400 YEAST OR FERMENT. 



Like wood in the state of eremacausis, yeast converts the 

 oxygen of the surrounding air into carbonic 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 car. 

 bonic acid, accompanied 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 water is quite necessary for sustaining the proper- 

 ties of ferment, for by simple pressure its power to excite fer- 

 mentation is much diminished, and is completely 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, substances, all 

 of which possess antiseptic properties. 



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 fer- 

 mentation. 



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 insoluble 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 pro- 



