iodine n a bluish-black element of the halogen group that sublimates into a violet irritating gas. Its compounds are used in medicine and photography and in dyes. The radioisotope iodine-131 (radioiodine), with a half-life of 8 days, is used in the diagnosis and treatment of thyroid disease. Symbol: I; atomic no.: 53; atomic wt.: 126.90447; valency: 1, 3, 5, or 7; relative density: 4.93; melting pt.: 113.5°C; boiling pt.: 184.35°C (C19: from French iode, from Greek iodes rust-coloured, but taken to mean violet-coloured, through a mistaken derivation from ion violet)
quartz-iodine lamp , quartz lamp
n a type of tungsten-halogen lamp containing small amounts of iodine and having a quartz envelope, operating at high temperature and producing an intense light for use in car headlamps, etc.