566 REPORT— 1902. 



The ground really occupied by chemical composition in theoretical chemistry is 

 now greatly limited ; for with the full acceptance of the idea of the molecule and 

 of the atom as a derivative of it, its place has been taken by chemical constitution 

 to an extent hardly realised. The useful and practically necessary expression 

 of the results of the quantitative analysis of a new substance gravimetrically 

 is all that can strictly receive the name of its chemical composition. When 

 the term is applied more widely it is used for what are really the simpler forms of 

 chemical constitution. It was otherwise before the conception of the molecule 

 had become current and the atom had become a derived function of the molecule. 

 Chemical composition as expressed b}^ Dalton in atoms is indeed that and nothing 

 else. Carbonic anhydride is composed, according to him, of two atoms of oxygen to 

 one of carbon, as against carbonic oxide which is composed of one ; marsh gas of 

 two atoms of hydrogen to one of carbon, as against olefiant gas composed of one. 

 But then it was only numerical necessity which led him to adopt such a mode of 

 expressing the facts. The same necessity, it is true, affects us also in the matter 

 of carbon dioxide, of water, and of ammonia, but how little it does so is shown by 

 the many cases in which the empirical or simple composition is expressed in 

 multiples. The atomic chemical composition of ethylene is two of hydrogen to 

 one of carbon, and that of benzene one of hydrogen to one of carbon. AVhen we 

 say, as we always do, that the one substance is ' composed ' of four atoms of hydro- 

 gen to two of carbon, and the other of six of hydrogen to six of carbon, we give 

 what is information concerning the constitution of these substances. Call it the 

 composition of the molecule as we may, it is evident that by composition we 

 can here mean only constitution. As with polymerism, so with isomerism, and 

 in a more marked way. Mercurous sulphate and mercuric oxysulphite, quite 

 distinct salts, have yet the same composition. 



In the great reformation wrought by the chemists to whom I have referred, 

 but by Gerhardt in particular, the new light set up in chemistry was the notion 

 of what came to be called ' double decomposition ' in chemical change. The phrase 

 is not, perhaps, happily constructed, but it has the merit of needing some expla- 

 nation of its meaning before it can be understood, and troubles, therefore, through 

 a too simple apprehension of the sense of the word ' composition ' are hardly to be 

 feared. Its introduction into chemistry marked the ascendency of the idea of 

 the molecule as the factor in chemical change whose interactions with other 

 molecules were to be considered, instead of those additions which, as chemical 

 phenomena, never take place. It led also to new conceptions of the nature of the 

 atom and the compound radical as being the quantitative and qualitative expres- 

 sions of the powers possessed by substances to change into others, and to the 

 conception of the valency of atoms and radicals as expressing the nature of the 

 connection of successive chemical changes. The zeal with which it was attempted 

 to force all chemical changes into the form of double decomposition interfered, 

 perhaps, with the full recognition of its importance ; but the fact remains that, 

 with hardly an exception, all that is stated concerning the nature of those 

 chemical changes in which two or three substances become one, or one becomes 

 two or more, is based upon notions derived from the study of double decomposition. 



The fundamental value of double decomposition consists in its displaying 

 threads running through chemical transformations which can be followed up. 

 When two substances change into two others, and only then, there can be found, 

 in most cases relations of resemblance, both physical and chemical, between the 

 before and after of a chemical change. Instead of the striking unlikenesses shown 

 by the substances formed by quasi-addition to those from which they are formed, 

 there are here met with the similarities of the outcoming to the interacting sub- 

 stances, and the similarities between the products of different interactions in which 

 the acting substances are similar. Chemists had been for very long familiar with 

 acids, bases, salts, without becoming deeply impressed with the significance of the 

 resemblances which these class-names imply, and also with the facts that acids 

 beget acids, bases bases, and salts salts, or in more general terms, that substances 

 in interaction produce others like them, and that differences between the products 

 and the agents in one change are distinctly repeated in a similar change in which 



