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ELEMENTS OF MATERIA MEDICA.

two salts, an iodate and an iodide, but by supersaturating with acid thecolour is restored.

The blue compound of iodine and starch is usually designated theiodide of starch, but Raspail, ( Nouveau Systeme Chimie Organique)objects to the term, as grains of starch consist of two parts—an externalenvelope, within which is a soluble gummy substance. Now the iodine,says Raspail, attaches itself to the envelope only, giving it a blue colour,just as it gives a yellow colour to other organic textures.

Iodine forms a blue compound with narceine (see Opium). The mine-ral acids (sulphuric, nitric, and hydrochloric) have the same effect onnarceine.

When iodine is in combination with oxygen, starch will not recognizeit. For example, if a little starch be added to a solution of iodic acid,no change of colour is observed; but if some deoxidating substance benow employed (such as sulphurous acid or morphia) the blue colour isimmediately produced. The iodates give out oxygen when heated, andare converted into iodides. They deflagrate when thrown on red hotcoals. The soluble iodates produce with a solution of the nitrate ofsilver, a white precipitate of iodate of silver soluble in ammonia If theiodine be combined with a base (as with potassium, or sodium), chlorineor sulphuric or nitric acid must be employed to remove the latter; and theiodine being then set free, will react on the starch. This is the mode ofproceeding to detect iodine in the urine of a patient; for the mere additionof starch will not suffice. Nitric or sulphuric acid or chlorine must be em-ployed to remove the base with which the iodine is combined.

The soluble iodides produce, with a solution of nitrate of silver, a yel-lowish precipitate (iodide of silver) very slightly soluble only in ammonia.They precipitate the salts of lead yellow, and bichloride of mercuryscarlet.

Purity. —We are told that iodine is much adulterated, but I doubt it.There are two properties which will, in most instances, determine its pu-rity—namely, its solubility in alcohol, and, when heated, its conversioninto violet vapour, leaving no residuum. Coal, plumbago, peroxide ofmanganese, sand, and charcoal (all of which, it is said, have been foundin iodine), would be in this way readily distinguished. But Dr. O’Shaugh-nessy ( Translation of Lugol's Essays, p. 210) states that he met with onespecimen so artfully adulterated, that the foreign ingredients were at thesame time soluble in alcohol and volatilizible by heat. A little imposi-tion may be, and indeed is, practised by some dealers in iodine, by sellingit in a moist state. An ounce, if very moist, may contain one drachm, orperhaps even a drachm and a half, of water. The easiest way of detectingthis is by compression between folds of blotting paper.

Physiological Effects. — (a.) On vegetables. Cantu slates that seedsplaced in pure sand and moistened with a solution of iodine, germinatemore readily than seeds sown in the usual way. Vogel, however, assertsthat iodine, so far from promoting, actually checks or stops germination.( Decandolle , Physiolog . VIgit. tom. 3 me . p. 1337).

(b). On animals generally. —Hitherto no examination has been madeof the effects of iodine on the different classes of animals, for, with theexception of man, the only animals on which experiments have hithertobeen made with it, are horses, dogs, and rabbits. On these it operatesas an irritant and caustic poison, though not of a very energetic kind.