These three metals give, like magnesium, oxides RO, which form feebly energetic bases, and like magnesium they are volatile. The volatility increases with the atomic weight. Magnesium can be distilled at a white heat, zinc at a temperature of about 930°, cadmium about 770°, and mercury about 351°. Their oxides, RO, are more easily reducible than magnesia, and mercuric oxide is the most easily reducible. The properties of their salts RX_{2} are very similar to the properties of MgX_{2}. Their solubility, power of forming double and basic salts, and many other qualities are in many respects identical with those of MgX_{2}. The greater or less ease with which they are oxidised, the instability of their compounds, the density of the metals and their compounds, their scarcity in nature, and many other properties gradually change with the increase of atomic weight, as might be expected from the periodicity of the elements. Their principal characteristics, as contrasted with magnesium, find a general expression in the fact that zinc, cadmium, and mercury are heavy metals. _Zinc_ stands nearest to magnesium in atomic weight and in properties. Thus zinc sulphate, or white vitriol, easily crystallises with seven molecules of water, ZnSO_{4},7H_{2}O. It is isomorphous with Epsom salts, and parts with difficulty with the last molecule of water; it forms double salts--for instance, ZnK_{2}(SO_{4})_{2},6H_{2}O--exactly as magnesium sulphate does.[1] _Zinc oxide_, ZnO, is a white powder, almost insoluble in water,[2] like magnesia, from which, however, it is distinguished by its solubility in solutions of sodium and potassium hydroxides.[3] Zinc chloride[4] is decomposed by water, combines with ammonium chloride, potassium chloride, &c., just like magnesium chloride, forms an oxychloride, and also combines with zinc oxide.[4 bis] [1] Zinc sulphate is often obtained as a by-product--for instance, in the action of galvanic batteries containing zinc and sulphuric acid. When the anhydrous salt is heated it forms zinc oxide, sulphurous anhydride, and oxygen. The solubility in 100 parts of water at O° = 43, 20° = 53, 40° = 63-1/2, 60° = 74, 80° = 84-1/2, 100° = 95 parts of anhydrous zinc sulphate--that is to say, it is closely expressed by the formula 43 + 0·52_t_. An admixture of iron is often found in ordinary sulphate of zinc in the form of ferrous sulphate, FeSO_{4}, isomorphous with the zinc sulphate. In order to separate it, chlorine is passed through the solution of the impure salt (when the ferrous salt is converted into ferric), the solution is then boiled, and zinc oxide is afterwards added, which, after some time has elapsed, precipitates all the ferric oxide. Ferric oxide of the form R_{2}O_{3} is displaced by zinc oxide of the form RO.