VAPORIZATION. 55 



boil in a vacuum at the temperature of 100, but the steam 

 produced is much more expanded and rare than that produced 

 at 212, and has a greater latent heat. Hence there is no fuel 

 saved by distilling in vacuo. It has been shown, by Mr. 

 Sharpe of Manchester, that whatever be the temperature of 

 steam, from 212 upwards, if the same weight of it be con- 

 densed by water, the temperature of the water will always be 

 raised the same number of degrees ; or the latent and sensible 

 heat of steam, added together, amount to a constant quantity. 

 We may hence deduce a simple rule for ascertaining the latent 

 heat of steam at any particular temperature. The sensible heat 

 of steam at 212 may be assumed as 212 degrees neglecting 

 the heat which it has below zero Fahrenheit, and the latent 

 heat of such steam is 1000 degrees, of which the sum is 

 1212 degrees. To calculate the latent heat of steam at any 

 particular temperature above 212, subtract the sensible heat 

 from this constant number 1212. Thus the latent heat of 

 steam at 300 is 1212300, or 912 degrees. The same re- 

 lation between the latent and sensible heat of vapour appears 

 to exist at temperatures below 212, and we may, therefore, 

 calculate the latent heat of vapour, below that temperature, by 

 the same rule. 



Temperature. Latent heat of equal weights of steam. 



. . . 1212 degrees 



32 ... 1184 



100 ... 1112 



150 ... 1062 



242 ... 1000 



250 . . . 962 



The latent heat of other vapours, such as that of alcohol, 

 ether, and oil of turpentine, has been found by Despretz to 

 vary according to the same law. 



From the large quantity of heat which steam possesses, and 

 the facility with which it imparts it to bodies colder than itself, 

 it is much used as a vehicle for the communication of heat. 

 The temperature of bodies heated by it can never be raised 

 above 212; so that it is much preferable to an open fire for 

 heating extracts and organic substances, all danger of empyreu- 

 ma being avoided. When applied to the cooking of food, the 

 steam is generally passed into a shallow tin box, in the upper 

 surface of which are cut several round apertures, of such sizes 

 as admit exactly the pans with the materials to be heated. The 

 pans are thus surrounded by steam, which condenses upon them 



