as takes
place if they are removed from the muffle before it has
cooled down to the room temperature, may give rise to
microscopic cracks and flaws which will effectually destroy
their efficiency.

_Hot Air._--Hot air at 150 deg. C. destroys all bacteria, spores, etc:, in
about thirty minutes; a momentary exposure to a temperature of 175 deg. to
180 deg. C. will effect the same result and offers the more convenient
method of sterilisation. This method is only applicable to glass and
metallic substances, and the small bulk of cotton-wool comprised in the
test-tube plugs, etc. Large masses of fabric are not effectually
sterilised by dry heat--short of charring--as its power of penetration
is not great.

Sterilisation by hot air is effected in the hot-air oven (Fig. 18). This
is a rectangular, double-walled metal box, mounted on a stand and heated
from below by a large Bunsen burner. The interior of the oven is
provided with loose shelves upon which the articles to be sterilised are
arranged, either singly or packed in square wire baskets or crates, kept
specially for this purpose. One of the sides is hinged to form a door.
The central portion of the metal bottom, on which the Bunsen flame would
play, is cut away, and replaced by firebrick plates, which slide in
metal grooves and are easily replaced when broken or worn out. The top
of the oven is provided with a perforated ventilator slide and two
tubulures, the one for the reception of a centigrade thermometer
graduated to 200 deg. or 250 deg. C., the other for a thermo-regulator. An
ordinary mercurial thermo-regulator may be used but it is preferable to
employ a regulating capsule of the Hearson type (see p. 219) with a
spring arm adjusted to the lever so that when the boiling-point of the
capsule (e. g., 175 deg. C.) is reached the gas supply is absolutely cut
off and the jet cannot again be lighted until the spring-arm has been
readjusted by hand. The thermo-regulator is by no means a necessity, and
may be replaced by a large bore thermometer with a sliding platinum
point, connected with an electric bell, which can be easily adjusted to
ring at any given temperature. Even if the steriliser is provided with
the capsule regulator above described the contact thermometer should
also be fitted.

[Illustration: FIG. 26.--Hot-air oven.]

TO USE THE HOT-AIR OVEN.--

1. Place the crates of test-tubes, metal cases containing plates and
pipettes, loo