2. It is probable, that all bodies are more or less perfect conductors, as they have less or more of either of the electric ethers combined with them; as mentioned in Preliminary Proposition, No. VI. as they may then less resist the passage of either of the ethers through them. Whence some conducting bodies admit the junction of these spontaneous electric atmospheres, in which the proportions or quantities of the two ethers are not very different, with greater facility than others.
Thus in the common experiments, where the vitreous or resinous ether is accumulated by art, metallic bodies have been esteemed the best conductors, and next to these water, and all other moist bodies; but it was lately discovered, that dry charcoal, recently burnt, was a more perfect conductor than metals; and it appears from the experiments discovered by Galvani, which have thence the name of Galvanism, that animal flesh, and particularly perhaps the nerves of animals, both which are composed of much carbon and water, are the most perfect conductors yet discovered; that is, that they give the least resistance to the junction of the spontaneous electric atmospheres, which exist round metallic bodies, and which differ very little in respect to the proportions of their vitreous and resinous ingredients.
Thus also, though where the accumulated electricities are dense, as in charging a coated glass-jar, the glass, which intervenes, may be of considerable thickness, and may still become charged by the stronger attraction of the secondary electric ethers; but where the spontaneous adhesive electric atmospheres are employed to charge plates of air, as in the Galvanic pile, or probably to charge thin animal membranes or cuticles, as perhaps in the shock given by the torpedo or gymnotus, it seems necessary that the intervening nonconducting plate must be extremely thin, that it may become charged by the weaker attraction of these small quantities or difference of the spontaneous electric atmospheres; and in this circumstance only, I suppose, the shocks from the Galvanic pile, and from the torpedo and gymnotus, differ from those of the coated jar.
3. When atmospheres of electricity, which do not differ much in the quantity or proportion of their vitreous and resinous ethers, approach each other, they are not easily or rapidly united; but the predominant vitreous or resinous ether of one of them repels the similar ether of the opposed atmosphere, and attracts the contrary kind of ether.
The slowness or difficulty with, which atmospheres, which differ but little in kind or in density, unite with each other, appears not only from the experiment of Mr. Canton above related, but also from the repeated smaller shocks, which may be taken from a charged coated jar after the first or principal discharge, if the conducting medium has not been quickly removed, as is also mentioned above.
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 adhering 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 conductor; 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 conductors 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 beneath 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 accumulated electricity, as it passes with greater velocity than the spontaneous accumulations of it, will readily permeate the muscles or other moist parts of animal bodies; whereas the spontaneous accumulations of electricity seem to require the best of all conductors, as animal nerves, to facilitate their passage.
4. In the Galvanic pile of Volta this electric shock becomes so much increased, as to pass by less perfect conductors, and to give shocks to the arms of the conducting person, if the cuticle of his hands be moistened, and even to show sparks like the coated jar; which appears to be effected in this manner. When a plate of silver is laid horizontally on a plate of zinc, the plate of air between them becomes charged like a coated jar; as the silver, naturally possessing more vitreous electric ether, repels the vitreous ether, which the zinc possesses in less quantity, and attracts the resinous ether of the zinc. Whence the inferior surface of the plate of zinc abounds now with vitreous ether, and its upper surface with resinous ether. Beneath this pair of plates lay a cloth moistened with water, or with some better conductor, as salt and water, or a slight acid mixed with water, or volatile alcali of ammoniac mixed with water, and this vitreous electric ether on the lower surface of the zinc plate will be given to the second silver plate which lies beneath it; and thus this second silver plate will possess not only its own natural vitreous atmosphere, which was denser or in greater quantity than that of the zinc plate next beneath it, but now acquires an addition of vitreous ether from the zinc plate above it, conducted to it through the moist cloth.