Page:The American Cyclopædia (1879) Volume XII.djvu/476

 462 NITRATES takes place slowly, the crystals are almost all rhombohedrons ; but if these are touched with a prismatic crystal, the solution becomes tur- bid with formation of prismatic crystals. These prismatic crystals may be again converted into rhombohedrons by heating to near the fusing point. Nitrate of potash dissolves in 3 parts of water at 64 F., and in one third its weight of boiling water. It is insoluble in absolute alcohol, is not subject to deliquesce, has a sharp biting taste, and is without action on vegeta- ble colors. It fuses without decomposition at 674*4, and when cast into moulds solidifies to a white fibrous radiated mass known as sal prunelle. At a red heat it decomposes with formation of nitrite and evolution of oxygen, and at a higher heat the nitrite is also decom- posed, with evolution of oxygen and nitro- gen and formation of potassic oxide and per- oxide. In large crystals it is apt to contain water mechanically held, which retains in solu- tion some of the foreign salts derived from the mother liquor, as sulphate of soda and the chlorides of potassium and sodium. Small- er crystals are commonly purer ; but common salt is almost always present to some extent, and is particularly injurious to saltpetre de- signed for the manufacture of gunpowder. A mixture of saltpetre with carbonaceous mat- ters is explosive when highly heated; and when this mixture is intimately made, the pro- duct is gunpowder; sulphur, which is com- monly introduced, not being essential for this property. (See GUNPOWDER.) Saltpetre is ob- tained both as a natural and artificial product. It is generated wherever nitrogenous animal and vegetable matters undergo decomposition in the presence of moist calcareous earth con- taining potash, the temperature being generally above 60 F. Ammonia is produced, which is decomposed, its nitrogen forming with oxygen nitric acid, which combines with the alkaline earths present. Nitre forms naturally upon the walls of cellars and of caves as an efflores- cence. On the surface of some soils in warm countries it also appears in this form after the rainy season, and in sufficient quantity to ren- der its collection profitable. In Hindostan it is thus produced so abundantly and cheaply, that our own market is largely supplied from Cal- cutta. To separate the nitre from the earth which contains it, this is lixiviated with water, which dissolves out the soluble salts ; and in the large vats into which the liquid is conveyed the salts crystallize as the water is evaporated by solar or artificial heat. The first crystals that form are crude saltpetre ; nitrates of lime and magnesia mostly remain in the mother liquor, and are either thrown away with it, or in some cases are decomposed by treatment with car- bonate of potash, and their nitric acid is thus recovered in new quantities of nitre. Natural saltpetre beds are also worked in Hungary, Egypt, Spain, and in various warm countries. In hot countries it does not appear that the im- mediate presence of decomposing animal mat- ters is essential to the production of saltpetre ; but ammonia thus derived and existing in the atmosphere is no doubt brought to the potash. In temperate climates the salt is artificially pro- duced in what are called in Europe saltpetre plantations. These are compost heaps of ani- mal and vegetable matters intermixed with earth, and with potash, lime, and magnesia, presented in porous form, as in ashes, marl, chalk, and old mortar. The heaps are exposed to the air, but it is better to protect them from the rain. Gutters are excavated around them, and in these are kept liquids from the cattle stalls and other similar fluids rich in nitrogen, with which the materials are occasionally moist- ened. In Sweden, the heaps are worked over once a week in summer and once a month in winter, and twigs are introduced to keep them open. The work is generally continued three years, until the product of saltpetre amounts to about 5 oz. in 1,000 cubic inches. For an an- nual product of 10 cwt. it is necessary to work over full 120 cubic fathoms of earth, of which one third becomes ripe each year, and is re- moved from time to time from the outermost layers to be lixiviated. The crude product ob- tained is afterward purified by repeated solu- tions and crystallizations. Saltpetre is now extensively manufactured by the double de- composition of the nitrate of soda from Chili and the chloride of potassium from the salt mines of Stassfurt, Germany. Besides its use for making gunpowder, nitre is employed in the manufacture of nitric acid. It is also a useful oxidizing flux in metallurgical opera- tions, and in medicine is much used for its cooling properties in inflammatory affections, and also as a promoter of perspiration and the secretions of the liver. In acute rheumatism it is sometimes administered in doses, largely diluted with water, to the extent of from one to two ounces, though half an ounce in con- centrated solution causes heat and pain in the stomach, which may be followed by convul- sions and death. When taken in poisonous quantities there is no antidote known, and the only relief is by the use of the stomach pump, laudanum to allay the pain, and mucilaginous drinks and cordials. Nitre is also a powerful antiseptic, and is used in the preservation of meats, as for curing hams. 2. Nitrate of Soda.. Another variety of nitre, called cubic nitre, is the salt nitrate of soda or sodic nitrate (Na NO 3 ). It crystallizes in obtuse rhombohedrons of specific gravity 2*26. It is deliquescent, sol- uble in about twice its weight of cold water, and has a cooling saline taste. It fuses at 591, and is decomposed at a higher temperature. It is found in beds among the hills in the province of Tarapaca which skirt the coast of Peru, and at their base on the W. side of the pampa over an extent of not less than 150 m. Under the nitrate of soda is marl impregnated with saline matter and mixed with fragments of shells. The nitrate of soda, as quarried, is very variable in quality, some yielding not