THE FILTRATION OF MEDIA.

Fluid media are usually filtered through stout Swedish filter paper (occasionally through a porcelain filter candle), and in order to accelerate the rate of filtration the filter paper should be folded in that form which is known as the "physiological filter," not in the ordinary "quadrant" shape, as by this means a large surface is available for filtration and a smaller area in contact with the glass funnel supporting it.

To fold the filter proceed thus:

1. Take a circular piece of filter paper and fold it exactly through its centre to form a semicircle.

2. Fold the semicircle exactly in half to form a quadrant; make the crease distinct by running the thumbnail along it, then open the filter out to a semicircle again.

3. Fold each end of the semicircle in to the centre and so form another quadrant; smooth down the two new creases thus formed and again open out to a semicircle.

4. The semicircle now appears as in figure 82, a, the dark lines indicating the creases already formed.

5. Fold the point 1 over to the point 3, and 1a to 3a, to form the creases 4 and 4a, indicated in the diagram by the light lines. Fold point 1 over to 3, and Ia to 3a, to form the creases 5 and 5a.

6. Thus far the creases have all been made on the same side of the paper. Now subdivide each of the eight sectors by a crease through its centre on the opposite side of the paper, indicated by the broken line in the diagram. Fold up the filter gradually as each crease is made, and when finished the filter has assumed the shape of a wedge, as in figure 82, b.

When opened out the filter assumes the shape represented in figure 82, c.

The folded filter is next placed inside a glass funnel

supported on a retort stand and moistened with hot distilled water before the filtration of the medium is commenced.

Solid media are filtered through a specially made filter paper,-"papier Chardin," which is sold in boxes of twenty-five ready-folded filters.

Fig. 82.-Filter folding: a, Filter folded in half, showing creases; b, appearance of filter on completion of folding; c, filter opened out ready for

use.

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 hotwater 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

Fig. 83.-Ring burner and hot-water filter.

filtration of the agar is

carried on, by a hot

water jacket. This is done by placing the glass funnel inside a doublewalled copper funnelthe space between the walls being filled with water at about 90° C.and supporting the latter on a ring gas burner fixed to a retort stand

(Fig. 83). The gas is lighted and the water jacket maintained at a high temperature until filtration is completed.

TUBING NUTRIENT MEDIA.

After the final filtration, the nutrient medium is "tubed"-i. e., filled into sterile tubes in definite measured quantities, usually 10 c.c.; or "flasked"-i. e., filled into sterile flasks in fairly large quantities. 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. 84).

A Geissler's three-way stop-cock has the tube on one side of the tap ground obliquely at its extremity,

the tube on the opposite side cut off within 3 c.c. of the tap. The short tube is connected by means of a perforated rubber cork with a 10 c.c. length of stout glass tubing (1.5 c.c. bore). The third channel of the threeway 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

Fig. 84.-Three-way tap, home-made.

Fig. 85.-Gas tube (Durham).

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. 14); or into ordinary media tubes which already have smaller tubes, inverted, inside them (Fig. 85), 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.

Storing "Tubed" Media.-Media after being tubed are best stored by packing, in the vertical position, in oblong boxes having an internal measurement of 37 cm. long by 12 cm. wide by 10 cm. deep. Each box (Fig. 86) has a movable partition formed by the vertical face of a weighted triangular block of wood, sliding free on the bottom (Fig. 86, A); or by a flat

Fig. 86.-Medium box, showing alternative partitions A and B.

piece of wood sliding in a metal groove in the bottom of the box, which can be fixed at any spot by tightening the thumbscrew of a brass guide rod which transfixes the partition (Fig. 86, B). The front of the box is provided with a handle and a celluloid label for the name of the contained medium. These boxes are arranged upon shelves in a dark cupboard, or preferably an iron safe, which should be rendered as nearly air-tight as possible, and should have the words "media stores" painted on its doors.