466 PRINCIPLES OP CHEMISTRY 



CO(OH)CO(OH), which generally .crystallises with 2H,0, following thus the 

 normal type of hydration characteristic of ethane. 8 



Thus, by applying the principle of substitution, we can, in the simplest 

 manner, derive not only every kind of hydrocarbon compound, such as the 

 alcohols, the aldehyde-alcohols, aldehydes, alcohol-acids, and the acids, but 

 also combinations analogous to hydrated crystals which usually are dis 

 regarded. 



But even those unsaturated substances, of which ethylene, CH,CH,, and 

 acetylene, CHCH, are types, may be evolved with equal simplicity. With 

 respect to the. phenomena of isomerism, there are many possibilities among 

 the hydrocarbon compounds conta ; ning two atoms of carbon, and without 

 going into details it will be sufficient to indicate that the following formulae, 

 though not identical, will be isomeric substantially among themselves : 

 CH 8 CHX 2 and CH^XCH^X, although both contain C. 2 H 4 X 3 ; or CH,CX, and 

 CHXCHX, although both contain CjELjXj, if by X we indicate chlorine or 

 generally an element capable of replacing one atom of hydrogen, or capable 

 of uniting with it. To isomerism of this kind belongs the case of aldehyde 

 and the oxide of ethylene, to which we have already referred, because both 

 have the composition C 2 H 4 0. 



What I have said appears to me sufficient to show that the principle of 

 substitution adequately explains the composition, the isomerism, and all the 

 diversity of combination of the hydrocarbons, and I shall limit the further 

 development of these views to preparing a complete list of every possible 

 hydrocarbon compound containing three atoms of carbon in the molecule. 

 There are eight in all, of which only five are known at present. 3 



Among those possible for C 3 H 6 there should be two isomerides, propylene 

 and trimethyleue, and they are both already known. For C 3 H 4 there should 

 be three isomerides : allylene and allene are known, but the third has not 

 yet been discovered ; and for C 3 H,j there should be two isomerides-, though 

 neither of them is. known as yet. Their composition and structure are easily 



9 One more isomeride, CH 2 CH(OH), is possible that is, secondary vinyl alcohol, 

 which is related to etbylene, CH 3 CH 2 , but derived by the principle of substitution from 

 CH 4 . Other isomerides, of the composition C^O, such, for example, AS CCH 3 (OH),, 

 are impossible, because it would correspond with the hydrocarbon CHCH 3 =C 2 H 4 , which 

 is isomeric with ethylene, and it cannot be derived from methane. If such an isomeride 

 existed it would be derived from CH^but such products are, up to the present, unknown. 

 In such cases the insufficiency of the points of departure of the statical structural teach- 

 ing is shown. It first admits constant atomicity and then rejects it, the facts serving to 

 establish either one or the other view ; and. therefore it seems to me that we must come 

 to the conclusion that the structural method of reasoning, having done a service to 

 science, has outlived the age, and must be regenerated, as in their time was the teaching 

 of the electro-chemists, the radicalists, and the adherents of- the doctrine of types. As 

 we cannot now lean on the views above stated, it is time to abandon the structural 

 theory They will all be united in chemical mechanics, and the principle of substitution 

 must be looked on only as a preparation for the coming epoch in chemistry, where 

 such cases as the isomerism of f umaric and maleic acids, when explained dynamically, ae 

 proposed by Le Bel and, Van't Hoff, may yield points of departure. 



5 Conceding variable atomicity, the structuralists must expect an incomparably larger 

 Dumber of isomerides, and they cannot now decline to acknowledge the change of 

 atomicity, were it only for the examples HgCl and HgCl a , CO and COa, PC1 3 and PCfe, 



