l8o HISTORY OF CHEMISTRY. [LECTURE X. 



In my opinion, the labours of Laurent and of Gerhardt 

 chiefly contributed to make the radical what it still is. Laurent, 

 by advancing the nucleus theory, emphasised the variability of 

 the radicals, a matter that was afterwards brought forward by 

 Dumas also. 13 But it was Gerhardt who first indicated the 

 possibility of the assumption of two radicals in one compound, 

 and thereby destroyed all idea of the actual existence of sepa- 

 rated groups. It is with the history of this part of the develop- 

 ment of organic chemistry that we are now concerned. 



The influence? of his gifted teacher, Liebig, can scarcely 

 fail to be observed in Gerhardt's earliest publications. We are 

 aware that Liebig contests the presence of water in the acids. 

 By a very happy extension of the idea, Gerhardt negatives the 

 pre-existence of water in the majority of organic compounds. 

 Its presence in alcohol, especially, appears to him just as 

 unlikely as that of ammonia in the substances containing 

 nitrogen from which ammonia is evolved by means of potash. 

 He knows that there is a class of substances of simple com- 

 position and of extraordinary stability, such as water, carbonic 

 acid, hydrochloric acid, and ammonia, some of which are pro- 

 duced in almost every, organic decomposition, without, however, 

 our being able to recompound, from these substances, the sub- 

 stances originally decomposed. 14 



The formation of one substance out of another was not, 

 with Gerhardt, any ground for the assumption that the first 

 was present in the second, ready formed : substances do not 

 require to contain any water in order that water may be 

 separated from them in certain reactions. The reason for the 

 frequent formation of water and similar substances is to be 

 found in their stability, and in the great affinity which their 

 constituents possess for one another. Now this view was of 

 essential significance, and it led Gerhardt, in 1839, to the 

 theory of residues and of coupled compounds. 15 He says 16 

 that when two substances react upon each other, an element 



13 Compare pp. 146 and 167. 14 J. pr. Chem. 15, 37. l5 Ann, 



Chim. [2] 72, 184. I6 Comptes Rendus, 20, 1031, 



