mercurous chloride, mercury (I) chloride, or calomel, chemical compound, Hg2Cl2, a white crystalline powder, very slightly soluble in water. It was once used medicinally as a purgative, cathartic, liver stimulant, and to eliminate parasitic worms, but is rarely so used today because it is readily decomposed into metallic mercury and the very poisonous mercuric chloride on exposure to sunlight or if heated in the presence of moisture. Mercurous chloride is a less dangerous poison than mercuric chloride chiefly because it is much less soluble; it is highly toxic if retained in the body. Mercurous chloride is prepared by sublimation from a mixture of mercury and mercuric chloride or by precipitation from a mercurous chloride solution on adding chloride ion. It is also found in nature as horn quicksilver. The calomel electrode, often used as a reference in determining electric potentials and for measuring the p H of solutions, contains mercurous chloride, mercury metal, and potassium chloride solution.