It would be hard to find, in any community, a person so ignorant as not to know that the lungs require good air. “Oh, yes, of course, I know we must have pure air.” Yes, indeed. Nevertheless, ninety-five families in every hundred, in city and country, though always ready to say this, suffer every day of their lives for want of it. This arises from a lack of definite knowledge (1) as to the true office of air——of the fact that it supplies the major portion of the body’s nourishment, since an ordinary person could live six weeks or more without eating, and as many days without liquids of any sort; while as many _minutes_ without oxygen is certain death; and (2) as to what constitutes “pure air in the home.” Says Prof. Huxley: “But the deprivation of oxygen, and the accumulation of carbonic acid, cause injury long before the asphyxiating point is reached. Uneasiness and headache arise when less than one per cent. of the oxygen of the air is replaced by other matters; while the persistent breathing of such air tends to lower all kinds of vital energy, and predisposes to disease. Hence the necessity of sufficient air, and of ventilation for every human being. To be supplied with respiratory air in a _fair state_ of purity, every man ought to have at least eight hundred cubic feet of space to himself, and that space ought to be freely accessible, by direct or indirect channels, to the atmosphere.” A room ten feet square, and eight feet high, if “freely accessible” to the outer air during the entire 24 hours, will, according to the high authority quoted, supply the necessary respiratory rations, so to say, for one adult person. In so far, then, as this space _per capita_ is diminished, its accessibility to the outer air must be increased; that is, the ventilation (which should in all cases be constant) must be freer, in proportion as the size of the room is diminished or the number of its occupants increased. No room built with hands will ever be large enough to supply the “breath of life,” in default of free communication with the outer air. WINTER VENTILATION.