226 WRITINGS OF JOSEPH HENRY. [1855- 



melting of ice,)* is called latent heat. In thus condensing 

 a given quantity of vapor, from water at different tempera- 

 tures in a given quantity of cold water and noting the 

 elevation of temperature of the latter, it has been shown by 

 Dalton and others that an ounce of vapor at all tem- 

 peratures contains very nearly the same amount of heat, 

 adding the latent and sensible heat together. 



This constancy of the amount of heat arises from the fact 

 that as we increase the thermometric heat a new portion of 

 vapor is forced into the same space, its density increases, and 

 the amount of latent heat is diminished ; hence if the at- 

 tenuated vapor from ice were received in a syringe and sud- 

 denly condensed until its density became equal to that of 

 boiling water, its temperature would be 212°. 



On account of the great amount of latent heat of vapor, 

 heat must be absorbed from all surrounding bodies during 

 the process of evaporation ; and in all cases of the reverse 

 process, that is of the conversion of vapor into water, an 

 equal amount of heat must be given out. This absorption 

 of heat by vapor at the place of its formation, and the evolu- 

 tion of an equal amount at the place where it is condensed 

 into water is one of the most efficient means of varying the 

 temperature of different portions of the earth from that which 

 they would naturally acquire under the regular periodical 

 variation due to the changes of declination of the sun. 



In the evaporation of a cubic foot of water it is known 

 from experiment that an amount of heat is absorbed equal 

 to that evolved from the combustion of 20 pounds of dry 

 pine wood, and consequently every cubic foot of rain water 

 which falls from the clouds leaves in the air above an equal 

 amount of extraneous heat, which tends to abnormally raise 

 the temperature due to the elevation, and to produce power- 

 ful upward currents above, and horizontal motions of the air 

 below. We may also recall in this place the fact that water, 

 in passing from the state of ice to that of a liquid absorbs 

 140° of heat, which is again evolved in the act of freezing, 



*[See ante, p. 195.] 



