POTASSIUM. 31S ing the resulting mass, and crystallizing. Tins salt crystallizes in white cubes that have a sharp taste, and is used chietly in pliotography and in medicine. Potassium nitrate, which is found na- tive as nitre, or saltpetre, is described under Salti'Ktke. Potassium silicate is prepared by heating potassium carbonate -nith white sand in a reverberatory furnace, usually with a small amount of charcoal, by means of which a compound is obtained that is put on the market as a thick solution, and is used as a substitute for sodium silicate in the manufacture of soaps and in fresco painting. ( See Water-Glass. ) Potassium sulphalr is found in large quantities at Stassfurt, principally as kainite, which is a potassium and magnesium sulphate with nuig- ncsium cliloridc. This mineral, when allowed to remain for some time exposed to the air, de- liquesces, and as soon as the soluble magnesium chloride has run ofl' the remaining salt is decom- posed by boiling water, so that on cooling the sulphate crystallizes out. It is also obtained by the action of sulphuric acid on a potassium chlo- ride. It is a crystalline salt with a bitter saline taste, and finds use in medicine as a purgative, while large quantities are used for the manufac- ture of potash-alum and potassium carbonate. It W'as formerly called sal pohjrhnst. The acid sulphate, or bisulphate, the sal cnixuin of the older chemists, may be formed by heating jiotas- sium sulphate with sulphuric acid, or by heating potassium nitrate with sulphuric acid. It is a white crystalline compound, occasionally used as a flux. Other potassium salts ma.v be found described under the names of the acids contained in them. Consult : Pfeitl'er, "Die Stassf urter Kali-Industrie." in vol. ii. of Bolley's Handbucli drr chcmischcn Tcclnwlo(/ir (Brunswick, 1887); Lunge, A Theoretical and Practical Treatise on the Manufacture of fiulphuric Acid and Alkali (London, 1891); id.. The Alkali-Maker's Hand- book (ib., 18S11). Consult also the dictionaries and industrial works recommended in the article Chemistry. POTASSIUM CYANIDE. See HTDROcyA^-IC Acid. POTATO (from Sp. -patata, white potato, ialata, sweet potato, from Haytian batata, sweet potato). An iinp<irtant cultivated plant, raised in temperate climates for its esculeat tubers. It is often called Irish potato becau.se of its general cultivation and use in Ireland. It belongs to the order Solanacefe. or nightshade family, which also includes tobacco, belladonna, tomato, egg- plant, and pepper. The potato {Solanum tuber- osum) is a native of the mountainous districts of tropical and subtropical America, from Chile to ^Mexico, a form of it even occurring as far north as southern Colorado. It is difficult, how- ever, to determine where it is really indigenous and where it has been introduced by man. Like maize, it was cultivated and its tubers used for food before the discovery of America. It seems to have been first brought to Europe from Peru hy the Spaniards early in the sixteenth cen- tury, and to have spread from Spain into Hol- land, Burgundy, and Italy, but only to be culti- vated in a few gardens as a curiosity. In nearly all European countries it was called batata, by which name sweet potato is designated by Eng- lish writers down to the middle of the seven- teenth century. The data concerning its intro- POTATO. duetion into Europe are not very definite. It appears to have been brought to Ireland from Virginia by Hawkins, a slave trader, in 1565, and to England by Sir Francis Drake in 1585. Sir Walter Ralegh is said to have taken some tubers to England in 15SG and brought them to the at- tention of Queen Elizabeth. It was not until a long time after its introduction that the cul- ture of the potato became general. Gerard in his Herball. published in 1597, described it under the name of Batata Virginiana, but so little were its merits appreciated that it was not even men- tioned in The Complete Gardener, a work pub- lished in 1719. At first it was regarded chietly as a food for swine and cattle, but later on it was thought that on account of its great yield it might be useful as food for poor people and for the prevention of famine due to failures of the grain crops. The Royal Society of London in 1063 adopted measures for the extension of its culture in accordance with this idea. Its culti- vation first became most general in Ireland, but it was not until about the middle of the eighteentij century that it acquired any real importance on the Continent of Europe, and not until the end of that century did it become important as a field crop in Germany and France, which are to-day two of the greatest potato-producing countries of the world. In France the extension of potato culture was mainly due to the efl'orts of Parmen- tier, a prominent agriculturist and economist. Historical data concerning the development of the potato as a crop in Xorth America are even nmre meagre. In 1771 only a white and a red variety were mentioned in the most important English work on gardening, while to-day at some of the experiment stations in Europe and America tests are made of 150 to 200 varieties at one time. Apart from the abnormal development of the tubers and the very much reduced production of seed, it is believed that there have not been great changes in the potato plant since its cultivation became general. The potato is a perennial plant with smooth herbaceous stems from one to three feet high, pinnate leaves, and wlrite or purple fiowers about one inch wide, and producing a globular, purplish fruit or seed-ball of the size of a gooseberry. See Plate of Vegetables. The tubers are distinct from the roots, being underground stems of consiiler- able size even when the plant is in its native state; under the influence of cultivation they have become enlarged through the accumulation of starch for the use of the plants grown from the eyes, or buds. Owing to its wide distribution, from the cooler tropics to the cooler temperate zones, it is grown on a great variety of soils, but the soil best suited to the crop is a rich, sandy loam, well supplied with organic matter and well drained. It responds to liberal manuring, but since a direct application often injures the quality of the tubers, barnyard manure is preferably ap- plied the previous year: otherwise complete com- mercial fertilizers, containing nitrogen, phos- phoric acid, and potash, are applied. The land should be plowed as deeply as possible without turning up the subsoil. The tubers are generally planted in drills wide enough apart to admit of cultivation with the horse hoe. or culti- vator, and from 12 to 10 inches apart in the row. The pieces of the tuber used for planting, called sets, cuttings, or seed pieces, are covered about 4 inches deep. Planting is generally done
