Second Letter from Dr. Hare to Prof. Faraday. 5 



those which may be considered in the one case as ordinary, in the 

 other, extraordinary. Thus, take the case cited in your reply, 

 (viii, ix, X.) Does the interposition of the spheres alter the char- 

 acter of the inductive power in the sphere A? 



Either the force exercised by the charge in A, is like that 

 of gravitation, altogether independent of the influence of inter- 

 vening bodies ; or, like that of light, it is dependent on the agency 

 of an intervening matter. Agreeably to one doctrine, the matter, 

 by means of which luminous bodies act, operates by its transmis- 

 sion from the luminous surface to that illumined. Agreeably to 

 another doctrine, the illuminating matter operates by its undula- 

 tions. If the inductive power of electrified bodies be not anal- 

 ogous to gravitation, it must be analogous to the power by which 

 light is produced so far as to be dependent on intervening matter. 

 But were it to resemble gravitation, like that force it would be 

 uninfluenced by such matter. If your experiments prove that 

 electrical induction is liable to be modified by intervening matter, 

 it is demonstrated that in its mode of operation it is analogous to 

 light, not to gravitation. It is then proved, that, agreeably to 

 your doctrine, electrical induction requires the intervention of 

 matter, but you admit that it acts across a vacuum, and of course, 

 acts without the presence of ponderable matter. Yet it requires 

 intervening matter of some kind, and, since that matter is not 

 ponderable, it must of necessity be imponderable. When light 

 is communicated from a luminous body in the centre of an ex- 

 hausted sphere, agreeably to the undulatory hypothesis, its efficacy 

 is dependent on the waves excited in an intervening impondera- 

 ble medium. Agreeably to your electropolar hypothesis, the in- 

 ductive efficacy of an electrified body in an exhausted sphere 

 would be due to a derangement of electric equilibrium, by which 

 an electric state opposite to that at the centre would be produced 

 at the surface of the containing sphere, (xxvi, xxvii.) This 

 case you consider as one of extraordinary induction, but when air 

 is admitted into the hollow sphere, or when concentric spheres 

 are interposed, you hold it to be a case of ordinary induction. 

 Let us then, in the case of the luminous body, imagine that con- 

 centric spheres of glass are interposed, of which the surfaces are 

 roughened by grinding. In consequence of the roughness thus 

 produced, the rays instead of proceeding in radii from the central 



