Iodine is a peculiar and compounded principle; it was discovered in Paris in 1811 by M. Courtois, a saltpetre manufacturer, who observed a rapid corrosion of his vessels in his processes for obtaining soda from the ashes of sea weeds; and in searching for the cause of the corrosion, he made the discovery of this important substance. It is from sea weeds alone that iodine can be obtained; it has not yet been decomposed; it is of a greyish-black colour, and metallic lustre; soft and friable to the touch; taste acrid, and a deadly poison. It gives a brown stain to the skin, which speedily vanishes by evaporation. The specific gravity of iodine at 62°, is 4.948: it dissolves in 7,000 parts of water, and the solution is of an orange yellow colour, and in small quantities tinges raw starch of a purple hue. Iodine is incombustible; but with azote it forms a detonating compound, and in combining with several bodies it produces the phenomenon of combustion. The oxide of sodium, and the subcarbonate of soda are completely decomposed by it. It forms with sulphur a compound of greyish black, radiated like sulphuret of antimony.
Hydrogen, whether dry or moist, does not seem to have any action on iodine at the ordinary temperature; but if we expose a mixture of hydrogen and iodine to a red heat in a tube, they unite together, and hydriodic acid is produced, which gives a reddish brown colour to water. Charcoal has no action upon iodine. Several of the metals, as zinc, iron, tin, mercury, attack it readily, even at a low temperature, provided they be in a divided state. Iron is acted upon by iodine in the same way as zinc, and a brown iodine results. Antimony presents with iodine the same phenomena as tin. The iodines of lead, copper, bismuth, silver, and mercury, are insoluble in water; tin's is at least the case with the above-mentioned metals. There are two iodines of mercury; the one yellow, the other red; both are fusible and volatile. When iodine and oxides act upon each other in contact with water, its hydrogen unites with iodine to form hydriodic acid; while its oxygen, on the other hand, produces with iodine iodic acid. Iodine of mercury has been proposed for a pigment. Iodine has been most successfully applied medicinally for reducing the goitre and glandular swellings.
See Ure's Dictionary.