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Gas, Air. Van Helmont first designated by this name, - the etymology of which is unknown, - the carbonic acid developed in the vinous fermentation. Afterwards, the term was appropriated to every permanently elastic fluid: that is, which preserves its aeriform state at all temperatures; and ultimately it was extended to all aeriform bodies; - which were divided into permanent and non-permanent gazes. The latter are generally termed vapours: - they return to the liquid state, when a portion of their caloric is abstracted. The permanent gazes, or gazes properly so called, are numerous, and may be divided into three sections with regard to their effects on the animal economy.It is proper to remark that the term respirable has been very differently employed by different writers. Sometimes it has meant the power of supporting life when applied to the blood in the lungs. At others, all gazes have been deemed irrespirable, which are incapable of being introduced into the lungs by voluntary efforts,-without any relation to their power of maintaining vitality; and this is perhaps the best sense. The gazes were, at one time, employed in medicine, under great expectations, -especially by the enthusiastic Beddoes; but they are now scarcely ever bad recourse to. They differ, considerably, In their effects on the animal economy. Some, as oxygen, are exciting; others, as azote, depressing; whilst others, again, as the Protoxide of azote or laughing gas, produce the most singular effects.