fill the bulb with dry
cotton-wool (Fig. 32), wrap a layer of cotton-wool around each end of
the tube, and secure in position with a turn of thin copper wire or
string; then sterilise the piece of apparatus in the hot-air oven.

2. Prepare the cultivation in a Ruffer or Woodhead flask (Fig. 33) the
inlet tube of which has its free extremity enveloped in a layer of
cotton-wool, secured by thread or wire, whilst the exit tube is plugged
in the usual manner.

[Illustration: FIG. 33.--Ruffer's flask.]

3. Sterilise a short length of rubber tubing by boiling. Transfer it
from the boiling water to a beaker of absolute alcohol.

4. When all is ready remove the rubber tube from the alcohol by means of
a pair of forceps, drain it thoroughly, and pass through the flame of a
Bunsen burner to burn off the last traces of alcohol.

5. Remove the cotton-wool wraps from the entry tube of the flask and
from one end of the filter tube and rapidly couple them up by means of
the sterile rubber tubing.

6. Connect the other end of the bulb tube with the delivery tube from
the gas reservoir.

The gas in its passage through the dry sterile cotton-wool in the bulb
of the filter tube will be freed from any contained micro-organisms and
will enter the flask in a sterile condition.

(b) _Porcelain Filter._--The sterilisation of liquids by filtration is
effected by passing them through a cylindrical vessel, closed at one end
like a test-tube, and made either of porous "biscuit" porcelain,
hard-burnt and unglazed (Chamberland system), or of Kieselguhr, a fine
diatomaceous earth (Berkefeld system), and termed a "bougie" or "candle"
(Fig. 34).

NOTE.--In selecting candles for use in the laboratory avoid
those with metal fittings, since during sterilisation cracks
develop at the junction of the metal and the siliceous
material owing to the unequal expansion.

In this method the bacteria are retained in the pores of the filter
while the liquid passes through in a germ-free condition.

It is obvious that to be effective the pores of the filter must be
extremely minute, and therefore the rate of filtration will usually be
slow. Chamberland filter candles possess finer channels than Berkefeld
candles and consequently filter much more slowly. To overcome this
disadvantage, either aspiration or pressure, or a combination of these
two forces, may be employed to hasten the process.

Doultons white porcelain filters it may b