Iodine

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.

Iodine's name captures the striking violet of its vapour, the colour that first drew Courtois's eye as it curled up from the seaweed ash. The element has no ancient or alchemical identity, being a discovery of the Napoleonic era, but it swiftly became one of the most useful substances in medicine. As tincture of iodine it served as a front-line antiseptic for wounds; as the corrective for dietary deficiency it banished the goitre and developmental harm that had plagued populations far from the sea, so that iodised salt became one of the quiet triumphs of public health. Its symbolism is entirely modern and medical, built on a colour and on the discovery that a trace of this violet element is indispensable to the body.

Long before anyone knew why it worked, physicians in ancient China and in Greece, including reportedly the school of Hippocrates, treated goitre with burnt sea sponge, a folk remedy that unknowingly delivered concentrated iodine to sufferers and that nineteenth-century chemists eventually recognised as the same principle behind iodine's modern deficiency treatments. Lugol's solution, a mixture of iodine and potassium iodide devised by the French physician Jean Lugol in 1829, remains in use today for thyroid conditions and emergency radiation protection, a two-century-old formulation that has outlived most other remedies of its era.