264< Prof. Apjohn on the Specific Heats of the Gases, 

 convert into vapour having the temperature t' an amount of 



7 



the same fluid represented by grains. Hence one grain 



of the gas in cooling through d degrees evolves heat adequate 

 to the evaporation of grains of water at the temperature 



shown by the wet thermometer ; or, if we divide by — , — 



■^ e a 



grains of the gas in cooling through d degrees extricates heat 

 which will vaporize d grains of moisture at /'. For m' there- 

 fore in the formula f 



d may be substituted. 



Now, supposing the gas to be atmospheric air, and that 

 100 cubic inches of this weigh 31 grs., the volume in cubic 



e 

 inches of — grains of it at 60° and under a pressure of 30 



will obviously be 



_e_ 100 

 a ^ 31 

 and at any other temperature t\ and pressure j), 



e 100 44<8 + ^' 30 



« ^ "sT ^ ~508~ ^ y 



an expression which, as the volumes are reciprocally as the 

 densities, will, for any gas whose specific gravity is 5, become 



e 100 448 + 1' 30 

 as ^ S\ ^ SOS ^ 'p'' 



Let this be multiplied by ,,^,j x 4;7 x '625 x 31, the 



448 + ^ 30 



weight of a cubic inch of aqueous vapour of maximum tension 

 at temperature t\ and the product, viz. 



e 100 448 + ^' 30 508 /' 



— X -rt- X — -„Q^ X X ■ ■- , ^, X •(— X'625X'31 = 



as 31 508 p 448 + r 30 



e f 



X ^^ X -625, 



as p 



will be the maximum amount of moisture which can be con- 



, e 

 tained in — grains of the gas at temperature t', and pressure 



p, and is therefore the value of m. 



