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Summary.—The food cycle is now complete. Starting from the soil, we have followed the transference of matter successively through plants, animals, bacteria, and back again to the soil. It is interesting to note the remarkable division of labour among soil bacteria, by which complex organic compounds are first broken up and then elaborated into nitrates. The loss of nitrogen which occurs during this process is prevented by the specialisation of function of certain bacteria. These organisms bring the errant nitrogen back to the soil, and thereby tend to balance the input and output of the soil food.

And so the food-stuffs ceaselessly move on in this eternal cycle. And as long as the sun yields the necessary energy there is no reason why life should ever come to a standstill. But, as we have just seen, the continuation of food supply is largely dependent upon these ubiquitous bacteria. These “subtle artisans of nature are constantly at work breaking up and rebuilding materials, and thus form the last link in the cyclical course of matter.

Nor does their importance end here, for they are of immense benefit to us in several other ways. And first let us look at the very disposition of soil bacteria. In the upper layers of the earth they are constantly oxidising off the organic matter and keeping the soil in a pure state. Lower down are anærobic organisms, which still further disintegrate the products filtered from above. Still lower down (at about twelve feet from the surface) no organisms are present, because the altered organic matter contains no nutriment for their growth.

The importance of these facts in relation to water supply is self-evident. This explains why superficial wells are generally impure, and deep wells pure. The sanitary engineer brings these organisms into his service,

and utilises them for the purification of our water supplies. He makes them grow on the surface of sand filters, so that the latter become not only mechanical, but biological filters as well. The bacterial purification of sewage is also brought about by the action of putrefactive bacteria.

Soil bacteria, however, play a more useful part in agriculture. From a study of their life-history we can deduce certain facts which are well worth the study of the scientific agriculturist. Since nitrification needs a great supply of oxygen, it is important that the soil should be well stirred, so as to admit air. Further, by the proper alternation of leguminous plants (e.g., peas), with those which cannot fix nitrogen but simply use it (e.g., wheat), it is possible to maintain the fertility of the soil, so far as it concerns nitrogen. Again, the advance of agricultural bacteriology has rendered it possible to replace artificial manures by the use of pure cultures of nitrogen-fixing bacilli. These, when introduced into the soil with the seed, are found to increase the produce in some cases.

But the results, on the whole, are not satisfactory. This is probably due to the fact that “ nitragin(under which name cultures of nodule bacteria are sold in the market) contains only one variety of the organism, which is not equally useful for every species of legumes. Perhaps a better method would be the inoculation of a barren soil with leguminous earth, i.e., a soil in which legumes flourish, and in which, therefore, many forms of nitrogen-fixing bacteria would be present. The subject, however, requires further elucidation.

CHAPTER VII.

ANTISEPTICS AND DISINFECTANTS.

It often becomes desirable to get rid of bacteria from certain media. A simple removal often suffices; as when air is passed through cotton wool, or when water is filtered through a porcelain filter. But in most cases this is not possible, and the destruction of the organisms becomes imperative. Fire is the best of germicides, but owing to its obvious disadvantage recourse is often had to various chemical agents. Those substances which retard the development of organisms without actually destroying them are called antiseptics. The complete destruction of their vitality is brought about by disinfectants or germicides. It, therefore, follows that all disinfectants are also antiseptics; but all antiseptics are not germicides. The division between antiseptics and disinfectants, although convenient, is purely arbitrary. The same substance in different degrees of concentration may act now as disinfectant, now as antiseptic. Thus, 1:200 of carbolic acid kills ; and 1:400 of the same acid restrains the bacillus of typhoid fever.

The method of determining the activity of a germicidal agent is comparatively simple. Organisms possessing a high degree of resistance (preferably anthrax spores) are first dried on sterile silk threads, and then introduced into

1 It must be pointed out, however, that the term “antiseptic" is frequently used in the sense of “ germicide".

a solution of the agent for varying lengths of time. By subsequent inoculations of nutrient media, it is noted whether the organisms have been destroyed or not.

It must be clearly understood, however, that the germicidal powers of an agent vary with the number and resistance of bacteria, the nature of the associated material, and

many other conditions. Therefore the statement that a certain substance is disinfectant in a certain proportion is valueless, unless we are informed of the conditions under which its germicidal powers have been determined.

It would obviously be impossible to detail the numerous substances which have been employed as bactericidal agents, but the following are those most commonly used in practical disinfection :

Acids.—Bacteria are more susceptible to the action of mineral acids, than to that of vegetable acids. It is for this reason that cholera vibrios, for example, are more readily destroyed by the hydrochloric acid in the stomach, than by the acids of fruits.

Potassium permanganate, in order to be of any value, must be used in sufficient quantity to destroy both the infective agent and its medium. According to Koch, as much as 5 per cent of the salt is required to destroy anthrax spores in one day.

Ferrous sulphate is an antiseptic of moderate powers, largely used for the disinfection of privies.

Sulphur dioxide is extensively used for the disinfection of houses, but can hardly be called efficient. It is more effective in association with moisture, than in the anhydrous state. Unfortunately it bleaches vegetable colours, attacks iron, and injures cloth and leather.

Formaldehyde possesses none of the drawbacks of sulphur dioxide, and, being more potent in its action, has largely replaced this gas as a practical disinfectant. It

can be used either as a vapour, for which there are numerous forms of apparatus on the market, or as a solution. A 40 per cent. solution of formaldehyde is called “formalin. Formaldehyde is fatal to most bacteria in solutions of 1:1,000; and can be safely used for the disinfection of rooms.

Mercuric chloride, in the proportion of 1:1,000, may be relied upon to kill spores, provided, of course, no albumin is present. Plague bacilli are killed by two minutes' exposure to 1:3,000. Corrosive sublimate forms insoluble compounds with albuminoids, and then loses its bactericidal powers. This reaction is prevented by adding some sodium chloride, which forms, with the sublimate, a double salt soluble in water.

Zinc mercuric cyanide is one of the most powerful germicides known. A solution of 1:40,000 is sufficient to destroy the vitality of anthrax bacilli.

Carbolic acid, in a 5 per cent. solution, is recommended by Koch for the disinfection of cholera stools. Pus cocci are destroyed by a solution of 1:125 after two hours' exposure. Oily solutions of carbolic acid are useless, because the oil cannot penetrate into the organisms. On the other hand, the addition of hydrochloric acid decidedly increases the germicidal powers of carbolic acid.

In crude carbolic acid, besides phenol, cresol is present in large quantities, and is said to have greater disinfecting powers than phenol. Cresols, however, are not soluble in water, and various devices have been employed to secure their presence in solutions. Cresol is easily soluble in soap solutions; and advantage is taken of this fact in the preparation of various disinfectants which are advertised under the names of Jeyes' fluid, Izal, Lysol, etc. They are all slightly superior to carbolic acid in actual germicidal value.

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