(Gr. iodes, violet) Discovered by Courtois in 1811, Iodine, a halogen, occurs sparinglyin the form of iodides in sea water from which it is assimilated by seaweeds, in Chileansaltpeter and nitrate-bearing earth, known as caliche in brines from old sea deposits, andin brackish waters from oil and salt wells.

Ultrapure iodine can be obtained from the reaction of potassium iodide with coppersulfate. Several other methods of isolating the element are known.