Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/561

NITROGEN 517 sulphide as FeS, and the cyanogen of the cyanide as prussiate, (NC) 6 Fe . (NH 4 ) 4 ), mixed with slaked lime and distilled. The vapours if condensed as they come off, would yield a very dilute liquor contaminated largely with volatile carbon compounds. To obtain a relatively pure gas, the vapour is subjected to a succession of partial condensations by making it pass through the several com partments of an iron apparatus, similar in its action to the " Coffi-y s still" which is used for the strengthening and refining of alcoholic liquors (see DISTILLATION, vol. vii. p. 265). The almost pure gas which leaves the last condenser is passed into a mass of water contained in a refrigerated close lead vessel, and thus converted into liquor ammonue of the requisite strength. In the majority of large establishments, however, the ammonia is converted into sulphate, for which purpose it need not be so elaborately purified. The ammoniacal vapour obtained from the crude liquor by simple distillation, or by distillation with lime from out of a steam-boiler, is passed into a quantity of chamber-acid con tained in a leaden tank, until the acid is almost but not quite neutralized. An excess of alkali would induce the formation of sulphide of iron from the sulphide of ammonium in the vapour and the traces of iron-salt adventitiously present, and lead to a discolora tion of the salt. Matters are generally arranged so that the bulk of the sulphate formed crystallizes out, on cooling, in the form of a granular magma, which is allowed to drain and to dry, and either sold as it is, or first purified by recrystallization. The pure salt, (XII 4 ).,S0 4 , forms anhydrous, colourless, transparent crystals, iso- morphous with those of the corresponding potash salt, and, like it, belonging to the rhombic system ; even when produced on a large scale they are generally of small dimensions ; when allowed to grow, they assume forms which strikingly remind one of sugar-candy, although the latter is clino-rhombic. The salt is insoluble in alcohol, like most sulphates, 100 parts of water at 0, 20, 100 C. dissolve 71 0, 76 "3, 97 "5 parts of salt. The solution is neutral to litmus. The salt readily unites with another equivalent of sul phuric acid into crystallizable acid sulphate, S0 4 . (N H 4 ) . H, soluble in alcohol. The neutral salt melts at 140 C. Above 280 it emits ammonia and leaves acid salt, which latter then breaks up with formation of acid sulphite S0 3 (NH 4 )H 1 , nitrogen, and water. At a red heat it breaks up into sulphur, nitrogen, and water : (NH.).,SO. = Of other ammonia salts only the hydrochloride and the carbon ates are industrially important. The hydrochloride, HC1 . NH 3 = XH 4 C1, better known as sal- ammoniac (see vol. i. p. 741-2), is made sometimes by sublimation of a mixture of the sulphate with common salt, but it is more con veniently produced direct from gas-liquor ammonia by passing it into muriatic acid until the latter is almost neutralized. The liquor, when sufficiently concentrated by evaporation, deposits, on cooling, part of its salt in feathery crystals, which are customarily purified by sublimation. The subliming apparatus consists of two parts, (1) a hemispherical stoneware basin placed within a close fitting iron one, or an enamelled iron basin, and (2) a hemispherical lead or stoneware lid, or dome, placed on the top of the basin and cemented on to prevent leakage. The dome has a small aperture in the top which remains open to preclude accumulation of pressure. The carefully dried crystallized salt is pressed into the basin, and, after the lid has been fitted on, is exposed to a long- lasting moderate heat. The salt volatilizes (mostly in the form of a mixed vapour of the two components, which reunite on cooling), and condenses in the dome in the form of a characteristically fibrous and tough crust. The salt readily dissolves in water, with consider able absorption of heat; 30 parts of salt with 100 parts of water at 13 3 give a mixture of the temperature of -5 l C. One hundred parts of water at 0, 10, 110 dissolve 28 4, 32 8, 77 2 parts of the salt. From its hot saturated solution it crystallizes on cooling in feathery groups of colourless needles. By slow evaporation of the solution it is possible to produce well-developed crystals which belong to the regular system, but look irregular on account of the predominance of the (hemihedric) faces of the trapezohedron. Of the carbonates of ammonia there are a large number, and their chemistry still lacks definiteness. The normal salt C0 3 (XH 4 ) 2 is so unstable that it can hardly be said to exist. The acid salt C0 3 (XH 4 )H is easily produced by passing carbonic acid into a saturated solution of the commercial salt, when it comes down as a crystalline precipitate. The commercial salt (important as a medicinal agent and as a chemical reagent) is obtained by subliming a mixture of sal-ammoniac and chalk from an iron "retort pro vided with a lead dome and receiver. It forms hard fibrous crusts or cakes, smelling strongly of ammonia. The salt has a variable composition. The greater part, as a rule, consists of " sesqui- carbonate," 2(XH 4 ) 2 . 3C0 2 + H 2 = (NH 4 ) 2 C0 3 + 2(XH 4 ) . HC0 3 . But it also contains carbamate of ammonia, " as obtainable by the direct union of carbonic anhydride and ammonia. Of the several ammonia compounds which we have referred to, the sulphate is by far the most important in an industrial sense. Immense quantities of the crude salt are being used as a manure the German sugar-beet growers alone consume a considerable fraction of the British produce while to the technical chemist generally it serves as the most convenient starting-point for the manufacture of ammonia, or of other ammonia salts." 1 In the mode of distilling coal customarily carried on in gas works, only about one-third of the nitrogen is obtained as ammonia in the tar- water, the remaining two-thirds being lost by evaporation into the air, or remaining in the coke in the carbide form. What used to go into the gas is now mostly recovered by efficient scrubbers. But the more efficient condensation of the ammonia actually formed is a matter of chemical engineering which cannot be more than touched on here. According to Bilbey the nitrogen of the coke can be recovered, partly at least, by distilling it at a very high temperature in a current of steam. Bilbey s process, however, has hitherto failed practically to give satisfaction, because the intense heat required means a great expense for fuel, and destroys the retorts at an alarming rate. The analytical chemist has no difficulty in extracting the whole of the nitrogen in a given sample of coal as ammonia by mixing it with soda- lime and heating the mixture in a combustion-tube to redness, and possibly the technical chemist will one day bring this process into a remunera tive form. What, however, is meanwhile more easy of attainment is the recovery of the large quantities of ammonia which are being produced in the manufacture of coke and in iron smelting (as far as carried out with coal), and which hitherto have been allowed to go to waste. Quite a number of chemists and engineers have tried their hands at this problem. The apparatus proposed, generally speaking, all come to this, that the coal-smoke produced in the furnace, instead of being allowed to have its own way, is sucked out by exhausters, made to pass through refrigerators to deposit at least part of its tar and ammonia water, and the uncondensed combustible gases are led away to be used as fuel for steam-boilers, or, what in the case of coking is far better, led back to the coke oven and consumed there to increase the temperature, and thus improve the qualities of both coke and tar. The very high tem perature of the oven or furnace smoke throws great difficulties in the way of a perfect condensation of the ammonia. These, in a Scottish iron-work, have been turned most ingeniously by mixing the smoke with the sulphureous vapour formed by the roasting of pyritic shale or coal, whereby the ammonia is converted into sulphite and sulphate, which can easily be condensed in even hot water. Should it not be possible to produce ammonia synthetically from atmospheric nitrogen ? This question is still waiting for an industrial solution ; scientifically it may be answered in more than one way. Magnesium, boron, and a number of other solid and non-volatile elementary substances, when kept in nitrogen gas at the proper temperature, unite with the nitrogen into solid non volatile nitrides ; and these, when heated in steam, yield ammonia and the corresponding oxide. Thus we have 3Mg+N., = X.,Mg 3 and X.,Mg 3 + 3H 2 = 2XH 3 + 3MgO. Unfortunately the reagents are all" expensive, and there is no economical method for their regeneration. The following method is not subject to this one objection. A mixture of baryta or carbonate of baryta with char coal, when heated intensely in nitrogen gas, yields cyanide of barium, Ba(XC).,, and this salt when heated in steam gives off ammonia while carbonate of baryta is left, which latter can be used for starting de novo, BaX 2 C 2 + 4H 2 = 2XH 3 + BaC0 3 + CO + H 2 . Where this process fails we are unable to say ; what we do know is that nobody has as yet succeeded in working it profitably even as a means for obtaining cyanides, whose value, per unit of nitrogen, is higher than that of ammonia. Nitrates. Nitrates (the generic term for nitric acid, HX0 3 , and its salts) are produced naturally by the electric discharges in the atmo sphere, and in the processes of "nitrification," a fermentative oxidation which always sets in when moist nitrogenous animal or vegetable matter is left to itself in the presence of air and some basic substance (see FERMENTATION, vol. ix. p. 98). This process in former times used to be carried out for the production of saltpetre, but as an industrial operation is now obsolete. The deposits of native nitre in India and elsewhere which nature has produced for us by the same method are, of course, still being utilized as far as they go. But they amount to very little compared with the immense masses of native nitrate of soda which exist in South America, and which, at present, constitute by far the most im portant raw material for the nitrate industry. This native nitrate of soda forms part of a salty earth known to