ἰοειδής

Bernard Courtois was not looking for a new element. He was manufacturing saltpeter — potassium nitrate — for Napoleon's gunpowder during the wars of 1811. The process required potash, which Courtois extracted from seaweed ash collected on the Normandy coast. One day, he added too much sulfuric acid to the ash. A cloud of violet vapor rose from the vessel and condensed into dark, lustrous crystals on cold surfaces. Courtois knew he was looking at something unknown.

Courtois lacked the resources to analyze his discovery properly. He gave samples to Charles Bernard Desormes and Nicolas Clément, who presented preliminary findings to the Institut de France in 1813. Joseph Louis Gay-Lussac conducted the full analysis and proposed the name iode, from the Greek ioeidḗs, meaning violet-like — from ion (violet flower) and eidos (form). Humphry Davy, independently analyzing samples in London, confirmed it was a new element. Gay-Lussac and Davy both published in the same month, and the priority dispute between France and England was immediate.

Iodine deficiency causes goiter — the swelling of the thyroid gland — and severe developmental impairment. This was known empirically for centuries; Chinese physicians prescribed seaweed and burnt sponge for goiter treatment as early as the Tang Dynasty, around 650 CE. In 1924, David Murray Cowie persuaded Morton Salt Company to add potassium iodide to table salt in the United States. Iodized salt virtually eliminated goiter in the developed world within a generation.

The element discovered in seaweed ash came full circle: the iodine that Courtois found by accident in marine plant residue was the same iodine that marine organisms had been concentrating from seawater for millions of years. Kelp accumulates iodine at concentrations 30,000 times higher than the surrounding ocean. Courtois did not discover iodine; the seaweed had been hoarding it all along.