Source. - A non-metallic element, obtained principally from Kelp, the ashes of sea-weed.

Characters. - Laminar crystals of a dark colour and lustre, and peculiar odour. Solubility, 1 in 7,000 of water, 1 in 12 of rectified spirit, 1 in 4 of ether, sparingly in glycerine, freely in a solution of iodide of potassium or chloride of sodium. Seldom given as pure iodine.

Impurities. - Iodide of cyanogen; subliming as colourless pungent prisms. Iron; not volatile. Water; as moisture. Deficient iodine; detected by hyposulphite of soda.

Incompatibles. - Ammonia, metallic Kilts, mineral acids, vegetable alkaloids.

Preparations.

a. Linimentum Iodi. - Iodine, 5; Iodide of Potassium, 2; Camphor, 1; Spirit, 40.

b. Liquor Iodi. - Iodine, 20 gr.; Iodide of Potassium, 30 gr.;Water, 1 oz.

c. Tinctura Iodi. - Iodine, 1/2; Iodide of Potassium, 1/4; Spirit, 20. Dose, 5_to 20 min.

From Tinctura Iodi is prepared:

Vapor Iodi. - Tincture of Iodine. 1 fl.dr.; Water, 1 fl.oz.

d. Unguentum Iodi. - Iodine, 32 gr.; Iodide of Potassium, 32 gr.; Spirit, 1 fl.dr.; Lard, 2 oz.

From Iodum is made:

Potassii Iodidum. - See Potassium, for source and characters. Dose, 2 to 10 gr. or more.

Preparations.

a. Linimentum Potassii Iodidi cum Sapone. -Iodide of Potassium, 1 1/2; Hard Soap, 1 1/2; Glycerine, 1; Oil of Lemon, 1/8; Water, 10.

. Unguentum Potassii Iodidi. - Iodide of Potassium, 64 gr.; Carbonate of Potash, 4 gr.; Water, 1 fl.dr.; Lard, 1 oz.

γ. Also all preparations of Iodum.

Iodine is also used in the production of the Iodides of Cadmium, Ferrum, Hydrargyrum (2), Plumbum, and Sulphur.

Solutions of Iodine may be decolorised by Hyposulphite of Soda.

Action And Uses - 1. Immediate Local Action And Uses

Externally applied, iodine is a powerful irritant and vesicant, decomposing organic molecules, and entering into loose chemical combination with the albuminous constituents of the parts. At the same time it stains the epidermis of a deep brown. causes considerable pain; and is absorbed into the blood, partly by the skin and partly by the air of respiration in the form of vapour. It is also a powerful antiseptic and disinfectant.

The tincture, liniment, and ointment of iodine are extensively used as stimulants and disinfectants to foul, callous ulcers, much like nitrate of silver; as vegetable parasiticides in ringworm; and as counter-irritants in subacute or chronic inflammation of joints, periosteum, lymphatic glands, the pleura, and the lungs. In these instances the chief effect is doubtless stimulation, but a certain amount of the iodine is absorbed, and acts specifically, as will be presently described. Iodine in solution is injected into cysts, goitres, hydrocele, etc., with much success.

Iodide of potassium applied to the unbroken skin is neither irritant nor capable of being absorbed, unless decomposed by the sweat. It is readily taken up from the exposed mucous membranes. How much specific value can be attached to the iodide liniment is doubtful.

Internally, the local action of free iodine is also irritant, and it is successfully applied to the gums in periosteal toothache. Inhaled into the respiratory passages, it gives rise to cough, sneezing, severe pain over the frontal sinuses, distressing pains in the chest, and dyspnoea. Combinations of iodine with creasote and various soothing volatile substances, such as chloroform and ether, have lately come into repute as continuous inhalations in the so-called "antiseptic" treatment of phthisis, bronchitis, and other forms of chronic lung disease.

In the stomach and bowels, although it is gradually converted into the iodide or iodate of sodium, the irritant effects of free iodine are continued, with abdominal pain, sickness, and diarrhoea as the result. The iodides of potassium and sodium have rarely this effect, and it is only in the form of a salt that iodine is now administered internally. Iodide of potassium is also decomposed in the stomach, the sodium salt and albuminate being formed from it.

2. Action On The Blood

Iodine is freely absorbed into the blood from mucous surfaces, and the sodium iodide quickly enters from the alimentary canal. In the blood the element is at first combined with sodium; but this salt appears to be decomposed, the iodine for a time set free, some of the red corpuscles broken down (if the amount of iodine be large), and bloody effusions and bloody urine make their appearance. Such results are to be carefully avoided in practice; and, as far as we know, less degrees of the same cannot be usefully applied to therapeutical purposes, unless the tendency to coagulation of the blood be somewhat increased by it.

3. Specific Action And Uses

The iodide of sodium and albuminous compounds pass from the blood into the tissues with remarkable rapidity, and may be found in all of them, especially the excreting organs and lymphatic glands, whilst they appear very scantily in the nervous centres. Almost as quickly the iodine leaves the tissues; and in thus passing rapidly through the protoplasm of the body, and sharing in its metabolism by combining (probably very loosely) with the albuminous molecules, it no doubt accelerates tissue change. As no increase of urea accompanies this effect, nor bodily wasting, the iodine must either spare the liver (which is the chief source of urea), or accelerate the metabolism of the plasma, rather than of the tissue elements themselves. (See Metabolism, Part III.) However this may be, the following are the principal directions in which iodine affects nutrition, and their applications:

(1) The lymphatic glands are reduced in size by iodine, which is extensively used for scrofulous and other chronic enlargements of the glands, whether applied locally as iodine, or internally as the iodides.

(2) Certain poisons, which have intimately associated themselves with the albuminous structures, are disengaged from this combination by iodine. Lead and mercury may be swept out of the tissues by iodide of potassium administered for plumbism and hydrargyrism respectively. The principal application, however, of iodine is in the treatment of syphilis. Either the poison of this disease is thus eliminated from the system, or iodine hastens the life and disappearance of the small-celled growth by which syphilis is characterised. It is specially valuable in the tertiary forms of syphilis, when mercury cannot be longer given with advantage; and nodes and other superficial enlargements, gummata in the viscera, and certain forms of skin disease may be very successfully treated by the potassium salt. The same precautions must be observed with respect to the general health, and especially the preservation of digestion in a course of iodide, as were laid down under the head of mercury.