LECTURE XII.] HISTORY OF CHEMISTRY. 221 



the substances obtained by the action of chlorine, bromine, 

 iodine, nitric acid, sulphuric acid, etc., upon organic materials. 

 Accordingly, the conjugated radicals were, as we should now 

 say, substituted radicals, and they embraced, besides, the 

 atomic groups containing a metal, such as cacodyl, etc. 



Whilst Gerhardt placed chloracetic acid C 2 C1 3^ I Q) picric 



acid C 6 H 2(N0 2 ) 8 | 0j sulphobenz oic acid C ? H 4( S0 4?1(X, etc. 

 " ) *1 2 J 



in this class of substances, other chemists, Mendius for example, 3 

 only cared to call substances of the latter kind conjugated ; 

 whereas others still, such as Limpricht and Uslar, 4 would 

 have wished to see almost all organic compounds placed in 

 this category. A discussion took place in connection with this 

 subject which ended with the introduction of the mixed types 

 and the abandonment of the conjugated compounds. 



Gerhardt had already referred the amid-acids, which he 

 places in his text-book amongst the "acides conjuges," to the 

 type ammonia + water 5 in 1853. Returning to this idea, but, 

 at the same time giving it an important extension, Kekule 

 shows, in 1857, upon the assumption of mixed types, how a 

 distinction between conjugated and other compounds becomes 

 quite unnecessary. 6 The possibility of this hypothesis rested 

 upon the conception of the polybasic radicals introduced in 

 1851 by Williamson, and by means of this conception it be- 

 came comprehensible how two molecules, previously separate, 

 may be united into a single one. Williamson had explained the 

 joining together of two molecules of water as depending upon 

 the nature of the radical SO 2 , and in this way the condensed 

 types arose. Kekule employs this view in establishing the 

 mixed types. With respect to this he expresses himself as 

 follows : "A union of several molecules of the types can only 

 occur when, by the entrance of a polyatomic radical in place 



3 Annalen. 103, 39. 4 Ibid. 102. 239. 5 Compare p. 215. 



6 Annalen. 104, 129. 



