252 YEAST OR FERMENT, 



mentation of beer, wine, and vegetable juices, may 

 be first studied. 



This substance, which has been called yeast or 

 ferment, from the power which it possesses of 

 causing fermentation in sugar, or saccharine vege- 

 table juices, possesses all the characters of a com- 

 pound of nitrogen in the state of putrefaction and 

 eremacausis. 



Like wood in the state of eremacausis, yeast con- 

 verts the oxygen of the surrounding air into carbo- 

 nic 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, 

 accompanied by gases of an offensive smell 

 (Thenard), and is at last converted into a sub- 

 stance 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 sus- 

 taining the properties of ferment, for by simple 

 pressure its power to excite fermentation 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 by all antiseptic sub- 

 stances. 



The insoluble part of the substance called ferment 

 does not cause fermentation. For when the yeast 



