Among the substances most fascinating to the alchemists besides
gold, silver, mercury, and arsenic were sulphur and phosphorus. This
sign is the most commonly used for sulphur. In the symbolism of the Middle Ages the
upward pointing triangle was the sign for the element of fire.
Sulphur was associated with Hell, and Hell with the Underworld. This is the reason for the arrow pointing down to the underworld. A horizontal bar has been added to the arrow to make a cross. The alchemists tried to include a cross in most of their sign structures.
After the alchemists, has been used for the asteroid Pallas. For more signs for Pallas, see the above entry.