Nitre, or SALTPETRE (Lat. sal petra, 'salt of the rock,' through Old French), is the nitrate of potash, . It usually occurs in long, colourless, striated, six-sided prisms; its taste is cooling, and very saline; it is soluble in seven times its weight of water at (.), and in less than one-third of its weight of boiling water, but is insoluble in alcohol. When heated to about (.) it fuses without decomposition into a thin liquid, which, when cast in moulds, solidifies into a white, fibrous, translucent mass, known as sal prunelle. At a higher temperature part of the oxygen is evolved, and nitrate of potash is formed. Owing to the facility with which nitre parts with its oxygen, it is much employed as an oxidising agent. Mixtures of nitre and carbon, or of nitre and sulphur, or of nitre, carbon, and sulphur, deflagrate on the application of heat with great energy; and if nitre be thrown on glowing coals it produces a brisk scintillation. Touch-paper is formed by dipping paper in a solution of nitre, and drying it.
Nitre occurs as a natural product in India and Persia, where it is found sometimes as an efflorescence upon the soil, and sometimes disseminated through its upper stratum. The crude salt is obtained by lixiviating the soil, and allowing the solution to crystallise. Much nitre used to be artificially formed in Europe by imitating the conditions under which it is naturally produced. Animal matter, mingled with ashes and lime rubbish, is placed in loosely aggregated heaps, exposed to the air, but sheltered from rain. The heaps are watered from time to time with urine or stable runnings; at suitable intervals the earth is lixiviated, and the salt crystallised. As there is always a considerable quantity of the nitrates of lime and magnesia present, which will not crystallise, carbonate of potash, in the shape of wood-ashes, is added so long as any precipitate occurs. The nitrate of lime is decomposed, and the insoluble carbonate of lime separated. The clear liquor is then evaporated and crystallised. But the bulk of the nitre of commerce is now made from the Chilian nitrate of soda by double decomposition. The common varieties of Indian nitre, which have a dirty yellowish appearance, are termed rough or crude saltpetre, while the purer kinds are called East India refined. The purification or refining of nitre is effected by dissolving it in water, boiling the solution, removing the scum, straining it while hot, and setting it aside to crystallise. The most common impurities are sulphate of potash, chlorides of sodium and potassium, and nitrate of lime. Chloride of barium will detect the first of these impurities, nitrate of silver the second, and oxalate of ammonia the third.
Nitre is employed in the manufacture of sulphuric acid, in the preparation of nitric acid, as an oxidising agent in numerous chemical processes, as an ingredient of fireworks, and especially in the manufacture of gunpowder. It is used in medicine. In moderate doses (from ten grains to a scruple) it acts as a refrigerant, diuretic, and diaphoretic, and hence its use is indicated when we wish to diminish abnormal heat, and to reduce the action of the pulse, as in febrile disorders and hemorrhages. In acute rheumatism it is given in large doses with great benefit. It is a popular remedy in sore throat, in the form of nitre balls, which should be retained in the mouth till it melts, and the saliva impregnated with it gently swallowed. The inhalation of the fumes produced by the ignition of touch-paper often gives speedy relief in cases of spasmodic asthma.
Cubic Nitre, or Nitrate of Soda, , occurs abundantly on the surface of the soil in Chili and Peru (especially Atacama and Tarapacá). It derives its name from its crystallising in cube-like rhombohedrons. In most of its properties it resembles ordinary nitre, but, in consequence of its greater deliquescence, it cannot be substituted for that salt in the preparation of gunpowder. Being considerably cheaper than the potash-salt, cubic nitre is often substituted for it in the manufacture of nitric and sulphuric acids; and it is largely used in agriculture (see MANURE). In 1882 the amount shipped from South American ports was stated at 476,000 tons; in 1889, at 930,000. See W. H. Russell's Visit to Chili and the Nitrate Fields of Tarapacá (1890). For the Sweet Spirits of Nitre, see NITROUS ETHER.