d with the germs of these organisms,
you can understand how easy it is to fall into error in studying the
action of any one of them. Indeed it is only the most accomplished
experimenter, who, moreover, avails himself of every means of
checking his conclusions, that can walk without tripping through this
land of pitfalls. Such a man the French chemist Pasteur has hitherto
proved himself to be. He has taught us how to separate the commingled
ferments of our air, and to study their pure individual action. Guided
by him, let us fix our attention more particularly upon the growth and
action of the true yeast-plant under different conditions. Let it be
sown in a fermentable liquid, which is supplied with plenty of pure
air. The plant will flourish in the aerated infusion, and produce
large quantities of carbonic acid gas--a compound, as you know, of
carbon and oxygen. The oxygen thus consumed by the plant is the free
oxygen of the air, which we suppose to be abundantly supplied to the
liquid. The action is so far similar to the respiration of animals,
which inspire oxygen and expire carbonic acid. If we examine the
liquid even when the vigour of the plant has reached its maximum, we
hardly find in it a trace of alcohol. The yeast has grown and
flourished, but it has almost ceased to act as a ferment. And could
every individual yeast-cell seize, without any impediment, free oxygen
from the surrounding liquid, it is certain that it would cease to act
as a ferment altogether.

What, then, are the conditions under which the yeast-plant must be
placed so that it may display its characteristic quality? Reflection
on the facts already referred to suggests a reply, and rigid
experiment confirms the suggestion. Consider the Alpine cherries in
their closed vessel. Consider the beer in its barrel, with a single
small aperture open to the air, through which it is observed not to
imbibe oxygen, but to pour forth carbonic acid. Whence come the
volumes of oxygen necessary to the production of this latter gas? The
small quantity of atmospheric air dissolved in the wort and overlying
it would be totally incompetent to supply the necessary oxygen. In no
other way can the yeast-plant obtain the gas necessary for its
respiration than by wrenching it from surrounding substances in which
the oxygen exists, not free, but in a state of combination. It
decomposes the sugar of the solution in which it grows, produces heat,
breathes fo