132 



Dielectrics and Inductive Capacity 



[CH. v 



This gives at once a method of determining 6 for substances for which 

 6 is small e.g. gases. It should, however, be noticed that, owing to the 

 unwarranted assumption that the molecules are spherical, the results will be 

 true as regards order of magnitude only. If the dielectric is a gas at 

 atmospheric pressure, the value of n is known, being roughly 4 x 10 19 , and 

 this enables us to calculate the value of a. 



K 

 150. The following table gives series of values of j=- for gases at atmo- 



"0 



spheric pressure : 



The last two columns give respectively the values of a calculated from 

 equation (75), and the value of a given by the Theory of Gases. The two 

 sets of values do not agree exactly this could not be expected when we 

 remember the magnitude of the errors introduced in treating the molecules 

 as spherical. But what agreement there is supplies very significant evidence 

 as to the truth of the theory of molecular polarisation. 



151. It still remains to explain what physical property of the molecule 

 justifies us in treating its surface as a perfect conductor. It has already 

 been explained that all matter has associated with it or perhaps entirely 



* Authorities : 1. Boltzmann, Wiener Sitzungsber. 69, p. 795. 



2. J. KlemenciS, Wiener Sitzungsber. 91, p. 712. 



3. These values are calculated from the refractive indices for Sodium Light, 

 t Jeans, Dynamical Theory of Gases, p. 340. 



