6 A letter to Prof. Faraday. 



leave the element of perfection at one extremity, we involve the 

 element of perfection at the opposite." Might it not be said of 

 light and darkness, or of opaqueness and translucency ; in which 

 case to resort to your language again, it might be added " espe 

 cially as we have not in nature, a case of perfection at one ex- 

 tremity or the other." But if there be not in nature, any two 

 bodies of which one possesses the property of perfectly resisting 

 the passage of electricity, while the other is endowed with the 

 faculty of permitting its passage without any resistance ; does this 

 affect the propriety of considering the qualities of insulation and 

 conduction in the abstract, as perfectly distinct, and inferring that 

 so far as matter may be endowed with the one property, it must 

 be wanting in the other ? 



Have yoLi ever known electricity to pass through a pane of 

 sound glass? My knowledge and experience create an impres- 

 sion that a coated pane is never discharged through the glass un- 

 less it be cracked or perforated. That the property by which 

 glass resists the passage of electricity, can be confounded with 

 that which enables a metallic wire to permit of its transfer, agree- 

 ably to Wheatstone's experiments, with a velocity greater than 

 that of the solar rays, is to my mind inconceivable. 



You infer that the residual charge of a battery arises from the 

 partial penetration of the glass by the opposite excitements. But 

 if glass be penetrable by electricity, why does it not pass through 

 it without a fracture or perforation ? 



According to your doctrine, induction consists "in a forced state 

 of polarization in contiguous rows of the particles of the glass" 

 (1.300) ; and since this is propagated from one side to the other, 

 it must of course exist equally at all depths. Yet the partial 

 penetration suggested by you, supposes a collateral affection of 

 the same kind, extending only to a limited depth. Is this con- 

 sistent ? Is it not more reasonable to suppose that the air in the 

 vicinity of the coating gradually relinquishes to it a portion of 

 free electricity, conveyed into it by what you call ^^ convection.^^ 

 The coating being equally in contact with the air and glass, it 

 appears to me more easy to conceive that the air might be pene- 

 trated by the excitement, than the glass. 



In paragraph 1300, I observe the following statement : ''When 

 a Leyden Jar is charged, the particles of the glass are forced 

 into this polarized and co?istrained condition by the electricity of 



