288 HISTORY OF CHEMISTRY. [LECTURE XIV. 



anthrene 1() - in which an atom of nitrogen has taken the place 



of CH: 



I have already stated (p. 1 1 7) that all these investigations 

 as to the constitution of organic compounds were occasioned 

 by the numerous cases of isomerism which meet the chemist 

 at almost every step, and the existence of which seems to 

 require some explanation. It must be acknowledged, that the 

 theory of the valency or atomicity of the elements fulfilled this 

 requirement to a large extent, and in this lies the great import- 

 ance of that theory ; whereas, on the other hand, it cannot be 

 denied that the principles of the theory are far from being 

 clearly and precisely worked out a matter into which I intend 

 to enter more fully in the next lecture. But attention may 

 here be called to the fact that not merely is the possibility 

 of an explanation of these isomerisms supplied to us by the 

 advancement of our theoretical knowledge, but this explanation 

 chiefly depends upon the much more extensive experimental 

 material at our disposal. And this material has, in great part, 

 been obtained by the application of a method which, even 

 although it has been recognised for a long time as a possible 

 one, has only attained to pre-eminent importance within com- 

 paratively recent times.. I refer to the method of synthesis, 

 which is, moreover, in many cases, not merely a means to an 

 end, but is itself the aim of the experiments. 



In an earlier lecture (p. 116) the synthesis by Wohler of 

 an organic compound (urea) was mentioned, and also the 

 importance of this synthesis in regard to our whole conception 

 of nature. Similar results were only obtained in the cases of 

 other substances long afterwards, and the value of this method 

 was shown in a proper light by Berthelot's comprehensive 

 work. 103 The syntheses of some specially important substances 

 marsh gas, ethylene, alcohol, formic acid, benzene, etc. 

 also originated with Berthelot. 



It has been found in many cases that the earlier analytical 



102 Ladenburg, Berichte, 16, 2063; Grabe, ibid. 17, 1370. 3 Chimie 

 organique fondee sur la Synthese, Paris 1860. 



