As I began with the illustration of the magnet introducing order and harmony into the confused mass of iron filings, let me take this other illustration from the same source. If we place an iron bar in contact with the pole of a magnet, the bar becomes itself a magnet with opposite poles to the original one, so that as opposite poles attract, the iron bar adheres to it. Bring a lump of nickel in contact with the further end or free pole of the iron bar, and the nickel also will be magnetised and adhere. Let the lump of nickel be as large as the pole of the iron bar is able to support, and now bring a lump of soft iron near this pole. It will drop the nickel and take the iron. This is exactly similar to those cases of chemical affinity in which a molecule drops one of its factors and takes on another to which its attraction is stronger. If iron rusts in water it is because the oxygen atom drops hydrogen to take iron just as the magnet dropped nickel. The polarity of chemical elements is attested by the fact that when compounds are decomposed by the electric current, the different elementary substances appear at different poles of the battery. Thus, oxygen, chlorine, and non-metallic substances appear at the positive pole; while hydrogen, potassium, and metals generally, appear at the negative one. The inference is irresistible that the atoms had in each case an opposite polarity to that of the poles to which they were attracted. This is confirmed by the fact that the radicals, i.e. the elementary atoms or groups of atoms which have opposite polarities, combine readily; while those which have the same polarity, as two metals, have but slight affinity for each other. Like therefore attracts unlike, as in all cases of polarity, and the greater the degree of unlikeness the stronger is the attraction. Thus, the radicals of all alkalies are electro-positive, and appear at the negative pole of a battery; while those of acids are all electro-negative, and the higher each stands in its respective scale of polarity the more strongly does it show the peculiar qualities of acid or alkali and the more eagerly does it combine with its opposite. Acids and alkalies are, in fact, all members of the same class of compounds called _hydrates_, because a single atom of hydrogen is a common feature in their composition. This atom is coupled with a single atom of oxygen, which may be conceived of as the central magnet holding the hydrogen atom at one pole, while at the other it holds either a single atom of some metallic element, such as potassium or sodium, or a group consisting of such an element together with atoms of oxygen, so constituted as to present a single pole to the attraction of the central oxygen atom. Thus, if K stands for kali or potassium, N for nitrogen, O for oxygen, and H for hydrogen, we may have the compounds H—O—K and { O} { / } H—O—{N }. { \ } { O}