f>2 VAPORIZATION. 



The depression of the mercurial column produced by water 

 at every degree of temperature, between 32 and 212% was 

 carefully determined by Dr. Dalton, and his results have been 

 confirmed by Dr. Ure. The following selected observations 

 prove that the elasticity increases at a very rapid rate with the 

 temperature. 



Tension of the vapour of water in inches of mercury. Temperature. 

 0.2 inch at . . . . 32 



0.263 . . . . 40 



0.375 50 



0.524 ... 60 



0.721 ... 70 



1 80. 



1.36 . 90 



1.86 . . . 100 



7.42 . . 150 



23.64 . 200 



30 . 2J2 



The vapours of other liquids increase in density and elastic 

 force with the temperature, as well as the vapour of water ; but 

 each vapour appears to follow a rate of progression peculiar to 

 itself. 



The assumption of latent heat by such vapours is evinced in 

 some processes for producing cold. Water may be frozen by 

 the evaporation of ether in the air-pump, and a cold produced 

 of 55 degrees under the zero of Fahrenheit by the evaporation 

 of that fluid. The ether- vapour derives its store of latent heat 

 from the remaining fluid, and contiguous bodies, which are 

 robbed of their heat, and suffer a great refrigeration. To sus- 

 tain the evaporation of this fluid, it is necessary to withdraw 

 the vapour as it is produced by continual pumping. The vola- 

 tile liquid, sulphuret of carbon, substituted for ether, pro- 

 duces even greater effects. 



On the same principle is founded Leslie's elegant process 

 for the freezing of water by its own evaporation, within 

 the exhausted receiver of an air-pump, the evaporation being 

 kept up by the absorbent power of sulphuric acid. A little 

 water, in a cup of porous stone-ware, is supported over a 

 shallow basin containing sulphuric acid. All that is necessary 

 is to produce a good exhaustion at first : the process of evapo- 

 ration and absorption then go on spontaneously, in an uninter- 



