One fact has been for some time known which demonstrates the oxygen in the red oxide to be double that in the black. Corrosive sublimate may be reduced to calomel by adding to it as much mercury as the sublimate contains, and triturating the mixture well, the oxygen of the red oxide (as well as the acid) becomes equally divided amongst the mercury and forms the black oxide, which is a constituent of calomel. Hence it follows that if the oxygen in one oxide can be ascertained, that of the other becomes known. Or if we can find how much oxygen must be added to the black oxide to change it to the red, we shall know the oxygen in both. Conformably with this last idea I have found a very accurate and elegant method of ascertaining the oxygen required to convert the black to the red oxide by treating protomuriate of mercury, mixed with water and a little muriatic acid, with oxymuriate of lime in solution; this must be gradually added till the protomuriate is dissolved, or rather converted to the deutomuriate. The quantity of oxygen in a given solution of oxymuriate of lime is most conveniently found by a solution of green sulphate of iron, as will be shewn under the oxides of that metal. The oxides of mercury may be investigated by the nitrous gas produced during solution. When mercury is dissolved without heat, as mentioned above, no nitrous gas is liberated. The solution has a strong nitrous smell and requires a great quantity of oxymuriate of lime to saturate both the oxide and the acid. When heat is employed to accelerate the solution, nitrous gas is liberated. I dissolved 154 grains of mercury in nitric acid, 1.2 sp. gr., by the application of a gentle heat from a lamp. About ⅒ excess of acid remained in the solution; the nitrous gas obtained was 12 oz. measures = 7.5 grains, corresponding to 6.5 oxygen, which gives nearly 4 oxygen or 100 mercury. This would have led me to suppose I had obtained the _black_ oxide in solution; it was however entirely the _red_, as it gave no precipitate by common salt, and exhibited the red oxide by lime water; but it required as much oxymuriate of lime as contained 6.5 oxygen to saturate the _nitrous gas_ in the solution before any oxymuriatic acid was liberated. It was clear therefore that only ½ of the nitrous gas was evolved, and the other ½ was retained in the solution, though it had been exposed to boiling heat. The following are the proportions assigned by the several authors for the oxides of mercury.