Characters. - A dull-white, heavy and nearly tasteless powder, rendered yellowish by trituration in a mortar.

Solubility. - It is insoluble in water, spirit, or ether.

Reactions. - It is very heavy, and can be distinguished by its weight from almost every other white powder. Its weight is noticed more distinctly by giving the bottle an up-and-down shake. Digested with solution of potash it becomes black (mercurous oxide); and the clear solution, acidulated with nitric acid, gives a copious white precipitate with nitrate of silver (chloride). Contact with hydrocyanic acid also darkens its colour.

Preparation. - Calomel is prepared by rubbing up mercury with sulphate of mercury moistened with water till globules are no longer visible, adding sodium chloride, mixing the whole by trituration, and subliming the mixture into a large chamber.

The mercury and mercuric sulphate form mercurous sulphate, and this, with sodium chloride, forms calomel and sulphate of sodium, HgSO4 + Hg + 2NaCl = Hg.CL, + Na2SO4.

When the calomel is sublimed into a small receiver it forms a thin crystalline crust which adheres to the sides, but when sublimed into a large chamber, as directed in the B.P., it falls as a powder on the floor. As some corrosive sublimate is often formed, the powdered calomel is washed with water till all the sublimate is removed, as shown by the water no longer giving a precipitate with ammonium sulphide.

It is then dried under 212° F., and kept in a well-stoppered and dark bottle.

Adulterations. - Chalk, sulphate of calcium, sulphate of barium, carbonate of lead, corrosive sublimate.

Tests. - It is entirely volatilised by a sufficient heat (no earthy impurities). Warm ether which has been shaken with it in a bottle leaves, on evaporation, no residue (no corrosive sublimate). Dose. - 1/2 grain to 5 grains. Preparations in which Subchloride of Mercury is used.

B.P.

Dose.

Lotio Hydrargyri Nigra

(3 grains to 1 fluid ounce)..........................................

Pilula Hydrargyri Subchloridi Composita

(1 part in 5, v. p. 522).

5-10 grs.

Unguentum Hydrargyri Subchloridi,

Calomel Ointment (with prepared lard)

(1 part in 6 1/2, nearly)...............

U.S.P

Pilulae Antimonii Compositae (p. 523). Pilulae Catharticae Compositae (p. 523).

Pilula Hydrargyri Subchloridi Composita, B.P. Pilulae Antimonii Compositae , U.S.P.; Compound Pill of Subchloride of Mercury, B.P.; Compoundae Pills of Antimony, U.S.P. Compound Calomel Pill. Plummer's Pill (p. 522).

B.P. Lotio Hydrargyri Nigra. Black Mercurial Lotion. Black Wash. Consists of half a drachm of calomel mixed with half a pint of lime-water. It contains suboxide of mercury.

Uses. - Calomel may be employed as a dusting powder to remove condylomata from the skin, and condylomatous patches from the tongue, throat, and larynx; it is also recommended in the following powder - calomel, six parts, boric acid, three parts, salicylic acid one part.1 As an ointment it may be applied to relieve the itching in pruritus and and pruritus scroti, and pityriasis of the scalp, and to heal strumous sores and lupus in children. In pruritus pudendi it is also of service, though not quite so much as in the other cases (Ringer). It should not be applied in large quantities, lest so much of it be absorbed as to cause its physiological action. Calomel ointment (1/2 to 1 drachm to the ounce) is useful in the treatment of small patches of vesicular eczema; and in psoriasis Rochard's ointment, which contains one part of iodine and one and a half part of calomel to seventy parts of simple ointment, is beneficial in some cases. Black wash is a good application to varicose ulcers, and is used as an application to syphilitic ulcerations, as a wash to the mouth in syphilitic sore-throat and in cancrum oris.

1 Philadelphia Medical Reporter, June 14, 1884.

Internally calomel may be given in cases of biliousness, and followed by a saline purgative in the same manner as is recommended under ' Blue Pill.' In some cases of diarrhoea it is very useful in combination with opium (p. 106).

It may also be used to produce the general action of mercury in syphilitic patients, and for this purpose may either be given internally, in combination with opium, or applied to the skin in the form of calomel fumigations (p. 471).1

The compound pill of subchloride of mercury may be used in cases of biliousness, gout or rheumatism.

Calomel is a useful diuretic in some cases of dropsy (pp. 432 and 686), especially when due to heart-disease. It must be given in doses of 4 or 5 grains, repeated when necessary, salivation being prevented by a chlorate of potassium gargle, and diarrhoea by small doses of opium.2