ed in a metal box,
or boxes, and heated to 130 deg. C. and retained at that temperature for
about thirty minutes.

~Moist Heat.~--_Water at 56 deg. C._--This temperature, if maintained for
thirty minutes, is sufficient to destroy the vegetative forms of
bacteria, but has practically no effect on spores. Its use is limited to
the sterilisation of such albuminous "fluid" media as would coagulate at
a higher temperature.

METHOD.--

1. Fit up a water-bath, heated by a Bunsen flame which is controlled by
a thermo-regulator, so that the temperature of the water remains at 56
deg. C.

2. Immerse the tubes or flasks containing the albuminous fluid in the
water-bath so that the upper level of such fluid is at least 2 cm. below
the level of the water. (The temperature of the bath will now fall
somewhat, but after a few minutes will again rise to 56 deg. C).

3. After thirty minutes' exposure to 56 deg. C, extinguish the gas, remove
the tubes or flasks from the bath, and subject them to the action of
running water so that their contents are rapidly cooled.

4. The vegetative forms of bacteria present in the liquid being killed,
stand it for twenty-four hours in a cool, dark place; at the end of that
time some at least of such spores as may be present will have germinated
and assumed the vegetative form.

5. Destroy these new vegetative forms by a similar exposure to 56 deg. C.
on the second day, whilst others, of slower germination, may be caught on
the third day, and so on.

6. In order to ensure thorough sterilisation, repeat the process on each
of six successive days.

This method of exposing liquids to a temperature of 56 deg. C. in a
water-bath for half an hour on each of six successive days is termed
_fractional sterilisation_.

_Water at 100 deg. C._ destroys the vegetative forms of bacteria almost
instantaneously, and spores in from five to fifteen minutes. This method
of sterilisation is applicable to the metal instruments, such as knives,
forceps, etc., used in animal experiments; syringes, rubber corks,
rubber and glass tubing, and other small apparatus, and is effected in
what is usually spoken of as the "water steriliser" (Fig. 27).

[Illustration: FIG. 27.--Water sterilizer.]

This is a rectangular copper box, 26 cm. long, 18 cm. wide, and 12 cm.
deep, mounted on legs, heated from below by a Bunsen or radial gas
burner, and containing a movable copper wire tray, 2 cm. smaller in
every dimension than