Eledtricity in Equilibrium. 50 



contact. Then, denoting by Q, Q' the quantities of electricity- 

 constituting the charges before, and q, 9' after contact, we shall 

 have, by the principles already developed, 



Also 



Q + Q!=:q + q'. 

 Hence we deduce -. 



_ kp + k'p' 

 ""^ k + k' ' 



In the experiment described, one of the dielectrics is air. Hence, 

 to obtain the required formula, we may put k' = 1> in this equa- 

 tion, and then resolve for k. 

 Thus we find 



p-a 

 If only one of the apparatus be originally charged, according 

 as it is the first or the second, we shall have 



or 



If the substance examined (the dielectric of the first appa- 

 ratus) be any gas, or air in a different state as to pressure or 

 temperature from the air of the second apparatus, Faraday 

 always finds the intensity after contact to be half the original 

 intensity, and hence for every gaseous body k = l. 



If the dielectric of the first apparatus be solid, the intensity 

 after contact is found to 1)3 greater than half the original inten- 

 sity when the first, and less than half when the second is the 

 apparatus originally charged. Hence for a solid dielectric ^> 1. 

 For sulphur Faraday finds the value to be rather more than 2'2; 

 for shell-lac, about 2; and for flint-glass, greater than 1*76. 



The commonly received ideas of attraction and repulsion 

 eiercised at a distance, independently of any intervening me- 

 dium, are quite consistent with all the phsenomena of electrical 

 action which have been here adduced. Thus we may consider 

 the particles of air in the neighbourhood of electrified bodies to 

 be entirely uninfluenced, and therefore to produce no effect in 

 the resultant action on any point : but the particles of a solid 

 non-conductor must be considered as assuming a polarized state 

 when under the influence of free electricity, so as to exercise 

 attractions or repulsions on points at a distance, which, with the 



