grasped between the fourth, or third and fourth fingers, and the palm of the hand.

For a large flask a similar but larger strip of wool must be taken; the method of making and inserting the plug is identical

ПI. METHODS OF STERILISATION.

STERILISING AGENTS.

STERILISATION-i. e., the removal or the destruction of germ life—may be effected by the aid of various agents. As applied to the requirements of the bacteriological laboratory, many of these agents, such as electricity, sunlight, etc., are practically useless; whilst others are limited in their applications, or are so well suited to particular purposes as to be almost entirely restricted to such.

The sterilising agents in common use are:

Chemical Reagents.-Disinfectants (for the disinfection of glass and metal apparatus and of morbid tissues).

1. Naked flame (for the sterilisation of platinum needles, etc.).

2. Muffle furnace (for the sterilisation of filter candles, and for the destruction of morbid tissues).

3. Hot air (for the sterilisation of all glassware and of metallic substances).

(b) Moist Heat:

1. Water at 56° C. (for the sterilisation of certain albuminous fluids).

2. Water at 100° C. (for the sterilisation of surgical instruments, rubber tubing, and stoppers, etc.).

3. Streaming steam at 100° C. (for the sterilisation of media).

4. Superheated steam at 115° C. or 120° C. (for the disinfection of contaminated articles and old cultivations of bacteria).

Filters.

1. Cotton-wool filters (for the sterilisation of air and gases).

2. Porcelain filters (for the sterilisation of various liquids).

METHODS OF APPLICATION.

Chemical Reagents, such as belong to the class known as antiseptics (i. e., substances which inhibit the growth of, but do not destroy, bacterial life), are obviously useless. Disinfectants or germicides (i. e., substances which destroy bacterial life), on the other hand, are of value in the disinfection of morbid material, and also of various pieces of apparatus, such as pipettes, pending their cleansing and complete sterilisation by other processes. To this class belong:

1. Lysol, 2 per cent. solution;

2. Perchloride of mercury, o.1 per cent. solution; 3. Carbolic acid, 5 per cent. solution;

4. Absolute alcohol;

5. Ether;

6. Chloroform;

7. Volatile oils, such as oil of mustard, oil of garlic, arranged in order of general utility. Formaldehyde is perhaps a more powerful germicide than any of the above, but its penetrating vapor restricts its use. These disinfectants are but little used in the final sterilisation of apparatus, chiefly on account of the difficulty of completely removing such substances, for even traces of these chemicals are sufficient to so inhibit or alter the growth of bacteria brought into contact with them, as to vitiate subsequent experiments conducted by the aid of apparatus sterilised in this manner.

NOTE.-Tubes, flasks, filter flasks, pipettes, glass tubing, etc., may be rapidly sterilised, in case of emergency, by washing, in turn, with distilled water, perchloride of mercury solution, alcohol, and ether, drain

ing, and finally gently heating over a gas flame to completely drive off the ether vapor. Chloroform or volatile oils may be added to various fluids in order to effect the destruction of contained bacteria, and when this has been done, may be completely driven off from the fluid by the application of gentle heat.

Dry Heat. The naked flame of the Bunsen burner is used for sterilising the platinum needles, the points of forceps, or other small instruments, cover-glasses, etc., a very short exposure to this heat being sufficient to ensure sterilisation. Muffle Furnace (Fig.

17). This form of heat is chiefly used for the destruction of the dead bodies of small infected animals, morbid tissues, etc., but is also employed for the sterilisation of porcelain filter candles.

Filter candles are disinfected immediately after use by boiling in a beaker of water for some

twenty minutes.

fifteen or

This treat

ment, however, leaves the dead bodies of the bacteria

upon the surface and blocking the interstices of the filter.

To destroy the organic matter and prepare the filter candle for further use proceed as follows:

1. Roll each bougie up in a piece of asbestos cloth, secure the ends with a few turns of copper wire, and place inside the muffle (a small muffle 76 by 88 by 163 mm. will hold perhaps four small Berkfeld candles).

2. Light the gas and raise the contents of the muffle to a white heat; maintain this temperature for five minutes.

3. Extinguish the gas, and when the muffle and its

contents have become quite cold, remove the filter candles from the interior of the muffle, and store without removing the asbestos wrappings, in sterile metal boxes.

Hot Air.-Hot air at 150° C. destroys all bacteria, spores, etc., in about thirty minutes, whilst a few minutes' exposure to a temperature of 170° to 180° C. will effect the same result. This method is only applicable to glass and metallic substances, and the small bulk of cotton-wool comprised in the test-tube plugs, etc. Large masses of

fabric are not effectually sterilised by dry

heat, as its power of penetration is not great.

Sterilisation by hot air is effected in the

hot-air oven (Fig. 18). This is a rectangular, double-walled

metal

box, mounted on a stand and heated from below by a large Bunsen burner. One of

Fig. 18.-Hot-air oven.

the sides is hinged to form a door. The central portion of the metal bottom, on which the Bunsen flame would play, is cut away, and replaced by firebrick plates, which slide in metal grooves and are easily replaced when broken or worn out. The top of the oven is provided with a perforated ventilator slide and two tubulures, the one for the reception of a centigrade thermometer graduated to 200° or 250° C., the other for a thermoregulator. The thermo-regulator is by no means a necessity, and it is more convenient to replace it by a