A metallic substance, the base of potash: it was discovered by Sir H. Davy, in 1807. It was prepared by causing hydrate of potash, slightly moistened for the purpose of increasing its conducting power, to communicate with the opposite poles of a galvanic battery of 200 double plates; when the oxygen, both of the water and the potash, passed over to the positive pole, while the hydrogen and the potassium appeared at the negative. In this way only small quantities can be procured; but it may be formed more abundantly by themethod of Guy Lussac and Thenard. This consists in bringing fused hydrate of potash in contact with turnings of iron heated to whiteness in a gun-barrel. The iron deprives the water and potash of oxygen; hydrogen gas, combined with a little potassium, is evolved, and pure potassium sublimes, and may be collected in a cool part of the apparatus. Potassium may also be prepared by mixing dry carbonate of potash with half its weight of powdered charcoal, and exposing the mixture in an iron bottle to a strong heat: these methods have been improved by M. Brunner, who decomposes potash by means of iron and charcoal.

From eight ounces of fused carbonate of potash, six ounces of iron filings, and two ounces of charcoal, mixed intimately, and heated in an iron bottle, he obtained 140 grains of potassium. If required to be quite pure, it must be re-distilled in a green glass retort. Potassium is solid at the ordinary temperature of the atmosphere; at 70° it is somewhat fluid, though its fluidity is imperfect till heated to 150°; at 50° it is soft and malleable, and yields like wax to the pressure of the fingers, but it becomes brittle when cooled to 32°; it sublimes at a low red heat, without undergoing any change, provided atmospheric air be completely excluded. Its texture is crystalline, as may be seen by breaking it when cold. In colour and lustre it is precisely similar to mercury. At 60° its specific gravity is 0.865, so that it is considerably lighter than water. It is completely opaque, and is a good conductor of heat and electricity. As this metal oxidizes rapidly in the air, or in fluids containing oxygen, it must be preserved either in glass tubes, hermetically sealed, or under the surface of liquids, like naphtha, which contain no oxygen.

If heated in the open air it takes fire, and burns with a purple flame; it decomposes water instantly, and so much heat is disengaged that the potassium is inflamed, and burns vividly while swimming on the surface: the hydrogen unites with a little potassium at the moment of separation, and this compound takes fire, and augments the brilliancy of the combustion. Under water, a violent action ensues, without the emission of light, and pure hydrogen is evolved: it is also inflamed when placed upon ice, burning a little hole, which becomes filled with solution of potash. Besides uniting with oxygen, to form the protoxide and peroxide of potassium, it combines with chlorine, iodine, hydrogen, sulphur, and phosphorus. When potassium is placed in an atmosphere of chlorine, it spontaneously takes fire, and burns with greater brilliancy than in oxygen; the result is the chloride of potassium, which is also produced when chlorate of potash is decomposed by heat. Iodide of potassium is formed with emission of light when potassium is heated in contact with iodine. Hydrogen and potassium unite in two proportions, forming, in one case, a solid, and in the other a gaseous compound.

The solid hydruret was made by heating potassium in hydrogen gas: it is a grey solid substance, easily decomposed by heat, or contact with water. The gaseous compound is formed when hydrate of potash is decomposed by iron, at a white heat, and it appears also to be generated when potassium burns on the surface of water. Sulphur unites readily by means of heat, and the compound sulphuret of potassium becomes incandescent at the moment of union. In like manner, phosphorus combines with potassium, forming phosphuret of potassium.