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i8o                PATHOGENIC BACTERIA.

the fifth-, 30; on the sixth, 150; on the seventh, 1000,
etc.; indicating that the serum exerted a destructive
action upon some but not all of the bacteria, and that
this power disappeared after the lapse of a certain time,
allowing the bacteria to develop ad libitum.

When the germicide to be studied is a gas, as in the
case of sulphurous acid or formaldehyd, a different
method must, of course, be adopted.

It may be sufficient simply to place a few test-tube cul-
tures of various bacteria, some with plugs in, some with
plugs out, in a closed room in which the gas is afterward
evolved. The germicidal action is shown by the failure
of the cultures to grow upon transplantation to fresh cul-
ture-media. This crude method may be supplemented
by an examination of the dust of the room. Pledgets
of sterile cotton are rubbed upon the floor, washboard,
or any dust-collecting surface present, and subsequently
dropped into culture-media. Failure of growth under
such circumstances is very certain evidence of good dis-
infection. These tests are, however, very severe, for in
the cultures there are immense numbers of bacteria in
the deeper portions of the bacterial mass upon which the
gas has no opportunity to act, and in the dust there are
many sporogenous organisms of extreme resisting power.
Failure to kill all the germs exposed in such manner is
no indication that the vapor cannot destroy all the ordi-
nary pathogenic organisms.

More refined is the method of saturating sterile sand
or fragments of blotting-paper or absorbent cotton with
cultures and exposing them, moist or dry, to the actior
of the gas. Such materials are best made ready in Petr:
dishes, which are opened immediately before and closec
immediately after the experiment. A piece of cotton 01
blotting-paper or a little sand transferred to fresh culture
media will not give any growth where the disinfection ha:
been thorough. By transplanting from different depths
the sand may be used incidently to show to what deptl
the gas is capable of penetrating.