Saltpetre may be made to act as a nitrite by dissolving it in water in the strength of about fifty grains to the ounce, soaking blotting-paper in the solution and letting the paper dry. Pieces about 2 in. square are then successively put into a jar and lighted. The patient inhales the fumes, which contain a considerable pro portion of nitrogen oxides. This treatment is frequently very suc cessful indeed in relaxing the bronchial spasm upon which the most obvious features of an attack depend.
2. Chile Saltpetre, Cubic Nitre or sodium nitrate, occurs under the same conditions as ordinary saltpetre in deposits covering immense areas in South America, which are known locally as caliche or terra salitrosa, and abound especially in the provinces of Tarapaca and Antofagasta in Chile, the fields being confined to a narrow strip of country 21 miles in width and 26o miles in length. The nitrate forms beds, varying in thick ness from 6 in. to 12 ft., under a covering of conglomerate locally known as lostra, which is itself overlaid by a loose sandy soil. The conglomerate consists of rock fragments, sodium chloride and various sulphates, cemented together by gypsum to form a hard compact mass 6 to io ft. in thickness. The caliche has often a granular structure, and is yellowish-white, bright lemon-yellow, brownish or violet in colour. It contains from 48 to 75% of sodium nitrate and from 20 to 40% of common salt, which are associated with various minor saline components, including sodium iodate and more or less insoluble mineral, and also some organic matter, e.g., guano, which suggests the idea that the nitrate was formed by the nitrification of this kind of excremental matter. The caliche is worked up in loco for crude nitrate by extracting the salts with hot water, allowing the suspended earth to settle, and then transferring the clarified liquor, first to a cistern where it deposits part of its sodium chloride at a high temperature, and then to another where, on cooling, it yields a crop of crystals of purified nitrate. The nitre thus refined is exported chiefly from
Valparaiso, whence the name of "Chile Saltpetre." The mother liquors used to be thrown away, but are now utilized for the extraction of their iodine (q.v.).
Chemically pure sodium nitrate can be obtained by repeated recrystallization of Chile saltpetre or by synthesis. It forms colourless, transparent rhombohedra, like those of Iceland spar; the crystals are almost cubic : hence the name of "cubic salt petre." One hundred parts of water at o° and at oo° dissolve 72.9 and 18o parts of the salt ; at 120°, the boiling-point of the saturated solution, 216 parts. The salt fuses at 316° ; at higher temperatures it loses oxygen (more readily than the corresponding potassium salt) with the formation of nitrite, which, at very high temperatures, is reduced ultimately to a mixture of peroxide, and •oxide, The chief applications of Chile salt petre are in the nitric acid industry, in the manufacture of ordinary saltpetre, and particularly as a fertilizer. When quite pure, it is only very slightly hygroscopic.
3. Wall-saltpetre or Lime Saltpetre, calcium nitrate, Ca(NO3)2, is found as an efflorescence on the walls of stables ; it is now manufactured in large quantities from atmospheric nitro gen. (See NITROGEN, FIXATION OF.) Its chief applications are as a manure and in the nitric acid industry.