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an autoclave (Fig. 12). Here under a pressure of two or
three atmospheres sufficient heat is generated to destroy
the spores. The objections to this method are that it
sometimes turns the agar-agar dark, and that it is likely
to destroy the gelatinizing power of the gelatin, which
after sterilization remains liquid.

Liquids may also be sterilized by filtration—i. e. by
passing them through unglazed porcelain or some other
material whose interstices are sufficiently fine to resist the
passage of the bacteria. This method is largely employed

FIG. 15.—Kitasato's filter: a, por-
celain bougie; £, attachment for suc-
tion-pump; <r, reservoir; d, sterile

FIG, 16.—Reichel's bacteriologic filter
of unglazed porcelain: A^ sterile re-
ceiver; J3, porcelain filter; c, rf, attach-
ments for pump.

for the sterilization of the unstable toxins and anti-
toxins, which are destroyed by heat. Various substances
have been used for filtration, as stone, sand, powdered
glass, etc., but experimentation has shown porcelain to
be the only reliable filter against bacteria. Even this
material, whose interstices are so small as to allow the
liquid to pass through with great slowness, is only cer-
tain in its action for a time, for after it has been used
considerably the bacteria seem able to work their way