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 ELECTRIC SMELTING AND REFINING : A Practical Manual of 
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 Part II. THE EARTH METALS : Aluminium, Cerium, Lanthanum, Didymium. 
 
 Part III. THE HEAVY METALS : Copper, Silver, Gold, Zinc and Cadmium, Mercury, 
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A TREATISE 
 
 ON 
 
 ELECTRO -METALLURGY : 
 
 EMBRACING 
 
 THE APPLICATION OF ELECTROLYSIS TO THE PLATING, DEPOSITING 
 
 SMELTING, AND REFINING OF VARIOUS METALS, AND TO 
 
 THE REPRODUCTION OF PRINTING SURFACES AND 
 
 ART-WORK, ETC. 
 
 BY 
 
 WALTER a. M C MILLAN, F.I.C., M.INST.M.M., 
 
 LATE LECTURER ON METALLURGY IN MASON UNIVERSITY COLLEGE, BIRMINGHAM, 
 
 AND FORMERLY CHEMIST AND METALLURGIST TO THE COSSIPORE 
 
 ORDNANCE FACTORIES. 
 
 REVISED BY 
 
 W. R COOPER, M.A., B.Sc., A.I.C., 
 
 MEMBER OF THE INSTITUTION OF ELECTRICAL ENGINEERS, ASSOCIATE MEMBER 
 OF THE INSTITUTION OF CIVIL ENGINEERS. 
 
 THIRD EDITION. REVISED AND ENLARGED 
 
 IKflitb numerous 3Uustratfon0. 
 
 LONDON: 
 
 CHARLES GRIFFIN AND COMPANY, LIMITED, 
 
 EXETER STREET, STRAND. 
 
 1910. 
 
 [All Eights Reserved.] 
 
70 
 
PREFACE TO THE THIRD EDITION. 
 
 IN revising this Treatise for a third edition I have adhered 
 as far as possible to the lines followed by the late Mr W. G. 
 M c Millan, though it has been found necessary to make 
 considerable alterations. Thus, I have thought it well to 
 omit the greater part of the section dealing with the 
 dynamo, as this is a specialised subject which cannot be 
 treated satisfactorily with brevity. Moreover, there are many 
 text-books available where such information is easily found, 
 and therefore the space in the present volume can be used to 
 better advantage in other ways. The table giving particulars 
 of copper refineries has also been omitted, because it is 
 almost impossible to obtain trustworthy data of equipment 
 and methods in such cases. Generally speaking, electro- 
 deposition has not made any very marked advance during 
 the last few years ; nevertheless, many minor alterations and 
 additions have been found necessary to bring the work up 
 to date. 
 
 As in previous editions, the field of electric smelting and 
 refining as a whole is dealt with but briefly. It was felt, 
 however, that the subject of electric smelting and refining of 
 iron, which was non-existent at the time of the last edition, 
 is becoming so important that considerable space should 
 be devoted to an account of the present position of this 
 new field. 
 
 W. E. C. 
 
 82 VICTORIA STREET, 
 LONDON, S.W. 
 
 210341 
 
PREFACE TO THE FIRST EDITION. 
 
 IN the following pages I have endeavoured to systematise 
 and explain the various processes of Electro-Metallurgy as 
 far as possible. Believing fully that in teaching and 
 writing upon such subjects a technological rather than a 
 technical treatment is required, I have tried so to set the 
 matter before the reader that, even if he be a novice, he 
 may be led to take an intelligent interest in any practical 
 work upon which he may be engaged ; but I have avoided 
 the accumulation of a mass of unnecessary descriptive detail, 
 which would only tend towards confusion, and which would 
 be dictated by common-sense to any who have grasped 
 the principles involved. In many cases, however, success 
 is in a large measure dependent upon strict attention to 
 mechanical detail ; and here I have not hesitated to introduce 
 such instructions as I believed needful to guide the worker 
 in the special operations in hand, while indicating the 
 reasons which should enable him to apply them to processes 
 of kindred character. In short, I have aimed at a combina- 
 tion of theory and practice. 
 
 The necessity for at least a fair knowledge of Chemistry 
 and Electricity has led to the introduction of a chapter 
 dealing in an elementary fashion with such laws as are 
 required for an understanding of our subject ; but it is not 
 pretended that this chapter shall in any degree supersede 
 the text-books upon these sciences ; it is rather intended to 
 lead up to them. In treating of the sources of current, 
 especially the dynamo -electric machine, I have dwelt longer 
 
 vii 
 
Viii PREFACE. 
 
 upon the general theory of construction and use as applicable 
 to all than upon the special modifications adopted by different 
 inventors and manufacturers. 
 
 In addition to the journals, the following works among 
 others have been consulted, and my general indebtedness 
 to these authors must here be thankfully recorded: 
 Fontaine's Electrolyse, Gore's Art of Electro-Metallurgy, 
 Japing's Elektrolyse Galvano-plastik und Reinmetallgewin- 
 nung, Napier's Manual of Electro-Metallurgy, Roseleur's 
 Manipulations Hydroplastiques, Schaschl's Galvanostegie, 
 Thompson's Dynamo Electric Machinery, Urquhart's Electro- 
 typing and Electro-plating, Volkmer's Betrieb der Galvano- 
 plastik mit Dynamo - Elektrisclien Maschinen zu Zwecken 
 der Graphischen Kunste, Wahl's Practical Guide for the 
 Gold and Silver Electro -plater arid the Galvano- plastic 
 Operator, Watt's Electro-deposition, Weiss's Galvano-plastik 
 and Wilson's Stereotyping and Electrotyping. My thanks 
 are also due to the Brush Electric Light Corporation, 
 Messrs Hoe & Company, Messrs Siemens Brothers, and 
 Messrs Townson & Mercer for diagrams of apparatus. 
 I must further acknowledge my obligations to Professor 
 Silvanus Thompson for his kind permission to describe 
 and figure a special form of switch which he has in use. 
 
 WALTER G. M C MILLAN. 
 
 COSSIPORE, CALCUTTA, 
 September 1890. 
 
CONTENTS. 
 
 CHAPTER I. 
 INTRODUCTORY AND HISTORICAL. 
 
 Definition of the term Electro-metallurgy Scope of the Art The Germ of 
 the Process in Primitive Times The Growth of Electrical Knowledge 
 The Invention of the Thermopile, Battery and Magneto-electric and 
 Dynamo-electric Machines First Attempts at the Electro-deposition of 
 Metals The Work of Nicholson and Carlisle, Cruickshank, Wollaston, 
 Brugnatelli, Davy, and Bessemer Becquerel's Experiments on the 
 Electrolytic Treatment of Ores The Invention of the Daniell -cell the first 
 step towards Electrotypy ; De la Rue's Observation, and the Development 
 of Electrotyping by Jacobi, Spencer, and Jordan The question of 
 precedence among the three Rival Inventors Murray's discovery of 
 Blackleading Non-conductive Electrotype Moulds Leeson's and Mont- 
 gomery's first Elastic Moulds The later Advances in the Art ; the value 
 of the Dynamo and its effect upon the Industry Electrolytic Ore 
 treatment Electrolytic Metal-refining Electrolysis of Fused Substances 
 Electro-smelting and Refining Recent Development of the Theory of 
 Electrolysis, Pages 1-13 
 
 CHAPTER II. 
 
 THEORETICAL AND GENERAL. 
 
 (See also Chapter XIX. ) 
 
 Matter and Force Conditions of Matter Constitution of Matter Molecules 
 and Atoms Elements and Compounds Relative Weights of Atoms- 
 Meaning of the Symbols and of Chemical Formulae Valency Laws of 
 Definite Chemical Combination List of Elementary Substances Energy 
 Displayed by Chemical Combination Effect of Heats of Combination in 
 Determining the Occurrence of Chemical Reactions The Elements placed 
 in Electro- chemical Series Transformations of Energy The Conversion 
 of Chemical into Electrical Energy, and its application in the Galvanic 
 Battery Re-conversion of Electrical Energy into Chemical Energy The 
 Theory of Electrolysis or Electro-deposition Laws Governing Pressure or 
 Electro- motive Force of Battery Currents The Relation of Current- 
 pressure to the Electro-deposition of Metals from Solutions under varying 
 
X CONTENTS. 
 
 Conditions Electric Conduction Electrolytic Conduction Electrolysis 
 of mixed Solutions The Deposition of Alloys Quantity and Potential of 
 Currents Units employed in Measurements, . . . Pages 14-36 
 
 CHAPTER III. 
 SOURCES OF CURRENT. 
 
 The Galvanic Battery The Use of Impure Zinc ; Local Action ; Amalgamation 
 The Polarisation of Battery Cells, and its Remedies Definition of 
 Terms used in speaking of Batteries The various Single- and Two-fluid 
 Cells Smee's, Daniell's, Grove's, Bunsen's, the Bichromate, Edison- 
 Lalande, and Leclanche Practical Hints on the Use of Batteries The 
 Fittings and Connections of a Battery Various ways of arranging several 
 Cells ; the application of Ohm's Law Thermo-electric Batteries The 
 Direct Conversion of Heat into Electricity The Thermo-electro-motive 
 Force of various Combinations of Metals at different Temperatures ; the 
 choice of Metals for Thermo-electric Coupler Clamond's and Giilcher's 
 Thermopiles The Conversion of Mechanical into Electrical Energy The 
 Elementary Theory and parts of the Dynamo- and Magneto-electrical 
 Machines Various Types Conditions to be observed in using the 
 Dynamo Accumulators or Secondary Batteries Use of Public Electricity 
 Supply for Electro-metallurgical Work Conversion of Alternating 
 Currents into Continuous Currents, and of High-pressure Currents into 
 Low-pressure Currents, . Pages 37-76 
 
 CHAPTER IV. 
 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. 
 
 Necessity for Cleanliness The Proportioning of Current to the Electrolytic 
 Separation to be Effected The Relation of Weight and Thickness of 
 Deposit to Current-strength and Duration of Process The Effect of, 
 and means for, altering the Electrical Resistance of the Circuit The 
 use of Measuring Instruments for Determining Current-strength 
 The Detector, Galvanometer, Ammeter, and Voltmeter The Arrange- 
 ment of Plating- Vats according to Current and Pressure, and other 
 Considerations; Connection in Series and in Parallel The Choice of 
 Anodes ; and the Regulation of the Distance between the Electrodes in 
 the Vat Necessity for Stirring the Plating Solutions Motion to be 
 imparted to the Solution, Pages 77-91 
 
 CHAPTER V. 
 PLATING ADJUNCTS AND DISPOSITION OF PLANT. 
 
 Necessity for Light and Air in Work-rooms Apportionment of Rooms to 
 Departments of Work Requirements in the Disposition of Shops- 
 Drainage and Ventilation Arrangement of Plant for Electro -plating 
 The Material for, and Form of, Vats, and Manner of Introducing the 
 Connecting Wires Relative Positions of Electrodes for different kinds 
 
CONTENTS. XI 
 
 of Work The Method of supporting Anode Plates The Systems of 
 Stirring Plating Liquids, and of imparting Motion to the Suspended 
 Objects The Weighing of the Deposited Metal ; Plating - balances 
 Corrections to be made in using the Plating-balance The Coating of 
 Wires Electro-plating large Surfaces The " Doctor "The Electro- 
 plating of small Goods in Rotating Drums Smith and Deakin's 
 Apparatus The Joints of Lead-lined Vats, . . . Pages 92-107 
 
 CHAPTER VI. 
 
 THE CLEANSING AND PREPARATION OF WORK FOR THE DEPOSITING -VAT, 
 AND SUBSEQUENT POLISHING OF PLATED GOODS. 
 
 The Different Methods of removing Grease and thick incrustations of Dirt 
 The Chemical Action of hot Alkali on Grease The Cleansing of 
 Objects from Mineral Oils The Construction and Use of the Potash 
 Tank Necessity for dipping in Acid before immersion in the Plating- 
 Vat The Cleansing of Copper, Brass, German Silver, Iron and Steel, 
 Zinc, Lead, Tin, Britannia Metal, and Aluminium Electrolytic 
 Cleaning The Quicking of Metallic Surfaces The Mechanical Treat- 
 ment df Metals to be Cleansed, and Methods of ensuring a Smooth and 
 Polished Surface The Polishing Lathe The Manner of Bobbing and 
 Mopping The Uses of Scratch-brushing by Hand and by Lathe The 
 Process of Burnishing The Preliminary Treatment of Hard Steel 
 The Preparation of Objects for Nickeling, . . . Pages 108-123 
 
 CHAPTER VII. 
 THE ELECTRO-DEPOSITION OF COPPER. 
 
 Objects with which Electro-Deposition of Copper is applied Coating by 
 Simple Immersion Coppering of small Steel Articles The Single-cell 
 Process The Manner of Coating Flat and Spherical Surfaces by Single- 
 cell Deposition The Battery Process of Coppering The Battery 
 employed The various Acid and Alkaline Plating-baths The Char- 
 acter, Suspension, and Use of the Anodes The Character of the Copper 
 Deposited ; the Strength, Hardness, and General Fitness of the Metal 
 precipitated from various strengths of Solution by different Current- 
 densities The Process of Deposition The Manner of preparing the 
 Articles for the Bath, immersing them, noting the Progress of the 
 Action, withdrawing and finally cleansing them The Coppering of 
 Printing - rollers Electrolytically formed Tubes Elmore's Process 
 Cowper-Coles' Process Copper Wire by Deposition Other Applications 
 of the Process, Pages 124-141 
 
 CHAPTER VIII. 
 ELECTROTYPING. 
 
 First Principles of Electrotyping Moulding Materials for Printers' and 
 Art Work in Solid and Elastic Compositions, applicable to Plain or 
 
xii CONTENTS. 
 
 Undercut Designs, and the manner of applying them Gutta-percha, 
 Bees' -wax, Plaster of Paris, Fusible Metal, Gelatine, and Sealing-wax ; 
 and Mixtures in which these are employed The Manner of rendering 
 the Mould Conductive Plumbago, Silvered and Gilded Plumbago, 
 Tin- and Copper-powders Printers' Electrotyping The Reproduction 
 of Steel Plates ; the Arrangement of Baths and Distribution of Current, 
 and General Conduct of the Process Typographical Matter The 
 Preparation of the Type ; Moulding ; Black-leading of the Mould, and 
 Deposition of Copper ; the final Backing and Treatment of the Electro- 
 type Treatment of Wood-blocks Art Electrotyping The Moulding 
 and Reproduction of Medals, of Busts and Statues, and of Natural 
 Objects The Special Treatment of Large Statues, in regard to Mould- 
 ing, Disposition of Anodes and Application of Solution The manner 
 of ensuring Equalisation of Deposit upon the Moulds Sundry Applica- 
 tions of Electrotyping Glyphography, Stylography, Galvanography, 
 Electro-etching, and the like Manufacture of Copper Reflectors by 
 Electrotyping, . . . . Pages 142-174 
 
 CHAPTER IX. 
 THE ELECTRO-DEPOSITION OF SILVER. 
 
 The Deposition of Silver by Simple Immersion ; the Composition and Use 
 of various Plating Solutions and Pastes The Single-cell Process The 
 Separate -current Process The Battery The Constitution and Pre- 
 paration of various Silver-baths The Conditions to be avoided in Making 
 up and in Tending Plating-solutions Bright Plating-liquids The 
 Anodes The Character of the Metal Deposited under different 
 Conditions The Stripping of old Silver Coats from Copper, Brass, or 
 German Silver, and from Zinc, Iron, Tin, Lead, and their Alloys The 
 Final Preparation of the Objects for the Bath The Suspension of the 
 Goods in the Vats, and Manner of Depositing the Metal Precautions 
 to be adopted in certain cases The Use of the Striking-bath 
 Difficulties encountered in Plating The Local Thickening of the Film 
 to withstand Wear The Thickness of Deposit "Gal vanit" Silver 
 Electrotyping The Ornamentation of Silver Surfaces by Dead Lustre, 
 and as Oxidised Silver, Antique Silver, or by Satin Finish Niello 
 Work, . , .. ;. v- , Pages 175-197 
 
 CHAPTER X. 
 THE ELECTRO-DEPOSITION OF GOLD. 
 
 Advantages of Gold-plating Solutions and Process for Deposition by 
 Simple Immersion The Single-cell Process Battery Methods of 
 Deposition The Preparation and Characteristics of various Solutions 
 The Production of Coloured Gold The Necessary Properties of the 
 Anode The Character of the Metal Deposited by different Current- 
 densities The Stripping of old Gold-deposits The Requirements and 
 Conduct of the Depositing Process The Gilding of Interior Surfaces 
 Plating with the " Doctor "The Thickness of the Films Dead Gild- 
 
CONTENTS. XU1 
 
 ing The Treatment of the more Electro-positive Metals The Gilding of 
 Soldered Goods Parcel-gilding The Ornamentation and Treatment of 
 Gilt Surfaces The Contrasting of Coloured Gilding The Colouring of 
 Deposited Gold The Gilding of Watch Mechanisms The Mechanical 
 Production of thb Surface-grain, Pages 198-215 
 
 CHAPTER XI. 
 
 THE ELECTRO-DEPOSITION OF NICKEL AND COBALT. 
 
 The Application and Advantages of Nickel-plating Difficulties in Electro- 
 nickeling The Character of the Deposited Metal The Battery Process 
 for Depositing Nickel The Nature and Preparation of the Various 
 Nickeling Solutions Cast and Rolled, Nickel and Carbon Anodes 
 The Necessity for Stripping old Nickel Coats The Preparation of Objects 
 for the Bath The Manner of effecting Electrolysis Precautions to be 
 Observed The Finishing of the Plated Goods Dead Nickel Surfaces 
 The Electro-deposition of Cobalt The Solutions used, and the Method of 
 Applying them The Details of the Process, and the Nature of the Pre- 
 cipitated Metal, . .... . . . . Pages 216-229 
 
 CHAPTER XII. 
 THE ELECTRO-DEPOSITION OF IRON. 
 
 The Advantages of Iron-faced Engraved Copper Plates The Depositing 
 Solutions, their Preparation, Maintenance, and Use The Physical 
 Properties of Deposited Iron Use of the Term Steel-facing Strip- 
 ping old Coats; the Plating Process and Final Treatment of the 
 Work, . . . ..''. . . ; -. Pages 230-236 
 
 CHAPTER XIII. 
 
 THE ELECTRO-DEPOSITION OF PLATINUM, ZINC, CHROMIUM, CADMIUM, TIN, 
 LEAD, ANTIMONY, BISMUTH, AND PALLADIUM ; ELECTRO-CHROMY. 
 
 Platinising by Simple Immersion Deposition by Single-cell Process The 
 Platinising of Silver plates of Smee's Battery Platinating by a Separate 
 Current Removing Old Deposits The Plating-solutions and manner of 
 applying them The Deposition of Zinc Comparison of Electro-zinced 
 Metal with so-called Galvanised Iron The Solutions and their Use ; Use 
 of Zinc Dust in the Solution ; Anomalies in the Application of the Current 
 The Character of the Zinc Deposit under various Conditions The 
 Electro -deposition of Chromium The Electro-deposition of Cadmium 
 Electro-tinning ; Electrolytically-coated Goods contrasted with Tin-plate 
 Tinning of Brass Pins and small objects by Simple Immersion Deposi- 
 tion of Tin by Single-cell and Separate-current Processes The Electro- 
 deposition of Lead The Coating of Bodies with Antimony Disqualifica- 
 tions of the Metal for Plating thin articles liable to be bent Simple 
 Immersion and Battery Methods of Deposition The Plating-solutions 
 and the Metals precipitated from them The Preparation and Properties 
 
xiv CONTENTS. 
 
 of Explosive Antimony The Electro-deposition of Bismuth The Electro- 
 deposition of Palladium Attempts to deposit Aluminium The Colouring 
 of Metallic Surfaces Electro-chromy Pages 237-256 
 
 CHAPTER XIV. 
 THE ELECTRO-DEPOSITION OF ALLOYS. 
 
 The Electro-deposition of Brass The Solution ; its Composition and Use 
 The Choice of Anodes The Influence of the various Conditions upon the 
 Character of the Deposited Brass The Deposition of Bronze and of 
 German Silver The Deposition of other Alloys, . . Pages 257-266 
 
 CHAPTER XV. 
 
 ELECTRO-METALLURGICAL EXTRACTION AND REFINING PROCESSES. 
 
 Conditions under which the Industry exists The Electro-refining of 
 Copper The Theory of the Process The Behaviour of the various 
 Impurities at the Anode, in the Solution, and at the Cathode The 
 Solution, Current-density, and General Conduct of the Process The 
 Disposition of Vats according to the Current employed Loss of Energy 
 Modern Systems of Refining The Electrolytic Extraction of Copper from 
 Ores and Products Classification of Extraction Processes Outlines of 
 the Methods employed The Electrolytic Refining of Lead Treatment 
 of Base Bullion The Electrolytic Extraction and Refining of Gold The 
 Electrolytic Refining of Silver The Electrolytic Extraction of Zinc 
 The Electro-reduction of Antimony The Electrolytic Extraction and 
 Refining of Iron Recovery of Tin from Tin Scrap Electro-smelting 
 The Electric Furnaces of Siemens, Cowles, and others The Electrolysis 
 of Fused Compounds The Electric Smelting of Aluminium, Magnesium, 
 and Sodium The Electro-thermal Production and Refining of Iron- 
 Electrical v. Chemical Methods Advantages of the Electric Furnace 
 Energy required Types of Electric Furnace The Furnaces of Heroult, 
 Keller, Girod, Stassano, Kjellin, and Rochling and Rodenhauser Electric 
 Welding and Annealing, . . . ... Pages 267-317 
 
 CHAPTER XVI. 
 
 THE RECOVERY OF CERTAIN METALS FROM THEIR SOLUTIONS 
 OR FROM WASTE SUBSTANCES. 
 
 Sketch of the Treatment of Residues of Cobalt, Copper, Gold, Lead, Mercury, 
 Nickel, Platinum, and Silver, Pages 318-323 
 
 CHAPTER XVII. 
 
 THE DETERMINATION OF THE PROPORTION OF METAL IN CERTAIN 
 DEPOSITING SOLUTIONS. 
 
 Outline of Methods for conducting Electrolytic Quantitative Analysis, and 
 for determining Antimony, Cobalt, Copper, Gold, Lead, Nickel, Plati- 
 num, and Silver, Pages 324-328 
 
CONTENTS. XV 
 
 CHAPTER XVIII. 
 POWER REQUIRED FOR ELECTROLYTIC WORK. 
 
 Calculation of Power required for Electrolytic Work Horse-Power required 
 per Unit Area of Electrode Surface for different Solutions and different 
 Metals Board of Trade Units of Electrical Energy (Kilowatt-hours) 
 required for the Deposition of Various Metals from different Solutions 
 Cost of Electricity generated from Batteries and from Dynamos Cost of 
 Steam Power and of Water Power on the large Scale Absorption of 
 Power in Conductors Loss of Electrical Energy by Transformation into 
 Heat in Conductors of different Materials and Sizes, . Pages 329-337 
 
 CHAPTER XIX. 
 
 MODERN THEORIES OF ELECTROLYSIS. 
 
 Development of the Modern Theories of Electrolysis Position of the Theory 
 of Ionic Dissociation The Theories of Solution Pressure and of Osmotic 
 Pressure Analogy between the Laws governing Gaseous Pressure and 
 Osmotic Pressure in Dilute Solutions Behaviour of Electrolytes in 
 respect to Osmotic Pressure The Theory of Dissociation of Electrolytes 
 into Ions when in Solution The Explanation of Electrolytic Conduction 
 in the Light of the Theory of lonisation The Meaning of Electrolytic 
 Solution Pressure The Application of Modern Theories to the Explana- 
 tion of the Simple Exchange of Metals, of Simple Galvanic Cells, of 
 Two-fluid Cells, of the Current produced when one Piece of Metal is 
 immersed in a non-homogeneous Solution, and of Electrolysis with 
 Insoluble and Soluble Anodes The Effect of Secondary Actions in 
 Electrolysis The Electrolysis of Mixed Solutions and Double Salts, 
 and of the Salts of Complex Acids, Potassium Aurocyanide and Silver 
 Cyanide, etc. The Effect of Unequal Rates of Migration of Ions in 
 Solutions on the Relative Concentration of the Electrolytes at the two 
 Electrodes Conditions of Electrolysis, . . j . Pages 338-362 
 
 CHAPTER XX. 
 
 A GLOSSARY OF SUBSTANCES COMMONLY EMPLOYED IN 
 ELECTRO-METALLURGY. 
 
 Rules to be observed in dealing with Acids and other Chemical Reagents 
 Glossary of Common Substances, . . . Pages 363-389 
 
 ADDENDA. 
 
 Various useful Tables The Bronzing of Copper and Brass Surfaces Anti- 
 dotes to Poisons, Pages 390-404 
 
 INDEX, Page 405 
 
A TREATISE 
 
 ON 
 
 ELECTRO-METALLURGY. 
 
 CHAPTER I. 
 
 INTRODUCTORY AND HISTORICAL. 
 
 THE word metallurgy is understood to mean the art of working- 
 metals extracting them from their ores and preparing them for 
 application to the varied uses of daily life. By analogy the 
 term electro -metallurgy, originally suggested by Smee, might 
 reasonably be expected to imply such extraction and preparation 
 effected with the aid of electricity. This, however, is, strictly 
 speaking, but one section of the subject, and, indeed, regarded 
 from the standpoint of commercial practicability, it is one of the 
 most recent developments of the art ; for the economical applica- 
 tion of electricity to the recovery of metals from their ores by 
 electrolysis was scarcely possible commercially until the invention 
 of the dynamo-electric machine had placed a cheap source of 
 electric energy at the disposal of the metallurgist. Just as the 
 science of metallurgy also is but a branch of that of chemistry, 
 and becomes elevated from an art to a science in proportion 
 as the laws of chemistry are made to regulate its processes, so 
 the science of electro-metallurgy is dependent on the laws of 
 chemistry and electricity, and will make more rapid progress 
 as the accurate study and application of these laws are made to 
 take the place of the 'rule of thumb' methods, which are the 
 inevitable outcome of the tentative experiments made in the early 
 dawn of an art. 
 
 Accepting, then, the broader use of the term, electro-metallurgy 
 may be denned as the science which treats of the application of 
 electrical methods to the separation or to the solution of metals 
 from substances containing them, and (perhaps we may add) to 
 the treatment of metals for certain specific purposes in the arts. 
 
 1 
 
2 INTRODUCTORY AND HISTORICAL. 
 
 Scope. Thus the electro-metallurgist may be called upon to 
 deposit metals with any of the following objects: (1) To obtain 
 a coherent and removable deposit on a mould, the form of which 
 it is desired to reproduce with accuracy ; this process is termed 
 electrotyping : (2) To obtain a thin, but perfect and adhesive, 
 film upon a metal of different character, in order to impart to 
 it acid- or air-resisting, or aesthetic properties, in which it was 
 naturally deficient ; this is known as electro-plating : (3) To 
 obtain the whole of a given metal from a substance containing it, 
 the method being used as a substitute for extraction by smelt- 
 ing, or for analytical or refining purposes. It will be evident 
 that in the first two of these, the interest centres in securing the 
 exact condition of deposit which is best suited to the work in 
 hand ; whilst in the third, it is of paramount importance that the 
 metal shall be completely separated, leaving no residue in the 
 material from which it was to be extracted. Finally (4), he may 
 be required to dissolve metals, either to remove an existing coat 
 of one metal from the surface of another, or to effect the complete 
 or partial solution of a homogeneous body superficially, as in the 
 case of electro-etching. 
 
 Early History. The history of the art is interesting, but 
 perhaps too much involved to render anything more than the 
 following brief sketch of value to the readers of this volume. 
 
 The fact that certain metals become superficially coated with 
 other metals when plunged into suitable solutions was known to 
 the ancients, and such a covering of iron swords and shields with 
 copper by immersion in copper solutions was described by the 
 Greek historian Zosimus in the fifth century. Paracelsus, who 
 lived in the beginning of the sixteenth century (1493-1541), 
 ascribed the apparent change of iron into copper, when dipped 
 into the blue waters of Schmollnitz in Hungary, to an actual 
 transmutation of metals, a view which found favour even at a 
 much later period. But although these may be considered as the 
 beginnings of electro-metallurgy on the chemical side, it was not 
 until the lapse of two centuries and a half from the latter date 
 that the application of electricity to the deposition of metals 
 became possible. Let us then glance at the gradual growth of 
 electrical knowledge and its adaptation to the requirements of 
 4 electro-deposition.' Such a retrospect cannot embrace any long 
 term of years ; for, although the attractive force of rubbed amber 
 was known to the ancients, it awoke only a wondering interest 
 until 1 647, when Otto von Guericke first constructed a machine 
 which exhibited the phenomenon in an intensified degree ; the un- 
 known force received the name electricity (from electron = amber), 
 electrical machines were gradually improved, and in 1752 Franklin 
 demonstrated the identity of the electric spark with the lightning 
 flash. But, in spite of the marvellous disruptive effect of these 
 'frictional' machines, the actual quantity of electricity which 
 
INVENTION OF THE ELECTRIC BATTERY. 3 
 
 could thus be generated was very minute, and could not avail 
 for the deposition of metals from solutions. Its destructive 
 power was derived from the enormous potential or ' pressure ' at 
 which it acted, and no electrolytic effect could possibly have 
 been observed except by a most careful experimenter actually 
 searching for such a manifestation. 
 
 In the year 1786 Galvani made his celebrated discovery that 
 a metal wire at one end touching the lumbar nerves of a recently- 
 killed frog, and at the other the muscles of its leg or thigh, 
 caused a rapid muscular contraction. Finding the same pheno- 
 menon producible with the aid of a frictional machine, he was led 
 to connect the two incidents, and to ascribe the former to the 
 action of electricity resident in the animal itself. Volta, on the 
 contrary, finding as, indeed, Galvani had done before him 
 that if two wires of different metals touched a single muscle 
 the result was similar, concluded that the electrical energy was 
 due rather to the action of the wires than to any property in- 
 herent in the animal tissue. Led on by this assumed production 
 of electricity by contact of dissimilar metals, he constructed in 
 1799, and described in 1800, the series of zinc and copper discs, 
 separated by moist cloth, which bears the name of the Voltaic 
 Pile. With this pile, for the first time, large currents, compared 
 with those from frictional machines, though of low potential, 
 were obtainable, such as might be used to produce electrolysis. 
 Subsequently, Fabroni in Italy, and Wollaston, Davy, and others 
 in England, showed that corrosion, oxidation, or ' rusting ' of the 
 zinc invariably attended the production of electricity in this way, 
 and ascribed the latter to chemical action. 
 
 First Electrical Battery. In 1800 Volta replaced the pile 
 by his 'crown of cups,' in which each pair of copper and zinc 
 plates was separated, not by damp cloth but by salt water placed 
 in a series of vessels, the copper of each intermediate vessel being 
 connected by a wire with the zinc of the next, leaving a free or 
 unattached copper plate at one end of the series and a corre- 
 sponding zinc plate at the other, these terminal plates being, of 
 course, equivalent to those of the 'pile. 5 This, then, was the 
 original electric battery, the discovery of which has led to the 
 invention of the art of electro-metallurgy. 
 
 Separation of Metals. In the same year Nicholson and Carlisle 
 succeeded in decomposing water, or, in other words, depositing 
 hydrogen, by means of the source of electricity thus placed at 
 their disposal ; and in 1803, Cruickshank, of Woolwich, con- 
 structed a large battery of considerable power, with which he 
 deposited, or ' revived,' as he termed it, many metals from their 
 solutions, and even proposed the use of an electrolysing current in 
 quantitative chemical analysis. Meanwhile, in 1801, Wollaston 
 had obtained a coating of copper on silver, sufficiently adherent 
 to allow of burnishing, by introducing the latter metal, in contact 
 
4 INTRODUCTORY AND HISTORICAL. 
 
 with one more oxidisable, into a solution of copper, thus forming 
 a small electric battery in the depositing liquid itself. 
 
 In the Philosophical Magazine, Brugnatelli, in 1805, described 
 the gilding of two large silver medals by means of the Voltaic 
 pile and a solution of * ammoniuret of gold,' and also the silver- 
 ing of platinum surfaces, at the same time directing attention to 
 the gradual solution of the plate through which the electric 
 current entered the liquids. Then Davy, in 1807, made his grand 
 discovery of the alkali-metals, potassium and sodium, by electro- 
 lytic isolation. 
 
 Magneto- and Dynamo-Electric Machines. The knowledge of 
 the relation between electricity and magnetism gained in 1820, 
 both by Oersted's researches on the action of the electric current 
 upon a compass-needle, and by the success of Arago in magnetising 
 a steel needle by means of the current, may perhaps be regarded 
 as the primary step towards the invention of magneto-electric 
 machines, the first of which was constructed by Faraday in 1831 ; 
 it consisted of a copper disc rotated between the poles of a 
 horse-shoe magnet, with the necessary fittings for taking off the 
 current thus generated. In the same year Faraday observed the 
 mutual action of electric currents, and the conditions governing 
 the formation of induced currents, and thus, as it were, paved 
 the way for the subsequent invention of the dynamo-electric 
 machine. Faraday's magneto-electric machine was not suffi- 
 ciently powerful to have any practical value, but in the following 
 year Pixii produced a machine of this character ; and this may 
 perhaps be regarded as the prototype from which the subsequent 
 generators of this class were evolved. 
 
 Thermopile. The thermopile, another source of electrical 
 energy which has been more or less largely used in electro- 
 metallurgical work, especially abroad, owes its origin to Seebeck's 
 observations in 1821 or 1822, that a current is produced by heat- 
 ing a compound bar of bismuth and copper at the junction of the 
 metals, provided that the free ends are connected by a metallic 
 wire. 
 
 Ohm's Law. In 1827, Ohm enunciated his great fundamental 
 law, which governs all electrical work, formulating, as it does, 
 the relation between strength or volume of current, electrical 
 'pressure,' and the resistance of bodies to the passage of the 
 current. Seven years later, Faraday demonstrated the relation 
 between the strength of the current and the amount of any 
 metal electrolytically deposited by it, and proved that the 
 quantity of electricity flowing in a given circuit could be 
 measured by the amount of metal which it could deposit in a 
 known period of time. It is by the systematic and intelligent 
 application of these laws that the electro-metallurgist of to-day 
 is able to arrange his plant with scientific accuracy, instead of by 
 mere rule of thumb. 
 
EARLY EXPERIMENTS IN ELECTROLYSIS. 5 
 
 Copper-coating by Bessemer. In 1831, Bessemer had coated 
 articles composed of an alloy of lead, tin, iron, and antimony 
 with a film of copper by simple immersion in a solution of a 
 copper salt ; but finding, as he describes in a letter published by 
 Watt and Philip (Electro-Plating and Electro-Refininy, p. 88), that 
 the metal was not adherent, he tried for, and obtained, better 
 results by placing the objects on a copper, iron, or, better still, 
 zinc tray, and then sinking them in the liquid. In this way he 
 formed a small battery in situ, as we have seen Wollaston had 
 done in 1801. 
 
 Becquerel's Electrolysis Works. Becquerel, in 1836, was the 
 first actually to apply the principles of electrolysis to the treat- 
 ment of natural products for the recovery of the metal contained 
 in them. He even planned out works upon a commercial scale 
 for the treatment of complex minerals containing copper and 
 silver, but they were never erected, owing to the prohibitive 
 expenditure of battery zinc involved in the process. 
 
 Daniell Battery. In the same year a new era was started by 
 Daniell's introduction of his two-fluid battery, which placed a 
 very constant current at the disposal of the electro-metallurgist, 
 and almost immediately produced a ripe harvest of results. 
 De la Rue at once took the first unconscioiis step in the direc- 
 tion of electrotyping, when he observed that the copper, which 
 is deposited in the cells of the Daniell battery whilst in use, 
 exactly reproduces upon its surface every line or scratch upon 
 the copper plate on which it forms. Intent, however, on 
 other objects, he failed to follow up the line of research thus 
 indicated. 
 
 Elkington's Process. In 1838, the Patent Office Records show 
 that Elkington, who had two years previously protected a process 
 of gilding for copper or brass objects by simple immersion in a 
 solution containing gold, produced a method of zinc plating, 
 analogous in principle to that of Wollaston's, by which the 
 copper, brass, or iron to be coated was immersed in contact with 
 a more oxidisable metal in a solution of that which it was 
 desired to deposit, thus forming a galvanic cell in the depositing 
 bath itself ; and so for the first time the deposition of one metal 
 upon another, through the galvanic action produced by the solu- 
 tion of a third, became the subject of a patent specification. 
 
 Earliest E lectroty per s Their Rival Claims. De la Rue, as 
 we have just seen, had already, in 1836, indicated the possibility 
 of copying uneven surfaces by electro-deposition, but had missed 
 the practical application of his discovery. But three years later 
 three individuals almost simultaneously, and it would seem 
 quite independently, publicly described processes of electro- 
 typing. These three were Jacobi of St Petersburg, and Spencer 
 and Jordan in England. The tale of these rival inventors has 
 often been told ; it is briefly as follows : Professor Jacobi 
 
6 INTRODUCTORY AND HISTORICAL. 
 
 published a method of converting into relief, by galvanic means, 
 even the finest lines engraved upon a copper plate, thus pro- 
 ducing a printing surface suitable to the requirements of the 
 printer. An account of this process found its way into the 
 pages of the Athenaeum on May 4, 1839. On the 8th of May 
 following, Spencer gave notice to the Liverpool Polytechnic 
 Institution of his intention to read a paper before that body on 
 the * electrotype process ' ; but this paper was not read until 
 September of the same year. Meanwhile, however, the account 
 of Jacobi's discovery had been copied into the London Mechanic's 
 Magazine, and had called forth a letter from Jordan, dated 
 May 22nd, but not published until June 8th, in which he described 
 his experiments in the same field, which were begun in the 
 summer of 1838. He clearly set forth here the method which 
 has since been known as the single-cell process of electrotyping, 
 and claimed the possibility of multiplying engraved plates, 
 typographical matter or medals, by forming galvano-plastic 
 matrices on the object, and using the ' negative ' copy thus ob- 
 tained to reproduce the original form ; and he even suggested 
 making tubes by depositing copper around a wire or metallic 
 core which could subsequently be removed. 
 
 Strangely enough, neither of these accounts received public 
 attention, and the matter remained unnoticed until the end of 
 September, when Spencer's paper was read. This paper is 
 especially interesting, because it shows how the process of 
 electrotyping was gradually developed in his hands, mainly by 
 an attentive and patient examination into the causes of a series 
 of apparently minor phenomena observed in September 1837, 
 while experimenting with a single voltaic cell, consisting of a 
 copper plate in copper sulphate solution connected by copper 
 wire to a zinc plate immersed in a solution of common salt. 
 The starting-point on the road to the new discovery was the 
 observation that certain spots on the copper plate, which had 
 accidentally been overlaid with molten sealing-wax, received no 
 metallic deposit when placed in the cell ; and he was thence led 
 to attempt the formation of designs in relief, for use in the 
 printing press, by coating a copper plate with an insulating 
 varnish and then tracing the desired pattern by scratching the 
 varnish completely away at the required points, and finally 
 building up a deposit of copper upon the portions of the metallic 
 plate thus exposed. While experimenting in this direction, he 
 made the important observation that the nature of the deposited 
 copper was dependent on the degree of ' intensity of the electro- 
 chemical action,' or, as we should say, on the current-density, 
 high densities giving rapidly deposited but highly crystalline 
 and friable metal. He now met with difficulties, which, however, 
 were not insurmountable, and even led to further triumphs ; he 
 found that the deposited copper would adhere perfectly only to 
 
RIVAL RESEARCHES IN ELECTROTYPING. 7 
 
 an absolutely clean surface of the same metal. Thus, when he 
 required to obtain perfect adhesion between the metals, he first 
 cleaned the copper surface with nitric acid, whereas if he wished 
 afterwards to separate the deposited metal from the original plate 
 he coated the latter with the thinnest possible film of bees' wax, 
 previous to exposing it in the battery-cell. The subsequent 
 application of a gentle heat enabled the two plates to be separated 
 with the greatest facility. Next, when using a penny-piece 
 instead of a copper plate, he observed that the inner surface of 
 the copper sheath with which it had been coated bore a perfectly 
 sharp copy, in intaglio, of the image and letters which were in 
 relief on the coin itself ; in this way he obtained matrices from 
 which the original could be faithfully reproduced. Now, as- 
 certaining that copper would deposit as readily upon lead as upon 
 itself, he secured exact copies of coins, of set-up type, or even of 
 wood-blocks, by pressing them upon sheets of lead and depositing 
 copper upon the indented lead matrix so prepared. And finally 
 he found that clay, plaster of Paris, wood, or other non-conducting 
 materials could be covered electrolytically with copper, if they 
 were first coated with a conductive film of bronze-powder or 
 gold-leaf. 
 
 It would appear hopeless to determine the question of real 
 priority between the three inventors. Jacobi seems to have 
 been the first to publish an account of his researches, and so 
 far his claim is good ; Spencer next declared his intentions to 
 describe his experiments, but was forestalled by Jordan ; on the 
 other hand, Spencer claims to be the earliest experimenter in 
 the field, and his investigations appear to have been deeper 
 and more fully developed than those of either Jacobi or Jordan. 
 It is doubtless one of those frequently recurring instances, 
 wherein the progress of knowledge has led several men to a 
 simultaneous but independent development of the same line of 
 thought ; and in such cases credit must be ascribed to each, but 
 the palm awarded to the most thorough and painstaking. 
 
 Murray's Blackleading Process. The immediate result of 
 Spencer's paper was the creation of a sudden mania for electro- 
 typing, the simple and inexpensive character of the necessary 
 apparatus enabling amateurs of all grades to vie with the fresh 
 race of operatives which sprang up at the birth of a new industry. 
 Evidence of this is to be seen in the rapidly increasing number 
 of patents which were now applied for in this branch of the arts. 
 With so many workers in the same field, it would indeed be 
 strange if the scope of the work were not quickly and widely 
 enlarged, and existing processes much improved; the year 1840 
 was accordingly destined to see many improvements effected. 
 The application of the art to the requirements of the printer was 
 in this year made practicable by Murray's discovery that moulds 
 of non-conducting material could be made to take the deposit of 
 
8 INTRODUCTORY AND HISTORICAL. 
 
 copper by brushing them over with plumbago, so that metallic 
 moulds were no longer essential. In the same year the first- 
 published newspaper print from an electrotype block is believed 
 by Smee to have appeared in the London Journal. Nevertheless, 
 Savage's Dictionary of Printing, which appeared in the following 
 year, although it contained many good engravings from electro- 
 types, exhibited a page of * diamond ' type, also printed from an 
 electro-deposited block, but so imperfect that, as Wilson has 
 suggested, we may infer that the art of electrotypirig formes of 
 small type had not yet attained sufficient excellence to warrant 
 its general application to this purpose. 
 
 In 1840, Mason endeavoured to utilise the current generated 
 by the single-cell electrotyping arrangement in a second depositing 
 cell, and thus to carry on two operations simultaneously ; and 
 although this method was not practically adopted, it nevertheless 
 pointed to the possibility of applying a separate current from 
 sources other than Darnell's battery. 
 
 Cyanide Baths. In this year Wright, after experimenting 
 with many solutions, discovered the use of the cyanide bath for 
 the production of thick deposits of gold and silver, in place of 
 the thin films obtainable by simple immersion. The invention 
 was patented and at once put in operation by the Messrs 
 Elkington, who were foremost in this field at the time. At the 
 end of the same year, de Ruolz patented in France the use of 
 similar solutions, not only for gold and silver, but for platinum, 
 copper, lead, tin, cobalt, nickel, and zinc. The following year 
 witnessed the publication of a very complete work on electro- 
 metallurgy by Smee, and this was followed by several others in 
 rapid succession. 
 
 Elastic Moulds. Leeson, in 1842, greatly advanced the appli- 
 cation of electrotyping to the reproduction of works of art by 
 the use of elastic moulds made of glue and gurn, which thus 
 enabled objects of intricate or undercut design to be faithfully 
 copied and indefinitely multiplied ; and by the insertion of 
 leading wires in the mould to distribute the current more uni- 
 formly, and hence, to facilitate a higher degree of simultaneity 
 in the deposition. In the following year Montgomery proposed 
 the application of gutta-percha as a moulding medium for slightly 
 undercut objects. 
 
 Bright Silver. From that time the inventions for many 
 years, although numerous enough, had not sufficient novelty to 
 render them worth recording in detail, excepting, perhaps, the 
 important discovery by Mil ward, in 1847, that the addition of a 
 small quantity of carbon bisulphide to the silver plating baths 
 caused the deposited silver to show no longer a dead or frosted 
 surface, but to exhibit greater lustre and brilliancy. 
 
 Cheap Sources of Electricity. In 1842 and 1843 respectively, 
 Woolrich patented the use of magneto-electric machines, and 
 
LATER PROGRESS IN THE ART. 9 
 
 Poole, that of thermopiles, for depositing metals ; but neither of 
 these seem to have been at the time successfully applied in 
 practice. But the introduction of Pacinotti's dynamo-electric 
 machine, first described by him in II Nuovo Cimento in 1864, of 
 Wilde's magneto-electric machines in the following year, of Siemens' 
 and Wheatstone's more perfect dynamos, simultaneously invented 
 in 1867, and Gramme's machine in 1870, profoundly modified the 
 scope of the art by affording a far cheaper source of electricity 
 than had hitherto been possible. From this time, with the more 
 careful study of the theory of the dynamo, and the consequent 
 improvements in its mechanical and electrical efficiency, there 
 have arisen a host of new machines, constructed especially to 
 satisfy certain specific objects, and approaching much nearer to 
 perfection than did their original progenitors. Dynamo-electric 
 machines are now made to suit the needs of the electro- 
 metallurgist, and thus new fields of labour have been opened, 
 more particularly in the domain of metal refining and smelting ; 
 and the readiness with which mechanical energy may now be 
 converted into electrical, renders the utilisation of natural waste 
 water-power thoroughly applicable to these purposes. 
 
 Ore-Treatment by Electricity. The later history of our sub- 
 ject is, in its more important branches, intimately associated 
 with the application of dynamo-electric machinery, and of 
 powerful currents to the treatment of ores, furnace-products, or 
 solutions. Becquerel, as we have seen, failed practically to apply 
 his process for the treatment of ores on account of the expense of 
 the zinc; it therefore remained dormant until 1868, when it was 
 tried in San Francisco by Wolfe and Pioche, who seem, however, 
 to have effected but little. Marchese's method of treating copper 
 mattes (impure fused copper sulphide), by using them as anodes, 
 started a new epoch in 1882, and many modifications of this and 
 analogous processes have since been carried into practice. How- 
 far this type of ore-treatment may be able to compete with the 
 older smelting methods, can only be considered in connection 
 with the particular circumstances of each individual case, and will 
 be more fully dealt with later. 
 
 Up to the present time, the direct treatment of ores and mattes 
 by electrolysis has not proved successful, chiefly on account of the 
 large proportion of sulphur and other insoluble material which has 
 to be separated. This soon contaminates the bath to an excessive 
 extent, and, moreover, the dissolution of the anode becomes very 
 irregular. The difficulties, however, are probably not insuperable, 
 inasmuch as they are industrial rather than scientific. The case, 
 indeed, forms an apt illustration of the necessity to make long 
 and careful experiments on a fairly large scale before embarking 
 in a costly electro-metallurgical enterprise, which must stand or 
 fall by its financial possibilities. The electrolytic extraction of 
 the copper from matte is quite possible, and may be effected V 
 
10 INTRODUCTORY AND HISTORICAL. 
 
 readily in the laboratory ; and it may proceed satisfactorily even 
 on the large scale for a time, but on prolonged use the processes 
 which have been tried have usually been found to be too costly to 
 allow of their competing with the improved methods introduced 
 by modern chemical or metallurgical science. It is therefore 
 customary to prepare an impure copper by metallurgical means, 
 and then to refine this copper electrolytically. 
 
 Metal Refining. The refining of copper by electrolysis is now 
 one of the most important applications of electro-metallurgy. It 
 is practically, however, a modification of the process of electro- 
 typing, and the two arts, therefore, up to a certain point, have a 
 common history. The earliest process of practical importance 
 was that of Elkington, patented in 1865. In principle it is the 
 same as that used at the present time, and is specially interesting 
 as being the first to employ the dynamo for the purpose. Among 
 those who have applied this process on a large scale are Siemens 
 and Halske and Borchers in Germany, and Thofehrn in America ; 
 and the magnitude of the industry may be gauged by the fact 
 that in one works alone, the Raritan Copper Works (U.S.A.), the 
 capacity of the plant is 3000 kilowatts, capable of an output of 
 200 ' short ' tons per day, while that of the Anaconda Company 
 (U.S.A.) has a capacity of 1790 kilowatts, and is capable of 
 producing 150 tons of refined copper per diem. The electrolytic 
 refining of other metals has been attempted with varying success, 
 but in few cases has it succeeded in displacing the older methods. 
 The Moebius process for treating impure silver in an electrolyte 
 of silver nitrate has been largely employed in America and 
 Germany. The commercial refining of nickel by electrolysis is 
 being conducted on a relatively small scale, and the same may be 
 said of zinc, but the inherent difficulties in the electrolysis of the 
 last-named metal have prevented any very great development up 
 to the present time. 
 
 Gold has been deposited from solutions obtained by the action 
 of dilute potassium cyanide liquors on gold ores, and this process, 
 the electrolytic part of which is due to Siemens and Halske, is 
 now a recognised method of gold recovery largely used for the 
 treatment of poor ores and 'tailings' (ores which have already 
 been treated by another process). 
 
 But in no case has the electric refining of metals been so 
 successful as in the case of copper. This, no doubt, is largely due 
 to several contributory causes. In the first place, the solution 
 required is simple, cheap, and easily managed ; secondly, the 
 precious metals contained in the copper are recovered practically 
 completely as by-products ; thirdly, the rate of deposition is com- 
 paratively rapid, so that the amount of capital lying idle is not 
 excessive, though much larger than is desirable ; fourthly, the 
 copper may be deposited in a reguliiie condition and almost 
 absolutely pure ; and, lastly, there is a very great and ever- 
 
ELECTROLYSIS OF FUSED SUBSTANCES. 11 
 
 ^ 
 
 increasing demand for pure copper for conductors for electrical 
 purposes. In addition to these a negative cause may perhaps be 
 found in the relative difficulty and expense involved in obtaining 
 equally pure copper by metallurgical means. 
 
 The industrial questions in connection with these processes will 
 be touched upon in Chapter XV. 
 
 Electrolysis of Fused Substances. In another direction, 
 electricity has been applied to the extraction of metals by the 
 passage of a current through a fused salt ; it was in this way, in 
 1854, that Bunsen and Deville reduced aluminium from its com- 
 bination with chlorine, and recently many arrangements purport- 
 ing to effect a similar result have appeared in the records of the 
 Patent Office. Electrolytic processes have entirely replaced the 
 older metallurgical methods of extracting aluminium, with the 
 result that the price of the metal fell in twenty-three years from 
 <! to Is. 3Jd. per pound (in 1901). During recent years the 
 price has sometimes risen considerably. The processes of Hall 
 (patented in 1886), Heroult (1887), and others, now operating so 
 successfully, pass a current of electricity through a bath of a fused 
 salt (usually a double fluoride of aluminium and an alkali metal), 
 in which pure oxide of aluminium is dissolved. The bath is main- 
 tained at a red heat without external firing by the passage of the 
 electric current through it, and the alumina is decomposed into 
 aluminium, which is collected, and oxygen, which combines with 
 the carbon block through which the current is passed into the 
 bath, forming gaseous oxides of carbon which escape into the air. 
 The alumina is replaced from time to time as it is decomposed, so 
 that the process is strictly analogous to the electrolysis of an 
 aqueous solution of copper sulphate with an insoluble anode and 
 a copper kathode. In the same way metallic sodium is now 
 almost entirely prepared by electrolysis, Castner's process 
 (patented in 1890) for the electrolysis of fused caustic soda being 
 largely employed in Great Britain, America and Germany, whilst 
 other processes for the electrolysis of melted sodium chloride are \ 
 also in use. A proposal by Pichou, in 1853, to reduce an ore 
 admixed with a small percentage of charcoal in the electric 
 arc passing between two large electrodes, found a later develop- 
 ment in the electric furnace of the brothers Cowles, patented 
 in 1885, for the treatment of ores containing aluminium and 
 other metals. 
 
 Latest Advances. Among the more interesting modern de- 
 velopments in the field of electro-metallurgy are those in which 
 electricity is employed merely as a heating agent, and not in 
 electrolysis. One of the earliest experiments on a small scale in 
 this direction was made by Depretz in 1849 ; Pichou used his 
 electric furnace for smelting in 1853, and Siemens elaborated a 
 furnace which was used on a comparatively large experimental 
 scale in 1880. In all these the heat of the electric arc was used. 
 
12 INTRODUCTORY AND HISTORICAL. 
 
 Another type of furnace in which the heat was obtained by the 
 incandescence of a rod of carbon, due to the resistance interposed 
 by it in the circuit of a powerful electric current, was successfully 
 introduced by Borchers in 1880. Since then, the electric furnace 
 has been developed in two directions, first in purely scientific work, 
 where, in the hands of Siemens, Moissan and others, it has been 
 the means of melting metals hitherto regarded as infusible, and 
 of studying the properties of such materials after fusion and 
 solidification ; secondly, it has been extended in size and capacity, 
 and has become a valuable industrial agent, as, for example, in the 
 manufacture of calcium carbide for use in acetylene generators 
 and as a metallurgical reagent for the production of steel, and 
 also in the production of phosphorus. 
 
 Nearly allied to this is the use of the current for electric weld- 
 ing and for the local annealing of hard steel plates and the like. 
 Here the metal is raised to a welding temperature mainly by the 
 use of the arc, althoiigh in the Lagrange-Hoho and Burton electric 
 forges the heat is largely generated by the passage of the current 
 through a very high resistance, aided in the last-named case by 
 the combustion of the hydrogen set free by the electrolysis of the 
 water in which the pieces are immersed. Several systems of 
 electric welding are now in use, and although they cannot com- 
 pete with the smith's forge for ordinary work, they have found 
 many applications for special purposes. 
 
 The applications of electrolysis to the manufacture of alkali, 
 bleaching powder, chlorate of potash, and in the field of organic 
 chemistry are now increasingly numerous, but these belong rather 
 to the sphere of electro-chemistry than to that of electro-metal- 
 lurgy, and need not be treated of in this work. 
 
 Recent Developments of the Theory of Electrolysis. In the 
 theory of the subject great changes have been wrought of late 
 years, and the theory evolved by Grotthus, in 1805, and universally 
 accepted until 1887, has since then rapidly lost ground, and is now- 
 replaced by explanations based on the investigations of modern 
 electro-chemists. The work of Van't Hoff on the osmotic pressure 
 of substances dissolved in liquid solvents was published in 1887, 
 and showed that, at least in dilute solutions, the dissolved material 
 was subject to laws similar to those governing the pressure and 
 volume of gases. This led to Arrhenius, in the same year, advanc- 
 ing the theory of the dissociation of electrolytes, when in solution, 
 into separate ions, each carrying a charge either of positive or of 
 negative electricity ; and this theory, with the earlier discovery by 
 Hittorf (in 1853) of the actual movement, or migration, of the 
 ions from one electrode to the other, forms the basis of the modern 
 explanations of which a short elementary account is attempted in 
 ^ Chapter XIX. 
 
 Conclusion. The growth of the art on the whole has been 
 rapidly progressive ; a few processes may have been superseded 
 
GROWTH OF THE ART. 13 
 
 by furnace-methods, which have proved to be less costly, but the 
 various branches have for the most part steadily gained ground as 
 the work became more reliable and more economically conducted, 
 until the electrotyper and the electroplater in gold, silver, and 
 nickel occupy a quite important position among the manufac- \ 
 turers of the world ; whilst the electro-refiner and the electro- 
 smelter, if one may use the terms, have, in certain branches of 
 industry, competed successfully with the chemist and metallurgist, 
 and in some few cases have even obtained a monopoly of the work. 
 
CHAPTER II. 
 
 THEORETICAL AND GENERAL. 
 
 (See also Chapter XIX.) 
 
 IT has already been hinted that a right understanding of all the 
 problems involved in the science of electro-metallurgy demands an 
 acquaintance, not only with the manner in which certain forces act 
 upon matter, but with the constitution of matter itself. A brief 
 review, therefore, of a few of the fundamental laws and theories of 
 electrical and chemical science naturally finds a place at this point ; 
 although for a full explanation of these subjects reference must, 
 of course, be made to the text-books devoted specially to them. 
 
 Matter Force. It must be clearly understood, then, that 
 matter (that is, anything which possesses mass) is variously con- 
 stituted. It is within our common experience that different 
 kinds of matter exhibit different properties and characteristics, 
 or, as we say, are made of different materials ; and it is the 
 object of chemical science to teach us what the materials are and 
 how they behave when brought into contact with one another. 
 Force has no material constitution, and therefore no weight, and 
 is only made known to us by its action on matter. Physical (or 
 mechanical) forces, as they are termed, may affect the relative 
 position or the outward shape and appearance of material sub- 
 stances, but chemical forces affect the very ingredients of which 
 the substance is composed, and govern the more intimate mutual 
 Relationship between different bodies. 
 
 Conditions of Matter. We are conscious that various kinds 
 of matter may exist at different times in certain distinct forms 
 solid, liquid, and gaseous but these are solely physical, not 
 chemical, differences (the constitution or component parts of the 
 substance remaining unchanged throughout), and are brought 
 about by physical means, such as alteration of heat or pressure, so 
 that a return to the original conditions is accompanied by a repro- 
 duction of the body in its first form. For example pure water 
 at 15 C. is a liquid, but cooled to C. it becomes solid ice, or 
 heated to 100 C. it is converted into gaseous steam ; nevertheless 
 if the ice and the steam be respectively brought back to 15 C. 
 
 14 
 
CONSTITUTION OF MATTER. 15 
 
 they will again form a liquid quite undistinguishable from that 
 originally experimented with. We may imagine, then, that water 
 is made up of a vast number of almost infinitesimal particles, all 
 of which are alike and are rapidly vibrating to and fro ; and that 
 in ice they are so packed together with shorter paths of vibration 
 that they will not readily separate, thus causing solidity ; but 
 that when heat is applied to them, each particle vibrates through 
 a longer distance, and the different units are farther apart and 
 more free to move among themselves, so that they present the 
 mobile characteristics of a liquid ; while above the boiling-point 
 the freedom is so great that they actually move away as a gas. 
 
 Constitution of Matter. If now we imagine these minute 
 similar particles to be so small that further subdivision by 
 physical means is impossible, we are figuring to ourselves those 
 penultimate particles which, in the language of the atomic 
 theory, are termed molecules. It is with the molecule that the 
 physicist has to deal, and it is on the molecule that physical 
 forces act ; but the chemist is able to break up each molecule into 
 a certain limited number of smaller particles, which are supposed 
 by this theory to be indivisible, and are hence called atoms (from 
 the Greek a = not, temno = I cut). Any molecule may contain two, 
 or a larger number, of these atoms, and the atoms themselves may 
 be similar or dissimilar ; there are, indeed, two classes of bodies, 
 the first of which includes those molecules in which all the atoms 
 are alike, while the other group includes those whose molecules 
 are made up of unlike atoms, and the number of these bodies is 
 almost unlimited, because of the endless combinations possible 
 between different varieties of atoms. In the first class both mole- 
 cules and atoms are of the same ultimate material, so that such a 
 substance can contain but one kind of matter, and is hence termed 
 an element. In the other group, although the molecules of any 
 substance are similar, the atoms are not; but each atom being 
 indivisible, and consisting of one body only, is an element, and, 
 therefore, the substance is said to be a compound of such and 
 such elements. Thus, if a molecule of water could be made to 
 yield its atoms, it would be found to contain two of a gaseous 
 element, hydrogen, and one of another gas, oxygen, each quite 
 unlike water, and unlike the other ; and if two molecules could 
 be so treated at the same time, the two liberated oxygen atoms 
 would, if kept apart from the hydrogen, unite to form a molecule 
 of oxygen, while the four hydrogen atoms would form two mole- 
 cules of hydrogen. 
 
 Atomic Weight. Now, if it were possible to isolate and weigh 
 a number of atoms, it would be found that all those of the same 
 element would possess equal weight, but that atoms of different 
 elements would have unlike weights ; and for all chemical and 
 electrolytical calculations this must be thoroughly understood. 
 Although the actual weighing of an atom is still an impossible 
 
16 THEORETICAL AND GENERAL. 
 
 feat, yet by studying the mutual relations of the elements the 
 chemist is able to estimate the relative, though not the absolute, 
 weights of different atoms. 
 
 Chemical Symbols and Formulas. Hydrogen, which is the 
 lightest substance known, being regarded as unity, the atomic 
 weight of each element is expressed as a multiple of that of 
 hydrogen. Thus, if an atom of hydrogen be regarded as weigh- 
 ing 1, it is found that an atom of oxygen will weigh 16. We do 
 not know in what unit of weight atomic weights are expressed, 
 because we cannot weigh a known number of atoms ; but what- 
 ever the unit may be we know that if an atom of hydrogen weighs 
 one such unit that of oxygen weighs 16. Since each molecule of 
 water has been shown to consist of two atoms of hydrogen com- 
 bined with one of oxygen, it is evident that it must contain 2 
 parts by weight of the former with 1 6 parts of the latter ; and as 
 all molecules of water are alike in composition, it follows that in 
 every 18 parts by weight of pure water there are 2 of hydrogen 
 and 1 6 of oxygen. 
 
 It should be clearly remembered that every true compound, no 
 matter how it is produced, not only contains always the same 
 elements, but contains them in the same proportions. Provided, 
 then, that we are able to determine the nature of the different 
 atoms present in a molecule of any substance, and the propor- 
 tion in which they are there, we can calculate with precision 
 the percentage of each element contained in it ; and, conversely, 
 if we know the proportionate weights of the constituents of a 
 given compound, we can at once determine the number of each 
 kind of atom in the molecule. Hence it is possible to assign a 
 definite chemical formula to every compound ; that of water 
 might be written ' 2 hydrogen : 1 oxygen,' implying that there 
 are two atoms of the former to one of the latter ; but in chemical 
 work to write down the names of the elements in full would be 
 both tiresome and clumsy, so that chemists are in the habit of 
 using a system of shorthand notation, which is both simpler and 
 more scientific. It consists in selecting a symbol, usually the 
 first letter, or the first with some specially suggestive subsequent 
 letter, of either the English or Latin name of the substance, to 
 represent each element ; and this is understood to stand for, not 
 an indefinite amount, but for 1 atom, and, therefore, so many 
 known parts by weight of the element. Thus * H ' implies 1 atom 
 or 1 part by weight of hydrogen; '0,' 1 atom or 16 parts by 
 weight of oxygen ; ' Fe ' (Ferrum), 1 atom or 56 parts of iron ; 
 * Sn ' (Stannum), 1 atom or 119 parts of tin ; and so on. By 
 such a system of notation, then, we are able to express both the 
 nature and the proportionate composition of any substance, and 
 so, for example, to ascribe the formula * H 2 ' to water. 
 
 It should now be evident that elements can combine with 
 others only in proportions which are multiples of their respective 
 
CHEMICAL COMBINATION. 17 
 
 atomic weights, because no fraction of an atom can enter into the 
 constitution of a molecule. But further than this, the propor- 
 tionate numbers of each kind of atom in any molecule are no 
 mere arbitrary or accidental figures, but are regulated by definite 
 laws ; and it is the fixedness of these laws which enables us to 
 predict the exact weight of any metal which should be deposited 
 by a given current in a known time. 
 
 Valency. We have seen that water contains 1 atom of oxygen 
 united with 2 of hydrogen ; but in hydrochloric acid (HC1) there 
 is 1 of chlorine, not with 2, but with only 1 of hydrogen, while 
 in ammonia (NH 3 ) we find 1 of nitrogen with 3 of hydrogen, 
 and in marsh gas (CH 4 ) 1 of carbon requires 4 of hydrogen. 
 Here we observe chlorine to be typical of a class of elements 
 which combine with hydrogen in equal atomic proportion, oxygen 
 typical of a class where the ratio is 1 : 2, nitrogen of one where 
 it is 1 : 3, and carbon of one where it is 1 : 4. The terms mono- 
 valent, divalent, trivalent, and tetravalent are applied to the 
 elements included in these classes respectively. The words 
 monatomic, diatomic, etc., are sometimes used, but those indicat- 
 ing valency are preferable. All elements, however, are not 
 capable of combining with hydrogen ; and to determine the 
 valency of these, it is simply necessary to ascertain how many 
 atoms of hydrogen are replaced in any compound by the element 
 in question. For example, common salt is a compound of the 
 metal sodium (Na) with the gas chlorine in the atomic propor- 
 tion of 1 : 1, the formula being NaCl. But 1 atom of chlorine 
 combines with 1 atom of hydrogen in hydrochloric acid, and, 
 therefore, 1 atom of sodium is equivalent to 1 atom of hydrogen, 
 inasmuch as each requires 1 atom of chlorine to combine with 
 it thus, sodium, like hydrogen and chlorine, is monovalent. 
 This equivalency is clearly seen by the following reaction : 
 
 Ichtrife"} added to (lsodiu m)yi e,d { { *- }a n d (1 hydrogen, ; 
 
 Hydrochloric acid ,, ,, ,, common salt ,, ,, 
 
 or putting it in form of an equation, 
 
 Again, in oxide of sodium, Na 2 0, 1 atom of oxygen combines 
 with 2 of the metal, just as it does with 2 of hydrogen ; and by 
 throwing metallic sodium into water, the latter is decomposed 
 and part of the hydrogen is liberated as a gas, while an equivalent 
 of sodium takes its place to form caustic soda. This exchange 
 is thus represented * : 
 
 Na + OH 2 NaOH + H 
 
 1 sodium added to 1 water yields 1 caustic soda and 1 hydrogen. 
 
 1 In writing equations, a figure written before u, group of symbols means 
 that all the elements symbolised up to the first stop are to be multiplied by 
 the number represented by that figure ; while a small figure below the line, to 
 
 2 
 
18 THEORETICAL AND GENERAL. 
 
 In both these instances also, therefore, we find evidence that 
 sodium replaces an equal number of hydrogen atoms, and is 
 monovalent. 
 
 The atom of the metal zinc (Zn) replaces 2 hydrogen atoms, as 
 is seen in the following equations representing the action of 
 hydrochloric acid and of water respectively upon metallic zinc : 
 
 Zn + H 2 = ZnO + H 2 
 
 1 zinc added to 1 water yields 1 zinc oxide and 2 hydrogen. 
 
 Zn + 2HC1 = ZnCl 2 + H 2 
 
 1 zinc added to 2 hydrochloric acid yields 1 zinc chloride and 2 hydrogen. 
 
 Zinc is therefore a divalent element. 
 
 Similarly, aluminium may be shown to be trivalent, 1 of the 
 metal replacing 3 of hydrogen ; or 2 of the metal replacing 6 of 
 hydrogen as indicated 
 
 6HC1 + 2A1 = AloCl 6 + 3H 2 
 3H 2 + 2A1 = Al a 3 + 3H 2 . 
 
 Other elements are tetravalent, pentavalent or hexavalent, 
 according as they replace 4, 5 or 6 atoms of hydrogen in .com- 
 pounds. To save the repetition of the full term monovalent or 
 divalent elements, etc., it is often more convenient to classify the 
 elements as monads, dyads, triads, tetrads, pentads, or hexads. 
 
 Now it sometimes happens that a single element may form two 
 classes of compounds, in which the valency of the element is 
 different ; but these classes are quite distinct from one another, 
 for although they are interconvertible, yet they do not arbitrarily 
 pass over from the one to the other, but tend to remain separate, 
 even, perhaps, through a long cycle of chemical changes ; but one 
 or other of these classes is generally more stable than the other 
 that is, less liable to change and is, therefore, the one more 
 commonly met with. Thus, copper (Cu = cuprum) forms the more 
 usual group of compounds in which it replaces 2 of hydrogen 
 (CuO = cupric 1 oxide, CuCl 2 = cupric chloride) and is generally 
 recognised as a dyad, but it may, under certain circumstances, 
 behave as a monad and form the group of compounds of which 
 cuprous l oxide = Cu 2 0, and cuprous chloride = Cu. 2 Cl 2 , are typical 
 members. To express the valency of an element, it is therefore 
 necessary to know with which class of compounds we are dealing. 
 
 Elements. In the following table will be found the names of 
 
 the right of a symbol, describes the number of those atoms to be taken into 
 consideration. 2HC1 means 2 molecules of hydrochloric acid (2 atoms of 
 hydrogen and 2 of chlorine). H 2 means 2 atoms 'of H with 1 of ; while 
 3H. 2 means 3 molecules of water containing 6 atoms of H and 3 of 0. 
 
 1 When an element forms two classes of compounds, that class in which the 
 greatest proportion of oxygen or other equivalent substance is combined with 
 it is distinguished by the suffix -ic attached to the name of the element, while 
 the other class takes the termination -ous. When the name alone is used, the 
 more common group is usually understood for example, copper oxide would 
 imply cupric oxide, CuO. 
 
CHEMICAL COMBINATION. 
 
 19 
 
 llfl 
 rill 
 
 l*il 
 
 Ss s 
 
 2* It; 
 
 l^l'a 
 * .i c 
 
 Jl ti 
 i 1 1> * 
 
 llli 
 
 itted as they may at present be disregard 
 t of the International Committee on A tomic 
 tly unity ; this is because it is found that oth 
 ole number, namely, 16, and is thus regarded 
 alency, and are here given to the nearest decima 
 
 e.A number of newly discovered elements have been o 
 standpoint. The atomic weights are taken from the Repor 
 that the atomic weight of hydrogen is not given as exa 
 whole numbers if that of oxygen is taken as an exact wh 
 weights are found by dividing the atomic weight by the v 
 
 
 El 
 
 
 OO5^r-MCOO 
 .-IO3 CO (M!-< 
 
 i I O O CO lO O 
 
 l>p O'P PpT 1 'P 1 ^ 1 ?''!* 1 ?*'?' 1 ?*? *P T 1 'P T 1 *P ^ ^ T*"P 
 
 cS 
 <M CO 
 
 rH(N -^ 
 
 - 
 
 O5 OS OS OS 
 
20 THEORETICAL AND GENERAL. 
 
 the elements at present known, together with their symbols and 
 valencies, their atomic weights, and their equivalent weights. By 
 the latter term is meant the number of parts by weight of an 
 element which are required to take the place of 1 part of hydro- 
 gen, or of 1 equivalent of any other element ; and these numbers 
 are evidently found by dividing the atomic weight (i.e., the weight 
 of an atom in our relative scale) of the element by the number 
 of hydrogen atoms which are equivalent to it. Thus, when any 
 element interchanges with another in a compound, it is always in 
 the proportion of a multiple of their equivalent weights. 
 
 In this table, the names of the more common metallic elements 
 and of those metals which, as such or in compounds, are most 
 largely used in electro-metallurgy, are printed in small capitals, 
 while the non-metallic elements are distinguished by italics. 
 
 Heat-evolution of Combinations. We have used the term 
 chemical forces in the earlier part of this chapter, and we must 
 now devote a short time to the observation of the comparative 
 effects produced by various chemical changes and combinations. 
 
 It must first be understood that when any chemical combination 
 occurs, heat is usually evolved, owing to the conversion of chemi- 
 cal energy into its equivalent of heat-energy; and that just as 
 given substances always combine in perfectly definite proportions, 
 so each of these combinations is attended by the evolution of a 
 perfectly definite amount of heat-energy. The amounts of heat 
 evolved by different combinations are extremely varied, but that 
 given out during one particular combination is quite constant, no 
 matter in what way the union is effected. Then by comparing 
 all the elements except one as to their behaviour in combining 
 with that one other element, it is possible to arrange them in a 
 series in which the first member evolves the greatest amount of 
 heat during its combination, and the last gives out the lowest 
 quantity. Further, if different individual group elements be 
 taken, and the remaining elements be combined with them, one 
 with one, several of these lists may be compiled, each of which 
 represents the heat-value of the various combinations of the single 
 elements with one of the others. On comparing these lists, it 
 will be found that the main order of the different substances will 
 be approximately the same in all, but that one or two elements 
 may alter their relative position in certain lists, which represent 
 the effect of their combination with special elements for which 
 they have 'great affinity.' Thus, certain elements maybe said 
 to be more energetic than others in all combinations, while others 
 may be regarded as, so to speak, more sluggish evolving far less 
 heat in any reaction in which they may play a part. 
 
 By way of example, the following table illustrates the number 
 of heat-units emitted during the combination of certain of the 
 more common elements with oxygen and chlorine respectively. 
 In using this table the formula of the resulting compound is 
 
CHEMICAL COMBINATION. 
 
 21 
 
 taken, and the molecular weight is taken in grammes or pounds, 
 or whatever unit may be convenient. Thus in the case of hydro- 
 gen we have H 2 as the result. What is termed a gramme 
 molecule of water, 2 + 16, i.e., 18 grammes, may be used. Simi- 
 larly a pound molecule would be 18 pounds. The heat evolved in 
 the formation of a gramme molecule of water, according to the 
 table, would raise 68,360 grammes of water through 1 degree 
 centigrade. Similarly, the heat evolved in the formation of a 
 gramme pound of water would raise 68,360 pounds of water 
 through 1 degree centigrade. Whatever unit of weight is chosen 
 must be used all through, and the scale of temperature in each 
 case is the centigrade. As another example we see that 65 pounds 
 of zinc combining with 16 pounds of oxygen evolve sufficient heat 
 to raise the temperature of 86,400 pounds of water 1 degree 
 centigrade. 
 
 TABLE II. SHOWING THE NUMBER OF HEAT-UNITS EVOLVED 
 IN CERTAIN COMBINATIONS. 
 
 
 
 Heat-units evolved in 
 
 
 
 Combination with 
 
 Name of Element, and 
 
 Number 
 
 
 of Atoms combining. 
 
 
 
 
 
 1 Atom of Oxygen, 
 
 2 Atoms of Chlorine, 
 
 
 
 0. 
 
 C1 2 . 
 
 Potassium, 
 
 K 2 , 
 
 97,200 
 
 211,200 
 
 Sodium, 
 
 Na 2 , 
 
 100,200 
 
 195,380 
 
 Calcium, 
 
 Ca, 
 
 131,360 
 
 170,230 
 
 Strontium, 
 
 Sr, 
 
 130,980 
 
 184,550 
 
 Barium, 
 
 Ba, 
 
 130,380 
 
 194,250 
 
 Manganese, 
 
 Mn, 
 
 *94,770 
 
 111,990 
 
 Zinc, . 
 
 Zn, 
 
 86,400 
 
 97,210 
 
 Hydrogen, . 
 
 H 2 , 
 
 68,360 
 
 t44,000 
 
 Iron, . <f! 
 
 Fe, 
 
 *68,280 
 
 82,050 
 
 Tin, . 
 
 Sn, 
 
 *68,090 
 
 80,790 
 
 Cadmium, 
 
 Cd, 
 
 *65,680 
 
 93,240 
 
 Cobalt, 
 
 Co, 
 
 *63,400 
 
 76,480 
 
 Nickel, 
 
 Ni, 
 
 *60,840 
 
 74,530 
 
 Lead, . 
 
 Pb, 
 
 50,300 
 
 82,770 
 
 Copper, 
 
 Cu, 
 
 37,160 
 
 51,630 
 
 Mercury, 
 
 Hg, 
 
 30,660 
 
 63,160 
 
 Silver, 
 
 Ag 2 , 
 
 5,900 
 
 58,760 
 
 Gold, . 
 
 xAu, 
 
 *- 8,790 
 
 15,210 
 
 * These oxides are hydrated ; the remaining c^ 
 t This number refers to gaseous hydrogen chloride. 
 
 lydrous. 
 
 Law of Selection in Chemical Union. This table shows that 
 the strongest combination of chlorine with any metal (i.e., the one 
 which evolves most heat in its production) is that with potassium, 
 next is ranked sodium, while zinc stands lower, and silver lower 
 
22 THEORETICAL AND GENERAL. 
 
 still, on the list. Now, speaking generally, if there be a limited 
 amount only of an element such as oxygen in a condition to 
 combine with an excess of various metals, the strongest combina- 
 tion will always form first ; then, if there be any of the limited 
 element left, it will combine with the element which affords the 
 next strongest combination, and so on. In other words, if a 
 choice of combinations be presented, that which produces the 
 greatest evolution of heat will usually take precedence of all 
 others. By way of example, let us suppose that a limited amount 
 of chlorine is brought into contact in a closed space with an 
 excess of potassium, sodium, zinc and copper. The potassium 
 will have the highest tendency to combine with chlorine, because, 
 in doing so, the greatest amount of heat will be evolved, and 
 potassium chloride will be the chief result of the reaction. When 
 all the potassium is used up, and not until then, if all the metals 
 may be supposed finely divided and well mixed together, sodium 
 chloride will be formed, then zinc chloride, and finally, if there 
 be any chlorine left, copper chloride. Further than this, if any 
 of the pure elements be placed in a fused compound of any 
 element occupying a lower position in the above list, it will tend 
 to liberate the latter element in the metallic state, and itself to 
 take its place in the compound, because in this exchange heat 
 will be evolved ; thus, metallic zinc dipped into fused silver 
 chloride forms zinc chloride (because the affinity of chlorine for 
 zinc is greater than for silver, heat being thus evolved in the 
 change) and sets free metallic silver, while, on the contrary, 
 metallic silver is, of course, without action on fused zinc chloride. 
 Electro-Chemical Series. A list of elements arranged in such 
 order that any one will thus displace from its compounds any 
 other lower than itself on the list, but will be without effect on 
 any compound of an element occupying a higher place, is termed 
 an electro-chemical series. For every group of compounds (oxides, 
 chlorides, sulphates, etc.) such a list may be formed; and the 
 elements at the head of the list are termed electro-positive, ex- 
 pressed by the algebraic sign for plus, ' + ,' while those at the 
 lower end are electro-negative and are represented by the minus 
 sign, ' - ' ; but these terms are purely relative. For example, 
 in the following list, which expresses the electro-chemical order 
 of the elements in relation to sulphuric acid, zinc is less readily 
 acted upon by the acid than potassium, but more so than copper ; 
 it, therefore, occupies an intermediate position, and while it is 
 electro-positive in regard to copper, it is' electro-negative as 
 compared with potassium. This fact can be readily demonstrated 
 by dipping a piece of zinc into separate solutions of potassium, 
 zinc and copper sulphates ; in the two former no action will be 
 observed, while in the latter zinc will slowly be dissolved, and a 
 corresponding deposit of copper will be found on the remainder 
 of the zinc. And it should be noted that for every equivalent 
 
COMBINATION. 23 
 
 (32'7 parts by weight) of zinc dissolved, an equivalent (31*8) of 
 copper will be separated, as we have already indicated. The 
 metals as a class are electro-positive, the non-metals electro- 
 negative. The order given below may vary with the condi 
 tions ; for example, it is not the same for all electrolytes. 
 
 TABLE III. ARRANGEMENT OF THE COMMON METALS IN ELECTRO- 
 CHEMICAL SERIES. 
 
 Potassium. Cobalt. Silver. 
 
 Sodium. Nickel. Platinum. 
 
 Magnesium. Lead. Gold. 
 
 Manganese. Tin. Hydrogen. 
 
 Zinc. Bismuth. Antimony. 
 
 Iron. Copper. The metalloids or 
 
 Cadmium. Mercury. non-metals. 
 
 It should be observed that the order of the metals in this list 
 is nearly in accordance with that in the table of heat-evolutions 
 on p. 21, as indeed would be expected from our knowledge 
 of the laws alluded to at that point. The relation of the electro- 
 chemical arrangement of the metals to their heats of combination 
 readily explains also the fact, that in different solutions the 
 order of sequence is not always absolutely identical. It is of 
 great importance to remember this in electro-metallurgical work, 
 especially of an experimental nature, as the numbers repre- 
 senting these ' heats of combination ' have in many cases been 
 accurately determined and are published in works on thermo- 
 chemistry ; and since, as we shall presently see, electrolytic 
 deposits on metals, which are themselves capable of decomposing 
 the plating liquid, are generally of inferior character, the cause 
 of many a failure might be explained by a reference to thermo- 
 chemical data. 
 
 Transformations of Energy. A few pages back we saw that, 
 during chemical combination, chemical energy is converted into 
 and rendered evident as heat-energy ; and the amount of heat 
 thus evolved may be regarded as an exact measure of the chemical 
 energy of the combining elements. All forms of energy are 
 interconvertible, but it is no more possible to create energy than 
 it is to produce or to destroy matter ; for the manifestation of 
 any form of energy such as heat or electricity, is merely a 
 conversion into it of some other form of energy. In the dynamo- 
 electric machine, electricity is ' generated,' but it is at the expense 
 of an exact equivalent of mechanical energy supplied by the steam 
 engine, and the current of electricity produced may in turn be 
 converted into heat-energy in overcoming the resistance of the 
 incandescent lamps, or into chemical energy by decomposing 
 chemical compounds and precipitating the constituents in the 
 electrolytic bath in circuit. We are riot able to convert the whole 
 
24 
 
 THEORETICAL AND GENERAL. 
 
 of the amount of one kind of energy into any other, where and 
 when we require it, as some portion is generally lost to us in a 
 form which we cannot utilise as, for example, that which is 
 converted into heat by the friction of the bearings or moving 
 parts of the machinery ; nevertheless, no portion of the energy is 
 destroyed, it is simply converted into another form, which happens 
 at the time to be useless to us. In this way, for example, a small 
 fraction of the electrical energy supplied to the plating-vats is 
 lost as heat in overcoming the resistance of the wires carrying 
 the current. 
 
 Intercon version of Chemical and Electrical Energy. Let us 
 suppose that a plate of ordinary commercial zinc is placed in 
 dilute sulphuric acid ; it will dissolve, and as it slowly dissolves 
 away, a rise of temperature is observable, due to the evolution of 
 heat, which may be shown by careful experiment to be precisely 
 proportional to the weight of zinc dissolved. If a plate of pure 
 
 zinc or amalgamated zinc is simi- 
 larly placed in dilute sulphuric 
 acid, it will not dissolve ; if now 
 a plate of copper be immersed in 
 the same liquid, but not touching 
 this pure zinc strip, as in fig. 1, 
 it will neither be attacked by the 
 acid itself, nor will it in any way 
 affect the relations between the 
 zinc and the liquid. But if the 
 FIG. l.-Copper- FIG. 2. -Copper- two metals be connected by a wire 
 zinc cell, un- zinc cell, con- as shown m fig. 2, the zinc will 
 
 connected. nected. dissolve ; yet the heat-measuring 
 
 apparatus will show practically no 
 
 evolution of heat. Either then the heat is not being emitted, 
 or it is being dissipated in some way. But the wire is found 
 to possess new properties ; if it is in the form of a spiral, it 
 attracts light particles of iron brought near to its ends, while 
 it influences the set of a compass-needle, and further, if it is 
 very fine, it becomes distinctly warmer than the surrounding 
 air. Here then is the solution of the problem : directly the two 
 plates are connected by the wire a current of electricity is set up, 
 which is regarded as flowing from the zinc through the solution 
 to the copper, and thence from the copper through the wire coil 
 to the zinc again ; and this current, meeting with resistance in 
 all parts of the circuit, causes heat to be evol-ved in all parts by 
 the conversion of electrical energy, but chiefly in the portion 
 which opposes the greatest resistance, namely, in the thin wire. 
 The proof of the existence of this electric current is seen in the 
 magnetisation of the spiral. Two similar pieces of zinc connected 
 in this way would produce no current, but any two unlike metals 
 would give results analogous to those from the copper and zinc. 
 
POTENTIAL DIFFERENCE. 25 
 
 Hence it may be generally stated that when two different metals 
 are metallically connected together in a solution which is capable 
 of acting chemically upon at least one of them, an electric current 
 is set up which will pass in the liquid from the more electro-positive 
 metal to the other, and complete the circuit by returning along the 
 metallic connection, in the inverse direction, to the positive plate. 
 It is further noticeable that the greater the electro-chemical 
 difference between the two metals used, the more vigorous will 
 be the action, the more rapid the solution of the electro-positive 
 element, and consequently the greater will be the heat and 
 electrical manifestation in the connecting wire. This is due to 
 the greater difference of potential between any pair of metals 
 widely separated in the electro-chemical series than that between 
 any pair more nearly adjacent. 
 
 Electro-motive Force. The electric force that causes the 
 current to flow when the plates of a voltaic cell are connected by 
 an external wire, as just described, is called the electro-motive force, 
 or, more briefly, the E.M.F. of the cell. There has been much 
 discussion as to where the electro-motive force in a cell is situated, 
 but into this point we need not enter. It will be sufficient here 
 to state that any voltaic cell has an electro-motive force tending 
 to force a current through any external circuit connecting the 
 poles or plates. 
 
 Potential Difference. If a current is flowing through a wire 
 there is a certain electric force or electric pressure between the 
 ends of the wire. In such cases, it is often said that a difference 
 of potential exists between the ends of the wire, just as there is 
 a difference of water pressure between two points along a water- 
 pipe when water is flowing. Such potential difference or electric 
 pressure is of the same nature as electro-motive force, but the 
 latter term is restricted to a source or generator of electric energy, 
 whereas potential difference or electric pressure is due to flow of 
 current, and therefore arises from an electro-motive force as its 
 primary cause. Potential in electricity is akin to pressure in 
 hydraulics, but we need not here inquire into the exact meaning 
 of the term ; difference of potential is what we are concerned with 
 in practice, just as difference of pressure is generally more im- 
 portant than absolute pressures in hydraulics, and in many ways 
 it is simpler to use the term electric 'pressure,' rather than poten- 
 tial difference. When the plates of a voltaic cell are not con- 
 nected by a wire, the pressure between the terminals is equal to 
 the electro-motive force of the cell ; if a wire is connected across 
 the terminals, the E.M.F. may remain the same (this depends on 
 the character of the cell), but the pressure between the terminals 
 will fall, its value depending on the resistance offered by the wire 
 and by the cell to the flow of current. There will be a potential 
 difference, or a certain pressure, between every two points on this 
 wire, and also between any two points within the cell along the 
 
26 THEORETICAL AND GENERAL. 
 
 path of the current. In fact, the E.M.F. is used up, as it were, 
 in giving rise to these potential differences all round the circuit. 
 
 Electrolysis. Following up the investigation of the pair of 
 metals referred to in figs. 1 and 2, it will be remarked that in the 
 case of commercial or impure zinc, bubbles of hydrogen gas are 
 constantly formed upon the surface of the zinc, and that these 
 rapidly find their way to the surface of the liquid and escape into 
 the air ; they are due to the decomposition of the sulphuric acid 
 by the zinc, expressed in the following equation : 
 
 Zn + H 2 S0 4 ZnS0 4 + H 2 
 
 Zinc added to sulphuric acid yields zinc sulphate and hydrogen. 
 
 The exchange of zinc and hydrogen can readily take place 
 because heat is evolved in the process. If the zinc is pure, there 
 is no such evolution of hydrogen until it is connected by a wire 
 to the copper. But there is this further important difference ; in 
 the first case the hydrogen is evolved on the zinc, whereas in the 
 second case it is evolved on the copper. This effect, which clearly 
 marks the introduction of new conditions, is a phenomenon con- 
 nected with the passage of the current through the sulphuric 
 acid. It may be regarded as a simple case of electrolysis or 
 electro-deposition. 
 
 We have seen that a current of electricity passed through a 
 coil of wire endues it with new magnetic properties ; but when it 
 is passed through certain compound liquids it tends to break up 
 the compounds, and to deposit certain of their constituents on 
 the surfaces by which it enters and leaves the solution. Thus 
 in the cell to which we are referring, when the current began to 
 flow, the elements of each molecule of acid (H 2 S0 4 ) that was 
 decomposed were distributed, the hydrogen (H 2 ) to the copper 
 plate, from which it escaped, and the S0 4 to the zinc plate, with 
 which it at once combined to form zinc sulphate, and this in turn 
 dissolved in the acid liquid as zinc sulphate. It is universally 
 true that when any solution is electrolysed (that is, decomposed 
 by the electric current), the electro-positive element, or metal, is 
 deposited on the plates towards which the current flows in the 
 bath, and by which it leaves the solution ; while the electro- 
 negative portion is thrown down on that by which the current 
 enters the liquid. Within the copper-zinc cell the current flows 
 from the zinc to the copper ; and, as hydrogen is the electro- 
 positive element of sulphuric acid, it is on the copper that this 
 gas is liberated, while the S0 4 or electro-negative group of 
 elements is set free upon the surface of the zinc. 
 
 Had the zinc plate been immersed in a solution of a copper salt 
 (e.g., copper sulphate) instead of in sulphuric acid, not hydrogen 
 but copper would have been deposited upon its surface. Any 
 metal will thus substitute itself for any less positive metal in a 
 suitable solution, and will deposit that metal upon its surface, or 
 
ELECTROLYTIC ACTION. 27 
 
 upon any other less electro-positive metal which may stand in 
 contact with it in the same solution. Generally speaking, when 
 two metals, one being electro-positive to the other, are placed 
 separately in a solution, each will dissolve irrespectively of the 
 other, supposing, of course, that the solution is capable of 
 attacking both metals. If the two metals are placed in contact, 
 the dissolving of the electro-positive metal not only continues but 
 is increased. 1 It is for this reason that the presence of platinum 
 assists in dissolving tin in hydrochloric acid. On the other hand, 
 the electro-negative metal does not dissolve so quickly as it did 
 before contact was made ; solution may still continue, but it is 
 more or less protected by the tendency of the voltaic couple so 
 formed to deposit hydrogen or a metal upon the electro-negative 
 of the two metals. It is for this reason that the zinc coating in 
 the case of galvanised iron affords some protection even when 
 worn away in parts ; any exposed iron is still protected, more or 
 less, by the action of the voltaic couple formed when moisture is 
 present. 
 
 But the same law of deposition holds good when a current of 
 electricity is passed through a separate solution, independent of 
 the dissolving cell ; we then have a voltaic cell or cells passing 
 current through an electrolysing cell. If the current available 
 have a high electro-motive force, it will effect decomposition in 
 the external liquid as readily as (or more readily than) in the 
 original solution, and will deposit the electro-negative substance 
 on the surface by which it enters that liquid or anode, as it is 
 termed, and the electro-positive body, or metal, at the cathode or 
 surface towards which it flows, and through which it leaves the 
 solution. In carrying the current, the molecules in the solution 
 split up into two parts called ions; one ion, called the anion, 
 travels in the opposite direction to that of the current and is 
 deposited at the anode ; the other ion, called the cation, travels 
 with the current and is deposited at the cathode. The metals, 
 therefore, as a class would be cations, the non-metals anions. 
 The anode and cathode form the electrodes. 
 
 When a number of different metals are connected together in 
 the dissolving cell, the most electro-positive metal will be acted 
 upon in preference to the others, these latter being more or less 
 protected, as we have just seen. When the most electro-positive 
 metal has been dissolved away, the next electro-positive metal 
 will be more readily attacked, and so on, because the most positive 
 metal evolves most heat during combination, and because that 
 reaction usually occurs which is accompanied by the greatest 
 heat-evolution. Conversely, a current passing through a mixed 
 
 1 It is sometimes wrongly stated that the chemical action previously taking 
 place on the electro-positive metal stops when the two metals are placed in 
 contact. If this were so there would be no local action in a battery when 
 supplying current. 
 
28 
 
 THEORETICAL AND GENERAL. 
 
 solution will generally l deposit first the least electro-positive 
 metal, then the next higher in the series, and finally the most 
 electro-positive : for it should be observed that to decompose a 
 given substance requires an expenditure of energy equivalent to 
 the heat generated in its formation ; and j ust as with a choice of 
 reactions, that combination will occur which sets free the greatest 
 amount of heat-energy, so when there is a choice of decomposi- 
 tions, that will be effected which requires the lowest expenditure 
 of energy ; and in electrolysing (i.e., decomposing by the electric 
 current) a mixed solution, least energy is, of course, needed to 
 break up the compound which evolved least heat during its 
 formation. Zinc, iron, lead, copper, and silver, bound together 
 by wire and placed in a bath of acid, would, therefore, dissolve in 
 the order in which they are enumerated, while a current passing 
 through a solution containing all these metals would deposit first 
 
 the silver, then the copper, and, finally, 
 the others in inverse order. 
 
 In electrolysing a solution, then, 
 by means of a current derived from 
 chemical action, the chemical energy 
 of the battery is converted into elec- 
 trical energy, and is pumped, as it 
 were, by the electro-motive force of 
 the battery, along a wire to the anode, 
 through the solution to the cathode 
 and thence back to the battery. In 
 
 FIG. 3. Copper-zinc cell con- the solution it is reconverted into 
 nected with electrolysing cell, chemical energy during electrolysis, by 
 
 the separation of the elements con- 
 tained in the liquid. Let us imagine that the plates by which the 
 current enters and leaves the electrolyte (or liquid which is being 
 electrolysed) are made of platinum or any other metal which 
 cannot be attacked by the solution or by the liberated elements ; 
 then, if the chemical energy required to dissociate (or drive 
 apart) the elements which are combined in the electrolyte is 
 greater than the chemical energy of the battery-cell, it is evident 
 that no dissociation can occur. Hence there is a practical limit 
 to electrolytic decomposition. There is another point of view 
 from which this phenomenon may be observed, which may serve 
 to make it more clearly understood. Let us suppose that a 
 certain pair of metals, C and Z (fig. 3), are placed in a battery- 
 cell, B, and joined with wires to the platinum plates, and M, 
 respectively, of an electrolysing cell, E, the solution in this cell 
 being that of some metallic salt. Here Z is the electro-positive 
 metal ; so that the current flows in the cell, B, from Z to C, and 
 thence through the wire to the plate, 0, on which the negative 
 
 1 Under certain conditions, which will be considered hereafter, p. 32, it 
 is possible to deposit alloys or mixtures of metals from a compound solution. 
 
ELECTROLYTIC ACTION. 29 
 
 constituent of the electrolyte would be separated, and then 
 through the liquid to M, which should receive the deposit of 
 metal, and finally returns to the plate, Z, of the battery. But 
 it is evident that as soon as the cathode, M, is completely covered 
 with metal, the electrolytic cell itself will become practically 
 converted into a battery, of which the deposit on M may be 
 supposed the positive metal, and the platinum, 0, the negative. 
 The result of this would be a tendency to send a current, pro- 
 duced by the dissolving of the deposit on M, and urged by the 
 electro-motive force of this cell, from M through the liquid E to 
 0, thence through the wire to C, and through the battery 
 solution B to Z, and then again finally to M ; in other words, the 
 tendency would be for a current to flow in the opposite direction 
 to that produced by the battery ; and if the opposing electro- 
 motive force of the new electrolytic cell were greater than the 
 electro-motive force of the battery, the action of the whole 
 arrangement would be reversed until the deposit on M had re- 
 dissolved again. It is, therefore, easy to understand that if the 
 counter E.M.F. (or opposing electro-motive force) of the electro- 
 lytic cell be equal to or greater than the original E.M.F. of the 
 battery, no decomposition can occur, for any metal deposited 
 would redissolve at once, and would tend to produce a reverse 
 current. Thus the same conclusion is arrived at, that' in order to 
 produce an electrolytic deposit, the decomposing current must have 
 a higher electro-motive force than that ivhich is set up betiveen the 
 deposited metal and the metal of which the anode is made. 
 
 If, instead of insoluble platinum strips, pieces of the same 
 metal that is to be deposited are used, then any current what- 
 ever or rather any E.M.F. may suffice to decompose the 
 solution ; because, although metal is being deposited, and there- 
 fore a certain amount of energy is absorbed at the cathode, yet 
 the same weight of metal is being constantly dissolved by com- 
 bination with the non-metal deposited at the anode ; and the 
 energy that is absorbed at the cathode by decomposition is thus 
 exactly counterbalanced by that which is produced at the anode 
 by the re-formation of the original substance. To take the other 
 view of the case, the metal which is deposited is of the same 
 nature as the anode itself, and between strips of the same metal 
 no opposing E.M.F. is produced. The function of the current is 
 simply to provide the means for destroying the equilibrium of 
 the electrolyte, and to transfer, as it were, the metal by degrees 
 from the anode to the cathode. Finally, to illustrate the matter 
 by a practical example : a solution of copper sulphate electro- 
 lysed between a copper anode and cathode. Copper will be 
 deposited on the copper cathode and will still be opposed to a 
 copper anode ; no current can thus be produced by the two plates, 
 no counter E.M.F. is set up, and the feeblest external E.M.F. 
 suffices to effect the electrolysis. But if platinum plates be used, 
 
30 THEORETICAL AND GENERAL. 
 
 then copper is deposited on the cathode, and thus practically 
 forms a sheet of copper which is now opposed to the platinum 
 anode ; at once an opposing E.M.F. is produced, due to copper 
 and platinum in a copper sulphate solution. Now, if the battery 
 producing the current have a higher E.M.F. than this, it will 
 overcome the opposition, and copper will be deposited continu- 
 ously ; but if it have a lower electro-motive force, no deposition 
 can possibly occur, and the platinum cathode remains un- 
 coppered. 
 
 When soluble anodes of a different metal from that which is to 
 be deposited are used the case is less simple, owing to the gradual 
 introduction of the metal dissolved from the anode into the 
 electrolyte. Here two classes of problems may arise. 
 
 (1) If the metal of the anode be more electro-positive than that 
 undergoing deposition. In this case, provided the cathode is 
 made of the depositing metal, no separate battery is needed, be- 
 cause a simple dissolving cell is formed, and the mere connection 
 of the two plates causes the anode to dissolve, and to deposit the 
 metal from the solution upon the cathode, as in the case of zinc, 
 iron, etc., in an acid solution, which we have instanced on p. 28. 
 But if a separate battery be employed, its current is reinforced by 
 that generated in the electrolytic cell, and the action proceeds 
 more rapidly than it could otherwise do, until the solution is 
 exhausted of the metal which is being separated. When this 
 point is reached the current of the battery will begin to deposit 
 the metal which has passed into solution from the anode, pro- 
 vided the E.M.F. be sufficiently high ; but if, on the other hand, 
 no battery be applied, all deposition must cease as soon as the 
 whole of the first metal has been thrown down. 
 
 (2) If the anode be less electro-positive than the metal undergoing 
 deposition. Here, provided the E.M.F. of the battery is suffi- 
 cient to initiate electrolytic action, the required metal will be 
 deposited for a time, but gradually a more electro-negative body 
 diffuses through the liquid as it dissolves from the anode, and 
 will, of course, be ultimately deposited instead of that of which 
 the solution was originally composed. 
 
 Electric Conduction. We have spoken freely of electricity 
 passing through matter, but have not yet noted that it cannot 
 do so with equal readiness in different media. There are many 
 substances which absolutely check the progress of the electric 
 current, and are hence termed non-conductors or insulators ; they 
 comprise such bodies as sulphur, pitch, shellac, glass, ebonite, 
 sealing-wax, india-rubber, gutta-percha, paraffin-wax, oils, and the 
 like; others are poor conductors, such as wood, and allow a 
 certain amount of electricity especially, of course, that at high 
 potential to pass through them ; while others transmit electricity 
 with facility, and are termed conductors. But there is an infinity 
 of grades even among conductors. The metals as a class are the 
 
ELECTRIC CONDUCTION. 
 
 31 
 
 best, and second, but after a long interval, are acidulated water 
 and aqueous solutions. Of the metals, silver is the best conductor, 
 and perfectly pure copper is a few per cent, lower; regarding 
 the conductivity of these as represented by 100, the relative 
 conductivity of the principal metals is given in the following 
 Table: 
 
 TABLE IV. SHOWING THE RELATIVE ELECTRIC CONDUCTIVITY 
 
 OF CERTAIN METALS. 
 
 Name of 
 Metal. 
 
 if 
 
 Authorities. 
 
 Name of Metal. 
 
 O 
 
 C 'J3 
 
 
 
 Authorities. 
 
 Silver . . 
 
 100-0 
 
 B,D,L,M,P,W. 
 
 Antimony . . 
 
 4'2 
 
 M,W. 
 
 Copper . 
 Gold . . 
 
 100-0 
 
 80-6 
 
 B,D,L,M,0,P,W. 
 B,D,L,M,0,P,W. 
 
 Mercury . . . 
 Bismuth . . . 
 
 2-5 
 1-2 
 
 B,M,P. 
 M. 
 
 Aluminium 
 
 55-1 
 
 M,W. 
 
 Graphite . . . 
 
 0-07 
 
 M. 
 
 Sodium 
 
 37'4 
 
 M. 
 
 Alloys, &c. 
 
 
 
 Zinc . . 
 
 30-2 
 
 B,M,0,W. 
 
 Cuwith 4% Si 
 
 75-0 
 
 W. 
 
 Cadmium . 
 
 23-7 
 
 M. 
 
 Cu 12,, Si 
 
 547 
 
 W. 
 
 Potassium . 
 
 20-8 
 
 M. 
 
 Cu 9,,P 
 
 4-9 
 
 W. 
 
 Platinum . 
 
 167 
 
 B,D,L,M,0,P,W. 
 
 Cu 10,, Pb 
 
 30-0 
 
 W. 
 
 Palladium . 
 
 16-4 
 
 D. 
 
 Cu 10,, Al 
 
 12-6 
 
 W. 
 
 Iron . . 
 
 16-4 
 
 B,D,L,M,0,P,W. 
 
 Cu 10,, As 
 
 9-1 
 
 W. 
 
 Tin . . . 
 
 15-2 
 
 B,M,0,W. 
 
 Cu 20,, Sn 
 
 8-4 
 
 W. 
 
 Thallium . 
 
 9'2 
 
 M. 
 
 Cu 35,, Zn 
 
 21-1 
 
 W. 
 
 Lead . . 
 
 8'8 
 
 B,M,0,W. 
 
 Cu 50,, Ag 
 
 86-6 
 
 W. 
 
 Nickel . . 
 
 7-9 
 
 W. 
 
 Au 50,, Ag 
 
 16-1 
 
 W. 
 
 Arsenic 
 
 4-8 
 
 M. 
 
 Sn 12,, Na 
 
 46-9 
 
 W. 
 
 Physicists have obtained very varied results in measuring the 
 conductivity of metals, probably owing to want of care in select- 
 ing perfectly pure specimens; for, remembering that a mere 
 trace of impurity is often sufficient to lower the conductivity (or, 
 in other words, to increase the resistivity) of a metal by an alto- 
 gether disproportionate amount, it is to be regretted that careful 
 analyses of the samples operated upon were not made, and the 
 nature and quantities of impurities published with the result of 
 the physical observations. In this Table, whenever possible, the 
 mean results obtained by Becquerel, Davy, Lenz, Matthiessen, 
 Ohm, Pouillet, and Weiller have been taken ; in many cases, how- 
 ever, measurements have been made by only one or two of these, 
 and in others a single result obtained by one of them is so 
 divergent from the corresponding numbers quoted by the others 
 as to point to an error of observation, and to render its omission 
 desirable in calculating the mean : for this reason we have in- 
 dicated the sources from which the figures have been derived by 
 the initial letters of the observers' names. 
 
THEORETICAL AND GENERAL. 
 
 The conductivity of liquids is much lower, that of dilute 
 sulphuric acid being '000 13, and copper sulphate solution 
 0-0000054, that of silver being taken as 100 : while pure water 
 itself is a non-conductor. In passing through any substance, 
 electric energy must always suffer loss, owing to partial trans- 
 formation into heat in overcoming the resistance of the conductor, 
 which is in its effect similar to that of friction on mechanical 
 energy; hence, for conducting wires, only that material which 
 offers least resistance (or friction) to the path of the current 
 should be used. Silver is, of course, placed out of the field on 
 account of its costliness ; but copper, which is practically as good, 
 is readily obtainable, and should, therefore, have the preference 
 over all metals for this purpose unless the circumstances are 
 unusual. 
 
 Electrolytic Conduction. Up to this point we have been 
 dealing with metallic conduction, which means that the electric 
 energy is merely transmitted through a substance without pro- 
 ducing any other effect than the conversion of a small proportion 
 into heat, due to the resistance of the conductor. But there is 
 another or electrolytic conduction, which has only been observed 
 to take place in compound liquids, and in this case the passage of 
 the electric current is accompanied by a decomposition of the liquid; 
 but this decomposition is only made apparent by the separation 
 of the constituents of the electrolyte at the electrodes (i.e., the 
 anode and cathode). This electrolytic conduction may occur in 
 solutions of solid substances (e.g., copper sulphate) or in fused 
 bodies, but it is essential that they shall be compounds (mercury 
 is a liquid conductor, but being an element it cannot be electrolysed 
 and, therefore, conducts metallically), and that they shall be 
 able to conduct the electricity. In electrolysing fused salts it 
 may sometimes happen that the ions dissolve instantaneously in 
 the liquid and, diffusing through it, reunite as rapidly as they 
 are separated ; and thus to all appearances the fluid is conducting 
 metallically rather than electrolytically. Fused alloys of metals 
 appear actually to conduct metallically only, as up to the present 
 no attempt to separate the constituents by electrolysis has proved 
 successful, though it is, of course, possible that the re-diffusion of 
 the separated metals just described may account for the failure of 
 these attempts. 
 
 The conditions, then, for electrolysis by the battery or dynamo 
 are, that the solution should be able to conduct electricity and 
 that the electro-motive force of the battery used should be in 
 excess of that set up by the separated ions ; and, further, that 
 the ions, or at least the cations, should be able to exist as such in 
 the electrolyte without bringing about secondary actions or 
 decompositions. 
 
 Deposition of Alloys. It has elsewhere been stated that when 
 a current is passed through a mixed solution, all the electro- 
 
DEPOSITION OF ALLOYS. 33 
 
 positive ions appear to take part in conducting the current 
 through the liquid, but that usually only the least electro-positive 
 metal present is deposited, either because the others require a 
 greater expenditure of energy, and are not precipitated at all, or 
 because, if deposited, they exchange places with less electro- 
 positive metals still in the electrolyte surrounding the cathode. 
 Thus, in a solution of zinc and copper it is possible that both 
 metals are deposited ; but that the zinc instantaneously acts 
 on the copper solution around and re-dissolves, whilst it precipi- 
 tates at the same time an equivalent of copper in its place, so 
 that only the latter metal is finally separated. But the deposi- 
 tion of the alloy will appear feasible in any of the following 
 cases: (1) If the current be so powerful that the secondary 
 action of the more electro-positive metal on the solution of the 
 other have not time to perfect itself; (2) if the particular 
 solution used be of such a character that the more electro- 
 positive metal, when it is separated, cannot chemically attack 
 the solution of the more electro-negative, because the heats of 
 formation of the two salts are so nearly identical that the 
 same electro-motive force is needed to deposit each ; (3) if the 
 proportion of the more electro-negative metal in the liquid be so 
 small, compared with that of the other, that the solution around 
 the cathode is exhausted of the former more rapidly than its 
 replacement is possible through diffusion ; so that the electro- 
 positive metal, having no possibility of exchanging places with 
 it, remains undissolved. Thus, from a mixed solution of copper 
 and zinc, brass might be deposited if the current were so strong, 
 and, therefore, the decomposition so rapid, that the separated 
 zinc had not time to precipitate the copper around it before it 
 was protected by a further electro-deposit of copper ; or if such 
 compounds were selected, that the copper salt could not be 
 decomposed by metallic zinc ; or if the quantity of zinc in the 
 solution were so great as compared with the copper that there 
 was insufficient copper in the solution around the anode to take 
 the place of the mass of zinc deposited. It should further be 
 remarked that probably a small proportion of heat is generated 
 in the alloying of certain metals, and that this would provide 
 an additional tendency to deposit the alloy, sufficient, perhaps, to 
 determine its production in certain cases. 
 
 The comparative electro-positiveness of two metals in a given 
 solution may be ascertained by connecting the metals by wires 
 to corresponding slips of copper immersed in copper sulphate 
 solution, and noting on which strip a further copper deposit is 
 produced ; the slip on which the copper is precipitated will, of 
 course, be that which is joined to the more positive metal ; or it 
 may be determined by connecting the plate with a wire, and 
 finding, with the aid of a galvanometer, in which direction the 
 current flows, remembering that outside the solution it must 
 
 3 
 
34 THEORETICAL AND GENERAL. 
 
 always pass from the electro-negative to the electro-positive 
 metal. 
 
 Units of Measurement. Before closing this chapter we will 
 refer briefly to some of the units of measurement employed in 
 practical work. 
 
 In dealing with electric currents there are two distinct factors 
 to be taken into account the quantity flowing and pressure. 
 The pressure, we have already seen, is the force under which the 
 current flows. The quantity flowing in electricity is generally 
 termed 'current,' simply. These terms, * pressure ' and 'quantity 
 flowing,' perhaps explain better than any others the sense of the 
 two words in question, inasmuch as they effect a comparison with 
 hydraulic power, -which is more tangible, so to speak, and, there- 
 "fore, better understood. The power of overcoming resistance 
 increases with the pressure in electrical as in hydraulic work ; 
 thousands of cubic feet of water may be flowing in a given time 
 through a large pipe (i.e., a large current) with but a slight fall 
 or pressure, and yet but little force is required to check the flow ; 
 while a few cubic inches per second conveyed along a small pipe, 
 if only from a sufficient height (i.e., at sufficient pressure), may 
 sweep away every obstacle placed in the path and will thus cause 
 a current to flow, even through a great resistance. Disregarding 
 loss due to heating, the amount of work done by a current at a 
 given pressure is exactly proportional to the flow of electricity in 
 a given time (i.e., to the product of the current into the time 
 during which it is allowed to flow) ; and this follows naturally 
 from the laws of current-production. A given amount of zinc 
 dissolved in the battery produces a given amount of heat, which 
 is converted into electricity, and can then, in the electrolyte, 
 only yield, at most, its equivalent of chemical energy. 
 
 The unit of electro-motive force (pressure), then, is the volt, 
 and is roughly equal to the E.M.F. of a single Daniell's cell. 
 
 The unit of resistance is the ohm, which is equal to that of a 
 column of mercury at centigrade, 106 '3 centimetres long and 
 having a mass of 14'4521 grammes, to an unvarying current. 
 
 The unit of current is the ampere, and is that produced by a 
 pressure of 1 volt when applied to the terminals of a wire having 
 a resistance of 1 ohm. A current of 1 ampere is such that when 
 flowing uniformly in one direction it will deposit 1*118 milli- 
 grammes of silver per second if passed through a solution of 
 silver nitrate. 
 
 In scientific work the metrical system of .weights and measures 
 is adopted. 
 
 The unit of weight is the gramme ( = 15'432 grains); ten, 
 hundred, and a thousand grammes being termed dekagramme, 
 hectogramme, and kilogramme; and -rfrth, T J<j-th, and -nnnrth 
 being denominated decigramme, centigramme, and milligramme, 
 respectively. 
 
UNITS OF MEASUREMENT. 35 
 
 The unit of length is the metre ( = 39'37 inches), and the 
 multiples and fractions have the same Greek and Latin prefixes 
 respectively as those of the unit of weight. 
 
 In cubic measurement a special term litre is used to denote the 
 cubic decimeter, or 1000 cubic centimetres ( = 1*76 pints). 
 
 The unit of heat is the calorie, and represents the amount of 
 heat-energy which must be expended in order to raise the tem- 
 perature of 1 gramme of cold water 1 centigrade (or, to be exact, 
 from 4 to 5 centigrade) : some authorities use a kilogramme- 
 degree unit, but the gramme-degree system is more general, and 
 is the one adopted in this work whenever it may be necessary 
 to use it. Other units are used in different countries, as, .for 
 example, that of the pound- Fahrenheit-degree/ intended to adapt 
 itself to the English systems of measurement. 
 
 Temperature is generally measured in scientific work by the 
 centigrade scale (indicated by the letter C) which denominates 
 the freezing-point of water zero, and the boiling-point 100, the 
 space between these points being divided into 100 equal degrees. 
 In the Fahrenheit scale (indicated by the letter F), largely used 
 in England, the freezing-point is 32, the boiling-point 212, the 
 space between them having 180 degrees. In the Reaumur system, 
 adopted widely on the Continent, the freezing-point is zero, and 
 the boiling-point 80. The first two of these systems alone will 
 be referred to in this book, and on page 399 will be found a Table 
 of Comparison of Temperatures recorded by each. It is often 
 convenient to remember that 9 F. are equal to 5 C. 
 
 In addition to these units there are several of practical utility 
 which may be well mentioned. 
 
 The electrical unit of quantity is the coulomb ; if the number 
 of coulombs flowing past a given point in a circuit is large, the 
 current is large. Thus when an ampere is flowing in a circuit 
 one coulomb per second flows past any point in the circuit; if 
 the current is 50 amperes then the number of coulombs per second 
 is 50, and so on. 
 
 Neither current alone nor ..pressure alone is a measure of the 
 power given to any circuit, but the product of the current and 
 the pressure (in continuous currents) in any particular case is a 
 measure of the power. If the current is expressed in amperes 
 and the pressure in volts, the power obtained by the product is 
 expressed in watts. If in any circuit a current of one ampere is 
 flowing due to a pressure of one volt, the power in that circuit is 
 one watt ; this is equally true over any part of the circuit where 
 the drop in pressure is one volt when one ampere is flowing. 
 
 With alternating currents (i.e., with currents which reverse 
 their direction many times a second) the relation is more com- 
 plicated, because the current and pressure are liable to be what 
 is termed "out of phase." In that case the power is not given 
 by the product of the volts into the amperes. This product 
 
36 THEORETICAL AND GENERAL. 
 
 gives the apparent watts, but the actual watts are less, and the 
 ratio of the latter to the former is termed the poiver factor of the 
 circuit. For example, if the true watts were 90 and the apparent 
 watts were 100, the power factor would be -f^ or 0'9. Since 
 the apparent power is larger than the real power, it follows that 
 the current or the pressure must be larger than would be the 
 case with continuous current This is a disadvantage, necessitating 
 larger conductors or larger plant, and is of importance in electric 
 furnaces worked by alternating currents. 
 
 The English horse-poiver (H.P.) is equal to work done at the 
 rate of 33,000 foot-pounds per minute, or 550 foot-pounds per 
 second, and is thus equivalent to raising 550 pounds through one 
 foot, or one pound through 550 feet in a second. (The French 
 H.P. is 542-48 foot-pounds per second.) The British H.P. is 
 equivalent to 746 watts ; or, in other words, 1 kilowatt is equal 
 to about 1J H.P. 
 
 Finally, to connect heat-energy with mechanical work, Joule 
 found that 1 calorie (gramme) is approximately equal to 0'424 
 kilogramme-metre or 3'066 foot-pounds i.e., 1 calorie liberated 
 per second is equal to 0*00557 H.P. 
 
 The term current density is used in electrolysis to represent the 
 current flowing per unifc surface of electrode. In England this 
 is sometimes expressed in amperes per sq. ft., sometimes in 
 amperes per sq. in., of anode (or cathode) surface exposed to the 
 current. On the Continent it is more often quoted in terms of 
 amperes per sq. decimetre. In electrolytic analysis amperes per 
 sq. decimetre are commonly quoted, and the symbol N.D. 100 
 is often used to express this : thus a current N.D. 100 = 0'5 would 
 mean a current density of 0'5 ampere per sq. decimetre. 
 
 As already explained, the unit of power is the watt. If a circuit 
 is supplied with power to the extent of one watt, then the work 
 done in the circuit per second is termed a watt-second or a joule. 
 It will thus be seen that a joule is also the work done by a 
 coulomb in passing between two points of a circuit differing in 
 pressure by one volt. The watt is too small a unit of power for 
 many practical purposes, and therefore a unit of 1000 watts, 
 termed a kilowatt, is often used. Similarily, the watt-second is 
 too small for general purposes, and thus the kilowatt-hour (i.e., 
 the work done by a kilowatt working for one hour) is adopted, 
 and is known as the Board of Trade Unit, frequently abbreviated 
 to the letters B.T.U. A kilowatt-hour may obviously be the work 
 done by a current of 1000 amperes at 1 volt passing for an hour, 
 or by 1 ampere at 1000 volts, or 20 amperes at 50 volts (or any 
 other combination provided that the number of amperes multi- 
 plied by the number of volts is 1000) passing for a like period. 
 
CHAPTER III. 
 
 SOURCES OF CURRENT. 
 
 Relative Efficiency and Cheapness. From what has been said 
 in the last chapter, it will be apparent that some other form 
 of energy must be converted into electrical energy in order 
 to yield the current required for electro-plating; and we can 
 now understand why the only electrical current originally 
 available, which was generated from mechanical energy through 
 friction between unlike bodies, although capable of exhibiting 
 wonderful disruptive effects, owing to its high electro-motive 
 force, was useless for electro-metallurgical work by reason of 
 its deficiency in quantity or volume. Then with the invention 
 of the battery, which aimed at the conversion of chemical into 
 electrical energy, currents of low electro-motive force but of 
 comparatively great ' volume ' were producible, and these soon 
 found an application in plating and electrotyping. But in nearly 
 all batteries zinc is used up, and as a given weight of zinc can 
 only produce a given (in many instances not very different) weight 
 of deposited metal, it is too costly to be used in some branches of 
 electro-metallurgy. The dynamo-electric machine, which is able 
 to convert mechanical energy directly into a current of large 
 * volume ' but low electro-motive force, is now largely used, not 
 only in this direction, but also as a substitute for battery-power 
 generally, because the combustion of the coal in the boiler of 
 the steam engine takes the place of the oxidation or solution of 
 the zinc in the cell of the battery, and is a far cheaper agent. 
 Nevertheless, as the steam engine utilises only a very small frac- 
 tion of the total heat produced by the combustion of the coal, 
 some means of directly converting heat into electricity is even 
 now a desideratum ; at present, the thermopile does indeed effect 
 this object, but it is not economical, and is not used for the 
 generation of large currents, nor is it extensively employed in 
 any way. 
 
 The manner of producing currents by the battery, the 
 dynamo, and the thermopile will be shortly discussed in this 
 chapter. 
 
 37 
 
38 SOURCES OF CURRENT. 
 
 THE VOLTAIC OR GALVANIC BATTERY (named after Volta 
 and Galvani). 
 
 Principle. The principle of the galvanic battery has already 
 been explained, namely: That when any two nietals are ( placed 
 in a fluid which can attack at least one of them, and are connected 
 by any conductor or set of conductors, an electric current is at 
 once set up, which flows within the cell from the more positive 
 metal to the other, and in the opposite direction through the 
 conductors ; and that the greater the distance between the two 
 metals on the electro-chemical scale, the higher will be the electro- 
 motive force of the cell. Zinc being a common metal, compara- 
 tively inexpensive, and, at the same time, very fairly electro- 
 positive, is almost universally adopted as the positive element. 
 The electro-negative element and the exciting fluid are, however, 
 frequently varied, such modifications constituting the chief differ- 
 ences between the types of cell commonly employed The electro- 
 lyte should be capable, preferably, of only attacking the electro- 
 positive metal, and of attacking only when the cell is supplying 
 current, i.e., when the circuit is closed ; otherwise there will be 
 much waste of material. 
 
 Parts of a Cell. It must here be explained that in describing 
 a cell, the more oxidisable or electro-positive metal (that, in fact, 
 which by dissolving produces the current) is termed the positive, 
 or preferably electro-positive, plate ; but as the current outside 
 the solution starts from the other plate, the terminal of the latter 
 from which the wire is taken to the external circuit is called 
 the positive pole. In the copper-zinc cell, the zinc is the positive 
 plate, but the wire attached to it is the negative pole ; and the 
 copper, while it is termed the negative plate, becomes the positive 
 pole outside the liquid. This is not likely to be forgotten if the 
 facts be clearly grasped: (1) That the plate from which the 
 current starts, whether within the solution or without, is in- 
 variably designated positive ; (2) that the current always starts 
 from the electro-positive element to pass through the invervening 
 liquid of the cell, and always, therefore, starts from the electro- 
 negative element to pass through the wires and outside-con- 
 nections of the battery ; and (3) that the starting and finishing 
 points within the liquid are termed the positive or negative 
 plates, metals, or elements, and the starting and finishing points 
 outside the solution are called the poles. This nomenclature is 
 not followed in the case of secondary cells, as it has been found 
 more convenient to call that plate with the positive pole the 
 positive plate, and similarly in regard to the negative pole and 
 plate. The whole path of the current is called the circuit that 
 in the cell being the internal, and that outside, the external 
 portion ; and any conductive connection between the two plates 
 is said to complete the circuit. When the two plates themselves 
 
THE GALVANIC BATTERY. 39 
 
 come into contact within the cell, or they become united, either 
 in the interior of the cell or close to the poles, by any metal which 
 presents practically no resistance to the current, the battery is 
 said to be short-circuited. When this short-circuiting takes place 
 in any way while the current is flowing through the ordinary 
 circuit, which probably presents a much higher resistance, nearly 
 the whole of the current is diverted from the desired path, and 
 passes through the easier passage, thereby causing a cessation of 
 work and a great waste of battery-zinc ; for when there are two 
 possible paths open to a current, it will divide itself between the 
 two in inverse proportion to their resistances. Thus let us 
 suppose a current of 200 amperes to arrive at a point in the 
 circuit at which a junction is reached, so that it may continue to 
 flow to another point in the circuit, either by one channel with 
 a resistance of 1 ohm, or by a second with a resistance of 99 
 ohms ; the result will be that the path having 1 ohm resistance 
 
 99 
 will carry th of the current, or 198 amperes, while the 
 
 \) 7 ~T* J. 
 
 other takes the remaining T jy^th, or 2 amperes. 
 
 Local Action on Zinc. Whenever commercial zinc is used as 
 the positive plate in an acid solution, it must be well amalgamated 
 with mercury. Impure zinc dipped alone into acid evolves clouds 
 of hydrogen gas from its surface. This is to be explained by the 
 presence of minute specks on the surface of the zinc of more 
 electro-negative bodies (e.g., lead and iron), which being impurities 
 in the body of the metal and, therefore, in contact with the mass 
 of the zinc, and also exposed to the acid liquid, act (each for itself) 
 as minute independent batteries, with the zinc as the dissolving 
 element, so that from each of them hydrogen is continuously 
 evolved. 
 
 This local action, as it is termed, is objectionable, because it 
 entails a loss of zinc without any equivalent of work done outside 
 the battery, all the energy being expended in heating the materials 
 of each of these minute and local circuits within the cell ; this, of 
 course, represents so much waste. Further, the phenomenon 
 is not merely temporary, for the electro-negative impurities, 
 although perhaps almost invisible, are not attacked by the liquid ; 
 and the gradual removal of the zinc by solution only results in 
 laying bare a greater number of them, thus increasing rather than 
 diminishing the extent of the local action during the progress of 
 the discharge. The most obvious remedy is to use only the purest 
 zinc, but this is costly ; while the protection afforded to the 
 metal of commerce, by a thin coat of mercury is found to impart 
 equal efficiency and to be more economical. To amalgamate the 
 zincs, clean them well ; then with a piece of flannel or sponge tied 
 to the end of a stick, wash them with dilute sulphuric acid (1 of 
 acid to 15 or 20 of water) ; then pouring a few drops of mercury 
 upon the centre of each, rub it cautiously over the plate until the 
 
40 SOURCES OF CURRENT. 
 
 whole surface presents a silvery brightness. Mercury will adhere 
 only to perfectly clean metallic surfaces, hence the necessity for 
 the preliminary washing with acid ; if, however, the zinc be very 
 dirty or at all greasy, this treatment will not suffice, and it should 
 be first immersed in warm caustic soda or potash solution. This 
 will remove all grease ; then, after washing thoroughly in water, 
 the plate should be flooded with the acid and rubbed with mercury 
 as before. An excess of mercury is to be avoided, as it soon pene- 
 trates the zinc and renders it brittle. The plate should be often 
 examined, and re-amalgamated as soon as dark spots are seen 
 upon the surface, or when gas is evolved from them during the 
 working of the battery. The acid should not be allowed to touch 
 the hands more than necessary ; and if it fall upon the clothes it 
 should be at once neutralised by a drop of strong ammonia, or it 
 will in course of time produce a red stain, and finally destroy the 
 cloth. 
 
 Polarisation. As already mentioned, the earliest form of 
 voltaic cell was the dry pile, which was very ineffective as a 
 source of current owing to its high internal resistance. This 
 was followed by Volta's 'simple cell' consisting of zinc and 
 copper plates in pairs immersed in an electrolyte : for this 
 purpose Volta used salt water, but Davy and others used acid, 
 which gives a much better result. By a * simple cell ' is now 
 generally understood a cell consisting of two metal plates, or a 
 metal plate and carbon, standing in acid, such as dilute sulphuric 
 acid. This is the simplest form of voltaic cell. One of the chief 
 objections to cells of this class is that the hydrogen gas evolved 
 at the copper plate clings to it and cannot readily escape. This 
 formation of hydrogen on the surface of the electro-negative 
 plate is termed polarisation, and presents a double disadvantage : 
 first, the mere formation of a film of any gas upon the metal 
 prevents the contact of its whole surface with the liquid, and, 
 by interposing a layer of an insulating medium, reduces the 
 surface area of the plate as much as if a portion of it were coated 
 with an acid-resisting varnish ; and in the second place, a far 
 more serious difficulty is found in the tendency of the separating 
 (positive) hydrogen to set up a cell on its own account, in co- 
 operation with any oxygen in solution near the zinc plate, 
 forming what is termed a 'gas cell.' Since hydrogen is electro- 
 positive to oxygen, the E.M.F. so created tends to send a current 
 from the hydrogen on the copper plate through the electrolyte 
 to the zinc; that is, the E.M.F. so set up is in the opposite 
 direction to that of the cell. Thus a simple copper-zinc cell may 
 for the first few seconds of use give a fairly strong current ; but 
 as soon as the action has proceeded sufficiently far to give rise 
 to a production of hydrogen on the copper plate, it becomes 
 polarised, and the opposing electro-motive force which is set up 
 greatly impairs the efficiency of the cell. 
 
THE GALVANIC BATTERY. 41 
 
 Reduction of Polarisation. A large number of voltaic cells 
 have been invented to meet the different requirements of practice, 
 but for the most part with the object of producing either a more 
 or less constant current (that is, a current of uniform strength 
 for a long time), or an intermittent current. In any case, 
 polarisation must be minimised, and this is effected either by 
 mechanical or by chemical means. The mechanical method 
 consists in so arranging the cell that the hydrogen shall escape 
 from the surface of the negative plate immediately it forms ; the 
 chemical method seeks to prevent the separation of the hydrogen 
 altogether, by means of substances which combine with it to form 
 harmless compounds, or which cause the deposition of another 
 suitable metal in its place. The latter, or chemical system, of 
 overcoming polarisation has the added advantage that it increases 
 the chemical activity in the cell, and thus adds to the electro- 
 motive force. The substances employed for converting the 
 hydrogen into a neutral substance are usually of an oxidising 
 character and thus effect their object by the formation of water ; 
 and this operation may be conducted either by immersing both 
 plates in a single oxidising solution, or by dipping the negative 
 metal only in the oxidant, which must then be contained in a 
 porous vessel placed within that holding the dilute acid and the 
 positive element. The former class are termed single-fluid cells, 
 the latter two-fluid cells. Other arrangements are possible, but 
 are not used in practice, and will not be dealt with here for 
 example, those with two liquids (in different compartments) and 
 one metal, or the gas battery of Grove. 
 
 Smee's Cell. Of single-fluid cells in which polarisation is 
 mechanically remedied, the most noteworthy is that invented by 
 Smee. In this battery a plate of silver is placed in dilute sul- 
 phuric acid between two plates of zinc, but the silver plate is 
 coated superficially with a very thin layer of platinum, so that 
 the hydrogen may escape with comparative readiness from the 
 slightly roughened surface which is thus produced. But even 
 with this device a certain amount of hydrogen must exist on the 
 surface of the silver so long as the cell remains in action. The 
 separation and escape of the gas, however, occur with fair regu- 
 larity, so that although the electro-motive force rapidly decreases 
 during the first few seconds of use, it then remains constant at a 
 point considerably higher than it would if the zinc were opposed 
 to a pure silver plate, and yet higher than if it were opposed to 
 copper, on account of the greater electro-chemical difference 
 between the two former metals than between the latter. But 
 the mechanical depolariser is palliative only, not remedial. 
 
 The annexed sketch of the Smee-cell (fig. 4), shown in section, 
 illustrates its construction. The zinc plate, Z, is doubled, so that 
 it may surround the silver on two sides and thus utilise both 
 surfaces of the latter. It may be formed of two zinc plates 
 
42 
 
 SOURCES OF CURRENT. 
 
 mounted with the platinised silver between them in a wooden 
 frame, which being a very feeble conductor may carry away a 
 minute fraction of the current, but serves to hold the metals in 
 position, so that a quite thin sheet of silver may be employed 
 without fear of its bending out of shape and making a short 
 circuit. The zinc alone dissolves, and requires, therefore, to be 
 initially of fair thickness ; the silver plate may theoretically be 
 only sufficiently stout to carry the current without presenting 
 undue resistance, for the E.M.F. of a cell is independent of the 
 dimensions of its elements. The acid, which is prepared by 
 carefully (vide p. 366) adding 1 volume of strong sulphuric acid 
 with constant stirring to 10 parts of water, is contained in a glass 
 or stoneware vessel of suitable shape. These vessels are generally 
 rectangular, and may be made to hold one pair or any greater 
 number of the zinc and silver plates, which are 
 supported by resting the wooden framework upon 
 3 x?^- the tops of the side walls of the acid trough. 
 *" The Smee-cells are very weak, the electro-motive 
 
 force of one being only about 0*47 volt. ; and 
 they evolve much hydrogen, the escape of which 
 into the air in a constant succession of minute 
 bubbles, each mechanically carrying with it a trace 
 of sulphuric acid, causes a slightly unpleasant 
 choking sensation in breathing the atmosphere of 
 the room in which they are working. Bat, on 
 the other hand, they are compact and very simple 
 in construction, and if kept clean are not liable 
 to become disordered ; while the escape of the 
 hydrogen produces a peculiar hissing sound, which 
 to the practised ear, may indicate by its varying 
 intensity any accidental derangement either in 
 the cells themselves or in the baths which they are electrolysing. 
 It will cease altogether if the circuit is broken and the current 
 has ceased to flow, or it will grow louder if a short circuit has 
 formed at any point, and so diminished the resistance and in- 
 creased the activity in the battery, thereby causing a needless 
 waste of current. This cell is still largely used by electrotypers on 
 account of its regularity and simplicity when properly supervised. 
 Daniell-Cell. The form of battery invented by Professor 
 Daniell is one which, by reason of its constancy, is well suited to 
 electro-depositing work. In it hydrogen is not deposited at all, 
 but is made to displace metallic copper from its solution, by 
 immersing the negative plate in a saturated solution of copper 
 sulphate. If both plates were placed in this liquid, the zinc, 
 being more electro-positive than copper, would at once commence 
 to exchange with that element contained in the solution, a part 
 of the zinc forming soluble zinc sulphate, and an equivalent 
 amount of copper precipitating in a pulverulent or spongy form 
 
 FIG. 4. Smee- 
 cell. 
 
THE GALVANIC BATTERY. 43 
 
 on the surface of the remaining zinc. This film, being pervious 
 to liquid, would still allow the copper solution to attack the zinc 
 core, and by ' local action ' would even facilitate the exchange 
 of the metals. The battery would be rendered wasteful of zinc, 
 and, indeed, useless. To obviate this, the copper solution with its 
 electro-negative plate of copper is separated from the simple acid 
 liquid containing the zinc by a porous partition of some kind, 
 generally of baked but unglazed earthenware, which will permit 
 diffusion, but not free interchange, of liquids through its pores. 
 This is usually effected by placing the zinc in a porous vessel 
 within the outer cell containing the copper ; thus, a * two-fluid 
 cell ' is produced. The action which takes place may be expressed 
 as follows number 1 representing the condition of the cell when 
 it is standing inactive, number 2 that when the circuit is com- 
 pleted and a current is being generated : 
 
 1. ( - Plate) Cu, Cu.S0 4 . Cu. S0 4 : : : H 2 .S0 4 . H^.S0 4 , Zn (+ Plate). 
 
 2. Cu, Cu. S0 4 .Cu. S0 4 .H 2 : : : S0 4 .H 2 . S0 4 .Zn. 
 
 In outer cell. Within porous cell. 
 
 These changes or reactions may be described in the form of 
 equations thus, 
 
 1. In porous cell : Zn + H 2 S0 4 = ZnS0 4 + H 2 . 
 
 2. In outer cell : H 2 + CuS0 4 = H 2 S0 4 + Cu, 
 
 or the ultimate reaction of the battery may be summed up in one 
 equation, 
 
 Zn + CuS0 4 = ZnS0 4 + Cu. 
 
 Thus, the hydrogen finds its way, as an ion, through the liquid 
 in the pores of the inner cell-wall, exchanges place with the 
 copper, and produces a practical transference of sulphuric acid 
 from the outer to the inner cell ; and, as the acid on the one side 
 of the porous division becomes neutralised by zinc, the strength 
 of the copper solution on the other side is rapidly diminished by 
 the deposition of its metallic constituent. The neutralisation of 
 the acid is immaterial, but the complete exhaustion of the copper 
 sulphate sets a limit to the utility of the cell ; long before this 
 point is reached, however, the power of the battery is much 
 lessened. As it is chiefly essential that copper alone, and no 
 hydrogen, should be separated, care must be taken to keep the 
 liquid in the inner cell saturated with copper sulphate, by 
 suspending crystals of the solid copper salt (blue vitriol) just 
 below the surface of the liquid, so that the solution may be 
 replenished as fast as it is impoverished. 
 
 The usual form of the Daniell-cell is indicated in fig. 5. It 
 consists of a cylindrical copper vessel holding a saturated solution 
 of copper sulphate, and acts both as a containing vessel and as 
 
44 
 
 SOURCES OF CURRENT. 
 
 FIG. 5. Daniell-cell. 
 
 the negative plate of the battery. Around the top is a perforated 
 copper, ring-shaped trough, T, which is below the level of the 
 liquid, and is always kept filled with crystals of the copper salt. 
 Within this vessel is the porous clay-cell, P, closed at the bottom 
 but open above, and in this is the amalgamated 
 zinc rod or plate, Z, immersed in dilute sul- 
 phuric acid (1 acid : 10 water), or in a solution 
 of zinc sulphate. Instead of using a copper 
 external vessel, a thin sheet of rolled copper 
 may be bent into cylindrical shape and placed 
 in a stoneware or glass jar ; the spare crystals 
 are then suspended in muslin bags within the 
 outer cell. The prime cost of this latter arrange- 
 ment is less ; but it must be remembered that 
 in the former the copper is the negative ele- 
 ment, and the cylinders are, therefore, in no 
 way injured, but on the contrary are thickened 
 by the gradual deposition of metal while the 
 current is passing, and are not appreciably 
 attacked by the dilute acid contained in them, 
 Modified Daniell-Cells. The arrangements for maintaining 
 the supply of copper sulphate crystals are very numerous ; for 
 example, Breguet and others have adopted a globular receptacle 
 of glass, which is charged with the salt, filled up with water, and, 
 the neck being closed by a cork with two perforations, is inverted 
 above the inner porous cell, so that the bottom of the cork lies 
 beneath the fluid level within the latter : then as the battery 
 liquid deposits copper it becomes lighter, and 
 floating upwards, becomes displaced by the heavy 
 saturated solution in the flask, while the poorer 
 solution, which has found its way into the re- 
 ceiver, again becomes enriched, and is in turn 
 ready to take the place of a further quantity of 
 impoverished liquor. In this cell the copper is 
 placed in the inner compartment, as in fig. 6 : 
 the globe, G, is supported by the neck of the 
 porous pot, while between the two is inserted 
 a copper tape, C, which serves for the negative 
 element. Around the porous cell is a bent j' 1G IT DanTeR-cell 
 cylinder of zinc in sulphuric acid ; the current is (Breguet's form), 
 led away by the wires marked + and - attached 
 to the copper and zinc respectively. Somewhat similar to this 
 is the Meidinger-cell, in which a flask containing the crystals 
 dips into a small cup containing the copper plate, resting on 
 the bottom of a glass vessel, the diameter of which is enlarged 
 to contain the zinc cylinder at a point somewhat above the 
 upper portion of the cup. In Kuhlo's cell a copper vessel is 
 used which carries the perforated trough outside instead of 
 
THE GALVANIC BATTERY. 45 
 
 inside, and the zinc is enclosed in parchment paper instead of 
 in a porous cell. 
 
 Other modifications in shape may be made. In the pattern 
 adopted by the Post Office authorities, a long teak-wood trough, 
 coated inside with some water-resisting varnish, such as marine 
 glue, is divided into a number of compartments by alternate 
 divisions of slate and porous earthenware. It contains the zinc 
 and dilute acid in divisions 1, 3, 5, and the remaining odd 
 numbers ; and the copper as copper sulphate in the even 
 divisions, 2, 4, 6, etc. It is simply a conveniently compact and 
 portable arrangement of a battery of several cells. Large Daniell- 
 cells may be made horizontal instead of upright, and without 
 porous divisions, provided they are allowed to rest undisturbed 
 in a position where they will not be subjected to any considerable 
 amount of vibration, the difference in the specific gravities of the 
 solutions employed sufficing to keep them in their required 
 relative positions, more particularly if the cells are in constant use. 
 If a cell of this kind is kept 
 out of use the copper sulphate 
 gradually diffuses to the zinc. 
 A simple form of such a gravity 
 battery is shown in fig. 7. A 
 wooden trough of convenient 
 size, say 14 ins. long by 10 
 wide and 3 high, is lined with 
 insulating varnish, and provided 
 
 with a glass partition, G, near FIG. 7. -Gravity battery, 
 
 to one end, which must reach 
 
 to the top of the trough, but only to within a quarter of an 
 inch of the bottom. On the bottom rests a large sheet of 
 thin copper plate with a narrow strip attached to it, which, 
 passing under the glass partition, is bent up within the smaller 
 compartment to form the positive pole, C. Above this plate, 
 and resting on wooden brackets in the sides of the trough, 
 is a stout grid or plate of cast zinc with perforations to allow 
 of the escape of any gas which might be formed through in- 
 correct working ; this should be well insulated from the copper 
 and kept at a minimum distance of a half an inch from it. A 
 zinc strip attached to the grid serves for the negative pole, Z. A 
 dilute solution of zinc sulphate, to which a few drops of sulphuric 
 acid have been added, is poured into the trough until the zinc is 
 covered ; the small compartment is then filled with copper 
 sulphate crystals, and these, gradually dissolving, form a heavy 
 solution which, finding its way beneath the glass partition, covers 
 the floor of the vessel to the height of an eighth or a quarter of 
 an inch. The zinc is, therefore, now in a dilute zinc sulphate 
 solution, the copper in a strong copper sulphate solution, and the 
 battery is ready for use. The crystals in the smaller division 
 
46 SOURCES OF CURRENT. 
 
 must, of course, be renewed, as they gradually dissolve away. If 
 well attended to, this form of the Daniell-cell will yield good 
 results, but it is essential to avoid all agitation which would tend 
 to mix the solutions and bring copper-bearing liquids into contact 
 with the zinc ; a parchment-paper tray to receive the zinc plate 
 will minimise this risk, but in any case the grid should be care- 
 fully removed from time to time and cleansed from any copper 
 deposited upon it. 
 
 Indeed, in all forms of the Daniell-cell the zinc plates should 
 be constantly inspected, as not infrequently splashes of copper 
 liquids find their way into the zinc compartment ; then, a portion 
 of the copper is deposited on the zinc by simple electro-chemical 
 exchange, and local action being set up, a certain amount of zinc 
 dissolves and more copper is separated. As soon, therefore, as 
 a blackish deposit is observed upon any portion of the zinc, it must 
 be removed, and the surrounding liquid examined ; even the 
 merest shade of blue colour in the solution indicates the presence 
 of copper, and points to the necessity for its renewal. When 
 copper is thus found in the zinc compartment the porous cell 
 should be most carefully examined, as it may have become 
 cracked, and would thus permit a comparatively free interchange 
 of liquids. 
 
 Kelvin's (Thomson's) Tray Battery is a gravity Daniell-battery 
 somewhat similar to the above, but so arranged that a number of 
 trays can be placed on top of one another. The zinc grid is made 
 with upwardly projecting lugs on which the next tray of the series 
 is stood. The zinc itself stands on porcelain insulators, and there 
 is no compartment (such as that shown in fig. 7) for copper 
 sulphate crystals. Such batteries are used largely in telegraphy. 
 
 The Daniell-cell has an electro-motive force of about 1 volt, 
 varying with the strength of the acid, the nature of the solvent 
 and the like, from 0'98 to 1 '08 volts. The E.M.F. usually accepted 
 is 1*079 volts. It is well suited for electrolytic work, especially 
 on account of the extreme constancy of the current, but requires 
 greater care and attention than the Smee-cell, to which, however, 
 it is in many respects preferable. 
 
 Grove- Cell. In the Grove-cell a different depolarising system 
 is adopted. Instead of substituting metal for the gaseous 
 hydrogen, Grove used a medium containing an excess of oxygen 
 which oxidises the hydrogen into the innocuous compound, water, as 
 rapidly as it is deposited upon the negative plate. This medium 
 is nitric acid, the chemical action being represented as follows : 
 
 1. In outer cell : Zn + H 2 S0 4 = ZnS0 4 + H 2 . 
 
 2. In inner cell: H 2 + 2HN0 3 = N 2 4 +2H 2 0. 
 
 Or in one equation 
 
 Zn + H 2 S9 4 + 2HNO ? = ZnS0 4 + N 2 4 + 2H 2 0. 
 
 Zinc. Sulphuric acid. Nitric acid. Zinc sulphate. Nitric peroxide. Water. 
 
THE GALVANIC BATTERY. 
 
 47 
 
 The nitrogen peroxide (N 2 4 ) is a gas which at once dissolves 
 in the nitric acid, imparting to it a red tint at first, and after- 
 wards a deep green colour. It is this gas which, after the acid 
 is saturated with it by the long-continued action of the battery, 
 is seen to come off in the form of dense red-brown fumes possess- 
 ing a suffocating odour. 
 
 The negative plate is not made of copper, which would be 
 vigorously attacked by the nitric acid, but of platinum in the 
 form of foil, as this metal entirely resists the action of the acid. 
 Fig. 8 shows (in section) two Grove-cells set up in series to 
 illustrate the method of connecting them. Within a glass or 
 glazed earthenware outer vessel is the dilute sulphuric acid con- 
 taining the zinc plate bent into the shape indicated, in order to 
 give a larger surface by surrounding the negative plate, and for 
 
 FIG. 8. Mode of connecting a 
 pair of Grove-cells. 
 
 FIG. 9. Grove-cell (external 
 
 view). 
 
 convenience in connecting with other cells. Enfolded by the zinc 
 is the porous compartment containing the strip of platinum foil, 
 P, in strong nitric acid. The platinum is not attacked when in 
 use, and is only deteriorated by repeated mechanical manipulation 
 in setting up or taking down the battery ; but its high value adds 
 greatly to the prime cost of the cell, and for this reason the 
 Bunsen-cell, with its block of carbon in lieu of platinum, is 
 generally preferred for commercial purposes. 
 
 Fig. 9 gives a general view of the external appearance of the 
 Grove-cell, showing one element complete, with the attachments 
 of the next platinum plate on the one side, and of the zinc with 
 its porous compartment on the other. 
 
 Bunsen-Cell. The Bunsen-cell has the same reaction and, there- 
 fore, practically the same electro-motive force as Grove's, viz., 
 nearly twice that of Daniell's (i.e., 1'8 to 1*9 volts). In a circular 
 stoneware jar (fig. 10) is contained the porous cell with its rod 
 of gas-carbon and strong nitric acid, while around the porous pot 
 
48 
 
 SOURCES OF CUKRENT. 
 
 is a bent cylinder of zinc immersed in dilute sulphuric acid. The 
 carbons are usually cut from the blocks of the deposit which 
 gradually forms on the interior of gas-retorts, and is known as 
 gas-carbon or retort-carbon. It is extremely dense, strong, and 
 durable ; but is sufficiently porous to allow the acid to pass 
 upward by capillary attraction, and so to attack the brass binding 
 screws, which form the connection between the battery plates 
 and the wires through which the current is conveyed to its place 
 of application. To prevent this action, D'Arsonval recommends 
 steeping the extreme upper end of the carbon for a short time 
 in melted paraffin, then depositing a thin film of metallic copper 
 on its surface by electrolysis, and plunging the whole of this 
 portion beneath melted type-metal ; the type-metal 
 will thus form a covering which is in contact with 
 the carbon at the edges of the joint, and must 
 effectually prevent the corrosion of the binding 
 screw, while maintaining perfect electrical con- 
 nection between the parts. 
 
 In both Grove's and Bunsen's cells the exciting 
 fluid may be prepared by adding 1 part (by 
 volume) of sulphuric acid to 10 parts (by volume) 
 of water. The nitric acid must initially be 
 highly concentrated, the specific gravity being 
 not less than 1-30 ( = 34 Baume). 
 
 The tendency of the reaction is to produce 
 nitric peroxide, which finally escapes as a gas, 
 and water, as we have already seen ; thus, not 
 only is the nitric acid constantly being weakened 
 ___________________________ by decomposition, but the remainder is at the 
 
 ' same time diluted by the water which is formed. 
 
 FIG. 10. Bunsen- T ,. ,, J , . . . , . ,, 
 
 In this manner the strength ot the acid rapidly 
 
 falls off, and its oxidising efficiency is lessened. 
 
 By the time it has been reduced to a density of 30 
 (sp. gr. = 1-26) the electro-motive force of the cell becomes 
 quickly diminished, and when it has reached a strength of 28 
 Baume (sp. gr. = 1-23) it is no longer serviceable, and should 
 be at once renewed. These cells present many advantages ; they 
 have a high electro-motive force, and are fairly constant for a few 
 hours, but they require to be filled with fresh acids daily. The 
 nitric acid, however, is a constant source of danger in inex- 
 perienced hands on account of its extreme corrosiveness ; and the 
 red nitrate peroxide gas, which is constantly evolved, is not only 
 disagreeable, but if inhaled for any time or in any quantity is 
 positively injurious. Whenever either of these forms of cell is 
 used it should be placed in a well-ventilated space outside the 
 workroom, outside a window, or in a cupboard provided with 
 the means of discharging the gases into a flue or directly into the 
 outer air. In spite of these drawbacks they are largely used for 
 
THE GALVANIC BATTERY. 49 
 
 nickel-plating and for some other classes of work, especially, 
 perhaps, for those of an experimental nature. 
 
 Bichromate-Cells. Some operators have substituted chromic 
 acid (in the form of a chromate) for nitric acid, because the pro- 
 ducts of decomposition are solid and remain dissolved in the 
 liquid instead of passing into and contaminating the atmosphere ; 
 a suitable solution, proposed by Higgins, consists of 1 part of 
 potassium bichromate dissolved in a mixture of 3 parts of sul- 
 phuric acid with 9 parts of water. The disposition of such a 
 cell is similar to those which we have just described. 
 
 But it is more usual to use the same liquid throughout, i.e., to 
 use single-fluid bichromate-cells in which no porous division is 
 needed, and these have the advantage of a lower internal resist- 
 ance, though the current is less constant. The best proportions 
 for the exciting fluid are, perhaps, those which correspond to the 
 equation representing its action as follows : 
 
 K 2 Cr 2 7 + 7H 2 S04 + 3Zn = 3ZnS0 4 + K 2 S0 4 + Cr 2 (S0 4 ) 3 + 7H 2 0. 
 Potassium Sulphuric Zinc. Zinc Potassium Chromium Water. 
 bichromate. acid. sulphate, sulphate. sulphate. 
 
 This would require about 3| oz. of potassium bichromate 
 and 8J oz. of sulphuric acid to be dissolved in a quart of 
 water. A much more easily prepared solution, 
 however, is obtained by using chromic acid instead 
 of a chromate ; for this purpose 1 1 pints of water 
 may be poured upon 12 oz. of chromic acid, 
 which will be dissolved immediately, and adding, 
 with constant stirring, 1 pint of concentrated 
 sulphuric acid. As the zinc is here actually in 
 the strongly-oxidising solution during the action 
 of the battery, and as a very notable amount of 
 solvent action would take place even when the 
 battery is at rest (though by no means com- 
 parable with that which would be set up in a 
 single-fluid nitric acid cell), means must be de- n (bottlebrm) 1 
 vised for removing the zincs from the liquid 
 directly the current is broken, as, indeed, is desirable in all acid 
 cells. This is generally accomplished by drawing or winding 
 them up out of the acid. In the bottle-form of this cell (fig. 11) 
 two long strips of carbon, united by a metallic connection 
 above, are fastened (parallel to one another) to a vulcanite 
 stopper, and are there connected with the binding screw + ; 
 these form the negative element and pass to the bottom of the 
 bottle ; between them is a short thick strip of zinc attached to 
 a brass rod passing stiffly through the centre of the ebonite cork 
 and connected with the binding screw - . The zinc is entirely 
 insulated from the carbon by the ebonite, and may be drawn out 
 of the solution by means of the brass rod as soon as its services 
 
 4 
 
50 
 
 SOURCES OF CURRENT. 
 
 are no longer required. In the trough-form of battery (fig. 1 2) a 
 series of carbon-and-zinc couples are immersed in the acid mixture 
 contained in jars held in a long rectangular trough, from which 
 the couples can be withdrawn when the battery is not in use. 
 These batteries are simple, give a high electro-motive force (1'9 to 
 2 volts), and have a very low internal resistance ; but the chromic 
 solution, although emitting no fumes, is highly corrosive, and, 
 moreover, must be constantly watched, for as it becomes weakened 
 by use the E.M.F. is increasingly affected by polarisation. 
 
 Edison-Lalande-Cell. Another form of cell, known as the 
 Edison-Lalande, has a great advantage over those just described 
 (at least for experimental work), because the current is very con- 
 stant and the battery is unaffected by standing ; in other words, 
 
 FIG. 12. Bichromate of potash battery (trough-form). 
 
 there is no local action, and therefore the zinc need not be 
 removed when the battery is out of use. There is, however, one 
 considerable disadvantage ; namely, the E.M.F. is low, only about 
 0*75 volt when in action, and the cost of materials is rather high. 
 The cell consists simply of a plate of copper oxide held in a suit- 
 able copper frame between two zinc plates and immersed in a 25 
 per cent, solution of caustic soda. The plates are conveniently 
 attached to the cover of the jar containing the electrolyte, and 
 the latter is covered with a layer of heavy paraffin oil to prevent 
 the formation of carbonate by the action of atmospheric carbon 
 dioxide. When the cell is in action the zinc is attacked by the 
 caustic soda, and the polarising hydrogen reduces the copper 
 oxide to metallic copper. Such cells are made by the General 
 Electric Company (London) in sizes ranging from 150 to 600 
 ampere-hours ; thus the largest size would give, for example, a 
 current of about 10 amperes for 60 hours. 
 
 Leclanch6-Cell. The number of other batteries in use for 
 various purposes is immensely large and is yearly increasing, but 
 
THE GALVANIC BATTERY. 51 
 
 only a few are of practical use to the electro-metallurgist. Many 
 are too weak ; others are strong at first, but speedily lose their 
 power ; while others again, which by their strength and constancy 
 are well adapted to our purpose, are too expensive or too trouble- 
 some for daily use under the conditions of the workshop. Thus, 
 to take a single example, the Leclanche'-cell, economical enough 
 as to prime cost and in use, gives a current which, at first power- 
 ful, rapidly declines owing to polarisation, but almost as rapidly 
 recovers itself when standing at rest ; it is thus very suitable for 
 intermittent or irregular work, such as the ringing of electric 
 bells, but is useless for electro-plating. 
 
 Practical Hints on the Use of Batteries. In all dealings with 
 batteries cleanliness in every detail is very desirable. The cells 
 and the metals must be thoroughly cleansed before commencing 
 work ; and it is of the highest importance that all metallic con- 
 nections in the external circuit, such as the junctions of the plates 
 with the binding screws or with one another (when placing them 
 in series) be kept perfectly bright. A film of oxide or tarnish 
 between two surfaces through which a current must pass increases 
 the resistance of the circuit to the passage of the current, and so, 
 while retarding the action of the battery, adds to the amount of 
 electrical energy converted into useless heat, and thus practically 
 wasted. Porous cells must be kept clean ; they are apt to become 
 choked with insoluble deposits which add greatly to their resist- 
 ance ; and in the Daniell-battery nodules of copper frequently 
 form upon their surfaces these must be watched for and removed. 
 When taking a battery to pieces after use, in order to replenish 
 and re-fit it, it is well to soak the porous cells for some time in 
 water, in order to extract the salts contained in them, and never 
 to allow them to become dry until they are thus purified ; other- 
 wise they crack. 
 
 A fairly pure zinc should always be used; it may be in the 
 condition of rolled sheet, but is more usually cast into the desired 
 shape ; in any case the whole surface must be thoroughly, but not 
 excessively, amalgamated. As soon as bubbles of gas are ob- 
 served to form upon the zinc, it should be carefully examined, as 
 this is a certain sign that some more electro-negative substance is 
 present, which may either be an impurity in the metal itself, when 
 re-amalgamation will remedy the defect, or may be due, as in a 
 Daniell-cell, to a metal deposited from the battery solutions, in 
 which case it must be removed by cutting, scraping, or filing. 
 Old or worn-out zincs should be saved, as they contain much 
 mercury which is recoverable (see p. 320). 
 
 No metallic connection between the two plates of a battery, 
 either within or outside the electrolyte, is permissible, because 
 such a connection forms a short circuit, which stops current 
 flowing in the external circuit. Absolute insulation (or electrical 
 separation) between the two poles, and between the wires at all 
 
52 
 
 SOURCES OF CURRENT. 
 
 intermediate points in the circuit, must be preserved ; and the 
 higher the electro-motive force of the battery, the greater is its 
 power of overcoming resistance, and, therefore, the more carefully 
 must the insulation be ensured. Dry wood, which is a poor 
 insulator for electricity of very high potential, is a good insulator 
 for ordinary battery currents ; but when moistened with acid or 
 with any of the battery solutions, it becomes capable of causing 
 very considerable leakage of current. 
 
 The battery solutions require careful watching. Any tendency 
 to crystallise or deposit solid matter upon the sides or bottom of 
 the cell must be checked ; it may be due to the use of too con- 
 centrated solutions at the outset, which is curable by the addition 
 of water, or it may be that they have become saturated and worn 
 
 c out, when they, of course, 
 
 require to be renewed. 
 The quantities to be used 
 in making up solutions 
 should be weighed or 
 measured ; this occupies 
 but little more time, and 
 in the result is more 
 reliable than rule-of- 
 thumb work or mere guess 
 measurements. If the 
 solutions made up from a 
 crystalline salt be turbid, 
 they should be filtered 
 through a blotting-paper 
 cone inserted in a glass 
 funnel (metallic funnels 
 should not be used, as 
 
 FIG. 13. Mode of folding filter-paper. 
 
 they are liable to be corroded by the fluids passing through 
 them). A circular filter-paper is readily made to fit the funnel 
 by folding it first across one diameter, as shown at A B in 
 division 1 of fig. 13 ; then on folding it again at right angles, 
 as at C D in No. 2, it has the form of No. 3; now on inserting 
 the finger between the folds of the paper it may be opened out 
 to the conical shape depicted in No. 4, and is thus ready to 
 place in the funnel. If, however, the paper should not fit 
 well into the cone of the latter, it may be refolded along the 
 line, E F, as in No. 5, or along any other suitable line, and 
 may thus be adapted to suit a funnel ' constructed with any 
 angle at its apex. Strongly-acid solutions, such as those used 
 in the bichromate battery, cannot be thus filtered, as they 
 destroy the paper; but the solution of the potassium bichro- 
 mate may be passed through a filter before adding the acid to 
 it. If it be necessary to clear any solution which attacks 
 paper, a plug of spun glass or of asbestos may be lightly rammed 
 
THE GALVANIC BATTERY. 53 
 
 into the apex of the funnel, and will form an efficient filtering 
 medium in lieu of paper. 
 
 It is a good plan to place the battery in a chamber apart from, 
 but adjoining, the depositing room, remembering that the greater 
 the length of copper wire to be traversed by the current between 
 the battery and the vats, the greater will be the loss of energy 
 through the resistance of the circuit. By keeping it outside, 
 however, the operator is not annoyed by the fumes or acid spray 
 produced by certain forms of cell ; and there is less danger of 
 mistake or of accident in manipulating the battery and depositing 
 solutions. If circumstances compel the double use of the same 
 room, the battery should on no account be placed above or too 
 near the electrolytic vats, but should be fitted up in a place easy 
 of access, and preferably in a ventilated cupboard, as already 
 explained. It will be found convenient to set up the battery in 
 proximity to a sink provided with a good water supply, so that 
 every facility may be afforded for cleansing the different parts 
 when the work of the day is over. In charging two-fluid cells, 
 no drops of the depolarising fluid must be permitted to enter the 
 zinc compartment. 
 
 On the Fittings and Connections of a Battery. It will be 
 remembered that the electro-motive force of a voltaic cell is 
 measured by the heat-energy produced by the combinations 
 taking place within it ; hence the size and shape of a cell may 
 greatly modify the volume of current generated by it, but are 
 without influence on the pressure or E.M.F. Other things being 
 equal, the volume of any current may be increased by raising 
 the electro-motive force or by lowering the total resistance of 
 the circuit, as will be more fully explained in the next chapter 
 (infra, p. 80); now, the internal resistance of a battery is in- 
 creased, either by increasing the distance between the two 
 plates, because the current has to traverse a longer distance 
 through an inferior conductor ; or by using smaller battery 
 plates, because the cross section of the liquid conductor is thus 
 reduced, and the current has to flow through a narrower channel. 
 By increasing the resistance by either of these methods, the total 
 current must be diminished while the electro-motive force is 
 constant. Conversely, the use of larger plates (that is, of larger 
 cells) or the nearer approach of the plates in the cell, while 
 leaving the pressure unchanged, increases the volume of current 
 by diminishing the resistance to its path in other words, the 
 output in amperes is increased, but the E.M.F. in volts is constant. 
 With any given type of cell, then, it is possible to alter the 
 current strength but not the electro-motive force ; to effect the 
 latter, changes in the chemical conditions are necessary. 
 
 Modes of Arranging Cells. By the use of more than one cell 
 the electro-motive force as well as the current-intensity may be 
 altered at will. If two similar cells be joined, as shown in fig. 14, 
 
54 
 
 SOURCES OF CURRENT. 
 
 copper to copper and zinc to zinc, the electro-motive force is no 
 more altered than would be the pressure per square inch produced 
 by placing two pails of water side by side upon a level floor in 
 place of one ; for both the cells are yielding the same pressure of 
 electricity, and the mere coupling them in parallel, as it is termed, 
 is only equivalent in effect to increasing the size of a single cell, 
 which as we have seen is without influence on the E.M.F. But 
 if the cells be disposed as in fig. 15, with the 
 copper of one joined to the zinc of the next, 
 and the free elements connected to the main 
 circuit, the current generated in the first cell 
 has to flow through the second, and that of 
 the second through the first, in order to com- 
 plete the whole circuit, with the result that 
 the total electro-motive force is doubled. This 
 arrangement, which is termed coupling in series, 
 is exactly analogous to lifting the one pail of 
 water above referred to and placing it upon the 
 other, when the pressure per square inch on 
 the floor is, of course, doubled. In parallel all 
 with 4 Tke W pl C a e tes S the Chemical energy of cells is placed upon 
 connected. the same level, and can, as it were, pump the 
 
 electricity only to the same height ; but in 
 series the energy of one cell pumps the electricity to one level, 
 and then that of the second cell starting from this point raises 
 it as high again. And so in setting up any number of cells, 
 if placed all in parallel, the E.M.F. is only that of one cell, but 
 the internal resistance is reduced, as it would be in one large 
 cell of the same type ; while if all are arranged in series the 
 E.M.F. will be raised in direct proportion to the number of cells 
 
 FIG. 15. Two cells with unlike plates connected. 
 
 in use. Intermediate dispositions to suit special requirements may 
 be made by grouping several cells together in parallel, and then 
 uniting this group as a whole in series with other similar groups. 
 Let us with the aid of the diagram, fig. 16 t consider the possible 
 groupings of the six cells, A, B, C, D, E, F, each of which has, let 
 us say, an electro-motive force of 1 volt and an internal resistance 
 of 1 ohm. One such cell, short-circuited by a piece of metal 
 having practically no resistance, would give a current of 1 ampere ; 
 for Ohm has enunciated the law that the current in a given 
 circuit is proportional to its electro-motive force, and inversely 
 
BATTERY CONNECTIONS. 
 
 55 
 
 proportional to the total resistance, which is expressed by saying 
 
 that the current is equal to the electro-motive force divided by 
 
 -pi 
 
 the resistance, or more shortly by the formula C = . In other 
 
 II . 
 
 words, Ohm's law may be written for any circuit as amperes 
 
 When the six cells in fig. 16 are placed in parallel, as 
 ohms 
 
 shown in position 1, all the coppers are united together, and as 
 a group are opposed to 
 all the zincs similarly 
 united ; the E.M.F. is 
 unaltered, and still 
 stands for the whole 
 group at only 1 volt ; 
 but the resistance is 
 divided by 6, because 2. 
 there are now six equal 
 internal passages for the 
 current instead of 1, and 
 it is therefore ^ ohm in- 
 stead of 1 ohm ; so that A 
 now the volume of the 
 current flowing through 
 
 , . . ., . 1 (volt) 
 
 a short circuit is .-4-. ' 
 
 i (ohm) 
 
 = 6 amperes. But if 
 the six cells are joined 
 in series, as in position 2, 
 the E.M.F. is multiplied 
 by 6, and is 6x1 = 6 
 
 FIG. 16. Modes of grouping six cells. 
 
 volts, while the resistance is also multiplied by 6, and is now 
 6x1 = 6 ohms, because now the current has to flow through 
 all the cells in series, and is, therefore, checked by the resist- 
 ance of each ; the current yielded on short-circuiting, is, therefore, 
 
 ' = 1 ampere, or the same as that of a single cell ! 
 
 6 (ohms) 
 
 (Note, however, that this is only on short-circuiting vide 
 infra.) Now, by joining the pairs A and B, C and D, E and 
 F, each respectively in parallel, and connecting the groups A B, 
 C D, and E F in series, as in position 3, the electro-motive 
 force of each parallel pair is only 1 volt; but there are three 
 pairs in series, so that the total electro-motive force is 3 volts ; 
 while the resistance of each pair is, of course, half that of one 
 cell, or J ohm, and the sum of the three resistances is f ohm, so 
 
 o 
 
 that the ultimate current on short circuit is = 2 amperes. 
 
 f 
 And, finally, by arranging the cells, as in position 4, with the 
 
56 SOURCES OF CURRENT. 
 
 three cells A B C in parallel, and united to form a series with the 
 group D E F also placed in parallel, the electro-motive force of 
 each group is 1 volt and the total pressure is 2 volts, while the 
 resistance of each is J ohm and the total resistance f ohm, so that 
 
 9 
 
 the resultant current is -^- = 3 amperes. The current quoted in 
 
 3 
 
 each case is, as we have stated, that produced when the external 
 resistance is nil ; but this condition never obtains in practice, and 
 the anomaly of six cells giving only the same current as would be 
 afforded by one is not met with when the external resistance is high, 
 for then the value of the increased electro-motive force is felt. 
 Thus the current from one of these cells acting through an 
 external resistance of 100 ohms (making with the internal resist- 
 ance of the cell a total resistance of 100 + 1 = 101 ohms) gives an 
 
 ampereage equal to \ ' = ampere, whereas the six cells 
 101 (ohms) 101 
 
 u 6 (volt) 6 
 
 in series would give ^ \ , -^ = ^-,^, ampere ; thus the current 
 106 (ohms) 106 
 
 1 /J 
 
 ratio in the two cases is - : - = 106 : 606 ; that is to say, 
 
 there is now 5*72 times as much current produced from the six 
 cells in series as there is from one alone. On the other hand, the 
 arrangement of the six cells in parallel, so advantageous on short 
 circuit, is little superior to a single cell when the current is to be 
 passed through so high a resistance as 100 ohms. In this case 
 the total resistance is 100 + J ohms ; the total E.M.F. is 1 volt; 
 
 1 fi 
 
 the total current , = ampere ; and the ratio of efficiency 
 10U + g bUl 
 
 of 1 cell to 6 in parallel is only JL ; JL = 601 : 606, or 1 : I'Ol 
 
 instead of 1 : 5 '7 2 as when they are set up in series. 
 
 From this it will be seen that to obtain the highest effect from 
 a number of cells, they must be united in series when the 
 external resistance is very high, or in parallel when it is very low. 
 The highest efficiency of all, as regards power, is obtained when 
 the cells are so grouped that the internal resistance of the battery is 
 equal to the external resistance of the circuit. But in electrolytic 
 work there are other considerations to which we will now refer. 
 
 Although the coupling-up of cells in series may be desirable as 
 sometimes affording the greatest possible current, such a disposi- 
 tion is by no means the most economical. ,A certain weight of 
 zinc, in dissolving, produces a certain E.M.F., and in one cell 
 yields a certain current capable of doing a certain quantum of 
 work. And where one cell only is employed, a given weight of 
 zinc, in dissolving, deposits electrolytically a definite equivalent 
 weight of any metal in the plating-baths. But when cells are 
 multiplied in series, an equal amount of zinc dissolves in each : 
 
BATTERY CONNECTIONS. 57 
 
 yet only the one equivalent of deposited metal is yielded, the 
 energy developed in the dissolving of the remaining zincs being 
 used in raising the electro-motive force in pumping the current 
 to a higher level. For example, in the case of the six cells in 
 series in short circuit, the same ultimate current is produced as 
 would be yielded by one cell ; but in the former case six times 
 the quantity of zinc is dissolved to produce the same effect. It 
 is clear that economically, the best result is obtained when the 
 smallest number of cells is placed in series which is compatible 
 with the production of the minimum electro-motive force required 
 for the electrolysis; and the disposition of the cells must be 
 governed by the balance of the two considerations time and 
 economy. In any case when two or more groups of similar cells 
 are coupled in parallel, each group should consist of the same 
 number of cells. 
 
 Connecting Screws. In making electrical connections between 
 the different parts of the circuit, the use of brass binding-screws 
 
 HHl 
 
 FIG. 17. FIG. 18. FIG. 19. FIG. 20. FIG. 21. FIG. 22. 
 Forms of binding-screws for batteries. 
 
 is advisable ; these may be procured of any apparatus-maker and 
 of any shape that may be required. Figs. 17, 18, 19, 20, 21, and 
 22 illustrate six of the more useful forms. 
 
 Figs. 17 to 19 represent the commoner connectors for uniting 
 plates or bars to wires. Fig. 17 is employed in the Bunsen 
 battery to join the carbon block to the terminal wire ; but it is 
 equally applicable to the uniting of any thick substance to a wire, 
 the former being held by the screw at the side of the large clamp, 
 the latter by that in the spherical portion above. Fig. 18 may 
 be screwed into a metal block, and is commonly used to join the 
 negative wire to the zinc of the Daniell-cell. Fig. 19 is employed 
 as the terminal binding-screw in a Grove-cell, and will serve to 
 connect any thin sheet to a wire. Fig. 20 illustrates the arrange- 
 ment for uniting two wires end to end. Figs. 21 and 22 represent 
 systems of coupling plates or blocks, the former as applied to two 
 thin sheets, the latter to a thick block with a thin plate. Other 
 forms may, of course, be devised to suit special requirements, but 
 these will answer most of the purposes to which they may be put. 
 
 Switch-Boards. Often for practical purposes, but for experi- 
 mental work more especially, it is desirable to have at hand a 
 
58 
 
 SOURCES OF CURRENT. 
 
 rapid method of altering the arrangement of a group of battery 
 cells, so that any desired combination in series and parallel may 
 be obtained. The constant alteration of battery connections is 
 tedious and clumsy; while a switch-board used by Professor 
 Silvanus Thompson is at once simple and efficacious. This 
 instrument is made with a series of binding-screws, and of 
 accurately-ground brass plugs fitting into cavities between ad- 
 jacent brass blocks, such as those employed in the manufacture 
 of resistance-coils for electrical measurement. Fig. 23 represents 
 a modification of Thompson's switch-board ; it is precisely the 
 same as the other in principle, but is of cheaper construction, and 
 may be made by any carpenter. A is a plan, and B a sectional 
 elevation of the board. Two parallel strips of brass plate, p and 
 n, are let into the surface of a varnished board, and are connected 
 
 each with a terminal binding- 
 screw at the end ( + and - ). 
 At even distances along the 
 surface of each brass rod is a 
 series of upright split brass 
 tubes, as at s', s in the sec- 
 tion B ; the tubes on p be- 
 ing spaced midway between 
 those on n, as shown. Along 
 the lines, P and N, are similar 
 rows of upright split tubes, 
 s, s, each of which is con- 
 nected by metal wire, with 
 a corresponding horizontal 
 split brass tube, s", s", as at 
 a b c a b' c' etc. The 
 
 c 
 
 FIG. 23.- 
 
 -Thompson's switch, modified. 
 
 connection is made as shown in the figure, B ; every pair of split 
 tubes, s' and s", must be perfectly insulated from every other pair, 
 and from the series, p and n ; there is, therefore, no brass rod 
 running along the lines, P and N. The upright tubes on the 
 row P correspond exactly to those in the row p ; those in N to 
 the others in n ; and the space between any corresponding tubes, 
 s and s, and also the diagonal space between s of row P and s of 
 row N, must be equal in every case to those between the tubes on 
 rows p and n. Twelve U-shaped pieces of brass rod, A, h, are 
 prepared of such size that they fit with each limb into one of any 
 adjacent pair of vertical split tubes (the object of the splitting is 
 to give sufficient spring to ensure a tight fit^and therefore good 
 contact) ; a like number of short straight brass rods are prepared 
 for the horizontal tubes, s", s". The necessity for even spacing 
 of the tubes is evident, if the handles, h, h, are to be inter- 
 changeable. 
 
 Each cell of the battery is now connected by a wire to the 
 apparatus, the positiye (copper) pole of the first to the s" tube, 
 
BATTERY CONNECTIONS. 59 
 
 marked a, the negative (zinc) pole to that marked a ; the second 
 is similarly attached to b and 6', the third to c and c', and so on, 
 taking care that the positive poles are all connected to one side, 
 the negative to the other, and that the two poles of every cell are 
 attached to corresponding tubes (a and a ; b and b' etc.). The 
 battery wires may be soldered, each to one of the above-mentioned 
 short straight brass rods, to be inserted in horizontal split tubes. 
 Supposing six cells to be thus connected up, no current can flow 
 even if the terminal wires marked ' + ' and * ' are joined, because 
 each of the tubes, s, s, is isolated ; but by inserting the handles, 
 h, h, in a suitable manner, the various connections are made in 
 any required fashion, and the current passes through the circuit 
 from the terminal + to that denoted by - . 
 
 By connecting every s in the P line with the corresponding s 
 in the p line, and similarly those in the N row with the others 
 in 7i, the positive pole of every cell is placed in direct contact 
 with the brass strip p, the negative with the strip n, so that 
 all the cells are in parallel, and can discharge thus through the 
 circuit. This arrangement is shown diagrammatically in plan C. 
 But if only the positive wire of cell a is placed in connection with 
 p, and only the negative wire of /with n, and the inner lines of 
 tubes are joined-up diagonally (a of line P with b' of N ; b of 
 P with c of N ; and so on), the whole six cells are in series, as 
 represented in diagram D ; because the copper pole of the first is 
 connected to the outer circuit, while the zinc pole is joined to 
 the copper pole of the next, and so on until the zinc pole of 
 the last is free and is united to the wire of the outer circuit. 
 Diagram E shows the cells, a, b, and c, in series connected in 
 parallel with d, e, /, also in series. Diagram F shows each 
 pair, a and b, c and d, e and /, in series, but the three couples in 
 parallel. Thus, diagrams C, D, E and F illustrate respectively 
 the four possible methods of combining 6 cells, all in parallel ; all 
 in series ; two parallel groups of 3 cells in series ; and three 
 parallel groups of 2 cells in series ; and these connections may 
 be altered at will in a few seconds. 
 
 A similar board can be made more simply by taking a piece of 
 board, cutting troughs, closed at each end, in place of the brass 
 strips, and cups or holes in place of the split tubes, s, s. The 
 main terminals are run into the troughs, and the cell terminals 
 through the board from underneath into the cups. Mercury is 
 then run into the troughs and cups, and connections are made by 
 dropping copper bridge pieces, similar to , into the mercury 
 when desired. 
 
 THERMO-ELECTRIC BATTERIES. 
 
 The fact, already alluded to in the introductory chapter, that 
 a compound bar, made up by soldering or jointing two unlike 
 metal strips end to end, is capable of producing an electric 
 
60 SOURCES OF CURRENT. 
 
 current when the junction is alone heated, renders possible the 
 direct conversion of heat into electrical energy. Instruments 
 used to effect this change are termed thermopiles or thermo-electric 
 batteries. They are simple, but they are not used to any large 
 extent, partly because they are wasteful and partly because they 
 are not very permanent. 
 
 Thermo-Electric Series. Any two metal bars joined as in fig. 
 24 will give a current always in the same direction when the 
 point of union is heated, or in the opposite direction when it is 
 cooled, more than the rest of the bars. In respect of their 
 behaviour when thus treated the various metals behave very 
 differently, and may be arranged in a thermo-electric series 
 similar to that indicative of their electro-chemical relations. Any 
 single pair of metals gives a constant electro-motive force so long 
 as the difference between the temperature of the junction and 
 that of the remainder of the circuit is con- 
 stant ; and, moreover, in many instances 
 the E.M.F. produced is nearly proportional 
 to this difference in temperature. The 
 actual electro-motive force of any pair of 
 metals is extremely minute compared with 
 that of a galvanic battery, so that a large 
 number of couples must be used to pro- 
 duce an equal effect. 
 
 In the following table are grouped some 
 of the principal metals with the electro- 
 motive force produced by heating a single 
 FIG 24" Simple couple made by heating them respectively 
 thermopile. a ^ their juncture with metallic lead to 
 
 a temperature 1 C. higher than the rest 
 
 of the circuit. The E.M.F. is given not in volts but in micro- 
 volts (1 micro-volt is the one-millionth part of 1 volt). In 
 harmony with the electro-chemical system of nomenclature, 
 which designates that metal electro-positive from which the 
 current starts to pass through the liquid in the voltaic cell, a 
 substance is said to be thermo-electro-positive to another when 
 the current passes from it to the second through the heated 
 juncture. In the table, lead is taken as the basis for comparing 
 the others, so that all the metals standing above lead are marked 
 with the + sign, which indicates that the current would flow 
 from them to the lead throught the joint, while with the negative 
 elements the current starts from the lead. The numbers cannot 
 be accepted as absolutely accurate, or universally applicable, 
 because the behaviour of metals in respect to these currents is 
 greatly affected by the presence of impurities, and because the 
 relations between them are disturbed, and in some cases reversed, 
 at different temperatures. The letters M, B refer to Matthiessen 
 and Becquerel, from whose results these numbers are taken. 
 
THERMO-ELECTRIC COUPLES. 
 
 61 
 
 TABLE V. SHOWING THE THERMO-ELECTRO-MOTIVE FORCE OF VARIOUS 
 ELEMENTS IN RELATION TO LEAD, EXPRESSED IN MICRO-VOLTS PER 
 DEGREE CENTIGRADE. 
 
 Metal. 
 
 Observer. 
 
 Micro- Volts. 
 
 Bismuth, pressed wire . . 
 
 M 
 
 + 89 to 97 
 
 Bismuth, crystals . . . . 
 
 M 
 
 + 65 to 45 
 
 Bismuth, ordinary . . , . 
 
 B 
 
 + 40 
 
 Bismuth- Antimony Alloy (10:1) . . 
 
 B 
 
 + 64-5 
 
 Cobalt 
 
 B& M 
 
 + 22 
 
 Nickel . . . . . s ; ' 
 
 B 
 
 + 15-5 
 
 German Silver 
 
 BM 
 
 + 11-6 
 
 Palladium . . . . 
 
 B 
 
 + 6-8 
 
 Mercury . . . . ' . . 
 
 B 
 
 + 3-2 
 
 Lead 
 
 ... 
 
 Zero 
 
 Tin, pure pressed wire .... 
 
 M 
 
 o-i 
 
 Tin, ordinary ..... 
 
 B 
 
 0'4 
 
 Copper, commercial . ._''.- 
 
 M 
 
 - 0-1 
 
 Platinum . . . Y . 
 
 BM 
 
 0-9 
 
 Gold ....... 
 
 M 
 
 - 1-2 
 
 Cadmium ...... 
 
 B 
 
 - 2-4 
 
 Antimony, pure pressed wire 
 Antimony, commercial pressed wire 
 
 M 
 M 
 
 - 2-8 
 6-0 
 
 Antimony, ordinary . . . ' . 
 
 B 
 
 - 17-1 
 
 Antimony, crystals . . . . 
 
 M 
 
 - 22-6 to 26 -4 
 
 Silver . . . . . 
 
 M 
 
 - 3'0 
 
 Zinc 
 
 M 
 
 - 37 
 
 Copper, electrolytic . . 
 
 M 
 
 - 3-8 
 
 
 M 
 
 - 13'6 
 
 Iron, pianoforte wire .... 
 
 M 
 
 - 22-6 
 
 Red phosphorus . . . ; 
 
 M 
 
 - 297 
 
 Antimony-Zinc Alloy (2:1). . . 
 
 B 
 
 - 99-0 
 
 Copper sulphide ..... 
 Antimony-Cadmium Alloy (1:1). 
 
 B 
 B 
 
 -196-7 
 -231-9 
 
 Tellurium ...... 
 
 M 
 
 -502 
 
 
 B 
 
 -807 
 
 
 
 
 Thus, for example, a cobalt-lead junction might be expected 
 to give an E.M.F. of 22 micro- volts for each degree through 
 which the junction was superheated ; and similarly a lead-iron 
 junction would give almost the same E.M.F., but the current 
 would flow in the opposite direction. To determine from these 
 figures the difference of potential between any other pair of 
 metals, it is only necessary to deduct the stated E.M.F. of the 
 more negative metal from that of the other, for example : 
 (1) When both metals are positive : a nickel-palladium pair 
 should give a current of 15-5 -6*8 = 8*7 micro-volts per degree, 
 passing from nickel to palladium through the junction ; (2) when 
 one is positive and the other negative : a nickel-zinc junction 
 should give 15'5 - ( - 3*7) = 15-5 + 3-7 = 19-2 micro-volts, flowing 
 from nickel to zinc ; and (3) when both are negative : a platinum- 
 silver couple should show -0'9 -(- 3-0) = 3'0-0'9 = 2'1 micro- 
 
62 
 
 SOURCES OF CURRENT. 
 
 volts, the current starting from the platinum. If the junction 
 be superheated 100 instead of 1, these numbers must of 
 course be multiplied by 100; thus the nickel-zinc pair should 
 yield an E.M.F. of 1920 micro-volts or 0-001920 volt, so that it 
 would require 521 of these couples heated to 100 C. to give the 
 E.M.F. equal to that of a single Daniell-cell. Some authors 
 arrange the metals in the opposite sense, designating those 
 negative which are here termed positive and vice versa. 
 
 Neutral Point. A peculiarity of these couples has already 
 been mentioned, namely, that the E.M.F.'s often vary dispropor- 
 tionately to the rise of temperature. It may so happen that a 
 
 5O" IOO" ISO" 2OO" 25O V 3OO" 35O" 4OO" 45O" 5OO" 55O 
 DEGREES CENTIGRADE 
 
 FIG. 25. Thermal E.M.F. of metals, per degree C., or thermo-electric 
 powers at different temperatures. 
 
 pair of metals giving a very low E.M.F. per degree at ordinary 
 temperatures will give a comparatively high E.M.F. at high tem- 
 peratures, while another couple which started well may gradually 
 fall off as heat is applied, until at a certain temperature (varying 
 with the metals, and known as the neutral point) there is no 
 E.M.F., arid, therefore, no current ; yet, on continuing the appli- 
 cation of heat, an E.M.F. is again set up, but it now tends to 
 drive the current in the reverse direction. The behaviour of 
 metals in this respect is best seen by reference to fig. 25. Lead 
 is here adopted as the standard of comparison, because, unlike 
 most other metals, hot lead in contact with cold lead produces no 
 difference of potential. To ascertain from this table the electro- 
 motive force produced by a difference of 1 C. between the hot 
 and cold junctions at any given temperature of the circuit as a 
 whole, it is only necessary to find the vertical line corresponding 
 
THERMO-ELECTRIC COUPLES. 63 
 
 to the required temperature, and then, noting the points at which 
 the sloping lines representing the two metals cross it, to read off 
 the difference in micro-volts between them with the aid of the 
 scale on the left-hand margin. Thus, for example, an increase in 
 temperature from to 1 in the warmer junction of an iron- 
 copper couple, the rest of the circuit being at 0, gives an 
 electro-motive force of -l-(-15) = 14 micro-volts, but a 
 similar rise from 100 to 101, the circuit being at 100, only 
 gives -2-(-ll) = 9 micro-volts, and from 200 to 201 only 
 _ 3 _ ( _ 6) = 3 micro- volts, in all these cases the current flowing from 
 copper to iron through the juncture. But a rise of temperature 
 from 260 to 261 produces no current, this being the neutral point 
 of these two metals (their respective lines cross here), while if 
 the two junctions of the copper-iron pair were at the temperatures 
 of 350 and 351, there would again be an E.M.F. of - ( - 5) = 5 
 micro-volts, but the current would now be flowing from iron to 
 copper. With some pairs of metals there is practically no 
 neutral point ; palladium and iron give a constant rise in poten- 
 tial for each increment of temperature, while with others the 
 neutral point is below the zero on the Centigrade scale, so 
 that the higher the temperature to which the one joint is heated, 
 the greater the efficiency of the pair e.g.) palladium and 
 cadmium. 
 
 Further, the total E.M.F. of a circuit may be readily calculated 
 from this diagram. Of course, if the rise in potential were strictly 
 proportional to the increase in temperature, it would be only 
 necessary to discover the difference in temperature between the 
 two junctions of the metal and multiply it by the number of 
 micro-volts stated per degree, but such a procedure would evi- 
 dently give very misleading results. To estimate the total E.M.F. 
 from the table, find the diagonal lines representing the two metals, 
 ascertain the mean temperature of the two junctions of the couple, 
 and mark the points at which the two diagonal lines respectively 
 cross the vertical line representing the mean temperature, then 
 multiply the number of micro- volts between these two points by 
 the number of degrees difference between the two temperatures, 
 and the result is the required E.M.F. Thus, it may be required 
 to know the actual E.M.F. of an iron-copper couple of which one 
 junction is at 200 C., the other at 100 C. The mean tempera- 
 ture is 150 C., and the difference of potential between the metals 
 at this point is (per degree) - 2 '5 - ( - 8 -5) = 6 micro- volts. The 
 required E.M.F. is, therefore, 6 x (200- 100) = 600 micro- volts or 
 0-0006 volt. 
 
 Another most important point, clearly brought out by this dia- 
 gram, is that when the neutral point of any pair of metals is above 
 zero Centigrade, not only does an increase of temperature yield 
 no proportionate return in the value of the E.M.F., but as soon as 
 the temperature of the hot end has exceeded that of the neutral 
 
64 
 
 SOURCES OF CURRENT. 
 
 point, there is an actual decrease of current, because the E.M.F.'s 
 at the two junctions are tending in opposite directions. A single 
 instance that of lead and iron will serve both to illustrate and 
 explain this phenomenon. The neutral point of these two bodies 
 is approximately 350 C. 
 
 TABLE VI. ILLUSTRATING THE TOTAL E.M.F. PRODUCED BY AN IRON- 
 
 AND-LEAD COUPLE AT DIFFERENT TEMPERATURES. 
 
 Temperature. 
 
 Total Micro-Volts 
 
 Increase of 
 
 
 J T 
 
 m ^ I TTI -\fi TJI 
 
 At Cool 
 
 At Hot 
 
 under these conditions. 
 
 per 100. 
 
 Junction. 
 
 Junction. 
 
 
 
 Centigrade. 
 
 Centigrade. 
 100 
 
 12-8x100 = 1280 
 
 Micro- Volts. 
 1280 
 
 
 
 200 
 
 107x200 = 2140 
 
 860 
 
 
 
 300 
 
 8-6x300 = 2580 
 
 440 
 
 
 
 350 
 
 7-5x350 = 2625 
 
 45 (per 50) 
 
 
 
 400 
 
 6-4x400 = 2560 
 
 -65 (per 50) 
 
 
 
 500 
 
 4-3x500 = 2150 
 
 - 410 
 
 
 
 600 
 
 2-1x600 = 1260 
 
 - 890 
 
 
 
 700 
 
 0x700= 
 
 -1260 
 
 The maximum current is thus produced at 350, thence it 
 decreases to zero at 700, and would then begin to increase again, 
 but in an inverse direction. The important bearing of this in- 
 version upon the choice of couples for the production of thermo- 
 electric currents and on the temperature to which they should be 
 submitted is at once evident. 
 
 Mode of Arranging Thermo Couples. A variety of thermo- 
 piles has been proposed, differing in the metals employed, in the 
 construction, and in the manner of applying the heat. The 
 essentials are that the two metals shall be as far apart in the 
 thermo-electric series as possible, that there shall be no chemical 
 or physical reason (too great fusibility or oxidisability) against the 
 use of either, and that they shall be conveniently arranged so that 
 the one point of juncture may be superheated as compared with 
 the other. The simplest method of arranging a series of couples, 
 so necessary where each pair produces but an infinitesimal electro- 
 motive force, is that shown in fig. 26. The rules which apply to 
 the fitting up of galvanic batteries in parallel or in series hold 
 good equally with thermo-electric batteries. 
 
 Clamond's Thermopile. A thermopile at one time popular 
 was that of Clcunond, in which metallic iron is united to an alloy 
 of antimony and zinc (combined preferably in the ratio of their 
 atomic weights, viz.: 122:65 or nearly 2:1). Fig. 27 shows 
 the arrangement of ten couples in circular form : strips of tinplate, 
 P, are generally employed for the iron element, and good contact 
 
THERMO-ELECTRIC COUPLES. 65 
 
 between the two metals is ensured by bending the strip into a 
 narrow loop at one end, placing this portion in a mould, and 
 pouring the melted alloy around it, so that it is actually embedded 
 in the casting ; thin plates of mica are inserted externally between 
 the metals to insulate them at all except the desired points of 
 contact. Each block, N, may be about 2J inches long by f inch 
 thick, but the dimensions may, of course, be greatly varied ; they 
 are arranged radially with a central space, H, between them 
 through which the heat is applied to the junctions, J. The 
 different pairs are arranged in series by bending the free end of 
 each tin-plate strip and soldering it to the external edge of the 
 adjoining block next in regular succession, a break, B, being left 
 at one point by which connection with the external circuit is made. 
 
 Fig. 26. Simple thermo- Fio. 27. Ten thermo-electric couples in 
 
 electric couples. circular form (Clamond's pile). 
 
 By this arrangement the points to be heated are brought to a 
 central focus, while the cool junctures are placed at the outside, 
 where they are most free to radiate away all excess of heat con- 
 ducted through the metal blocks. Generally there are ten pairs 
 in the circle, and several tiers of similar circles are piled one upon 
 another, but insulated from each other by a layer of cement com- 
 posed of powdered asbestos moistened with a solution of potassium 
 silicate. All the pairs in each row are in series, and each tier is 
 placed in series with the preceding one, so that the resulting 
 electro-motive force of a pile of ten tiers of ten blocks each would 
 be 100 times that of a single pair. The source of heat is usually 
 coal-gas, which is burned at jets from a perforated earthenware 
 tube placed upright in the central cavity. In some forms of this 
 pile the top of the cavity is closed, and a thin sheet-iron tube is 
 inserted between the centre pipe and the metal couples, leaving a 
 space above, so that the products of combustion are led down 
 through the annular space outside the iron lining, and are passed 
 
 5 
 
66 
 
 SOURCES OF CURRENT. 
 
 away through a flue at the bottom, the object being the economy 
 of a greater proportion of the heat from the burning gas. Char- 
 coal or coke may be substituted for coal-gas when necessary. A 
 battery of 6000 pairs, arranged in series and heated by coke, 
 has been found to give an electro-motive force of 109 volts (equal 
 to 100 Daniell-cells) with a total internal resistance of 15J ohms. 
 This description illustrates the principles on which a practical 
 thermopile depends ; but, owing to the fact that more convenient 
 sources of electrical energy are easily obtainable, the Clamond 
 pile has gone completely out of use, and this remark applies 
 also to Noe"'s pile. 
 
 Gtilcher's Thermopile. The only thermopile that is at present 
 obtainable, as far as the author is aware, is that due to Giilcher. 
 As seen in fig. 28, it consists of two rows of small bunsen burners 
 (noticeable at the right-hand end) above which are the elements, 
 on either side of the burners, clamped together by screws bearing 
 against a bracket at each end. The elements are supported by 
 sheet metal radiators which serve to keep down the temperature 
 of the cool junctions. On the right-hand side is seen the gas 
 connection for supplying the burners. These thermopiles are 
 made in three sizes, and their characteristics vary in accordance 
 with the accompanying table, which is due to the makers : 
 
 Size. 
 
 1 
 
 2 
 
 3 
 
 No. of elements 
 
 26 
 
 50 
 
 66 
 
 E.M.F Volts 
 
 1| 
 
 3 
 
 4 
 
 Intensity of current (when^j 
 the external resistance 1 A 
 is equal to the internal f Am P er 
 
 3 
 
 3 
 
 3 
 
 one) . . . J 
 
 
 
 
 Internal resistance . . Ohms 
 
 25 
 
 50 
 
 65 
 
 Approx. consumption of gas 
 per hour, about . . Cubic feet 
 
 2'6 
 
 4'9 
 
 6-4 
 
 Price 
 
 676 
 
 12 
 
 14 15 
 
 But even at best the thermopile is very wasteful, and it is 
 questionable whether more than 1 per cent, of the heat-energy 
 expended is recovered in the form of electricity ; indeed, Fischer 
 found by experiments, conducted in 1882,, that only 0'3 to 0'5 
 per cent, was utilised. Nevertheless, the study of the subject is 
 very fascinating, because there is a direct conversion of heat into 
 electricity; and, although at present an almost prohibitive loss 
 is involved, increase of knowledge may be attended by the pro- 
 duction of a higher efficiency ; the simplicity and convenience of 
 the arrangement would probably then ensure to it a wide applica- 
 
THE DYNAMO. 
 
 67 
 
 tion as a source of electricity for electro-metallurgical purposes 
 where only a small amount of power was required. 
 
 THE DYNAMO. 
 
 In the generation of electric energy the choice lies between 
 the oxidation or solution of zinc in a battery, and the oxidation 
 of coal, gas or oil, with the conversion of chemical energy first 
 into mechanical energy, and thence by dynamo-electric machinery 
 into electricity; but here also a large percentage of loss is in- 
 curred in the first stage of the conversion, owing to the waste 
 which is inevitable even in the best constructed steam-, gas- or 
 
 FIG. 28. Giilcher's thermopile. 
 
 oil-engine. Where, however, a steady and constant water-supply 
 is available for actuating water-wheels or turbines, the dynamo 
 is an economical machine, which may convert a very large 
 proportion of an otherwise waste form of energy into useful 
 electricity. 
 
 The prime phenomenon on which the dynamo depends is the 
 production of electro-motive force in electric circuits which are 
 caused to move in a magnetic field. 
 
 Principle of the Dynamo. It is customary to regard the space 
 in the neighbourhood of a magnet as being filled with magnetic 
 lines of force. Such lines are not tangible; they are but a 
 conception, due to Faraday, but nevertheless an extremely useful 
 conception. If iron filings are sprinkled on a sheet of' paper and 
 this is laid on a bar magnet, the paper being then tapped lightly, 
 the filings will arrange themselves in curved lines from pole to 
 pole, and at once indicate the magnetic lines of force as shown 
 
68 
 
 SOURCES OF CURRENT. 
 
 in fig. 29. This does not mean that there is necessarily a line of 
 force to each line of filings, but the filings show the general 
 curvature that such lines would take, and in order to obtain a 
 mental picture of what is going on we consider the space to be 
 filled with such lines, crowded together where the field is strong, 
 and more or less separated where the field is weak. 
 
 Now, it was found by Faraday that when a coil, of wire was 
 moved or rotated in a magnetic field so as to cut the lines of force, 
 
 and so that the number of lines 
 embraced by the coil varied, an 
 E.M.F. was produced in , the coil. 
 The direction of the E.M.F. de- 
 pends on the direction of the field 
 and on the direction of motion of 
 the coil. Suppose now that we 
 have a rectangular coil of wire 
 A B C D (fig. 30) between two 
 magnet poles, a north pole N and 
 FIG. 29. Bar-magnet lines of force. a soutn Ple S. The usual con- 
 ception is that the magnetic lines 
 
 pass from the north pole across to the south pole (in this case 
 we may say practically straight across). Now, if the coil is 
 vertical it will embrace a maximum number of lines, but a very 
 slight movement either way round the axis E F will not cause 
 any lines to be cut; there will therefore be no E.M.F. pro- 
 duced by motion just where the coil is vertical. But as it 
 moves round it will cut lines more and more rapidly for a 
 given speed of rotation, until it 
 becomes horizontal (when the rate 
 of cutting will be a maximum), 
 and will then cut them less rapidly 
 until a vertical position is again 
 reached after half a revolution. 
 In this particular case, then, the 
 E.M.F. becomes zero twice every 
 revolution, and has maximum 
 values twice every revolution, at 
 times midway between the times of zero values. It must be 
 remarked, however, that the E.M.F. is not in the same 
 direction during both the half-revolutions ; thus if it is in the 
 direction from A to B during one half-revolution, it will have the 
 direction B to A during the next half-revolution. Consequently 
 the E.M.F. reverses twice every revolution and is known as an 
 alternating E.M.F., and would give rise to an alternating current 
 in any circuit connected to the coil. Since alternating currents 
 cannot be used in ordinary electrolysis, this alternating E.M.F. 
 in the dynamo is converted into a continuous E.M.F. by a device 
 called a commutator which we shall now describe briefly in its 
 
 FIG. 30. Production of E.M.F. 
 in magnetic field. 
 
THE DYNAMO. 69 
 
 most elementary form. The commutator for a single coil, 
 C D E F, is arranged as shown in fig. 31, and consists of a metal 
 tube split into two equal parts lengthways, the two halves being 
 fastened around a small cylinder, but well insulated from one 
 another; one half is attached to one end of the loop wire, the 
 second half to the other. Copper strips or brushes, B B', are 
 fixed to the frame of the dynamo, so that they press lightly upon 
 the split cylinder at points diametrically opposite to one another ; 
 as the tube is divided equally, and as the brushes are parallel, it 
 follows that whatever the position of the cylinder, both brushes 
 are never in contact with the same section simultaneously. To 
 the brushes are fastened the wires connecting with the external 
 circuit, so that any current generated in the coil flows first to 
 one section of the split tube, thence to the brush in contact with 
 it, and so around the outer circuit to the other brush, then 
 through the second section of the 
 tube to the other end of the coil 
 of wire, and so completes the cir- 
 cuit. Now, provided the rotation 
 of the coil around its axis, A A, be 
 always in the same direction, and 
 both the brushes and the breaks 
 in the commutator tube be at the 
 highest and lowest points in the 
 circle, at the moment when the 
 reversal of current takes place, the 
 current in the main circuit flows 
 
 continually in the same direction ; 
 
 3f . i FIG. 31. Commutator for a 
 
 because the brushes come in con- single coil, 
 
 tact with different sections of the 
 
 commutator directly the current is reversed, and the same brush 
 is always connected with the descending side of the coil, the 
 other brush always with the ascending portion, no matter whether 
 it be C D or E F. Supposing, for example, the coil is rotating 
 from left to right as indicated, C D is now descending and brush 
 B is touching section G, and the direction of current may be 
 supposed to be (C-D-E-F-H)-B'-circuit-B-(G-C), etc. ; this direction 
 continues until a half-revolution is completed and the side E F is 
 uppermost ; now as E F begins to descend, the current tends to 
 flow in the coil in the reverse direction, thus, F E D C ; but at 
 this instant the commutator also shifts, and section G is now in 
 contact with the brush B' (instead of with B) and H with B 
 now, then, the current flows through the circuit in the direction 
 (F-E-D-C-G)-B'-circuit-B-(H-F), etc., and so on. It is very clear, 
 then, that in spite of alterations in direction of the current in 
 the coil, the current invariably flows from the brush B' to the 
 outer circuit, and from the outer circuit through the brush B, to 
 whichever section of the commutator happens to be touching it ; 
 
70 SOURCES OF CURRENT. 
 
 B' is, therefore, as much the positive pole of the system we have 
 been describing as the binding-screw attached to the platinum 
 plate is the positive pole of the Grove-battery. 
 
 Parts of a Dynamo. In the actual dynamo the number of 
 these coils is multiplied, and the methods of arranging them 
 various, the whole system being known as the armature. Thus, 
 in electric generators of this type, there are two main parts, 
 the field-magnet and the armature (including the commutator) ; 
 and the different types of machine are made by varying the 
 construction of one or both of these. It should be noted here 
 that the electro-motive force of a dynamo may be raised (1) by 
 increasing the number of lines of force cut by the rotating rings 
 (i.e., by using a stronger magnet), (2) by increasing the number of 
 coils in the armature, or (3) by increasing the speed of its rotation. 
 
 Types of Dynamo. In the earliest types of dynamo the 
 magnets for producing the field in which the armature rotates 
 were permanent magnets ; but as the dynamo was developed 
 these were replaced by electro-magnets in order to obtain more 
 powerful effects, and the current for these was supplied either by 
 an outside source or by the dynamo itself. As to which method 
 of supplying the current is the better is a matter of convenience, 
 depending on circumstances. If the current is supplied by the 
 dynamo itself, there are two main methods in which the current 
 may be used. (1) The whole of the current passing through the 
 armature and through the external circuit may also be passed 
 round the field magnets, so that the greater the current required 
 the greater also is the magnetic field produced. Consequently 
 the E.M.F. of such a dynamo varies with the current that is taken 
 from it. For this and other reasons series dynamos, as they are 
 termed, are unsuitable for electrolytic work. (2) The other 
 method is to let the dynamo supply current to the field circuit as 
 though it were an external circuit in parallel with any other 
 circuit to which we wish to supply current. The field circuit is 
 simply connected up to the two brushes, and such a machine is 
 said to be a shunt dynamo. This kind of dynamo gives a more 
 or less constant E.M F., whatever the load, any drop in the E.M.F. 
 when the current required is heavy being generally compensated 
 by a 'field regulator' which varies the resistance in the field 
 circuit. Dynamos of this kind are therefore suitable for electro- 
 lytic work, and they are always used for this purpose. 
 
 The usual characteristic of dynamos for electro-metallurgical 
 work is that they are required to give large currents at low 
 pressures as compared with dynamos for electric light and power ; 
 for example, hundreds or thousands of amperes may be required 
 at pressures ranging from quite a low figure, say 10 volts, up to 
 200 volts or more, the pressure depending entirely on the purpose 
 for which current is required and whether vats can be conveni- 
 ently run in series. Fig. 32 illustrates an interesting example, 
 
THE DYNAMO. 
 
 71 
 
 a 
 
 1! 
 
 il 
 
72 
 
 SOURCES OF CURRENT. 
 
 and refers to a turbine set made by Messrs Brown, Boveri & Co. 
 for the Austrian Chemical and Metallurgical Company in Aussig, 
 Bohemia. It is a steam turbine, and drives a 1000-kilowatt 
 three-phase generator, as well as two generators for electrolytic 
 work. The two latter are seen more particularly in the illustra- 
 tion; they are each of 540 kilowatts at 120 volts and thus give 
 
 FIG. 33. Water-driven electrolytic generator giving 1700 kilowatts at 
 220 volts. (Brown, Boveri & Co.) 
 
 4500 amperes. It will be noticed that the brush gear is very 
 heavy so as to collect this relatively large current without 
 sparking or undue heating. In fig. 33 is illustrated a dynamo 
 supplied by the same firm to the Kellner Partington Paper Pulp 
 Company of Norway. This machine gives 1700 kilowatts at 220 
 volts, which is equivalent to 7700 amperes. It is coupled to a 
 vertical water turbine and is proportionately much more massive 
 than the other machine, because it runs at 150 revolutions per 
 minute as compared with 1260 revolutions in the case of the 
 steam turbine set just mentioned. 
 
THE DYNAMO. 73 
 
 We do not propose to go further into the details of dynamos, 
 because it is impossible to treat the subject sufficiently in a few 
 pages to serve any useful purpose ; the reader is therefore 
 referred to the many treatises now obtainable on dynamo-electric 
 machinery, or text-books on electrical engineering, if he desires 
 further information. 
 
 Instructions as to the Management of Dynamos. In itself the 
 dynamo is a simple machine and should give but little trouble 
 or difficulty, provided that a due amount of attention is paid to 
 its installation and superintendence. It should be firmly set on 
 good level foundations, in a position where it is not likely to 
 be exposed to contact with any of the liquids employed in the 
 shops, nor to acid fumes or dust. It is best located in a dry 
 place, enclosed in a box of wood or glass which may be easily 
 removed for purposes of inspection, and which has openings cut 
 in its side to provide for the passage of the engine belt or for 
 the shaft of the pulley; it should be in a room adjoining the 
 plating-room, but as near as possible to the vats, in order to 
 minimise the loss of energy due to the resistance of the wires. 
 If erected in the shop, it must be in a place removed from splash- 
 ings, and must be constantly examined. It must be kept 
 scrupulously clean in every part; the bearings must always be 
 well lubricated with good oil, as the speed of shaft rotation is 
 usually very high (500 to 1000 revolutions per minute), but 
 excess of oil, and especially leakage of oil into the armature or 
 on to the commutator and brushes, must be rigidly guarded 
 against. By the action of internal currents, as well as by 
 friction, the machine is liable to become overheated, and may 
 even be seriously damaged through the burning of the insulating 
 material upon the wires ; the temperature of the fixed portions 
 should, therefore, be ascertained while current is being generated 
 by feeling them carefully with the hand. With fair usage the 
 only part needing special attention is the commutator or collector. 
 The brushes must be perfectly flat ; if they are of copper gauze, 
 when they become ragged or turned up at the tips they should be 
 clamped in a wooden holder and filed carefully; they should 
 rest with a gentle pressure upon the commutator ; insufficient 
 pressure combined with an uneven commutator will induce 
 sparking at the brushes, which causes much wear and tear of 
 the parts, whilst excessive friction will wear away the collecting 
 bars unevenly and ruin the brushes, at the same time producing 
 more or less copper dust. The brushes should also touch the 
 commutator at opposite ends of a diameter ; they are generally 
 set on a frame or * rocker ' by which they are maintained exactly 
 opposite to one another, and by which they may be kept in any 
 desired position. Should the dynamo spark at the commutator, 
 the position of the brushes may be altered backwards or forwards 
 by shifting the rocker until a neutral or sparkless position for the 
 
74 SOURCES OF CURRENT. 
 
 brushes is found ; this position is liable to vary according to the 
 current taken from the dynamo. The commutator may be very 
 slightly greased before commencing the day's work, preferably 
 with a little grease or vaseline applied with the finger ; cloth or 
 cotton should not be used, as fibres may be left behind, and it is 
 essential that no foreign substance find its way to this portion of 
 the machine. Oil must never be applied in any quantity, and 
 the grease only in very minute proportions (they add to the 
 resistance), while black lead or mercury, which some operators 
 have applied to the surfaces, must be avoided altogether, as the 
 latter gradually amalgamates the copper and renders it very 
 brittle, while the former gives a slightly conductive film to the 
 insulating space between the bars of the collecting ring, and so 
 impairs the electrical efficiency of the machines. The brushes 
 must never be lifted out of contact while the dynamo is running 
 and the current passing ; the insulation may be, and the com- 
 mutator certainly will be, damaged by the sparking caused by 
 such treatment, which, moreover, may be a source of danger if 
 the machine is provided with but a single pair of brushes. If 
 there are several pairs of brushes a single brush may be removed 
 or replaced by a skilled attendant without difficulty. Irregulari- 
 ties in the commutator bars, or undue pressure, or sparking at 
 the brushes will gradually wear the shaft to an oval or uneven 
 shape ; this fault should be watched for and rectified at once by 
 turning in the lathe to true circular section ; such a course should 
 not, however, be necessary for several years. 
 
 The dynamo may be driven by any regular source of mechani- 
 cal energy ; either the steam-, gas-, hot air-, or petroleum-engine 
 (or, of course, water-power) may be used, the only requirements 
 being sufficiency of power and uniformity of speed. Frequently 
 spare power is obtainable from an engine used for other purposes, 
 and may be applied with advantage, unless great irregularities in 
 speed are caused by frequent variations in the amount of power 
 absorbed by the machinery to which it was originally adapted. 
 
 ACCUMULATORS or Secondary Batteries, which are so largely 
 used in other branches of electrical engineering, may be also 
 employed in electro-metallurgical installations ; but, generally 
 speaking, little benefit will be derived from their use. 
 
 Secondary Batteries. The great advantage of the accumu- 
 lator is that it converts the electrical energy of the dynamo, at 
 times when it cannot be directly applied, into another form of 
 energy (chemical) which may be, as it were, stored up until a 
 more convenient moment has arrived for its re-conversion into 
 electricity. Several kinds of secondary batteries have been 
 brought before the public, but almost the only one in general 
 requisition is that originated by Plante and afterwards modified 
 by Faure and others. It consists of two lead plates opposed to 
 
ELECTRICAL ACCUMULATORS. 75 
 
 one another in dilute sulphuric acid. On passing a current 
 through this constantly in the same direction, the anode plate, 
 by which the current enters the solution, becomes superficially 
 converted into lead peroxide (Pb0 2 ) by the oxygen liberated at 
 its surface, and this, being insoluble in sulphuric acid, remains 
 in situ, the action penetrating deeper and deeper into the plate 
 by proper treatment. The cathode is unaltered by the hydrogen, 
 but in practice it consists largely of spongy lead. On the 
 cessation of the current, two plates are opposed in the solution, 
 one of metallic lead, the other practically of lead peroxide ; on 
 completing the circuit these act like a primary (or ordinary 
 voltaic) battery, the clean lead plate (electro-positive) is super- 
 ficially converted into sulphate, which, like the peroxide, is in- 
 soluble, while the peroxide (electro-negative) gives up its excess 
 oxygen and becomes also converted into sulphate. When both 
 peroxide and metal have attained to the same condition the action 
 ceases, but on re-charging by a dynamo, the original arrangement 
 is restored, the sulphate at the anode taking up oxygen and be- 
 coming peroxide, that at the cathode being reduced to the state 
 of metal by the hydrogen liberated upon it. It is thus rehabili- 
 tated and is ready for use again, and this cycle of changes may 
 be repeated indefinitely. Now, instead of forming the cell from 
 two clean plates, the positive and negative plates are generally 
 coated with pastes of red lead and sulphuric acid and of litharge 
 and sulphuric acid respectively ; cavities are frequently made on 
 the surface of the lead with the object of affording a larger 
 surface, and, more particularly, of maintaining the oxide pastes 
 in position. Plates thus prepared are quickly converted electro- 
 lytically into spongy lead and lead peroxide respectively on the 
 surface, and do not require the frequent repetition and reversal 
 of charging which is found necessary to ensure sufficient penetra- 
 tion of the oxide into the substance of the pure metallic sheets. 
 In use, these cells are exactly analogous to ordinary batteries and 
 obey the same laws as to generation and distribution of current 
 indeed, they are practically nothing but an unusual form of 
 galvanic battery ; but they should not be allowed to discharge 
 themselves completely nor should they be subjected to unduly 
 heavy currents, such as would occur if they became suddenly 
 short-circuited. The electro-motive force of each cell is equal to 
 about 2 volts. 
 
 Use of Public Electricity Supply. Most towns have now a 
 public electricity supply for lighting purposes and for power ; but 
 this supply is usually at a pressure of 200 volts or more, which 
 is far too high for direct use in electrolytic vats, unless many 
 are coupled in series, which, for plating purposes, is practically 
 out of the question. Moreover, in many towns the current is 
 alternating, and therefore useless to the electro-metallurgist. 
 Such currents may, however, be readily utilised. For example, 
 
76 SOURCES OF CURRENT. 
 
 if the current from the supply mains be continuous (not alter- 
 nating) and at a pressure of 200 volts, it may be passed through a 
 number of accumulator cells (say 80) joined up in series. The 
 cells, being thus charged together, may be separated and used 
 individually, so that a pressure of 2, 4, 6, 8 or more volts may be 
 obtained for any purpose by using 1, 2, 3, 4 or more cells. Such 
 a method, however, is not very desirable, because the cells are apt 
 not to get uniform treatment and therefore some of them may 
 deteriorate. 
 
 A simpler method than this may be used with advantage, and 
 is applicable alike to continuous and alternate current systems ; 
 it depends on the fact that a dynamo when reversed becomes a 
 motor ; that is to say, if a current of electricity be sent through 
 the brushes, and thus through the armature of a dynamo, the 
 electrical energy will become re-converted into mechanical energy, 
 and the armature will rotate and will act as a motor. Motors 
 specially wound to suit any continuous or alternating current can 
 be procured, and may be placed on the main town circuit, and 
 used to drive an electro-plating dynamo by means of a belt ; or 
 the two dynamos (motor and generator) may be mounted on the 
 same shaft, in which case the couple are frequently called a motor 
 generator or they may even be made, by special winding, to use 
 the same field magnets, and are then practically one machine, 
 often known as a rotary converter. With an arrangement such 
 as any of the above, the current from the town supply mains 
 may be converted into a current of low E.M.F., suitable for 
 electro-plating, and of correspondingly greater volume. Except 
 for the loss in conversion, the total watts remain the same, so 
 that what is lost in volts is gained in amperes. It should be un- 
 necessary to point out that a plating dynamo cannot be used as 
 a motor on the town supply mains as it would have been made for 
 too low a voltage. 
 
CHAPTER IV. 
 
 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. 
 
 Absolute Cleanliness Essential. In electro-metallurgical work 
 there is one prime necessity which cannot be too often or too 
 strongly insisted upon, and that is, absolute cleanliness in every 
 particular. Neglect of cleanliness in the electric generator, and 
 in the connections throughout, causes the current to be deficient 
 and the resistance of the circuit to be increased; neglect of 
 cleanliness in making up the baths introduces impurities into 
 the solutions, and the character of the deposit suffers accord- 
 ingly; neglect of cleanliness in preparing the articles to be 
 plated for their plunge into the depositing - vat ensures an 
 irregular and non-adhesive coating ; neglect of cleanliness after 
 the plating is accomplished is likely to cause rapid tarnishing 
 or rusting of the deposit. In short, want of cleanliness is the 
 cause to which the largest proportion of electrotypers' and platers' 
 troubles may be referred. And in respect of non-adhesiveness 
 of a deposit, it should be noted that the articles to be treated 
 must be chemically clean ; a trace of grease, producible by mere 
 handling, suffices to ruin the coating, because the precipitated 
 metal will adhere only to perfectly pure metallic surfaces, and 
 the merest film of foreign matter, invisible perhaps to the eye, 
 prevents this adhesion, or weakens it to so great an extent that 
 very slight friction may cause separation to take place. 
 
 As also Careful Adjustment of Currents. It is further essential 
 that the current shall be correctly proportioned to the work re- 
 quired from it, and, if for this reason alone, it is to be regretted 
 that old ' rule-of -thumb ' methods find very general favour. 
 Apparatus for measuring the current is comparatively inex- 
 pensive, and the first outlay in instruments will soon be found 
 to repay itself in the greater security insured against accidents 
 due to careless work or imperfect knowledge, and in the im- 
 mediate and certain indication of a failure in any part of the 
 circuit, which may prove most detrimental to the work if allowed 
 to remain unremedied, but which, if attended to in time, might 
 exert no evil influence. An ammeter ( = amperemeter) for 
 measuring the strength of current to every bath, or to every 
 
 77 
 
78 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. 
 
 series of baths placed in parallel, with a voltmeter for determining 
 the fall in the pressure of the current between the electrodes, 
 should be used in all large installations in which it is desired 
 to produce good work of uniform character, or where varied 
 work is undertaken. It is true that an experienced workman 
 may know by inspection whether his baths are in good order; 
 but the best workman is not infallible, and the use of measuring 
 apparatus substitutes certainty for uncertainty, besides enabling 
 the foreman to ascertain at a glance the conditions of work at 
 any moment. Where, too, a partial failure has occurred, the 
 remedial measure to be adopted may often be indicated, and the 
 current be at once restored to its original strength by adjusting 
 the resistance of the circuit until the ammeter returns to its 
 first position. 
 
 TABLE VII. SHOWING THE AVERAGE CURRENT-DENSITIES SUITABLE 
 FOR DEPOSITING CERTAIN METALS. 
 
 
 Amperes. 
 
 Vnlfo 
 
 Metal. 
 
 Per Sq. 
 Decimetre 
 of Cathode 
 Surface. 
 
 Per Sq. Inch 
 of Cathode 
 Surface. 
 
 V OltS 
 
 between 
 Anode and 
 Cathode. 
 
 Antimony . 
 
 0-4-0-5 
 
 0-020-0-030 
 
 1-0 -1-2 
 
 Brass ... 
 
 0-5-0-8 
 
 0-030-0-050 
 
 3-0 -4-0 
 
 Copper, acid bath 
 
 1-0-1-5 
 
 0-065-0-100 
 
 0-5 -1-5 
 
 ,, alkaline bath 
 
 0-3-0-5 
 
 0-020-0-030 
 
 3-0 -5-0 
 
 Gold ... 
 
 o-i 
 
 0-006 
 
 0-5 -4-0 
 
 Iron ... . 
 
 0-5 
 
 0-030 
 
 1-0 
 
 Nickel, at first . 
 
 1-4-1-5 
 
 0-09 -0-10 
 
 5-0 
 
 after . .... 
 
 0-2-0-3 
 
 0-015-0-02 
 
 1-5 -2-0 
 
 ,, on zinc . 
 
 0-4 
 
 0-025 
 
 4-0 -5-0 
 
 Silver . . . 
 
 0-2-0-5 
 
 0-015-0-030 
 
 075-1-0 
 
 Zinc. . . . . . k 
 
 0-3-0-6 
 
 0-02 -0-04 
 
 2'5 -3-0 
 
 
 A current which is either too strong or too weak gives un- 
 satisfactory deposits, the most suitable strength in any instance 
 depending upon the nature of the metal which is being deposited 
 and that of the bath from which it is separated. A very weak 
 current causes a slow and in some cases a bad deposit, while 
 a very intense current renders the metal non-adhesive, and may 
 even reduce it to a pulverulent form. In the preceding table 
 are given the density and electro-motive force of the currents 
 which have been found by careful operators to be best suited 
 for depositing the commoner metals; but such general state- 
 ments must be regarded only in the light of a guide ; every bath 
 will be found to have its own most suitable current value, which 
 
USE OF ELECTRO-CHEMICAL EQUIVALENTS. 79 
 
 will probably differ but little from those given above, but which 
 may be readily determined, once and for all, by the use of the bath. 
 Electro-Chemical Equivalents. In dealing with electro-de- 
 positing arrangements, Ohm's fundamental law that the current 
 in amperes is proportional to the electro-motive force in volts 
 
 TT 
 divided by the resistance in ohrns (C = , see p. 55) must be 
 
 R 
 
 borne in mind. The weight of metal deposited in a given time 
 is dependent solely on the volume (amperes) of current passing 
 through the solution. A coulomb of electricity (i.e., 1 ampere 
 passing for the space of one second of time), according to 
 measurements made by Lord Rayleigh, deposits invariably 
 0*000010352 gramme of hydrogen ; of any other metal it will 
 deposit this fraction of a gramme multiplied by the equivalent 
 weight of the metal, that is, by the atomic weight divided by 
 the valency of the metal as it exists in the solution. Thus, a 
 coulomb of electricity precipitates per second from silver cyanide 
 
 0-000010352 x 1^=0-001118 gramme of silver, or 
 
 0-000010352x5?^ = 0-000328675 gramme of copper from a 
 
 2 
 
 solution of copper sulphate ; while from a solution of a cuprous 
 salt it would deposit twice the last-named weight of copper, 
 because in this class of compounds, copper is monovalent, and 
 the atomic weight is therefore divided by 1 instead of by 2. 
 The figures obtained by thus multiplying the coulomb-weight- 
 value of hydrogen by the equivalent weights of the elements 
 are termed their electro-chemical equivalents, and afford the data 
 from which the electro-plater may determine the coulombs which 
 he will require to deposit a known weight of any metal, or to 
 obtain a given thickness of coating upon a known area of surface. 
 These numbers are collected together in a tabular form on p. 390 ; 
 where also will be found the weights of the commoner metals, 
 expressed both in grammes and in grains, which should be 
 deposited in one hour by a current of 1 ampere, together with 
 the thickness of the deposits produced in the same period of time 
 by a current of 1 ampere per square decimetre, and per square 
 inch of anode- and cathode-surface. The number of grammes 
 precipitated per hour is calculated by multiplying the electro- 
 chemical equivalent, or grammes per second, by 3600 (the 
 number of seconds in the hour). The thickness of deposit is 
 found from the weight deposited per hour, taken in connection 
 with its volume ; a cubic centimetre of any metal weighing, in 
 grammes, the number which represents its specific gravity (for 
 example, the specific gravity of electrolytic copper being 8-914, 
 1 cubic centimetre weighs 8'914 grammes). From this table, 
 then, may be found approximately the time required to produce 
 
80 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. 
 
 either a given weight or a given thickness of deposit, provided 
 that in the former case the strength of current, and in the second 
 both current-strength and total area of cathode be known. The 
 most accurately known electro-chemical equivalent is that of 
 silver, namely 0'001118, and it is therefore best to use this figure 
 in calculating other equivalents. 
 
 Alteration of Resistance. Since the current depends upon the 
 E.M.F. and the resistance, to increase the strength of current 
 (and hence, also, the rapidity of deposition) either the electro- 
 motive force must be increased or the resistance reduced. Of 
 these alternatives, the latter is more readily modified than the 
 former, and it is usual, for this reason, to introduce into the path 
 of the current an arrangement of wires forming an added resist- 
 ance, which may be thrown in or out of circuit at pleasure, and 
 will thus produce a commensurate effect on the current-strength. 
 Any alteration of the relative positions of the electrodes in the 
 bath produces a change in the electrical resistance and, therefore, 
 demands thoughtful attention, especially when measuring instru- 
 ments are not available to indicate the extent of the derangement. 
 
 Whenever an anode (or cathode) is removed from the bath, the 
 conducting surface through which the current enters (or leaves) 
 the solution is diminished, and the resistance is consequently 
 increased ; or when the anode is removed to a greater distance 
 from the object being plated the current has to traverse a more 
 extended length of the solution, which is a very inferior conductor, 
 and the resistance is again increased. In each case the volume 
 of current is correspondingly diminished, and the alteration is at 
 once detected by the retrogression of the pointer upon the scale 
 of the ammeter, or current-measurer. It sometimes happens 
 that the electrodes touch beneath the liquid, or become connected 
 by a fragment of metal, broken off perhaps from a faulty anode 
 or a bad cathode-deposit ; the current then ceases to pass through 
 the solution, and finds its way through the short circuit ; electro- 
 lytic action is of course stopped, but outwardly there may be 
 nothing to indicate the casualty, except in a few instances the 
 behaviour of the battery, which may show signs of unwonted 
 activity ; here again the ammeter at once givps the alarm, because 
 the diminution of resistance, owing to the substitution of a 
 metallic for a liquid conductor, causes a great increase of current ; 
 it should be observed that this increase of current is accompanied 
 by a greater consumption of battery-zinc, of which none is doing 
 useful work ; not only is the whole energy wasted, but the deposit 
 may even be seriously damaged. All this serves to emphasise 
 the necessity for applying measuring-apparatus at any time, but 
 especially when dynamos are used, as they are more liable than 
 batteries to be injured by short-circuiting or unfair treatment. 
 
 Again, since increase of resistance is attended by a decrease in 
 current-strength, the resistance of the circuit must be minimised 
 
USE OF RESISTANCE BOARDS. 
 
 81 
 
 in every possible way ; the generator must be placed as near to 
 the vats as may be convenient, the copper connecting-rods or 
 leads must be as thick and as pure as possible, and all surfaces 
 of contact, through which the current has to flow, must be 
 perfectly clean and bright, while the electrolyte-solution itself 
 should be chosen with a view to high conductivity. Conversely, 
 all wires must be prevented from mutual contact, except at the 
 desired points of connection, otherwise a short circuit or leak may 
 be set up which permits practically the whole, or at best a large 
 portion, of the current to return by the negative wire to the 
 generator, not only without having done its appointed work, but 
 with introduction of inconvenience and waste by its conversion 
 into heat principally within the 
 battery itself, which now imposes 
 the principal resistance. 
 
 Resistance Boards. The ar- 
 rangements which are employed 
 to introduce additional resistance 
 into the circuit are usually made 
 of wire, which must not be too 
 thin, or they will become over- 
 heated by the passage of the cur- 
 rent; they may be constructed of MC 
 copper, brass, or German-silver 
 wire, of electric-light carbons, or 
 (for large currents) of moderately 
 thin hoop-iron. Of the three 
 former, German-silver is the least 
 conductive and therefore the best, 
 
 brass standing second. A length of 20 inches of German-silver 
 wire, or 10 yards of copper wire, either of No. 20 Birmingham 
 wire-gauge, or a foot length of a carbon T 3 ^ of an inch in diameter, 
 should each afford a resistance equal to about the quarter of an 
 ohm. Fig. 34 illustrates a convenient method of arranging 
 several coils of wire. The circuit is broken at one point, and the 
 two ends of the conductor are connected to the system of resis- 
 tance wires at MC and MC' ; the wire is mounted on a wooden 
 frame (if the E.M.F. is only low) by stretching it alternately over 
 the metal pins A, G, B, H, C, I, D, J, E, K, and F, of which A is 
 attached to MC. The pins are so arranged that the handle H p 
 which alone is attached to MC', may be moved upon its axis from 
 contact with the MC rod at A, until it touches the buttons B, C 
 D, E, or F successively. Each section of the wire from A to B, 
 from B to C, and so on, should have a resistance of (say) half an 
 ohm. H! consists of a flat brass rod with a wooden holder around 
 the free end ; while it rests at A, the current flows through it 
 directly from MC to MC' without meeting with any appreciable 
 resistance, but as soon as it is shifted to B, the current has to pass 
 
 6 
 
 FIG. 34. Mode of arranging 
 resistance board. 
 
82 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. 
 
 through the section of wire passing from A over G to B, and so 
 meets with a resistance of J ohm ; by moving the handle until it 
 rests upon C, the resistance of the length of wire B H C is added 
 to that of A G B, and thus a total of 1 ohm is now inserted ; 
 similarly each successive move adds an extra J ohm until, when 
 H is resting upon F, the added resistance is 2 J ohms in all ; 
 finally, by shifting the handle beyond the position F, the current 
 is broken altogether. This instrument serves, 
 therefore, as a switch to start or break the cur- 
 rent, and as a regulator to control its strength. 
 Platinoid and 'Eureka' wire are other alloys used 
 for resistance. 
 
 G-alvanoscope. Of the measuring and detect- 
 ing instruments available, the galvanoscope or 
 detector is one of the simplest : it is simply a 
 magnetic needle surrounded with a coil of in- 
 sulated wire through which the current is made 
 to pass ; it cannot well be employed for actual 
 measurement, but may sometimes be useful in 
 _ indicating the direction of the current, and thus 
 
 Qf . , enabling the operator to determine at a glance, 
 
 FIG. 35. Simplest n .,? \ . , , , & 
 
 galvanoscope. an( ^ without reference to the battery or dynamo, 
 which of two wires should be connected to the 
 anode of the depositing- vat. This it does in obedience to the law, 
 indirectly due to Oersted, that a current flowing in a circuit, 
 placed around a magnetic needle in a plane perpendicular to that 
 in which the latter is free to turn, causes the needle to set itself 
 at right angles to the coil (fig. 35), the south pole being on that 
 side of the coil from which the current appears to circulate in 
 the direction of the hands of a watch. This arrangement will, 
 perhaps, be better understood by the self- 
 explanatory diagram (fig. 36). But for 
 ordinary electro-plating currents even a 
 detector is unnecessary; a common com- 
 pass-needle held in its case above or below 
 the wire through which the current is 
 passing suffices to indicate its direction by 
 
 FIG. 36. Illustrating 
 polarity of coils. 
 
 turning on its pivot with the north pole facing to the left or 
 to the right. . For this experiment the wire should extend from 
 north to south so as to be parallel to the normal direction of 
 the magnetic needle. Ampere's rule by which the direction of 
 current may be determined (and this rule applies equally to 
 the last-considered example of a coil of wire) is, supposing a 
 man to be swimming with the electric current, inside the wire, 
 head first, and with his face turned towards the magnetic 
 needle, the north pole of the latter will set so as to be on 
 his left hand. Prof. Jamieson's mnemonic rule is also simple 
 and reliable here ; it will be readily understood in reference 
 
DETERMINING DIRECTION OF CURRENT. 
 
 83 
 
 to figs. 37 and 38. If a compass-needle be placed on one wire 
 through which a current of unknown direction is flowing and 
 'the right hand be placed on the other side of, and with the 
 palm next to, the wire, so that the thumb points in the direction 
 taken by the north pole of the needle ; then the current will 
 always be found flowing (from positive to negative) from the 
 wrist towards the fingers. Thus, if in a wire the current run 
 from north to south, the compass-needle will place itself with its 
 north-seeking end pointing east when the compass is held beneath 
 the wire, and pointing west when it is above it. 
 
 Galvanometers. For rough measurements, elaborations of 
 the compass-needle detector are often employed, and are termed 
 galvanometers, though ammeters and voltmeters are much more 
 convenient and should be preferred. As the galvanometer, 
 however, still survives, we 
 will give some description 
 of this instrument. These 
 are of many kinds, but all 
 depend on the fact that 
 a stronger current always 
 causes a greater deflection 
 of the needle than does a 
 weak current. In its 
 simplest form the galvano- 
 meter is like the galvano- 
 scope ; it consists of a 
 number of coils of wire sur- 
 rounding a delicately-poised 
 compass - needle, which is 
 attached to a thin vertical 
 wire passing through a cir- 
 cular card, and carrying above the surface of the latter a light 
 horizontal index-needle or pointer ; the card is firmly attached to 
 the coils, and is divided up into degrees, so that the angular 
 motion of the pointer produced by a given current may be measured 
 accurately. The angle traversed by the index is not, however, 
 proportional to the strength of the current, and the instrument 
 must be graduated by ascertaining the varying angles of deflection 
 produced by currents of known strength ; for the same volume 
 of current always registers the same number of degrees upon the 
 scale. The extent of the deflection is regulated by the resultant 
 of the two forces one the directive force of the earth, which tends 
 to set the needle north and south, and the other that of the 
 current, which tends to place it equatorially. In using this in- 
 strument the index must first be allowed to come to rest under 
 the action of the earth's magnetism alone, the coils (and the card 
 with them) are then gradually shifted until the index points 
 to the zero of the scale, then the current is passed around the 
 
 I 
 
 FIG. 37. FIG. 38. 
 
 Direction of electric currents. 
 
84 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. 
 
 coils, and the angle through which the pointer has turned is 
 measured as soon as equilibrium is restored. It is often incon- 
 venient to be obliged to alter the position of the galvanometer, 
 so that the needle is initially north and south ; and an astatic 
 pair is with advantage substituted for the single needle ; that is 
 to say, two needles equally magnetised are fixed, one at a short 
 distance above the other, but pointing in reverse directions, so 
 that the north pole of the upper one lies above the south pole of 
 the lower. The wire coil is wound around the lower needle 
 only; the object of this arrangement being to eliminate terres- 
 trial magnetism, by causing its opposite directive action on the 
 two needles respectively to produce equilibrium, while it does not 
 interfere with the relation of the needles to the current, because, 
 although they are reversed in position, one lies above and the 
 other beneath the upper portion of the 
 coil, so that the effect of the current 
 is also reversed. Since the earth's field 
 is ineffective, a permanent field magnet 
 is placed so as to affect one of the 
 needles more than the other, and thus 
 to give the necessary control. In addi- 
 tion to eliminating the action of the 
 earth's field and weakening the control, 
 the astatic needle has the advantage of 
 providing two needles instead of one 
 to be acted upon by the magnetism 
 of the current, and the instrument is 
 thus made more sensitive. The method 
 of fixing the astatic galvanometer is 
 shown in fig. 39 ; the two needles are 
 attached to a pointer which rotates 
 above the fixed cardboard scale, the whole system of moving 
 parts being suspended by a single filament of raw (unspun) silk. 
 This galvanometer must also be graduated. 
 
 The type of galvanometer known as the ' tangent galvano- 
 meter' gives deflections following a perfectly definite law, but 
 this instrument is never now used commercially, and, therefore, 
 we shall not describe it. 
 
 Ammeters and Voltmeters. Where actual measurements of 
 current or pressure are required, ammeters and voltmeters are 
 now universally used. They depend for their action upon the 
 magnetic effect of a current passing through -a coil, more or less 
 like the galvanometer just described, or sometimes upon the 
 heating effect of a current. An instrument for measuring current 
 is called an ammeter (or amperemeter) and is graduated in 
 amperes ; the resistance of such an instrument is made as low as 
 possible so as to avoid loss of pressure in the ammeter. An 
 instrument of the same kind, wound so as to give the same effect 
 
 FIG. 39. Astatic galvano- 
 meter. 
 
CURRENT-MEASURERS. 85 
 
 with as small a current as is practicable, and thus wound to have 
 a high resistance, is used to measure pressure and is known as a 
 voltmeter. It is graduated in volts, and the object of high 
 resistance is to avoid the waste of energy that would take place 
 by using a large current merely to indicate pressure. These two 
 instruments are now so simple, compact, and inexpensive that 
 they are within the reach of all users of electricity, and should 
 certainly be included in the plant of every electro-metallurgical 
 works. The ammeter is placed in the main circuit, and thus the 
 whole of the current passes through it, and may be read off at 
 any time ; where several baths are arranged in parallel with one 
 another, and the current is divided between them, an ammeter 
 should be included in each section, in order that the operator may 
 be sure that every vat is receiving its due proportion of current. 
 The voltmeter will indicate the difference of potential between 
 any two points of the circuit, but it is usually connected as a 
 shunt, or across from one wire to the other (* parallel ' with the 
 vats) in close proximity to the electrodes in the vats ; the re- 
 sistance of the coils is so great, as compared with that of the 
 baths, that only an infinitesimal portion of the current passes 
 through them, and the distribution to the baths is unaffected. It 
 is customary to attach a small press button on a switch to the 
 stand of the instrument, so that it is only thrown into circuit 
 at the moment when the observation is made, and thus the 
 instrument is not kept in use unnecessarily. A single voltmeter 
 may be made to suffice for a small installation. 
 
 Arrangement of Baths in Electro-plating. In arranging the 
 baths and selecting solutions the operator must be guided entirely 
 by the class of work which he proposes to undertake. The 
 necessity for pure solutions of high conductivity has already been 
 insisted upon. The chemicals employed should be the purest 
 obtainable, and the water should, if possible, be distilled, other- 
 wise rain-water must be used. The solutions must be made up 
 carefully to the required strength, and watched well, and 
 occasionally tested while in use to ensure that they do not 
 sensibly vary in composition, or become excessively alkaline or 
 acid. Moreover, they must be suited to the nature of the 
 substance which is receiving the deposit ; if this latter metal 
 should of itself decompose the solution, unaided by any external 
 current, the resulting deposit will probably be non-adhesive. 
 Hence if a very electro-positive metal is to receive a coating of 
 one which is highly electro-negative, it should first be covered 
 with an intermediate metal from some solution which it cannot 
 readily decompose, this metal, in turn, being of such nature that 
 it will not break up the electrolyte of the ultimate metal to be 
 precipitated. For example, in coating iron with silver, a pre- 
 liminary film of copper may be given in the copper cyanide bath, 
 and the coating of precious metal is readily deposited upon this. 
 
86 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. 
 
 The vats should be thoroughly cleaned out from time to time to 
 prevent the accumulation of slime, which always tends to collect, 
 owing to dust and accidental impurities derived from the air, 
 and to insoluble impurities contained in the anodes, that remain 
 as a precipitate in the liquid after the rest of the metal has 
 dissolved. The current should be switched on as soon as the 
 objects are introduced, or there will be a tendency for the basis 
 metal to dissolve into and contaminate the bath before it can be 
 covered with a protecting film ; this frequently renders the 
 introduction of resistances necessary when first placing goods in 
 the vat, in order to avoid using an excessive current, as will be 
 explained in dealing with silver (see p. 194). 
 
 Arrangement of Baths in Series or Parallel. When several 
 baths are used on the same circuit, they may be arranged either 
 in series or in parallel with one another ; but for miscellaneous 
 plating the latter method is superior, although, in a few instances, 
 when the work is quite uniform, as, for example, in electrotyping 
 plates for printers, it may sometimes be advantageous to couple 
 the vats in series, especially if a dynamo be used as a generator. 
 The two systems may, of course, be combined, two or more in 
 series, with two or three baths in each, being arranged in parallel. 
 When insoluble anodes are employed, it must be remembered 
 that the electro-motive force required for the decomposition will 
 be as many times higher than is required for one couple as there 
 are baths in series. Two vats in series require twice the pressure 
 needed for one, three vats demand thrice the pressure, and so on. 
 When the anode is soluble and is made of the metal which is 
 being deposited, the increase of E.M.F. is not great, but the 
 resistance is, of course, multiplied. Placed in parallel, the re- 
 sistance is diminished as the number of vats is increased, while 
 the pressure required is constant. Figs. 40 and 41 show dia- 
 grammatically the arrangement of vats in parallel and in series 
 respectively. B is the battery ; Y is the voltmeter, which may 
 be thrown into circuit when required for use by means of the 
 switch, S ; A is the ammeter, of which there is one in each 
 parallel circuit (with it may conveniently be a set of resistance 
 wires) ; and E represents the electrolyte or plating- vat. 
 
 It was shown on p. 56 that battery-cells coupled in series 
 caused the solution of equal quantities of zinc in each cell, 
 although altogether they could not deposit more than one equi- 
 valent of metal in the plating-bath, so that five times as much 
 zinc was dissolved to precipitate one pound 'of silver when five 
 cells were placed in series as when one cell alone was made to do 
 the work. Conversely, in electro-deposition a given current with 
 sufficient electro-motive force will deposit in each vat as much 
 metal per unit of zinc dissolved in the battery as would be pre- 
 cipitated in a single vat. Just, therefore, as in the battery, 
 coupling in series gives an increase of pressure and of power to 
 
ARRANGEMENT OF BATHS. 
 
 87 
 
 do work rapidly, so a similar arrangement of plating-baths in- 
 volves a great absorption of pressure, and to a corresponding 
 extent increases the time required to deposit a given thickness of 
 
 FIG. 40. Parallel arrangement 
 of vats. 
 
 FIG. 41. Vats arranj 
 series. 
 
 3. In these two figures one voltmeter is so arranged that, with the 
 aid of the switch, S, it may be used for either vat at will, and will 
 thus show the difference of potential between either pair of electrodes. In 
 fig. 41 two switches are necessary (as shown) unless the voltmeter used 
 is capable of measuring deflections on either side of zero. One ammeter 
 is used to each bath. 
 
 metal. The economy of power, apparently promised by the 
 precipitation of an increased weight of metal per unit of zinc 
 dissolved, is thus discounted 
 by the inconvenience of a 
 slow deposit. A further ob- 
 jection to the series system 
 as applied to the plating- 
 batbs is that they become 
 inconveniently inter-depen- 
 dent, for any increase in the 
 resistance of one bath, as 
 when a large article is re- 
 moved from it, reduces the 
 current- volume in the whole 
 
 circuit. It is true that even Fm> 42i _ Platillg . bath . Parallel a . 
 when the parallel arrange- ment of articles, 
 
 ment is adopted, an altera- 
 tion of resistance in one vat also influences the current-strength 
 of the whole system, but the effect is scarcely appreciable owing 
 to the large aggregate surface presented in the different electro- 
 lytes ; the added resistance in one bath merely causes a greater 
 volume of current to flow through the others, while on the other 
 
88 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. 
 
 hand it tends to diminish the total current. Where, then, the 
 articles to be plated may be of every conceivable size, and the 
 superficial areas are difficult to estimate exactly, the parallel 
 system is preferable ; but when the articles present a fair uni- 
 formity of surface, and the current has sufficient potential, the 
 alternative method may be substituted. For similar reasons it 
 is best to hang the various articles in each bath in parallel with 
 
 FIG. 43. Plating-bath. Parallel 
 arrangement of articles. 
 
 FIG. 44. Plating-bath. Arrange- 
 ment of articles in series. 
 
 one another, as shown in figs. 42 and 43, not in series, as in figs. 
 44 and 45, in which the current enters the electrolyte by one 
 anode, passes to its corresponding cathode, and thence by a wire 
 connection to the next anode, and so on. 
 
 Choice of Anodes. In the matter of anodes stress has 
 already been laid upon the necessity for their absolute purity and 
 complete solubility in the electrolyte ; this latter condition is, 
 however, a question to be more particularly regarded in the 
 
 FIG. 45. Plating-bath. Arrangement of articles in series. 
 
 selection of the constituents of the bath. Unless under the 
 action of the current the anodes dissolve freely in the liquid, the 
 latter must become impoverished and change rapidly in composi- 
 tion ; this is always a source of trouble and annoyance, and 
 where the use of an insoluble or imperfectly soluble anode is 
 unavoidable, small quantities of that salt of the metal which is 
 undergoing electrolysis (or of metallic oxide) must be added from 
 time to time to make good the loss due to deposition. Cast 
 anodes will often be found more soluble than the rolled metal, as 
 they are more porous and open in grain, and, therefore, more 
 
THE ANODES. 89 
 
 readily attacked by the liquid ; in some cases they may even be 
 found to become spongy and friable, a condition which should of 
 course be avoided, and which indicates the desirability of substi- 
 tuting the rolled sheet. When there is any difficulty in obtaining 
 pure metal for anodes, the rolled material may generally be 
 preferred, because the cast plates may contain a large percentage 
 of foreign substances, which would escape detection on merely 
 examining the exterior of the block, whereas any considerable 
 addition of impurities would in many instances cause the metal 
 to break up as it passed through the rolling machinery, so that 
 the mere fact of a metal being in the form of rolled sheet is often 
 a guarantee of at least a fair degree of purity. Cast-iron should 
 on no account be used ; it always contains three or four per cent, 
 of the insoluble substances, carbon and silicon, with other bodies, 
 such as manganese, phosphorus and sulphur ; some at least of 
 these are necessary to render it sufficiently fusible to melt in the 
 cupola-furnace. Anodes should usually be of larger size than 
 the cathodes to which they are opposed, so that the greater 
 surface exposed to the solvent action of the electrolyte may 
 compensate for the slower rate of solution as compared with that 
 of deposition. 
 
 Spacing of Electrodes Polished Cathodes. It has been seen 
 that the resistance of a bath is higher when the electrodes are 
 small, and that it increases as the distance between them is 
 more extended ; thus economy is effected by plating many 
 objects simultaneously in parallel, and also by minimising the 
 distance between them and their corresponding anodes. But 
 there are objections to approximating them too closely first, 
 they are more liable to come in contact or to be united by a 
 fragment of metal, and thus to produce short-circuiting ; and, 
 secondly, if the surface of the object is irregular the deposited 
 metal is liable to be of unequal thickness, because a current 
 passing between points at unequal distances tends to deposit 
 most rapidly upon those portions of the cathode which most 
 nearly approach the anode. By increasing the space separating 
 the two surfaces the irregularities of either have less influence on 
 the deposit, because they are small as compared with the mean 
 distance between them. This difficulty is chiefly experienced in 
 electrotyping, where strong deposits of uniform thickness are 
 required. In depositing copper when the solution is strong and 
 at rest, small projections and striated markings may be observed, 
 which increase in extent as the current is maintained. Marks 
 and scratches on the cathode do not become obliterated, but 
 rather accentuated, as the metal is deposited over them; great 
 care must, therefore, be taken that the surface to be coated is not 
 only thoroughly cleansed, but that all tool- or file-marks are 
 completely removed, as there is no remedy when once deposition 
 has begun. 
 
90 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. 
 
 Homogeneous Solutions necessary. Another difficulty in- 
 herent in the process is, that a current long continued with the 
 solution at rest produces a gradual local alteration in the density 
 of the latter. At the anode, metal is constantly dissolving into 
 the surrounding liquid, which thus becomes heavier, bulk for 
 bulk, than the remainder of the bath, and sinks to the bottom ; 
 while at the cathode the liquid is denuded of metal, and from its 
 lower specific gravity rises to the surface. In this manner a very 
 gentle but sure circulation occurs in the vat, producing an undue 
 proportion of metal in solution at the bottom, and of acid at the 
 top. The effect of this is that thicker deposits form on the lower 
 portions of goods immersed in the bath, owing largely to the 
 higher conductivity of the strong solution and the greater pro- 
 portion of current flowing through it ; moreover, a kind of local 
 action may be set up in the deposited copper plate, which is rest- 
 ing in two practically different solutions, with the result that the 
 upper portion of the plate in the acid solution tends to dissolve, 
 and to deposit a corresponding proportion of copper upon the 
 lower half. It is probably the steady flow of liquid over the 
 surface of the cathode which gives rise to the striated markings 
 above referred to. The only remedy is to keep the solutions 
 thoroughly mixed by stirring or gently shaking them in any 
 suitable way, and this precaution should never be omitted when 
 thick deposits are required, which necessitate a comparatively 
 long exposure in the bath. The solutions must also be kept free 
 from suspended particles such as are apt to become detached in 
 the form of slime from the anode, because the settling of these 
 particles on the cathode is one of the principal causes of the 
 rough nodular formation on the surface of electro - deposited 
 metals. 
 
 Having seen the causes which operate to produce failure in 
 electro-plating, it remains to be seen how they are avoided in 
 practice. 
 
 Motion of Solution. The circulation of the electrolyte not 
 only ensures the homogeneity of the solution, but enables a 
 higher current-density, and therefore a higher rate of deposition, 
 to be used, by bringing a greater number of metal ions into 
 contact with the cathode in a given time. By employing a very 
 rapid flow of the solution around the cathode, the rate of de- 
 position (for example, of copper in refining or in electrotyping) 
 may be enormously increased. Thus, in certain copper deposit- 
 ing processes, recently introduced, jets of- the electrolyte are 
 directed upon the surface during the whole period of electrolysis, 
 with the result that a current density of over 100 amperes per 
 sq. ft. may be used, as against 10 amperes per sq. ft., which not 
 long since was, with ordinary methods of depositing, considered a 
 high density. By using strong solutions and a proportionately 
 high current-density, with rapid motion of the electrolyte, Sir 
 
MOTION OF SOLUTION. 91 
 
 J. W. Swan has deposited good copper with current- den si ties 
 higher than 1000 amperes per sq. ft. There are various methods 
 of causing circulation of an electrolyte, such as purely mechanical 
 means, or by pumping air into the electrolyte, or by actually 
 pumping the electrolyte itself from one part of the vat and re 
 turning it to another. Another method of obtaining a similar 
 end is the rotation of the cathode, to which further reference will 
 be made when the electro-deposition of copper is considered in 
 detail. 
 
CHAPTER V. 
 
 PLATING ADJUNCTS AND DISPOSITION OF PLANT. 
 
 Light and Pure Air. In arranging an electro-plating establish- 
 ment, due regard must be had for light and ventilation ; with in- 
 sufficiency of the former bad work is almost sure to result, as it 
 is not easy to judge when the pieces are sufficiently stripped, 
 polished, cleaned, or quicked, and the progress of the deposition 
 cannot be watched with the requisite amount of care ; it is often 
 necessary to stop a process immediately upon the appearance of 
 certain signs, indicative of imperfect cleansing or the like, and it 
 is of the highest importance that these characteristics should be 
 noted at once, which cannot be done if the light be deficient. 
 Badly-ventilated rooms are productive of ill-health and disease to 
 the workmen ; this maxim, applicable, indeed, to all rooms in 
 which men live or work, must be specially regarded in rooms 
 where batteries or cyanide plating-solutions are in use ; the acid 
 fume or spray given off by most batteries is most penetrating and 
 injurious to health, even when considerably diluted with air, as it 
 is likely to be found even in a large room, if it be insufficiently 
 provided with means to carry off vitiated air and supply fresh in 
 its place ; moreover, the cyanide solution becomes slowly de- 
 composed by the carbonic-acid-laden air of towns, and evolves the 
 deadly prussic acid gas in minute proportion, it is true, but 
 amply sufficient to become prejudicial to the well-being and 
 comfort of the operator, when breathed continuously for any 
 considerable period of time. 
 
 Arrangement of Booms. At least three rooms should be 
 available, if possible, for the purposes of the art. In the first of 
 these the mechanical operations of cleansing and polishing are 
 carried out ; these give rise to the production of more or less dust, 
 arid should not, therefore, be conducted in the same room with the 
 plating-vats. Again, the engine driving the machinery should be 
 in a separate chamber, apart from the dust of the polishing-room 
 on the one hand, and from the fumes of the vat-room on the other, 
 the power being communicated to the lathes and other machine 
 tools by shafting running between the two rooms. When the 
 dynamo is the source of electric energy, it should be placed in the 
 
 92 
 
ARRANGEMENT OF ROOMS. 93 
 
 engine-room, but as close as possible to the baths in the adjoining 
 chamber, so that there may be no great loss of energy owing to 
 the resistance of the copper conducting wires or leads to the 
 passage of the electric current. But if batteries be employed, 
 they should be carefully isolated from each of the three rooms 
 above mentioned ; if a fourth be not available (a small one will 
 suffice), a corner of the vat-room should be partitioned off, the 
 chamber or cupboard thus formed being provided with a separate 
 outlet into the open air, or, better still, into a chimney, so that all 
 fumes may be at once and completely removed ; a sink and a 
 supply of fresh water may be fitted in this room with advantage. 
 Care should be taken to have sufficient room and good light to 
 ensure easy attention to the cells, and this remark applies 
 particularly to batteries of accumulators, which only prove un- 
 satisfactory if not carefully looked after. In the third or vat-room 
 are the potash and all other baths used in chemically cleansing 
 the pieces, together with those devoted to plating ; an ample 
 supply of water must be available in this department, and a 
 steam-pipe should convey steam from the boiler if one be used 
 to the potash-tank and steam-heated vessels. Here the various 
 pieces of furniture should not be crowded together, but an ample 
 margin of space should be left so that the operator may not be 
 cramped for want of room. When more than one plating process 
 is employed, each should be kept to itself, and in large establish- 
 ments a separate room may be set apart for each. 
 
 It is, of course, impossible to lay down any general plan for the 
 disposition of the plant of an electro-plating shop, because it must 
 be arranged and modified, not only to suit the work to be per- 
 formed, but also the floor-space, shape, and position of the rooms 
 available. But, to sum up, it may be taken as a general rule that 
 mechanical works should be isolated from chemical, that the battery 
 in the one case, or the dynamo and motor in the other, should be 
 separated from both, that in each room the various classes of work 
 should be kept distinct, but that where consecutive processes have 
 to be followed, the arrangements for conducting them should be 
 so placed that, when one stage of the work is completed, the 
 articles may be conveniently transferred into position for the 
 next operation. 
 
 Drainage of Floors. The floors should be of stone, asphalt, or 
 concrete, or they may be covered with lead sheet so that they 
 may readily be kept clean, and be non-absorbent of the acids and 
 chemical reagents splashed upon them : wood, besides being con- 
 stantly wet, is liable to become rotten by the action of these sub- 
 stances. Trapped gullies or sinks should be placed at suitable 
 points flush with the floor ; they should not communicate with 
 the house-drains directly, but should discharge into a pipe which 
 runs outside the house, and delivers into the open air above a 
 second trapped gully communicating with the sewers or drains. 
 
94 PLATING ADJUNCTS AND DISPOSITION OF PLANT. 
 
 In this way the floor may be kept from accumulations of 
 liquid, and may be readily and perfectly cleansed by flooding it 
 with water, and then sweeping it into the gullies by means of 
 india-rubber * squeegees ' or brooms ; at the same time, there is no 
 danger of sewer-air contaminating the atmosphere of the room (a 
 fertile source of danger), because there is no direct communication 
 with the drain. Pipes from sinks should discharge into the open 
 air after a like manner. 
 
 Ventilation. To ensure the purity of air, it is not well to 
 trust simply to the ventilation of the room by doors and windows, 
 but a systematic arrangement should be adopted, such as that of 
 Tobin. Several ventilators should be made immediately below the 
 ceiling of the room, by removing a brick, passing a tube through 
 the wall, and bending it upwards in the open air (it may be 
 shielded from rain by a cap placed a few inches above the exit) ; 
 if possible, one of these ventilating-tubes should pass into a disused 
 chimney-shaft. The vitiated air is thus carried away, and pure 
 air must be admitted at the floor level to take its place ; this may 
 be done by carrying two or three pipes through the wall close to 
 the floor, and bending them up in the interior of the room to a 
 height of five or six feet. Fresh air is delivered by them in a 
 manner which does not give rise to draughts. Entering cold, it 
 flows up these pipes and gradually distributes itself over the 
 chamber, where, becoming heated, it rises to the roof and finds an 
 exit through the upper row of ventilators. 
 
 The guiding principles for the sanitary and safe conduct of an 
 otherwise unhealthy occupation are expressed in a few words by 
 saying ensure abundance of light, water, and fresh air. 
 
 ARRANGEMENT OP PLANT FOR ELECTRO-PLATING. 
 
 The vats and apparatus used in cleansing are described in 
 Chapter VI. 
 
 Vats. The vats employed to hold the solution for electro- 
 plating should be considerably larger than the largest object to 
 be coated in them, and must be made of, or at least lined with, 
 some material which will resist the acid liquid that may be placed 
 in them. 
 
 Glass is by far the cleanest and best, but is rarely used, except 
 for very small work, on account of the initial expense and the risk 
 of fracture. For very small objects glass vessels may be had in 
 one piece, as circular trays or jars ; but for large articles, the bath 
 should be made by joining five plates of sheet glass of the re- 
 quisite sizes with marine glue, or white lead, or other cement, 
 protected on the inside by a varnish made of asphalt dissolved in 
 benzoline ; or of gutta-percha in benzene or in carbon bisulphide ; 
 or, in fact, by any water- and acid-proof mixture. The glass 
 should be supported by an outer frame of wood. Slate similarly 
 
ARRANGEMENT OF PLANT FOR ELECTRO-PLATING. 
 
 95 
 
 arranged is a good substitute for glass ; but though less brittle, it 
 must still be used with care. For small work stone or earthen- 
 ware glazed troughs are readily obtainable and are very con- 
 venient. 
 
 Lead presents the advantage that it is readily formed into any 
 shape. A tank made of this material should be supported 
 beneath and around by a wooden case, so that it will not be 
 subjected to the stress of a mass of liquid within. All the joints 
 in the lead-lining l should be made by autogenous soldering that 
 is, by melting together the two edges of the lead sheet, instead 
 of uniting them with soft solder, which would set up galvanic 
 action with the lead if it came into contact with the electrolytic 
 solutions. But even when united into one continuous leaden 
 trough, the metal should be 
 completely protected in every 
 part by a good layer of acid- 
 resisting varnish, to prevent 
 the decomposition of the liquid 
 by the lead through simple ex- 
 change. Iron tanks also are 
 very largely used, and, indeed, 
 almost universally so for hot 
 solutions. These also, being 
 constructed of a highly elec- 
 tro-positive material, must be 
 carefully preserved from attack 
 by the solutions, either by var- 
 nish, or, better, by a good 
 coating of enamel, which, since 
 it is a fused complex silicate, 
 forms practically a tank of 
 glass, so long as it remains 
 intact ; but as soon as the enamel is chipped and the surface 
 of the iron is laid bare, its use must be discontinued until a new 
 coating can be given. 
 
 Wood is abundantly used, and, being a cheap material, easily 
 worked, is especially well adapted for vats of unusual shape which 
 may have to be constructed for one particular class of temporary 
 work. These tanks are best secured at the ends by bolts and 
 nuts, as shown in fig. 46, which serve to hold the sides firmly 
 pressed against the end pieces. As wood alone is very absorbent, 
 they should be lined with gutta-percha or any other water-proof 
 material, and must be carefully watched so that they may be 
 re- lined as soon as leakage into the wood is observed. Wooden 
 vats are sometimes lined with thin lead sheet autogenously 
 soldered, and this inner case may be varnished, or may be again 
 lined with varnished wood. A mixture of 10 oz. of gutta-percha 
 1 But see also page 107. 
 
 FIG. 46. Plating- vat. 
 
96 
 
 PLATING ADJUNCTS AND DISPOSITION OF PLANT. 
 
 FIG. 47. Rim of plating- vat. 
 
 with 3 oz. of pitch and 1J oz. each of stearin and linseed oil, 
 melted together and well incorporated, has been found to afford a 
 good protective covering to lead or wood. 
 
 The tanks for hot solutions are best made of enamelled iron, 
 and may be set over a small fire-grate in which charcoal is burned, 
 or better because the heat is more under control over a series 
 of Bunsen burners of the ordinary upright form, or in the shape 
 
 of horizontal rings ; or they 
 may each be surrounded with 
 an outer jacket, the inter- 
 vening space being filled with 
 waste steam, which is often 
 available in large works and 
 may be economically applied. 
 Iron tanks are often made of 
 thin metal, and, if of large 
 size, should be supported by 
 strong iron bearing bars beneath, to prevent them bulging when 
 the weight of the liquid is applied. 
 
 Vat-Connections. Of whatever material the vat is constructed 
 it should be provided with an insulated rim around the top, to 
 carry the wires which conduct the current to the objects in the 
 bath. This rim is best made of well-painted wood fitting on to the 
 top of the bath, and the outer portion should be at a higher level 
 than the inner, as indicated in fig. 47, which with fig. 48 illustrate 
 the general arrangement as adapted to an iron vat. Around three 
 sides of the raised por- 
 tion there runs a short 
 brass or copper rod, 
 A, ending in a bind- 
 ing screw, B, attached 
 to the positive pole of 
 the battery. Around 
 the corresponding three 
 sides of the inner or Br^ D 
 
 -A 
 
 FIG. 48. Rim of plating- vat. 
 
 lower level platform is 
 a similar rod, C, insu- 
 lated completely from 
 A and from the bath by the woodwork of the frame, and ter- 
 minating in the binding-screw, D, attached to the negative (zinc) 
 pole of the battery. Resting on the two sides of the rod, A, 
 may be placed any number of cross-rods of, brass or copper, E, 
 which can be held firmly in position by the screws, F F ; from 
 these are suspended the anodes, which are thus placed in direct 
 connection with the battery. Lying upon the lower rods, C, 
 similar cross-bars serve to support the cathodes or objects to be 
 coated. Any reasonable number of electrodes may be thus 
 suspended in the same bath, the current flowing always from the 
 
CONNECTION OF ELECTRODES. 
 
 97 
 
 anodes to the cathodes in parallel. When both sides of an object 
 are to be coated, the anodes and cathodes must be placed alter- 
 nately; such an arrangement has been shown in fig. 43, where 
 A A represent anodes and C C cathodes ; here both sides of the 
 anode are used and dissolved, as the current flows from them in 
 either direction to the cathode next adjoining. But where a 
 deposit is required on one side only of a plate, as in printers' 
 electrotyping, one anode may be placed between each pair of 
 cathodes set back to back (fig. 49), so that both sides of the 
 former are used, but only that 
 side of the latter which is 
 nearest to its corresponding 
 anode. 
 
 Connection of Electrodes. 
 Other methods of connecting 
 the electrodes with the battery 
 are used, but that just de- 
 scribed presents many ad van- FIG. 49. -Arrangement of plates for 
 tages : thus the distance be- electrotyping. 
 
 tween an anode and cathode 
 
 may be adjusted at will, and either may be removed from the 
 solution, examined and returned without disturbing the remainder, 
 and without any manipulation of binding-screws ; the anode may 
 be instantaneously transferred to a cathode rod at the beginning 
 or end of an operation, if desired, to control the current (see 
 p. 1 94) ; and the anode supporting-rods are held firmly in 
 position by the external screws a similar arrangement, but of 
 internal screws, may be applied to the cathode rods if required. 
 
 Suspension of Anode Plates. The anode plates are suspended 
 from the cross-rods by suitable hooks. The anodes may con- 
 veniently have a perforation in each of the upper corners, through 
 which the suspending hooks are passed ; but as they are liable to 
 dissolve irregularly, even when every precaution is used to ensure 
 uniformity of liquid, the lower corners should also be perforated, 
 so that the plates may be suspended alternately from opposite 
 sides. They are sometimes hung so that they project above the 
 surface of the liquid, and the supporting wires, not being im- 
 mersed, are, therefore, not liable to corrosion and ultimate de- 
 struction by solution ; but the plates themselves will then have 
 a shorter life, for they are most vigorously attacked at the line of 
 uppermost contact with the liquid, and will be worn away at this 
 point while the remainder of the plate is still sound ; moreover, 
 the portion of metal above the water line is practically wasted. 
 They are frequently made, therefore, with projecting perforated 
 lugs, either at each corner, or only at two, as in fig. 50 ; these 
 alone project above the solution, and, while protecting the sup- 
 porting wire from destruction, minimise the amount of useless 
 anode surface outside the liquid. To obviate the destruction of 
 
 7 
 
98 
 
 PLATING ADJUNCTS AND DISPOSITION OF PLANT. 
 
 the suspending hook, without permitting any portion of the anode 
 to remain above the bath, some operators use but one side of the 
 anode to face the objects which are being coated. On the back 
 reversed hooks are fastened by which the plate is suspended, as in 
 fig. 51 ; the hooks are thus protected from dissolving by the anode 
 which intervenes between them and the cathode, except in the 
 space between the top of the plate and the surface 
 of the liquid, which space should, therefore, be 
 made as short as possible. 
 
 Agitators. The necessity for keeping the elec- 
 trolyte in motion to ensure uniformity in composi- 
 tion when a thick deposit is required has been 
 dealt with already ; the manner of effecting it 
 
 must depend upon the appliances at command. 
 FIG. 50. Mode .. . , r , f. A r I. j i j. *. 
 
 of suspending Stirring by hand is frequently relied upon, but it is 
 
 anode plate, liable to be accidentally omitted, and, being neces- 
 sarily intermittent, allows time for partial separa- 
 tion to occur between two consecutive stirrings. Mechanical 
 agitation, which is more certain in its effect, may be applied by 
 such devices as working a small screw-propeller slowly at one end 
 of the bath ; or by blowing air into the solution constantly, 
 
 n 
 
 e 
 
 FIG. 51. Mode of suspending anode. FIG. 52. Von Hiibl's agitator. 
 
 through a tube passing to the bottom of the vat, by means of a 
 fan-blower, or by an agitating arrangement such as that used by 
 v. Hiibl in the electrotype baths of the Austrian Military 
 Geographical Institute. 
 
 In this system a glass rod, A (fig. 52), is fastened to a crank- 
 shaft connected with an eccentric or with any suitable device 
 for imparting a reciprocating or to-and-fro motion, so that at 
 each reciprocation the rod is moved through the arc of a circle, 
 
MOTION OF CATHODES. 
 
 99 
 
 from its original position at A to that represented by the dotted 
 line A', and is then returned to its first place at A. Such a rod 
 is placed between each pair of anodes and cathodes, all the rods 
 or beaters being attached to the same crank-shaft which runs the 
 whole length of the vat, so that they may all be actuated by 
 the same mechanism. The 
 motion of the beaters need 
 not be very rapid from 
 10 to 30 strokes a minute 
 amply sufficing. The use 
 of this or any similar device 
 presupposes the existence of 
 steam- or water-power and 
 
 FIG. 53. Mode of attaching sliding-frame. 
 
 TROUGH OR VAT 
 
 machinery in the shops; where this is not available, manual 
 power must be substituted in connection with any suitable mixing 
 appliance, which must be set in motion at frequent intervals. 
 Whatever motion is given must be sufficiently vigorous to ensure 
 
 thorough mixture of the 
 solution, but without dis- 
 turbing the relative posi- 
 tions of anode and cathode, 
 and the mechanism must 
 be so applied that it in no 
 way lessens the facilities for 
 examining the progress of 
 J~3 deposition. 
 
 Motion of Cathodes. 
 Silver and some other 
 metals require a gentle 
 motion to be imparted to 
 the objects upon which they 
 are being deposited. This 
 may be done by enclosing 
 the suspending rods of the 
 objects within a wooden 
 frame which is caused to 
 move to and fro above the 
 solution by means of an 
 attachment to an eccentric. 
 The frame may be sus- 
 
 FRAME 
 
 FIGS. 54 and 55. Mode of attaching 
 sliding-frame. 
 
 pended above the vat, or it may be caused to slide upon the 
 edge of the bath. 
 
 Figs. 53, 54, and 55 illustrate a convenient method of fixing 
 the sliding frame ; it is supported on wheels placed at the 
 corners, each wheel rolling upon an inclined plane E (fig. 55), 
 which may be set at any angle by means of a screw. The rod R 
 thus imparts the necessary backwards and forwards motion to the 
 system, so that, at every stroke, a double action occurs, and the 
 
100 PLATING ADJUNCTS AND DISPOSITION OF PLANT. 
 
 frame with the objects suspended from it moves in a horizontal 
 direction by virtue of the pull of the rod R, and in a vertical 
 direction on account of the inclined plane, the extent of the latter 
 motion being controlled by the screw which determines its angle 
 of inclination. The frame with the objects should be caused to 
 slide backwards or forwards once in about two or three minutes, 
 through a distance of 2 or 3 inches. 
 
 Plating-Balances. In plating with precious metals it is fre- 
 quently required to deposit only a given weight upon the various 
 articles ; and, although this may be approximately accomplished by 
 calculating the time required to deposit a given weight of metal 
 with a known current-strength and upon a known superficial area, 1 
 with the aid of the table given on p. 390, it is safer when absolute 
 accuracy is required to use a plating-balance, by which the weight 
 of metal deposited may be determined as the operation proceeds. 
 In this instrument, a metal frame for carrying the cathode-rods is 
 substituted for one scale-pan of a large balance of the ordinary 
 description. The frame is supported from the beam of the balance 
 by metallic connection, while the pillar of the scale is connected 
 with the negative pole of the battery, so that electric communica- 
 tion is made through the parts of the balance. Having attached 
 the objects to the frame and immersed them in the solution, 
 they are counterpoised by placing weights in the opposite pan of 
 the balance until equilibrium is restored, and the frame and the 
 objects are suspended freely, the former, of course, above and 
 the latter in the solution ; an extra weight, equal to that of the 
 metal which is to be deposited upon all the objects in the 
 aggregate, is now added to those already in the scale-pan, and 
 the current is switched on until the deposited metal, just over- 
 balancing the added weights in the pan, turns the scale in the 
 opposite direction an action which may be indicated automati- 
 cally by causing the pointer or beam of the balance to release 
 the hammer of a small gong or to make contact with an electric 
 bell. 
 
 Roseleur's Plating-Balance. Roseleur has introduced a more 
 elaborate balance by which the current is automatically cut off 
 as soon as the beam of the scale is turned, so that the electrolytic 
 action ceases directly the required amount of metal has been 
 deposited ; this was effected by making electrical contact, not 
 with the pillar, but by attaching a platinum wire to the arm of 
 the balance which supports the weights, and arranging underneath 
 it a cup of mercury connected with the negative pole of the 
 generator, into which cup it dips to such a distance that, as 
 soon as the arm is raised by the reversal of the beam, the wire is 
 
 1 When it is only desired to deposit a total weight of metal, and not a 
 certain weight per square inch, the superficial area of the objects may be 
 neglected ; all that is required to be known is the mean strength of current 
 applied, in amperes. 
 
USE OF THE PLATING-BALANCE. 
 
 101 
 
 lifted out of the mercury and connection between cathode and 
 battery is permanently broken. All the knife-edges of this 
 balance work under mercury, in order to prevent overheating of 
 these parts by the current flowing through them, and at the same 
 time to lessen friction and obviate the corrosive action of acid 
 fumes. 
 
 In the balance diagrammatically indicated in fig. 56 the current 
 does not traverse the beam at all, but enters by a contact screw 
 attached to the supporting-rod of the cathode-frame. The objects 
 are suspended in the bath from the flat cathode-rod, C, in the 
 usual way ; then when these have been counterpoised by intro- 
 ducing weights into the pan, P, and the extra weights represent- 
 ing the total mass of metal to be deposited have also been added, 
 the beam will turn, so that the 
 arm, A', rests on the stop, R', 
 which is rigidly attached to the 
 pillar of the balance to prevent 
 an excessive amount of play ; at 
 the same time the point of the 
 screw, S, in the supporting rod 
 of the cathode-frame, should 
 just make good contact with the 
 block, M, at the end of a spring, 
 attached to a suitable fixed sup- 
 port, and connected with the 
 negative pole of the battery ; 
 the spring must, of course, be 
 insulated from the pillar and all 
 parts of the balance. Through 
 
 this connection the current FIG. 56. Balance, 
 
 passes from the bath to the 
 
 return-wire of the battery ; both S and M should, therefore, be 
 tipped with platinum, which remains untarnished under all condi- 
 tions, and, therefore, secures good contact. If the extremity of S do 
 not actually touch M, or if it press so hard upon it that the spring 
 is bent, adjustment may readily be made by turning the milled 
 head of the screw : the adjustment should be so made that when 
 the balance is in exact equilibrium the points are just touching ; 
 so that when the beam rests upon the stop, R', the spring becomes 
 slightly bent and ensures perfect contact ; but when it rests upon 
 the corresponding stop, R, contact is entirely broken, and a space 
 of at least the T \- of an inch separates the two platinum surfaces. 
 The current now flows through the circuit from the positive pole 
 of the battery to the anodes, which rests as usual upon the sides 
 of the vat, thence through the solution to the cathodes upon which 
 it deposits the precious metal ; and from the cathodes it traverses 
 the supporting-rod, the screw, S, and the spring, M, and returns 
 to the negative pole by the wire, W. As soon as the weight of 
 
102 PLATING ADJUNCTS AND DISPOSITION OF PLANT. 
 
 metal precipitated is equal to that added to the pan, P, the 
 balance comes to equilibrium and remains poised between the 
 stops R and R' ; but the current is still flowing because S is 
 not yet withdrawn from M ; then, as soon as a slight additional 
 amount of metal is deposited, the beam comes over and rests upon 
 the stop, R, and, contact being broken, no further electrolytic 
 action can ensue. The cathode supporting-rod is made in two 
 pieces joined by a ball-and-socket joint, B ; any disturbance of 
 the knife-edge of the balance caused by the necessary slow recipro- 
 cating motion imparted to the frame is thus obviated. The 
 motion is imparted by an inverted fork, running between horizontal 
 guide-rollers, which spans the edge of the frame at the central 
 point on one of its sides, the fork being, of course, attached to an 
 eccentric rod ; the length of the fork may be so arranged that as 
 soon as the balance rests upon the stop, R, the frame falls out of 
 reach of its action. 
 
 Weight Corrections. In using any of these balances, it must 
 be remembered that the weight of a substance in water is less 
 than its weight in air, and that the plated article will appear to 
 weigh more after removal from the solution than it did when 
 immersed; the actual weight of silver deposited by balance is, 
 therefore, in excess of that indicated by the weights in the pan. 
 It is quite possible to rectify this error in making the needful 
 calculation. The initial weight of the objects to be plated may 
 be left out of account, because it is counterpoised while they are 
 in the solution at the beginning of the operation, and they remain 
 in the same liquid to the end. But the weight of silver (or gold) 
 deposited upon them will be less while it is in the solution than 
 it would be outside, by the weight of an equal volume of the 
 liquid. For example, the specific gravity of silver may be taken 
 at 10'6 ; that is to say, 1 cub. in. of silver weighs 10*6 times as 
 much as 1 cub. in. of water; thus 10*6 oz. of silver, weighed in 
 the air in the usual manner, would show only 10'6 - 1'0 = 9'6 oz. 
 if it were weighed while immersed in water. But the specific 
 gravity of the solution is more than 1 as compared with 
 water; regarding it as I'l, the weight of the 10*6 oz. of 
 silver becomes 10*6 -I'l, or only 9:5 'oz., if counterpoised while 
 suspended in this liquid, and this is equivalent to a loss of 
 over 10 per cent. Therefore, strictly speaking, to obtain an 
 aggregate deposit of 10 oz. of silver upon a batch of articles, 
 only 9 oz. need be placed upon the scale-pan. For gold, 
 similar calculations may be made, but the loss is not so great 
 owing to the higher specific gravity of gold ( = 19'3), so that the 
 ratio of its weigh w in air to that in water is 19*3 : 18 '3. In the 
 same way, for other metals the weight in the pan should be 
 divided by the fraction : 
 
 specific gravity of the metal 
 
 ~~ specific gravity of metal - specific gravity of solution* 
 
USE OF THE PLATING-BALANCE. 
 
 103 
 
 We are not aware that these calculations are often made in 
 practice, and the thickness of silver deposited must, therefore, 
 always be perceptibly greater than that intended certainly an 
 error on the right side from the consumer's point of view. A 
 certain loss may be incurred in subsequent polishing processes, 
 but this should not be greater than would be compensated for by 
 the small excess of metal which is required to overcome the 
 friction of the balance and bring the beam over to the opposite 
 side. 
 
 Should any articles or anodes accidentally fall into the plating- 
 vat, they should be carefully picked out by means of a long wire, 
 bent at one end into hook-shape, or by a pair of light tongs, 
 which may be made of brass, previously coated with the metal 
 which is being deposited, or with some more electro-negative 
 
 FIG. 57. Roseleur's wire-gilding arrangement. 
 
 metal ; the bare arm should not be introduced, because of the 
 risk of blood-poisoning which may be caused by the contact of 
 recent wounds with the plating-liquid. 
 
 For special classes of work special vats and special arrange- 
 ments of all kinds may have to be made, arid herein lies the scope 
 for the inventive skill of the operator. But, with a knowledge of 
 the principles laid down in works on electro-metallurgy, there 
 should be no serious difficulty in dealing with the various problems 
 which may be presented. 
 
 Roseleur's Wire-Gilding Process. Among these special ar- 
 rangements, the method adopted by Roseleur for gilding iron by 
 a single continuous process is especially interesting ; a diagram 
 of the plant is given in fig. 57. The tin-coated and well-cleaned 
 wire is slowly uncoiled from a reel, R, and passed to the drum, 
 D, on which the finished wire is ultimately wound, and whose 
 rotation causes the wire to travel through the various stages of 
 the process. First it is passed into the hot gold-bath, E, heated 
 by the furnace, F, and is maintained beneath the liquid by the 
 
104 PLATING ADJUNCTS AND DISPOSITION OF PLANT. 
 
 glass rollers, G G ; here, by the current supplied through the 
 platinum-wire anodes, A A, and pass through the wire to the 
 connection with the battery at B, the gold is deposited upon it. 
 Passing thence, the gilt wire is guided by two wooden rollers, 
 W, to a cleansing-bath of potassium cyanide solution ; from this 
 it is lead into a vessel of clean wash-water, V, and finally between 
 the calico-covered draining rollers, C, to the drying and annealing 
 tube, T, which is maintained at a dull-red heat by a charcoal- 
 furnace. Several parallel wires may, of course, be passed through 
 the same process simultaneously. 
 
 Wire gauze, or fabrics of any kind, provided that they conduct 
 electricity, may be similarly coated by passing 
 them beneath a roller in the depositing trough, 
 and winding them finally upon a reel or drum. 
 Wagener and Netto's Doctor. A device 
 invented by Wagener arid Netto for coating 
 surfaces which are too extensive to immerse 
 in a solution may well be rioted as an applica- 
 tion of ingenuity to the solution of a difficult 
 problem, although it is really only a modifica- 
 tion of the apparatus long since known as a 
 ' doctor,' which is used for gilding the lips of 
 ewers and the like (see p. 208). To a hollow 
 wooden handle, H (fig. 58), is attached a cir- 
 cular anode, A, of the required metal per- 
 forated in the centre, and connected by a wire, 
 W, with the positive pole of the battery ; in 
 close contact with the lower surface of A is 
 a flannel pad, E, held in place by the brass 
 tube, T, which passes through the length 
 of the handle, and, being connected with the 
 india-rubber tube, R, conveys the liquid elec- 
 trolyte from any convenient receptacle to the 
 pad, E. This pad must be kept constantly wet, and the flow 
 of solution is regulated by the clip, C, upon the india-rubber 
 tube. The surface to be coated is connected with the negative 
 pole of the battery; now, by brushing the apparatus over the 
 surface, electrolytic connection is made between it and the anode, 
 A, through the solution with which the pad, E, is wetted ; thus 
 the electrolyte is decomposed and deposits its metal upon the 
 required object, the thickness of film being regulated by the time 
 during which the handle is held over each portion in succession. 
 As the electrolyte is used up, fresh liquid is supplied through the 
 central tube. For very irregular surfaces a long-haired brush, 
 with short anode-wires admixed with the hairs, may be substituted 
 for the sheet-anode and flannel-pad. Care must be taken, of 
 course, as in all electro-depositing work, that the surfaces are 
 perfectly clean before attempting to coat them. 
 
 FIG. 58. Wagener 
 and Netto's ' doctor. 
 
SPECIAL PLATING ARRANGEMENTS. 
 
 105 
 
 Smith and Deakin's Rotatory Plating Apparatus. In the 
 
 plating, especially in the electro-nickeling and brassing of small 
 goods, much time is lost in attaching the wires required to sus- 
 
 FIG. 59. Smith and Deakin's rotatory plating apparatus. 
 
 pend them in the bath, and in polishing the goods after deposi- 
 tion. By placing the objects to be plated in a revolving drum 
 and at the same time connecting them with the negative con- 
 ductor from the dynamo, the difficulty has been obviated by 
 Smith and Deakin in their 1896 patent. Fig 59, taken from a 
 
106 PLATING ADJUNCTS AND DISPOSITION OF PLANT. 
 
 drawing supplied by the Electrolytic Plating Apparatus Company, 
 shows the arrangement in use. V is an ordinary vat (part of the 
 side being supposed cut away for the purpose of the illustration, to 
 show the internal arrangements) ; within V is a hexagonal drum, D, 
 mounted horizontally in the bath, and supported at each end from 
 the longitudinal iron bar, B B. This bar is held at either end, 
 in clamps which may be opened at will ; when they are opened, the 
 bar with all its attachments may be raised to any desired height 
 above the vat by the cord, R R, passing over the pulleys, P P, 
 attached to the roof immediately over the vat, and may be held 
 in such position by bending the cord over the cleat, C. 
 
 Passing transversely over the centre of the vat is the shaft, S, 
 normally running at 100 revolutions per minute, carrying the 
 pulley, C P, from which a belt is passed over the lower pulley, 
 L P, attached to the rod, R ; this pulley, in turn, serves to 
 rotate the drum, D, by means of a strap passing round the latter, 
 as shown in the figure. 
 
 The drum is made of wood, so perforated throughout that the 
 area of the perforations is about equal to that of the solid wood 
 between them, and the size of the holes used depends upon that 
 of the goods to be plated, being, of course, larger when the goods 
 are of larger size. One side of the drum is removable, so that the 
 objects to be plated may be introduced and removed. The drum 
 itself is mounted on a hollow spindle or sleeve with copper con- 
 nections at intervals, which serve to make contact with the goods 
 within, and which are connected by means of the sleeve and 
 thence by conductors within the supporting brackets to the bar, 
 B B, and so to the negative lead - . The anodes, A A, are sus- 
 pended on either side of the drum and are connected with the 
 positive lead + . 
 
 In use the bar, B B, with the drum and attachments, is raised 
 out of the vat, and supported just above the top rim ; the drum 
 is then half filled with the goods to be plated, which must, of 
 course, be thoroughly cleaned with potash, and dipped as usual. 
 The removable side is then fastened in place, and the whole is 
 lowered until the drum is completely immersed in the bath, and 
 the bar, B B, is gripped in its clamps. The belt being started, 
 rotation is commenced and kept up until the operation is finished. 
 For ordinary small goods, without sharp corners, the drum should 
 rotate at about 55 revolutions per minute, but with heavy 
 goods, especially those with square or sharp corners, it is preferred 
 to reduce the speed to from 3 to 6 revolutions. The current- 
 density and E.M.F. used, and hence the time required for plating, 
 are the same as in the ordinary process. When the coat is 
 sufficiently thick, the rod, B B, and the drum are again raised and 
 supported above the vat ; a tray is placed under the drum, the 
 loose side is removed, the goods are turned out on to the tray, and 
 are then washed and dried out without further treatment. 
 
SPECIAL PLATING ARRANGEMENTS. 107 
 
 In this process the use of a polygonal instead of a circular 
 rotating barrel causes the goods to be constantly turning over 
 upon themselves so long as they are in the bath ; it is therefore 
 practically impossible that they should show anything corre- 
 sponding to wiring-marks, such as are caused by surfaces re- 
 maining in contact and not becoming properly coated ; moreover, 
 as their relative positions are constantly changing, there is no 
 fear of a permanent bad contact causing one piece to receive an 
 insufficient contact. But it is not only in this direction that 
 the apparatus is useful : the constant friction of surface against 
 surface as the pieces tumble one upon another causes them to be 
 constantly burnished, and in the end the goods are delivered in 
 such a condition that they do not require to be polished. It is 
 obvious that, in order to produce a good burnish, the goods must 
 have freedom to move as the barrel rotates. If the drum be more 
 than half filled, therefore, or if it be run at the lower rate of rota- 
 tion, they will come out in such a condition that they will require 
 
 FIGS. 60 and 61. Joints of lead-lined vats. 
 
 to be coloured that is, finished by mopping, as explained on 
 p. 122. When it is not desired that the pieces should be finished 
 during the plating process, the barrel may be completely filled. 
 
 It should be unnecessary to remark that in any one operation 
 all the goods under treatment should be of the same metal ; if, for 
 example, brass and iron ware be placed in the barrel together, there 
 would be a danger of the iron becoming attacked to some extent 
 before it is sufficiently protected by a film of metal. It is recom- 
 mended in using this process to deposit nickel or brass directly 
 upon iron or steel without an intermediate coppering. In nickel- 
 ing, the patentees prefer to employ a somewhat alkaline solution 
 at a density of T057 (8 Be), and at a temperature of over 60 F. 
 
 Joints of Lead-lined Vats. Figs. 60 and 61 illustrate a 
 simple method of making the joints of lead-lined wooden vats 
 without having recourse to autogenous soldering. It was used 
 successfully at the Casarza Copper Extraction Works. It will be 
 seen that one sheet of lead is bent over at right angles outside 
 the wooden side of the vat, whilst the sheet on the cross side is 
 extended beyond the wood. Iron angles are attached to the 
 wood, and the wood and lead are held firmly together by a series 
 of bolts and nuts. 
 
CHAPTER VI. 
 
 THE CLEANSING AND PREPARATION OF WORK FOR THE DEPOS1TING- 
 VAT, AND SUBSEQUENT POLISHING OF PLATED GOODS. 
 
 IN this chapter it is proposed to treat of those adjunctory 
 processes and arrangements which, not being purely electrolytic, 
 and being common, moreover, to all branches of deposition, are 
 less conveniently dealt with in the chapters devoted to electro- 
 plating with the metals individually. 
 
 Objects must be Clean. We have constantly urged that too 
 much stress cannot be laid upon the necessity for absolute 
 cleanliness in all operations, but this is especially to be observed 
 in the preparation of the plate or object to receive the deposit, 
 because the merest speck of tarnish, oxide, or grease such as 
 may result from merely fingering it suffices to prevent the 
 adhesion of the coating-metal at the points affected ; the whole 
 work may be ruined, the deposit may have to be removed, and 
 the precipitation repeated from the beginning. Cleansing pro- 
 cesses are, therefore, essential in every case ; frequently, as when 
 a bright deposit of a hard metal (e.g., nickel) is sought for, it is 
 necessary also to give the highest possible polish to the articles 
 before immersing them in the depositing-vat. There are two 
 systems of cleansing, chemical and mechanical, which must be 
 varied to suit the metal which is to be coated. 
 
 Removal of Grease. This is usually the first operation to be 
 performed. Large amounts of oily or fatty matter should be 
 removed by rinsing thoroughly in two washes of benzene. The 
 benzene must be kept in tightly-closed vessels maintained in a 
 cool position, as far as possible from any furnace or from arti- 
 ficial lights (excepting, of course, incandescent electric lamps) on 
 account of its rapid evaporativeness and dangerously-inflammable 
 nature. Benzene and benzoline exert merely a solvent action 
 upon the grease, and so extract the greater portion of it. On 
 removal from this liquid the goods should be well rubbed with 
 a soft cotton cloth if they were originally very dirty, and may 
 often receive a further dip into fresher benzene with advantage. 
 The last trace of grease is removed by a more chemical treatment, 
 such as immersion in boiling caustic potash solution. If the pieces 
 
 108 
 
CLEANSING BY POTASH LIQUIDS. 109 
 
 were not in bad condition at the outset, the mere application of 
 the potash may suffice ; indeed, it may be used alone in any case, 
 but the time required for the treatment must then be extended in 
 proportion to the amount of fatty matter to be removed. 
 
 The action of hot alkali on animal or vegetable oil, fat, or 
 grease is to convert it into a soap. The fatty matter is a com- 
 bination of a fatty acid with glycerine, and is not soluble in water ; 
 but, on heating with a caustic alkali, such as potash or soda, the 
 fat is decomposed, the fatty acid combines with the alkali to form 
 a soap, and the glycerine is set free. As both the soap and 
 glycerine are capable of dissolving in water, it is obvious that the 
 greasy mass, after saponification (as the soap-making process is 
 termed), can be completely washed away in the excess water of 
 the dip. The soap or soapy water must, however, be thoroughly 
 rinsed off in fresh water before immersing the object in an acid 
 dip, because acids decompose soap, combining with the alkali, and 
 setting free the fatty acid which, being practically insoluble, would 
 be almost as objectionable as the 
 original grease. Hence the swilling 
 away of the soap is as important as 
 is the dip in potash itself. 
 
 It must be remembered that the 
 mineral oils and greases (paraffin, 
 petroleum, vaseline, and the like) are 
 chemically quite unlike the fatty oils, F IG. 62. Muffle-furnace, 
 and are not decomposed by alkali, 
 
 so that a potash or soda dip is useless for the cleansing of objects 
 coated with such an oil, or with a mixture containing a mineral 
 oil. Benzene or mineral naphtha dissolve them perfectly, and 
 this treatment, in conjunction with scrubbing with lime, should 
 be resorted to whenever this difficulty is met with. 
 
 The organic dirt is sometimes removed by heating the objects 
 to dull redness on charcoal, or, preferably, in a mw^Ze-furnace, 
 which consists of a small fireclay oven (fig. 62), about 6 to 12 
 ins. long by 4 or 5 ins. wide and high, in which the objects are 
 placed, so that they do not actually come into contact with the 
 fuel, the burning charcoal or coke being built up around the 
 muffle in a suitable furnace, and there is, therefore, no liability of 
 their becoming injured by the impurities from the heating agent. 
 All grease and other organic matter is thus completely destroyed, 
 but the surface becomes covered with a film of oxide which must 
 be thoroughly removed in the subsequent acid dip ; this usually 
 imparts a dull or frosted surface to the metal, so that polishing 
 should be resorted to before plating if a bright surface is ulti- 
 mately required, especially when the covering metal is so hard 
 that it will not itself admit of polishing. After this polishing the 
 objects must be rapidly passed through the potash- vat, and again 
 through the acid-baths, to ensure perfect cleanliness prior to the 
 
110 PREPARATION FOR DEPOSITING- VAT POLISHING. 
 
 actual plating operation. The heating process being really one 
 of annealing, rolled, hammered or worked metal becomes softened 
 and so far altered in character that many articles, such as spoons, 
 forks, or dessert-knives, which are purposely left unannealed, and, 
 therefore, hard, are rendered practically useless ; this alteration 
 is indicated by the loss of the characteristic metallic ring that 
 should be emitted by striking the object in its hardened con- 
 dition. It is, of course, unnecessary to remark that bodies which 
 have a low fusing-point, such as tin, lead, pewter, Britannia 
 metal, and the like, cannot be submitted to the fire treatment. 
 This method is not, therefore, of universal application, and, 
 indeed, is rarely, if ever, to be preferred to the other processes 
 here described. 
 
 The potash-tank is best constructed of wrought-iron, which may 
 be heated by setting it over a gas stove or charcoal fire, but more 
 conveniently, when a supply of steam is available, by forcing the 
 steam through a spiral or series of iron pipes placed within the 
 vessel and close to the bottom. As the final stages of the cleans- 
 ing process should be conducted as close to the depositing-vats as 
 possible, so that no time may be wasted between the operations, 
 it is frequently convenient to utilise steam, which has been 
 previously employed in a steam-jacket or coil of pipes, for heating 
 the latter vessels whenever the plating-solution is to be worked 
 hot. In this case the pipes in the potash-tub may be perforated, 
 so that the steam and condensed water may pass into the liquid 
 itself, which is thus maintained in constant motion, while the 
 amount of water lost by gradual evaporation is compensated for. 
 Such a system of heating compares favourably with the direct 
 application of solid, liquid, or gaseous fuel, inasmuch as there is a 
 greater choice of materials from which to construct the containing 
 vessel. When external heating arrangements are adopted, the 
 vat must be of fairly thin metal, to minimise the loss of heat- 
 energy, and must be carefully supported to avoid strains from 
 the weight of the liquid ; but with internal steam-heating by 
 coils of pipes, whether closed or perforated, any material which 
 will resist the chemical action of the hot alkali is available, and 
 the tank may have any convenient size, thickness, or shape. An 
 extension of the steaming system, much to be recommended, is to 
 continue the solid-walled pipe from the potash-vat into a second 
 vessel, there discharging the steam from the orifices of a perfor- 
 ated pipe into the water, which is used for rinsing the pieces after 
 removal from the alkaline solution. 
 
 The cleansing liquid is a 5 or 10 per cent, solution of caustic 
 potash or caustic soda in water ; if these be not available, sodium 
 carbonate (washing soda) may be substituted, but its effects are 
 not comparable with those of the caustic alkali, and it demands 
 a far more protracted steeping of the articles to be cleansed. 
 Since the caustic alkalies gradually become carbonated by the 
 
CLEANSING BY POTASH LIQUIDS. 
 
 Ill 
 
 FIG. 63. Copper 
 dipping-wire. 
 
 absorption of carbonic acid from the air, and since they also 
 become gradually saturated with greasy matter, forming soapy 
 substances by combination with them, fresh potash (or soda) 
 must frequently be added to restore the strength of the bath ; 
 this precaution must be most carefully observed, as the bath soon 
 becomes useless for its work, and the process then begins to give 
 unsatisfactory results. To check this forma- 
 tion of carbonates by exposure to the air, the 
 potash-vats should be closed with a tightly- 
 fitting cover whenever they are not in use, 
 and are not being heated. The caustic alkalies 
 readily attack tin and lead and allied metals, 
 and for this reason a prolonged potash dip 
 must never be given to any objects made 
 entirely, or in large part, of these metals 
 (pewter, Britannia metal, and similar alloys), 
 or to any which are joined by tinman's soft 
 solder, which is an alloy of tin and lead; if 
 applied at all to these bodies, the dip must 
 be momentary ; but whenever practicable it 
 should be avoided altogether, inasmuch as a 
 rapid dip does not afford sufficient security 
 for that absolute cleanliness and freedom from 
 grease which is quite indispensable. For such 
 metals, and for any which it is undesirable to heat to the 
 temperature of the boiling alkali bath, a very thorough treat- 
 ment with benzene or any simple solvent of fat may suffice ; or 
 the pieces may be carefully rubbed by means of a brush with 
 a paste of very finely-crushed whiting and water, taking care 
 that neither the fingers nor any greasy material be allowed to 
 
 come into contact with the cleaned 
 surfaces. 
 
 In using the potash dip, the small 
 pieces should be slung upon copper 
 wires if convenient (fig. 63), or held 
 in perforated bowls of glazed earthen- 
 FIG. 64. Earthenware dipper, ware (fig. 64), or in a copper wire 
 
 sieve (larger articles are suspended 
 
 singly from suitable hooks), and must be allowed to remain in 
 the bath, with frequent agitation, for a space of time ranging 
 from half a minute to ten minutes or a quarter of an hour, 
 according to the amount of impurity to be removed. On com- 
 pletion of this treatment the articles are immediately plunged 
 into a large volume of hot water; and, after this preliminary 
 rinsing, are rapidly and thoroughly washed by passing through 
 successive wash- waters, which may be used cold ; they should 
 then be transferred at once to the acid-baths, and thence to the 
 plating-vats. From the moment they enter the potash-tank, they 
 
112 PREPARATION FOR DEPOSITING-VAT POLISHING. 
 
 must on no account be touched with the fingers or with any 
 surface that may be oily or greasy. The reason is not far to 
 seek. Oil and water are not miscible, and if even the smallest 
 patch be ever so slightly greased, water will be repelled from 
 that point, so that the acids cannot effect the work required of 
 them, nor will the electrolytic solution be able to come into 
 contact with the metal surface, with the result that this portion 
 will be left uncoated, or at best covered with a non-adherent 
 deposit which forms on the greasy matter. 
 
 The object being removed from the potash -vat, and now 
 unstained with grease, the liquid in the plating-baths can make 
 the most intimate contact with its surface in every part ; and, 
 therefore, when the current is passed, the required metal may 
 be deposited uniformly over the whole article ; but it would 
 even then be found to have little or no adhesion to the surface 
 covered and would strip off when subjected to a very moderate 
 amount of rubbing most probably during the final polishing 
 process. All objects which have been exposed to the air for 
 any appreciable time after polishing are covered with a film of 
 tarnish or rust, which is insoluble in potash and may be so thin 
 as to be imperceptible, but which, by interposition between the 
 two surfaces, prevents true metallic contact and consequently 
 destroys adhesion. This film must, therefore, be most carefully 
 detached, which is best accomplished by immersion in a bath of 
 acid, or of a solvent suitable to the metal under treatment. 
 The acid dip constitutes the final preparation for the plating- 
 bath, and should leave a chemically-clean surface for the re- 
 ception of the coating metal. After washing, tarnishing again 
 takes place by unnecessary exposure to the air ; and it is thus 
 important that the object should be transferred with the highest 
 rapidity from the acid-bath to the wash-waters, and from these to 
 the electrolytic solution. 
 
 Copper, brass, German silver, and other alloys, of which the 
 chief constituent is copper, are, as already indicated, cleaned and 
 brightened by dipping into acid after removal from the potash 
 solution. The common nitric acid of commerce, which emits red 
 fumes and is known in the trade as nitrous acid, is very generally 
 used for the acid dip ; it consists of nitric acid in which lower 
 oxides of nitrogen are dissolved, and the waste nitric acids from 
 used Grove's or Bunsen's battery cells are not unsuitable for the 
 work. It is used undiluted ; the articles to be dipped are slung 
 on wires or in the baskets already described ; baskets of platinum- 
 wire mesh are sometimes used for small articles, and as they are 
 quite unattacked by the acid, and therefore neither weaken nor 
 contaminate the liquid, they are to be preferred to all others ; the 
 prime cost alone is against them, but they soon repay this 
 expenditure by their indestructibility and by the greater comfort 
 and saving in their use. Since the acid exerts a powerful action 
 
SPECIAL CLEANSING PROCESSES. 113 
 
 upon the metal of which the articles are made, and the film of 
 oxide is instantaneously cleared from the surface, the plunge into 
 acid must be but momentary ; the objects are then rapidly 
 transferred to a vessel containing a large volume of water, and 
 are afterwards washed again and again, the last time in quite 
 clean water. It is false economy in electro-plating to stint wash- 
 waters at any stage. Another acid dip, sometimes used in 
 preference to the above, is made by mixing together, little by 
 little and with the utmost care, equal volumes of strong sulphuric 
 acid and water, and adding to each gallon of the mixture one 
 pint of the common aquafortis (nitric acid) of commerce. The 
 subsequent manipulations are similar to those already described. 
 A dead or dull surface may be given by plunging the article, 
 previously dipped in nitric acid, into Roseleur's mixture of 200 
 parts of yellow nitric acid, 100 of oil of vitriol, and 1 part of 
 common salt, together with from 1 to 5 parts of zinc sulphate 
 (white vitriol), which should be made up the day before it is 
 required for use. After remaining in this liquid for several 
 minutes (from live to fifteen or even twenty) the articles are 
 removed, plunged momentarily into a bright-dipping bath, to 
 restore a certain amount of the brilliancy which is generally too 
 thoroughly removed in the dead dip, and are then rinsed as usual. 
 
 A bright lustre is given by placing for a few seconds in a 
 mixture of the yellow nitric acid and oil of vitriol in equal parts, 
 with 0'5 per cent, of common salt ; like the last, this mixture 
 should be made up the day before it is required, or earlier, in 
 order that it may be quite cold when required. Instead of this solu- 
 tion, a mixture of 1J to 2 parts of sulphuric acid with 1 -part of 
 an old nitric acid dip is frequently used ; after mixing these 
 acids, they should be put aside to cool, and decanted from any 
 crystals of copper sulphate which may be formed by the action 
 of the vitriol upon the copper contained in the old aquafortis 
 (due to the partial solution of the objects that have been 
 immersed in it). Small proportions of hydrochloric acid or of 
 sodium chloride, which by contact with the vitriol liberates 
 hydrochloric acid, or of lamp-black are sometimes added to this 
 solution. 
 
 Potassium cyanide, dissolved in ten times its weight of water, 
 is often used instead of the acid dip for brass, especially when 
 it is essential that the original polish upon the article should 
 not be destroyed, as in the preparation of the objects for nickel- 
 plating. A longer immersion in this liquid is to be recom- 
 mended because the metallic oxides are far less readily soluble 
 in this than in the acid dips. In all cases the final cleansing in 
 water must be observed. 
 
 Iron and steel articles, which have been thoroughly polished, 
 are dipped first into the boiling potash solution, and then, when 
 thoroughly cleansed from grease, into a pickle consisting of water 
 
 8 
 
114 PREPARATION FOR DEPOSITING-VAT POLISHING. 
 
 containing 10 per cent, of sulphuric or nitric acid or 25 per cent, 
 of hydrochloric acid. Iron is vigorously attacked by acid, and by 
 long immersion becomes unevenly dissolved and pitted. Cast- 
 iron and steel also are liable to have a film of carbon and other 
 insoluble impurities on the surface after pickling. Some operators 
 therefore prefer, when possible, to substitute for pickling a 
 mechanical process, such as that described on p. 117. When 
 the metal is covered with oxide or scale, as after forging or heating 
 to redness, more vigorous measures are needed to effect their 
 removal ; the objects are first suspended for two or three hours in 
 a bath of dilute sulphuric acid (2 or 3 per cent, of acid for 
 wrought-iron or steel, about 1 per cent, for cast-iron), which dis- 
 solves a little of the oxide and loosens much of the remainder, so 
 that, after washing well with water, it may, for the most part, be 
 detached by scouring with very fine sand. A second dip into the 
 acid usually removes the last portions of scale ; but, if necessary, 
 the process must be repeated until the pieces are perfectly clean. 
 
 Zinc is first passed through the potash-bath, which exerts a 
 distinct solvent action upon it, so that the process must be 
 expedited, and is next dipped for a few minutes into water con- 
 taining 10 per cent, of sulphuric acid ; the mixture of acids used 
 for bright-dipping brass (50 sulphuric acid, 50 nitric acid, and 0'5 
 salt) is sometimes used, but as it violently attacks the zinc, the 
 operation of dipping must be rapidly effected, and the subsequent 
 washing must be immediate and thorough. After treatment by 
 either of these processes, scouring with fine sand and clean water 
 must be resorted to, which has the effect, inter alia, of obliterating 
 the black lines that indicate the position of solder after dipping 
 in the acids. 
 
 Lead, tin, Britannia metal, and the like are very rapidly passed 
 through the potash-bath, or, as already described, are scoured 
 with whiting and water subsequent to cleansing in benzene, if 
 this be necessary by reason of extreme greasiness. They should 
 be polished finally by rubbing with lime, even after the potash 
 dip ; they then require only to be well rinsed in water to be ready 
 for the electrolytic vat. 
 
 Aluminium articles are not easily plated. Burgess and Ham- 
 buechen recommend that the article be first cleaned in dilute hydro- 
 fluoric acid until it is suitably roughened ; then rinse in running 
 water, dip for a few seconds in a mixture of 100 parts sulphuric 
 acid and 75 parts of nitric acid (both concentrated) ; rinse again 
 and transfer to a zinc-plating solution. The latter consists of a 
 mixture of zinc and aluminium sulphates, slightly acidified, to 
 which is added 1 per cent, of hydrofluoric acid in the equivalent 
 amount of potassium fluoride. A current-density of 10 to 20 
 amperes per sq. ft. is used. After a deposit of zinc has been 
 thus obtained, a coating of any other metal may be deposited. 
 
 AH acid pickles used for different classes of work should be 
 
SPECIAL CLEANSING PROCESSES. 115 
 
 kept distinct from each other, so that one metal may not be 
 dipped into a solution containing a more electro-negative metal, 
 which would deposit upon it by chemical exchange. For example, 
 zinc or iron must not be immersed in a pickle which is used for 
 cleansing copper articles, because a certain amount of copper 
 gradually dissolves into the liquid as successive objects are dipped, 
 and this copper tends to deposit upon the more electro-positive 
 metals afterwards brought into contact with it. 
 
 Electrolytic Cleaning. During recent years electrolytic 
 methods of cleaning have come into use. The method of the 
 Vereinigte Elektricitats Aktien Gesellschaft of Vienna is to use 
 a solution of a salt of one of the alkali metals ; if the object 
 to be cleaned is of aluminium or zinc it is made a cathode, 
 whereas if it is of iron or copper it is made an anode, carbon 
 being the other electrode in each case. In the former case an 
 aluminate or zincate is formed, and when this diffuses sufficiently 
 to meet the acid formed at the anode it is decomposed, and the 
 hydroxide of the metal is precipitated ; in the second case, a salt 
 of the metal is formed, and this diffuses until it meets the caustic 
 formed at the cathode, when precipitation again occurs. Thus 
 the metal is regenerated, and the dissolved metal is collected as 
 a precipitated hydroxide. 
 
 C. J. Reed has found the electrolytic method of pickling iron 
 as cathode in dilute sulphuric acid effective under the following 
 conditions : temperature 60 C., specific gravity of acid 1'25, 
 and current-density about 70 amperes per sq. ft. If the current- 
 density is low the iron becomes attacked, and the scale con- 
 sisting of Fe 3 4 is reduced but slowly ; whereas if the current- 
 density is high the solvent action on the scale becomes very 
 rapid and the chemical action on the iron is entirely prevented. 
 Under these conditions the heaviest scale is removed in two or 
 three minutes. 
 
 For cleaning iron and brass articles, H. S. Coleman recommends 
 a potash-bath, preferably hot. This may be contained in an iron 
 tank which is made the cathode, the articles to be cleaned being 
 the anode. With a current-density of 8 amperes per sq. ft., 
 any grease and dirt is readily removed in five to ten minutes, 
 but a stain is left ; the latter is removed by reversing the current 
 for a short time (thirty to forty seconds) as soon as the stain 
 appears. It was found that work was cleaned thoroughly in all 
 parts in one-third the time occupied by the old method of scouring 
 by hand, and thus the electrolytic method was adopted. This 
 method has the further advantage that the articles are not 
 handled after the cleaning, but being already wired up they are 
 merely passed through water and dipping solutions, and are at 
 once ready for the plating bath. 
 
 Quicking. Many articles are ' quicked ' before being subjected 
 to the operation of depositing other metals, especially silver and 
 
116 PREPARATION FOR DEPOSITING-VAT POLISHING. 
 
 gold, upon their surfaces. This simply consists in giving them 
 a superficial amalgamation by the deposition of a thin film of 
 mercury, in order that many metals, which alone would deposit 
 the coating-metal from the plating-liquors by simple immersion, 
 may be rendered practically incapable of so doing, the resulting 
 deposit being more adhesive and of better quality in consequence. 
 
 But quicking is also often resorted to in order to increase the 
 adhesiveness of deposited metals on objects which would have 
 no action on the bath ; for the mercury, being but little liable to 
 tarnish by oxidation, retains a bright surface when exposed to 
 the air for a period which would suffice to produce a film of oxide 
 upon an unquicked surface, and thus prevent adhesion. More- 
 over, the solvent action of the mercury on both surfaces (especi- 
 ally on gold and silver) tends to unite the two metals in the 
 most intimate contact, and may even, by interamalgamation, 
 form a superficial alloy which would thus make adhesion perfect. 
 
 The principal metals so treated are copper, brass, German 
 silver, and the like, previous to gold- and silver-plating, and zinc 
 prior to nickeling. The quicking-solutions more commonly used 
 are : the per-nitrate or proto-nitrate of mercury, the strength 
 ranging from 1 to 2 oz. per gallon (Roseleur recommends 1 
 part of mercuric (per-)nitrate and 2 of sulphuric acid to 1000 
 parts of water) ; or the cyanide of mercury, which is made either 
 by adding a solution of potassium cyanide to one of a mercury 
 salt (nitrate, chloride, or sulphate), until no further precipitate 
 is produced, allowing the cyanide of mercury to subside, washing 
 it two or three times with water by alternately stirring it up, 
 allowing it to settle and pouring off the clear liquor from the 
 precipitate, then dissolving it in a further quantity of potassium 
 cyanide and diluting with water ; or by dissolving mercuric (per-) 
 oxide directly in potassium cyanide solution. 
 
 The objects are merely dipped into these solutions, when metal 
 is superficially dissolved from them and mercury is deposited in 
 its place by simple chemical exchange. The duration of the dip, 
 never much more than momentary, is governed by the amount of 
 mercury to be deposited, which in turn depends upon the thick- 
 ness of the object to be plated and that of the coat to be applied. 
 Usually a thick object and a thick coating demand, or at least 
 permit, a heavier mercury deposit than thinner ones, which are 
 more liable to become brittle, and for which a mere momentary 
 immersion will suffice. The character of the basis-metal } also 
 influences the time required for mercury-deposition, inasmuch as 
 the electro-positive metals have a more rapid action than those 
 which are more electro-negative. Zinc, especially, requires 
 
 1 The word basis-metal is here applied to the metal which forms the object 
 or base upon which an electro-deposit is ultimately to be given ; the term 
 base-metal, though more euphonious, has a second signification which might 
 prove to be misleading. 
 
ECHANICAL TREATMENT. 117 
 
 careful quicking, as it not only deposits the mercury with rapidity, 
 but is very readily penetrated by it, and is rendered brittle in 
 consequence. The quicking-bath should be of such a strength 
 that copper plunged into it becomes immediately covered with a 
 silvery-white metallic film. The use of old or dilute solutions 
 should be discontinued when they begin to yield a dark or almost 
 black deposit of mercury, which is worse than useless, because 
 the electro-deposited metal refuses to adhere to it. If the liquid 
 be too strong, or contain too large an excess of free nitric acid, 
 a similar result obtains ; it is, however, easy to decide to which 
 cause failure is to be attributed. 
 
 If the pieces have not been properly cleansed, the quicking- 
 solution will give an irregular, patchy or discoloured film, instead 
 of a clear silver-like uniform coat, owing to the presence of foreign 
 bodies such as grease or oxide. 
 
 Mechanical Treatment. Scouring with sand or pumice is best 
 conducted on a wooden board placed above a tub containing the 
 water or liquid to be used. The scouring brush should be made 
 with moderately hard bristles (hogs' hair is generally preferred), 
 and is used by plunging it into the water, withdrawing it, shaking 
 gently to remove the excess of the liquid, dipping in the powdered 
 pumice-stone, and at once rubbing it over the whole surface of 
 the object. To yield good results, the brush must be constantly 
 charged with the powder, but while keeping it thoroughly moist, 
 excess of water is to be avoided ; the dipping into water and 
 powder may thus have to be frequently repeated if the object be 
 at all large. When the scouring is not to be followed by a potash 
 dip, the pieces should not be touched with the bare hands, and 
 both brushes and powder must be examined to see that there is 
 no trace of greasy matter attached to them. 
 
 In preparing metal for the chemical treatment previous to 
 actual electro-deposition, the pieces should be polished at least in 
 part ; as a rule, however, it will be found that the coating-metal 
 adheres less satisfactorily to a surface which is perfectly polished 
 than to one which is in a slight degree, it may be almost imper- 
 ceptibly, roughened. Nevertheless, the polishing must be so far 
 completed that all marks of the file or tool are obliterated, and 
 the whole surface has only a regular, and hence almost invisible, 
 roughness ; for it is difficult to remove file-marks or scratches 
 afterwards from the plated article without cutting through the 
 coating, or at least rendering it dangerously thin. 
 
 Deep irregularities must first be removed, and this may necessi- 
 tate the use of a file ; the marks of the latter must now be erased 
 by rubbing with some material such as emery ; and this in turn 
 leaves finer markings, still too coarse to be left untouched, and 
 which necessitate the use of polishing tools. Of these the most 
 successful are those which are caused to revolve rapidly in one 
 plane by suitable mechanishi, and which not only economise time, 
 
118 PREPARATION FOR DEPOSITING- VAT POLISHING. 
 
 but have a perfect regularity of action and produce true parallelism 
 of the fine lines scratched by them. It is well known that the 
 best surface is always obtained when the polishing tool is passed 
 over it uniformly in the same direction, and that any motion 
 which produces cross-lines, no matter how fine they be, and 
 thus gives rise to cross-reflections of light, destroys the evenness 
 of the appearance. Hand-polishing is especially liable to produce 
 these cross-lines, and thus entails a greater expenditure of time 
 and care. 
 
 Polishing-Lathe and Dolly. The lathe-action is generally used 
 for polishing ; discs or bobs of stout leather, usually hippopotamus- 
 or walrus-hide, about half-an-inch to one inch in thickness, and 
 four or six inches in diameter, are rotated rapidly on the lathe- 
 spindle by means of a treadle like that of a lathe or of a grind- 
 stone, and while thus in motion the workman with one hand 
 firmly presses the object to be polished against the lower side of 
 the leather bob, while with the other he allows a gentle stream 
 
 of fine sand to fall upon the 
 
 B D A C D S B' top of the disc as it revolves 
 
 towards him from above down- 
 wards. Trent sand is usually 
 deemed most suitable for the 
 work; it may be used re- 
 peatedly. The pieces may be 
 first treated with fresh rough 
 FIG. 65. Bench power-spindle. sand to obliterate the deeper 
 
 markings, and afterwards with 
 
 worn sand, which has been used many times. For this purpose 
 the bobs should make 1500 or 2000 revolutions per minute, 
 but this is a high rate of rotation to be maintained by a treadle 
 action, and is, therefore, more satisfactorily communicated by 
 steam-power. Fig. 65 shows a bench power-spindle suitable 
 for this purpose ; the base of the stand is bolted firmly to a 
 strong table or bench ; the spindle, S, has a screw at either 
 end, to which the bobs, B B', are firmly attached ; and between 
 the forked arms of the stand, which carry the bearings, D D, 
 of the rotating spindle, are fast-and-loose pulleys, one of which 
 is keyed firmly to S at A, so that when connected with a 
 large pulley on the main shafting in the shop by means of 
 a leather belt, it acts as a driven pulley and imparts the 
 required motion to the bobs ; the other is free to turn loosely 
 upon S at C, so that when the belt is shifted to it from the 
 driving pulley, it alone rotates, and the spindle remains at rest. 
 A lever must be conveniently placed to shift the belt from the 
 one pulley to the other at a moment's notice. Two workmen 
 may use such a tool simultaneously ; one standing at B applies 
 the coarse sand only, and when the object is thus sufficiently 
 treated, hands it on to the second operator at B', who finishes it 
 
THE USE OF THE SCRATCH-BRUSH. 119 
 
 with the finer sand. Even now the metal is not absolutely 
 bright, but requires a final polish with a finer material, which 
 may be given by bobbing the article with a little fresh, finely- 
 crushed quicklime (Sheffield lime is particularly well suited to the 
 work) mixed with a little oil. The lime should be thoroughly 
 caustic, and as it rapidly absorbs both carbonic acid and moisture 
 from the air, it must be stored in air-tight closed boxes as soon 
 as possible after burning, and should only be removed from these 
 a little at a time as required for use. The last polish of all is 
 given by a small quantity of lime applied without oil by the pro- 
 jecting edges of a series of calico rings clamped one upon another 
 in a wooden holder with a central hole, by which it is screwed to 
 the lathe-spindle in place of one of the bobs. This instrument is 
 known as a dolly. Or a 
 mop may be used for colour- 
 ing the goods (see p. 122). 
 Scratch - brushing con- 
 sists in submitting the sur- FIG. 66. Short wire brush, 
 faces of articles to the 
 
 polishing action of a number of fine wires set on end. The 
 wire selected for this purpose must be harder than the metal to 
 be treated by it, or it will have little or no action, and may even 
 cover its surface with a thin film of metal worn off from the wires 
 by attrition ; when, for example, nickel is scratch-brushed with 
 brass wire the surface becomes quite yellow in tint ; it must not 
 only be relatively hard, but must also be actually and intrinsically 
 
 rigid and stiff, so that 
 the points shall not be 
 readily bent over out 
 of shape when in use. 
 FIG. 67. Long wire brush. Hard-drawn thin brass 
 
 wire, which may be made 
 
 partially or wholly soft if required, by suitable annealing, is the 
 usual material for scratch-brushes, but occasionally steel, or even 
 spun-glass, may be employed for treating extremely hard surfaces. 
 Hand scratch-brushes are about 6 or 8 ins. long, and are 
 made by firmly binding a large number of wires in the middle so 
 that they form a compact bundle, with the ends free for the space 
 of half an inch to an inch. One end is then dipped into soldering 
 fluid (hydrochloric acid, in which as much zinc as possible has 
 been dissolved) and then into a ladle of melted soft solder, firmly 
 to unite the various wires and so form a solid brush ; the whole 
 is then mounted on a wooden handle for convenience, with the 
 free ends of the wires extending beyond the handle, as in fig. 66. 
 Occasionally a double length of the wires is taken, and they are 
 simply bent over upon themselves and bound round the centre, 
 leaving a loop at one end, and are afterwards mounted as before 
 on a wooden handle (fig. 67) ; but it is less easy to produce 
 
120 
 
 PREPARATION FOR DEPOSITING-VAT POLISHING. 
 
 regularity in the laying of the wires by this means. Either of 
 these brushes may be used upon small surfaces ; for large areas, 
 
 FIG. 68. Wire scrubbing-brush. 
 
 FIG. 69. --Machine brush. 
 
 FIG. 70. Rotary 
 machine brush. 
 
 the wires are mounted in handles in the form of a scrubbing-brush, 
 as shown in fig. 68. 
 
 When the motion is to be applied to the brushes by machine 
 power and this method is to be preferred to hand labour a 
 number (4, 6, 8 or more) of hand-brushes may be mounted on 
 the periphery of a wooden drum, as indicated 
 in fig. 69 ; or a better form is made by setting 
 the wires radially in a circular handle, so as 
 to form a disc of from 5 to 6 ins. in diameter 
 (fig. 70). Both forms of circular brush are 
 mounted on the spindle of the lathe, or on 
 that driven by machine power, shown in 
 fig. 65. 
 
 For surfacing the interiors of vessels, a 
 brush of the shape indicated in fig. 71 is use- 
 ful, or on an emergency an old hand scratch- 
 brush may suffice, the wires of which have become turned over 
 at the points. 
 
 In using any of these brushes, a liquid lubricating medium is 
 employed ; this is most generally stale beer, but many other 
 liquids, such as crude tartar dissolved in water, diluted vinegar, 
 or decoction of soap-wort, are supposed by some 
 operators to produce a better effect, and are, 
 accordingly, substituted for it. The brushes are 
 useless when the ends of the wires have turned 
 over upon themselves ; if they cannot then be 
 straightened by means of a wooden mallet, the ex- 
 treme tips must be cut off by a sharp metal chisel. 
 The circular lathe-brush should be mounted upon 
 the spindle, sometimes on one side, sometimes 
 on the other, so that the direction of rotation is 7 . 
 
 reversed, and the wires strike the object alter- ^rush fo7inner 
 nately with different sides of their surfaces ; thus surfaces. 
 the latter are not unduly bent in one direction. 
 It is essential that the wires be kept in good order, and an 
 occasional dip into potash to remove grease, or into the acid 
 dip to remove oxide, may have to be resorted to. 
 
 The hand-brush is used by holding it in the same manner as, 
 
POLISHING HARD METALS. 121 
 
 and imparting to it somewhat the motion of, a paint-brush. The 
 lathe-brush is mounted upon a spindle, and should be arranged 
 with a small reservoir above to contain the lubricating fluid, a 
 small pipe with a tap serving to conduct the solution from this 
 to a point immediately above the rotating brush, upon which 
 the drops fall at intervals ; the piece is held firmly underneath the 
 brush, but slightly on the side nearer the operator, so as to meet 
 the wires as they descend. Around the brush is a metal screen 
 to prevent splashings produced by the rapid rotation, and beneath 
 it is a tray with an overflow pipe conducting to a receptacle placed 
 below, to retain the waste solution for use again. 
 
 The operation of scratch-brushing is had recourse to after 
 deposition, in order to brighten the dull deposit ; sometimes even 
 at intervals during the process to secure a good coating ; some- 
 times beforehand to brighten the object finally before immersion 
 in the plating- vat. Whenever it is used prior to or during 
 deposition it is obvious that every trace of the lubricating liquid 
 must be washed away before placing, or replacing, the article in 
 the bath. 
 
 When a large amount of work has to be handled, more 
 particularly if the objects to be cleaned are of iron, or if the 
 surfaces are so shaped that they cannot be conveniently reached 
 with brushes, sand blast is preferable. Wooden wheels, covered 
 on the edge with leather which is coated with emery, are also used 
 for grinding. 
 
 Burnishing. This is a process finally applied to polishing 
 silver and some other deposited metals, and consists in rubbing 
 the whole surface under considerable pressure by a very hard and, 
 at the same time, highly-polished surface ; it may be effected after 
 scratch-brushing the articles, or is often used as a substitute for 
 this latter operation. The burnishing tools are usually made of 
 steel for the first or grounding process, and of a very hard stone, 
 such as agate or blood-stone, for finishing. 
 
 These tools must be kept in the highest degree polished by 
 rubbing them vigorously with very finely-crushed crocus- or rouge- 
 powder on a strip of leather, fastened upon a piece of wood which 
 is placed in a convenient position upon the working bench. The 
 burnishers are of various shapes to suit the requirements of 
 different kinds of work, the first rough burnishing being often 
 accomplished by instruments with comparatively sharp edges, 
 while the finishing stages are accomplished with rounded ones. 
 The annexed sketch (fig. 72) illustrates a few of the patterns 
 commonly employed. Soap suds may be used to lubricate and 
 moisten the burnishers. 
 
 Silver-plated goods may be readily polished by submitting 
 them to the action of the lathe-bobs, such as those already de- 
 scribed, or of wood covered with leather, or of brushes, upon which 
 is maintained a small quantity of tripoli-powder mixed with a few 
 
122 
 
 PREPARATION FOR DEPOSITING-VAT POLISHING. 
 
 drops of oil. The last polish is given, either by the application 
 of rouge by constant rubbing with the fleshy portions of the hand, 
 or by a dolly, termed a mop, in which swan's-down is substituted 
 for the calico between the wooden clamps (see p. 119), using with 
 it the finest possible paste of rouge-powder, entirely free from 
 gritty matter, which would destroy rather than improve the 
 existing polish. This mopping is commonly known as finishing 
 or colouring, and gives the final perfect polish. 
 
 The results of scratch-brushing and burnishing are quite dif- 
 ferent, and each system has its own special advantage. Electro- 
 deposited metal is always crystalline, however close the texture 
 may be ; and being thus made up of an aggregation of minute 
 crystals, the light falling upon it is not evenly reflected, but is 
 more or less scattered by the varying facets of the crystalline 
 surface; and thus, although metallic, it has a dead lustre. 
 Scratch-brushing followed by buffing, or bobbing, has the effect of 
 
 very slightly flattening the 
 projecting portions down- 
 wards upon the surface, but 
 mainly of grinding them off 
 until they are level with the 
 lowest portions, and so, a 
 perfectly even and uniform 
 surface being produced, light 
 is reflected as it were from 
 a mirror; but no practical 
 alteration of the physical 
 condition of the coating re- 
 sults. Burnishing, on the 
 
 contrary, scarcely effects any grinding of the irregularities, but 
 rather produces the level surface by flattening the raised portions 
 into adjacent cavities, so that the pressure exerted tends to fill 
 up any pores or inter-crystalline spaces, and so to yield a more 
 solid coat. Thus burnishing produces a denser, more durable, 
 and more solid covering, but the colour and general appearance 
 is somewhat less satisfactory, possibly because the irregularities 
 are merely rounded off and not entirely effaced, so that the 
 surface is not so absolutely true as that yielded by good buffing 
 and dollying. 
 
 Steel, which is too hard to be polished by the methods given 
 above, should be rough-polished with the , emery-wheels, then 
 glazed by the action of bobs of wood covered with leather, to which 
 a mixture of the finest emery-powder with oil is applied. A strip 
 of a soft alloy of lead and tin is sometimes substituted for the 
 leather upon the bob. The finishing polish is administered with 
 the best crocus-powder. 
 
 For nickel deposits the object should be thoroughly polished so 
 as to obliterate tool marks. At the same time, as above pointed 
 
 FIG. 72. Burnishers. 
 
POLISHING HARD METALS. 123 
 
 out, the polish must not be too good, or the nickel will not adhere 
 properly, and will be liable to strip. Iron or steel objects to be 
 nickel-plated may with advantage be placed in boiling potash, 
 removed, and allowed to cool without swilling. They may then 
 be polished well with fine sand and water, which cleanses them 
 mechanically from soapy matter, and leaves very fine scratches 
 into which the preliminary coating of copper keys so that it 
 adheres well. The objects may be coppered without an acid 
 pickle, and after coppering they are well swilled, polished again 
 with sand and water, and nickeled immediately. 
 
CHAPTER VII. 
 
 THE ELECTRO-DEPOSITION OF COPPER. 
 
 IT is frequently necessary to give a coating of copper to metals, 
 chiefly to those, such as iron, which are more electro-positive, 
 occasionally with the object of imparting to them the external 
 characteristics of copper, but more often in order to enable them 
 to receive a good deposit of a less electro-positive metal. But by 
 far the most extensive application of electro-plating with copper 
 is to be found in electrotyping, or obtaining facsimile copies of 
 various objects for the use of the printer or sculptor. The (acid) 
 copper solutions present fewer difficulties in management than 
 perhaps those of any metal, permitting at once a wider range of 
 current-strength and a greater variation of bath-composition. 
 
 COATING BY SIMPLE IMMERSION. 
 
 Iron. Iron is practically the only metal that is coated with 
 copper by simple immersion, and only small articles of this body 
 are usually so treated. An acid solution of copper sulphate, made 
 by dissolving about 2 oz. of the blue salt in a gallon of water, 
 and adding about 1 to 2 oz. of sulphuric acid, may be em- 
 ployed with advantage, but considerable latitude is permissible 
 in the proportions adopted. The deposition of the copper upon 
 the surface of the iron is almost instantaneous, and, indeed, a 
 long exposure in the solution produces a slimy precipitate which 
 has almost no adhesion to the basis-metal; such a deposit, 
 Roseleur recommends, should be mechanically consolidated and 
 attached by rolling, if the metal be in the form of sheet, or by 
 passing through the dies of a wiredrawer's plate, if it be in the 
 condition of wire. Before dipping any iron or steel article into 
 the copper solution, it must be thoroughly cleansed by plunging 
 it consecutively into the caustic alkali liquids and the suitable 
 acid dips, or by an alkali-dip followed by scouring, as described in 
 the last chapter ; then, when thoroughly cleansed, it is immersed 
 in the copper-bath. 
 
 Steel Pens. Steel pens may be coppered superficially by treat- 
 ment in the liquid already described, but are more satisfactorily 
 
 124 
 
COATING BY SIMPLE IMMERSION. 
 
 125 
 
 coated by thoroughly stirring them, after cleansing, in sawdust 
 moistened with a solution of half an ounce of copper sulphate 
 with a like weight of sulphuric acid per gallon of water. The 
 mixture is usually effected in a barrel or drum mounted upon a 
 horizontal axis. The long hexagonal drum outlined in fig. 73 is 
 a convenient arrangement for this purpose. It is mounted so 
 that it may be turned on its horizontal axis, the pins at either 
 end resting in bearings upon the upright supports; one side is 
 hinged, so that it may be opened to admit or discharge the damp 
 sawdust and pens, and when closed is held in position by a suit- 
 able catch. An improvement upon this form may be made by 
 substituting short lengths of tube for the pins at the ends of the 
 drum, and instead of causing them to rotate within bearings, 
 passing a fixed rod completely through the drum, so that the 
 tubes turn upon this rod which is held 
 firmly by the uprights, and which carries 
 fixed arms within the drum. Thus, on 
 rotating the latter, the contents are turned 
 over, and are more thoroughly mixed to- 
 gether by the arms or beaters stationary 
 within it. The fixed rod and beaters should 
 be made of brass or of iron completely 
 sheathed in copper. In using the apparatus, 
 it is first half-filled with the moistened saw- 
 dust, then the pens are introduced ; the 
 lid is closed and fastened in place, and the 
 drum is rapidly rotated on its axis for a 
 few minutes, until it is judged that every 
 pen has been thoroughly coated, when it 
 is stopped with the door at the lowest Fio. 73. Mixing-drum, 
 point, and this being opened allows the 
 
 contents to fall upon the floor. The mixture is now placed on 
 brass sieves, the mesh of which is of such size that it passes 
 the sawdust through, but retains even the smallest articles that 
 have been treated ; the sieves containing the latter are now 
 plunged twice or thrice into fresh water, and the washed pens 
 are transferred to a second rotating drum, in which they are dried 
 by contact with hot, clean, and dry sawdust, which is subse- 
 quently separated from the finished nibs by means of sieves. 
 
 Other solutions for coppering by simple immersion have been 
 recommended, and notably those of Kopp, who coats iron in 
 cupric chloride containing a little nitric or hydrochloric acids ; 
 and Puscher, who treats brass by exposing it to a solution of 
 copper sulphate and ammonium chloride. 
 
 Obviously in all these processes the deposition of the copper is 
 due to an exchange of a more electro-positive metal (e.g., iron) for 
 the copper contained in the solution ; thus the latter gradually 
 accumulates a large quantity of iron, while it loses a corresponding 
 
126 ELECTRO-DEPOSITION OF COPPER. 
 
 amount of copper (56 of iron being equivalent to 63'5 of copper) ; 
 and for this reason the bath must be watched to ensure that it is 
 maintained at approximately the right strength. 
 
 The simple immersion process is not strictly electrolytic, but 
 merges into a single-cell process when, as by Weil's method, a 
 piece of zinc is placed in contact with the metal to be coated, 
 to facilitate the deposition of the required metal from a solution 
 which is tardy or inactive. 
 
 SINGLE-CELL PROCESS. 
 
 Weil's Process. Weil's process, which he has used for cop- 
 pering cast-iron pieces, even of large size, consists in dissolving 5 
 oz. of copper sulphate, 13 oz. of soda-lime (containing 50 per 
 cent, of caustic soda), and 24 oz. of potassium-sodium tartrate in 
 each gallon of water, and in submitting each 
 piece of iron, with a fragment of zinc attached 
 to it, to the action of this solution. The 
 zinc, being in metallic connection with the 
 iron, sets up a current as it dissolves in 
 the liquid, and deposits the copper, there- 
 fore, not on itself but upon the iron, to 
 which it is electro-positive ; the duration of 
 the immersion may range from a few hours 
 to several days, as the deposition proceeds 
 very slowly. 
 
 The f g' e -- Process was actually the 
 
 for one object. source of all the others, tor by its aid the 
 
 art of electrotyping was first accomplished. 
 In its simplest form it is well represented by the arrangement of 
 Weil's which we have just described, but a porous cell is almost 
 everywhere used to contain the zinc, so that it shall not be im- 
 mersed in the copper liquid. Fig. 74 illustrates a depositing 
 apparatus of this type ; the outer jar, a, which may be made of 
 glass or earthenware, is filled to about two-thirds of its height 
 with a nearly saturated solution of copper sulphate, and contains 
 an inner cell of porous earthenware, b, closed at the bottom, within 
 which is a plate of zinc, z, standing in a moderately strong solution 
 of common salt or sal-ammoniac, or, preferably, of dilute sulphuric 
 acid. In the latter case the zinc must be amalgamated ; the 
 liquids in the two cells should stand at the same level. The zinc 
 plate should project above the porous pot, and have soldered to it 
 a piece of copper wire, which serves to connect it with the object 
 to be electrotyped, c. Thus a species of Daniell-cell is formed, in 
 which the zinc, dissolving in the acid liquid of 'the porous cell, 
 deposits copper upon the conductor in the outer jar ; and crystals 
 of copper sulphate should be suspended in the liquid at the 
 upper portion of the outer cell, to replace the metal deposited 
 
SINGLE-CELL PROCESS. 
 
 127 
 
 from the solution upon the negative plate, just as they are in 
 the ordinary battery-cell. 
 
 Several objects may be coated simultaneously without detri- 
 ment to the working of the cell ; all must, of course, be attached 
 to the zinc, and they may be suspended around the central zinc 
 plate, as indicated in fig. 75 ; the arrangement here figured on 
 a small scale may be made of any 
 required size by substituting wooden 
 vats for the glass containing-vessel, 
 and using porous cells and zinc plates 
 of corresponding dimensions. In all 
 the methods of deposition which we 
 have been considering, one face only 
 of the object is turned towards the 
 zinc, and that face alone will receive 
 a deposit of copper ; this is suitable 
 enough when it is only required to 
 produce an electrotype from a coin, 
 
 the two sides of which are separately FlG 7 5. -Single-cell depositing 
 treated ; but when an object is to apparatus for two objects. 
 be completely covered with copper, 
 
 prior to receiving a coating of a different metal, some such arrange- 
 ment as that sketched in plan in fig. 76 is to be recommended. 
 The object is suspended in the centre of the tub containing the 
 copper solution from two cross-rods which rest on a circular wire 
 connecting all the zincs in their separate porous cells, these being 
 arranged around the circumference of the containing-vessel. In 
 this manner the object to be coppered is com- 
 pletely surrounded with zincs, and the .de- 
 position proceeds with equal regularity on all 
 portions. Porous diaphragms may be made of 
 parchment-paper or of plaster of Paris, but are 
 less satisfactory in use, and should only be 
 adopted as a temporary substitute for the un- 
 glazed earthenware cells upon emergency. 
 
 ment fbr eSo- The sin le cel1 would seldom be used in 
 typing all surfaces practice when a separate battery-plant could 
 at once. be obtained, because it is more clumsy in its 
 
 arrangements, the process is less under con- 
 trol, and the solution gradually becomes exhausted of copper 
 unless well tended. 
 
 DEPOSITION BY BATTERY; OR SEPARATE-CURRENT PROCESS. 
 
 The principle 'of this process has already been fully explained ; 
 a current of electricity is passed from a copper plate (anode) to 
 the object which is to be coated (cathode), both being immersed 
 in a solution containing copper ; a quantity of copper, depending 
 
128 ELECTRO-DEPOSITION OF COPPER. 
 
 entirely on the strength of the current, is thus dissolved from 
 the anode, and an equal amount is deposited upon the cathode. 
 Such details as strength of current, duration of process, com- 
 position of bath, and disposition of plant must be determined 
 by the character of the work under treatment. In the remainder 
 of the chapter it is proposed to treat first of the electro-deposition 
 of copper generally, then as applied to the covering of iron, 
 brass, or other metals for protective, ornamental, or other 
 purposes ; while the electrotyping of printers' plates and art- 
 electrotypy will be dealt with in a separate chapter. 
 
 The Battery. The battery employed is very frequently that 
 of Smee, which is a favourite with printers' electrotypers ; the 
 Daniell and bichromate, or modifications of them, are, however, 
 also largely used. For the acid copper-baths a comparatively 
 weak current of low electro-motive force is required, and any 
 attempt to hasten the deposit by increasing the battery-power 
 will result in defeat, owing to the production of brittle and 
 crystalline or spongy copper. The alkaline bath requires a 
 higher electro-motive force, such as would be provided by two, 
 or even three, Bunsen- or bichromate-cells in series ; but the 
 volume of current must not be excessive, on account of the 
 lower solubility of the anodes in the solution, which would lead 
 to a portion of the ions deposited at the anode escaping with- 
 out combining with copper, and this in turn would result in 
 a lower rate of solution than of deposition, and so to a gradual 
 impoverishing of the liquid. The number of cells to be used 
 must depend upon the quantity of the work ; with the acid 
 copper-solution they will all be arranged in parallel and should 
 be increased in number as the area of cathode surface is multi- 
 plied. A dynamo may, with great advantage, be substituted 
 for the battery, but it must have a very low electro-motive 
 force, and must, of course, be selected to suit this class of 
 work. 
 
 The Solutions. For coating metals which are less electro- 
 positive than copper, and for the production of electrotype-plates, 
 a simple solution of 1J pounds of copper sulphate and \ pound 
 of concentrated sulphuric acid in each gallon of water, will 
 be found to give excellent results with a current of about 1 
 ampere per square decimetre ( = 0*064 ampere per sq. in., or 9'3 
 amperes per sq. ft.) of cathode surface. The bath should be 
 made up by placing the weighed quantity of crystallised copper 
 salt in a suitable vessel, and pouring upon it about four or 
 five pints of boiling distilled or rain water, and stirring until 
 the crystals have quite dissolved. If the solution be not now 
 perfectly clear, owing to the presence of insoluble impurities in 
 the copper sulphate, it must be filtered by passing it through 
 a cone of blotting-paper fitted into a glass funnel (see p. 52), 
 which will remove all mechanical impurities. The remainder of 
 
THE SOLUTIONS. 129 
 
 the water, necessary to make up the solution to the volume of 
 one gallon, is now added cold ; and when the mixture is thoroughly 
 cool, the sulphuric acid is cautiously added in a gentle stream, 
 while the liquid is briskly stirred with a glass rod, or if glass be 
 not at hand, with a clean wooden stick or a length of copper rod. 
 Iron must on no account be used, nor may iron or zinc contain- 
 ing-vessels be employed to hold copper solutions, because these 
 metals deposit a portion of the copper and contaminate the liquid 
 by passing into solution themselves. Iron vessels, protected in- 
 ternally by a sound coating of enamel, may, of course, be used, 
 but glass, glazed stoneware, or even wood is preferable, unless 
 the enamel is frequently examined, to ensure that the iron is 
 nowhere exposed to the solution. 
 
 For treating metals such as zinc and iron which, being more 
 electro-positive than copper, would take a non-adhesive deposit 
 in the acid solution we have just described, recourse must be had 
 to a special bath. In the following table are given the percentage 
 compositions of a number of different copper solutions which have 
 been advocated by various authorities. It will be seen that the 
 majority of these take advantage of the solubility of copper 
 cyanide in the solution of potassium cyanide, while the remainder, 
 for the most part, use copper oxide dissolved in alkaline liquids 
 containing salts of tartaric acid. The chief variations are due to 
 the substitution of copper acetate or of verdigris for the sulphate, 
 and of the single tartrate of potash or soda for the double tartrate 
 (of the two metals together), and in the addition of varying pro- 
 portions of other substances which play a minor part in the action 
 of the bath. 
 
 Of all these liquids, the bath of Roseleur is, perhaps, the most 
 generally useful, as it is equally applicable to all metals, and may 
 be worked at any desired temperature. It is best prepared by 
 working up 3J avoirdupois oz. of copper acetate into a thick 
 paste with a little water; then an equal weight of sodium 
 carbonate crystals is added, with about 1J pints of water, and 
 the whole is well stirred for a few minutes. An exchange thus 
 takes place between the sodium carbonate and the copper acetate, 
 with the result that the water contains copper carbonate in 
 suspension and the sodium acetate in solution ; 3J oz. of sodium 
 bisulphite are now added in a second 1| pints of water; 
 and finally the remaining 5 pints of water, containing 3J oz. 
 of potassium cyanide, are introduced. The pale yellow-coloured 
 precipitate produced on the addition of the bisulphite will be 
 gradually dissolved on stirring with the cyanide, and should 
 completely disappear after a few minutes, leaving a practically 
 colourless solution. Should it not be so, a little more of the 
 potassium cyanide must be added by degrees, until decolorisation 
 is perfect. Failure in the first case is probably due to the use of 
 more than usually impure potassium cyanide. If necessary, the 
 
 9 
 
130 
 
 ELECTRO-DEPOSITION OF COPPER. 
 
 
 Special Method of Preparation. 
 
 CO --.XtA-Sui M^w" 2t2 * -' 
 
 S ?i222 |"s2s ::i 
 
 * .2"^ol. Srffi* 0-2^ 
 
 3 S/gJflfl^'ii ^-S .|afi| 
 a" o 8*irrjggl .2^-s-= rt g o-c 
 
 -S -TS"^*^ I H ^ c^^ g ?. 
 
 ^o 2 ^>^g'?2S2 ^o^?^^^-'^^ 
 
 S"" S S"*S ^'^^ B B S wwl1 
 
 
 Pure Water. 
 
 || || 1 || || 
 
 
 Cupro-Cupric 
 Sulphite. 
 
 : : : : : : :J :::::: : : : : 
 
 
 Sulphuric Acid. 
 
 g : ::::::::::: : : : : 
 
 Q 
 
 Sodium-Potassium 
 Tartrate. 
 
 : : :::::::::::: : : : S 
 
 I 
 
 Potassium Car- 
 bonate. 
 
 
 S 
 
 Sodium Carbonate. 
 
 : : : : : S s :5S : : : : ^ ^ : 
 
 ^ 
 
 Caustic Soda 
 
 -o 
 
 
 
 
 a 
 
 Ammonia (-880). 
 
 GO CO -(M .Tt<(MCOOg fa 53 ** 
 
 25 
 
 Sodium Bisulphite. 
 
 : : ::::" ^SS 00 S "* : : S S : 
 
 
 Q j. Q 1 V"4- 
 
 . IM '00 
 
 
 
 
 x 
 
 Potassium 
 Cyanide. 
 
 1 
 O ^O r ^ OO <MCQOCOCSICOC<IIO lO 1C 10 
 (M SO (M _g (M (M (M <N rH (M i-l (N (M ' (N :M 
 
 (0 *~* CO 
 
 2 
 
 Copper Sulphate. 
 
 | : : : g ::::::: :g : g 8 
 
 
 
 
 
 
 00 
 
 
 Copper Carbonate. 
 
 
 
 
 
 
 Copper Acetate. 
 
 . T* !< .07*" o os o -^H so o - 
 
 r-< -i-l IM Jj (M i-H <N i-H i-H -CM 
 
 
 || 
 
 !! e 
 
 EH 
 
 '~^~^ 6C 
 
 ' ^' ' ' 8 | SS |f 
 
 
 zc a*o 
 
 'S "3 ^ ^~- O / *~v 
 
 |1 : - - : 1 : :1 Jfl . 1 i '*? . 
 
 +3^ e -gJSN ~ J N W-e 
 
 w 8 3 *S a " crt 
 
 Is I 1 "2^ &p 
 
 
 j 
 
 "1 1 1 s 1 ! 1 " " " 1 = | "-I 
 
 
 
 
 i-H C3 CO * 1O (O fr- OO OS O i-H CO CO ^ IO tO l^ 00 
 rH r-t rH rH i-H rH rH rH 1-1 
 
THE SOLUTIONS. 131 
 
 bath should be finally filtered. When the bath is used hot (say 
 at 65 C.), the quantity of copper may be increased and a stronger 
 current may be used. Very good hot coppering baths may be 
 made by Gore's formula (preparing the copper cyanide chemically), 
 or by simply dissolving copper carbonate (which may be pur- 
 chased) in a solution of potassium cyanide. 
 
 The alkaline baths require more careful watching than the acid 
 liquors, owing to the comparative insolubility of the anode; if, 
 on examination after use, the strength is found to be greatly 
 diminished, it may be restored by adding to it a sufficient 
 quantity of a copper cyanide solution prepared as follows : A 
 solution of sodium carbonate is added to one of copper sulphate 
 until no further precipitate or cloudiness is observed on the 
 addition of another drop of the soda solution. The mixture is 
 allowed to stand until the copper carbonate which forms has 
 subsided; the clear liquor is now poured off, and potassium 
 cyanide solution is added until the whole of the slimy powder is 
 redissolved to a colourless fluid. Or the cyanide solution may 
 be added to bought sodium carbonate made into a paste with 
 water. If, on the other hand, the use of too large anodes has 
 charged the bath too highly with copper, it will have a bluish 
 colour, which must be discharged by the addition of a sufficient 
 quantity of potassium cyanide. The appearance of a white or 
 pale brown precipitate on the anode indicates the necessity for 
 a further addition of copper salt, which Roseleur recommends 
 should in this case consist of the ammonium-copper acetate, 
 formed by adding excess of ammonia to a solution of copper 
 acetate, so that a perfectly clear intense blue liquid results ; this 
 liquid should be added little by little, until the blue shade pro- 
 duced by it in the solution disappears only with difficulty ; any 
 excess of copper may, of course, be neutralised by adding potassium 
 cyanide. The use of much ammonia in hot baths is to be avoided 
 on account of the pungent ammonia gas that is evolved. Where 
 possible soda or potash should be substituted. 
 
 The acid baths are almost invariably used cold, for if allowed 
 to become warm the resistance is diminished and the current 
 may then be too intense to give a good deposit; the others are 
 with advantage warmed some even to boiling, as indicated in 
 Table VIII. See pp. 98, 99, as to the necessity for stirring the 
 solution and for the methods of stirring. 
 
 The Anodes. These should be of the purest quality only ; for 
 electrotyping in all its branches it is well to use only electrotype- 
 copper that is, metal which has already been electro-deposited 
 from a pure solution. If this cannot be obtained when required, 
 the best rolled copper should be used, as it is likely to contain 
 fewer impurities than the cast metal. The impurities in com- 
 mercial copper are for the most part insoluble in the bath, and, 
 therefore, gradually cover the anodes with a dark grey'or black 
 
132 ELECTRO-DEPOSITION OF COPPER. 
 
 slime, which accumulates on the surface as the copper dissolves, 
 and by degrees becomes detached and sinks to the bottom of the 
 bath ; but remaining for a time suspended in the solution, 
 especially if the bath is agitated as it should be, it tends partly 
 to become attached to the cathode, and so to give rise to diffi- 
 culties in deposition. The composition of the slime will mani- 
 festly depend upon the brand of copper used and the impurities 
 which it contains ; an analysis of a specimen by Maximilian, Duke 
 of Leuchtenberg, in 1848, gave 33 per cent, of tin; 25 of 
 oxygen ; 9 J of antimony, and an equal percentage of copper ; 7J 
 of arsenic ; 4 J of silver ; 2J of sulphur ; 2 J of nickel ; 2 of silica ; 
 1J of selenium; and 1 of gold, with smaller proportions of cobalt, 
 vanadium, platinum, iron, and lead. Other samples would con- 
 tain bismuth in addition to the above. When this slime appears 
 upon the anodes, they must from time to time be removed from 
 the bath, rapidly washed with water, and returned. 
 
 The anodes in the case of copper should present approximately 
 the same surface as the cathodes ; any considerable departure 
 from this rule will, on the one hand, cause an excess of copper to 
 pass into the bath, especially if a very acid solution be employed ; 
 or, on the other, it will give rise to a decrease in the strength of 
 the solution. 
 
 The Character of the Copper Deposited. The copper deposited 
 electrolytically may vary according to the conditions of working, 
 from a loose black powdery deposit such as results from using 
 a strong current or weak solutions, or whenever hydrogen is 
 deposited simultaneously with the copper to a minutely crystal- 
 line, highly tenacious reguline metal, when the current is well 
 proportioned to a solution of the right strength. The physical 
 condition of the copper is a matter of prime importance to the 
 electrotyper who, unlike the electroplater, is required to produce 
 a thick deposit which must be sufficiently tough, pliant, and 
 tenacious to withstand a considerable amount of rough usage, 
 receiving at first no support from backing metal. The experiments 
 of v. Hiibl, to whom reference has been made in an earlier chapter, 
 are of extreme interest in consequence of the light which they 
 throw upon the relation of strength and toughness of electrotype- 
 copper to the conditions of the depositing-bath. 
 
 Several points are clearly brought out by these tables, notably 
 the disadvantages attending the use of weak and neutral solutions 
 and of very weak or very strong currents. Thus, with a 5 per 
 cent, solution, no current higher than 1 ampere per square deci- 
 metre may be used on account of the resulting deposition of hydro- 
 gen, while with a 20 per cent, solution the current-density may 
 even rise to 3 or 4 amperes per square decimetre without ruining 
 the deposit ; further, the addition of acid to the weaker solution 
 can alone render it fit for use, while even with the stronger solution 
 it enables a smaller current to be employed successfully. 
 
THE NATURE OF DEPOSITED COPPER. 
 
 133 
 
 TABLE IX. SHOWING THE EFFECT OF VARYING CURRENT-STRENGTHS 
 ON THE COPPER DEPOSITED FROM NEUTRAL COPPER SULPHATE 
 SOLUTIONS (von Hubl}. 
 
 Intensity of Cur- 
 rent in 
 
 Deposit from Solution with 5 per 
 cent. Copper Sulphate. 
 
 Deposit from Solution with 
 20 per cent. Copper Sulphate. 
 
 Amperes 
 per sq. 
 inch. 
 
 Amperes 
 per sq. 
 decimetre. 
 
 Appearance. 
 
 Character. 
 
 Appearance. 
 
 Character. 
 
 0-013 
 
 0-026 
 0-052 
 
 0-193 
 
 0-2 
 
 0-4 
 0-8 
 
 3-0 
 
 Very coarse 
 crystals. 
 Coarse crystals. 
 Rather coarse 
 crystals. 
 * 
 
 Very brittle. 
 
 Brittle. 
 Fairly good. 
 
 * 
 
 Very coarse 
 crystals. 
 Coarse crystals. 
 Rather coarse 
 crystals. 
 Exceedingly 
 fine grain. 
 
 Exceedingly 
 brittle. 
 Very brittle. 
 Very, brittle. 
 
 Excellent. 
 
 TABLE X. SHOWING THE EFFECT OF VARYING CURRENT-STRENGTHS ON 
 THE COPPER DEPOSITED FROM COPPER SULPHATE SOLUTIONS CON- 
 TAINING 2 PER CENT. OF FREE SULPHURIC ACID (von Hubl). 
 
 Intensity of Cur- 
 rent in 
 
 Deposit from Acid Solution with 
 5 per cent. Copper Sulphate. 
 
 Deposit from Acid Solution 
 with 20 per cent. Copper 
 Sulphate. 
 
 Amperes 
 per sq. 
 inch. 
 
 Amperes 
 per sq. 
 decimetre. 
 
 Appearance. 
 
 Character. 
 
 Appearance. 
 
 Character. 
 
 0-013 
 
 0-026 
 0-052 
 
 0-077 
 ' 0-258 
 
 0-2 
 
 0-4 
 0-8 
 
 1-2 
 4-0 
 
 Small crystals. 
 
 Small crystals. 
 Very small 
 crystals. 
 
 Somewhat brittle. 
 
 Good. 
 Good. 
 
 Small crystals. 
 
 Small crystals. 
 Very small 
 crystals. 
 Exceedingly 
 fine grain. 
 Exceedingly 
 fine grain. 
 
 Somewhat 
 brittle. 
 Good. 
 Good. 
 
 Excellent. 
 Excellent. 
 
 * Under these conditions hydrogen is evolved with the copper, and the deposit of the latter 
 is, therefore, worthless. 
 
 Very weak currents are clearly to be avoided, because they 
 give a coarsely-crystalline and brittle deposit, while dilute solu- 
 tions are more readily overburdened with current, so that the 
 copper is spoiled. 
 
 In a second series of experiments, v. Hubl determined the 
 maximum current-density which may be safely applied to the 
 acid copper-solutions under certain varying conditions. See 
 Table XL, page 134. 
 
 In a third series, the breaking weight, elasticity, and stretching- 
 power under tensile (or pulling) stress were determined : new 
 acid baths containing from 10 to 20 per cent, of copper sulphate, 
 and old baths with from 5 to 20 per cent., were alike submitted 
 
134 
 
 ELECTRO-DEPOSITION OF COPPER. 
 
 to the action of currents of different strengths. The conclusions 
 which may be drawn from his results are : 
 
 (1) That the actual strength of the deposited copper increases 
 as the current-density rises from 0'61 to 3 amperes per square 
 decimetre. 
 
 (2) That the amount of extension at the moment of rupture, 
 under tensile stress, diminishes as the intensity of the current is 
 increased between these limits. 
 
 (3) That the highest limit of elasticity is exhibited by copper 
 deposited by a current of from 1 to 1 '5 amperes per square deci- 
 metre. 
 
 (4) That the elasticity alone is practically affected by varia- 
 tions in the strength of the solution between 10 and 20 per cent, 
 of copper sulphate, the limit of elasticity of the deposited metal 
 increasing with the percentage of copper in the bath. 
 
 TABLE XL SHOWING THE MAXIMUM CURRENT-DENSITY FOR 
 ELECTROTYPING (von ffilbl). 
 
 Composition of Solution. 
 
 With Solution undis- 
 turbed, Amperes 
 
 With Solution in gentle 
 motion, Amperes. 
 
 Per sq. 
 deci- 
 metre. 
 
 Per sq. inch. 
 
 Per sq. 
 deci- 
 metre. 
 
 Per sq. inch. 
 
 15 per cent, copper sulphate, neutral 
 
 2-6 to 3-9 
 
 0-168 to 0-252 
 
 3-9 to 5-2 
 
 0-252 to 0-335 
 
 15 per cent, copper sulphate with from 
 2 to 8 per cent, sulphuric acid . . 
 
 1-5 2-3 
 
 0-097 ,, 0-148 
 
 2-3 3-0 
 
 0-148 0-193 
 
 20 per cent, copper sulphate, neutral 
 
 3-4 5-1 
 
 0-219 ,, 0-329 
 
 5-1 6-8 
 
 0-329 0-439 
 
 20 per cent, copper sulphate with from 
 2 to 8 per cent, sulphuric acid . . 
 
 2-0 ,, 3-0 
 
 0-129 0-193 
 
 3-0 4-0 
 
 0-193 0-258 
 
 Thus, to quote actual figures, in a new 20 per cent, bath con- 
 taining 4 per cent, of acid, a current of 0'61 ampere per square 
 decimetre yielded a copper which broke under a stress of 18'0 
 tons per square inch after stretching 27 per cent, of its original 
 length; while a current of 2-22 amperes per square decimetre 
 deposited a metal which required 23 -6 tons per square inch to 
 fracture it, but which only extended 16 per cent. In an old 
 solution of equal strength, a current of 1 ampere per square deci- 
 metre produced a deposit which gave way at -17 '2 tons per square 
 inch, with 26'4 per cent, elongation ; while the copper thrown 
 down by a current of 2 -5 amperes ruptured at 18-7 tons per 
 square inch, at 25 '1 per cent, increase of length ; and that pre- 
 cipitated by 4 amperes per square decimetre yielded at only 15*3 
 tons per square inch under a stretch of 19 '4 per cent. 
 
 The highest breaking stress in the whole series of trials was 
 that of 2 3 '6 tons per square inch above recorded ; the greatest 
 
THE PLATING PROCESS. 135 
 
 extension was 33 per cent., shown by a metal deposited by a 
 current of 1 ampere per square decimetre from a solution contain- 
 ing 15 per cent, of copper sulphate and 3 per cent, of sulphuric 
 acid; but the breaking stress of this specimen was only 17 '3 
 tons per square inch. 
 
 Some of these samples of copper may be said to compare 
 not unfavourably even with good rolled copper, such as would 
 ordinarily be used for engraved plates, which are required to 
 withstand severe stress in the various processes to which they are 
 to be applied. 
 
 Electro-plating with Copper. The objects to be coated are 
 first thoroughly cleansed by the methods indicated in the last 
 chapter. Cast-iron articles are especially liable to contain 
 particles of sand and other non-metallic impurities embedded in 
 the surface ; these must be carefully looked for and removed 
 before the treatment there described. On removal from the last 
 acid dip, the pieces must be thoroughly washed and at once 
 suspended in the copper-vat, by means of copper wires or hooks 
 slung from the cross-bars attached to the negative (zinc) pole of 
 the battery. The method of suspension will, however, depend 
 upon the size and shape of the pieces, and upon the skill and 
 ingenuity of the operator. Large objects may be hung individu- 
 ally from the cathode-rods, but the wires must be attached to 
 portions of the object where they will not leave permanent marks 
 upon the finished work \ or they may be rested in wire-slings ; or, 
 if very heavy, they may be allowed to stand on the floor of the 
 vat with conducting wires attached underneath ; but in this latter 
 case no deposit can, of course, form upon the bottom of the object, 
 which must be treated subsequently, if necessary ; light articles 
 which are perforated in any part may be slung on copper wires. 
 The methods of supporting the pieces are obviously innumerable ; 
 but it must always be remembered that whenever they are hung 
 from wires, their position must be slightly shifted from time to 
 time to prevent the formation of wire-marks due to imperfect 
 coating at the points of contact. The objects should be moment- 
 arily removed from the vat for examination soon after the deposi- 
 tion has commenced, for at this time imperfect cleansing will be 
 clearly evident, so that if the coating be incomplete the cause may 
 be at once detected and the defect remedied. Finger- or-grease- 
 marks, produced by handling after cleansing, can thus be removed 
 by washing the pieces in water, immersing for a moment in the 
 potash-vat, rinsing, dipping in the acid liquor, once more rinsing, 
 and finally replacing in the copper-bath, when, the clean surface 
 being restored, the action proceeds in due course. Small articles 
 may also be coated by placing them in a perforated porcelain 
 ladle (fig. 64, p. Ill), on the bottom of which rests a coil of 
 copper wire attached to the negative pole of the battery, so that 
 they form a continuous cathode by mutual contact ; in this ladle 
 
136 ELECTRO-DEPOSITION OF COPPER. 
 
 they are plunged into the copper-vat and constantly agitated until 
 the required thickness of metal is obtained ; the anode should in 
 this case take the form of a cylindrical sheet of copper surrounding 
 the ladle, or it may be a disc of copper-plate, which is placed 
 above the ladle after its introduction into the bath. 
 
 All portions of any object which are not to receive a coating of 
 copper should be painted over with an insulating varnish (p. 388), 
 which may be removed subsequently. 
 
 The duration of the process depends upon the thickness of 
 deposit required ; for most objects treated in this way whether 
 iron or zinc to be subsequently silvered, silver to be gilded, or 
 iron which is to receive a coating, either protective or ornamental 
 a very thin wash will suffice, such as might be imparted in a 
 period ranging from a few minutes to an hour. The alkaline 
 baths that would generally be used for this class of work are 
 slower in action than the acid baths. 
 
 On removing the work from the vat, it must be dipped into 
 three or four successive wash-waters ; as each piece is transferred 
 to the washing-tank it carries with it a notable quantity of copper 
 solution from the bath, so that the water in the first tub should 
 not be frequently renewed, but should be allowed to remain until, 
 many articles having been treated, it has become gradually 
 charged with copper ; it is then poured into a different vessel, so 
 that the dissolved metal may be recovered from it. Even the 
 second tank receives a gradual addition of copper, and it is well to 
 allow this also to accumulate until the solution in the first tank 
 is discharged, when this liquor may be substituted for it. The 
 water in the third and fourth tanks should be very frequently re- 
 newed, and, since they should contain no more than a trace of 
 copper, may be discarded. On leaving the last wash-water the 
 objects should be free from copper solution, and, after drying, are 
 ready for the market. They may be dried in ovens heated to the 
 temperature of boiling water, or in hot box-wood sawdust if they 
 be small enough to render such a process practicable. When the 
 articles are receiving the copper coat merely as a preliminary to 
 the deposition of other metals, they should, if possible, be trans- 
 ferred directly from the final washing-tank of the coppering 
 process to the depositing- vat of the metal next to be deposited. 
 This system is to be recommended because, exposed to the air of 
 towns, copper is most liable to receive a sulphur-tarnish which 
 ordinary potash and acid dips might fail to remove completely. 
 
 The difficulties likely to arise in the process of deposition, and 
 the methods by which they may be evercome, have been treated 
 of in the earlier portion of this chapter, dealing with the solutions 
 and anodes. 
 
 Sundry Applications of Copper Deposition. Thick deposits 
 of copper for various purposes are now made by means of the 
 dynamo-electric machine. 
 
VAHIOUS APPLICATIONS OF PEOCESS. 137 
 
 An interesting example of such a process is seen in Wilde's 
 system of applying a thick copper coat to iron printing-rollers. 
 The roller is first covered with a thin film of copper in the 
 alkaline bath, and being thus protected it may afterwards be 
 safely treated in the more rapidly-acting acid solution. For this 
 purpose it is placed on end, mounted upon a vertical insulated 
 axis in the acid copper- vat, and is connected at top and bottom 
 with the negative pole of the dynamo ; within a short distance 
 from it is mounted a copper cylinder of about the same size, which 
 is connected with the positive pole, and, therefore, plays the part 
 of anode. The two cylinders are simultaneously rotated upon 
 their axes at a moderate pace, and a gradual transference of 
 copper from anode to cathode takes place. In this way an even 
 thickness of deposit is obtained over the whole surface ; uniform 
 along the length of the roller, by reason of the motion imparted 
 to the bath, which maintains an equal density of solution through- 
 out ; and uniform as to its diameter, because the rotation con- 
 stantly brings fresh surfaces opposite the anode. Moreover, a 
 high rate of deposition, or, in other words, rapid work, is permis- 
 sible, because of the motion in the bath ; for we have seen 
 (p. 90) that a bath which is well agitated will take a stronger 
 current than one which remains tranquil. Additional means for 
 securing a thorough mixture may be employed by adapting a 
 small propeller-blade to a shaft run by the same machinery that 
 actuates the rollers. 
 
 Watt, in his treatise on Electro-Deposition, describes a plant 
 which was used for a similar purpose by the Electro-Metallurgical 
 Company of London. The rollers were dipped into potash solu- 
 tion contained in a steam-jacketed cylinder 15 feet high and 3 
 feet in diameter, and a preliminary wash of copper was imparted 
 by an alkaline copper bath in a similar tank. The anodes used 
 in the acid bath consisted of copper billets 4 inches square and 
 12 feet long, while the tank itself was capable of holding 60,000 
 gallons of solution. 
 
 A third method which has been successfully utilised for copper- 
 ing rollers consists in slowly rotating them on a horizonal axis 
 about 12 or 18 inches below the surface of the bath, and between 
 two curved copper anodes fixed on either side and nearly meeting 
 beneath the cylinder. 
 
 Tubes of any size, shape, or section may be gradually built up 
 in a similar manner, and, if the current be properly proportioned 
 to the strength of the solution, should not be greatly inferior to 
 ordinary solid-drawn copper tubes, and would, doubtless, be, on 
 the average, stronger than the common brazed tubes so frequently 
 employed. Straight tubes may be built up upon a well-polished 
 iron mandrel or core, which may be subsequently withdrawn, the 
 surface of the iron being black-leaded to prevent the adhesion of 
 the two metals; bends and unusual shapes may, of course, be 
 
138 ELECTKO-DEPOSITION OF COPPER. 
 
 formed by employing a moulded core, such as is described in the 
 next chapter, made of fusible materials, which can afterwards 
 be removed by the application of a gentle heat. When, however, 
 the tubes are required to withstand a high fluid-pressure either 
 from within or from without, the utmost care is required to guard 
 against the formation of a largely-crystalline deposit, because, 
 the grain of the metal being 'open,' it would be more or less 
 porous, and would give rise to leakage of liquid through the tube 
 walls themselves. 
 
 The Elmore process for making copper tubes, which is now 
 worked on a large scale at Leeds, as also in Germany and France, 
 is of interest on account of the method adopted and the fact that 
 high current-densities may be used. In this process the copper 
 is deposited on to a rotating mandrel of a diameter equal to the 
 internal diameter of the desired tube. Simple deposition in this 
 way would produce but a poor result. The essential feature of 
 the process is that the deposit is submitted to a burnishing action, 
 an agate burnisher being mechanically pressed upon the tube as 
 it rotates, and travelling up and down the tube as the process 
 proceeds. Every part of the deposit is therefore acted upon, and 
 the layer of copper at any point only increases about -^ of a 
 millimetre before it again passes beneath the burnisher. The 
 effect of the burnisher is not merely to maintain a good deposit 
 but to increase the tensile strength very materially in much the 
 same way as rolling and drawing. The resulting copper has 
 ordinarily a tensile strength of 25*5 tons per square inch, 
 but this figure may become as high as 42 tons by suitably 
 adjusting the conditions. A further advantage of the process is 
 that the current-density, which in refining is usually about 10 to 
 20 amperes per square foot (1 to 2 amperes per square decimetre) 
 may be 30 amperes per square foot when the electrolyte is not 
 circulated, and as high as 200 amperes per square foot with 
 circulation. 
 
 A process of another kind for the production of seamless copper 
 tubes has been developed by Sherrard 0. Cowper-Coles. This 
 consists in rotating a mandrel rapidly in a vertical position while 
 deposition is taking place upon it. Rotation in this way was 
 patented by Henry Wilde so long ago as 1875, but Cowper-Coles 
 has emphasised the fact that a certain critical speed is necessary 
 for the best effect. The peripheral speed should not be less than 
 1000 feet per minute, but it is not clear whether centrifugal force 
 has anything to do with the effect or not. Briefly, the advantages 
 of rotation are : (1) agitation of the electrolyte so that its im- 
 poverishment is avoided, (2) a burnishing effect is produced due 
 to the friction between the cathode and the surrounding liquid, 
 
 (3) foreign matter is prevented from settling on the cathode, 
 
 (4) air-bubbles, and consequent nodules, cannot form on the 
 cathode, and (5) a uniform thickness of copper is ensured. High 
 
VARIOUS APPLICATIONS OF PROCESS. 
 
 139 
 
 current-density is permissible ; in fact, 200 amperes per square 
 foot (21*5 amperes per square decimetre) is the most economical 
 current-density, and this may be increased up to 500 amperes per 
 square foot by increasing the rate of rotation, though there is then 
 greater loss of energy. 
 
 The method of carrying out this process is indicated in fig. 77. 
 The vat is annular, and the cylindrical mandrel is so supported 
 that there are no bearings in the electrolyte. The mandrel is 
 copper coated, highly burnished and treated chemically so as to 
 ensure the easy removal of the deposited tube. The anodes, of 
 
 FIG. 77. rSection of vat for depositing copper sheets on rotating mandrel 
 (Cowper-Coles' process). 
 
 crude copper, are placed round the mandrel with intervening 
 spaces, and are seen with wedges attached in fig. 77 ; they are 
 fed forward as the copper dissolves away by acting mechanically 
 on these wedges, as shown in fig. 78. The distance between anode 
 and cathode varies from J inch to \ inch or thereabouts, and, this 
 distance being small, a voltage of only 0*8 volt at the terminals 
 is necessary when working at 200 amperes per square foot. The 
 copper tubes are removed by slightly expanding them by rolling ; 
 but in the case of sheets a narrow insulating strip is fixed down 
 the mandrel so that a tool can be inserted. 
 
 Messrs Langbein & Company of Leipzig appear to be using a 
 process in which a mandrel is rotated at about 20 revolutions per 
 minute, and a frictional effect is obtained by the attrition of 
 siliceous matter suspended in the electrolyte. 
 
140 
 
 ELECTRO-DEPOSITION OF COPPER. 
 
 An application of copper deposition which has suggested itself 
 to the electrician is the coating of an iron 
 or steel wire with any desirable thick- 
 ness of copper, for overhead telegraph- 
 wires, so that the full advantages of the 
 greater strength of the steel may be 
 combined with the higher conductivity 
 of the copper. To this end the American 
 Postal Telegraph Company have arranged 
 a series of copper baths at their works, 
 through which the iron wire is passed 
 by means of rotating drums until suffi- 
 cient metal has been deposited. Twenty- 
 five large dynamos were used to actuate 
 200 baths, and these were capable of 
 depositing daily 500 Ibs. of copper per 
 mile upon 20 miles of steel wire, weigh- 
 ing 200 Ibs. per mile, the time occupied 
 by any point upon the surface of the 
 wire in traversing the whole chain of 
 baths being about 60 hours. 
 
 Several attempts have been made to 
 obtain copper wire direct by deposition. 
 The Elmore Company make wire by 
 cutting up their electrolytic copper tube 
 spirally. Sherard Cowper-Coles uses the 
 following ingenious method. A spiral 
 scratch is made the whole length of a 
 mandrel which is to receive the deposit 
 of copper. Copper is then deposited to 
 the desired thickness ; the scratch just 
 referred to is reproduced on the deposits 
 and causes weakness in the copper all 
 along this line, with the result that the 
 copper can be pulled off quite easily in 
 the form of strip as shown in fig. 79. 
 By making the scratch of suitable pitch, 
 the strip can be obtained of any desired 
 width ; the pitch is so chosen as to give 
 a square section for the strip, which is 
 finally drawn into wire. 
 
 The number of uses for thick de- 
 posits of copper is endless, and very 
 Son oFan an anode S Tn manv other applications unenumerated 
 Cowper-Coles' process, nere nave been successfully carried into 
 showing arrangement effect as, to take a solitary example, 
 for adjusting distance the formation of copper rings upon small 
 cylindrical shells (projectiles) to enable 
 
 from mandrel. 
 
VARIOUS APPLICATIONS OF PROCESS. 141 
 
 them to take the grooves of the gun. The process for pro- 
 ducing these thick coatings is, as we have seen, very simple, 
 but presents the extreme disadvantage of requiring a great 
 expenditure of time to yield any considerable depth of deposit. 
 
 FIG. 79. Copper strips being unwound from mandrel after deposition 
 (Cowper-Coles 5 process). 
 
 The use of the dynamo is almost an essential to the work 
 the continual wear and tear of the batteries, and the worry 
 of continual inspection and renewal, added to the expenditure 
 of costly zinc for producing the current, renders the application 
 of the galvanic cell practically impossible. 
 
 Notes to Table VIII. Hossauer (formula 3) dissolves the 
 copper cyanide in the potassium cyanide dissolved in 300 parts 
 of the water, niters, and then dilutes with the remaining water. 
 This bath is stated to work well at 113-122F., but if used 
 cold it requires a strong current. 
 
 Langbein strongly recommends formula 7. To prepare the 
 bath, dissolve the crystallised sodium carbonate in 700 parts of 
 the water (warm) r add the crystallised bisulphite gradually to 
 prevent violent effervescence, and then add with vigorous stirring 
 the neutral acetate of copper. Dissolve the cyanide in the 
 remaining 300 parts of water (cold), and mix the two solutions 
 when the first is cold. Siphon off the clear liquid. If the bath 
 is not colourless, add a little more potassium cyanide. A pressure 
 of 3 volts is used with electrodes 10 cms. apart, and a current- 
 density of 0*35 ampere per square decimetre. 
 
 Langbein has introduced the use of cupro-cupric sulphite, and 
 states that it gives a beautiful warm colour and a very adherent 
 and dense deposit on iron and steel. For formula 8 dissolve the 
 cyanide in half the quantity of water given, then the cupro- 
 cupric sulphite, and add remaining water. As an alternative 
 3J parts of ammonia-soda may be added. 
 
CHAPTER VIII. 
 
 ELECTROTYPING. 
 
 THE object of electrotyping is the production of an exact facsimile 
 of any object having an irregular surface, whether it be an 
 engraved steel- or copper-plate, a wood-cut, or a forme of set-up 
 type, to be used for printing ; or a medal, medallion, statue, bust, 
 or even a natural object, for art-purposes. 
 
 In all cases a reversed mould of the object is first obtained, and 
 upon this the copper is electrolytically deposited to a sufficient 
 thickness. It was very early discovered (p. 5) that metal de- 
 posited upon an uneven surface, and then wrenched apart from it, 
 exhibited in reverse an absolutely perfect reproduction of every 
 irregularity or line upon the surface of the original, and that by 
 depositing a second coat upon this new surface, a re-reversed 
 image of the first surface was obtained which, as to minuteness 
 of detail, defied distinction from the original. Hence impressions 
 from a single printing-plate or block may be repeated an indefinite 
 number of times by retaining the original plate merely as a means 
 for producing any number of duplicate copies, and not taking 
 press-copies directly from itself. Works of art from the chisel of 
 the sculptor or the tools of the moulder may thus be similarly 
 multiplied, or the process may even be substituted for that of the 
 . foundry in obtaining statues from the sculptor's model. Sc 
 delicate, accurate, and refined is the copy, that the finest grain of 
 wood, even the peculiar sheen of satin wood, is perfectly imitated, 
 and it is even possible to reproduce the daguerreotype photo- 
 graphic image with its infinitely minute gradations of light and 
 shade. 
 
 In all classes of electrotyping, then, the first step is to obtain 
 a cast, intaglio, or negative impression of the object, in which the 
 projecting portions of the original become depressions, and vice 
 versa. Occasionally it is possible, and often it is preferable, to 
 effect this by rendering- the surface of the object somewhat dirty, 
 and depositing a sufficient thickness of copper or silver upon it 
 directly, to enable the two surfaces to be separated without 
 fracture or distortion of the deposited plate. Thus a faithful 
 negative is obtained which is not adhesive to the unclean surface 
 
 142 
 
GUTTA-PERCHA MOULDING COMPOSITIONS. 143 
 
 and may be readily detached ; it is then ready to receive a coat- 
 ing upon itself, and this on removal is found to be a true copy of 
 the original in every respect, so that it may be substituted for 
 it as, for example, in the printing-press. But it frequently 
 happens that the original subject would be spoiled if placed in the 
 depositing-bath, or that it has undercut surfaces, which would 
 prevent the deposited coat from being detached ; in such cases, 
 and, indeed, more usually, casts of the object are first taken in a 
 suitable moulding material, which, having been rendered a con- 
 ductor of electricity, at least superficially, receives the deposit in 
 the form that is to be substituted for the original. 
 
 MOULDING MATERIALS. 
 
 For moulding, any material may be used which is capable of 
 taking accurate impressions of an object, and which is not so 
 brittle or so soft at ordinary temperatures as to be broken or bent 
 out of shape during subsequent processes, provided that it is a 
 conductor of electricity, or may be made so superficially ; and 
 provided, also, that it neither injures the object to be moulded 
 from nor is affected in any way by the solutions in which it will 
 be placed. The principal materials employed are gutta-percha, 
 bees'-wax, plaster of Paris, fusible metal, gelatine, or sealing-wax, 
 or compositions in which these bodies are used, in conjunction 
 with others added to modify their properties in some desired 
 manner. 
 
 Gutta-percha and Compositions in which it is used. Gutta- 
 percha is usually obtained in the market in the form of stout 
 sheet, into which it has been manufactured from the crude masses 
 imported from abroad. It should be of good quality, otherwise 
 the foreign impurities present in it are liable to ruin a cast, if at 
 least any of them should be present on the surface which is to 
 receive the impression. It is a non-conductor of electricity, and 
 possesses the property of becoming so plastic at the temperature- 
 of boiling water that it may be pressed into the finest lines of an 
 uneven surface ; on cooling in situ it again becomes rigid, and 
 thus preserves a faithful and permanent impression of them, 
 while remaining sufficiently elastic to allow of its being used for 
 reproducing work that is slightly 'undercut.' Heated, to a 
 temperature of 95 to 100 C. (203 to 212 F.) in water, or, 
 better, in an oven warmed by a hot-water jacket, a piece of gutta- 
 percha may be pressed on to any surface which it is desired to 
 reproduce. The surface of the cast thus obtained requires only 
 to be rendered conductive, and is ready to receive the metal by 
 electro-deposition. Gutta-percha contracts considerably on cool- 
 ing ; in order to ensure a good impression, it should, therefore, 
 be allowed to cool in contact with the original object, so that no 
 superficial contraction can possibly occur by reason of the intimate 
 
144 ELECTROTYPING 
 
 contact between the two irregular surfaces, which prevents the 
 sliding of the one upon the other. 
 
 Where the very finest lines are to be reproduced with precision, 
 as in the copying of engraved steel-plates, a mixture of gutta- 
 percha with not more than about half its weight of fatty matter 
 is frequently used ; this is * thinner ' than the gutta-percha alone, 
 and may even be melted and simply poured over the plate. Such 
 a mixture is : 
 
 Gutta-percha, . ... . . .66 parts by weight, 
 
 Lard, . . .' . .33 
 
 Russian tallow, ' . . . . . 1 ,, ., 
 
 The lard should be refined by melting in an earthen pot or pipkin 
 and pouring it into hot water, when the fat remains fluid upon the 
 surface of the water, while the mineral impurities sink to the 
 bottom of the vessel ; on cooling, the lard solidifies and may be 
 removed from the water. The tallow should be similarly treated. 
 Then the gutta-percha is cut into small strips, mixed with the 
 requisite quantities of the other materials and melted slowly in 
 a porcelain dish, or in an oven, which should be maintained at 
 the lowest temperature compatible with complete fusion of the 
 mixture. The mass is thoroughly incorporated by stirring, and is 
 ready for use at once. These mixtures must not be overheated, as 
 the gutta-percha at high temperatures becomes brittle and useless. 
 
 The Russian tallow is often omitted from the mixture, and 
 many operators use linseed or other oils in preference to lard. 
 The addition of a little plumbago is to be recommended; it 
 facilitates the production of the conductive film required for 
 electro-deposition. Gore recommends, for moulding from medals 
 and coins, a mixture of two parts of gutta-percha and one of 
 marine glue, which must be thoroughly incorporated, and is then 
 used in the plastic state like gutta-percha alone. 
 
 Gutta-percha is best adapted to copying plates, medallions, 
 coins, or medals which are not too deeply undercut. To effect 
 this, it is made plastic by heating to 100 C. (212 F.) as described 
 above ; a sufficiently large fragment is now carefully rolled and 
 kneaded in the hand until it forms a quite uniform sphere with- 
 out visible lines or seams ; then, while still plastic, the ball is 
 rested on the centre of the medal, and pressed into place with 
 the thumb, while the fingers gradually flatten it and extend the 
 material outwards and downwards, beginning at the centre, until 
 it completely covers the whole surface ; it is then placed under 
 gentle pressure until cool, when it will be found that the surfaces 
 will part without effort, and the gutta-percha intaglio will be 
 ready for the next process. The medal should be well brushed 
 with plumbago before moulding to prevent adhesion, and the 
 fingers should be thoroughly moistened or rendered slightly oily, 
 or they will be liable to cling to the gutta-percha. Very large 
 
WAX MOULDING COMPOSITIONS. 145 
 
 surfaces cannot satisfactorily be treated in this manner, and it is 
 expedient in dealing with them to render plastic a sheet of gutta- 
 percha, slightly larger than necessary to cover the plate ; then, 
 placing it upon the surface, it is pressed into intimate contact 
 throughout, beginning from the centre and working outwards 
 as before, in order to prevent the entanglement of air-bubbles 
 between the two surfaces, which would prove fatal to the sharp- 
 ness of the impression. The plate and mould are finally allowed 
 to cool under a gentle and even pressure. 
 
 When the fluid mixture is employed, the medal or plate is 
 surrounded with a narrow rim of paper or thin metal placed on 
 edge, beyond the limits of the part to be copied, and the melted 
 material is poured slowly on to the centre and allowed to flow 
 gently from this point over the whole surface, where it should be 
 permitted to remain for several hours to ensure that it is com- 
 pletely set. 
 
 Either the gutta-percha alone, or the composition made from it, 
 may be used over and over again, but after a long period of use, 
 when so much plumbago has accumulated in it that it becomes 
 hard and brittle, it is necessary to add a sufficient quantity of 
 fresh gutta-percha (or composition) to regenerate it for use. 
 
 Bees'- wax and Compositions in which it is used. Bees'- wax 
 is very largely used in various compositions, and, indeed, is the 
 usual basis of mixtures for electrotyping set-up printers' type. As 
 a general rule, the ordinary yellow bees'-wax of commerce is suffi- 
 ciently good for the purpose ; but as the presence of some of the 
 commoner adulterants increases its tendency to crack on cooling, 
 especially in cold weather, some operators have preferred to melt 
 the wax in a pipkin and to add to it about one-tenth of its weight 
 of white lead (lead carbonate); after stirring, the excess of lead 
 is allowed to separate and collect at the bottom, when the melted 
 wax, still retaining a small quantity of lead, is poured into slabs 
 and may be remelted at will. The presence of the lead has the 
 twofold advantage of checking the liability to crack and of 
 increasing the specific gravity of the composition, so that it sinks 
 readily in the depositing-vats. As a general rule, however, this 
 precaution is unnecessary. 
 
 Usually the wax is not employed alone, but is mixed with 
 stearine, Venice turpentine, or other bodies which tend to prevent 
 the cracking of the moulds. For taking electrotypes of printers' 
 formes in the usual way with the aid of pressure, an excellent 
 mixture is made by melting together and stirring well : 
 
 Bees'-wax, 85 parts by weight. 
 
 Venice turpentine, . . . . .13 ,, ,, 
 Plumbago .2 ,, ,, 
 
 The composition is then poured into flat trays, and, when 
 nearly set, the typographical plate is pressed upon its surface 
 
 10 
 
146 ELECTROTYPING. 
 
 in a manner to be described hereafter (p. 159). Many other 
 mixtures are, of course, used, different workers adopting their 
 own formulae. Thus, Urquhart recommends the same ingre- 
 dients as above, but mixed in the proportion of 87 '3 : 9'7 : 3 
 for this class of work, or in the proportion of 80'5 of wax, 
 27 '9 of mutton-suet, and 1*7 of graphite for moulding when 
 pressure may not be applied, on account of the fragility of the 
 original object. Good mixtures may be made, according to the 
 formula used by Volkmer, by melting together 70 parts of wax 
 and 30 of stearine, or to that preferred by Watt, consisting of 
 70 of wax, 26 of stearine, and 4 of litharge or flake-white. For 
 medal-work Walker was in the habit of using 69| parts of 
 spermaceti with 15^- each of mutton-suet and bees'-wax. For 
 certain uses many operators have introduced rosin into the 
 composition ; Weiss employs a mixture of 62 of bees'-wax, 28^ 
 of mutton-suet, and 9J of rosin to produce a replica from a 
 statue, the first cast of which has been made in an elastic 
 composition attackable by water, as will be explained later ; 
 and a mixture of 50 parts of wax, with 50 of rosin, and from 
 3f to 5 of a solution of phosphorus in carbon bisulphide (1 of 
 phosphorus in 15 of bisulphide), was recommended by Parkes 
 for the preparation of specially-conductive moulds. 
 
 In using melted wax to prepare matrices from medals and 
 the like, it should not be poured on until it is nearly solidifying ; 
 then (having previously slightly oiled the surface of the object, 
 surrounded it with a strip of paper or metal placed on edge, 
 and rested the whole upon a gentle slope) the wax should be 
 gently poured upon the lowest part and allowed to flow with an 
 even motion over the whole surface ; as soon as it is set, the 
 containing rim should be removed, because the composition 
 adheres to it somewhat strongly, and the contraction due to 
 cooling may cause the wax to fracture at the centre. Then, after 
 standing for one or two hours, the impression may be safely 
 detached from the original. 
 
 Plaster of Paris. Only the finest quality of plaster should 
 be used, which has not been overburnt, is very finely ground, 
 and has been stored in a well-covered jar or bottle, so that it 
 shall not have deteriorated by exposure to moist air. But, even 
 at the best, it is not to be recommended in comparison with the 
 materials enumerated above. In applying it, the object is first 
 thoroughly well covered with an almost imperceptible film of 
 oil, by means of a soft rag or a tuft of cotton-wool ; then, having 
 surrounded it, if necessary, with a paper rim, the plaster is made 
 into a thick cream by sprinkling it into a sufficient body of 
 water, and without loss of time a small quantity is poured upon 
 the surface of the object, and with great rapidity brushed into 
 every conrer or line by means of a moderately hard brush to 
 prevent the formation of air-holes by entanglement of air in the 
 
MOULDING IN FUSIBLE METAL. 
 
 147 
 
 liquid ; then, immediately, the remainder of the paste is poured 
 over it and allowed to remain until it has completely set, when 
 it may be removed in the usual way. The plaster must be free 
 from grit, and must be well mixed with sufficient water, or it 
 will set too rapidly to allow of the treatment above recom- 
 mended ; but in any case the operations must be conducted very 
 quickly, as the cream speedily assumes a pasty and semi-solid 
 condition. 
 
 The matrix so prepared is porous and absorbent, and is not 
 ready to be placed in the bath of copper-liquor until it has been 
 waterproofed, which is usually effected by saturating the whole 
 surface with wax or solid paraffin. The mould, if small, may 
 be simply rested on its back in a shallow tray of melted wax, 
 which must be insufficient to cover it completely, until the 
 latter has penetrated through to the face by capillary action ; 
 it is then removed and cooled. Large moulds may be super- 
 ficially covered by pouring the melted wax over the whole 
 surface and then transferring them to an oven or hot closet, 
 until the layer of wax just melts and becomes absorbed by the 
 plaster. 
 
 Fusible Metal. Certain alloys melt at a temperature below 
 that of boiling water, and may, therefore, be used to obtain casts 
 from the most delicate objects, or even from organic bodies, such 
 as animals, fruits, and flowers ; but, although the casts obtained 
 in this manner are very sharp and accurate, the process is rarely 
 employed because of the expense entailed by the waste, however 
 slight, incurred in the constant use of a costly metal. The 
 principal so-called fusible metals are compounded as follows : 
 
 TABLE XII. SHOWING THE COMPOSITIONS OF CERTAIN 
 FUSIBLE ALLOYS. 
 
 
 Percentage Composition. 
 
 Approxi- 
 
 Name of Alloy. 
 
 
 mate Fus- 
 
 
 
 
 
 
 Bismuth. 
 
 Lead. 
 
 Tin. 
 
 Cadmium. 
 
 ing-point. 
 
 Newton's 
 
 50 
 
 31-25 
 
 1875 
 
 
 202 F. 
 
 Rose's . 
 
 50 
 
 25 
 
 25 
 
 
 201 
 
 Lichtenberg's . 
 
 50 
 
 30 
 
 20 
 
 
 197 
 
 Darcet's 
 
 50 
 
 20 
 
 30 
 
 
 197 
 
 Wood's 
 
 50 
 
 25 
 
 12-5 
 
 12 5 
 
 141 
 
 Lipowitz's 
 
 50 
 
 27 
 
 13 
 
 10 
 
 140 
 
 Any of these may be used, but those with the lower melting- 
 point are naturally to be preferred. The metals should be 
 thoroughly mixed by long stirring while still fluid ; and they may 
 
148 ELECTROTYPING. 
 
 even with advantage be poured into slabs and remelted several 
 times with the same object in view ; the necessity for perfect 
 alloying will be recognised when it is remembered that the 
 melting-point of any of the four components alone is far higher 
 than that even of Newton's alloy, the numbers being approxi- 
 mately, lead 533, bismuth 515, tin 442, cadmium 608 F. The 
 melted alloy may be poured upon the surface of the object to be 
 copied ; or, for the treatment of medals and coins, it may be run 
 into a shallow tray ; then, when nearly solidifying, any dross upon 
 the surface is removed by skimming with a piece of card, and at 
 once the medal is dropped upon the surface and pressed in this 
 position until the fluid has completely set, which it should do 
 almost immediately. No difficulty should be experienced in 
 separating the two surfaces or in obtaining successful results 
 after a few trials. The addition of mercury to reduce the fusing- 
 point of the alloy, which is recommended by some operators, should 
 not be practised, but it is especially to be avoided when gold or 
 silver objects are being treated, as both these metals have a very 
 strong tendency to amalgamate or take up mercury, and in so 
 doing to alter in appearance. The special recommendation to the 
 use of matrices of fusible alloy is that they are metallic, and are, 
 therefore, conductors of electricity, requiring no treatment before 
 placing in the bath beyond rendering them slightly dirty on the 
 surface to prevent adhesion of the deposit. The metal of old 
 moulds may, of course, be remelted as often as desired. 
 
 Elastic Moulding Material. When objects in high relief 
 are deeply undercut, none of the moulding materials hitherto 
 mentioned can be successfully applied, because it is not possible 
 to detach the cast from the original without fracturing the one 
 or the other. A special material, which shall possess so much 
 elasticity that it may be drawn over projecting portions unhurt, 
 and yet on its release return completely to its original form, 
 must, therefore, be sought. Such a material may be made by 
 mixing gelatine with sugar or treacle, the latter substances 
 preventing the cracking of the substance on drying. 
 
 The mixture originally used by Parkes was made by soaking 
 80 parts of glue in cold water over night, or for several hours, 
 until it became perfectly soft; then, pouring off the water, it 
 was melted in a hot-water jacketed arrangement, similar to an 
 ordinary glue-pot, and, when quite fluid, 20 parts of common 
 treacle were stirred in. Busts and statues 'may be readily copied 
 with the aid of this mixture, but it has the inconvenience that 
 it swells up when wetted, and cannot, therefore, be safely placed 
 in the depositing-vat without special preparation. To render it 
 capable of resisting the liquid, the hardening power of potassium 
 bichromate on gelatine is utilised, the matrix being immersed for 
 a short time in a 10 per cent, solution of the bichromate and then 
 placed in direct sunlight until dry ; or 2 per cent, of tannin may 
 
SUPERFICIAL CONDUCTIVITY OF MOULD. 149 
 
 be added to the moulding-mixture before casting, to render it 
 waterproof, in which case the bichromate process may be dispensed 
 with. It is not, however, safe to rely upon these waterproofing 
 mixtures, but when the elastic composition must of necessity be 
 used, a reproduction of the original object should be made in wax 
 composition, and then from this a second matrix should be pre- 
 pared in plaster of Paris, from which the wax may be melted 
 away without injuring the delicacy of undercut portions. The 
 manner of accomplishing this is described on p. 167. 
 
 Sealing- Wax. Sealing-wax is only employed for very small 
 work, such as the reproduction of seals or signets, and is, there- 
 fore, rarely required in practice. It is applied by melting a portion 
 on to a sheet of card or metal, and, after breathing lightly on the 
 signet, taking an impression as in sealing a letter. 
 
 Rendering the Mould Conductive. Having obtained a mould 
 in any of the above materials (with the exception of the fusible 
 alloys), it is necessary to cover it with a film, which will enable 
 the surface to conduct electricity sufficiently well to receive the 
 first deposit of copper uniformly. If it were convenient, the 
 covering of the surface with a superficial metallic layer would 
 obviously be the most satisfactory method, and where it is desired 
 to produce an especially fine reproduction of a delicate design, 
 this is frequently done. Parkes' system of metallising the moulds 
 consisted in preparing the matrix with the special material, con- 
 taining phosphorus dissolved in carbon bisulphide, described on 
 p. 146; then, on dipping this mould into a solution of silver 
 nitrate, the phosphorus present on the surface reduces silver to 
 the metallic state, and thus coats the whole with a superficial 
 but continuous layer of metallic silver. Now, on dipping the 
 matrix into a solution of gold chloride, a partial exchange of 
 gold for silver takes place, and the whole design is covered with 
 an almost imperceptible film of metal, which is capable of con- 
 ducting the current and depositing copper in the finest lines and 
 interstices. 
 
 The more usual practically the universal method is to brush 
 the best and most finely-ground plumbago thoroughly into every 
 detail of the design. Plumbago or black-lead of the best quality 
 is a sufficiently good conductor, but the lower grades are pro- 
 portionately inferior in this respect. The utmost care must be 
 taken to rub the plumbago with a fairly hard brush into every 
 corner of the mould, for it must be remembered that any point 
 which remains untouched can receive no deposit of metal, with 
 the result that the electrotype-copper will prove defective or 
 covered with pin holes. The plumbago is best applied by 
 sprinkling the fine powder over the whole surface to be covered, 
 and then rubbing it persistently in with circular strokes from the 
 brush until the whole presents the uniform characteristic sub- 
 metallic dead-black appearance of black-leaded articles. 
 
150 ELECTROTYPING. 
 
 A wet process has been introduced by Knight in America, so as 
 to avoid the dirt and dust of the usual dry process, an emulsion of 
 graphite and water being pumped over the mould. 
 
 By soaking 100 parts of the plumbago in 10 parts of silver 
 nitrate dissolved in 200 of distilled water, drying and exposing 
 the mixture to a red heat in a crucible, well protected from the 
 action of the air by a fire-clay cover, the silver nitrate is first 
 absorbed by the plumbago, and by the ignition is converted into 
 metallic silver, and thus the black-lead becomes most intimately 
 mixed with the best conductor known, in the finest state of 
 subdivision. It may be similarly gilt by treating 100 parts 
 with 2 parts of gold chloride dissolved in 200 of ether; simple 
 exposure to light followed by gentle heating in an oven then 
 suffices to metallise the gold. But these methods are costly, 
 arid the materials can be used only once, for as soon as they 
 are spread over the surface of the matrix, it is practically im- 
 possible to recover the precious metal ; they are, therefore, rarely 
 met with in practice. 
 
 Other metals may be obtained in a finely-divided state. Adams 
 proposed to use powdered tin upon the surface of the moulds. 
 This is prepared by briskly stirring melted tin with an iron rod 
 just as it reaches its solidify ing-point ; so that the tin is broken 
 up into globules, each of which is coated with a pellicle of oxide 
 that entirely prevents its union with contiguous particles. The 
 whole of the metal is now poured on to a sieve of the finest muslin 
 or wire mesh, the portion which passes through being retained 
 for use, while the remainder is remelted and again treated in 
 the same way. A still later plan is to stir up the globules with 
 water, allow them to stand for one or two seconds in order to give 
 the heavier particles time to subside, and decant the water, with 
 the finest grains still in suspension, into a second vessel, where it 
 is allowed finally to subside. The clear liquid is then poured 
 away, and the tin powder is dried for use. 
 
 A process invented by Knight is used in America. The mould 
 is well cleaned with water delivered from a rose jet, and is then 
 flooded with a solution of copper sulphate. A small quantity of 
 the finest iron filings is now sprinkled over the surface with the 
 aid of a pepper-castor, with the result that a spongy deposit of 
 copper is reduced over the whole area by exchange with the 
 metallic iron, and may be made to form a continuous film by 
 brushing the surface during the time of precipitation. Wahl has 
 accidentally found that pure iron, obtained by reducing ferric 
 oxide in hydrogen, does not give an adhesive copper, owing, as he 
 thinks, to the extreme rapidity of the exchange, the comparatively 
 large though still minute grains of the filings giving a slower 
 reaction than the chemically reduced iron, so that the copper is 
 reduced in actual contact with the surface of the matrix itself 
 during the time of brushing. 
 
ENGRAVED PLATES. 151 
 
 PRINTERS' ELECTROTYPING. 
 
 The principal applications of electrolysis to the requirements 
 of the printing-trade are to be found in the reproduction of 
 engraved steel or copper plates, of wood-blocks or set-up type. 
 For the last-named purpose, electrotyping has a keen rival in 
 the process of stereotyping, which is more largely in favour in 
 England for heavy newspaper or book work ; but for copying 
 wood-blocks or engraved plates, electrotyping stands alone in the 
 field, the other processes being inapplicable, or productive of 
 inferior results. 
 
 ENGRAVED PLATES. 
 
 Steel Plates. Steel plates carefully engraved have a very high 
 value, and as they are slowly worn away by repeated use in the 
 press, in spite of the hardness of the material, their faithful 
 reproduction by electrolytic means effects an immense saving 
 when a large number of impressions are required from one design. 
 Thus, the steel plate may be preserved intact, and any number 
 of copper replicas may be formed, each of which, when steel-faced, 
 is as lasting and as clear as the original, so that impressions may 
 be multiplied to any degree. 
 
 Steel plates which are to be stored are usually coated with 
 wax to preserve them from oxidation, and the first step towards 
 obtaining an electrotype is, therefore, the complete removal of 
 the wax. This may be ensured by boiling the plate for ten 
 minutes in a strong solution of caustic potash (but not, of course, 
 in the same bath that is used for cleaning plates previous to 
 electro-deposition), then rinsing thoroughly with water, and 
 finally rubbing with a soft rag moistened with benzene. The 
 plate may now itself be made the cathode in an alkaline copper- 
 bath, by which means a reversed plate or negative will be pro- 
 duced; or a suitable moulding material, preferably the mixture 
 of gutta-percha, lard, and tallow recommended on p. 144, may be 
 used to produce a non-metallic matrix, which is then rendered 
 conductive as to its surface. The mould, whether made by 
 casting or by electrolysis, is finally suspended as the cathode in 
 an acid copper-vat, until a sufficient thickness of metal has been 
 deposited to fulfil the requirements of the original plate. 
 
 In making metallic matrices from valuable steel plates, it is 
 safer not to undergo the risk of spoiling them by suspending 
 them at once even in an alkaline copper-bath, because the 
 slightest trace of solvent action on the steel will be clearly shown 
 by the rounding-off of the finer lines, and by the consequent 
 want of sharpness and brilliancy of texture in the resulting 
 engravings. This may be avoided by the expedient of taking a 
 silver matrix from the plate by electro-deposition (see p. 196), 
 and then detaching the silver and depositing copper upon it so 
 
152 ELECTROTYPING. 
 
 as to form a reproduction of the original plate ; then, from this 
 in turn, a second matrix is obtainable, but this time in copper. 
 The steel plate may not be required again, except to renew its 
 replica in copper, in case of accident ; and the silver plate, having 
 fulfilled its mission, may be brought again into use at once by 
 remelting, so that there need be no large stock of a costly 
 material lying idle. 
 
 Copper Plates. Whenever a copper plate is to be electrotyped 
 with the same metal, care must be taken to prevent adhesion 
 between the two surfaces, the process being, in fact, the exact 
 reverse to that of covering one metal with a firm coating of 
 another. A film of foreign matter must be interposed, sufficient 
 to prevent adhesion without so far impairing the electrical con- 
 ductivity of the surface that it refuses to receive any deposit. 
 The surest method of accomplishing this is to impart the thinnest 
 possible wash of silver to the surface by a momentary immersion 
 in a silver-bath, which may be done without sensibly affecting 
 even the finest lines of the engraving ; then by pouring over the 
 silvered surface a small quantity of water containing sufficient 
 tincture of iodine to give to it a pale sherry colour, and rubbing 
 lightly with a cloth or with cotton wool, a scarcely visible film 
 of silver iodide is formed upon the surface, which will guarantee 
 an easy parting of the plates. When the plate to be copied is 
 not particularly valuable, and the silver treatment is deemed 
 unnecessary, it should be rubbed gently with turpentine contain- 
 ing a small quantity of bees'-wax in solution ; on the spontaneous 
 evaporation of the liquid, a mere trace of residual wax will be 
 distributed evenly over the whole plate, and will usually suffice 
 to prevent adhesions; but if the engraved lines are numerous 
 and fine, the former method is to be preferred. 
 
 Having prepared the plate to receive the deposit upon its face, 
 the back must be well painted with a water-resisting and non- 
 conductive varnish, so that there shall be no tendency for copper 
 to deposit where it is not required ; common copal- varnish 
 answers the purpose very well, and is readily removed at any 
 time by means of turpentine. The plate is now ready to be 
 suspended in the bath, and must be gripped by some metal 
 support which will serve to suspend it from the cross-rods of the 
 bath, and thus to open electrical communication with the battery. 
 Rings or bent wires may be soldered lightly to the back (before 
 coating it with the insulating varnish) ; or, when there is danger 
 of buckling the plate by the heat necessary to this treatment, 
 the plate may be rested on two or more S-shaped copper strips, 
 bent upwards into hook-shape at the bottom to receive it, and 
 downwards at the top that it may be hung upon the cathode- 
 rods. Or a sliding-frame, such as that sketched in fig. 80, may 
 be made to grip the plate. This consists of an upright of 
 varnished wood, with a fixed cross-piece of the same material 
 
THE COPYING OF ENGRAVED PLATES. 153 
 
 near the lower end ; the cross-piece is made with a longitudinal 
 groove to support the plate ; and along the bottom of the groove 
 is a strip of clean copper, to which are soldered at the ends two 
 copper wires insulated, except at the joint with the strip, by 
 means of an india-rubber sheath, and these passing out of the 
 solution above are unsheathed and bent into hook-form, thus 
 serving at once to support the whole arrangement, and to make 
 connection between the plate and the battery. Sliding on the 
 upright rod is a similar grooved cross-bar above the former, but 
 with groove reversed ; this may be fixed in any position by a 
 screw at the back, and is merely intended to clamp the plate 
 firmly in position, and to press it into contact with the copper 
 strip in the lower groove : the insulated wires pass through the 
 upper support, and thus act as guides. In using this apparatus 
 no wires need be soldered to the plate ; the upper 
 cross-bar is made to slide upwards, the plate is 
 placed with its lower edge upon the copper strip, 
 and the upper groove is brought down upon its 
 upper edge, and is then clamped in position. The 
 whole frame is now suspended in the bath from the 
 cathode-rods above ; connection is thus made with 
 the battery, and deposition at once commences. 
 
 All pairs of electrodes placed in the bath in 
 parallel must be approximately equidistant, for 
 reasons which will shortly be explained. 
 
 Arrangement of Baths and Plates. With 
 metallic plates of known area, most of which are 
 probably of the same size, the plates in any bath, Fl ^; 80. Elec- 
 or the baths themselves, may be arranged either in supporter & 
 parallel or in series, according to the strength of the 
 current and at the will of the operator. If the dynamo be giving, 
 let us say, 5 volts pressure (the electro-motive force of batteries 
 may be altered by adjusting the arrangement), then as each pair 
 of electrodes requires only from 0'7 to TO volt, either five baths, 
 each with all its plates in parallel, may be placed in series ; or 
 five pairs of plates in each bath may be arranged series-fashion, 
 all the baths being now in parallel ; but if the electro-motive 
 force at the brushes of the dynamo be 2 volts, two baths would 
 be placed in series, and so on. Having determined the number 
 of couples which should be connected in series to suit the avail- 
 able current-pressure, the distribution of plates in parallel will 
 depend upon the relation of the aggregate surface of such plates 
 to the volume of the current. It has been seen that a current 
 of from 1'7 to 2 amperes per square decimetre (or from O'll to 
 0'13 ampere per square inch) is most suitable for copper-deposition 
 in the acid bath (when the liquid is kept in motion and of correct 
 density). If there be, let us say, 20 equal pairs of plates with a 
 total cathode area of 400 square inches, and all the couples are 
 
154 
 
 ELECTROTYPING. 
 
 FIG. 81. 
 
 placed in parallel, the best current ranges from 400 x O'll = 44 to 
 400x0-13 = 52 amperes; but if the E.M.F. be 2 volts, so that 
 the plates are divided into two groups, each consisting of ten pairs 
 of electrodes, and these two groups are placed in series, a current 
 of only 22 to 26 amperes is needed, because only half the total 
 surface is arranged in parallel, while if the voltage be 4, the 
 20 pairs of plates are subdivided into four groups in series 
 with five parallel couples in each ; and the current required will 
 be only 1 1 to 1 3 amperes, and so forth. All this follows, of course, 
 from the law explained on p. 86, that a given current deposits 
 ( < < _ T . the same weight of metal in 
 
 RO RO RO RO I every cell placed in series. 
 
 1 It will be observed that the 
 product of amperes multi- 
 plied by volts is in all these 
 cases the same, showing that 
 the total number of watts 
 evolved (in other words, the 
 work done) is practically 
 identical under all condi- 
 tions, and the different dis- 
 positions are only to be 
 made in order to suit the 
 current to the work. 
 
 To make this quite clear, 
 the following diagrams are 
 constructed to illustrate the 
 disposition to be observed 
 in the cases of four cathode- 
 plates, each measuring 40 
 square inches, this being 
 equivalent to four plates 
 each 8 inches by 5. 
 
 In fig. 81, the electro- 
 motive force at the dynamo brushes is 1 volt ; all four plates are, 
 therefore, arranged in parallel, presenting a total of 4 x 40 = 160 
 square inches of surface, which will require (say) 20 amperes (at 
 0*125 ampere per square inch). 
 
 In fig. 82, the difference of potential is supposed to be 2 volts. 
 The plates are now placed in two series of two parallel pairs each : 
 the total surface in each group is now 2 x 40.= 80 square inches, 
 and the current required is 10 amperes. 
 
 In fig. 83, the initial pressure is 4 volts, so that all plates are 
 disposed in series, each one presenting an area of 40 square inches ; 
 the total current demanded is thus only 5 amperes. 
 
 The arrangement of ammeters, resistance frames, and volt- 
 meters, marked respectively A, R, and V, is shown for each of the 
 above dispositions of baths. 
 
 FIG. 82. 
 
 FIG. 83. 
 
 FIGS. 81 to 83. Modes of arranging plates. 
 
DETERMINING THICKNESS OF DEPOSIT. 155 
 
 When batteries are used, it is best to arrange them in parallel, 
 which will also require a parallel grouping of the plates, because 
 of the low electro-motive force of a single voltaic cell ; neverthe- 
 less with Bunsen- or bichromate-cells, each of which gives an 
 E.M.F. of nearly two volts, the battery- elements should be placed 
 as before in parallel, but the electrodes in two large-series groups. 
 It is impossible to lay down any fixed rule for the number of cells 
 of any battery required to accomplish a given work, because the 
 volume of current depends upon the condition of the battery, 
 upon the temperature, and upon the internal resistance of the 
 battery itself, and the latter varies greatly according to the strength 
 of the exciting liquid (and the depolariser, when one is used) 
 as well as on the external resistance in the circuit, due to that 
 of the copper-baths and connecting wires. By the use of an 
 ammeter all doubts upon questions of current-strength may be 
 solved, and the operator will know precisely the value of the 
 current with which he is working. When an ammeter is not 
 available, recourse should be had to the table given on p. 390 ; 
 from this it will be found that 1 ampere should deposit 18 '26 
 grains of copper in an hour ; a current of 0'12 amperes per square 
 inch of cathode-surface should therefore deposit about 2 '2 grains 
 of copper per hour on every square inch of plate, or 35 grains 
 upon a plate about 4 inches long by 4 inches wide. To ascertain 
 the average current-strength by this means, a deposited electro- 
 type-plate should be weighed carefully after drying ; the weight, 
 expressed in grains, divided by the number of hours which were 
 required for its deposition gives, of course, the weight deposited 
 per hour ; and this number, divided by the superficial area of the 
 plate in square inches, gives the weight of copper deposited per 
 square inch per hour. Finally, this last number, divided by 
 18 26 (the weight Of copper per ampere per hour), is the average 
 strength of the current per square inch expressed in amperes. 
 
 To consider an example : a plate 5 inches by 4 ( = 20 square 
 inches area) is deposited in 10 hours, and has then increased by 500 
 grains ; find the average current-density per square inch of surface. 
 
 The weight divided by the number of hours 
 
 ~~10 
 This number divided by the area in square inches 
 
 20 
 
 So that 2-5 grains were deposited on each square inch every 
 hour. 
 
 This number divided by the copper equivalent of 1 ampere in grains 
 
 2-5 , = 0-137. 
 
 18-26 
 Therefore, the current-density = 0'137 ampere per square inch. 
 
156 ELECTROTYPING. 
 
 From a simple calculation of this description, at least a rough 
 estimate of the relation of current-density to cathode-area may 
 be made, and the disposition of the apparatus in subsequent work 
 governed accordingly. 
 
 Character of Copper Deposits. Adjustment of Current. 
 Further indications are made to an experienced observer by the 
 character and rapidity of the deposit. A film of copper should be 
 observed to cover the whole surface of the metal plate immediately 
 it is placed in the bath, and this copper should have a rich reddish- 
 yellow colour ; a dark-brown deposit usually indicates a more or 
 less spongy copper and too strong a current ; while any indication! 
 of gas evolution at the cathode is a sign of the current being far 
 too powerful, and will certainly be accompanied by a pulverulent 
 coating; on the other hand, a tardy formation, and a copper 
 possessing an extremely pale shade of colour, shows that the 
 current is too weak. With measuring apparatus at command, it 
 is only necessary to divide the indicated current by the total 
 area of the cathode-plates placed in parallel, and to compare the 
 quotient with the prescribed number of amperes per unit of surface, 
 to know in which direction and to what extent there is error in 
 the pressure applied. 
 
 To regulate the current is then a simple matter ; if the current 
 is too strong, greater electrical resistance must be added to the 
 circuit, either by increasing the distance between each pair of elec- 
 trodes, or by introducing one or more wires of the resistance coil. 
 The latter is the safer method ; if the former be adopted, all the 
 anodes (in parallel) must be shifted equally, or some of the cathode- 
 plates will be nearer to their respective anodes than others, with 
 the result that the local resistance is decreased, and the propor- 
 tion of current passing through them is increased, and with it the 
 amount of deposit ; some plates may thus be receiving an ex- 
 cessive current, while others are insufficiently served. Should the 
 current be too weak, either the anodes must be brought nearer to 
 the cathodes, if this be consistent with safety, or a plate may be 
 removed from the bath, thus distributing a slightly diminished 
 current over a much smaller area, and so giving greater current- 
 volume per square inch ; or the battery-power must be increased, 
 unless there were at first added resistance in the circuit from the 
 wire coils, when it will, of course, suffice to remove or reduce this 
 extra resistance. In causing the plates to approach, care must be 
 taken to prevent their forming a short circuit by coming into 
 contact. 
 
 The current having been adjusted, the electrolytic action must 
 be allowed to proceed steadily, with frequent agitation of the 
 liquid in the bath, until a sufficient thickness of deposit has been 
 attained. This may be ascertained by removing the plate from 
 the bath and rapidly turning up one extreme corner very slightly, 
 by carefully inserting the edge of a thin-bladed penknife between 
 
TREATMENT OF TYPOGRAPHIC FORMES. 157 
 
 the two surfaces. The actual thickness required depends upon 
 the treatment which the plate is to receive subsequently ; if it is 
 to be backed or strengthened by another metal, the thickness of 
 stout writing-paper will suffice : but if the plate should be required 
 to depend upon its own unsupported strength, then the depositing 
 should be continued until it is from -^ to ~ of an inch thick, 
 which may require from ten days to a fortnight to accomplish. 
 
 The precipitation completed, the plates must be separated by 
 gently inserting the edge of a fine chisel between them on each 
 side, and applying gentle pressure ; if they should not part 
 readily, the chisels must be removed and re-inserted at fresh 
 points at the ends, for example and this operation must be 
 repeated until success is achieved. Often the rapid warming of 
 one plate by the sudden application of hot water, followed by an 
 immediate application of the chisels, will effect the separation of 
 a refractory pair of plates. Force must on no account be used ; it 
 will only result in bending and spoiling one or both of the plates. 
 But usually there should be little difficulty encountered in re- 
 moving the deposit from the original surface. 
 
 The plate has now only to be cut square and finished by 
 mechanical means; it may also be coated with a thin film of 
 hard iron, nickel, or brass by processes to be described in the 
 chapters dealing with these metals, in order to impart to it higher 
 resisting qualities than the softer copper alone possesses. 
 
 TYPOGRAPHICAL MATTER. 
 
 When 'formes' of type set up for printing are sent to be 
 electrotyped, they must first be carefully examined to see that 
 they are properly prepared for moulding ; for success in this 
 branch of work is only secured by patient attention to details. 
 
 Preparation of Forme. In the preparation of the forme by the 
 compositor, due regard must be had to the requirements of the 
 process. In the first place, only strong wrought-iron chases must 
 be used for holding the type in place, thin, cast-iron, or wooden 
 chases being liable to give way under the intense pressure applied 
 in moulding ; screw chases are to be preferred if available. The 
 type and rules should be made with a fair amount of bevel, so 
 that a better impression is given to the wax. All the spaces 
 between the letters and all furniture should be high ; low spaces 
 cause much trouble, because the wax is forced deeply into them 
 and, on withdrawal from the mould, stands unprotected, and 
 because the cavities formed by the letters are so deep in the wax 
 matrix, that it is only with difficulty that they receive a good 
 coating of copper ; and this is a worse evil than the former, inas- 
 much as it causes weakness of deposit at the very point where 
 strength is most needed. And, lastly, the leads between the lines 
 should be square, not bevelled, as the wax becomes forced into 
 
158 ELECTROTYPING. 
 
 the fine space made by the bevel, and cannot be removed cleanly, 
 so that extra labour is demanded in the subsequent distribution 
 of the type. If wood-blocks are present in the page they must 
 be exactly type high ; if less than this they must be brought up 
 to the required level by packing slips of paper beneath them ; it 
 is preferable that they should be if anything a trifle higher than 
 the rest of the page, so that they may give a sharper impression 
 in the printing-press. 
 
 When the type has been set up in weak chases, they must be 
 carefully removed by * dropping ' upon an imposing-surface, arid 
 others must be substituted. If the spaces are low, the forme 
 must be planed (made perfectly level) and floated by pouring over 
 it a thick cream of plaster of Paris, which must be well rubbed 
 into the spaces by hand, and the excess removed by means of a 
 hard brush applied while it is still pasty ; then, after thoroughly 
 drying, it is again brushed to remove every trace of plaster from 
 the angles of the letters. The forme must now be 
 firmly locked ; and at this stage a proof should be 
 taken, that accidental slips or errors may be finally 
 detected and rectified. The surface of the type 
 is then thoroughly cleansed from dirt and stale 
 ink by brushing with potash solution (followed by 
 water), or with benzene, while wood-blocks are 
 similarly cleansed with benzene or turpentine alone. 
 The forme is now dried and brushed over with fine 
 plumbago applied with a soft brush : the whole 
 surface should thus be perfectly glazed, but all 
 Moulding-box, excess must be carefully removed from the finer 
 lines and from the beards of the letters. The 
 plumbago coat not only assists the separation of the type from 
 the wax-mould, but by leaving a trace of the black-lead on the 
 latter, helps to ensure it being thoroughly coated before it is 
 placed in the copper-vat. 
 
 Preparation of the Wax Surface. Meanwhile the wax should 
 have been prepared to receive the impression. A cast-iron 
 moulding-box (fig. 84) is gently heated ; it is in reality only a 
 tray of sufficiently great external dimensions with shallow sides 
 of about a quarter of an inch in depth all round, and with a con- 
 tinuation of the bottom-plate on one of the shorter sides for about 
 3 inches beyond the box, to allow of its being supported by hooks 
 from the conducting rods of the bath. It is then filled with the 
 wax-and-turpentine composition formulated on p. 144 ; this should 
 be quietly melted in a steam-heated vessel, and poured into the 
 box placed upon a level surface until the latter is filled to the 
 brim. Air-bells forming on the surface during cooling are re- 
 moved at once by a touch with a hot iron rod. If the contraction 
 which occurs during solidification reduces the surface-level, more 
 wax must be added before the former is quite set, as it is better 
 
THE TOGGLE-PRESS. 
 
 159 
 
 that the composition should overflow than that insufficient should 
 be used, while, if the contraction cause the material to crack away 
 from the side of the box at any point, the fault may be made 
 good by the careful application of a warm iron rod. In short, it 
 is necessary that a perfectly smooth, level, and unbroken surface 
 of wax shall be presented after solidification. This surface is 
 then dusted over with the finest plumbago, which is lightly 
 brushed in with a soft brush ; the excess being removed, the 
 box is ready for taking the impression of the type. In place 
 of these iron boxes, brass cases of the same size and shape have 
 come also into use ; these have a special ' electric connection 
 gripper' which serves to suspend the mould in the solution, and 
 at the same time to make connection with the surface of the 
 mould only, and not with the metal box containing the wax, 
 the back of which, therefore, does not require the protection 
 of stopping-out varnish, as 
 does the iron tray above 
 described, to prevent the 
 deposition of copper where 
 it is not required. Steam- 
 heat should be applied to 
 melt the wax, as it affords 
 less danger of overheating 
 and burning the composi- 
 tion ; this is readily done 
 by using two concentric 
 pots joined air-tight at the 
 rim, the inner one contain- 
 ing the wax, and the space 
 between the two being util- 
 
 FIG. 85. Toggle-press. 
 
 ised for the steam ; two pipes, therefore, communicate with the 
 outer jacket, one to introduce the steam, the other to carry it 
 off. Waste steam may often be utilised thus. 
 
 Taking the Wax Impression. The next process consists in 
 carefully placing the forme of type, face downwards, upon the 
 centre of the wax surface and submitting it to intense pressure. 
 For this purpose, either a hydraulic or a toggle press is usually 
 employed ; the action of the former is too well known to need , 
 further description, that of the latter is indicated in the sectional 
 diagram (fig. 85). The counter-balanced swing-cover, C, closed in 
 the drawing, is indicated by dotted lines in its open position for 
 the purpose of introducing the mould and type between it and 
 the rising pressure-plate, P ; these having been brought into 
 place, the cover, C, is brought down into a horizontal position and 
 retained there by the movable bar, B. On turning the hand- 
 wheel, W, the pressure is applied to the two >-shaped toggle- 
 joints, T T, which are pivoted at their centre and held by the 
 framework, beneath at F F, and attached to the plate at D D, 
 
160 EL'ECTROTYPING. 
 
 the result of the pressure being that the >-shaped jointed bars tend 
 to straighten, and being constrained from moving at their lower 
 ends, thrust the plate carrying the mould with great force upwards 
 against the cover, and in so doing press the face of the type into 
 the wax. The pressure must be gradually, steadily, and evenly 
 applied, until, for large surfaces, it may amount in the aggregate 
 to several tons. Excessive pressure renders it almost impossible 
 to separate the forme from the wax without damaging the latter, 
 whilst an insufficient pressure does not give the depth necessary 
 for printing to prevent the inking of the paper in the spaces 
 
 FIG. 86. Hoe's toggle-press. 
 
 between the letters ; the mean between the two pressures is soon 
 learnt by experience, and may be judged with tolerable accuracy 
 by the force applied on the hand-wheel or pump of the press. 
 
 When the pressure is released and the press opened, the 
 surfaces of mould and type must be separated by inserting bent 
 screw-drivers gently between them at either end, and applying 
 slight leverage until they are almost disengaged ; after similar 
 assistance, applied if necessary at the sides also, and when the 
 forme is quite free of the matrix, the latter is removed by lifting 
 it vertically upwards ; it must be inspected to ensure that it is 
 perfectly sound in every part. 
 
 Trimming the )jVax Impression. All the wax which has been 
 forced up around the sides or into deep spaces must now be 
 
TRIMMING THE WAX IMPRESSION. 
 
 161 
 
 carefully pared away with a sharp knife, and other spaces, the 
 levels of which are so dangerously near to that of the type 
 face that there is danger of their printing off with the type when 
 they have been electrotyped, are filled up by means of a heated 
 knife (fig. 87) or a building-tool (fig. 88). These tools are heated 
 to a temperature a little above the boiling-point of water, so 
 that a fragment of wax placed against them melts, and passing 
 down to the point of the tool, 
 may be run on to any de- 
 sired point. This is a criti- 
 cal operation, which requires 
 much care and no little skill FIG. 87. Building-knife, 
 
 to accomplish satisfactorily. 
 
 Black-leading the Impression. The surface must now be most 
 completely black-leaded. This is done by sprinkling a quantity 
 of the finest plumbago over the whole, and then gently stippling 
 and beating it into the wax by means of goats'-hair brushes. The 
 great volume of black dust produced is very unpleasant, and 
 many operators use black-leading machines, which not only 
 prevent the scattering of dust, and consequent loss of valuable 
 material, but effect a more certain covering of the matrix, which 
 will be recognised as a matter of vital importance when it is 
 remembered that a small area of surface insufficiently coated will 
 give rise to a flaw in the electrotype plate, while even a speck will 
 produce a pin-hole. The black-leading machine (fig. 89) is a large 
 rectangular frame with a box beneath, a cover over the whole, 
 
 and a trellis-table to support 
 the wax matrix ; a reciprocat- 
 ing motion is imparted to the 
 table by a hand-wheel placed 
 outside the case, but connected 
 with the necessary mechanism 
 within ; and this serves also to 
 produce a vertical reciprocating 
 or dabbing motion to a brush 
 which extends across the whole 
 table at its centre. On sprink- 
 ling the mould with a fair supply of plumbago, placing it face 
 upwards on the table, and actuating the hand-wheel, the wax 
 cast will be drawn to and fro beneath the moving brush, which 
 will force the plumbago into every interstice ; the excess of 
 black-lead falls into the box, while the cover prevents the escape 
 of dust into the air. The cover may be made of glass, so that 
 the progress of the operation may be watched, as far as the dust 
 will permit; occasionally even the hand-brushing is conducted 
 under a shade, but this is not altogether satisfactory, as the 
 operator is somewhat cramped in his position, and, therefore, 
 less able to ensure thorough work. 
 
 11 
 
 FIG. 88. Building-tools. 
 
162 
 
 ELECTROTYPING. 
 
 The silvered or gilt plumbago, the tin-powder or the precipi- 
 tated copper process for metallising the mould, may be used here 
 if desired. Any system which necessitates the use of phosphorus 
 is to be avoided, because this element may render the copper 
 superficially brittle, and this defect is fatal to work that has to 
 withstand the wear of the printing-press. 
 
 When the mould is thus rendered superficially a conductor of 
 electricity, every portion which is not required to receive a 
 coating of copper for example, the back of the frame and the 
 edges of the wax is painted over with melted wax by means of a 
 soft brush. 
 
 FIG. 89. Hoe's black-leading machine. 
 
 Making Electrical Contact. It is now ready for the electro- 
 depositing process. Electrical connection must first be arranged 
 for by embedding a framework of warm copper wire around the 
 wax edge of the mould, then black-leading the surface of the wire 
 to ensure good contact with the plumbago coating upon the wax ; 
 and attaching the end of the wire itself to the cathode-rod of the 
 bath. Sometimes the end of the wire only is embedded, but 
 then the depositing action of the current has to spread itself over 
 the whole of the plumbago surface from one point ; whereas, by 
 using the frame, it starts simultaneously from the whole circum- 
 ference and gradually covers the surface towards the centre. 
 When the electric contact gripper is used no further trouble 
 need be taken. 
 
 The smaller cavities in the wax would remain filled with air 
 if plunged at once into the copper- vat, especially as the plumbago 
 
MAKING ELECTRICAL CONTACT. 163 
 
 has a somewhat repellent action upon water until it is once 
 wetted; and as any air space of this kind prevents deposition 
 locally by destroying contact between the wax and the solution, 
 the mould is finally prepared for the bath by placing it in a 
 tray and flowing an ounce or two of spirits of wine over it, to 
 facilitate the wetting of the plumbago, then filling the tray to 
 a depth of about 3 inches with water, and directing a high- 
 pressure jet of water upon the surface from a rose held at a 
 height of a few inches above the surface. After washing in 
 this way for a minute or two, the surface should be inspected in 
 various lights while still under water ; any air-bells yet adhering 
 to the surface of the wax are thus at once seen, and the washing 
 must be continued until they have disappeared. 
 
 Depositing the Copper. The tray of wax is at once trans- 
 ferred to the acid copper-bath, in which it may be suspended 
 after the manner recommended for steel-engravings. It is 
 advisable to increase the electro-motive force of the current 
 employed beyond the normal at first, in order to force the 
 copper deposit over the comparatively weak-conducting surface 
 afforded by the plumbago. Copper should be deposited immedi- 
 ately on the exposed metallic surfaces of the conducting wire, 
 and should gradually spread from this until the whole surface 
 is covered, when the potential of the current should again be 
 reduced; the metal should then continue to precipitate evenly 
 over the entire plate until it has attained to a thickness of the 
 j-J-Q- up to the ^- of an inch, which is generally adequate. 
 Progress is tested as in depositing upon metallic plates, by gently 
 lifting one corner with a penknife ; from four to fifteen hours 
 usually suffice. When sufficient copper has been deposited, the 
 frame is removed, rinsed with water, rested upon a level or 
 slightly-sloping board, and suddenly flooded with hot water on 
 the back of the newly-deposited metal ; this immediately releases 
 the latter from the wax so that it may be detached at once (every 
 precaution being taken against bending it) and examined by 
 holding it up to the light. If many holes be visible the plate 
 should be discarded ; if only a few, and these small ones, they 
 may be made good in the next process. The wax matrix can 
 rarely, if ever, be safely used a second time. These electrotype- 
 plates will be subsequently strengthened by 'backing -metal,' 
 hence they need not be so strong intrinsically as those which 
 have to bear the strain of the press unsupported ; and, therefore, 
 a more intense current may be used than is permissible for 
 example, in the reproduction of engraved plates ; but on no 
 account must the current be so intense that hydrogen is deposited 
 with the copper, for not only is the metal itself weak (even if it 
 be coherent at all) under these circumstances, but the clinging 
 of the bubbles of gas to the work is certain to produce pin-holes 
 in the plate. With a 20 per cent, solution of copper sulphate, 
 
164 ELECTROTYPING. 
 
 slightly acidified and constantly agitated, a current of 0'2 to 
 0*225 ampere per square inch (3 or 3*5 amperes per square deci- 
 metre) is quite the maximum that should be adopted. For work 
 which will be carefully used a thin deposit may suffice, such as 
 might be deposited by O13 to (H6 ampere per square inch (2 or 
 2 '5 amperes per square decimetre) in three or four hours; but 
 for plates which may have to withstand rough treatment or long 
 wear, fifteen or twenty hours may be given. 
 
 Backing the Copper Sheet. After examination, the trace of 
 wax which adheres to the copper is removed by a rinsing with 
 caustic potash solution, followed by a thorough washing with 
 water. The next operation is to protect the thin shell with a 
 strengthening metal. The back of the copper sheet is painted 
 over with a solution of zinc chloride containing a little sal- 
 ammoniac (ammonium chloride) or borax; it is then rested on 
 an iron tray which is suspended in contact with the surface of 
 melted backing-metal. Granulated tin-lead alloy or foil con- 
 taining 50 per cent, of each of these metals is then placed upon 
 the copper sheet, and the heat is continued until the white metal 
 has just melted not higher, lest the copper become oxidised. 
 The tray is now removed to a level place and a small ladle full 
 of backing-metal, which has been well skimmed, is slowly poured 
 over the surface, commencing at one corner, until a depth of 
 about one-eighth of an inch is attained. It is then allowed to 
 cool. The backing-metal is an alloy of 91 per cent, of lead, 5 of 
 antimony, and 4 of tin (by weight) ; it should not be overheated 
 a temperature of about 600 F. is suitable. A rough practical 
 test is to dip a scrap of white paper into the molten bath, when 
 it should become only just discoloured, any stronger signs of 
 scorching showing that the temperature is too high. The object 
 of the preliminary coating with tin and lead is to ensure a sound 
 union between the copper and the backing-metal, such as could 
 not otherwise be guaranteed. 
 
 After subdividing with a circular saw, if necessary by reason 
 of the treatment of separate blocks or pages upon the same plate, 
 the copper is examined with a steel straight-edge, and if not truly 
 level, the positions of defective portions are marked on the back : 
 it is then straightened by gentle blows with a polished hammer, 
 taking every care that the face be not damaged. After obtain- 
 ing a plane surface, the excess of backing-metal is shaved off in 
 a specially constructed lathe or hand shaving-machine ; it is then 
 trimmed and again tested with the straight-edge ; irregularities 
 are again rectified, and it is finally reduced to exactly the 
 required thickness (usually that of a small pica) by a hand 
 planing-machine ; after finally bevelling at the edges it is 
 mounted on wood, type high. 
 
 Any backing-metal which has found its way to the surface 
 through pin-holes in the copper may generally be removed, 
 
BACKING THE COPPER SHEET. 
 
 165 
 
 unless the soldering-fluid has also penetrated, and so caused the 
 two clean metallic surfaces to unite. Unevennesses and defects 
 of this character must be set right by a competent workman with 
 a knowledge of engraving, to whom the final examination of the 
 finished plate should be entrusted. 
 
 Gutta-percha composition-moulds from type-formes are treated 
 in the same manner as wax-impressions. 
 
 WOOD-BLOCKS. 
 
 Wood-blocks, like typographical matter, may be copied by 
 wax ; but, since they are liable to be damaged by extreme 
 pressure, it is safer to mould them in gutta- 
 percha rendered plastic by heat, or by pouring 
 the melted gutta-percha and lard mixture over 
 them as described on pp. 144, 145; then, after 
 rendering them conductive, they are electro- 
 typed, trimmed, backed, planed, and mounted 
 in the same way as those produced from wax- 
 matrices. 
 
 ART ELECTROTYPING. 
 
 In this group may be arranged the reproduc- 
 tion of medals, medallions, busts, and statues, 
 or objects of vegetable and animal origin and 
 the like. Among the principal points to be 
 observed are the choice of a suitable moulding- 
 material, and the carrying out of the casting on the one hand, 
 and the arrangement of the anodes in the bath on the other. 
 
 FIG. 90. Copper 
 trays. 
 
 MOULDING. 
 
 Medals. Medals, coins (or medallions if metallic) may be 
 coated directly with copper after the manner of copying engraved 
 steel-plates, the copper-matrix then being used to electrotype 
 upon, provided that they are not in any degree undercut. Only 
 one side can be treated at a time, and the back must be protected 
 by a stopping-off varnish, while the face is brushed over with 
 the solution of wax in turpentine, already described, to prevent 
 adhesion. If possible, the connecting wire should be soldered 
 lightly to the rim of the medal ; but as this is rarely allowable, 
 the object may be slung in a wire loop, or it may be placed in a 
 copper tray as described by Urquhart, and depicted in fig. 90 ; 
 these trays are made of thin sheet copper painted on the outside 
 with Japan black to prevent local deposition, but left bright 
 inside to make connection with the medal ; they are supported 
 in the bath by the hook shown above, the medal being merely 
 
166 ELECTROT YPING. 
 
 fitted into the bottom of the tray. They may be made of various 
 sizes to take any coin or medal of which a copy may be required. 
 When the object is slung from a wire loop, its position must be 
 shifted from time to time to prevent the formation of wire-marks. 
 Medallions made of a nonconducting material must be made 
 conductive by plumbago or thin metal 'leaf,' a process which is 
 rarely admissible. 
 
 It is usually safer, in any case, to prepare a mould from the 
 medal in preference to taking an electrotype-matrix. To this 
 end the medal is rubbed lightly over with plumbago by means 
 of a brush to which a circular movement is given ; it is then 
 placed upon a flat surface, preferably protected on the under 
 side by a disc of chamois leather, if there be designs on both 
 sides and the ' relief ' of the lower one is nearly as high as its 
 surrounding rim; it is then moulded with plastic gutta-percha 
 or with the fluid composition, as explained in the section dealing 
 with moulding materials. Either of these methods can be 
 recommended, but any of the other materials described in the 
 beginning of this chapter may be employed as there directed. 
 The mould is then rendered conductive with plumbago or metal 
 and is ready to be electrotyped, the methods of doing which have 
 been already dealt with in full. 
 
 Busts and Statues. The moulding may be first effected in 
 the elastic composition (see p. 148) by placing the object, slightly 
 oiled on the surface, if permissible, within a box with tapering 
 sides slightly greased, and gently pouring in the warm liquefied 
 mixture, until the object is covered and the box completely 
 filled, taking care that no air-bells form upon the surface of the 
 former during the operation. The whole is now allowed to 
 stand in a cool place until solidification is complete, when the 
 box is removed, and the composition is cut through to the 
 statue or bust from top to bottom, with the aid of a sharp knife, 
 along a line previously determined by the shape of the object to 
 be the most convenient. Being elastic, the mould may now be 
 opened out and withdrawn from the object, even if it be some- 
 what sharply undercut ; once removed, it returns to its original 
 shape by virtue of its elasticity. The interior cavity, which, of 
 course, has taken the form of the moulded object, may be 
 hardened and waterproofed as above described, and then coated 
 with a conductive film and subjected to electrolysis; but, owing 
 to the difficulty in rendering it completely water-resisting, and 
 to the fact that wherever liquid may penetrate the mould will 
 swell out of shape, it is not advisable to adopt this plan. It is 
 better to prepare a special wax-composition by melting together 
 a mixture of bees'-wax, rosin, and Russian tallow, in the propor- 
 tions of 50 : 40 : 10 respectively, arid pouring this, just at the 
 moment before it sets, into the hollow space within the elastic 
 mould, which should be re-closed for the time in its original 
 
THE MOULDING OF STATUARY. 167 
 
 containing-box ; the wax-mixture must not be too hot, or the 
 two compositions will unite and the whole operation will be 
 ruined. When the wax has solidified throughout, the mould is 
 stripped from it, and an exact wax-reproduction 6f the original 
 bust should result. This in turn is placed in a suitable box, and 
 the space around is filled with a cream of plaster of Paris, which 
 must be allowed to harden; it is then removed from the box, 
 dried and heated over a trough, in an oven or stove, to a 
 temperature sufficient to melt the composition from the interior : 
 the side of the plaster block, which had been in contact with the 
 bottom of the box, and upon the surface of which the base of the 
 wax-object is visible, is, of course, placed downwards, so that as 
 the wax melts it runs into the tray prepared for its reception. 
 A small proportion of the wax is absorbed by the plaster, and 
 thus renders it non-absorbent. The interior of the cavity in the 
 plaster is now rendered conductive by black-leading or metallisa- 
 tion, and is ready for the electrolytic process. When, however, 
 the whole plaster-mould is to be immersed in the vat, the outside 
 must also be waterproofed by painting it with wax and subse- 
 quently applying heat ; and it is desirable also in this case to cut 
 a small aperture through the plaster at the highest part of the 
 object (the top of the head in the case of a bust) so that a con- 
 stant circulation of the electrolyte may be effected during the time 
 of deposition ; this channel also must be made impervious to water. 
 
 A similar but shortened process is applicable to the copying 
 of models moulded in wax, if the original may be destroyed 
 the operation is taken up at the second stage of the above cycle, 
 the plaster being poured around the object at once, leaving only 
 an opening at a convenient point, through which the composition 
 may be melted out, and the copper solution and anode intro- 
 duced. When the original wax-model may not be sacrificed, 
 the longer process of taking a first impression in elastic com- 
 position must be resorted to, but the fracture of projecting portions 
 of the brittle wax must be carefully guarded against. 
 
 Other systems of moulding are also in vogue. Lenoir's 
 method for reproducing statues in a manner approaches in 
 principle to that of the foundry. He moulds the figure in 
 gutta-percha in a sufficient number of different parts, the sections 
 being so disposed and marked that when united together they 
 form a complete mould of the object ; the different internal 
 surfaces are black-leaded and then fitted around a skeleton-anode 
 of wire ; a convenient number of apertures are made above and 
 below to afford communication between the exterior and interior 
 of the mould, for connecting the anodes with the battery, and for 
 the circulation of the solution ; and the arrangement is ready for 
 electrolysis. A knowledge of the moulder's art is very valuable, 
 if not indispensable, in determining the most suitable method of 
 dividing up the surface of the statue into sections. 
 
168 ELECTROTYPING. 
 
 Large statues moulded in plaster have been copied by render- 
 ing them impermeable to liquid, coating the whole exterior with 
 plumbago, and then immersing them as cathodes in an acid 
 copper sulphate bath, until a thickness of about one-sixteenth 
 of an inch of copper has been deposited. They are then cut 
 through at suitable points, where the marks of the joins will be 
 least conspicuous on the finished reproduction, and the plaster 
 being completely removed, the outside is joined to connecting 
 wires and covered with stopping-out varnish, and the inside is 
 rendered dirty by the turpentine solution of wax, or by painting 
 with dilute ammonium sulphide, which gives a superficial tarnish 
 of copper sulphide; the excess of the ammonium sulphide must 
 be washed away, and copper is then deposited upon the different 
 sections of the copper matrix individually ; when a thickness of 
 copper of at least one-sixth of an inch, but preferably a quarter 
 or even a third of an inch, has been acquired, the thin copper 
 mould is stripped away, and the separate portions of the electro- 
 typed statue are mechanically finished off and fitted together to 
 form the complete figure. 
 
 The mould having been prepared and rendered conductive by 
 any of these processes, it is finally arranged to receive the de- 
 posit of copper ; the main point now to be observed is that the 
 anode shall be as nearly as possible equidistant from every part 
 of the cathode-surface. If this be not attended to for example, 
 in electrotyping statue-moulds the chief recesses in the mould 
 will receive the thinnest deposit of metal, whereas they will 
 afterwards be subjected to the greatest wear, being the most 
 prominent portions of the finished surface, and should, therefore, 
 by preference have increased rather than diminished thickness. 
 This matter needs careful consideration, and the ingenuity of the 
 workmen may often be taxed to find the best possible arrange- 
 ment. For shallow-cut metals or plane surfaces with no design 
 in high relief, a flat anode placed at some little distance may 
 suffice ; for reasons fully given on p. 89 the electrodes must not 
 be allowed to approximate too closely. But for surfaces which 
 are raised at any point, and which, therefore, produce deeply-cut 
 moulds, the anode-surface should be dished out into an approxi- 
 mate representation of the mean lines of the original object. It 
 may then be placed nearer to the cathode, and thus impose less 
 resistance in the circuit. But in dealing with statue-moulds, 
 the problem is more difficult. Lenoir meets it by using an anode 
 of thin platinum wire, bent backwards and forwards into a frame- 
 work or skeleton of the figure of course, of smaller size, so that 
 there shall be no danger of contact with the plumbagoed mould. 
 The mould is then built up around this (vide supra), and the 
 whole of the cavity is filled with the copper-solution ; the metal 
 is deposited, and the wire-skeleton is finally removed by with- 
 drawal through one of the cavities in the plaster. Plante used a 
 
THE TREATMENT OF STATUARY. 169 
 
 similar skeleton composed of perforated lead sheet, fashioned 
 roughly into the required shape, and this, being comparatively 
 inexpensive, was left within the statue when the process was 
 finished. When the statue is moulded in sections, there is, of 
 course, less difficulty in adapting a suitable anode. 
 
 The lead and platinum anodes do not dissolve in the solution ; 
 the strength of the latter must be kept up by adding crystals of 
 copper sulphate from time to time, as the copper which it 
 contains initially becomes exhausted. It is mainly for this 
 reason that apertures must be provided in the mould-walls for 
 the circulation of the liquid, which may be maintained by placing 
 a muslin bag or copper-wire box, containing crystals of copper 
 sulphate, above the head-aperture, as this produces a gradual 
 downward flow of heavy liquid containing fresh supplies of copper 
 salt. Another result of using insoluble anodes is that a current 
 with higher electro-motive force is necessary to effect the deposition 
 (see p. 29) ; and, again, when platinum is used, oxygen gas is 
 evolved, and for this reason the head-aperture must be at the 
 very highest point to allow the gas to escape, otherwise an 
 accumulation of gas forms at the summit of the figure, so that 
 the mould will not be in contact with the solution, and from that 
 time can there receive no further deposit of metal. The lead 
 anode, especially at first, combines with and thus absorbs a large 
 proportion of the oxygen to form lead peroxide; and, in pro- 
 portion as this is formed, less electro-motive force is required, 
 because the heat of the lead undergoing oxidation is a substitute 
 for that of the copper dissolving at the anode in ordinary electro- 
 typy. When copper anodes are used it is more than ever of im- 
 portance that they should be of the purest electrotype-copper, to 
 prevent the formation of insoluble mud, which would deposit 
 upon the interior surfaces of statue-moulds, and give rise to much 
 inconvenience 
 
 Care is needed to ensure that the anodes at no time short- 
 circuit the current, and stop the process by coming in contact 
 with the cathodes. This is especially liable to happen in electro- 
 typing statues or busts, because the slightest movement may 
 alter the relative positions of the surfaces inside the moulds, and 
 it is impossible to watch the progress of the deposition. Lenoir 
 has suggested that the outside wires of his platinum skeleton 
 should be encircled by an extended spiral of india-rubber 
 filament, which is an insulator ; but, although for a time this 
 would be successful, it is probable that by the gradual growth 
 of the deposit the precipitated copper might creep up to the 
 anode and effect contact at some point at which it happened 
 originally to approach the cathode too nearly. Such short-cir- 
 cuiting would, of course, be fatal to the deposition upon all the 
 moulds which might happen to be in the same circuit ; Lenoir, 
 therefore, introduced into the circuit of each individual mould 
 
170 ELECTROTYPING. 
 
 a short length of thin iron wire sufficient to carry the com- 
 paratively small current required for the electrolysis, but which 
 would heat, and almost immediately fuse, by reason of its high 
 electrical resistance, if subjected to the much stronger current 
 entailed by a short circuit. In this way the iron acts as a safety- 
 valve, automatically breaking the circuit in the branch in which 
 the accident has occurred, and restoring it to the remaining 
 electrotypes in the bath. It is, in fact, what is known as a fusible 
 cut-out, such as are now usually made of lead foil or wire for 
 electric-lighting circuits. The more modern form would answer 
 the same purpose, being made to melt and break the circuit as 
 soon as it is subjected to an undue intensity of current, the 
 thickness of the lead being determined by the strength of the 
 current which is to call it into use. 
 
 For this class of work the ammeter should always be em- 
 ployed ; it would not only indicate short-circuiting as soon as 
 it occurred by registering the greatly-increased current flowing 
 in the circuit, but it would also show whether the process was 
 taking its normal course. Any undue approach of the electrodes 
 would diminish the resistance and give rise to an increased 
 current, while any break or defect in the wires would be shown 
 by the diminished ampereage recorded by the instrument. It, 
 alone, affords an opportunity of judging of the progress of the 
 work within a closed mould. 
 
 Having decided upon the best arrangement of anodes, the 
 method of ensuring the most rapid covering of the mould 
 demands attention. When the matrix or mould is of metal, no 
 difficulty arises, because of its high conductivity; it is only 
 necessary to connect any part of the mould with the generator 
 to ensure an immediate deposition over the whole surface exposed. 
 But when connection is made between the battery and cathode at 
 only one point in a large non-conductive mould, a long time must 
 elapse before the deposit will spread to the more distant portions 
 of the plumbagoed surfaces ; but the deposit is more uniform, 
 and the result, therefore, more satisfactory, in proportion to the 
 rapidity with which the mould is initially covered with copper. 
 In order to convey the current to several parts of the mould at 
 once, light guiding-wires may sometimes be temporarily arranged 
 so that their points rest lightly on the plumbagoed surface ; these 
 act as so many nuclei or starting-points for the deposit, and, as 
 soon as the copper has spread from them arid covered the inter- 
 vening surfaces, they are no longer required and may be removed. 
 These guiding- wires are specially useful in carrying the deposit 
 into the deeper or under-cut portions of the mould, into which it 
 is often difficult to drive it at the outset, but which continue to 
 receive a deposit when once they have been covered by a better 
 conducting surface than the plumbago. The wires cannot be well 
 arranged in the internal cavities required for reproducing busts 
 
THE USE OF GUIDING-WIRES. 171 
 
 and statues, as they are liable to make contact with, and to 
 disturb the position of, the anodes. They may, however, be some- 
 times passed permanently through the walls of the mould itself by 
 adjusting them within the casting-box, so that their points rest 
 very lightly upon raised portions of the wax-model (preferably at 
 such points that they may not mar a flat surface on the finished 
 figure by any mark indicative of their position). The several 
 wires are collected into a bundle together outside the mould, 
 and are then connected with the negative wire from the battery. 
 The tip of the wire should be flush with the internal surface 
 of the plaster or composition in the finished matrix, and, being 
 black-leaded, will not adhere to the deposited metal. The deposit 
 may often be coaxed into a refractory corner by using a supple- 
 mentary anode of stout copper wire or thin sheet, which, being 
 connected with the battery (positive pole), is held temporarily 
 with its surface very close to, but not touching, the part to be 
 covered ; thus the local resistance of the solution is much 
 diminished and the deposit is readily started, and, when once 
 formed, will continue to increase without difficulty. 
 
 In all art-work of the description to which we have been latterly 
 referring, the conductive film must be of the finest quality in 
 order to transmit the current with the utmost rapidity from the 
 points of original contact to the remainder of the surface. The 
 silvered plumbago offers great advantages in this connection ; 
 and if ordinary plumbago be employed, only the best description 
 is permissible. 
 
 The finished electrotype generally presents a dirty appearance, 
 owing to the black-leaded surface with which it has been in con- 
 tact ; it may, however, be cleaned by rubbing with turpentine or 
 benzene, sometimes after a preliminary plunge into boiling oil. 
 
 Reproducing Natural Objects. Animal or vegetable objects 
 are often simply coated with a thin film of copper, and used in 
 this condition for ornamental purposes, an additional deposit of 
 gold or silver being added to that of the copper. To effect this, a 
 conductive surface is first formed upon the object to be coated. 
 Warm spirits of wine are shaken with crystals of silver nitrate 
 until no more of the solid is dissolved. The object is then painted 
 superficially with this solution, and placed under a glass bell-jar, 
 or clock-shade, together with a saucer containing a few drops of a 
 solution made by dissolving a small fragment of vitreous phos- 
 phorus in an ounce of carbon bisulphide. The vapour of phos- 
 phorus evolved reduces the silver nitrate to the metallic state, 
 and thus covers the whole surface of the object with a thin but 
 continuous conductive film of silver, and enables it to receive an 
 electro-deposit of copper of any desired thickness by merely sus- 
 pending it as the cathode in an acid copper-bath. The greatest 
 care is required in using this solution of phosphorus ; it is liable 
 to produce painful sores if it fall upon the skin of the operator and 
 
172 ELECTRO! YPING. 
 
 be not immediately washed off, and to cause spontaneous ignition, 
 even after the lapse of a considerable time, if it evaporate in con- 
 tact with organic fabrics. 
 
 Instead of merely covering the object with a film of copper, it 
 may be reproduced by taking a mould in suitable material, black- 
 leading its internal surface, and electrotyping it in the manner of 
 statues or busts. The copying of any insect or leaf becomes thus 
 a question of moulding. 
 
 Many other applications of electro-metallurgy in connection 
 with copper are used, especially in connection with the multi- 
 plication of drawings and designs : among processes of this kind 
 that have been proposed may be mentioned 
 
 Glyptography, which requires a flat copper plate to be either 
 coated with two layers of composition, one black, the other white, 
 or, preferably, to be itself rendered black by exposure to ammonium 
 sulphide solution ; it is then coated with a white material. The 
 required design is scratched through the wax until the black 
 surface of the copper is visible ; when the drawing is complete 
 which is readily seen, because the lines appear black upon a white 
 ground the whole surface is coated with plumbago and electro- 
 typed to a thickness of about the -^ of an inch. This is supported 
 with backing-material, like an ordinary electrotype-block, and is 
 ready for printing in the typographical printing-press, the lines of 
 the drawing being, of course, in relief upon a flat surface in the 
 finished plate. 
 
 Stylography is somewhat similar as to the mechanical part of 
 the process. The copper plate being covered with a mixture of 
 67 per cent, of shellac and 33 of stearine, with sufficient lamp- 
 black to render it black, is varnished and sprinkled lightly with 
 silver dust. The latter is then removed along the lines of the 
 intended design until the black composition is seen beneath : 
 the whole is then plumbagoed and electrolytically coated with 
 copper. The lines are not sufficiently raised for ordinary type- 
 printing ; a second plate must, therefore, be taken from the first, 
 reproducing the etched lines of the original, and this is used for 
 printing as from an engraved surface. 
 
 Galvanography consists in building up a picture in coloured 
 varnish, the gradation of light and shade being given by varying 
 the thickness of this film. After black-leading, the surface is 
 coppered, and, being cleaned with oil of turpentine, is used like 
 the last as an engraved plate. 
 
 It is obvious that any of the photo-mechanical printing pro- 
 cesses of the present day, in which the printing surfaces in relief 
 are obtained from photographic reproductions of any drawing or 
 suitable object, may also be aided by the sister art of electro- 
 typy : the electrolytic part of the process is practically the same 
 in all, and does not differ from those already instanced, so that 
 it is unnecessary to describe any of them in further detail. 
 
ELECTROLYTIC ETCHING. 173 
 
 Electrolytic Etching. By the reverse of these processes the 
 same result is attained. The design is traced on the waxed 
 surface of a copper plate, taking care that etching-tools lay bare 
 the metal in all the lines. The plate is now introduced into the 
 copper-vat in connection with the anode wire instead of the 
 cathode, and the copper dissolves at all places where it is exposed 
 to the action of the solution ; but since the whole plate is in- 
 sulated with the exception of the lines of the etching, it follows 
 that along these only the copper is attacked. The lines may 
 thus be bitten-in to any required depth ; the depth may be 
 determined at the will of the operator by adjusting the distance 
 between the electrodes, the points of nearest approach being 
 those which receive the deepest cut. The resulting plate is used 
 precisely as an ordinary etched copper plate ; and, indeed, it is 
 such, the processes employed to produce them being identical 
 except in the method of biting-in the lines. To obtain anything 
 but crude results, however, by these processes demands much 
 experience and attention, as it is frequently necessary to stop-out 
 some of the finer lines to prevent further action at different 
 periods of the process, and practice and artistic skill alone can 
 guide the operator in this matter. 
 
 An ingenious process for obtaining nature-prints of leaves and 
 similar bodies has sometimes been used. The leaf is placed 
 between two plates, one of polished steel, the other of soft lead, 
 and is then passed between rollers which exert a considerable 
 pressure. The leaf thus imparts an exact impression of itself, 
 and of all its veins and markings, to the surface of the lead; 
 and this impression may be electrotyped and the produced copper 
 plate used for printing in the ordinary way. 
 
 The subject of reducing copper from its ores and refining the 
 crude metal will be dealt with in Chapter XV. 
 
 Manufacture of Reflectors. The increasing use of powerful 
 electric search-lights for naval and military purposes has led to a 
 demand for a substitute for the heavy, costly and frangible glass 
 lenses hitherto employed. The substitute patented by Cowper- 
 Coles J is interesting, not only as an example of a special process 
 of copper deposition, but as illustrating a successful application 
 of electro-metallurgy in a new field. Instead of being refracted 
 through a glass lens, the rays of light are rendered parallel by 
 reflection from an accurately formed parabolic reflector. 
 
 To accomplish this, a convex glass matrix is prepared of such 
 shape that its convexity would exactly fit into the concavity of 
 the reflector required. This matrix may be made mathemati- 
 cally true by grinding and polishing, and is ready to serve as the 
 mould for a whole series of reflectors. It is first coated with 
 silver by immersion for half an hour, face downwards, in a silver- 
 ing liquid, made up of equal quantities of a 0'5 per cent, solution 
 1 Journ. Inst. of Electrical Engineers^ 1898, vol. xxvii. p. 99. 
 
174 ELECTROTYPING. 
 
 of silver nitrate, a 0*5 per cent, solution of caustic potash, and a 
 0*25 per cent, solution of glucose. The film of silver is then 
 burnished with cotton-wool and chemically precipitated peroxide 
 of iron, and the mould, which is handled by means of a ' sucker ' 
 attached to the reverse side, is clamped to a metal ring, through 
 which connection is made between the silver film and the cathode 
 wires. The ring is supported horizontally by a rotating frame, 
 so that the mould is plunged face downwards in the electrolytic 
 tank containing an acid copper-sulphate bath (copper-sulphate 
 13, sulphuric acid 3, water 84. per cent.) and suitable anodes. 
 The frame is so attached to its support that the mould may be 
 temporarily tilted, in order that the periphery of the silver shall 
 be immersed before the centre portion. Here it is rotated at 
 first with an applied E.M.F. of 9 volts, and then with a 
 current-density of 19 amperes per square foot, until the silver 
 is well covered with a layer of conducting copper. The edges 
 of the mould are then stopped off by contact with a ring, which 
 prevents deposition on the copper beyond it, and so gives a 
 sharply-defined border to the reflector. When the layer of 
 copper is sufficiently thick, the mould with the reflector adhering 
 to it is removed from the bath, and placed in cold water, the 
 temperature of which is gradually raised to 120 F., whereupon 
 the difference in the expansion causes the separation of the 
 copper reflector, which is then electro-plated on the face with 
 palladium for use, leaving the mould ready for a fresh coating of 
 silver and popper. Palladium chloride is used for the electro- 
 lyte ; for the deposition a carbon plate, with the edge shaped 
 to the same curvature as the mirror, is used as an anode, and this 
 is oscillated during the process so as to ensure even deposition 
 and agitation of the electrolyte. 
 
CHAPTER IX. 
 
 THE ELECTRO-DEPOSITION OP SILVER. 
 
 THE electro-plating of articles with silver was one of the earliest 
 applications of electrolysis, because it produced a material 
 analogous to, but cheaper than, the older 'silver-plate,' in 
 which the base metal was covered mechanically with a layer of 
 silver ; and even at the present day, when electrolysis is used to 
 obtain coatings of so many different metals for such varied pur- 
 poses, the deposition of silver must, perhaps, take the foremost 
 place, both in respect of universality of practice and value of 
 results. 
 
 DEPOSITION BY SIMPLE IMMERSION, OR WHITENING. 
 
 Silver, as compared with most metals, is very electro-negative, 
 and hence all the base metals are capable of exchanging places 
 with it when dipped into a solution of one of its salts. This 
 process is, however, used only to impart the thinnest possible 
 wash of silver, more especially to small articles such as nails and 
 hooks ; so thin, indeed, is the film that the name whitening is 
 thoroughly descriptive of the process. It is clear that the coat 
 of silver can be but of the thinnest, because, as soon as the metal 
 is covered with the slightest covering of silver, it becomes pro- 
 tected, partially at least, from further contact with the solution. 
 
 There are two principal methods of silvering by simple im- 
 mersion first, by dipping the article into a solution of silver, 
 either hot or cold ; second, by rubbing a semi-solid paste of a 
 silver compound over the surface of the object. For both pro- 
 cesses the objects must be clean, and must present bright metallic 
 surfaces to the action of the depositing compound. Formulae for 
 making-up such silver mixtures are numerous ; those principally 
 used are included in the following table, in which they are arranged 
 under the respective class-headings of solutions and pastes. 
 
 Immersion Solutions. It is obvious that since the deposition 
 of the silver is due (and is also proportional) to the amount of the 
 base-metal which dissolves from the object under treatment, the 
 solution gradually becomes exhausted of the former and con- 
 
 175 
 
176 
 
 ELECTRO-DEPOSITION OF SILVER. 
 
 S 
 1 
 
 5zs 
 
 i 
 
 Special Method of Preparation. 
 
 Dissolve 1 in 14 and add to rest. 
 Filter. 
 
 Pour 2, in 100 of 17, into 3 in 
 900 of 17. 
 Dissolve 5 and 9 in 17, boil, and 
 addl. 
 
 Add enough 17 to make paste. 
 
 Prepare as required, a'nd apply 
 with rag. 
 
 Mix 1, 3, and 17, add enough 16 
 to make paste. 
 Add enough 17 to make paste. 
 
 * 5 
 
 i 
 
 Water. 
 
 1 |s.| 1 || 
 
 s.3! 1 1 S 
 
 S 
 
 Levigated Chalk. 
 
 : : : : : : : 
 
 : :g : : : 
 
 M to 
 
 Ammonium 
 
 
 .0 
 
 i ^ 
 
 Chloride. 
 
 
 
 -<M 
 
 * * 
 
 Ammonia. 
 
 S : : : : : : 
 
 : : : : : : : 
 
 CO 
 
 S ^ 
 
 g Sodium 
 ^ Thiosulphate. 
 
 
 :|S : : : : 
 
 w . 
 
 S rH 
 
 M 
 
 q Sodium Chloride. 
 
 IS :S : : : S 
 
 QO . . . 
 * 
 
 a - 
 
 & Sodium Carbonate. 
 
 CO 
 
 : : : : : : : 
 
 * s 
 
 f^ Alum. 
 
 : : : : : : : 
 
 : : : : : : 
 
 02 g 
 
 Potassium 
 
 o 
 
 
 S 
 
 S Ferrocyanide. 
 
 : : : : ^j : : 
 
 
 
 ^5 
 
 Potassium 
 Bitartrate. 
 
 : :S : : : 
 
 " : : | : : => 
 
 CQ ta 
 
 ! " 
 
 Potassium 
 g Binoxalate. 
 
 
 ::: | : : : 
 
 O . 
 
 g Potas. Bicarbonate. 
 
 : : : : : : 
 
 : : : : : : : 
 
 H 
 
 ^ Potas. Carbonate. 
 
 : : : i : : 
 
 
 ^ 
 
 """ Caustic Potash. 
 
 : : : : : g : 
 
 : : : : : : : 
 
 CO 
 
 Potassium Cyanide. 
 
 g S :S : 3 : 
 
 : = : 1 " : 
 
 S 
 
 Silver Nitrate. 
 
 : S :S : -* : 
 
 : :S : : M : 
 
 H 
 
 o ^ 
 
 }z; 
 
 Silver Chloride. 
 
 ! -: : 
 
 CQ 
 
 S 
 
 11 ll 
 
 3 0> oS 
 
 bo to 
 .S -g .S * 
 
 
 1 
 
 h-H 
 S 
 
 1 
 
 <j 
 
 3 
 
 1 : :::}?: 
 S IS, 
 
 ^ ft 
 
 M S 
 
 cc ftO ft 
 m ''' ' '<^S 
 
 M 
 
 H 
 
 | 
 
 ^ ;s ; ; 
 
 |j| = |J 
 
 
 I 
 
 r-4 (NCO-* 10 COI> 
 
 OOOSO r-j 0^ CO 2J 
 
IMMERSION SOLUTIONS. 177 
 
 taminated with the latter ; and if the articles are of copper or brass, 
 as they most frequently are, the fact of the contamination, and in 
 some degree its extent, are rendered apparent by the blue colour 
 imparted to the solution by the dissolved copper. Most of the 
 liquids are used hot, and a momentary dip suffices to effect the 
 required purpose. One of the best is No. 3 (Roseleur's), prepared 
 by making up into a paste 1 ounce of silver chloride with 4 
 pounds each of powdered potassium bitartrate (cream of tartar) 
 and sodium chloride (common salt), then adding a proportion of 
 this to boiling water, contained in a copper vessel, immediately 
 before it is required for use. The articles, held in a copper sieve 
 or porcelain colander, are plunged into the solution, where they 
 become coated instantaneously ; but for the sake of security they 
 should be stirred around with a piece of wood or with a porcelain 
 or glass rod ; they may then be removed, thoroughly washed by 
 rinsing in two or three vats of water, and dried in hot boxwood 
 sawdust. As this bath works best when old, and consequently 
 highly charged with copper, care must be taken that no pieces of 
 iron or zinc or other very electro-positive metal be clinging to the 
 goods, or a certain proportion of copper will be deposited with 
 the silver, which will in consequence acquire a pinkish coloration. 
 
 Cyanide solutions may be made to give a good whitening effect, 
 as indeed may any of those specified in the above table. An 
 interesting process of Roseleur's is not included in this table ; the 
 liquid is prepared by slowly adding a solution of silver nitrate to 
 one of sodium bisulphite, until the precipitate, which forms upon 
 admixture, begins to dissolve but slowly in the solution on shak- 
 ing. The copper or brass objects are dipped into the bath, cold, 
 and immediately become covered with silver by simple exchange ; 
 but after this, unlike the behaviour of other solutions, the film 
 continues to increase in thickness, not, however, on account of 
 any further solution of the base-metal, but owing to a chemical 
 action inherent in the bath itself, which causes the deposition of 
 the silver, not only on the metallic objects immersed, but even 
 on the walls of the bath, on glass, or on any substance introduced. 
 This is due to the ready decomposability of the silver salt em- 
 ployed, and to the tendency of surphurous acid to absorb oxygen, 
 which it does at the expense of a portion of the silver oxide, 
 depositing an amount of silver corresponding to that of the oxygen 
 used up. This reaction occurs but slowly in the cold, so that 
 there is time for a gradual building up of the silver into a 
 coherent and adhesive deposit. If the liquid be heated, the action 
 becomes too rapid, and the quality of the coat suffers accordingly. 
 
 Pastes. The use of pastes is especially applicable to the 
 wash-silvering of comparatively large and flat surfaces, such as 
 the dials of barometers, and for the application of local deposits, 
 or even of preliminary protective films to bodies which are subse- 
 quently to be plated with an electro-negative metal. The simplest 
 
 12 
 
178 ELECTRO-DEPOSITION OF SILVER. 
 
 paste is that made by rubbing together 1 part of silver chloride 
 with 2 or 3 parts of potassium bitartrate (cream of tartar) until 
 they are in a condition of the finest powder, and then working 
 the mixture into a creamy paste by the addition of water. Many 
 operators vary these proportions, or add other ingredients, but 
 the mixture, as it stands, will be found to give excellent results. 
 Roseleur's paste for silvering lamp-reflectors (No. 12 on above 
 list) is rubbed on to the surface with a wad of soft rag, allowed 
 to dry in situ, and is then rapidly removed with a fresh piece of 
 soft linen. 
 
 In applying the pastes generally, a piece of soft cork or a pad 
 of wash-leather may conveniently be employed. Many of the so- 
 called * plate-restoring powders ' used for restoring a white colour 
 to worn electro-plate, which shows the brass foundation in places, 
 consists of one or other of these mixtures or of modifications of 
 them. Occasionally plate-powders containing mercury are sold ; 
 they are, however, fraudulent, for they purport to give a film of 
 silver to the discoloured object, but instead impart one of the less 
 expensive mercury, which is in every way to be condemned, for 
 not only is the mercury itself objectionable, but it is gradually 
 absorbed by the base-metal, leaving the surface dull, while re- 
 peated applications cause the object to become brittle and useless. 
 
 The thickness of silver on whitened goods is usually so infini- 
 tesimal that they will not bear scratch-brushing or any of the 
 ordinary methods of polishing ; but friction by contact with dry 
 sawdust in a rotating barrel may be satisfactorily substituted. 
 
 SINGLE-CELL PROCESS. 
 
 This process is not largely used for silver-deposition, and is 
 quite unsuitable to establishments where there is much work in 
 hand. It may, however, be effected by using an ordinary cyanide 
 plating-solution, containing a porous cell with a zinc rod or plate 
 immersed in potassium cyanide solution, with the usual connec- 
 tions between the zinc and the objects which are being coated in 
 the outer cell. Steele prepared a solution for single-cell work by 
 converting 1 part of silver into silver chloride, washing and dis- 
 solving it in 60 parts of water, in which was also placed the 
 mass resulting from the fusion of 6 parts of potassium ferro- 
 cyanide with 3 of potassium carbonate. No porous cell was used ; 
 the object to be plated was simply connected with a plate of zinc, 
 and both together were plunged into the prepared solution. In 
 a similar manner articles immersed in hot silver-baths have been 
 sometimes treated by simply binding zinc wire around them, so 
 that a greater thickness of deposit would be given than that impar- 
 tible by simple immersion. The separate-current process may be 
 said to be universally applied to electro-silvering, as the plant 
 may be made of any size, and the process is under perfect control. 
 
SEPARATE-CURRENT PROCESS. 179 
 
 THE SEPARATE-CURRENT PROCESS. 
 
 In working silver solutions with a battery or dynamo- electric 
 machine, the solutions must be well watched, and the current 
 prevented from becoming excessive, as a good fine-grained 
 minutely-crystalline deposit can never be yielded with a high 
 current-density. Resistance-coils should, therefore, be at hand, 
 or some other suitable means of regulating the current under all 
 conditions of the bath, and under all dispositions of the electrodes 
 within it. 
 
 The Battery. The Smee- or Daniell-cells are, perhaps, to be 
 most recommended ; the former being arranged in groups of two 
 in series, when more than one cell is employed, so that the electro- 
 motive force may be twice that given by a single pair of the 
 plates. A single Daniell-element gives an electro-motive force 
 very suitable to the work (1 volt), and if several cells are used 
 they should be placed in parallel. Some operators prefer the 
 original copper-zinc cell, probably because its prime cost is less 
 than that of Smee's, owing to the absence of platinised silver. It 
 is, however, less effective, and becomes very badly polarised as 
 soon as its action commences ; but, by using a number of couples 
 and plates of large size, it is quite possible to obtain excellent 
 results with it. If a dynamo be used, it should have a very low 
 electro-motive force, because it is less convenient to arrange the 
 silver-baths or the individual electrodes in each series-fashion, 
 than it is in the electrotype-copper vats ; the current, moreover, 
 must be under absolute control by the use of measuring-apparatus 
 and resistance. 
 
 The Solution. Most of the solutions used largely in practice 
 have the double cyanide of silver and potassium for their basis ; 
 and doubtless the solution of this body in a liquid containing an 
 excess of potassium cyanide constitutes the simplest and best 
 plating-bath for general work. The composition of the principal 
 mixtures suggested is embodied in the following table. 
 
 Silver-Baths. The silver-baths are generally prepared, as re- 
 quired, by dissolving metallic silver in nitric acid, precipitating 
 it with potassium cyanide, washing thoroughly, and dissolving 
 it in excess of the potassium salt. Ten parts of pure silver yield 
 12 '4 parts of pure silver cyanide. The water and all the chemicals 
 used in preparing the solutions must be pure, as the presence of 
 much foreign matter acts injuriously upon the deposit; the 
 potassium cyanide especially should be examined, as it frequently 
 contains only 50 or 60 per cent, of the pure salt (see p. 386). 
 For a like reason it is better to prepare the silver cyanide 
 separately, and to wash it thoroughly, before mixing it with the 
 remaining ingredients of the solution ; by simply adding potas- 
 sium cyanide in excess to the nitrate or chloride of silver, a clear 
 bath is prepared, but it contains, in addition to the silver cyanide, 
 
180 
 
 ELECTRO-DEPOSITION OF SILVER. 
 
 
 Special Method of Preparation, etc. 
 
 Dissolve 4 in 250 of 12 ; add 7, and then 
 10 in 750 of 12. 
 Dissolve 4 in part of 12, 7 in rest of 12; 
 mix. 
 
 (Use 3 to 10 Smee-cells in series.) 
 ( 3 to 10 ,, parallel.) 
 
 Dissolve 4 in 500 of 12 ; and 7 in rest ; 
 mix. Filter, if necessary. 
 
 Dissolve 7 in 12, and 2 in mixture. 
 
 | (Rogers Plating Co., U.S.A.) 
 
 jfMeriteiis Plating Co., U.S.A.) 
 
 Mix 6 in 12 ; add just sufficient 7 to dis- 
 solve 6, and then slight excess. 
 (Requires no quicking.) 
 
 Dissolve 1 (freshly precipitated from 7 
 Ag.) in 11 ; add rest of ingredients. 
 
 Dissolve 4 ; add 8 ; use weak current. 
 
 Water. 
 
 8 | j 
 
 Hill || 
 
 | | || 
 
 
 
 
 
 . 
 
 
 
 . 
 
 
 
 
 * 
 
 Sodium 
 
 CD 
 
 
 
 Chloride. 
 
 
 
 
 Sodium 
 
 
 10 
 
 
 EH Carbonate. 
 
 
 
 
 & Potassium 
 
 
 
 
 5 Iodide. 
 
 
 
 
 P3 Potassium 
 c5 Cyanide 
 % (95 %) 
 
 S ; 
 
 H O O if) 34 ,_( ud 
 
 ,008 888' 
 
 ^ fcfcji**^ =0 
 
 $y* Ss* $y* 
 
 Silver 
 
 
 
 
 S Carbonate. 
 
 
 
 
 H Silver 
 
 
 ^ 
 
 
 jg? Oxide. 
 
 
 -CO ' 
 
 
 fc Silver 
 
 50 33 
 
 
 ^ 
 
 Nitrate. 
 
 IO rH 
 
 
 CO 
 
 2 Silver 
 
 
 
 <N 
 
 Iodide. 
 
 " 
 
 
 lO 
 
 Silver 
 Cyanide. 
 
 
 KjCOO CNCN 
 
 m^, ^10 :::::: 
 
 M 
 
 Silver 
 
 . 
 
 
 
 io -T 3 . . . eo co r 1 . 
 
 Chloride. 
 
 
 
 c > S 
 
 ill 
 
 l|5 
 
 to pi's 
 M| 
 
 Cast-iron 
 
 f| 
 
 :|1:: :: 
 
 ^ 
 
 5 : |S| : i|^| : 1 : 
 
 .g 
 
 
 .""T' . . . . 
 
 . .""T'^r' . . Xl ^r^ . . . 
 
 
 
 H 
 
 h 
 
 I 
 
 ii, 
 
 .' 'll =1 
 
 ?fc * "t^ " "s> "-S 
 
 >*^ .P ? CS3 
 
 d 
 
 
 o * m co i-- oo 
 
 OJ O -H (M CO * 10 CO t^ 00 
 
 5 
 
 
 
 r-l rH rH r-l r-l rH I-H rH r-l 
 
ELECTRO-SILVERING BATHS. 181 
 
 a quantity of the potassium salt corresponding to the silver 
 compound used, and this is generally objectionable. 1 Thus, for 
 example, on adding potassium cyanide to silver chloride, the silver 
 cyanide is formed which is required for plating, but with it is an 
 equivalent of potassium chloride produced by exchange. 
 
 AgCl + KCN = KCL + AgCN. 
 
 Silver chloride. Potass, cyanide. Potass, chloride. Silver cyanide. 
 
 Again, the proportion of potassium cyanide to silver in the 
 bath, although variable between wide limits, is by no means an 
 indeterminate quantity. Having produced the insoluble silver 
 cyanide, as in the above equation, by the use of one equivalent of 
 potassium cyanide, a second equivalent of the latter is necessary 
 to form the double cyanide of silver and potassium, which alone 
 is soluble in the bath ; and in addition to this an extra proportion 
 of the potassium salt (free cyanide) must be employed to ensure 
 the perfect solution of the anodes, for a reason which may be 
 stated as follows : In passing the electric current through a solu- 
 tion of silver-potassium cyanide KAg(CN) 2 , the double cyanide is 
 broken up into the cation, K, and the anion, Ag(CN) 2 . The 
 potassium is deposited at the cathode, but, by chemical exchange, 
 displaces from the surrounding solution of the double cyanide 
 an equivalent of silver (which deposits on the cathode) and 
 forms potassium cyanide in the liquid [thus : KAg(CN) 2 + K = 
 Ag + 2KCN]. Thus there is a concentration of potassium 
 cyanide around the objects which are being plated. The silver 
 travels in the anion, Ag(CN) 2 , to the anode, where the ion decom- 
 poses into AgCN + ON, and the cyanogen (CN) set free attacks the 
 silver anode to form another molecule of AgCN. Hence at the 
 anode there is formed a double quantity of the insoluble compound 
 AgCN. It is therefore necessary that a good excess of free 
 potassium cyanide be present to dissolve the silver cyanide, and 
 prevent incrustation on the anode. The following two equations 
 show the requirement of the minimum two equivalents of potas- 
 sium cyanide : 
 
 1. AgN0 3 + KCN = AgCN + KN0 3 (washed away). 
 = KAg(CN) 2 . 
 
 Thus 108 parts of silver, or 108 + 14 + 48 (AgN0 3 ) = 170 of 
 silver nitrate, require 2(39 + 12 + 14)= 130 parts of potassium 
 cyanide (two equivalents = 2 CKN) ; while 108 + 12 + 14 = 134 
 parts of the pure silver cyanide (AgCN) require one equivalent, 
 or 39 + 12 + 14 = 65 parts of the potassium salt, to form the 
 double compound. Thus 108 parts of silver require a minimum 
 of 130 parts of potassium cyanide, and should have, in addition, 
 at least 50 to 75 per cent, extra cyanide, supposing the latter to 
 
 1 Langbein states that experiments show the same results with AgCl as 
 with AgCN. The resistance is less with AgCl, but the bath should be 
 refreshed with AgCN to prevent thick solution and coarse deposit. 
 
182 ELECTRO-DEPOSITION OF SILVER. 
 
 be pure \ when the commercial salt, containing (say) from 50 to 
 70 of pure KCN, is employed, the minimum would range from 
 180 to 250 parts, and the added quantity from 70 to 150. So 
 also 134 parts of silver cyanide call for 65 of the pure potassium 
 cyanide as the minimum allowance. 
 
 But, although a certain amount of free cyanide is necessary, a 
 great excess must be avoided, because it would dissolve the anode 
 too freely and increase the strength of the bath, and, worse than 
 that, would tend to produce a somewhat scaly and non-adhesive 
 deposit upon the cathode. 
 
 The condition of the bath in respect of free cyanide may be 
 readily tested by withdrawing a little of the liquid in a glass 
 vessel and adding to it a few drops of silver nitrate solution ; a 
 precipitate is thus produced which should at once redissolve in 
 the liquid. If it dissolve but slowly even on stirring, it is an in- 
 dication of a deficiency of cyanide, and the time that elapses before 
 it vanishes completely affords a rough gauge of the amount of free 
 cyanide present. In practice the appearance of the anodes is 
 itself indicative of the condition of the bath ; if they are covered 
 with a black deposit during the passage of the current, there is 
 insufficient cyanide present, while if they are quite bright and 
 white, the cyanide is in excess. The best results are obtained 
 when the anodes present a greyish appearance while the current 
 passes, but immediately become white and brilliant when it ceases, 
 showing a complete solution of the thin film upon the surface. A 
 further precaution which may be taken as a check upon the work- 
 ing of the bath is occasionally to weigh the electrodes separately 
 both before and after the process ; the loss of weight shown by 
 the anode at its second weighing should be just balanced by the 
 gain upon the plated objects or cathodes. Any departure from 
 this equilibrium indicates either an incorrect ratio of anode to 
 cathode surface, or a wrong proportion of cyanide in the bath ; or, 
 thirdly, an unsatisfactory adjustment of the one to the other. 
 But if the surfaces of the electrodes are well arranged (see further 
 on, under anodes), a loss of anode-weight unaccounted for by the 
 increase in cathode-weight clearly points to an excess of cyanide 
 in the bath, or vice versa. Such an abnormal action brings about 
 an alteration in the character of the bath, and must be rectified 
 by the addition of cyanide of silver or potassium, as the case may 
 be. Another very useful test may be made by dipping a strip of 
 bright copper into the bath ; if it become coated with silver by 
 simple immersion, the liquid contains too much cyanide, and the 
 deposit yielded by it will be bad, especially upon copper articles, 
 or upon coppered objects. 
 
 Baths are also liable to alteration by exposure to the air, 
 gradually absorbing carbonic acid, which takes the place of an 
 equivalent of hydrocyanic acid in the bath, so that a certain pro- 
 portion of the cyanide is removed, and the electrical resistance 
 
ELECTRO- SILVERING BATHS. 183 
 
 of the solution is increased. Fresh cyanide must, therefore, be 
 added from time to time, and these frequent additions, coupled 
 with the gradual accumulation of other substances dissolved 
 from impure anodes, or from the cathodes, give rise to a corre- 
 sponding increase in the density of the solution. Accompanying 
 the increased density is a greater sluggishness of the solution, 
 and hence a greater tendency to separate into layers during 
 electrolysis, the heavy silver-laden liquid from the anode sinking 
 to the bottom of the vat and accumulating there, while the 
 lighter potassium cyanide finds its way to the top. Thus the 
 electrolytic action becomes irregular ; a greater quantity of silver 
 is deposited upon the lower portions of the cathodes while the 
 coating upon the upper part is not increased, but may even be 
 dissolved, after the manner described on p. 90. A similar 
 action is observed in working concentrated, and, therefore, 
 sluggish baths, with an exceedingly weak current from a battery 
 which has 'run down.' Under these circumstances the anode 
 is immersed in a liquid saturated with silver, the cathode in one 
 containing little silver but much free cyanide, and an opposing 
 electro-motive force is thus set up which tends to re-dissolve the 
 deposit. Nevertheless, a moderately (one or two years) old 
 solution will be generally found to give a better deposit than 
 one newly made up, provided that the ratio of free cyanide to 
 silver be rightly maintained; hence a certain proportion of an 
 old plating-liquid is commonly used in making-up a new bath. 
 When this is impracticable, the effect of age may be imitated by 
 boiling the liquid for two or three hours, or by the addition of a 
 few drops of ammonia solution. The evils attending excessive 
 concentration may be remedied by appropriate dilution, except 
 when the extreme density is due to the accumulation of foreign 
 matter; or, within certain limits, by maintaining the cathode- 
 objects in gentle motion, which exposes changing surfaces to the 
 liquid, and also prevents the separation of the latter into layers ; 
 or, thirdly, by stirring the liquid well every night after work is 
 over. Even a concentrated solution, however, may conduct well, 
 and may be made to give a good deposit ; but a dilute bath has 
 a lower conductivity and is, therefore, more tardy in action 
 while the metal will have a characteristic dead-white lustre. The 
 specific gravity of the solution should lie between 1*05 and 1*10, 
 pure water being regarded as unity ; Gore lays down the limits 
 between which a good deposit is attainable as 1*036 and 1*116. 
 But the dissolved bodies are not the only impurities which find 
 their way into the solution ; insoluble matter from the anodes and 
 dust from the air gradually collect, and when the liquid is kept 
 well stirred, remain in suspension in it, and, becoming entangled 
 in the precipitating silver, produce an uneven deposit. It is, 
 therefore, advisable to filter the solution through blotting-paper 
 from time to time as required. 
 
184 ELECTRO-DEPOSITION OF SILVER. 
 
 The worst enemy of the plating-solution is organic matter : 
 little by little it accumulates, and, although exerting no pre- 
 judicial influence in small quantities, it is fatal to successful 
 work when a certain limit is reached. The cyanide solution, 
 most unfortunately, is capable of readily dissolving many organic 
 bodies, and the most jealous examination must be made of all 
 objects which are to be introduced into it. Gutta-percha in any 
 form is especially to be avoided \ moulds or stopping-out varnishes 
 containing this body should, therefore, be excluded when silver- 
 plating is to be effected in the cyanide bath. 
 
 The cyanide solution is generally used cold ; for coating small 
 articles, however, or objects made of iron, tin, zinc, or lead, upon 
 which a film of copper has been first deposited, it is occasionally 
 heated. 
 
 The bath may be prepared electrolytically, although it is rarely 
 so treated on a large scale, by dissolving \\ to 2 oz. of pure 
 potassium cyanide in a gallon of distilled or rain water, and passing 
 a current through it from a large weighed silver anode to a small 
 silver or platinum cathode, the weight of which also should be 
 known, until the excess of silver dissolved into the bath from the 
 former over that deposited upon the latter shows the bath to be 
 sufficiently charged with the precious metal. For this purpose 
 the electrodes are removed from time to time, rinsed, dried, and 
 weighed, until at last the desired strength of solution is reached. 
 Here the large anode is used with a small cathode that the action 
 of the current upon them respectively may be disproportionate ; it 
 is required to add to the weight of silver in solution, so that the 
 case is different from that of an electro-plating bath, in which the 
 solution is to be maintained of uniform density, and which, there- 
 fore, demands approximate equality of electrode-surfaces. 
 
 There is no doubt that the cyanide solution possesses many 
 advantages over other possible baths, and with ordinary care will 
 give but little trouble. The main objection to its use is its highly 
 poisonous character, which always involves risk to the operator, 
 and which renders the atmosphere unwholesome, even in fairly- 
 ventilated rooms. Many attempts have been made to substitute 
 safer solutions, but in no case yet with sufficient success to pro- 
 claim the introduction of a serious rival to the cyanide bath. 
 One inventor has used a silver salt dissolved in thiosulphate 
 solution ; but this bath, although it is said to give good results, 
 gradually decomposes, especially on exposure to light, and, 
 becoming brown at first, gradually deposits its silver in the form 
 of a black sulphide. Zinin has more recently found that the 
 solution of silver iodide in potassium iodide (No. 18 in the table 
 of solutions) could give very good results with a small current of 
 low electro-motive force. He recommends it especially for the 
 production of thick deposits, as in electrotypy. One practical 
 objection to its use is, of course, the high price of the iodides. 
 
ELECTRO-SILVERING BATHS. 185 
 
 This is not a fatal objection, if the process be a good one, but is 
 certainly adverse to its general adoption. 
 
 The metal obtained from the solutions named has a frosted 
 appearance, due to its being built up of an incalculable number 
 of minute crystals, the facets of which disperse the rays of light 
 falling upon them, instead of reflecting them uniformly ; but the 
 slightest friction upon the surface suffices to unite the crystals 
 into an even plane, which reflects the light perfectly, arid has the 
 lustre of polished silver. 
 
 It was early found, however, that the presence of a minute 
 quantity of carbon bisulphide in the plating-vat caused the pre- 
 cipitation of the metal in the bright condition, and although the 
 use of the bright plating-solution entails greater difficulties than 
 are met with in the ordinary processes, yet it is largely used for 
 certain classes of work ; for example, in those to which it is not 
 easy to apply friction, such as those carrying remote or sharp 
 angles or interior surfaces. In no case, however, is the whole 
 plating-process conducted in the brightening-vat, but the bright 
 deposit is given finally, when an almost sufficient weight of silver 
 has been deposited in the usual way. 
 
 Of all the substances which have been recommended as 
 brightening agents for the silver-bath (and these include, inter 
 alia, silver sulphide, collodion, a solution of iodine and gutta- 
 percha in chloroform, chloride of carbon, and chloride of sulphur), 
 the only reagent practically employed is carbon bisulphide. A 
 mere trace of this suffices to effect its object, while a slight excess 
 produces spotted deposits and brown stains, probably of silver 
 sulphide, and a large excess overshoots the mark altogether, and 
 often gives a dead-white film. 
 
 To prepare the bright-bath, place a quart of an old plating- 
 solution in a large bottle (a Winchester quart bottle, for instance) 
 and add to it 3 oz. of carbon bisulphide with, or without, 
 1 or 2 oz. of ether, shake vigorously for a minute, and add 
 1J pints more of the old solution. Again agitate thoroughly, 
 and allow it to stand for two or three days ; there is, at most, 
 ^5 of an ounce of carbon bisulphide in each ounce of this 
 liquid. A separate plating-bath must be used for brightening ; 
 then every night, after the day's work is done, 1 oz. of the 
 mixture just described is added to every 10 gallons of an ordinary 
 plating-solution, specially devoted to this class of work and con- 
 tained in the special vat. An ounce of old plating-liquid may be 
 added to the mixture in the Winchester bottle in place of that 
 removed. This quantity (1 oz. per 10 gallons) is the maximum 
 amount of brightening-mixture permissible. If the required effect 
 can be produced with less, so much the better ; but on no account 
 should a larger proportion be used, as the risk of spoiling the 
 whole bath would amount almost to a certainty. 
 
 The bright-bath requires a stronger current than the ordinary 
 
186 ELECTRO-DEPOSITION OF SILVER. 
 
 solution, and it deposits a harder metal, the bright film beginning 
 to show itself at the bottom and gradually extending upwards. 
 Any disturbance of the solution during electrolysis may cause it 
 to yield a dull deposit; a whole batch of objects should, there- 
 fore, be prepared for immersion simultaneously, and introduced 
 consecutively with the utmost rapidity possible; then, when a 
 sufficient deposit has been given, which usually requires from 
 ten to twenty minutes, the current is stopped, and the pieces are 
 removed and are at once well washed in clean water. 
 
 These liquids have a great tendency to deposit the black sul- 
 phide of silver, to check which the cyanide solution is added, as 
 described, to replace the portion added to the bath ; the brown 
 stains above alluded to are doubtless traceable to the same cause. 
 If the pieces are not thoroughly washed immediately upon removal 
 from the brightening-bath, they will become rapidly tarnished. 
 Gore has shown, too, that the deposited metal contains appreci- 
 able quantities of sulphur, which may, in part, account for the 
 variation in the physical characteristics of the metal. 
 
 The Anodes. The silver anodes must be of the purest silver 
 obtainable ; the ordinary standard silver for coinage contains 7*5 
 per cent, of copper (most foreign currency has even a larger pro- 
 portion of base metal) ; it should not, therefore, be used as such, 
 but if it be the only form readily available at any time, fine silver 
 should be prepared by the method described on p. 384. The 
 anode may be of cast or rolled metal, but if the latter be selected, 
 it should be annealed by heating to a dull-red heat, and subse- 
 quently cooling it before immersion in the vat, so that it may be 
 softer and more readily soluble. As a general rule, the anode 
 should present an area of surface equal to that of the cathode ; 
 but, as will now be readily understood, a somewhat smaller anode 
 surface must be employed when the bath contains excess of 
 cyanide, and a greater area when the cyanide is deficient, the 
 object being always to equalise the action at the electrodes, in so 
 far as it is represented by solution or deposition of metal. The 
 anode should never be suspended in the solution by means of 
 copper-wires (unless they can be so arranged that the copper 
 never comes into contact with the bath), because they dissolve 
 under the action of the current, and, passing into the solution, 
 render it impure. Silver wire has not the same objection ; but 
 as it dissolves rapidly, especially at the surface of the liquid, 
 it gradually becomes weakened until it is 'no longer capable of 
 supporting the weight of the anode. Platinum wire, being quite 
 insoluble, is free from both these objections, and is the best 
 material to use. If copper or even silver supports are adopted, 
 they should be kept from contact with the liquid in the manner 
 explained in Chapter V. (p. 97). 
 
 The Vat. Any of the ordinary vats described in Chapter V. 
 may be employed, except those lined with gutta-percha mixtures, 
 
CHARACTER OF THE METAL DEPOSITED. 187 
 
 which are more or less soluble in the cyanide liquid. Enamelled 
 iron lined with thin wood is, perhaps, mostly to be preferred, 
 necessarily so if the solution be heated. The disposition of con- 
 ducting wires and the manner of imparting a reciprocating motion 
 to a frame, from which the various objects are suspended, so that 
 they may be kept in constant motion, has also been explained in 
 Chapter V. The vats should be considerably larger than the 
 objects to be plated, and may indeed be made of any reasonable 
 size, remembering that a large bulk of solution generally gives 
 a better deposit than a small one, especially in the case of bright- 
 plating baths. When several vats are to be worked from the 
 same battery or dynamo, they should be coupled in parallel, 
 because, as a rule, the work to be silvered is very irregular in 
 shape and size, and under these conditions the series arrangement 
 is less satisfactory. A well-fitting cover may be made for the 
 vat, to preserve it from atmospheric dust when it is not in actual 
 use. 
 
 The Character of the Metal Deposited. Like most other 
 metals, silver, which is deposited by a current strong enough to 
 evolve hydrogen simultaneously, is dark in colour, powdery, and 
 non-adherent ; it is in the spongy condition, and is useless as a 
 coating. A weak current, on the contrary, gives a strong, malle- 
 able metal, adherent and coherent, and minutely crystalline. 
 Some operators consider the commonly-employed current-density 
 of 0'032 ampere per square inch (0'5 ampere per square deci- 
 metre) too high, and prefer to reduce it to 0'013 ampere 
 (0*2 ampere per square decimetre) ; but for all ordinary work 
 the larger current-density will be found satisfactory, and will, 
 of course, deposit a given weight of metal in a shorter period 
 of time. 
 
 The metal should have a pure white colour, any departure from 
 this indicating the presence of impurities. A pinkish shade prob- 
 ably points to the existence of copper in the precipitate. A 
 yellowish shade or tarnish, which is apt to appear upon surfaces 
 that have been for some time exposed after removal from the 
 vat, is probably due to a small percentage of a sub-cyanide of 
 silver deposited with the metal, which gradually changes colour 
 on exposure to light. It is found that a dip into potassium- 
 cyanide solution, or even a stay of two or three minutes in the 
 plating-bath after the current is cut off, suffices to prevent this, 
 doubtless by dissolving the objectionable sub-salt. It has already 
 been stated that the silver deposited by the carbon bisulphide 
 brightening-solution contains a small proportion of sulphur, which 
 is possibly accountable for the alteration of structure indicated by 
 the different nature of the deposit. 
 
 The thickness of a coating of silver may vary from an almost 
 imperceptible film to a depth of -^ of an inch on electro-plate, or 
 of y 1 ^ of an inch on silver electrotypes. 
 
188 ELECTRO-DEPOSITION OF SILVER. 
 
 Owing to its open crystalline nature, the deposited silver, if 
 peeled from the surface on which it is precipitated, lacks the 
 metallic ring emitted by the rolled metal when struck. 
 
 THE PROCESS OP ELECTRO-SILVERING. 
 
 Brass, copper, bronze, German silver, and similar alloys are 
 best adapted to the electro-silvering treatment ; the softer metals 
 lead, tin, Britannia metal and pewter though sometimes 
 plated, are less well suited because they are not structurally so 
 capable of resisting the final mechanical treatment of polishing 
 and burnishing ; iron and steel, zinc and other metals may also 
 be silvered. But whenever the metal is attacked by the cyanide 
 bath, so that silver is deposited by simple exchange and without 
 the aid of the current, it should receive a thin coating of copper 
 or be subjected to the process of quicking ; this coating with 
 mercury is, however, often used, even when the metal has no 
 action on the bath, to render adhesion doubly sure. The explana- 
 tion of the process is given on p. 116. 
 
 Organic matter must as far as possible be eliminated for 
 reasons already given ; hollow sheet-metal objects, therefore 
 (brass candlesticks, for example), which are often filled up with 
 pitch-composition, as a support to the thin metal of which they 
 are made, must be gently heated to effect its thorough removal 
 prior to electro-plating ; this operation must be conducted with 
 care, because cheap articles are frequently made in several pieces, 
 which are held in place by the composition, and, therefore, 
 become separated when it is removed. All non-metallic handles 
 or appurtenances should be, if possible, detached from objects 
 before plating, because there is not only a risk of their being 
 damaged by the solution, but liquid is sure to penetrate into the 
 sockets and interstices, from which it can afterwards be removed 
 only at the expense of much trouble. 
 
 The processes preliminary to the actual electro-deposition are : 
 
 (a) Stripping, or removal of an old coat of silver, if any exists. 
 
 (b) Polishing, if necessary. 
 
 (c) Cleansing, consisting of 1, boiling in caustic potash to 
 remove grease ; 2, dipping in sulphuric acid to remove oxide ; 
 and 3, scouring with sand or a dip into a mixture containing nitric 
 acid according to the nature of the metal. (See Chapter VI.) 
 
 (d) Preliminary coating with copper, if necessary. 
 
 (e) Quicking, if required. 
 
 Stripping. When old goods are to be re-plated, every trace of 
 the original coating must be removed, in order that the new 
 deposit shall be regular and uniformly adhesive. In the choice 
 of a stripping solution, the operator must be guided by the 
 character of the basis-metal from the surface of which the 
 silver is to be dissolved, as it is essential that the liquid should 
 
STRIPPING. 189 
 
 not be able to attack this to any serious extent, when it is laid 
 bare by the removal of the precious metal. 
 
 For brass, copper, or German silver, a mixture of concentrated 
 sulphuric and nitric acids is generally employed. The most 
 rapid method consists in heating a sufficiently large quantity of 
 strong sulphuric acid in a stoneware vessel, and adding to it, 
 immediately before use, a small quantity of potassium nitrate 
 (saltpetre) or sodium nitrate (Chili saltpetre) ; this is at once 
 decomposed, a small proportion of the sulphuric acid being 
 neutralised and a corresponding quantity of nitric acid being 
 liberated in the liquid. Such a mixture when used hot is 
 capable of dissolving the silver from the articles, which should 
 be suspended in it by copper hooks or preferably by copper tongs, 
 but should show no very great corrosive effect on the copper or 
 basis metal. Nevertheless, it is not entirely without action : and 
 the process must, therefore, be watched most carefully, especially 
 towards the end, when most of the silver has been dissolved, so 
 that on the disappearance of the last trace of covering metal the 
 article may be removed and plunged into a large volume of water 
 without loss of time. With this object in view, the pieces under 
 treatment should be frequently removed from the liquid for 
 inspection. The extremities of long articles which have not 
 been properly reversed during their first electro-silvering, and 
 the more prominent portions of every object having a thicker 
 coat than the remainder, are denuded last ; and it is often 
 advisable so to place the goods towards the end of the stripping- 
 process that only these portions are immersed in the liquid. It 
 is essential that the concentration of the bath be well maintained ; 
 any dilution increases its tendency to attack the base-metal, a 
 comparatively small addition of water sufficing to render the 
 action even violent. For this reason the mixture, which absorbs 
 water vapour from the air with great avidity, must be stored in 
 tightly-closed vessels when not actually in use, and the objects 
 to be stripped must be dry when placed in the vat ; indeed the 
 introduction of water in any way into the hot sulphuric acid 
 would cause a sudden generation of steam, almost explosive in 
 its violence, so that it is alike dangerous to the operator and 
 destructive to the bath. In course of time the accumulation of 
 potassium bisulphate in the bath (from the decomposition of the 
 saltpetre added each time before use) is rendered evident by the 
 deposition of cr3 T stals. When this is observed it will generally be 
 found that so much acid is neutralised that the liquid is no longer 
 serviceable ; a fresh quantity of sulphuric acid should then be 
 prepared, the old bath being reserved to recover from it the silver 
 which it contains. 
 
 On account of the great care necessary in conducting this 
 process, only one object should be treated at a time, if at least 
 it be of moderate size, for the operation proceeds with great 
 
190 ELECTRO-DEPOSITION OF SILVER. 
 
 rapidity. When this is inconvenient, by reason of the number 
 of pieces to be treated, extra precautions must be taken to guard 
 against too prolonged action in any individual case. 
 
 With a cold solution the action is slower, and consequently 
 under better control ; hence a mixture of 10 parts of concentrated 
 sulphuric acid (specific gravity 1 -84) and 1 part of strong nitric 
 acid (specific gravity = 1 '39) is often preferred, this being applied 
 at the ordinary temperature of the room. Except that it is not 
 heated, and that a greater number of pieces may be treated with 
 safety, the method of use is the same as that just described ; and 
 water and moisture must be equally rigorously excluded. 
 
 A different stripping-process must be employed for zinc, iron, 
 lead, tin, Britannia metal or pewter, or for any alloys of these, 
 which would be vigorously attacked by the acid mixture suitable 
 for copper. 
 
 This process consists in suspending the articles as the anodes 
 in a strong solution (say 10 per cent.) of potassium cyanide, 
 opposite a plate of platinum, copper, or brass, and connecting the 
 former with the positive or copper pole of the battery, so that 
 the process of electro-plating is reversed and the current flows 
 in the electrolyte from, instead of to, the pieces. Thus, the 
 goods being the anode, the silver is dissolved from them and 
 deposited upon the platinum cathode after a time, at a rate 
 depending upon the strength of the current which is being applied. 
 An old silver-bath may be utilised for this purpose, the metal 
 deposited upon the cathode plate being, of course, recoverable. 
 In any case the same solution may be used repeatedly ; and the 
 current may be stronger than that permissible for plating, 
 because the object is no longer to produce a good deposit, but to 
 dissolve an old one with the utmost rapidity. But when a strong 
 current is employed, the silver may be deposited in the pulveru- 
 lent condition, so that particles frequently become detached and 
 fall into the liquid, to prevent which the cathode plate may be 
 enveloped in a case of parchment paper or even of fine muslin. 
 Silver baths in current use must never be employed as stripping- 
 solutions, because they would gradually dissolve small quantities 
 of the base metals from which the silver has been removed, and 
 would thus become too impure to yield a good deposit; only 
 disused baths are permissible. As soon as the pieces are com- 
 pletely stripped, they are removed from the vat, plunged into 
 water and well washed. 
 
 The process is, of course, equally applicable to copper and 
 those alloys which are often treated by the more rapid acid 
 method. 
 
 Polishing, Washing, and Copper Coating. After the original 
 silver case has been removed, it is often necessary to pass the 
 goods to the polishers to buff and finish, prior to the cleansing 
 and quicking operations, after which they are transferred to 
 
SUSPENSION OF OBJECTS IN THE BATH. 191 
 
 the plating-vat without loss of time. Iron or steel cannot be 
 quicked, because this metal is one of the few which refuse to 
 amalgamate or alloy with mercury ; Britannia metal also is not 
 usually quicked ; but copper, brass, or nickel-silver are fitted in 
 this way to receive an adherent deposit. Zinc should receive a 
 preliminary wash of copper in the alkaline copper-bath, and it 
 was at one time customary to submit the tin-lead alloys (pewter, 
 Britannia metal, and the like) to the same treatment, but there 
 is no great difficulty in directly silvering them with good results. 
 Steel, which is, in some hands, still coppered before silvering, 
 may also take a perfectly sound and adhesive deposit by dipping 
 the cleaned articles at once into a striking-solution. 
 
 Suspension of Objects in the Bath. The suspension of objects 
 in the silver-bath is effected by thin copper wires 
 (commonly of about No. 20 of the standard wire- 
 gauge). New wire should be used each time, because 
 the deposit upon an old wire is apt to be loosened 
 by re-bending, and to crumble off in the bath. The 
 manner of attaching the wire depends upon the nature 
 and shape of the goods. Some articles afford natural 
 places of vantage from which they can be slung ; such 
 as cream ewers, cups, and the like, which carry 
 metallic handles ; or perforated objects ; or those 
 which, being unfinished, have rivet holes, through 
 which the wire may be threaded. Spoons and forks 
 are best supported in slings made by forming the 
 wire into a loop around the shank, or by bending 
 it into three-quarters of a circle at the end, and at 
 right angles to the wire itself, leaving a horizontal 
 space (the remaining quarter of the circle) through sling for 
 which the shank may be slipped, but which is not spoons, 
 wide enough to allow the handle or bowl to pass 
 (fig. 91). Plates, salvers, and the like should be hung by wires 
 bent lightly around them, these having their ends joined by 
 twisting together. Obviously the methods of attaching wires 
 are innumerable, and must be determined by the circumstances 
 of the case ; the guiding rule in making the connection is that 
 the wire shall be so arranged that the object cannot escape from 
 its hold ; yet, on the other hand, it should be so loosely fixed 
 that the relative position of wire and object may be shifted at 
 any moment without difficulty, so that fresh surfaces are brought 
 in contact, and wire-marks are not formed on the deposited metal. 
 
 The wiring is best done before the final potash and acid- 
 cleansing processes. Many firms place a piece of glass-tubing 
 over that portion of the wire which is in contact with the solution 
 between the cross cathode-rod of the vat and the suspended object, 
 so that silver may not be uselessly deposited upon it. Others use 
 gutta-percha or india-rubber as an isolating medium ; but, as these 
 
192 ELECTRO-DEPOSITION OF SILVER. 
 
 are slowly attacked by the cyanide liquor, glass is preferable, and 
 there is no difficulty in adapting it. Having ascertained the 
 length necessary to protect the portion of wire which is to be 
 immersed, this distance is measured off on a piece of narrow glass- 
 tubing ; a fairly deep mark is then made at the desired point with 
 a triangular file ; now, placing a hand on either side of the nick, 
 with the thumbs immediately beneath it on the other side of the 
 tube, a steady bending pressure is so applied that the file-mark is 
 on the outside, the thumbs on the inside of the bend ; almost 
 immediately the tube should break with a clean even fracture at 
 the place of the file-mark. In experienced hands accidents are 
 not likely to happen, but in early attempts at breaking tube in 
 this way it is perhaps safer, though even then scarcely necessary, 
 to envelop the hands with a thick cloth. 
 
 Arrangement of Objects. The objects must be introduced into 
 the bath gently without disturbing the sediment ; they should 
 be arranged in rows alternating with, and midway between, the 
 anode plates, so that each side will receive the same weight 
 of deposit ; thus, whatever the size of the bath, the number 
 of anodes will always be one in excess of that of the rows of 
 cathodes, and all will be in parallel circuit. If there be only 
 one row of cathodes there will be two anodes; if two rows, 
 then three anodes, and so on. Several objects may, of course, 
 be suspended side by side from the same cathode-rod, and will 
 be influenced by the same pair of anode-plates provided that 
 free space is left between adjacent objects. The anode- and 
 cathode-rods should be parallel to one another, in order that the 
 spaces of conducting liquid between the different pairs of electrodes 
 may be equal. It is preferable also that, as far as practicable, only 
 goods of the same shape, or, at least, of the same diameter, should 
 be suspended from the same rod. On account of the great irregu- 
 larities in form of objects to be plated, a considerable distance 
 must be left between the electrodes, so that the portions nearest 
 to the anode may not be so near, as compared with those more 
 remote, that they receive an undue share of the deposited metal. 
 Allowance must also be made for the motion imparted to the 
 objects in the bath, so that the opposing electrodes may not make 
 contact at the end of each swing. Again, two different kinds of 
 metal should not be suspended from the same rod (for example, 
 copper and Britannia metal), as the local current set up between 
 them, both being immersed in the same exciting liquid, and being 
 in metallic connection through the suspending-rod, will tend to 
 cause the gradual solution of the more electro-positive metal, and 
 will diminish the deposition of silver upon it until it is quite pro- 
 tected by a perfect layer of the precious metal. The solution is 
 thus injured by the introduction of foreign matter. If the electro- 
 chemical difference between the two metals be so great as to cause 
 an electro-motive force greater than that of the depositing current 
 
THE STRIKING-BATH. 193 
 
 (which could rarely, if ever, happen were ordinary care bestowed 
 on the process), no deposit would occur on the more positive 
 metal, which would rapidly dissolve and cause a double thickness 
 of coating to be given to the object made of the more negative 
 metal. Otherwise the local back electro-motive force simply 
 retards the action of the current in regard to the positive 
 metal, until it is sufficiently covered to prevent further action ; 
 and unless this retardation be very protracted, the difference 
 between the weights of metal deposited on the plates and that 
 which it was desired to precipitate will not be very appreciable, 
 so that the chief injury is done to the bath. 
 
 Use of the Striking-Bath. An additional reason for guarding 
 against this contingency is that lead and its alloys conduct electri- 
 city less satisfactorily than brass or nickel-silver, and far less so than 
 copper, and hence they need a somewhat greater length of time 
 to acquire the thin wash of silver which suffices to protect them. 
 
 Many operators, therefore, prefer as a preliminary step to dip the 
 articles, immediately after quicking, into a silver-bath worked by 
 a stronger current until, almost immediately, a thin film of this 
 metal has been imparted, when they may be transferred to the ordi- 
 nary vat, in which the remainder of the deposit is to be built up. 
 The first, or striking-bath, may contain less silver than the usual 
 solutions (half an ounce to the gallon commonly suffices), but the 
 proportion of free cyanide is often greater. Large silver anodes 
 are used ; and, indeed, everything must be done which tends to 
 reduce the resistance and increase the rapidity of deposit, in order 
 that the action may be almost instantaneous, and that a momen- 
 tary dip into the vat may be sufficient to give the required deposit. 
 But, on the other hand, it need scarcely be remarked that the 
 volume of current must not be so great that a pulverulent or 
 spongy deposit results. The electrical connections of the striking- 
 vat may be similar to those recommended for the plating-bath ; 
 but a smaller bath with two large anodes, one at either end, and 
 with a single cathode-rod, to which the negative battery-wire is 
 attached, and which is lowered into the bath by hand, is really all 
 that is required. 
 
 How to Ensure Uniformity of Coating. After the transference 
 of the goods to the plating-vats, they may be left with less con- 
 stant attention until a sufficient thickness of film has been ob- 
 tained, provided that the current is constant and that the arrange- 
 ment for imparting motion to them during the action is working 
 satisfactorily. All that is necessary is slightly to shift the position 
 of each piece relatively to its supporting wires from time to time, 
 to ensure uniformity of deposit at these points, with an occasional 
 momentary removal from the bath for an examination as to the 
 regularity of the action. Should spots appear upon the surface, 
 the article must be removed from the bath, rinsed, scratch-brushed, 
 and then cleansed by a dip into hot potassium cyanide or caustic 
 
 13 
 
194 ELECTRO-DEPOSITION OF SILVER. 
 
 potash solution. Finally, after rinsing once more, they are re- 
 quicked and introduced again into the bath. Since, in spite of 
 the gentle motion imparted to the objects, the solution is certain to 
 vary in density, and to produce a thicker deposit upon the lower 
 portions of articles, long objects, such as spoons and forks, sus- 
 pended upright in the bath, should be reversed at intervals of 
 (say) half an hour, so that if the bowls were downwards at first, 
 the handles would be so after the first shift. 
 
 In immersing the quicked and struck articles into an empty 
 vat, that is, into one which contains only anodes suspended 
 within it, those first immersed would receive too strong a current, 
 unless their superficial area were very considerable, and would be 
 covered with a spongy silver precipitate, until, at last, the cathode - 
 surface had been increased by the introduction of more objects, 
 sufficient to produce the right proportion of current-strength 
 per unit of area. Meanwhile, however, the original pieces would 
 have suffered serious injury. To obviate this, either the current- 
 strength may be reduced at first by the interposition of wire- 
 resistances, which are gradually lessened as fresh objects are 
 introduced, or one or more of the anode-plates (according to the 
 size of the vat) are hung upon the cathode-rods at the outset, and 
 are transferred to their proper places, one by one, as each batch of 
 objects is immersed which presents a total area equal to that of 
 one plate. In this way a large proportion of the current is at first 
 occupied in transferring silver from one anode-plate to another ; 
 and the bath from the very beginning is under the same conditions 
 as it is when filled with goods undergoing the silvering process. 
 Thus even a small object introduced alone should receive a normal 
 current throughout. 
 
 Some electro-platers, in order to secure a more perfect coat, are 
 in the habit of removing the articles after a certain amount of 
 silver has been deposited and submitting them to a preliminary 
 scratch-brushing, after which they are well rinsed and cleansed 
 and again returned to the bath ; but it is very doubtful whether 
 any real advantage accrues from this practice. 
 
 When a sufficient thickness of metal has been deposited, which 
 may be known, as explained in Chapter V., by ascertaining the 
 mean strength of current, the total cathode-area and the time 
 occupied, or by the use of the plating-balance, the pieces are re- 
 moved from the vat, transferred (if necessary) to the brightening- 
 bath, where they are left undisturbed for a few minutes, and are 
 then plunged into a slightly warm solution of potassium cyanide 
 to remove any silver subcyanide left in the pores of the metal, and 
 thoroughly washed in several waters held in successive tubs (vide 
 instructions for washing coppered goods on p. 136) ; they are next 
 dipped momentarily into a vat containing water mixed with 2 or 
 3 per cent, of sulphuric acid, and are again rinsed in water, and 
 taken to the scratch-brush for preliminary polishing, and to the 
 
UNIFOEMITY OF COATING. 195 
 
 burnishers for the final treatment. The potassium cyanide dip 
 may be dispensed with, if the objects are left in the plating-solution 
 for a few minutes after disconnecting the current. 
 
 How to Thicken the Coat Locally. An extra thick coating of 
 silver may sometimes be imparted to those portions of goods 
 which will have to stand the chief amount of wear in use. This 
 may be effected in many ways according to the appliances and 
 ingenuity of the plater. The application of stopping-out varnish 
 after a certain time to the parts which are to receive a thinner 
 coat is rarely admissible, because, although it would have the 
 desired effect, it gives too defined an outline of the thicker deposit, 
 and this has to be obliterated mechanically, an imperceptible 
 gradation being generally required. This method would be 
 suitable if the whole of one side of the object had to be thickened, 
 but an equally good result would be attainable by the use of only 
 one anode (adjacent to this side). Another plan is to introduce, 
 towards the end of the operation, a subsidiary anode, correspond- 
 ing in shape to the part which is to receive the greater deposit, 
 and which is placed in greater proximity to it, in proportion to 
 the increase of substance to be acquired. Yet another system 
 may be adopted in plating the bowls of spoons and similar objects, 
 which are worn most largely in use at the most prominent parts 
 of the curve. After plating in the usual way the spoons are so 
 placed in a shallow bath that only the convexity of the bowl is 
 immersed, and receives a coating ; the depth of immersion must 
 be frequently altered in a slight degree to ensure that no distinct 
 boundary marks are produced on the surface. It is true that 
 these marks may be mechanically removed, but there is no reason 
 why they should occur at all. 
 
 Thickness of Deposit. The thickness of the deposit and, con- 
 sequently, the duration of the process is, of course, governed solely 
 by the class of work under treatment. Many common goods, 
 especially those made of white metal, receive a film so thin as to 
 be beyond the range of practical measurement. This is naturally 
 useless to resist even the slightest wear ; it gives simply an orna- 
 mental covering for a short time. As a general rule for ordinary 
 electro-plating, a deposit of from 1 to 2 oz. per sq. ft. of surface- 
 area may be deemed a good well- wearing coating ; a single page 
 of this book represents approximately the thickness of a film equal 
 to 1 oz, per sq. ft. This will occupy from three to nine hours in 
 coating, according to the current-density used. A thinner deposit 
 than that of 1 oz. per sq. ft. is not to be recommended, as even 
 two or three years of ordinary wear would suffice to lay bare the 
 base metal at the edges and in all the more prominent parts. In 
 working, however, for the trade, the craftsman is rarely allowed 
 to decide what, in his judgment, is best fitted for the work, but 
 must do as he is ordered by his customer, and will be paid at the 
 rate of so much per unit weight of silver deposited. 
 
196 ELECTRO-DEPOSITION OF SILVER. 
 
 ' Galvanit.' A. Rosenberg has introduced a method of plating 
 by means of powders, which are known by the trade name of 
 'Galvanit,' and which are moistened with water when used. 
 The powders consist of a salt of the metal to be deposited mixed 
 with a fine powder of a more electro-positive metal. On rubbing 
 the wet powder on the metal surface to be plated, the latter forms 
 a voltaic couple with the grains of powdered electro-positive metal 
 (acting as anodes), and deposition takes places from the paste, just 
 as in a single -cell process. It is not a ' simple - immersion ' 
 process, as there is not an exchange of metals on the surface 
 plated. Instead of using a salt as mentioned above, the metal to 
 be deposited may also be in the form of a powder, and the two 
 powdered metals may be mixed with an ammonium salt. The 
 moist powder is rubbed on to the surface to be plated, and this 
 rubbing action renders previous cleaning unnecessary. It is 
 claimed that even stripping is unnecessary. The process cer- 
 tainly provides a simple means of plating articles with silver 
 and other metals, though it is not likely to supersede the electro- 
 lytic vat in commercial work. 
 
 SILVER ELECTROTYPING. 
 
 Silver is occasionally used in special cases for copying works 
 of art or even valuable engraved steel-plates. Ordinary wax 
 and gutta-percha moulds, such as are used for copper electro- 
 typing, are not admissible for silvering, because they are to some 
 extent attacked by the cyanide solutions. The simplest method 
 of obtaining replicas of works of art in silver is to obtain first 
 a thin electrotype shell of copper from the intaglio-mould, and 
 then to deposit silver upon this in the cyanide-bath. The copper 
 protecting-film may be of the thinnest, so that it shall not destroy 
 the sharpness of the lines ; but it must, of course, be subsequently 
 removed, after the required thickness of silver has been deposited, 
 and the whole electro separated from the mould. This solution 
 of the copper may be effected by treatment with warm hydro- 
 chloric acid, or (better) with a warm solution of iron perchloride, 
 either of which will attack the copper but leave the silver un- 
 touched. On the removal of the copper, the pure silver surface 
 has the required form in practically undiminished sharpness and 
 brilliancy. The silver may be built up to a thickness of one- 
 eighth of an inch or more. It is rarely, however, that this process 
 is required, and practically the sole application of electro-silvering 
 is to be found in the coating of other metals to endow them with 
 properties which they do not of themselves possess. 
 
 ORNAMENTING SILVER SURFACES. 
 
 There are many ways of altering the appearance of electro- 
 silvered goods ; but to give a description of these, many of which 
 
ORNAMENTING SILVER SURFACES. 197 
 
 are purely mechanical, is beyond the scope of this work. Let it 
 suffice then to say that 
 
 A dead lustre may be obtained by depositing upon the silver 
 a thin film of copper, which has a slightly roughened surface of 
 excessively fine grain, and then again upon this a thin layer of 
 silver. 
 
 Oxidised silver, which is an entirely misleading term, inasmuch 
 as oxygen plays no part in its formation, is made by dipping the 
 object into, or painting it with, either a solution of platinum, 
 which covers the whole surface with a thin layer of that metal 
 by 'simple immersion,' or one containing sulphides which imparts 
 to the silver a superficial film of black silver sulphide. This 
 latter solution is made up by dissolving three-quarters of an ounce 
 of potassium polysulphide ('liver of sulphur'), or of ammonium 
 sulphide, in each gallon of water ; it is applied to the silver at a 
 temperature of 150 F. The potassium compound is to be pre- 
 ferred ; some operators add to it about twice its weight of 
 ammonium carbonate. A few seconds' immersion in either of 
 these liquids usually suffices ; the articles are then rinsed in water 
 and dried. 
 
 Antique silver is produced by rubbing into, and leaving upon, 
 the parts of an object which are not in relief a thin layer of 
 black-lead, finely crushed and stirred into spirit of turpentine ; 
 some prefer to add a little ochre to the mixture in order to pro- 
 duce a warmer ground-tone of colour. 
 
 Niello-work is prepared by tracing a pattern upon bright 
 silver with silver sulphide or with mixtures of lead, copper, and 
 silver sulphides, prepared artificially; when placed in position 
 the object is heated to their fusing-point in order to ensure 
 adhesion. It is, in fact, a process of enamelling. Clearly, 
 however, it is quite inapplicable to many classes of electro-plate, 
 while to any it must be applied with the utmost care, in order 
 to avoid the stripping or buckling of the coated object or the 
 fusing of the basis metal. 
 
 Satin finish may be produced, according to Wahl, by the 
 application of fine sand propelled forcibly upon the surface of an 
 object with the aid of an air-blast a process analogous to that 
 largely used at present for decorating glass. Any process which 
 will destroy the polish upon the silver with equal fineness and 
 regularity would, of course, answer the same purpose ; but the 
 sand-blast is probably the simplest and most economical extant. 
 
 Obviously the enumeration of the methods of decorating silver 
 surfaces is by no means exhausted in the few words given above ; 
 they are innumerable, and capable of infinite variation according 
 to the taste and skill of the artificer. 
 
CHAPTER X. 
 
 THE ELECTRO-DEPOSITION OF GOLD. 
 
 Advantages of Gold-Plating. Owing to its high power of resisting 
 atmospheric influences, combined with the richness of its colour, 
 and the brilliancy of the polish which it is capable of receiving, 
 and to the fact that all these properties are manifested even by 
 the thinnest imaginable film of the metal, gold is very frequently 
 deposited ; none the less so, perhaps, because, being a costly 
 material, gilt objects of low value may pass for articles of much 
 higher worth. 
 
 Gold is a very electro-negative element, so that any of the 
 common metals is capable of replacing it in any of its compounds. 
 It may, therefore, be readily deposited by simple immersion, 
 although the electrolytic process is more satisfactory. 
 
 DEPOSITION BY SIMPLE IMMERSION. 
 
 Solutions. Many baths have been used at various times, the 
 principal of which are included in the following table. 
 
 Eoseleur's Process. Of all these, Roseleur's solution (No. 4) 
 is the best for treating small articles of jewellery, for example 
 made of copper, bronze, or brass. The gold chloride crystals 
 should be dissolved in a small proportion of the water, and added 
 to the solution of sodium pyrophosphate in the remainder, and 
 the mixture warmed until the yellow colour of the liquid has dis- 
 appeared. The solution thus made up, however, is too readily 
 decomposable, as, indeed, is indicated by the gradual change of 
 colour to a dark red purple which it undergoes on standing ; 
 hence the hydrocyanic (prussic) acid is added as a check upon 
 the rapidity of the spontaneous reduction. It is omitted by 
 some gilders, but the bath is under better control when it is 
 used ; when working too slowly, more gold chloride is added, or 
 when it becomes deep purple in colour, fresh hydrocyanic acid is 
 introduced. 
 
 In using this bath, the object must present a clean bright 
 surface such as may be imparted to it by pickling, scratch- 
 brushing, and cleaning ; it is then quicked by a momentary 
 
 198 
 
IMMERSION GOLD SOLUTIONS. 
 
 199 
 
 
 
 i 
 
 
 
 d 
 
 M 
 
 
 
 
 
 j 
 
 
 
 1 
 
 o "s a 
 
 it? 
 
 1 
 
 
 
 
 ** 
 
 
 o 
 
 '" S -^ 
 
 sc 
 
 
 1 
 
 1 1= i i i 
 
 1 
 
 
 i 
 
 I !l M 1 
 
 1 
 
 
 
 5 if i . i 
 
 1 
 
 
 
 ^^ rH ^ 2 ^ 
 
 
 
 02 
 
 M u 'ft i ""^ * 
 
 
 
 
 1 1- 1 * I 
 
 1 
 
 
 Water. 
 
 1 1 
 
 
 
 
 
 
 
 B 
 
 S 
 
 Ammonium 
 Sulphide. 
 
 : : : : ':- 
 
 3 
 
 3 
 
 w 
 
 Sodium 
 Pyrophosphate. 
 
 : : : : : 
 
 3 
 o3 
 
 PH 
 
 
 
 P 
 
 CD 
 
 to 
 
 H 
 
 Potassium 
 Bicarbonate. 
 
 : a 8 : S g 
 
 1 
 
 i 
 
 
 eo us ko 
 
 c 
 
 
 
 H 
 
 Caustic 
 Potash. 
 
 : : : : | : 
 
 1 
 
 w 
 
 
 
 a 
 
 M 
 
 Potassium 
 
 
 3 
 
 
 
 Cyanide. 
 
 0> 
 
 1 
 
 S 
 
 E 
 
 Hydrocyanic 
 Acid. 
 
 oo 
 6 
 
 
 1 
 
 Gold 
 Sulphide. 
 
 . : : : : : 
 
 E 
 
 G 
 
 S 
 
 
 Gold, as 
 
 ' 10 ^ "* 
 
 8 
 
 
 Gold Chloride. 
 
 
 1 
 
 
 ts!3 o> oj 
 
 s 
 
 
 
 o^ a 
 
 rt O O 
 
 
 
 S||i 
 
 f i i 
 
 'i 
 
 
 -I 
 
 . ^ |2 
 
 
 
 S cS ^ 
 
 : : III sl : 
 
 i 
 
 <D 
 
 
 ^ P* ^ 
 
 O O U Q tiC 
 
 A 
 
 
 
 
 P-l 
 
 
 
 ** 
 
 I 
 
 
 >, 
 
 . . . ^X^ ^r" . 
 
 H 
 
 
 i 
 
 
 R 
 
 
 1 
 
 1 s 
 
 
 
 -3 
 
 fl 5P r^ ^H 
 
 ^ 
 
 
 -* 
 
 I 1 I I = 1 
 
 * 
 
 
 o 
 
 tH (N CO -* 10 
 
 
 
 fc 
 
 
 
200 ELECTRO-DEPOSITION OF GOLD. 
 
 plunge into a dilute solution of mercuric nitrate, rinsed in water, 
 and immediately transferred to the gold-bath, which should be 
 nearly boiling. It is more economical and satisfactory to use 
 three gold dips in succession, each solution being richer in gold 
 than that previously applied; this is readily effected by using 
 old baths for the first two operations, and by arranging a system 
 in which, as soon as the final vat ceases to yield a good deposit, 
 it is made the second instead of the third bath ; that which had 
 been the second being now the first, and the old first, now 
 practically exhausted, being discarded. Thus the pieces are most 
 thoroughly washed before they enter the last bath ; and no gold 
 is lost, as the small quantity left in the third solution, when it 
 is no longer serviceable, is used up during the time that it is 
 acting as the second and the first. An immersion of a few 
 seconds in each liquid should suffice ; and the resulting deposit, 
 which is, of course, very thin, should have a good yellow colour 
 and require only slightly scratch-brushing or burnishing to 
 impart a final polish, rinsing, and lastly, drying in hot white 
 wood sawdust; for this resinous woods, oak, or walnut, which 
 tend to discolour the work, are to be avoided. It may sometimes 
 be necessary to improve the appearance of the gold by colouring 
 methods, which will be described at the end of this chapter. 
 
 If it be desired to obtain a thicker coating by this method, it 
 is only necessary to repeat the process several times, re-quicking 
 each time before passing the articles through the gold-baths. 
 The deposit is thus gradually built up, because at each quicking- 
 stage a small proportion of mercury deposits upon the surface, 
 and then exchanges for an equivalent of gold, when placed in the 
 gilding-solution, the latter gradually accumulating mercury in 
 place of the more precious metal. A really thick coating, how- 
 ever, cannot well be built up by this tedious process, and the 
 electrolytic process is more convenient and more expeditious. 
 
 Elkington's Process. Elkington's process of water-gilding 
 (No. 2) employed potassium bicarbonate in place of sodium 
 phosphate ; but its use is more troublesome, and it permits only 
 a semi-exhaustion of the bath, leaving the remainder of the gold 
 to be recovered from the residual liquids by chemical means. 
 
 Roseleur's Process for Large Objects. Roseleur's bath (No. 5) 
 rapidly precipitates gold upon articles which have not been 
 previously quicked ; the deposit is not of high quality, but the 
 process is well adapted and largely used for coating large objects 
 with a wash of gold, prior to the electrolytic process. 
 
 Other Solutions. Of other solutions for simple-immersion 
 gilding, perhaps the most interesting are : that of gold chloride 
 in ether, which is applied to gilding iron and steel goods; and 
 that of Braun, a solution of gold sulphide in ammonium sulphide, 
 which is adapted to the direct gilding of zinc, because the latter 
 metal would dissolve but slowly in such a liquid, and the coating 
 
IMMERSION GOLD SOLUTIONS. 201 
 
 is, therefore, the more likely to be adherent. This sulphide 
 solution is quickly oxidised, and should be preserved from 
 unnecessary exposure to the air. 
 
 DEPOSITION BY THE SINGLE-CELL PROCESS. 
 
 The Elkington bicarbonate process, above alluded to, when 
 used to deposit upon silver or German silver, demanded that a 
 piece of zinc or copper should be attached to the objects, and 
 thus became practically a single-cell process. Steele also, in the 
 specification of a patent granted to him, claimed the use of a 
 cyanide bath in which the object to be coated was immersed in 
 contact with a piece of zinc ; but, inasmuch as a considerable 
 proportion of the gold was found to deposit upon the zinc itself, 
 owing to the wide difference between the electro-chemical relations 
 of the two metals, zinc and gold, the method is not practically used. 
 
 DEPOSITION BY THE SEPARATE-CURRENT PROCESS. 
 
 The Battery. Almost any of the ordinary battery-cells may be 
 used. A current of fairly high potential is required, and a high 
 current-density is essential. The Bunsen-cell is well adapted for 
 the work, but the resistance in the circuit should be sufficient to 
 reduce the current-density to 0*006 ampere per square inch (0*1 
 ampere per square decimetre). 
 
 The Solution. As with silver, the double cyanide solution 
 will generally be found to give the best results, provided that 
 due care is paid to all the details of the process. The number of 
 other solutions prepared with the object of supplanting the 
 poisonous cyanide compounds, and of the modifications of the 
 cyanide-bath itself are, as usual, innumerable. The chief of them 
 are included in the following table. 
 
 For ordinary use, a bath containing three quarters of a troy 
 ounce of gold, dissolved and converted into cyanide, together 
 with about 7 oz. of good potassium cyanide in every gallon 
 of solution, should give a good deposit at a temperature of 120 
 to 140 F. ; it should be boiled prior to use. The proportions, 
 however, may be widely altered without greatly prejudicing the 
 character of the work ; and it is well to vary the composition of 
 the bath with any change in the conditions of depositing, 
 [ndeed, it is best not to adhere slavishly to any given formula, 
 but rather to modify it by dilution or strengthening, by adding 
 cyanide or gold, according to the work in hand and the methods 
 of the operator. A solution to be used for cold gilding should 
 contain at least two or three times as much gold, and proportion- 
 ately cyanide, as one which is to be employed when heated. A 
 weak solution, however, will usually give a better deposit than 
 a stronger one at the same temperature. 
 
202 
 
 ELECTRO-DEPOSITION OF GOLD. 
 
 I s 
 
 
 
 Special Method of Preparation. 
 
 s I if g . I || || a . I y 
 
 * |2 g C o5 "^ " .^- 2-sS 
 
 c -g M 2 ^.' ?3 s ^s sg |s^ 
 
 |1 |-S5 2 ^-S |i || || l^'ll 
 
 arious ingredients. The weight 
 aed. Ppt.= precipitate. 
 
 10 
 
 iH 
 
 
 Water. 
 
 III 1 1 1 II 1 1 II 
 
 ll 
 
 3 
 
 
 Ammonium 
 Sulphide. 
 
 :::::::: : : : : : ::::::::: J : : 
 
 11 
 
 CO 
 
 
 Ammonium 
 
 -co o 
 
 
 rH 
 
 
 Chloride. 
 
 
 c g* 
 
 (N 
 
 iH 
 
 B 
 
 Sodium 
 Phosphate. 
 
 :::::::: : : : : : g :::::::::: 
 
 S'8 
 
 iH 
 rH 
 
 P 1 "" 1 
 
 Sodium 
 Hyposulphite. 
 
 :::::::: * : : : : :::::::::::: 
 
 3 
 
 
 
 g 
 
 Sodium 
 
 : : : : : : : : : : : : : 2 g ::::::: : : : 
 
 
 i 1 
 
 05 
 
 Bisulphite. 
 
 
 ^"2 
 
 05 
 
 Sq 
 
 Potassium 
 Carbonate. 
 
 
 ^ 
 
 00 
 
 PH 
 
 o 
 
 Potash. 
 
 . to 
 
 Is 
 
 1> 
 
 H 
 
 w 
 
 Potassium 
 Ferrocyanide. 
 
 oo; :;;;; ; -iS 'e^ <& ' ' ' 
 
 It 
 
 
 m 
 
 Potassium 
 
 
 J3~ 
 
 <o 
 
 
 Sulpho- 
 cyanide. 
 
 ' ' ' ' 
 
 il 
 
 10 
 
 i 
 
 Potassium 
 Cyanide. 
 
 to * 
 
 OCOiO- W 4< ^ M O' rH rH HH^ S M Jj 
 
 Jl 
 
 <f 
 
 PH 
 
 "3 niuret. 
 
 : * g 
 
 * M 
 
 
 
 g~ 
 
 
 
 CO 
 
 
 Oxide. 
 
 
 a a 
 
 c^ 
 
 
 a Cyanide. 
 
 . . . . <N . rH . . 
 
 13 
 
 TH 
 
 
 < Chloride. 
 
 
 {i 
 
 
 
 Its 
 
 ,S| - II ^ . . h .'g . o . b 
 
 S| 
 
 
 
 "Hi 
 
 ^ ^ ^ s s ^ " " 
 
 1; 
 
 
 
 ll.il 
 
 CQ^*^ 
 
 i ^|sr-s|1I 
 
 -The heavy 
 n columns 1 
 
 
 
 
 V^^ >^-Y^^ ^,_ 
 
 . 
 
 
 
 ti 
 
 2 
 
 g2 
 
 
 
 
 - h S fa N 
 
 H- 
 
 
 
 
 
 M ^o 1 ? - 11 = = = =^Is 81 
 
 DIES TO 
 
 
 
 
 O 
 
 M M CO .*^^00 p. NCO j* JOO SSgSS ^^ 
 
 * 
 
 a - 
 
ELECTRO-GILDING SOLUTIONS. 203 
 
 Langbein mentions that the following formula (used cold) gives 
 a beautiful bright deposit on all metals, even on iron and steel : 
 0'5 oz. of potassium ferrocyanide, 0*5 oz. of sodium carbonate, 
 30*75 grains of fine gold (as chloride), and one quart of water. 
 The solution is made up by heating the mixed ferrocyanide and 
 carbonate solutions to boiling, adding the gold salt, boiling for a 
 quarter of an hour till the smell of ammonia disappears, and 
 making up with distilled water. This bath is specially suitable 
 for clock gilding. Langbein states that potassium ferrocyanide 
 baths, although deservedly popular for decorative gilding, where 
 deposits of different colours are desired, cannot be recommended 
 otherwise on account of secondary decompositions during plating, ^ 
 and because they do not dissolve the gold anodes. 
 
 The cyanide gilding-solution is comparable with the correspond- 
 ing silver-bath ; a certain amount of free cyanide is required 
 to promote the solution of the anodes, a deficiency of this salt 
 being indicated by the formation of a slimy deposit upon the 
 anode-surface. Too great an excess of cyanide, however, causes 
 the anode to dissolve too rapidly, and thus to yield too strong a 
 bath; but it also tends to attack gold without the aid of the 
 current, with the result that the deposit is produced excessively 
 slowly, and may not even be formed at all in the deeper recesses 
 of an irregularly-surfaced article, the simple solvent action of 
 the bath neutralising the depositing power of a weak current at 
 these more distant points ; moreover, when each side of an 
 article is coated singly, the film of gold upon the side more 
 remote from the anode may be redissolved during the covering 
 of the second surface. When such actions as these are perceived 
 they must be neutralised by the addition of more gold cyanide. 
 
 The gradual accumulation of dirt and various impurities, 
 soluble and insoluble, renders the bath unfit for use after a time. 
 The use of the liquid for depositing gold upon articles made of 
 base metals causes a slow absorption of these metals into it by 
 simple exchange in the few seconds during which they are 
 immersed before a protective cover of gold is imparted to them ; 
 and as soon as the proportion of silver or copper thus introduced 
 becomes appreciable, the colour of the gold precipitated by the 
 bath becomes influenced, as will be explained hereafter. But 
 these impure baths may, of course, be successfully applied to the 
 production of a deposit, when the particular shade of colour 
 which they yield is sought, or they may be used for deeper 
 colours by adding to them a further quantity of one of these 
 colouring metals as may be desired. The presence of much 
 organic matter gives a dark colour to the solution, and causes 
 it to yield a brown deposit, which can never be converted into 
 a good coat ; such a bath should, therefore, be discarded. 
 
 The cyanide- bath is often made up by simply adding the 
 chloride or some other salt of gold to the solution of potassium 
 
204 ELECTRO-DEPOSITION OF GOLD. 
 
 cyanide, but this is an objectionable practice, because it need- 
 lessly introduces impurities into the solution (vide p. 179). Gold 
 cyanide should, therefore, be prepared and well washed so that only 
 the pure salt is introduced into the bath. The bath rnay also be 
 prepared electrolytically by passing a fairly strong current from 
 a large pure gold anode to a small gold or platinum cathode 
 through a 3 or 4 per cent, solution of potassium cyanide heated 
 to a temperature of 120 to 140 F., until, as in the case of silver- 
 baths similarly made up, the difference between the loss of 
 weight of the one electrode and the gain of the other 
 indicates that sufficient metal has passed into the solution. 
 
 Heated solutions suffer a gradual loss of water by evaporation, 
 which must be made good from time to time, preferably every 
 evening after the day's work is over. 
 
 In order to obtain certain shades of colour upon the gold 
 deposit, solutions of certain metals are added to the gold-bath, 
 which metals, by being precipitated simultaneously with the more 
 precious metal, influence its tint. The effect of varying the 
 strength of either current or solution and of modifying the 
 working of the bath will be discussed in the section relating to 
 the character of the deposit; but it should be noted at this 
 point that a red colour, or a greenish shade merging almost into 
 white, may be produced by adding to the gold-bath (preferably 
 a cold one) a sufficient quantity of a copper cyanide solution on 
 the one hand, or of silver cyanide upon the other. The pro- 
 portions cannot, of course, be rigorously prescribed, because they 
 will vary with the shade of colour to be produced, a few trial 
 experiments sufficing to indicate the amounts suitable for any 
 given tint. In preparing such baths the added metal must be 
 introduced very gradually, so that an excess may be avoided, 
 remembering that it is generally more convenient to add a 
 little more copper or silver, if required, than to reduce the 
 relative proportion by the introduction of a further quantity 
 of gold. It has been noted previously that an old gold-bath, 
 which is beginning to yield a coloured deposit, may with 
 advantage be used as the basis for a solution intended to produce 
 the same shade intensified. 
 
 The Anode. The anode should be made of the purest gold 
 obtainable ; the presence of silver and copper which, either singly 
 or together, are nearly always alloyed with gold in the arts to 
 render it harder and more durable, is fatal, because these also 
 dissolve under the combined influence of the electrolyte and the 
 current, and the bath then deposits a coloured gold. 
 
 When pure gold is not to be had, and the means for preparing 
 it from the alloys of commerce are not available, it is better to 
 substitute a platinum sheet as anode, which will not be attacked 
 by the solution. In this case the bath rapidly decreases in 
 strength, and must be replenished from time to time by the 
 
CHARACTER OF THE METAL DEPOSITED. 205 
 
 addition of gold cyanide, until so great a quantity of foreign 
 matter has accumulated that a good deposit is no longer pro- 
 duced. The gold anode, however, is to be preferred, as it main- 
 tains an even constitution of solution. 
 
 For large articles which require a thick covering, the surfaces of 
 the two electrodes should be approximately equal in area ; the 
 anode should be completely immersed in the liquid by means of 
 platinum suspending wires, to obviate unequal corrosion of the 
 plate. For small objects, which need but a few minutes' 
 exposure to impart a sufficient film, a smaller anode is often 
 used ; but the bath should be examined at intervals, when it is 
 much worked, and, if necessary, a further amount of gold cyanide 
 must be added. It is convenient to have ready means for alter- 
 ing the position of the anode in the liquid, so that a greater or 
 less surface may be immersed at will, and hence also for regulat- 
 ing the strength of current and with this the rate and character 
 of deposit. To impart the almost imaginary film of gold which 
 is to be found on cheap jewellery, Roseleur recommends the use 
 of a platinum anode, maintaining the strength of the bath by 
 adding to it crystals of gold chloride. 
 
 The Vat. Earthenware or porcelain vats are best suited to 
 cold solutions ; and a deep porcelain evaporating-basin, obtainable 
 from any chemical-apparatus dealer, and from many druggists, is 
 the best containing vessel for hot plating-baths, provided that 
 only small objects are to be treated. For larger work enamelled 
 iron should be employed, but it is more important than ever that 
 the enamel should be perfectly sound. A cover should be made 
 to exclude dust, or the liquid may be stored in stoppered bottles 
 when not in use. Generally speaking, the duration of the gilding 
 is so short, and the objects are often so small, that it is unneces- 
 sary to arrange conducting-wires around the vats : they may simply 
 be attached to the anode-plate and the pieces respectively. 
 
 Character of the Metal Deposited. The deposit of gold is, 
 perhaps, more susceptible of change by varying external condi- 
 tions than that of any other metal. A large proportion of the 
 value of gilding depends upon the colour of the metal precipitated, 
 and this is most readily affected not only by the presence of 
 foreign matter, as recently explained, but by changes in the 
 strength of the current or of the bath. 
 
 A current which is too strong will, of course, deposit the gold 
 as a black powder ; but within the limits between which coherent 
 and adhesive deposits are yielded, a stronger current produces a 
 deeper coloured coating than a weak current. Hence, speaking 
 generally, any influence which tends to increase the current- 
 density gives rise to a metal possessing a warmer hue. A feeble 
 current, a small anode, and a cold solution, alike give pale yellow 
 deposits ; but a stronger battery, an increase of anode-surface 
 (and hence less resistance), or warming the liquid, increase the 
 
206 ELECTKO-DEPOSITION OF GOLD. 
 
 current-strength, and a deeper yellow tone prevails. Motion 
 imparted to the pieces also causes a lighter colour. 
 
 The best colour for the pieces to present on removal from the 
 bath is a very deep yellow, inclining to brown ; a pure gold 
 colour will become too pale when the lightening and polishing 
 action of scratch-brushing or burnishing has done its work. A 
 dark, almost black, colour, produced by an excess of gold or too 
 powerful a current, will never yield a good tint subsequently, 
 nor indeed will even a brown deposit do so. 
 
 The Electro-Plating Process for Gold. Stripping. As with 
 silver, so with gold, old coatings should be entirely removed 
 before attempting to deposit a fresh layer of the precious metal. 
 For a simple stripping-bath, some workers recommend a mixture 
 of nitric acid with a little common salt, which produces by 
 chemical reaction nitro-hydrochloric acid or aqua regia ; but if 
 this mixture be used, the utmost care must be taken to stop the 
 action immediately the gold is removed, as the basis metal would 
 be most powerfully attacked by the mixture. Others use strong 
 sulphuric acid, to which about one-tenth of its volume of strong 
 nitric acid and one-fifth of strong hydrochloric acid have been 
 added. Equally with the other, however, great care is necessary 
 to prevent the attack upon the basis metal by this solution, and 
 it must be most scrupulously preserved from contact with water, 
 for very moderate dilution would render its action upon the 
 basis metals intensely vigorous (see p. 189). Large articles may 
 be stripped, according to Wahl, by making them anodes in a bath 
 of the strongest sulphuric acid, the cathodes being of copper. 
 But the simplest plan is to make the old plated goods the anodes 
 in a 10 per cent, solution of potassium cyanide ; such a solution 
 per se may even suffice to dissolve a mere wash of gold, but 
 for any appreciable depth of deposit the solvent action should 
 be aided by attaching the pieces to the positive wire of a 
 battery; the action must not be unnecessarily prolonged, as 
 many of the basis metals are dissolved in this way silver 
 especially so. 
 
 The Process. The articles, whether new or old (but in the 
 latter case after stripping), are well polished and cleansed by 
 potash and acid dips, and after a thorough rinsing are placed in 
 the gold-bath. Many platers quick the objects previously to this 
 by passing them through a weak solution of mercurous nitrate ; 
 but if they are clean and the baths are well prepared, they should 
 take a good coat of gold in the vat without this preliminary pro- 
 cess, and as gold is prone to amalgamate or alloy with mercury, 
 and thus to become discoloured, it is well to guard as far as 
 possible against damage to finished goods by avoiding the use of 
 mercury in all operations connected with gilding. 
 
 Small wares, when only a few pieces are to be dipped at a time, 
 may be slung upon a copper wire attached to the negative pole of 
 
ELECTRO-PLATING PROCESS FOR GOLD. 207 
 
 the battery, and are thus plunged into the gold-bath (usually 
 hot), the wire being still held by the right hand so that they 
 may be gently moved about in the solution ; the left hand mean- 
 while grasps the wire attached to the anode, and is thus able to 
 regulate the proportion of its surface which is immersed in the 
 liquid, and hence also to alter the strength of current at will. 
 The deposit should take place immediately, and as soon as the 
 articles are completely covered, the anode may be partially with- 
 drawn from the liquid to reduce the current-density, but not 
 sufficiently to give rise to a yellow coating of gold. This method 
 of working has the merit of affording control over the colour 
 of the deposit; if too pale it may be brought to the slightly 
 brownish yellow which ultimately gives the richest tone, by 
 lowering the anode and thus increasing its surface ; or if too 
 dark it may be correspondingly improved by decreasing the area 
 of anode exposed. 
 
 After a few seconds' immersion, the pieces should be lifted from 
 the solution and examined. If the coating be sound and of good 
 colour, the position of the wire-support should be shifted slightly 
 by a gentle shake, and the goods returned to the bath. They 
 should be re-examined from time to time, and finally removed, 
 washed, and scratch-brushed lightly or burnished. The washing 
 should be effected in several waters as explained in reference to 
 copper (p. 136). If at the first inspection the colour is found to 
 be wrong, the fault must be remedied by altering the position of 
 the anode to a proportionate extent ; if the object is imperfectly 
 coated owing to the presence of grease-marks (which is scarcely 
 likely to be the case owing to the comparatively ready solubility 
 of grease in the hot cyanide of the bath itself), it must be re- 
 moved, rinsed, and re-dipped in potash, again rinsed and returned 
 to the plating-vat. Bad solutions sometimes cause discoloured 
 patches on the surface of the article, and these should be obliter- 
 ated by scratch-brushing before continuing the deposition. The 
 beer or other organic matter from the liquid used in scratch- 
 brushing must, of course, be most carefully washed away before 
 replacing the goods in the gold-bath. 
 
 The continuance of a black or dark-brown deposit must not be 
 permitted, as it is impossible to produce a good coloured lustre by 
 polishing such a coating. If the colour be due to excess of gold in 
 the solution, or to too strong a current, the remedy is obvious ; 
 but if it be due to organic matter contained in an old bath, the 
 use of the latter must be discontinued. For some classes of 
 work, however, such as the coating of interior surfaces of 
 tankards and the like a slightly dark colour is often preferable, 
 and an old bath which is not absolutely past use may find an 
 application here. The duration of the gilding-process rarely 
 exceeds a few minutes, as a comparatively thin film of the metal 
 suffices for most purposes. When thick deposits are required, the 
 
208 ELECTRO-DEPOSITION OF GOLD. 
 
 pieces may be removed from the bath two or three times during 
 the process, and be scratch-brushed, well washed, and returned. 
 
 Large objects cannot, of course, be treated in this manner, but 
 should be suspended in the bath as in ordinary plating -vats, but, 
 if possible, they should be gently moved from time to time ; 
 very large pieces are more often treated in cold baths because 
 of their greater convenience of application. Small perforated 
 articles are best hung upon a wire on which they are separated 
 from one another by small glass beads ; but the wire should now 
 and again be sharply shaken, to prevent the formation of 
 permanent marks upon the goods at the points of contact; so 
 also in slinging chains in the gold-bath, the relative positions of 
 the links should be shifted from time to time with the same 
 object in view. To gild the interior of a vessel it must be well 
 cleansed and prepared to receive the deposit, and then very 
 thoroughly dried on the exterior, especially around the rim. 
 Having been rested upon a level surface, and attached by a wire 
 to the negative pole of the battery, it is carefully filled to the 
 brim with gold solution at a temperature of 120 to 140 F., so 
 that none of the liquid splashes or creeps over the margin (hence 
 the necessity for absolute dryness outside). A gold anode 
 attached to the positive pole of the battery is dipped to a 
 sufficient distance in the liquid and gently stirred around within 
 it; in a few seconds the interior surface will be completely 
 covered with gold, and in four or five minutes a sufficient 
 deposit will have been effected. The anode is then removed, 
 the liquid returned to the heated gold-plating vat, and the 
 article thoroughly washed, polished, and dried. Attention is 
 chiefly to be bestowed in securing an even margin at the edge of 
 the vessel; if it be not sufficiently dried initially, the gilding- 
 solution will gradually creep up the damp portions ; and where- 
 ever the liquid has penetrated, gold will be deposited, and thus a 
 wavy line instead of a straight one will mark the junction between 
 the outside of the vessel and the lining ; splashings which are in 
 liquid connection with the main portion of the solution will, of 
 course, bring about a similar result. 
 
 It often happens that the vessels which are to be gilt interiorly 
 have an irregular outline at the top, so that some parts of the 
 surface which should receive a coating are above the surface of 
 the liquid, as, for example, in the case of ewers and other lipped 
 wares. All these portions may, however, be covered by means of 
 a doctor ; this consists of a piece of soft rag folded several times 
 around a thin strip of gold connected as an anode. On saturating 
 the rag with gilding-solution and applying it as a brush to the 
 parts which are to be treated, the object being, of course, con- 
 nected up as a cathode, the current passes and electrolyses the 
 liquid in the rag, depositing the gold upon the metallic surface 
 and dissolving it from the anode. As, however, there is not free 
 
DEAD-GILDING. 209 
 
 motion in such an absorbed solution, it is advisable to re-moisten 
 the rag in the gold-bath from time to time. The doctor is best 
 applied during the time of gilding the rest of the interior, as there 
 is then less likelihood of a line forming, which would indicate the 
 level of the liquid in the vessel. Such a line, if at all pronounced, 
 is not easy to obliterate without risk to the gilding around. 
 Nevertheless, with care, the rag-gilding may be applied either 
 before or after the other process without evil consequences. The 
 more elaborate device proposed by Wagener and Netto for 
 other purposes (p. 104) could be applied to this work if desired. 
 
 The time required for gilding is far less than that demanded 
 for silvering, because, as a rule, the coated articles are not 
 required to withstand such severe wear, and a much thinner 
 deposit is sufficient ; but thimbles, pencil- or watch-cases, or any 
 object which will be put to the test of rougher usage, must receive 
 a fair proportion of gold. For other goods which are to receive 
 but a thin film, the actual thickness must be regulated by the 
 colour, which depends largely in the early stages of gilding upon 
 the colour of the basis metal ; thus brass, copper, or bronze, 
 which are themselves yellow or reddish metals, become thoroughly 
 gold-like after an immersion of a few seconds, while silver, which 
 is a white metal, pales the colour of the gold precipitated upon 
 it until sufficient has been deposited to form a film completely 
 opaque. Occasionally silver is covered with a preliminary wash 
 of copper, in order to enhance the colour of a mere wash of de- 
 posited gold. It must be borne in mind, however, that a mere 
 film of gold will not entirely protect the silver or other basis metal 
 from the action of the atmosphere ; so that such surfaces are very 
 liable to tarnish when brought into large towns, or wherever the 
 air is at all polluted with hydrogen sulphide. 
 
 Dead-Gilding. It is sometimes desired to produce a surface 
 with that dead lustre which in richness of effect is so charming 
 to the eye. This may always be accomplished by ensuring that 
 the surface is dead before gilding ; if it be not, it must be 
 rendered so, either mechanically, by rubbing with a fine powder, 
 such as that of Bath-brick, which will impart the necessary 
 degree of roughness without causing deep marks or scratches ; 
 or chemically, as in the case of copper, by dipping momentarily 
 into a strong acid ; or electrolytically, by imparting a preliminary 
 frosted, film to the article before gilding. Of these processes the 
 first is self-explanatory. The second is convenient for small 
 wares, being especially adapted to copper, brass, or bronze goods, 
 and depends for success upon the microscopically uneven etching 
 of the surface of a minutely crystalline, or not absolutely homo- 
 geneous, material. It is effected by plunging the goods, sus- 
 pended from a wire, or contained in a perforated porcelain or 
 platinum-wire basket, into a bath containing 100 parts of nitric 
 acid (specific gravity = 1 *33), a like quantity of sulphuric acid 
 
 14 
 
210 ELECTRO-DEPOSITION OF GOLD. 
 
 (specific gravity = 1 '84), and one part of common salt. They are 
 almost immediately removed and plunged into a large volume of 
 water, so that the acid clinging to them is at once washed away 
 (an insufficient volume of wash-water would, for the first moment, 
 only dilute the acid, and cause it to attack the metal too violently). 
 If sufficiently frosted, they are very thoroughly washed, and, with 
 or without quicking, according to the practice of the establishment, 
 are passed on to the gilding-vat. If still too bright, the acid dip 
 is again and again repeated, until the requisite degree of dulness 
 has been imparted. It need hardly be observed that previous to 
 this process the goods must have been thoroughly cleansed by 
 the usual dips. The third method is equally convenient and 
 simple, and is especially suitable to silver articles. Advantage is 
 taken of the beautiful dead lustre of electro-deposited copper ; a 
 thin film of this metal is deposited upon the cleansed silver surface, 
 and without scratch-brushing or burnishing, but after washing 
 well, is at once protected by the gold deposit. Occasionally the 
 copper is made still more dead by very rapidly passing the pieces 
 through the acid dip, immediately before gilding ; but in this 
 case great care must be taken that the copper is not entirely 
 removed at any point, because at such places an irregularity 
 of gilt surface would be apparent. The addition of aurate of 
 ammonia to the gold-bath also tends in the direction of giving a 
 good dead lustre. 
 
 Dead-gilded work must never be exposed to friction either in 
 manufacture or in use or it will rapidly become brightened ; 
 this class of gilding is, therefore, not applicable to general work, 
 but is well suited to surfaces which will be kept under glass, 
 such as the dials of clocks or philosophical instruments, or for 
 the groundwork of a raised design which will protect it from 
 being rubbed, and which, being itself burnished, may be made 
 to produce a very fine effect by the skilful contrasting of the two 
 styles of gilding. When the mechanical method of producing 
 the dead surface before gilding is resorted to, as is often done 
 with sword mountings and the like, or where the two kinds of 
 gilding are blended on the same object, the Bath-brick treatment 
 should be confined, as far as possible, to those portions which are 
 to be dead-surfaced, as the labour of brightening the others after- 
 wards would be greatly increased. So, too, in imparting the 
 final polish to the bright portions, the tools must not be allowed 
 to touch the other surfaces, or the latter will at once lose their 
 characteristic appearance. 
 
 Some difficulty is at times experienced in giving the first 
 coating of gold to dead surfaces, and the progress of the 
 deposition must be carefully watched. Should such difficulties 
 arise, the current-strength and that of the solution may be 
 increased ; indeed with filigree work, in which several depths of 
 surface are presented, there may be difficulty in forcing the 
 
GILDING ELECTRO-POSITIVE METALS. 211 
 
 deposit into the deeper interstices, unless this treatment is 
 resorted to. But as these alterations of conditions both tend in 
 the direction of giving a brown deposit, which it is quite impos- 
 sible to remedy if the surface is to remain dull, and almost so if 
 it should be brightened (because of the difficulty in causing the 
 polishing tool to penetrate to all parts), the countervailing 
 precautions of using a new bath and keeping the pieces in active 
 motion in the liquid must be taken, because cceteris paribus 
 either of these measures tends to lighten the colour of the deposit. 
 When the surfaces are once covered with gold, the progress should 
 be steady. 
 
 Gilding the more Electro-positive Metals. Any metal which 
 will itself deposit gold from the cyanide solution demands especial 
 care in its treatment. When it is possible to effect the gilding 
 without preliminary protection by copper, as, for example, in the 
 case of nickel-silver, iron, or steel, the rapidity of deposition must 
 be checked by iising a weaker solution, a less intense current, and 
 a lower temperature during the operation : unless these points 
 are attended to, the deposit will probably not be sufficiently 
 adhesive to withstand the subsequent treatment of polishing. 
 Very commonly these metals are first protected by a coat of 
 copper or brass, imparted by the alkaline (cyanide) bath ; lead, 
 Britannia metal, zinc, and similar metals are always so treated. 
 A thin film of copper is deposited upon the well-cleaned surface in 
 the cyanide-vat (p. 129); then, if wished, this may be slightly 
 thickened in the acid copper-bath, and having been scratch- 
 brushed (and quicked if desired) it is ready for gilding. 
 
 The dead-gilding of large surfaces of zinc is frequently demanded 
 by the requirements of art, and is readily accomplished by a modi- 
 fication of the last process. The metal being well prepared is 
 coppered thinly in the cyanide- vat, washed, scratch-brushed to 
 ensure an even and adherent deposit, and a slight increase of 
 copper is given in the same bath ; then, after washing, a thicker 
 frosted deposit is made in the acid copper-bath, and now the 
 surface is ready for the gold- vat, which should be used almost at 
 the boiling-point. A sufficient amount of gold having been thrown 
 down, the plate is thoroughly washed and dried, preferably in a 
 drying-oven. It is necessary to avoid handling the dead-coated 
 portions at any stage of the process, as the slightest mark becomes 
 visible upon the finished surface. Many operators, according to 
 Roseleur, give the coppered objects a thin coating of silver by 
 simple immersion before gilding, and, after drying and burnishing 
 those portions which are to be brightened, impart a second thin 
 coat of gold, wash, and again dry it. 
 
 When silver objects have been joined with tinman's (soft) 
 solder, which is an alloy of tin and lead, the gold frequently 
 refuses to deposit upon it, especially if the bath is in bad order 
 or the current weak. Watt obviates this difficulty by painting 
 
212 ELECTRO-DEPOSITION OF GOLD. 
 
 the line of the solder with a solution containing 5 per cent, of 
 copper sulphate crystals and a like proportion of sulphuric acid, 
 and lightly resting a piece of iron in the liquid in contact with 
 the solder ; galvanic action is immediately set up, iron dissolves, 
 and an equivalent of copper is precipitated over the whole 
 surface covered by the solution. Thus, a copper surface is 
 substituted for one of solder, and the entire object may be gilt 
 in the usual way. When only a wash of gold is to be given, the 
 coppered line should preferably be silvered before gilding, to 
 ensure uniformity of colour. 
 
 Ornamentation and Treatment of Gilt Surfaces. It is suffi- 
 ciently obvious that, from the number of varied tints which may 
 be imparted to electro-deposited gold, as well as from the different 
 lustres obtainable, the means of ornamentation depending upon 
 the combination of these alone, or of part- silvering and part-gild- 
 ing, are almost infinite in number and variation of effect. Thus, 
 a silver (or silvered) article may be in part gilded, by painting 
 with a stopping-off varnish (see p. 388) those portions of the 
 design that are to remain as silver; then, after drying and 
 hardening the varnish by heating gently in a stove, the re- 
 mainder may be caused to receive a deposit of gold of any desired 
 colour. Or a pattern may be left blank upon the varnished 
 surface, to receive a coat of red gold ; the varnish may then be 
 washed off the remaining portion by the application of turpentine 
 and subsequently spirits of wine, and the gilt pattern being in 
 turn protected, the groundwork may be covered with a green or 
 yellow gold. This class of work is known as Par eel -gilding. But 
 the various combinations of flat and relief surfaces, of red, green, 
 and yellow gilding, with silver, and with gold of bright or dead 
 lustre, are better discussed in a manual of decorative art. Here 
 we simply indicate the means by which the ideas of the artist 
 may be carried into effect. 
 
 It occasionally happens that the gold deposit has not a good 
 colour, so that a special colouring process must be resorted to. 
 Usually the electro-deposited metal, being under better control 
 at the time of precipitation, does not require this treatment, but 
 the gilding produced by simple immersion may do so. The 
 required shade may, of course, be imparted by electro-gilding; 
 but when the layer of gold is not too thin, the older jeweller's 
 method may be adopted, by which the subjection of the object 
 to the action of oxidising agents in a state -of fusion, or in hot 
 concentrated solution, effects the oxidation and removal of the 
 alloyed metal upon the surface, leaving the gold unaltered. Such 
 a mixture is made by fusing together in an earthen pipkin equal 
 parts of alum, nitre, ferrous sulphate, and zinc sulphate ; when 
 thoroughly melted it is mixed by stirring, and is painted over 
 the surfaces of the objects to be coloured. These are then 
 suspended in the central space of a specially-constructed circular 
 
TREATMENT OF GILT SURFACES. 
 
 213 
 
 furnace, which has an inner concentric lining of vertical fire bars 
 (fig. 92 shows this furnace in vertical cross-section), the annular 
 space between the two being filled with incandescent fuel. Here 
 they should remain until a moistened surface, caused to touch 
 any of the pieces, produces a slight hissing sound, by which time 
 the colouring mixture will have fused ; they are then removed, 
 and at once pickled in dilute sulphuric acid (water containing 2 
 or 3 per cent, of the acid), until the solid crust and the oxide of 
 copper, produced by the oxygen of the nitre, have dissolved and 
 left a clear gold surface. Any copper uncovered, or any basis 
 metal insufficiently protected, will, of course, be corroded and 
 the piece spoiled ; copper surfaces will have become covered with 
 red cuprous oxide, which is insoluble in the weak acid. If results 
 of this nature have been obtained, the only remedy is to strip off 
 any gold which may remain, and re-cleanse and re-gild the objects 
 with a thicker coating. 
 
 A mixture made from 2 parts of potassium nitrate and 1 part 
 each of alum, sodium chloride, and zinc sulphate by rubbing 
 them into a thick paste may be applied in the 
 same way ; it is painted over the surfaces to 
 be covered, and the pieces are heated on a 
 clean iron plate over a charcoal fire until the 
 mixture has darkened in colour, when it is 
 removed by dipping the articles into dilute 
 sulphuric acid. A third mixture, which is 
 often similarly employed, consists of 6 parts 
 of potassium nitrate, with 2 of ferrous sul- 
 phate, and 1 of zinc sulphate. But, however 
 useful these mixtures may be in imparting a 
 good colour to inferior but solid alloys of gold 
 with copper, they are certainly not generally 
 suitable to the treatment of the thin films of 
 
 FIG. 92. Colouring- 
 furnace. 
 
 gold imparted by electrolysis to articles of base metal ; and 
 the electro-colouring process, by judiciously combining gold and 
 copper or gold and silver solutions, or by merely adjusting 
 the strength of the plating-current, is far more reliable and 
 satisfactory. 
 
 Gilding of Watch-Mechanisms. The peculiar semi-dead lustre 
 noticeable in the internal movements of watches is produced by 
 a preliminary mechanical process known as graining, which has 
 been fully described by Roseleur. It carries sufficient intrinsic 
 interest to warrant the insertion of an outline sketch of the 
 process at this point. 
 
 The various parts are carefully polished to obliterate com- 
 pletely all file- and tool-marks; they are next strung upon a 
 brass wire and boiled in a 10 per cent, solution of caustic potash 
 or soda, to remove grease ; and, being then well rinsed with 
 water, should show no marks of imperfect wetting, which would 
 
214 ELECTRO-DEPOSITION OF GOLD. 
 
 indicate the presence of unremoved greasy matter. (This would 
 point to the necessity of further treatment with potash.) All 
 iron or steel portions are now protected by covering them with a 
 stopping-off varnish, applied in the melted condition by means of 
 a thin warm glass rod, and consisting of 
 
 Clear rosin, . . 10 parts. Best red sealing-wax, . 4 parts. 
 Yellow bees'-wax, . 6 ,, Finest polishing-rouge, 3 ,, 
 
 The rosin and sealing-wax are melted together, the bees'-wax 
 is added, and, finally, the rouge is stiried in and thoroughly in- 
 corporated. The pieces may be momentarily immersed in an 
 acid dip, but this stage is frequently omitted ; in either case they 
 are now fastened by flat-headed brass pins to a level surface of 
 cork, cavities being made when necessary, to allow for spindles 
 or projections upon the articles. Held thus in position, they are 
 rubbed well by a rotary motion, with a brush dipped in powdered 
 pumice, and wetted with water; after perfect rinsing they are 
 (with the cork-support) passed rapidly through a weak quicking- 
 solution containing 1 part of nitrate of mercury and 2 of 
 sulphuric acid in 5000 parts of water, and are ready for the 
 process of graining. Impalpable silver-powder must be procured 
 for this purpose ; it may be prepared either by grinding the finest 
 leaf -silver (thin silver-foil) with honey upon a ground-glass slab 
 with the aid of an artist's muller, and then washing away the 
 honey with boiling water, the mixture being placed upon a good 
 blotting-paper filter (p. 52) ; or by placing strips of clean copper 
 in a very dilute solution of silver nitrate, collecting the spongy 
 silver precipitated by 'simple immersion,' washing it free from 
 adhering copper solution, and drying it. The silver-powder is 
 then most intimately mixed with finely - powdered and -sieved 
 tartar (potassium bitartrate) and common salt, in the proportions 
 of 3 of silver with 10 to 30 of -tartar and 40 to 100 of salt; the 
 ingredients should of preference be dried individually at a 
 temperature slightly above 212 F., and mixed warm. The 
 mixture is now made into a thin paste with water and spread 
 evenly over the surfaces of the pieces, and is rubbed persistently 
 over the whole by an oval brush with stout hard bristles; a 
 circular motion should meanwhile be imparted both to the brush 
 and to the cork, but in opposite directions. The use of a large 
 quantity of paste or of much salt produces a large grain, while 
 less paste and more tartar yield a smaller- grain ; the desired 
 effect of roundness is imparted to the grain in direct proportion 
 to the extent of the circular motion applied to the work. After 
 this operation and subsequent washing, the surfaces are scratch- 
 brushed with a straight brush of very thin brass wires more or 
 less annealed. It is recommended to keep three brushes : one of 
 which has been heated to dull redness and cooled, and is very 
 soft in consequence; one somewhat less heated, and so only 
 
GILDING WATCH MECHANISMS. 215 
 
 moderately hard ; and one but slightly annealed, and, therefore, 
 much harder. The scratch-brushing must also be effected under 
 the influence of a double rotary motion, applied to cork and 
 brush, and is aided by a decoction of liquorice or soapwort as a 
 lubricant. The grain should finally be perfectly uniform, even 
 when viewed under a magnifying lens. 
 
 The pieces are now removed from the cork, and being attached 
 singly to suitable holders, are suspended in the gold-bath, for 
 which Roseleur recommends the use of the solution No. 1 7 (quoted 
 on p. 202), containing gold fulminate. The gold should be slowly 
 deposited by a moderate current and with platinum anodes. The 
 pieces are finally re-scratch-brushed with either of the lubricants 
 above named ; and the varnish is removed from the steel portions 
 by the application of warm oil, benzene, or turpentine, followed 
 by immersion in an alkaline solution almost boiling. 
 
 Platinum- or gold-powders may be substituted for the silver 
 in the production of grain, but, as their use presents no advantage 
 and they are far more costly, they are rarely so employed. 
 
CHAPTER XL 
 
 THE ELECTRO-DEPOSITION OF NICKEL AND COBALT. 
 
 Advantages of Nickel. The introduction of nickel-plating is 
 one of the more recent applications of electrolysis. Until about 
 the year 1870 or later, the high cost of metallic nickel, combined 
 with the impurity and unsuitability of the metal which was then 
 available, rendered nugatory all attempts at electro-nickeling on 
 a large scale. But with the improvements in the metallurgy 
 and manufacture of nickel, which have placed comparatively pure 
 anodes on the market at a reasonable rate, arose the new industry 
 of plating with nickel, which has perhaps advanced with more 
 rapid strides than any of its numerous rivals in the same field. 
 The extreme hardness of deposited nickel, which enables even 
 a thin coating to resist so well the wear and tear of hard use ; 
 the brilliant polish which from its hardness it is capable of taking ; 
 combined with its pure white colour, almost rivalling that of 
 silver, and its non-liability to tarnish under ordinary atmospheric 
 conditions all tend to popularise the use of the metal, not only 
 as a protective coating for more oxidisable metals, but as an 
 ornamental addition to those less attractive in appearance. In 
 the first-named capacity it is applied to exposed portions of 
 machinery which do not actually present working surfaces 
 that are liable to friction, especially in domestic appliances, 
 such as the sewing machine, accessories, or in bicycles, etc., in 
 which it plays the dual part of protecting and decorating. In 
 the second capacity it is applied to small articles of brass or 
 zinc, such as pencil-cases and umbrella-fittings, as well as to 
 larger surfaces, such as restaurant coffee-urns and the like. A 
 thousand other applications might be named, but the above 
 sufficiently represent the classes of work to which electro-nickel- 
 ing is daily applied, and which are constantly extending in all 
 directions. 
 
 The nickel-plater, therefore, has mainly three classes of work 
 to treat iron or steel, brass, and zinc ; none of them are likely 
 to give trouble, provided attention to detail is rigorously ob- 
 served. More than any other metal, perhaps, nickel requires 
 
 216 
 
ADVANTAGES OF NICKEL. 217 
 
 the most punctilious care in order to obtain an adhesive de- 
 posit; no metal is so sensitive to any undue change in the 
 manner of treatment, but, on the contrary, none repays the 
 operator so well for his attention to the preliminaries and re- 
 quirements of working. The solutions must, of course, be pure, 
 and the presence of even traces of a more electro-negative metal 
 (such as copper) must be carefully avoided, as the latter would 
 tend to be deposited first, and would alter the character, and 
 possibly even the colour, of the nickel deposit. But care is 
 most largely needed in the re-nickeling of old work, for it is 
 found that the metal cannot be induced to give a deposit in 
 any degree adhesive upon an old surface of nickel. (See, how- 
 ever, p. 233.) 
 
 Nickel well deposited is extremely hard, so hard that it cannot 
 be burnished, and is somewhat brittle. Thick coatings are 
 especially liable to flake off in use, unless exceptionally well 
 deposited, and even the thinnest films will part from surfaces 
 which are not chemically clean. Fortunately, however, thick 
 deposits are rarely required, on account of the high resistance of 
 the metal to wear, which enables even a thin film to rival in 
 durability a thick coat of any other metal. Nevertheless, the 
 opposite extreme to which manufacturers tend, is greatly to be 
 deprecated, because a non-durable film brings discredit upon the 
 process and upon the manufacturers who use it. It must be 
 remembered that the cost of a slight increase of thickness is by 
 no means proportional to the extra metal and to the battery- 
 power used ; inasmuch as one of the largest items of expenditure 
 is to be found in the preparation of the object to receive the 
 deposit, and this is constant, whatever the thickness of the 
 metal precipitated. 
 
 The nickel precipitated by too strong a current is grey and 
 pulverulent, and is said to be ' burnt ' ; but, on the other hand, 
 a feeble current produces a hard but very brittle deposit, which 
 will probably become separated from the plate during the final 
 process of polishing. Schaschl, using Pfanhauser's citrated bath, 
 has found that a film 0*2 millimetre thick deposited on thin 
 sheet-iron by a current of about 1*0 ampere per square decimetre, 
 became torn along the line of the bend when the plate was 
 sharply bent over upon itself; but that by first annealing the 
 plate at a dull-red heat, the nickel was so far softened that the 
 plate could be hammered down to a quarter of its original 
 thickness without incurring the slightest injury. Copper and 
 brass, however, on which nickel had been deposited by the 
 normal current, withstood this treatment without any preliminary 
 heating. 
 
 Nickel is practically never deposited by simple immersion or 
 by the single-cell process; the battery deposition is, therefore, 
 the only method which demands attention. 
 
218 
 
 DEPOSITION OF NICKEL AND COBALT. 
 
 TABLE XVII. SHOWING THE COMPOSITION OF NICKEL-BATHS FOR 
 1234567 8 9 10 11 12 13 
 
 No. 
 
 Authority. 
 
 Special 
 Application 
 of Bath. 
 
 PARTS BY WEIGHT 
 
 Nickel 
 Acetate. 
 
 j5| 
 
 . . Nickel 
 Chloride. 
 
 
 
 "s-2 
 
 o s-> 
 
 Nickel 
 Phosphate. 
 
 CO 
 
 JjLij 
 
 Potassium 
 Citrate. 
 
 Sodium 
 Bicarbonate. 
 
 it 
 
 8 
 
 Sodium 
 Phosphate. 
 
 
 g jf 
 
 HI 
 
 JJ02 
 
 1 
 
 2 
 3 
 
 4 
 
 5 
 6 
 
 7 
 8 
 9 
 10 
 11 
 12 
 13 
 14 
 15 
 
 16 
 17 
 
 18 
 19 
 
 20 
 21 
 22 
 23 
 24 
 25 
 
 Adams . . 
 
 Boden . . 
 Desmur . . 
 
 ' Electricias ' 
 
 Hospitalier 
 Langbein . 
 
 Nagel . . 
 Pfanhauser. 
 
 Potts . . . 
 Powell . . 
 >i 
 
 Roseleur . 
 
 Volkmer . 
 Watt. . . 
 
 Weiss . . 
 Weston . . 
 
 Small goods 
 
 Printing 
 surfaces 
 
 Tin, Britannia 
 metal, etc. 
 
 Iron 
 
 Iron and steel 
 Zinc 
 
 Hard deposit 
 
 
 
 
 26-7 
 
 
 ... 
 
 50-80 
 
 
 
 
 
 
 
 
 
 33 
 
 ... 
 
 
 
 70 
 50 
 100 
 
 50-60 
 
 ... 
 
 ... 
 
 8 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 60-72 
 
 ... 
 
 4-5 
 
 
 
 
 
 
 
 
 
 54 
 50 
 50 
 
 
 
 
 
 
 
 
 50 
 
 ... 
 
 
 27-5 
 
 
 
 
 
 
 
 
 
 
 
 ... 
 
 15 
 
 15 
 
 
 15 
 25 
 
 30 
 111 
 
 50 
 
 
 
 
 
 
 
 
 
 
 
 40 
 
 
 ... 
 
 
 3 
 
 26 
 
 
 
 
 
 
 
 33-3 
 40 
 
 50 
 50 
 
 ... 
 
 
 
 
 ... 
 
 
 
 
 
 
 
 50 
 
 
 
 
 
 
 
 ... 
 
 17 
 
 
 
 
 
 
 
 
 
 
 
 ... 
 
 42 
 
 
 ... 
 
 
 42 
 
 
 
 
 40 
 50-67 
 
 10 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 NOTES TO TABLE. The heavy figures in the last column refer to the numbers of the 
 
 which may be used nearly boiling, are generally 
 
NICKELING SOLUTIONS. 
 
 219 
 
 SEPARATE-CURRENT PROCESS, AS RECOMMENDED BY VARIOUS AUTHORITIES. 
 14 15 16 17 18 19 20 21 22 23 24 25 
 
 OF INGREDIENTS. 
 
 Special Method of 
 Preparation. 
 
 
 Ammonia. 
 
 Ammonium 
 Carbonate. 
 
 Ammonium 
 Chloride. 
 
 Ammonium 
 Sulphate. 
 
 Ammonium 
 Tartrate. 
 
 Calcium 
 Acetate. 
 
 d 
 
 3 
 o 
 IB 
 j 
 
 T3 
 
 1 
 
 o 
 o> 
 
 PQ 
 
 Boric Acid. 
 
 Citric Acid. 
 
 13 
 5 
 
 <$ 
 _o 
 
 1 
 
 1 
 
 
 
 
 
 
 
 
 
 
 
 0-25 
 
 1000 
 
 1000 
 1000 
 
 1000 
 1000 
 
 1000 
 1000 
 1000 
 1000 
 1000 
 1000 
 1000 
 1000 
 1000 
 1000 
 
 1000 
 
 1000 
 1000 
 1000 
 
 1000 
 1000 
 1000 
 1000 
 1000 
 1000 
 
 Neutralise, if necessary, with am- 
 monia. 
 
 Dissolve 12 in 25 ; then add rest. 
 
 Warm sol. of 8 in 25; add 11 
 slowly. 
 
 Stir all with 150 of 25 ; then add 
 rest of 25. 
 
 Boil, cool, and lilter. 
 
 Add 15 last, till just neutral. 
 
 Pour sol. of 15 into sol. of 8 till 
 just neutral ; avoid alkalinity. 
 
 Boil 7 and 17 with 25, add 15 till 
 neutral, then 23 till just acid. 
 
 (Mixed cobalt-nickel precipitate.) 
 
 26-7 
 
 
 
 
 
 
 
 
 
 
 ... 
 
 
 36-5 
 
 
 
 ... 
 
 ... 
 
 
 
 
 
 
 
 
 19-22 
 
 ... 
 
 
 
 
 25-30 
 
 4-5 
 
 
 190 
 37 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 q.s. 
 
 50 
 
 
 
 
 
 
 
 
 20 
 
 
 25 
 
 q.s. 
 
 
 
 
 
 7-5 
 
 
 5 
 7-5 
 
 
 n g 
 
 
 
 
 
 
 
 
 
 
 
 ... 
 
 q.s. 
 
 22-33 
 
 25 
 42 
 
 
 
 
 
 
 
 
 
 6-6 
 6 
 
 50 
 
 15 
 25 
 
 
 
 
 
 
 
 
 
 
 
 
 ... 
 
 ... 
 
 ... 
 
 
 ... 
 
 ... 
 
 q.s. 
 
 q.s. 
 5 
 
 
 
 
 
 
 ... 
 
 ... 
 
 
 
 
 
 17 
 
 
 
 
 
 
 
 
 
 
 
 
 15-30 
 
 
 
 
 
 
 
 
 
 
 vertical columns representing the various reagents. All the solutions except No. 3, 
 employed at the ordinary temperature. 
 
220 DEPOSITION OF NICKEL AND COBALT. 
 
 NICKEL-PLATING BY THE SEPARATE-CURRENT PROCESS. 
 
 The Battery. The Bunsen- or bichromate-cells are well suited 
 for this work, the electro-motive force best adapted for nickeling 
 being higher than that required for most metals. Two or even 
 three Bunsen-cells coupled in series may be employed, and these 
 should be found to last for one day's work, being renewed every 
 morning. The chief objection to this battery is to be found in 
 the red fumes of nitrogen peroxide evolved during use ; but this 
 is of small consequence if the cells be kept, as recommended, in 
 a separate and well-ventilated chamber. The current should be 
 somewhat stronger at first, until the whole surface is just flashed 
 over with nickel, when it should be reduced to the normal 
 strength. Thus, at the outset, it may conveniently be O'l 
 ampere per square inch, or 1'5 amperes per square decimetre at 
 5 volts' pressure, and should be subsequently reduced to 0*02 
 ampere per square inch at 2 volts' pressure. 
 
 The Solution. The number of solutions which have been 
 successfully employed is very great, and any of them may be 
 made to give good results : that being so, the simplest is prob- 
 ably the best. The list in Table XVII. includes some of the 
 principal formulae recommended. 
 
 For general work, the solution made by dissolving 8 pounds 
 of the nickel-ammonium sulphate in each gallon of water, with 
 the addition of just so much ammonia if it be acid, or of citric or 
 sulphuric acid if it be alkaline, as will suffice to render it exactly 
 neutral, will be found to give good results. Nickel-platers should 
 always be supplied with blue and red litmus papers, which by 
 turning red or blue respectively on immersion in the fluid, 
 indicate acidity or alkalinity ; when neutral the solution should 
 not change the colour of either paper. Although, theoretically, 
 the bath should be neutral, in practice it is found better to 
 maintain it in the faintest degree acid, 1 because secondary re- 
 actions, which constantly take place during electrolysis, tend to 
 liberate ammonia, and thus render it alkaline, and alkaline-baths 
 give trouble by depositing a basic compound of nickel in the form 
 of a greenish powder; moreover they tend, other things being 
 equal, to give a darker deposit of nickel. On the other hand, 
 excessive acidity must be avoided, as it may entirely prevent the 
 deposition of nickel upon the cathode, and it is liable to make 
 the deposit peel off. The cause of the increasing alkalinity is, 
 no doubt, to be ascribed to the simultaneous decomposition of the 
 nickel sulphate with a small proportion of the ammonium sulphate 
 as would be predicted. The electrolysis of the nickel salt alone 
 deposits upon the cathode a weight of metal equal to that dis- 
 
 1 Some operators, however, prefer an alkaline nickel-bath, and among them 
 (for use with their arrangement) the inventors of the Smith and Deakin 
 apparatus, described on p. 105. 
 
NICKELING SOLUTIONS. 221 
 
 solved from the anode, and is thus without influence upon the 
 composition of the bath. But the ammonium salt under elec- 
 trolysis deposits sulphuric acid upon the anode, which, by com- 
 bining with it, adds an equivalent weight of nickel to the bath, 
 while it throws down the elements of the hypothetical body 
 ammonium (NH 4 ) upon the cathode. Thus the sulphuric acid 
 is neutralised by the nickel, and the bath is enriched to that 
 extent by the fresh nickel introduced, while the ammonium is 
 broken up into ammonia (NH 3 ) and hydrogen (H), the former 
 dissolving in the solution and tending to alkalise it, the latter 
 escaping as a gas from the surface of the object being plated. 
 In depositing nickel, hydrogen is usually deposited to some 
 extent with the metal (in part due to the reaction above con- 
 sidered) ; but this must be minimised as far as possible by 
 regulating the strength of the current and of the solution, re- 
 membering that a strong current and a weak solution alike 
 favour the evolution of hydrogen. Whenever much hydrogen 
 is given off, the metal becomes grey and pulverulent, and the 
 absorption of hydrogen undoubtedly tends to brittleness. 
 
 The reactions in the nickel-bath may, therefore, be thus 
 
 62 
 
 At cathode. At anode. Forming with Ni anode. 
 Main reaction, Ni S0 4 NiS0 4 
 
 Subsidiary reaction, NH 3 + H S0 4 NiS0 4 
 
 While the bath is in use it should be frequently tested with 
 litmus paper, and rendered faintly acid, if necessary, with sul- 
 phuric acid. It is advisable to agitate the bath at intervals in 
 order to maintain an equality of density throughout ; but this is 
 not so necessary as in the case of copper deposition, because the 
 duration of the nickeling process is so much less, rarely requiring 
 an exposure for more than two or three hours. 
 
 Some operators have endeavoured, with considerable success, 
 to prevent the formation of the basic salt of nickel by the addition 
 of a small proportion of an organic acid (citric or tannic), or of a 
 feeble inorganic acid, such as boric acid. This last-named solu- 
 tion, as formulated by Weston (No. 25), certainly gives admirable 
 results, and although the simple solution previously recommended 
 may be made to yield a most excellent deposit without difficulty 
 by careful attention to the working of the vat, Weston's bath 
 allows more latitude in working, and may perhaps be preferred 
 by many. It is made by dissolving 8 or 10 oz. of a double 
 sulphate of nickel and ammonia per gallon of water, and adding 
 2J to 5 oz. of boric acid. 
 
 Various other nickel salts have been substituted for the double 
 sulphate, among them the chloride and acetate ; but in nearly 
 every case a double rather than a single salt is preferred. When 
 the single salt is used in making up the bath, a quantity of the 
 
222 DEPOSITION OF NICKEL AND COBALT. 
 
 corresponding compound of ammonia is added at the same time. 
 In one bath (No. 24) a small proportion of cobalt is added, which 
 causes a joint precipitate of the two metals that is said to possess 
 greater hardness than nickel alone. Langbein states that baths 
 made with the addition of chlorides, or made with nickel chloride 
 or nitrate, are not suitable for solid nickeling of iron, though 
 well adapted to the rapid light nickeling of cheap brass articles. 
 
 The Anodes. The nickel anodes must be as pure as it is 
 possible to obtain them. They are to be had either cast or 
 rolled, of almost any shape. The cast plates are less dense and, 
 as a rule, are more readily soluble than the others ; either kind 
 may be used, but the latter may be procured thinner than the 
 former, and the prime cost of the nickeling plant is thus reduced, 
 moreover, they are more uniform and reliable in composition, 
 and are less liable to become spongy during treatment owing to 
 unequal solution. They should present a total area in excess of 
 that of the cathodes, in order that the bath may be kept saturated 
 with nickel, thus allowing for the inferior solubility of the metal. 
 
 A bath that requires a difference of potential of 2 volts between 
 cast anodes will probably work well and give even better results 
 with a difference of 3*5 or 4 volts between rolled anodes. 
 
 Cast anodes being the more readily soluble are more likely to 
 neutralise the acid set free by electrolysis at the anode, and hence 
 the natural tendency of the (ammoniacal) nickel bath to become 
 alkaline asserts itself. Rolled anodes are more likely by insuffi- 
 ciently neutralising the acid to cause the bath to become acid. 
 Langbein states that baths containing boric acid require a 
 definite proportion between rolled and cast anodes. 
 
 Carbon anodes are not to be recommended, partly because, 
 sooner or later, the carbon is sure to disintegrate into the bath, 
 and partly because, as it does not dissolve in the solution, the 
 acid radical deposited at the anode is not neutralised, and there- 
 fore causes the bath rapidly to become acid. 
 
 The anodes should be supported by nickel hooks, and may 
 with advantage be made with lugs at the upper corners, as 
 indicated in fig. 50, so that the hooks do not enter the solution ; 
 even, however, with these anodes, the use of brass or copper 
 supports is to be avoided, for, becoming splashed with the solu- 
 tion, they would in time become corroded, and the copper passing 
 into the vat would seriously damage the nickel : bath. 
 
 The Vats. Any glass, earthenware, enamelled iron, or lined 
 wood tank may be used. If the solution is to be heated, the 
 enamelled iron is, of course, preferable. The vat should always 
 exceed in size that of the work treated by 15 or 20 per cent. 
 
 The Process of Electro-Nickeling. Stripping. It is even 
 more important in nickeling than in silvering or gilding that an 
 existing film of nickel shall be entirely removed, or the new 
 deposit will most certainly lack adhesive properties. 
 
ELECTRO-NICKELING. 223 
 
 R. C. Snowdon, 1 however, has found that the nickel need not 
 be removed if the article is made the cathode in an acid bath 
 and current is passed so as to produce a vigorous evolution of 
 hydrogen on the nickel to remove all trace of oxide ; the article 
 is then washed rapidly and is plunged into the usual plating solu- 
 tion. The acid solution used was a 3-normal solution of hydro- 
 chloric acid, with a current-density of 8 amperes per square 
 decimetre. The plating solution was nickel-ammonium sulphate, 
 and current-density 2 amperes per square decimetre. In what 
 follows, however, we shall describe the more usual treatment. 
 
 Small articles may be treated by persistently rubbing them 
 with fine emery-cloth until the desired end is accomplished. 
 More often a chemical method is used, which consists in dipping 
 the articles for a brief period into an acid bath that will readily 
 attack nickel. Such a bath may be prepared, as recommended 
 by Watt, by gradually and carefully adding one volume of nitric 
 acid and two of sulphuric acid to one volume of water, constantly 
 stirring meanwhile with a porcelain or wooden rod to prevent the 
 action from becoming too violent. The liquid is allowed to cool, 
 and should be transferred for use to a glazed earthenware pan or 
 dish sufficiently large to hold any object which it will be required 
 to treat in it. It is employed cold or only very slightly warm, 
 and should be placed outside the operating-room, in a well- 
 ventilated place, for example, in the battery - cupboard, be- 
 cause unwholesome and irritating acid fumes are evolved during 
 the process. The author has frequently used this bath with 
 success. 
 
 The plated article is suspended from a copper wire and plunged 
 beneath the liquid in the bath, where, however, it should not be 
 permitted to remain for more than a few seconds at a time, for a 
 thin coating is almost instantaneously dissolved, and the acid is 
 then free to attack the basis metal beneath. It is, therefore, 
 frequently removed from the vat and closely examined, so that 
 the action may be stayed at the moment when the last trace of 
 nickel has disappeared ; it is then transferred to a large volume 
 of cold water, and after washing twice or thrice in fresh water, is 
 ready for the subsequent stages of the process. 
 
 Some operators prefer to strip by electrolysis, by making the 
 object the anode in an old nickel-bath. Attention is equally 
 necessary in conducting this process to guard against any attack 
 upon the basis metal ; but, since it is impossible entirely to 
 prevent all action, no bath which is to be afterwards employed 
 for depositing the metal should be used for this purpose, as it 
 will become gradually charged with impurities. A 10 per cent, 
 solution of sulphuric acid may be used for electrolytic stripping. 
 
 Iron or steel articles are best treated by either of the first two 
 processes, brass or copper by either of the two last-named, yet 
 1 Trans. Amer. Electro-chem. Soe., vol. vii. p. 301. 
 
224 DEPOSITION OF NICKEL AND COBALT. 
 
 with due care any of the three methods may be applied to all 
 classes of work. 
 
 Preliminary Preparation for the Bath. It has already been 
 pointed out that, by reason of its extreme hardness, the nickel 
 deposit cannot be burnished. Ordinary methods for imparting 
 the final polish to electro-plated goods are not, therefore, appli- 
 cable to nickel-coated wares. It is thus essential that the highest 
 possible polish 1 should be given to the objects prior to immersion 
 in the plating- vat, remembering that as they are when placed in 
 the bath so they will be when finished. Even traces of pre- 
 existing scratches, or tool-marks, cannot be obliterated, except 
 with the greatest difficulty, when once nickel has been precipi- 
 tated upon the surface. If, therefore, the goods passing into the 
 hands of the plater are in any degree rough or unfinished, they 
 must be most carefully polished until every scratch has ceased 
 to show ; extra care should be taken with large blank areas of 
 surface, unbroken by the lines of a design or by a change of 
 shape in the article itself, because even the slightest flaw becomes 
 more visible on such surfaces than upon smaller articles. 
 
 More than usual care must also be bestowed upon the cleans- 
 ing operations. In silver- and gold-plating, especially with warm 
 solutions, the cyanide liquor compensates for any slight inade- 
 quacy of cleansing by its power of dissolving the offending 
 grease, so that the surface is finally cleansed by the electrolytic 
 bath itself (which, however, is gradually spoiled by the absorp- 
 tion of organic matter), but the nickel-bath has no such solvent 
 action ; so that it cannot be too strongly impressed upon beginners 
 that the success of their work is dependent upon the absolute 
 chemical cleanliness of the pieces to be plated. After polishing, 
 every trace of grease is first removed in the potash-vat, and of 
 tarnish in the acid dip (for iron goods) or the cyanide-bath (for 
 brass, copper, or zinc). Then after a thorough rinsing in water, 
 the goods are transferred without loss of time to the plating-vat. 
 After passing through the potash-bath the surface of the article 
 should be handled with brass tongs or with clean rags, and 
 must on no account be touched with the hands. 
 
 Copper and brass articles always, wrought-iron and steel com- 
 monly, are at once nickeled without further preliminary treatment ; 
 zinc is also occasionally treated in the same way, but, inasmuch 
 as it is readily attacked by the nickel solution, and the latter is 
 rendered worthless when contaminated with zinc, it is advisable 
 to protect the objects with a covering of copper before immersion. 
 Meidinger has suggested a covering of the zinc sheet with mer- 
 cury, which would answer the same purpose, but care is neces- 
 sary to guard against over-amalgamation, which only renders the 
 plate very brittle without affording any corresponding advantage. 
 Cast-iron also should be covered with copper; it is a common 
 1 See, however, p. 123. 
 
ELECTRO-NICKELING. 225 
 
 practice first to bestow upon the surface a wash of tin, then upon 
 this one of copper, and, finally, the layer of nickel. The articles 
 having been made perfectly bright and clean, and, if necessary, 
 covered with copper, are ready for suspension in the bath. 
 
 Nickel-Depositing. It has been pointed out that the current 
 should be somewhat stronger at first than subsequently j but it 
 must not be so intense that the metal becomes burnt, as explained 
 on p. 217, a fault which is by far the more serious of the two. 
 Therefore, in introducing the cleaned objects, although the cur- 
 rent must pass immediately they enter the bath, the objects first 
 suspended must be protected from receiving an excessive current 
 by the interposition of resistances, or by hanging one or more 
 anodes from the cathode rods, as explained in speaking of electro- 
 silvering on p. 194. The surface of the article should almost 
 immediately be completely covered with a grey deposit of nickel. 
 The goods are suspended by copper wires, which should be used 
 only once, because from the want of adhesion of nickel to old 
 nickel surfaces the metal deposited upon old wires is liable to 
 strip off in flakes, which, falling into the solution, may, in course 
 of time, form a metallic bridge or connection between the elec- 
 trodes, and thus produce a short circuit, or they may adhere to 
 projecting portions of the cathode surface and interfere with the 
 regularity of the coating. An alkaline bath is apt to give a 
 darker tinge to the deposited nickel than is the acid bath ; the 
 same effect, however, is produced if the current-density is not 
 suitable. 
 
 The nickel solutions are usually inferior conductors of elec- 
 tricity, and there is in consequence a more marked difference 
 than usual in the rate of deposition upon portions of a given 
 object placed at different distances from the anode ; and there is 
 even less tendency for a current to pass through great lengths 
 of solution when the basis metal is also a poor conductor of 
 electricity ; coating bad conductors with copper is, therefore, to 
 be recommended as a distinct assistance in starting a deposit of 
 nickel. Objects which are to be coated on all sides with nickel 
 should therefore be quite surrounded with anodes, and should be 
 placed as nearly as possible equidistant from them ; and if they 
 have an irregular form, they should be systematically inspected 
 to ensure that all the deeper hollows are covered at once. While, 
 then, on the one hand, the pieces must be very carefully examined 
 after they have been struck (i.e., first completely covered with 
 nickel), they must not, on the other hand, be kept too long out 
 of the solution, so that they tend to become dry, because in that 
 time they will have acquired an imperceptible film of oxide, which 
 will effectually prevent the adhesion of the nickel afterwards 
 deposited. 
 
 The thickness of the nickel need not, as a rule, be very great 
 on account of its extreme hardness. Generally speaking, from 
 
 15 
 
226 DEPOSITION OF NICKEL AND COBALT. 
 
 half an hour to four hours will suffice for the deposition. Thick 
 deposits are very liable to peel off, occasionally spontaneously 
 in the bath, but more often during the period of administering 
 the final polish ; this is especially the case with iron and steel 
 goods, which take a thick deposit less satisfactorily than those 
 made of brass or copper. This peeling of the metal, whenever 
 it happens, is annoying, because it necessitates stripping the 
 remainder of the deposit, with a recommencement of the process 
 de novo; but if it occur in the bath, the separation of loose 
 fragments may give trouble in a manner already described. 
 
 When the thickness of coating is sufficient, the pieces are 
 removed from the bath and thoroughly washed in cold water, 
 then plunged into boiling water, so that evaporation may take 
 place more rapidly, and dried completely small objects in hot 
 sawdust, large articles in a stove heated to, or very slightly 
 above, the boiling-point of water. They must then receive their 
 final polish, and are ready for the market. 
 
 The nickeling of larger or irregular surfaces is conducted after 
 the same manner as that of smaller objects : the conditions to 
 be observed most particularly are that the goods shall be 
 thoroughly polished and absolutely clean ; that they shall be 
 as far as possible surrounded by anodes, and equidistant from 
 them; that the whole surface is in fair conductive connection 
 with the negative pole of the battery ; and that the solution is 
 in good order, being neither alkaline nor more than feebly acid. 
 To secure good connection it is often desirable to employ more 
 than one wire, especially when considerable lengths, such as 
 chains or rods of a feeble conductor, are under treatment ; these 
 should be supported from the cathode-rods at intervals by copper 
 hooks, so that several starting-points are offered, instead of one, 
 for the formation and spread of the deposit: there is thus a 
 greater uniformity of coat. The hooks must be shifted from 
 time to time, to avoid the formation of surface markings. 
 
 Small objects should not be coated in the perforated porcelain 
 pans recommendable in plating with other metals, because of the 
 difficulty in arranging the anodes. It is, indeed, possible to 
 effect the nickeling in this manner, and the method is sometimes 
 practically adopted ; but it is safer either to attach each article 
 individually to a copper wire, or to rest several together on a 
 very shallow and narrow metal tray which may be suspended 
 betweeen the two anodes. The Smith Deakin process (p. 105) may 
 with advantage be employed. Small articles are especially liable 
 to receive a burnt deposit when first placed in an empty vat, and 
 for this reason either a large number of articles should be intro- 
 duced into the solution simultaneously, or one of the anodes 
 should be made a cathode for the time being, as previously 
 explained, or small pieces may be immersed while the current is 
 already coating objects with larger surfaces. 
 
ELECTRO-COBALTING SOLUTIONS. 22? 
 
 Finishing. When the goods are to be left as they come from 
 the bath without further polishing, and, therefore, with a slightly 
 deadened surface, they must not be touched with the hands upon 
 any exposed surface, as the coating in this condition is peculiarly 
 susceptible to grease-markings, and the stains will inevitably 
 show after drying. 
 
 The pieces should be lifted from the vat by the suspending 
 wires, plunged first into two or three cold wash- waters, and then 
 into hot clean water. If the pieces are at all thick or substantial, 
 the heat energy stored up in them, by a short immersion in the 
 boiling water, will suffice on removal to evaporate the small 
 proportion of liquid clinging to them ; but if the surface be large 
 as compared with the mass, it may be necessary to finish the 
 drying in a stove. To this end the objects are placed on a tray, 
 the suspending wires unhooked, and the tray transferred to the 
 oven, so that from first to last they are not touched with the 
 fingers. So treated the dead surface presents an extremely 
 attractive appearance. Distilled water should be used for the 
 final washing-bath (heated) if possible, because it leaves no residue. 
 
 Britannia metal and zinc should be coppered before nickeling, 
 and some prefer thus to treat even German silver; this done, 
 the objects are coated with nickel in the manner described. 
 
 Applications. Among the many useful applications of electro- 
 nickeling, that of coating the comparatively soft copper printing 
 surfaces demands especial notice. A thin film of nickel, so thin 
 that the size of the printing surface is not affected, will increase 
 the hardness, and consequently the life of the plate enormously ; 
 indeed it would appear that a nickel-coated copper plate will 
 give about four times as many impressions as one coated even 
 in the usual way with iron. A special advantage also attaches 
 to its use it enables copper type to be used with a red pigment 
 (vermilion) which cannot be done without such protection, 
 because the copper alone decomposes the mercury sulphide, 
 which is the basis of the pigment, and thus destroys its colour, 
 and at the same time tends to become brittle by the absorption 
 of the reduced mercury. 
 
 ELECTRO-DEPOSITION OF COBALT. 
 
 The chemical properties of nickel and cobalt are so nearly 
 allied that this chapter would appear to be the most appropriate 
 place for introducing the subject of electro-plating with the latter 
 metal. The deposit of cobalt is similar to that of nickel ; it is 
 equally brilliant, but is somewhat harder, and has been found by 
 Professor Silvanus Thompson to possess a higher resisting power 
 for organic acids, which renders it more suitable for the internal 
 coating of copper or other cooking utensils. It is only lately, 
 however, that cobalt anodes have been procurable, and have thus 
 
228 
 
 DEPOSITION OF NICKEL AND COBALT. 
 
 BE 
 
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ELECTRO-COBALTING SOLUTIONS. 229 
 
 enabled the process to enter practically upon the field of electro- 
 metallurgical competition. Even now, its comparatively high 
 price tends to check its adaptation to many purposes to which 
 its application may be desirable. Its good qualities have not been 
 seriously verified, and very little commercial progress has been 
 made. 
 
 The battery should consist of two Bunsen-cells, and it is well 
 to place an ammeter in the circuit and to have resistance-coils at 
 hand, because the deposit yielded by a powerful current is 
 defective in adhesion ; a fairly high electro-motive force but a 
 low density of current is required. Many solutions have now 
 been used, of which some are indicated in the preceding table. 
 
 The double sulphate of cobalt and potash, corresponding to 
 the nickel-potassium sulphate, above recommended, may also be 
 used in 10 per cent, solution. Professor Thompson, however, 
 has obtained perfect results from the solution No. 5, made by 
 mixing 20 volumes of a nearly saturated solution of magnesium 
 sulphate with 1 volume of a similar solution of cobalt sulphate 
 or chloride. This bath should be used hot, and will then yield 
 a film which, if deposited with all due precautions on iron, brass, 
 or German silver (inter alia), is extremely adherent and good. 
 Instead of making up this solution in the manner described, it 
 may be prepared by electrolysis, if the current be passed from a 
 large cobalt anode into a magnesium sulphate solution with a 
 temporary cobalt cathode, which should be removed as soon as 
 sufficient metal has dissolved in the electrolyte to yield a good 
 deposit. 
 
 Langbein remarks that cobalt deposited from the chloride is not 
 hard, and he therefore recommends the following solution : 
 21 oz. of cobalt-ammonium sulphate and 10 J oz. of crystallised 
 boric acid in 10 quarts of water. A potential difference of 2*5 
 to 2 '75 volts is required with a current-density of 0'4 ampere 
 per square decimetre. 
 
 Anodes of pure rolled cobalt are now obtainable, and should 
 be used like those of nickel, of large superficial area as compared 
 with that of the cathodes : they must not be supported by 
 copper wire. The vats and the general treatment, as well as 
 the character of metal deposited, are, indeed, regulated exactly 
 as in the case of nickel. And, because the film is harder even 
 than that of the sister metal, it follows that the same precautions 
 must be taken in regard to previous polishing as well as cleans- 
 ing. It is even more difficult to impart a smooth polish to a 
 rough cobalt surface than it is to one of nickel. So also the 
 same care is necessary in stripping old deposits previous to 
 plating ; corresponding methods may be employed. 
 
CHAPTER XII. 
 
 THE ELECTRO-DEPOSITION OF IRON. 
 
 THE electro-deposition of iron (or of steel, as it is sometimes 
 wrongly termed) upon the surface of engraved copper plates has 
 long been practised, in order that its superior hardness may 
 enable the printer to obtain a greater number of sharp impres- 
 sions from the same plate ; and the extreme ease with which the 
 worn film may be removed from the copper renders it possible 
 to renew the protective coating again and again. All that is 
 necessary is to suspend the printing operations as soon as the 
 first tinge of the red foundation metal appears through any 
 portion of the iron covering, then remove the iron by means 
 of dilute acid, and re-immerse it in the iron-plating vat. In this 
 manner the copper need scarcely be appreciably worn, and may 
 be made to yield many thousand impressions without losing any 
 of the sharpness or accuracy in definition of the lines. This, 
 however, is practically the only use for electro-deposited iron ; 
 as a metal it is too readily oxidisable to render its use as an 
 external protective coating serviceable in any other way. Its 
 hardness qualifies it for printers' work, and enables copper to 
 compete with steel as a material for steel-plate engraving ; but 
 even in this field it is outrun by nickel, by which it will probably 
 be superseded in course of time. Nevertheless, for plates which 
 may require alteration from time to time (maps, for example) the 
 iron is preferable, on account of the greater difficulty experienced 
 in removing the nickel coat ; yet, if the alterations are likely to 
 occur frequently, it may be better not even to coat the plate 
 with iron, although the stripping is in this case so comparatively 
 simple. 
 
 Iron is never deposited by immersion, but always by the 
 separate-current process, for which purpose the Bunsen-cell is 
 best adapted. 
 
 The Solution. Two kinds of iron salts are commonly known 
 the ferric or per-salts, and the ferrous or jorofo-salts, which are 
 combinations of the metal (Fe) with a greater (Fe 2 3 ) or smaller 
 (FeO) proportion of non-metal respectively ; and, as the heat of 
 formation of any ferric compound is considerably higher than 
 
 230 
 
IRON SOLUTIONS. 231 
 
 that of the corresponding ferrous body, the proto-salts exhibit 
 a great tendency to absorb oxygen from the air, or from any 
 substance in which it is loosely combined, and thus to become 
 converted into the per-salt of the same class. But the ferrous 
 salts are alone suited for electro-depositing the metal ; hence 
 the iron solutions employed must be carefully protected from 
 the action of the air by retaining them in closed vessels when 
 not in actual use. While the current is flowing, it tends to 
 correct any peroxidising action ; because the newly-deposited 
 iron upon the cathode is attacked by ferric salts in the solution 
 in its immediate vicinity, and is re-dissolved, while the ferric salt 
 is simultaneously reduced to the ferrous condition, thus 
 
 Fe 2 Cl 6 + Fe 3FeCl 2 
 
 Ferric chloride with iron give ferrous chloride. 
 
 And newly-precipitated iron or, if we may use the expression, 
 nascent iron, exhibits a much greater activity in this respect 
 than metal which has been cast or rolled, and which is usually 
 in a more dense, as well as a more stable, condition. The oxygen 
 which, meanwhile, is being deposited upon the other pole by a 
 moderate current, such as is required for iron coating, acting 
 upon a sufficiently large anode and, therefore, upon an excess 
 of metal, forms only the protoxide. Thus, in course of time, 
 the peroxide originally present will become reduced, and the 
 bath will be brought into the best condition for yielding a 
 good deposit; but in effecting this, a considerable amount of 
 time and of battery-power may have been wasted. 
 
 Another effect of oxidation of ferrous salts in neutral solutions 
 is the formation of basic salts (containing an excess of iron), 
 which being insoluble in the liquid give rise to a rust-coloured 
 precipitate or turbidity. This happens because the iron in the 
 ferric condition requires not only more oxygen, but more acid 
 to form salts, than it does in the ferrous state ; for example, 
 2 atoms of iron in the state of ferric oxide require 3 mole- 
 cules of sulphuric acid to dissolve them and form the sul- 
 phate (Fe 2 3 + 3H 2 S0 4 = Fe 2 (S0 4 ) 3 + 3H 2 0), while in the form 
 of ferrous oxide only 2 molecules of the acid are needed 
 (Fe 2 2 + 2H 2 S0 4 = 2FeS0 4 +2H 2 0). The addition of oxygen 
 from the air may thus suffice to produce peroxide, but there 
 may not then be sufficient acid in the bath to combine with it, 
 and it thus appears as a solid precipitate. The following equa- 
 tion sums up the reaction in a general way, and shows how per- 
 oxide is formed along with the persulphate, by the action of 
 oxygen upon the protosulphates : 
 
 6FeS0 4 + 30 = 2Fe 2 (S0 4 ) 3 + Fe 2 3 . 
 
 The remedy for this precipitate or cloudiness is obviously the 
 addition of a sufficient quantity of free acid to combine with the 
 
232 ELECTRO-DEPOSITION OF IRON. 
 
 peroxide or basic precipitate, and form the soluble persulphate. 
 When much of the ferric compound has formed, just sufficient 
 acid should be added to dissolve the precipitate, and a current of 
 electricity should be passed through it between iron electrodes, 
 until the pure pale green colour of the ferrous liquid has been 
 restored and the metal is depositing well upon the cathode. But 
 it must be remembered that all acid which is added to dissolve 
 the rust-coloured precipitate becomes free again in the solution 
 as soon as the bath is reduced to the ferrous condition, which 
 requires less acid to form its compounds. Prevention in this 
 case is, therefore, better than cure, and the bath should be kept 
 as far as possible out of contact with air ; Klein adds a small 
 proportion of glycerin to the liquid to hinder the formation of 
 ferric compounds. 
 
 The compositions of the principal baths used in depositing are 
 quoted in Table XIX. 
 
 One of the best of these solutions is that of Klein (No. 3), 
 which is especially suitable for the production of thick deposits. 
 A solution of ferrous sulphate in water is first prepared in a 
 sufficiently large jar ; a solution of ammonium carbonate is now 
 gradually added to it until no further quantity of the precipitate 
 which forms at first is produced ; the precipitate is now allowed 
 to subside, the liquid is poured off, fresh water is added, from 
 which the ferrous carbonate is again allowed to separate by 
 subsidence. The water is once more poured away, and sulphuric 
 acid, diluted with twice its volume of water, is added little by 
 little, with constant stirring, until the precipitate is exactly 
 re-dissolved and yet no excess of free acid is present. Towards 
 the end, the acid must be added by a few drops only at a time, 
 after which a few seconds' pause must be made to give oppor- 
 tunity for it to attack the precipitate before introducing a 
 further quantity. A blue litmus paper suspended in the solution 
 will indicate the condition of the liquid at any time ; it should 
 become purple, but never red. Only recently-boiled water 
 should be employed to dissolve and wash the ferrous sulphate 
 and precipitate, for natural water contains a quantity of dissolved 
 oxygen, which would peroxidise the iron, but which is expelled 
 by boiling. This solution should be used as concentrated as 
 possible and preferably warm ; it must never be allowed to 
 become acid. In order to guard against this latter evil, Klein 
 recommended the use of anodes presenting an aggregate area 
 equal to eight times that of the cathode surface, so that any free 
 acid should have every facility for becoming saturated with iron ; 
 and he even attached slips of a more electro-negative element, such 
 as platinum or copper, to the anode plates, so that a slight local 
 current might be set up, which would assist in the solution of 
 the iron, without affecting the current passing through the bath. 
 The deposit from Klein's solution should not show any tendency 
 
IRON SOLUTIONS. 
 
 233 
 
 H a; e* 
 
 H? 63 rH 
 
 ll 
 
 O 525 
 PH W 
 
 s s 
 
 S3 
 
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 g 
 
 I 
 
 M 
 
 OQ 
 
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 1 
 
 IM 
 
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 'S 
 
 -5 
 
 0) 
 
 is 
 
 05 
 
 Dissolve 2 in 12 ; just precipitate with 
 9, and just re-dissolve with 11 ; use as 
 concentrated as possible. 
 
 Stand two days ; always filter before use. 
 
 Form electrolytically. 
 
 Dissolve 2 in 250 of 12, and add to 7 in 
 750 of 12 ; then add 10. 
 
 Form electrolytically. 
 
 
 CSl 
 
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 Water. 
 
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 Ammonium 
 Chloride. 
 
 
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 Ammonium 
 
 
 
 
 
 
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 Carbonate. 
 
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 00 
 
 i ^ 
 
 Caustic Soda. 
 
 
 
 . 
 
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 . 
 
 
 
 
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 Rochelle 
 Salt. 
 
 O 
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 rp 
 
 r*i 
 
 Potassium 
 
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 Ferrocyanide. 
 
 
 
 
 
 
 
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 10 
 
 ^ 
 
 Iron Alum. 
 
 
 
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 Ammonio- 
 Ferrous 
 
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 Sulphate. 
 
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 Ferrous 
 Sulphate. 
 
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 Varrentrapp 
 
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 o 
 
234 ELECTRO-DEPOSITION OF IRON. 
 
 to crack upon the surface and peel off in the form of spangles, 
 as its inventor observed happened frequently when other baths 
 were used for producing thick coatings, especially the double 
 chloride of iron and ammonium. 
 
 The double sulphate of iron and ammonia, which is obtainable 
 in the market in a very pure condition, is capable of yielding 
 extremely good films upon engraved copper plates. If it be at 
 all acid a little chalk should be added, or, better still, a little 
 washed ferrous carbonate prepared as above described, until no 
 further quantity is dissolved by the liquid. 
 
 Langbein remarks that a heavy and hard deposit is obtained 
 from a solution made up of 1J oz. of ammonio-ferrous sulphate 
 and O88 oz. of crystallised citric acid in one quart of water. 
 Enough ammonia should be added to render the solution neutral, 
 or slightly acid. 
 
 Perhaps the best solution of all is one recommended by Klein, 
 and made by dissolving sufficient equal proportions of ferrous sul- 
 phate and magnesium sulphate to form a concentrated solution, 
 neutralising acidity, not by means of chalk, but by means of 
 magnesium carbonate suspended in a tray. The bath should be 
 used with a very low current-density. 
 
 In using any of these iron-baths, the separation of hydrogen 
 with the iron upon the object is to be strenuously avoided, 
 because the gas bells clinging to the surface form pin-holes which 
 are fatal to the impression taken in the press from a plate so 
 affected ; and, further than this, the character of the iron is pre- 
 judicially influenced by the absorbed hydrogen. The conditions, 
 therefore, which are least favourable to hydrogen production 
 must be fulfilled ; these are mainly the use of a concentrated 
 solution, the absence of free acid, and the application of a 
 sufficiently weak current. 
 
 The Anodes. The anodes should be made of the purest iron 
 available. Electro-deposited iron would theoretically be most 
 suitable ; next to this the softest wrought-iron, or so-called mild 
 steel sheet, is preferable. Hard steel and, above all, cast-iron 
 plates are to be avoided, because they contain comparatively 
 large percentages of carbon and other impurities which, being 
 insoluble in the liquid, remain for a time suspended in the bath 
 and are apt to attach themselves to the objects under treatment. 
 Cast-iron may contain as little as 93 per cent, of iron (or some- 
 times even less). The anode surface should be much larger than 
 that of the cathodes (eight times as large) for reasons already 
 given. The anodes should be removed from the vat now and 
 again, brushed to detach the insoluble matter which is left upon 
 the surface in a spongy or pulverulent condition, and returned 
 to their position. 
 
 The Vat. The vat is best constructed of iron, which may be 
 heated when the solution is to be used warm ; it must be kept 
 
ELECTRO-PLATING WITH IRON. 235 
 
 thoroughly well cleaned, and, for this cause, enamelled iron is to be 
 preferred. It must, of course, be larger than the work to be plated 
 on account of the increased surface which is given to the anodes. 
 
 The Character of the Deposited Metal. The iron deposited in 
 thick coatings is of a bright grey-white colour, is extremely hard 
 and brittle, and demands the most careful attention if it is to be 
 removed from the matrix. After annealing at a low red heat it 
 becomes softer, and after a fair cherry- red it is as soft as steel 
 which has been similarly treated. The fracture of unannealed 
 deposited iron resembles that of cast-iron ; it, of course, contains 
 no carbon, but is usually highly charged with hydrogen, which it 
 occludes during the process of formation. Cailletet has found as 
 much as 240 volumes of this gas in 1 volume of a sample of iron, 
 which was sufficiently hard to scratch glass, and was extremely 
 brittle. The annealing has the double effect of softening the 
 deposited iron and of removing the hydrogen which it had previ- 
 ously contained ; but the inference is not safe that the hydrogen 
 is the sole cause of the brittleness, which is more probably 
 mainly due to the particular arrangement of the molecules of the 
 metal. Its extreme hardness has procured for the application of 
 deposited iron to engraved copper plates the misleading title of 
 steel-facing a designation which implies the presence of combined 
 carbon within it, whereas, excepting the hydrogen contained in it 
 (which may be removed by heating), it is one of the purest forms 
 of iron obtainable. 
 
 In its relation to magnetism, deposited iron is comparable 
 with mild steel ; but Beetz has shown that when deposited 
 under powerful magnetic influence, as between the poles of a 
 strong electro-magnet, and from solutions containing ammonium 
 chloride, it will itself act as a powerful magnet, retaining its 
 magnetism for a considerable period of time. 
 
 Iron is so electro-positive a metal that it has a great tendency 
 to combine with oxygen, that is, to rust; the electro-deposited 
 film must, therefore, be dried very thoroughly as soon as possible. 
 It, however, generally contains within its pores a distinct quantity 
 of the solution from which it was precipitated, and this must be 
 perfectly removed by washing two or three times in boiling 
 water, or the liability to rust will be greatly increased. On the 
 other hand, A. Neuburger has made the observation that electro- 
 lytic iron (presumably unannealed) resists rust in a remarkable 
 way. 
 
 The Process of Electro-Deposition. In coating copper plates 
 with iron (which is the chief application of the process), the 
 copper must first be cleansed carefully, so that the sharpness of 
 the lines may not be diminished. Klein dips the plate first into 
 benzene and then into potash to remove grease, but it is usually 
 sufficient first to rinse and then to boil the plate in a solution of 
 caustic potash ; then, after washing twice or thrice in clean water, 
 
236 ELECTRO-DEPOSITION OF IRON. 
 
 to pass it through a bath of dilute sulphuric acid (containing 
 from 2 to 5 per cent, of the acid), and after a second thorough 
 wash to transfer it at once to the iron-vat, without touching the 
 surface at any point. In the bath it is suspended by suitable 
 hooks or by holders such as those mentioned on p. 97 ; two 
 plates may be used with one anode by placing the CAigraved 
 faces of the copper fronting the latter. 
 
 The current from the Bunsen-cell (or cells, arranged in 
 parallel, if much work is in hand) is then passed through the 
 solution ; an ammeter and set of resistance-coils should be placed 
 in position. Usually from five to six minutes suffice for the 
 process of deposition; but if a thicker coating be required, 
 remembering that the last portion deposited forms the printing 
 surface, and hence must be perfect in character, it is advisable 
 to remove the plate, rinse, rapidly examine, and brush it well with 
 a hard brush under water, so that no extraneous matter may 
 cling to the surface ; then replacing it for five or six minutes in 
 the iron-bath, the alternation is again and again repeated until 
 the desired thickness is obtained. After the final removal from 
 the bath, the plate is dipped into a large volume of cold water, 
 is then immersed in boiling water for the space of half a minute, 
 and is again rinsed in cold water. It may then be lightly rubbed 
 with dilute potash or soda solution, sponged dry, and rubbed 
 with oil, the excess of which is subsequently removed by means 
 of benzene. Thus treated, the plate will not be greatly liable to 
 rust ; but if it is to be stored for any length of time, it must be 
 treated like an ordinary engraved steel plate and covered with a 
 protective film of wax. 
 
 Stripping. When an old iron-coated plate is to be re-plated, 
 the residue of the first coat must be stripped, after removing all 
 grease in a potash-bath, by immersing it in dilute sulphuric acid 
 (5 to 10 per cent.) until the copper surface is left completely bare ; 
 the plate, after washing, is then ready for the iron-bath as usual. 
 
 Electrotyping with Iron. Thick deposits also may be made, 
 and may be obtained direct from the mould, but in this case the 
 matrix should be first covered with a thin sheath of copper, upon 
 which the iron is precipitated, because iron refuses to deposit 
 well upon the black-leaded surface of the gutta-percha or other 
 non-conducting mould. The copper may be afterwards dissolved 
 away by making the iron sheet the anode in a copper cyanide 
 bath, or (but with greater risk) by simple treatment with the 
 strongest nitric acid, the excess of which must be quite washed 
 away immediately the copper is removed. 
 
CHAPTER XIII. 
 
 THE ELECTRO-DEPOSITION OP PLATINUM, ZINC, CHROMIUM, CADMIUM, TIN, 
 LEAD, ANTIMONY, BISMUTH, AND PALLADIUM ; ELECTRO-CHROMY. 
 
 ELECTRO-DEPOSITION OF PLATINUM. 
 
 PLATINUM, one of the most insoluble and acid-resisting metals 
 known, would form an excellent protective coating to metals 
 could it be readily applied ; Roseleur, indeed, has stated that he 
 had twenty times evaporated nitric and sulphuric acids alter- 
 nately in a platinum-plated copper crucible without finding the 
 basis metal to be sensibly attacked until the last operation. 
 But the very insolubility of the metal constitutes one of the 
 difficulties in electro-plating with it ; for the anodes resist the 
 solvent action of any solution which may be safely used as an 
 electrolyte without injuring the objects suspended as the cathode. 
 It is very rarely used, however, as a covering metal. 
 
 Platinising. It is so electro-negative an element that nearly 
 all the other metals are able to decompose its solution, and thus 
 deposit the platinum by simple immersion ; but the coating so 
 formed is usually black, granular, and non-adherent. Silver, 
 copper, and brass are the most readily treated, while lead, tin, 
 zinc, iron, Britannia metal, and the like, present great difficulty 
 unless previously protected by a substantial film of copper. For 
 platinising copper by simple immersion, Roseleur recommends the 
 use of a boiling solution containing 10 parts of platinum converted 
 into the neutral chloride, and 120 parts of caustic soda in 1000 of 
 pure (distilled) water. Another solution may be made by dissolv- 
 ing 25 parts of the double chloride of platinum and ammonium, 
 and 250 of ammonium chloride in 1000 parts of water; this also 
 is used at the boiling temperature. When silver is platinised 
 and a simple solution (not too strong) of platinum tetrachloride 
 in water will suffice to effect this the object should be afterwards 
 rinsed, first in dilute ammonia and then in water, because silver 
 chloride is formed by the exchange of metal with the platinum 
 chloride ; and this silver compound, being insoluble in water, 
 requires the treatment with ammonia, in which it readily dis- 
 solves, to effect its complete removal from the plated goods. 
 
 237 
 
238 ELECTRO-DEPOSITION OF PLATINUM. 
 
 Tin, brass, bronze, copper, and tin plate have also been platinised 
 by rubbing upon their surface, with a woollen or linen rag, a 
 solution of 1 part of platinum chloride in 15 parts of spirits of 
 wine and 50 of ether, and then washing well with water, after 
 allowing the ether to evaporate. 
 
 Platinum may also be deposited by the single-cell process. 
 Lesmondes' method consisted in placing the articles in a per- 
 forated zinc tray and immersing the whole in a solution made by 
 adding sodium carbonate, in the first place, to a strong solution 
 of platinic chloride until effervescence ceases, then a little glucose, 
 and afterwards sufficient sodium chloride to yield a white pre- 
 cipitate. This bath is used at a temperature of 140 F., and is 
 most suitable for treating copper and brass, the deposition being 
 mainly due to the current set up by the solution of the zinc tray. 
 
 The method adopted by Smee for coating the silver plates 
 required for use in his battery is also a single-cell process. The 
 plate, slightly roughened by mechanical means or by a momentary 
 immersion in nitric acid, is placed in a solution containing dilute 
 sulphuric acid with a few drops of platinum chloride solution 
 added to it. A porous cell containing a rod of zinc standing in 
 dilute sulphuric acid is then introduced into the bath ; on 
 making metallic connection between the silver and the zinc, a 
 current is set up, the zinc dissolves, and a proportionate amount 
 of platinum is deposited as a dark-grey or black powder, which, 
 nevertheless, holds fairly tenaciously to the silver. 
 
 Platinating. But for general purposes the separate-current 
 process should be employed. Of the various solutions recorded in 
 the appended table, those of Langbein (No. 3) and Roseleur (No. 4) 
 are the most reliable. To prepare a gallon of the latter, three- 
 quarters of an ounce of platinum is dissolved in aqua regia and 
 converted into chloride, which is then dissolved in a quart of dis- 
 tilled water ; in the meantime half a pound of ammonium 
 phosphate should have been dissolved in a quart of pure water in 
 one vessel, and two and a half pounds of sodium phosphate in the 
 remaining two quarts contained in a second vessel. The solution 
 of the ammonium salt is now to be added to the platinum liquid, 
 with which it produces a dense precipitate ; disregarding this, the 
 sodium phosphate solution is next added with constant stirring, 
 and the whole bath is boiled until no more smell of ammonia is 
 observed, but on the contrary it is shown to be faintly acid by 
 blue litmus paper. During the period of boiling, water will have 
 been evaporated, which must be restored before using the liquid. 
 When in active use, a little of the fresh solution must be added 
 at intervals to supply the place of the platinum which has been 
 lost by deposition upon the cathode ; because, as already ex- 
 plained, the anodes are not attacked, and cannot, therefore, 
 replenish the exhausted solution. A process similar to this, but 
 with the addition of a small proportion of common salt, has been 
 
PLATINATING SOLUTIONS. 
 
 239 
 
 S S 
 
 H B 
 
 Special Method of Preparation. 
 
 Dissolve 1 as platinic chloride in 15 ; add 
 12. (For silver.) 
 
 Dissolve 14 and 10 in 400 of 15 ; neutralise 
 with 4 ; add 2, and stir ; then 600 of 15. 
 
 Vide text. 
 
 Dissolve 1 in 15 ; add first 6, then 13 ; render 
 alkaline with 4; warm and filter. 
 
 V Good for thin deposits. 
 
 Water. 
 
 iH iH |H i-l 
 
 1 I 1 1 I 
 
 
 o 
 
 , 
 
 
 rH 
 
 
 Acetic Acid. 
 
 : : : : 
 
 S : : : : 
 
 Sulphuric Acid. 
 
 3 
 
 : | : : 
 
 : : : : : 
 
 ^ Ammonium 
 ^ Phosphate. 
 
 : : : g 
 
 
 eq Ammonium 
 
 id 
 
 
 m Chloride. 
 
 
 
 fc Sodium Citrate. 
 
 a> : : : 
 
 
 O Sodium 
 H Pyrophosphate. 
 
 : : : : 
 
 : : : 1 : 
 
 g Di-Sodium 
 ; Orthophosphate. 
 
 S 
 
 (M 
 
 : : | : : 
 
 fn Sodium Chloride. 
 
 o 
 
 : : : : 
 
 oo 
 
 te Sodium 
 Carbonate. 
 
 
 : 8 : : : 
 
 OH Caustic Soda. 
 % 
 
 GO 
 
 oe .... 
 
 pi Potassium 
 PM Cyanide. 
 
 : : : : 
 
 : : : : | 
 
 Ammonium 
 
 
 "^ 
 
 Platinic Chloride. 
 
 
 
 Platinum as 
 Platinic Chloride. 
 
 . o : 
 
 t~ 
 
 
 : : : I 
 
 
 
 J? 
 
 fi 
 
 
 
 ' "S 
 
 
 : ^ 
 
 O C fcH 
 
 t-> G '33 s 
 
 1 1 1 1 
 
 8 S S & 
 
 qj .... 
 
 1 2 * 
 II 1 
 
 
 
 r-l <M CO * 
 
 io <o t- <x> a 
 
240 ELECTRO-DEPOSITION OF PLATINUM. 
 
 patented by Thomas. The method of preparing Langbein's 
 excellent solution is given in the table. Platinum solutions 
 should be used hot, and gas should appear at both anode and 
 cathode. Copper and brass may be plated direct, but iron and 
 other metals must be coppered first. 
 
 The objects to be plated should be well polished before de- 
 positing, because electrolytic platinum is hard, so that greater 
 difficulty is involved in polishing after deposition than before. 
 The platinum-coated surface may be left dead, or it may be 
 brightened by means of iron-wire scratch-brushes (brass is too 
 soft and, itself becoming rubbed, leaves a yellow stain upon the 
 goods) or by careful rubbing with very finely-powdered pumice. 
 
 When old platinum-covered goods are to be re-plated, the 
 stripping of the previous coat presents a difficult problem ; it 
 cannot well be removed electro! ytically, because the baths do not 
 attack the metal, although a long exposure in a gold-stripping 
 bath may sometimes effect the desired object, especially if the 
 platinum be not in its densest and hardest condition; but at 
 best it is a very tedious operation. The chief solvent for 
 platinum is aqua regia, but this cannot be applied because it 
 would vigorously attack the basis metal beneath j and, in fact, 
 as soon as any of the latter metal became uncovered, it would 
 dissolve all the more rapidly on account of its contact with the 
 remainder of the platinum coat, and, being more electro-positive, 
 would even serve to protect the latter from further action. The 
 surest and most rapid method, whenever practicable, is to apply 
 mechanical means and simply rub off the platinum by means of 
 emery-cloth, and then re-polish the metal beneath. This system 
 cannot, of course, be applied when any delicate pattern or design 
 is traced upon the object, in which case the chemical methods 
 must be tried. A greater loss is caused by the use of emery, 
 but this may be minimised by saving the dust produced, and 
 subsequently working it up to recover the platinum. 
 
 The name usually applied to platinum-coating by simple immer- 
 sion is platinising, as in the case of the Smee-battery silver plate, 
 while the electrolytically- covered object is said to be platinated. 
 Watt, however, raises an objection to this latter term, and 
 suggests the use of the expression ' platined ' to denominate 
 this class of work. His objections to the other term are doubt- 
 less worthy of consideration, but the older word is perhaps more 
 euphonious ; while if it be regarded as a contraction of the 
 compound word 'platinumplating,' it is not, after all, unscientific. 
 
 ELECTRO-DEPOSITION OP ZINC. 
 
 Only in special cases is electrolytic zinc deposition resorted to ; 
 the metal has not a fine colour or lustre (except when certain 
 organic bodies are added to the solution) and is readily dulled 
 
USE OF DEPOSITED ZINC. 241 
 
 with the thin film of tarnish, which forms very soon upon exposure 
 to the atmosphere. Its highly electro-positive character certainly 
 renders it suitable to the protection of iron surfaces from destruc- 
 tion by rust ; because when submitted together (in metallic con- 
 tact) to the same corrosive influence^ the zinc is the earlier of the 
 two metals to be attacked. But, like tin, zinc is usually more satis- 
 factorily deposited upon iron by dipping the latter into a bath of 
 the molten metal. Such a process yields a perfectly homogeneous 
 and continuous coat, which is applied at such a temperature that 
 it is impossible for water to exist between the two surfaces, or in 
 cavities ; while the electrolytic method deposits a crystalline, and, 
 therefore, to some (however slight) extent, porous cover, in which 
 small quantities of the solution from which it has been deposited 
 may be locked up, and facilitate the oxidising action. Thus the 
 dry or fusion method of coating the iron is preferable whenever 
 its application is possible ; the meaningless title galvanised iron 
 given to this product is manifestly a misnomer, and is distinctly 
 misleading. But for internal or other surfaces which cannot well 
 be coated by the fusion method, electro-deposition is frequently 
 used with great advantage ; it is especially useful in coating 
 hardened steelware, which is to be used in the hardened condition, 
 and of which the temper would be drawn by the heating necessary 
 for the treatment with metallic zinc. 
 
 Another method of galvanising (non-electric) due to Cowper- 
 Coles, and known by the name of ' sherardising,' has come into 
 use during the last few years. Metals are heated in contact with 
 zinc dust at a temperature below that of fusion, when the zinc 
 alloys superficially with the iron or other metal, the thickness of 
 the coating depending on the time of the operation. 
 
 Battery Process. On account of its very electro-positive nature, 
 zinc cannot be precipitated upon ordinary metals by simple 
 immersion, nor is it practically deposited by single-cell methods. 
 By separate current it may be obtained from the neutral sulphate, 
 chloride, acetate, or other soluble salt of zinc (except the nitrate), 
 or from the corresponding double salt of zinc and ammonia. The 
 current-strength required for most zinc solutions is considerable, 
 and demands the use of two or three Bunsen-cells. 
 
 The solutions are, as usual, various, but good results may be 
 obtained from a 10 per cent, solution of zinc sulphate with a 
 current of from 0'06 to 0'13 ampere per sq. in., or 1 to 2 ampere 
 per sq. decimetre j other formulae are given in Table XXI. 
 
 For small work perhaps the best solution is that patented by 
 Watt in 1885 (No. 5): 200 oz. of potassium cyanide are to be 
 dissolved in 20 gallons of water; 80 fluid oz. of the strongest 
 liquor ammonice are then stirred into the liquid, and the mixture 
 is transferred to a large vessel containing pure rolled zinc anodes ; 
 in this vessel are also several large porous battery-cells, which 
 must be filled up with the solution to the level of the surrounding 
 
 16 
 
242 
 
 ELECTKO-DEPOSITION OF ZINC. 
 
 s 
 
 2 & < 
 
 CO 
 
 ^ 
 
 Special Method of Preparation. 
 
 (For cast-iron.) 
 Dissolve 4 in 13, add 5 until precipi- 
 tate at first formed is re- dissolved. 
 
 Precipitate the zinc hydroxide from 
 sulphate by potash. 
 Dissolve 6 in 13, add 9 ; mix ; add 1 
 electrolytically ; and finally add 7. 
 
 .5 o 1| 
 
 ^T3 
 
 : aj 
 
 g N 
 t M 
 
 T3 S 
 
 Ssa'S* 
 ill 
 
 rj CO -^ ^ 
 
 sf-S,S / S 
 
 g >H Ttl S re 
 
 aSl^.l 
 
 "8 e D S 
 
 o S w 53 cJ 
 
 fH _CO _ CD 
 
 H Q 
 
 Water. 
 
 O O O O O 
 
 o o o o o 
 o o o o o 
 
 i-t i-H rH 
 
 o o o 
 o o o 
 o o o 
 
 
 
 
 Sulphuric 
 
 r2 
 
 
 Acid. 
 
 11 
 
 
 i? Ammonium 
 
 QnlWkofn 
 
 : : : 
 
 o 
 
 
 
 
 ^ Ammonium 
 Chloride. 
 
 ^ i : 
 
 o o 
 
 <N <N 
 
 ^ Ammonia. 
 
 ^ 
 
 <M 
 
 
 Alum. 
 
 : 
 i i 
 
 
 ^ Potassium 
 H Carbonate. 
 
 ; s 
 
 
 Potassium 
 ^ Cyanide. 
 
 <r> 
 
 
 ^ Potash. 
 
 
 
 B 
 
 O 
 
 02 Zinc 
 g Sulphate. 
 
 18 1 i 
 
 o 2 
 
 00 : 
 
 <! Zinc 
 
 
 :o 
 
 Chloride. 
 
 
 - 1) 
 
 Zinc 
 
 o 
 
 . . 
 
 Hydroxide. 
 
 . r-l 
 
 . . 
 
 Zinc. 
 
 : : : : oo 
 
 i i 
 
 : : : 
 
 1 
 
 . . ._g . 
 
 ' ' jg ' 
 . . ^ . 
 O^- 
 
 
 S3 9 
 
 G S >> ^ 
 
 <n g^-| 
 ^ = 1 
 
 6 
 
 I-H (N CO TJ U3 
 
 ^O l> CO 
 
 'o'C 
 
CHARACTER OF DEPOSITED ZINC. 243 
 
 liquid, and in which are cathodes of metallic copper. On passing 
 a current from a Bunsen-battery of several cells, the zinc anode 
 gradually dissolves in the solution until, when it has reached the 
 strength of 3 oz. per gallon (i.e. a loss of weight on the part 
 of the anodes of 60 oz. in the aggregate), the battery is dis- 
 connected, and 80 oz. of potassium carbonate are finally added 
 to the liquid little by little, by dissolving each additional portion 
 in a fraction of the liquid, and then returning it to the vat and 
 stirring well. After a final rest of twelve hours, for subsidence, 
 the clear liquid is poured off, and the sedimentary matter at the 
 bottom filtered from the contained liquid ; the whole is then 
 ready for use with a battery of three Bunsen-cells, or, better, with 
 a dynamo. 
 
 The anodes for all solutions should be made of pure rolled zinc 
 only. It is more likely to be pure than the cast commercial zinc 
 or spelter of the market, which frequently contains much lead, 
 iron, or arsenic, as well as other impurities, and readily crumbles. 
 
 But for large work, the cyanide bath is costly and inconvenient. 
 The bath No. 6 in the above table may be made to give good 
 results with a current of about O'l ampere per sq. in. and 3 volts' 
 pressure. But it should always contain a trace, but scarcely 
 more than a trace, of free acid. 
 
 Cowper-Coles' process (No. 8) is largely used for producing zinc 
 coatings on iron plates, tubes, and heavy iron work generally. 
 One of the difficulties in working with high current-densities is 
 the tendency of the anode to resist solution at a rate proportional 
 to the current supplied, which entails a gradual acidification of the 
 bath. For this reason only about 50 per cent, of the zinc anode 
 goes into solution. Cowper-Coles therefore prefers to use in- 
 soluble anodes (of lead) and to circulate the electrolyte through 
 a bed of zinc dust mixed with carbon. As the electrolyte becomes 
 acid through deposition of zinc, the zinc dust dissolves, being aided 
 voltaically in the process by the carbon. Zinc dust has the 
 advantage of being a cheap by-product in the manufacture of zinc, 
 the only disadvantage in using lead for the anodes being that it 
 becomes peroxidised and thus sets up a considerable back E.M.F. 
 This disadvantage is reduced as far as possible by reducing the 
 distance between the electrodes to a minimum. If zinc anodes 
 are used, a large proportion crumbles away, and should be caught 
 on a filter bed of carbon, zinc dust being then unnecessary. Lead 
 electrodes, however, appear to be preferable. 
 
 Character of Deposit. The metal should be reguline. If 
 produced by a current which causes a simultaneous separation of 
 hydrogen, it is, of course, spongy ; but Kiliani has made the 
 remarkable observation that with a strong solution of zinc (of 
 specific gravity = 1 '38) a very weak current causes an evolution 
 of hydrogen, which diminishes in extent as the current increases in 
 volume up to a certain point. For example, a current of about 
 
244 ELECTRO-DEPOSITION OF ZINC. 
 
 0*07 ampere per sq. decimetre yielded 1'6 cubic centimetres of 
 hydrogen for every gramme of zinc deposited ; but as the current 
 was increased to 0*2, 0'4, 1'6, and 3'2 amperes per sq. decimetre, 
 so the hydrogen evolution per gramme of deposited zinc was 
 reduced to 1*5, 0'37, 0'29, and 0*22 cubic centimetres respectively. 
 In all these cases the zinc was in a spongy condition, but less 
 markedly so in the last two instances. When the current-density 
 was increased to 18*5 amperes per sq. decimetre, the gas separa- 
 tion ceased, and the metal appeared lustrous and adherent. This 
 anomalous action may perhaps be ascribed to the ready oxidisa- 
 bility of the zinc, which, deposited by a weak current, is attacked 
 by the solution in its freshly precipitated state, with the formation 
 of zinc oxide and evolution of hydrogen (Zn -f H 2 = ZnO + H 2 ) ; 
 but as the current-strength increases, so also the quantity of zinc 
 thrown down is increased, until at length the action may perhaps 
 be so hurried that the liquid has no chance of attacking the 
 metal at the moment of deposition. The current-strength could, 
 of course, be greatly increased in so strong a solution without 
 causing hydrogen to be evolved to any extent by the simultaneous 
 electrolysis of the metallic salt in contact with the cathode, 
 and a certain proportion of the acid or water. The same observer 
 also noted that a current of 0*4 ampere at 17 volts, in passing 
 through a one per cent, solution, threw down zinc oxide with the 
 metal upon the cathode. 
 
 It is clear, then, that the current-density must not be too low 
 in dealing with zinc solutions. The final washing of electro-zinced 
 goods should be in hot water, and the drying, if necessary, effected 
 in a stove, in order that as little time as possible may be given for 
 oxidation of the deposit. 
 
 The method of operating in depositing zinc should require no 
 further detailed explanation for those who are acquainted with 
 the matter in the previous chapters of this work. The same care 
 must be expended upon cleansing and deoxidising at first, and 
 upon washing and finishing subsequently, as has been insisted 
 upon throughout. The vats, suspension arrangements, and the 
 like call for no special comment. 
 
 There is a very great tendency with zinc to deposit in a spongy 
 state, forming loose tree-like forms and excrescences, especially 
 around the edges of the work or cathode. This action has been 
 ascribed by some to the formation of a compound of zinc and 
 hydrogen, but by Mylius and Fromm and others to the formation 
 of oxide of zinc, owing to the presence in the solution, for example, 
 of oxidising substances, or of copper, arsenic, or other elements 
 more electro-negative than zinc, which, being deposited with the 
 latter metal, cause gradual oxidation of the zinc by local action. 
 The use of the zinc dust in the Cowper-Coles process is clearly an 
 advantage from this point of view, inasmuch as it tends to check 
 the formation of the zinc oxide. 
 
ELECTRO-DEPOSITION OF CHROMIUM, ETC. 245 
 
 ELECTRO-DEPOSITION OF CHROMIUM. 
 
 Chromium is a very hard infusible metal, which so far has not 
 received commercial application. Le Blanc 1 has deposited the 
 metal from a solution of chromium sulphate, specific gravity 1 '25, 
 with half a gramme of boric acid per 100 cubic centimetres of 
 solution. A lead anode and copper cathode were used. A good 
 smooth deposit was obtained, but it was found impossible to get a 
 thick deposit owing to the formation of cracks. 
 
 ELECTRO-DEPOSITION OF CADMIUM. 
 
 Cadmium is a metal nearly allied to zinc, but less commonly met 
 with and more costly ; its electro-deposition is rarely effected. 
 
 Smee found that a liquid made by adding a solution of ammonia 
 to one of cadmium sulphate, until the precipitate at first formed 
 is just re-dissolved, readily yields a good deposit, while the 
 simple solutions of the cadmium sulphate or chloride are difficult 
 to work ; Bertrand, however, appears to have been more successful 
 with the sulphate. This experimenter has also used a bath of 
 cadmium bromide slightly acidified with sulphuric acid. 
 
 The only solution remaining to be noted is that of Russell and 
 Woolrich, made by dissolving 40 parts of the metal in dilute 
 nitric acid, adding a ten per cent, solution of sodium carbonate 
 until no further precipitate is produced, allowing the precipitate 
 to subside, pouring fresh tepid water upon it, settling it again, and 
 repeating the washing process four or five times, then just dis- 
 solving it in sufficient strong potassium cyanide solution, adding 
 ten per cent, excess of the latter, and sufficient additional water 
 to render the total weight of water in the solution equal to 1000 
 parts. The bath is used at a temperature of 100 F., and, with a 
 current at 3 or 4 volts produced by a like number of Daniell-cells 
 placed in series, may be made to give a white reguline metal. 
 
 ELECTRO-DEPOSITION OF TIN. 
 
 The process of coating articles of copper or wrought-iron by 
 merely bringing their cleansed surfaces into contact with melted 
 tin is so simple, and, moreover, gives such a sound and perfect 
 covering, that there is but little room for an electro-tinning 
 method, which must usually entail greater trouble and expense. 
 Notwithstanding this, the wet deposition is to some extent 
 practised, largely indeed for whitening small objects of brass wire, 
 such as pins, hooks and eyes, and the like, for which it is to be 
 preferred ; and for giving a preliminary coating to certain metals, 
 like cast-iron, which are subsequently to receive an electro-deposit 
 of any more electro-negative metal that may not be deposited 
 
 1 Trans. Amer. Electro-chemical Society, vol. ix. p. 315. 
 
246 
 
 ELECTRO-DEPOSITION OF TIN. 
 
 OQ B 
 
 
 10 J 
 
 'dk 
 
 
 41 
 
 
 
 
 
 ID 
 
 '43 
 
 1 
 
 ^J 
 
 o3 
 
 2 
 
 4T.s 
 
 O 
 
 
 
 a 
 
 1 s 
 
 J 
 
 rt 
 
 CO 
 S 
 
 g 
 
 
 
 
 O 
 
 PM 
 
 00 a 
 
 
 
 S 
 
 S 
 ^ 
 
 *s S^ 
 
 M 
 
 i 
 
 ,3 
 
 "+j 
 
 li 
 
 s 
 
 1*1 
 
 C< .S 
 
 4J 
 
 'i 
 
 1 
 
 ^ 
 
 la 
 
 a a 
 
 
 St- 
 
 fin ^O t^S 
 
 9 
 
 ^f, 
 
 
 
 
 
 "'S 
 
 o fl 
 
 02 
 
 P 
 
 
 
 H 
 
 Is 
 
 .. 
 
 Water. 
 
 
 O O O O 
 
 o o o 
 
 O O 
 
 o o o 
 
 
 . 
 
 
 
 s a" 
 
 U Sodium 
 H Pyrophosphate. 
 
 : : : : 
 
 8 I ; 
 
 ^ ^ 
 
 11 
 
 P 
 
 
 
 n 
 
 Alum. 
 
 * t i 
 
 
 li 
 
 Potassium 
 
 
 
 
 ^ 
 
 O Bitartrate. 
 
 : ' 3 2 
 
 
 l^ 
 
 H 
 ffi Caustic 
 
 
 
 .p 
 
 .. n3 
 
 2 Potash. 
 
 ^ : : : 
 
 : : : 
 
 11 
 
 ^ Tin 
 !x Tetrachloride. 
 
 o 
 : : : 
 
 : i : 
 
 II 
 
 g w 
 
 g Tin 
 
 10 
 
 CO CO 
 
 
 A3 Bichloride. 
 
 o r ~ t 
 
 ^-1 CO 
 
 ^ S 
 
 <1 
 
 
 
 ^,3 
 
 P-i 
 
 
 . 
 
 -j cS 
 
 
 
 . . . 
 
 co O 
 
 
 
 
 g,o 
 
 ill 
 HI 
 
 c3 < S 
 
 ^l s 
 p... . * 
 
 
 
 s 
 
 ft 
 
 02 P<0 
 
 
 
 11 
 
 6 
 
 <Vn 
 
 . 
 
 g. fa 
 
 s 
 
 o ,_, 
 
 
 ^ 
 
 o 
 
 
 
 
 | 
 
 1 ^ 
 
 " 2 2 
 
 
 
 5 
 
 1 2 3 J 
 
 * 
 
 1 
 
 d 
 
 r-l d CO <* 
 
 CO ^ 
 
 o 
 
TINNING BY IMMERSION. 24? 
 
 directly upon them. Like many other metals, tin may be de- 
 posited either by simple-immersion, by single-cell, or by separate- 
 current methods. This is, of course, a consequence of its relatively 
 low position in the electro-chemical series of metals. 
 
 Tinning by Simple Immersion. This is the commonest method 
 of dealing with small copper or brass objects. Of the various 
 processes, the oldest and simplest is to place them in a strong 
 solution of potassium bitartrate (cream of tartar), with which may 
 be mixed a small proportion of stannous chloride (tin proto- 
 chloride) to quicken the action. They are then covered with a 
 layer of pure tin, either in small pieces, in the form of foil, or as 
 cast plate ; a second layer of brass pieces is now placed above 
 this, then a fresh stratum of tin, and so on. On boiling for a 
 short time, it will be found that an exchange has taken place 
 superficially upon the articles, so that they have become covered 
 with a white film of tin. Iron and steel articles to be treated in 
 this way must first receive a coating of copper, which will enable 
 them to take the tin satisfactorily. This coating of tin is very 
 thin a mere wash and will not withstand much wear, but 
 suffices for the purposes for which it is required. On removal 
 from the bath, the articles are thoroughly washed, and are 
 polished by shaking with bran in a revolving drum (e.g. fig. 73), 
 or by any other mechanical expedient. Large objects may be 
 polished by scratch-brushing. An alternative method is to dis- 
 solve stannic oxide (putty powder) in caustic potash solution, 
 and immerse the articles in it, in contact with fragments of tin, 
 the subsequent processes being identical with those practised in 
 the first process. 
 
 Zinc, placed in a solution of stannous chloride, precipitates the 
 tin in a spongy and useless condition (analogous to that of the 
 ' lead tree ') ; but by using a very dilute solution, to which a certain 
 proportion of alum (potash- or ammonia-alum) is added, a good 
 adhesive deposit may be obtained. A similar solution of stannous 
 chlorine and ammonia-alum may be applied to the tinning of iron 
 by simple immersion. The whole process is very simple; and 
 the principal solutions are enumerated in the preceding table. 
 
 Deposition by Single-Cell Process. Solutions recommended 
 by Roseleur are given in Table XXII. (Nos. 4 and 5). In use, 
 small articles are most conveniently placed upon a tray of per- 
 forated zinc and lowered into the liquid, which should be kept 
 warm. The tray should be lightly shaken from time to time in 
 order to bring different points into contact with the zinc, and the 
 surface of the latter must be scraped at intervals to remove the 
 white incrustation which forms upon it and destroys metallic 
 contact. Large objects are immersed in the liquid in contact 
 with fragments of zinc, which should have a surface equal in the 
 aggregate to about the one-thirtieth part of that which is to be 
 coated with tin. At the end of an immersion lasting from one 
 
248 ELECTRO-DEPOSITION OF TIN. 
 
 to three hours, Roseleur recommends that the pieces should be 
 removed and scratch-brushed, while at the same time the bath, 
 which has been impoverished by the deposition of tin, is re- 
 generated by the addition of fresh tin and alkaline bitartrate or 
 pyrophosphate. After a further immersion of about equal dura- 
 tion, the goods are well washed, scratch-brushed, and dried. 
 
 Deposition by Separate Current. For this purpose the current 
 should have a fairly high electro-motive force, such as would be 
 yielded by two Bunsen-cells in series. Of all the baths quoted 
 in Table XXIII. , that of Roseleur will probably be found to give 
 the most universally good results. 
 
 Roseleur's bath (No. 8, Table XXIII.) is made by placing the 
 pyrophosphate and water in a tin-lined wooden tank, the lining 
 serving also as anode ; afterwards the stannous chloride is sus- 
 pended in the liquid in a copper sieve. The first result is to 
 produce a cloudiness in the liquid, which, however, subsequently 
 becomes clear; and after complete solution of the tin salt is 
 ready for use. The liquid may have a yellowish colour, but 
 should be perfectly transparent and clear. This bath requires 
 an addition of tin from time to time, because the metal is de- 
 posited from it at a greater rate than that at which the solution 
 of the anodes replenishes it, in spite of the relatively large 
 surface of the latter exposed. The solution already mentioned 
 as being made by dissolving the binoxide of tin in caustic potash 
 may be used as an electrolytic bath for coating iron. Maistrasse 
 ensures the complete continuity of the tin-covering given by his 
 bath (No. 6) by heating the coated object to the melting-point 
 of the tin, thus causing the latter to fuse over the surface and 
 alloy with the basis metal ; this solution is said to be especially 
 applicable to the tinning of cast-iron. 
 
 No special remarks are called for upon the practical electro- 
 deposition of tin. A current of moderate density at from 3 to 5 
 volts' pressure is required. The time to be expended varies with 
 the process and with the thickness of metal to be deposited, from 
 one or two up to twenty-four hours, which latter period is 
 recommended for Maistrasse's solution. The objects, as usual, 
 are carefully cleansed before immersion, and are subsequently 
 most thoroughly washed. They may be left in the ' dead ' con- 
 dition, if preferred, but are more generally polished, either by 
 scratch-brushing or by friction with bran. 
 
 The anodes should be of pure metal. So-called tin-plate, which 
 is only sheet-iron covered (in the dry way) with the thinnest 
 possible coating of metallic tin, is, of course, useless ; and tin-foil 
 must be used with caution, for many samples of the foil are 
 made from alloys of lead and tin, but these are generally duller 
 in their outer aspect ; while others may consist of the thinnest 
 lead-foil, covered on one or both sides with tin, the two surfaces 
 being united together by rolling ; and such samples in external 
 
COMPOSITION OF ELECTRO-TINNING BATHS. 
 
 249 
 
 o 2 
 
 I o 
 
 
 Special Method of Preparation. 
 
 QJ rrj <> <-H 
 
 1 ^"1 ^3' 
 
 ^d ^ - s i 
 
 3 > ^S ^^ 
 
 -2 J*J rn ON 
 
 si * !s 
 
 S p 
 
 -J S.B 
 
 l||rflr. 
 
 53 Q 
 
 O 
 0) 
 
 JH* 
 
 1 
 
 .2 
 
 1 
 
 8 
 
 .s 
 
 CO 
 
 a 
 <N| 
 
 S CO 
 
 s 
 
 Dissolve 2 in 14 ; add sufficient 9 to pre- 
 cipitate it ; and just re-dissolve in 12. 
 
 Form electrolytically. 
 Dissolve 4. 5, and 9 in water ; filter ; add 
 1 and 11 ; stir till dissolved. 
 
 
 
 Water. 
 
 o o o 
 o o o 
 o o o 
 
 o o o 
 o o o 
 o o o 
 
 
 
 O O 
 
 II 
 
 | 
 
 
 
 
 
 
 
 
 
 Tartaric 
 Acid. 
 
 1 i S 
 
 rH ; 
 
 ; i 
 
 i 
 
 
 | 
 
 Sulphuric 
 Acid. 
 
 : : : 
 
 : : 
 
 ^ ! 
 
 : : 
 
 : 
 
 H 1 
 
 Q 
 
 Zinc 
 Acetate. 
 
 : : : 
 
 i i 
 
 : : 
 
 . OS 
 
 : o 
 
 i 
 
 tf 
 
 Sodium Pyro- 
 phosphate. 
 
 ! 5 : 
 
 : i 
 
 : o 
 
 rH 
 
 ! : 
 
 
 
 
 Sodium 
 
 . 
 
 . . . 
 
 CO 
 
 to 
 
 . 
 
 
 Carbonate. 
 
 . 
 
 
 
 5^ 
 
 .to 
 
 
 o 
 
 Caustic 
 Soda. 
 
 : : : 
 
 o oo c5 
 
 CO rH ^ 
 
 ; i 
 
 : : 
 
 
 E 
 
 Potassium 
 
 . 
 
 . . . 
 
 . 
 
 > . 
 
 
 M 
 
 Bitartrate. 
 
 . 
 
 . 
 
 
 
 rH 
 
 
 W 
 * 
 
 Potassium 
 
 r-> 
 
 . T* CO 
 
 
 . OS 
 
 . 
 
 
 Cyanide. 
 
 r-t 
 
 o o 
 
 
 o 
 
 
 
 ^H 
 
 Potassium 
 
 
 
 
 iO 
 
 
 PQ 
 
 
 
 
 ... 
 
 
 to 
 
 I 
 
 
 Carbonate. 
 
 
 
 
 r-t 
 
 
 1 
 
 Caustic 
 Potash. 
 
 S^ rH ^ 
 
 : : : 
 
 : : 
 
 :J 
 
 ': 
 
 PH 
 
 Tin Tetra- 
 
 T* 
 
 . CO . 
 
 . 
 
 . . 
 
 
 
 chloride. 
 
 <N 
 
 o 
 
 
 
 
 
 
 Fused Tin 
 Bichloride. 
 
 <N CO 
 
 o : -r 1 
 
 CO - o 
 
 H - 
 
 N : 
 
 r-t 
 
 
 Tin 
 Binoxide. 
 
 ; ; 
 
 : : i 
 
 i : 
 
 : ^ 
 
 : 
 
 
 I? 
 
 ^ 
 
 CP fl 
 fH fH 
 
 Hern . . 
 
 Lobstein . 
 Maistrasse 
 
 Munro . . 
 Roseleur . 
 
 || 
 
 02 GO 
 
 J 
 
 
 6 
 
 rH (N CO 
 
 -* 10 V> 
 
 t 1 ^ OO 
 
 OS O 
 
 rH 
 
 i I 
 
250 ELECTRO-DEPOSITION OF LEAD. 
 
 appearance have all the characteristic appearance and brightness 
 of the pure metal. Only pure tin-foil, or plates cast from the 
 best grain tin, should be employed. 
 
 ELECTRO-DEPOSITION OF LEAD. 
 
 With lead, as with tin, the low fusing-point renders the coating 
 of an object more simply effected by immersion in the melted 
 metal than by electro-deposition. The old experiment of grow- 
 ing a 'lead tree' by suspending a fragment of metallic zinc in 
 a dilute solution of lead acetate (sugar of lead) is simply a case 
 of deposition by ' simple immersion ' ; the peculiar, largely- 
 crystalline, spongy formation of the resulting lead illustrates very 
 well the difficulty of getting a good solid adherent metal by 
 simple exchange with a more electro-positive element. When, 
 however, it is necessary to deposit lead in the wet way, a simple 
 dilute solution of the acetate may be electrolysed by separate 
 current ; but the alkaline bath, prepared by boiling 5 parts of 
 lead oxide (litharge) in a solution of 50 parts of caustic potash 
 in 1000 of water until it is completely dissolved, is preferable. 
 
 With either liquid lead anodes are used, and the objects are 
 carefully prepared for the bath, and polished afterwards as usual. 
 The methods cannot, however, be relied upon to give a thick 
 deposit, nor are they largely used in practice. 
 
 Many lead solutions tend to form an insoluble higher oxide 
 (a peroxide, Pb0 2 ) at the anode, which thus receives a coating 
 as well as the cathode, but of a different kind ; and the principal 
 interest attaching to the process of lead electrolysis centres in 
 the possibility of producing films of oxide which present different 
 colours to the eye by reason of their extreme tenuity. 
 
 ELECTRO-DEPOSITION OF ANTIMONY. 
 
 The deposition of antimony, again, is a process of no commercial 
 importance, although the metal, which has a fairly bright lustre 
 when polished, but is rather grey in colour, resists well the 
 tarnishing action of the atmosphere. It is a very brittle metal, 
 and would be useless as a coating upon any thin article, or 
 upon one which is liable to be bent in any degree when in use. 
 
 Immersion Process. It is fairly electro-negative, and will, 
 therefore, give a deposit upon many metals by simple immersion. 
 For example, brass will receive a lilac-coloured surface tint, 
 varying in depth of shade according to the time of immersion, 
 by dipping it in a boiling dilute solution of antimony terchloride 
 (butter of antimony), made by adding much water to a little 
 of the antimony compound, and boiling until the dense white 
 precipitate formed on mixture has re-dissolved ; and then, after 
 
ELECTKO-DEPOSITION OF ANTIMONY. 
 
 251 
 
 a further addition of water and a second boiling, filtering and 
 
 heating for use. The coated pieces must be well dried in hot 
 
 sawdust or in a stove, and must be protected by a varnish of 
 lacquer, if the lilac colour is to be preserved. 
 
 TABLE XXIV. SHOWING THE COMPOSITION OF SOLUTIONS FOR THE 
 ELECTRO - DEPOSITION OF ANTIMONY, RECOMMENDED BY VARIOUS 
 AUTHORITIES. 
 
 1 2 
 
 45678 9 
 
 No. 
 
 Authority. 
 
 PARTS BY WEIGHT OF INGREDIENTS. 
 
 Special Method of 
 Preparation. 
 
 Antimony. 
 
 Antimony 
 Terchloride. 
 
 Antimony- 
 Potassium 
 Tartrate. 
 
 Antimony 
 Tersulphide. 
 
 Sodium 
 Carbonate. 
 
 Ammonium 
 Chloride. 
 
 Hydrochloric 
 Acid. 
 
 Tartaric 
 Acid. 
 
 1 
 
 1 
 
 2 
 
 Gore . . 
 
 q.s. 
 
 X 
 
 
 
 
 y 
 
 (1 8. 
 
 
 1000 
 1000 
 
 Electrolyse pure strong 
 hydrochloric acid with 
 antimony anode until 
 
 
 
 
 
 
 
 
 3 
 
 4 
 5 
 
 >i 
 
 
 
 4000 
 83 
 
 50 
 
 100 
 
 
 2000 
 124 
 
 83 
 
 1000 
 1000 
 1000 
 
 (Use boiling ; it deposits 
 kermes mineral on 
 cooling.) 
 
 Roseleur 
 
 
 
 Battery Process. But, as usual, the separate-current process 
 is to be preferred, for which the solutions given in Table XXIV., 
 inter alia, have been recommended. 
 
 Of these solutions No. 3 will probably give the best results in 
 workshop practice. It is made by dissolving four pounds of the 
 double potassium-antimony tartrate (tartar emetic) in a mixture 
 of two pounds of strong hydrochloric acid with one of water. 
 This solution is particularly useful for producing thick deposits, 
 as considerable latitude in current-strength is permissible. A 
 current of 0'06 to 0*1 ampere per sq. in., or 1 to 1J amperes 
 per square decimetre, will be found most suitable, but the 
 current may be greatly increased, and the rate of deposition 
 correspondingly hurried, without danger. Tartar emetic itself 
 is a feeble conductor, and cannot alone be made to give good de- 
 posits, for, as Gore has shown, even a very weak current brings 
 down the metal in a pulverulent form; hence the addition of 
 hydrochloric acid, which sufficiently increases the conductivity. 
 The other solution containing tartar emetic has less hydro- 
 chloric acid, is a poorer conductor, and must be used with a 
 weaker current, not exceeding O013 ampere per sq. in. or 0-2 
 ampere per square decimetre. The bath prepared by Roseleur 
 
252 ELECTRO-DEPOSITION OF ANTIMONY. 
 
 by boiling together for the space of one hour, and subsequently 
 filtering the solution, 1 ounce of antimony tersulphide, 2 of 
 sodium carbonate, and 1 pint of water, tends to deposit antimony 
 oxysulphide (kermes mineral) on cooling, as above noted ; and 
 must, therefore, be always used hot. The pale orange precipitate 
 of oxysulphide is soluble in the mother-liquor as soon as the 
 boiling-point is reached. 
 
 The anodes may be made of platinum, but preferably of 
 antimony, which must be cast into slabs of the required shape 
 and size, as it is far too brittle to allow of mechanical work, such 
 as rolling or hammering. 
 
 No special treatment is required in depositing antimony ; the 
 pieces must be cleansed thoroughly, and the strength of the bath 
 must be maintained by adding a further quantity of the solution 
 from time to time. After coating, the pieces are rinsed, dried 
 in a stove (or in hot sawdust), and brightened in the usual way. 
 But if the chloride solution has been employed, the object must 
 be rinsed once or twice in hydrochloric acid immediately it is 
 removed from the bath, and then in water, because water added 
 to the original solution produces a dense curdy- white precipitate 
 of antimony oxychloride. So that, if the pieces, with a portion of 
 the bath liquor clinging to them, were dipped into water at the 
 outset, they would be covered with this white deposit; but if 
 first washed in a menstruum, such as hydrochloric acid, with 
 which the solution mixes without decomposition, the original 
 liquid is safely removed, and the final cleansing may be effected 
 in water without risk. 
 
 Character of Deposit. The metal deposited by too strong a 
 current is, as usual, black, powdery, and non-adherent ; and that 
 yielded by some solutions may be so, even when a weak current is 
 employed. But the metal is capable of being thrown down in two 
 different modifications of the reguline or solid form one of a grey- 
 slate colour in the dull condition, but taking a good polish and 
 resembling cast-iron when scratch-brushed, having a crystalline 
 fracture, and being hard and very brittle ; while the other is 
 darker and more steely, but somewhat softer, non-crystalline, or 
 amorphous, and with a lustre which resists atmospheric influences 
 for quite a lengthened period. 
 
 Explosive Antimony. The most curious and interesting pheno- 
 menon in connection with antimony deposition- is the production 
 of an explosive variety, which has been fully studied and described 
 by Gore. He found that the amorphous antimony deposited from 
 a solution of 1 part of antimony terchloride in 5 or 6 parts of 
 hydrochloric acid (of specific gravity 1*12), or in 10 of hydro- 
 bromic acid (specific gravity l - 3), or in 15 parts of hydriodic acid 
 (specific gravity 1*25), would, under certain conditions, undergo 
 a physical change and become crystalline ; and that this change 
 was attended by an increase of density, and with an evolution 
 
EXPLOSIVE ANTIMONY DEPOSIT. 253 
 
 of heat so considerable that, if evolved instantaneously by a con- 
 siderable mass, it may develop almost explosive violence. The 
 heat is so great that, if sufficient metal undergo the change, 
 paper in contact with it is burned, and wood is scorched brown ; 
 the ' explosion ' is often accompanied by a flash of light, but 
 always by a slight cloud of vapour expelled from the interior. 
 
 The three varieties (from the chloride, bromide, and iodide) 
 differ in their sensitiveness, as well as in other particulars. None 
 of them are pure, but retain within their pores about 6, 20, and 
 22 per cent, respectively of the depositing liquor, which may be 
 expelled by heating, as, for example, at the moment of explosion ; 
 the cloud of vapour observed is thus accounted for. The presence 
 of this liquor in the metal gives rise to an apparently abnormal 
 excess of deposited metal over that which should be yielded 
 according to the electro-chemical equivalent. The alteration of 
 condition proceeds gradually on keeping, but more quickly in the 
 case of powder or of thin pieces than with larger masses of metal ; 
 and the heat is then evolved almost imperceptibly. But freshly- 
 deposited material may be caused to undergo the change, in a 
 rapid or explosive manner, by any physical means, which is cap- 
 able of sufficiently affecting the molecular arrangement of the body. 
 With the chloride variety the action begins when it is heated 
 to 170 F., becoming sudden and complete at about 205 F.; 
 with the bromide deposit the explosion occurs at 320 F.; with 
 that from the iodide at a still higher temperature. A similar 
 descending order of sensitiveness was observed when other means 
 were employed to initiate the action ; a touch with a red-hot wire 
 caused immediate conversion of the chloride variety, while the 
 bromide metal was merely locally affected by contact with the hot 
 wire, the action, only spreading through the whole when it was 
 raised to 250 F. throughout, and through the iodine specimen 
 when it was heated at 338 F. The heat developed by the 
 alteration in the first-named case was so great that a thin rod 
 Jth of an inch in diameter, upon which the amorphous anti- 
 mony was electrolytically built up to a total diameter of half an 
 inch, melted, flowed away from the antimony, and remained fluid 
 for some time. The explosiveness appears to be due to the 
 content of chloride, bromide, or iodide, as the case may be. 
 
 A sudden blow, or even rubbing with glass or metal, is liable 
 to convert the amorphous into the crystalline variety, so that if 
 it is required to break up the unexploded metal into smaller 
 pieces, it should be fractured under cold water by a comparatively 
 soft material, such as wood. Provided that they are kept under 
 iced water meanwhile, very thin pieces may even be crushed to 
 a fine powder in a mortar ; and this powder may be dried in the 
 cold over sulphuric acid ; and, remaining in the original condi- 
 tion, will evolve subsequently the same proportion of heat as the 
 thicker untouched deposits. 
 
254 ELECTRO-DEPOSITION OF BISMUTH, ETC. 
 
 THE ELECTRO-DEPOSITION OF BISMUTH. 
 
 The deposition of this metal possesses at present little beside 
 a scientific or theoretical interest. 
 
 It may be thrown down from a weak and very slightly acidi- 
 fied solution of the nitrate, either by simple immersion upon 
 certain more electro-positive metals, such as tin, or by the 
 separate-current process. Bertrand uses for the latter method a 
 solution of 30 parts of the double chloride of bismuth and 
 ammonium in 1000 of water, containing a small proportion of 
 hydrochloric acid. With one Bunsen-cell he succeeded in obtain- 
 ing a coat which, although black exteriorly, exhibited the well- 
 known slightly pink shade of the metal, and was susceptible 
 of a very high polish. Like antimony, the brittle nature of the 
 metal renders it unfit for coating objects which are at all strained 
 or altered in shape subsequently. 
 
 THE ELECTRO-DEPOSITION OF PALLADIUM. 
 
 Palladium is one of the rarer metals, belonging to the platinum 
 group. Having a silver-white colour and lustre, and being also 
 untarnishable at ordinary temperatures by oxygen or (unlike 
 silver) by sulphur compounds in the air, it is sometimes substituted 
 for silver. Occasionally, but very rarely, silver-plated goods are 
 given a thin final coat of palladium. Cowper-Coles, 1 in his 
 interesting electrolytic process for the manufacture of parabolic 
 reflectors, which are required to be always bright, and which, if 
 used for electric search-lights may be exposed to high temperatures 
 and to influences which would rapidly tarnish silver, covers the 
 copper of which the reflectors are made with a coat of palladium 
 having a thickness corresponding to 70 to 80 grains of palladium 
 per square foot. 
 
 The bath that he uses is that recommended by Bertrand, 
 namely, a solution of the double chloride of ammonium and 
 palladium but with excess of ammonium chloride added. He 
 dissolves 6'2 parts of this compound with ten parts of ammonium 
 chloride in 1000 of water, and uses it at a temperature of 75 F. 
 with a current-density of about 0'15 ampere per square foot and 
 an E.M.F. between the electrodes of 4 to 5 volts. He uses an 
 anode of carbon ; Bertrand, however, employs one of palladium. 
 
 Of the remaining metals there are none which render necessary 
 in this work a description of the means by which they may be 
 electrolysed. With regard to some of them, indeed, many 
 published processes would appear on thermo-chemical grounds to 
 be visionary.. 
 
 In regard to aluminium especially, the extreme popularity of 
 
 1 Jour. Inst. of Electrical Engineers, 1898 (xxvii.), p. 105. 
 
PRODUCTION OF METALLO-CHROMES. 255 
 
 this metal, combined with a great want of knowledge, on the 
 part of the public, as to its properties, have led to a demand for 
 its electro-deposition. Many solutions have been proposed which 
 it was claimed should give good deposits of the metal, but have 
 been found by various experimenters to be worthless. In our 
 own experience, the brilliant grey deposit, which has been 
 afforded by some of these methods, but which has never exceeded 
 in thickness that of a mere film, has consisted principally of iron, 
 a metal which is almost universally present in commercial 
 aluminium compounds. The deposit has been often found, on 
 testing, to contain aluminium ; this may have been due to traces 
 of the solution remaining in the pores of the coat, or it may 
 have resulted from aluminium which had actually been deposited 
 with the iron as an alloy ; but in all cases the iron was found 
 to be vastly preponderating. That it is possible to deposit 
 aluminium by the electrolysis of fused compounds is no doubt 
 true, but further evidence is necessary to prove the satisfactory 
 deposition of the pure metal from aqueous solutions. 
 
 COLOURING OF METALLIC SURFACES. 
 
 Advantage has been taken of the fact that lead and certain 
 other metals tend to deposit as peroxide upon the anode, instead 
 of, or sometimes in addition to, precipitating as metal upon the 
 cathode, to obtain certain colours upon metal surfaces. The 
 most interesting application of this is to be seen in the formation 
 of metallo-chromes by the deposition of an infinitesimal film of 
 lead peroxide upon a polished steel surface. It has long been 
 known that colourless transparent substances, if sufficiently thin, 
 are capable of displaying a series of colours by reflected light by 
 the optical phenomenon known as the interference of luminous 
 waves (where the wave of light reflected from one side of the 
 film is so similar to that reflected after refraction from the 
 other that their respective vibratory influences interfere with 
 one another). The play of colours upon the soap-bubble or upon 
 oil floating on water are instances of this phenomenon, which 
 was first studied by Newton. A momentary immersion of a 
 bright steel or platinum plate as anode in a lead solution suffices 
 to deposit a film of peroxide, which answers the requirements for 
 the production of these iridescent colours. Nobili was the first 
 to observe this action with acetate of lead. Becquerel's solution 
 is now used for this purpose; it is made by dissolving 14 oz. 
 of caustic potash in half a gallon of water, adding to this 10 J oz. 
 of lead oxide (litharge) and boiling for from half an hour 
 to an hour, allowing it to stand for some time, then decanting 
 the clear liquid from the subsided precipitate, and making up 
 the whole to a gallon in volume. The electrolytic action 
 must be continued for exactly the right period of time; an 
 
256 PRODUCTION OF METALLO-CHROMES. 
 
 insufficient exposure does not give time for the development of 
 sufficient thickness to allow of interference, while an excessive 
 action causes an opaque, dirty brown deposit ; intermediately 
 between the two, a very beautiful play of colours may be secured 
 The cathode may be of copper-sheet. Gassiot produced patterns 
 upon the anode by interposing a cardboard disc, with a perforated 
 design upon it, between the electrodes, so that the deposit chiefly 
 occurred on the portions unshielded by the solid portions of the 
 card. Watt used copper wire bent into various shapes ; this 
 system has the advantage that there are varying distances between 
 the different portions of the anode and the cathode, and, therefore, 
 a varying thickness of film is produced, which adds to the beauty 
 of the iridescence. The film is fairly adhesive, but should not be 
 handled more than is necessary. 
 
CHAPTER XIV. 
 
 THE ELECTRO-DEPOSITION OP ALLOYS. 
 
 THE principles upon which the possibility of electro-depositing 
 alloys may be said to depend have already been explained in 
 Chapter II. (p. 33). Brass, bronze, and German silver are practi- 
 cally the only alloys deposited, if we except the mixtures used in 
 producing coloured gold coatings, and of these the first-named 
 alone has any widespread use. 
 
 THE ELECTRO-DEPOSITION OP BRASS (COPPER AND ZINC). 
 
 A brass coating may be given to a copper article by cover- 
 ing it electrolytically with a thin film of zinc, and then, after 
 washing and drying, applying to it a heat just sufficient to cause 
 the two metals to form an alloy superficially ; and similarly an 
 object made of any other material, which will withstand the 
 necessary heating, may be brass-surfaced by depositing alternate 
 layers of copper and zinc, and alloying them in situ as before. 
 But in practice this could not well be done. Nor is brassing 
 usually effected by simple immersion, although Watt has shown 
 that a zinc rod, dipped into a mixed solution of copper and zinc 
 acetate, becomes covered with a yellow deposit of the alloy. But 
 for practical purposes the production by the separate-battery 
 process is alone adopted. The Bunsen form of battery is the 
 best, and should generally be arranged with two cells in series. 
 
 The Solution. The solution may be greatly varied, and, indeed, 
 no absolute and unalterable rules can be laid down as to its con- 
 stitution. The basis of most of the liquids is the mixed cyanides 
 of copper and zinc, as in this combination zinc does not dis- 
 place copper from its solution, and there is in consequence a 
 better chance of obtaining a simultaneous coating of the two 
 bodies ; but the relative proportions of these may require to be 
 varied in working, according to the behaviour of the solution, 
 which depends upon several inconstant quantities strength of 
 current, resistance of solution, and the like. The following table 
 (XXV.) summarises the principal electro-brassing solutions. 
 
 257 17 
 
258 
 
 ELECTRO-DEPOSITION OF ALLOYS. 
 
 TABLE XXV. SHOWING THE COMPOSITION OF ELECTRO- 
 1 2 3 4 5 6 7 8 9 10 11 12 13 
 
 
 
 PARTS BY WEIGHT 
 
 No. 
 
 Authority. 
 
 1 
 
 m 
 
 Copper 
 Acetate. 
 
 Copper 
 Carbonate. 
 
 Copper 
 Chloride. 
 
 II 
 ! 
 ^ 
 
 = 
 ^ 
 
 o| 
 
 Is 
 
 <J 
 
 Zinc 
 1 Carbonate. 
 
 Zinc 
 Chloride. 
 
 o| 
 
 il 
 
 A 
 
 2-5 
 N 3 
 
 02 
 
 Caustic 
 1 Potash. 
 
 Potassium 
 Carbonate. 
 
 
 1 
 
 Brunei . . - . 
 
 
 
 
 10 
 
 
 
 
 
 
 
 20 
 
 
 '250 
 
 
 2 
 
 jj . 
 
 
 
 ... 
 
 5 
 
 ... 
 
 
 
 
 
 
 10 
 
 
 80 
 
 
 3 
 
 Gore . . . . 
 
 X 
 
 
 
 
 ... 
 
 
 ... 
 
 
 
 
 
 
 
 
 4 
 
 Heeren 
 
 ... 
 
 
 
 
 
 3 
 
 
 
 
 
 26 
 
 
 ... 
 
 
 5 
 
 Hess .... 
 
 X 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Q 
 
 Japinsr 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 7 
 
 Morris and Johnson . 
 
 
 
 
 
 12-5 
 
 
 
 
 
 6-2 
 
 
 ... 
 
 
 
 8 
 
 Roseleur 
 
 
 ... 
 
 10 
 
 
 
 
 ... 
 
 10 
 
 
 
 
 
 
 
 g 
 
 
 
 1?-5 
 
 
 
 
 
 
 
 10 
 
 
 
 
 
 
 10 
 
 
 
 14 
 
 
 
 
 
 
 
 14 
 
 
 
 
 
 
 H 
 
 Russell and AVoolrich. 
 
 
 lf0 
 
 
 
 
 
 15 
 
 
 
 
 
 
 
 
 12 
 
 De la Salzede 
 
 
 
 
 5 
 
 
 
 
 
 
 
 9-5 
 
 
 120 
 
 
 13 
 
 
 
 
 
 a 
 
 
 
 
 
 
 
 7 
 
 
 100 
 
 
 14 
 
 Volkmer 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 15 
 
 Watt . . 
 
 
 4 
 
 
 
 
 
 
 ... 
 
 
 
 8 
 
 56 
 
 
 
 16 
 
 Weiss 
 
 
 4 
 
 
 
 
 
 
 
 
 
 6-8 
 
 40 
 
 
 
 17 
 
 Wood . . . . 
 
 
 
 
 
 14 
 
 
 
 
 
 7 
 
 
 
 
 
 
 
 
 
 NOTES TO TABLE. The heavy figures in the last column refer to the numbers of the vertical 
 to zinc ; Nos. 9 and 17 to iron and steel. No. 8 is recommended to be used cold ; 
 
ELECTRO-BRASSING SOLUTIONS. 
 
 259 
 
 BRASSING SOLUTIONS, AS RECOMMENDED BY VARIOUS AUTHORITIES. 
 14 15 16 17 18 19 20 21 22 23 24 
 
 OF INGREDIENTS. 
 
 
 
 Bj 
 
 jM 
 
 4 
 
 ^ 
 
 j 
 
 j 
 
 1* 
 
 1 
 
 
 
 =3 aj 
 
 1 
 
 
 Special Method of Preparation. 
 
 
 .2 ^ 
 
 o? '3 
 
 
 ff a '; 5 
 
 'gJS-e 
 
 || 
 
 3 
 
 1 
 
 || 
 S- 
 
 11 
 
 11 
 
 || 
 
 K.^. 
 
 
 
 | 
 
 I* 
 
 p ?-) 
 
 S 
 M 
 
 1 
 
 s 
 
 ^ 
 
 ia 
 
 I s 
 
 S 
 ^ 
 
 * 
 
 ^ 
 
 
 
 
 
 
 
 
 
 
 
 125 
 
 
 1000 
 
 
 
 12 
 
 
 
 
 
 
 
 
 
 
 1000 
 
 ( Dissolve together in 24 ; add 14 
 t last. 
 
 
 
 
 
 
 
 
 
 
 
 125 
 
 
 
 
 
 62-5 
 
 
 
 
 
 1000 
 
 Form electrolytically. 
 ( Dissolve 6 in 12 of 24 ; 11 in 52 
 
 
 60 
 
 
 
 
 
 
 
 
 
 
 1000 
 
 < of 24 ; 14 in 120 of 24 ; mix ; 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ( add rest of 24. 
 
 
 6-5 
 
 
 
 42 
 
 
 
 
 07 
 
 
 
 1000 
 
 ( Form electrolytically with brass 
 \ anode. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / Form electrolytically, with brass 
 
 
 q.s. 
 
 
 
 
 
 
 
 
 
 
 1000 
 
 ) anode suppl emented by copper 
 j or zinc anodes, if necessary, to 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ improve colour. 
 
 
 100 
 
 
 
 
 
 
 100 
 
 
 
 
 1000 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 {Precipitate 3 and 8 from CuS0 4 
 and ZnS0 4 with Na 2 C0 3 , and 
 
 
 20 
 
 
 20 
 
 
 20 
 
 
 
 
 ... 
 
 0-2 
 
 1000 
 
 wash ; add 16 and 18 in 900 
 of 24 ; add 14 and 23 in 100 
 
 
 
 
 
 
 
 
 
 
 
 
 
 of 24; filter. Add more 14 
 
 
 
 
 
 
 
 
 
 
 
 
 
 if solution remain blue. 
 
 
 40 
 
 
 100 
 
 
 20 
 
 
 
 
 
 
 1000 
 
 f Dissolve 14, 16, and 18 in 800 of 
 1 24 ; add 2 and 9 in 200 of 24. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 [ Dissolve 14 and 18 in 800 of 24 ; 
 
 
 30 
 
 
 
 
 28 
 
 16 
 
 
 
 
 
 1000 
 
 < add to solution of 2, 9, and 19 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ( in 200 of 24. 
 
 
 excess 
 
 150 
 
 
 ... 
 
 
 
 
 
 
 
 1000 
 
 f Add 14 last, until precipitate re- 
 ( dissolves. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ( Dissolve 14 in 24 of 24 ; dissolve 
 
 
 2-5 
 
 
 
 
 
 
 
 
 61 
 
 
 1000 
 
 < 4, 11, 13 in rest of 24 ; add 22 ; 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ( stand for some days and decant. 
 
 
 10 
 
 
 
 
 
 
 
 
 
 
 1000 
 
 
 
 125 
 
 
 
 
 
 
 125 
 
 
 
 
 1000 
 
 / Form electrolytically ; with cop- 
 ) per anode till saturated ; then 
 j with zinc anode till deposit is 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ brass colour. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ^Dissolve 2 in 65 of 24 and add 15 
 
 
 
 
 
 
 
 
 
 
 
 
 
 I of 19 ; dissolve 11 in 125 of 24 
 
 
 6-5 
 
 
 
 ... 
 
 
 31 
 
 
 
 
 
 1000 
 
 I and add 16 of 19 ; mix ; add 
 \ 12 in 125 of 24, then 14 in 125 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1 of 24. Stir, add rest of 24; 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ^ stand and decant. 
 
 
 3-4 
 
 
 
 
 
 24 
 
 ... 
 
 
 
 
 1000 
 
 ( Dissolve 2 in 19 ; add 12 ; then 
 ( add 11, 14, and 24. 
 
 
 
 
 
 
 82 
 
 
 
 
 
 
 
 14 
 
 
 
 1000 
 
 
 
 
 
 
 
 
 columns indicating the various reagents. Solutions Nos. 6 and 10 are especially applicable 
 No. 14 at 86 to 100 ; No. 7 at 150 ; No. 17 at 160 F. ; and Nos. 3 and 4 boiling. 
 
260 ELECTRO-DEPOSITION OF ALLOYS. 
 
 One of the best of these solutions is that recommended by 
 Roseleur (No. 8), which is somewhat complex, but will be found 
 to give good results to prepare 1 gallon, 2J oz. of copper 
 sulphate and a like weight of zinc sulphate are dissolved in 
 water ; and a solution of 6 J oz. of sodium carbonate in a con- 
 venient quantity of water is added to the mixture. A dense 
 precipitate of copper and zinc carbonates is thus formed. It is 
 allowed to settle, water is poured on, and it is thus washed 
 several times by decantation ; the clear liquor is finally siphoned 
 away from the precipitate, which is then filtered off from the 
 residual liquid, and mixed with a solution of 3J oz. of sodium 
 carbonate, and 3J oz. of sodium bisulphite, in 7J pints 
 of water. To this is added 3J oz. of potassium cyanide 
 and 20 grains of arsenious acid (white arsenic) in J pint 
 of water. After filtering, the clear liquid contains the copper 
 and zinc, and should be quite decolorised ; any blue colour 
 indicates the presence of unaltered copper salt, and calls for the 
 addition of a further quantity of potassium cyanide solution. It 
 may be taken as a general rule that all the cyanide brass- 
 baths should be free from blue colour ; and that if they are not so 
 initially, they must be made so by the introduction of sufficient 
 extra potassium cyanide, while if they become blue when in use, 
 the same remedy is to be applied. It is to be recommended that 
 a 10 per cent, solution of the cyanide should be kept for this 
 purpose, so that a little may be added whenever necessary. 
 The bath (Roseleur's) is used cold and with brass anodes, but as 
 the composition is liable to variation by unequal solution of the 
 brass, a further addition of copper or zinc may be required from 
 time to time. This should be effected by adding separate solu- 
 tions of copper or of zinc cyanides, made by dissolving their 
 carbonates in solutions of potassium cyanide. Sodium or potas- 
 sium arsenite may be used in place of arsenious acid, but a 
 proportionately larger quantity must, of course, be employed. 
 With the addition of a little arsenic the solution gives a brighter 
 deposit than is yielded by the copper-zinc solution alone ; a 
 large quantity, however, is found to give to the deposit a tem- 
 porary increase in whiteness, which is objectionable. 
 
 Baths may be prepared electrolytically either, as in Gore's 
 solution (No. 3), by passing the current through a suitable 
 solution from a brass anode, so that the constituents of the 
 anode alloy may dissolve simultaneously into the liquid, or, as 
 in Yolkmer's bath (No. 14), by passing the current from a 
 copper anode alone at first, until sufficient of that metal has 
 been taken up ; and next from a zinc anode alone, until the 
 requisite shade of brass-colour appears on the sheet metal 
 cathode ; a brass anode is then substituted for that of zinc, 
 and the plate at the cathode pole is replaced by the object 
 to be coated. 
 
NATUEE OF DEPOSIT. 261 
 
 Anodes. The anode used for the various solutions above given 
 may be made of brass, and this should have approximately the 
 same composition as the metal which it is proposed to deposit, 
 and should be prepared from the pure virgin metals ; but, instead 
 of using the copper and the zinc combined together in the form of 
 an alloy, they may be used separately by suspending alternate 
 strips of the two metals from the anode rod ; and this method, 
 although less usually adopted, presents the advantage that the 
 composition of the bath may be controlled by altering the 
 relative numbers of the two kinds of strips, so that a greater 
 area of copper anode surface may be presented to the liquid 
 when the deposit is becoming too pale in colour, or of zinc when 
 it grows too red or yellow. The anodes are supported in the 
 usual way, either completely immersed and supported by stout 
 brass hooks, or partially immersed only. The vats for cold and 
 for hot solutions are similar to those used for the copper (cyanide) 
 depositing process. 
 
 Nature of Deposit. The character of the metal deposited is 
 entirely dependent upon the conditions of current and of bath, 
 but chiefly upon the composition of the solution. If the liquid 
 contain an excess of either constituent beyond the normal, the 
 deposited metal will also contain an excessive percentage of that 
 metal, and its properties and colour will be influenced accord- 
 ingly. The composition of the anodes vastly influences the 
 deposit yielded by a solution, inasmuch as it modifies the con- 
 stitution of the bath itself. 
 
 A weak current, or the imparting of motion to the suspended 
 objects (which is in some respects equivalent to a reduction in 
 current-density), tends to produce a deposit containing a greater 
 proportion of the more electro-negative element copper, while 
 a stronger current yields a larger percentage of zinc. Then 
 again given the same battery, solution, and anodes heating 
 the bath, by increasing its conductivity, raises the current- 
 strength, and thus also tends to increase the percentage of zinc. 
 The deposition of hydrogen must as usual be avoided, in order 
 to obtain a good adherent deposit. But, to sum up the preceding 
 observations, within the limits of current-density that permit the 
 production of a good coat, the conditions which favour the pre- 
 cipitation of an alloy rich in copper, and, therefore, a red or 
 yellow colour, are a solution and anode containing a high per- 
 centage of copper, a weak current, a cold bath, and the movement 
 of the articles under treatment ; while the opposite effects of a 
 greater proportion of zinc with a corresponding whitening of the 
 deposit are yielded by a strong current, by anodes and solution 
 richer in zinc than in copper, and by maintaining the objects 
 motionless in a hot liquid. 
 
 It will now be understood that the various relations of current- 
 density and other conditions of work are mutually so inter- 
 
262 ELECTRO-DEPOSITION OF ALLOYS. 
 
 dependent that it is impossible to lay down inviolable rules for 
 working brass solutions; but with the requisite constant obser- 
 vation and care, there is no difficulty in obtaining any desired 
 nature of deposit. If the colour of the metal is too red, an 
 excess of copper is indicated, which may be rectified by increasing 
 the current-strength or adding more zinc to the liquid ; if it is 
 too white, showing an excess of zinc, the current is reduced or 
 copper is added to the bath. The alteration of current may 
 often be conveniently effected, as mentioned in another chapter, 
 by increasing or decreasing the surface of the anodes according 
 as its strength is required to be greater or less. It is evident, 
 therefore, that baths of even comparatively widely-differing 
 compositions may be caused to yield deposits of the same alloy 
 by adjusting the various conditions of work. But since the 
 proportion of the constituents in the deposited metal, and hence 
 its colour, are so susceptible of alteration by a change in any 
 of the conditions, it becomes necessary to watch the progress of 
 the electrolysis most carefully, not only to prevent the variation 
 of the alloy, but to prevent local alterations, which may give 
 rise to spotted or unevenly coloured deposits, such as would be 
 produced if any portion of the cathode object were receiving 
 more or less current than the remainder, or if the solution were 
 not properly mixed. It is, therefore, advisable to stir the solu- 
 tion very thoroughly before commencing work, and at intervals 
 while the deposition is in progress ; and again to observe that 
 the pieces forming the opposite electrodes are as nearly as 
 possible equidistant from one another. The observance of this 
 latter precaution, which is necessary enough in depositing single 
 metals, where the main question is one of thickness of deposit, 
 becomes greatly exalted in importance when it is seen that not 
 only the thickness but also the colour of the coating is influenced, 
 the portions more remote from the anodes receiving less current, 
 and, therefore, having a redder shade than those in closer 
 proximity. Similarly, the difference in specific gravity of the 
 various strata in imperfectly mixed liquids produces a variation 
 in colour between the deposit at the top and that at the bottom 
 of an article. 
 
 The Process. In the practical application of the process, the 
 objects to be brassed must be first carefully cleansed and polished 
 after the orthodox fashion ; they are then immersed in the 
 electrolytic bath until the required thickness of metal has been 
 attained, and, provided that it be of the right colour, it is then 
 rinsed, scratch-brushed, well washed in hot water, and dried in 
 hot sawdust or in a stove. In some of the liquids used the 
 cyanide solution does not exert sufficient solvent action upon zinc 
 oxide, which thus becomes separated in the solid form upon the 
 surface of the anode, finally crumbling away and collecting on the 
 bottom of the bath. Such a formation is objectionable, first, 
 
THE PROCESS. 263 
 
 because it impedes the action by yielding a film upon the 
 electrode surface ; then, because it becomes detached, and so 
 introduces into the bath solid matter that may be held for a 
 time in suspension in the liquid, which is always dangerous, 
 because fragments are liable to become attached to the surface 
 of the object being plated ; and, thirdly, because a deficiency of 
 zinc passing into solution from the anode is equivalent to a 
 relative increase in the amount of copper in the bath. In cases 
 where such a precipitate is observed upon the anode (which 
 should, therefore, be inspected from time to time), a small 
 quantity of liquor ammonice mixed with a slight excess of 
 potassium cyanide should be added to the liquid; any oxide 
 already formed will thus be dissolved, and the anode surface 
 will be kept clean, owing to the non-formation of fresh quantities 
 of the substance. When ammonia is one of the constituents of 
 the bath, a further quantity must be added at intervals, because 
 it is constantly evaporating by exposure to the air. 
 
 When an object is found to be taking a bad deposit at first, it 
 should be removed from the vat, scratch-brushed and returned, 
 the defect in the electrolytic arrangements having been made 
 good in the meantime. A dirty yellowish or earthy-looking 
 deposit is often the result of insufficiency of cyanide, and may be 
 corrected accordingly. 
 
 The chief use of brass composition is to coat zinc or iron 
 surfaces with a rich-coloured material that may be subsequently 
 bronzed or otherwise ornamented, or which may be simply 
 lacquered and left with the original brass colour. Large quanti- 
 ties of iron tube for bedstead, fender, and other work are thus 
 coated electrolytically with brass. It is sometimes employed for 
 facing typographic matter, but presents no advantage over nickel 
 for this purpose ; it is especially applicable to coating book- 
 binders' type, which should have a hard face, and which must 
 bear heating, as these tools are frequently used hot. 
 
 THE ELECTRO-DEPOSITION OP BRONZE (COPPER AND TIN). 
 
 Maistrasse has obtained a superficial bronze coating upon 
 copper objects by first electro-tinning them, and then heating 
 them for some time above the melting-point of tin, so that it 
 may fuse and alloy with the base metal upon the surface. But 
 in regard to the more purely electrolytic methods, the general 
 remarks which were made in reference to brass apply with equal 
 force to the less commonly employed deposition of bronze. The 
 principles underlying the two processes are identical, and the 
 methods similar. The following short table of solutions will show 
 that there is practically only a substitution of tin for zinc as com- 
 pared with brassing liquids. 
 
264 
 
 ELECTRO-DEPOSITION OF ALLOYS. 
 
 (OLUT10NS, EECOMMBNDED BY 
 
 Special Method of Preparation. 
 
 Q, - , 
 
 q3 co^ 
 
 r ~ < r- 1 T 1 
 88 II ^ 
 
 s ^i 
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 rH 1 ^ '^ 
 
 > p? >'o^ 
 
 -3 g ^ go 
 
 Q M s^ 
 
 Dissolve 1 in 14 ; add 11 ; when dis- 
 solved, add 5 in little solution of 9. 
 
 Add sufficient 8 to rest of solution to 
 give required deposit. 
 
 i 2 
 
 a 
 
 Water. 
 
 O 
 O 
 o 
 
 I 1 I 1 
 
 o o o o 
 
 O O O 
 
 O O O 
 
 CO 
 rH 
 
 PQ 
 
 Rochelle Salt. 
 
 : 
 
 o 
 : : : * 
 
 . S 
 
 H 01 
 o w 
 
 g Soda-lime. 
 
 : : 
 
 : : : o 
 
 CO 
 
 II - 
 
 Potassium 
 g Cyanide. 
 
 : 2, 
 
 o o go 
 
 1-H I-( ^ 
 
 5 S o 
 
 << |!j rH 
 
 pq Potassium 
 * Carbonate. 
 
 
 o o 
 o o 
 
 rH i 1 
 
 og * 
 
 ^ Caustic 
 Potash. 
 
 co ; 
 
 1 ! SH 
 
 O 3 00 
 
 O Sodium 
 r, Stannate. 
 
 : i 
 
 : : =* 
 
 E ^ 
 
 Tin Binoxide. 
 
 : ^ 
 
 ; i 
 
 1 " 
 
 g Tin 
 Bichloride. 
 
 : : 
 
 c^> o 
 
 1 1 r ~ ( 
 
 
 
 W ^ 
 
 fe Tin 
 w Tetrachloride. 
 
 00 | 
 
 
 
 H 
 g * 
 
 H Cuprous 
 P^ Cyanide. 
 
 : o 
 
 : : : 
 
 1 - 
 
 pu Cuprous 
 Chloride. 
 
 : : 
 
 T 1 ' " 
 
 w 
 
 T 
 
 Cupric 
 Chloride. 
 
 ; ; 
 
 ; : 
 
 - 
 
 Copper 
 Sulphate. 
 
 g 
 
 -^ 10 
 
 : ' : CD co 
 
 X 
 
 1 
 
 H 
 
 45 
 fi 
 
 o 
 
 CD t>3 
 
 1 1 
 fiq p^ 
 
 <D 
 
 'N 
 
 II s ! 
 
 
 Q 
 
 
 CO "* O CO 
 
DEPOSITION OF VARIOUS ALLOYS. 265 
 
 Bronze anodes are used, varying in composition with the char- 
 acter of the metal to be deposited. A ternary alloy of copper, 
 tin, and zinc may be obtained by suitably combining brassing and 
 bronzing solutions ; but, like bronzing itself, this process has but 
 little practical utility. 
 
 THE ELECTRO-DEPOSITION OF GERMAN SILVER (COPPER, 
 NICKEL, AND ZINC). 
 
 This alloy is recommended by Watt as a substitute for nickel 
 in coating certain small articles, such as revolvers, on account of 
 the colour, which has a somewhat redder shade than the pure 
 white of nickel, and is sometimes considered to be more pleasing. 
 The bath which he recommends is made by dissolving 1 oz. 
 of German silver, of the required composition, in nitric acid, 
 diluted with an equal volume of water, taking care that no 
 excess of the acid is present, by adding it very gradually, and so 
 that a small portion of the metal remains undissolved in the 
 liquid when all chemical action has ceased (this is indicated by 
 the cessation of gas evolution) ; about 4 oz. of potassium 
 carbonate are dissolved in a fairly large quantity of water, and 
 this solution is added to that of the alloy, until no further 
 precipitate is produced by the addition of another drop of the car- 
 bonate. The precipitate is then allowed to subside, the liquid 
 is poured away from it, and fresh water is added several times 
 successively. A strong solution of potassium cyanide with 
 1 oz. of the strongest ammonia solution (liquor ammonice fortiss.), 
 filtered clear, if necessary, is now added with constant stirring, 
 until the precipitated carbonates are just dissolved; a slight 
 excess of potassium cyanide is now introduced, with sufficient 
 water to make up the bulk of the liquid to one gallon. After 
 filtering, the liquid is allowed to stand for twelve hours, the 
 clear liquid is decanted off, and is used as an electrolyte with a 
 German-silver anode, similar in composition to that from which 
 the bath is prepared, the current being produced by a Bunsen- 
 battery. 
 
 Morris and Johnson, whose electro-brassing bath has already 
 been given, protected, by their patent of 1852, the deposition of 
 German silver from a solution of the cyanides of copper; zinc, 
 and nickel (in the correct proportions of the alloy) in one of 
 ammonium carbonate and potassium cyanide dissolved in 10 
 parts of water. An excess of potassium cyanide produces an 
 alloy containing a larger percentage of copper, and imparts to 
 it a reddish shade in consequence ; but this may be rectified by 
 the addition of ammonium carbonate, while, on the other hand, 
 if the deposit be too white, fresh cyanide is added by degrees 
 until the desired shade is produced. This bath is to be used with 
 a high current-density at a temperature of 160 F. 
 
266 ELECTRO-DEPOSITION OF ALLOYS. 
 
 Other alloys may also be deposited by adopting suitable solu- 
 tions prepared to satisfy the requirements set forth in an earlier 
 chapter. At present there is no demand for any of these, and 
 they have generally been designed to imitate the qualities of a 
 more costly unalloyed metal. For example, Round proposed to 
 precipitate an alloy of tin and silver to take the place of the 
 latter metal, by electrolysing a clear solution of 4 oz. of potas- 
 sium cyanide, 5 of the strongest liquor ammonice, and J oz. of 
 silver, to which a suitable proportion of any soluble tin compound, 
 and subsequently about 2J oz. of potassium carbonate, had been 
 added, and from which sediment had been separated by decanta- 
 tion. He used a dual anode, consisting of a large sheet of tin 
 with a small plate of silver in juxtaposition. 
 
 Alloys have also been prepared to resist the action of acids or 
 other corrosive fluids, such as that of platinum and silver, which 
 Campbell has patented, and made by depositing from a solution 
 containing silver and platinum in the proportion of 70 : 30, with 
 an anode made from an alloy of similar percentage composition. 
 The solution was prepared by dissolving the mixed chlorides, 
 obtained by the addition of ammonium chloride to the conjoint 
 solution, in potassium cyanide solution. An alloy of aluminium 
 and nickel is said to have been deposited in Philadelphia and 
 denominated alu-ni, to take the place of the nickel, but it is more 
 costly while presenting no commensurate advantages. 
 
 Another group of alloys sometimes prepared includes those of 
 magnesium and aluminium, which it is difficult, if not impossible, 
 to deposit alone. 
 
CHAPTER XV. 
 
 ELECTRO-METALLURGICAL EXTRACTION AND REFINING PROCESSES. 
 
 Preliminary. As we have already explained in the first chapter, 
 the term electro-metallurgy is commonly applied to all classes of 
 work which necessitate the deposition of metals by means of the 
 electric current. Some authorities are disposed to reserve it for 
 those processes by which metals are extracted from the ore with 
 the aid of electricity. It would seem that technically the first 
 and wider definition is admissible, and it is certainly more con- 
 venient to group together under one name all the kindred 
 processes of electro-extraction, refining, and plating. The term 
 electro-metallurgy may, then, embrace all these, and the different 
 subsections may receive special designations such as electro- 
 typing, electro-plating, electro-smelting, electro-refining, and the 
 like. 
 
 In the more restricted sense of the term there are two prin- 
 cipal divisions the extraction of metal from the ore, and the 
 refining of metals already produced, either in this way or by 
 other methods. 1 Only comparatively few ores and metals are 
 thus dealt with at present of these the principal are copper, 
 gold, silver, nickel, zinc, aluminium, and sodium, whilst, incident- 
 ally, gold and silver are extracted from other metals, in which 
 they had previously existed in minute quantities ; indeed, one 
 advantage of electrolytic refining is that usually the precious 
 metals are completely and readily recovered by its application. 
 
 The laws which govern the electro-deposition of metals in 
 plating or typing are equally applicable in this case, and well 
 repay the closest study ; but there are financial questions enter- 
 ing into the problem as here presented which are not so clearly 
 placed before the electro-plater. This is due to the fact that the 
 principal object in plating or typing is to produce a good and 
 reliable deposit, and failing this, all attempts at economy result 
 in extravagance ; whereas a slight additional expenditure may 
 repay itself again and again by the enhanced excellence of the 
 
 1 In the short space here available, allusion can only be made to a few 
 typical processes. For a detailed account the reader is referred to Electric 
 Smelting and Refining, by Borchers and M'Millan. 
 
 267 
 
268 EXTRACTION AND REFINING PROCESSES. 
 
 work ; thus, although there may be great competition, and, there- 
 fore, necessity to minimise expenditure, yet the competition is 
 in the same field, and all rivals are working under like con- 
 ditions. On the other hand, the electro-reduction of ores or 
 refining of metals has to be measured against other purely 
 metallurgical or chemical processes, both as to purity of product 
 and cost of production. And although there may occasionally 
 be a demand for a pure article at any price for certain specific 
 purposes, and although the electrolytic refining is perhaps best 
 fitted to meet this, yet the demand is comparatively small. If, 
 therefore, the field is to be widened so that the electrical system 
 enters into competition with metallurgical methods upon common, 
 ground, the greatest attention must be paid to every detail of 
 the work, and every effort must be made to ensure the highest 
 efficiency at the lowest cost. But as the main object in electro- 
 refining is the production of a metal having the maximum 
 purity, it may often be necessary to sacrifice the false economy of 
 saving in the conduct of the process for the true economy of obtain- 
 ing a pure metal, although at a slightly increased expenditure. 
 
 THE ELECTRO-REFINING OF COPPER. 
 
 The refining of crude metallic copper produced by other 
 processes is one of the chief applications of this branch of the 
 electro-metallurgist's work. The impure metal may contain, in 
 addition to the copper, which is generally present to the extent 
 of 98 to 99'5 per cent., such substances as gold, silver, lead, 
 bismuth, tin, zinc, iron, manganese, nickel, antimony ; arsenic, 
 sulphur, and silicious matter or slag in varying proportions ; and 
 the problem is, so to remove these that perfectly pure copper is 
 obtained. Less pure copper is also used, and attempts have been 
 made to avoid part of the preliminary refining by working with 
 copper matte as anodes. Thus, at Mansfeld, in Germany, the 
 Gunther process is used, in which the anodes are of 75 per cent, 
 matte. The success of the process appears to depend on the cast- 
 ing of the anodes. The electrolyte is heated to 70 C. and aerated, 
 the pressure is '5 to '7 5 volt between electrodes, and the current- 
 density is 30 amperes per square foot. Many experiments have 
 been tried and recorded, both on a small and on a large scale, by 
 such authorities as Sprague, Gramme, Becquerel, Kiliani, Keith, 
 and others, whose observations have borne results of extreme 
 value, in fixing the most suitable conditions under which the 
 copper-baths should be worked. They have been freely quoted 
 in this chapter, more especially those of Kiliani. 
 
 Principles. The universal principle underlying the various 
 processes in use for this purpose may be said to consist in passing 
 a current of electricity, of suitable strength, from an anode of the 
 metal to be refined through a solution of copper sulphate, acidu- 
 
THEORY OF COPPER-REFINING. 269 
 
 lated with sulphuric acid, and collecting the pure metal upon a 
 convenient cathode ; at the same time allowing for the removal 
 of such insoluble impurities as gradually form a slimy sediment 
 upon the bottom of the vat, and guarding against the excessive 
 accumulation of soluble impurities in the liquid itself. It is 
 clear that the precise manner of applying this principle is capable 
 of almost infinite variation; and so it is scarcely a matter for 
 surprise that almost every electro-refinery has its own special 
 method (concerning which in many cases secrecy is most jealously 
 preserved), differing from others either in the current-density 
 or in the disposition of the baths, or in some other matter at 
 first sight trivial, perhaps, but in reality vastly influencing the 
 economical use of the system for good or for evil. 
 
 In order to obtain a complete understanding of the process 
 itself, it is desirable that the behaviour of each of the elements 
 likely to occur in the crude copper should be examined both at 
 the anode in relation to the action of the solvent employed, and 
 at the cathode in regard to its tendency to deposit from copper 
 solutions of various degrees of dilution, and with different current- 
 densities. 
 
 In accordance with the principle explained in an earlier chapter, 
 given an alloy of all known metals in equal proportions, a suitable 
 electrolyte, and only a weak current to effect the electrolysis, the 
 most electro-positive metal should first dissolve, then the next on 
 the scale, and so on, until at last only the most electro-negative 
 element remains, and this would itself dissolve in turn ; or, on the 
 other hand, if all were together in solution, the order would be 
 reversed, and the most electro-negative element would first deposit 
 upon the application of a current. In the copper anode, however, 
 and with the strength of current employed, such a reaction is not 
 likely to occur, because of the comparatively small amount of the 
 total impurities present in it and the still smaller proportion of 
 any one taken singly : there is only the tendency to follow in this 
 order of sequence. Actually, there is an attack at first on such 
 molecules of the most electro-positive metals as are upon the 
 surface of the anode, to an extent proportionately greater than on 
 those of copper ; but the current-volume is so great, as compared 
 with the amount of these impurities, that the last-named metal 
 also dissolves. In this manner a fresh series of molecules is ex- 
 posed by the removal of the outermost layer, and these are in 
 turn attacked, the more positive metals in greater proportion 
 than the copper (in relation to their respective percentages) and 
 so forth ; thus, there is a tendency to form corrosion-pittings or 
 hollows, due to excessive action locally upon superposed electro- 
 positive molecules ; then, as the acid penetrates these, the action 
 presses more and more unevenly into the body of the anode, until, 
 in course of time, the latter becomes deeply honeycombed, or it 
 may be even a mass of sponge, if originally it were very impure. 
 
270 EXTRACTION AND REFINING PROCESSES. 
 
 This sponge consists mainly of copper and of metals more electro- 
 negative than itself, together with any oxidised bodies which may 
 be insoluble in the liquid, the greater proportion of the more 
 electro-positive elements having been removed by solution. This 
 sponge, being very frangible, is constantly becoming broken up to 
 a slight extent by any motion of the liquid, and the detached 
 portions fall to the bottom in the form of mud. When the per- 
 centage of impurities in the copper is not excessive, the anode is 
 less readily penetrated by the solution, the action proceeds more 
 evenly, a greater proportion of copper dissolves superficially, and 
 the spongy deposit on the surface consists almost entirely of the 
 bodies which are not oxidisable in the bath, or whose oxides are 
 insoluble in it ; and this being very limited in quantity is not 
 coherent, and is more readily detached, forming a mud com- 
 paratively poor in copper. 
 
 Dealing first with the solution of the anode, copper is a 
 very electro-negative element (see p. 22), and thus nearly every 
 metal, other than the precious metals, tends to become oxidised, 
 or to pass into solution, before it. 
 
 Behaviour of Individual Impurities. Zinc, iron, nickel, cobalt 
 and manganese dissolve and remain in the solution, and by taking 
 the place of the equivalent of copper which should have dissolved 
 in their place had the anode been pure bring about a gradual 
 impoverishment of the liquid in regard to copper ; and ultimately, 
 by continued solution of small quantities, they accumulate to so 
 great an extent as to render the bath unworkable. This relates 
 to the electrolytic solution of the anode ; but it should be remarked 
 that acid baths tend also to act upon these bodies by simple solu- 
 tion (or by local action see p. 39 with the more electro-negative 
 copper) when they are present in any quantity, and thus the 
 acidity is reduced, whilst the weight of metallic substances 
 present is increased, and this is objectionable because neutral 
 baths afford less satisfactory results than do acid solutions. With 
 a neutral liquid, silver must be added to the list of the substances 
 which dissolve in the bath. 
 
 Bismuth and antimony in part dissolve in the solution, in part 
 remain upon the anode, and thence pass into the mud as insoluble 
 basic sulphates (salts containing less than the due proportion of 
 acid), but even the portion which dissolves is more or less com- 
 pletely precipitated on standing. Tin behaves similarly, but a 
 greater proportion remains as an insoluble oxide at the anode 
 when the latter is rich in this metal. Arsenic also dissolves at 
 first, but as soon as the liquid is saturated with it the remaining 
 proportions are found in the slime. 
 
 Gold, platinum, lead, and, in acid solutions, silver, remain in 
 the anode mud, the two former as undissolved metal, and the 
 latter as lead sulphate, a compound which is quite insoluble in 
 the acid liquid. 
 
THEORY OF COPPER-REFINING. 271 
 
 Crude copper generally contains a certain percentage of ad- 
 mixed slag, cuprous oxide (i.e., suboxide of copper), and cuprous 
 sulphide ; and these, with the usual intensity of current, being 
 but feeble conductors of electricity, pass unchanged into the 
 slime, from which, however, the oxide may gradually be dissolved 
 out in the free acid of the bath by a purely chemical reaction. , 
 
 Thus, the composition of the mud varies with the nature and 
 quantity of the impurities in the unrefined copper and may 
 embrace gold, platinum, silver, lead sulphate, basic sulphates of 
 bismuth, tin, and antimony, arseniates and antimoniates, slag 
 (chiefly silicate of copper and iron), cuprous oxide and sulphide, 
 and metallic copper. 
 
 The solution may contain besides copper, iron, zinc, cadmium, 
 nickel, cobalt, manganese and small proportions of arsenic, tin, 
 and antimony, together with silver if the solution be neutral. It 
 is evident that, so far as regards the refined copper at the cathode, 
 the bodies which remain undissolved at the anode are ipso facto 
 removed, as it were, from the sphere of action, and are perfectly 
 eliminated, unless, through bad working, minute fragments obtain 
 access and cling to the cathode surface. The only impurities 
 which can possibly be deposited upon the cathode, and thus 
 contaminate the refined metal under normal working, are those 
 which have become dissolved in the solution, and diffused 
 through it, so that they are actually brought into contact with 
 the surface of the cathode plate. Of these, silver is always pre- 
 cipitated with the copper, together with gold and platinum if, 
 from any unusual cause, either of them be present in the liquid. 
 In neutral solutions, and even in acid solutions, if they be weak in 
 copper, antimony and arsenic are deposited, and thus tend to make 
 the refined metal brittle and unserviceable. The other constituents 
 of the bath, being far more electro-positive, do not precipitate 
 when the electrolytic conditions are satisfactory, and require no 
 consideration, except in so far as they take the place of copper in 
 the solution, and thus reduce the strength of the bath in regard 
 to its principal element. 
 
 American refineries now add a small percentage of hydrochloric 
 acid to the electrolyte to guard against loss of silver and to hinder 
 deposition of arsenic and antimony. 
 
 In the Montana (U.S.A.) refineries the electrolyte is refined by 
 electrolysing at a high current-density with lead anodes and copper 
 cathodes. By this means, the impurities, chiefly arsenic and 
 antimony, are loosely deposited, being partly retained on the 
 cathode and partly dropping to the bottom of the vat. The 
 electrolyte is then brought up to standard strength and again 
 used. In due time it becomes so far contaminated with iron as to 
 blacken the copper deposits ; it is then withdrawn from use, the 
 blue vitriol is crystallised out, and after this is removed the liquor 
 is still further evaporated, when ferrous sulphate crystallises. 
 
272 EXTRACTION AND REFINING PROCESSES. 
 
 Current Regulation. The source of the current was originally 
 the voltaic battery, but the high price of the zinc was prohibitive, 
 so that the electro-refining or extraction of metals remained merely 
 an interesting laboratory experiment, the practical application of 
 which was beyond the range of economical solution until the 
 invention and multiplication of dynamo-electric machinery intro- 
 duced the possibility of unlimited electric power at a moderate 
 cost. Now, however, the readiness with which other forces may 
 be converted into electricity renders the electrolytic process a 
 formidable competitor with other methods, especially when 
 natural power is economically applicable, as, for example, in the 
 neighbourhood of some hill torrents or continually running water 
 of any kind, and the loss of energy inseparable from the use of the 
 steam-engine as a motor is thus excluded. The dynamo used 
 is similar to those which are employed for electro-plating, though 
 usually, of course, of considerably higher E.M.F. and of increased 
 size and output (see Chapter III.). 
 
 The current-density used in American refineries varies generally 
 from 12 to 35 amperes per sq. ft. (1*3 to 3*8 amperes per square 
 decimetre). In the Eastern States the most usual current-density \ 
 is 17 to 20 amperes per sq. ft. At Great Falls Refinery (U.S.A.) 
 current-densities are used up to 45 amperes per sq. ft. On the 
 other hand, in Europe the current-density is more usually not 
 higher than 10 amperes per sq. ft., and Titus Ulke mentions that 
 the average of twenty-three European refineries is less than 
 6 amperes per sq. ft. The disadvantage of low current-density 
 is that a long time is required to turn out a given quantity of 
 copper, and more money is locked up. The electro-motive force 
 or pressure required depends upon the circumstances of the case 
 the nature of the copper under treatment, the distances between 
 the electrodes, and the number of baths or pairs of plates in 
 series. Thus, two identical sheets of pure copper, opposed to one 
 another, and joined by metallic connection in a homogeneous 
 depositing solution, show no difference of potential ; but any 
 variation between them, even in the degree of compression of the 
 metal, though more markedly, of course, in the chemical com- 
 position, gives rise to a certain development of electro-motive force ; 
 and thus, by producing a current (however feeble), either aids or 
 retards the electrolysing current, according to the direction in 
 which it is flowing. It is obvious that the crude copper anode 
 and the pure refined metal cathode thus set up an electro-motive 
 force between them, which must to a small extent influence the 
 decision as to the pressure that is required from the electric 
 generator. In the second place, an increase in the distance 
 between the electrodes involves also an increase in the resistance 
 of the bath, and demands a corresponding development of current- 
 pressure, in order to overcome it ; while, thirdly, the multiplica- 
 tion of the number of baths in series intensifies the interfering 
 
COPPER REFINING. 273 
 
 action of the unlike electrodes, and of the varying distances 
 between them. 
 
 The Electrodes. The space most usually allowed between 
 anode and cathode varies from 2 to 2J ins. ; less than this is not 
 advisable, because of the facility with which detached fragments 
 of copper and mud from the anode, or excrescent growths, which 
 are common enough on the cathode especially when powerful 
 currents are employed tend to bridge over the space and short- 
 circuit the current (see p. 39), thus entirely stopping the action, 
 while at the same time wasting energy and even perhaps 
 damaging the dynamo. With the inter-electrode distance above 
 recommended, an allowance of 0'2'to 0*3 volt E.M.F. for every 
 pair of electrodes placed in series will be found suitable. These 
 remarks apply to the multiple system ; in the series system the 
 distance between electrodes is only about 0*5 to 0*8 in. (see 
 p. 275). 
 
 The cathode plates, on which the metal is to be deposited, may 
 be of thin sheet copper, but in this case the surface should be 
 slightly greased and well covered with black lead, to prevent the 
 uniting of the surface with that of the precipitated metal ; but 
 it is in many respects better thus to form a plate up to a 
 thickness of ^ of an inch, and then, stripping it from the 
 original sheet, to use it in turn as cathode for the remainder of 
 the process. At the Buffalo (U.S.A.) works {he cathode sheets 
 are washed with a dilute solution of iodine in naphtha instead of 
 greasing them. The anode consists usually of a cast slab of the 
 metal to be refined, of convenient size for the bath employed ; it 
 may range, for example, from 2 to 2 J ft. in length, from 1 to 2 ft. 
 in width, and from 1 to 2 or 3 ins. in thickness. The distance 
 between the two plates should not be less than 1J ins., as shown 
 above ; any great increase beyond this lessens, it is true, the 
 risk of short-circuiting, but it also lessens the current for a given 
 P.D., and, in consequence, not only adds to the time required for 
 depositing a given weight of metal, but also gives rise to an 
 increased loss of electrical energy, by conversion into useless heat, 
 in the bath, so that the efficiency of the installation is diminished. 
 The distance which best strikes the mean between the two 
 opposing sources of danger may be taken to be 2 ins. ; but with 
 high current-densities, with which there is a greater risk of pro- 
 jections forming upon the cathode, or with very impure copper, 
 which yields unduly large volumes of spongy matter, this distance 
 may with advantage be extended to 2J ins. 
 
 The Solution. The solution employed as electrolyte should 
 contain from 1J to 2 Ibs. crystallised copper sulphate, and 
 from 4 to 10 oz. of sulphuric acid, in each gallon of water. 
 The baths are made after the manner already described, and the 
 fittings of each bath may be conveniently arranged like those 
 in use for electro-plating or electrotyping, all plates in the 
 
 18 
 
274 EXTRACTION AND REFINING PROCESSES. 
 
 same bath being placed in parallel, not in series, so that all 
 the anode plates are in direct connection with the positive lead 
 from the dynamos, and all the cathodes with the negative lead. 
 The series system, however, is also used to a large extent, as 
 described later. 
 
 Arrangement of Baths. To consider the question of the dis- 
 position of the baths in series or in parallel. First, in regard to 
 the number of groups in series. Since each group requires a 
 certain expenditure of electro-motive force, which we have taken 
 at O3 volt, it is clear that the maximum number of vats which 
 it is convenient to place in series will be found by dividing the 
 electro-motive force of the generator, expressed in volts, by O3. 
 Thus, if the generator can give only 6 volts, the highest number 
 
 / 
 
 of baths permissible in series is - = 20. If a larger number 
 
 (jo 
 
 were grouped in series, the voltage per group would be less than 
 that recommended, and although metal might be deposited 
 (though much more slowly) to a certain point the further increase 
 of the number of groups placed in tandem fashion would practically 
 stop the current from flowing through them at all. By using a 
 dynamo, giving a higher pressure than 6 volts at the brushes, the 
 number of baths in series may be proportionately increased. In 
 the second place, in regard to the number of cells in parallel in 
 each group, if these be multiplied, the plate surface is increased 
 to a corresponding extent, and, with only a given current available, 
 the proportion of current-strength to superficial electrode-area will 
 be reduced to a point at which the character of the deposit will 
 be greatly deteriorated, and at which the thickness deposited in 
 a given time will be so small that the duration of the process must 
 be abnormally extended. These two factors then the current- 
 density per unit of electrode surface and the electro-motive force 
 form the scientific limits to the extension of plant in the directions 
 shown ; and within these limits the refiner must be guided by the 
 considerations of time, space, and capital under which he is called 
 upon to work. 
 
 Particulars are often published of copper refineries, but they 
 must be accepted with caution, as great secrecy is maintained, and 
 such details are generally out of date, if not actually misleading. 
 The following particulars of the working of the American Smelting 
 and Refining Company's works at Maurer, where the daily output 
 is 160 tons, are given by J. N. Pring, 1 and are typical of copper 
 refining as carried on in the United States. The crude blister 
 copper contains about 96 per cent, of copper. This is melted down, 
 partially refined, and cast into anodes by means of a machine 
 which carries a number of moulds on a wheel ; this is rotated so 
 as to bring each mould in turn into the required position for 
 
 1 Some Electro-chemical Centres. 
 
COPPER REFINING. 275 
 
 casting. The anodes so cast, which are 3 ft. high, 2 ft. wide, and 
 1| ins. thick, weighing 275 Ibs., are rapidly cooled by water and 
 removed. The cathode-starting sheets are formed electrolytic- 
 ally by depositing copper on a plate with an oxidised surface 
 and then stripping it off. Each vat is 10 ft. long, 3 ft. deep, 
 and 2 ft. 6 ins. wide, and contains 22 anodes and 23 cathodes. 
 The electrolyte contains 16 per cent, of copper sulphate and 
 6 per cent, of sulphuric acid. The slime contains about 15 per 
 cent, of metallic copper, 45 per cent, of silver, and 5 per cent, 
 of gold, from which, after suitable treatment, dore bars are 
 obtained, and the silver and gold in this are parted by the 
 Moebius process. 
 
 Loss of Electrical Energy due to Resistance. Although the 
 electrical connections are always very large so as to minimise loss 
 due to resistance of the conductors as far as possible, the loss due 
 to this cause is nevertheless a large proportion of the total energy 
 used. B. Magnus has published the following results of measure- 
 ments at a copper refinery in which the current was 4000 amperes, 
 the pressure per tank was 0'230 volt, and the current-density 
 1 1 amperes per square foot : 
 
 Volts. 
 Drop between anode rod and 'bus bar . .0 '0270 
 
 cathode rod and 'bus bar 
 anode rod and and anode hanger 
 cathode rod and cathode hanger 
 anode hook and anode . 
 
 0-0135 
 0-0060 
 0-0045 
 0-0008 
 
 Total losses . . 0'0518 
 
 Thus the total losses amounted to 22*5 per cent, of the pressure 
 across each tank. 
 
 Modern Systems of Refining. The processes at present in use 
 are sometimes divided into two groups, known respectively as the 
 Multiple and Series systems. In the former, all the anodes in any 
 one tank are hung in parallel, that is, in connection with the same 
 anode rod ; whilst in the latter the first anode in any one bath is 
 connected to the positive lead or conductor from the dynamo, 
 and the last cathode is connected to the negative lead, the inter- 
 mediate plates being suspended in the solution independent of 
 electrical connections. Thus every plate, except the two outside 
 electrodes, acts both as anode and cathode as cathode on the 
 side facing the positive end of the bath, and as anode on the other 
 side ; so, while the impure copper is dissolving from the plate on 
 one face, a deposit of pure copper is gradiially being built up on 
 the reverse side, until at length the whole of the copper originally 
 in the plate, with the more electro-positive metals present in it, 
 have passed into the solution, and the insoluble impurities have 
 been washed off the surface into the bath. By this time the plate 
 of refined copper built up in its place will be nearly as thick as 
 
276 EXTRACTION AND REFINING PROCESSES. 
 
 the original plate (not quite as thick, owing to the fact that im- 
 purities have passed electrolytically into the bath with the copper 
 and are not re-deposited) ; the electrolysis must then be stopped, 
 because, if continued, the refined copper itself would dissolve and 
 cause a needless waste of current. Several modifications of the 
 series system have been introduced by Haydn, Smith, Stalmann 
 and others ; but great care is required in working most of them, 
 as irregular solution of the anodes gives more trouble than in the 
 other system, and there is risk in some of them of a waste of 
 power by re-solution of the refined copper. Besides having more 
 trouble with the cathodes in maintaining their quality, it is also 
 impossible to use lead-lined tanks in the series system. Many 
 more contacts are required in the multiple system, and thus the 
 energy used is about one-half more than in the series system. 
 Also in the series system less copper is locked up in the process, 
 the distance between electrodes may be reduced to O5 to 0*8 in. 
 as compared with 2 to 2J ins., and the volume of the electrolyte 
 used is only about one-fourth that in the multiple system. Large 
 refineries are run on both systems, but the multiple system is 
 more commonly used. 
 
 One of the chief necessities in copper-refining is the thorough cir- 
 culation of the solution through the vats, so that the baths may be 
 homogeneous, and the free acid evenly distributed. This is usually 
 accomplished by arranging each series of baths on an inclined sur- 
 face, so that a gentle but constant stream of the electrolyte solu- 
 tion is run from a cistern into the vat at the highest level, and, 
 after passing through this vat, leaves it by a pipe, connecting the 
 bottom of the vat with the top of the next, and so on through the 
 series, the liquor drawn from the bottom of one tank being always 
 delivered into the top of the next, and that from the last of the 
 series being pumped up again to the reservoir. In the Siemens- 
 Halske tanks the solution is caused to flow from the cistern to a 
 pipe running along the whole row of vats (on a level floor), and 
 through this to a cross pipe communicating with a number of small 
 perforated pipes placed parallel in each bath. Each tank has a 
 leaden tray forming a loosely-fitting false bottom just above the 
 actual bottom, and beneath this is a horizontal pipe leading into a 
 vertical siphon pipe in one corner of the vat. The solution passes 
 freely around the margin of the false bottom into the space be- 
 neath, and is thence removed by the siphon and delivered into a 
 channel running the whole length of the vats, and discharging 
 into a well from which the liquor may be pumped to the reservoir 
 for further use, or, when necessary, to the purifiers. Borchers and 
 Schneider and Szontagh have employed methods which, at least 
 in part, render such circulation unnecessary. The vertical pipe 
 connected with that lying horizontally beneath the false bottom 
 terminates a little above the level of the solution in the vat, and 
 within it is a narrow tube extending from the top half-way to the 
 
COPPER REFINING. 277 
 
 bottom. Air is constantly blown through the inner tube by means 
 of a fan, and the air bubbling up through the liquid in the outer 
 tube displaces some of it, and causes it to overflow into the vat j 
 thus so long as air is blown in, liquor is transferred from the bottom 
 to the top of each vat. At the same time, the air has to some extent 
 a purifying action on the bath. 
 
 Other systems are now being introduced, in which a more 
 rapid circulation is adopted, and in one of these the electrolyte is 
 pumped with some force on to the surface of a revolving drum- 
 shaped cathode through a series of nozzles placed close together. 
 In addition to ensuring the thorough circulation of the electro- 
 lyte, such a system as this allows of a greatly increased current- 
 density for reasons which are elsewhere explained. As an off-set 
 against the extra cost of working such a process, therefore, 
 there is to be placed the gain in point of time, and the saving 
 in space and plant necessary for the production of a given 
 output. 
 
 Precautions. In working this process the usual attention must 
 be paid to perfect cleanliness throughout. All connections and all 
 cathode surfaces (unless the deposit is to be detached afterwards) 
 must be clean initially ; the mud should be frequently removed, 
 since it becomes slowly attacked by the acid liquor of the bath 
 and thus gives rise to an alteration in the composition of the 
 latter, and also because the adhesion of the mud particles to the 
 cathode tends to re-introduce into the refined metal impurities 
 which, by acting as nuclei, lead to an increased unevenness and 
 irregularity of surface. But as the slime contains practically the 
 whole of the precious metals originally present in the copper, it 
 must be preserved, so that they may be subsequently extracted, 
 for this recovery of gold and silver may form no inconsiderable 
 source of profit. By the gradual accumulation of impurities in 
 the liquid, the percentage of copper is greatly reduced. This 
 should be rectified by partially evaporating the solution, and 
 collecting the crystals of iron sulphate which separate out, and 
 which constitute the chief foreign matter introduced during the 
 process. The residual liquid, still containing iron, however, but 
 in reduced proportion, may be diluted sufficiently, and mixed 
 with a fresh quantity of copper sulphate, and is then ready for 
 further use. The copper may be deposited in thick or in thin 
 sheets, but conveniently about T \- to -J of an inch in thickness, 
 which may require from two to four weeks' to produce ; it should 
 be almost absolutely pure, and may be sent into the market as 
 electro-deposited plate, or it may be remelted and poured into 
 ingots of the ordinary size and shape. It should be finely 
 crystalline, with clear close grain, solid and free from pin-holes 
 or blisters in the sheet form, and as such is the purest copper 
 known commercially. 
 
278 EXTRACTION AND REFINING PROCESSES. 
 
 THE ELECTROLYTIC EXTRACTION OF COPPER FROM ORES AND 
 PRODUCTS. 
 
 Progress. The first step in the direction of electro-extraction 
 was, no doubt, the discovery alluded to in the first chapter of this 
 treatise, that metallic iron dipped into liquors containing copper 
 became covered with a deposit of the latter metal a reaction 
 known, but misunderstood, even in the fourth or fifth centuries 
 of the Christian era, and applied in the fifteenth century to the 
 extraction of copper from the cement water at Schmollnitz in 
 Hungary. This precipitation by a system of ' simple immersion ' 
 is still very largely used in the treatment of many thousands of 
 tons of copper ore annually, and is especially applicable to the 
 recovery of the small proportion of this metal (2 to 4 per cent.) 
 present in the burnt Spanish pyrites, which has been used and 
 discarded by the vitriol maker. 
 
 The next step was that taken in 1846 by Dechaud and 
 Gaultier, who mixed crude copper oxide (either the natural ore 
 or a roasted sulphide) with sulphate of iron, and strongly heated 
 the two in an air-current until the copper had become converted 
 into sulphate at the expense of the iron salt, which thus became 
 changed to peroxide. After cooling, the burnt charge, on treat- 
 ing with water, or leaching, gave up its copper sulphate into the 
 solution, which was then ready for precipitation with iron. 
 Instead, however, of merely inserting scrap-iron, which deposited 
 the copper in a spongy condition, and left it commingled with all 
 other metals which iron was capable of precipitating by simple 
 immersion, and with all the carbon and other impurities con- 
 tained in the iron itself, plates of the last-named metal were 
 placed in the solution, parallel with copper plates, and joined to 
 them externally by copper wires, but separated from them in 
 the liquid by cotton cloth, to prevent the contamination of the 
 copper by impurities from the precipitating element. Thus the 
 iron dissolved, and, producing a current of electricity, caused the 
 deposition of the copper to take place upon the copper plates (or 
 upon the lead plates which were sometimes substituted for the 
 latter). To compensate for the gradual exhaustion of the liquors, 
 in regard to copper, a continuous gentle flow of saturated extract 
 from the roasted product was introduced into the bottom of the 
 bath, while simultaneously the decopperised liquid, rich in ferrous 
 sulphate, was removed from above. 
 
 So also in 1867, Patera treated the Schmolmitz cement liquors 
 by immersing in the copper fluid porous clay cells, containing 
 fragments of iron in a strong solution of sodium chloride, and 
 connecting this* metal with pieces of coke immersed in the 
 cement water ; here, too, a * single-cell ' arrangement resulted, 
 and the solution of the iron in the porous vessels caused the 
 deposition of the copper upon the coke fragments without. 
 
COPPEft-EXT&ACTlON METHODS. 279 
 
 Later still, the copper liquors obtained from an ore by any treat- 
 ment such as that above alluded to, or by attacking oxidised 
 copper products with sulphuric acid, have been treated with the 
 aid of dynamo-electric machinery. 
 
 Yet another method of procedure has been employed. A 
 regulus of copper (that is, an impure cuprous sulphide) is 
 obtained in the usual way by smelting sulphide ores in the 
 furnace ; and this regulus is cast into slabs, which are then used 
 at once as anodes, in a solution of copper sulphate, a powerful 
 dynamo-current being employed to effect the electrolysis. The 
 compound becomes broken up, the copper of the copper sulphide 
 is oxidised and dissolved, and the sulphur, which is separated and 
 remains practically unaltered, even in presence of the nascent 
 oxygen at the anode, together with the precious metals, and all 
 the other bodies that were mentioned on p. 271 as being left in 
 the slime during refining, remain undissolved at the anode, which 
 is usually enclosed in a bag of porous material, so that the 
 pulverulent deposit may not become mixed with the solution 
 and the deposited copper. 
 
 The Processes Available. Thus the general methods may be 
 classified as those in which (1) the ore is treated by ordinary 
 furnace processes with the object of rendering certain constituents 
 (chiefly copper) soluble, and these are extracted electrolytically 
 from their solution in water by processes of simple immersion, 
 single cell, or separate current with insoluble anode ; (2) the ore 
 is simply melted, or is partly refined, and then run into slabs, 
 which are used as anodes in a bath of a suitable copper salt with 
 a separate current for electrolysis. 
 
 By the first method any copper ore may be successfully treated, 
 provided that due attention be paid to metallurgical details in 
 the roasting, into which it would be out of place to enter here. 
 The second is applicable to native copper ores, such as those of 
 the Lake Superior district, which are already in the metallic 
 state, and therefore need refining only, and to sulphide ores, or 
 to those in which the copper may be converted metallurgically 
 into regulus or sulphide. 
 
 Becquerel in 1836 made a series of experiments on the treat- 
 ment of complex ores containing copper, lead, silver, and iron, 
 by converting them into chlorides or sulphates, and subsequently 
 electrolysing these solutions ; but although they were successful 
 enough in their result, the high cost of producing electrical 
 energy at that time proved an insuperable bar to their com- 
 mercial adaptation. 
 
 In 1871 Elkington dealt with copper mattes and solutions 
 with the aid of dynamo- or magneto-electric machinery, and 
 thus placed the process at once upon a practical footing. Cobley 
 in 1880 electrolysed copper solutions obtained by lixiviating 
 roasted copper ores with water or sulphuric acid, obtaining, if 
 
280 EXTRACTION AND REFINING PROCESSES. 
 
 possible, a solution containing 18 per cent, of copper salt and 
 1 per cent, of free acid. A year later Deligny described experi- 
 ments in which he had successfully used sulphide ores of copper 
 packed around carbon rods as anodes in a weak solution of 
 copper sulphate, with a copper cathode, and with two Bunsen- 
 cells as the source of current. In carrying out this process he 
 took advantage of the comparatively high electrical conductivity 
 of many natural sulphides, such as those of copper, iron, lead, 
 silver, etc., conductivity being quite essential to the success of 
 the process in order that the current may be transmitted through 
 the anode at all. It may be noted that most oxide ores and 
 oxidised compounds, and the sulphides of zinc and antimony, are 
 bad conductors, and are, therefore, unsuited to such treatment. 
 
 In 1882, Bias and Miest elaborated a process intended to deal 
 with many complex ores, but especially with those containing 
 copper. First, in order to ensure better conductivity than 
 may be obtained with a loose powder, the crushed mineral is 
 moulded into cakes at a pressure of about 40 atmospheres (600 
 Ibs. per square inch), and is heated to a temperature of from 
 950 to 1100 F., bad conductors being formed into slabs of greater 
 thickness than those which exhibit a superior conductivity, in 
 order that a greater area may be allowed for the passage of the 
 current. The plates are then used as conductors in a suitable 
 solution copper sulphate for copper ores, lead nitrate for lead 
 ores, and so forth when the metals soluble in the electrolyte are 
 gradually removed under the action of the current, and certain of 
 them are deposited upon a sheet metal cathode. The whole of 
 the sulphur remains with the gangue or earthy matter, and 
 other insoluble constituents of the anode ; and these, after a time, 
 being weakened by the honeycombing process of solution, tend to 
 fall to the bottom of the bath ; it is well, therefore, to remove and 
 re-agglomerate them from time to time before this can take place. 
 The cathode-metals are finally electro-refined, while the spent 
 anodes containing the sulphur in a recoverable form, are treated 
 accordingly. In this process, and in that of Marchese, proposed a 
 year later, the number of soluble constituents, and principally the 
 amount of iron, which are constantly passing into the solution 
 and thus altering its composition, militate greatly against the 
 economical success of the treatment. 
 
 The two chief processes at present available are the Siemens 
 and Halske process and the Hoepfner process, but neither of them 
 appears to have made much progress during the last few years. 
 In the Siemens and Halske process the ore is roasted, finely 
 powdered, and then leached with a solution of ferric sulphate 
 containing free sulphuric acid. Copper is thus taken up as 
 sulphate, the ferric sulphate being reduced to ferrous sulphate. 
 The resulting solution is treated in a diaphragm cell, passing first 
 through the cathode compartment where the copper is deposited, 
 
COPPER-EXTRACTION METHODS. 281 
 
 and then through the anode compartment where the ferrous 
 sulphate is re-oxidised to ferric sulphate to be used once more 
 for leaching ore. In the Hoepfner process the ore is crushed and 
 then leached with a solution of cupric chloride containing calcium 
 chloride or sodium chloride. The cupric chloride reacts with the 
 sulphide of the ore, copper is taken up, changing the cupric 
 chloride into cuprous chloride, which keeps in solution on account 
 of the calcium chloride. The resulting solution is passed through 
 a diaphragm cell, one portion going to the cathode and the other 
 to the anode ; from the part going to the cathode the copper is 
 deposited, whilst in the part going to the anode the cuprous 
 chloride becomes cupric chloride, and thus when these two 
 portions reunite after the electrolytic treatment, we have cupric 
 chloride and calcium chloride as at first, so that the process can 
 be repeated. One advantage claimed for the process is that a 
 cuprous salt is used instead of a cupric salt for the electrolysis, 
 and therefore, theoretically, the amount of copper deposited by 
 a given quantity of electricity should be double that in the usual 
 process of copper refining. 
 
 A consideration of the subject shows that it is question- 
 able whether the use of raw ores as anodes can compete with 
 a mixed process of preparing a less impure compound for 
 example, blister copper by metallurgical or furnace methods, 
 and then refining this product by electrolytic means. Up to 
 the present time most of the processes in which sulphide ores, 
 or mattes smelted from them, have been used as anodes have not 
 been commercially successful. In some instances such processes 
 have given good results for a short time, and have then, as the 
 copper was extracted from the anodes, required more and more 
 current, and have at last been abandoned as being too costly. 
 Other processes, using a porous diaphragm, and different solutions 
 at anode and cathode, have been more successful, but the difficulty 
 still is to find a material for the diaphragm which would be 
 durable. 
 
 THE ELECTROLYTIC REFINING AND EXTRACTION OF LEAD. 
 
 Base Bullion. The principal lead product which offers itself 
 for electrolytic treatment is that known as base bullion, in which 
 the lead usually exists to the amount of 90 per cent, or more, 
 and is accompanied by varying proportions of silver and gold with 
 antimony, arsenic, zinc, copper, iron, tin, etc., the main object 
 being the recovery of the precious metals, together with the 
 production of a good soft lead, free from antimony and arsenic. 
 This problem was taken in hand by Keith ; difficulties were 
 encountered at the outset in the selection of an electrolyte which, 
 while dissolving the lead, should not at the same time dissolve 
 silver, as lead nitrate tends to do, nor encourage the formation of 
 crystalline growths of spongy character resembling the 'lead tree,' 
 
282 EXTRACTION AND REFINING PROCESSES. 
 
 which may form a metallic bridge between the electrodes as lead 
 acetate would do ; the simple sulphuric acid solution is useless, 
 because lead sulphate is practically insoluble in it, and there is also 
 a tendency to form lead peroxide upon the anode, which is still 
 less soluble than the sulphate in this menstruum. A solution of 
 lead sulphate in sodium acetate is found, however, to satisfy the 
 requirements of the process. 
 
 The bullion is fused and run into thin plates (about -J of an 
 inch thick), which are enclosed in muslin bags and immersed as 
 anodes in a solution of 1J pounds of sodium acetate and 2J to 
 3 oz. of lead sulphate in every gallon of water. The solution 
 must not be allowed to become alkaline, or the anode will be 
 covered with a film of lead peroxide, which will protect it from 
 further action ; it should be gently agitated to ensure thorough 
 admixture and to guard against polarisation due to unequal dis- 
 tribution of lead, and it may with advantage be heated to about 
 100" F. with the same object in view. 
 
 By the application of 0'016 ampere per square inch ( = 0'25 
 ampere per square decimetre, or 2 '3 amperes per square foot), the 
 greater proportion of the lead will be gradually transferred to 
 the cathode plate, where, however, it may be mixed with mere 
 traces of silver, copper, tin, antimony, and zinc, and a somewhat 
 larger proportion of the total bismuth present in the bullion. 
 The percentage of lead, however, in the refined metal should 
 exceed 99 '99 per cent, indicating a total impurity of less than 
 O'Ol per cent. The remaining substances are usually found in 
 the anode bag, often in the form of a spongy mass retaining the 
 shape of the original anode. After drying this sponge, it is fused 
 with nitre and poured into a small ingot-mould, by which means 
 the silver and gold are obtained in the form of a button of alloy, 
 the other constituents having passed into the slag, from which 
 they may be more or less readily recovered. The lead frequently 
 crumbles into the solution from the cathode plate, but is readily 
 collected and fused the use of a containing bag for the anode 
 prevents any of the slime becoming mixed with the detached lead 
 in the bath. A modification of the process has been suggested by 
 Keith, by which the bullion is first melted with 2 per cent, of zinc, 
 as in Parkes' process for desilverizing the metal, and then electro- 
 lysing only the zinc skimmings, which may contain about 20 per 
 cent, of the lead originally used, with practically the whole of the 
 precious metal ; thus, the period of time necessary for electrolysis 
 is considerably lessened. Keith's process appears to be no longer 
 in use. 
 
 Borchers also has a process for the refining of lead by the 
 electrolysis of fused lead in a bath of fused sodium and potassium 
 chlorides. 
 
 In the Betts process, in operation at Trail, in British Columbia, 
 an aqueous solution of lead fluosilicide (PbSiF 6 ) with 8 per cent. 
 
ELECTROLYTIC KEFINING OF LEAD. 283 
 
 of lead and 1 1 per cent, of free hydrofluoric acid is used as the 
 electrolyte. This is prepared by diluting hydrofluoric acid with 
 water and saturating it with powdered quartz. The base bullion 
 is cast into anodes ; the cathodes are made of sheet steel which is 
 first coated with copper and then with* lead, the latter being painted 
 with a benzine solution of paraffin. The current-density is 14 
 amperes per square foot of cathode. Gelatine or glue is added to 
 the electrolyte to render the deposit solid and to prevent the 
 formation of 'lead trees' which would otherwise cause short cir- 
 cuits. During electrolysis the lead is transferred, and if tin, iron, 
 zinc, nickel, and cobalt are present they also go into solution. Any 
 gold, silver, copper, antimony, arsenic, and bismuth remain in 
 the anode mud. Since tin is deposited under almost the same 
 conditions as lead, it must be removed previous to electrolysis. 
 Apparently iron remains' chiefly in the anode mud, which is 
 worked up to recover the gold and silver on similar lines to 
 those adopted in copper refining. The Betts process is also in 
 use at Newcastle, where it is said to be more economical than 
 the Parkes process for rich bullion, but if the bullion is poor the 
 reverse holds good. 
 
 Electrolytic reduction of galena has been carried on at Niagara 
 Falls. This is effected by placing the finely crushed galena in 
 sulphuric acid contained in hard lead conical pans. These pans 
 are placed one above another, so that the bottom of each pan dips 
 into the sulphuric acid contained in the one below. The bottom of 
 each pan forms an anode, and the galena below it the correspond- 
 ing cathode, all the pans being thus in series. Upon passing a 
 current, hydrogen is liberated at the cathodes, reducing the galena 
 to spongy lead and forming sulphuretted hydrogen which is 
 evolved. 
 
 This appears to be the only electrical process worth consideration 
 for the electrolytic extraction of lead from its ores, and whether 
 this will come into general use is doubtful. It must be re- 
 membered that lead, in contradistinction to copper, is treated 
 without much difficulty by ordinary metallurgical methods, and 
 therefore there is not the same opening for an electrolytic process. 
 The position in refining is similar. Commercial lead, as obtained 
 by the usual metallurgical methods, is very pure, and there is not 
 the same need for an electrolytic refining process to recover the 
 precious metals as there is in copper refining. 
 
 THE ELECTROLYTIC EXTRACTION AND REFINING OF GOLD. 
 
 Generally speaking, gold does not occur in what may be termed 
 a mechanically free state, but embedded as small particles in 
 quartz. By crushing the quartz to a fine powder, and treating 
 it with a suitable solvent, the gold can be removed in solution. 
 In Siemens and Halske's process, which is used extensively and 
 
284 EXTRACTION AND REFINING PROCESSES. 
 
 which is the only electrolytic process that need be taken seriously 
 into account, a dilute solution of potassium cyanide is employed 
 as the solvent. Electrolysis does not enter into the process of 
 solution ; in fact, the bulk of ore that has to be treated is so large 
 in comparison with the gold that any electrolytic solvent action 
 is impracticable. Thus electrolysis is merely concerned with the 
 deposition of gold from the leaching solution. When so much 
 of the gold has been deposited as to render further electrolysis 
 unprofitable the solution is again used for leaching to bring up 
 its strength as regards the gold, after which it is electrolysed once 
 more, and so on. The anodes are of iron enclosed in linen bags 
 to retain the Prussian blue that is formed, and the cathodes are of 
 sheet lead, the gold being removed from these finally by cupella- 
 tion. The advantage of the electrolytic process is that weaker 
 solutions can be treated at a profit, and therefore lower grade ores 
 can be handled, than by purely chemical methods. 
 
 The Greenawalt process, in which the ore is treated with 
 chlorine and bromine in solution, and the resulting chlorides 
 subsequently electrolysed, is in use in Colorado. 
 
 In the Butters plant in Mexico and Nevada, peroxidised lead 
 plates are used as anodes and tin plates for cathodes, the advan- 
 tage of this being that the deposit is in the form of a mud which 
 falls to the bottom of the vat. 
 
 Electrolytic refining of gold is generally carried out on 
 Wohlwill's method, in which gold chloride acidulated with hydro- 
 chloric acid is used as the electrolyte ; this is heated to 50-55 C., 
 and a high current-density, such as 140 amperes per square foot, 
 is employed. Under these circumstances the impure gold anodes 
 are dissolved, the gold passes into solution and is deposited on 
 the cathode, whereas any silver is precipitated as chloride. This 
 method, therefore, forms a convenient 'parting' process. The 
 other usual impurities are either not deposited on the cathode, 
 or collect in the slime, and thus the resulting deposited gold is 
 very pure. This process is in use at the U.S.A. Mint. 
 
 THE ELECTROLYTIC REFINING OF SILVER. 
 
 The Moebius Process. It is only necessary in this short sketch 
 to refer to the Moebius process in its later development. The 
 problem is to refine bar containing perhaps 90 per cent., or more, of 
 silver, with, it may be, J or 1 per cent, of gold, and some copper. 
 The electrolyte used is a 0*13 per cent, solution of silver nitrate, 
 containing about O'l per cent, of free nitric acid, in which the 
 silver and copper dissolve, whilst the gold is left insoluble; so 
 long as there is sufficient silver and free nitric acid in the bath 
 there is no fear of any appreciable amount of copper being 
 deposited with the silver at the cathode, so that the insoluble 
 gold may be collected on filters and melted, whilst the silver is 
 
ELECTROLYTIC REFINING OF SILVER. 
 
 285 
 
 recovered at the cathode, and the copper gradually accumulates 
 in the solution until it is present in undesirable quantities, when 
 the liquid is withdrawn and treated to recover both the copper 
 and the residual silver contained in it. 
 
 Fig. 93 represents a sectional view of one form of the plant. 
 A is the tank, about 14 feet long, containing the electrolyte; the 
 anodes G are held horizontally at the top of the liquid in trays E, 
 of which the bottoms are of porous material, such as a closely 
 woven filter cloth, connection with the positive leads being made 
 by the copper conductor K, and the rotatable bent copper rod M, 
 which may be pressed into contact with the anode plates. The 
 ends of the copper rods M immersed in the liquid are tipped with 
 platinum, and the joint between the metals, and the copper above 
 it up to a point considerably above the level of the solution, are 
 protected with a caoutchouc sleeve to prevent the nitric acid dis- 
 solving the copper rod. The cathode consists of an endless band 
 
 FIG. 93. Sectional view of the'plant. 
 
 of sheet-silver, C, in. in thickness, which is caused to revolve 
 continuously in the direction indicated by the white arrow over 
 the rollers B, 6, at the rate of about 3 ft. per minute, connection 
 with the negative conductor being made by the brush L. The 
 current-density employed may be higher at first, when little or no 
 copper is present in the electrolyte, than it is later, when there 
 would be danger of copper being co-precipitated ; it may be about 
 30 amperes per sq. ft. (0'2 ampere per sq. in.), but often has 
 to be reduced to half this amount after the operation has been 
 in progress for some time. The E.M.F. is about 1J to 2 volts. 
 With the relatively high current-density used, the rapidity of 
 electrolysis is considerable, so that the amount of costly material 
 lying idle in anode and cathode is relatively small. But the silver 
 is deposited in the form of a crystalline powder instead of in a 
 coherent film. A special method of treatment is therefore used : 
 the silver belt moving over the right-hand roller B comes in 
 contact with the ascending belt d, which detaches the crystals 
 from the surface and carries them forward out of the solution and 
 over the roller 0, where a part falls by gravity into the receptacle 
 
286 EXTRACTION AND REFINING PROCESSES. 
 
 R; any crystals still clinging to the belt are removed by the 
 scraper S, and thence fall into the same vessel. The gradual 
 accumulation of copper in the solution causes the neutralisa- 
 tion of a portion of the nitric acid, inasmuch as the copper 
 nitrate is not electrolysed and so broken up again as the silver 
 nitrate is ; it is therefore necessary to add nitric acid from time 
 to time. To ensure the non-adhesion of the deposited silver to 
 the cathode, the surface of the belt is well black-leaded from 
 time to time. 
 
 At Perth Amboy (U.S.A.), difficulty was experienced owing to 
 the crystals adhering to the silver band. G. Nebel overcame the 
 difficulty by using oil. 
 
 In the U.S.A. Mint, where the Moebius process is in use, 
 the anodes contain 3 parts of gold and 7 of silver ; the solution 
 contains 3 per cent, of silver nitrate and 1*5 of nitric acid. 
 The current-density is considerably lower than that mentioned 
 above, being 0'05 ampere per sq. in. Dr Tuttle, the refiner 
 of the Mint, has introduced the improvement of adding gelatine 
 to the electrolyte.' This has the effect of making the silver 
 crystals adherent so that they may be taken out with the 
 cathodes. 
 
 In the Balbach refinery (U.S.A.) the vat is different. The 
 bullion to be refined is contained in a cloth case supported 
 on a wooden frame which is suspended over the tank, and the 
 cathode is made of graphite slabs. 
 
 THE ELECTROLYTIC EXTRACTION OF ZINC. 
 
 Methods Available. The principal ores of zinc are blende 
 (sulphide) and calamine (carbonate). The methods proposed for 
 dealing with them either require a preliminary roasting in the 
 case of blende, to obtain the zinc in a soluble form, or they use 
 the ore itself as anode. 
 
 Luckow adopted the latter method, and enclosed the ore, mixed 
 with coke, in open chests, which were used as anodes in an 
 electrolyte of a somewhat strong and acid solution of common 
 salt or of zinc chloride (20 to 30 per cent, of zinc), with a 
 similar case filled .with coke, or else a zinc plate, as cathode. 
 These were held in large rectangular troughs of wood or stone- 
 ware, while beneath the cathode case wa's a wooden vessel 
 covered with webbing or basket-work, to retain the zinc which 
 becomes detached during the deposition. The process had to be 
 well watched so that no short-circuiting should result from the 
 formation of arborescent growths from the cathode, that the 
 scum forming upon the surface might be frequently removed, and 
 that the bath did not become acid, especially if the sulphate was 
 employed instead of the chloride. A current of somewhat high 
 electro-motive force was necessary. 
 
ELECTROLYTIC EXTRACTION OF ZINC. 287 
 
 Lambotte and Doucet chose the first alternative, and formed 
 a neutral solution of zinc chloride by treating the roasted ore 
 with crude hydrochloric acid, and freeing it from iron by the 
 addition of chloride of lime and zinc oxide, which effected the 
 precipitation of the latter as insoluble peroxide ; were the iron, 
 which is universally present in zinc ores, allowed to remain in 
 the solution, it would be deposited with the zinc a result to be 
 carefully avoided. The electrolysis was conducted between in- 
 soluble (carbon) anodes and zinc cathodes. 
 
 Letrange first roasted sulphides (blendes) carefully at a low 
 temperature a very low red heat so that as much of the 
 sulphide as possible should form soluble zinc sulphate, for this, 
 at a high temperature, would be completely decomposed into 
 insoluble zinc oxide, oxygen, and sulphur dioxide, the last two 
 passing away in the gaseous form. This sulphate was extracted 
 with water, and, being neutral, formed the electrolytic solution. 
 The residual zinc oxide, left after lixiviation, he proposed to 
 place in a moist condition with other oxide, in towers through 
 which passed the waste gases from the roasting furnace, so 
 that sulphur dioxide formed during the calcination of a fresh 
 batch of ore might be absorbed by the zinc oxide which it would 
 convert into zinc sulphite, and this salt, itself soluble, and, 
 therefore, capable of electrolysis, was to be gradually converted 
 into sulphate by absorption of oxygen from the air. 
 
 The zinc solution was electrolysed between lead anodes, which 
 are insoluble, and zinc cathodes. The solution was thus gradually 
 deprived of zinc, and also became richer in acid, because, as the 
 zinc sulphate was decomposed, the metal was deposited and the 
 acid left free in the solution. But as zinc is not precipitable 
 electrolytically from acid solutions, a point was soon reached at 
 which action ceased; the solution partially spent, but still 
 carrying much zinc, was, therefore, caused to flow over fresh zinc 
 oxide (roasted blende or calamine), so that it might take up a 
 further quantity of metal which, at the same time, neutralised 
 the free acid ; it was then ready to be passed once more through 
 the electrolytic tanks. This cycle of operations was constantly 
 maintained, so that the same acid was used again and again, until 
 it had accumulated so large a proportion of foreign matter that 
 the solutions were useless. The current to be employed must 
 have a high electro-motive force, because, when the lead becomes 
 peroxidised at the surface, which occurs almost immediately, the 
 opposing electro-motive force set up between the surface covering 
 and the zinc cathode is very considerable. 
 
 Some of the difficulties inherent in the deposition of zinc have 
 already been described (see p. 244). But those attending the 
 production of pure solutions of zinc from the ore at a moderate 
 cost, for the purpose of electrolysis, are equally great. 
 
 Many other processes have been proposed and tried, but 
 
288 EXTRACTION AND REFINING PROCESSES. 
 
 the problem of electrolytic zinc extraction is a troublesome 
 one. Electrolytic zinc is, however, upon the market, and it 
 would appear that the difficulties which wrecked the earlier 
 processes are in a fair way to being overcome. 
 
 The most successful process appears to be that due to Hoepfner, 
 as worked by Brunner, Mond & Co. The roasted zinc ore is 
 treated with a solution of calcium chloride and carbon dioxide ; 
 this gives a precipitate of calcium carbonate and a solution of zinc 
 chloride. From this solution the zinc is deposited electrolytically, 
 and the chlorine, which is obtained at the same time, is used in 
 the alkali works. 
 
 One of the most recent processes for dealing with sulphide ores 
 is that of Swinburne and Ashcroft, in which the crushed ore is 
 mixed with fused zinc chloride, the mixture being heated in a 
 converter and chlorine blown through it. The chlorine displaces 
 the sulphur, which is given off in the free state, and the metals, 
 which were previously in the form of sulphides, are converted into 
 chlorides. The fused chlorides are run off and treated with zinc 
 to precipitate lead, gold, and silver ; or the silver can be extracted 
 by agitating with metallic lead, giving base bullion. The zinc 
 may be precipitated by fractional electrolysis of the fused 
 chlorides, the chlorine being collected for further use. This 
 ingenious process has been worked on a large experimental scale, 
 but so far it has not come into commercial use. 
 
 Another process for sulphide ores is that due to Siemens and 
 Halske, in which the ore is roasted to convert the sulphides into 
 oxides and sulphates. The ore is then treated with dilute 
 sulphuric acid, and the resulting solution after purification is 
 electrolysed to deposit the zinc. Any silver is left in the ore, 
 which requires further treatment for its recovery. 
 
 Both zinc and iron ores are now treated somewhat extensively 
 in the United States by electromagnetic and electrostatic methods 
 so as to concentrate them for further treatment. The former 
 method is used for dealing with magnetic ores, such particles as 
 can be magnetised being attracted by electromagnets and thus 
 separated from the remainder. In the electrostatic method, 
 which is applicable to non-magnetic ores, the powdered ore is 
 passed over a metal plate, or roller, which is highly electrified 
 (at, say, 350,000 volts) ; the conducting particles become elec- 
 trified and are repelled from the plate, whereas the non-conducting 
 particles (such as quartz, silicates, etc.) merely glide over the 
 surface. 
 
 It may be said that there is a distinct opening for the electro- 
 lytic extraction of zinc from its ores ; but the refining of zinc, 
 apart from its extraction, is accomplished so easily by ordinary 
 metallurgical methods that there is not much scope for an 
 electrolytic method. 
 
ELECTKO-KEDUCTION OF ANTIMONY. 289 
 
 THE ELECTRO-REDUCTION OP ANTIMONY. 
 
 Borchers in 1887 appears to have obtained favourable results 
 from the electrolysis of a solution of antimony sulphide in sodium 
 sulphide. The sodium compound may contain poly sulphides, but 
 should be added in such a proportion that there is only one equi- 
 valent of sulphur present in any form to each equivalent of sodium, 
 as it is found that any increase in the amount of sodium produces 
 a less conductive fluid, while on the other hand a reduction is 
 attended by the precipitation of sulphur. The ore, or product 
 containing antimony tersulphide, is treated with a solution of 
 sodium sulphide in water, until the liquid has a density equal to 
 12 Baume; it is then electrolysed in iron tanks, the walls of 
 which serve as cathodes to receive the deposited metal ; leaden 
 anodes, being insoluble, are to be preferred. A current-pressure 
 of 2 to 2J volts per vat is recommended. 
 
 The antimony is deposited in the pulverulent condition, or in 
 the form of shining scales ; any portion clinging to the iron tank- 
 walls is readily removed by steel brushes, and the whole is washed 
 free from admixed solution, first with water containing a small 
 quantity of sodium sulphide and ammonia or caustic soda, then 
 several times with water, next with very dilute hydrochloric acid, 
 and, finally, with water again, after which it is dried and fused 
 together under glass of antimony. 
 
 This process, like most others of the kind, has to compete with 
 a fairly economical extraction method by the dry way ; and as 
 a fusion is, after all, necessary to unite together the powdered 
 deposit, it is a question whether it could compete with the older 
 processes, unless a very cheap supply of electrical energy be 
 available. Indeed there is not much opening for electrolysis in 
 either the refining or winning of antimony. 
 
 Antimony has been produced commercially from the ore by 
 Messrs Siemens & Halske, who treat the sulphide ore with a 
 solution of sodium sulphide. This dissolves out the antimony 
 sulphide, and the solution is then electrolysed in the cathode 
 compartment of a diaphragm cell, the antimony being deposited 
 on iron cathodes. The anode compartment contains a solution 
 of sodium chloride. 
 
 THE ELECTROLYTIC EXTRACTION AND REFINING OF IRON. 
 
 Very little has been done in electrolytic extraction or refining 
 of iron. In 1908, however, Sherard Cowper-Coles described a 
 process by which he had obtained iron plates and tubes by one 
 operation from crude iron, or even from iron ore. The electrolyte 
 employed consists of a 20 per cent, solution of sulpho-crysilic acid 
 saturated with iron, to which is sometimes added a little carbon 
 bisulphide. This solution is kept charged with iron oxide, which 
 is maintained in suspension by stirring, and the electrolyte so 
 
 19 
 
290 EXTRACTION AND REFINING PROCESSES. 
 
 charged has a specific gravity of about 1'32. If the ore is being 
 treated, it is leached with the acid solution, and the leaching is 
 assisted electrically by a current of low current-density. In this 
 case graphite anodes are used, but if iron is being refined the iron 
 itself is used as the anode. The temperature should be 70 C., 
 and a current density of 100 amperes per sq. ft. may be used. 
 By this means iron is obtained containing only about 0'05 per cent. 
 of carbon and small quantities of other impurities. Owing to the 
 large amount of hydrogen generally present in electrolytic iron, 
 the iron so obtained must be annealed to remove the brittleness, 
 unless the soft variety is produced (by adjusting the E.M.F.). As 
 far as the author is aware, the process is not yet in commercial use. 
 
 RECOVERY OF TIN FROM TIN SCRAP. 
 
 As already explained, what is popularly termed ' tin ' is 
 generally sheet-iron with a coating of tin upon it. During recent 
 years an industry has sprung up, having for its object the re- 
 covery of tin from tin-scrap and from disused tins of all kinds. 
 In Germany and the United States many thousands of tons of 
 such scrap are treated annually, the tin being recovered electro- 
 lytically. The scrap is made the anode in a solution of caustic 
 soda with iron cathodes ; by this means the tin is dissolved as 
 sodium stannate and can be deposited at the cathode in a spongy 
 state without fear of contamination by iron. In some processes 
 a solution of stannic chloride is used as the electrolyte. In the 
 Brown-Neil process ferric chloride is used ; this acts without the 
 aid of electrolysis, current being used merely for the recovery of 
 tin from the solution. 
 
 Although a very considerable industry has sprung up on these 
 lines, certain difficulties are encountered. The caustic soda 
 absorbs carbonic acid from the atmosphere, and the tin dissolves 
 more rapidly than it is deposited, so that the solution becomes 
 unworkable unless caustic soda is added at intervals. There is 
 also a considerable loss of electrolyte when the scrap is removed 
 from the vats, and this cannot be avoided simply by washing with 
 water, because this causes a precipitate. 
 
 The electrolytic process is now being supplanted largely by a 
 purely chemical method, chlorine being used to remove the tin. 
 For this purpose the tin scrap is carefully dried, and freed from 
 any organic substances, and is then treated with dry chlorine 
 under a pressure of several atmospheres. So long as the chlorine 
 is dry, the iron is not attacked, but the tin is converted into 
 tetrachloride, which is a heavy, fuming liquid. 
 
 ELECTRO-SMELTING. 
 
 Siemens' Electric Furnace. A process which is in no degree 
 dependent upon electrolysis, but purely upon the reducing action 
 
ELECTRO-SMELTING. 
 
 291 
 
 of carbon at excessively high temperatures, was introduced by 
 the two brothers E. H. and A. H. Cowles. Before dealing with 
 this, however, reference should be made to the Siemens electrical 
 furnace, with which a number of experiments were made (and 
 recorded in 1882 in a paper before the British Association) by 
 the inventor, conjointly with Professor Huntington, in the Metal- 
 lurgical Department of King's College, London. This furnace of 
 Sir William Siemens was the first embodiment of the principle 
 subsequently adopted in modified form by Cowles. 
 
 The furnace depends for its power upon the intense heat of 
 the electric arc. A crucible is bored at the bottom to receive 
 a tightly-fitting stout carbon rod (fig. 94), such as is used in 
 large electric arc-light projectors. This forms the positive pole 
 and is connected with a wire 
 which passes from the dynamo 
 or other generator. The nega- 
 tive pole consists of a similar 
 carbon rod, connected with the 
 negative brush of the dynamo, 
 and is suspended from above, 
 centrally within the crucible ; 
 an arrangement of a solenoid 
 upon the same base plate con- 
 trols the current, so that im- 
 mediately the upper of the 
 two carbon rods is sufficiently 
 lowered to make contact with 
 the other, the current flows, 
 and, passing around the sole- 
 noid, automatically separates 
 the poles and maintains them 
 at a practically equal distance 
 from one another during the 
 whole time of fusion, in spite of the gradual wearing away of the 
 carbon. Thus at the moment when the rods break contact, an 
 intensely powerful electric arc is formed between them, and this 
 is maintained evenly within the crucible so long as the current 
 is continued ; any substance packed around the lower carbon is 
 in this manner rapidly heated to the highest temperature attain- 
 able. Comparatively large quantities of steel and of platinum 
 (several pounds) were rapidly melted ; and even tungsten, one of 
 the most refractory metals k nown, could be fused in the arc, but 
 only in small quantities and with the greatest difficulty, by placing 
 it in a hollow scooped out of the lower carbon itself instead of in 
 a crucible. Setting aside all questions of prime cost of plant and 
 working expenses, an electric furnace on these lines no doubt 
 renders available the highest temperature that is to be obtained 
 with the means now at command. 
 
 FIG. 94. Siemens' electric furnace. 
 
292 EXTRACTION AND REFINING PROCESSES. 
 
 Cowles' Electric Furnace. In the Cowles furnace for the electro- 
 thermal production of aluminium alloys from alumina, the two 
 carbons are passed through opposite ends of a fire-brick chamber 
 lined with charcoal dust, which is selected as a lining on account 
 of its high resistance to fusion ; but as the charcoal, itself but 
 a poor conductor of electricity, rapidly becomes converted into 
 graphite, which is vastly superior in this respect, and thus causes 
 a leakage of electricity unutilised between the poles, it was found 
 necessary to soak the charcoal in milk of lime before use. On 
 drying this product each grain of charcoal is coated with a deposit 
 of lime, which effectually destroys the conductivity. All around 
 the carbons, and within the space thus formed, is packed the 
 mixture of the ore or compound to be reduced with fragments of 
 carbon, and with pieces of the metal which it is desired to alloy 
 with the aluminium ; the whole is then covered with fine charcoal, 
 and finally with a luted iron lid, lined with fire-brick, and provided 
 with an aperture for the escape of gases. The carbon rods are 
 attached to stout copper cables leading to the dynamo, but placed 
 in circuit with an ammeter, a switch by which the current may 
 be broken, an automatic cut-out to prevent the passage of an 
 excessive current, and a set of resistances which may be interposed 
 if necessary at any time. Placing first the resistances in circuit, 
 the current is turned on, a short space only existing between 
 the ends of the carbon electrodes, which are then withdrawn by 
 degrees through the furnace walls as the operation proceeds, 
 until at last the whole interior of the furnace is filled with a 
 glowing mass. 
 
 The Cowles Process. A furnace of this type is shown in 
 vertical cross-section, as well as in part elevation, part longi- 
 tudinal vertical section, in fig. 95 ; it is constructed of fire-brick, 
 as described, with a cast-iron cover, and measures internally 5 ft. 
 by 3 ft. by 1 ft. 8 ins. ; through each end- wall is built a cast- 
 iron tube of sufficient diameter to pass the bundles of electrodes. 
 The latter consist of nine carbon rods, each 2J ins. in diameter 
 (or of five 3-in. rods), held together by a cast-copper or cast-iron 
 head (the former if a copper alloy is being made, the latter for 
 iron alloys) attached to a rod, actuated by gearing suitable for 
 the withdrawal of the carbons from the furnace, and connected 
 with one of the poles of the dynamo. The walls having been 
 lined with finely-crushed oak charcoal, insulated by the lime 
 process, the charge of aluminium compound, charcoal and alloying 
 metal in the form of small turnings or granules, is introduced 
 and covered with fine charcoal, broken fragments of carbon 
 having been previously placed in position, in order to start the 
 arc as soon as the current is switched on. The source of electrical 
 energy is a large dynamo capable of furnishing a current of from 
 5000 to 6000 amperes at a pressure of from 50 to 60 volts. 
 The current at first is about 3000 amperes, but is gradually 
 
ELECTRO-SMELTING. 
 
 293 
 
 increased to about 5000 amperes in the space of half an hour, the 
 whole process lasting about 1J hours. Gases, due to the reduc- 
 tion of the aluminium compound, and therefore consisting mainly 
 of carbonic oxide, as explained by the annexed equation, pour 
 from the aperture in the cast-iron cover, and burn with a long 
 flame. 
 
 Al a 3 + 30 A1 2 - + 3CO 
 
 Alumina with carbon yield aluminium and carbonic oxide gas. 
 
 When the operation is complete, the current is broken, and 
 switched on to an adjoining furnace, while the contents of the first 
 are either allowed to settle upon the hearth, or are tapped out 
 into a receptacle in front, through a tap-hole, which is plugged 
 during the passage of the current. The expenditure of power to 
 
 FIG. 95. Cowles' electric furnace. 
 
 produce a given weight of metal depends upon the nature of the 
 alloy which is being produced, but is said to average eighteen 
 horse-power hours per pound of copper. 
 
 It is difficult, if not impossible, to produce pure aluminium by 
 such a process as this, because this metal is very liable to combine 
 with carbon at high temperatures, and as, under the conditions 
 named, these two substances are in intimate contact under the 
 circumstances most favourable to combination, the metal or such 
 of it as escapes volatilisation in the furnace will contain more or 
 less carbide, which ruins its working properties. The process is, 
 however, available for the production of alloys such as aluminium 
 bronze, consisting of copper containing various proportions of 
 aluminium, or ferro-aluminium, which is an alloy of the latter 
 metal with iron. The presence of the alloying metal no doubt 
 favours the reduction of the aluminium ; just as in the case of 
 heating together iron or copper with carbon and silica, even at 
 
294 EXTRACTION AND REFINING PROCESSES. 
 
 temperatures obtainable with furnaces fed by solid or gaseous fuel, 
 the silicon is reduced and combines with the assisting metal, 
 although the heating of silica and carbon alone is barren of result ; 
 but the Cowles furnace is able to reduce alumina to the metallic 
 state, even in the absence of alloying elements, by the direct 
 action of carbon upon alumina, a reaction which cannot occur at 
 ordinary furnace temperatures, but which is possible at the 
 enormously high temperature of the electric arc. As already 
 explained, however, the aluminium is not sufficiently pure for 
 use. The Cowles process, although interesting as illustrating 
 what may be done by such means, is no longer in use for the 
 manufacture of aluminium alloys, because aluminium is now so 
 readily obtained by other methods, as described below, that it is 
 used directly in the metallic state for alloying with other metals, 
 thus avoiding impurities. 
 
 Until the electric furnace had placed these high tempera- 
 tures at the disposal of the chemist, it was believed that many 
 oxides alumina among them were incapable of reduction by 
 carbon ; but Borchers has demonstrated, not only with the aid of 
 the electric arc, but with a furnace in which the heat is derived 
 from the incandescence of carbon heated, as in the glow-lamp, by 
 the passage of an electric current, that all oxides are so reducible. 
 Such reduction-processes may be known as electro-thermal ; they are 
 not electrolytic, and the use of electricity is simply as a source of 
 heat. Phosphorus is thus produced on a large scale. A consider- 
 able industry has also sprung up in the manufacture of calcium 
 carbide in electric furnaces, similar in principle to that of Siemens. 
 This is effected by heating together at the temperature of the arc 
 a mixture of lime and coke dust, using an excess of the latter 
 material. On heating, a part of the carbon removes the oxygen of 
 the lime, whilst another portion of the carbon combines with the 
 liberated calcium to form calcium carbide, thus 
 
 The semi-fused mass of carbide is cooled out of contact with the 
 air, and is largely employed in the manufacture of acetylene for 
 illuminating purposes. 
 
 Electrolysis of Fused Compounds. The electrolysis of fused 
 metallic compounds instead of aqueous solutions has frequently 
 been suggested, and several of such processes are now in con- 
 stant use. At first it was considered necessary to use ordinary 
 fuel externally applied to melt the substance to be used as an 
 electrolyte. But now the current employed for electrolysis is 
 also made the means of bringing the solid substance to a state of 
 fusion, arid of so maintaining it. By this means magnesium, 
 sodium, and aluminium are constantly produced, and, in fact, the 
 whole of the aluminium now smelted is obtained by the elec- 
 trolysis of fused aluminium compounds. 
 
ELECTRO-SMELTING. 
 
 295 
 
 Aluminium Smelting. There are several processes in use for 
 this purpose, but they are identical in principle. A metal case 
 lined with carbon or alumina (but preferably alumina) is so 
 arranged that a carbon block attached to a stout copper cable 
 may be raised or lowered within it, as shown diagrammatically 
 in fig. 96. At one side is an outlet or tap-hole, and at the 
 bottom is a plate marked - connected with another copper cable 
 leading to the negative pole of the dynamo. This plate may with 
 advantage be cooled by boring the narrow part of the plug be- 
 neath the plate and introducing a flow of water into the tube 
 thus formed. Even the smallest trace of water must, of course, 
 be rigorously excluded from 
 the interior of the fur- 
 nace. The carbon block 
 is joined up by its cable 
 to the positive pole of the 
 dynamo. The general ar- 
 rangement of the furnace 
 is thus very similar to 
 that of the Siemens fur- 
 nace (fig. 94). 
 
 To start the furnace, the 
 carbon block is lowered into 
 contact with the plate, a 
 current flows, the block is 
 then raised just out of con- 
 tact, a powerful electric 
 arc is formed between the 
 two surfaces ; finely divided 
 cryolite (double fluoride of 
 aluminium and sodium), 
 
 FIG. 96. Aluminium smelting process. 
 
 with or without an addition 
 of some other fluoride or 
 chloride, such as calcium fluoride or chloride, is then introduced, 
 and as it becomes fused by the heat of the current, more and 
 more is added, and with it alumina, until the crucible is nearly 
 full, the carbon block being gradually raised until it is at some 
 distance above the plate. The action now is comparable with 
 that of the electrolytic vat : the alumina dissolves in the melted 
 fluoride bath, as copper sulphate dissolves in water, and is electro- 
 lysed in the same way by the current ; the reduced aluminium 
 collects at the bottom of the receptacle, and is run off from time 
 to time, care being taken that it is never allowed to rise to the 
 level of the carbon anode-block, and so cause a short circuit, while 
 the anion, oxygen, is deposited on the carbon anode, which is thus 
 gradually burned away, forming carbonic oxide, which escapes at 
 the top of the furnace. The electrolyte is maintained at a red- 
 heat (about 750 to 850 C.), owing to the fact that the powerful 
 
296 EXTRACTION AND REFINING PROCESSES. 
 
 current passing through the bath meets with ohmic resistance, so 
 that a portion of it (sufficient for the purpose if the installation 
 be large enough) is converted into heat. This process is identified 
 with the names of Heroult in Europe and of Hall in America. 
 
 The reduced aluminium is very liable to combine chemically 
 with non-metals, so that contact with carbon is to be avoided, and 
 alumina is, therefore, better than carbon as a furnace-lining 
 (although carbon is often used), and to alloy with other metals, 
 so that the - plate is better water-cooled, if possible, to such an 
 extent that there is always a frozen "Jay er of aluminium in contact 
 with it to protect it from contact with melted aluminium at a red- 
 heat. The alumina lining gradually dissolves in the cryolite, but 
 a limit is reached when the walls have become so thin that the 
 heat lost by radiation causes a film of the electrolytic bath to 
 freeze in contact with and to protect it, as the aluminium protected 
 the - plate. The anodes are consumed, and must be replaced 
 from time to time. The cryolite, being unaltered, and forming 
 only a vehicle for the electrolysis of the alumina, may last a long 
 time, until, in fact, the accumulated impurities from the alumina 
 added render it unserviceable. For this reason the alumina 
 employed should be as pure as possible. Alumina is constantly 
 added to replace that which is electrolysed, and should always be 
 present to the extent of 20 per cent, of the cryolite. 
 
 A current of several thousand amperes at about 5 or 7 volts 
 is used (the current-density being frequently higher than 1*5 
 amperes per sq. in. of cathode surface), and it is found that 
 about 1 oz. of aluminium is produced by the expenditure of 1 
 electrical horse-power for 1 hour, while' about 1 oz. of carbon 
 anode is consumed in the same time. According to the most 
 recent practice, it would seem that 1 2 electrical horse-power hours 
 will now suffice to yield f Ib. to 1 Ib. of aluminium. 
 
 Magnesium Smelting. Bunsen heated the chloride of the metal 
 to its fusing-point by placing it in a crucible in a suitably 
 arranged fire, and then passed a powerful current through the 
 melted mass. In this manner he obtained magnesium by urging 
 a current from 10 cells of his battery between gas-carbon elec- 
 trodes, passed through the clay cover of a porcelain crucible, 
 in which at the upper part they were separated by a porcelain 
 partition, while at the lower they both dipped into the fused mag- 
 nesium chloride. The anode carbon was plain, but the cathode 
 was made with inverted steps upon the surface, so that it would 
 retain the globules of melted (metallic) magnesium, which, being 
 specifically lighter than the liquid, would otherwise float to the 
 surface and become re-oxidised. Matthiessen, by melting three 
 equivalents of potassium chloride and a little ammonium chloride 
 with the magnesium salt, obtained a bath which was of lower 
 specific gravity than the metal, so that the special arrangement 
 of the cathode was rendered unnecessary. Carnallite, a natural 
 
ELECTRO-SMELTING. 297 
 
 double chloride of magnesium and potassium, is now commonly 
 used for the extraction. 
 
 Sodium Smelting. It was by passing an electric current 
 through fused common salt (sodium chloride) that the metal 
 sodium was discovered by Sir Humphrey Davy, but until quite 
 recently it was always obtained industrially by a purely metal- 
 lurgical process. Now, however, the improvements in electric 
 smelting have enabled the manufacturer to obtain sodium most 
 economically by the electrolysis of fused compounds. Sodium 
 chloride is used in some processes. In Castner's process, which 
 is now in extensive operation, fused caustic soda is electrolysed, 
 whereby sodium, with some hydrogen, is deposited at the cathode 
 and oxygen at the anode. The soda is melted in a suitably con- 
 structed iron pot (heated by a ring of gas burners beneath) with 
 a cylindrical cathode passing through the bottom of the vessel, 
 to within a short distance below the upper level of the fused 
 material in the bath. The cathode is, of course, insulated from 
 the metal of the containing vessel. Metal anodes are suspended 
 around the cathode from the cover of the pot, and these also 
 are well insulated. One of the peculiarities in sodium extrac- 
 tion is that the metal is lighter than the electrolyte, so that 
 after deposition it floats off the surface of the cathode to the 
 top of the liquid, and, as its fusing-point is lower than that of 
 soda, it forms a layer of melted metal on the surface of the bath. 
 If this were permitted to continue it would cause a short circuit 
 between the electrodes, and special provision has therefore to be 
 made to guard against the danger. A tube, open at the bottom 
 and of somewhat greater diameter than the cathode, is suspended 
 above the top of the latter, and the globules of sodium, as they 
 become detached from the cathode surface, float up within the 
 tube and collect there. The tube is provided with a movable 
 cover to exclude air, and the sodium is removed as soon as 
 sufficient has accumulated, by taking off the cover and ladling 
 out the melted metal in a ladle perforated with small holes of 
 such size that the sodium (by reason of its high surface tension) 
 cannot pass through, but the caustic soda can do so. The melted 
 sodium is then poured into moulds for the market. 
 
 THE ELECTRO-THERMAL PRODUCTION AND REFINING OF IRON. 
 
 Electrical v. Chemical Methods. In considering the com- 
 mercial possibilities of the electric reduction of iron ores it is 
 necessary to remember that the usual chemical process is very 
 simple. Coal is used as the reducing agent, and the ore is further 
 mixed with limestone to give a fusible slag. The coal by its 
 combustion not only causes the desired reduction but also pro- 
 vides the necessary heat, and as the ore, the limestone, and the 
 coal are obtained in the same localities, at least in this country 
 
298 EXTRACTION AND REFINING PROCESSES. 
 
 and many others, the essentials are cheap. Moreover, as the 
 process itself is quite simple unlike those necessary in the pro- 
 duction of certain other metals, such as copper, from their ores 
 the cost of producing iron by purely chemical methods is low. 
 
 By means of an electric furnace heat can be applied under the 
 very best conditions without the waste which accompanies the 
 operation of the blast furnace ; but, on the other hand, the 
 required electric energy is generally obtained by the combustion 
 of coal through the intermediary of the boiler and the steam- 
 engine. The two methods therefore resolve themselves into 
 (1) the direct utilisation of the heat from coal, and (2) the 
 indirect use of such heat. The direct use of coal in the blast 
 furnace involves serious waste (largely because the air contains 
 so great a proportion of nitrogen, which is inert), but the indirect 
 use of the heat, by converting first into electric energy, is subject 
 to a still more serious loss. This is due to the fact that it is 
 impossible, for thermo-dynamic reasons, to convert fully into 
 mechanical energy all the heat imparted by the coal to the steam. 
 The loss on this account is so serious that the overall efficiency 
 in converting the energy of coal into electric energy in practice 
 is usually well under 10 per cent. With gas-engines instead of 
 steam-engines the efficiency is higher, and if water-power is the 
 primary source of energy this question does not, of course, arise. 
 But even with water-power, the first cost of the hydraulic work 
 is often very heavy, and it is quite possible that the elimination 
 of the fuel item by this means in a locality where coal is cheap 
 will not lead to any very great cheapening of the total cost of 
 electric energy. For this reason it is found that the electric 
 furnace for the reduction of iron from its ores cannot compete 
 commercially with the blast furnace if the electric energy is 
 generated from coal ; in fact, for the electrical method to be 
 commercially successful it follows that the price of electric energy 
 must be very low and the cost of fuel very high. The whole 
 question was considered very fully in the report of the Com- 
 mission appointed by the Canadian Government to investigate 
 the electro-thermic processes for the smelting of iron ores and 
 the making of steel in operation in Europe (published in 1904). 
 One of the conclusions reached in this report is that the two 
 methods of producing pig-iron by the electric furnace and by 
 the blast furnace are about equal in cost if electric energy is 
 obtainable at $10 (41s. 8d.) per electrical horse-power year and the 
 price of coke is f 7 (29s. 2d.) per ton. This price for electric energy 
 is very low, just as that for the coke is very high. These figures 
 may be modified in the future by improvements in the electrical 
 efficiency on the one hand, and greater efficiency in the blast 
 furnace on the other hand, by utilising the waste heat in the 
 furnace gases, which latter are of value for driving gas engines or 
 for raising steam. 
 
ELECTRIC PRODUCTION AND REFINING OF IRON. 299 
 
 When we come to consider the cost of producing steel from pig- 
 iron by electrical methods, as distinct from the reduction of iron 
 ores to produce pig-iron itself, the case is very different. Here 
 it is a question of refining an impure metal instead of producing 
 impure metal itself, or of producing iron containing definite 
 proportions of carbon or other ingredients so as to give steels of 
 definite composition. As the proportions of these ingredients 
 are small the operations require care, and if the process is such 
 that impurities cannot be easily introduced the advantage is 
 obvious. It is in this respect that the electric furnace is to be 
 preferred to the older methods. The manipulation is more 
 certain. If impurities from electrodes through resistance-heating 
 introduce a difficulty, arc-heating may be used, or, as we shall 
 see later, electrodes may be entirely avoided, and the heat may 
 be produced by induction. 
 
 It follows therefore that the electric furnace may be commer- 
 cially valuable for making steel, though its field for the production 
 of pig-iron from the ore is limited to special circumstances. With 
 regard to the latter, however, it must be remembered that the 
 blast furnace is the product of generations, whereas the electric 
 furnace is in its first youth. It is possible that further improve- 
 ments will render the electrical method a much more efficient 
 rival of the blast furnace than it is at present. For the time 
 being, however, only special cases can be considered. Canada, 
 for example, is a case of this kind, and the Canadian Government 
 have recently carried out experiments which show that magnetite 
 can be reduced by using, in place of coke, the wood charcoal so 
 readily obtainable in that country. 
 
 While the electric production of pig is in abeyance the electric 
 furnace is in regular use for producing steel, ferro, and other 
 alloys. Thus the Societe Anonyme Electrometallurgique Precedes 
 (P. Girod) produce regularly ferro-chrome, cupro- vanadium, cupro- 
 silicon, ferro - silicon, silico - manganese, ferro - tungsten, ferro- 
 manganese, ferro- vanadium, ferro - molybdenum, silico - chrome, 
 and ferro-titanium by the electric furnace. 
 
 As the result of their investigations in Europe, the Canadian 
 Commission came to the following conclusions : 
 
 1. Steel, equal in all respects to the best Sheffield crucible 
 steel, can be produced, either by the Kjellin or Heroult or Keller 
 processes, at a cost considerably less than the cost of producing 
 a high-class crucible steel. 
 
 2. At present, structural steel to compete with Siemens or 
 Bessemer steel cannot be economically produced in the electric 
 furnaces, and such furnaces can be used commercially for the 
 production of only very high-class steel for special purposes. 
 
 3. Speaking generally, the reactions in the electric smelting 
 furnace as regards the reduction and combination of iron with 
 silicon, sulphur, phosphorus, and manganese are similar to those 
 
300 EXTRACTION AND REFINING PROCESSES. 
 
 taking place in the blast furnace. By altering the burden and 
 regulating the temperature by varying the electric current, any 
 grade of iron, grey or white, can be obtained, and the change 
 from one grade to another is effected more rapidly than in the 
 blast furnace. 
 
 4. Grey pig-iron, suitable in all respects for acid steel manu- 
 facture, either by Bessemer or Siemens processes, can be produced 
 in the electric furnace. 
 
 5. Grey pig-iron, suitable for foundry purposes, can be readily 
 produced. 
 
 6. Pig-iron low in silicon and sulphur, suitable either for the 
 basic Bessemer or the basic Siemens process, can be produced, 
 provided that the ore mixture contains oxide of manganese, and 
 that a basic slag is maintained by suitable additions of lime. 
 
 7. It has not been experimentally demonstrated, but from 
 general considerations there is every reason to believe, that pig- 
 iron low in silicon and sulphur can be produced even in the 
 absence of manganese oxide in the iron mixture, provided a fluid 
 and basic slag be maintained. 
 
 8. Pig-iron can be produced on a commercial scale at a price 
 to compete with the blast furnace, only when electric energy is 
 very cheap and fuel very dear. On the basis taken in this report, 
 with electric energy at $10 (41s. 8d.) per E.H.P.-year, and coke 
 at $7 (29s. 2d.) per ton, the cost of production is approximately the 
 same as the cost of producing pig-iron in a modern blast furnace. 
 
 9. Under ordinary conditions, where blast furnaces are an 
 established industry, electric smelting cannot compete ; but in 
 special cases, where ample water-power is available, and blast- 
 furnace coke is not readily obtainable, electric smelting may be 
 commercially successful. 
 
 Some further Advantages of the Electric Furnace. Apart 
 from the cost of operation, which will certainly diminish as 
 improvements are introduced in design, it may be noted that 
 the electric furnace has, in comparison with the blast furnace, 
 some very distinct advantages which have been emphasised by 
 Dr. Haanel. Thus, the first cost is small ; the charging machinery 
 is not costly ; the expense involved in a breakdown is small ; and 
 repairs can be made easily, because the electric furnace would be 
 used in small units, whereas the blast furnace, to be most econo- 
 mical, must be large, say 90 ft. in height. 'Owing to the com- 
 paratively small size of the electric furnace, the loss due to 
 wrong composition of a charge is reduced to a minimum, and, 
 lastly, there is perfect control of the temperature in the reducing 
 and melting zone. 
 
 Recently it has been found that the nitrogen in the air pro- 
 duces in the blast furnace, under certain conditions, nitride of 
 iron, which has a bad effect on iron and steel, rendering them 
 brittle. Thus great brittleness has been found in iron and steels 
 
ELECTKIC PRODUCTION AND REFINING OF IRON. 301 
 
 with low sulphur and phosphorus. Consequently, the elimination 
 of nitrogen by the electric furnace not only prevents inefficiency, 
 but allows certain grades of ore to be handled which would not 
 submit to the usual treatment by the blast furnace. This is 
 particularly important in the case of Canada, where the magnetites 
 which there occur contain too much sulphur for blast furnace 
 treatment, but can be economically smelted electrothermally. 
 Experiments at Sault Ste. Marie showed that in the electric 
 furnace sulphur and silicon can be varied as desired, and that 
 titaniferous iron, containing up to a certain proportion of titanic 
 acid, can also be reduced. As a field, therefore, for the electric 
 furnace Canada is particularly well situated, as not only is there 
 ample water-power and coal is expensive, but the nature of the 
 ores renders them well suited to electrical treatment in preference 
 to the ordinary methods. For similar reasons the electric smelt- 
 ing of iron ores is being taken up extensively in Sweden. 
 
 Amount of Electrical Energy Required. There are not many 
 authoritative figures as to the amount of energy required by the 
 electric furnace per ton of metal produced. From the figures 
 obtained by the Canadian Commission for the Heroult, Keller and 
 Kjellin furnaces, it appears that the production of steel from cold 
 pig and scrap requires about 1000 kilowatt-hours per ton. If 
 the raw material is first fused before running into the electric 
 furnace, so that the electric energy is not required for fusing the 
 metal, but is used merely for maintaining the temperature during 
 the process of refining, the energy required is, of course, very 
 much reduced. 
 
 In Table XXVII. are given a number of results published by 
 well-known makers of electric furnaces, showing the amount of 
 energy required per ton of steel, and certain other particulars. 
 From this it will be seen that the amount of electric energy used 
 per ton, merely for refining, is comparatively small, and will, no 
 doubt, be further reduced as furnaces are made larger and more 
 experience is gained. It may be said that in all probability the 
 electric refining furnace will shortly take up a prominent position 
 beside the blast furnace for the refining of iron, which, under 
 certain conditions, will be taken conveniently from the latter in 
 the molten condition, and refined without being cast into pig. 
 Or under certain conditions an ordinary smelting furnace may be 
 used as a preliminary to the electric refining furnace. 
 
 Reduction from the ore is quite another matter. Figures ob- 
 tained by the Commission as to the production of pig-iron from the 
 ore in the case of the Heroult and Keller furnaces indicate that 
 about 3000 kilowatt-hours are required per ton of pig produced. It 
 is quite possible, however, that further experience will materially 
 reduce this figure, which must be considered as somewhat tentative. 
 In fact, experiments carried out by Dr. Haanel at Sault Ste. 
 Marie, for the Canadian Government in 1906, showed that a ton 
 
302 
 
 EXTRACTION AND REFINING PROCESSES. 
 
 of pig could be produced with an expenditure of about 1700 kilo- 
 watt-hours. If arrangements were adopted to utilise the heat 
 obtainable from the waste gases effectually to preheat the charge, 
 there is no doubt that even this improved figure would be consider- 
 ably reduced. 
 
 TABLE XXVJL- GIVING PARTICULARS OF WORKING OF ELECTRIC FURNACES 
 
 FOR REFINING IRON. 
 
 
 
 Energy in kilowatt-hours 
 
 
 
 
 
 used in refining, per ton. 
 
 Consump- 
 
 Life of 
 
 Authority. 
 
 Conditions. 
 
 
 tion of 
 Electrodes 
 per ton. 
 
 Furnace 
 Linings. 
 
 Starting 
 with cold 
 metal. 
 
 Starting with 
 fused metal. 
 
 P. Girod . 
 
 In 2-ton furnace 
 In 8-12-ton fur- 
 
 900 
 
 700 
 
 
 35-40 Ibs. 
 13-15 Ibs. 
 
 i 40-50 heats. 
 
 
 nace 
 
 
 
 
 
 P. L. T. Heroult 
 
 In 5-ton furnace 
 
 700 
 
 140-180 
 
 60-65 Ibs. 
 
 May last a year, 
 
 
 
 
 
 starting cold 
 
 but roof must 
 
 j j 
 
 In 15-ton furnace 
 
 
 100 
 
 10-15 Ibs. 
 
 be renewed 
 
 
 
 
 
 starting hot 
 
 once a month. 
 
 C. A. Keller . 
 
 In 7 J-ton furnace 
 
 
 275 
 
 
 
 F. A. Kjellin . 
 
 In 3^-ton furnace 
 
 800 
 
 In 25 heats, varied 
 
 
 
 
 at Rdchlings' 
 
 
 from 175 300 ; 
 
 nil 
 
 
 
 works 
 
 
 mean, say, 250. 
 
 
 
 E. Stassano 
 
 In small furnaces 
 In large furnaces 
 
 1,000 
 700-300 
 
 | 200 
 
 17^-26 Ibs. 
 
 
 Note. The tons here given are presumably metric tons of 1000 kilogrammes 
 (2200 Ibs.). The figures given for electrodes include waste. 
 
 Dr. Haanel, working in conjunction with Dr. Heroult, came to 
 the conclusion that the largest furnace on the lines of the one 
 used at Sault Ste. Marie would not exceed 2000 H.-P., and certain 
 alterations would be necessary, namely, the introduction of labour- 
 saving machinery, provision for collecting and using the carbon 
 monoxide produced, automatic regulation of the electrode, and 
 the use of a higher shaft to permit the heated carbon monoxide 
 to effect the maximum reduction, the electrode being placed in a 
 side chamber. 
 
 Types of Electric Furnace. Electric furnaces may be classi- 
 fied according to the methods adopted in converting the electric 
 energy into heat, into the following classes : 
 
 1. Resistance furnaces. 
 
 "2. Arc furnaces. 
 
 3. Combined resistance and arc furnaces. 
 
 4. Induction furnaces. 
 
 5. Combined induction and resistance furnaces. 
 
 In Class 1, heat is produced by the resistance offered by the 
 charge to the passage of the current, the carbon electrodes dipping 
 
ELECTRIC PRODUCTION AND REFINING OF IRON. 303 
 
 into the charge ; or one electrode may be used above, the other 
 electrode being made the hearth of the furnace, and the current 
 regulated by varying the distance between the electrodes. In the 
 second class, the two electrodes are maintained above the surface 
 of the charge, and an arc is formed between them, the heat being 
 radiated to the charge. In Class 3, two electrodes are used, and 
 are first lowered to dip into the charge so as to start the heating 
 by resistance ; subsequently, however, the carbons are withdrawn 
 somewhat, so that an arc is formed from each electrode to the 
 charge, the current flowing through the charge between the two 
 arcs. The disadvantage of allowing electrodes to dip into the 
 charge is that impurities may be introduced, and, even if the 
 electrodes are pure carbon, the introduction of carbon is often very 
 undesirable, as, for example, in converting pig into steel. If arc- 
 heating is adopted, volatile products are formed, and these are 
 comparatively harmless, though sometimes undesirable. The 
 objection to arc-heating is that the heat is not generated within 
 the body of the charge as in resistance-heating. In order to get 
 over this difficulty of introducing impurities by the electrodes, Class 
 4 has been devised, in which the heating is really by resistance, 
 but the currents are ' induced ' by the variation of a magnetic 
 field, just as in an electric transformer. The reader will find this 
 method explained in greater detail in the description of the Kjellin 
 furnace. By its means electrodes are entirely obviated, but the 
 charge must be fairly conductive to start with, and the temperature 
 attainable is not so high as by the other methods ; it has been 
 found particularly adaptable to the making of steels and alloys, 
 but is obviously useless for the reduction of ores, nor are its 
 virtues in any way necessary to the latter. Owing to certain 
 difficulties experienced with the simple induction furnace this has 
 been modified so as to be worked on the induction principle and 
 at the same time with electrodes, thus giving Class 5. 
 
 H6roult's Furnace. The Heroult furnace has been used both 
 for making steel and for producing iron from the ore. In the 
 following description reference is first made to the crucible form 
 of furnace suitable in connection with the crucible-, and possibly 
 Siemens, methods which are now so largely in use. As seen in 
 figs. 97 and 98, the furnace consists of a crucible-shaped vessel 
 cased in with iron, which is lined with dolomite brick H, H, and 
 magnesite brick round the openings. The bottom is further lined 
 with crushed dolomite K, which is rammed on top of the brick and 
 forms the hearth. Two electrodes E, E, pass through the roof of 
 the furnace, and are sometimes water-jacketed above the openings 
 and to some extent below them. These electrodes are raised and 
 lowered by the rack and gear seen on the right of fig. 97, and by 
 this means the alternating current, which may be thousands of 
 amperes at, say, 100 volts, may be regulated. The current jumps 
 from one electrode to the slag, passes through the molten metal, 
 
304 
 
 EXTRACTION AND EEFINING PROCESSES. 
 
ELECTRIC PRODUCTION AND REFINING OF IRON. 
 
 305 
 
 and returns by jumping to the other electrode, so that there is 
 both arc- and resistance-heating, though mainly the latter, and 
 this resistance-heating is chiefly effected in the slag, which has a 
 much higher resistivity than has the metal below it. Owing to 
 the comparatively high conductivity of molten iron, purely 
 resistance-heating is not much used in refining, or in the produc- 
 tion of alloys (except in induction furnaces), because the currents 
 required would be so much larger than where arcs are introduced, 
 and the carbon would be deleterious. 
 
 Heroult's furnace for producing pig-iron from the ore is shown 
 in fig. 99. It consists of a vertical 
 shaft H through which the coal gravi- 
 tates, coming in contact with the carbon 
 electrode G in its progress. The other 
 electrode B forms the hearth of the 
 furnace. The ore gravitates down a 
 separate inclined shaft A, up which the 
 hot gases are allowed to escape ; in so 
 doing they heat the ore, and thus greater 
 economy is obtained. The metal is 
 tapped off at D, and the slag run off 
 at E. This furnace belongs to Class 1. 
 
 The advantages claimed for the 
 Heroult furnace are : (1) absence of 
 electrical parts in the furnace proper, 
 it being merely a modified open hearth ; 
 no trouble can arise from a bottom 
 electrode, as there is none ; (2) the 
 current is only half that necessary if 
 one electrode and arc are used with a 
 bottom electrode, because there are two 
 arcs in series ; (3) the slag being the 
 hottest part of the charge, all impuri- 
 ties can be removed by special slags. 
 
 Keller's Furnace. In the Keller furnace, shown diagram- 
 matically in fig. 100, 'resistance-heating' is adopted. The 
 furnace shown has a plurality of hearths, and above each of these 
 is a carbon electrode, which can be raised or lowered as necessary. 
 These furnaces can be used either for producing steel and alloys, 
 such as ferro-silicon, which is thus produced in large quantities 
 by Messrs Keller, Leleux & Cie., or for obtaining iron direct from 
 the ore. For this purpose the ore is fed in at the top of each 
 individual furnace, and the fused metal collects in the common 
 central well. Normally the current flows from one furnace, 
 through the well to the opposite furnace, these two furnaces being 
 connected in parallel with the other pair. The centre electrode 
 is brought into use if the metal in the well becomes chilled. To 
 prevent any interruption of the current when the metal is run off, 
 
 20 
 
 FIG. 99. Heroult furnace for 
 producing pig from the ore. 
 
306 EXTKACTION AND REFINING PROCESSES. 
 
 C. - 
 
 -D 
 
 Section C.D. 
 
 FIG. 100. The Keller furnace. 
 
ELECTRIC PRODUCTION AND REFINING OF IRON. 
 
 307' 
 
 a carbon block is built into the sole of each furnace, and these 
 are connected by copper conductors as indicated, so that an alter- 
 
 3 Meterg 
 
 FIG. 101. Keller electric high furnace. 
 
 native path is provided. The latter carries but a small part of 
 the current when the well is fully charged. New electrodes are 
 easily replaced by overhead travellers, and, as two pairs are run 
 
308 
 
 EXTRACTION AND REFINING PROCESSES. 
 
 in parallel, any single electrode can be replaced without interfering 
 with the working of the furnace. If the furnace is used merely 
 for alloys or for refining them, it is used as a combined arc and 
 resistance furnace. 
 
 An electric 'high furnace' is shown in fig. 101, and, in 
 principle, is just the same as the furnace just described in that 
 the four shafts are simply replaced by a single central shaft 
 which distributes the charge over the four hearths. The electrodes 
 
 FIG. 102. The Girod crucible furnace. 
 
 used in the experiments for the Commission were made up by 
 assembling four carbons into a single mass having a square cross 
 section, 85 cms. per side and 1 '4 metres long. The life of such 
 an electrode is stated to be twenty days'. Such a furnace is 
 purely for the reduction of iron ores. 
 
 The most recent form of Keller refining furnace is made up with 
 a conducting hearth, consisting of vertical iron bars, the spaces 
 between which are filled by ramming in magnesia, or some such 
 refractory material, which becomes a conductor when sufficiently 
 heated. This hearth forms one electrode, the other electrode 
 being a vertical carbon which is brought sufficiently near to the 
 charge for an arc to be maintained. Such a furnace has the 
 
ELECTRIC PRODUCTION AND REFINING OF IRON. 309 
 
 advantage that it is more easy to start up than the He'roult 
 furnace ; but it is less simple, and the water cooling which is 
 adopted for the hearth may very possibly be regarded as a 
 disadvantage. 
 
 G-irod's Furnace. The Girod refining furnace consists of a 
 crucible chamber as shown in fig. 102, with one or more carbon 
 electrodes of like polarity, suspended above the charge. The 
 other pole consists of a number of pieces of soft steel buried in 
 the refractory material of the hearth at its periphery ; the upper 
 ends of these bars are in contact with the fused metal, and the 
 lower ends are water-cooled to prevent excessive fusion of the 
 upper ends, and to protect the lining. An arc is formed between 
 the upper electrode and the surface of the charge. This furnace, 
 which somewhat resembles the Keller furnace, has been used 
 largely by the Societe Anonyme Electrome'tallurgique Precedes, 
 Paul Girod for the manufacture of ferro-alloys. Its chief 
 advantage is simplicity and ease of operation. 
 
 Stassano's Furnace. In the Stassano furnace, heating is 
 obtained entirely from arcs above the surface of the charge, so 
 that no solid impurities are introduced. A single arc may be 
 used, or, for convenience in regulating the temperature, two or 
 more arcs may be used in parallel, and the number varied 
 according to requirements. In the furnace illustrated in figs. 
 103 and 104 there are three carbons which nearly meet at the 
 axis of the furnace, and as these are supplied with three-phase 
 current, there are three arcs, one between each pair of carbons, 
 taking the carbons two and two. The holders for the carbons 
 are water-jacketed and are adjusted hydraulically, the holders 
 being attached to pistons working in hydraulic cylinders for this 
 purpose. The axis of the furnace is inclined to the vertical 
 through an angle of about 7, and the furnace is slowly rotated 
 when in operation so as to mix the charge and thus render the 
 resulting metal quite uniform. The gearing for this purpose is 
 seen below the furnace. On account of this rotation the current 
 cannot be led in as simply as in other furnaces ; rings and brushes 
 are provided to give the necessary sliding contacts. A special 
 feature of this furnace is the exclusion of air; the furnace 
 terminates in a tube which is connected by a special form of cup- 
 joint to a fixed tube, and the latter terminates in an hydraulic 
 valve. Volatile products are thus expelled and may be collected. 
 
 The advantages claimed for the Stassano furnace are: (1) the 
 furnace being air-tight, a neutral atmosphere is obtained, and the 
 arcs are very stable ; (2) the furnace can be worked with a 
 minimum of slag ; and (3) thorough mechanical mixing. Against 
 these advantages must be set the disadvantage that the furnace 
 is comparatively complicated, and that the lining works under 
 more severe conditions than in furnaces of the combined arc and 
 resistance type. 
 
310 
 
 EXTRACTION AND REFINING PROCESSES. 
 
 FIG. 103, The Stassano furnace, Vertical section. 
 
ELECTRIC PRODUCTION AND REFINING OF IRON. 
 
 311 
 
 Kjellin's Furnace. The Kjellin furnace was the first com- 
 mercially of the class depending upon electromagnetic induction 
 for the production of the heat, though such furnaces had been 
 suggested by Ferranti some years before. If a closed metallic 
 circuit is suitably placed in a varying magnetic field it will be 
 traversed by electric currents depending upon the rate of variation 
 and the strength of the field. Thus, if the metal in a furnace is 
 disposed in a trough forming a ring, and a magnetic field through 
 this ring is varied rapidly, currents will flow through the metal, 
 
 FIG. 104. The Stassano furnace. Horizontal section. 
 
 and under suitable conditions they will be sufficiently intense to 
 cause fusion. By this means the use of electrodes can be entirely 
 avoided, and thus impurities can be eliminated. The method is 
 unsuitable for the smelting of ores, because these would not be 
 sufficiently conducting ; but for the production of alloys from 
 constituents of known purity the method is very advantageous, 
 there being no question as to the composition of the product. 
 Even when such a furnace is charged with broken pieces of 
 pig-iron or scrap- the resistance is too high for starting, and 
 therefore it is necessary to use a continuous ring of iron for this 
 purpose. In ordinary working it is usual to leave sufficient metal 
 in the furnace to avoid any difficulty in starting up. 
 
312 
 
 EXTRACTION AND REFINING PROCESSES. 
 
 The construction of the Kjellin furnace, in which this principle 
 is utilised, is shown in fig. 105. The charge is contained in the 
 annular groove A A. D is a coil of insulated copper wire, and 
 
 Yertical section. 
 
 Plan. 
 Yin. 105. The Kjellin furnace. 
 
 CC consists of sheets of soft iron to form a magnetic circuit. 
 Thus, D forms what is termed the primary of an electric trans- 
 former, A A forming the secondary ; and when alternating current 
 is supplied to D, a much larger alternating current, though at 
 lower pressure, is induced in the metal in A A. 
 
ELECTRIC PRODUCTION AND REFINING OF IRON. 313 
 
 The furnace is made of an iron casing resting on brickwork. 
 The casing is filled in with firebrick except the part in the im- 
 mediate neighbourhood of the channel A A, this part being made 
 of magnesite or silica brick, according as a basic or acid lining is 
 required. The primary is cooled by an air draft, or, if necessary, 
 by water circulation in the space surrounding it. 
 
 Rb'chling and Rodenhauser's Furnace. Although the Kjellin 
 furnace is very suitable as a refining or crucible furnace in 
 comparatively small sizes, yet if the size is much increased it 
 becomes somewhat inefficient from the electrical point of view. 
 If, for example, such a furnace is supplied with 200 amperes at 
 500 volts it might be supposed that the power so supplied would 
 be 200 x 500 watts, or 100 kilowatts. But, since the current is 
 alternating, the power is really less, and consequently the furnace 
 is said to have a 'power factor' less than unity. The power 
 factor is merely the ratio of the actual power to the apparent 
 power. For example, if the power absorbed in the above case is 
 only 80 kilowatts, whereas apparently the power is 100 kilowatts, 
 then the power factor is 0'8. In large induction furnaces it 
 might be only 0'5. All alternating current furnaces have a 
 power factor less than unity, but the induction furnace is inferior 
 to others in respect to this quantity. The objection to a low 
 power factor is that for a given pressure the current has to be 
 correspondingly larger than if the power factor were unity, and 
 consequently the generating plant must be larger than would be 
 otherwise necessary (see page 35). 
 
 This difficulty has been overcome to a considerable extent in 
 the modified form of Kjellin's furnace due to Rochling and 
 Rodenhauser. In this form the transformer not merely induces 
 current in the molten metal but also in a secondary winding 
 from which current is taken and is used for resistance-heating, 
 so that this type of furnace is often called the ' combination type,' 
 which is thus referred to in Class 5. 
 
 Such a furnace is shown in figs. 106 and 107. Unlike the 
 Kjellin furnace, the iron core H (fig. 107) has a coil, A, on each 
 of the vertical legs, and outside these coils are the additional 
 secondary coils, as seen at B. The iron core is seen in section in 
 fig. 106, passing over the top and also beneath the furnace, and 
 the masonry is built round the core and coils with a small space 
 between. The channel for the molten metal passes round each 
 leg of the core, as indicated by the small arrows on each side of 
 fig. 107, and these two channels unite into a single channel, D, 
 between the legs, thus forming roughly a figure 8. It will be 
 noticed that the channels on the outside are narrow, and in the 
 centre they combine to form a wide channel ; in this channel of 
 molten metal currents are induced, as in the Kjellin furnace. 
 This action is also assisted by the secondary coils, which are con- 
 nected up to four corrugated steel plates ; two of these are seen 
 
314 
 
 EXTRACTION AND REFINING PROCESSES. 
 
 at E, and there are two others indicated at the other side of the 
 furnace. These plates are built into the furnace lining with a 
 
 FIG. 106. Vertical section of Rochling-Rodenhauser furnace. 
 
 FIG. 107. Sectional plan of Rochling-Rodenhauser furnace. 
 
 compound of magnesite, dolomite, and tar. When the furnace 
 is cold this material does not conduct, and therefore, in starting 
 
Electro- Metallurgy.'] 
 
 [To face p. 314. 
 
ELECTRIC PRODUCTION AND REFINING OF IRON. 315 
 
 up, these plates do not help the heating, which is thus purely 
 inductive. But when the furnace heats up, the compound con- 
 ducts electrolytically as a conductor of the 'second class,' and 
 currents flow between the plates on opposite sides of the furnace, 
 as indicated by the long arrows. The lower part of fig. 106 
 shows the arrangement for tilting the furnace. 
 
 A photograph of such a furnace being tilted to discharge the 
 molten metal is reproduced in fig. 108. Here the top of the 
 iron core traversing the furnace is clearly seen, and also the 
 curved strips connecting the secondary winding to the plates at 
 either end. 
 
 It is stated that this furnace has a further advantage over the 
 purely induction type in that a higher temperature is obtained, 
 and this is important in enabling the slag to be maintained 
 sufficiently liquid for efficient refining purposes. 
 
 ELECTRIC WELDING AND ANNEALING. 
 
 Electric Welding. Of late years the high temperatures obtain- 
 able by the electric current have been used very successfully for 
 welding together iron, steel and copper surfaces. There may be 
 said to be three principal processes : 
 
 (1) That in which the metal is heated, not by an arc but by 
 incandescence, as in the Elihu Thomson process, in which the two 
 pieces to be united are connected by heavy clamps with the opposite 
 poles of a generator and are then brought into contact. As, 
 owing to the unevenness of the surface, contact occurs only at a 
 few points, the current (which must be equal to about 10,000 
 amperes per square inch of sectional area of the iron near the joint) 
 is conducted through a restricted area, with the result that these 
 portions are raised to incandescence, and, becoming softened, allow 
 the two pieces to be pressed together somewhat ; hence a larger 
 area of conductor is provided, but this, being still far less than 
 that of the bar, is still raised to welding heat. Meanwhile 
 surrounding surfaces become superficially oxidised by heat, and 
 so covered with a film of an inferior conductor, with the result 
 that even when these are pressed together the whole mass is raised 
 to the necessary temperature owing to the added resistance in the 
 path of the current. The effect is increased by the fact that the 
 resistance of the iron increases rapidly with rise of temperature. 
 In a space of time varying from a few seconds to two or three 
 minutes, according to the size of the joint, the metal will have 
 been heated sufficiently to allow of the two surfaces being united 
 by pressure. A very low E.M.F., produced by an alternating 
 current transformer, is used, and it is found that to join two 
 1-inch bars end to end requires the expenditure of about 22 
 H.P. for one minute. 
 
 (2) In a process of which Burton's liquid forge and Lagrange and 
 
316 EXTRACTION AND REFINING PROCESSES. 
 
 Hoho's systems are types, the metal to be heated is connected with 
 the negative pole of the generator (which must give a current of 
 over 110 volts) and is then plunged into a lead-lined vat containing 
 dilute sulphuric acid, or of a suitable saline solution (e.g., sodium 
 carbonate), the lead being connected with the positive pole. The 
 result is that a very powerful current passes through the bath 
 from the lead which acts as anode, to the immersed object, on 
 which hydrogen is generated in large quantities. The resistance 
 at the surface, in part covered as it is with hydrogen, becomes so 
 great that the metal is very rapidly raised to a welding heat ; it 
 may then be removed and welded on the anvil in the usual way. 
 In Burton's process the piece is only in part immersed, so that the 
 evolved hydrogen burns and contributes to the heating of the bar. 
 It is obvious that these processes, rather than Thomson's, are the 
 equivalent of the forge, and they may be employed for heating 
 metal otherwise than for welding for example, for bending or for 
 annealing. But it is questionable whether there are many cases 
 in which they would prove more economical than ordinary forge- 
 heating. It must be remembered, however, that the metal is 
 perfectly clean, being protected from oxidation by the hydrogen 
 in contact with it whilst in the bath. 
 
 (3) That in which the electric arc is used. This may be done, 
 as in the Benardos process, by placing together the surfaces to be 
 welded and connecting both with the positive pole of a generator 
 giving a current at 110-120 volts, and from 200 to 300 amperes 
 or more (200 amperes for joining metal | inch thick), while the 
 negative pole is connected to a rod of carbon which may be held 
 by insulated tongs, and brought into contact with the surfaces. 
 The moment contact is made the current passes, and the carbon 
 is withdrawn, so that an arc is formed, which is commonly 2 or 2J 
 inches in length. The metal in the immediate neighbourhood 
 of the arc is rapidly melted, and the surfaces are thus burned 
 together (as in the autogenous soldering of lead) ; the carbon is 
 then shifted along the joint until the next part is similarly 
 united, and so on, the metal being hammered where possible, 
 in order to improve its quality, but in any case some hammer- 
 ing is given by means of a 'former' adapted to the joint in 
 hand. In the Zerener process, an ingenious adaptation of the 
 deflection of the arc by magnetic action is used : two carbons 
 form the electrodes, and a very powerful electro-magnet, actuated 
 by the current used in welding, is placed with its two poles so 
 arranged that they would be on either side of the arc that 
 forms between the carbons. The result is that the arc is de- 
 flected downwards, and may thus be made to play upon the 
 surfaces to be joined. The whole arrangement forms a kind 
 of electric blow-pipe. A current at 60-65 volts pressure and 
 from 3 to 250 amperes may be used, according to the character 
 of the work. 
 
ELECTKIC WELDING AND ANNEALING. 317 
 
 Electric Annealing or Softening. In the use of hardened 
 steel plate it is frequently necessary to make additional rivet 
 holes, which cannot ordinarily be done without softening the plate 
 and re-hardening. The Thomson Welding Company of America 
 have adapted an electrical heating process for drawing the temper 
 of such plates locally. This is accomplished by resting a couple 
 of copper bars on the same surface of the plate on either side of 
 the part to be softened, and passing through them (and so through 
 the stretch of plate between them) a current of about 3500 
 amperes at 4 volts. Gradually the current is increased to 6000 
 amperes. The steel is thus rapidly raised to a dull red heat 
 (sufficient for the purpose), and the current is then gradually re- 
 duced again, so that the steel may cool very slowly until it is 
 below about 1100F., or else it may chill by contact with the 
 mass of cold metal around it, and so become hardened again. 
 
 Small furnaces are now coming into use, consisting of baths of 
 metallic salts maintained in the fused condition by passing through 
 the salt an alternating electric current. Such furnaces are found 
 to be very useful for hardening tools, etc., as a very uniform tem- 
 perature can be maintained. 
 
CHAPTER XVI. 
 
 THE RECOVERY OF CERTAIN METALS FROM THEIR SOLUTIONS 
 OR FROM WASTE SUBSTANCES. 
 
 WHEN a solution has become spent, and is too highly charged 
 with impurities to be any longer serviceable for electrolytic 
 work, it is desirable so to treat the liquid that the valuable 
 metal is recovered in a form suitable for re-application to the 
 plating work. Such methods as are applicable to the more usual 
 solutions are briefly sketched in this chapter; but it must be 
 understood that they are merely general methods, and modifica- 
 tions of baths by the addition of fresh substances may render 
 recovery by these means incomplete or perhaps impossible ; and 
 in many cases the metal may not be entirely regained, even 
 under ordinary circumstances, the amount that is left depending 
 upon the solubility of the precipitated compound in the solution 
 from which it is to be separated. 
 
 COBALT. 
 
 The solution should be boiled down until fairly concentrated ; 
 if acid (reddens blue litmus-paper), sodium carbonate should be 
 added until it is neutral, or nearly so, but it must still remain 
 clear. The addition now of finely-divided barium carbonate, 
 stirred up in water, will cause the separation of all the iron as per- 
 oxide ; this is allowed to subside, and the clear liquid containing 
 nearly the whole of the cobalt is poured off. This liquid must be 
 neutral or slightly acidified with acetic acid, and to it must be 
 added an excess of strong solution of potassium nitrite mixed 
 with sufficient acetic acid to prevent it from turning red 
 litmus-paper blue ; the whole should be left in a warm place for 
 one or two days. The precipitated double nitrite of cobalt 
 and potassium is then separated from the supernatant liquid, and 
 dissolved in hydrochloric acid; the addition of caustic alkali 
 (potash or soda, not ammonia) to this solution brings down the 
 cobalt practically pure as hydrated oxide, which may be filtered 
 off and dissolved in any acid, the cobalt salt of which it is 
 desired to use. From a simple solution of cobalt, free from iron 
 
 318 
 
GOLD. 319 
 
 and other metals, the pure cobalt may be at once precipitated as 
 hydrated oxide by caustic alkali ; but-the solution must be boiled, 
 if necessary, until all odour of ammonia has vanished, as the 
 presence of this body interferes with complete precipitation. 
 
 COPPER. 
 
 From acid solutions, fragments of metallic iron suspended in 
 the solution will precipitate all the copper in the metallic state, 
 by simple exchange. Subsequently the copper may be dissolved 
 in warm dilute sulphuric acid containing a small proportion of 
 nitric acid, more of the latter being added by degrees as the 
 action is observed to become less ; and in this way it will 
 become converted into copper sulphate, which may be afterwards 
 separated in the form of pure crystals by slow evaporation. The 
 precipitation by iron may be effected by single-cell deposition, as 
 explained in an earlier chapter, by connecting the fragments of 
 iron contained in sulphuric acid within a porous cell, with copper 
 strips placed in the copper solution. This will cause the precipi- 
 tated metal to deposit upon the copper instead of upon the iron, 
 so that there is no risk of introducing impurities into the spongy 
 copper. 
 
 GOLD. 
 
 From the cyanide solution the gold may be recovered as 
 follows : Hydrochloric acid is first added in excess, until no 
 further precipitate of gold cyanide is produced ; the precipitate 
 is allowed to subside, and is washed twice by pouring water upon 
 it, allowing it to settle, and then pouring away and renewing the 
 water again ; then, after filtering, it is dried and fused with an 
 excess of dry sodium carbonate in a clay crucible. But this 
 process is not to be recommended on account of the intensely 
 poisonous nature of the hydrocyanic acid gas, so abundantly 
 evolved during the first treatment with hydrochloric acid ; but if 
 it be adopted, this portion of the process must be conducted in a 
 special draught-cupboard, or in the open air with the operator 
 well to the windward of the vessel. 
 
 Bottger's method is preferable ; he evaporates the whole solu- 
 tion to dryness in a porcelain or enamelled dish, placed over a 
 saucepan containing boiling water. The residue is then to be 
 crushed in a mortar and mixed with an equal weight of lead 
 oxide (litharge) and a thirtieth part of its weight of charcoal 
 powder introduced into a fire-clay crucible and heated to bright 
 redness in a small pot-furnace ; a portion of the lead is reduced 
 to the metallic state, and with it will be alloyed all the gold 
 contained in the charge. This alloy is then boiled with nitric 
 acid, in which the lead will dissolve together with any silver or 
 copper that may be present ; the gold is left in a finely divided 
 
320 EECOVERY OF CERTAIN METALS FROM THEIR SOLUTIONS. 
 
 condition, and after washing may be fused into a single homo- 
 geneous mass, or it may be re-dissolved at once to form a fresh 
 gold-bath. 
 
 A plate of zinc attached by wire to one of gold, and placed in 
 the original gold-bath, gradually deposits the precious metal 
 upon the gold strip, forming, in fact, a single-cell arrangement, 
 which may be used for recovering the gold directly from the 
 spent solution ; but the operation occupies a long time, and other 
 metals are liable to be precipitated subsequently. These latter, 
 however, may be partly separated by boiling with nitric acid, 
 in which gold is insoluble (the separation can never be absolute, 
 because portions of them are locked up within the gold so as to 
 be out of the reach of the acid solvent). 
 
 LEAD. 
 
 From the solution of the oxide in potash, insoluble lead 
 carbonate is produced by bubbling carbonic acid gas through it ; 
 this gas is evolved by the action of hydrochloric (muriatic) acid 
 upon chalk or marble contained in a separate vessel which must 
 be closed with a cork through which a tube is passed to conduct 
 the gas to the required spot. The addition of acetic acid to the 
 liquid until it is nearly neutral, followed by the introduction of 
 sodium bicarbonate, will produce the same effect. From the 
 lead acetate solution, sodium carbonate precipitates lead car- 
 bonate. This latter substance, however produced, may, after 
 washing, be dissolved in acetic acid, or in any other solvent 
 suited to the particular form of bath which is to be prepared. 
 
 MERCURY. 
 
 This metal is used largely in amalgamating the zinc plates of 
 the galvanic battery, from the residue of which it may sub- 
 sequently be recovered by distillation. To obtain it pure, it 
 should be distilled in vacua, or at least under reduced pressure ; 
 but to recover it fairly clean, in a condition suitable for applica- 
 tion once more to amalgamation, the fragments of zinc may be 
 introduced into an iron retort, A (fig. 109), with a tube passing 
 air-tight through one neck of a Woulffe's bottle, B, into water ; 
 the other neck of the bottle is connected with a second tube, 
 also dipping beneath water in a second vessel, d, as a safeguard 
 against the escape of mercury. On applying heat to the retort 
 by a suitable fire the mercury slowly vaporises, and distilling 
 over, should be entirely condensed in B, any escaping this being 
 collected in d. The residue in the retort still contains a little 
 mercury, together with the bulk of the lead and copper originally 
 present in the whole zinc plate, for being more electro-negative 
 than the zinc, the latter dissolves in preference, and leaves the 
 
NICKEL. 
 
 321 
 
 other metals to accumulate by the amalgamating effect of the 
 mercury. Care should be taken to remove the tube dipping into 
 the water in the Woulffe's bottle before removing the flame from 
 the retort, otherwise water may pass back into the latter. 
 
 NICKEL. 
 
 From solutions of the double sulphate of nickel and ammonia 
 the green double salt itself is precipitated in a practically pure 
 condition in the form of minute crystals by the addition of succes- 
 sive quantities of a saturated solution of ammonium sulphate, 
 until on standing, and after complete subsidence of the precipi- 
 tate, the solution is quite decolorised. Advantage is thus taken 
 of the insolubility of the double salt in strong solutions of am- 
 monium sulphate, a fact which was first observed by Unwin. 
 The crystals have only to be filtered, drained free from the liquid 
 
 FIG. 109. Mercury distillation apparatus. 
 
 clinging to them, and washed once or twice with a strong solu- 
 tion of ammonium sulphate, and they are ready to form a new 
 bath. The original liquid may with advantage be boiled down to 
 half its bulk, or even less, before applying this treatment. 
 
 PLATINUM. 
 
 By adding to the bath an excess of a ferrous sulphate solution, 
 and then a caustic alkali, the resulting dark-green coloured 
 precipitate of hydrated ferrous oxide (FeO) will reduce the 
 platinum to the metallic state, and itself become converted to a 
 proportionate extent into ferric oxide (Fe 2 3 ). This precipitation 
 should be effected in a covered or corked flask, which should then 
 be allowed to stand for some hours in a warm place, with occa- 
 sional agitation. On the addition subsequently of an excess of 
 hydrochloric acid, the iron oxides will re-dissolve, but the plati- 
 num powder precipitated with them will remain untouched. 
 This finely-divided platinum may then be filtered, washed, and 
 re-dissolved in aqua regia for further use. 
 
 21 
 
322 EECOVERY OF CERTAIN METALS FROM THEIR SOLUTIONS. 
 
 SILVER. 
 
 As in the case of gold solutions, the addition of hydrochloric 
 acid precipitates silver cyanide ; but the use of an excess of the 
 acid effects its further conversion into silver chloride. If this 
 precipitate be red in colour the presence of copper cyanide is in- 
 dicated, which may, however, be effectually removed by boiling 
 it with moderately concentrated hydrochloric acid. The washed 
 silver chloride should then be dried, mixed with soda ash or 
 dried sodium carbonate, and fused in a clay crucible. The 
 acidifying process has the same objections on the score of danger 
 to health, and, therefore, requires the same precautions, as the 
 similar method above described for the treatment of gold solutions. 
 
 A second method, allied to the dry process for recovering gold, 
 is also applicable to silver residues. The solution is evaporated 
 to dryness, and the residue is transferred to a crucible, but alone, 
 not mixed with litharge, and fused at a bright red heat. If 
 sufficiently heated, the silver compound, mixed with excess of 
 potassium cyanide and carbonate, will be broken up, and the 
 liquid silver will melt and sink through the fluid slag to the 
 bottom of the pot. When effervescence has ceased, the crucible 
 is removed from the fire, and its contents poured into a hemi- 
 spherical cast-iron ingot-mould, from which the solidified mass is 
 detached when cold, and broken by a light blow with a hammer, 
 to separate the metal from the slag. The crucible may be used 
 again and again, provided that an examination after each opera- 
 tion reveals no sign of a crack. The fluid contents of the pots 
 may, of course, be allowed to cool in situ, by setting the crucible 
 aside on a level surface ; but to extract the silver button in this 
 case involves the loss of the pot by fracture. If the heat is 
 insufficient, the silver will remain as a grey spongy mass, from 
 which the slag may be removed by boiling with water containing 
 a small proportion of hydrochloric acid. 
 
 The silver should be practically 'fine'; but to ensure the 
 perfect purity of the product, it may be re-fused with carbonate 
 of soda mixed with about one-tenth 'of its volume of nitre, which 
 will attack the base metals (but not the silver), and cause them 
 to pass into the slag ; it will not affect gold or platinum, which 
 are scarcely likely, however, to be present. If small quantities 
 of these metals should by any chance become mixed with the 
 silver, the button must be boiled with pure nitric acid (free 
 from hydrochloric acid), which dissolves silver (and even platinum 
 when present in small proportion with much silver), but leaves 
 the gold as a dark-brown or black powder. But if more than 
 40 per cent, of gold be present, a certain proportion of silver 
 (increasing with the percentage of gold) will be left with the 
 latter. The silver solution is then filtered from the residue of 
 gold, and is mixed with an excess of hydrochloric acid. In this 
 
SILVER. 323 
 
 way the insoluble silver chloride is formed, which must be 
 filtered, washed, dried, mixed with an equal bulk of sodium 
 carbonate and a little charcoal, and fused at a red heat. Pure 
 silver will thus result, the gold having been left undissolved, 
 while the platinum which had passed into the solution would not 
 be precipitated by the hydrochloric acid, and must, therefore, 
 remain to be extracted from the liquid filtered from the silver 
 chloride precipitate. 
 
CHAPTER XVII. 
 
 THE DETERMINATION OF THE PROPORTION OF METAL IN CERTAIN 
 DEPOSITING SOLUTIONS. 
 
 IN this chapter it is proposed to give outlines of methods by 
 which the depositing solutions of the more important metals in 
 electro-metallurgical use may be assayed, to give at least an 
 
 FIG. 110. Balance. 
 
 approximate idea of the quantities contained. For a full ex- 
 planation of this subject, or for methods of dealing with compli- 
 cated analytical problems, works on quantitative analysis must 
 be consulted, as it is impossible and undesirable to treat of so 
 wide a subject in detail within the limits of this book. 
 
 For this purpose, a fairly delicate balance is essential. It 
 should be enclosed within a glass case, as in fig. 110, to protect 
 it from dust and corrosive fumes, and should be capable of indi- 
 cating a weight of T ^-Q of a grain. For electrolytic analysis, a 
 
 324 
 
ANTIMONY. 325 
 
 platinum dish, about three-quarters of an inch to an inch in 
 height, and about 3 ins. in diameter, forms a convenient cathode, 
 at once holding the solution and receiving the deposited metal. 
 The anode consists of a circular plate of stout platinum foil about 
 2J ins. in diameter, with several perforations to allow gas to 
 escape from beneath it. The platinum sheet is fastened horizon- 
 tally without solder to the end of a vertical platinum wire attached 
 to the positive pole of the battery, the platinum dish making con- 
 tact externally with a copper wire attached to the negative pole. 
 Instead of this, a cylinder of platinum foil may be used as cathode, 
 being suspended, with its main axis vertical, within a small 
 beaker, the anode consisting of a coil of platinum wire placed 
 within the cathode. The object of the electrolytic method is to 
 continue the action of the current until every trace of the required 
 metal is precipitated on the platinum cathode ; and, as the latter 
 should have been weighed previously, the increase of weight 
 shown after the deposition gives the number of grains of the 
 metal in the quantity of solution taken. The platinum dish 
 should be lightly covered with the two halves of a broken clock 
 glass, 4 ins. in width that no splashings be lost these glasses 
 should be rinsed into the solution about a couple of hours before 
 stopping the current. After deposition, the metal must be washed 
 well with water, then rinsed with alcohol, and dried rapidly by 
 heating to the temperature of boiling water. 
 
 ANTIMONY. 
 
 For the sulphide solution, measure out very accurately half 
 a fluid-ounce of the liquid, transfer it to the weighed platinum 
 dish ; dilute it with from 1 to 2 oz. of water ; introduce the 
 platinum anode, and pass a current from two Bunsen- cells 
 through the liquid, until a drop of the solution, removed by a 
 glass rod and placed upon a watch-glass, gives no yellow-coloured 
 precipitate (but only white), when mixed with a few drops 
 of hydrochloric acid. Electrolytic experiments of this nature 
 may be conveniently started in the evening, and will be found 
 to be finished on the following morning. When completed, the 
 current is stopped, the anode is removed, the contents of the 
 dish poured into a beaker ; the dish rinsed four or five times 
 with pure water, then with alcohol, and finally with a few drops 
 of ether ; it is, lastly, deposited in a warm place for a few 
 minutes until the ether has quite evaporated, and is then re- 
 weighed. The increase in weight shows the number of grains 
 of metallic antimony in half an ounce of the original solution. 
 
 For the tartar emetic solutions, half an ounce should be taken 
 and mixed with 1 ounce of yellow ammonium sulphide and a 
 like volume of water. It is then electrolysed as before. 
 
326 DETERMINATION OF METALS IN DEPOSITING SOLUTIONS. 
 
 COBALT. 
 
 This is a difficult assay for untrained hands and, indeed, 
 even the simplest methods of dealing with any metal can scarcely 
 be expected to give more than approximate results unless the 
 operator has had some previous training. Probably the best 
 method will be to measure half an ounce of the solution and pro- 
 ceed to obtain the cobalt as the cobalt-potassium nitrite, after 
 the manner described in the last chapter (p. 318). The experi- 
 ment must, of course, be conducted with great care ; the solutions 
 should be maintained fairly concentrated, and a considerable 
 excess of the potassium nitrite must be used for the precipitation, 
 because the precipitate is less soluble in a liquid containing this 
 salt than in pure water, so that the time required for it to rest 
 in a warm place is shortened usually one night suffices. The 
 precipitate may then be treated as described, and the second 
 precipitate of hydrated cobalt oxide may be filtered through 
 blotting-paper (see p. 52), washed well, dried on the paper, 
 brushed off from the filter, by means of a camel's-hair brush, 
 into a small weighed porcelain crucible (about an inch in height), 
 heated to dull redness in a smokeless Bunsen-burner or spirit 
 lamp, cooled and weighed in the crucible. The excess of weight 
 over that of the clean crucible gives the weight of the precipitate, 
 which is an oxide of cobalt having the formula Co 3 4 . Multi- 
 plying this weight by 0*73 gives the weight of metallic cobalt in 
 the half fluid-ounce of solution taken. If preferred, the nitrite 
 precipitate may be dissolved in a little hydrochloric acid, the 
 solution rendered neutral with ammonia, and a good excess of 
 ammonium oxalate added to produce a clear solution of the 
 double cobalt-ammonium oxalate, which may then be electrolysed 
 in the platinum dish, and the deposited cobalt washed, dried, and 
 weighed as above described for antimony. The solution should 
 be kept warm during electrolysis by resting the dish upon a 
 saucepan of water heated over a gentle flame. The electrolysis 
 must be continued until a drop of the solution gives no black 
 precipitate, or even brown colour, when mixed with a drop of 
 ammonium sulphide in a watch-glass. 
 
 COPPER. 
 
 The simple acid copper solution lends itself so readily to the 
 electrolytic assay, that this would certainly seem to be the most 
 suitable. It is possible to separate every trace of copper from 
 such a solution, so that the method may be made to give 
 absolutely accurate results. The cyanide solution should be 
 acidified with sulphuric acid and boiled until there is no longer 
 any smell of prussic acid, resembling that of bitter almonds. 
 Both these operations must be conducted cautiously and in a 
 
GOLD. 327 
 
 well-ventilated place, on account of the poisonous character of 
 the evolved gas. The latter requires all the more careful hand- 
 ling, because some persons cannot detect it readily by the smell, 
 but only by a sensation in the throat. The liquid is then ready 
 for electrolysis. Half an ounce of either solution may be employed, 
 and electrolysis is continued until the liquid is decolorised, and 
 a drop removed from it strikes no blue colour with an excess of 
 ammonia. The excess- weight of the platinum dish is that of 
 metallic copper in the volume of solution taken. 
 
 GOLD. 
 
 One fluid-ounce of the solution is boiled with an excess of 
 hydrochloric acid in a well-ventilated spot until there is no 
 further smell of hydrocyanic acid ; an excess of a clear solution 
 of ferrous sulphate is then added, and the mixture is allowed to 
 stand all night in a warm place. The precipitated gold powder 
 is then filtered from the solution, which should now be quite 
 free from precious metal. It is washed many times on the filter 
 with pure water, and, after drying, is placed with the filter-paper 
 in a small weighed porcelain crucible, and heated over a spirit- 
 lamp or a non-luminous gas-flame, until the paper is completely 
 burnt to a white ash. The crucible is re-weighed with its con- 
 tents, on cooling, and thus the weight of metallic gold in one 
 fluid-ounce of the liquid is determined. The precipitate is pure 
 gold, so that no further calculation is necessary. 
 
 LEAD. 
 
 Add to half an ounce of the solution 3 ounces of pure water, an 
 excess of sulphuric acid (until a further addition no longer pro- 
 duces a white precipitate) and an equal volume of spirits of wine. 
 Allow the mixture to stand for a few hours ; filter off the white 
 lead sulphate precipitate, and wash the precipitate on the filter 
 several times with a mixture of equal volumes of spirits of wine 
 and water. This washing must be very thorough, or the trace of 
 residual acid left in the precipitate will char or weaken the paper 
 when it is dried. A drop from the last washing but one should 
 not redden blue litmus-paper. The paper and contents are dried, 
 the heavy white powder is brushed into a weighed porcelain 
 crucible, heated over a non-luminous flame, cooled and re-weighed. 
 The increase of weight multiplied by 0'683 gives the weight of 
 metallic lead in the sample taken. 
 
 NICKEL. 
 
 To half an ounce of the solution add a fair excess of ammonium 
 sulphate, together with about 4 oz. of water, and then oxalic 
 acid. If this should produce a precipitate, more ammonium sul- 
 
328 DETERMINATION OF METALS IN DEPOSITING SOLUTIONS. 
 
 phate must be added until the solution is clear. Excess of am- 
 monia is now introduced, any precipitate is filtered off and washed 
 with water containing ammonia, the washings being added to the 
 filtrate. The filtered solution and washings are then mixed with 
 a further small portion of ammonium oxalate, evaporated to con- 
 venient bulk in a porcelain dish, and electrolysed for metallic 
 nickel as above described. 
 
 PLATINUM. 
 
 To half an ounce of the solution add an excess of a ferrous sul- 
 phate solution, and then excess of caustic potash. The mixture 
 is kept in a warm place for an hour or two with occasional stir- 
 ring ; an excess of hydrochloric acid is now added, so that the 
 liquid reddens blue litmus-paper, and the precipitated metallic 
 platinum, which remains undissolved, is filtered and burned with 
 the paper after the manner described under the heading of Gold 
 in this chapter. The final precipitate should be pure metallic 
 platinum. 
 
 SILVER. 
 
 The solutions may be treated electrolytically, using the cur- 
 rent from one Bunsen-cell only, as a stronger current tends to 
 deposit pulverulent or flaky metal which peels off during the pro- 
 cess of deposition. The silver is, of course, deposited and weighed 
 in the metallic state. 
 
CHAPTER XVIII. 
 
 POWER REQUIRED FOR ELECTROLYTIC WORK. 
 
 Calculation of Power required for Electrolysis. It has been 
 shown in Chapter II. that the power expressed in watts absorbed 
 in electrical work is found by multiplying the strength of current 
 used, expressed in amperes, by its pressure in volts. Hence the 
 power absorbed in any electrolytic vat may be readily deter- 
 mined, provided the current conditions are known. Thus, suppos- 
 ing a P.D. of 0*3 volt is necessary to force an electric current 
 through a certain copper electrotype bath, with a current-density 
 of 10 amperes per sq. ft. of cathode surface, an expenditure of 
 10 x 0'3 or 3 watts will be required per sq. ft. of surface, and if 
 the total area of the cathode were 249 sq. ft. there would be 
 needed 3x249. or 747 watts, which is practically 1 H.P. In 
 other words, in such a copper bath as that described, 1 H.P. will 
 be absorbed in the vat itself in the deposition of ten times the 
 electro-chemical equivalent of copper per second on each of the 
 249 sq. ft. of cathode surfaces used. So that 
 
 H.P. required per unit area of cathode surface = '- - 
 
 where P.D. is the potential difference expressed in volts between 
 the anode and cathode, and C.D. is the current density expressed 
 as the number of amperes employed per unit area of cathode. 
 
 Using this formula for an alkaline copper bath, in which a 
 P.D. of 4 volts is driving a current of 0*025 amperes per sq. in. 
 of cathode surface through the bath, 
 
 the H. P. required per sq. in. of surface would be -^- ; 
 
 but, as the square inch is a very small unit for calculations of 
 this magnitude, the equivalent in amperes per sq. ft. may be 
 more conveniently taken (see Table XXIX., 'p. 391) ; 
 
 the H.P. required per sq. ft. of surface would then be i^A?=li!l , 
 
 746 746 
 
 which is approximately 0'02 H.P. ; but as the H.P. required per 
 sq. ft. of the acid copper bath is ^49 H.P. ( = 0'004 H.P.), it is 
 
 329 
 
330 POWER REQUIRED FOR ELECTROLYTIC WORK. 
 
 evident that the alkaline bath absorbs almost exactly five times 
 as much power as the acid bath. 1 
 
 In the same way it may be shown that in nickel baths using a 
 P.D. of 2 volts and a C.D. of 3 amperes per sq. ft., 
 
 9 v q a 
 
 the H.P required per sq. ft. of cathode is ^=-1. , 
 
 746 746 
 
 or about 0*008 H.P., which is approximately double that used in 
 the acid copper bath. 
 
 In silver baths with a P.D. of 0'6 volt and a C.D. of 3 amperes 
 per sq. ft., 
 
 the H.P. per sq. ft. of cathode is 2^1^ = ^, or about 0'0024 H.P. 
 
 746 746 
 
 In all these calculations only the actual power required for the 
 bath has been taken into account. The power required from the 
 generator will be greater, inasmuch as the loss of power in over- 
 coming the resistance of the copper cables and wires used to con- 
 duct the current to the baths and that of the various joints and 
 electrical connections in the circuit have been neglected. But 
 with a good arrangement of the plant, so that stout copper leads 
 are used and the baths are in close proximity to the generators, 
 and all joints and connections are thoroughly clean, this loss may 
 be practically negligible. Where, however, a dynamo is used, the 
 conversion of mechanical into electrical energy absorbs an amount 
 of power varying with the size and construction of the dynamo. 
 As a rule the efficiency of the small dynamos used in electro- 
 plating is less than that of the larger machines employed in the 
 heavier work of smelting or refining. If the efficiency be taken 
 in round numbers at 85 per cent., it is obvious that the H.P. re- 
 quired from the engine driving the dynamo will have to be in 
 excess of that required for the vat itself (neglecting resistance of 
 leads) in the proportion of 100 : 85, or, in other words, the H.P. 
 of the engine will have to be lij- times as great as that found by 
 the calculations indicated above. 
 
 Board of Trade Units required for Electrolysis. Electricity 
 is charged for by supply authorities by the Board of Trade unit, 
 which, as already stated, is one kilowatt-hour or a power of 1000 
 watts passing for one hour. Since the horse-power hour is equal 
 to 746 watt-hours, it is evident that the number of Board of Trade 
 
 1 It is obviously unnecessary to work out these calculations to obtain 
 simply a comparison of the power required to work different baths : it suffices 
 to compare the respective products of P.D. xC.D. Thus the H.P. required 
 for the acid is to that for the alkaline bath in the ratio 0'3 x 10 : 4 x 37 = 
 3 : 14-8. 
 
 Generally, in electro-chemical work, only continuous current has to be 
 considered. If alternating currents are in question, it must be remembered 
 that the power is not necessarily given by the product of voltage and current ; 
 this gives the apparent power, but the actual power may be considerably less, 
 as mentioned in Chapter II. (p. 35). 
 
BOARD OF TRADE UNITS REQUIRED FOR ELECTROLYSIS. 331 
 
 units required per hour for any given work will be approximately 
 1J times the horse-power hours required for the same work; and 
 the calculation may be made by the formulae given above, if 1000 
 be substituted for 746 in the denominator of the fraction. 
 
 The watt is the unit of activity or power, and expresses the 
 rate of doing electrical work, just as the horse-power is the 
 mechanical unit of activity, and it is easy to eliminate, if required, 
 the question of time. The term * horse- power ' conveys the idea 
 that work is being done at a rate equivalent to the raising of 
 550 Ibs. through the height of 1 ft. in each second of time during 
 which the power is applied; or, of course, to the lifting of 
 1 Ib. through 550 ft. in each second. Hence, a 'horse-power 
 hour ' means that an amount of work has been done equivalent 
 to 550 Ibs. being raised through the space of 1 foot during 
 every second for an hour ; i.e., since there are 3600 seconds in an 
 hour, to 550 Ibs. being lifted through 3600 ft., or 550 x 3600 Ibs. 
 being lifted through 1 ft. during that interval of time. Hence 
 550 x 3600 ft. Ibs. represents the total energy equivalent to 1 
 H. P. -hour. Now a current of 1 ampere is one in which 1 coulomb 
 of electricity passes every second. Hence a watt is used to 
 indicate a current in which 1 coulomb of electricity flows per 
 second under a pressure of 1 volt, or one in which 2 coulombs 
 flow per second at a pressure of \ volt, or 3 coulombs at J volt, 
 \ coulomb at 4 volts, or any number of coulombs at such a 
 pressure that the number of coulombs per second multiplied by 
 the number of volts is unity. The term * kilowatt-hour ' expresses 
 the idea that a current of 1000 amperes, i.e., of 1000 coulombs 
 per second, has been flowing for an hour under a pressure of 
 1 volt, so that it is equivalent to 1000 x 3600 coulombs of electri- 
 city at a pressure of 1 volt, or 3,600,000 volt-coulombs. It may 
 therefore be 1,800,000 coulombs at 2 volts, 7,200,000 coulombs 
 at J volt, 3600 coulombs at 1000 volts, or any other number 
 of coulombs, provided that the product of coulombs x volts = 
 3,600,000. 
 
 From this the quantity of any metal that may be deposited by 
 a Board of Trade unit of electricity may be found as follows : 
 
 Weight in grammes of any metal deposited by B. T. unit=~ q ' X MOO,000^ 
 
 where Eq. is the electro-chemical equivalent in grammes and P.D. 
 is the voltage between the electrodes of the vat. 
 
 Since the last column but two in Table XXVIII. (p. 390) shows 
 the weight in grains of each element deposited by a current of 
 1 ampere in an hour, the figures there given may be utilised for the 
 purpose of calculation to obtain the required result in pounds in- 
 stead of in grammes. These numbers (which we will term Dep.) 
 are found by converting the electro-chemical equivalent from 
 grammes into grains and multiplying the product by 3600 ; hence 
 
332 POWER REQUIRED FOR ELECTROLYTIC WORK. 
 
 Dep. is the equivalent of Eq. x 3600 in the above formula, and so 
 weight in grains of any metal deposited by B. T. unit= ^p^L - > r 
 weight in Ibs. deposited by B. T. umt- 
 
 Thus, in the case of the acid copper bath, where a voltage of 0*3 
 is used, 
 
 Ibs. of Cu deposited from the acid bath by 1 B. T. unit= p-S.=87. 
 
 0-3x7 
 
 With the alkaline bath worked at a pressure of 4 volts, and 
 remembering that the copper is in the cuprous condition, so that 
 its electro-chemical equivalent is double that of the cupric copper 
 in the acid bath, 
 
 Ibs. of Cu deposited from the alkaline bath by 1 B. T. unit ??! = 1*3. 
 
 From this it is seen that a given amount of electrical energy 
 
 0,17 
 
 will deposit , ( = 6'7) times as much copper from the acid bath 
 
 as from the alkaline bath. That this must be so is obvious from 
 the fact that although the E.M.F.'s applied respectively in the two 
 cases are in the ratio of 0-3 : 4 (or 1 : 13'3), yet a given quantity of 
 electricity deposits twice as much copper from the alkaline as it does 
 from the acid bath, so that the ratio of the energies required is 
 
 13'3 
 1 : ~o~' or nearly 1 : 6'7. At first sight, however, this may seem 
 
 to be in contradiction to the results of the calculations at the 
 beginning of the chapter, by which it was shown that the power 
 absorbed per sq. ft. of cathode surface in the alkaline bath was 5 
 (not 6 *7) times that required in the acid bath. But the discrepancy 
 is apparent, not real. It must be remembered that the earlier 
 numbers (ratio 5:1) represent power absorbed per unit of 
 electrode area, while the later (ratio 6 '7 : 1) represent energy 
 required per unit of weight of metal deposited, and the rate of 
 deposition in the acid bath was taken as 10 amp. per sq. ft., whilst 
 in the alkaline bath it was only 3 '7 amp. per sq. ft. Making 
 allowance for the fact that in the latter case each ampere deposits 
 twice as much copper as in the former case, the ratio of copper 
 deposited in the two baths is 10 : 7*4 (not 10 : 3'7). That this is 
 correct may be seen from the fact that the ratio 10 : 7 '4 is practi- 
 cally the same as the ratio 6'7 : 5. 
 
 It must not be forgotten that in these latter calculations, as in 
 the former, the loss of energy in the dynamo or in the motor- 
 dynamo and in the leads has been neglected ; and due allowance 
 must be made, as before, for these losses in estimating the gross 
 energy that would be required. It should also be noted that, on 
 account of subsidiary reactions, the amount of metal deposited by 
 
COST OF ELECTRICITY. 333 
 
 a given quantity of current is never quite in accordance with 
 theory, although, under favourable conditions, it should be 
 nearly so. 
 
 Cost of Electricity. The actual cost of electricity varies 
 enormously, according to the system of generation and distribution. 
 The battery is one of the most costly methods of producing 
 electricity, and by estimating the consumption of material in the 
 battery and calculating the cost there is no difficulty in determining 
 the expenditure required per unit. Professor Ayrton, in his 
 Practical Electricity, has worked this out for several cells, taking 
 the lowest cost of materials, at wholesale rates, and excluding 
 prime cost of battery or expenditure on renewal of porous cells or 
 other relatively durable parts. The numbers are as follows, the 
 value of the copper deposited in the battery whilst in use being 
 deducted in the case of the Daniell-cell : 
 
 COST OF 1 BOARD OF TRADE UNIT (Kilowatt-hour). 
 
 Using Daniell-cells, . < . ... lid. 
 
 ,, Grove-cells, . . . . ( . .Is. 
 
 ,, Potassium bichromate cells, . . .Is. 3d. 
 
 ,, Leclanche-cells, . . . . .Is. 5d. 
 
 When the materials are bought at retail prices, the cost per unit 
 of electricity delivered at the terminals of the battery would range 
 from Is. 6d. to 2s., or even higher. 
 
 W. R. Cooper, in his Primary Batteries, gives the theoretical 
 cost per unit from a Daniell-cell as 8'4d. and from a chromic acid 
 cell as 14*ld., but it must be remembered that such figures take 
 no account of local action, impurities, handling, manufacture of 
 plates, interest on capital, etc., so that in practice the cost would 
 be very much greater. 
 
 The cost per unit of public electricity supply at the consumers' 
 premises varies in different towns, and in some towns according 
 to the quantity required and the purpose to which it is applied. 
 Under favourable circumstances it may be as low as Id. or 2d. 
 per unit, in other cases it may be 6d. per unit (or more) ; but in 
 the worst case it is far less costly than battery-current. It must 
 be remembered that a considerable addition is made to the cost by 
 the necessity to convert the current at the town pressure to that 
 at which it is to be used. Using a combination of a small motor 
 and dynamo with, let us say, an efficiency of 80 per cent, each, 
 the combined efficiency is only 64 per cent., and the cost of 
 the current as supplied must be multiplied by say (roughly) 1*5, 
 in order to find the cost per kilowatt-hour at a pressure suitable 
 for distribution to the vats. 
 
 In public electricity supply the cost of distribution (that is, the 
 cost of the mains and of their upkeep and the loss of energy by 
 resistance) forms a very large item in the consumer's bill ; more- 
 
334 POWER REQUIRED FOR ELECTROLYTIC WORK. 
 
 over, at present, while a large portion of the current generated 
 is still used for lighting purposes, the cost of generation is 
 enormously enhanced by the necessity to put down a plant 
 sufficiently large to cope with the maximum demand for current 
 in the early hours of the evening, when light is most wanted, 
 much of the plant standing idle for the remainder of the day. 
 If the load could be equalised, so that it should be practically con- 
 stant at all hours of the day, a much smaller plant could be made 
 to yield the same total output. Hence the charge for power is 
 usually less than the charge for lighting purposes. 
 
 In large electrical works, where current is constantly required 
 (night and day) as, for example, in copper refineries- the condi- 
 tions are much more favourable. In large establishments the 
 generating plant may be larger than that of many of the smaller 
 * central stations ' giving a public supply, so that the greater 
 economy of large plant is utilised to the full, and at the same 
 time a steady current is in continuous use, and the cost of distri- 
 bution is minimised, because the generating station is close to the 
 place where the current is to be used. Under such circumstances 
 the cost of production per unit need not exceed Jd. where coal is 
 cheap and good, or, say, |d., including capital charges and depre- 
 ciation. It has been calculated that with continuous (night and 
 day) running, where coal is about 4s. or 5s. a ton, the cost per 
 kilowatt-year need not, with the best possible plant, exceed .10, 
 which is equal to 0*27d. per unit. 
 
 The price at which electric power can be purchased depends very 
 much upon the size of the undertaking by which it is produced, 
 and also on how many hours per day the energy is used con- 
 tinuously. In the case of electro-chemical work, current is often 
 used day and night continuously, and therefore current is obtain- 
 able at very favourable rates for loads of this kind. The electricity 
 undertakings in this country supply a great deal of energy for 
 lighting as well as for power, but since current for lighting is 
 required for a comparatively small number of hours per day (in 
 other words, what is termed the * load factor ' is low), the gen- 
 erating plant is not used continuously to its full extent for light 
 so much as for power, and thus the cost per unit is higher for 
 light owing to capital and other charges. Nevertheless, very low 
 average costs are obtainable, and in some of the largest under- 
 takings the average cost per unit, including capital charges, is 
 about IJd. or less, and thus it follows that the total cost of pro- 
 ducing a unit for power purposes, particularly if the load is more 
 or less continuous, is considerably less than even this low figure. 
 On the other hand, in the case of small undertakings, and where 
 the load is chiefly a lighting one, the cost is much higher, and less 
 favourable rates are obtainable. 
 
 Where water-power is available, the price of power depends 
 chiefly upon the cost of developing the power, that is, upon the 
 
ABSORPTION OF POWER IN CONDUCTORS. 335 
 
 cost of the hydraulic works and plant involved. At Niagara the 
 charge per H.P.-year is from 3, 10s.; at Sault Ste. Marie, in 
 Canada, it is a little over 2 ; whilst in parts of Norway, where 
 conditions are particularly favourable, the cost per H.P.-year is 
 less than ,1. E. A. Ashcroft 1 estimates the cost per H.P.-year 
 to be 6, 9s. 6d. by steam plant, <5, 5s. by gas plant, and <!, 10s. 
 to <4 by water-power. 
 
 Absorption of Power in Conductors. In order to ascertain the 
 actual loss of power in conductors, and so to form a true estimate 
 of the waste due to the employment of unnecessarily long or thin 
 leads, it is only necessary to know the resistance of the conductor 
 and the strength of current flowing. When a current flows through 
 a circuit, it is well known that the loss of pressure in any section 
 due to overcoming electrical resistance is proportional to that 
 resistance, so that the amount of power absorbed in any part of 
 the circuit may be found (as in the case of the electrolytic vats 
 described earlier in the chapter) by multiplying the current- 
 strength in amperes by the potential difference at the two ends of 
 the section under examination. So if the difference of potential 
 between the two ends of a wire (say between one terminal of a 
 dynamo and the corresponding terminal of the vat) be known, 
 the loss of power will be C x E, where C = amperes flowing, and E 
 the potential difference in volts between the two ends of the wire. 
 
 in 
 
 But, since, according to Ohm's law, C = , and since, therefore, 
 
 R 
 
 E - CR, E may be eliminated from the expression C x E, and the 
 expression CR may be substituted for it. Thus the loss of power 
 in watts is equal to C x CR or to C 2 R, where R is resistance in 
 ohms j or, the loss in any rod or wire is found by multiplying its 
 resistance in ohms by the square of the current passing through it. 
 
 Thus if a current of 100 amperes be passed through 50 yards 
 of No. 00 (Imperial standard wire gauge) copper wire, with an 
 area of 0'095 sq. in. in cross section, and a resistance of 
 0-0128 ohm in all, the loss of power will be 100 2 x 0'0128 = 128 
 watts, or almost exactly ^th of a horse-power. 
 
 Again with a current of 30 amperes passing through 50 yards 
 of No. 8 (standard W.G.) copper wire with a total resistance of 
 0'06 ohm, the loss of power will be 30 2 x 0'06 = 54 watts, or about 
 0-07 H.P. 
 
 In the case of alternating currents the relation is apt to be 
 more complicated and the drop of pressure greater than with the 
 continuous current (see p. 36). 
 
 Influence of Size of Conductor on Absorption of Power. It 
 will be noticed that the loss of power in conductors varies as the 
 square of the current-strength. Independently, therefore, of the 
 danger that would accrue through the overheating of a conductor if 
 
 1 Transactions of the Faraday Society, vol. iv., 1908. 
 
336 POWER REQUIRED FOR ELECTROLYTIC WORK. 
 
 it be used for too large a current, there is a great and increasing 
 loss of power. Thus, to take two concrete examples : 
 
 It has been shown in the last paragraph that when a current 
 of 30 amperes is passed through 50 yards of No. 8 copper wire, 
 the loss of power is 54 watts or 0'07 H.P.; if 60 amperes were 
 passed, the loss would not be 108 watts or 0'14 H.P., but 60 2 x 0'06 
 or 216 watts or 0'28 H.P.; and if 90 amperes were passed 
 the loss would be, not 158 watts or 0'21 H.P., but 486 watts or 
 0-65 H.P. 
 
 As has been mentioned above, the overloading of a wire with 
 current means a rise of temperature which, if the current were in 
 very great excess of normal, might cause risk of fire ; but even 
 where there is no such danger to be apprehended, there is not only 
 the greater loss of power already described, but there is an in- 
 creased resistance and therefore further loss of power, because the 
 conductivity of a wire when warm is inferior to that of the same 
 wire when cold. A general rule at one time adopted was that the 
 sectional area of copper conductors should be calculated for the 
 current which they were to carry in the proportion of 1 sq. 
 in. per 1000 amperes. The Institution of Electrical Engineers, 
 in their revised rules for wiring for the supply of electrical energy, 
 have recommended a formula by which the sectional area of copper 
 wire or cable may be calculated, according to the current to be 
 transmitted, provided the external temperature does not exceed 
 100 F., the conductors being insulated and laid in casing or 
 tubing bare wire, being more free to radiate heat, could, of course, 
 take a higher current-density. The formula given by them as 
 the result of careful experiment is 
 
 Log = 0-82 log A + 0-415, 
 or 
 
 = 2-6 A ' 82 , 
 
 where C = current in amperes, and A = area in thousandths of 
 a square inch. 
 
 Table XXXVIlA on page 398 shows in Column 2 the maximum 
 current in amperes allowable for wires or cables of different sizes, 
 according to this formula. Column 4 gives the total length of wire 
 or conductor of the size specified in Column 1, along which there 
 will be a P.D. of 1 volt, when the maximum current allowed in 
 Column 4 is passing. From this the loss of, power is readily found, 
 because the number of amperes in Column 4 multiplied by 1 volt 
 gives the number of watts converted into heat in overcoming the 
 electrical resistance of the wire along the length named in Column 
 5. Column 3 gives the corresponding length for the same wire 
 with the current given in Column 2. 
 
 To take two examples by way of illustration : 
 (1) A No. 18 wire (standard wire gauge) may, under normal 
 conditions of temperature, be allowed to take a current of 4 '2 
 
INFLUENCE OF MATERIAL ON ABSORPTION OF POWER. 337 
 
 amperes, and with this current passing, the potential of the wire 
 at any one point will be 1 volt higher than it is at a point 18 
 yards nearer the negative pole of the dynamo or battery ; and in 
 every 18 yards of the wire there will be a loss of 4 "2 watts with 
 the current of 4*2 amperes flowing. 
 
 (2) A cable of 19 strands of No. 20 wire may carry a current, at 
 normal temperature, of 30 amperes, and there will be a P.D. of 
 1 volt between any two points 26 yards apart, so that the loss of 
 power will be 30 watts for every 26 yards of cable with the 
 maximum current allowable. 
 
 Influence of Material of Conductor on Absorption of Power. 
 It is obvious that, if the loss of electrical energy in a conductor 
 when a constant current is flowing through it varies with the 
 resistance of the conductor, the absorption of power in wires of 
 the same sectional area but made of different metals will vary 
 in inverse proportion to the conductivities of the metals used. 
 Thus, for example, since the relative conductances (see p. 31) of 
 copper and iron are approximately 100 : 16, an iron wire will 
 
 absorb or rather more than 6 times as much power as a 
 16 
 
 copper wire of the same diameter and length ; and the current 
 of 100 amperes which causes a loss of nearly Jth H.P. in 50 yards 
 of copper wire of No. 00 gauge (see pp. 335, 336) will cause a loss 
 of about 1 H.P. in 50 yards of an iron wire of the same gauge. 
 
 22 
 
CHAPTER XIX. 
 
 MODERN THEORIES OF ELECTROLYSIS. 
 
 Modern Theories of Electrolysis. Within the last few years 
 much work has been done in the field of electro-chemistry, tend- 
 ing to explain the phenomena of electrolysis. It would be im- 
 possible, even if it were desirable, in the short space here avail- 
 able to describe this work in detail ; and they who would study 
 the subject exhaustively must, therefore, be referred to some of 
 the larger text-books dealing with the theories of electro-chemistry. 
 A short survey of one or two of the principal observations and 
 theories is, however, here necessary. 
 
 It should be observed that although the theory of ionic dis- 
 sociation is very widely accepted as the true explanation of 
 electrolysis, or at least as a very useful working hypothesis, it is 
 based upon premises which are distasteful, and even repugnant, 
 to a number of eminent physical chemists. It has done so much, 
 however, to advance electro-chemical knowledge, and is accepted 
 by so large a number of thinkers, that the author has endeavoured 
 to give a few broad outlines of the theory. 
 
 Solution Pressure. It is well known that many substances, 
 such as gold and silver, are insoluble in water; whilst others, 
 such as copper sulphate, silver nitrate, and potassium cyanide are 
 readily soluble, and that the degrees of solubility of the different 
 members of the latter class are very varied. Each soluble com- 
 pound has its own constant degree of solubility under constant 
 conditions thus 37 parts of crystallised copper sulphate, and no 
 more, are always soluble in 100 parts of pure water at 10 C. 
 But the solubility of each substance is largely dependent on 
 temperature. 
 
 Now this phenomenon is comparable with certain phenomena 
 connected with vaporisation. Water, at all temperatures, tends 
 to evaporate, and the higher the temperature the greater is the 
 tendency to do so. Nevertheless, at any one temperature the 
 tendency is always constant and can be measured ; it is known 
 as the vapour pressure of water. The 'vapour pressure' of a 
 liquid expresses, then, its tendency to pass into the form of 
 vapour. The corresponding tendency of a solid to pass into 
 
 338 
 
OSMOTIC PRESSURE. 
 
 339 
 
 solution, when immersed in a solvent such as water, is known as 
 its solution pressure, and this solution pressure is constant under 
 any given set of conditions, but varies definitely with a definite 
 change of the conditions. 
 
 Osmotic Pressure. It is well known that when a liquid is 
 placed in contact with air in a closed vessel, evaporation goes on 
 steadily until the air becomes ' saturated ' thus, water intro- 
 duced into the bottom of a dry bottle evaporates until the air in 
 the bottle is saturated with water vapour, and then the evapora- 
 tion ceases. As the vapour passes into 
 the air, it exerts a pressure in the 
 atmosphere within the bottle, and the 
 partial pressure of this vapour tends to 
 prevent further evaporation, until at 
 last the pressure of the accumulated 
 vapour in the air is equal to the pres- 
 sure or tendency of the water particles 
 to vaporise, and equilibrium is attained. 
 Similarly, when a soluble substance is 
 placed in (say) pure water, it passes, 
 at first, rapidly into solution, owing to 
 its solution pressure, but, as the dis- 
 solved molecules accumulate in the 
 water, they exert a back pressure, simi- 
 lar to the back pressure of the water 
 vapour in air, and so gradually lessen 
 the tendency of the solid to dissolve, 
 until at last the point of equilibrium 
 is reached, when the back pressure of 
 the dissolved particles equals the solu- 
 tion pressure of the substance. This 
 back pressure is known as Osmotic 
 Pressure, and is susceptible of measure- 
 ment. Just as the motion of the 
 particles of any gas, when mixed with 
 another gas, causes the one to diffuse 
 into the other, until they are uniformly mixed, so does the 
 motion of the molecules of a dissolved substance cause diffusion 
 until the mixture is uniform ; but in the case of solutions, owing 
 to the greater density of the solvent, diffusion is vastly more 
 slow than in the case of gases. 
 
 If a cylinder with a counterpoised piston (fig. Ill), sliding air- 
 tight within it, be so arranged that there is air in the space A 
 beneath the piston, while the space B above it is rendered vacuous, 
 the pressure of the air in A will cause the balanced piston to rise 
 until it touches the top of the cylinder, but if weights are placed 
 on the tray C, they will counteract the expansive pressure of the 
 air, and the piston will be forced downward to some extent, the 
 
 FIG. 111. 
 
340 MODERN THEORIES OF ELECTROLYSIS. 
 
 greater the weight on C the greater being the compression of the 
 air in A. Equilibrium will be attained only when the expansive 
 pressure in A exactly equals the compressive pressure exerted 
 by the weights on the piston, for the rising of the piston is due 
 to the expansive force of the air in A. 
 
 If, instead of air, a solution, say of sugar in water, is placed in 
 A, and B is left vacuous, there can be no expansion, because the 
 liquid in A is incapable of expanding like a gas ; yet the dissolved 
 sugar has the same tendency to expand that is, to diffuse through 
 a larger volume as had the air in the former experiment. To 
 show this tendency, it is only necessary to provide, in place of the 
 vacuum, a substitute which will enable the sugar to diffuse as 
 air expands into a vacuum. This can be effected by filling the 
 space B with pure water and using a piston made of a porous 
 material, through which water, but not dissolved sugar particles, 
 can pass freely. Water can now pass from A into B or vice versa, 
 but the sugar, being unable to pass through the piston, must all 
 remain in A. The expansive force (i.e., the osmotic pressure) of 
 the sugar in the solution in A now becomes effective (but, of 
 course, very slowly), because water passes freely through the 
 piston as the latter rises, and the pure water passing into A from 
 above plays the part of the vacuum through which the sugar 
 particles can diffuse ; the diffusion will be found to take place 
 gradually until the material is evenly distributed through the 
 liquid and the piston has risen to the top of the cylinder. By 
 placing weights on C, the osmotic pressure of the dissolved sugar 
 could be counterbalanced, and this pressure, at any position of 
 the piston, i.e., at any degree of dilution, could be measured. 
 Not exactly in this way, but by an analogous method, the osmotic 
 pressures of various substances have been determined. 
 
 Speaking generally, and referring mainly to very dilute and 
 therefore unsaturated solutions, it is worthy of note that such 
 substances when in solution seem to obey the ordinary laws of 
 gases. It is even true that the actual pressure exerted by each 
 molecule of the substance in the dilute solution is the same as if 
 it were in the gaseous state diffused through a space equal to the 
 volume of the solution. Further, Boyle's law is obeyed, for the 
 osmotic pressure exerted in a given volume of the solution is 
 proportional to the number of molecules present. Thus, the 
 osmotic pressure exerted by 20 grains of sugar in a cubic inch of 
 water is just double that exerted by 10 grains in the same volume. 
 So also is Avogadro's law applicable, for equal numbers of mole- 
 cules of different substances exert equal osmotic pressures ; thus 
 the pressure exerted by 500 molecules of sugar in a given volume 
 of water would be the same as that caused by 500 molecules of 
 alcohol, although the actual weights present in the two cases 
 would be in the proportion of 342 : 46, which are the respective 
 molecular weights of the two compounds. 
 
ELECTROLYTES. 341 
 
 Electrolytes. The osmotic pressures exerted by certain 
 substances do not, at first sight, obey the law last quoted. 
 Solutions of organic substances in solvents such as benzene or 
 ether, or of many soluble substances in water, behave generally 
 in accordance with the law. But the whole class of substances 
 known as electrolytes, when dissolved in water, behave abnor- 
 mally. The osmotic pressures of their solutions are in excess 
 of the normal. In very dilute solutions, the pressure exerted by 
 such substances as sodium chloride (NaCl), hydrochloric acid (HC1), 
 potassium nitrate (KN0 3 ) and silver nitrate (AgN0 3 ), give about 
 twice the pressure that an equal number of molecules of (say) sugar 
 or alcohol dissolved in the same volume of liquid would show, and 
 it is remarkable that these substances are the salts or compounds 
 of monovalent atoms. Similarly, such substances as copper chloride 
 (CuCl 2 ) and potassium sulphate (K 2 S0 4 ) give nearly three times 
 the normal pressure ; and these are the compounds of divalent 
 metals or acids. It is supposed, then, that each molecule of the 
 salt in the very dilute solution breaks up, in the case of those 
 substances which exhibit twice the normal pressure into two parts, 
 and in the case in which the pressure is thrice the normal into 
 three parts, each of the dissociated parts then producing as high 
 an osmotic pressure as the whole molecule would produce. 
 Analogies for this are to be found for example, the vapour 
 density of heated ammonium chloride (NH 4 C1) is only half 
 the normal, owing to the fact that the vapour of the salt dis- 
 sociates at high temperatures into HC1 and NH 3 , and thus 
 occupies twice the volume that would be required for the un- 
 dissociated vapour at the same temperature. 
 
 Ions. The theory, then, assumes that sodium chloride, NaCl, 
 when dissolved in a large volume of water, becomes broken up 
 into the two separate parts Na and Cl ; that hydrochloric acid 
 dissociates into H and Cl ; potassmm nitrate into K and N0 3 , and 
 silver nitrate into Ag and N0 3 , each molecule forming two separate 
 simpler entities, which are known as Ions, and are recognisable as 
 the materials that are deposited at the electrodes of a solution 
 undergoing electrolysis. Again copper chloride (CuCl 2 ) breaks up 
 into three parts (Cu + Cl + Cl), as does also potassium sulphate 
 K 2 S0 4 (K + K + S0 4 ). At first sight it seems contrary to all 
 the teachings of chemistry that two such substances as Na 
 (sodium) and Cl (chlorine) should exist uncombined in the same 
 aqueous solution, especially as sodium is known to decompose 
 water with great violence, and chlorine is distinguished by a most 
 unpleasant smell that is entirely lacking in solutions of pure 
 sodium chloride (common salt). But it is certain that even if 
 they be diffused as ions through the liquid, they do not exist in 
 the condition familiar to chemists. It is assumed that the ions 
 carry heavy charges of electricity, those of metals carrying 
 positive charges, and those of non-metals (chlorine and the like) or 
 
342 MODERN THEORIES OF ELECTROLYSIS. 
 
 acid radicals i.e., certain non-metallic groups of atoms (such as 
 N0 3 or S0 4 ) carrying negative charges. It is supposed that the 
 charges of all monovalent ions are equal, and that the charge of 
 every divalent ion is double, and that of each trivalent ion is 
 three times, that of a monovalent ion, and so on. Hence sodium 
 chloride is assumed to dissociate into the sodium ion, Na, with a 
 positive charge, and the chlorine ion, Cl, with an exactly equal 
 (but opposite) negative charge, the resulting electrical effect in the 
 solution being nil, because the free ions are diffused equally 
 through the solution, and the positive electricity of the one set 
 exactly neutralises the negative electricity of the other. So also 
 with silver nitrate, the positive charges of the silver ions, Ag, 
 exactly neutralise the negative charges of the N0 3 ions. In the 
 case of copper chloride, the divalent copper ion, Cu, carries a 
 double charge of positive electricity, which is neutralised by the 
 two singly charged monovalent chlorine ions (Cl + Cl), separated 
 simultaneously with the Cu at the moment of dissociation. 
 
 The actual weight of an ion must be exceedingly small, but 
 can only be roughly derived. Since, however, its chemical formula 
 or symbol is known, it is convenient in making calculations to 
 think of it as having a gramme-equivalent weight ; the weight of 
 a silver gramme ion would thus be its atomic weight in grammes, 
 viz., 108 grammes (nearly), and that of a N0 3 gramme ion would 
 be (N = 14) + (0 3 = 48) = 62 grammes. The actual electrical 
 charge of such a monovalent ion is regarded as 96,540 coulombs, 
 which is the exact quantity of electricity necessary to deposit 
 precisely one equivalent weight of the ion in grammes, viz., 1 
 gramme of hydrogen or 108 grammes of silver. 
 
 Electrolytic Conduction. It is especially to be noted that 
 solutions of substances which do not show some sign of dissociation 
 in solution are non-conductors of electricity. Water alone does 
 not dissociate, and is, therefore, when pure, practically a non-con- 
 ductor ; and the addition of sugar (which does not dissociate) 
 cannot render it a better conductor, whereas the presence of a 
 mere trace of an acid or a salt (such as sodium chloride, or copper 
 sulphate) capable of dissociation into ions, renders it to some extent 
 a conductor at once. There would appear, therefore, to be a direct 
 connection between the separation of the salt into ions, or lonisa- 
 tion as it is termed, and its power of conducting electricity. It 
 would seem that when a current of electricity is led through a 
 solution, it is conducted onward by the separate ions existing in 
 the solution, each carrying its own charge and conveying the elec- 
 tricity piecemeal, so to speak, from one electrode to the other. 
 Every ion takes a definite charge, and the transfer of electricity is 
 accompanied by a migration of the ions. As the ions carry definite 
 charges and have definite weights, it is evident that the transport 
 of electricity is accompanied by, and proportional to, the transport 
 of matter, every 96,540 coulombs of electricity being accompanied 
 
ELECTROLYTIC SOLUTION PRESSURE. 343 
 
 by 1 gramme ion (or atom) of a monovalent element such as sodium 
 (weighing 23 grammes), or of silver (weighing 108 grammes), or 
 of chlorine (weighing 35 '5 grammes) ; or by one gramme equiva- 
 
 CZ Q K 
 
 lent of a divalent ion such as copper (weighing grammes) or 
 
 Z 
 q/> 
 
 of sulphur tetroxide (S0 4 ) weighing grammes. Thus Faraday's 
 
 2 
 
 laws of electrolysis follow as a matter of course from the ionisa- 
 tion theory. The electrical charge is essential to the existence 
 of an ion, and if it be removed, the ion at once becomes converted 
 into the corresponding element (or group of elements) with the 
 properties so familiar in chemistry. 
 
 Electrolytic Solution Pressure. It has been stated above that 
 a soluble salt passes into solution when placed in a solvent, and 
 that it has a certain solution pressure. A. salt is made up of a 
 combination of a positive with a negative ion, and the solution of 
 the salt, accompanied by a partial ionisation, does not affect the 
 apparent electrical condition of the solution. If, however, a metal 
 such as zinc be placed in water, and zinc ions be formed in the 
 solution, each of such ions must take a definite charge of electricity, 
 so that positively charged ions would then exist in the solution 
 without any balance of negatively charged ions. But if any 
 portion of zinc did so pass into solution as an ion, there would 
 necessarily be liberated at the same moment an equal and opposite 
 charge of negative electricity in the zinc plate, because it is not 
 possible to generate either positive or negative electricity alone. 
 The solution would thus be positively charged, and the zinc 
 negatively, and this would counteract any tendency for more zinc 
 ions to take up charges and pass into solution. The special 
 tendency of any metal to ionisation when placed in a liquid is 
 known as the Electrolytic Solution Pressure, to distinguish it 
 from the ' solution pressure ' of salts. But, as in the case of 
 solution pressure, electrolytic solution pressure is opposed by 
 osmotic pressure ; thus, if a metal were placed in a solution of one 
 of its own salts, the osmotic pressure of the ions of that metal 
 already in the solution would tend to prevent the passage of fresh 
 ions of the same kind into the liquid. If the osmotic pressure 
 were less than the electrolytic solution pressure of the metal, ions 
 would tend to pass into solution and to charge the solution 
 positively, while the metal itself would, up to a certain point, be 
 charged negatively, as already shown ; but if the osmotic pressure 
 were greater than the electrolytic solution pressure, it would not 
 only prevent the passage of fresh metallic ions into the solution, 
 but would contrariwise tend to deposit some of the positive ions from 
 the liquid on to the metal, so that the metal would be positively 
 charged, and the negatively charged ion set free in the solution 
 would impart to the liquid a negative charge ; whilst, if the 
 osmotic pressure were exactly equal to the electrolytic solution 
 
344 
 
 MODERN THEORIES OF ELECTROLYSIS. 
 
 pressure, no change at all would be observable, as the system 
 would be in stable equilibrium. It has actually been observed 
 that the so-called electro-positive metals, such as potassium and 
 sodium, zinc, cadmium, and iron, are charged negatively when 
 placed in solutions of their salts, whilst such metals as copper, 
 mercury, gold, and platinum, having electrolytic solution pressures 
 that are low as compared with their osmotic pressures, usually 
 become charged with positive electricity. 
 
 It is thus possible to arrange the metals in an order cor- 
 responding to their behaviour in this respect. In this way 
 we obtain again the electro-chemical series, as it has been 
 given above, and find a new explanation of the series. Some 
 idea of the actual electrolytic solution pressures of certain 
 common metals is gained from the following numbers, taken from 
 Le Blanc : 
 
 Atmospheres. 
 Zinc, z=9'9xl0 18 
 
 Cadmium, = 27xl0 6 
 Iron, =l'2xl0 4 
 
 Cobalt, =1'9 
 Nickel, =1-3 
 
 Atmospheres. 
 Lead, =l'lxlO- 3 
 
 Hydrogen, =9'9xlO- 4 
 Copper, =4 < 8xlO- 20 
 Mercury, =l'lxlO- 16 
 Silver, = 2'3xlO- 17 
 
 It will be noticed that some of these values are enormously high, 
 and some extremely small. It should be remembered, however, 
 that such figures only represent the striving of the molecules to pass 
 into the ionic state, or the osmotic pressures which the metallic ions 
 already in solution would have to exert to prevent ionisation of the 
 metal. No such pressures have been directly or indirectly observed. 
 
 But there is a very close connection between the heat that would 
 be generated by the chemical changes taking place in such a cell, 
 if no current were produced, and the electro-motive force of any 
 current that is generated. It thus appears that, in the case of 
 electro-positive metals, heat is generated when a metal passes into 
 the ionic condition, and this is termed the heat of ionisation. 
 It is not rendered sensible in electro-chemical operations because 
 it is converted into electrical energy, and this is expended to a 
 large extent outside the cell in which the action has taken place. 
 The heat of ionisation for certain metals has been calculated by 
 Ostwald, as shown (expressed in calories) in the following Table: 
 
 Metal. 
 
 Symbol 
 and 
 Valency. 
 
 Heat of 
 Ionisation of 
 1 Equivalent 
 Weight. 
 
 Metal. ' 
 
 Symbol 
 and 
 Valency. 
 
 Heat of 
 Ionisation of 
 1 Equivalent 
 Weight. 
 
 Potassium . 
 Sodium 
 Magnesium . 
 Iron . ." 
 Zinc . 
 
 K' 
 
 Na' 
 Mg" 
 Fe 7 ' 
 Zn" 
 
 Grin. cals. 
 61,000 
 56,300 
 53,400 
 10,000 
 16,300 
 
 Cadmium 
 Cobalt 
 Nickel 
 Copper 
 Silver. 
 
 Cd" 
 Co" 
 Ni" 
 Cu" 
 Ag' 
 
 Grm. cals. 
 8,100 
 7,300 
 6,800 
 - 8,800 
 -26,200 
 
SIMPLE EXCHANGE OF METALS. 345 
 
 Thus a given quantity of electricity is conducted equally by 
 equivalent weights of all metals alike, but the electro-motive force 
 necessary to set the particles in continuous motion varies with 
 different ions, and depends upon conditions intimately connected 
 with the heats of formation of the compounds in use. So, also, 
 the force with which an ion clings to its electrical charge varies 
 with, and is measurable by, the heat of ionisation, those metals 
 which evolve the most heat in becoming converted into ions 
 requiring the greatest expenditure of energy to make them relin- 
 quish their charges and re-assume the metallic state. This must 
 evidently be so, the heat absorbed in the latter process being 
 exactly equal to that evolved in the former. 
 
 Simple Exchange of Metals. It has been shown that when a 
 metal (e.g., zinc) is placed in water it tends to form ions, but that 
 the tendency is checked by the fact that the positive charges 
 necessary to the independent existence of the ions can only be 
 derived from the rest of the zinc, which must, therefore, become 
 charged negatively, so that ionisation stops almost as soon as it 
 has begun. 
 
 It is evident also that no appreciable proportion of any ions, 
 whether positive or negative, can exist in a solution without being 
 accompanied by a number of ions of the opposite kind, carrying 
 in all a charge equal and opposite to their own, otherwise the 
 solution would give evidence of strong electrification. Water is 
 but slightly dissociated into its ions hydrogen (H) and hydroxyl 
 (OH). If now zinc be placed in water, and if we suppose it to dis- 
 place hydrogen (as we shall shortly see that it does in sulphuric 
 acid), then zinc (Zn) and OH ions would exist side by side in 
 the solution. The chemical compound, zinc hydroxide Zn(OH) 2 , 
 represented by this combination, is, however, one which is com- 
 paratively insoluble in water, that is to say, it scarcely dissociates 
 into ions at all. Hence, if zinc is to have any action on water in 
 which it is immersed, it must rob hydrogen ions of their charges 
 and set free the hydrogen as a gas and form, instead, free Zn 
 and OH ions ; but the action must stop almost at once, because 
 solid undissolved and insoluble zinc hydroxide Zn(OH) 2 would 
 be formed on the surface of the zinc, and so gradually prevent 
 further contact between the water and the zinc. But when an 
 acid, sulphuric acid for example, is substituted for water, 
 the action is different, for zinc sulphate (ZnS0 4 ) is soluble 
 in water. Sulphuric acid in aqueous solution is largely dis- 
 sociated into the ions H and S0 4 ; and hydrogen is an element 
 which has a much lower electrolytic solution pressure than 
 zinc or, in other words, its ions cling to their charges of 
 positive electricity far less tenaciously than do the zinc ions. 
 Hence the greater electrolytic solution pressure of the zinc tends 
 to carry zinc ions into the solution ; and this is now possible, 
 because, by contact with hydrogen ions already in the solution, the 
 
346 MODERN THEORIES OF ELECTROLYSIS. 
 
 metallic zinc particles are able to take up the charges of positive 
 electricity necessary to convert them into ions. In this process 
 the hydrogen ions lose their charges, and, therefore, their existence 
 as ions, and the hydrogen is deposited in the gaseous state. This, 
 then, is the explanation of the observed fact that zinc, when 
 placed in sulphuric acid, evolves bubbles of hydrogen gas. 
 Similarly, zinc in copper sulphate solution deposits copper by 
 exchange ; and any metal tends to pass into solution as an ion 
 when it has the opportunity of taking the charges from the ions 
 of another metal of lower electrolytic solution pressure contained 
 in the solution. Only those metals whose electrolytic solution 
 pressure is higher than that of hydrogen are able to evolve the 
 latter element as a gas, when dipped into a solution containing 
 free hydrogen ions, i.e., into an acid. It follows, also, that the 
 acids which are the most completely ionised in solution (that is 
 to say, the acids whose solutions contain the greatest number of 
 free ions) are those in which the zinc and hydrogen exchange 
 places most readily because in such solutions there must be the 
 largest number of free hydrogen ions in contact with the metal. 
 It is found that the so-called strong acids sulphuric, hydrochloric, 
 and nitric acids are those which are most completely dissociated 
 into ions when mixed with water, and this, of course, explains the 
 readiness with which they act upon metals, such as zinc. Acetic 
 acid and the organic acids, as a class, dissociate but little, and 
 therefore have proportionately slight action on zinc. 
 
 It is, then, obvious that the heat evolved by the action of an 
 acid upon a metal is equal to the difference between the heat of 
 ionisation of the metal (as it passes into the ionic state) in solution, 
 and that of the hydrogen ions as they assume the gaseous or 
 elementary condition). 
 
 In such a solution zinc will continue to dissolve until the solu- 
 tion is saturated with it; but the action will go on more and 
 more slowly, because the accumulation of zinc ions in the liquid 
 will cause an increasing osmotic pressure, which opposes the 
 entrance of more zinc ions into the bath, and because the free 
 hydrogen ions are gradually replaced by zinc ions. 
 
 The Various Cases of Solution. It is well to distinguish 
 between the different kinds of solution which commonly occur. 
 From what has been said it will be gathered that, first, there is 
 the solution of a non-electrolyte, such as sugar, which dissolves in, 
 say, water with a certain solution pressure. Secondly, there is 
 the case of a compound solid, such as sodium chloride, which 
 dissolves in water, also with a certain solution pressure, and which 
 in the process of solution dissociates into ions. In neither of 
 these cases is there ordinary chemical change as expressed by an 
 equation. Thirdly, we have the case of a simple metallic body, 
 such as sodium, which can dissolve only by decomposing the 
 solvent, and, since solution takes place in the ionised condition 
 
- SIMPLE CELLS. 347 
 
 (single ions being produced instead of pairs of combined ions 
 being separated), the solvent action can only be effected by driving 
 another ion, such as hydrogen, having a lower electrolytic solution 
 pressure, on to the metal, where it is liberated. If the electrolytic 
 solution pressure is sufficiently great, solution continues until the 
 water is all decomposed. Fourthly, there is the case of a metal 
 such as zinc, whose solution pressure is sufficient to enable the 
 zinc to replace hydrogen, but only to a comparatively small 
 extent. In thus speaking of zinc, reference is made to pure, or 
 practically pure, zinc. The hydrogen thrown down on to the 
 zinc prevents any considerable action ; but if the zinc is coupled 
 up in the sulphuric acid to an electro-negative metal, such as 
 platinum or copper, solution of the zinc continues and the 
 hydrogen is then liberated on the electro-negative element. It 
 is this class of solution which is important for voltaic action. 
 If the electrolytic solution pressure of a metal is so high that it 
 decomposes the solvent readily, then it is useless for voltaic 
 purposes, because solution takes place whether the circuit is closed 
 or not, and thus the chemical energy cannot be converted into 
 electrical energy. 
 
 Simple Cells. If two isolated plates of metal are immersed in 
 an electrolyte, each will exert its own electrolytic solution pressure 
 and will become negatively or positively charged, according to 
 the nature of the metal. Possibly neither will have a sufficiently 
 high solution pressure to dissolve to an appreciable extent. But 
 if the two plates are joined externally by a wire, a different state 
 of things is set up. The metal with the greater solution pressure, 
 and therefore more highly charged negatively, will receive positive 
 electricity from the other plate, and its charge being thus reduced, 
 it will be free to pass more ions into solution ; on the other hand, 
 the plate with the smaller solution pressure, owing to its altered 
 electrical condition, will be free to have hydrogen ions precipitated 
 upon it and thus to have its electrical state maintained. It 
 should be noticed that in such a case the hydrogen no longer 
 appears on the metal that is passing into solution, but on the 
 other plate. 
 
 Thus there will be set up a flow of positively charged ions 
 through the solution from the metal which has the higher solu- 
 tion pressure to the other plate, and of negatively charged ions 
 in the opposite direction ; and the electro-motive force which 
 causes this circulation will depend mainly upon the difference 
 between the electrolytic solution pressures of the two electrode- 
 metals, or in other words, the difference between their heats of 
 ionisation. 
 
 To take a concrete example. Copper and zinc plates are 
 separately immersed in sulphuric acid. The electrolytic solution 
 pressure of zinc is higher than that of hydrogen, so that simple 
 exchange takes place : hydrogen ions are converted into hydrogen 
 
348 MODERN THEORIES OF ELECTROLYSIS. 
 
 molecules which escape in the form of gas at the copper plate if 
 the circuit is closed (as we have just seen in the preceding section), 
 and zinc molecules are changed into ions, each (divalent) zinc ion 
 taking its positive charge from two hydrogen ions and passing 
 into the solution, whilst the excess energy of this change is rendered 
 evident in the form of heat, so that the metal and solution become 
 sensibly warmer. The copper has a negative solution pressure, 
 and so does not displace any hydrogen. Perfectly pure zinc does 
 not deposit hydrogen (i.e., decompose dilute acid) when simply 
 immersed, the action no doubt being stopped after the first in- 
 stant by the negative charge of the metal as already described, 
 and ionisation can only proceed if by some means sufficient charges 
 of positive electricity can be supplied to satisfy the ions of zinc. 
 This is effected when the zinc and copper plates are united by a 
 wire. The copper has a positive charge when immersed in the 
 acid, and can now give up some of its charge through the wire to 
 the zinc. The positive charge of the copper is renewed at the 
 expense of hydrogen ions on its surface, which thus become con- 
 verted into free hydrogen gas. In this way it is possible to 
 picture the higher solution pressure of the zinc tending to force 
 the atoms of this metal to take up their proper charges of 
 positive electricity and so to become ions, and to pass through 
 the solution towards the copper, to which the weaker hydrogen 
 ions in front of them give up their charges ; the positive charges 
 of the hydrogen are thus imparted to the copper plate, and, 
 passing through the wire, assist in the ionisation of zinc. This 
 action can evidently go on until all the free hydrogen ions have 
 given up their charges through the copper plate and the wire to 
 zinc ions, that is, until all the free acid is neutralised and only 
 zinc sulphate exists in solution. It is obvious that the action 
 could not proceed if zinc ions were to give up their charges at the 
 surface of the copper and be there deposited, because the expendi- 
 ture of energy required to form zinc ions at one plate would be 
 exactly equalled by that required to discharge the ions and 
 deposit metallic zinc at the other plate ; and there would be no 
 force available to cause the circulation of the ions. The falling 
 off in the strength of a copper-zinc battery can, of course, be ex- 
 plained partly by the gradual diminution in the number of free 
 hydrogen ions available at the surface of the copper plate, as 
 they are, little by little, replaced by zinc ions, but chiefly by the 
 polarisation caused by the gaseous hydrogen deposited on the 
 copper having a higher solution pressure than that metal, so 
 that the difference between the solution pressures at the two 
 plates is much lower than it was when the zinc was opposed to 
 copper alone. Thus, at first, the potential difference between 
 zinc and copper would be represented by the difference between 
 the electrolytic solution pressures of zinc and copper. But as 
 this pressure is in each case measured by the potential difference 
 
SIMPLE CELLS. 349 
 
 between the metal and the solution in which it is immersed, the 
 potential difference (written P.D. for the sake of brevity) between 
 copper and zinc in sulphuric acid would be : P.D. between 
 copper and sulphuric acid minus P.D. between zinc and sulphuric 
 acid. The P.D. of copper and sulphuric acid is about 0*5, or, as 
 it may be written, +0'5, because the copper becomes charged 
 with positive electricity, and the P.D. of zinc and sulphuric 
 acid is about 0'6, or, as it must now be written, - O6, because 
 the zinc is charged negatively. Hence the initial P.D. between 
 copper and zinc is +0'5 -(- 0'6) = 0'5 + 0'6 = I'l volts. Owing, 
 however, to polarisation, the P.D. almost immediately falls off, so 
 that usually only 0'7 or 0'8 volt is observed. 
 
 If cadmium had been used instead of copper, the P.D. of the 
 two metals would have been much less, because the electrolytic 
 solution pressure of cadmium is relatively high. 
 
 The P.D. of cadmium and sulphuric acid is about O2 
 (cadmium being charged negatively), so that the P.D. of cad- 
 mium and zinc is - 0'2 - ( - 0-6) = - 0'2 + 0'6= +0-4. In this 
 case each metal becomes charged negatively when immersed 
 alone, but the cadmium, having the lower electrolytic solution 
 pressure, takes the place of the copper and acts as the positive 
 pole or cathode. If silver were substituted for copper, the 
 E.M.F. would be even greater than that between zinc and copper, 
 because the solution pressure of silver is lower than that of 
 copper. The E.M.F. of silver and zinc in sulphuric acid would 
 then, at first, be about +07 -(- 0'6) = 07 + 0-6 = 1 -3 volts. In 
 every case, however, the P.D. would rapidly fall off owing to 
 polarisation. 
 
 Local Action. No further explanation of impure zinc dissolv- 
 ing in acid without being opposed to a plate of a metal of lower 
 electrolytic solution pressure is necessary, as the effect of local 
 action described on p. 39 can obviously be as readily harmonised 
 with the new theories as with the old. Local action is seen to be 
 due to electro-negative impurities having a lower electrolytic 
 solution pressure than that of zinc. 
 
 Two-fluid Cells. It has been seen that no action could be 
 anticipated if pure zinc and copper be opposed to one another in 
 a normal solution of zinc sulphate containing no free hydrogen ions. 
 If, now, a porous partition be placed across the cell between the 
 two plates, and sulphuric acid be poured into the portion contain- 
 ing the zinc, and normal zinc sulphate solution, free from acid, 
 into the half containing the copper, no appreciable action could 
 be expected, because, although the zinc is immersed in acid, and 
 there is no osmotic pressure tending to retard its solution, yet the 
 solution or ionisation of any of the zinc could only occur if deposi- 
 tion of ions took place at the surface of the copper, and the only 
 ions that could be there deposited would be zinc, which it has 
 already been shown will not be thrown down. If, however, the 
 
350 MODEEN THEOEIES OF ELECTROLYSIS. 
 
 sulphate of zinc be placed in the zinc compartment, and the sul- 
 phuric acid on the copper side, action will take place freely : the 
 tendency of zinc to ionise will be accompanied by the tendency 
 of the hydrogen ions to give up their charges to the copper, 
 and so the action will proceed, zinc ions from the zinc sulphate 
 travelling through the porous partition into the copper compart- 
 ment, forcing the hydrogen ions before them into contact with 
 the copper, and making room for fresh zinc ions behind. 
 
 Other two-fluid cells may also be explained according to the 
 newer theories. Thus in the Daniell-cell, the zinc is immersed in 
 sulphuric acid or in zinc sulphate contained in a porous pot, the 
 copper in copper sulphate. Here, in the former case, the zinc can 
 be readily ionised, and can displace some of the hydrogen ions in 
 the acid, which pass through the porous cell into the copper 
 sulphate solution, and there displace copper ions which deposit 
 on the copper plate, and impart their charges through the con- 
 necting wire to the zinc, and so enable it to become ionised. 
 The zinc at the outset has not to contend against any back 
 osmotic pressure, seeing that there are no zinc ions in the solu- 
 tion initially, whilst the deposition of the copper ions is actually 
 assisted by the osmotic pressure of the copper ions in the copper 
 sulphate solution outside the porous cell. The battery is ob- 
 viously constant, as the element which is de-ionised is the same 
 as that on which it is deposited ; and it is, moreover, clear that 
 the solution outside the porous pot should be kept saturated with 
 copper sulphate, not only because there will then be always in 
 contact with the copper a sufficiency of free copper ions to give 
 up the quantity of electricity necessary to charge the zinc as it is 
 ionised, without calling upon any free hydrogen ions that may be 
 at hand (and which are more reluctant to give up their charges 
 than are copper ions), but because the greatest assistance is to 
 be derived from the osmotic pressure of the copper salt when the 
 solution is most nearly saturated. 
 
 It is most important here to observe that the newer theories 
 afford an explanation of the fact that if two pieces of the same 
 metal (connected together) be immersed on different sides of a 
 porous division, in solutions of the same substance but of different 
 strengths, a current is produced. Thus, suppose a strip of copper, 
 bent into the shape of a fl, be immersed in a cell divided by 
 a porous partition into two compartments, so that one of the 
 limbs rests in a strong solution, and the 'other in a weak solu- 
 tion of copper sulphate, the electrolytic solution pressure of the 
 copper is, of course, the same in both fluids, but in a stronger 
 solution it is opposed by a greater osmotic back pressure than it 
 is in the weaker solution. Hence there will be a tendency for the 
 copper to dissolve in the compartment containing the weaker 
 solution and to send its ions through this solution to the porous 
 partition, and through this to the stronger side. Thus the current 
 
TWO-FLUID CELLS. 351 
 
 flows through the solution from the copper plate in the weak 
 liquid to that in the strong, and thence through the copper strip 
 outside the cell back to the original plate. Meanwhile, anions 
 charged with negative electricity are supposed to be passing from 
 the cathode through the solution towards the anode, and are so 
 migrating from the cathode cell through the porous division to 
 the anode cell (vide inf.). It is true that, even if one solution were 
 one hundred times as strong as the other, the potential difference 
 between the two plates could not amount to one quarter of a volt. 
 But it is a fact that must be borne in mind in practical working, 
 as it shows the necessity for keeping the solutions uniform in 
 strength, and explains why, if a plated article be left in a bath 
 which is not well mixed, the deposit may dissolve off the portion 
 that is immersed in the weaker solution and thicken on the other 
 portions. This action may, of course, be greatly assisted by any 
 concentration of acid in the one portion, and by the difference in 
 conductance of the solutions, but it must be remembered that it 
 will occur wherever there is a difference in the concentration of 
 ions in different parts of the same liquid, even though only a mere 
 trace of acid may be present. The use of a porous pot is not, of 
 course, essential ; provided the two solutions of unequal concentra- 
 tion are in contact with one another, and each with one of the copper 
 strips, the latter being joined by a metallic connection. The 
 phenomenon is therefore observable if a single piece of copper be so 
 immersed in a solution of copper that one part of the metal rests 
 in a stronger portion of the liquid than does the rest of the metal. 
 Electrolysis. After what has been said concerning the theory 
 of batteries, the elementary explanation of the mechanism of 
 electrolysis according to these hypotheses should be simple. Two 
 plates are immersed in a uniform solution. The solution must be 
 one in which a neutral salt is wholly or in part dissociated into 
 ions, charged respectively with positive and negative electricity, 
 otherwise it could not act as a conductor of electricity. The plates 
 are connected to the opposite poles of a generator of electricity, 
 so that one, the anode, is kept supplied with positive electricity 
 at a certain potential, and the other with negative electricity. 
 Immediately the positively charged ions, which had previously 
 been in motion without any uniformity of direction, will commence 
 to move in a constant direction from the plate which is receiving 
 a constant supply of positive electricity towards that the cathode 
 which is supplied with negative electricity, and the ions which 
 carry negative charges will also have imparted to them uniformity 
 of direction, but opposite to that of the positive ions, namely, 
 towards the positively charged anode. Thus there will be a 
 stream of positive ions moving towards the cathode and there 
 giving up their charges and being converted from the ionic to 
 the neutral or elementary condition, and a stream of negative ions 
 moving towards and giving up their charges to the anode. 
 
352 MODERN THEORIES OF ELECTROLYSIS. 
 
 Thus to picture the simplest case first : Copper sulphate dis- 
 solved in water is partly dissociated into positively charged Cu 
 ions and negatively charged S0 4 ions homogeneously distributed 
 through the liquid. Two copper plates are immersed in the 
 solution ; the plates, being alike, have equal tendencies to dis- 
 solve (or the same electrolytic solution pressure), and the copper 
 sulphate solution, being homogeneous, exerts the same osmotic 
 back pressure on the two plates. Even, therefore, if the plates 
 be joined by a wire, no action can take place. Meanwhile it 
 may be supposed that the ions of both kinds have more or less 
 motion in all directions throughout the solution ; possibly even 
 the free ions may be constantly changing places with atoms of 
 the same kind existing in as yet undissociated molecules. Now 
 it may be supposed that the copper plates are connected severally 
 to the two poles of a battery. One plate becomes an anode, the 
 other a cathode. At once there is an increase in the potential 
 difference between the positively charged anode and the surround- 
 ing solution,. and this is equivalent to establishing an increased 
 electrolytic solution pressure for the anode plate. Thus the copper 
 tends to be converted into ions, which derive their charges from 
 the positive electricity of the anode plate, and so pass into the 
 solution as positive ions. The supplying battery being supposed 
 of constant E.M.F., the potential at the anode remains the same, 
 or, in other words, as fast as a part of the positive charge is 
 withdrawn from it by the newly-formed ions, it receives an 
 additional supply from the battery, sufficient to maintain the 
 same potential difference between the anode and the solution. 
 At the same time the potential difference between the cathode 
 connected with the negative pole of the battery and the surround- 
 ing solution is altered, but, of course, in the opposite direction to 
 that of the anode, for while the anode becomes markedly positive 
 to the solution, the cathode becomes markedly negative. This is 
 equivalent to the creation of a strong negative solution pressure, 
 that is to say, of a tendency, not for the copper of the plate to be 
 ionised into the solution, but to attract to itself positively charged 
 ions from the solution. Thus while one portion of copper is receiving 
 charges from the battery, and so being ionised, at the anode, another 
 portion is giving up charges, and so becoming neutral metallic 
 copper at the cathode, and there is a constant flow of copper ions 
 in a steady stream from the anode to the cathode ; moreover, each 
 ion is of definite weight and is charged with a definite quantity 
 of electricity. So copper dissolves into the solution at the anode 
 and is deposited at the cathode ; the positive electricity led 
 in, so to speak, from the battery at the anode is carried by 
 the copper ions through the solution to the cathode, and the 
 quantity dissolved from the one must be equal to the quantity 
 deposited at the other, and each quantity must be dependent 
 entirely on the quantity of electricity passing through the 
 
ELECTROLYSIS. 353 
 
 solution. The current carried depends partly on the number of 
 free ions in the solution a very weak solution cannot conduct as 
 well as a stronger solution, because it contains fewer of the ions 
 at any moment in contact with the electrode and partly on the 
 potential difference between the electrodes, that is, between the 
 anode and the solution at one side, and the cathode and the 
 solution at the other because the higher the P.D. the greater 
 the added solution pressure at the one pole and negative pressure 
 at the other. But while Cu ions are travelling from anode to 
 cathode, negative S0 4 ions must be passing in the opposite 
 direction, delivering up their negative charges at the anode. 
 Since there is no electrification of the solution observable any- 
 where except at the surfaces of the electrodes, it follows that 
 although the Cu ions are migrating steadily from anode to cathode, 
 there are an equal number of oppositely charged S0 4 ions near 
 them, otherwise the solution at some places would show an excess 
 of positive electricity owing to the excess of Cu ions at those 
 points, and at other places an excess of negative electricity due to 
 free S0 4 ions. Simultaneously with the formation of one Cu ion 
 at the anode, another is discharged at the cathode ; at the same 
 time an S0 4 ion is delivered at the anode, where its negative 
 charge is exactly equal to the positive charge imparted to the 
 copper ion at that moment launched from the anode into the 
 solution, so that the two constituents of the molecule Cu and S0 4 
 become free ions in the liquid at the anode surface at the same 
 instant, and the result is the same as if a molecule of copper 
 sulphate, CuS0 4 , were there added to the solution in the dis- 
 sociated condition ; whilst the Cu ion deposited at the cathode 
 leaves there an unneutralised S0 4 ion at the very moment when 
 it is required to take the place of an S0 4 ion which has set 
 off on its migration towards the anode. Hence the transport 
 of ions, and, therefore, of electricity, through the solution is 
 continuous, and practically the only resistance to the flow of 
 the current is the frictional resistance of the solutions to the 
 ions passing through it. As solutions of salts become more 
 mobile when they are heated, they then offer less frictional 
 resistance to the ions, and so the conductivity of salt solutions is 
 higher as the temperature is raised. The balancing of the equal 
 and opposite pressures at the anode and cathode, when anodes of 
 the same metal as that undergoing deposition are used, accounts 
 for the fact that with pure copper solutions an exceedingly low 
 E.M.F. suffices to cause copper to be deposited. 
 
 Electrolysis with Insoluble Anodes. The principal difference 
 between this case and that last described is that with insoluble 
 anodes the anion does not meet with a positively charged metal 
 ion passing into the solution from the anode. Its negative 
 charge is therefore given up to the anode, and the ion ceases to 
 be an ion and becomes an ordinary uncharged element or group 
 
 23 
 
354 MODEKN THEOEIES OF ELECTROLYSIS. 
 
 of elements. Groups of elements which are supposed to exist 
 together in the ionic state are usually unstable when their charges 
 are removed, so that a decomposition is then observed. Thus 
 the anion, S0 4 , of sulphuric acid and sulphates cannot exist as an 
 uncharged chemical group, and so breaks up in contact with the 
 water, forming free oxygen and sulphuric acid thus : 
 
 But since no metal is being ionised at the anode, the liquid, at 
 least so long as metal is being deposited at the cathode, is 
 gradually becoming weaker. Hence electrolysis with insoluble 
 anodes, in cases where metals are deposited at the cathode, is 
 characterised by the deposition at the anode of the element con- 
 stituting the anion, or the evolution of oxygen gas or one of the 
 constituents of the anion, and also by the gradual removal of the 
 cations from the solution. 
 
 If sulphate of copper solution be electrolysed between a 
 platinum anode and a copper cathode, copper will be deposited 
 at the latter, and, since the platinum does not dissolve, S0 4 will 
 be deposited at the anode, and will there break up, in the presence 
 of water, into sulphuric acid and oxygen ; the nett result of the 
 experiment will be the deposition of metallic copper on the copper 
 plate and bubbles of oxygen on the platinum, and the accumulation 
 of sulphuric acid in the solution. Gradually all the copper will 
 be deposited, and then, if the applied pressure be sufficient to 
 cause hydrogen ions to give up their charges at the cathode, the 
 sulphuric acid will itself be electrolysed, and hydrogen ions will 
 be caused to discharge at one pole, and oxygen will, as before, 
 appear at the other, re-forming sulphuric acid, so that the result 
 is the same as if water only were decomposed. In this case the 
 copper and the platinum exert their own solution pressures, and 
 these in such a way that the copper tends to become ionised and to 
 pass into the solution, so that the action of electrolysis is opposed to 
 the tendency of the metals forming the electrolytic cell, and therefore 
 the current passed into the latter must be able to exert sufficient 
 pressure to overcome the difference between the solution pressures 
 of platinum and copper in a solution of copper sulphate, and to 
 deposit oxygen upon the platinum electrode. Even if electrolysis 
 be commenced with two platinum electrodes, the cathode soon 
 receives a sufficient deposit of copper to cause it to act as a copper 
 plate, and after that time the electrolysis is carried on as between 
 a platinum anode and a copper cathode. 
 
 Secondary Actions. It has been stated above that when an 
 unstable complex anion, such as the group S0 4 , is deposited, it 
 breaks up by chemical action unless it is re-absorbed into the 
 solution along with an equivalent cation detached from a soluble 
 anode. But, to consider the electrolysis of a sulphate with an 
 insoluble anode (for example, platinum) : some water may be 
 
SECONDARY ACTIONS. 355 
 
 electrolysed (but not much, as it is but slightly dissociated or 
 ionised), and there is in consequence a little hydroxyl formed, but 
 chiefly S0 4 is deposited, and this forms free oxygen, as alread 
 described. Should there be contained in the liquid in contact 
 with the anode a substance that is capable of a higher degree of 
 oxidation, as, for example, ferrous sulphate, some or all of the free 
 oxygen will be absorbed in converting this substance into the 
 corresponding compound containing a larger proportion of oxygen, 
 ferric sulphate, or, if suitable organic substances are present*, they 
 may be oxidised, or, as it were, burned by a process of liquid com- 
 bustion. The evolution of oxygen would then be wholly or in 
 part suppressed, and the heat that, in an ordinary chemical 
 experiment, would be produced by this oxidation, would be 
 rendered available for reducing the total E.M.F. required for the 
 reactions at the electrodes. These anode reactions are actually 
 employed in many electro-chemical operations. But chemical 
 changes may also occur at the cathode, if any metal or cation 
 be there deposited that is really capable of decomposing water. 
 Thus when potassium or sodium salts in solution in water are 
 electrolysed, potassium or sodium ions become converted into 
 metal at the cathode ; and as either metal is capable of attacking 
 .water with great energy, the metal itself is not seen, but only 
 the hydrogen that results from its action on water. When 
 mercury is employed as cathode, the deposited alkali metal 
 becomes dissolved to some extent in, and diffused through, the 
 mercury, and so removed from the possibility of attack by water 
 except on the exposed surface of the mercury. The formation of 
 a true amalgam of mercury and the alkali metal testifies to the 
 reality of this deposition, and the reaction is utilised in certain 
 electrolytic processes for the production of caustic soda from 
 solutions of common salt and the like. 
 
 Electrolysis of Mixed Solutions and Double Salts. When a 
 current is passed through a mixed solution of several electrolytes, 
 the conductivity of the liquid is found still to be dependent on the 
 number of free ions. It thus appears that all the free ions in the 
 liquid are engaged in the transport of electricity from one pole 
 to the other, cations from anode to cathode, and anions in the 
 opposite direction. But it is well known that if a current of 
 moderate density be used with a solution containing several 
 metals, only the one with the lowest electrolytic solution pressure 
 (the most electro-negative) will be deposited at first, and then the 
 others in turn, in the reverse order of their solution pressures. 
 Thus in a solution of cadmium, copper, and silver, silver alone 
 would be deposited first, then silver with an increasing percentage 
 of copper, then copper, next copper with an increasing percentage 
 of cadmium, and lastly cadmium alone, supposing always that the 
 E.M.F. applied is sufficient to allow of the deposition of the 
 cadmium with its relatively high solution pressure. Remembering 
 
356 MODERN THEORIES OF ELECTROLYSIS. 
 
 that a metal with high solution pressure is able to become ionised 
 at the expense of the ions of metals with lower solution pressure, 
 this action may be thus explained : All the free ions alike 
 cadmium, copper and silver are migrating and carrying charges 
 from anode to cathode, but at the cathode surface the current 
 deposits that ion which requires least E.M.F. to overcome its 
 electrolytic solution pressure and to cause it to give up its 
 positive charge to the cathode ; even if the ion of another metal 
 with higher solution pressure were deposited, the deposited metal 
 would exchange with an equivalent of the metal with lower 
 solution pressure, and would pass again into solution. So long, 
 then, as there is a sufficient number of ions of the metal with 
 lowest solution pressure (e.g., silver) in contact with the cathode 
 to carry to the cathode the whole volume of the current passing, 
 only that metal, silver, will be deposited. But as, in course of 
 time, more and more of the silver in the solution is deposited at the 
 cathode, the quantity left will sooner or later be insufficient to carry 
 the whole of the current to the cathode surface ; then ions of the 
 metal with the next lowest solution pressure (copper) will begin to 
 be deposited. At first only very little copper will deposit with 
 the silver, but as the silver ions become fewer, the proportion 
 of the copper ions discharged must increase, until, at last, all the 
 silver is thrown down, and only copper and cadmium remain in 
 the solution. Just in the same way the gradual exhaustion of 
 the copper ions will lead to the co-deposition of cadmium, and, 
 finally, when all the silver and all the copper are deposited, 
 cadmium ions only will be available to carry the electricity 
 through the solution, and give up their charges at the cathode 
 plate. 
 
 Electrolysis of Solutions of Complex Acids. Where, however, 
 the solution is not merely a mixture of two electrolytes or a solu- 
 tion of a double salt, but a chemical compound, the action of the 
 current is different. A mixture of copper sulphate and nickel 
 sulphate would act as described above ; copper first, and then (if 
 the solution were kept neutral) nickel, would be deposited at the 
 cathode, and S0 4 (and hence oxygen) only at the anode. But 
 if gold cyanide be dissolved in potassium cyanide solution, the 
 resulting liquid does not behave in the same way. It may be 
 shown that the gold travels with the ion which migrates from 
 cathode to anode, and only the potassium migrates from anode to 
 cathode ; yet the gold is deposited only at the cathode. This is 
 due to chemical reaction. The ions of such a cyanide (KAuCy 2 ) 
 are apparently K and AuCy 2 , the K ion giving up its charge at 
 the cathode, but immediately attacking the solution in contact 
 with it and depositing not hydrogen but gold from the liquid 
 around, because potassium can break up the complex aurocyanide 
 of potassium that is present. 
 
 KAuCy 2 + K + H 2 = 2KCy + Au + H 2 0. 
 
ELECTROLYSIS OF SOLUTIONS OF COMPLEX ACIDS. 357 
 
 Thus gold is deposited at the cathode, not because ions of 
 free gold exist in the liquid, but because potassium is deposited 
 and exchanges with the gold in some of the complex substance 
 KAuCy 2 in contact with the cathode. At the same time gold does 
 not form at the anode, because the ion AuCy 2 cannot exist alone 
 in the liquid, but breaks up into cyanogen, Cy, and gold cyanide 
 AuCy, which re-dissolves in the free potassium cyanide in the 
 solution, re-forming potassium aurocyanide. The free cyanogen 
 is then able, with its negative charge, to allow the passage of 
 another atom of gold which withdraws its positive charge from the 
 anode and passes into the ionic condition. It will thus be under- 
 stood that in a gold bath there is a great tendency for gold 
 cyanide to deposit on the anode, and also that, as there is no con- 
 stant migration of gold to the cathode, but rather contrariwise, 
 the deposition of potassium must be made to take place so slowly 
 that the natural diffusion of the salts in the solution may ensure 
 that there is always sufficient of the aurocyanide in contact with 
 the cathode to allow of the exchange of the whole of the potassium 
 for gold without water being decomposed and hydrogen deposited. 
 Thus, also, silver travels in the anion in the electrolysis of the 
 double cyanide of silver and potassium, and the silver is deposited 
 by exchange with the potassium which, as cation, is deposited at 
 the cathode. 
 
 The double chloride of platinum and sodium Na 2 PtCl 6 behaves 
 similarly, breaking up into the anion PtCl 6 (which decom- 
 poses) and the cation Na ; so that it is not merely a double 
 salt, but a platino-chloride of sodium, and platinum is deposited 
 at the cathode only by secondary action. The ferrocyanides 
 and ferricyanides, and many other complex salts, behave simi- 
 larly, the iron or other metal being present in a complex 
 anion. 
 
 If it be true that in the case of these complex salts some 
 of the metal to be deposited is travelling in the anion, and, 
 therefore, away from the cathode, it is obvious that the current- 
 density used for electrolysis should be low, otherwise the liquid 
 around the cathode might become exhausted and the deposited 
 cation would fail to find in the solution in contact with it any 
 of the metal with which it is intended to exchange places. 
 In the case of potassium or sodium compounds, hydrogen would 
 then be deposited by exchange with the alkali-metal. The 
 danger is, of course, greatest in the case of very dilute 
 solutions. 
 
 Effects of the Migration of the Ions. It has already been 
 mentioned that different ions travel at different velocities. Thus, 
 if the potassium ion move through a solution so weak that it 
 may be regarded as infinitely dilute with a velocity of -00006 
 centimetres per second, when there is a fall of electrical potential 
 of 1 volt per centimetre traversed, and the liquid is at 18 C., 
 
358 
 
 MODERN THEORIES OF ELECTROLYSIS. 
 
 Kohlrausch has shown that some of the principal elements will 
 travel at the velocities given in the following table : 
 
 Table, showing the velocities of individual ions, expressed as the 
 number of centimetres traversed by the ions in 100,000 seconds. 
 
 Ion. 
 
 Cms. traversed in 
 1x100, 000 sees. 
 
 Ion. 
 
 Cms. traversed in 
 1 x 100,000 sees. 
 
 H 
 
 320 
 
 OH 
 
 165 
 
 NH 4 
 
 66 
 
 1 
 
 69 
 
 K 
 
 66 
 
 Cl 
 
 69 
 
 Ag 
 
 57 
 
 N0 3 
 
 64 
 
 Na 
 Li 
 
 45 
 36 
 
 CIO, 
 
 C 2 H S 2 
 
 57 
 36 
 
 The conductivity of the solution depends then partly on the 
 number of free ions, because only free ions are able to conduct ; 
 partly upon the nature of the ions, because each monovalent ion 
 carries half as much electricity as a divalent, one-third as much as 
 a trivalent ion, and so on ; and partly upon the combined rates at 
 which the different ions in the solution migrate through it in one 
 direction or the other. Obviously, other things being equal, the 
 faster the charged ions can travel the greater will be the con- 
 ductivity. Hence the solutions which contain most free hydrogen 
 or hydroxyl ions are those which conduct best. Water, which 
 would contain both of these ions, if it were appreciably dissociated, 
 would be one of the best of liquid conductors ; but as it is not 
 ionised, no advantage is derived from the ions, and it is almost a 
 non-conductor. The acids, however, which all contain free H ions 
 are good conductors, and the hydroxides are fair conductors. 
 Thus, on comparing solutions containing equal numbers of free 
 ions in a given volume, it will be found that, if hydrochloric acid, 
 let us say, have a conductivity of 320 + 69, or 389, potassium 
 chloride would have a conductivity of 66 + 69, or 135 (i.e., it is 
 only one-third as good a conductor as a hydrochloric acid solution 
 of equivalent strength), sodium chloride would have a conductivity 
 of 45 + 69, or 114, sodium nitrate a conductivity of 45 + 64 = 109, 
 and so on. This is evident from the above table, because there it 
 is shown that each ion has its own rate 'of migration, which is 
 independent of the nature of the compound of which it had formed 
 a part. The caution must again be given that such numbers as 
 these are only strictly comparable when the substances are 
 practically completely dissociated, that is to say, when the solu- 
 tions are exceedingly dilute, or at least when the degree of disso- 
 ciation is known. 
 
 Now, seeing that the ions move with varying velocity, and 
 
EFFECTS OF THE MIGEATION OF THE IONS. 
 
 359 
 
 I, 
 
 that in a given solution the number of anions giving up their 
 charges to the anode in a 
 given time may, therefore, 
 be greater (or less) than the 
 number of cations striking 
 the cathode in the same 
 time, the question may be 
 asked Why are there not 
 disproportionate amounts 
 of the ions discharged and 
 deposited at the two elec- 
 trodes? The answer may, 
 perhaps, be best given dia- 
 grammatically. 
 
 To take first the case 
 of a solution in which the 
 two ions travel with equal 
 velocities : Let A and C 
 be the (insoluble) anode 
 and cathode respectively, 
 in an electrolyte (let us 
 say dilute solution of potas- 
 sium chloride) of uniform 
 strength contained in a 
 vessel divided into two 
 by the porous partition P. 
 The uppermost section re- 
 presents the distribution of 
 the ions in the solution 
 before the current passes; 
 the second section the same 
 solution after 2 x 96540 
 coulombs of current have 
 passed, that is, after suffi- 
 cient current to deposit 
 twice the equivalent weight 
 of potassium and chlorine 
 have been conducted 
 through the solution ; and 
 the , lowest division the 
 same after 4 x 96540 cou- 
 lombs have passed. 
 
 For the sake of clear- 
 ness there are supposed to 
 be only a limited number 
 of ions, and these are only 
 shown in the centre of the 
 cell, in order to allow for 
 
 
 
 
 
 
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360 MODERN THEORIES OF ELECTROLYSIS. 
 
 towards one or other electrode, and the rates of migration of K 
 and Cl ions are supposed to be equal, which is very nearly, but 
 not precisely, accurate. In the uppermost cell there are supposed to 
 be five (dissociated) molecules in each compartment, the positive 
 charges of the five K ions being exactly neutralised by the nega- 
 tive charges of the five Cl ions. But now 2 x 96540 coulombs 
 of electricity are passed, migration takes place, one coulomb, con- 
 veyed by a potassium ion, moves through the porous partition 
 as the ion migrates from the anode to the cathode side, and one 
 coulomb is carried by a chlorine ion through the partition in the 
 opposite direction, so that there are now 6 Cl ions instead of 5, 
 and 4 K ions instead of 5 on the anode side, and 4 Cl ions and 6 
 K ions on the cathode side. Hence there are, in each compartment, 
 4 ions of each kind with charges neutralising one another, and on 
 the anode side an excess of 2 negatively charged Cl ions, and on 
 the cathode side an excess of 2 positively charged K ions. These 
 charged ions, of course, lose their charges in contact with the 
 electrodes, and so, ceasing to be ions, they deposit in the ordinary 
 uncharged conditions (metal or non-metal, as the case may be). 
 Thus the passage of 2 coulombs of electricity is accompanied by 
 the migration of 1 ion in each direction, and by the separation of 
 2 equivalents of an element at each electrode. Similarly, the 
 passage of 4 coulombs means the transfer of 2 anions from the 
 cathode compartment to the other, and of 2 cations in the 
 opposite direction, with the separation of 4 equivalents at each 
 electrode. But there are still 3 ions of each kind neutralising 
 each other's charges on either side of the partition, or, in other 
 words, there are as many molecules of KC1 (dissociated though 
 they may be) in the anode compartment as there are in the 
 cathode division. This is equivalent to saying that when the 
 velocities of migration of the two ions are equal, the distribution 
 of the molecules is unaffected. 
 
 Effect of Unequal Ionic Velocities on the Concentration of 
 the Electrolyte at the Electrodes. Next, a case may be con- 
 sidered in which one ion travels at twice the rate of the other. 
 This is approximately the case with copper sulphate, in which the 
 S0 4 ion travels at about double the rate of the Cu ion. Using 
 the same method of diagrammatic illustration as before, the upper- 
 most section represents a dilute solution of copper sulphate before 
 any current is passed, the second section the same solution after 
 6 x 96540 coulombs have passed, and the' lowest the same after 
 12 x 96540 coulombs have been passed. Here it must be remem- 
 bered that Cu and S0 4 are both divalent, so that each ion trans- 
 ports a double charge of electricity, namely, 2 x 96540 coulombs. 
 
 In this case the middle section of the figure shows that the migra- 
 tion of 1 Cu ion from anode to cathode chamber is accompanied 
 by the migration of 2 S0 4 ions in the opposite direction, each of 
 which carries a double charge or 2 coulombs of electricity, so 
 
EFFECT OF UNEQUAL IONIC VELOCITIES. 
 
 361 
 
 T T t 
 
 | i ! 
 
 that 4 coulombs are transported by 2 ions in one direction, and 
 
 2 coulombs by 1 ion in the 
 opposite direction. It will 
 be seen that in spite of the 
 uneven rate of travel, there 
 are 3 Cu ions discharging 
 their double loads of elec- 
 tricity at the cathode, and 
 
 3 S0 4 ions giving up their 
 negative charges at the 
 anode, and hence the 6 cou- 
 lombs of electricity deposit 
 3 atoms of Cu and 3 S0 4 
 ions. But it will be noted 
 that while there remain on 
 the anode side 5 Cu ions 
 electrically neutralised by 
 5 S0 4 ions, or 5 dissociated 
 molecules of copper sulph- 
 ate, there are only 4 sets of 
 mutually neutralised ions, 
 or 4 molecules of dissoci- 
 ated copper sulphate in 
 the cathode compartment. 
 This inequality is still more 
 clearly shown in the lowest 
 section of the diagram, 
 where, as it will be seen, 
 there are 6 S0 4 ions dis- 
 charged and 4 dissociated 
 CuS0 4 molecules left in 
 the anode cell, while there 
 are 6 Cu ions discharged 
 and 2 dissociated CuS0 4 
 molecules left in the cathode 
 cell. 
 
 The effect, then, of the 
 unequal rate of migration 
 of the ions is to cause an 
 accumulation or concentra- 
 tion of molecules of the elec- 
 trolyte immediately around 
 the electrode, towards which 
 are migrating the ions that 
 move with the greater ra- 
 pidity. Thus, as has been 
 shown, there is a tendency for the solution around the anode 
 in a copper sulphate bath to become stronger than that around 
 
 
 
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362 MODERN THEORIES OF ELECTROLYSIS. 
 
 the cathode. In electro-plating, soluble anodes are generally 
 used, and the tendency is even more marked than in the above 
 instance, where the anode was supposed to be insoluble, because 
 each anion liberated at the anode carries back with it into the 
 solution a cation from the anode. If no migration of anions 
 occurred (say of S0 4 in copper sulphate), the copper deposited 
 at the cathode would leave a gradually increasing excess of acid 
 ion in the neighbourhood ; but as migration always occurs, there 
 must always be a tendency for the molecules of the electrolyte 
 to accumulate on the anode side, and the greater the relative 
 velocity of the anion, the greater this accumulation will be. The 
 lesson to be learned is that the baths must be kept very thoroughly 
 stirred, in order to keep the composition uniform. Attention is 
 especially necessary, however, when double cells are used with 
 porous partitions between, as in some electro-metallurgical opera- 
 tions. The problem is also of special importance in electro- 
 chemical work, such as that of caustic soda manufacture by 
 electrolysis from common salt. 
 
 Conditions of Electrolysis. By the light of modern theories, 
 it is obvious that the conditions most favourable to electrolysis 
 are that 
 
 (1) The liquid around the (soluble) anode should contain as 
 little of the anode metal as possible in solution, because in that 
 case the back osmotic pressure opposing the solution of the anode 
 is minimised, whilst 
 
 (2) The liquid around the cathode should contain as much as 
 possible in solution of the metal to be deposited, as the osmotic 
 pressure at this electrode is favourable to deposition. 
 
 (3) The solution should be kept as uniform as possible, so that 
 interfering local actions due to solutions of different density or of 
 different degrees of acidity may be prevented. 
 
 It is obvious that these three conditions are incompatible 
 unless porous diaphragms are employed to keep the anode and 
 cathode solutions separate, which is usually a course to be 
 avoided, as there are no materials that can generally be 
 economically employed for such diaphragms in practical work, 
 and the loss in their use more than compensates for the gain 
 otherwise. 
 
 (4) The E.M.F. used must be sufficient when insoluble anodes 
 are used to neutralise the back solution pressure of the metal de- 
 posited. For this reason it is commonly possible in a mixed 
 solution of two or more metals to separate one metal from the 
 other electrolytically by applying an E.M.F. at the electrodes 
 sufficient to overcome the solution pressure of the most negative 
 metal, but insufficient to neutralise that of the other metal 
 present. - 
 
CHAPTER XX. 
 
 A GLOSSARY OF SUBSTANCES COMMONLY EMPLOYED IN 
 ELECTRO-METALLURGY. 
 
 IN using the various chemical preparations, it is impossible to be 
 too careful, as in all operations connected with electro-metallurgy 
 the most scrupulous cleanliness and thoughtful attention are 
 necessary, especially on the part of one who is unaccustomed to 
 handling chemical apparatus and reagents. 
 
 In opening fresh bottles of acid or of ammonia, especially in hot 
 weather, the stopper must be first carefully cleansed externally, 
 and must then be covered with a stout cloth before attempting to 
 remove it, because it is frequently wetted with the liquid contained 
 in the bottle, and if there be any pressure from within, drops of the 
 fluid may be scattered in all directions when the stopper is loosened, 
 and may fall upon the clothes, hands, or face, and are likely to 
 cause blindness if they should happen to penetrate to the eye. 
 In tropical climates the strong ammonia solution boils violently 
 as soon as it is uncorked ; and as this is evidence of a strong 
 internal pressure, it is safer to surround the bottle in several 
 folds of cloth before attempting to open it, lest the application of 
 the slight force, sometimes required to loosen the stopper, may 
 cause the bottle itself to burst should it be defective. 
 
 The following simple rules should be carefully observed : 
 
 In opening any bottle, first dust the whole surface and then 
 clear away from the neck all dirt or loose particles of sealing-wax, 
 cork, or luting ; see that the corkscrew does not break off particles 
 of cork into the bottle. 
 
 In loosening the refractory stopper of a bottle containing any 
 inflammable substance, on no account apply the heat of a flame ; 
 and in decanting any such liquid, or opening any such bottle, 
 see that it is not in the vicinity of a lamp or flame of any kind, 
 or even of any heated substance if the liquid be carbon bisulphide. 
 
 On removing the cork or stopper, never place it with the smaller 
 end downwards upon the table, as it is very likely to pick up 
 foreign matter from the surface and transfer it to the bottle as 
 soon as it is re-inserted. 
 
 Never allow a bottle to remain open to the air longer than 
 
 363 
 
364 A GLOSSARY OF SUBSTANCES 
 
 necessary, and always see that it is closed with its original 
 stopper. 
 
 If the solid contents of a bottle require loosening, always apply 
 a stout glass or glazed procelain rod, carefully cleaned before 
 insertion. On no account use a metal rod, unless it be made of 
 platinum or other material which cannot be corroded by the sub- 
 stance with which it is brought in contact. 
 
 Never pour the contents of a bottle into a dirty vessel, or upon 
 an unclean surface. 
 
 Never place chemical substances directly upon the metal pan of 
 a balance, but always upon a tared plate of glass (i.e. one whose 
 weight is known, or compensated for), though a sheet of clean 
 glazed paper may suffice if the substance is dry and not deli- 
 quescent (that is, does not tend to become moist by exposure 
 to damp air). 
 
 In returning an excess of any substance into store, see that it 
 is placed in the right bottle or vessel. 
 
 Never mix a strong acid with a strong alkali, and even when 
 both are diluted, let the mixture be effected gradually. 
 
 Never add water to strong sulphuric acid, but if it be desired 
 to dilute the acid, let it be added little by little, with constant 
 stirring, to the required bulk of water. 
 
 Never employ vessels of a kind which are used for domestic 
 purposes to contain chemical reagents. Dangerous results may 
 follow the leaving of acids or poisonous substances in ordinary 
 tumblers or wine glasses. Let the vessels used for chemical work 
 be restricted to this duty, and let no others be employed. 
 
 Note. In the following glossary, the specific gravity of a substance 
 refers to its weight as compared with that of an equal bulk of 
 pure distilled water at the same temperature. The acids are 
 given first. 
 
 Acetic Acid, CH 3 . C0 2 H, or C 2 H 4 2 . A monobasic and some- 
 what feeble acid, the chief acid constituent of vinegar. It may 
 be bought as glacial acetic acid, which is practically the pure 
 substance, and is a crystalline solid at ordinary temperatures. It 
 is more usually obtained somewhat diluted with water. The 
 acidum aceticum of the British Pharmacopoaia contains from 32 to 
 33 per cent, of the pure acid, and has a specific gravity of 1*044. 
 With bases it forms acetates. 
 
 Benzoic Acid, C 6 H 5 . C0 2 H, or C 7 H 6 2 . A monobasic weak 
 organic acid, obtained as colourless plates very slightly soluble 
 in water, which should leave little or no residue when burnt 
 upon a sheet of thin platinum held over a flame. Its salts are 
 termed benzoates. 
 
 Boric Acid, H 3 B0 3 . Commonly known as boracic acid; a 
 mineral body and the acid basis of borax ; it is a white crystal- 
 
COMMONLY EMPLOYED IN ELECTRO-METALLURGY. 365 
 
 line solid, fairly soluble in water. Dissolved in warm spirits of wine 
 and ignited, the liquid burns with a brilliant and characteristic 
 green flame. It is tribasic, and its salts are called borates. 
 
 Citric Acid, C 3 H 4 (OH) . (C0 2 H) 3 , or C 6 H 8 7 . A white trans- 
 lucently-crystalline, solid, tribasic, organic acid, readily soluble in 
 water (100 parts in 75 of cold or 50 of hot water). It forms 
 citrates with metallic oxides. 
 
 Hydrochloric Acid, HC1. The pure body is a gas under 
 ordinary conditions. It is intensely soluble in water, and its 
 solution is sold under the above name or that of muriatic acid. 
 The stronger solutions emit pungent white fumes in air, owing 
 to a partial evaporation of the contained gas, which recondenses 
 in the form of clouds in contact with atmospheric moisture. A 
 saturated solution contains about 40 per cent, of the pure gas, 
 and has a specific gravity of 1*2; the actual saturation percent- 
 age depends upon the temperature, as the acid is less soluble in 
 hot water than in cold. The acid of commerce has usually a 
 specific gravity of about 1*150, equal to 30 per cent, of pure 
 HC1 ; the acidum hydrochloricum of the British Pharmacopoeia 
 contains about 32 per cent., with a specific gravity equal to 1*16. 
 The liquid should be colourless, but the commercial muriatic 
 acid is generally yellow, owing to the presence of iron and other 
 impurities ; only the pure acid, however, should be used for most 
 electro-metallurgical processes, though the crude liquid may often 
 be employed for cleansing purposes. The acid is monobasic, and 
 its salts are termed chlorides. 
 
 Hydrocyanic Acid, HCN. Commonly known as prussic acid. 
 This also is bought as an aqueous solution of HCN, which is a 
 liquid of low boiling-point. It smells strongly of bitter almonds 
 and is a deadly poison, so that its use in the arts is to be strongly 
 deprecated whenever it can be avoided. The British Pharma- 
 copoeia solution contains 2 per cent, of the pure acid. When used, 
 the greatest care must be exercised, as the vapour evolved by 
 a strong solution is itself extremely poisonous, and even when 
 diluted considerably with air, produces giddiness and headache. 
 Its salts are designated cyanides ; the acid is monobasic. 
 
 Nitric Acid, HN0 3 . Commercially known as aquafortis. It 
 is a very powerful and corrosive monobasic acid, which must be 
 handled with great care. It stains the skin and other animal 
 substances a bright yellow, which becomes intensified by the 
 application of an alkali or of soap ; its oxidising power is so 
 intense that the accidental fracture of a carboy of the acid has 
 been known to set fire to straw which happened to surround it. 
 The pure acid (HN0 3 ) has a specific gravity of 1 '52. The acid 
 
366 A GLOSSARY OF SUBSTANCES 
 
 commonly sold in commerce has a specific gravity of 1*45, equal 
 to 77 per cent, of pure HN0 3 , while a weaker acid, containing 
 70 per cent, (specific gravity = 1 '42) is also to be had, and con- 
 stitutes in fact the acidum nitricum of the British Pharmacopoeia, 
 while others even less concentrated are likewise to be procured. 
 The stronger solutions fume in the air. When pure, the liquid 
 should be colourless, but owing to partial decomposition into lower 
 oxides of nitrogen, it frequently possesses a straw-coloured or 
 yellow tint, becoming in the commoner kinds, orange and, finally, 
 green, when it is commercially termed nitrous acid. The salts of 
 nitric acid are called nitrates. 
 
 When mixed with hydrochloric acid in the proportion of 1 to 3 
 (HN0 3 : HC1) the liquid is known as aqua regia, and assumes an 
 orange colour due to the presence of the gas nitrosyl chloride 
 (NOC1) formed by the union. This liquid is endowed with the 
 highest oxidising powers, dissolving even the precious metals 
 which resist the attack of either acid singly. 
 
 Oxalic Acid (C0. 2 H) 9 or C 2 H 2 4 . A dibasic organic acid, which 
 forms a white crystalline solid containing 2 molecules of water 
 (C 2 H 2 4 . 2H 2 0). It is readily soluble in water or alcohol, and is 
 a powerful poison. It forms oxalates. 
 
 Sulphuric Acid, H 2 S0 4 . Known as oil of vitriol. A dibasic 
 and most powerful mineral acid. When perfectly pure, it is a 
 colourless and odourless, oily liquid of specific gravity 1/842. It 
 combines most energetically with water, and in doing so generates 
 much heat, so that dilution even of the ordinary commercial 
 acid with water must be effected with the greatest care. Large 
 volumes must never be thoughtlessly mixed, nor should the water 
 be added to the acid, lest a sudden generation of steam of ex- 
 plosive violence result, and the dangerously corrosive liquid be 
 scattered in all directions. The acid is in all cases to be added 
 to the water in a very gentle stream, and with constant stirring. 
 The acid of commerce is diluted in various degrees, but is always 
 concentrated. The salts formed from this acid are sulphates. 
 
 Tannic Acid, C 14 H 10 9 . A pale yellow solid and weak organic 
 acid, readily soluble in water. It burns completely away when 
 heated upon a metallic plate, and yields a blue-black colour when 
 added to solutions containing iron. It forms tannates. 
 
 Tartaric Acid, C 2 H 2 (OH) 2 . (C0 2 H) 2 or C 4 H 6 6 . It is a colour- 
 less crystalline solid, which readily dissolves in water. It is a weak 
 dibasic organic acid, forming salts which are known as tartrates. 
 
 Alcohol, C 2 H 5 (OH) or C 2 H 6 0. Pure or absolute alcohol has 
 a strong affinity for water, so that the alcohol usually bought 
 
COMMONLY EMPLOYED IN ELECTRO-METALLURGY. 367 
 
 generally contains a small quantity of the diluent. It should be 
 colourless and have a specific gravity of 0*7939. It is commonly 
 sold under the name of spirits of 'wine , or rectified spirit, which 
 contains about 16 per cent, of water, and has a specific gravity of 
 O838. Proof spirit is a mixture carrying 49'24 per cent, of pure 
 alcohol and is the standard with which alcoholic liquids are com- 
 pared in commerce. Methylated spirit contains a certain amount 
 of methylic alcohol or wood spirit, and frequently has a quantity 
 of resinous matter added to it to render it unfit for drinking, 
 so that it may not be liable to excise duties, and shall yet be 
 available for most of the purposes for which spirit is required in 
 the arts. Such a product should on no account be used for 
 electro-metallurgical work without previous thought as to the 
 probable consequences of using it. For example, the effect of 
 washing an object in the sophisticated spirit and then plunging 
 it into the electrolytic bath would be the introduction of undesir- 
 able organic impurities into the latter, and even the formation of 
 a non-conductive coating upon the surface of the article itself ; 
 because water added to the impure alcohol throws down the 
 resinous substances in the form of a white precipitate. Such 
 spirit tends to burn with a smoky flame. Alcohol should, 
 therefore, be tested by burning, when the flame should be almost 
 non-luminous, and by the addition of water to a sample, which 
 addition should produce no turbidity. 
 
 Aluminium, Al. A white silvery element with an almost 
 imperceptible bluish shade. It is extremely light (the specific 
 gravity being only 2 '58), is very malleable and ductile, takes a 
 high polish, and is not liable to tarnish in air. It melts at about 
 1160 F. It is largely used in alloys, and for manufacture of 
 objects where combined lightness and strength are required. Its 
 principal common impurities are iron and silicon. 
 
 Aluminium Chloride, A1 2 C1 6 . A white, sometimes yellowish 
 substance, which in the anhydrous condition absorbs water with 
 great avidity ; and, having once absorbed it, cannot be induced 
 to part with it, the action of heat upon the hydrated crystalline 
 salt (A1 2 C1 6 . 12H 2 0) causing decomposition, with the formation 
 of alumina and hydrochloric acid. It must, therefore, be stored 
 in perfectly air-tight jars. In small quantities it volatilises at 
 356 to 365 F. without fusion, but in a large bulk it may be 
 induced to melt first. 
 
 Aluminium - Sodium Chloride, A1 2 C1 6 . 2NaCl. This salt is 
 made by heating the simple aluminium chloride with common 
 salt. It is more useful than the latter for electro-reduction by the 
 fusion method, because, melting at 365 F., it volatilises only at a 
 red heat ; moreover, it is less readily attacked by aqueous vapour. 
 
368 A GLOSSARY OF SUBSTANCES 
 
 Aluminium-Potassium Sulphate, A1 2 (S0 4 ) 3 . K 2 S0 4 . 24H 2 0. 
 Commonly known as potash alum. It is a crystalline substance, 
 with an astringent taste, and is readily soluble in water, 12 parts 
 of alum dissolving in 100 parts of water at the ordinary tempera- 
 ture, 357 parts at the boiling-point. It is a double sulphate of 
 alumina and potash. Ammonia alum is exactly analogous, the 
 potassium sulphate being simply replaced by ammonium sulphate 
 (A1 2 (S0 4 ) 3 .(NH 4 ) 2 S0 4 .24H 2 0), and is for most purposes inter- 
 changeable with potash alum. Soda alum is similar, but is more 
 readily soluble in water. 
 
 Ammonium Hydrate, NH 3 . H 2 0. Commonly termed ammonia 
 or spirits of hartshorn. It is simply water saturated with 
 ammonia gas (NH 8 ). It is a powerful alkali, and is, therefore, 
 useful to neutralise the acid properties of any substance. As it 
 has an overpoweringly pungent odour, care must be taken in 
 using the stronger solutions. It is obtainable in the market as 
 ammonia fortiss., with a specific gravity of 0'880, which is almost 
 saturated with the gas and contains about 36 per cent, of pure 
 NH 3 . Bottles of the liquid must be opened cautiously in hot 
 weather, because the warmer water cannot dissolve so large a 
 volume except under pressure ; and the excess is given off, 
 sometimes with considerable violence, directly the pressure is 
 released by the removal of the stopper. A weaker solution, 
 ammonice liquor fortior, containing 32 '5 per cent, of NH 8 (specific 
 gravity = 0*891) is that recognised by the British Pharmacopoeia, 
 and is safer for use in the heat of summer. By exposure, 
 ammonia gas is gradually evolved, so that it must be stored in 
 closely-stoppered bottles in order to preserve the strength of the 
 solution unimpaired. By regarding the formula as NH 4 . OH, it 
 becomes the hydrate of the hypothetical metal-like substance 
 ammonium (NH 4 ). This radical or group of elements, NH 4 , 
 behaves in its chemical relations exactly as a monovalent metal, 
 combining with acids to form ammonium salts. 
 
 Ammonium Carbonate, (NH 4 ) 2 C0 3 , and the Bicarbonate, 
 (NH 4 )HC0 3 , are white solid substances, soluble in water and smell- 
 ing strongly of ammonia. The commercial salt is not a pure car- 
 bonate, but is in part carbamate ; it is, however, suitable for 
 the purposes to which it is generally applied. As an alkaline 
 carbonate it takes the place of ammonia itself in neutralising 
 acids, carbonic acid gas being evolved while the ammonium salt 
 of the stronger acid is formed. 
 
 Ammonium Chloride, NH 4 C1. A white substance occurring 
 in commerce in the form of tough fibrous crystals, odourless, and 
 soluble in water (100 parts of cold water dissolve 35 parts, and 
 of boiling water 77 parts of the salt). 
 
COMMONLY EMPLOYED IN ELECTRO-METALLURGY. 369 
 
 Ammonium Nitrate, NH 4 N0 3 . A colourless crystalline body, 
 200 parts of which dissolve in 100 parts of cold water. Heated 
 gently it melts and is afterwards decomposed into nitrous oxide 
 gas (laughing gas = N 2 0) and water, leaving no residue. 
 
 Ammonium Sulphide, (NH 4 ) 2 S, and Hydrosulphide, NH 4 HS, 
 may be prepared by passing hydrogen sulphide gas into a 
 solution of ammonia. It is at first colourless, but by gradual 
 decomposition becomes yellow. It is generally bought as an 
 amber-coloured, frequently almost orange, fluid. The many 
 products of decomposition do not seriously interfere with its 
 general utility except in so far as they weaken the solution of 
 the sulphide itself. 
 
 Ammonium Sulphate, (NH 4 ) 2 S0 4 . A white crystalline solid of 
 which 100 parts of cold water dissolve 50, of hot water 100 parts. 
 
 Ammonium Salts of the acids just described should leave no 
 fixed residue when heated over a flame upon a piece of sheet 
 metal. 
 
 Antimony, Sb. A white, highly crystalline and extremely 
 brittle metal. The commercial ingots have usually a very 
 crystalline surface, which resembles the fern-like appearance of 
 frost upon window-glass, from which the name star antimony is 
 derived, and which is regarded by many as an infallible criterion 
 of the purity of the metal, but which must not be relied upon 
 too closely. This metal has a specific gravity of 6*8, and melts 
 at 800 F. The principal objectionable impurities likely to 
 occur in ordinary antimony are sulphur, arsenic, tin, lead, silver, 
 bismuth, and iron. It is only procurable in the cast condition, 
 because on account of its brittleness it cannot be rolled. 
 
 Antimony Chlorides. There are two chlorides the trichloride 
 or antimonious chloride, SbCl 3 , known as butter of antimony, 
 which is that more usually required for electro-metallurgical 
 work ; and the pentachloride or antimonic chloride, SbCl 5 . The 
 former is a colourless crystalline substance melting at 164 F. 
 It may be prepared in crude form, mixed with excess of acid, by 
 dissolving antimony, or the sulphide, in hydrochloric acid to 
 which a little nitric acid has been added to increase its oxidising 
 action. The pentachloride is a colourless fuming liquid having a 
 most unpleasant odour. 
 
 Antimony Sulphide, Sb 2 S 3 , occurs in nature as stibnite or 
 antimony glance. It is usually bought as grey needle-shaped 
 crystals sub-metallic in lustre. The pentasulphide, Sb 2 S 5 , 
 requires no notice here. 
 
 24 
 
370 A GLOSSARY OF SUBSTANCES 
 
 Antimony-Potassium Tartrate, KSbC 4 H 4 0, r , commonly known 
 as tartar emetic. A white crystalline substance, of which 100 
 parts of cold water dissolve 5 parts, while a like volume of hot 
 water dissolves 50 parts. 
 
 Aqua Fortis. See Acid, Nitric. 
 Aqua Regia. See under Acid, Nitric. 
 
 Bees'-wax. The substance of which the honeycomb is built 
 up. It is usually of a yellow or brown colour, the specific 
 gravity ranging from 0*958 to 0*960, and the melting-point from 
 144 to 156 F. in different samples. It dissolves readily in oils 
 and in ether, but not in water. Its chief adulterants are 
 mineral matter, starch or flour, and water, the presence of 
 these being detected on melting the sample ; and resinous or 
 fatty substances, vegetable waxes and paraffin, which influence 
 the specific gravity and the melting-point. When melted it 
 should give a clear liquid free from any cloudiness, and should 
 not indicate the presence of water by the formation of two 
 layers of fluid. 
 
 Bismuth, Bi. A highly crystalline and very brittle metal, 
 resembling antimony, but having a faint pinkish colour. Like 
 antimony it cannot be rolled or drawn into a wire, but, on 
 the contrary, may be crushed into a powder with an ordinary 
 pestle and mortar. It melts at 515 F., and has a specific 
 gravity of 9*759. It is one of the most useful constituents of 
 fusible metal. The commonest objectionable impurities are 
 sulphur, iron, lead, copper, arsenic, and silver. 
 
 Bismuth Nitrate, Bi(N0 3 )3 + 3H 2 0. Made by dissolving the 
 metal in dilute nitric acid ; it is precipitated as a white basic 
 bismuth sub-nitrate by the addition of much water. 
 
 Black Lead. See Plumbago. 
 
 Brass is an alloy of copper and zinc, the percentage of the 
 former varying from 70 to 60. Special alloys are made for 
 certain purposes, many containing tin and lead. Sterro metal is a 
 brass to which a small percentage of iron has been added, while 
 other complex alloys are made containing iron and manganese in 
 addition to other bodies, to give additional strength or stiffness 
 to the metal. Ordinary brass, unless specially made from the 
 purest virgin metal, generally contains notable proportions of 
 iron and lead and sometimes of tin. 
 
 Bronze. This term embraces the class of alloys of copper and 
 tin, and includes bell-metal and gun-metal. The proportion of 
 
COMMONLY EMPLOYED IN ELECTRO-METALLURGY. 371 
 
 tin varies from 5 to 20, the average sample containing about 10 
 per cent. Some samples have a small percentage of zinc added, 
 others manganese and iron ; in fact the remarks made in regard 
 to brass apply equally to bronze in this matter. Certain alloys 
 are wrongly called by this term, for example, aluminium bronze, 
 which contains 10 per cent, of aluminium, but no tin. Some 
 forms of manganese bronze also contain only a nominal percentage 
 of tin, and the alloy is then really a complex brass with a very 
 small, but sufficient, percentage of manganese. 
 
 Cadmium, Cd. A soft and very malleable bluish-white metal 
 not unlike zinc, with which element it is commonly associated in 
 nature. Its specific gravity should lie between 8'6 and 8'8, while 
 its melting-point is about 608 F. It is rarely used in the arts 
 in the metallic form, except in the manufacture of fusible alloys. 
 
 Cadmium Bromide, CdBr 2 . A white crystalline substance 
 soluble in water. 
 
 Cadmium Chloride, CdCl 2 . 2H 2 0. A similar body, of which 
 140 parts are soluble in 100 of water. It is made by dissolving 
 the metal in hydrochloric acid and evaporating. 
 
 Cadmium Sulphate, 3CdS0 4 . 8H 2 0. A white crystalline sub- 
 stance, 59 parts dissolving in 100 parts of water. 
 
 Calcium Carbonate. See Chalk. 
 
 Chalk (CaC0 3 ) is a natural rock composed of calcium oxide (lime) 
 and carbonic acid. On strongly heating it the carbon dioxide 
 (C0 2 ) is driven off, and the pure calcium oxide remains as quick- 
 lime (CaO). Limestone has the same composition as chalk, and 
 behaves similarly. A particular kind of lime, selected from that 
 burnt in the neighbourhood of Sheffield, has found especial 
 favour among metal-polishers. As the burnt lime, by contact 
 with the air, rapidly absorbs first water, and thus becomes slaked 
 (Ca(OH) 2 ), and then carbon dioxide, and so becomes reconverted 
 into calcium carbonate, lime which is to be stored for any 
 length of time must be preserved in air-tight cases until it is 
 required for use. For polishing purposes it must be uniformly 
 soft and free from gritty particles, which would give rise to 
 scratches ; treated with dilute hydrochloric acid, a sample of 
 quicklime should dissolve with but slight effervescence, and leave 
 no residue undissolved. Chalk or whiting should dissolve with 
 brisk effervescence, but this also should leave no appreciable 
 residue. 
 
372 A GLOSSARY OF SUBSTANCES 
 
 Cobalt, Co. A metal similar to, and generally occurring in 
 nature with, nickel. It has a specific gravity of from 85 to 8*7, 
 and a high melting-point, approximating that of iron. It is 
 readily soluble in sulphuric (dilute), hydrochloric, and nitric acids, 
 forming cobalt sulphate, chloride, and nitrate respectively. 
 
 Cobalt Chloride, CoCl 2 .6H 2 0. A dark red crystalline body 
 readily soluble in water ; it may be prepared by dissolving the 
 metal, its oxide or carbonate, in just sufficient hydrochloric acid. 
 It is well to use a deficiency of the latter in order to ensure the 
 neutrality of the solution. 
 
 Cobalt Nitrate, Co(N0 3 ) 2 . 6H 2 0. A pink crystalline soluble 
 substance prepared like the chloride but with nitric acid. It is 
 readily procurable in the market. 
 
 Cobalt Sulphate, CoS0 4 . 7H 2 0. It resembles the two last- 
 named salts in the manner of preparation (but using sulphuric 
 acid) ; 100 parts of cold water dissolve 35 parts of the salt. 
 
 Cobalt-Ammonium Sulphate, CoS0 4 . (NH 4 ) 2 S0 4 . 6H 2 0. A 
 pink salt which may be made by adding the right proportion of 
 ammonium sulphate to cobalt sulphate (47 : 100), and then 
 dissolving them and evaporating them together until a good crop 
 of crystals has separated. 
 
 Copper, Cu. A red metal with a fusing-point of about 1920 
 F., and a specific gravity of 8'9 to 8'95 according to its condition 
 whether it is simply cast or has been afterwards rolled or 
 hammered. By reason of its extreme malleability and ductility 
 it may be readily obtained in the form of rolled plate or foil, and 
 of rod or wire. It is most readily attacked by nitric acid, but is 
 slowly dissolved when immersed in heated hydrochloric or sul- 
 phuric acids. The metal is frequently found native (that is, in 
 the metallic state in nature), but is most usually smelted from 
 ores in which it is combined with sulphur as sulphide, or with 
 oxygen and perhaps other acid substances as oxide, or oxidised 
 compounds. Such metal often contains small percentages of 
 sulphur, lead, bismuth, arsenic, antimony,, and iron, with some- 
 times traces of silver and gold, and, more rarely, of nickel, 
 cobalt, and tin. There are two classes of copper salts one, 
 cupric, containing more oxygen or other electro-negative ele- 
 ment, formed from the oxide CuO, and yielding generally blue- or 
 green-coloured salts and solutions ; the other, cuprous, prepared 
 from the sub-oxide, Cu 2 0, and giving nearly colourless solutions. 
 The former, which are more usual, alone need be referred to 
 here. 
 
COMMONLY EMPLOYED IN ELECTRO-METALLURGY. 373 
 
 Copper Acetate, Cu(C 2 H 3 2 ) 2 . H 2 0. Dark green crystals 
 moderately soluble in water, formable by dissolving cupric oxide 
 or carbonate in acetic acid. 
 
 Copper Carbonate, CuC0 3 . Occurs in nature as malachite 
 and allied minerals. The artificial carbonate is a green substance, 
 insoluble in water, but readily decomposed by mineral acids (as 
 well as by many of organic origin) yielding the copper compound 
 of the added acid, and carbon dioxide gas, the evolution of which 
 gives rise to great effervescence. The so-called carbonate is 
 usually mixed with hydrated oxide and has the formula 
 CuC0 3 .Cu(OH) 2 . 
 
 Copper Chloride, CuCl 2 . 2H 2 O. Blue-green crystals, readily 
 soluble in water. May be prepared by treating an excess of oxide 
 or carbonate with hydrochloric acid, filtering, and evaporating 
 the resulting solution. 
 
 Copper Cyanides.- The cupric cyanide, Cu(CN) 2 , precipitated 
 by potassium cyanide from copper sulphate solutions, is very un- 
 stable, rapidly changing by exposure into cupro-cupric cyanide, 
 Cu(CN) 2 . Cu 2 (CN) 2 , and cyanogen gas ; the former, when solid, 
 forms green crystals, which are again decomposed at the tempera- 
 ture of boiling water into cuprous cyanide, Cu 2 cyanogen ; 
 and this latter forms several double cyanides with (CN) 2 , and 
 potassium for example, Cu 2 (CN) 2 .KCN.H 2 0, Cu 2 (CN) 2 .2KCN 
 and others, which are for the most part verys oluble in water. 
 
 Copper Nitrate, Cu(N0 3 ) 2 . 3H 2 0. Blue crystals, very soluble 
 in water, and rapidly absorbing moisture from the air. Excess o 
 metal, oxide or carbonate treated with nitric acid yields the salt. 
 
 Copper Sulphate, CuS0 4 . 5H 2 0. Commercially known as blue 
 vitriol } it is the commonest compound of copper. It forms blue 
 crystals, of which 100 parts of cold water dissolve about 40, and 
 of hot water about 200 parts. It is so largely used in the arts 
 that it may be procured everywhere ; it may be made the starting- 
 point for making other compounds of the metal. By adding to 
 an aqueous solution of the salt a quantity of sodium carbonate, 
 dissolved in water, a green solid precipitate of copper carbonate 
 is produced; this may be allowed to subside, filtered, washed 
 well, arid dissolved in any acid which will produce the required 
 salt. 
 
 Glue. For moulding, the ordinary best glue in the market, 
 made from bones, should be used. It should be quite trans- 
 parent, although perhaps dark in colour, hard, and brittle when 
 sharply struck, and must be free from particles of foreign matter. 
 
374 A GLOSSARY OF SUBSTANCES 
 
 When soaked in water it should swell and absorb about five or 
 six times its weight of the water. Gelatine is only a specially 
 pure and clean form of glue, and isinglass is similar in composition. 
 
 Glue, Marine. There are several descriptions of this useful 
 cementing material. A commonly employed glue is made by 
 dissolving a little india-rubber very carefully and with the aid of 
 heat in twelve times its weight of coal-tar naphtha, adding to it 
 twenty times its weight of shellac, and finally pouring it upon a 
 flat cold surface to solidify and harden. It is only necessary to 
 warm the glue and to apply it to the gently-heated surfaces 
 that are to be united. It also makes a good waterproof and 
 non-conductive lining when painted thickly upon the interior 
 surfaces of tanks for cold solutions. 
 
 G-old, An. A yellow metal of high specific gravity (19*26) and 
 fusing-point (1915 F.). It is the most malleable and ductile of 
 metals, and combines with these properties that of a very good 
 electric conductivity. In nature it occurs in the metallic state, 
 almost invariably associated with silver, and often with copper 
 and iron. In commerce it is met with as fine gold, and in 
 various alloys of which the principal has the standard value of 
 the British sovereign gold 91 '67 of gold to 8*33 of copper. 
 These alloys are described as being so many carats fine ; thus, if 
 an alloy contain 22 parts of pure gold in 24 it is said to be 
 22-carat gold, if it contain ^| of its weight of the pure metal it 
 is 18-carat gold, and so forth ; the remaining metal may be 
 copper or silver, separately or together, according to the colour 
 which the metal is required to have; the sovereign is made of 
 
 22-carat gold = 
 
 Gold is insoluble in nitric, hydrochloric, or sulphuric acid 
 ilone, but readily dissolves in a mixture of the two former 
 (aqua regia), and in a very finely-divided condition may be made 
 to dissolve in a mixture of the first and third. To prepare pure 
 gold from the alloy on a small scale is a simple matter. If the 
 alloy contain less than 40 (or more safely 30) per cent, of the 
 precious metal, mere prolonged boiling in nitric acid will dissolve 
 the copper and silver, but leave the gold untouched in the form 
 of a black powder, which is very heavy, and requires only to be 
 washed several times, by stirring it up with repeated additions 
 of cold water, allowing it to settle and pouring off one batch 
 of water each time before adding fresh, and then to be dried and 
 fused to yield the metal practically in a state of purity. The 
 solution must be effected in a glass or glazed earthen- or stone- 
 ware vessel, which will not be attacked by the strong acid ; and 
 should be carried on in the open air or in a well-ventilated fume- 
 cupboard; But if the alloy contain a larger percentage of gold, 
 
COMMONLY EMPLOYED IN ELECTRO-METALLURGY. 375 
 
 the other metals are not completely removed by the acid, and the 
 original mixture must be treated with aqua regia. The gold will 
 now be in solution ; any undissolved white residue is probably 
 silver chloride, which is formed by the agency of the hydrochloric 
 acid and is insoluble in the liquid. It should be allowed to 
 subside, and should be washed once or twice by decantation and 
 filtered. The solution should now be transferred to an evaporat- 
 ing dish or other vessel, in which it may be evaporated to the 
 consistency of a thick syrup, by placing it over a saucepan of 
 boiling water. By this time the bulk of the nitric acid will have 
 been boiled away, and the residue will be a strong, but more or 
 less impure, solution of gold chloride containing hydrochloric acid. 
 The liquid is now diluted, and a quantity of a solution of ferrous 
 sulphate is added, and the mixture is allowed to stand for a day 
 or two in a warm place. The ferrous salt becomes converted 
 into a ferric compound at the expense of the gold oxide, and the 
 gold should thus be liberated completely as pure precipitated 
 metal, of dark brown or black colour, which may be washed, 
 dried, and fused as before. The fusion may be made in a small 
 clay crucible under a cover of a few grains of borax by way of 
 flux for residual impurities. Oxalic acid is sometimes substituted 
 for ferrous sulphate as a precipitant. 
 
 Gold Chloride, AuCl 3 . 2H 2 0. A most soluble and deliquescent 
 yellow crystalline substance, which may be prepared as described 
 in the latter portion of the last paragraph, but using pure gold 
 instead of alloyed metal as the basis. 
 
 Gold Cyanides. On adding a neutral gold chloride solution to 
 one of potassium cyanide, there is produced potassium auricyanide, 
 2KAu(CN) 4 . 3H 2 0, which in the solid condition forms colourless 
 tabular crystals, that are very soluble in hot water, but decom- 
 pose at about 400 into potassium aurous cyanide, AuCN . KCN, 
 and cyanogen gas. This latter body, potassium aurous cyanide, 
 is formed by dissolving aurous oxide, Au 2 0, or even finely- 
 divided gold in potassium cyanide solution ; it is a colourless, 
 crystalline,- and very soluble salt. Simple aurous cyanide, AuCN, 
 is an insoluble lemon-yellow substance. 
 
 Gold, Fulminating, Au 2 3 . (NH 3 ) 4 . A brown or green powder, 
 obtainable by adding ammonia or ammonium carbonate to a solu- 
 tion of gold chloride. It should never be allowed to become dry, 
 for in this condition it is liable to explode with great violence. 
 So long as it is moist, there is no danger attending its use. 
 
 Gold Sulphide, Au 2 S 3 . Obtained by passing sulphuretted 
 hydrogen gas into a solution of the chloride ; it then appears 
 as a black precipitate, soluble in alkaline sulphides. 
 
376 A GLOSSARY OF SUBSTANCES 
 
 Graphite. See Plumbago. 
 
 Gutta-percha. A gum prepared from the exudation of certain 
 trees in the Malay Peninsula and Islands. It is usually pro- 
 curable in sheets. As a moulding material it is valuable, because 
 at the temperature of boiling water it is extremely plastic and 
 may be worked into any required shape, which it will retain on 
 cooling, when it again becomes hard yet somewhat elastic. It is 
 a non-conductor of electricity. 
 
 Iron, Fe. The fusing-point of the pure metal is very high. 
 The iron of commerce is never pure. In the condition in which 
 it is melted from the ore as pig-iron or cast-iron it is more 
 readily fusible, but is highly charged with impurities, derived 
 from the ore, fuel, and flux, and contains varying proportions of 
 carbon, sulphur, phosphorus, silicon, and manganese, frequently 
 accompanied by other elements also, the foreign matter in the 
 aggregate amounting to from 4 to 7 or more per cent. In this 
 condition it is hard, brittle, and unworkable. By refining away 
 the greater proportion of these impurities, the melting-point is 
 greatly raised, and at the same time the metal becomes soft, 
 ductile, and malleable, and is known as malleable- or wrought- 
 iron, which may be rolled into sheets of any degree of thinness. 
 Wrought-iron is the purest form of marketable iron, but even 
 this is not pure, containing perhaps 0*5 per cent, of foreign 
 substances. Between wrought- and cast-iron is another form 
 steel the characteristics of which are chiefly governed by the 
 percentage of carbon, which may range from 1J per cent, in the 
 harder varieties of tool steel to practically nothing in mild-steel. 
 
 The latter of these alone, from its greater purity, enters into com- 
 petition with wrought-iron as a rival in the manufacture of anodes. 
 
 The metal is soluble in either of the three common mineral 
 acids, and forms two classes of salts, one (ferric) with more oxy- 
 gen, of which the peroxide, Fe 2 3 , is typical, the other (ferrous) of 
 which the protoxide, FeO, is the basis. The latter are readily 
 converted into the former by the addition of oxygen, even by 
 absorption from the air ; but unless there be an excess of acid in 
 the bath the effect of the peroxidisation will be the precipitation of 
 basic salt (see p. 231). It is for this reason that neutral ferrous 
 solutions rapidly become turbid with a yellowish slimy deposit. 
 
 Iron Chlorides. Ferrous Chloride, FeCl 2 . 4H 2 0, and ferric 
 chloride, Fe 2 Cl 6 . 12H 2 0, are both very soluble crystalline bodies 
 the former bluish, the latter yellow in colour. 
 
 Iron Sulphate. Iron protosulphate, ferrous sulphate, or green 
 vitriol, FeS0 4 . 7H 2 0, is a green crystalline substance, often 
 yellowish on the exterior, owing to the formation of ferric com- 
 
OF THE 
 UNIVERSITY 
 
 OF 
 
 COMMONLY EMPLOYED IN ELECTSQ 5 ifiXALL^Y^/ 377 
 
 pounds with the aid of atmospheric oxygen. On account of this 
 tendency to peroxidation, this and other ferrous compounds 
 should not be exposed more than necessary to the air. 100 parts 
 of cold water dissolve about 70 parts of the salt, of hot water 
 330 parts. The ferric sulphate, Fe 2 (S0 4 ) 3 , demands only casual 
 men tion in this place. 
 
 Iron- Ammonium Sulphate, FeS0 4 .(NH 4 ) 2 S0 4 .6H 2 0, is a body 
 similar to the last, but with a bluer shade of colour, and is 
 much less liable to alteration by exposure to air, and is there- 
 fore preferable to the former for many reasons. 100 parts of 
 cold water dissolve 16 parts of this salt. 
 
 Lard. The pure white lard is used ; as it is frequently 
 adulterated with water and solid substances, it should be melted 
 and allowed to stand for some time in this condition. The bulk 
 of the water and heavier matter will sink to the bottom, and 
 the purified fat, which should now be quite transparent, may be 
 drawn off from above, or removed by ladles into vessels wherein 
 it may be allowed to solidify. It should melt at a temperature 
 of about 110-5 F. 
 
 Lead, Pb. One of the softest of metals, it is very malleable, 
 but, having a low tenacity, is deficient in ductility; it may be 
 rolled into sheet, but not drawn into wire. Its fusing-point is 
 625 F., and its specific gravity 11 '25 in the cast state, or 11 '39 
 when it has been condensed by rolling. On account of its ready 
 fusibility it may be cast into slabs, or it may be rolled into sheet 
 for use as anodes. Commercial lead, frequently very nearly 
 pure, is never absolutely so. It always contains at least a trace 
 of silver, often with varying proportions of antimony, tin, copper, 
 iron, and sulphur. It is readily dissolved in nitric acid, and 
 slowly in boiling hydrochloric acid. Sulphuric acid, except of the 
 most concentrated description, is almost without action on the 
 metal. Both chloride and sulphate of lead are practically insol- 
 uble in their respective acids, so that very soon the metallic surface 
 becomes coated with a deposit which prevents further action. 
 The salts of lead are formed from the basis of the monoxide 
 (litharge = PbO), in which the metal is divalent, although two 
 other oxides, red lead, Pb 3 4 , and peroxide, Pb0 2 , are known. 
 
 Lead Acetate, Pb(C 2 H 3 2 ) 2 . A readily soluble white crystal- 
 line substance, easily formed by dissolving lead oxide or carbonate 
 in acetic acid. It is very commonly known as sugar of lead. 
 
 Lead Nitrate, Pb(N0 3 ) 2 , forms soluble white crystals (100 
 parts of water dissolve about 54 parts in the cold, or 135 parts 
 when heated). 
 
378 A GLOSSARY OF SUBSTANCES 
 
 Lime. See Chalk. 
 
 Magnesium, Mg. A white divalent metal, readily becoming 
 dull in moist air. When ignited at a slightly elevated tempera- 
 ture, it continues to burn with a most brilliant white light, until 
 it is completely converted into oxide. It has a very low specific 
 gravity (1*75), and fuses at 1380 F. It forms one oxide, mag- 
 nesia, MgO, which is the basis of the various salts of the metal. 
 
 Magnesium Chloride, MgCl 2 .6H 2 0. A most soluble and 
 deliquescent crystalline salt (100 parts of water dissolve 280 parts 
 in the cold, or 782 parts of the body when heated). It must be 
 stored in a closely-stoppered bottle. 
 
 Magnesium Sulphate, MgS0 4 .7H 2 0. Commonly known as 
 Epsom salts. It forms white crystals, easily procurable, of which 
 100 parts of cold water dissolve about 70. 
 
 Mercury, Hg. Frequently called quicksilver. It is the only 
 known metal which is liquid at ordinary temperatures ; it 
 solidifies at - 38'9 F., and boils at 680 F. Its specific gravity 
 at the normal temperature is 13 '5 9. Mercury has a great 
 tendency to dissolve other metals, and so to form amalgams ; it 
 must not, therefore, be stored in, or allowed to come into contact 
 with, clean surfaces of any metal commonly in use except iron or 
 platinum, with which it does not combine. Gold and silver are 
 especially liable to be dissolved, and as articles of jewellery are 
 thus readily spoiled by mercury, the greatest care must be taken 
 in using it. Gold becomes white and dulled by it, and requires 
 the application of strong heat to effect its removal ; and the 
 surface is then left ' dead,' so that it must be re-polished. 
 Mercury, therefore, should not unnecessarily be introduced into 
 the workroom containing electro-plated goods awaiting treatment ; 
 but if used for any purpose it should be carefully preserved 
 from contact with any article liable to be spoiled by it. In 
 consequence of its proneness to combine with other metals, 
 mercury is rarely quite pure. If it be required clean, it may be 
 spread in a shallow dish and covered with dilute nitric acid, with 
 which it should be stirred from time to time. The base metals, 
 such as zinc, copper, and lead, being more electro-positive than 
 mercury, tend to dissolve first; but a certain amount of the 
 mercury itself dissolves also, and forms m'ercurous nitrate. This 
 sub-nitrate assists in the removal of the other metals by simple 
 exchange ; gold and silver, which are more negative than 
 mercury, are, of course, unaffected by the treatment. The solu- 
 tion which has been used for cleaning mercury may be used 
 again and again to treat fresh samples, so long as it contains 
 either an excess of acid, or an appreciable quantity of mercury 
 in solution. This rnay be ascertained by the blue litmus-paper 
 
COMMONLY EMPLOYED IN ELECTRO-METALLURGY. 379 
 
 test in the former case, or in the latter, by adding a drop of 
 hydrochloric acid to a little of the solution placed in a test tube, 
 when a dense white precipitate of mercurous chloride (calomel) 
 is at once produced if mercury be present. Mercurous nitrate 
 solution may be substituted for nitric acid at the outset if pre- 
 ferred. The most satisfactory method of purification, however, 
 is to distil the mercury from a glass retort, and, preferably, under 
 diminished atmospheric pressure, effected by adopting a system 
 of hermetically-joined retort and condenser connected to an air 
 pump. In this way the boiling-point of the mercury may be 
 greatly lowered, and the probability of simultaneous distillation 
 of small quantities of zinc and lead is diminished. A rough test, 
 commonly applied to indicate the presence of any considerable 
 percentage of base metal, is conducted by placing a drop of the 
 mercury upon an inclined surface of smooth glass or glazed 
 porcelain ; if pure, it should retain its spherical shape and roll 
 over the surface, leaving no trace behind ; but if impure, it 
 assumes an elongated form and tends to leave a grey trail behind 
 it, or, in other words, it is said to tail. Mercury may be mono- 
 valent or divalent, and thus forms two oxides, mercurous (Hg 2 0) 
 and mercuric (HgO), with their corresponding salts. As these 
 salts deposit mercury on base metal by simple immersion, and 
 the reduced mercury then amalgamates with the remainder of the 
 other metal, their solutions must be used in the operating room 
 with as much circumspection as quicksilver itself. 
 
 Mercury Chlorides. Mercurous chloride or calomel, Hg 2 Cl 2 , 
 and mercuric chloride or corrosive sublimate, HgCl 2 . The former 
 is a heavy white powder insoluble in water ; the latter an ex- 
 tremely poisonous, white, crystalline body, of which about 7 parts 
 dissolve in 100 parts of cold, 53 in a like weight of hot water. 
 
 Mercurous Nitrate, Hg 2 (N0 8 ) 2 . 2H 2 0. A white, crystalline, 
 very poisonous substance, which may deposit basic salt when 
 treated with water, but is readily soluble in water containing 
 a little nitric acid. It is best made by treating an excess of 
 mercury with cold dilute nitric acid. The hot concentrated acid 
 tends to produce mercuric nitrate, Hg(N0 3 ) 2 . 
 
 Nickel, Ni. A white metal of specific gravity 8'9 and very 
 high fusing-point. Formerly it could be obtained only in the 
 cast condition, but by improved methods of treatment it is now 
 readily procurable rolled into sheet of any required size. The 
 chief impurities affecting its use are iron, copper, cobalt, and 
 arsenic. It forms two oxides, but the chief salts belong to the 
 monoxide group (NiO), in which it is divalent. Nickel is slowly 
 dissolved by sulphuric or hydrochloric acids, rapidly by nitric 
 acid, the attack being always favoured by heating. 
 
380 A GLOSSARY OF SUBSTANCES 
 
 Nickel Carbonate, NiC0 3 . An insoluble, pale apple-green 
 powder. An impure carbonate containing an excess of oxide is 
 produced by adding potassium or sodium carbonate to the solution 
 of a nickel salt. 
 
 Nickel Chloride, NiCl 2 . 6H 2 0. Green soluble crystals, resulting 
 from the solution of oxide, metal, or carbonate in hydrochloric acid. 
 
 Nickel Citrate, Ni(C 6 H 5 7 ) 2 . UH 2 0. A soluble green body 
 formed by dissolving nickel oxide or carbonate in citric acid. 
 
 Nickel Nitrate, Ni(N0 3 ) 2 . 6H 2 0. Green crystals, of which 50 
 parts are soluble in 100 of cold water. May be formed like the 
 chloride, substituting nitric for hydrochloric acid. 
 
 Nickel Sulphate, NiS0 4 . 7H 2 0. The most generally known 
 and used salt of nickel. It is full green in colour, and is soluble 
 in water to the extent of 37 parts in 100. 
 
 Nickel- Ammonium Sulphate, NiS0 4 . (NH 4 ) 2 S0 4 . 6H 2 0. Re- 
 sembles the last, but 100 parts of water dissolve only 5 '5 parts 
 of the salt. It may be made by dissolving together nickel sul- 
 phate and ammonium sulphate, and evaporating the solution 
 until crystals are obtained. 
 
 Phosphorus, P. A non-metallic elementary substance procur- 
 able in two modifications vitreous and amorphous. The vitreous 
 phosphorus is sold in colourless or yellowish translucent sticks 
 which gradually become slightly opaque, especially upon the sur 
 face. It is poisonous, and is insoluble in water, but dissolves 
 very readily in certain liquids, of which carbon bisulphide is a 
 type. It is a most oxidisable body, and takes fire spontaneously 
 when exposed to the air ; it must, therefore, be preserved under 
 water, and should only be removed from it when required for 
 use, and then all operations must be conducted rapidly and 
 carefully. If it is required to cut the blocks into smaller frag- 
 ments, they should be placed singly in a shallow dish containing 
 sufficient water to cover them completely ; they may then be cut 
 with a penknife, but on no account should they be so cut except 
 under water, as the friction of the knife may suffice to inflame 
 the phosphorus when in contact with ' air. Fragments must 
 be prevented from clinging under the finger nail, as should they 
 inflame subsequently, very troublesome sores may be produced. 
 The pieces should be rapidly dried between pieces of blotting- 
 paper, and used without delay, being handled as little as possible ; 
 it is safer for those unaccustomed to work with chemical sub- 
 stances to hold them with light brass tongs. 
 
 Phosphorus is soluble in oils and in carbon bisulphide ; its 
 
COMMONLY EMPLOYED IN ELECTRO- METALLURGY. 381 
 
 solution in the latter substance is used occasionally to assist in 
 the metallisation of electrotype moulds (see pp. 146, 149), but 
 it is a most dangerous liquid to work with, owing to the readi- 
 ness with which the solvent evaporates and leaves upon any 
 object a thin film of phosphorus which often takes fire spontane- 
 ously. This solution and its destructive properties have long 
 been known under the name of Greek fire. This, and indeed all 
 operations involving the use of stick-phosphorus, should be under- 
 taken only by experienced persons, and should, if possible, be 
 excluded from common workshop use. 
 
 The amorphous (or red) phosphorus, which is prepared by 
 heating the vitreous variety to 464 F. with suitable precautions 
 for the exclusion of air, is not spontaneously inflammable at 
 ordinary temperatures, and is not poisonous ; but as it is 
 insoluble in carbon bisulphide, it is useless for the purposes 
 to which this element is usually applied in electro-metallurgy. 
 
 Plaster of Paris, from the mineral gypsum. This is a more 
 or less pure calcium sulphate, CaS0 4 . Its use as a plaster 
 depends upon the property possessed by the substance after 
 heating (two-thirds of the water having been expelled by this 
 heating) of taking to itself a quantity of water in chemical 
 combination, to form the fully hydrated salt, CaS0 4 . 2H 2 0. In 
 doing this a considerable amount of heat is evolved, expansion 
 ensues, and the cream formed by the admixture of water and the 
 powdered material sets into a substance which rapidly hardens 
 as the combination becomes complete and the excess of liquid is 
 absorbed. Since the value of the plaster is dependent upon its 
 power of absorbing water, it must never be allowed to remain 
 exposed to the moisture of the air, from which it would slowly 
 extract its full measure of water of hydration, but must be 
 preserved in well-closed vessels. Gypsum which has been over- 
 burnt refuses to absorb water, and is, therefore, useless. A 
 sample of the plaster when made into a cream with water 
 should become warm, and in the course of half an hour set into 
 a firm, solid, but porous mass. For moulding purposes the 
 plaster must be free from foreign matter, especially from gritty 
 particles. 
 
 Platinum, Pt. A heavy, brilliantly- white metal, unalterable 
 in air, very ductile and malleable, and of extremely high fusing- 
 point. Its specific gravity is 21*5. It dissolves only in aqua 
 regia, being unaffected by either hydrochloric or nitric acid 
 alone, and forms two sets of salts, corresponding to the oxides 
 PtO and Pt0 2 respectively. 
 
 Platinic Chloride, PtCl 4 . 5H 2 0. Red crystals soluble in 
 water ; but the substance usually known by this name is hydro- 
 
382 A GLOSSARY OF SUBSTANCES 
 
 platinic chloride, PtH 2 Cl 6 . 6H 2 0. This results from evaporating 
 a solution of the metal in aqua regia, together with a good 
 excess of hydrochloric acid, and thus forms red-brown, very 
 soluble and, indeed, deliquescent crystals. 
 
 Plumbago, sometimes known as graphite or black-lead. It is 
 an impure natural variety of carbon ; and is found very abund- 
 antly in Cumberland and in Ceylon. Being a conductor of 
 electricity, it is largely used for facing non-conductive surfaces, 
 which are to receive an electro-deposit of any metal. It should 
 be very finely crushed, even to an impalpable powder. As some 
 varieties are very inferior conductors, samples should be tested 
 for efficiency in this respect before final selection for use. 
 
 Potassium Acetate, KC 2 H 3 2 . White soluble crystals; 100 
 parts of cold water dissolving about 230 of the salt. 
 
 Potassium Carbonate, 2K 2 C0 3 . 3H 2 0. White crystals very 
 soluble in water; often used in the anhydrous state, when 100 
 parts of water dissolve about 105 parts of the solid. It is 
 decomposed, with effervescence, by the addition of acid. 
 
 Potassium Bicarbonate, KHC0 3 . A much less soluble salt 
 (25 in 100) which may be formed by passing carbon dioxide 
 (carbonic acid gas) through a strong solution of the normal 
 carbonate. 
 
 Potassium Citrate. White soluble crystals formed by just 
 neutralising citric acid with potassium carbonate. 
 
 Potassium Cyanide, KCN. A white opaque solid, generally 
 bought in irregular lumps or in sticks. It is very soluble in 
 water; and, owing to its becoming decomposed by even the 
 weakest acids, carbonic acid among the number, it gradually 
 alters by exposure to the air, especially in large towns where 
 the atmosphere is laden with carbon dioxide, slowly evolving 
 hydrocyanic acid, which imparts to it the peculiar and charac- 
 teristic faint smell of bitter almonds. It is a deadly poison, and 
 must be used with the utmost caution. Taken internally in 
 minute quantities it may cause instant death, while the solution 
 passing into the blood through cuts in the hand gives rise to 
 painful sores and blood-poisoning; even the fumes, in a badly- 
 ventilated room, cause headache and depression. The commercial 
 cyanide is rarely pure ; that known as gold cyanide is the best, 
 the silver cyanide is inferior. It should always be tested before 
 use, as it frequently contains less than half its weight of the 
 pure salt. 
 
COMMONLY EMPLOYED IN ELECTRO-METALLURGY. 383 
 
 Potassium Ferrocyanide, K 4 FeC 6 N 6 . 3H 2 0. Yellow prussiate 
 of potash. Yellow crystals, of which 25 parts dissolve in 100 of 
 water. A very commonly procurable substance. It gives a deep 
 blue precipitate of Prussian blue when mixed with a solution of 
 ferric chloride. 
 
 Potassium Hydroxide (Potassium Hydrate), KHO. Caustic 
 Potash. A most powerful caustic alkali bought, like the cyanide, 
 in sticks or cakes. It is soluble in water with evolution of 
 much heat ; and substances, moistened with the solution, give 
 rise to a peculiar slimy sensation of the skin when touched. 
 It should never be allowed to enter the mouth, as even dilute 
 solutions almost immediately remove the lining of tender skin. 
 Should such an event happen, the mouth should be at once rinsed 
 several times with water and then with very dilute acetic acid. 
 This body, whether in the solid state or in solution, must be 
 carefully stored in well-closed vessels, as it rapidly becomes 
 converted into carbonate by absorption of carbonic acid from 
 the air, and thus loses its caustic properties. 
 
 Potassium Iodide, KI. An intensely soluble, white crystalline 
 substance, 150 parts of which dissolve in 100 of cold water. It 
 is decomposed by nitric acid with separation of iodine, which 
 colours the solution yellow if dilute, or produces a dark, almost 
 black precipitate if it be concentrated. Strong sulphuric acid 
 has a similar effect. 
 
 Potassium Binoxalate, KC 2 H0 4 Salt of Sorrel. White 
 crystals, not largely soluble in cold water, but imparting to it an 
 acid reaction, turning blue litmus-paper red. 
 
 Potassium Thiocyanate (Potassium Sulphocyanide), KCNS. 
 A very soluble white salt, absorbing much heat (or, as it is 
 more commonly said, producing great cold) when dissolved in 
 water. Its solution gives a blood-red colour when mixed with 
 ferric chloride. 
 
 Potassium Bitartrate, KC 4 H 5 6 Cream of Tartar. A. some- 
 what insoluble acid salt, 100 parts of water dissolving only 0*5 
 parts in the cold or 7 at the boiling temperature. It is colourless 
 when pure, but the commercial crude tartar or argol, which is 
 a by-product in the wine industry, is usually stained purple. 
 The pure salt may be made from this by dissolving it in water, 
 filtering it and allowing it to crystallise on cooling. 
 
 Potassium-Sodium Tartrate, KNaC 4 H 4 6 . 4H 2 Rochelle- or 
 Seignette-salt. A very soluble, white, crystalline substance, which 
 may be made by adding 4 parts of potassium bitartrate and 3 of 
 
384 A GLOSSARY OF SUBSTANCES 
 
 crystallised sodium bicarbonate, little by little, to 12 parts of 
 boiling water, and then cooling in order to allow crystals to 
 deposit. 
 
 Rosin, or Colophony. One of a large series of bodies termed 
 resins, exuded by certain trees. Common rosin is deep amber 
 to brown in colour, and should be translucent and brittle. It 
 becomes slightly but distinctly softened at a temperature of 
 120 F., and as the temperature rises increases in softness until 
 it becomes viscous, and, finally, semi-liquid at about the tempera- 
 ture of boiling water. 
 
 Silver, Ag. A very white and unalterable metal with a 
 specific gravity of 10*4 to 10 '5 and a fusing-point of 1740 F. It 
 is extremely malleable and ductile, and is at the same time the 
 best known conductor of heat and electricity. It combines 
 readily with sulphur, and is thus rapidly covered with a black 
 tarnish of silver sulphide in the atmosphere of towns. Alloyed 
 with copper it is used for silver coinage, the amount of alloy 
 varying in different countries, the English standard being 92*5 
 of silver to 7*5 of copper. To prepare fine silver (i.e., pure silver) 
 from such an alloy, the metal should be dissolved in nitric acid 
 in a glass or glazed earthenware vessel ; any black residue is 
 gold, of which there is frequently a small quantity present, and 
 must be filtered off. To the solution (which is blue, owing to the 
 presence of copper) a common salt solution, or better, dilute 
 hydrochloric acid, is slowly added, so long as it continues to 
 produce a white curdy precipitate. The liquid is stirred well to 
 promote the subsidence of the latter, and then allowed to settle ; 
 a little more of the salt or acid is now added, which should 
 produce no further precipitate (if it should do so, more must be 
 added, until the whole of the silver has thus been thrown down). 
 Any addition of common salt beyond that necessary for complete 
 precipitation only tends to re-dissolve the silver chloride formed, 
 which is fairly soluble in brine ; thus, hydrochloric acid is to be 
 preferred as a precipitant, because a moderate excess is without 
 action on the silver salt. The blue copper solution is now 
 poured away from the heavy silver chloride, which is then stirred 
 up with fresh water, and allowed to subside. This washing 
 by decantation is repeated several times, the wash waters being 
 disregarded. The chloride is then collected on a filter, dried, and 
 mixed with an equal bulk of dried sodium carbonate, transferred 
 to a fire-clay crucible, and heated to a bright-red heat in a clear 
 charcoal- or coke-fire. As soon as fusion commences, effervescence 
 will be observed, due to the mutual decomposition which occurs 
 between the silver chloride and sodium carbonate, whereby 
 sodium chloride and silver carbonate are produced, the latter body 
 being dissociated at the temperature of the operation into carbon 
 
COMMONLY EMPLOYED IN ELECTRO- METALLURGY. 385 
 
 dioxide, oxygen, and metallic silver, the two former escaping in 
 the gaseous state, the latter sinking through the slag by virtue 
 of its higher specific gravity, and collecting into a fused mass at 
 the bottom of the pot. When the contents of the crucible are 
 tranquil they are poured, with the aid of a pair of large bent 
 iron tongs, into an iron ingot-mould of cup-shape, from which, 
 when cold, the silver and slag (that is, the fused salt) are readily 
 removed and separated one from the other. Silver forms one set 
 of salts derived from the oxide, Ag 2 0, in which the metal is 
 monovalent. Moist silver salts should not be allowed to come 
 into contact with clean surfaces of base metals, which will de- 
 compose them by simple exchange ; nor should they be exposed 
 unnecessarily to white light, by which many of them are 
 gradually decomposed and darkened in colour. 
 
 Silver Carbonate, Ag 2 C0 3 . A pale yellow, insoluble substance, 
 formed by adding a carbonate of soda solution to one of a soluble 
 silver salt such as the nitrate. 
 
 Silver Chloride, AgCl Horn silver. A white substance grad- 
 ually passing through a gradation of shade from violet to black 
 by exposure to white light. It is practically insoluble in water, 
 but dissolves to some extent in solutions of sodium chloride, and 
 readily in ammonia, and in sodium thiosulphate (hyposulphite) 
 or potassium cyanide solutions. It is formed, as described above 
 under the head of fine silver, by adding hydrochloric acid or 
 common salt to a solution of silver nitrate. 
 
 Silver Cyanide, AgCN. A white insoluble salt, best formed 
 by gradually adding a potassium cyanide solution to one of silver 
 nitrate, carefully watching the formation of the precipitate, and 
 allowing it to subside after each addition of cya'nide, so that, 
 immediately another drop of the potash salt fails to produce a 
 further precipitate or cloudiness in the liquid, all further addition 
 is stopped, otherwise the silver cyanide will begin to re-dissolve 
 in the excess of the precipitant. The liquid is then washed 
 several times by decantation, as in the case of the chloride. To 
 obtain the pure cyanide, only distilled water must be used ; 
 ordinary spring- or river-water, or even rain-water, contains 
 chlorides, which cause the contamination of the cyanide by silver 
 chloride. The essentials for success are pure substances, and pre- 
 cisely the right proportion between the silver nitrate and potassium 
 cyanide. The silver cyanide dissolves readily in ammonia and 
 sodium thiosulphate as well as in potassium cyanide. 
 
 Silver Iodide, Agl. Has a pale yellow colour; it is readily 
 formed by adding potassium iodide to silver nitrate solutions. 
 It is insoluble in water, and practically even in strong ammonia ; 
 
 25 
 
386 A GLOSSARY OF SUBSTANCES 
 
 strong potassium iodide liquor, however, dissolves a fair propor- 
 tion of the salt. 
 
 Silver Nitrate, AgN0 3 Lunar Caustic. A white crystalline 
 body, obtainable readily in crystals, but sometimes fused into 
 sticks. It dissolves readily in water. In making solutions of 
 this or of any other silver salt, only distilled water should be 
 employed ; all other waters, owing to the presence of chlorine, 
 produce a cloudiness or even a distinct precipitate of silver 
 chloride. 
 
 Silver Oxide, Ag 2 0. A deep brown, or almost black, insoluble 
 powder, obtained by adding caustic soda or potash to silver 
 nitrate solution. 
 
 Silver Sulphate, Ag 2 S0 4 . Brilliant white crystals, only 
 slightly soluble in cold water, but more so in boiling water : 
 they are also soluble in strong sulphuric acid, from which they 
 are partly reprecipitated by the addition of water. 
 
 Sodium Carbonate, Na 2 C0 3 . 10H 2 Washing Soda or Soda 
 Crystals. Very soluble, colourless, alkaline crystals. It behaves 
 chemically like potassium carbonate. An impure kind, contain- 
 ing, inter alia, caustic soda and various foreign salts, is sold as a 
 non-crystalline powder under the name of soda ash, which is 
 suitable for fluxing in obtaining fine silver or gold, but should 
 not be emplo} T ed in making up electrolytic baths. A similar 
 variety, commonly known as refined alkali, is purer, but still not 
 always safe. 
 
 Sodium Bicarbonate, NaHC0 3 . A white soluble powder, 
 whose relation to the carbonate is analogous to that between the 
 corresponding potassium salts. 
 
 Sodium Chloride, NaCl Common Salt ; Table Salt Rock Salt ; 
 or Bay Salt ; the latter are not always pure. The pure salt 
 should form white cubical crystals, of which 100 parts of cold 
 water dissolve 36 parts, hot water taking up slightly more. 
 The natural varieties, or rock salt, frequently contain a con- 
 siderable percentage of iron, which imparts a brown or purple 
 tint to the body ; while salt obtained from sea-water is often 
 found to contain magnesium compounds and other bodies. 
 
 Sodium Citrate. Soluble colourless crystals formed by neutra- 
 lising citric acid with sodium carbonate. 
 
 Sodium Hydroxide (Sodium Hydrate], NaOH Caustic Soda. 
 White soluble lumps of a highly caustic character resembling 
 potassium hydrate (which see) in properties and effects. 
 
COMMONLY EMPLOYED IN ELECTRO-METALLURGY. 387 
 
 Sodium Phosphate. There are three principal phosphates 
 the orthophosphate, Na 3 P0 4 . 12H 2 ; the pyrophosphate, 
 Na 4 P 2 7 . 10H 2 ; and the metaphosphate, NaP0 3 . All of 
 them are white bodies soluble in water ; but the orthophosphate, 
 or rather the disodium-orthophosphate, Na 2 HP0 4 . 12H 2 0, in which 
 one atom of hydrogen takes the place of one of sodium, is that 
 more commonly met with. The pyrophosphate is sometimes used 
 in making up baths. 
 
 Sodium Sulphite, Na 2 S0 3 . 7H 2 0. White soluble crystals, with 
 an alkaline reaction. 
 
 Sodium Bisulphide, NaHS0 3 , is a similar body, but with an 
 acid reaction. Both are compounds of soda with sulphurous acid. 
 
 Sodium Thiosulphate (Sodium Hyposulphite), Na 2 S 2 3 . 5H 2 0. 
 Colourless soluble crystals, which have the property of dissolving 
 silver salts by forming a soluble double thiosulphate of silver and 
 soda. 
 
 Sodium Stannate, Na 2 Sn0 3 . A white substance soluble in 
 water, formed by fusing either stannic oxide (the dressed ore, 
 tin stone, or cassiterite may be used) with caustic soda ; or the 
 metal itself with caustic soda to which sodium nitrate has been 
 added. The aqueous solution, when evaporated, yields crystals 
 containing water of crystallisation. 
 
 Sodium Tartrate, Na 2 C 4 H 4 6 . 2H 2 0. White and soluble 
 crystals, much more soluble in hot than in cold water. 
 
 Sulphur, S, formerly better known as brimstone. Obtainable 
 as flowers of sulphur and as stick or roll sulphur, both of which 
 are pale yellow in colour, but the latter variety is crystalline 
 and soluble in carbon bisulphide, while the former is amorphous 
 and insoluble. It melts at 238 F., and will yield sharp impres- 
 sions of objects, so that it is occasionally useful in obtaining 
 casts. Mixed with iron sulphide it forms the basis of Spence's 
 metal, of which medallions and plaques have been sometimes 
 made, and may thus come into the hands of the electrotyper. 
 Being very fragile, objects made of sulphur or Spence's composi- 
 tion must not be subjected to pressure in taking casts from them 
 with other moulding materials. Sulphur is, of course, very 
 inflammable, and, therefore, requires care in melting. 
 
 Tin, Sn. A white, soft, very fusible and malleable metal, too 
 weak to possess any great ductility, with specific gravity 7 '29, 
 and fusing - point 450 F. It is not readily tarnishable, and 
 therefore retains its brilliancy for a long time when exposed to 
 the air. Easily soluble in hydrochloric acid or aqua regia, but 
 
388 A GLOSSARY OF SUBSTANCES 
 
 converted into an insoluble white oxide by nitric acid. It forms 
 two series of salts, corresponding to the oxides, stannic, Sn0 2 , in 
 which it is tetravalent, and stannous, SnO, where it is divalent. 
 Commercially, tin frequently contains traces of lead, tungsten, 
 iron, copper, antimony, or arsenic. 
 
 Tin Tetrachloride, SnCl 4 Stannic Chloride. A colourless, 
 fuming liquid, boiling at 248 F. It forms several solid hydrates, 
 mostly crystalline, by the addition of water ; such are the butter 
 of tin and oxymuriate of tin. 
 
 Tin Bichloride, SnCl 2 . 2H 2 Stannous Chloride or Tin Salt. 
 A white soluble crystalline substance, formed by dissolving 
 tin in hydrochloric acid. 
 
 Varnish Lacquer Varnish. The formulae for this varnish, 
 which is used for protecting metallic surfaces from tarnish, are 
 almost innumerable. Perhaps the best are those in which seed 
 lac is dissolved in from 8 to 10 parts of the strongest spirits of 
 wine, freed as far as possible from water, and coloured by the 
 addition of dragon's blood or gamboge (say from J to J of a part), 
 or by mixtures of these, according to the particular tint that it is 
 desired to obtain. 
 
 Stopping-off Varnish. Since the object of this class of varnish 
 is to prevent the formation of an electrolytic deposit upon any 
 desired portion of an article, the requirements are evidently 
 a non-conductive material, easily applied and with facility remov- 
 able, which shall not be attacked by the solution in which it is 
 to be immersed, and which should of preference be coloured in 
 such a way that the portions of the surface protected by it may 
 be seen at a glance, for convenience of application. To resist 
 ordinary bath-solutions used cold, almost any varnish is appli- 
 cable, and common copal varnish, mixed with a colouring medium, 
 will be found suitable; or asphalt varnish may be used, as for 
 the back of electrotype plates. But for hot cyanide solutions 
 other materials are required, although copal varnish is often 
 used, and for this work, a mixture of 44 per cent, rosin with 
 26 of bees'-wax, 17 of sealing-wax, and 13 of jeweller's rouge, 
 may be applied with advantage, provided that the best materials 
 alone are employed in preparing it. 
 
 Zinc, Zn. A bluish-white divalent metal, fusible at 783 F., 
 easily distilled at a higher temperature. Specific gravity, 7'1. 
 It is brittle in the cold, and above 250 F., but near the tempera- 
 ture of boiling water it is sufficiently malleable to admit of roll- 
 ing into thin sheets. It forms one class of salts only, from the 
 monoxide, ZnO. The chief impurities in the commercial metal 
 are lead, iron, cadmium, and arsenic. 
 
COMMONLY EMPLOYED IN ELECTRO-METALLURGY. 389 
 
 Zinc Acetate, Zn(C 2 H 3 2 ) 2 . 3H 2 0. Very soluble white 
 crystals. 
 
 Zinc Carbonate, ZnC0 3 . A white insoluble substance, pre- 
 pared by adding sodium carbonate in excess to a solution of 
 any zinc salt. It is decomposed by acids with effervescence. 
 
 Zinc Chloride, ZnCl 2 . A very soluble and deliquescent 
 white, opaque, soft mass ; may be prepared by dissolving zinc 
 or its oxide or carbonate in hydrochloric acid. 
 
 Zinc Cyanide, Zn(CN) 2 . A white substance insoluble in 
 water, but readily so in potassium cyanide solutions. Prepared 
 by precipitating a solution of zinc sulphate with one of potassium 
 cyanide ; of course, carefully avoiding excess of the latter. 
 
 Zinc Oxide, ZnO. A white substance, becoming yellowish 
 when strongly heated, but returning to its original colour on 
 cooling. The hydroxide, or hydrated oxide, Zn(OH) 2 , is preci- 
 pitated when an exact equivalent of caustic alkali is added to a 
 solution of a zinc salt ; an excess of the alkali tends to redissolve 
 the precipitate. 
 
 Zinc Sulphate, ZnS0 4 . 7H 2 White Vitriol. Soluble white 
 crystals ; the commonest salt of zinc. It may be prepared like 
 the chloride, of course substituting sulphuric for hydrochloric 
 acid. 100 parts of water dissolve about 50 of the salt in the cold, 
 and nearly 100 at the boiling-point. 
 
ADDENDA. 
 
 TABLE XX VI 1 1. GIVING DATA FOR CALCULATING THE WEIGHT AND THICK- 
 NESS OF DEPOSIT PRODUCED BY A KNOWN CURRENT OR CURRENT-DENSITY 
 IN A GIVEN TIME FOR CERTAIN OF THE COMMONER METALS. 
 
 
 
 
 
 
 Weight de- 
 
 Thickness of 
 
 
 
 ^ 
 
 
 
 posited pei- 
 
 Deposit pro- 
 
 
 +j 
 
 JS 
 
 >> 
 
 'el 
 
 hour by 
 
 duced in 
 
 
 -a 
 
 ' 
 
 "S 
 
 O 
 
 current of 
 
 1 hour by 
 
 
 '8 
 
 ^ 
 
 t 
 
 fl 
 
 1 ampere. 
 
 current of : 
 
 Metal. 
 
 ^ 
 
 -u 
 
 
 
 'S'rt 
 
 
 
 
 o 
 
 
 o> 
 
 o 
 
 o -* 
 
 
 
 
 
 a 
 
 | 
 
 S 
 
 o* 
 
 
 
 
 90 g 9 
 
 2 2 
 
 
 4 
 
 1 
 
 I 
 
 GO 
 
 S- W 
 
 Gramme 
 
 1 
 
 S h 
 
 2Z% 
 
 5 <.- 
 * U 
 
 ft 
 
 nfc 
 
 
 
 
 
 
 
 
 mm. 
 
 inch. 
 
 Aluminium 
 
 27 
 
 9-0 
 
 2-6 
 
 0-00009317 
 
 0-3354 
 
 5-175 
 
 0-0129 
 
 0-007875 
 
 Antimony 
 
 122 
 
 40-6 
 
 6-8 
 
 00042025 
 
 1-5130 
 
 23-350 
 
 0222 
 
 013584 
 
 Arsenic 
 
 75 
 
 25 
 
 5-8 
 
 00025880 
 
 0-9317 
 
 14-378 
 
 0161 
 
 009806 
 
 Bismuth . 
 
 210 
 
 70 
 
 9-8 
 
 00072464 
 
 2-6087 
 
 40-258 
 
 0266 
 
 016251 
 
 Cadmium . 
 
 112 
 
 56 
 
 8-6 
 
 00057971 
 
 2-0870 
 
 32-207 
 
 0243 
 
 014811 
 
 Chromium . 
 
 52-5 
 
 17-5 
 
 7-0 
 
 00018116 
 
 0-6522 
 
 10-052 
 
 0093 
 
 005687 
 
 Cobalt ' . . v - . 
 
 59 
 
 29-5 
 
 8-7 
 
 00030538 
 
 1-0994 
 
 16-966 
 
 0126 
 
 007715 
 
 Copper (Monovalent) 
 
 63-5 
 
 63-5 
 
 8-9 
 
 00065735 
 
 2-3665 
 
 36-520 
 
 0265 
 
 016208 
 
 ,, (Divalent) . 
 
 63-5 
 
 31-7 
 
 8-9 
 
 00032867 
 
 1-1832 
 
 18-260 
 
 0133 
 
 008104 
 
 Gold . . . 
 
 196-6 
 
 65-5 
 
 19-3 
 
 00067806 
 
 2-4410 
 
 37-670 
 
 0127 
 
 007721 
 
 Iridium . 
 
 197 
 
 49-2 
 
 21-1 
 
 00050932 
 
 1-8335 
 
 28-296 
 
 0087 
 
 005291 
 
 Iron (Divalent) 
 
 56 
 
 28 
 
 8-1 
 
 00028986 
 
 1-0435 
 
 16-103 
 
 0128 
 
 007826 
 
 Lead 
 
 207 
 
 103-5 
 
 11-4 
 
 00107140 
 
 3-8571 
 
 59-525 
 
 0338 
 
 021134 
 
 Magnesium 
 
 24-3 
 
 12-1 
 
 1-7 
 
 00012526 
 
 0-4509 
 
 6-959 
 
 0265 
 
 016190 
 
 Manganese 
 
 55 
 
 27'5 
 
 8-0 
 
 00028468 
 
 1-0248 
 
 15-816 
 
 0128 
 
 007821 
 
 Nickel . "'.' . 
 
 59 
 
 29-5 
 
 8-5 
 
 00030538 
 
 1-0994 
 
 16-966 
 
 0129 
 
 007894 
 
 Palladium 
 
 106-5 
 
 26-6 
 
 11-4 
 
 00027536 
 
 0:9913 
 
 15-298 
 
 0087 
 
 005291 
 
 Platinum . 
 
 197 
 
 44-3 
 
 21-2 
 
 00045859 
 
 1-6509 
 
 25-478 
 
 0078 
 
 004743 
 
 Silver . . . 
 
 108 
 
 108 
 
 10-6 
 
 00111800 
 
 4-0249 
 
 62-113 
 
 0380 
 
 023142 
 
 Tin (Divalent) . 
 
 118 
 
 59 
 
 7-3 
 
 00061077 
 
 2-1988 
 
 33-932 
 
 0302 
 
 018414 
 
 Zinc. 
 
 65 
 
 32-5 
 
 7-1 
 
 00033644 
 
 1-2112 
 
 18-691 
 
 0171 
 
 010415 
 
 NOTE. These figures are based on Lord Rayleigh's number for the electro- 
 chemical equivalent of hydrogen =0-00001 0352. This (0-000010352 gramme H 
 per second) is equal to 6-9503 c.c. H or 10-4255 c.c. mixed gas (H 2 +0) per minute. 
 
 390 
 
ADDENDA. 
 
 391 
 
 TABLE XXIX. SHOWING THE VALUE OP EQUAL CURRENT-DENSITIES AS EX- 
 PRESSED IN AMPERES PER SQUARE DECIMETRE, PER SQUARE FOOT, AND 
 PER SQUARE INCH OF ELECTRODE SURFACE. 
 
 |is 
 
 a n-s 
 
 <J <D <U 
 
 ^ &.T3 
 
 = Amperes 
 per square 
 foot. 
 
 2 
 
 Ht 
 
 a * a 
 
 "4 "" 
 II ft 
 
 . 2 
 eS ^3 
 
 IU 
 
 S t-, o 
 
 ^ 
 
 = Amperes 
 per square 
 foot. 
 
 8 2 
 
 a Et^' 
 
 aSs 
 
 <ri f-c "-" 
 
 n& 
 
 Amperes 
 per square 
 decimetre. 
 
 = Amperes 
 per square 
 foot. 
 
 CO <JJ 
 
 Hi 
 
 4 "" 
 II a 
 
 0-05 
 
 0-46 
 
 0-0032 
 
 0-8 
 
 7-43 
 
 0-0516 
 
 6-20 
 
 57-6 
 
 0-4 
 
 0-054 
 
 0-5 
 
 0-0035 
 
 0-86 
 
 8 
 
 0-0555 
 
 6-46 
 
 60 
 
 0-4167 
 
 0-077 
 
 0-72 
 
 0-005 
 
 0-9 
 
 8-36 
 
 0-0581 
 
 7 
 
 65-0 
 
 0-4516 
 
 01 
 
 0-93 
 
 0-0064 
 
 0-93 
 
 8-64 
 
 0-06 
 
 7-53 
 
 70 
 
 0-4861 
 
 0-11 
 
 1 
 
 0-0069 
 
 0-97 
 
 9 
 
 0-0625 
 
 7-75 
 
 72-0 
 
 0-5 
 
 0-15 
 
 1-44 
 
 o-oi 
 
 1 
 
 9-29 
 
 0-0645 
 
 8 
 
 74-3 
 
 0-5161 
 
 0-2 
 
 1-86 
 
 0-0129 
 
 1-08 
 
 10 
 
 0-0694 
 
 8-61 
 
 80 
 
 0-5555 
 
 0-22 
 
 2 
 
 0-0139 
 
 1-09 
 
 10-08 
 
 0-07 
 
 9 
 
 83-6 
 
 0-5806 
 
 0-3 
 
 2-79 
 
 0-0193 
 
 1-24 
 
 11-52 
 
 0-08 
 
 9-30 
 
 86-4 
 
 0-6 
 
 0-31 
 
 2-88 
 
 0-02 
 
 1-39 
 
 12-96 
 
 0-09 
 
 9-69 
 
 90 
 
 0-6250 
 
 0-32 
 
 3 
 
 0-0208 
 
 1-55 
 
 14-4 
 
 01 
 
 10 
 
 92-9 
 
 0-6452 
 
 0'4 
 
 3-71 
 
 0-0258 
 
 2 
 
 18-6 
 
 0-1290 
 
 10-76 
 
 100 
 
 0-6944 
 
 0-43 
 
 4 
 
 0-0278 
 
 2-15 
 
 20 
 
 0-1389 
 
 10-85 
 
 100-8 
 
 0-7 
 
 0-46 
 
 4-32 
 
 0-03 
 
 3 
 
 27-9 
 
 0-1935 
 
 12-40 
 
 115-2 
 
 08 
 
 0-5 
 
 4-64 
 
 0-0323 
 
 3-10 
 
 28-8 
 
 0-2 
 
 13-95 
 
 129-6 
 
 0-9 
 
 0-54 
 
 5 
 
 0-0348 
 
 3-23 
 
 30 
 
 0-2083 
 
 15-50 
 
 144-0 
 
 1 
 
 0-6 
 
 5-57 
 
 0-0387 
 
 4 
 
 37-1 
 
 0-2581 
 
 20 
 
 185-8 
 
 1-2903 
 
 0-62 
 
 5-76 
 
 0'04 
 
 4-30 
 
 40 
 
 0-2778 
 
 21-53 
 
 200 
 
 1-3889 
 
 0-65 
 
 6 
 
 0-0417 
 
 4-60 
 
 43-2 
 
 0-3 
 
 30 
 
 278-7 
 
 1-9355 
 
 0-7 
 
 6-50 
 
 0-0452 
 
 5 
 
 46-4 
 
 0-3:226 
 
 31-0 
 
 288 
 
 2 
 
 075 
 
 7 
 
 0-0486 
 
 5-38 
 
 50 
 
 0-3478 
 
 32-3 
 
 300 
 
 2-0833 
 
 0-77 
 
 7-20 
 
 00-5 
 
 6 
 
 55-7 
 
 0-3871 
 
 46-5 
 
 432-0 
 
 3 
 
 By this table the current-density may be expressed in amperes per square deci- 
 metre, square foot, or square inch, any one of them being given. Thus a current of 
 1 ampere per square decimetre has the same electrolytic value as one of 9'29 amperes 
 per square foot, or 0'0645 per square inch. To find the value of intermediate 
 numbers not shown above, add together the various numbers representing the 
 hundreds, tens, units, and decimals of the given quantity. Thus 27 '5 amperes per 
 square decimetre (=20 + 7 + 0'5) is equivalent to 185-8 + 65 + 4-64=255-44 amperes 
 per square foot, or 1-2903 + 0'4516 +0-0323=17742 amperes per square inch. 
 
392 
 
 ADDENDA. 
 
 TABLE XXX. SHOWING THE SPECIFIC GRAVITIES OF SULPHURIC, NITRIC, AND 
 HYDROCHLORIC ACIDS CORRESPONDING TO VARYING PERCENTAGES OF PURE 
 H 2 S0 4 , HN0 3> AND HC1 IN THE LIQUIDS RESPECTIVELY. 
 
 % 
 
 H 2 S0 4 . 
 
 HN0 3 . 
 
 HC1. 
 
 % 
 
 H 2 S0 4 . 
 
 HNOg. 
 
 HC1. 
 
 % 
 
 H 2 S0 4 . 
 
 HN0 3 . 
 
 HC1. 
 
 100 
 
 ] -8426 
 
 1-500 
 
 
 66 
 
 1-568 
 
 1-3783 
 
 
 33 
 
 1-247 
 
 1-1895 
 
 1-1640 
 
 99 
 
 1-8420 
 
 1-498 
 
 
 65 
 
 1-557 
 
 1-3732 
 
 
 32 
 
 1-239 
 
 1-1833 
 
 1-1584 
 
 98 
 
 1-8406 
 
 1-496 
 
 
 64 
 
 1-545 
 
 1-3681 
 
 
 81 
 
 1-231 
 
 1-1770 
 
 1-1536 
 
 97 
 
 1-8400 
 
 1-494 
 
 
 63 
 
 1-534 
 
 1-3630 
 
 
 30 
 
 1-223 
 
 1-1709 
 
 1-1484 
 
 96 
 
 1-8384 
 
 1-491 
 
 
 62 
 
 1-523 
 
 1-3579 
 
 
 29 
 
 1-215 
 
 1-1648 
 
 1-1433 
 
 95 
 
 1-8376 
 
 1-488 
 
 
 61 
 
 1-512 
 
 1-3529 
 
 
 28 
 
 1-2066 
 
 1-1587 
 
 1-1382 
 
 94 
 
 1-8356 
 
 1-485 
 
 
 60 
 
 1-501 
 
 1-3477 
 
 
 27 
 
 1-1980 
 
 1-1515 
 
 1-1333 
 
 93 
 
 1-8340 
 
 1-482 
 
 
 59 
 
 1-490 
 
 1-3427 
 
 
 26 
 
 1-190 
 
 1 -1467 
 
 1-1282 
 
 92 
 
 1-8310 
 
 1-479 
 
 
 58 
 
 1-480 
 
 1-3376 
 
 
 25 
 
 1-182 
 
 1-1403 
 
 1-1232 
 
 91 
 
 1-8270 
 
 1-476 
 
 
 57 
 
 1-469 
 
 1-3323 
 
 ... 
 
 24 
 
 1-174 
 
 1-1345 
 
 1-1182 
 
 90 
 
 1 -8220 
 
 1-473 
 
 
 56 
 
 1-4586 
 
 1 -3270 
 
 
 23 
 
 1-167 
 
 1-1286 
 
 1-1131 
 
 89 
 
 1-8160 
 
 1-470 
 
 
 55 
 
 1-448 
 
 1-3216 
 
 
 22 
 
 1-159 
 
 1-1227 
 
 1-1081 
 
 88 
 
 1-8090 
 
 1-467 
 
 
 54 
 
 1-438 
 
 1-3163 
 
 
 21 
 
 1-1516 
 
 1-1168 
 
 1-1031 
 
 87 
 
 1-802 
 
 1-464 
 
 
 53 
 
 1-428 
 
 1-3110 
 
 
 20 
 
 1-144 
 
 1-1109 
 
 1-0981 
 
 86 
 
 1-794 
 
 1-460 
 
 
 52 
 
 1-418 
 
 1-3056 
 
 
 19 
 
 1-136 
 
 1-1051 
 
 1-0931 
 
 85 
 
 1-786 
 
 1-457 
 
 
 51 
 
 1-408 
 
 1-3001 
 
 
 18 
 
 1-129 
 
 1-0993 
 
 1-0882 
 
 84 1-777 
 
 1-453 
 
 
 50 1-398 
 
 1-2947 
 
 
 17 
 
 1-121 
 
 1-0935 
 
 1-0832 
 
 83 
 
 1767 
 
 1-450 
 
 
 49 
 
 1-3886 
 
 1-2887 
 
 
 16 
 
 1-1136 
 
 1-0878 
 
 1-0783 
 
 82 
 
 1-756 
 
 1-446 
 
 
 48 
 
 1-379 
 
 1-2826 
 
 
 15 
 
 1-106 
 
 1-0821 
 
 1-0734 
 
 81 
 
 1-745 
 
 1-4424 
 
 
 47 
 
 1-370 
 
 1-2765 
 
 
 14 
 
 1-098 
 
 1-0764 
 
 1-0684 
 
 80 
 
 1-734 
 
 1-4385 
 
 
 46 
 
 1-361 
 
 1-2705 
 
 
 13 
 
 1-091 
 
 1 -0708 
 
 1-0635 
 
 79 
 
 1-722 
 
 1-4346 
 
 
 45 
 
 1-351 
 
 1-2644 
 
 
 12 
 
 1-083 
 
 1-0651 
 
 1-0586 
 
 78 
 
 1-710 
 
 1-4306 
 
 
 44 
 
 1-342 
 
 1-2583 
 
 
 11 
 
 1 -0756 
 
 1-0595 
 
 1-0537 
 
 77 
 
 1-698 
 
 1-4269 
 
 
 43 
 
 1-333 
 
 1-2523 
 
 
 10 
 
 1-068 
 
 1-0540 
 
 1-0487 
 
 76 
 
 1-686 
 
 1-4228 
 
 
 42 
 
 1-324 
 
 1-2464 
 
 
 9 
 
 1-061 
 
 1-0485 
 
 1-0438 
 
 75 
 
 1-675 
 
 1-4189 
 
 
 41 
 
 1-315 
 
 1-2402 
 
 
 8 
 
 1-0536 
 
 1-0430 
 
 1-0389 
 
 74 
 
 1-663 
 
 1-4147 
 
 
 40 
 
 1-306 
 
 1-2341 
 
 1-1966 
 
 7 
 
 1-0464 
 
 1-0375 
 
 1-0340 
 
 73 
 
 1-651 
 
 1-4107 
 
 
 39 
 
 1-297 
 
 1-2277 
 
 1-1922 
 
 6 
 
 1-039 
 
 1-0320 
 
 1-0292 
 
 72 
 
 1-639 
 
 1-4065 
 
 
 38 
 
 1-289 
 
 1-2212 
 
 1-1878 
 
 5 
 
 1-032 
 
 1-0267 
 
 1-0244 
 
 71 
 
 1-627 
 
 1-4023 
 
 
 37 
 
 1-281 
 
 1-2148 
 
 1-1840 
 
 4 
 
 1-0256 
 
 1-0212 
 
 1-0196 
 
 70 
 
 1-615 
 
 1-3978 
 
 
 36 
 
 1-272 
 
 1-2084 
 
 1-1786 
 
 3 
 
 1-019 
 
 1-0159 
 
 1-0148 
 
 69 
 
 1-604 
 
 1-3945 
 
 
 35 
 
 1-264 
 
 1 -2019 
 
 1-1738 
 
 2 
 
 1-013 
 
 1-0106 
 
 1-0098 
 
 68 
 
 1-592 
 
 1-3882 
 
 
 34 
 
 1-256 
 
 1-1958 
 
 1-1689 
 
 1 
 
 1-0064 
 
 1-0053 
 
 1-0049 
 
 67 
 
 1-580 
 
 1-3833 
 
 
 
 
 
 
 
 
 
 
 NOTE. The sulphuric acid numbers are quoted from Otto, those for nitric 
 acid from Ure ; while the hydrochloric acid figures are compiled by interpolation 
 from Ure. Liquid hydrochloric acid is practically saturated with 40 per cent, 
 of HC1 gas. 
 
ADDENDA. 
 
 393 
 
 TABLE XXXI. SHOWING THE SPECIFIC GRAVITIES OF SOLUTIONS CORRE- 
 SPONDING TO THE DEGREES OF THE BAUME HYDROMETER. 
 
 Degree 
 Baume. 
 
 = Specific 
 Gravity. 
 
 Degree 
 Baume. 
 
 = Specific 
 Gravity. 
 
 Degree 
 Baume. 
 
 = Specific 
 Gravity. 
 
 Degree 
 Baume. 
 
 = Specific 
 Gravity. 
 
 
 
 1-000 
 
 19 
 
 1-147 
 
 37 
 
 1-337 
 
 55 
 
 1-596 
 
 1 
 
 1-007 
 
 20 
 
 1-157 
 
 38 
 
 1-349 
 
 56 
 
 1-615 
 
 2 
 
 1-014 
 
 21 
 
 1-166 
 
 39 
 
 1-361 
 
 57 
 
 1-634 
 
 3 
 
 1-020 
 
 22 
 
 1-176 
 
 40 
 
 1-375 
 
 58 
 
 1-653 
 
 4 
 
 1-028 
 
 23 
 
 1-185 
 
 41 
 
 1-388 
 
 59 
 
 1-671 
 
 5 
 
 1-034 
 
 24 
 
 1-195 
 
 42 
 
 1-401 
 
 60 
 
 1-690 
 
 6 
 
 1-041 
 
 25 
 
 1-205 
 
 43 
 
 1-414 
 
 61 
 
 1-709 
 
 7 
 
 1-049 
 
 26 
 
 1-215 
 
 44 
 
 1-428 
 
 62 
 
 1-729 
 
 8 
 
 1-057 
 
 27 
 
 1-225 
 
 45 
 
 1-442 
 
 63 
 
 1-750 
 
 9 
 
 1-064 
 
 28 
 
 1-235 
 
 46 
 
 1-456 
 
 64 
 
 1-771 
 
 10 
 
 1-072 
 
 29 
 
 1-245 
 
 47 
 
 1-470 
 
 -65 
 
 1-793 
 
 11 
 
 1-080 
 
 30 
 
 1-256 
 
 48 
 
 1-485 
 
 66 
 
 1-815 
 
 12 
 
 1-088 
 
 31 
 
 1-267 
 
 49 
 
 1-500 
 
 67 
 
 1-839 
 
 13 
 
 1-096 
 
 32 
 
 1-278 
 
 50 
 
 1-515 
 
 68 
 
 1-864 
 
 14 
 
 1-104 
 
 33 
 
 1-289 
 
 51 
 
 1-531 
 
 69 
 
 1-885 
 
 15 
 
 1-113 
 
 34 
 
 1-300 
 
 52 
 
 1-546 
 
 70 
 
 1-909 
 
 16 
 
 1-121 
 
 35 
 
 1-312 
 
 53 
 
 1-562 
 
 71 
 
 1-935 
 
 17 
 
 1-130 
 
 36 
 
 1-324 
 
 54 
 
 1-578 
 
 72 
 
 1-960 
 
 18 
 
 1-138 
 
 
 
 
 
 
 
 NOTE. The specific gravity of a solution is rapidly ascertained by floating in it 
 a weighted glass tube closed at both ends, with a bulb in the centre and a long, 
 thin glass tube above, which carries a graduated scale upon it. This instrument 
 sinks deeper in solutions of low density than in those of high gravity ; and the 
 actual specific gravity is found by the level at which the liquid stands on the 
 graduated portion when the apparatus is floating freely in it. Hydrometers of this 
 kind are sometimes graduated so that the specific gravity is read off direct from the 
 scale, others are graduated by Baume's method ; and the reading may then be con- 
 verted into the number representing the true density, by reference to the above table. 
 
 Specific gravities are sometimes expressed according to Twaddell's hydrometer. 
 This scale can be converted into ordinary figures by multiplying by 5, dividing by 
 1000, and adding unity. Thus 1 Twaddell is specific gravity 1-005, 2 is I'OIO, 
 and so on. 
 
 TABLE XXXII. DENSITIES OF SOLUTIONS OF CRYSTALLISED COPPER 
 AND ZINC SULPHATES. 
 
 Copper Sulphate. 
 
 Zinc Sulphate. 
 
 <3 
 
 
 -kg^L 
 
 
 -kg 4 
 
 
 Xo 
 
 
 So W 
 
 
 fa04J^ 
 
 
 bD-n W 
 
 
 o W 
 
 
 ^IJ^- 
 
 Density. 
 
 ""1 
 
 Density. 
 
 -g'S)^ 
 
 Density. 
 
 g'So^ 
 
 Density. 
 
 o> "S O 
 
 
 t> ^ 
 
 
 g O 
 
 
 a? a> O 
 
 
 
 
 
 PM 5 
 
 
 Irl 
 
 
 P^ N 
 
 
 2 
 
 1-0126 
 
 14 
 
 1-0923 
 
 5 
 
 1-0289 
 
 35 
 
 1-2285 
 
 4 
 
 1-0254 
 
 16 
 
 1-1063 
 
 10 
 
 1-0588 
 
 40 
 
 1-2674 
 
 6 
 
 1-0384 
 
 18 
 
 1-1208 
 
 15 
 
 1 -0899 
 
 45 
 
 1-3083 
 
 8 
 
 1-0516 
 
 20 
 
 1-1354 
 
 20 
 
 1-1222 
 
 50 
 
 1-3511 
 
 10 
 
 1-0649 
 
 22 
 
 1-1501 
 
 25 
 
 1-1560 
 
 55 
 
 1-3964 
 
 12 
 
 1-0785 
 
 24 
 
 1-1659 
 
 30 
 
 1-1914 
 
 60 
 
 1-4451 
 
 NOTE. The pure salt is supposed to be dissolved in pure distilled water. 
 
394 
 
 ADDENDA. 
 
 TABLE XXXIII. SHOWING THE SPECIFIC ELECTRICAL RESISTANCES! OP DIF- 
 FERENT SULPHURIC ACID SOLUTIONS AT VARIOUS TEMPERATURES (Fleeming 
 Jenkin). 
 
 Specific 
 Gravity of 
 Acid. 
 
 Temperatures (Fahrenheit). 
 
 32 
 
 39 -2 
 
 46 -4 
 
 53-6 
 
 60-8 
 
 68 
 
 75 -2 
 
 82 4 
 
 MO 
 
 1-37 
 
 1-17 
 
 1-04 
 
 0-92 
 
 0-84 
 
 0-79 
 
 0-74 
 
 071 
 
 1-20 
 
 1-33 
 
 1-11 
 
 0-93 
 
 079 
 
 0-67 
 
 0-57 
 
 0-49 
 
 0-41 
 
 1-25 
 
 1-31 
 
 1-09 
 
 0-90 
 
 0-74 
 
 0-62 
 
 0-51 
 
 0-43 
 
 0-36 
 
 1-30 
 
 1-36 
 
 1-13 
 
 0-94 
 
 0-79 
 
 0-66 
 
 0-56 
 
 0-47 
 
 0-39 
 
 1-40 
 
 1-69 
 
 1-47 
 
 1-30 
 
 1-16 
 
 1-05 
 
 0-96 
 
 0-89 
 
 0-84 
 
 1-50 
 
 274 
 
 2-41 
 
 2-13 
 
 1-89 
 
 1-72 
 
 1-61 
 
 1-32 
 
 1-43 
 
 1-60 
 
 4-32 
 
 4-16 
 
 3-62 
 
 3-11 
 
 275 
 
 2-46 
 
 2-21 
 
 2-02 
 
 170 
 
 9-41 
 
 7-67 
 
 6-25 
 
 5-12 
 
 4-23 
 
 3-57 
 
 3-07 
 
 271 
 
 TABLE XXXIV. SHOWING THE SPECIFIC ELECTRICAL RESISTANCES! OF DIF- 
 FERENT COPPKR SULPHATE SOLUTIONS AT VARIOUS TEMPERATURES (Fleeming 
 Jenkin). 
 
 No. of parts 
 of Copper 
 Sulphate 
 dissolved in 
 100 parts of 
 water. 
 
 Temperatures (Fahrenheit). 
 
 57 '2 
 
 60-8 
 
 64 -4 
 
 68 
 
 75 -2 
 
 82 -4 
 
 86 
 
 8 
 
 457 
 
 437 
 
 41-9 
 
 40'2 
 
 37-1 
 
 34-2 
 
 32-9 
 
 12 
 
 36-3 
 
 34-9 
 
 33-5 
 
 32-2 
 
 29-9 
 
 27-9 
 
 27-0 
 
 16 
 
 31-2 
 
 30-0 
 
 28-9 
 
 27-9 
 
 26-1 
 
 24-6 
 
 24-0 
 
 20 
 
 28-5 
 
 27-5 
 
 26-5 
 
 25-6 
 
 . 24-1 
 
 227 
 
 22-2 
 
 24 
 
 26-9 
 
 25-9 
 
 24-8 
 
 23-9 
 
 22-2 
 
 207 
 
 20-0 
 
 28 
 
 247 
 
 23-4 
 
 22-1 
 
 21-0 
 
 18-8 
 
 16-9 
 
 16-0 
 
 1 By the term Specific Resistance in the above tables is meant the resistance 
 in ohms between the opposite faces of a centimetre cube of the substance in 
 question. The diminution of resistance accompanying a rise of temperature 
 should be especially remarked. 
 
ADDENDA. 
 
 395 
 
 Tji os kp 
 
 00 O CO 
 
 r o co co i 
 
 O i-H rH i-( C^ 
 
 j c ?< 
 
 s^ 1 a" 
 SJl-^-g 
 
 il* 8 g 1 ^ 
 
 i ( <N CO > i 
 CO 01 t-H rH 
 
 rH Od CD l_-- O 
 
 CO CO OO O 
 OS !>. iO ^Ji 
 
 II 
 
 r& 
 
 P P P P P P P 
 
 
 ^"S ?*S aZi! 
 
 S 18 
 
 CO (N 
 
 ip O p i-~ p p op 
 
 CO CO rH OO CO ^ CO 
 
 s s s 
 
 p p p 
 
 o 
 
 s 
 
 OO OJ O T-H 
 
 d 
 
 r;H rH OS OS CO 
 O O OS ITS O 
 O CO CO S OO 
 
 
 iij 
 
 CO OS l>- Cfl 
 rHOOCO(NOO<MOSOO 
 
 rH CO >& OO 
 
 O rH (N 
 
396 
 
 ADDENDA. 
 
 TABLE XXXVI. ACTUAL DIAMETERS CORRESPONDING TO THE NUMBERS OF 
 THE OLD BIRMINGHAM WIRE GAUGE. 
 
 
 Diameter. 
 
 
 Diameter. 
 
 
 Diameter. 
 
 
 
 H 
 
 
 ll 
 
 c5 c 
 
 
 i! 
 
 
 
 
 
 
 
 
 * 
 
 Inch. 
 
 Milli- 
 metres. 
 
 * 
 
 Inch. 
 
 Milli- 
 metres. 
 
 *! 
 
 Inch. 
 
 Milli- 
 metres. 
 
 0000 
 
 0-454 
 
 11-53 
 
 11 
 
 0-120 
 
 3-05 
 
 24 
 
 0-022 
 
 0-56 
 
 000 
 
 425 
 
 10-79 
 
 12 
 
 109 
 
 2-77 
 
 25 
 
 020 
 
 51 
 
 00 
 
 380 
 
 9-65 
 
 13 
 
 095 
 
 2-41 
 
 26 
 
 018 
 
 46 
 
 
 
 340 
 
 8-63 
 
 14 
 
 083 
 
 2-11 
 
 27 
 
 016 
 
 41 
 
 1 
 
 300 
 
 7-62 
 
 15 
 
 072 
 
 1-83 
 
 28 
 
 014 
 
 36 
 
 2 
 
 284 
 
 7-21 
 
 16 
 
 065 
 
 1-65 
 
 29 
 
 013 
 
 33 
 
 3 
 
 259 
 
 6-58 
 
 17 
 
 058 
 
 1-47 
 
 30 
 
 012 
 
 305 
 
 4 
 
 238 
 
 6-04 
 
 18 
 
 049 
 
 1-24 
 
 31 
 
 010 
 
 254 
 
 5 
 
 220 
 
 5-59 
 
 19 
 
 042 
 
 1-07 
 
 32 
 
 009 
 
 229 
 
 6 
 
 203 
 
 5-16 
 
 20 
 
 035 
 
 0-89 
 
 33 
 
 008 
 
 203 
 
 7 
 
 180 
 
 4-57 
 
 21 
 
 032 
 
 81 
 
 34 
 
 007 
 
 178 
 
 8 
 
 165 
 
 4-19 
 
 22 
 
 028 
 
 71 
 
 35 
 
 005 
 
 127 
 
 9 
 
 148 
 
 3-76 
 
 23 
 
 025 
 
 63 
 
 36 
 
 004 
 
 102 
 
 10 
 
 134 
 
 3-40 
 
 
 
 
 
 
 
 TABLE XXX VI A. DIAMETERS COKRESPONDING TO THE NUMBERS OF THE 
 AMERICAN (BROWN & SHARPE'S) STANDARD WIRE GAUGE. 
 
 
 Diameter. 
 
 
 Diameter. 
 
 
 Diameter. 
 
 
 
 6D,5 
 
 
 sj 
 
 
 >!' 
 
 
 n 
 
 Inch. 
 
 Milli- 
 metres. 
 
 fl 
 
 Inch. 
 
 Milli- 
 metres. 
 
 rK & 
 
 & 
 
 Inch. 
 
 Milli- 
 metres. 
 
 0000 
 
 0'46 
 
 11-68 
 
 12 
 
 0-0808 
 
 2-05 
 
 27 
 
 0-0142 
 
 361 
 
 000 
 
 4096 
 
 10-44 
 
 13 
 
 0720 
 
 1-83 
 
 28 
 
 0126 
 
 320 
 
 00 
 
 3648 
 
 9-26 
 
 14 
 
 0641 
 
 1-63 
 
 29 
 
 0113 
 
 287 
 
 
 
 3249 
 
 8-24 
 
 15 
 
 0571 
 
 1-45 
 
 30 
 
 0100 
 
 254 
 
 1 
 
 2893 
 
 7-35 
 
 16 
 
 0508 
 
 1-29 
 
 31 
 
 00893 
 
 227 
 
 2 
 
 2576 
 
 6-54 
 
 17 
 
 0453 
 
 1-15 
 
 . 32 
 
 00795 
 
 202 
 
 3 
 
 2294 
 
 5-83 
 
 18 
 
 0403 
 
 1-02 
 
 33 
 
 00708 
 
 180 
 
 4 
 
 2043 
 
 5-19 
 
 19 
 
 0359 
 
 0-912 
 
 34 
 
 0063 
 
 160 
 
 5 
 
 1819 
 
 4-62 
 
 20 
 
 0320 
 
 813 
 
 35 
 
 00561 
 
 142 
 
 6 
 
 1620 
 
 4-11 
 
 21 
 
 0285 
 
 724 
 
 36 
 
 005 
 
 127 
 
 7 
 
 1443 
 
 3-66 
 
 22 
 
 0253 
 
 642 
 
 37 
 
 00445 
 
 113 
 
 8 
 
 1285 
 
 3-26 
 
 23 
 
 0226 
 
 574 
 
 38 
 
 00397 
 
 101 
 
 9 
 
 1144 
 
 2-90 
 
 24 
 
 0201 
 
 510 
 
 39 
 
 00353 
 
 09 
 
 10 
 
 1019 
 
 2-59 
 
 25 
 
 0179 
 
 455 
 
 40 
 
 00314 
 
 08 
 
 11 
 
 0907 
 
 2-30 
 
 26 
 
 0159 
 
 404 
 
 
 
 
ADDENDA. 
 
 397 
 
 iO 1O -^ CO <M OJ i I i I r-l < 
 ppppOpppp. 
 
 
 iSS: 
 
 b ' 
 
 O O O O O O O ' 
 
 p p p o p p p 
 
 SSS 
 
 n 
 
 >-rt<COr^OSt^T-ICO<MOOS 
 
 i cb <N c<i AH r-i b 
 
 t^ !O CO rH CO C^l 
 OC*;ftOOdOCOMjCOOQC9<Nr-l!-HrH^H 
 
 ooooooooooooooo 
 o o o o o o o o o o o o o o o 
 
 g 
 
 .2 
 
 ef 1 
 
 
 ii iCOt^-f--! (^t^-OOOSCOCOOCOi ICOOSCD 
 OS CO lO GO kO C^ O OS OS C<J 1O OS "^ O CO C^l O ' 
 
 o>t-~om^co<N!-iooiooooi^o?oioir5^ 
 
 -- t^ O4 r^ -^ OO (N O O * QO CO GO -^ O CO C<) OS CD 
 
 !55S^; 
 
 __^ 
 t^ CO O rji CO (M 
 
398 
 
 ADDENDA. 
 
 TABLE XXXVIlA. 1 SHOWING MAXIMUM CURRENTS FOR COPPER 
 CONDUCTORS INSULATED AND LAID IN CASING OR TUBING. 
 
 1 
 
 Wire or Cable. 
 
 2 
 
 Maximum 
 Current in 
 amperes for 
 high external 
 Temperature. 
 
 3 
 
 Approximate 
 amperes per 
 square inch 
 in col. 2. 
 
 4 
 
 Total Length 
 in yards of 
 Lead and 
 Return giving 
 1 volt drop. 3 
 
 Size S.W.G.2 
 
 Approximate 
 Area in 
 square inch. 
 
 18 or 62/38 or 97/40 
 
 < 0-0018 
 
 4-2 
 
 2,300 
 
 18 
 
 3/22 
 
 > 0-0018 
 
 4-2 
 
 2,300 
 
 18 
 
 17 or 130/40 
 
 0-0024 
 
 5-4 
 
 2,200 
 
 19 
 
 3/20 
 
 0-0031 
 
 6-4 
 
 2,150 
 
 19 
 
 16 or 110/38 or 172/40 
 
 0-0032 
 
 6-8 
 
 2,100 
 
 20 
 
 15 
 
 0-0041 
 
 8-2 
 
 2,000 
 
 21 
 
 7/22 
 
 0-0043 
 
 8-5 
 
 2,000 
 
 21 
 
 14 or 172/38 or 7/2 1J 
 
 0-0050 
 
 9-8 
 
 1,950 
 
 21 
 
 3/18 
 
 0-0054 
 
 10-3 
 
 1,950 
 
 21 
 
 7/20 
 
 0-0071 
 
 13-0 
 
 1,850 
 
 22 
 
 7/18 
 
 0-013 
 
 21-0 
 
 1,700 
 
 25 
 
 19/20 
 
 0-019 
 
 29-0 
 
 1,550 
 
 27 
 
 7/16 
 
 0-022 
 
 33-0 
 
 1,500 
 
 28 
 
 19/18 
 
 0-034 
 
 47-0 
 
 1,400 
 
 30 
 
 7/14 
 
 0-035 
 
 48-0 
 
 1,400 
 
 30 
 
 19/16 
 
 0-061 
 
 75-0 
 
 1,250 
 
 33 
 
 19/14 
 
 0-096 
 
 108-0 
 
 1,150 
 
 36 
 
 37/16 
 
 0-12 
 
 1300 
 
 1,100 
 
 37 
 
 19/13 
 
 0-16 
 
 1360 
 
 1,100 
 
 38 
 
 37/14 
 
 0-19 
 
 187-0 
 
 1,050 
 
 40 
 
 61/13 
 
 0-31 
 
 350-0 
 
 '. . 900 
 
 47 
 
 91/12 
 
 077 
 
 625-0 
 
 800 
 
 51 
 
 1 This table, with the exception of the approximate areas in column 1, which 
 have been added, forms part of a table given in the Wiring Rides of the 
 Institution of Electrical Engineers. 
 
 2 The double figures in this column refer to the number of strands of the given 
 number of wire in a cable : thus 3/22 means a cable of 3 strands of No. 22 wire. 
 
 3 For example, 18 yards means that the distance between the two points is 9 yards. 
 
ADDENDA. 
 
 399 
 
 TABLE XXXVIII. COMPARISON OP CENTIGRADE AND FAHRENHEIT 
 THERMOMETERS. 
 
 Fah. 
 
 Cent. 
 
 Fah. 
 
 Cent. 
 
 Fah. 
 
 Cent. 
 
 Fah. 
 
 Cent. 
 
 Fah. 
 
 Cent. 
 
 Fah. 
 
 Cent. 
 
 Deg. 
 
 Deg. 
 
 Deg. 
 
 Deg. 
 
 Deg. 
 
 Deg. 
 
 Deg. 
 
 Deg. 
 
 Deg. 
 
 Deg. 
 
 Deg. 
 
 Deg. 
 
 32 
 
 
 
 66 
 
 18-9 
 
 100 
 
 37-8 
 
 133 
 
 56-1 
 
 166 
 
 74-4 
 
 199-4 
 
 93 
 
 33 
 
 0-5 
 
 66-2 
 
 19 
 
 100-4 
 
 38 
 
 134 
 
 56-7 
 
 167 
 
 75 
 
 200 
 
 93-3 
 
 33-8 
 
 1 
 
 67 
 
 19-4 
 
 101 
 
 38-3 
 
 134-6 
 
 57 
 
 168 
 
 75-5 
 
 201 
 
 93-9 
 
 34 
 
 1-1 
 
 68 
 
 20 
 
 102 
 
 38-9 
 
 135 
 
 57-2 
 
 168-8 
 
 76 
 
 201-2 
 
 94 
 
 35 
 
 17 
 
 69 
 
 20-5 
 
 102-2 
 
 39 
 
 136 
 
 57-8 
 
 169 
 
 76-1 
 
 202 
 
 94-4 
 
 35-6 
 
 2 
 
 69-8 
 
 21 
 
 103 
 
 39-4 
 
 136-4 
 
 58 
 
 170 
 
 76-7 
 
 203 
 
 95 
 
 36 
 
 2-2 
 
 70 
 
 2M 
 
 104 
 
 40 
 
 137 
 
 58-3 
 
 170-6 
 
 77 
 
 204 
 
 95-5 
 
 37 
 
 2-8 
 
 71 
 
 217 
 
 105 
 
 40-5 
 
 138 
 
 58-9 
 
 171 
 
 77-2 
 
 204-8 
 
 96 
 
 37-4 
 
 3 
 
 71-6 
 
 22 
 
 105-8 
 
 41 
 
 138-2 
 
 59 
 
 172 
 
 77-8 
 
 205 
 
 96-1 
 
 38 
 
 3-3 
 
 72 
 
 22-2 
 
 106 
 
 41-1 
 
 139 
 
 59-4 
 
 172-4 
 
 78 
 
 206 
 
 967 
 
 39 
 
 3-9 
 
 73 
 
 22-8 
 
 107 
 
 41-7 
 
 140 
 
 60 
 
 173 
 
 78-3 
 
 206-6 
 
 97 
 
 39'2 
 
 4 
 
 73-4 
 
 23 
 
 107-6 
 
 42 
 
 141 
 
 60-5 
 
 174 
 
 78-9 
 
 207 
 
 97-2 
 
 40 
 
 4-4 
 
 74 
 
 23-3 
 
 108 
 
 42-2 
 
 141-8 
 
 61 
 
 174-2 
 
 79 
 
 208 
 
 97-8 
 
 41 
 
 5 
 
 75 
 
 23-9 
 
 109 
 
 42-8 
 
 142 
 
 61-1 
 
 175 
 
 79-4 
 
 208-4 
 
 98 
 
 42 
 
 5-5 
 
 75-2 
 
 24 
 
 109-4 
 
 43 
 
 143 
 
 617 
 
 176 
 
 80 
 
 209 
 
 98-3 
 
 42-8 
 
 6 
 
 76 
 
 24-4 
 
 110 
 
 43-3 
 
 1436 
 
 62 
 
 177 
 
 80-5 
 
 210 
 
 98-9 
 
 43 
 
 6-1 
 
 77 
 
 25 
 
 111 
 
 43-9 
 
 144 
 
 62-2 
 
 177-8 
 
 81 
 
 2102 
 
 99 
 
 44 
 
 67 
 
 78 
 
 25-5 
 
 111-2 
 
 44 
 
 145 
 
 62-8 
 
 178 
 
 81-1 
 
 211 
 
 99-4 
 
 44-6 
 
 7 
 
 78-8 
 
 26 
 
 112 
 
 44-4 
 
 145-4 
 
 63 
 
 179 
 
 817 
 
 212 
 
 100 
 
 45 
 
 7-2 
 
 79 
 
 26-1 
 
 113 
 
 45 
 
 146 
 
 63-3 
 
 179-6 
 
 82 
 
 213 
 
 100-5 
 
 46 
 
 7-8 
 
 80 
 
 26-7 
 
 114 
 
 45-5 
 
 147 
 
 63-9 
 
 180 
 
 82-2 
 
 213-8 
 
 101 
 
 46-4 
 
 8 
 
 80-6 
 
 27 
 
 114-8 
 
 46 
 
 147-2 
 
 64 
 
 181 
 
 82-8 
 
 214 
 
 101-1 
 
 47 
 
 8-3 
 
 81 
 
 27-2 
 
 115 
 
 46-1 
 
 148 
 
 64-4 
 
 181-4 
 
 83 
 
 215 
 
 1017 
 
 48 
 
 8-9 
 
 82 
 
 27-8 
 
 116 
 
 46-7 
 
 149 
 
 65 
 
 182 
 
 83-3 
 
 215-6 
 
 102 
 
 48-2 
 
 9 
 
 82-4 
 
 28 
 
 116-6 
 
 47 
 
 150 
 
 65-5 
 
 183 
 
 83-9 
 
 216 
 
 102-2 
 
 49 
 
 9-4 
 
 83 
 
 28-3 
 
 117 
 
 47-2 
 
 150-8 
 
 66 
 
 183-2 
 
 84 
 
 217 
 
 102-8 
 
 50 
 
 10 
 
 84 
 
 28-9 
 
 118 
 
 47-8 
 
 151 
 
 66-1 
 
 184 
 
 84-4 
 
 217-4 
 
 103 
 
 51 
 
 10-5 
 
 84-2 
 
 29 
 
 118-4 
 
 48 
 
 152 
 
 66-7 
 
 185 
 
 85 
 
 218 
 
 103-3 
 
 51-8 
 
 11 
 
 85 
 
 29-4 
 
 119 
 
 48-3 
 
 152-6 
 
 67 
 
 186 
 
 85-5 
 
 219 
 
 103-9 
 
 52 
 
 11-1 
 
 86 
 
 30 
 
 120 
 
 48-9 
 
 153 
 
 67-2 
 
 186-8 
 
 86 
 
 219-2 
 
 ]04 
 
 53 
 
 117 
 
 87 
 
 30-5 
 
 120-2 
 
 49 
 
 154 
 
 67-8 
 
 187 
 
 86-1 
 
 220 
 
 104-4 
 
 53'6 
 
 12 
 
 87-8 
 
 31 
 
 121 
 
 49-4 
 
 154-4 
 
 68 
 
 188 
 
 867 
 
 221 
 
 105 
 
 54 
 
 12-2 
 
 88 
 
 3M 
 
 122 
 
 50 
 
 155 
 
 68-3 
 
 188-6 
 
 87 
 
 250 
 
 121 
 
 55 
 
 12-8 
 
 89 
 
 31-7 
 
 123 
 
 50-5 
 
 156 
 
 68-9 
 
 189 
 
 87-2 
 
 302 
 
 150 
 
 55-4 
 
 13 
 
 89-6 
 
 32 
 
 123-8 
 
 51 
 
 156-2 
 
 69 
 
 190 
 
 87-8 
 
 400 
 
 204 
 
 56 
 
 13-3 
 
 90 
 
 32-2 
 
 124 
 
 51-1 
 
 157 
 
 69-4 
 
 190-4 
 
 88 
 
 482 
 
 250 
 
 57 
 
 13-9 
 
 91 
 
 32-8 
 
 125 
 
 51-7 
 
 158 
 
 70 
 
 191 
 
 88-3 
 
 572 
 
 300 
 
 57-2 
 
 14 
 
 91-4 
 
 33 
 
 125-6 
 
 52 
 
 159 
 
 70-5 
 
 192 
 
 88-9 
 
 752 
 
 400 
 
 58 
 
 14-4 
 
 92 
 
 33-3 
 
 126 
 
 52-2 
 
 159-8 
 
 71 
 
 192-2 
 
 89 
 
 932 
 
 500 
 
 59 
 
 15 
 
 93 
 
 33-9 
 
 127 
 
 52-8 
 
 160 
 
 71-1 
 
 193 
 
 89-4 
 
 1112 
 
 600 
 
 60 
 
 15-5 
 
 93-2 
 
 34 
 
 127-4 
 
 53 
 
 161 
 
 717 
 
 194 
 
 90 
 
 1292 
 
 700 
 
 60-8 
 
 16 
 
 94 
 
 34-4 
 
 128 
 
 53-3 
 
 161-6 
 
 72 
 
 195 
 
 90-5 
 
 1472 
 
 800 
 
 61 
 
 ,16-1 
 
 95 
 
 35 
 
 129 
 
 53-9 
 
 162 
 
 72-2 
 
 195-8 
 
 91 
 
 1652 
 
 900 
 
 62 
 
 16-7 
 
 96 
 
 35-5 
 
 129-2 
 
 54 
 
 163 
 
 72-8 
 
 196 
 
 91-1 
 
 1832 
 
 1000 
 
 62-6 
 
 17 
 
 96-8 
 
 36 
 
 130 
 
 54-4 
 
 163-4 
 
 73 
 
 197 
 
 91-7 
 
 2282 
 
 1250 
 
 63 
 
 17-2 
 
 97 
 
 36-1 
 
 131 
 
 55 
 
 164 
 
 73-3 
 
 197-6 
 
 92 
 
 2732 
 
 1500 
 
 64 
 
 17-8 
 
 98 
 
 36-7 
 
 132 
 
 55-5 
 
 165 
 
 73-9 
 
 198 
 
 92-2 
 
 3182 
 
 1750 
 
 64-4 
 
 18 
 
 98-6 
 
 37 
 
 132-8 56 
 
 165-2 
 
 74 
 
 199 
 
 92-8 
 
 3632 
 
 20UO 
 
 65 
 
 18-3 
 
 99 
 
 37-2 
 
 
 
 
 
 
 
 
400 
 
 ADDENDA. 
 
 TABLE XXXIX. AVOIRDUPOIS WEIGHT. 
 
 
 = Ounces. 
 
 = Drachms. 
 
 = Grains. 
 
 = Grammes. 
 
 1 Pound .... 
 
 16 
 
 256 
 
 7,000 
 
 453-25 
 
 1 Ounce .... 
 
 1 
 
 16 
 
 437-5 
 
 28-33 
 
 1 Drachm .... 
 
 0-062 
 
 1 
 
 27-34 
 
 1-77 
 
 TABLE XL. TROY WEIGHT. 
 
 
 = Ounces. 
 
 = Penny weights. 
 
 = Grains. 
 
 = Grammes. 
 
 1 Pound .... 
 
 12 
 
 240 
 
 5,760 
 
 372-96 
 
 1 Ounce .... 
 
 1 
 
 20 
 
 480 
 
 31-08 
 
 1 Pennyweight . . 
 
 0-05 
 
 1 
 
 24 
 
 1-55 
 
 TABLE XLL APOTHECARIES' WEIGHT. 
 
 
 = Ounces. 
 
 = Drachms. 
 
 = Scruples. 
 
 = Grains. 
 
 = Grammes. 
 
 1 Pound . . . 
 
 12 
 
 96 
 
 288 
 
 5,760 
 
 372-96 
 
 1 Ounce . 
 
 1 
 
 8 
 
 24 
 
 480 
 
 31-08 
 
 1 Drachm . . 
 
 0-125 
 
 1 
 
 3 
 
 60 
 
 3-38 
 
 1 Scruple . . 
 
 0-042 
 
 0-33 
 
 1 
 
 20 
 
 1-29 
 
 TABLE XLII. IMPERIAL FLUID MEASURE. 
 
 
 03 
 
 I 
 II 
 
 1 
 
 
 
 II 
 
 "2 " 
 
 3$ 
 
 Sg 
 II o 
 
 = Fluid 
 Drachms. 
 
 = Minims. 
 
 u 
 
 > 2 
 
 
 H.2 
 
 o . 
 
 2 % 
 
 3rT5 
 
 
 
 li 
 
 1 
 >3 
 
 II 
 
 = Cubic 
 Centimetres. 
 
 1 Gallon . . 
 
 4 
 
 8 
 
 160 
 
 1,280 
 
 76,800 
 
 70,000 
 
 277-276 
 
 4-541 
 
 4,541 
 
 1 Quart . . . 
 
 1 
 
 2 
 
 40 
 
 320 
 
 19,200 
 
 17,500 
 
 69-319 
 
 1-135 
 
 1,135-2 
 
 1 Pint . . . 
 
 0-5 
 
 1 
 
 20 
 
 160 
 
 9,600 
 
 8,750 
 
 34-659 
 
 0-567 
 
 567-6 
 
 1 Fluid ounce . 
 
 0-025 
 
 0-05 
 
 1 
 
 8 
 
 480 
 
 437-5 
 
 1-733 
 
 0-0284 
 
 28-34 
 
 1 Fluid drachm 
 
 0-0031 
 
 0-0062 
 
 0-125 
 
 1 
 
 60 
 
 54-7 
 
 0-217 
 
 0-0035 
 
 3-55 
 
 1 Minim . . 
 
 0-00005 0-0001 
 
 0-0021 
 
 0-0167 
 
 1 
 
 0-91 
 
 0-0036 0-00006 
 
 0-059 
 
ADDENDA. 
 
 401 
 
 a -3 
 
 3-38 
 
 Ml 
 
 O to 
 
 
 > < OJ 10 IN ** 
 
 PH i-H <N CO 5 
 
 "? r^ tr ? oo 
 
 sll 
 
 ^OiCO^ 
 
 . 
 
 t^O^t^-TH^ 
 
 c<i QO * 
 
 iHM^eb<NOTib 1 HOT4jiAH^iw^i^ 
 
 
 Ills 
 
 MH o 
 
 ll 
 
 > CO (M OS US i 
 
 cq eo * i us c (^ 05 
 
 (^ 05 in N * 
 
 !!M 
 
 IdCO 
 
 26 
 
402 ADDENDA. 
 
 From this table any ordinary conversions up to 2000 units may be readily 
 made. For example : It is required to find the number of cubic centimetres 
 equal to 1728 cubic inches. 
 
 1728 = 1000 + 700 + 20 + 8 cubic inches. 
 
 But a reference to the sixth column shows that 
 
 1000 cubic inches 16,385*92 cubic centimetres 
 *700 ,, = 11,470-01 ,, 
 20 , ,, 32772 ,, 
 
 8 , = 131-09 . 
 
 Add together: 1728 ,, ,, = 28,31474 
 
 THE BRONZING OF COPPER AND BRASS SURFACES. 
 
 It is often desired to give newly deposited copper the appearance of age 
 and to destroy the brilliant metallic lustre which it possesses at first. The 
 methods of accomplishing this end are numerous. In all cases it is desirable 
 to start with a clean metallic surface, freed from grease by immersion in 
 potash, or by any other suitable cleansing process. To obtain a red bronze 
 tone, the metal is brushed over with finely-powdered crocus, or a mixture 
 of crocus and black-lead, made up into a paste with a little water, and is 
 then heated on a metal plate above a clear fire until the powder has become 
 dark. After cooling, the whole surface is thoroughly brushed ; if necessary 
 the process may be repeated to produce a darker colour. The bronzing is 
 due to the oxidation of the copper superficially by the heated crocus ; a 
 better lustre is obtained by finally rubbing persistently with a brush which 
 is from time to time passed over the surface of a cake of bees' -wax. 
 Slightly wetting the clean surface of copper articles with very dilute nitric 
 acid, or with a solution of ferric chloride and nitrate, or with a solution of 
 copper nitrate, followed by drying and heating, also effects the required 
 oxidation and produces a brown bronze. 
 
 Dark brown or black bronzing has sometimes been effected by merely 
 brushing the surface with plumbago or vegetable black, conveyed in a 
 suitable medium followed by a varnish or lacquer. The formation of the 
 black copper sulphide on the surface of the metal, by painting with dilute 
 alkaline sulphide solution (such as ammonium sulphide), gives the same 
 appearance. The superficial precipitation of other metals, such as platinum, 
 gold, or arsenic, is often adopted also ; a very weak solution of platinic 
 chloride, or of gold chloride, or a solution of 1 oz. of arsenious acid (white 
 arsenic), and 1 oz. of ferrous sulphate in 12 oz. of water, answering the pur- 
 pose well. After aplying any of these solutions, the object must be well 
 washed and dried,_ and finally lacquered. A mere bronze-coloured varnish, 
 recommended by Hutton for bronzing brass work, is made by dissolving 5 
 parts of aniline purple and 10 parts of fuchsine in 100 parts of methylated 
 spirit, and then adding 5 parts of benzoic acid, and boiling until the liquid 
 has attained the desired colour. 
 
 An excellent black bronzing for brass is obtained by dissolving copper 
 carbonate in an aqueous solution of ammonium carbonate. The best results 
 are obtained with gilding metal, which after cleansing thoroughly need only 
 be immersed for a short time in the liquid, after which the object is swilled 
 in water and dried out. One of the secrets of success is to avoid the presence 
 of even traces of chlorides in the bronzing solution. In their presence the 
 black coating is liable to scale or to lighten in colour after a short time. 
 
 Green bronzes are made by converting the surface of the article into the 
 
 * Note that the equivalent of 700 cubic ins. is found by multiplying the figure for 
 70 by 10. 
 
ADDENDA. 403 
 
 green basic acetate or carbonate of copper, and may be produced by exposing 
 the article for a time to the vapour of acetic acid. Any acid vapour in 
 moderation will afford the same result. One method, recommended by 
 Napier, consists in enclosing the object immediately above a little dry 
 bleaching powder contained in a closed vessel, until the required effect is 
 produced. 
 
 ANTIDOTES TO POISONS. 
 
 Most of the metallic-plating and the cleansing solutions are extremely 
 poisonous, and stress has already been laid upon the dangers both of using 
 domestic drinking utensils for any purposes connected with the work 
 of electro-plating, and of dipping the bare arm or hand into any of the 
 depositing liquids. Unforeseen accidents, however, may occur and may 
 demand the application of speedy remedial measures. Amateur doctoring 
 is to be strongly deprecated, and medical aid should be sought at once ; but 
 upon a sudden emergency it may be necessary to administer relief, pending 
 the arrival of the physician. In any case of poisoning by swallowing, simple 
 emetics should at once be given for example, lukewarm water, mustard 
 and water, ipecacuanha, or even zinc sulphate (of the latter from 10 to 30 
 grains are often given), the two first-named are better for domestic applica- 
 tion ; while these are preparing, the patient may often induce vomiting by 
 thrusting the forefinger as far as possible down the entrance to the throat. 
 The nature of the subsequent remedies will depend upon the character of the 
 poison, thus : 
 
 Acids. Mineral acids, such as sulphuric, nitric, hydrochloric, or glacial 
 acetic acids, require an alkali to neutralise them ; magnesia, chalk, whiting, 
 lime water, or carbonate of soda may be administered stirred up with water. 
 Failing these, the acid must be diluted by copious draughts of water ; olive 
 oil, milk, or white of egg may then be given. 
 
 Alkalies. Caustic alkalies demand neutralisation with a mild acid, such 
 as vinegar, or the juice of an acid fruit, such as the lemon or lime, or by 
 extremely dilute acetic, citric, or tartaric acids. Then oil or white of egg 
 may be taken. 
 
 Antimony. For the chloride solution, magnesia or sodium carbonate are 
 used ; for tartar emetic, a vegetable astringent is to be applied ; very strong 
 tea may answer the purpose ; then barley water or the like ; small doses of 
 stimulants being given from time to time. 
 
 Arsenic. Freshly made hydrated ferric oxide with magnesia is often 
 employed. 
 
 Copper. White of egg mixed with water, plenty of milk, water, or barley- 
 water or the like should be taken. Some have used calcined magnesia 
 stirred with water. 
 
 Cyanides. Freshly precipitated peroxide of iron with an alkaline car- 
 bonate, such as potassium carbonate ; plenty of fresh air should be available ; 
 the coldest possible water should be poured over the head and down the 
 spine ; and the atmosphere around the patient may with advantage contain 
 a little chlorine ; for example, a little dilute acid may be poured upon 
 bleaching powder in a saucer placed at some distance to windward of the 
 patient. 
 
 Lead. A very dilute solution of sulphuric acid or a solution of magnesium 
 or sodium sulphate may be administered ; some use sodium phosphate ; the 
 object in each case being the formation of an insoluble salt of lead. This 
 should be followed by an active purgative. Milk or white of egg may be 
 plentifully taken. 
 
 Mercury. Albuminous fluids (white of egg) should be given in sufficient 
 quantity, mixed preferably with milk ; a large excess of the albumen is not 
 advisable, the quantity generally recommended being the white of one egg 
 to each 4 grains (about) of mercuric chloride taken. Then barley-water or 
 its equivalent is allowable. 
 
404 ADDENDA. 
 
 Oxalic Acid and Oxalates. Lime-water or chalk may be used ; but 
 alkaline carbonates should not be applied, because they form intensely 
 poisonous oxalates. 
 
 Silver Nitrate. Common salt in solution forms insoluble silver chloride. 
 
 Zinc Salts. Warm demulcent drinks, such as barley-water, to be given. 
 
 In all the above cases the application of the special remedy must be pre- 
 ceded by the use of strong emetics, except perhaps in the case of strong 
 acids, when water should be taken to effect dilution before inducing the 
 vomiting. 
 
 Acids which have been spilled upon the hands or upon the floor of the 
 room should be neutralised with chalk after dilution. The vapour of acid 
 in the atmosphere of a room may be neutralised by the vapour of ammonia. 
 
INDEX. 
 
 ACCUMULATORS, electrical, 74. 
 
 Acetic acid, 364. 
 
 Acid, effect of, in copper-baths, 133. 
 
 final cleansing in, 112. 
 
 nitric, as depolariser, 46. 
 
 strength for Grove's battery, 48. 
 Acid-bath for copper deposit, 128. 
 
 -resisting composition (vat-lining), 
 
 96. 
 Acids, opening bottles of, 363. 
 
 organic, resistance of cobalt to, 227. 
 use in nickeling, 221. 
 
 specific-gravity tables, 392. 
 
 various, 390-401. 
 Adams, metallisation of moulds, 150. 
 
 nickeling solution, 218. 
 Adhesion of copper surfaces prevented, 
 152. 
 
 low, of nickel deposits (cause), 225. 
 
 non-, of deposits (cause), 76. 
 Agate burnishers, 121. 
 Ageing of silver-baths, 183. 
 Agitation of solutions, necessary, 90. 
 Agitator, von Hiibl's, 98. 
 
 for baths, 98. 
 Air, effect of, on iron-baths, 231. 
 
 pure, need of, 92. 
 
 use of, in circulating solutions, 277. 
 Alcohol, 366. 
 Alkali, action of, on grease, 109. 
 
 without action on mineral oils, 109. 
 Alkaline cleansing liquid, 110. 
 
 copper-baths, 129. 
 Alkalinity of nickel-baths, cause of, 
 
 220. 
 Alloy, backing-, electrotypers', 164. 
 
 lead-tin-, for polishing steel, 122. 
 
 standard silver-coinage, 186. 
 Alloys, aluminium, produced, 293. 
 
 copper, cleansing of, 112. 
 
 deposition of, 32, 256. 
 
 fused, electrolysis of, 32. 
 
 fusible, 147. 
 
 gold, 204. 
 
 lead, stripping silver from, 190. 
 
 Alloys, nature of deposit, 261. 
 
 tin, cleansing of, 114. 
 Alternate-current dynamo, 68. 
 Aluminium and its compounds, 367. 
 
 carbide, 293. 
 
 chloride, 367. 
 
 deposition of, 254. 
 
 deposition on, 114. 
 
 -nickel alloy, 266. 
 
 reduction of, 11, 293, 294. 
 
 smelting of, 295. 
 Alums, 368. 
 Alu-ni, 266. 
 Amalgamation of battery zincs, 39. 
 
 of gold, 206. 
 Amalgams, 378. 
 American Postal Telegraph Co.'s 
 
 plant, 140. 
 American Smelting and Refining Co., 
 
 274. 
 Ammeter, 84. 
 
 advantages of, 78, 80. 
 
 position of, in electrotype circuit, 
 l)4c 
 
 use of, in art- electro typing, 170. 
 Ammonia, 368. 
 
 alum, 368. 
 
 solution, opening bottles of, 368. 
 
 use of, in hot copper-baths, 131. 
 Ammonium compounds, 368. 
 
 sulphide, use in electrotyping, 168. 
 Amorphous phosphorus, 381. 
 Amperage, best, for electro-deposi- 
 tion, 78. 
 
 surface-, interconversion of units, 
 
 391. 
 
 Ampere, value of the, 34. 
 Animal forms reproduced in copper, 
 
 An ion, meaning of term, 27. 
 Annealing, effect of, on hammered 
 
 metals, 110. 
 on iron deposit, 235. 
 on nickel deposit, 217. 
 electric, 317. 
 
 405 
 
406 
 
 INDEX. 
 
 Anode, meaning of term, 27. 
 
 plate, form of, 97. 
 
 slime (copper), 132. 
 Anodes, antimony, 252. 
 
 arrangement of, in art- work, 168. 
 
 brassing, 261. 
 
 carbon, use of, 222. 
 
 choice of, 88. 
 
 coating on, by lead, 250. 
 
 cobalt, 229. 
 
 copper, 131. 
 
 behaviour in refining, 269. 
 refinery, 274. 
 -regulus, use of, 279. 
 size for, 132. 
 
 gold, 204. 
 
 incrustation on, in brassing, 263. 
 
 insoluble, use of, 28, 29, 89, 353. 
 
 iron, 234. 
 
 lead, effect of, in copper-bath, 169. 
 
 nickel, 222. 
 
 arrangement of, 252. 
 
 silver, 186. 
 
 appearance during electrolysis, 
 
 182. 
 arrangement of, 192. 
 
 size of, 89. 
 
 soluble, effect of, 27. 
 use of, 29, 88. 
 
 supplementary, use of, 171. 
 
 suspension of, 96, 97. 
 
 tin, 248. 
 
 various, effect of, 30. 
 
 wire-skeleton, for statuary, 168. 
 
 zinc, 243. 
 
 Antidotes to poisons, 403. 
 Antimony and its compounds, 369. 
 
 anodes, 252. 
 
 behaviour in copper-refining, 270. 
 
 deposited, nature of, 252. 
 
 depositing solutions, 251. 
 
 deposition of, 250. 
 
 explosive, 252. 
 
 extraction of, 289. 
 
 solutions, assay of, 325. 
 Antique silver, 197. 
 Apothecaries' weight, 400. 
 Aqua fortis, 113, 365. 
 
 regia, 366. 
 Argol, 383. 
 Armature, dynamo, 70. 
 
 direction of current in, 69. 
 
 varieties of, 70. 
 
 Arsenic, behaviour in copper-refining, 
 271. 
 
 effect of, in brassing-bath, 260. 
 Art-electrotyping, 165. 
 Ashcroft and Swinburne, zinc extrac- 
 tion process, 288. 
 Assay of depositing solutions, 324. 
 
 Atomic weight, definition of, 15. 
 
 weights of elements, 19. 
 Atoms, meaning of term, 15. 
 Autogenous soldering of lead, 95. 
 Avoirdupois weight, 400. 
 
 BACKING of copper electrotypes, 164. 
 
 -metal for electrotypes, 164. 
 Balance, plating-, 100. 
 
 correction in use of, 102. 
 sensitive, 324. 
 Barometer dials, dead-gilding of, 210. 
 
 silvering of, 177. 
 Barrel, rotating-, for plating small 
 
 goods, 105. 
 
 Base-bullion, refining of, 281. 
 Basis-metal, influence on colour in 
 
 gilding, 209. 
 use of term, 116. 
 Baths (see Solutions), 
 arrangement of, in copper-refining, 
 
 274. 
 cyanide, spontaneous alteration of, 
 
 182. 
 
 electrotype, arrangement of, 153. 
 old, recovery of metal from, 318. 
 plating, arrangement of, 85. 
 Battery, arrangement of, 53. 
 bichromate, 49. 
 Bunsen's, 47. 
 costliness of, 37, 333. 
 Cruickshank's, 3. 
 DanielPs, 42. 
 Breguet's, 44. 
 gravity, 45. 
 Kuhlo's, 44. 
 Meidinger's, 44. 
 post-office, 45. 
 depolarisation of, 41. 
 direction of current in, 24. 
 economical arrangement of cells, 54. 
 Edison-Lalande, 50. 
 effect of size of plates, 53. 
 for brassing, 257. 
 for cadmium-plating, 245. 
 for cobalt-plating, 229. 
 for copper-depositing, 128. 
 for electrotype, 154. 
 for gilding, 201. 
 for iron-depositing, 236. 
 for nickel-plating, 220. 
 for silvering, 179. 
 for tinning, 248. 
 for zinc-depositing, 241. 
 Grove's, 46. 
 
 injurious fumes from, 48, 92. 
 invention of, 3. 
 Leclanch6's, 50. 
 local action on zinc, 39. 
 maximum efficiency of, 56. 
 
INDEX. 
 
 407 
 
 Battery, parts of, 38. 
 polarisation of, 40. 
 position of, in plant, 93. 
 practical hints on, 51. 
 principle of, 24, 38. 
 screws, 57. 
 secondary, 74. 
 
 single- and two-fluid, 41, 347, 349. 
 size of, effect on current, 53. 
 Smee's, 41. 
 switch-board for, 57. 
 theory of, 347. 
 thermo-electric, 59. 
 (Diamond's, 64. 
 direction of current in, 60. 
 Giilcher's, 66. 
 reversal of current in, 62. 
 wastefulness of, 66. 
 weakening of, 40. 
 -zincs, amalgamation of, 39. 
 
 need for purity, 39. 
 Baume's hydrometer, value of degrees, 
 
 393. 
 
 Bay salt, 386. 
 
 Beardslee's cobalting solution, 228. 
 Becquerel's cobalting solution, 228. 
 electro-chromy, 255. 
 
 -gilding, 202. 
 electrolytic works, 5. 
 ore-treatment, 279. 
 Bedstead tubes, brass-coated, 263. 
 Bees'-wax, 370. 
 
 cracking of, on cooling, 145. 
 use of, in moulding, 145. 
 Beuardos electric welding process, 316. 
 Benzene, use of, in cleansing, 108. 
 Benzoic acid, 364. 
 Benzoline, use of, in cleansing, 108. 
 Bertrand's bismuth solution, 254. 
 cadmium solution, 245. 
 palladium solution, 254. 
 Bessemer's copper-plating, 5. 
 Betts' process for refining lead, 282. 
 Bichromate battery, 49. 
 Binding-screws, 57. 
 Birmingham wire-gauge, value of 
 
 numbers, 396. 
 Bismuth, 370. 
 
 behaviour of, in copper-refining, 270. 
 deposition of, 254. 
 use of, in fusible alloys, 147. 
 Bisulphide of carbon for bright-plating, 
 
 8, 185. 
 Black deposit in bright-silver bath, 1 86. 
 
 on silver anodes, 182. 
 gold deposit, 205. 
 Black-lead, 382. 
 
 application to moulds, 149. 
 water-repelling action of, overcome, 
 163. 
 
 Black-leading machine, 162. 
 process, invention of, 7. 
 wax (type) moulds, 161. 
 Bias and Miest's copper ore-treatment, 
 
 280. 
 Blende, roasting of, 287. 
 
 treatment of, 287. 
 
 Blocks, wood-, electrotyping of, 165. 
 Blood-poisoning from plating solu- 
 tions, 103. 
 Board of Trade Unit, 36, 330. 
 
 used in electrolysis, 331. 
 Bobs for polishing, 118. 
 Boden's nickeling solution, 218. 
 Bookbinders' type, brassing of, 263. 
 Boracic acid, 364. 
 
 Borcher's antimony extraction, 289. 
 refining of lead, 282. 
 system of copper- refining, 276. 
 Boric acid, 364. 
 
 use of, in nickeling, 221. 
 Bottger's cobalting solution, 228. 
 iron-plating solution, 233. 
 platinating solution, 239. 
 silvering solution, 180. 
 Box- wood sawdust, use of, 136. 
 Brass, 370. 
 anode, 261. 
 
 cause of incrustation on, 263. 
 bronzing of, 402. 
 cobalting of, 229. 
 coppered by immersion, 125. 
 deposit, colour of, controlled, 261. 
 
 nature of, 257. 
 
 depositing solutions, 257, 258. 
 deposition of, 32, 257. 
 final cleansing of, 112. 
 gilding of, by immersion, 199. 
 nickeling of, 223. 
 platinising of, 237, 238. 
 silvering of, 176, 191. 
 stripping of nickel from, 222. 
 
 of silver from, 189. 
 wire scratch-brushes, 119. 
 Brassing of small goods, 105. 
 Braun's immersion gilding, 199. 
 Breguet's Daniell-cell, 44. 
 Briant (de), gilding solution, 202. 
 Bright-dipping of metals (in acid), 113. 
 -plating, 8, 185. 
 -silver bath, use of, 185. 
 Brightness of silver anodes during 
 
 electrolysis, 182. 
 Brimstone, 387. 
 
 Britannia metal, cleansing of, 114. 
 gilding of, 211. 
 nickeling of, 218, 227. 
 silvering of, 191. 
 stripping silver from, 190. 
 unsuited for silvering, 188. 
 
408 
 
 INDEX. 
 
 Bronze, 370. 
 
 depositing solutions, 264. 
 
 deposition of, 263. 
 gilding of, by immersion, 199. 
 Bronzing of copper and brass, 402. 
 Brown & Sharpe wire-gauge, 396. 
 Brown copper-deposit, cause of, 156. 
 Brown gold, cause of, 203, 206. 
 Brown-Neil process of recovering tin 
 
 from tin scrap, 290. 
 Brunei's brassing solutions, 258. 
 Brushes of dynamo, 69. 
 position of, 73. 
 regulation of, 73. 
 
 scratch-, 119. 
 
 Building-tools for wax moulds, 161. 
 Bullion, base-, refining of, 281. 
 Bunsen's cell, 48. 
 
 electro-smelting of magnesium, 296. 
 Burnishing, 121. 
 
 and scratch-brushing contrasted, 
 122. 
 
 deposited copper, 138. 
 
 effect of, on metals, 122. 
 
 nickel, difficulty in, 224. 
 Burnt nickel, 217. 
 Burton's liquid forge, 315. 
 Busts, moulding from, 148, 166. 
 Butter of antimony, 369. 
 
 of tin, 388. 
 Buttons, silvering of, 176. 
 
 CADMIUM, 371. 
 deposition of, 245. 
 use of, in fusible alloys, 147. 
 Calamine, treatment of, 286. 
 Calcium carbide, production of, 294. 
 
 sulphate, 381. 
 Calculations as to disposition of vats 
 
 154. 
 
 as to thickness of deposit, 79. 
 Calomel, 379. 
 
 Calorie, value of the, 35, 36. 
 Campbell's platinum-silver bath, 266. 
 Canadian Government Commission 
 appointed to report on 
 electro - thermic iron and 
 steel processes, 299. 
 Carbon anodes, use of, 222. 
 bisulphide for bright-plating, 8, 
 
 185. 
 
 chloride, use in bright silvering, 185. 
 Cast-iron, 376. 
 as anode, 89, 234. 
 nickeling of, 225. 
 preliminary cleansing of, 135. 
 refining (electrolytic), 289. 
 
 (electro-thermal), 297. 
 tinning of, 248. 
 Cast-metal anodes, 89. 
 
 Cast-nickel anodes, 222. 
 
 Oastner's sodium smelting process, 
 
 297. 
 
 Casts, electrotype, 143. 
 Cathode for copper-refining, 273. 
 meaning of term, 27. 
 motion imparted to, 99. 
 secondary actions at, 354. 
 suspension of, 97, 152. 
 Cathodes, unlike, on same rod, effect 
 
 of, 192. 
 
 Cation, meaning of term, 27. 
 Caustic alkali cleansing-baths, 110. 
 lunar, 386. 
 potash, 383. 
 soda, 386. 
 Cell (see Battery), 
 direction of current in, 24, 347. 
 economical arrangement of, 56. 
 principles of voltaic, 24, 347. 
 size of, effect on current, 53. 
 Cells, porous, preservation of, 51. 
 simple, 347. 
 
 single-fluid and two-fluid, 349. 
 Centigrade thermometer scale, 35. 
 and Fahrenheit scales compared, 
 
 399. 
 
 Chalk, 371. 
 Charcoal rendered non-conductive, 
 
 292. 
 
 Chases for use in electrotyping, 157. 
 Chemical combination, heat of, 21, 22, 
 
 344. 
 energy, relation of, to electrical, 23, 
 
 344. 
 
 formulae, 16. 
 
 symbols, 16. 
 
 Chlorides, 365. 
 
 of carbon and sulphur for bright- 
 plating, 185. 
 
 Chromium, deposition of, 245. 
 Circuit, short, 39. 
 
 divided-, distribution of current in, 
 
 39. 
 Citric acid, 365. 
 
 use of, in nickeling, 221. 
 Clamond's thermo-electric battery, 64. 
 Cleanliness, necessity for, 77, 108. 
 Cleansing, electrolytic, 115. 
 finished electrotypes, 171. 
 for nickeling, care in, 224. 
 liquids, acid, 112. 
 alkaline, 109, 111. 
 cyanide, 113. 
 objects for plating, 108. 
 Clock-dials, dead-gilding of, 210. 
 Coal and zinc as electrical generators, 
 
 37. 
 
 Cobalt and its compounds, 372. 
 anodes, 229. 
 
INDEX. 
 
 409 
 
 Cobalt and its compounds, behaviour 
 
 of, in copper-refining, 270. 
 characteristics of, 227. 
 depositing solutions, 228. 
 deposition of, 229. 
 -nickel alloy deposited, 219. 
 recovery of, from old baths, 318. 
 resistance of, to organic acids, 227. 
 solutions, assay of, 326. 
 Cobley's copper ore treatment, 279. 
 Coils, resistance-, 81. 
 Coinage, standard silver, 186. 
 Coins, electrotyping of, 165. 
 
 moulding from, 144. 
 Collodion, use in bright-silvering, 185. 
 Colophony, 384. 
 Colour of brass-deposit controlled, 
 
 262. 
 of gold, 204, 205. 
 
 affected by impurities, 203. 
 -deposit controlled, 205. 
 Coloured silver-deposit, cause of, 187. 
 Colouring, 122. 
 
 of gold (dry), 212. 
 Colours, iridescent, produced, 255. 
 Combination, chemical, heat of, 20, 
 
 21, 344. 
 
 Commission appointed by Canadian 
 Government to report on 
 electro-thermic iron and steel 
 processes, 299. 
 Commutator, dynamo-, 69. 
 sparking of, 73. 
 tending of, 73. 
 Compass-needle, use of, 83. 
 Composition, moulding-, conductive, 
 
 146, 149. 
 elastic, 148, 166. 
 fluid, moulding with, 145. 
 gutta-percha, 143. 
 wax, use of, 145. 
 
 use of, in elastic, 166. 
 Compound-wound dynamo, 70. 
 Compounds, definition of term, 15. 
 Conditions of electrolysis, 362. 
 Conduction, electrolytic, 32, 342, 358. 
 Conductivity, electrical, 30, 358. 
 of metals, 31. 
 
 of oxides and sulphides, 379, 380. 
 of mould ensured, 149. 
 Conductors, choice of metals for, 31, 
 
 337. 
 
 copper, maximum currents for, 398. 
 loss of power in, 335. 
 maximum current for, 336, 398. 
 size of, 335. 
 Connecting screws, 57. 
 Continuous current, conversion of 
 alternating current into, 76. 
 dynamo, 67* 
 
 Converter, rotary, 76. 
 Copper and its compounds, 372. 
 and its alloys, final cleansing of, 112. 
 anodes, 131. 
 
 behaviour in refining, 269. 
 
 for art electrotyping, 167. 
 
 slime on, 132. 
 
 use of, in brassing, 261. 
 -baths, acid, 128. 
 
 alkaline, management of, 131. 
 
 effect of anode-size on, 132. 
 of lead anode on, 169. 
 
 temperature for, 130, 131. 
 bronzing of, 402. 
 conductors, maximum current for. 
 
 336, 398. 
 
 crude, impurities in, 268. 
 deposit, character of, 132, 156. 
 
 rel ation to cur rent strength ,133 
 to nature of solution, 133. 
 
 colours of, 156. 
 
 drying of, 136. 
 
 spread of, 170. 
 
 strength of, 133. 
 
 tenacity of, 134, 138. 
 depositing iron upon, 236. 
 -depositing prior to silvering, 191. 
 deposition by battery, 127. 
 
 by immersion, 124. 
 
 consolidation of coat, 124. 
 
 by single-cell process, 126. 
 
 of, on copper, 152. 
 on iron rollers, 137. 
 on wax, 163. 
 
 power absorbed in, 329. 
 
 solutions for, 128, 130. 
 effect of, on colour of gold, 204. 
 
 on colour of silver, 187. 
 electrolytic moulds of, 142. 
 electro-plating with, 135. 
 electrotype, backing of, 164. 
 
 nature of metal required, 133. 
 
 plates, suspension of, 152. 
 
 separated from plate, 157. 
 
 from wax, 163. 
 extraction from ores, 278. 
 
 Hoepfner, 280. 
 
 Siemens and Halske, 280. 
 gilding by immersion, 198. 
 native, treatment of, 279. 
 -nickel-zinc alloys deposited, 265. 
 nickeling of, 224. 
 -plates, iron-facing of, 235. 
 
 reproduction of, 152. 
 platinising of, 237. 
 printing-surfaces, nickeled, 227. 
 recovery of, from old baths, 319. 
 
 from wash-waters, 136. 
 refined, foreign metals in, 271. 
 
 form of, 277. 
 
410 
 
 INDEX. 
 
 Copper- refining, bath arrangement, 
 
 274. 
 
 behaviour of foreign metals, 270. 
 current- strength for, 272. 
 electrolytic, 268. 
 loss of energy in, 275. 
 relation of E.M.F. to inter-elec- 
 trode space, 273. 
 renewing old bath, 277. 
 solution for, 273. 
 spacing of electrodes, 273. 
 systems of, 275. 
 reflectors, manufacture of, 173. 
 -regulus, use as anode, 279. 
 silvering of, 176, 191. 
 solutions, assay of, 326. 
 spongy, used, 150. 
 stripped from iron, 236. 
 stripping of nickel from, 223. 
 
 of silver from, 189. 
 sulphate as a depolariser, 42. 
 
 solutions, specific gravity of, 393. 
 
 specific resistance of, 394. 
 thick deposits of, 136, 137. 
 thickness of coat required, 136, 
 
 168. 
 -tin alloys, deposition of, 263. 
 
 -zinc alloys, deposition of, 265. 
 tubes deposited, 137. 
 wires, resistance of. 395. 
 -zinc alloys, deposition of, 257. 
 Coppering metals before gilding, 211. 
 Correction for plating-balance, 102. 
 Corrosion of anodes in refining, 269. 
 Corrosive sublimate, 379. 
 Cost of electricity, 333. 
 Coulomb, value of, 35, 79. 
 Couples, thermo-, arrangement of, 64. 
 Cowles' aluminium-reduction, 11, 292. 
 Cowper- Coles' extraction and refining 
 
 of iron, 289. 
 process for manufacture of copper 
 
 wire, 140. 
 
 process for manufacture of re- 
 flectors, 173. 
 
 process for manufacture of seam- 
 less tubes and sheets of 
 copper, 138. 
 rotating cathodes, 138. 
 zinc bath, 243. 
 Cream of tartar, 383. 
 Crown of cups, Volta's, 3. 
 Cruickshank's electrolytic experi- 
 ments, 3. 
 Cryolite, 295. 
 
 Crystalline character of deposits, 138. 
 Crystallisation of battery fluids, 52. 
 Cub. ins. and cub. cms., intercon ver- 
 sion of, 401. 
 and pints, interconversion of, 401. 
 
 Current, alternating, converted into 
 
 continuous, 76. 
 -density, 36. 
 
 effect of, in electrotyping, 156. 
 on brass-deposit, 261. 
 on deposits, 78. 
 on silver-deposit, 187. 
 excessive, safe-guarded, 170. 
 for aluminium smelting, 296. 
 for copper-refining, 272. 
 for lead-refining, 282. 
 for nickeling, 220. 
 for zinc-deposition, 244. 
 maximum, for electrotyping, 134. 
 
 for wax-moulds, 164. 
 measured without instruments, 
 
 155. 
 permissible effect of motion of 
 
 solution on, 90. 
 
 relation to nature of copper- 
 deposit, 133. 
 unit of measurement, 34. 
 densities, equivalents in different 
 
 units, 391. 
 -detector, 82. 
 direction of, found, 83. 
 in armature, 69. 
 in battery, 24, 347. 
 in thermal battery, 60. 
 distribution of, in divided circuit, 
 
 39. 
 
 from public supply, use of, 75. 
 in coils rotating near magnet, 68. 
 maximum, for copper conductors, 
 
 336, 398. 
 
 measurement of, 83, 84, 154. 
 regulated by anode, 205. 
 regulation of, in electrotyping, 156. 
 reversal of, in dynamo, 68. 
 
 in thermopiles, 62. 
 short-circuiting of, indicated, 80. 
 sources of, 37. 
 weight and thickness of deposit by, 
 
 390. 
 
 Cut-out for check on current, 170. 
 Cyanide-bath, discovery of, 8. 
 poisonous fumes from, 92. 
 spontaneous alteration of, 182. 
 Cyanide cleansing-liquid, 113. 
 copper-baths, 129. 
 free, in gold-bath, 203. 
 
 in silver-bath, 181. 
 gold-bath, made up, 203. 
 of potassium, 382. 
 silver-baths, 180. 
 Cyanides, 365. 
 
 solution of organic matter by, 
 
 184, 207. 
 
 Cylinders, iron, coppering of, 137. 
 mixing-, 125. 
 
INDEX. 
 
 411 
 
 DANIELL-OELL, 5, 42. 
 
 Darcet's fusible alloy, 147. 
 
 De la Rue's discovery of electrotyp- 
 
 ing, 5. 
 Dead-dipping of objects, 113. 
 
 -gilding, 209. 
 
 -lustre on silver, 197. 
 
 -nickeling, 227. 
 Deakin and Smith's rotatory plating 
 
 apparatus, 105. 
 Decantation, washing by, 384. 
 Dechaud and Gaultier's ore treatment, 
 
 278. 
 
 Deligny's copper ore treatment, 280. 
 Densities of copper and zinc sulphates 
 
 in solution, 393. 
 Density, current, 36. 
 
 of silver-bath, effect of increasing, 
 201. 
 
 unequal, in solutions, 212, 365. 
 Depierre's copper-bath, 130. 
 Depolarisation of battery, 41. 
 
 by chromic acid, 49. 
 
 by copper sulphate, 43. 
 
 by nitric acid, 46. 
 
 mechanical, 41. 
 Deposition, electro-, early, 4, 5, 8. 
 
 of alloys, 32, 257. 
 
 of aluminium, 254. 
 
 of antimony, 250. 
 
 of bismuth, 254. 
 
 of brass, 257. 
 
 of bronze, 263. 
 
 of cadmium, 245. 
 
 of chromium, 245. 
 
 of cobalt, 227. 
 
 of copper, 124. 
 
 of German-silver, 265. 
 
 of gold, 198. 
 
 of iron, 230. 
 
 of lead, 250. 
 
 of nickel, 216. 
 
 of palladium, 254. 
 
 of platinum, 237. 
 
 of silver, 175. 
 (bright), 185. 
 (non-electrolytic), 177. 
 
 of tin, 245. 
 
 of zinc, 240. 
 
 on electro-positive metals, 85, 345. 
 Deposits, conditions of formation, 28. 
 
 crystalline character of, 138. 
 
 effect of varying currents on, 78. 
 
 non-adhesion of, cause of, 77, 78. 
 
 relation to current-density, 79. 
 
 roughness, cause of, 277. 
 
 ruined by want of cleanliness, 108. 
 
 slimy, on copper-anodes, 132. 
 
 striated, cause of, 89. 
 
 thickness, calculation of, 79. 
 
 Deposits, time required for given, 79. 
 
 uneven, cause of, 89, 277. 
 
 weighing of, in bath, 100. 
 
 weight and thickness of, 390. 
 
 weight of, calculated, 79. 
 
 per B.T. unit, 330. 
 Desmur's nickeling solution, 218. 
 Detector, current-, 82. 
 Diameters, actual, of wire-gauges, 
 
 396, 397. 
 Dilute solutions, effect of (coppering), 
 
 132. 
 Dipping in acid, need for, 112. 
 
 in potash-vat, 111. 
 Direction of current found, 83. 
 in battery, 24, 347. 
 in dynamo-armature, 68. 
 in thermal battery, 60. 
 
 of lines of magnetic force, 68. 
 Dirt, effect of, in gold-bath, 203. 
 Dirty brass-deposit, cause of, 263. 
 Distance between electrodes, 89. 
 Divalent, meaning of term, 17. 
 Divided - circuit, current - distribution 
 
 in, 39. 
 Doctor, use of, in gilding, 208. 
 
 Wagener and Netto's, 104. 
 Dolly for polishing, 118. 
 Double salt solutions, electrolysis of, 
 
 355. 
 Doucet and Lambotte's zinc -extraction, 
 
 287. 
 
 Drainage, system of, 93. 
 Drum for coating small goods, 105, 
 
 125. 
 Dry colouring of gold, 212. 
 
 pile, 3. 
 Drying of coppered goods, 136. 
 
 of nickeled goods, 226, 227. 
 
 of zinced goods, 244. 
 Ductility of deposited copper, 134. 
 Dynamo armature, 70. 
 
 direction of current in, 68. 
 
 classed by magnet-windings, 70. 
 
 driving-power for, 74. 
 
 efficiency of, 330. 
 
 field-magnets of, 70. 
 
 for copper-depositing, 128. 
 
 for copper-refining, 272. 
 
 invention of, 4, 8. 
 
 management of, 73. 
 
 necessity of, for large works, 141. 
 
 position of brushes in, 69. 
 
 position of, in works, 93. 
 
 reversal of current in, 69. 
 
 sparking of, 73. 
 
 theory of, 67. 
 
 EDISON-LALANDE-CELL, 50. 
 Efficiency, maximum, of battery, 56. 
 
412 
 
 INDEX. 
 
 Elastic moulding-composition, 148, 
 
 166. 
 
 moulds, invention of, 8. 
 Elasticity of deposited-copper, 134. 
 Electric and chemical energy, relation 
 
 of, 23, 24. 
 
 conductivity of metals, 31. 
 connection-gripper, 159. 
 current, direction found, 83. 
 resistance, unit of, 34. 
 ' Electricias ' nickeling solution, 218. 
 Electricity, cost of, 77, 333. 
 derivation of term, 2. 
 generated, 24. 
 supply, public, use of, 75. 
 voltaic and static, 25. 
 Electro-chemical equivalents, 390. 
 
 series, 22. 
 -chromy, 255. 
 -deposition, 26. 
 arrangement of vats, 86. 
 current-strength for, 78. 
 early experiments, 3, 5, 8. 
 limit of E.M.F. for, 29. 
 relation of current and time, 79. 
 -etching, 2, 173. 
 -metallurgy, definition of, 1. 
 
 scope of, 2. 
 -motive force, best for depositing, 
 
 78. 
 
 counter-, 29. 
 
 limit of, in depositing, 29. 
 meaning of term, 25. 
 unit of, 34. 
 
 -negative, meaning of term, 22. 
 -ore-extraction, scope for, 267. 
 -plating, definition of, 2. 
 essentials for baths, 86. 
 of positive metals, 85. 
 -positive, meaning of term, 22. 
 -thermal refining of iron and steel, 
 
 scope for, 299. 
 -smelting, 290. 
 of aluminium, 295. 
 of iron, 297. 
 of magnesium, 296. 
 of sodium, 297. 
 Electrodes, altering position of, at 
 
 first, 194. 
 
 distance between, 89. 
 manner of connecting, 97. 
 
 of suspending, 97. 
 meaning of term, 27. 
 Electrolysis, conditions for, 32, 351, 
 
 362. 
 
 meaning of term, 26. 
 of complex acids, 356. 
 of double salts, 355. 
 of ferric solutions, 231. 
 of mixed solutions, 355. 
 
 Electrolysis of silver - potassium 
 
 cyanide, 181. 
 power absorbed in, 329. 
 theories of (modern), 338. 
 with insoluble anodes, 353. 
 Electrolyte, agitation of, 90, 98. 
 
 meaning of term, 28, 341. 
 Electrolytic assay, 325. 
 cleaning, 115. 
 conduction, 32, 342, 358. 
 etching, 173. 
 extraction of antimony, 289. 
 
 of copper, 278. 
 
 of gold, 283. 
 
 of iron, 289. 
 
 of lead, 283. 
 
 of zinc, 286. 
 moulding, 142. 
 preparation of brass-bath, 260. 
 
 of silver-bath, 184. 
 recovery of tin, 290. 
 refining of antimony, 289. 
 
 of copper, 268. 
 
 of gold, 284. 
 
 of iron, 289. 
 
 of lead, 281. 
 
 of silver, 284. 
 
 scope for, 267. 
 stripping of gold, 206. 
 
 of nickel, 223. 
 
 of silver, 190. 
 
 Electro-thermal processes, 294. 
 Electro type -baths, arrangement, 153. 
 -copper for anodes, 169. 
 
 separation from plate, 157. 
 
 thickness for, 157, 164. 
 -plate, backing of, 164. 
 
 final cleansing of, 171. 
 preparation of, 164. 
 
 -supporters, 152. 
 Electrotyping, 142. 
 
 arrangement of vats, 86, 153. 
 art-, anodes in, 169. 
 
 excess current prevented, 170. 
 calculation of current-strength, 154. 
 chases, type, etc., to be used for, 
 
 157. 
 
 coins, medals, etc., 165. 
 definition of term, 2. 
 deposit on wax, 163. 
 
 irregular, 89. 
 earliest experiments in, 5. 
 maximum cur rent- strength for, 133. 
 mechanical finish of plates, 157. 
 moulding-compositions, 143. 
 nature of copper required, 132. 
 printers', 151. 
 regulation of current, 156. 
 separating copper from matrix, 
 
INDEX. 
 
 413 
 
 Eleetrotyping, statuary, 166. 
 
 supporting of plate in bath, 152. 
 
 use of guiding-wires, 170. 
 
 in various printing processes, 172. 
 measuring instruments, 154. 
 
 wood-blocks, 165. 
 Elements, definition of term, 18. 
 
 electro-positive and -negative, 22. 
 
 list of, 19. 
 Elkington's copper ore treatment, 279. 
 
 early patents, 5. 
 
 gilding solution, 199, 200. 
 Elmore's solid-deposited tubes, 138. 
 Eisner's bronzing solution, 264. 
 
 silver-bath (battery), 180. 
 (immersion), 176. 
 
 tinning-bath, 249. 
 
 zincing-bath, 242. 
 Emery-wheels, use of, 122. 
 Emetic, tartar, 370. 
 E.M.F. (see Electro-motive force). 
 Enamelled iron for tanks, 95. 
 Energy, transformations of, 23. 
 Engraved steel plates copied, 151. 
 Epsom salts, 378. 
 Equations, chemical, use of, 17. 
 Equivalent weights, 19. 
 Equivalents, electro - chemical, 79, 
 
 390. 
 
 Etching, electrolytic, 173. 
 Ewers, gilding lips of, 208. 
 Exchange, simple, of metals, 345. 
 Extensibility of deposited copper, 134. 
 
 FAHRENHEIT and Centigrade scales 
 compared, 399. 
 
 thermometer-scale, 35. 
 Faraday's laws of deposition, 4. 
 Faure's accumulator, 74. 
 Fearn's tinning solutions, 249. 
 Ferric and ferrous compounds, 230, 
 376. 
 
 salts, result of electrolysing, 231. 
 File-marks, removal of, 117. 
 Filigree -work, gilding of, 210. 
 Filter-paper, folding of, 52. 
 Fine gold, preparation of, 374. 
 
 silver, preparation of, 384. 
 Finishing, 122. 
 Fizeau's gilding solution, 202. 
 Floating typographic formes, 158. 
 Floors, suitable, 92. 
 Force, electro-motive, 25. 
 
 physical, 14. 
 
 Forces, correlation of, 23. 
 Forge, liquid, Burton's, 315. 
 Forks, supported in silver- vat, 191. 
 Formes, printers', electrotyping of, 
 
 floating of, 158. 
 
 Formes, printers', moulding from, 
 
 159. 
 
 preparation of, 157. 
 Formulae, chemical, 16. 
 French weights and measures, 400, 
 
 401. 
 
 Frosted gold, 209. 
 Fulminating gold, 375. 
 Fumes from batteries injurious, 42, 
 
 48, 92. 
 
 cyanide-bath dangerous, 92. 
 Furnace, electric, Cowles', 292. 
 Girod's, 309. 
 Heroult's, 303. 
 Keller's, 305. 
 Kjellin's, 311. 
 
 Rochling and Rodenhauser's, 313. 
 Siemens', 290. 
 Stassano's, 309. 
 for colouring gold, 213. 
 muffle-, for cleansing, 109. 
 Furnaces, electric, advantages of, 300. 
 
 for iron and steel, types of, 302. 
 Fused alloys, electrolysis of, 32. 
 salts, electrolysis of, 32, 294. 
 Fusible metals, 147. 
 Fusion by electricity, 290. 
 
 GALLONS and litres, intercon version, 
 
 401. 
 Galvanic battery, principles of, 24, 
 
 38. 
 
 Galvani's experiments, 3. 
 Galvanised iron, 27, 241. 
 'Galvanit,'196. 
 Galvanography, 172. 
 Galvanometers, 83. 
 
 astatic, 84. 
 Galvanoscope, 82. 
 Gaultier and Dechaud's ore treatment, 
 
 278. 
 
 Gauze, wire-, plating of, 108. 
 Gelatine, 374. 
 
 hardening of, 148. 
 
 use of, in moulding, 148. 
 German -silver, cobalting of, 229. 
 
 deposition of, 265. 
 
 final cleansing of, 112. 
 
 nickeling of, 227. 
 
 silvering of, 180. 
 
 stripping silver from, 189. 
 Gilding by battery, solutions for, 201. 
 
 by immersion, 198. 
 
 colour of gold in, 204, 205. 
 
 dead-, 209. 
 
 starting of deposit, 210. 
 
 discoloured patches in, 207. 
 
 irregular surfaces, 208. 
 
 management of process, 206. 
 
 of electro-positive metals, 211. 
 
414 
 
 INDEX. 
 
 Gilding of watch-movements, 213. 
 
 of wire, 103. 
 
 parcel, 212. 
 
 quicking prior to, 206. 
 
 stripping of gold before, 206. 
 
 thin, failure of, 209. 
 
 use of ' doctor' in, 208. 
 old baths, 204, 207. 
 
 -vat, 205. 
 
 water-, 200. 
 Gilt plumbago, 150, 162. 
 
 surfaces, ornamentation of, 212. 
 Girod electric refining furnace for iron, 
 
 309. 
 
 Glass, silvering of, 173. 
 Glass- tube insulator for wires, 191. 
 
 -vats, cement for, 94. 
 Glazing of steel, 122. 
 Glossary of substances used, 363. 
 Glue, 373. 
 
 marine, 374. 
 
 use of, in moulding, 144. 
 Glycerin, use of, in iron-bath, 232. 
 Glyphography, 172. 
 Gold and its compounds, 374. 
 
 amalgamation by mercury, 206. 
 
 anode, 204. 
 
 behaviour of, in copper-refining, 270. 
 
 character of deposit, 204, 205. 
 
 coin, 374. 
 
 colour, control of, 204, 205. 
 affected by basis metal, 209. 
 by impurities, 203. 
 
 coloured, 204, 205. 
 
 colouring of (dry), 212. 
 
 -cyanide of potassium, 382. 
 
 dead-, 209. 
 
 deposition of (see Gilding). 
 
 extraction, 283. 
 
 fulminating, 375. 
 
 properties of, 198. 
 
 pure, preparation of, 374. 
 
 recovered in copper-refining, 277. 
 in lead-refining, 282. 
 
 recovery from old baths, 319. 
 
 refining, 284. 
 
 solubility of, in cyanide-bath, 203. 
 
 -solutions, assay of, 327. 
 
 standard, 374. 
 
 stripping of old coat, 206. 
 Gore's antimony-bath, 251. 
 
 brassing-bath, 258. 
 
 experiments with explosive anti- 
 mony, 252. 
 
 gilding-bath (battery), 202. 
 (immersion), 199. 
 
 silvering-bath (battery), 180. 
 (immersion), 176. 
 
 silvering-pastes, 176. 
 
 tinning-bath, 246. 
 
 Graining of surfaces (watch - move- 
 ments), 213. 
 Grains and grammes, interconversion 
 
 of, 401. 
 
 Gramme, value of, 34, 401. 
 Gramme's dynamo, 9. 
 Graphite, 382. 
 Gravity, Daniell's, cell, 45. 
 Grease, removal from objects, 108. 
 
 solubility in cyanides, 207. 
 Greek fire, 381. 
 Greenawalt gold extraction process, 
 
 284. 
 Green -bronzing, 402. 
 
 gold, 204. 
 
 Grippers, electric connection, 159. 
 Grounding in burnishing, 121. 
 Grouping of battery-cells, 54. 
 Grove's cell, 46. 
 
 Guericke's electrical machine, 2. 
 Guiding wires for art-moulds, 170. 
 Gutta-percha, 376. 
 
 action of cyanides upon, 184. 
 
 compositions, moulding, 144. 
 
 moulding by, 143, 144. 
 Gypsum, 381. 
 
 HAANEL, advantages of electric smelt- 
 ing of iron, 300. 
 Hall's aluminium extraction process, 
 
 296. 
 
 Halske (see Siemens). 
 Handling of cleansed objects, 112. 
 Hand-polishing, 118. 
 Hardening of gelatine, 148. 
 Hardness of deposited iron, 235. 
 of deposited nickel, 216. 
 of deposited platinum, 240. 
 Hartmann and Weiss, cobalting by, 
 
 228. 
 
 Hartshorn, spirits of, 385. 
 Haydn's system of copper - refining, 
 
 276. 
 Heat evolved in chemical union, 20. 
 
 unit of, 35. 
 Heating of potash- vat, 110. 
 
 of solutions, 96. 
 Heeren's brassing solution, 258. 
 Hern's tinning solution, 249. 
 Heroult's aluminium extraction pro- 
 cess, 296. 
 electric crucible furnace for iron, 
 
 303. 
 
 smelting furnace for iron, 305. 
 Hess' brassing solution, 258. 
 Hides for polishing, 118. 
 Higgins' bichromate cell, 49. 
 Hippopotamus hide for polishing, 118. 
 Hoe's black-leading machine, 162. 
 toggle-press, 160. 
 
INDEX. 
 
 415 
 
 Hoepfner's copper extraction process, 
 
 280. 
 
 zinc extraction process, 288. 
 Hoho-Lagrange electric welding, 316. 
 Hook for anode suspension, 97. 
 Hooks and eyes, tinning of, 245. 
 Horse-power hour, 331. 
 
 cost of, 333. 
 
 relation to kilowatt, 36, 331. 
 unit, 35, 331. 
 
 Hospitaller's nickeling-bath, 218. 
 Hossauer's coppering-bath, 130, 141. 
 Hot solutions, stopping-off varnish for, 
 
 388. 
 
 vats for, 96. 
 Hiibl (von), experiments on copper 
 
 depositing, 133. 
 solution agitator of, 98. 
 Hydrochloric acid, 365. 
 
 sp. gr. table, 392. 
 Hydrocyanic acid, 365. 
 Hydrogen, 16. 
 
 absorbed by iron-deposit, 235. 
 co-deposit of, effect of, 132, 187. 
 deposition of, in zincing, 243. 
 Hydrometer, Baume's, value of de- 
 grees, 393. 
 
 Twaddell's, value of degrees, 393. 
 Hygienic precautions to be observed, 
 
 92. 
 Hyposulphite silver-bath, 184. 
 
 IMMERSION, antimony deposited by, 
 
 250. 
 
 copper extraction by, 278. 
 coppering by, 124. 
 gilding by, 198. 
 platinising by, 237. . 
 silvering by, 175. 
 tinning by, 247. 
 Imperial fluid measure, 400. 
 
 wire-gauge numbers, value of, 397. 
 Impurities, behaviour in copper-refin- 
 ing, 270. 
 
 effect on gold-bath, 203. 
 in copper anode, effect of, 131. 
 in refined copper, 271. 
 in silver-bath, effect of, 183. 
 Inches and millimetres, interconver- 
 
 sion of, 401. 
 
 Incrustation on anode in brassing, 263. 
 Installation, arrangement of, 92. 
 Institution of Electrical Engineers, 
 rules for copper conductors, 
 336, 398. 
 Instruments, current - measurement 
 
 without, 155. 
 
 Insulation of suspending wires, 190. 
 Insulators, electrical, 30. 
 Intaglios, production of, 142. 
 
 Intensity of current, meaning of term, 
 
 34. 
 Intercon version of amperes per sq. ft. , 
 
 sq. in., and sq. dm., 391. 
 of thermometer scales, 399. 
 of weights and measures, 401. 
 Internal surfaces, gilding of, 208. 
 Iodide silver-bath, 184. 
 Ionic velocities, effect of unequal, 360. 
 lonisation, 342. 
 heat of, 344. 
 Ions, charges on, 341. 
 meaning of term, 27, 341. 
 migration of, 342, 357. 
 
 rate of, 358. 
 
 Iron and its compounds, 376. 
 anodes, 234. 
 behaviour of, in copper-refining, 
 
 270. 
 cast, cleansing of, 135. 
 
 unsuited for anodes, 89. 
 character of deposit, 235. 
 cleansing of, 113. 
 cobalting of, 229. 
 coppering, by immersion, 2, 125. 
 coppering-baths for, 129, 130. 
 deposit, absorption of hydrogen by, 
 
 235. 
 
 preservation from rust, 236. 
 deposition of, 237. 
 electro- thermal production and re- 
 fining of, 297. 
 electrotyping, 236. 
 extraction, 289. 
 
 electro-thermal, energy required 
 
 for, 301. 
 
 facing of copper-plates, 230. 
 galvanised, 241. 
 gilding of, by battery, 211. 
 
 by immersion, 200. 
 nickeling of, 224. 
 refining, 289. 
 rollers, coppering of, 137. 
 silvering of, 176, 180. 
 solutions, 230. 
 
 red precipitate in, 231. 
 stripping of nickel from, 223. 
 of old coat, 236. 
 of silver from, 190. 
 tinning of, 246. 
 vats, use of, 95, 96. 
 Isinglass, 374. 
 
 JACOBI, electrotyping by, 5. 
 Jamieson's rule for current-direction, 
 
 82. 
 Japing's brassing-bath, 258. 
 
 copper-baths, 130. 
 
 zinc-bath, 242. 
 Jewreinoff's platinating-bath, 239. 
 
416 
 
 INDEX. 
 
 Johnson and Morris' brassing-bath, 
 
 258. 
 
 German-silver-bath, 265. 
 Jointing of lead-lined vats, 107. 
 Jordan, electrotyping by, 5. 
 Joule, 36. 
 Jugs, gilding lips of, 208. 
 
 KASALOWSKY'S copper-bath, 130. 
 
 Keith, refining of lead, 282. 
 
 Keller's electric refining furnace for 
 
 iron, 305. 
 
 electric smelting furnace for iron, 
 308. 
 
 Kermes mineral from antimony-bath, 
 252. 
 
 Kick's gilding-bath, 202. 
 
 Kiliani's experiments on zinc-deposi- 
 tion, 243. 
 
 Kilowatt, 36, 333. 
 -hour, 36, 333. 
 
 Kjellin's electric refining furnace for 
 iron, 311. 
 
 Klein's iron-bath, 232, 233. 
 
 Knight's metallisation of moulds, 150. 
 
 Kopp's immersion copper-bath, 125. 
 
 Kuhlo's Daniell-cell, 44. 
 
 Kiihn's silvering- paste, 176. 
 
 LACQUER varnish, 388. 
 Lagrange-Hoho, electric welding by, 
 
 315. 
 Lake Superior copper, treatment of, 
 
 279. 
 
 Lambotte and Doucet's zinc-extrac- 
 tion, 287. 
 
 Lamp-reflectors, silvering of, 173. 
 Lanaux and Roseleur's platinating- 
 
 bath, 239. 
 Langbein's cobalting, 229. 
 
 coppering-bath, 130, 141. 
 
 gilding-bath, 203. 
 
 iron-bath, 234. 
 
 nickeling-bath, 218. 
 
 platinating bath, 239. 
 
 use of rotating cathodes for deposit- 
 ing copper, 139. 
 Lard, 377. 
 
 clarification of, 144. 
 Large surfaces, plating of, 104. 
 Lathe for polishing, 118. 
 Lead and its compounds, 377- 
 
 anode, effect of, in copper-bath, 169. 
 
 behaviour in copper-refining, 270. 
 
 -carbonate used in wax-moulding, 
 145. 
 
 cleansing of, 114. 
 
 coppering-bath for, 130. 
 
 crude, impurities in, 281. 
 
 deposition of, 250. 
 
 Lead extraction, 283. 
 
 gilding of, 211. 
 
 -lined vats, jointing of, 107. 
 
 -peroxide, colours of films, 255. 
 deposit on anode, 250. 
 
 recovery from old baths, 320. 
 
 refined, impurities in, 282. 
 
 refining of, 281. 
 
 sheet-, anodes for statuary, 169. 
 
 solutions, assay of, 327. 
 
 stripping of silver from, 190. 
 
 -tree, 250. 
 
 unsuited for silvering, 188. 
 
 use of, in accumulators, 74. 
 in fusible alloys, 147. 
 
 vats, jointing of, 95, 107. 
 Lead, black-, application to moulds, 
 
 149. 
 
 Leather bobs for polishing, 118. 
 Leaves, nature-prints from, 172. 
 Leclanche-cell, 50. 
 Leeson's elastic moulds, 8. 
 Length, unit of, 35. 
 Lenoir's automatic cut-out, 169. 
 
 statuary-moulding, 167. 
 Lerebour's gilding solution, 202. 
 Lesmonde's platinising cell, 238. 
 Letrange's zinc-extraction, 287. 
 Level's gilding solution, 202. 
 Lichtenberg's fusible alloy, 147. 
 Light, need for, 92. 
 Lime, quick and slaked, 371. 
 
 Sheffield, use in polishing, 119. 
 Lines of force, magnetic, 72. 
 Lipowitz's fusible alloy, 147. 
 Lips of ewers, gilding of, 208. 
 Liquid forge, Burton's, 315. 
 Liquids, conductivity of, 32. 
 Litre, value of the. 35. 
 Litres and gallons, interconversion of, 
 
 401. 
 
 Lobstein's tinning-bath, 249. 
 Local action, meaning and effect of, 
 39, 349. 
 
 thickening of silver-deposits, 195. 
 Lonyet's zinc-bath, 242. 
 Looseness of deposit, causes of, 77. 
 Lubrication in scratch- brushing, 120. 
 Luckow's zinc-extraction, 286. 
 Liidersdorf s tinning-bath, 246. 
 Lunar caustic, 386. 
 
 MAGNESIUM and its compounds, 378. 
 
 electro-smelting of, 296. 
 Magnet, dynamo-, exciting of, 70. 
 
 field-, of dynamo, 70. 
 
 lines of force around, 68. 
 Magnetic iron-deposit, 235. 
 Magneto-electric machines, invention 
 of, 4. 
 
INDEX. 
 
 417 
 
 iMaistrasse's tinning-bath, 249. 
 Manganese, behaviour in copper- 
 refining, 271. 
 Manufactured goods prepared for 
 
 silvering, 188. 
 
 Marchese's copper ore treatment, 280. 
 Marine glue, 374. 
 
 use in moulding, 144. 
 Marks, striated, cause of, on deposits, 
 
 89. 
 
 Materials for moulding, 143. 
 Matter, definition of, 14. 
 Matthiessen's magnesium - smelting, 
 
 296. 
 Measurement of current, 83. 
 
 without instruments, 1 55. 
 units of, 34. 
 
 Measures and weights, 400. 
 Measuring apparatus, electrical need 
 
 for, 77. 
 
 instruments, position in circuit, 154. 
 Medals and medallions, electrotyping 
 
 of, 144, 165. 
 
 Meidinger's Daniell-cell, 44. 
 Melting-points of fusible alloys, 147. 
 Mercury and its compounds, 378. 
 danger to gold, 206. 
 in plate powders, 178. 
 protection of battery-zincs by, 39. 
 quicking by, 115. 
 recovery of, from old zincs, 320. 
 use of, in fusible alloys, 148. 
 Meritens Plating Co.'s silver-baths, 
 
 180. 
 Metal, backing-, for electrotypes, 164. 
 
 Spence's, 387. 
 Metallisation of moulds, 149, 150, 
 
 162. 
 
 Metallo-chromes, 255. 
 Metals and metalloids, 19, 20. 
 best current for depositing, 78. 
 conductance of, 31. 
 electro-chemical series, 22. 
 -positive and -negative, 22. 
 
 coating of, 85. 
 
 fusible, use in moulding, 147- 
 list of, 19. 
 
 precious, recovered in copper-refin- 
 ing, 277. 
 
 recovered in lead-refining, 281. 
 simple exchange of, 345. 
 thermo-electric series of, 60. 
 thermo-electro-motive force of, 61. 
 unlike, effect of hanging, on same 
 
 cathode-rod, 192. 
 Methylated spirit, 367. 
 Metre, value of the, 35. 
 Micro-volts, 60. 
 
 Miest and Bias, copper ore treatment 
 by, 280. 
 
 Migration of ions, 342, 357. 
 rate of, 358. 
 
 varying velocity of, 360. 
 Millimetres and inches, interconver- 
 
 sion of, 401. 
 
 Mil ward's bright-plating solution, 8. 
 Mineral oils, cleansing from, 109. 
 Mixed solutions, electrolysis of, 33, 
 
 355. 
 
 Mixing-drum, 125. 
 Mixture of solution, necessary in 
 
 brassing, 262. 
 
 Modern theories of electrolysis, 338. 
 Moebius silver-refining process, 284. 
 Molecule, definition of term, 15. 
 Monovalent, meaning of term, 17. 
 Mop ('mopping], 122. 
 Morris and Johnson's brassing-bath, 
 
 258. 
 
 German-silver-bath, 265. 
 Motion of bath effected, 98. 
 
 of cathodes, 99. 
 Motor-generator, 76. 
 Moulding-box for wax, 158. 
 by electrolysis, 1 42. 
 -composition, conductive, 146, 149. 
 
 elastic, 148, 166. 
 from coins and medals, 144, 165. 
 natural objects, 171. 
 statuary, 166. 
 steel-plates, 144. 
 type, 145, 157. 
 undercut models, 143. 
 wax-models, 167. 
 wood-blocks, 165. 
 in sections, 167, 168. 
 materials, 143. 
 
 with elastic composition, 148. 
 gutta-percha, 143. 
 
 mixtures, 144. 
 plaster of Paris, 146. 
 sealing-wax, 149. 
 wax, 158. 
 
 -compositions, 145. 
 Moulds, elastic, invention of, 8. 
 guiding-wires in, 170. 
 metallisation of, 149, 150, 162. 
 rendered conductive, 149. 
 wax, electrical connection with, 
 
 162. 
 
 maximum current for, 164. 
 parting of copper from, 164. 
 plumbagoing, 161. 
 trimming and building up, 160. 
 wetting of surface, 163. 
 Mud in copper-refining, composition 
 
 0&271. 
 
 Muffle-fuiifcce for cleansing, 109. 
 Multiple system of copper-refining, 
 
 * 
 
418 
 
 INDEX. 
 
 Munro's tinning solution, 249. 
 Murray's black-leading process, 7. 
 
 NAGEL's.nickeling-bath, 218. 
 Native copper, treatment of, 279. 
 Natural objects, reproduction of, 171. 
 Nature-prints of leaves, 172. 
 Negative plate and pole of battery. 
 
 38. 
 
 Net, wire-, plating of, 104. 
 Netto and Wagoner's ' doctor,' 104. 
 Neutral point, thermo-electric, 62. 
 Newton's fusible alloy, 147. 
 Nickel and its compounds, 379. 
 anodes, 222. 
 -baths, 218. 
 
 cause of alkalinity in, 220. 
 behaviour in copper-refining, 270. 
 burnt, production of, 217. 
 character of metal, 216. 
 -cobalt alloy deposited, 218. 
 -copper-zinc alloy deposited, 265. 
 -deposit, advantages of, 216. 
 cause of peeling, 226. 
 dark coloured, cause of, 225. 
 hardness of, 217. 
 polishing of, 122. 
 thickness of, 225. 
 -plating, uses of, 216. 
 recovery from old baths, 321. 
 solutions, assay of, 327. 
 stripping old coats, 222. 
 Nickeling, arrangement of anodes, 
 
 222, 225. 
 battery for, 220. 
 careful preparation needed, 122, 
 
 224. 
 
 finishing, 227. 
 
 rotatory plating apparatus for, 105. 
 small goods, 105. 
 solutions, 218, 220. 
 suspension of objects for, 105, 225. 
 time required for, 226. 
 -vats, 222. 
 Nickel-silver, gilding of, 211. 
 
 silvering of, 191. 
 Niello work, 197. 
 Nitric acid, 365. 
 
 as depolariser, 46. 
 dip, 112. 
 
 specific gravity tables, 392. 
 Nitrous acid, 112, 366. 
 Non-conductors, electrical, 30. 
 Non-metals and metals, 19, 20. 
 
 OBERNETTER'S iron-bath, 233. 
 Ohm, value of the, 34. 
 Ohm's law, 4, 54. 
 Oil of vitriol, 366. 
 Oil, removal of, 109. 
 
 Ore treatment, Becquerel's, 5. 
 early experiments in, 9. 
 scope for, 268. 
 
 Ores, antimony-, treatment of, 289. 
 copper-, treatment of, 278. 
 zinc-, treatment of, 286. 
 Organic acids, resistance of cobalt to, 
 
 229. 
 
 use of, in nickeling, 221. 
 dirt, destruction of, 109. 
 matter, danger of, to silver-baths, 
 
 184. ' 
 
 effect of, on gold-bath, 203. 
 Ornamentation of gilt surfaces, 212. 
 
 of silver sufaces, 196. 
 Osmotic pressure, 339. 
 Oxalic acid, 366. 
 Oxidised silver, 196. 
 
 PALLADIUM deposit, advantages of, 
 254. 
 
 deposition of, 254. 
 
 properties of, 254. 
 Paracelsus on coppering iron, 2. 
 Paraffin, cleansing from, 109. 
 Parallel arrangement of battery-cells, 
 
 55. 
 
 of plating- vats, 86. 
 of refining- vats, 274. 
 Parcel-gilding, 212. 
 Parchment-paper for porous partition, 
 
 127. 
 
 Paris, plaster of, 381. 
 Parkes' elastic moulding-composition, 
 148. 
 
 metallisation of moulds, 149. 
 
 silvering-bath, 180. 
 
 wax moulding-composition, 146, 149. 
 Patera's treatment of copper-liquors, 
 
 278. 
 
 P.D., 349. 
 
 Peeling of nickel-deposits, cause of, 226. 
 Pens, steel, coppering of, 124. 
 Person and Sire's zinc-bath, 242. 
 Petroleum, cleansing from, 109. 
 Pewter, stripping of silver from, 190. 
 
 unsuited to silvering, 188. 
 Pfanhauser's gilding-bath, 202. 
 
 nickeling- bath, 218. 
 
 silvering-bath, 180. 
 Phosphorus, 380. 
 
 compositions in type-moulding, 162. 
 
 production of, 294. 
 
 solutions, danger of, 171. 
 
 use of, in moulding, 146, 149. 
 Pickles for cleansing, 114. 
 Pile, dry, 3. 
 
 Pinholes in copper-deposit cause of, 
 164. 
 
 in iron-deposit, 234, 
 
INDEX. 
 
 419 
 
 Pins, immersion tinning of, 247. 
 Pints and cubic inches, intercon- 
 
 version of, 401. 
 Plante accumulator, 74. 
 statuary anodes, 168. 
 Plaster of Paris, 381. 
 
 as porous partition, 127. 
 made waterproof, 147. 
 use of, in moulding, 146, 168. 
 Plastic moulding materials, 144. 
 Plate-restoring powders, 178. 
 Plates, anode-, form of, 97. 
 battery-, 38. 
 
 copper-, reproduced, 152. 
 steel-, copied, 161. 
 Platinating, 238, 240. 
 Plating apparatus, rotatory, 105. 
 -balances, 100. 
 bright-, invention of, 8. 
 Platining, 240. 
 Platinising, 237, 240. 
 
 silver, 196. 
 
 Platinum and its compounds, 381. 
 behaviour in copper-refining, 271. 
 characteristics of, 237. 
 deposition of, 237. 
 dipping-baskets, 112. 
 recovery from old baths, 321. 
 resisting power of deposit, 237. 
 scratch-brushing of, 240. 
 -silver alloy deposited, 266. 
 solutions, assay of, 328. 
 stripping of, 240. 
 -wire anodes for statuary, 168. 
 Plumbago, 382. 
 
 application to moulds, 149, 161. 
 increasing conductivity of, 150. 
 water-repelling action of, overcome, 
 
 163. 
 
 Plumbagoing wax moulds, 161. 
 Poisoning of blood by plating-baths, 
 
 103. 
 
 Poisons, antidotes to, 403. 
 Polarisation of battery, 40. 
 
 prevention of, 41. 
 of electrolyte, 29. 
 Poles of battery, 38. 
 Polishing before nickeling, 224. 
 hides for, 118. 
 -lathe, 118. 
 of surfaces, 117. 
 Porosity of deposits, 138. 
 Porous cell of battery, 42. 
 
 preservation of, 51. 
 Positive metals, plating of, 85. 
 plate and pole of battery, 38. 
 Post Office Daniell-cell, 45. 
 Potash alum, 368. 
 cleansing solution, 109, 110. 
 -vat, 110. 
 
 Potassium compounds, 382. 
 
 cyanide, cleansing by, 113. 
 use in silver-baths, 179. 
 Potential difference, 349. 
 Potential, meaning of term, 25. 
 Pott's nickeling-bath, 218. 
 Powder, silver-, obtained, 214. 
 
 plate-restoring, 178. 
 Powdery copper-deposit, cause of, 132. 
 Powell's nickeling solutions, 218. 
 Power, cost of, 333. 
 
 factor, 36, 313. 
 
 loss of, in conductors, 335. 
 
 required for electrolysis, calculation 
 of, 329. 
 
 unit of, 35, 331. 
 
 Press for moulding from type, 159. 
 Pressure, electrolytic solution-, 343. 
 
 of electric current, 25. 
 
 osmotic-, 339. 
 
 solution-, 338. 
 Primary battery, 75. 
 Printers' electrotyping, 151. 
 
 formes, moulding from, 157. 
 Printing-plates, copper, iron-faced, 
 
 230. 
 nickeled copper, life of, 227. 
 
 -surfaces, nickeled, 227. 
 
 various, production of, 172. 
 Projections on deposit, cause of, 90. 
 Proof-spirit, 367. 
 Prussiate of potash, yellow, 383. 
 Prussic acid gas evolved from cyanide- 
 baths, 92. 
 
 Pumice, scouring with, 117. 
 Puscher's immersion copper-bath, 125. 
 Pyrites, burnt Spanish, treatment of, 
 278. 
 
 QUANTITY and intensity of current, 
 
 35. 
 
 Quicking of objects, 115. 
 before gilding, 206. 
 Quicklime, 371. 
 Quicksilver, 378. 
 
 RAG-GILDING, 209. 
 Reaumur thermometer and scale, 35. 
 Red-lead, 385. 
 
 Refining, electro-, scope for, 267. 
 of copper, electrolytic, 268. 
 of iron (electrolytic), 289. 
 
 electro-thermal, 297. 
 of lead, 281. 
 
 of silver, electrolytic, 284. 
 Reflectors, electrolytic manufacture 
 
 of, 173. 
 
 silvering of, 176, 178. 
 Regulation of current in electrotyping, 
 156. 
 
420 
 
 INDEX. 
 
 Regulus, copper-, use as anode, 279. 
 Resistance, effect of varying, on de- 
 posit, 80. 
 
 electrical, of copper wires, 395. 
 relation of, to power required, 
 
 336. 
 
 specific, of copper- sulphate solu- 
 tions, 394. 
 
 of sulphuric acid, 394. 
 unit of measurement, 34. 
 Resistance-coils, 81. 
 Resistances, use of, 81. 
 Rochelle salt, 383. 
 
 Rochling and Rodenhauser's electric 
 refining furnace for iron, 313. 
 Rock salt, 386. 
 
 Rodenhauser and Rochling's electric 
 
 refining furnace for iron, 313. 
 
 Rogers Plating Co.'s silver - baths, 
 
 180. 
 
 Rollers, iron, coppering of, 137. 
 Rooms for plating, arrangement of, 
 
 92. 
 
 Rose's fusible alloy, 147. 
 Roseleur's antimony-bath, 251. 
 brassing-bath, 258, 260. 
 coppering-bath, 129, 130. 
 gilding-bath (battery), 202. 
 
 (immersion), 198, 199, 200. 
 nickeling-bath, 218. 
 plating-balance, 100. 
 platinising-bath, 238. 
 quicking-bath, 116. 
 silvering-bath (battery), 180. 
 
 (immersion), 176. 
 silvering-pastes, 178. 
 tinning-bath (battery), 248. 
 
 (immersion), 246. 
 wire-gilding process, 103. 
 Roseleur and Lanaux's platinating- 
 
 bath, 239. 
 Rosin, 384. 
 
 use of, in moulding, 146, 166. 
 Rotary converter, 76. 
 Rotatory plating apparatus, 105. 
 Rouge, use of, 122. 
 Round's tin-silver-bath, 266. 
 Rue (de la), discovery of electro- 
 typing, 5. 
 
 Rules for handling chemicals, 263. 
 Ruolz (de) bronzing solution, 264. 
 
 gilding- bath, 202. 
 Russell and Woolrich's brassing-bath, 
 
 258. 
 
 cadmium-bath, 245. 
 Rust, preservation of iron-deposit from, 
 
 232. 
 Rust-coloured precipitate in iron vats, 
 
 231. 
 Ryhiner's iron-plating solution, 233. 
 
 SALT, common, 386. 
 
 Salts, fused, electrolysis of, 32. 
 
 Salzede's (De la) brassing-bath, 258. 
 
 bronzing-bath, 264. 
 Sand, scouring with, 117. 
 
 Trent, use of, in polishing, 118. 
 Sartoria's tinning-bath, 249. 
 Satin finish to silver, 197. 
 Sawdust for drying, 136. 
 Schmollnitz waters, coppering iron 
 
 by, 2. 
 
 treatment of, 278. 
 Schneider and Szontagh system of 
 
 circulating electrolyte, 276. 
 Scratch-brushes, 119. 
 -brushing, 119. 
 during deposition, 194, 207. 
 effect of, 122. 
 of platinum, 240. 
 Scratches, unobliterated, 89. 
 Screws, binding-, for batteries, 57. 
 Sealing-wax, use of, in moulding, 149. 
 Secondary actions in electrolysis, 354. 
 
 batteries, 74. 
 Sectional moulding, 167. 
 Seebeck's discovery of thermo-elec- 
 tricity, 4. 
 
 Seignette salt, 383. 
 Selective chemical union, 21. 
 Series arrangement of battery cells, 55. 
 of plating- vats, 86. 
 of refining- vats, 274. 
 electro-chemical, 22. 
 system of copper-refining, 275. 
 thermo-electric, of metals, 60. 
 Shape of object, effect on thickness of 
 
 coat, 89. 
 Sheffield lime, 371. 
 
 used in polishing, 119. 
 'Sherardising,'241. 
 Short-circuiting of current, 39. 
 avoided in art-electrotyping, 170. 
 indicated, 80. 
 Shunt-wires, distribution of current 
 
 in, 39. 
 
 Siemens' electric furnace, 290. 
 Siemens and .Halske's antimony-ex- 
 traction process, 289. 
 copper ore treatment process, 280. 
 gold-extraction process, 283. 
 zinc-extraction process, 288. 
 Signets, moulding from, 149. 
 Silver and its compounds, 384. 
 anodes, 186. 
 
 appearance during electrolysis, 
 
 182. 
 
 antique, 197. 
 -bath, bright, use of, 194. 
 controlled by anode appearance, 
 182. 
 
INDEX. 
 
 421 
 
 Silver-bath cyanide, 181. 
 
 effect of age on, 183. 
 
 electrolytic preparation of, 184. 
 
 iodide, 184. 
 
 limits of density for, 183. 
 
 suspension of objects in, 191. 
 
 thiosulphate, 184. 
 behaviour of, in copper-refining, 
 
 271. 
 bright, deposit, 8, 185. 
 
 presence of sulphur in, 186. 
 coin, standard, 186, 384. 
 -cyanide of potassium, 382. 
 cyanide, preparation of, 179, 385. 
 
 use of, in silver-bath, 179. 
 dead lustre produced, 197. 
 -deposit, character of, 187. 
 
 final treatment of, 194. 
 
 non-adhesive, cause of, 182. 
 
 thickening of, locally, 195. 
 
 thickness of, 187, 196. 
 deposition of, by battery, 179. 
 by immersion, 175. 
 from pastes, 175, 177. 
 on glass, 173. 
 
 electrode arrangement, 192. 
 
 non-electrolytic, 177. 
 effect of, on colour of gold, 203. 
 electro-refining of, 284. 
 electrotype, use of, 151. 
 electrotyping with, 196. 
 German-, deposition of, 265. 
 
 final cleansing of, 112. 
 gilding of soft - soldered goods, 
 
 211. 
 
 gilt, dead lustre on, 209. 
 niello- work, 197. 
 oxidised, 197. 
 plate, polishing of, 121. 
 platinised by immersion, 237. 
 -platinum alloy deposited, 266. 
 -potassium cyanide, electrolysis of, 
 
 181. 
 
 -powder obtained, 214. 
 pure, preparation of, 384. 
 recovered in copper-refining, 277. 
 
 in lead -refining, 281. 
 
 from old baths, 322. 
 refining of, 284. 
 solutions, assay of, 328. 
 
 for separate current, 180. 
 
 preparation of, 179. 
 spongy, prepared, 214. 
 standard, 186, 384. 
 striking-bath, use of, 193. 
 stripping of old coat, 188. 
 sub-cyanide dissolved, 194. 
 -surfaces, ornamentation of, 196. 
 -tin alloy deposited, 266. 
 Silvered plumbago used, 150, 162. 
 
 Silvering of glass, 173. 
 Silvering- vat, 186. 
 Single-cell deposition of copper, 126. 
 extraction of copper, 278. 
 gilding by, 201. 
 platinising by, 238. 
 silvering by, 178. 
 tinning by, 247. 
 -fluid battery, 41. 
 Sire and Person's zinc-bath, 242. 
 Skeleton wire statuary anodes, 1 68. 
 Slaked lime, 371. 
 Slate vats, 94. 
 Slime on copper anode, 132. 
 rusty, in iron-baths, 231. 
 Slimes, copper-refining, composition of, 
 
 271. 
 Slinging of objects in silver- vat, 
 
 191. 
 
 Small objects, coating of, 105, 135. 
 Smee's battery cell, 41. 
 
 book on electro-metallurgy, 8. 
 platinising, 238. 
 Smelting, electro-, 290. 
 of aluminium, 295. 
 of iron, 297. 
 of magnesium, 296. 
 of sodium, 297. 
 Smith's system of copper - refining, 
 
 276. 
 Smith and Deakin's rotatory plating 
 
 apparatus, 105. 
 Snowdon, nickeling, 223. 
 Soda alum, 368. 
 ash, 386. 
 
 cleansing- vat, 110. 
 Sodium compounds, 386. 
 
 smelting, 297. 
 
 Softening of deposited iron, 235. 
 Soldering of lead-vats, 95. 
 Solder-lines, gilding of, 211. 
 Solution of anode in copper-refining, 
 
 269. 
 
 pressure, 338. 
 electrolytic, 343. 
 various cases of, 346. 
 Solutions, agitation of, effected, 98. 
 
 necessary, 90. 
 circulation of, 90, 98, 276. 
 copper and zinc sulphates, sp. gr., 
 
 393. 
 
 depositing-, assay of, 324. 
 heating of, 96. 
 
 homogeneity of, need for, 90. 
 potash, vat for, 110. 
 quicking-, 115. 
 recovery of metals from, 318. 
 Sorrel, salt of, 383. 
 
 Sparking of dynamo - commutator, 
 73. 
 
422 
 
 INDEX. 
 
 Specific gravity of silver-baths, 183. 
 tables for acids, 392. 
 
 for copper and zinc sulphate 
 
 solutions, 393. 
 
 resistance of copper sulphate solu- 
 tion, 394. 
 
 of sulphuric acid, 394. 
 Spence's metal, 387. 
 Spencer, electrotyping by, 6. 
 Spermaceti, use in moulding, 146. 
 Spirits of hartshorn, 368. 
 
 of wine, 367. 
 
 Sponginess of anode in refining, 270. 
 Spongy copper-deposit, cause of, 156. 
 
 silver prepared, 214. 
 Spoons supported in vat, 191. 
 
 thickness on bowls increased, 195. 
 Spots on silver-deposit, 193. 
 Sprague's platinating-bath. 239. 
 Sq. ins. and dms. , intercon version of, 
 
 401. 
 Stalmann's system of copper-refining, 
 
 276. 
 
 Stannate of soda, 387. 
 Stannous and stannic compounds, 
 
 387. 
 
 Star antimony, 369. 
 Stassano's electric refining furnace for 
 
 iron, 309. 
 
 Statuary, moulding from, 148, 166. 
 Steam-heated plating-vat, 96. 
 
 potash -vat, 110. 
 
 Stearine, use of, in moulding, 146. 
 Steel, 376. 
 
 burnishers, 121. 
 cleansing of, 113. 
 coppering of, 151. 
 electro-thermal production, 297. 
 electro - thermal energy required 
 
 for, 301. 
 
 -facing of copper-plate, 230, 235. 
 gilding of, 199, 200, 211. 
 hardened, zincing of, 241. 
 nickeling of, 224. 
 pens, coppering of, 124. 
 -plates, copying of, 151. 
 moulding from, 144. 
 preservation of, 151. 
 polishing of, 122. 
 silvering of, 180. 
 stripping nickel from, 223. 
 Steele's gilding-bath, 201. 
 
 tinning-bath, 249. 
 Stein's silver-paste, 176. 
 Stereotyping, 151. 
 Stibnite, 369. 
 Stirring of bath effected, 98. 
 
 need for, 90. 
 
 Stopping-out varnish, 388. 
 Straightening of electrotypes, 164. 
 
 I Striation marks on deposits, 89. 
 Striking of nickel, 225. 
 Striking-bath for silver, 193. 
 Stripping of old gold coats, 206. 
 of iron coats, 236. 
 of nickel coats, 222. 
 of platinum coats, 240. 
 of silver coats, 188. 
 Stylography, 172. 
 Sub-cyanide of silver in silver-deposit, 
 
 187. 
 
 Sublimate, corrosive, 379. 
 Suet, use of, in moulding, 146. 
 Sugar, use of, in moulding, 148. 
 
 of lead, 377. 
 Sulphide of ammonium, use of, in 
 
 electrotyping, 168. 
 Sulphides as anodes, 279, 281. 
 Sulphur, 387. 
 
 chloride for bright- plating, 185. 
 presence in bright-silver deposits, 
 
 186. 
 Sulphuric acid, 366. 
 
 specific gravity tables, 392. 
 specific resistance of, 394. 
 Supply, public electricity, use of, 75. 
 Surface amperage, units intercon- 
 
 verted, 391. 
 Suspension of electrodes, 98. 
 
 of objects in depositing-vats, 105, 
 
 135, 191. 
 
 of objects in potash- vat, 111. 
 Swan's-down used in dollying, 122. 
 Swinburne and Ashcroft's zinc-extrac- 
 tion process, 288. 
 Switch-board for battery, 57. 
 Symbols, chemical, 16. 
 
 of elements, 19. 
 
 Szontagh and Schneider's system of 
 circulating electrolyte, 276. 
 
 TABLE-SALT, 386. 
 
 Tallow, clarification of, 144. 
 
 used in moulding, 166. 
 Tanks for solutions, 94. 
 Tannic acid, 366. 
 Tarnish to be removed from objects, 
 
 108. . 
 Tartar, cream of, 383. 
 
 emetic, 370. 
 Tartaric acid, 366. 
 Telegraph-wires, coppering of, 1 40. 
 Temperature, effect of, on resistance 
 of baths, 394. 
 
 unit of measurement, 35. 
 Tenacity of deposited copper, 134, 
 
 138. 
 
 Tetravalent, meaning of term, 17. 
 Theories of electrolysis, modern, 338. 
 Thermal (electro-) processes, 294. 
 
INDEX. 
 
 423 
 
 Thermo-electric battery, 59. 
 diamond's, 64. 
 direction of current in, 60. 
 Giilcher's, 66. 
 invention of, 4. 
 reversal of current in, 62. 
 wastefulness of, 66. 
 neutral-point, 62. 
 series of metals, 60. 
 Thermo-electro-motive force of metals, 
 
 61. 
 Thermometer - scales, intercon version 
 
 of, 399. 
 
 Thermopile (see Thermo-electric bat- 
 tery). 
 
 Thick copper-deposits on iron, 137. 
 iron-deposits, 236. 
 tin-deposits, 249. 
 Thickening of deposits at bottom, 90. 
 
 of silver coat locally, 195. 
 Thickness of deposit, calculation of, 
 
 79. 
 
 of film determined, 156. 
 of metal deposited by any known 
 
 current, 390. 
 for copper- deposit, 136. 
 for copper electrotype, 157. 
 for copper in statuary, 168. 
 for gold, 209. 
 for nickel, 225. 
 for silver, 187, 195. 
 unequal, on long articles, 194. 
 Thiosulphate silver-bath, 184. 
 Thompson's cobalting-bath, 228. 
 
 switch -board, 58. . 
 Thomson's electric annealing process, 
 
 317. 
 
 electric welding process, 315. 
 Thorn's platinating-bath, 240. 
 Tin and its compounds, 387. 
 anodes, 248. 
 
 behaviour in copper-refining, 270. 
 cleansing of, 114. 
 -copper alloys, deposition of, 263. 
 coppering-bath for, 130. 
 -foil, grades of, 248. 
 nickeling of, 218. 
 -plate, 248. 
 platinising of, 238. 
 -powder as conductor, 150, 162. 
 recovery from tin scrap, 290. 
 -salt, 388. 
 
 -silver alloy, deposition of, 266. 
 stripping of silver from, 190. 
 unsuited for silvering, 188. 
 use in fusible alloys, 147. 
 Tinning solutions for battery, 249. 
 for immersion, 246. 
 for single-cell process, 246, 247. 
 Toggle-press, 160. 
 
 Tools for burnishing, 122. 
 Treacle, use of, in moulding, 148. 
 Trent sand for polishing, 118. 
 Tripoli-powder, use of, 121. 
 Trivalent, meaning of term, 17. 
 Troy-weight, 400. 
 Tubes, formation of (copper), 137. 
 Turpentine, Venice, use of, 145. 
 Tuttle, refining silver, 286. 
 Twaddell's hydrometer, value of 
 
 degrees, 393. 
 
 Two-fluid battery, 41, 349. 
 Type, bookbinders', brassing of, 263. 
 
 cleansing and preparation of, 158. 
 
 moulding from, 157. 
 
 to be used for electrotyping, 157. 
 Typographical matter, electrotyping 
 of, 157. 
 
 UNDERCUT moulding, 143, 148. 
 Uneven deposits, cause of, 90. 
 Units of measurement, 34. 
 Urquhart's electrotyping of medals, 
 
 165. 
 wax-moulding, 146. 
 
 VALENCY, meaning of term, 17. 
 Varnish, acid-resisting, 214. 
 
 for interior of vats, 95. 
 
 lacquer, 388. 
 
 non- conductive, 152, 388. 
 use of, 6, 136. 
 
 stopping-off, 152, 214, 388. 
 Varrentrapp's iron-bath, 233. 
 Vat for gilding, 105. 
 
 iron-depositing, 234. 
 
 nickeling, 222. 
 
 potash solutions, 110. 
 
 silvering, 186. 
 
 Vats, arrangement of, for copper- 
 refining, 274. 
 for plating, 85. 
 
 description of, 94. 
 
 lead-lined, jointing of, 107. 
 
 supporting objects in, 97. 
 Vegetable forms reproduced, 171. 
 Ventilation, need for, 92, 94. 
 
 system of, 94. 
 
 Vermilion pigments, nickeled sur- 
 faces for printing with, 227. 
 Vitriol, blue-, 373. 
 
 green-, 376. 
 
 oil of, 366. 
 
 white-, 389. 
 Volkmer's brassing-bath, 258. 
 
 iron-bath, 233. 
 
 nickeling-bath, 218. 
 
 silvering-bath, 180. 
 
 wax moulding-composition, 146. 
 Volt, value of the, 34. 
 
424 
 
 INDEX. 
 
 Volta's experiments, 3. 
 Voltage, best, for metal - deposition, 
 78. 
 
 reduction of, 76. 
 Voltaic cell, principles of, 24, 38. 
 
 pile, 3. 
 Voltmeter, 84. 
 
 advantages of, 78. 
 
 position of, in circuit, 154. 
 
 WAGENEE, and Netto's ' doctor,' 
 
 104. 
 
 Wagner's gilding-bath, 202. 
 Wahl's gilding-bath, 199. 
 platinating-baths, 239. 
 silvering-baths (battery), 180. 
 
 (immersion), 176. 
 tinning-bath, 246. 
 Walenn's iron-bath, 233. 
 Walker's wax moulding-composition, 
 
 146. 
 
 Walrus-hide for polishing, 118. 
 Wash-waters, recovery of copper from, 
 
 136. 
 
 Watch-movements, gilding of, 213. 
 Water, composition of, 16. 
 -gilding, 200. 
 power, cost of, 334. 
 supply, good, need for, 93. 
 to be used in operations, 85. 
 waste, removal of, 93. 
 Watt, value of the, 35. 
 Watt's brassing-bath, 258. 
 cobalting-bath, 228. 
 copper-baths, 130. 
 German-silver-bath, 265. 
 gilding-baths, 202. 
 nickeling-baths, 218. 
 platinating-bath, 239. 
 silvering-baths, 176, 180. 
 silvering-paste, 176. 
 wax moulding-composition, 146. 
 zincing solution, 242. 
 Wax, bees'-, 370. 
 compositions, use of, 145. 
 
 in elastic moulding, 166. 
 cracking of, on cooling, 145. 
 melting of, 159. 
 models, moulding from, 167. 
 moulding with, 146. 
 
 with pressure, 159. 
 moulds, copper deposited on, 163. 
 electrical contact with, 162. 
 maximum current-strength for, 
 
 164. 
 
 parting of copper from, 164. 
 plumbagoing of, 161. 
 trimming and building up, 160. 
 wetting of surface, 163. 
 sealing-, moulding with, 149. 
 
 Weak currents, effect in coppering, 
 
 133. 
 effect in electrotyping, 156. 
 
 solutions, effect in coppering, 132. 
 Weighing deposit in bath, 100. 
 
 correction for, 102. 
 Weight, atomic, definition of, 16. 
 
 of deposit, calculation of, 79. 
 
 unit of, 34. 
 
 Weights and measures, 400. 
 intercon version of, 401. 
 
 atomic, of elements, 19. 
 
 equivalent, 19, 20. 
 Weil's bronzing-bath, 264. 
 
 copper-baths, 126, 130. 
 Weiss and Hartmann's cobalt-bath, 
 
 228. 
 Weiss' brassing-bath, 258. 
 
 bronzing-bath, 264. 
 
 copper-bath, 130. 
 
 gilding-bath, 202. 
 
 iron-bath, 233. 
 
 nickel-baths, 218. 
 
 silvering-baths, 180. 
 
 tinning-bath, 249. 
 
 wax moulding-composition, 146. 
 
 zinc-bath, 242. 
 Welding, electric, 315. 
 Weston's nickeling solutions, 218, 221. 
 White-lead, use in wax moulding, 145. 
 Whitening of small goods, 175. 
 Whiting, 371. 
 
 cleansing by, 111. 
 Wilde's first dynamo, 9. 
 
 coppering of iron rollers, 137. 
 Wire anode for statuary, 168. 
 
 -gauges, diameters of, 396, 397. 
 
 -gauze, plating of, 104. 
 
 -gilding process, 103. 
 
 marks, to avoid, 135. 
 
 scratch-brushes, 119. 
 Wires, conducting, loss of power in, 335. 
 maximum current for, 335. 
 
 copper, resistances of, 395. 
 
 guiding-, for art moulds, 170. 
 
 telegraph, coppering of, 140. 
 Wiring of goods for nickeling, 225. 
 
 for silvering, 191. 
 
 Wohlwill's gold-refining process, 284. 
 Wolfe and Pioche's ore-treatment, 9. 
 Wollaston's coppering of silver, 3. 
 Wood-blocks, electrotyping of, 158, 
 
 165. 
 Wood's brassing-bath, 258. 
 
 fusible alloy, 147. 
 
 gilding-bath, 202. 
 Wooden vats, use of, 95. 
 Woolrich and Russell's brassing-bath, 
 
 258. 
 cadmium-bath, 245. 
 
INDEX. 
 
 425 
 
 Wright's cyanide-bath, 8. 
 Wrought iron, 376. 
 
 YELLOW stain on scratch - brushed 
 platinum, 240. 
 
 ZERENER'S electric welding process, 
 
 316. 
 
 Zinc and its compounds, 388. 
 anodes, 243. 
 
 used in brassing, 261. 
 battery-, amalgamation of, 39. 
 costliness of, 37, 333. 
 mercury from, 51. 
 behaviour of, in copper-refining, 270. 
 characteristics of, 241. 
 cleansing of, 114. 
 -copper alloys, deposition of, 257. 
 -copper-nickel alloy deposited, 265. 
 coppering baths for, 130. 
 
 Zinc, dead-gilding of, 211. 
 
 deposition of, 241. 
 
 effect of current-density, 243. 
 
 dust, use of, in zinc-bath, 243, 244. 
 
 extraction of, 286. 
 
 gilding of, by battery, 211. 
 by immersion, 199. 
 
 local action on, 39. 
 
 nickeling of, 218, 224, 227. 
 
 ores, electro-magnetic and electro- 
 static treatment of, 288. 
 
 quicking of, 116. 
 
 silvering of, 191. 
 
 solutions for depositing, 242. 
 
 spongy deposits, cause of, 244. 
 
 stripping of silver from, 190. 
 
 sulphate solutions, sp. gr. of, 393. 
 
 tinning of, 246. 
 
 Zinin's silvering-bath, 180, 184. 
 Zosimus on the coppering of iron, 2. 
 
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 8vo, with numerous Illustrations and Micro- Photographic 
 
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 BY 
 
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 Late Chemist and Assayer of the Royal Mint, and Professor of Metallurgy 
 in the Royal College of Science. 
 
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 GENERAL CONTENTS. The Relation of Metallurgy to Chemistry. Physical Properties 
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 to Furnaces. Thermo- Chemistry. Typical Metallurgical Processes. The Micro-Structure 
 of Metals and Alloys. Economic Considerations. 
 
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 Chemist and Assayer of the Royal Mint. 
 
 
 GENERAL CONTENTS. The Properties of Gold and its Alloys. Chemistry of the- 
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METALLURGICAL WORKS. 49 
 
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 .A. m, m. o -y s 
 
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 BY EDWARD F. LAW, A.R.S.M. 
 
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METALLURGICAL WORKS. 51 
 
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 THE ART OF THE GOLDSMITH AND JEWELLER. 
 
 A Manual on the Manipulation of Gold and the 
 Manufacture of Personal Ornaments. 
 
 BY THOS. B. WIGLEY, 
 
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 I 1ST TWO V O L. U IVI E S. 
 
 A TR E AT!S E ON 
 
 BY SIR BOVERTON REDWOOD, 
 
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 CONTENTS. SECTION I. : Historical Account of the Petroleum Industry. SECTION II. : 
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