Mercury is the metal that is liquid at ordinary room temperature and was traditionally used in thermometers. Its chemical symbol is Hg, from the Latin name hydrargyrum, meaning “liquid silver.”
Mercury expands predictably when heated, making the height of a mercury column useful for measuring temperature. It was also used in barometers and other scientific instruments because it is dense and has a relatively low freezing point.
Many countries have restricted or phased out mercury thermometers because mercury is toxic and can harm the nervous system and other organs. Digital instruments and alcohol-based thermometers are common alternatives.
Mercury is the only metallic element that is liquid under standard room conditions. Gallium and cesium can melt near room temperature, but they are normally solid at typical indoor temperatures.