That some substances conduct or convey the electric fluid to a distance better than others, is a fact known to the earliest electricians; but on what power or property of the body this superiority depends, is a question on which different opinions still seem to prevail. We constantly hear the expressions “electricity is attracted by metals; the lightning is attracted by the metallic points of a conducting rod,” and other expressions of similar import,—all signifying that a powerful attraction does exist between metals and the electric fluid. Now the contrary is really the fact, those bodies being the best conductors which have the least attraction for the electric fluid. From the profound mathematical investigations of M. Poisson, and the luminous writings of M. Biot, it appears that these philosophers consider the metals merely as forming the passive interior of a vessel, of which the exterior surface is the ambient air; and that the electric fluid rushes along between the atmospheric boundary and the surface of the metal, where it finds an easy passage. We are therefore to consider the metals as quite passive in the conduction of the electric fluid, and that the prime mover is the repulsive energy existing between similar atoms of the compound electric fluid. When a metallic ball connected with the earth is placed near the prime conductor, the vitreous electricity surrounding the conductor repels the vitreous electricity of the ball, and forces it to glide along to a greater distance, whilst the ball will now be surrounded by a thin film of the resinous fluid. The vitreous electricity of the conductor thus finding an easier passage in the direction of the ball, and being in a high state of tension, will, like every other elastic fluid, glide along in the direction of the ball as if it had actually been attracted by that body. The reason why it does not strike off with equal facility to a vitreous body is, not because it is less attracted by that body, but simply because it is unable to decompose with the same facility the natural electricity belonging to the glass, on account of the powerful attraction existing between the atoms of the glass and those of the electric fluid. If the glass be thin and a metallic conductor placed in its interior, the vitreous electricity will act through the glass, decompose the fluid in the metallic conductor, and then actually strike through the glass in the direction of the metal where the resistance is least. Exp. I. On the ends of two thermometer tubes I blew two balls of extreme tenuity. I then introduced two pieces of brass wire into the tubes till the ends reached within a small distance of the interior surface of the balls. Having brought the other ends of the tubes together, I joined them at the flame of the blowpipe, so that I had now a metallic conductor completely surrounded with glass. This being placed on a stand, and one of the balls brought near the prime conductor, I found I could take sparks, for any length of time, from the other end, in the same manner as if the glass had not been interposed. When the bulbs were about the thickness of those of a common thermometer, I observed that if sparks were taken for any length of time from the same place, they afterwards chose the same tract. I naturally concluded that the glass had been pierced, though I could not determine it by the naked eye. I found, however, that if the tubes were again separated and the air partially expelled from one of the balls by heat, and the open end of the tube placed in a vessel containing mercury, the mercury rose in the tube, but after a short time it again sunk to its proper level; clearly showing that the bulb had been pierced, though the aperture was extremely minute. I now began to suspect that in every case in which glass seemed to have been freely permeated by the electric fluid, that the fluid had been either silently conducted through it, or that, if carefully examined, it would have been found to have forced out some of the atoms of the glass. I therefore repeated the experiment with glass as thin as it could be blown without bursting, and found that the electric fluid would in that case freely permeate it; and that by no known method could I detect the smallest aperture in the glass.
The Country Cart of Ancient Greece
