Iodine was discovered in 1811 by the French manufacturer Bernard Courtois, who noticed violet fumes rising when he treated the ash of seaweed with acid during the production of saltpetre for gunpowder. Joseph Louis Gay-Lussac confirmed and named it, from the Greek iodes, violet-coloured, after the vapour. Iodine is a lustrous, purple-black solid that sublimes to a violet gas, a halogen essential to human health through the thyroid gland. The thyroid uses iodine to make the hormones that regulate metabolism, and the element is also valued as an antiseptic, in medicines and dyes, in photographic chemistry, and as a tracer and contrast agent in medical imaging.
Courtois's discovery had a wartime backdrop: Napoleonic France, blockaded by the British navy and cut off from Chilean saltpetre, turned to extracting potassium nitrate from the ashes of burned seaweed, and it was in this substitute saltpetre industry that the strange violet fumes first appeared. Once Courtois shared samples, both Humphry Davy, then travelling in France despite the war, and Gay-Lussac raced to characterise the new substance in 1813 and 1814, each publishing swiftly and each claiming priority in establishing it as a true element, a minor Anglo-French scientific rivalry playing out against the larger backdrop of the Napoleonic conflict itself.