However it may be compressed, air returns, on liberation, to its original volume, and while thus perfectly elastic it is also the most compressible of bodies. This elasticity arises from the repulsive force of its particles, and is always equal to the compressive force which it balances. A glass vessel full of air, placed under a receiver and then exhausted by the air-pump, will burst into atoms. Water, on the other hand, is almost the reverse. Twenty cubic inches, introduced into a cannon whose sides are three inches thick, cannot be compressed into nineteen inches without bursting it. This non-elastic property of water, with another, that of communicating, when under the action of any force, an equal pressure in all directions, led to the invention of the hydraulic press. The elasticity of the air enables fishes to rise and sink in water, through the action of the air-bladder. The sudden compression of air liberates its latent heat, and produces fire. On this principle the pneumatic tinder-box is constructed. Brockhaus says that air has as yet been compressed only into one-eighth of its original bulk. For every degree of heat between the freezing-point and the boiling-point, 32° and 212°, the expansion of air is about 1/490th part, so that any invention which seeks to use rarefied air as a motive power must employ a very intense degree of heat, enough to fuse many kinds of metals. To the celebrated Mr. Boyle and to Henry Cavendish, both of Great Britain, we are indebted for most of what we know of this particular property of the air. * * * * * Density, or closeness, is another quality of the atmosphere. It has been found to be 770 times less than that of water, and 770 cubic inches of air weigh as much as a cubic inch of water. It is in direct ratio with its elasticity, and there are tables by which it may be determined at different altitudes. At the surface of the earth, this density is indicated as 1; at 2-1/2 miles, as 1/2; at 5 miles, as 1/4; and so on, the difference being in a geometrical progression. As we proceed in the consideration of our general subject, we shall find, under the appropriate heads, that density is not without material influence on reflection and refraction, on transparency and the transmission of light, the presence or absence of moisture, and the amount of heat at the earth's surface,--and we might add, on health, and the increase or diminution of the vital energies. * * * * * Temperature is another branch of our subject, and one involving a series of subordinate topics on which volumes have been written, and to which are still devoted the labors of the most learned men of our day. In this place, merely an out-line can be attempted.