Mercury is a chemical element with the symbol Hg and atomic number 80 on the Periodic table. It is a heavy metal that is liquid under standard temperature and pressure.
The remaining debris of the Caretaker's array contained mercury, hydrogen, helium, and argon. (VOY: "The Voyager Conspiracy")
Mercury was used in an instrument called a barometer to measure gaseous pressure. The atmospheric pressure of Taurus II measured 70 milimeters of mercury. (TOS: "The Galileo Seven")