iodine itself, in the solid state, is now seldom if ever prescribed, and never ought to be. Some acquaintance, however, with its physical properties and chemical relations is essential to the physician. For the mode of its preparation, I must be content with referring to the U. S. Dispensatory, as also for the methods of detecting its presence, and the signs of purity of its various preparations.

iodine is an elementary body. it is a heavy, opaque solid, of a bluish-black colour, with a lustre approaching the metallic, and not unlike plumbago in appearance. it has a peculiar odour, analogous to that of chlorine, and an acrid taste. Though nearly insoluble in water, it is capable of imparting a slight flavour to that liquid; but a watery solution of iodide of potassium dissolves it very freely. it is readily dissolved by alcohol and ether; and all its solutions have, when not feeble, a deep yellowish-brown colour, approaching to black. At ordinary temperatures, it is very slowly volatilized; but, with the application of heat, it readily rises in vapours, of a beautiful purple or violet colour, which has given origin to its name. its chemical relations are so numerous, and affinities so strong, that it is scarcely possible that it should remain long in the stomach in its pure state, at least when used in medicinal doses.

The probabilities are that, when swallowed, or when brought in any other way into contact with the fluids of the body, it undergoes changes, through the reagency of the alkaline salts which it encounters, by which a portion of it is converted into iodide of potassium or sodium, and possibly into iodates of the alkalies of these metals respectively. Through the influence of the alkaline iodides thus formed, the iodine itself becomes soluble in the liquids, and thus enabled to exercise its own immediate influence. This results in the coagulation of the albumen present, supposing it to have been deprived of its alkaline matter in the previous changes; so that it would seem impossible for iodine to enter the blood in its uncombined state. For, so long as there is alkaline matter present, it converts the iodine into soluble iodides or iodates; and, should it all have been consumed, and a portion of iodine remain, the albumen coagulates. Hence it follows that, admitted into the circulation, iodine must either undergo the change referred to, or check the circulation by solidifying the albuminous constituent of the blood.

But, as before stated, iodine is highly irritant, and sometimes even corrosive in its uncombined state; and may exercise this property in the stomach, if not carefully guarded. The inference from all this is that, for internal use, when the object is not to produce an irritant impression on the mucous membrane, but to affect the system, and, on the same principle, when the medicine is applied externally with an exclusive view to absorption, the soluble iodides or iodates should be used preferably to iodine itself; for, whenever the latter is employed, it is admitted into the circulation only in the saline state referred to, while the inconvenience of its local irritant effect is experienced. All its good effects, therefore, are obtained by the direct use of the salts, and the inconvenience avoided.

But there is another reason for not employing uncombined iodine. I have stated that, with the alkaline matter of the blood, it forms both iodides and iodates. Now, the iodates, by the facility with which they part with oxygen to substances having an affinity for it, are liable to be decomposed, thus rendering the iodine free again to exercise its irritant influence. it is better, therefore, to use one of the iodides, by which this inconvenience is avoided, while all the effects desired are obtained. The iodide of potassium is the one almost universally preferred. To M. Mialhe is due the credit of having developed these important practical views. {Chim. Appliq. ci la Physiol., Paris, 1856, pp. 221-4.) Should iodine ever be given internally in the solid form, the dose must not exceed one-quarter or half a grain at once.

In treating of the individual preparations of iodine, I shall consider first those in which it exists wholly or in part in the free state, and afterwards its compounds.