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within their own plasm or micro-protein. The material used in “ vaccination” for typhoid fever consists of such intra-cellular poison of the typhoid bacilli.

The inter-cellular and more soluble toxines appear nearly related to the digestive enzymes of animal glands, such as trypsine and pepsine in their method of action, and it is extremely probable that the nerve degeneration of diphtheria and the solution of fibrin by digestive ferment proceed along exactly comparable lines. Under such an hypothesis it is easy to understand why a small continued dosage of a given bacterial poison will produce such profound effect, and how it comes about that such minute quantities are relatively so potent. Probably the change is the same process of hydration that we have seen occurs in the carbohydrate transformation, and that when the molecule has become enlarged by the addition of water to a given extent it breaks up along new planes of cleavage.

It is of course possible that the true toxic bodies are definite chemical compounds which are precipitated along with the albumoses in the alcohol method adopted. So far, however, all attempts to obtain definite crystalline bodies have failed, and all we are able to state is that the toxine, whatever it may be, is found in the precipitate thrown down by alcohol from cultivations of bacteriaforming toxines, and that the precipitate thus found certainly contains albumoses.

For further information on toxines see chapter on immunity.

CHAPTER III.

Sterilization and Disinfection.

BACTERIA are the most widely distributed of living things; they teem in the dust of cities, in hospital wards, they are to be found in countless numbers in the soil, in the air we breathe, in common articles of food, in water, and particularly in the dusty air of streets and living rooms.

The air of high mountains and mid-ocean are generally practically free from organisms, whereas city air may contain as many as 100,000 or more per cubic foot. They have recently been found in glacier ice.

The organisms present in air are by no means all pathogenic, but at the same time many pathogenic bacteria are frequently present; amongst them the pyogenic cocci are common. The source of the organisms in the air is for the most part dust, and where dust contains the dried expectoration of tuberculous persons the tubercle bacillus is invariably present. During damp and wet weather the number of organisms present diminishes considerably, the falling rain freeing the air from suspended matter and bacteria, which are carried away with the surface water in properly drained places, or remain in the mud of pools to be wafted into the air as dust when the water evaporates.

Bacteria of the air are, for the most part, simple' saprophytes, and although not disease-producers in the ordinary way are capable of setting up profound changes in organic fluid exposed to their advent, producing “disease ” in such articles as milk, meat, &c. Many of the spores of the higher fungi are air-borne as well as yeasts and torula. A gelatin plate exposed to the air for a few moments will generally develop a number of colonies when incubated. I have already referred to the minute size of these micro-organisms, and it is not difficult to understand that almost anything with which we commonly have to deal in bacteriological work is contaminated

with numbers of unseen organisms ready to develop the moment we make the conditions favourable for them. And what is true of bacteriological apparatus is still more true of dental and other instruments, for with these latter not only are air-borne organisms present, but also those from infected wounds, oral secretions and decaying dentine, septic pulps, &c., with which they have been in contact.

Owing to their minute size bacteria are carried about by the slightest currents and motion of the air, but in still air they gradually sink to the lower strata. Tyndall proved that when the dust in a specially constructed room had been allowed to settle till the polariscope showed no trace of suspended matter, sterile open vessels of nutrient solution could be freely exposed without decomposition taking place as long as the dust remained quiescent. When, however, the dust was again made to rise the fluids quickly became putrescent.

It is evident that any inaterials with which we wish to conduct bacteriological experiments must be first of all freed from the organisms naturally present, otherwise we shall be unable to determine if the particular fermentation or growth we are examining is the product of a single species or of a mixture of species, or in mycological parlance know if we are dealing with a pure culture.

Pure cultures, consisting of members of one given species only, are the means by which determinative bacteriology has been rendered possible. Although much of the earlier work was conducted with what are now known to have been mixtures, it must not be supposed that the combined action or symbiosis of bacteria is to be disregarded, many of the most interesting of natural fermentations belonging to symbiotic phenomena.

But to properly study the combined activity of two or more bacteria we must first have pure cultures of each from which to make our mixture. A considerable portion of bacteriological technique is directed towards obtaining pure cultures, and the process of excluding adventitious organisms is termed sterilization. Heat in some form is commonly used, of such a temperature that the articles sterilized are not injured while the organisms present are killed. The difference in resisting power has been already noted, and the reader is advised to take particular note of the “ resisting power” of various organisms, as it has large practical bearing upon the question of sterilization.

Heat is applied in two forms: (1) Dry heat ; (2) Moist heat.

(1) Sterilization by Hot Air.—The various pieces of apparatus used in bacteriological work, such as flasks, test tubes, Petri dishes, and the like, are sterilized by heating to 150° C. for three-quarters

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Fig. 4.-HOT-AIR STERILIZER WITH APPARATUS READY FOR STERILIZATION.

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Fig. 5.-COPPER Box AND RACK FOR STERILIZING PETRI DISHES AND CAPSULES.

of an hour in a hot-air sterilizer; the flasks, &c., are first plugged with cotton wool plugs, which so long as they remain dry prevent the passage of bacteria. The hot-air sterilizer (fig. 4) consists of a copper or sheet iron box with hollow walls and a fire-brick bottom, placed upon a stand to admit of a large gas burner underneath. There is a hinged door opening the whole width of the sterilizer. In the roof are two tubes communicating with the inner chamber, through which a thermometer is placed to register the temperature of the interior.

Petri dishes, capsules (figs. 6 and 7) small Petri dishes, 5 cm. wide), pipettes, &c., are placed in special copper boxes with a central movable rack from which the plates may be lifted out when required (fig. 5).

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Test tubes, flasks, &c., are first plugged with cotton-wool. A piece of wool is folded up and twisted into a firm plug and forced into the mouth of the tube, about a third left projecting. The testtubes are placed in wire crates which fit into the sterilizer.

A convenient addition is a “ contact alarm,” so arranged that a bell rings when the temperature reaches the point required.

The temperature is allowed to rise slowly to 170° C., when the gas is turned out and the apparatus allowed to cool down. The door must not be opened till the temperature has fallen to 60° C.

The temperature here suggested is that which is found to destroy spores, the vegetative forms succumbing at a much lower temperature (68° C.). For the various liquid and solid media used 170° C. is too high, and would evaporate and char the tube contents. Streaming steam in the steam sterilizer is therefore used.

(2) Sterilization by Streaming Steam.- Although the spores of most bacteria resist the application of 100° C. for a considerable

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