are derived, the means of effecting their removal therefrom will next be discussed.
The chief factors in carrying on ventilation are (a) the difference in temperature between the outside air and that within the room, and (b) the diffusibility of gases.
It is the difference in temperature that produces a draught up a flue or chimney when a fire is lighted below; for the products of combustion have a very much higher temperature (several hundred degrees Fahr.) than the surrounding atmosphere. Being so much warmer, they are lighter in consequence (as will be explained presently), and therefore have a constant tendency to ascend—being compelled by the force of gravity—till, after cooling little by little, they reach a layer of their own temperature. Upon the same principle an inflated balloon ascends and a cork immersed in water constantly tends to rise to the surface. As the coefficient of expansion for gases equals about 1273—i. e., they increase about 1273 of their bulk for every degree centigrade increase in temperature, thus becoming lighter in proportion to their volume, and, becoming lighter (some being originally lighter) than the atmosphere, are compelled by gravitation to ascend. It is important that the pipe or flue, in rooms heated by stoves or grates, should be vertical or nearly so; also that it be not too wide, otherwise downward currents will be produced, and these interfere with the draught, and cause the gases of combustion to escape into the room. In a stove-pipe the elbows should be as few in number as possible, and rounded rather than acute; for a sharp or abrupt bend materially diminishes the velocity of the draught. Two or more pipes opening into the same chimney should have separate flues; when they open into the same flue, the pipe that draws best will interfere with the draught in the others, and set up downward currents.
The air consumed by combustion escapes by the chimney, and tends to create a vacuum in the room; but it is steadily replaced by the atmosphere which rushes in at every available opening. This rush is strongest at the lowest openings (those nearest the earth), and here the whole amount enters if the space is sufficient. On the other hand, and for the reasons before given, the warmer (lighter) and fouled air within has a constant tendency to escape at the highest points; and it is here, therefore, that ventilators should be placed to allow its exit. Thus it is that, when a door is opened, the warmer (foul) air escapes in a current at the top, and the colder (fresh) from the outside rushes in at the bottom. This may be shown by a lighted taper held in these situations—the flame in each case taking the direction of the current. When the outside air is the warmer, and per consequence the lighter, as on a very warm summer day, the direction of the currents, other things being the same, will be reversed—the fresh air coming in above, and the cool air within escaping below. But, owing to the large amount of heat radiated from the pupils—the normal temperature of
