110 CHANGE OF VOLUME AND TEMPERATURE. 



$ie temperature be increased so that all the water is turned 

 into vapor, then for this and all higher temperatures the 

 pressure of the vapor will change in accordance with the 

 same law which regulates the connection between the press- 

 ure and temperature of gases (Art. 53). 



The atmosphere always contains more or less aqueous 

 vapor, and if p be the pressure of dry air, and TT of the 

 vapor in the atmosphere at any time, the actual pressure of 

 the atmosphere is 



61. Volume of Atmospheric Air without its Va- 

 por. Having given the pressures of a volume v of 

 atmospheric air, and of the vapor it contains, to find 

 the volume of the air without its vapor at the same 

 pressure and temperature. 



Let P be the pressure of the atmosphere and p that of 

 the vapor; and let v' be the required volume of the air 

 without its vapor, at the pressure P. Then P p is the 

 pressure of the air alone when its volume is v. Hence we 

 have (Art. 48), 



P : P p :: v : v' ; 



P-p 



.-. v' = ^- v. 



62. Pressure of Gas when Volume and Temper- 

 ature are Changed. A gas contained in a closed 

 vessel of volume v is in contact with water, and its 

 pressure at the temperature t is P ; it is required to 

 determine its pressure when v is changed to v' and t 

 to t'. 



Let p and p' be the pressures of the vapor at the temper- 

 atures t and t', respectively, and P' the required pressure. 



Then P p and P' p' are the pressures of the gas 

 alone, under the two sets of conditions stated. Hence, 



