Iodine is named from the Greek iodes, violet and was discovered in 1811. Iodine, a halogen, occurs sparingly in the form of iodides in sea water from which it is assimilated by seaweeds, in Chilean saltpeter and nitrate bearing earth, known as caliche, in brines from old sea deposits, and in brackish waters from oil and salt wells. Ultrapure iodine can be obtained from the reaction of potassium iodide with copper sulfate. Iodine is a bluish-black, lustrous solid, volatilizing at ordinary temperatures into a blue-violet gas with an irritating odor. It forms compounds wiht many elements, but is less active than the other halogens, which displace it from iodides. Iodine compounds are very important in organic chemistry and in medicine.