_Compressibility_ and _elasticity_ are evident properties of air. Its elasticity was first ascertained by some experiments of Lord Bacon. The air nearest the earth is in a state of compression, occupying a smaller space than it otherwise would do, were it not compressed by the superincumbent air. It must therefore be in a state something resembling that of a quantity of fine carded wool thrown loosely into a deep pit; the lower strata supporting the weight of the upper strata, and being compressed by them; and so much the more compressed as they are further down, while the upper stratum only is in its unconstrained and most expanded state. If we should suppose this wool thrown in by a hundred weight at a time, it will be divided into strata of equal weights, but of unequal thickness, the lowest being the thinnest, and the superior strata gradually increasing in thickness.[61] When the air is in a state of compression, we find that the same force with which we compressed it is necessary to keep it in its bulk; and that if we cease to press it together, it will swell out and regain its natural dimensions, which shows its elasticity. This distinguishes it essentially from such a body as a mass of flour, salt, and such like, which remains in the compressed state to which we reduce it. There is something therefore which opposes the compression of air, different from its simple impenetrability, and produces motion, by repelling the compressing body. As an arrow is gradually accelerated by the bow-string pressing it forward, and at the moment of its discharge is brought to a state of rapid motion; so the ball from a pop-gun or wind-gun is gradually accelerated along the barrel by the pressure of the air during its expansion from its compressed state, and finally quits it with an accumulated velocity. These two motions are indications perfectly similar to the elasticity of the bow and of the air. Mr. Parkes observes, that atmospheric air in all states, and in all seasons, is _permanently_ elastic. This elasticity arises from caloric being chemically combined with the solid substances of which it is composed. I say _solid_, because we have abundant evidence that oxygen and nitrogen are both capable of taking a solid form, and actually do, in many instances, exist in a state of solidity. Nitrogen is a component part of all animal substances, and exists in a solid state in all the ammoniacal salts. Oxygen takes the same state when it combines with metals and other combustibles; and in the composition of the nitrous salts they both take the same state of solidity. These facts surely evince that atmospheric air owes its fluidity to caloric.