Chemically, nitrogen gas is characterized by perfect inertness towards all ordinary reagents under ordinary conditions. But at certain higher temperatures boron, magnesium, vanadium, and titanium combine with it directly into nitrides. Nitrogen is capable even of uniting with ordinary oxygen. A mixture of the two gases, it is true, remains unchanged when exposed en masse to any temperature, but when it is subjected to a succession of electric sparks a small proportion of the two gases, no doubt through local dissociation into isolated atoms N and O, does unite into nitric oxide, NO, which then combines with more oxygen into red fumes of peroxide, N 2 O 4 .

The part which the nitrogen gas in the atmosphere plays in the economy of nature is as yet a mystery. It certainly is not susceptible of being taken up directly by the plants and utilized in their synthesis of nitrogenous compounds. It plays no active part in the processes of combustion and of animal respiration; in either it appears to act only as an inert diluent of the oxygen.

In the case of respiration, however, this particular diluent seems to be essential; no animal could live healthily for any considerable period of time in pure oxygen, and we know of no other diluent which could be substituted for the nitrogen without producing poisonous effects. Besides, there can be no doubt that atmospheric nitrogen, in an indirect way, contributes towards the building up of nitrogenous organic matter. Every process of ordinary combustion probably, and every electric discharge in the atmosphere certainly, induces the formation of some nitric acid, which by combining with the atmospheric ammonia becomes nitrate of ammonia, and from certain experiments of Schönbein’s it would appear that nitrogen gas and water are capable of uniting directly into nitrite of ammonia (N 2 + 2H 2 O = NO 2 . NH 4 ), which, supposing it to be produced in the atmosphere, would promptly be oxidized into nitrate. The nitrate produced by either process is carried down by the rain and conveyed to the roots of the plants, which assimilate it as part of their nitrogenous organic ma tter. However small the scale may appear on which these processes of atmospheric nitrification go on when measured by the mass of nitrogen which remains unchanged, as this mass is immense, their absolute effect must be very considerable, and may form an important item in the economy of nature.

The compounds of nitrogen may be arranged under the heads of ammonia, nitrates, nitro-compounds, organic nitrogen compounds, and cyanides. As all the several classes and their most important members are treated of under Chemistry (vol. v. p. 509-514), we confine ourselves here, in the main, to supplementing that article by such details as are of practical or general scientific interest.

Ammonia.

This, the only known compound of hydrogen with nitrogen, is a gas of the molecular formula NH 3 . The most convenient process for the preparation of the pure gas is to mix powdered sal-ammoniac with powdered quicklime in a flask and to heat the mixture in a sand-bath. Torrents of ammonia come off, which must be dried by passing it through a closely packed column of solid caustic potash or soda (chloride of calcium absorbs the gas chemically) and collected over mercury, as the gas dissolves most abundantly in water. In addition to what has been said under Chemistry of the properties of ammonia, it may here be mentioned that, though uninflammable in air, it burns brilliantly in oxygen, and that it is liable to the following peculiar kind of oxidation. Pour some strong liquor ammoniae into a large flask, so as to produce a moist mixture of the gas and of air, and suspend in this atmosphere a recently ignited spiral of thin platinum wire. The wire continues glowing and the flask soon fills with dense white fumes of nitrite and nitrate of ammonia, formed according to equation: 2NH 3 + (40 or 30) = H 2 O + N0 3 H . NH 3 or NO 2 H.NH 3 . The platinum suffers no permanent change; its mode of action probably consists in this that it alternately absorbs (i.e., combines with) oxygen and hands it over to the ammonia. In any case the reaction is interesting as throwing some light upon the process of nitrification {vide infra}.

Aqueous ammonia (liquor ammoniae), being in constant requisition as a reagent, and also used in medicine and in the arts, is being manufactured industrially. Fresenius recommends the following process. A cast-iron pot, fitted up as a retort, is charged with alternate layers of slaked lime (10 kilos of quicklime plus 4 kilos of water) and a powdered and sifted mixture of 6’5 kilos of chloride and 3'5 kilos of sulphate of ammonium. Eight litres of water are then added and well incorporated with the solids. The retort is now closed, and the outlet-tube joined on to the lower end of an inverted condenser, the upper end of which communicates with a set of Woulfe’s bottles charged with water. The ammonia is driven off by judicious application of heat, the inverted condenser serving to make the gas relatively dry before being absorbed. In this operation the tubes conveying the gas to the water must go to the bottom of the bottles, as the solution produced is lighter than pure water; and, of course, the bottles must be kept cool by immersion in a cold water-bath. The strength of aqueous ammonia, for commercial purposes, is readily ascertained by means of an hydrometer ranging from 0’85 to 1’0 specific gravity. The relation between specific gravity S at 14° C. (water of 14 d = l) and percentage of ammonia NH 3 (p), according to experiments by Carius, is as follows :— [table]

Aqueous ammonia is generally sent out as “liquor fortissimus” of 30 to 35 per cent. For ordinary laboratory purposes it is usually diluted down to 9 to 10 per cent. This explains the discontinuities in the table.

Two natural sources of ammonia are at present in industrial use. (1) The gaseous exhalations of volcanoes always include ammonia, hydrochloric acid, and sulphurous acid, of which the first two are formed no doubt by the action of steam on deposits of nitrides and chlorides in the interior of the earth. This explains the existence in the fumaroles (smoke-holes), and in the clefts of the lava of Vesuvius, Hecla, and other volcanoes, of deposits of (chiefly) sal-ammoniac. This volcanic ammonia salt is highly valued as a material for the preparation of pure liquor ammoniae; but its supply hardly comes up to the demands of even this small industry. (2) More important are the masses of ammonia formed in the processes of putrefaction which are going on constantly in nature, and of which a mere fraction would satisfy all the demands of industry, if the recovery of such ammonia were not, as a rule, beset with insuperable difficulties. Thus, for instance, all the immense mass of the ammonia of the sewage of our large cities must be allowed to go to waste because we have no economical method for its extraction. Urine, when undiluted, is an easily handled raw material, and in former times actually formed the principal source of ammonia. Human urine contains from 2 to 3 per cent, of urea, or carbamide as it is called in systematic chemistry, because it is the anhydride of carbonate of ammonia. When urine putrefies, this carbamide takes up the elements of water and becomes carbonate of ammonia, CO . (N 2 H 4 ) + 2H 2 O = CO 3 (NH 4 ) 2 , A prompter mode of conversion is to evaporate the urine with a small proportion of vitriol, and heat the residue to near the boiling-point of the acid, when the nitrogen of the urea passes at once into the form of sulphate of ammonia. This latter process would apply also to the urine of horses and cattle, which, instead of urea, contains hippuric acid, a compound which, when taken conjointly with water, contains the elements of ammonia and benzoic and acetic acids. At the present time urine plays hardly any part in the ammonia industry; but it may be mentioned that the produce of the urinals of Glasgow is, or lately was, wrought for carbonate of ammonia.

Preparation of Ammonia from Nitrogenous Organic Matter.— All such matter when subjected to dry distillation gives up part of its nitrogen as ammonia, of which the greater part condenses with the vapour of water produced, and is thus obtained as part of the aqueous portion of the “tar.” Large quantities of such tarwater are being produced incidentally in the manufacture of coal-gas, and it is this material which at present forms the principal source for the industrial production of ammonia and ammonia salts.

It may also be mentioned that the tar-water obtained as a byeproduct in the distillation of shale for the production of paraffin oil is rich in ammonia, and has long come to be worked up for sulphate like gas-liquors.

Crude tar-water contains about 1 per cent, of ammonia (more or less according to the quality of coal used, and the way it has been manipulated), mostly in the form of carbonate, part as cyanide, sulphocyanate, and sulphide of ammonium; and this ammonia, of course, is associated with traces of hydrocarbons and other organic matter dissolved, or suspended, in the liquor. In some establishments the ammonia is extracted directly in the form of liquor ammoniae. The liquor is run into a large iron boiler, and after addition of some ferrous and ferric salt (for fixing the sulphur of the 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 compartments of an iron apparatus, similar in its action to the “Coffey’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 ammoniae 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 contained 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 discoloration 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, (NH 4 ) 2 SO 4 , forms anhydrous, colourless, transparent crystals, isomorphous 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 sulphuric acid into crystallizable acid sulphate, SO 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 SO 3 (NH 4 )H 1 , nitrogen, and water. At a red heat it breaks up into sulphur, nitrogen, and water: (NH 4 ).,SO 4 = S + N., + 4H.,O.

Of other ammonia salts only the hydrochloride and the carbonates are industrially important.

The hydrochloride, HC1. NH 3 = NH 4 C1, better known as salammoniac (see vol. i. p. 741-2), is made sometimes by sublimation of a mixture of the sulphate with common salt, but it is more conveniently 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 considerable absorption of heat; 30 parts of salt with 100 parts of water at 13 o- 3 give a mixture of the temperature of -5°'1 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 CO 3 (NH 4 ) 2 is so unstable that it can hardly be said to exist. The acid salt CO 3 (NH 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 provided 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(NH 4 ) 2 O . 3CO 2 + H 2 O = (NH 4 ) 2 CO 3 + 2(NH 4 ) . HCO 3 . But it also contains carbamate of ammonia, ^

co ) S-(NH,)- co ≈ +2nh ∙∙ 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.

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 remunerative 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 temperature 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 nonvolatile nitrides; and these, when heated in steam, yield ammonia and the corresponding oxide. Thus we have 3Mg + N 2 = N 2 Mg 3 and N 2 Mg 3 + 3H 2 O = 2NH 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 charcoal, when heated intensely in nitrogen gas, yields cyanide of barium, Ba(NC) 2 , and this salt when heated in steam gives off ammonia while carbonate of baryta is left, which...