The Violet Vapour

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.