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NIPIGON 469 NITRIFICATION Rochelle and 109 S. W. of Tours. An important railway junction, it has an old castle, a city hall (1530), a fine public garden, and the 16th-century church of Notre Dame, with a spire 246 feet high. The dressing of leather and the manu- facture of gloves are the leading indus- tries. Niort in the 14th century was held for 18 years by the English. It was the birthplace of Madame de Maintenon. Pop. about 23,000. NIPIGON, a lake in the province of Ontario, Canada ; 30 miles N. W. of Lake Superior, with which it is connected by the Nipigon river; is about 70 miles long, but its deeply indented coast line measures 580 miles; greatest depth 540 feet. The lake is studded ^vith hundreds of islands. NIPISSING, a lake in the province of Ontario, Canada; N. E. of Lake Huron, into which (Georgian Bay) it drains through French river (55 miles) ; is about 50 by 28 miles. NIRGUA, town in Yaracuy, Venezuela, 40 miles S. W. of Valencia. Region is fertile, coffee, sugar, tobacco, cocoa be- ing abundant. Considerable cattle breed- ing, and some mining. Pop. about 11,000. NISH, the chief town and commercial center of southern Serbia; in a vine- growing district; 130 miles S. E. of Bel- grade; is the seat of a Greek bishop; and has a fairly strong citadel (1737). The place played a conspicuous part in the Turkish wars from 1375 to 1878, when it was occupied by Serbia. Here, on Sept. 23, 1689, the Austrians defeated the Turks. At the outbreak of the World Wa.r the Serbian Government was tem- porarily transferred to Nish. It was captured by the forces of the Central Powers when they overran Serbia in the winter of 1915. See World Wak. Pop. about 25,000. NISHAPTTB, a town of Persia; capi- tal of the province of the same name; 63 miles W. of Meshed, in a beautiful and fertile valley; was the birthplace and contains the grave of Omar Khay- yam. Pop. about 15,000. NITER, or NITRE, an orthorhombic mineral occurring in crusts, silky fibers, acicular crystals, or diffused through cer- tain soils. Composition: Nitric acid, 53.4; potash, 46.6 = 100, corresponding to the formula, KNOa. In pharmacy it is exhibited in small doses as a refrigerant and diuretic, and in large doses as a vascular sedative in fever, especially in that of acute rheu- matism. It has been found useful also in dropsy. NITRATE, a salt of nitric acid. NITRATE OF SILVER, lunar caustic; sjmibol, AgNOs; prepared by dissolving silver in nitric acid. It crystallizes in colorless trimetric crystals, which dis- solve in one part of cold water, and melt at 198° C. It is extensively used in pho- tography and medicine. NITRATE OP SODIUM, Chile salt- peter; symbol NaNOa. Occurs abun- dantly as a natural product in Chile. Its chief use is as a fertilizer. NITRIC ACID, the most important of the five compounds formed by oxygen with nitrogen ; symbol HNO3. When pure it is a colorless liquid, very strong and disagreeable to the smell, and so acrid that it cannot be safely tasted without being much diluted. It is known in the arts as "aqua fortis," and is commonly obtained by distilling niter (potassium nitrate) or Chile saltpeter (sodium ni- trate) with strong sulphuric acid. Ni- tric acid contains about 76 per cent, of oxygen, a great part of which it readily gives up to other substances, acting thus as a powerful oxidizer. Thus many met- als — such as copper, tin, silver, etc. — when brought into contact with this acid are oxidized at the expense of the acid with the production of lower oxides of nitrogen and an oxygenated metallic salt. Nitric acid, when moderately dilute, acts on organic bodies so as to produce a series of most useful substances, notably acetic, oxalic, and pierid acids, isatin or white indigo. Nitric acid is employed in etching on steel or copper; as a solvent of tin to form with that metal a mordant for some of the finest dyes; in metal- lurgy and assaying; also in medicine, in a diluted state, as a tonic and as a sub- stitute for mercurial preparations in syphilis and affections of the liver; and also in form of vapor to destroy conta- gion. Nitric acid is one of the most impor- tant essentials in the manufacture of ex- plosives, and thus became of great im- portance during the World War. Its manufacture through artificial sources was developed in the various countries. The most important of these was the ex- traction of nitrate from the air. See Nitrate. NITRIDE, a compound of nitrogen with phosphorus, boron, silicon, and the metals, e. g., boron nitride, BN. NITRIFICATION, the changing of nitrogenous organic matter or ammonia compounds into nitrates under certain conditions. First, the temperature must be suitable, for at about 5' C. the process is stopped; but with a rise of