Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/560

516 NITROGEN posing 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 matter. However small the scale may appear on which these processes of atmospheric nitrifica tion 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 our selves 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 nitro gen, 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 ammonia? into a large flask, so as to produce a moist mixture of the gas and of air, and suspend in this atmo sphere 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 am monia, formed according to equation : 2NH 3 + (40 or 30) = H 9 + NO,H . NH, or 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 ascer tained by means of an hydrometer ranging from 85 to TO specific gravity. The relation between specific gravity S at 14 C. (water of 14" = 1) and percentage of ammonia NH 3 (p), according to experiments by Carius, is as follows : P S jj p S P S 36 -8844 25 9106 10 9593 35 -8856 20 9251 9 9631 34 -8868 15 9414 8 9670 33 -8907 14 9449 7 9709 32 -8929 13 9484 6 9749 31 8953 12 9520 5 9790 30 -8976 11 9556 Aqueous ammonia is generally sent out as "liquor fortissimus " of 30 to 35 per cent. For ordinary labora tory 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 ammonias ; 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 ele ments of water and becomes carbonate of ammonia, CO . (N 2 H 4 ) + 2H 2 O = CO 3 (NH 4 )o. A prompter mode of conversion is to evaporate the urine with a small propor tion 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 car bonate 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 tar- water 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 bye- product 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 establish ments the ammonia is extracted directly in the form of liquor arnmonise. The liquor is run into a large iron boiler, and after addition of some ferrous and ferric salt (for fixing the sulphur of the