LECTURE XL] HISTORY OF ' CHEMISTRY. 211 



this with the same exactness as in the cases considered above. 

 It became manifest, however, that with few exceptions, to 

 which we shall afterwards return, the chemical molecule 

 agrees with the molecular weight deduced from the observed 

 volume, on the basis of Avogadro's hypothesis. This led to 

 the assumption of the general identity of the physical and 

 the chemical molecule, whereby a new means, and an almost 

 always sufficient one, is furnished for ascertaining the highly 

 important molecular weight. 



But Williamson's experiments and opinions also exerted an 

 influence in another direction ; that is, with respect to the 

 views concerning the constitution of compounds. The way was 

 now prepared for a fusion of the newer radical theory or theory 

 of residues with Dumas' theory of types, from which Gerhardt's 

 theory of types arose. In this development the labours of 

 other chemists were, however, of at least as great importance, 

 especially as these had already been partially carried out. 

 Accordingly, we shall now direct our attention more particularly 

 to the latter. 



In 1849, Wurtz, by treating cyanic ether, cyanuric ether, 

 and the substituted ureas prepared from these by himself, with 

 potash, obtained bases extremely like ammonia, which he 

 compared with the latter inasmuch as he regarded them as 

 ammonia in which an atom of hydrogen is replaced by a 

 radical such as methyl, ethyl, amyl, etc. 45 This way of 

 regarding these substances involved an important step in 

 advance, since it was the first successful attempt to introduce 

 radicals into the types. 46 The fact that Liebig, as early as 

 1839, expressed a similar view concerning these substances, 

 which were then only hypothetical, 47 satisfactorily proves the 

 clear perception of this gifted scientist, but cannot detract from 

 the merit of Wurtz. 



The views of Wurtz respecting the constitution of these 



45 Comptes Rendus. 28, 224, 323 ; 29, 169, 186 ; Annalen. 71, 330. 

 46 Compare, however, Laurent, Ann. Chim. [3] 18, 266. 47 Hand- 

 worterbuch der Chemie, by Liebig, Poggendorff, and Wohler, I, 698. 



