CHAPTER XI.
THE  BACTERIOLOGIC EXAMINATION OF THE AIR.
IT has been repeatedly emphasized—and indeed at the
present time almost every one knows—that micro-organ-
isms float almost everywhere in the air, and that their
presence there is a constant source of danger, not only
of contamination in our bacteriologic researches, but
also a menace to our health.
Such micro-organisms are neither ubiquitous nor equally
disseminated, but are much more numerous where the air
is dusty than where it is pure—much more so where men
and animals are accustomed to live, than upon the ocean
or upon high mountain-tops. The purity of the atmo-
sphere bears a distinct relation to the purity of the soil
over which its currents blow.
The micro-organisms that occur in the air are for the
most part harmless saprophytes which have been sepa-
rated from their nutrient birthplace and carried about by
the wind. They are almost always taken up from dried
materials, experiment having shown that they arise from
the surfaces of liquids in which they grow with much dif-
ficulty. They are by no means all bacteria, and a plate
of sterile gelatin exposed for a brief time to the air will
generally grow moulds and yeasts as well as bacteria.
The bacteria present are occasionally pathogenic, espe-
cially in localities where the discharges of diseased animals
have been allowed to collect and dry. For this reason the
atmosphere of the wards of hospitals and of rooms in
which infectious cases are being treated is much more
apt to contain them than the air of the street. However,
the dried expectoration of cases of tuberculosis, of in-
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