~Indirect measure of valence.~ Many elements, especially among the metals, do not readily form compounds with hydrogen, and their valence is not easy to determine by direct comparison with the standard element. These elements, however, combine with other univalent elements, such as chlorine, and their valence can be determined from the compounds so formed. ~Variable valence.~ Many elements are able to exert different valences under differing circumstances. Thus we have the compounds Cu_{2}O and CuO, CO and CO_{2}, FeCl_{2} and FeCl_{3}. It is not always possible to assign a fixed valence to an element. Nevertheless each element tends to exert some normal valence, and the compounds in which it has a valence different from this are apt to be unstable and easily changed into compounds in which the valence of the element is normal. The valences of the various elements will become familiar as the elements are studied in detail. ~Valence and combining ratios.~ When elements combine to form compounds, the ratio in which they combine will be determined by their valences. In those compounds which consist of two elements directly combined, the union is between such numbers of the two atoms as have equal valences. Elements of the same valence will therefore combine atom for atom. Designating the valence of the atoms by Roman numerals placed above their symbols, we have the formulas II II II III III IV IV HCl, ZnO, BN, CSi. A divalent element, on the other hand, will combine with two atoms of a univalent element. Thus we have II II II II ZnCl_{2} and H_{2}O (the numerals above each symbol representing the sum of the valences of the atoms of the element present). A trivalent atom will combine with three atoms of a univalent element, as in the compound III III H_{3}N. If a trivalent element combines with a divalent element, the union will be between two atoms of the trivalent element and three of the divalent element, since these numbers are the smallest which have equal valences. Thus the oxide of the trivalent metal aluminium has the formula Al_{2}O_{3}. Finally one atom of a tetravalent element such as carbon will combine with four atoms of a univalent element, as in the compound CH_{4}, or with two atoms of a divalent element, as in the compound CO_{2}. We have no knowledge as to why elements differ in their combining power, and there is no way to determine their valences save by experiment.