Though discarded from the Pharmacopoeia, and now little used, this preparation demands notice from its former reputation, and real efficiency. Various modes of making it have been employed; but perhaps the best, on the whole, is that of the former U. S. Pharmacopoeia, which consists in decomposing calomel by means of solution of potassa, washing the resulting oxide, and drying it with a gentle heat. An interchange of principles takes place between the protochloride of mercury or calomel, and the oxide of potassium or potassa, by which the chloride of potassium is formed in solution, and the protoxide of mercury is left undissolved.

As first prepared, the oxide is blackish; but it very soon becomes olive-coloured, and at length not unfrequently presents a slightly reddish tint. it is inodorous, tasteless, insoluble in water and alkaline solutions, but soluble in acetic and nitric acids, and forms with muriatic acid calomel and water. Though a protoxide, consisting of one eq. of mercury and one of oxygen, it is readily decomposed; and, under the influence of light, is resolved partially into deutoxide and metallic mercury, of which the globules may be seen on close inspection. it often also contains a portion of undecomposed calomel. Exposed to a strong heat, it is volatilized, and condenses in metallic globules.

Its operation upon the system resembles that of calomel, though perhaps more variable. According to Mialhe, it is changed into corrosive sublimate or the bichloride in the primae viae. The readiest explanation of the change is that one part of the protoxide gives up its oxygen to the other part, forming the deutoxide or binoxide, which then reacts with any alkaline chloride or muriatic acid contained in the stomach.

The black oxide is capable of producing, according to its dose, an alterative, sialagogue, or purgative effect. it occasionally causes considerable irritation, which may sometimes be ascribed to the presence of the deutoxide. The dose as an alterative is from one-quarter of a grain to a grain daily; with a view to general mercurialization, from one to three grains twice or thrice a day. Before being given in larger doses, its freedom from deutoxide should be well ascertained; and at best it can scarcely be considered as suitable for use as a purgative.

Mr. Abernethy employed it for mercurial fumigation. The patient, protected by a complete suit of underclothing, was placed at night in a vapour bath, with the head projecting, and the neck guarded by a cloth, and was exposed, for fifteen or twenty minutes, to the vapours of two drachms of the oxide, put on a heated iron within the bath. He was then transferred to bed, and passed the night in the same garments. Salivation has thus been induced in forty-eight hours; but the cases in which the proceeding would be desirable must be extremely few.

The Black Wash or Lotto Nigra is made by agitating about a drachm of calomel in a pint of lime-water. By reaction between the calomel and lime, chloride of calcium is formed, which remains in solution, and protoxide of mercury or the black oxide, which subsides. But, with the proportions mentioned, there must be a large amount of undecomposed calomel mixed with the oxide. The preparation is used as an application to all kinds of syphilitic ulcers, being always well shaken when applied.