THE HEAT OF EVAPORATION. The temperature of pure boiling water remains constant; yet heat must be continually supplied to it in order that evaporation may go ; the heat does not appear as sensible heat, but is required to produce the change of state. The heat required to convert one gram of a substance into vapor is called its heat of evaporation; that required to convert one 'mot' into vapor is called the molecular heat of erapo. ration. (By a 'mot,' or gram-molecule, is meant number of grams equal to the chemical reacting weight of the substance; thus a Inc)] of water is IS grams, IS being the molecular weight of water, 110.) The number of substances whose heats of evaporation have been actually determined is as yet rather small. On the other hand, the laws of thermodynamics lead to methods that permit of calculating the heat of evaporation of a sub stance if certain other numerical data in connec tion with the substance are known. Thus, if the volume occupied by one gram of a liquid is U', and the volume occupied by one gram of a sat. orated vapor is V, the 'absolute temperature' be ing T (by t he absolute temperature is meant the ordinary centigrade temperature increased by 273; thus the normal boiling-temperature of water on the absolute scale is 100° + 273° = 373°), and if the increase in the vapo•-pressure produced by a rise of I° in temperature is (lc dI noted by the symbol then the heat of evapora tion, /, may, according to Clausius, be calculated by the following formula: / = :1! (V—NP) t/T At the critical temperature (see Ctu•lem, there is no difference whatever between a liquid and its vapor; the difference between the volumes occupied by one g,ratif of liquid and one gram of vapor is naught, and the heat of evapora tion is likewise naught, as is shown by the above formula.
Cuoaiac BY EVAPORATION. Since the transfor mation of liquids into vapors involves the ab sorption of heat, a liquid may be cooled by allow ing it to ovaporate, while no heat is applied to it ; the evaporation will then take place at the ex pense of the sensible beat of the liquid itself, and as a result the temperature will he lowered. Thus the rapid evaporation of liquid ammonia or of sulphurous acid produces temperatures low enough for water to freeze, and is employed in making artificial ice. Liquid ethylene evaporates rapidly and produces temperatures low enough for compressed air to be liquefied; the evapora tion of the liquefied air thus obtained produces still lower temperatures, and in this manner the most refractory gases may be liquefied. When liquefied carbonic acid is forced, by the pressure of its own vapor, in a fine stream into the air, it evaporates so rapidly that a portion of the stream is frozen, forming a delicate snow-like mass.
Evaporation is continually going on in nature on a most gigantic scale. Vapor from the ocean is continually rising invisible in the air. ing, with a colder stream of the atmosphere, or with a cold mountain ridge, it beeomes condensed into mists or clouds. The fine particles of these unite into larger masses and fall as rain, hail, or snow, to be again evaporated by heat from the moist ground, or from rivers, lakes, and seas. See BOILING-POINT; VAPOR.