Whenever a family-room is heated by an open fire, it is duly ventilated, as the impure air is constantly passing off through the heated 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. This is especially the case where air-tight stoves are used. A common mode of warming is by heated air from a furnace. The chief objection to this is the loss of moisture and of all radiated heat, and the consequent necessity of breathing air which is debilitating, both from its heat and also from being usually deprived of the requisite moisture provided by the Creator in all outdoor air. Another objection is the fact that it is important to health to preserve an equal circulation of the blood, and the greatest impediment to this is a mode of heating which keeps the head in warmer air than the feet. This is especially deleterious in an age and country where active brains are constantly drawing blood from the extremities to the head. All furnace-heated rooms have coldest air at the feet, and warmest around the head. What follows illustrates the principles on which several modes of ventilation are practiced. 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: Take a glass jar about a foot high and three inches in diameter, and with a wire to aid in placing it aright, sink a small bit of lighted candle so as to stand in the centre at the bottom. (Fig. 34.) The candle will heat the air of the jar, which will rise a little on one side, while the colder air without will begin falling on the other side. These two currents will so conflict as finally to cease, and then the candle, having no supply of oxygen from fresh air, will begin to go out. Insert a bit of stiff paper so as to divide the mouth of the jar, and instantly the cold and warm air are not in conflict as before, because a current is formed each side of the paper; the cold air descending on one side and the warm air ascending the other side, as indicated by the arrows. As long as the paper remains, the candle will burn, and as soon as it is removed, it will begin to go out, and can be restored by again inserting the paper. [Illustration: Fig. 34.] [Illustration: Fig. 35.]