candle from the mouth of the flask, flame the
mouth, and plug the neck with sterile cotton-wool.

9. Disinfect the filter candle and separatory funnel by boiling.

If it is found necessary to employ pressure in addition to or in place
of suction, insert a perforated rubber stopper into the mouth of the
separatory funnel and secure in position with copper wire; next fit a
piece of glass tubing through the stopper, and connect the external
orifice with an air-pressure pump of some kind (an ordinary foot pump
such as is employed for inflating bicycle tyres is one of the most
generally useful, for this purpose) or with a cylinder of compressed air
or other gas.

In order to filter a large bulk of fluid very rapidly it is necessary to
use a higher pressure than glass would stand, and in these cases the
metal receptacle designed by Pakes (Fig. 38, a), to hold the filter
candle itself as well as the fluid to be filtered, should be employed.
(A vacuum must also be maintained in the filter flask, by means of an
exhaust pump, during the entire process.)

This piece of apparatus consists of a brass cylinder, capacity 2500
c.c., with two shoulders; and an opening in the neck at each end,
provided with screw threads.

A nut carrying a pressure gauge fits into the top screw; and into the
bottom is fitted a brass cylinder carrying the filter candle and
prolonged downwards into a delivery tube. Leakage is prevented by means
of rubber washers.

Into the top shoulder a tube is inserted, bent at right angles and
provided with a tap. All the brass-work is tinned inside (Fig. 38, a).
In use the reservoir is generally mounted on a tripod stand.

~To Sterilise.~--

1. Insert the filter candle into its cylinder and screw this loosely on.

[Illustration: FIG. 38.--Pakes' filtering reservoir--pressure and
aspiration.]

2. Wrap a layer of cotton-wool around the delivery tube and fasten in
position.

3. Remove the nut carrying the pressure gauge and plug the neck with
cotton-wool.

4. Heat the whole apparatus in the autoclave at 120 deg. C. for twenty
minutes.

METHOD.--

1. Remove the apparatus from the autoclave, and allow it to cool.

2. Screw home the box carrying the bougie.

3. Set the apparatus up in position, with its delivery tube (from which
the cotton-wool wrapping has been removed) passing through a perforated
rubber stopper in the neck of a filter flask.

[Illustration: FIG. 39.--Closed candle arranged for filte