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Iodine is a non-metallic, solid, halogen element usually obtained as a heavy, shining, blackish-grey crystalline form usually obtained in the form of brilliant, rhomboidal plates or in elongated octahedrons, and is used especially in medicine, in the preparation of aniline colours, in photography and analysis. It has the symbol I and an atomic weight of 127.

Iodine exists in the form of metallic iodides in sea-water, various mineral waters, in marine molluscs, and in sea-weeds, from the ashes of which it is traditionally chiefly procured. Iodine exists also in certain land-plants and in cod-liver oil. The preparation of iodine was traditionally associated with Glasgow.

Iodine is a non-conductor of electricity. Its vapour has a characteristic violet colour, and hence the name. This vapour is remarkably dense, almost nine times that of air. Iodine has a very acrid taste, and its odour resembles that of chlorine.

In most of its chemical properties iodine resembles chlorine, but is less chemically active; it combines with phosphorus or sodium, and bleaches vegetable colouring matters, but not so readily as chlorine. It is an irritant poison; but in small doses has been of great service in certain forms of glandular disease., and in other ways.

Iodine is very sparingly soluble in water, but dissolves copiously in alcohol and in ether, forming dark-brown liquids. Iodine possesses strong powers of combination, and forms, with the metals, and many of the simple non-metallic substances, compounds which are termed iodides, the commonest being potassium iodide. With hydrogen and oxygen it forms iodic acid; combined with hydrogen it forms hydriodic acid. This is a colourless gas, which strongly reddens litmus.

Starch is a characteristic test of iodine, forming with it a compound of a deep-blue colour. This test is so delicate that a solution of starch dropped into water containing less than a millionth part of iodine is tinged blue by it. The great consumption of iodine was in medicine; it was employed in its pure state, but much more frequently in the form of potassium iodide, which had been found of great benefit in goitre, scrofula, disease of the liver and spleen, in syphilitic affections, rheumatism, etc., as well as in lead-poisoning. Iodide of iron was another useful medicine, being employed in chlorosis, anaemia, scrofula, and glandular affections. Since the start of the 20th century considerable quantities of iodine were prepared from crude Chili saltpetre.
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