o replace the air in the sterilising compartment, as shown by the steam
issuing in a steady, continuous stream from the tubulure in the lid.

3. Remove the lid, quickly lower the wire basket containing media tubes,
etc., into the sterilising compartment until it rests on the diaphragm,
and replace the lid.

4. After an interval of twenty minutes in the case of fluid media, or
thirty minutes in the case of solid media, take off the lid and remove
the basket with its contents.

5. Now, but not before, extinguish the gas.

NOTE.--After removing tubes, flasks, etc., from the steam
steriliser, they should be at once separated freely in order
to prevent moisture condensing upon the cotton-wool plugs
and soaking through into the interior of the tubes.

This treatment will destroy any vegetative forms of bacteria; during the
hours of cooling any spores present will germinate, and the young
organisms will be destroyed by repeating the process twenty-four hours
later; a third sterilisation after a similar interval makes assurance
doubly sure.

The method of sterilising by exposure to streaming steam at 100 deg. C.
for twenty minutes on each of three consecutive days is termed
_discontinuous_ or _intermittent sterilisation_.

Exposure to steam at 100 deg. C. for a period of one or two hours, or
_continuous sterilisation_, cannot always be depended upon and is
therefore not to be recommended.

_Superheated steam_--i. e., steam under pressure (see
Pressure-temperature table, Appendix, page 500) in sealed vessels at a
temperature of 115 deg. C.--will destroy both the vegetative and the
sporing forms of bacteria within fifteen minutes; if the pressure is
increased, and the temperature raised to 120 deg. C., the same end is
attained in ten minutes. This method was formerly employed for the
sterilisation of media (and indeed is so used in some laboratories
still), but most workers now realise that media subjected to this high
temperature undergo hydrolytic changes which render them unsuitable for
the cultivation of the more delicate micro-organisms. The use of
superheated steam should be restricted almost entirely to the
disinfection of such contaminated articles, old cultivations, etc., as
cannot be dealt with by dry heat or the actual furnace. Sterilisation
by means of superheated steam is carried out in a special
boiler--Chamberland's autoclave (Fig. 30). The autoclave consists of a
stout copper cylinder