1892.] ; Some Uses of Bacteria. 903 
two compounds; one of them is alcohol, and the other one is 
the gas which we commonly call carbonic acid (CO,). We 
make use of yeasts for various purposes along two directions. 
We may use them either for the purpose of getting the alco- 
hol or for the purpose of getting the carbonic acid. For 
instance, you want to bake a loaf of bread; you take your 
dough, you plant yeast in it and set it in a warm place; now, 
there is always a little sugar in the dough, and the yeast 
begins to grow, breaking the sugar to pieces, as I have just 
stated, and produce from it alcohol and carbonic acid. The 
carbonic acid is a gas, and as the yeast grows and the carbonic 
acid makes its appearance in the bread, little bubbles are seen 
_in the dough until presently it becomes filled with these little 
bubbles of carbonic acid gas which render it lighter. Of 
course, as the gas accumulates the dough swells, or, as we say, 
it “rises.” Then you bake it, and when you take it out of the 
oven and cut it open you find that the bread is full of little 
holes. Those little holes are the remains of the bubbles of 
carbonic acid gas which the yeast produced, and the object of 
growing the yeast was simply to make those holes in the 
bread. The bread is light, and the object of the introduction 
of the yeast is thus accomplished. You cannot bake a loaf of 
bread, then, without the agency of microscopic organisms. 
In the baking of bread we have an instance of the use of 
carbonic acid alone. In the manufacture of wine the object 
of the vintner is to get the other product of yeasts, namely, 
the alcohol. He grows yeasts in his grape juice, usually 
depending on those from the air. Again there are carbonic 
acid and alcohol produced and the carbonic acid in this case 
passes off into the air during the fermentation, while the alco- 
hol remains behind; when the fermentation has continued 
long enough a considerable amount of alcohol remains in the 
grape juice, and thus produces the wine. Similarly, in the 
manufacture of alcohol or of ‘any of the other alcoholic 
liquors, such as rum or whisky, the same process is made use 
of; that is, the little yeasts are planted in some sort of sugar 
solution, it may be molasses, it may be barley; they grow 
there; there they produce carbonic acid and alcohol ; the car- 
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