Mercury (element)

• 1. Introduction

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  Mercury is the only metallic element known to be liquid at standard temperature and pressure. Cool it below 4 K and it became the first known superconductor, discovered in 1911 by Heike Kamerlingh Onnes. Pour it into a wooden statue, and according to Aristotle, the inventor Daedalus made a figure of Aphrodite appear to move. This heavy, silvery substance, carrying the symbol Hg and atomic number 80, has fascinated and poisoned people for thousands of years. The ancient Greeks called it hydrargyros. English speakers came to call it quicksilver, because it behaves like silver that is alive. How does a single metal end up in Egyptian tombs, in felt hats, in fluorescent tubes, and in the bodies of swordfish? Why did the first emperor of a unified China reportedly drink it, hoping to live forever? And how did a metal prized for its strange beauty become one of the most regulated toxins on Earth? The answers run from alchemists' workbenches to the bottom of Minamata Bay.

• 2. A Metal That Misbehaves

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  Mercury has the lowest boiling point of any metal, 356.73 C, and the lowest freezing point too, giving it the narrowest liquid range of any metal under standard conditions. This oddity traces to lanthanide contraction and relativistic effects that pull in the outermost electrons and weaken the metallic bonding. Compared with other metals it conducts heat poorly, though it carries electricity fairly well. Bromine is the only other element that is liquid at the same conditions, while caesium, gallium, and rubidium melt just above room temperature. When mercury freezes, its volume drops by 3.59%, and its density rises from 13.69 grams per cubic centimetre as a liquid to 14.184 when solid. Solid mercury is soft enough to cut with a knife, malleable and ductile. Chemically it is reluctant to react. Dilute sulfuric acid leaves it alone, though oxidizing acids such as nitric acid or aqua regia will dissolve it. It refuses to mix with iron, which is why iron flasks have traditionally carried it. With aluminium the story is different. The two metals form an amalgam that eats through the protective oxide layer, so even a little mercury can badly corrode aluminium. That single reaction is why mercury is barred from most aircraft.

• 3. Planets And Alchemy

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  Hg, the chemical symbol, abbreviates hydrargyrum, a romanized form of the ancient Greek hydrargyros, a compound meaning water-silver. The modern English name comes from the planet Mercury. Medieval alchemists tied the seven known metals to the seven classical planets, and quicksilver went to the fastest of them, named for the Roman god of speed and mobility. Mercury is the only metal whose alchemical planetary name has survived, chosen over quicksilver as the formal chemical term. To alchemists, mercury was the First Matter from which all metals were formed. They believed varying the sulfur held within it produced different metals, the purest being gold. So mercury was central to attempts at transmuting base metals into gold, the great goal of the craft. The metal already had a long working life by then. Cinnabar, its most common natural source, has been in use since the Neolithic Age. By 500 BC mercury was being used to make amalgams with other metals, the word amalgama meaning an alloy of mercury.

• 4. Rivers Of Quicksilver

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  Mercury was found in Egyptian tombs dating from 1500 BC. In China and Tibet it was thought to prolong life, heal fractures, and keep a person healthy, a belief now known to be deadly wrong. The first emperor of a unified China, Qin Shi Huang Di, was allegedly buried in a tomb holding rivers of flowing mercury laid out as a model of his lands. He was reportedly killed by drinking a mixture of mercury and powdered jade prepared as an elixir of immortality. In Egypt, the Tulunid ruler Khumarawayh ibn Ahmad ibn Tulun, known for extravagance, reportedly built a basin of mercury and floated on air-filled cushions to be rocked to sleep. The metal turns up in the Americas too. Between 90 and 600 grams of liquid mercury were recovered from a chamber sixty feet below the Temple of the Feathered Serpent at Teotihuacan in Mexico, alongside jade statues, jaguar remains, and carved shells. At Lamanai, once a major Maya city, a pool of mercury sat beneath a marker in a ballcourt, perhaps used in bowls as mirrors for divination. Five such finds date to the Classic Period of Maya civilization, roughly 250 to 900, while one predates it.

• 5. Silver, Hats, And Madness

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  The mines at Almaden in Spain, Monte Amiata in Italy, and Idrija in present-day Slovenia dominated mercury production for centuries. Mining at Almaden began about 2500 years ago. Then in 1558 the invention of the patio process, which used mercury to extract silver from ore, turned the metal into a pillar of the Spanish economy and its American colonies. After deposits were found at Huancavelica in Peru in 1563, more than 100,000 tons of mercury were mined there over three centuries. In 1786 the main Huancavelica mine collapsed, killing over 100 people and cutting its output. Mercury poisoning left many miners disabled, though it was not the chief cause of deaths in the mines. A different kind of damage came from felt hats. From the mid-18th to the mid-19th centuries, hatters used a process called carroting, rinsing animal skins in an orange solution of mercuric nitrate to separate and mat the fur. The vapors were highly toxic. The psychological symptoms inspired the phrase mad as a hatter. The United States Public Health Service banned mercury in the felt industry in December 1941.

• 6. From Thermometers To Ion Engines

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  Daniel Gabriel Fahrenheit invented the mercury thermometer in the early 18th century, building on an earlier alcohol design and finding mercury far more accurate. Mercury's near-linear thermal expansion and dense, opaque body made it ideal for barometers, sphygmomanometers, and diffusion pumps. In fluorescent lamps, electricity through mercury vapor produces short-wave ultraviolet light, which makes the phosphor coating glow. Its electrical reach went further still. Because mercury conducts as a liquid, it served in tilt switches and old fire detectors, and its acoustic properties made it the medium in delay-line memory for early computers like the UNIVAC I and SEAC. Mercury even left the planet. It fueled early ion engines, prized for high molecular weight and low ionization energy. The first spaceflight to use electric propulsion carried a mercury-fueled ion thruster from NASA Glenn Research Center aboard SERT-1, launched from Wallops Flight Facility in 1964, with SERT-2 following in 1970. Xenon later replaced it. The triple point of mercury, minus 38.8344 C, is still a fixed temperature standard for the International Temperature Scale. And in the largest industrial use of the late 20th century, the mercury cell process formed sodium amalgam to make chlorine and caustic soda.

• 7. Poison In The Water

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  Liquid mercury barely passes through skin or the gut, but its vapor is the primary hazard, and its organic compounds are the deadliest of all. Methylmercury, formed by biomethylation, collects in polluted water and works its way up the food chain. Fish high on that chain, such as shark, swordfish, king mackerel, and bluefin tuna, can carry mercury burdens ten times higher than the prey they eat, a process called biomagnification. The worst case unfolded at Minamata Bay in Japan. Between 1932 and 1968, waste mercury compounds were dumped there, and over 3,000 people suffered deformities, severe poisoning, or death from what became Minamata disease. Symptoms of mercury poisoning include tremors, impaired memory, sleep disturbance, and a loss of coordination, alongside diseases such as acrodynia and Hunter-Russell syndrome. The sources of atmospheric mercury are sobering. Coal-fired power plants are the single largest aggregate source, with stationary combustion accounting for about 65% of human emissions around the year 2000. Volcanic eruptions and other natural sources supply roughly the other half of all atmospheric mercury.

• 8. The World Pulls Back

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  On the 10th of October 2013, the Minamata Convention on Mercury brought 140 countries together under the United Nations Environment Programme to curb mercury vapor emissions. The retreat had been building for decades. Norway enacted a total ban on manufacturing and trading mercury products from the 1st of January 2008, with its environment minister Erik Solheim calling mercury among the most dangerous environmental toxins. Sweden banned mercury products in 2009, and Denmark restricted elemental mercury from 2008. In the European Union, the RoHS directive bars mercury from many electrical products, and in July 2007 the bloc banned it in non-electrical measuring devices like thermometers and barometers. The United States has leaned on the Clean Air Act, the Clean Water Act, and others, with the Mercury-Containing and Rechargeable Battery Management Act of 1996 phasing it out of batteries. Yet the metal has not vanished. It still sits in dental amalgam, in some fluorescent lighting, and in scientific research. The Minamata Convention caps mercury in skin-whitening creams at 1 part per million, but as of 2022 many whiteners on sale still exceed that limit and remain toxic.