BACTERIOLOGIC EXAMINATION OF AIR. 167
the upper filter is then carefully mixed with sterile
melted gelatin and poured into sterile Petri dishes, where
the colonies develop and can be counted. Sternberg re-
marks that the chief objection to the method is the pres-
ence in the gelatin of the slightly opaque sand, which
interferes with the recognition and count-
ing of the colonies. This objection has, f^rr^
however, been removed by Sedgwick and
Miquel, who use a soluble material — granu-
lated or pulverized sugar — instead of the
sand. The apparatus used for the sugar-
experiments differs a little from the original
of Petri, but the principle is the same, and
can be modified to suit the experimenter.
Petri points out in relation to his method
that the filter catches a relatively greater
number of bacteria in proportion to moulds
than the Hesse apparatus, which depends
upon sedimentation.
A particularly useful form of apparatus
is a granulated sugar-filter suggested by
Sedgwick and Tucker, which has an ex-
pansion above the filter, so that as soon as
the sugar is dissolved in the melted gela-
tin it can be rolled out into a lining like
that of an Esmarch tube. This cylindrical
expansion is divided into squares which
make the counting of the colonies very easy
(Fig- 45)-
The number of germs in the atmosphere
will naturally be very variable. Roughly,
the number may be estimated at from 100
to 1000 per cubic meter.
In reality, the bacteriologic examination of air is
of very little value, as so many possibilities of error
may occur. Thus, when the air of a room is quiescent
there may be very few bacteria in it ; let some one walk
across the floor and dust at once rises, and the number
FIG. 45.— Sedg-