24 



BOYS AND GIRLS IN BIOLOGY. 



planted in some nest, or jstidus. When the cook dries 

 her yeast-cakes, she puts all the little tortile to sleep, 

 and then they go into winter quarters, or hibernate in 

 their cells like the bears in their caves, and some of 

 them sleep longer than old Kip Van Winkle himself. 



There is another appearance of yeast about which I 

 have not told you ; if you let your cup of yeast stand 

 long enough, and do not add any more sugar or water 

 to it, you will notice that the bubbling or fermentation 

 stops, the torul^e cells settle to the bottom, and the 

 fluid comes to the top. The fluid has a strong or biting 

 instead of a sweet taste, like the fluid into which you 

 first placed the yeast. The fermentation has changed 

 its nature — the torul^:, like so many little fairies with 

 their wands, have turned the sugar into carbonic acid, 

 alcohol, glycerine, and succinic acid. These are 

 called the products of fermentation. The carbonic 

 acid, you know, passed off through the bubbles; the 

 other products are still in the fluid. If you taste a lit- 

 tle of this fluid it will make you merry, if you take 

 much of it you will become intoxicated ; this is due to 

 the alcohol, and the value of yeast depends upon its 

 power to make alcohol. The distiller takes this fluid, 

 and separates the alcohol by a process called distilla- 

 tion. If you had a retort, you might distill some for 

 yourselves. You may know that the fluid is alcohol, 



