POTASSIUM (K). This metal was discovered in 1807 by Sir Humphry Davy. Most of its compounds had been known for years prior to that date, and that the basis of them was a metal, that they were in fact compounds of a metallic oxide, had long been suspected. In that year, however, Davy succeeded in isolating the metal by sub mitting fused hydrate of potash to the action of a powerful voltaic battery. Potassium has since been obtained by more facile methods, and the following is the one now generally adopted.
Crude bitartrate of potash (argol) is heated in a covered iron pot till gases cease to be evolved. The product—a mixture of carbonate of potash and finely divided carbon—is triturated in a mortar, while still hot, with some coarsely powdered charcoal, and the porous mass promptly transferred to an iron mercury bottle, or other wrought-iron retort. A neck is formed to the bottle or retort by simply screwing into the mouth a piece of iron tube about six inches long, and the whole is then so Imbedded in a furnace• as that the open end shall project about a quarter of an inch through an opening in that side of the furnace that is purposely formed of plate iron. The fire is now lighted and fed with a mixture of coke and charcoal ; an intense white heat is thus produced, and vapours of potassium soon appear at the mouth of the apparatus, and burn with a brilliant purple flame. A receiver is now fitted on, the construction of which needs some care; it may be so formed that the potassium shall drop into naphtha the moment it solidifies ; or it may merely be an iron box five inches wide, twelve long, and a quarter of an inch deep, foi-med by clamping together two very shallow wrought-iron dishes made of plate-iron one sixth of an inch in thickness. In either case an opening must be made exactly opposite the neck of the bottle, in order to allow of the latter being cleared by a stout steel wire in the event of its becoming stopped up by condensed potassium, &c.; such an opening also allows of the escape of the carbonic oxide gas that is abundantly evolved during the process. The receiver must be kept cool by ice, or other convenient means, and, in the case of the flat box, must, at the close of the operation, be plunged under the surface of rectified Persian naphtha, as otherwise the potassium would quickly become oxidised. The
metal thus obtained is not pure, and must be redistilled in a similarly arranged apparatus.
The manufacture of potassium is a somewhat dangerous operation, owing to the frequent formation of explosive compounds. The apparatus in which it has been performed should be immersed in water as soon as cold, for a black detonating compound soon forms by exposure of the residual potassium to the air, and explodes by the slightest friction.
The decomposition that takes place during the manufacture of potassium is primarily a simple case of deoxidation, as expressed in the following equation :— Unfortunately, however, the liberated potassium has a great ten dency to unite with carbonic oxide, and form a very explosive com pound of a grayish colour ; it is termed croroweeeets matter, and when dissolved In water yields croconate and rAodisosatc of potash.
Potaarium has so Intense an affinity for other elements that it does not occur in nature in the free state. In combination, however, it is frequently met with. Nearly all land plants contain it, derived mainly from disintegration of feldspar—their ashes consequently yield much carbonate of potash ; sea-water also contains a small quantity of potash salts, and nitrate of potash occurs native in large quantities as an efflorescence from the soil In warm climates. (Porassiey, in NAT. Hiss. Div.] Potassium is a bluish-white metal of sp. g. 0'8135. At 32' Falls. it is brittle, and has a crystalline fracture; at ordinary temperatures It is quite soft, and at 130' Fehr. it is liquid. A freshly cut sur face possesses high metallic lustre, but it almost instantly become covered with a coating of white rust. It may be preserved in the bright state by melting it in one end of a glass tube, from which the air has been expelled by a current of hydrogen, or coal gas, and then allowing It to flow down to the other end of the tube, passing in its course through a small piece of wire gauze ; the dross is thus filtered away and the metal obtained in a brilliant lustrous condition. On fusing off, by the aid of a blow-pipe, that part of the tube con taining the gauze, kc., then melting the potassium and allowing it to cool very slowly, and when partly solidified rapidly pouring off the liquid portion, the solid potasaiuni is obtained crystallised in quadratic octahedra.