing has an alkaline reaction. 
 
POSITIVE FEINTING. 297 
 
 and alkalies are found to diminish redness, just as acids in- 
 crease redness. 
 
 Many additional observations might be made on the colour of 
 the image yielded by the various commercial qualities of Photo- 
 graphic paper, but the above general division into Gelatine-sized 
 paper and Starch-sized paper will be sufficient. At the same 
 time, it should be borne in mind that the manufacturer has it in 
 his power, by adding small quantities of organic bodies to the 
 size, to modify the tone given by the paper, even when employed 
 in the albuminized state. Hence, although the general characters 
 of a paper may be those of a Starch-sized paper, yet the print 
 may assume a brick-red colour in place of a violet tone, in con- 
 sequence of some addition made to the size. 
 
 Rendering the Paper Sensitive. Under this head we speak 
 first of the salting solution, and second of the Nitrate Bath. 
 Observe, in the first place, that the strength of both solutions 
 should be properly adjusted, so that when the amount of Chloride 
 of Silver in the paper increases, the excess of free Nitrate of Silver 
 may increase also. Theoretically, three parts by weight of Nitrate 
 of Silver will precipitate nearly one part by weight of Salt, and 
 a slight excess of the Nitrate of Silver will remain. These pro- 
 portions, however, are not always adhered to, because in Photo- 
 graphic printing there are many disturbing conditions. The 
 sizing of the paper, and the Albumen glaze, appropriate a quantity 
 of free Nitrate of Silver, as already shown ; and not only so, 
 but the proportions of Nitrate and of Chloride left in the paper, 
 vary with the method of applying the solutions. There are three 
 modes of spreading solutions on photographic paper, by brush- 
 ing, by floating, and by total immersion. The first leaves a small, 
 and the third a large amount of solid matter upon the paper, 
 whilst the second gives a variable result according to the length 
 of time the paper is left floating upon the Bath. Experience 
 shows that for an Albuminized paper, both solutions being ap- 
 plied by floating, the Nitrate Bath should be about six times as 
 strong as the Salting Bath, and should be left twice as long in 
 contact with the paper. In the case of a paper floated upon the 
 Salting Bath, and 'brushed with the Nitrate of Silver, the latter 
 may be twelve times stronger than the former ; and when the 
 papers are salted by total immersion, and sensitized by brushing, 
 the Salt may conveniently be reduced to a fifth of the ordinary 
 
298 THEORY OF PHOTOGRAPHY. 
 
 weight, and the Nitrate of Silver left as before, so that the pro- 
 portions may be nearly as thirty of Nitrate to one of Salt. 
 
 The strength of the Nitrate Bath in Photographic printing 
 .must also be regulated partly by the mode of sizing adopted. A 
 hard-sized English paper keeps all the chemicals comparatively 
 upon the surface, and does not require such concentrated solu- 
 tions as a paper which is more soft and spongy. Also, when 
 any organic matter like Gelatine or Albumen is added to the 
 salting bath, the amount of Salt must be lessened, because the 
 glutinous character of such fluids causes more to be retained 
 upon the surface of the paper. The difference in the atomic 
 weights of the various soluble Chlorides used in salting must also 
 be borne in mind ; 100 grains of Chloride of Ammonium con- 
 tain as much Chlorine as 109 grains of Chloride of Sodium, 
 or as 228 grains of Chloride of Barium (see the Vocabulary, 
 Part I.). 
 
 The Salting Eath. The foregoing remarks apply both to the 
 Saline and to the Sensitizing bath ; in those which follow, the 
 two will be considered separately. The sensibility of Photographic 
 paper is regulated up to a certain point by the amount of salt 
 used in its preparation. The quantity of alkaline Chloride de- 
 termines the amount of Chloride of Silver ; and with a proper 
 excess of Nitrate of Silver, papers are, up to a certain point, more 
 sensitive in proportion as they contain more of the Chloride. 
 Highly sensitized papers darken rapidly, and pass very com- 
 pletely into the bronze stage. Those containing less Chloride 
 darken more slowly, and do not become bronzed with the same 
 intensity of light. A Photographic print formed upon a paper 
 highly salted and sensitized, is usually vigorous, with great con- 
 trast of light and shade ; particularly so when the printing is 
 conducted in a strong light. Hence it will be an advantage, 
 with a feeble Negative, and in dull weather, to double the ordi- 
 nary quantity of Salt ; whereas in the case of an intense Nega- 
 tive, and with direct sunlight, the deep shadows will be too much 
 bronzed unless the quantity of Chloride of Silver in the paper is 
 kept low, so as to stop the action of the strong lights at a certain 
 point, and thus to allow the feeble rays time to come forward. 
 
 With regard to the effect which the amount of Chloride in the 
 paper exerts upon the colour and general appearance of the print, 
 the following statements may be made : Highly salted and sen- 
 
POSITIVE FEINTING. 299 
 
 sitized papers give a picture more nearly approaching to black, 
 than those which, containing a small proportion of Chloride of 
 Silver, are less sensitive to light. Hence in printing upon paper 
 weakly sensitized, in order to bring out the finer details of a 
 highly intense Negative, we find the image unusually red after 
 fixing, and of a brown or mulberry colour when toned. 
 
 It is possible to carry the proportion of Salt too far in Photo- 
 graphic printing ; in that case, even though the excess of Nitrate 
 be properly maintained, the print appears cold and dull, because 
 the Chloride of Silver is in too large quantity with reference to 
 the organic matter. A reduction in the amount of Salt, on the 
 other hand, simply leaves the image of less contrast, but does not 
 destroy its velvety softness : transparency and warmth depending 
 upon the organic matter, and not xipon the Chloride. 
 
 The Nitrate Bath. The compound on which a positive print 
 is formed is a Chloride, or an organic Salt of Silver, with an ex- 
 cess of Nitrate of Silver : hence nothing is gained by increasing 
 the proportion of Chloride of Sodium, unless at the same time an 
 addition be made to the quantity of free Nitrate in the sensitizing 
 bath. 
 
 Let us consider more minutely the appearances which present 
 themselves when the Nitrate Bath is too weak. If a sample of 
 the ordinary salted and albuminized paper be floated for two or 
 three minutes upon a solution of Nitrate of Silver of the strength 
 of twenty grains to the ounce of water, the quantity of Nitrate 
 left upon the surface will be insufficient, and the following defects 
 will appear : The paper darkens on exposure, but it does not 
 reach the bronzed stage, the action appearing to stop at a certain 
 point. On placing the print in Hyposulphite of Soda, it becomes 
 very pale, and when tinted, looks cold and slaty, without depth 
 of shadow. In almost all cases there exist on pictures of this 
 kind large spots or patches of a paler colour than the surrounding 
 parts, since the capillarity of the paper is unequal, and some por- 
 tions absorb slowly. The spots are most abundant at that edge 
 of the paper which is uppermost in drying, and are nearly absent 
 at the lower part where the excess of liquid drains down and be- 
 comes concentrated by evaporation. In a second experiment, the 
 salted paper may be left as long as ten or fifteen minutes upon 
 the same weak Bath : the result will be improved thereby, for 
 the Albumen is allowed time to draw to itself more of the Nitrate 
 
300 THEORY OF PHOTOGRAPHY. 
 
 of Silver. A third experiment may consist in dissolving forty 
 grains of Nitrate of Silver in an ounce of water, and floating 
 portions of the same paper upon it : the shadows will be deeper 
 than before, and the colour warmer, but in all probability there 
 will be room for further improvement, since a forty-grain Bath 
 is scarcely strong enough for the foreign papers albuminized with 
 ft ten-grain Salt solution, although sufficiently so for the English 
 albuminized paper of the same strength. Lastly, dissolve sixty 
 grains, eighty grains, and a hundred grains of Nitrate of Silver 
 in three separate ounces of water and float upon each, when it 
 will be found that the pictures are all good, and differ very little 
 in appearance. If, however, the time of floating be reduced to 
 a single minute, the Nitrate Bath of a hundred grains will prove 
 the best ; and some artists consider that, in Photographic printing, 
 both time and money are saved by employing a highly-concen- 
 trated Nitrate Bath, and floating the papers upon it for not more 
 than a minute. The defect which the Writer would apprehend 
 under such circumstances would be a tendency to spottiness from 
 uneven absorption by the paper, since a lengthened floating is 
 certainly favourable to even precipitation of the Chloride. 
 
 Those precautions which are observed in making the Nitrate 
 Bath for Collodion Negative Photography, are unnecessary in the 
 case of the Bath for printing. We saturate the former Bath with 
 Iodide of Silver, but the printing bath need not be saturated with 
 Chloride of Silver, since this compound, although not absolutely 
 insoluble in solution of the Nitrate, dissolves in a proportion so 
 small, that it may be disregarded. Neither is it actually necessary 
 to examine the crystals of Nitrate of Silver for free Nitric Acid, 
 for unless the sample of Nitrate be very impure, the retarding 
 efiect of the Nitric Acid will be inappreciable, and especially so 
 in the case of albuminized paper, which possesses usually a 
 slightly alkaline reaction. 
 
 A Nitrate Bath containing free Oxide of Silver, however, and 
 giving an alkaline reaction to Litmus, would in some cases be 
 injurious, since Alkaline Nitrate of Silver does not properly co- 
 agulate Albumen, and in consequence a Bath of this kind soon 
 exhibits a white turbidity when the papers are floated upon it. 
 There are certain qualities of albuminized paper sold in com- 
 merce, which tend to precipitate the same white substance in the 
 sensitizing Bath, and the Writer believes this to be due in part 
 
POSITIVE FEINTING. 301 
 
 to alkalinity. The strength of the Bath, however, must always be 
 noted ; for the weaker it becomes, the greater the tendency to dis- 
 solve away the Albumen without coagulating it. In such a case 
 the greater part of the Nitrate of Silver is converted into Chloride, 
 and not being properly retained upon the surface of the paper by 
 the coagulated Albumen, falls away into the solution. 
 
 Papers rendered sensitive upon a Nitrate Bath faintly acid 
 with Nitric Acid, or with Acetic Acid, are less liable to sponta- 
 neous reduction in the dark : whereas papers prepared upon a 
 Bath which has become alkaline from continued employment of 
 an alkaline albuminized paper, or other causes, soon change on 
 keeping. 
 
 Acetate of Silver exercises an important effect in the Photo- 
 graphic Negative Bath, but in the Bath for printing, its action is 
 not very remarkable. The only perceivable difference is a little 
 extra bronzing in the shadows, and an increased difficulty of 
 keeping the paper without discoloration in the dark. Organic 
 matter is also mentioned as greatly influencing the action of the 
 Negative Nitrate Bath, but in the printing bath its effect is 
 inappreciable, and even when the Bath has become highly co- 
 loured by the Albumen, its action is nearly the same as at first. 
 The brown coloration above alluded to is probably due to the 
 gradual formation of a SwS-Albummate of Silver, partially solu- 
 ble in solution of Nitrate of Silver. 
 
 Before leaving this subject, we must advert for a moment to the 
 employment of the compound known as Ammonio-Nitrate of 
 Silver* in Photography. The advantage derived from its use is 
 an increase of sensitiveness, and also of intensity in the image. 
 Another advantage of the Ammonio-Nitrate is that the colour of 
 the print is improved, the redness being diminished and a soft 
 velvety aspect being given to the image such as would be diffi- 
 cult to secure, on plain paper, with simple Nitrate of Silver. 
 
 There are, however, disadvantages attending the use of Am- 
 monio-Nitrate of Silver, which prevent it from being generally 
 adopted. In the first place it does not coagulate Albumen, so 
 that albuminized paper floated upon Ammonio-Nitrate of Silver 
 loses its surface- varnish, and appears dead, like the plain paper. 
 Secondly, it is more liable to spontaneous change, and to disco- 
 
 * The Chemistry of Ammonio-Nitrate of Silver is explained in the Voca- 
 bulary, Part I. 
 
302 THEOEY OF PHOTOGBAPHY. 
 
 lo ration by traces of organic matter, than simple Nitrate of Silver; 
 and hence when used as a Bath, it becomes perfectly black in the 
 course of a few days, from sizing, etc., dissolved out of the salted 
 paper. Thirdly, the action of the salted paper upon Ammonio- 
 Nitrate of Silver liberates free ^Ammonia, as will be seen by re- 
 ferring to the Vocabulary, page 95, and this free Ammonia being 
 a solvent of Chloride of Silver, attacks the sensitive coating 
 and dissolves it, thus producing white lines and transparent 
 markings. The latter objection is the most formidable of all, 
 and in consequence of it the Ammonio-Nitrate has nearly fallen 
 into disuse, excepting in the case of papers purposely prepared 
 with a very small quantity of salt, so as to avoid the production 
 of free Ammonia as far as is possible. Such papers are economical, 
 because a comparatively weak Silver solution suffices to sensitize 
 them, and to give the requisite vigour to the shadows. The colour 
 of the finished picture, however, on Ammonio-Nitrate paper feebly 
 salted, is not black, but rather of a chocolate-brown, since the 
 diminution in the quantity of Chloride increases redness, and 
 the effect of the Ammonio-Nitrate of Silver in an opposite direc- 
 tion is not sufficient to counteract this tendency. 
 
 Organic bodies used in Printing. The most important of 
 these is Albumen. Albumen is remarkable for producing a 
 smooth and homogeneous layer upon the very surface of the 
 paper, so that every detail of the Negative is rendered with an 
 amount of distinctness which cannot be obtained in any other 
 way. The prints are clear and brilliant, retaining even when 
 dry much of the transparency which plain paper pictures exhibit 
 only whilst in the water. 
 
 The main obstacle to the general adoption of Albumen is the 
 difficulty of applying it evenly to the surface of the paper. Being 
 a glutinous fluid, and not immediately amalgamating with the 
 sizing, it is apt to run into lines when the paper is floated only 
 for a short time, such as a few seconds ; whilst on the other 
 hand, if the paper be left for several minutes upon the Albumen, 
 a portion of the size is dissolved, and the Albumen in conse- 
 quence sinks into the paper, and does not impart the proper 
 amount of gloss. In order to understand this, we must bear in 
 mind that Albumen is not a neutral fluid, but possesses an alka- 
 line reaction, due to the presence of a small quantity of Soda ; 
 hence, on adding Chloride of Ammonium to Albumen a develop- 
 
POSITIVE FEINTING. 303 
 
 ment of free Ammonia takes place, easily perceptible to the smell, 
 and Ammonia is a solvent of the materials used in sizing paper. 
 To overcome this difficulty of applying the Albumen, and to ob- 
 tain a greater amount of gloss, makers of albuminized paper have 
 found it advantageous to expose the Albumen in an open vessel 
 to the air, until a considerable amount of evaporation has taken 
 place. This evaporation does not always render the Albumen 
 more glairy ; on the other hand, the fluid often becomes gradually 
 more limpid, acquiring a rather offensive odour, and an add re- 
 action to Litmus paper. Albumen so prepared runs upon the 
 paper very easily, and does not dissolve the size, but it possesses 
 some objectionable qualities, to be pointed out under the head of 
 " Toning." That defects should arise from decomposition of Al- 
 bumen, is not to be wondered at when we consider that one of 
 the constituents of this substance is Sulphur, which during the 
 putrefaction passes into the state of Sulphuretted Hydrogen, and 
 is the cause of the offensive smell. The Nitrate Bath is soon ren- 
 dered turbid by the use of stale albuminized paper, and the sen- 
 sitiveness to light is injured, so that the half- tones of the picture 
 do not appear until after a prolonged exposure. In preparing a 
 highly albuminized paper, the requisite amount of evaporation 
 may be effected over hot water at a temperature short of the co- 
 agulating point of Albumen ; or what in most cases will be found 
 sufficient, the wet Albuminized sheet may be rapidly dried by an 
 ascending current of hot air. Pressing between heated rollers 
 will still further condense the finished sheet, and give it the ap- 
 pearance of possessing a hard and glossy surface. 
 
 The reddening action of Gelatine, although greater than that 
 of Starch, is less than that produced by Albumen, and the surface 
 brilliancy is also less. Caseine, the animal principle of Milk, gives 
 good definition, and a red colour like Albumen, but is destitute of 
 gloss. Were it not for the difficulty of preparing soluble Caseine, 
 it would probably come into more extended use, since the tone, 
 when modified by a deposit of Gold, is very agreeable. Serum 
 of Milk is a convenient form of employing a dilute solution of 
 Caseine ; since the rennet used in coagulating the milk does not 
 separate the whole of the Caseine, but leaves a little dissolved in 
 the liquid : by agitation with white of eggs, and subsequently 
 boiling, the suspended oil-globules may be entangled, and the 
 Serum thus clarified. 
 
304 THEORY OF PHOTOGBAPHY. 
 
 Citrate and Tartrate dissolved in the Salting Bath exercise an 
 effect upon the colour of the print quite as remarkable as that of 
 Albumen. Paper prepared with Citrate, in addition to Chloride 
 of Silver, darkens to a fine purple colour, which becomes brick- 
 red in the fixing bath. Oxalate, however, has not the same 
 action; paper prepared with Oxalate and Chloride of Silver 
 darkening to a violet- blue colour, resembling that of the ordinary 
 Subchloride. These facts will enable the reader to understand 
 the remarks previously made on the composition of materials for 
 sizing paper ; and to see that the maker might, if so desired, 
 introduce small quantities of organic substances capable of mo- 
 difying the colour and general aspect of the print. 
 
 Amount of Silver in Sensitive Sheets. To determine this point 
 roughly, fifty whole sheets of Saxe paper, 18 X 22, albuminized 
 with nearly pure Albumen, containing ten grains of Salt to the 
 ounce, were floated for three or four minutes each, on a fifty- 
 five-grain Nitrate Bath measuring 137 ounces. The sheets were 
 found to remove 17 ounces of liquid from the Bath, and to im- 
 poverish the remainder to the extent of five grains of Nitrate of 
 Silver per ounce. The whole quantity of Nitrate of Silver ab- 
 sorbed by each sheet must, therefore, have been about half a 
 drachm. In a second experiment, a quarter-sheet of albiiminized 
 Papier Saxe was dried and weighed ; it was then floated for four 
 minutes upon a forty-five-grain solution of Nitrate of Silver, dried 
 and weighed a second time : the gain in weight amounted to seven 
 grains. 
 
 Preservation of Sensitive Sheets. The discoloration of Sensi- 
 tive Paper in the dark is due to a slow reducing action of organic 
 matter upon the free Nitrate of Silver. The rapidity of the change 
 varies much with the nature of the organic matter, but more so 
 with the state of the Nitrate of Silver. Alkalies in the size of 
 the papers or in the sensitizing bath, facilitate the discoloration, 
 whilst free acid and acid salts, like Alum, etc., retard it : hence 
 French papers darken more quickly than English papers as a 
 rule, and Ammomo-Nitrate paper is more unstable than that 
 prepared with simple Nitrate of Silver. 
 
 The degree of dryness of the atmosphere in which the papers 
 are kept, also affects their rate of change, since moisture appears 
 essential to the reduction. Hence the various contrivances devised 
 for drying the paper, the best of which appears to be that of Mr. 
 
TONING BY SULPHITE. 305 
 
 Spiller, in which a box is constructed with a false bottom pierced 
 by holes, and lumps of quick-lime are placed beneath. To all 
 such plans, however, there are some objections, for it has been 
 found that papers, when rendered absolutely dry, do not darken 
 vigorously in the light, and that it is necessary, in consequence, 
 to leave them for a short time in close proximity to a damp cloth, 
 until the requisite amount of water has been absorbed. Other?, 
 again, have observed that, although Sensitive Papers, kept for 
 many months in a drying Box, may print to a sufficiently deep 
 shade, yet that the image does not tone in the alkaline Gold Bath 
 so readily as a print upon newly-sensitized paper. 
 
 SECTION II. 
 Toning the Proof. 
 
 By the term "toning " we understand the removal of the 
 reddish colour described in the first Section as the proper 
 tint of a Photographic Positive, after the compounds of 
 Silver unacted upon by light have been extracted by an 
 appropriate fixing bath. Colours approaching to brick- 
 red are in themselves so unartistic that from the very first 
 discovery of Photography means were tried of removing 
 them ; and the employment of Sulphur was the plan ori- 
 ginally adopted for that purpose. 
 
 TONING BY SULPHUR. 
 
 It is well known that articles of Silver plate become 
 darkened by exposure to the fumes of Sulphur, or to those 
 of Sulphuretted Hydrogen, of which minute traces are 
 always present in the atmosphere. If the stopper of a 
 bottle of Sulphuretted Hydrogen water be removed, and 
 a simply fixed Photographic Positive suspended over it, 
 the picture will lose its characteristic red tone and become 
 nearly black. The black colour is even more intense than 
 an experienced chemist would have anticipated, because 
 analysis teaches us that the actual quantity of Silver pre- 
 sent in a Photographic picture on paper is infinitesimally 
 
 x 
 
306 THEOBY OF PHOTOGRAPHY. 
 
 small, and it is well known that Sulphide of Silver, although 
 of a deep brown colour, approaching to black, when in mass, 
 exhibits a pale-yellow tint in thin layers, so that a mere 
 film of Silver converted into Sulphide possesses very little 
 depth of colour. To explain the difficulty it has been sug- 
 gested that the toning action of Sulphur on a red print is 
 probably due to the production of a Sub- Sulphide possess- 
 ing an intense colorific power, like the Sub-Oxide and 
 Sub-Chloride of Silver. When the toned picture is sub- 
 jected to the further action of Sulphur, it is converted into 
 the ordinary Protosulphide of Silver, and becomes yellow 
 and faded. 
 
 It is not necessary to enter into any details of the va- 
 rious processes originally recommended for toning prints 
 by means of Sulphur. The principle was the same in all, 
 but the mode in which the Sulphur was set free and 
 brought to bear upon the proof varied. Commonly, a so- 
 lution of Hyposulphite of Soda was mixed with some 
 chemical which gradually decomposed it. Everything re- 
 quiring explanation under this head will be treated in the 
 next Section, when we speak of the properties of the Hy- 
 posulphite fixing Bath. 
 
 TONING BY GOLD. 
 
 After the Sulphur toning process had been discarded, 
 the Salts of Gold were used for improving the normal co- 
 lour of the Paper Photograph, and the merit of first in- 
 troducing them is due to M. Le Grey, of Paris. These 
 methods, being still in vogue, will require a detailed de- 
 scription. 
 
 The single fixing and toning Bath. A simple mode and 
 one even now sometimes followed, is to add Chloride of 
 Gold to the ordinary fixing bath of Hyposulphite of Soda. 
 The prints are immersed in the resulting liquid immediately 
 on taking them from the frame, and the first action pf the 
 *' fixing and toning bath " is to dissolve out the unchanged 
 Silver salts, and to leave the image of the usual red colour. 
 
TONING BY GOLD. 307 
 
 Shortly however the red colour begins to pass gradually 
 into blue or black, and the toning is complete. 
 
 At first sight the above process appeared correct both 
 as regards theory and practice. A more extended experi- 
 ence however led to its condemnation, for it was found 
 that the improvement of the colour was not due to a 
 simple precipitation of Gold upon the surface of the image, 
 as had been at first supposed, but partly to a deposit of 
 Gold and partly to a communication of Sulphur. When 
 a solution of Chloride of Gold is added to Hyposulphite 
 of Soda, we have in the liquid not only the double Hypo- 
 sulphite of Gold and Soda known by the name of Sel d'Or, 
 but also a portion of the unstable Tetrathionate of Soda, 
 prone to liberate Sulphur. The action of the Bath is there- 
 fore complex from the very first, but becomes more so on 
 keeping the solution for a time, since spontaneous decom- 
 position ensues, as will be more fully shown in the next 
 Section. Practical Photographers were not slow in disco- 
 vering that the fixing and toning Bath was inconstant in 
 its action, and that although the prints were coloured with 
 as much rapidity in an old as in a newly-mixed Bath, yet 
 that the tints were more fugitive in the former case than in 
 the latter. The reason was that whereas the newly -prepared 
 Bath acted mostly by depositing Gold, and only partially 
 by communication of Sulphur, the old Bath, on the other 
 hand, toned the prints entirely by Sulphuration. The 
 reader will doubtless be surprised to hear that the colours 
 produced upon the Photographic picture by means of Gold 
 closely resemble those obtained by Sulphur ; yet such is 
 the case, for although shades of blue are characteristic of 
 Gold, and shades of brown of Sulphur, yet it requires 
 a practised eye to distinguish between them, and prints 
 toned by Sulphur often possess the fine purple-black which 
 many suppose to be due to a deposit of Gold. 
 
 In the Gold toning process, as now employed, all danger of 
 Sulphuretting the prints is avoided. Hyposulphite of Soda 
 having been proved to suffer decomposition in presence of 
 Chloride of Gold, we no longer mix the two solutions, but 
 
308 THEORY OF PHOTOGRAPHY. 
 
 prefer to tone the picture first, and fix it subsequently. The 
 colouring action of a simple solution of Chloride of Gold 
 upon the Photographic image may be thus explained : The 
 Chlorine previously in combination with Gold passes to 
 the reduced Silver salt, bleaching the lighter shades by 
 converting them into the white Protochloride of Silver, 
 and imparting to the shadows a deep violet tint due to the 
 production of Subchloride of Silver ; at the same time me- 
 tallic Gold is deposited, but its effect is not very clearly 
 seen at this stage of the process, since a deep violet colour 
 of nearly equal intensity may be obtained by using a so- 
 lution of Chlorine in place of Chloride of Gold. If how- 
 ever the toned proof be acted upon by a fixing bath of 
 Hyposulphite of Soda, all that portion of the violet co- 
 lour which depends upon Subchloride of Silver will be de- 
 stroyed, since the Subchloride is decomposed by fixing 
 agents. Another portion of the colour will resist the ac- 
 tion of the Hyposulphite, and this is probably Metallic 
 Gold. 
 
 During the whole process of toning in solution of Chlo- 
 ride of Gold we observe a gradual lowering of intensity in 
 the image ; it has been found that this destructive effect is 
 greater when the solution of the Chloride contains free 
 Hydrochloric Acid, and that it is lessened by the addition 
 of an alkali : hence the employment of alkaline solutions 
 of Chloride of Gold, first introduced in this country by 
 Mr. Waterhouse, of Halifax, must be deemed an improve- 
 ment. 
 
 The question as to the manner in which the alkali acts 
 in preventing the image from being eaten away has been 
 variously answered. The first effect of adding Carbonate 
 of Soda to Chloride of Gold is to neutralize the free Hydro- 
 chloric Acid which that compound invariably contains, and 
 thus to produce a double Chloride of Gold and Sodium. 
 Now it has been suggested that this Chloride of Gold and 
 Sodium is the real toning agent ; that the process consists 
 essentially in the substitution of Gold for Silver, and that 
 a double Chloride of Gold and Sodium becomes a double 
 
TONING BY GOLD. 309 
 
 Chloride of Silver and Sodium, Metallic Gold being at the 
 same time thrown down, and taking the place of the Silver 
 of the proof. Further, it has been urged that in the alka- 
 line toning process lies the real key to the production of 
 permanent Photographic pictures, since the image of Sil- 
 ver, proved to be unstable, is by it converted into an image 
 of Gold, not prone to change. These views are ingenious, 
 but it is doubtful whether they express the truth ; for on 
 subjecting the image to the action of Chloride of Gold, we 
 do not find that an atom of Silver is dissolved for each 
 atom of Gold deposited, whilst as regards the entire con- 
 version of the image into Metallic Gold, it may be shown 
 to be impracticable. MM. Davanne and Girard have sug- 
 gested that the alkaline addition to the Chloride of Gold 
 is useful in neutralizing traces of free Nitric Acid, always 
 liberated when the organic matter reduces the Nitrate of 
 Silver in the process of printing. Undoubtedly the Posi- 
 tive, on its removal from the printing-frame, has an acid re- 
 action, and the Carbonate of Soda in the toning bath must 
 operate beneficially in neutralizing it. This, however, is 
 not the sole use of the alkali, according to the Writer's ex- 
 perience : it probably acts also by converting a portion of 
 the Chloride of Gold into an Oxide of Gold, which is an 
 unstable substance, and may be made to yield up Metallic 
 Gold to the proof. 
 
 When Carbonate of Soda is added to Chloride of Gold 
 the change into Oxide is at first only partial, and an Oxy- 
 chloride of Gold is probably formed. The Oxide of Gold, 
 or Auric Acid, does not appear of itself to possess the 
 power of toning the picture ; and hence the total conversion 
 of the Chloride under the influence of the alkali renders 
 the Bath useless to the photographer. If, however, a por- 
 tion of the Gold still remains in the form of Chloride, and 
 the toning action can once be started, a spontaneous de- 
 composition of the Oxide of Gold will take place by cata- 
 lysis, and a larger quantity of Gold will thus be thrown 
 down without the same injury to the print from communi- 
 cation of Chlorine. These views may appear complex, but 
 
310 THEOEY OF PHOTOGRAPHY. 
 
 they have been adopted by the Author after a careful study 
 of the properties of the alkaline Aurates. 
 
 The alkaline substance usually employed for admixture 
 with Chloride of Gold is the Carbonate of Soda, but it has 
 been shown by Mr. Maxwell Lyte that an alkaline Phos- 
 phate or Borate produces the same effect. Others again 
 have used an Acetate, and it will probably be found that 
 any salt in which the alkaline constituent is combined with 
 a weak acid will answer the purpose. In such cases the solu- 
 tion of the Chloride of Gold gradually becomes colourless 
 by an interchange of elements, as above described. 
 
 CONDITIONS WHICH AFFECT THE ACTION OF THE ALKA- 
 LINE GOLD TONING BATH. 
 
 a. Degree of Concentration of the BatTi. The exact amount 
 of dilution with water in the alkaline toning bath is not of much 
 consequence. A strong bath acts quickly, but is more liable to 
 destroy the middle tints than a weaker solution left upon the 
 image for a longer time. The proportion of Carbonate of Soda 
 is also of minor importance, but it is desirable to use the mini- 
 mum quantity, since a highly alkaline liquid dissolves the size of 
 the paper, and causes a blistering of the Albumen when the pic- 
 ture is laid in the fixing bath. 
 
 b. The length of time after mixing. The two solutions of Chlo- 
 ride of Gold and Carbonate of Soda may be kept separately for 
 any length of time, but on mixing them a chemical change en- 
 sues, and the liquid gradually loses its yellow colour. When the 
 solution has become nearly colourless it is in the best state for 
 use, and will remain so for many hours, but after a longer keeping 
 of days or weeks it loses its activity, and deposits Gold in the 
 metallic state on the sides of the bottle : alkaline solutions of 
 Chloride of Gold being easily reduced by traces of organic matter. 
 
 c. Presence of free Nitrate of Silver on the surface of the 
 Proof. It is quite necessary to wash away the free Nitrate of 
 Silver from the pictures before they are immersed in the toning 
 bath, otherwise the Bath will become turbid and discoloured. 
 Nitrate of Silver added to an alkaline solution of Chloride of 
 Gold throws down not only Carbonate of Silver and Chloride of 
 Silver, but also a brown powder, consisting of Peroxide of Gold, 
 
TONING BY GOLD. 31 1 
 
 or Metallic Gold, and the supernatant liquid contains little else 
 than Nitrate of Soda. 
 
 d. Temperature of the Solution. The action of the Bath is 
 much increased by carefully heating it to about 120 Fahr. before 
 the prints are immersed, and this method has also the advantage 
 of bringing the solution at once to the nearly colourless condi- 
 tion, in which it contains more Oxide and less Chloride of Gold. 
 The plan of warming the Bath, however, is not often adopted by 
 operators, since a similar result can generally be obtained by con- 
 tinuing the action of the cold solution for a longer time. 
 
 e. Presence of Iodide of Silver. When iodide of Silver is pre- 
 sent in a proof, the toning action of the alkaline Chloride appears 
 to be less rapid than when the image is formed on Chloride only. 
 Hence sensitive paper is not improved, but rather the contrary, 
 by associating Iodide with the Chloride in salting, as some au- 
 thors have advised. It is also unwise to employ a Nitrate Bath 
 which has been previously used for Negatives, and contains Io- 
 dide of Silver dissolved. On floating the Paper upon such a Bath, 
 the Albumen abstracts the Iodide from it in virtue of a chemical 
 affinity, and the printed proof upon such paper remains for a com- 
 paratively long time in the toning solution without being coloured 
 by the Gold. 
 
 f. Mode of Preparing the Paper. The preparation of the 
 paper is a matter of greater nicety in the alkaline Gold toning 
 process than m the old mode of toning prints by Sulphur, since 
 the affinity which secures the colouring action is stronger in the 
 latter case than in the former. By adding an alkali to Chloride 
 of Gold you improve the colour of the print, but retard the pro- 
 cess of colouring, since, as before shown, the Oxide of Gold pro- 
 duced by the alkali is of itself nearly or quite inert, and the Chlo- 
 rine is the element which possesses the affinity for Silver, and 
 thus determines the change. Hence, although a defective sample 
 of paper may tone evenly in a Sulphur Bath, in the case of the 
 alkaline Gold Bath, there would be either spots or markings,which 
 would remain of a red colour and refuse to tone. 
 
 One cause of unevenness of toning is an improper mode of 
 applying the Albumen to the paper ; for Albumen being a glairy 
 substance, and having a strong tendency to run into lines, much 
 will necessarily depend upon the manipulation. We might in- 
 deed remedy this defect by avoiding the use of Albumen, but it 
 
312 THEQEY OF PHOTOGKAPHY. 
 
 is found that in the Chloride of Gold toning process, the proofs 
 acquire an inky shade, unless the image is printed upon the 
 maximum quantity of organic matter, so as to encourage redness 
 as much as possible. A hard-sized gelatinous English paper, 
 with a minimum of Chloride in the salting bath, or a foreign 
 starch-sized paper, salted with a mixed Citrate and Chloride, will 
 produce an agreeable effect. Mr. Waterhouse, of Halifax, gives 
 a formula in which the organic matter is Caseine dissolved in 
 highly dilute potash ; there is no gloss, and the finished pictures 
 are of a purple tone. If Albumen be used, the quality of the 
 paper itself wih 1 be the principal point to attend to ; but, in addi- 
 tion to this, the state of the Albumen must be noted, since sour- 
 ness and decomposition are injurious not only to the sensitiveness 
 of the paper, but also to the rapidity of toning. 
 
 Additional experiments on the conditions of the paper affecting 
 the toning action are much needed. To further their prosecution 
 the Writer ventures to give his experience, as follows : the more 
 metallic the image, the greater the rapidity of toning by Chloride 
 of Gold, but the more inky the colour. Organic bodies which 
 encourage the production of a subsalt of Silver under the influ- 
 ence of the light, hinder the deposition of Gold, but improve the 
 colour by imparting a shade of red. There are however differ- 
 ences between these organic bodies, ascertainable only by expe- 
 riment, for although Albumen and Citrate both favour the re- 
 duction of the Silver to the state of a subsalt, it will be found 
 on trial that Citrate papers tone with greater facility than albu- 
 minized papers. Mr. Mabley, the Honorary Secretary of the 
 Manchester Photographic Society, states that the image toned 
 more rapidly when the paper was excited on a Silver Bath con- 
 taining a little free Nitric Acid : this accords with theory, because 
 in presence of a strong acid, the reduction approaches more nearly 
 to the state of metal nevertheless the colour so obtained is not 
 agreeable, and a warmer shade will usually be produced by albu- 
 minized paper which on its removal from the printing frame exhi- 
 bits an image of a brick-red, and not of a violet-blue tone. 
 
 g. Time of keeping the Paper. If a print be formed upon 
 albuminized paper, and kept for a long time before toning, there 
 will commonly be some difficulty in securing a strong deposit of 
 Gold. This is probably due to a change which takes place in 
 the paper, from the organic matter reacting by degrees upon the 
 
FIXING THE PEOOF. 313 
 
 Nitrate of Silver, and it has been stated that the same change is 
 more gradually produced when the sensitive papers are preserved 
 for many months in a dry state by means of Chloride of Calcium 
 or Sulphuric Acid ; such papers darken in the sun, but if the 
 print be subjected to the action of a Gold toning bath at the same 
 time with one on paper newly sensitized, the latter will be found 
 to tone with the greater rapidity. 
 
 SECTION III. 
 Fixing the Proof. 
 
 The conditions for a proper fixing of the proof are not 
 always understood by operators, and consequently they 
 have no certain guide as to how long the prints should re- 
 main in the fixing Bath. 
 
 The time occupied in fixing will of course vary with the 
 strength of the solution employed : but there are simple 
 rules which may be followed. In the act of dissolving the 
 unaltered Chloride of Silver in the proof, the fixing solu- 
 tion of Hyposulphite of Soda converts it into Hyposul- 
 phite of Silver (p. 168), which is soluble in an excess of 
 Hyposulphite of Soda. But if there be an insufficient 
 excess, that is, if the Bath be too weak, or the print 
 removed from it too speedily, then the Hyposulphite of 
 Silver is not perfectly dissolved, and begins by degrees to 
 decompose, producing a brown deposit in the tissue of the 
 paper. This deposit, called measles, which has the appear- 
 ance of yellow spots and patches, is not usually seen upon 
 the surface of the print, but becomes very evident when 
 the print is held up to the light, or if it be split in half, 
 which can readily be done by glueing it between two flat 
 surfaces of deal, and then forcing them asunder. 
 
 The reaction of Hyposulphite of Soda with Nitrate of 
 Silver. In order to understand more fully how decom- 
 position of Hyposulphite of Silver may disturb the process 
 of fixing, the peculiar properties of this salt should be 
 studied. With this view Nitrate of Silver and Hyposul- 
 phite of Soda may be mixed in equivalent proportions, 
 
314 THEQEY OF PHOTOGRAPHY. 
 
 viz. about twenty-one grains of the former salt to sixteen 
 grains of the latter, first dissolving each in separate vessels 
 in half an ounce of distilled water. These solutions are 
 to be added to each other and well agitated ; immediately 
 a dense deposit forms, which is Hyposulphite of Silver. 
 
 At this point a curious series of changes commences. 
 The precipitate, at first white and curdy, soon alters in 
 colour : it becomes canary-yellow, then of a rich orange- 
 yellow, afterwards liver-colour, and finally black. The 
 rationale of these changes is explained to a certain extent 
 by studying the composition of the Hyposulphite of Silver. 
 
 The formula for this substance is as follows : 
 
 AgO,S 2 2 . 
 
 But AgO, S 2 O 2 plainly equals AgS, or Sulphide of Silver, 
 and SO 3 , or Sulphuric Acid. The acid reaction assumed 
 by the supernatant liquid is due therefore to Sulphuric 
 Acid, and the black substance formed is Sulphide of Sil- 
 ver. The yellow and orange-yellow compounds are earlier 
 stages of the decomposition, but their exact nature is un- 
 certain. 
 
 The instability of Hyposulphite of Silver is principally 
 seen when it is in an isolated state : the presence of an ex- 
 cess of Hyposulphite of Soda renders it more permanent, 
 by forming a double salt, as already described at p. 168. 
 
 These facts explain the reason why, in fixing Photographic 
 prints, a brown deposit of Sulphide of Silver sometimes 
 forms in the Bath and upon the picture. To obviate it, ob- 
 serve the following directions : It is especially in the reac- 
 tion between Nitrate of Silver and Hyposulphite of Soda 
 that the blackening is seen, the Chloride and other insoluble 
 Salts of Silver being dissolved, even to saturation, without 
 any decomposition of the Hyposulphite formed. Hence if 
 the print be carefully washed in water to remove the so- 
 luble Nitrate, a comparatively weak fixing bath may be 
 employed. But if the proofs are taken at once from the 
 printing frame and immersed in a dilute Bath of Hypo- 
 sulphite (one part of the salt to six or eight of water) with- 
 out any previous washing, a shade of brown will be observed 
 
FIXING THE PROOF. 315 
 
 to pass over the surface of the print, and a large deposit 
 of Sulphide of Silver will form as the result of the decompo- 
 sition. On the other hand, with a strong Hyposulphite 
 Bath there will be little or no discoloration, and the black 
 deposit will be absent even when the prints are immersed 
 with the free Nitrate of Silver still upon the surface. 
 
 The print must also be left for a sufficient time in the 
 fixing bath, or some appearance of brown patches, visible 
 only on looking through the paper, may occur. Each 
 atom of Nitrate of Silver requires three atoms of Hypo- 
 sulphite of Soda to form the sweet and soluble double salt, 
 and hence, if the action be not continued sufficiently long, 
 another compound will be formed almost insoluble (p. 168) 
 and with a greater tendency to decompose into Sulphide 
 of Silver. Even immersion in a new Bath of Hyposul- 
 phite of Soda will not fix the print when once decomposi- 
 tion of Hyposulphite of Silver has commenced. The yellow 
 or brown compound is not entirely soluble in Hyposulphite 
 of Soda, and consequently remains in the paper. 
 
 Another important matter to observe in fixing Photo- 
 graphic prints is the temperature of the fixing bath. When 
 Hyposulphite of Soda is first dissolved in water, much 
 heat is rendered latent, and the solution in consequence 
 is almost at the freezing point. In this state of things 
 prints are fixed slowly and with difficulty, whereas at 
 70 or 80 Fahr. the action is more rapid. In the cold 
 winter months Positives should be left in the Bath at least 
 twice as long as in the summer months, to prevent the oc- 
 currence of brown patches of imperfect fixation in the 
 substance of the Paper. 
 
 The nature of the sensitive surface to be cleared of its 
 superfluous Silver salt must also be considered in estima- 
 ting the strength of Bath and the time of immersion likely 
 to be required. Albuminized paper, from the horny na- 
 ture of its surface coating, requires a longer treatment 
 with the Hyposulphite of Soda than plain paper requires ; 
 and not only so, but it must be borne in mind that the 
 Albumen has a property of combining with Nitrate of 
 
316 THEORY OF PHOTOGRAPHY. 
 
 Silver and forming an insoluble salt which is more difficult 
 of solution than Chloride of Silver. Gelatine also com- 
 bines with Nitrate of Silver, and therefore the mere wash- 
 ing of a gelatinized sensitive paper for a few minutes in 
 cold water does not remove the whole of the free Nitrate. 
 It is better in all such cases to make the fixing bath far 
 stronger than theory would indicate as sufficient for the 
 solution of simple Chloride of Silver, otherwise it will be 
 found that the fixing will be insufficient, and that there 
 will be mealy spots in the print ; although on adding re- 
 cently precipitated Chloride of Silver to the Bath of Hypo- 
 sulphite it immediately dissolves. 
 
 The Writer has noticed that when sensitive paper is 
 kept for some time before being used for printing, yellow 
 patches of imperfect fixation are very liable to occur. The 
 Nitrate of Silver appears gradually to undergo a partial 
 reduction by the organic matter, and cannot then be so 
 easily extracted by the fixing bath. It has also been ob- 
 served that albuminized and salted paper, when kept in a 
 damp place, or exported to warm and damp climates, is 
 liable to undergo a decomposition, in consequence of which 
 the fixing bath fails to extract the whole of the superfluous 
 Silver after printing, and the whites remain discoloured 
 and spotty. 
 
 It has been recommended by some writers on Photo- 
 graphic chemistry to saturate the fixing bath with com- 
 mon Salt, in order to convert any Nitrate of Silrer left in 
 the proof by imperfect washing, into Chloride, and thus to 
 prevent blackening from decomposition of Hyposulphite 
 of Silver. An excess of salt undoubtedly has the effect 
 thus ascribed to it, but it appears to the Writer that its use 
 is contra-indicated by the fact that Hyposulphite of Soda 
 in presence of Chloride of Sodium is less active as a fixing 
 agent, since the Chloride of Sodium exercises an oppos- 
 ing affinity and tends to keep the Silver in the state of 
 Chloride. 
 
 For a reason similar to that above assigned, the fixing 
 bath employed for Negatives should not be used for fixing 
 

 FIXING THE PROOF. 317 
 
 positive prints, since the Iodide of Sodium present in the 
 former tends to produce yellow patches on the surface of 
 the print ; which patches consist of Iodide of Silver in 
 union with Albuminate of Silver or other analogous sub- 
 stances, and these compounds are very difficult of removal 
 by the action of the fixing bath. 
 
 ON CERTAIN INJURIOUS CONDITIONS OP THE 
 FIXING BATH. 
 
 The object of using the Hyposulphite Bath is to fix the proof 
 by dissolving the unaltered Silver salts, without in any way af- 
 fecting the image. 3ut it is a fact familiar to the Photographic 
 chemist, that the Hyposulphite of Soda is a substance which very 
 readily yields Sulphur to any bodies which possess an affinity for 
 that element ; and as the reduced Silver compound in the print 
 has su.ch an affinity, there is a tendency to absorption of Sulphur 
 when the proofs are immersed in the Bath. Hence in many 
 cases a sulphur-toning process is set up, and as the picture is im- 
 proved by it in appearance, assuming a more purple shade, it 
 is often encouraged by Photographers. Experience, however, has . 
 shown that colours brightened in this way are rendered less per- 
 manent, and therefore the careful operator will avoid sulphuration 
 as far as possible. Some of the conditions which facilitate a sul- 
 phuretting action upon the proof, and are therefore injurious, are 
 as follows : 
 
 a. The addition of an Acid to the Bath. It was at one time 
 common to add a few drops of Acetic Acid to the fixing Bath of 
 Hyposulphite of Soda, immediately before immersing the proofs. 
 The Bath then assumes an opalescent appearance in the course 
 of a few mimites, and, when this milkmess is perceptible, the 
 print is rendered darker in colour. The chemical changes pro- 
 duced in a Hyposulphite Bath by addition of acid, may be ex- 
 plained thus : The acid first displaces the feeble Hyposulphu- 
 rous Acid from its combination with Soda. Then the Hyposul- 
 phurous Acid, not being a stable substance when isolated, begins 
 spontaneously to decompose, and splits up into Sulphurous Acid 
 remaining dissolved in the liquid, and communicating the cha- 
 racteristic odour of burning Sulphur and Sulphur, which sepa- 
 
318 THEOEY OF PHOTOGRAPHY. 
 
 rates in a finely divided state and forms a milky deposit. In 
 chemical symbols, S 2 O 2 = SO 2 and S. 
 
 Observe therefore that free acids of all kinds must be excluded 
 from the fixing Bath, or, if inadvertently added, the liquid must 
 afterwards be neutralized by Carbonate of Soda or Chalk. 
 
 b. Decomposition of the Bath by constant use. It has long 
 been known that Hyposulphite of Soda undergoes a peculiar 
 change in properties when much used in fixing. The solution 
 when first prepared leaves the image of a red tone, which is the 
 characteristic colour of the reduced Silver Salt, but it soon ac- 
 quires the property of darkening this red colour by a subsequent 
 communication of Sulphur. Hence a simple fixing bath becomes 
 at last a toning bath, without any addition of Gold. This change 
 of properties has been shown by the Author to be due to a de- 
 composition of Hyposulphite of Silver, and to the consequent 
 formation of a sulphuretting body analogous in its properties to 
 the Tetrathionates.* During the progress of the change Sulphide 
 of Silver is thrown down, forming a black deposit, and the super- 
 natant liquid becomes acid and effervesces on the addition of 
 Chalk. The exact composition of the acid body above mentioned 
 as a product of the spontaneous decomposition of old fixing baths 
 of Hyposulphite is not known, but the effects it produces on the 
 prints are well understood, and can be shown to be most inju- 
 rious ; for it not only blackens the shadows of the picture by 
 communicating Sulphur, but causes a loss of the middle tints, 
 and destroys the pure white colour of the high lights. 
 
 In order to avoid the formation of this acid substance in the 
 Bath, it is most important to wash away all the free Nitrate of 
 Silver; hence arises one advantage of the alkaline Gold toning 
 process over the old methods, since this process cannot be 
 carried on unless the free Nitrate of Silver be first washed away. 
 Another precaution is to keep the Bath shielded from a very 
 strong light, and to put it away in a cool place : light and heat 
 both favour the decomposition of Hyposulphite of Silver and 
 cause a black deposit to be thrown down. 
 
 A more practical means of preserving a fixing bath for a long 
 time in a state in which it communicates but little Sulphur to the 
 proof is the addition of an alkali. The Author showed in his 
 early papers on the Tetrathionates and their reaction with Hypo- 
 * See 'Journal of the Photographic Society/ September and October 1854. 
 
FADING OF FEINTS. 319 
 
 sulphite of Soda that alkalies decompose the unstable sulphuret- 
 ting principle. Hence if the Bath be treated with Potash or Car- 
 bonate of Soda, an alkaline Sulphide is gradually formed, which 
 precipitates Sulphide of Silver, and in the course of a few days 
 the liquid returns to its original condition and ceases to act as 
 a toning agent upon the proof. Here we see another point of 
 excellence in the alkaline Gold toning process, since each print 
 carries with it small doses of Carbonate of Soda, which effectually 
 prevent the formation of acid in the Bath. 
 
 SECTION IY. 
 On the Fading of Photographic Prints. 
 
 For many years subsequent to the discovery of the pro- 
 cess of Photographic Printing by Mr. Fox Talbot, it was 
 not generally known that pictures so produced were easily 
 susceptible of injury from various causes, and in particular 
 from traces of the fixing-agent remaining in the paper. 
 Hence, due care not being taken in the proper cleansing 
 and preservation of the proofs, the majority of them faded. 
 
 The present Section is intended to explain practically 
 and in a concise manner the causes of the fading of Photo- 
 graphic Prints, and the precautions which should be taken 
 to ensure their permanency. 
 
 a. Imperfect washing. This is perhaps the most impor- 
 tant cause of fading and the most frequent. When Hypo- 
 sulphite of Soda is allowed to remain in the paper, even in 
 minute quantity, it gradually decomposes, with liberation 
 of Sulphur, and destroys the print in the same way and 
 quite as effectually as a solution of Sulphuretted Hydrogen 
 or an alkaline Sulphide.* 
 
 Imperfect washing may be suspected, if the Photograph 
 within a few months from the date of its preparation, be- 
 gins to get darker in colour : the half-tints, which are the 
 first to show the action, afterwards passing into the yellow 
 
 * The reason why the long-continued action of Sulphur fades the print, 
 has already been indicated at p. 306. It is because the amount of real Silver 
 in the image is very small, and Sulphide of Silver in minute quantity ap- 
 pears pale and yellow. 
 
320 THEORY OF PHOTOGRAPHY. 
 
 stage, whilst the dark shadows remain black or brown for 
 a longer time. 
 
 The proper mode of washing Photographs is sometimes 
 misunderstood. The length of time during which the print 
 lies in the water is a point of less importance than the con- 
 tinual changing of the water. When a number of Posi- 
 tives are placed together in a pan, and a tap turned upon 
 them, the circulation of fluid does not necessarily extend 
 to the bottom. This is proved by the addition of a little 
 colouring matter, which shows that the stream flows ac- 
 tively above, but at the lower part of the vessel, and be- 
 tween the prints, there is a stationary layer of water which 
 is of little use in washing out the Hyposulphite. Care 
 should therefore be taken that the pictures be kept as far 
 as possible separate from each other, and when running 
 water cannot be had, that they be frequently moved and 
 turned over, fresh water being constantly added. When 
 this is done, and especially if the size be removed from the 
 paper in the manner presently to be advised, four or five 
 hours' washing will be sufficient. It is a mistake to allow 
 the pictures to remain in the water for several days ; which 
 produces no good effect, and may tend to encourage a 
 putrefactive fermentation, or the formation of a white de- 
 posit upon the image when the water contains Carbonate 
 of Lime. 
 
 Mr. Maxwell Lyte states that traces of soluble Chloride 
 left in the paper are injurious to the permanency of Photo- 
 graphic prints : this indicates the propriety of avoiding 
 the use of water too highly charged with saline matter, and 
 of finishing the washing with a bath of rain-water. 
 
 b. Atmospheric Oxidation. Upon examining collections 
 of old Photographs, it is not uncommon to find prints which 
 are stated to have remained unaltered for a long while after 
 their first production, but which in the course of time have 
 lost their brilliancy, and become pale and indistinct. This 
 kind of fading often commences at the corners and edges 
 of the paper, and works inwards towards the centre ; it is 
 principally caused by a slow process of oxidation. 
 
FADING OF FEINTS. 321 
 
 Sulphuration in the process of toning, which has before 
 been shown to occur when old and decomposed solutions 
 of Hyposulphite of Soda are employed, always facilitates 
 subsequent oxidation and yellowness. It is indeed difficult 
 to avoid slight sulphuration unless fresh solutions of Hypo- 
 sulphite are employed in fixing, but the remarks now made 
 on the injury caused by Sulphur, refer to its full action. 
 
 Moisture greatly hastens the yellow change due to at- 
 mospheric oxidation, and hence a state of comparative dry- 
 ness may be said to be essential to the preservation of all 
 Photographs. In collecting evidence upon the subject, 
 " wet " and " damp " are frequently alleged as having been 
 causes of fading ; the prints were hung against a damp 
 wall during frosty weather, in a room without a fire ; or 
 the rain had been allowed to penetrate the frame ! 
 
 Acid matters, and also other bodies which dissolve Oxide 
 of Silver, such as Ammonia, etc., favour oxidation when 
 left for a long time in contact with the image. The ma- 
 terials often used in sizing papers, such as Alum and Hesin, 
 being of an acid nature, are ultimately injurious to the 
 image ; and the removal of the size, which may easily be 
 effected by means of hot water, without much injury to 
 the tint, has the additional advantage of carrying out the 
 last traces of Hyposulphite of Soda, and also the germs of 
 fungi, which, if allowed to remain, would vegetate and pro- 
 duce a destructive mouldiness. 
 
 The fact that acids facilitate oxidation of the image sug- 
 gests also that Photographic Prints should not be handled 
 too frequently, or touched with the finger more than is 
 necessary ; the warm hand may leave behind a trace of 
 acid* which would tend in time to produce a yellow mark. 
 For the same cause they ought not to be laid down care- 
 lessly in any place likely to communicate impurity. 
 
 The employment of albuminized paper confers a positive 
 advantage in protecting the image from oxidation : this 
 was proved by careful experiments performed with various 
 
 * This acid reaction of perspiration is due to Lactic, Acetic, and Formic 
 Acids. 
 
322 THEOBY OF PHOTOGRAPHY. 
 
 oxidizers. The use of Albumen indeed gives the main 
 chance of preventing a print which has been toned to 
 blackness by sulphur, from gradually becoming pale and 
 yellow by atmospheric oxidation. 
 
 c. Improper modes of Mounting the Proof. This cause 
 of change might have been included with the last, since 
 the fading of badly mounted prints is usually due to at- 
 mospheric oxidation. All cements which are of an- acid 
 nature, or which are liable to become sour by acetous fer- 
 mentation, should be avoided. Flour paste is especially 
 injurious, and many cases of fading have been traced to 
 this cause. The addition of Bichloride of Mercury, which 
 is often made to prevent the paste from becoming mouldy, 
 still more unfits it for Photographic use. No substance 
 appears better than Gelatine, which does not readily de- 
 compose, and shows no tendency to absorb atmospheric 
 moisture. The deliquescent nature of many bodies is a 
 point which should be borne in mind in mounting Photo- 
 graphs, and hence the use of a salt like Carbonate of Pot- 
 ask, which the Writer has known to be added to paste to 
 prevent the formation of acid, would be unadvisable. 
 
 d. The effect of Imperfect Miration. The earlier Photo- 
 graphers did not always succeed in properly fixing then* 
 prints, and old Photographs are often found thickly stud- 
 ded with spots and blotches in the tissue of the paper. 
 These prints, however, are not invariably faded upon the 
 surface, and hence it cannot be said that imperfect fixation 
 will certainly end in the total destruction of the picture. 
 Still a notice of the subject may properly be introduced in 
 this place, and the attention of the reader be once more 
 drawn to the importance of examining each print by hold- 
 ing it against the light : imperfect fixation will be shown 
 by yellow patches in the body of the paper. 
 
 e. Mr. Spiller has shown that Hyposulphite of Soda is 
 not a perfect solvent of Albuminate and some other organic 
 compounds of Silver. Hence some of these must remain 
 in the paper in spite of all care taken to the contrary, and 
 may be acted on by the air in a manner similar to that de- 
 scribed in the next paragraph. 
 
FADING OF FEINTS. 323 
 
 f. Impurities in the Air. A Photograph carefully pre- 
 pared may suffer injury from deleterious matters often 
 present in the atmosphere. The air of large cities, and 
 particularly that emanating from sewers and drains, con- 
 tains Sulphuretted Hydrogen, and hence articles of silver- 
 plate become tarnished unless placed beneath glass. The 
 injury which a print sustains by exposure to air contami- 
 nated with Sulphuretted Hydrogen, is less than the tarnish 
 produced upon the bright surface of a silver plate ; but it 
 is recommended as a precautionary measure, that Photo- 
 graphic Pictures should be protected by glass or kept in a 
 portfolio. 
 
 The products of the combustion of coal-gas are probably 
 more likely than the cause last named, to be a source of 
 injury to Photographs suspended without any covering. 
 The sulphur compounds in gas burn into Sulphurous and 
 Sulphuric Acid, the latter of which, in combination with 
 Ammonia, produces the sparkling crystals often observed 
 upon the shop windows. 
 
 The question as to the manner in which the Photographic 
 image may best be protected from these extraneous causes 
 of fading has been mooted, and many plans of coating 
 prints with some impervious material have been devised. 
 If the pictures are to be glazed or kept in a portfolio, this 
 of itself will be sufficient, but in other cases it may perhaps 
 be useful to apply a layer of spirit or gutta-percha varnish. 
 The use of wax, resin, and such bodies is likely, by intro- 
 ducing impurities, to act injuriously rather than otherwise. 
 
 g. Decomposition of Pyroxyline a source of injury to 
 Collodion Photographs. Although the present Section 
 refers more especially to Photographic prints on paper, yet 
 a few remarks upon Collodion images may not be out of 
 place. Collodion Positives and Negatives are usually es- 
 teemed permanent ; but some have been exhibited which, 
 having been put away in a damp place, gradually became 
 pale and indistinct. The change commences at rough 
 edges and isolated points, leaving the centre, as a rule, the 
 last affected. On examination, numerous cracks are often 
 
324 THEORY OF PHOTOGEAPHY. 
 
 visible, tlms indicating that the Collodion film has under- 
 gone decomposition, and has liberated corrosive Oxides of 
 Nitrogen. Substitution compounds containing Peroxide 
 of Nitrogen are known to be liable to spontaneous change. 
 The bitter resin produced by acting upon white sugar with. 
 Nitro- Sulphuric Acid, if not kept perfectly dry, will some- 
 times evolve enough gas to destroy the cork of the bottle 
 in which it is placed ; the solution of the resin has then a 
 strong acid reaction, and rapidly fades an ordinary Positive 
 Print. 
 
 These facts are interesting, and indicate that Collodion 
 Pictures, containing in themselves the elements of their 
 destruction, should be protected from moisture by a- coat- 
 ing of varnish. 
 
 Comparative Permanency. The Photographic prints 
 do not necessarily fade, in the same manner as fugitive 
 colours, by a simple exposure to light and air. Bottles 
 containing Photographs suspended in damp air were placed 
 outside the window of a house with a southern aspect, for 
 nearly three months, but no difference whatever could be 
 detected between Positives so treated and others kept in 
 total darkness. Supposing a case, however, which is the 
 common one, of injurious influences which cannot alto- 
 gether be removed, it may be useful to inquire what mode 
 of printing gives the greatest amount of stability. 
 
 Positives produced by a short exposure to light, and sub- 
 sequent development with Gallic Acid, may be expected 
 to be more permanent than ordinary sun-prints ; not that 
 there is any reason to suppose the chemical composition 
 of a developed image to be necessarily peculiar, but that 
 the use of Gallic Acid enables us to increase the intensity 
 of the red -picture first formed, and to add to its stability 
 by precipitating fresh Silver upon it. This point has not 
 always been attended to ; it has been recommended to 
 remove the print from the developing solution whilst in 
 the red and early stage of development, and to produce 
 the dark tones subsequently by means of gold ; but this 
 plan, although giving very good results as regards colour 
 
FADING OF PEINTS. 325 
 
 and gradation of tone, appears to lessen the advantage 
 which would otherwise accrue from the adoption of a Ne- 
 gative process, and to leave the picture, as regards perma- 
 nency, much in the condition of an ordinary print obtained 
 by direct action of light. 
 
 The original Talbotype process, in which the latent 
 image is formed upon Iodide of Silver, produces, next to 
 Collodion, the most stable image ; but the difficulty of ob- 
 taining bright and warm tints on Iodide of Silver, will 
 stand in the way of its adoption. 
 
 Great stress has been laid upon the superior perma- 
 nency given by the use of gold in toning, but the fact 
 appears to be that the Gold is precipitated in so small a 
 proportion upon the lighter shades of the proof, that it can- 
 not be expected to preserve them from any destructive 
 agencies. It will be sufficient if the toning and fixing can 
 be fully effected without any real injury from communica- 
 tion of Sulphur. 
 
 The prints which are least stable are such as have been 
 toned in acid Hyposulphite Baths, without Gold ; and the 
 difficulty of preserving such pictures from becoming yellow 
 in the half-tones is very great. All those plans of toning 
 in which Acetic or Hydrochloric Acid is mixed with Hy- 
 posulphite of Soda, and the Positive immersed whilst the 
 liquid is in a milky state from precipitation of Sulphur, 
 ought studiously to be avoided. 
 
 Mode of testing the permanence of Positives. Perman- 
 ganate of Potash in very dilute solutions and slightly 
 acidified with Sulphuric Acid, is a very delicate test for 
 Hyposulphite in the droppings from the corner of a print ; 
 a mere trace will instantly destroy the red tint of the 
 solution. 
 
 A dilute solution of this Salt, prepared by dissolving 
 half a grain, or from that to two grains of the salt, accord- 
 ing to its purity, in one gallon of distilled water, affords 
 a convenient mode of testing Positives as regards their 
 power of resisting oxidation. The Positives must be 
 moved occasionally, as the first effect is to decolorize a 
 
326 THEOEY OF PHOTOGRAPHY. 
 
 portion of the liquid, the Permanganate oxidizing the size 
 and organic tissue of the paper. After an immersion of 
 twenty minutes to half an hour, varying with the degree 
 of dilution, the half-tones of the picture begin to die out, 
 and the full shadows become darker in colour. The 
 bronzed portions of the print withstand the action longer, 
 but at length the whole is changed to a yellow image 
 much resembling in appearance the Photograph faded by 
 Sulphur. 
 
 The most available and simple plan of testing perma- 
 nence is to enclose the pictures in a stoppered glass bottle 
 with a small quantity of water. If they retain their half- 
 tones after a course of three months of this treatment, 
 and do not become mouldy, the mode of printing followed 
 is satisfactory. 
 
 Boiling water will also be found useful in distinguishing 
 the unstable colours produced by Sulphur from those fol- 
 lowing the judicious employment of Gold ; in all cases the 
 image will at first be reddened by the hot water, but if 
 toned without Sulphur it will, as a rule, recover much of 
 its dark colour on drying. 
 
 A solution of Chromic Acid may also be applied to dis- 
 tinguish prints toned by Sulphur from others toned by 
 Gold ; the presence of metallic Gold protecting the shadows 
 of the picture in some measure from the action of the acid. 
 
 The solution should be prepared as follows : 
 Bichromate of Potash ..... 6 grains. 
 
 Strong Sulphuric Acid 4 minims. 
 
 Water 12 ounces. 
 
 The characteristic appearance of prints which have been 
 much sulphuretted, and are very liable to fade, should be 
 known. A yellow colour in the lights is a bad sign ; and 
 if the half-tones are at all faint and indistinct, with an 
 aspect of commencing yellowness, it is almost certain that 
 the Positive will not last for any considerable length of 
 time. 
 
327 
 
 CHAPTER IX. 
 
 COLLODION PEESERVATIYE AND DET COLLODION 
 PBOCESSES. 
 
 THE object of the "Collodion Preservative" and dry Col- 
 lodion Processes is to maintain the sensitiveness of the film 
 for a certain length of time after it has been excited in the 
 Bath. There is some difficulty in effecting this, because 
 if the plate be allowed to dry spontaneously, the solution 
 of free Nitrate of Silver upon its surface, becoming con- 
 centrated by evaporation, eats away the Iodide of Silver, 
 and produces transparent spots (p. 24S). 
 
 Some operators have attempted to use a second plate of 
 glass in such a way as to enclose the sensitive film, with 
 an intervening stratum of liquid. The difficulty however 
 of separating the glasses again without tearing the film, is 
 considerable. 
 
 In the process of Messrs. Spill er and Crookes, the pro- 
 perty possessed by certain saline substances of remaining 
 for a long time in a moist condition was turned to account. 
 Such salts are termed " deliquescent," and many of them 
 have so great an attraction for water that they absorb it 
 eagerly from the air ; the solution having been formed, the 
 water cannot entirely be driven off except by the applica- 
 tion of heat. In selecting a deliquescent salt for a Pre- 
 servative Process, it is necessary to choose one which pro- 
 duces no precipitate in solution of Nitrate of Silver, such 
 for instance as the Nitrate of Magnesia, or the Nitrate 
 of Zinc. Deliquescent bodies like Chloride of Calcium or 
 
328 THEORY OF PHOTOGBAPHY. 
 
 Carbonate of Potash, which decompose Nitrate of Silver, 
 are inadmissible, since it is found that they destroy the 
 sensitiveness of the film, as will presently be shown. 
 
 The process of Spiller and Crookes, although it had the 
 great merit of originality, was soon superseded by others, 
 since it appeared that plates preserved by Nitrate of Mag- 
 nesia were liable to yield feeble and imperfect Negatives, 
 unless the Collodion and Bath happened to be in a pecu- 
 liar condition. This defect, together with occasional fog- 
 ging of the transparent parts, the Author believes to have 
 resulted from the employment of an inorganic instead of 
 an organic preservative agent. 
 
 The attention of experimenters in the Preservative Pro- 
 cesses was also directed to Glycerine, as a substance with 
 little or no action on Nitrate of Silver, and therefore one 
 likely to succeed. Glycerine failed however to give any 
 certain results, and often produced a weak and foggy pic- 
 ture, like the inorganic salts before alluded to. 
 
 The first use of Honey as a preservative agent was pro- 
 posed by Mr. Shadbolt. This substance was deemed by 
 some to be unsuitable, since Grape Sugar, always pre- 
 sent in it, reduces Nitrate of Silver to the metallic state : 
 the results however showed that plates prepared with 
 Honey, yielded more vigorous, and also cleaner Negatives 
 than those made with Glycerine or Nitrate of Magnesia. 
 The reason of such an apparent anomaly will be under- 
 stood, when it has been shown, a few pages in advance, 
 that a preservative substance possessing reducing power 
 on Salts of Silver is precisely what is required, and that a 
 neutral body like Glycerine, which does not reduce Nitrate 
 of Silver, is incapable of acting as a stimulus to the actinic 
 
 Honey employed as a preservative substance varies much 
 in its action with the extent to which the plates are washed 
 from the Nitrate of Silver before the Honey is laid on. 
 In proportion as the quantity of free Nitrate is large, 
 both sensitiveness and intensity are increased, but the 
 keeping qualities of the plates are diminished. This ia 
 
PEESEEVATIVE PEOCESSES. 329 
 
 especially the case in hot weather, and it will then fre- 
 quently happen that in the course even of a few hours, an 
 incipient decomposition will be set up, so that after the 
 exposure of the film to the Camera, the developing solu- 
 tion brings out an intensely red and solarized picture, with 
 a universal deposit of Silver upon its surface, sufficient to 
 render the picture invisible by reflected light. Honey in 
 fact being a slow reducing agent, although it exerts in the 
 first instance a positive action in keeping the shadows of 
 the image clean, yet afterwards sets up decomposition, and 
 gives rise to fogging. 
 
 In order to employ the preservative Honey in an advan- 
 tageous manner as regards keeping properties of the plate, 
 the Collodion film should be freed more or less completely 
 from the soluble Nitrate of Silver ; the Grape Sugar has 
 then only the pure Iodide and Bromide of Silver to act 
 upon, and no decomposition takes place. This is the plan 
 followed in the " Oxymel " process of Mr. Llewelyn, in 
 which also the Honey is acidified by Acetic Acid, in order 
 still further to lessen the chances of failure from incipient 
 reduction of Nitrate of Silver. 
 
 The Oxymel process, although free from the peculiar de- 
 fects of the original Honey process, is however open to 
 others of an opposite kind, for it is found that the presence 
 of small quantities of free Nitrate of Silver is useful in the 
 Honey process in increasing sensitiveness and intensity 
 of image ; hence when the plates are fully washed, the 
 film not only takes the Camera impression very slowly, 
 but the image developes of a blue or slaty tone instead 
 of a brown or red tone. Thus it is perceived that long 
 keeping properties are, in this process, irreconcilable with 
 other good qualities, and therefore we naturally turn to 
 the consideration of a purely dry process : in this case 
 all moisture being removed, and the chemical decomposi- 
 tions being in consequence nearly suspended, we are en- 
 abled to avail ourselves of those conditions which secure a 
 rapid and intense action, without at the same time en- 
 dangering the stability of the excited film. 
 
330 THEORY OF PHOTOGRAPHY. 
 
 Dry Collodion processes. Formulae for operating with a 
 dried film of Collodion have been given at various times 
 in the foreign journals ; in some of which it is stated that 
 nothing is required but mere washing with water after 
 taking the film from the Bath. Success by this mode, 
 however, is quite problematical, and can only be expected 
 when either the Collodion or Bath is in a particular state of 
 organic change. It has now been clearly determined that a 
 film of Iodide of Silver upon Collodion will not uniformly 
 yield a perfect Negative after removal of the Nitrate of 
 Silver and desiccation. On applying the reducing agent 
 to such a plate, previously exposed in the Camera, we see 
 at once that the particles of Iodide have not been thrown 
 into the exact condition requisite to obtain a rapid precipi- 
 tation of Silver in the black form, and consequently the 
 action is irregular and fogging ensues : Organic matter in 
 some shape appears to be necessary in a dry process mani- 
 pulated in the ordinary way. 
 
 The most simple dry Collodion process in which the ne- 
 cessity for organic matter is recognized, is that of the 
 Abbe Despratz, who dissolves common rosin in iodized 
 Collodion, and washes the excited film with water after re- 
 moving it from the Bath. An exposure in the Camera ot 
 not more than twice the length required for wet Collodion 
 is then given, and the plate is re-dipped for an instant in 
 the Nitrate Bath : after this the image comes out on flood- 
 ing the plate with Pyrogallic Acid in the usual way. Al- 
 though the Writer is not able to state that this process will 
 yield clear Negatives with certainty, yet he is convinced 
 that a remarkable effect is produced by this second dip in 
 the Nitrate Bath. Theory might at first suggest that the 
 restoration of the Nitrate of Silver, previously removed by 
 washing, was the only object attained, but it is not so, for 
 the image cannot be developed after so short an exposure 
 when Pyrogallic Acid and Nitrate of Silver are mixed and 
 applied simultaneously to the plate. It seems probable 
 therefore that the exposure in the Camera sets up a mole- 
 cular disturbance, not in itself sufficiently decided to pro- 
 
DEY PEOCESSES. 331 
 
 duce quick development, but capable of being increased, 
 or carried into a further stage by contact with Nitrate of 
 Silver. It has however been shown, on the other hand, 
 that the application of Nitrate of Silver will nearly destroy 
 the highest lights of an invisible image on a plate left in 
 the Camera for a longer time than in the last experiment, 
 and hence we can only conjecture that there exists an 
 alternating action of the light, and that the same chemical 
 agent may produce opposite effects according to the stage 
 at which it is applied to the plate (p. 149). This subject 
 is still involved in much obscurity, and deserves further 
 investigation. 
 
 To all dry processes like that of the Abbe Despratz, in 
 which the organic matter is added to the Collodion, there 
 exists one serious objection, viz. that the Nitrate Bath 
 gradually suffers deterioration in consequence of the or- 
 ganic substance dissolving out of the film (p. 253). The 
 Photographic properties of the solution are not immedi- 
 ately injured thereby, if a bromo-iodized Collodion be em- 
 ployed, but with a simply iodized Collodion used in the 
 ordinary wet process, either a general fogging of the film, 
 or some other disturbance of the development, will soon 
 be perceived. It appears therefore more desirable, as far 
 as the integrity of the Nitrate of Silver is concerned, to 
 apply the requisite organic body to the excited film sub- 
 sequently to its removal from the Bath. 
 
 Tauperiot's Collodio- Albumen process follows next in 
 order of discovery. It is not altogether a Collodion pro- 
 cess, nor is it entirely a process on Albumen, but rather a 
 union of the two. The manipulations are double through- 
 out, a sensitive film of Iodide of Silver in Albumen being 
 placed upon a film of Iodide of Silver in Collodion. The 
 two together offer advantages which cannot be obtained by 
 the use of either separately, for the presence of the Albu- 
 men secures a vigorous development, whilst the substratum 
 of Collodio-Iodide adds to the sensitiveness. There are 
 two Nitrate Baths in this process, one for the Collodion, 
 and the other for sensitizing the dry film of iodized Albu- 
 
332 THEORY OF PHOTOGRAPHY. 
 
 men. There are also two operations of washing ; the first, 
 as before, for the Collodion, and the other for removing 
 the excess of free Nitrate from the Albumen. 
 
 The Collodio-Albumen process of M. Taupenot is un- 
 doubtedly a troublesome one, but it is perhaps more to be 
 depended on than any other known, inasmuch as it is in- 
 dependent of the exact physical and chemical condition of 
 the Collodion.. In dry processes generally, the state of the 
 Collodion must be carefully attended to, as will presently 
 be shown, but the Taupenot method is rather an Albumen 
 than a Collodion process, since the substratum of Iodide 
 upon Collodion is used merely for the purpose of increasing 
 the sensitiveness. A facility in receiving the latent im- 
 pression is indeed a matter of importance in any dry pro- 
 cess, but less so than a correct development, because in 
 the one case a few minutes longer in the Camera prove a 
 remedy, whereas in the other no simple remedy is known. 
 Albumen, therefore, still maintains its ground, because the 
 Photographer finds by experience than he can rely upon it 
 for controlling the action of his reducing agent, and pro- 
 ducing the chemically opaque image. 
 
 Our knowledge of the theory of dry Collodion has been 
 much assisted by the researches of Dr. Hill N orris. His 
 view of the function of a preservative substance applied 
 to the excited film was that it acted by keeping open 
 the pores of the Collodion : Pyroxyline forms a gelatinous 
 layer, which is soft and spongy when wet, but shrinks on 
 drying, and after it has been once dried, it does not return 
 to its former pappy condition on being a second time wet- 
 ted with water. In order therefore to secure that pene- 
 tration by the developer which is requisite for the rapid 
 bringing out of the picture, some such substance as Gela- 
 tine or Albumen is required, capable of insinuating itself 
 between the particles of the Iodide, and so of preventing 
 them from collapsing and producing a surface impervious 
 to liquids. N orris did not consider the presence of free 
 Nitrate of Silver to be necessary in a dry process, but ra- 
 ther detrimental, and advised that the plates should be 
 
DEY PEOCESSES. 333 
 
 perfectly washed with distilled water previous to the ap- 
 plication of the Gelatine. Another point on which great 
 stress was laid was the physical condition of the Collodion : 
 it ought not to be of the kind which gives a horny film, but 
 rather of the powdery variety, the properties of which are 
 described at p. 228 ; less difficulty would then be experi- 
 enced in maintaining the conditions previously insisted on. 
 
 The above views on the theory of dry processes have 
 never been disputed, but it was soon found that they em- 
 braced only a portion of the truth; for the same certainty 
 was not obtained by the method of Norris as by that of 
 Taupenot, and with some samples of Collodion, apparently 
 in the proper physical state, only faint Negatives could be 
 produced. 
 
 The next decided step in the theory of dry Collodion 
 was the discovery of the method known as the " Fother- 
 gill process," in which diluted Albumen was poured over 
 the plate, and afterwards washed off again with water. 
 This process proved to be less dependent upon the par- 
 ticular state of the Collodion than that of Dr. Norris, since 
 intense Negatives were obtainable both with horny and 
 porous Collodion films. It was found advisable, however, 
 as a rule to avoid washing the film too closely with water 
 before the Albumen was applied ; or, in other words, to 
 leave a little free Nitrate of Silver with which the or- 
 ganic substance might combine. The theory therefore was 
 stated as follows : In the wet Collodion process the pre- 
 sence of free Nitrate of Silver imparts an increased sensi- 
 tiveness to the film of Iodide of Silver : in the Fothergill 
 process a portion of this free Nitrate is converted into Al- 
 buminate of Silver, and the remainder is removed by wash- 
 ing with water. 
 
 Observe certain points of difference between the dry 
 process of Norris and that of Fothergill. In the former 
 you remove the whole, and in the latter only a part, of the 
 free Nitrate of Silver. In the Norris process also, the 
 Gelatine preservative is left upon the film, whereas in the 
 Fothergill process the greater part of the Albumen pre- 
 
334 THEORY OF PHOTOGRAPHY. 
 
 servative is washed away. Hence a N orris plate has the 
 pores of its collodionized surface filled up with the organic 
 substance, so that although the film contracts on drying, 
 it softens and swells up again when it is wetted previous 
 to development ; but in the Fothergill plate the quantity 
 of Albumen which remains upon the Collodion after the 
 second washing is so small, that the film once dried does 
 not again expand fully on being wetted, and the action of 
 the developer is in consequence confined nearly to the sur- 
 face. For this reason a Fothergill plate must be under a 
 disadvantage as regards rapidity of development, since the 
 solution of Pyrogallic Acid cannot penetrate deeply, and 
 the image has to be built up by superposition of metallic 
 particles. Porosity being wanting in this process, we have 
 to trust to the extraordinary capabilities of Albumen before 
 alluded to, to supply the deficiency ; and since the con- 
 trolling action of Albumen upon the development is more 
 decidedly seen when it is previously combined with Nitrate 
 of Silver, a little free Nitrate should always be left upon 
 the plates before albuminizing. 
 
 The views above expressed will not perhaps tally with 
 the experience of all who practise the Fothergill process, 
 since it is recommended by some to remove the whole of 
 the Nitrate of Silver from the plate before applying the 
 Albumen. This plan is indeed favourable as regards long- 
 keeping properties, because Albumen when combined with 
 Nitrate of Silver, tends to reduce it by degrees to a Sub- 
 Albuminate even in the dark. It is, however, unfavourable 
 as regards the sensitiveness, and also as regards the inten- 
 sity of the developed image. If any should assert the con- 
 trary, we invite them to examine more closely the condi- 
 tions under which their experiments were performed : pro- 
 bably they will find that they have employed a Collodion 
 in a state of organic decomposition ; and such a Collodion, 
 as we shall presently show, is capable of supplying in a 
 measure the place of the Albumen, and, even when dried, 
 of yielding a Negative without any preservative, 
 
 If the Fothergill process is to be assimilated to that ot 
 
DRY PEOCESSES. 335 
 
 Taupenot, and to be considered as an Albumen process, 
 nearly independent of the state of the Collodion, it cannot 
 be too strongly insisted on that the Albumen left upon the 
 film must be coagulated with Nitrate of Silver. Herein, 
 however, lies the practical objection to this process, for it 
 is not by any means an easy matter to apply Albumen 
 evenly to the surface of a large plate, unless the whole of 
 the Nitrate of Silver be previously removed by washing. 
 Albumen precipitates Nitrate of Silver, and hence in the 
 ordinary mode of manipulating Fothergill plates, it is a 
 common circumstance to encounter stains and markings of 
 shapes varying with the structure of the Collodion and the 
 manner of pouring on the Albumen. 
 
 Messrs. Petschler and Mann, members of the Man- 
 chester Photographic Society, have lately described a mo- 
 dified dry method which appears to occupy an interme- 
 diate position between the process of Taupenot and that 
 of Fothergill. The excited Collodion film, after a- careful 
 washing in water, is coated with diluted Albumen contain- 
 ing Chloride of Sodium or Ammonium, and is then dried 
 with the Albumen still remaining upon its surface. In 
 this state light has but little action upon it, since it ap- 
 pears that dry Iodide of Silver, in contact with Chloride of 
 Sodium, is comparatively insensitive. To render the plates 
 again sensitive to light, they are abundantly washed with 
 water, and then dried a second time. 
 
 The above process differs from that of Taupenot in the 
 omission of the second Bath of Nitrate of Silver ; the ob- 
 jection to the use of which is, that it soon decomposes 
 under the reducing influence of Albumen dissolved out of 
 the plates, and a surface-scum in consequence forms, thus 
 originating stains. Another effect due to decomposition 
 of Nitrate of Silver by Albumen, is a tendency to rapid 
 discoloration in the mixed developer, and a difficulty in 
 keeping the shadows clean, and free from deposited Silver. 
 In discarding the second Bath we gain therefore important 
 advantages, but the fact cannot be concealed that as re- 
 gards a constant intensity of the image we are more de- 
 
336 THEORY OF PHOTOGRAPHY. 
 
 pendent upon the state of the Collodion than before, since 
 Albumen appears to increase the vigour of the impression 
 in the Camera more decidedly when it has been coagulated 
 by Nitrate of Silver. 
 
 The new method resembles that of Fothergill in the 
 .plates being washed with water after the application of the 
 Albumen, but differs from it in the fact that the Albumen 
 is allowed to become dry before the water is again applied. 
 This difference is important, because the effect of drying is 
 to -render the Albumen less soluble, so that the second ap- 
 plication of water removes only a portion of the organic 
 substance. The fact of the plates regaining a considerable 
 sensitiveness after the second washing cannot easily be ex- 
 plained, except on the supposition that the porosity of the 
 film is preserved in a measure by a part of the Albumen 
 remaining upon the Iodide, for we know that in the 
 Fothergill process, if the Albumen be washed away whilst 
 w r et, and no free Nitrate of Silver be present to coagulate 
 and retain a portion of it, the sensitiveness will be lessened. 
 
 In Major Russell's dry process we recognize a new prin- 
 ciple. The preservative material is Tannin, and the plates 
 are entirely freed from Nitrate of Silver, in the same man- 
 ner as in the process of Norris. Tannin is very soluble in 
 water, and its aqueous solution on evaporation leaves a 
 film of varnish which is not sticky to the touch. When 
 applied to the surface of the excited film the Tannin keeps 
 open the pores of the Collodion as before described, and 
 the mechanical condition of the film appears to be inter- 
 mediate between that of Norris and Fothergill, being 
 slightly more permeable than the latter, but considerably 
 less so than the former.* Independently of this, however, 
 Tannin has a very remarkable effect in stimulating the 
 sensitive Iodide, and causing it to take a much deeper and 
 
 * When the porosity of a film of dried Collodion is fully maintained by the 
 application of a glutinous preservative, it changes colour on being wetted 
 with water, losing the pale opaline appearance which it had acquired by dry- 
 ing, and returning again to the yellow creamy aspect of a recently prepared 
 plate. A film of collodion not kept open by a preservative becomes pale and 
 blue on drying, and remains so even after wetting. 
 
DRY PROCESSES. 337 
 
 more decided impression in the Camera. It would not be 
 correct to speak of this acceleration as an increased sensi- 
 tiveness .imparted to the Iodide, but rather as an augmen- 
 tation in the force of development, in consequence of which 
 the image starts out quickly, and with more contrast be- 
 tween the extreme tints : the colour of the image is also 
 different, the particles which compose it being no longer 
 grey and crystalline, but finely divided and opaque, so that 
 the Negative possesses all those qualities which the Pho- 
 tographer requires. 
 
 It would involve the reader in endless perplexity were 
 we to attempt to trace the exact origin of the process now 
 under discussion, for it cannot be denied that Tannin and 
 Gallic Acid are photographically similar, and that Gallic 
 Acid upon the film was employed by Talbot, and also by 
 Archer and others who followed him. In the " Talbotype," 
 however, the Gallic Acid was conjoined with Nitrate of 
 Silver, and also \vith Acetic Acid, the latter being used to 
 maintain the balance and prevent a too speedy reduction. 
 Russell, in applying the method to Collodion, modifies it in 
 such a way as to suit the altered requirements of the pro- 
 cess, eliminating tiie Nitrate of Silver and dispensing with 
 the Acetic Acid : then in order to overcome the tendency 
 which Gallic Acid exhibits to crystallize on drying, he sub- 
 stitutes Tannic Acid, and thus secures the mechanical con- 
 ditions needful for permeability by aqueous liquids. 
 
 Let us explain at greater length the mode in which we 
 suppose this new Photographic chemical to act. Tannin is 
 a reducing agent : if you mix it with solution of Nitrate of 
 Silver in a test-tube, in the course of an hour or two the 
 sides of the tube will be silvered like a mirror. Now it is 
 known that moist Iodide of Silver on Collodion is active 
 in presence of free Nitrate of Silver, but that dried Iodide 
 of Silver on Collodion is actinically sluggish: although 
 capable of receiving an impression in the Camera, the la- 
 tent image is so faint that it cannot be developed in a satis- 
 factory manner. The addition of free Nitrate of Silver in 
 anything like quantity is not admissible in a dry process, 
 
 z 
 
338 THEORY OF PHOTOGRAPHY. 
 
 since it dissolves the sensitive Iodide and converts it into 
 an lodo-Nitrate of Silver. An organic reducing agent ap- 
 pears to be capable of supplying the place of free Nitrate 
 of Silver, and of causing the Iodide to assume that peculiar 
 condition in the Camera which leads to a precipitation of 
 Silver in the black form on the application of the mixed 
 developer. 
 
 It had been supposed by many that the employment of 
 substances like Gallic Acid or Tannin in a dry process 
 would lead to a general clouding of the film : so far, how- 
 ever, from such being the case, it is a fact that they pre- 
 serve the transparency of the shadowy portion of the Ne- 
 gative. The phenomenon known as fogging may, as we 
 have shown at page 157, be originated by opposite causes. 
 One form of this defect is due to an excess of action of the 
 chemicals, and is remedied by the use of a retarding acid ; 
 but another kind of fogging is caused by defective action, 
 and a remedy for this is the use of a reducing agent applied 
 to the film before exposure. Every observant Photographer 
 knows that in the wet process too much Nitric Acid in the 
 Negative Bath is a cause of fogging almost as decided as 
 an excess of the alkali Ammonia : and so in the dry pro- 
 cess, if the Iodide be used in too languid a state, so to 
 speak, it will be impossible to obtain a clean picture, be- 
 cause the latent image being imperfectly formed, the de- 
 veloper finds no definite surface upon which to exhaust its 
 energies. Those who pay but little regard to theoreti- 
 cal considerations in Photography, and are guided solely 
 by practice, will be convinced on trying the following ex- 
 periment : Take a dry Collodion plate prepared with a 
 simple preservative solution of four grains of Gelatine to 
 the ounce of Water : then apply to one-half of this plate a 
 saturated solution of Gallic Acid ; dry and expose as usual. 
 A single trial will be sufficient to indicate a surprising dif- 
 ference between the two halves, not only in depth and 
 rapidity of development, but also in the brilliancy and 
 sharpness of the whole picture. In the one case there 
 will probably be indistinctness in the lines, and fringes or 
 
COLLODION FOE DRY PROCESSES. 339 
 
 halation round the shadows, whilst in the other the deposit 
 of Silver forming the image will be regular and uniform, 
 being limited exactly to those parts which have been 
 touched by light. This effect could not be obtained by 
 the use of a body like Glycerine, which is miscible with 
 Nitrate of Silver without change, but according to the ex- 
 perience of the Writer is confined to the organic substances 
 which give intensity in the dry processes, and are cha- 
 racterized by decomposing Nitrate of Silver, or by entering 
 into some form of combination with it. 
 
 Another inconvenience originally anticipated from the 
 use of reducing agents in dry Collodion Photography was 
 a deficiency of keeping properties. This however has, like 
 the last, proved to be an unfounded apprehension. It would 
 not be safe to leave Gallic Acid and Nitrate of Silver to- 
 gether upon the excited film, but when the last portions of 
 free Nitrate of Silver have been removed by a Bath of 
 Salt and Water, the plates will retain their properties un- 
 changed for a length of time amply sufficient for all prac- 
 tical purposes. 
 
 Collodion for the Dry Processes. The above sketch of 
 the theory of dry Collodion would be incomplete without a 
 few remarks upon the composition of the Collodion itself. 
 What is the reason of the difficulty which has always been 
 experienced in employing films of dry Collodion ? Iodide 
 of Silver upon desiccated Albumen was used successfully 
 from the first : why then should it not be so when sup- 
 ported by Pyroxyline? Doubtless the close and parch- 
 ment-like texture of dry Collodion and its impermeability 
 by aqueous liquids constitutes a difficulty, but another and 
 more serious impediment lies in the fact that Collodion is 
 too similar to Glycerine and bodies of that class, before 
 mentioned as being neutral and indifferent to the Salts of 
 Silver. When iodized Albumen is dipped in a Nitrate 
 Bath, the substance precipitated in the film is not pure 
 Iodide of Silver, but Iodide of Silver associated with Al- 
 buminate of Silver, probably in the form of a double com- 
 pound. This body is highly photographic, and takes a 
 
340 THEORY OF PHOTOGBAPHY. 
 
 strong invisible impression in the Camera, even after the 
 free Nitrate of Silver has been washed away. Iodized 
 Collodion, on the other hand, throws down a purer form 
 of Iodide in its passage through the Bath, and this Iodide 
 when washed and dried, does not take a strong impression 
 in the Camera. Comparing the Collodion film with the 
 Albumen film, we find that the luminous image affects the 
 former with greater rapidity, but not with equal force. 
 The depth of the latent impression on pure dry Collodion, 
 never increases uniformly with the intensity of the light, 
 and hence there is an imperfect gradation of tone ; also 
 the image is easily reversed by over-exposure, so that on 
 developing, a transmitted Positive is obtained instead of 
 a. Negative. It may be taken to be an established fact 
 that a body possessing organic reactions towards Salts of 
 Silver ought to be in contact with the Iodide during its 
 formation in the Bath, if that Iodide is to be precipitated 
 in a state photographically available after drying. For- 
 tunately it is in our power to modify Pyroxyline, and to 
 impart to it a certain measure of those organic reactions 
 which it lacks when existing in a pure form. Let us ex- 
 plain briefly how this may be effected. Cellulose possesses 
 an appreciable affinity for Nitrate of Silver, although less 
 than that of Albumen or Gelatine : this affinity it loses 
 when converted into Pyroxyline. If however the propor- 
 tions of the constituents of the Nitrosulphuric Acid be 
 purposely varied, and the temperature of the acid at the 
 same time raised, a partial conversion of the Cellulose 
 into vegetable parchment or into Grape Sugar takes place, 
 and it is then found that the resulting Pyroxyline is no 
 longer indifferent to the Salts of Silver, but has acquired 
 the property of retaining a small portion of Nitrate of 
 Silver after passing through the Bath. Thus the modified 
 Pyroxyline becomes assimilated to Albumen in its Photo- 
 graphic reactions, and when iodized throws down no longer 
 a pure and isolated Iodide of Silver in the Bath, but an 
 Iodide of Silver associated with traces of an organic com- 
 pound of Silver. If the reader should fail in comprehend- 
 
COLLODION FOE DRY PROCESSES. 341 
 
 ing how the presence of mere traces of an organic Salt of 
 Silver in a Collodion film can have any effect upon the 
 image, let him make the simple experiment of dissolving 
 Glycyrrhizine, or Sugar of Liquorice, in iodized Collodion, 
 and of testing the Photographic effect before and after : he 
 will find that a fractional part of a grain of the Sugar will 
 make all the difference between a feeble and an intense 
 Negative. 
 
 Another means of altering Collodion in such a manner 
 as to render it available in the dry processes, is by keeping 
 the Collodion for a time in the iodized state. We have al- 
 ready shown, in the Chapter on Negatives, that Collodion, 
 however iodized, may be expected to produce a more vigo- 
 rous picture after keeping ; but in the dry process this effect 
 is even more decidedly seen than in the wet process. In 
 both cases there is a slow and gradual decomposition of 
 the Pyroxyline under the influence of the base of the Io- 
 dide, the effect of which is to give the former an increased 
 power of uniting with Nitrate of Silver. In the dry pro- 
 cess however the efficiency of the film is still further aug- 
 mented by the fact that iodized Collodion loses by age a 
 portion of its horny quality, setting less firmly upon the 
 glass, and presenting a more open and spongy surface, so 
 as to admit of the glutinous organic substance penetrating 
 between the separate particles of Iodide, and thus preserv- 
 ing a passage for the developer. 
 
 Bromide in the Collodion. In all dry processes in which 
 the free Nitrate has been entirely removed from the plate, 
 the accelerating influence of a Bromide in conjunction with 
 an Iodide is very remarkable. It is so also in the Taupenot 
 process, but here if the Collodion contains a Bromide, the 
 plates must not be exposed to light, as is sometimes recom-. 
 mended, after the application of the iodized Albumen. 
 In respect of the accelerating effects of a Bromide, in dry 
 Collodion processes, it boars a close resemblance to its 
 action on the Daguerreotype plate. In the wet Collodion 
 process, on the other hand, no increase of sensitiveness is 
 gained by its use. In looking for an explanation of this 
 
342 THEORY OF PHOTOGEAPHY. 
 
 seeming anomaly, it is probable it will be found in tlie fact, 
 that Iodide of Silver, in the absence of free Nitrate, is 
 totally insensitive to light, but that Bromide of Silver 
 under the same circumstances is sensitive. It may be 
 presumed therefore, with some show of reason, that the 
 Bromide receives the impression in the Camera, forms, as 
 it were, the nucleus of the picture, and by its mere pre- 
 sence in contact with the Iodide and an organic element, 
 helps to produce the requisite density on the application 
 of the developer. Whatever be the explanation, its acce- 
 lerating influence is beyond dispute ; but unless the organic 
 element be also present, the Negative at the best would be 
 but thin and feeble. 
 
 Our general conclusions on the subject of Collodion for 
 the dry processes may be recapitulated as follows : A 
 Bromide in conjunction with an Iodide (except perhaps in 
 the Taupenot process) is desirable, and the organic ele- 
 ment is always necessary in the sensitive film. If the 
 latter exist already in the Collodion itself through decom- 
 position after iodising or other causes, it will scarcely be 
 required in an extraneous form ;. and in such a case it will 
 only be necessary to wash and dry the film, supposing it 
 to be a porous Collodion, or to flood it with a simple gluti- 
 nous solution if it be a horny Collodion. When however 
 the Collodion is more pure, and hence deficient in those 
 qualities which give intensity to the development, the lack- 
 ing element must be supplied in the form of a preserva- 
 tive, and of all the substances commonly employed for that 
 purpose, Albumen and Tannin are the most decided in 
 their action, the former being supposed to be coagulated by 
 Nitrate of Silver; other preservatives, such as Dextrine, 
 Cane Sugar, and Gum, are only effective when the Collo- 
 dion is able of itself to supply a part of the lacking ele- 
 ment, and merely requires assistance to enable it to com- 
 plete the remainder. 
 
 The Development of Dry Plates. Wet Collodion is de- 
 veloped by the simple application of a reducing agent to 
 the surface of the film. In a dry process, however, Ni- 
 
DEVELOPMENT OF DRY PLATES. 343 
 
 trate of Silver must be added to the developer before the 
 action commences. The difference depends upon the fact 
 that in the wet process you haA r e free Nitrate of Silver in 
 contact with the Iodide, but in the dry process this Nitrate 
 has been washed away, and hence it is necessary to add it 
 to the Pyrogallic Acid, which when used alone is not a 
 developer in the proper sense of the term. 
 
 Skilful Photographers find by experience that they are 
 able to control the development of dry plates by varying 
 the relative quantities of Nitrate of Silver and of the 
 reducing agent. This proceeding is quite necessary, be- 
 cause the quality of the resulting Negative is much in- 
 fluenced by the length of the exposure in the Camera, and 
 it constantly happens in a dry process that too much or 
 too little exposure has been allowed. Under-exposure is 
 favourable to excessive contrast in the Negative, some 
 parts being highly opaque and others translucent. Over- 
 exposure, on the other hand, produces too much unifor- 
 mity, the half-tones and the high lights being nearly of 
 the same depth. Now it is found that a developer con- 
 taining a large relative proportion of Nitrate of Silver 
 adds to the contrast between the tones, whilst, on the other 
 hand, a diminished proportion of the same compound les- 
 sens the contrast. The Artist therefore keeps both solu- 
 tions conveniently near at hand, and observes the beha- 
 viour of the image. If the light has acted for too short a 
 time, and hence has failed in impressing the shadows deci- 
 dedly, he compensates for the defect as far as possible by 
 increasing the strength of his Pyrogallic Acid solution ; 
 whereas when he sees any evidence of over-action of light 
 in the diminished intensity of the fully-exposed portions 
 of the film, he then encourages a vigorous deposition of 
 Silver by dropping in the Nitrate more freely. 
 
 An improved developer for dry processes has lately been 
 devised by Major Russell. The modification consists in 
 mixing the retarding acid with the Nitrate of Silver, instead 
 of adding it to the reducing agent in the usual way. Sup- 
 posing the balance between the Acid and the Silver to be 
 
344 THEOEY OF PHOTOGEAPHY. 
 
 properly adjusted, the developer will seldom become tur- 
 bid, because if an excess of the Nitrate be inadvertently 
 added to the solution of Pyrogallic Acid, a corresponding 
 excess of Citric Acid will be introduced with it and will 
 counteract the effect. The suggestion is one which has 
 been found to possess a practical value, and to assist much 
 in securing brilliancy and freedom from fogging. 
 
 Comparative Sensitiveness and keeping qualities of Dry 
 Plates. The comparative sensitiveness of wet and dry 
 Collodion has been variously estimated at from four to 
 forty times in favour of the former. The discrepancy de- 
 pends partly upon the fact that the most sensitive variety 
 of wet Collodion has rarely been employed in the experi- 
 ments, and partly also upon the difference of opinion as to 
 what constitutes a fully-exposed plate in dry Collodion Pho- 
 tography. If you take four pictures upon the same glass, 
 and allow two, four, six, and eight minutes respectively in 
 the Camera, there will be less difference between the re- 
 sulting Photographs than might have been anticipated, the 
 principal effect of the prolonged exposure appearing in the 
 form of a few additional details in the shadows, and of a 
 better and more harmonious Negative. Taking ordinary 
 wet Collodion, as it is generally used, iodized with simple 
 Iodides only, and supposing twenty seconds to be the cor- 
 rect time of exposure, the Writer would allow two minutes 
 and a half for a dry plate preserved with Tannin. He has 
 succeeded in making dry plates considerably more sensitive 
 than here described, but not without losing in intensity : 
 and when such is the case, the contrast and intensity of 
 development being too much reduced, it will be found very 
 difficult to prevent the high lights, such as the sky, from 
 passing beyond their proper boundaries and encroaching 
 upon the shadows. There will also be a want of sharpness 
 and decision in the image under such circumstances, so 
 that fine lines in the picture will coalesce, the space be- 
 tween them being obliterated by deposited Silver. 
 
 The keeping qualities of dry plates have also been va- 
 riously stated. The Writer himself has not been success- 
 
DRY PROCESSES. 345 
 
 ful with long-kept plates, and has usually found spots 
 during development, arising, as he supposes, from minute 
 particles in the film having become centres of chemical 
 action. After the plates have been exposed in the Camera, 
 the chances of obtaining a perfect picture are still further 
 diminished by long keeping, since the image will in some 
 cases dev elope spontaneously, and in others will partially 
 disappear so as not to admit of a perfect development. 
 
 GENERAL OBSERVATIONS ON DRY COLLODION PROCESSES. 
 
 a. Different forms of Iodide of Silver. It was shown at page 
 248 that Iodide of Silver forms a crystalline compound with Ni- 
 trate of Silver, known as lodo-Nitrate of Silver ; the crystals of 
 which are soluble without change in a concentrated solution of 
 Nitrate of Silver, but are decomposed into free Iodide of Silver 
 and free Nitrate of Silver by treatment with water. This lodo- 
 Nitrate of Silver, however, is not of much importance in Photo- 
 graphy, for iu the wet process it exists in the film only in minute 
 quantities, and iu the dry process it is altogether absent, the plates 
 being invariably washed in water before drying. 
 
 Iodide of Silver possesses another peculiarity, which is more 
 important to the Photographer than its property of combining 
 with Nitrate of Silver. We refer to its existence in two states, 
 which may be designated by the terms sensitive and insensitive. 
 Sensitive Iodide of Silver is formed when a soluble Iodide is added 
 to Nitrate of Silver, the latter salt being in excess. Insensitive 
 Iodide is formed by the admixture of the same salts, but with the 
 soluble Iodide in excess instead of the Nitrate. The two forms 
 of Iodide differ from each other in colour and general appear- 
 ance, the sensitive being of a greenish-yellow and rather dense, 
 the insensitive nearly white and comparatively bulky : in che- 
 mical composition, however, they are believed to be the same, 
 provided they have been thoroughly washed with distilled water 
 to remove the precipitating salts left in excess. 
 
 Let us notice the conditions under which these two forms of 
 Iodide are mutually convertible. The application of a solution 
 of Iodide of Potassium or of any other soluble Iodide immediately 
 renders the sensitive Iodide insensitive, and no amount of wash- 
 ing with water will restore to it the property of being impressed 
 
346 THEORY OF PHOTOGRAPHY. 
 
 in the Camera. On the other hand, a solution of Nitrate of Silver 
 imparts sensitiveness to the form of Iodide which lacks it, and 
 although the excess of Nitrate be afterwards removed by abun- 
 dant ablutions, sensitiveness still remains. Sensitive Iodide of 
 Silver placed in contact with Chloride of Sodium and dried, either 
 receives no impression at all in the Camera, or at all events an 
 imperfect one ; but if it be washed with water to remove the 
 Chloride of Sodium, its sensitiveness is in a measure restored. 
 
 b. Organic Compounds containing Iodide, Bromide, and Chlo- 
 ride of Silver. The study of this class of bodies is necessary to a 
 perfect comprehension of the phenomena of dry Collodion. If 
 the Albumen of an egg be added by degrees to the ordinary Pho- 
 tographic Nitrate Bath containing Iodide of Silver dissolved, the 
 first addition causes a white precipitate, which is not simple Al- 
 buminate of Silver described at page 131, but a compound of Al- 
 buminate of Silver with Iodide of Silver. One property of this 
 substance is, that it is capable of existing in a perfectly trans- 
 parent condition, in lumps resembling calf's-foot jelly or gela- 
 tinous silica : this we mention because it explains the cause of the 
 pale opaline appearance of an excited Albumen film as compared 
 with the opaque and creamy aspect of a Collodion film. 
 
 Glycyrrhizine, or Sugar of Liquorice, added to the Photo- 
 graphic Bath in small quantity throws down a yellow precipitate 
 of Glycyrrhizinate of Silver with Iodide of Silver. This sub- 
 stance has a strong acid reaction, the Glycyrrhizine having united 
 with the Oxide of Silver and displaced the Nitric Acid. When 
 treated with Ammonia it behaves in a manner quite peculiar : 
 the whole deposit appears at once to dissolve, yet on close exami- 
 nation it is seen that the solution is not complete, but that the 
 liquid contains suspended matter in a form nearly invisible. 
 Bearing this in mind, we comprehend the phenomena produced 
 by adding Glycyrrhizine to Collodion, for it is found that if this 
 addition be carried beyond a certain limited point, the film will 
 remain perfectly clear on dipping it in the Bath, and will exhibit 
 no deposit visible to the eye. 
 
 Gelatine combines with Nitrate of Silver, and the combination 
 again unites with Iodide of Silver. The substance so formed is 
 soluble to a certain extent in a solution of the Gelatino-Nitrate of 
 Silver, and no precipitation of Iodide of Silver is produced by 
 dilution with water. If, however, the solution be boiled with a 
 
DEY PROCESSES. 347 
 
 little free Nitric Acid in order to decompose the organic com- 
 pound, then the Iodide will be thrown down in a milky state. 
 The above explains the reason that commercial Gelatine, although 
 nearly always containing common Salt, seldom gives any preci- 
 pitate with Nitrate of Silver : it will be sufficient in such a case 
 to acidify with a few drops of Nitric Acid and heat to 212 Fahr., 
 to obtain a decided turbidity. We have spoken already of combi- 
 nations of Iodide of Silver with Nitrate of Silver, and shown that 
 corresponding compounds of Bromide and Chloride with Nitrate 
 of Silver cannot be formed. Bromide of Silver and Chloride of 
 Silver, however, surpass the Iodide of Silver in affinity for the 
 organic compounds of Silver, and it will be instructive to exa- 
 mine two or three of the more remarkable of the properties of the 
 combinations so produced. 
 
 Chloride of Silver is insoluble in water, and comparatively so in 
 a strong solution of Nitrate of Silver. When, however, Nitrate 
 of Silver is mixed with organic matter, an appreciable quantity of 
 Chloride will be dissolved by it. Hence it follows that in wash- 
 ing a Collodion film, after taking it from the Bath, a solution of 
 common Salt cannot be depended on as a test to show when the 
 whole of the Nitrate of Silver has been removed. The washings 
 from such a plate may give no immediate precipitate with Salt, 
 and may yet contain enough Nitrate of Silver to have a distinctly 
 bitter and metallic taste. 
 
 We gain a correct notion of the relative affinities of Iodide, 
 Bromide, and Chloride of Silver for organic compounds of Silver, 
 by selecting a plain Collodion, the Pyroxyline of which is so pre- 
 pared as to possess a portion of what we term the organic reac- 
 tions, and by dissolving in it separately an Iodide, a Bromide, and 
 a Chloride : on coating glasses with these Collodions and dipping 
 in the Bath, the first will produce a yellow film, the second a pale 
 opaline-blue film, and the third will remain clear, exhibiting no 
 precipitate whatever upon the glass. Unless the experiments of 
 the Author have led him to erroneous conclusions, this gradual 
 diminution of opacity is a measure of the extent to which the 
 affinity reaches. 
 
 Observe the importance of remembering the preceding reactions 
 in experimenting with dry Collodion. Free Iodide of Potassium 
 has been shown to convert the sensitive form of Iodide of Silver 
 into the insensitive, and this in so complete a manner that the 
 
348 THEORY OF PHOTOGRAPHY. 
 
 sensitiveness cannot be restored by washing with water. It is 
 possible, however, to apply Iodide of Potassium to a Collodion 
 film, and yet afterwards to succeed in obtaining a feeble picture, 
 if the excess of Iodide be perfectly removed by hot water : the 
 explanation probably is that the Collodion film is not simply an 
 Iodide of Silver, but contains a portion of an organic compound 
 not immediately decomposable by Iodide of Potassium. Suppose 
 again that a film of sensitive Iodide of Silver with a trace of Ni- 
 trate of Silver adhering to it be washed with Iodide of Potassium 
 dissolved in Albumen. The sensitiveness will be destroyed there- 
 by, as long as the excess of Iodide of Potassium remains upon 
 the film, but it will return in a measure on washing the film with 
 water, because a portion of the Albumiiiate of Silver remains un- 
 decomposed. The affinity of Albumen for Nitrate of Silver ap- 
 peal's to be very great. When an Iodide and a Chloride exist 
 together in aqueous solution, and a limited amount of Nitrate of 
 Silver is added, the Iodide appropriates it to the exclusion of the 
 Chloride. But when Albumen is mixed with an Iodide, the 
 most careful addition of Nitrate of Silver will throw down a com- 
 pound precipitate from the very first. 
 
 Organic matter of certain kinds present in the Nitrate Bath 
 will modify the Photographic properties of a film excited in that 
 Bath. For just as an organic body withdraws in insoluble com- 
 bination Iodide of Silver from its solution in Nitrate of Silver, so 
 conversely does Iodide of Silver in falling carry down with it 
 traces of organic matters previously dissolved in Nitrate of Silver ; 
 whilst Bromide of Silver and Chloride of Silver effect the same 
 in a manner still more decided. The precipitate formed when 
 Chloride is added to Nitrate of Silver previously mixed with or- 
 ganic matter is often in a state of excessively fine division, so that 
 it occupies a long time in subsiding. On analysis it appears that 
 the organic matter is present in very small quantity, scarcely 
 more than what the chemist would term " a trace," yet the action 
 of light is much controlled by it, and we find that the Chloride 
 of Silver thrown down from a solution of Nitrate of Silver con- 
 taining a little Gelatine always darkens in the sun to a brick-red 
 instead of the usual violet-blue. The Iodide of Silver also which 
 is precipitated in a Bath containing certain organic matters, gives, 
 even in a Preservative process, a Negative image with greater 
 contrast, and one which developes more quickly than usual. 
 
DRY PROCESSES. 349 
 
 c. Manufacture of Collodion for the Dry Processes. Much 
 that it would otherwise have been necessary to describe under 
 this head, has been already given in the Sixth Chapter. We re- 
 quire in dry Collodion certain physical qualities, such as adhesive- 
 ness to the glass, and a moderate amount of porosity. Preserva- 
 tive organic bodies tend to loosen the film from the glass, and 
 hence the necessity of making the Collodion as adherent as pos- 
 sible. In effecting this, attention must be paid to the temperature 
 of the Acids, which ought not to be under 150 Fahr., and may 
 sometimes with advantage be raised to 160 Fahr. Hot acids 
 disintegrate Pyroxyline and render it short and friable, whereas 
 cold acids give a strong and tough film, which leaves the glass in 
 a connected skin. Secondly, a considerable quantity of water 
 must be used in the Nitrosulphuric Acid, because this assists in 
 breaking up the structure of the Pyroxyline and in rendering it 
 adhesive. 
 
 Here we pause to make an observation which is important as 
 regards Collodion for dry processes. It has been shown that the 
 fibre of the Pyroxyline must be considerably broken up to ensure 
 an adhesive Collodion, and that high temperature and abundance 
 of water in the acids are both favourable. Now in effecting the 
 object proposed, it will be proper to avail ourselves of both con- 
 ditions, and not to trust too much to the latter, for it is found 
 that a high temperature increases the intensity of the Collodion, 
 whereas a dilution of the acids lessens it. Collodions have been 
 made with which it is difficult to succeed in obtaining opaque 
 skies to landscapes, in consequence of the Nitrosulphuric Acid 
 having been mixed with too large a quantity of water. You may 
 often recognize such Collodions by their requiring an unusually 
 long time to set upon the glass, the reason of which has been 
 already explained. To obviate the occurrence of this difficulty, 
 it is advised that the Nitrosulphuric Acid should contain at least 
 two measures of Oil of Vitriol to one of Nitric Acid of 1'45 sp. gr., 
 and that the temperature should be maintained steadily at 160 
 Fahr. No fear need then be entertained of the water being in 
 excess, because the immediate effect of making the acids too weak 
 in that formula would be to dissolve the whole of the Pyroxyline. 
 If, however, equal bulks of Oil of Vitriol and Nitric Acid were 
 used with the maximum of water, and the temperature allowed 
 to fall to 14,0 Fahr., you might obtain a Pyroxyline, apparently 
 
350 THEORY OF PHOTOGRAPHY. 
 
 disintegrated to the correct point, and also one giving a porous 
 and adhesive film, but the Collodion would produce an excessively 
 feeble Negative, unless free Nitrate of Silver were left upon the 
 film, in addition to the organic preservative. 
 
 Although a difference of opinion exists as to whether Negative 
 Collodion for the wet process ought to be simply iodized or 
 bromo-iodized, all are agreed that Collodion for the dry process 
 should contain a portion of Bromide. The Writer has before 
 shown that the effect of Bromide in Collodion varies with the 
 presence or absence of organic matter. An affinity exists between 
 Bromide of Silver and organic matters coagulated by Nitrate of 
 Silver, and this affinity is more marked than that between Iodide 
 of Silver and organic matters coagulated in the same way ; hence 
 we find that Bromide of Silver in a dry process interferes neither 
 with the rapidity nor with the intensity of the development, but 
 often increases both. The dry process is also peculiar as regards 
 the increased length of exposure given in the Camera, and hence 
 we have an additional reason for dismissing from our minds all 
 preconceived ideas as to the action of Bromide, derived from the 
 use of that substance in the ordinary wet Collodion process. 
 
 Collodion for the dry processes should always contain a suffici- 
 ency of Iodide to produce an opaque film in the Bath, since so- 
 larization or over-action of light in the sky is more marked when 
 the film is transparent. Mr. Moxham has shown that in such 
 a case the actinic rays, after acting for a certain time, pass through 
 the film, and may be received on a second plate placed behind 
 it : this observation is very important. 
 
 In Collodion for the dry processes we are accustomed to rely 
 much upon changes produced by iodizing with an alkaline Iodide 
 and subsequent keeping, which have been shown to result in the 
 formation of Nitrite, and also of an organic substance which 
 combines with Nitrate of Silver ; the adhesiveness and porosity 
 of the Collodion being at the same time augmented. At present 
 we need only indicate the propriety of bearing these changes in 
 mind, and making allowance for them at the time of manufactu- 
 ring the Collodion. For example, it would be possible to prepare 
 a Pyroxyline so hard and contractile as to be quite unsuitable, 
 and yet to bring the Collodion at once into the proper state by 
 agitating it with powdered Carbonate of Potash until liquefaction 
 was produced. Conversely, the temperature and degree of dilu- 
 
DEY PEOCESSES. 351 
 
 tion of the acids might be so adjusted as to render the Pyroxyline 
 independent of any assistance from decomposition after iodizing. 
 Far better, however, would be an intermediate method, in which 
 the newly-iodized Collodion was purposely left a little deficient 
 in porosity, and then kept for a time in the iodized condition, to 
 bring it exactly into the proper state. 
 
 Old Linen rags, or paper from the same, have been used for 
 dry Collodion. The Author sums up his experience of such ma- 
 terials as follows : They produce a Pyroxyline very adhesive and 
 porous, and with stronger organic reactions than that from Cot- 
 ton. This Pyroxyline is, however, unstable both before and after 
 iodizing, and although giving great intensity, often fails in yield- 
 ing the details of foliage and other dark objects. If any should 
 adopt the use of Linen Pyroxyline, let it be remembered that the 
 proportion and strength of the acids, the temperature, the rela- 
 tive bulks of Alcohol and Ether, the nature of the Iodide, etc., 
 must all be varied from the usual formulae. 
 
 Dry Collodion may be made porous by other methods not yet 
 described. Mr. Hockin* has shown that lodoform will destroy 
 the parchment quality common in recently-prepared Collodion ; 
 and even if we add no extraneous substance whatever, we find 
 as a rule that plain Collodion becomes more porous by simple 
 keeping, especially when made from Methylated Ether. 
 
 * ' Practical Hints on Photography, its Chemistry and its Manipulations,' 
 third edition, p. 7. 
 
352 
 
 CHAPTER X. 
 
 ON THE THEORY OF THE DAGUERREOTYPE 
 TALBOTYPE PROCESSES, ETC. 
 
 SECTION I. 
 
 The Daguerreotype. 
 
 IT was not the original intention of the Author to include 
 a description of the Daguerreotype Process within the li- 
 mits of the present Work. The Daguerreotype is a branch 
 of the Photographic Art so distinct from the others, that, 
 in manipulatory details, it bears very little analogy to them; 
 a slight sketch of the theory of the process may not, how- 
 ever, be unacceptable. 
 
 All necessary remarks will fall under three heads : The 
 preparation of the Daguerreotype film ; the means by which 
 the lament image is developed ; and the strengthening of 
 the image by the Hyposulphite of Gold. 
 
 The Preparation of the Daguerreotype Film. The sen- 
 sitive film of the Daguerreotype is in many respects dif- 
 ferent from that of the Calotype or Collodiotype. The 
 latter may be termed wet processes, in contradistinction to 
 the former, where aqueous solutions are not employed. 
 The Daguerreotype film is a pure and isolated Iodide of 
 Silver, formed by the direct action of Iodine upon the 
 metal, and it is important to remember that Iodide of 
 Silver so prepared is different in its Photographic action 
 from the yellow salt obtained by double decomposition 
 
DAGUEREEOTYPE. 353 
 
 between Iodide of Potassium and Nitrate of Silver. A 
 Daguerreotype film, when exposed to a bright light, first 
 darkens to an ash-grey colour and then becomes nearly 
 white ; the solubility in Hyposulphite of Soda being at the 
 same time lessened. A Collodion film, on the other hand, 
 if the excess of Nitrate of Silver be washed off, although 
 still capable of receiving the radiant impression in the 
 Camera, does not alter either in colour or in solubility by 
 exposure to the sun's rays. 
 
 Details of the process for preparing a Daguerreotype 
 Plate. A copper plate of moderate thickness is coated 
 upon the surface with a layer of pure Silver, either by the 
 electrotype or in any other convenient manner. It is then 
 polished with great care, until the surface assumes a bril- 
 liant metallic lustre. This preliminary operation of po- 
 lishing is one of great practical importance, and the trou- 
 blesome details attending it constitute one of the main 
 difficulties to be overcome. 
 
 After the polishing is complete, the plate is ready to 
 receive the sensitive coating. This part of the process is 
 conducted in a peculiar manner. A simple piece of card- 
 board or a thin sheet of wood, previously soaked in solu- 
 tion of Iodine, evolves enough of the vapour to attack the 
 silver plate ; which being placed immediately above, and 
 allowed to remain for a short time, acquires a pale-violet 
 hue, due to the formation of an excessively delicate layer 
 of Iodide of Silver. By prolonging the action of the 
 Iodine the violet tint disappears and a variety of pris- 
 matic colours are produced, much in the same way as when 
 light is decomposed by thin plates of mica or the surface 
 of mother-of-pearl. From violet the plate becomes of a 
 straw-yellow, then rose-colour, and afterwards steel-grey. 
 As the Iodine acts further, this sequence of tints is re- 
 peated ; the steel-grey disappears, and the yellow and 
 rose-colours recur. The deposit of Iodide of Silver gra- 
 dually increases in thickness during these changes ; but 
 to the end it remains excessively thin and delicate. In 
 this respect it contrasts strongly with the dense and creamy 
 
 2 A 
 
354 THEORY OF PHOTOGRAPHY. 
 
 layer often employed in the Collodion process, and shows 
 that a large proportion of the Iodide of Silver must in 
 such a case be superfluous, as far as any influence pro- 
 duced by the light is concerned. An inspection of a sen- 
 sitive Daguerreotype plate reveals the microscopic nature 
 of the actinic changes involved in the Photographic Art, 
 and teaches a useful lesson. 
 
 Increase of sensibility obtained by combining the joint 
 action of Bromine and Iodine. The original process of 
 Daguerre was conducted with the vapour of Iodine only ; 
 but in the year 1840 it was discovered by Mr. JohnGoddard 
 that the sensibility of the plate was greatly promoted by 
 exposing it to the vapours of Iodine and Bromine in suc- 
 cession, the proper time for each being regulated by the 
 tints assumed. 
 
 The composition of this Sro mo -Iodide of Silver, so called, 
 is uncertain, and has not been proved to be the same as 
 that of the mixed salt obtained by decomposing a solu- 
 tion of Iodide and Bromide of Potassium with Nitrate of 
 Silver. Observe also that the Bromo-Iodide of Silver is 
 more sensitive than the simple Iodide only when the va- 
 pour of Mercury is employed as a developer. M. Claudet 
 proves that if the image be formed by the direct action of 
 light alone (see next page) the usual condition is reversed, 
 and that the use of Bromide under such circumstances 
 retards the effect. 
 
 TJie development and properties of the Image. The la- 
 tent image of the Daguerreotype is developed in a manner 
 different from that of the humid processes generally, viz. 
 by the action of Mercurial vapour. The cup containing 
 the Quicksilver is previously heated by means of a spirit- 
 lamp to about 140 Fahrenheit : although the amount of 
 Mercurial vapour evolved at this temperature is very small, 
 it is sufficient for the purpose, and after continuing the ac- 
 tion for a short time the image is perfectly developed. 
 
 There are few questions which have given rise to greater 
 discussion amongst chemists than the nature of the Da- 
 guerreotype image. Unfortunately, the quantity of mate- 
 
DAGUEEEEOTYPE. 355 
 
 rial to be operated on is so small, that it becomes almost 
 impossible to ascertain its composition by direct analysis. 
 Some suppose it to consist of mercury alone. Others have 
 thought that the Mercury is in combination with metallic 
 Silver. The presence of the former metal is certain, since 
 M. Claudet shows that, by the application of a strong heat, 
 it can actually be volatilized from the image in sufficient 
 quantity to develope a second impression immediately su- 
 perimposed. 
 
 It is a remarkable fact that an image more or less re- 
 sembling that developed by Mercury can be obtained by 
 the prolonged action of light alone upon the iodized plate. 
 The substance so formed is a white powder, insoluble in 
 solution of Hyposulphite of Soda ; amorphous to the eye, 
 but presenting the appearance of minute reflecting crys- 
 tals when highly magnified. Its composition is uncertain; 
 For all practical purposes the production of the Daguer- 
 reotype image by light alone is useless, on account of the 
 length of time required to effect it. This was alluded to 
 in the Third Chapter, where it was shown that in the case 
 of the Bromo-Iodide of Silver an intensity of light 3000 
 times greater would be required, if the use of the Mercu- 
 rial vapour were omitted. 
 
 M. Ed. BecquereTs discovery of the continuing action of 
 rays of yellow light. Pure homogeneous yellow light has 
 no action upon the Daguerreotype plate ; but if the iodized 
 surface bo first exposed to white light for a sufficient time 
 to impress a latent image, and then afterwards to the yel- 
 low light, the action already commenced is continued, and 
 even to the extent of forming the peculiar white deposit, 
 insoluble in Hyposulphite of Soda, already alluded to. 
 
 Yellow light may therefore in one sense be spoken of as 
 a developing agent, since it produces the same effect as 
 the Mercurial vapour in bringing out to view the latent 
 image. 
 
 A singular anomaly however requires notice, viz. that if 
 the plate be prepared with the mixed vapours of Bromine 
 and Iodine, in place of Iodine alone, then the yellow light 
 
356 THEORY OF PHOTOGRAPHY. 
 
 cannot be made to develope the image. In fact, the same 
 coloured ray which continues the action of white light 
 upon the surface of Iodide of Silver actually destroys it, 
 and restores the particles to their original condition, with 
 a surface of Bromo-Iodide of Silver. 
 
 These facts, although not of great practical importance, 
 are interesting in illustration of the delicate and complex 
 nature of the chemical changes produced by light. 
 
 The Strengthening of the Daguerreotype Image by means 
 of Hyposulphite of Gold. The use of the Hyposulphite 
 of Gold to whiten the Daguerreotype image, and render 
 it more lasting and indestructible, was introduced by M. 
 Fizeau, subsequent to the original discovery of the process. 
 
 After removal of the unaltered Iodide of Silver by 
 means of Hyposulphite of Soda, the plate is placed upon 
 a levelling-stand and covered with a solution of Hyposul- 
 phite of Gold, containing about one part of the salt dis- 
 solved in 500 parts of water. The flame of a spirit-lamp 
 is then applied until the liquid begins to boil. Shortly 
 a change is seen to take place in the appearance of the 
 image ; it becomes whiter than before, and acquires great 
 force. This fact seems to prove conclusively that metallic 
 Mercury enters into its composition, since a surface of 
 Silver such, for instance, as that of the Collodion image 
 is darkened by Hyposulphite of Gold. 
 
 The difference in the action of the gilding solution upon 
 the image and the pure Silver surrounding it illustrates 
 the same fact. This Silver, which appears of a dark colour 
 and forms the shadows of the image, is rendered still 
 darker ; a very delicate crust of metallic Gold gradually 
 forms upon it, whereas in the case of the image itself the 
 whitening effect is immediate and striking. 
 
 SECTION II. 
 Theory of the Talbotype and Albumen Processes. 
 
 The Talbotype or Calotype. This process, as practised 
 at the present time, is almost identical with that originally 
 
CALOTYPE. 357 
 
 described by Mr. Fox Talbot. The object is to obtain an 
 even and finely divided layer of Iodide of Silver upon the 
 surface of a sheet of paper ; the particles of the Iodide 
 being left in contact with an excess of Nitrate of Silver, 
 and usually with a small proportion of G-allic Acid. 
 
 The English papers sized with Gelatine are commonly 
 used for the Calotype process : they retain the film more 
 perfectly at the surface, and the Gelatine in all probability 
 assists in forming the image. With a foreign starch-paper, 
 unless it be re-sized with some organic substance, the so- 
 lutions sink in too deeply, and the picture is wanting in 
 clearness and definition. 
 
 There are two modes of iodizing and sensitizing the 
 sheets : first, by floating alternately upon Iodide of Potas- 
 sium and Nitrate of Silver, in the same manner as in the 
 preparation of papers for Positive Printing ; and second, 
 by what is termed " the single wash," which is thought by 
 many to give superior results as regards sensitiveness and 
 intensity of image. To iodize by this mode, the yellow 
 Iodide of Silver, prepared by mixing solutions of Iodide 
 of Potassium and Nitrate of Silver, is dissolved in a strong 
 solution of Iodide of Potassium ; the sheets are floated 
 for an instant upon this liquid and dried ; they are then 
 removed to a dish of water, by the action of which the 
 Iodide of Silver is precipitated upon the surface of the 
 paper in a finely divided state. 
 
 The properties of a solution of Iodide of Silver in Iodide 
 of Potassium, or of the double Iodide of Potassium and 
 Silver, are described at page 167, a reference to which will 
 show that the double salt is decomposed by a large quantity 
 of water, with precipitation of the Iodide of Silver, this 
 substance being less soluble in a dilute solution of Iodide 
 of Potassium than in a strong solution. 
 
 Paper coated with Iodide of Silver by this mode, after 
 proper washing in water to remove soluble salts, will keep 
 good for a long time. The layer of Iodide appears of a 
 pale primrose-colour, and is perfectly insensitive to light, 
 so that exposure to the sun's rays produces no change. 
 
358 THEORY OF PHOTOGRAPHY. 
 
 To render Calotype paper sensitive to light, it is brushed 
 with a solution of Nitrate of Silver containing both Acetic 
 and Gallic Acid, termed " Aceto-Nitrate " and " Gallo- 
 Nitrate " solution. The Gallic Acid lessens the keeping 
 qualities of the paper, but increases the sensitiveness. 
 The Acetic Acid prevents the paper from blackening all 
 over during the development, and preserves the clearness 
 of the white parts ; its employment is indispensable. 
 
 The paper is commonly excited upon the morning of the 
 day upon which it is intended to be used ; and the longer 
 it is kept, the less active and certain it becomes. An ex- 
 posure of five to eight minutes in the Camera is the ave- 
 rage time with an ordinary view Lens. 
 
 The picture is developed with a saturated solution of 
 Gallic Acid, to which a portion of Aceto-Nitrate of Silver 
 has been added to heighten the intensity. Both Sulphate 
 of Iron and Pyrogallic Acid have also been used, but the 
 gradual action of a more feeble developer is preferable in 
 this process. 
 
 After fixing the Negative by removing the unaltered 
 Iodide of Silver with Hyposulphite of Soda, it is well 
 washed and dried. White wax is then melted in with a 
 hot iron, so as to render the paper transparent, and to faci- 
 litate the after-process of printing. 
 
 The Calotype cannot be compared with the Collodion 
 process for sensitiveness and delicacy of detail, but it pos- 
 sesses advantages for tourists and those who do not wish 
 to be encumbered with large glass plates. The principal 
 difficulty appears to be in obtaining a uniformly good 
 paper, many samples giving a speckled appearance in the 
 black parts of the Negative. 
 
 The Waxed Paper process of Le Grey. This is a useful 
 modification of the Talbotype, introduced by M. Le Grey. 
 The paper is waxed before iodizing, by which, without in- 
 volving any additional operation, a very fine surface layer 
 of Iodide of Silver can be obtained. The Waxed Paper 
 Process is well adapted for tourists, from its extreme sim- 
 plicity, and the length of time which the film may be kept 
 in a sensitive condition. 
 
THE WAXED PAPER PROCESS. 359 
 
 Both English and foreign papers are employed ; but the 
 former take the wax with difficulty. Mr. Crookes, who 
 has devoted his attention to this process, gives clear direc- 
 tions for waxing paper ; it is essential that pure white wax 
 should be obtained direct from the bleachers, since the 
 flat cakes sold in the shops are commonly adulterated. 
 The temperature must also be carefully kept below that 
 point at which decomposition of the wax takes place ; the 
 use of too hot an iron being a common source of failure 
 (see 'Photographic Journal,' vol. ii. p. 231). 
 
 The sheets of paper, having been properly waxed, are 
 soaked for two hours in a solution containing Iodide and 
 Bromide of Potassium, with enough free Iodine to tinge 
 the liquid of a port-wine colour. The greasy nature of 
 wax impedes the entry of liquids, and hence a long immer- 
 sion is required. The iodizing formulae of the French Pho- 
 tographers have been encumbered by the addition of a 
 variety of substances which appear to introduce compli- 
 cations without giving proportional advantage, and it is 
 now known that the Iodide and Bromide of Potassium, 
 with free Iodine, are sufficient. This latter ingredient was 
 first used by Mr. Crookes ; it seems to add to the clearness 
 and sharpness of the Negatives ; and as the papers are co- 
 loured by the Iodine, air-bubbles cannot escape detection. 
 The process of exciting with Nitrate of Silver is also ren- 
 dered more certain by the employment of free Iodine, the 
 action of the Bath being continued until the purple colour 
 gives place to the characteristic yellow tint of the Iodide 
 of Silver.* 
 
 Waxed Paper is rendered sensitive by immersion -in a 
 Bath of Nitrate of Silver containing Acetic Acid ; the 
 quantity of which latter ingredient should be increased 
 when the papers are to be long kept. As the excess of 
 Nitrate is subsequently removed, the solution may be 
 used weaker than in the Calotype or Collodion process. 
 
 * The later experience of the Writer induces him to believe that organic 
 matter may be used with advantage in the waxed paper process, and that 
 serum of milk is a better vehicle for the chemicals than pure water. 
 
360 THEOBY OF PHOTOGRAPHY. 
 
 After exciting, the papers are washed with water, to re- 
 duce the amount of free Nitrate of Silver to a minimum. 
 This lessens the sensitiveness, but greatly increases the 
 keeping qualities, and the paper will often remain good 
 for ten days or longer. 
 
 It is a very important point, in operating with Waxed 
 Paper, to. keep the developing dishes clean. The develop- 
 ment is conducted by immersion in a Bath of Gallic Acid, 
 containing Acetic Acid and Nitrate of Silver ; and being 
 retarded by the superficial coating of wax, there is always 
 a tendency to an irregular reduction of Silver upon the 
 white portions of the Negative. When the developer 
 becomes brown and discoloured, this is more likely to 
 happen ; and it is well known to chemists that the length 
 of time during which G-allic Acid and Nitrate of Silver 
 may remain mixed without decomposing, is lessened by 
 using vessels which are dirty from having been before em- 
 ployed for a similar purpose. The black deposit of silver 
 exercises a catalytic action (KaraXva-is, decomposition by 
 contact) upon the freshly -mixed portion, and hastens its 
 discoloration. 
 
 The Waxed Paper process is exceedingly simple and in- 
 expensive, very suitable for tourists, as requiring but little 
 experience, and a minimum of apparatus. It is, however, 
 tedious in all its stages, the sensitive papers frequently 
 taking an exposure of twenty minutes in the Camera, and 
 the development extending over an hour or an hour and a 
 half. Several Negatives, however, may be developed at 
 the same time ; and as the removal of the free Nitrate of 
 Silver gives the process a great advantage during hot 
 weather, it will in all probability continue to be followed. 
 The prints which have been sent to the exhibition of the 
 Photographic Society, show that waxed papei 1 , in the hands 
 of a skilful operator, may be made to delineate architectural 
 subjects with great fidelity, and also to give the details of 
 foliage and landscape Photography with tolerable distinct- 
 ness. 
 
 The Albumen Process upon Glass. The process with 
 
ALBUMEN UPON GLASS. 361 
 
 Albumen originated in a desire to obtain a more even sur- 
 face layer of Iodide of Silver than the coarse structure of 
 the tissue of paper will allow. It is conducted with sim- 
 ple Albumen, or " white of eggs," diluted with a conve- 
 nient quantity of water. In this glutinous liquid Iodide 
 of Potassium is dissolved ; and the solution, having been 
 thoroughly shaken, is set aside, the upper portion being 
 drawn off for use, in the same manner as in the prepara- 
 tion of Albuminized paper for printing. 
 
 The glasses are coated with the iodized Albumen, and 
 are then placed horizontally in a box to dry. This part of 
 the process is considered the most troublesome, the moist 
 Albumen easily attracting particles of dust, and being apt 
 to blister and separate from the glass. If an even layer 
 of the dried and Iodized material can be obtained, the 
 chief difficulty of the process has been overcome. 
 
 The plates are rendered sensitive by immersion in a 
 Bath of Nitrate of Silver, with Acetic Acid added, and are 
 then washed in water and dried. They may be kept for a 
 long time in an excited state. 
 
 The exposure in the Camera must be unusually long ; 
 the free titrate of Silver having been removed by washing, 
 and the Albumen exercising a direct retarding influence 
 upon the sensitiveness of Iodide of Silver (p. 155). 
 
 The development is conducted in the ordinary way by a 
 mixture of Gallic Acid and Nitrate of Silver, with Acetic 
 Acid added to preserve the clearness of the lights. It 
 usually requires one hour or more, but may be accelerated 
 by the gentle application of heat. 
 
 Albumen pictures are remarkable for elaborate distinct- 
 ness, and are admirably adapted for viewing in the Stereo- 
 scope ; but they do not often possess the peculiar and 
 characteristic softness of the Photograph upon Collodion. 
 The process is well adapted for hot climates, being very 
 little prone to the cloudiness and irregular reduction of 
 Silver, which are often complained of with moist Collodion 
 under such circumstances. 
 
 Amateurs practising the Albumen process for the first 
 
362 THEORY OF PHOTOGRAPHY. 
 
 time are often surprised at the pale transparent appear- 
 ance of the plates on being raised from the' Nitrate Bath, 
 giving the idea that too little Iodide of Potassium had been 
 used. This peculiarity, however, depends upon the chemi- 
 cal nature of the Albumen itself. It has an affinity for 
 Iodide of Silver, and unites with it, producing a film of 
 a different character from the Collodio-Iodide of Silver. 
 G-lycyrrliizine, the sugar of Liquorice, added in excess to 
 Collodion, diminishes the opacity of the sensitive film in a 
 similar manner, by uniting chemically with the Iodide of 
 Silver. 
 
 SECTION III. 
 
 Photographic Processes by Sir John Herschel and others. 
 
 Under this head may be included the processes known 
 by the name of Cyanotype, Chrysotype, etc., together with 
 the Photo-galvanographic, Photo-lithographic, Photo-gly- 
 phic, and Carbon Printing Processes, all of which involve 
 chemical reactions of the greatest interest. 
 
 Processes with Persalts of Iron. Sir John Herschel 
 discovered, about 1840, that certain of the Persalts of Iron 
 when exposed to sunlight in contact with organic matter, 
 were reduced to the state of Protosalts. The Perchloride 
 of Iron has this property when accurately neutralized, but 
 the Ammonio-Tartrate and Potassio-Tartrate of Iron are 
 more easily affected by light, whilst the Ammonio-Citrate 
 is sensitive to a degree greater than either. All these pro- 
 cesses are slower than the ordinary modes of printing in 
 which the Chloride of Silver is employed. 
 
 In the manipulations, a sheet of photographic paper is 
 brushed with a solution of the yellow Persalt, and when dry 
 placed in the sun in the usual way. The image so formed 
 is always faint, but it may be intensified by either of the 
 following methods : 
 
 a. By the Ferridcyanide of Potassium. This process 
 was named by the inventor, the " Cyanotype." Ferrid- 
 cyanide of Potassium, also known as " Red Prussiate of 
 
CYANOTYPE AND OTHEE PROCESSES. 363 
 
 Potash," produces no precipitate with the Persalts of Iron, 
 but throws down a variety of Prussian blue (TurnbulTs 
 blue) when added to the Protosalts. Consequently on 
 treating the Iron-print with a solution of Ferridcyanide of 
 Potassium, those parts which have been sunned, and there- 
 by reduced to the lower form of oxide, assume a blue tint.* 
 
 b. Intensifying by Chloride of Gold. When this plan 
 of intensifying the image on the Ammonio-Citrate paper 
 is adopted, the inventor terms the process " Chrysotype " 
 (from xpuo-os-, gold}. The image must be exposed for a 
 brief time only and should be very faint. On brushing 
 it with a dilute neutral solution of Chloride of Gold, the 
 Protosalt of Iron, on the parts touched by light, reduces 
 the Gold, and a purple image is the result ; this image con- 
 tinues to darken by precipitation of fresh particles until it 
 is nearly black. 
 
 c. Intensifying by other Metallic Compounds admitting 
 of Deoxidation. Nitrate of Silver, when applied to the 
 image, is slowly reduced, and darkens the tone consider- 
 ably. Proto-Nitrate of Mercury acts in the same way, 
 but pictures containing Mercury or its Oxide are liable to 
 fade. Bichromate of Potash placed in contact with those 
 parts of the Ammonio-Citrate paper which have been 
 sunned, likewise gives up Oxygen and is precipitated upon 
 the image in the form of a lower oxide of Chromium. 
 
 Processes with Salts of Uranium. Mr. Burnett, follow- 
 ing in the steps of Sir John Herschel, ascertained that the 
 Persalts of Uranium had similar Photographic properties 
 to the Persalts of Iron, and were reduced to Protosalts by 
 light and organic matter acting conjointly. 
 
 Uranium is a rare metal, forming a coloured series of 
 salts somewhat analogous in composition to the Proto- and 
 Persalts of Iron. Paper saturated with a solution of the 
 
 * 111 place of the Ferridcyanide, the Ferrocyauide of Potassium, known as 
 yellow Prussiate of Potash, may be employed, if the prints are afterwards- 
 thrown into water. The reaction, however, is not so simple, since the Fer- 
 rocyanide of Potassium forms a white precipitate with Protosalts of Iron, 
 which becomes blue by oxidation. With unchanged Ammonio-Citrate of 
 Iron, Ferrocyanide of Potassium produces no precipitation. 
 
364 THEOBY OF PHOTOGRAPHY. 
 
 Nitrate of Uranium, a salt always obtainable in commerce, 
 receives a very distinct image on exposure to light, which 
 may be further intensified either by Chloride of Gold or 
 Nitrate of Silver, as above described. In either case the 
 Uranium compound, previously reduced under the influ- 
 ence of light, in its turn reduces the Salts of Gold or Silver, 
 and the noble metal is in consequence precipitated upon 
 the image. The colour of these prints is better than those 
 produced by Sir John Herschel's process above described, 
 and may be further improved by different processes of 
 toning. 
 
 Processes with Bichromate of Potash. These are the 
 invention of Mr. Mungo Ponton, and were first announced 
 some years since in the ' Edinburgh New Philosophical 
 Journal.' 
 
 Bichromate of Potash contains Chromic Acid, a yellow 
 crystalline body, which is a high Oxide of the metal Chro- 
 mium. Light alone produces no effect upon Chromic Acid, 
 but if organic substances are also present, the Chromic 
 Acid is reduced to the condition of a lower Oxide of Chro- 
 mium, and the Oxygen unites with the organic body. 
 Some kinds of organic matter act more decidedly than 
 others, and especially Gelatine, which reduces the Bichro- 
 mate rapidly in presence of sunlight ; the Gelatine itself 
 being oxidized into a resinous substance, which remains in 
 union with Oxide of Chromium. 
 
 The image obtained upon gelatinized paper soaked in 
 Bichromate of Potash is of a tawny colour, and not very 
 vigorous, but Mr. Hunt and others succeeded in getting a 
 better effect by associating certain coloured salts, such as 
 those of Copper and Nickel, with the Bichromate, and 
 afterwards toning with Nitrate of Silver, etc. The Chro- 
 mates of many of the metals are most brilliant in colour, 
 and quite equal in that respect to the Ferrocyanides and 
 Ferridcyanides before alluded to. 
 
 Another mode of toning Photographic prints prepared 
 by the above method, has been described by Mr. Perry, and 
 also by M. Sella, of Piedmont. The pictures, after having 
 
CAEBON PRINTING-. 365 
 
 been washed in water to remove the unaltered Bichromate, 
 are immersed in a solution of Sulphate of Iron. They are 
 then washed a second time in water, and treated with 
 Gallic or Pyrogallic Acid, the effect of which is to change 
 the shadows to a bluish-black colour, having a composi- 
 tion similiar to common writing-ink. This process depends 
 upon the image retaining the Salt of Iron, so that when 
 once applied it cannot afterwards be removed by washing 
 with water. 
 
 CABBON FEINTING, PHOTO-LITHOGRAPHY, PHOTO-GALVA- 
 NOGEAPHY, AND PHOTO-GLYPHY. 
 
 In all these processes the peculiar Photographic proper- 
 ties of the Bichromate of Potash, before mentioned as the 
 discovery of Mungo Ponton, are turned to account, 
 
 Carbon Printing. M. Poitevin patented this process in 
 1855. Finely-divided charcoal is mixed with solutions of 
 Bichromate of Potash and Gum Arabic, and dabbed on a 
 sheet of paper. The Photographic impression is produced 
 upon this surface by the action of light passing through 
 a Negative picture, and the sheet is then washed with a 
 sponge and a large quantity of water. The organic matter 
 having been rendered insoluble by the light, the colour re- 
 mains upon the sunned parts ; but from the other portions 
 of the paper, not touched by light, it is removed by the 
 washing, leaving the surface white. The Oxide of Chro- 
 mium, which is the product of Bichromate of Potash re- 
 duced by organic matter, is a mordant substance, having 
 affinity for colours. Hence pigments of various kinds may 
 be successfully employed in this process, and will remain 
 fixed upon the image. 
 
 M. Testud de Beauregard proposed what he considered 
 as an improvement on the above method, by applying only 
 the Gum and Bichromate in the first instance, and putting 
 on the Carbon when the paper had become dry. The pro- 
 cess, however, although not at present patented, has never 
 come into general use, because the delicacy and gradation 
 of tone are not equal to that given by the ordinary Silver 
 
366 THEORY OF PHOTOGRAPHY. 
 
 printing on albuminized paper. For the delineation of 
 maps and line-engraving the Carbon printing is more suit- 
 able, but even here it has been superseded by the Photo- 
 lithographic methods now to be described. 
 
 Photo-lithography. In ordinary lithographic printing 
 an absorbent stone is drawn upon with chalks purposely 
 made of a waxy or fatty nature. The stone is then damped, 
 so as to fill its pores with water in the parts not covered 
 with drawing. A greasy ink is next laid on, which ad- 
 heres to the waxy drawing, but is repelled by the watery 
 surface of the stone. M. Poitevin patented a process in 
 1856, in which the required waxy or resinous design was 
 produced by Photography in the following manner: A 
 stone covered with a layer of Bichromate of Potash and 
 Gum, or Gelatine, was exposed to a luminous image, and 
 then wetted with water. The parts acted on b}- light were 
 found to have become greasy or resinous, and to refuse the 
 water, so that the lithographic ink, afterwards applied, 
 attached itself to them in preference to the moistened 
 shadows. 
 
 The defects of M. Poitevin's process were, that if he ex- 
 posed in the Camera, or under a transparent Positive; a 
 Negative picture was produced on the stone. If, on the 
 other hand, he exposed under a Negative, the picture, al- 
 though Positive, was necessarily reversed, right-hand ob- 
 jects being on the left side, and vice versa. In addition 
 to these defects there were others relating to matters of 
 practical detail, and hence the patent was eventually al- 
 lowed to lapse. 
 
 In November, 1859, Mr. Osborne communicated a paper 
 to the Melbourne Institute, on a Photo-lithographic pro- 
 cess differing from that of M. Poitevin in some important 
 particulars. The Bichromate of Potash and Gelatine were 
 applied to paper in the first instance, and the image sub- 
 sequently transferred to the stone : the objection of *' re- 
 versed Positives" was thus removed, and other advantages 
 were secured to the lithographer. Another peculiarity of 
 Mr. Osborne's method is in his mode of inking. He does 
 
PHOTO-LITHOGEAPHY. 367 
 
 not begin by wetting the surface to make the unaltered 
 Gelatine refuse the ink, but first inks-in the whole of the 
 Photographic print, and then, by a subsequent washing 
 process, removes the Gelatine from the unchanged parts 
 after the manner described under the head of Carbon 
 printing. Mr. Osborne, in 1862, communicated to the 
 British Association some improvements in his process, 
 which is now as follows : 
 
 800 grs. of Gelatine and 440 of Bichromate of Potash 
 are dissolved in 8 ounces of warm water. When cooled to 
 about 110 F., 2 ounces of Albumen from perfectly fresh 
 eggs are added, and the whole is well mixed. The sheets 
 of paper are then coated with this mixture on one side and 
 hung up to dry in the dark, and when dry glazed by pres- 
 sure. 
 
 The paper thus prepared receives the impression from 
 a Negative in from half a minute to a minute, after which 
 it is inked all over with lithographic ink, and then floated 
 on boiling water for about an hour. 
 
 The soaking being completed, it is next rubbed with a 
 sponge, when the parts unacted on by light, together with 
 the ink attached, are completely removed, and all the de- 
 tails come out ; it is then well treated with a stream of 
 boiling water, dried, and finally transferred to the Litho- 
 graphic stone in the usual way. Wood-cuts and engravings 
 are copied by this process with wonderful sharpness and 
 perfection. 
 
 M. Asser, of Amsterdam, was an independent discoverer 
 of a method which, in its essential features, resembles that 
 of Mr. Osborne. In the early part of the year 1860 this 
 method was adopted in the Ordnance Survey Department 
 at Southampton, and improved in many of its minor details. 
 The Photograph, as before, is made on paper, and trans- 
 ferred to a surface of zinc or stone. The ink is applied 
 to the whole surface of the sheet of paper, in quantity 
 more or less according to the boldness or fineness of the 
 subject, and the unchanged parts are afterwards cleaned 
 with a sponge dipped in Gum-water. The method of trans- 
 
368 THEORY OF PHOTOGRAPHY. 
 
 ferring to Zinc varies according to the quantity of ink in 
 the Photograph. Supposing this to be small, the " ana- 
 static " process is adopted, in which the surface of the Zinc 
 is slightly roughened by dilate Nitric Acid, and the draw- 
 ing made to adhere to it by passing through a press ; but 
 when a larger quantity of ink was used, the Zinc is simply 
 grained by rubbing with fine wet sand and a muller. In 
 either case the prominence of the lines is further increased 
 by acting on the interspaces with a very weak etching 
 liquid, consisting of dilute Phosphoric Acid in Gum Water, 
 and decoction of galls.* 
 
 Photo-galvanography, introduced into this country by 
 Herr Pretsch, of Vienna, is a mode of producing, from a 
 Photograph, an electrotype Copperplate in a state fit for 
 printing. A glass is first coated with a mixture of Bi- 
 chromate of Potash and Gelatine, other ingredients, not 
 essential to the theory, being at the same time added for 
 the purpose of giving what artists term " a grain." An 
 image is next impressed upon this surface by the action 
 of light. The object of the engraver is to make from the 
 image a surface partly raised and partly depressed, such 
 as would be required for carrying the printer's ink, and 
 this is done by simply pouring water over the film. A cha- 
 racteristic property of Gelatine is to swell up by imbibing 
 water, but Gelatine which has been altered in composition 
 by light and Bichromate of Potash loses this property. 
 Hence when water is applied to the film, those parts on 
 which the light has not acted swell, but those on which 
 it has acted remain as before : consequently the image 
 becomes sunk in, and assumes the state required. It re- 
 mains therefore only to make a gutta-percha mould of 
 the image, to render that mould conducting by means of 
 Carbon powder, and to deposit Copper upon it by the 
 electrotype to any required thickness. The Copperplate 
 so obtained is used for printing in the ordinary way. 
 
 Photo-glyphy. Mr. Fox Talbot has invented a mode of 
 
 * See ' Photo-Zincography,' by Colonel Sir Henry James. Longman and 
 Co. Second edition. 
 
PHOTO-LITHOGEAPHY. 369 
 
 biting a Photograph into a steel plate. The Photographic 
 impression is made upon a surface of Bichromate of Potash 
 and Gelatine, as before. At first it was thought necessary 
 to wash the plate afterwards in hot water, so as to dissolve 
 away the unaltered Gelatine, and expose the bare surface 
 of the metal. Subsequently, however, Mr. Talbot found 
 that the removal of the unchanged parts of the film was 
 not required, but that an aqueous solution of Perchloride 
 of Iron would penetrate them and bite into the steel be- 
 neath. The Gelatine altered by light, however, does not 
 absorb the aqueous liquid, but repels it, thus protecting 
 the metal from the action of the solvent. It is very im- 
 portant that the etching liquid should be exactly of a 
 certain strength, since a dilute solution of Perchloride of 
 Iron soaks into a film of Gelatine more readily than a 
 strong solution. 
 
 2B 
 
PART III. 
 
 THE PRACTICE OF PHOTOGRAPHY UPON 
 COLLODION. 
 
373 
 
 PRACTICE OF PHOTOGRAPHY UPON COLLODION. 
 
 CHAPTER I. 
 
 PHOTO GBAPHIC CHEMICALS. 
 
 SECTION I. Preparation of Plain Collodion. 
 
 SECTION II. Formulas for solutions for Collodion Positives. 
 
 SECTION III. Formulae for solutions for Negatives. 
 
 SECTION I. 
 
 Preparation of Plain Collodion. 
 
 THIS includes the manufacture of the Pyroxyline and the 
 solution of the same in mixed Ether and Alcohol. 
 
 Photographic Pyroxyline may be made either from 
 Linen, Calico, Paper, or Cotton-wool. For reasons which 
 are more fully stated in the Second Part of the Work 
 (p. 232 et seq.), the Writer proposes to confine his obser- 
 vations to the use of the latter material. 
 
 The manufacture of Collodion in large quantities, and 
 of a perfectly uniform quality, is a delicate operation, and 
 one which requires great nicety in conducting it. For the 
 amateur who wishes to prepare only as much as will suffice 
 for his immediate wants, it will be better to describe a pro- 
 cess more easy of execution, and capable of yielding a 
 moderately good product without liability to failure. The 
 
374 PRACTICE OF PHOTOGRAPHY. 
 
 " Nitre process," as it is termed, is a convenient method 
 of the latter kind, and the process "by mixed acids," which 
 will be found at page 377, will fulfil the requirements of 
 the commercial manufacturer. In either case it will be 
 desirable to consult the Vocabulary, Part I., for the mode 
 of testing the purity of the chemicals. 
 
 THE NITRE PROCESS FOR PYROXYLINE. 
 
 Use only the best Nitre, as described in the Vocabulary, 
 article " Nitrate of Potash," and having reduced it to 
 powder, dry it on a hot metal plate, or in an oven heated 
 to a temperature above that of boiling water ; then pul- 
 verize a second time, to prevent the formation of lumps 
 on adding the Oil of Vitriol. Take of 
 
 Oil of Vitriol 6 fluid ounces. 
 
 Dried Nitre 3^ ounces (avoirdupois). 
 
 Water 1 fluid ounce. 
 
 Best Cotton Wool ... 60 grains. 
 
 Mix the Acid and Water in a teacup, or any porcelain 
 or stoneware vessel, and throw in the pulverized Nitre by 
 degrees, stirring meanwhile with a glass rod, until lumps 
 disappear, and a transparent viscid liquid is obtained. This 
 will occupy several minutes. 
 
 Pull out the Cotton into separate balls of the size of a 
 walnut, and when a mercurial thermometer,* dipped in 
 the acid and stirred backwards and forwards, remains 
 stationary at 145 to 150 Fahr., immerse the balls singly 
 with a glass rod, and press each against the sides of the 
 cup. This part of the operation ought not to take more 
 than two minutes. Supposing the acid to have cooled 
 down below 140 during the pulling out of the Cotton, 
 float the cup upon boiling water for a few minutes until 
 the exact temperature is attained. Leave the Cotton in 
 ten minutes, and afterwards pour off the excess of the 
 
 * In the preparation of soluble cotton, a thermometer is indispensable. 
 Instruments of sufficient delicacy for common purposes are sold in Hatton 
 Garden and elsewhere, at a low price. The bulb should be uncovered, to 
 admit of being dipped in acids, etc., without injury to the acale. 
 
PBEPABATION OF NEGATIVE PYBOXYLINE. 375 
 
 acids and squeeze the Pyroxyline with the glass rod. Do 
 this quickly, and then dash the cup and its contents sud- 
 denly into cold water, moving the Cotton backwards and 
 forwards with the hand until it feels perfectly cold. 
 
 Complete the washing of the Cotton, either by leaving 
 it for twenty-four hours in a slowly-running stream of 
 water, or by squeezing it with the hand like a sponge, 
 for twenty minutes or half an hour; after which it may be 
 spread out upon a cloth, and allowed to dry spontaneously. 
 
 In this process the chances of failure are very slight, if 
 the Sulphuric Acid be sufficiently strong, and the sample 
 of Nitre not too much contaminated with Chloride of 
 Potassium. If however a failure should occur from the 
 Cotton dissolving in the acid, the operation must be re- 
 peated with a drachm less Water. 
 
 .COMMEECIAL MANUFACTURE OF PYBOXYLINE BY THE 
 PEOCESS WITH MIXED ACIDS. 
 
 This process will be described with more minuteness than the 
 last, since it is intended for the use of the manufacturer, who has 
 to satisfy the requirements of the professional artist, and to pro- 
 duce an article as nearly uniform as possible. 
 
 The formula for the Nitrosulphuric Acid will vary according as 
 the Pyroxyline is intended for Positive or Negative Collodion, 
 and the proportions of the solvents also will be different in the 
 two cases ; the general details of the manufacture however will be 
 the same. 
 
 The Apparatus. The following articles will be found useful 
 in preparing Collodion on a large scale : 
 
 a. Porcelain pots for the mixed acids. These should be made 
 of the best glazed porcelain, and provided with handles and 
 covers. In shape, they should be deep in proportion to their 
 width, so as to expose as little surface to the atmosphere as is 
 compatible with easy working of the spatulas in pressing the 
 Cotton. Considerable thickness of materid is also important to 
 prevent the temperature from falling too low while the cotton is 
 being acted on by the acids. 
 
 b. Glass spatulas. These are male of thick plate-glass, of 
 
376 PEACTICE OF PHOTOGEAPHY. 
 
 sufficient strength to allow of considerable pressure without 
 breakage. 
 
 c. Hot Air Bath. This is a simple apparatus for warming the 
 pots containing the acid mixture, when the temperature is inad- 
 vertently allowed to fall too low. It consists of an open vessel 
 like a saucepan, made of sheet-iron fastened by rivets, standing 
 on legs over a Bunsen's burner. In the part corresponding to 
 the cover of the saucepan is a round hole, into which the porce- 
 lain pot drops, until it is caught and supported by the rim. The 
 pot must not touch the bottom, but may approach very near to 
 it, and hence ah 1 danger of cracking is avoided, whilst sufficient 
 heat can be obtained in a few minutes. 
 
 d. Thermometer. Select an instrument with a wide column 
 of Mercury, so as to be easily seen by gaslight, and with a large 
 bulb, that it may be sufficiently sensitive. It is quite necessary 
 to compare the thermometer with a standard instrument, since 
 the cheaper thermometers are frequently incorrect, to the extent 
 of several degrees. It is useful also to have a second thermo- 
 meter with which to compare the first, since the column of 
 Mercury sometimes separates, leaving a vacant space, and thus 
 indicating too high a temperature. 
 
 e. India-rubier gauntlets. These are indispensable, as a pro- 
 tection to the hands in making large quantities of Pyroxyline ; 
 they may be obtained at any India-rubber warehouse. 
 
 f. Trough for preliminary washing. It consists of a strong 
 deal framework, three feet long by two feet wide, lined inside 
 with gutta-percha. The water passes in through two tubes, one 
 at the end and the other near the centre, each being pierced with 
 fine holes, so as to deliver the water in a series of jets, and thus 
 to keep up a constant current at every part of the tray. 
 
 g. Cylinder for final washing. This consists of a glazed 
 earthenware cylindrical vessel about nine inches in diameter by 
 2 feet high, fitted with a false or perforated bottom about 2 
 inches above the real one. In the intermediate space a hole an 
 inch or more in diameter is driUed, and fitted with a gutta-percha 
 tube to convey awty to the sink, a stream of water entering by 
 the top and passing through the Cotton in the body of the 
 cylinder. 
 
 h. Steam Bath for dryng Cotton, etc. This may be made by 
 first constructing a flat zinc tray, two feet wide by four feet long, 
 
PEEPARATION OF NEGATIVE PYBOXYLINE. 377 
 
 with a tightly soldered cover, and standing on short legs. The 
 water is boiled in an ordinary tin saucepan, and the steam con- 
 ducted by a pipe about three feet long, well rolled in list or 
 flannel, into the closed tray, the condensed water being allowed 
 to drop out at the end. Pyroxyline may be dried with safety 
 upon this bath, when the flame is placed at a sufficient distance, 
 since the heat can be regulated at will by adjusting the supply 
 of gas. If the upper zinc plate be covered with flannel or calico, 
 the temperature does not rise higher than 130 Fahr. 
 
 THE MANIPULATIONS. 
 
 We commence with Pyroxyline for Negative Collodion, and at 
 the end of this Section will add the few remaining particulars 
 which apply to Positive Collodion. 
 
 a. The Pyroxyline. The Writer always adopted the plan of 
 laying in a large stock of acids at one time, since it was some- 
 what troublesome to ascertain the exact strength, and no mode of 
 analysis seemed perfectly satisfactory. Three carboys of Oil of 
 Vitriol were sent in, holding six gallons each (the strongest ob- 
 tainable), and one carboy of strong Nitric Acid of the same size. 
 These were bottled off into Winchester quarts, or half-gallon stop- 
 pered bottles (labelled No. 1 for the first Carboy, No. 2 for the 
 second, and No. 3 for the third), for the sake of greater conve- 
 nience in handling, and to lessen the chance of the acid absorb- 
 ing water from the atmosphere. As the strength of the Oil of 
 Vitriol in each carboy differed, a mixture was made of No. 1, 
 No. 2, and No. 3, taking a single bottle of each. Then, sup- 
 posing the specific gravity of the acids to be nearly as before 
 given, the formula will stand as follows : 
 
 Oil of Vitriol, T845 at 60 Fahr. 18 fluid ounces. 
 Nitric Acid, T457 at 60 Fahr. . 6 fluid ounces. 
 
 Water* 5i fluid ounces. 
 
 Pour in first the water, then the Nitric Acid, and lastly the Oil 
 of Vitriol : obtain a perfect admixture by stirring, and take the 
 temperature. If the thermometer rises to 165 Fahr. or 170, 
 
 * Most who have tried this formula complain that the Cotton dissolves, 
 unless the quantity of water be reduced to five ounces. The Writer how- 
 ever finds that by a careful attention to the purity of the chemicals, he is 
 always able to work with more water than that given, so that he prefers to 
 leave the formula as it stands. 
 
378 PEACTICE OF PHOTOGBAPHY. 
 
 the acid must be allowed to cool until it stands exactly at 150 
 Fahr. Then immerse the Cotton in pieces well pulled out, and 
 weighing thirty grains each, continuing to put them in singly 
 until ten have been introduced, making 300 grains in all. This 
 operation, together with the pressing against the sides of the 
 vessel, etc., to be alluded to again presently, will occupy about 
 a minute and a half, after which the vessel may be covered up 
 and left for nine minutes. The process is complete in half that 
 time, but the extra five minutes, although producing a little more 
 solution of the Pyroxyliue, and consequently diminishing the 
 weight of the product by about 15 per cent., produces a Collodion 
 which is less likely to show glutinous markings upon the glass, 
 or to dry up speedily and repel the developer. Lastly, take out 
 the Pyroxyline in one lump with glass spatulas ; squeeze out as 
 much of the acids as possible in a porcelain capsule, and dasli 
 the whole into a large quantity of water. 
 
 With regard to the temperature at which the Pyroxyline should 
 be made, the Author worked at first at 140 Fahr. ; but was even- 
 tually induced to raise it ten degrees, in consequence of repre- 
 sentations (hat the Collodion was somewhat deficient in fluidity. 
 This increase of temperature, however, assists in generating traces 
 of a body (probably Nitro- Glucose) which causes the Collodion 
 to lose its sensitiveness more rapidly after iodizing. 
 
 An experienced person will be able to judge on lifting the 
 Cotton out of the acid whether he has hit the right point. If, 
 on attempting to lift out the whole mass of Pyroxyline at once 
 with the glass spatulas, it seems rather small in quantity and 
 very rotten, so that little pieces break away and are left behind 
 in the acid, then the temperature was too high, or the acids 
 too weak, and in repeating the operation the water must be di- 
 minished by two or three drachms in the quantity of acid given 
 above. If, on the other hand, the mass of Pyroxyline appears 
 large, sticks well together, and sliows no tendency to tear, either 
 the temperature has fallen several degrees, or it will be advanta- 
 geous to work with a few drachms more of water. 
 
 Whilst the Pyroxyline is washing in the tray, it is still more 
 easy to judge of its quality ; for if the ten separate pieces, in 
 which the Cotton was originally weighed, are seen floating about, 
 and can be separated arid counted, the acids were certainly too 
 strong ; whilst if there be an evident aspect of commencing so- 
 
PBEPABATION OF NEGATIVE PYEOXYLINE. 379 
 
 lution a piece of cotton here and there scarcely changed, but 
 the others in a measure broken up, and tearing easily under the 
 finger, the operation is probably successful ; but when the whole 
 is so mixed up together that nothing but fragments of the ten 
 pieces can be detected, then the acids were too weak, unless the 
 temperature was inadvertently allowed to fall. 
 
 The Pyroxyline should remain in the tray with the tap running, 
 for twenty-four hours. After that it is to be transferred to the 
 washing cylinder, where a strong stream of water is allowed to 
 run through it for two or three hours. The last washing acts 
 effectually in removing the slight trace of acid which is always 
 left in a large bulk of Cotton even after a longer washing than 
 twenty-four hours in the first tray. 
 
 When thoroughly washed, the Pyroxy line is squeezed in the 
 hand, and then picked out to dry upon a cloth. If it resembles 
 the original Cotton in appearance, and feels strong and tough, 
 the amount of water in the acids must be increased ; but when 
 it breaks up into little bits, it is correctly made, or else is some- 
 what too weak, in which case the fragments will mat together, 
 so as to increase the difficulty of picking them out. As the 
 Pyroxyline dries upon the cloth, it will be well to examine it 
 and give directions accordingly, separating any piece which ap- 
 pears less acted on than the rest ; this piece was probably the 
 piece put last into the acid and left at the upper part of the 
 liquid. 
 
 Two or three days' exposure to the air will render the soluble 
 Cotton sufficiently dry ; but it is convenient to finish it off on 
 the hot steam bath before described, and the temperature in which 
 must not be allowed to rise higher than 120 Fahr. When dry, 
 proceed to weigh it in the scales, and form your estimate of its 
 value accordingly. A long experience tends to show that, sup- 
 posing nothing to be lost in the washing, the weight of the re- 
 sulting Pyroxyline is a certain and safe guide in this process, and 
 it is always possible to tell what the quality of the Collodion will 
 be by using the scales. If 300 grains of Cotton yield 450 grains 
 of Pyroxyline, it is certain that complaints will be made of the 
 resulting Collodion being thick, and giving streaky pictures : four 
 or five additional drachms of water in the above quantity of Ni- 
 tre-Sulphuric Acid will be the remedy. When the weight of the 
 Pyroxyline is the same as that of the original Cotton, viz. 300 
 
380 PRACTICE OF PHOTOGRAPHY. 
 
 grains, there will be a sediment on dissolving it in the mixed 
 Ether and Alcohol ; nevertheless the Collodion, although lessened 
 in quantity, will be good, very limpid and structureless, with 
 great adhesion to the glass, less tendency to markings of all 
 kinds, and considerable softness of Negative, with sensitiveness to 
 dark rays. 
 
 The weight which on the whole appears to be best is 375 
 grains, that is to say, exactly 25 per cent, of increase : this gives 
 sufficient fluidity of Collodion, and at the same time leaves very 
 little sediment in dissolving. 
 
 The above facts are quite reliable, since they have been verified 
 by repeated observation, extending over a long time. It must, 
 however, be distinctly understood that the weight of the Pyroxy- 
 line can be taken as a criterion of quality only under the condi- 
 tions stated, the fibre of the Cotton must be cleaned by Potash 
 and quite dry, the Nitric Acid nearly free from Chlorine, the time 
 of immersion always the same, and, most important of all, the 
 temperature correctly ascertained, otherwise the weight will be 
 so variable that nothing can be deduced from it, and the Cotton 
 will be considerably acted on, even when the acids are strong 
 enough to produce an explosive variety of Pyroxyline. The whole 
 process, in fact, requires care, because it is conducted with the 
 maximum quantity of water, and at a high temperature. At 
 least twenty per cent, of the Pyroxyline is dissolved in any case ; 
 and the acids having once begun to act, will readily destroy the 
 remaining portion of the fibre, if an error be permitted. 
 
 Before passing on to the preparation of the Collodion, it may 
 be mentioned that the quantity of Pyroxyline which it is conve- 
 nient to make at one operation is four times that stated. A 
 double quantity of acids is mixed in a jug (thirty-six ounces of 
 Sulphuric and twelve of Nitric), and poured out into the porce- 
 lain pots before mentioned. The first three hundred grains of 
 Cotton are then immersed, and left digesting whilst the second 
 similar portion of Cotton is put into vessel No. 2. When the 
 Pyroxyline has been removed, the acids are emptied out, and a 
 double quantity again mixed as before. At one time the plan was 
 adopted of using the old acids again, by adding Oil of Vitriol to 
 restore the strength, but afterwards it was discontinued as caus- 
 ing uncertainty. 
 
 b. The Plain Collodion. ^This is made by introducing half a 
 
PLAIN NEGATIVE COLLODION. 381 
 
 gallon of Alcohol of "805 into a two-gallon stoppered bottle, and 
 adding 1900 grains of dry Pyroxyline. When the soluble Cotton 
 has become thoroughly saturated with the Alcohol, pour in half 
 a gallon of Ether of '725, and agitate for two or three minutes ; 
 next add another half-gallon of Ether, and again shake the bottle 
 for a few minutes. After this the Collodion may be allowed to 
 settle for about a week or ten days, when it will be sufficiently 
 clear for use : no plans for filtering have been found to answer. 
 The quantity of Pyroxyline may be increased to 2000 grains when 
 a Collodion of some body is required, or reduced to 1800 for a 
 thin Collodion suitable for large plates. Something, however, in 
 this respect depends upon the proportion of undissolved sedi- 
 ment, which is always greater when the acids are used with the 
 full quantity of water. Supposing the height of the column of 
 Collodion as it stands in the bottle to be ten inches, the sediment 
 when measured twenty-four hours after mixing is often about 
 half an inch, but it settles down more closely at the expiration of 
 a week or ten days. If the sediment should stand as high as two 
 inches in the bottle, the Collodion is probably of that kind which 
 has been described as giving a soft Negative with a tendency to 
 white spots. The above point being of importance in a com- 
 mercial point of view, pains were taken to collect the sediment 
 from more than two hundred gallons of Collodion, and it was 
 found that the loss did not exceed one pint in fifty. It appeared 
 at first to be much greater than this, but the residue continued 
 to settle for many months, the clear Collodion being occasionally 
 drawn off from the upper part and mixed in with the general 
 stock, a little at a time. 
 
 The proportions of Ether and Alcohol in the above Collodion 
 will be found to be those best adapted for general purposes. 
 With less Alcohol the film is more contractile, and more prone 
 to dry up after sensitizing. With a larger proportion of Alcohol, 
 say five parts of Alcohol of '805 at 60 to three of Ether of 725 
 (these proportions are for the Iodized and not the plain Collo- 
 dion), the sensitiveness is impaired ; as also is the adhesion to the 
 glass. The Alcoholic formula is well adapted for coating large 
 plates, and is sufficiently sensitive for copying works of art or 
 landscapes : nevertheless it is not recommended in preference to 
 the other for a normal Collodion, seeing that we have no means 
 of increasing sensitiveness when it proves deficient. 
 
382 PRACTICE OF PHOTOGRAPHY. 
 
 On the day following the preparation of the Collodion, about 
 half of a fluid ounce may be drawn off by a pipette from the 
 upper clear portion, and a small piece of red and blue test-paper 
 immersed for twelve hours. If, at the expiration of that time, 
 the blue paper appears reddened, the Pyroxyline was imperfectly 
 washed, and the standard alkaline solution, which will be imme- 
 diately described, is dropped into the plain Collodion in the pro- 
 portion of one full-sized drop to each half-gallon. The addition 
 of Alkali will never be required, if a heavy column of water is 
 forced through the cotton in the cylinder, after twenty-four 
 hours' washing in the tray. To make the standard solutions 
 which are required, dilute the Nitric Acid of T45 with an equal 
 bulk of water for the acid, and then dilute down the strong Am- 
 monia of commerce also with distilled water, until a fluid drachm 
 exactly neutralizes a corresponding bulk of the standard acid. 
 With these two liquids at hand no further trouble will be ex- 
 perienced, since the quantity of acid left in the Pyroxyline varies 
 very little ; and if any number of drops of Ammonia be added in 
 excess, a corresponding number of the acid liquid will neutralize 
 them. 
 
 PRECAUTIONS TO BE OBSERVED. 
 
 Attention may be directed to certain minor details of manipu- 
 lation, which are in themselves simple, but may be unknown to 
 some who desire to carry out the instructions contained in tin's 
 paper. 
 
 Beginning with the Cotton, it will be necessary to cleanse it by 
 boiling with Potash as described in the Vocabulary, Part I. ; and 
 if this process of cleansing be properly performed, a second boil- 
 ing with fresh Potash ought not to produce any yellow discolora- 
 tion of the liquid. The washed Cotton must be dried by arti- 
 ficial heat, and if not to be used immediately, may afterwards be 
 put away in a drying-box, which can be economically constructed 
 'by making a false bottom to an ordinary trunk, piercing it with 
 holes, and placing beneath it a shallow dish containing Oil of 
 Yitriol : it will then be sufficient to lift the cover and throw in 
 the balls of dried Cotton to ensure their remaining perfectly 
 dry ; whereas on the other hand, if the Cotton be left in a damp 
 place, it will absorb water from the atmosphere. 
 
 In mixing the acids, it simplifies matters to begin by selecting 
 
PRECAUTIONS. 383 
 
 a stoppered bottle which holds the proper quantity of Oil of 
 Vitriol when filled quite up to the neck. The Nitric Acid and 
 the water may be measured in a narrow cylindrical hydrometer 
 glass ; a mark being made for the former with black varnish, at 
 a height corresponding to one-third of the bulk of the Oil of 
 Vitriol, and a second mark lower down for the water, the quan- 
 tity of which will vary according to the strength of the acids. 
 Before using these measures, always invert them, and allow the 
 drainings from the last operation, consisting of acid diluted by 
 absorption of atmospheric moisture, to flow out. Then measure 
 the Oil of Vitriol, and make a leaden counterpoise for it in the 
 scales, bottle included. This is necessary when perfect accuracy 
 is desired, since otherwise the Nitro-Sulphuric Acid will be 
 stronger in winter than in summer, which the Writer always 
 found to be the case to a noticeable extent ; the product of Py- 
 roxyline obtained from three hundred grains of Cotton being fifty 
 grains heavier in frosty weather than during the hot months of 
 June and July. It is not absolutely necessary to weigh either 
 the Nitric Acid or the water ; and, with regard to the Sulphuric 
 Acid, the plan usually adopted was to measure it as a rule, but 
 to put the bottle afterwards into the scales, if a sudden change of 
 atmospheric temperature took place. 
 
 A failure would certainly be produced if the three constituents 
 of the Nitro-Sulphuric Acid were not properly mixed ; but there 
 is no difficulty in effecting this by stirring with the broad spatula 
 for half a minute in a shallow vessel. In the deep porcelain pots 
 before spoken of, it may not be quite so easy, and hence it is ad- 
 vantageous to adopt the plan of mixing a double quantity of acids 
 in a jug, and pouring it afterwards into the pots. The sides of 
 the jug, however, must not be too thick, or the temperature will 
 sink below 150 in very cold weather, especially when the Sul- 
 phuric Acid is a little weaker than usual, or the mixture is kept 
 too long in the jug. 
 
 Some may perhaps be inclined to keep a portion of the Nitro- 
 Sulphuric Acid ready prepared, and to obtain the correct tem- 
 perature by mixing cold acid with the hot. If so, bear in mind 
 that a stoppered bottle must be used, since Nitro-Sulphuric Acid, 
 like Oil of Vitriol, absorbs water from the atmosphere. On one 
 occasion some experimental results were completely spoilt, by 
 leaving the acid for a few days in a beaker covered by a glass 
 
384 PRACTICE OF PHOTOGRAPHY. 
 
 plate ; the upper part became so far weakened that, on putting 
 in the Cotton, it instantly dissolved. 
 
 Taking the temperature of the acids is an operation of some 
 nicety, and especially so in cold weather. Begin by stirring 
 briskly with the glass spatula in a circular direction ; then dip 
 the thermometer exactly into the centre of the liquid, and hold it 
 in that position for at least a minute, since the rise of the mer- 
 cury, although rapid at first, is very slow towards the end. If 
 the acid is too hot, it can be cooled two degrees by taking a cold 
 spatula and stirring it for a few seconds ; therefore it is of con- 
 sequence that the spatula which is used to immerse the cotton 
 should be previously warmed by dipping it in the liquid. The 
 acids also must be at least at 165 Fahr. when they are first 
 placed in the porcelain vessel, otherwise its thick sides will re- 
 duce the temperature so rapidly that, although the thermometer 
 may indicate 1 50 Fahr. at first, it will soon fall, and the weight 
 of the resulting Pyroxyline will be greater than that indicated 
 for a given strength of acids. In order to obtain a uniform tem- 
 perature during the time that the Pyroxyline remains immersed, 
 large jars may be inverted upon the porcelain pots, so as to pre- 
 vent them from being cooled by contact with the external air. 
 
 The Cotton should be weighed out into pieces of 30 grains 
 each, and each piece worked with the fingers into a circular form, 
 so as to facilitate absorption of the acid. In pursuance of the 
 same object, each piece, as it is placed in the acid, is carefully 
 pressed with the spatula against the side of the vessel ; and in 
 order that the last pieces may not be at the top (in which case 
 they always come out less broken than the others), a well of acid 
 is kept free by means of the spatula, and these last pieces are 
 pushed down nearly to the bottom. When all have been im- 
 mersed, the mass is squeezed against the vessel, first on one 
 side and then on the other, after which the whole is loosened 
 by putting the spatula down to the bottom, and raising it up 
 until the Pyroxyline nearly fills the liquid j the vessel is then co- 
 vered up and left for nine minutes, as before said. It is better 
 not to finish the process of putting in the Cotton by pressing it 
 down to the bottom in a hard mass, because a good deal of solu- 
 tion always takes place in the acids, and this is attended with an 
 evolution of heat, which increases the disintegrating action on 
 the Cotton. The object, therefore, is to prevent the mass from 
 
PRECAUTIONS. 385 
 
 " heating," as far as possible, by loosening it out with the spatula 
 and diffusing it through the liquid. Observe, however, that the 
 Cotton must not be permitted to project above the surface into 
 the air, or evolution of red fumes will take place. 
 
 Both spatulas are employed in removing the Pyroxyline from 
 the acids by forcing them down to the bottom on opposite sides, 
 and then bringing them together so as to pinch the mass and 
 lift it out entire. In squeezing the acids away, no time must be 
 lost, or the action of the air will produce red fumes. A few se- 
 conds will be sufficient, and especially so if great pains be taken 
 to distribute the Pyroxyline through the water by catching it 
 with the gloved hand. A sensation of heat is felt at first, due to 
 the dilution of the Oil of Yitriol ; but this soon ceases, and the 
 chance of failure from that cause is very slight. . If the material, 
 however, were simply thrown into a small quantity of water and 
 allowed to remain, the rise of temperature might be sufficient to 
 cause solution. 
 
 It was conjectured at first that the waste Nitre-Sulphuric Acid 
 which if thrown down the sink would be likely to act upon the 
 leaden pipes might be useful for some other process. This ex- 
 pectation, however, proved fallacious, since the Pyroxyline which 
 it contains in solution appears to interfere with its application to 
 any such purpose as dissolving metals, etc. 
 
 At first it was thought sufficient to place the Pyroxyline in 
 the gutta-percha washing-dish immediately on taking it from 
 the acids ; but finding that the heat and acid together gradually 
 decomposed the gutta-percha and made it sticky, the plan was 
 afterwards adopted of throwing the soluble Cotton first into a 
 leaden sink, and when the greater part of the acid had been re- 
 moved by a few hours' washing, lifting it out into the gutta- 
 percha dish. 
 
 The gutta-percha washing-tray will require cleansing after a 
 week or so ; a deposit adheres to the bottom, which seems to 
 consist of matted fibres of partially dissolved Pyroxyline. This 
 material having been some time in the water, would be likely to 
 decompose and liberate Oxides of Nitrogen in the Collodion. It is 
 better therefore not to disturb it at first ; and when sufficient has 
 collected, it may be scraped out, and the tray washed with water. 
 
 It is advisable not to complete the washing of the Pyroxyline 
 with boiling water, nor to use any carbonated or caustic alkali to 
 
 2c 
 
386 PRACTICE OF PHOTOGRAPHY. 
 
 remove the last traces of acid. All alkalies tend to decompose 
 Pyroxyline, and remove a portion of the Peroxide of Nitrogen 
 in the form of Nitrite ; and although dilute Ammonia is commonly 
 employed to neutralize the acid, it will be found that some varie- 
 ties of Pyroxyline assume a yellow colour, and become more un- 
 stable in Collodion after this treatment. 
 
 The Pyroxyline may be prepared in small quantities at a time, 
 as required for use ; but if it be necessary to keep it in stock, it 
 should be dried either over Oil of Vitriol, or at a temperature 
 below 120 Fahr. : at 140 Fahr. it begins slowly to decompose 
 and lose weight. Probably something depends upon the parti- 
 cular variety of Pyroxyline; but with some kinds red fumes 
 will be given off, on placing the material in a covered tin vessel 
 surrounded by boiling water. Pyroxyline for keeping ought also 
 to be put away in a dry place, and excluded from light, since 
 this substance is known to be liable to spontaneous change, and 
 unfortunately the exact conditions of permanency have not been 
 ascertained. 
 
 Having completed the preparation of the Pyroxyline, this list 
 of " Precautions " is nearly at an end : a few words on the sub- 
 ject of plain Collodion will close it. For holding the Collodion, 
 narrow-mouthed stoppered bottles may be used, containing two 
 gallons each. 
 
 There is a decided advantage in placing the Alcohol in the bottle 
 before the Ether, not only in facilitating the solution of the Py- 
 roxyline, but also in enabling the operator, by shaking the bottle, 
 to remove a flocculent deposit, which otherwise is apt to adhere, 
 and to be drawn over with the Collodion ; the mass of Pyroxy- 
 line wetted by the spirit acts effectually as a mop, and cleanses 
 the sides. 
 
 In drawing off the Collodion, place the bottle in such a position 
 jhat the end of the siphon comes between the eye and a strong 
 light ; any flocculi which appear likely to be drawn into the end 
 of the siphon will then be seen, and may be avoided. When it 
 is not intended to refill the bottle immediately, pour out the sedi- 
 ment and introduce half a gallon of absolute Alcohol, which will 
 absorb the remaining Ether vapour, and prevent it from being 
 oxidized into Acetic Acid, and afterwards forming Acetic Ether. 
 On one occasion a two-gallon bottle having been used for Col- 
 lodion and left empty, was put away in a dark place for about 
 
PEECAUTIONS, 387 
 
 three months. It was then washed out with ahout a pint of plain 
 Collodion and refilled. The result, however, was unsatisfactory ; 
 for on adding the iodizer to the newly-made batch, it at once 
 became yeUow, which was probably due to portions of Collodion 
 left at the bottom of the bottle having decomposed and ozonized 
 the Ether. Mere washing with plain Collodion was not suffi- 
 cient in this case, and a thorough cleansing with shot and water 
 should have been resorted to. 
 
 No attempt must be made to utilize the sediment of the plain 
 Collodion by re-distilling the Ether from it. This has been tried, 
 but with indifferent success ; for although the Ether so recovered 
 appeared tolerably good at first, it soon acquired the property of 
 liberating Iodine from Iodide of Potassium, and the Collodion 
 then became unfit for any purpose except for copying objects of 
 still life, where extreme sensitiveness is not required. 
 
 Whilst the Collodion is settling down, the bottle should be 
 covered over, to exclude the light. The room which the Author 
 used was a vault lighted by gas, and whilst plain Collodion re- 
 mained in that room, it continued good ; but if any portions were 
 taken up into the glass-house and left exposed, they gradually de- 
 teriorated, colouring at once on adding the Potassium iodizer, and 
 losing sensitiveness. 
 
 In manufacturing Collodion in large quantities, mistakes will 
 be prevented if separate measures, scales, funnels, etc., are kept 
 for each purpose ; and in cases where two operations are being 
 carried on at the same time such as picking out wet Pyroxy- 
 line, and filtering iodizing solution a basin of water may be 
 placed near at hand, into which the fingers may be dipped in pass- 
 ing from one process to the other. The mention of matters so 
 trivial may appear unnecessary ; but the importance of being 
 very methodical in all matters relating to Photography is some- 
 times overlooked. 
 
 Minor precautions to prevent accidents. Several cases of ac- 
 cident in making Collodion having come to the Writer's know- 
 ledge, he feels it a duty to give plain instructions to those who 
 are not practical chemists. 
 
 Beginning with the preparation of the Pyroxyline, it may be 
 observed that the vessel containing the acids must be placed 
 either in the open air, or beneath a flue, to carry off the fumes ; 
 and as Nitro- Sulphuric Acid, especially when heated, is a most 
 
388 PEACTICE or PHOTOGRAPHY. 
 
 corrosive substance, the hands of the operator should be protected 
 by the gloves of vulcanized india-rubber before described ; other- 
 wise, after repeating the process a few times, the skin and nails 
 will become rough and yellow from the action of the fumes. Of 
 the two acids the Nitric Acid is the most destructive, and it 
 sometimes happens that a minute drop spirts into the face. To 
 avoid this, be careful when pouring acids from a full bottle to 
 hold the neck very near to the recipient vessel, and to allow the 
 acid to flow gently against the sides without any gurgling ; and 
 .when removing the stopper, throw an old cloth over the bottle, 
 to prevent a sudden rush of vapour from projecting the acid. 
 Have at hand a small quantity of tow to wipe up any acid which 
 may run over the sides, and if the fingers are touched, wash them 
 immediately in water before they become yellow. 
 
 A drop of either acid may fall upon the clothes ; if Sulphuric, 
 & red spot is at once produced on black cloth or leather, but it 
 disappears on treating with strong Ammonia and subsequent 
 washing ; Nitric Acid or Nitro-Sulphuric Acid, however, will 
 almost certainly make the cloth rotten and yellow, unless the 
 Ammonia be applied instantly. 
 
 In mixing the acids, avoid by all means the use of any kind of 
 vessel which is likely to crack under a sudden rise of tempera- 
 ture, and 'in particular of a stoppered vessel, on account of the 
 disengagement of vapour which takes place. In one instance 
 where this precaution was neglected, a stoppered Winchester 
 quart, containing thirty ounces, was instantly blown to pieces. 
 
 In disposing of the waste acids, it is necessary to dilute very 
 freely with water before throwing them down the sink, or the 
 leaden pipes will soon be destroyed. If they are stowed away 
 for a time in bottles, see that the bottles have not previously 
 been used for Ether or Alcohol, since warm Nitro-Sulphuric Acid 
 inadvertently poured into a bottle with a little Ether at the 
 bottom, produces a violent effervescence of red fumes. For this 
 reason also, the same glass measure ought not to be employed 
 both for Collodion and acids, unless carefully cleansed ; for or- 
 ganic matters of a kind so easily susceptible of oxidation, will 
 invariably produce decomposition and spirting with hot Nitric 
 Acid. 
 
 Fears are sometimes entertained of the Pyroxyline itself ex- 
 ploding during the process of manufacture ; but these are ground- 
 
PRECAUTIONS. 389 
 
 less. It may, indeed, decompose rather suddenly and evolve red 
 fumes, as before shown, but in that case the action would be 
 stopped by throwing it into a pan of water. The use of artificial 
 heat in drying Pyroxyline, however, has been the cause of very 
 severe explosions, when from evaporation of the water in the 
 boiler, or from other causes, the heat has been allowed to rise. 
 
 Accidents result sometimes from carelessly handling Ether, 
 and especially from attempting to conduct the distillation of 
 Ether without the proper apparatus. Those who have seen an 
 Ether distillery, know that extraordinary precautions are always 
 taken to prevent the vapour ..from flowing along the ground and 
 reaching the source of heat ; and when we consider the very low 
 temperature at which Ether boils, and the chance of the retort 
 bursting from sudden evolutions of vapour, it is, evident that the 
 process ought not to be attempted, excepting by a practised 
 chemist. 
 
 When Ether is kept in bottles only partially full, the stopper 
 will. sometimes blow out suddenly in hot weather, and may fall 
 upon another bottle near at hand. Avoid this, by purposely 
 fixing each stopper with a twist, or passing over it a slip of 
 vulcanized rubber. In shaking a bottle containing Collodion, 
 examine it first for cracks, and if it is only partially full, fix the 
 stopper with the finger, and also loosen it between each shake, 
 so as to permit of the escape of the pent-up vapour. 
 
 Collodion ought not to be prepared in large vessels like car- 
 boys, which are often made of badly annealed glass. A carboy 
 of Collodion, containing about three gallons, has been known to 
 burst suddenly, when the temperature of the atmosphere has risen 
 to the summer heat. 
 
 In entering a dark room or a cellar containing much Collodion, 
 if a candle be absolutely required, hold it high above the head, 
 inasmuch as the vapour of Ether is very dense, and soon falls to 
 the ground. Amateurs sometimes meet with mishaps in coating 
 glasses with Collodion, from forgetting this fact, and placing a 
 naked candle below the level of the plate. 
 
 If a beaker or any other vessel containing Ether or Alcohol 
 should catch fire, do not attempt to extinguish it with water, but 
 throw a thick cloth over the mouth of the vessel, so as to cut off 
 the supply of air. 
 
390 PRACTICE OF PHOTOGRAPHY. 
 
 PREPARATION OF POSITIVE COLLODION. 
 
 The plain Collodion described in the preceding pages may be 
 used for Positive as well as Negative pictures, if the iodizer be 
 made with mixed Iodide and Bromide ; but it has been deemed 
 advisable to give a formula for a Pyroxyline producing a less 
 vigorous image than that prepared in the Nitro-Sulphuric Acid 
 containing an excess of the diluted Sulphuric Acid. Take of 
 Oil of Vitriol T845 at 60 Fahr. . . 12 fluid ounces. 
 Nitric Acid T45 at 60 Fahr. ... 12 
 
 Water 3* 
 
 Mix these ingredients, and allow the temperature to fall to 140 
 Fahr. The Cotton may be the same as before, and the Writer 
 adopts the plan of cleansing it with Potash precisely as described 
 in the Vocabulary; also, of drying it artificially before immersion. 
 Supposing these precautions to be neglected, it is more than pro- 
 bable that it will be necessary to reduce the quantity of Water in 
 the formula from 3^ down to 3 ounces, as the Nitro-Sulphuric 
 Acid is undoubtedly very weak, and would not bear any further 
 dilution. 
 
 The Cotton when immersed in the above mixture behaves dif- 
 ferently from that dipped in the Nitro-Sulphuric Acid containing 
 the excess of diluted Sulphuric Acid, and the appearances de- 
 scribed at page 378 as indicating the strength of the acid cannot 
 in this case be relied on. When the acid of the formula now 
 given is too weak, the Cotton sinks down almost to the bottom, 
 occupying but little space, and matting together until it forms a 
 mass like "felt ;" it also becomes very opaque, and gelatinizes in 
 the acid. On lifting it out into the water it sticks together so 
 that the fibres cannot be separated from each other, and the ac- 
 tion of the water in washing out the acid is interfered with. 
 Supposing the acids, on the other hand, to be too concentrated, 
 the Cotton is little altered in appearance, and is very strong and 
 tough, so that it may be well handled without tearing. These 
 appearances are described from experiments in which the Cotton 
 had been both boiled with Potash and dried ; they may not 
 accord exactly with the experience of those who use ordinary 
 Cotton, the effect of which would be to favour disintegration and 
 solution in the Nitro-Sulphuric Acid. 
 
 The plain Collodion. This may be made with the best Methy- 
 
POSITIVE IODIZED COLLODION. 391 
 
 lated Ether, or by preference with the pure Ether of '728. The 
 Alcohol should be that of '805 at 60 Fahr., sold as absolute : 
 if absolute Alcohol cannot be obtained, a spirit of '815 rectified 
 from Carbonate of Potash, or even of '825, may be substituted : 
 in the latter case the relative quantity of Ether must be increased 
 from 4^ to 5 ounces, and that of the spirit diminished from 1^ to 
 1 ounce. Take of 
 
 Ether of '725 . 4 fluid ounces. 
 
 Alcohol of -805 1 fluid ounce. 
 
 Pyroxyline 48 grains. 
 
 Shake up the Pyroxyline first with the Alcohol, and afterwards 
 add the Ether : a little more agitation may be required than in 
 the case of the Negative Collodion, as this Pyroxyline is more apt 
 to form gummy masses at the bottom of the bottle. The Collo- 
 dion ought to be clearer and more translucent than the Negative 
 Collodion, and to settle in less than one-half of the time ; if a 
 thick deposit forms, the acids contained too much water, and the 
 Collodion will probably yield a cloudy film on drying. 
 
 The iodizing compound for the Collodion will be found in the 
 formulae for Positives in the next Section. 
 
 SECTION II. 
 
 Formula for Collodion Positives. 
 
 The solutions are taken in the following order: The 
 Collodion. The Nitrate Bath. Developing Fluids. Fix- 
 ing Liquid. Varnishes. 
 
 THE IODIZED COLLODION. 
 
 The plain Collodion having been made according to the 
 directions given in tlie last Section, it remains only to pre- 
 pare the iodizing solution. For this purpose take of 
 
 Iodide of Ammonium .... 1^ drachm. 
 
 Iodide of Cadmium H drachm. 
 
 Bromide of Ammonium ... 40 grains. 
 
 Alcohol -805 to '816 10 fluid ounces. 
 
 Eead the Vocabulary for the mode of testing the purity 
 of these compounds. The Iodide of Ammonium especially, 
 is liable to vary in quality. 
 
392 .PRACTICE or PHOTOGRAPHY. 
 
 Seduce the salts to powder, and shake them in a bottle 
 with the Alcohol : no heat will be required. 
 
 This iodizer becomes yellow on keeping, from liberation 
 of Iodine, but not to an extent calculated to do injury. 
 It must not be exposed to the light, and should be kept in 
 a cool place. 
 
 The plain Collodion ought to be kept in the dark, or the 
 Ether will rapidly.be ozonizerd by the Pyroxyline (p. 224). 
 A cool cellar or cupboard will answer" the purpose. 
 
 The proportions for mixing are three 'parts of Collodion 
 to one of iodizer ; that is to say, six fluid drachms of the 
 former, to two fluid drachms of the latter. 
 
 The film produced by the Collodion, on dipping in the 
 Bath, ought to be tolerably dense and creamy, and if it be 
 opaline or pale-blue, the plain Collodion is too thin. In 
 that case add more Pyroxyline, which in most cases will 
 be sufficient to increase the opacity to the desired extent. 
 Bear in mind however that the presence of Bromide always 
 tends to make the film appear rather less opaque, and there- 
 fore it will be better to try a picture before condemning the 
 Collodion. 
 
 The newly iodized Collodion is exquisitely sensitive, but 
 sometimes fails in giving a vigorous picture with clear 
 shadows. In that case dissolve five grains of Iodine in an 
 ounce of Spirits of Wine, and add a few drops until the 
 fluid assumes a golden-yellow colour. 
 
 The Collodion may be expected to produce good Posi- 
 tives for six weeks or two months after iodizing, in cool 
 weather. The exposure is longer than at first, but the 
 pictures are more free from blemish. Positive Collodion, 
 containing Bromide, always suffers less from keeping than 
 Negative Collodion ;. and hence it may safely be put away 
 in stock, ready iodized 
 
 A mixture of the best. Ether with absolute Alcohol, 
 5 drachms of the former. to. 3 drachms of the latter, is 
 proper for diluting this Collodion, as it becomes thick at 
 the bottom of the bottle. 
 
 In order to convert old brown, residues of Iodized Nega- 
 
POSITIVE NITEATE BATH. 393 
 
 tive Collodion into Collodion for Positives, proceed as fol- 
 lows : Weigh out a grain of Bromide of Ammonium for 
 each ounce of Collodion, rub it up in a mortar with the small- 
 est possible quantity of spirit until dissolved, and add it. 
 A turbidity and precipitation of Bromide of Potassium will 
 take place (unless the Collodion contain an unusually large 
 quantity of water), but in twenty-four hours the liquid 
 will clear itself, and may be poured off for use. An addi- 
 tional quantity of Iodide will also be required, if the old 
 brown Collodion gives a blue film, as is frequently the case : 
 one grain of Iodide of Cadmium may be added to each 
 ounce. 
 
 THE NITEATE BATH. 
 
 Ee- crystallized Nitrate of Silver . . 30 grains. 
 
 Nitric Acid minim. 
 
 Alcohol . 15 minims. 
 
 Distilled Water 1 fluid ounce. 
 
 In the Vocabulary, Part I., directions are given for the 
 purification of Nitrate of Silver for the Bath ; also for the 
 testing of distilled water, and the best substitutes when it 
 cannot be obtained. 
 
 The Bath must be saturated with Iodide of Silver, and 
 Nitric Acid neutralized if it be present. Weigh out the 
 total quantity of crystals of Nitrate required for the 
 Bath, and dissolve in about twice their weight of water. 
 Then take a quarter of a grain of Iodide of Potassium to 
 each 100 grains of Nitrate, dissolve in half a drachm of 
 water, and add to the strong solution ; a yellow deposit 
 of Iodide of Silver first forms, but on stirring is com- 
 pletely redissolved. When the liquid is clear, test for free 
 Nitric Acid by dropping a piece of blue litmus-paper into 
 the strong solution. If at the expiration of two minutes 
 the paper appears reddened, Nitric Acid is present, to neu- 
 tralize which, add a few drops of solution of Carbonate 
 of Soda (ten grains to the ounce of water), until a distinct 
 white turbidity, remaining after agitation, is produced. 
 Then dilute down the concentrated solution with the re- 
 
394 PEACTICE OF PHOTOGBAPHY. 
 
 maining portion of the water, stirring all the time, and 
 filter out the milky deposit. If the liquid does not at first 
 run clear, it will probably do so on passing it again through 
 the same filter. 
 
 Lastly, add the Nitric Acid to the clear filtered liquid, 
 and also the Alcohol. 
 
 It may appear unnecessary to commence by neutrali- 
 zing Nitric Acid in a Bath, when the formula directs that 
 the same acid should afterwards be added ; but inasmuch 
 as the amount of free acid in commercial Nitrate of Silver 
 varies considerably, a normal solution could not otherwise 
 be obtained. Some employ Ammonia, for neutralizing the 
 Nitric Acid, but Nitrate of Ammonia is then formed in 
 the Bath ; and this substance being a solvent of Oxide of 
 Silver, the latter remains in solution instead of being pre- 
 cipitated as with Carbonate of Soda, and thus the Bath 
 might unconsciously be rendered highly alkaline. 
 
 Bear in mind that all the common metals decompose 
 Nitrate of Silver, and consequently, that the Bath must 
 be kept in glass or porcelain, out of contact with iron, 
 brass, etc. 
 
 The object of the Alcohol in the formula is to enable the 
 Bath to penetrate the film more readily, and thus to hasten 
 the process of sensitizing. 
 
 As the bulk of the Bath becomes lessened by use, fill 
 up with a solution containing 35 grains of plain Nitrate of 
 Silver to the ounce, without any Iodide or Alcohol ; this 
 will be found sufficient to maintain the strength nearly at 
 the original point. 
 
 When the Bath becomes very old and saturated with 
 Ether, it may yield Positives which are streaked and 
 slightly foggy ; if so, it will be advisable to precipitate it 
 with a Chloride and prepare a new one. 
 
 THE DEVELOPING SOLUTIONS. 
 
 Two formulae are recommended : the first being the most 
 simple in its preparation, and easy to manipulate with ; 
 the second producing a more metallic picture nearly re- 
 sembling a Daguerreotype. 
 
POSITIVE DEVELOPING SOLUTIONS. 395 
 
 FOEMULA No. 1. 
 
 Pure Sulphate of Iron ... 12 grains. 
 
 Glacial Acetic Acid . . , . | fluid draclim. 
 
 Water 1 ounce. 
 
 Consult the Vocabulary for purity of chemicals, and best 
 substitutes for distilled water. No filtering needed, as a 
 rule. 
 
 An addition of Nitric Acid, half a minim to the ounce, 
 makes the image whiter and more metallic ; but if too much 
 be used, the development proceeds irregularly, and span- 
 gles of Silver are formed. 
 
 The Acetic Acid renders the development uniform, by 
 causing the solution of Protosulphate to combine more 
 readily with the film ; it has also an effect in whitening 
 the image and increasing its brightness. 
 
 This solution of Sulphate of Iron becomes red on keep- 
 ing, from a gradual formation of per salt. When it is too 
 weak, add more of the Protosulphate, but the experience of 
 the Write? is in favour of working with the newly-mixed 
 liquid, since Acetate of Iron and Acetic Ether appear to 
 formed by decomposition. 
 
 Some operators add pure Nitrate of Potash to this deve- 
 loping solution, to form a small portion of Protonitrate of 
 Iron. It is said to improve the colour slightly. The pro- 
 portions are 10 grains of Nitrate of Potash to about 14 or 
 15 grains of Protosulphate of Iron. 
 
 FOEMULA No. a. 
 
 This is a mixture of Sulphate and Nitrate of Iron, em- 
 ployed with much success by Mr. Keith, of Liverpool, and 
 other operators. Take of 
 
 Sulphate of Iron 1| ounce. 
 
 Nitrate of Baryta ...... 1 ounce. 
 
 Alcohol 1 ounce. 
 
 Pure Nitric Acid . . . . . .40 minims. 
 
 Water 16 ounces. 
 
 Reduce the Nitrate of Baryta to fine powder, in a mortar, 
 and dissolve it in the water by the aid of heat. This may be 
 
396 PEACTICE OF PHOTOGEAPHY. 
 
 done in a porcelain capsule standing on a tripod, with a 
 spirit-lamp below, or the water may be boiled in a kettle 
 and poured upon the pulverized salt with constant stirring. 
 Next, reduce the Sulphate of Iron also to powder, and when 
 the whole of the Nitrate of Baryta has dissolved, stir the 
 Sulphate into the warm liquid, and continue to stir for 
 three minutes, to ensure a perfect decomposition. A dense 
 white deposit of Sulphate of Baryta forms, rendering the 
 whole milky, but on leaving the capsule for ten minutes, 
 the heavy powder settles to the bottom, leaving a clear 
 liquid of an apple-green colour, with occasionally a little 
 reddening at the edges, from oxidation and conversion into 
 Pernitrate of Iron. Throw the whole' on a paper filter, 
 and if the liquid at first runs muddy, pass it through the 
 same filter a second time. Drain away as much as pos- 
 sible, and when it has become quite cold, drop in first the 
 Nitric Acid, and then the Alcohol. 
 
 The above operation is very simple, but the Amateur 
 must be guarded against two or three causes of failure. 
 The Nitric Acid will sometimes oxidize the Iron, and make 
 it red, and it may also act upon the Alcohol. Do not 
 therefore mix the Nitric Acid with the Alcohol, but add 
 them separately, and drop the required quantity of Nitric 
 Acid first into a few drachms of water, to dilute it down 
 and prevent any chance of oxidation. Lastly, fill up with 
 distilled water to the proper bulk. 
 
 The solution so formed will contain in each ounce about 
 30 grains of Nitrate of Iron, and 10 grains of Sulphate of 
 Iron ; but the Nitrate is so feeble a developer, that 5 or 10 
 grains additional of the Sulphate may be needed to prevent 
 the formation of spangles on the shadows, when the weather 
 is cold or the Bath very acid. 
 
 This developer will keep for two months in cold weather, 
 and probably longer, if the- bottles are filled to the neck ; 
 hence a stock can be prepared beforehand. It becomes 
 weakened by a partial decomposition, but produces suffi- 
 ciently white pictures, if a little Sulphate be added. Acetic 
 Acid is purposely omitted from this formula. 
 
POSITIVE VABNISHES. 397 
 
 THE FIXING SOLUTION. 
 
 Cyanide of Potassium . 1 ... 10 grains. 
 Common Water 1 ounce. 
 
 The percentage of Carbonate of Potash in commercial 
 Cyanide of Potassium is so variable that no exact directions 
 can be given for the formula. It is best, however, to use it 
 rather dilute of such a strength that the plate is cleared 
 gradually in from half a minute to a minute. 
 
 Solution of Cyanide of Potassium decomposes slowly on 
 keeping, but it will usually retain its solvent power for 
 several weeks. In order to escape inconvenience from the 
 pungent odour evolved by this salt, many employ a ver- 
 tical Bath to hold the solution ;< but in that case the plates 
 must be carefully washed before fixing, as the Iron salts 
 decompose the Cyanide and produce a blue deposit. 
 
 VAENISHES. 
 
 Two kinds of varnish .are usually employed for glass 
 Positives ; the clear varnish for protecting the film, and 
 the black varnish for backing up. 
 
 The objection to the use of black japan is its liability to 
 crack, in consequence of which the film splits away from 
 the glass after keeping the picture a few months. To pre- 
 vent this cracking and to impart elasticity, it is advised to 
 add a portion of Caoutchouc to the solvent. Take of 
 
 Caoutchouc \ drachm. 
 
 Asphaltum . . 4 ounces. 
 
 Coal Naphtha 10 ounces. 
 
 Dissolve the Caoutchouc first, and the Asphaltum sub- 
 sequently ; apply heat if necessary. 
 For the clear Varnish, take 
 
 Soft Copal 40 grains. 
 
 Benzole 1 fluid ounce. 
 
 The solution filters easily through paper, and forms a 
 colourless Varnish, which is brilliant, and dries without 
 heat. 
 . The Writer cannot speak from experience of the merits 
 
398 PRACTICE OF PHOTOGRAPHY. 
 
 of the above receipts. The composition of the best com- 
 mercial Varnishes is secret. 
 
 SECTION III. 
 
 Formulae, for Negative Solutions. 
 
 THE IODIZED COLLODION. 
 
 The plain Collodion having been made according to di- 
 rections given in the First Section (p. 381), it remains only 
 to prepare the iodizer. 
 
 FOKMULA No. 1. (Potassium lodizer.) 
 
 Iodide of Potassium 135 grains. 
 
 Alcohol of -816 10 ounces. 
 
 Pulverize, dissolve, and filter. 
 
 FORMULA No. 2. (Cadmium lodizer.) 
 
 Iodide of Cadmium 170 grains. 
 
 Alcohol of '816 10 ounces. 
 
 Dissolve in the cold and filter ; no pulverizing required. 
 
 Consult the Vocabulary on the purity of the Iodides, 
 and the varying strength of commercial Alcohol. The 
 proportion in which these iodizing solutions are to be 
 added to the plain Collodion is the same for all, viz. two 
 fluid drachms of iodizer to six fluid drachms of plain 
 Collodion. 
 
 Remarks on Formula 1. This iodizer is the one which 
 the majority prefer : many however mix it with Formula 
 No. 2, half-and-half. Reduce the Iodide of Potassium to 
 an impalpable powder, and shake in a bottle. Perfect solu- 
 tion will scarcely be obtained unless the Alcohol be pre- 
 viously warmed to about 140 Fahrenheit, or the bottle 
 dipped in hot water. The latter plan is simple, but there 
 is a chance of cracking the glass. 
 
 The quantity of Iodide of Potassium given is the maxi- 
 mum which the Collodion will carry without producing 
 marks like watered silk at the lower edge of the film. 
 
 The film produced in the Bath ought to be tolerably 
 
NEGATIVE IODIZED COLLODION AND BATH. 399 
 
 dense, and if it comes out very pale and blue, the plain 
 Collodion may be too thin, and if so, must be thickened 
 by addition of Pyroxyline. Supposing the plain Collodion 
 to be sufficiently thick, and the film too opaline, add to 
 each ounce a single grain of Iodide of Cadmium. 
 
 This iodizer usually remains colourless if kept in a dark 
 place, but on exposure to light a trace of Iodine is liberated, 
 tinging it yellow. Re-absorption of the Iodine will occa- 
 sionally take place on again removing the iodizer from the 
 light. 
 
 The plain Collodion must also be kept in the dark, or 
 the Ether will be rapidly ozonized by the Pyroxyline. A 
 cool cellar or cupboard will answer the purpose. 
 
 On adding the iodizer to the plain Collodion, no imme- 
 diate change of colour ought to result, except in very hot 
 weather, or in the case of plain Collodion which has been 
 long kept. By the following day, a straw-yellow tint may 
 be expected ; in a fortnight, a full yellow ; and in a month, 
 a brown tint. These changes, however, will vary with the 
 purity of the Ether, and with the temperature. 
 
 Formula No. 2. This is a very stable iodizer, and the 
 best adapted for Collodion, which has to be kept long in 
 the Iodized state. It has some tendency to render the Col- 
 lodion glutinous, thus impairing its flowing qualities, and 
 also to favour solarization. It is however very sensitive, 
 and retains its good qualities for a very long time unin- 
 jured. In coating large plates, it is advisable to mix it in 
 equal proportions with the Potassium Iodizer to render 
 the Collodion more fluid, and also to diminish its setting 
 power. 
 
 THE NITRATE BATH. 
 
 This solution is the same as that given for Positives at 
 page 393, Acetic Acid however being used in acidifying 
 instead of Nitric Acid. Take of 
 
 Hecrystallized Nitrate of Silver . 30 grains. 
 
 Glacial Acetic Acid minim. 
 
 Alcohol . 15 minims. 
 
 Distilled Water 1 fluid ounce. 
 
400 FBACTICE OF PHOTOGRAPHY. 
 
 In the Vocabulary, Part I., directions are given for the 
 purification of Nitrate of Silver ; also for the testing of 
 distilled water, and the best substitutes when it cannot be 
 obtained. 
 
 The Bath must be saturated with Iodide of Silver, and 
 Nitric Acid neutralized if it be present. Weigh out the 
 total quantity of crystals of Nitrate required for the Bath, 
 and dissolve in about twice their weight of water. Then 
 take a quarter of a grain of Iodide of Potassium to each 
 100 grains of Nitrate, dissolve in half a drachm of water, 
 and add to the strong solution ; a yellow deposit of Iodide 
 of Silver first forms, but on stirring is completely redis- 
 solved. When the liquid is clear, test for free Nitric Acid 
 by dropping a piece of blue litmus-paper into the strong 
 solution. If at the expiration of two minutes the paper 
 appears reddened, Nitric Acid is present, to neutralize 
 which add a few drops of solution of Carbonate of Soda 
 ,(10 grains, more or less, to the ounce of water) until a 
 distinct turbidity, remaining after agitation, is produced. 
 Then dilute down the concentrated solution with the re- 
 maining portion of the water, stirring all the time, and 
 filter out the milky deposit. If the liquid does not at first 
 run clear, it will probably do so on passing it again through 
 the same filter. 
 
 Lastly, add the Acetic Acid and the Alcohol to the clear 
 filtered liquid. The object of the Alcohol is to accelerate 
 the process of sensitizing, by making the film take the Bath 
 more quickly. 
 
 Some employ Ammonia for neutralizing the Nitric Acid 
 contained in the crystals, but by so doing Nitrate of Am- 
 monia is formed, which is capable of dissolving Oxide of 
 Silver, and any excess of Ammonia would consequent^ 
 render the Bath highly alkaline. 
 
 Bear in mind that all the common metals decompose 
 Nitrate of Silver,, and consequently that the Bath must 
 be kept in glass or porcelain, out of contact with iron, 
 brass, etc. .... 
 
 The Writer finds that the Bath does not become weak- 
 
DEVELOPERS FOE IODIZED COLLODION. 401 
 
 ened by use to the extent that would be anticipated. Very 
 rarely does the cover of the trough fit tight enough to 
 prevent all evaporation, and consequently it is sufficient in 
 most cases to fill up as it becomes exhausted, with a fresh 
 portion of the same, or with a thirty-five grain solution of 
 Nitrate of Silver. To be continually adding crystals of 
 Nitrate is an error, and is likely to favour staining and 
 excessive density of image. 
 
 Negative Baths are generally laid aside from having 
 acquired the property of giving streaky films. This re- 
 sults from the accumulation of Ether and other organic 
 matters as the Bath becomes very old, and it is then better 
 to begin afresh and prepare a new one. 
 
 THE DEVELOPING SOLUTIONS. 
 FORMULA No. 1. 
 
 Pyrogallic Acid 1 grain. 
 
 Glacial Acetic Acid 20 minims. 
 
 Distilled Water 1 ounce. 
 
 FORMULA No. 2. 
 
 Pyrogallic Acid 1| grain. 
 
 Citric Acid 1 grain. 
 
 Alcohol 20 minims. 
 
 Water 1 ounce. 
 
 FORMULA No. 3. 
 
 Sulphate of Iron 12 grains. 
 
 Glacial Acetic Acid 30 minims. 
 
 Distilled Water 1 ounce. 
 
 Remarks on Formula 1. This is the form commonly 
 employed, and will seldom require any modification; in 
 cold and damp weather, however, if the Negative should 
 develope slowly and be wanting in bloom and depth, reduce 
 the Acetic Acid to ten minims and increase the Pyrogallic 
 Acid to 1^ grain. No filtering of this solution will be re- 
 quired, as a rule. 
 
 Sometimes an addition of twenty minims of Alcohol to 
 the ounce of developer will be an improvement, when, from 
 
402 PRACTICE OF PHOTOGEAPHY. 
 
 age of the Bath, and accumulation of Ether, the solution 
 flows over the film in an oily manner. It is better, how- 
 ever, to dispense with the Alcohol if possible. 
 
 If the solution of Pyrogallic Acid be kept for a few days, 
 it becomes yellow. In this state it will still develope the 
 picture, but is apt to produce a feeble Negative, and to 
 discolour rapidly on touching the film. 
 
 Remarks on Formula 2. Useful in very hot weather 
 for objects well lighted, or in the case of Collodion which 
 solarizes the sky and gives a red picture. The Pyrogallic 
 Acid and Citric Acid may be weighed out and carried in 
 packets ready mixed for a journey, but the solution loses 
 a great part of its reducing power after keeping for a few 
 days. No filtering needed as a rule. 
 
 Formula No. 3 may be used with advantage when, 
 through peculiar conditions of the Bath or Collodion, the 
 Pyrogallic Acid developer gives . Negatives with violent 
 contrasts and too much density. Useful also when work- 
 ing in a feeble light. 
 
 BEOMO-IODIZED COLLODION. 
 
 The plain Collodion having been prepared as previously 
 directed at p. 381, take of 
 
 Iodide of Ammonium 80 grains. 
 
 Iodide of Cadmium 80 grains. 
 
 Bromide of Cadmium or Ammonium 40 grains. 
 
 Alcohol of '816 10 fluid ounces. 
 
 Dissolve without heat ; filter. 
 
 Bromide of Cadmium may be employed when the Collo- 
 dion has to be kept long in the iodized state ; it tends 
 much to stability. The Bromide of Ammonium is better, 
 when the Collodion is to be used soon after iodizing. The 
 proportion of the iodizer to add is the same as before, viz. 
 two fluid drachms to six of the plain Collodion. 
 
 THE NITRATE BATH. 
 
 Hecrystallized Nitrate of Silver . . 30 grains. 
 Distilled Water .... 1 fluid ounce. 
 
BATH FOR BEOMO- IODIZED COLLODION. 403 
 
 Dissolve and saturate with Iodide of Silver as directed 
 at p. 393. Most specimens of commercial recrystallized 
 Nitrate of Silver contain a trace of free Nitric Acid, which 
 may not be sufficient to affect Litmus-paper. A slight ex- 
 cess is, however, necessary to the clean working of this 
 Bath with a Bromo-iodized Collodion. It is better, there- 
 fore, to filter out the undissolved Iodide of Silver and ex- 
 cite a collodionized plate at once, before subjecting the Bath 
 to any doctoring operations, which are generally unneces- 
 sary and very often hurtful. Should the plate on develop- 
 ment turn out fogged or veiled by a deposit of Silver on 
 the deep shadows, the Bath contains either free Oxide of 
 Silver, too much acid, or organic matter, the appearance 
 in each case being somewhat similar. The remedies are 
 as under : 
 
 First, Alkalinity. If a piece of Litmus-paper previ- 
 ously reddened by a very weak acid, slowly resumes its 
 blue colour, the Bath is too alkaline. To correct it, dilute 
 Nitric Acid with ten times its bulk of distilled water, and 
 add one drop to every ten ounces of Bath solution ; mix 
 well and allow to stand for some minutes, then try another 
 plate. Probably, another drop or two may be needed, if 
 fused, instead of recrystallized, Nitrate of Silver has been 
 used. When the point of best working of a Bath has been 
 nearly attained, that is to say, the point which combines 
 the greatest sensitiveness with density and clearness of 
 negative, Litmus-paper ceases to give satisfactory indica- 
 tions. The safest plan is to try a plate between each addi- 
 tion of acid, and to add the latter gradually. 
 
 Second. The best remedy for over-acidity is moist Oxide 
 of Silver. It may be made by adding a solution of Caustic 
 Potash drop by drop to a solution of Nitrate of Silver till 
 the Oxide ceases to fall. Throw the precipitate on a filter, 
 and wash three or four times in distilled water. A few 
 grains of this an excess not being injurious will give the 
 Bath a feebly alkaline reaction. Filter and acidify as 
 above. 
 
 Third, Organic Matter. When a piece of blue Litmus- 
 
404 PRACTICE OF PHOTOGRAPHY. 
 
 paper shaken up with the Bath, begins at the end of five 
 or six minutes to redden slightly, and at the same time 
 the resulting pictures are fogged, the fault is probably 
 due to organic matter. A remedy proposed by Mr. 
 Barber, and generally efficacious, is to render the Bath 
 slightly alkaline by the addition of a few drops of Ammo- 
 nia, sufficient to restore the blue colour to reddened Lit- 
 mus, and then to expose it for some time to sunshine in a 
 transparent vessel. The action of light precipitates the 
 organic Silver compound, which may then be filtered out 
 and the Bath acidified as before. Sometimes five or six 
 days will be needed for complete precipitation. 
 
 DEVELOPING SOLUTION FOE BKOMO-IODIZED COLLODION. 
 
 Protosulphate of Iron 20 grains. 
 
 Glacial Acetic Acid 30 minims. 
 
 Distilled Water 1 ounce. 
 
 REDEVELOPING SOLUTIONS. 
 
 A. 
 
 Pyrogallic Acid 1 grain. 
 
 Citric Acid 1 grain. 
 
 Distilled Water 1 ounce. 
 
 B. 
 
 Nitrate of Silver 30 grains. 
 
 Glacial Acetic Acid 2 drachms. 
 
 Distilled Water 1 ounce. 
 
 FIXING SOLUTIONS. 
 
 No. 1. 
 
 Take of Hyposulphite of Soda . . 4 ounces. 
 Common Water .... 4 ounces. 
 Dissolve the crystals in the water with or without the 
 aid of heat. No pulverizing or filtering will be required. 
 Use the liquid over and over again until it becomes ex- 
 hausted, when it may be thrown away. Traces of the 
 
FIXING SOLUTIONS. 405 
 
 Pyrogallic Acid from the Negatives soon discolour the fix- 
 ing solution, but this is immaterial. 
 
 If a jug be employed to hold this liquid, it must be made 
 of good ware, or the Hyposulphite will pass through and 
 form an efflorescence on the exterior. As a rule, no vessel 
 in which solution of Hyposulphite has been long kept 
 ought to be employed for any other purpose. 
 
 No. 2. 
 
 Cyanide of Potassium 15 grains. 
 
 Common Water 1 ounce. 
 
 Formula No. 1 is applicable to any Collodion developed 
 either with Pyrogallic Acid or Iron. Formula No. 2 can- 
 not be employed with safety in fixing an iodized film deve- 
 loped with Pyrogallic Acid only, because of its great ten- 
 dency to weaken the half-tones, but it may be used for a 
 Bromo-iodized plate developed with Iron ; it need not be 
 thrown away till it ceases to dissolve the Iodide readily. 
 
 VAENISH. 
 
 It is of the greatest importance to use a Varnish which 
 will not crack, and one of considerable body, so as to pro- 
 tect the Negative in the printing process. The Writer 
 selects the following as the best Varnish of which the for- 
 mula has been given to the public : 
 
 Alcohol of '815 40 ounces. 
 
 White Stick Lac 3 ounces. 
 
 Picked Sandrac 3 drachms. 
 
 Place in a flask, and raise the temperature by plunging 
 the flask into hot water, and in a few minutes the solution 
 will be effected, with the exception of a few light filaments 
 of insoluble lac. Filter through bibulous paper, and the 
 light-yellowish liquid will be fit for use. 
 
 Warm the Negative gently (to 140 or 160) before a fire, 
 and pour the Varnish on and off, like Collodion. If the 
 glass be made too hot, it will be difficult to apply the Var- 
 nish, and it will also dry very rapidly, producing veins. 
 
406 PBACTICE OT 1 PHOTOGRAPHY. 
 
 Never attempt to give a second coating, as the Negative 
 would then inevitably be spoiled. 
 
 These particulars are given on the authority of M onck- 
 hoven. 
 
 Some of the varieties of Pyroxyline used for Collodion 
 are soluble in Alcohol, and consequently the above Var- 
 nish may be found occasionally to attack the film, in such 
 a case the clear Varnish recommended for Negatives (p. 
 397) may be substituted for it. 
 
407 
 
 CHAPTER II. 
 
 MANIPULATIONS AND PROCESSES. 
 
 THE contents of this Chapter may be arranged in seven 
 distinct Sections. 
 
 SECTION I. Ordinary Manipulations of the Collodion 
 
 Process. 
 
 SECTION II. Positive and Negative Portraiture. 
 SECTION III. Landscape Photography. 
 SECTION IV. Copying Works of Art, etc. 
 SECTION Y. Stereoscopic and Instantaneous Pictures. 
 SECTION VI. Micro-Photography. 
 SECTION VII. Photography in hot climates. 
 
 SECTION I. 
 
 Manipulations of the Collodion Process. 
 
 These include the cleaning of the plates, coating them 
 with Collodion, exposing in the camera, developing, fixing, 
 and varnishing. 
 
 CLEANING- THE GLASS PLATES. 
 
 Care should be taken in selecting glass for Photography. 
 The ordinary window-glass is inferior, having scratches 
 upon the surface, which often cause an irregular action of 
 the developing fluid ; and the squares are seldom flat, so 
 that they are apt to be broken in compression during the 
 printing process. 
 
408 PEACTICE OF PHOTOGRAPHY. 
 
 The patent plate answers better than any other descrip- 
 tion of glass, especially for large sizes ; but if it cannot be 
 procured, the " flatted crown glass " may be substituted. 
 
 Before washing the glasses, each square should be rough- 
 ened on the edges and at the corners by means of a file or 
 a sheet of emery-paper, or more simply by drawing the 
 edges of two plates across each other. If this precaution 
 is omitted, the fingers are liable to injury, and particles 
 of flue are cut from the cloths used in wiping. When the 
 Collodion is at all of a contractile nature, the surface of 
 large plates should be roughened to the depth of \ inch 
 from the edge, to prevent the film from drawing itself 
 away, and allowing the fixing solution to pass beneath. 
 Place the glass upon a cloth at the end of a table, and rub 
 it sharply with a piece of soft stone, sold for whetting 
 scythes. A groove will soon be formed in the stone, 
 and both the edge and the surface of the glass will be 
 roughened at the same time, the latter to the extent of 
 \ inch all round. 
 
 In cleaning glasses, it is not sufficient, as a rule, to wash 
 them simply with water. This may be proved by putting 
 the glass under a tap, and observing how the fluid runs off 
 from the surface without wetting it, showing the presence 
 of grease or other organic impurity. A cream of Tripoli 
 powder and Spirits of Wine, with a little Ammonia added, 
 is commonly employed. A tuft of cotton is to be dipped in 
 this mixture, and the glasses well rubbed with it for a few 
 minutes. They are then rinsed in plain water and wiped 
 with a dry cloth. Do not rear them up on end to dry 
 spontaneously, because the evaporation would leave a film 
 upon the surface, and dust might rise by capillary attrac- 
 tion from the table on which they were standing. 
 
 The cloths used for cleaning glasses should be kept ex- 
 pressly for that purpose ; they are best made of a mate- 
 rial sold as fine " diaper," and very free from flocculi and 
 loosely- adhering fibres. They are not to be washed in 
 soap and water, but always in pure water, or in water con- 
 taining a little Carbonate of Soda. Never lay these cloths 
 
CLEANING GLASS PLATES. 409 
 
 upon the operating-table, but place them, when not in use, 
 across the back of a chair, or in a bag kept for the pur- 
 pose. If the hands are warm, wear a pair of gloves, that 
 the cloths may be kept clean. 
 
 After wiping the glass carefully, complete the process 
 by polishing with an old silk handkerchief, avoiding con- 
 tact with the skin of the hand. Some object to silk, as 
 tending to render the glass electrical, and so to attract 
 particles of dust, but in practice no inconvenience will be 
 experienced from this source. Wash-leathers may be 
 used instead of silk for the final polishing. They should 
 be first soaked for twelve hours in a strong solution of 
 Carbonate of Soda in cold water, then washed in many 
 waters to get rid of the " dressing " and the last trace of 
 soda, dried, and finally, well rubbed or beaten till they are 
 soft and pliable. 
 
 Before deciding that the glass is clean, hold it in an an- 
 gular position against a dark background, and breathe 
 gently upon it ; the condensed moisture should fly off 
 evenly arid quickly. The importance of attending to this 
 simple rule will be at once seen by referring to the re- 
 marks made at page 160. Stow the cleaned plates away 
 in grooved boxes; the plan of packing them between 
 sheets of paper, as is sometimes done, is not to be com- 
 mended, since the letters and marks upon the paper are 
 gradually impressed upon the glass. 
 
 Other modes of cleaning glasses may be adopted with 
 advantage. One of the most simple and efficacious is the 
 use of residues of old Collodion ; thin down the Collodion 
 with an equal bulk of ordinary Spirits of Wine or weak 
 Alcohol, add an excess of Iodide of Potassium, and shake 
 till the solution is saturated, a few crystals left in the 
 bottom of the bottle will not be injurious. Then pour a 
 little on the glass, and rub with a tuft of cotton-wool or 
 piece of clean wash-leather ; clean off with a second piece, 
 and finish with the silk or leathers, as before. If the plates 
 have not been previously used, no preliminary washing 
 will be required, for the grease and other impurities will 
 
410 PRACTICE OF PHOTOGRAPHY. 
 
 be effectually removed by the Ether and Iodide of Potas- 
 sium, and even should a trace of the latter be left adhering 
 to the plate, it will do no harm. 
 
 Caustic Potash, sold by the druggist under the name of 
 " Liquor Potassse," is very efficacious in cleaning glasses, 
 or in place of it, a warm solution of " washing Soda" (Car- 
 bonate of Soda). Liquor Potassae, being a caustic and 
 alkaline liquid, softens the skin and dissolves it ; it must 
 therefore be diluted with about four parts of water, and 
 applied to the glass by means of a cylindrical roll of flan- 
 nel. After wetting both sides thoroughly, allow the glass 
 to stand for a time until several have been treated in the 
 same way ; then wash with water and rub dry in a cloth. 
 Observe, in adding either Potash, Carbonate of Soda, or 
 Ammonia to common hard Water, that a deposit of Car- 
 bonate of Lime may be formed. In that case, if the glasses 
 are left in the liquid, they will soon be covered with little 
 crystals, which adhere tightly. Cyanide of Potassium, 
 which some employ as a cleansing agent, will also pre- 
 cipitate Lime salts from hard water, since it is almost in- 
 variably contaminated with Carbonate of Potash. 
 
 New glass plates are frequently dotted on the surface 
 with little gritty particles, which consist, as above, of Car- 
 bonate of Lime. They are not removed by Potash or any 
 alkali, but dissolve readily in a diluted acid, such as Oil 
 of Vitriol, or Nitric Acid, diluted with about four parts of 
 water. 
 
 The objection to the use of Nitric Acid is, that if al- 
 lowed to come in contact with the dress, it produces stains 
 which cannot be removed unless immediately treated with 
 an alkali. A drop of Ammonia should be applied to the 
 spot before it becomes yellow and faded. 
 
 To clean old plates. When a Negative has been once 
 varnished, mere soaking and washing in ordinary water 
 will not be sufficient to remove it completely from the glass. 
 The plates should be placed for some hours in a warm and 
 strong solution of Carbonate of Soda in rain-water till the 
 film is loosened and comes off easily. They may then be 
 
CLEANING GLASS PLATES. 411 
 
 washed in abundance of common water, wiped dry and 
 laid aside for future use. Even after this treatment the 
 surface of the glass will often not assume the bright 
 polished appearance of a new plate, and will be tarnished 
 with spots of iron, etc., insoluble in alkalies. In that 
 case, after all the soda has been w r ashed off, an additional 
 soaking in dilute Nitric Acid or better still, in the waste 
 acids used in making Pyroxyline will be necessary before 
 use ; the glasses should be finished with old Collodion. 
 
 General remarks. Amateurs often waste time in need- 
 lessly rubbing and breathing upon glasses, the only result 
 of which is to render them warm and electric, so that the 
 Collodion will not adhere. Particles of saliva are also pro- 
 jected upon the glass, and being imperfectly removed by 
 the leather, produce transparent markings, like the barb 
 of a feather. The condition of the cloths is commonly too 
 much neglected ; they are thrown down, and continually 
 handled with warm hands. Hence no sooner is one mark 
 rubbed off, than another makes its appearance. Cleaning 
 should be done with a quick and light movement, such as 
 the cabinet-maker uses in polishing ; and in the case of 
 plates of a large size, a screw-press should be used to give 
 a firm purchase. If possible, leave the glass for a short time 
 after cleaning it, and before pouring the Collodion. 
 
 The Darlc, Room. When practicable, it is better that 
 the dark room should be large and well ventilated, other- 
 wise the continued breathing of the pent-up fumes of 
 Ether, etc,, by the operator, would eventually be injurious 
 to health. It should be provided with a water-tap and 
 sink, or where these are not available, a large pouring-jug 
 and basin for the reception of wastes and washings. Pro- 
 vide also two or three cloths, one for wiping the fingers 
 after sensitizing, and the other after developing. All white 
 light must be carefully excluded ; it will therefore be 
 necessary to cover over the window with three or four folds 
 of orange-coloured calico, or have it glazed with a deep 
 orange non-actinic glass, which can now be readily procured 
 from the Photographic dealers. Either of these modes 
 
412 PRACTICE OF PHOTOGRAPHY. 
 
 will admit sufficient visual light to enable the operator to 
 work with ease and certainty. When a window is not 
 available, a candle or gas-flame placed in a convenient posi- 
 tion in the room, and shielded by yellow calico or glass, 
 will answer the purpose. The following description of a 
 Photographic Lantern is extracted from Professor Dela- 
 motte's pamphlet on the ' Oxymel Preservative Process : ' 
 
 " Take three millboards, about twelve inches high by nine 
 inches wide, and cut out a large piece, say eight inches by 
 five inches, from the centre of each. Lay all three boards 
 close together, side by side, and having dabbed on paste 
 or glue, cover them entirely with two thicknesses of yellow 
 calico. Ornament them in any way you please on the 
 other side. When set up so as to form a triangle, with 
 the calico side inwards, a candle may be placed in the 
 centre, and when not in use, the lantern will fold up and 
 make a good portfolio for paper, etc." 
 
 The operator should accustom himself to work in a mi- 
 nimum of light, for the orange-coloured glasses and calico 
 are never perfectly non-actinic, and will admit sufficient 
 light to act feebly after a time on a highly-sensitive iodized 
 plate. The fumes of Ammonia and of many other sub- 
 stances are also frequent causes of failure ; everything, 
 therefore, that is not absolutely necessary for the purpose 
 of exciting, developing, fixing, and washing the collo- 
 
COATING THE PLATE. 
 
 413 
 
 dionized plates, should be excluded from the dark room. 
 It is important also to observe the rule, " A place for every- 
 thing, and everything in its place;" so that if occasion 
 should arise, the operator may be able to lay his hand on 
 what he wants in the dark. 
 
 Coating the Plate. Before coating the plate with Collo- 
 dion, see that the fluid is perfectly clear and transparent, 
 and that all particles have settled to the bottom ; also that 
 the neck of the bottle is free from hard and dry crusts, 
 which, if allowed to remain, would partially dissolve and 
 produce striae upon the film. Before taking the first pic- 
 ture, pour out a few drops of Collodion on the floor ; this 
 will remove stray particles, and clear the way for the re- 
 mainder. 
 
 A useful piece of apparatus for decanting Collodion con- 
 taining undissolved Pyroxyline is represented in the fol- 
 lowing woodcut. 
 
 The Collodion, having been previously iodized, is allowed 
 to settle down in this bottle ; then by gently blowing at 
 the point of the shorter tube, the small glass siphon is 
 filled, and the fluid drawn off more closely than could be 
 done by simply pouring from one bottle to another. An 
 apparatus for the same purpose, but contrived so as to fit 
 any bottle, may be made by passing the two tubes through 
 a larger cork than that above shown, and then pressing 
 down the lower surface of this cork upon the open neck of 
 the bottle. 
 
414 PRACTICE OF PHOTOGKAPHY. 
 
 When the Collodion is properly cleared from sediment, 
 take a glass plate, previously cleaned, and wipe it gently 
 with a broad camel's-hair brush, in order to remove any 
 particles of dust which may have subsequently collected. 
 If it be a plate of moderate size, it may be held by the 
 corners in a horizontal position, between the forefinger 
 and the thumb of the left hand. The Collodion is to be 
 poured on steadily until a circular pool is formed, extend- 
 ing nearly to the edges of the glass. 
 
 By a slight inclination of the plate the fluid is made to 
 flow towards the corner marked 1, in the above diagram, 
 until it nearly touches the thumb by which the glass is 
 held ; from corner 1 it is passed to corner 2, held by the 
 forefinger ; from 2 to 3, and lastly, the excess poured 
 back into the bottle from the corner marked No. 4. It 
 is next to be held over the bottle for a moment, until it 
 nearly ceases to drip, and then, by raising the thumb a 
 little, the direction of the plate is changed, so as to give a 
 rocking movement, which makes the diagonal lines coalesce 
 and produces a smooth surface. The operation of coating 
 a plate with Collodion must not be done hurriedly, and 
 nothing is required to ensure success but steadiness of 
 hand and a sufficiency of the fluid poured in the first in- 
 stance upon the plate. 
 
 In coating larger plates, the pneumatic holder, which 
 fixes itself by suction, will be found the most simple and 
 useful. 
 
 The presence of white light in the room does no injury 
 until the plate has been placed in the Bath, and therefore 
 the door may remain open during the operation of coating, 
 
SENSITIZING THE FILM. 415 
 
 since it would be difficult to apply the fluid evenly without 
 plenty of light. Draughts from an open door must, how- 
 ever, be avoided, as promoting too rapid evaporation, and 
 carrying along particles of dust which will adhere to the 
 plate. If the light of a candle, not shielded by yellow 
 glass or calico, is employed in coating, let the wick burn 
 long and smoky, to produce a yellow flame ; also be care- 
 ful not to bring the glass too near, and especially not to 
 hold it above the flame, or the vapour of the Ether will 
 fall and the whole will be in a blaze (p. 389). 
 
 The common error in coating glasses is to keep the eye 
 too much upon the neck of the bottle, and not sufliciently 
 upon the glass. The pool of Collodion is itself the guide 
 to the right position of the plate, and if it begins to flow 
 away to one corner, that corner is the lowest, and must be 
 raised a little. Apply the whole quantity of Collodion first 
 and incline the plate afterwards ; but never tilt and pour at 
 the same time. Do not be economical as regards quantity, 
 and if a little should flow away at the edges it will be of 
 no consequence. 
 
 Sensitizing the Film. Examine first the state of the 
 Bath solution. This liquid is most conveniently kept in 
 a glass trough mounted in a wooden box, with a loosely- 
 fitting cover. It is an advantage to leave the Bath always 
 in the trough, that it may settle down and clear itself after 
 the day's work is done ; but if the plan of keeping it in 
 a separate bottle be adopted, remember to wash out the 
 trough with water, and drain it before putting in the 
 Nitrate solution ; otherwise there will be dust and scum 
 upon the surface when it is again filled. Porcelain troughs 
 are sometimes ^used, but they should be glazed with extra 
 care, or the Nitrate of Silver will percolate through to the 
 exterior. 
 
 Before commencing operations bring the Bath out to 
 the light for an instant, and examine the surface. If 
 there be any greasy scum, remove it carefully with blot- 
 ting-paper. In using a new sample of Collodion, ascer- 
 tain that it does not leave a floating film of Iodide upon 
 
416 PEACTICE OF PHOTOGEAPHY. 
 
 the surface of the Nitrate in dipping, which often hap- 
 pens with over-iodized Collodion, deficient in setting 
 qualities. 
 
 Lastly, arrange the Bath upon layers of clean blotting- 
 paper for draining, and all will be ready for immersion. 
 
 With regard to the time which ought to elapse between 
 coating and dipping, observe the following : After expo- 
 sing a layer of Collodion to the air for a short time, the 
 greater part of the Ether evaporates, and leaves the Py- 
 roxyline in a state in which it is neither wet nor dry, but 
 receives the impression of the finger without adhering to 
 it. This is termed setting, and when it takes place, the 
 time has come for submitting the plate to the action of 
 the Bath. 
 
 If the film be lowered into the Nitrate before it has set, 
 the effect is the same as that produced by adding Water 
 to Collodion. The Pyroxyline is precipitated in part, and 
 consequently there are cracks at the lower edge of the 
 plate ; the film is also easily loosened, and is very liable to 
 wrinkle at the bottom part. On the other hand, if it be 
 allowed to become too dry, the Iodide of Silver does not 
 form perfectly at the top of the plate, and the film, on 
 being washed and brought out to the light, exhibits a pe- 
 culiar iridescent appearance, and is paler in some parts 
 than in others ; pictures taken on a film of this kind are 
 always weak, and lie very much upon the surface of the 
 Collodion. 
 
 No rule can be given as to the exact time which ought 
 to elapse before dipping : it varies with the temperature 
 of the atmosphere, and with the proportions of Ether and 
 of Pyroxyline ; thin Collodion, containing but little Alco- 
 hol, requiring to be immersed more speedily. On the other 
 hand, Collodion containing Bromide sets more slowly than 
 one containing only an Iodide, although the proportions of 
 Ether and Alcohol in each should be the same. Twenty 
 seconds in ordinary temperatures, or ten seconds in hot 
 weather, will be found an average time. The correct mode 
 of proceeding is to make a trial with the first plate coated, 
 
COATING THE PLATE. 417 
 
 and not to dip it until you find by the touch that the 
 lower corner has become firm and solid. 
 
 When the plate is ready, rest it upon the glass dipper, 
 Collodion side uppermost, and lower it into the solution 
 by a slow and steady movement ; if any pause be made, 
 a horizontal line corresponding to the surface of the liquid 
 will be formed. Then place the cover upon the vertical 
 trough and darken the room, if this has not already been 
 done. 
 
 Whilst the plate remains in the Bath, the operator may 
 occupy himself in wiping out the corners and lower edge 
 of the slide with blotting-paper ; next, in measuring a 
 sufficiency of the developing solution, and in focussing the 
 object. Then dip the fingers an instant in water, to cleanse 
 them from any traces of Pyrogallic Acid, and return to the 
 Bath. 
 
 The light ought to fall upon the plate at a sharp angle 
 whilst it is lifted from the Bath, that the operator may see 
 the greasy lines upon the surface. An immersion of from 
 two to three minutes will usually be sufficient to remove 
 them in warm weather ; but when the temperature falls, 
 the time must be prolonged. Something will depend upon 
 the number of times the plate is moved up and down, and 
 many adopt the plan of leaving it in five minutes, and then 
 taking it out without any movement. When the liquid 
 flows off in a uniform sheet, the decomposition may be 
 considered to be perfect. The principal impediment in 
 this part of the process lies in the difficulty with which 
 Ether and Water mix together, which causes the Collo- 
 dion surface on its first immersion to appear oily and co- 
 vered with streaks. By gentle motion the Ether is washed 
 away, and a smooth layer obtained. 
 
 When the Bath is in perfect order no harm will result 
 from leaving the plate in it for a quarter of an hour or 
 longer, but sometimes the film will eventually become 
 covered with little projecting crystals, each of which 
 produces a white spot ; and in the case of extreme heat 
 of the weather, or of a bath contaminated with organic 
 
 2E 
 
418 PRACTICE OF PHOTOGEAPHY. 
 
 matter, the longer the film is left in, the greater the chance 
 of fogging. The experience of most practical operators is 
 not in favour of leaving plates in the Bath longer than is 
 absolutely necessary for complete sensitizing. 
 
 The plate is next removed from the dipper, and held ver- 
 tically over the trough for a minute to drain, and then 
 placed with its lower edge resting on a piece of clean blot- 
 ting-paper for about half a minute, and the back wiped 
 with a sponge or piece of blotting-paper. It may now be 
 placed in the dark slide, and is ready for the Camera. 
 
 Success will much depend upon the careful draining of 
 the film, and hence the importance of having the develop- 
 ing room thoroughly darkened, in order to do this efficiently 
 without fear of producing fogging. The older the Bath 
 the greater the care required in draining, because the pre- 
 sence of the Ether gives a tendency to run into narrow 
 lines and channels upon the surface of the film. Especially 
 avoid altering the direction of the plate after draining and 
 previously to putting it into the slide. In the case of 
 Stereoscopic glasses, the direction of the film is often so 
 changed, and consequently there are lines on the lower 
 picture, from the wave of the Nitrate altering its course. 
 Transparent curtain markings at the bottom of the film, 
 and a blurring of the image downwards, are also defects 
 likely to follow imperfect draining. 
 
 The Writer objects to another change of direction which 
 is sometimes given to the plate, viz. a complete turn on 
 taking it out of the Bath, so as to bring the top edge 
 downwards on the blotting-paper. This is a fruitful source 
 of stains, since it is very difficult to drain away the excess 
 of Nitrate under such circumstances. 
 
 The amateur is advised not to proceed to the process of 
 taking pictures in the Camera until, by a little practice, 
 he is able to produce a perfect film, which is uniform in 
 every part, and will bear inspection when washed and 
 brought out to the light. 
 
 Such a film may be expected to present the following 
 appearance : Smooth and uniform, both by reflected and 
 
EXPOSUEE OF THE PLATE, 419 
 
 transmitted light ; free from crapy lines, such as would 
 be caused by not rocking the plate properly whilst coating 
 it, and from opaque dots, due to small particles in suspen- 
 sion ; also from marginal thickenings of the Iodide, which 
 may result from the neck of the bottle not having been 
 drawn along the lower edge of the glass whilst the Collo- 
 dion was setting. 
 
 The evidences of a too rapid immersion in the Bath are 
 sought for on the side of the plate from which the Collo- 
 dion was poured off. This part remains wet longer than 
 the other, and always suffers the most. On the other 
 hand, the upper part of the plate must be examined for 
 the pale colour or the zigzag markings characteristic of a 
 film which had become too dry before immersion, since the 
 Collodion is thinner there than at any other point. 
 
 EXPOSURE OF THE PLATE IN THE CAMERA. 
 
 The proper time for exposure will depend upon the kind 
 of picture to be taken, and directions on that head must 
 be sought in the Second and following Sections of this 
 Chapter. Although perfect cleanliness is very essential 
 to success, yet the Camera and apparatus ought not to be 
 wiped out immediately before use, as the effect of doing 
 so would be to stir up dust : the cloth employed may also 
 be slightly damped with advantage. Examine the Lens 
 and see that the glasses are free from cobweb markings, 
 and from condensed moisture. 
 
 After the plate has been taken out of the Bath, it should 
 be exposed and developed with all convenient dispatch ; 
 otherwise the film will become partially dry, the develop- 
 ing solutions will not flow easily, and the Negative will be 
 weak and metallic. There are often white spots upon 
 plates long kept, from small particles of dust, etc., becom- 
 ing centres of chemical action. 
 
 In carrying the slide to and from the Camera, and indeed 
 on all occasions when the excited plate is inside, care must 
 be taken to keep the same edge of the glass downwards, 
 to prevent the accumulated wave of Nitrate on the lower 
 
420 PRACTICE OF PHOTOGRAPHY. 
 
 edge from flowing back over the plate. The collodionized 
 side of the glass also should never be turned upwards, 
 otherwise any dust or dirt which may have settled on the 
 slide will fall on its surface, causing opaque or transparent 
 spots. 
 
 DEVELOPMENT OF A NEGATIVE IODIZED FILM WITH PYRO- 
 GALLIC ACID. 
 
 The details of this part of the process differ so much in 
 the case of Positive and Negative pictures, that it will 
 perhaps be better to confine ourselves to the latter, and to 
 defer the consideration of the development of glass Posi- 
 tives until the next Section. 
 
 On taking the plate out of the slide there will be some 
 accumulation of Bath solution at the lower edge, which, 
 if the glass be held horizontally for a short time, will be 
 seen to work its way along the surface of the film ; and 
 the effect will be to produce a transparent mark on apply- 
 ing the Pyrogallic Acid. To prevent this annoyance, the 
 film may be placed vertically on absorbent blotting-paper, 
 and the liquid drawn off by suction. 
 
 The Pyrogallic Acid solution having been previously 
 measured out (about three drachms for a plate 5x4, one 
 ounce for a 9 X 7, and twelve drachms for a plate of 
 10 X 8), hold the glass in the hand in the same manner as 
 when coating it with Collodion, and throw the liquid on 
 evenly. It must not be poured from a height on to one 
 single spot, or the whole of the Nitrate of Silver would be 
 displaced from that spot, leaving a transparent mark of 
 non-reduction. The lip of the developing glass should be 
 depressed until it nearly touches the film, so as to apply 
 the liquid close to the edge of the plate, or to some part of 
 the image which is of minor importance. Always pour 
 out the whole of the measured quantity of developer, and 
 then move the plate so as to keep it waving backwards 
 and forwards upon the film. It will be quite necessary to 
 have the source of light so arranged that it falls nicely 
 upon the Collodion surface, because the operator has to 
 
DEVELOPMENT WITH PYROGALLIC ACID. 421 
 
 lilt the glass until the developer runs into each of the 
 corners, and to keep his eye upon the wave as it moves 
 backwards and forwards, in order to prevent it from flowing 
 off at the edges of the plate, or trickling down his sleeve. 
 
 The freedom with which the developer flows depends 
 much upon the Collodion, and the length of time it was 
 held before dipping, since if the film retain too much Ether, 
 it will repel aqueous liquids. It also depends upon the 
 Bath in a measure, for when this solution is newly mixed, 
 and comparatively free from Alcohol, it will be sometimes 
 necessary to give the plate a sudden jerk, to prevent the 
 Pyrogallic Acid solution from stopping short of the edge. 
 
 Notice whether the developer remains bright and clear, 
 or becomes turbid before the image is fully brought out. 
 In the latter case the heat is too high, or the Chemicals are 
 in fault, and the Chapter on " Failures " must be consulted. 
 
 Watch the course of the development for about thirty 
 or forty seconds, and especially the behaviour of those parts 
 of the film which at first remain yellow, but at length 
 begin to evolve fine details corresponding to the shadows. 
 Keep the Pyrogallic Acid on the plate until nothing more 
 appears in these yellow parts, and then pour it off into a 
 measure, and hold the plate for an instant against the light, 
 so as to look through it and see the appearance of the Ne- 
 gative image. 
 
 At this point a failure very commonly occurs, from the 
 operator being too tardy in his movements, and allowing 
 the developer to run into oily lines, so as to produce diago- 
 nal black streaks upon the film. 
 
 Having decided that the image is sufficiently intense, 
 proceed to the fixing ; but if it appears too weak (and al- 
 lowance must be made for the lowering action of the Hypo- 
 sulphite), carry the development into the second stage. To 
 do this, pour away the discoloured Pyrogallic Acid, and 
 wash out the glass rapidly with water ; then measure out 
 a fresh portion, add to each drachm about five drops of a 
 20-grain solution of Nitrate of Silver, and apply it a second 
 time to the film, until the desired intensity is obtained. 
 
422 PRACTICE OF PHOTOGRAPHY. 
 
 It is not recommended to add a few drops of the Nitrate 
 Bath to the Pyrogallic Acid, but to employ a weaker so- 
 lution of Nitrate of Silver, not saturated with Iodide. This 
 Nitrate of Silver must also be very pure, or the developer 
 will soon discolour and the shadows become stained. 
 
 As time ought to be economized as much as possible, 
 observe the following: If the image is well out before 
 the Pyrogallic Acid becomes turbid, it will be unneces- 
 sary to throw away the first portion, but any amount of 
 strengthening may be obtained by pouring the same deve- 
 loper back into the measure, and adding the Nitrate of 
 Silver. Neither is it necessary to wash out the developing 
 glass with water when it contains Pyrogallic Acid simply 
 discoloured, but anything approaching turbidity will sug- 
 gest an immediate change of solution, and a washing with 
 water, both of the film and the glass. The addition of 
 fresh Nitrate of Silver to a developer in such a state, is 
 improper : the Silver then falls irregularly, and the chance 
 of staining is increased. 
 
 With regard to the quantity of Nitrate of Silver to be 
 added to the developer, nothing positive can be stated. At 
 least three times as much will be required in cold as in hot 
 weather, and something will depend upon the condition of 
 the developer itself. The rapidity of discoloration or tur- 
 bidity in the Pyrogallic Acid is the proper guide to follow ; 
 and the two liquids may be mixed even in equal bulks if 
 it is found that the mixture remains 
 clear for twenty or thirty seconds, 
 so as to give time for pouring it 
 over the image. As a rule, how- 
 ever, the less the quantity of Silver 
 in proportion to the Pyrogallic Acid 
 the better. 
 
 The accompanying diagram shows 
 the best form of dropping-bottle for 
 Nitrate of Silver that the Writer 
 has seen. 
 The bottle being partially full of solution, incline it 
 
APPEARANCE OF THE NEGATIVE. 423 
 
 gently, when the air will pass in at the wide mouth, and 
 the liquid will drop out at the fine-drawn end. In travel- 
 ling, both orifices are closed with corks, 
 
 Appearance of the Negative image developed with Pyro- 
 gallic Acid, as a guide to the exposure to Light. An 
 under-exposed plate developes slowly. By continuing the 
 action of the Pyrogallic Acid, the high lights become very 
 black, but the shadows are usually defective, nothing but 
 the yellow Iodide being seen on those portions of the plate. 
 After treatment with the Hyposulphite, the picture shows 
 well as a Positive, but by transmitted light all the minor 
 details are invisible ; the image is black and white, without 
 any half-tone. 
 
 An over-exposed Negative developes rapidly at first, but 
 soon appears to blacken slightly at every part of the plate. 
 After the fixing is completed, the image is indistinct, and 
 very little can be seen by reflected light but a uniform grey 
 surface of metallic Silver. By transmitted light the plates 
 often show a red or brown colour, and the image is faint 
 and flat. The half-shadows having acted so long as nearly 
 to overtake the lights, there is a want of proper contrast; 
 hence the over-exposed plate is the exact converse of the 
 under-exposed, where the contrast between lights and sha- 
 dows is too well marked, from the absence of intermediate 
 tints. 
 
 A Negative which has received the proper amount of 
 exposure, usually possesses the following characters after 
 fixing: The image is partially but not fully seen by 
 reflected light. In the case of a portrait, any dark por- 
 tions of drapery show well as a Positive, but the features 
 of the sitter are scarcely to be discerned. By transmitted 
 light the figure is bright, and appears to stand out from 
 the glass ; the dark shadows are clear, without any misty 
 deposit of metallic Silver ; the high lights black almost to 
 complete opacity. 
 
 Collodion with strong organic reactions (p. 270) gives a 
 Negative which often shows upon the surface of the glass 
 nearly as well as by transmission. And if the light be at 
 
424 PEACTICE OF PHOTOGRAPHY. 
 
 all good, the yellow creamy appearance which Photogra- 
 phers term bloom ought to be seen upon the image. Its 
 absence in the case of a simply iodized Collodion, not con- 
 taining Bromide, usually implies that some of the chemi- 
 cals are out of order. 
 
 Development with Sulphate of Iron. This developer 
 should not be used with a simply iodized Collodion, unless 
 under the exceptional conditions referred to at p. 281, or 
 when the light is feeble and the picture under-exposed. 
 If the Collodion and Bath are in the most sensitive state, 
 the action of Sulphate of Iron is too energetic, and is apt 
 to give rise to stains and markings of all sorts. Its mode 
 of application is described in the next paragraph. 
 
 Development of a Negative Bromo-iodized film with Sul- 
 phate of Iron. This is the ordinary developer for a Collo- 
 dion containing Bromide, inasmuch as it acts in a more 
 gradual and uniform manner than when the Collodion con- 
 tains only an Iodide. The solution should be thrown on 
 the plate with a quick even sweep, and the plate should 
 be tilted and moved about so as to cause the developer 
 to flow round and round in an unbroken wave until the 
 whole of the details have appeared. It must then be 
 poured off and the plate washed in a stream of water. 
 The development should not be continued after the solu- 
 tion becomes turbid, otherwise the shadows will be misty 
 from adherent particles of Silver, and the picture will 
 present the appearance of what is generally denominated 
 " fogging." 
 
 When well washed, the plate may be examined by the 
 transmitted yellow light of the dark room, in order to 
 judge of its density. It will rarely be found sufficiently in- 
 tense in the first instance ; a second development will there- 
 fore be necessary. Mix in a clean glass four or five drops 
 of the redeveloping solution B. (p. 404), with a sufficient 
 portion of A. to cover the plate well, and continue the deve- 
 lopment by pouring on and off till the requisite intensity 
 has been attained. If the solution becomes much decom- 
 posed and turbid before that point is reached, wash both 
 
DEVELOPMENT WITH SULPHATE OF IEOX. 425 
 
 the plate and the developing glass, and repeat the opera- 
 tion. It will be observed that the redevelopment with 
 Pyrogallic Acid and Nitrate of Silver, brings out no new 
 details in the Negative, but only strengthens those pro- 
 duced by the Sulphate of Iron. 
 
 Another method of intensifying Negatives developed by 
 Sulphate of Iron is by means of a second, and, when neces- 
 sary, of a third, application of a fresh portion of the same 
 developer. This plan has been adopted with the highest 
 success by some Photographers. The plate should be well 
 washed, and freed from all trace of the first developer ; it 
 is then to be flooded with a 20-grain solution of Nitrate of 
 Silver, and the Sulphate of Iron applied as before. Very 
 delicate manipulation is required to ensure complete suc- 
 cess, irregular development being invariably the result of 
 the least clumsiness on the part of the operator. 
 
 This method of redevelopment is extremely useful when 
 the plate has been under-exposed in the Camera, for not 
 only does it intensify the picture produced by the first 
 development, but brings out fresh details at each applica- 
 tion. In cold weather also it is of advantage in shortening 
 the time of exposure, for if the plate be dipped in warm 
 water for a minute or two before the second developer is 
 poured on : an extraordinary increase of vigour and detail 
 is thereby obtained, which otherwise would have been 
 wanting. 
 
 Fixing and varnishing the Negative. Wash the film 
 gently with water, and pour the solution of Hyposulphite 
 or Cyanide on and off until the whole of the Iodide has 
 been cleared away. A thorough washing after the Hypo- 
 sulphite will be essential, and it is from neglecting this 
 point that beginners frequently find their plates covered 
 with crystals on drying. Even supposing the washing to 
 have been sufficient to prevent the occurrence of crystal- 
 lization, there may yet be a blueuess and loss of intensity 
 at the lower part of the film, visible after drying, from 
 traces of the fixing agent remaining in the Collodion, and 
 when such is the case, the varnish does not always adhere 
 
426 PRACTICE OF PHOTOGRAPHY. 
 
 properly, but spirts out when applied to the heated plate, 
 leaving minute holes. These Negatives generally absorb 
 atmospheric moisture and remain continually damp, fading 
 away after a time from the edges towards the centre. 
 
 The proper mode of washing is as follows : If a tap is 
 at hand, turn it gently and allow the water to now in a 
 stream upon the centre, inclining the plate so that it runs 
 off from each of the corners in succession ; continue this 
 for two or three minutes, when the fixing agent may be 
 considered as removed. If running water cannot be had, 
 first wash off the excess of the fixing agent by water 
 poured from a jug, and then lay the plate film upwards 
 in a dish and leave it for three or four hours, giving se- 
 veral changes of water. When the Collodion appears to be 
 tender, and inclined to separate from the edges, see that the 
 liquid is made to flow aivay from the glass at that point, so 
 as to keep the film down and prevent the water from passing 
 beneath. This observation applies in a still more decided 
 manner to the Hyposulphite, because if the fixing solution 
 works beneath the film, there is a chance of its escaping 
 the washing water, and if left, the Negative will lose inten- 
 sity on drying. 
 
 After the Negative has been washed, stand it on clean 
 blotting-paper to dry ; otherwise dust will ascend the film 
 by capillary action, and give an appearance as if impuri- 
 ties had drained down from above. 
 
 It is important that the plate should be dried by arti- 
 ficial heat before varnishing, and the Negative will also 
 look more neat if a damp cloth be first run along the edge 
 with the finger and thumb, so as to scrape away the Collo- 
 dion and leave a clear margin of an eighth to a quarter of 
 an inch all round. After doing so, remove with the nail or 
 a camel's-hair brush any loose or detached particles of Col- 
 lodion, which otherwise might be washed on to the image 
 by the varnish and produce a spot. 
 
POETEAITUEE. 4-27 
 
 SECTION II. 
 Portraiture Positive and Negative. 
 
 We commence this Section by remarking upon the ad- 
 vantages, or the contrary, to be expected from the use of a 
 glass-house in Collodion Portraiture ; adding at the same 
 time a few hints on the mode of its construction. Those 
 who are accustomed to work beneath glass, are often at 
 fault on trying to take a picture in the open air, and ob- 
 tain under such circumstances a hard and unpleasing por- 
 trait, with exaggerated contrast of light and shade. It is, 
 without doubt, somewhat difficult to secure fine gradation 
 of tone when the object is brilliantly illuminated, on account 
 of the actinic power of the Lenses which are employed 
 in portraiture ; and therefore, unless the Chemicals are 
 prepared purposely, the subdued light of a glass studio 
 is likely to give the best effect. 
 
 Great diversity of opinion prevails as to the best mode 
 of constructing a glass-house. There are, however, some 
 principles generally agreed upon which may be here stated. 
 The glass-house should, when practicable, have its greatest 
 length in the direction of North and South, so that the sit- 
 ter may be placed facing the North. It should be glazed 
 with a pure colourless glass, as window-glass has usually 
 a green tint, and when exposed for many months to the 
 air becomes also more or less yellow, and absorbs the ac- 
 tinic rays. The glass should extend nearly to the ground. 
 This is not so necessary for half-length portraits as for the 
 carte de visite form, where equal illumination down to the 
 feet is required. A complete system of black and white 
 curtains must be provided both for the roof and sides, to 
 throw the light on that side of the sitter which seems to be 
 most desirable. The roof over the sitter and the sides, to 
 the distance of about three feet from the background, must 
 also be opaque, since a vertical light on the head is apt to 
 solarize the hair, so as to make it appear grey, and will also 
 
428 PEACTICE OF PHOTOGEAPHY. 
 
 give strong shadows underneath the eye. The end of the 
 room where the Camera is placed had better be darkened, 
 to prevent diffused light from falling on the lens, and also 
 to relieve the eye of the sitter from the painful expression 
 often caused by too much light falling on the retinae. 
 
 The background may be varied according to the taste 
 of the operator, but to produce a neutral tint, the colour 
 should be somewhat like that of common brown paper. 
 Unless, however, the surface is quite dead, the effect of the 
 background will depend upon the amount of light which 
 shines upon it, so that it will produce a different colour 
 in the open air from that given in the glass-house. 
 
 Proper ventilation is of the utmost importance, the heat 
 being excessive in the summer months. 
 
 The developing room is frequently made smaller than is 
 advisable, the consequence of which is, that it becomes al- 
 most unbearable in hot weather. It ought also to be more 
 generally known that the vapour of Ether, when continu- 
 ally inhaled, has an extremely depressing effect upon the 
 nervous system. 
 
 It may be observed that a horizontal light destroys the 
 shadows entirely, and is suitable in the case of deep sunken 
 features ; but a light falling obliquely, partly from above 
 and partly from the sides, and striking more upon one side 
 of the face than the other, is the best in the majority of 
 cases. 
 
 The following diagram and description of a glass-house 
 by Mr. Matheson* is given, as combining efficiency, con- 
 venience, and economy. It has been found to answer 
 better than the ordinary form, both in this country and 
 in India. 
 
 " The tallest end for the background, B, is square a 
 great advantage in taking groups while all the rest of 
 the room not actually wanted for light may be built of 
 any opaque material, to keep out the heat. The quantity 
 of glass surface need bear very little proportion to the size 
 
 * ' Practical Hints to Amateur Photographers.' London : James How, 
 2, Foster Lane. 
 
PORTEAITUEE IN GLASS-HOUSE. 
 
 429 
 
 of the room, as ten feet of glass from the sitter (or from 
 A to C) is sufficient, no matter how long the room may be. 
 If the room is sufficiently wide, there need be no side-lights 
 at all. About three or four feet from the background of 
 the under surface of glass, from A to B, may be blackened 
 or built of opaque material, as shown in the drawing. Sup- 
 posing the room to be twelve feet wide, the highest part of 
 the roof, A, should be about ten, sloping down for about 
 eight or ten feet from the sitter, or A, to the lowest part, 
 C, which may be a flat roof, leaded or tiles, or other 
 opaque material, and which need be no higher than con- 
 venient to walk under. The sloping part just over the 
 head of the sitter, A, B, may be hinged as a flap, to be 
 
 lifted up when the weather will admit, as the warm air 
 ascends to this the highest part. This makes an admirable 
 ventilator, even in this country, in summer. The back, 
 B, should be placed against a tall house, if possible, while 
 the rest should be placed so as to have no obstructions. 
 The interior of the room should be coloured a pale green 
 colour, without pattern of any kind, as a pattern would 
 distract the eye of the sitter. 
 
 " The blinds on the top lights, E, may be made to puU 
 
430 PRACTICE OF PHOTOGEAPHY. 
 
 down from A to C ; spring blinds are best, if good. The 
 side-lights, D, may be managed with curtains." 
 
 Directions for working in the open air. When a 
 glass-house is not at command, portraits may be taken in 
 the open air. Indeed, a glass-house is only necessary as 
 a protection from the wind, dust, and the inclemency of 
 the weather. By selecting a proper situation, and by a 
 judicious arrangement of screens, portraits with as much 
 roundness and delicacy of detail can be taken out of doors 
 as in. Mr. Shadbolt,* who has given much attention to 
 this subject, writes : 
 
 " If we can find a spot where an angle is formed by two 
 walls of a building the walls standing respectively north- 
 east and north-west we have in fact nearly all we re- 
 quire ; for by making a light wooden frame of about eight 
 feet square, and covering it with stout unbleached calico- 
 sheeting, we form a roof or canopy that can be readily 
 fitted into the angle formed by the two walls, and that will 
 give the requisite shade over the heads of our subjects. 
 This square frame can also be readily raised or lowered at 
 pleasure by a little ingenuity ; and further, by posing our 
 subjects so as to look towards either the north-west or 
 north-east that is, making each wall alternately the back- 
 ground either side of our subject can be the most strongly 
 illuminated, and the walls may either be painted of a suit- 
 able colour for backgrounds, or covered temporarily with 
 such materials as we may select for the purpose. The as- 
 pect indicated is not only preferable on account of unifor- 
 mity of the light, but, as a rule, it will also be one most 
 sheltered from wind in the summer time, when amateurs 
 are most addicted to photographic pursuits. 
 
 " With regard to the point of holding the canopy in po- 
 sition, we have only to attach cords to each corner and 
 gather them to a central point above it, so as to convert 
 the frame into a kind of scale. To the point of conver- 
 gence of the cords from the angles we attach a somewhat 
 stouter single cord, and this, drawn though a small pulley, 
 
 * ' British Journal of Photography,' vol. x. p. 224, 
 
PORTRAITURE IN THE OPEN AIR. 431 
 
 or even a ring attached to the wall above, will enable us 
 readily to raise it. The cord can be run through a second 
 ring or pulley to enable us to draw it conveniently with- 
 out interfering with the canopy itself; and the weight of 
 the same may be counterpoised by a piece of iron, or even 
 by a large stone. Lastly, a couple of long nails partly 
 driven into the wall at the proper height may be so ar- 
 ranged as to prevent the frame from assuming other than 
 a horizontal position. It is evident that with the aspect 
 above indicated it will never happen that the sitter is ex- 
 posed to the direct rays of the sun ; but when from va- 
 riation in the aspect this inconvenience does arise, it is 
 so difficult a matter to apply an appropriate screen out of 
 doors, that it will generally be better to modify the ar- 
 rangement altogether." 
 
 When the desired conditions of a high building cannot 
 be obtained, portraits may be taken in an open courtyard 
 or garden by attending to the following precautions : Al- 
 ways back up the sitter, if possible, by a dark ground, not 
 less than seven feet square, to prevent diffused light from 
 entering the Lens. To make an effective screen, construct 
 in the first instance a strong deal framework, consisting of 
 a central portion, with two lateral ones hinged to it. Cover 
 the whole with oil-cloth, painted of a suitable colour, and 
 as free from gloss as possible. By bringing one of the two 
 sides forward, you are enabled to throw a shadow on the 
 face of the sitter, and by turning the other side back the 
 whole arrangement may be fixed and rendered firm. Pro- 
 bably a screen will be required above to cut off the vertical 
 light ; and if so, the apparatus may be still further strength- 
 ened by constructing this part of a deal framework made to 
 bolt down against the side which projects forward. 
 
 When everything is complete, stand in front of the Lens 
 and look in through the glasses from below upwards, to 
 see if there be any reflection from the sky, or other bright 
 objects. Unless the Lens be placed very near indeed to the 
 sitter, it is almost certain that there will be something of 
 this kind, and if so a foggy picture will result. A large 
 
432 PRACTICE OF PHOTOGRAPHY. 
 
 funnel of cardboard, lined with black velvet, should in such 
 a case be carried out to a distance of about a foot in front of 
 the Lens, and the mouth of this funnel should be contracted 
 as far as possible until it begins to cut off the corners of 
 the field. This will render the image very clear, and will 
 wonderfully improve the quality of the picture. Before 
 exposing the plate, throw a black silk handkerchief over 
 the end of the funnel, which will be found sufficient to ex- 
 clude the light. 
 
 It is on account of the size of the glasses in a portrait 
 combination, and the fact of their presenting so large a 
 reflecting surface, that the above precautions are required. 
 If small diaphragms were used, the sitter might be ar- 
 ranged with the open sky for a background without pro- 
 ducing fogging. 
 
 Sow to test the correctness of the Camera and Lens. 
 This point must always be attended to in purchasing a new 
 portrait apparatus ; from neglecting it many fail in obtain- 
 ing as good pictures as the Lens might yield. 
 
 First ascertain that the prepared sensitive plate falls 
 precisely in the plane occupied by the ground glass. Sus- 
 pend a newspaper or a small engraving at a distance of 
 about three feet from the Camera, and focus the letters oc- 
 cupying the centre of the field ; then insert the slide, with 
 a square of ground glass substituted for the ordinary plate 
 (the rough surface of the glass looking inwards), and ob- 
 serve if the letters are still distinct. In place of the ground 
 glass, a transparent plate smeared with glazier's putty may 
 be used, but the former is preferable. 
 
 Another method, more simple than the last, is to gauge 
 the slide with a strip of card. Begin by inserting a glass 
 plate in the slide, and then lay it down upon the table side 
 by side with the focussing glass. Raise the door of the 
 slide, and having applied a flat rule, measure the exact dis- 
 tance between the edge of the rule and the surface of the 
 glass plate. Now place the rule upon a corresponding por- 
 tion of the focussing glass, when it will be at once evident 
 whether the two planes coincide. 
 
TESTING THE LENS. 433 
 
 If the result of these trials seems to show that the 
 Camera is good, proceed to test the correctness of the 
 Lens. 
 
 Take a Photograph with the full aperture of the portrait 
 Lens, the central letters of the newspaper being carefully 
 focussed as before. Then examine at what part of the 
 plate the greatest amount of distinctness of outline is to 
 be found. It will sometimes happen, that whereas the 
 exact centre was focussed visually, the letters on a spot 
 midway between the centre and edge are the sharpest in 
 the Photograph. In that case the chemical focus is longer 
 than the other, and by a distance equivalent to, bat in the 
 opposite direction of, the space through which the Lens has 
 to be moved, in order to define those particular letters 
 sharply to the eye. 
 
 When the chemical focus is the shorter of the two, the 
 letters in the Photograph are indistinct at every portion of 
 the plate ; the experiment must therefore be repeated, the 
 Lens being shifted an eighth of an inch or less. Indeed, it 
 will be proper to take many Photographs at minute varia- 
 tions of focal distance before the capabilities of the Lens 
 will be fully shown. 
 
 The object of finding the point at which the sharpest 
 image is obtained will also be assisted by placing several 
 small figures in different planes, and focussing those in the 
 centre. This being done, if the more distant figures come 
 out distinctly in the Photograph, the chemical focus is 
 longer than the visual, vice versa when the nearest ones 
 are most sharply defined. 
 
 Mr. Shadbolt has given very lucid instructions for test- 
 ing a portrait Lens, which the Writer copies by his per- 
 mission : 
 
 " Put the Lens in its place and test for coincidence of 
 chemical and visual foci as follows : take any paper with 
 a number of fine lines drawn upon it, for instance, that 
 upon which patterns are printed for ladies' Berlin wool 
 work, or else some small but clear printed matter, and 
 paste either upon stout cards, with a large distinctive letter 
 
 2F 
 
434 PEACTICE OF PHOTOGRAPHY. 
 
 or number upon each one : about ten or a dozen will be 
 enough. Place a slip of deal at an angle of 45, and with 
 a thin saw make a series of cuts perpendicular to the ho- 
 rizon, one behind another, at about an inch distance apart, 
 a card with the printed matter or lines being slipped into 
 each. All will then be visible at once from a single point, 
 just in the same way as would be the case with the faces 
 of a number of people standing one behind another upon 
 a flight of stairs. Now carefully focus for the fine lines 
 or small letters on one of the cards in the centre of the" 
 group, taking special note at the same time of its distinct- 
 tive letter or number, and having done so, expose a sensi- 
 tive plate and develope it. If the card upon which the 
 focus was taken be most distinct and sharp, the actinic 
 correction is perfect : if, however, the best definition be 
 upon one nearer to the Lens than that focussed, it is 
 under-corrected ; if further from it, over-corrected." 
 
 The above directions are abundantly sufficient for testing 
 the Camera and Lens. White light, however, occasionally 
 enters the instrument between the two sliding portions of 
 the body, and this will be seen on throwing a black cloth 
 over the back and looking into the interior. Or some 
 scattered rays may pass in through the chink when the 
 door of the slide is raised. It is therefore customary 
 always to throw a large black cotton velvet focussing cloth 
 over the Camera, and to introduce the arm beneath this 
 cloth whilst opening the slide. 
 
 Portrait Lenses magnify those parts of the object which 
 are nearest to the glass, and hence the complaints of en- 
 largement of the hands and lower portions of the figure. 
 In sitting portraits, this may, to some extent, be obviated 
 by pointing the Lens a little downwards ; but when taking 
 a full-length portrait, such as a carte-de-visite, the Ca- 
 mera must be placed quite horizontal, and pointing at 
 right angles to the object. If the whole figure is not in 
 correct focus, which will seldom be the case if the object 
 is near, the Camera must be placed further away, or the 
 Lens stopped down with a Waterhouse diaphragm, which 
 
POETEAIT LENSES. 435 
 
 both flattens the field and gives greater depth of focus, 
 but lengthens the time of exposure. 
 
 Swinging backs to portrait Cameras are useful in focus- 
 sing near and distant parts of the object at one time, sup- 
 posing the figure to be arranged in a diagonal position with 
 reference to the Lens ; but they are very expensive, and 
 add much to the bulk of the instrument. 
 
 The Lens must be chosen according to the size of picture 
 which is required, always bearing in mind that the smallest 
 Lenses are, as a rule, the most perfect, both as regards ra- 
 pidity of action, sharpness of image, depth of focus from, 
 before backwards, and freedom from distortion. Those of 
 a larger size are not only very much more expensive, but 
 are more liable to vary in quality ; and it often happens 
 that the operator fails in getting a really serviceable in- 
 strument, even after having paid the full price. 
 
 A double combination (about If inches in diameter) of 
 4| inches focal length, measured from the back Lens, 
 which is equivalent to about 6 inches real focus, is adapted 
 for portraits of the stereoscopic size. A Lens (about 2^ 
 inches in diameter) of 6 inches apparent focus, equal to 
 nearly 8 inches equivalent or true focus, will cover a 
 plate 5x4, and is much used for card portraits. A Lens 
 (about 3^ inches in diameter) nearly 10 inches equivalent 
 focus, is the one commonly used for portraits on plates 
 6x4f. 
 
 Lenses are constructed for taking both portraits and 
 views, the back combination being removed, and the front 
 glass turned round and inserted in its place when the Lens is 
 employed for the latter purpose. Such arrangements have 
 of % late been much improved, and when economy is an ob- 
 ject, the amateur will find it worth his while to make trial 
 of an instrument of this kind. 
 
 In taking groups, Lenses of rather long focus are the 
 best, since they admit of being placed at a greater distance 
 from the object, and therefore give more depth of focus. 
 Lenses of the same diameter may usually be bought of two 
 different focal lengths. Arrange the group pyramidally, 
 
436 PRACTICE OF PHOTOGRAPHY. 
 
 the tallest towards the centre, and stop down the Lens 
 until everything is rendered equally sharp. 
 
 In cleaning the glasses of a Lens, which will sometimes 
 be necessary before taking a picture, use wash-leather, 
 as being less likely to scratch the glass than silk ; and ob- 
 serve in replacing the Lenses that the front one, which 
 consists of two Lenses cemented together, must be placed 
 with its convex side towards the sitter. The back combi- 
 nation also consists of two Lenses, not cemented together, 
 but separated by a brass ring. Put in the double-convex 
 Lens first into the cell, then the ring, and lastly, the other 
 glass, with its concave side towards the brass ring. 
 
 DIRECT POSITIVES ON GLASS. 
 
 The remarks on the arrangement of apparatus which 
 have been made in the preceding pages refer both to Posi- 
 tives and Negatives, but in the few remaining observations 
 the two processes must be considered separately. 
 
 In the Second Division of this Work the theory of 
 Collodion Positives has been explained, and in the Chapter 
 immediately preceding the present, formulae for the Collo- 
 dion, Bath, and developers, are given at length. The first 
 Section of this Chapter describes those manipulations which 
 are common both to Positives and Negatives , but omits 
 such details of development, etc., as are peculiar to the 
 former. 
 
 In developing a glass Positive, the solution of Sulphate 
 of Iron should be scattered evenly over the film, and in 
 some quantity, so as to wash off a portion of Nitrate of 
 Silver into the sink. In the case of Negatives it is an 
 object to save every trace of Nitrate, and precipitate it 
 upon the image, in order to increase the density ; but with 
 Positives there is a fear of getting an excess of intensity, 
 and if the Collodion film be tolerably creamy, it will always 
 retain more than enough of the Nitrate to give a Positive 
 free from the green or blue marks characteristic of deficient 
 reduction. White stains at the margins of the plate are 
 
DIRECT POSITIVES ON GLASS. 437 
 
 produced in great measure by the developer coming into 
 contact with an excess of Silver. 
 
 Tilt the developing fluid backwards and forwards upon 
 the film for about thirty seconds, or a minute, until the 
 darkest shadows begin to be visible. Supposing the film 
 to be well lighted with yellow light, there will be no diffi- 
 culty whatever in ascertaining when the image is fully 
 out ; the developer must then be poured off immediately, 
 and the plate washed with water, or the image will be 
 rendered too dense by fresh precipitation of Silver, and 
 the middle tints in the face will be lost in consequence. 
 The blacks are rarely quite pure when the plate has been 
 too much developed, but show either little spangles of Sil- 
 ver, or a general clouding. 
 
 When the Sulphate of Iron is washed off before the 
 proper development is complete, the whole image looks 
 very thin and weak, with a blue or greenish tint : the de- 
 tails in the shadows may perhaps be visible, if the plate was 
 fully exposed, but usually they are more or less defective. 
 The difficulty with the beginner is to distinguish the effects 
 produced by wrong exposure in the Camera, from others 
 due to faulty development. A little care, however, will 
 usually enable him to do so. If no details appear in the 
 dark shadows, the image is either under-exposed or under- 
 developed, but in the latter case the lights would be very 
 poor and thin, as before described ; whereas in the former 
 they would probably be vigorous, especially if the operator 
 kept the Sulphate of Iron for a long time upon the film 
 with a hope of bringing out the shadows. 
 
 The finished picture in another case, the reverse of the 
 last, may appear altogether too white and flat, without any 
 deep shadows, every portion of the plate showing more or 
 less of a deposit of Silver. In this instance it is either 
 over-exposed or over-developed, but probably the former, 
 and particularly so if the lights are not of a very good 
 colour, but appear grey and feeble, and if the whole image 
 shoivs very fairly as a Negative when held against the 
 light. You may always calculate upon diminishing ex- 
 
438 PEACTICE OF PHOTOGRAPHY. 
 
 cessive density of the face and light parts by over-expo- 
 sing in the Camera, but over-development makes them 
 quite opaque, so that the black varnish, when placed be- 
 neath, does not show through. On the other hand, a pro- 
 longed exposure produces a far greater effect in clouding 
 over the shadows, and giving a grey colour to black dra- 
 pery, than any amount of over-development. 
 
 When all the Chemicals are in good working order, the 
 finest Positives are obtained by giving a rather short ex- 
 posure in the Camera, because the lights are then of a pure 
 white, and the shadows transparent. With a Collodion 
 not suitable for Positives, the image would be too intense 
 when developed after a short exposure. 
 
 Wash the plate with Water, to remove the whole of the 
 Iron, before putting on the Cyanide, or a blue deposit 
 will often be formed. The Cyanide is used over and over 
 again until exhausted. 
 
 In mounting glass Positives it has become a common 
 practice to cover the back of the glass with black varnish, 
 and to mount the picture with the Collodion side unvar- 
 nished and towards the eye. The image is necessarily re- 
 versed, but the whites are very bright, and the shadows 
 sufficiently clear if the Collodion be of the transparent 
 kind. The only question is, whether such a plan can be 
 considered a safe one as regards permanency, since it has 
 been ascertained that unvarnished films of Collodion will 
 occasionally decompose from the edges to the centre, unless 
 carefully washed and well protected from moisture (p. 324). 
 
 NEGATIVE PORTE AITUEE. 
 
 In the Seventh Chapter of Part II. the general mode 
 of producing Negatives has been explained ; and the for- 
 mulae for solutions and details of manipulation, with the 
 necessary apparatus and mode of arrangement of sitter, 
 have been described at pages 398, 407, arid 427. It only 
 remains therefore to give advice on the proper state of 
 the Chemicals. 
 
 Collodion. The circumstances under which Negative 
 
 
NEGATIVE POETEAITUEE. 439 
 
 portraits are taken vary so much, that no uniformly appli- 
 cable directions can be given. The normal Collodion of 
 page 377, iodized with the Potassium Salt, and developed 
 with Pyrogallic Acid, has been extensively used for large 
 portraits. Some have spoken of it as being highly sensi- 
 tive, but somewhat wanting in contrast and intensity; 
 others, as possessing full intensity, but lacking extreme 
 sensitiveness. One cause of the discrepancy arises from 
 the fact that plain Collodion kept for some time gradually 
 acquires the property of liberating free Iodine from the 
 Potassium Salt, and hence is more insensitive and gives 
 violent contrasts. The state of the Bath, too, has much 
 to do with the question, see p. 281. When the Collodion 
 colours very slowly after iodizing, it will be likely, cceteris 
 paribus, to yield a soft Negative, and if so, Pyrogallic Acid 
 will be indicated. A rapid liberation of Iodine, on the 
 other hand, will show that some decomposition has taken 
 place, and if so, Sulphate of Iron will probably be required 
 to keep down the intensity of the Negative. 
 
 For a Pyrogallic developer, the same Collodion admits 
 of being iodized with Iodide of Cadmium, and is indeed 
 highly suitable for that purpose, except at high tempera- 
 tures, when it sets too rapidly on the glass and also becomes 
 surface-dry too speedily. To counteract this disposition of 
 the normal Collodion with a simple Cadmium lodizer, it 
 was deemed important to produce a Pyroxyline of a some- 
 what less horny character, and one of which the film would 
 remain moist sufficiently long to invite the progress of the 
 developer instead of repelling it. It was also equally im- 
 portant to hit the exact point as regards contrast, because 
 if this quality is in excess, the portrait will be wanting in 
 harmony and gradation ; and it must be borne in mind that 
 in the case of a Cadmium Collodion the operator loses that 
 control over the intensity which he ordinarily gains by 
 keeping the Collodion after iodizing. Another point was 
 that the half-tint of the background should be perfectly 
 even, and free from mottling or hair-like lines. How far 
 success has been obtained in each of the above particulars 
 
440 PRACTICE OF PHOTOGRAPHY. 
 
 the operator himself will decide when he has prepared the 
 Collodion by the accompanying formula,* and given it an 
 extended trial. 
 
 p 
 
 * Formula for a Portrait Cadmium Collodion. In the following directions, 
 the reader is supposed to have studied the theory of the manufacture of 
 Collodion in Part II., and also its practical details in the First Chapter of 
 Part III. .Take of 
 
 Oil of Vitriol, sp. gr. 1'845 ... 2 fluid ounces. 
 
 Pure Nitric Acid 1'45 1 fluid ounce. 
 
 Water 5 fluid drachms. 
 
 Best Cotton 40 grains. 
 
 The Nitric Acid should be colourless and free from Chlorine, the Cotton 
 purified by dilute Potash and dried. Time of immersion in the acids, ten 
 minutes. Temperature, 150 Fahr. 
 
 Th rule for bringing the acid mixture to the proper strength is to continue 
 the addition of Water until the Pyroxyline is only partially soluble in Alco- 
 holized Ether, and leaves a whitish sediment in the Collodion. It would not 
 be safe to continue the addition of water until the acids dissolved a con- 
 siderable portion of the Pyroxyline, since in this formula the tendency to 
 solution in the acids is not so great as when the proportions of Sulphuric 
 Acid and Nitric Acid are as 3 to 1 instead of 2 to 1 : hence acids weak enough 
 to dissolve thirty per cent, of the product, would yield a Pyroxyline leaving 
 too much sediment in the Collodion. The Pyroxyline ought to be decidedly 
 disintegrated in appearance, but less so than that for the Normal Collodion 
 of page 377 ; if 40 grains of Cotton are put in, 50 grains of Pyroxyline should 
 be taken out. 
 
 It will be far easier to work with the aoids stronger than here indicated, 
 but the Writer does not advise it, because he finds that when the Pyroxy. 
 line leaves the acids in a tough and unbroken condition, the quality of the re- 
 sulting Collodion, as regards fluidity and perfect smoothness of the portrait 
 background, is injured. There is, it is true, a more marked intensity of image 
 and less liability to spots, but this is more than counter-balanced by a woolli- 
 ness of the background, and a glutinous state of the Collodion as it nears 
 the bottom of the bottle. On the other hand, it must not be attempted to use 
 the acids weaker than here indicated, because in that case the loss of Collo- 
 dion from sediment would be considerable, and not only so, but the image 
 would be deficient in intensity, and would exhibit a blurring of the lines, or 
 " halation," due to irregular deposit of Silver in the development. 
 To prepare the Collodion, take of 
 
 Ether of -725 4 fluid drachms. 
 
 Alcohol of -805 4 fluid drachms. 
 
 Iodide of Cadmium 4| grains. 
 
 Pyroxyliue 5 grains. 
 
 The whole of the Alcohol is used in the so-called absolute state, because 
 the Collodion, being more porous the the normal Collodion of p. 377, and 
 setting less rapidly, is not improved, but rather the contrary, by the presence 
 of water. The sedimentary layer of fluid in the Collodion ought not to mea- 
 sure more than ^-th of the total bulk. Allow to stand for a fortnight before 
 
NEGATIVE POETEAIT COLLODION. 441 
 
 The Writer's own opinion as regards the excellencies 
 and defects of this Collodion is as follows : It does not 
 produce an intense image, but rather the contrary, and 
 hence certain precautions must be taken. In the first 
 place, the operator must obtain the best quality of Nitrate 
 of Silver, since the slightest trace of the impurity men- 
 tioned at p. 252 would produce indistinctness of image and 
 blurring or halation. In a rather dull light, a trace of 
 Acetate in the Bath will be an improvement, and in any 
 case the solution should be nearly neutral, since excess of 
 Acetic Acid lowers the intensity. Saturate the Bath as 
 fully as possible with Iodide of Silver, or there may be 
 defects, in the shape of transparent markings. The state 
 of the developer must also be attended to when this Cad- 
 mium Collodion gives too little intensity ; the proportion 
 of Pyrogallic Acid may be increased to 1^ grains, and that 
 of the Acetic Acid diminished to 10 minims. Half a 
 drachm of spirit may also be required to make the deve- 
 loper flow, since, on account of the great strength of the 
 Alcohol in the Collodion, there is still some tendency to 
 repel liquids when the Nitrate Bath is newly prepared, 
 and nearly free from Alcohol. Lastly, let a stock of the 
 Collodion be always kept on hand, since age increases the 
 intensity ; and mix the new with the old as circumstances 
 dictate. With a long-focus Lens the image will usually 
 require a little strengthening by extra development, and 
 for this purpose the solutions described at page 343 answer 
 admirably. 
 
 The above-mentioned Cadmium Collodion, although pro- 
 ducing a rather feeble image, is remarkable for its sensi- 
 tiveness. The want of intensity may probably be one 
 cause of its great rapidity, since organic reactions of Py- 
 roxyline appear incompatible with excessive sensitiveness. 
 Not only is the Negative obtained by a short exposure 
 
 decanting, as the fluid will then have time to clear itself, and the sensitive- 
 ness will be greater than at first. 
 
 If it be desired to impart to this Collodion the property of changing slightly 
 by long keeping, and of giving greater intensity, about a quarter of a grain 
 of Iodide of Magnesium may be added to each ounce. 
 
442 PRACTICE OF PHOTOGRAPHY. 
 
 with this Collodion, but it is remarkable for its roundness 
 or stereoscopic effect. On viewing the image, it seems to 
 stand out from the glass, and to be raised above the sur- 
 face. An intense or organic Collodion could not give such 
 an effect, because the exposure required to impress the 
 shadows would be so long that the lights would be flat- 
 tened. Every ray must make a distinct and simultaneous 
 impression upon the plate, to constitute a good portrait 
 Collodion. If the Chemicals are in that perfect order that 
 the Writer is now supposing, the whole of the image will 
 develope even after an insufficient exposure : an inferior 
 sample of Collodion gives no detail in the shadows when 
 under-exposed, but this Cadmium Collodion will exhibit 
 under such circumstances an image perfect in itself, al- 
 though too faint to admit of strengthening. 
 
 Spots and fogging are often complained of with the 
 Collodion now under discussion. The only remedy is 
 great care and cleanliness, since the exquisite sensitive- 
 ness of the Collodion implies a corresponding delicacy. 
 Specks of dust, moisture on the glass, and impurity of 
 Chemicals, are ruinous to the working of the Collodion, 
 and the operator, unless well armed, will be unable to 
 succeed. 
 
 Experience has shown that for large portraits a Bromo- 
 iodized Collodion developed with Sulphate of Iron, and 
 strengthened by the formula given at p. 404, is at least not 
 inferior in its results to those produced by the simply 
 iodized Collodion developed with Pyrogallic Acid, and for 
 small portraits it is superior. This mode also gives great 
 certainty, the delicate balancing of the chemicals appear- 
 ing not to be of so much importance as with an iodized 
 Collodion. The normal Collodion, bromo-iodized by the 
 formula (p. 402), may be depended on as working in a satis- 
 factory manner when the Bath is slightly acid and free 
 from excess of organic matter. It is very uniform in sen- 
 sitiveness, free from all tendency to spots and fogging, and 
 produces harmonious Negatives. The amount of acid re- 
 quired in the Bath, in order to obtain the highest sensi- 
 
CARTE-DE-VISITE PORTRAITS. ' 443 
 
 tiveness, will depend in a great measure, 1st, on the depth 
 of colour in the Collodion arising from free Iodine ; and, 
 2ndly, on the temperature. In the first case, if the Collo- 
 dion has become very red through decomposition of the 
 iodizer, a neutral Bath may be used ; but each succeeding 
 plate will be Jess sensitive than the preceding one, in con- 
 sequence of the liberation of Nitric Acid by the chemical 
 changes going on in the Nitrate solution. In this state it 
 gives rise to much uncertainty. The normal Collodion, 
 bromo-iodized as directed, should not assume a deep-red 
 colour till after several months' keeping. A predisposition 
 in that direction indicates that there is something wrong 
 with the Ether or Pyroxyline. 
 
 As to temperature, it will be found that when the ther- 
 mometer stands high, more acid will be required in the 
 Bath than in the cold winter months ; but no rule can be 
 laid down to suit all circumstances. The indications given 
 by the action of the developer on the plate will be the 
 safest guide. 
 
 Card or Carte-de-Visite Portraits. For this class of 
 pictures, which have recently come much into vogue, a 
 Bromo-iodized Collodion is superior to a simply iodized, 
 as giving greater softness and less violent contrasts. The 
 development should in the first instance be conducted with 
 Iron, and the Negative redeveloped as directed at p. 424. 
 Some operators have succeeded perfectly by dispensing 
 with the redevelopment, and adopting one or other of the 
 intensifying processes mentioned at p. 285. The former 
 plan is easier and more certain when the Collodion and 
 Bath are of that kind which give sufficient intensity. Oc- 
 casionally, however, great difficulty is found in obtaining 
 a dense Negative by the ordinary redeveloping processes ; 
 in that case, recourse must be had to the latter. Iodized 
 Collodion, followed by Pyrogallic Acid as a preliminary 
 developer, is not recommended, for the reason that it fa- 
 vours hardness, unless extraordinary care be taken to have 
 the chemicals of the purest kind, and to keep them in that 
 condition. In conjunction with Formic Acid, however, it 
 
444 * PBACTICE OF PHOTOGRAPHY. 
 
 has recently been used for this class of work, and is stated 
 to have the effect of shortening considerably the time of 
 exposure, and also of producing fine soft Negatives. 
 
 SECTION III. 
 Landscape Photography with Wet Collodion. 
 
 The Apparatus. As this Work is addressed principally 
 to one about to commence the practice of Photography, 
 let the Writer advise him not to be induced, by the hope of 
 obtaining large pictures, to purchase a photographic appa- 
 ratus of a very considerable size ; he will find that plates 
 above ten inches by eight will involve an amount of bag- 
 gage which in travelling becomes a burden. 
 
 The expanding and folding Camera, made from well- 
 seasoned wood, is a useful instrument, and although many 
 prefer employing a rigid Camera with the Chemicals packed 
 in the interior, yet the experience of the majority is in 
 favour of the former. Cameras with cloth or accordion- 
 shaped bodies are very portable, and useful when made in 
 a proper manner so as not to crack ; but the back of the 
 Camera which carries the slide must be immovably fixed 
 by a screw, to prevent it from shaking in a high wind. 
 Cloth and leather contract dust, and require occasional 
 cleansing. 
 
 The Lens for plates of the size above-named may be an 
 actinic meniscus of fifteen inches focal length, with three 
 diaphragms mounted square, rather than in a cone, which 
 is liable to give reflection from the sides. It is recom- 
 mended to paint a narrow black circle of about an eighth 
 of an inch wide at the posterior surface of the Lens, to pre- 
 vent the edge of the glass from reflecting ; also to put in a 
 second diaphragm rather larger in size and made of black- 
 ened cardboard, behind the first, in order that a portion of 
 the oblique rays, which otherwise would strike off' from the 
 interior of the brass mounting, may be intercepted. These 
 precautions are not absolutely necessary, but they have been 
 
APPARATUS FOE LANDSCAPE PHOTOGRAPHY. 445 
 
 adopted by many of our most successful landscape Photo- 
 graphers. 
 
 A thin partition in the body of the Camera itself has 
 also been found to improve the brilliancy of the image : it 
 should be placed about midway between the Lens and the 
 ground-glass, and the aperture ought to be of such a size 
 as to admit only those rays which fall directly upon the 
 plate. A more simple plan, and one nearly as effective, is to 
 line the interior of the Camera with black cotton-velvet, 
 taking care to place the nap of the velvet towards the 
 Lens. Tripod stands should be firm and unyielding, and 
 it is recommended that they should not fold in the middle. 
 
 For architectural subjects the triplet form of Lens (p. 190) 
 is the best adapted, being free from distortion, and there- 
 fore refracting, each portion of the view in its correct re- 
 lative position. It should be well stopped down to give 
 greater depth of focus, and to improve the marginal de- 
 finition. With equal apertures, it is slower than the ordi- 
 nary view Lens ; the exposure must therefore be longer, 
 and in consequence the Negatives are generally feebler. 
 The Orthographic or Orthoscopic Lens has also been re- 
 commended for the same purpose ; but it possesses some 
 amount of distortion. It is only superior to the triplet 
 when, from the building being too near, it is necessary to 
 tilt the Camera a little to include its height. In this case, 
 the perpendiculars, which, when refracted by another 
 Lens, would seem on the focussing-screen to fall inwards, 
 are by the hour-glass distortion of the Orthoscopic Lens 
 thrown outwards at their extremities, and will thus ap- 
 pear straight. For general landscape views no form of 
 Lens yet introduced is equal to the single actinic meniscus 
 Lens. 
 
 The Nitrate Bath may be carried in a glass trough, with 
 a tight top of Caoutchouc, and mounted in a wooden case 
 for protection. Fasten a little sheet rubber round the 
 bottom of the dipper to prevent it from cracking the trough 
 if inadvertently dropped. Gutta-percha is not recom- 
 mended, for, although some troughs made from it will 
 
446 PRACTICE OF PHOTOGRAPHY. 
 
 work well for some time, they eventually give rise to 
 fogging and other annoyances. 
 
 The late Mr. Archer contrived an ingenious form of 
 Camera, in which the operations of sensitizing and deve- 
 loping the plate could be carried on in the interior of the 
 instrument. Very good pictures have been taken in this 
 Camera, and the Author has seen it used by the inventor 
 himself with perfect success. Nevertheless, he is disposed 
 to recommend some form of tent in preference, unless the 
 operator be possessed of considerable tact and dexterity. 
 
 There are several tents which have been well spoken of 
 for field work. The following, by Mr. Levi, is very simple, 
 and may be constructed at a small expense :* 
 
 The tent consists of: 1st. Two boards, each 30 inches in 
 length by 15 in breadth, and T ^- of an inch thickness, hinged to- 
 gether by strong hinges. Each of these boards is formed of a 
 frame of hard wood, such as nutwood, to give it strength, with 
 a panel of deal for lightness. In the outer corners of each are 
 holes 1 inch in diameter, a small iron plate with a corresponding 
 opening being screwed over each hole. Each of these plates is 
 2 inches square. 
 
 2ndly. Four poles of light strong wood Jf of an inch in diame- 
 ter. Each pole is formed of two parts fitted together by a brass 
 friction tube fixed on the top part of the pole. The bottom part, 
 41 inches in length, has an iron point at the end (fixed on by a 
 ring), to plant in the ground if necessary ; the top part, 35 inches 
 long, has a small iron point at the end, 1 inch long and i inch 
 diameter. The two poles fitted together have an entire length of 
 6 feet 1 inch. 
 
 3rdly. The covering of the tent, formed of two thicknesses of 
 ordinary calico, dyed yellow, and sewn in such a manner that the 
 seams do not correspond. This covering lias the form of a cube, 
 open on one side, that side continuing in the shape of a sack, 
 with a cord passing round the opening, so that the operator may 
 be able to tighten it round his waist. In the bottom part of this 
 cube are four openings, fitting exactly the poles, and corresponding 
 to the openings in the table ; these holes are bound with leather. 
 
 * ' London Photographic Journal,' vol. iv. p. 43. 
 
THE TENT. 447 
 
 On the opposite face of the cube are four smaller holes, equally 
 trimmed with leather, and fitting exactly on the four small pins 
 on the poles. 
 
 The tent is packed by putting the eight half-poles on the closed 
 table, surrounding them by the covering doubled up, and strap- 
 ping the whole together. To mount it, the table is inserted in 
 the covering, and opened (with the hinges downwards), so that 
 the holes correspond with the bottom holes in the covering ; the 
 bottom half-poles are passed through these openings, a small peg 
 fitted on them, preventing them from passing too far, and forming 
 a rest for the table when in place. Two persons taking hold of 
 the corners of the board and the poles at the same time, place 
 them in position, that is to say, stick the points in the ground 
 not vertically, but bringing them outwards slightly on the dia- 
 gonal of the table, which thus remains horizontal and perfectly 
 firm. The four top poles are then put inside the covering and 
 each one fitted on the corresponding half (the two parts of each 
 
 pole being numbered to correspond), the iron spike at the tops 
 being first inserted in the corresponding hole. 
 
 It only remains to put in the photographic implements ; when 
 the plate is ready to be dipped in the Nitrate Bath, or developed, 
 the operator puts the top part of his body through the opening, 
 the assistant ties the cord tightly round his waist, and the arms 
 and upper part of the body remain perfectly free, and in no ways 
 cramped for room. 
 
 It takes three minutes to mount, or to undo the tent; its 
 weight when strapped together is thirteen pounds, and the total 
 
448 PEACTICE OF PHOTOGEAPHY. 
 
 cost 24*. 6d., viz. 18-y. for purchase of calico, dyeing, and making 
 up the covering, 3*. for carpenter's work, and 3*. 6d. for smith's 
 work. 
 
 In working with this tent in a strong light, it would be 
 advisable to add one thickness of black calico inside the 
 yellow, leaving an aperture of a foot square for a window ; 
 and also to attach strong cords to the top of the poles, and 
 peg them down to the ground. 
 
 Glass plates may be carried in a grooved-box with a 
 brass handle affixed, and water for washing the plates, 
 either in a gutta-percha bottle, or in a tubular vessel made 
 of Mackintosh cloth, which is suspended by a ring to the 
 interior of the tent, and carries a flexible tap. 
 
 Thus there will be three principal packages to be carried 
 into the field 1st. The Camera folded in its leathern case. 
 2nd. The tent. 3rd. A box of leather strengthened at the 
 corners and lined with green baize, containing the Bath, 
 glass plates, and chemicals : a square wicker basket also 
 answers well for this purpose. In addition to this may be 
 mentioned the Tripod legs, and the Lens in a leather case 
 slung across the shoulder. 
 
 The full list of apparatus and chemicals required for a 
 few days' trip of landscape Photography, on plates 8x5, 
 is as follows : Camera, slide, and focussing screen. Large 
 focussing cloth with strings in front, and focussing glass. 
 Tripod-stand with screw bolt. Lens in case. Nitrate 
 Bath filled with solution. Half-pint solution extra in 
 glass bottle. Dipper in case. Carbonate of Soda, one 
 drachm. Glass plates in box, with any number extra. 
 Linen cloth and leather for polishing. Broad camel's-hair 
 brush. Pneumatic plate-holder. Twenty ounces Iodized 
 or Bromo-iodized Collodion in a stock bottle. Two four- 
 ounce stoppered bottles for decanting Collodion. Two or 
 three spare ditto, various sizes. Bottle of Varnish. Four 
 ounces of Ether and Alcohol, mixed. Ditto, of Spirits of 
 Wine. Two-ounce measure. Half a quire of blotting- 
 paper. Two hand-cloths, with sponge or tow for wiping. 
 Half an ounce of Pyrogallic Acid. Eight ounces of glacial 
 
APPARATUS AND CHEMICALS. 449 
 
 Acetic Acid. Half an ounce Citric Acid. Twelve-ounce 
 bottle for solution of Pyrogallic Acid. Sulphate of Iron, 
 four ounces. Twelve-ounce bottle for solution of ditto. 
 Cyanide of Potassium for cleaning fingers and fixing. 
 Two or three small developing glasses, holding each twelve 
 drachms. A dropping-bottle filled with thirty-grain solu- 
 tion of Nitrate of Silver. Hyposulphite of Soda one 
 pound. Six-ounce bottle of saturated solution of ditto. 
 Photographic tent, with table, poles, covering, and cords 
 for fastening to the ground. Gutta-percha tray, 12 X 10. 
 Water-bottle. Scales and weights. Glass measures. Glass 
 or porcelain funnel for filtering the Bath. A yard of yel- 
 low tammy or calico. Ditto of black calico. A ball of 
 string. Paper of pins. 
 
 The leathern case or basket which is actually carried 
 into the field contains only a portion of these Chemicals, 
 viz. two four-ounce bottles of Collodion ; developing solu- 
 tions ; dropping-bottle and small glasses ; Fixing solution ; 
 Ether and Alcohol for diluting, etc. : all of which should 
 be in turned wooden boxes, or well wrapped up to prevent 
 breakage. 
 
 Before starting, it will be well to ascertain that the 
 chemicals and apparatus are in working order, and in par- 
 ticular, that the tent does not let in the light. Prepare a 
 plate therefore in the tent, and rear it up on blotting- 
 paper immediately opposite to the yellow window, for at 
 least two minutes. It ought, after this, to stand the ac- 
 tion of the developer without any staining. This point 
 is of importance, because it will be required sometimes to 
 give the plates a long draining, and therefore there must 
 be security against the presence of even a trace of white 
 light. 
 
 Having proved the^tent to be tight, next see to the Ca- 
 mera, whether light gets in between the two portions of 
 the body ; if it does so, the focussing cloth must invariably 
 be thrown over the instrument during exposure. 
 
 Collodion, Nitrate Bath, and Developer for Landscapes. 
 It is the present almost universal practice of Photo- 
 
 2G 
 
450 PEACTICE OF PHOTOGRAPHY. 
 
 graphers to use a simply iodized Collodion for Pyrogallic 
 development, and a Bromo-iodized for a Sulphate of Iron 
 developer, and since the best conditions of Bath for these 
 are different, it will be well to treat each case separately. 
 
 Iodized Collodion. The normal Collodion of p. 377, 
 iodized either by the first or second formula (p. 398), or by 
 equal parts of both, may be safely recommended for gene- 
 ral landscape work. The film has a little too much ten- 
 dency to repel the developer when the Nitrate Bath is 
 entirely aqueous, but this disappears as Alcohol accumu- 
 lates in it. The Negatives are also rather too hard and 
 intense in sunny weather, especially so when the plain 
 Collodion has undergone partial decomposition by long 
 keeping before iodizing, or if the sample of Ether was 
 impure. These defects the operator may have to encounter, 
 as also a tendency to white spots if the Collodion is very 
 new and sensitive. Develope with Pyrogallic Acid and 
 Acetic Acid as a rule. 
 
 It will be well to iodize a portion of the normal Collo- 
 dion about three weeks or a month before use. When the 
 colour becomes of an amber-yellow, you will find that al- 
 though slow in action, it is admirably adapted for distant 
 views uniformly lighted. New Collodion would produce a 
 thin red Negative under such circumstances, and would 
 probably fog during development : age increases the con- 
 trast, and favours clearness and brilliancy. For architec- 
 tural subjects the old Collodion is also better as a rule, 
 since it gives greater decision and sharpness to the lines. 
 
 The rapidly-acting Cadmium Collodion of p. 398 is use- 
 ful on occasion for glens and other views lighted from 
 above, but it is liable to solarization and other defects al- 
 luded to at p. 399, if employed too frequently alone. Mixed 
 with the normal Collodion, it assists in bringing out the 
 shadows when the organic reactions due to decomposition 
 are excessive. Pyrogallic Acid with Citric Acid is a suit- 
 able developer for this Collodion in a strong light. 
 
 In some exceptional instances, a small proportion of 
 Bromide in the Collodion is useful in keeping back the 
 
COLLODION AND BATH FOR LANDSCAPES. 451 
 
 shadows when a highly-sensitive iodized Collodion gives 
 too flat a picture, at other times in remedying defects of 
 the Pyroxyline, such as excessive hardness and insensitive- 
 ness due to decomposition, or again in obviating over- 
 action of light, and thus harmonizing the sky with the 
 landscape beneath. A Nitrate Bath prepared from the 
 purest neutral Nitrate of Silver is the only one that will 
 impart the highest quickness and certainty of action to a 
 simply iodized Collodion. Great care must be taken not 
 to introduce organic matter by handling the plate with 
 warm hands while coating with Collodion, or by any other 
 means, otherwise the Bath will soon cease to work well, 
 but may still be used, when slightly acid, to excite Bromo- 
 iodized films. Develope with Pyrogallic Acid. 
 
 Bromo -iodized Collodion. The normal Collodion of 
 p. 377, and also Collodion prepared from the Pyroxyline of 
 p. 440, are well adapted for a Bromo-iodizer. The former 
 gives the greater intensity, the latter a higher sensitiveness. 
 Protosulphate of Iron, followed by Pyrogallic Acid as di- 
 rected at p. 424, is the proper developer ; but in hot weather, 
 and when working with large plates in a small confined 
 space, it will often be a matter of great difficulty to keep 
 the developer under sufficient control so as to avoid stains 
 and marks of irregularity. In that case it may be deemed 
 advisable to prolong the exposure, and develope with the 
 ordinary Pyrogallic solution only. With small plates, in 
 the hottest weather, this will hardly ever be necessary. 
 
 The Bath should be slightly acid with Nitric Acid only, 
 and made as directed at p. 403. 
 
 Manipulations, etc. It is of some importance, and more 
 particularly so in hot weather, and when the Collodion 
 and Bath are in the most sensitive working order, not to 
 leave the plate in the Nitrate solution longer than is abso- 
 lutely necessary. To promote quick sensitizing, move the 
 plate up and down a few times before removing it from 
 the Bath. The chances of fogging are much increased 
 by leaving it in the liquid after the Nitrate solution has 
 ceased to run in greasy lines on the iodized film. 
 
452 PRACTICE OF PHOTOGRAPHY. 
 
 Let the dark frame be well dusted before putting in the 
 plate. The tent should be pitched near to the Camera ; 
 but if that be inconvenient, and the plate has to be carried 
 some distance, such as five or ten minutes' walk from the 
 developing-chamber, fold the focussing-cloth round it, and 
 be particularly careful to keep the lower side of the frame 
 towards .the ground, not perpendicularly, but inclined in 
 such a way that any particles of dust, etc., which may be 
 detached from the inner part of the slide, cannot fall on 
 the sensitive surface. To avoid failures from specks of 
 dust, it is also important that the slide should not be vio- 
 lently shaken or tapped against the Camera, etc., while 
 the plate is inside. 
 
 Arrangement of the Camera. It is not the object of the 
 present Work to enter into the artistic principles involved 
 in Photography, but it may be remarked that it is in the 
 arrangement of the Camera that the distinction chiefly lies 
 . between the artist-photographer and the mere manipulator. 
 The former selects his point of view with the object of 
 obtaining the best effects of light and shade of which the 
 subject is capable ; and having once selected it, is ready to 
 sacrifice time and patience in waiting for the best moment 
 of exposure. The manipulator, on the other hand, places 
 his Camera anyhow so as to include the subject, without 
 reference to that which constitutes the charm of a picture, 
 viz. a nice balancing of light and shade. Supposing that 
 the photographer is an artist, and has selected the best 
 point of view, it will be necessary for him to observe 
 whether his Camera is carefully levelled, since it is only 
 with a Petzval Lens that he can venture to point the in- 
 strument upwards without destroying perpendiculars. If 
 perpendicular lines do not form a prominent part of the 
 view, the precaution of levelling need not be rigidly ad- 
 hered to. It will however be found a convenient plan to 
 rule the focussing screen with a number of vertical lines, 
 parallel to each other, by which the position and appear- 
 ance of the principal objects, as they will be depicted on 
 the Negative, may be judged of. As a rule, work with 
 
EXPOSUEE AND DEVELOPMENT FOE LANDSCAPES. 453 
 
 the light, that is to say, with the sun, or the principal light, 
 coming from behind the Camera towards the object. At 
 the same time, except in rare instances, the sun, the Ca- 
 mera, and the object, should not be in one line, a state of 
 things which would necessarily yield a picture without 
 contrast. The principal light ought to fall at an angle on 
 the object from either side of the Camera, so as to give 
 sufficient variety of light and shade. 
 
 Exposure. With respect to the time of exposure no 
 definite rule can be laid down, so much depending on the 
 lights, state of chemicals, and the aperture and focal length 
 of the Lens. For instance, in the case of a 4^-inch focus 
 lens, with ^--inch stop, the exposure may vary from less 
 than a second, for a well illuminated, bright object, to half 
 an hour for a feebly-lighted interior. Beyond the latter 
 time it is scarcely possible to keep a plate without injury, 
 unless the sensitive film, after removal from the Bath, is 
 covered with honey, or with a solution of some deliques- 
 cent inert salt. This plan, however, is not recommended, 
 for besides giving rise to much uncertainty, by diluting 
 the free Nitrate on the surface of the plate, it detracts 
 from its sensitiveness. Some kinds of Collodion will keep 
 surface-wet under long exposures better than others ; and, 
 as a rule, Collodion iodized with the alkaline and earthy 
 iodizers, such as Potassium, Ammonium, or Magnesium, 
 are the best in this respect. The intelligent operator will 
 soon be able, by watching the action of the developer, to 
 distinguish between the effects of under- and over-expo- 
 sure (see p. 423), and to acquire that experience which is 
 the only safe guide. 
 
 Developing, etc. Before removing the plate from the 
 slide for development, see that all the solutions are near 
 at hand, and ready to be applied. If the development 
 is to be carried out with Pyrogallic Acid only, few so- 
 lutions will be required, and the small space in the tent 
 need not jbe encumbered with unnecessary bottles. The 
 Bath should be carefully covered over to protect it from 
 splashes of the developer, and for the same reason the 
 
454 PRACTICE OF PHOTOGEAPHT. 
 
 Collodion bottles should be put away in a quiet corner, as 
 far off as possible from the spot where the development is 
 to be performed. The only other vessels required inside 
 are the Pyrogallic solution, developing-glass, dropping- 
 bottle containing Nitrate of Silver, and a jug of water. 
 Having applied the Pyrogallic Acid to the film, keep it 
 waving backwards and forwards, and observe its behaviour. 
 If it turns muddy, and the image comes out rapidly, this 
 may be due to the great heat of the tent, and it can 
 scarcely be hoped that the picture will be free from stains 
 after fixing. Proceed then to bridle the reduction by the 
 use of a stronger acid, as advised at p. 280. 
 
 Sometimes, when the plate has been long kept, the de- 
 veloper will flow over its surface with difficulty, and give 
 rise to stains and irregularities. A drachm of Alcohol, 
 added to each ounce of the developing solution, in most 
 cases corrects this. Another plan has been recommended, 
 to redip the plate in the Nitrate Bath after it has been 
 taken from the slide. Unfortunately, this often gives rise 
 to fogging, and has, moreover, the disadvantage of carry- 
 ing impurities into the Bath, contracted while the plate 
 was in the frame. 
 
 When the development is conducted with Sulphate of 
 Iron, the Negative has seldom sufficient intensity in the 
 first instance. The Iron should then be washed off, and 
 the development continued with Pyrogallic Acid, as di- 
 rected at p. 424, or with a fresh portion of Sulphate of 
 Iron after the plate has been covered with weak Nitrate 
 of Silver (see p. 425). 
 
 The developer must be washed off inside the tent, and 
 the plate may then be fixed outside (but not in the direct 
 rays of the sun), either with a 15-grain solution of Cya- 
 nide of Potassium or a saturated solution of Hyposulphite 
 of Soda. It has then to be washed in abundance of water, 
 and put aside in a convenient place to dry, or at once 
 stowed away in the plate-box. 
 
 In out-of-door Photography, where a plentiful supply of 
 water cannot be had, it is not necessary to complete the 
 
COPYING ENGRAVINGS AND PAINTINGS. 455 
 
 washing of tlie plate at the moment ; but in that case it 
 ought not to be fixed with Cyanide of Potassium, which, 
 if allowed to remain on the plate, even in very dilute pro- 
 portions, will weaken the half-tones. The plate may be 
 slightly washed, and immediately stowed away in a close 
 box, so as to preserve it in a moist state. The washing 
 may be finished afterwards, but if the plates have be- 
 come dry before the second application of the water, it 
 will be a wise precaution to pass a camel's-hair brush, 
 dipped in spirit-varnish, round the edge of the film, to 
 about inch inwards, to prevent its being detached from 
 the glass. 
 
 This division of the Chapter would not be complete with- 
 out some remarks on the substitutes for distilled water 
 when it cannot be obtained ; but as this subject is fully 
 discussed in the Vocabulary, Part I., the reader may there 
 seek the required information. 
 
 SECTION IV. 
 
 How to Copy Engravings, Oil Paintings, etc. 
 
 In this branch of Photography we have to consider in 
 the first place whether the copy is required to be of a re- 
 duced size, or of the same dimensions as the original, be- 
 cause in the latter case the operator works with a less 
 amount of light, and hence the mode of proceeding must be 
 different. The colour of the object should also be taken 
 into account, inasmuch as the yellow faded tint of old 
 prints and manuscripts is very detrimental to Photographic 
 action. 
 
 To Photograph a full-sized steel engraving on a plate 
 not larger than 7 X 6 or 5 X 4, is a very simple operation, 
 and no special directions will be needed. Hemove the en- 
 graving from its frame (the glass would cause irregular re- 
 flection) and suspend it vertically and in a reversed posi- 
 tion, in a good diffused light, placing a black cloth behind 
 it, if any bright reflecting surface be presented to the Lens. 
 Point at it a Camera mounted with a portrait Lens, and 
 
456 PRACTICE OF PHOTOGBAPHY. 
 
 stop it down with diaphragms until the field is quite flat, 
 and the whole is in focus.* Whilst the plate is being ex- 
 posed, the Lens should be shielded from diffused light by 
 a projecting funnel or otherwise. 
 
 Instead of a portrait Lens an ordinary view Lens may 
 be used, and as the field to be covered is small, a compa- 
 ratively large diaphragm will suffice. 
 
 The exact state of the Collodion and Nitrate Bath is not 
 very material ; but a sample of Collodion, somewhat dis- 
 coloured from long keeping, is to be preferred. Be careful 
 not to over-expose. Develope and fix in the usual way. 
 
 The copying of old prints, manuscripts, maps, and dia- 
 grams, on plates of 10x12, 15x12, 20x17, etc., is an 
 operation requiring greater care, and the different steps 
 may be described successively : 
 
 The Lens. The old form of view Lens possesses certain 
 advantages. The number of glasses and of reflecting sur- 
 faces is small, so that the light is not much intercepted in 
 passing through the Lens ; hence the image is bright, and 
 an intense Negative is more easily obtained : in addition 
 to this, the construction of the Lens is simple, so that the 
 optician has less difficulty in mastering it. But, on the 
 other hand, the distortion of the common view Lens is a 
 great defect, for it is more decided than that of any other 
 form, and especially so if the Lens be at all strained by 
 bringing it very near to the object. The best mode of ob- 
 viating this is to employ a Lens of a tolerably long focus, 
 and not to work it at the extreme limit. Thus if the in- 
 strument be of 20 inches focal length, and be constructed 
 to cover a plate of 15 inches by 12, it may be used for 
 copies of the full size up to 10 inches by 8. Beyond 
 that, the copy, although sufficiently correct for many pur- 
 poses, would not be likely to bear accurate measurement. 
 Nothing will be gained as regards distortion by diminish- 
 
 * If the Lens is not fitted with Waterhouse's diaphragms, a temporary one 
 may be made by cutting a circle of cardboard to fit into the mounting in 
 front of the anterior Lens, then cutting another smaller circle within it of 
 the size of the aperture required. Black the diaphragm with Indian ink on 
 both sides. 
 
LENSES FOE COPYING. 457 
 
 ing the size of the diaphragm, and the plan of pushing 
 the stop nearer to the glass, although rendering the lines 
 straighter, cannot often be adopted, because it makes the 
 field concave, and thus destroys the definition at the edges. 
 The difficulty of covering a large field perfectly flat is in- 
 deed severely felt with the common view Lens ; so much 
 so that the operator, after reducing the size of the stop as 
 far as practicable, is often compelled to remove it to a 
 greater distance from the Lens, even although the distor- 
 tion be thereby increased. 
 
 The portrait Lens of the ordinary form is remarkably 
 well adapted for giving straight lines, and has also the 
 merit of producing great sharpness with a large aperture. 
 The objection is, that the image is so concave as to be out 
 of focus at the edges. 
 
 The Orthographic or Orthoscopic Lens produces a flat 
 image on a large plate, and throws the light well up into 
 the corners of the picture. The distortion, however, is 
 greater than with the portrait Lens, although somewhat 
 less than that produced by the old form of view Lens. In 
 sharpness of definition it is not equal to the portrait Lens, 
 unless a small stop be used, and the Negatives are then 
 less easily intensified than when taken by the view Lens* 
 since there are more reflecting surfaces to weaken the 
 light. But where great accuracy is required, the best 
 Lens for copying purposes is unquestionably the Triplet. 
 It has a flat field, and when well stopped down gives fine 
 definition over a large angle. It is absolutely without dis- 
 tortion, so that with this instrument maps, large docu- 
 ments, etc., may be copied piecemeal, and the parts after- 
 wards placed in junction. The only objection to its use is 
 the great number of reflecting surfaces, which weakens 
 the light, and disposes to fogginess of Negatives, more es- 
 pecially when the Lens is not well shielded from diffused 
 light. Its flatness of field and freedom from "distortion, 
 however, more than counterbalance this defect ; hence its 
 almost universal adoption for copying maps, diagrams, etc., 
 where absolute correctness is required. 
 
458 PRACTICE OF PHOTOGRAPHY. 
 
 The Camera. In a copying Camera, lightness and por- 
 tability should be sacrificed to rigidity, since the least 
 vibration would produce a blurring of the lines of the 
 picture. For ordinary purposes it will be sufficient to 
 lengthen out a Camera of the common sort by a light 
 cone, made to slide on at the front ; but for copying maps 
 and plans on a large scale, a more substantial instrument 
 should be provided, and should be placed upon a strong 
 and heavy table standing on four legs. If the instrument 
 is to stand in the open air, it may be covered in and pro- 
 tected from wind and rain by a movable " Punch and 
 Judy box," with curtains at the front, sliding vertically 
 and laterally, so as to shield the Lens from diffused light ; 
 an additional curtain being placed at the back to facilitate 
 the darkening of the chamber around the Camera, that 
 the operator may see the image upon the greyed glass. 
 The importance of fixing the Camera very firmly cannot 
 be over-rated, and the difficulty of effecting this in a satis- 
 factory manner with a large Camera, is considerable. This 
 you will see if you make a pencil-mark upon the greyed 
 glass, and focus a fine line upon it : keeping the head quite 
 still, the line will be found to vibrate from side to side, 
 unless unusual precautions have been taken. 
 
 A flange should be fitted to the cone in front of the 
 Camera, of a size to take the largest Lens which it is pro- 
 posed to use, and for each smaller Lens an adapter screw- 
 ing into the flange. Sometimes Cameras are made with 
 the Lens working back into the body of the instrument 
 by a perpetual screw ; but this plan, although excellent as 
 regards protecting the Lens from diffused light, has the 
 disadvantage of throwing the front of the Camera so near 
 to the object, that sometimes after the focussing is com- 
 plete, the light is prevented from falling properly upon 
 the lower part of the picture. 
 
 Supposing it be desired to copy the picture of the full 
 size, fasten it to a black board, and bring the Camera up 
 to it, until the distance equals twice the focal length of the- 
 Lens. Now, with a piece of string measure the length 
 
CAMERA FOB COPYING. 459 
 
 from the centre of the front part of the Lens to each 
 corner of the picture ; if the four measurements accurately 
 correspond, the print is properly placed, in the middle. 
 Next focus carefully upon the ground glass, and with a 
 foot rule or a string ascertain that the image is of the right 
 size, and also that it is perfectly rectangular. To do this 
 compare the marginal lines with the edges of the greyed 
 focussing glass, when the eye will at once detect any error. 
 The picture will not be rectangular unless the Camera be 
 absolutely level, and pointed quite at right angles to the 
 picture ; if the vertical lines, for instance, fall inwards, the 
 front of the instrument must be a little depressed. 
 
 An inquiry is often made as to whether the sun should 
 shine upon the picture P This question can only be an- 
 swered conditionally. In the case of an old manuscript, 
 yellow and faded, of a sepia drawing, or of other subjects 
 of that kind, the stronger the light the better. These 
 colours are sometimes so non-actinic that they make but 
 little impression upon the sensitive film, and can only be 
 copied in direct sunlight. Other prints will show the tex- 
 ture of the paper when copied in sunlight, and if the white 
 parts are pure, they throw off quite enough light to pro- 
 duce an excellent Photograph in the shade, A horizontal 
 light is in all cases to be preferred, and vertical rays may 
 be cut off by a narrow curtain of blue calico. The opera- 
 tor may also regulate the light according to the rapidity 
 of development of his picture, which will vary with the 
 focal length and aperture of the Lens. 
 
 It is correct in theory to increase the amount of illumi- 
 nation by using a portrait Lens when the image appears 
 iin usually dark upon the focussing screen, as will often 
 be the case in copying old sepia drawings. For if a view 
 Lens or Triplet be employed, the amount of light may be 
 too small to make any impression upon the film when the 
 shadows of the picture are heavy ; whereas, by opening 
 up the aperture of the Lens and admitting all the rays 
 which can be collected, a certain amount of detail will be 
 obtained even in those parts of the picture which appear 
 most unpromising. 
 
460 PEACTICE OF PHOTOGRAPHY. 
 
 The Collodion for copying. Both the Collodion and the 
 Nitrate Bath must be in the best possible working order, 
 since they will be put to a severe trial in this process if the 
 copies are to be of the full size, and if the Lens has a very 
 considerable focal length. Collodion made from weak Ne- 
 gative Pyroxyline (p. 270) will not answer the purpose, the 
 image developing in the grey metallic form, and being diffi- 
 cult to strengthen. No amount of keeping after iodizing 
 will bring it into the proper state ; and when it becomes 
 much coloured from free Iodine, the amount of Nitric Acid 
 liberated in the Bath will tend further to weaken the image 
 and produce stains in developing. 
 
 The formula for a normal Collodion given at p. 277, has 
 been much used in copying, and this preparation undoubt- 
 edly possesses the merits of being very limpid and ad- 
 herent to the glass. The Writer, however, finds it to be 
 somewhat deficient as regards the required intensity, and 
 likewise to be too sensitive for copying black and white 
 objects without middle tints. Excess of sensitiveness is an 
 objection in copying maps : it is difficult to preserve the 
 transparency of lines and dots in such a case, seeing that 
 even black surfaces throw off a small amount of light, 
 and a very sensitive Collodion is affected thereby. Hence 
 it becomes convenient to prepare a second Collodion with 
 more contrast and clearness of developing than the last, 
 and to mix them together or use them separately as occa- 
 sion may require.* 
 
 Collodions for copying will usually improve by keeping 
 
 * Formula for a Copying Collodion. In the directions now given the reader 
 is presumed to be acquainted with the manufacture of Collodion as described 
 in Part II., and also in the First Chapter of the present Part. Take of 
 
 Oil of Vitriol, sp. gr. 1-845 2 fluid ounces. 
 
 Pure Nitric Acid, sp. gr. 1'45 1 fluid ounce. 
 
 Water 4 fluid ounce. 
 
 Best Cotton 30 grains. 
 
 Purify and dry the Cotton in the usual way. Temperature, 170 to 1?5 
 Fahr. Time of immersion, five minutes. 
 
 In the above formula the proportion of diluted Oil of Vitriol is reduced in 
 order to render the Pyroxyline less horny. The Water is also reduced in 
 
COLLODION AND BATH FOR COPYING. 461 
 
 after iodizing, but no definite rule can be stated. The 
 normal Collodion of p. 377 may usually be kept for a 
 month or six weeks in cool weather, or for three months 
 if iodized with mixed Potassium and Cadmium. The 
 Collodion described in the foot-note loses sensitiveness 
 more rapidly on account of the high temperature em- 
 ployed in making the Pyroxyline. 
 
 In place of simple iodides, the Collodion of the foot- 
 note may be iodized with Bromide and Iodide conjoined. 
 Many operators esteemed skilful in this branch of the art, 
 attribute their success to the use of the Bromo-iodide, 
 which imparts sharpness of outline, with freedom from 
 fogging and spots. A want of intensity constitutes its 
 only defect, to overcome which the picture may be hung 
 in a good light, and the plate be fixed with Cyanide, and 
 subsequently treated by Davanne's Iodine method, pre- 
 sently to be described (p. 463). In copying maps and 
 plans of full size by Lenses of four or six feet solar focus, 
 the light is, according to the Author's experience, too much 
 reduced to yield a dense Negative with any Collodion ex- 
 cepting the simply iodized. 
 
 The Bath Solution for Copying purposes must be pre- 
 pared with recrystallized Nitrate of Silver, perfectly free 
 from that impurity mentioned at p. 252, which weakens 
 the reduction, destroys the bloom, and produces spots and 
 stains. A trace of Acetate of Silver will be favourable 
 rather than otherwise ; about as much as would be pro- 
 duced by dropping a single drop of Ammonia into ten 
 ounces of the solution, and supersaturating with Acetic 
 
 quantity with a view of adding to the intensity, and the temperature is raised 
 twenty degrees to lessen sensitiveness and improve the flowing qualities. 
 
 The perfection of this Collodion will depend entirely upon the Pyroxyline 
 being exactly in the proper state ; it must be broken up and small in quantity 
 on leaving the acids, and inclined to be dusty on drying; otherwise more 
 water should be added, or the time of immersion increased from five to ten 
 minutes. In washing the Pyroxyline a final bath of very dilute Ammonia 
 may be used, since a trace of alkali appears to assist in giving intensity. 
 
 The Ether and Alcohol are to be in the usual proportions, and the iodizer 
 may consist of Iodide of Potassium simply, or may contain an extra grain of 
 Iodide of Cadmium to each ounce, if the film appears too opalescent. 
 
462 PRACTICE OF PHOTOGRAPHY. 
 
 Acid. The Bath will improve by moderate use if all 
 the chemicals are pure ; the film becoming less repellent 
 to the developer, giving rather more intensity, and yield- 
 ing cleaner Negatives. 
 
 Exposure of tlie Plates. No special directions will be 
 needed under this head, excepting when the object to be 
 copied presents merely a black and white surface. In such 
 a case the ordinary rules for exposure fail, the middle tints 
 being absent. When a photographic copy of a map or 
 diagram has received an insufficient amount of exposure in 
 the Camera, the parts of the Negative corresponding to 
 the black lines remain very clear, without showing any 
 disposition to fog under the action of the developer, but 
 the lights develope slowly, and are not sufficiently intense. 
 Slightly over-exposed Negatives, on the other hand, deve- 
 lope more quickly, and with a deeper shade of colour ; but 
 it is difficult to prevent the developer from clouding over 
 the lines, and thus rendering the design invisible by re- 
 flected light. 
 
 The time of exposure in copying pictures will vary much 
 with the focal length of the Lens and the sensitiveness of 
 the Collodion ; but in the case of a Triplet Lens, stopped 
 down, as much as five minutes may be given for an oil 
 painting, even in the direct rays of the sun. A black and 
 white object would require about a minute or a minute 
 and a half under similar circumstances. 
 
 Development. This is often a tedious operation, and 
 when the Negatives are taken in a weak light, from five 
 to ten minutes are commonly spent in getting up the in- 
 tensity to the proper point. In a strong light, Pyrogallic 
 Acid will be found the best developer as a rule, when the 
 Collodion contains Iodide only ; and it will be often advis- 
 able to reduce the quantity of retarding Acetic Acid to 
 one-half, otherwise the Silver will be thrown down in a 
 too crystalline and metallic form. If the diminution in 
 the amount of Acetic Acid makes the developer flow with 
 difficulty, remedy it by adding twenty minims of Alcohol 
 to the ounce. In a feeble light, whether the Collodion 
 
DEVELOPING- AND STRENGTHENING COPIES. 463 
 
 contains Bromide or not, Protosulphate of Iron as a prelimi- 
 nary developer may be employed with marked advantage. 
 
 In the case of engravings, paintings, sepia drawings, 
 etc., a careful process of pushing the development in the 
 ordinary way (p. 424) will usually suffice to give intensity, 
 if the Collodion be in the proper state. 
 
 Negatives of maps, plans, printed matter, etc., ought in- 
 variably to be under-developed 'in the first instance, so as 
 to obviate all chance of depositing Silver on the lines. The 
 Pyrogallic Acid must be poured oft' as soon as the image 
 appears in all its parts, and the Iodide of Silver cleared 
 away by weak Cyanide of Potassium, after which the re- 
 development may be carried on as before described. It 
 will occasionally happen in this process of intensifying 
 that an opaline deposit of Silver will be thrown down in a 
 very regular manner upon the clear parts of the Negative, 
 stopping abruptly at the points where the true image com- 
 mences. When this happens, a stronger acid may be used 
 in the intensifying developer, viz. Citric Acid instead of 
 Acetic Acid, and for this purpose nothing answers better 
 than the admirable developer described under " Russell's 
 Tannin Process." 
 
 Artificial modes of strengthening Negatives. These will 
 be required in all cases where the actinic intensity of the 
 light falls below a certain point. In making a full-sized Ne- 
 gative of a Collodion Positive on glass, by means of a Por- 
 trait Lens stopped down, the method of Davanne will often 
 be found sufficient. Throw a crystal of Iodine into Spirits 
 of Wine until the liquid becomes very dark and opaque, 
 then add a few drops of the solution to distilled water, 
 enough to render the water of an amber-yellow. Too much 
 Iodine causes a blackish turbidity in the water, but this 
 is instantly removed by a little Spirit. After fixing and 
 washing the picture, pour on the aqueous Iodine solution 
 for about a minute, then wash under a tap, and expose for 
 an instant to diffused daylight. Carry the plate back again 
 into the dark room, and intensify with Russell's developer 
 (see p. 343) : no second fixing will be needed. 
 
464 PRACTICE OF PHOTOGRAPHY. 
 
 The additional deposit obtained as above described will 
 often be found sufficient, and when such is the case the 
 method is to be preferred, because the half-tones of a Pho- 
 tograph are easily obliterated by too much intensifying. 
 In the case of large copies of maps taken with Triplet 
 Lenses of long focus, the Iodine method proves insufficient, 
 and the fine lines of the drawing become partly obliterated 
 during the prolonged development. In such a case it is 
 advised to develope the plate only partially in the first in- 
 stance, and to fix it with Cyanide of Potassium ; then to 
 intensify twice with Pyrogallic Acid, Citric Acid, and Ni- 
 trate of Silver, and lastly to treat the plate with the two 
 following liquids : 
 
 /Iodine 6 grains. 
 
 No. 1. .5 Iodide Potassium .... 12 grains. 
 
 (.Water 6 ounces. 
 
 ~ C Sulphide Potassium .... 1 drachm. 
 
 " 1 Water 6 ounces. 
 
 Apply No. 1 either in the yellow room or in the daylight, 
 until the whole of the image is converted into Iodide of 
 Silver and the deposit appears yellow throughout. Then 
 pour water on the image from a jug, and apply No. 2, 
 which must be allowed to remain until the yellow colour 
 changes to a deep reddish-brown. Lastly, dry the plate, 
 and varnish in the usual way. The theory of this process 
 is explained at page 286. 
 
 If the opacity of the Negative is not complete after the 
 above treatment, it will indicate that the amount of real 
 Silver in the image was insufficient to produce opacity when 
 converted into Sulphide. A second Negative must there- 
 fore be taken, and the intensifying with Pyrogallic Acid, 
 Citric Acid, and Nitrate of Silver continued for a longer 
 time. Even if this continuation of the development should 
 not appear to add much to the density, it must still be per- 
 severed with, since after the application of the Sulphide of 
 Potassium the difference will be manifest. 
 
STEEEOGKAPHY. 465 
 
 SECTION Y. 
 
 Rules for taking Stereoscopic and Instantaneous 
 Photographs. 
 
 Binocular pictures of a large size, for the reflecting Ste- 
 reoscope, may be taken with, an ordinary View Lens of 
 about 15 inches focus. The ground glass of the Camera 
 having been ruled with lines in the manner described at 
 page 452, the position of some prominent part is marked 
 upon one of the lines with a pencil, and the first view is 
 taken. The stand is then moved laterally, to the proper 
 distance, and the Camera adjusted to its second position 
 by shifting it until the marked object occupies the same 
 place as before. The distance between the two positions 
 should be about one foot when the foreground of the pic- 
 ture is twenty-five feet from the instrument, or four feet 
 when it is at thirty or forty yards. But, as before shown 
 at page 213, this rule is not to be followed implicitly, much 
 depending upon the character of the picture and the effect 
 desired. 
 
 Photographs for the refracting Stereoscope are taken 
 with small Lenses of from 4| to 6 inches focus. For por- 
 traits, a Camera may advantageously be fitted with two 
 double-combination Lenses, of If inches diameter, exactly 
 equal in focal length and in rapidity of action. The caps 
 are removed simultaneously, and the pictures impressed at 
 the same instant. The centres of the Lenses may be sepa- 
 rated by three inches when the Camera is placed at about 
 six or eight feet from the sitter. 
 
 Pictures taken with a binocular Camera of this kind, re- 
 quire to be mounted in a reversed position to that which 
 they occupy on the glass, or a pseudoscopic effect will be 
 produced. The Negatives may be cut in half, the right 
 half being printed on the left side, or the finished prints 
 may be reversed before mounting. 
 
 Mr. Latimer Clark has devised an arrangement for ta- 
 king stereoscopic pictures with a single Camera, which is 
 
466 
 
 PEACTICE OF PHOTOGRAPHY. 
 
 exceedingly ingenious. Its most important feature is a 
 contrivance for rapidly moving the Camera in a lateral 
 
 a. 
 
 direction without disturbing the centre of the image upon 
 the ground glass. This will be understood by a reference 
 to the above woodcut. 
 
 "A strongly-framed Camera-stand carries a flat table 
 about 20 inches wide by 16, furnished with the usual ad- 
 justments. Upon this are laid two flat bars of wood in the 
 direction of the object, and parallel, and about the width 
 of the Camera asunder. They are 18 inches in length ; 
 their front ends carry stout pins, which descend into the 
 table and form centres upon which they turn. Their op- 
 posite ends also carry similar pins, but these are directed 
 upwards, and fit into two corresponding holes in the tail- 
 board of the Camera. 
 
 " Now when the Camera is placed upon these pins, and 
 moved to and fro laterally, the whole system exactly re- 
 sembles the common parallel ruler. The two bars form 
 the guides, and the Camera, although capable of free late- 
 ral motion, always maintains a parallel position. In this 
 condition of things it is only suited to take stereoscopic 
 pictures of an object at an infinite distance ; but to make 
 it move in an arc, converging on an object at any nearer 
 distance, it is only necessary to make the two guide-bars 
 
STEREOGRAPHY. 467 
 
 approximate at their nearer end so as to converge slightly 
 towards the object ; and by a few trials some degree of 
 convergence will be readily found at which the image will 
 remain as it were fixed on the focussing glass while the 
 Camera is moved to and fro. To admit of this adjustment, 
 one of the pins descends through a slot in the table, and 
 carries a clamping-screw, by means of which it is readily 
 fixed in any required position. 
 
 " In order however to render the motion of the Camera 
 smoother, it is advisable not to place it directly upon the 
 two guides, but to interpose two thin slips of wood, lying 
 across them at right angles, beneath the front and back of 
 the Camera respectively (and which may be fixed to the 
 Camera if preferred), and to dust the surfaces with pow- 
 dered soap-stone or French chalk." 
 
 In addition to this arrangement for moving the Camera 
 laterally, the slide for holding the sensitive plates must be 
 modified from the common form. It is oblong in shape, 
 and being about ten or eleven inches long, requires some 
 little adaptation to fit it to the end of an ordinary Camera. 
 The glasses are cut to the requisite size ; and when coated 
 with Iodide of Silver, the two images are impressed side 
 by side, the plate being shifted laterally about 2| inches, 
 at the same time and in the same direction as the Camera 
 itself. 
 
 The operation of taking a portrait is thus performed. 
 The focus having been adjusted, and the bars approxi- 
 mated about a quarter of an inch in front to give the right 
 convergence, the Camera and the slide are both drawn to the 
 left-hand. The door is then raised and the plate exposed. 
 Next, the Camera, and after it the slide, is shifted to the 
 right-hand, and the plate in its new position having been 
 again exposed, the door is closed and the operation com- 
 pleted. 
 
 Pictures taken with this instrument do not require to 
 be reversed in mounting, the left picture being purposely 
 formed on the right-hand side of the glass. 
 
 A Camera with twin Lenses placed about 2\ or 3 inches 
 
468 PEACTICE OF PHOTOGKAPHY. 
 
 apart, lias now almost entirely superseded the old form, 
 and although theoretically not quite so perfect as the sys- 
 tem of Latimer Clarke, where the Camera is moved so as 
 to converge towards the centre of the view, it possesses other 
 and more important advantages. Two separate Lenses 
 are indispensable when moving or changeable objects have 
 to be included in the picture, and this, in ordinary land- 
 scape and portrait work, is almost invariably the case. 
 Another advantage attending the use of the double Camera, 
 is, that by removing the front containing the Stereoscopic 
 Lenses, and the central partition, it may be made available 
 with a longer-focused Lens for single views up to 8x5. 
 Portrait Lenses of 6 inches or less equivalent focus, fitted 
 with Waterhouse's diaphragms, are often employed for 
 Stereoscopic Landscape Photography, instead of view 
 Lenses. They do not answer quite so well as the latter 
 for views, but as they may also be used for portraits, their 
 use is considered by some an advantage. 
 
 The Manipulations and Chemicals. Any extended di- 
 rections on this head would be superfluous. Photography 
 on plates of the Stereoscopic size is the most simple branch 
 of the art, and for that reason is usually recommended to 
 beginners. The amateur may commence by reading the 
 general theory of Positive or Negative Photography, as 
 the case may be, in the Seventh Chapter of Part II. He 
 may next practise the manipulations described in the First 
 Section of this Chapter, and afterwards read such observa- 
 tions on Collodion portraiture and landscape Photography 
 as he will find in the two preceding Sections. The Collo- 
 dion and other chemicals are made by the formulae given 
 in Chapter I. of the present Part, and the list of apparatus 
 and bottles is taken from page 448. 
 
 Nevertheless, to attain to first-rate excellence in Stereo- 
 scopic Photography requires skill and practice, and it is a 
 common remark that these pictures are often defective in 
 just balance of light and shade. The defect of hardness 
 is generally apparent in Negatives produced from a simply 
 Iodized Collodion, developed with Pyrogallic Acid. The 
 
.INSTANTANEOUS PICTUEES. 469. 
 
 ISFegatives are developed too strongly, which gives to the 
 print an appearance of chalk or snow. The brilliancy of 
 the Camera image is in fact so great with Lenses of 4| inch 
 focus, that several appliances are often required to subdue 
 the force of the development. The most efficacious remedy 
 is the use of a Bromo-Iodized Collodion and Sulphate of 
 Iron developer, followed by Pyrogallic Acid or Iron, as 
 described at p. 424, which gives very fine gradations of 
 tone and much softness of negative. 
 
 The state of the Nitrate Bath will require a little extra 
 attention in Stereoscopic Photography, when the troughs 
 are made narrow, and their capacity is small. Two or three 
 days' hard work in hot weather will occasionally throw the 
 Bath out of order, either from Acidity or Alkalinity, ac- 
 cording to the state of the Collodion : and on account of 
 accumulation of Ether, etc., the liquid will begin to run. 
 into lines as above described. 
 
 INSTANTANEOUS PICTUEES. 
 
 Pictures shown as Instantaneous are, as a rule, taken 
 by a brief exposure, but yet one occupying an appreciable 
 amount of time, as is evidenced by the fact that the out- 
 lines of moving objects are more or less indistinct. The 
 conditions necessary for success in this beautiful but diffi-. 
 cult branch of Photography are various, and must all be 
 attended to, before satisfactory pictures can be obtained. 
 Each will be described separately. 
 
 The Collodion should be Bromo-iodized and not too old. 
 The normal Collodion of p. 377 and 440, Bromo-iodized 
 some three or four days before use, has been found to 
 answer well ; the formula of p. 440 giving perhaps the 
 higher sensitiveness. When the Bromo-iodizer has been 
 added, it should be well shaken up and allowed to settle 
 before being decanted into clean bottles for use. The 
 chance of spots from sediment is particularly great in in- 
 stantaneous photography. Care therefore must be taken 
 to guard against them and to have the Collodion as pure 
 and limpid as possible. The use of Bromide in the Collo- 
 
470 PRACTICE OF PHOTOGRAPHY. 
 
 dion is not as some have supposed to render it more sensi- 
 tive, but to control the action of the powerful developer 
 and to allow it to remain longer on the film without indu- 
 cing fogging. It enables the operator to use a stronger 
 Nitrate Bath in hot weather and also assists in the redeve- 
 lopment by rendering it more uniform and progressive. 
 
 The Bath must be made of the purest recrystallized 
 Nitrate of Silver, and may be used with advantage of the 
 strength of forty grains to the ounce of distilled water, and 
 must be nearly neutral. It will have of course to be satu- 
 rated with Iodide of Silver in the usual way, see p. 393. 
 It will not work long for instantaneous photographs, for 
 it has been found practically, that a Bath, however care- 
 fully prepared, soon loses its power of conferring the 
 highest sensibility on a Bromo-iodized film ; but it may 
 still remain useful for ordinary work. 
 
 The strength of the preliminary developer is of much 
 importance. It should have from thirty to fifty grains of 
 Protosulphate of Iron and thirty minims of Glacial Acetic 
 Acid to one ounce of water. Twenty minims of Alcohol 
 added, is useful if the Collodion surface is of a repellent 
 nature. The Negative will invariably require redevelop- 
 ment ; it is better to do this at once by either of the me- 
 thods directed at p. 424. 
 
 The actinic power of the Lens employed is of quite as 
 much importance as the purity of the chemicals. Com- 
 pound lenses of short focus, fitted with diaphragms, are 
 generally used. They act very powerfully in a strong 
 light, but are deficient in " depth of focus," so that if the 
 middle distance be well defined, the foreground and dis- 
 tance are indistinct in proportion to the greater or smaller 
 aperture of the diaphragm. The finest specimens of in- 
 stantaneous pictures hitherto produced have been by single 
 lenses, carefully constructed so as to admit of a tolerably 
 large aperture with the requisite sharpness of definition. 
 The future advance of instantaneous photography will pro- 
 bably depend as much on the improvement of the Lens as 
 on an increase in the sensitiveness of the chemicals. 
 
INSTANTANEOUS PICTURES. 471 
 
 The class of subjects fitted for Instantaneous exposures is 
 limited to that wherein there are no violent contrasts of 
 light and shade. Foliage combined with clouds, can hardly 
 be rendered with good effect by the same amount of ex- 
 posure, for while the former may be secured almost in- 
 tantaneously, the latter, under the most favourable circum- 
 stances, takes an appreciable time to affect the sensitive 
 plate. Moving objects in the immediate foreground must 
 be avoided. 
 
 The time selected for operating should be when the 
 weather is warm and the light of the most actinic cha- 
 racter. The former under all circumstances promotes 
 energetic action of the chemicals ; the latter condition can 
 only be accurately ascertained by experiment at the time. 
 A clear atmosphere with light fleecy clouds (known under 
 the name of the mackerel-back sky) in the zenith, and 
 banks of cirro-cumulus in the horizon, combined with a 
 gentle south-west breeze, are almost always conditions of 
 the highest actinic power in the light. A cloudless sky 
 with haze in the distance, are indications of the opposite 
 kind. 
 
 When the chemicals and light are in that state of ac- 
 tivity which fits them for rapid exposures, it will be neces- 
 sary to examine carefully the state of the Camera, to see 
 that no light is admitted except through the Lens. The 
 latter should be so shielded during exposure, that no light, 
 except that from the object, falls on its outer surface, 
 which, being reflected into the Camera, would cause fog- 
 ging. As a further precaution against the same defect, the 
 dark room or tent must be looked to with special care, to 
 make sure that no white light gains admission. Most of 
 the so-called non-actinic glass admits enough of the chemi- 
 cal rays to act injuriously on the plate when in a highly 
 sensitive condition. The additional precaution will there- 
 fore be useful, of fastening over the orange glass two or 
 three folds of yellow calico and working in the minimum 
 of light. If an excited plate held about six inches from 
 the window for two or three minutes, shows no deposit of 
 
472 PRACTICE OF PHOTOGBAPHY. 
 
 Silver on applying the developer, there will be little danger 
 of fogging from that cause. 
 
 The exposure may be effected either by an apparatus in 
 front of the Lens, or still better, by a shutter placed be- 
 tween the Lens and the sensitive plate. They may be 
 procured of various kinds at the photographic instrument- 
 makers. A black cloth hanging over the Lens may, by 
 an adroit movement ,be raised for an instant to admit the 
 light, and will sometimes answer well, but it is uncertain. 
 The exposure should take place within three minutes after 
 the plate has been taken from the Bath. After that time 
 it begins to lose its extreme sensitiveness very rapidly. 
 If, therefore, the operator has to wait for some time with 
 the plate in the Camera till moving objects have arranged 
 themselves to his liking, or while a passing cloud obscures 
 the landscape, failure is almost certain, and he will have 
 to make another attempt with a fresh plate. 
 
 A good arrangement for a first attempt at instantaneous 
 Photography, is a Stereoscopic Camera mounted with twin 
 portrait Lenses. A light outside casing of mahogany, 
 blackened in the interior, slides over the Camera when not 
 in use, and draws out so as to project several inches be- 
 yond the Lenses when the pictures are taken ; the open- 
 ing of the box in front being still further contracted, if ne- 
 cessary, by a blackened card, with two circular apertures 
 corresponding to the Lenses. There are no caps to the 
 Lenses, but a black cloth hangs down in front of the loose 
 box just described, and is raised for an instant to admit 
 the light. Begin by adjusting the two Lenses to a cor- 
 responding focus from the front, then draw out the sliding 
 box, and (the rack work of the Lenses being now out of 
 reach) complete the focussing from the back. 
 
 In order still further to secure such an amount of suc- 
 cess as will be encouraging, make the first trials on a 
 clear spring morning, and at the seaside, if practicable. 
 The few instantaneous pictures which are exhibited have 
 doubtless been selected from a number of failures ; for the 
 operator soon finds that even with apparatus and chemicals 
 
MICEO-PHOTOGEAPHY. 473 
 
 in the best order, he depends much upon the light, and 
 cannot always obtain the same success. 
 
 SECTION VI. 
 
 On the Photographic Delineation of Microscopic Objects. 
 
 Many specimens of Micro-photography which have been 
 exhibited are exceedingly beautiful ; and their production 
 is not difficult to one thoroughly acquainted with the use 
 of the Microscope and with the manipulations of the Col- 
 lodion Process. It is important, however, to possess a 
 good apparatus, and to have it properly arranged. 
 
 The object-glass of the ordinary compound Microscope 
 is the only part actually required in Photography, but it 
 is useful to retain the body for the sake of the adjustments, 
 and also the mirrors used in the illumination. The eye- 
 piece, however, which simply magnifies the image formed 
 by the object-glass, is not actually necessary, since the same 
 effect of enlargement may be obtained by lengthening out 
 the dark chamber, and throwing the image further off. 
 
 Arrangement of the Apparatus. The Microscope is 
 placed with its body in a horizontal position, and the eye- 
 piece being removed, a tube of paper, properly blackened 
 in the interior, or lined with black velvet, is inserted into 
 the instrument, to prevent irregular reflection of light from 
 the sides. 
 
 A dark chamber of about two feet in length, having at 
 one end an aperture for the insertion of the eye-piece end 
 of the body, and at the other a groove for carrying the 
 slide containing the sensitive plate, is then attached ; care 
 being taken to stop all crevices likely to admit diffused 
 light. An ordinary Camera may be employed as the dark 
 chamber, the Lens being removed, and the body lengthened 
 out if required by a conical tube of gutta-percha, made to 
 fasten into the flange of the Lens in front. The whole ap- 
 paratus should be placed exactly in a straight line, that the 
 ground glass used in focussing may fall at right angles to 
 the axis of the Microscope. 
 
474 PRACTICE OF PHOTOGBAPHY. 
 
 The length of chamber which has been recommended, 
 measuring from the object- glass, is from two to three feet, 
 according to the size of image required : if extended beyond 
 this, the pencil of light transmitted by the object-glass 
 will be diffused over too large a surface, and may produce 
 a faint picture. It will be shown, however, as we proceed, 
 that some operators have produced good results with a 
 Camera of double the length here mentioned. The object 
 should be illuminated by sunlight if it can be obtained, 
 but a bright diffused daylight will succeed with low-power 
 glasses, and especially when Positives are taken. Employ 
 the concave mirror for reflecting the light on the object in 
 the latter case ; but in the former the plane mirror is 
 better, except with powers exceeding a quarter of an inch 
 and of large angular aperture. 
 
 The time of exposure must be varied according to the 
 intensity of the light, the degree of magnifying power, etc. 
 At this point a difficulty will probably occur from the 
 plane of the chemical focus not corresponding, as a rule, 
 with that of the visual focus. This arises from the fact 
 that the object-glasses of Microscopes are "over-corrected" 
 for colour, in order to compensate for a little chromatic 
 aberration in the eye-piece. The violet rays, in con- 
 sequence of the over-correction, are projected beyond the 
 yellow, and hence the focus of chemical action is further 
 from the glass than the visible image. 
 
 The allowance may be made by shifting the sensitive 
 plate, or, what amounts to the same thing, by removing 
 the object-glass a little away from the object with the fine 
 adjustment screw ; the latter is the more convenient. The 
 exact distance must be determined by careful experiment 
 for each glass ; but it is greatest with the low powers, and 
 decreases as they ascend. 
 
 Mr. Shadbolt gives the following as a guide : " An inch 
 and a half objective, of Smith and Beck's make, required 
 to be shifted l-50th of an inch, or two turns of their fine 
 adjustment ; a 2-3rds of an inch, l-200th of an inch, or 
 half a turn ; and a 4-10ths of an inch, l-1000th of an inch, 
 
MICRO-PHOTOGKAPHY. 475 
 
 or about two divisions of the adjustment. With the l-4th 
 and higher powers, the difference between the foci was so 
 small as to be practically unimportant." 
 
 There is some reason to think that the kind of light 
 employed may have an influence upon the separation of the 
 foci. Mr. Delves found that with sunlight the difference 
 between them was small even with the low powers, and 
 inappreciable with the higher ; whereas in using diffused 
 daylight which had undergone a previous reflection from 
 white clouds, it appeared considerable. 
 
 The object-glasses of the same maker, and particularly 
 those of different makers, also vary much ; so that it will 
 be necessary to test each glass separately, and to register 
 the allowance which is required. 
 
 Mr. Beeves Traer, who has been very successful in this 
 branch of Photography, has kindly favoured the Author 
 with his experience. In attempting to arrange his Micro- 
 scope according to the directions above given, he failed in 
 fixing it as steadily as he could desire, and was induced in 
 consequence to have a brass apparatus made for uniting 
 together the object-glass, stage, mirror, and adjustments, 
 and screwing them bodily into the Camera. The whole 
 then formed only one piece, and became perfectly under 
 control.* 
 
 The Camera which he employs is about four feet in 
 length and ten inches square. He uses a concave mirror 
 of two and a half inches diameter, made purposely, the or- 
 dinary mirror having been found to be too small ; and in 
 the track of the rays of light, he places an inverted object- 
 glass of a lower power than the one employed in magni- 
 fying, to serve as an achromatic condenser. The stage is 
 firmly fixed in a vertical position, and has two spring clips 
 to hold the slip of glass on which the object is mounted. 
 There are no stage movements : but two adjustments are 
 employed as usual, a coarse for focussing with low powers, 
 
 * A diagram of this arrangement will be found in the fifth volume of the 
 ' Journal of the Photographic Society.' The price charged for it by the 
 Microscope-maker was five pounds. 
 
476 PRACTICE OF PHOTOGRAPHY. 
 
 and a fine for the higher powers. The milled head of the 
 fine adjustment is marked into a certain number of divi- 
 sions, to allow for the difference between the visual and 
 chemical focus by a graduated rotation. 
 
 The Camera with its brass mountings affixed to it is 
 placed upon a table in the open air, its long axis in a line 
 with the sun's rays, so that the source of light is behind 
 the operator as he looks into the instrument. The mirror 
 is then shifted until the field appears equally illuminated, 
 and the position it ought to occupy will easily be found by 
 moving it backwards and forwards until a bright white 
 spot of reflected light is seen on some portion of the field. 
 When this is the case, the object-glass used in condensing 
 is probably exactly in the focus of the mirror, which it 
 ought not to be, and therefore the mirror must be pushed 
 back a little, so as to allow the reflected rays to cross be- 
 fore entering the condenser, after which the white spot 
 will disappear. With glasses lower than the ^-inch a plane 
 mirror may be used. 
 
 The exact power of glass which ought to be employed 
 will depend upon the nature of the object, and not so much 
 upon the size of image required. A large image can 
 always be obtained with a low power by prolonging the 
 Camera and throwing the focus further off; but to exhibit 
 the peculiar structure of some objects it is essential to em- 
 ploy a high power, and when low powers are substituted, 
 surface-markings, dots, etc., are in a measure lost. In 
 place of throwing the image further off when a large re- 
 presentation is desired, an eye-piece may be used, thus 
 saving additional length of Camera. Theoretically the cur- 
 vature of the field is increased by so doing, but nothing 
 peculiar was observed in the specimens taken in this way 
 excepting a little loss of definition. The eye-piece of least 
 magnifying power is the proper one to employ, and it may 
 be pushed into the brass tube from the interior of the 
 Camera. 
 
 It is important to work in a bright light, and rather 
 quickly ; because if many seconds are allowed to elapse, the 
 
MICEO-PHOTOGBAPHY. 477 
 
 sun shifts its position sufficiently to disturb the circle of 
 light formed by the mirror. Mr. Traer has not been able 
 to confirm the observations of Mr. Delves above quoted, 
 that the chemical foci of miscroscopic objectives vary with 
 the quality of the light. 
 
 The late Mr. Hewlett exhibited enlarged Micro-Photo- 
 graphs of a very superior quality. His experience was as 
 follows : It is a waste of time to work in a bad light. 
 The sun's rays may be condensed upon the object by means 
 of a large plane reflector and a double convex Lens of six 
 inches diameter. The employment of low-power glasses 
 is most successful, and when the higher powers are used, 
 the difficulties increase at a rapid rate. A f-inch object- 
 glass is useful, the difference between the foci of which 
 was found to be y^ of an inch. It is better not to use a 
 larger angular aperture than is actually required to display 
 the structure of the object, but to cut off the excess of 
 aperture in some instances by a stop placed against the 
 posterior Lens of the object-glass. The use of other Lenses 
 added to the object-glass for the purpose of making the 
 visual and chemical foci coincide, is not recommended.* 
 
 Chemicals and Manipulations for Micro- Photography. 
 The image when formed by the direct rays of the sun 
 is remarkably brilliant, and the Negative may be obtained 
 under favourable circumstances in one or two seconds. 
 There are usually strong contrasts of light and shade, some 
 parts of the image being very dark and others extremely 
 light. The remarks therefore on Collodion and Bath made 
 under the head of Portrait Photography will apply in this 
 case. Intense Collodion prepared from Linen, or Cotton 
 at high temperatures of Nitro- Sulphuric Acid, will become 
 red in the lights before the shadows can be brought out ; 
 and if either Collodion or Bath contain organic matter, 
 such as Glycyrrhizine, Grape Sugar, etc., the evil will be 
 increased. Acetate of Soda in the Bath is not to be re- 
 commended, for the same reason. Iodide of Potassium will 
 form a better iodizer than Iodide of Cadmium, as giving a 
 
 * ' Photographic Notes/ vol. iii. p. 168. 
 
478 PBACTICE OF PHOTOGEAPHY. 
 
 film less liable to solarize under a strong light. And for 
 some objects the presence of Bromide will be an improve- 
 ment ; since Bromide always gives the power of exposing 
 for a long time without producing over-action, and in a 
 very bright light it does not interfere in getting up the in- 
 tensity of the Negative. 
 
 In the case of a simply iodized Collodion, the ordinary 
 developing solution of Acetic Acid and Pyrogallic is re- 
 commended in preference to Citric Acid and Pyrogallic 
 Acid, the former being more adapted for bringing out the 
 heavy shadows, which are so abundant in some Micro- 
 scopic objects. 
 
 SECTION VII. 
 
 Photography in Hot Climates. 
 
 The Photographic Art is now so extensively practised 
 in India and other parts of the globe, where the climate is 
 more or less unfavourable, that the Author has been in- 
 duced to devote a distinct Section to the subject. Those 
 points which have already been explained in the First 
 Division of the Work, or in the separate Sections of the 
 present Chapter, will not again be brought forward, the 
 reader being simply referred to them for the details of 
 manipulation. 
 
 The Calotype and waxed-paper processes have met with 
 much favour in India and other hot climates, and have 
 been practised with considerable success. In the early 
 days of Collodion, when the conditions of its successful 
 manufacture were not so well understood as they are at 
 present, great uncertainty was experienced by those who 
 practised the process, arising from the instability of the 
 Collodion, and from other causes. The Collodion, being 
 necessarily sent from Europe, suffered decomposition in 
 its transit to such an extent as sometimes to be thin and 
 limpid like water, and highly insensitive ; at other times, 
 from the evaporation of the Ether, it became semi-gela- 
 tinous and useless. Often also, on adding the lodizer, a 
 
PHOTOGRAPHY IN HOT CLIMATES. 479 
 
 blood- red colour was immediately developed, and pictures 
 could not be taken with it except by an impracticable 
 amount of exposure. At other times, from the high tem- 
 perature and the consequent rapid evaporation of the 
 Ether, it was impossible to cover a moderately-sized glass 
 without the upper part of the plate having set too strongly 
 before the excess of Collodion had been poured from the 
 opposite corner. A plate of this kind, when taken from 
 the exciting bath, will appear thin and opalescent at one 
 end, creamy and covered with ridges of unequal thick- 
 ness at the other ; and no amount of care in the develop- 
 ment will enable the operator to obtain an equally dense 
 gradation of tone all over his Negative. The indications 
 given by these and similar appearances point to the pre- 
 paration of a Collodion which sets slowly at high tempe- 
 ratures, and to the use of the more stable Iodides. For- 
 tunately both the conditions are to a great extent under 
 control. 
 
 To begin with the former. The Pyroxyline must be of 
 that kind which will bear a large dose of alcohol in pro- 
 portion to the Ether, without losing tenacity, and be- 
 coming semi-gelatinous. Alcohol being less volatile than 
 Ether gives time for the film to set evenly, and for the 
 operator to perform the necessary manipulations with ease 
 before it becomes too dry for the Bath. The Pyroxyline 
 at p. 377 will bear to be dissolved in 5 parts Alcohol, 
 sp. gr. 805, and 3 of Ether, sp. gr. 725 (these proportions 
 refer to the Collodion when iodized), and will give an ex- 
 tremely smooth structureless film, setting very slowly. If 
 in the same formula the temperature of the mixed acids 
 be raised to 160, and the quantity of water decreased by 
 four fluid drachms, the Pyroxyline will bear a still larger 
 proportion of alcohol, without injury to its flowing quali- 
 ties, and the film will be sufficiently tenacious. But it 
 must be borne in mind that .the Alcohol must always be of 
 the most highly rectified kind, not under '805, for it is not 
 the Alcohol but the water it contains that renders the Col- 
 lodion tender and gelatinous. 
 
480 PEACTICE OF PHOTOGBAPHY. 
 
 Besides the great benefit conferred by an excess of Al- 
 cohol over the Ether, in retarding the setting of the Col- 
 lodion on the glass, and in facilitating its even spreading 
 at high temperatures, it also renders the Collodion more 
 stable by reducing its tendency to become ozonized. 
 
 The alkaline Iodides, such as those of Ammonium, Po- 
 tassium, etc., per se, have been found quite unfitted for 
 iodizing Collodion that has been kept for a long time and 
 become ozonized, or that has undergone the process of 
 " baking," as it is called, in its transit to India. The 
 effect produced immediately on the addition of an alkaline 
 lodizer is the decomposition of the Iodide, and a copious 
 liberation of free Iodine, a state of things fatal to sensi- 
 tiveness. At the same time, the Collodion itself often un- 
 dergoes some sort of decomposition, which renders it thin 
 and limpid like water, and causes a white precipitate to 
 collect in the bottom of the bottle. 
 
 The Iodide of Cadmium is, so to speak, the " sheet an- 
 chor" of the photographer in hot climates. It is condu- 
 cive to stability and sensitiveness ; but when used alone, 
 without the addition of a Bromide or an alkaline Iodide 
 to correct its bad propensities, it is apt to render the Col- 
 lodion glutinous, and cause it to flow in an irregular man- 
 ner over the plate. This defect is not so observable when 
 the Pyroxyline has been prepared in acids at high tempe- 
 ratures ; but if the cotton has been immersed when the 
 thermometer stood at 120 or under, Iodide of Cadmium 
 alone is a most unfit lodizer. Another drawback to the 
 use of Iodide of Cadmium, in hot climates and in a strong 
 light, is its extreme liability to red solarization when de- 
 veloped with Pyrogallic acid, and to fogging when deve- 
 loped with Sulphate of Iron. Lastly, the plate, when re- 
 moved from the sensitizing bath, becomes surface-dry, 
 sooner than if the Collodion had been iodized with an 
 alkaline Iodide. Notwithstanding all these drawbacks, 
 which, however, are not without a remedy, Iodide of 
 Cadmium must still form the foundation of an iodizer for 
 a stable Collodion. 
 
 
PHOTOGBAPHY IN HOT CLIMATES. 481 
 
 It would seem that a judicious combination of Iodide 
 of Cadmium with an alkaline Iodide and with a Bromide, 
 fulfils in the most complete manner the requirements of 
 the photographer in hot climates, always supposing that 
 the plain Collodion has been properly prepared. 
 
 The Bromo-iodizer of p. 402 with the Bromide of Cad- 
 mium instead of Ammonium, renders the Collodion ex- 
 tremely stable, and may be used in hot climates, accord- 
 ing to circumstances, either with a pyrogallic or an iron 
 developer. If on adding this Bromo-iodizer to the plain 
 Collodion, a considerable liberation of free Iodine takes 
 place in the course of a few minutes, either the Ether has 
 become ozonized, or the Collodion has been badly pre- 
 pared. In the former case, it may still work well with a 
 preliminary iron developer, and subsequent redevelop- 
 ment with Pyrogallic Acid ; in the latter case, the Collo- 
 dion itself will probably be decomposed, and will only be 
 fit for Positives. If, on the other hand, the Collodion, 
 after adding the Bromo-iodizer, remains colourless, the 
 addition of two or three drops of Tincture of Iodine will 
 be beneficial. 
 
 With respect to the Bath, it has been sometimes found 
 necessary to reduce the strength as low as 20 or 15 grains 
 of Nitrate of Silver to the ounce of distilled water ; but 
 this could hardly occur, except under peculiar conditions 
 of temperature or chemicals. As a rule, more acid will 
 be required in the Bath than is usual in this country ; but 
 this also will depend in a great measure on the state of 
 the Collodion, a badly decomposed sample requiring less 
 than a nearly colourless Collodion. 
 
 In regard to the developer, the operator may choose 
 between Pyrogallic Acid and Iron, according to his habits 
 or the requirements of the Collodion. In no part of the 
 Collodion process is the effect of high temperature so 
 marked as on the developer, in increasing its energy to a 
 surprising degree. It is noticeable more particularly with 
 Pyrogallic Acid, the plate often developing fully imme- 
 diately on its application. In order, therefore, to guard 
 
 2 i 
 
482 PRACTICE OF PHOTOGEAPHY. 
 
 against stains and marks of irregularity, the action of the 
 developer must be checked by reducing the quantity of 
 Iron or Pyrogallic Acid, and by increasing the Acetic or 
 Citric Acid ; the latter acting very effectually in subduing 
 the energy of Pyrogallic Acid, but less powerfully on the 
 Iron. In order further, if necessary, to restrain excessive 
 rapidity of development, it is sometimes recommended 
 to dip the exposed plate for a minute or two in a weak 
 5-grain solution of Nitrate of Silver before applying the 
 developer. 
 
 Another difficulty that the photographer in tropical 
 climates has to contend with is the ever-varying actinic 
 nature of the light, often changing within an hour, while 
 no seeming difference can be detected by the eye. This 
 difficulty is incurable, and the photographer must submit 
 and get over it the best way he can. 
 
 Dry Plate Photography. Little need be added to what 
 is fully described in Chapter V. of this Part. The Taupe- 
 not process has been practised in India with great suc- 
 cess ; so also has the Tannin. The latter appears to be 
 now generally preferred, on account of its greater sim- 
 plicity, and at least equal certainty. The plates must be 
 washed with extra care, and the Tannin solution need not 
 be so strong as is generally used in this country. 
 
 Apparatus and Package. In selecting a Camera for a 
 tropical climate, strength and thorough seasoning of the 
 wood are of the first importance. Teak wood is preferable 
 to mahogany, being less liable to be attacked by the white 
 ants. It should be brass bound, and the dovetail joints 
 secured by screws. The sliding adjustments, which move 
 within each other, must be very accurately constructed, so 
 as to work very easily, for as tropical climates are gene- 
 rally at some seasons of the year very moist, the slides du- 
 ring those seasons are liable to jam and become immov- 
 able. Special care, also, should be taken with the dark 
 frames, to prevent the rays of the vertical sun from pene- 
 trating at the top when the slide is being drawn up to ex- 
 pose the plate. 
 
CHEMICALS AND APPAEATUS FOE HOT CLIMATES. 483 
 
 The ordinary bellows Cameras, which, on account of 
 their lightness, are much in vogue in this country, are soon 
 rendered useless in India by the ravages of cockroaches 
 and other insects. These animals, however, do not seem 
 to have much appetite for the strong-scented Russia lea- 
 ther ; and it is believed that, if the bellows part be made 
 of this instead of the ordinary material, the advantages of 
 lightness and portability can with safety be secured. 
 
 Collodion should be sent in bottles well corked, and then 
 tied over with bladder, and dipped in hard cement ; the 
 stopper being fastened round the neck. This plan has 
 been found to answer better than the most careful stop- 
 pering, or even capping outside the stopper with glass. 
 The bottles ought not to be quite full, the heat causing 
 expansion. As an additional security, fit each bottle into 
 a tin box and pack it with sawdust. 
 
 In opening these bottles, be very careful to clean away 
 the cement with the blade of a knife, else a portion of 
 it may find its way into the Collodion ; and put them 
 away at once in a dark place, since the fluid is probably 
 in a state to ozonize rapidly by exposure to light, and in 
 that case would immediately strike a brown colour upon 
 adding the lodizer. 
 
 It has often been remarked that expensive sets of ap- 
 paratus have been sent abroad with an ill-assorted supply 
 of chemicals, altogether inadequate to the work intended 
 to be done ; some being omitted altogether, or in very 
 short quantity, others being in great excess. 
 
 The following list comprehends a well-balanced stock of 
 chemicals and apparatus for a Camera adapted for plates 
 8x5, and fitted with Stereoscopic Lenses and also with a 
 single or Triplet View Lens for the full-sized plate : 
 
 One gross, No. 2 patent plate-glasses, roughened at the 
 edges. Plate-box for twelve plates. Ditto for twenty-four 
 plates. Two pneumatic plate-holders. Broad camel's-hair 
 brush. Three wash-leathers, very soft. Five pints plain 
 Collodion. Iodizing or Bromo-iodizing solution for ditto. 
 Sulphuric Ether, sp. gr. 725, one pint ; and Alcohol, sp. gr. 
 
484 PEACTICE OF PHOTOGEAPHY. 
 
 805, one pint, for thinning Collodion when necessary. 
 Spirits of Wine for adding to the developer, etc., two 
 pints. Watertight Bath enclosed in well-seasoned wood, 
 brass-bound or dovetails fastened with screws. Spare 
 glass bath for ditto. Two dippers in case. Fifteen ounces 
 of best recrystallized Nitrate of Silver for making bath 
 solution. Twenty ounces of common Nitrate of Silver for 
 printing. Pyrogallic Acid, one and a half ounce. Eight 
 ounces of Protosulphate of Iron. Glacial Acetic Acid, two 
 pounds. Citric Acid, four ounces. Cyanide of Potassium, 
 commercial, two pounds. Hyposulphite of Soda, eight 
 pounds. Varnish for Negatives, one pint. Five flat-bot- 
 tomed porcelain dishes, 10 X 8. Two quires of plain Saxe or 
 Bive paper (medium thickness) for printing. Four quires 
 albuminized ditto. Chloride of Gold, three drachms. 
 Acetate of Soda, eight ounces. Four ounces of Bicarbo- 
 nate of Soda. Tannin, four ounces. Half an ounce of 
 Iodide of Cadmium. Bromide of Cadmium, half an ounce. 
 Iodide of Ammonium, half an ounce. Iodide of Potas- 
 sium, one ounce. Iodine, a quarter of an ounce. Kaolin, 
 two ounces. Two quires filtering paper. Two glass or 
 porcelain funnels. Scales and weights. Minim and 4- 
 ounce glass measures. Pure Nitric Acid, one ounce. A 
 few stoppered bottles of different sizes for decanting Col- 
 lodion and holding solutions. Tripoli, two ounces. Emery 
 powder, fine, one ounce. Two printing frames with thick 
 felt pad 9x7, inside measurement. Litmus paper. Dark 
 tent if required. 
 
 If the focussing screen should by mischance get broken, 
 one may be made quite as serviceable as any that can 
 be bought at the camera makers, by rubbing two of the 
 large glass plates together with some fine emery-powder 
 and a few drops of water placed between, and fixing one of 
 them, when ground fine enough, in the focussing frame. 
 
 Albuminized paper will soon deteriorate unless kept in 
 a dry place. It is usually sent out in an hermetically 
 sealed tin case, which, unless the weather is dry, should 
 not be opened till it is required for use. 
 
CHEMICALS AND APPAEATUS FOE HOT CLIMATES. 485 
 
 Porcelain dishes are liable to be broken in transit. 
 Gutta-percha ones will answer quite as well for all photo- 
 graphic work, except for holding solutions of Nitrate of 
 Silver. A good substitute for holding the latter may be 
 made by sinking a piece of plate glass in a wooden frame- 
 work, and daubing the latter with a hard varnish to pre- 
 vent absorption of the liquid. The dish should be kept 
 for Nitrate of Silver alone. The porcelain or gutta-percha 
 tray for holding Hyposulphite of Soda, should also be 
 reserved for that purpose only 
 
486 
 
 CHAPTER III. 
 
 THE PRACTICAL DETAILS OF PHOTOGRAPHIC 
 PRINTING. 
 
 THIS Chapter is divided as follows : 
 
 SECTION I. The ordinary process of Positive printing. 
 
 SECTION II. Positive printing by development, inclu- 
 ding the production of transparencies for the Stereo- 
 scope. 
 
 SECTION I. 
 
 Positive Printing by the direct action of Light. 
 
 The ordinary varieties of paper sold in commerce are not 
 sufficiently smooth and uniform to be well adapted for the 
 production of Positive prints, and even the finest papers 
 prepared purposely for Photography are often defective. 
 Hence each sheet should be examined separately by holding 
 it against the light, and if spots or irregularities of texture 
 are seen, it should be rejected. These spots usually con- 
 sist of small particles of brass or iron, which, when the 
 paper is rendered sensitive, decompose the Nitrate of Sil- 
 ver and leave a circular mark very noticeable after fixing. 
 
 The foreign papers, French and German, differ from the 
 English in the mode of sizing adopted, and hence the two 
 varieties will not produce similar tones, as already shown 
 in Part II. In all cases there is a difference in smooth- 
 ness between the two sides of the paper, which may be 
 detected by holding each sheet in such a manner that the 
 
ALBUMINIZING PAPEE FOE FEINTING. 487 
 
 light strikes it at an angle ; on the wrong side will be seen 
 dark wavy bands, of an inch to an inch and a half in 
 breadth, caused by the strips of felt on which the paper 
 was dried. With most qualities of paper no difficulty 
 whatever will be experienced in detecting the broad and 
 regular bands above referred to ; but when they cannot 
 be seen, the wrong side of the sheet may be known by 
 gauze-wire markings ; or, in default of such, the paper 
 may be wetted at the corner, when one side will appear 
 smoother than the other. 
 
 PEEPAEATION OF SENSITIVE PAPEE. 
 
 There are three formulae for sensitive paper, viz. the Al- 
 buminized, the plain, and the Ammonio-Nitrate paper. 
 
 Formula I. Preparation of Albuminized Paper. 
 Take of 
 
 Chloride of Ammonium . . . 200 grains. 
 
 Water 5 fluid ounces. 
 
 Albumen 15 fluid ounces. 
 
 Chloride of Barium, sometimes used in salting paper 
 instead of Chloride of Ammonium, is contra-indicated when 
 the Alkaline Gold-toning process is adopted, since the 
 Carbonate of Soda would throw down Carbonate of Baryta 
 in the paper. 
 
 When pure Albumen is used without water, from 5 to 
 8 grains of Salt to each ounce will be sufficient. The less 
 the quantity of Salt the warmer the colour, but it must 
 not be so far reduced as to injure the contrast and depth 
 of shadow in the print. 
 
 If distilled water cannot be procured, rain-water, or even 
 common spring- water, will often answer the purpose. For 
 the Albumen, use eggs nearly fresh, and be careful that in 
 opening the shell the yolk is not broken ; each egg will 
 yield about one fluid ounce of Albumen. 
 
 When the ingredients are mixed, take a bundle of quills 
 or a fork, and beat the whole into a perfect froth. As the 
 froth forms, it is to be skimmed off and placed in a flat 
 
488 PEACTICE OF PHOTOGRAPHY. 
 
 dish to subside. The success of the operation depends 
 very much upon this part of the process, for if the Albu- 
 men be not thoroughly beaten, flakes of animal membrane 
 will be left in the liquid, and will cause streaks upon the 
 paper. When the froth has partially subsided, transfer it 
 to a tall and narrow jar, and allow to stand for several 
 hours, that the membranous shreds may settle to the bot- 
 tom. Then pour off the upper clear portion, which will be 
 fit for use. Albuminous liquids are too glutinous to run 
 well through a paper filter, and are better cleared by sub- 
 sidence. 
 
 Another plan is to fill a bottle to about three parts with 
 the salted mixture of Albumen and Water, and to shake it 
 well for ten minutes or a quarter of an hour until it loses 
 its glutinosity and can be poured out smoothly from the 
 neck of the bottle : it is then to be transferred to an open 
 jar, and allowed to settle as before. This method is more 
 simple than the last, but is not so much used by makers of 
 Albuminized paper. 
 
 Albumen alone, without any addition of water, gives a 
 more highly varnished appearance, but much in this respect 
 will depend upon the kind of paper employed, certain va- 
 rieties of paper taking more gloss than others : on this sub- 
 ject consult the remarks made at page 296. 
 
 The principal difficulty in Albuminizing paper, is to 
 avoid the occurrence of streaky lines, which, when the 
 paper is rendered sensitive, bronze strongly under the in- 
 fluence of the light. To avoid them, lower the paper on 
 the liquid by one steady movement, since if a pause be 
 made, a line will be formed. Some papers are not readily 
 wetted by the Albumen, and when such is the case, a few 
 drops of spirituous solution of bile, or a fragment of the 
 prepared Ox-Grail sold by the artists'-colourmen, will be 
 found a useful adjunct. Care must be taken however not 
 to add an excess, or the Albumen will be rendered too 
 fluid, and will sink into the paper, leaving no gloss. The 
 Ox-Gall has been thought to make the paper tone more 
 slowly in the Gold Bath, and consequently the Writer 
 
ALBUMINIZING PAPEE FOE FEINTING. 489 
 
 now omits it, and adds two drachms of Spirits of Wine 
 (previously diluted with water to prevent coagulation) to 
 each four ounces of Albumen. This removes the greasi- 
 ness in a great measure, and lessens the chance of streaks. 
 
 In salting and Albuminizing Photographic paper by the 
 formula above given, it is found that each quarter-sheet, 
 measuring eleven inches by nine inches, removes one fluid 
 drachm and a half from the Bath, equivalent to about one 
 grain and three quarters of salt (including droppings). 
 In salting plain paper, each quarter-sheet takes up only 
 one drachm ; so that the glutinous nature of the Albu- 
 men causes a third part more of Salt to be retained by the 
 paper. 
 
 The Hive and Saxe papers, of which there are several 
 varieties, are those which are chiefly used for Albumin- 
 izing. The former takes the higher gloss, and being less 
 strong in texture, is therefore generally used for small 
 pictures. It also gives a fine rich clear tone, suitable for 
 Stereoscopic prints and cartes-de-visite. The Saxe paper 
 is very strong in texture, takes the Albumen very evenly, 
 and is generally very free from metallic spots. 
 
 To apply Albumen, pour a portion of the solution into 
 a flat dish to the depth of half an inch. Then, having 
 previously cut the paper to the proper size, take a sheet 
 by the two corners, bend it into a curved form, convexity 
 downwards, and lay it upon the Albumen, the centre part 
 first touching the liquid, and the corners being lowered 
 gradually. In this way bubbles of air will be pushed 
 forwards and excluded. One side only of the paper is 
 wetted : the other remains dry. Allow the sheet to rest 
 upon the solution for one minute and a half, and then 
 raise it off, and pin it up by two corners. If any circular 
 spots, free from Albumen, are seen, caused by bubbles of 
 air, replace the sheet for the same length of time as at 
 first. 
 
 The paper must not be allowed to rest upon the Salting 
 Bath much longer than the time specified, because the 
 solution of Albumen being alkaline (p. 302), tends to re- 
 
490 PEACTICE OP PHOTOGRAPHY. 
 
 move the size from the paper and to sink in too deeply : 
 thus losing its surface gloss. 
 
 Albuminized paper will keep a long time in a dry place. 
 Some have recommended to press it with a heated iron, 
 in order to coagulate the layer of Albumen upon the sur- 
 face ; but this precaution is unnecessary, since the coagu- 
 lation is perfectly effected by the Nitrate of Silver used in 
 sensitizing ; and it is doubtful whether a layer of dry 
 Albumen admits of coagulation by the simple application 
 of a heated iron. 
 
 In cutting the paper to the required sizes use a lone 
 spatula, and avoid touching the Albuminized surface with 
 the fingers. Press the sheets flat between boards, or in a 
 printing frame, and, if possible, avoid rolling them. 
 
 To render the paper sensitive. This operation must be 
 conducted by the light of a candle, or by yellow light. 
 Take of 
 
 Nitrate of Silver 90 grains. 
 
 Distilled Water 1 ounce. 
 
 Prepare a sufficient quantity of this solution, and pour it 
 out into a porcelain dish. After it has been a short time 
 in use, the Albumen dissolved out of the papers will cause 
 a greasy scum to form upon the liquid, which if allowed to 
 remain produces marbled stains upon the sensitive paper ; 
 it must therefore be removed by folding a strip of blotting- 
 paper the exact breadth of the dish, and drawing it lightly 
 along the surface. Lay the sheet upon the solution in the 
 same manner as above described for the Albumen. Three 
 minutes' contact will be sufficient with thin paper, but if a 
 thick paper be used, four or five minutes must be allowed 
 for the decomposition. The papers are raised from the 
 solution by a pair of bone forceps, or common tweezers 
 tipped with sealing-wax ; or a pin may be used to lift up 
 the corner, which is to be held by the finger and thumb 
 and allowed to drain a little before again putting in the 
 pin, otherwise a white mark will be produced upon the 
 paper, from decomposition of the Nitrate of Silver. When 
 the sheet is hung up, a small strip of blotting-paper sus- 
 
SENSITIZING ALBUMINIZED FAPEE. 
 
 491 
 
 pended from the lower edge of the paper will serve to 
 drain off the last drop of liquid. A convenient arrange- 
 ment for hanging up papers to dry is by means of the 
 ordinary American clips, attached to a string stretched 
 across the room. They can be purchased for a mere trifle 
 at the oil and colour shops. 
 
 A Bath prepared by the above formula is stronger than 
 is actually necessary. But it has been found that paper 
 floated on weak solutions is always more or less deficient 
 in vigour. The strength of the Bath decreases rapidly by 
 use, and hence an addition of Nitrate of Silver must occa- 
 sionally be made. 
 
 The following convenient little instrument, called a 
 " Silver Meter," is now almost universally employed to 
 ascertain the strength of Silver solutions. It is not quite 
 accurate, but near enough for practical purposes. When 
 greater accuracy is required, consult the sp. gr. table given 
 in the Appendix. 
 
 It consists of a cylindrical vessel 
 holding some two or three ounces, with 
 an hydrometer of the common form. 
 The Sensitizing Bath to be tested is 
 poured into the glass, and the bulb 
 floated in it, when the number of grains 
 per ounce will be indicated by the part 
 of the scale corresponding to the sur- 
 face of the liquid. As these instruments 
 are sold for a few shillings, and conse- 
 quently are not made with much care, 
 it is advisable to commence by preparing a 90-grain solu- 
 tion of pure Nitrate of Silver, to verify the correctness of 
 the scale. Observe also that the indications of the hydro- 
 meter will cease to be trustworthy if the Bath contain 
 Alcohol or Ether ; hence an old Negative Bath, properly 
 analysed, may be found to yield three or four grains of Ni- 
 trate per ounce, over the quantity given by floating. 
 
 The solution of Nitrate of Silver becomes after a time 
 discoloured by the Albumen, but may be used for sensi- 
 
492 PRACTICE OF PHOTOGRAPHY. 
 
 tizing until it is nearly black. The colour can be removed 
 by Animal Charcoal,* but a better plan is to use the 
 "Kaolin," or pure white china clay. Shake up twenty 
 ounces of Bath with a quarter of an ounce of finely pul- 
 verized Kaolin, and filter through paper. Kaolin often 
 contains Carbonate of Lime, and effervesces with acids : 
 if so, it must be purified by washing in dilute vinegar, or 
 the Bath will become alkaline, and dissolve off the Albumen. 
 
 When Kaolin cannot be obtained, the Bath may be de- 
 colorized by shaking it up with recently precipitated Chlo- 
 ride of Silver in the curdy state. This substance has an 
 affinity for the brown Sub-Albuminate of Silver which pro- 
 duces the colour, and gradually carries it down, leaving the 
 supernatant liquid clear ; but it is inferior to Kaolin as a 
 decolorizer. 
 
 Old printing Nitrate Baths often become somewhat al- 
 kaline from the reaction of the Albumen ; and the pro- 
 portion of Silver falls so low that the coagulation of the 
 Albumen is imperfect, and a white precipitate falls away 
 into the Bath. In such a case, add to each ounce ten 
 grains of Nitrate of Silver, and half a drop of Glacial 
 Acetic Acid. 
 
 Sensitive albuminized paper will usually remain good for 
 several days, if protected from the light, but it afterwards 
 turns yellow from partial decomposition. To preserve the 
 sheets white for a longer time, dry them in a warm place, 
 and then pile them up under pressure in a printing-frame : 
 this excludes the air, and is a better plan than that of 
 placing the papers between the leaves of a book, since the 
 printers' ink may gradually produce an effect upon the 
 surface. Eor a description of a drying apparatus intended 
 to preserve sensitized papers for an indefinite time, see 
 page 305. Papers so kept however can scarcely be consi- 
 dered equal to those recently sensitized. 
 
 * Common Animal Charcoal contains Carbonate and Phosphate of Lime, 
 the former of which renders the Nitrate of Silver alkaline, if Salts of Am- 
 monia are present ; purified Animal Charcoal 13 usually acid from Hydro- 
 chloric Acid. 
 
SALTING AND SENSITIZING PLAIN PAPEE. 493 
 
 Formula II. Preparation of Plain Paper. Take of 
 
 Chloride of Ammonium . . . 200 grains. 
 
 Citrate of Soda 200 grains. 
 
 Gelatine 20 grains. 
 
 Water 20 ounces. 
 
 To prepare the Citrate of Soda, dissolve 112 grains of 
 Citric Acid in the 20 ounces of Water, and add 133 grains 
 of the dried Bicarbonate of Soda used for effervescing 
 draughts. Supposing the Citric Acid to be adulterated with 
 Tartaric Acid, the above quantity of Carbonate of Soda 
 wouid be too great, and free alkaline Carbonate would then 
 remain in the liquid after the neutralization was complete. 
 The size of the paper would be liable to suffer in such a 
 case, and the print would not be clean and bright. Ex- 
 cess of Citric Acid, on the other hand, gives very clean 
 pictures, but they are too pale and red, without depth of 
 shadow. Amateurs desirous of simplifying the formula 
 may substitute an equal weight of " Eochelle Salt" for 
 the Citrate of Soda : this substance is a Tartrate of Potash 
 and Soda, and is sold by druggists in large crystals. A 
 reference to Part II., page 304, will show that both Tar- 
 trates and Citrates are used for the purpose of giving a red 
 and warm tone to the prints. 
 
 Use Papier Saxe, or Towgood's or Hollingworth's paper, 
 for the above formula, each sheet being floated for one 
 minute, and then suspended to dry as directed for albumi- 
 nized paper. It is doubtful whether salted papers contain- 
 ing Tartrate and Citrate are to be depended upon for long 
 keeping, since in presence of moisture these organic salts 
 become mouldy, and absorb Oxygen from the air. 
 
 Eender sensitive by floating for two or three minutes 
 upon the solution of Nitrate of Silver employed for the 
 Albuminized paper (p. 490). 
 
 Formula III. Ammonio-Nitrate Paper. This is always 
 prepared without Albumen, which is dissolved by Am- 
 raonio-Nitrate of Silver. Take of 
 
 Chloride of Ammonium ... 40 grains. 
 
 Gelatine 20 grains. 
 
 Water . 20 ounces. 
 
494 PRACTICE OF PHOTOGRAPHY. 
 
 Dissolve by the aid of heat, and filter when cold. 
 
 Take ten or a dozen sheets of Saxe's or Towgood's 
 paper, and having marked the right side, immerse them 
 bodily in the liquid, one by one, with care to remove 
 air-bubbles. Then turn the batch over and remove them 
 singly, beginning with the sheet first immersed. Each 
 paper will thus be a similar length of time in the salting 
 liquid. 
 
 This salting solution is very weak, but it must be borne 
 in mind that the papers being immersed will take up a 
 larger quantity, and also that the Ammonio-Nitrate pro- 
 cess requires less salt, inasmuch as the Silver solution is 
 to be laid on with a brush. 
 
 An Ammonio-Nitrate paper yielding a very rich colour 
 is made by salting Towgood's paper, or Papier Saxe, with 
 a mixed Chloride and Citrate, in quantity exactly one-half 
 of that advised in the last page, and afterwards sensitizing 
 it with an 80-grain solution of Ammonio-Nitrate. The 
 Writer however gives a formula with a smaller quantity 
 of salt in preference ; having found that beginners cannot 
 brush a highly-salted paper sufficiently evenly to prevent 
 marks on the finished picture. 
 
 Render sensitive by a solution of Ammonio-Nitrate of 
 Silver, 60 grains to the ounce of water, prepared as fol- 
 lows : 
 
 Dissolve the Nitrate of Silver in one-half of the total 
 quantity of water. Then take a pure solution of Ammonia 
 and drop it in carefully, stirring meanwhile with a glass 
 rod. A brown precipitate of Oxide of Silver first forms, 
 but on the addition of more Ammonia it is redissolved.* 
 When the liquid appears to be clearing up, add the Am- 
 monia very cautiously, so as not to incur an excess. In 
 order still further to secure the absence of free Ammonia, 
 it is usual to direct that, when the liquid becomes per- 
 fectly clear, a drop or two of solution of Nitrate of Sil- 
 ver should be added until a slight turbidity is again pro- 
 
 * If the excess of Ammonia does not readily dissolve the precipitate, the 
 Nitrate of Silver or the Ammonia is impure. 
 
PEEPAEATION OF AMMONIO-NITEATE PAPEE. 495 
 
 duced. Lastly, dilute with water to tlie proper bulk. If 
 the crystals of Nitrate of Silver employed contain a large 
 excess of free Nitric Acid, no precipitate will be formed 
 on the first addition of Ammonia : the free Nitric Acid, 
 producing Nitrate of Ammonia with the alkali, keeps the 
 Oxide of Silver in solution. This cause of error however 
 is not likely to happen frequently, since the amount of 
 Nitrate of Ammonia required to prevent all precipitation 
 would be considerable. From the same reason, viz. the pre- 
 sence of Nitrate of Ammonia, it is often useless to attempt 
 to convert an old Nitrate Bath already used for sensitizing 
 into Ammonio-Nitrate. The great objection to the use of 
 Ammonio- Nitrate of Silver, as above prepared, is the libe- 
 ration of free Ammonia in sensitizing the salted papers. 
 To obviate this in a measure, the Author employs a solu- 
 tion prepared as follows : Dissolve 60 grains of Nitrate 
 of Silver in half an ounce of water, and drop in Ammonia 
 until the precipitated Oxide of Silver is exactly re-dissolved. 
 Then divide this solution of Ammonio-Nitrate of Silver 
 into two equal parts, to one of which add Nitric Acid cau- 
 tiously, until a piece of immersed litmus- paper is reddened 
 by an excess of the acid ; then mix the two together, fill 
 up to one ounce with water, and filter from the milky de- 
 posit of Chloride or Carbonate of Silver, if any be formed. 
 
 Ammonio-Nitrate of Silver should be kept in a dark 
 place, being more prone to reduction than the Nitrate of 
 Silver. 
 
 To apply the liquid. It is not usual to float the paper 
 when the Ammonio-Nitrate of Silver is used, for reasons 
 given at page 301. Brushes are manufactured purposely 
 for applying Silver solutions, but the hair is soon destroyed 
 unless the brush is kept scrupulously clean. Lay the salted 
 sheet upon blotting-paper, and wet it thoroughly by draw- 
 ing the brush first lengthways and then across. Allow it 
 to remain flat for a minute or so, in order that a sufficient 
 quantity of the solution may be absorbed (you will see 
 when it is evenly wet by looking along the surface), and 
 then pin up by the corner in the usual way. If, on drying, 
 
496 PEACTICE OF PHOTOGEAPHY. 
 
 
 white lines appear at the points last touched by the brush, 
 it is probable that the paper was too highly salted, or that 
 the Ammonio-Nitrate contained free Ammonia. 
 
 Ammonio-Nitrate paper is more prone to spontaneous 
 decomposition than either Albuminized or plain paper ; 
 hence it cannot be kept many hours after sensitizing with- 
 out turning yellow. 
 
 PREPARATION OF THE TONING BATH. 
 
 Three formulae may be given, differing but little in their 
 mode of action, or in the tones which they yield. 
 
 No. 1. 
 
 Solution of Chloride of Gold . 1 fluid drachm. 
 Bicarbonate of Soda .... 10 grains. 
 Distilled Water 10 fluid ounces. 
 
 No. 2. 
 
 Solution of Chloride of Gold . 1 fluid drachm. 
 Phosphate of Soda .... 100 grains. 
 Distilled Water 10 fluid ounces. 
 
 No. 3. 
 Solution of Chloride of Gold . 1 fluid drachm. 
 
 Acetate of Soda 100 grains. 
 
 Distilled Water 10 fluid ounces. 
 
 Formula No. 1 should be mixed immediately before 
 use, as the Carbonate of Soda has a disposition to precipi- 
 tate the Gold on the bottom of the dish in the metallic 
 form. For the same reason, after the Gold has become 
 exhausted by the immersion of a succession of prints, it 
 is not advisable to add more Gold and keep the Bath 
 for future toning, but on each occasion to make a fresh 
 Bath. 
 
 Formula No. 2 and 3 are very similar in their composi- 
 tion and action. They should always be mixed several 
 hours before use, and when the Gold has been exhausted, 
 they may be bottled up and kept for a future occasion, 
 taking care to add more Gold as needed. Indeed, if care- 
 
TONING WITH GOLD AND FIXING FEINTS. 497 
 
 fully kept, it has been found that Baths made by these two 
 formula improve from use, yield brighter and cleaner 
 tones, and the prints are usually free from those small 
 white and red spots called " mealiness," which give them 
 a feeble appearance. 
 
 It would be impossible to say which of these toning 
 Baths is the best under all circumstances, much depend- 
 ing on the quality of the paper, and the composition of the 
 albuminizing fluid. It has been practically found that 
 with different samples of paper sometimes one and some- 
 times another formula is to be preferred ; hence it is 
 important that the operator, having once hit on the formula 
 which suits a particular paper, should, as far as possible, 
 continue to use the same sample of paper, albuminized 
 also in the same manner. The best Chloride of Gold to 
 use for these formulae is that from which the excess of 
 Hydrochloric Acid has been nearly driven off by repeated 
 evaporations to dryness on a water Bath. 
 
 The solution of Chloride of Gold contains a grain to each 
 fluid drachm of water, and will keep for an unlimited time 
 without appreciable change, previously to the addition of 
 the Carbonate of Soda. A useful simplification, substitu- 
 ting measure for weight, consists in having always on hand 
 an aqueous solution of Carbonate of Soda containing ten 
 grains to the ounce, or, for the second and third formula, 
 a solution of Phosphate or Acetate of Soda containing a 
 hundred grains to the ounce. An ounce of the alkaline 
 salt would then in each case require a fluid drachm of 
 the solution of Chloride of Gold, and a subsequent dilution 
 with water to the full amount given in the formula. 
 
 THE FIXING BATH. 
 
 Take of' 
 
 Hyposulphite of Soda .... 1 ounce. 
 
 Water 6 ounces. 
 
 Dissolve without filtering, and preserve the solution in 
 a stock bottle ready for use. 
 
 2K 
 
498 PEACTICE OF PHOTOGBAPHY. 
 
 MANIPULATOEY DETAILS OF PHOTOGEAPHIC FEINTING. 
 
 Sensitive papers ought not to be exposed in the frame 
 until they are quite dry, on account of the danger of 
 staining the Negative with Nitrate of Silver. It will often 
 be convenient to dry the paper rapidly by the heat of a 
 fire, or by a metal plate placed over a lamp, which may be 
 done without fear even in ordinary daylight. Ammonio- 
 Nitrate paper, however, will require a little extra care in 
 this respect. 
 
 Paper thus dried should always be allowed ten minutes 
 or longer to become cold before putting it into the printing- 
 frame, otherwise it will cockle up and form ridges beneath 
 the Negative. 
 
 The Exposure to Light. For this purpose reversing- 
 frames are sold, which admit of being opened at the back, 
 in order to examine the progress of the darkening by light, 
 without producing any disturbance of position. Simple 
 squares of glass however succeed equally well, when a little 
 experience has been acquired. They maybe held together 
 by the wooden clips sold at the American warehouses at a 
 shilling per dozen. The lower plate should be covered 
 with black cloth or velvet. 
 
 Supposing the frame to be employed, the shutter at the 
 back is removed, and the Negative laid flat upon the glass, 
 Collodion side uppermost. A sheet of sensitive paper is 
 then placed upon the Negative, sensitive side downwards ; 
 next comes a layer of thick felt ; and the whole is then 
 tightly compressed by replacing and bolting down the 
 shutter. The amount of pressure required is not very con- 
 siderable, but if the springs of the frame become too weak 
 after a time, a few pieces of mill-board may be placed 
 beneath them. 
 
 This operation may be conducted in the dark room ; but 
 unless the light is very strong, such a precaution will be 
 needless. The time of exposure to light varies much with 
 the density of the Negative and the power of the actinic 
 rays, as influenced by the season of the year and other 
 
. DETAILS OF FEINTING. 499 
 
 obvious considerations. As a general rule, the best Nega- 
 tives print rather slowly ; whereas Negatives which have 
 been under-exposed and under-developed, print quickly. 
 
 In the early spring or summer, when the light is power- 
 ful, about ten to fifteen minutes may be required ; but 
 from three-quarters of an hour to an hour and a half may 
 be allowed in the winter months, even in the direct rays 
 of the sun. It is always easy to judge of the length of 
 time which will be sufficient, by exposing a small slip of 
 the sensitive paper, unshielded, to the sun's rays, and 
 observing how long it takes to reach the coppery stage of 
 reduction. Whatever that time may be, nearly the same 
 will be occupied in the printing, if the Negative be a good 
 one. 
 
 In a dull London light, the Writer has seen four days 
 spent in getting one impression from a Negative ; but pic- 
 tures so obtained are not equal to others printed by a 
 stronger light. The organic matter of the size reacts more 
 or less upon the Nitrate of Silver, and causes yellowness 
 of the whites of the paper ; the toning process is also inter- 
 fered with, as will presently be shown. 
 
 A light of excessive brilliancy is objectionable in print- 
 ing, and especially so when the thermometer stands high. 
 Complaints of unmanageable bronzing, with obliteration 
 of details in the shadows, are frequent at such times, the 
 reducing process being carried on with too much activity. 
 Hence it is sometimes advisable, in the summer months, 
 not to print by the direct rays of the Sun. This point is 
 further important, because the excessive heat of the Sun's 
 rays often cracks the glasses by unequal expansion, and 
 glues the Negative firmly down to the sensitive paper. 
 An exception may be made in the case of Negatives of 
 great intensity, which are printed most successfully upon 
 a weakly sensitized paper exposed to the full rays of 
 the Sun; a feeble light not fully penetrating the dark 
 parts. 
 
 When the darkening of the paper appears to have pro- 
 ceeded to a considerable extent, the frame is taken in and 
 
500 PRACTICE OF PHOTOGRAPHY. 
 
 the picture examined. This, however, may be done in the 
 open air, with care and expedition. If squares of plate- 
 glass are used to keep the Negative and sensitive paper in 
 contact, some difficulty may be experienced at first in re- 
 turning it precisely to the same position after the examina- 
 tion is complete, but it will be overcome by practice. The 
 finger and thumb should be fixed on the lower corners or 
 edge, and the plate raised evenly and quickly. 
 
 If the exposure to light has been corrrect, the print ap- 
 pears slightly darker than it is intended to remain. The 
 toning Bath dissolves away the lighter shades, and reduces 
 the intensity, for which allowance is made in the exposure 
 to light. A little experience soon teaches the proper point ; 
 but much will depend upon the state of the Toning Bath, 
 and albuminized paper will require to be printed some- 
 what more deeply than plain paper. 
 
 If, on removal from the printing- frame, a peculiar spot- 
 ted appearance is seen, produced by unequal darkening of 
 the Chloride of Silver, either the Nitrate Bath is too weak, 
 the sheet removed from its surface too speedily, or the 
 paper is of inferior quality. 
 
 On the other hand, if the general aspect of the print is 
 a rich chocolate-brown in the case of Albumen, a dark 
 slate-blue with Ammonio- Nitrate Paper, or a reddish-pur- 
 ple with paper prepared with Chloride and Citrate of Sil- 
 ver, or Chloride and Serum of Milk, the subsequent parts 
 of the process will probably proceed well. 
 
 If, in the exposure to ordinary diffused daylight, the 
 shadows of the proof became very decidedly coppery be- 
 fore the lights are sufficiently printed, the Negative is in 
 fault. Ammonio-Nitrate paper highly salted is particularly 
 liable to this excess of reduction, and especially so if the 
 light is powerful. 
 
 The Toning of the Proof. No injury results from post- 
 poning this part of the process for some hours, provided 
 the print be kept in a dark place. But it is not advisable 
 to leave the print for several days before toning, since a 
 chemical change may take place, the effect of which will be 
 
 
DETAILS OF TONING FEINTS. 501 
 
 to interfere with the deposition of the Gold in the toning 
 process. It is considered that more harm is done by keep- 
 ing paper after printing, than by keeping it between sen- 
 sitizing and exposure. 
 
 The print should be first washed in common water until 
 the soluble titrate of Silver is removed. This is known 
 to be the case when the liquid flows away clear ; the first 
 milkiness being caused by the soluble Carbonates and 
 Chlorides in the water precipitating the Nitrate of Silver. 
 Ten minutes in water running slowly from a tap will be 
 sufficient to cleanse a print from Nitrate of Silver; or 
 three or four changes in a dish, pouring off quite dry be- 
 tween each change. A difference of opinion exists as to 
 the use of a final Bath of Salt and Water to convert the 
 last traces of Nitrate into Chloride of Silver. Most ope- 
 rators content themselves with simple Water, but the 
 Writer believes that it is an advantage to finish off with 
 solution of Salt (two grains to the ounce), in cases where 
 the printing has not been carried far enough ; the half- tints 
 are better preserved thereby from the destructive action 
 of the Chlorine contained in the Gold solution, and the 
 colour is not so likely to disappear in the fixing Bath. 
 
 A white sediment often forms upon the prints if they 
 are left too long in hard water without moving. The 
 sediment consists of Carbonate and Chloride of Silver, and 
 would be dissolved by treatment with water smelling very 
 faintly of free Ammonia. The fixing Bath, however, will 
 clear it away in great measure, leaving only a slight want 
 of brilliancy on drying. 
 
 Pour the toning Bath out into a flat dish and put the 
 prints into it two or three at a time, waving the dish mean- 
 while backwards and forwards to secure a constant move- 
 ment. The danger is, that the prints may project above the 
 surface of the liquid, or that they may stick together ; the 
 result in both cases being a large red spot of imperfect 
 toning. Continue to keep the prints moving, and watch 
 the changes in colour. At first the toning will be com- 
 plete in two or three minutes ; then, as the proportion of 
 
502 PRACTICE OF PHOTOGRAPHY. 
 
 Gold falls, in five minutes ; afterwards in ten minutes or a 
 quarter of an hour. 
 
 The action of the Bath must be regulated according to 
 the colour desired. If the prints are removed as soon as 
 the blue colour of the Gold is seen, they will usually 
 change in the fixing Bath to a warm shade of brown ; but 
 when left for two or three minutes longer in the toning 
 Bath, the darker tint becomes permanent. Hold them up 
 against the light, and when they cease to appear red by 
 transmission, the colour is well fixed upon the image and 
 they are ready for the Hyposulphite. 
 
 Overprinted proofs always yield the blackest colours, 
 because they may be kept for a longer time in the Gold 
 without losing the half-tones ; and, indeed, the state of the 
 lighter shades of the proof is a good criterion of the time 
 for removing it, since the Chlorine previously combined 
 with the Gold has a bleaching action. 
 
 Do not attempt to get pure black and white tones on 
 pictures printed from a feeble Negative. Unless there be 
 a perceptible amount of bronzing, the deep blacks cannot 
 be obtained on albuminized paper. The ultimate colour 
 of the Print will also vary much with the density of the 
 Negative and the character of the subject : copies of line 
 engravings having but little half-tone, are easily obtained 
 of a dark shade resembling the original impression. 
 
 Each grain of Chloride of Gold ought to tone seven or 
 eight pictures of the stereoscopic size without any diffi- 
 culty, or even a larger number if the Bath be used until 
 it is entirely exhausted. Mr. C. Jabez Hughes, who has de- 
 voted much attention to this process, states that one grain 
 of Chloride of Gold will tone five hundred square inches, 
 of twenty stereoscopic pictures. 
 
 The most common error with beginners is to leave the 
 prints too long in the toning Bath, in which case they ap- 
 pear blue and inky after fixing, without any depth of sha- 
 dow. Sometimes however the opposite error prevails, and 
 the prints having been removed too speedily, the Hypo- 
 sulphite leaves them of a brick -red hue. 
 
DETAILS OF TONING AND FIXING. 603 
 
 The observations given above apply principally to albu- 
 minized paper, which is far more generally employed than, 
 plain paper. Plain paper prints tone with greater facility 
 than prints upon Albumen, and hence it may be desirable 
 to dilute the alkaline Chloride of Gold with water, if the 
 action of the Bath should prove uncontrollable. 
 
 In toning prints by alkaline Chloride of Gold, it is not 
 necessary to work entirely in the yellow room, but a sub- 
 dued light is usually recommended. 
 
 Washing in water between the toning and fixing is not 
 required ; the proofs may be passed from the one Bath to the 
 other. In doing this, however, there is great danger that 
 the fingers will convey a little of the Hyposulphite fixing 
 Bath into the toning Bath, and if so, the Chloride of Gold 
 will be decomposed, and the toning action thereby sus- 
 pended. A pair of horn forceps should be kept in the 
 fixing Bath, and each print as it is removed from the toning 
 solution with the left hand should be seized by the for- 
 ceps held in the right hand, and be immediately immersed 
 in the Hyposulphite solution. 
 
 An ounce of Hyposulphite of Soda dissolved in six ounces 
 of Water would fix two batches of stereoscopic prints, 
 twenty in each batch. The solution however may be 
 thrown away when twenty prints have been immersed in 
 each six ounces of Bath, since the price of Hyposulphite of 
 Soda is very small, and it is not worth while to risk the oc- 
 currence of yellow patches of imperfect fixation. Allow 
 the prints to remain in the fixing Bath for twenty minutes, 
 with occasional movement, after which they may be trans- 
 ferred to a dish of clean water. 
 
 A newly-prepared fixing Bath will usually lower the 
 tone of the prints more or less, and this even although 
 a strong deposit of Gold exists upon the image. The 
 weaker the solution of the Hyposulphite, the less the 
 lowering action, and the greater the probability that the 
 slight loss of colour at first perceived, will return on 
 drying the print. Solutions of Hyposulphite which are 
 used over and over again (fresh crystals of Hyposulphite 
 
504 PRACTICE OF PHOTOGRAPHY. 
 
 being occasionally added, until the specific gravity is found 
 to be correct by the hydrometer), acquire by degrees a 
 slight sulphuretting property which, much improves the 
 colour of the print. This mode of working is adopted by 
 some ; but, on theoretical grounds, the plan of making a 
 fresh bath each day is better. 
 
 The washing, drying, and mounting of the Positive 
 Proofs. It is essential to wash out every trace of Hypo- 
 sulphite of Soda from the Print if it is to be preserved 
 from fading, and to do this properly requires care. 
 
 Always wash with running water when it can be ob- 
 tained, and choose a large shallow vessel exposing a con- 
 siderable surface in preference to one of lesser diameter. 
 A constant dribbling of water must be maintained for four 
 or five hours, and the prints should not lie together too 
 closely, or the water will not find its way between them. 
 
 The most perfect apparatus which the Writer has seen 
 for washing prints on a large scale, consisted of a series of 
 circular troughs, about a yard wide and twelve inches deep, 
 with a small syphon in each, so arranged that when the 
 water rose to the bend of the tube, the syphon at once 
 began to act, and gradually drew it off dry. Twice during 
 the day, when the water came on from the main, it was 
 allowed to flow freely through the troughs, and being di- 
 rected by a tube against the sides, the prints were kept 
 spinning round, and a constant change of water was thus 
 secured. It would not be safe to make such troughs of 
 Zinc or any common metal, unless a glaze were applied to 
 the interior, since Hyposulphite charged with Salts of Sil- 
 ver gradually acts upon Zinc. 
 
 When running water cannot be obtained, proceed as 
 follows: First wash the Prints gently, to remove the 
 greater part of the Hyposulphite solution. Then transfer 
 them to a large shallow pan, in which may be placed as 
 many Prints as it will conveniently hold. Leave them 
 in for about a quarter of an hour, with occasional move- 
 ment, and then pour off the water quite dry. This point 
 is important, viz. to drain off the last portion of liquid 
 
DRYING AND MOUNTING FEINTS. 505 
 
 completely before adding fresh water. Repeat this process 
 every half- hour, giving five or six changes or more, accord- 
 ing to the bulk of water, number of prints, and degree of 
 attention paid to them. 
 
 Dabbing with a large sponge has been recommended for 
 cleansing prints from Hyposulphite. A quarter of an 
 hour's application of this kind goes further than two or 
 three hours of washing, since the suction power of a sponge 
 draws out the liquid from the substance of the paper. 
 
 Lastly, proceed to remove the size from the print by im- 
 mersion in hot water.* Ammonio-Nitrate and plain paper 
 Prints prepared on foreign papers may be expected to stand 
 the test of boiling water ; Albumen Prints and Positives 
 on English paper are a little reddened, although not to an 
 objectionable degree. 
 
 The treatment of prints with hot water is a plan which 
 all approve theoretically, but few adopt ; the Writer how- 
 ever does not esteem a print secure unless it has been im- 
 mersed in water nearly at the boiling point. 
 
 When the prints have been thoroughly washed, blot 
 them off between sheets of porous paper and hang them 
 up to dry. Some press them, with a hot iron, which 
 darkens the colour slightly, but not in an injurious manner, 
 unless Hyposulphite of Soda remains in the paper. This 
 darkening by dry heat is most evidently seen in the case 
 of English papers sized with Gelatine, and sensitized by 
 Ammonio-Nitrate of Silver. 
 
 Albumen proofs when dry are sufficiently bright without 
 further treatment ; but in the case of plain paper, salted 
 simply, the effect is improved by laying the Print face 
 downwards upon a square of plate-glass and rubbing the 
 back with an agate burnisher, sold at the artists '-colour- 
 man's. This hardens the grain of the paper and brings 
 out the details of the picture. Hot-pressing has a similar 
 effect, and is often employed. 
 
 * The Print must be well washed in cold water, to remove the Hyposul- 
 phite, before using the hot water ; otherwise the half-tones will be liable to 
 be darkened, or changed to incipient yellowness, by sulphuration. This 
 point is important as regards the permanency. 
 
506 PRACTICE OF PHOTOGRAPHY. 
 
 Mount the proofs with a solution of Gelatine in hot 
 water, freshly made : the best Scotch glue answers well. 
 Gum-water prepared from the finest commercial gum, 
 and free from acidity, may also be used ; but it should be 
 made very thick, so as not to sink into the paper, or pro- 
 duce " cockling up " of the cardboard on drying. 
 
 Caoutchouc dissolved in mineral Naphtha to the consist- 
 ence of thick glue or gold-beaters' size, is employed by 
 some for mounting Photographic Prints ; it may be ob- 
 tained at the varnish-shops, and is sold in tin boxes. The 
 mode of using it is as follows : With a broad brush made 
 of stiff bristles, apply the cement to the back of the pic- 
 ture ; then take a strip of glass with a straight edge, and 
 by drawing it across the paper, scrape off as much as 
 possible of the excess. The print will then be found to 
 adhere very readily to the cardboard, without causing ex- 
 pansion or cockling ; and any portion of the cement which 
 oozes out during the pressing, may, when dry, be removed 
 with a penknife without leaving a stain. 
 
 VIGNETTING. 
 
 The mode of printing, by which the edges of the Photo- 
 graph are softened away, produces a very agreeable effect. 
 Attempts have been made to obtain the result by placing 
 in front of the Negative a plate of tinted glass, having the 
 colour stippled in gradually from the edges, so as to leave 
 the centre quite transparent. Some of the glasses made in 
 that way are very good, but often the gradation is imper- 
 fect, and the print in consequence shows an abrupt margin. 
 
 The use of Cotton- wool has been recommended for print- 
 ing vignettes, but it requires dexterity to arrange it. The 
 following plan is more simple. Take a sheet of mill-board 
 or coloured card, and cut out from the centre an aperture 
 decidedly smaller than the picture required. The shape 
 of this aperture may be varied at pleasure, but for por- 
 traits nothing is better than an oval, or the same expanded 
 below into a triangular-shaped outline. Fasten this screen 
 to the front part of the printing-frame, at a distance of 
 
 
PRINTING BY DEVELOPMENT ON IODIDE. 507 
 
 about three-quarters of an inch, and expose in the ordinary 
 way. The direct rays of the sun must be avoided, and the 
 print should be placed in a shady place, when the light will 
 diffuse itself evenly, and a perfect softening will be pro- 
 duced to an extent varying with the size of the aperture 
 and its distance from the front glass. The strength of the 
 light is to be considered in determining the size of the 
 aperture ; and in whatever direction the light falls, the 
 frame should be placed at right angles to it. 
 
 SECTION II. 
 
 Positive Printing by Development, 
 
 Negative printing processes will be found useful during 
 the dull winter months, and at other times when the light 
 is feeble, or when it is required to produce a large number 
 of impressions from, a Negative in a short space of time. 
 The proofs however as thus obtained are not equal to direct 
 sun-prints in beauty and gradation of tone. 
 
 In the present Edition two methods are described, of 
 which the first is more suitable when a luminous image of 
 low intensity is to be projected upon the prepared paper, 
 and the second may be preferred for printing beneath a 
 Negative in the ordinary way. 
 
 A NEGATIVE FEINTING PROCESS UPON IODIDE 
 AND BROMIDE OF SILVER. 
 
 Iodide of Silver is more sensitive to the reception of the 
 invisible image than the other compounds of that metal ; 
 and hence it is usefully employed in printing enlarged Po- 
 sitives from small Negatives, by means of the solar Ca- 
 mera. The great stability of the proofs upon Iodide of 
 Silver will also be a recommendation of this process when 
 unusual permanency is required. 
 Take of 
 
 Iodide of Potassium . . . 120 grains. 
 
 Bromide of Ammonium . . 30 grains. 
 
 Water . 20 fluid ounces. 
 
508 PBACTICE OF PHOTOGRAPHY. 
 
 The best paper to use will be either Turner's Calotype, 
 or Whatman's or Hollingworth's Negative ; the foreign 
 papers do not succeed well with the above formula. 
 
 Float the paper on the iodizing Bath, until it ceases to 
 curl up, and lies flat upon the liquid ; then pin up to dry 
 in the usual way. 
 
 Render sensitive upon a Bath of Aceto-Nitrate of Silver 
 containing 30 grains of Nitrate of Silver with 30 minims 
 of Glacial Acetic Acid to each ounce of water. 
 
 When the process is to be used for printing beneath a 
 Negative, place the dried sheet in contact with the Nega- 
 tive in a pressure frame, and expose to a feeble light. 
 About 30 seconds will be an average time upon a dull 
 winter's day, when it would be impossible to print in the 
 ordinary way. On removing the Negative nothing can be 
 seen upon the paper, the image being strictly invisible in 
 this process, unless the exposure has been carried too far. 
 
 Develope by immersion in a saturated solution of Gallic 
 Acid prepared as described under the head of " Collodio- 
 Albumen dry process." The image appears slowly, and 
 the process may last from fifteen minutes to half an hour. 
 If the exposure has been correctly timed, the Gallic Acid 
 appears at length almost to cease acting ; but when the 
 proof has been over-exposed, the development goes on un- 
 interruptedly, and the image becomes too dark, partaking 
 more of the character of a Negative than a Positive, and 
 disappearing from the surface. The usual rule, that under- 
 exposed proofs develope slowly, but show no half-tones, and 
 that the over-exposed develope with unusual rapidity, is 
 observed in the process with Iodide of Silver. 
 
 After the picture is fully brought out, wash in cold, and 
 subsequently in warm water, to remove the Gallic Acid, 
 which, if allowed to remain, would discolour the Hyposul- 
 phite Bath. Fix the print in a solution of Hyposulphite 
 of Soda, one part to two of water, continuing the action 
 until the yellow colour of the Iodide disappears. The fix- 
 ing Bath ought not to produce much change in the tint. 
 If the Positive loses its dark colour on immersion in the 
 
FEINTING BY DEVELOPMENT ON CHLORIDE. 509 
 
 Hyposulphite, and becomes pale and red, it has been in- 
 sufficiently developed and probably too much exposed. 
 
 The colour of Positives developed upon Iodide and 
 Bromide of Silver is not very agreeable, but by fixing the 
 proof in Hyposulphite of Soda which has been long used 
 and has acquired sulphuretting properties, the tint is some- 
 what improved. 
 
 NEGATIVE PRINTING PROCESS UPON CHLORIDE OF 
 SILVER. 
 
 Positives may be obtained by exposing ordinary Chlo- 
 ride of Silver Paper to the action #f light until a faint 
 image is perceptible, and subsequently developing it by 
 Gallic Acid. It is difficult however to obtain sufficient con- 
 trast of light and shade in this way ; the impression, if 
 sufficiently exposed and not too much developed, being 
 feeble, with a want of intensity in the dark parts. By as- 
 sociating with the Chloride an organic salt of Silver, such 
 as the Citrate, this difficulty may be overcome, and the 
 shadows brought out with great depth and distinctness. 
 
 The formula now given is the best known to the Writer, 
 but he cannot recommend it with full confidence. All Ne- 
 gative paper processes appear to be greatly dependent upon 
 the quality of the paper used, and it is not easy to obtain 
 Photographic paper uniformly good. Hence such defects 
 result as a sunk-in appearance of the picture, and a po- 
 verty of tone, which would not occur with a paper sized in 
 a different way. It will be better that the operator should 
 not attempt printing by development until he feels himself 
 to be a perfect master of the Positive process. 
 
 To salt the papers, prepare the following solution : 
 
 Chloride of Ammonium .... 100 grains. 
 
 Citric Acid 56 grains. 
 
 Bicarbonate of Soda ..... 66 grains. 
 
 Gelatine 40 grains. 
 
 Distilled Water 20 ounces. 
 
 The effervescing Carbonate of Soda sold by druggists 
 
510 PRACTICE OF PHOTOGEAPHY. 
 
 is the compound to be used, and the Citric Acid must be 
 genuine, not adulterated with Tartaric Acid (see Vocabu- 
 lary). Choose the Papier Saxe in preference to any other 
 paper, and float the sheets for one minute, after which they 
 may be pinned up in the usual way. 
 
 This salting solution is liable to decompose by keeping, 
 since Citrates become mouldy in presence of Gelatine. 
 Alcohol lj ounce with two drops of Oil of Cloves dissolved 
 would tend to preserve it. Complaints have also been 
 made of the salted paper decomposing by keeping. To 
 obviate this, hold each sheet to the fire before packing it 
 away, and keep the paper in a very dry place. 
 
 Render sensitive uon a Bath of Aceto-Nitrate of Silver 
 prepared in the same way as for the Collodio%Albumen dry 
 process ; and the proper management of which is described 
 in the Fifth Chapter. 
 
 Take of 
 
 Nitrate of Silver 30 grains. 
 
 Glacial Acetic Acid .... 30 minims. 
 Water 1 fluid ounce. 
 
 Float the papers upon this Bath for three minutes, and 
 suspend them to dry in a room from which actinic rays are 
 perfectly excluded. 
 
 The exposure to light is conducted in the ordinary print- 
 ing-frame, the Negative and sensitive paper being laid in 
 contact in the usual way, and it will seldom be longer than 
 three or four minutes, even upon a dull day, It may be 
 regulated by the colour assumed by the projecting margin 
 of the paper ; but it is quite possible to tell by the appear- 
 ance of the image when it has received a sufficient amount 
 of exposure. The whole of the picture should be seen, 
 excepting the lightest shades, and it will be found that 
 comparatively few details can be brought out in the deve- 
 lopment which were altogether invisible before the Gallic 
 Acid was applied. 
 
 The developing solution is prepared as follows : 
 Gallic Acid . . . . . . . . 2 grains. 
 
 Water .1 fluid ounce. 
 
 
PRINTING BY DEVELOPMENT ON CHLORIDE. 511 
 
 Filter through blotting-paper to remove floating parti- 
 cles, which would produce spots in this process. 
 
 To facilitate the solution of the Gallic Acid, place the 
 bottle in a warm situation by a fire. A lump of Camphor 
 floated in the liquid, or a drop of Oil of Cloves added, will 
 to a great extent prevent it from becoming mouldy by 
 keeping ; but if once mould has formed, the bottle must 
 be well cleansed with Nitric Acid, or the decomposition of 
 the fresh Gallic Acid will be hastened. 
 
 When the weather is cold and the light bad, the deve- 
 lopment of the image proceeds slowly, the Gallie Acid Bath 
 remains clear, and good half-tones are obtained; but under 
 opposite conditions, the developer may become turbid and 
 the shadows be lost by excessive deposit of Silver. This 
 over-development will be remedied by printing the Ne- 
 gative in a feeble light (near to the open window of a 
 room), and by adding Acetic Acid to the developer, about 
 five or ten minims to the ounce, so as to bring out the 
 image more slowly. 
 
 Pour the solution of Gallic Acid into a flat dish, and 
 immerse the prints two or three at a time, moving them 
 about, and using a glass rod to remove air-bubbles. The 
 development will probably be completed in from three to 
 six minutes. If the Print developes slowly, becoming after 
 a time very dark in colour, but showing no half-tones, it has 
 not been exposed sufficiently long to the light. An over- 
 exposed proof, on the other hand, developes with unusual 
 rapidity, and it is necessary to remove it speedily from the 
 Bath in order to preserve the clearness of the white parts ; 
 when taken out to the light, it appears pale and red, with 
 no depth of shadow. 
 
 A saving of Gallic Acid may be effected by adopting a 
 plan of which the Writer was informed by Mr. Sutton, of 
 Jersey. It consists in turning up the edges of the paper 
 all round, so as to make the print its own developing-dish. 
 No pinning at the corners will be needed, and the solution 
 of Gallic Acid may be spread by means of a glass-rod bent 
 at right-angles. Lay each print upon a plate of glass, to 
 
512 PEACTICE OF PHOTOGRAPHY. 
 
 prevent the back from being soiled. The paper ought not 
 to be of a kind which is given to cockling, or portions of 
 the image will rise above the level of the liquid. Bear in 
 mind that the development will be unusually rapid and in- 
 tense when this mode of manipulating is followed ; inas- 
 much as the Nitrate of Silver is all retained in the picture, 
 and the quantity of Gallic Acid is relatively small. Hence 
 an addition of Acetic Acid may be needed, as before ad- 
 vised. 
 
 The extent to which the development should be carried 
 depends upon the kind of Print desired. By pushing the 
 action of the Gallic Acid, a dark picture not much altered 
 by the fixing Bath will be produced. But a better result 
 as regards colour and gradation of tone will be obtained 
 by removing the Print from the developing solution whilst 
 in the light-red stage, and toning it subsequently by means 
 of Gold ; in which case it will correspond very much both 
 in appearance and stability to a Positive obtained by the 
 direct action of light. 
 
 When it is intended to follow the latter plan, the action 
 of the developer must be stopped at a point when the proof 
 appears lighter than it is to remain ; since the Gold Bath 
 adds a little to the intensity, and the image becomes some- 
 what more vigorous on drying. 
 
 Wash the Prints for several minutes in one or two 
 changes of water, in order to extract the Gallic Acid; 
 which if allowed to remain will discolour the toning Bath. 
 Sometimes the developing action of the Gallic Acid will 
 continue whilst the pictures are washing in the water, and 
 the whites will in consequence be discoloured. This may 
 be obviated by washing them in weak salt and water, so 
 as to decompose the remaining traces of Nitrate of Silver ; 
 but the proportion of Salt must not be greater than two or 
 three grains to the ounce, or the image will become red 
 and feeble. Ammonia would not be proper for washing, 
 since it would at once discolour the Gallo -Nitrate. 
 
 The formula for a toning Bath may be taken from page 
 496, but it is questionable whether much Alkali ought to 
 
 
FEINTING BY DEVELOPMENT. 513 
 
 be used, since the Gallic Acid left in the paper would turn 
 brown in presence of free Carbonate of Soda. Add, there- 
 fore, to each grain of Chloride of Gold not more than half 
 a grain of the alkaline Carbonate. Avoid also a prolonged 
 action of the toning Bath, since the tendency to a cold and 
 inky colour is greater in developed Prints than in Prints 
 obtained without development. 
 
 Upon comparing the developed Prints with others ob- 
 tained by the direct action of light, it is evident that the 
 advantage is on the side of the latter ; but the difference 
 is not great, and would be overlooked in printing large 
 subjects, for which the Negative process is more especially 
 adapted. The colour of both kinds of Positives is the same, 
 or perhaps a shade darker in the developed proofs, which 
 are usually of a violet-purple tone, but sometimes of a dark 
 chocolate-brown. 
 
 Failures. Printing by development must not be at- 
 tempted until the operator is quite conversant with the 
 Negative process upon paper or Collodion. He will then 
 be aware of the importance of excluding scattered rays of 
 white light from the developing room, and of filtering all 
 the liquids, points which are not of so much importance in 
 the ordinary mode of printing by direct action of light. 
 
 Citrate of Soda is used in the above process, and the dif- 
 ficulty of obtaining this Salt commercially is an objection. 
 In making it extemporaneously by neutralizing Carbonate 
 of Soda with Citric Acid, when Tartaric Acid is present 
 as an adulteration, the Soda is in excess, Citric Acid having 
 more neutralizing power than Tartaric Acid. Test papers 
 would be of little use in examining the liquid, because 
 Citrate of Soda itself has an alkaline reaction. It may, 
 therefore, be desirable to notice the results which are likely 
 to follow when the salting Bath contains free Carbonate 
 of Soda. In the first place it would tend to dissolve the 
 size from the paper, and to give the resulting Print a mealy 
 appearance. Obviate the chance of this by not floating the 
 papers longer than one minute. Secondly, Citrate papers 
 with excess of Alkali remove by degrees the Acetic Acid 
 
 2L 
 
514 PRACTICE OF PHOTOGEAPHY. 
 
 from tine exciting Bath, the effect of which would be at 
 first to increase the rapidity- of development and intensity 
 of image ; but afterwards, when the Bath became nearly 
 neutral, to produce a foggy picture. 
 
 Experimenters in this process will do well to try the 
 effect of varying the mode of development, by using the 
 solution of Pyrogallic Acid and Citric Acid given at page 
 401, in place of Gallic Acid. 
 
 Transparencies on Glass. The finest transparencies on 
 glass have been obtained by the Continental Photographers 
 on dried albumen films. Some of them leave nothing to 
 be desired in their beautiful richness and softness of tone. 
 The details of the process are, however, kept secret, and 
 most of those who have attempted it in thia country, while 
 succeeding in obtaining sharpness of definition, fail in ob- 
 taining the richness of tone, so characteristic of the works 
 of Ferrier and Soullier, of Paris. Dried albumen, on ac- 
 count of its structureless character, is eminently fitted for 
 giving sharp definition and general delicacy of detail, but 
 when developed by the ordinary method, and viewed as a 
 transparency, the^image is usually characterized by a cold, 
 dull-reddish or yellowish-brown, lacking brilliancy and 
 tone. Citric Acid in the developer, to some extent, modi- 
 fies this for the better, and it is probably by a judicious 
 use of this acid, and the toning action of gold, that the 
 finest specimens have been produced. 
 
 Good transparencies may be obtained by most of the 
 dry Collodion processes, but by far the best, and almost 
 rivalling the finest on Albumen, have been produced by a 
 slight modification of Russell's Tannin process, which is 
 thus described : 
 
 The Collodion should be structureless and limpid, and 
 setting on the glass without lines or markings of any kind. 
 The normal Collodion of p. 377, thinned down with one- 
 third of its bulk of pure Ether and Alcohol in equal pro- 
 portions, and iodized with not more than 3 grains of Iodide 
 of Potassium to the ounce, works satisfactorily. It is 
 better if kept till decomposition has fully set in, and the 
 
TRANSPARENT PRINTS ON GLASS. 515 
 
 colour has become of a light port- wine tint by the libera- 
 tion of Iodine. Bromide seems to be disadvantageous 
 
 O 
 
 rather than otherwise, having a tendency to injure the 
 tone considerably, rendering it dull and cold. 
 
 Excite in a bath slightly acidified by Acetic in prefer- 
 ence to Nitric Acid ; wash and cover with a five-grain 
 solution of Tannin in the usual way, and stand up to dry 
 (see Tannin process). A stronger solution of Tannin is 
 not desirable, as favouring too great intensity and hardness. 
 
 Printing. The Negatives to be used in printing trans- 
 parencies ought not to be very dense, otherwise there will 
 be a danger of the shadows of the transparency losing 
 depth by over-action of light, and presenting in conse- 
 quence a pale translucent appearance. Negatives de- 
 veloped by Sulphate of Iron, without further intensifying, 
 are often remarkably well adapted for the purpose. 
 
 Place the Negative and the prepared plate in contact, 
 and squeeze them together in an ordinary pressure frame : 
 not too strongly, however, or it will probably be found, 
 after throwing off a few impressions, that the Negative 
 has been scratched. Lay a strip of black velvet behind 
 the sensitive film, to absorb stray light. 
 
 The printing may be effected by an exposure of three or 
 four minutes to gas-light, or of a few seconds to ordinary 
 daylight. The subdued light of a room is very good for 
 the purpose, and will probably form the latent image in 
 about twenty seconds. 
 
 Development. Before developing, attend to the pre- 
 cautions mentioned at p. 455, to prevent the film from 
 leaving the glass, also moisten the surface with about a 
 drachm of Alcohol in an ounce of distilled water. 
 
 Three developing solutions will be required, and should 
 be placed near at hand. 
 
 f Pyrogallic Acid ..... 1 grain. 
 
 No. 1. < Citric Acid ....... 1 grain. 
 
 L Distilled water ...... 1 ounce. 
 
 f Pyrogallic Acid ..... 1 grain. 
 
 No. 2. < Acetic Acid 30 minims. 
 
 I Distilled water 1 ounce. 
 
516 PEACTICE OF PKOTOGKAPHY. 
 
 f Nitrate of Silver 20 grains. 
 
 No. 3. < Citric Acid 5 grains. 
 
 L Distilled water 1 ounce. 
 
 The object of the two Pyrogallic solutions is to enable 
 the operator to control the colour and tone of the deposit ; 
 bearing in mind that the Acetic Acid developer imparts a 
 strong reddish-brown tone to the picture, and the Citric 
 Acid a blue colour. By a mixture of the two, in propor- 
 tions which the circumstances of the case only can deter- 
 mine, any intermediate tone can readily be obtained. 
 Much in this respect also will depend on the amount of 
 exposure, an over-exposed Negative sometimes showing a 
 reddish tone when developed with the Citro-pyrogallic 
 alone. As a rule it is better rather to over-expose and un- 
 der-develope, as a safeguard against too much opacity. For 
 other details of development not mentioned, see p. 420. 
 
 Fix with Hyposulphite of Soda, and wash as usual. 
 
517 
 
 CHAPTER IV. 
 
 CLASSIFICATION OF CAUSES OF FAILTJKE IN THE 
 COLLODION PROCESS. 
 
 SECTION I. Imperfections in Collodion Photographs. 
 SECTION II. Imperfections in Paper Positives. 
 
 SECTION I. 
 
 Imperfections in Negative and Positive Collodion 
 Photographs. 
 
 FOGGING OF COLLODION PLATES. 
 
 1. Over-exposure. The image in the Camera is rarely 
 pure, but exhibits enough diffused light to produce uni- 
 versal clouding when the plate is at all over-exposed. 
 Collodion newly iodized being more sensitive to a weak 
 light, shows the fogging more than old coloured Collodion. 
 When the objects are at a great distance, and brightly 
 illuminated, or when Lenses of large aperture are employed, 
 without a diaphragm, beginners are especially likely to 
 over-expose the plates. 
 
 2. Diffused Light. a. In the developing room. This 
 is a frequent cause of fogging, and especially so when a 
 naked candle is employed to illuminate, or the common 
 yellow calico is used, which is apt to fade. Examine for 
 cracks and crevices in the sides and floor of the room. 
 b. Mist in the atmosphere. When there is any vapour in 
 
518 PEACTICE OF PHOTOGRAPHY. 
 
 the air, the shadows will be foggy, unless the object be 
 very near to the Camera. c. Diffused light in the Ca- 
 mera : light may get in between the two portions of the 
 body (p. 434). d. Diffused light from the Lens. If a 
 Portrait Lens, see how to protect it at p. 432. In the case 
 of a View Lens producing a false glare of light, deal with 
 it as described at p. 444. Orthoscopic Lenses sometimes 
 give a foggy image from imperfect arrangement of the 
 separate glasses, only to be corrected by the maker of the 
 Lens. e. Diffused light in instantaneous Photography. 
 See page 471. 
 
 3. Alkalinity of the Bath. This condition, explained at 
 p. 249, may be suspected, if the operator has added Am- 
 monia to neutralize Nitric Acid: in all probability he has 
 used too much. If the Bath has been shaken up with 
 Chalk or Carbonate of Soda to remove excess of Acid, 
 Acetic or Nitric Acid must afterwards be used (p. 250). 
 
 4. Acidity of the Bath. Too much Nitric Acid in a 
 weak Bath will produce fogging (p. 158). Glass Positives 
 are often cloudy from this cause. 
 
 5. Organic Matter in the Bath. The Nitrate of Silver 
 not freed from organic impurity by re-crystallization 
 (p. 252) its solution decomposed by being kept in new 
 Baths of Gutta-percha (p. 252), or by use of foul-smelling 
 rain-water, or distilled water collected from condensed 
 water of steam-boilers, and containing oily and ernpy- 
 reumatic matters. When a Bath gives foggy pictures 
 from these causes, the longer the plates are left in it the 
 worse will be the result. 
 
 6. Use of colourless Collodion. Collodion iodized with 
 Cadmium, and used within a week, or iodized with the 
 Iodide of Potassium and used immediately, will often give 
 foggy Negatives if the other solutions are not in the best 
 working order. Mix the new Collodion with a little old 
 Collodion, or drop in Alcoholic Tincture of Iodine until 
 the Collodion becomes straw-yellow (p. 481). 
 
 7. Heat of the Weather. I? the Bath and Collodion 
 are both pure, clear Negatives will be obtained at any 
 
CAUSES OF FOGGING OF COLLODION PLATES. 519 
 
 temperature short of 90 Fahrenheit. Above that point 
 proceed to modify the developer, as described at p. 279. 
 
 8. Sulphate of Iron as a Developer. If fogging results, 
 the Bath and Collodion give an insufficient amount of in- 
 tensity (p. 281), or there is diffused light in quantity so 
 small as not to be detected by Pyrogallic Acid, which is a 
 less active reducing agent than Sulphate of Iron. 
 
 9. Sundry other causes of fogging. a. Vapour of Am- 
 monia or the products of the combustion of coal-gas, es- 
 caping into the developing room. b. Development of the 
 image by immersion in solution of Sulphate of Iron : it is 
 better to pour the fluid over the plate, and not to use the 
 same portion twice. c. Redipping the plate in the Bath, 
 before development : this is apt to give a foggy picture 
 when using an old Bath. d. Omission of the Acetic Acid 
 in the solution of Pyrogallic : this will produce a universal 
 blackness (p. 157). e. Developing solution inadvertently 
 dropped into the Nitrate Bath. -f. Metallic Iron left in 
 contact with the Bath solution. g. Using too strong a 
 developer in hot weather, or keeping any developer, and 
 especially Sulphate of Iron, on the film for too long a 
 time. 
 
 Systematic plan of proceeding to detect the cause of the 
 fogging. If the operator has had but little experience in 
 the Collodion process, and is using Collodion of moderate 
 sensitiveness and a new Bath, the probability is that the 
 fogging is caused by over-exposure. Having obviated this, 
 proceed to test the Bath ; if it was made from pure mate- 
 rials, and does not restore the blue colour of a piece of 
 litmus-paper previously reddened by holding it over the 
 mouth of a Glacial Acetic Acid bottle (p. 250), it may be 
 considered in working order; but in the case of an old 
 Bath this does not apply, because the Bath may have been 
 spoilt by organic matter. 
 
 Next prepare a sensitive plate, and after draining it for 
 two or three minutes on blotting-paper, pour on the deve- 
 loper : wash, fix, and bring out to the light ; if any misti- 
 ness is perceptible, either the developing-room is not suffi- 
 
520 PRACTICE OF PHOTOGRAPHY. 
 
 ciently dark, or the Bath was prepared with a bad sample 
 of Nitrate of Silver, or with impure Alcohol, or impure 
 water. 
 
 On the other hand, if the plate remains absolutely clear 
 under these circumstances, the cause of error may be in 
 the Camera; therefore prepare another sensitive film, 
 place it in the Camera, and proceed exactly as if taking a 
 picture, with the exception of not removing the brass cap 
 of the Lens : allow to remain for two or three minutes, 
 and then remove and develope as usual. 
 
 If no indication of the cause of the fogging is thus ob- 
 tained, there is every reason to suppose that it is due to 
 vapour in the atmosphere, or to diffused light gaining en- 
 trance through the Lens. 
 
 For directions how to deal with a Nitrate Bath which 
 has been made foggy by organic decomposition, see p. 253. 
 
 Foggy Negatives, if sufficiently intense, should not be 
 rejected as useless : they may often be cleared by the ap- 
 plication of a weak solution of Iodine, followed by a fixing 
 Bath of Hyposulphite of Soda. 
 
 SPOTS UPON COLLODION PLATES. 
 
 1. Newly iodized Collodion. a. Containing Bromide of 
 Potassium in quantity more than the Collodion will dis- 
 solve : in this case the spots will be round and transparent, 
 thickly studding the plate. Allow twenty-four hours to sub- 
 side, and draw off the upper part. b. The Collodion is free 
 from undissolved particles of Iodide, but contains a fine 
 sediment of Pyroxyline ; specks from this cause are ex- 
 tremely minute and abundant. Three or four days' set- 
 tling will clear the Collodion ; but very often the specks 
 will disappear, if a portion of a more intense Collodion be 
 mixed with the newly iodized (p. 158). c. The Collodion is 
 free from suspended Iodide or undissolved Pyroxyline, but 
 nevertheless the plate shows transparent specks. In this 
 case the specks may be due to dust, but the remedy last 
 given is the most likely to be successful. 
 
 2. Faults of the Bath. a. Newly made from impure 
 
CAUSES OF SPOTS ON COLLODION PLATES. 521 
 
 Nitrate of Silver (p. 252). In this case the image will be 
 very weak as well as spotty, and the Bath will require a 
 trace of Acetate, as described at p. 529. The reason why 
 a bath in this state produces spots is given at page 158. 
 b. Iodide of Silver previously dissolved in the Bath crys- 
 tallizing upon the film. The film in such a case exhibits 
 little projecting points upon the surface of the Iodide be- 
 fore exposure in the Camera, and transparent pin-holes 
 after fixing. Add to the Bath a sixth part of a thirty-grain 
 solution of Nitrate of Silver, not containing any Iodide of 
 Silver, and remove the plate from the solution after an im- 
 mersion of two minutes. Or leave a sensitive plate in the 
 solution all night, that the excess of Iodide of Silver may 
 gradually crystallize upon its surface, and so be removed. 
 c. A floating film of Iodide of Silver upon the surface 
 of the Bath. These spots are not universally distributed, 
 and are larger than the last. Remedy : change the Col- 
 lodion, or the time of dipping (p. 416). d. Deposits of re- 
 duced Silver on the sides of the Bath and dipper or frag- 
 ments of Iodized Collodion which have fallen into the so- 
 lution. Spots, so produced, are easily recognized, and the 
 appearance of the picture at once shows that the Bath re- 
 quires filtering. 
 
 3. Dust as a cause of Spots. a. New apparatus is ge- 
 nerally dusty, and the hinges full of brass-filings. Tents 
 are the same, and so are dark boxes for working in the 
 open air, or leather-bodied Cameras, especially when used 
 for the first time in the season. Wipe them out with a 
 damp cloth, but not immediately before working, if it can 
 be avoided. b. Hard rubbing of glasses in cleaning ren- 
 ders them electrical, and attracts dust. An electrical con- 
 dition of glass tends also, as before shown (p. 161), to pro- 
 duce irregular action of the developer. <?. Dropping the 
 plate suddenly into the slide, or tapping the slide against 
 the ground, is likely to stir up dust. 
 
 4. Faults of the Slide. Sometimes a small hole exists, 
 which admits a pencil of light, and produces a spot, known 
 by its being always in the same part of the plate ; or the 
 
522 PRACTICE OF PHOTOGRAPHY. 
 
 door works tightly, so that small particles of French polish 
 are scraped off, and projected against the plate when it is 
 raised; or the operator, after the exposure is finished, 
 shuts down the door with a jerk, and so causes a splash in 
 the liquid which has drained down and accumulated in the 
 groove below. The hinges of the slide also frequently pro- 
 duce a little row of white spots when brass-filings are left 
 or dust accumulates. 
 
 In the experience of the Writer, spots upon the plate 
 have been more frequently due to defects in the slide than 
 to any other cause, and a simple rubbing with a piece of 
 soap has often sufficed to remove them. 
 
 5. Larger spots from concentration of the Nitrate of Sil- 
 ver. When the film becomes too dry after removal from 
 the Bath, the solvent power of the Nitrate increases so 
 much that it eats away the Iodide and produces transpa- 
 rent marks. 
 
 6. Use of glasses improperly cleansed. After glasses 
 have been long used, it is often difficult to clean them so 
 thoroughly that the breath lies smoothly ; spots due to 
 this cause are of a large size, and always come in the same 
 place. 
 
 MARKINGS OF VARIOUS KINDS. 
 
 1. Transparent marks like Curtains. These occur at 
 the edge of the plate which is most depending in the 
 Camera, and are produced by the Bath solution draining 
 down, or sometimes ascending by capillary attraction from 
 the corners of a dirty slide. They are most common with 
 a Gelatinous simply iodized Collodion, and are seen in cold 
 and damp weather more than in hot and dry weather. To 
 obviate their occurrence, allow the Ether to evaporate 
 from the film before dipping, so that the Bath solution. 
 may run completely off; or turn the plate round, that the 
 upper and dryer part may be below ; or place it across the 
 dipper, so as to bring the curtain mark to the side of the 
 picture, where it will be less visible. 
 
 2. Wavy Streaks. Due to the Bath solution running 
 
TAEIOUS MARKS ON COLLODION PLATES. 523 
 
 into lines upon the surface of the film. The rivulet of 
 liquid often acts like a Lens, and refracts the light which 
 
 falls upon it in the Camera, producing an appearance like 
 that shown in the woodcut. 
 
 Collodion of a tough and contractile structure, made 
 with spirits comparatively anhydrous, and with an excess 
 of Ether, favours these markings ; a loose and powdery 
 film showing them less frequently. Much however depends 
 upon the Bath. Newly-mixed Batfrs can be used success- 
 fully, but when Alcohol and Ether accumulate, and the 
 Bath is slightly contaminated with organic matter, the 
 marks appear in profusion. As a remedy, first try the 
 effect. of leaving the plate double the usual time in the 
 Bath, and moving it several times up and down. Then 
 drain with extra care on blotting-paper, especially so when 
 the direction of the glass is to be changed on putting it 
 into the slide ; as for instance with stereoscopic plates, 
 which nearly always show the marks on the picture which 
 comes at the part lowest in draining. Supposing these 
 plans to be insufficient, turn the plate round after coating 
 it with Collodion, so as to bring the top downwards and to 
 immerse the driest part of the film first in the Bath. 
 
 3. Straight lines like hairs, running in the direction of 
 the dip in the Bath, are frequently observed when using a 
 horny Collodion which sets rapidly upon the glass. The 
 solution of Nitrate of Silver hangs aboiit its surface, and 
 either a lateral motion must be given to the plate imme- 
 diately after putting it into the Bath, or the Nitrate must 
 be prevented from causing excess of reduction by using 
 more acid in the developing solution. 
 
 When large black straight lines are abundant upon the 
 film, the Bath is often in too reducible a state, being either 
 alkaline, or loaded with Acetate of Silver. Frequently 
 also the plate is under-exposed when such marks occur. 
 
524 PRACTICE OF PHOTOGEAPHY. 
 
 4. Structural crapy lines. Marks from glutinosity will 
 sometimes occur when using Cadmium as an iodizing so- 
 lution. A sample of plain Collodion which gives these 
 markings, will probably be free from them after keeping 
 for a few months, without any further treatment (p. 239). 
 Structural lines on the film also frequently depend upon 
 the plate not being properly rocked whilst pouring off the 
 Collodion. Some operators dilute glutinous Collodion with 
 Ether, but in doing so there is always a danger of pre- 
 cipitating the Iodide of Potassium, and if the Pyroxyiine 
 be of that kind which sets very rapidly, the use of too 
 large a proportion of Ether will produce markings of a 
 different kind, viz. such as are alluded to in paragraph 9 in 
 the next page. 
 
 5. A netivork appearance on the film after developing. 
 When this is universal, and accompanied by rottenness of 
 film, it often depends upon the employment of Collodion 
 containing water. Or, if not due to this cause, the plate 
 may have been immersed too quickly in the Bath, and the 
 soluble Pyroxyiine partially precipitated. This is very 
 likely to happen when using Collodion containing a suffi- 
 cient excess of Alcohol to interfere with the setting pro- 
 perties of the Pyroxyiine, especially in cold and damp 
 weather. 
 
 6. Oily Lines. a. Lifting the plate out of the Nitrate 
 Bath before it has been immersed sufficiently long to be 
 thoroughly wetted, or before the Ether upon the surface 
 is washed away. b. Hedipping the plate in the IVitrate 
 Bath after exposure in the Camera, and pouring on the 
 developer immediately ; if a few seconds be not allowed for 
 the purpose of draining off the excess of Nitrate, the 
 Pyrogallic Acid will not readily flow over the film. 
 c. From too small a quantity of fluid having been employed 
 to develope the image. In this case, the whole plate not 
 being thoroughly covered during the development, the 
 action does not proceed with regularity. d. Opaque oily 
 diagonal markings are very commonly produced by pour- 
 ing off the developer and examining the plate by trans- 
 
VAEIOUS MABKS ON COLLODION PLATES. 525 
 
 mitted light without washing the film : the solution of 
 Pyrogallic Acid runs into lines which show after fixing. 
 e. Addition of old Collodion in making a new Bath. The 
 Ether mixes with the Bath, and gives a greasy appearance 
 to the immersed plates. 
 
 7. Straight lines traversing the film horizontally. A 
 check in immersing the plate in the Bath (p. 417). 
 
 8. Curved, lines of over-development. The developer too 
 concentrated ; or not poured on sufficiently quickly to 
 cover the surface before the action begins. 
 
 9. Marks like the 
 woodcut are exaggera- 
 ted by holding the i 
 plate a long time be- 
 fore dipping it in the Bath. Excess of Ether in Collodion 
 increases these markings, but dilution with Alcohol lessens 
 them by preventing the rapid setting of the Collodion. 
 The same Collodion which produces the marks when iodized 
 with Iodide of Ammonium, and especially with Iodide of 
 Ammonium containing free Ammonia, shows none when 
 iodized partially or entirely with Iodide of Cadmium. Pro- 
 bably the difference depends upon the fact that Iodide of 
 Cadmium thickens Collodion, whilst Iodide of Ammonium 
 renders it limpid (p. 242). Chloroform in Collodion also 
 modifies the structure, and a fluid sample of Collodion 
 giving marks resembling vegetation, flashes of lightning, 
 or watered silk, may often be cured by adding ten minims 
 of Chloroform to each ounce, and allowing it to stand for 
 twelve hours. 
 
 The marks can usually be seen upon the film by a close 
 inspection after sensitizing, but if the Bath is pure and 
 newly prepared, they will not invariably show in the de- 
 velopment. With a Bath, however, which is prone to give 
 foggy films and irregular reduction, they are exaggerated 
 during development. 
 
 10. Opaque yellow or white streaks at the part of the 
 film from which the Collodion was poured off. These, 
 
 when seen upon the surface of the Iodide as it is lifted 
 
526 PEACTICE or PHOTOGEAPHY. 
 
 from the Bath, may depend upon the presence of too large 
 a quantity of Iodide. In this case, the film appearing yel- 
 low and creamy, make a mixture of equal parts of highly 
 rectified Alcohol and pure Ether, and slightly dilute the 
 Collodion, or add a little plain Collodion to the iodized 
 Collodion. Supposing these marks to appear, and the film 
 to be rather opaline than creamy, add to each ounce a grain 
 of Iodide of Cadmium. 
 
 11. Thickenings of the Sensitive Iodide. These, when 
 visible on taking the plate out of the Bath, are often caused 
 by pouring the Collodion from a full bottle, and holding it 
 too high, so that the liquid spatters on the glass and pro- 
 duces air-bubbles, which course across the plate. Secondly, 
 by employing a Collodion containing too little Alcohol : it 
 cannot easily be applied to a large plate without leaving a 
 band of thickening at one end. Try the effect of pouring 
 less of the Collodion, so that the fluid shall not twice tra- 
 verse the same part of the plate. Thirdly, thickenings are 
 caused by fragments of dried Collodion accumulating in 
 the neck of the bottle. 
 
 12. Silver stains of varied shape upon the finished Pic- 
 ture. a. From altering the direction of the plate after re- 
 moving it from the Bath, and placing the top edge down- 
 wards. This throws a wave of Nitrate back over the film, 
 and often produces stains. b. Scum upon the Bath. c. 
 Developer poured out of a dirty glass containing the scum 
 of a previous reduction. 
 
 13. A Hue line at upper edge of film, seen on the Iodide 
 before exposure. a. Often caused by using a Collodion 
 made from highly rectified spirits, the Ether being in ex- 
 cess : the film in hot weather dries and shows iridescent 
 tints at the upper part before it can be immersed in the 
 Bath. b. From use of too much Alcohol in Collodion, thus 
 retarding the setting of the Collodion at the lower edge, to 
 such an extent that the top part becomes dry. 
 
 14. Transparent marks of various shapes, seen on the 
 finished pictures. a. From wiping the glasses with a damp 
 
 cloth. b. From employing a thin Collodion very sensitive 
 and neutral. 
 
DEFECTS IN COLLODION NEGATIVES. 527 
 
 15. Patches of a reddish colour by transmitted light on 
 the shadows of the finished Negatives. Try a second ap- 
 plication of the Hyposulphite, which will clear them away 
 if they consist of Iodide of Silver ; but sometimes a de- 
 posit of this kind is due to irregular action of the deve- 
 loper, or to impurities on the glass. 
 
 16. The picture vanishes when the plate is fixed. In this 
 case the Iodide was upon the film, and not in it. Pyroxy- 
 line made in acids weak enough to destroy the power of 
 setting will produce this effect (p. 228) ; and the only re- 
 medy is to use more Ether and less Alcohol in dissolving 
 such Pyroxyline. 
 
 17. The picture survives the fixing, but disappears in the 
 varnishing. Pyroxyline prepared in very weak acids is 
 soluble in absolute Alcohol, and hence Alcohol varnishes 
 will attack it. Use the clear Benzole varnish of p. 397. 
 
 18. A scum left upon the surf ace of the Bath in dipping. 
 a. Over-iodized Collodion. b. Want of proper setting 
 properties in the Pyroxyline ; mix the Collodion with a 
 sample of a different kind. 
 
 IMPEEFECTIONS IN COLLODION NEGATIVES. 
 
 1. TJie image weak and slaty-blue. a. Damp weather 
 with the air full of moisture (p. 268). b. The Nitrate 
 Bath newly made from impure crystals (p. 252). c. Excess 
 of Nitric Acid in the Bath (p. 158). d. Too much free 
 Iodine in the Collodion (p. 249). In this case the colour 
 of the Collodion will be nearly brown. e. Bromide in the 
 Collodion (p. 272). -f. Use of Sulphate of Iron as a deve- 
 loper for a Collodion not adapted for it (p. 281). a. Ca- 
 mera-image very weak, as in copying old manuscripts, 
 etc., of full size (p. 269). h. Use of a Negative Collodion 
 made from weak Pyroxyline (p. 233). i. Sulphuric Acid 
 left in the Pyroxyline from imperfect washing. In this 
 case a piece of blue test-paper left in the plain Collodion 
 one hour will show reddening at the edges. Jc. Coating 
 large plates too leisurely in hot weather ; the film dries, 
 
528 , PEACTICE OF PHOTOGKAPHY. 
 
 and there is no penetration by the developer (p. 454). 
 I. Over-exposure. 
 
 2. The image black and white without half-tones. a. 
 Under-exposure in the Camera (p. 423). b. Yellow va- 
 pours in the atmosphere (p. 282). c. The Nitrate Bath 
 decomposed by long use, and containing organic matters in- 
 jurious- to sensitiveness (p. 252). d. Collodion made at high 
 temperatures, and kept after iodizing (pp. 234 and 239). 
 e. Non-actinic colours in the object to be copied (p. 459). 
 f. Too rapid liberation of Iodine by Collodion (p. 439) ; 
 especially when the film contains only a small portion of 
 Iodide, and is semi-transparent (p. 259). g. Depression of 
 temperature in the developing-room (p. 281). h. Impure 
 Pyrogallic Acid. Bad samples sometimes develope the 
 half-shadows imperfectly. 
 
 From whatever cause the above defect may arise, try 
 the action of Sulphate of Iron as a developer : and give a 
 longer exposure in the Camera. 
 
 3. The image intensely red when developed with Pyro- 
 gallic Acid. a. The development not sufficiently pushed 
 (p. 150). b. The Collodion film under-iodized, and too 
 transparent for Negatives. c. Over-exposure in the Ca- 
 mera (p. 149). d. Use of Collodion made from a highly 
 intense kind of Pyroxyline (p. 234), and kept for some 
 time in the plain state (p. 239) : the redness is most 
 marked when the Collodion is used newly iodized and 
 colourless, since free Iodine tends to make the image blue. 
 e. Acetate in the Nitrate Bath (p. 278). -f. Alkalinity 
 of the Nitrate Bath produced by shaking up with chalk, etc. 
 g. The light unusually brilliant, as in Photographing 
 distant views and marine subjects. h. The Camera image 
 very bright, as in the case of short-focus Lenses (p. 269). 
 i. Fixing with Cyanide of Potassium (p. 285). 
 
 When redness of the image is an objection, remedy it 
 by use of Citric Acid in the developing solution, as de- 
 scribed at page 280. 
 
 4. Polarization of Negatives occurs generally when using 
 an Iodized Collodion with Pyrogallic developer. Over- 
 
DEFECTS IN COLLODION NEGATIVES. 529 
 
 action of light may produce either grey or red solarizatioa 
 (p. 160). In the former the image developes very slowly, 
 and the shadows are wanting in detail. The plate shows 
 no creaminess on looking down upon it, and the high lights, 
 instead of being opaque, are nearly or quite transparent^ 
 and bounded by dark outlines. The causes have been 
 principally mentioned under the first heading in page 527. 
 a. Excess of Iodine in Collodion, from keeping several 
 weeks or months after iodizing. b. Excess of Nitric Acid 
 in the Bath. <?. Use of a new Bath made from impure 
 crystallized Nitrate of Silver (p. 252). 
 
 In Red solarization the plates develope rapidly, and 
 often give a fair amount of detail in the shadows. The 
 superficial bloom is well marked, and the image brown by 
 transmitted light ; but the high lights are very pale and 
 red, with a steel-blue tint by reflected light. The causes 
 have been already indicated under the head of red Ne- 
 gatives. They are, a. Use of Pyroxyline made at high 
 temperatures, and kept for a long time in the plain state, 
 but employed soon after iodizing. b. Acetate in the Bath 
 (p. 278). c. Alkalinity of Bath. d. Use of short-focus 
 Lenses in a bright light. N.B. Collodion of that kind 
 which produces red solarization when newly iodized, often 
 gives grey solarization when kept until much Iodine is 
 eliminated ; and a Nitrate Bath which solarizes red from 
 the presence of organic matter, may; by keeping and addi- 
 tion of Acid, be made to solarize grey, and produce Nega- 
 tives with reversed light and shade, such as are often 
 termed transmitted Positives. 
 
 The proper remedy for grey solarization is the addition 
 of Acetate to the Bath. Red solarization may be corrected 
 by the use of Citric Acid in the developer (p. 280), or by 
 the employment of Bromo-iodized Collodion. 
 
 Acetate may conveniently be added to the Bath in the 
 form of crystallized Acetate of Soda, which can always be 
 purchased. Dissolve three grains in a drachm of water, 
 and pour the solution into twenty ounces of Bath ; the 
 white precipitate first formed will be taken up again on 
 
 2M 
 
530 PRACTICE OF PHOTC-GRAT-HY. 
 
 agitation. To prepare the Acetate extemporaneously, dis- 
 solve two grains of Bicarbonate of Soda in four minims of 
 Glacial Acetic Acid, diluted with a drachm of Water. 
 
 5. Blurring of the Image. An irregular 
 action of the developing fluid, giving an 
 appearance as if the image had drained 
 away to the bottom of the plate, often 
 happens when some conditions are present 
 which interfere with rapid and continuous 
 development, such as, a. The state of 
 Bath described at page 252. b. Bad light 
 from presence of vapour in the atmosphere. 
 c. Weak Camera Image. d. Depression 
 
 of temperature. As a remedy, try the effects of draining 
 the plate more carefully before putting it into the slide ; 
 also, of halving the quantity of Acid in the developer, and 
 doubling that of the Pyrogallic Acid ; or, as a last re- 
 source, of adding Acetate to the Bath as above advised. 
 ]S".B. Pyroxyline of the most sensitive but least intense 
 kind, prepared by the use of weak acids, is especially liable 
 to this defect when the other conditions are unfavourable. 
 
 6. An opaque halo at the edges of the high lights. 
 This defect is essentially different from that last 
 described, and is caused by an excess of light. It 
 
 may often be seen round the sky of a landscape Negative, 
 or the windows of an interior. The less the intensity of 
 the Collodion, the more marked the halo. Albumen in 
 the film, in a dry process, exercises a favourable action, 
 and tends to prevent the reduction of the Silver from ex- 
 tending beyond its proper bounds. 
 
 7. Falling away of the film. This depends much upon 
 the Collodion : the best kind for adhering tightly is thaj; 
 made in weak acids. When the Pyroxyline is not in a 
 favourable state, the amount of Alcohol in the Collodion 
 must be diminished (p. 237) ; but this observation does not 
 apply to a very hard and tough Collodion, because excess 
 of Ether increases the contractility of such a film to so 
 great an extent that it shrinks away from the edges of the 
 
DEFECTS IN COLLODION NEGATIVES. 531 
 
 plate. Any film, kept too long before dipping in the Bath, 
 will slip from the glass at the upper part, and if dipped 
 very quickly it is liable to peel away from below. Koughen. 
 the glasses at the surface as well as the edges, and pour 
 on the water for washing at the centre of the plate. Be 
 careful that the fixing solution of Hyposulphite does not 
 work underneath, and allow it to flow off* at that corner 
 which appears loosened. JST.B. A very acid state of the 
 Bath, and the use of Iodide of Cadmium as an iodizer, are 
 both liable to cause gelatinization of the Collodion, and 
 consequent slipping. 
 
 8. The Collodion curls away from the glass on drying. 
 This may occur with samples of Pyroxyline made in strong 
 acids, especially when the Collodion is iodized with Cad- 
 mium. Long development also occasions it. Coating the 
 plate after it has been washed, with a thin solution of 
 Gum. Arabic, is an effectual preventive. 
 
 9. Woolliness in the skies. Employment of Pyroxyline 
 made in strong acids with Cadmium as the iodizer. Try 
 the mixed iodizer of Potassium and Cadmium, and substi- 
 tute Alcohol of -805 for Alcohol of '820. 
 
 10. Turbidity of Developer. When the Pyrogallic Acid 
 developer becomes muddy on touching the film, instead of 
 assuming by degrees the colour of sherry wine, the Bath 
 may be in fault. New Baths made from impure Nitrate 
 of Silver will often produce the effect in a marked manner. 
 Another cause is decomposition of the Pyrogallic Acid, 
 by keeping for several days in aqueous solution. Also 
 extreme heat of the weather. Also a dirty developing 
 glass. 
 
 11. Blueness of the Iodide Film. When a Collodion 
 containing the proper quantity of Iodine produces an opa- 
 lescent instead of a creamy film, an organic impurity is 
 often present. Ether distilled from plain Collodion will 
 often produce the effect, or Pyroxyline made at high tem- 
 peratures, and kept for a time in solution. The Bath ia 
 sometimes in fault, organic matter having been introduced 
 purposely or otherwise. 
 
532 PEACTICE OF PHOTOGBAPHY. 
 
 12. Intensity of Image injured by Fixing. a. From the 
 Collodion being too highly iodized. In this case the film 
 will be dense and creamy, and will probably leave a scum 
 on the surface of the Bath in dipping. b. Insufficient de- 
 velopment. The longer the action of the Pyrogallic Acid 
 is continued, the less the chance of the image losing inten- 
 sity in fixing. c. Use of Cyanide of Potassium as a fixing 
 agent (p. 285). 
 
 13. Violet- Blue stains on Image after Drying. Hypo- 
 sulphite of Soda left in the film in small quantity : the 
 stains are mostly seen at the lower edge of the plate. 
 
 14. Crystals on Image after Drying. Hyposulphite of 
 Soda as before, but in larger quantity. 
 
 . 15. The developer flows in a greasy manner. a. From 
 the Bath being old and containing much Ether and Alcohol. 
 In this case a purely aqueous liquid is not capable of 
 flowing at once over the film, and a portion of spirit must 
 be added to the developer (p. 401). b. From use of too 
 much spirit in the developer when the Bath is newly made 
 .and nearly free from Alcohol : if the surface liquid of the 
 film is entirely aqueous in character, the developer should 
 be aqueous also. 
 
 16. The developer refuses to wet certain parts of the film, 
 and will not flow up to the edge. A gelatinous Collodion 
 often repels the developer, and especially so with a newly - 
 prepared Bath. Old Baths containing Ether and Alcohol 
 render the surface of the film less gelatinous, and remedy 
 the defect. The plate should not be dipped too quickly, 
 or the difficulty will be increased. Acetic Acid and spirit 
 added to the developer are serviceable. The stronger the 
 Alcohol in the Collodion, the more marked the defect. 
 
 In hot weather, when large plates become partially dry 
 and repel the developer, redip them, in the Bath, or use a 
 developing solution greatly diluted with water. 
 
 17. The Collodion changes quickly after iodizing. A 
 rapid elimination of Iodine on adding Iodide of Potassium, 
 may be due to the plain Collodion having been spoilt by 
 exposure to light (p. 224), or to the use of Pyroxy line made 
 
DEFECTS IN COLLODION NEGATIVES. 533 
 
 at very high temperatures (p. 240) : in this case, the longer 
 the Collodion is kept in the plain state the worse it will 
 become. Thirdly, to the employment of ozonized Ether, 
 or of Pyroxyline left too long in the Nitro- Sulphuric Acid 
 (p. 240). As a remedy, try the effect of iodizing the Col- 
 lodion with Cadmium only, and keeping for a day or two. 
 If this should fail, iodize with Iodide and Bromide of Am- 
 monium (p. 402), and develope with Sulphate of Iron. For 
 directions how to bromize an old coloured Collodion, and 
 thus to render it again useful, see page 393. 
 
 Discoloration of Collodion after iodizing may also de- 
 pend upon traces of acid left in the Pyroxyline from im- 
 perfect washing ; but in this case the liberation of Iodine is 
 slower than in the last, and may not be fully seen until the 
 following day. Blue Litmus-paper left in the plain Collo- 
 dion for six hours becomes red. Try as a remedy, agita- 
 tion of the Plain Collodion with powdered Chalk. 
 
 18. An opalescent deposit limited exactly to those parts 
 of the Negative which ought to be clear. This defect is 
 due to a reversed action of the developer, and often indi- 
 cates the existence of white light in the developing-room. 
 When such is not the case, consult page 463. 
 
 19. Transparent lines enclosing diamond- shaped Spaces. 
 Collodion made by the formula of page 377 will some- 
 times exhibit these 
 
 markings, and espe- 
 cially when the Bath 
 is not fully saturated 
 with Iodide of Silver. Any Collodion will be liable to 
 them when Glycyrrhizine is added to the Bath, or organic 
 substances are present upon the surface of the film. Be- 
 yond re-saturating the Bath with Iodide of Silver, nothing 
 can be done to remedy the defect. 
 
 20. Irregular Surf ace -Blotches on the Negative. This 
 defect in Negatives which w r as almost unknown in the 
 earlier days of Collodion, when purer Ether and Alcohol 
 were used in its manufacture, has recently become but too 
 familiar to most Photographers, and is, indeed, the most 
 
534 PEACT1CE OF PHOTOGEAPHY. 
 
 common cause of failure in the wet process in the field, 
 and is by no means rare also in indoor photography. It 
 consists of thin irregular patches, apparently of scum, but 
 in reality of a thin organic reduction of Silver, on the sur- 
 face of the Negative sometimes interfering with its print- 
 ing qualities, and sometimes not materially so, according 
 to its thickness. It is readily observable on looking at 
 the Negative by reflected light, and often appears traceable 
 to the corner of the plate, or to some point where it has 
 been in contact with the wood of the frame, spreading 
 from thence over the surface of the Negative in a string of 
 irregular blotches. On the other hand, it is sometimes 
 found on the centre, while all the rest of the plate is per- 
 fectly clear from it. New frames, more particularly when 
 varnished in the inside, are more liable to this defect than 
 old ones that have been much used. Indeed, so observa- 
 ble in this respect is the difference, that it has often been 
 attributed entirely to that cause. It is probable that the 
 resinous vapours from new wood and varnish, and also 
 from the Methylic Alcohol in which the latter is dissolved, 
 exercise a predisposing influence, but experiments would 
 seem to point to Methylic Alcohol in the Collodion, or per- 
 haps rather to the oily impurities which it always contains, 
 as the true cause. A Collodion prepared from pure Ether 
 and Alcohol is not subject to these defects if excited in a 
 Bath kept for itself alone, but when dipped in a Bath used 
 for Methylated Collodion, it also will show the patches, but 
 in a less marked degree. Again, it does not originate from 
 the developer, for the nucleus or foundation may often be 
 observed faintly on the surface of the film, before apply- 
 ing the developer. Possibly it may be in reality a scum 
 of essential oil floating in the Bath, and derived from im- 
 purities in the Collodion. If this explanation be correct, 
 the remedy is obvious use a Collodion made from pure 
 Ether and Alcohol. 
 
 21. Destruction of the Negative on applying the var- 
 nish. Some kinds of Pyroxyline are perfectly soluble in 
 strong Alcohol, and hence, when a strong Alcoholic varnish 
 
DEFECTS IN COLLODION POSITIVES. 535 
 
 is applied, the film begins partly to dissolve, if unfor- 
 tunately consisting of such a sample of Pyroxyline. The 
 appearance presented is usually that of crapy lines in the, 
 direction of the flow of the varnish when poured on the 
 plate. The remedy will be either to obtain Collodion from 
 a different source, or to use a varnish containing Chloro- 
 form, Benzole, or Coal-Naphtha as the solvent, in the place 
 of rectified Spirit (p. 397). 
 
 IMPERFECTIONS IN COLLODION POSITIVES. 
 
 1. The Shadows dark and heavy. The plate has not 
 received sufficient exposure in the Camera; or the film 
 being transparent and the Silver solution weak, too much 
 Nitric Acid is present in the Bath ; in the latter case, make 
 the Collodion a little thicker, and develope with Sulphate 
 of Iron containing only Acetic Acid (p. 404). Sometimes 
 this .defect depends upon the use of too little Bromide, the 
 Collodion being decomposed by keeping after iodizing (p. 
 224) : if so, increase the proportion of Bromide to a grain 
 and a half per ounce. 
 
 2. The Shadows good, the Lights vigorous but without 
 detail. The developing fluid may have been kept on too 
 long ; or the object is too strongly illuminated (p. 268) ; or 
 the Collodion is not adapted for Positives, giving a creamy 
 film, and not containing much Bromide (p. 273). 
 
 3. The high Lights pale and fiat, the Shadows misty. 
 The plate is over-exposed. Indistinctness caused by over- 
 exposure is distinguished from that produced by fogging 
 by holding the plate up to the light ; in the former case 
 the image shows as a Negative. 
 
 If the Collodion is colourless, clearer shadows will pro- 
 bably be obtained by dropping in Tincture of Iodine (ten 
 grains to the ounce .of Spirit), until a yellow colour is pro- 
 duced. But a still better plan will be to keep on hand a 
 stock of old iodized Collodion for mixing with the newty- 
 iodized. Too little attention is usually paid to the quality 
 of the Nitrate of Silver for the Bath : this has much to do 
 with clearness of shadow. 
 
536 PRACTICE OF PHOTOGRAPHY. 
 
 4. The picture developes slowly; spangles of metallic 
 Silver are formed. Too much Nitric Acid is present, in 
 proportion to the strength of the Bath, to the amount of 
 Iodide in the film, and to the quantity of Protosalt of Iron 
 in the developer (pp. 158 and 262). 
 
 5. Circular Spots of a black colour after backing up 
 with the varnish. These are often caused by pouring on 
 the developer at one spot, so as to wash away the Nitrate 
 of Silver ; or by the use of glasses imperfectly cleaned, 
 
 6. Silver Stains at the edges of the Plate. They often 
 occur when using a forty-grain Bath strongly acidified. 
 Pour on a large quantity of developer, so as to wash off 
 some of the Nitrate of Silver into the sink, the stains being 
 caused by excess of Nitrate. 
 
 7. A green or blue Tint in certain parts of the Image. 
 This is caused by the deposit of Silver being too scanty, 
 which may happen from under-development, from too much 
 exposure in the Camera, or from the film of Pyroxyline 
 being very thin ; if the Collodion be diluted down beyond 
 a certain point, the same quantity of free Nitrate of Silver 
 will not be retained upon the surface of the film. Add a 
 few drops of the Bath to the developer before pouring it 
 on the plate. 
 
 8. Vertical Lines and Mistiness on the Image. If the 
 Bath has been much used, add to it a third part of a simple 
 solution of Nitrate of Silver in water, without any Alcohol 
 or Iodide. Also prepare the developer with addition of 
 Alcohol, to make it flow more readily (p. 401). 
 
 9. Opacity of the Film on drying. Due to the use of 
 Pyroxyline prepared in too weak acids (p. 226). 
 
 10. Had Colour of Image. The Pyroxyline too much 
 of the Negative kind (p. 233). The Bath decomposed by 
 use, and containing organic impurities. 
 
 SECTION II. 
 
 Imperfections in Paper Positives. 
 1. The Print marbled and spotty on taking it out of the 
 
 
DEFECTS IN FEINTS ON PAPEB. 537 
 
 printing-frame. The quality of the paper is often inferior, 
 which causes it to imbibe liquids unevenly ; or the amount 
 of Silver in the titrate Bath is insufficient. In this case 
 the spots are often absent at the lower and most depending 
 part of the sheet, where the excess of liquid drains off. 
 
 2. TJie Print marbled and spotty after toning but not 
 before. a. Badly albuminized paper. b. The Print al- 
 lowed to rise to the surface of the toning Bath. c. Two 
 or more Prints adhering, or touching each other so as to 
 prevent the circulation of the toning liquid. 
 
 3. The Print clean on the Surface, but spotted when held 
 up to the Light. In this case the spots are probably due 
 to imperfect fixation (p. 313). 
 
 4. TJie Print has a cold and faded appearance when 
 finished. a. The Chloride of Silver in the paper in excess 
 
 with regard to the free Nitrate of Silver. This may be 
 suspected when no bronzing can be obtained by prolonged 
 action of the light, or when a weak solution of Nitrate of 
 Silver was laid on with a brush, or by a glass rod (p. 297). 
 Prints formed on paper which has been kept too long after 
 sensitizing present the same appearance, the free Nitrate 
 of Silver having entered into combination with the organic 
 matter. b. Over-action of the Gold Bath imparts a cold 
 and grey tone to the picture. 
 
 5. Yelloioness of the light parts of the Proof. The fol- 
 lowing causes are likely to produce yellowness : Acidity 
 of the fixing Bath (p. 317) ; its action continued for too 
 long a time ; the first washings of the fixed proof not per- 
 formed quickly ; the fixing Bath laid aside until decom- 
 posed ; the paper kept for several days after sensitizing. 
 
 6. Intense bronzing of the deep shadows. Perhaps the 
 Negative is in fault. Hemedy it as far as possible by 
 printing on paper containing but little salt ; or sensitize 
 the paper on a 30-grain solution of Nitrate of Silver in- 
 stead of a 60-grain solution. 
 
 7. The definition of the Print imperfect, the Negative 
 being a good one. Much will depend upon the quality of 
 the paper and the amount of Albumen used in preparing 
 it (p. 295). 
 
 
538 PEACTICE OF PHOTOGRAPHY. 
 
 8. Markings of a yellow tint in the dark portions of the 
 Positive. Care should be taken not to handle the paper 
 too much, either before or after sensitizing ; to wash the 
 prints in a clean vessel ; and not to lay them down whilst 
 wet on a wooden table, or in contact with anything likely 
 to communicate impurities. 
 
 9. Small specks and spots of different kinds. These, 
 when not corresponding to similar marks upon the Nega- 
 tive, are usually due to metallic specks in the paper ; or to 
 minute insoluble particles in the Albumen which refuse to 
 tone. Try the effect of halving the quantity of Carbonate 
 or Acetate of Soda in the toning Bath. 
 
 10. Markings of the brush in Ammonio-Nitrate Pictures. 
 In this case there is probably an excess of Ammonia, 
 which dissolves the Chloride of Silver. Add a little fresh 
 Nitrate of Silver, or use a Bath prepared according to 
 the directions given (p, 495), and prepare the paper with 
 only a small quantity of salt. 
 
 11. Marbled stains on the surface of the sensitive Paper. 
 Draw a strip of blotting-paper gently over the surface 
 of the Nitrate Bath ; and see that the sheet does not touch 
 the bottom of the dish. 
 
 12. Removal of the Albumen from the Paper during sen- 
 sitizing, and white deposit in the sensitizing Bath. The 
 Bath is probably alkaline, and contains too little Nitrate 
 of Silver (p. 301). Add twenty grains of fresh Nitrate of 
 Silver to each ounce, with a quarter of a minim of Glacial 
 Acetic Acid. 
 
 13. The Print refuses to tone. a. Often the fault of the 
 Albuminized paper. b. Long keeping of the proof after 
 printing (p. 312). c. The toning Bath kept until it has 
 become perfectly colourless (p. 309). d. A trace of Hypo- 
 sulphite of Soda carried from the fixing Bath into the toning 
 Bath. 
 
 14. Accidental over-printing .If the pictures when 
 toned and fixed appear too dark, try Mr. Shadbolt's plan 
 of agitating a pint of water with a minute quantity of li- 
 quid Bromine until it smells decidedly, and of then im- 
 
DEFECTS IN PEINTS ON PAPER. 539 
 
 mersing the print until sufficiently reduced. Afterwards 
 wash with water to remove the Bromine. This process 
 eats away the print too much at the edges, but no better 
 is known. Over-printed proofs may be reduced by toning 
 in a Bath of Chloride of Gold from which the alkali has 
 been omitted. 
 
 15. Skies printing through. This is often a fault of the 
 Negative, and may be corrected as follows : Dip a fine 
 camel's-hair brush in Gamboge, and trace round the horizon 
 of the Negative to about the sixteenth of an inch inwards 
 on the defective sky. Then print an impression, and cut 
 out the sky with a penknife. Blacken the paper sky by a 
 proper exposure to light, and paste it on the Negative : 
 the Gamboge tracing will prevent the line of junction from 
 being seen. If the sky is defective only at the upper edge 
 or at the corners, it will be sufficient to place a piece of 
 brown paper in front of the printing-frame, and to pack 
 Cotton-wool beneath it. 
 
 16. Difficulties in albuminizing paper. When the Al- 
 bumen dries in streaks upon the paper, try the effect of 
 adding two minims of Glacial Acetic Acid (previously di- 
 luted with a little water) to each egg : increasing the pro- 
 portion of salt from ten to twelve grains per ounce, and 
 that of the Nitrate of Silver in the Bath from sixty to 
 eighty grains per ounce, if the addition of the acid be 
 found to make the paper print too red, or to diminish the 
 depth of the shadows to too great an extent. 
 
510 
 
 CHAPTER V. 
 
 DRY COLLODIOK, AND COLLODIO-A.LBUMEN PROCESSES. 
 
 OF the various methods alluded to in discussing the theory 
 of dry Collodion in Part II., the Author selects four for more 
 especial consideration. Being an advocate for the simpli- 
 fication of formulae, he would have confined himself to a 
 single process, had absolute certainty been obtainable. At 
 present however this can scarcely be said to be the case, 
 and hence it would be a mistake to limit the operator in his 
 choice of resources. 
 
 Russell's Tannin process stands first upon the list, being 
 the most closely allied to wet Collodion, both as regards 
 the composition of the sensitive film and the quality of the 
 picture. The manipulations are easy, and success is at 
 least as certain as by any other dry process. Fothergill's 
 process is very simple, and enjoys a high degree of popu- 
 larity : the Author himself, however, has not been uni- 
 formly successful with it, the Negatives being often solar- 
 ized in the sky, and presenting other defects presently to 
 be alluded to. Taupenot's Collodio-Albumen process is 
 allowed by all to be valuable, although many are deterred 
 from practising it by the labour involved in preparing the 
 plates. In the case of long-focus Lenses, however, the 
 process possesses so many advantages that no obstacle of 
 the above kind ought to be considered. Lastly, the pro- 
 cess of Messrs. Petschler and Mann may be recommended 
 to such as desire to carry a large stock of plates with them 
 on an excursion, and to leave all their baths at home. 
 
.DRY COLLODION PROCESSES. 541 
 
 Bulky solutions of Nitrate of Silver are unpleasant com- 
 panions on a journey, and are very liable to suffer injury 
 before it is ended. 
 
 EUSSELL'S TANNIN PROCESS. 
 
 A simply-iodized Collodion will succeed in this process, 
 but the presence of a Bromide is highly conducive to sen- 
 sitiveness. The normal Collodion of page 377, bromo-iodized 
 in the manner described at page 402, has been found suc- 
 cessful, and when pure Ether is employed, this Collodion 
 will remain in perfect working order for twelve months 
 after iodizing. At first it is scarcely as porous as could be 
 desired, but acquires this condition by degrees under the 
 influence of the alkaline Iodide. A Collodion of a more 
 spongy character would be superior when newly iodized, 
 but it would scarcely bear the same length of keeping, be- 
 cause as the film became by degrees powdery and rotten, 
 the sensitiveness to dark shadows would be injured. 
 
 In this process the tendency to blistering of the film is 
 not seen, but the Collodion will sometimes wrinkle and peel 
 away from the glass at the lower edge. No preliminary 
 coating to the glass will be required when the Collodion 
 is in the most suitable state for adhering tightly, and when 
 the glasses are small. Glasses of 10 X 8 and upwards may 
 require the use of Gelatine, and even Stereoscopic glasses 
 must be coated with it when the Collodion shows a ten- 
 dency to separate, or if the operator should fail to observe 
 the directions given for ensuring adhesion. 
 
 Clean the surface of the glass with great care, and for 
 that purpose use a liquid capable of removing grease, such 
 as the Tripoli mixture of page 408, or the old Collodion de- 
 scribed at page 409. The glasses must be quite dry at the 
 time of coating with Collodion ; and hence if the room is 
 damp and cold a fire must be lighted. When the Gelatine 
 solution is used, the above precautions are less needful, 
 but even in that case a greasy plate would probably cause 
 slipping of the film. 
 
 Major Russell's instructions for preparing the Gelatine 
 
542 PRACTICE OF PHOTOGRAPHY. 
 
 solution are as follows : " Soak twenty grains of Gelatine 
 in eight ounces of distilled water, and four drops of Glacial 
 Acetic Acid. When thoroughly swelled and transparent, 
 dissolve by warming. Should the water be heated before 
 the Gelatine has had time to swell, the latter will adhere 
 to the bottom of the vessel containing it, in a stiff glutinous 
 state, and will dissolve with great difficulty. Dissolve 
 three grains of Iodide of Cadmium, three grains of Bro- 
 mide of Cadmium, and a small piece of Iodine, in a few 
 drops of water, and mix with the solution ; then filter two 
 or three times through white filtering paper in a warm 
 place." The object of the Acetic Acid is to coagulate a 
 white impurity in the Gelatine, which collects into fila- 
 ments, and is removed by filtering. It is better not to 
 add Alcohol. No gelatinization occurs on cooling, and the 
 solution ought to remain good for a long time. A solution 
 of Isinglass about half the above strength will probably 
 answer better in very hot weather. It may be filtered 
 through very thin filtering-paper, or through a conical 
 piece of sponge placed in the neck of the funnel. 
 
 The application of the Gelatine solution to the glass is 
 more troublesome than that of the Collodion, since it flows 
 with difficulty, and if the glass be at all greasy, will not 
 readily wet it. Having gently warmed the plate, pour a 
 large pool of the Gelatine liquid on its centre, and if it 
 does not run, guide it with the finger or a glass rod, 
 taking care to keep the back of the glass clean. Allo\v 
 the excess to flow back into the bottle, and when particles 
 of fluff are seen floating, filter it through blotting-paper 
 before using it a second time. Rear the plates on end to 
 dry, with blotting-paper beneath, and when they are well 
 drained, draw a slip of blotting-paper along the lower 
 edge, to remove an accumulation of the gelatinous liquid. 
 This precaution is quite necessary, since it will be found 
 that if too thick a film of Gelatine be left on evaporation, 
 the film will remain perfectly transparent on passing 
 through the Nitrate Bath (p. 346), and no image will be 
 developed on applying the Pyrogallic Acid. The plates 
 
RUSSELL'S TANXIN PROCESS. 543 
 
 should be dried spontaneously in a place free from dust, 
 and if the attempt be made to dry them hurriedly, the 
 Gelatine solution will often collect into ridges, and will 
 leave a series of transparent marks on the surface of the 
 excited Collodion as above described. This irregular dry- 
 ing almost invariably occurs in the case of a gelatinous 
 liquid, but the Writer has seen glasses coated with di- 
 luted Albumen, and dried at a fire, without any such in- 
 equalities being produced. When the coated plates are 
 surface-dry, warm them gently, and put them away in a 
 grooved box, where they will remain good for a long time. 
 Before packing them, however, it will be desirable to ex- 
 amine the back of each glass, and cleanse it from any ad- 
 hering Gelatine, which, if allowed to remain, would affect 
 the Nitrate Bath injuriously (p. 253). 
 
 Coat with Collodion exactly as usual : the presence or 
 absence of the Gelatine film will scarcely be noticed. 
 Allow the full length of time to elapse before dipping, 
 otherwise the Collodion will certainly lie loose at the lower 
 corner. Beginners frequently fall into the opposite error, 
 and produce plates which are pale and opalescent, with a 
 blue line running round the upper edge. The proper rule 
 to follow is to touch the lower corner of the film with the 
 finger after it has ceased to drip, and thus to ascertain how 
 much setting power the Collodion possesses. If the Gela- 
 tine was applied as a preliminary coating, be very careful 
 to cover the whole plate with Collodion. 
 
 The Nitrate Bath is an ordinary 30-grain solution, tested 
 arid found to be in working order for the wet process. 
 Many will use a stronger Bath by preference, and in pre- 
 paring the solution purposely, it is perhaps as well to make 
 it of 35 or 40 grains to the ounce. The immersion ought 
 to be longer than that often adopted for wet Collodion : 
 from three to five minutes being an average time. 
 
 Having placed two gutta-percha or porcelain trays con- 
 taining distilled or filtered rain-water, and two others con- 
 taining ordinary water, in a convenient position near at 
 hand, lift the sensitive plate from the Bath, drain for a 
 
544 PEACTICE OF PHOTOGRAPHY. 
 
 second or two against the dipper, and place it, Collodion 
 side upwards, in tray No. 1, containing distilled water. 
 Dip the fingers in water, wipe with a clean cloth, and coat 
 another plate with Collodion and immerse in the Bath. 
 When it is nearly ready for removal, promote the first plate 
 from dish No. 1 to dish No. 2, also containing distilled 
 water ; then remove the second plate from the Bath, and 
 place in the dish recently vacated by the first plate. Pro- 
 ceed in the same way throughout, till all the plates have 
 been passed into No. 4, where they may remain for hours 
 without injury, or may be coated at once with Tannin. 
 After five or six plates have been passed through the 
 dishes, pour off the water and renew it ; indeed, if the 
 plates have to be kept for some time, more frequent 
 changing will be required, or the plates must be passed 
 through a greater number of dishes. If the last washing 
 has been performed in very hard water, it will be found 
 beneficial to pour over each plate an ounce or two of dis- 
 tilled water, immediately before applying the Tannin. 
 
 The preservative solution of Tannin may vary according 
 to circumstances, from 2 to 30 grains to the ounce of dis- 
 tilled water. A strong solution has a tendency to lessen 
 sensitiveness, but usually gives great vigour, tending to 
 harshness. Again, a simply-iodized Collodion, or one con- 
 taining little Bromide, requires a weaker solution than a 
 highly -bromized sample. The amount of Tannin that will 
 give the best results will depend also on the temperature, 
 the nature of the subject, the light and the aperture of the 
 lens. The operator must, therefore, be guided by the ap- 
 pearance of his Negatives, whether he has hit the right 
 proportions, and treasure his experience accordingly. For 
 the bromo-iodized Collodion of page 402, the following has 
 been found a good average strength of solution : 
 
 Tannin 60 grains. 
 
 Distilled water 9 ounces. 
 
 Dissolve and filter. 
 
 Should the filter become clogged by resinous matter, 
 which some samples of Tannin contain in large quantities, 
 
RUSSELL'S TANNIN PROCESS. 545 
 
 pass it through a fresh paper, then add one ounce of 
 Alcohol. This solution will keep good for some months in 
 stoppered or well-corked bottles. 
 
 Measure out two separate portions of the Tannin 
 solution, containing, for a plate 8x5, about 4 fluid 
 drachms each. Apply the first two or three times to the 
 washed film, to remove the adherent water ; then pour the 
 second portion on and off several times, and rear up the 
 plate to dry on blotting-paper in an absolutely dark place. 
 It is better to allow the plates to dry spontaneously at a 
 moderate temperature, since plates dried by artificial heat 
 are generally more uncertain. They should be quite dry 
 before exposure, otherwise irregularities of development 
 will be the consequence. When the drying is complete, 
 the surface of the plate may be examined by a taper light, 
 or by the orange light of the dark room, previous to their 
 being stowed away in a grooved dark plate-box, for use. 
 If all the operations have been performed successfully, the 
 film will present a bright, highly-polished surface, hardly 
 distinguishable from glass ; if patches of dulness be ob- 
 served, the plate will be worthless. 
 
 In the second edition of Major Russell's ' Tannin Pro- 
 cess,' a modification of treatment, after applying the Tan- 
 nin, has been recommended. He says, " Before setting 
 up the plates to dry, the Tannin may be thoroughly washed 
 off; a decided increase of sensitiveness is thus produced in 
 all cases, even with a very horny Collodion freshly iodized. 
 The use of Bromide alone in the Collodion, and washing 
 off the Tannin, will increase sensitiveness at least three 
 times. The chemical effect of the Tannin is not lost, nor 
 even diminished by the washing ; a plate prepared with 
 an 8-grain solution will show, after development, the same 
 difference from one prepared with a two-grain solution, 
 which would appear if the Tannin had been dried on the 
 film in both cases. By washing off the Tannin, the greater 
 vigour and intensity given by a strong solution may, if re- 
 quired, be obtained up to a certain point without the loss 
 of sensitiveness which would otherwise be incurred." 
 
546 PRACTICE OF PHOTOGRAPHY. 
 
 Exposure. Directions as to time of exposure will be 
 found under the head of Fothergill Process, and Taupenot 
 Process. The method now under consideration is, accord- 
 ing to the experience of the Writer, more sensitive than 
 an Albumen process by at least one-third, but the Citric 
 Acid in the developer, although improving the colour, ap- 
 pears to add something to the required exposure, and since 
 a powdery structure of the Collodion film has the same 
 effect, the directions given under the head of the Collodio- 
 Albumen method may be adhered to. 
 
 Development. Develope as soon after exposure as pos- 
 sible. The evening of the same day is usually a convenient 
 time for this part of the process. If kept for a day or two 
 after exposure, they are apt to develope feebly and rather 
 foggily, although sometimes they may be left for even a 
 longer time without injury. 
 
 If the glasses were not coated with Gelatine before the 
 Collodion, the chance of wrinkling and contraction during 
 development, will be much lessened by dipping a camel's- 
 hair brush in any quickly-drying Negative Varnish, and 
 running it round the edge of the film, to about an eighth 
 of an inch inwards on the surface. For plates up to 8 X 5 
 this plan will usually be sufficient to fix the Collodion. 
 
 The developer is made from the two following solu- 
 tions : 
 
 ]N T o. 1. 
 
 Pyrogallic Acid 72 grains. 
 
 Alcohol 1 fluid ounce. 
 
 Dissolve without filtering, unless there be evident tur- 
 bidity, add two or three drops of Ether, and preserve in a 
 stoppered bottle. The solution will keep for months with- 
 out much change. 
 
 No. 2. 
 
 Nitrate of Silver 20 grains. 
 
 Citric Acid 40 grains. 
 
 Distilled Water 1 fluid ounce. 
 
 No filtering needed unless a white precipitate of Citrate of 
 Silver should fall ; as w T ill sometimes be the case when the 
 
DEVELOPMENT OF TANNIN PLATES. 547 
 
 Nitrate of Silver was previously fused, or when the Citric 
 Acid contains impurities. 
 
 Begin by diluting 30 minims of No. 1 (equivalent to 4| 
 grains of Pyrogallic Acid) with three ounces of distilled 
 water. The resulting solution will keep for 24 hours, but 
 eventually turns yellow from the absorption, of oxygen. 
 Next place the diluted liquid upon the table, side by side 
 with the concentrated alcoholic solution, and all will be 
 ready for the development. 
 
 To develope, moisten the plate with a small quantity of 
 distilled water and spirits of wine, mixed in equal propor- 
 tions, and afterwards with a larger quantity of distilled 
 water without the Spirits of Wine. Then for a plate 
 8x5, measure out five drachms of the diluted No. 1, and 
 pour over the plate two or three times, till it runs evenly : 
 afterwards pour back into the developing glass, and add 
 four or five drops of No. 2, mixing them well. Apply this 
 mixed developer, and keep it gently moving on the plate. 
 Allow it to act for a short time, and watch the process of 
 the reduction. If the sky developes strongly, but is not 
 gradually followed by the darker portions of the object, 
 the plate was under-exposed, and the picture will then be 
 harsh, unless the developer be immediately strengthened 
 by pouring it back into the measure, and adding ten addi- 
 tional minims of No. 1 to the three drachms, so as to raise 
 the proportion of Pyrogallic Acid to four or five grains to 
 the ounce. If, on the other hand, the whole image ap- 
 pears simultaneously, add a few more drops of the solution 
 No. 2, in order to increase the opacity of the sky. The 
 theory of this part of the process will be found fully de- 
 scribed at page 343 of the Second Part of the work. 
 
 The ordinary solution of Pyrogallic Acid and Acetic 
 Acid employed in the wet process (p. 401) may be used in 
 this dry method, but it does not answer so well. 
 
 Observe that in Russell's Tannin process the porosity 
 of the Collodion is very little maintained, the preservative 
 solution being thin and limpid. Hence these plates would 
 certainly develope slowly, were it not for the reducing pro- 
 
548 PRACTICE OF PHOTOGRAPHY. 
 
 parties of the Tannin. Those who desire to prepare their 
 plates in such a way that the film will expand on being 
 wetted, are advised to mix Gum Arabic with the Tannin, 
 taking from 15 to 30 grains of Gum to the ounce of Water. 
 The image having been developed to the proper point, 
 fix the plate with solution of Hyposulphite of Soda, and 
 varnish in the usual way. 
 
 FOTHERGILL'S DRY PROCESS. 
 
 The Collodion. This process is more dependent upon 
 the state of the Collodion than that last described, and un- 
 less a certain amount of porosity co-exist with the organic 
 reactions which are necessary to intensity, the developed 
 image will often be feeble. The normal Collodion of page 
 377, even when Bromo-iodized, is not so suitable for 
 Fothergill's process as some other kinds, being too dense 
 in structure, and too feeble in developing unless after a 
 long keeping in the iodized state. Although very tolerable 
 pictures have been obtained with this Normal Collodion, 
 yet the formula of page 460 affords better promise of suc- 
 cess, the contrast of image being greater, and the film less 
 liable to solarize. It should be Bromo-iodized and kept 
 for some time till it has suffered partial decomposition. A 
 simply Iodized Collodion which has undergone the same 
 decomposition may also be used, but it is less sensitive. 
 
 The glasses will scarcely require any preliminary coat- 
 ing of Gelatine or Albumen as described at page 542, un- 
 less they are of a large size. It will be sufficient to clean 
 them carefully with old Collodion or Tripoli, and to see 
 that no moisture is present at the time of applying the 
 Collodion. The directions for coating and dipping in the 
 Bath already given under the head of " Russell's Process," 
 page 543, will apply also in this method, and the strength 
 of the Bath may be the same, viz. from 30 to 40 grains to 
 the ounce of Water. 
 
 Mode of washing the Film. This, as already shown 
 when explaining the theory in Part II., is the most impor- 
 tant part of the process, and the one on which success or 
 
 
FOTHERGILL'S DEY PROCESS. 549 
 
 failure mainly depends. A small quantity of Nitrate of 
 Silver must always be left for the purpose of coagulating 
 the Albumen and combining with it ; and when the Collo- 
 dion is deficient in those qualities which give intensity to 
 the development, a still larger quantity of Nitrate of Sil- 
 ver will be required to fix the Albumen abundantly upon 
 the film, and thus to secure the desired effect. Further 
 than this, it has been shown * that the same quantity of 
 water applied to two Collodion films differing in physical 
 structure will often cleanse one more perfectly than the 
 other. Therefore no universally applicable rule can be 
 given, but it will save the operator many failures and dis- 
 appointments if he attend to the following general prin- 
 ciples, which have been practically verified. A Bromo- 
 iodized Collodion under all circumstances, and an Iodized 
 with strong organic reactions, will with advantage bear 
 washing for about ten minutes, in a large shallow porcelain 
 or gutta-percha tray, half filled with distilled or filtered 
 rain-water, before applying the Albumen solution. An 
 Iodized Collodion with feeble organic reactions will gene- 
 rally be sufficiently washed by pouring on and off a few 
 drachms (or ounces according to the size of the plate) of 
 distilled water, till all greasiness has disappeared. 
 
 When a Collodion which ought to be washed very slightly 
 before albuminizing, is thoroughly washed, the image de- 
 velopes slowly, exhibits no red translucency at the early 
 stage of the development, usually fogs more or less, and 
 is wanting in contrast. When, on the other hand, a Collo- 
 dion requiring considerable washing, is not much washed, 
 the image developes rapidly, shows a red tone at first, and 
 is strong in contrast. Defects however soon arise, such as 
 markings of varied form due to a clotty coagulation of the 
 Albumen, and perhaps fogging from incipient reduction of 
 the Albuminate of Silver. The more the plates are washed, 
 the longer they will keep in the sensitive condition, and 
 hence the washing should always be carried as far as pos- 
 sible without sacrificing the contrast of the Negative. 
 
 * Acldand's ' Hints on FothergilTs Piocess,' published by Messrs. Home 
 and Thornthwaite, Newgate Street, London. 
 
550 PRACTICE OF PHOTOGRAPHY. 
 
 The Albumen solution. Take the white of a fresh egg, 
 and add to it an ounce of distilled water, with three drops 
 of strong Ammonia. The Ammonia preserves the Albu- 
 men from becoming sour and mouldy, makes it run more 
 easily through the filter, and slightly increases the inten- 
 sity of development. The solution will usually keep for 
 several weeks. 
 
 Shake for ten minutes in a bottle, and pour out the 
 liquid on a filter previously wetted. If the paper is suffi- 
 ciently thin (Papier Joseph), the liquid ought to pass 
 through drop by drop ; but as eggs vary in glutinosity, 
 there may be a difficulty, and in this case more water must 
 be added, or fine muslin substituted for paper. 
 
 Apply the filtered Albumen twice or thrice to the film 
 in the same manner as Collodion. Then pour distilled 
 water on and off the plate for a minute to remove the ex- 
 cess of Albumen, afterwards let it soak for another minute 
 or two in a tray of distilled w-ater, and stand up on end to 
 dry on blotting-paper. 
 
 When the plates are perfectly dry, they are ready for 
 exposure, but as the sensitiveness is lessened by adhering 
 moisture, it is recommended to use artificial heat in damp 
 weather. Warm the contained air of a deal box, by means 
 of a common flat-iron, and stand the glasses at the sides 
 after removing the iron. The plates may be placed in a 
 box of this kind whilst still wet, and the moisture steamed 
 off by artificial heat ; but this is a hazardous plan, and it 
 will be better to get them surface-dry before using the iron. 
 
 Exposure. The sensitiveness has been stated to be 
 double that of the Taupenot process ; but the pictures 
 shown to the Writer in corroboration w T ere more or less 
 imperfect in the shadows, and he believes that a gain of 
 one-third is nearer the mark. From two to four minutes 
 may be allowed on a favourable day, according to the dis- 
 tance of the view, and the amount of foliage ; or from five 
 to ten minutes in a dull light. The above exposures are 
 for a stereoscopic Camera mounted with a single lens : fur- 
 ther particulars are given in Taupenot's process, page 556. 
 
DEVELOPMENT OF FOTIIERGILI/S PLATES. 551 
 
 Development. This process is not quick as regards de- 
 velopment. When a full quantity of Nitrate of Silver is 
 left upon the film before applying the Albumen, it will be 
 somewhat more rapid, but as a rule, two of Russell's Tan- 
 nin plates may be developed to one of Fothergill's ; the 
 same reducing agent being employed in each case. Theory 
 teaches us to expect a slow development in Fothergill's 
 process, because the porosity of the Collodion is not well 
 maintained by the preservative (p. 334) ; and in addition 
 to this it can be shown that the employment of Albumen 
 always diminishes the rapidity of development, although it 
 increases the intensity. 
 
 The advocates of the Fothergill process mostly recom- 
 mend the ordinary formula of Pyrogallic Acid and Acetic 
 Acid (p. 401), but for Stereoscopic transparencies the 
 Writer prefers the second formula of the same page, viz. 
 Pyrogallic Acid and Citric Acid, which preserves the clear- 
 ness of the shadows : the Alcohol in the formula must be 
 omitted. The use of Gallic Acid for dry plates is also 
 much to be recommended, the Negatives being developed 
 face downwards, upon a saturated solution of Gallic Acid, 
 to each ounce of which is added 15 drops of a 20-grain 
 solution of Nitrate of Silver. 
 
 When Pyrogallic Acid is used, each drachm of the liquid 
 is previously mixed with about five minims of a solution 
 of Nitrate of Silver containing twenty grains to the ounce 
 of Water. The film is first wetted, and the mixed deve- 
 loper poured on and off from a glass measure. This mea- 
 sure must be scrupulously clean, or defects will result. 
 In two or three minutes the developing solution may 
 turn brown, and if so, a second portion must be used to 
 complete the reduction. From five to ten minutes will be 
 occupied in bringing the Negative up to the proper point. 
 
 Fixing. Hyposulphite of Soda is better than Cyanide 
 in all cases where albumen is employed, as being less likely 
 to dissolve the delicate details of the Negative. 
 
 Failures. Those which the Writer has met with have 
 depended either upon the plate being held at the corners 
 
552 PKACTICE OF PHOTOGKAPHY. 
 
 by dirty fingers, or upon the use of newly iodized Collo- 
 dion too copiously washed before putting on the Albumen. 
 In the latter case everything goes wrong, and there are 
 spots, stains, and marbled markings, such as would not 
 occur if the particles of Iodide had been in the state to 
 receive a decided impression in the Camera (p. 158). 
 
 Markings like those on watered silk are also a common 
 cause of failure ; they probably depend partly upon the 
 structure of the Collodion, but are exaggerated by the use 
 of Albumen fully coagulated by excess of Nitrate of Silver. 
 The remedy suggested is to keep the Collodion for a longer 
 time in the iodized state, or to mix it with another sample 
 which is free from the markings. 
 
 TAUPENOT'S COLLODIO-ALBUMEN PKOCESS. 
 
 Cleaning the Glasses. This part of the process must be 
 conducted with care, since the Albumen which is laid upon 
 the Collodion, tends to raise the latter in blisters : one 
 mode of obviating which is to clean the glasses in such a 
 way that the film adheres with tenacity. 
 
 Old Collodion may be used for cleaning, or in place of it 
 the mixture of Tripoli powder and Ammonia described at 
 page 408. 
 
 The Collodion. All necessary information on Collodion 
 will be found in the Ninth Chapter of the Second Part of 
 the Work. One great advantage of this process in fact 
 consists in its being comparatively independent of the Col- 
 lodion. Blistering of the film during development is the 
 principal matter which requires attention, and for this a 
 suitable Collodion is the most available remedy. Old Col- 
 lodion, iodized with alkaline Iodides, and which has be- 
 come soft and porous in structure by gradual decomposi- 
 tion, is the best kind to employ ; but it will frequently 
 happen on attempting to coat the plate with such Collo- 
 dion, that it proves to be too limpid, and leaves an uneven 
 coating upon the glass, thicker below than above. In such 
 a case a grain and a half of Iodide of Cadmium to the ounce 
 will often thicken it sufficiently ; if not, a little newly 
 
TAUPENOT'S DET PROCESS. 553 
 
 iodized Collodion must be mixed with it, or two grains of 
 Pyroxyline dissolved in each ounce. The normal Collo- 
 dion of page 377 will usually require this treatment after 
 six months' keeping, but in other respects it is peculiarly 
 well adapted for Taupenot's process, since the large excess 
 of Sulphuric Acid, and of water, employed in the formula 
 for the Nitro-Sulphuric Acid, are both highly favourable 
 to adhesiveness of Collodion. 
 
 Sensitizing and Washing. It is usual to employ two Ni- 
 trate Baths in this process, one for the Collodion, and the 
 other for the second film, viz. the Albumen. The Albumen 
 Nitrate Bath must contain free Acetic Acid, and therefore 
 if one solution be employed for both films it should be a 
 Bath of Aceto-Nltrate. Very good results have been ob- 
 tained with a single Bath, and it is more convenient in tra- 
 velling ; but it has always appeared to the Writer that the 
 Collodion sticks better to the glass when sensitized in a 
 clean and neutral Bath, and therefore, when portability is 
 not an object, he advises to keep the Baths distinct. 
 
 Sensitize the Collodion in the ordinary way, holding it 
 rather longer than usual before dipping, to favour perfect 
 adhesion and so to prevent blistering. Then having ar- 
 ranged two dishes side by side, No. 1 containing distilled 
 and No. 2 common water, lay the plate face uppermost in 
 the first dish, and wave the water backwards and forwards 
 for about thirty seconds. Then put the plate into the se- 
 cond dish and leave it whilst another glass is being coated 
 and immersed in the sensitizing Bath. Now drain plate 
 No. 1 closely on blotting-paper, and it will be ready for 
 the iodized Albumen. Plate No. 2 remains in the sensiti- 
 zing Bath until the first glass has been coated with Albu- 
 men and placed to dry. 
 
 Preparation of the iodized Albumen. Procure eggs 
 fresh laid, or not more than a week old. Take of 
 
 Albumen 3 fluid ounces. 
 
 Distilled Water .... 1 fluid ounce. 
 
 Strong Ammonia .... 10 minims. 
 
 Iodide of Potassium ... 10 grains. 
 
 Bromide of Ammonium . . 10 grains. 
 
551 PRACTICE OF PHOTOGEAPHT. 
 
 First mix the Ammonia and the Water, then add the 
 other ingredients, and shake together in a bottle. This 
 mode of proceeding will be found more fully described 
 under the head of Albuminized Paper (p. 487). 
 
 In preparing the Albumen solution it must be borne in 
 mind that the viscidity of different eggs varies : hence it 
 will be occasionally necessary to introduce a little more 
 Water, in order to make the liquid run through the filter. 
 The filtering-paper ought not to be as close in texture as 
 Swedish filtering-paper, but Papier Joseph will usually 
 answer. Some operators filter through a piece of sponge, 
 but the difficulty is to push the sponge down with sufficient 
 firmness to stop back floating particles without choking the 
 neck of the funnel : a piece of fine cambric appears to the 
 Writer to be better, but he prefers paper when the Albu- 
 men can be made to run through it. After Albumen has 
 once been filtered it will run more easily a second time, 
 and it will also flow better after a few days' keeping, since 
 the Ammonia gradually liquefies it, and removes much of 
 the glutinosity. 
 
 Instead of Iodide of Potassium, Iodide of Ammonium 
 may advantageously be used when it can be obtained pure, 
 being a more soluble salt, and thus less likely to cause pin- 
 holes in the Negative by crystallizing in the Albumen. 
 Iodide and Bromide of Cadmium cannot be used, since 
 they have the property of coagulating Albumen. 
 
 Another addition is sometimes made to the Albumen so- 
 lution, viz. common white Sugar, in the proportion of one 
 drachm to each ounce of Albumen. It must be added 
 after filtration, as it increases the viscidity. The use of 
 the Sugar is to lessen the chance of the blisters before re- 
 ferred to, by mechanically altering the Albumen. Nitrate 
 of Magnesia and other deliquescent Salts have been em- 
 ployed with success for a similar purpose. 
 
 The keeping properties of the prepared Albumen vary. 
 It will often remain good for a month, if a piece of Cam- 
 phor the size of a pea be left floating on the surface ; but 
 sometimes mucous threads form, or a white deposit falls, 
 which must be filtered out. 
 
TATJPENOT'S DRY PEOCESS. 555 
 
 To apply the Albumen, For a glass 8x5, measure out 
 two drachms of the Albumen and pour it on and off twice, 
 to displace the surface water of the washed Collodion film. 
 Then throw it down the sink, and apply a second quan- 
 tity, which will serve as the preliminary portion for the suc- 
 ceeding plate. The stock of Albumen is thus kept pure 
 and undiluted. 
 
 Place the Albuminized plates vertically on blotting- 
 paper to drain and dry. The process, when nearly com- 
 plete, may be hastened by artificial heat (p. 550), or the 
 plates may be dried off at once, whilst still wet. 
 
 It is generally believed that the plates will now bear 
 exposure to daylight without injury. On theoretical 
 grounds, the opinion may to some extent be correct, since 
 pure Iodide of Silver, in the presence of an excess of 
 Iodide of Potassium, is not sensitive to light. If the Col- 
 lodion, however contains a Bromide or is made from Py- 
 roxyline with strong organic reactions, an excess of Iodide 
 of Potassium does not entirely destroy the sensitiveness. 
 Hence, therefore, in order to guard against fogging, it is 
 advisable not to subject them to light, but to stow them 
 away at once in a dark cupboard or box for future use. 
 If the place is dry, they will keep well for months. 
 
 The Aceto-Nitrate Bath. Take an ordinary Collodion 
 Negative Bath, and add to each fluid ounce thirty minims 
 of Glacial Acetic Acid ; keep it in glass or gutta-percha, 
 and continue to use it until it has become blackened by 
 the action of the Albumen (p. 492). Then decolorize by 
 Kaolin in the usual way. It may require some weeks or 
 months to bring the solution to the colour of claret, and 
 the Writer has printed transparencies quite clean with a 
 Bath of this kind, so that it is a mistake to purify it too 
 frequently. An occasional filtration or cleansing of scum 
 from the surface will be required. 
 
 Complaints have sometimes been made that the Aceto- 
 Nitrate Bath loses all its acid after a time, and produces 
 foggy pictures. Albumen is an alkaline substance, and will 
 render a neutral solution of Nitrate of Silver alkaline by 
 
556 PEACTICE OF PHOTOGRAPHY. 
 
 degrees (p. 302) ; but a portion of the Acetic Acid undoubt- 
 edly disappears in the form of Acetic Ether. Old Collo- 
 dion Baths contain abundance of Alcohol, and when they 
 are converted into Aceto-Nitrate, the characteristic sweet 
 smell of Acetic Ether soon becomes perceptible. The 
 film of Albumen must be rendered quite dry by holding it 
 to the fire before it is dipped in the Aceto-Nitrate Bath. 
 All agree that blisters are less frequent, both when the 
 glasses are well dried before putting on the Collodion, and 
 again after coating with Albumen. Leave them in the 
 Bath any time from thirty seconds to two minutes, and 
 then remove and wash with water, 
 
 Use two dishes for this washing, the first of distilled, 
 and the second of common water, and allow twenty or 
 thirty seconds in each dish ; then rear up again to dry, and 
 the plates will be ready for exposure. Artificial heat may 
 be used with advantage as before ; a common flat-iron 
 being placed in the centre of a box, and the glasses ar- 
 ranged at the sides. If little blisters appear on the sur- 
 face of the Albumen when it is half wet and half dry, they 
 will probably come out with increased strength in the de- 
 velopment. 
 
 Exposure in the Camera. Carry the plates in a grooved 
 tin box, or a box of mahogany. Pine or deal is not re- 
 commended, on account of the volatile resin it contains, 
 but a box of this kind may answer when no better can be 
 obtained. Change the plates after exposure by means of 
 a large bag, made of two thicknesses of yellow and one of 
 black calico, with a window cut out of the latter at the 
 side, and an elastic band to draw round the waist. 
 
 Dry -plate Cameras are made purposely, very portable ; 
 also dark-boxes for sliding the glasses backwards and for- 
 wards as required. The objection to the dark-box is, that 
 unless the glasses are cut with unusual care, they will not 
 always slip readily, and the Writer prefers either the black 
 bag above described, or separate backs for the Camera, 
 each enclosing a prepared plate. 
 
 The most successful operators in Taupenot's process 
 
DEVELOPMENT OP TAUPENOl's PLATES. 557 
 
 .give a very long exposure, which has the effect of softening 
 the picture, and obviating that excessive contrast or hard- 
 ness which is often complained of in Albumen Negatives. 
 The chances of fogging in development, however, are in- 
 creased by lengthened exposure, when the film is not in 
 very good order, or more especially when the Camera admits 
 any diffused light. Three minutes may be mentioned as 
 an average time for a Stereoscopic view in a good light, 
 eight minutes for a picture on a plate 10 X 8 under the 
 same circumstances, or twenty minutes for a group of trees 
 rather near to the Camera, the Lens being of fifteen-inch 
 focus, and half-inch diaphragm. In other instances, how- 
 ever, where from hardness of washing water, etc., over- 
 action of light is encouraged, the skies will be nearly free 
 from deposited silver, if too much time be allowed in the 
 Camera. 
 
 Development. It was formerly customary to develope 
 C olio dio- Album en plates with Gallic Acid, but this plan 
 has now been laid aside as unnecessarily tedious, and offer- 
 ing no marked advantages of any kind. The best mode of 
 development is the same as directed for Tannin plates ; see 
 p. 546. 
 
 Fixing solution. Fix with a saturated solution of Hy- 
 posulphite in preference to Cyanide, and wash for double 
 the time required in the case of a Collodion Negative 
 (p. 426) ; after which the plate may be varnished in the 
 usual way. Small blisters forming at this stage of the pro- 
 cess will disappear on drying. 
 
 Failures. The common failures are blistering hard- 
 ness of Negative, which makes it print black and white 
 without half-tone and markings upon the Negative. 
 
 The cause of blistering has already been partially ex- 
 plained at page 552, and the remedy on which experienced 
 operators lay the most stress is perfect dryness of the film 
 at every stage ; it may be held to the fire before dipping 
 in the Aceto-Nitrate Bath, and again previous to its expo- 
 sure in the Camera. Those who attempt to economize 
 waste Collodion and " bottoms " of bottles by using them 
 
558 PRACTICE OF PHOTOGEAPHY. 
 
 in Taupenot's process must be prepared for wrinkling or. 
 blistering of the film, since an increase in the amount of 
 Alcohol beyond a certain point appears to lessen the adhe- 
 sion of the Collodion to the glass. Perfect security against 
 blisters can only be obtained by coating the glasses with di- 
 lute Gelatine or Albumen before Collodionizing (p. 542). 
 
 Hardness of Negative is due to over-development, to the 
 addition of too much Nitrate of Silver to the developer, or 
 to under-exposure. The use of very powerful lenses should 
 be mentioned as another cause. Taupenot's process is 
 better fitted for long-focus lenses than for lenses of the Ste- 
 reoscopic size. 
 
 Markings like those in the figure may be traced to the 
 
 Collodion in the first instance, but the presence of Albu- 
 men exaggerates them. The Pyroxyline was made in too 
 strong acids, and hence the structural arrangement. Much 
 however may be done to remedy the markings by keeping 
 the developing solution moving, and not allowing a sta- 
 tionary layer to rest upon the plate. 
 
 Marbled markings are often produced by an oily scum 
 floating upon the Aceto-Nitrate Bath, and carried down 
 by the film. A rapid discoloration and turbidity of the 
 developer, causing a deposit on the shadows, may be due 
 to chalk in the water used for the Gallic Acid, or to de- 
 composition in the Aceto-Nitrate Bath by long contact 
 with dissolved Albumen. 
 
 THE MODIFICATION OF MESSES. PETSCHLEE AND MANN. 
 
 It will not be advisable to attempt this modification with 
 an unsuitable Collodion. The tendency to blistering is 
 less than with Taupenot's process, but the image is more 
 feeble, and hence greater attention must be paid to the 
 state of the Collodion. Read the remarks at page 548, 
 
MODIFICATION OF TAUPENOT's PROCESS. 559 
 
 since it will be found that a Collodion in good order for 
 Fothergill's process will answer likewise for the process 
 now under discussion. 
 
 Clean the glasses, coat them with Collodion, and render 
 them sensitive exactly as in Fothergill's process. Then 
 wash freely under a tap, or with two or three changes of 
 common water in a dish, after which the film will be ready 
 fcr the Albumen. 
 
 To prepare the Albumen, take of 
 
 Albumen 1 fluid ounce. 
 
 Water 1 fluid ounce. 
 
 Chloride of Ammonium ... 3 grains. 
 
 Beat the whole to a perfect froth, and wh,en it has again 
 liquefied, filter it through paper, if possible ; but when it 
 refuses to run through the paper (which in the absence of 
 Ammonia is not unlikely to happen) it must be passed two 
 or three times through a double thickness of fine muslin, 
 until all suspended particles are removed. 
 
 Apply this Albumen to the washed film, exactly as in 
 Taupenot's process, p. 555, and then dry the plates spon- 
 taneously, finishing with artificial heat, p. 550, after which 
 they may be stowed away in a box. 
 
 The plates are at this stage nearly insensitive to light, 
 and are well fitted for long keeping ; but inasmuch as they 
 do not appear to be quite insensitive, it will be better to 
 prepare them by yellow light, and to keep them in a dark 
 place until they are required for use. 
 
 In order to restore the sensitiveness, which the presence 
 of free Chloride of Ammonium or Sodium in the film re- 
 duces to a minimum, wash the plates in two or three 
 changes of common water, and then flood them gently with 
 distilled water, after which they may be reared up to dry. 
 A safe plan will be to leave each plate for a few minutes 
 in the first washing w 7 ater, so as to soften the Albumen, 
 and thus to facilitate the removal of soluble matters. 
 
 From this point proceed in every respect as in Fother- 
 gill's process ; but if the Negative should prove to be too 
 metallic, and too deficient in contrast, it will be well in the 
 
560 PRACTICE OF PHOTOGRAPHY. 
 
 next experiment to give the Albumen film a final wash of 
 weak aqueous solution of Tannin. Dissolve two grains of 
 Tannin in. half an ounce of distilled water, and pour it on 
 and off the plate twice or thrice ; this will add much to 
 the vigour of the Negative. 
 
 The inquiry has been made why stains during develop- 
 ment are less frequently met with in this process than in 
 the original one of M. Taupenot. The reason appears to 
 be that in the latter the Nitrate of Silver is always slightly 
 in excess, since the plates receive a final dip in the Aceto- 
 Nitrate Bath : in the former however the Chloride of So- 
 dium is the last application, and hence, although the rapi- 
 dity of reduction is restrained, the developer remains for 
 a longer time without becoming turbid. The same ob- 
 servation applies in a measure to the Fothergill process, 
 in which also turbidity of developer is more rare than in 
 some other dry processes. 
 
561 
 
 APPENDIX. 
 
 TO FIND THE EQUIVALENT FOCUS OF A LENS, AND THE 
 ANGLE OF VIEW INCLUDED. 
 
 IT is often of importance, and always of interest to the Photo- 
 grapher to be able to ascertain the angle of picture included on 
 a plate of a given size, and also to determine the " equivalent" 
 or correct focal length of his lens. Mr. Shadbolt has published 
 a very simple and correct method of determining both these 
 points, which, with his permission, we here reproduce. 
 
 " If we place a theodolite upon the spot whence we are about 
 to take a photograph, and having ascertained by inspection what 
 portion of the view will be included upon our plate, \ye turn the 
 instrument first to one extremity of the subject and then to the 
 other, we ascertain by the number of degrees marked on the 
 scale that have been passed over by the telescope what ' angle of 
 picture' we are to have. If we move nearer to or further from 
 the subject, we shall of course include less or more of it respec- 
 tively upon our plate or ground-glass. But for all that, we do 
 not alter the angle of picture included ; for if we perform the 
 same operations as before with the theodolite, at the nearer and 
 more distant stations respectively, we find just the same number 
 of degrees on the scale of the instrument passed over in each 
 6ase as before. 
 
 " If we imagine a line to be drawn from between the eyes of 
 the spectator directly towards the centre of our subject, the 
 * angle of picture' means the distance to the right and left of 
 the central spot to which other imaginary lines from the ob- 
 
 2o 
 
562 APPENDIX^ 
 
 server might be drawn, so as just to limit the view to that which 
 our lens would include from the same spot. 
 
 "Mr. Grubb has already pointed out that the camera itself 
 may be made to do duty as a theodolite, by placing it upon a 
 level table on a sheet of paper fastened down upon the latter ; 
 then, having ascertained by inspection the limits of the subject 
 delineated upon the focussing-screen, turn the camera round 
 until the object at one extreme occupies the centre of the 
 ground-glass, and accurately coincides with a pencil line made 
 thereon. Now with a pencil draw a line on the paper beneath 
 the camera, parallel with one side of the latter ; turn the camei'a 
 round until the object at the other extremity of the subject coin- 
 cides with the pencil line on the focussing-screen, and draw 
 another line on the paper as before. The angle formed by the 
 two lines thus drawn will show the ' angle of picture' included. 
 But this is not all ; for if we draw a line opposite to the angle 
 thus found, at such a distance therefrom as shall allow the third 
 line, bounded by the two previous ones, to be of the same length 
 as the focussing-screen, a perpendicular let fall from the angle 
 upon the third line will give the equivalent focal length of the 
 lens employed. This equivalent focal distance may or may not 
 agree with the distance of the screen from the lens most pro- 
 bably not, especially if it be a double or triple combination. 
 
 " To simplify this matter, we append the following diagram : 
 
 Let A B represent a moderately distant view, and C the place 
 
APPENDIX. 563 
 
 of the camera. On focussing, we find the portion included on 
 the ground-glass to be from D to E only ; this is the ' angle of 
 picture.' If we now turn the camera towards D, so as to make 
 its image fall on a line in the centre of the focussing-screen, and 
 draw a line parallel with the side of the camera, we shall have a 
 line parallel to C D, viz. c d. On turning the camera towards 
 E, and making its image fall on the same mark on the screen, 
 another line drawn parallel with the same side of the camera 
 will also be parallel with C E, viz. c e, and these two lines are 
 inclined to one another exactly in the same ratio as are C D 
 and C E. 
 
 " By measurement upon the ground-glass, we find the distance 
 between the images of the objects D and E to be equal to g Ti ; 
 and if we now place this line exactly opposite the angle c, so as 
 to be bounded by the lines e d, ce, and let fall the perpendicular 
 cf, the distance of is the equivalent focus of the lens, and this 
 holds good whether the marginal rays suffer deviation by the 
 lens or not ; only in the latter case the equivalent focus of the 
 lens may vary to a very trifling extent as the rays of light fall 
 upon it perpendicularly or obliquely. This is not of any great 
 importance in practice, and we merely name it for the sake of 
 accuracy. Any one possessing a lens and camera need there- 
 fore be under no uncertainty about the equivalent focus of the 
 latter; and this once ascertained, he can always calculate the 
 angle of any view he may take with it, no matter upon what 
 sized plate he may operate."* 
 
 QUANTITATIVE TESTING OF SOLUTIONS OF NITRATE OF 
 SILVER. 
 
 The amount of Nitrate of Silver contained in solutions of that 
 salt may be estimated with sufficient delicacy for ordinary Pho- 
 tographic operations by the following simple process : 
 
 Take the pure crystallized Chloride of Sodium, which opera- 
 tive chemists make purposely for analysis by dissolving the best 
 Carbonate of Soda in pure Hydrochloric Acid, and either dry 
 it strongly or fuse it at a moderate heat, in order to drive off 
 
 * See ' British Journal of Photography,' vol. x. pp. 281 and 319, for a 
 more extended explanation of this interesting subject. 
 
564 4-PPENDIX, 
 
 any water which may be retained between the interstices of the 
 crystals ; then dissolve in distilled water, in the proportion of 8| 
 grains to 6 fluid ounces. 
 
 In this way, a standard solution of salt is formed, each drachm 
 of which (containing slightly more than one- sixth of a grain of 
 salt) will precipitate half a grain of Nitrate of Silver. 
 
 To use it, measure out accurately one drachm of the Bath in a 
 minim measure, and place it in a two-ounce stoppered phial, tak- 
 ing care to rinse out the measure with a drachm of distilled water, 
 and to add the rinsings to the fluid drachm of Bath ; then pour in 
 the salt solution, in the proportion of a drachm for every 4 grains 
 of Nitrate known to have been present in an ounce of the Bath 
 which is to be tested ; shake the contents of the bottle briskly, 
 until the white curds have perfectly separated, and the superr 
 natant liquid is clear and colourless ; then add fresh portions of 
 the standard solution of salt by 30 minims at a time, with con- 
 stant shaking. When the last addition causes no milkiness, read 
 off the total number of drachms employed (the last half-drachm 
 being subtracted), and multiply that number by 4 for the weight 
 in grains of the Nitrate of Silver present in an ounce of the Bath. 
 
 In this manner the strength of the Bath will be indicated 
 within two grains to the ounce, or even to a single grain if the 
 last additions of standard salt-solution be made in portions of 15, 
 instead of 30 minims. 
 
 Supposing the Bath to be tested is thought to contain about 28 
 grains of Nitrate to the ounce, it will be convenient to begin by 
 adding to the measured drachm 6 drachms of the standard solu- 
 tion ; afterwards, as the milkiness and precipitation become less 
 marked, the process must be carried on more cautiously, and the 
 bottle shaken violently for several minutes, in order to obtain a 
 clear solution. A few drops of Nitric Acid added to the Nitrate 
 of Silver facilitate the deposition of the Chloride ; but care must 
 be taken that the sample of Nitric Acid employed is pure and 
 free from Chlorine, the presence of which would cause an error. 
 The delicacy of this mode of testing may be increased by adding 
 to the Silver solution a little Bichromate of Potash, omitting the 
 Nitric Acid, a deep red precipitate of Chromate of Silver is thus 
 formed, which tinges the Chloride of Silver produced on adding 
 the standard solution, until the end, when all the Silver having 
 been precipitated, the Chloride of Sodium finally decomposes the 
 
APPENDIX. 565 
 
 red Chromate of Silver into the white Chloride, and the comple- 
 tion of the operation is evident from the change of colour in the 
 precipitate. 
 
 The Photographer may perhaps require to perform these ope- 
 rations when pure Chloride of Sodium is not immediately ob- 
 tainable. In that case the ordinary commercial Chloride of 
 Ammonium may be substituted, 7f grains being dissolved in 
 6 fluid ounces of water. It is advisable however, when using a 
 Chloride of doubtful purity, to take the precaution of trying the 
 strength of the standard saline solution, by testing it upon 30 
 grains of pure dried Nitrate of Silver dissolved in an ounce of 
 water. 
 
 Ordinary fluid measures are seldom graduated with perfect 
 accuracy, and hence one of the various forms of " argentometer" 
 which have been devised* purposely for bath-testing is prefer- 
 able : by its employment the necessity for calculations will be 
 avoided, since the number of measures are made to indicate 
 grains per ounce. Another very correct method of ascertaining 
 the strength of pure solutions of Nitrate of Silver is by the an- 
 nexed Table of Specific Gravities, which, if the correction be 
 made for temperature, will indicate the number of grains in each 
 ounce of solution to within a small fraction of a grain. If the 
 solution to be tested has been used for some time in sensitizing 
 Positive paper, Nitrate of Potash or Ammonia will have accumu- 
 lated to a slight degree, and two on an average must, therefore, 
 be deducted from the tabulated number of grains. To test an 
 old Nitrate of Silver Collodion Bath, 2 grains will require to be 
 added, on account of the Ether and Alcohol which have become 
 mixed up therewith, and which reduce the specific gravity more 
 than the Nitrate of Potash ; Ammonia, etc., increase it. See 
 page 573 on taking of Specific Gravities. 
 
 * For information as to the merits of the various forms of argentometer, 
 see a paper by Jabez Hughes, Phot. News, 1861, p. 51. 
 
566 
 
 APPENDIX. 
 
 TABLE FOE ASCERTAINING FROM THE SPECIFIC GRAVITY 
 THE AMOUNT OF NITRATE OF SILVER CONTAINED IN ONE 
 FLUID OUNCE OF ANY PURE SOLUTION, AT SIXTY DEGREES 
 FAHRENHEIT. 
 
 Grs. 
 
 perfl. oz. 
 
 Sp. Gr. 
 
 Grs. 
 perfl.oz. 
 
 Sp. Gr. 
 
 Grs. 
 perfl.oz. 
 
 Sp. Gr. 
 
 Grs. 
 perfl.oz. 
 
 Sp. Gr. 
 
 10 
 
 1-021 
 
 43 
 
 1-084 
 
 76 
 
 1-146 
 
 109 
 
 1-207 
 
 11 
 
 1-023 
 
 44 
 
 1-086 
 
 77 
 
 1-148 
 
 110 
 
 1-209 
 
 12 
 
 1-025 
 
 45 
 
 1-088 
 
 78 
 
 1-150 
 
 111 
 
 1-211 
 
 13 
 
 1-027 
 
 46 
 
 1-089 
 
 79 
 
 1-152 
 
 112 
 
 1-213 
 
 14 
 
 1-029 
 
 47 
 
 1-091 
 
 80 
 
 1-153 
 
 113 
 
 1-215 
 
 15 
 
 1-031 
 
 48 
 
 1-093 
 
 81 
 
 1-155 
 
 114 
 
 1-216 
 
 16 
 
 1-032 
 
 49 
 
 1-095 
 
 82 
 
 1-157 
 
 115 
 
 1-218 
 
 17 
 
 1-034 
 
 50 
 
 1-097 
 
 83 
 
 1-159 
 
 116 
 
 1-220 
 
 18 
 
 1-036 
 
 51 
 
 1-099 
 
 84 
 
 1-161 
 
 117 
 
 1-222 
 
 19 
 
 1-038 
 
 52 
 
 1-10] 
 
 85 
 
 1-163 
 
 118 
 
 1-224 
 
 20 
 
 1-040 
 
 53 
 
 1-103 
 
 86 
 
 1-165 
 
 119 
 
 226 
 
 21 
 
 1-042 
 
 54 
 
 1-105 
 
 87 
 
 1-167 
 
 120 
 
 227 
 
 22 
 
 1-044 
 
 55 
 
 1-106 
 
 88 
 
 1-168 
 
 121 
 
 229 
 
 23 
 
 1-046 
 
 56 
 
 108 
 
 89 
 
 1-170 
 
 122 
 
 231 
 
 24 
 
 1-048 
 
 57 
 
 110 
 
 90 
 
 1-172 
 
 123 
 
 233 
 
 25 
 
 1-050 
 
 58 
 
 112 
 
 91 
 
 1-174 
 
 124 
 
 235 
 
 26 
 
 1-051 
 
 59 
 
 114 
 
 92 
 
 1-176 
 
 125 
 
 236' 
 
 27 
 
 1-053 
 
 60 
 
 116 
 
 93 
 
 1-178 
 
 126 
 
 238 
 
 28 
 
 1-055 
 
 61 
 
 118 
 
 94 
 
 1-180 
 
 127 
 
 240 
 
 29 
 
 1057 
 
 62 
 
 120 
 
 95 
 
 1-181 
 
 128 
 
 1-242 
 
 30 
 
 1-059 
 
 63 
 
 122 
 
 96 
 
 1-183 
 
 129 
 
 1-244 
 
 31 
 
 1-061 
 
 64 
 
 123 
 
 97 
 
 1-185 
 
 130 
 
 1-245 
 
 32 
 
 1-063 
 
 65 
 
 125 
 
 98 
 
 1-187 
 
 131 
 
 1-247 
 
 33 
 
 1-065 
 
 66 
 
 127 
 
 99 
 
 1-189 
 
 132 
 
 1-249 
 
 34 
 
 1-067 
 
 67 
 
 129 
 
 100 
 
 1-191 
 
 133 
 
 1-251 
 
 35 
 
 1-069 
 
 68 
 
 131 
 
 101 
 
 1-193 
 
 134 
 
 1-252 
 
 36 
 
 1-070 
 
 69 
 
 133 
 
 ]02 
 
 1-194 
 
 135 
 
 1-254 
 
 37 
 
 1-072 
 
 70 
 
 135 
 
 103 
 
 1-196 
 
 136 
 
 1-256 
 
 38 
 
 1-074 
 
 71 
 
 1-137 
 
 104 
 
 1-198 
 
 137 
 
 1-258 
 
 39 
 
 1-076 
 
 72 
 
 1-138 
 
 105 
 
 1-200 
 
 138 
 
 1-259 
 
 40 
 
 1-078 
 
 73 
 
 1-140 
 
 106 
 
 1-202 
 
 139 
 
 1-261 
 
 41 
 
 1-080 
 
 74 
 
 1-142 
 
 107 
 
 1-204 
 
 140 
 
 1-263 
 
 42 
 
 1-082 
 
 75 
 
 1-144 
 
 108 
 
 1-205 
 
 
 
 CORRECTION FOR TEMPERATURE, ETC. For every 10 below 
 60 deduct one grain from the number quoted by the Table, and 
 for every 10 above 60 add one grain to the number tabulated. 
 In testing old Printing Solutions deduct two, and for old Collo- 
 dion Baths add two grains to the number quoted. 
 
APPENDIX. 567 
 
 KECOVEBY OF SILVER AND GOLD PROM WASTE SOLUTIONS 
 AND EESIDTJES. 
 
 a. Separation of metallic Silver from old Nitrate Baths. The 
 Silver contained in solutions of the Nitrate, Acetate, etc., may 
 easily be precipitated by suspending a strip of Sheet Copper in 
 the liquid ; the action is complete in two or three days, the whole 
 of the Nitric Acid and Oxygen passing to the Copper, and form- 
 ing a blue solution of the Nitrate of Copper. The metallic Silver 
 however, separated in this manner, always contains a portion of 
 Copper, and gives a blue solution when dissolved in Nitric Acid ; . 
 and in the case of a Negative Bath, there will also be present the 
 Iodide of Silver that had been in solution in the Bath. 
 
 The Writer has tried, on a large scale, the process of separating 
 Silver from old Baths by suspending bars of metaUic Zinc in the 
 liquid, but the operation was not so successful as could be desired. 
 A large tree of metallic Silver formed, and when the vessel was 
 tapped, fell to the bottom, carrying down with it detached por- 
 tions of Zinc or Oxide of Zinc. In addition to this, a white pre- 
 cipitate was noticed towards the end of the process, and the re- 
 sult was, that in spite of very careful washing with warm dilute 
 Sulphuric Acid, a sufficient quantity of Zinc remained to form 
 a bulky slag, and prevent the Silver from running onto a clear 
 liquid in the crucible. 
 
 A better process is to commence by precipitating the Silver en- 
 tirely in the form of Chloride of Silver, by adding common Salt 
 until no further milkiness can be produced, and then acidifying 
 with Hydrochloric or Nitric Acid. If the liquid be well stirred, 
 the Chloride of Silver will sink to the bottom, and may be washed 
 by repeatedly filling the vessel with common water, and pouring 
 off the upper clear portion when the clots have again settled. 
 The Chloride of Silver thus formed may afterwards be dried 
 and reduced to metallic Silver by a process presently to be 
 described. 
 
 b. Washings from Positive Prints. In a Photographic labora- 
 tory a place should be provided for large wide-mouthed two-gallon 
 bottles, into which washings of dishes, prints, etc., should be 
 thrown, when they contain nothing but free Nitrate of Silver. A 
 lump of salt will serve to keep the liquid free from Silver, and a 
 few drops of common Muriatic Acid will acidify it, and encourage 
 
568 APPENDIX. 
 
 the deposition of the Chloride of Silver. The upper part of the 
 liquid may be poured off from time to time, and when the deposit 
 of Chloride is sufficiently thick to make it worth collecting, it may 
 be drained on a cloth, and treated as presently to be described. 
 
 c. Cuttings of sensitive paper. These should all be thrown into 
 a waste basket, and eventually burnt with free access of air : the 
 ash will probably contain from 50 to 70 per cent, of metallic 
 Silver, and may be treated as described at page 571 under the 
 head of reduction of Sulphide of Silver. Cuttings of prints pre- 
 viously toned and fixed contain so little Silver as scarcely to 
 repay the trouble of collecting. 
 
 d. Waste Developing Solution. Amateurs would scarcely find 
 it worth their while to economize these liquids ; but for the pro- 
 fessional Photographer who uses larger quantities, and works on 
 plates which require much pushing of the development with Ni- 
 trate of Silver, it will be advantageous to fit up a gutta-percha 
 sink, and to conduct away all the waste products, both developing 
 and hypo-fixing, into one vessel. Afterwards proceed as advised 
 below. 
 
 e. Hyposulphite Baths. The Silver cannot be separated from 
 Hypo-Baths in the same manner as from solution of the Nitrate, 
 since both Chloride and Iodide of Silver are soluble in Hyposul- 
 phite of Soda. 
 
 In former editions the employment of Sulphuretted Hydrogen 
 was advised ; but this plan is very slow and tedious, and requires 
 a long time to cany out. Another method is to decompose the 
 whole of the Hyposulphite by means of the waste Nitrosulphuric 
 Acid employed in making Pyroxyline, and so to throw down a 
 mixture of Sulphide of Silver and free Sulphur, leaving Sulphate 
 and Nitrate of Soda in solution. A third process, which has 
 superseded both of these modes, is to warm the solution with 
 a bar of suspended Zinc. After two or three hours' boiling, a 
 portion of the liquid is filtered, after which it is examined for 
 Silver by Sulphuretted Hydrogen. If there be no precipitation, 
 or simply a white turbidity, the whole of the noble metals have 
 been thrown down by the Zinc. Collect the precipitate therefore 
 upon a cloth, wash it with water until a drop evaporated on plati- 
 num foil leaves no residue, dry by artificial heat, and run it down 
 into a button with a mixture of Borax and dry Carbonate of Soda, 
 as advised in works on Analytical Chemistry. Hydrosiilphate 
 
 
APPENDIX. 569 
 
 of Ammonia cannot be used instead of Sulphuretted Hydrogen to 
 ascertain when the warm solution of Hyposulphite is freed from 
 Silver, because it precipitates the white Sulphide of Zinc, and as 
 a trace of Iron is almost invariably present, a brown colour is im- 
 parted to the deposit, which would then be mistaken for Sulphide 
 of Silver. Collect the deposited Silver and redissolve it in Nitric 
 Acid, afterwards throwing it down again as Chloride ; this will 
 free it from impurities, which, if present in any quantity, prevent 
 the Silver from running in the crucible, and occasion much loss 
 in the form of slag. The residuary part not taken up by the 
 Nitric Acid would contain any Gold which might be present, 
 and must be dealt with as advised in page 60. 
 
 Many practical photographers, objecting to the trouble involved 
 in boiling with Metallic Zinc, precipitate the Silver from their 
 Hypo-fixing baths by means of the Sulphide of Potassium, which 
 the operative chemists now sell in lumps for that purpose. Dis- 
 solve a drachm of the Sulphide in an ounce of water, and add it 
 until a fresh portion causes no further precipitate : it ought to 
 decompose at least twice its weight of Nitrate of Silver, but no 
 certain directions can be given ; an excess however should be 
 avoided if possible, since Sulphide of Gold is soluble in excess of 
 Sulphide of Potassium, and hence, if the Silver contained Gold, it 
 would not be thrown down under such circumstances. For the 
 mode of dealing with the Sulphide of Silver, jee page 571. 
 
 f. Alkaline toning Solution of Chloride of Gold. Throw the 
 exhausted toning Bath into a large bottle, and when a sufficiency 
 has been collected, add a solution of Sulphate of Iron in the pro- 
 portion of two grains of the Sulphate for each grain of Chloride 
 of Gold originally present in the Bath. A black precipitate will 
 fall, consisting of Carbonate and Oxide of Iron, mixed with metal- 
 lic Gold ; it may be dried and sent to the assayer, or may be 
 purified as follows : Digest twenty grains in a flask with a fluid 
 drachm of Hydrochloric Acid, fifteen minims of Nitric Acid, 
 and two drachms of Water. After the lapse of a quarter of an 
 hour, boil the mixture, add two ounces more of Water, and filter 
 out the insoluble residue, which will consist principally of Chloride 
 of Silver and organic matter. One hundred grains of Proto- 
 sulphate of Iron will then throw down the whole of the Gold in a 
 pure state. To convert the metallic Gold into Chloride of Gold, 
 see the Vocabulary, page 60. 
 
570 APPENDIX. 
 
 g. Solution of Cyanide of Potassium. The liquid may be diluted 
 largely with water, and Sulphide of Potassium added as above 
 described, until no further precipitation of black Sulphide ensues. 
 Collect the deposit, and treat it as advised in the next page. 
 
 h. Reduction of Chloride of Silver to the metallic state. The 
 Chloride of Silver is first to be washed, by filling up the vessel which 
 contains it, two or three times with water, stirring it up -well each 
 time, allowing it to settle down, and then pouring off the liquid, 
 or drawing it off close with a siphon. It may next be dried at a 
 gentle heat, and fused with twice its weight of dry Carbonate of 
 Potash, or, better still, with a mixture of the Carbonates of Potash 
 and Soda, which is more fusible than either of the separate salts. 
 A Cornish crucible may be used to contain the mixture, and the 
 heat may be raised to bright redness, after which, on allowing the 
 crucible to cool, and breaking it, a button of Silver will be found 
 at the bottom. 
 
 The process for reducing Chloride of Silver in the moist way, 
 by metallic Zinc and Sulphuric Acid, is conducted as follows : 
 The Chloride, after having been well washed as before, is placed in 
 a large flat dish, and a bar of metallic Zinc is laid in contact with 
 it. A small quantity of Oil of Vitriol, diluted with four parts of 
 water, is then added, until a slight effervescence of Hydrogen gas 
 begins to take place. The vessel is set aside for two or three 
 days, and is not to be disturbed, either by stirring or moving the 
 bar. The reduction begins with the Chloride immediately in 
 contact with the Zinc, and radiates in all directions. When the 
 whole mass has become of a grey colour, the bar is to be carefully 
 removed and the adhering Silver washed off with a stream of 
 water ; the Zinc usually presents a honeycombed appearance, 
 with irregularities upon the surface, which however are not me- 
 tallic Silver : they consist of metallic Lead, mixed with some 
 basic Sulphate or Oxychloride of Zinc. 
 
 In order to ensure the purity of the Silver, a fresh addition of 
 dilute Sulphuric Acid must be made, after the Zinc bar has been 
 removed, and the digestion continued for several hours, in order 
 to dissolve any fragments of metallic Zinc which may have been 
 inadvertently detached. The grey powder must be repeatedly 
 washed, first with Sulphuric Acid and water (this is necessary to 
 dissolve any insoluble Salt of Zinc), and then with water alone, 
 until the liquid runs away neutral, and gives no precipitate with 
 
 
APPENDIX. 571 
 
 Carbonate of Soda ; it should then be fused into a button, to burn 
 off organic matter if present, and subsequently converted into 
 Nitrate of Silver by boiling with Nitric Acid diluted with two 
 parts of water. 
 
 In reducing Chloride of Silver precipitated from old Nitrate 
 Baths containing Iodide of Silver, the grey metallic powder is con- 
 taminated with unreduced Iodide of Silver, which prevents a part 
 of the metal from readily fusing in the crucible, and so increases 
 the amount of slag. Be careful therefore to save all the scoriae, 
 and to run them down separately with a mixture of Borax and 
 Carbonate of Soda : the quantity of Silver recovered by so doing 
 is sometimes considerable. 
 
 In some of the earlier editions of this Work the importance of 
 fusing the metallic Silver before reconverting it into Nitrate was 
 not alluded to. This omission the Writer now supplies, having 
 found that the Chloride, both before and after its reduction, may 
 retain traces of organic matter. 
 
 i. Conversion of Sulphide into Metallic Silver. The black Sul- 
 phide of Silver may easily be converted into Metallic Silver, as 
 follows : dry and subsequently pulverize it, after which it is to 
 be rubbed up in a mortar with about twice its weight of common 
 Nitre. Then place a clay crucible upon the fire, and when the 
 bottom part is heated nearly to redness, throw in the black mix- 
 ture a little at a time, when immediate deflagration will ensue, 
 and the Sulphur will be burnt away, leaving the Silver in the 
 form of a network of metal. This Silver is sufficiently pure for 
 dissolving hi Nitric Acid if it be first cleansed by boiling in water 
 twice, and then in a little dilute Hydrochloric Acid, and lastly 
 washing with water once more, but when the requisite appliances 
 are at hand, it will be better to run it into a button with Borax 
 and Carbonate of Soda. 
 
 The ashes from burnt sensitive paper may be treated in a simi- 
 lar way, and any Chloride of Silver present in them will be re 
 duced by the Potash of the Nitre. 
 
 k. Separation of Metallic Gold from Silver. The deposit pro- 
 duced from Hyposulphite fixing and toning Baths by long boiling 
 with Zinc, or by Sulphide of Potassium, may contain a portion of 
 Gold as well as Silver. To separate these metals when the latter 
 is in excess, dissolve the button in dilute Nitric Acid, and collect 
 the small portion of insoluble matter which subsides. Wash this 
 
572 APPENDIX. 
 
 precipitate with dilute Ammonia, to remove Chloride of Silver, 
 if present, and reconvert it into Chloride of Gold by the mode 
 described in the Vocabulary, Art. " Gold, Chloride of." 
 
 PURIFICATION OF OLD COLLODION NITBATE BATHS FOE 
 EMPLOYMENT IN FEINTING. 
 
 This operation is required in order to separate the dissolved 
 Iodide of Silver, which if allowed to remain interferes with the 
 rapidity of the toning action of the Gold-solutions (p. 311). 
 
 Make a solution of Citric Acid, sixteen grains in an ounce of 
 water, and drop it into the Bath in the proportion of a fluid 
 drachm for each eight ounces of the solution of Nitrate of Silver. 
 This addition may or may not cause a white precipitate, but the 
 subsequent steps are the same in either case. Next immerse a 
 strip of blue litmus-paper, which will immediately be reddened ; 
 then add Liquor Ammonias drop by drop, until, after vigorous 
 stirring, the blue colour of the reddened paper appears perma- 
 nently restored. Now throw the whole on a paper filter, and 
 when the liquid has run through, drop in an excess of pure diluted 
 Nitric Acid, just sufficient to redden the test-paper decidedly. 
 Lastly, ascertain the strength of the liquid, and make it up to 
 sixty grains to the ounce of water, by adding the proper quantity 
 of crystallized Nitrate of Silver. 
 
 No exact directions can be given as to the quantities of Am- 
 monia and Nitric Acid ; but with the strongest Liquor Ammoniee 
 obtainable about two drops will be required for each drachm of 
 the Citric Acid solution. The number of drops of Nitric Acid 
 must be varied according to the amount of the excess of Am- 
 monia. 
 
 The rationale of the process is as follows : The Citric Acid, 
 with the aid of the Ammonia, precipitates white Citrate of Silver 
 in the Bath, and this insoluble substance carries down the Iodide 
 of Silver with it. Excess of Critic Acid must not be permitted 
 to remain in the Bath, since it would cause redness in the prints. 
 Excess of Ammonia, on the other hand, would favour discoloration 
 of the Bath by use, and would lessen the keeping qualities of the 
 paper in hot weather ; hence a trace of Nitric Acid will be re- 
 quired to neutralize the alkali.* 
 
 * Photographic Journal, vol. v. p. 138. F. G. Eliot. 
 
APPENDIX. 573 
 
 MODE OF TAKING THE SPECIFIC GRAVITY OF LIQUIDS. 
 
 Instruments are sold, termed " Hydrometers," which indicate 
 specific gravity by the extent to which a glass bulb containing 
 air, and properly balanced, rises or sinks in the liquid ; but a 
 more exact process is by the use of the specific-gravity bottle. 
 
 These bottles are made to contain exactly 1000 grains of dis- 
 tilled water, and with each is sold a brass weight, which counter- 
 balances it when filled with pure water. 
 
 In taking the specific gravity of a liquid, fill the bottle quite 
 full and insert the stopper, which being pierced through by a fine 
 capillary tube allows the excess to escape. Then, having wiped 
 the bottle quite dry, place it in the scale-pan, and ascertain the 
 number of grains required to produce equilibrium ; this number 
 added to, or subtracted from, unity (the assumed specific gravity 
 of water), will give the density of the liquid. 
 
 Thus, supposing the bottle filled with rectified Ether to require 
 250 grains to enable it to counterbalance the brass weight, then 
 1* minus '250, or '750, is the specific gravity ; but in the case of Oil 
 of Vitriol the bottle, when full, will be heavier than the counter- 
 poise by perhaps 836 grains ; therefore 1' plus '836, idest, T836, 
 is the density of the sample examined. 
 
 Sometimes the bottle is made to hold only 500 grains of dis- 
 tilled water in place of 1000 : in this case the number of grains 
 to be added or subtracted must be multiplied by 2. 
 
 In taking specific gravities, observe that the temperature be 
 within a few degrees of 60 Fahrenheit (if higher or lower, im- 
 merse the bottle in warm or cold water, in the latter case, it will 
 need to have the stopper removed-, and a little more liquid added 
 to fill it perfectly); and wash out the bottle thoroughly with 
 water each time after use. 
 
 ON FILTEATION AND WASHING PRECIPITATES. 
 
 In preparing filters, cut the paper into squares of a sufficient 
 size, and fold each square neatly upon itself, first into a half-square, 
 and then again, at right angles, into a quarter-square ; round 
 off the corners with a pair of scissors, and open out the filter into 
 a conical form, when it will be found to drop exactly into the 
 
574 APPENDIX. 
 
 funnel if the latter is properly shaped, and to be uniformly sup- 
 ported throughout. 
 
 Before pouring in the liquid, always moisten the filter with dis- 
 tilled water, in order to expand the fibres ; if this precaution be 
 neglected, the pores are apt to become choked in filtering liquids 
 which contain finely-divided matter in suspension. The solution 
 to be filtered may be poured gently down a glass rod, held in the 
 left hand, and directed against the side of the funnel, near to the 
 upper part. If it does not immediately run clear, it will usually 
 do so on returning it into the filter and allowing it to pass through 
 a second time. When glass stirring-rods are not at hand, sub- 
 stitute slate pencils, which are without action upon the solutions 
 employed by photographers. 
 
 Mode of Washing Precipitates. Collect the precipitate upon a 
 filter and drain off as much of the mother-liquor as possible ; then 
 pour in distilled water by small portions at a time, allowing each 
 to percolate through the deposit before adding a fresh quantity. 
 When the water passes through perfectly pure, the washing is 
 complete ; in testing it, a single drop may be laid upon a strip of 
 glass and aUowed to evaporate spontaneously in a warm place, or 
 the proper chemical reagents may be applied, and the washing 
 continued until no impurity can be detected. Thus, for example, 
 in washing the Sulphide of Silver precipitated from a solution of 
 Hyposulphite of Soda by means of Sulphide of Potassium, the pro- 
 cess will be completed when the water which runs through causes 
 no discoloration with a drop of Nitrate of Silver solution. 
 
 ON THE USE OF TEST-PAPERS. 
 
 The nature of the colouring matter which is employed in the 
 preparation of litmus-paper has already been described at page 
 72. 
 
 In testing for the alkalies and basic oxides generally, the blue 
 litmus-paper which has been reddened by an acid may be used, 
 or, in place of it, the turmeric paper. Turmeric is a yellow vege- 
 table substance which possesses the property of becoming brown 
 when treated with an alkali ; it is however less sensitive than the 
 reddened litmus, and is scarcely aifected by the weaker bases, 
 such as Oxide of Silver. 
 
 In using test-papers, observe the following precautions : They 
 
APPENDIX. 575 
 
 should be kept in a dark place, and protected from the action of 
 the air, or they soon become purple from Carbonic Acid, always 
 present in the atmosphere in small quantity. By immersion in 
 water containing about one drop of Liquor Potassse, or a grain of 
 Carbonate of Soda to four ounces, the blue colour is restored. As 
 the quantities which are tested for in Photography are often in- 
 finitesimally small, it is essential that the litmus-paper should be 
 in good condition ; and test-papers prepared with porous paper 
 will be found to show the colour more decidedly than those upon 
 glazed or strongly-sized paper. The mode of employing the paper 
 is as follows : Place a small strip in the liquid to be examined : 
 if it becomes at once bright red, a strong acid is present ; but if it 
 changes slo^vIy to a wine-red tint, a weak acid, such as Carbonic, 
 or a very minute amount of a strong one is indicated, or the 
 change may be due to a chemically neutral salt having an acid 
 reaction. In the case of the Photographic Nitrate Bath faintly 
 acidified with Acetic Acid, a purple colour only may be expected, 
 and a decided red colour would suggest the presence of Nitric 
 Acid. In the Hypo-fixing Bath which has acquired acidity, the 
 litmus-paper will perhaps redden slightly in about three or four 
 minutes. 
 
 Blue litmus-paper may be changed to the red papers used for 
 alkalies by soaking in water acidified with Sulphuric Acid, one 
 drop to half a pint ; or by holding for an instant near the mouth 
 of a bottle containing Glacial Acetic Acid. In examining a Ni- 
 trate Bath for alkalinity by means of the reddened litmus-paper, 
 at least five or ten minutes should be allowed for the action, 
 since the change of colour from red to blue takes place very slowly 
 (p. 250) ; and if test papers are left floating in any liquid whilst 
 acids or alkalies are being added for the purpose of neutralizing, 
 be careful not to drop the neutralizing solution upon the paper. 
 
 REMOVAL OF SILVER STAINS FROM THE HANDS, 
 LINEN, ETC. 
 
 The black stains upon the hands caused by Nitrate of Silver, 
 may readily be removed by rubbing them with a moistened lump 
 of Cyanide of Potassium, leaving it on the hands for a little time, 
 and then washing well with water. A solution of Iodide of Po- 
 tassium allowed to dry on the hands will also after a time change 
 
576 APPENDIX* 
 
 the black stains into yellow Iodide of Silver, which may then be 
 removed by Hyposulphite of Soda. 
 
 Stains upon white linen may be easily removed by brushing 
 them with a solution of Iodine in Iodide of Potassium, and after- 
 wards washing with water and soaking in Hyposulphite of Soda, or 
 Cyanide of Potassium, until the yellow Iodide of Silver dissolves 
 out ; the Bichloride of Mercury (neutral solution) also answers 
 well in many cases, changing the dark spot to white (p. 266). 
 
 The following liquid, when other means fail, is an energetic re- 
 mover of Silver stains : Cyanide of Potassium, 100 grains ; Io- 
 dine, 10 grains ; Water, 1 ounce : the solution should be colourless, 
 
 BEMOVAL OF STOPPERS AND CLEANSING OF BOTTLES. 
 
 Substances which exert a solvent action on Silica are likely 
 to fix the stoppers of bottles ; such, for instance, as Caustic Pot- 
 ash and Carbonate of Potash ; also iodizing solutions containing 
 Iodide of Potassium, etc. To remove the stopper, begin by invert- 
 ing the bottle for about half an hour, that the liquid may gravi- 
 tate into the neck, then tap it gently, pressing the side of the 
 stopper, opposite to the part tapped, pretty strongly with the 
 thumb, when it will probably be found to yield ; if not, continue 
 the tapping with the wooden handle of a spatula, on each side 
 successively, so as to keep up a constant vibration. Next fix a 
 string to the table, and having twisted it round the neck, hold 
 the other end of the string in the hand, and move the bottle ra- 
 pidly backwards and forwards, until a considerable amount of 
 heat has been generated, after which the gentle tapping may be 
 resumed. Lastly, heat the neck with a spirit-lamp applied gra- 
 dually and with constant movement, and when sufficient expan- 
 sion has been produced, tap as before, or tie a cloth round the 
 stopper, and twist it out by aid of a key. 
 
 In cleansing dirty bottles, consider first the nature of the im- 
 purity to be removed. If it be soluble in water, a careful rins- 
 ing two or three times repeated will be sufficient, but in other cases 
 the proper solvent must be used, or mechanical friction resorted to. 
 Greasy bottles are always troublesome, and are likely to soil both 
 the bottle-brushes and the washing water. Liquor Potassse or a 
 strong solution of washing Soda will generally remove the grease ; 
 if not, try a little strong Sulphuric Acid, turning the bottle round 
 
APPENDIX. 577 
 
 so as to bring it into contact with every part, and it will generally 
 be found that on washing it out the bottle will be left perfectly 
 clean. Red stains of Peroxide of Iron are removed by a few 
 drops of Hydrochloric Acid ; crusts of Carbonate of Lime by the 
 same. Brown stains of Sulphide of Silver dissolve in Nitric Acid. 
 Yarnish bottles are often most eifectually cleansed by Spirit of 
 Wine, followed by shot and water ; Collodion bottles, by shot 
 assisted by a pointed wire, to scrape away the precipitated Py- 
 roxyline. 
 
 In using shot for cleaning, avoid those bottles which contain 
 grease or resinous matter likely to adhere to the shot ; bottle- 
 brushes would also be soiled by using them for cleaning away any 
 organic substance insoluble in water. When a considerable num- 
 ber of bottles require cleansing, begin by marking each of the 
 stoppers, or by tying them securely to the necks, in order to avoid 
 confusion. 
 
 REMOVAL OF VARNISH FROM GLASS PLATES. 
 
 The black varnish may often be removed from Collodion posi- 
 tives by means of Chloroform or Benzole, neither of which dis- 
 solves the Pyroxyline constituting the film. 
 
 Glass plates varnished with Negative varnish, may sometimes 
 be cleaned with Chloroform or Benzole ; but when the preserva- 
 tion of the picture is not an object, the plates may be left soaking 
 in a solution of washing "Soda" for several hours, after which 
 the film will be removable by friction. A few drachms of waste 
 Nitro-Stilpburic Acid poured over the plate, will oxidize the var- 
 nish and loosen the film in the course of half an hour. 
 
 VARNISH FOR BLACKING BRASS-WORK. 
 
 Take the ordinary spirit-lacquer of the shops, and rub it up 
 with Lampblack into a thin, cream, afterwards filtering through 
 muslin. The brass-work must be heated before the black is 
 applied, or it will give a shining instead of a lustreless surface. 
 
 2 P 
 
578 
 
 APPENDIX. 
 
 A TABLE SHOWING THE QUANTITY OF ANHYDROUS ACID 
 IN DILUTE SULPHUEIC ACID OF DIFFERENT SPECIFIC 
 GRAVITIES, AT 60 F. (URE.) 
 
 Specific 
 Gravity. 
 
 SO 3 
 in 100 
 parts of the 
 Liquid. 
 
 Specific 
 Gravity. 
 
 S0 3 
 in 100 
 parts of the 
 Liquid. 
 
 Specific 
 Gravity. 
 
 so, 
 
 in 100 
 parts of the 
 Liquid. 
 
 1-8460 
 
 81-54 
 
 1-7080 
 
 65-23 
 
 1-4860 
 
 48-92 
 
 1-8415 
 
 79-90 
 
 1-6860 
 
 63-60 
 
 1-4660 
 
 47-29 
 
 1-8366 
 
 78-28 
 
 1-6624 
 
 61-97 
 
 1-4460 
 
 45-66 
 
 1-8288 
 
 76-65 
 
 1-6415 
 
 60-34 
 
 1-4265 
 
 44-03 
 
 1-8181 
 
 75-02 
 
 1-6204 
 
 58-71 
 
 1-4073 
 
 42-40 
 
 1-8070 
 
 73-39 
 
 1-5975 
 
 57-08 
 
 1-3884 
 
 40-77 
 
 1-7901 
 
 71-75 
 
 1-5760 
 
 55-45 
 
 1-3697 
 
 39-14 
 
 1-7728 
 
 70-12 
 
 1-5503 
 
 53-82 
 
 1-3530 
 
 37-51 
 
 1-7540 
 
 68-49 
 
 1-5280 
 
 52-18 
 
 1-3345 
 
 35-88 
 
 1-7315 
 
 66-86 
 
 1-5066 
 
 50-55 
 
 1-3165 
 
 34-25 
 
 A TABLE SHOWING THE QUANTITY OF ANHYDROUS ACID 
 IN THE LIQUID NITRIC ACID OF DIFFERENT SPECIFIC 
 GRAVITIES. (URE.) 
 
 Specific 
 Gravity. 
 
 N0 5 | 
 in 100 
 parts ot the 
 Liquid. 
 
 Specific 
 Gravity. 
 
 NO S 
 
 in 100 
 parts of the 
 Liquid. 
 
 Specific 
 Gravity. 
 
 NO 5 
 
 in 100 
 parts of the 
 Liquid. 
 
 1-5000 
 
 79-700 
 
 1 -4640 
 
 69-339 
 
 1-4147 
 
 58-978 
 
 1-4980 
 
 78-903 
 
 1-4600 
 
 68-542 
 
 1-4107 
 
 58-181 
 
 1-4960 
 
 78-106 
 
 1-4570 
 
 67-745 
 
 1-4065 
 
 57-384 
 
 1-4940 
 
 77-309 
 
 1-4530 
 
 66-948 
 
 1-4023 
 
 56-587 
 
 1-4910 
 
 76-512 
 
 1-4500 
 
 66-155 
 
 1-31)78 
 
 55-790 
 
 1-4880 
 
 75-715 
 
 1-4460 
 
 65-354 
 
 1-3945 
 
 54-993 
 
 1-4850 
 
 74-918 
 
 1-4424 
 
 64-557 
 
 1-3882 
 
 54-196 
 
 1-4820 
 
 74-121 
 
 1-4385 
 
 63-760 
 
 1-3833 
 
 53-399 
 
 1-4790 
 
 73-324 
 
 1-4346 
 
 62-963 
 
 1-3783 
 
 52-602 
 
 1-4760 
 
 72-527 
 
 1-4306 
 
 62-166 
 
 1-3732 
 
 51-805 
 
 1-4730 
 
 71-730 
 
 1-4269 
 
 61-369 
 
 1-3681 
 
 51-068 
 
 1-4700 
 
 70-933 
 
 1-4228 
 
 60-572 
 
 1-3630 
 
 50-211 
 
 1-4670 
 
 70-136 
 
 1-4189 
 
 59-775 
 
 1-3579 
 
 49-414 
 
APPENDIX. 
 
 579 
 
 THE PEOPOETION OF ABSOLUTE ALCOHOL BY WEIGHT IN 
 100 PAETS OF SPIEIT, OF DIFFEEENT SPECIFIC GEAVI- 
 TIES, AT 60 FAHE. 
 
 Alcohol 
 per cent. 
 
 Specific 
 Gravity. 
 
 Alcohol 
 per cent. 
 
 Specific 
 Gravity. 
 
 Alcohol 
 per cent. 
 
 Specific 
 Gravity. 
 
 48 
 
 9228 
 
 69 
 
 8745 
 
 85 
 
 8357 
 
 50 
 
 9184 
 
 70 
 
 8721 
 
 86 
 
 8331 
 
 52 
 
 9135 
 
 71 
 
 8696 
 
 87 
 
 8305 
 
 54 
 
 9090 
 
 72 
 
 8672 
 
 88 
 
 8279 
 
 56 
 
 9047 
 
 73 
 
 8649 
 
 89 
 
 8254 
 
 58 
 
 9001 
 
 74 
 
 8625 
 
 90 
 
 8228 
 
 59 
 
 8979 
 
 75 
 
 8503 
 
 91 
 
 '8199 
 
 60 
 
 8956 
 
 76 
 
 8581 
 
 92 
 
 8172 
 
 61 
 
 8932 
 
 77 
 
 8557 
 
 93 
 
 8145 
 
 62 
 
 8908 
 
 78 
 
 8533 
 
 94 
 
 8118 
 
 63 
 
 8886 
 
 79 
 
 8508 
 
 95 
 
 8089 
 
 64 
 
 8863 
 
 80 
 
 8483 
 
 96 
 
 8061 
 
 65 
 
 8840 
 
 81 
 
 8459 
 
 97 
 
 8031 
 
 66 
 
 8816 
 
 82 
 
 8434 
 
 98 
 
 8001 
 
 67 
 
 8793 
 
 83 
 
 8408 
 
 99 
 
 7969 
 
 68 
 
 8769 
 
 84 
 
 8382 
 
 100 
 
 7938 
 
580 
 
 APPENDIX. 
 
 BAUME'S HYDEOMETER SCALE. 
 
 A portion of the above Scale is given with a view of facilitating 
 the translation of foreign papers, in which degrees of Baume are 
 often put instead of specific gravities. 
 
 For Liquids heavier than Water. 
 
 Degrees. 
 
 Specific 
 Gravity. 
 
 Degrees. 
 
 Specific 
 Gravity. 
 
 Degrees. 
 
 Specific 
 Gravity. 
 
 29 
 
 1-235 
 
 43 
 
 1-395 
 
 57 
 
 1-600 
 
 30 
 
 1-245 
 
 44 
 
 1-407 
 
 58 
 
 1-617 
 
 31 
 
 1-256 
 
 45 
 
 1-420 
 
 59 
 
 1-634 
 
 32 
 
 1-267 
 
 46 
 
 1-434 
 
 60 
 
 1-652 
 
 33 
 
 1-277 
 
 47 
 
 1-448 
 
 61 
 
 1-670 
 
 34 
 
 1-288 
 
 48 
 
 1-462 
 
 62 
 
 1-689 
 
 35 
 
 1-299 
 
 49 
 
 1-476 
 
 63 
 
 1-708 
 
 36 
 
 1-310 
 
 50 
 
 1-490 
 
 64 
 
 1-727 
 
 37 
 
 1-321 
 
 5] 
 
 1-495 
 
 65 
 
 1-747 
 
 38 
 
 1-333 
 
 52 
 
 1-520 
 
 66 
 
 1-767 
 
 39 
 
 1-345 
 
 53 
 
 1-535 
 
 67 
 
 1-788 
 
 40 
 
 1-357 
 
 54 
 
 1-551 
 
 68 
 
 1-809 
 
 41 
 
 1-369 
 
 55 
 
 1-567 
 
 69 
 
 1-831 
 
 42 
 
 1-381 
 
 56 
 
 1-583 
 
 70 
 
 1-854 
 
 For Liquids lighter than Water. 
 
 Degrees. 
 
 Specific 
 Gravity. 
 
 Degrees. 
 
 Specific 
 Gravity. 
 
 Degrees. 
 
 Specific 
 Gravity. 
 
 13 
 
 0-980 
 
 29 
 
 0-885 
 
 45 
 
 0-807 
 
 14 
 
 0-973 
 
 30 
 
 0-880 
 
 46 
 
 0-802 
 
 15 
 
 0-967 
 
 31 
 
 0-874 
 
 47 
 
 0-798 
 
 16 
 
 0-960 
 
 32 
 
 0-869 
 
 48 
 
 0-794 
 
 17 
 
 0-954 
 
 33 
 
 0-864 
 
 49 
 
 0-789 
 
 18 
 
 0-948 
 
 34 
 
 0-859 
 
 50 
 
 0-785 
 
 19 
 
 0-942 
 
 35 
 
 0-854 
 
 51 
 
 0-781 
 
 20 
 
 0-936 
 
 36 
 
 0-849 
 
 52 
 
 0777 
 
 21 
 
 0-930 
 
 37 
 
 0-844 
 
 53 
 
 0-773 
 
 22 
 
 0-924 
 
 38 
 
 0-839 
 
 54 
 
 0-768 
 
 23 
 
 0-918 
 
 39 
 
 0-834 
 
 55 
 
 0-764 
 
 24 
 
 0-913 
 
 40 
 
 0-830 
 
 56 
 
 0-760 
 
 25 
 
 0-907 
 
 41 
 
 0-825 
 
 57 
 
 0-757 
 
 26 
 
 0-901 
 
 42 
 
 0-820 
 
 58 
 
 0-753 
 
 27 
 
 0-896 
 
 43 
 
 0-816 
 
 59 
 
 0-749 
 
 28 
 
 0-890 
 
 44 
 
 0-811 
 
 60 
 
 0-745 
 
APPENDIX. 
 
 581 
 
 DEGREES CENTIGEADE, COMPARED WITH THE SCALE OF 
 FAHRENHEIT. 
 
 The Thermometric Scale in common use in France is the Cen- 
 tigrade. The following Table will serve for its conversion into 
 degrees of Fahrenheit, from the freezing to the boiling point of 
 water. 
 
 Cent. 
 
 Fahr. 
 
 Cent. 
 
 Fahr. 
 
 Cent. 
 
 Fahr. 
 
 Cent. 
 
 Fahr. 
 
 
 
 32-0 
 
 26 
 
 78-8 
 
 51 
 
 123-8 
 
 76 
 
 168-8 
 
 1 
 
 33-8 
 
 27 
 
 80-6 
 
 52 
 
 125-6 
 
 77 
 
 170-6 
 
 2 
 
 35-6 
 
 28 
 
 82-4 
 
 53 
 
 127-4 
 
 78 
 
 172-4 
 
 3 
 
 37-4 
 
 29 
 
 84-2 
 
 54 
 
 129-2 ] 
 
 79 
 
 174-2 
 
 4 
 
 39'2 
 
 30 
 
 86-0 
 
 55 
 
 131-0 ! 
 
 80 
 
 176-0 
 
 5 
 
 41-0 
 
 31 
 
 87-8 
 
 56 
 
 132-8 
 
 81 
 
 177-8 
 
 6 
 
 42-8 
 
 32 
 
 89-6 
 
 57 
 
 134-6 
 
 82 
 
 179-6 
 
 7 
 
 44-6 
 
 33 
 
 91-4 
 
 58 
 
 136-4 ! 
 
 83 
 
 181-4 
 
 8 
 
 46-4 
 
 34 
 
 932 
 
 59 
 
 138-2 
 
 84 
 
 183-2 
 
 9 
 
 48-2 
 
 35 
 
 95-0 
 
 60 
 
 140-0 
 
 85 
 
 185-0 
 
 10 
 
 50'0 
 
 36 
 
 96-8 
 
 61 
 
 141-8 
 
 86 
 
 186-8 
 
 11 
 
 51-8 
 
 37 
 
 98-6 
 
 62 
 
 143-6 
 
 87 
 
 188-6 
 
 12 
 
 53-6 
 
 38 
 
 100-4 
 
 63 
 
 145-4 
 
 88 
 
 190-4 
 
 13 
 
 55-4 
 
 39 
 
 102-2 
 
 64 
 
 147-2 
 
 89 
 
 192-2 
 
 14 
 
 57-2 
 
 40 
 
 104-0 
 
 65 
 
 149-0 ! 
 
 90 
 
 1940 
 
 15 
 
 59-0 
 
 41 
 
 105-8 
 
 66 
 
 150-8 ! 
 
 91 
 
 195-8 
 
 16 
 
 60-8 
 
 42 
 
 107-6 
 
 67 
 
 152'6 
 
 92 
 
 197-6 
 
 17 
 
 62-6 
 
 43 
 
 109-4 
 
 68 
 
 154-4 
 
 93 
 
 199-4 
 
 18 
 
 64-4 
 
 ! 44 
 
 111-2 
 
 69 
 
 156-2 
 
 94 
 
 201-2 
 
 19 
 
 66-2 
 
 45 
 
 113-0 
 
 70 
 
 158-0 
 
 95 
 
 203-0 
 
 20 
 
 68-0 
 
 46 
 
 114-8 
 
 71 
 
 159*8 
 
 96 
 
 204-8 
 
 21 
 
 69-8 
 
 47 
 
 116-6 
 
 72 
 
 161'6 
 
 97 
 
 206-6 
 
 22 
 
 71-6 
 
 48 
 
 118-4 
 
 73 
 
 163'4 
 
 98 
 
 208-4 
 
 23 
 
 734 
 
 ! 49 
 
 120-2 
 
 74 
 
 165'2 
 
 99 
 
 210-2 
 
 24 
 
 75-2 
 
 50 
 
 122-0 
 
 75 
 
 167-0 
 
 100 
 
 212-0 
 
 25 
 
 77-0 
 
 
 
 
 
 
 
 [Reaumur's Scale is occasionally employed in Germany, Russia, 
 etc. : in this the freezing point of water is termed 0, and the 
 boiling point 80. Each degree of Fahrenheit's scale is equal to 
 four-ninths of a degree on Reaumur's : if therefore the number of 
 degrees of Reaumur be multiplied by 9 and divided by 4, the 
 quotient plus 32 will be the corresponding degree of Fahrenheit. 
 Thus 80 (Reaumur) x 9 = 720 -h 4 = 180 + 32 = 212 (Fahren- 
 heit).] 
 
582 APPENDIX. 
 
 WEIGHTS AND MEASURES. 
 
 Troy or Apothecaries' Weight. 
 
 1 Pound = 12 Ounces. 1 Ounce = 8 Drachms. 1 Drachm 
 = 3 Scruples. 1 Scruple = 20 Grains. (1 Ounce Troy = 480 
 Grains, or = 1 Ounce Avoirdupois plus 42'5 grains.) 
 
 Avoirdupois Weight. 
 
 1 Pound = 16 Ounces. 1 Ounce = 16 Drachms. 1 Drachm 
 = 27'343 grains. (1 Ounce Avoirdupois = 437'5 grains.) (1 
 Pound Avoirdupois = 7000 Grains, or = 1 Pound Troy plus 2j? 
 Troy Ounces plus 40 grains.) 
 
 Imperial Measure. 
 
 1 Gallon = 8 Pints. 1 Pint = 20 Ounces. 1 Ounce = 8 
 Drachms. 1 Drachm = 60 Minims. (A Wine Pint of water 
 measures 16 Ounces, and weighs a Pound.) 
 
 An Imperial Gallon of water u-eighs 10 Pounds Avoirdupois, or 
 70,000 Grains. An Imperial Pint of water weighs li Pound 
 Avoirdupois. A fluid Ounce of water weighs 1 Ounce Avoirdu- 
 pois, or 437'5 Grains. A fluid Drachm of water weighs 54'7 
 Grains. A Minim iveighs 0'91 Grains. 
 
 French Measures of Weight. 
 
 1 Kilogramme 1000 Grammes = something less than 2 
 Pounds Avoirdupois. 
 
 1 Gramme == 10 Decigrammes = 100 Centigrammes = 1000 
 Milligrammes = 15'433 English Grains. 
 
 A Gramme of water measu res 1 Cubic Centimetre, or 17 English 
 ' Minims, nearly. 1000 Grammes of water measure 35J English 
 fluid Ounces. 
 
 French Measures of Volume. 
 
 1 Litre = 10 Decilitres 100 Centilitres = 1000 Millilitres 
 = 35 English fluid Ounces. 
 
 1 Litre = Cubic Decimetre = 1000 Cubic Centimetres. 
 
 1 Cubic Centimetre = 17 English Minims. 
 
 A Litre of water weighs a Kilogramme, or something less than 
 2 Pounds Avoirdupois. A Cubic Centimetre of water weighs a 
 Gramme. 
 
APPENDIX. 
 
 583 
 
 French Measures of Length. 
 
 1 Metre = 10 Decimetres = 100 Centimetres = 1000 Milli- 
 metres = 39*37079 English inches. 
 
 A metre is equivalent to the ten-millionth part of the arc of 
 the meridian, extending from the Equator to the Pole. 
 
 TwaddelVs Hydrometer. 
 
 Degrees. 
 
 Specific 
 Gravity. 
 
 Degrees. 
 
 Specific 
 Gravity. 
 
 Degrees. 
 
 Specific 
 Gravity. 
 
 1 
 
 1-005 
 
 11 
 
 1-055 
 
 20 
 
 1-100 
 
 2 ' 
 
 1-010 
 
 12 
 
 1-060 
 
 21 
 
 1-105 
 
 3 
 
 rois 
 
 13 
 
 1-065 
 
 22 
 
 1-110 
 
 4 
 
 1-020 
 
 14 
 
 1-070 
 
 23 
 
 1-115 
 
 5 
 
 1-025 
 
 15 
 
 1-075 
 
 24 
 
 1-120 
 
 6 
 
 1-030 
 
 16 
 
 1-080 
 
 25 
 
 1-125 
 
 7 
 
 1-035 
 
 17 
 
 1-085 
 
 26 
 
 1-130 
 
 8 
 
 1-040 
 
 18 
 
 1-090 
 
 27 
 
 1-135 
 
 9 
 
 1-015 
 
 19 
 
 1-095 
 
 28 
 
 1-140 
 
 10 
 
 1-050 
 
 
 
 
 
585 
 
 INDEX. 
 
 Page 
 . 181 
 . 183 
 . 330 
 . 94 
 . 249 
 . 251 
 . 277 
 . 529 
 
 Aberration, chromatic . . . 
 spherical 
 
 Abbe Despratz, his dry process 
 
 Acetate of Silver 
 
 in Nitrate Bath .... 
 its formation explained . . 
 effect of, in Bath .... 
 
 how to add it 
 
 useful in copying 4-61 
 
 remedy for grey solarization . 529 
 injurious in Positive Bath . . 261 
 
 Acetate of Soda 101 
 
 used in Toning Bath . . . .496 
 
 Acetic Acid 29 
 
 retards reduction of Silver . 144 
 renders development uniform 
 
 395, 279 
 
 use in Nitrate Bath .... 399 
 essential in Calotype . . . . 35 - 
 albumen and waxed paper . . 360 
 
 Aceto-Nitrate of Silver, for print- 
 ing by development . . . 508 
 for Collodio- Albumen . . .555 
 
 Acidity of Nitrate, Bath explained 248 
 of Hyposulphite of Soda Bath 317 
 
 Acids, nature of 12 
 
 effect of, in developing . 144, 157 
 
 Actinism explained 198 
 
 Actinic and visual focus .... 181 
 
 Actinic focus of lens, to find . . 433 
 in microsc-opic Objectives . . 474 
 Lenses, construction of . . 181 
 properties of transparent media 204 
 
 Albumen 31 
 
 combines with Oxide of Silver 131 
 used for Negatives on glass . 360 
 used in Positive printing . . 302 
 affects the colour of prints . . 303 
 retards toning and fixing 312, 315 
 protects the image .... 321 
 discolours Nitrate Bath . . .492 
 theory of, in dry processes . . '331 
 how applied in Eothergiil pro- 
 cess 550 
 
 how applied in Taupenot pro- 
 cess 553 
 
 Page 
 
 Albuminate of Silver ... 32, 131 
 not entirely soluble in Hypo- 
 sulphite of Soda .... 322 
 Albuminized paper, use of, ex- 
 plained 292, 302 
 
 how to make it 487 
 
 difficulties removed .... 539 
 cannot be used with Ammonio- 
 
 nitrate of Silver 301 
 
 Alcohol 32 
 
 action of, in Collodion . 223, 236 
 
 effect of excess of 238 
 
 in Collodion for hot climates . 479 
 sometimes useful in developer 401 
 
 Methylic 35 
 
 Aldehyda 27 
 
 Alkalies 12 
 
 effect of, in development . .157 
 Alkaline Gold Toning-Bath. (See 
 
 Toning-Bath.) 
 Alkalinity of Nitrate-Bath explained 
 
 249, 403 
 Ammonia 
 
 onia ......... 36 
 
 e as a fixing agent .... 166 
 
 tion on Chloride of Gold . . 61 
 
 38 
 
 use 
 action 
 
 on Nitrate of Silver 
 Carbonate of 
 Hydrosulphate of 
 Nitrate of 
 Ammonium, Bromide of 
 
 Chloride of ....... 39 
 
 Irdide of ........ 39 
 
 Ammonio-Nitrate of Silver . . 94 
 use in Positive printing . . 301 
 formula for preparation . . 494 
 objection to, and how to obviate 495 
 cannot be used with Albumen 301 
 Ammonio-Nitrate Paper, how to 
 
 prepare ....... 495 
 
 Ang.lar pencils of light .... 186 
 
 Angle of view, how to estimate . 561 
 Anhydrides ........ 13 
 
 Aplanatic Lens ....... 188 
 
 Apparatus for portraiture . . .432 
 tor landscapes ...... 4.44 
 
 for copying ...... '.453 
 
586 
 
 INDEX. 
 
 Page 
 
 Apparatus for stereographs . . 465 
 for instantaneous pictures . . 469 
 for micro-photographs . . . 473 
 
 for hot climates 482 
 
 for manufacture of Collodion . 375 
 
 for washing prints 504 
 
 Arsenic, Bromide of 40 
 
 Artificial parchment 221 
 
 Asser, M., his photo-lithographic 
 
 process ..367 
 
 Astigmation 196 
 
 Atmosphere, actinic state of . . 200 
 
 ever varying in hot climates . 482 
 
 indications of actinism in . 471 
 
 Atomic theory explained ... 17 
 
 Atomic weights, table of ... 9 
 
 Axis, optic, explained .... 207 
 
 Backgrounds 428, 430 
 
 Barium, Chloride of 41 
 
 Baryta, Nitrate of 41 
 
 Bases explained 11 
 
 Beaufoy's Acetic Acid .... 30 
 
 Benzole 42 
 
 Bichloride of Mercury .... 73 
 
 Bichromate of Potash .... 84 
 
 printing processes with . . 364 
 
 Binocular vision 205 
 
 Bitumen of Judaea 42 
 
 used by Nie"pce 11*5 
 
 Blistering of film 557 
 
 Blue stains on glass Positives . . 263 
 Bromide of Silver .... 95, 128 
 its sensibility to coloured light 201 
 compared with Chloride and 
 
 Iodide . 130 
 
 less capable of forming invi- 
 sible image than Iodide . . 152 
 in Positive Collodion . . . .259 
 in Negative Collodion . . .271 
 diminishes intensity .... 274 
 
 for portraiture 442 
 
 for card pictures 443 
 
 for micro-photographs . . . 477 
 
 for landscapes 451 
 
 for instantaneous photographs 469 
 for Collodion in hot climates . 481 
 in dry processes, theory of, 341, 350 
 chemical spectrum on ... 201 
 
 Bromine 42 
 
 Bromo-Iodide of Silver in Daguer- 
 reotype 354 
 
 Bromo-Iodide for Positives, the- 
 ory of 259 
 
 formula for solution of . . .39 
 for negatives, theory of . . . 27i 
 formula for solution of ... 402 
 
 (See also Bromide of Silver.) 
 Busk, on invisible image . . . 162 
 
 Cadmium 43 
 
 Cadmium Collodion 242 
 
 Page 
 
 Cadmium Collodion, formula for . 40 
 useful in portraiture .... 439 
 useful in hot climates . . . 480 
 iodizing solution for .... 440 
 
 Cadmium Iodide 43 
 
 Calotype process, theory of . . 356 
 
 Calcium, Chloride of 43 
 
 Iodide of 44 
 
 Camera, invention of 118 
 
 construction of 180 
 
 testing accuracy of .... 432 
 
 for landscapes 444 
 
 arrangement of 452 
 
 arrangement of, for portraiture 431 
 
 for copying 458 
 
 for stereography (Clarke's) . 465 
 
 Binocular 4';7 
 
 for micro-photography . . . 473 
 
 for hot climates 482 
 
 Camphor 44 
 
 use in detecting grease . . .111 
 
 Caoutchouc 44 
 
 used in varnish for Positives . 3'J? 
 for mounting prints .... 500 
 
 Carbon 45 
 
 Bisulphide 45 
 
 printing 365 
 
 Carte-de-visite portraits .... 443 
 
 Cellulose 45,217 
 
 use of, for Collodion .... 231 
 
 parchmentized 221 
 
 Charcoal, animal 46 
 
 Chemical changes explained . . 23 
 Chemical combination .... 10 
 
 laws of 16 
 
 Chemical Spectrum 198 
 
 Chemicals, Photographic, Voca- 
 bulary of 29 
 
 list of, for landscapes . . . 448 
 for sending abroad .... 483 
 
 Chloride of Silver 95-125 
 
 decomposed by light . . 130, 133 
 with Nitrate and organic matter 130 
 agents which dissolve . . .166 
 mode of reducing to metallic 
 
 state 5G7 
 
 Chloride of Gold 60 
 
 action of Ammonia on . . . 61 
 used for toning prints . 308, 496 
 with Hyposulphite of Soda . 306 
 with Chloride of Sodium . . 103 
 
 Chlorine 47 
 
 Chloroform 48 
 
 Chromatic aberration .... 182 
 
 Chromium 48 
 
 Chromic Acid 326 
 
 Chrysotype 363 
 
 Citrate of Silver 9 
 
 action of light on 131 
 
 used in printing . . . 304, 493 
 
 Citrate of Soda 102 
 
 formula for preparation of . . 493 
 
INDEX. 
 
 587 
 
 Page 
 
 Citric Acid 49 
 
 used to restrain development . 279 
 
 in developer 401 
 
 in redeveloping solution . . 404 
 in developer, for hot climates . 482 
 in printing by development . 509 
 
 Cleaning glass plates 160 
 
 details of 407 
 
 Collodion, its discovery . . . .121 
 
 plain Negative 222 
 
 theory of manufacture of . . 225 
 
 practical details 375 
 
 Nitre process 374 
 
 precautions 382 
 
 decomposition of, by keeping 239 
 
 Collodion, Iodized . . . 398, 440 
 
 Bromo-Iodized 402 
 
 for dry processes . . 339, 3-49 
 for Tannin process . . . 541 
 for Fothergill process . . 548 
 for Taupenot process . . . 552 
 for portraiture . . . 438, 440 
 for card pictures . . . . 443 
 
 for landscapes 449 
 
 for copying 460 
 
 for stereographs . . . . . 468 
 for instantaneous pictures . . 469 
 for micro -photography . . . 467 
 
 for hot climates 478 
 
 changes in ... 223, 247, 274 
 to remove brown colour in . . 224 
 
 Collodion, Positive, theory of . . 258 
 preparation of 390 
 
 Collodion film, time for immersion 416 
 
 toughness of 227 
 
 contractility of 223 
 
 porosity of 228-242 
 
 glutinosity of 231 
 
 adhesiveness 349 
 
 opacity of 260 
 
 spots and markings on, 158, 520, 522 
 
 Collodion preservative processes, 
 
 theory of 327 
 
 practice of 540 
 
 Collodio-albumen process, theory 
 
 of 331 
 
 practice of 552 
 
 Colours, nature of 173 
 
 chemical action of ... 198-203 
 
 Combination, chemical .... 11 
 
 laws of 16 
 
 their practical application . . 18 
 
 Conjugate foci 178 
 
 Copying engravings, etc. . . . 455 
 
 Cotton 49 
 
 Crookes, on the waxed paper pro- 
 cess 359 
 
 on a preservative process . . 327 
 
 Curvature of field 184 
 
 corrected by stop 187 
 
 Cyanide of Potassium .... 87 
 a fixing agent 109 
 
 Page 
 Cyanide of Potassium for Positives 266 
 
 formula for 397 
 
 for Negatives 284 
 
 formula for 405 
 
 for removing stains .... 575 
 
 Cyanogen 51 
 
 Cyanotype 362 
 
 Daguerreotype, invention of . . 119 
 
 theory of 352 
 
 Depth of focus explained . . . 186 
 Dark or developing room, glass for 200 
 
 how to fit up 411 
 
 Decantation 24 
 
 Decomposition, single and double 25 
 Development of invisible image, 
 
 theory of 145 
 
 second stage, theory of . . .150 
 
 abnormal 156 
 
 of glass Positives 436 
 
 of Negatives 420,424 
 
 of Positives on paper . . . . 507 
 
 of Tannin plates 547 
 
 of Fothergill plates . . . .551 
 of Collodio-albumen .... 557 
 Developers, chemical nature of . 141 
 comparative strength of ... 144 
 Positive, theory of .... 263 
 
 formulae for 394 
 
 Negative, theory of . . 278, 284 
 
 formulae for 401, 404 
 
 Eussell's, for Tannin plates . 546 
 Diaphragms. (See Stops.) 
 Diffused light in camera . . 428, 431 
 
 Dispersion by Lenses 176 
 
 Distortion in Lenses 196 
 
 Double decomposition . . 25, 125 
 Drapery for portraiture .... 203 
 
 Dropping-bottle 422 
 
 Dry Collodion process, theory of . 330 
 
 practice of 540 
 
 Drying-box, Spiller's, for sensitive 
 
 paper 304 
 
 Electrical images, Grove's . . .161 
 
 Elementary bodies 9 
 
 Engravings, etc., mode of copying 455 
 
 Ether 52 
 
 ozonized by air and light . . 223 
 not to be distilled from old re- 
 sidues 387 
 
 methylated prone to decompo- 
 sition 241 
 
 defects in Negatives caused by 534 
 
 acetic 55 
 
 methylic 56 
 
 Experiments on action of light 
 
 on Chloride of Silver . . .131 
 on development of invisible 
 
 image 137 
 
 on chemical effects of coloured 
 
 light 199 
 
 on parchmentizing action of oil 
 of vitriol 226 
 
588 
 
 INDEX. 
 
 Page 
 
 Experiments on changes in iodized 
 Collodion 246 
 
 Exposure in camera for Posi- 
 tives 437 
 
 for Negatives 453 
 
 for developed Positives . 508-510 
 
 for Tannin plates 546 
 
 for Fothergill plates . . . .550 
 for Taupenot plates .... 556 
 for micro-photographs . . . 477 
 for copying maps, etc. . . . 462 
 
 Fading of Positives, causes of . . 319 
 
 Failures in Negatives and Posi- 
 tives on glass 517 
 
 in paper prints 536 
 
 in Fothergill process .... 551 
 in Taupenot process .... 557 
 
 Filtration 24, 573 
 
 Fixing solutions, theory of ... 166 
 for Positives on glass, theory of 2(>6 
 
 formula for ditto 397 
 
 for Negatives on glass, theory 
 
 of 284 
 
 formula; for ditto 404 
 
 for paper prints, theory of . .313 
 
 formula for ditto 497 
 
 injurious conditions of ... 317 
 
 Focal length of Lens 178 
 
 how to find 180, 56 1 
 
 Focus, actinic and visual . . . 18[ 
 
 variation of focus 185 
 
 corrected by stop 186 
 
 conjugate foci 178 
 
 actinic, to find 4'33 
 
 actinic, in microscopic objec- 
 tives 474 
 
 equivalent focus, how to find . 561 
 
 Fogging, theory of 156 
 
 detecting causes of .... 617 
 
 Formic acid 56 
 
 use of, in developer .... 443 
 
 Formula;, mode of writing ... 20 
 
 FothergilTs dry process, theory of 333 
 practice of 548 
 
 Gallic acid . 57, 138 
 
 compared with other developers 144 
 used in Calotype process . . 853 
 formula for developing paper 
 
 Positives 510 
 
 how to preserve solution of . 139 
 
 Gallo-Nitrate of Silver . . . .358 
 decomposes rapidly in dirty 
 dishes 360 
 
 Gelatine 58 
 
 combines with Oxide of Silver 
 
 131, 346 
 
 used in Positive printing 294, 493 
 used in sizing paper .... 296 
 used in mounting prints . . . 505 
 used in dry processes . . . 218 
 preparation of,for Tannin plates 542 
 
 Glass-house, hints in erecting . 427 
 
 
 Page 
 
 Glass-house, Matheson's . . . 428 
 Glass, photographic transparency 204 
 
 for a dark room 204 
 
 plates to clean . . . . . . 160 
 
 full details for cleaning . . 407 
 
 Globe Lens 193 
 
 Glycerine 58 
 
 Glycyrrhizine 59 
 
 action of, in Collodion . 341-346 
 Gold (see Chloride of). 
 
 Grape sugar 62 
 
 Ground glass for focussing . . . 484 
 
 Gun-cotton 91, 217 
 
 Gum Arabic 63 
 
 Haloid Salts 14 
 
 Hard Negatives, causes of ... 528 
 in Taupenot process . . . .558 
 
 how to print from 298 
 
 Heliography, invention of ... 118 
 Herschel's printing processes . . 362 
 Historical sketch of Photography 117 
 
 Honey .....' 63 
 
 preservative process .... 328 
 Hot climates, Photography iu . . 478 
 
 Hjdriodic Acid (54 
 
 Hydrochloric 65 
 
 Hydrogen 65 
 
 Acids 13 
 
 Hydrometer, use of 573 
 
 for testing Silver solutions . . 491 
 
 Baume"'s scale of 580 
 
 Twaddell's . . 583 
 
 Hvdrosulphuric Acid 65 
 
 Hypo Bath. (See Fixing Solutions.) 
 
 Hyposulphite of Silver . . 96, 313 
 
 with Hyposulphite of Soda, 168, 313 
 
 brown patches of, in prints 315, 322 
 
 Hyposulphite of Soda . . 102, 167 
 
 theory of fixing action . . . 168 
 
 decomposed by acids . . . 317 
 
 decomposed by frequent use 318 
 
 with Silver 168, 313 
 
 reaction of, with Chloride of 
 
 Gold 307 
 
 test for presence of .... 325 
 to recover Silver from old 
 
 baths of 5G8 
 
 Iceland Moss 66 
 
 Image, photographic. (See Photo- 
 graphic image.) 
 
 Imperfection in Collodion Nega- 
 tives 527 
 
 in Collodion Positives . . . 535 
 
 in paper Positives 5-S6 
 
 Ink printing process (Perry's) . 364 
 Instantaneous Photography . . 469 
 Intensity of Negative increased 
 
 by redevelopment . . 150, 424 
 by Acetate of Silver in Bath . 277 
 by parchmentized Pyroxyline . 233 
 by decomposition of Pvroxyline 235 
 by excess of Alcohol .... 239 
 
INDEX. 
 
 589 
 
 Page 
 
 Intensity of Negative increased by 
 decomposition of Collodion245, 275 
 by artificial methods . . 285, 464. 
 in developed paper Positives . 509 
 
 Intensity diminished 
 
 by Bromide in Collodion . . 272 
 
 by Iron developer 281 
 
 in glass Positives 260 
 
 condition affecting ditto . . 258 
 
 Intensity of paper Positives 291, 298 
 
 Invisible images, nature of . . . 145 
 theory of development of . . 150 
 illustrative experiments . . . 137 
 
 Iodide of Ammonium .... 39 
 when to be used in Collodion . 244 
 use in Positive Collodion . . 391 
 use in Negative Collodion . . 402 
 
 Iodide of Cadmium 43 
 
 when to be used in Collodion . 242 
 useful in hot climates . . . 480 
 plain Collodion for . . . .440 
 formulae of solutions of . 398, 440 
 
 Iodide of Potassium 88 
 
 use as fixing agent .... 167 
 solubility in Alcohol .... 243 
 when to be used in Collodion . 243 
 formula for solution of ... 398 
 use of, for Calotype and waxed 
 papers 357 
 
 Iodide of Iron 69 
 
 injurious in Collodion . . . 245 
 
 Iodides in Collodion, physical 
 
 effects of 241 
 
 chemical effects of .... 245 
 
 Iodide of Silver .... 97, 127 
 
 effect of light on 130 
 
 molecularly affected by light . 153 
 hypothesis of invisible image on 145 
 theory of development of . . 150 
 with Nitrate of Silver . . .150 
 chemical Spectrum on ... 198 
 
 solvents cf 167 
 
 soluble in Nitrate Bath . . .247 
 sensitive and insensitive forms 
 
 of 345 
 
 with organic compounds . . 346 
 causes affecting sensitiveness of 154 
 causes affecting development of 156 
 less soluble than Chloride . . 169 
 to remove from old Nitrate 
 Baths 572 
 
 Iodine 66 
 
 liberates Nitric Acid in Bath . 248 
 often lowers the intensity of 
 
 image 275 
 
 useful in Positive Collodion . 261 
 how to remove from Collodion 225 
 
 Iodizing solutions for Negatives 
 
 398-440 
 
 theory of .241 
 
 (See also Bromo-Iodide.) 
 
 lodo-Nitrate of Silver . . 247, 345 
 
 Page 
 
 lodoform 67 
 
 renders Collodion porous . . 351 
 
 lodo-Cyanide of Potassium . .170 
 
 removes Silver stains . . . 575 
 
 Iron, Protacetate of 68 
 
 Ammonio-Citrate of .... 68 
 
 Perchloride of 69 
 
 Iodide of 69 
 
 Proto-Nitrate of 70 
 
 Proto-Sulphate of 70 
 
 Kaolin 70 
 
 decolorizes Silver solutions . 492 
 Keith's formula for Positive Deve- 
 loper 395 
 
 Landscape Photography on wet 
 
 plates 444 
 
 on dry Collodion 540 
 
 Lantern for dark room (Dela- 
 
 motte's) 412 
 
 Latent image. (See Invisible image.) 
 Laws of Combination ... . . 16 
 Leathers for polishing glasses . . 409 
 
 Lead, Acetate of 71 
 
 Nitrate of 71 
 
 Le Grey's waxed paper process . 358 
 Lenses, various forms of ... 177 
 
 foci of 177 
 
 image formed by 178 
 
 focal length of 180 
 
 visual and actinic foci of . . 181 
 spherical aberration of ... 183 
 curvature of field in . . . .184 
 variation of focus in ... . 185 
 use of stops or diaphragms in 
 
 186 et seq. 
 
 Lens, Aplanatic 188 
 
 Globe 193 
 
 Orthoscopic 188 
 
 Panoramic 192 
 
 Portrait 194 
 
 Triplet 190 
 
 View 187 
 
 to find actinic focus of ... 433 
 single, to find focal length of . 180 
 to find equivalent focus of any 561 
 
 for architecture 445 
 
 for copying 456 
 
 for landscapes 44 1 
 
 for portraits 434 
 
 Light, action of, on Silver Salts 130, 131 
 compound nature of .... 172 
 
 properties of 174 
 
 refraction of 175 
 
 chemical action of coloured . 19S 
 
 Linen or Paper Collodion, limpid 232 
 
 produces intense Negatives . 235 
 
 unstable 240, 351 
 
 Lithium, Iodide of 72 
 
 Litmus Paper, how to prepare . 12 
 how to use 574 
 
 Magnesium, Iodide of .... 72 
 
590 
 
 INDEX. 
 
 Page 
 
 Manipulations of Collodion pro- 
 cess . 407 
 
 of Photographic printing . . 498 
 Manufacture of Collodion, theory 
 
 of 225 
 
 Manuscripts, mode of copying . 455 
 Markings on Collodion pictures . 522 
 Measures and weights .... 582 
 
 Meniscus Lens ....... 177 
 
 Mercury, Chloride of 73 
 
 whitens Positives 266 
 
 Micro-photography 473 
 
 Milk, serum of 73 
 
 use in Positive printing . . . 303 
 Miller, Dr., on transparent media 204 
 
 Mordants 292 
 
 Moser's researches 159 
 
 Mounting proofs 506 
 
 substances to be avoided in . 322 
 
 Naphtha 75 
 
 Negative processes for printing . 507 
 Negative Collodion, theory of. . 216 
 
 manufacture of . . . . . . 373 
 
 Negative Nitrate Bath, theory of 
 
 275, 278 
 
 preparation of .... 399-402 
 N egative developers, theory of 278-284 
 
 preparation of .... 401-404 
 Negative fixing solutions, theory of 284 
 
 preparation of 404 
 
 Negatives, definition of .... 255 
 
 on wet Collodion, theory of . 267 
 
 F reduction of 373 
 'alotype, theory of .... 356 
 waxed paper, theory of ... 358 
 Albumen, theory of .... 3(iO 
 on dry Collodion, theory of . 327 
 ,, production of 540 
 spots and markings on ... 520 
 
 Nipce, M., his discovery of " He- 
 liography" 118 
 
 Niepce, de St. Victor, his Albumen 
 
 process 121 
 
 Nitrate of Iron 141,70 
 
 developer for Positives on glass 264 
 Keith's formula for .... 395 
 
 Nitrate of Potash 85 
 
 used for Pyroxy line . . . .218 
 
 details of process 374 
 
 used in Positive developer . . 395 
 
 Nitrate of Silver 93, 97 
 
 preparation, etc., of . . . .123 
 organic impurities in ... 252 
 other impurities in .... 98 
 may contain Nitrite . 97, 125, 254 
 reduced by developers . . . 142 
 essential in development . .150 
 dissolves Iodide of Silver . . 247 
 unacted on by light .... 130 
 indifferent to Glycerine . 328, 393 
 mode of recovering Silver from 567 
 
 Page 
 
 Nitrate Bath, chemistry of ... 247 
 for Collodion Positives, theory 
 
 of .261 
 
 how to prepare 393 
 
 for Collodion Negatives, theory 
 
 of 275-278 
 
 how to prepare .... 399-402 
 dissolves Iodide of Silver . . 247 
 alkalinity and acidity of ex- 
 plained 248 
 
 how to acidify a Positive Bath 
 
 261, 393 
 how to acidify a Negative Bath 
 
 for Iodized Collodion . 276, 400 
 for Bromo-Iodized Collodion 
 
 278, 403 
 
 to neutralize .... 400, 403 
 to saturate with Iodide of Silver 
 
 393 
 
 Acetate of Silver in .... 251 
 how to add Acetate to ... 529 
 
 organic matter in 252 
 
 to remove organic matter from 253 
 may contain Nitrite .... 254 
 changes by use . . . . 394, 401 
 
 to cure fogging in 403 
 
 to ascertain strength of . . . 561 
 Negative, to prepare for print- 
 ing 572 
 
 recovery of silver from . . . 567 
 Nitrate Ba"(h for printing, theory 
 
 of 289,297-299 
 
 mode of preparation . . 490, 494 
 meter for testing strength of 491 
 
 to decolorize 492 
 
 Nitrite of Silver 100 
 
 in Nitrate 97, 254 
 
 effect of, in development . . 254 
 
 Nitric Acid 75 
 
 oxidizing properties of ... . 124 
 retards formation of latent image 
 
 154 
 checks development . . . .144 
 
 prevents fogging 157 
 
 excess of, causes fogging . .158 
 use in Positive Bath .... 2iil 
 use, with Bromo-Iodide . . 278 
 accumulates in Nitrate Bath . 249 
 to remove excess of . . 393^103 
 for Pyroxyline. (See Pyroxyline. ) 
 Ure's table of strength of . . 578 
 
 Nitrogen 77 
 
 Binoxide of 78 
 
 Peroxide of 78 
 
 Nitro-glucose 78 
 
 action of, on Collodion . 240, 2l6 
 Nitro-hydrochloric Acid ... 79 
 
 Nitro-sulphuric Acid 217 
 
 observations on proper strength 
 
 of 225 
 
 formulae for mixed acids 
 
 377, 390, 440 
 
INDEX. 
 
 591 
 
 Page 
 Nitre-sulphuric Acid, formula for 
 
 Nitre and Sulphuric Acid . 374 
 should be used hot .... 231 
 precautions in using .... 387 
 
 Nitrous Acid 79 
 
 Nomenclature 20 
 
 N orris, Dr., on dry Collodion . . 332 
 
 Opacity of Collodion film . . .226 
 injurious in Positives on glass 260 
 
 Organic matter 80 
 
 Organic substances, chemistry of 26 
 effects of, in Nitrate Bath . . 252 
 how to remove from Bath 253, 404 
 effects of, on Positive printing 291 
 necessary in dry Collodion pro- 
 cesses 330 
 
 effects of, in dry processes . . 340 
 to distinguish from inorganic . 28 
 
 Orthoscopic or Orthographic Lens 188 
 useful when camera is tilted . 445 
 distortion of 197 
 
 Osborne's Photo-lithographic pro- 
 cess 366 
 
 Over-exposure, effect on develop- 
 ment 160 
 
 effect on Collodion Positives . 437 
 effects on Collodion Negatives 423 
 effects in printing by develop- 
 ment 511 
 
 effects in Taupenot process . 557 
 
 Oxide of Silver 100, 128 
 
 soluble in Nitrate Bath . . .249 
 for acidity of Bath .... 403 
 dissolves in Ammonia . . . 301 
 
 preservative 
 
 Oxides 
 Oxygen 
 Oxymel, preparation of . 
 
 as a Collodion preservi 
 Ozone 
 
 its effect on Collodion 
 Paper, Photographic . . . 295, 486 
 
 sensitive. (See Sensitive paper.) 
 Parchmentizing action of Sulphu- 
 ric Acid 221 
 
 Permanganate of Potash, use of . 325 
 Permanence of Positives, to test . 325 
 Petschler and Mann's dry piocess 335 
 
 details of 558 
 
 Petzval Lens. (See Orthoscopic 
 
 Lens.) 
 Petzval introduces Negative Lens 
 
 188, 192 
 
 Phosphorus 82 
 
 Phosphoric Acid 83 
 
 Phosphate of Soda 103 
 
 Photo-galvanography . . . 368 
 
 Photo-glyphy 368 
 
 Photo-lithography 366 
 
 Photo-zincography 367 
 
 Photographic chemicals, vocabu 
 
 laryof 29 
 
 Page 
 
 Photographic Chemicals, list of, 
 
 for landscapes 448 
 
 list of, for sending abroad . . 483 
 
 Photographic image, nature of 
 
 133, 163, 292 
 
 action of destructive tests on . 325 
 (See also Development.) 
 
 Photographic properties of Salts 
 
 of Silver 129 
 
 Photography, historical sketch of 117 
 in hot climates 478 
 
 Plain Collodion. (See Collodion.) 
 
 Portrait Lenses, construction of . 194 
 
 how to use 431 
 
 to test correctness of . . . . 432 
 
 Portraiture, drapery for .... 203 
 practical directions for . . . 427 
 
 Positive 436 
 
 Negative 438 
 
 Positive Collodion, theory of . . 258 
 details of manufacture . . . 390 
 Bromo-Iodizer for .... 391 
 
 Positive Nitrate Bath, theory of . 261 
 preparation of 393 
 
 Positive developing solutions, the- 
 ory of 263 
 
 preparation of 394 
 
 Positive fixing solutions, theory of 266 
 preparation of 397 
 
 Positive printing, theory of . . 287 
 practice of, on albuminized 
 
 paper 487 
 
 practice of, on plain paper . . 493 
 practice of, on Ammonio- Ni- 
 trate paper 493 
 
 by development on Iodized 
 
 paper 507 
 
 by development on Chloride of 
 
 . Silver 509 
 
 on glass . 514 
 
 by vignetting 506 
 
 failures in 536 
 
 Positive prints defined .... 255 
 action of sulphur on .... 305 
 effect of Permanganate of Po- 
 tash on 325 
 
 effect of Chromic Acid on . . 326 
 fading of, explained .... 324 
 comparative permanency of .324 
 
 how to wash 504 
 
 effects of imperfect washing on 319 
 
 how to mount 500 
 
 improper modes of mounting . 321 
 to test permanency of ... 325 
 imperfections in 536 
 
 Positives on Collodion . . . .257 
 
 formulae for 391 
 
 practical details of .... 436 
 mode of whitening .... 266 
 
 varnishes for 397 
 
 how to apply the varnishes to . 438 
 imperfections in 35 
 
592 
 
 INDEX. 
 
 Page 
 
 Potash 83 
 
 Bichromate of 84 
 
 Carbonate of 85 
 
 Nitrate of 85 
 
 and Soda, Tartrate of ... 86 
 Potassium, Bromide of . . . .86 
 
 Cyanide of 87 
 
 Fluoride of 87 
 
 Iodide of 88 
 
 Sulphide of . . 90 
 
 Sulphocyanide of 105 
 
 Precipitate explained 24 
 
 Preservation of sensitive paper . 304 
 
 Spiller's box for 305 
 
 Preservative processes, theory of 327 
 practical details of .... 540 
 Prints. (See Positive Prints.) 
 
 Printing, theory of 287 
 
 practical details of .... 486 
 Prism, decomposition of light by 172 
 diagrams of spectrum by . 173, 198 
 Proto-Nitrate of Iron. (See Nitrate 
 
 of Iron.) 
 
 Protosulphate of Iron . . 70 S 141 
 theory of its developing pro- 
 perties 141 
 
 a developer for Positives, the- 
 ory of 263 
 
 formulae for solutions of . . 394 
 how to apply the solutions . . 436 
 a developer for Negatives, the- 
 ory of 280,284 
 
 formulae for solutions . 401-404 
 how to apply developer . . . 424 
 
 Pyrogallic Acid 90, 139 
 
 theory of developing properties 1 42 
 
 use for Positives 263 
 
 a developer for Negatives, the- 
 ory of 278 
 
 formulaa for ditto . . . 401-404 
 mode of applying the developer 420 
 mode of applyingforintensifying 424 
 formula for Tannin plates . . 546 
 mode of applying to Tannin 
 
 plates 547 
 
 Pyroxyline, varieties of .... 91 
 theory of its preparation . . 217 
 theory more fully considered . 225 
 practical details of Nitre pro- 
 cess 374 
 
 practical details of mixed-acids 
 
 process 375 et seq. 
 
 preparation of, for Positives . 390 
 preparation of, for Cadmium 
 
 lodizer 440 
 
 preparation of, for copying Col- 
 lodion . . . .... .460 
 
 theory of preparation for dry 
 
 processes 339 
 
 precautions in preparation . . 382 
 physical and photographic pro- 
 perties of 226 
 
 Page 
 
 Pyroxyline, decomposition of . . 203 
 organic reactions in, effect of . 270 
 Eecovery of Silver from solutions 5(>7 
 Eed Glass, non-actinic . . . .200 
 Keduction of Silver Salts by deve- 
 lopers 141 
 
 to the metallic state . . . .507 
 Eussell's Tannin process . . . 33<'i 
 practical details of .... 5 II 
 developer, theory of .... 343 
 formula? for developer . . . 540 
 
 Salts, nature of 13 
 
 Salts of Silver, chemistry of . . 123 
 
 photographic properties of . .129 
 
 reduced by developers . . . 141 
 
 recovery of Silver trom . . . 567 
 
 Sensitiveness, causes affecting, 154, 
 
 238, 272. 
 
 Sensitive paper, theory of prepa- 
 ration 287 
 
 more fully considered . . . 295 
 preparation of, practical details 487 
 requires excess of Nitrate of 
 
 Silver 2f;9 
 
 mode of preserving .... 304 
 amount of Silver in .... 304 
 
 (See also Positive Printing.) 
 Serum of Milk, (flee Milk.) 
 Setting of Collodion film . . . .416 
 Shadbolt, his Honey process . 328 
 remedy for over-printing . . 538 
 on out-ofJ-door portraiture . .430 
 on testing Portrait lens . . 433 
 on finding equivalent focus of 
 
 lens 561 
 
 Silver 93 
 
 Acetate of 94 
 
 Albuminateof ...... 94 
 
 Ammonio-nitrate of . . . .94 
 
 Bromide of 95 
 
 Carbonate of 95 
 
 Chloride of 95 
 
 Citrate of 96 
 
 Huorideof 96 
 
 Hyposulphite of 96 
 
 Iodide of 97 
 
 Nitrate of 97 
 
 Nitrite of 100 
 
 Oxide of 100 
 
 Organic compounds of . . .131 
 
 Sulphide of 101 
 
 recovery of, from wastes . . 567 
 
 removal of stains of .... 575 
 
 Silver Meter for testing baths . . 491 
 
 Size in paper 205 
 
 photographic properties of . . 296 
 to remove from prints . . . 505 
 
 Soda, Acetate of 101 
 
 Carbonate of 101 
 
 Citrate of 102 
 
 Hyposulphite of 102 
 
 Phosphate of ...... 103 
 
INDEX. 
 
 593 
 
 Page 
 Sodium, Auro-chloride of ... 103 
 
 Chloride of 104 
 
 Solar Camera, principle of ... 179 
 
 Solar Spectrum 173 
 
 on Iodide of Silver . . . .198 
 on Bromide of Silver . . . .201 
 
 Solarization 160 
 
 condition of bath favouring . 253 
 depends partly on light . . . 268 
 depends partly on lens . . . 269 
 Bromide a remedy for . . . 272 
 Citric Acid a remedy for . . 279 
 remedied by mode of develop- 
 ment 283 
 
 remedy in dry plates .... 342 
 
 Solution explained 24 
 
 Specific Gravities, method of ta- 
 king 573 
 
 solutions of Silver, table of. . 566 
 Sulphuric Acid, table of . . . 578 
 Nitric Acid, table of . . . .578 
 
 Alcohol, table of 579 
 
 Spherical Aberration 183 
 
 corrected by stops .... 184 
 Spirits of Wine. (See Alcohol.) 
 Spots on Negatives, theory of . .158 
 
 causes of 520 
 
 on Positive prints 537 
 
 Stains, Silver, removal of ... 575 
 Stereoscope, principle of ... 205 
 
 reflecting 211 
 
 refracting 211 
 
 Stereoscopic Photography, theory 
 
 of 213 
 
 practical details of .... 465 
 
 Stops, use of. . . . 184,186,187 
 
 position of, in View Lens . . 187 
 
 in Portrait Lens 195 
 
 simple method of making . .456 
 Strength of Acids, tables of . . 578 
 
 Subchloride of Silver 127 
 
 formed by light 134 
 
 Suboxide of Silver 129 
 
 compounds of, formed by light 134 
 Substitution Compounds . . 26, 219 
 Sulphate of Iron. (See Protosulphate.) 
 Sulphate of Quinine, action on 
 
 chemical rays 202 
 
 Sulphocyanide of Potassium . . 105 
 of Ammonium ...... 105 
 
 Sulphocyanides as fixing agents . 170 
 
 Sulphuric Acid 106 
 
 use in making Pyroxyline . . 217 
 
 absorbs water 218 
 
 parchmentizing, action of . . 221 
 
 used for drying cotton . . . 382 
 
 table of strength of . . . .578 
 
 Surface reflection, actinic effect of 203 
 
 Button's Panoramic Lens . . . 192 
 
 Symmetrical Triplet . . . .190 
 
 mode of developing Positives 
 
 on paper 511 
 
 Page 
 
 Swing-backs to Cameras . . 435 
 
 Symbols of Elements ... .9 
 
 Symbols and Formulae . . .19 
 
 Talbot's discoveries ... .120 
 
 Calotype process ... . 356 
 
 Photoglyphic process . . 368 
 
 Tannin 108 
 
 process, theory of . . .336 
 
 practical details of . . . 541 
 
 Tartaric Acid ....... 109 
 
 Taupenot's process, theory of . 331 
 practical details of .... 552 
 
 Temperature, effect of, on deve- 
 lopment 144 
 
 on fogging 518 
 
 on Collodion 479 
 
 on developer 481 
 
 on hypo-bath 315 
 
 Tent, Photographic (Levi's) . . 446 
 
 Test paper 72 
 
 Tetrathionic Acid 109 
 
 Tomlinson's test for grease on 
 
 water Ill 
 
 Toning bath with sulphur . . . 305 
 Chloride of Gold in hypo . . 306 
 Alkaline Gold, theory of . . 308 
 conditions affecting .... .S10 
 
 formulae for 496 
 
 Transparencies on glass . . . 514 
 Triplet lens, construction of . . 190 
 
 used for copying 457 
 
 used for architecture . . . 445 
 
 Turmeric 110 
 
 Uranium, Nitrate of Ill 
 
 Uranium printing process . . . 363 
 Varnishes for Positives .... 397 
 
 how to apply 438 
 
 for Negatives 405 
 
 how to apply 426 
 
 Velvet used to line Camera . . 445 
 View Lens, construction of . . 187 
 
 distortion of 196 
 
 to prevent fogging from . . 444 
 to measure focal length of . . 180 
 
 Vignetting 506 
 
 Vocabulary of Chemicals ... 29 
 Washing prints, rules for . . . 319 
 
 details of 504 
 
 Water, distilled Ill 
 
 to detect oily matter in . . Ill 
 
 different kinds of 112 
 
 Waterhouse's diaphragms . . . 195 
 Waxed paper process .... 358 
 Weights and Measures .... 582 
 Woolliness of Collodion film . . 226 
 
 Xyloidine 220, 226 
 
 Yellow glass for dark room . . 200 
 Yellowness in prints, how caused 537 
 Young's experiments on develop- 
 ment 152 
 
 explanation of 153 
 
594 
 
 CORKIGIEKDA. 
 
 Page 23, 25 lines from top, for " Pyrosphate" read " Pjrophosphate." 
 ,, 38, 4 lines from bottom, insert " a" before " solvent." 
 ,, 129, 14 lines from top, for " latter" read "former." 
 198, 15 lines from top, innert Section Til. 
 
 204. It should have been stated here that the numbers representing the 
 comparative lengths of the Spectra impressed on a sensitive 
 Collodion film by light transmitted through various media, are 
 true only of Electric light, which has rays of far higher re- 
 frangibility than any contained in Solar light ; so that when the 
 latter is the source of Photographic power, it would be far less 
 affected by the media named than the corresponding numbers 
 would represent.' 
 
 212, 3 lines from bottom, for " an" read "in." 
 216, first line, for "hapter" read " chapter." 
 227, bottom line, for "a" read " af-" 
 230, 7 lines from top, after "with" insert "the." 
 460, 13 lines from top, for "277" read "377." 
 471, 3 lines from top,./or " foliage combined with clouds," read " clouds 
 
 combined with foliage." 
 
 494, 2 lines from top, for " Sale's" read " Saxe." 
 508, 19 lines from top, for " under the head of Collodio- Albumen Pro- 
 cess," read " at page 510." 
 
 JOHN EDWARD TAYLOR. PRINTER, 
 LtTTLE QUEEN STREET, LINCOLN'S I&'N.FIELPS. 
 

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