Faraday. 207 



through it : it was called by Faraday the specific inductive 

 capacity of the insulator.* 



The discovery of this property of insulating substances or 

 dielectrics raised the question as to whether it could be 

 harmonized with the old ideas of electrostatic action. Consider, 

 for example, the force of attraction or repulsion between two 

 small electrically- charged bodies. So long as they are in air, 

 the force is proportional to the inverse square of the distance ; 

 but if the medium in which they are immersed be partly 

 changed e.g., if a globe of sulphur be inserted in the intervening 

 space this law is no longer valid : the change in the dielectric 

 affects the distribution of electric intensity throughout the 

 entire field. 



The problem could be satisfactorily solved only by forming 

 a physical conception of the action of dielectrics : and such a 

 conception Faraday now put forward. 



The original idea had been promulgated long before by his 

 master Davy. Davy, it will be remembered,f in his explanation 

 of the voltaic pile, had supposed that at first, before chemical 

 decompositions take place, the liquid plays a part analogous to 

 that of the glass in a Leyden jar, and that in this is involved an 

 electric polarization of the liquid molecules.^ This hypothesis 

 was now developed by Faraday. Keferring first to his own work 

 on electrolysis, he asserted that the behaviour of a dielectric is 

 exactly the same as that of an electrolyte, up to the point at 

 which the electrolyte breaks down under the electric stress ; a 

 dielectric being, in fact, a body which is capable of sustaining 

 the stress without suffering decomposition. 



" For," he argued,|| " let the electrolyte be water, a plate of 

 ice being coated with platina foil on its two surfaces, and these 



* Exp. Res., 1252 (1837). Cavendish had discovered specific inductive capacity 

 long before, but his papers were still unpublished. 



t Cf. p. 77. 



\ This is expressly stated in Davy's Elements of Chemical Philosophy (1812), 

 Div. i, 7, where he lays it down that an essential " property of non-conductors" 

 is "to receive electrical polarities." 



$ Exp. Res., 1164, 1338, 1343, 1621. 



|| Exp. Res., 1164. 



