POTASSIUM (from upot-ashes,)) wood ashes being one of the best known sources of the carbonate of the metal), a metallic element, many of whose compounds have been known from antiquity, but which was first prepared in the metallic form by Davy, in 1807, by the electrolysis of fused potassium hydrate—a sub stance which had previously been supposed to be itself elementary. In 1808 Gay-Lussac and Thenard showed that the hydrate can be re duced by heating it with finely divided iron and in the same year Curaudan showed that the reduction can also be effected by carbon. Sev eral methods of preparing metallic potassium are now in use. The best known of these and the only one that was in practical service for many years consists in the direct reduction of the carbonate of potassium by carbon, the re action being carried out in an iron retort heated to redness. The chief products of the distilla tion are carbon monoxide gas and metallic potas sium, the latter being volatile at a red heat. The production of potassium by this method is attended by more or less danger, not only because the vapor of potassium takes fire spon taneously upon contact with air or with water, but also because under certain conditions potas sium and carbon monoxide combine to form a black, explosive compound, whose production has caused a number of fatal accidents. The pot assium vapor should be permitted to condense before it issues from the neck of the retort, a cooling device being provided for effecting its liquefaction, and the molten metal should then be allowed to drip into a receiver contain ing petroleum oil or some other similar fluid that is free from oxygen. In Castner's process,
the carbonate or (hydrate) of potassium is mixed with carbon and finely divided iron, or with carbon and a metallic carbide and the re duction is effected by the agency of heat, as before. A considerable part of the commercial supply of metallic potassium is now produced, however, by the electrolysis of the melted hydrate or chloride of potassium.
Potassium is a lustrous, silvery-white metal, brittle at 32° F., but soft and wax-like at ordi nary temperatures, so that it can be easily cut with a knife. It melts at about 144° F., and boils at about 1,250° F. (when heated in an at mosphere of hydrogen), its vapor being green. It has a specific gravity of about 0.87 and, with the exception of lithium, is the lightest metal known. Its specific heat is 0.166 and its coeffi cient of linear expansion (Fahrenheit scale) is about 0.0000467. It conducts electricity much better than mercury does and it is soluble in anhydrous liquid ammonia (NH,), forming a blue solution from which metallic potassium is again deposited upon the evaporation of the ammonia.
Chemically, potassium has the symbol K (from kalium, Latin for potash), and the atomic weight 39.15. It oxidizes in the air with great readiness, a freshly cut surface becoming almost instantly covered with a film of oxide. It also decomposes water vigorously, uniting with it to form potassium hydrate, KOH, in accordance with the equation