Gas
A name given to all permanently elastic fluids and airs. Gases have no cohesion, in consequence of which, their particles tend to recede from each other and would expand into space--so far as is known--if they were not restrained by the pressure exerted by the atmosphere upon the earth's surface. In gas, as in liquid, the particles are in a condition of equilibrium,--with this difference, that in a liquid the equilibrium exists between the attractive and repulsive forces in the liquid itself, but in the gas between the excess of the repulsive forces in the body and an external pressure. Gases are therefore fluids in consequence of this condition of equilibrium which endows the particles with perfect freedom of motion. Gases are also compressible and elastic. The solid, liquid, and gaseous conditions of bodies depend upon temperature and pressure: for instance, mercury becomes solid at 40 deg. below zero F.; from that temperature to 662 deg. F. it is a liquid, and a gas when the temperature exceeds that. If sufficiently cooled and pressed all gases would probably become liquids, as many of them which are permanent at ordinary atmospheric pressure and temperature become liquids on increasing the pressure and diminishing the temperature, and some even solidify when cooled sufficiently. But some gases, such as oxygen, hydrogen, nitrogen, carbonic oxide, and nitrous oxide, cannot be liquefied. See AIR; THERMOMETER; FLUID.