Chemistry . a heavy, silver-white, highly toxic metallic element, the only one that is liquid at room temperature; quicksilver: used in barometers, thermometers, pesticides, pharmaceutical preparations, reflecting surfaces of mirrors, and dental fillings, in certain switches, lamps, and other electric apparatus, and as a laboratory catalyst. Symbol: Hg; atomic weight: 200.59; atomic number: 80; specific gravity: 13.546 at 20?C; freezing point: ?38.9?C; boiling point

357?C.

Pharmacology . this metal as used in medicine, in the form of various organic and inorganic compounds, usually for skin infections.

( initial capital letter ) the ancient Roman god who served as messenger of the gods and was also the god of commerce, thievery, eloquence, and science, identified with the Greek god Hermes.

( initial capital letter ) Astronomy . the planet nearest the sun, having a diameter of 3031 mi. (4878 km), a mean distance from the sun of 36 million mi. (57.9 million km), and a period of revolution of 87.96 days, and having no satellites

the smallest planet in the solar system.

a messenger, esp. a carrier of news.

any plant belonging to the genus Mercurialis, of the spurge family, esp. the poisonous, weedy M. perennis of Europe.

Good-King-Henry.

( initial capital letter ) Aerospace . one of a series of U.S. spacecraft, carrying one astronaut, that achieved the first U.S. suborbital and orbital manned spaceflights.