the wire B is laid aside, and the gas is conducted into the chamber by the tube a', and escapes by the tube a.

Application of Electricity.-To study the effect of electricity we may prepare a drop-culture in the moist chamber (Fig. 41). The cover-glass to be used is provided with two strips of tinfoil,

which are isolated from the brass of the microscope, and so arranged that a current of electricity may be passed through them (Fig. 42).

A much simpler plan, which may also be employed, is to take an ordinary glass slide and coat the surface with gold-size. The slide is then pressed firmly down on gold-leaf or tinfoil and allowed to dry. When dry, the metal is scraped away, leaving

two triangles with a small interval between them, as in Fig. 43.

FIG. 43.-SLIDE WITH GOLD-LEAF ELECTROdes.

The liquid containing the micro-organisms is placed between the electrodes, covered with a coverglass, and then subjected to the electric current.

CHAPTER V.

EXAMINATION OF AIR, SOIL, AND WATER.

AIR.

THE air, as is well known, contains in suspension mineral, animal, and vegetable substances. The mineral world is represented by such substances as silica, silicate of aluminium, carbonate and phosphate of calcium, which may be raised from the soil by the wind, and particles of carbon, etc., which gain access from accidental sources. Belonging to the animal kingdom we find the débris of perished creatures, as well as sometimes living animals. The vegetable world supplies micrococci, bacilli, and other forms of the great family of bacteria, spores of other fungi, pollen seeds, parts of flowers, and so forth. The air of hospitals and sick rooms has been found to be especially rich in vegetable forms, e.g., fungi and spores have been observed as present in particularly large numbers in cholera wards, spores of tricophyton have been discovered in the air of hospitals for diseases of the skin, and achorioni n wards with cases of favus. The tubercle-bacillus is said to have been detected in the breath of patients suffering from phthisis.

These points indicate that, in addition to the interest for the microbiologist, considerable importance, from a hygienic point of view, must be attached to the systematic examination of the air. Especially a knowledge of the microbes which are found in the air of marshy and other unhealthy districts, and in the air of towns, dwellings, hospitals, workshops, factories, and mines, will be of practical value.

Miquel, who has particularly studied the bacteria in the air, has found that their number varies considerably. The average number per cubic metre of air for the autumn quarter at Montsouris is given at 142, winter quarter 49, spring quarter 85, and summer quarter 105. In air collected 2,000 to 4,000 metres above the sea-level, not a single bacterium or fungus spore was furnished, while in 10 cubic metres of air from the Rue de Rivoli (Paris) the number was computed at 55,000.

The simplest method for examining the organisms in air consists in exposing plates of glass or microscopic slides coated with glycerine, or a mixture of glycerine and glucose, which is stable, colourless, and transparent. Nutrient gelatine spread out on glass plates (p. 91), may be exposed to the air for a certain time, and then put aside in damp chambers for the colonies to develop. Sterilised potatoes, prepared in the usual way (p. 99), may be similarly exposed. In both the last-mentioned methods

* Miquel, Organismes Vivantes de l'Atmosphère.

separate colonies develop, which may be isolated as already described, and pure cultivations carried on in various other nutrient media (p. 97). Nutrient gelatine has also been employed in the special methods of Koch and Hesse.

Koch's Apparatus.-This consists of a glass jar, about six inches high, the neck of which is plugged with cotton-wool. In the interior is a shallow glass capsule, which can be removed by means of a brass lifter. The whole is sterilised by exposure to 150° C. for an hour in the hot-air steriliser. The nutrient gelatine in a stock-tube is liquefied, and the contents emptied into the glass capsule. The jar is exposed to the air to be examined for a definite time, the cotton-wool plug replaced, and the apparatus set aside for the colonies to develop.

Hesse's Apparatus (Fig. 44).-The advantage of this apparatus consists in that a known volume of air can be examined. A glass cylinder, 70 cm. long and 35 cm. in diameter, is closed at one end by an india-rubber cap, perforated in the centre. Over this fits another cap, which is not perforated. The opposite end of the cylinder is closed with a caoutchouc stopper, perforated to admit a glass tube plugged with cotton-wool. The tube can be connected by means of india-rubber tubing with an aspirating apparatus. This apparatus consists of a couple of litre-flasks, suspended by hooks from the tripod-stand which supports the whole apparatus.