EVAPORATION, the conversion of a fluid or solid into vapor. Steam, vapors of alcohol, camphor, iodine, etc., are familiar instances. All fluids arc surrounded by vapor at common temperatures; but for every substance there is a limiting temperature, below which no E. takes place. The pressure, or tension, of a vapor depends mainly upon the nature of the substance evaporated, and the temperature to which it is raised. The full amount of vapor, however, is not produced instantaneously, and therefore, in general, time is an element in the question as well as temperature. See DIFFUSION.
The boiling-point (q.v.) is the temperature at which vapor is freely given off—i.e., at which the tension of the vapor of a substance is equal to the atmospheric pressure. Dalton gave an empirical law, which, however, is only at all approximate for tempera tures near the boiling-point: " The tension of the vapor of a substance rises in geometric, as the temperature rises in arithmetic, progression." It is sufficient for our present pur pose to notice, that the tension increases very rapidly with the tempelature. Some curious consequences result from this. Thus, water boils at 212° F., under a pressure of 30 in., or at that temperature the tension of its vapor is one atmosphere. At 162° F., or 50° below its boili?g-point, its vapor has a tension of 10 in. of mercury, and it will therefore boil, if placed in the receiver of an air-pump, as soon as two thirds of the air have been extracted.
If a little water be boiled in an open flask till the steam has displaced a great part of the contained air, and the flask be then tightly corked, the water will gradually cool. If the flask be now dipped in cold water, boiling recommences, the cold water having condensed some of the vapor, and so diminished the pressure on the contained liquid. Dip the flask in hot water, and the boiling ceases. These appearances may be obtained several times in succession.
A fluid cannot be heated above its boiling-point, at the ordinary pressure of the atmosphere; but if it be heated in a closed vessel, the tension of the vapor produced is to be taken in addition to the former pressure, and the boiling-point rises with it.
Thus, when the pressure is equivalent to 2 atmospheres, the boiling-point of water is i raised 40° F. At such temperatures, its solvent powers are greatly increased. Many minerals which are found in fine crystals are supposed to have been deposited from water which had dissolved them in large quantities, under the combined influences of pressure and temperature. Papin's digester (q.v.) depends upon this principle.
The amount of E. from a fluid depends upon many circumstances. As, except in the case of actual boiling, it takes place only at the surface, the amount of surface exposed is an important consideration where rapid and copious E. is required, as in steam-boilers, salt pans, etc. When, on the contrary, it is desirable to prevent E. as much as possible, a layer of oil, preventing direct contact with the air, is of great use. The rate of E. depends also on the pressure, and varies, according to Daniell, nearly inversely as the latter. His experiments, which appear trustworthy, were made in an exhausted receiver, and the vapor was removed as it was formed.
In the conversion of a fluid into vapor, a quantity of heat disappears; i.e., is required to produce and maintain the gaseous state. Thus, the temperature of steam at 30 in. is the same (to the thermometer) as that of the boiling water from which it comes off; but the heat necessary to convert a pound of water at 212° into steam at 212°, would raise nearly 1000 pounds of water from 60° to 61°. See HEAT. When, therefore a fluid evaporates, the vapor carries off heat from the fluid, and thus E. pro duces cold. This, of course, is matter of daily observation. Porous earthenware jars are employed to cool water in summer in this climate; and in India ice is procured by exposing water in shallow pans, laid on straw, to the combined effects of E. and radia tion at night.