PNEUMATICS (Gr. pncuma, spirit or breath, air) is the science which treats of the mechanical properties of aeriform fluids, such as their weight, pressure, elasticity, motion. etc. The great representative of the aeriform fluids is the atmosphere. The atmosphere is very frequently called "air," to distinguish it from the others, \Odell are known as "gases." The fact of air having weight, and generally exercising pressure and resistance, was unsuspected by most of the ancients, though they were aware of the latter property in particular cases, from seeing and. feeling the effects of the wind; but the idea that air in a state of rest exerts pressure on a body immersed iu it, never seems to have occurred to them. Aristotle, however, asserted that air had weight, and so did several subsequent philosophers; but the truth of this opinion was not established till the time of Torriceili (q.v.), who not only showed that it had weight and exerted pres sure, but also found the amount off this weight or pressure. See Arstosmilinu. Pascal (q.v.) completed the investigation, and invented the barometer (q.v.). The experiments of these philosophers proved that what is called "suction" is nothing more than an effect of the pressure of the air on one side of a body, unbalanced by an equal pressure of air on the opposite side of it. To this property of air we owe the working of the various kinds of pumps (q.v.), the barometer (q.v ), the siphon, cupping-glass, etc. But the great distinguishing feature of aeriform bodies is the repulsive force which their molecules exercise over each other. and the constant expansion of these bodies when pressure is re
moved, or compression when pressure is increased. The investigation of the expansibility and compressibility of air was carried on by means of the air-pump (q.v.), an invention of Guerieke (q.v.), and soon resulted in the discovery of a law by Boyle (about 1650), and Mariotte (1670), called Hariottc's law, (q.v.), which affirms, that "at a given tem perature the volume of a gas is inversely as the pressure." See GASES. The second great law of tension and pressure is that of Dalton mid Gay-Lussac (1504 which states, that "when the tension remains the same, the density of a gas varies inversely as the temperature"—that is to say, when the temperature is increased by equal increments, the bulk is increased by equal increments. The motion of gases is subject to the same laws with that of liquids, the laws which regulate the motion of liquids depending for their efficacy not on the liquidity, but on the fluidity (see FLUID) of these bodies. The flow of gases in tubes seems to be retarded by friction against the sides, in the same way as that of water is. and the diminished efflux at an orifice shows that the unit wntracta exists for gases as well as for liquids. Abundance of examples and further explanations of the properties of air will be found under such heads as ATMOSPHERE, BALLOON, BAROMETER, DIVING-BELL, MAGDEBURG HEMISPHERES, etc.