ALCOHOL. 209 



Yeast added to such sweet liquids hastens, or makes more 

 certain, this form of fermentation. 



Yeast cells are not killed by drying. They become dry 

 and float as part of the common dust of the air. They are 

 still alive, and if they fall into a sweet liquid, especially if 

 the liquid is not saturated with sugar, they begin to grow. 

 In their growth they break up, or decompose, sugar and 

 form at least two substances, carbon dioxid and alcohol. 

 Yeast, therefore, is called a ferment, and this change is 

 called alcoholic fermentation. A small quantity of yeast 

 has the power of changing a large amount of sugar into 

 carbon dioxid and alcohol. Then, too, we must remem- 

 ber that its growth is so rapid that a small quantity of 

 yeast soon becomes a large quantity. 



Ferments. Besides yeast there are many other ferments 

 which, when introduced into liquids, cause various changes, 

 i.e. there are many sorts of fermentation. For instance, 

 putrefaction is a kind of fermentation of substances con- 

 taining nitrogen, during which process offensive gases are 

 given off. Most of the ferments belong to a group of very 

 simple, one-celled plants called Bacteria. (Yeast is an 

 exception, not belonging to the Bacteria.) 



Fermented Drinks. All the alcoholic liquors are the 

 result of alcoholic fermentation of various substances. 

 Such liquors may be classed in three groups, wines, 

 malt liquors, and distilled liquors. 



Wines. The wines are the result of fermentation in 

 the juice of the grape or other fruit which is rich in 

 sugar. This fermentation was, in all probability, dis- 

 covered very early by the human race, for we find it in 

 use among nearly all races of men, and accounts of it in 



