640 Sir J. A. Ewing on Specific Heat of Saturated Vapour 



Here for the greater part of its course, from about 20° to 

 150° or thereabouts, the saturation line slopes up to the 

 rio-ht showing that within these limits the specific heat of 



Fio\ 3.— Ether. 



















1 



A 









/ 





















B 



0-2 

 Entropy 



the saturated vapour is positive. Below 20° it is negative, 

 and it of course becomes negative again as the critical point 

 is approached. 



Another substance which gives a diagram of the same 

 type is Benzene (fig. 4). In drawing the diagram for 

 Benzene I have taken SchifFs formula for the specific heat 

 of the liquid, namely *, 



0-3834 + 0-001043*, 

 from which 



^=0-2270 log 10 T + 0-001043 T-0'8380. 



Young's values of L are used f, but as they do not extend 

 below 70° I have determined L for 0°, 20°, 40°, and 60° by 

 taking Regnault's values of the total heat of formation of 

 the vapour (under constant pressure, from liquid at 0° C.) 



™d deducting from that the quantity \ C p dt, which repre- 

 sents the heat required to warm the liquid up to the 

 temperature of vaporization. The values of L below 70° 

 so determined fit in pretty smoothly with those found by 



* R. Schiff, Liebig's Annalen, vol. 234. p. 319. 

 f S. Young, loc. cit. p. 422. 



