§ 2. Potassium and its Compounds.

POTASSIUM.-Symbol, K. (Kalium). Atomic Weight, 39; Quantivalence, I., III., and V.; Specific Gravity, 0.86.

326. History and Occurrence. This metal was discovered by Sir Humphry Davy in 1807. He first obtained it by subjecting moistened potash to the action of a powerful voltaic battery; the positive pole gave off oxygen, and metallic globules of pure potassium appeared at the negative pole. It is found abundantly in Nature, but never uncombined. It is obtained by the action of charcoal upon potassic carbonate at a very high temperature. The potassic carbonate is decomposed with liberation of potassium, carbonic monoxide, and small quantities of other compounds but little known. Leaving these out of view, the reaction may be represented by the equation:

FIG. 189.

K,CO, +2C3CO + K ̧.

327. Metallic Potassium (Molecule) (K-K), at common temperatures, is a silver-white metal, and so soft that it may be moulded like wax. If thrown upon the surface of water, instant decomposition takes place (Fig. 139), potassic oxide being at first formed. The liberated hydrogen, together with a small quantity of volatilized metal, is ignited by the heat evolved during the decomposition, and burns with a beautiful lilac flame as the globule floats about on the surface of the liquid. At the close of this reaction a second change ensues: the potassic oxide, which had been kept above the surface of the water, coming in contact with the liquid, gives rise to the formation of potassic hydrate, which becomes red-hot, and scatters with a violent explosion. Potassium decomposes nearly all compounds

Combustion of Potassium.

containing oxygen, if brought in contact with them at high temperatures, and many even at ordinary temperatures. Hence, to preserve it pure, it is kept in naphtha, a liquid containing no oxygen.

328. Potassic Monoxide (K,O).-This compound is obtained by exposing metallic potassium to perfectly dry air, free from carbonic dioxide, at ordinary temperatures. It is a white solid, which melts at a red heat, and volatilizes at higher temperatures. It is very deliquescent; brought in contact with water it becomes incandescent, potassic hydrate being produced.

329. Potassic Hydrate (KHO) (Caustic Potash).-The method of obtaining this important substance is similar to that of obtaining caustic soda. The solution of potassic hydrate, after being boiled until the temperature of the liquid is near a red heat, flows without ebullition, and may. be run into moulds, solidifying on cooling to a white, hard, brittle substance, which melts, below redness, to an oily liquid, and volatilizes at a full red heat in white, pungent vapors. Potassic hydrate dissolves freely in water, with great evolution of heat. It has a peculiar nauseous odor, and acrid taste. It decomposes acids with formation of corresponding potassic salts and elimination of water, changes vegetable yellows to brown, restores the blues discharged by acids, and decomposes animal and vegetable substances, whether living or dead. It is used in medicine to cauterize and cleanse ulcers and foul sores; hence its name, caustic potash. If a solution of potash be shaken in a bottle with any fixed oil, the two unite, forming a soap. This accounts for the soft, greasy feel it has when touched by the fingers, as it decomposes the skin and forms a soap with its oily elements. When taken into the system, potash acts as a powerfully corrosive poison. Its active chemical character renders it an indispensable reagent in the laboratory.

330. Potassic Chloride (KCl) is known as the mineral sylvite, and is isomorphous with NaCl. Potassic iodide

(KI) may be formed by adding iodine to a solution of potash, and gently warming until the solution assumes a brown tint. It is a very soluble, white solid, which crystallizes in cubes, and is much used in medicine.

331. Potassic Carbonate (K,CO,).—Potassic salts of various kinds exist in the juices of plants. By the combustion of the plants, most of these are decomposed with the formation of potassic carbonate, which may be obtained from their ashes. This is a highly alkaline, deliquescent salt, and is used largely in the manufacture of soap and glass, in preparing caustic potash, etc. It is also an important reagent in the laboratory, and a most valuable fertilizer. This salt rarely forms less than 20 per cent., and sometimes more than 50 per cent., of the weight of wood-ashes. The ashes of different plants, and even different parts of the same plant, yield it in varying quantities. Wood ashes furnish the principal source of the potassic carbonate of commerce, from which it is obtained by leaching them and boiling the solution to dryness in iron pots. The residue is called potashes, and these, when calcined, afford the impure carbonate known as pearlash. Potash, or pearlash, therefore represents the readily soluble portion of wood-ashes, and consists chiefly of potassic carbonate with small amounts of sodic carbonate and common salt.

332. Hydro-Potassic Carbonate (KHCO,). This is formed by passing carbonic dioxide through a strong solution of potassic carbonate. It is employed as a source of potassium in the formation of many of its other compounds, and is also used for making effervescing draughts, by adding citric or tartaric acid to its solution.

333. Potassic Nitrate (KNO,) (Nitre, Saltpetre).-This salt occurs as a native product in the earth of various districts in the East Indies, and is separated therefrom by leaching the soil, and allowing the nitre to crystallize. It is artificially formed by heaping up organic matter with lime, ashes, and soil, and keeping the mass well moistened

with urine for a period of two or three years, when the heap is lixiviated and the salt crystallized out. Besides these sources, nitre occurs in the sap of certain plants, such as the sunflower, tobacco-plant, etc. Nitre dissolves in about three times its weight of cold and one-third its weight of boiling water. When thrown upon burning charcoal it is decomposed and deflagrates violently. Paper dipped in a solution of sodic nitrate, and dried, forms what is known as touch-paper. When ignited, it burns slowly and steadily until consumed; hence its use in lighting trains of gunpowder, fireworks, etc. Nitre has a cooling, saline taste and strong antiseptic powers. Owing to the latter quality it is used extensively in packing meat, to which it imparts a ruddy color. It is chiefly consumed, however, in the manufacture of gunpowder.

334. Gunpowder is an intimate mechanical mixture of about 1 part nitre, 1 part sulphur, and 3 parts charcoal. These proportions, however, vary somewhat in different countries, as well as in different sorts of powder. More charcoal adds to its power, but also causes it to attract moisture from the air, which of course injures its quality. For blasting rocks, where a sustained force, rather than an instantaneous one, is required, the powder contains more sulphur, and is even then often mixed with sawdust to retard the explosion. The nitre, sulphur, and charcoal, having been ground and sifted separately, are thoroughly mixed and then made into a thick paste with water. This is ground for some hours under edge stones, after which it is subjected to immense pressure between gun-metal plates, forming what is known as press-cake. These cakes are then submitted to the action of toothed rollers, whereby the granulation of the powder is effected. The grains thus formed are sorted by means of a series of sieves, and thoroughly dried at a steam heat. The last operation, that of polishing, is accomplished in revolving barrels, after which the powder is ready for market.

The

heavier the powder, the greater is its explosive power. Good powder should resist pressure between the fingers, giving no dust when rubbed, and have a slightly glossy aspect. The explosive power of gunpowder is due to a sudden formation of a large volume of nitrogen and carbonic dioxide; one volume of the powder giving about 1,800 volumes of vapor. Fireworks contain nitre as a chief ingredient, mixed with charcoal, sulphur, ground gunpowder, and various coloring substances.

335. Potassic Sulphate (K,SO,) is obtained in the manufacture of hydric nitrate, and is of limited use in the arts. Potassic chlorate (KCIO;) may be formed by passing chlorine gas through a solution of potassic carbonate (K,CO,). Potassic chlorate is soluble in water, has a taste resembling that of nitre, melts at about 700°, and, if heated above that temperature, decomposes with formation of potassic chloride and perchlorate, and oxygen gas. It is used in the manufacture of lucifer matches, in certain operations of calico-printing, and as a source of oxygen.

336. Sodic and Potassic Silicates.-If 8 or 10 parts of sodic or potassic carbonate are mixed with 12 or 15 parts of sand and 1 of charcoal, on being heated they melt, and form a mass resembling ordinary glass; but it entirely dissolves in hot water. This is known as soluble glass, and when applied to wood and other substances answers the protective purpose of a varnish or paint.

337. Soap. When caustic potash, or soda, acts upon certain organic acid radicles, chiefly oleine and stearine, which are present in fats and oils, the resulting salts are termed soaps, and the process by which they are produced is called saponification. The consistence of soap depends chiefly upon its alkali. Hard soaps are made of soda, or a mixture of soda and potash, while in soft soaps potash alone is used, the soaps made with this base being deliquescent and consequently attracting water, which renders the soap liquid. The quality of hardness is due to the con