SILVER WATER

Back in the days of Ancient Rome, each of the seven known heavenly bodies was associated with a metal: the sun with gold, the moon with silver, Venus with copper, Mars with iron, Jupiter with tin, Saturn with lead, and Mercury with, well, mercury. The normally liquid element was associated with the planet and the god it was named after because all three had an association with being mobile and quickly changing (this is also how we got the word mercurial). The god's name probably comes from the Latin word merx, meaning "merchandise", because Mercury was the god of tradesmen.  The Greek word for the element, hydrargyros, is the reason that it's represented with an Hg on the Periodic Table. That name comes from hydor, the word for "water", and argyros, the word for "silver" (so, together, mercury was "silver water").