Second Letter from Dr. Hare to Prof. Faraday. 9 



If as you suggest, the interposition of ponderable particles have 

 any tendency to promote inductive influence, (xiv,) there must 

 be some number of such particles by which this effect will be 

 best attained. That number being interposed, I cannot imagine 

 how the intensity of any electro-polarity, thus created in the 

 intervening particles, can, by a diminution of their number, ac- 

 quire a proportional increase ; evidently in no case can the excite- 

 ment in the particles exceed that of the " inductric" surfaces 

 whence the derangement of electrical equilibrium arises. 



l^'he repulsive power of electricity being admitted to be in- 

 versely as the squares of the distances, you correctly infer that 

 the aggregate influence of an electrified ball, B, situated at the 

 centre of a hollow sphere, C, will be a constant quantity, what- 

 ever may be the diameter of C. This is perfectly analogous to 

 the illuminating influence of a luminous body situated at the 

 centre of a hollow sphere, which would of course receive the 

 whole of the light emitted whatever might be its diameter, pro- 

 vided that there were nothing interposed to intercept any portion 

 of the rays. But in order to answer the objection which I have 

 advanced, that the diminution of the density of a ^^ dielectric^'' 

 cannot be compensated by any consequetit increase of inductive 

 intensity, it must be shown in the case of several similar hollow 

 spheres, in which various numbers of electrified equidistant balls 

 should exist, that the influence of such balls upon each other, and 

 upon the surfaces of the spheres, would not be directly as the 

 number of the balls and inversely as the size of the containing 

 spaces. Were gas lights substituted for the balls, it must be ev- 

 ident that the intensity of the light, in any oile of the spheres, 

 would be as the number of lights which it might contain. Now 

 one of your illustrations (viii,) above noticed makes light and 

 electrical induction, obey the same law as respects the influence 

 of distance upon the respective intensities. 



From these considerations, and others above stated, I infer, 

 that if electrical induction were an action of particles in prox- 

 imity operating reciprocally with forces varying in intensity with 

 the squares of the distances, their aggregate influence upon any 

 surfaces, between which they might be situated, would be pro- 

 portionable to their number ; and since experience demonstrates 

 that the inductive power is not diminished by the reduction of 

 the number of the intervening particles, I conclude that it is in- 



Vol. xLi, No. 1.— April-June, 1641. 2 



