zero 



114 On the Joale- Thomson Thermal Effect. 



(1*406), and by Jamin and Richard (1*41), who worke 

 totally different methods. 



Had air been a perfect gas, ( -^— 1 would have been 



and our formula would become 



Kp _ K p 



In this case the same values of K p , p, cr, and p would give 



K 



^ =1-400, which is the value assigned by the kinetic theory 



for a perfect gas consisting of diatomic molecules. The 

 Joule-Thomson effect, then, completely accounts for the 

 difference between the " observed " and " theoretical " values 

 of this important ratio ; and verifies the suggestion, made 

 long ago, that the " imperfection " of the gas is probably the 

 source of this difference. 



(5) The Maximum Density Point. 



Some interesting thermodynamic properties of substances 

 at their temperature of maximum density are deducible from 

 the equation, employed in the last section, 



K '- K -iH(fX + *} ! 



as the present writer does-not remember to have seen them 

 stated anywhere, he ventures to insert them here. Lord 

 Kelvin * has shown that the following equation holds good for 

 all substances, • 



K P- Kf 



K CT 2 



^ K p -T.f .^ 



(where e<i denotes the isothermal elasticity, and the notation 

 is changed to that used in this paper) ; hence we get 



Kp — K„=T . -=— . ei; 

 Op 



combining this equation w r ith the previous one we obtain 



for all substances. 



* Encyc. Brit. art. " Elasticity." 



