(Chemical symbol; Hg). Mercury is a heavy silver-white, shiny, toxic metallic element, the only common metal that is liquid at room temperature. It is a fair conductor of heat and electricity. Mercury occurs free in nature, but the chief source is the sulfide (cinnabar, HgS), from which it may be obtained by heating in air. Mercury is used extensively in electrical apparatus, and because of its linear coefficient of thermal expansion, it is used in laboratory thermometers, barometers, and many other instruments. Atomic weight 200.61; atomic number 80; melting point -38.87oC (-39 OF); boiling point 356.9°C (673°F).