By "disinfection" is meant the destruction of the specific causes of disease, and more especially of the infectious or spreading diseases. A disinfectant is not necessarily an antiseptic or a deodorant, nor are these last necessarily disinfectants. The best practical antiseptic for sanitary purposes is cleanliness; the best disinfectants are heat, bichloride of mercury, sulphate of iron, chloride of zinc, sulphurous acid, chlorine, sunlight, and pure air, and, for yellow fever, cold. With our present very imperfect knowledge of the nature of specific causes of disease which we wish to destroy, we have no means of determining the presence of these causes in or on an article of clothing or of furniture, or in a room or other locality, except by the production of their specific effects on man or by inductive reasoning; in other words, we can only say that it is more or less probable that such causes are present. This makes it necessary, or at least expedient, to employ disinfectants in many cases where the presence of such causes is doubtful. The practical difficulties are, first, to bring the disinfecting agent into such relation with the causes of disease that it can act upon them, and act upon all of them; second, to avoid unnecessary destruction or injury of things which should be preserved. The majority of the causes of disease upon which we wish to act by disinfectants are probably minute particles of solid or semi-solid matter which are living, and may be conveniently designated by the word "_germs_." In the presence of moisture the destruction of the vitality of these germs can be effected with comparative ease and rapidity, but when they have become dried, or, as in the case of the bacilli, are in the form of spores, it is a more difficult matter.