Source : Webster's Revised Unabridged Dictionary (1913)

Gas \Gas\ (g[a^]s), n.; pl. {Gases} (-[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.

   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.