Mercury is a metallic chemical element identified by the symbol Hg. It is silvery in appearance and, unlike all other metals, is liquid at room temperature. The ancient name for mercury was quicksilver, meaning living silver. This name reflected mercury’s unusually lively behavior: when it is poured onto a smooth surface, it forms beads that roll rapidly around at a touch. The element’s modern name comes from Mercury (or Mercurius), the fleet-footed messenger of the gods in Roman mythology.
Many ancient civilizations were familiar with this element. As long ago as Roman times, people had learned to extract mercury from ore and used it to purify gold and silver. Ore containing gold or silver would be crushed and treated with mercury, which rejects impurities, to form a mercury alloy, called an amalgam. When the amalgam is heated, the mercury vaporizes, leaving pure gold or silver.
Mercury’s overall presence in Earth’s crust is relatively low compared to other elements. However, mercury is not considered rare because it is found in large, highly concentrated deposits. Nearly all mercury exists in the form of a red ore called cinnabar, which consists primarily of mercury and sulfur. Sometimes shiny globules of mercury appear among outcrops of cinnabar, which is probably why mercury was discovered so long ago. The metal is relatively easy to extract from the ore by applying heat and a filtration process. First the ore is heated in an oxygen furnace. The mercury is released as fumes and those fumes condense into soot on a water-cooled metal condenser. The mercury is then removed from the soot by a filter system and purified in a vacuum. Much of the world’s mercury has traditionally been mined in Spain and Italy, though several other countries also produce commercial quantities.
Mercury’s atomic number is 80 and its atomic weight is 200.59. It has a boiling point of 674°F (356.7°C) and a melting point of –38°F (–38.89°C). Mercury is stable (it does not react) in air and water, as well as in acids and alkalis. The surface tension of mercury is six times higher than that of water. Because of this, even when mercury is in liquid form, it does not wet the surfaces it contacts.
Like some other metals, mercury exhibits unusual behavior at extremely low temperatures. In 1911, Dutch physicist Heike Kamerlingh Onnes discovered the phenomenon of superconductivity by freezing mercury to only a few degrees above absolute zero. At that temperature, mercury loses all of its natural resistance to the flow of electricity and becomes superconductive.
Mercury is uniquely suited for measuring temperatures. When heated or cooled, mercury expands or contracts at a rate that is more constant than most other substances. Also, it has a wide range of temperatures between its boiling and freezing points. In 1714, German-Dutch physicist Gabriel Daniel Fahrenheit developed the mercury thermometer. (Previous fluid thermometers had used alcohol or alcohol-water mixtures.) With mercury as the measuring fluid, temperatures could be recorded well above water’s boiling point and below its freezing point. Using mercury also allowed the degrees to be marked more accurately in finer subdivisions.
Mercury and all of its compounds are extremely poisonous, and mercury is one of the few substances known to have no natural function in the human body. Classified as a heavy metal, mercury is difficult for the body to eliminate. This means that even small amounts can act as a cumulative poison, collecting over a long period of time until they reach dangerous levels. Humans can absorb mercury through any mucous membrane and through the skin. Its vapor can be inhaled, and mercury can be ingested in foods such as fish, eggs, meat, and grain. In the body, mercury primarily affects the nervous system, liver, and kidneys. Mercury poisoning symptoms include tremors, tunnel vision, loss of balance, slurred speech, and unpredictable emotions. The phrase “mad as a hatter” owes its origin to symptoms of mercury poisoning that afflicted hatmakers in the 1800s, when a mercury compound was used to prepare beaver fur and felt materials.
The toxic qualities of mercury have been known for hundreds of years. In the seventeenth century, Swiss toxicologist Johann-Jakob Wepfer studied the characteristics of mercury poisoning. In the early 1920s, German chemist Alfred stock discovered that he had been suffering from undiagnosed mercury poisoning for most of his adult life. His case was probably caused by years of exposure to mercury vapors in poorly ventilated laboratories. Stock analyzed the pathology of mercury poisoning and devised techniques for detecting very small amounts of mercury. Often using himself as an experimental subject, Stock traced mercury’s path through the body and its accumulation in various organs. He published numerous articles warning of mercury’s dangers and suggesting safety precautions.
Until recently, scientists thought that inorganic mercury was relatively harmless, so industrial wastes containing it were routinely discharged into large bodies of water. Then in the 1950s, more than 100 people in Japan were poisoned by fish containing mercury; 43 people died, dozens more were horribly crippled, and babies born after the outbreak developed irreversible damage. It was found that inorganic mercury in industrial wastes had been converted to a much more harmful organic form—methyl mercury. As this substance works its way up the food chain, its quantities accumulate to dangerous levels in larger fish. Today, the dumping of mercury-containing wastes has been largely banned, and many of its industrial uses have been halted. However, mercury is still used in electrical switches and relays, fluorescent lamps, flat-screen computer screens and televisions, and electrolytic cells for manufacturing chlorine. Tiny amounts are also present in dental fillings.