Page:The American Cyclopædia (1879) Volume XIII.djvu/783

 POTASSIUM 759 with a cold water drip. If any obstruction forms, it must be immediately removed by thrusting an iron rod through the receiver and into the tube b. Failing in this, the fire should be immediately withdrawn. When the opera- tion is completed, the receiver containing the potassium is detached and plunged into a ves- sel of rectified Persian naphtha, having a cover and kept cool by immersion in water. When cooled, the receiver may be taken apart, and the potassium detached and preserved under naphtha. When required absolutely pure, 1 it must be redistilled in an iron retort from which the air has been expelled by the intro- duction of naphtha vapor. The purified metal usually amounts to about two thirds of the cruder material first taken from the receiver. Potassium is a brilliant bluish white metal, hav- ing a specific gravity of 0*865, being the light- est of all the metals except lithium. Its sym- bol is K (kalium), and its atomic weight 39'1. It is monatomic, belonging to the group which includes the other alkali metals, caesium, rubi- dinum, lithium, and sodium, together with sil- ver. At 32 F. it is brittle, having a crystal- line fracture, and at a little higher temperature it becomes malleable. At 59 it may be easily moulded ; at a few degrees higher it becomes pasty, and at 144'5 it is liquid. When soft its clean surfaces may be welded together like iron, and at a red heat it distils as a beauti- ful green vapor. If exposed to the air at or- dinary temperatures, it rapidly becomes cov- ered with a film of oxide ; and when raised to the point of volatilization it bursts into a bright violet flame. In contact with water, upon which it floats, it combines with its oxygen so powerfully as to produce sufficient heat to inflame the liberated hydrogen. The melted globule spins around upon the surface of the water, emitting a violet flame caused by the admixture of a small quantity of volatilized potassium with the hydrogen gas. A film of vapor is formed between it and the water, and the decomposition of this vapor supplies the oxygen to the burning potassium. When the melted globule of potassic hydrate formed by the combustion becomes sufficiently cooled to come into contact with the water, it is scat- tered with an explosive burst of steam. Po- tassium decomposes nearly all the gases con- taining oxygen when heated in contact with them, and at a high temperature will remove that element from almost all bodies contain- ing it. At a heat a little below redness it ab- sorbs hydrogen and is converted into a gray- ish hydride, from which a higher heat expels the hydrogen. It forms alloys with most oth- er metals, the combination being generally ef- fected by fusion. Antimonide, arsenide, and bismuthide may also be formed by heating the metals with cream of tartar. Potassium is very widely diffused in the mineral, vegeta- ble, and animal kingdoms. It occurs as^a silicate in several minerals, particularly in feldspar (orthoclase) and mica, and is there- fore an important constituent of most grani- j tic rocks. As a sulphate it is combined with i sulphate of alumina in alum stone; as chlo- ! ride, bromide, and iodide, in sea water and ! in tropical countries. It enters largely into the composition of the tissues and juices of land plants, especially of the grape, the apple, and other fruits, and of esculent vegetables, particularly the potato. The ashes of most forest trees abound with it. In marine plants, and in several land plants growing near the sea, the potassium is replaced by sodium ; and in animals, although the tissues and fluids con- tain more or less potassium salts, the alkaline constituent is chiefly sodium, mostly in the form of chloride or common salt. There are three well defined oxides of potassium: a basic or dipotassic oxide (the potash of the chemist, K 2 O), which furnishes the salts of the alkali, a deutoxide (K 2 O 2 ), and a peroxide (K 3 4 ). The two latter do not form cor- responding salts with acids. The dipotassic oxide is formed by allowing thin slices of me- tallic potassium to become oxidized in air per- fectly free from moisture or carbonic acid, or by heating potassium with an equivalent quan- tity of hydrate (hydroxide), KHO + K=K 2 O + H. When moistened it combines with wa- ter, with incandescence, being reconverted into hydrate. Potassic hydrate, hydroxide of potassium, or caustic potash, known in com- merce as potash, is described in the preceding article. When metallic potassium is gently heated in a current of dry air, it absorbs more oxygen than at ordinary temperatures, and the peroxide is formed; and the deutoxide is formed at a certain stage of the process. Salts. The salts of potassium are numer- ous and important. 1. Chloride of potassium, KOI (digestive salt, sal febrifugum Sylmi), may be formed by the spontaneous combustion of potassium in chlorine gas ; also by passing chlorine over red-hot hydrate or iodide of potassium, by gently heating potassium in hy- drochloric acid gas, by dissolving hydrate or carbonate of potash in aqueous hydrochloric acid, and in other ways. It occurs native, sometimes pure, as in sylvine, which is found in cubic crystals about the fumaroles of Ve- suvius and in thin layers in the salt beds of Stassfurt near Magdeburg; but it is more fre- quently found mixed or in combination with other chlorides, in mineral waters, in kelp, and in mineral deposits. At Stassfurt it occurs not only as sylvine, but also as carnallite, a hy- drated double chloride of magnesium and po- tassium (KCl,Mg01 2 + 6H 2 0), named from its pink color, in a bed of clay lying immediately above a bed of rock salt ; and it is also found in the rock salt formation of Maman in Persia. Its position with regard to the rock salt is ex- actly that which would result from the grad- ual "drying up of an inland sea, the common salt, from being less soluble, crystallizing out first. Chloride of potassium crystallizes in
 * salt deposits; and as nitrate, in various soils