2. Tube, or flask and sterilise as for Pasteur's solution.

2. Tube or flask; sterilise as for Pasteur's solution.

Asparagin Medium (Uschinsky).— 1. Weigh out and mix

2. Dissolve the mixture in distilled water, 1000 c.c.

3. Add glycerine, 40 c.c.

4. Tube, and sterilise by the discontinuous method (three days).

Asparagin Medium (Frankel and Voges).—

1. Weigh out and mix

2. Tube, and sterilise by the discontinuous method (three days).

NOTE.-Either of the above asparagin media, after the addition of 10 per cent. gelatine or 1.5 per cent. agar, may be advantageously employed in the solid condition.

2. Fill into flasks, in quantities of 20 c.c., and add to each a small quantity of freshly washed magnesium carbonate.

3. Sterilise in the steamer at 100° C. for twenty minutes on each of three consecutive days.

4. Add to each flask containing 20 c.c. solution, 2 c.c. of a sterile 2 per cent. solution of ammonium sulphate. 5. Incubate at 37° C. for forty-eight hours and eliminate any contaminated culture flasks.

Winogradsky's Solution (for Nitrous Organisms).1. Weigh out and mix

2. Add magnesium carbonate, previously sterilised by boiling.

3. Fill into flasks and sterilise as for previous solution. Silicate Jelly (Winogradsky).—

1. Weigh out and mix.

Ammonium sulphate

Magnesium sulphate

Calcium chloride.

and dissolve in

Distilled water.

4. Pour the silicic acid solution into a large porcelain basin.

5. Mix equal quantities of the solutions A and B; then add successive small quantities of the mixed salts to the silicic acid solution, stirring continuously with a glass rod, until a jelly of sufficiently firm consistence has been formed. 6. Spread a layer of this jelly over the bottom of each of several large capsules or "plates."

7. Sterilise in the steamer for twenty minutes on each of three consecutive days.

Plaster-of-Paris Discs.—

1. Take large corks, 2.5 cm. diameter, and roll a piece of stiff note-paper round each, so that about a centimeter projects as a ridge above the upper sur

face of the cork, and secure in position with a pin (Fig. 93).

2. Mix plaster-of-Paris into a stiff paste with distilled water, and fill each of the cork moulds with the paste.

3. When the plaster has set, remove the paper from the corks, and lift up the plaster discs.

4. Place the plaster discs on a piece of asbestos board and sterilise by exposing in the hot-air oven to 150° C. for half an hour.

Fig. 93.-Cork and paper mould for plas

ter-of-Paris disc.

5. Remove the sterile discs from the oven by means of sterile forceps, place each inside a sterile capsule, and moisten with a little sterile water.

6. Sterilise in the steamer at 100° C. for twenty minutes on each of three consecutive days.

XI. INCUBATORS.

AN incubator consists essentially of a chamber for the reception of cultivations, etc., surrounded by a water jacket, the walls of which are of metal, usually copper, and outside all an asbestos or felt jacket, or

wooden casing. The water in the jacket is heated by gas and maintained at some constant temperature by a thermo-regulator.

Two incubators at least are required for the cultivation of bacteria in the laboratory, the one regulated

to maintain a temperature of 37° C., and known as the "hot" incubator; the other, 22° C., and known as the "cool" or "cold" incubator.

Thermo-regulators. The thermo-regulator is the most essential portion of the incubator, as upon its efficient working depends the maintenance of a constant temperature in the cultivation chamber. It is also used in the fitting up of water and paraffin baths, and for many other purposes.

Of the many forms and varieties of thermo-regulator (other than electrical), two only are of sufficiently general use to need mention. In one of these the flow of gas to the gas-jet is controlled by the expansion or contraction of mercury within a glass bulb; in the other, by alterations in the position of the walls of a hollow metallic capsule. They are:

B

...C

D

Fig. 95.-Reichert's thermo-regulator.

(a) Reichert's (Fig. 95), which consists of a bulb containing mercury. Gas enters at A, and passes out to the jet by B. As the temperature rises the mercury expands and cuts off the main gas supply, thereby reducing the temperature. As the temperature falls the mercury contracts and reopens the narrow tube C. By means of a thumbscrew D (which mechanically raises or lowers the column of mercury irrespective of the temperature) and the aid of a thermometer the apparatus can be set to keep the incubator at any desired temperature. With this form a special gas burner is required, with separate supply of gas to a pilot jet at the side.

(b) Capsule regulator consists of a metal capsule filled with a liquid which boils at the required temperature, and hermetically sealed; this is adjusted in the interior of the incubator. Soldered to the upper side of the