air when microbes do not intervene; but the acetifica-
tion in the German process is very rapid. It is true
that it was not immediately plain just where the micro-
organisms could intervene in this mass of shavings,
which always remain unchanged; but there was some-
thing which resembled it in the factory of Orléans, a
village which, for a long time, has had a merited reputa-
tion for its vinegars.

There they carry on operations in casks lying on end
in piles and filled about two-thirds full of a mixture of
fermented vinegar and unfermented wine. Now, on the
surface of the liquid, in the casks which behave properly,
there is a fragile pellicle which the vinegar-maker takes
great pains not to disturb and not to submerge, because
he considers it a precious ally. Experience having
taught him that it needs air, he has opened for it a large
window in the top end of the cask, above the surface of
the liquid. He watches this pellicle and cares for it.
As long as it remains spread over the surface of the
liquid, all goes well; if it is broken and falls in fragments,
all is lost. It is then necessary to produce a new one;
and sometimes, God knows, with how much trouble,
expense and groping about! A blast of heat, a blast of
cold, may suddenly interrupt all manufacture.

What then is this pellicle which is so precious and so
delicate? Pasteur had been asking himself this question
for a long time, but he only felt himself ripe for the study
of this question after he had carried out his studies on
the nutrition of micro-organisms and on the spontaneous
generations which we have reviewed. He was hence-
forth armed and equipped, and less than a year sufficed
him to make on this subject one of those researches a la
Lavoisier, which immediately become classic because of
their fullness, their elegance and their simplicity.

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