POTASSIUM [symbol K (from kalium), atomic number 19, atomic weight 39.105], a metallic chemical element, belonging to the group termed the metals of the alkalis. Although never found free in nature, in combination the metal is abundantly and widely distributed. In the oceans alone there are estimated to be i,141 X tons of sulphate, K2SO4, but this inexhaustible store is not much drawn upon; and the "salt gardens" on the coast of France lost their industrial importance as potash-producers when the deposits at Stassfurt in Germany came to be worked. These deposits, in addition to common salt, include the follow ing minerals: sylvine, KC1; carnallite, (trans parent, deliquescent crystals, often red with diffused oxide of iron) ; kainite, (hard crystalline masses, permanent in the air) ; kieserite MgSO4.H20 (only very slowly dissolved by water) ; besides polyhalite, and anhydrite, CaSO4; salt, NaCl, and some minor components. These potassium minerals are not confined to Stassfurt ; larger quantities of sylvine and kainite are met with in the salt mines of Kalusz in the eastern Carpathian Mountains. Important de posits are being worked in Thuringia and Baden, and others occur at Wittelsheim (Alsace) and at Suria (Catalonia, Spain). There are also undeveloped resources in Chile, Peru and Brazil. The Stassfurt minerals owe their industrial importance to their solu bility in water and consequent ready amenability to chemical operations. In point of absolute mass they are insignificant com pared with the abundance and variety of potassiferous silicates, which occur everywhere in the earth's crust; orthoclase (potash felspar) and potash mica may be quoted as prominent examples. Such potassiferous silicates are found in almost all rocks, both as normal and as accessory components; and their disintegration fur nishes the soluble potassium salts which are found in all fertile soils. These salts are sucked up by the roots of plants, and by tak ing part in the process of nutrition are partly converted into oxalate, tartrate, and other organic salts, which, when the plants are burned, are converted into the carbonate, The "vinasse" of beet-sugar factories, i.e., the material left in the re torts after the distillation of the fermented molasses, also consists largely of potassium salts (chiefly the carbonate) and the potash is either utilized directly as a manure or in the manufacture of soft soaps, or it is refined by fractional crystallization. It is a remark able fact that, although in a given soil the soda-content may pre dominate largely over the potash salts, the plants growing in the soil take up the latter: in the ashes of most land plants the potash (calculated as forms upwards of 90% of the total alkali.
The proposition holds, in its general sense, for sea plants like wise. In ocean water the ratio of soda to potash is oo :3.23*(Dittmar) ; in kelp it is, on the average, 100:5.26 (Richardson). Ashes particularly rich in potash are those of burning nettles, wormwood (Artemisia absinthium), tansy (Tan acetum vulgare), fumitory (Fumaria officinalis) and tobacco. According to Liebig, potassium is the essential alkali of the animal body; and it may be noted that sheep excrete most of the potassium which they take from the land as sweat, one-third of the weight of raw merino consisting of potassium compounds.
Sir Humphry Davy's Experiment.—To Sir Humphry Davy belongs the merit of isolating this element from potash, which itself had previously been considered an element. On placing a piece of potash on a platinum plate, connected to the negative of a powerful electric battery, and bringing a platinum wire, connected to the positive of the battery, to the surface of the potassium a vivid action was observed : gas was evolved at the upper surface of the fused globule of potash, whilst at the lower surface, adjacent to the platinum plate, minute metallic globules were formed, some of which immediately inflamed, whilst others merely tarnished. In 1808 Gay-Lussac and Thenard obtained the metal by passing melted potash down a clay tube contain ing iron turnings or wire heated to whiteness, and Caradau effected the same decomposition with charcoal at a white heat. Electrolytic methods are now generally employed for the manufac ture of potassium. The Castner process used for sodium (q.v.) is somewhat unsatisfactory for potassium, but has been rendered more suitable by various modifications. Fused potassium hydrox ide is electrolysed by means of a sheet-iron anode and an iron wire cathode, the latter being surrounded by a cylinder of magne site in order to prevent metallic potassium diffusing into the hydroxide in which it tends to dissolve. The temperature is kept as low as possible and air is excluded. The molten hydrox ide may be replaced by a readily fusible mixture of potassium chloride and fluoride. The metal, however, is not in great demand, for it is generally found that sodium (q.v.), which is cheaper, and, weight for weight, more reactive, will fulfil any purpose for which potassium may be desired.