A Leyden phial,--so called from its inventor, Musschenbroek, having resided at Leyden,--is merely a glass bottle lined within and without, to within a few inches of the top, with a metal coating. If a wire or chain, carrying an electric current, is allowed to dip to the bottom of the bottle, the inner coat of the jar becomes charged, or gathers an excess, whilst the outer one is in its natural condition--one is said to be in a _positive_, and the other in a _negative_ state. If the two coatings are now connected by a good conductor, as a piece of copper wire, passing from one to the other, the outside to the inside, a discharge, arising from the establishment of the equilibrium of the two coatings, takes place; and, if the connection is made through the medium of our bodies, we are sensible of a severe disturbance of the nervous system. The cause of the conducting and non-conducting powers of bodies we know not; they bear some relation to their conducting powers for caloric; but they are not in exact obedience to the same laws. When we consider that resin, a comparatively soft body, in which, consequently, cohesive attraction is not very strong, is an imperfect conductor, and that copper, in which cohesion is much more powerful, is a good conductor, we may be disposed to consider that it is regulated by the closer approximation of the particles of matter. But in platinum the corpuscular arrangement must be much more dense than it is in copper, and yet it is, compared with it, a very bad conductor.[138] We have now learnt that we may, by friction, excite the electricity in a vitreous substance; but it must not be forgotten that we cannot increase the quantity which is, under ordinary conditions, natural to the electric; to do so, we must in some way establish a channel of communication with the earth, from which, through the medium we excite, we draw our supply. We have the means of confining this mighty force within certain limits of quantity and of time. If we place bodies which are susceptible of electrical excitation in a sensible degree upon insulating ones, we may retain for a considerable time the evidences of the excitement, in the same way as with the Leyden jar; but there is a constant effort to maintain a balance of conditions, and the body in which we have accumulated any extraordinary quantity by conduction soon returns to its natural state.