The unpleasant symptoms which have sometimes followed the rather free use of iodide of potassium may be explained in two ways. in the first place, it sometimes contains injurious impurities, particularly the iodate of potassa, which, in large doses, possesses poisonous properties. it is, therefore, highly important that, when freely used, its purity should be well ascertained beforehand. Secondly, in a mode already explained, the iodide is supposed to have the effect of liberating the compounds of mercury and lead which may have become in some degree fixed in the system, and restoring them to the circulation in a soluble and active form; so that the characteristic effects of these metals may apparently result from the direct action of the medicine administered. The inference from this view is, that iodide of potassium should be given in small doses, and carefully watched, whenever there is reason to suppose that any of the compounds of mercury or lead may exist in the tissues of the body.

Another important point in regard to the salt, according to M. Mel-sens, is that, though it may be given successively with chlorate of potassa with impunity, yet the simultaneous exhibition of these two salts is hazardous. When they are mixed in solution out of the body, no chemical change takes place; but, if heated together in the dry state to the melting point, they are decomposed, with the formation of iodate of potassa. Now it is supposed that a similar reaction may take place in the system when the two salts are administered simultaneously; and poisonous results may be experienced. To a dog iodide of potassium was given daily for a month without any unpleasant effect. Afterwards the chlorate of potassa was similarly given with the same result. The two were then exhibited conjointly in the same quantities; when the animal soon began to languish, and died about the twenty-fifth day; and the same experiments were repeatedly performed, with the same issue. (See Am. Journ. of Pharm., Nov. 1866, p. 522-3.)

Therapeutic Application

This is the preparation now most commonly used, when the effects of iodine on the system are desired. it is probably capable of producing whatever desirable systemic effects can be obtained from any other preparation of that substance, so far as the iodine itself is concerned. I have myself long been in the habit of using it almost exclusively. The diseases, and special conditions of disease in which it may be expected to do good, have already been sufficiently detailed.*

The dose is for an adult from five to twenty grains three times a day. I have almost invariably found five grains to be well borne by the stomach; and have generally been able to procure all the expected therapeutic effects from this dose, which I therefore seldom exceed. Sometimes, however, a much larger quantity may be found necessary; and a case which has advanced favourably under the dose mentioned for some time, and then ceased to make progress, may again improve with an increase of the dose. Under these circumstances, it is occasionally useful to add to the iodide a little of the compound solution or tincture of iodine, instead of augmenting the dose of the salt itself. in cases of great susceptibility of the system, or unusual delicacy of stomach, the dose may be reduced. it should be given in a wineglassful of water, sweetened water, or some demulcent liquid; and, if increased, the proportion of liquid should be increased accordingly. To cover the taste, and render it more acceptable to the stomach, it may be given in carbonic acid water, sweetened with syrup of lemon or ginger. The quantities which are said to have sometimes been taken with entire impunity are enormous, amounting even to an ounce in twenty-four hours.* iodide of potassium may also be used externally, in the form of lotion, cataplasm, ointment, or bath, and in this way is capable of producing the effects of iodine on the system, and probably not less decidedly than the iodine itself, employed in the same way. it has here also the advantage, over uncombined iodine, in reference to the constitutional impression, that it is less irritant, and may, therefore, be used largely with less unpleasant local effects.

* The following conclusions of Dr. Rosenthal, of Vienna, founded on observations in the hospital of that city, are practically interesting.

1. iodide of potassium, taken in large doses with little fluid, remains long in the system, but if liquids are given copiously at the same time, it is quickly washed away with the secretions.

2. Taken internally, it is discoverable not only in the saliva, urine, and other secretions, but in from 4 to 7 hours may also be detected in the stools, whether solid or liquid.

3. When iodide of iron or of mercury is swallowed, a partial separation of the iodine takes place; a considerable proportion of it being found in the urine, and a little in the alvine evacuations.

4. Rubbed on the skin in the form of ointment, iodide of potassium may be detected in the saliva and urine.

5. Administered by means of a bath, the iodide is found in the urine; while, in the bath, the quantity is considerably diminished.

6. injected into the lower bowels in weak solution, it is rapidly taken up by the mucous membrane.

7. Large doses, or small doses long continued, are not well borne in certain morbid states of the system; and large quantities in concentrated solution are hurtful under all circumstances. (See Boston Med. and Surg. Journ., Nov. 1863, p. 287.) - Note to the third edition.

The Ointment of iodide of Potassium (Unguentum Potassii iodidi, U.S., Br.) is prepared by dissolving a drachm of iodide of potassium in a fluidrachm of boiling water, and mixing the solution with a troyounce of lard.

In this preparation, the salt has the advantage of being in solution, so that it probably acts better than if the powder merely were mixed with the lard. it is preferable to the ointment of iodine from this circumstance, if nothing else, that it does not discolour the skin.