THE USES OF FUNGI.
193
It has been proved that ordinary vinous fermentation always involves
the formation of yeast: the more yeast, the quicker the fermentation.
How by their growth these living organisms become the sources for
the ferments proper, which are considered by scientists to be really
only chemical substances pure and simple, has not yet been ascer-
tained. All we know is that, in the presence of certain substances,
yeast appears; and that with its multiplication—by simple "gem-
mation," or budding, be it noticed, and not by sporification, which
is never observed—fermentation takes place. Whence the yeast
comes it is beyond our power to say, save that germs capable of its
reproduction must be carried by the air. According to Pasteur's
experiments and observations, the yeast which forms spontaneously
in grape-juice is derived chiefly from certain germs which abound
about harvest-time on the grapes, and still more on the grape-stalks.
These germs are largely diffused also through the atmosphere of
breweries, wine-cellars, and laboratories where fermentation experi-
ments are carried on ; but they are not by any means widely diffused
through the atmosphere generally.
It is not, however, only as the means by which mankind obtains
wine and beer and whiskey that yeast is such an important economical
factor. It is to the properties of yeast that we owe almost all the
bread which we ordinarily eat. The generation of carbonic acid gas
caused by yeast has indeed been imitated by art, and dough can be
impregnated with the gas by purely mechanical means, as in the
so-called "aerated bread," or by the interaction of certain chemicals
in the form of "baking-powders." But by far the larger proportion
of the bread consumed in England is spongified and made light and
digestible by the action either of brewers' yeast, or of the "German
yeast" obtained from the fermentation of wine.
Before I have done with yeast, I must call your attention to
the fact that it has a place in the British Pharmacopoeia, where
directions are given for making a "yeast poultice," cataplasma
fermenti, as it is there called. In Dr. Garrod's classical work
on Materia Medica the therapeutical uses of yeast are thus
summarized: "Yeast, when externally applied, acts as a stimulant
and antiseptic, and in the form of cataplasm or poultice is
employed to correct the discharges of indolent ulcers. Internally
it has been used in low states of the system, in which it is stated
to clean the tongue and correct the foetor of the alvine discharges,
to prevent the formation of boils and carbuncles, and as a remedy
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