REESE LIBRARY 
 
 OF THK 
 
 UNIVERSITY OF CALIFORNIA. 
 
 Deceived 
 Accession No. $ 
 
 _n_n_n, 
 
SCIENTIFIC CHEMISTRY 
 
 IN OUR OWN TIMES 
 
A SHORT HISTORY 
 
 OF THE PROGRESS OF 
 
 SCIENTIFIC CHEMISTRY 
 
 IN OUR OWN TIMES 
 
 BY 
 
 WILLIAM, A. TILDEN, 
 
 D.Sc. LOND., D.Sc. DUB., F.R.S. 
 
 FELLOW OF THE UNIVERSITY OF LONDON 
 
 PROFESSOR OF CHEMISTRY IN THE ROYAL COLLEGE OF 
 
 SCIENCE, LONDON 
 
 LONGMANS, GREEN, AND CO. 
 
 39 PATERNOSTER ROW, LONDON 
 
 NEW YORK AND BOMBAY 
 
 1899 
 
 All rights reserved 
 
7 I* 
 
 
PKEFACE 
 
 IT fell to iny lot, in the spring of last year, to be 
 called upon to deliver one of the courses of " Lec- 
 tures to Working Men " which have been given by 
 the Professors of the Royal School of Mines and the 
 Royal College of Science annually for the last five- 
 and-thirty years. The celebration of the sixtieth 
 year of the Queen's reign had taken place a few 
 months previously, and it occurred to me that it 
 would be appropriate to the occasion to attempt a 
 survey of the progress made in the science and 
 practice of chemistry during the preceding sixty 
 years. The difficulty of the task lay chiefly in 
 making such a selection from the immense range 
 of material which at once presented itself to the 
 mind, as to give to the audience a tolerably clear 
 view of those discoveries which may be regarded as 
 fundamentally important, without creating confusion 
 by the introduction of too much detail. 
 
 It is obvious that, in the time at my disposal in 
 six lectures, it was not possible to do' more than 
 sketch in very broad outline the general features of 
 
vi PKEFACE 
 
 each subject, and the extent of progress which has 
 been made in each direction. I was desirous of 
 assisting my audience, most of whom, I believe I 
 was justified in assuming, possessed at least an ele- 
 mentary knowledge of physical science, by referring 
 them to some book by the aid of which they could 
 verify and amplify such notes as they had been able 
 to take of the lectures. But I could find no single 
 book of moderate size which afforded a historical 
 survey of the succession of events which has led up 
 to the system of theory in chemistry accepted at the 
 present day. 
 
 In the following pages I have endeavoured to 
 provide for the student such information as will 
 enable him to understand clearly how the system 
 of chemistry as it now is arose out of the previous 
 order of things ; and for the general reader, who is 
 not a systematic student, but who possesses a slight 
 acquaintance with the elementary facts of the sub- 
 ject, a survey of the progress of chemistry as a 
 branch of science during the period covered by the 
 lives of those chemists, a few of whom only remain 
 among us, who were young when Queen Victoria 
 came to the throne. 
 
 If justification in a more strictly chemical sense 
 were required for beginning such a story with the 
 year 1837 or thereabout, it would be provided by 
 recalling the fact that at this time the influence of 
 
PKEFACE Vll 
 
 Liebig's teaching was beginning to be felt. The new 
 organic chemistry, and the system of practical in- 
 struction in research inaugurated by Liebig, were 
 attracting students from every part of Europe. 
 
 A large proportion of the progress in discovery, 
 which has gone forward with increasing rapidity since 
 that time, may be attributed to the spirit they there 
 and then imbibed, and which continued to animate 
 so many of them when afterwards they were called 
 as teachers to other schools. 
 
 A retrospect over so long a period as sixty years 
 necessarily includes the consideration of the claims 
 of many workers to a place among the founders of 
 our science. It is not always possible, however, 
 to determine exactly how new ideas originated, or to 
 assign to every contributor to their establishment or 
 overthrow his just share of credit. With reference 
 to this matter, all I can say is that I have used my 
 best judgment upon the recorded evidence, and I 
 have endeavoured in all cases to be completely 
 impartial. My own recollections carry me back 
 over more than half the period referred to, and 
 I am therefore in a position to speak with direct 
 knowledge of some of the subjects concerning 
 which, in times to come, uncertainty would be likely 
 to arise. 
 
 Finally, I desire to point out that this does not 
 profess to be a text-book giving a complete picture 
 
viii PREFACE 
 
 of the state of knowledge and of theory at the 
 moment. Its object, as already stated, is to show 
 by what principal roads we have arrived at the 
 present position, in regard to questions of general 
 and fundamental importance. 
 
 KOYAL COLLEGE OF SCIENCE, 
 LONDON, 1899. 
 
CONTENTS 
 
 PAGE 
 PREFACE v 
 
 INTRODUCTION . 1 
 
 CHAPTER I 
 
 MATTER AND ENERGY , . . . . . . 17 
 
 CHAPTER II 
 
 THE CHEMICAL ELEMENTS : THEIR DISTRIBUTION IN 
 
 NATURE, AND RECOGNITION BY THE CHEMIST . . 37 
 
 CHAPTER III 
 
 RECTIFICATION AND STANDARDISATION OF ATOMIC WEIGHTS 60 
 
 CHAPTER IV 
 
 NUMERICAL RELATIONS AMONG THE ATOMIC WEIGHTS : 
 
 CLASSIFICATION OF THE ELEMENTS .... 83 
 
 CHAPTER V 
 
 ORIGIN AND DEVELOPMENT OF THE IDEAS OF VALENCY 
 
 AND THE LINKING OF ATOMS 108 
 
CONTENTS 
 
 PAGE 
 
 CHAPTER VI 
 
 THE DEVELOPMENT OF SYNTHETICAL CHEMISTRY 144 
 
 CHAPTER VII 
 
 THE ORIGIN OF STEREO-CHEMISTRY CONSTITUTIONAL FOR- 
 MULA IN SPACE 181 
 
 CHAPTER VIII 
 
 ELECTRICITY AND CHEMICAL AFFINITY .... 209 
 
 CHAPTER IX 
 
 DISCOVERIES RELATING TO THE LIQUEFACTION OF GASES . 238 
 
 CHAPTER X 
 
 SUMMARY AND CONCLUSION 258 
 
 INDEX 271 
 
A SHORT HISTORY OF THE 
 
 PROGRESS OF SCIENTIFIC CHEMISTRY 
 
 INTRODUCTION 
 
 WHEN the French revolutionaries in 1792 determined 
 that the dead past might bury its dead, that for them 
 the world should begin over again with a new calendar 
 and the year 1, a new era was indeed about to com- 
 mence, though in a fashion and with results which 
 they little dreamed of. Lavoisier's head fell to the 
 blade of the guillotine, but there remained active 
 minds among the chemists in France and England, 
 and the time was near at hand when the conse- 
 quences of their discoveries would prove to be more 
 momentous than even that great convulsion which 
 changed so much in France and other countries of 
 Europe. Looking back a century, it is easy to see 
 the general features of a social state wholly different 
 from that which prevails in all civilised countries at 
 the present day. And it would be no exaggeration 
 to say that the greater part of the changes, advan- 
 tageous to rich and poor alike, which have accrued 
 in the course of this hundred years, arose directly or 
 
 A 
 
2 A SHORT HISTORY OF THE 
 
 indirectly out of the discoveries in physical science, 
 the full tide of which set in with the opening of the 
 nineteenth century. Even the progress in biological 
 knowledge, and the study of the relations of man to 
 his surroundings, may fairly be counted among the 
 consequences of the intellectual activity promoted 
 and stimulated by the successful physical and chemi- 
 cal investigations of this period. Some testimony as 
 to the recognition in our own time of the idea that 
 the world in relation to science was then passing from 
 the old to a new order of things, is to be found in the 
 fact that the Royal Society, the oldest of the scientific 
 societies in Great Britain, and one of the oldest in the 
 world, commences its great " Catalogue of Scientific 
 Papers " with the year 1800. Previously to this time, 
 the only branch of science, apart from pure mathe- 
 matics, which may be said to have made substantial 
 progress was astronomy ; and even in that case, while 
 the mechanism of the heavens was pretty well known, 
 observers had yet to wait half a century longer for 
 that wonderful instrument the spectroscope, which 
 would inform them as to the composition as well as 
 the movements of the heavenly bodies. In mechanics, 
 again, much was known as to the theory of the 
 mechanical powers the theory of latent heat and its 
 application to the steam-engine, the pressure of the 
 atmosphere and the use of the barometer but the 
 properties of gases in relation to temperature, and 
 the phenomena of gaseous diffusion, were yet to be 
 discovered, and the doctrine of the conservation of 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 3 
 
 energy, one of the really great discoveries of our age, 
 had yet to be recognised and established. 
 
 Electricity was in its cradle in 1800, and as to 
 chemistry, the composition of air and water, and the 
 general characters of acids, bases, salts, and the com- 
 position of a few minerals, were matters of knowledge 
 so recently acquired, that they had hardly become 
 familiar enough to be fully realised. The doctrine of 
 phlogiston was, of course, by this time defunct, and 
 Davy, Dalton, Gay-Lussac, Berthollet, and Berzelius 
 were hard at work in their several countries erecting 
 the system of chemical theory which lasted almost 
 down to our own time, and of which the essential 
 features have subsequently been developed, not 
 abandoned. 
 
 The object and chief business of scientific chemistry 
 has always been to find out what things are made 
 of, to study their properties, and to discover the 
 relation of composition to properties. But scientific 
 chemistry did not begin till the middle of the 
 seventeenth century, and its founder was an Eng- 
 lishman, Robert Boyle. Previously, what was called 
 chemistry, or alchemy, was mainly a confused mass 
 of observation largely erroneous, and of hypothesis 
 mainly groundless. Two objects had been set 
 before the inquirer, one the production of gold 
 from base metal, the other the production of a medi- 
 cine to cure all disease, including old age. Those 
 days are now long gone by, and all the substantial 
 practical advances and inventions of modern times 
 
4 A SHORT HISTORY OF THE 
 
 may be said to have arisen out of the adoption of a 
 new principle, the pursuit of knowledge for its own 
 sake. The cultivation of exact observation of nature, 
 the practice of experiment (which is the substitution 
 of prearranged for natural conditions), and the use 
 of common sense in the construction of theories, these 
 are the steps which have led to real progress, to a 
 better knowledge by man of his relation to the uni- 
 verse in which he is placed, and the adaptation of the 
 forces of nature to his needs and desires. 
 
 The study of chemistry leads us at once to a 
 classification of the materials of which the world, 
 with its atmosphere, ocean, and inhabitants, animal 
 and vegetable, are composed. We recognise, first, 
 two great divisions in these materials, viz. simple or 
 single stuffs, and compound. From the former we 
 can extract only one sort of substance, and this we 
 call an element. From iron, for example, we can get 
 only iron, while iron rust is a compound, because we 
 can get from it both iron and oxygen. 
 
 Observation of this kind, however, to be complete, 
 must be not only qualitative but quantitative. We 
 must know whether the compound, recognised by a 
 certain association of qualities which distinguishes it 
 from all other kinds of matter, is invariably composed 
 of the same materials united in the same proportion. 
 This has been done for us in a very large number of 
 cases, and the proposition that a given compound is 
 definite in its nature, and always contains the same 
 ingredients combined in the same proportions, is a 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 5 
 
 recognised principle which, though many times 
 threatened, survives triumphant. To take an example, 
 water is a common substance obtainable from many 
 natural sources, and exhibiting variations of colour, 
 density, and so forth. But these variations are only 
 apparent, and are really due to the presence of other 
 things mixed with it. It is most pure as it falls from 
 the sky, but even then it brings with it gases from 
 the air. When it falls upon the ground it meets 
 with salts which dissolve, and which, when accumu- 
 lated in quantity as in sea-water, give it a taste ; or it 
 may dissolve vegetable remains and become green, 
 brown, yellow, according to circumstances. Pure 
 water can also be formed by the chemist by uniting 
 together hydrogen and oxygen gases, as first shown 
 by Cavendish in 1784, and in other ways. But 
 when separated from everything else not an easy 
 matter, but still possible we know water as a nearly 
 colourless 1 liquid, which crystallises into a solid at 
 C. ( = 32 F.), and boils at 100 C. ( = 212 F.), 
 under ordinary atmospheric pressure. It is always 
 composed of eight 2 parts of oxygen to one part of 
 hydrogen by weight, or one part of oxygen to two 
 parts of hydrogen by measure, and this admits of no 
 variation whatever. 
 
 It follows from all this that the idea of the inde- 
 structibility of matter is taken as an axiom. The 
 mass of a compound is the sum of the masses of 
 
 1 It has a blue colour when seen in mass. 
 
 2 The exact ratio is 15-88 to 2 or 7 '99 to 1. 
 
6 A SHOUT HISTORY OF THE 
 
 the elements which enter into it, and though a sub- 
 stance may change its form, it is never diminished 
 in amount. From a series of chemical transformations 
 a given element may be extracted at any stage, 
 possessing the same properties and having the same 
 weight as at first. This was probably a hard thing 
 to believe and understand before the days when the 
 materiality of gases (gas = geist = spirit) had been 
 established, and practical methods for the manage- 
 ment and manipulation of such light and bulky stuff 
 had been introduced. 
 
 Chemists have been industriously testing all sorts 
 of materials : the minerals which form the solid earth, 
 the gases of the air, the waters of the ocean, and the 
 strange, complex, and multitudinous forms of matter 
 which make up the bones and muscles, the blood and 
 nerves of animals, and the wood, pulp, and juices of 
 vegetables; and out of this highly diversified mass 
 they have extracted, not an infinite number, but a 
 strictly limited and comparatively small number of 
 simple things. In 1837, the year from which our 
 story must commence, fifty-four elements were known. 
 A question at once arises: If definite compound 
 substances have a fixed composition, the elements in 
 them being always present in the same proportion, 
 what will be the consequence of bringing together 
 two or three elements and making them unite ? Will 
 the same elements in the same proportion always 
 produce the same compound? The answer is that 
 this is not always so. The same elements combined 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 7 
 
 in exactly the same proportions may produce two or 
 more entirely different substances. For example, 
 starch and cotton are both composed of carbon, 
 hydrogen, and oxygen in the same proportions 
 (C 4444, H 617, 49-38 per cent.), and the sugar of 
 honey or fruit, and the lactic acid of sour milk, form 
 another pair of compounds which contain the same 
 ingredients in the same proportion. But while starch 
 forms a jelly with hot water, and is useful as a food, 
 cotton is quite insoluble in water, and is indigestible ; 
 while grape sugar is crystalline, sweet, and neutral, 
 lactic acid is liquid, sour, and strongly acid. Here 
 is a phenomenon, then, which must be accounted for, 
 not by the nature of the elements present, but by 
 some other hypothesis. 
 
 The facts of definite combination and of multiple 
 .combination, where two elements are joined in several 
 proportions to form a series of distinct compounds, 
 were long ago accounted for by the atomic theory 
 of Dalton. According to this doctrine, which has 
 been the fundamental hypothesis of chemistry for 
 nearly a century, chemical combination is due to 
 the close approximation of the separate particles of 
 the substances uniting. Dalton has expressed the 
 whole idea very clearly in the following passage 
 taken from his "Chemical Philosophy" (1808): 
 "Chemical analysis and synthesis go no farther 
 than to the separation of particles one from another, 
 and to their reunion. No new creation or destruc- 
 tion of matter is within the reach of chemical agency. 
 
8 A SHORT HISTORY OF THE 
 
 We might as well attempt to introduce a new planet 
 into the solar system, or to annihilate one already 
 in existence, as to create or destroy a particle of 
 hydrogen. All the changes we can produce, consist 
 in separating particles that are in a state of cohesion 
 or combination, and joining those that were previously 
 at a distance." 
 
 If, then, two elements combining in the same pro- 
 portion may produce two or more distinct compounds, 
 this can only be explained, according to the atomic 
 theory, by the assumption that the atoms in the 
 different compounds are the same in number, but 
 differently arranged. But why should atoms com- 
 bine together at all? This has been, and probably 
 will always remain, one of the fundamental but 
 unsolved problems of chemistry. Two bodies at a 
 distance from each other are drawn together with 
 a force which is directly as the product of their 
 masses, and inversely as the square of the distance 
 between them. This is the cause of weight, in which 
 the earth as the larger body seems to draw all things 
 to it, though the action is always mutual. So also a 
 magnet attracts iron and some other metals ; a stick 
 of glass or resin, which has been rubbed, attracts 
 light bodies, such as a feather. In the last case, as 
 in the others, the action is reciprocal, the glass attract- 
 ing the feather and the feather the glass, and the 
 cause of the attraction is said to be the charge of 
 electricity which has been developed upon both 
 bodies. But after all, every one of these cases of 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 9 
 
 attraction is only a degree less obscure than that 
 which we call " chemical affinity," which operates at 
 distances so small as to be immeasurably beyond 
 recognition by our senses, and even probably by any 
 direct means of measurement which we can experi- 
 mentally apply. For although we know the LAW of 
 gravitation as already stated, and can make quan- 
 titative expressions of the force with which bodies 
 are drawn together in the other cases, we can only 
 make guesses as to the essential nature of gravitation, 
 and of the attraction due to magnetism or to electrical 
 induction. In the case of chemical affinity, we not 
 only do not yet know why substances unite together, 
 but there is at present no measure of the law as 
 to the distance through which particles of uniting 
 substances may act upon each other. Nevertheless, 
 speculation has of course not been wanting, and 
 among the several hypotheses concerning the nature 
 of chemical affinity, none has attracted more atten- 
 tion, or shown signs of greater vitality, than the one 
 attributing chemical combination to the existence 
 upon the atoms of charges of electricity of opposite 
 kinds, in virtue of which they unite to form more or 
 less completely neutral products. This idea is de- 
 rived mainly from the discoveries in connection with 
 chemical decomposition by an electric current, begun 
 by Nicholson and Carlisle, when in 1800 they for 
 the first time decomposed water into oxygen and 
 hydrogen ; continued by Davy, who discovered potas- 
 sium and sodium by the same agency; employed by 
 
10 A SHORT HISTORY OF THE 
 
 Berzelius as the basis of his celebrated theory; and 
 studied lastly and chiefly by Faraday, who enunci- 
 ated the quantitative statements which are known as 
 Faraday's laws of electrolysis. In the decomposition 
 of compounds by the current, the elements range 
 themselves under two classes, namely, those which in 
 electrolysis go to the anode 1 or positive electrode, 
 and those which appear at the cathode or negative 
 electrode. The former of these two classes includes 
 oxygen, chlorine, bromine, sulphur, and others which 
 are called negative elements ; and the latter includes 
 hydrogen and the metals which are called positive. 
 But while it is true that a strongly positive element 
 unites with a strongly negative element to form a 
 very stable combination, there are many facts which 
 render very difficult the application of this electro- 
 chemical hypothesis to all cases of chemical com- 
 bination. 
 
 In 1837 the great Swedish chemist Berzelius was 
 still living, and his views about the composition of 
 salts and acids were still predominant. Lavoisier had 
 taught that the compounds of oxygen with metals 
 formed bases, while the compounds of oxygen with 
 non-metals, such as sulphur, were acids. By union 
 of a base with an acid a salt was formed. Thus 
 sulphate of soda was composed of soda or oxide of 
 sodium, and sulphuric acid or trioxide of sulphur. 
 Later, when it was found that salts may be formed by 
 
 1 These terms were introduced by Faraday, and are in common 
 use. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 11 
 
 the union of elements like chlorine with metals, the 
 name halogen 1 was given to such elements by Berze- 
 lius, and two classes of salts, called respectively haloid 
 and amphid salts, were recognised. But in each class 
 the proximate constituents of the salt, that is, the 
 sodium and chlorine of common salt, and the soda 
 and sulphuric acid of sulphate of soda, were, accord- 
 ing to the Berzelian classification, respectively electro- 
 positive and electro-negative, and in the compound 
 were united by electric attraction. Faraday before 
 this time had definitely declared his belief in the 
 identity of electricity and chemical affinity; and 
 Daniell, in his well-known "Chemical Philosophy," 
 published soon after this time, refers constantly to 
 "current affinity" in describing the effects of the 
 electric current. 
 
 Such, then, were in general terms the views com- 
 monly accepted in inorganic chemistry. Faraday 
 having long abandoned the pursuit of pure chemistry, 
 the most famous by far among English chemists was 
 Thomas Graham, then Professor of Chemistry in 
 University College (" The University of London "), to 
 whom science is indebted for his discoveries in con- 
 nection with gaseous and liquid diffusion, and for his 
 advanced views on the constitution of such acids as 
 phosphoric acid, which are now called polybasic. 
 
 At the beginning of the century Germany had few 
 chemists, and none of the first rank. In 1837 Liebig 
 was in the height of his fame. But this distinguished 
 
 1 tfXs m. sea-salt, or simply salt. 
 
12 A SHORT HISTORY OF THE 
 
 man, whose work and teaching were destined to have 
 so great an influence on the scientific and industrial 
 future of his own country, and so powerfully to stimu- 
 late the development of scientific chemistry in Eng- 
 land, had been compelled in his own youth to seek 
 the instruction he wanted in the laboratory of a 
 French chemist. His first investigation was made 
 under the direction of Gay-Lussac, and his first 
 paper was published in the Annales de Chimie et de 
 Physique. Liebig did much to point the way to the 
 physiologist, the agriculturist, the manufacturer, in the 
 practical application of chemistry to useful purposes ; 
 but the great work which he had achieved in 1837 
 was to inaugurate the systematisation of what was 
 then rightly called organic chemistry, as distinguished 
 from the chemistry of minerals, salts, and inorganic 
 nature in general. Previously to his time the very 
 composition, to say nothing of the " constitution " of 
 such substances as alcohol, sugar, and the vegetable 
 acids, was almost unknown. Various imperfect and 
 difficult methods of analysis of such substances had 
 been used successively by Lavoisier, by Gay-Lussac 
 and Thenard, by Berzelius and others; but it was 
 only after an investigation extending over seven or 
 eight years that Liebig succeeded in devising the 
 process which, in principle and with modifications 
 relating only to details, is used in every laboratory 
 for the same purpose at the present day. All the 
 compounds in question contain carbon, associated with 
 hydrogen, oxygen, nitrogen, sulphur or other elements, 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 13 
 
 one or more of them ; but the great majority contain 
 carbon, hydrogen, and oxygen with or without nitrogen. 
 When such a compound is burnt in excess of oxygen, 
 or in contact with a substance which yields oxygen, 
 the carbon is wholly converted into carbon dioxide, 
 the hydrogen is converted into water, the nitrogen is 
 set free. These products are easily collected and 
 their weight determined, and inasmuch as the com- 
 position of carbon dioxide and water is accurately 
 known, the proportion of carbon and hydrogen in the 
 substance burnt is easily determined. The exact 
 determination of the composition of a large number 
 of compounds of organic origin, such as those already 
 mentioned, and a study of some of their transforma- 
 tions and of the products obtainable from them by 
 oxidation and otherwise, led to very important con- 
 sequences ; for in the end a definite system of 
 organic chemistry was established, and this was based 
 upon the idea that whereas inorganic compounds are 
 formed of elements united together hi different pro- 
 portions, organic compounds were formed of groups 
 of elements which were capable of passing from one 
 combination to another, and of appearing as com- 
 ponents of many distinct compounds, as though they 
 were simple. And just as in inorganic chemistry 
 there are metals and non-metals which stand in anti- 
 thesis to each other, so there are organic " radicals," 
 as these groups were called, some of which play 
 the part of metals, others of elements like sulphur, 
 chlorine, or oxygen. Organic chemistry, then, came 
 
14 A SHORT HISTORY OF THE 
 
 to be regarded as the chemistry of compound 
 " radicals." 
 
 Ideas of this kind, however, rarely receive general 
 adoption immediately. It had already been shown 
 by Gay-Lussac that cyanogen, a compound of carbon 
 and nitrogen, habitually imitated the element chlorine 
 in its combinations. But the establishment of the 
 idea of compound radicles l and their relation to the 
 properties of series of compounds was undoubtedly 
 due to the investigation by Liebig and Wb'hler into 
 the compounds of benzoyl [C 14 H 6 2 ], existing in 
 bitter-almond oil, in benzoic acid, and in the com- 
 pounds immediately derivable from them. The pro- 
 pagation of this doctrine was, however, not left to 
 the efforts of Liebig and Wohler alone, for by 1837 
 Dumas in France had so fully adopted the idea, that 
 in a communication made jointly with Liebig to the 
 Academy of Sciences, 2 he announced formally his 
 adhesion to the doctrine. In mineral chemistry, he 
 says, the radicals are simple, in organic chemistry the 
 radicals are compound : " Voila toute la difference." 
 
 Dumas himself, senior to Liebig by about three 
 years, had made by this time very important dis- 
 coveries. Of these the most striking, whether con- 
 sidered in reference to its influence on the progress 
 of ".organic " chemistry, or its relation to the electro- 
 chemical theory of combination, is the discovery of 
 the phenomenon of "substitution." The story goes 
 
 1 This is the more usual modern form of this word. 
 
 2 Comptes Rendus, v. 567 (23rd October 1837). 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 
 
 that attention was drawn to the action of chlorine 
 upon wax, by the annoyance caused at a soiree at 
 the Tuileries by the irritating fumes emitted by 
 the candles, which burned with a smoky flame. It 
 turned out on inquiry that the wax had been 
 bleached by chlorine, and the fumes emitted on 
 burning were due to hydrogen chloride. Researches 
 undertaken by Dumas led to the discovery in 1834, 
 that in contact with many organic compounds 
 chlorine is capable of replacing hydrogen atom by 
 atom, so that for every atom of hydrogen removed, 
 an atom of chlorine takes its place. It may be easily 
 imagined how distasteful such a discovery would be 
 to Berzelius and the school of electro-chemists, in- 
 volving as it does the idea that a negative element 
 may be exchanged for a positive element, without a 
 fundamental alteration in the chemical character of 
 the resulting compound. That such is the case was 
 not admitted without a controversy which extended 
 over many years. It has long been known that the 
 property displayed by chlorine is possessed by bro- 
 mine, and under proper conditions by iodine, and 
 even by compound groups, such as the ra'dicle of 
 nitric acid. 
 
 Such then was the general condition of knowledge 
 relating to chemistry, and to those branches of science 
 immediately connected with it, which had been es- 
 tablished when Queen Victoria came to the throne. 
 The position of science, however, will be better 
 appreciated if it is remembered that at this date 
 
16 A SHOET HISTORY OF THE 
 
 none of the special societies which exist for the 
 cultivation of pure or applied chemistry and physics 
 had come into existence. 1 The Royal Society of 
 London (chartered by Charles II. in 1663) and the 
 Royal Society of Edinburgh (founded 1783) were the 
 only important Societies to which communications 
 relating to discoveries in chemistry could be appro- 
 priately addressed in this country, and in the Philo- 
 sophical Transactions of the Royal Society of London 
 are to be found the greater part of the discoveries 
 made by Davy and Faraday. But after the death 
 of Davy in 1829 it seems probable that there was 
 considerable justification for complaint as to mis- 
 management in the Royal Society, such as took shape 
 in Babbage's " Reflections on the Decline of Science 
 in England," published in 1830. This, however, was 
 a state of things destined soon to pass away, for not 
 only was Faraday at work at the Royal Institution, 
 but in 1831 the British Association for the Advance- 
 ment of Science started upon its prosperous career. 
 
 We may now proceed to trace the course of dis- 
 covery in chemistry, but as this necessarily advanced 
 upon several lines, which were in the beginning at 
 any rate quite distinct from each other, it will be 
 advantageous to follow these separately, taking one 
 at a time. 
 
 1 The Chemical Society of London was founded 1841 ; Phar- 
 maceutical Society of Great Britain, 1841 ; Socie"te Chimique de 
 Paris, 1858; the Berlin Chemical Society, 1867; the Physical 
 Society, 1874 ; American Chemical Society, 1876 ; the Society of 
 Chemical Industry, 1881. 
 
PKOGKESS OF SCIENTIFIC CHEMISTRY 17 
 
 CHAPTER I 
 
 MATTER AND ENERGY 
 
 IT has already been- pointed out in the Introduction 
 that chemical changes are attended by transmuta- 
 tions in the form of matter, but never by any gain or 
 loss in its amount. The doctrine of the conservation 
 of matter teaches that ponderable things are inde- 
 structible, and that the amount of matter in the 
 universe, so far as its conditions are yet known, 
 is absolutely constant and invariable. Without this 
 fundamental postulate no system of chemistry could 
 exist, nor indeed could the present order of things 
 endure. This was practically acknowledged so soon 
 as the minds and the writings of chemists were freed 
 from the influence of the old theory of phlogiston, 
 and hence we may say that it was adopted from the 
 time when Lavoisier's explanation of combustion was 
 accepted. 
 
 A second equally important principle is, however, 
 necessary, though its full recognition was delayed for 
 another half-century. This is the doctrine which 
 affirms the indestructibility of energy ; but it required 
 for its establishment the thought, observation, and 
 experiment of many generations of men. Newton 
 
18 A SHOET HISTORY OF THE 
 
 had, no doubt, clear views of his own upon the sub- 
 ject, and these received expression in his statement 
 of the laws of motion ; but the experimental work of 
 Rumford and Davy upon the production of heat by 
 friction, were required to prove that such a form of 
 energy as heat is not matter, as had been previously 
 supposed. Measurements of the correlation of heat 
 and mechanical work were not made till forty years 
 later, chiefly by Dr. James Prescott Joule, of Man- 
 chester. If the determination of the first precise 
 quantitative relations be regarded as the best foun- 
 dation of exact knowledge, and if, as we now believe, 
 this kind of knowledge is essential to the formation 
 of correct ideas concerning chemical changes, then 
 the name of Joule deserves to be ranked along 
 with those of Boyle, Lavoisier, and Dalton, who 
 have successively, at different times, by different 
 writers, and for different reasons, been invested 
 with the title of Father or Founder of modern 
 chemistry. Since the recognition of the funda- 
 mental idea that all chemical changes involve re- 
 distribution but no destruction of energy as well 
 as of matter, and that the same matter associated 
 with different amounts of energy assumes very 
 different aspects and properties, the progress of 
 chemistry has been more rapid than in any pre- 
 vious period. 
 
 It is, however, time that some explanation should 
 be offered as to the modern use of the word energy, 
 and its application in connection with chemistry as 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 19 
 
 well as other departments of physical science. When 
 hydrogen combines with oxygen in due proportions 
 the sole material product is water ; but another and 
 very significant effect is produced during the act 
 of combination, and that is the evolution of heat. 
 This heat is communicated to the water formed and 
 to the walls of the tube or other vessel in which 
 the gases are brought together, and it is thus soon 
 dissipated. But the amount of heat thus pro(^u 
 can be measured, and this is usually done by obser^ 
 vation of the rise of temperature in a given quantity 
 of water into which the heat is conveyed, or what 
 comes to the same thing, the determination of the 
 amount of water, the temperature of which is raised 
 one degree from zero. That amount of heat which 
 will raise the temperature of ]/part by weight of 
 water, 1 degree is spoken of as 1 calorie or 1 unit 
 of heat. The same result can be attained by observ- 
 ing other thermal effects, as, for example, by noting 
 the amount of ice melted. 
 
 Now when 1 part by weight, say 1 gram 1 of 
 hydrogen, is burned in oxygen, and the water formed 
 is collected in the liquid state, the amount of heat 
 evolved is sufficient to raise the temperature of about 
 34,000 grams of water 1, or, hi other words, 34,000 
 calories or units of heat are evolved. This includes 
 a certain quantity of heat, about 5500 units, which 
 
 1 Throughout the book weights and measures of the metric 
 system and degrees of the Centigrade scale will be used. 
 
20 A SHORT HISTORY OF THE 
 
 is given out in the change of the water from the state 
 of vapour, in which it is formed, to the state of liquid, 
 in which it is collected. The amount of heat pro- 
 duced in this way is constant for unit weight of 
 either hydrogen or oxygen, and whether combination 
 takes place quickly, by exploding a mixture of the 
 two gases in a strong vessel, or slowly, by burning one 
 of the gases at a jet in an atmosphere of the other, or 
 
 banging them together in the presence of spongy 
 platinum which makes them combine, the amount ot 
 heat given out is always 34,000 calories per gram of 
 hydrogen. From this we learn that when hydrogen 
 combines with oxygen a definite quantity of some- 
 thing is lost by the elements, and passes out of them 
 in the form of heat : this something is called energy. 
 The water which has been formed may be made to 
 yield up the hydrogen and oxygen which have com- 
 bined for its formation, but this can only be brought 
 about on condition that that energy is restored to the 
 two elements. This result may be reached in several 
 ways, as by the action of a high temperature, or by 
 the use of. an electric current, or indirectly by the 
 use of chemical agents. 
 
 If a penny is rubbed hard upon a board it becomes 
 heated, and it is said that by hammering a piece of 
 cold iron a skilful blacksmith can make it even red 
 hot. In both such cases a sense of fatigue in the 
 arm is soon felt, and there is a consciousness that 
 work has been done. If we substitute for human 
 effort the falling of a weight, similar effects can be 
 
PROGRESS OF SCIENTIFIC CHEMISTRY' 21 
 
 produced ; but just as fatigue puts an end to the work 
 done by the arm, so the fall from a higher to a lower 
 level corresponds to work performed which cannot be 
 repeated with the same mass until it is lifted up to its 
 original height. The power of doing work, called in 
 physical science energy, is capable of measurement, 
 and . may take many different forms. For example, 
 the arm whose muscular strength may be used to 
 heat a mass of metal by hammering or by rubbing, 
 may be otherwise employed to turn the handle of a 
 machine by which a coil of copper wire is made to 
 rotate in the field of a magnet, and thus an electric 
 current may be produced in the wire. The current 
 flows so long as the work is being done, and it 
 ceases immediately upon the cessation of the motion. 
 Further, it is a familiar fact that when a current 
 of electricity flows through an imperfect conductor 
 electricity disappears, and the conductor becomes 
 heated. An example of this is seen in the com- 
 mon incandescent electric lamp, in which a thread 
 of carbon is heated till it gives out a brilliant 
 light. 
 
 Observations of such a kind must, however, be 
 supplemented by measurements of the amount of 
 heat or of electricity produced when a given amount 
 of work is done. The problem is to find a suitable 
 unit of work, and this is provided by gravitation. 
 The question is, first, what amount of work must be 
 done in order to produce heat enough to raise the 
 temperature of unit mass water one degree? and 
 
22 A SHORT HISTORY OF THE 
 
 secondly, supposing the water allowed to cool down 
 again, and the heat which passes from it made to 
 do work, how much work will be done ? Answers to 
 these questions are supplied by the investigations 
 made by Joule, and published in 1843 and following 
 years. The results which are considered most trust- 
 worthy were obtained by the following process. The 
 work done was the descent of a mass of lead from a 
 certain measured height to a lower level, and the 
 heat corresponding to this was generated by causing 
 this weight in its descent to move a paddle working 
 in a vessel containing a measured quantity of water, 
 and provided with fixed projections within, so as to 
 prevent the water from being whirled bodily round. 
 The friction thus caused gave rise to heat, and so 
 the temperature of the water was raised through a 
 certain number of degrees which could be determined 
 by delicate thermometers immersed in the water. 
 By this means it was found, as the mean result of a 
 number of successive concordant experiments, that 
 the descent of a weight of 424 grams through a dis- 
 tance of 1 metre, or 1 gram falling 424 metres, gene- 
 rates heat enough to raise the temperature of 1 gram 
 of water 1 C. The same facts may be expressed 
 in English weights and measures, by saying that the 
 fall of 772 Ibs. through 1 foot, or of 1 Ib. 772 feet, 
 gives a rise of 1 Fahr. in 1 Ib. of water. The num- 
 bers, 424 gram-metres or 772 foot-pounds, according 
 to the system chosen, represent the mechanical 
 equivalent of heat, and are often spoken of as Joule's 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 23 
 
 equivalent, and represented for mathematical purposes 
 by the letter J. 1 
 
 Other methods have been used, both by Joule and 
 others since his time, such for example as the 
 measurement of the heat generated by the friction 
 of two metallic surfaces upon each other under deter- 
 mined conditions, and with approximately the same 
 result. 
 
 Bearing in mind, then, that a definite quantitative 
 relation is now established between mechanical work 
 performed and heat generated when the work is all 
 expended in friction, and that the same relation can 
 be traced whether the work is transformed directly 
 into heat, or is first made to generate an electric 
 current, which is afterwards converted into heat, it is 
 obvious that heat is not a substance but a mechanical 
 effect, and, as now universally believed, the effect of 
 vibratory or other motion in the particles of the 
 heated body (see Chap. X.). We may now return to 
 the question involved in the phenomena of chemical 
 combination and decomposition. 
 
 1 The value of the mechanical equivalent given in the text applies 
 to the short range of temperature, which was, of course, near to 
 common air temperatures, employed in the experiments. The 
 specific heat of water increases as the temperature is raised, and 
 the value for higher temperatures would therefore be greater. Pro- 
 fessor Osborne Reynolds has made an elaborate series of experi- 
 ments, in which the work done in raising the temperature of water 
 from freezing to boiling point has been determined. The mean 
 value deduced from these experiments for this range of tempera- 
 ture is about 777, and this corresponds to a value slightly higher 
 than Joule's, namely, 773'7 for 1 Fahr. at 60 Fahr. Phil. Trans., 
 1897, A. 
 
24 A SHORT HISTORY OF THE 
 
 It is clear, from what has already been said, that an 
 element usually differs from a compound not only in 
 containing only one form of matter, but in having a 
 power of doing work ; that is, a store of energy which 
 is more or less dissipated and lost when the element 
 enters into a state of chemical combination. 1 The 
 process -of separating an element or other substance 
 from a compound in which it is held by chemical 
 affinity is comparable with the operation of raising a 
 weight. In each case work has to be expended from 
 some source human, animal, or mechanical power, 
 the falling of water, the pressure of the wind, the 
 rising of the tide, the heat of the sun, or the chemi- 
 cal process involved in the burning of coal. Hence, 
 to revert to the case of water, which is formed 
 when hydrogen and oxygen unite together, it must 
 now be obvious that, to separate the united hy- 
 drogen and oxygen, work must be done equivalent 
 at least to the heat generated in the act of combi- 
 nation. 
 
 An element, then, is matter combined with poten- 
 tial energy. This energy, by appropriate means, 
 may be allowed to run down, either all at once or by 
 stages, as for example in the case of sulphur, the 
 burning of which in oxygen may be arranged to 
 produce sulphur dioxide, S0 2 , or sulphur trioxide, 
 
 1 This is, of course, not true in those, not rare, but much less 
 frequent cases in which " endothermic " compounds, such as carbon 
 bisulphide, are formed, the production of which is attended by 
 absorption of heat, and their decomposition therefore by evolution 
 of heat. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 25 
 
 S0 3 . An element which has been separated from 
 chemical combination with other elements may, 
 however, in some cases be compelled to take up an 
 additional store of energy. This change may often 
 be accomplished by the application of heat. The 
 product is spoken of as an " allotropic " modifica- 
 tion, or simply as an allotrope of the element. If 
 sulphur, for example, is melted, and the resulting 
 liquid raised only a few degrees above the melting 
 point of common sulphur, a new substance is formed 
 which crystallises in rhombic prisms, having a dif- 
 ferent form and a different density from common 
 sulphur, the crystals of which are rhombic octa- 
 hedrons. Or by heating the liquefied sulphur still 
 further, it may be obtained in the form of a plastic 
 mass wholly devoid of crystalline structure. But 
 both these modifications revert in time to the 
 common kind, and the change is attended by evolu- 
 tion of heat. Similarly, red phosphorus may be got 
 from common white phosphorus by heat, ozone from 
 oxygen by electricity, graphite or the diamond from 
 common charcoal, by dissolving it at a high tempera- 
 ture in melted iron, and allowing it to crystallise 
 under pressure when cooling. In all these cases the 
 same matter is concerned, but different amounts of 
 energy are bound up with it, and different amounts 
 of heat are therefore generated when equal quan- 
 tities of the various allotropes of any one element 
 enter into chemical combination. Thus, 1 gram of 
 each of the three chief varieties of carbon burnt com- 
 
26 A SHOET HISTORY OF THE 
 
 pletely in oxygen give the following amounts of heat 
 expressed in ordinary units : 
 
 Diamond . . . . . . .7770 
 
 Graphite (natural) 7797 
 
 Graphite (from iron) . . . . . 7762 
 Wood-charcoal 8080 
 
 The possibility of the conversion of one element 
 into another, as, for example, the transmutation of 
 silver into gold, is a question which even in these 
 days, so remote from alchemical times, has not been 
 entirely set aside. But although the idea that the 
 elements may have had a common origin, or may 
 contain common constituents, may be admitted as 
 worthy of discussion, evidence of a direct kind for 
 either of these propositions is absolutely wanting. 
 Such considerations as have been brought into the 
 discussion have been derived chiefly from a com- 
 parison of the atomic weights, which will form the 
 subject of a later chapter. 
 
 The combination of hydrogen with oxygen is 
 attended, as already stated, by the formation of a 
 definite amount of water and the evolution of a 
 definite amount of heat. The determination of the 
 quantity of heat thus disengaged has occupied at 
 different periods many of the most distinguished 
 chemists, from the time of Lavoisier onwards. The 
 experiments of Lavoisier were made with his cele- 
 brated ice calorimeter, and subsequent determina- 
 tions, in which the heat of union was communicated 
 to water, were made by Dulong, by Favre and 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 27 
 
 Silbermann, and by Andrews. The examination of 
 other cases of combination have led to the establish- 
 ment of the important general principle, that every 
 chemical change, whether of combination or decom- 
 position, is accompanied by the evolution of a definite 
 amount of heat, which is the same as saying that a 
 definite amount of energy, previously existent in the 
 bodies concerned, becomes dissipated. This is the 
 first principle of that department of science which is 
 called " thermo-chemistry." 
 
 One of the first serious workers in this field was 
 Dr. Thomas Andrews of Belfast, and though later 
 experimenters have improved upon his methods and 
 results, his name deserves to be remembered as a 
 pioneer in this difficult department of experimental 
 inquiry. One of the most important series of experi- 
 ments carried out by Andrews relates to the heat 
 developed during the combination of acids and bases 
 in aqueous solution. 1 From these experiments he 
 drew the conclusion that the heat developed during 
 the union of acids and bases is determined by the 
 base and not by the acid. In this he was mistaken, 
 for it appears that the heat produced by the neutrali- 
 sation of an acid by a base is an effect to which both 
 contribute, though in proportions which depend upon 
 their chemical nature, and are not yet determinable. 
 One of his general laws, 2 which states that "an 
 
 1 Trans. R. Irish Academy, 1841 ; reprinted in the volume of his 
 collected " Scientific Papers." 
 
 2 British Association Report for 1849, p. 69. 
 
28 A SHORT HISTOEY OF THE 
 
 equivalent of the same base combined with different 
 acids produces nearly the same quantity of heat," 
 has been confirmed by later researches, and has been 
 explained by a hypothesis, of which an account will 
 be given in a later chapter. Andrews also made 
 determinations of the heat produced in other chemi- 
 cal changes, including the amount of heat disengaged 
 during the combination of various substances with 
 oxygen and with chlorine. 
 
 About the same time thermo-chemical researches 
 of considerable importance were being carried on by 
 the Russian chemist Hermann Hess. As the result 
 of his experiments on the neutralisation of acids 
 diluted with different proportions of water, he was led 
 to the enunciation of the principle that the sum of 
 the several amounts of heat evolved during the suc- 
 cessive stages of a process are the same in whatever 
 order they follow one another; in other words, the 
 effect is dependent on the relation of the final to the 
 initial state of the system, and not upon the inter- 
 mediate stages. On neutralising an aqueous solution 
 of ammonia with sulphuric acid containing one, two, 
 three, and six proportions of water there is a different 
 development of heat in each case ; but by adding to 
 the results found by experiment in the three last 
 cases the quantity of heat evolved when the mono- 
 hydrated acid combines with one, two, and five pro- 
 portions of water, nearly the same value is obtained 
 in each case. Andrews thought this principle correct 
 but self-evident. It is, however, of such importance 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 29 
 
 that its experimental verification was very desirable. 
 By the application of this principle it is possible to 
 calculate thermal changes which are not capable of 
 direct experimental determination. The heat evolved 
 by the formation of carbon monoxide, for example, 
 cannot be ascertained by burning carbon in oxygen ; 
 but by observing the heat evolved in the production 
 of carbon dioxide, and then determining the heat 
 produced by the combustion of carbon monoxide 
 itself, the difference between the two affords the 
 number required. One part by weight of carbon 
 burnt to carbon dioxide gas gives 7797 units of heat, 
 but 2-33 parts of carbon monoxide gas, which con- 
 tains the same quantity of carbon, gives 5607 units. 
 Now, if the amount of heat given out when the first 
 atom of oxygen unites with the carbon were the same 
 as that produced by the second atom, the total amount 
 of heat evolved when carbon burns into carbon 
 dioxide would be 5607 x 2, or 11,214 units. But the 
 actual amount observed is only 7797 units ; the union 
 of the first atom of oxygen forming carbon monoxide 
 is attended by the evolution of 2190 units ; the 
 difference, 3417, therefore must represent the heat 
 rendered latent when solid carbon is converted into 
 the gaseous oxide. 
 
 Nearly all chemical changes are associated in a 
 similar way with physical changes, and hence in the 
 great majority of cases the interpretation of the 
 results is encumbered with serious difficulties. This 
 is the case even when all the products remain in the 
 
30 A SHORT HISTORY OF THE 
 
 same state, gaseous, or liquid, or solid, as the materials 
 employed, for there is usually a separation as well as 
 a combination of atoms even in cases which appear 
 most simple. The union of hydrogen with oxygen, 
 for example, is not merely 
 
 H 2 + = H 2 
 but 
 
 from which it appears that the oxygen molecule is 
 divided in this process into two parts. 
 
 Since the days of Favre and Silbermann, of 
 Andrews and of Hess, the problems presented by 
 the thermal changes which accompany chemical 
 changes have been studied in great detail by Pro- 
 fessor Julius Thomsen in Copenhagen, and later by 
 Professor Berthelot in Paris. 
 
 Thomsen's first memoir appeared in 1853, and 
 from that date down to the time when more than 
 thirty years later he completed his great work in 
 four volumes, Thermochemische Untersuchungen, the 
 author pursued without interruption the systematic in- 
 vestigation of which he had laid down the plan so long 
 ago. In this work the phenomena of neutralisation 
 are first investigated, the succeeding volumes being 
 devoted to the determination of the heats of forma- 
 tion of the more important oxides, chlorides, hydrides 
 of the non-metals, and then of the metals, the consti- 
 tution of aqueous solutions, the heats of combustion 
 of organic compounds, and the thermal phenomena 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 31 
 
 attending isomeric change; in fact, a body of data 
 is provided which relates to all kinds of chemical 
 changes, and is available for the use of theory in 
 every department of the subject. 
 
 M. Berthelot tells us that it was in 1864 that 
 he began to study thermo-chemistry. In 1879 he 
 brought out his treatise entitled Essai de Mechanique 
 Chimique, fondee sur la Thermochimie. This work 
 embodies the chief results of all his researches on the 
 subject which had been from time to time communi- 
 cated to the Annales de Chimie et de Physique, where 
 they occupy more than two thousand pages. The 
 author sets forth in his introduction the three 
 propositions which he regards as fundamental. The 
 first of these states that the heat disengaged in any 
 reaction is a measure of the chemical and physical 
 work accomplished in that reaction. The second 
 affirms the dependence of the thermal change upon 
 the relation of the final to the initial state of the 
 system; it is in fact the principle established by 
 Hess, as already explained. The third proposition 
 was introduced by Berthelot himself, under the title 
 of the " Principle of Maximum Work." This it will 
 be well to state as nearly as possible in the words of 
 the author himself, inasmuch as it has been the 
 subject of very severe criticism. 
 
 " Every chemical change accomplished without 
 the aid of external energy tends towards the pro- 
 duction of a body or system of bodies which gives 
 out the greatest amount of heat." As a corollary 
 
32 A SHORT HISTORY OF THE 
 
 from this, he adds that " every chemical reaction 
 which can be accomplished without the assistance 
 of preliminary work, or of energy external to the 
 system of bodies concerned, proceeds necessarily if it 
 is attended by evolution of heat." 1 
 
 Here is a doctrine at first sight attractive in no 
 ordinary degree, and which, if established, would 
 appear to account for much that would be otherwise 
 obscure. But unfortunately it is expressed in terms 
 which are far too general, and is manifestly in oppo- 
 sition to well-recognised facts. In the first place, 
 " endothermic " compounds are sometimes formed 
 spontaneously, and in most cases of combination 
 which is attended by evolution of heat the process 
 is retarded and ultimately stopped, unless the heat 
 generated by the union of the first portions of sub- 
 stance is conducted away out of the system. Am- 
 monia and hydrogen chloride, for example, unite at 
 common temperatures to form solid sal-ammoniac, 
 and in this process heat is evolved. The consequence 
 is that until the whole is allowed to cool below the 
 temperature at which it is known that sal-ammoniac 
 is resolved into these two gases, a portion of the 
 materials remain separate, notwithstanding that their 
 
 1 " Tout changement chimique accompli sans 1'intervention d'une 
 energie etrangere tend vers la production du corps ou du systeme 
 de corps qui de"gage le plus de chaleur." And as a corollary from 
 this, " toute reaction chimique susceptible d'etre accomplie sans le 
 concours d'un travail preliminaire et en dehors de 1'intervention 
 d'une energie Etrangere h, celle des corps presents dans le systeme, 
 se produit ne'cessairement, si elle degage de la chaleur." Essai, 
 vol. i., Introduction, xxix. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 33 
 
 union would be attended by evolution of heat. The 
 same kind of thing is true of all reversible reactions, 
 that is, all chemical combinations which are pre- 
 vented by rise of temperature and promoted by fall 
 of temperature. 
 
 Nevertheless Berthelot's principle, if applied with 
 due limitation, does seem to accord with a number 
 of familiar facts. The displacement, for example, of 
 iodine by bromine, and of bromine by chlorine, in the 
 combinations of these elements with hydrogen and 
 the metals, and their apparent order of affinity in such 
 compounds, is in accordance with the thermal rela- 
 tions of these elements, as shown by the following 
 statement of the heat produced by the combination of 
 hydrogen with equivalent quantities of each of them : 
 
 Formula-weights in grams. Calories. 
 
 H + C1+4H 2 give 39320 
 
 H + Br + 4H 2 28380 
 
 H + I +4H 2 O 13170 
 
 Similar relations may be noticed among the metals. 
 In such a series as iron, copper, silver, mercury, for 
 example, where the apparent affinities for chlorine or 
 oxygen are manifested in the order in which they 
 are written, on the one hand by the displacement by 
 each one of those which follow it in the list, and on 
 the other by the heat of formation of their salts, 
 which stands in the same succession. 
 
 The chief object of quantitative therrno-chemical 
 investigation is, according to Julius Thomson, 1 the 
 
 1 Therm. Unterschimgen, vol. i. 3, 
 
 C 
 
34 A SHORT HISTORY OF THE 
 
 establishment of dynamical laws relating to chemical 
 processes ; but how little progress towards this object 
 has been at present accomplished is manifest from 
 the brief sketch which alone it is possible to give 
 in these pages. Notwithstanding the labours of half 
 a century, thermo-chemistry remains for the most 
 part a mass of experimental results, which still 
 await interpretation. For, apart from the assistance 
 which such results afford towards the establishment 
 of the doctrine of the conservation of energy, it 
 is plain that successive attempts at generalisation 
 have been unsuccessful when considered in a strictly 
 scientific sense. 
 
 One direction, indeed, in which the relation of 
 heat to chemical affinity has been studied with 
 much advantage, is in the examination of those 
 reversible chemical changes which are commonly 
 brought about by change of temperature. The term 
 dissociation l was introduced by Sainte-Claire Deville, 
 in 1857, to designate decompositions of this kind, 
 and, though it was but slowly accepted by the 
 chemical world, it has now become firmly embedded 
 in the language of science. 
 
 So long ago as 1846 it was discovered by Grove 2 
 that water is decomposed into a mixture of oxygen 
 and hydrogen by contact with intensely heated 
 platinum. The experiment was afterwards repeated 
 by Deville, who found that decomposition begins at 
 a temperature of 960 to 1000, but that it proceeds 
 
 1 " De la dissociation ou decomposition spontanee des corps sous 
 1'influence de la chaleur," Compt. Rend., xlv. 857. 
 
 2 Phil, Trans. Bakerian Lecture for 1846. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 35 
 
 to only a limited extent. If, however, any means 
 be taken to separate the resulting gases from each 
 other, the decomposition may be carried much fur- 
 ther. Regnault found, for example, that steam passed 
 over melted silver is decomposed more freely, ap- 
 parently because the oxygen is absorbed by the 
 melted silver, which gives it off again on solidifying. 
 These effects are not due to any chemical action 
 on the part of the metal, but are the result of the 
 high temperature to which the vapour is exposed. 
 Hydrogen and oxygen combine together completely 
 at lower temperatures. Results so remarkable as 
 these did not attract the attention they deserved 
 till some years later, when the systematic study of 
 vapour- densities led to the discovery that a large 
 number of familiar compounds, commonly reputed 
 stable, are decomposible at various temperatures in 
 a similar manner. In each case the products of 
 dissociation reunite on cooling, producing the original 
 compound. Sulphuric acid, for example, converted 
 into vapour is no longer sulphuric acid, but a mix- 
 ture of vapours of water and sulphur trioxide, which 
 on cooling reunite. By conducting the operation in 
 a flask with a long drawn-out capillary neck, the 
 water vapour, being the lighter of the two, diffuses 
 away more rapidly, leaving a preponderant quantity 
 of sulphur trioxide behind, which, if the process is 
 continued long enough, crystallises from the residue 
 when allowed to cool. In a similar manner it has 
 been shown that sal-ammoniac when vaporised yields 
 
36 A SHORT HISTORY OF THE 
 
 a mixture of ammonia and hydrogen chloride ; that 
 phosphoric chloride is split up into phosphorous 
 chloride and chlorine ; that ammonium carbonate 
 is resolved into ammonia, water, and carbon dioxide. 
 In all these and many other cases the dissocia- 
 tion proceeds gradually as the temperature is raised, 
 till it becomes complete at a temperature which is 
 peculiar to each case ; it is promoted by reduction of 
 pressure, and diminished by increase of pressure, or 
 what is practically the same thing, by heating in an 
 atmosphere consisting of one of the products of 
 dissociation. 
 
 A history of the progress of thermo-chemistry, 
 though professedly brief, would be in some danger 
 of conveying an erroneous impression if a passing 
 reference were not made to the nature of the work 
 undertaken by other chemists and physicists who 
 have occupied themselves with the subject, Among 
 the rest, we owe to Marignac the determination of 
 the specific heats of a number of saline solutions, 
 and the thermal phenomena which ensue on diluting 
 them (187076); to A. Horstmann a careful study 
 of the progress of dissociation of a number of com- 
 pounds, such as hydrogen iodide, ammonium chloride, 
 ammonium carbonate (1868-78); to Alexander Nau- 
 mann one of the earliest and most instructive syste- 
 matic treatises on thermo-chemistry (Grundriss der 
 Thermochemie, 1869); and lastly to Professor J. 
 Willard Gibbs, the application of the principles of 
 thermo-dynamics to many thermo-chemical problems. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 37 
 
 CHAPTER II 
 
 THE CHEMICAL ELEMENTS: THEIR DISTRIBUTION IN 
 NATURE, AND RECOGNITION BY THE CHEMIST 
 
 THE word " element," apart from poetical usage, is 
 now universally understood to mean a substance 
 which, though it may pass through many trans- 
 formations, is always recoverable undiminished in 
 quantity from any chemical combination into which 
 it may enter, and is not by any known means re- 
 solvable into two or more distinct kinds of matter. 
 Sulphur, for example, is regarded as an element, 
 notwithstanding the allotropic changes which it 
 undergoes under the influence of heat, because from 
 sulphur in any of its known forms nothing can be 
 abstracted which is not sulphur ; in other words, it 
 is homogeneous, and consists in its most minute 
 parts of one kind of substance. On the other hand, 
 water and iron rust are regarded as compounds be- 
 cause in proportion as, by suitable means, either of 
 them is destroyed two kinds of new matter make 
 their appearance, and the united weights of the 
 products of "decomposition" are equal to the weight 
 of the "compound" body from which they are 
 educed. These ideas date from the time of Robert 
 
38 A SHORT HISTORY OF THE 
 
 Boyle. In his "Sceptical Chymist" (1680), he 
 demonstrated the inconsistencies not only of the 
 ancient Aristotelian doctrine of the Four Elements, 
 but showed how little of foundation in fact and 
 how much of imagination was to be found in the 
 alchemical doctrine of the Tria prima current in his 
 day. 1 Boyle not only insisted upon homogeneity 
 as a characteristic of a true " element," but refused 
 to admit any arguments but such as were based 
 upon experiment. 
 
 The materials of which the earth and its in- 
 habitants consist are chiefly compound, but they 
 are resolvable into a limited number of substances, 
 regarded for the present as elementary, because 
 they have never yet been decompounded. Of these 
 several oxygen, nitrogen, and some other gases 
 are found in the atmosphere in the elemental form. 
 The rest occur in proportions which are very un- 
 equal ; some, as oxygen, silicon, carbon, and a few 
 metals like aluminium, iron, calcium, magnesium, 
 constituting in their various combinations the stuff 
 out of which the greater part of the solid earth is 
 formed, while others, though very necessary to the 
 constitution of vegetable or animal tissue, occur in 
 much smaller amount, and others again are found 
 
 1 The complete title of Boyle's work sufficiently explains its ob- 
 ject : " The Sceptical Chymist : or Chemico-physical Doubts and 
 Paradoxes, touching the Experiments whereby vulgar Spagirists 
 are wont to endeavour to evince their Salt, Sulphur, and Mercury 
 to be the true Principles of Things, to which are subjoined divers 
 Experiments and Notes about the Producibleness of Chemical Prin- 
 ciples" 
 
PROGEESS OF SCIENTIFIC CHEMISTRY 39 
 
 only locally, or even in such minute quantity as to 
 require special methods for their detection. 
 
 The recognition of new elementary substances 
 does not necessarily imply, as often popularly sup- 
 posed, the isolation of the element itself. Fluor- 
 spar, for example, was known in the middle of the 
 last century to yield an acid analogous to muriatic 
 acid (Scheele), but the element fluorine was not 
 isolated till Moissan announced the results of his 
 experiments in 1 8 8 6 . Similarly, alumina was known 
 as a distinct earth in 1754 (Marggraff), but the 
 metal was not obtained till 1828 (Wohler). Potash, 
 soda, lime, and magnesia were recognised and dis- 
 tinguished from one another long before their com- 
 pound nature was even suspected, and so in many 
 other cases. On the other hand, examples have been 
 known of substances which, produced by methods 
 calculated to afford the element, have been supposed 
 to be simple, till long afterwards they have been 
 found to be compound. This was the case with the 
 inetalloidal element titanium. A peculiar crystalline 
 copper- coloured substance of metallic aspect, some- 
 times found in the bottom of blast-furnaces in which 
 iron is reduced, was for a long time supposed to be 
 metallic, that is elemental, titanium, till it was 
 discovered by Wohler to contain not only titanium, 
 but carbon and nitrogen. 
 
 In 1837 fifty-four elements were known. In 1897 we 
 recognise nearly eighty distinct substances believed 
 to be elementary, notwithstanding that about ten of 
 
40 A SHORT HISTORY OF THE 
 
 these have as yet been very imperfectly studied. 
 From time to time new elements are announced, 
 and while some of these " come like shadows " and 
 " so depart, " there is a tolerably steady addition of 
 a permanently established member of the series on 
 an average every three or four years. For these 
 additions to our knowledge science is mainly 
 indebted on the one hand to the introduction of 
 previously unknown methods of experiment, and 
 on the other hand to a closer attention to resi- 
 dual phenomena, previously neglected or imperfectly 
 studied. As to the former, we need only refer to 
 the application by Humphry Davy, in 1807, of the 
 then recently discovered chemical effects of the 
 electric current, by which he was led to the isola- 
 tion of potassium and sodium ; while the use of 
 these metals in their turn afforded the means of 
 decomposing compounds of boron, silicon, and alu- 
 minium with liberation of those elements. 
 
 In recent times the most fertile method of dis- 
 covery of new elements has been the process called 
 Spectrum Analysis. This was introduced as a 
 definite method of experimental inquiry by Bunsen 
 and KirchhofF about 1859, and in Bunsen's hands 
 led at once to the recognition of two previously 
 unknown metals of the alkali group, to which he 
 gave the names rubidium and csesium. This dis- 
 covery was followed by the isolation of thallium by 
 Crookes in 1861, of indium by Reich and Richter 
 in 1863, of gallium by De Boisbaudran in 1875 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 41 
 
 and of the oxide of an element called scandium by 
 Nilson in 1879. 
 
 The story of Newton's experiments made in 1675 
 with the spectrum of the sun's rays is almost too 
 familiar to require repetition. However, it is neces- 
 sary to recall these experiments to mind in this 
 place, because they not only form the basis upon 
 which all subsequent discoveries with the prism 
 were made, but they show what very important 
 results often arise from apparently slight modifica- 
 tions in the mode of operating, or in the form of 
 apparatus. Newton gives the following account of 
 his procedure : l " In a very dark chamber, at a 
 round hole about one-third part of an inch broad 
 made in the shut of a window, I placed a glass 
 prism, whereby the beam of the sun's light which 
 came in at that hole might be refracted upwards 
 towards the opposite wall of the chamber, and there 
 form a coloured image of the sun. . . . This image 
 was oblong and not oval, but terminated with two 
 rectilinear and parallel sides and two semicircular 
 ends. On its sides it was bounded pretty distinctly, 
 but on its ends very confusedly and indistinctly, the 
 light there decaying and vanishing by degrees. The 
 breadth of this image answered to the sun's dia- 
 meter, and was about two inches and the eighth 
 part of an inch, including the penumbra. For the 
 image was eighteen feet and a half distant from 
 the prism ; and at this distance that breadth, if 
 
 1 Newton's "Opticks," Book i. 
 
42 A SHORT HISTORY OF THE 
 
 diminished by the diameter of the hole in window- 
 shut, that is by a quarter of an inch, subtended 
 an angle at the prism of about half a degree, 
 which is the sun's apparent diameter. But the 
 length of the image was about ten inches and a 
 quarter, and the length of the rectilinear sides about 
 eight inches, and the refracting angle of the prism 
 whereby so great a length was made was 6 4. With 
 a less angle the length of the image was less, the 
 breadth remaining the same. . . . Now, the different 
 magnitude of the hole in the window- shut and dif- 
 ferent thickness of the prism where the rays passed 
 through it, and different inclinations of the prism to 
 the horizon, made no sensible changes in the length 
 of the image. . . . 
 
 " This image or spectrum PT was coloured, being 
 red at its least refracted end, T, and violet at its 
 most refracted end, P, and yellow, green, and blue in 
 the intermediate spaces, which agrees with the first 
 proposition, that lights which differ in colour do 
 also differ in refrangibility." 
 
 Newton further proved that " whiteness, and all 
 grey colours between white and black, may be 
 compounded of colours," and that " all homogeneal 
 light has its proper colour answering to its degree 
 of refrangibility, and that colour cannot be changed 
 by reflections or refractions." 
 
 This represents the extent of knowledge regard- 
 ing the nature of sunlight which remained for 
 upwards of a century. What Newton saw in the 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 43 
 
 spectrum upon the wall was a series of images of 
 the sun so close together that they overlapped at 
 their edges, forming a continuous band, having, as 
 he says, parallel sides and circular ends. But the 
 several rays which in sunlight are blended so as to 
 give to the eye the sense of whiteness are separated 
 in passing through the prism, so that the images 
 overlapping give to the eye the impression of colour, 
 those at the least refracted end being pure red, and 
 those at the most refracted end being pure violet, 
 while the intermediate spaces are filled by imper- 
 fectly separated rays of different degrees of refrangi- 
 bility. If, however, a very narrow slit is used for 
 the admission of the light, and a lens is interposed 
 so as to throw a clear image of the slit upon the 
 first face of the prism, which must be placed with 
 its edges parallel to the slit, a new phenomenon 
 may be observed when sunlight is used, and that 
 is the appearance of fine black lines crossing the 
 spectrum. These were first seen by Wollaston in 
 1802, 1 but were studied and mapped in 1814 by a 
 German optician at Munich, Fraunhofer, and are 
 generally known as Fraunhofer's lines. The physi- 
 cal cause of these black lines will be explained a 
 little later ; they are seen in the spectrum of sun- 
 
 1 Wollaston (Phil. Trans. 1802) used for admission of the light a 
 crevice ^ inch broad. The beam was received by the eye close to 
 the interposed prism, and four colours only were perceived, namely, 
 red, yellowish green, blue, and violet. Wollaston seems to have 
 regarded the few dark lines he saw as simply boundaries of these 
 regions of colour. 
 
44 A SHORT HISTORY OF THE 
 
 light and of reflected sunlight, such as that which 
 reaches us from the moon and the planets, but are 
 not seen in the light derived from a heated solid, 
 such as the lime in an oxy hydrogen lamp, or the 
 carbon of an electric lamp. It has, however, long 
 been known that flame may be coloured by putting 
 into it various metals, salts, and other vaporisable 
 substances, and the light thus obtained, when seen 
 through a prism, gives separate bright lines standing 
 in the order of their refrangibilities, but separated 
 by dark spaces. Some of these were described by 
 Sir John Herschel in 1822, and again by Professor 
 W. A. Miller in 1845. It was not until 1859, how- 
 ever, that the position and colour of the lines seen 
 in the spectra of metallic salts vaporised in a flame 
 were employed systematically for the recognition of 
 such substances, and the foundation of the spectro- 
 scopic method of analysis was laid by Bunsen and 
 KirchhofF. The apparatus employed is in principle 
 very simple. The light to be examined passes 
 throuah a narrow slit, the edges of which are 
 parallel with the edges of the prism. To concen- 
 trate the light after entering the slit, it passes 
 through a tube containing a pair of lenses by which 
 the rays are made parallel before entering the prism. 
 On leaving the prism the spectrum is seen through 
 a telescope, which gives a magnified image of it. 
 The arrangement commonly adopted is shown in 
 the figures which are to be found in nearly all text- 
 books of physics. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 45 
 
 Now, when the flame of a Bunsen lamp is placed 
 before the slit, and a platinum wire dipped into a 
 solution of, say, common salt, is introduced into the 
 flame, a bright yellow light is seen, and looking into 
 the telescope a bright yellow line is seen, and 
 nothing else. 1 If for a sodium compound we sub- 
 stitute a salt of potassium, then a red line is seen 
 near the less refrangible end ; if a lithium com- 
 pound, a red line is seen nearer to the yellow than 
 the red potassium line, and also a yellow line, which 
 is not far from the sodium line, but not coincident 
 with it. In like manner other metals give colora- 
 tions to flame, or their compounds give colorations, 
 which in the spectroscope are resolved into bright 
 lines separated usually by dark spaces, each line hav- 
 ing an invariable position relatively to the others. 
 
 The method of spectrum analysis is distinguished 
 by its extraordinary delicacy from the ordinary 
 chemical methods, which are chiefly based upon the 
 production of colours or precipitates in liquids. 
 Bunsen and Kirchhoff in their Memoir 2 show that, 
 for example, in the case of sodium the eye can 
 recognise the presence of one three-millionth of a 
 milligramme of the metal. Hence the common pro- 
 duction of the yellow light in the Bunsen flame, 
 when a platinum wire, apparently clean, is held in 
 it, or when the air is but slightly agitated, so as to 
 
 1 This really consists of two yellow lines so close together that the 
 simple spectroscope is usually incompetent to show them separate. 
 
 2 Translated into several English journals; e.g. Phil. Mag., vol. 
 xx. (1860), and Jour. Chem. Soc., xiii. (1861), 270. 
 
 ' 
 
 SITY 
 
46 A SHORT HISTORY OF THE 
 
 raise a little common dust. Sodium, in the form 
 chiefly of common salt, is to be found in minute 
 quantity distributed in the atmosphere everywhere ; 
 it is present in all common water, and in nearly all 
 animal and vegetable substances, and the frequent 
 appearance of the yellow light, and the correspond- 
 ing yellow double line, was a source of much per- 
 plexity to the earlier observers, who, finding nothing 
 else to account for it, attributed it not unnaturally 
 to the presence of water. It was not till 1856 that 
 Professor Swan of St. Andrews recognised in sodium 
 the cause of the yellow line. 
 
 Thus far reference has been made only to the 
 effect of introducing into a flame substances which 
 are capable of being converted into vapour by the 
 heat, and so carried up in that state into the upper 
 part of the flame, where the vapour becomes incan- 
 descent and gives out light. In some cases, how- 
 ever, the temperature of a common flame is not 
 sufficiently high to give this effect. For example, 
 many metals are not vaporised by such a source of 
 heat, and it is necessary to resort to the much hotter 
 electric " arc," or to the electric spark obtained from 
 an induction coil, in order to produce vapour from 
 them, and cause this vapour to emit light. The in- 
 duction coil is specially serviceable in such cases, for 
 sparks may be taken between the poles tipped with 
 the metal under examination, or may be made to 
 pass between one pole and a solution of the sub- 
 stance to be tested without any appreciable loss of 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 47 
 
 material. It must, however, be remembered that 
 the spectra observed under these circumstances are 
 not identical with those which would be obtained 
 from the same element at the lower temperature 
 of a flame ; the spectrum afforded by a given ele- 
 mentary substance in the arc or spark is in almost 
 all cases more complex, that is, it exhibits a greater 
 number of lines than when a flame is used. The 
 spectrum observable when a flame is coloured by 
 the introduction of a salt is, in many cases, made up 
 of comparatively broad bands, and these disappear 
 when the temperature is raised, being replaced by 
 bright narrower lines in different positions, not co- 
 incident with those of the bands. These lines usually 
 remain unchanged at still more elevated tempera- 
 tures, but other additional lines frequently make 
 their appearance. The spectrum derived from the 
 flame is, in such cases, usually attributed to the 
 glowing vapour of a compound, while the line 
 spectrum obtained by the use of the arc or the 
 spark is supposed to be that of the metal present. 
 These are quite distinct from each other, and at 
 present there is no recognisable relation of the 
 spectrum of a metallic element to that of its com- 
 pounds, such as the oxide or chloride. 
 
 As to the non-metallic elements, and especially 
 the gases hydrogen, oxygen, nitrogen, the light 
 which they give out at high temperatures is less 
 intense than that emitted by metallic vapours ; and 
 they are usually observed most conveniently by 
 
48 A SHORT HISTORY OF THE 
 
 allowing an electric discharge to pass through the 
 gas confined in a glass tube, and expanded by 
 means of an air-pump till the pressure of the gas 
 is reduced to something very small. The dis- 
 charge under such conditions passes through a much 
 longer column of the gas, which becomes incandes- 
 cent throughout. As with the metals, the character 
 of the spectrum varies according to conditions : at 
 comparatively low temperatures bands of light or 
 closely grouped lines are seen ; at higher tempera- 
 tures these change in position, ultimately disappear- 
 ing as the temperature is raised, and giving place 
 to separate fine lines, the relative intensities of 
 which, however, change with altered conditions in a 
 manner which is often very difficult to explain. It 
 seems established that the number and positions of 
 the lines forming a given spectrum are dependent 
 partly upon the composition or molecular constitu- 
 tion of the substance employed, partly upon the 
 quantity or density of its vapour, and partly upon 
 the temperature to which it is exposed. 
 
 These variations, however, perplexing as they are 
 to the inexperienced observer, do not prevent the 
 application of the spectroscope to the recognition of 
 a great many elements, and, as already stated, have 
 led to the discovery of several. The presence of 
 several elements together does not interfere under 
 ordinary circumstances with the exhibition by each 
 of its own special array of lines, and hence complex 
 mixtures, of minerals for example, may be submitted 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 49 
 
 to examination by the spectroscope, with the cer- 
 tainty that those constituents which are capable of 
 yielding vapour will be recognisable, notwithstand- 
 ing that they are present in only minute quantity. 
 Naturally, however, when heat is applied, the more 
 volatile constituents will pass off in vapour first, and 
 will therefore afford their spectra more readily than 
 the less volatile. 
 
 Reference has already been made to the fact that 
 sunlight differs from the light emitted by heated 
 solid bodies, inasmuch as the band of colour is not 
 continuous from end to end, but exhibits a large 
 number of fine black lines crossing it transversely, 
 which are known as Fraunhofer's lines. Notwith- 
 standing that Fraunhofer counted and mapped some 
 hundreds of these lines, he did not give any ex- 
 planation of their occurrence, and it was only many 
 years later that the solution of the mystery was 
 supplied by Kirchhoff. When a vapour is heated 
 strongly enough it gives out light which consists of 
 rays possessing definite refrangibility, and capable of 
 exciting the sensation of colour in the eye, as already 
 stated ; at lower temperatures, however, the vapour 
 is capable of stopping the same radiations. Hence, 
 if a sufficiently thick layer of such vapour is inter- 
 posed in the path of a ray of light from a source 
 which supplies a continuous spectrum, a series of 
 black lines would appear in the same position as 
 the bright lines which would be seen if the vapour 
 itself gave out light and this was viewed through a 
 
 D 
 
50 A SHORT HISTORY OF THE 
 
 prism. The sun is supposed to be a very hot, solid, 
 or fluid body, the light from which, if uninterrupted, 
 would afford a continuous spectrum, like that given 
 by a heated solid metal, or by heated lime. But this 
 luminescent nucleus is surrounded by an atmosphere 
 of vapours less hot, and therefore capable of stop- 
 ping certain of the radiations ; and so black lines 
 appear in the spectrum of the sun's light, and these 
 correspond in position to the bright lines given by 
 such metals as iron, calcium, sodium, and other 
 terrestrial elements when their vapours are heated 
 to incandescence. Similar observations have led to 
 the belief that a large number of elements are 
 common to the earth, and to the sun, and many of 
 the stars. 
 
 With regard to the recognition of elemental bodies, 
 however, a distinction must be drawn between the 
 recognition of wholly unknown and unsuspected new 
 substances, and the discovery of methods for isolat- 
 ing from known compounds elements the existence 
 of which is already well established. As already 
 stated, potash and soda were distinguished from 
 each other , many years before their compound nature 
 was demonstrated, and potassium and sodium ob- 
 tained in the metallic state ; alumina and silica were 
 familiar long before the elements aluminium and 
 silicon were separated from their associated oxygen. 
 One of the most interesting cases of this kind is 
 afforded by the non-metallic element fluorine. Fluo- 
 ride of calcium is widely diffused in nature. It occurs 
 
PKOGRESS OF SCIENTIFIC CHEMISTRY 51 
 
 in many minerals, and, in small quantity, as a con- 
 stituent of the tissues of plants and animals. It is 
 well known in the crystalline form as the beautiful 
 fluor or Derbyshire spar. The action of sulphuric 
 acid upon this substance was studied by Scheele in 
 the middle of the last century, and the acid so pro- 
 duced, long employed for etching glass, has been 
 recognised since the time of Davy as a compound of 
 the same nature as muriatic acid ; that is to say, 
 as constituted of hydrogen associated with an ele- 
 ment having properties similar to those of chlorine. 
 Nevertheless the separation of this element, long 
 called fluorine, from the compounds in which it is 
 known to reside has only been accomplished after a 
 long series of fruitless attempts. Fluorine in the 
 so-called free state is the most energetic chemical 
 agent known. It decomposes water in consequence 
 of its affinity for hydrogen, and the displaced 
 oxygen is partly converted into ozone. It combines 
 with metals of all kinds, and the heat generated by 
 its union with silicon, with sulphur, with iodine, and 
 even with carbon is so great as to cause ignition of 
 those substances. It is not displaced from its com- 
 pounds by the action of any other known element, 
 and the statements concerning its liberation from 
 mercuric or silver fluorides by the action of chlorine 
 were erroneously based upon imperfect experiments. 
 It is obtainable, though with difficulty, by heating 
 certain fluorides, notably eerie fluoride, CeF 4 , and 
 plumbic fluoride, PbF 4 (Brauner), which thus be- 
 
52 A SHOKT HISTORY OF THE 
 
 come reduced to lower fluorides. But the know- 
 ledge of this remarkable substance would have 
 remained extremely imperfect but for Moissan's ex- 
 periments in 1886 on the electrolysis of anhydrous 
 hydrogen fluoride. This liquid is not an electro- 
 lyte, but on the addition of dry potassium hydrogen 
 fluoride it conducts, and the salt is resolved into fluo- 
 rine and potassium. The latter liberates hydrogen 
 which escapes from the surface of the cathode, 
 while the former is set free at the anode in the 
 form of a pale greenish gas which possesses all the 
 chemical activity of chlorine in an exalted degree. 
 It is liquefiable at about 190, and its boiling point 
 under atmospheric pressure is very close to 187 C. 
 It forms at this temperature a pale yellow liquid, 
 which no longer exhibits the energetic chemical 
 properties displayed by the gas, for at this low 
 temperature it does not even displace iodine from 
 iodides, though it retains the power of seizing the 
 hydrogen in benzene or oil of turpentine with in- 
 candescence. The powerful affinity of fluorine for 
 hydrogen is the last to disappear. 
 
 We may now turn by way of contrast to the dis- 
 covery in the atmosphere of a new gas, or rather a 
 mixture of gases, the existence of which there had 
 been no reason for suspecting, and the strange 
 characters of which could never have been pre- 
 dicted from any consideration within the range of 
 recognised chemical philosophy. The history of the 
 discovery of " argon " is one of the most interesting 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 53 
 
 and instructive chapters in the records of natural 
 science. For some time previously to 1893 Lord 
 Rayleigb had been making determinations of the 
 densities of the principal gases, 1 nitrogen among 
 the rest, and his attention was early attracted to 
 a curious anomaly observed in the case of this 
 element. When the gas was made from ammonia 
 it was found to be decidedly lighter than when ob- 
 tained from air, and as it seemed " certain that the 
 abnormal lightness cannot be explained by contami- 
 nation with hydrogen, or with ammonia, or with 
 water," everything seemed to suggest " that the 
 explanation is to be sought in a dissociated state 
 of the nitrogen itself." And by successive experi- 
 ments it was shown that whether the oxygen of air 
 was removed by red-hot copper, by red-hot iron, or 
 by cold ferrous hydrate, the superior density of 
 atmospheric nitrogen remained undiminished, while 
 the density of nitrogen obtained by various chemical 
 processes from nitrous oxide and from nitric oxide 
 was the same as that from ammonia. The mean 
 weights of nitrogen held by a certain globe were as 
 follows : 
 
 From nitric oxide . ' . . 
 
 . 2-3001 '] 
 
 1. 
 
 
 From nitrous oxide . . 
 
 . 2-2990 
 
 >Mean 
 
 2-2993 
 
 From ammonium nitrite . 
 
 . 2-2987 ] 
 
 [ : 
 
 
 From air by hot copper 
 
 . 2-3103 ] 
 
 Mean 
 
 9-3102 
 
 From air by hot iron 
 
 . 2-3100 
 
 > Different -0109 
 
 From air by ferrous hydrate 
 
 . 2-3102 j 
 
 
 
 1 Proc. R. S., liii. 134. 
 
54 A SHORT HISTORY OF THE 
 
 Hence, after due corrections, one litre of chemical 
 nitrogen weighs 1'2 5 05 gram; atmospheric nitrogen 
 weighs 1*2572 gram. A review of all these facts 
 led to the conclusion that the lightness of chemical 
 nitrogen was not to be attributed to the presence 
 of any familiar impurity, or to the existence of 
 two forms of nitrogen, but rather that the greater 
 density of atmospheric nitrogen was due to its 
 association with a heavier gas existing in the air 
 in small quantity, and hitherto unrecognised. The 
 question as to the homogeneousness of the gaseous 
 residue left when the oxygen of air, the moisture, 
 and the carbon dioxide have all been withdrawn, 
 was long ago considered by Cavendish in his 
 " Experiments on Air," published in the Philosophical 
 Transactions for 1785. Here he says, "As far as the 
 experiments hitherto published extend, we scarcely 
 know more of the nature of the phlogisticated * part 
 of our atmosphere, than that it is not diminished by 
 lime-water, caustic alkalis, or nitrous air ; that it is 
 unfit to support fire or maintain life in animals; 
 and that its specific gravity is not much less than 
 that of common air ; so that though the nitrous acid, 
 by being united to phlogiston, 2 is converted into air 
 possessed of these properties, and, consequently, 
 though it was reasonable to suppose that part at 
 least of the phlogisticated air of the atmosphere 
 
 1 Phlogisticated air is the term, in the language of the theory of 
 phlogiston, for nitrogen ; dephlogisticated air is oxygen. 
 
 2 i.e. deprived of oxygen. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 55 
 
 consists of this acid united to phlogiston, yet it 
 might fairly be doubted whether the whole is of 
 this kind, or whether there are not in reality many 
 different substances compounded together by us 
 under the name of phlogisticated air. I therefore 
 made an experiment to determine whether the whole 
 of a given portion of the phlogisticated air of the 
 atmosphere could be reduced to nitrous acid, or 
 whether there was not a part of a different nature 
 from the rest which would refuse to undergo that 
 change." Cavendish then proceeds to describe his 
 experiment, from which he concludes that " if there 
 is any part of the phlogisticated air of our atmo- 
 sphere which differs from the rest, and cannot be 
 reduced to nitrous acid, we may safely conclude 
 that it is not more than T ^ part of the whole." 
 There the question was left more than a century 
 ago. In August 1894 it was answered by the 
 announcement by Lord Rayleigh and Professor 1 
 Ramsay of the discovery of a new constituent of 
 the atmosphere, a gas having a density nearly 
 half as large again as that of nitrogen, and distin- 
 guished from all then known gases by absolute 
 chemical inertness, being, so far as at present known, 
 incapable of entering into any form of chemical 
 combination. 
 
 Two methods have been employed by the dis- 
 coverers for the removal of the nitrogen and the 
 isolation of the argon. The first was the method 
 used by Cavendish, though with the advantage of 
 
56 A SHOKT HISTORY OF THE 
 
 modern appliances. This consists in adding oxygen 
 to the air confined over a solution of caustic potash, 
 and then passing electric sparks through the gaseous 
 mixture. The nitrogen is thus made to unite with 
 oxygen, and the resulting oxide of nitrogen is ab- 
 sorbed by the potash, and is converted into nitrite 
 and nitrate. At the end of the experiment the 
 residual oxygen is easily removable by red-hot 
 copper or otherwise. The other process consists in 
 first absorbing the oxygen from the air operated 
 on by means of red-hot copper, and then getting 
 rid of the nitrogen by passing the gas over the 
 surface of magnesium, or better still, a mixture of 
 lime and metallic magnesium. Either of these 
 agents absorbs nitrogen, forming solid magnesium 
 or calcium nitride, leaving the argon as a colour- 
 
 The discovery of argon among the atmospheric 
 gases naturally led to a search for a more produc- 
 tive source of the element, and the attention of 
 Professor Ramsay was drawn to the statement that 
 certain minerals containing uranium evolve under 
 the action of dilute sulphuric acid a gas which 
 was supposed to be nitrogen. On submitting some 
 of the gas thus obtained to the process of spark- 
 ing in admixture with oxygen very little contrac- 
 tion occurred, and it was manifest that the amount 
 of nitrogen present was insignificant. On exami- 
 nation of the light afforded by the expanded gas 
 exposed to the spark discharge, it showed at once a 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 57 
 
 feature which gave a clue to the character of the new 
 substance. In addition to lines due to hydrogen and 
 argon, present in the gas, a brilliant yellow line was 
 observed, nearly, but not quite coincident with the 
 yellow line D X of sodium. The wave-length of this 
 line is 587*49 millionths of a millimetre, and it is 
 exactly coincident with the line D 3 in the solar 
 chromosphere attributed to the solar element, which 
 has been named by Lockyer helium. The complete 
 spectrum is characterised by five very brilliant lines 
 in the red, yellow, blue-green, blue, and violet respec- 
 tively. The gas is chemically inert like argon, and, 
 like that element, in molecular constitution it appears 
 to be monatomic. The difficulty of complete separa- 
 tion from argon renders the determinations which 
 have been made of its density somewhat uncertain, 
 but helium is undoubtedly, next to hydrogen, the 
 lightest gas known, and its specific gravity has been 
 observed as somewhat less than 2. Whether it will 
 be further reduced is a matter for future experi- 
 ment. 
 
 This remarkable history, however, does not end 
 here, for early in June 1898 Professor Ramsay and 
 Mr. M. W. Travers communicated to the Royal 
 Society an account of their examination of liquid 
 air, in which they announced the discovery of a new 
 constituent, to which they have given the name 
 krypton (hidden). Ten days later, in a further 
 paper, they described two other gases, named respec- 
 tively r) con (new) and metaryon, among the " Com- 
 
58 A SHOUT HISTOKY OF THE 
 
 panions of Argon." The method employed consisted 
 in liquefying a large amount nearly 18 litres 
 of " argon," obtained from atmospheric air by 
 absorbing the oxygen by red-hot copper, and the 
 nitrogen by magnesium. The argon in the liquid 
 state was accompanied by a considerable quan- 
 tity of a solid substance. When the temperature 
 was allowed to rise, the liquid evaporated away, 
 the first portions of gas being collected separately, 
 as likely to contain any substance lighter than 
 argon. The old argon then boiled off, and was 
 collected in a gas-holder, while the solid, of which 
 mention has been made, evaporated last, and was 
 collected by itself. The lightest and most volatile 
 ingredient of this mixture, called neon, is a gas 
 whose density is believed to be about 11, while 
 argon has a density approaching 20. The solid 
 matter gave a gas having a density nearly identical 
 with that of argon, and since it has been found 
 to be, like argon, both monatomic and chemically 
 inert, though exhibiting a different spectrum, it has 
 been named met argon. The gases thus isolated 
 from air seem to agree in the common character- 
 istic of chemical inactivity, or inability to form 
 compounds. They are believed to be monatomic, 
 that is, that, like mercury vapour, their molecules 
 contain one atom only ; but as their densities have 
 only been estimated very roughly, in consequence 
 of the difficulty of separating them completely from 
 one another, the atomic weights of the most recently 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 59 
 
 discovered are still very doubtful. The values as- 
 signed to them provisionally are as follows : 
 
 Name. Symbol. Atomic ^ i ^ 1 t lecular 
 Helium He 4 
 
 Neon Ne 22 
 
 Argon A 40 
 
 Metargon A/z 40 
 
 Krypton Kr 80 
 
 Quite recently small quantities of hydrogen are 
 said to have been detected in atmospheric air by 
 M. Gautier, so that our atmosphere is a mixture even 
 more complex than had ever been previously sus- 
 pected. The new gases are probably physiologically 
 as well as chemically inert ; but whether there may 
 not be still lurking, undetected in the vast ocean of 
 gas at the bottom of which all human affairs are 
 transacted, some small quantity of hitherto unre- 
 cognised but physiologically active ingredients, is 
 a question to which for the present there is no 
 answer. 
 
60 A SHORT HISTORY OF THE 
 
 CHAPTER III 
 
 RECTIFICATION AND STANDARDISATION OF 
 ATOMIC WEIGHTS 
 
 THE necessity for preserving the distinction between 
 fact established by observation or experiment, and 
 hypothesis which suggests an explanation of the facts, 
 has not always been clearly recognised in chemistry. 
 We know, for example, that oxygen and hydrogen 
 will combine together in certain proportions, and in 
 no others. This is explained by the assumption that 
 atoms of oxygen unite with atoms of hydrogen to form 
 compounds, and that the atoms of oxygen are all of 
 equal mass, and are each nearly sixteen times heavier 
 than an atom of hydrogen. It is therefore impos- 
 sible that there can be compounds made up of com- 
 plex proportions of these two elements, unless we 
 assume that which is very improbable, namely, that 
 the atoms combine in large numbers and uneven pro- 
 portions, say, for example, thirty of one kind to thirty- 
 one of the other. Dalton was the first to apply the 
 " atomic theory " to chemistry, and his ideas re- 
 specting chemical combination were expressed some- 
 what in the following manner: l "When two elements 
 
 1 Thomson's "System of Chemistry," 3rd edition, vol. iii. 1807. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 
 
 combine to form a third substance, it is to be 
 sumed that one atom of one joins to one atom of the 
 other, unless when some reason can be assigned for 
 supposing the contrary. Thus oxygen and hydrogen 
 unite together and form water. We are to presume 
 that an atom of water is formed by the combination 
 of one atom of oxygen with one atom of hydrogen. 
 In like manner one atom of ammonia is formed by 
 the combination of one atom of azote with one atom 
 of hydrogen." It is obvious that if such hypothetical 
 ideas are superimposed upon the acknowledged facts 
 as to the composition of water, an artificial rule is 
 established for estimating the relative atomic weights; 
 and in Dalton's time, and chiefly as the outcome of 
 his experiments, the values attributed to the atomic 
 weights of the three elements referred to above were 
 actually based on this combination of ideas. Dalton, 
 however, and all his successors, were obliged to admit 
 that this simple hypothesis is not applicable to all 
 cases, and is manifestly often opposed to well-estab- 
 lished facts. The composition of water, for example, 
 was represented by nearly all chemists during the 
 former half of this century by the formula HO, in 
 which H stands for 1 part by weight of hydro- 
 gen, and for 8 parts by weight of oxygep. The 
 change which has resulted in the universal adoption 
 of the formula H 2 0, in which is approximately 
 twice 8, was the result of a protracted contro- 
 versy beginning from the time of Dalton himself. 
 Gay-Lussac's celebrated " Memoir on the combina- 
 
62 A SHORT HISTOKY OF THE 
 
 tion of Gaseous Substances with each other," 1 was 
 published in 1809, and the Essay 1 by Avogadro, 
 on " A Manner of determining the relative Masses of 
 the Elementary Molecules of Bodies, and the Pro- 
 portions in which they enter into these Compounds," 
 in 1811. But while Gay-Lussac's facts have always 
 been admitted, except for a time by Dalton, the 
 hypothesis of Avogadro has been generally adopted 
 only within the last thirty years. 
 
 The nature of the problem will be shown most 
 clearly if we consider closely a single example, that 
 of oxygen. Nearly eight parts of oxygen unite with 
 one part of hydrogen to form water, but the question 
 is, whether in the smallest existing particle, that is 
 the molecule, of water there is but one atom of 
 oxygen with one atom of hydrogen, as Dalton 
 supposed, or whether water may not contain more 
 than one atom of either or both these elements ; 
 and whether, taking the atomic weight of hydro- 
 gen as the unit, that of oxygen should be 8 or 
 some multiple of 8 ? The question began to be 
 seriously discussed when, in 1843, Gerhardt pro- 
 posed to use the hypothesis of Avogadro as the basis 
 for a common measure of molecules. If it be true, 
 as Avogadro taught, that " equal volumes of different 
 gases at the same temperature and pressure contain 
 the same number of molecules," then those quanti- 
 ties of all substances which fill the same volume in 
 
 1 English translations of both are included in No. 4 of the 
 " Alembic Club Reprints." 
 
PKOGKESS OF SCIENTIFIC CHEMISTRY 63 
 
 the state of gas must be taken as molecular propor- 
 tions under the same conditions. 
 
 Previously to this time there had been no rule 
 commonly recognised and applied to this purpose. 
 Thus, if the symbol H stand for one volume of 
 hydrogen gas, HO (0 == 8) represents also one volume 
 of water vapour, and HC1 represents two volumes of 
 hydrogen chloride gas. Or if HO stand for two 
 volumes, then H must also stand for two volumes, 
 and HC1 for four volumes. Gerhardt proposed to 
 take water as the standard of comparison; that is, 
 as the unit of molecular magnitude. He repre- 
 sented the molecule of water by the formula H 2 
 (H=l and = 16), reviving the relative value of 
 the atomic weight attributed to oxygen by Berzelius, 
 though now reduced to the scale in which H is 
 taken as 1, instead of as 100. Accordingly the 
 formulae H 2 0, HC1, NH 3 , C0 2 represent equal volumes 
 of, the several compounds in the gaseous state, and 
 under the same conditions of temperature and pres- 
 sure. And it is necessary to observe that these for- 
 mulae do not only agree in regard to the hypothetical, 
 physical constitution of the gases which they repre- 
 sent ; that is, they comply with the hypothesis of 
 Avogadro, but they actually represent the chemically 
 reactive units or molecules of these substances. But 
 these formulas imply that the atomic weights of 
 oxygen and carbon must be assumed to be double 
 of those commonly adopted by chemistry at the 
 time, and require a corresponding change in the 
 
64 A SHORT HISTORY OF THE 
 
 formulae of all the oxides, acids, bases, and other 
 compounds in which these elements exist. Great 
 support for these views was derived from the suc- 
 cessive discoveries of the constitution of ether by 
 Williamson (1850), and of many acid anhydrides 
 by Gerhardt himself. 1 For so long as ether was 
 regarded as the oxide of ethyl, C 4 H 5 0, while alcohol 
 represented a compound of this oxide with water, 
 namely, as hydrated oxide of ethyl, C 4 H 5 O + HO 
 (0=6, 0=8), the relation was not perceptible. 
 But when Williamson showed that the conversion 
 of alcohol into ether is accomplished, not by the 
 simple withdrawal of water from alcohol, but as a 
 consequence of the exchange of an atom of hydrogen 
 in alcohol for another ethyl group, C 4 H 5 , the result- 
 ing compound forming a volume of vapour equal to 
 the standard volume, namely, to the volume occu- 
 pied by a molecular proportion of water, H 2 0, a 
 new view of the constitution of alcohol followed 
 as a necessary consequence. The following com- 
 parison will show clearly the nature of the change 
 involved : 
 
 
 OLD STYLE. 
 
 NEW STYLE. 
 
 Formula : 
 
 Corresponding 
 Volume 
 
 Formula : 
 
 Corresponding 
 Volume 
 
 [0 = 8, C=6] 
 
 of Vapour. 
 
 [0-16, C = 12] 
 
 of Vapour. 
 
 HO 
 
 1 vol. 
 
 H 2 O 
 
 2 vols. 
 
 HC1 
 
 2 vols. 
 
 HC1 
 
 2 
 
 CO 
 
 1 vol. 
 
 CO 
 
 2 
 
 C0 2 
 NH 3 
 C 4 H 5 C 
 
 1 vol. 
 2 vols. 
 t 1 vol. 
 
 C0 2 
 NH 3 
 C 4 H 10 
 
 2 
 
 2 
 2 
 
 C 4 H 6 C 
 
 + HO 2 vols. C 2 H 6 
 
 2 
 
 1 See further on, Chap. V. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 65 
 
 It was, however, long before such views as these 
 received the general assent of the chemical world, 
 and it required the support of evidence drawn 
 from various apparently distinct lines of inquiry, to 
 establish firmly the new doctrine. In order to ascer- 
 tain the volume of vapour which corresponds to a 
 given formula, it is only necessary to determine the 
 specific gravity of the vapour, that is, the weight of 
 one unit volume, hydrogen or air being usually taken 
 as the standard. But though text-books of this period 
 usually gave an account of the methods of Dumas and 
 Gay-Lussac for such experimental determinations, the 
 results were treated as isolated physical facts, and 
 were seldom or never applied to the correction of 
 formulae, and their reduction to a common standard. 
 
 In 1834 Dumas discovered the remarkable fact 
 that chlorine is capable of replacing an equal volume 
 of hydrogen in many organic compounds, the pro- 
 cess being afterwards known as substitution, or, 
 as Dumas called it, metalepsy. Acetic acid, for 
 example, is a monobasic acid, for it affords with 
 each metal, as a rule, only one salt, in the formation 
 of which one equivalent of the acid was known to 
 interact with one equivalent of such a base as potash. 
 But three-fourths of the hydrogen of acetic acid is 
 exchangeable in three successive stages for equiva- 
 lent quantities of chlorine, giving rise to mono-, di-, 
 and trichloracetic acids, thus : 
 
 Acetic acid . . . C 4 H 3 3 HO (0 = 6,0 = 8) 
 
 Monochloracetic acid . C 4 H 2 C1O 3 HO 
 
 Dichloracetic acid . O^CLjOgHO 
 
 Trichloracetic acid . C 4 CLj0 3 HO 
 
66 A SHORT HISTORY OF THE 
 
 These chlorinated acids are monobasic, and bear a 
 strong resemblance to acetic acid, from which they 
 are derived. 1 
 
 Dumas also pointed out, 2 in reply to criticisms 
 from Berzelius, that while chloracetic acid heated 
 with potash splits into carbon dioxide and chloro- 
 form, acetic acid heated with baryta gives carbon 
 dioxide and a gas which he identified with marsh gas, 
 of which chloroform is the trichloro-derivative, and 
 therefore an analogue. And taking the principle of 
 isomorphism as a guide, he declared that compounds 
 like acetic acid and chloracetic acid belong to the 
 same chemical type, just as all the different varieties 
 of alum belong to the same crystallographic or 
 mechanical type. A little later he studied the 
 action of chlorine upon marsh gas, and though he 
 did not succeed in isolating all the successive pro- 
 ducts of the substitute of chlorine for hydrogen in 
 this compound, he obtained and analysed the per- 
 chloride, which is the final product of the action, 
 and showed that the production of this compound 
 and chloroform represented two stages in the same 
 process. The complete series of substitution pro- 
 ducts would be expressed by the following names 
 and formulae : 
 
 Methane or marsh gas CHHHH. 
 
 Chloromethane or methyl chloride . . . CHHHC1. 
 Dichloromethane or methylene dichloride CHHC1C1. 
 Trichloromethane or chloroform .... CHC1C1C1. 
 Tetrachloromethane or carbon tetrachloride CC1C1C1C1. 
 
 1 Compt. Rend., vii. 474 (1838). 
 2 Annalen d. Chem. u. Pharm., xxxiii. 179, 259 (1840). 
 
OF THK >' 
 
 UNIVERSIT' 
 PROGRESS OF SCIENTIFIC CHEMISTBtOyt 67 
 
 These successive steps have since that time been 
 completely traced, and each of these compounds is 
 now well known and characterised. Dumas also, 
 together with Stas, investigated the action of potash 
 upon a number of alcohols and their principal ethers, 
 and showed l that all true alcohols can produce a 
 corresponding acid. Thus he was led to a classifica- 
 tion of carbon compounds according to the nature 
 of the typical substance, alcohol, ether, acetic acid, 
 aldehyd, &c., from which the compound could be 
 derived, actually or hypothetically, by a process of 
 substitution. 
 
 As will be shown in a later chapter, the molecules 
 of water, ammonia, hydrochloric acid, hydrogen, and 
 marsh gas were afterwards successively adopted as 
 types of the various classes of chemical compounds, 
 and thus a classification was effected of the other- 
 wise miscellaneous products of successive discoveries, 
 especially in the domain of what has so long been 
 called " organic " chemistry. Though it was many 
 years before this doctrine was generally accepted, 
 the important fact came ultimately to be recognised 
 that certain elements are distinguished by the power 
 of holding together two or more atoms of other 
 elements, or of " residues " consisting of several ele- 
 ments united into a group. 2 
 
 In water, for example, and in all the immediate 
 
 1 Ann. Chim. Phys. [2], Ixxiii. 73 (1840). 
 
 2 A fuller account of these developments is reserved for a later 
 chapter (Chap. V.). 
 
68 A SHORT HISTORY OF THE 
 
 derivatives of water, such as caustic potash, alcohol, 
 ether, silver oxide, hypochlorous acid, it was per- 
 ceived that while the electro-positive elements, such 
 as hydrogen, potassium, ethyl, &c., could be sepa- 
 rately replaced, and that in water the hydrogen is 
 divisible into two exactly equal parts, the oxygen 
 maintains its indivisibility throughout. This impor- 
 tant distinction is recorded in the formulae of the 
 water type : 
 
 1 K io K fo G -^\o 
 
 Hf U H f U K U H U ' 
 
 AgU Hf 
 
 111 
 
 A g ; u a ; 
 
 0. 
 
 Gerhardt pointed out that the formula H 2 for 
 water is preferable to the formula HO (0 = 8) then 
 in use, because it is consistent with the fact that 
 each monatomic (in modern language, monad or 
 univalent) radicle gives two derivatives of the water 
 type, that is, by successive replacement of the two 
 atoms of hydrogen it forms two oxides, 1 like caustic 
 potash and oxide of potassium, as shown by the 
 above formulae. The same radicle, however, gives 
 only one chloride, one bromide, and one iodide. 
 
 Precisely similar considerations can be applied to 
 compounds containing carbon. Marsh gas is com- 
 posed of this element united with hydrogen, but 
 whereas the latter can be replaced by chlorine, or 
 other agents, in four separate and equal portions, 
 the carbon is not only not so divisible, but it has 
 
 1 Traitt de Chimie Organique, t. iv. 589. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 69 
 
 the power of linking together in one definite and 
 homogeneous compound the chlorine which has been 
 introduced in exchange for a part of the hydrogen, 
 and the residue of hydrogen which is left after such 
 operation. Hence such compounds as CH 3 C1 and 
 CHC1 3 are producible from marsh gas, and are by 
 an inverse operation transformable back again into 
 that compound. These relations are sufficiently in- 
 dicated by the formulae already given (p. 66). 
 
 Further, if we examine the now familiar series 
 of homologous hydrocarbons, alcohols, aldehyds, 
 acids, &c., containing carbon in progressively increas- 
 ing proportion, it becomes obvious that each term 
 of such series differs from the next below and the 
 next above by a quantity of carbon which is never 
 less than 12 parts by weight, the quantity of 
 oxygen, if present, remaining the same throughout 
 the series. The following formulae, for example, 
 represent all the known members of the several 
 series of paraffins, alcohols, and fatty acids : 
 
 Paraffins. 
 
 C X H 4 
 C 2X H 6 
 C 3X H 8 
 
 Alcohols. 
 
 Fatty Acids. 
 
 C X H 4 
 
 C X H 2 2 
 
 C 2X H 6 
 
 C 2X H 4 9 
 
 C 3X H 8 
 
 C 3X H 6 2 
 
 C 4X H 10 
 
 C 4X H 8 2 
 
 C 5X H 12 
 
 C 5X H 10 2 
 
 &c. 
 
 &c. 
 
 &c. 
 
 It is obvious that in such series x must be equal 
 to 1, and the atomic weight of carbon must be 12, 
 and not a smaller number such as 6. For sup- 
 posing C = 6, then each formula would represent 
 
70 A SHORT HISTORY OF THE 
 
 the proportion of carbon present as divisible into 
 two equal parts, for which there is no justification 
 in fact. Moreover, in such a series as the paraffins, 
 there would be on that hypothesis a gas composed 
 of carbon and hydrogen having half the density of 
 marsh gas, and the formula CH 9 . It is almost 
 needless to say that such a gas is not known. A 
 similar argument may be based upon the well-estab- 
 lished fact that in the numerous definite decom- 
 positions in which the oxides of carbon are evolved, 
 none are known in which carbon is eliminated in 
 this form in quantity less than would be expressed 
 by 12 parts by weight. Take formic acid, for ex- 
 ample 
 
 C X H 2 2 -H 2 = C X 
 
 C X H 2 2 + O = C x 2 -fH 2 
 
 If C = 6 these quantities would have to be repre- 
 sented as C 2 and C 2 O 2 respectively, which would 
 be unnecessary and illogical, so long as C 2 is known 
 to represent an indivisible quantity. 
 
 Throughout this long discussion, extending over 
 twenty years or more, two chief considerations were 
 gradually brought to one common focus; the one 
 based upon the hypothesis of Avogadro provides a 
 uniform measure of molecular magnitudes, the other 
 indicates the limited combining capacity of each 
 elementary atom, and of each group of atoms form- 
 ing a radicle. If to this is superadded the strictly 
 chemical process which consists in ascertaining by 
 experiment whether the quantity of any given ele- 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 71 
 
 ment, found in a series of molecules of which it is 
 a common ingredient, is divisible into several equal 
 parts, or is not so divisible, we arrive at the con- 
 clusion that the indivisible or atomic proportions 
 of each element can be determined. And the end 
 of it is, that taking one part by weight of hydrogen 
 as the unit for the scale of atomic weights, and find- 
 ing that not less than 16 such parts by weight of 
 oxygen ever enter into or leave a molecule of a com- 
 pound, 16 must be regarded as the atomic weight 
 of this element. By a similar course of reasoning 
 applied to the compounds of carbon, silicon, sulphur 
 and its allies, it is now agreed that the value 
 assigned to the atomic weights of each of these 
 elements must be, as in the case of oxygen, double 
 the value previously assumed, so that henceforth C 
 stands for 12 parts of carbon, S for 32 parts of 
 sulphur, and so forth. 
 
 The new atomic weights and the simultaneous 
 changes in the system of chemical formulae were, 
 however, not generally adopted in text-books or in 
 scientific memoirs till long after 1860. The new 
 system was employed for the first time by Hofmann 
 in his lectures at the Royal College of Chemistry in 
 the year 1861, and this example doubtless assisted 
 greatly to promote the recognition of the new doc- 
 trine in England. In 1864 Dr. Odling, as President 
 of the Chemical Section of the British Association, 
 was able to congratulate the section upon " the 
 substantial agreement which now prevails among 
 
72 A SHORT HISTOEY OF THE 
 
 English chemists as to the combining proportions of 
 the elementary bodies and the molecular weights of 
 their most important compounds." But in France 
 the formula for water continued very generally to be 
 written HO, or occasionally, with equal impropriety, 
 H 2 2 , down to a period at least five-and-twenty 
 years later. So great, even in science, is the in- 
 fluence of habit and of authority in retarding the 
 modification of long settled ideas. 
 
 The application of the " law " of Avogadro to the 
 settlement of atomic weights is, however, limited 
 by the fact that certain elements, metals, appear 
 to be incapable of producing compounds which are 
 vaporisable without decomposition. Thus silver, 
 gold, platinum, copper, and cobalt form no volatile 
 chloride or other compound of which the vapour 
 density could be ascertained. In such cases recourse 
 must be had to other methods, whereof the most 
 important is the application of the specific heat, in 
 accordance with the discovery of Petit and Dulong 
 published in 1819. These physicists found that 
 when the number expressing the specific heat of a 
 solid element is multiplied by the atomic weight 
 of the same, the numerical value of the product is 
 nearly constant. This is shown in the following 
 table : 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 
 
 73 
 
 COPY OF TABLE BY PETIT AND DuLONG. 1 
 
 
 Specific Heats. 
 
 Atomic Weights 
 (0=1). 
 
 Atomic Weight 
 X Specific Heat. 
 
 Bismuth 
 
 0288 
 
 13-30 
 
 3830 
 
 Lead 
 
 0293 
 
 12-95 
 
 3794 
 
 Gold 
 
 0298 
 
 12-43 
 
 3704 
 
 Platinum 
 
 0314 
 
 11-16 
 
 3740 
 
 Tin . 
 
 0514 
 
 7-35 
 
 3779 
 
 Silver 
 
 0557 
 
 6-75 
 
 3759 
 
 Zinc. 
 
 0927 
 
 4-03 
 
 3736 
 
 Tellurium 
 
 0912 
 
 4-03 
 
 3675 
 
 Copper 
 
 0949 
 
 3-957 
 
 3755 
 
 Nickel 
 
 1035 
 
 3-69 
 
 3819 
 
 Iron . 
 
 1100 
 
 3-392 
 
 3731 
 
 Cobalt 
 
 1498 
 
 2-46 
 
 3685 
 
 Sulphur 
 
 1880 
 
 2-011 
 
 3780 
 
 The statement of the relation indicated in the last 
 column of figures is expressed in the following words 
 of the authors, p. 405: "Les atonies de tous les 
 corps simples ont exactement la inerne capacite pour 
 la chaleur." 
 
 Of course several of the values inserted in this 
 table have since been proved to be exceedingly 
 inaccurate, but more modern researches have estab- 
 lished the general truth of the principle here enun- 
 ciated. The system of atomic weights at present in 
 use is referred to hydrogen as unity, but the scale 
 upon which the atomic weight is calculated makes 
 no difference except in the absolute value of the pro- 
 duct, approximately constant, obtained by multiply- 
 ing together the specific heat and the atomic weight. 
 
 1 Ann. Chim. Phys., 1819, x. 403. 
 
74 A SHOUT HISTORY OF THE 
 
 On the hydrogen scale this product is about 6*4, 
 and the process of settling an atomic weight is very 
 simply based on the equation- 
 
 Atomic Wt. x Spec. Heat = 6 -4, 
 
 from which, if the specific heat is known, the atomic 
 weight can be at once roughly calculated. Since, 
 however, the experimental difficulties attending the 
 determination of the specific heat are greater than 
 those which are involved in the determination of the 
 combining proportion of an element, the numbers 
 expressing specific heats are less exact than those 
 which express combining weight. The atomic 
 weight is either identical with the combining pro- 
 portion, or is some multiple of it. So that, Tn order 
 to fix the atomic weight of a metal, we take that 
 multiple of the equivalent or combining propor- 
 tion which comes nearest to the value of this pro- 
 duct. The specific heat of tin, for example, is 
 0559 (Bunsen), and 29*75 parts of the metal com- 
 bine with an equivalent of chlorine. Then, since 
 
 114 ' 5 ' the at mic 
 
 taken to be 29*75x4, or 119'0, or thereabouts, 
 rather than 29*75 or any smaller multiple of this 
 number. 
 
 It is interesting and important to note that when- 
 ever the two methods, based on the use of the law 
 of Avogadro on the one hand and that of Petit and 
 Dulong on the other, can be applied to the same 
 element the results agree. Thus nickel is known 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 75 
 
 to have an atomic weight which approaches 5 9, for 
 the specific heat is '108 and ; ,^ g =59'2. Also the 
 
 vapour density of its carbonyl compound is 86*5 
 compared with hydrogen. Hence the molecular 
 weight of this compound is 173, and it is found by 
 analysis to contain 33*3 per cent, of nickel; 173 
 parts therefore contain 57*6 parts of nickel, which 
 is in practical agreement with the value derived 
 from the specific heat. But just as the law of 
 Avogadro had to wait nearly fifty years for general 
 recognition, so the principle asserted by Petit and 
 Dulong remained unapplied and almost unnoticed, 
 save casually as a matter of curiosity, down to the 
 present generation. It is true that Regnault, as a 
 result of his researches, commenced in 1840, 1 was 
 led to regard the law as universally applicable, but 
 Kopp, who resumed the question a quarter of a cen- 
 tury later, came to the conclusion 2 that the law of 
 Dulong and Petit is not strictly valid, even when the 
 exceptional cases of boron, carbon, and silicon are ex- 
 cluded. The want of exact concordance among the pro- 
 ducts of the multiplication of specific heat by atomic 
 weight does not, however, prevent the very general 
 application of the law for the purpose of controlling 
 atomic weights in the manner already described. 
 And chemistry is indebted chiefly to the representa- 
 tions of Cannizzaro 3 in 1858 for the recognition of 
 this most important use of the observed relations. 
 
 1 Especially Ann. Oh. Phys. [2], Ixxiii. 66, and [3] xxvi. 261, 
 and xlvi. 257. 
 
 2 Phil. Trans., 1865. 3 11 Nuovo Cimento, vii. 321. 
 
76 A SHORT HISTORY OF THE 
 
 The system of atomic weights most generally 
 adopted at the present day takes hydrogen as unity, 
 though unfortunately there is not a universal agree- 
 ment as to this matter, for a considerable number 
 of chemists prefer to use the round number 16 for 
 oxygen, on the ground that if this number is used 
 instead of the somewhat smaller value which more 
 exactly represents the atomic weight of oxygen when 
 hydrogen is taken as the unit, the atomic weights of 
 many of the more common of the elements may 
 also be represented by whole numbers without appre- 
 ciable error. Thus, if is 16, we have As =7 5, 
 Br=80, Ca = 40, 0=12, F = 19 r Fe=56, 1=127, 
 Hg = 200, N=14, P = 31, Na = 23, S = 32, 
 Sn=119, very approximately. It must not be 
 forgotten, however, that if 0= 16, the value TO 08 
 must be assigned for all exact purposes to hydrogen. 
 Quite recently a committee of the German Chemical 
 Society has drawn up a table of atomic weights in 
 accordance with the best available evidence, in which 
 they recommend the use of the number 16 for 
 oxygen, and 1*01 for hydrogen, as sufficiently near 
 for all practical purposes. 
 
 Whether the one scale or the other is used, how- 
 ever, is a matter of small importance in comparison 
 with the immense advantages which have accrued 
 from a general agreement as to the methods by 
 which the atomic weights may be calculated from 
 the chemical equivalents. As a consequence of this 
 agreement certain relations among the numerical 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 77 
 
 values of the atomic weights have been discovered, 
 and the nature of the elements themselves set in an 
 entirely new light (see next chapter). 
 
 It seems proper to recall at this point the names 
 of a few of the more prominent among the workers 
 who have laboured to introduce accuracy into the 
 experimental estimations of the combining pro- 
 portions of the elements, from which the atomic 
 weights of the same are, as already explained, de- 
 rived. The first to make experiments explicitly 
 directed towards the estimation of the relative 
 weights of atoms was, of course, John Dalton, but 
 the numbers he obtained were in many cases so far 
 from the truth, that his results have at the present 
 day no interest, except from the historical point of 
 view. The same may be said of the " equivalents " 
 calculated later by Wollaston (Phil. Trans., 1814), 
 and the first chemist to whom science is indebted 
 for estimating these ratios with a tolerable approach 
 to accuracy was the Swedish professor Berzelius 
 (born 1779, died 1848). To this business, indeed, 
 he devoted the greater part of a laborious life. His 
 example was to a certain extent followed, and a 
 number of very exact estimations were due to the 
 labours of Dumas, Pelouze, De Marignac, and 
 others. Later, the most eminent among the 
 numerous workers in this field was J. S. Stas, 
 who, in a series of papers of which the first was 
 published in 1860, gave the results of his experi- 
 ments on the atomic weights of ten elements, con- 
 
78 A SHORT HISTORY OF THE 
 
 ducted with precautions more elaborate, and with a 
 skill more refined than anything previously known 
 in researches of this kind. 1 
 
 The object aimed at is to determine exactly the 
 proportion of each element which enters into com- 
 bination with the unit weight of some one element 
 taken as the standard. During the first quarter of 
 this century oxygen was used as the standard for 
 comparison, and its combining unit was assumed to 
 be 100. But inasmuch as hydrogen enters into 
 combination in the smallest proportion of all, it was 
 soon found more convenient to take hydrogen as 
 the standard, and refer all other combining weights 
 to that of hydrogen, assumed to be 1. The hypo- 
 thesis suggested by Prout in 1815, that the atomic 
 weights of the elements are multiples of the atomic 
 weight of hydrogen by whole numbers, doubtless 
 assisted in promoting the adoption of hydrogen as 
 the unit. This hypothesis in its original form has 
 long since been abandoned. 
 
 The methods actually employed for the purpose 
 contemplated are very diverse. It is not possible in 
 all cases to obtain compounds of the elements with 
 hydrogen. The metals, for example, afford but few 
 examples of such compounds. On the other hand, the 
 metals form oxides which, as a class, are remarkably 
 definite and stable substances. Analytical difficulties 
 
 1 A complete account of the object, scope, and results of Stas' 
 work is given in the Memorial Lecture by Professor J. W. Mallet, 
 read before the Chemical Society, Dec. 1892. Trans. Chem. Soc., 
 Ixiii. 1 (1893). 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 79 
 
 of a practical kind, however, also stand in the way of 
 directly ascertaining the proportions of the elements 
 in such compounds, and chemists resort, therefore, to 
 the chlorides, bromides, sulphates, and other com- 
 pounds, as well as to the direct examination of the 
 hydrides or oxides for the information desired. 
 The ratio in which hydrogen and oxygen stand to 
 each other in water is a matter of such funda- 
 mental importance, and the experimental processes 
 employed are so instructive, that a short account of 
 them may be given here. 
 
 All the early determinations of the composition 
 of water by weight were based upon the fact that 
 copper oxide may be heated to redness by itself 
 without decomposition or loss of weight, but that 
 in presence of hydrogen it yields copper, which 
 remains behind, and water in vapour, which may 
 be condensed and collected in suitable apparatus, so 
 that its weight can be determined. Hence the loss of 
 weight sustained by oxide of copper heated in a stream 
 of pure hydrogen would give the weight of oxygen in 
 the water which is formed. The difference between 
 the weight of water and that of the oxygen in it gives 
 the hydrogen. The first results of real value were 
 obtained by Dumas, and were published in 1842. 
 
 The figure, given in Dumas' paper in the Annales 
 de Chimie, shows the vessel in which hydrogen was 
 generated, tubes containing materials for purifying 
 and drying the gas, a bulb containing pure cupric 
 oxide, and a second bulb with connected tubes, in 
 
80 A SHORT HISTORY OF THE 
 
 which the water formed is collected without loss 
 of vapour, carried away by the escaping excess of 
 hydrogen. The weight of the bulb containing the 
 oxide is determined with great care before the ex- 
 periment begins, and again at its close. The weight 
 of the bulb and of the connected tubes is also deter- 
 mined when empty, and after the collection of the 
 water. Many precautions are necessary, and many 
 were actually adopted by Dumas, but all sources of 
 error could not be avoided at that day, even if they 
 were recognised. Some of the experimental diffi- 
 culties are obvious enough, such as the impurities 
 present in hydrogen obtained by the customary 
 methods, the difficulty of removing moisture from 
 the gas, and the intrusion of air by leakage through 
 the joints of the apparatus, the presence of impurities 
 in the copper oxide, the uncertainty of the weigh- 
 ings performed in atmospheric air, the condition of 
 which as to moisture, pressure, and temperature varies 
 from day to day. These and others, unsuspected in 
 Dumas' time, such as the retention of hydrogen by 
 the reduced metallic copper, have been considered, 
 and more or less completely met by later investiga- 
 tions. The results of these successive inquiries into 
 the application of this method are given below : 
 
 Names of Experimenters. Combining Weight of Oxygen. 
 
 Probable Error. 
 
 Dumas 15*9607 '0070 
 
 Erdmann and Marcliand . . 15*975 '0113 
 
 Cook and Richards . . . 15'869 '0020 
 
 Keiser . . . / . 15*9514 -0011 
 
 Dittmar and Henderson . . 15*8667 + *0046 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 81 
 
 An entirely distinct method, involving the diffi- 
 cult task of uniting oxygen to hydrogen, and weigh- 
 ing not only the water produced, but the gases 
 themselves before combination, was undertaken 
 by Professor E. W. Morley. The hydrogen was 
 absorbed by palladium, and the metal with the 
 " occluded " gas weighed separately. The oxygen 
 was weighed in the gaseous form in compensated 
 globes, and the combination of the hydrogen with 
 the oxygen was effected by means of electric sparks, 
 in an apparatus in which the resulting water could 
 be collected and weighed, while the unconsumed 
 residue, whether of hydrogen or. oxygen, could be 
 collected apart and determined. The result of a 
 series of such experiments gave for the combining 
 
 weight the value 
 
 15-8790 '00028. 
 
 One other important method, involving again a 
 different principle, must not be omitted : this is a 
 comparison of the densities of the two gases, hydro- 
 gen and oxygen, with the assumption, fully justified 
 by abundant evidence which cannot be discussed 
 at this point, that supposing them to be true gases 
 (see Chapter IX.), their densities would be pro- 
 portional to their combining weights. Oxygen and 
 hydrogen are not, however, perfect gases, uniting in 
 the exact ratio of one volume to two volumes. As 
 the combined result of very elaborate experiments 
 conducted by Dr. Alexander Scott 1 and by E. W. 
 
 1 Phil. Trans., 1893. 
 
 F 
 
82 A SHOKT HISTORY OF THE 
 
 Morley, 1 the ratio is actually 1 to 2'0028, with a 
 small probable error. This fact has to be taken 
 into account in estimating the chemical combining 
 weight of the gases -from their relative densities. 
 
 A large number of weighings of these gases have 
 been made by successive generations of chemists 
 from the times of Cavendish and Lavoisier onwards, 
 but the first determinations which attained to any 
 considerable degree of accuracy were those made by 
 Regnault about 1845. The subject has been taken 
 up again in recent years by Lord Rayleigh, 2 and by 
 Morley, whose work has already been referred to, 
 also by the late Professor J. P. Cooke, and others; 
 the result being that the number finally adopted, as 
 expressing the density of oxygen, is appreciably less 
 than the number resulting from Regnault's and the 
 other earlier estimations. In the end the results 
 stand as follows : the value of the symbol from 
 the synthesis of water is 15*8796 ; from the densi- 
 ties of the gases it is 15-8769. The number 15'88 
 may therefore be taken for all practical purposes as 
 the combining proportion of oxygen. 3 
 
 1 Amer. Journ. Sci. [3], xl i. 220, 276. 
 
 2 "Qn the Densities of the Principal Gases" (Proc. Roy. Soc., 
 liii. 134). 
 
 3 These are the values calculated by Professor F. W. Clarke from 
 all the best data combined. See "Smithsonian Constants of Nature. 
 A Recalculation of the Atomic Weights," 1897. 
 
PBOGRESS OF SCIENTIFIC CHEMISTRY 83 
 
 CHAPTER IV 
 
 NUMERICAL RELATIONS AMONG THE ATOMIC WEIGHTS: 
 CLASSIFICATION OF THE ELEMENTS 
 
 IT inay be inferred, from what has been stated in 
 the preceding chapter, that the process of deter- 
 mining an atomic weight resolves itself into two 
 parts, namely, the exact determination of the com- 
 bining proportion, or, as it was formerly called, the 
 equivalent, and the multiplication of the equivalent 
 by a factor, 1, 2, 3, or 4, derived from the applica- 
 tion of the law of Avogadro, the law of Dulong and 
 Petit, or from some other consideration, according to 
 the circumstances of the case. 
 
 A complete digest of all the determinations of 
 modern times has been prepared by Professor F. W. 
 Clarke, and published as a volume of the " Constants 
 of Nature," by the Smithsonian Institution, Wash- 
 ington, and the numbers adopted by Professor Clarke 
 will be used in the discussion which occupies the latter 
 part of this chapter. Some of these values are ob- 
 viously inexact, but they represent the best estimate 
 which can be made in the present state of knowledge. 
 
 The hypothesis put forward by Prout early in the 
 century has been a fruitful source of discussion, and 
 
84 A SHORT HISTOKY OF THE 
 
 even at the present day is regarded by some chemists 
 as hardly yet disposed of. Prout supposed that the 
 atomic weights of the elements are multiples by 
 whole numbers of the atomic weight of hydrogen ; 
 but in consequence of the atomic weight of chlorine, 
 according to the experiments of many chemists, 
 invariably coming out midway between 35 and 36, 
 it is obvious that the principle thus expressed is 
 untenable. Consequently it was suggested, first, that 
 the hypothesis might be modified by making one 
 half the atomic weight of hydrogen, and subse- 
 quently one fourth the atomic weight of that element, 
 the unit. Stas began his researches with a strong 
 prepossession in favour of Prout's hypothesis, but 
 the results of his protracted labours, by far the most 
 trustworthy of all the experimental investigations of 
 the subject which we possess, have only tended to 
 render it more improbable that any such relation 
 among the atomic weights really subsists. 
 
 A large part of the interest attaching to this sub- 
 ject arises from its association with the question as 
 to the probable nature and origin of the chemical 
 elements. On the one hand, each of the elemen- 
 tary bodies may represent a separate creation inde- 
 pendent of all the rest, and having nothing in 
 common with them. On the other hand, supposing 
 a relation can be traced between the masses of 
 the atoms of which different elements are composed, 
 then it is open to inquiry whether they may not 
 have had a common origin ; whether they may not 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 85 
 
 represent several stages in a formative or evolutionary 
 process, operating upon a primitive simple material ; 
 and whether in that case it may not be possible to 
 transform one into another by the operation of 
 agencies within the range of practicable experiment. 
 It had long been noticed, and specially pointed 
 out by Dobereiner in 1829, that when families of 
 closely allied elements are examined they are com- 
 monly found to consist of three members, for ex- 
 ample chlorine, bromine, iodine or sulphur, selenion, 
 tellurium or lithium, sodium, potassium ; and that 
 in such cases the values of the atomic weights are 
 so related that the middle term of the series is 
 nearly the arithmetical mean of the two other 
 terms. For example : 
 
 But no general discussion of the subject possessing 
 much interest appeared until 1858, when Dumas 
 published a most interesting Mtmoire sur Us Equi- 
 valents des Corps Simples} 
 
 In this memoir Dumas drew attention to the 
 analogy which may be recognised between series 
 of closely related elements and the known series 
 of compound radicles, such as methyl, ethyl, propyl, 
 &c., of which so many examples occur among carbon 
 compounds. Thus, regarding hydrogen as the first 
 member of the series, and writing equivalent quantities 
 
 1 Ann. Chim. Phys. [3], Iv. 129. 
 
86 A SHORT HISTORY OF THE 
 
 of the hydrocarbon radicles in succession with their 
 combining weights, we have 
 
 Hydrogen H 1 
 
 Methyl CH 3 15 
 
 Ethyl C 2 H 5 29 
 
 Propyl C 3 Hj. 43 
 
 Butyl C 4 H 9 57 
 
 Amyl C 5 H n 71 
 
 &c. &c. &c. 
 
 Here it is obvious that in passing from term to 
 term there is a common difference of 14 units, and 
 representing the value of the first term as a, and the 
 difference as d, the value of any single term may be 
 expressed by a + nd. Hence, on taking three terms 
 at equal distances in the series, it is found that the 
 combining weight of the intermediate term is the 
 arithmetical mean of the combining weights of the 
 other two. For example, 43 is equal to the sum of 
 29 and 57 divided by 2. Dumas also drew atten- 
 tion to the fact that in such a series combining 
 proportions represented by such numbers as 141 
 and 281, 127 and 253, stand so nearly in the 
 relation of 1 to 2 that if they belonged to substances 
 supposed to be elementary, and not known to be 
 compound, it would almost certainly be inferred 
 that this actually represented the ratio of the one 
 to the other. Relations very similar to these are 
 traceable among the members of the several recog- 
 nised natural families of the elements, and Dumas 
 was able, with the atomic weights accepted at that 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 87 
 
 time, many of which had been corrected by his own 
 experiments, to represent several series by formulae 
 of this kind. He further showed that, on placing 
 certain of these groups side by side, a common 
 difference ran through the successive terms of the 
 parallel series. For example : 
 
 Fluorine, F . . 19 Nitrogen, N . . 14 
 
 Chlorine, 01 . . 35'5 Phosphorus, P . . 31 
 
 Bromine, Br . . 80 Arsenic, As . . . 75 
 
 Iodine, I . . .127 Antimony, Sb . .122 
 
 Here the difference between each term of the 
 halogens and the corresponding member of the 
 nitrogen series is, with one exception, exactly 5. 
 Later determinations have, however, so modified 
 these numbers that the difference is no longer 
 constant throughout. The question of the numerical 
 relations among the atomic weights was not long 
 suffered to remain at rest. Many writers had drawn 
 attention to various cases of individual or serial 
 peculiarities, but so long as the atomic weights of 
 the elements remained uncoordinated with a com- 
 mon standard, it is obvious that no substantial 
 progress could be made. We have seen in the last 
 chapter how, by the recognition of the law of 
 Avogadro and of the law of Dulong and Petit, the 
 atomic weights were ultimately systematised ; and 
 almost so soon as this was accomplished a very 
 remarkable discovery was made, which ultimately 
 brought the whole of the known elements within 
 one comprehensive scheme. This scheme, which is 
 
88 A SHORT HISTORY OF THE 
 
 usually incorrectly referred to as the " periodic law," 
 is based upon a principle which would more appro- 
 priately receive that designation. The principle 
 which is now generally recognised may be expressed 
 as follows : If the elements are arranged in the order 
 of the numerical value of their atomic weights, their 
 properties, physical and chemical, vary in a recurrent 
 or periodic manner. 
 
 We will now endeavour to trace the successive steps 
 which have led up to this generalisation. The exist- 
 ence of triads of closely related elements, as already 
 stated, had long been recognised ; and an extension of 
 this idea, recalling the properties of homologous series, 
 had been discussed by Dumas, with results already de- 
 scribed. But the various series of elements remained 
 as separate series, disconnected one from another. 1 
 
 1 A claim has been put forward by Messieurs Lecoq de Bois- 
 baudran and Lapparent in favour of A. E. B. de Chancourtois, for 
 a share in the credit of having originated the idea of the periodic 
 relation of properties to atomic weights among the chemical ele- 
 ments. In 1862-63 M. de Chancourtois, a geologist and engineer, 
 presented a series of papers to the French Academic des Sciences, 
 which were collected together in 1863 under one title, "Le Vis 
 Tellurique, classement naturel des corps simples ou radicaux obtenu 
 au moyen d'un Systeme de Classification helicoidal et numerique." 
 By coiling a helix with an angle of 45 round a cylinder divided 
 vertically into sixteen equal parts by lines drawn from the circular 
 base, the helix cuts these lines at equal distances in its ascent, and 
 the points of intersection were supposed to represent the atomic 
 weights of elements which differed from one another by 16, or 
 multiples of 16. The author seems to have had a dim idea that 
 properties were in some way related to atomic weight, but this 
 idea is so confused by fantastic notions of his own, that it is im- 
 possible to be sure that he really recognised anything like periodicity 
 in this relation. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 89 
 
 In July 1864 a paper was communicated to the 
 Chemical News by Mr. John A. R. Newlands, in which, 
 in the course of discussing certain supposed regulari- 
 ties in the atomic weights of the elements, he drew 
 up a list of all the then known elements in the order 
 of the numerical value of their atomic weights, and 
 in the month following he was led to announce, 
 with reference to this table, the existence of a 
 simple relation among the elements so arranged. 
 Numbering the elements in order, hydrogen 1, 
 lithium 2, glucinum 3, boron 4, and so on, he pointed 
 out that " the eighth element, starting from a given 
 one, was a sort of repetition of the first, or that 
 elements belonging to the same group stood to each 
 other in a relation similar to that between the ex- 
 tremes of one or more octaves in music." 
 
 Almost immediately after this, namely, in October 
 of the same year, an interesting paper appeared in 
 the Quarterly Journal of Science (vol. i. p. 642), by 
 Dr. Odling, on " The Proportional Numbers of the 
 Elements," in which he pointed to the marked con- 
 tinuity in the arithmetical series, resulting from an 
 arrangement of the whole of the then known ele- 
 ments in the order of their atomic weights, or 
 "proportional numbers," as he preferred to call 
 them, the only exceptions to the very gradual in- 
 crease in value of the consecutive terms being 
 manifested between the numbers 40 and 50 (Ca 
 and Ti), 65 and 75 (Zn and As), 96 and 104 
 (Mo and Ro), 138 and 184 (Ta and W), 184 and 
 
90 
 
 A SHORT HISTORY OF THE 
 
 195 (W and Nb), and 210 and 231-5 (Bi and Th). 1 
 In this paper Odling showed that this purely arith- 
 metical seriation may be made to agree with an 
 arrangement of the elements according to their 
 generally recognised affinities, and he drew up a 
 table, of which the following is a revised version, 
 published in Watts' Dictionary of Chemistry (vol. 
 iii. p. 975), only a few months later : 
 
 
 
 
 Mo 96 
 Pd 106-5 
 
 W 184 
 Au 196-5 
 Pt 197 
 
 L 7 
 G 9 
 B 11 
 C 12 
 N 14 
 16 
 F 19 
 
 Na 23 
 Mg 24 
 Al 27-5 
 Si 28 
 P 31 
 S 32 
 Cl 35-5 
 
 Zn 65 
 
 As 75 
 Se 79-5 
 Br 80 
 
 Ag 108 
 Cd 112 
 
 Sn 118 
 Sb 122 
 Te 129 
 Te 127 
 
 Hg 200 
 Tl 203 
 Pb 207 
 Bi 210 
 
 
 K 39 
 Ca 40 
 Ti 48 
 Cr 52-5 
 Mn 55 
 
 Rb 85 
 Sr 87'5 
 Zr 89-5 
 
 Cs 133 
 Ba 137 
 
 V 138 
 
 Th 231 
 
 Here manganese stands proxy for the iron metals, 
 and platinum and palladium for their respective 
 congeners. 
 
 Towards the end of this paper, the author drew 
 attention to the large number of instances in which 
 
 1 Most of these breaks are now accounted for by the existence of 
 elements discovered since that time, viz. Sc = 44, Ga-69, and 
 Ge = 72; the earth metals, La = 137, Ce = 139, Neo-dymium = Nd 
 = 140, Praseo-dymium = Pr=142, Sm = 149, Tb = 159, Eb = 165, while 
 Ta is now 181, and Nb 94 (?). 
 
PEOGEESS OF SCIENTIFIC CHEMISTEY 
 
 91 
 
 the atomic weights of proximate elements differ 
 from one another by 48 or 44, or 40 or 16, and 
 remarks : " We cannot help looking wistfully at the 
 number 4 as embodying, somehow or other, the 
 unit of a common difference." l From this it would 
 appear that something analogous to the idea of 
 homology, which had attracted Dumas, was hover- 
 ing in his mind. In conclusion, the pregnant ob- 
 servation occurs that " among the members of every 
 well-defined group, the sequence of properties and 
 sequence of atomic weights are strictly parallel to 
 one another." 
 
 In August 1865 Mr. Newlands again wrote to the 
 Chemical News (vol. xii. p. 83), as follows: "If the 
 elements are arranged in the order of their equiva- 
 lents, with a few slight transpositions, as in the 
 accompanying table, it will be observed that elements 
 belonging to the same group usually appear in the 
 same horizontal line. 
 
 No. 
 
 No. 
 
 No. 
 
 No. 
 
 No. 
 
 No. 
 
 No. 
 
 No. 
 
 H 1 
 
 F 8 
 
 Cl 15 
 
 Co & Ni 22 
 
 Br . 29 
 
 Pd 36 
 
 I 42 
 
 Pt & Ir 50 
 
 Li 2 
 
 Na 9 
 
 K 16 
 
 Cu 23 
 
 Kb . 30 
 
 Ag 37 
 
 Cs 44 
 
 Tl 53 
 
 G 3 
 
 MglO 
 
 Ca 17 
 
 Zn 25 
 
 Sr . 31 
 
 Cd 38 
 
 Ba & V 45 
 
 Pb 54 
 
 Bo 4 
 
 Al 11 
 
 Cr 19 
 
 Y 24 
 
 Ce & La 33 
 
 U 40 
 
 Ta 46 
 
 Th 56 
 
 C 5 
 
 Si 12 
 
 Ti 18 
 
 In 26 
 
 Zr . 32 
 
 Sn 39 
 
 W 47 
 
 Hg 52 
 
 N 6 
 
 P 13 
 
 Mn20 
 
 As 27 
 
 Di & Mo 34 
 
 Sb 41 
 
 Nb 48 
 
 Bi 55 
 
 7 
 
 S 14 
 
 Fe 21 
 
 Se 28 
 
 Ro & Ru 35 
 
 Te 43 
 
 Au 49 
 
 Os 51 
 
 Note. When two elements happen to have the same equivalent, 
 both are designated by the same number. 
 
 1 From the more exact values for the atomic weights to be given 
 later, it will be seen that these differences are but roughly repre- 
 sented by these whole numbers. 
 
92 A SHORT HISTORY OF THE 
 
 It will also be seen that the numbers of analogous 
 elements generally differ either by 7 or by some 
 multiple of 7 ; in other words, members of the same 
 group stand to each other in the same relation as 
 the extremities of one or more octaves in music. 
 This relationship Newlands proposed to call the 
 " Law of Octaves." 
 
 The following year the same chemist brought the 
 relations which he had observed to the notice of 
 the Chemical Society of London, and produced a 
 table similar to the above, with a few alterations, by 
 which mercury was brought into the same line with 
 cadmium, lead into the same line with tin. He also 
 used atomic weights calculated on Cannizzaro's sys- 
 tem, so far as known facts would permit. The time, 
 however, had not arrived for the general acceptance 
 of ideas of this kind, obscured as they necessarily 
 were by imperfect knowledge, both of atomic weights 
 and of the inter-relations of the elements as to pro- 
 perties. The Chemical Society in 1866 were dis- 
 posed to laugh at Newlands and his "law." Twenty- 
 one years later the Royal Society awarded him the 
 Davy Medal for his discovery. 
 
 Others, however, soon took up the question in 
 a very serious spirit, and with less of the hesita- 
 tion which had characterised the treatment of the 
 subject up to this time. This arose concurrently 
 with a feeling of greater confidence in the re- 
 vised system of atomic weights, which by the 
 end of the decade 186070 were quite gener- 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 
 
 93 
 
 ally adopted, at any rate in England and Ger- 
 many ; and probably out of this confidence was 
 gradually developed some idea that the relations 
 already noticed did indeed correspond to some pro- 
 found physical law relating to the nature of the 
 elements. In March 1869, Professor D. Mendeleeff 
 communicated a paper to the Russian Chemical 
 Society, 1 in which he set out an arrangement of the 
 elements in a table having considerable resemblance 
 to the table drawn up by Odling five years before. 
 
 MENDELEEF'S TABLE OF THE ELEMENTS, 1869. 
 
 
 
 
 Ti = 50 
 
 Zr = 90 
 
 ? =180 
 
 
 
 
 V = 51 
 
 Nb = 94 
 
 Ta=182 
 
 
 
 
 Or = 52 
 
 Mo= 96 
 
 W=186 
 
 
 
 
 Mn= 55 
 
 Rh =104-4 
 
 Pt=197'4 
 
 
 
 
 Fe = 56 
 
 Ru=104'4 
 
 Ir =198 
 
 
 
 Ni= 
 
 Co = 59 
 
 Pd =106-6 
 
 Os=199 
 
 H=l 
 
 
 
 Cu = 63-4 
 
 Ag =108 
 
 Hg=200 
 
 
 Be = 9-4 
 B =11 
 
 Mg=24 
 Al =27-4 
 
 Zn = 65-2 
 
 ? = 68 
 
 Cd =112 
 Ur =116 
 
 Au=197? 
 
 
 C =12 
 
 Si =28 
 
 ? = 70 
 
 Sn =118 
 
 
 
 N -14 
 
 P =31 
 
 As = 75 
 
 Sb =122 
 
 Bi =210 
 
 
 = 16 
 
 S =32 
 
 Se = 79-4 
 
 Te =128? 
 
 
 
 F =19 
 
 Cl =35-5 
 
 Br = 80 
 
 I =127 
 
 
 Li=7 
 
 Na=23 
 
 K =39 
 
 Rb = 85-4 
 
 Cs =133 
 
 Tl =204 
 
 
 
 Ca =40 
 
 Sr = 87'6 
 
 Ba =137 
 
 Pb=207 
 
 
 
 ? =45 
 
 Ce = 92 
 
 
 
 
 
 ?Er =56 
 
 La = 94 
 
 
 
 
 
 ?Yt =60 
 
 Di = 95 
 
 
 
 
 
 ?In =75-6 
 
 Th =118? 
 
 
 
 In commenting upon this table the German ab- 
 stract, 1 which is alone accessible to English, and 
 presumably also to most German readers, repre- 
 sents the author's view by the following words : 
 
 1 Zeitschrift fur Chemie, 1869, p. 405. 
 
94 A SHOUT HISTORY OF THE 
 
 " 1. Die nach der Grosse des Atomgewichts geord- 
 neten Elemente zeigen eine Stufenwejse Abandoning 
 in den Eigenschaften." This appears to contain an 
 important error of translation, for Mendeleeff, in 
 a communication made to the Berlin Chemical 
 Society l upon the history of the question, explains 
 in a footnote that the word which in the Russian 
 original means periodische-, has been rendered stufen- 
 weise (gradual, or by degrees). It must, however, be 
 remarked that the table does not so obviously sug- 
 gest periodicity of properties in the elements so 
 arranged, as does Newlands' earlier, though con- 
 fessedly imperfect, attempt. 
 
 In this paper Mendeleeff pointed out that the 
 discovery and properties of elements then unknown, 
 as for example analogues of silicon and aluminium, 
 might be predicted ; and further expressed the con- 
 viction that such a system might be applied to the 
 correction of atomic weights, citing as an example 
 the case of tellurium, which, according to this view, 
 ought to have an atomic weight smaller than that 
 L of iodine. 
 
 In December 1869, Lothar Meyer, then professor 
 in the Polytechnicum at Carlsruhe, contributed a 
 paper to Liebig's Annalcn, 2 entitled " Die Natur der 
 Chemischen Elemente als Function ihrer Atom- 
 gewichte," in which, after giving a table of elements, 
 with their atomic weights, which he described as 
 
 1 tierichte der Deutsch Chem. Ges., iv. 351 (1871). 
 2 Supplement, vi. and vii., 1870, p. 354. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 
 
 95 
 
 substantially identical with that of Mendeleeff, he 
 pointed out that from the table it may be deduced 
 that the properties of the elements are generally 
 a periodic function of the atomic weight. This obser- 
 vation seems to show that the periodicity did not 
 become apparent to Meyer, at any rate, till the table 
 of Mendeleeff had been subjected to the modifications 
 adopted by him, and shown below : 
 
 I. 
 
 II. HI. 
 
 IV. 
 
 V. 
 
 VI. 
 
 VII. 
 
 VIII. 
 
 IX. 
 
 
 B 11 
 
 Al 27-3 
 
 
 - 
 
 
 ?In 113-4 
 
 Tl 202-7 
 
 
 C 12 
 
 Si 28 
 
 
 
 
 
 Sn 117-8 
 
 
 Pb 206-4 
 
 
 
 
 Ti 48 
 
 
 Zr 89-7 
 
 
 
 
 
 
 N 14 
 
 P 30-9 
 
 
 As 74-9 
 
 
 Sb 122-1 
 
 
 Bi 207-5 
 
 
 
 
 V 51"2 
 
 
 Nb 93-7 
 
 
 Ta 182-2 
 
 
 
 16 
 
 S 32 
 
 
 Se 78 
 
 
 Te 128? 
 
 
 
 
 
 
 Cr 52-4 
 
 
 Mo 95-6 
 
 
 W 183-5 
 
 
 
 F 19-1 
 
 Cl 35-4 
 
 
 Br 79-75 
 
 
 I 126-5 
 
 
 
 
 
 
 Mn 54-8 
 
 
 Ru 103-5 
 
 
 Os 198 "6? 
 
 
 
 
 
 Fe 55-9 
 
 
 Rh 104-1 
 
 
 Ir 196-7 
 
 
 
 
 Co = 
 
 Ni 58-6 
 
 
 Pd 106-2 
 
 
 Pt 196-7 
 
 
 Li 7-0 
 
 Na 22-9 SK 39 
 
 
 Rb 85-2 
 
 
 Cs 132-7 
 
 
 
 
 
 Cu 63-3 
 
 
 Ag 107-7 
 
 
 Au 192-2 
 
 
 ?Be 9-3 
 
 Mg 23-9 Ca 39'9 
 
 
 Sr 87 
 
 
 Ba 136-8 
 
 
 
 
 | 
 
 Zn 64-9 
 
 
 Cd 111-6 
 
 
 Hg 199-8 
 
 
 Obviously in this scheme the same or similar 
 properties recur when the atomic weight is increased 
 by a certain amount, which is first 16, then about 
 46, and afterwards amounts to 88 to 92 units. 
 This had been noticed by Odling and others pre- 
 viously, but Meyer got a step further when he 
 pointed out (loc. cit. p. 388) that the combining 
 capacity of the atoms rises and falls regularly and 
 equally in two such series as the following : 
 
 Univalent. Biv. Triv. Ouadriv. Triv. Biv. Univ. 
 Li Be B C N O F 
 
 Na Mg Al Si P S Cl 
 
96 A SHORT HISTORY OF THE 
 
 A complete and triumphant vindication of the 
 principle of periodicity is provided by the graphic 
 representation of the relation of atomic volume to 
 atomic weight, drawn up for the first time by Meyer 
 at the end of this paper. The diagram he gave, and 
 of which a portion is reproduced on the opposite 
 page, speaks for itself. 
 
 It is evident, then, that the conception of the 
 periodic relation gradually and independently took 
 shape in the mind of more than one chemist during 
 the period we have had under review. But it would 
 be only just to point out that a depth of conviction, 
 which almost amounts to inspiration, carried Men- 
 deleeff further in the study and application of the 
 principle than any of his predecessors or contempo- 
 raries. In August 1871 he drew up a complete 
 exposition 1 of the principle, which he henceforth 
 calls the periodic law, and of the deductions which 
 may be made from it. And here for the first time 
 appeared the table which, in some form or other, is 
 now to be found in the pages of nearly every text- 
 book of theoretical chemistry, and which is now 
 employed as the most generally received basis of 
 classification of the elements. The table is repro- 
 duced in this original form in order to show how 
 great an advance it represents in the development 
 of the fundamental idea. 
 
 1 Annalen, Supplem., viii. p. 133 (1872). 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 
 
 97 
 
 
 OOi OO I 
 
 O)O rH rH 
 
 II II 
 
 &Z 
 
 sl i 
 
 H II 
 
 s H g ii | 
 
 II 3 II m || 
 
 I I 
 
 S3 
 
 gas 
 
 5 
 
 3 
 
 (Nl II CO 
 
 rH .^ * 
 II <tt II 
 
 n 9 
 
 
 
 
 8 
 
 - " o 
 
 05 jf ^ S3 00 
 
 n * H N ii 
 
 PQ 
 
 M 
 
 15, 
 
 r 
 
 " ^ 
 II 
 
 CO 
 05 || 
 
 I? 5 
 
 H 
 
 -' I 
 
 CO OS O rH 
 
 G 
 
98 A SHORT HISTORY OF THE 
 
 In connection with the discussion of this scheme, 
 MendeleefFs most brilliant achievement was its appli- 
 cation to the prediction of the properties of elements 
 yet to be discovered. " When," he says, in one of 
 the characteristic footnotes in his " Principles of 
 Chemistry" (vol. ii. p. 25), "in 1871 I wrote a paper 
 on the application of the periodic law to the determi- 
 nation of the properties of yet undiscovered elements, 
 I did not think I should live to see the verification 
 of this consequence of the law, but such was to be 
 the case. Three elements were described, eka-boron, 
 eka-aluminium, and eka-silicon ; and now, after the 
 lapse of twenty years, I have had the great pleasure 
 of seeing them discovered and named after those 
 countries where the rare minerals containing them 
 are found, and where they were discovered Gallia, 
 Scandinavia, and Germany." Mendeleeff s eka-boron 
 has been called by Nilson, its discoverer, Scandium ; 
 the representative of eka-aluminium was discovered 
 in the zinc-blende of the Pyrenees by Lecoq de Bois- 
 baudran in 1875, and named Gallium; while a new 
 silver ore at Freiberg yielded, in the hands of 
 Clemens Winkler, a new metal, which he recog- 
 nised as the representative of eka-silicon, and to 
 which he patriotically gave the name Germanium. 
 
 The process which led Mendeleeff to this remark- 
 able result consists, naturally, in the careful study 
 of the properties, not only of the series in which the 
 unoccupied place occurred, but also of the proper- 
 ties of the neighbouring series, and of their mutual 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 99 
 
 relations. A few facts may be given to show the 
 nature of the conclusions to which he was led in one 
 case, and the extent to which they were afterwards 
 justified. 
 
 To eka-silicon Mendeleeff assigned, in 1871, the 
 atomic weight 72. He described it as a difficultly 
 fusible, dark grey, metallic substance, which in the 
 state of powder would pass at a red heat into the 
 difficultly fusible oxide Es0 2 . The specific gravity 
 of the oxide would be about 4*7, and similar in 
 appearance, perhaps in crystalline form, to titanic 
 oxide. The metal would act on steam only with 
 difficulty, and on acids slightly, though more easily 
 on alkalis, while the oxide would display more 
 decided acidifying power than titanic oxide. The 
 fluoride, EsF 4 , would be volatile, but not gaseous at 
 common temperatures. The chloride, EsCl 4 , would 
 be a volatile liquid, boiling at a temperature about 
 100, probably somewhat lower; and there would 
 be a tetrethide, EsEt 4 , with a boiling point about 
 160. 
 
 Germanium is described as a greyish white, 
 lustrous, very brittle metal, which melts at about 
 900, and crystallises in regular octahedrons on 
 cooling. It is unchanged in air at ordinary tem- 
 peratures, but is oxidised when heated in a state 
 of powder. It dissolves in sulphuric acid, but 
 not in hydrochloric acid. The oxide, Ge0 2 , is a 
 dense white powder of specific gravity 4' 7, slightly 
 soluble in water, producing a solution which has a 
 
100 A SHORT HISTORY OF THE 
 
 sour taste. The oxide dissolves readily in alkalis. 
 The fluoride is a volatile substance, probably solid. 
 The chloride, GeCl 4 , is a colourless liquid, which 
 boils at 86, and the tetrethide, GeEt 4 , is a liquid 
 which boils at 160. The atomic weight of ger- 
 manium is 71*93 when H= 1. 
 
 That the periodic system of the elements stands 
 for something which is actually based on natural 
 physical relations, no one can now be supposed to 
 doubt. It brings into view a number of facts in 
 the chemical history of the elements which would 
 otherwise be less apparent, and it does undoubtedly 
 support very strongly the idea that all the elements 
 included in Mendeleeff's and Meyer's synopsis belong 
 to one system of things, and perhaps have common 
 constituents, or may have arisen from a common 
 origin. Nevertheless, such a synopsis is encumbered 
 with some difficulties, which have not yet been satis- 
 factorily accounted for. The fundamental idea of 
 the scheme is the periodic " law," which asserts that 
 the properties of the elements are dependent upon 
 their atomic weights. If that were strictly true, 
 the existence of two elements having the same 
 atomic weight, but different properties, would be 
 impossible. Hence the relations of such elements 
 as cobalt and nickel, ruthenium and rhodium, are 
 far from clear in the original table. And even 
 if we substitute for the round numbers used by 
 Mendeleeff in 1871 the values for the atomic weight 
 which result from all the best recorded experiments, 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 101 
 
 so that these coincidences cease to be exact, the 
 difficulty still remains that several elements iron, 
 nickel, cobalt, ruthenium, rhodium, palladium, 
 osmium, iridium, platinum are not provided for. 
 In some cases the peculiarities of individual elements, 
 and their relations to others, could never have been 
 suspected from their position in the table. The 
 case of thallium is one of the most notable. This 
 metal has a very strong resemblance to lead, but its 
 sulphate, and several other salts, are entirely unlike 
 the corresponding compounds of lead, but, on the 
 other hand, agree closely in characters with the 
 compounds of the alkali metals. Lead itself stands 
 in a similar relation to the metals of the alkaline 
 earths. The positions of thallium and lead in the 
 table would scarcely have suggested these characters, 
 if these two elements had been previously unknown. 
 And now we have the newly discovered helium, 
 argon, and the rest, with their extraordinary charac- 
 teristic of chemical inactivity, which marks them off 
 from all other known substances, elementary or 
 compound, and of which no prognostication is to be 
 found in the almost innumerable memoirs relating 
 to the atomic weights of the elements published 
 previously to the discovery of argon. It must of 
 course be remembered, however, that since the dis- 
 covery of argon and helium, the gases of the atmos- 
 phere and those obtained from minerals have been 
 submitted by Ramsay to a special search, with the 
 object of discovering other elements of intermediate 
 
102 A SHORT HISTORY OF THE 
 
 and higher atomic weight, and his labour has been 
 rewarded by the recognition of neon, rnetargon, and. 
 krypton, as already mentioned. But the endeavour 
 to introduce these new substances into the periodic 
 scheme has, up to the present, been but imperfectly 
 successful; for such arrangements as the figure of 
 eight proposed by Crookes 1 involve some violence 
 to natural associations, hydrogen, for example, being- 
 separated from all the metals, and made the first 
 member of a group in which the halogens follow. 
 
 Another point is worthy of notice in a discussion 
 of these figures. The numerical amount of the 
 differences observable between elements which are 
 consecutive in the list vary very much. The smallest 
 difference appears to lie between nickel and cobalt, 
 and to be equal to about 0'25 ; while there are many 
 cases where the difference amounts to between 4 
 and 5 units or more, as, for example, Cu Co = 
 4-63, Sn- In =5-1 6, Cs-I = 6'00, and, Rb-Br- 
 5*44. In the last case it may be that the newly 
 discovered krypton stands between, with an atomic 
 weight about 80. The point, however, is that the 
 transition from the even to the odd series, as, for 
 example, from fluorine to sodium, or from chlorine 
 to potassium, or from bromine to rubidium, is not 
 marked by a difference noticeably greater, or less, 
 than is observable in many other parts of the list 
 of elements. The ninth series in Mendeleeff's table 
 will doubtless be filled up by many of the elements 
 
 1 Proc. Hoy. Soc., Ixiii. 408 (1898). 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 103 
 
 derived from the so-called Swedish or rare earths. 
 The atomic weights of some of these have already 
 been determined with sufficient show of probability 
 to allow of their finding provisional places in the 
 table. Among these may be mentioned neodymium 
 139*7, praseodymium 142*5, samarium 149*1, gado- 
 linium 155*6, terbium 158*8, erbium 165*0, thulium 
 169*4, ytterbium 171*9. 
 
 From all that has gone before, it may readily be 
 supposed that many speculations have been put 
 forward as to the origin of the elements, or in ex- 
 planation of their assumed complex nature. Of 
 these, one of the most serious, though by no means 
 the earliest, is embodied in a paper communicated 
 to the British Association by the late Professor 
 Carnelley. 1 It is typical of a whole group of these 
 speculative compositions, but is more carefully worked 
 out than many of the rest. The author proceeds 
 upon the assumption of cut least two primal elements, 
 A and B, which, by their combination in various 
 proportions, give masses approximately equal to the 
 atomic weights of the known elements, which are 
 regarded as analogous to hydrocarbon radicles ; but 
 even this scheme, attractive as it is on the whole, 
 requires us to assume for B a negative weight equal 
 to something between 1*99 and 2*00. It can- 
 not be forgotten that a somewhat similar device 
 
 1 " Suggestions as to the cause of the Periodic Law and the Nature 
 of the Chemical Elements." British Association, 1885; Chemical 
 News, 1886. 
 
104 A SHOKT HISTORY OF THE 
 
 was the last refuge of the declining theory of phlo- 
 giston more than a century ago. 
 
 There seems to be a general disposition at the 
 present time to revive and rehabilitate the ancient 
 notion of the unity of matter, and its derivation 
 from a common prothyl (irpcorr], first ; v\tj } stuff or 
 matter). 
 
 " All things the world which fill 
 Of but one stuff are spun." 
 
 This idea has been made the text of an interesting 
 address by Sir William Crookes, on the " Genesis of 
 the Chemical Elements." * It has also for many 
 years been the guiding principle of a remarkable 
 series of researches, by Sir Norman Lockyer, on the 
 spectroscopy of the sun and stars. According to 
 Lockyer, the elements, as known to terrestrial 
 chemistry, are more or less completely dissociated 
 into substances of simpler constitution at the high 
 temperatures prevailing in these bodies and their 
 gaseous atmospheres. It seems fully established 
 that the greater number of the lines exhibited by 
 the spectra of common metals, such as iron, calcium, 
 manganese, are wanting in the spectra of the hotter 
 stars; while a small number of such lines remain, 
 accompanied by other lines not recognisable as be- 
 longing to any known terrestrial element. And 
 when the spectra of certain stars are ranged in the 
 order of what is supposed to be ascending tempera- 
 
 1 British Association Reports, 1886. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 105 
 
 ture in the series of stars, there is a progressive 
 disappearance of the old, and appearance of the new 
 lines, corresponding to this progressive dissociation. 
 What is much needed, is some experimental evi- 
 dence that dissociation of the elements does actually 
 occur at the high temperatures now obtainable under 
 laboratory conditions. 
 
 The association of the ultimate particles of 
 prothyl must be supposed to be accomplished by 
 a process comparable with that which in ordinary 
 chemistry is called " polymerisation." This implies 
 the association together of a number of particles, 
 elementary or compound, into groups. No considera- 
 tion, based on the extreme minuteness of atoms, 1 
 need deter us from admitting such a conception ; 
 because, after all, what we call size, great or small, 
 is relative only to the range of human measure- 
 ments. But if there is any objection to the intro- 
 duction of the idea into serious physics, it will 
 probably be found in the relations of some of the 
 atomic ratios, commonly called atomic weights. It 
 is usually conceded that the atomic weights of 
 silver and chlorine, for example, have been deter- 
 mined with very great accuracy, the probable error 
 
 1 The estimate of Lord Kelvin, though familiar, may be recalled 
 to the recollection of the reader : " Imagine a globe of water, or 
 glass, as large as a football (or say a globe of 16 centimetres dia- 
 meter), to be magnified up to the size of the earth, each constituent 
 molecule being magnified in the same proportion. The magnified 
 structure would be more coarse-grained than a heap of small shot, 
 but probably less coarse-grained than a heap of footballs." Lecture 
 at Royal Inst., Feb. 1883. 
 
106 A SHORT HISTOBY OF THE 
 
 being less than 1 part in 10,000. Taking the 
 ratio between these two, as given by Stas, to be 
 
 Ag : Cl : : 107'930 : 35'457, 
 
 there is no common measure for these two numbers, 
 nor can any two numbers near to them be found 
 having a common measure, unless they are altered 
 by addition or subtraction of amounts much greater 
 than the experimental error. Suppose even such 
 round numbers as 108 and 3 5 '5 be taken, there is 
 no common measure for them greater than *5 ; and 
 if these two forms of matter be assumed to be made 
 up of the same " prothyl," the number of units of 
 such primordial stuff required to compose one atom 
 of chlorine and of silver, would be at least 355 and 
 1080 respectively. On this ground alone it would 
 be difficult to find any known mechanical principle 
 to account for the formation of such a highly stable 
 thing as an atom of silver, by the congregation of 
 1080 equal and similar particles. And supposing 
 this difficulty got over, another at once presents 
 itself as to the nature of the primitive particles : 
 are these really atoms, that is indivisibles, or does 
 not a similar hypothesis seem to be required to 
 account for their existence ? Thus visions of sub- 
 atoms of sub-atoms seem to arise in the mind, 
 suggestive of hypotheses worthy only of a Ptolemaic 
 philosophy. 
 
 The alternatives seern to lie between the Bos- 
 covichian doctrine of centres of force, which cannot 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 107 
 
 be discussed here, and the theory of vortices, which 
 we owe to Lord Kelvin. Fascinating as is the idea 
 of a universal frictionless fluid, in which vortices of 
 variously knotted forms subsist for ever, inalterable 
 and indestructible, and yield to our senses and in- 
 struments all those effects which we ascribe to 
 matter, the theory in its application to chemistry 
 has not yet been brought into harmony with all the 
 facts. But supposing such a theory to be accepted, 
 one consequence would be that the present notion 
 of the dissociability of the chemical elements would 
 have to be put aside, for each kind of vortex would 
 represent a special creative act which could not be 
 undone. There would be, similarly, an end of trans- 
 mutation of one element into another, and to some 
 extent also of the supposed significance of atomic 
 weights. 1 
 
 In such a position it is the duty and the best 
 interest of the chemist to preserve a perfectly un- 
 biassed niind, and to pursue the safer path of experi- 
 mental inquiry. 
 
 1 Mr. J. Larmor has recently propounded an "Electro-dynamical 
 Theory of the Electric and Luminiferous Medium," which gives 
 a new view of inter-atomic action, and the nature of the ether. 
 Phil. Trans., 1897, 205. 
 
108 A SHORT HISTORY OF THE 
 
 CHAPTER V 
 
 ORIGIN AND DEVELOPMENT OF THE IDEAS OF 
 VALENCY AND THE LINKING OF ATOMS 
 
 IT has already been explained in Chapter III. that 
 the discovery by Dumas of the fact that chlorine 
 may be introduced into organic compounds in 
 exchange for hydrogen, equivalent for equivalent, 
 led to very important changes in the then current 
 ideas of chemical combination. Up to that time 
 the electro-chemical theory of Berzelius had been 
 almost entirely predominant, and, naturally, the 
 notion that an electro-negative element like chlorine 
 could be exchanged for an electro-positive element 
 such as hydrogen, without fundamentally altering 
 the characters of the resulting compound, was com- 
 pletely at variance with the canons of that theory. 
 But facts remain, while theories must be left either 
 to be adapted to the new state of knowledge, or to 
 be abandoned altogether. In this case the theory 
 has been modified, but even to the present day the 
 interchange of negative chlorine and positive hydro- 
 gen is a phenomenon which still affords room for 
 speculation. 
 
 One important consequence of the establishment 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 109 
 
 of the facts of substitution was the inception of 
 the idea of types, a theory which, as remarked by 
 Williamson many years later, was " the vehicle of 
 many an important discovery," and led to pro- 
 found changes in the way of regarding chemical 
 combination. A most instructive review of the 
 whole position was given by Dumas himself in 
 1840. 1 He then stated clearly that compounds 
 which contain the . same number of equivalents 
 united in the same manner, and which exhibit the 
 same fundamental chemical properties, belong to 
 the same chemical type. 
 
 This view, though opposed by Berzelius, and at 
 first by Liebig, was afterwards supported by the 
 latter, who cited as analogous the case of chlorine 
 and manganese, which were known in perchlorates 
 and permanganates to be capable of replacing each 
 other without altering the crystalline form of the 
 salt, adding that " a reciprocal substitution of simple 
 or compound bodies, acting in the same way as 
 isomorphous bodies, must be regarded as a veritable 
 law of nature." 
 
 Dumas' theory of types, then, expressly recognised 
 the idea of arrangement among the constituent par- 
 ticles united together in a compound, and he saw, 
 and stated clearly, the distinction between the older 
 Berzelian system, which attributed to the nature of 
 the elements concerned the principal share in deter- 
 
 1 " M&noire sur la loi des substitutions et la thdorie des types " 
 (Compt. Rend., x. 149). 
 
110 A SHORT HISTORY OF THE 
 
 mining the nature of the compound, and the newer 
 doctrine, according to which the number and ar- 
 rangement of the atoms (or equivalents, as he called 
 them) played the most important part. Dumas, 
 however, does not seem to have perceived so clearly 
 that chemical combination in organic compounds, 
 that is, in compounds of carbon, is governed by the 
 same laws as inorganic compounds ; and save for the 
 use which he made of the analogy derived from 
 isomorphism, he did not bridge over the gulf which 
 then separated organic from inorganic chemistry. 
 Hence, while he spoke of alcohol, aldehyd, acetic 
 acid as representing different types, these were not 
 referred by him to substances of a simpler constitu- 
 tion, such as were afterwards employed in the famous 
 system of Gerhardt. 
 
 The word " radical " or " radicle " has long held 
 a place in the language of chemistry, but very 
 different meanings have at different times been 
 attached to it. Dismissing as unimportant the use 
 which was made of this word by Lavoisier, and 
 afterwards by Berzelius, we must not omit to recall 
 the fact that in 1832 it received a very definite 
 application in consequence of the discoveries of 
 Liebig and Wb'hler. In that year l the two great 
 German chemists showed that bitter almond oil, 
 benzoic acid, and many substances derived from 
 them, might all be represented as compounds 
 
 1 " Untersuchungen uber das Radical der Benzoesaure " ( Liebig' s 
 Annalen, iii. 249). 
 
PKOGRESS OF SCIENTIFIC CHEMISTRY 111 
 
 of one and the same radicle, benzoyl, C 14 H 5 O 2 
 (0 = 6, O = 8), thus: 
 
 Hydride of benzoyl .... C 14 H 5 2 H 
 
 (bitter almond oil). 
 Oxide of benzoyl .... C 14 H 5 O 2 
 
 (anhydrous benzoic acid). 
 Hydrated oxide of benzoyl . . . C 14 H 5 2 0, HO 
 
 (benzoic acid). 
 
 Chloride of benzoyl .... C 14 H 5 2 G1 
 Hydrate of benzoyl .... C 14 H 5 0, HO 
 
 (benzoic alcohol). 
 
 Henceforward it became the object of many 
 chemical investigations to separate and isolate com- 
 pounds of this kind, which seemed to play the part 
 of elements, and the recognition of which served as 
 the basis for the definition by Liebig of organic 
 chemistry as the chemistry of compound radicles, while 
 inorganic chemistry was the chemistry of elementary 
 or simple bodies. One of the most notable dis- 
 coveries resulting from such investigations was the 
 discovery of ethyl by Frankland in 1848. This 
 substance was obtained by the action of zinc upon 
 ethyl iodide, from which compound it was originally 
 supp'osed that the metal simply withdrew the iodine, 
 setting the ethyl free. The gas thus obtained was 
 for some years supposed to be the true radicle of 
 ether and alcohol, which were respectively con- 
 sidered to be its oxide and hydrated oxide : 
 
 Ethyl . . . C 4 H 5 (0 = 6) 
 
 Ether . . . C 4 H 5 (0 = 8) 
 
 Alcohol . . . C 4 H 6 OHO 
 
112 A SHOKT HISTORY OF THE 
 
 It turned out, however, that this compound really 
 contains in the molecule double these proportions of 
 carbon and hydrogen, and is therefore more correctly 
 expressed by the formula C 4 H 5 .C 4 H 5 , or C 8 H 1Q . 
 (0=6, O=8). 
 
 A corresponding revision awaited many others of 
 the isolable radicles, so soon as the law of Avogadro 
 came to be applied, as it was by Gerhardt, to the 
 adjustment of molecular weights. Cyanogen, for 
 example, was not simply C 2 N, but (C 2 N) 2 . Kakodyl, 
 in like manner, discovered by Bunsen in 1839, 
 affords another example of a well-defined radicle, 
 which, by union with oxygen, sulphur, and the halo- 
 gens, affords a long series of compounds. At first, 
 and for many years, represented as (C 2 H 3 ) 2 As, that 
 is, as arsenious methide, containing two equivalents 
 of methyl, it was afterwards proved to have double 
 that molecular weight. 
 
 Gradually, then, it began to be recognised that a 
 " radical " was nothing more than a group of symbols 
 which represented a constituent common to a series 
 of compounds, but not necessarily capable of existence 
 in the free or isolated state. In place of " radical " 
 Gerhardt introduced the word residue, defining its 
 application to those portions of a compound which can 
 be transferred to another compound by the process of 
 double decomposition or exchange, and which are not 
 necessarily capable of being isolated. Accordingly, 
 H, 01, or H0 2 would be entitled to rank as a radical or 
 residue equally with groups such as benzoyl, C 4 H 5 2 . 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 113 
 
 The classification of known carbon compounds, 
 which in a few years had become very numerous, 
 was now a matter of urgent necessity. A system 
 based upon the recognition of radicles like methyl, 
 ethyl, benzoyl, &c., had been attempted by Liebig, 
 but without full success, partly because the relations 
 of many of the newly discovered compounds were 
 obscure, and partly because each series was inde- 
 pendent and isolated from all others. 
 
 The existence of a relation among the successive 
 members of the series of alcohols was first pointed 
 out by Schiel hi 1842, and a corresponding rela- 
 tion having been detected by Dumas among the 
 fatty acids, Gerhardt called them " homologous " 
 series. Such a series includes compounds of the 
 same chemical character, the successive terms of 
 which increase by an addition of C 2 H 2 (C=6) or 
 CH 2 (C= 12), this addition being attended by cor- 
 responding rise in the density, boiling point, and, 
 frequently, in the melting points. The arrangement 
 of carbon compounds in homologous series is one 
 which has stood the test of time, but at the period 
 now referred to few such series were known, and 
 these usually imperfectly. 
 
 A remarkable effort to bring some system into 
 the chaos then existing was made by Auguste 
 Laurent, whose views were first embodied in a 
 memoir published in 1844, 1 but afterwards in a 
 more elaborate and extended form in a volume 
 
 1 "Classification Chimique " (Compt. Rend., xix. 1089). 
 
 H 
 
114 A SHORT HISTORY OF THE 
 
 which appeared in English only after his death. 1 
 In this system Laurent supposed every organic 
 compound to contain a nucleus composed usually of 
 carbon and hydrogen, and from this fundamental 
 nucleus derived nuclei could be formed by substitu- 
 tion. Thus upon the nucleus, C 4 H 4 , called etherene, 
 acetic acid, C 4 H 4 + 4 , was formed, and from the 
 derived nucleus chloretherene, C^HgCl), chloracetic 
 acid, C 4 H 3 C1 + 4 , was formed. Laurent represented 
 all organic compounds as containing an even num- 
 ber of atoms, and hence was led to employ for many 
 compounds formulae which were the double of those 
 commonly received. The nucleus theory, though in 
 reality a sort of extension of the theory of radicles, 
 was practically of little use, and was adopted as the 
 basis of classification of organic compounds only 
 in one notable book, namely, in Gmelin's well- 
 known and comprehensive " Handbook of Organic 
 Chemistry." The writings of Laurent, .however, 
 must have had a great influence upon the chemical 
 thought of his day, and even now there are pages 
 in his " Chemical Method " which afford refreshing 
 and instructive reading. 
 
 But though the theory of nuclei did not provide 
 the system of classification so urgently needed, 
 especially for organic compounds, it happened fortu- 
 nately for the progress of science that the young 
 Laurent became associated with a still younger col- 
 
 1 Translated by Odling, 1855 (Publications of the Cavendish 
 Society), under the title " Chemical Method." 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 115 
 
 league, Charles Gerhardt, who joined him in opposi- 
 tion to the dualistic system of Berzelius. Gerhardt's 
 scientific career may be said to have begun when in 
 1842, at the age of about twenty-six, he published 
 an important series of papers on the classification of 
 organic substances. 1 Here he examined the founda- 
 tion of the system of equivalents then in use, and 
 showed how a great simplification might be effected, 
 while reducing the whole system of formulae to a 
 common standard, as already explained (Chap. III.). 
 Laurent finally adopted Gerhardt's system of for- 
 mulae, and made use of it in his " Chemical Method." 
 According to this system the formula of each sub- 
 stance represents that quantity of it which, in the 
 state of vapour, would occupy two volumes, the unit 
 volume being the space occupied by one part by 
 weight of hydrogen gas at standard temperature and 
 pressure. Hence, the formulae of the compounds 
 HC1, H 2 0, NH 3 , C0 2 , C 2 H 6 0, &c, represent two 
 volumes, while to express the same bulk of the 
 elements and radicles H, Cl, C 2 H., CN, &c., these 
 atomic symbols must be doubled, thus 
 
 HH or H 2 , C1C1 or C1 2 , C 2 H 5 C 2 H 6 or (C 2 H 6 ) 2 , CNCN or (CN) 2 . 
 
 The problem involved in the classification of 
 organic compounds was not, however, yet solved. 
 It required a better knowledge of the relations and 
 properties of many compounds, which was only to 
 
 1 "Ueber die Chemische Classification der Organischen Sub- 
 stanzen " (Erdmann's Jour. f. Prakt. Chemie, 1842-43). 
 
116 A SHORT HISTORY OF THE 
 
 be obtained by further experimental investigations. 
 To these Laurent and Gerhardt both contributed 
 their full share : Laurent more especially by his 
 work on the hydrocarbon naphthalene, and the 
 numerous derivatives formed from it and from other 
 compounds by the action of chlorine ; Gerhardt by 
 his discovery, though much later, of secondary and 
 tertiary amides, and of the anhydrides of acetic and 
 other acids. 
 
 The next important step appears to have been 
 taken when in 1849 Wurtz, 1 in studying the action 
 of potash on cyanic and cyanuric ethers, was led to 
 the discovery of the compound ammonias. 
 
 Many vegetable matters which possess strong 
 physiological properties have long been known to 
 owe their activity to the presence of definite crys- 
 talline or volatile liquid substances, which have the 
 property, in common with ammonia, of combining 
 with acids to form salts. Thus opium contains 
 morphine and other similar compounds, nux-vomica 
 contains strychnine, cinchona bark quinine, while 
 tobacco yields nicotine. In consequence of this 
 basic property these active substances were long 
 ago called alkaloids. So far back as 1837, Berzelius, 
 applying to these substances his own views con- 
 cerning chemical constitution, expressed the opinion 
 that they consisted of ammonia, which gave them 
 
 1 " Sur une Serie d'alcalis organiques homologues avec I'ammo- 
 nique" (Compt. Rend., xxviii. 223, 1849); "Recherches sur les 
 ammoniaques composes" (Compt. Rend., xxix. 169, 1849). 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 117 
 
 their basic character, united with a " conjunct " 
 composed of carbon with hydrogen or oxygen, by 
 which the character of the ammonia was modified. 
 On the other hand, Liebig in 1840 had expressed 
 the view that if "we were able to replace by ami- 
 dogen the oxygen in the oxides of methyl and 
 ethyl, in the oxides of two basic radicles, we should, 
 without the slightest doubt, obtain a series of com- 
 pounds exhibiting a deportment similar in every 
 respect to that of ammonia. Expressed in sym- 
 bols, a compound of the formula C 4 H 5 H 9 ]Xr = EAd 
 would be endowed with basic properties." 
 
 Wurtz regarded ammonia as a link between in- 
 organic and organic chemistry, and suggested that 
 if it contained carbon it would be considered as the 
 simplest representative of the organic bases. In his 
 newly discovered bases, C 2 H 5 N, and C 4 H 7 N (C = 6), 
 he saw organic compounds which might be viewed 
 according to the hypotheses just explained, either 
 as methyl ether, C 2 H 3 0, or common ether, C 4 H 5 0, 
 in which 1 equivalent of oxygen was replaced by 
 1 equivalent of amidogen, NH 2 , or as ammonia, in 
 which 1 equivalent of hydrogen is replaced by 
 methyl, C 2 H 3 , or ethyl C 4 H 5 . 
 
 The following formulae show these relations : 
 
 H 3 N Ammonia, or NH 2 H Hydramide. 
 
 C 2 H 5 N Methyl ammonia, or NH 2 C 2 H 3 Methlyamide. 
 
 C 4 H 7 N Ethyl ammonia, or NH 2 C 4 H 5 Ethylamide. 
 
 He adds, " Je me servirai de preference des mots 
 
118 A SHORT HISTORY OF THE 
 
 methylamide, ethylamide," but in the second paper 
 he at once uses, without preface or explanation, the 
 terms methylamine, ethylamine, valeramine, to which 
 we have been so long accustomed. 
 
 Here, then, it is evident that the great question of 
 chemical " constitution " was beginning seriously to" 
 occupy the attention of chemists. Indeed, there is 
 proof that this problem must have arisen already, 
 for the theory of compound radicles introduced by 
 Liebig and Wohler in 1832, and publicly promul- 
 gated by Dumas in 1837, 1 and the still earlier 
 ammonium theory of Berzelius, all implied, if they 
 did not explicitly declare, that distinct functions 
 were performed by the different elements composing 
 a given compound. In a paper 2 of Hofmann's a 
 year before Wurtz's discovery, the following passage 
 occurs in reference to the constitution of alkaloids : 
 " A relation between the nitrogen and the satu- 
 rating capacity remained extremely probable, and 
 chemists now commenced to assume the nitrogen 
 as existing in these bases under two forms. In 
 almost all cases that portion of this element to 
 which the basic properties were referred was be- 
 lieved to be in the form of amidogen, ammonia, or 
 oxide of ammonium, while the views respecting the 
 other portions were for the most part less decided." 
 
 Later on, in his memoir on the action of cyanogen 
 chloride, bromide, and iodide on aniline, he expressed 
 
 1 Compt. Rend., v. 567 ; Brit. Ass. Rep. 
 2 Jour. Chem. Soc., i. 172 (1848). 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 119 
 
 the view that it is " exceedingly probable that the 
 organic bases are indeed conjugated ammonia 
 compounds." This view, however, he soon modified, 
 for almost immediately afterwards, in discussing the 
 results of experiments made with the object of with- 
 drawing the elements of water from oxalate of aniline, 
 and so obtaining a nitril corresponding to cyanogen, 
 he remarks that " the impossibility of obtaining 
 an anilo -cyanogen throws some doubt on the pre- 
 existence of ammonia in aniline. It is probably 
 more in conformity with truth to consider aniline 
 as a substitution product, as ammonia, in which part 
 of the hydrogen is replaced by phenyl." The ethyl 
 and methyl derivatives of aniline were then pro- 
 duced by the action of ethyl and methyl bromides 
 on aniline, 1 and shown to have the relation to 
 aniline which has ever since been recognised, and 
 which is displayed in the following formulae : 
 
 Aniline. ' Ethylaniline. Diethylaniline. 
 
 18 H 6 ) C 12 H 6) C 12 H 5 ) 
 
 H U C 4 H 6 N G 4 H 8 lN 
 
 H ) H ) C 4 H 5 ) 
 
 Here, then, was a definite recognition of ammonia 
 as a type, upon which was modelled, not only the 
 new artificial compound ammonias, ethylamine, 
 methylamine, and the rest, but all nitrogen com- 
 pounds possessing basic properties. The acknow- 
 ledgment of the type ammonia was followed very 
 soon by a corresponding scheme for bringing into 
 
 1 Compt. Rend., xxix. 184 (1849). 
 
120 A SHORT HISTORY OF THE 
 
 one class the numerous and very various compounds 
 known as salts, together with a number of other 
 substances containing oxygen. This conception we 
 owe to Williamson. In 1 8 5 1 he wrote as follows : 
 " I believe that throughout inorganic chemistry, and 
 for the best known organic compounds, one single 
 type will be sufficient it is that of water, repre- 
 sented as containing two atoms of hydrogen to one 
 of oxygen, thus jjO-" 1 In the course of this paper 
 he gave the following formulae : 
 
 Potash **0 
 
 t\. 
 
 T7- 
 
 Oxide of potassium ^0 
 
 Methyl alcohol pjr 
 
 U1 3 
 
 Ethyl alcohol n ^ 
 
 Potassium acetate 
 
 Anhydrous acetic acid .... 
 
 Nitrate of potash 
 
 Hydric chlorite 
 
 Hydric chlorate ..... 
 
 xx 
 
 Hydric perchlorate ^i? 3 
 
 In many cases, however, a multiple of this for- 
 mula, H 0, must be used, as in expressing the com- 
 
 1 " Constitution of Salts" (Chem. Gaz., 1851, and/owr. Chem, Soc., 
 jv, 1852, 350). 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 121 
 
 position of bibasic acids and salts like carbonates, 
 sulphates, oxalates. He then explained the action 
 of potash on cyanic ether discovered by Wurtz, and 
 expressed the change by the following diagram : 
 
 K 9 K 2 
 
 2 2 
 
 (H 2 ) (CO) 
 
 C 2 H 5 C 2 H 5 
 
 CO (H 2 ) 
 
 N N 
 
 " One atom of carbonic oxide is here equivalent to 
 two atoms of hydrogen, and, by replacing them, holds 
 together the two atoms of hydrate in which they were 
 contained, thus necessarily forming a bibasic com- 
 pound 2 , carbonate of potash." This passage is 
 especially noteworthy, for, if not the very first, it is 
 one of the earliest definite expressions of the idea of 
 linkage ; that is, of two portions of the same mole- 
 cule being held together by the agency of an atom 
 or group of atoms which serves as a link between 
 them. 
 
 Not long afterwards Williamson also discovered 
 what he called tribasic formic ether, which was ob- 
 tained by heating together chloroform and sodium 
 ethylate. 
 
 Cl NaOC 2 H 6 ( OC 2 H 5 
 
 CHC1 + NaOC 2 H 5 = OH \ OC 2 H 5 + 3NaCl 
 
 Cl NaOC 2 H 5 I OC 2 H, 
 
 Here manifestly the residue, CH, of chloroform 
 links together three residues of alcohol, OC 2 H 5 , and 
 
122 A SHORT HISTORY OF THE 
 
 combines them in one molecule. It is true that 
 about 1839 Gerhardt had introduced the use of 
 the term " copulated " or " conjugated " as applied 
 to certain compounds, such as the product of the 
 action of sulphuric acid upon benzene or upon benzoic 
 acid, in which sulphuric acid cannot be recognised 
 by the usual tests, and was therefore supposed to 
 be in a state of more intimate union than is found 
 in the sulphates. Berzelius also adopted the same 
 expression, but with a somewhat different meaning, 
 for he seems to have applied the term " copula " to 
 neutral or passive substances supposed to be asso- 
 ciated with an active body. Thus acetic acid was 
 regarded by Berzelius as owing its acid properties 
 and chemical activity to oxalic acid, but united with 
 the copula methyl. Evidently these ideas are en- 
 tirely different, and the copula of Berzelius did not 
 connote, as the word might seem to imply, the idea 
 of holding together two things which would otherwise 
 separate. 
 
 The idea of classifying according to types, then, 
 belongs to Dumas; but for the extension of the 
 idea and modifications by which it was converted 
 into the really serviceable system which it con- 
 tinued to be for many years, it needed the co-opera- 
 tion of many active minds. We have seen how 
 Williamson introduced the water type, and how the 
 discoveries of Wurtz and of Hofmann led to the 
 establishment of the ammonia type. A large 
 number of compounds remained, however, which 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 123 
 
 these two substances, water and ammonia, scarcely 
 seemed to represent. Gerhardt therefore added 
 the hydrochloric acid type and the hydrogen type, 
 and in the fourth volume of his celebrated Traits 
 de Chimie Organique he supplied a tabular scheme, 
 showing at a glance how these four types might 
 be made the basis for a system of classification. 
 
 To these, as more appropriate to the compounds 
 of carbon, Kolbe added the carbonic acid type ; but 
 he used the double value, 12, for carbon, while he 
 retained the equivalent value, 8, for the symbol of 
 oxygen. Hence the formula, C0 4 , which he used 
 for the type, represented the oxygen as divisible into 
 four equal parts, whereas there is no reason for 
 believing it to be divisible into more than two 
 parts. The marsh gas type, CH 4 , introduced a little 
 later by Kekule, was of far greater importance. 
 
 Further, in 1854 Williamson showed that by the 
 action of phosphorus pentachloride on sulphuric 
 acid a compound is formed, which may be regarded 
 as formed on the conjoint or mixed type of hydro- 
 chloric acid and water by the introduction of the 
 residue S0 2 in place of two atoms of hydrogen, one 
 derived from each of these compounds, thus : 
 
 Cl) Cl 
 
 H 
 
 H 2 | 
 
 This chlorhydrin of sulphuric acid, or chlorhydrated 
 sulphuric acid, evidently owes its existence to the 
 
124 A SHORT HISTORY OF THE 
 
 property possessed by the S0 2 of binding together 
 the Cl and the HO, thus retaining the properties of 
 a chloride joined to those of a hydrate. 
 
 In like manner sulphuric acid itself contains the 
 radicle S0 2 , which in this compound holds together 
 the residues of two molecules of water. Hence, to 
 represent it, a condensed type must be assumed in 
 which two molecules of water are concerned, thus : 
 
 Type H ? Sulphuric Acid H > n 
 
 H > > 
 
 
 
 H \ 
 
 A similar condensed type must be used to repre- 
 sent such acids as phosphoric and arsenic acids, 1 
 and compounds such as glycol 2 and glycerin, 3 which 
 are not acids but are alcoholic in character. 
 
 i Graham in 1833 first demonstrated the true nature of arsenic 
 and phosphoric acids, and so laid the foundation of the idea of the 
 " basicity " of acids. The following formulae show the composition 
 of the phosphoric acids, according to the binary system, using equi- 
 valents, and for comparison the unitary formulas, using atomic 
 weights : 
 
 Orthophosphoric acid . . . PO 5 3HO 
 Pyrophosphoric acid . . . PO 5 2HO H 4 P 2 O 7 
 
 Metaphosphoric acid . . . PO 5 HO HPO 3 
 
 See Alembic Club Reprints, No. 10. 
 
 2 Glycol was discovered by Wurtz in 1856, as a result of the 
 application of the theory of condensed types to a consideration of 
 the case of glycerin. " Sur le Glycol ou alcool diatomique " (Compt. 
 Rend., xliii. 199). 
 
 3 "Berthelot was the first to demonstrate the true nature of 
 glycerin as an alcoholic body capable of interacting with three 
 molecules of such acids as acetic and palmitic." Ann. Chim., xli. 
 266 (1854). 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 125 
 
 Triple Water Type. Phosphoric Acid. Glycerin. 
 
 
 
 ri 
 
 H 
 
 
 
 H 
 
 O 
 
 O 
 
 H 
 
 
 
 In such compounds as these three residues of water 
 are linked together in one molecule by the poly- 
 atomic radicle. 
 
 But while adopting the idea, Gerhardt imposed a 
 strict limitation upon the signification of the word 
 type, as used in his system. For, while the hypo- 
 thesis of Dumas implied that compounds grouped 
 together were composed of elements arranged in a 
 similar order within the molecule, Gerhardt insisted 
 that any knowledge of what we should now call 
 " constitution " was inaccessible to experiment. The 
 four bodies selected by him as types, namely water, 
 hydrogen chloride, ammonia, and hydrogen, in equal 
 volumes in the state of gas, that is, H 2 0, HC1, H 3 N, 
 and H , he called types of double decomposition} 
 Water, for example, in a great variety of trans- 
 formations, can exchange its hydrogen or oxygen 
 for other elements, giving rise to all the innumer- 
 able oxides or sulphides appearing as bases, acids, 
 salts, alcohols, and so forth. 
 
 It is difficult for us now to understand why 
 Gerhardt insisted so strongly upon the distinction 
 which he thought he saw between the significance 
 
 1 Traite, de Chim. Org., tome iv. 286. 
 
126 A SHORT HISTORY OF THE 
 
 of his own type formulae and those of Dumas. For 
 when substitution occurs it must be assumed, accord- 
 ing to either system, that the radicle which is intro- 
 duced, whether elementary or compound, takes the 
 same place as the hydrogen or other element which 
 is removed. So that, although it might be asserted 
 that nothing was known, for example, of the arrange- 
 ment of the atoms in a molecule of water, whatever 
 that arrangement might be assumed to be, it would 
 be maintained in the molecules of all the substances 
 which are fairly to be regarded as derivatives of water. 
 In the meantime, however, facts were being accu- 
 mulated in other directions, which subsequently 
 served a most important purpose in the develop- 
 ment of the ideas of more modern times in reference 
 to constitution. As already mentioned, Frankland 
 discovered in 1848 the compound which was re- 
 garded by all chemists of that time as the free 
 radicle ethyl. In the course of the investigation 
 it was discovered that the zinc does not only com- 
 bine with the iodine, setting the ethyl free, but that 
 it also combines with a portion of ethyl, producing 
 a definite compound possessed of very remarkable 
 properties. Zinc ethyl, or zinc ethide, ZnC 4 H 5 , or 
 in modern symbols, Zn(C 2 H 5 ) 2 , was the first of a 
 long series of " organo-metallic " compounds, the 
 majority of which were prepared and examined by 
 Frankland himself. Writing on the subject many 
 years later, he says : l " I had not proceeded far in the 
 
 1 " Experimental Researches " (collected 1877), p. 145. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 127 
 
 investigation of these compounds, before the facts 
 brought to light began to impress upon me the 
 existence of a fixity in the maximum combining 
 value or capacity of saturation in the metallic ele- 
 ments which had not before been suspected. That 
 stannous ethide refused to combine with more than 
 the complementary number of atoms of chlorine, &c., 
 necessary to form a molecule symmetrical with 
 stannic chloride, surprised me greatly at first ; but 
 such behaviour in this and other organo-metallic 
 bodies scarcely permitted of misinterpretation. It 
 was evident that the atoms of zinc, tin, arsenic, 
 antimony, &c., had only room, so to speak, for the 
 attachment of a fixed and definite number of the 
 atoms of other elements ; or, as I should now express 
 it, of the bonds of other elements. This hypothesis 
 constitutes the basis of what has since been called 
 the doctrine of atomicity or equivalence of elements, 
 and it was, so far as I am aware, the first announce- 
 ment of that doctrine." This statement is justified 
 by reference to the original paper 1 (dated May 10, 
 1852), in which occurs a clear expression of the 
 view, that in the several compounds of a given 
 element, " no matter what the character of the uniting 
 atoms may be, the combining power of the attracting 
 element, if I may be allowed the term, is always 
 satisfied by the same number of these atoms." In 
 other words, the combining capacity of an element 
 does not depend upon the nature of the atoms with 
 
 1 Phil. Trans., cxlii. 417. 
 
128 A SHOKT HISTORY OF THE 
 
 which it is united, but is determined by the special 
 character of the element itself. 
 
 It appears, then, that while the distinction be- 
 tween univalent and polyvalent atoms and groups 
 of atoms was first perceived by Williamson, we owe 
 to Frankland the first enunciation of the doctrine 
 that each atom possesses a capacity for combination 
 peculiar to itself, and usually limited according to a 
 definite rule. 
 
 At this time and henceforward for some years 
 chemists were much occupied with the business of 
 ranging compounds, old as well as new, under their 
 appropriate types, with the natural result that much 
 wrangling ensued upon questions which, when 
 answered, seemed to lose their importance. To 
 what type, for example, should such a compound 
 as nitrous oxide be referred to the water type or to 
 the ammonia type ? Or, again, should chloroform 
 be regarded as a derivative of condensed hydro- 
 chloric acid, seeing that in its want of reactivity 
 with silver salts it gave no sign of being a chloride ? 
 Gradually it became obvious to all, that the system 
 of types was a merely artificial scheme of classifica- 
 tion, as Gerhardt himself had pointed out long ago : 
 " Comme je 1'ai souvent dit, mes radicaux et mes 
 types ne sont que des symboles, destines a concreter 
 en quelque sorte certains rapports de composition 
 et de transformation." l 
 
 But though types of themselves could serve only 
 
 1 Traitd, tome iv. 611. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 129 
 
 a temporary purpose, and afford merely one point 
 of view, the ideas which arose out of their employ- 
 ment were of the utmost importance ; for, as we have 
 seen, they led to the notion of atomic combining 
 power, or " atomicity," and from this sprang the 
 doctrine of the linkage of atoms and modern views 
 as to chemical structure. For this immense stride 
 in advance the world is indebted chiefly to the writ- 
 ings of Kekule. The question whether the credit 
 of initiating the idea of atomicity belongs to this 
 chemist need not now be discussed, notwithstanding 
 the claim which he put forward some years later, 1 
 inasmuch as it has already been shown that the 
 idea was first conceived and expressed by Frankland 
 in 1852. 2 Kekule", however, deserves to be regarded 
 as the founder of modern structural chemistry, inas- 
 much as the linkage of carbon to carbon is first 
 clearly set forth in his paper " On the Constitution 
 and Metamorphoses of Chemical Compounds, and on 
 the Chemical Nature of Carbon," published in 1858. 3 
 In this paper he pointed out that the disposition of 
 the atoms constituting a radicle can be represented 
 after sufficient study of its reactions, and in the 
 case of carbon by a study of the compounds of that 
 element. If we consider the simplest compounds of 
 carbon, CH 4 , CH 8 C1, CC1 4 , CHC1 3 , CO 2 , COC1 2 , CS 2 , 
 
 1 "Sur 1'atomicitd des elements" (Compt. Rend., Iviii. 510, 1864). 
 
 2 This point has been very carefully discussed by Professor Japp 
 in the " Kekule Memorial Lecture " (Jour. Chem, ^c.,lxxiii. 108-120, 
 1898). 
 
 * Liebig's Annalen, cvi. 129. 
 
130 A SHORT HISTORY OF THE 
 
 CNH, &c., it is obvious, he says, that " th.e sum of the 
 chemical units which are combined with one atom of 
 carbon is equal to 4. ... In the case of substances 
 which contain several atoms of carbon, it must be 
 assumed that some of the atoms at least are held in 
 the compound in the same way by the affinity of 
 the carbon, and that the carbon atoms themselves 
 are united together, whereby, of course, a part of the 
 affinity of one is combined with an equal part of the 
 affinity of another. The simplest, and therefore the 
 most probable case of such a union of two carbon 
 atoms, is that in which one unit of affinity of one 
 atom is combined with one unit of the other. Of 
 the 2x4 units of affinity of the two carbon atoms, 
 two are employed in holding the two atoms together ; 
 there remain, therefore, six which may be united with 
 atoms of other elements. In other words, a group 
 of two atoms of carbon, C 2 , is hexatomic ; it may 
 form a compound with six atoms of a monatomic 
 element, or generally with so many atoms that the 
 sum of the chemical units of these is six. (Examples : 
 C 2 H 6 , C 2 H 5 C1, C 2 H 4 C1 2 , C 2 C1 6 , C 2 H 3 N, C 2 N 2 , C 9 H 4 0, 
 C 2 H 3 OC1, C 2 H 2 2 , &c.) 
 
 " If more than two carbon atoms unite in the same 
 way, the basicity of the carbon group is increased by 
 two units for each additional atom. The number 
 of atoms of hydrogen (chemical units) which are 
 united in this way with n atoms of carbon, may be 
 expressed as follows : 
 
PEOGRESS OF SCIENTIFIC CHEMISTRY 131 
 
 Suppose n .= 5, the basicity is therefore 12. (Ex- 
 amples: C 5 H 12 , C^Cl, C 5 H 10 C1 2 , C 6 H 9 N, &c.) So 
 far it has been assumed that all the atoms associ- 
 ated with the carbon are held by the affinity of the 
 carbon. It may just as well be conceived, that in 
 the case of polyatomic elements, 0, N, &c., only a 
 part of the affinity of these, only one of the two 
 units of the oxygen, for example, or only one of the 
 three units of the nitrogen, is combined with the 
 carbon; so that one of the units of affinity of the 
 oxygen, or two out of the three units of affinity of 
 the nitrogen, remain over, and may be united with 
 other elements. These other elements are thus only 
 indirectly combined with the carbon, as indicated 
 by the typical manner of writing the formulae 
 
 H N 3 
 
 H ) C A 
 
 Similarly, by means of the oxygen or the nitrogen, 
 different carbon groups are held together." 
 
 Further on he refers to the fact that while in a 
 very large number of organic compounds such simple 
 combination of the carbon atoms may be assumed, 
 others exist which contain so much carbon in the 
 molecule, that for them a closer combination of the 
 carbon must be supposed ; and then he mentions 
 benzol and its derivatives and homologues, as well 
 as naphthalene, as examples of compounds richer in 
 carbon. 
 
 Looking at the position of Gerhardt's system of 
 
132 A SHORT HISTORY OF THE 
 
 types of double decomposition about this time, it 
 is clear that the chief advance which had been 
 made resulted from the recognition of polyatomic 
 radicles; and the power they possess of linking 
 together two or more residues or radicles. As to 
 typical formulae themselves, though so recently in- 
 troduced, their importance was already diminishing; 
 for " typical formulae being representations of reac- 
 tions, it follows that if a substance affords two or 
 more distinct kinds of reactions, either of formation 
 or of decomposition, it may be consistently repre- 
 sented by formulae deriving from a corresponding 
 number of distinct types." l Hence in a rational 
 formula of the highest possible degree of generality, 
 that is, one which would express all the possible 
 reactions of a body, the constituent radicles must 
 be reduced to the greatest possible simplicity they 
 must, in fact, be reduced to their elementary atoms. 
 Such a generalised type formula becomes equivalent 
 to a modern constitutional formula. Kekule himself 
 seems for some time to have hesitated to accept the 
 full consequences of his own conclusions, for while 
 in the paper just quoted he definitely proclaimed 
 the tetratomic character of carbon, and showed how 
 the atoms of this element might combine together, 
 he was writing in his famous Lehrbuch 2 " that 
 
 1 See British Association Report on the "Recent Progress and 
 Present State of Organic Chemistry," by G. C. Foster (Report for 
 1859, p. 1). 
 
 2 Lehrbuch der Organischen Chcmie oder der Chemie der Kohlenstoff- 
 Verbindungen, i. 157. 
 
PKOGRESS OF SCIENTIFIC CHEMISTRY 133 
 
 rational formulae are only formulae representing the 
 reactions and not the constitution of a body ; they 
 are nothing more than expressions for the meta- 
 morphoses of a body, and a comparison of different 
 bodies, and are in nowise intended as an expression 
 of the constitution, that is of the arrangement, of 
 the atoms in the actual substance." The reasons 
 he goes on to express for this hesitating position are 
 not quite satisfactory. 
 
 Almost at the same time, however, another young 
 chemist, A. S. Couper, in a paper which appeared 
 in all the chief chemical journals, put forth inde- 
 pendently quite similar views as to the peculiarities 
 of the element carbon, and by using a system of 
 graphic formulae had even gone a step further. 
 Gouper showed, in the course of his paper, that the 
 quantity of carbon represented by C 2 (C = 6) is never 
 divided during chemical changes; hence he remarks: 
 " It is only consequent to write, with Gerhardt, C 2 
 simply as C, it being understood that the equivalent 
 of carbon is 12." 1 In consequence of peculiar views 
 of his own, however, he retains 0=8, and hence all 
 his formulae contain O 2 , or O...O in place of 0, but 
 otherwise they resemble modern structural formulae. 
 As Couper's graphic formulae are the first symbols 
 of the kind, and are so remarkably like those in 
 common use at the present day, they deserve to be 
 kept in remembrance. One example will suffice; 
 
 1 A. S. Couper, " Sur une nouvelle theorie chimique" (Ann. 
 Chim., liii. 469 ; Phil. Mag. [4], xvi. 104). 
 
134 
 
 A SHORT HISTORY OF THE 
 
 propyl alcohol, C 3 H g O, he expressed by the follow- 
 ing scheme : 
 
 . OH 
 
 C . , . H, 
 
 C . . . H 3 
 
 This was in 1858; in the following year Kekule's 
 Lehrbuch appeared, and in a footnote (p. 160) he 
 introduced a system of graphic formulae, in which 
 the basicity (i.e. valency) of each atom is expressed by 
 the size of the symbol in the following manner : 
 
 CD 
 
 (HIM) 
 
 Hydrogen w . f -_ 
 Chloride. w er. 
 
 Ammonia. 
 
 Oxygen. 
 
 Sulphur 
 Dioxide. 
 
 Nitric Acid. 
 
 Kekule thought it necessary to state in a foot- 
 note that by these symbols the size of the atoms 
 is not intended, but only the respective number of 
 chemical units of each element. Symbols somewhat 
 similar to these were afterwards used by Naquet 
 and other writers. 
 
 Little use of these methods of notation was, how- 
 ever, made for some years, but a system was intro- 
 duced in 1865 by Professor Crum Brown, which 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 135 
 
 with slight modification has stood its ground and 
 served a valuable purpose. The symbols explain 
 themselves, but the author thought it advisable to 
 state that by them he did " not mean to indicate 
 the physical, but merely the chemical position of 
 the atoms." l The formula for ethane, as an example, 
 is on this system as follows : 
 
 
 
 - 
 
 In 1866 Dr. Frankland adopted practically the 
 same system, merely omitting the circles round the 
 symbols of the elements, and henceforward graphic 
 formulae came freely into general use. By this time 
 the doctrine of atomicity or valency was fully estab- 
 lished, and the reluctance which was at first felt 
 by many to use any kind of symbol which seemed 
 to suggest, even remotely, a pictorial representation 
 of a molecule, gradually wore off, in proportion as 
 belief became general in the linking of atoms in 
 definite order as a necessary consequence of the 
 doctrine of atomicity. 
 
 From this time forward chemical theory has ad- 
 vanced upon the same road, and mechanical ideas 
 of constitution have more and more permeated the 
 views of chemists as to the mutual relations of 
 
 1 Jour. Chem. Soc., xviii. 232 (1865). 
 
136 A SHORT HISTORY OF THE 
 
 chemically united atoms. This will be developed 
 to some extent in a later chapter, but the subject 
 cannot be relinquished at this point without a brief 
 reference to one important consequence of Kekule's 
 further study of the question. 
 
 The story carries us back to about 1825, when 
 Faraday, in examining the volatile liquid which was 
 wont to collect in the receivers then in use, contain- 
 ing compressed oil gas, discovered benzol, or, as it is 
 now written, benzene. 1 The same liquid was obtained by 
 Mitscherlich in 1833 as the sole volatile product of 
 the distillation of benzoic acid with lime, the elements 
 of carbon dioxide being abstracted from the acid 
 and retained by the lime in the form of carbonate. 
 Benzoic acid itself was at that time known chiefly 
 as a constituent of a fragrant medicinal resin ob- 
 tained from a tree, the Styrax benzoin, growing in 
 Sumatra. Hence the name of the new compound, 
 which Faraday had analysed and shown to consist 
 of carbon and hydrogen. Coal-tar naphtha was not 
 investigated till about 1847, when Mansfield found 
 in it a more abundant and convenient source of the 
 new hydrocarbon, which henceforward was always 
 manufactured from that liquid. Mitscherlich had 
 discovered that by mixing benzol with strong nitric 
 acid a peculiar aromatic-scented volatile nitro-com- 
 
 1 This is not to be confounded with the volatile mixture of liquids 
 known as benzine or benzoline, distilled from the lightest parts of 
 petroleum, and which consists of hydrocarbons of the paraffin series, 
 
V OF THK ' ' 
 
 XJNIVERSr 
 
 PROGRESS OF SCIENTIFIC 
 
 pound was formed, and this in the hands of the 
 Russian chemist Zinin was converted into the liquid 
 which has long been known as aniline, and which 
 has since become famous as the material from which 
 the earliest of the artificial dyes were produced. 
 Benzol, or benzene, contains carbon and hydrogen 
 in proportions which are most simply represented 
 by the formula CH ; but the density of the vapour 
 as compared with that of hydrogen, is such that 
 the molecule must be expressed by six times this 
 formula, or C 6 H fi . This compound is the first term 
 of a series of hydrocarbons, of which we owe to the 
 labours of Mansfield the discovery of several mem- 
 bers which stand in the relation of homologues to 
 benzene. Thus we have 
 
 Benzene .- C 6 H 6 
 
 Toluene . . .... . C 7 H 8 
 
 Xylene . . . . . . C 8 H 10 
 
 Cumene . . . . . C 9 H 12 
 
 Cymene ...... C 10 H 14 , &c. 
 
 Some years later Fittig and Tollens proved that 
 all these hydrocarbons can be formed from benzene, 
 by the substitution of methyl CH 3 , ethyl C 2 H 5 , and 
 so forth, for one or more atoms of hydrogen in benzene, 
 and they supplied a method by which this exchange 
 can be actually effected. It soon became obvious that 
 these compounds stood in some very intimate relation- 
 ship towards essential oil of almonds, essence of cinna- 
 mon, essence of cummin, and other fragrant or aromatic 
 
138 A SHORT HISTORY OF THE 
 
 compounds, among which could be counted hydro- 
 carbons, alcohols, aldehyds, acids, &c. Further, it 
 was recognised that these aromatic compounds pre- 
 sented certain chemical characteristics which dis- 
 tinguish them from the corresponding series derived 
 from fats, and the acids, &c., connected with them. 
 And thus carbon compounds began to be ranged under 
 the two great divisions of Fatty and Aromatic com- 
 pounds, which are recognised at the present day. 
 These facts were by 1865 sufficiently established to 
 provide material for reflection, and problems which 
 would have to be encountered by the promoters of 
 the new doctrine of atomic linkage. Kekule ob- 
 served that benzene is the first term of the series 
 to which it belongs, and the supposed lower homo- 
 logue, C 5 H 4 , was shown to have no existence. The 
 group of six atoms of carbon seems to form a unit 
 which continues to subsist undivided in all the 
 numerous compounds into which the molecule of 
 benzene, more or less stripped of hydrogen, enters 
 as a constituent. Further, the hydrogen in this 
 molecule shows no signs of being gathered round 
 any one or more of the carbon atoms to the disad- 
 vantage of the rest. In other words, it is equally 
 distributed among them, so that every atom of 
 carbon has an atom of hydrogen to itself. We 
 have seen how the idea became established, that an 
 atom of carbon is capable of linking itself to a 
 second atom by means of one unit of its atomicity, 
 or valency, as it has been variously called, so that 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 139 
 
 each of the two conjoined atoms loses one-fourth of 
 the power it possesses by itself of becoming attached 
 to other atoms. This may be diagrammatically ex- 
 
 pressed as follows : 
 
 I i 
 
 -C-C- 
 i i 
 
 By the application of the same idea to an indefinite 
 number of atoms, it is easy to conceive of a long 
 chain of carbon atoms linked together in a similar 
 manner, thus : 
 
 ill ill 
 
 -C-C-C- ... &c. ... -C-C-C- 
 
 iil III 
 
 Or again, these atoms may be supposed to be joined 
 in consequence of the suppression of two units of 
 valency between two neighbouring atoms, which 
 
 may be expressed in a similar manner 
 
 f 
 
 =C = C=C=,&c. 
 
 \ . ''-. 
 
 Or again, by suppression of one and two units 
 
 alternately 
 
 i i i i i 
 
 All these are examples of what are now called 
 open chains, in which the terminal atoms have their 
 affinities satisfied by means of hydrogen, chlorine, 
 or some other element which ends the series. The 
 idea which Kekule introduced is derived very simply 
 
140 A SHORT HISTORY OF THE 
 
 from this. He explained the peculiar composition 
 and properties of benzene, by supposing that six 
 atoms of carbon are joined together by alternate 
 single and double linkages, and that the terminal 
 atoms are united into a closed chain or ring. This 
 may be expressed by writing the six carbon atoms 
 in a straight line as above, and joining the extremi- 
 ties; but since the molecule of benzene is symmetrical 
 in its chemical behaviour, it has long been the 
 custom to represent it by a symmetrical figure, the 
 hexagon, thus : 
 
 H c 
 
 In a large proportion of cases the fourth unit of 
 valency, here, by a guess, represented as linking the 
 carbons together, is simply unaccounted for, and 
 exercised in a way not yet understood; it may, 
 therefore, be left unexpressed. In like manner it 
 is unnecessary to write all the symbols for hydrogen 
 every time the formula is required, and in practice 
 a skeleton symbol has come into use which expresses 
 at once all that is wanted. Further, for reasons 
 which may readily be understood, it is convenient 
 to number the carbon atoms so as to distinguish 
 one from another in the ring, and to signify their 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 141 
 
 relative positions ; and so the formula may be re- 
 duced to 
 
 Or more simply as follows, the numbers being 
 understood 
 
 The idea involved in this formula of a chain closed 
 by the interlocking of its terminal atoms has been 
 largely applied, and there is reason now to think 
 that there are groups of compounds formed upon 
 nuclei consisting of 3, 4, and 5, as well as 6 atoms 
 of carbon, and that other elements, such as nitrogen, 
 oxygen, and sulphur, may take a place in the ring. 
 The comparative ease with which rings of six carbons 
 may be formed, and the much greater difficulty 
 experienced in producing rings made up of three or 
 four atoms of that element, have been explained by 
 hypotheses of a remarkable mechanical character 
 
142 A SHOKT HISTORY OF THE 
 
 which will be explained in a later chapter. Kekule's 
 formula, in the simple shape to which it has been 
 reduced by omitting the debatable and much 
 debated question of what becomes of the fourth 
 valency, is accepted by all chemists. Indeed, it 
 may now be regarded as established upon a basis of 
 experimental evidence, comparable with that upon 
 which rests the Atomic Theory itself. It requires 
 in this form no assumptions that have not been 
 fully verified by experiment ; * and it has rendered 
 such service in the development of " organic " 
 chemistry generally, that it would be no exaggera- 
 tion to assert, that without it the greater part of 
 modern knowledge in this field could never have 
 been established. Without its guidance a large 
 proportion of the familiar and beautiful artificial 
 colouring matters would probably never have been 
 discovered, and the greater number of the numerous 
 modern synthetical medicinal agents could never* 
 have been produced, save, possibly, by accident. 
 The beauty and fertility of Kekule's theory of the 
 " aromatic " compounds can only be fully appreci- 
 ated by those who have made a study of the subject, 
 
 1 Probably the most important single contribution to the subject 
 ever made is the remarkable memoir by W. Kb'rner, published in 
 1876, in which he applied the principle, indicated in general terms 
 by Kekule", by which the problem of the position of radicles intro- 
 duced by substitution into the molecule of benzene was solved. In 
 working out the application of the method, he described about 120 
 new compounds. (Gazzetta Chimica Italiana, iv., translated and 
 abstracted into the Journal of the Chemical Society, 1876, vol. i.) 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 143 
 
 but the literature relating to this department of 
 chemistry, even for purposes of exposition, is very 
 extensive. It is not possible to enter into more 
 detail in these pages, but the student will readily 
 find in every text-book of " organic " chemistry a 
 statement more or less complete of the applications 
 of the theory. 
 
 It has also been well remarked that " Kekule's 
 structural formulas cleared away at one stroke the 
 entire brood of pseudo-constitutional formulae. If 
 organic chemists no longer waste their time in 
 wrangling over the question whether, for example, 
 methylamine is methane, in which one atom of 
 hydrogen is replaced by the amido group, or am- 
 monia, in which one atom of hydrogen is replaced 
 by methyl, the merit is Kekule's. 
 
 " The accuracy of Kekule's predictions has done 
 more to inspire a belief in the utility of legitimate 
 hypothesis in chemistry, and has, therefore, done 
 more for the deductive side of the science, than that 
 of almost any other investigator. His work stands 
 pre-eminent as an example of the power of ideas." ] 
 
 1 Professor Japp's "Kekul Memorial Lecture" (Trans. Chem. 
 Soc., Feb. 1898). 
 
144 A SHORT HISTORY OF THE 
 
 CHAPTER VI 
 
 THE DEVELOPMENT OF SYNTHETICAL CHEMISTRY 
 
 FROM the time of Lavoisier the subject-matter of 
 chemistry has been divided, in nearly all general 
 treatises on the science, into two chief departments, 
 the mineral or inorganic, and the organic. In the 
 older text-books vegetable substances were described 
 separately from those of animal origin ; but down 
 to comparatively recent times the idea commonly 
 prevailed that the composition and properties of 
 both these classes of compounds were governed by 
 laws differing essentially from those which were 
 found to prevail among substances of mineral 
 nature. It was recognised that organic compounds 
 are usually more complex in composition, and more 
 easily decomposed by heat than minerals, and that 
 " we cannot always proceed, as with materials de- 
 rived from the mineral kingdom, from a knowledge 
 of their components to the actual formation of the 
 substances themselves. It is not probable," it was 
 said, " that we shall ever attain the power of imi- 
 tating nature in these operations. For in the 
 functions of a living plant a directing vital prin- 
 ciple appears to be concerned peculiar to animated 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 145 
 
 bodies, and superior to and differing from the cause 
 which has been termed chemical affinity " (Henry's 
 "Elements of Experimental Chemistry," 1829). 
 Another purely hypothetical distinction, the result 
 of ignorance of the constitution of the majority of 
 organic compounds, was based upon the assumption 
 of a binary plan of combination in inorganic com- 
 pounds not observable in organic compounds. So 
 late as 1863 the ninth edition of Fownes' " Manual 
 of Chemistry," with Hofmann as joint editor, con- 
 tains a passage in which it is explained that " copper 
 and oxygen combine to oxide of copper, potassium 
 and oxygen to potassa, sulphur and oxygen to sul- 
 phuric acid ; sulphuric acid in its turn combines 
 both with oxide of copper and oxide of potas- 
 sium, generating a pair of salts, which are again 
 capable of uniting to form the double compound 
 CuO, S0 3 + K0, S0 3 . The most complicated pro- 
 ducts of inorganic chemistry may be thus shown to 
 be built up by this repeated pairing on the part of 
 their constituents. With organic bodies, however, 
 the case is strikingly different ; no such arrange- 
 ment can be traced." 
 
 Organic chemistry then originally, and for a long 
 time, was understood to mean the study of com- 
 pounds derived from organic sources ; but as to the 
 constitution of such compounds, opinion has passed 
 through many successive phases of modification. In 
 1837 Liebig, in conjunction with Dumas, defined 
 organic chemistry as the chemistry of compound 
 
146 A SHORT HISTORY OF THE 
 
 radicles. Gmelin, ten years later, included in the 
 organic division of his " Handbook " compounds 
 which contained more than one atom (C=6) 
 of carbon. Gerhardt's definition is as follows : 
 " La chimie organique s'occupe de 1'etude des lois 
 d'apres lesquelles se met amor phosent les matieres 
 qui constituent les plantes et les animaux ; elle a 
 pour but la connaissance des moyens propres a 
 composer les substances organiques en dehors de 
 Teconomie vivante " (Traitt, i. 7). But recognising 
 carbon as the essential and characteristic element, 
 Gerhardt made no distinction between those com- 
 pounds which contain only one atom and those 
 which contain more than one atom of this element. 
 Hence he described as fully the oxides and sul- 
 phides of carbon and the carbonates as the more 
 complex compounds which follow in the book. 
 
 Kekule defined organic chemistry as the Chem- 
 istry of the Carbon Compounds, 1 and pointed out 
 that the separate treatment of such compounds is 
 chiefly a matter of convenience, and is rendered 
 necessary in consequence of their very large num- 
 ber, and the great practical as well as theoretical 
 importance of so many of them. 2 
 
 
 
 1 The title of his well-known treatise expresses this association 
 of ideas: "Lehrbuch der organischen Chemie, oder der Chemie 
 der Kohlenstoff-Verbindungen," 1859. 
 
 2 " Wir definiren also die organische Chemie als die Chemie der 
 Kohlenstoff-Verbindungen. Wir sehen dabei keinen Gegensatz 
 zwischen unorganischen und organischen Verbindungen " (Lehr- 
 buck, i. 11). 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 147 
 
 Kekule, like Gerhardt, included in his book a 
 description of the oxides and other simple com- 
 pounds of carbon. Schorlemmer, recognising the 
 important part played by hydrogen in the com- 
 pounds of carbon, defined Organic Chemistry as the 
 " Chemistry of the Hydrocarbons and their Deriva- 
 tives." This, however, is a definition belonging to 
 more recent times (Lehrbuch der Kohlenstoff- Verbin- 
 dungen oder der Organiscken Chemie, 1872 ; in Eng- 
 lish, " A Manual of the Chemistry of the Carbon 
 Compounds or Organic Chemistry," 1874). 
 
 The definition of the province of " Organic " 
 Chemistry and investigation of the nature and 
 constitution of " organic " compounds are matters 
 of more than merely technical interest. They con- 
 cern the great question as to the sources and dis- 
 tribution of energy in nature, and the origin and 
 operation of life itself. 
 
 Previously to the publication of Berthelot's Chimie 
 Organique fondle sur la Synthese (1860) no syste- 
 matic research had been attempted in the direction 
 of building up compounds of carbon, comparable 
 with natural organic compounds, by the union of the 
 elements of which they are composed. Two notable 
 though isolated examples of the production of or- 
 ganic compounds by total synthesis are afforded 
 by Wohler's formation of urea in 1828, and Kolbe's 
 synthesis of acetic acid in 1845. That Wohler's 
 discovery should not have attracted more attention 
 than it did for many years is all the more remark- 
 
148 A SHORT HISTORY OF THE 
 
 able, because the author himself seems to have been 
 fully aware of its significance. His words are as 
 follows : l " J'obtins le resultat inattendu que par le 
 combinaison de 1'acide cyanique avec Fammoniaque 
 il se produit de 1'uree ; fait d'autant plus remarquable 
 qu'il offre un exemple de la formation artificielle 
 d'une matiere organique et merne de nature animale 
 au moyen de principes inorganiques. . . . Je ne 
 parlerai pas davantage des proprietes de cette uree 
 artificielle puisqu'elles sont tout-a-fait semblables a 
 celles que Ton peut trouver dans les ecrits de Proust, 
 Prout, et autres sur 1'uree." 
 
 Kolbe's process was more complicated. By the 
 action of chlorine upon carbon bisulphide, which is 
 formed by the union of its two elements, carbon 
 tetrachloride is obtained, and at a red heat this 
 compound is decomposed into chlorine and tetra- 
 chlorethylene, C 2 C1 4 . In the presence of water, 
 chlorine, and sunlight, this compound yields tri- 
 chloracetic acid, probably through the intermediate 
 formation of hexchlorethane 
 
 C 2 C1 6 + 2H 2 = CC1 3 . C0 2 H + 3HC1. 
 
 Trichloracetic acid mixed with water can be reduced 
 to acetic acid by the action of sodium amalgam. 
 It is almost needless to add that the acetic acid 
 prepared by this synthetical process is identical in 
 all respects with the acetic acid obtained from vinegar, 
 which is the product of a peculiar fermentation. It 
 
 1 Ann. Chim. Phys, [2], xxxvii. 330 (1828). 
 
PROGEESS OF SCIENTIFIC CHEMISTRY 149 
 
 may be added that Melsens also succeeded, in 1846, 
 in reducing carbon tetrachloride to marsh gas by the 
 action of the same reagents. 
 
 The methods employed by Berthelot were for the 
 most part simple and direct. Starting from carbon 
 or one of its oxides, he obtained several hydro- 
 carbons from which, as well known even at that 
 time, more complex compounds can be built up. 
 The following are a few examples of his processes. 
 
 At the temperature of the electric arc, carbon 
 and hydrogen unite directly to form acetylene, C 2 H 2 . 
 The same compound is produced by the action of 
 the electric spark on a mixture of hydrogen with 
 carbonic oxide, with carbon bisulphide, or cyanogen. 
 From acetylene, by acting upon its peculiar copper 
 compound by hydrogen in the nascent state, ethylene 
 is produced. Thus 
 
 2xC+ccH 2 = xC 2 H 2 acetylene. 
 C 2 H 2 + H 2 =C 2 H 4 ethylene. 
 
 Ethylene united with the elements of water con- 
 stitutes common alcohol. To effect this union the 
 gas may be made to combine with a hydracid, 
 especially with hydrogen iodide 
 
 C 2 H 4 +HI=C 2 H 6 I, 
 
 and the resulting compound heated with potassium 
 acetate gives ethyl acetate, from which, by the action 
 of potash, alcohol may be obtained 
 
 C 2 H 6 I + KC a H 3 O 2 =C 2 H 6 C 2 H 3 O 2 + KI, and 
 C 2 H 5 C 2 H 3 2 + KHO = KC 2 H 3 2 + C 2 H 5 HO alcohol. 
 
150 A SHORT HISTORY OF THE 
 
 Alcohol may of course be employed as the starting 
 point for the production not only of aldehyd, acetic 
 acid, or acetone, but of a large number of com- 
 pounds of more complex composition. 
 
 Alcohol, however, is obtainable from ethylene by 
 the simpler process of dissolving the gas in hot 
 sulphuric acid, whereby sulphovinic acid is formed, 
 and subsequently decomposing this compound by 
 distilling it with water. 
 
 4 and 
 
 Acetylene may be employed as the material from 
 which benzene and all its multitudinous train of 
 derivatives may be formed, for by the simple appli- 
 cation of a moderate heat to the gas it suffers 
 condensation almost completely into benzene 
 
 3C 2 H 2 =C 6 H 6 . 
 
 At higher temperatures more complex hydrocar- 
 bons, such as napthalene, C 10 H g , and anthracene, 
 C U H 10 , are produced. But beside the direct union 
 of carbon with hydrogen at the temperature of the 
 electric arc, the formation of hydrocarbons from 
 these two elements may be accomplished by the 
 addition of one preliminary stage to the series of 
 operations. Thus carbon may be combined with 
 sulphur, forming carbon bisulphide, and hydrogen 
 with sulphur, forming hydrogen sulphide ; if, then, 
 these two compounds be transmitted simultaneously 
 through a tube containing heated metallic copper, 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 151 
 
 the sulphur is withdrawn by the metal, and the 
 other elements unite at the moment of their libera- 
 tion in the presence of each other. A mixture 
 chiefly of marsh gas and ethylene results 
 
 &c. 
 
 Or carbon monoxide may be used as the parent 
 material. This gas is not affected by caustic potash 
 at the common temperature of the air, but at the 
 temperature of 100 and upwards it is absorbed by 
 a concentrated solution of potash with formation of 
 potassium formate 
 
 CO + KHO = KCH0 2 . 
 
 From this compound formic acid itself may be 
 obtained, a substance originally procured by the 
 distillation of ants with water, and in more recent 
 times by the oxidation of various materials of 
 vegetable origin. 
 
 Such examples as these are sufficient to prove 
 that compounds identical in every respect with 
 the products of animal and vegetable life may be 
 formed from dead mineral matter. Berthelot was so 
 anxious to establish this point beyond the possibility 
 of dispute, that he gives in detail one series of 
 experiments in which the carbon employed was ob- 
 tained in the form of carbon dioxide from barium 
 carbonate ; it was then made to pass successively 
 through the forms of carbonic oxide, formic acid, 
 barium formate, ethylene, ethylene bromide, ethy- 
 lene again, and finally into ethylsulphuric acid, 
 
152 A SHORT HISTORY OF THE 
 
 and its crystallised barium salt from which alcohol, 
 the ultimate object of these experiments, was gene- 
 rated. Water and carbon dioxide, then, were the 
 only compounds from which the elements of this 
 alcohol were derived. 
 
 Chemists, then, were long ago completely con- 
 vinced that so-called " organic " compounds, though 
 frequently more complex than inorganic compounds, 
 such as metallic oxides and salts, owe their existence 
 to the operation of the same chemical affinity which 
 governs the formation and transformation of these 
 compounds. The peculiarities of their constitution 
 arise from the facts pointed out nearly forty years 
 ago by KekuM and by Couper (see Chapter V.), 
 namely that carbon, the essential element in all 
 such combinations, possesses the remarkable power 
 of uniting with itself atom to atom ; and secondly, 
 that all the combining units of such an atom or 
 group of atoms may be saturated by hydrogen. 
 
 Considering the now universal recognition of the 
 true province of Organic Chemistry, it is unfortunate 
 that the names Inorganic and Organic should be still 
 retained for the two co-ordinate departments of the 
 science, and that the division between them, though 
 practically necessary, should be maintained in so 
 absolute and arbitrary a manner. To speak of 
 Organic chemistry at all, is only one of the many 
 examples which might be given of the etymological 
 confusion which everywhere prevails in the language 
 of chemistry. There are organic beings, and there 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 153 
 
 may be a chemistry peculiar to their functions, but 
 this is what would be rightly comprehended under 
 the term physiological chemistry, or rather chemical 
 physiology. All chemists, however, now agree that 
 there is but one chemistry so far as principles are 
 concerned, no matter how various may be its ap- 
 plications. The sharp distinction and separation of 
 inorganic and organic chemistry is in teaching and 
 learning a source of great loss and inconvenience ; 
 for until a student has become acquainted with 
 the properties of at least a few carefully selected 
 carbon compounds, he can have no true idea of the 
 relation of composition and constitution to physical 
 properties, which is only to be acquired by the 
 study of the phenomena of isomerism and of series. 
 Among metallic and mineral compounds there is 
 nothing corresponding to homologous series, unless 
 we admit the relations which have been traced (see 
 Chap. IV.) between the atomic weights of certain 
 elements and their properties. But these are far 
 less regular than the relations observable among 
 the members of a series like the acetic series of 
 acids. Moreover, a student who is limited to the 
 study of salts and other metallic compounds has 
 few opportunities of observing the methods by 
 which " constitution " is established, and even the 
 processes and effects of oxidation and reduction can 
 be but imperfectly understood. 
 
 Since the time of Berthelot's experimental investi- 
 gation of the conditions under which such carbon 
 
154 A SHORT HISTORY OF THE 
 
 compounds may be formed, the art of chemical 
 synthesis, the building up of complex from simple 
 materials, has made remarkable progress. Not only 
 the simple formic and acetic acids, but complex 
 vegetable acids, such as tartaric, citric, salicylic, 
 gallic, cinnamic ; not marsh gas and ethylic alcohol 
 only, but phenols, indigo, alizarin, sugars, and even 
 alkaloids identical with those extracted from the 
 tissues of plants, are now producible by purely 
 chemical processes in the laboratory. It might 
 appear that such triumphs would justify anticipa- 
 tions of still greater advances, by which it might 
 become possible to penetrate into the citadel of 
 life itself. Nevertheless the warning that a limit, 
 though distant yet, is certainly set in this direction 
 to the powers of man, appears to be as justifiable 
 now, and even as necessary, as in the days when all 
 these definite organic compounds were supposed to 
 be producible only through the agency of a " vital 
 force." Never yet has any compound approaching 
 the character and composition of albumen or any 
 proteid been formed by artificial methods, and it is 
 at least improbable that it ever will be without the 
 assistance of living organisms. But even supposing 
 the secret of the chemical constitution of all the 
 colloidal proteid substances fully understood, the 
 eloquent words of Gerhardt are just as true as ever : 
 " Jamais le chimiste ne saura produire dans son 
 laboratoire ni un muscle, ni un nerf, ni une feuille, 
 ni une fleur, ni la plus legere fibre ; car a supposer 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 155 
 
 meme qu'il apprenne a composer toutes les matieres 
 qui constituent les muscles, les nerfs, les feuilles, les 
 fleurs, les fibres vege tales, il manquera toujours de 
 la libre disposition de cet agent inconnu qui co- 
 ordonne ces matieres en organes doues de vie, c'est-a- 
 dire doues d'un mouvement propre, different de celui 
 qu'impriment a la matiere les attractions chimiques " 
 (Traitt, L, Introduction, iv.). 
 
 Emphasis has been recently l laid upon a physical 
 distinction between living and dead matter, which 
 has for many years been suspected. Notwithstanding 
 the remarkable advances in synthetical chemistry, 
 of which a few salient examples are described in 
 this chapter, one result has never yet been achieved, 
 and that is the production by purely chemical pro- 
 cesses, and without the aid of living matter, of a 
 compound possessing the power of rotating the plane 
 of polarisation of a ray of polarised light, in the 
 manner which is so characteristic of many of the 
 proximate constituents of animal and vegetable 
 structures, such as the proteids, the sugars, and 
 various hydrocarbons, acids, and alkaloids. This 
 question will be discussed in the next chapter. It 
 is sufficient to remark here that this distinction is, 
 after all, arbitrary and artificial, and possibly, nay, 
 even probably, will disappear before the light of 
 more knowledge derived from further experimental 
 investigation, just as the " vital force " of the older 
 school was shown, full forty years ago, to be unneces- 
 
 1 See "Address to the Chemical Section of the British Association," 
 Bristol, 1898, by Professor F. R. Japp, President of the Section. 
 
156 A SHORT HISTORY OF THE 
 
 sary to the formation of even the most complicated 
 among definite compounds of carbon. 
 
 The methods employed by the modern chemist in 
 the construction of carbon compounds, the mole- 
 cules of which are known to contain many atoms 
 of carbon, are so numerous that it is not possible 
 in such a sketch as this to do more than indicate 
 broadly their general nature. It has already been 
 shown how Berthelot and others succeeded in uniting 
 the elements carbon, hydrogen, oxygen, and nitrogen 
 into compounds previously believed to be derivable 
 only from organic sources, but such substances as 
 formic acid, alcohol, and acetic acid are after all 
 very simple in constitution, and between such com- 
 pounds as these and the common constituents of 
 vegetable and animal juices there is a wide interval. 
 Tartaric acid, for example, contains four atoms of 
 carbon, citric acid six atoms, common sugar twelve 
 atoms of the same element, while caffeine, the alka- 
 loid of tea and coffee, contains eight atoms of carbon 
 with four atoms of nitrogen ; and the blue colouring 
 matter of indigo contains sixteen atoms of carbon 
 and two atoms of nitrogen in the molecule. The 
 art of uniting carbon to carbon has now become so 
 familiar, that chemists are apt to forget that its dis- 
 covery and application is so recent that it really 
 belongs to the present generation. Before attempt- 
 ing to illustrate by an example or two the nature of 
 the methods employed, it is necessary to remind the 
 reader that advances in the direction referred to 
 postulate certain fundamental ideas, the origin and 
 
PEOGRESS OF SCIENTIFIC CHEMISTRY 157 
 
 development of which has already (Chap. V.) been 
 described. We believe now that in a molecule the 
 constituent atoms are not thrown together confusedly 
 in a general jumble, but that a definite order is 
 maintained, and that this order can be, at least with 
 great probability, inferred from the properties, modes 
 of formation, and decomposition of the compound. 
 In such a system it is recognised that some elements 
 are united together directly and some indirectly, 
 according to their respective valencies, into a struc- 
 ture which, though it doubtless possesses great elas- 
 ticity, is more or less permanent. So long as the 
 compound retains its identity its constituent atoms 
 do not wander about, but retain their relative posi- 
 tions ; whereas, if a change of relative position does 
 occur, this is immediately manifested by a change 
 in the properties or chemical behaviour of the sub- 
 stance. The knowledge which we now possess of 
 the " constitution " of so large a number of chemical 
 compounds is, of course, the outcome of an immense 
 amount of patient labour, the utility of which has 
 not always been obvious to the unlearned. 
 
 One method of uniting carbon to carbon is based 
 upon the peculiar properties of cyanogen and its 
 compound with hydrogen, hydrocyanic or prussic 
 acid. Cyanogen is very familiar as a compound 
 radicle which is capable of playing the same kind 
 of part as chlorine or bromine, and of being ex- 
 changed for either of those elements. If, then, 
 such a substance as ethylene, which, as already 
 explained, can be prepared from its elements, is 
 
158 
 
 A SHORT HISTORY OF THE 
 
 first converted into its bromide, C 2 H 4 Br 2 , the bro- 
 mine may be exchanged for cyanogen by simply 
 heating it with potassium cyanide. The compound 
 ethylene cyanide, C 2 H 4 (CN) 2 , results, and if this is 
 boiled with an acid or an alkali the nitrogen is 
 removed in the form of ammonia, while an equi- 
 valent quantity of oxygen and hydrogen is intro- 
 duced, and succinic acid, C 2 H 4 (CO 2 H) 2 , is obtained. 
 This was accomplished by Maxwell Simpson in 
 1861, and since the means of converting succinic 
 acid into raceinic acid had been previously made 
 known by Perkin and Duppa, here was a method of 
 building up a complex vegetable acid by a process 
 of purely chemical synthesis. 
 
 This succession of operations may be traced in 
 the following series of formulae, which serve to show 
 how the chemical constitution of tartaric acid has 
 been determined : 
 
 CH 2 
 
 II 
 
 CH 2 
 
 CH 2 .CN 
 CH 2 .CN 
 CH 2 .CO.OH 
 CH 2 .CO.OH 
 
 CHBr.CO.OH 
 
 i 
 
 CHBr.CO.OH 
 
 CH(OH).CO.OH 
 
 l 
 
 CH(OH).CO.OH 
 
 Ethylene. 
 
 Ethylene cyanide. 
 
 Succinic acid. 
 
 Dibromo-succinic acid. 
 
 Tartaric or racemic acid. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 159 
 
 Hydrocyanic acid has the power of uniting with 
 many compounds, especially with aldehyds and 
 ketones, in such a manner that its carbon becomes 
 attached to the carbon of the aldehyd or ketone, 
 while the nitrogen can afterwards be eliminated, if 
 desired, by the action of dilute acids or alkalis, as 
 already explained. In this way, for example, lactic 
 acid, C 3 H 6 3 , the acid of sour milk, may be formed 
 from aldehyd, C 2 H 4 0. 
 
 Another method of producing more complex 
 from simple carbon compounds is founded on the 
 tendency of many of them to undergo the process 
 of polymerisation, or formation of new molecules 
 by the union of several into one. The polymerisa- 
 tion of acetylene into benzene has been already 
 mentioned, and to this may be added such cases as 
 the conversion of aldehyd into paralaehyd, 
 
 3C 2 H 4 = C 6 H 12 3 , 
 
 and the production of the sugar-like substances 
 formose from formaldehyd, 
 
 6CH 2 = C 6 H 12 6 , 
 and acrose from glyceric aldehyd, 
 
 2C 3 H 6 = C 6 H 12 6 . 
 
 Many of such changes occur spontaneously in course 
 of time or under the influence of heat. 
 
 In other cases condensation is effected by the 
 use of agents which have a tendency to unite with 
 water or with ammonia, which may be separated 
 
 ! ' 
 UKTTVT 
 
160 A SHORT HISTOKY OF THE 
 
 from the elements of the parent substance as a by- 
 product. For example, acetone mixed with strong 
 sulphuric acid yields water and trirnethyl-benzene 
 or mesitylene 
 
 Another very interesting method of joining carbon 
 to carbon arises out of the remarkable influence 
 exercised by oxygen upon the properties of hydro- 
 gen atoms attached, not to the oxygen itself but 
 to carbon atoms near to it within the molecule. 
 Malonic acid, for example, contains a series of three 
 carbon atoms, of which the central is united with 
 two atoms of hydrogen, while the two lateral are 
 combined with oxygen, thus : 
 
 -CO-CH 2 -CO- 
 
 Now, when carbon is united to hydrogen only, the 
 hydrogen is incapable of being disturbed by the 
 action of sodium, and in compounds which contain 
 three carbon atoms thus united, but all combined 
 with hydrogen, contact with sodium or a sodium 
 compound would have no effect. But if the ethereal 
 salt of malonic acid is mixed with sodium ethylate, 
 one of the two atoms of hydrogen is immediately 
 replaced by sodium, thus : 
 
 -CO -CHNa -00- 
 
 The sodium thus introduced may be easily ex- 
 changed for a hydrocarbon radicle ethyl, for 
 example by bringing the new compound into 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 161 
 
 contact with, the iodide, while the sodium is elimi- 
 nated in the form of sodium iodide. A compound 
 thus results, in which the carbon of the ethyl is 
 attached to the carbon which was previously pro- 
 vided with hydrogen only, 
 
 -CO -CH -CO 
 
 C 2 H 5 
 
 The synthetical formation of large numbers of 
 complex compounds has been effected by making 
 use of this principle. 
 
 Enough has now been said to indicate the lines 
 upon which research has travelled during the last 
 thirty years or more, but it must be obvious that 
 the practical success of such operations is greatly 
 dependent upon providing the right physical condi- 
 tions, and these can only be arrived at as the result 
 of much experience in the laboratory. 
 
 The artificial production of complex carbon com- 
 pounds, possessing properties which render them 
 applicable to a great variety of practical purposes, 
 may be justly regarded as one of the triumphs of 
 modern chemistry. Many of these compounds, such, 
 for example, as salicylic acid, used extensively as 
 an antiseptic and as a remedial agent in medicine, 
 indigo and alizarin as dyes, coumarin and vanillin 
 as perfumes, are identical with the compounds pre- 
 viously known only as products of vegetable life, 
 and obtainable only from the substance of the 
 several plants which yield them. Some of these 
 
 L 
 
162 A SHORT HISTORY OF THE 
 
 discoveries, in consequence of which it has become 
 possible to dispense with the cultivation or collection 
 of large quantities of a plant, have been followed 
 by economic results of far-reaching effect. One of 
 the most notable instances of this kind is supplied 
 by the case of alizarin, the chief red colouring 
 matter of the madder root. The cultivation of this 
 plant, the Rubia tinctoria of the botanist, of which 
 the wild variety is found commonly in hedges in 
 Britain, has been for centuries carried on in the 
 south of Europe. It was introduced into the south 
 of France in 1766 by Jean Alt hen, to whom a 
 statue was erected at Avignon, in recognition of the 
 value of this service to the district. But in 1868 
 the relation of alizarin to anthracene, a hydro- 
 carbon present in the less volatile portion of coal- 
 tar oil, was established by Graebe and Liebermann. 
 Methods were immediately devised by W. H. 
 Perkin in this country, and by Caro, Graebe, and 
 Liebermann in Germany, by which the manufacture 
 of alizarin from anthracene became commercially 
 possible. Henceforth the cultivation of the madder 
 plant in the countries in which previously it had 
 been a crop of considerable money value, and 
 occupying large tracts of land, became unnecessary, 
 and it therefore speedily declined, and has now 
 almost disappeared. Dr. Perkin states 1 that the 
 value of the imports of madder root into the 
 United Kingdom had been previously about one 
 
 1 Lectures before the Society of Arts, 1879. 
 
PROGEESS OF SCIENTIFIC CHEMISTRY 163 
 
 million pounds sterling per annum ; and when we 
 reflect upon this, and upon the influence which is 
 imposed upon the inhabitants of a country district 
 by the necessity of learning new methods of cultiva- 
 tion, and of finding new markets, as the consequence 
 of the exchange upon so large a scale of one kind 
 of crop for others, perhaps untried, the great im- 
 portance of such a discovery becomes obvious. 
 
 Of the very numerous substances now emanating 
 from the chemical laboratory, many others, like ali- 
 zarin and indigo, are applicable as dyes, either to 
 cotton on the one hand, or to wool and silk on the 
 other. Substances like alizarin require the previous 
 application to the fabric of some substance, called 
 a "mordant," with which they can unite, forming 
 a compound which is not only insoluble in water, 
 and therefore is not washed out by water, but also 
 exhibits a brighter and characteristic colour, the tint 
 of which is determined by the mordant employed. 
 Thus with alizarin a red colour is produced by 
 alumina, a purple by peroxide of iron. The most 
 famous of these artificial substances of strong tinc- 
 torial power are, of course, the so-called " aniline 
 dyes," and these for the most part require no mor- 
 dant when applied to wool and silk. The story of 
 the discovery of the first of these colours, the sub- 
 stance originally called mauve, or aniline purple, 
 has become so familiar, and has been followed by 
 so many other wonders, that its interest may be 
 thought to have faded; but to Englishmen it 
 
164 A SHORT HISTORY OF THE 
 
 ought always to serve both as a source of justifiable 
 pride and as a warning for the present and the 
 future. This discovery was made and was worked 
 out into a practical process of manufacture in 1856 
 by our distinguished countryman, Dr. W. H. Perkin, 
 whose name has already been mentioned in con- 
 nection with alizarin. But the manufacture of the 
 vast series of colouring matters of every shade and 
 tint, of which mauve must be regarded as the 
 ancestor, has been gradually transferred to Germany, 
 where the cultivation of "organic" chemistry has 
 been fostered in the universities and technical high 
 schools, while it has been comparatively neglected in 
 the corresponding institutions of this country. 
 
 The history of the development of the coal-tar 
 colour industry would alone be sufficient to occupy 
 several volumes, and it is therefore impossible to do 
 justice to it in these pages ; l but it should be men- 
 tioned that the name " aniline," applied popularly to 
 these colours, is in a great many cases entirely a mis- 
 nomer. Mauve and magenta were the colours first 
 obtained, and they were formed by the action of oxi- 
 dising agents upon aniline, or rather upon a mixture 
 of aniline with some of its homologues, especially tolu- 
 idine. A large proportion of the colours now manufac- 
 tured are produced by chemical changes from other 
 
 1 An exceedingly interesting account was given by Dr. Perkin 
 of his own career, and of the discovery of mauve and other colours, 
 in connection with the Hofmann Memorial Lecture, May 1893, of 
 which a full report appears in the Transactions of the Chemical 
 Society [for 1896. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 165 
 
 substances obtained from the constituents of coal-tar, 
 for example, naphthalene, and by the application of 
 wholly different methods, of which the most im- 
 portant is the process known as " diazotisation," 
 discovered by P. Griess in 1865. This consists in 
 the introduction of two atoms of nitrogen, combined 
 on the one hand with carbon, and on the other with 
 an acid radicle or other group. Most of these com- 
 pounds are unstable, and decompose with explosion 
 when heated or struck, and are often rapidly affected 
 by light. 
 
 The problem presented by the intense colour 
 and tinctorial power of the " organic " colouring 
 matters has been much debated during the last 
 five-and-twenty years, but so far without the estab- 
 lishment of any general theory. In fact, it seems 
 improbable that substances so diverse in composi- 
 tion and in constitution should agree in any one 
 peculiarity of structure, to which the property of 
 selective absorption of light, that is, of colour, can 
 be fairly attributed. And if, for example, a particu- 
 lar molecular configuration is indeed connected with 
 the production of a red or yellow colour, it seems 
 likely that not the same, but some other configuration 
 would be required to transmit or to absorb vibrations 
 of very different wave-length belonging to the blue 
 or violet order of colours. Many of the facts ob- 
 served must, however, be admitted to be tempting. 
 For example, the very general development of yellow 
 colour by the introduction of the nitro group N0 2 
 
166 A SHORT HISTORY OF THE 
 
 into compounds, which are themselves colourless. 
 Phenol, C 6 H 5 OH, is perfectly white, nitric acid and 
 nitrates generally are white ; but the product of the 
 interaction of phenol and nitric acid is picric acid, 
 C 6 H 2 (N0 2 ) 3 OH, a common yellow dye. The diazo 
 group is also connected with the production of colour. 
 The influence of molecular weight in modifying 
 the shade of colour was one of the first observa- 
 tions of this kind made, and it was attended with 
 practical results of great importance. The red dye 
 of aniline, fuchsine, magenta, or rosaniline, as it 
 has been variously called, was converted into a series 
 of other dyes, in which the red was gradually sup- 
 pressed and blue developed, by the introduction of 
 methyl, ethyl, phenyl, and naphthyl groups, which 
 are simply composed of carbon and hydrogen, in 
 place of one or more atoms of hydrogen in the 
 original dye stuff. We thus arrive at the following 
 series : 
 
 C 20 H 21 N 3 Red. 
 
 C 20 H 18 (CH 3 ) 3 N 3 .... Keddish violet. 
 
 C 20 H 18 (C 2 H 6 ) 3 N 3 .... Pure violet. 
 
 C 20 H 18 (C 6 H 5 ) 3 N 3 .... Blue. 
 
 A remarkable fact about these compounds is that 
 the bases themselves, of which the formulae are 
 given above, are colourless, and it is only in the 
 form of salts that they behave as dyes. 
 
 Turning from the production of colouring matters, 
 a survey of the applications of " organic " chemistry 
 to useful purposes reveals such a variety and wealth 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 167 
 
 of material that the pages of no single book could 
 contain even a superficial sketch of the whole. And 
 it is not possible, therefore, in this place to do more 
 than point out the most important of the directions 
 in which the greatly enlarged knowledge of these 
 modern times has been applied. 
 
 The arts of peace and of war have alike profited by 
 the discoveries of the chemist. In medicine the physi- 
 cian is now provided with a bewildering host of new 
 agents. The introduction of antiseptics by Lister 
 produced a revolution in the practice of surgery as 
 great as that which resulted from the use of the anaes- 
 thetics by which pain is abolished, and both these 
 classes of agents are obtained from the laboratory of 
 the chemist. And now we have a choice of a great 
 variety of chemical compounds produced by synthe- 
 tical processes, and of which the physiological action 
 has been more or less completely investigated and 
 shown to be applicable to the treatment of dis- 
 ease. It is only necessary to recall a few out of 
 the scores of substances which have been pro- 
 posed for use. Among antipyretics there are anti- 
 febrine (acetanilide, C 6 H 5 .NH.C 2 H 3 0), phenacetine 
 (aceto - para - phenetidine, C 6 H 4 (OC 2 H 5 ) . NH . C 2 H 3 0), 
 and antipyrine or phenazone (phenyl- dimethyl - 
 isopyrazolone, CH CO 
 
 C.CH 3 -N.CH 3 
 
 Among anodynes and hypnotics there are par- 
 aldehyd (C 6 H 12 O 3 ), chloral (CC1 3 .COH) and its com- 
 
168 A SHORT HISTORY OF THE 
 
 binations, sulphonal (dimethyl - methane - diethyl 
 sulphone (CH 3 ) 2 C(S0 2 C 2 H 5 ) 2 ), and others. Among 
 anaesthetics there are not only the long familiar 
 chloroform and ether, but many substances of value 
 for the production of local insensibility to pain. Of 
 these, the alkaloid cocaine and its various deriva- 
 tives and substitutes are the most remarkable. 
 The employment of antiseptics has extended beyond 
 the application made by the surgeon in the treat- 
 ment of wounds, and the sanitarian as disinfectants, 
 to the preservation of milk, meat, fish, and various 
 other articles of food, until it has now become a 
 question whether the use of these substances in an 
 indiscriminate manner may not before long require 
 some legislative restriction. 
 
 The use of explosives is not now confined to their 
 applications to warlike purposes. The discovery of 
 nitro -glycerin and its employment in the form of 
 " dynamite " have contributed in no small degree to 
 the assistance of work which makes for peace, in 
 road and tunnel making, in quarrying, in shattering 
 rocky obstructions in rivers, and generally to the pur- 
 poses of the engineer. Discovered by Pelouze about 
 1838, gun-cotton has long been a famous explosive; 
 but the difficulties attending its manufacture and 
 storage at first interfered with its production on a 
 large scale, while the rapidity of its explosion, as 
 compared with that of the old black gunpowder, 
 prevented for a long time its use for artillery pur- 
 poses. By attention to certain details in the purifi- 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 169 
 
 cation of the cotton, both before and after its 
 immersion in the nitric acid, the stability of the 
 product is now insured ; and by mixing it with other 
 nitrates, and with various combustible, but not 
 explosible, substances which serve to diminish the 
 rapidity of its combustion, and so damp the violence 
 of its action upon the gun, explosives are now freely 
 manufactured which are applicable to sporting as 
 well as to warlike purposes. These mixtures, known 
 under the names of Schultze's powder, cordite, &c., 
 are valued for their smokeless combustion. 
 
 It is unnecessary to add further to the list of 
 applications which have been made of the " chemic 
 art" so far as concerns compounds of which the 
 atomic framework is composed of carbon. The 
 development of the industrial production of organic 
 dye-stuffs, drugs, antiseptics, explosives, illuminat- 
 ing oils and gases, perfumes, artificial substitutes 
 for india-rubber, ivory, parchment, and many other 
 things familiar in daily life, proceeds with increasing 
 rapidity, and the pages of journals of chemistry are 
 crowded with the description of new compounds. 
 The fertility of the methods employed seems to 
 show that for the present the commonly accepted 
 views of structural chemistry are sufficient, and will 
 perhaps prevail for some years to come. There are, 
 however, indications that ideas of valency require 
 considerable modification, and when that modifica- 
 tion has been agreed upon changes in formulae will 
 undoubtedly follow. 
 
170 A SHORT HISTORY OF THE 
 
 Some earlier pages of this chapter were occupied 
 with the consideration of the successive discoveries 
 by which it has been shown that many of the 
 definite chemical compounds, which were formerly 
 derived solely from organic sources, have been 
 successively produced by the operations of the 
 chemist, independently of animal and of plant. 
 Any one who attentively considers the details of such 
 laboratory processes as have been described, must 
 at once perceive that the chemist and the organism 
 proceed by very different ways to the attainment of 
 the same result. The methods of the laboratory 
 commonly require the employment of strong chemical 
 agents, caustic alkalis, acids, and the like, as well 
 as a high temperature. The range of temperature 
 within which processes of growth, of secretion, or of 
 excretion go on in the plant or animal is restricted 
 to a few degrees ; and the chemical changes occur 
 within a medium, the sap or blood, the composition 
 of which is extremely complicated, and altogether 
 unlike any reagent employed by the chemist. 
 Whether it will ever be possible to discover the 
 precise nature and order of the changes by which 
 a plant, for example, produces sugar or starch out 
 of carbonic acid and water, is a question which does 
 not admit of profitable discussion in the present 
 state of knowledge ; but the study of the remark- 
 able changes which go on in the long familiar 
 process of alcoholic fermentation has led to a great 
 extension of our knowledge of one class of agents 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 171 
 
 employed in the living organism, and a brief outline 
 of the successive theories which have been advanced 
 in regard to the nature of the fermentive process 
 itself will not be out of place. As every one knows, 
 wine is made from grape-juice, beer from solution of 
 malt or sugar, cider from the juice of apples, and so 
 forth. It is also familiar knowledge that the bever- 
 ages which result agree in containing alcohol, which 
 is formed, together with carbonic acid, out of the 
 elements of the sugar originally present in the liquid, 
 and which after fermentation is much reduced, or 
 altogether disappears. But every one does not 
 know what are the conditions which are essential to 
 this transformation, and what products, if any, are 
 formed along with the alcohol and carbonic acid. 
 Thanks to the researches of Pasteur, these condi- 
 tions are now pretty well established. A solution 
 of pure sugar in water may be kept without change 
 for an indefinite length of time, but if to this liquid 
 is added a minute amount of a phosphate, together 
 with a little nitrogenous matter, even in the inorganic 
 form of an ammonium salt, fermentation will set in 
 almost immediately on addition of a small quantity 
 of yeast, or, after a longer and variable interval of 
 time, if the liquid is exposed freely to the air. If 
 yeast has not been added, it will nevertheless be 
 found in the liquid as soon as fermentation has 
 manifestly commenced, and its presence has been 
 traced to the admission of stray yeast cells or spores, 
 which are now known to exist along with other 
 
172 A SHOKT HISTORY OF THE 
 
 organisms, in countless numbers, floating in the air. 
 During the process the temperature must not be 
 allowed to fall below about 40 F., nor to rise much 
 above 80 F. 
 
 The destruction of the sugar is indicated by the 
 gradual loss of sweetness by the solution, carbon 
 dioxide gas makes its escape with effervescence, and 
 the liquid retains alcohol with a small quantity of 
 glycerin and succinic acid, which are always pro- 
 duced, though the source of them is still a little 
 uncertain. If a definite amount of yeast has been 
 added, it will be found to have increased in quantity, 
 and the cells of which it is composed show under 
 the microscope the process of multiplication by 
 budding. 
 
 Reduced to its simplest form, this is the pheno- 
 menon exhibited during the change of sweet vegetable 
 juices into wine, an art which has been practised 
 from the earliest times of which tradition brings us 
 an account. Notwithstanding its great antiquity, 
 however, no definite knowledge concerning the 
 nature of the process was secured until times well 
 within the period to which this book refers, and 
 although various theories were propounded at suc- 
 cessive periods, from alchemical times onwards, they 
 were for this very reason all beside the mark. 
 The changes which have occurred within the last 
 fifty years in the hypotheses relating to alcoholic 
 fermentation, have been brought about in conse- 
 quence of the gradual recognition of the essential 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 173 
 
 part played in the process by the yeast which is 
 always present. 
 
 Of the several theories in the field forty years 
 ago, the most generally accepted was that of Liebig. 
 Regarding yeast merely as a putrescent mass, he 
 supposed the peculiar state of atomic motion, 
 hypothetically prevailing in all substances in that 
 condition, to be transmitted by contact with the 
 sugar to the atoms of that compound, which were 
 thus shaken asunder so as to give rise to new 
 products more stable than itself. This kinetic idea, 
 not objectionable in itself, but only because it paid 
 no regard to established facts, Liebig maintained in 
 some form or other to the end of his life. But in 
 science fact stands before authority, and notwith- 
 standing the influence of the great German chemist, 
 his theory was on the point of being finally over- 
 thrown at the very time when, at the head of one 
 of his celebrated " Letters on Chemistry," he was 
 declaring the " theory which ascribes fermentation 
 to fungi refuted." ] For in 1857 Pasteur began the 
 long series of researches on fermentation upon which 
 so large a part of his great fame rests. Reviving 
 almost forgotten observations of Cagniard de la Tour 
 and of Schwann, who had established the true nature' 
 of yeast as a unicellular organism of spheroidal form, 
 invariably associated with alcoholic fermentation, and 
 the life and fermentive activity of which was de- 
 stroyed by heat, Pasteur completely established the 
 
 1 Fourth edition, 1859, Letter xxi. 
 
174 A SHORT HISTORY OF THE 
 
 vitalistic theory of the process. According to this 
 doctrine the change of sugar into alcohol and carbonic 
 acid is a consequence of the multiplication in the 
 solution of the cells of the yeast which, for the purposes 
 of its own growth, apparently uses the sugar as its 
 food ; while the alcohol and carbonic acid are to be 
 regarded as excretory products, the glycerin and 
 succinic acid being perhaps products of metabolism in 
 the constituents of the organism itself. Here is, then, 
 an example of chemical changes which accompany 
 the development of a specific organism under certain 
 definite conditions. If the organism is changed, or 
 the conditions are changed, different effects ensue. 
 
 But the decomposition of sugar into alcohol and 
 carbonic acid is not the only change which may 
 be spoken of as fermentation. The lactic ferment 
 is another organism more minute than yeast, and 
 presenting a different rod-like form, which has the 
 power of changing sugar into lactic acid. In this 
 case the action soon comes to an end if the liquid 
 is allowed to become acid, but this is easily prevented 
 by stirring into the liquid a sufficient quantity of 
 chalk, which neutralises the acid as fast as it is 
 produced. A third organism is endowed with the 
 specific function of breaking up lactic acid into 
 butyric acid, carbon dioxide, and hydrogen. In this 
 case a peculiarity of the process consists in the fact 
 that the presence of air is unfavourable to the 
 development of the organism, and is even capable 
 of suspending the process of fermentation. In like 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 175 
 
 manner it has come to be recognised that a con- 
 siderable number of changes, formerly supposed to 
 be purely chemical, are brought about by the influ- 
 ence of minute cellular organisms, some of which are 
 known as bacteria (fiaKTrjpia, a stick or staff) or bacilli 
 (bacillum, a little stick), from their cylindrical, rod- 
 like, or spindle-shaped forms. 
 
 The vitalistic theory of fermentation connects the 
 chemical changes, of which alike the materials and 
 the products have in many cases long been known, 
 with the existence of certain lowly forms of life. 
 The presence of these organisms in contact with the 
 liquid, under proper conditions, determines the decom- 
 position which in their absence does not take place. 
 This theory was practically established by the re- 
 searches of Pasteur by the year 1861, and though 
 in some minor particulars further knowledge is very 
 desirable, the recognition of the main principle has 
 been attended with consequences of importance so 
 great as to be inestimable ; for by means of these 
 earlier discoveries Pasteur was led to the still more 
 valuable pathological investigations which followed 
 continuously down to the close of his life. To say 
 that Pasteur effectually and finally disposed of the 
 doctrine which affirmed the possibility of spontaneous 
 generation, and that from the organic theory of fer- 
 mentation he was led to the "germ theory" of disease, 
 is to repeat what is familiar to all the world ; but 
 it is not possible in this place to follow the course 
 of the marvellous discoveries connected with the 
 
176 A SHORT HISTORY OF THE 
 
 chemical and physiological effects of micro-organisms, 
 or ferments, as they may all be called. And refer- 
 ence is made to this subject only in order to lead 
 back to a series of facts which have gradually come 
 to light concerning the chemical properties and re- 
 actions of certain nitrogenous constituents of animal 
 and of vegetable tissues. The compounds referred 
 to are soluble in water, are coagulable, and in any 
 case rendered inert by the application of heat, as 
 well as by contact with strong chemical agents. 
 Such compounds, which doubtless originate in some, 
 at present mysterious, change in the " protoplasm," 
 or living substance, are endowed with the power of 
 transforming a relatively large quantity of some 
 other compound into simpler material. They are 
 generally referred to as enzymes, or sometimes less 
 appropriately as soluble ferments. These substances 
 are very widely diffused in both the animal and 
 vegetable kingdoms, and many of them are concerned 
 in processes which have been long familiar. Thus 
 it has been known for a century or more that malt 
 contains a soluble material to which the name 
 diastase has been given, which has the power of 
 rendering starch soluble in water by converting it 
 into a kind of sugar. One of the most familiar of 
 enzymes is the substance contained in rennet, a 
 fluid obtained from the stomach of the calf, which 
 has the power of coagulating the casein of milk, and 
 is for this purpose employed extensively in the manu- 
 facture of cheese. It has also long been known that 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 177 
 
 sweet and bitter almonds in the dry state are both 
 without special odour, but that when the bitter 
 almond is crushed in the presence of water, the 
 characteristic volatile essence, consisting of benzal- 
 dehyd, begins immediately to be formed. It was 
 Liebig who discovered that the bitter almond con- 
 tains a crystalline substance, amygdalin, C 20 H 27 NO n , 
 which, in contact with a peculiar soluble albuminous 
 matter existing in both sweet and bitter almonds, is 
 resolved by the assumption of the elements of water 
 into glucose, prussic acid, and benzaldehyd. The 
 pungent oil of mustard is developed in an entirely 
 similar way. In the " pepsin " of the animal 
 stomach there is another example of a soluble 
 enzyme, which in this case is specially active in 
 causing the degradation and simplification of the 
 complex albuminoids of food, converting them into 
 soluble materials called "peptones," which probably 
 pass directly into the blood and are assimilated. 
 Great attention has been given of late years to the 
 recognition of these enzymes, and to the study of 
 the changes which they bring about. Their re- 
 markable activity is still a mystery, but the rapidity 
 and energy of their special effects are often greater 
 than the corresponding effects produced by the 
 recognised chemical agents of the laboratory. 
 
 It has been stated that the peculiar function of 
 these compounds is " hydrolytic " ; that is, they are 
 believed to act by causing the addition of the 
 elements of water to a great variety of compounds 
 
178 A SHORT HISTORY OF THE 
 
 which are then resolved into simpler molecules. 
 This does, indeed, appear to be the usual mode of 
 action exercised by these remarkable and complex 
 substances. Nevertheless there is evidence that in 
 some cases at any rate the process is reversible, in 
 the same sense that so many other chemical changes 
 are reversible, in consequence of the interfering in- 
 fluence of the accumulation of the products of 
 change. 
 
 Cane sugar under the influence of an enzyme 
 extracted from yeast yields " invert sugar," a mix- 
 ture of equal numbers of molecules of glucose and 
 fructose 
 
 Ci 2 H 22 O n + H 2 = C 6 H 12 6 + C 6 H 12 6 . 
 
 Maltose, which is isomeric with cane sugar, splits 
 under similar conditions into two molecules of 
 
 glucose 
 
 C 12 H aa O u +H 2 0=2C 6 H 13 O fl . 
 
 It has been lately discovered 1 that this process 
 in the case of maltose is hindered by adding glucose 
 to the liquid, and that when the enzyme is added to 
 a strong solution of glucose some of it is converted 
 into maltose. Whether a solution of maltose or 
 a solution of glucose of the same concentration is 
 employed, the tendency is to the production of a 
 state of equilibrium among the products of the 
 change, so that the liquid ultimately contains 
 
 i A. C. Hill, "Reversible Zymohydrolysis " (Trans. Chem. Soc 
 1898, p. 634). 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 179 
 
 maltose and glucose in the same proportions to 
 each other. 
 
 But some of these soluble enzymes appear to be 
 capable of acting in an altogether different way, 
 for it has been found quite recently (1897) by E. 
 Buchner * that yeast cells, when ruptured by grind- 
 ing with sand and a little water, yield a liquid which, 
 after nitration, has the power of producing the 
 fermentation of sugar, although it appears to be 
 quite free from yeast cells. The soluble substance 
 so obtained is apparently allied to the proteids, and 
 its power is destroyed by heating to about 50 C., 
 whereby an albuminous substance is precipitated. 
 It may, however, be evaporated to dryness and re- 
 dissolved in water without destruction of its activity, 
 provided the temperature has not been allowed to 
 rise too high. 
 
 We cannot doubt that substances of the nature 
 of enzymes are generated abundantly in the tissues 
 of both plants and animals, and that the secretions 
 which are so intimately associated with the opera- 
 tions of digestion and other functions of the body, 
 owe their special characters to the presence of 
 peculiar substances of this order. There is also very 
 little doubt that they are very similar in composi- 
 tion, in constitution, and in chemical properties, and 
 probably they have a similar, or nearly similar, 
 origin ; but of their differences we know practically 
 nothing, except by observation of their different 
 
 1 Berichte, 1897, xxx. 117. 
 
180 A SHORT HISTORY OF THE 
 
 effects; and as to their origin and modes of action, 
 there can be in the present state of knowledge 
 nothing beyond conjecture. That they are also 
 connected with the manifestation of disease there 
 is great probability, and Jenner's great discovery 
 and Pasteur's extension of the same principle, in- 
 volving the use of attenuated or modified virus to 
 neutralise the effects of the morbid secretions in 
 the body, are doubtless dependent upon the special 
 chemical effects of complex substances having the 
 character of enzymes. 
 
 But here we reach the borderland where chemistry 
 and physiology meet. Each has something to learn 
 from the other. The chemist finds in the enzymes, 
 which for the present are procurable only from the 
 living organism which probably no laboratory syn- 
 thesis will ever replace, agents which are often indis- 
 pensable in his study of the more complex carbon 
 compounds. The physiologist, on the other hand, 
 must acknowledge that structural chemistry has 
 given him the clue to many otherwise inexplicable 
 transformations taking place in the body ; while the 
 pharmacologist and the physician, who are familiar 
 with the history of the scientific labours of Pasteur 
 from beginning to end, will admit that the discipline 
 of the chemical laboratory is no bad preparation for 
 the business of the scientific pathologist. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 1.81 
 
 CHAPTER VII 
 
 THE ORIGIN OF STEREO-CHEMISTRY -CONSTI- 
 TUTIONAL FORMULAE IN SPACE 
 
 WHEN a ray of light passes through a crystal of 
 Iceland spar it exhibits the familiar phenomenon 
 of double refraction ; that is to say, the ray in 
 entering the crystal divides into two rays, which 
 emerging separately, give rise to two distinct images. 
 These two rays are polarised at right angles to each 
 other. Light may also be polarised by reflection ; 
 in that case, one half the light is reflected, the other 
 half passes into the reflecting surface, and is either 
 stopped and ceases to produce the effect of light, 
 or it is transmitted. 
 
 The discovery, however, which in connection with 
 the subject about to be discussed possesses the 
 greatest interest, was made early in the present 
 century by the French physicist Biot. He found 
 that a ray of light polarised in one plane has that 
 plane twisted to the right or to the left in passing 
 through certain substances of organic origin, such 
 as sugar, camphor, and oil of turpentine when in the 
 liquid state. He also observed that the angle of 
 rotation of the plane of polarisation differs in dif- 
 
182 A SHORT HISTORY OF THE 
 
 ferent substances, and is directly proportional to 
 the thickness of the layer of transparent substance 
 through which the ray passes. 
 
 Now there are two classes of materials which 
 have the power of rotating the polarised ray. One 
 class, represented by quartz or sodium chlorate, are 
 crystalline solids, and the optical power which they 
 possess in the solid state is lost when they are 
 liquefied by fusion or solution in a solvent. In 
 such cases it would appear that the optical activity 
 of the crystals is attributable to a peculiarity in the 
 arrangement of their molecules one upon another, 
 and not to any want of symmetry in the internal 
 constitution of the molecules themselves. The other 
 class includes those substances already mentioned, 
 which exhibit their characteristic properties in the 
 liquid state. In such cases it is fair to infer that 
 the arrangement of the molecules has nothing to 
 do with the phenomenon, which must be due to a 
 peculiarity inherent in each molecule. 
 
 Common tartaric acid, obtained from the " tartar " 
 deposited in the fermentation of wine, when dis- 
 solved in water rotates the polarised ray very 
 strongly to the right. Its salts have the same 
 property in different degrees. But the tartar got 
 from grapes grown in certain districts (it was origin- 
 ally observed in tartar from the Vosges) yields an 
 acid called racemic acid, which, while agreeing with 
 tartaric acid in composition and in chemical pro- 
 perties, differs from it in being optically inactive, for 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 183 
 
 a ray of polarised light passes through a solution of 
 the acid, or of one of its salts, unaltered. Racemic 
 acid, however, has been shown to consist of a mix- 
 ture of two kinds of tartaric acid in equal quantities, 
 and having equal but opposite effects on the polarised 
 ray. Further, the property of rotating a polarised 
 ray to the right or to the left is associated in crystal- 
 lisable substances with a peculiarity of crystalline 
 habit, in consequence of which they produce crystals, 
 the fundamental form of which is modified by the 
 development of small faces on one side or other of 
 the crystals. When the sodium ammonium race- 
 mate is dissolved in water, and the solution is con- 
 centrated so as to deposit crystals, these crystals are 
 found to be of two kinds, distinguished from each 
 other by the position of the hemihedral faces re- 
 ferred to ; some having these faces disposed on 
 one side of the prism, and some an equal number 
 on the opposite side, so that the two forms 
 differ from each other only as an object differs 
 from its image in a mirror, or as the right hand 
 from the left. Consequently, such forms are not 
 in any position superposable on one another. On 
 separating these crystals, some will be found to agree 
 with the crystals formed under similar circumstances 
 from common tartaric acid, and like these, to turn 
 the plane of polarisation to the right. The others 
 having similar faces, but on the other side of the 
 crystal, turn the plane of polarisation to the left, 
 and the acid recovered from these crystals presents 
 
184 A SHORT HISTORY OF THE 
 
 a corresponding property. Common tartaric acid, 
 being called dextro-taxtstfic acid, the complementary 
 compound is called te?o-tartaric acid ; mixed to- 
 gether in equal quantities they reproduced racemic 
 acid. Many similar cases are now known. 
 
 The discovery of all these important facts is due to 
 Pasteur. Having been attracted at a very early age 
 to the study of crystallography, he was led to repeat 
 an examination of tartaric acid and the tartrates, 
 published by De la Provostaye many years previously. 
 In the first of a series of memoirs, in which he 
 describes his observations and experiments on the 
 relations subsisting between crystalline form, chemical 
 composition, and direction of rotatory polarisation, 
 he found that while common tartaric acid and all 
 the common tartrates exhibit hemihedral forms in 
 their crystals, and that the hemihedrism is always 
 of the same kind, in paratartaric (racemic) acid and 
 its salts hemihedral forms could not be detected. 
 In the examination of these compounds he made 
 the capital discovery already referred to. It will be 
 worth while to quote his own words, written in 1848 : 
 "Lorsque j'eus decouvert 1'hemiedrie de tous les 
 tartrates je me hatai d'etudier avec soin le para- 
 tar trate double de soude et d'ammoniaque ; mais je 
 vis que les facettes tetraedriques correspondant a 
 celles des tartrates isomorphes, etaient placees rela- 
 tivement aux faces principales du cristal, tantot a 
 droite, tantot a gauche, sur les differents cristaux 
 que j'avais obtenus. Prolongees respectivement ces 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 185 
 
 facettes donnaient les deux tdtraedres symetriques 
 dont nous parlions precedeminent. Je se'parai avec 
 soin les cristaux hemiedres a droite, les cristaux 
 hemiedres a gauche ; j'observai separement leurs dis- 
 solutions dans 1'appareil de polarisation de M. Biot, 
 et je vis, avec surprise et bonheur que les cristaux 
 hemiedres a gauche deViaient a gauche le plan de 
 polarisation. . . . Les deux especes de cristaux sont 
 isomorphes et isomorphes avec le tartrate corres- 
 pondant ; mais I'isomorphisme se presente la avec 
 une particularity jusqu'ici sans exemple ; c'est Tiso- 
 morphism de deux cristaux dissymetriques qui se 
 regardent dans un miroir." l 
 
 Pasteur was, of course, not content to let his 
 investigations stop here. He proceeded to investigate 
 the questions whether the connection between rota- 
 tory polarisation and hemihedral crystallisation are 
 in all cases connected together ; in fact, whether 
 from observation of the one property the other may 
 always be predicted. These inquiries led him to 
 the experimental study of a number of substances 
 other than the tartaric acids, among the rest the 
 important vegetable principle asparagine, which is 
 obtainable from the juice of a number of plants, 
 such as asparagus, marshmallow, and various species 
 of leguminosse, especially, as shown by Pasteur, the 
 juice of vetches blanched by exclusion of daylight. 
 Having assured himself of the hemihedrism of the 
 crystals of asparagine, Pasteur proceeded to examine 
 
 1 Ann. Chim., 3rd series, xxiv. 456 (1848). 
 
186 A SHORT HISTOEY OF THE 
 
 the action of a solution of this substance on the 
 polarised ray, and found that when dissolved in 
 water or alkalis it deviates the plane of polarisation 
 to the left. At this time only one kind of asparagine 
 was known, but so confident was Pasteur in his 
 belief in the correctness of the principles deduced 
 from his observations on tartaric acid, that he did 
 not hesitate to predict the discovery of a comple- 
 mentary dextro-rotatory form, such that between 
 these two kinds of asparagine the same relation 
 would be found to exist as between right- and left- 
 handed tartaric acid. 1 Dextro-rotatory and inactive 
 asparagine have, in fact, been since discovered. In 
 the examination of one of the formates, the strontium 
 salt, he encountered a different phenomenon. In 
 this case, while the crystals exhibit hemihedrisrn, the 
 two opposite forms are always simultaneously pro- 
 duced ; and when the crystals are separated, neither 
 the right-hand nor the left-hand form dissolved in 
 water exhibits any rotatory power. Moreover, while 
 the dextro-tartrate or Isevo-tartrate, when recrys- 
 tallised, never yield crystals of the opposite form, 
 the formate, whether dextro or laevo, always gives 
 a mixture of both kinds of crystal. 2 The explana- 
 tion of the difference between these two cases was 
 supplied by Pasteur himself. He says, 3 " Les faits 
 precedents conduisent a supposer que 1'hemie'drie du 
 
 1 Ann. Chim., 3rd series, xxxi. 72 (1851). 
 
 2 Ann. Chim., 3rd series, xxxi. 100 (1851). 
 
 3 LOG. cit., p. 101. 
 
PROGKESS OF SCIENTIFIC CHEMISTRY 187 
 
 formiate de strontiane ne tient pas a 1'arrangement 
 des atomes dans la molecule chimique, mais a 
 1'arrangement des molecules physiques dans le cristal 
 total, de telle maniere que la structure cristalline 
 une fois disparue dans 1'acte de la dissolution il 
 n'y a plus de dissymetrie ; a peu pres comme si Ton 
 construisait un edifice ayant la forme exterieure 
 d'un polyedre qui offrirait VMmUdrie non superposable 
 et que Ton detruirait ensuite." 
 
 It appears, then, that if a substance in the liquid 
 form possesses the power of rotating the polar- 
 ised ray, it will produce in crystallising hemi- 
 hedral forms 1 which are not superposable, and 
 which have the mutual relation of an object and 
 its image as seen in a mirror. On the other hand, 
 optical activity cannot be inferred for the liquid 
 state from the existence of hemihedral crystalline 
 forms. 
 
 The first to perceive the connection between all 
 these phenomena and the question as to the internal 
 structure of molecules possessing such peculiarities 
 was Pasteur himself; and so far back as 1853 he 
 was able to anticipate to some extent the views 
 accepted later by all chemists. If in two substances 
 composed of the same elements, united in the same 
 
 1 In some of these cases the hemihedral form only makes its 
 appearance when a particular condition is established in the solu- 
 tion. Thus Pasteur himself showed that calcium bimalate crystal- 
 lises in hemihedral forms from nitric acid but not from water. 
 Ann. Chim., 3rd series, xxxviii. 437 (1853). 
 
188 A SHORT HISTORY OF THE 
 
 proportions, and having the same chemical pro- 
 perties, we can perceive only the two differences 
 already described (i.e. direction of rotation and 
 hemihedral crystallisation), and in many cases only 
 one of them, namely, the optical activity, we are 
 driven to the conclusion that their peculiarities must 
 be connected with some peculiarity of construction 
 in the molecule. The atoms composing the mole- 
 cule in one of these compounds must be arranged 
 in some way which is repeated in the other, but 
 in inverse order, so that the two would be related in 
 the same way as an object and its image in a mirror. 
 " Are the atoms of the dextro-acid grouped on the 
 spirals of a right-handed helix, or placed at the 
 solid angles of an irregular tetrahedron, or disposed 
 according to some particular asymmetric arrange- 
 ment ? " If dextro- or Isevo-tartaric acid is com- 
 bined with some substance, such as potash, ammonia, 
 aniline, which is inactive, and hence devoid of 
 asymmetry, the inactive substance affects to the 
 same extent the activity of both varieties of the 
 acid ; but if the two acids are combined with an 
 active substance, such as the alkaloid cinchonine, 
 Pasteur found that a pair of compounds results 
 which differ in form, in solubility, and in other pro- 
 perties. While in the one case the rotatory power 
 is the sum of the rotatory powers of the acid and 
 the base, in the other case the rotatory power of the 
 compound is the difference between the two. On 
 these facts Pasteur based a method which has been 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 189 
 
 since freely employed for the separation of the con- 
 stituents of racemoid compounds. If racemic acid, 
 for example, be saturated with the active base cin- 
 chonicine, the first crystals obtained from the solution 
 consist of pure cinchonicine Isevo-tartrate, while the 
 whole of the dextro-tartrate remains in the mother 
 liquor, and can afterwards be obtained in forms 
 distinct from those of the Isevo-tartrate. 
 
 The ideas put forth quite clearly, but in general 
 terms, by Pasteur were long afterwards developed 
 into a definite theory of structure, which forms the 
 foundation of that large department or aspect of our 
 science which is called Stereo-chemistry, or chemistry 
 in space of three dimensions. 
 
 But while the merit of creating the basis of this 
 system of ideas, by providing the facts and showing 
 clearly the direction in which further investigation 
 should travel, belongs to Pasteur, it is only just to 
 recall the fact that the necessity for a theory of the 
 kind was perceived long before Pasteur's researches 
 gave the clue which led ultimately to the conception 
 of the doctrine of symmetry and asymmetry in 
 carbon compounds. 
 
 The fundamental idea of stereo -chemistry arises 
 immediately out of the doctrine of atoms. Dalton 
 himself and others of his time, promoters of the 
 atomic system, were led to consider, though without 
 giving the subject much attention, the question of 
 the arrangement which united atoms must occupy 
 in space. Dalton's diagrams (" Chemical Philo- 
 
190 A SHOUT HISTORY OF THE 
 
 sophy," Part I.) represent all the atoms lying in one 
 plane, but the question was raised very definitely by 
 Wollaston, so early as 1808, at the conclusion of a 
 paper in which he brought forward experiments in 
 support of the atomic doctrine. 1 He says, " I am 
 further inclined to think that when our views are 
 sufficiently extended to enable us to reason with 
 precision concerning the proportions of elementary 
 atoms, we shall find the arithmetical relation alone 
 will not be sufficient to explain their mutual action, 
 and that we shall be obliged to acquire a geometrical 
 conception of their relative arrangement in all the 
 three dimensions of solid extension." But after 
 giving some examples of possible arrangements, he 
 goes on to say that as this geometrical arrangement 
 of the primary elements of matter is altogether 
 conjectural, it must rely for its confirmation or 
 rejection upon future inquiry, and he adds, " It is 
 perhaps too much to hope that the geometrical 
 arrangement of primary particles will ever be per- 
 fectly known." 
 
 Leopold Ginelin seems to have been more in- 
 clined to be hopeful, for in 1848 we find in his 
 great "Handbook of Organic Chemistry" (vol. i., 
 Cavendish Society Publications) : " Suggestions re- 
 specting the Kelative Position of the Elementary 
 Atoms in a Compound Organic Atom, assuming the 
 
 1 "On Superacid and Subacid Salts," by William Hyde Wollas- 
 ton (Phil. Trans., xcviii. (1808), pp. 96-102). Keprinted by Alembic 
 Club, No. 2. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 191 
 
 truth of the Nucleus Theory." "Nearly all chemists," 
 he says, "adopt the Atomic Theory. They deter- 
 mine the relative weights of the atoms, and their 
 relative distances one from the other, or the relative 
 space occupied by each atom of the combined 
 substances, including the surrounding calorific en- 
 velope; hypotheses are also made respecting the 
 form of the atoms, &c. Why, then, should we not 
 likewise throw out suggestions with regard to their 
 relative positions ? " This he proceeds to do, and 
 with the aid of certain & priori principles, he arrives 
 at the conclusion that potassium sulphate, for ex- 
 ample, must have the form of a double four- sided 
 pyramid, that ethylene has probably the form of a 
 cube, alcohol and acetic acid other forms. All 
 these forms, however, require the assumption of a 
 certain number of atoms in each molecule which 
 would be dependent upon the atomic weights as- 
 signed to each. As these have in many cases been 
 changed since Gmelin's time, his assumptions neces- 
 sarily fall to the ground; but the arguments employed 
 are interesting, as affording an example of an early 
 and serious attempt to attack problems of this kind. 
 At the conclusion of the discussion he continues as 
 follows : " Even if the data of this investigation are 
 defective or erroneous, I am yet convinced that all 
 theories on the constitution of organic compounds, 
 and all controversies as to this or that mode of 
 writing rational formulae, if not supported by a 
 plausible arrangement of the compound atom, will 
 
192 A SHORT HISTORY OF THE 
 
 aid us but little in the acquisition of correct ideas. 
 Look, for instance, at the controversy respecting the 
 constitution of ether and alcohol between Durnas 
 and Boullay on the one side, and Berzelius and 
 Liebig on the other. According to the former, ether 
 is a compound of etherine with water = C 4 H 4 Aq, 
 and alcohol = C 4 H 4 2 Aq ; according to the latter, the 
 hypothetical radical ethyl = C 4 H 5 forms with the 
 oxide of ethyl = ether ; and this with the addition 
 of lAt. water forms the hydrated oxide of ethyl 
 = alcohol = C 4 H 5 + Aq. Now, on comparing these 
 views with the explanation given, it appears probable 
 that neither of them is right. At all events, neither 
 ether nor alcohol can be supposed to contain water 
 ready formed. They are not hydrates; if so, they 
 would surely give up this water to burnt lime or 
 baryta, which, however, is not the case. Neither is 
 ether converted into alcohol by solution in water. 
 On the other hand, ethyl is a fictitious compound, 
 supposed to combine like a metal with oxygen and 
 with chlorine, forming compounds analogous to the 
 metallic oxides and chlorides. Thus hydrochloric 
 ether = C 4 H 5 C1 ( = C 4 H 8 Cl,H 2 .0m) is regarded as 
 chloride of ethyl ; but it does not precipitate silver 
 solutions, &c." 
 
 Long afterwards, in 1872, the study of the 
 lactic acids led Wislicenus again to perceive that 
 the then existing conceptions of atoms and their 
 union together were insufficient to explain the 
 facts, and to suggest that they might be accounted 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 193 
 
 for " durch verschiedene Lagerung ihrer Atome im 
 Raume." 1 But there the matter rested till the 
 theory of Van't Hoff and Le Bel was enunciated two 
 years later. 2 What Van't Hoff and Le Bel dis- 
 covered was the remarkable fact that, in carbon 
 compounds which exhibit the property of rotating 
 the polarised ray in either direction, the molecule 
 in every case contains at least one atom of carbon 
 combined in four different ways; that is, having 
 its four units of valency occupied with radicles 
 of different composition, and therefore usually of 
 different combining weights. Thus in succinic acid, 
 an example of an optically inactive compound, there 
 are four atoms of carbon, the affinities of which are 
 disposed of according to the order displayed in the 
 following diagram : 
 
 SUCCINIC ACID. 
 
 O H H 
 
 II i I II 
 
 H C C C C O- H 
 I i 
 H H 
 
 In chloro-succinic acid, we have a compound in 
 which the carbon atoms are linked together in the 
 
 1 Ann. Chim., clxvi. 3, clxvii. 302, 346. 
 
 2 Voorstel tot uitbreiding der structuurformules in de ruimte, pam- 
 phlet published by J. H. Van't Hoff, Sept. 1874 : republished under 
 the title Sur les formules de structure dans I'espace. Archives Neer- 
 land, ix., 1874, pp. 445-454 ; Bulletin Soc. Chim., Paris, xxiii., 1875, 
 pp. 295-301. 
 
 Sur les relations qui existent entre les formules atomiques des corps 
 organiques, et le pouvoir rotatoire de leurs dissolutions. By J. A. Le 
 Bel, bulletin Soc. Chim., Paris, xxii., 1874, pp. 337-347. 
 
 N 
 
194 A SHORT HISTORY OF THE 
 
 same order, but one of them has exchanged an atom 
 of hydrogen for an atom of chlorine 
 
 CHLORO-SUCCINIC ACID. 
 
 O H Cl 
 
 II i I II 
 
 H C C C C H 
 
 I i 
 H H 
 
 In this molecule, then, one atom of carbon is attached 
 to four different atoms or groups, for it is united to 
 (1) H, (2) (Cl), (3) CH 2 , and (4) CO.O. This com- 
 pound is known in two optically active forms, which 
 show the same kind of relation to each other which 
 has already been observed in the case of tartaric acid. 
 Tartaric acid itself contains two atoms of carbon 
 in the same kind of condition, each united with 
 (1) H, (2) OH, (3) CH 2 , and (4) CO.O. To explain 
 the effect of this upon the optical properties of the 
 compound, a further hypothesis is required. In all 
 the early speculations regarding the nature of atoms, 
 it seems to have been assumed, as it was by Dalton 
 and Wollaston, that the " virtual extent " of each 
 atom is spherical. Since that day several other 
 hypotheses have been suggested concerning the 
 nature of the atom, of which one of the most im- 
 portant is Lord Kelvin's notion of vortices; and it 
 will be obvious to every one who considers the sub- 
 ject, that our views of the process of chemical 
 combination must be seriously affected by the idea 
 
PEOGRESS OF SCIENTIFIC CHEMISTRY 195 
 
 in the mind of the form and nature of the atoms 
 supposed to be in the act of union. 
 
 The idea which has proved most fertile is one 
 which appeared for the first time in a paper by 
 Kekule in 1867. 1 At that time he was still engaged 
 in the elaboration of the great doctrine of the link- 
 ing of atoms, of which, as already shown (Chap. V.), 
 we owe the chief development to his insight. And the 
 use of models, as well as of " graphic " formulae, was 
 beginning to be freely practised by the more ad- 
 vanced chemists. At the end of a paper on the con- 
 stitution of mesitylene, in a kind of supplementary 
 note, Kekule pointed out that the models then most 
 in favour, consisting of spherical balls joined together 
 by rods, were no better than diagrams, since it was 
 impossible to display combination between two atoms 
 by more than one unit of valency; and, moreover, 
 everything was represented as lying in the same 
 plane. But by using a sphere to represent the atom 
 of carbon, and four rods to represent its four affinities, 
 placed in the directions of four hexhedral axes ending in the 
 faces of a tetrahedron,th.GSQ difficulties could be got over, 
 and two such models placed together could be used 
 to represent union by one, two, or three units. Van't 
 Hoff adopted and has pursued with most brilliant 
 success the consequences of this remarkable idea. 2 
 
 1 Zeitschrift f. Chem. (1867), N.F. iii. 217. 
 
 2 For details concerning the verification of the theory, and the 
 removal of difficulties in its way, arising chiefly out of erroneous 
 observations, see Van't Hoff' s Dix Annees dans Vhistoire d'une Thtorie, 
 of which an English edition has been prepared by Mr. J. E. Marsh. 
 Clarendon Press. 
 
196 
 
 A SHORT HISTORY OF THE 
 
 An atom of carbon, then, is generally believed to be 
 capable of combining with four other atoms and no 
 more, and is therefore said to be tetrad or quadri- 
 valent. The carbon atom is supposed by this 
 hypothesis to be accessible only in four different 
 directions, which are representable by the four 
 straight lines which may be drawn from the centre 
 of a regular tetrahedron to its solid angles. The 
 centre of mass of the carbon atom is supposed to be 
 situated at the intersection of these lines, or at the 
 centre of the figure. A model to represent such an 
 arrangement could be easily made by means of a 
 ball of wood and four wires of equal length. If two 
 such models be constructed, and the wires marked 
 by tipping them with beads of different colours, or 
 in some other way, it can easily be shown that two 
 distinct arrangements become possible. In the fol- 
 lowing figures the letters a, b, c, d represent the atoms, 
 all different, which we may imagine to be united, as in 
 chloro-succinic acid, to the same atom of carbon : 
 
 A consideration of these figures will show that the 
 one is not superposable upon the other so that the 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 19*7 
 
 same letters come together. Any change in the 
 plane of polarisation produced by one of these struc- 
 tures would be also produced, but in the inverse 
 sense, by the other structure. The exact mechanism 
 of this process cannot at present be described, be- 
 cause we have no more positive knowledge as to the 
 construction of a molecule than we have of the 
 atoms composing it. Pasteur, as already stated, 
 frequently used the analogy of a spiral with a twist 
 to the right or to the left, according as the mole- 
 cule possessed dextro- or Igevo-rotatory powers. 
 
 The use of models assists materially in the con- 
 sideration of the problems arising out of this hypo- 
 thesis. One of the first questions which arise relates 
 to the direction in which the several valencies of an 
 atom of carbon may be supposed to be exerted. If 
 the direction of these be supposed to be absolutely 
 fixed, then it can be shown that (1) two carbon 
 atoms cannot unite by two bonds, nor by three, 
 because that would involve the distortion of the 
 atom ; and (2) three or more carbon atoms cannot 
 unite to form a ring, for the same reason. 
 
 Hence it seems that if the direction of the attrac- 
 tions of carbon for other atoms can be determined 
 at all, the line of attraction must be rather easily 
 displaceable from the normal, or it operates some- 
 what like the pole of a magnet, that is, there is a 
 certain field. The analogy between the attractions 
 of two atoms for each other, and the attraction 
 exercised between the opposite poles of two magnets, 
 
198 A SHORT HISTORY OF THE 
 
 or between a magnet and a piece of iron, may in 
 fact be considered as extending even further. For 
 just as the presence of a mass of iron in the neigh- 
 bourhood of a magnetic pole seems to absorb the 
 lines of magnetic induction and so reduce the action 
 of the magnet upon other bodies placed near, so 
 the addition of one element to another in chemical 
 union diminishes the tendency of either to combine 
 with a third element, but does not, in the majority 
 of cases, absolutely extinguish this tendency. Hence 
 we have what has been called " residual valency," 
 which, whatever may be the fate of the various 
 hypotheses concerning it, gives rise to very well 
 marked phenomena of combination. It is, however, 
 usually believed that two carbon atoms may actu- 
 ally be united by two or more units of valency, but 
 that in all such cases the combination is not only 
 not more secure, but is decidedly more easily broken 
 up than where one bond only of each is employed. 
 To account for this difference, so contrary to what 
 might be at first sight expected, two chief hypotheses 
 have been proposed. The first, which originated 
 with Von Baeyer, 1 is based upon the same hypo- 
 thesis with regard to the carbon atom as that of 
 Le Bel and Van't Hoff. If the valencies of the 
 carbon atom act along the lines drawn from the 
 centre to the solid angles of the regular tetrahedron, 
 these lines form with each other angles of 109 28'. 
 If two carbon atoms are so situated towards each 
 
 1 Ber., xviii. 2277. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 
 
 199 
 
 other that two valencies of each are united to- 
 gether so that the directions of the two are 
 parallel to each other, each is turned out of its 
 normal position by an angle of 54 44'; and if two 
 carbon atoms are similarly joined by three units of 
 valency, each of these must deviate 70 32' from its 
 normal position. It is known that combination by 
 double or triple bonds is easily converted into union 
 by single bonds in all cases of this kind. Such 
 substances as acetylene and ethylene are saturated 
 with great readiness, not only by bromine, but by 
 hydracids, and even by iodine, and the tension is 
 thus relieved. 
 
 CH 2 
 
 II 
 
 CH 2 
 Ethylene. 
 
 CH 2 H 
 
 I 
 
 CH 2 Br 
 Ethyl bromide. 
 
 CH 2 Br 
 
 CH 2 Br 
 
 Ethylene bromide, &c. 
 
 V. Baeyer has also pointed out that in the forma- 
 tion of rings of carbon atoms the distortion of the 
 atoms diminishes as the number of carbon atoms 
 increases up to five. In a ring of six carbon atoms 
 united by single bonds the distortion is a little 
 greater. The following angles represent the extent to 
 which the direction of each valency is disturbed : 
 
 CH 2 
 CH 2 |CH 2 
 
 r 
 
 CH 2 CH 2 CH 2 -CH 2 
 24 44' 9 34' 
 
 CH 2 OH 2 
 44' + 5 16' 
 
200 A SHORT HISTORY OF THE 
 
 The other hypothesis, proposed by Wunderlich, 1 
 is based upon the idea that the relative force of 
 attraction between two units of valency depends 
 upon the relative distances through which they 
 have to act. Briefly, the hypothesis is somewhat 
 as follows. The carbon atom is a sphere from which 
 four equal symmetrically placed segments have been 
 removed, and the circular faces thus formed are 
 situated in the planes of the four faces of a regular 
 tetrahedron. When combination takes place between 
 two carbon atoms the most intimate union is that 
 in which two of these faces are placed parallel to, 
 and probably very near, each other. A less inti- 
 mate union occurs when the centres of gravity of 
 two faces of one atom attracts two faces of another. 
 The two tetrahedra have then a common edge, the 
 two pairs of faces forming equal angles with each 
 other. And, lastly, three faces of one may attract 
 equally three faces of the other, and so cause the 
 two tetrahedra to be applied to each other by one of 
 their solid angles. These three positions correspond 
 to combination by single, double, and triple bonds. 
 
 1 Configuration Organischer MokJciile, Wurtzburg, 1886. Abstract 
 in Ber., xix. 592c. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 
 
 201 
 
 According to this assumption it is only possible 
 for the atoms to touch each other when united by 
 the single bond, as shown at a. 
 
 When two asymmetric carbon atoms are united 
 together the number of possible isomeric forms is 
 greater. Tartaric acid is a case of this kind. Four 
 isomeric acids of the same composition are known, 
 namely : dextro-tartaric acid, Isevo-tartaric, meso- 
 tartaric, and racemic acids. Racemic acid is known 
 by the researches of Pasteur, made forty years ago, 
 to consist of a mixture or molecular compound of 
 dextro- and laevo-tartaric acids. These two latter 
 are supposed to be related to each other in the 
 manner indicated in the following formulae : 
 
 CO. OH 
 
 OH 
 
 CO. OH 
 
 CO. OH 
 
 H OH 
 
 which may be more briefly written : 
 
 CO. OH 
 
 r I 
 
 and 
 r I 
 
 where the letters r and / represent an arrangement 
 which results in right- or left-handed rotation of the 
 
202 
 
 A SHOKT HISTORY OF THE 
 
 polarised ray. Meso-tartaric acid, which, as already 
 explained, is optically inactive, though containing 
 two asymmetric carbons, is by an extension of the 
 same hypothesis represented as 
 
 CO. OH 
 
 or 
 
 CO. OH 
 
 OH 
 
 Van't Hoff long ago indicated that when two 
 carbon atoms are united together by a single bond 
 they may be supposed to be free to rotate about an 
 axis which is in the line representing the direction 
 of the uniting valencies, and that if two carbons are 
 joined by two or more bonds rotation becomes im- 
 possible. This hypothesis was first made use of by 
 Wislicenus in 1886, and has been the subject of a 
 good deal of discussion since. 
 
 It may be assumed that the radicles united to 
 two adjacent atoms of carbon will be likely to in- 
 fluence each other, 1 and, according as they attract 
 or repel each other, rotation may or may not occur. 
 Thus it may be supposed that in Dutch liquid the 
 
 1 The relative masses (atomic weights) of the atoms or atomic 
 groups may have something to do with this. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 
 
 203 
 
 chlorine and hydrogen atoms probably attract each 
 other. Hence there can be only one stable form of 
 this compound, viz. : 
 
 H 
 
 other arrangements passing spontaneously by rota- 
 tion into this. 1 Succinic acid also is known in only 
 one form, which probably has the following struc- 
 ture : 
 
 CO.OH 
 
 CO. OH 
 
 1 That atoms which are not directly united do influence each other 
 is certain from such cases as the of the carboxyl group - CO.OH, 
 the characters of bromnitroethane, the chloranilines, &c. 
 
204 A SHORT HISTORY OF THE 
 
 If this be so, it seems to follow that rotation must 
 occur when water is lost and the anhydride is formed 
 under the influence of heat : 
 
 H 
 H | 
 
 - C C = O 
 
 C G = O 
 H | 
 H 
 
 From other similar cases it appears probable that 
 the formation of rings, in which four carbon atoms, 
 or four carbon atoms and one oxygen atom are 
 concerned, probably occurs in one plane. And Von 
 Baeyer has pointed out that the interior angles of a 
 regular pentagon are very nearly equal to the angle 
 which the valencies of carbon in their normal position 
 form with one another (see diagram, p. 199). This 
 appears to explain such facts as the ready production 
 of anhydrides from dibasic acids, such as succinic 
 acid, already referred to, and phthalic acid, &c. ; also 
 the formation of lactones by loss of the elements of 
 water by the 7 hydroxy-acids of the fatty series, 
 such as oxybutyric acid. It is certainly interesting 
 to compare the action of heat upon a /3 acid with 
 that of a 7 acid of the same series ; for example : 
 
 CHoOH CH 2 
 
 I II ' 
 
 CH 2 yields CH 
 
 CO. OH CO. OH 
 
 Hydracrylic acid. Acrylic acid. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 205 
 
 y 
 
 CH 2 OH CH 2 
 
 CH 2 CH 2 
 
 j yields | 
 
 CH 2 CH 2 X 
 
 I I X 
 
 CO. OH CO 
 
 Oxybutyric acid. Butyrolactone. 
 
 One or two other questions relating to the origin 
 or constitution of optically active bodies have been 
 the subject of discussion and much experiment. As 
 they are still unsettled, they may be dismissed 
 briefly. Attempts have been made to trace a re- 
 lation between the degree or amount of rotation 
 produced by a compound and the masses of the 
 radicles which enter into its composition in union 
 with the asymmetric carbon atom it contains. It 
 has been found in some cases which have been 
 examined that two of the four groups may have 
 the same weight, and yet the compound is still 
 active. Guye therefore supposes 1 that not merely 
 the masses of the atomic groups but their relative 
 distances from the carbon to which they are attached 
 may influence the rotatory power. Some regula- 
 rities which seem to support these views have 
 actually been observed by Guye ; but on the other 
 hand there are many cases which are incompatible 
 with them. 
 
 There appears to be some reason for thinking 
 that it may be possible to trace the effect of each 
 
 1 Compt. Rend. (1893), cxvi. 1451. 
 
206 A SHORT HISTORY OF THE 
 
 constituent of an optically active body upon its 
 rotatory power, but at present no determinations 
 have been placed on record which are sufficient to 
 set the question at rest. 1 It would appear probable 
 that this problem will assume and retain for a long 
 time the position and aspect presented by the some- 
 what similar questions which arise in connection 
 with many of the physical characters of chemical 
 compounds. The specific or molecular volumes of 
 liquids and solids, their magnetic rotation, their re- 
 fractive power, and even their boiling points, may 
 each be represented by numerical values which 
 are respectively the sum of terms which are nearly 
 but not absolutely constant, and which may be 
 attributed to the several constituents of the com- 
 pound. But just as it is impossible even now to 
 assign an invariable value to the specific volume, 
 for example, of an elementary atom, so it is not 
 very likely that the specific rotation produced by 
 a particular asymmetric group of atoms will be 
 found constant ; and in both cases for the same 
 reason, namely, that notwithstanding the accepted 
 doctrine of atom-linking, based on the idea of 
 valency, there can be no doubt that in a molecule 
 every atom exerts an influence, greater or less, on 
 every other atom. 
 
 Closely connected with the inquiry as to the effect 
 upon the plane of polarisation of the constituent 
 radicles in a compound is another which has arisen 
 
 1 See J. W. Walker, Jour. Chem. Soc. (1895), Ixvii. 914. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 207 
 
 out of the researches of Walden. Compounds 
 which contain asymmetric carbon, and hence are 
 capable of existing in two stereo -isomeric forms, 
 when produced from inactive materials by ordi- 
 nary laboratory processes, invariably appear in the 
 raceinic condition ; that is, an equal quantity of 
 each oppositely active variety is produced. It 
 now appears, however, that starting from one and 
 the same active compound, a given derivative 
 formed from it may exhibit a rotatory power the 
 same or the reverse of that of the parent substance. 
 From Icevo-malic acid, according to Walden, 1 is 
 produced by the action of pentachloride of phos- 
 phorus, a chloro - succinic acid which is dextro- 
 rotatory. Replacing the chlorine in this compound 
 by hydroxyl, a malic acid is reproduced, which is 
 also d&^ro-rotatory. This malic acid, treated with 
 phosphorus pentachloride, gives Zo^o-chloro-succinic 
 acid, from which tow-malic acid identical with the 
 original substance may be regenerated. Phosphoric 
 chloride appears therefore to have a peculiar power 
 in such cases of causing optical inversion, by which 
 it is distinguished from all other reagents. Natural 
 tovo-asparagine and the aspartic acid formed from it 
 yield, by the action of nitrosyl chloride, Icevo-chloro- 
 succinic acid ; 2 while, as stated above, the malic acid 
 derived from the same substances yields, by the 
 action of phosphoric chloride, a chloro-succinic acid 
 
 i Ber., xxix. 133 (1896). 
 2 Tilden and Marshall, Jour. Ckem. Soc., Ixvii. 494 (1895). 
 
208 A SHORT HISTORY OF THE 
 
 of opposite sign. Walden's production of optical 
 antipodes may be represented by the following 
 diagram : 
 
 I. Malic + PC1 5 . 
 
 1. Chlor-succ. V Id, Chlor-succ. 
 
 d. Malic + PC1 5 . 
 
 The hypotheses of which an outline has been 
 given relate to the atom of the element carbon ; but 
 considerations of this kind, if valid in the case of 
 one element, must be applicable to others. Hitherto, 
 however, but little progress has been made in this 
 direction, for no other element is yet known which 
 possesses equally with carbon the power of uniting 
 with other atoms of the same kind (i.e. carbon to 
 carbon, &c.), and at the same time of combining 
 with several different atoms forming compounds 
 which exhibit optical activity or some other mani- 
 festation of asymmetry. 
 
PEOGEESS OF SCIENTIFIC CHEMISTEY 209 
 
 
 CHAPTER VIII 
 
 ELECTEICITY AND CHEMICAL AFFINITY 
 
 IN the Philosophical Transactions for 1784 will be 
 found a paper by the Hon. Henry Cavendish, which 
 embodies those results of his " Experiments on Air " 
 which led to the famous discovery of the composi- 
 tion of water. The proposition that water consists 
 or is composed of two elementary gases is, however, 
 all the more firmly established when, to the state- 
 ment of Cavendish that from two measures of 
 " inflammable air " (hydrogen) exploded with one 
 measure of " dephlogisticated air " (oxygen) water 
 is produced, we are in a position to add that from 
 the decomposition of water these two gases, and 
 nothing else, can be reproduced. This latter result 
 was observed for the first time by Nicholson and 
 Carlisle in 1800, only sixteen years later than the 
 announcement of Cavendish's discovery. 
 
 In the meantime a new philosophical instrument 
 and a new field of research had been brought within 
 the reach of chemists and physicists by the dis- 
 coveries of Galvani and Volta, which culminated in 
 the invention of the Voltaic Pile and the Voltaic 
 Battery shortly before this time. Electric currents 
 
 o 
 
210 A SHOKT HISTORY OF THE 
 
 could thus be procured at will, and the voltaic 
 decomposition of acidified water was followed by a 
 host of observations of a similar kind. Solutions of 
 various salts submitted to the action of the current 
 yielded in some cases acids and alkalis, in others 
 acid and oxygen at one pole, while metal and 
 hydrogen appeared at the opposite pole. By this 
 same agency Davy, in 1807, succeeded in proving 
 that potash and soda are oxides containing a pair of 
 very remarkable metals. 
 
 The attention of chemists in this way became 
 attracted to the fascinating phenomena of electro- 
 lytic or electro - chemical decomposition, and not 
 only were many facts discovered of greater or less 
 practical utility, but hypotheses were necessarily 
 framed to account for the facts. Davy himself, 
 after some years occupied more or less with the 
 study of the galvanic phenomena, seems to have 
 arrived at the conclusion that chemical combination 
 is produced by the union of atoms charged with 
 opposite kinds of electricity ; and, in fact, to have 
 identified chemical " affinity " with electrical attrac- 
 tion between particles, or, at any rate, to have 
 regarded chemical affinity and electrical attraction 
 as dependent upon the same cause. A similar 
 theory was formulated by Berzelius about 1818, 
 but the Swedish chemist considered it probable that 
 " simple and compound atoms are electro-polar ; in 
 the majority of them one of the poles is endowed 
 with a preponderant force, the intensity of which 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 211 
 
 varies according to the nature of the body. Those in 
 which the + pole is preponderant are called electro- 
 positive, those in which the pole predominates are 
 electro-negative." l Hence, according to Berzelius, 
 each atom contains both positive and negative elec- 
 tricity ; but the atom of oxygen, for example, has a 
 large amount of negative electricity accumulated 
 upon one pole, with a relatively small amount of 
 positive electricity at the other, while the atom of 
 such an element as potassium possesses a strong 
 charge of positive with a small charge of negative 
 electricity. During the act of combination the 
 positive pole of one atom is turned toward the 
 negative pole of the atom with which it is about to 
 unite, and the opposite electricities neutralise each 
 other with production of the phenomena of heat 
 and light, as in the production of sparks between 
 oppositely charged masses. 
 
 From this time forward little further progress 
 was made, so far as electro-chemical phenomena 
 were concerned, till the subject was taken up by 
 Faraday about 1832 ; and as the result of his 
 researches the two important quantitative state- 
 ments which are usually known as Faraday's Laws 
 of Electrolysis, together with a vast body of other 
 important observations, were established. The state- 
 ments referred to may, for the sake of completeness, 
 be recalled to the recollection of the reader. The 
 first of these may be given in the original words of 
 
 1 Berzelius, Traite de Chimie, vol. i. (1842). 
 
212 A SHORT HISTORY OF THE 
 
 the discoverer : " The chemical power of a current 
 of electricity is in direct proportion to the absolute 
 quantity of electricity which passes." 1 Faraday's 
 own language may also be used to lead up to and 
 express the second law. " Compound bodies," he 
 says, 2 "may be separated into two great classes, 
 namely, those which are decomposible by the 
 electric current and those which are not. Of the 
 latter some are conductors, others non-conductors, 
 of voltaic electricity. ... I propose to call bodies of 
 the decomposible class electrolytes. Then, again, the 
 substances into which these divide, under the in- 
 fluence of the electric current, form an exceedingly 
 important general class. They are combining bodies, 
 are directly associated with the fundamental parts of 
 the doctrine of chemical affinity, and have each a 
 definite proportion in which they are always evolved 
 during electrolytic action. I have proposed to call 
 these bodies generally ions, or particularly anions 
 and cations, according as they appear at the anode 
 or cathode, and the numbers representing the pro- 
 portions in which they are evolved electro-chemical 
 equivalents. . Thus oxygen, chlorine, iodine, hydro- 
 gen, lead, tin, are ions ; the three former are anions, 
 hydrogen and the two metals are cations, and 8, 36, 
 125, 1, 104, 58, are their electro-chemical equiva- 
 lents nearly." 3 We learn from this, and from other 
 
 1 " Experimental Researches in Electricity," vol. i. p. 241. 
 
 2 LOG. cit., pp. 242, 243. 
 
 3 In this passage there is in the original a slight verbal slip, which 
 is corrected above. 
 
PROGKESS OF SCIENTIFIC CHEMISTRY 213 
 
 experiments described in the course of these re- 
 searches, that the electro- chemical equivalents of 
 ions are the same as their ordinary chemical equi- 
 valents or combining proportions. 
 
 Faraday's view as to the cause of electrolytic 
 phenomena led him to reject the notion enter- 
 tained by some of his predecessors that electro- 
 chemical decomposition was the result of the 
 electrical attraction or repulsion of the poles acting 
 upon the constituents of the electrolyte. He states 
 expressly that he believed that the effect is due to 
 a force acting internally upon the ions, and " either 
 superadded to or giving direction to the ordinary 
 chemical affinity of the bodies present." 
 
 Nevertheless, while correcting some of the experi- 
 mental errors in Davy's work, Faraday seems to 
 have entertained nearly the same idea with regard 
 to the nature of the phenomena of chemical com- 
 bination and electro -chemical decomposition, for 
 (loc. dt. t p. 248) he refers, with evident sympathy, 
 to " the beautiful idea that ordinary chemical 
 affinity is a mere consequence of the electrical 
 attractions of the particles of different kinds of 
 matter " ; and a little further on he states his 
 "conviction that the power which governs electro- 
 chemical decomposition and ordinary chemical at- 
 tractions is the same." This idea evidently became 
 established in the mind of Faraday, for in a later 
 paper l the following passage occurs : " All the 
 
 1 " Experimental Researches in Electricity," vol. i. p. 272. 
 
214 A SHORT HISTORY OF THE 
 
 facts show us that that power commonly called 
 chemical affinity can be communicated to a distance 
 through the metals and certain forms of carbon ; 
 that the electric current is only another form of 
 the forces of chemical affinity ; that its power is in 
 proportion to the chemical affinities producing it ; 
 that when it is deficient in force it may be helped 
 by calling in chemical aid, the want in the former 
 being made up by an equivalent of the latter ; that 
 in other words the forces termed chemical affinity and 
 electricity are one and the same." This view was 
 adopted by other writers. Daniell, for example, the 
 inventor of the constant cell, in his " Chemical 
 Philosophy," heads the paragraphs relating to the 
 battery by the title, " Circulating Affinity or Elec- 
 tricity." 
 
 The phenomena of electro-chemical decomposition 
 present a great many questions of difficulty, the 
 interpretation of which have led in the past and 
 are likely still to lead to much difference of opinion 
 and controversy. When, for example, an electric 
 current is sent through a solution of copper sul- 
 phate, the ordinary visible products which accumu- 
 late at the poles are metallic copper at the cathode, 
 and oxygen gas at the anode. If the current is 
 strong, hydrogen is also given off with the copper. 
 When sodium sulphate is used instead of copper 
 sulphate, the products are gaseous hydrogen and 
 oxygen; while the solution, originally neutral, 
 becomes alkaline from the production of soda round 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 215 
 
 the cathode, and acid from the accumulation of 
 sulphuric acid round the anode. The difference 
 between these two results has been explained in 
 the terms of two distinct hypotheses, the earlier of 
 which was based on the dualistic theory of salts. In 
 the system inaugurated by Lavoisier, and approved 
 by Berzelius, every salt was made up of two oxides 
 the one of a metal, the other of a non-metal ; thus 
 cupric sulphate was supposed to consist of copper 
 oxide united to sulphur trioxide or sulphuric acid, 
 while sodium sulphate was composed of soda or 
 oxide of sodium united to the same sulphuric acid. 
 Expressed in symbols, they stood thus : 
 
 Cupric sulphate .... CuO.S0 3 
 Sodium sulphate . ' . . . NaO.S0 3 
 
 Berzelius and his school supposed that when these 
 compounds are decomposed by the electric current 
 the ions are CuO and S0 3 in one case, and NaO 
 and S0 3 in the other ; but that in the case of the 
 copper salt the metal separates in consequence of the 
 simultaneous decomposition of water, the hydrogen 
 of which reduces the metallic oxide uniting with 
 the oxygen and setting the metal free. 
 
 The other hypothesis was based on the unitary 
 system, of which the germ perhaps may be found 
 in the writings of Davy, but which was formally 
 introduced by Daniell about 1842, 1 and afterwards 
 adopted by Gerhardt upon evidence of a different 
 
 1 Darnell's " Chemical Philosophy," 2nd edit. pp. 432-440. 
 
216 A SHOKT HISTORY OF THE 
 
 kind. According to this view the two salts are 
 resolved on electrolysis into the ions Cu and S0 4 , 
 Na and S0 4 respectively. The decomposition of 
 the copper compound therefore gives metal as 
 the primary cation, while at the anode the radicle 
 S0 4 is resolved into 0, which escapes, and S0 3 , 
 which forms sulphuric acid with the water. In the 
 case of the sodium salt, the alkali and the acid 
 observed are alike secondary products ; the former 
 resulting from the action of the liberated sodium 
 upon the water with simultaneous escape of hydro- 
 gen gas. 
 
 Another question which required investigation 
 relates to the course of the current through the 
 solution. It is a remarkable fact that water alone 
 appears to be almost destitute of conducting power, 
 its resistance increasing in proportion as it is more 
 free from dissolved salts or gases. But the addition 
 of a very small quantity of sulphuric acid seems to 
 make it not only conduct freely, but undergo electro- 
 chemical decomposition into its constituents oxygen 
 and hydrogen. When the current passes, is it really 
 conveyed by the water, by the acid, or by some 
 compound of the two ? That is a very important 
 question ; but in order to supply the answer to it, 
 which is provided by commonly received theories of 
 the present day, it is necessary to know something 
 of the course of observation and experiment which 
 has led within the last ten years to the adoption of 
 rather remarkable views concerning the state of 
 
PKOGKESS OF SCIENTIFIC CHEMISTRY 217 
 
 dissolved electrolytes, and the nature of solutions 
 generally. 
 
 So long ago as 1788 Sir Charles Blagden pub- 
 lished in the Philosophical Transactions a paper 
 dealing with the familiar fact that salt water does 
 not freeze so easily as fresh water. In that paper 
 he showed that various salts added to water cause 
 a depression of the freezing point, and that the 
 depression is in each case practically proportional to 
 the amount of the salt present. There the ques- 
 tion remained for nearly a century. The next steps 
 in advance could only be taken when Blagden's 
 conclusions had been established upon a much 
 firmer basis of exact experiment than he had been 
 able to supply; and, as in so many other cases, it 
 required the successive efforts of many experimenters 
 to provide material upon which to build theory 
 with any prospect of success. Among the workers 
 who took up the question must be mentioned 
 Riidorff, 1 De Coppet, 2 and Guthrie. 3 De Coppet 
 got so far as to show that the " co-efficient of de- 
 pression " of the freezing point is constant for the 
 same substance, and that it is equal for similar 
 substances when added to the same quantity of 
 water in amounts proportional to their molecular 
 weights. It is, however, to the important researches 
 of Professor F. M. Raoult 4 of Grenoble that the 
 
 1 Pogg., cxiv. 63 (1861), cxvi. 55 (1862), &c. 
 
 z Ann. Chim., xxiii. 366 (1871), xxv. 502 (1872), xxvi. 98 (1872). 
 
 3 Phil. Mag., 1875, 1876, 1878. 
 
 4 Ann. Chim., xxviii. 133 (1883), &c. 
 
218 A SHORT HISTORY OF THE 
 
 establishment of this generalisation is due, and as a 
 result a new method has been provided for the 
 determination of the molecular weights of those 
 substances to which the vapour-density method is 
 not applicable. Raoult's law may be stated as fol- 
 lows : the depression of the freezing point of a 
 liquid, caused by the solution in it of a liquid or 
 solid, is proportional to the absolute amount of dis- 
 solved substance, and is inversely proportional to its 
 molecular weight. Consequently, if a number of 
 different substances be taken in the proportions of 
 their molecular weights, and dissolved in a liquid 
 which is capable of solidifying at a determinable 
 temperature, the resulting reduction of the freezing 
 point will be the same in each case. This molecular 
 depression is then a constant for any given liquid, 
 though of course it differs in different liquids. The 
 solvents recommended by Raoult are water, acetic 
 acid, and benzene. If we take A as the reduction 
 of freezing point caused by the solution of 1 gram 
 of substance in 1 grams of water, for example, M 
 the molecular weight of the substance, and T the 
 lowering of the freezing point caused by the solution 
 of a molecular proportion of the substance in 100 
 parts of liquid, then, if the solution is dilute, 
 
 MA=T. 
 
 The value of T for water has been found to be 19 
 in reference to a great variety of neutral carbon 
 compounds, but it rises to about 37, or double the 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 219 
 
 value, when inorganic salts are employed. By 1888 
 the method of estimating the molecular weights 
 based upon these observations was generally adopted, 
 and consequently it was recognised, not only that 
 the freezing point of a solution is related to the 
 molecular weight of the dissolved substance, but 
 that metallic salts produce an effect per molecule 
 about twice as great as the effect produced by 
 neutral carbon compounds such as sugar. 
 
 The presence of a substance dissolved in a liquid 
 affects the other properties of the liquid, such as 
 its viscosity, or, if volatile, its boiling point and 
 the pressure of its vapour. The reduction of vapour 
 pressure, and the raising of the boiling points of a 
 number of liquids, have been subjects of repeated 
 experiment during the last half-century ; but the 
 connection between the magnitude of the effect 
 produced and the molecular weight of the dissolved 
 substance was discovered so recently as 1887 by 
 Raoult, whose work upon the freezing points of 
 solutions has been already referred to. The law 
 relating to vapour pressure has the same form as 
 that which applies to freezing points; that is, the 
 effect produced by molecular proportions of all sub- 
 stances in the same liquid is the same provided 
 the solution is dilute. Hence the equation already 
 given for the freezing point expresses the relation 
 for the vapour pressure equally well, if we let A 
 stand for the lowering of pressure produced by 
 1 gram of substance in 1 grams of solvent, and T 
 
220 A SHORT HISTORY OF THE 
 
 for the effect produced by a molecular proportion of 
 the substance in the same quantity of solvent. 
 
 The methods practically employed for estimating 
 the effect of dissolving a definite quantity of sub- 
 stance in a volatile liquid, may be based upon the 
 actual measurement of the pressure of the vapour 
 of the solution when in a barometric vacuum, or 
 upon the loss of weight sustained by a given 
 quantity of the solution, as compared with the loss 
 of weight of the same quantity of the pure solvent 
 when a current of air is passed under the same 
 conditions through both of them ; or the method 
 may be based upon observation of the boiling point 
 at atmospheric pressure of the liquid before and 
 after the dissolution in it of definite quantities of 
 substance. 
 
 Again, if a solution of sugar or of a salt is separated 
 from pure water by certain kinds of membrane, the 
 volume of the solution increases by the entrance of 
 water through the membrane, and a difference of 
 pressure will be immediately established on the two 
 sides of the membrane, which will go on increasing 
 up to a certain maximum for each kind of dissolved 
 substance. The pressure thus established is called 
 the osmotic pressure of the dissolved substance. It 
 appears to arise from the " semi-permeable " character 
 of the membrane, which allows water to pass in 
 either direction, but not the solute ; hence water 
 passes in the direction of the solution until the 
 pressure it alone exerts per unit of surface is equal 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 221 
 
 on both sides the septum, while on the side of the 
 solution there is the additional pressure due to the 
 solute. This can be measured and expressed in 
 millimetres of mercury or otherwise. The cells of 
 plant tissues are semi-permeable in this sense, and 
 membranes formed by the precipitation of certain 
 salts, especially cupric ferrocyanide within the pores 
 of unglazed earthenware, have been used for the 
 purpose of experiments on osmotic pressure. The 
 earliest and best determinations of osmotic pressure, 
 for which science is indebted to Professor W. Pfeffer 
 the botanist, 1 have led to conclusions which stand in 
 evident relation to the facts which have already been 
 stated concerning the influence of dissolved sub- 
 stances on the properties of liquids, and they may 
 be expressed nearly in the same terms. For it is 
 found that the effect is again proportional to the 
 strength of the solution, and when solutions of two 
 distinct substances are compared, the osmotic pres- 
 sure is found to be the same when each contains for 
 100 parts of the liquid molecular proportions of the 
 dissolved substance. 
 
 In 1 8 8 7 a theory was conceived which provided a 
 beautiful explanation of all these various facts and 
 relations. The first volume of the Zeitschrift fur 
 physikalische Chemie (pp. 481-508) contained a paper 
 by Professor J. H. Van't Hoff which threw a flood 
 
 1 " Osmotisehe Untersuchungen. Studien zur Zellenmechanik " 
 (BdUatter, ii., 1878, 182-193). See also Adie, Trans. Chem. Soc. 
 1891, 368. 
 
222 A SHORT HISTORY OF THE 
 
 of light upon the whole subject of solutions. 1 The 
 theory, then for the first time brought forward, is 
 based upon the recognition of the analogy between 
 the state of substances in solution and the same in 
 the state of gas. Pfeffer's results seem to have 
 established the fact that osmotic pressure in dilute 
 solutions is proportional to the amount of dissolved 
 substance in unit mass, and that the pressure is 
 directly proportional to the absolute temperature of 
 the liquid. The laws of osmotic pressure, then, 
 assume the same form as the law of Boyle, which 
 connects pressure and volume, and the law of Gay- 
 Lussac, which connects temperature and volume, for 
 gases. And further, since molecular proportions of 
 dissolved substances produce at the same tempera- 
 ture equal osmotic pressures, equal volumes of different 
 solutions, which give the same osmotic pressures, 
 contain the same number of molecules. This is equiva- 
 lent to asserting that the law of Avogadro applies 
 to solutions as well as to gases. 
 
 For the sake of clearness it may be as well, before 
 proceeding further, to recall the main features and 
 history of the kinetic theory of gases. From the 
 phenomena of gaseous diffusion there seems to be 
 very direct proof that the particles or molecules of 
 gases are always moving about. This idea was long 
 ago employed to explain their pressure and elasticity, 
 but only in a general and rather indefinite way, and 
 
 1 For an interesting summary of the course of events which led 
 Van't Hoff to the theory, see Berichte, xxvii. (1894), 6. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 223 
 
 it was in 1848 that Joule for the first time made 
 some calculations as to the velocity which the 
 particles of hydrogen and of oxygen must have in 
 order to produce the observed pressure. He found 
 that the particles of hydrogen, when at C. and 
 atmospheric pressure, move at the rate of more 
 than 6000 feet per second, which is much faster 
 than any ordinary projectile. 1 
 
 A paper had been communicated to the Royal 
 Society by J. J. Waterston in 1845, but unfor- 
 tunately was not thought fit for publication at the 
 time. Nearly half a century afterwards it was dis- 
 covered by Lord Rayleigh in the archives of the 
 Society, and was found to contain a nearly complete 
 development of the theory which has since become 
 so famous. This paper has been printed in the 
 Philosophical Transactions for 1892. Views essen- 
 tially the same as Waterston's were afterwards 
 published by Kronig in 1856, 2 and by Clausius 3 in 
 1857. 
 
 The kinetic theory supposes that all the par- 
 ticles in a gas subsist in a state of perpetual 
 movement with great velocity, but since they are 
 very small, and much crowded together, no single 
 molecule proceeds far in a straight line before 
 it approaches another molecule, and its course is 
 
 1 " On the Mechanical Equivalent of Heat, and on the Constitu- 
 tion of Elastic Fluids " (Brit. Assoc. Rep., 1848, Ft. II. 21-22). 
 
 2 " Grundziige einer Theorie der Gase " (Pogg. Ann., xcix. 315-322). 
 
 3 "Ueber die Art der Bewegung welche wir Warme nennen" 
 (Pogg. Ann., 1857, 353-380, and Phil. Mag., xiv. 108-127). 
 
224 A SHORT HISTOKY OF THE 
 
 altered, much in the same way that an elastic ball 
 falling upon any surface rebounds and passes off in 
 a new direction. The constitution of a liquid is 
 supposed to be somewhat similar, the difference be- 
 tween a gas and a liquid being attributed to the 
 association of a certain proportion of the moving 
 molecules into groups ; so that while in a true 
 gas each molecule moves independently of the 
 rest, except for collisions, in a liquid there is 
 a mixture of free molecules and aggregations of 
 molecules. 
 
 We may now return to the questions presented 
 by the phenomena of electrolysis. It has already 
 been stated that metallic salts generally, as well as 
 acids and alkalis, when dissolved in a large quantity of 
 water, cause a depression of the freezing point of the 
 water nearly twice as great as the depression caused 
 by the majority of carbon compounds. All these 
 substances are electrolytes when in aqueous solution, 
 while neutral carbon compounds are not. Now, by 
 carrying the analogy between dilute solutions and 
 the gaseous state a step further, an explanation 
 is provided of the existence of these exceptions. 
 This explanation is afforded by the doctrine of ionic 
 dissociation introduced by Arrhenius in 1888. The 
 abnormal vapour pressures of ammonium chloride, 
 phosphoric chloride, sulphuric acid, and many other 
 compounds are accounted for by the hypothesis that 
 each molecule of such a compound when vaporised 
 dissociates into two or more separate molecules, and 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 225 
 
 in like manner the new theory supposes that sub- 
 stances which become electrolytes when dissolved owe 
 the assumption of that character to a process of 
 dissociation. The dissociation of sulphuric acid, for 
 example, by the action of heat is, however, different in 
 its cause as in its results, from the dissociation which 
 is assumed to occur when it is mixed with water. 
 Vapour of sulphuric acid is made up of equal num- 
 bers of molecules of water and sulphur trioxide, 
 which, if cooled so that liquefaction occurs, unite 
 together again, almost completely reproducing the 
 original compound. Sulphuric acid diluted with 
 water is supposed to yield a mixture of the ions 
 H and S0 4 , the number of molecules of the com- 
 pound so resolved increasing in proportion to the 
 dilution up to a limit. 
 
 A view which, at first sight, appears somewhat 
 similar to this was proposed some forty years ago 
 by Clausius, 1 almost immediately after his enuncia- 
 tion and discussion of the kinetic theory of gases. 
 Clausius appears to have regarded a solution as 
 consisting of a mixture of entire molecules moving 
 about more or less rapidly, according to the tem- 
 perature of the liquid, together with positive and 
 negative partial molecules or ions, which owe their 
 separation from one another to the encounters which 
 result from the motion of heat. These partial mole- 
 cules, moving irregularly through the liquid, meet 
 
 1 R. Clausius, " Ueber die Elektricitatsleitung in Elektrolyten " 
 (Pogg., cL, 1857, 338 ; and Phil. Mag., xv., 1858, 94). 
 
226 A SHORT HISTORY OF THE 
 
 now and then with complementary partial molecules, 
 with which, when conditions are favourable, they 
 reunite and reproduce the original compound, the 
 number of free or ionised particles being, according 
 to this view, dependent upon the temperature, in- 
 creasing in numbers as the temperature is higher ; 
 while if the temperature is lowered the recombina- 
 tions occur more frequently than the separations in 
 unit time, so that, on the whole, there are fewer 
 free ions in the liquid, while under no circum- 
 stances is it assumed that the number of partial 
 molecules is more than a very small percentage of 
 the number of entire molecules. A doctrine of the 
 same kind had been enunciated by Williamson some 
 years earlier in his memoir l on the " Theory of the 
 Formation of ^Ether." The following passage ex- 
 plains his view quite clearly : " We are thus forced 
 to admit that in an aggregate of molecules of any 
 compound there is an exchange constantly going on 
 between the elements which are contained in it. 
 For instance, a drop of hydrochloric acid being sup- 
 posed to be made up of a great number of mole- 
 cules of the composition C1H, the proposition at 
 which we have just arrived would lead us to believe 
 that each atom of hydrogen does not remain quietly 
 in juxtaposition with the atom of chlorine with 
 which it first united, but on the contrary, is con- 
 stantly changing places with other atoms of hydro- 
 gen, or, what is the same thing, changing chlorine.' 
 
 1 Phil. Mag., 1850. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 227 
 
 Clausius, remarking upon this quotation, points out 
 that Williamson's theory assumes a more frequent 
 exchange of one atom for another than appears to 
 him to be necessary for the explanation of electro- 
 lytic conduction, in which case he observes, " it is 
 sufficient if the impact between complete molecules 
 is occasionally and perhaps, comparatively speaking, 
 rarely accompanied by an interchange of partial 
 molecules." 
 
 Since these views were expressed by Clausius, 
 facts have become known which seem to show that, 
 if the comparison of the dissolved with the gaseous 
 state is accepted, electrolytic conduction should 
 depend upon and be proportional to a dissociated 
 state in the dissolved electrolyte, which increases as 
 solution is more dilute. It has been stated, for 
 example, that the depression of the freezing point 
 of a solution of such a neutral compound as sugar, 
 a non-electrolyte, is only about half the depression 
 observed in the solution of an acid or a metallic 
 salt which is an electrolyte. Arrhenius 1 supposes 
 that when an electrolyte is dissolved its ions separate 
 from each other, not only, as assumed by Clausius, 
 to a small extent, but to a large extent which 
 increases with dilution ; so that in an infinitely dilute 
 solution none of the original molecules of the com- 
 pound exist, but only the electrolytically active 
 fragments of molecules or ions. When any electro- 
 motive force is applied to an electrolyte, therefore, 
 
 1 Zeitschr. Phys. Ckem., i. (1887), 631. 
 
228 A SHORT HISTORY OF THE 
 
 the current which passes is proportional to the 
 number of ions ready to convey the electricity. 
 
 There is therefore a fundamental difference be- 
 tween the earlier and the later views of the process 
 of electrolysis. According to the well-known hypo- 
 thesis of Grotthus, introduced in 1805, and to be 
 found in all text-books of electricity to the present 
 day, the molecules of an electrolyte upon which no 
 electro-motive force is acting must be supposed 
 to be distributed at random throughout the liquid, 
 and to be arranged in no sort of order. When two 
 metallic plates connected with a source of electricity 
 are dipped into the liquid, say dilute sulphuric 
 acid, the positive electricity of one plate attracts 
 the oxygen, while the negative electricity of the 
 other plate attracts the hydrogen, with the result 
 that the molecules of acid range themselves in a 
 multitude of polar chains across the space between 
 the electrodes. Then, if the electro -motive force is 
 sufficient, atoms of hydrogen are detached from 
 one end of all these chains, while the residues of the 
 molecules left take hydrogen from the adjoining 
 molecules, and so the transfer of hydrogen from 
 molecule to molecule occurs throughout each chain. 
 Oxygen is liberated in a similar way at the surface 
 of the opposite electrode, with a similar transfer of 
 oxygen from molecule to molecule throughout the 
 series of molecules forming each chain. And thus, 
 while hydrogen and oxygen are liberated in visible 
 bubbles at the surfaces of the electrodes, no action 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 229 
 
 is perceptible within the liquid which fills the space 
 between. A difficulty about this hypothesis, which 
 has become apparent only within recent years, is 
 that it assumes that the electrolyte is torn asunder 
 into its ions by the action of the current. If 
 that were the case, each chemical compound would 
 require the application of a certain definite mini- 
 mum electro-motive force peculiar to itself before 
 electrolysis would begin. But decomposition occurs 
 even when the electro-motive force is extremely 
 weak, and it is therefore now more generally be- 
 lieved that the current does nothing more than 
 direct to the respective electrodes the already 
 separated ions. According to the Clausius hypo- 
 thesis these are few at any one instant, but as fast 
 as they are driven to the electrodes and are libe- 
 rated, others are produced by dissociation within 
 the liquid. According to the doctrine of Arrhenius, 
 on the other hand, good electrolytic conductors are 
 in a state of ionisation or dissociation, which is 
 much more extensive, and which is increased by 
 dilution up to a certain limit, when it may be nearly 
 complete. The electric conductivity of a consider- 
 able number of electrolytes, including all the most 
 important acids, has been determined chiefly by a 
 method introduced by Kohlrausch, which is based 
 on the employment of an alternating current, thus 
 avoiding the difficulties arising from " polarisation " 
 of the electrodes and other causes. The results of 
 these experiments have led to the remarkable con- 
 
230 A SHORT HISTOKY OF THE 
 
 elusion that electric conductivity is directly related 
 to chemical activity. Thus the numbers expressing 
 the electric conductivities of a series of acids of any 
 given degree of dilution, also represent very closely 
 the relative powers of the same acids to effect such 
 chemical changes as the inversion of cane-sugar. 
 The chemical activity of electrolytes is therefore 
 directly related to the extent of the ionic dissocia- 
 tion of the substance, if we accept the hypothesis, 
 and at the present time it must be admitted that 
 there is no escape by way of any better explanation. 
 The same hypothesis accords also with the well- 
 known fact that many of the strongest acids, 
 hydrogen chloride for example, are, when apart 
 from water in the liquid state, both non-conductors 
 and non-electrolytes, and at the same time almost 
 destitute of chemical activity. The most charac- 
 teristic of all the interactions produced by acids is 
 that which follows from their contact with bases. 
 This results in the general formation of salts and 
 water, but an anomaly is here noticed which is 
 almost inexplicable without the ionisation theory. 
 The neutralisation of an acid by a base is attended 
 by the evolution of heat, and it has been generally 
 supposed that the amount of heat evolved in a 
 given reaction is a measure of the activity of the 
 affinities concerned, and of the amount of energy 
 which escapes or runs down in the process. But 
 when chemically equivalent quantities of different 
 acids are neutralised by the same basic hydroxide, 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 231 
 
 say soda NaHO, the amount of heat evolved by strong 
 acids is not much greater than that afforded by 
 acids reputed weak. Many experimental investiga- 
 tions of this subject have been undertaken, but the 
 results of the work of Professor Julius Thomsen, 
 of Copenhagen, may be regarded as the most 
 exact and trustworthy. He found that when one 
 molecule of soda, NaHO, dissolved in water is 
 neutralised by different acids mixed with the same 
 proportion of water, the amount of heat evolved 
 may be represented by thermal units, expressed in 
 the successive cases by the following numbers taken 
 as examples : 
 
 Name of acid. ^by^Ha^ Heat evolved. 
 
 Hydrochloric acid . . HC1 13740 
 
 Nitric acid . . . . HN0 3 13617 
 
 Sulphuric acid . '. .' |H 2 S0 4 15690 
 
 Phosphoric acid : . . . JH 3 P0 4 11343 
 
 Formic acid . . . . HCH0 2 13450 
 
 Acetic acid . . . . HC 2 H 3 2 13400 
 
 These examples suffice to show that the heat de- 
 veloped when an acid is neutralised by a basic 
 hydroxide bears no very obvious relation to the 
 chemical activity of the acid, as indicated by other 
 chemical reactions in which it is capable of taking 
 part. The ionisation hypothesis affords an explana- 
 tion of this which is consistent with all the facts 
 known. The heat produced in such changes as 
 these is chiefly due to the formation of water, and 
 not to the production of the several salts, which 
 like the acids and bases are electrolytes, and are 
 
232 A SHORT HISTORY OF THE 
 
 therefore, according to the hypothesis, dissociated to 
 a considerable extent when in aqueous solution. 
 The change which occurs when soda and hydro- 
 chloric acid are brought together is therefore not to 
 be represented by the familiar equation 
 
 HC1 + NaHO = NaCl + H 2 0. 
 
 The expression must take rather the following 
 form : 
 
 And from this it appears that water is a compound 
 which differs from caustic soda and nearly all other 
 soluble oxides, chlorides, and salts, inasmuch as it is 
 not under ordinary circumstances ionised to any 
 appreciable extent. This agrees with what is known 
 of the properties of water as an electrolyte. It 
 results from the experiments of Davy, and all who 
 have followed since his time, that water is a very 
 bad conductor, and its conductivity diminishes in 
 proportion as it is deprived of dissolved salts or 
 other substances. Pure water is very difficult to 
 make, and still more difficult to preserve ; but when 
 every precaution has been taken to avoid contamina- 
 tion from the vessel in which it is kept, and from 
 the atmosphere surrounding it, the conductivity is 
 so small that it has never been determined with 
 accuracy. It is perhaps owing to this remarkable 
 peculiarity that water so well plays its part in 
 nature as a liquid which is almost absolutely neutral, 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 233 
 
 and save as a solvent usually exhibits very little 
 chemical activity. 
 
 The heat of neutralisation of acids per equivalent 
 is from 13,000 to 14,000 calories on the average. 
 The differences noticeable in the values given above 
 are attributed to the different extent of ionisation 
 in the different acids when mixed with the same 
 amounts of water. They are also partly due to the 
 fact that in some cases, notably that of phosphoric 
 acid, the normal sodium salt supposed to be formed 
 is partially decomposed in the presence of water 
 into soda and acid, and hence the interaction be- 
 tween equivalent quantities of base and acid is in 
 such cases incomplete. 
 
 Such in broad outline are the views accepted by, 
 we must suppose, the majority of chemists at the 
 present day. They are consistent enough among 
 themselves, but while they help to connect together 
 many facts and phenomena they are still attended 
 by many difficulties, and leave much that has always 
 been obscure still unexplained. 
 
 The ions, according to Arrhenius, are associated 
 with electric charges, but whence these charges are 
 derived is far from intelligible. Taking common 
 salt, for example, this substance in the solid state is \ 
 universally supposed to be made up of atoms of j 
 sodium in juxtaposition with atoms of chlorine; 
 when the salt is dissolved in water a large propor- 
 tion of the whole number of molecules are at once 
 resolved into ions of sodium with a positive charge, 
 
234 A SHORT HISTORY OF THE 
 
 and ions of chlorine with an equal negative charge, 
 and if the liquid is diluted the number of these dis- 
 sociated and charged particles increases till there are 
 very few, if any, molecules of salt left. It does not 
 appear necessary to the ionic doctrine, however, to 
 assume the permanent residence of charges of elec- 
 tricity upon the separated atoms. It is only neces- 
 sary to state what is the fact, that there are two 
 classes of elementary atoms. One of these includes 
 hydrogen and the metals which, in virtue of some 
 peculiarity of their structure, are capable of becoming 
 associated with a unit charge of positive electricity, 
 and conveying this from the positive electrode to 
 the negative in the process of electrolysis. The 
 other class includes oxygen, the halogens, sulphur, 
 and perhaps some others, which are similarly en- 
 dowed with the power of conveying negative elec- 
 tricity only, and in the process of electrolysis travel 
 with their unit charges from the negative to the 
 positive surface. On such a hypothesis the initial 
 difficulty as to the determining cause of ionisation 
 when a substance like salt is dissolved in water, is 
 certainly not greater than that which attends the 
 ordinary view ; and, further, it may be pointed out 
 that it is not yet proved that all the elements come 
 within the two classes just mentioned. Carbon 
 notably seems to be incapable of assuming the ionic 
 state. Its chlorides are non-electrolytes, as are all 
 its hydrides, and when electrolysis of a compound of 
 carbon like acetic acid occurs, the hydrogen ion 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 235 
 
 goes to the cathode as usual, while the carbon, carrying 
 also some hydrogen with it, accompanies the oxygen 
 to the cathode. The old question is, in fact, not yet 
 finally answered: is chemical combination due to 
 the joining together of electrically charged atoms ? 
 in other words, is " chemical affinity " identical with 
 electricity ? This may be true of acids, bases, and 
 salts, but there is nothing to lead us to suspect that 
 the atoms of carbon, hydrogen, and oxygen in sugar 
 are held together in any such way. If the chemical 
 energy of the sodium and chlorine becomes electrical 
 when common salt dissolves, there is no obvious 
 reason why the chemical energy of the sugar should 
 not, at least in part, undergo a corresponding change, 
 and give an electrolyte, of which the hydroxyl 
 present in the sugar molecule would naturally be 
 the negative ion. Sugar, however, shows a normal 
 osmotic pressure, and does not conduct electrolyti- 
 cally. It seems improbable that the complex mole- 
 cules formed by carbon compounds, in which are 
 frequently concerned forty or fifty atoms of carbon, 
 perhaps twice that number of atoms of hydrogen 
 and oxygen, beside nitrogen, are held together by 
 the operation of any agency which requires opposition 
 of quality, a positive and a negative, in juxtaposition 
 to account for the union of any two atoms together. 
 The only compounds of carbon which are capable 
 of pure electrolytic decomposition are those which 
 play the part of acids, and in such cases the carbonyl 
 group, CO. OH, is usually present, and is resolved 
 
236 A SHORT HISTORY OF THE 
 
 into H, which goes to the cathode, while the CO.O 
 passes to the anode, dragging with it all the remain- 
 ing carbon, hydrogen, and other atoms which are 
 attached to it in the compound. It is easy enough 
 to see why the conductivity of trichloracetic acid, 
 CClg.COOH, so greatly exceeds that of acetic acid, 
 for the anion has not the burden of hydrogen atoms 
 to carry to the electrode, but has in the chlorine 
 a passenger which actually helps to row the boat 
 across. 
 
 Nitrogen is another element about the capability 
 of which to form an ion there is some room for 
 doubt. Ammonia is not an electrolyte unless dis- 
 solved in water, and then it behaves as a hydroxide. 
 Nitric acid is resolved into H and NCL which is 
 
 o 
 
 further broken up by electrolysis. The only com- 
 pound in which nitrogen alone figures as an ion 
 appears to be the remarkable hydrazoic acid, or 
 diazo-imide, of which the ions must be H and N>. 
 
 o 
 
 This seems to deserve some further investigation. 
 
 In 1880 Professor H. B. Dixon 1 discovered the 
 remarkable fact that a mixture of carbonic oxide 
 and oxygen completely dried is not inflamed by 
 the passage of an electric spark in contact with the 
 gas. This attracted attention to other facts pre- 
 viously known, which seemed to prove that chlorine 
 in the absence of moisture was very far from being 
 the active substance commonly supposed. Chlorine, 
 it is now known, may be kept in contact with 
 
 1 Brit. Assoc. Report (1880), 503. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 237 
 
 sodium, or with copper, for years without tarnishing 
 the lustre of the metal, provided both have been 
 most carefully dried, while on the introduction of a 
 drop of water instantaneous combination occurs. 
 Other experiments by W. H. Brereton Baker 
 have demonstrated that the combustion of carbon, 
 sulphur, and even phosphorus in oxygen is pre- 
 vented at temperatures considerably above the 
 ordinary igniting points of these substances if 
 moisture is removed as completely as possible, 1 and 
 that even ammonia and hydrogen chloride, 2 and 
 nitric oxide and oxygen do not combine when both 
 gases are perfectly dry. On these and some other 
 results attempts have been made to construct a new 
 electro- chemical theory of combination, which, how- 
 ever, requires that in all cases of union a small 
 quantity of some third substance, not necessarily 
 water, must be present. Facts, however, are at 
 present in opposition to the general application of 
 such a view, and it will be necessary to study the 
 conditions of chemical change yet more fully and 
 completely before anything more than partial and 
 tentative hypothesis will be within reach. 
 
 l Proc. Roy. Soc., xlv. 1-3. 
 
 2 Trans. Chem. Soc. (1894), 611-624. 
 
238 A SHORT HISTORY OF THE 
 
 CHAPTER IX 
 
 DISCOVERIES RELATING TO THE LIQUEFACTION 
 OF GASES 
 
 IT is perhaps not surprising that the older chemists, 
 down to the middle of the seventeenth century, 
 should have been almost entirely ignorant as to the 
 relation of " volatile spirits " and air to other kinds 
 of matter, and that no clear distinctions could be 
 drawn between the different kinds of air, whether 
 as to chemical composition or physical properties, 
 till the experiments of Black, Priestley, Cavendish, 
 and Lavoisier in the middle of the eighteenth century 
 brought so much new light upon this difficult sub- 
 ject. For, down to our own day, notwithstanding 
 exact knowledge as to their composition, a purely 
 arbitrary distinction prevailed between " permanent " 
 gases and other kinds of vaporisable substance. We 
 owe the abolition of this artificial and baseless dis- 
 tinction, and our present convictions as to the unity 
 of the essential nature of all terrestrial matter, to a 
 long course of experimental inquiry, the history of 
 which during these later years forms a tangled web, 
 the several threads of which are very difficult to 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 239 
 
 follow clearly, and the pursuit of which would not 
 be very profitable to the student. 1 
 
 The first recorded reference to the liquefaction of 
 a commonly recognised gas occurs in " Fourcroy's 
 Chemistry," vol. ii. p. 74, where it is stated, without 
 any description of the process, that Citizens Monge 
 and Clouet have liquefied sulphurous acid (sulphur 
 dioxide). The next experiment of the same kind 
 was made by Northmore, who, in 1805, reduced 
 chlorine and probably also sulphurous acid to the 
 liquid state by compressing the gas, by means of 
 a brass condensing syringe, into a pear-shaped glass 
 receiver. 2 From this time till the subject was taken 
 up by Faraday no gases were reduced to liquid, but 
 in the interval Cagniard de la Tour carried out his 
 remarkable investigation into the action of heat 
 upon volatile liquids. 3 By heating to various tem- 
 peratures water, or ether, or alcohol, contained in a 
 gun-barrel stopped at each end, he was able to 
 prove that such liquid may be wholly changed into 
 vapour, notwithstanding the existence of an enor- 
 mous pressure, and in the case of ether the vapour 
 thus formed occupies a volume less than twice the 
 volume of the liquid from which it is produced. By 
 enclosing a stone ball in the tube along with the 
 
 1 A tolerably complete and impartial statement of the contribu- 
 tions made to the subject of the liquefaction of gases by various 
 experimenters who have devoted themselves to it, is given by 
 Prince Kropotkin in the Nineteenth Century for Aug. 1898. 
 
 2 Nicholson's Journal, xii. and xiii. 
 
 3 Ann.[Chim., 2nd series, xxi. 127, 178, and xxii. 410. 
 
240 A SHORT HISTORY OF THE 
 
 liquid, he was able to tell when the liquid had 
 entirely evaporated by the character of the sound 
 produced within by rolling the ball to and fro, and 
 he even succeeded in making some rough estima- 
 tions of the pressure of the vapour within. Similar 
 results were afterwards obtained in glass tubes. 
 
 The researches which ultimately led to the lique- 
 faction of all known gases were begun by Faraday 
 and Davy in 1823. The example of steam, which 
 is known to be condensable to liquid water either 
 by cooling or by pressure, would lead naturally to 
 the belief that some at least of the substances called 
 gases might be vapours of very volatile liquids con- 
 densable like steam to liquid. In dealing with a 
 gas two methods present themselves when the object 
 is to subject it to pressure : the one already used 
 by Northmore consists in the direct application of 
 mechanical pressure by means of a pump ; the other 
 consists in enclosing materials from which the gas 
 can be generated within a tube strong enough to 
 resist the pressure of the gas as it accumulates. 
 The latter was the method used by Faraday. The 
 first case examined was that of chlorine, which had 
 been found by Faraday himself to form a crystalline 
 compound with water. It is now a little doubtful 
 what idea led to the heating of these crystals in a 
 closed tube, and whether it occurred first to Davy 
 or to his assistant. Faraday had been previously 
 occupied with chlorine, and had discovered two 
 chlorides of carbon in 1820. But Davy seems to 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 241 
 
 have suggested the experiment which led to the 
 observation of liquid chlorine, and to have regarded 
 the condensation of the gases as his own subject 
 (see Faraday's "Researches," p. 139). Immediately 
 after this muriatic acid was liquefied by a similar 
 method, in which the materials used were sal-am- 
 moniac and sulphuric acid, and Faraday, there can 
 be no doubt, was the operator. Faraday also lique- 
 fied in glass tubes sulphurous acid, sulphuretted 
 hydrogen, carbonic acid, nitrous oxide, euchlorine, 
 cyanogen, and ammonia. There then remained only 
 the gases of the atmosphere, namely, oxygen and 
 nitrogen, beside hydrogen, marsh gas, carbonic oxide, 
 and nitric oxide, which resisted all attempts by this 
 method to change their state, and arsenetted hydro- 
 gen, hydriodic and hydrobromic acids, which were 
 easily overcome by Faraday when some twenty 
 years later he resumed his experiments upon the 
 subject. In the meantime Thilorier in Paris, acting 
 upon the same principle, with the substitution of 
 metallic cylinders for glass tubes, prepared large 
 quantities of liquid carbonic acid, and was the first 
 to obtain this substance in a solid state. 1 For this 
 purpose he used the now familiar method of allowing 
 a jet of the liquid to escape through a fine orifice 
 into a box of peculiar construction, where, in con- 
 sequence of the evaporation of a portion of the 
 liquid, the rest is chilled below its freezing point, 
 and accumulates in the form of a fine snow-like 
 
 1 Ann. Chim., Ix. (1335), 432. 
 
242 A SHORT HISTORY OF THE 
 
 powder. This solid material has ever since played 
 a prominent part in many experiments requiring a 
 low temperature, and though it is not by the aid of 
 solid carbon dioxide that the most remarkable results 
 have been obtained, it remains to this day an ex- 
 tremely valuable cryogenic agent. In 1845 Faraday 
 published l the results of further attempts to liquefy 
 the gases remaining unsubdued by his earlier method. 
 He now employed two pumps for compression, and 
 glass tubes fitted with stop-cocks as receivers. The 
 latter he cooled to a temperature of 166 F. below 
 zero, by means of solid carbonic acid and ether boil- 
 ing under reduced pressure. In stating the con- 
 siderations which led him to employ this agent, and 
 in discussing the want of success in dealing with 
 the six so-called permanent gases, Faraday evidently 
 had ideas which came very near to an anticipation 
 of the important principle established twenty years 
 later by Andrews. With regard to the experimental 
 conditions he says, " As my hopes of success, beyond 
 that heretofore obtained, depended more upon de- 
 pression of temperature than on the pressure which 
 I could employ in these tubes, I endeavoured to 
 obtain a still greater degree of cold. There are, in 
 fact, some results producible by cold which no 
 pressure may be able to effect. Again, that beauti- 
 ful condition which Cagniard de la Tour has made 
 known, and which comes on with liquids at a certain 
 heat, may have its point of temperature for some of 
 
 1 Phil. Trans. (1845), 155, and " Collected Works," p. 96. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 243 
 
 the bodies to be experimented with, as oxygen, 
 hydrogen, nitrogen, &c., below that belonging to the 
 bath of carbonic acid and ether ; and in that case 
 no pressure which any apparatus could bear would 
 be able to. bring them into the liquid or solid state." 
 And later on he observes that " the temperature of 
 166 F. below 0, low as it is, is probably above 
 this point of temperature for hydrogen, and perhaps 
 oxygen and nitrogen ; and then no compression, 
 without the conjoint application of a degree of cold 
 below that we have as yet obtained, can be expected 
 to take from them their gaseous state." 
 
 Here, then, the resources of the physical labora- 
 tory seemed to have been exhausted, and it is 
 probable that, slowly as successive steps toward 
 the desired end, the reduction of the remaining 
 intractable gases, seemed to be accomplished, pro- 
 gress would have been still further delayed but 
 for the important experiments of Andrews, which, 
 though carried on during many years, were not 
 published in extenso till 1869. 1 Previously to 1863 
 Dr. Andrews had observed that " on partially lique- 
 fying carbonic acid by pressure alone, and gradually 
 raising at the same time the temperature to 88 F., 
 the surface of demarcation between the liquid and 
 gas became fainter, lost its curvature, and at last 
 disappeared. The space was then occupied by a 
 homogeneous fluid, which exhibited, when the 
 
 1 "On the Continuity of the Gaseous and Liquid States of 
 Matter : " The Bakerian Lecture (Phil. Trans., 1869, ii. 575). 
 
244 A SHOKT HISTORY OF THE 
 
 pressure was suddenly diminished or the temperature 
 slightly lowered, a peculiar appearance of moving 
 or flickering striae throughout its entire mass. At 
 temperatures above 8 8 no apparent liquefaction * of 
 carbonic acid, or separation into two distinct forms 
 of matter, could be effected even when a pressure of 
 300 or 400 atmospheres was applied." Andrews 
 then proceeded to make a series of exact compari- 
 sons of the volume assumed by carbonic acid and 
 air when submitted to pressure at successive degrees 
 of temperatures starting from that of the air. The 
 results are most easily intelligible with the aid of 
 the diagram given in the Bakerian Lecture. Here 
 the curves are drawn with reference to two axes 
 of rectangular co-ordinates; the volumes occupied 
 by the gases being the ordinates, and the pressures 
 the abscissae, while the temperatures marked on each 
 curve are maintained constant. 
 
 The difference in the behaviour of carbonic acid 
 and air when submitted to gradually increasing 
 pressure at the temperature of the air is well shown 
 by the curves. While air is steadily reduced in 
 volume as pressure increases, from 1 up to 110 
 atmospheres, carbonic acid contracts at all tempera- 
 tures more rapidly than would be indicated by 
 Boyle's law, and at the pressure of 49*89 atmos- 
 pheres liquefaction begins. This is shown in the 
 
 1 It is interesting here to compare Faraday's remark in 1845: 
 "I am inclined to think that at 90 Cagniard de la Tour's state 
 comes on with carbonic acid." Collected Works, p. 109. 
 
PROGKESS OF SCIENTIFIC CHEMISTRY 
 
 245 
 
 curve marked 13'l by the sudden change from a 
 slope to a vertical direction. The other curves 
 show the nature of the volume changes which ensue 
 when the same increase of pressure is applied at 
 higher temperatures. Above 30'92 C. or 87'7 F. 
 Andrews found that no pressure was capable of 
 producing visible liquefaction. This temperature, 
 
 HE: 
 
 13 / 
 
 TToJ 
 
 then, is called the critical point ; below this lique- 
 faction occurs when sufficient pressure is applied, 
 above it carbonic acid behaves more nearly like 
 a permanent gas in proportion as the tempera- 
 ture is raised. All other gases behave in a similar 
 way. 
 
 On this series of observations Andrews was able 
 to base- an interesting distinction between a " gas " 
 
246 A SHORT HISTORY OF THE 
 
 and a "vapour," terms which up to this time 
 had been used in an uncertain and arbitrary way. 
 " Ether," he says, " in the .state of gas is called a 
 vapour, while sulphurous acid in the same state is 
 called a gas ; yet they are both vapours, the one 
 derived from a liquid boiling at 35, the other 
 from a liquid boiling at 10. . . . Many of the 
 properties of vapours depend on the gas and liquid 
 being present in contact with one another, and this 
 we have seen can only occur at temperatures below 
 the critical point. We may accordingly define a 
 vapour to be a gas at any temperature under its 
 critical point. ... If this definition be accepted, 
 carbonic acid will be a vapour below 31, a gas 
 above that temperature ; ether a vapour below 
 200, a gas above that temperature." 
 
 The most important deduction from the results of 
 these experiments, then, supplies a clear explanation 
 of the difficulty encountered in attempting the 
 liquefaction of the six exceptional gases. Up to 
 this time the lowest temperatures attainable had 
 been above the critical points of all of them. It is 
 now known that the critical temperature of oxygen, 
 for example, is about 118 C '8C., and that of nitrogen 
 146 0< C. It was not till towards the close of the 
 year 1877, that two experimenters working along 
 distinct lines arrived by the use of two different 
 methods at substantially the same result. On the 
 24th December 1877, at the meeting of the 
 French Academy, two communications announcing 
 
PKOGRESS OF SCIENTIFIC CHEMISTRY 247 
 
 the liquefaction of oxygen were received, the one 
 from M. Raoul Pictet of Geneva, the other from M. 
 Louis Cailletet 1 of Chatillon-sur-Seine. Pictet 2 em- 
 ployed what was essentially the method of Faraday, 
 that is, he generated the gas within a closed vessel, 
 where by its accumulation pressure was generated, 
 and he cooled the tube containing the gas. He 
 attained the success which was denied to Faraday, 
 by the more efficient cooling of the gas. By means 
 of the evaporation of liquid sulphur dioxide, the 
 temperature of 6 5 C. is reached, and at this point 
 carbon dioxide is easily liquefied. By the rapid 
 boiling of the liquid thus produced, the temperature 
 of 140 C. is attained. This is below the critical 
 point for oxygen, and the pressure employed by 
 Pictet in his first experiments, amounting to about 
 475 atmospheres, was, therefore, excessive and 
 unnecessary. Liquid oxygen was formed in con- 
 siderable quantity, but the announcement in the 
 following month of the liquefaction and solidification 
 of hydrogen was evidently based upon some error 
 of observation, for we now know that the critical 
 temperature of hydrogen is nearly 120 lower in 
 the scale. 
 
 Cailletet employed an apparatus, ever since 
 familiarly known as a laboratory appliance under 
 the name of the Cailletet pump, whereby a gas 
 
 1 " De la condensation de 1'oxygene et de 1'oxyde de carbone," 
 L. Cailletet (Compt. Rend., Ixxxv. 1213). 
 
 2 " Experiences sur la liquefaction de 1'oxygene," R, Pictet (Compt. 
 Rend., Ixxxv. 1214). 
 
248 A SHORT HISTORY OF THE 
 
 can be submitted to considerable pressure, and 
 when greatly reduced in volume the pressure can 
 be suddenly relieved. Under these conditions the 
 expansion of the gas produces cooling, in consequence 
 of which a portion of it appears in the form of 
 minute droplets, of which a part remains suspended 
 in the gas, giving the appearance of a cloud, and 
 part usually collects in visible streams upon the 
 side of the tube containing it. With this apparatus 
 Cailletet reduced to the liquid state oxygen and 
 carbonic oxide, beside ethylene and acetylene, marsh 
 gas and nitric oxide. 
 
 Two now remained of the original six uncon- 
 densable gases, namely, nitrogen and hydrogen. 
 Nitrogen yielded in 1883, in the hands of the 
 Polish Professors Wroblewski and Olszewski 1 ; but 
 hydrogen resisted for many years the almost con- 
 tinuous efforts which were made to collect it in 
 the liquid form, though in 1884 Wroblewski 2 
 announced that he had observed an appearance of 
 ebullition as of liquid in the gas under experiment. 
 This was contradicted by Olszewski, who in his 
 turn almost immediately afterwards 3 declared that 
 he had obtained a similar effect under somewhat 
 different conditions. The difficulties of the investi- 
 gation now increased enormously, and it is not 
 surprising that progress was slow, considering both 
 the great pecuniary cost of the work, involving as it 
 
 1 Compt. Rend., xcvi. 1140 and 1225. 
 
 2 Ibid., xcviii. 304. 3 Ibid., xcviii. 365. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 249 
 
 did the construction of much expensive apparatus, 
 and the use of large quantities of liquefied gases, as 
 well as the considerable personal risk involved in 
 the employment of the very high pressures which 
 the various reservoirs of gas were required to 
 sustain. Henceforward, for nearly twenty years no 
 new principle was introduced. Cailletet in 1882 
 recommended the use of liquid ethylene for the 
 production of low temperatures, and when by the 
 aid of ethylene oxygen in a liquid state could be 
 obtained in fairly large quantity, this also was 
 employed as a refrigerant. The application of 
 external cold to the vessel containing the highly 
 compressed gas was also associated with the cool- 
 ing effect produced by expansion as in Cailletet's 
 method, and it was in this way that nitrogen was 
 first liquefied by Wroblewski and Olszewskiin 1883, 
 as already mentioned. By a similar process Olszewski 
 got evidence of the liquefaction of hydrogen, and 
 was able a few years later to determine its critical 
 temperature and boiling point 1 with some consider- 
 able approach to accuracy. 
 
 About this time, namely, in 1884, the production 
 of low temperatures and the liquefaction of gases 
 became a subject of research in the laboratory of 
 the Royal Institution, under Professor Dewar. He, 
 like the Russian chemists, employed liquid nitrous 
 oxide and ethylene as cooling agents, but save in 
 the dimensions of the apparatus, no essential differ- 
 
 1 Phil. Mag., Aug. 1895. 
 
250 A SHORT HISTORY OF THE 
 
 ence is apparent in the published accounts of the 
 methods of procedure, the principles involved being 
 exactly the same. The collection of large quanti- 
 ties of liquid oxygen and liquid air has, however, 
 provided the means of producing and maintaining a 
 low temperature for a length of time, sufficient to 
 allow a number of important investigations to be 
 carried on, which have resulted in the discovery of 
 many interesting facts relating to the physical and 
 chemical properties of matter at .temperatures not 
 far above the absolute zero. Some of these will be 
 referred to later on. 
 
 In the course of this work an ingenious device of 
 Professor Dewar's has provided the means of avoid- 
 ing one serious difficulty. Of course all objects at 
 the common temperature of a room are at some 
 200 degrees Centigrade above the boiling points of 
 these very volatile liquids, and hence any glass or 
 other vessel used for their collection is relatively 
 very hot. When the liquid is poured into such a 
 vessel violent boiling at first occurs until the tem- 
 perature of the glass is reduced to that of the 
 liquid. But even then heat passes from the air 
 into the walls of the vessel, and so to the liquid, fast 
 enough to cause very rapid evaporation and loss. 
 This is avoided by immersing the vessel in which 
 the liquid is to be collected in a second vessel, also of 
 glass, united with it at the mouth, and completely 
 removing the air from the space between the two. 
 Across such a vacuous space no heat can be brought 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 251 
 
 by convection of air, and as under ordinary circum- 
 stances the amount of heat which is radiated to the 
 contents of such a jacketed receiver is relatively 
 small, vessels of this kind afford the means of storing 
 liquid air, or oxygen, even for hours, while the rate 
 of evaporation becomes exceedingly slow. 
 
 The year 1895 will be memorable for the introduc- 
 tion of a principle which, though previously known, 
 had never before been made the basis of a method 
 for effecting the cooling of a compressed gas. It 
 has, of course, long been known that when a gas is 
 compressed it becomes heated, and if the operation 
 is performed quickly, so that there is no time for 
 much loss by radiation or conduction, the tempera- 
 ture may be raised very considerably. This is often 
 demonstrated by the use of the so-called " fire 
 syringe," which consists of a strong glass tube closed 
 at one end and fitted with a piston. By introducing 
 a drop of an inflammable liquid, such as ether or 
 carbon bisulphide, and then suddenly forcing down 
 the piston, so as to squeeze the air into a relatively 
 small volume, a flash of light is seen, which is pro- 
 duced by the ignition of the mixed air and vapour 
 in the tube. Supposing a quantity of air compressed 
 by a piston in a similar manner, and the heat dis- 
 engaged is allowed to pass away, upon allowing the 
 gas to expand again, and so to lift the piston against 
 the pressure of the atmosphere, a corresponding 
 cooling effect will result, and the temperature of the 
 air will be lowered. This is, in fact, the principle 
 
252 A SHORT HISTORY OF THE 
 
 made use of in the most ordinary refrigerating 
 machines, which are used for making ice for pre- 
 serving meat, and for other purposes. 
 
 About 1844 Joule made a number of experiments 
 upon this subject, and demonstrated that the cold 
 produced by the dilatation of a gas results from the 
 conversion of heat into work in accordance with the 
 universal principle. He thought at first that if the 
 dilatation was so arranged that the gas did no work, 
 then no cooling would result. But it was pointed 
 out later by Professor W. Thomson 1 (now Lord 
 Kelvin), that this is only approximately true for 
 ordinary gases, which do not strictly comply with 
 the gaseous laws connecting volume with pressure 
 and temperature, and that some cooling would occur 
 under such conditions, the effect being the greater 
 in gases which, like carbonic acid gas, were less 
 perfect than others, like air. This was verified by 
 experiment, gases under considerable pressure being 
 allowed to escape through a porous plug. Air at 
 16 C. was found to be reduced in temperature 
 about *26 of a degree Centigrade for each atmos- 
 phere of release, oxygen at was cooled '316 C., 
 and carbon dioxide 1*252 C. per atmosphere. The 
 amount of such cooling is approximately inversely 
 as the square of the absolute temperature, so that 
 the colder the gas is while under pressure the more 
 it is cooled by release. At about 130 C. com- 
 
 1 Thomson's "Mathematical and Physical Papers," vol. i. 
 (" Thermal Effects of Fluids "). 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 253 
 
 pressed air is cooled to the extent of about 1 per 
 atmosphere taken off. 
 
 This principle has been applied not only by 
 Cailletet in the apparatus successfully used in the 
 liquefaction of oxygen, but by later workers in the 
 various attempts to reduce hydrogen to the liquid 
 state. Thus Olszewski, in 1884, exposed hydrogen 
 gas under a pressure of about 150 atmospheres to 
 the temperature of 2 1 1 by means of liquid oxygen 
 boiling under reduced pressure. The pressure of the 
 hydrogen was then reduced to about 20 atmos- 
 pheres, and being thus reduced in temperature below 
 its critical point a portion of it was liquefied. 
 
 Siemens, in 1857, seems to have had the idea of 
 " regenerating " cold by applying the same principle 
 which is used in his well-known regenerative fur- 
 nace for the storage of heat, but this idea was 
 never carried into practice. Later, in 1885, Solvay 
 patented a process based upon the same principle, 
 which involved the use of an expansion cylinder. 
 By such methods, however, the difficulty of exclud- 
 ing access of heat from without puts a rather narrow 
 limit upon the amount of cooling actually attainable. 
 
 More recently a method has been devised by 
 which such cooling effect can be made practically 
 cumulative, the gas while still under pressure being 
 cooled by another portion of the same immediately 
 after release. A description of such apparatus by 
 which practical results upon a large scale were ob- 
 tainable was given for the first time by Herr Linde, 
 
254 A SHORT HISTORY OF THE 
 
 an engineer, of Munich ; 1 but a patent for an appli- 
 cation of the same principle had been obtained pre- 
 viously (May 23, 1895) in England by Dr. W. 
 Hampson. Both machines have since undergone 
 modification in detail, and are now employed suc- 
 cessfully on a manufacturing scale for the lique- 
 faction of air for use as a refrigerating agent, and for 
 other purposes. It is impossible to describe the ap- 
 paratus in detail without the use of many diagrams, 
 but from what has already been said it will be easily 
 anticipated that it is only necessary to provide a 
 spiral copper tube having a fine hole at its extremity 
 for the escape of the compressed gas, and a metal 
 cylinder surrounding the coil through which the gas 
 enters, so that the gas cooled by expansion is made 
 to return over the coils of pipe before escaping into 
 the air. The entering gas thus has its temperature 
 continuously brought lower and lower, till at last it 
 is reduced below the critical point of the gas, and 
 the pressure being sufficient a portion of the gas 
 liquefies, and is blown in drops and spray out of the 
 hole at the end of the spiral. Of course the whole 
 arrangement requires to be very efficiently protected 
 by non-conducting material against the entrance of 
 heat from the outside air. 
 
 By such means, then, all those substances formerly 
 spoken of as " permanent " gases have been seen in 
 
 1 An account of these experiments was given in the Engineer, 
 Oct. 4, 1895 ; and later more fully in the Howard Lectures, by Pro- 
 fessor Ewing (Journal of the Society of Arts, 1897, p. 1091). 
 
PROGRESS. OF SCIENTIFIC CHEMISTRY 255 
 
 the liquid state, and most of them also in the form 
 of solids. Nevertheless hydrogen alone remained 
 unobtainable in quantity to allow of its more com- 
 plete study. The boiling point and critical tem- 
 perature were known approximately from the 
 experiments of Olszewski, but all attempts had 
 failed to liquefy the element in such quantity that 
 it could be retained in an open vessel under atmos- 
 pheric pressure. This important and interesting 
 result has now been achieved in the laboratory of 
 the Royal Institution under the direction of Pro- 
 fessor Dewar. The apparatus employed has not 
 been described in detail, but it is understood to be 
 constructed upon the principle already indicated of 
 cumulative cooling by expansion from pressure. The 
 difficulties to be overcome were unquestionably very 
 great, and success could only be hoped for in the 
 hands of an operator prepared for the task by years 
 of experience in the management of the complicated 
 machinery necessary in work of the kind. It is 
 both interesting and gratifying that the final victory 
 which crowns the long series of successful attacks 
 upon the apparently impregnable position of the 
 permanent gases should have been recorded in the 
 laboratory of the Royal Institution, where the first 
 successes in this field were won by Faraday. The 
 facts are as follows : 
 
 On May 10, 1898, hydrogen was for the first 
 time seen to drop from the nozzle of the apparatus 
 into a specially constructed receiver, where some 
 
256 A SHOUT HISTORY OF THE 
 
 20 cubic centimetres, or nearly three-quarters of a 
 fluid ounce, collected in about five minutes. Larger 
 quantities have since been obtained, but the propor- 
 tion of the whole gas reduced to the liquid state is 
 only about 1 per cent. Hydrogen in the liquid state 
 is a clear, colourless liquid which exhibits a well- 
 defined surface. It is remarkable for its low density. 
 This was determined by measuring the volume of the 
 gas obtained by evaporating 10 cubic centimetres, 
 and was found to be rather less than 0*07, or about 
 one-fourteenth the density of liquid water at 0. One 
 interesting point as to the relations of hydrogen 
 seems to have been settled by these results. The 
 favourite idea, probably originating with Graham, 
 and generally held for many years, has been that 
 hydrogen was the vapour of a very volatile metal. 
 Its association with the metals in chemical and elec- 
 trolytic decompositions, and the absorption of large 
 quantities of the gas by palladium without loss of 
 its metallic properties, always seemed to be consis- 
 tent with this notion. But it is now clear that 
 hydrogen in the liquid state does not exhibit the 
 characteristics of a metal, but seems rather to find 
 its nearest analogues in the gases of the paraffin 
 series, marsh gas and the rest, though liquid 
 hydrogen has only about one sixth the density of 
 liquid marsh gas. 
 
 The boiling point of this wonderful liquid was 
 found to be about 238 below the Centigrade zero, 
 and therefore about 35 above the hypothetical 
 
PROGRESS, OF SCIENTIFIC CHEMISTRY 257 
 
 absolute zero. The question so often debated as to 
 what would happen if a gas could be cooled to this 
 absolute zero is already partly answered. All gases 
 change to liquids before the zero is reached, and 
 now what would ensue if the liquid could be deprived 
 completely of heat seemed within the possibility of 
 settlement. We know that when a liquid is made 
 to boil it carries off heat so-called latent heat in 
 the resulting vapour, and if changed into vapour by 
 reduction of pressure only, the latent heat is supplied 
 at the expense of its own sensible heat, and cooling 
 is the result. Professor Dewar has, of course, lost 
 no time in making an experiment of this kind with 
 liquid hydrogen. The result, however, was disap- 
 pointing. 
 
 Under a pressure equal to about 25 millimetres 
 of mercury the liquid hydrogen evaporated quietly 
 and steadily, remaining clear and colourless to the 
 eye ; but the temperature fell only about one degree 
 below the boiling point under atmospheric pressure, 
 whereas it was expected that it would have been 
 some ten degrees lower. In such cases, of course, 
 all ordinary thermometers are useless, and tempera- 
 tures have to be determined by a platinum resist- 
 ance coil. It is possible that the thermometer is at 
 fault in this case, but the cause of this unexpected 
 result is not at present known. It seems likely, 
 however, that the absolute zero of the thermometric 
 scale will remain for some time a subject upon which 
 the scientific imagination can continue to Ibe exercised, 
 
 R 
 
258 A SHORT HISTORY OF THE 
 
 CHAPTER X 
 
 SUMMARY AND CONCLUSION 
 
 IT must now be evident to the reader that ideas of 
 the present day relating to the act of chemical 
 combination and the nature and constitution of 
 chemical compounds are very different from those 
 of a hundred or even fifty years ago. The Atomic 
 Theory has been not only received as affording a 
 plausible explanation of the familiar quantitative 
 laws of chemical combination, but the theory has 
 been enthroned as the predominant and indispen- 
 sable doctrine to which every question in modern 
 chemistry is referred, and of which the triumphs 
 of modern theory supply the justification. Without 
 the Atomic Theory and the doctrine of the orderly 
 linking of atoms, the natural outcome of the recog- 
 nition of that property of atoms which is now called 
 their valency, " organic " chemistry would be a heap 
 of confusion, and progress very slow if not im- 
 possible. The Atomic Theory, however, was not 
 established without a struggle. The early crude 
 results of quantitative analysis were sufficient for the 
 genius of Dalton; but the fixity of combining pro- 
 portions can hardly be said to have been fully 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 259 
 
 established till the researches of Stas (p. 77) sup- 
 plied the necessary facts. The problem which led 
 Berthollet in the early years of the century to 
 dispute the truth of this fundamental proposition is 
 one which has ever since afforded material both 
 for experimental inquiry and theoretical discussion. 
 We are quite satisfied now that every chemical 
 compound is definite in its nature, and that its 
 constituents are joined together in proportions 
 which cannot be varied except per salturn, and then 
 a new substance is produced. But the youngest 
 student in practical chemistry soon finds out that 
 the production of any given compound is largely 
 dependent upon the conditions of the experiment. 
 Of these conditions, one very important is the 
 relation between the quantities of the acting mate- 
 rials, another is the nature of the products of their 
 interaction, whether solid, liquid, or gaseous, soluble 
 or not soluble in the menstruum employed, and 
 so forth. A beautiful experiment, illustrating the 
 effect of mass or relative quantity, may be seen 
 by mixing together a dilute solution of ferric 
 chloride in water with a similar solution of potas- 
 sium thiocyanate, when the familiar red coloration 
 due to the production of ferric thiocyanate appears. 
 If now this liquid be divided into two equal parts, 
 and to one is added some more of the ferric 
 chloride, a deepening of tint will be observed, which 
 seems to indicate that the iron was deficient in the 
 original mixture. But if to the other portion of the 
 
260 A SHORT HISTORY OF THE 
 
 red solution a further dose of the thiocyanate is added, 
 a similar deepening at once results. This, on the 
 other hand, seems to show that the thiocyanate was 
 added to the original in insufficient amount. These 
 results seem contradictory, and at first sight the 
 explanation is not apparent ; but on trial it will 
 be found that, in order to produce the maximum 
 effect, one or the other of the acting materials 
 must be used in very great excess over and above 
 the quantity indicated by the theoretical equa- 
 tion which would be used to express the change. 
 And so in many other instances we have to 
 seek the conditions under which, in any system of 
 bodies capable of acting upon one another, equili- 
 brium can be established. In most of these cases 
 the changes which occur are reversible ; that is, 
 they proceed in one direction till certain quantities 
 of .the products of interaction have accumulated, 
 and the action then comes to an end. This we can 
 now explain by a hypothesis which in its original form 
 we owe to Williamson (see p. 226). It is now clear 
 that we must exchange the older statical views of 
 chemical compounds, and their modes of interaction 
 with other compounds, for others which involve the 
 idea of motion among the atoms. 
 
 In every such system as that described above we 
 have two changes going on simultaneously, the one 
 between the two original substances, in this case 
 ferric chloride and potassium thiocyanate, and the 
 other between the products of their interaction, 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 261 
 
 namely, potassium chloride and ferric thiocyanate. 
 If these two changes take place at different rates, 
 more and more of one of these pairs of compounds 
 will accumulate ; when they go on at the same rate, 
 then action appears to be at an end, because equili- 
 brium has been established. The action, however, 
 must be supposed to continue, and the equilibrium 
 results from the two opposite interactions proceed- 
 ing at the same rate. Such changes are generally 
 represented by an equation in which the sign of 
 equality is replaced by a pair of arrows pointing in 
 opposite directions, to signify that the equation may 
 be read backwards or forwards, thus : 
 
 FeCl 3 + 3KCNS ^ Fe(CNS) 3 + 3KCl. 
 
 If now into such a system in equilibrium a larger 
 quantity of any one of the substances present is 
 introduced, a disturbance is set up which leads to 
 the redistribution of its elements to a greater or less 
 extent. This disturbance, there is reason to believe, 
 is dependent upon the increased opportunities which 
 are afforded to the constituents of the added sub- 
 stance of meeting and reacting with the other 
 elements present ; in the case taken by way of 
 illustration, the addition of more of the iron salt 
 gives increased chances for the iron to find out the 
 unchanged thiocyanate present in the liquid, and 
 vice versd. Of course, hi all such cases the extent of 
 the change is also largely dependent upon tempera- 
 ture, and where gaseous products are formed it is also 
 
262 A SHORT HISTORY OF THE 
 
 much dependent upon pressure. The nature of the 
 solvent employed is important, for in the event of 
 gas escaping, or of a solid precipitating, a part of the 
 material is eliminated from the sphere of action, 
 and a disturbance of proportions ensues. In such 
 cases the theoretically possible change is usually 
 completely accomplished. 
 
 So early as 1852 experiments were made by 
 Bunsen, 1 with the object of testing the " Law of 
 Mass." He exploded mixtures of hydrogen and 
 carbonic oxide with oxygen in different proportions, 
 insufficient to burn both the gases ; but the conclu- 
 sions he arrived at were afterwards shown to be 
 erroneous. 
 
 Studies relating to the rate at which chemical 
 change proceeds in particular cases have been 
 undertaken by many chemists during the last forty 
 years. One of the earliest was the investigation of 
 the formation of compound ethers, by the inter- 
 action of alcohol with acids, by Berthelot. 1 He 
 found, as we should now expect, that the interaction 
 proceeds more and more slowly as the ether and 
 water are formed, until ultimately it comes to an 
 end, although both acid and alcohol remain in the 
 liquid. Another important research was carried out 
 by A. Vernon Harcourt and W. Esson, 2 in which they 
 demonstrated the influence upon the rate of change 
 of varying the proportions of the materials in the 
 
 1 Ann. Chim. Phys., 1862 [3], Ixv. 385 ; Ixvi. 110. 
 2 Jour. Chem. Soc., 1867, v. 459. 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 263 
 
 reactions they examined. Other special cases have 
 been examined by Gladstone, Horstmann, Dixon, 
 Deville, Ostwald, Thornsen, and others, some of 
 whom have been referred to in a previous chapter. 
 But the merit of formulating the fundamental prin- 
 ciple of the action of mass, and thus bringing into 
 scientific form previous vague notions about affinity, 
 belongs to the Norwegian physicists Guldberg and 
 Waage. Their book, entitled Etudes sur Us affiniUs 
 chimiques, published by the University of Christiania 
 in 1867, contains an investigation of the law of mass 
 action, which has been of great service to theoretical 
 chemistry. A discussion of the theorem would be 
 unsuitable to these pages, but sufficient has been 
 said to indicate the general nature of the inquiry. 
 
 Temperature, as already explained, is a very im- 
 portant factor in the circumstances which determine 
 the rate of chemical change. At the low tempera- 
 tures now obtainable by liquid air or oxygen all 
 chemical activity seems to be suspended, while at 
 the opposite end of the scale, at the temperatures of 
 the electric arc or spark, all ordinary chemical com- 
 pounds seem to be broken up into their elements. 
 
 Chemical combination, then, is an affair of atoms, 
 and their joining together or separating is regulated 
 by " affinity," by temperature and pressure, and by 
 the mass or relative quantities of the materials pre- 
 sented to each other. 
 
 Of the internal construction of the molecules 
 which result from the union of atoms, it has been 
 
264 A SHORT HISTORY OF THE 
 
 shown that we have some reason to believe that 
 a certain degree of knowledge has been attained. 
 Further information seems likely to result from the 
 more careful attention which is now being given to 
 the interrelation of chemical constitution and physi- 
 cal properties. The discoveries which have resulted 
 from the careful study of the action of certain com- 
 pounds on polarised light, encourages the belief that 
 an equally careful investigation of the refractivity 
 and dispersive power, of the electric conductivity, 
 the viscosity, the specific volume, and the specific 
 heat of pure substances of known composition may 
 help in the further elaboration of those ideas of con- 
 stitution which, imperfectly expressed in current 
 chemical formulae, have been derived from the obser- 
 vation of the modes of formation and of decomposi- 
 tion of compounds chiefly of one element, carbon. 
 Valuable pioneering work of this kind has been 
 already accomplished by Gladstone and by Briihl, by 
 Hermann Kopp and by Thorpe, by Ostwald, Kohl- 
 rausch, and others. In the future one lesson derived 
 from the past will doubtless be always borne in 
 mind. The serious influence of small quantities of 
 other substances in the materials which are the sub- 
 ject of experiment is now recognised, and the labour 
 of physical measurements will surely in future be 
 expended only upon substances which have been 
 purified with the most scrupulous care, or of which 
 the composition is accurately known. 
 
 But the exchange of the past tense for the future 
 
PROGRESS OF SCIENTIFIC CHEMISTRY 265 
 
 is a sign that the story is drawing to a close. No 
 Englishman, however, will consider the account com- 
 plete without some attempt to form an estimate of 
 the share which has been contributed to the progress 
 of chemistry by his own countrymen. It is some- 
 times said that England has fallen behind in the 
 race, and British chemists are reproached with 
 inactivity, indolence, ignorance, and neglect. The 
 days of Davy and Faraday are long past. " Those 
 suns are set," the voice of dejection may be some- 
 times heard to exclaim, and 
 
 " All that we have left is empty talk 
 Of old achievements, and despair of new." 
 
 How far such assertions have any foundation in 
 fact, the reader of this little volume may judge for 
 himself; or if he desires categorical assurance, let 
 him glance through the Philosophical Transactions of 
 the Royal Society and the Transactions of the Chemical 
 Society for the last thirty or forty years. In those 
 volumes he will find proof that the chemists of 
 Britain have not been negligent of the traditions 
 which have come down to them from the famous 
 days of old. If Dalton with his Atomic Theory 
 laid the foundation of all the ideas of more recent 
 times, the superstructure has been raised and 
 adorned by the work and writings of Graham and 
 Faraday, of Williamson and Frankland, of Odling 
 and Gladstone, in friendly rivalry with their contem- 
 poraries in France and Germany. 
 
266 THE PROGRESS OF SCIENTIFIC CHEMISTRY 
 
 If the discoveries of elements are to be enume- 
 rated, we can boast not only of oxygen and of potas- 
 sium and sodium, but of thallium, one of the earliest 
 and most remarkable trophies won by the spectro- 
 scope, and the whole of the new and strange com- 
 panions of argon, while the principle of periodicity 
 among the atomic weights was beyond question first 
 enunciated by an Englishman. . And if, unfortu- 
 nately, it must be admitted that the cultivation of 
 " organic " chemistry was pursued with but a lan- 
 guid interest for many years in England, the 
 researches of Perkin alone, by their number, their 
 variety, and their importance, go far towards making 
 up for the deficiencies of others. This, however, can 
 no longer be said, and English chemists can point 
 with justifiable pride to the annual volume of Trans- 
 actions issued by the Chemical Society, which, for 
 excellence and quantity of matter, will bear com- 
 parison with the proceedings of any other chemical 
 society in the world. 
 
 In the ancient universities of Great Britain, where 
 chemistry has not flourished with the vigour shown 
 by the older studies, the spirit of mediaeval ism is no 
 longer wholly predominant. We may therefore look 
 forward hopefully to the day, not far distant, when 
 Science and Letters, no longer fierce competitors for 
 academic vantage, may walk hand in hand, each 
 conscious of her own dignity, but ready to yield to 
 the other her due share of honour. 
 
 UNIVERSITY J 
 
IMPOKTANT EVENTS 
 
 ARRANGED IN CHRONOLOGICAL ORDER 
 
 Jean Baptiste Dumas born . . . . . 1800 
 
 Friedrich Wohler born ... . . . 1800 
 
 Electrolytic decomposition of water by Nicholson 
 
 and Carlisle . . .' . . . . 1800 
 
 Justus Liebig born . . . . . . 1803 
 
 Joseph Priestley (born 1733) died .... 1804 
 
 Thomas Graham born . . " . . . . 1805 
 
 J. B. Richter (born 1762) died .... 1807 
 
 Atomic Theory of Dalton, published by Thomas 
 
 Thomson . > . . . . . . 1807 
 
 Isolation of potassium and sodium . . . * . 1807 
 
 Auguste Laurent born . . * . . . "... 1807 
 
 Henry Cavendish (born 1731) died . . . 1810 
 
 Elementary nature of chlorine established . . 1810 
 
 Hypothesis of Avogadro ..... 1811 
 
 Jean Servais Stas born . . . . . . 1813 
 
 Charles Gerhardt born 1816 
 
 Adolph Wurtz born 1817 
 
 Hermann Kopp born . . . . . .1817 
 
 Jean Charles Galissard de Marignac born . . 1817 
 James Prescott Joule born . . . ". .1818 
 Hermann Kolbe born . . . . . .1818 
 
 August Wilhelm Hofmaim born . . . .1818 
 
 Law of Dulong and Petit 1819 
 
 Mitscherlich's first work on isomorphism . . 1819 
 
 267 
 
268 IMPORTANT EVENTS 
 
 Claude Louis Bertho.llet (born 1748) died . .1822 
 Louis Pasteur born . . . . . .1822 
 
 Faraday's first experiments on liquefaction of gases . 1823 
 Berzelius' electro- chemical theory .... 1827 
 
 Wohler's synthesis of urea . . . . .1828 
 
 Humphry Davy (born 1778) died .... 1829 
 
 William Hyde Wollaston (born 1766) died . . 1829 
 August Kekule born . . . . . .1829 
 
 Julius Lothar Meyer born . . . . .1830 
 
 Graham's law of gaseous diffusion ,. N . . .1831 
 Faraday's work in electricity begun . . . 1831 
 
 British Association for the Advancement of Science 
 
 founded 1831 
 
 Liebig and Wohler on the "Radical of Benzoic 
 
 Acid" 1832 
 
 Graham on arseniates and phosphates (recognition of 
 
 basicity of acids) . . . . x . .1833 
 Faraday's first law of electro-chemical decomposition 1834 
 Dumas' discovery of chlorine substitution . . 1834 
 Carbon dioxide solidified by Thilorier . . . 1835 
 Laurent's theory of nuclei used by Gmelin . . 1836 
 
 Dumas' theory of types 1839 
 
 Bunsen's discovery of cacodyl . . . . .1841 
 Chemical Society of London founded . . .1841 
 Homologous series recognised by Schiel . . . 1842 
 Mechanical equivalent of heat determined by Joule . 1842 
 John Dalton (born 1766) died . . . . 1844 
 Racernic acid dissected by Pasteur .... 1845 
 Synthesis of acetic acid by Kolbe .... 1845 
 College of Chemistry, London, founded . . . 1845 
 Dissociation of water by heat (W. R. Grove) . . 1846 
 Frankland's discovery of ethyl . . . .1848 
 Jons Jakob Berzelius (born 1779) died . . . 1848 
 Frankland's discovery of zinc-ethyl .... 1849 
 Wurtz's discovery of compound ammonias . . 1849 
 
IMPOKTANT EVENTS 269 
 
 Hofmann's synthetical formation of compound 
 
 ammonias . . . . . . . 1849 
 
 Louis Joseph Gay-Lussac (born 1778) died . . 1850 
 
 Constitution of ether established by Williamson . 1850 
 Water-type proposed by Williamson . . .1851 
 
 Principle of "atomicity" recognised by Frankland . 1852 
 
 Auguste Laurent died ...... 1853 
 
 Law of Avogadro applied by Gerhardt . . . 1853 
 
 Charles Gerhardt died 1856 
 
 Perkiu discovers " Mauve " . . . . . 1856 
 
 Clausius' theory of electrolysis .... 1857 
 
 Dissociation studied by Deville .... 1857 
 
 Linkage of atoms recognised by A. S. Couper . . 1858 
 
 Synthesis of acetylene (Berthelot) .... 1859 
 
 Spectrum analysis introduced by Bunsen and Kirchoff 1859 
 
 Pasteur's work on fermentation begun . . . 1861 
 
 Eilhert Mitscherlich (born 1794) died . . . 1863 
 
 Andrews' experiments on liquefaction of gases begun 1863 
 Atomicity applied to the explanation of isomerism 
 
 by Crum Brown 1864 
 
 Newland's " Law of Octaves " .... 1864 
 
 Sprengel's mercury pump invented ... . . 1864 
 
 Kekule's formula for benzene . - * 1865 
 
 Bunsen's gas-burner invented * 1866 
 
 Michael Faraday (born 1791) died . . > . . 1867 
 
 German Chemical Society founded .... 1867 
 
 Synthesis of alizarin (Graebe and Liebermann, Perkin) 1868 
 
 Thomas Graham died 1869 
 
 Mendel6effs first table of the elements . | . . 1869 
 
 Lothar Meyer's periodic curve . . . . 1870 
 
 Cannizzaro's Faraday Lecture . . . * . 1872 
 
 Justus Liebig died 1873 
 
 Theory of stereo-isomerism (Le Bel and Van't Hoff) 1875 
 
 Liquefaction of oxygen (Pictet and Cailletet) . . 1877 
 
 Society of Chemical Industry founded . . * 1881 
 
270 IMPORTANT EVENTS 
 
 Friedrich Wohler died 1882 
 
 Hermann Kolbe died 1884 
 
 Jean Baptiste Dumas died ..... 1884 
 
 Adolph Wurtz died 1884 
 
 Tautomerism or desmotropy recognised (Laar) . . 1885 
 Emil Fischer's synthesis of sugars begun . . .1 885 
 Gas theory of solutions (Van't Hot!) . . .1887 
 Theory of free ions (Arrhenius) .... 1887 
 
 James Prescott Joule died ..... 1889 
 
 Jean Servais Stas died . . . . . .1891 
 
 August Wilhelm Hofmann died .... 1892 
 
 Hermann Kopp died . . . . . .1892 
 
 Discovery of argon (Eayleigh and Ramsay) . . 1894 
 Discovery of terrestrial helium (Ramsay) . . 1894 
 
 Jean Charles Galissard de Marignac died . . 1894 
 
 Louis Pasteur died . . . . . .1895 
 
 Julius Lothar Meyer died . . . . .1895 
 
 Critical temperature and boiling point of hydrogen 
 
 determined by Olszewski . .... 1895 
 
 August Kekule died 1896 
 
 Liquefaction of hydrogen in quantity (De war). . 1898 
 
INDEX OF SUBJECTS 
 
 ALIZARIN from madder, 162 
 Allotropes, 25 
 
 American Chemical Society, 16 
 Ammonia type recognised, 119 
 Ammonium theory, 118 
 Anhydrides, formation of, 204 
 Argon, 52, 59 
 
 companions of, 57 
 
 Aromatic compounds, 138 
 Asymmetric carbon, 193, 196, 201 
 
 compounds, 188 
 
 Atomic theory, 60 
 
 weights, numerical relations 
 of, 83 
 
 rectified, 60 
 Atomicity or equivalence, 127 
 Avogadro's hypothesis, 62, 70, 72 
 
 BASES, organic, Berzelius' theory 
 of, 116 
 
 Hofmann's theory of, 119 
 
 Liebig's theory of, 117 
 
 Wurtz's theory of, 117 
 Benzene and its homologues, 137 
 Benzol discovered, 136 
 Benzoyl, compounds of, 111 
 British Association, 16 
 
 chemists defended, 265 
 
 CARBON compounds, 146 
 Cavendish on Air, 54 
 Chemical affinity, 209 
 
 elements, 37 
 
 Society of Berlin, 1 6 
 
 Chemical Society of London, 16 
 
 of Paris, 16 
 types, 66, 109 
 
 Clausius' theory of electrolysis, 
 
 225 
 Colouring matters, constitution 
 
 of, 165 
 Combining weight of oxygen, 80, 
 
 82 
 
 Compound radicles, 14 
 Condensed types, 124 
 Conditions of chemical action, 
 
 259 
 
 Constitution of chemical com- 
 pounds, 118 
 
 Constitutional formula, 181 
 Continuity of gaseous and liquid 
 
 states of matter, 243 
 Copulated compounds, 122 
 Critical points, 246 
 Cyanogen, a compound radicle, 
 
 14,112 
 
 D ALTON'S Atomic Theory, 60 
 
 Dissociation, 34, 224 
 
 ionic, 224 
 
 * 
 
 ELECTRICITY and chemical affin- 
 ity, 209 
 
 Electro-chemical theory of Ber- 
 zelius, 210 
 
 of Davy, 210 
 Electrolysis, laws of, 211 
 
 phenomena of, 214 
 
 271 
 
272 
 
 INDEX OF SUBJECTS 
 
 Electrolytes, ionic, dissociation of, 
 
 224 
 Elements, 6, 24, 37, 39 
 
 Aristotelian, four, 38 
 
 classified, 83 
 
 distribution of, 37 
 
 positive and negative, 10 
 
 recognition of, 37 
 
 Endothermic compounds, 24, 32 
 Energy, 17, 18, 21 
 Enzymes, 175 
 Ether, constitution of, 64 
 Ethyl, compounds of, 111 
 Explosives, 168 
 
 FARADAY'S Laws of Electrolysis, 
 
 211 
 
 Fatty compounds, 138 
 Fermentation, alcoholic, 171 
 
 butyric, 174 
 
 lactic, 174 
 
 Liebig's theory of, 173 
 
 Fluorine, 50, 51, 52 
 Fraunhofer's lines, 43, 49 
 Freezing points of solutions, 217 
 
 GAS and vapour defined, 246 
 Gases, liquefaction of, 238 
 Genesis of the chemical elements, 
 
 104 
 
 Gerhard t's types, 125 
 Germanium identified with eka- 
 
 silicon, 99 
 Graphic formulae of Crum Brown, 
 
 134 
 
 of Couper, 134 
 of Kekule", 134 
 
 HEAT, mechanical equivalent of, 
 
 22 
 Heat of neutralisation of acids, 
 
 231 
 
 Helium, 57, 59 
 Hemihedral crystals, 183 
 
 Homologous series, 113 
 Hydrochloric acid type, 123 
 Hydrogen liquefied, 248, 255 
 Hydrogen liquid, properties of, 
 
 256 
 Hydrogen type, 123 
 
 INDESTRUCTIBILITY of energy, 17 
 | Indestructibility of matter, 5 
 
 j KAKODYL, a compound radicle, 
 
 112 
 
 i Kekule's benzene formula, 140 
 j Kinetic theory of gases, 222 
 j Krypton, 57, 59 
 
 j LACTONES, formation of, 204 
 
 Law of Avogadro, 62, 70, 72 
 
 of Dulong and Petit, 73 
 
 of octaves, 92 
 
 periodic, 88 
 
 Laws of Electrolysis, 211 
 Laurent's chemical method, 114 
 
 theory of nuclei, 114 
 Linking of atoms, 108 
 
 -Kekuteon, 130 
 Liquefaction of gases, 238 
 
 MAGENTA discovered, 164 
 
 Mass, action of, 259 
 
 Matter and energy, 17 
 
 Mauve discovered, 163 
 j Maximum work, 31 
 | Mixed types, 123 
 
 Medicinal agents, synthetical, 167 
 
 Mendeleeffs eka-boron, &c., 98 
 
 table of elements, 97 
 
 Metalepsy, 65 
 
 Metargon, 57, 59 
 
 Meyer's curve, 96 
 
 Moisture, influence of on com- 
 bination, 237 
 
 Molecules, Kelvin's estimate of 
 size, 105 
 
INDEX OF SUBJECTS 
 
 273 
 
 NEON, 57, 59 
 Nitrogen, density of, 53 
 Nuclei, Laurent's theory of, 114 
 
 OPTICALLY active liquids, 181 
 Organic chemistry defined, 144 
 et seq. 
 
 compounds defined, 144 
 
 Osmotic pressure, 220 
 Oxygen liquefied, 247 
 
 PERIODIC Law, 88 
 
 cause of, 103 
 
 Pharmaceutical Society, 16 
 
 Physical Society, 16 
 
 Polarised light, 181 
 
 Principle of maximum work, 31 
 
 Prothyl, 104 
 
 Prout's hypothesis, 84 
 
 RADICALS or radicles, 110, 112 
 
 Residual valency, 198 
 
 Residues, 67, 112 
 
 Royal Society, 2, 16 
 
 Royal Society of Edinburgh, 16 
 
 SALTS, haloid and amphid, 11 
 Semi- permeable membranes, 220 
 Societe Chimique de Paris, 16 
 Society of Chemical Industry, 16 
 Solutions, gas theory of, 222 
 Solutions, properties of, 219 
 Space formulas, 181 
 Spectrum analysis, 40 
 Spectrum of sunlight, 41 
 Stereo-chemistry, 181, 189 
 Substitution, 14, 65 
 Synthesis of carbon compounds, 
 
 149 
 Synthetical chemistry, 144 
 
 THERMO-CHEMISTBY, 27 
 Triaprima, 38 
 
 UNITARY system of formulae, 215 
 
 VAPOUR pressures of solutions, 217 
 Valency, 108 
 
 WATER type recognised, 120 
 Williamson's theory of ether, 226 
 
 ZERO, absolute, 257 
 
INDEX OF PEESONS 
 
 ALTHEN, Jean, 162 
 Andrews, 27, 28, 30, 242, 243 
 Arrhenius, 224 
 Avogadro, 62 
 
 BABBAGE, 16 
 ~Bae>^von, 198, 204 
 Baker/V. H. B., 237 
 Bel, Le, 193 
 
 Berthelot, 30, 31, 124, 147, 262 
 Berthollet, 3, 259 
 Berzelius, 3, 10, 11, 66, 77, 109, 
 
 116, 210 
 Biot, 181 
 Blagden, 217 
 Boisbaudran, De, 41, 88 
 Boyle, 3, 18, 38 
 Brown, Crum, 134 
 Briihl, 264 
 Buchner, E., 179 
 Bjmsen, 40, 112, 262 
 
 CAILLETET, 247, 249 
 Cannizzaro, 75 
 Carnelley, 103 
 Caro, 162 
 
 Cavendish, 54, 55, 209 
 Chancourtois, De, 88 
 Clarke, F. W., 82, 83 
 Clausius, 223, 225 
 Cooke, J. P., 82 
 Coppet, De, 217 
 Couper, A. S., 133 
 Crookes, 40, 102, 104 
 
 DALTON, 3, 7, 18, 60, 77, 189 
 
 Dauiell, 11, 214, 215 
 
 Davy, 3, 9, 1(5, 17, 40, 210, 215, 
 
 232, 240 
 Deville, 34 
 Dewar, 249 
 Dixon, H. B., 236 
 Dobereiner, 85 
 Dulong, 26 
 Dulong and Petit, 72 
 Dumas, 14, 15, 65, 66, 67, 79, 85, 
 
 109, 122, 148 
 
 EWING, 254 
 
 FARADAY, 10, 11, 16, 136, 211, 240 
 Favre and Silbermann, 26, 30 
 Fittig and Tollens, 137 
 Foster, G. C., 132 
 Frankland, 111, 126, 135 
 Fraunhofer, 43, 49 
 
 GALVANI, 209 
 
 Gautier, 59 
 
 Gay-Lussac, 3, 12, 14, 61, 65 
 
 Gerhardt, 62, 63, 64, 68, 115, 
 
 125, 146, 154, 215 
 Gibbs, Willard, 36 
 Gladstone, 263, 264 
 Gmelin, 114, 146, 190 
 Graebe and Liebermann, 162 
 Graham, 11, 124 
 Griess, 165 
 Grotthus, 228 
 
 274 
 
INDEX OF PEKSONS 
 
 275 
 
 Grove, 34 
 
 Guldberg and Waage, 263 
 
 Guthrie, 217 
 
 Guye, 205 
 
 HAMPSON, 253 
 
 Harcourt and Esson, 262 
 
 Herschel, J., 44 
 
 Hess, 28, 30 
 
 Hill, A. C., 178 
 
 Hofmann, 71, 118 
 
 Hoff, Van't, 193, 195, 221, 222 
 
 Horstmann, 36 
 
 JAPP, 129, 143, 155 
 
 Jenner, 180 
 
 Joule, 18, 22, 23, 223, 252 
 
 ti, 123, 129, 133, 146, 195 
 Kelvin, 105, 107, 252 
 Kirch hoff, 40 
 Kohlrausch, 229, 264 
 Kolbe, 123, 147 
 Kopp, 75 
 Korner, 142 
 Kronig, 223 
 
 L ARMOR, 107 
 
 Lapparent, 88 
 
 Laurent, 113 
 
 Lavoisier, 1, 10, 12, 18, 26 
 
 Liebig, 11, 12, 109, 145, 173, 
 
 177 
 Liebig and Wohler, 14, 110, 
 
 118 
 
 Linde, 253 
 Lister, 167 
 Lockyer, 57, 104 
 
 MALLET, 78 
 Mansfield, 136 
 Marggraff, 39 
 Marignac, 36, 77 
 Melsens, 149 
 
 Mendele"eff, 93, 96, 98 
 Meyer, L., 94 
 Miller, W. A., 44 
 Mitscherlich, 136 
 Moissan, 39, 52 
 Monge and Clouet, 239 
 Morley, 81 
 
 NAUMANN, 36 
 
 Newlands, J. A. R., 89, 91, 92 
 
 Newton, 17, 41 
 
 Nicholson and Carlisle, 9, 209 
 
 Nilson, 41 
 
 Northmore, 239 
 
 ODLING, 71, 89 
 Olszewski, 248 
 
 PASTEUR, 171, 180, 184 
 Pelouze, 77, 168 
 Perkin, 162, 164, 266 
 Perkin and Duppa, 158 
 Petit and Dulong, 72 
 Pfeffer, 221 
 Pictet, 247 
 Prout, 78, 83 
 Provostaye, De la, 184 
 
 RAMSAY, 55, 56 
 Ramsay and Travers, 57 
 Raoult, 217 
 
 Rayleigh, 53, 55, 82, 223 
 Regnault, 35, 75, 82 
 Reich and Richter, 40 
 Reynolds, O., 23 
 Rudorff, 217 
 Rumford, 18 
 
 SCHEELE, 39 
 Schiel, 113 
 Schwann, 173 
 Scott, Alex., 81 
 Siemens, 253 
 Solvay, 253 
 
276 
 
 INDEX OF PERSONS 
 
 Stas, 67, 77 
 Swan, 46 
 
 TH^NAED, 12 
 
 Thilorier, 241 
 
 Thomsen, Julius, 30, 33, 231 
 
 Thomson, W. See Kelvin 
 
 Thorpe, 264 
 
 Tilden and Marshall, 207 
 
 Tour, De la, C., 173, 239 
 
 VOLTA, 209 
 
 WALDEN, 207 
 Walker, J. W., 206 
 Waterston, 223 
 Williamson, 64, 120, 123, 226 
 Wislicenus, 192, 202 
 Wohler, 39, 147 
 Wollaston, 43, 77, 190 
 Wroblewski, 248 
 Wunderlich, 200 
 Wurtz, 116, 124 
 
 ZININ, 137 
 
 THE END 
 
 Printed by'BALLANTYNE, HANSON &= Co. 
 Edinburgh &> London 
 
UNIVERSITY OP CALIFORNIA LIBRARY 
 
 THIS BOOK IS DUE ON THE LAST DATE 
 STAMPED BELOW 
 
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