.:, 
Second Letter from 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 permeability to the 
electric fluid, insulation that of impermeability ; and I am unable 
to understand how these irreconcilable properties can be produced 
by a difference 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 brief interval. Insulation does not prevent induction ; 
but, so far as it goes, it 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 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 electromoter, 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. 
