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From The Collaborative International Dictionary of English v.0.48:
Gas \Gas\ (g[a^]s), n.; pl. Gases (g[a^]s"[e^]z). [Invented by the chemist Van Helmont of Brussels, who died in 1644.] 1. An a["e]riform fluid; -- a term used at first by chemists as synonymous with air, but since restricted to fluids supposed to be permanently elastic, as oxygen, hydrogen, etc., in distinction from vapors, as steam, which become liquid on a reduction of temperature. In present usage, since all of the supposed permanent gases have been liquified by cold and pressure, the term has resumed nearly its original signification, and is applied to any substance in the elastic or a["e]riform state. [1913 Webster] 2. (Popular Usage) (a) A complex mixture of gases, of which the most important constituents are marsh gas, olefiant gas, and hydrogen, artificially produced by the destructive distillation of gas coal, or sometimes of peat, wood, oil, resin, etc. It gives a brilliant light when burned, and is the common gas used for illuminating purposes. (b) Laughing gas. (c) Any irrespirable a["e]riform fluid. [1913 Webster] 3. same as gasoline; -- a shortened form. Also, the accelerator pedal of a motor vehicle; used in the term " step on the gas". [PJC] 4. the accelerator pedal of a motor vehicle; used in the term " step on the gas". [PJC] 5. Same as natural gas. [PJC] 6. an exceptionally enjoyable event; a good time; as, The concert was a gas. [slang] [PJC] Note: Gas is often used adjectively or in combination; as, gas fitter or gasfitter; gas meter or gas-meter, etc. [1913 Webster] Air gas (Chem.), a kind of gas made by forcing air through some volatile hydrocarbon, as the lighter petroleums. The air is so saturated with combustible vapor as to be a convenient illuminating and heating agent. Gas battery (Elec.), a form of voltaic battery, in which gases, especially hydrogen and oxygen, are the active agents. Gas carbon, Gas coke, etc. See under Carbon, Coke, etc. Gas coal, a bituminous or hydrogenous coal yielding a high percentage of volatile matters, and therefore available for the manufacture of illuminating gas. --R. W. Raymond. Gas engine, an engine in which the motion of the piston is produced by the combustion or sudden production or expansion of gas; -- especially, an engine in which an explosive mixture of gas and air is forced into the working cylinder and ignited there by a gas flame or an electric spark. [1913 Webster]