_Protoxide._ To obtain the protoxide, mercury must be slowly dissolved in dilute nitric acid without heat, and an excess of mercury must be used. If to 1000 grain measures of nitric acid, 1.2 sp. gr. be put 500 grains of mercury, by occasional agitation the requisite solution will be obtained in 24 hours. A portion of this solution must be treated with a small excess of lime water or caustic alkali, when a black powder will be thrown down, which is the oxide containing a minimum of oxygen, and hence may be considered the protoxide. _Deutoxide._ If to 1000 measures of nitric acid, 1.2 sp. gr. be put 350 grains of mercury, and the mixture be boiled till the mercury disappear, a solution will be obtained containing the deutoxide. A portion of this being treated as beforementioned with lime water, a yellowish red powder is precipitated, which is the oxide of mercury containing a maximum of oxygen; all the later authors agree that it contains just double the quantity of oxygen to a given portion of mercury that the former does, and may therefore be called the deutoxide. These two oxides combine with most acids and form salts, some of which exhibit remarkable differences occasioned by the oxides; thus, muriatic acid with the protoxide forms protomuriate of mercury, commonly called _calomel_, an insoluble salt; with the deutoxide it forms deutomuriate of mercury, commonly called _corrosive sublimate_, a soluble salt. The proportions of metal and oxygen in the two oxides may be found by precipitating a known weight of mercury reduced by solution to either of the oxides, then drying and weighing the oxides, when the increase of weight by the addition of oxygen may be observed. This method is less accurate with regard to mercury than to other metals, on account of the very great weight of the atom, by which a small error in the gross weight of the oxide will be a great one as it respects the oxygen. This circumstance will partly account for the differences of authors on this subject.