LATENT HEAT.
It was formerly supposed that the air dissolved vapour and held it in solution as water bolds sugar or salt. This was the theory of Lo Roi, propounded in some otherwise ingenious papers,' Sur l'Elavation et la Suspension de l'Eau dans l'Air et sur la Rosati.' (`Diem. Acad. Royale dee Sciences,' 1752 ; and also in a separate volume, Melaoges de Physique et de Medicine,' 1771.1 This theory, however, was com pletely demolished by Dalton at the commencement of the present century (` Manchester Memoirs'), who showed that a vapour forms much more easily in a vacuum, and that the pressure of air retards and obstructs the evaporation of a liquid. Dalton's apparatus consisted chiefly of two barometers placed side by side. A drop of a given liquid was sent up into the vacuum of one of them, when it became converted into a transparent vapour, and, exerting a pressure on the mercurial column, lowered the mercury in the tube. The amount of this depression was measured by comparison with the adjacent baro meter, and the elasticity of tho vapour was expressed not by that depression, but by an equal quantity of mercury which the vapour would support. Thus, if the summit of the column of the mercury containing the vapour stood half an inch below the mercury in the adjacent barometer, the pressure of the vapour would be such as would support a column of mercury half an Inch In height. In esti mating this pressure, sufficient liquid must be sent Into the vacuum to saturate it, and it is then found that the elasticity of the vapour is directly as the temperature. By surrounding the tube with some vessel capable of containing hot water, with a thermometer for indi cating temperature, the temperature of the vapour can be known, and the relation between its temperature, pressure, and density ascertained. [EvAroneama.] Various forms of apparatus have been contrived by Dalton, Gay-Lussac, Arago, and Dulong, and by Regnault, for deter mining the prefigures and densities of vapours, and obtaining such results as are given in tho table under STEAM and STEAM-ENOINE.
The specific gravities of vapours, like those of gases, are referred to air as a standard at the temperature of 32° and a pressure of 30 inches; or the density of a vapour may merely express the ratio of a given volume to an equal volume of air of the same temperature and pressure. In the case of steam above 250° this ratio is invariable, and indeed tho ratio of the densities Is constant for the vapour of most liquids above their boiling points. Theao ratios have been thus determined by Gay Lussac When a gEs and a vapour which do not act chemically on each other are Inclosed in the same apace, they will exert separately on the sides of the vessel the same pressures that each would produce if it occupied the same space in the absence of the other, so that the total pressure of the mixture is equal to the sum of the separate pressures.
When vapour receives a supply of heat after it has been separated from the liquid it la called super-heated vapour. Such vapour, unlike ordinary vapour, may lose a portion of its beat, and still the whole of it continue to be vapour. If, after a vapour has been raised from a liquid, it bo compressed into a smaller space, its temperature will rise, and if expanded it will fall; but the temperature, pressure, and volume will always be such as the vapour would have had if it had been raised directly from the liquid at such temperature and pressure. Thus,
vapour raised from water at 68' has a volume 58.224 times greater than the water that produced it. If this vapour bo separated from the water, and its volume be compressed until it is only 1696 times that of the water, its temperature will rise to 212°, or that which it would have had it directly raised from the water under the increased pressure.
Evaporation, both spontaneous and artificial, and especially the Latter, is employed In numerous manufacturing and chemical processes. When, for example, common salt is prepared from sea-water, it is exposed in the first Instance to the air in shallow clay pits, by which spontaneous evaporation takes place ; and this occurs to the greatest extent In hot weather, and when the surface of tho brine is agitated by the win. It is found, however, that spontaneous evaporation can be carried on with advantage to a certain extent only ; and when this point is arrived at, the operation of salt-making Is finished by removing the concentrated brine to iron vessels, in which the evaporation is artificially conducted by the application of beat, the vaporisation being greater as the temperature is higher, till the boiling-point is arrived at, when it is greatest, Evaporation is used for numerous purposes and processes, and in different modes, according to the substances operated on and the objects to be attained. When contrivances are adopted for condensing the whole or any portion of an evaporated liquid, the process is termed DISTILLATION, and the ends accomplished by it are various. When, for example, water is distilled, it is for the purpose of separating the same and earthy impurities, which, not being vaporisable, remain in the body of the still, while the pure vapour of the water is condensed by cooling in the worm : so, again, when wine is submitted to distilla tion, it is for the purpose of evaporating and subsequently condensing the spirit or brandy from the water and tho colouring-matter. When herbs, as lavender, peppermint, &c., are heated with water in a still, the oil and water rise in vapour and are condensed; when turpentine is similarly treated, a volatile oil rises in vapour, while the resin or rosin, not being volatile, remains In the stilL Vaporization in the form Of distillation is also largely employed in the preparation of various :wide, such as the nitric acid, hydrochloric acid, kc.
When solid bodies are vaporised and subsequently condensed, the operation is termed sublimation, and it is resorted to with different intentions, as for the purification of camphor and the preparation of corrosive sublimate and calomel.
It will be evident on alight consideration that TOMO" of very different materials and construction must be employed in evaporation, distilla tion, and sublimation, and according to the nature cf the substance operated on. Thus the first stage of the concentration of sulphuric ru id is conducted in lead, the concluding one in glass or platinum ; saline solutions are evaporated to the crystallising point in lead or copper ; the caustic alkalies in iron or silver; the distillation of spirits in copper ; that of acids in iron, earthenware, or glass; while the preparation of common salt is completed in vessels of iron.
For further information we refer to EVAPORATION; DEW; HYGRO METRY ; BOILING or LIQUIDS; EOULLITION; DISTILLATION; STEAM and STRAM-ENOINE; LATEST HEAT; SPECIFIC HEAT; TRANSPIRATION.