iv.] YEAST. 81 



with the life and growth of the plant. In fact, whatever 

 arrests the vital activity of the plant also prevents it 

 from exciting fermentation. 



Such being the facts with regard to the nature of yeast, 

 and the changes which it effects in sugar, how are they 

 to be accounted for? Before modern chemistry had 

 come into existence, Stahl, stumbling, with the stride of 

 genius, upon the conception which lies at the bottom of 

 all modern views of the process, put forward the notion 

 that the ferment, being in a state of internal motion, 

 communicated that motion to the sugar, and thus caused 

 its resolution into new stubstances. And Lavoisier, as 

 we have seen, adopts substantially the same view. But 

 Fabroni, full of the then novel conception of acids and 

 bases and double decompositions, propounded the hypo- 

 thesis that sugar is an oxide with two bases, and the 

 ferment a carbonate with two bases ; that the carbon of 

 the ferment unites with the oxygen of the sugar, and 

 gives rise to carbonic acid ; while the sugar, uniting with 

 the nitrogen of the ferment, produces a new substance 

 analogous to opium. This is decomposed by distillation, 

 and gives rise to alcohol. Next, in 1803, The'nard pro- 

 pounded a hypothesis which partakes somewhat of the 

 nature of both Stahl's and Fabroni's views. " I do not 

 believe with Lavoisier," he says, " that all the carbonic 

 acid formed proceeds from the sugar. How, in that case, 

 could we conceive the action of the ferment on it ? I 

 think that the first portions of the acid are due to a 

 combination of the carbon of the ferment with the oxygen 

 of the sugar, and that it is by carrying off a portion of 

 oxygen from the last that the ferment causes the fer- 

 mentation to commence the equilibrium between the 

 principles of the sugar being disturbed, they combine 

 afresh to form carbonic acid and alcohol." 



The three views here before us may be familiarly 



