Thermodynamic Properties of Air. 33 



To obtain a graphical representation of the variations of 

 c v I constructed isothermals of that quantity, considering it at 



first as a function of the density - . These lines are curves, 



turning their concave sides to the axis of abscissse. After- 

 wards I made the remark that a- far simpler law results when 

 c v is considered as a function of the pressure p. As shown 

 in fig. 5 the isothermals of cv drawn on that supposition are 



Fig-. 5. 



0.5O 





/ 

















0.A5 





it 

















0,40 



/ 



' 



/ 















0.35 



/ 



1 / 



/ 















0,30 



/ ^ 





•/ 















125 / 





















/ < 

 0,20/ /A 



% S 



















0,15 













n.i_ ■ 



-3S' 





o" 



2 



3 



. A 



5 



o 6 



7 



8 



o g 



O ATM. 



very nearly straight lines. This means that the increments 

 of the specific heat at constant volume are proportional to the 

 increments of pressure caused by increased density, tempera- 

 ture being kept constant. This empirical relation holds at all 

 temperatures between 0° and — 140°, and for all densities up 

 to the hundredfold of the ordinary density. 



According to this diagram it is possible to express the values 

 of cv between the just mentioned limits by the following 

 linear equations : — 



Phil. Mag. S. 5. Vol. 42. No. 254. July 1896. D 



