Hydrargyri Nitrico-Oxidum. Lond. - Deutoxide of Mercury. - Binoxide of Mercury. - Peroxide of Mercury. - Red Precipitate.

This is made by dissolving mercury in nitric acid, evaporating the solution to dryness, and exposing the residue to heat, in shallow vessels, so long as red vapours escape. Both nitrate of the protoxide and that of the deutoxide of mercury are formed in the first step of the process, and are afterwards decomposed by heat; the metal being oxidized to the maximum at the expense of the acid, which is driven off. The resulting product is the deutoxide of mercury, with a very small proportion of nitrate not entirely decomposed. it is commonly called red precipitate.

This oxide is in minute shining scales, of a fine orange-red colour, and an acrid, metallic taste. it may be considered as virtually insoluble in water; requiring, according to Dr. Christison, Togo parts of that liquid for solution. it is dissolved by nitric and muriatic acids. With a moderate heat, it yields oxygen, and at a red heat is decomposed and dissipated. it should always be rubbed into very fine powder before being used.

Effects on the System

Externally, the red oxide of mercury is powerfully irritant, and even escharotic when applied to ulcerated or excoriated surfaces. internally it is capable of affecting the system like the other preparations of mercury; but it is liable to be very harsh in its action, sometimes causing vomiting and purging, in doses in which it produces little obvious effect at other times, and capable in over- doses of causing dangerous gastro-enteritis. There can be little doubt that it acts, whether externally or internally, mainly through conversion into corrosive sublimate. This conversion, depending on the presence of the alkaline chlorides or muriatic acid, is not constant in the same degree; and hence the inequality in the action of the medicine. Under favouring circumstances it takes place rapidly, and gives great energy to the preparation. Mialhe supposes that the change is effected by an interchange of principles between a portion of the alkaline chloride and the mercurial oxide, resulting in the formation of the deuto-chloride of mercury and the pure alkali, the former combining with a portion of undecomposed alkaline chloride to form a double chloride of the mercury and alkaline metal. Were it not for this latter combination, the deuto-chloride could not exist in the presence of the generated alkali.

Therapeutic Application

in consequence of its inequality of action, and occasional violence, the red oxide should not be used internally. if given, the dose should not exceed one-quarter or half a grain.

At present it is used exclusively as an external remedy, for its locally alterative, stimulant, or escharotic effect, in syphilitic ulcers, in old indolent, flabby, or fungous ulcers whether syphilitic or not, in various cutaneous eruptions, and in different diseases of the eye or its appendages. it is used in the form of powder, ointment, or lotion.

The powder is sometimes sprinkled, as an escharotic, on chancres, and indolent or flabby and fungous ulcers; and, diluted with eight or ten parts of very finely powdered sugar, is blown into the eye for the removal of opacity of the cornea. Great care should be taken that the oxide is very finely powdered.

The Ointment (Unguentum Hydrargyri Oxidi Rubri, U. S., Br.), commonly called red precipitate ointment, is made by rubbing a drachm of the red oxide, very finely powdered, with an ounce of the ointment of lard (simple ointment) previously softened by heat. it is at first of a fine orange-red colour, which gradually darkens by time into a dirty leaden hue, probably owing to the partial deoxidation of the deutoxide through the agency of the fatty matter. it should, therefore, not be kept long; but prepared as wanted.

This ointment is often used as a dressing for ulcers such as above referred to, and as an application to certain cutaneous diseases, particularly porrigo, the advanced stages of impetigo of the face and scalp, and the scaly affections. it may be used also in Psorophthalmia, being carefully applied to the edges of the affected lids; and, more or less diluted, is occasionally introduced into the eye in obstinate chronic ophthalmia, with thickening of the palpebral conjunctiva, and specks on the cornea.

The Phagedenic Lotion, or Yellow Wash {Aqua Phagedaenica, or Lotio Phagedaenica), is made by adding corrosive sublimate to lime-water, in the proportion of two grains to a fluidounce. By mutual decomposition between the bichloride of mercury and the oxide of calcium or lime, chloride of calcium and red oxide of mercury are generated, the former of which remains dissolved, and the latter is thrown down. Of course, whenever the preparation is used, it must be well shaken. it was formerly much employed, as a wash, in the ulcers and cutaneous eruptions above referred to; being applied to the former by means of lint, or folded linen wet with it, and to the latter by lotion. Trousseau speaks of it favourably as a wash in all cases in which solution of corrosive sublimate is used, and especially in pruritus of the vulva, diluted with from twice to four times its bulk of warm water.