EVAPORA′TION

The conversion of a fluid into vapour by means of heat, diminished atmospheric pre ure, or exposure to a dry atmosphere. Evaporation is had recourse to—1. For the vapour as a source of heat or power, as in the case of steam-boilers, .;—2. To separate volatile fluids from impurities or other bodies, which are either fixed or le volatile;—3. To recover solid bodies from their solutions, as in the preparation of extracts, chemical salts, .;—4. To concentrate or strengthen a solution by the expulsion of some of the fluid matter that forms the menstruum;—5. To purify liquids by the di ipation of the volatile matters which may contaminate them. It is found that, under ordinary circumstances, evaporation is confined to the surface of the heated liquid, and is therefore slower or quicker, in proportion to the extension of that surface. Hence has arisen the adoption of wide, shallow ve els for containing fluids during their exposure to heat for this purpose. Evaporation proceeds most rapidly when a current of air (especially hot and dry air) is made to pa over the surface of the fluid; as, in this ease, the vapour is prevented from resting upon the surface, and impeding the proce by its pre ure. For a similar reason, liquids evaporate more rapidly in ve els partially covered than in open ones. In the former case the cool incumbent air condenses and throws back a portion of the vapour, which thereupon, besides its cooling action, offers mechanical resistance to the diffusion of the vaporous particles as they arrive at the surface of the liquid. In the latter case these obstacles are avoided, and the impetus of the vapour pouring forth from a contracted orifice (or pipe), not only readily overcomes the pre ure of the atmosphere, but offers le surface for its cooling action, until it has pa ed much beyond the points at which it can exert any influence on the fluid from which it has escaped. In this way the chemical action of the atmosphere on the liquid operated on is also considerably le ened. On the small scale, shallow capsules of gla , wedgwood-ware, porcelain, or metal, are commonly employed as evaporating ve els, and these are exposed to heat by placing them over a lamp, or naked fire, or in a water bath, or sand bath, according to the temperature at which it is proper to conduct the proce . On the large scale, high-pre ure steam is usually employed as the source of heat. The term ‘spontaneous evaporation’ is applied to the di ipation of a fluid by mere exposure in open ve els, at the common temperature of the atmosphere, and without the application of artificial heat. The celerity of this species of evaporation wholly depends on the degree of humidity of the surrounding air, and differs from the former, in which the rate of evaporation is proportionate to the degree of heat at which the proce is conducted, and the amount of pre ure upon the surface of the liquid. Evaporation ‘ in vacuo ’ (as it is called) is conducted under the receiver of an air-pump, or in an attenuated air is expelled, when all communication with the external atmosphere is cut off, and the vapour condensed by the application of cold. Fluids are also evaporated in air-tight receivers over sulphuric acid, by which they are continually exposed to the action of a very dry atmosphere. When such a receiver is connected with an air-pump in action, evaporation proceeds with increased rapidity, and intense cold is produced. It appears, from the experiments of Dr Ure, that “if the bottom of a pan, and the portion of the sides immersed in a hot fluid medium (solution of chloride of calcium, for example), be corrugated, so as to contain a double expanse of metallic surface, that pan will evaporate exactly double the quantity of water, in a given time, which a like pan, with smooth bottom and sides, will do, immersed equally deep in the same bath. If the corrugation contain three times the quantity of metallic surface, the evaporation will be threefold in the above circumstances. But if the pan, with the same corrugated bottom and sides, be set over a fire, or in an oblong flue, so that the current of flame may sweep along the corrugations, it will evaporate no more water from its interior than a smooth pan of like shape and dimensions placed alongside it in the same flue, or over the same fire.” In the laboratory, steam heat is now almost exclusively employed. Copper, or tinned, glazed, or silvered coppered pans, boilers, and stills, are surrounded by a ‘jacket’ of cast iron, and high-pre ure steam admitted between the two. By due management of the supply-cock, a range of temperature may be thus obtained extending from about 90° to 325° Fahr. It is found that, under ordinary circumstances, 10 square feet of heated surface will evaporate fully 1 lb. of water per minute; and that a thin copper tube exposing 10 feet surface will condense about 3 lbs. of steam per minute, with a difference of temperature of about 90° Fahr. This is equal to 30° Fahr. per lb.; and, consequently, the heat of the steam employed to produce the evaporation should be 212° + 30° = 242° Fahr. An attention to the facts and principles thus briefly explained above will be found of great value in the laboratory.