ETHYL ALCOHOL. 43 



instead of sugar. The turbidness has settled in the 

 form of an ill-looking, grayish mass, which is called 

 yeast. 



A solution of pure sugar in water does not undergo 

 this change alone. If, however, a small quantity of 

 yeast he added to it, the phenomena observed in con- 

 nection with the plant-juice make their appearance, 

 though more slowly than in the former case. Cane- 

 sugar, grape-sugar, and fruit-sugar, according to all 

 appearances, conduct themselves in a similar manner. 

 Grape-sugar and fruit-sugar are in reality, however, 

 the only varieties capable of fermentation ; cane-sugar 

 only undergoes fermentation after having been previ- 

 ously converted into these varieties. From one mole- 

 cule of grape-sugar result two molecules of alcohol, and 

 two molecules of carbonic anhydride ; but in addition 

 to these are always formed small quantities of succinic 

 acid and glycerin. 



Yeast consists of microscopical vesicles (yeast-cells), 

 the walls of which are formed by an elastic membrane 

 consisting of cellulose. Their contents are, in the 

 young cells, a liquid, but in the older ones, a thick, 

 granular, nitrogenous mass. 



The germs of the yeast-cells come from the air. 

 Hence, contact of the plant-juice with the air is essen- 

 tial to the beginning of fermentation ; once begun, how- 

 ever, fermentation continues regularly even though 

 the air be excluded. The germs, which have fallen 

 from the air into the solution, develop when they meet 

 with the substances necessary to their growth. But, in 

 addition to the saccharine solution, nitrogenous sub- 

 stances and inorganic salts are essential. For this 

 reason albuminous substances aid fermentation mate- 

 rially, but they are not, as was supposed for a long 

 time, the real ferment which causes fermentation. As 

 these substances are not present in a pure solution 

 of sugar, the germs cannot develop in it. They are 

 contained in plant-juices, however, and hence in these 

 the development and rapid multiplication of the cells 

 by means of the formation of buds begin immediately. 

 The splitting up of sugar into alcohol and carbonic 



