"Take of iodine, one part; sulphuret of iron rubbed into coarse powder, five parts; sulphuric acid, seven parts; distilled water, forty-eight parts; solution of carbonate of potassa, a sufficient quantity; rectified spirit, six parts. Mix by rubbing the iodine with sixteen parts of water, and put the mixture into a glass vessel. The sulphuret being put into the matrass, pour over it the acid previously diluted with thirty-two parts of water; and a tube being adapted to the neck of the matrass, and dipping to the bottom of the vessel containing the iodine and water, pass the gas through the mixture until the iodine disappear. Evaporate the filtered solution with a high temperature, and again filter. Then add, gradually, the solution of carbonate of potassa to saturate the acid, which is known by the effervescence ceasing. Let the mixture be set aside in a warm place until a dry residuary salt of a white colour be obtained; on this pour the spirit, and dissolve with heat. Finally, pour off the liquor from the salt, evaporate to dryness, and preserve the residue in a close vessel."

This salt, correctly speaking, is the iodide of potassium, and only becomes a hydriodate when it is dissolved in water. In the above process the hydrogen of the sulphureted hydrogen unites with the iodine and forms hydriodic acid, which, when saturated with the potassa of the carbonate, forms a hydriodate. When the subsequent steps of the process are followed, the hydrogen is extricated by uniting with the oxygen of the base, and the iodine remains in combination with potassium, forming an iodide.

Qualities. - Iodide of potassium is obtained in white cubical crystals, inodorous, and having a penetrating, slightly bitter taste; it is deliquescent, and very soluble, 136 parts requiring only 100 of water at 60°; during solution, a portion of water is decomposed, and the iodide is changed into hydriodate of potassa. The iodide is a compound of 1 prop, of iodine = 126.3 + 1 of potassium=39.15, making the equivalent of the salt 165.45.

Medical properties and uses. - The iodide is seldom or never employed except to form the solution of the hydriodate, which is used for the same purposes as iodine; and also as the medium of holding the latter substance in solution. Of a solution consisting of thirty-six grains in a fluid ounce of water, the dose at first should not exceed x., but it may be gradually increased to lx., three times a day.

Officinal preparation. - Unguentum Potassae Hydriodatis, D.