Seco7id Letter fj'om Dr. Hare to Prof. Faraday. 11 



in as much as either prevails, the other must be counteracted. 

 Conduction conveys to my mind the idea of permeahility to the 

 electric fluid, insulation that of impermeability ; and I am unable 

 to understand how these irreconcilable properties can be produced 

 by a difterence of degree in any one property of electrics and 

 conductors. 



If, as your infer, glass have, comparatively with metals, an 

 almost infinitely minute degree of the conducting power, is it 

 this power which enables it to prevent conduction, or in other 

 words to insulate ? Let it be granted that you have correctly 

 supposed conduction to comprise both induction and discharge, 

 the one following the other in perfect conductors within an inex- 

 pressibly ' rief interval. Insulation does not prevent induction ; 

 but, so r as it goes, it prevents discharge. In practice this part 

 of the piocess of conduction does not take place through glass 

 during any time ordinarily allotted to our experiments, however 

 correct you may have been in supposing it to have ensued before 

 the expiration of a year or more in the case of the tubes which 

 you had sealed after charging them. But conceding it to have 

 been thus proved that glass has, comparatively with metals, an 

 infinitely small degree of the conducting power; is it this minute 

 degree of conducting power, which enables it to prevent conduc- 

 tion, or in other words to insulate ? 



Induction arises from one or more properties of electricity, in- 

 sulation from a property of ponderable matter; and although 

 there be no matter capable of preventing induction, as well as dis- 

 charge, were there such a matter, would that annihilate insula- 

 tion ? On the contrary would it not exhibit the property in the 

 highest perfection ? 



As respects the residual charge of a battery, is it not evi- 

 dent that any electrical charge which affects the surface of the 

 glass, must produce a corresponding effect upon the stratum of 

 air in contact with the coating of the glass? If we place one 

 coating between two panes, will it not enable us to a certain ex- 

 tent to charge or discharge both ? Substituting the air for one of 

 them, will it not, in some measure, be liable to an affection simi- 

 lar to that of the vitreous surface for which it is substituted ? In 

 the well known process of the condensing electromotor, the plate 

 of air interposed between the disks is, I believe, universally ad- 

 mitted to perform the part of an electric, and to be equivalent in 

 its properties to the glass in a coated pane. 



