Discovering iodine

This year is the bicentenary of the discovery of iodine by Paris saltpetre manufacturer Bernard Courtois, although it remained officially unrecorded for nearly two years. Fortunately, when details began to emerge late in 1813 the famous chemist Sir Humphry Davy and his assistant the young Michael Faraday were in Paris on a continental tour. Despite the Napoleonic wars (1796-1815) between Britain and France, they had been granted passports by the French government in recognition of Davy's fame. Faraday's journal for 1 December reads:

On this and the preceding day Sir H. Davy made many new experiments on the substance discovered by 

M. Courtois ... M. Clément has lately read a paper on it at the Institute, in which he says it is procured from the ashes of seaweeds by lixiviation and treatment with sulphuric acid: he conceives it to be a new supporter of combustion. 

The discovery of this substance, in matters so common and supposed so well known, must be a stimulus of no small force to the inquiring mind of modern chemists.¹