no PATHOGENIC BACTERIA.

a third time, lest a few spores remain alive and capable
of spoiling the material. When properly sterilized in
this way, culture-media will remain free from contamina-
tion until time and evaporation cause them to dry tip.

FIG. 14.—Pasteur-Chamberland filter arranged to filter under pressure.

If hermetically sealed, a sterile medium will keep indef-
initely.

If it should be necessary to sterilize culture-media at
once, not waiting the three days required by the inter-
mittent method, it may be done by superheated steam in