-1859] WRITINGS OP JOSEPH HENRY. 225 



B — Weight of vapor in a cubic foot of saturated air, in grains Troy. 



This table we shall see is of great importance in practi- 

 cal meteorology, as it enables us to ascertain the weight of 

 the vapor in a given portion of the atmosphere at different 

 temperatures. 



The latent heat of vapor. — There is another circumstance 

 in regard to vapor which is of essential importance in un- 

 derstanding the part which it plays in producing the diver- 

 sified changes of the weather, namely, the great amount of 

 heat which it contains at different temperatures. It is well 

 known that the quantity of heat that a body contains is not 

 actually measured by the thermometer or the temperature 

 which it exhibits ; for example, if a cubic foot of air at 60° be 

 expanded without receiving or losing heat its temperature 

 will be much diminished, because the same amount of heat 

 which was before contained in a given space is now dis- 

 tributed through a larger space. If an ounce of steam from 

 boiling water, which indicates a temperature of 212°, be 

 condensed in water at 60°, it will give out to the latter 

 enough heat to elevate six times the quantity of water to the 

 boiling temperature ; that is, six times as much water 

 through 152°, or the same amount of water 912° ; or in 

 other words after having given out more than 900° of heat 

 in the act of being converted from a vapor to a liquid, it 

 still retains a temperature of 212°. The heat which is thus 

 evolved, and is not indicated by the thermometer, (as has 

 been stated in our preceding article with reference to the 



15-2 



