Hence there exists for an instant a charged plate of air between the finger and the prime conductor, with an accumulation of vitreous ether on one side of it, and of resinous ether on the other side of it; and lastly these two kinds of electric ethers suddenly unite by their powerful attraction of each other, explode, and give out heat and light, and rupture the plate of nonconducting air, which separated them.
The rupture or disjunction of the plate of air is known by the sound of the spark, as of thunder; which shows that a vacuum of air was previously produced by the explosion of the electric fluids, and a vibration of the air in consequence of the sudden joining again of the sides of the vacuum.
The light which attends electric sparks and shocks, is not accounted for by the Theory of Dr. Franklin. I suspect that it is owing to the combination of the two electric ethers, from which as from all chemical explosions both light and heat are set at liberty, and because a smell is said to be perceptible from electric sparks, and even a taste which must be deduced from new combinations, or decompositions, as in other explosions: add to this that the same thing occurs, when electric shocks are passed through eggs in the dark, or through water, a luminous line is seen like the explosion of a train of gunpowder; lastly, whether light is really produced in the passage of the Galvanic electricity through the eyes, or that the sensation alone of light is perceived by its stimulating the optic nerve, has not yet been investigated; but I suspect the former, as it emits light from its explosion even in passing through eggs and through water, as mentioned above.
VIII. The shock from the coated jar, and of electric condensation.
1. When a glass jar is coated on both sides, and either vitreous or resinous electricity is thrown upon the coating on one side, and there is a communication to the earth from the other side, the same thing happens as in the plate of air between the finger and prime conductor above described; that is, the accumulated electricity, if it be of the vitreous kind, on one coating of the glass jar will attract the resinous part of the electricity, which surrounds or penetrates the coating on the other side of the jar, and also repel the vitreous part of it; but this occurs on a much more extensive surface than in the instance of the plate of air between the finger and prime conductor.
The difference between electric sparks and shocks consists in this circumstance, that in the former the insulating medium, whether of air, or of thin glass, is ruptured in one part, and thus a communication is made between the vitreous and resinous ethers, and they unite immediately, like globules of quicksilver, when pressed forcibly together: but in the electric shock a communication is made by some conducting body applied to the other extremities of the vitreous, and of the resinous atmospheres, through which they pass and unite, whether both sides of the coated jar are insulated, or only one side of it.
And in this line, as they reciprocally meet, they appear to explode and give out light and heat, and a new combination of the two ethers is produced, as a residuum after the explosion, which probably occupies much less space than either the vitreous or resinous ethers did separately before. At the same time there may be another unrestrainable ethereal fluid yet unobserved, given out from this explosion, which rends oak trees, bursts stone-walls, lights inflammable substances, and fuses metals, or dissipates them in a calciform smoak, along with which great light and much heat are emitted, or these effects are produced by the heat and light only thus set at liberty by their synchronous and sudden evolution.
2. The curious circumstance of electric condensation appears from the violence of the shock of the coated jar compared with the strongest spark from an insulated conductor, though the latter possesses a much greater surface; when vitreous electric ether is thrown on one side of a coated jar, it attracts the resinous electric ether of the other side of the coated jar; and the same occurs, when resinous ether is thrown on one side of it, it attracts the vitreous ether of the other side of it, and thus the vitreous electric ether on one side of the jar, and the resinous ether on the other side of it become condensed, that is accumulated in less space, by their reciprocal attraction of each other.
This condensation of the two electric ethers owing to their reciprocal attraction appears from another curious event, that the thinner the glass jar is, the stronger will the charge be on the same quantity of surface, as then the two ethers approaching nearer without their intermixing attract each other stronger, and consequently condense each other more. And when the glass jar is very thin the reciprocal attractive powers of the vitreous and resinous ether attract each other so violently as at length to pass through the glass by rupturing it, in the same manner as a less forcible attraction of them ruptures and passes through the plate of air in the production of sparks from the prime conductor.
As these two ethers on each side of a charged coated jar so powerfully attract each other, when a communication is made between them by some conducting substance as in the common mode of discharging an electrised coated jar, they reciprocally pass to each other for the purpose of combining, as some chemical fluids are known to do; as when nitrous gas and oxygen gas are mixed together; whence as these fluids pass both ways to intermix with each other, and then explode; a bur appears on each side of a quire of paper well pressed together, when a strong electric shock is passed through it; which is occasioned by their explosion, like a train of gunpowder, and consequent emission of some other ethereal fluid, either those of heat and light or of some new one not yet observed. Whence it becomes difficult to explain, according to the theory of Dr. Franklin, which way the electric fluid passed; and which side of the coated jar contained positive and which the negative charge according to that doctrine.