The temperature of air influences its elastic force. It is probable that air would become first liquid, and then solid, if it could be made sufficiently cold. Like all other substances, air gives out heat when it is compressed ; property strikingly illustrated by the fact that tinder can be set on fire when the air in which it is contained is suddenly and violently com pressed.
Air, like gases and vapours generally, en larges its bulk with every increase of tempera ture, or increases its elastic force if enlarge ment of bulk be prevented. The quantity of this expansion, when the temperature passes from the freezing to the boiling point of water (that is, from 32° to 212' of Fahren heit's thermometer, from to 80° of Reau mar's, and from 0° to 100° of the Centigrade), is 375 parts out of a thousand of the bulk which it had at the freezing point. And this enlargement is uniform; that is, whatever ex pansion arises from an increase of 12° of tem perature, half as much arises from an in crease of twice as much from one of and so on. From the different sys tems on which the Fahrenheit, Reaumur, and Centigrade thermometers are graduated, it follows that the increase of bulk, corres ponding to a rise of one degree of temperature in the air (the bulk of 32° F. being taken as
a standard) is equal to 8a, respect tively. In some few eases this regularity of expansion is not quite certain, but it is known to be very near the truth.
On the properties of air with regard to other bodies, we may notice that probably there is a slight adhesion of air to many, if not to all surfaces. A small needle may be made to swim on water, and in this state the water evidently retires from around it, leaving it, as it were, suspended over a hollow in the fluid. This is attributed to the adhesion of a coat of air, which, with the iron, makes the whole specifically lighter than the water. Recent experiments on the pendulum have led some to suspect, that, in addition to the resistance of the air, a slight coating of this substance travels with the pendulum, and thereby causes an irregular addition to its weight.
These few details concerning the general properties of air will meet with various illus trations under AIR-GUN; AIR-rusts; ATMOS• PHERE ; BALLOON; COMBUSTION; PNEUMATICS; STOVE ; VENTILATION, &C.