Bartholomew says: "Quicksilver is a watery substance, mixed with a subtle sort of earth in a strong and indissoluble mixture. This is on account of its great earthy dryness, which does not liquefy on a flat surface, “and therefore it does not adhere [to the surface]” like something watery. Its substance is bright, “from the brightness of subtle water” and from the “whiteness of earth” that is well refined and divided. It also has whiteness from the “admixture of air” with these components. Quicksilver naturally has this as its own property, and it cannot be thickened on its own, without sulfur, but with sulfur and with the substance of lead it may be congealed. So it is said in the same place [Avicenna’s On minerals (De mineralibus)] that quicksilver and sulfur are elements -- that is, they comprise the principle[s] and matter of everything liquefiable, namely metals. All of this is discussed in the book of Meteorology and indeed just as Aristotle put them forth. So quicksilver is said to be the principle of all metals, and thus with respect to the others to be a simple element."
One example that should still resonate today is the phrase “mad as a hatter.” We still remember this because of the Mad Hatter in Lewis Carroll’s Alice’s Adventures in Wonderland. Lewis Carroll doesn’t explain why the hatter is mad, but it was fairly common in the nineteenth century because hatters were susceptible to mercury poisoning.
That mercury is a poison was not always known. For example, in China, the presumed alchemical properties of mercury led to its use in pills intended to make the first emperor Qin Shi Huangdi immortal. The pills killed him instead (and flowing mercury was used to represent water in a map inside his necropolis, with its famous Terracotta Army).
So what is mercury anyway?
Mercury is a liquid made from the solid ore red cinnabar, which is mercury sulfide. When crushed and heated, liquid mercury separates from the sulfur and must be cooled and caught before it evaporates. This much is factual and was known in the Middle Ages, as it must also have been to the Romans, the Greeks, and at least the Persians before them.
The claim that quicksilver is the basis of all metals, however, is not in Aristotle’s Meteorology. That claim and an allied one that both quicksilver and sulfur are the basis of all metals appear later in alchemy, such as in The Secret Book of Artephius. So where did Bartholomew get this information, and his other explanations?
The Middle Ages ascribed a number of works to Aristotle that he did not write, and because we have not discovered their true authors we still say that some works are by Pseudo-Aristotle. What is likely here is that Bartholomew saw Richard Rufus’s lost commentary on Aristotle’s Meteorology.