Mercury combines with chlorine in two proportions, forming the subchloride or calomel, and the bichloride or corrosive sublimate, the one consisting of one equivalent of chlorine and two of mercury, Hg2Cl, and the other of one equivalent of chlorine and one of mercury, HgCl. The name calomel is probably derived from the Greek words Calomel 0300333 , fair, andCalomel 0300334 , black; a black mixture being produced in the process of preparing it by rubbing mercury with corrosive sublimate, and this, when subjected to heat, yielding the white sublimate calomel. It occurs as an ore of mercury in the quicksilver mines of Idria in Carniola, Almaden in Spain, and other localities. It is in the form of a crystalline sublimation, coating other substances, and of granular structure. It is also crystallized in quadrangular prisms, of yellowish gray and ash-gray colors. Its hardness is 1 to 2, and specific gravity 6.482. - As prepared for medicinal purposes, calomel is either obtained as a powder by precipitation, or reduced to a powdered state from the crystalline cake obtained by sublimation. It is a substance without taste or smell, insoluble in water, ether, and alcohol, and becomes black by exposure, without undergoing chemical change. For this reason it is necessary to keep it protected from the light. It requires a higher temperature than corrosive sublimate to volatilize it, and in the sublimation a portion is converted into mercury and the bichloride.

By its entirely subliming when pure, non-volatile substances that may have been mixed with it, such as salts of lime, barytes, or lead, may be detected. As calomel is liable to be contaminated with corrosive sublimate, by which mixture it may produce the most dangerous consequences, it is especially important to test it for this salt. A buff color is an indication of freedom from corrosive sublimate, but the very purest calomel, as that called Jewell's, is perfectly white. If calomel is washed in warm distilled water, and a white precipitate should fall on the addition of ammonia, this indicates the presence of corrosive sublimate. Caustic potash may also be used instead of ammonia, and will give when corrosive sublimate is present a yellow precipitate. - Various processes are given in the pharmacopoeias for this preparation. The most common method is by sublimation. This may be done by mixing four parts of corrosive sublimate with three parts of mercury, and rubbing them together until the metallic globules entirely disappear, and then subliming. The product should be powdered and washed with boiling water to free it from corrosive sublimate.

The process of the " United States Pharmacopoeia " is as follows: " Take of mercury 4 lbs., sulphuric acid 3 lbs., chloride of sodium 1 1/2 lb., distilled water a sufficient quantity. Boil 2 lbs. of the mercury with the sulphuric acid until a dry, white mass is left. Rub this, when cold, with the remainder of the mercury in an earthenware mortar, until they are thoroughly mixed; then add the chloride of sodium, and rub it with the other ingredients till all the globules disappear; afterward sublime. Reduce the sublimed matter to a very fine powder, and wash it frequently with boiling distilled water, till the washings afford no precipitate upon the addition of solution of ammonia; then dry it." A mode of preparation in the wet way is recommended by Prof. Wheeler in the "Chemical Gazette " of July, 1854. The commercial corrosive sublimate is dissolved in water heated to 122° F., and sulphurous acid gas, obtained by heating coarse charcoal powder with concentrated sulphuric acid, is passed through the hot saturated solution. Calomel in the form of a delicate powder and of a dazzling whiteness is precipitated. The liquid, when saturated with the gas, is digested for a time, and when cooled is filtered from the calomel, which is afterward washed.

This process has the advantage that it is easily available for making calomel in small quantities. The calomel of Joseph Jewell of London, sometimes called Howard's, which possesses the highest reputation, is prepared by causing the vapor to come in contact with steam in a large receiver. It is thus entirely washed from corrosive sublimate, at the same time that it is condensed into an impalpable powder. Its extreme fineness appears to give it more activity as a medicine than is possessed by the calomel obtained by levigation and elutriation. - Calomel is used in medicine to obtain many of the effects of mercury. It is administered in doses of one to ten or more grains, as a cathartic, being supposed to have a special action upon the liver. This is however not proved, and is doubted by many. In smaller doses more frequently repeated, it produces the constitutional effects of mercury; and if too long retained, or given to persons possessing a peculiar susceptibility, a single dose may give rise to disagreeable effects. Calomel is rendered soluble in the intestines by the albuminous secretions, and perhaps partly by the alkaline chlorides.

It has been supposed that these salts convert a portion of the protochloride or subchloride (calomel) into the bichloride of mercury (corrosive sublimate) at the temperature of the body; but experiments have shown that this does not take place, at least out of the body. This conversion may however take place from the action of nitro-muriatic acid, hydrocyanic acid, bitter almonds, or cherry laurel water, and it should therefore not be prescribed with these substances. Calomel is incompatible with alkalies, alkaline earths, alkaline carbonates, soaps, and hydrosulphates. - There are few diseases in which calomel has not been largely employed. Such various views are held in regard to its usefulness that it would be difficult to reconcile them, or even to properly state them within appropriate limits. The sedative and alterative actions of calomel, which seem-to be something distinct from its specific mercurial effect, are those which seem the most hypothetical. It is safe to say, however, that the use of calomel by the greater part of the medical profession has vastly diminished of late years, probably to the advantage of the community.