812 MV A TO RATION 



of -water, one degree of tho same scale. Wore the vessel charged with water so 

 heated, opened, it would bo instantaneously emptied by vaporisation, since the whole 

 caloric, equivalent to its constitution as <-teani. is present. When, upon the other hand, 

 steam is condensed by contact with cold substances, so much heat is set free as is 

 capable of heating about five times its weight of water from 32 to 212 F. 



Equal weights of vapour of any temperature contain equal quantities of heat ; for 

 example, the vapour exhaled from one pound of water, at '77 F., absorbs during its 

 formation, and will give out in its condensation, as much heat as tho steam produced by 

 one pound of water at 212 F. The first portion of vapour with a tension = 30 inches, 

 occupies a space of 27'31 cubic feet; the second, with a tension of 0'92 inch, occupies 

 a space of 890 cubic feet. 1 Suppose that these 890 volumes were to bo compressed 

 into 27*3 1 in a cylinder capable of confining the heat, tho temperature of the vapoui 

 would rise from 77 to 212, in virtue of the condensation, as air becomes so hot by 

 compression in a syringe, as to ignite amadou. The latent heat of steam at 212 F. is 

 1180-180 = 1000; that of vapour, at 77, is 1183 45 = 1135; so that, in fact, 

 the lower the temperature at which the vapour is exhaled, the greater is its latent heat, 

 as Joseph Black and James Watt long ago proved by experiments upon distillation 

 and. the steam-engine. 



From the preceding researches it follows, that evaporation may be effected upon 

 two different plans : 



1. Under the ordinary pressure of the atmosphere ; and that either, 



(A) By external application of heat to boilers, with (a) an open fire ; (6) steam ; 

 (c) hot liquid media. 



(B) By evaporation with air ; (a) at the ordinary temperature of tho atmosphere ; 

 (b) by currents of warm air. 



2. Under progressively lower degrees of pressure than the atmospheric, down to 

 evaporation in as perfect a vacuum as can be made. 



It is generally affirmed, that a thick metallic boiler obstructs the passage of the 

 heat through it so much more than a thin one as to make a considerable difference in 

 their relative powers of evaporating liquids. Dr. Ure states that he made a series of 

 experiments upon this subject. Two cylindrical copper pans, of equal dimensions, 

 were provided ; but the metal of the one was twelve times thicker than that of the 

 other. Each being charged with an equal volume of water, and placed either upon 

 the same hot plate of iron, or immersed, to a certain depth, in a hot solution of muriate 

 of lime, he found that the ebullition was greatly more vigorous in tho thick than in 

 the thin vessel, which he ascribed to the conducting substance up the sides, above the 

 contact of the source of heat, being 12 times greater in the former case than in tho latter. 



If tho bottom of a pan, and the portions of the sides, immersed in a hot fluid medium, 

 solution of caustic potash or muriate of lime, for example, be corrugated, so as to con- 

 tain 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 tho same bath. If the corrugations contain three 

 times tho quantity of metallic surface, the evaporation will be threefold in tho 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 cor- 

 rugations, it will evaporate no more water from its interior than a smooth pan of like 

 shape and dimensions placed alongside in the same flue, or over the same fire. This 

 curious fact Dr. Ure states he has verified upon models constructed with many modi- 

 fications. Among others, he caused a cylindrical pan, 10 inches diameter, and (> 

 inches deep, to be made of tin-plate, with a vertical plate soldered across its diameter ; 

 dividing it into two equal semi-cylindrical compartments. One of these was smooth 

 at the bottom, the other corrugated ; the former afforded as rapid an evaporation over 

 tho naked fire as the latter, but it was far outstripped by its neighbour when plunged 

 into the heated liquid medium. 



If a shallow pan of extensive surface be heated over a subjacent fire, by a liquid 

 medium, or a series a steam-pipes upon its bottom, it will givo off less vapour in tin- 

 same time when it is loft open, than when partially covered. In tho former case, the 

 cool incumbent air precipitates l.y eondemMtlOn a portion of tin- steam, ;md also op- 

 poses considerable mechanical resistance to the diffusion of the vaporous particles. 

 In tho latter case, as the steam issues with concentrated force and velocity from the 

 contracted orifice, the air must offer less proportional resistance, upon the known 

 hydrostatic principle of the pressure being as the areas of the respective bases of the 

 communicating vi 



In evaporating by surfaces heated witli ordinary steam., it must bo borne in mind 



1 One pound avoirdupois of wator contains 27-72 cubic inches ; one cnbic inch of water fi.rms 

 1C9C cubic inches of steam at 212 F. : therefore one jKmnd of water will form 27-ai cubic feet of such 

 steam ; and 0-93 : 30:: 27-81 : 8l>0 cubic feet. 



