732 Sir J. J. Thomson on the Application of the 



wave-lengths calculated by the equation (5), where cjd has 

 the value 1*7, so that mj» 2 = 5'224 e 2 A/M. 



Metal. 



A. 



(M/1-64X 10-24). 



X calculated. 



X observed. 



Sodium 



. -971 



23 



3234 



3400 



Potassium.. 



. -862 



39 



4457 



4400 



Rubidium ,. 



. 1-532 



85-45 



41)40 



4800 



It will be noticed that the agreement is very satisfactory. 



Specific Inductive Capacity of the Solid. 



Let us suppose that a vertical electric force F acts on the 

 solid, and that it displaces all the electrons through a 

 distance p, and all the atoms through a distance z, then by 

 equation (4) the force tending to restore the electron to 

 equilibrium is A(p—t) ; hence for equilibrium we must 

 have 



A(p-z)=¥e (6) 



If the distance between the electron and the atom is 

 increased by [p — z) each of the cells has an electric moment 

 equal to e(p—z), and the electric moment per unit volume 

 will be e(p — z)j8d 5 . If K be the specific inductive capacity 

 of the solid this moment is (K — l)F/47r, hence 



gQo-g) (K- l)F, 



8d 3 " ' 4tt ', 

 or by equation (6) 



47T6 2 47T 



K-l 



8d 3 A '6'07(c/d) 

 If p 2 is the limiting frequency given by (5) we see that 



9 _ 47ri 2 A 

 mF ~M(K-l)' 



an expression from which c/d has been eliminated. 



Potential Energy per unit volume of the Solid. 



If the forces between all the electrical charges varied 

 inversely as the square of the distance between them the 

 potential energy per unit volume would be iS^i^AO* where 

 <? l5 <? 2 are two charges separated by a distance r and the 

 summation is extended over all the charges in unit volume. 

 In addition to the forces varying inversely as the square of 

 the distance, there are forces between positive charges and 



