A column of air, having a base an inch square, and reaching to the top of the atmosphere, weighs about fifteen pounds. This pressure, like the pressure of liquids, is exerted equally in all directions.
The elasticity of air and other aëriform fluids is that property by which they are increased or diminished in extension, according as they are compressed. This property exists in a much greater degree in air and other similar fluids than in any other substance. In fact, it has no known limit, for, when the pressure is removed from any portion of air, it immediately expands to such a degree that the smallest quantity will diffuse itself over an indefinitely large space. And, on the contrary, when the pressure is increased, it will be compressed into indefinitely small dimensions.
The elasticity or pressure of air and all gases is in direct proportion to their density; or, what is the same thing, inversely proportional to the space which the fluid occupies. This law, which was discovered by Mariotte, is called “Mariotte’s Law.” This law may perhaps be better expressed in the following language; namely, the density of an elastic fluid is in direct proportion to the pressure which it sustains.
Air becomes a mechanical agent by means of its weight, its elasticity, its inertia and its fluidity.
The fluidity of air invests it, as it invests all other liquids, with the power of transmitting pressure; fluidity is a necessary consequence of the independent gravitation of the particles of a fluid. It may, therefore, be included among the effects of weight.
The inertia of air is exhibited in the resistance which it opposes to motion, which has already been noticed under the head of Mechanics. This is clearly seen in its effects upon falling bodies, as will be exemplified in the experiments with the air-pump.
The great degree of elasticity possessed by all aëriform fluids, renders them susceptible of compression and expansion to an almost unlimited extent. The repulsion of their particles causes them to expand, while within certain limits they are easily compressed. This materially affects the state of density and rarety under which they are at times exhibited.
It may here be stated that all the laws and properties of liquids (described under the heads of Hydrostatics and Hydraulics) belong also to aëriform fluids.
The chemical properties of both liquids and fluids belong peculiarly to the science of Chemistry, and are, therefore, not to any extent, considered in this volume.
The air which we breathe is an elastic fluid, surrounding the earth, and extending to an indefinite distance above its surface, and constantly decreasing upwards in density. It has already been stated that the air near the surface of the earth bears the weight of that which is above it.