616 BACTERIA IN THE AIR. 
mixture of glycerin and glucose has been placed, is adjusted near the 
opening of the funnel, at a distance of about three millimetres, so 
that the air escaping through the small orifice is projected against it. 
By this arrangement a considerable number of the microérganisms 
present in the air, as well as suspended particles of all kinds, are ar- 
rested upon the surface of the slide and can be examined under the 
microscope or studied by bacteriological methods. But an aéroscope 
of this kind gives no precise information as to the number of living 
germs contained in a definite quantity of air. The microscopical ex- 
amination also fails to differentiate the bacteria from particles of 
various kinds which resemble them in shape, and the microérgan- 
isms seen are for the most part spores of various fungi mingled with 
pollen grains, vegetable fibres, plant hairs, starch granules, and 
amorphous granular material. 
Another method, which has been employed by Cohn, Pasteur, 
Miquel, and others, consists in the aspiration of a definite quantity of 
air through a culture liquid, which is then placed in an incubating 
oven for the development of microédrganisms washed out of the air 
which has been passed through it. This method shows that bacteria 
of different species are present, but gives no information as to their 
relative number, and requires further researches by the plate method 
to determine the characters of the several species in pure cultures. 
A far simpler method consists in the exposure of a solid culture 
medium, which has been carefully sterilized and allowed to cool on a 
glass plate or in a Petvri’s dish, for a short time in the air to be ex- 
amined. Bacteria and mould fungi deposited from the air adhere to 
the surface of the moist culture medium, and form colonies when the 
plate, enclosed in a covered glass dish, is placed in the incubating oven. 
The number of these colonies which develop after exposure in the 
air for a given time enables us to estimate in a rough way the num- 
ber of microérganisms present in the air of the locality where the 
exposure was made ; and the variety of species is determined by ex- 
amining the separate colonies,each of which is, as a rule, developed 
from a single germ. By exposing a number of plates at different 
times this method enables us to determine what species are most 
abundant in a given locality and the comparative number in dif- 
ferent localities, as determined by counting the colonies after ex- 
posure for a definite time—e.g., ten minutes. Of course we will only 
obtain evidence of the presence of such aérobic bacteria as will 
grow in our culturemedium. The anaérobic bacteria may be studied 
by placing plates exposed in a similar way in an atmosphere of hydro- 
gen. Bacteria which grow slowly and only under special conditions, 
like the tubercle bacillus, would be likely to escape observation, as 
the mould fungi and common saprophytes would take complete pos- 
