f easily, as it has become friable from contact with the
chromic acid.

5. Return all the cover-slips to the beaker, fill in _fresh_ chromic
acid solution, and treat as new cover-slips.

NOTE.--_Test-tubes, plates, capsules_, etc., which, from
long use, have become scratched and hazy, or which cannot be
cleaned in any other way, may be dealt with by immersing
them in an enamelled iron bath, containing water acidulated
to 1 per cent. with hydrofluoric acid, for ten minutes,
rinsing thoroughly in water, drying, and polishing.

PLUGGING TEST-TUBES AND FLASKS.

Before sterilisation all test-tubes and flasks must be carefully plugged
with cotton-wool, and for this purpose best absorbent cotton-wool
(preferably that put up in cylindrical one-pound packets and interleaved
with tissue paper--known as surgeons' wool) should be employed.

1. For a test-tube or a small flask, tear a strip of cotton-wool some 10
cm. long by 2 cm. wide from the roll.

2. Turn in the ends neatly and roll the strip of wool lightly between
the thumb and fingers of both hands to form a long cylinder.

3. Double this at the centre and introduce the now rounded end into the
open mouth of the tube or flask.

4. Now, whilst supporting the wool between the thumb and fingers of the
right hand, rotate the test-tube between those of the left, and
gradually screw the plug of wool into its mouth for a distance of about
2.5 cm., leaving about the same length of wool projecting.

[Illustration: FIG 24..--Plugging test-tubes: a, cylinder of wool
being rolled; b, cylinder of wool being doubled; c, cylinder of wool
being inserted in tube.]

The plug must be firm and fit the tube or flask fairly tightly,
sufficiently tightly in fact to bear the weight of the glass plus the
amount of medium the vessel is intended to contain, but not so tightly
as to prevent it from being easily removed by a screwing motion when
grasped between the fourth, or third and fourth, fingers, and the palm
of the hand.

For a large flask a similar but larger strip of wool must be taken; the
method of making and inserting the plug is identical.

III. METHODS OF STERILISATION.

STERILISING AGENTS.

Sterilisation--i. e., the removal or the destruction of germ life--may
be effected by the use of various agents. As applied to the practical
requirements of the bacteriological laboratory, many of these agents,
such as electricity, sunlight, etc.