Mercury(I) chloride (chemical formula Hg2Cl2, often still known by its ancient name of calomel) is a chloride of mercury, a heavy, white or yellowish-white substance. It may also be called mercurous chloride.
Although previously used in medicine as a diuretic and purgative, calomel is no longer used in medicine because of the toxic risks of mercury poisoning.
It occurs native as the mineral horn quicksilver or horn mercury.
Calomel is sometimes used as a reference electrode in electrochemistry.
