Chemical Theory of Electricity and Magnetism. 65 



Hence those atmospheres of either kind of electric matter, which 

 differ but very little from each other in kind or quantity, require the 

 most perfect conductors to cause them to unite. Thus it appears by 

 Mr. Bennet's doubler, as mentioned in the Preliminary Proposition, 

 No. VI. that the natural adhesive atmosphere round silver contains 

 more vitreous electricity than that naturally round zinc; but when 

 thin plates of these metals, each about an ounce in weight, are laid 

 on each other, or moderately pressed together, their atmospheres do 

 not unite. For metallic plates, which when laid on each other, do not 

 adhere, cannot be said to be in real contact, of which their not ad- 

 hering is a proof; and in consequence a thin plate of air, or of their 

 own repulsive ethers exists between them. 



Hence when two plates of zinc and silver are thus brought in to the 

 vicinity of each other, the plate of air between them, as they are not 

 in adhesive contact, becomes like a charged coated jar; and if these 

 two metallic plates are touched by your dry hands, they do not unite 

 their electricities, as the dry cuticle is not a sufficiently good con- 

 ductor; but if one of the metals be put above, and another under the 

 tongue, the saliva and moist mucous membrane, muscular fibres, and 

 nerves, supply so good a conductor, that this very minute electric 

 shock is produced, and a kind- of pungent taste is perceived. 



When a plate or pencil of silver is put between the upper lip and 

 the gum, and a plate or pencil of zinc under the tongue, a sensation 

 of light is perceived in the eyes, as often as the exterior extremities 

 of these metals are brought into contact; which is owing in like 

 manner to the discharge of a very minute electric shock, which would 

 not have been produced but by the intervention of such good con- 

 ductors as moist membranes, muscular fibres, and nerves. 



In this situation, a sensation of light is produced in the eyes; 

 which seems to show, that these ethers pass through nerves more 

 easily, than through muscular flesh simply; since the passage of them 

 through the retina of the eyes from the upper gum to the parts be- 

 neath the tongue is a more distant one, than would otherwise appear 

 necessary. It is not so easy to give the sensation of light in the eyes 

 by passing a small shock of artificially accumulated electricity through 

 the eyes (though this may, I believe, be done) because this artificial 



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