The only thing which is compound in the composition of oxygen is that its molecules consist of two atoms linked together. This appears from the fact that while the weight of oxygen, and therefore that of its molecules, is sixteen times greater than that of an equal volume of hydrogen, and therefore of hydrogen molecules, it combines with it in the proportion not of sixteen, but of eight to one. If, therefore, the molecule were identical with the atom of oxygen, we must admit that the atom could be halved, which is contrary to its definition as the ultimate indivisible particle of the substance oxygen. But if the oxygen molecule consists of two linked atoms, O—O, and the hydrogen molecule equally of two, H—H, as can be proved by other considerations, everything is explained by assuming that the molecule of water consists of two atoms of hydrogen linked to one of oxygen, or H₂O, and that when this molecule is broken up by electricity, its constituents resolve themselves into atoms, which recombine so as to form twice as many molecules of hydrogen, H—H, as of oxygen, O,—i.e. into two volumes of hydrogen gas to one of oxygen. Taking the single hydrogen atom as the unit of weight as being the lightest known ponderable body, and calling this weight a microcrith, or standard of the smallest of this order of excessively small weights, this is equivalent to saying that the weight of an oxygen atom is equal to 16 microcriths, and as water is composed of one such atom plus two of hydrogen, the weight of its molecule ought to be 16 + 2 = 18, which is in fact the exact ratio in which the weight of a volume of steam, or water in the form of gas, is heavier than an equal volume of hydrogen. This key unlocks the whole secret of the chemical changes and combinations by which matter assumes all the various forms known to us in the universe. Thus oxygen enters into a great variety of combinations forming different substances, but always in the proportion which is either 16, or some multiple of 16, such as 32, 48, 64. That is, either 1, 2, 3, or 4 atoms of oxygen unite with other atoms to form the molecules from which these other substances are made. One atom of oxygen weighing 16 microcriths combines, as we have seen, with two atoms of hydrogen weighing 2, to form a molecule of water weighing 18 mc. In like manner one atom of oxygen, 16 mc., combines with one of carbon, which weighs 12 mc., to form a molecule of carbonic oxide weighing 28 mc.; and two of oxygen, 32 mc., with one of carbon, 12 mc., to form a molecule of carbonic dioxide weighing 44 mc.