Lussac had seen some inert must of grape begin to fer-
ment as soon as he placed it in contact with some bubbles
of external air. Men had concluded from this, with
some appearance of justice, that there was in each bubble
of air something capable of starting all the fermentations
or putrefactions which could take place in the most varied
liquids in contact with air. This was, it is true, a little
too liberal an interpretation given to an experiment which
had been performed only twice and had succeeded only
once. But if it accorded well with the hypothesis of
spontaneous generation which saw in the oxygen the
only cause of the appearance of life, it could not accom-
modate itself to the theory of germs. It seemed difficult
that there should be sufficient in each bubble of air to
populate the most varied liquids with the most varied
microbes.

What degree of credence and of generality could be
attributed to the experiment of Gay-Lussac? This was
what no one knew, and what Pasteur was obliged to
study. It is this part of his work which has attracted
the most attention, not that it is the best: all of it is
valuable; but this is the most easily understood, and the
experiments in it are as simple as they are convincing.
Pasteur took again his flasks with a straight neck drawn
out. He brought to a boil the organic infusion which
they contained, and after having driven out all the air
from the interior, through the open extremity of the neck,
he closed this at the moment when the steam was given
off by melting the glass in the flame of a blowpipe. The
flask is thus practically empty of air when it is cooled.

He then took 20 or 40 of these flasks to the place where
he wished to make a study of the air, and broke the necks
with a long pair of pincers, having first taken the precau-
tion to pass the necks and the pincers through the flame
of an alcohol lamp, in order to kill all the germs which

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