Mercury, along with sulfur and salt are the three essentials that get mentioned over and over. Mercury deserves it’s own page, which is here. The three essentials can also be called the Tria Prima, the Three Universals, the three humors, and many other things. It can be represented by a three headed dragon in illustrations. To alchemists all created things consist of Sulfur, Mercury and Salt, though they don’t always mean the same thing as one would expect. The alchemical meaning might be spelled out as sophic sulfur, sophic mercury and sophic salt, or just capitalized. They can have spiritual meanings, and sometimes even more than one precise meaning in the same text. I talk a little bit about that when discussing Michael Sendivogius. Basically to the philosopher’s stone (to some) was a re-combination of the three essentials: break them down, purify and build them back up the right way. A similar theory, that is often repeated is from Al-Jabir (Geber) which is that all metals are a mixture of sulfur and mercury, gold is just perfect balance of them. So to make you gold you simply break them down and balance them, alternatively the philosopher’s stone is a substance which instantly created that balance (therefore creating gold). To complete that theory of matter, the [theory of elements] is important too: Earth, Air, Fire, and Water.
The union of sulphur and mercury is often mentioned in alchemical recipes. It’s associated with the fire element.
Salt is the stuff of growth and involved in precipitation and crystallization processes. It is the principle of the earth element.