THE MINERAL REALGAR, IS COMPOSED OF THE ELEMENTS OF ARSENIC AND SULFUR. IT IS A MAJOR SOURCE OF ARSENIC. Arsenic is a metallic chemical element in Group VA of the periodic table. Its symbol is As, its atomic number 33, and its atomic weight 74.9216. The Earth's crust contains only about 5.5 parts arsenic per million. Arsenic occurs in numerous minerals, in particular realgar, orpiment, and arsenopyrite. Arsenic and some arsenic compounds have been known for a long time. Aristotle thought that arsenic was a kind of sulfur. In about 1250, Albertus Magnus became the first to describe a method of manufacturing arsenic. Since then the method has scarcely changed: the mineral arsenopyrite is heated and decomposes with the liberation of arsenic gas. The gas can be condensed on a cold surface.