might have been deposited there. Furthermore, through-
out the operation he kept the flasks as high as possible
above his head, so as to avoid the dust from his clothing.
When the necks were broken, there was a hissing sound:
this was the air entering. The flasks were then re-
sealed in the flame of a lamp, and carried back to the
thermostat.

In some cases, the air which entered contained viable
germs, and the infusion became populated with various
organisms; in others, the air contained nothing, and the
infusion remained sterile. There were always some flasks
which remained intact, although each had received from
200 to 300 cc. of external air. To say that there are
germs in the air is not, therefore, to say that they occur
everywhere, or even that they are very numerous: it is
saying that there are some here and none there, that we
find more in a low and humid place, favorable to crypto-
gamic vegetation; that we find fewer in air which is in
repose, like that of the cellar of the observatory; that they
will be the more rare the farther we go from cultivated
land, and the higher we ascend a mountain; that there
will be almost none in the midst of the Swiss glaciers
where no vegetation can live. Pasteur opened a great
number of flasks in the air of these various places, and
he always found that some of them remained sterile, and
and the greater the known purity of the air at the point
studied, the greater the number of these.

All the researches made since have confirmed the truth
of this conclusion. The air is much less populated with
germs than has been supposed, much less, even, than
Pasteur thought. To-day men carry on with security,
in this regard, either in the laboratories or in surgical
wards, operations which they would not have dared to
undertake in 1862, haunted as they were by the idea of
those germs in the air, to which Pasteur had just called

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