We know that certain substances when rubbed with silk or wool acquire the property of attracting other substances. But in the case of a rod of glass or stick of wax, the attraction will only be perceived when the rubbing has been applied. But metal will behave differently. Any part of the metal rod will continue to attract. So metals are CONDUCTORS of electricity; while glass, wax, silk, amber, sulphur, etc., are _bad_, or NON-CONDUCTORS. Metals are the best conductors we have, but trees, plants, liquids, and the bodies of animals, including men, are all good conductors of electricity. Dry air is a bad conductor. [Illustration: Fig. 204.—Attraction and repulsion.] There are two kinds of electricity, known as positive and negative (_plus_ or _minus_), vitreous or resinous. We saw in fig. 200 that we can attract a small ball of pith or cork by a piece of sealing-wax rubbed with flannel. If we then present a glass rod rubbed with silk to it, it will be equally attracted, but will be at once repelled; and after being so repelled, if we put the wax to it, it will be attracted to the sealing-wax again. So wax at first attracts then repels the ball, and so does glass, but either will attract the ball if presented alternately (fig. 204). The reason for this is as follows:— When we have rubbed the glass with silk, we charge it with positive electricity, and when the rod touches the ball, the latter imbibes that electricity, and flies away from the glass rod. The sealing-wax imparts negative electricity in the same way. The law is, that bodies charged with the same kind of electricity _repel_ each other, and those containing the opposite kinds _attract_ each other. Positive _repels_ positive; negative _repels_ negative. But positive _attracts_ negative, and negative _attracts_ positive. Opposite electricities unite, and so neutralize each other that no effect is perceived; but it must be borne in mind that all bodies possess both electricities in some quantity, greater or less. By rubbing we separate these electricities, the rubber becoming negative, the rub_bee_ positive. The friction of glass supplies positive electricity, and sealing-wax supplies negative electricity, or we can obtain the same effect by rubbing either with certain material. The manner in which a body is electrified depends upon its nature and condition; but we may accept as a general axiom,—but by no means as a law,—that when two bodies are rubbed together, that which gets the hotter in the process takes the negative kind of electricity. In the following list the substances have been so arranged that each is negatively electrified by those preceding, and positively by those succeeding it. 1, cat’s skin; 2, glass; 3, woollen stuffs; 4, feathers; 5, wood; 6, paper; 7, silk; 8, shellac; 9, rough glass. We append a list of conducting and non-conducting bodies in their order:— CONDUCTORS.