Temperature and the Tension of Vapours. 3 



If a be here, as in the case of the gases, a constant number, 

 we should, by starting with a given tension p and temperature t, 

 obtain by means of equation (3), 



log (273 + 1 + t) - log (273 + 1) 

 *~~ logP(273 + 1) -\ogp{273 + t + r) } 



and, by substituting various values for P and r, always find the 

 same value for a. This, however, is not the case ; for it furnishes 

 with increasing temperatures an appreciable though very slowly 

 augmenting value for a. 



In order to obtain an expression as generally applicable as 

 possible for the calculation of these numbers, I have made use 

 of five of the determinations directly resulting from the obser- 

 vations which Regnault has especially selected in his work, Rela- 

 tion des Experiences, fyc., premiere partie, p. 608, as data for the 

 calculation of a general formula for the determination of the 

 tensions of saturated vapours. They are as follow : — 



Temperatures of the air- 



Tensions in millims. of 



thermometer in degrees C. 



mercury. 



- 20 



0-91 



+ 40 



54-91 



100 



76000 



160 



464700 



220 



17390-00 



Of these tensions, the one at 100° C. and under a pressure of 

 760 millims. is determined with most accuracy. In the fore- 

 going equation, therefore, we put 



/=100, p = 760; 



and as an example, t + r=- —20, and P accordingly corresponds 

 to 0-91, &c. 



The values of a thus found may be calculated very exactly by 

 the equation 



a =006479 + 0-0001722 T - 0-0000001 T 2 , . . (5) 

 wherein T stands for t + r. 



The value of «, as calculated from this equation, for any chosen 

 temperature T signifies a mean value between T and 100°. 



It is evident that the heat which saturated aqueous vapour sets 

 free by the compression to a very small but an equal fractional 

 part of its volume at 100° C. certainly increases with the tem- 

 perature, but in a somewhat smaller ratio than the latter. If 

 we transform equation (3), which was formerly employed for the 

 determination of a, and give it the following form — 



lo g P = lo g gg + lo g(2 73 + T) + 1 °g( 273 + ^- lo g 373 > (4) 



B2 



