142	PATHOGENIC BACTERIA.
covering the ice-water of the levelling apparatus. The
plug of cotton closing the mouth of tube No. i is re-
moved, and to prevent contamination during the outflow
of the gelatin the mouth of the tube is held in the flame
of a Bunsen burner for a moment or two. The gelatin
is then cautiously poured out upon the plate, the mouth
of the tube, as well as the plate, being covered by tlie
bell-glass to prevent contamination by germs in the air.
The apparatus being level, the gelatin spreads out in an
even, thin layer, and, the plate being cold from the ice

beneath, it immediately solidi-
fies, and in a few moments can
be removed to the moist cliam-
_.    .     .	ber prepared to receive it.    As
fig. 25.—Glass bench.	i     i	^
soon as plate No. i is prepared,
the contents of tube No. 2 are poured upon plate No. 2,
allowed to spread out and solidify, and then superimposed
on plate No. i in the moist chamber, being separated from
the plate already in the chamber by small glass benches
(Fig. 25) made for the purpose and sterilized. After the
contents of all the tubes are thus distributed, the moist
chamber and its contents are allowed to stand for some
hours, to permit the bacteria to grow. Where each or-
ganism falls a colony develops, and the success of the
whole method depends upon the isolation of a colony
and its transfer to a tube of culture-medium where it
can grow unmixed and undisturbed.
The description must have made evident the fact that
only such culture-media can be used for plate-ciiltures as
can be melted and solidified at will—viz. gelatin, agar-
agar, and glycerin agar-agar. Blood-serum and L6ffler\s
mixture are entirely inappropriate.
The great drawback to this excellent method is tlie
cumbersome apparatus required and the comparative im-
possibility of making plate-cultures, as is often desirable,
in the clinic, at the bedside, or elsewhere than in the
laboratory. The method therefore soon underwent mod-
ifications, the most important being