mercury or quicksilver [from the Roman god Mercury], metallic chemical element; symbol Hg [Lat. hydrargyrum=liquid silver]; at. no. 80; at. wt. 200.59; m.p. -38.842°C;; b.p. 356.58°C;; sp. gr. 13.55 at 20°C;; valence +1 or +2. Mercury was discovered in antiquity, and was known to the ancient Chinese, Hindus, and Egyptians, but was not recognized as an element. It was used as a medicine by Paracelsus. It was first recognized as a chemical element (in the modern sense) by A. L. Lavoisier about the end of the 18th cent.


Mercury is the only common metal existing as a liquid at ordinary temperatures. The pure metal has a silver-white mirrorlike appearance. Mercury is below cadmium in Group 2 of the periodic table. It is relatively stable in dry air, but in moist air slowly forms a gray oxide coating. Mercury has high surface tension; when spilled, it breaks up into tiny beads which often become lodged in cracks.


Mercury forms numerous compounds, assuming +1 valence in mercurous compounds and +2 valence in mercuric compounds. Mercury is not attacked by dilute hydrochloric or sulfuric acid. It reacts with hot nitric acid to form mercuric nitrate, Hg(No3)2. An excess of mercury reacts with nitric acid to form mercurous nitrate, HgNO3. Mercury reacts with hot concentrated sulfuric acid to form mercuric sulfate, HgSO4; with excess mercury, mercurous sulfate, Hg2SO4, is formed. Mercury reacts directly with the halogens to form mercuric salts. At elevated temperatures mercury reacts slowly with oxygen to form mercuric oxide, HgO. A mercurous oxide may be formed chemically but is unstable, decomposing to a mixture of mercury and mercuric oxide.

Natural Occurrence and Uses

Mercury occurs uncombined in nature to a limited extent. The metal is obtained commercially from cinnabar, a mercuric sulfide ore; it is easily separated by roasting the ore in air. The metal is usually purified by repeated vacuum distillation.

Mercury metal has many uses. Because of its high density, it is used in barometers and manometers. Because it has a high rate of thermal expansion that is fairly constant over a wide temperature range, it is used extensively in thermometers. Mercury is important as a liquid contact material for electric switches. It is used in mercury-vapor lamps, which emit light rich in ultraviolet radiation; various kinds of such lamps are used for street lighting, as sun lamps, and in "black lights" (see lighting). Mercury is used as an electrode in the production of chlorine and sodium hydroxide. It is also used in certain electric batteries. With some other metals mercury forms a special type of alloy called an amalgam; a special amalgam (mostly mercury, silver, and tin) is used in dentistry for filling teeth.

Mercury compounds have many uses. Calomel (mercurous chloride, Hg2Cl2) is used as a standard in electrochemical measurements and in medicine as a purgative. Mercuric chloride (corrosive sublimate, HgCl2) is used as an insecticide, in rat poison, and as a disinfectant. Mercuric oxide is used in skin ointments. Mercuric sulfate is used as a catalyst in organic chemistry. Vermilion, a red pigment, is mercuric sulfide; another crystalline form of the sulfide (also used as a pigment) is black. Mercury fulminate, Hg(CNO)2, is used as a detonator. Mercury forms many organic compounds. Mercurochrome (in 2% aqueous solution) is used in medicine as a topical antiseptic. Mercury compounds were formerly used in the treatment of syphilis.

See also mercury poisoning.

Mercury, in astronomy, nearest planet to the sun, at a mean distance of 36 million mi (58 million km); its period of revolution is 88 days. Mercury passes through phases similar to those of the moon as it completes each revolution about the sun, although the visible disk varies in size with respect to its distance from the earth. Because its greatest elongation is 28°, it is seen only for a short time after sunset or before sunrise. Since observation of Mercury is particularly unfavorable when it is near the horizon, the planet has often been studied in full daylight, with the sun's light blocked off. Mercury has the most elliptic orbit of the planets in the solar system. Its great eccentricity of orbit and its great orbital speed provided one of the important tests of Einstein's general theory of relativity. Mercury's perihelion (its closest point to the sun) is observed to advance by 43″ each century more than can be explained from planetary perturbations using Newton's theory of gravitation, yet in nearly exact agreement with the prediction of the general theory. Mercury is the smallest planet in the solar system, having a diameter of 3,031 mi (4,878 km); both Jupiter's moon Ganymede and Saturn's moon Titan are larger. Its mean density is a little less than that of the earth. Its small mass and proximity to the sun prevent it from having an appreciable atmosphere, although a slight amount of carbon dioxide has been detected. The surface of Mercury is much like that of the moon, as was shown during the Mariner 10 flybys in 1974-75 and the Messenger flybys in 2008-9. Most of its craters were formed during a period of heavy bombardment by small asteroids early in the solar system's history. The Messenger space probe also found evidence of volcanism. It was long thought that Mercury's period of rotation on its axis was identical to its period of revolution, so that the same side of the planet always faced the sun. However, radar studies in 1965 showed a period of rotation of 58.6 days. This results in periods of daylight and night of 90 earth days each, with the daylight temperatures reaching as high as 800°F; (450°C;). Night temperatures are believed to drop as low as -300°F; (-184°C;).
Mercury, in Roman religion, god of commerce and messenger of the gods; identified with the Greek Hermes. He was honored at the Mercuralia, a festival held in May and attended primarily by traders and merchants.
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  • Edmonton Mercurys, a 1940s and 50s intermediate ice hockey team from Canada
  • Phoenix Mercury, a Women's National Basketball Association team from Arizona, United States
  • Toledo Mercurys, a defunct International Hockey League franchise from Ohio, United States



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