For Hermes said of this Science: Alchemy is a Corporal Science simply composed of one and by one, naturally conjoining things more precious, by knowledge and effect, and converting them by a natural commixtion into a better kind.
A certain other said: Alchemy is a Science, teaching how to transform any kind of metal into another: and that by a proper medicine, as it appeared by many Philosophers’ Books.
Alchemy therefore is a science teaching how to make and compound a certain medicine, which is called Elixir, the which when it is cast upon metals or imperfect bodies, does fully perfect them in the very projection.
“The Mirror Of Alchemy”, by Roger Bacon 1250
Concerning this admirable, excellent, divine, and most secret Art, it is a matter of no ordinary difficulty to satisfactorily resolve the question of the actuality thereof, but, as appears from Aristotle, it is absurd to prove the existence of Nature, or to argue the possibility of what is know. Our subject is the transmutation of metals into true gold and silver by the skill of art. Alchemy is the Art by which the principles, causes, activities, properties, and affections of metals are thoroughly apprehended; and by means of this knowledge those metals which are imperfect, incomplete, mixed, and corrupt, and therefore base, are transmuted into gold and silver. Alchemy is an operative science, and produces effects by supplying natural conditions, e.g., by the action of fire.
And the claims of the Art itself appear so miraculous, and so far exalted above the ordinary course of Nature, that the vulgar herd are of necessity led to regard the Alchemist as a kind of sorcerer or magician, and to place his pretensions in the same class with those of the man who professes to work signs and wonders. Nevertheless, I stoutly maintain that the Art of Alchemy is clear and true, and founded upon Nature.
The fact is that, in producing gold, the Art of Alchemy does not pretend to imitate in the whole work of Nature. It does not create metals, or even develop them out of the metallic first substance; it only takes up the unfinished handiwork of Nature and completes it.
As to the brief space of time required for the conversion in our Art, it must not be thought that we bring this about by exposing metals in the furnace to the sudden operation of fierce heat. If we did so, their metallic moisture would, of course, be destroyed and dried up. But we only just melt the imperfect metals over the fire, and then add to them the Philosopher’s Stone, which, in a moment of time, imparts to them the form of gold, thus changing and ennobling their nature.
“The New Pearl Of Great Price”, by Peter Bonus 1339