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has introduced little paper caps with which the cotton
stoppers are protected from the dust. These are easily
made by curling a small square of paper into a " cornu-
copia, '' fastening by turning up the edge or putting in a pin.
The paper is placed over the stopper before the sterilization,
after which no contamination of the cotton can occur.

Sterilization of Culture-media.—As almost all of the
culture-media contain about 80 per cent of water, which
would be evaporated in the hot-air closet, so that the
material would be destroyed, hot-air sterilization is not
appropriate for them. Sterilization by streaming steam
is the best and surest method. The prepared media are

placed in flasks or tubes care-
fully plugged with cotton and
previously sterilized with dry
heat, and then sterilized in what
is known as Koch's steam appa-
ratus (Fig. 10) or in Arnold's

FIG. 10.—Koch's steam sterilizer.

FIG. ii.—Arnold's steam sterilizer.

steam sterilizer (Fig. n), which is more convenient and
more generally useful.

The temperature of boiling water,  100° C., does not