The essential thing to do is, therefore, to find an agent which will bring about the maturing and perfecting of Mercury.

This agent, Philalethes calls "Our divine Arcanum." Although it appears to me impossible to translate the sayings of the alchemists concerning Elements and Principles into expressions which shall have definite and exact meanings for us to-day, still we may, perhaps, get an inkling of the meaning of such sentences as those I have quoted from Basil Valentine and Philalethes.
Take the terms _Fire_ and _Water_.

In former times all liquid substances were supposed to be liquid because they possessed something in common; this hypothetical something was called the _Element, Water_.

Similarly, the view prevailed until comparatively recent times, that burning substances burn because of the presence in them of a hypothetical imponderable fluid, called "_Caloric_"; the alchemists preferred to call this indefinable something an Element, and to name it _Fire_.
We are accustomed to-day to use the words _fire_ and _water_ with different meanings, according to the ideas we wish to express.

When we say "do not touch the fire," or "put your hand into the water," we are regarding fire and water as material things; when we say "the house is on fire," or speak of "a diamond of the first water," we are thinking of the condition or state of a burning body, or of a substance as transparent as water.