9 8 The Living Plant 



be provided with an outlet tube bent over to end at the bottom 

 of a vessel of clear limewater, the gas will come bubbling up, and 

 will soon turn the limewater milky, thus proving its identity. 

 And when the fermentation is ended the liquid left in the flask 

 has always that " sourish" smell distinctive of the presence of al- 

 cohol, which, indeed, can be separated for testing by distilling 

 the liquid. As to its quantity, however, it is important to know 

 that even when all the conditions for fermentation are most 

 favorable and the sugar is present in plenty, the Yeast neverthe- 

 less does not form more than a limited quantity of alcohol, 

 (about ten per cent of the liquid in round numbers), for then the 

 plant is rendered inactive and may finally be killed by the very 

 alcohol which it has produced. 



Such is the process of fermentation, which, as everybody knows, 

 is vastly important in the arts. Sometimes it is used for the sake 

 of its carbon dioxide and sometimes for the sake of its alcohol. 

 The conspicuous case of the former is found in the making of 

 bread, where the carbon dioxide released from the growth of the 

 yeast cells throughout the mass of the dough, forms the cavities 

 by which it is lightened and raised. When everything goes as it 

 should, the alcohol evaporates hi the baking, but sometimes 

 it does not, and then the bread goes "sour. " Of course other 

 methods of raising bread are in use, notably by aid of gases re- 

 leased in the dough from chemical action between the constit- 

 uents of suitable "baking powders," or other substances, and 

 also by use of air blown into the dough; but yeast fermentation 

 is much the most used of the methods. But far more extensive 

 is the employment of fermentation for the making of the various 

 kinds of alcoholic liquids. When the sweet juice of the grape is 

 allowed to ferment (by action of yeast blown as spores through 

 the air to the fruits), the carbon dioxide escapes to the air, and 

 the remaining admixture of alcohol, water, and flavors we call 

 wine. When the sweet pulp of the germinating grains of barley 

 is allowed to ferment (by Yeast which is added for the purpose), 



