Iodine was discovered in 1811, and was as an indirect result of Napoleon’s need for gunpowder. This is made from potassium nitrate, which was in short supply due to the Royal Navy blockade. It was extracted from human and animal dung from which it is formed by bacteria which converts ammonium ions to nitrate ions. These crystallise out as KNO3, the least soluble of the nitrates. The main supply had been the cesspits of the teeming cities of the East and especially India. To stimulate demand, Napoleon encouraged French producers. They used all kinds of rotting vegetable waste and manure.
One such entrepreneur was Bernard Courtois (1777–1838) who leached rotting seaweed. The solution from this was boiled down in large pans and gave a mixture of NaNO3 and KNO3, mainly the latter. While cleaning out the pans one day with sulfuric acid, he noticed purple fumes, which condensed as shiny black crystals. (Seaweed has up to 0.45% dry weight of iodide.) These he collected and gave to two chemist friends, Charles-Bernard Desormes and Nicolas Clément, who investigated them with the help of J.L Gay-Lussac. On 29 November 1813, Gay-Lussac revealed the new element at the Institut Imperial in Paris, naming if from the Greek iodes meaning violet.
At this point, Sir Humphry Davy comes into the picture. Despite the war with Britain, Napoleon said Davy could visit Paris, with Michael Faraday as his assistant. When they arrived, Davy was given a sample of the new material and set to work in his hotel room with his portable lab. Davy confirmed that iodine was indeed a new element. He addressed the Institut Imperial on 11 December and sent his results back to the Royal Institution in London.
When his report was read at the Royal Institution in January, the impression was that Davy was the first to realise that iodine was a new element. The idea took hold. Two years later, Britain won the war and the defeated French were in no position to put the record straight. The result was that for most of the 1800s, Davy was believed to be the discoverer of iodine.