ITS PROPERTIES. 253 



from wine or beer is carefully washed with water, 

 care being taken that it is always covered with 

 this fluid, the residue does not produce fermen- 

 tation. 



The soluble part of ferment likewise does not excite 

 fermentation. An aqueous infusion of yeast may be 

 mixed with a solution of sugar, and preserved in 

 vessels from which the air is excluded, without 

 either experiencing the slightest change. What 

 then, we may ask, is the matter in ferment which 

 excites fermentation, if neither the soluble nor in- 

 soluble parts possess the power ? This question has 

 been answered by Colin in the most satisfactory 

 manner. He has shown that in reality it is the 

 soluble part. But before it obtains this power, the 

 decanted infusion must be allowed to cool in 

 contact with the air, and to remain some time 

 exposed to its action. When introduced into a 

 solution of sugar in this state, it produces a brisk 

 fermentation ; but without previous exposure to 

 the air, it manifests no such property. 



The infusion absorbs oxygen during its exposure 

 to the air, and carbonic acid may be found in it 

 after a short time. 



Yeast produces fermentation in consequence of 

 the progressive decomposition which it suffers from 

 the action of air and water. 



Now when yeast is made to act on sugar, it 

 is found, that after the transformation of the latter 



