PIIlLOSOniY OF STOKMS. O 



urn, and in this way tlic tension may be very conveniently expressed. 

 Dalton, to whom we owe this method of investigation, has constructed 

 a table exhibiting this clastic force of vapor, and which may be found 

 in any of the recent works on chenii:jtry. Tiie following arc a few of 

 the results : — 



Again : — The point of maximum density of a vapor is dependent 

 upon the temperature ; it increases rapidly as the temperature rises. — 

 Tlius, taking the specific gravity of atmospheric air, at 212° = 1000, 

 that of aqueous vapor in its greatest possible slate of compression for 

 the temperature will be as follows : ' 



Evaporation into a space filled with air or gas follows the same law 

 as evaporation into a vacuum ; as much vapor rises, and the condition 

 of maximum density is assumed in the same manner as if the space 

 were perfectly empty. 



Now let us apply the foregoing principles to determine the quantity 

 of aqueous vapor in the air : 



Suppose the temperature of the air to be 70°, and that of the dew- 

 point 60°, as we have assumed above ; the elasticity of the watery va- 

 porj would correspond to a maximum density proper to 60°, and would 

 support a column of mercury .521 inch high. Therefore, if the ba- 

 rometer on the spot stood at 30 inch, 29.476 inches would be supported 

 by the pressure of the dry air, and the remaining .524 inch by the va- 

 por. Now a cubic foot, or 1728 cubic inches of vapor at 70'^ would 

 become reduced by contraction, according to the above law, to 1695.4 

 cubic niches at 60°. Thus : 



Mcas. at 70° Mcas. at UO" Mens, at 'S" Menu, at 60' 



530 -520 r; 1728 1695.4 



/' 



