rth carbonic acid gas, and one of the liquid products of
the decomposition is our familiar alcohol. The act of fermentation,
then, is a result of the effort of the little plant to maintain its
respiration by means of combined oxygen, when its supply of free
oxygen is cut off. As defined by Pasteur, fermentation is life
without air.

But here the knowledge of that thorough investigator comes to our aid
to warn us against errors which have 'been committed over and over
again. It is not all yeast-cells that can thus live without air and
provoke fermentation. They must be young cells which have caught
their vegetative vigour from contact with free oxygen. But once
possessed of this vigour the yeast may be transplanted into a
saccharine infusion absolutely purged of air, where it will continue
to live at the expense of the oxygen, carbon, and other constituents
of the infusion. Under these new conditions its life, as a plant,
will be by no means so vigorous as when it had a supply of free
oxygen, but its action as a ferment will be indefinitely greater.

Does the yeast-plant stand alone in its power of provoking alcoholic
fermentation? It would be singular if amid the multitude of low
vegetable forms no other could be found capable of acting in a similar
way. And here again we have occasion to marvel at that sagacity of
observation among the ancients to which we owe so vast a debt. Not
only did they discover the alcoholic ferment of yeast, but they had to
exercise a wise selection in picking it out from others, and giving it
special prominence. Place an old boot in a moist place, or expose
common paste or a pot of jam to the air; it soon becomes coated with a
blue-green mould, which is nothing else than the fructification of a
little plant called Penicillium glaucum. Do not imagine that the
mould has sprung spontaneously from boot, or paste, or jam; its germs,
which are abundant in the air, have been sown, and have germinated, in
as legal and legitimate a way as thistle-seeds wafted by the wind to a
proper soil. Let the minute spores of Penicillium be sown in a
fermentable liquid, which has been previously so boiled as to kill all
other spores or seeds which it may contain; let pure air have free
access to the mixture; the Penicillium will grow rapidly, striking
long filaments into the liquid, and fructifying at its surface. Test
the infusion at various stages of the plant's growth, you will never
find in it a