LECTURE VII.] HISTORY OF CHEMISTRY. Ill 



still proved insufficient, and chemical reasons were required in 

 order to convince chemists. Gerhardt, who received very 

 substantial support from Laurent, exerted himself in vain to 

 adduce decisive proofs of the accuracy of his ideas. At this 

 juncture, Williamson's investigations appeared, and they gave 

 a real foundation to the thoughts that had flitted before 

 Gerhardt's mind. This gifted chemist had shown the way; 

 and, in imitation of him, it was extensively followed, since it 

 permitted of a direct comparison of the quantities entering into 

 reaction. Thus there arose the conception of the chemical 

 molecule. In Gerhardt's system, which was now rapidly gain- 

 ing recognition, this conception found its formal expression in 

 the theory of types. 



I shall here conclude this sketch, in which I have indicated, 

 in general outline, the different phases of the historical develop- 

 ment ; and I shall now proceed to a detailed account. 



As early as the second half of the seventeenth century, 

 Lemery separated organic from inorganic chemistry. He 

 divided substances, according to their origin, into three classes, 

 viz., mineral, animal, and vegetable. 1 The phlogistians occupied 

 themselves chiefly with the first class. Scheele deserves to be 

 mentioned as the discoverer of an extensive series of organic 

 substances. 2 Lavoisier believed that compounds belonging to 

 this class consisted of carbon, hydrogen, and oxygen ; Berthollet 

 proved the presence of nitrogen in substances of animal 

 origin ; 3 at a later date, it was recognised that all the elements 

 can enter into organic combinations, but that carbon must 

 never be absent. 4 



It is difficult to say what substances were regarded as 

 organic compounds at the beginning of the present century. 

 This class naturally included all substances occurring in the 



1 Kopp, Geschichte. 4, 241. 2 See pp. 10-11. 3 Journ. de Phys. 28, 

 272. 4 Gmelin, Handbuch der Chemie. Fourth Edition, 4, 3 ; E. 7> 4- 



