Mercurius, philosophical mercury, is arguably the most important and most complex substance in the entire alchemical vocabulary. De Jong devotes extensive attention to its meaning in her introduction to Atalanta Fugiens, establishing that Mercury stretches from the common metallic element (argentum vivum, quicksilver) to primordial water from which all Creation arose, to the waters of destruction in which the soul perishes, to the Water of Life from which the purified soul and metals emerge transformed. In this last meaning, De Jong identifies, lies the association between Mercury and Christ: both are mediators, both descend and ascend, both die and are reborn.

The polyvalence of Mercury was recognized from the earliest period of alchemical writing. Zosimos of Panopolis, the Greek alchemist of the third century, wrote of mercury as "the divine water" and "the great mystery: it is the object one is looking for. It is the Universe. From it the Universe proceeds and by it the Universe exists. Two natures, one Being. It is the mercury and the philosophical mercury, it is the hermaphrodite, who is eternal." De Jong demonstrates that later alchemists developed this idea and connected Mercury with the water over which the Spirit of God hovered in Genesis, with the depths of the sea in which the king is drowned in Emblem XXXI of Atalanta Fugiens, and with the Fountain of Life.

In the Mercury-Sulphur theory transmitted from Jabir ibn Hayyan through the Latin Geber into Maier's work, mercury and sulphur are the two fundamental principles from which all metals are composed. De Jong emphasizes that by mercury and sulphur, Jabir did not mean common mercury or common sulphur but hypothetical substances whose nature was most closely approximated by those elements. The differences between metals were defined by the purity of their component mercury and sulphur and by the balanced equilibrium in which these principles occurred. This theoretical framework placed mercury at the very foundation of material reality.

Maier expresses this Mercury-Sulphur theory directly in the title and structure of Atalanta Fugiens. As De Jong states: "Maier wanted to express this sulphur-mercury theory, in the title of his emblem book, in the form of the legend of Atalanta and Hippomenes. Atalanta represents the volatile mercury, Hippomenes symbolizes the force of sulphur. It is the alchemist's task to unite these opposites." The entire fifty-emblem sequence can therefore be read as a sustained meditation on the pursuit and fixation of volatile mercury by active sulphur.

In the individual emblems, Mercury appears in multiple guises. In Emblem I, the Son of Philosophy is identified as sulphur carried in mercury, following Lullius. The common mercury (liquid quicksilver) is volatile and rises, while the philosophical mercury is heavy and fixed in solid metals. Mercury the god serves as the mythological cipher for this chemical principle: the winged messenger of the gods, mediating between heaven and earth, carrying the double caduceus that takes souls from bodies and brings them back again. Emblem VII and Emblem XVI deploy the winged-wingless contrast to represent the evaporation and condensation of mercury in the retort. The winged figures consistently signify the mercurial principle in its volatile, ascending mode.

De Jong connects Mercury to the broader theme of dissolution and reconstitution that structures the opus. Mercury is water because of its continuously liquid state, and in the meaning of water it is polyvalent: it is the primordial water, the water of death in which metals decompose, and the water of resurrection from which metals, converted into gold, rise again. This triple function of mercury as destroyer, medium, and regenerator makes it the central agent of alchemical transformation.

Maier also explored Mercury in his Lusus Serius (1616), an allegory about Mercury who reigned on earth as King over men and things, using all the motifs traditionally connected with mercury in alchemy. De Jong describes how Maier used this figure to express the entirety of mercury's meaning, from the practical metallic substance to the cosmic principle of mediation and transformation.

In chemical terms, mercury's extraordinary properties made it a natural focus for alchemical speculation. It is the only metal that is liquid at room temperature, it amalgamates readily with other metals (appearing to "dissolve" them), it volatilizes when heated and re-condenses when cooled (appearing to die and be reborn), and its compounds display a remarkable range of colors from white through yellow to vivid red. These empirical properties gave genuine observational support to the alchemists' philosophical elaborations.

Scholars have consistently identified Mercury as the key to understanding Maier's alchemical system. Tilton shows that Maier's own "mercurial medicine" was similar to Anthony's aurum potabile, both acting by fortifying the heart's calor innatus with the virtues of gold. De Jong demonstrates that the figure of Atalanta running across the frontispiece is Mercury made visual: the volatile principle that must be pursued, caught, and fixed if the Work is to succeed.

For the lay reader, Mercury is the shape-shifter of alchemy, the substance that is everything and nothing, that dissolves all things and becomes all things. Understanding Mercury is understanding why alchemy was not merely primitive chemistry but a comprehensive philosophy of transformation that operated simultaneously at the material, medical, psychological, and spiritual levels.

Meanings Across Registers

De Jong shows that alchemical terms operate simultaneously across material, medical, spiritual, and cosmological dimensions.

Alchemical

The philosophical Mercury: not common quicksilver but the universal solvent and volatile principle—the moist, cold, feminine component of the metallic dyad.

Medical

The vital spirit or pneuma that circulates through the body; the mercurial principle governing nerve function, respiration, and the subtle fluids.

Spiritual

The anima or world-soul; the mediating spirit that connects body and soul, matter and intellect—Hermes as psychopomp guiding transformation.

Cosmological

The planet Mercury as cosmic messenger between Sun and Moon; the principle of mediation and communication linking all levels of the great chain of being.