Heat of Saturated Vcepours. 539 



B. Experimental Evidence. 



As in the case of ordinary specific heats one might expect 

 the foregoing results to hold qualitatively for real substances; 

 but, although observations of inversion points are neither 

 sufficiently numerous nor accurate to afford a satisfactory 

 test, an attempt was made to obtain a curve more closely 

 adapted to experimental results. 



For the saturation constants the known properties of a 

 normal substance may be used instead of the equation of 

 state. Isopentane was taken as a typically normal substance. 

 Its properties have been fully investigated by S. Young*, 

 whose data have here been used. 



The equation to the curve of inversion points is 



The assumption was made that for all values of 0, n v 



along the saturation line is the same function of 0, so that 



C r = C r=00 $(#), 



The above equation may therefore be rewritten in terms 

 of 7, 



C. Dieterici f, as a result of calorimetric experiment, has 



determined G v for isopentane along the saturation line, and 



dv 



dT 



given by him have been used. <£(#) may therefore be 

 evaluated for various values of 0, if C r=00 is known. For 

 this purpose, C^ — (\. was calculated from Young's data J at a 

 temperature of 30° C, and as near the saturation volume as 

 possible. This gave a value C p —C v = 0*028. C v at that 

 temperature is given by Dieterici =0'361; hence, <y at that 

 point =1-08. This was assumed to be equal to <y^. From 

 the gas laws it follows that (C^ — C r )oo = 0*0276 for isopentane; 

 and since fy w = l-08, C r=00 must be 0*345. By using the 



* S. YouEjr, Proc. Phvs. Soc. xiii. pp. 602-657 (1894). 

 f C. Dieterici, Ann. d. Physik, xii. pp. 602-657 (1903). 

 \ Loc. cit. 



has also calculated Ti ~p) -^ ; in the following, the values 



