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Mercury (pronounced /ˈmɜrkjʊri/), also called quicksilver (/ˈkwɪksɪlvər/) or hydrargyrum (/haɪˈdrɑrdʒɨrəm/), is a chemical element with the symbol Hg (Latinized Greek: hydrargyrum, meaning watery or liquid silver) and atomic number 80. A heavy, silvery d-block metal, mercury is one of six elements that are liquid at or near room temperature and pressure,[1][2] the others being caesium, francium, gallium, bromine, and rubidium. Mercury is the only metal liquid at standard conditions for temperature and pressure, with a melting point of −38.83°C. Its boiling point of 356.73°C gives it one of the widest liquid ranges of any metal.

Mercury occurs in deposits throughout the world as cinnabar (mercuric sulfide), source of the red pigment vermilion. It is mostly obtained by reduction from cinnabar. It is highly toxic by ingestion or inhalation of the dust,[3] and mercury poisoning can also result from exposure to soluble forms (such as mercuric chloride or methylmercury), inhalation of mercury vapour, or eating fish contaminated with mercury.

Mercury is used in thermometers, barometers, manometers, sphygmomanometers, float valves, and other scientific apparatus, though concerns about the element's toxicity have led to mercury thermometers and sphygmomanometers being largely phased out in clinical environments in favor of alcohol-filled, digital, or thermistor-based instruments. It remains in use in a number of other ways in scientific and scientific research applications, and in amalgam material for dental restoration. It is used in lighting; electricity passed through mercury vapor in a phosphor tube produces short-wave ultraviolet light which then causes the phosphor to fluoresce, making visible light.

Applications
Mercury-in-glass thermometerMercury is used primarily for the manufacture of industrial chemicals or for electrical and electronic applications. It is used in some thermometers, especially ones which are used to measure high temperatures. A still increasing amount is used as gaseous mercury in fluorescent lamps, while most of the other applications are slowly phased out due to health and safety regulations and is in some applications replaced with less toxic but considerably more expensive galinstan alloy.

Present Use 

Dentistry
Amalgam fillingMain article: Amalgam (dentistry)
The element mercury is an ingredient in dental amalgams.

Cosmetics
Mercury, as thiomersal, is widely used in the manufacture of mascara. In 2008, Minnesota became the first state in the US to ban intentionally added mercury in cosmetics, giving it a tougher standard than the federal government.

Production of chlorine and caustic soda
Chlorine is produced from sodium chloride (common salt, NaCl) using electrolysis to separate the metallic sodium from the chlorine gas. Usually the salt is dissolved in water to produce a brine. By-products of any such chloralkali process are caustic soda (sodium hydroxide (NaOH)) and hydrogen (H2). By far the largest use of mercury in the late 1900s was in the mercury cell process (also called the Castner-Kellner process) where metallic sodium is formed as an amalgam at a cathode made from mercury; this sodium is then reacted with water to produce sodium hydroxide. Many of the industrial mercury releases of the 1900s came from this process, although modern plants claimed to be safe in this regard. After about 1985, all new chloralkali production facilities that were built in the United States used either membrane cell or diaphragm cell technologies to produce chlorine.

Medicine
Mercury manometer to measure pressureMercury and its compounds have been used in medicine, although they are much less common today than they once were, now that the toxic effects of mercury and its compounds are more widely understood. Thiomersal (called Thimerosal in the United States) is an organic compound used as a preservative in vaccines, though this use is in decline. Another mercury compound Merbromin (Mercurochrome) is a topical antiseptic used for minor cuts and scrapes is still in use in some countries.

Mercury(I) chloride (also known as calomel or mercurous chloride) has traditionally been used as a diuretic, topical disinfectant, and laxative. Mercury(II) chloride (also known as mercuric chloride or corrosive sublimate) was once used to treat syphilis (along with other mercury compounds), although it is so toxic that sometimes the symptoms of its toxicity were confused with those of the syphilis it was believed to treat.It was also used as a disinfectant. Blue mass, a pill or syrup in which mercury is the main ingredient, was prescribed throughout the 1800s for numerous conditions including constipation, depression, child-bearing and toothaches. In the early 20th century, mercury was administered to children yearly as a laxative and dewormer, and it was used in teething powders for infants. The mercury-containing organohalide merbromin (sometimes sold as Mercurochrome) is still widely used but has been banned in some countries such as the U.S.

Since the 1930s some vaccines have contained the preservative thiomersal, which is metabolized or degraded to ethyl mercury. Although it was widely speculated that this mercury-based preservative can cause or trigger autism in children, scientific studies showed no evidence supporting any such link. Nevertheless thiomersal has been removed from or reduced to trace amounts in all U.S. vaccines recommended for children 6 years of age and under, with the exception of inactivated influenza vaccine.

Mercury in the form of one of its common ores, cinnabar, remains an important component of Chinese, Tibetan, and Ayurvedic medicine. As problems may arise when these medicines are exported to countries that prohibit the use of mercury in medicines, in recent times, less toxic substitutes have been devised.

Today, the use of mercury in medicine has greatly declined in all respects, especially in developed countries. Thermometers and sphygmomanometers containing mercury were invented in the early 18th and late 19th centuries, respectively. In the early 21st century, their use is declining and has been banned in some countries, states and medical institutions. In 2002, the U.S. Senate passed legislation to phase out the sale of non-prescription mercury thermometers. In 2003, Washington and Maine became the first states to ban mercury blood pressure devices. Mercury compounds are found in some over-the-counter drugs, including topical antiseptics, stimulant laxatives, diaper-rash ointment, eye drops, and nasal sprays. The FDA has “inadequate data to establish general recognition of the safety and effectiveness,” of the mercury ingredients in these products. Mercury is still used in some diuretics, although substitutes now exist for most therapeutic uses.

Gold and Silver mining
Historically, mercury was used extensively in hydraulic gold mining in order to help the gold to sink through the flowing water-gravel mixture. Thin mercury particles may form mercury-gold amalgam and therefore increase the gold recovery rates. Large scale use of mercury stopped in the 1960s. However, mercury is still used in small scale, often clandestine, gold prospection. It is estimated that 45,000 metric tons of mercury used in California for placer mining have not been recovered.

Mercury was also used in silver mining.
Other present uses
Assorted types of fluorescent lamps.Gaseous mercury is used in mercury-vapour lamps and some "neon sign" type advertising signs and fluorescent lamps. Those low-pressure lamps emit very spectrally narrow lines, which are traditionally used in optical spectroscopy for calibration of spectral position. Commercial calibration lamps are sold for this purpose; however simply reflecting some of the fluorescent-lamp ceiling light into a spectrometer is a common calibration practice.
It is used in some thermometers, especially ones which are used to measure high temperatures. In the United States, non-prescription sale of mercury fever thermometers has been banned in 2003. 
The triple point of mercury, -38.8344 °C, is a fixed point used as a temperature standard for the International Temperature Scale (ITS-90).
In some gaseous electron tubes, including ignitrons, thyratrons, and mercury arc rectifiers
Mercury is used to construct liquid-mirror telescopes. The mirror is formed by rotating liquid mercury on a disk, the parabolic form of the liquid thus formed reflecting and focusing incident light. Such telescopes are cheaper than conventional large mirror telescopes by up to a factor of 100, but the mirror cannot be tilted and always points straight up.
Mercury is used in the amalgamation process of refining gold and silver ores. This polluting practice is still used by the garimpeiros (gold miners) of the Amazon basin in Brazil and by illegal miners in South Africa.
Mercury is still used in some cultures for folk medicine and ceremonial purposes which may involve ingestion, injection, or the sprinkling of elemental mercury around the home. The former two procedures, especially, are extremely hazardous and the latter is nearly as so because mercury spreads easily and is therefore ingested.
Used in electrochemistry as part of a secondary reference electrode called the calomel electrode as an alternative to the Standard Hydrogen Electrode. This is used to work out the electrode potential of half cells.
Used in cold cathode lighting to increase the ionization and electrical conductivity in argon filled lamps. An argon filled lamp without mercury will have dull spots and will fail to light correctly. Lighting containing mercury can be bombarded/oven pumped only once. When added to neon filled tubes the light produced will be inconsistent red/blue spots until the initial burning-in process is completed; eventually it will light a consistent dull off-blue color.

 Proposed uses
Liquid mercury has been proposed as a working fluid for a heat pipe type of cooling device for spacecraft heat rejection systems or radiation panels.
Experimental mercury vapour turbines were proposed to increase the efficiency of fossil-fuel electrical power plants.
A new type of atomic clock, using mercury instead of caesium, has been demonstrated. Accuracy is expected to be within one second in 100 million years.
Old mercury switches

Historic uses
The deep violet glow of a mercury vapor discharge in a germicidal lamp, whose spectrum is rich in invisible ultraviolet radiation.Mercury was used for preserving wood, developing daguerreotypes, silvering mirrors, anti-fouling paints (discontinued in 1990), herbicides (discontinued in 1995), handheld maze games, cleaning, and road levelling devices in cars. Mercury compounds have been used in antiseptics, laxatives, antidepressants, and in antisyphilitics. It was also allegedly used by allied spies to sabotage German planes. A mercury paste was applied to bare aluminium, causing the metal to rapidly corrode. This would cause structural failures.

Mercury switches (including home mercury light switches installed prior to 1970), tilt switches used in old fire detectors, tilt switches in many modern home thermostats, electrodes in some types of electrolysis, batteries (mercury cells), sodium hydroxide and chlorine production, handheld games, catalysts, insecticides, dental amalgams/preparations and liquid mirror telescopes.
In Islamic Spain it was used for filling decorative pools. Later the American artist Alexander Calder built a mercury fountain for the Spanish Pavilion at the 1937 World Exhibition in Paris. The fountain is now on display at the Fundació Joan Miró in Barcelona.

Mercury sphygmomanometers or blood pressure meter.
Mercury was used inside wobbler lures. Its heavy, liquid form made it useful since the lures made an attractive irregular movement when the mercury moved inside the plug. Such use was stopped due to environmental concerns, but illegal preparation of modern fishing plugs has occurred.
The Fresnel lenses of old lighthouses used to float and rotate in a bath of mercury which acted like a bearing.
Mercury barometers, diffusion pumps, coulometers, and many other laboratory instruments. As an opaque liquid with a high density and a nearly linear thermal expansion, it is ideal for this role.
Liquid mercury was sometimes used as a coolant for nuclear reactors; however, sodium is proposed for reactors cooled with liquid metal, because the high density of mercury requires much more energy to circulate as coolant.
Mercury was a propellant for early ion engines in electric propulsion systems. Advantages were mercury's high molecular weight, low ionization energy, low dual-ionization energy, high liquid density and liquid storability at room temperature. Disadvantages were concerns regarding environmental impact associated with ground testing and concerns about eventual cooling and condensation of some of the propellant on the spacecraft in long-duration operations. The first spaceflight to use electric propulsion was a mercury-fueled ion thruster SERT-1 launched by NASA at its Wallops Flight Facility in 1964. SERT stands for Space Electric Rocket Test. The SERT-1 flight was followed up by the SERT-2 flight in 1970. Mercury and caesium were preferred propellants for ion engines until Hughes Research Laboratory performed studies finding xenon gas to be a suitable replacement. Xenon is now the preferred propellant for ion engines as it has a high molecular weight, little or no reactivity due its noble gas nature, and has a high liquid density under mild cryogenic storage.
Mercury was also traded in flasks at trade centers, one flask = 76 pounds (lb).

Mercury was once used as a gun barrel bore cleaner.

Hat making
From the mid-18th to the mid-19th centuries, a process called "carroting" was used in the making of felt hats. Animal skins were rinsed in an orange solution (the term "carroting" arose from this color) of the mercury compound mercuric nitrate, Hg(NO3)2·2H2O. This process separated the fur from the pelt and matted it together. This solution and the vapors it produced were highly toxic. The United States Public Health Service banned the use of mercury in the felt industry in December 1941. The psychological symptoms associated with mercury poisoning are said by some to have inspired the phrase "mad as a hatter", though etymological study suggests that the phrase is actually much older and unrelated to hatters - see hatter for commentary on the origin of the phrase. Lewis Carroll's "Mad Hatter" in his book Alice's Adventures in Wonderland was a play on words based on the older phrase, but the character himself does not exhibit symptoms of mercury poisoning