Dr. Franklin’s theory has gained the greatest reputation, from the easy solution it affords of all the phenomena of the Leyden phial. The fluid is supposed to move with the greatest ease in bodies which are conductors; but with extreme difficulty in _electrics per se_; in so much that glass is absolutely impermeable to it. It is also supposed that all electrics, and particularly glass, on account of the smallness of their pores, do at all times contain an exceedingly great, and always an equal quantity of this fluid; so that no more can be thrown into any one part of any electric substance, except the same quantity go out at another, and the gain be exactly equal to the loss. These things being premised, the phenomena of charging and discharging a plate of glass, or a Leyden phial, may be easily solved. In the usual manner of electrifying by a smooth glass globe or cylinder, all the electric matter is supplied by the rubber, from all the bodies which communicate with it. If it be made to communicate with nothing but one of the coatings of a glass plate, while the prime-conductor is connected with the other, that side of the glass which communicates with the rubber, must necessarily be exhausted in order to supply the conductor, which must convey the whole of it to the coating with which it is connected. By this operation, therefore, the electric fluid becomes almost entirely exhausted from one side of the plate, while it is as much accumulated on the other; and the discharge is made by the electric fluid rushing, as soon as an opportunity is given it by means of proper conductors, from the side which was overloaded to that which was exhausted. It is not however necessary to this theory, that the same individual particles of electric matter which were thrown upon one side of the plate, should make the whole circuit of the intervening conductors, especially in very great distances, so as actually to arrive at the exhausted side. It may be sufficient to suppose, that the additional quantity of fluid displaces and occupies the place of an equal portion of the natural quantity of fluid, belonging to those conductors in the circuit which lay contiguous to the charged side of the glass. This displaced fluid may drive forwards an equal quantity of the same matter in the next conductor; and thus the progress may continue, till the exhausted side of the glass is supplied by the fluid naturally existing in the conductors contiguous to it. To account for the velocity with which electricity passes through good conductors, Dr. Franklin compares the electricity in the conductors, to a wire in the bore of a tube, which it exactly fills.—If one end of this wire be moved forward, every other part of it will move in the same direction, and at the same instant.