A symbol in alchemy of the sacred fire or Kundalini.
"Sulfur is the fire of Alchemy." - Samael Aun Weor, Treatise of Sexual Alchemy
Since we have lust, pride, envy, etc., that fire is modified, and is called Arsenic (i.e poisonous) Sulfur.
"When one thinks differently and positively about people, it is a sign that one is changing."