Mercury is the one metal that is liquid at ordinary temperature. It freezes only at minus 38.8 degrees C, and it flows, beads and shatters into rolling globules that recombine on contact. To anyone without a modern theory of matter this is a metal behaving as a liquid, and it was entirely reasonable to conclude that it was the metallic principle itself in its most fluid state.
That is precisely the role it plays in alchemical theory. In the sulfur-mercury account, current in Latin Europe from the twelfth century and inherited from Arabic sources, every metal is composed of a mercurial principle giving fusibility, lustre and volatility, and a sulfurous principle giving combustibility and fixity. Ordinary quicksilver was the visible embodiment of the first, and the texts distinguish it, sometimes carefully and sometimes not at all, from the philosophical mercury they are actually discussing. Paracelsus made it one of the three principles, the tria prima, in the sixteenth century.
Its planetary correspondence is Mercury, and the symbol combines a crescent above a circle above a cross: horns, disc and cross, one of the more elaborate of the seven. The link to the planet is usually explained by speed, Mercury being the fastest-moving of the planets and quicksilver the most mobile of the metals, and by the god's character as messenger, trickster and crosser of boundaries, which suits a substance that is neither properly solid nor properly liquid. The Latin name of the metal, hydrargyrum, water-silver, gives the symbol Hg; quicksilver, from the Old English for living silver, says the same thing.