LECTURE XI.] HISTORY OF CHEMISTRY. 2iy 



decomposition of the terms which are wanting in the chemical 

 classification can be stated beforehand. 



Gerhardt compares the members of one and the same 

 heterologous series (representative, therefore, of the various 

 homologous and isologous series) with four very minutely 

 studied inorganic substances, as prototypes, viz., water, hydro- 

 chloric acid, hydrogen, and ammonia all compounds of 

 hydrogen. A substance which was to be regarded as belonging 

 to one of these types, was necessarily capable of being con- 

 ceived as derived from it by the replacement of hydrogen 

 atoms by radicals. Thus Gerhardt refers alcohols, ethers, 

 acids, anhydrides, salts, aldehydes, ketones, etc., to the water 

 type, and to this type there also belong the mercaptans, sul- 

 phides, etc. The latter really correspond to the type of 

 sulphuretted hydrogen, but this is merely a subdivision of the 

 water type. Chlorides, bromides, iodides, and cyanides are 

 referred to hydrochloric acid. Ammonia was the prototype of 

 the amines, amides, imides, and nitriles, as well as of the 

 corresponding phosphorus compounds. Finally, the hydro- 

 carbons, the alcohol radicals and the radicals containing 

 metals were referred to the hydrogen type, H 2 . 



The great step had, accordingly, now been taken ; radicals 

 had been introduced into the mechanical types of Regnault 

 and of Dumas. If we look back and inquire to whom we are 

 chiefly indebted for this excellent extension of the earlier theory 

 of types, the names of Laurent and of Wurtz especially deserve 

 to be mentioned. As early as 1846, the former had referred 

 alcohol and ether to water ; three years later, Wurtz discovered 

 ethylamine which he regarded as a substituted ammonia. This 

 view met with acceptance all the more rapidly that the simi- 

 larity between the two substances is so startling. C5 I shall not 

 omit to observe here again, that the conception of the radical 

 was now adopted in the sense in which Gerhardt had defined 



65 With respect to the share which Hunt took in the development of 

 the theory of types, compare Silliman's Journal [2] 5, 265 ; 6, 173 ; 8, 92 ; 

 9, 65 ; also Phil. Mag. [4] 3, 392. His claim is printed in Comptes Rendus. 

 52, 247, and the reply of Wurtz in Repert. de Chimie pure. 3, 418. 



