YEAST.


IT has been known, from time immemorial, that the sweet liquids which
may be obtained by expressing the juices of the fruits and stems
of various plants, or by steeping malted barley in hot water, or
by mixing honey with water--are liable to undergo a series of very
singular changes, if freely exposed to the air and left to themselves,
in warm weather. However clear and pellucid the liquid may have been
when first prepared, however carefully it may have been freed, by
straining and filtration, from even the finest visible impurities, it
will not remain clear. After a time it will become cloudy and turbid;
little bubbles will be seen rising to the surface, and their abundance
will increase until the liquid hisses as if it were simmering on
the fire. By degrees, some of the solid particles which produce the
turbidity of the liquid collect at its surface into a scum, which
is blown up by the emerging air-bubbles into a thick, foamy froth.
Another moiety sinks to the bottom, and accumulates as a muddy
sediment, or "lees."

When this action has continued, with more or less violence, for
a certain time, it gradually moderates. The evolution of bubbles
slackens, and finally comes to an end; scum and lees alike settle at
the bottom, and the fluid is once more clear and transparent. But
it has acquired properties of which no trace existed in the original
liquid. Instead of being a mere sweet fluid, mainly composed of sugar
and water, the sugar has more or less completely disappeared, and it
has acquired that peculiar smell and taste which we call "spirituous."
Instead of being devoid of any obvious effect upon the animal economy,