VAPOUR. There are many substances, both fluid and solid, which when exposed to the air, or to the more powerful agency of heat, are gradually but totally dissipated, owing to their particles assuming the state of vapour by what is termed spontaneous evaporation. A vapour, then, consists of ponderable matter combined with sufficient specific heat to enable it to retain its aariform existence : we have already [GAS] given a similar definition of a gas. The question, then, natu rally arises,. In what do vapours differ from gases I The answer is, that the difference is a conventional one, being of degree only, and not of kind : thus, when atmospheric air containing, as It always does, the vapour of water, is suddenly cooled by exposure to a colder substance, the water which it contained In the state of Invisible vapour is depo sited in the state of palpable water on the colder body; we say then aqueous vapour or the vapour of water, and not aqueous gas. No similar change is produced, by this abstraction of heat, in the form of the constituents of the air, and they are therefore termed gaseous bodies or gases. The difference, however, we repeat, is one of degree
only ; for many gaseous bodies which had been, not many years since, considered as permanently elastic as atmospheric air, have been shown by the important investigations of Dr. Faraday to be reducible to liquids [OASES, LIQUEFACTION or]; and additional experiments have even shown that carbonic acid gas, which requires a pressure of 35 atmospheres to render it fluid, may by particular management be con verted into a solid. [Clnton° Am.] A practical difference between a vapour and a gas is illustrated by the use of the vapour of water, and its subsequent condensation, as a motive-power in the steam-engine. [STEAM and STEAM-ENGINE.] No known gaseous body could be employed with the same advantage, owing to the great degree of pressure and cold required for its con densation. Nor could the vapour of any other liquid than water be so profitably employed as a prime mover, for the reason given under