As the chemical disinfectants are usually employed in aqueous solutions, investigations have been undertaken with regard to the nature of solutions, and they have led to considerable modifications in our views about disinfectants. According to the modern theory of dissociation, in a solution of a salt some of the molecules of the dissolved substance remain unaltered, while others are broken up (dissociated) into electropositive and electronegative constituents-into their ions. Now, many of the physical properties of a solution depend upon ionization, and it is certain that the toxicity of a salt also varies with its degree of dissociation. Thus, cyanide of mercury, which is very slightly dissociated, is a less efficient germicide than mercuric chloride. Indeed, the sublimate is the most electrically active, that is to say, the most toxic, of all other mercurial salts. To express the matter in more general terms, solutions of mercurial salts are the more active the more mercury they contain, not by atomic weight, but in the form of ions.

But the relation between dissociation and the disinfecting powers becomes even more marked when we compare the same salt in different degrees of dissociation. It is known that in a solution of corrosive sublimate (HgCl,) the proportion of Cl ions to unchanged HgCl, molecules is constant. Now, if we add to this the strongly active solution of common salt (NaCl) more Cl ions are liberated, and the proportion of dissolved molecules to unaltered molecules is disturbed. The result is that the number

of Cl ions derived from HgCl, is reduced, its degree of dissociation diminished, and some of the free Hg and Cl ions are built up again into complete molecules. It has been proved experimentally that as the degree of dissociation is lessened so is the toxicity reduced.

From this it follows that in the disinfection of fluids

containing sodium chloride a larger quantity of the sublimate must be added than would otherwise be required. The presence of albumin will demand a still further addition of the mercurial salt, because some of it will be precipitated in the insoluble compound with proteids. In the case of media rich in albumin (e.g., fæces, sputum), the whole of the mercury may be precipitated, and its germicidal action entirely lost.

Besides the nature of the solvent, the temperature of the solution has an important influence on the degree of dissociation, that is to say, on the germicidal powers of the agent employed. Again, the knowledge that the action of germicides is chemical, and due to their combination with the albumin of bacteria, is apt to lessen still further our confidence in the permanence of their action.

It would serve no useful purpose to give a list of all the antiseptics, and the proportions in which they are most efficient. Such figures, obtained experimentally, have at best a theoretical interest, and may even be misleading. As has already been observed, there are a number of conditions which influence the germicidal powers of agents, and these must be carefully taken into consideration in the choice of a disinfectant. In the light of these facts the absurdity of sprinkling carbolic powders, and of pouring a deodorant down the gutter becomes apparent. It must be realised that the creation of a rival smell is no criterion of safety.

Klein has recently endeavoured to ascertain the value of various disinfectants by a more direct method. For this purpose he infected wood, cloth, and wall-paper with cultures of anthrax, tubercle, and cholera organisms, and then exposed them to the action of various germicides for twenty-four hours. It was found that carbolic acid,

corrosive sublimate, and sulphurous acid were the most efficacious, whereas Condy's fluid (1-53 to the pint) was the least reliable. Gaseous formalin, with only five hours' exposure, sufficed to destroy all organisms except when tubercle bacilli were present on the wood and cloth.

CHAPTER VIII.

THE PRESERVATION OF FOOD-STUFFS.

No better illustration could be furnished of the advantages of bacteriological study than the subject we are about to consider. This important industry owes its foundation to a Parisian, named Appert. While the scientists were discussing the theory of spontaneous generation, this enterprising confectioner perceived the possibilities of Spallanzani's experiment (p. 4), and perfected his process for preserving meat, vegetables, etc. To this end he exposed these substances to the temperature of boiling water for some time, and succeeded in obtaining the desired result. The modern methods of food preservation are essentially modifications of Appert's process. The use of borax, formalin, etc., as food preservatives is to be condemned, on account of their injurious effect on digestive enzymes. For it must be remembered that the problem which lies at the root of food preservation is this: how to destroy or suspend the vitality of microorganisms, without at the same time lowering the nutrient value of the medium? The answer to such a question will necessarily vary with the nature of the food, and its bacteria flora; and, therefore, different methods are employed for different foods.

Milk, when secreted from the gland, is practically germfree, but by the time it has entered the milk-pail it is extremely rich in bacterial contents. These come from

all sources: from the vessel, hands of the milker, air of the cowshed, and the dirt which adheres to the body of the cow. Of the non-pathogenic germs the lactic organisms are commonly found in milk, where they grow so rapidly as to check the growth of all other species. They may even prevent the growth of pathogenic forms, and would be most effective in preventing infection, were it not for the fact that milk is consumed long before this As is well known, milk is a frequent carrier of the infections of tuberculosis, cholera, and scarlet fever; and there is no doubt that various diarrhoeal disorders are due to the bacteria of the "milk dirt”.

occurs.

A short boiling suffices to kill lactic and most pathogenic germs. But there are other milk bacteria which form spores, and these are not destroyed by the comparatively low temperature of ebullition. A boiled

milk, therefore, may be rich in these bacteria, and yet remain unaltered to the naked eye. If, however, such a milk be consumed by a young infant, the spores develop into organisms, which rapidly decompose the milk and give rise to various gastro-intestinal disorders.

But this is not the only objection to boiling, for, as is well known, it alters the flavour and nutritive value of milk. The fat loses its emulsified condition separating out its cream, and the albuminoids are converted into a form very difficult of digestion.

Sterilisation by means of superheated steam has been tried to destroy spores, but without much success. It is found that this method is by no means efficient, as some spores still escape destruction.

A much better method is that of "pasteurisation," whereby milk is kept at 70° C. for fifteen minutes, and then rapidly cooled. The rationale of this process is simple. The high temperature is evidently below the