518 NITROGEN the natives as caliche or terra salitrosa, which abounds especially in the district of Atacama and the Peruvian province of Tarapaca. The caliche there lies from 25 to 1 5 metres deep, and stretches over a distance of forty leagues ; it is covered by a layer, from a half to two metres thick, of a hard conglomerate of sand, felspar, phosphates, and other mineral matters, which is designated "lostra." In other places the caliche forms part of a sandy deposit which some times comes to the surface and never goes down to a depth beyond 2 6 metres. The caliche contains from 48 to 75 per cent, of nitrate of soda and from 20 to 40 per cent, of common salt, which are associated with various minor saline components, including iodate of soda, and more or less of insoluble mineral, and also some organic matter, guano amongst other things, which suggests the idea that the nitrate was formed by the nitrification of this kind of excre- mental matter. The caliche is worked up in loco for crude nitrate of soda ; 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 chloride of sodium 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 imported chiefly from Valparaiso, whence the name of "Chili saltpetre." The mother-liquors used until a few years ago to be thrown away, but are now being utilized for the extraction of their iodine, which, although little in a relative sense, on account of the large masses of raw material wrought, amounts to a good deal absolutely, as is illustrated by the fact that Peruvian iodine has put an end to the kelp industry in Scotland. Chemically pure nitrate of soda can be obtained by repeated recrystallization of Chili saltpetre or by synthesis from pure nitric acid and pure car bonate of soda. It forms colourless transparent rhombohedra, like those of Iceland spar, only the angles are more nearly equal to right angles, so that the crystals look like cubes. Hence the name of cubic saltpetre, which is sometimes given to the salt. One hundred parts of water at 0, 20, 50, 100, 110 C. dissolve 72 "9, 87 5, 112, 180, 200 parts of the salt; at 120, the boiling-point of the saturated solution, 216 parts. It fuses at 330 C. (Carnelley); at higher temperatures it loses oxygen (more readily than the corresponding potash salt) with formation of nitrite N0 2 Na, which, at very high temperatures, is reduced ultimately to a mixture of peroxide, Na 2 2 , and oxide, Na 2 0. Industrially the salt is important as being the raw material for the manufacture of nitric acid and of nitrate of potash. Nitrate of potash (saltpetre), which forms the predominating component of gunpowder, occurs native in India and other parts of the world, and such native nitre has only to be purified by crystallization to become fit for the market. But the bulk of what occurs in commerce is made by double decomposition of Chili saltpetre with (a) caustic potash, (b) carbonate of potash, or (c) chloride of potassium, which processes yield (a) caustic soda, (b) carbonate of soda, (c) common salt as bye-products. The third form (c) of the method is most largely wrought, the necessary supplies of cheap chloride of potassium being furnished by the works at the Stassfurt deposits. The two raw materials are analysed, and quantities corresponding to NaN0 3 = 85 of nitrate of soda and KC1 = 74 5 parts of chloride of potassium respectively are dissolved together in an iron basin in the least quantity of hot water. The solution is boiled down to 1 5 specific gravity (hot), when the common salt formed gradually crystallizes out. It is fished out, allowed to drain, and the runnings are returned to the basin. The highly concentrated mother-liquor is allowed to cool with frequent agitation, so that the saltpetre, which crystallizes and assumes the form of a "meal," is more readily freed from mother-liquor, by judicious washing with cold water. The crude saltpetre which is thus obtained is recrystallized until it is almost chemically pure, because an even slightly contaminated salt is unfit for gunpowder making. Nitrate of potash is isomorphous with the soda salt in this sense that it is possible to obtain it in the form of rhombohedra ; but these rhombohedra have no great stability, and under ordinary conditions the salt always assumes the form of long six-sided prisms of the right-rhombic system. It is far less soluble in water than nitrate of soda, and, unlike it, is absolutely non-hygroscopic. One hundred parts of water at 0, 10, 20, 50, 80, 100 C. dissolve 13 3, 21-1, 31-2, 86, 172, 247 parts of salt. The boiling fully saturated solution is at 114, and contains 327 parts of salt per 100 of water (Mulder). It fuses at 352 C. (Carnelley) into a colourless liquid, which freezes into a hard compact mass exhibiting a coarse, fibrous fracture if the salt is pure. In an impure salt this structure be comes the less distinct the greater the proportion of impurities ; with a saltpetre which contains 3 5 per cent, of common salt, the fibres appear only at the edges of the surface of fracture. This is the basis of a now almost forgotten test for purity, and explains that to the present day the term "refraction" is used to designate the sum total of impurities contained in 100 parts of nitre analysed. When heated sufficiently beyond its fusing point it decomposes similarly to nitrate of soda, only it gives up its oxygen far less readily. A mixture of nitre with charcoal, sulphur, or other com bustible matter, when kindled, burns off with explosive violence. Hence its application for the manufacture of gunpowder and in pyrotechnics, and its use in the laboratory as a powerful oxidizing agent in operations of the dry way. Nitric Acid, HN0 3 (see CHEMISTRY, p. 511 sq. ), is prepared from nitrate of potash or soda by distillation with sulphuric acid. The scientific chemist prefers the potash salt because it is more easily purified ; the manufacturer uses nitrate of soda because it is cheaper and a lower temperature and a less excess of oil of vitriol (over and above 1H 2 S0 4 for 2RNO H ) suffice for its successful con version into acid. For manufacturing purposes the distillation is effected from out of horizontal cylindrical or deep hemispherical retorts made of cast iron (a material which is far less attacked by the acids than one might be inclined to think). The retort com municates with a series of Woulf e s bottles made of stoneware, which are sometimes provided with taps for letting off the distillate. The retort, after having been charged and connected with the receivers, is heated over a naked fire until all the acid is driven off, and nothing but a (more or less acid) sulphate of soda is left. The details of the manufacture vary according to the kind of acid which is intended to be produced. If the manufacture of ordinary aqua fortis (of 1 3 to 1 - 4 specific gravity) is aimed at the receivers are charged with the proper proportion of water, and less than 1H S0 4 may be used for the decomposition of lN0 3 Na, because the per oxide of nitrogen from the dissociated part of the acid, by the action of the water and the air in the receivers, is to a great extent recon verted into nitric acid. The preparation of acid of highest strength demands the full equivalent of vitriol. But even then the product, apart from the contents of the first few condenser bottles nearest to the retort, is strongly contaminated with dissolved peroxide, which imparts to the acid a deep red colour. This impurity can be brought down to about 2 per cent, by blowing air through the gently heated acid, which carries away the peroxide as a vapour. To utilize the latter, the mixture is made to ascend through a tower filled with coke kept moist by a constant rain of water running downwards against the stream of acid fumes. Water alone would convert two-thirds of the nitrogen into nitric acid, thus : 3N 2 4 ( + aqua) = 2NO + (2ISr 2 5 + aqua); but by the cooperation of the oxygen of the air a larger proportion of acid is being regenerated as aquafortis of about l - 3 specific gravity. Two kinds of acid are chiefly being produced, viz., (1) full-strength fuming acid of 1 5 to 1 52 specific gravity at 15 C. , which is largely used (in the tar-colour industry for the making of nitro-bodies, and also for the manu facture of gun-cotton and nitroglycerin), and (2) aquafortis of 1 35 to 1 42 specific gravity, for the charging of batteries, and as a re agent generally. Either acid is sold in two qualities, as colour less acid relatively free of peroxide of nitrogen, and as red acid charged with it, which admixture is not always an impurity in a technical sense, but desired for certain purposes to be present. Most commercial acid is contaminated with chlorine and sulphuric acid, which can both be removed by fractional distillation most easily from the strongest acid. The chlorine accumulates in the first runnings ; the sulphuric acid remains in the retort, if the dis tillation be stopped in time. For most purposes the strength of an acid can be determined with sufficient accuracy by means of a hydrometer. The following table gives the relation between the percentages p of HN0 3 and the specific gravity S according to Kolb: P S at 0" C. S at 10 C.
S at 0" C. Sat 15C. 100 1-559 1-530 30 1-200 1-185 90 1-522 1-495 20 1-132 1-120 80 1-4S4 1-460 15 1-099 1-089 70 1-444 1-423 10 1-070 1-060 60 1-393 1-374 5 1-031 1-029 50 1-334 1-317
1-000 1-000 40 1-267 1-251 Regarding the reactions of nitric acid with elementary substances and with inorganic compounds generally, see CHEMISTRY (ut sup. pp. 511-514). The general effect of the treatment of an organic substance with aquafortis is that the acid is reduced to lower oxides of nitrogen and water, while the organic substance undergoes some kind of oxidation, which sometimes assumes the character of a com bustion. With regard to the more immediately formed oxidation products it is difficult to speak in general terms; suffice it to say that, according to Carius, all organic substances when heated with nitric acid in sealed-up tubes to 150 to 300 C. are burned com pletely into caibonic acid, water, and nitrogen or nitrogen oxides. Any other element that may be present passes into its highest state of oxidation: sulphur, for instance, becomes sulphuric acid; phos phorus, phosphoric, &c. Very remarkable is the action of real nitric acid, when employed in the cold, and under conditions pre cluding the accumulation of water or lower oxides of nitrogen. The acid then, if it acts at all, unites with the respective substances, with elimination of water and formation from the body B of a pro duct B + nHN0 3 -nH 2 or B + nX0 2 -nH. These nitro-bodies, as they are sometimes called, divide themselves into two classes: (1) nitric esters, i.e., real nitrates formed from alcohols, which term
