Ventilation means the act of admitting air into any place, or of causing a draught or current of air to pass through it. When any one speaks of ventilating a room or building, it is understood to refer only to pure air. Unless we breathe pure air continually, our blood cannot be purified and invigorated; impure blood causes disease and death. The easiest and quickest way of ventilating ordinary rooms is to open the doors and windows, whereby the fresh air outside rushes through and purifies the place. But it is not always safe to ventilate in this way; for if a person is ill or heated it would be hurtful for him to feel a sudden rush of air from without; besides which, it is always more or less dangerous to sit in a draught. Unless in very warm weather, the door and window should only be opened now and then, and for a short time, and the persons in the room should take care to sit out of the draught. The best time for admitting air is in the morning, between eight and twelve, but earlier than this in dry sunshiny weather. People who have but one or two rooms to live in should make it a rule to ventilate their rooms thoroughly after every meal, because it is desirable to get rid of the smell of cooking as soon as possible. But the chief causes of impure air are the breath and perspiration of persons who live in the room, and the smoke and vapour of lamps and candles. Perspiration adds considerably to the impurity of apartments, for as it goes slowly away from the surface of the skin it mingles with the air and vitiates or spoils it. While in health, day or night, we are constantly perspiring. In hot weather, or after severe exertion, we can see it plainly ; but besides this, there is an invisible or insensible perspiration; this perspiration spoils the air rapidly, but more rapidly in summer than in winter. If a man were sitting in a room which contained sixty feet of air, the insensible perspiration from his body would spoil the whole in ten minutes and render it unfit to be breathed a second time, if pure air were not admitted. But there are other methods of ventilating rooms besides those already described; a sheet of wire gauze or zine pierced full of small holes may be put into the place of an upper pane in the window, so as to admit fresh air, or Arnot's chimney-ventilator may be used. The latter is thus constructed: An opening of the size of one or two bricks is made from the room quite through the breast of the chimney, as near to the ceiling as possible; into this opening a metal frame is tilted, which has a balance-door that opens inwardly of itself, so that heated or foul air passes from the room through the opening, and goes away with the smoke of the chimney. Ventilators ought to be fixed at the top of a room, because it is the nature of warm or breathed air to rise; and they are therefore fixed in or near the ceiling, so that the air on its ascent may pass off without interruption. Ten feet of fresh air is required for each person every minute. One fact is certain - we cannot retain our health, nor have any enjoyment of life, unless we continually breath pure air.