904 The American Naturalist. [November, 
bonic acid is allowed to go off into the air and the alcohol 
remains behind. Then by the processes of distillation the 
alcohol is separated from the fermenting mass. The carbonic 
acid is all given off into the air in these cases. 
In the manufacture of beer the attempt is made to get both 
products of the yeast growth. In the making of beer the 
yeast is cultivated in the same way in the malt; aleohol and 
carbonic acid both are produced. After some fermentation 
the beer is put into bottles. A certain amount of fermenta- 
tion takes place after the bottling. The carbonic acid thus 
produced is dissolved in the liquid and soon accumulates so 
as to produce considerable pressure. When the bottle is opened 
it is this gas which causes the froth at the top of the beer. It 
is the alcohol which produces the intoxicating quality in the 
beer, but it is the carbonic acid chiefly which gives the beer 
its sharp, pungent taste. The alcohol aids, of course, to a 
certain extent, but the carbonic acid is the chief factor in the 
taste of beer. It may be a little question whether it is proper 
to use yeasts in this way to produce rum, whisky, alcohol and 
beer, with the untold miseries which they involve; neverthe- 
less, yeasts are at the foundation of the gigantic industries 
connected with distilling and brewing dperations. 
The farmer makes use of them in the manufacture of cider. 
Yeast from the atmosphere is planted in his apple juice; it 
attacks the sugar that it finds there, breaks the sugar to pieces, 
and produces carbonic acid and alcohol as before. The car- 
bonic acid accumulates during the first day or two, and gives 
the sharp, pungent taste that is noticeable in sweet cider. 
Later on the alcohol accumulates in larger quantities, and 
that gives the taste to hard, sour cider. After the cider has 
fermented for several days the carbonic acid is of second 
importance ; the alcohol accumulates until you get the strong, 
sharp, intoxicating hard cider. So much, then, for the uses 
to which we put yeasts. 
Now, leaving yeasts, turn for a moment to the consideration 
of a few miscellaneous phenomena connected with bacteria. 
I may take as a starting point this very product that I men- 
tioned last, namely, hard cider. Your yeasts produce alcohol 
