Sulphur was thought of as Sol's terrestrial embodiment. The chemistry of sulphur is very relevant to its symbolism:
BURNING STONE
Most of us know sulphur as the so-called flowers of sulphurpowdered sulphurbut it shows up in nature in crystalline form as a hard yellow rock. Miraculously, as far as most rocks are concerned, this rock burns! That is why it was called the burning stone. The word sulphur is supposed to have derived from a Latin term meaning burning stone. And the word brimstone is the Anglo-Saxon term for burning stone. So brimstone is a synonym for sulphur
ML p.95SULPHUR EQUIVALENT TO FIRE
The fact that it is a brilliant yellow color connects it with Sol symbolism. And the fact that it is so readily combustible made it, in the minds of the alchemists, practically equivalent to fire itself
ML p.95CORRUPTING EFFECT OF SULPHUR
Both hydrogen sulphide and sulphur dioxide immediately tarnish metalsthey form sulphide compounds, many of which are black. If, for instance, you drop a penny into a solution that has a little hydrogen sulphide in it, it immediately turns black. So it is very easy to see how the idea of the corrupting effect of sulphur could come about. Also, sulphur is the central ingredient of gunpowder. So much for its chemical manifestations:
ML p.96Jung says:Sulphur represents the active substance of the sun or, in psychological language, the motive factor in consciousness: on the one hand the will, which can best be regarded as a dynamism subordinated to consciousness, and on the other hand compulsion, an involuntary motivation or impulse ranging from mere interest to possession proper. The unconscious dynamism would correspond to sulphur, for compulsion is the great mystery of human life. It is the thwarting of our conscious will and of our reason by an inflammable element within us, appearing now as a consuming fire and now as life-giving warmth. ( CW14: par. 151 )
SULPHUR AS BOTH GOOD AND EVIL
Jung brings in a great deal of imagery to demonstrate that sulphur was thought of as both good and evil. In its negative aspect it is associated with the devilbrimstone is a component of hell. And the fact that sulphur tends to corrupt or blacken whatever it touches connects it with the devil and negative factors
ML p.96POSITIVE ASPECT OF SULPHUR
The positive aspect of sulphur is its association with Christ and the Holy Ghost ( CW14: par. 145 ). It is described in CW14: par. 140, as the “painter of all colours”
ML p.96DOUBLE SYMBOLISM OF FIRE
These two aspects correspond to the double symbolism of fire: there's a heavenly or celestial aspect and a lower or infernal aspect. In the apocryphal Gospel of Thomas, Christ identifies himself with fire: “He who is near me is near the fire” ( Robert M. Grant, The Secret Sayings of Jesus p. 180 ). In Luke he identifies himself as an incendiary: “I came to cast fire upon the earth; and would that it were already kindled” ( Luke 12 : 49, Revised Standard Version ). He's calling himself a match to ignite the world, you see; he's identifying himself with the fire. But since for our aeon Christ is the most developed symbolic manifestation of the Self, his identification with the fire of sulphur also connects sulphur with the Self
ML p.96CONTRARY ASPECTS OF FIRE
The devil on the other hand represents the contrary aspects of fire: the torturing fire of hell and purgatory where brimstone is supposed to be the chief ingredient. So these speak to the imagery of sulphur duplex
ML p.97