Mercury in Roman mythology, the Roman god of eloquence, skill, trading, and thieving, herald and messenger of the gods, presider over roads, and conductor of departed souls to Hades, who was identified with Hermes. He is usually represented in art as a young man with winged sandals and a winged hat, and bearing the caduceus.

His function as a messenger gave rise to the use of his name in the titles of newspapers and journals, as The Scotch Mercury of 1643. (The ‘English Mercury (1588)’, sometimes cited as the earliest English newspaper, was in fact an 18th-century forgery.)

From late Middle English, mercury was used to denote the chemical element of atomic number 80, a heavy silvery-white metal (also called quicksilver) which is liquid at ordinary temperatures. This application probably arose from an analogy between the fluidity of the metal at room temperature and the rapid motion held to be characteristic of the classical deity.
Mercurial was formerly used to designate those born under the planet Mercury; having the qualities (identical with those assigned to or supposed to be inspired by the god Mercury) considered to be a consequence of this, as eloquence, ingenuity, aptitude for commerce. In current usage, it means subject to sudden or unpredictable changes of mood or mind; although these qualities were originally associated with the god, the allusion is now generally understood as referring to the properties of mercury as a metal.
ELIZABETH KNOWLES. "Mercury." The Oxford Dictionary of Phrase and Fable. 2006. Encyclopedia.com. 27 Nov. 2012 <http://www.encyclopedia.com>.
Hermes, Greek god, son of Zeus and the Pleiad Maia; often identified with the Roman Mercury and with Casmilus or Cadmilus, one of the Cabeiri. His name is probably derived from herma (see herm), the Greek word for a heap of stones, such as was used in the country to indicate boundaries or as a landmark. The earliest centre of his cult was probably Arcadia, where Mt. Cyllene was reputed to be his birthplace. There he was especially worshipped as the god of fertility, and his images were ithyphallic.
Both in literature and cult Hermes was constantly associated with the protection of cattle and sheep, and he was often closely connected with deities of vegetation, especially Pan and the nymphs. In the Odyssey, however, he appears mainly as the messenger of the gods and the conductor of the dead to Hades. Hermes was also a dream god, and the Greeks offered to him the last libation before sleep. As a messenger, he may also have become the god of roads and doorways, and he was the protector of travellers. Treasure casually found was his gift, and any stroke of good luck was attributed to him; this conception and his function as a deity of gain, honest or dishonest, are natural derivatives of his character as a god of fertility. In many respects he was Apollo’s counterpart; like him, Hermes was a patron of music and was credited with the invention of the kithara and sometimes of music itself. He was also god of eloquence and presided over some kinds of popular divination.
The sacred number of Hermes was four, and the fourth day of the month was his birthday. In archaic art, apart from the stylized herms, he was portrayed as a full-grown and bearded man, clothed in a long tunic and often wearing a cap and winged boots. Sometimes he was represented in his pastoral character, bearing a sheep on his shoulders; at other times he appeared as the messenger of the gods with the kērykeion, or herald’s staff (see caduceus), which was his most frequent attribute. From the latter part of the 5th century bc he was portrayed as a nude and beardless youth, a young athlete.
"Hermes". Encyclopædia Britannica. Encyclopædia Britannica Online.
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Hermes, in Greek religion and mythology
Hermes, in Greek religion and mythology, son of Zeus and Maia. His