316 . YEAST OR FERMENT. 



This substance, which has been called yeast ov fer- 

 ment, from the power which it possesses of causing 

 fermentation in sugar, or saccharine vegetable juices, 

 possesses all the characters of a compound of nitro- 

 gen in the state of 'putrefaction and eremacausis. 



Like wood in the state of eremacausis, yeast con- 

 verts the oxygen of the surrounding air into carbon- 

 ic acid, but it also evolves this gas from its own 

 mass, like bodies in the state of putrefaction. (Colin.) 

 When kept under water, it emits carbonic acid, ac- 

 companied by gases of an offensive smell, (Thenard,) 

 and is at last converted into a substance resembling 

 old cheese. (Proust.) But when its own putrefaction 

 is completed, it has no longer the power of inducing 

 fermentation in other bodies. The presence of wa- 

 ter is quite necessary for sustaining the properties 

 of ferment, for by simple pressure its power to ex- 

 cite fermentation is much diminished, and is com- 

 pletely destroyed by drying. Its action is arrested 

 also by the temperature of boiling water, by alcohol, 

 common salt, an excess of sugar, oxide of mercury, 

 corrosive sublimate, pyroligneous acid, sulphurous 

 acid, nitrate of silver, volatile oils, and in short by 

 all antiseptic substances. 



The insoluble part of the substance called ferment 

 does not cause fermentation. For when the yeast 

 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 fermentation. 



The soluble part offerm^ent 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, w^ithout eith- 

 er experiencing the slightest change. What then, 

 we may ask, is the matter in ferment which excites 

 fermentation, if neither the soluble nor insoluble 

 parts possess the power 1 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 infu- 



