trace of alcohol. But forcibly submerge the little
plant, push it down deep into the liquid, where the quantity of free
oxygen that can reach it is insufficient for its needs, it immediately
begins to act as a ferment, supplying itself with oxygen by the
decomposition of the sugar, and producing alcohol as one of the
results of the decomposition. Many other low microscopic plants act
in a similar manner. In aerated liquids they flourish without any
production of alcohol, but cut off from free oxygen they act as
ferments, producing alcohol exactly as the real alcoholic leaven
produces it, only less copiously. For the right apprehension of all
these facts we are indebted to Pasteur.

In the cases hitherto considered, the fermentation is proved to be the
invariable correlative of life, being produced by organisms foreign to
the fermentable substance. But the substance itself may also have
within it, to some extent, the motive power of fermentation. The
yeast-plant, as we have learned, is an assemblage of living cells; but
so at bottom, as shown by Schleiden and Schwann, are all living
organisms. Cherries, apples, peaches, pears, plums, and grapes, for
example, are composed of cells, each of which is a living unit. And
here I have to direct your attention to a point of extreme interest.
In 1821, the celebrated French chemist, Berard, established the
important fact that all ripening fruit, exposed to the free
atmosphere, absorbed the oxygen of the atmosphere and liberated an
approximately equal volume of carbonic acid. He also found that when
ripe fruits were placed in a confined atmosphere, the oxygen of the
atmosphere was first absorbed, and an equal volume of carbonic acid
given out. But the process did not end here. After the oxygen had
vanished, carbonic acid, in considerable quantities, continued to be
exhaled by the fruits, which at the same time lost a portion of their
sugar, becoming more acid to the taste, though the absolute quantity
of acid was not augmented. This was an observation of capital
importance, and Berard had the sagacity to remark that the process
might be regarded as a kind of fermentation.

Thus the living cells of fruits can absorb oxygen and breathe out
carbonic acid, exactly like the living cells of the leaven of beer.
Supposing the access of oxygen suddenly cut off, will the living
fruit-cells as suddenly die, or will they continue to live as yeast
lives, by extracting oxygen from