The term HG is an abbreviation for the element MERCURY, which is represented by the chemical symbol Hg. MERCURY is a silvery-white metal that is liquid at room temperature and is the only metal that is liquid at this temperature. It is commonly used in thermometers, barometers, and other scientific instruments. MERCURY is also found in some electrical switches and fluorescent lamps. It is highly toxic and can cause serious health problems if ingested or inhaled.