VAPOUR-BATH. 



VAPOUR-PLANE. 



681 



evaporation take* pUoe ; and thii ocoun to the greatest I 

 extent in hot weather, and when the surface of the brine is agitated by 

 the wind It is found, however, that spontaneous evaporation can be 

 carried on with advantage to a certain extent only ; and when this 

 point U arrived at, the operation of Bait-making U finiabed by removing 

 the concentrated brine to iron vessels, in which the evaporation it 

 artificially conducted by the application of heat, the vaporisation being 

 greater aa the temperature it higher, till the boiling-point U arrived at, 

 when it i* greatest. 



Evaporation it used for numerous purposes and processes, and in 

 different modes, according to the substances operated on and the 

 objects to be attained. When contrivance! are adopted for condensing 

 the whole or any portion of an evaporated liquid, the process U termed 

 i M TII.I.ATIOII, and the ends accomplished by it are various. When, 

 (or example, water is distilled, it is for the purpose of separating the 

 s-a'iine and earthy impurities, which, not being vaporisable, remain in 

 the body of the still, while the pure vapour of the water is condensed 

 liy 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 the colouring-matter. When 

 herbs, as lavender, peppermint, Ac., 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. Vaporisation in the form 

 illation is also largely employed in the preparation of various 

 acidit, such as the nitric acid, hydrochloric acid, Ac. 



When solid bodies are vaporised and subsequently condensed, the 

 operation is termed tuUimation, and it is resorted to with different 

 intentions, as for the purification of camphor and the preparation of 

 corrosive sublimate and calomel. 



Jt will be evident on slight consideration that vessels of very different 

 materials and construction must be employed in evaporation, distilla- 

 tion, and sublimation, and according to the nature if the substance 

 operated on. Thus the first stage of the concentration of sulphuric 

 iicid is conducted in lead, the concluding one in glass or platinum ; 

 i aline 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; HYGBO- 

 MLTIIT ; BOII.IXO OF LIQUIDS ; EBULLITION ; DISTILLATION ; STEAM 

 .mil STKAU-KNUIXB; LATENT HEAT; SPECIFIC HEAT ; TUANSPIRATIOS. 

 VAl'urU HATH. [BATH; BATHINO.] 



VAl'OUR, OPALESCENT. This appears to be the most con 

 vcnient appellation for what has sometimes been termed red or orange 

 steam. It is a condition of condensed and condensing aqueous vapour 

 hich was first distinctly recognised, and its optical properties in ves* 

 ligated by Dr. James D. Forbes, F.K.S., when Professor of Natural 

 Philosophy in the University of Edinburgh. IU effects in nature 

 have been observed from time immemorial, though ascribed to other 

 causes; and there can be no doubt that it would long ago have been 

 recognised and described by those practically conversant with steam and 

 had not its effect on luminous bodies been confounded 



with that of smoke, from which it is in fact (indistinguishable by the 

 eye. In the year 1838, Professor Forbes, standing near a locomotive 

 engine which was discharging a large quantity of high-pressure steam 

 by its safety-valve, chanced to look at the sun through the ascending 

 column of vapour, and was struck by seeing it of a very deep orange 

 colour, exactly similar to dense smoke, or to the colour imparted to the 

 sun when viewed through a common smoked glass. The same ho 

 found might be observed during the ordinary progress of the engine in 

 the steam thrown into the chimney, but the presence of smoke itsell 

 rendered the experiment less satisfactory. He afterwards observed 

 that while for some feet or yards from the safety-valve at which the 

 steam blows, its colour for transmitted light is the deep orange red, at 

 a greater distance, the steam being moro fully condensed, the effect 

 entirely cesses. Even at moderate thicknesses the steam cloud is 

 absolutely opaque to the direct solar rays, the shadow it throws being 

 as black as that of a dense body ; and when tho thickness is very small 

 it it translucent, but absolutely colourless, just like thin clouds passing 

 over the sun, which indeed, according to Professor Forbes, have a 

 perfect analogy of structure. When (he steam is in this state no in- 

 dication of colour is perceptible in passing from the thickness corres- 

 ponding to tranalucency to that which is absolutely opaque. 



or Forbes proceeded to investigate this novel subject by moans 

 <>t a high-pressure steam-boiler, and a theodolite with a good prism 

 placed in front of the telescope, and from the experiments he made 

 deduced the following conclusions : " 1. Steam in its purely gaseous 

 form U, as commonly supposed, colourless, at least in small thicknesses. 



orange red colour of steam by transmitted light appears to be 

 due to a particular stage of the condensing process. Before condensation 

 team is colourless and transparent; it is next transparent and smoke- 

 red ; finally it becomes colourless at small thicknesses, and 

 absolutely opaque at greater. 3. The state of tension of the steam 

 only to affect the phenomena so far at it renders the critical 

 colorific stage of condensation more or less completely observable. 

 4. The abmrptivs action of steam on the spectrum U not exerted in 

 ll(i> rune way as that of other gnssoas coloured bodies, such n i 



acid gu, and iodine vapour. It cuts oft", however, totally the same 

 part of the spectrum as nitrous acid does. Its phenomena perhaps 

 Lave a greater analogy to those of opalesoenoe than any other." ' 1'lul. 

 JUg.,' series 8, voL xiv., p. 1U1-126; the paper having been read 

 before the Royal Society of Edinburgh, on January 21, 1839. 



The combination of a variety of other fact* with those thus made 

 known by Professor Forbes, relating to the nature and properties of 

 condensing steam with respect to light, have subsequently led <>th. r 

 men of science to unite in the conclusion, that the structure of orange 

 steam is in reality that of opalescent bodies, with tho phenomena of 

 wliii-h, as we have seen, he recognised the analogy of those presented 

 by it. Innumerable globules of water are formed throughout the still 

 gaseous vapour, by the joint action of which on light the colour of the 

 aggregate moss is produced. It is on this account that its absorptive 

 action on the spectrum is not exerted in the same way aa nitron 

 gas and iodine vapour, which themselves possess true colour. Tho 

 other results obtained by Professor Forbes also harmonise with this 

 conclusion. 



But though we are thus obliged to relinquish the idea that aqueous 

 vapour in any state is itself truly coloured, the observation of tho 

 existence of this particular condition of a condensing volume of steam 

 is of great value; especially in its application to the phenomena of 

 nature. Professor Forbes at once inferred from his investigation of it, 

 that the condition of watery vapour he had observed " is the prim -ip.il 

 or only cause of the red colour observed in clouds;" and he U en 

 to the credit of being, in fact, the discoverer of the true cause of tho 

 colours of dawn and sunset. In a subsequent elaborate communication 

 ' On the Colours of the Atmosphere,' he investigated the history of 

 science on this subject, refuting the fallacious inferences which bad 

 prevailed, and applied to it his own observations. " Soon after the 

 maximum temperature of the day, and before sunset," he remarks, 

 " the surface of the ground, and likewise tho strata at different heights 

 in the atmosphere, begin to lose heat by radiation ; this is the cause of 

 the deposition of dew, and consequently in severe weather we have v.u-t 

 tracts of air containing moisture in that critical state which precedes 

 condensation," in other words, in the red opalescent condition, by 

 which the rays of the setting sun are coloured accordingly. For thu 

 details of this subject we must refer to Professor Forbes's paper in tho 

 Trans, of the Royal Society of Edinburgh,' voL xiv. But we caiin.it 

 wholly agree with him as to the colours of the morning sky, the 

 phenomena of which appear to us to be the same with those of thu 

 evening, but in the reverse order. Mr. Luke Howard had long 1 

 observed the connection of the presence in the sky of a stratum of 

 vapour having the peculiar red colour, with the coming or aetiul 

 formation of dew, and had given to it the name of the dew-band, of 

 which many observations will be found in his ' Climate of London.' 



\ 'A POUR-PLANE. This term, which was probably adopted by 

 Luko Howard from De Luc, denotes a region of the atmosphere hori- 

 zontal in its general direction, though subject to elovatim 

 depressions, at or on and above which clouds form by the condensation 

 of aqueous vapour, and on which, therefore, they appear to float. Its 

 position is most readily recognised by the eye, by observing the modi- 

 fication of cloud called cumulus, tho various aggregations of which, or 

 distinct clouds, have a common base-lino, or have their inferior K 

 at the same height. The subject has already been noticed in Uio 

 articles CLOUD (col. 981) and DEW-POINT ; but in the latter tho v. 

 plane is erroneously stated to be the superior, instead of the in: 

 limit of a certain stratum of the atmosphere, and also that cloud : 

 below instead of above it, when it is regarded as a mathematical plane. 

 But there are in fact as many vapour-planes in the atmosphere of any 

 locality on the earth's surface, and at any time, as there are strata of 

 clouds at different elevations; and in each cose the piano becomes 

 itself a physical plane, or stratum, throughout which cloud is produced. 

 These successive strata originate, of course, in alternations of tempe- 

 rature aud in the amount of aqueous vapour present in a given volume 

 of air. 



Tho following statement, founded upon tho principal results deduced 

 by the late Mr. J. Welsh from experiments made by him and described 

 in his account of meteorological observations in four balloon ascents 

 made in 1852 (' Phil. Trans.,' 1853, pp. 311-346), already cited nn.l.-r 

 CLOUD, gives a general view of the nature of the alternations in 

 question, which may bo compared with the particulars of the height of 

 the several strata of cloud passed through in those ascents, and stated 

 in that article, col. 983. 



Tho temperature of the sir decreases uniformly with the height 

 above the earth's surface, until, at a certain elevation, varyi 

 different days, the decrease is arrested, and for a space of from 2000 

 to 3000 feet the temperature remains nearly constant, or even increases 

 by a small amount, the regular diminution being afterwards resumed 

 aud generally maintained, at a rate slightly loss rapid than in the lower 

 part of tho atmosphere, and commencing from a higher tcmper.il urn 

 th.il! would have existed but for the interruption noticed. This inter- 

 ruption in the decrease of temperature is accompanied by a large and 

 :iln upt fall in the temperature of the dew-point, or by actual condensa- 

 ti"ii "f v.-ijiour, from which it may be inferred that the disturbance in 

 tho progression of temperature arises from a development of heat in 

 the neighbourhood of the plane of condensation, or rajmnr-flane, Tho 

 subxeqic nt falls in the temperature of the dew-point are generally of 



