LECTURE XIII.] HISTORY OF CHEMISTRY. 265 



of the latter. 02 Wislicenus afterwards followed up these reac- 

 tions, and elucidated them more fully. Numerous syntheses 

 were carried out by Wislicenus and his pupils according to this 

 method, and thus our knowledge of the constitution of acids 

 containing a number of carbon atoms was very considerably 

 advanced. 



The fact established by Schorlemmer, 03 that dimethyl is 

 identical with ethyl hydride, had a distinct value in all con- 

 siderations with respect to constitution ; and so also had 

 the recognition of the identity of carbonic ethers containing 

 two different alcohol radicals 64 (a point that was doubted at 

 first 5 ). It was only after these matters had been settled that 

 the similarity of the four valencies of carbon the first thing 

 necessary in order to inspire confidence in the " structural for- 

 mulae " now so commonly employed could be assumed. 



It will be apparent from the researches already mentioned 

 that it is really a part of the business of scientific chemistry to 

 explain the phenomena of isomerism. Cases of isomerism 

 occur so frequently that even the clearest head would not be 

 in a position to survey the facts, if these were simply enume- 

 rated without any theoretical assumptions. Experience has 

 shown, however, that graphic or structural formulae are ex- 

 tremely valuable in explaining known cases of isomerism and 

 in helping us to foresee new ones ; and it can thus be under- 

 stood why the efforts of chemists were more and more directed 

 towards establishing such formulae It is obvious that I cannot 

 here take notice of all these efforts, but I must give an exposi- 

 tion of the principle that is adopted in drawing conclusions from 

 the reactions of substances with respect to their constitutions. 

 This principle states that the mutual relations of the atoms 

 remain unchanged during transformations, with the exception 

 of those which are severed, and that with respect to the latter 

 the atoms or groups that enter into the new combinations re- 



tw Annalen. 135, 217; 138, 204 and 328. ra Ibid. 131, 76; com- 

 pare also Carius, ibid. 131, 173 ; and Schoyen, ibid. 130, 233. 4 Rose, 

 Annalen. 205, 227. tir ' J. pr. Chem. [2] 22, 353. 



