Characters. - A crystalline powder of vermilion colour, becoming yellow from an alteration in its crystalline form when gently heated over a lamp on a sheet of paper, and again becoming red when placed on a sheet of paper and rubbed with a smooth substance.

Solubility. - It is almost insoluble in water, dissolves sparingly in alcohol, but freely in ether, or in an aqueous solution of iodide of potassium.

Reactions. - When digested with solution of soda it assumes a reddish-brown colour (mercuric oxide); and the fluid, cleared by filtration and mixed with solution of starch, gives a blue precipitate on being acidulated with nitric acid (iodide). Entirely volatilised by a heat under redness.

Preparation. - By mixing solutions of corrosive sublimate with potassium iodide in the proper proportions.

Dose. - 1/16 to 1/4 grain.

Preparations. B.P.

Unguentum Hydrargyri Iodidi Rubri

(Ointment of Red Iodide of Mercury) ..........

1 part in 28.

With yellow wax and almond oil.

B. And U.S.P.

Dose.

Liquor Arsenici et Hydrargyri Iodidi ...............................................

10-30 min.

Uses. - It may be used for the same purposes as the green iodide, but, like all the mercuric salts, it is much more powerful than the corresponding mercurous one. It is a powerful local irritant, and is used in the form of ointment in cases of goitre. The mode of employing it is to rub the ointment upon the tumour, and afterwards to expose the patient either to the heat of the sun or of a fire as long as he can bear it. This treatment was first used in India. In this country, where the sun's rays are not so powerful, the heat of a fire may be employed, and I have found it efficacious when used in this way. Red iodide ointment is useful in obstinate skin-diseases, especially lupus erythematosus.

It is frequently given in syphilis, one of the most common ways of prescribing it being to give one-half to one drachm of the solution of the perchloride with several grains of potassium iodide. The per iodide is thus formed, and is dissolved in excess of the potassium iodide.