Mercury was considered by alchemists to be one of the principal substances along with salt and sulfur. This silver liquid has the Latin name hydrargyrum, literally water silver. This is the reason for the symbol Hg on today’s periodic table. It was also known by the name quicksilver.
The symbol for the metal mercury was sometimes the same as the astrological symbol for the planet Mercury. The element could be represented by a stylized snake or serpent. Alchemists would heat elemental mercury with nitric acid to prepare mercuric oxide. The reaction produced a thick red vapor which hovered over the surface of the solution, rather like a snake, while the mercuric oxide precipitated and fell to the bottom of the liquid in the form of bright red crystals.
Mercury was believed to transcend the liquid and solid states. The belief carried over into other areas, as mercury was thought to transcend life/death and heaven/earth. The element was believed to have feminine qualities, so it was common for a mercury alchemy symbol to reflect the female influences.
Here is a collection of mercury alchemy symbols.