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having first shown that when the mouths of flasks and
tubes are closed with plugs of sterile cotton no germs
can filter through. This observation has been of ines-
timable value to every bacteriologist.

Before sterilizing

FIG. 9.—Hot-air sterilizer.

flasks and, tubes we plug them with ordinary raw cotton,
and are sure that afterward their interiors will remain
free from the access of germs until opened. Instruments
may be sterilized wrapped in cotton, to be opened only
when ready for use; or instruments and rubber goods
sterilized by steam can subsequently be wrapped in
sterile cotton and kept for use. It is of the utmost
importance to carefully protect every sterilized object,
and to allow as little dust to collect upon it as possible,
in order that the object of the sterilization be not de-
feated. As the spores of moulds falling upon cotton
sometimes grow and allow their mycelia to work their
way through and drop into a culture-medium, Roux