" Specific Heat of Saturated Ether Vapour." 289 

 8Q=y^.dT+^.dp, 



and since by the fundamental principles of thermodynamics 



1 BtQ _ "dpv 

 T* dp ~ dT> 



we obtain for the specific heat along the path 



Now assuming the liquid line as reversible path, we find 



p BT • W/' 



where the differential quotient -^ is enclosed in brackets to 



indicate distinctly that it relates to the liquid line. Hence, 

 the question under consideration depends on the second term 

 of the right-hand member, and it is necessary to know the 



value of ^-. 

 O-L 

 Let us consider, for this purpose, a small triangle ABO in 

 the pressure-volume diagram, which is bounded by the liquid 

 line AB, the isothermal AC, and the isopiestic BC. Draw 

 the perpendicular AD from A on the isopiestic. We have 



BC=BD + DO, 



which expressed in thermodynamic terms becomes 



/dp\ 

 -d p v /dv\ \dT) 

 dT ~ [dTj _drp_ 



It is, therefore, necessary to know the courses of the liquid 

 line and the isothermals starting from that line in the pressure- 

 volume diagram of ether, and these can evidently be found 

 by experiments. 



Now, referring to Ramsay and Young's valuable paper * 



n the thermal properties of ether, we find that ( -^ ) and 



/dp\ . \ di -' 



f-^plare given for all temperatures from +40° to 4-190°, 



* Ramsay and Young, Phil. Trans, pt. i. 1887. 

 Phil. Mag. S. 5. Vol. 48. No. 292. Sept. 1899. Y 



