It relates to the four humours, and subsequently its connection with Alchemical research, if you're not already familiar; there were considered four each of the elements (fire, water, wind, air) bodily fluids or Humours (blood, phlegm, black bile, yellow bile) and temperaments (sanguinous, phlegmatic, bilious and melancholy), each one related to the other, but I don't fully remember each one, though I believe water=blood=sanguine (blood is obviously sanguine).

In Alchemical processes both sulfur and mercury were important, because they could cause a transmutation from one element to another, by way of adding aspects of hot, cold, wet or dry, it stood to reason, thus, that those same transmutational substances could cause a change in temperament.

A lot of this thinking was reintroduced from ancient greek sources to Europe, predominantly at the beginning of the Early Modern period, the corpus of Hippocrates and Galen were highly regarded, along with the natural philosophies of Plato and Aristotle.

I recommend picking up a copy of Lawrence M Princip, Secrets of Alchemy, it goes a lot more in depth about the connections between alchemy and medicine and the theories behind it.