Hence it is evident that isomorphism--that is, the analogy of forms and the property of inducing crystallisation--may serve as a means for the discovery of analogies in molecular composition. We will take an example in order to render this clear. If, instead of aluminium sulphate, we add magnesium sulphate to potassium sulphate, then, on evaporating the solution, the double salt K_{2}MgS_{2}O_{8},6H_{2}O (Chapter XIV., Note 28) separates instead of an alum, and the ratio of the component parts (in alums one atom of potassium per 2SO_{4}, and here two atoms) and the amount of water of crystallisation (in alums 12, and here 6 equivalents per 2SO_{4}) are quite different; nor is this double salt in any way isomorphous with the alums, nor capable of forming an isomorphous crystalline mixture with them, nor does the one salt provoke the crystallisation of the other. From this we must conclude that although alumina and magnesia, or aluminium and magnesium, resemble each other, they are not isomorphous, and that although they give partially similar double salts, these salts are not analogous to each other. And this is expressed in their chemical formulæ by the fact that the number of atoms in alumina or aluminium oxide, Al_{2}O_{3}, is different from the number in magnesia, MgO. Aluminium is trivalent and magnesium bivalent. Thus, having obtained a double salt from a given metal, it is possible to judge of the analogy of the given metal with aluminium or with magnesium, or of the absence of such an analogy, from the composition and form of this salt. Thus zinc, for example, does not form alums, but forms a double salt with potassium sulphate, which has a composition exactly like that of the corresponding salt of magnesium. It is often possible to distinguish the bivalent metals analogous to magnesium or calcium from the trivalent metals, like aluminium, by such a method. Furthermore, the specific heat and vapour density serve as guides. There are also indirect proofs. Thus iron gives ferrous compounds, FeX_{2}, which are isomorphous with the compounds of magnesium, and ferric compounds, FeX_{3}, which are isomorphous with the compounds of aluminium; in this instance the relative composition is directly determined by analysis, because, for a given amount of iron, FeCl_{2} only contains two-thirds of the amount of chlorine which occurs in FeCl_{3}, and the composition of the corresponding oxygen compounds, _i.e._ of ferrous oxide, FeO, and ferric oxide, Fe_{2}O_{3}, clearly indicates the analogy of the ferrous oxide with MgO and of the ferric oxide with Al_{2}O_{3}.