We have seen in the preceding pages the process through which the air is rendered unhealthful by close rooms and want of ventilation. Every person inspires air about twenty times each minute, using half a pint each time. At this rate, every pair of lungs vitiates one hogshead of air every hour. The membrane that lines the multitudinous air-cells of the lungs in which the capillaries are, should it be united in one sheet, would cover the floor of a room twelve feet square. Every breath brings a surface of air in contact with this extent of capillaries, by which the air inspired gives up most of its oxygen and receives carbonic acid in its stead. These facts furnish a guide for the proper ventilation of rooms. Just in proportion to the number of persons in a room or a house, should be the amount of air brought in and carried out by arrangements for ventilation. But how rarely is this rule regarded in building houses or in the care of families by housekeepers! The evils resulting from the substitution of stoves instead of the open fireplace, have led scientific and benevolent men to contrive various modes of supplying pure air to both public and private houses. But as yet little has been accomplished, except for a few of the more intelligent and wealthy. The great majority of the American people, owing to sheer ignorance, are, for want of pure air, being poisoned and starved; the result being weakened constitutions, frequent disease, and shortened life. Whenever a family-room is heated by an open fire, it is duly ventilated, as the impure air is constantly passing off through the chimney, while, to supply the vacated space, the pure air presses in through the cracks of doors, windows, and floors. No such supply is gained for rooms warmed by stoves. And yet, from mistaken motives of economy, as well as from ignorance of the resulting evils, multitudes of householders are thus destroying health and shortening life, especially in regard to women and children who spend most of their time within-doors. The most successful modes of making "a healthful home" by a full supply of pure air to every inmate, will now be described and illustrated. It is the common property of both air and water to expand, become lighter and rise, just in proportion as they are heated; and therefore it is the invariable law that cool air sinks, thus replacing the warmer air below. Thus, whenever cool air enters a warm room, it sinks downward and takes the place of an equal amount of the warmer air, which is constantly tending upward and outward. This principle of all fluids is illustrated by the following experiment: