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.
A very simple means may be adopted of showing what is thought to be one of the many evidences in favour of two electricities. If the wire carrying the current flowing from the machine, is passed over paper covered with nitrate of silver, it produces no change upon it; but if the wire which conveys the current to the instrument, when it is excited, is passed over the same paper, the silver salt is decomposed.[139] We may, however, explain this result in a satisfactory manner, upon the hypothesis that the decomposition is produced by the abstraction of electricity, rather than by any physical difference in the fluid itself. By frictional electricity we may produce curious molecular disturbances, and give rise to molecular re-arrangements, which have been called “electrical images,” in glass, in stone, and in the apparently less tractable metals: these images are rendered visible by the manner in which, according to their electrical states, some lines receive any particular powder, or vapour, which is repelled from other spaces. Many of the great natural phenomena, such as Lightning and Thunder, the Aurora Borealis, and Meteors, may be imitated in a curiously exact manner by the electrical machine and a few familiar arrangements.[140]