Calomel

Calomel is the popular name of one of the compounds of mercury, Hg, and chlorine, Cl, known to chemists as the subchloride of mercury or mercurous chloride, HgCl. It is prepared by taking two equal portions of mercury, dissolving one portion in hot concentrated sulphuric acid, $H\_2SO\_4$, which forms sulphate of mercury, $HgSO\_4$, thereafter adding the second part of the metal, and triturating the whole in a mortar till the metal becomes incorporated with the sulphate of mercury. This mixture is then added to about one-third of its weight of common salt, $NaCl$, and heated in a retort, when calomel sublimes, and condenses in the cool part of the receiver as a fine white powder. A minute portion of corrosive sublimate which accompanies it is removed by washing with water. Calomel is very dense. It is not soluble in water, and sparingly so in acids. It turns black on the addition of lime-water, potash, soda, or ammonia; and when heated in an iron spoon, or on a knife, it does not char, but rises in vapour, sublimes unaltered, and readily condenses again on any cool surface held near it. By these tests it may be readily distinguished from flour. Although calomel has been more used in British practice than any other preparation of mercury, it is not known to have been employed before the 17th century. It is the most valuable of the mercurial preparations, possessing, in addition to their general properties (see MERCURY), those of a vermifuge and purgative.