IX. 

 YEAST {Tonda or SaccJiaromyces Cera'istce). 



Yeast is a substance which has been long known on ac- 

 count of the power which it possesses of exciting the process 

 termed fenneniation in substances which contain sugar. 



If strained through a coarse filter, it appears to the naked 

 eye as a brownish liuid in which no solid particles can be 

 discerned. When some of this fluid is added to a solution 

 of sugar and kept warm, the mixture soon begins to dis- 

 engage bubbles of gas and become frothy ; its sweetness 

 gradually disappears ; it acquires a spirituous flavour and 

 intoxicating qualities ; and it yields by distillation a light 

 fluid — alcohol (or spirits of wine) which readily burns. 



"When dried slowly and at a low temperature, yeast is 

 reduced to a powdery mass, which retains its power of 

 exciting fermentation in a saccharine fluid for a considerable 

 period. If yeast is heated to the temperature of boiling 

 water, before it is added to the saccharine fluid, no ferment- 

 ation takes place ; and fermentation which has commenced 

 is stopped by boiling the saccharine liquid. 



A saccharine solution will not ferment spontaneously. If 

 it begins to ferment, yeast has undoubtedly got into it in 

 some way or other. 



If the yeast is not added directly to the saccharine fluid, 

 but is separated from it by a very fine filter, such as porous 

 earthenware, the saccharine fluid Avill not ferment, although 



