4'7'1; Dr. Hare's Second Letter to Prof, Faraday. 



begin to be exposed to a minute portion of the vitreous fluid; 

 would this be a sufficient reason for confounding the two 

 fluids, and treating the phaenomena to which they give rise as 

 the effect of one only ? 



()3. But the discussion into which your illustrations have 

 led me refers to things, whereas conduction and insulation, as 

 I understand them, are opposite and incompatible properties; 

 so that, inasmuch as either prevails, the other must be coun- 

 teracted. Conduction conveys to my mind the idea of jy^r- 

 meability to the electric fluid, insulation that oi impermeability, 

 and I am unable to understand how these irreconcilable pro- 

 perties can be produced by a difference of degree in any one 

 property of electrics and conductors. 



64. If, as you 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 inexpressibly brief interval: insulation does not pre- 

 vent induction, but, so far as it goes, prevents discharge. In 

 practice, this part of the process 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 conduction, or, in other 

 words, to insulate? 



65. Induction arises from one or more properties of elec- 

 tricity, insulation from a property of ponderable matter ; and 

 although there be no matter capable of preventing induction, 

 as well as discharge, were there such a matter, would that 

 annihilate insulation ? On the contrary, would it not exhibit 

 the property in the highest perfection? 



66. As respects the residual charge of a battery, is it not 

 evident that any electrical change which affects the surface of 

 the glass, must produce a corresponding effect upon the stra- 

 tum 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 extent, to charge or discharge both? Substituting 

 the air for one of them, will it not in some measure be liable 

 to an affection similar to that of the vitreous surface, for which 

 it is substituted ? In the well-known process of the conden- 



