In the third century, Zosimus the Theban discovered how to release oxygen from the red oxide of mercury. The Arabian alchemists of the Middle Ages thought that all metals were compounds, consisting of mercury and sulfur in varying proportions. Philippus Paracelsus, the 16th-century Swiss alchemist, believed that the elements of compound bodies were mercury, sulfur, and salt, representing respectively the elements of earth, air, and water. This liquid metal was once known as quicksilver, because its rapid, fluid mobility at room temperature. Mercury is most frequently found in the ore cinnabar, a sulfide that often forms around volcanic vents.
Most people associate mercury with the thermometer.
It is also found in other scientific equipment, including barometers, vacuum pumps, and switches. Ultraviolet lights contain mercury vapor. Although it is has many practical applications, mercury is extremely poisonous. Where mercury has polluted water in the form of water-soluble salts, it has entered the food chain. Over long periods, mercury contamination can cause permanent brain damage, as well as harming the liver and kidneys.