Forces between Atoms and Chemical Affinity. 763 

 But if N is the number o£ molecules per unit volume, 



JN=(Jl e sin Odd 



Jo 



"SlU J 



Substituting this value for we find that P, the electrical 

 moment parallel to the electric force per unit volume, is 

 given by the equation 



r € AXM i 6 -AXM j_ -| 



P = M i_ € ^M_ e -AXM - /7XM } * 



The contribution of this to the specific inductive capacity K 



477-P 



is ^7— ; hence K will contain the term 



47rNM j V XM + e- A XM 1 1 



X \ € *xm_ 6 -axm AXMJ* 



Now, except at very low temperatures, ZiXM will be an 

 exceedingly small quantity, for h is inversely proportional 

 to the average kinetic energy possessed by a molecule 

 at the temperature of the gas. At 0° C. this energy 

 is represented by the work done by the atomic charge 

 falling through a potential difference of 1/30 of a volt ; 

 XM is this atomic charge multiplied by the potential 

 difference between the poles of a doublet : this, in any 

 feasible electric field, will be exceedingly small compared 

 with 1/30. Hence, unless the absolute temperature of the gas 

 is almost vanishingly small, hXM is a very small quantity, 

 in this case equation gives 



K = ^N/iM 2 . 

 o 



Now h — -j57r, , where T is the absolute temperature of the 



gas and R is the gas constant 13*2 x 10~ 17 . Hence these 

 doublets contribute a term 



47TNM 2 _M 2 



S8 ~x 10 



36 



3RT T 



provided the density of the gas is constant and equal to the 

 density when the temperature is 0° C. and the pressure 

 760 mm., as in this case N = 2'8x 10 19 . 



