Joule- Thomson Thermal Effect. 113 



-jY are both small compared with v, we have 



(SX-K-"'-*)f-* • • <**•> 





 n 



.2-.J.K. 







"" IT 



. p . p . J . Ka. 



We should have obtained the same expression if we had 

 employed either the Boyle-Charles equation or that of 

 Clausius ; in the former case as the direct result, in the latter 

 as an approximation of the same order as that made above. 



We may now evaluate this quantity numerically for air at 

 standard pressure and temperature ; for this purpose we have 

 the approximate values = 092; p=U ; p = 0'00129 ; 

 K c = 17; J = 4-2xl0 7 ; whence 



(W) T = 8 ' 5xl0S <*rt 



(b) Ratio of the two Specific Heats of a Gas. — We have for 

 all bodies 



K„ K„ 



*~W,USW" 



dp 



If, therefore, we make use of the value of Isr— ) as deter- 



mined in the preceding subsection, we can determine this 

 ratio for ordinary oases. 



We give the result for air at standard pressure and tem- 

 perature ; here K p = 0'2375 ; <r = 0-003665 ; p = 0-001293 ; 



J=4-212xl0 7 ; /> = 1-0136x10 6 j(^2} = 8'5x 10 3 ; whence 



K„ 



^ = 1-408. 



This is exactly the value deduced from the velocity of 

 sound ; it is also the mean of the results given by Rontgen 

 Phil. Mag. S. 5. Vol. 48. No. 290. July 1899. I 



