212 Faraday. 



and this gives on averaging 



div E = pi + jo, 



where pi denotes the volume-density of free electric charge, 

 i.e. excluding that in the doublets ; or 



div (E + P) = Plt 

 or div (* E) = p lt 



This is the fundamental equation of electrostatics, as modified 

 in order to take into account the effect of the specific inductive 

 capacity . 



The conception of action propagated step by step through a 

 medium by the influence of contiguous particles had a firm hold 

 on Faraday's mind, and was applied by him in almost every 

 part of physics. " It appears to me possible," he wrote in 

 1838,* " and even probable, that magnetic action may be 

 communicated to a distance by the action of the intervening 

 particles, in a manner having a relation to the way in which 

 the inductive forces of static electricity are transferred to a 

 distance ; the intervening particles assuming for the time more 

 or less of a peculiar condition, which (though with a very 

 imperfect idea) I have several times expressed by the term 

 electro-tonic state."^ 



The same set of ideas sufficed to explain electric currents. 

 Conduction, Faraday suggested,* might be " an action of 

 contiguous particles, dependent on the forces developed in 

 electrical excitement ; these forces bring the particles into a 

 state of tension or polarity ; and being in this state the 

 contiguous particles have a power or capability of communicating 

 these forces, one to the other, by which they are lowered and 

 discharge occurs." 



* Exp Res., 1729. 



f This name had been devised in 1831 to express the state of matter subject to 

 magneto-electric induction ; cf. Exp.^Res., 60. 

 J Exp. Res. iii, p. 513. 

 As in electrostatic induction in dielectrics. 



