Prep. Four ounces of corrosive sublimate, and five ounces of iodide of potassium, are dissolved separately in water, and the two solutions mixed; the red iodide is precipitated, and is separated by decantation and filtration, and afterwards washed and dried. This is a simple case of double decomposition, chloride of mercury and iodide of potassium becoming chloride of potassium and iodide of mercury (Hg Cl + KI=Hg I + K Cl).

Prop. & Comp. A crystalline powder of a fine bright vermilion colour, becoming yellow when heated over a lamp on a sheet of paper; almost insoluble in water, dissolves sparingly in alcohol, but freely in ether or in an aqueous solution of iodide of potassium. When digested with a solution of soda, it assumes a reddish-brown colour, and the fluid cleared by filtration and mixed with a solution of starch, gives a blue precipitate on being acidulated with nitric acid. Entirely volatilized by a heat under redness, and completely soluble in ether.

Off. Prep. Unguentum Hydrargyri Iodidi Rubri. Ointment of Red Iodide of Mercury. [Not officinal in U. S. P.] (Red iodide of mercury, in fine powder, sixteen grains; simple ointment, one ounce.) This ointment contains one-fourth as much red iodide of mercury as Unguentum Hydrargyri Iodidi Rubri. Dub.

Therapeutics. The red iodide closely resembles corrosive sublimate in its action upon the system, and is far more irritant than the green iodide. It is chiefly used as an external application in the form of ointment to enlarged glands, and periosteal nodes of a syphilitic nature, also in goitre. Internally it may be administered in the same cases as corrosive sublimate.

Dose. 1/12 gr. to 1/3 gr. Hydrargyri Oxidum Rubrum. Red Oxide of Mercury.

Synonym. Hydrargyri Nitrico-Oxidum. Lond.

Prep. Mercury by weight, eight ounces; nitric acid, three fluid ounces; distilled water, two fluid ounces. Dissolve half the mercury in the nitric acid diluted with the water, evaporate the solution to dryness, and triturate the dry salt thus obtained with the remainder of the mercury. Put this into a porcelain capsule, then apply a slow fire, and gradually increase it until acid vapour ceases to arise.

Prop. & Comp. Red shining crystalline scales, insoluble in water, but entirely soluble in hydrochloric acid. Composition (Hg O). Entirely volatilized by a heat under redness, being at the same time decomposed into mercury and oxygen. If this be done in a test tube no orange vapours should be given off, indicating the absence of nitric acid. The solution in hydrochloric acid gives a yellow precipitate with caustic potash in excess, and a white precipitate with solution of ammonia.

Off. Prep. Unguentum Hydrargyri Oxidi Rubri. Ointment of Red Oxide of Mercury. (Red oxide of mercury, in very fine powder, sixty-four grains; simple ointment, one ounce.) About one grain of oxide in twenty-eight grains of the ointment.

Therapeutics. Used externally only, as a powerful irritant and escharotic; applied as an ointment to the eye in ophthalmia, to indolent ulcers, etc.; as an escharotic, in powder, alone or mixed with sugar, to specks in the cornea, over excrescences, chancres, and fungous ulcers.

Adulteration. Brick-dust, red-lead, and other red powders, detected by not being volatile; some undecomposed nitrate of mercury may be present, and then red fumes are given off when heated; the nitrate is also soluble in water.