Sulfur

Sulfur is one of a very small number of elements that occur native, pure and unbound, at the surface of the Earth. It crusts around volcanic vents in yellow crystals and accumulates in enormous deposits above salt domes. It could simply be picked up, and it was, long before anybody had a concept of an element.

Burned, it gives sulfur dioxide, a sharp choking gas that kills insects and moulds and bleaches wool. Egyptian, Greek and Roman sources describe fumigation with burning sulfur, and Homer refers to it. Its association with the underworld follows straightforwardly from the smell of volcanic ground and the sting of the fumes; brimstone in the King James Bible is simply the older English word for the substance.

Sulfur is one of the three ingredients of gunpowder, with charcoal and saltpetre, in the Chinese formulations of the ninth century onward and in every European recipe afterwards. Its acid was known to Islamic and Latin alchemists as oil of vitriol, obtained by strongly heating green vitriol, which is iron sulfate. Sulfuric acid production remains one of the standard measures of a country's industrial capacity; more of it is made by mass than any other manufactured chemical.

Charles Goodyear's discovery in 1839 that heating rubber with sulfur cross-links it into a tough elastic solid, a process named vulcanisation, turned rubber into a usable engineering material and remains among the element's largest applications.

In the body, sulfur appears in the amino acids cysteine and methionine. Disulfide bridges between cysteine residues hold proteins in shape, which is why hair and wool are strong, and why the smell of burning hair is the smell of sulfur.

The alchemical sulfur is not quite the same thing as the yellow powder, and confusing the two makes nonsense of the texts.

In the sulfur-mercury theory, which reached Latin Europe from Arabic sources and was current from the twelfth century, all metals were held to be composed of two principles: a sulfurous principle responsible for combustibility, colour and fixity, and a mercurial principle responsible for fusibility, lustre and volatility. Different proportions and purities of the two accounted for the differences between lead and gold. This was a real theory doing real explanatory work, and the transmutation project follows from it directly. If gold and lead differ only in the proportion and refinement of two constituents, then adjusting the proportion ought to convert one into the other.

Paracelsus added a third principle, salt, standing for solidity and incombustibility, in the early sixteenth century, giving the tria prima. In his medical writing the three become the basis of an entire physiology.

The crucial point is that philosophical sulfur was not ordinary brimstone, and the texts say so repeatedly. It was the quality of combustibility considered as a substance. Ordinary sulfur was its most obvious embodiment and was used as such in the laboratory, which is why the same word and the same mark, a triangle set above a cross, served for both.

None of the seven planetary correspondences applies to sulfur, because it was not one of the seven metals. Its place in the tradition is of a different kind, and arguably a more fundamental one, since the metals themselves were held to be made out of it.