LECTURE XI.] HISTORY OF CHEMISTRY. 215 



of dibasic acids. 58 Thus Gerhardt, conjointly with Chiozza, 

 had published, in June 1853 (and thus half a year before this 

 last paper of Williamson's appeared), investigations on the 

 derivatives of dibasic acids, and especially on anhydrides and 

 chlorides, in which he thought it was shown, amongst other 

 things, that the first action of phosphorus pentachloride con- 

 sists in the removal of water, and that it is only in the second 

 stage of the reaction that a substance containing chlorine is 

 produced. Gerhardt and Chiozza arrived at this time, however, 

 at very important results ; they regarded the dibasic anhydrides, 

 for the first time, as water in which both the hydrogen atoms 

 are replaced by a single radical, and they also showed how to 

 prepare succinyl chloride and similar chlorides. In two sub- 

 sequent papers 59 they deal with the investigation of the amides 

 corresponding to the polybasic acids. They show that these 

 are either derived from two molecules of ammonia which are 

 held together by the replacement, by a dibasic radical, of one 

 atom of hydrogen in each, or that they may be derived from 

 one molecule of ammonia. The amid-acids correspond to the 

 mixed type NH 3 + H 2 O, which can only be produced by the 

 polyatomic acid radical entering the molecule ; and the earlier 

 statement of Gerhardt that only dibasic acids could give rise 

 to the formation of amid-acids is thus explained. 



By these and similar experiments, but especially by adopt- 

 ing the ingenious conclusions that Williamson had drawn from 

 his investigations, Gerhardt is able to establish a complete 

 classification of organic compounds according to a new 

 principle, 60 and he expounds this classification in the fourth 

 volume of his excellent Hand-book. 



An important point in Gerhardt's system, consists in his 

 showing the connection between substances of opposite char- 

 acter by means of intermediate substances. 



Unlike the dualists, he does not contrast such substances 

 as potash and sulphuric acid as absolutely opposite in character, 



58 Comptes Rendus. 36, 1050. 59 Ibid. 37, 86 ; 38, 457. 60 Gerhardt, 

 Traite de Chimie organique, 4, quatrieme partie. 



