GASES, Compressed. In general it may be stated that all gases are included under this head, all being compressible to a greater or less extent. Specifically, however, the term has come into use in the industries where it is used to designate those gases which by compression are reducible to liquids and in this state are more easily transported and adapted to indus trial uses. The most characteristic feature of the gaseous state is the law or relation which obtains between the volume and tension of a mass or quantity of gas. When pressure is applied to such a mass its volume diminishes until its tension balances the pressure. Pro vided the temperature is constant, the general principle will be observed that the tension is in inverse proportion to the volume — the greater the tension the less the volume, and vice-versa, when expansion is permitted, the tension de creases in exact proportion to the increase of the volume. Here it is possible to state the general law that the product of the tension and the volume of a gas mass maintained at an invariable temperature remains a constant quan tity under moderate pressures; thus, if T,T' represent the tensions at different pressures and V,V' the volumes. Since the tension and vol ume are in inverse proportion, as above stated we have: T : V' : : T' : V, or TV Boyle discovered and stated this law in 1662 and it was subsequently verified by Mariotte.
Under high pressures, however, the law does not hold, but here a modification has been sug gested by Van der Waals. The law of Boyle indicates rather a typical condition of gaseous bodies and not a state which is met with in practice. No known gas conforms exactly to
the law. Generally the tension increases less rapidly than the law states, while in the case of hydrogen the tension increases more rap idly than the volume diminishes. In practice, however, the deviations are negligible in the case of hydrogen, oxygen, nitrogen, the hydro carbon gases, nitrous oxide, and a few others; while with other gases the deviations are very marked, and increase rapidly with the pressure. Through the means of pressure and cold all gases may be reduced to liquids when pressure is applied by means of a piston, to a mass of sulphur dioxide gas, for example, the volume is reduced and the tension increased. The latter, however, increases in a lessening ratio up to a certain value, upon attaining which, a further reduction in volume does not further increase the tension, but a part of the gas becomes a liquid, and with further pressure the remainder of the gas also assumes a liquid state.
The more important compressed gases used commercially and medicinally at the present time are the following: acetylene, anhydrous ammonia, argon, carbon acid gas, chlorine, hydrocarbon gases, hydrogen, nitrogen, nitrous oxide, oxygen, and sulphur dioxide. For the historic, scientific, commercial, and economic phases of these gases, also the manufacturing methods, accessories and appliances used in the industry and the distinctive legislation affecting it see LIQUEFIED AND COMPRESSED GASES.