POTASSIUM. 311 POTASSIUM. in reducing a potassium salt, such as the hydrate, by a mixture of carbon and a metallic carbide, or a mixture of very finely divided metal and car- bon, which is heated in an iron crucible with an exit tube passing through the lid. The potassium distills over into the receiver and at the end of the operation is placed imder petroleum so as to prevent oxidation. Potassium (symbol, K; atomic weight, 39.11) has, when freshly cut, a bright silvery metallic lustre and a specific gravity of 0.87.3 at 1.3° C, being lighter than any other metal except lithium. It is brittle at 0° C, and possesses a ci'ystalline fracture; at 15° C. it becomes soft like wax, and it melts at 62.5° C. (144.5° F.), forming a liquid that closely resembles mercury in appear- ance. With the exception of cesium and rubid- ium, it is the most electro-positive element kno^•Tl, and it acts as a powerful reducing agent. On exposure to the air it rapidly becomes con- verted into the hydrate, and finally into the car- bonate. It decomposes water with sufficient energy to ignite the liberated hydrogen. When brought into contact with the halogens and with sulphur, selenium, tellurium, and phosphorus, it unites with them. It also alloys with most metals, usually by being heated with them, and with sodium it forms an alloy of a low melting- point. For commercial purposes its use, especial- ly as a reducing agent, has been almost entirely superseded by sodium, owing to the cheapness of the latter. With oxygen potassium combines to form two oxides, a monoxide (K,0) and a per- oxide (KjO,). The former is formed when potas- sium hydrate is heated with metallic potassium ; it is a gray, brittle mass that has strong basic properties. 'hen dissolved in water this oxide forms potassium hydroxide (KOH), or caustic potash, which, however, is generally prepared com- mercially by decomposing a dilute solution of potassium carbonate with slaked lime. Potassium hydroxide is a hard, white, brittle, powerfully caustic substance, quickly destroying many ani- mal and vegetable substances, and finds extensive use in the manufacture of soap. It is the liquor potassw of the pharmacopteia, and is used as a caustic in surgical operations, and finds some application in analytical chemistry. Potassium peroxide is formed when metallic potassium is burned in the air. Potassium arsenate, called also 'Macquer's salt,' is prepared by fusing to- gether equal parts of potassium nitrate and arse- nious anhydride, then dissolving the resulting mass in water and evaporating. On cooling, the arsenate crystallizes out. It is used in calico printing for the purpose of fixing the mordant on the fibre of the material. Potassium bromide may be made by dissolving bromine in a solution of potassium hydroxide, evaporating the solution, and igniting the mixture, with the addition of a little char- coal, which reduces potassium bromate to bro- mide, and the mass is dissolved in water, filtered, and allowed to evaporate and crystallize. A more satisfactory method, however, consists in decomposing iron bromide with potassium car- bonate, and allowing the resulting liquid to crystallize. Potassium bromide crystallizes in the form of white cubes that have a strong saline taste, and find extensive application in photog- raphy and medicine. Potassium carbonate, which was known to the ancients and is described by Aristotle as being prepared by the burning of rushes, w-as long obtained by Ijurning plants in dry pits and dissolving tlie ashes in water, then evaporating till the sulphates, chlorides, etc., separated out by crj'stallization, and then boil- ing the mother liquor to dryness in iron pots, which was probably the origin of the name "pot- ashes.' The process subsequently introduced was similar to that used by Le Blanc for the manufac- ture of soda ash, and consisted in fusing potassium sulphate with calcium carbonate and coal. Another source is from the suint of sheep's wool. The raw wool on washing yields a mixture which is evaporated to dryness, and the solid residue is calcined in retorts. The resulting mass is dis- solved in water and allowed to cool, when the potassiurfi carbonate is deposited as a mass of crystals. This salt, which is a white solid with an alkaline and caustic taste, is used largely in the manufacture of soft soap, glass, potassium chromate, and potassium ferrocyanide. Potassium chloride, known commercially as muriate of pot- ash, is now largely obtained from the Stassfurt deposits, where it occurs native as sylvite, and in combination with magnesium chloride as car- nallite. The last-named is the principal source. The nuneral is dissolved in warm water, heated by steam to about 120° C. (248° F.), and then allowed to cool. Between 60° and 70° C. (140° to 158° F. ) the magnesium sulphate, calcium sul- phate, and sodium chloride separate, and on fur- ther cooling about 70 per cent, of the potassium chloride is obtained, the crystals of which are washed with a little cold water to remove any sodium or magnesium chloride, and a product is obtained containing about 05 per cent, of potas- sium chloride. Small quantities are also manufac- tured from the ashes of seaweed, which, previous to the working of the Stassfurt deposits, formed one of the principal sources of potassium salts. Potassium chloride is a white crystalline com- pound with a strong saline taste. It is used chiefly in the preparation of other potassium salts, such as the carbonate and the chlorate, and in an impure state it is employed as a fertilizer. Potassium bichromate is prepared by heating to- gether finely ground chromic iron ore with potas- sium carbonate and lime. The resulting mass is extracted with hot water and the calcium chro- mate precipitated out by means of potassium sul- phate, leaving in solution potassiiuu chromate, which is then converted into the bichromate by treatment with sulphuric acid. The resulting solution is evaporated to crystallization, yield- ing splendid garnet-red crystals. Potassium bi- chromate finds extensive use in the preparation of chromium compounds, in the manufacture of various colors, as an oxidizing agent, and in certain photo-engraving processes, owing to the fact that when mixed with gelatin it becomes insoluble when exposed to the light. This par- ticular property has also led to its employment in the manufacture of insoluble glue. Potassium cyanide may be prepared by melting potassium ferrocyanide with potassium carbonate in an iron crucible. It is a white crystalline compound, ex- ceedingly poisonous. It finds some use in photog- raphy, and as a reducing agent in chemical operations, especially in metallurgy. (See Hy- drocyanic Acid.) Potassium iodide may be prepared by dissolving iodine in potassium hy- droxide, in the same way as the bromide is made by the action of the hydroxide on bromine. It is also manufactured by decomposing ferrous iodide with potassium carbonate, evaporating, dissolv-
