Iodine (Gr.Iodine 0900517violet-colored), an elementary substance named from the color of its vapor, existing in various marine plants, the water of many mineral springs and of the ocean, the bittern of salt works, sponges, corals, and some rocks and minerals. It was discovered in manufacturing saltpetre by Courtois of Paris in 1812, and afterward examined and described by several chemists, but more particularly by Gay-Lussac (Annales de chimie, vols. Ixxxviii., xc, and xci). It is represented by the symbol I; its chemical equivalent is 127. In its preparation it crystallizes either from solution or by sublimation in scales like those of micaceous iron, and in regular crystals of elongated octa-hedrons with rhomboidal base. These are brittle, opaque, bluish black, and of metallic lustre; their specific gravity is 4.95 ; they fuse at 225° F. into a dark liquid, and boil at 347°, giving off deep purple and violet vapors. Iodine is also volatile at common temperatures, and when exposed to the air diffuses an odor like that of chlorine, the vapor irritating the nostrils and exciting cough. This is among the heaviest of aeriform bodies, its density being 8.7 times that of air.

Alcohol, ether, and carbon disulphide dissolve iodine freely; pure water takes up only about 1/7000 of its weight of it, and thus acquires a yellowish or brown tinge. By adding nitrate or chloride of ammonium, common salt, or any of the iodides, to the water, its power of dissolving iodine is greatly increased, and the solution then takes a very deep brown color. Iodine gives a yellow stain to the skin, which soon disappears. Though resembling chlorine in its combinations and some of its qualities, it has not the property of bleaching, and its chemical affinities are weaker. Its remarkable property of imparting a deep blue color to a mass or solution of starch serves as a distinguishing test of extreme delicacy. The starch solution, if cold, will sensibly indicate the pres,ence of iodine in solutions containing only 1/1,000,000 supposed that the iodine is merely mixed in a finely divided state with the starch. It must be free for the test to succeed; and to insure this, where the iodine may be in the state of an iodide, it is recommended to add to the solution a drop of sulphuric acid, and then a little vapor of chlorine, or instead of the chlorine a drop or two of nitric acid may be used. - Though iodine is detected in a multitude of organic bodies, principally those connected with the sea or in plants growing near the salt water, it is found in largest proportion in the fuci or common seaweeds, and other marine plants which grow at great depths.

The fucus palmatus and saccharinus are especially rich in it. The preparation of the iodine of commerce is principally carried on at Glasgow, Scotland, at Donegal, Ireland, and at Cherbourg, France, to which places are brought the half vitrified ashes produced by burning the seaweeds collected on the coast. These ashes, called kelp, or on the continent varec, being coarsely powdered, are digested some hours in water, and the solution is then drawn off upon a fresh portion, and from this upon a third, fourth, fifth, and it may be a sixth, until the liquor has attained a density of 1.257. The various soluble salts, including the iodides and bromides of all the alkalies, are thus taken up and separated from the earthy salts. The solution is then drawn off into broad evaporating pans, and concentrated to 60° Twaddell, or sp. gr. 1.30. At this point the sulphate of soda and chloride of sodium begin to crystallize; they are ladled out as they separate from the solution, and placed so that the drainings from them run back into the pans. When they cease to appear, the liquor is left to repose and to deposit more chloride in the pans. It is then drawn off into coolers, and left for five days for the sulphates to crystallize, as also chloride of potassium.

The liquor is then again evaporated in the pans, and at 68° T., or sp. gr. 1.34, deposits carbonate of soda, and more chloride of sodium and sulphate of soda. It is again run into a cooler to cause a further separation of chloride of potassium. The process is sometimes again repeated, and the liquor brought by evaporation to 74° T. After removing all the crystals that appear, there frequently remain in the solution some chlorides, hyposulphite of soda, and sulphide of sodium. Strong sulphuric acid in the proportion of one seventh of the whole is then added, and after agitation the mixture is left to stand for two days. The sulphurous compounds are in this time decomposed, and sulphates are produced with liberation of sulphur in a free state, in sulphuretted hydrogen, and in sulphurous acid. The iodine is fixed by its combination with sodium, and, unless too much acid has been added, cannot be liberated except by oxygen. The lye is now poured into an iron still lined with lead, and when heated in a sand bath to 140°, a quantity of manganese dioxide is introduced, and the still is luted and connected with its condensers. Iodine vapors come off at a temperature below the boiling point, and condense in the receivers.

Particular care is required that the temperature does not exceed 212°, in which case the iodine is apt to combine with chlorine with loss. Cyanide of iodine often collects in white, prismatic crystals in the receiver furthest from the retort. A portion of iodine remains in the retort in combination with lead and sodium, which is recovered by first converting it into an iodide of copper by the addition of sulphate of copper, and, when this is separated by filtration, decomposing it by sulphuric acid and oxide of manganese, and collecting the vapor. The process above given is somewhat modified at different localities. A method has been proposed by Dr. Kemp to dispense with the burning of the plants, by which much iodine is volatilized and lost, and to crush the roots in which the largest proportion of iodine is concentrated, and set them to ferment; after which the iodine with other salts may be dissolved out by water acidulated with hydrochloric acid, and finally separated by proper reagents. It is also proposed to distil the seaweeds instead of incinerating them.

Large quantities of iodine are now recovered from the nitrate of soda of Chili, according to a method invented by Thiercelin. The mother liquors resulting from the manufacture of saltpetre are treated with a mixture of sulphurous acid and sulphite of soda, and the iodine is precipitated as a black powder. This is placed in earthen jars, on the bottom of which are layers of quartz sand, fine at the top and coarse at the bottom; from these jars the iodine is removed by earthen spoons lined with gypsum, and the greater part of the water is thus separated. It is further purified by sublimation, but is often sold before undergoing the last named process. The amount of iodine thus reclaimed from Chili saltpetre in 1870 amounted to 30,000 lbs. - Iodine is useful as a test for starch, and also as an ingredient of various chemical reagents. Some of these are of great importance in the photographic art. From the iodide of potassium is prepared the iodide of silver, which.constitutes the sensitive film upon the plates. - Iodine forms two important acids, iodic acid, HIO3, and periodic, HIO4. Hydriodic acid closely resembles hydrochloric acid, as the oxygen acids correspond respectively to chloric and perchloric acids. - Iodine has been employed in medicine since 1819, although burnt sponge, which depends upon iodine for its efficacy, had been previously used with advantage in treatment of goitre.

A large number of preparations are employed both for external and internal application; the most important are solutions of iodine in alcohol, iodine dissolved in water by the aid of iodide of potassium, iodoform, the iodide of potassium, and iodides formed with mercury, iron, sulphur, sodium, and arsenic. Iodoform is a yellow salt in hexagonal flat crystals, which contains more than 96 per cent. of iodine. Iodine and many of its compounds are absorbed with considerable rapidity from the stomach, and reappear in the excretions, especially the urine, in a short time. It has also been found in the saliva and milk; this is particularly true of the iodide of potassium. It remains but a short time in the system. Iodine itself is an irritant, and is used to produce counter-irritation. If taken in considerable quantities internally, it becomes a poison by exciting inflammation of the stomach, oesophagus, and fauces. When it is used in small doses long continued, a condition called iodism may arise, consisting in fever, restlessness, disturbed sleep, gastro-intestinal irritation, and progressive emaciation. These symptoms are not likely to arise either from the iodide of potassium or from the iodide of sodium, and are indeed not very frequent from the cautious use of iodine itself.

The action of iodide of potassium in considerable quantity is sometimes marked by coryza, and a rash, like acne, upon the face and chest. Iodine has been principally used in diseases involving glandular enlargement, such as goitre and scrofula, and also with great benefit in syphilis and in chronic rheumatism. In chronic poisoning by mercury or by lead, the metal remaining in the system may be rendered more soluble, and removed by the iodide of potassium. This is shown not only by the improvement of the patient's condition, but by the detection of the metal by chemical tests in the urine. For these purposes it should be used in large doses of from 10 to 20 or 30 grains three times a day. Iodine, iodide of potassium, and iodides of sulphur, lead, and mercury are used externally in the shape of tincture and ointment. Some of these are useful counter-irritants. Any specific absorbent effect on the part of the ointment is not clearly proved. Injections of tincture of iodine have been made into diseased cavities, especially those lined with serous or synovial membrane, as the joints, tunica vaginalis, and ovarian cysts, to excite adhesive inflammation; and it has been injected into the pleura, and even the pericardium and peritoneum.

Inhalations of the vapor have been employed in pulmonary disease. Iodic alimentation, by introducing iodine into articles of food, as bread, has been proposed. Iodine has also been united with cod-liver oil. A solution of iodine in iodide of potassium is useful in the detection of many alkaloids, with which it forms more or less insoluble compounds. It has also been proposed as an antidote to alkaloid poisons; but it should be borne in mind that the compounds formed by iodine with strychnia, for instance, are probably only relatively harmless, and the absorption merely delayed, so that other means of treatment should not be neglected, although the iodine solution, if convenient, may be used to gain time. The special forms of disease in which iodine and its preparations have been found most useful are goitre, enlarged scrofulous glands, scrofulous ulcers and abscesses, secondary and tertiary syphilis, enlargement and induration of the ovaries, chronic affections of the os uteri, dysmenorrhoea, enlargement of the spleen, chronic rheumatism and gout, pleurisy with effusion, and tubercular affections of the head and chest.

The dose of iodine for internal use is from 1/10 to 1/2 gr. three times a day; of iodide of potassium, from 1 gr. to 12 or 15 grs.; of iodide of sodium, 5 to 20 grs.; of iodide of iron, 3 or 4 grs.; of iodide of mercury, 1/2 gr. to 1 gr.; of biniodide of mercury, 1/16 to -1/8 gr.; of iodide of sulphur, 2 or 3 grs.; of iodoform, 1/2 gr. to 2 grs. Iodine and the iodides are best given before or after eating; they are apt to irritate an empty stomach. A generous diet is usually advisable in connection with a therapeutic course of iodine or of the iodides.