Sulfur, chemical element, symbol S; for physical constants see Periodic Table. Sulfur has been known and used for thousands of years. It is referred to in the bible as brimstone. As a mineral, it occurs in iron pyrites, galena, sphalerite, Epsom salts, barite, and many others. It occurs free in nature in the vicinity of volcanoes and hot springs. Sulfur is also found in the atmosphere of Venus and in meteorites and interstellar clouds. It is produced commercially using the Frasch process in which heated water is used to melt underground sulfur that is then brought to the surface. Sulfur is a yellow, brittle, low melting, reactive nonmetal. It readily forms sulfides with many elements. Hydrogen sulfide is poisonous and can cause death by respiratory paralysis. Sulfur forms several allotropes amorphous and crystalline. Sulfur is essential to human and animal life. Sulfur and its compounds are used in gunpowder, in the vulcanization of rubber, as a fungicide, in making sulfite paper, as a fumigant, and in bleaching dried fruits. It is used to produce sulfuric acid, the most important manufactured chemical.