Salts of potassium (silicates and chlorides mainly) exist in several rocks, and in moat soils and mineral waters. The metal is prepared from the carbonate by reduction with carbon at a white heat. Acid potassium tartrate (crude tartar) is ignited in a covered iron crucible, and the porous charred mass is rapidly cooled by dipping the crucible into cold water, and then transferred to a retort. This latter was until lately represented by a wrought-iron mercury-bottle, but malleable-iron tubes covered with a coating of fire-clay are now substituted. The arrangement of the retort and furnace is shown in Fig. 158. At a white heat, the reduction takes place, a mired vapour of metallic potassium and carbon monoxide being generated. At a very high temperature these vapours combine to form a very explosive black substance, represented by the formula K2C202, rendering the process one of great danger. The risk of explosions may, however, be avoided by employing the- condenser introduced by Mareska and Donny,and shown in Fig. 159, which effects very rapid cooling of the vapours before the explosive combination can take place.
This condenser consists of a tray a and cover 6 of cast iron, which can be clamped together as in c, so as to form a shallow box almost 1/4 in. deep, 10 to 12 in. long, and 4 to 5 in. wide; the socket at d fits in the short tube emerging from the end of the retort or bottle, while the other end e is left open to give free passage to the vapours. When these hitter commence to appear at the exit tube of the retort, the condenser is attached, and the metal begins to condense into a liquid state, and flow into a vessel containing petroleum, placed below to receive it; any deposit obstructing the exit tube must be promptly removed by inserting a red-hot rod.
On the small scale, metallic potassium can be prepared by electrolysing the cyanide or the chloride. In the former case, the salt is melted and allowed to coot so as to form a solid crust, when a current from 3 or 4 Bunsen cells is passed through by means of gas-retort carbon poles. In the second case, a mixture of potassium and calcium chlorides, in equal molecular proportions, is fused in a small porcelain crucible over a lamp, with carbon poles from 8 or 8 Bunsen couples dipping into the fused salt; the lamp-name is adjusted so that the portion of salt around the negative pole may solidify, while that around the positive pole remains fluid, thus allowing the free escape of the chlorine; the current is passed for about 20 minutes, when the crucible is allowed to cool, and is opened under petroleum, affording a deposit of pure potassium around the negative pole.
The metal possesses some singular physical and chemical properties. White brittle and of crystalline fracture at 32° F. (0° C), it softens like wax at 59 1/2° F. (15° C), and can easily be cut by a knife, the clean surfaces being readily welded again; it melts at 144 1/2° F. (62 1/2° C), and boils at a red heat; it has a silvery lustrous appearance, and a sp. gr. of 0.875 at 55 1/2° F. (13° C); it dissolves in liquid ammonia, and is re-precipitated unchanged on evaporation of the solvent; in ordinary atmospheres, its surface is converted into caustic potash and carbonate, oxidation often proceeding so rapidly when the metal is in thin shreds that ignition ensues, and the metal burns, but in quite dry pure air it undergoes no change; in electro-positiveness it ranks next to caesium and rubidium; it decomposes water so energetically that sufficient heat is developed to ignite the hydrogen evolved; and its powerful reducing properties give it a use in chemical operations, where, however, it is generally replaced by its cheaper ally sodium. ___