CHAPTER VI 



ALCOHOL 



92. Yeast. A narcotic which man often takes into his 

 body is alcohol. When a little sugar in water stands a 

 while, bubbles of carbonic acid gas rise up through the 

 water, and the liquid turns sour. If one puts more sugar 

 in the water, the bubbles will rise, 



but the liquid has a strong odor and 

 a sharp taste, for some of the sugar 

 has turned to alcohol. We say that 

 the sugar ferments. When there is a 

 great deal of sugar in the water, it 

 does not ferment at all. Let us seek 

 the causes of these changes. 



In the air there float tiny germs of 

 a living plant called the yeast plant. 

 When these germs fall into sugar and 

 water they grow, and produce great 



numbers of yeast plants. Each yeast . ^ .. 



Yeast plant cells (xsoo). 



plant is only a single round or oval 

 cell, so small that a microscope is needed to see it. By its 

 growth it changes the sugar to alcohol by taking some car- 

 bonic acid gas away. But if there is a great deal of sugar 

 in the water, the germs will not grow at all. This is why 

 fruit preserved with a great deal of sugar keeps so well. 



93. Souring. If there is only a little sugar in water, a 

 smaller germ from the air grows with the yeast. This 



ov. PHYSIOL. (INTER.) 4 49 



