Also known as Quicksilver, mercury is a silvery white liquid that slowly tarnishes in moist air. Mercury was used by the ancient Chinese and Hindus and has been found in an Egyptian tomb dating back to around 1500 BC. During the European Middle Ages and in China, mercury was used to make potions for eternal life or to try and turn base metals into gold! Neither venture was successful. Mercury is a poison so it tended to shorten life, not extend it and turning base metals into gold requires the use of advanced nuclear physics, not available in the Middle Ages.

Occurrence
Mercury is rare in the Earth's crust. Its principal ore is the red mineral cinnabar called mercury sulfide. Native mercury occurs in droplets, usually in conjunction with cinnabar, near volcanoes or hot springs. Mercury is not a very reactive metal and hence it is reasonably easy to release from its ore. Mercury can be obtained by heating cinnabar and collecting the mercury vapours in a still.

Properties and uses
Mercury still holds great interest for the modern student of chemistry, being the only liquid metal. Its low melting point of -38.9°C and relatively high boiling point for a liquid of 356.9°C, gives mercury a large range of temperature as a liquid.

Mercury makes an excellent liquid to use in thermometers. This is because mercury expands uniformly as it is heated and has a high surface tension. Thus, when a thermometer is warmed, the mercury rises at a constant rate within the thermometer and does not wet or cling to glass.

Mercury's high density of 13.5 grams per cubic centimetre, means that most material will float on its surface. A person with density around 1 g/cm³ easily floats on mercury, not getting wet because of mercury's high surface tension.

The high density of mercury and a high surface tension makes mercury the perfect liquid for measuring gas pressures in barometers and manometers. For example, a column of mercury 76cm high exerts the same pressure as the atmosphere on the surface of the Earth at sea level. If water were used instead of mercury, a column 1026cm or 10.26 metres, high would be needed.

When the high electrical conductivity of mercury is added to its already unusual features, a whole new range of uses become possibilities. Mercury is used in electric switches, fluorescent and mercury-vapour lamps, mercury cell batteries, and as electrodes in the industrial production of chlorine and caustic soda.

Toxicity
Mercury is highly toxic. Care must be exercised in its handling and transport. Poisoning may result from inhalation of mercury vapour, ingestion of mercury compounds, or absorption through the skin.

The Mad Hatter in Alice and Wonderland had jitters, convulsions and stuttering all symptomatic of mercury poisoning. Hats at that time were made of felt, and mercury compounds were used in the felt making process.

Mercury poisoning
Mercury was involved in a mass poisoning during the 1950s in Minamata, Japan, where a whole community was poisoned by eating fish caught in the mercury polluted waters in Minamata Bay. Babies were born deformed, children and adults lost the use of their limbs and many died.
As a result of the Minamata poisonings and other cases, mercury has been withdrawn from many uses and industrial processes and is now used only sparingly in laboratories and specialised industrial processes.

Mercury pollution
The high density of liquid mercury means that if it escapes into the environment it will flow to, and accumulate in, low-lying places. In Minamata, the mercury escaped from an industrial plant and flowed through drainage outlets to accumulate in the mud in the bottom of the local bay.
In the mud, bacteria converted the metallic mercury into simple mercury organic compounds, which then accumulated in the crabs and shellfish living on the bottom of the bay. The shellfish were eaten by other fish and by the local community. The mercury then accumulated in the people attaching itself to the protein in the fastest growing parts of the victims' bodies, usually in the central nervous system. The unborn babies of pregnant women and children were the most at risk because their bodies were growing the fastest. Babies and children were often affected when adults in the same house showed no sign of mercury poisoning.

Mercury's ability to accumulate in one place and combine with protein in living things is typical of many other "heavy metals" like cadmium, lead and zinc. The only way to effectively stop poisoning from mercury and the other heavy metals is to stop them getting into the environment and cleaning up already polluted areas.