Many treatises define Mercurius simply as fire.
He is ignis elementarisj^ noster naturalis ignis certissimus, which again indicates his “philosophic” nature.
The aqua mercurialis is even a divine fire.^ This fire is “highly vaporous” (vaporosus)
Indeed, Mercurius is really the only fire in the whole procedure.
He is an “invisible fire, working in secret.”
One text says that the “heart” of Mercurius is at the North Pole and that he is like a fire (northern lights).
He is, in fact, as another text says, “the universal and scintillating fire of the light of nature, which carries the heavenly spirit within it.”
This passage is particularly important as it relates Mercurius to the lumen naturae, the source of mystical knowledge second only to the holy revelation of the Scriptures.
Once more we catch a glimpse of the ancient role of Hermes as the god of revelation.
Although the lumen naturae, as originally bestowed by God upon his creatures, is not by nature ungodly, its essence was nevertheless felt to be abysmal, since the ignis mercurialis was also connected with the fires of hell.
It seems, however, that the alchemists did not understand hell, or its fire, as absolutely outside of God or opposed to him, but rather as an internal component of the deity, which must indeed be so if God is held to be a coincidentia oppositorum.
The concept of an all-encompassing God must necessarily include his opposite.
The coincideniia, of course, must not be too radical or too extreme, otherwise God would cancel himself out.
The principle of the coincidence of opposites must therefore be completed by that of absolute opposition in order to attain full paradoxicality and hence psychological validity. ~Carl Jung, CW 13, Para 256