Substances which allow electricity to pass freely are called conductors, and those which do not are called non-conductors; while between the two extremes are many substances which are called partial conductors. It may be said here that no substance is quite perfect in either respect, for all conductors offer some resistance to the passage of electricity, while all non-conductors possess some conducting power. Amongst conductors are metals, acids, water, and the human body; cotton, linen, and paper are partial conductors; and air, resin, silk, glass, sealing-wax, and gutta-percha are non-conductors. When a conductor is guarded by a non-conductor so that its electricity cannot escape, it is said to be insulated, from Latin, _insula_, an island; and non-conductors are also called “insulators.” So far we have mentioned only the electric charge produced on the substance rubbed, but the material used as rubber also becomes electrified. The two charges, however, are not alike, but one is always positive and the other negative. For instance, if glass is rubbed with silk, the glass receives a positive, and the silk a negative charge. It also can be shown that the two opposite charges are always equal in quantity. The two kinds of electricity are generally represented by the signs + and -, the former standing for positive and the latter for negative electricity. The electricity produced by rubbing, or friction, is known as Static Electricity; that is, electricity in a state of rest, as distinguished from electricity in motion, or current electricity. The word static is derived from a Greek word meaning to stand. At the same time it must be understood that this electricity of friction is at rest only in the sense that it is a prisoner, unable to move. When we produce a charge of static electricity on a glass rod, by rubbing it, the electricity would escape fast enough if it could. It has only two possible ways of escape, along the rod and through the air, and as both glass and air are non-conductors it is obliged to remain at rest where it was produced. On the other hand, as we have seen, the electricity produced by rubbing a metal rod which is not protected by an insulating handle escapes instantly, because the metal is a good conductor.