DISINFECTANTS are agents or substances employed to prevent the spread of contagious or infectious disease. Recent investigations all tend to demonstrate that the efficiency of any disinfectant is due to its power of destroy-ing, or of rendering inert, specific poisons or disease germs which possess in themselves an independent existence ; and which, when introduced into the animal system, under favourable conditions, increase and multiply, thus pro-ducing the phenomena of special diseases. Therefore, antiseptic substances generally, which check or stop putrefactive decay in organic compounds, by preventing the growth of those minute organisms which produce putrefaction, are, on that account, disinfectants. So also the deodorizers, which act by oxidizine. or otherwise changing the chemical constitution of voratile substances disseminated in the air, or which prevent noxious exhala-tions from organic substances, are in virtue of these properties effective disinfectants in certain diseases. A knowledge of the value of disinfectants, and the use of some of the most valuable agents, can be traced to very remote times • and much of the Levitical law of cleansing, as well as tire oririn of numerous heathen ceremonial practices, are clearlybbased on a perception of the value of disinfection. The means of disinfection, and the substances employed, are very numerous, as are the classes and condi-tions of disease and contagion they are desiemed to meet. Nature, in the oxidizing influence of heel; circulatinc, atmospheric air, in the purifying effect of water, and in the powerful deodorizing properties of common earth, has pro-vided the most potent ever-present and acting disinfectine-media. Of the artificial disinfectants employed or available three classes may be recognized :-1st, volatile or vaporizable substances which attack impurities in the air ; 2d, -chemi-cal agents 'for acting on the diseased body or on the infec-tious discharges therefrom ; and 3d, the physical agencies of heat and cold. In some of these cases the destruction of the contagium is effected by the formation of new chemical compounds by oxidation, deoxidation, or other reaction, and in others the conditions favourable to life are removed or life is destroyed by high temperature. Of the first class - aerial disinfectants - those most employed are the gaseous sulphurous anhydride, the fumes of nitrous acid and other acid substances, including vaporized carbolic acid, with chlorine gas and the vapours of bromine and iodine. The use of sulphurous anhydride, obtained by burning sul-phur, is of great antiquity, and it still is unequalled as a disiufectant of air on account both of its convenience and general efficacy. Camphor and some volatile oils have also been employed as air disinfectants, but their virtues lie chiefly in masking, not destroying, noxious effluvia. In the 2d class - non-gaseous disinfecting compounds - all the numerous antiseptic substances may be reckoned ; but the substances principally employed in practice are oxidizing agents, as potassic manganates aud permang,anates (Condy's fluid), and solutions of the so-called chlorides of lime, soda, and potash, with the chlorides of aluminium and zinc, soluble sulphates and sulphites, solutions of sulphurous acid, and the tar products - carbolic, cresylic, and salicylic acids. Dr J. Dougall of Glasgow found the following sub-stmces the most powerful in destroying minute forms of life : - sulphato of copper, chloride of aluminium, chromic acid and bichromate of potassium, bichloride of mercury, benzoic: acid, brornal hydrate, chloral hydrate, hydrocyanic acid, alum, hydrochlorate of stryehnia, ferrous sulphate, arsenious acid, and picric acid, Of the physical agents heat and cold, the latter, though a powerful natural disin-fectant, is not practically available by artificial means ; heat is a power chiefly relied on for purifying and disinfect-ing clothes, bedding, and textile substances generally. Different degrees of temperature are required for the destruction of the virus of various diseases ; but as clothing, Sze., can be exposed to a heat of about 250° Fehr. 1-vithout injury, provision is made for submitting articles to nearly that temperature. For the thorough disinfection of a sick-room the employment of all three classes of disinfectants, for purifying the air, for destroying the virus at its point of origin, and for cleansing clothing, &c., may• be required.