As many of the compounds of mercury decompose on being heated--for instance, the oxide or carbonate[17]--and as zinc, cadmium, copper, iron, and other metals separate mercury from its salts,[18] it is evident that mercury has less chemical energy than the metals already described, even than zinc and cadmium. Nitric acid, when acting on _an excess_ of mercury at the ordinary temperature, gives mercurous nitrate, HgNO_{3}.[19] The same acid, under the influence of heat and when in excess (nitric oxide being liberated), forms mercuric nitrate, Hg(NO_{3})_{2}. This,[20] both in its composition and properties, resembles the salts of zinc and cadmium. Dilute sulphuric acid does not act on mercury, but strong sulphuric acid dissolves it, with evolution of _sulphurous anhydride_ (not hydrogen), and on being slightly heated with an excess of mercury it forms the sparingly soluble mercurous sulphate, Hg_{2}SO_{4}; but if mercury be strongly heated with an excess of the acid, the mercuric salt, HgSO_{4},[21] is formed. Alkalis do not act on mercury, but the non-metals chlorine, bromine, sulphur, and phosphorus easily combine with it. They form, like the acids, two series of compounds, HgX and HgX_{2}. The oxygen compound of the first series is the suboxide of mercury, or mercurous oxide, Hg_{2}O, and of the second order the oxide HgO, mercuric oxide. The chlorine compound corresponding with the suboxide is HgCl (calomel), and with the oxide HgCl_{2} (corrosive sublimate or mercuric chloride). In the compounds HgX, mercury resembles the metals of the first group, and more especially silver. In the mercuric compounds there is an evident resemblance to those of magnesium, cadmium, &c. Here the atom of mercury is bivalent, as in the type RX_{2}.[22] Every soluble mercurous compound (corresponding with the type of the suboxide of mercury), HgX, forms a white precipitate of calomel, HgCl, with hydrochloric acid or a metallic chloride, because HgCl is very slightly soluble in water, HgX + MCl = HgCl + MX. In soluble mercuric compounds, HgX_{2}, hydrochloric acid and metallic chlorides do not form a precipitate, because corrosive sublimate, HgCl_{2}, is soluble in water. Alkali hydroxides precipitate the yellow mercuric oxide from a solution of HgX_{2}, and the black mercurous oxide from HgX. Potassium iodide forms a dirty greenish precipitate, HgI, with mercurous salts, HgX, and a red precipitate, HgI_{2}, with the mercuric salts, HgX_{2}. These reactions distinguish the mercuric from the mercurous salts, which latter represent the transition from the mercuric salts to mercury itself, 2HgX = Hg + HgX_{2}. The salts, HgX, as well as HgX_{2}, are reduced by nascent hydrogen (_e.g._ from Zn + H_{2}SO_{4}), by such metals as zinc and copper, and also by many reducing agents--for example, hypophosphorous acid, the lowest grade of oxidation of phosphorus, by sulphurous anhydride, stannous chloride, &c. Under the action of these