FERMENTATION 97 
96. Alcohol Formed. If a large quantity of fermenting 
liquid as much as a pint or a quart is boiled, and the 
vapor that first comes off is condensed to a liquid, this 
liquid will have the characteristic odor of alcohol, and 
will burn with a pale, almost colorless flame. It is, in 
fact, alcohol, and it is on this account that this kind of 
fermentation is called alcoholic fermentation, to distinguish 
it from other kinds. Carbon dioxide and alcohol are, 
therefore, two products of the fermentation of sugar by 
yeast. If the proportions of yeast, sugar, and water, 
and the temperature are properly adjusted, and if the 
fermentation is allowed to continue long enough, it will 
be found that nearly all the sugar has disappeared, having 
been converted by the yeast into carbon dioxide and 
alcohol. Not all of the sugar will be converted for, as 
Pasteur, the great student of fermentation, demonstrated, 
small amounts of other substances such as, for example, suc- 
cinic acid and glycerine, are formed by fermentation, and 
these finally begin to check the activity of the ferment. 
97. Heat Produced. If a delicate and accurate ther- 
mometer is inserted into a fermenting yeast-mixture, and 
the temperature recorded from time to time, it will be 
found that the mixture grows gradually warmer, indicating 
that fermentation produces heat. The experiment will 
succeed best if the yeast mixture is placed in a Dewar flask, 
or, what amounts to the same thing, in a "thermos" 
bottle, which is double-walled, and thus retains more of 
the heat produced than does an ordinary, single-walled 
container. It is important to keep in mind the three 
major results of alcoholic fermentation: (i) the formation 
of alcohol; (2) the formation of carbon dioxide; (3) the in- 
crease of temperature. We shall see, in the next chapter, 
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