EVAPORATION is the transformation of a liquid into a gaseous state by the action of heat. If any liquid be placed in an open vessel, it gradually diminishes in quantity by evaporation, and at length disappears. The quantity of vapour produced in a given time is proportional to the area of the exposed sur face; but, with equal temperatures, the escape of vapour from different liquids varies in rapidity. Over the surface of an ocean the aqueous vapour held in the atmosphere amounts to an enormous quantity. The quan tity of vapour which rises from a liquid de , pends mainly upon the temperature ; but it is influenced by the state of the atmosphere with respect to dryness or moisture, a dry and warm atmosphere being favourable to it. At equal temperatures, in a close vessel, the eva poration is the same in quantity whether the vessel containing the liquid contain also air, or have the air exhausted from it, but more rapid in the latter case.
It was found by Dalton, that at Fahr., the evaporation of water from one square inch of surface is equal to 4.244 grains per minute ; and at 138° Fahr., one grain per minute. Dalton and Gay Lussac have found that the evaporation from ice is equal to that from water at the same temperature. Dalton also determined that the quantity of vapour raised from a given surface of any liquid, at a given temperature, is directly proportional to the elastic force of the vapour at that temperature. Solid substances and liquids of great specific gravity have vapours of small elastic force. When, by evaporation, a liquid is transformed into a vapour, heat is abstracted from the liquid, and a thermometer in it indicates a depression of temperature.