THE BEGINNINGS OF LIFE. 403 



may initiate molecular movements in even a large 

 amount of a less unstable substance with which it is 

 brought into contact ; and to this latter set of changes the 

 name fermentation' is applied. Liebig's explanation of 

 this process, which is accepted by Gerhardt and many 

 other chemists, is thus described in Gerhardt's Chimie 

 Organise 1 : c Every substance which decomposes or 

 enters into combination is in a state of movement its 

 molecules being agitated; but since friction, shock, 

 mechanical agitation, suffice to provoke the decompo- 

 sition of many substances (chlorous acid, chloride of 

 nitrogen, fulminating silver), there is all the more 

 reason why a chemical decomposition, in which the 

 molecular agitation is more complete, should produce 

 similar effects upon certain substances. In addition, 

 bodies are known which, when alone, are not decom- 

 posed by certain agents, but which are attacked when 

 they exist in contact with other bodies, incapable of 

 resisting the influence of these agents. Thus platinum 

 alone does not dissolve in nitric acid, but when allied 

 with silver, it is easily dissolved; pure copper is not 

 dissolved by sulphuric acid, but it does dissolve in this 

 when it is allied with zinc, &c. According to M. 

 Liebig it is the same with ferments and fermentable 

 substances; sugar, which does not change when it is 

 quite alone, changes that is to say, ferments when it 

 is in contact with a nitrogenous substance undergoing 

 change, that is, with a ferment.' 



1 Tom. iv. p. 539. 

 D d 2 



