36 HEAT OR CALOKIC. 



4 



state, or steam, is the true cause of the mechanical movements in liu-: 

 boiling fluid, and the cloud which we see in the air near the surface 

 is the vapor condensed into minute drops resembling a fog or mist. 

 The singing arises from the escape of innumerable air bubbles, and 

 the crackling noise, that precedes boiling, and ceases when it begins, 

 is owing to the formation of elastic vapor, and its immediate conden- 

 sation by the colder fluid above. 



(/.) Perfectly formed vapor is invisible. If water or other fluid 

 be boiled in a glass flask, the space above the water, appears as if 

 the vessel were empty, and the cloud at the mouth consisting of con- 

 densed steam, in the form of mist, would not be seen, if the air 

 were of the temperature, 212. 



Ether, in thin glass vessels, is easily vaporized by applying boiling 

 water, and condensed again by cold water. 



(G.) The latent heatof steam is about 950 orfromthatto 1000. 

 Dr. Ure adopts the latter number, which is probably correct. 

 This is proved, by distilling one gallon of water, and condensing the 

 vapor in a worm immersed in ten gallons of the same fluid, each of 

 which will receive nearly 100 of heat, and this multiplied by 10, 

 gives the above result very nearly.* Or one gallon of water in steam, 

 will heat six gallons from 50 to 212 ; 212^-50x6 = 972=: latent 

 heat of steam very nearly. 



Hence, steam is an excellent vehicle of heat, and is very useful in 

 cookery, in heating manufactories, drying gunpowder, and chemical 

 precipitates, in heating baths, dye vats, and apartments for invalids 5 

 in making pharmaceutical extracts, and in many other cases. Large 

 vessels of wood are employed with great economy because they can 

 be heated by steam. 



(H.) PRINCIPLE OF DISTILLATION. 



Caloric, combining ivith the more volatile part of a fluid, raises it 

 in vayor; it is again condensed by the cold water of the refrigeratory, 

 which thus becomes rapidly hot, and must be often changed ; this is 

 usually done by a stream of cold water conveyed into the condenser ; 

 on one side, hot water runs out, and on another, cold water runs in.f 



A retort and receiver is the simplest distilling apparatus ; the fluid 

 in the retort is made to boil, and the vapor is condensed in the re- 

 ceiver, which is kept cold for that purpose. Sublimation is the same 

 thing in principle, as distillation, but the vapor is condensed in the 

 solid form ; this is seen, in the case of camphor, sulphur, benzoic 

 acid, corrosive sublimate, calomel, arsenic, &c. 



* Due allowance being made for the sensible heat, and for waste. Henry, 10th 

 Lond. Ed. Vol. I. p. 127. 



t Col. Wm. Moseley of New Haven, ingeniously avails himself of the cold water 

 at the bottom and of the hot water at the top of the condensing tub, to supply baths 

 conveniently and economically. 



