276 APPLIED BIOLOGY 



from much plant material which, on the ordinary farm, goes 

 to waste (unmarketable fruits and grains) ; and it is probable 

 that before many years pass the making of alcohol for mechan- 

 ical purposes will be a vast industry in agricultural regions. 



Vinegar, which is used so extensively as a condiment and 

 for preserving certain foods for human use, is best made 

 from alcohol derived from fermented fruit juice. For ex- 

 ample, yeast plants ferment the sugar of apple cider and form 

 alcohol ; and the vinegar bacteria, of which there are millions 

 in " mother of vinegar," ferment the alcohol into acetic 

 acid, which gives vinegar its acid qualities. 



On the harmful side, we must mention the undesired fer- 

 mentation of fruits and sugary mixtures prepared for human 

 food. However, we have seen that heating to near boiling 

 will check such action of yeast. Sometimes yeasts cause 

 undesired fermentation in milk and cheese in dairies. 



Certain kinds of yeasts (not the varieties used commonly) 

 have caused skin diseases, but rarely. 



We must conclude, then, that on the whole, yeast plants 

 are decidedly useful as producers of alcohol and carbon 

 dioxide. Most of the bread-making since human civilization 

 reached a stage where attention began to be paid to preparing 

 the best possible foods, has depended upon yeast plants. 

 Quite apart from the question of the harmful use of alcoholic 

 liquors, to be discussed in the last part of this book, there can 

 be no question concerning the value of alcohol for many 

 other purposes. The usefulness of yeasts forms a decided 

 contrast to the harmfulness of the molds ( 246). 



Reading for pupils: Section II in Conn's "Bacteria, Yeasts, and 

 Molds in the Home." 



III. BACTERIA 



254. Bacteria, Germs, Micro-organisms, Microbes. 



The word bacteria (singular, bacterium) is the biological name 

 for a group of one-celled plants without chlorophyll, which 



