FERMENTATION 
95 
fundamental processes of plant physiology. Fermen- 
tation is most commonly associated in our minds with 
yeast. 
92. Yeast. Practically everyone is acquainted with 
yeast, which was the earliest recognized agent of fermen- 
tation. We are now most familiar with it in the form 
of small cakes, purchased at 
the grocer's for use in making 
bread and other "raised" 
dough. Our grandparents 
bought it in liquid form from 
the local baker; and in brew- 
eries and large bakeries it is 
used in the liquid form in mak- 
ing beer and bread. If a small 
piece of a "compressed yeast" 
cake, about the size of a pea 
seed, is placed with a little 
sugar and water in a fermenta- 
FIG. 66. Fermentation-tube, 
tion-tube (Fig. 66), and Set in I, level of fermenting liquid; 
warm rklarp tVip> TnivtiirP will *' s P ace ^ td with 8 as ( C 2 ) 
a warm place tne mixture will given O g by fermentation, 
soon begin to "work," and tiny 
bubbles of gas will be seen rising in increasing numbers 
to the top of the tube. The process which gives rise to 
these bubbles is alcoholic fermentation. 
93. Conditions Necessary for Alcoholic Fermentation. 
If two other tubes are prepared precisely like the first 
one, except that ice-water is used in one and boiling water 
in the other, and are set, the first in a cold place (e.g., the 
refrigerator), and the second in a very warm place, fer- 
mentation will occur either very tardily or not at all. 
If a third fermentation-tube is set up with sugar but no 
