bottle,
expanded by the heat, is driven out through the valvular aperture in the
solid portion of the stopper. On removing the bottle from the steam
chamber, the liquid contracts as it cools, and the pressure of the
external air drives the solid piece of rubber down into the neck of the
bottle, and forces together the lips of the slit (Fig. 102, b). Thus
sealed, the bottle will preserve its contents sterile for an indefinite
period without loss from evaporation.

TUBING NUTRIENT MEDIA.

After the final filtration, the nutrient medium is usually "tubed"--_i.
e._, filled into sterile tubes in definite measured quantities, usually
10 c.c. This process is sometimes carried out by means of a large
separator funnel fitted with a "three-way" tap which communicates with a
small graduated tube (capacity 20 c.c. and graduated in cubic
centimetres) attached to the side. The shape of this piece of apparatus,
known as Treskow's funnel, renders it particularly liable to damage. It
is better, therefore, to arrange a less expensive piece of apparatus
which will serve the purpose equally well (Fig. 103).

A Geissler's three-way stop-cock has the tube on one side of the tap
ground obliquely at its extremity, and the tube on the opposite side cut
off within 3 cm. of the tap. The short tube is connected by means of a
perforated rubber cork with a 10 cm. length of stout glass tubing (1.5
cm. bore). The third channel of the three-way tap is connected, by means
of rubber tubing, with the nozzle of an ordinary separator funnel.
Finally, the receiving cylinder above the three-way tap is graduated in
cubic centimetres up to 20, by pouring into it measured quantities of
water and marking the various levels on the outside with a writing
diamond.

Fluid media containing carbohydrates are filled into fermentation tubes
(_vide_ Fig. 21); or into ordinary media tubes which already have
smaller tubes, inverted, inside them (Fig. 104), to collect the products
of growth of gas-forming bacteria. When first filled, the small tubes
float on the surface of the medium after the first sterilisation nearly
all the air is replaced by the medium, and after the final sterilisation
the gas tubes will be submerged and completely filled with the medium.

[Illustration: FIG. 103.--Separatory funnel and three-way tap arranged
for tubing media.]

[Illustration: FIG. 104.--Gas tube (Durham).]

~Storing "Tubed" Media.~--Media after being tubed are best stored by