A French saltpetre manufacturer, Courtois, in 1811 discovered a strange substance in the soda obtained from sea plants; he told his discovery to Clément, who showed the body in question to Davy. Davy soon demonstrated its elementary nature and Gay Lussac, after a complete investigation of iodine, as he called it, and its compounds, succeeded in showing its marked likeness to chlorine.
Laurent, in 1837, striving after the real truth of substitution, had pictured the organic molecule as a prism, the angles of which were occupied by carbon atoms, the centres of its edges by hydrogen atoms, or failing these by chlorine, bromine, or iodine atoms.
In 1812 [Humphry Davy's] Elements of Chemical Philosophy was published, and next year he travelled on the Continent. During this journey, while in Paris, he examined iodine, and declared it to be an element.
Speaking broadly... we understand by the Atomicity of a chemical element its power of combining with other elements in equivalent proportions; by its Valency or quantivalence, the difference between that element and other elements in their respective powers of combining with hydrogen as a standard. Thus... Hydrogen, chlorine, bromine, iodine, and fluorine combine with hydrogen atom for atom and are termed Monovalent elements or Monads.
Iodine was discovered by Courtois of Paris in 1812 in the mother-liquors collected in the process of manufacturing the sodium salts from kelp or burnt sea-weeds. The name is derived from a Greek word meaning "violet" in allusion to the colour of the vapour of iodine. Its elementary character was established by Gay-Lussac in 1815.
In 1812 and 1813 iodine, discovered by Courtois, a French soapmaker, and investigated by Gay Lussac, was added to the list of acidifiers.
Gay-Lussac (1778 1850)... enriched chemical literature by many excellent investigations, working often in company with Thenard, Humbolt, and Liebig. His most noteworthy work was upon iodine, cyanogen (the first compound radical), the alkaline oxides, the isolation of boron, improved methods for organic analysis, and many similar studies.
The table of Mendeleeff was changed but little for thirty years. Its anomalies, as the omission of hydrogen and the rejection of the atomic weight as the deciding factor in such cases as cobalt and nickel, tellurium and iodine, etc., were recognized; but greater knowledge was needed before these could be explained or the underlying law grasped.