EVAPORATION. The conversion of substances into vapour is one of the most important and general effects of heat. During this process, a considerable quan tity of sensible heat passes into the latent or insensible state. When a vessel of water is placed upon the fire, its temper ature gradually rises till it attains 212° ; then, although it remains upon the fire, and of course receives heat as before, it does not become hotter, but is gradually converted into steam or vapor ; so that the effect of heat is not to elevate tem perature, but to change state or form : that is, in the case of water, to convert it into steam. Hence we assume that steam, though not hotter than water, contains a much larger quantity of heat, and this heat again makes its appearance when the steam is condensed or re-con verted into water. At whatever temper ature vapour is produced, it is similarly constituted ; and that which escapes from water at ordinary temperatures, by the process usually called spontaneous evapo ration, resembles the former in all re spects : hence it is that evaporation is to surrounding bodies a cooling process ; and that in the converse change, or the return of the vapor to the liquid state, heat is evolved and rendered sensible. The same general phenomena are ob served with all other liquids, and those which evaporate rapidly at common tem peratures often give rise to the produc tion of a great degree of cold. such as spirit of wine, or ether. If the latter flu id be suffered to dribble over the bulb of a thermometer, it will cause it to sink below the freezing point of water ; and by accelerating similar cases of evapora tion, we obtain most intense degrees of artificial cold.
The circumstances that principally- in fluence the process of evaporation are, extent of surface, and the state of the air as to temperature, dryness, stillness, and density.
In evaporating by surfaces heated with ordinary steam, it must be borne in mind that a surface of 10 square feet will eva porate fully one pound of water per min ute, or 725X10=-7250 gr., the same as
over a naked fire ; consequently the con densing surface must be equally exten sive. Suppose that the vessel is to re ceive of water 2500 lbs., which corres ponds to a boiler 5 feet long, 4 broad, and 2 deep, being 40 cubic feet by mea sure, and let there be laid over the bot tom of this vessel 8 connected tubes each 5 inches in diameter and 5 feet long, pos sessing therefore a surface of 5 feet square. If charged with steam, they will cause the evaporation of half a pound of wa ter per minute. The boiler to supply the steam for this purpose must expose a sur face of 5 square feet to the fire. It has been proved experimentally that 10 square feet snrfilee of thin copper can condense 3 lbs. of steam per minute, with a differ ence of temperature of 90 degrees Fehr. In the above example, 10 square feet eva porate 1 lb. of water per minute ; the temperature of the evaporating field be ing 212° F., consequently 3 : 1 : : 90: During this evaporation the difference of the temperature is therefore =30°. Consequently the heat of the steam placed in connection with the inte rior of the boiler, to produce the calcu lated evaporation, should be, 212-F3 242°, corresponding to an elastic force of 53.6 inches of mercury. Were the tern perature of the steam only 224. the same boiler in the same time would produce a diminished quantity of steam, in the pro portion of 12 to 80 ; or to produce the same quantity the boiler or tubular sur face should be enlarged in the proportion of 80 to 12. In general, however, steam boilers employed for this mode of evapo ration are of such capacity as to give an unfailing supply of steam.