Dr. Robert X...Alchemical Divorce: Prostituting The Feminine Essence

Alchemists were convinced that mercury transcended both the solid and liquid states, both earth and heaven, both life and death. Mercury is one of the seven metals of alchemy (gold, silver, mercury, copper, lead, iron & tin). ... The metal is often also represented by a serpent or snake.

Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn) was associated with one of the seven metals known to the classical world (silver, mercury/quicksilver, copper, gold, iron, tin and lead respectively).