Iodine, a chemical element, symbol I; for physical constants see Periodic Table. Iodine was discovered 1811 by Bernard Courtois. It occurs as iodides in sea water, in brines, and in brackish waters from oil wells. It is obtained commercially from caliche, Chilean nitrate-bearing earth and from seaweed ash. Iodine is prepared by displacement of an iodine compound with chlorine. It is a shiny, bluish-black solid, which volatilizes at ordinary temperatures into an irritating, blue-violet gas. Iodine is the least reactive of the halogens. Lack of iodine in humans is the cause of goiter. Radioactive iodine has been used in treating the thyroid gland. Iodine and its compounds are used in organic chemistry, medicine, and photography.