IODINE, is one of the simple chemical bodies which was discovered accidentally in 1812, by M. Courtois, a manufacturer of saltpetre, in the mother-waters of that salt. Its affinities for other substances are so powerful as to prevent it from ex isting in an insulated state. It occurs combined with potassium and sodium in many mineral waters, such as the brine spring of Ashby-de-la-Zonche, and other strongly saline springs. This combina tion exists sparingly in sca-water,. abun dantly in many species of focus or sea weed, and in the kelp made from them ; in sponges ; in several marine mollusece, such as the Soria, the cent's, oysters, &c.; in several polyparies and sea-plants, as the gargonta, the austere marina, &c. ; particularly in the mother-waters of the salt-works upon the Mecriterratean sea; and it has been found in combination with silver, in some ores brought from the neighborhood of Mexico.
It is an ingredient in the salt licks, sa line, and brine springs of this country, especially of those in the Valley of the Mississippi, and it has been found to be a constituent of coal. It seems to be be neficial to marine plants, and they have the power of abstracting it from sea-wa ter. It is from these plants that almost all the iodine of commerce is derived.
Kelp, or the half vitrified ashes of sea weeds, prepared by the inhabitants of the Western Islands and the northern shores of Scotland and Ireland, is treated with water, and the solution filtered. The li quid is then concentrated by evaporation until it is reduced to a very small volume, the chloride of sodium, carbonate of soda, chloride of potassium, and other salts, being removed as they successively crys tallize. The dark brown mother-liquor left, contains very nearly the whole of the iodide ; this is mixed with sulphuric acid and peroxide of manganese, and gently heated in a leaden retort, when the io dine distils over and condenses in the re ceiver. The theory of the operation is exactly analogous to that of the prepara tion of chlorine ; it requires in practice, however, careful management, otherwise the impurities present in the solution in terfere with the general result.
The manganese is not really essential ; the iodide of potassium or sodium, heated with an excess of sulphuric acid evolves iodine. It is probable that this effect is due to a secondary action between the hydriodic acid first produced and the re sidue of the sulphuric acid, in which both suffer decomposition, yielding iodine, water, and sulphurous acid.
Iodine crystallizes in plates or scales of a bluish black color and imperfect metal lic lustre, resembling that of plumbago ; the crystals are sometimes very large and brilliant. Its density is 4.948. At 225°
it fuses, and at 347° boils, the vapor hav ing an exceedingly beautiful violet color. It is slowly volatile, however, at common temperature, and exhales an odor much resembling that of chlorine. The density of the vapor is 8•716. Iodine requires for solution about 7000 parts of water, which nevertheless acquires a brown co lor in alcohol it is much more freely so luble. Solutions of hydriodic acid and the iodides of the alkaline metals also dis solve a large quantity ; these solutions are not decomposed by water, which is the case with the alcoholic tincture.
This substance stains the skin, but not permanently ; it has a very energetic ac tion upon the animal system, and is much used in medicine.
One of the most characteristic proper ties of iodine is the production of a splendid blue color by contact with the organic principle starch. The iodine for this purpose must be free or uncombined. It is easy, however, to make the test available for the purpose of recognising the presence of the element in question when a soluble iodide is suspected ; it is only necessary to add a very small quan tity of chlorine-water, when the iodine, being displaced from combination, be comes capable of acting upon the starch.
Iodine is now extensively used by the Daguerreotypist to coat the silver plate with so as to form a surface sensitive to light ; generally the pure iodine is used in the coating, occasionally the chloride of iodine is preferred. Iodine is also used by the French to produce a blue color adapted for dyeing cotton with.
IonnrE, CHLORIDE OF is a preparation used in daguerreotyping. It is made by passing chlorine gas through iodine until the whole becomes liquid. lodinereadily absorbs chlorine forming when the chlo rine is in excess a solid yellow compound, and when the iodine preponderates a brown liquid; the solid iodide is decom posed by water. The liquid is not, and is that which is used in the arts. It is a yellow, oily liquid, of a suffocating smell and astringent taste, soluble in water and alcohol. It consists of 1 equivalent of chlorine united with 1 equiv. of iodine. IRIDIUM is a metal discovered Descotils, in 1803, as also by Tenant, in 1804; and is so called because its differ ent solutions exhibit all the colors of the rainbow. It occurs only in the ore of platinum, being found there in two states; 1. united to that metal, and 2. as alloy of osmium and iridium, in the form of small, insulated, hardgrains. Iridium is the most refractory of all the metals ; and ap pears as a gray metallic powder. It is not fused by the flame of the hydro-oxy gen lamp.