1. (n.) An aeriform 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 liquefied by cold and pressure, the term has resumed nearly its original signification, and is applied to any substance in the elastic or aeriform state.

2. (n.) 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.

3. (n.) Laughing gas.

4. (n.) Any irrespirable aeriform fluid.

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