This polarity of chemical elements manifests itself in different ways. In some cases it appears like that of a magnet, in which there are two opposite poles, and two only, one at each end. Thus oxygen (O) is bipolar, and its atom holds together two atoms of hydrogen (H) in forming the molecule of water, which may be represented as H+-O+-H, which is equivalent to [Illustration]. Others again, like hydrogen and chlorine, seem to have only a single pole, as in the case of electricity in an excited glass rod, and have to create for themselves the opposite pole, which is the indispensable condition of all polarity, by induction in another body. Thus, muriatic or hydrochloric acid is formed by the union of a single atom of chlorine, which is strongly negative, with a single atom of hydrogen, in which it appears to have induced a positive pole: though the combination is not a very stable one, for if an element with a stronger positive pole of its own is presented to the chlorine, it drops the hydrogen, just as the magnet drops the nickel. Other atoms are multipolar, and seem as if made up of more than one magnet, or rather as if the atom had regular shape like a triangle, square, or pentagon, and each angle was a pole, thus enabling it to unite with three, four, five, or more atoms of other substances. Thus, one atom of nitrogen unites with three of hydrogen, one of carbon with four of hydrogen, and so on. Every substance has, therefore, what is called its ‘quantivalence,’ or power of uniting with it a greater or less quantity of other atoms, and conversely that of replacing in combinations other atoms, or groups of atoms, the sum of whose quantivalence equals its own. Thus, one atom of carbon, which has four poles, combines with four atoms of hydrogen or chlorine, which is unipolar, but with only two of oxygen, which are bipolar; while the oxygen atom combines with two of hydrogen, and that of chlorine with one atom only of hydrogen. The analogy between the single atomic and electrical poles on the one hand, and the dual and magnetic poles on the other, will be evident if we consider what occurs if a pith-ball, electrified positively, is brought near a similar ball electrified negatively. They attract each other, and the one becomes the pole of the other; but if separated, each carries with it its own electrical charge. But the separate balls or poles, though no longer influencing each other, are not isolated, for each draws by induction an electrical charge opposite to its own to the extremity of the nearest conductor, and thus creates for itself a new or second pole. Polarity, in fact, involves opposition of relations, or two poles, and electrical only differs from magnetic polarity in the fact that in the latter the two poles are in the same body, while in the former they are in separate bodies.