Page:The American Cyclopædia (1879) Volume XIII.djvu/784

 760 POTASSIUM cubes, rarely in octahedrons, composed in 100 parts of potassium 52-41 and chlorine 47*59. Its specific gravity is 1*994. It has the taste of common salt, decrepitates when heated, melts at a low red heat, and at a higher tem- perature volatilizes unchanged. It is spar- ingly soluble in alcohol. 2. Bromide of po- tassium, KBr. Potassium unites directly with bromine, with ignition and detonation. The salt may also be prepared by neutralizing hy- drobromic acid with potash, by decomposing bromide of iron with carbonate of potash, or by adding pure bromine to caustic potash till the liquid acquires a slight yellow color, a bro- mate being also formed, which may be decom- posed by a current of sulphuretted hydrogen gas. The excess of gas is expelled by a gentle heat, and the crystals of bromide obtained by evaporation. It crystallizes in brilliant cubes, which are sometimes elongated into prisms or flattened to plates, composed in 100 parts of po- tassium 32*83 and bromine 67*17 ; specific grav- ity 2*672. It has a sharp taste, decrepitates by heat, and melts without decomposition. It is very soluble in water, more so in hot than cold, and is slightly soluble in alcohol. At a red heat it is decomposed by chlorine. Bromide of potassium is an important article of the ma- teria medica. It has of late years been largely used as a remedy for wakefulness,- on account of its sedative action on the nervous system. In large doses, from 30 to 60 grains several times a day, it produces a peculiar intoxica- tion, with torpor, drowsiness, and slowness of the pulse. Its reputed antaphrodisiac proper- ties have been confirmed by Theilmann of St. Petersburg, Trousseau of Paris, Dr. Garrod, and others. It has been used with advantage by Sir Charles Locock in hysterical epilepsy and other nervous affections connected with uterine disorder. 3. Iodide of potassium, KI, may be prepared by several different methods, similar to those for preparing the bromide, but the best is that recommended by Liebig, which consists in decomposing iodide of barium by sulphate of potassium. The iodide of barium is prepared as follows: to 1 part of amor- phous phosphorus and 40 parts of warm water there is gradually added 20 parts of dry iodine, which is triturated with the phosphorus under the water. The resulting dark brown liquid is heated on a water bath till it becomes slight- ly alkaline. Insoluble phosphate and soluble iodide of barium are formed, and being sep- arated by filtration, the filtrate containing the iodide of barium is treated with sulphate of potassium. Double decomposition ensues, by which insoluble sulphate of baryta (heavy spar) and soluble iodide of potassium are formed. The iodide may then be obtained by filtration and evaporation. It crystallizes in cubes, which are translucent or milk-white and opaque, composed in 100 parts of potassium 23-54 and iodine 76-46. Sometimes it crystal- lizes in octahedrons. Its specific gravity is 3*056. It is not deliquescent in a moderately dry atmosphere, is very soluble in water, and in six parts of alcohol of specific gravity 0'83. It has a sharp, bitter taste, turns reddened lit- mus paper slightly blue, melts below a red heat, and volatilizes undecomposed at a moderate red heat. It is decomposed by chlorine with the aid of heat into chloride of potassium and iodine. An excess of chlorine causes the for- mation of a double salt of terchloride of iodine and chloride of potassium, which colors the liquid yellow and yields crystals by evapora- tion. Iodide of potassium is one of the most important agents of the materia medica. Its peculiar medicinal properties are principally conferred upon it by its iodine constituent. (See IODINE.) Its principal use is in reducing scrofulous and other tumors, in the treatment of skin diseases, of rheumatism, and of consti- tutional diseases, and in eliminating mineral poisons, particularly mercury and lead, from the system. Free iodine dissolves in a solu- tion of iodide of potassium, forming, in the proportion of one part of iodine to two of the salt, Lugol's solution. 5. Phosphide of potassium, KP, is formed when phosphorus and potassium are heated together in hydro- gen gas, the combination being attended with evolution of light and heat. It is decomposed by water, yielding hypophosphite of potassi- um, phosphuretted hydrogen gas, and solid phosphide of hydrogen. 6. Fluoride of potas- sium, KF, is formed by dissolving potash or its carbonate in hydrofluoric acid, evaporating, and heating strongly to expel the acid. It is deliquescent, soluble in water, and crystal- lizes in colorless cubes, often lengthened into prisms. It forms definite crystallizable com- pounds with many other fluorides. The fluo- ride of potassium and hydrogen, KHF 2, has been recommended by Prof. Wolcott Gibbs as a convenient reagent for decomposing refrac- tory silicates and other minerals. 7. Potassium unites with sulphur in five different propor- tions, forming five sulphides, K 2 S, KaSa, KaSa, K 2 S 4, and K 2 S 6. They all have an alkaline reaction to test paper, and an odor of sul- phuretted hydrogen. Liver of sulphur (hepar sulphuris), prepared by heating sulphur with carbonate of potash in a covered crucible, is composed of a trisulphide, a pentasulphide, and some of the intermediate sulphides, mixed with sulphate, and often with carbonate of potash. It is a local irritant, and is used in the form of the unguentum potassm sulphurates of the British pharmacopoeia. 8. There are two car- bonates of potassium, a normal or neutral car- bonate, or dipotassic carbonate, K 2 CO 3, and an acid salt, monopotassic carbonate, KIICOs, commonly called bicarbonate of potash. The normal carbonate is prepared as follows. Com- mercial pearlash, known also as crude carbon- ate of potash (see POTASH), which consists principally of carbonate, is agitated with an equal weight of cold water. The carbonate, being much more soluble than the other salts, is dissolved out. The solution is then boiled