The chemical element mercury is a shiny metallic liquid. Its chemical symbol, Hg, is derived from the Greek word hydrargyrum, meaning "liquid silver," or "quick silver." Although now obsolete, the word quicksilver has long been used as a synonym for mercury. The element shares group IIB of the periodic table with zinc and cadmium. The atomic number of mercury is 80; its atomic weight is 200.59. Mercury is very heavy, weighing 13.6 times as much as an equal volume of water. Stone, iron, and even lead can float on its surface. Mercury occurs in only trace amounts in igneous rocks; sedimentary rocks are slightly richer.

The element constitutes only 0.5 ppm of the earth's crust, making it scarcer than uranium but more plentiful than gold or silver. Mercury is found principally in the form of the ore cinnabar (mercury sulfide) and can also be found in the uncombined state. The preparation of mercury from its ores is fairly simple. The ore is ground up and heated to about 580 deg C in the presence of oxygen. Mercury vapor escapes from the ores and sulfur dioxide is removed. The metal is condensed and purified by washing with nitric acid, followed by distillation.

Mercury was among the first metals known, and its compounds have been used throughout history. Archaeologists found mercury in an Egyptian tomb dating from 1500 BC. The Egyptians and the Chinese may have been using cinnabar as a red pigment for centuries before the birth of Christ. In many civilizations mercury was used to placate or chase away evil spirits. The alchemists thought that mercury, which they associated with the planet Mercury, had mystical properties and used it in their attempts to transmute base metals into gold. The Greeks knew of mercury and used it as a medicine. Mercury and mercury compounds were used from about the 15th century to the mid 20th century to cure syphilis. Because mercury is extremely toxic and its curative effect is unproven, other syphilis medicines are now used. The usefulness of mercury is limited by its poisonous nature and scarcity.

Mercury is used in electrical switches; these consist of a small tube with two contacts at one end. If the tube is held in such a way that the mercury collects at this end, then contact is made and the circuit is completed. If the tube is tilted, contact is broken. Mercury switches are used in thermostats and some doze alarm type alarm clocks. Mercury is highly suitable for use in thermometers because it does not moisten glass and its thermal expansion is uniform. Although many liquids could be used in pressure measuring devices, mercury is used because its high density requires less space. Mercury will dissolve numerous metals to form amalgams and is thus used to extract gold dust from rocks by dissolving the gold and then boiling off the mercury. The amalgam used in dental fillings contains tin and silver (and sometimes gold) dissolved in mercury. Mercury vapor lamps are widely used because they are powerful and economical sources of ultraviolet and visible light.

Mercury is also used in a number of industrial applications, such as fluid bearing and fluid clutches that require a heavy liquid. Mercury is a fairly unreactive metal and is highly resistant to corrosion. When heated to near its boiling point (346.72 deg C/675 deg F), mercury oxidizes in air, and mercuric oxide is formed. At 500 deg C, mercuric oxide decomposes into mercury and oxygen, a phenomenon that led to the discovery of oxygen by Joseph Priestley and Karl Scheele. Mercuric oxide is a constituent of mercury batteries, which have been invaluable as compact, efficient power sources in exploration of outer space. The most useful mercury salts are the two mercury chlorides and mercury sulfide. Mercurous chloride, or calomel, is a white, relatively insoluble salt. It is used in calomel electrodes, which are commonly used in electrochemistry, and in medicine as a cathartic and diuretic. When calomel is used as a teething powder for young children, it can poison them. Mercuric chloride, or corrosive sublimate, is highly poisonous because it is so soluble. It was used for deliberate poisonings as early as the 14th century. It is now used as a disinfectant, in preparing other mercury compounds, and in antifungal skin ointments.

Mercuric sulfide occurs in a red form and an amorphous black form. The red form (vermilion) is used as a coloring material. Cinnabar is sometimes used to color tattoos red, but it causes significant skin irritations and obstructions of the lymphatic system. Mercuric fulminate is an explosive that is sensitive to impact and is used in percussion caps for munitions. Mercurochrome is an organic mercury compound that is used on wounds as an antibacterial agent. There are two types of mercury poisoning, acute and chronic. Acute mercury poisoning results from the ingestion of soluble mercury salts, which violently corrode skin and mucous membranes. Although cases have occurred in which persons have ingested elemental mercury without suffering permanent damage, mercury vapor aspirated into the lungs can cause severe pneumonia and death. Chronic mercury poisoning occurs through the regular absorption of small amounts of mercury. This condition is often a disease of workers in mercury mines, laboratories, and industries that use mercury. The most toxic mercury compounds are those that are fat soluble, because this property assists in their distribution throughout the body. Methyl mercury compounds, such as dimethyl mercury, are among the most dangerous. Mercury salts released into the environment may frequently be converted by anaerobic bacteria into such compounds, which can then be carried through the food chain to humans as in the disaster at Minamata Bay, Japan Other micro organisms can convert methyl mercury compounds into the insoluble, and therefore harmless, mercury sulfide.