Synonym. Hydrargyri Chloridum. Lond.

Prep. Sulphate of mercury, ten ounces; mercury, by weight, seven ounces; chloride of sodium, dried, five ounces; distilled water, a sufficiency. The sulphate of mercury and the mercury are rubbed together with a little water until globules are no longer visible; then the chloride of sodium is well triturated with them and sublimed. In this process the sulphate of mercury is, by being rubbed with a second equivalent of the metal, reduced to a sulphate of the suboxide; and this last, by the action of the common salt, is converted into the subchloride of mercury, sulphate of soda being produced at the same time. The decompositions may be thus represented:

1st part of process, Hg O, So3 + Hg=Hg2 O, So3. 2d part of process, Hg2 O, So8+Na Cl=Na O, So3 + Hg2 Cl.

Prop. & Comp. Calomel, when sublimed, occurs in cakes, with a crystalline structure; but as a drug it is met with in the form of a dull-white, heavy powder, rendered yellowish by trituration in a mortar, without odour or taste; sublimes with heat; it is blackened when treated with potash, arid the clear supernatant fluid, acidulated with nitric acid, gives a copious white precipitate with nitrate of silver. When calomel is boiled or washed in water, this liquid should afterwards give no precipitate with nitrate of silver, lime-water, or sulphuretted hydrogen. Calomel is insoluble in water, spirit, or ether; by the action of nitric and hydrochloric acids, it is converted slowly into the chloride. It is entirely volatilized by a sufficient heat. Warm ether shaken with it in a bottle leaves, on evaporation, no residue.

Off. Prep. Pilula Calomelanos Composita. Compound Pill of Calomel. (Calomel and sulphurated antimony, each one ounce; guaiac resin, in powder, two ounces; castor oil, one fluid ounce.) One grain of calomel is contained in five grains of the pill mass. [Pilulae Antimonii Compositae. Compound Pills of Antimony. Plummer's Pills. U. S. Sulphurated antimony and mild chloride of mercury, each one hundred and twenty grains; guaiac and molasses, each half a troy ounce. Divided into two hundred and forty pills; each pill contains half a grain of the mild chloride of mercury.]

Unguentum Calomelanos. Ointment of Calomel. [Not officinal in U. S. P.] (Calomel, eighty grains; prepared lard, one ounce.) About one grain of calomel is contained in forty grains of this ointment.

Therapeutics. Calomel when absorbed acts on the system in the manner noticed under Hydrargyrum. Its peculiarities are, that it produces little local irritant action; as a purgative, it increases the secretion of bile and other intestinal fluids: and hence forms a useful adjunct in affections of the liver, and obstructions to the portal circulation. It often produces in children the so-named calomel stools, or green-coloured faeces. The compound pill is employed chiefly as an alterative in chronic skin diseases, in which the antimony and guaiacum aid its operation.

The ointment is a clean and valuable local alterative application in some forms of skin disease; it can also be employed as an inunction.

Dose. Of calomel as a purgative, 2 gr. to 6 gr.; to affect the system, 1/2 gr. to 1 gr. or more, frequently repeated. Of pil. calo-melanos comp., as an alterative, 5 gr. to 10 gr.

Adulteration. Calomel is apt to contain a trace of corrosive sublimate (chloride of mercury) formed in the process of preparation: this can be detected by the water in which it is boiled, exhibiting reactions with the tests above given. Intentional impurities, as carbonate of lead, sulphate or carbonate of baryta, etc, are not volatilized by heat, and the carbonates effervesce with acids.