Mercurius, in alchemy, was the transformative substance which allows the process of transmutation to occur.
It was frequently called philosophical mercury, thus distinguishing it from ordinary mercury.
Mercurius represented the divine spirit of life and the prima materia and was formed though the union of sulphur and argent vive.
It was thought to be the mother of all metals-the mother from which all metals were created. Mercurius supposedly contained both male and female seeds, therefore hermaphroditic, and required to be present if the chemical marriage of King Sol and Queen Luna were to occur.
Other names for Mercurius were aqua permanens, divine water, or aqua vitae. A.G.H
Source:
Drury, Nevil. The Watkins Dictionary of Magic. London. Watkins Publishing. 2005. p. 194