208 Physiology. 



" hard," as we call it it usually passes on to become vine- 

 gar. This change is another form of fermentation, due 

 to another kind of ferment. This formation of vinegar is 

 likely to take place in any weak solution of alcohol-fer- 

 mented liquor. In this fermentation acetic acid is pro- 

 duced, hence it is called the acetous fermentation. It is 

 interesting to note that the word " vinegar " comes from 

 the French vin (wine) and aigre (sharp or sour), as vinegar 

 was formerly made by this secondary fermentation of the 

 lighter wines. 



"Temperance Drinks." Many well-meaning persons 

 use the various preparations called "root beer," perhaps 

 without realizing that most, if not all, of them are made 

 with yeast and in their preparation undergo fermentation, 

 thereby producing alcohol, though not ordinarily in large 

 amounts. By giving such drinks (often called "temper- 

 ance drinks ") to children, an appetite for alcohol may be 

 cultivated and the beginning of a terrible habit made. It 

 may be well here to call attention to the real meaning of 

 the word " habit," that which holds us. 



Ferments. There are many other ferments which, when 

 introduced into liquids, cause various changes, i.e. there 

 are many sorts of fermentation. For instance, putrefac- 

 tion is a kind of fermentation of substances containing 

 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. (The alcoholic 

 ferment is an exception, not belonging to the Bacteria.) 



Yeast. Yeast is used in bread-making for the purpose of producing 

 gas within the dough, and by this means causing the dough to puff up 

 and become light. The yeast acts on the free sugar in the flour, and 

 produces carbon dioxid and alcohol. The alcohol turns to vapor and 

 passes*out of the bread while it is baking. 



