101.--Hot-water filter funnel and ring burner.]

Gelatine, when properly made, filters through this paper as quickly as
bouillon does through the Swedish filter paper, and does _not_ require
the use of the hot-water funnel.

Agar, likewise, if properly made, filters readily, although not at so
rapid a rate as gelatine. If badly "egged," and also during the winter
months, it is necessary to surround the glass funnel, in which the
filtration of the agar is carried on, by a hot-water jacket. This is
done by placing the glass funnel inside a double-walled copper
funnel--the space between the walls being filled with water at about
90 deg. C.--and supporting the latter on a ring gas burner fixed to a
retort stand (Fig. 101). The gas is lighted and the water jacket
maintained at a high temperature until filtration is completed. If the
steam steriliser of the laboratory is sufficiently large, it is sometimes
more convenient to place the flask and filtering funnel bodily inside,
close the steriliser and allow filtration to proceed in an atmosphere of
live steam, than to use the gas ring and hot-water funnel.

STORING MEDIA IN BULK.

After filtration fill the medium into sterile litre flasks with
cotton-wool plugs and sterilise in the steamer for twenty minutes on
each of three consecutive days. After the third sterilisation, and when
the flasks and contents are cool, cut off the top of the cotton-wool
plug square with the mouth of the flask; push the plug a short distance
down into the neck of the flask and fill in with melted paraffin wax to
the level of the mouth. When the wax has set the flasks are stored in a
cool dark cupboard for future use.

[Illustration: FIG. 102.--Rubber cap closing store bottle. a, before,
and b, after sterilizing.]

This plan is not absolutely satisfactory, although very generally
employed on occasion, and it is preferable to fill the medium into
long-necked flint glass bottles (the quart size, holding nearly 1000
c.c., such as those in which Pasteurised milk is retailed) and to close
the neck of the bottle by a special rubber cap.[3] This cap is made of
soft rubber, the lower part, dome-shaped with thin walls, being slipped
over the neck of the bottle (Fig. 102, a). The upper part is solid,
but with a sharp clean-cut (made with a cataract or tenotomy knife)
running completely through its axis from the centre of the disc to the
top of the dome. During sterilisation the air in the neck of the