Mercury —(Planet Mercury), Hg (hydrargyrum, liquid silver); atomic weight 200.59; atomic number 80; melting point -38.842°C; boiling point 356.58°C; specific gravity 13.546 (20°C); valence 1 or 2.
Known to ancient Chinese and Hindus, found in Egyptian tombs of 1500 B.C., mercury is the only common metal liquid at ordinary temperatures. It only rarely occurs freely in nature. The chief ore is cinnabar (HgS). Spain and Italy produce about 50% of the world's supply of the metal. The commercial unit for handling mercury is the "flask," which weighs 76 lb. The metal is obtained by heating cinnabar in a current of air and by condensing the vapor.
Mercury is a heavy, silvery-white metal; a rather poor conductor of heat, as compared with other metals, and a fair conductor of electricity. It easily forms alloys with many metals, such as gold, silver, and tin, which are called amalgams. Its ease in amalgamating with gold is made use of in the recovery of gold from its ores. The metal is widely used in laboratory work for making thermometers, barometers, diffusion pumps, and many other instruments. It is used in making mercury-vapor lamps and advertising signs, etc. and is used in mercury switches and other electrical apparatus. Other uses are in making pesticides, mercury cells for caustic soda and chlorine production, dental preparations, antifouling paint, batteries, and catalysts.
The most important salts are mercuric chloride, HgCl2 (corrosive sublimate—a violent poison), mercurous chloride, Hg2Cl2 (calomel, occasionally still used in medicine), mercury fulminate, Hg(ONC)2, (a detonator widely used in explosives), and mercuric sulfide HgS, vermillion (a high-grade paint pigment). Organic mercury compounds are important. It has been found that an electrical discharge causes mercury vapor to combine with neon, argon, krypton, and xenon. These products, held together with van der Waals' forces, correspond to HgNe, HgAr, HgKr, and HgXe.
Mercury is a virulent poison and is readily absorbed through the respiratory tract, the gastrointestinal tract, or through unbroken skin. It acts as a cumulative poison since only small amounts of the element can be eliminated at a time by the human organism. Since mercury is a very volatile element, dangerous levels are readily attained in air. Air saturated with mercury vapor at 20°C contains a concentration that exceeds the toxic limit many times. The danger increases at higher temperatures. It is therefore important that mercury be handled with care. Containers of mercury should be securely covered and spillage should be avoided. If it is necessary to heat mercury or mercury compounds, it should be done in a well-ventilated hood. Methyl mercury is a dangerous pollutant and is now widely found in water and streams. The U.S. National Bureau of Standards has redetermined the triple point of mercury and found it to be -38.84168°C.