Evaporation, the dissipation of bodies by the volatile particles at their surface assuming the form of vapors and disappearing in the space around them. Liquids manifest this property most sensibly. Mercury exhibits it at temperatures exceeding 60° F., as is shown by the invisible fumes forming an amalgam on the surface of a bit of gold leaf, suspended for some days over the surface of the metal; and at temperatures far below this limit the vapor of mercury is present in the vacuum of the upper portions of the tubes of barometers and thermometers. Many solid bodies are subject to it; camphor, ice, snow, musk, etc, waste away by their particles being taken in invisible vapor into the surrounding atmosphere. It is a part of the process provided by nature for restoring to the earth, through the medium of the clouds, the waters which have drained from its surface into the sea, and those also held in the soil, or upon the leaves of the forest. Once having performed their office, they are recalled by the process of evaporation, purified by it of their earthy contaminations, and are again poured out for the refreshment of vegetable and animal life. (See Atmosphere, Dew, Heat, and Ice.) The following table gives some relations between the vapor of water and the liquid:
Volume of vapor containing a unit of water.
Mechanical force required to vaporize, in foot pounds.
Evaporation takes place in ordinary temperatures only from the surface of objects. It is greater in a warm dry air than when the temperature is low, or the atmosphere is already nearly filled with vapor. The more moisture is taken up into the same body of air, the more the process is retarded, until at length it is entirely checked. It is renewed by new supplies of dry air. The most favorable natural conditions for its rapid action are presented on the Atlantic ocean under the trade winds, which from the hot deserts of Africa blow across to the Cordilleras. The Amazon and the Orinoco are the fruits of the evaporation thus produced. The vapors that are continually ascending from moist surfaces are for the most part invisible, like those exhaled by breathing. Their existence is proved by instruments called hygro-scopes, hygrometers, and psychrometers; and at times they become visible, as when in clear frosty weather they rise copiously from the surface of pools fed by deep springs, and are seen congealed in white clouds, like the vapors of the breath under the same conditions.
But unless deprived of their heat they possess the properties of gaseous bodies; a given bulk of air or of other gases takes up the same quantity of a vapor as would be received in a vacant space of the same extent and temperature. This was conclusively proved from the experiments of Dr. Dalton. It results that no more vapor can be received into any space after the weight of that already there amounts to the elastic force of the vapor at the temperature of the surface which generates it; in proportion as the air approaches this state of complete saturation it is said to be more moist, and evaporation proceeds more slowly. Increase of temperature adds to the elasticity of the vapor and promotes evaporation: cold reduces the elasticity and promotes precipitation. Pressure does not affect the capacity of air to contain vapor; but evaporation proceeds more slowly by its increase. If it be removed, as when a liquid is placed in an exhausted receiver of an air pump, evaporation goes on with great rapidity. Ether may thus at ordinary temperatures be thrown into ebullition.
A difference is observed in the tendency of different liquids to pass into vapor; the lower their boiling point, the more rapid is their evaporation; but it is also observed that the vapor thus easily produced is correspondingly less rare, occupying less space than that requiring a greater expenditure of heat for its evolution. The density of alcoholic vapor is 2.5 times greater than that of water. Dalton discovered that the presence of air or any gas impeded evaporation by the resistance its particles opposed to the circulation of the vapor; but whether any gas were present or not, the same amount of vapor would always be formed at the same temperature. The effect of the air was seen in the longer time required to fill the space with the amount of vapor belonging to the temperature. Vapors have a greater capacity for heat than their particles when condensed into liquid or solid form. In their formation consequently they abstract heat from surrounding bodies, producing an amount of cold corresponding to the rapidity of the process. This principle is applied in the water and wine coolers used in hot countries. The water with which they are filled, and in which the wine bottles are placed, filters through the porous vessels and evaporates from their surface, cooling all the contents.
A similar effect is experienced in the animal body by rapid evaporation. The heat generated by the chemical actions going on within is taken off by the vapor formed at the surface. Damp clothes furnish the means for the production of much vapor and consequent reduction of temperature, often to an injurious extent. The heat abstracted by vapor in its formation is given out on its condensation. In low pressure steam engines it is economized by being transferred in the condensers to the water that is returned to the boilers. - Hygroscopes and hygrometers, already referred to, are instruments designed, the first for detecting the presence of moisture in the atmosphere, and the second for determining either the temperature at which the air under observation begins to shed its moisture, called the dew point, or else the temperature of evaporation. Either of these and the normal temperature of the air being known, the elastic tension of the atmospheric vapor, and the amount of moisture in a given quantity, are approximately ascertained by reference to tables constructed for this purpose. (See Dew Point, and Hygrometer.) Evaporation is accompanied with ebullition when the elastic force pressing upon the surface of a liquid is less than that due to the temperature of this liquid. (See Boiling Point.) - The principles developed by researches in the evaporation of liquids have been applied in a variety of ways to facilitate and render more economical several practical operations.
Sirups are evaporated, as in the refining of sugar, in vacuum pans, or vessels in which the atmospheric pressure may be partially taken off by air pumps. A low degree of heat only is thus required, and the risk of overheating and burning the sirup is avoided. Extracts are conveniently prepared on the same principle. But when it is desirable to effect the boiling at high temperatures, as for digesting bones and subjects difficult to dissolve, the evaporation is prevented by the vapor being confined, so as to exert its elastic force upon the surface of the fluid. Thus the escape of more steam is checked until, by greater heat, its elastic force is made greater than that upon the surface. By this method the temperature of the water has been raised to more than 400° F. Rapid evaporation has been promoted in salt works and in bleacheries by causing currents of air to blow over the extended surfaces of the liquids, thus constantly bringing new portions of dry air to absorb fresh quantities of moisture. - As evaporation cannot take place unless the elasticity of the vapor can overcome the superficial tension of the liquid, and as this tension depends to a large extent on the nature of the gas in contact with the free surface of the liquid, it follows that the boiling point must vary with every circumstance.
Thus Dufour, having dropped some pure water into a mixture of oils having nearly the same density, was able to raise the temperature to 356° F., and still saw drops of water swimming in the mixture, although the tension of the aqueous vapor at this temperature is about 147 lbs. to the square inch. When water falls upon red-hot surfaces it separates in spherical drops, which dance around upon the metal, apparently without touching it, and thus continue without evaporating much longer than the fluid would if exposed to the same degree of heat under other circumstances. A platinum crucible brought nearly to a white heat may be almost half filled with water introduced drop by drop, which will continue in this state for some minutes without perceptible evaporation. On cooling the crucible, the liquid suddenly begins to boil, and discharges a volume of vapor. While in the spheroidal state drops are seen to be supported on an atmosphere of vapor, which prevents their contact with the surface of the metal. Most liquids, except oils which are decomposed by the heat, display the same phenomena.
Their temperature while in this condition is not only much less than that of the surface upon which they rest, but is also below their own boiling point; and if they are already boiling when dropped upon the heated surface, the temperature falls to a certain point, which appears to be a fixed one for each liquid in this condition. Water remains at 205°; alcohol, which boils at 173°, falls at least 3°; ether, which boils at 95°, falls at least 5°. The ternperature of the heated surface at which liquids are caused to assume this condition has been found, for water, to be 340° or more; for alcohol, 273°; and for ether, 140°. The check upon evaporation is very remarkable. A quantity of water which would ordinarily disappear in vapor in one minute at the temperature of 212°, has been kept from total dispersion nearly an hour in a metallic vessel heated nearly red.