POTA'SSIUM ; POTASH. The remark able metal Potassium was discovered by Sir H. Davy in the year 1807. Its properties are the following :—In colour and lustre it strongly resembles mercury; it is solid at the usual temperature of the air : at 50° it is soft and malleable, and yields like wax to moderate pressure, and at 32° it becomes brittle; at 70° it is somewhat fluid, but not perfectly so till the temperature reaches 150°; if heated to low redness, out of the contact of the air, it sublimes, and condenses on cooling, un changed. Its texture when brittle is crystal line. Its specific gravity at 60° is only 0.865, thereby presenting a remarkable contrast to mercury. It is opaque, and a good conductor of heat and electricity.
The most remarkable chemical property of potassium is its great affinity for oxygen, which at common temperatures exceeds that of any other body for this elementary sub stance. It tarnishes rapidly and visibly by mere exposure to the air, and the more rapidly as the air contains moisture, which it decom poses, and by combining with its oxygen be comes oxide of potassium, or potash. On account of its powerful affinity for oxygen, it must be kept either in small glass tubes her metically sealed, or in a fluid, such as naphtha, which contains no oxygen. )Vhen heated in the air it takes fire, burns with a purple flame, the evolution of much heat, and is converted either into potash or peroxide of potash, or a mixture of them. It takes the oxygen from any mixture of gases containing it, and indeed from almost every substance with which that element happens to be combined ; so that when thrown upon water it decomposes it, burning vividly.
Oxygen and Potassium form two compounds, of which the chief is the well known substance Potash. This is white, solid, extremely caustic, and readily attracts moisture and carbonic acid from the air ; it fuses when heated, but bears a high temperature without being vola tilised or decomposed. The hydrate of potash is formed whenever an aqueous solution of potash is evaporated to dryness ; it is white, hard, and brittle, extremely caustic, very de liquescent, and very soluble both in water and in alcohol. Potash is most extensively em ployed in manufactures, and is an extremely valuable alkali. From a combination of cyan ogen with potassium Prussian Blue is obtained.
Nitrate of Potash, Nitre, or Saltpetre, is colour less, inodorous, has a cooling, sharp saline taste, and is readily soluble both in cold and in hot water. At about 616° of Fahr. nitre fuses, and at a high temperature it suffers decomposition. Nitre possesses powerful anti septic properties. It is largely employed in the manufacture of gunpowder and of nitric acid, and also in numerous processes in the chemical arts and manufactures. Chlorate of Potash is colourless, inodorous, and has a cooling austere taste. When triturated with certain inflammable bodies, as sulphur and phosphorus, combustion or explosion, or both, are produced. This salt is employed in chemi cal investigations on account of the purity of the oxygen gas which it yields; it is also employed in the manufacture of matches which aro flied by means of friction. [11.1ercuis.] Carbonate of Potash is known in its impure state by the name of Pearl Ash; when pure, it is colourless and inodorous, its taste is strong and disagreeable, it does not readily crystallise, and is never kept in crystals ; it is largely employed in the arts, and also in medicine. Bicarbonate of Potash is inodorcius, colourless, and crystalline; has scarcely any alkaline taste, and is largely employed in medicine and in chemical investigations. Sul phate of Potash is colourless, inodorous, bitter, and rather hard ; it is employed to a small extent in medicine and manufactures. Bi tartrate of Potash, Cream of Tartar, or Tartar, is colourless, rather hard, inodorous, and has a sour taste. It is very largely employed in the preparation of tartaric acid, in medicine, and some chemical arts. When an equivalent of potash is added to this salt, it becomes neutral tartrate of potash ; this salt is used in medicine, and being much more soluble in water than the bitartrate, was formerly called Soluble Tartar. Oxalic Acid forms three different compounds with potash, the oxalate, quadroxakete, and binoxalate ; this last is a natural product obtained from sorrel, and is commonly known by the name of salt ofsorrel. It is a colourless crystalline salt, and has a sour bitterish taste. There are many other valuable substances obtained from potassium.