

THE THEUMODYN. \MICAL PROPEUTIES OF SITKKHEATED STK.VM. 



27 



and hence by subtraction 



K. 

 



_ 



__ T ' 

 , J t 



K now being the mean specific heat at constant pressure between temperatures T, 

 and T,. 



This formula, which is easily applied to the experimental results just given, is, 

 however, not quite so accurate as its deduction from the general theory would lead 

 one to suppose, for it has been shown by HIRN that by neglecting to take account of 

 the energy existing in the liquid water at the temperature from which the total heat 

 of evaporation is measured, in this case 32 F., an error is introduced which may 

 under certain conditions become appreciable, and hence the formula just quoted will 

 be discarded in favour of the more complete expression given by HIRN in his ' TheV>rie 

 M4canique de la Chaleur' (Tome L, 3* 1 * Edition, Paris, 1875, p. 434), vi/. 



ir . 



H, - H, + 



(P, - P,) 



_ 

 T, - T, 



where the O'OIG in the numerator is the volume in cubic feet of 1 Ib. of liquid water, 

 0774 being JOULE'S equivalent, P, and P 2 being the pressures from which the wire- 

 drawing takes place, and the remainder, as in the previous formula, for K. 



By the aid of this formula the values of the mean specific heat at constant 

 pressure have been calculated for various pressures and between certain temperatures, 

 the results being tabulated in the following Table IV. 



TABLE IV. 



E 2 



