Air is the respirable fluid which surrounds the earth and forms its atmosphere. It is inodorous, invisible, insipid, colorless, elastic, possessed of gravity, easily moved, rarefied and condensed, essential to respiration and combustion, and is the medium of sound. It is composed by volume of 20.7 parts of oxygen and 79.3 of nitrogen, by weight, of 23 of oxygen and 77 of nitrogen. These gases are not chemically united, but are mixed mechanically. Air contains also 1⁄2000 of carbon dioxide, some aqueous vapor, about one per cent. of argon, and small varying amounts of ammonia, nitric acid, ozone, and organic matter. The specific gravity of the air at 32° F is to that of water as 1 to 773, and 100 cubic inches of air at mean temperature and pressure weighs 301⁄2 grains.
Aëriform fluids are those which have the form of air. Many of them are invisible, or nearly so, and all of them perform very important operations in the material world. But, notwithstanding that they are in most instances imperceptible to our sight, they are really material, and possess all the essential properties of matter. They possess, also, in an eminent degree, all the properties which have been ascribed to liquids in general, besides others by which they are distinguished from liquids.
Elastic fluids are divided into two classes, namely, 1, permanent gases and, 2, vapors. The gases cannot be easily converted into the liquid state by any known process of art;* but the vapors are readily reduced to the liquid form either by pressure or diminution of temperature. There is, however, no essential difference between the mechanical properties of both classes of fluids.
As the air which we breathe, and which surrounds us, is the most familiar of all this class of bodies, it is generally selected as the subject of Pneumatics. But it must be premised that the same laws, properties and effects, which belong to air, belong in common, also, to all aëriform fluids or gaseous bodies.
There are two principal properties of air, namely, gravity and elasticity. These are called the principal properties of this class of bodies, because they are the means by which their presence and mechanical agency are especially exhibited.
Although the aëriform fluids all have weight, they appear to possess no cohesive attraction.
The pressure of the atmosphere caused by its weight is exerted on all substances, internally and externally, and it is a necessary consequence of its fluidity. When the external pressure is artificially removed from any part, it is immediately felt by the reaction of the internal air.
Heat insinuates itself between the particles of bodies and forces them asunder, in opposition to the attraction of cohesion and of gravity. It therefore exerts its power against both the attraction of gravitation and the attraction of cohesion. But, as the attraction of cohesion does not exist in aëriform fluids, the expansive power of heat upon them has nothing to contend with but gravity. Hence, any increase of temperature, expands an elastic fluid prodigiously, and a diminution of heat condenses it.
*Note.—Carbonic acid gas forms an exception to this assertion. Water also is the union of oxygen and hydrogen gas.