DISINFECTANTS, substances employed to neutralize the action of pathogenic organisms and prevent the spread of infec tious disease. Putting on one side sunlight, perhaps the best of all disinfectants, these agents may be divided into three classes: 1st, volatile or vaporizable substances, which attack impurities in the air; 2nd, chemical agents, for acting on the diseased body or on the infectious discharges therefrom ; and 3rd, the physical agencies of heat and cold. Among the first class, formic aldehyde takes foremost place. It acts more rapidly than equal quantities of sulphurous acid, and does not affect colours. It is non-poisonous, though irritating to the eyes and throat. With the exception of iron and steel it does not attack metals. For destroying vermin sulphurous acid is more powerful than formic aldehyde. Camphor and some volatile oils have been employed as air disinfectants, but mask and do not destroy bad odours. In the second class all antiseptic substances may be reckoned; but the substances chiefly employed are oxidizing agents, as potassium permanganate ("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. Of the physical agents cold is not practically available by artificial means ; heat is used for purifying and disinfecting clothes, bedding and textile substances generally, a temperature of about 25o° being employed in the form of steam under pressure. For the thorough disinfection of a sick-room all three classes of disinfectants may be required.