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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-