Vapor

(va'pur,) or Vapour. [L. vapor.) (Phys). Vapors are the aëriform fluids into which volatile substances, such as ether, alcohol, water, and mercury, are changed by the absorption of heat. Volatile liquids are those which thus po e the property of pa ing into the aëriform state; and fixed liquids, those which do not form vapors at any temperature without undergoing chemical decomposition, such as the fatty oils. There are some solids,such as ice, arsenic, camphor, and in general all odoriferous solid substances, which can directly form vapors without first becoming liquid. Vapors are transparent like gases, and generally colorle : there are only a few colored liquids, which also give colored vapors. The pa age ofa liquid into the gaseous state is designated by the general teri vaporization; the term evapora. tion especially refers to the slow production of vapor at the free surface of a liquid; and boiling to its rapid production in the ma of the liquid itself. Like gases, vapors have a certain elastic force, in virtue of which they exert pre ures on the sides of ve els in which they are contained. The tension of vapors may be demonstrated by the following experiment. A quantity of mercury is placed in a bent gla tube (Fig. 664), the shorter leg of which is closed; a few drops of ether are then pa ed into the closed leg, and the tube im-Fig. 664. mer sed in a water bath at a temperature of about 45°. The mercury then sinks slowly in the short branch, and the space ab is filled with a gas which has all the appearance of air, and whose elastic force counterbalances the pre ure of the column of mercury cd, and the atmospheric pre ure on d. This gas is the vapor of ether. If the water be cooled, or if the tube be removed from the bath, the vapor which fills the space ab disappears, and the drop of ether is reproduced. If, on the contrary, the bath be heated still higher, the level of the mercury descends below b, indicating an increased tension. See STEAM. [s. 1]