82 PREPARATION OF CARBONIC OXIDE. 3.-If one of the formiates be treated with oil of vitriol, pure carbonic oxide is obtained; for instance : 2 Na¤HO2+H2SO1=2 €O+Na„SÐ ̧+2 H ̧Ð. 4.-Carbonic oxide may also be formed in several other ways. Half an ounce of the yellow prussiate of potash, if heated in a retort with 4 or 5 ounces of oil of vitriol, yields more than a gallon of the pure gas (Fownes). Care is requisite in applying the heat, because when the temperature rises to a certain point the extrication of the gas takes place with tumultuous rapidity. The reaction is in this case of a complicated nature, but is expressed by the annexed symbols : -- Carb. oxide. 6 Є0 2 = Sulph. potassium, Sulph. iron. Sulph. ammonium. + 2 K2SO4 + FeSO4 + 3 [(H2N)2SO] 5. Another method by which carbonic oxide may be obtained with facility consists in heating oxalic acid with 5 or 6 times its weight of oil of vitriol. The oxalic acid is thus deprived of water, and is resolved into a gaseous mixture consisting of equal measures of carbonic anhydride and carbonic oxide: by allowing the mixed gases to pass through a vessel filled with a solution of potash, or with milk of lime, the carbonic anhydride is absorbed, and the carbonic oxide may be collected in a state of purity. The decomposition may be thus explained : A convenient mode of washing the gas is shown at в, fig. 289: the bent tube is connected to the neck of the retort, A, and passes to the bottom of a wider tube, c, open both at top and bottom, which passes into the washing bottle, D: a moveable gas-tight joint, which can be mounted or dismounted in a moment, is thus obtained. Properties.-Carbonic oxide is a transparent colourless gas, with a faint oppressive odour. It is much lighter than carbonic acid, having a specific gravity of o'967 (Wrede.) All attempts at its liquefaction have as yet been unsuccessful. It is but very sparingly soluble in water, ico parts of this liquid dissolving 3.28 parts at 320, and 2:43 at 59° (Bunsen). When respired, even though largely diluted with air, it acts as a direct poison, producing a peculiar sensation of oppression and tightness of the head. It PROPERTIES AND COMPOSITION OF CARBONIC OXIDE. 83 does not support combustion, but burns itself with a beautiful pale blue light, producing by its combination with oxygen carbonic FIG. 289. B anhydride only. A solution of subchloride of copper in hydrochloric acid, or of a cupreous salt dissolved in ammonia, gradually absorbs carbonic oxide if agitated with it. The solution of this compound is not decomposed by dilution, but if the liquid be boiled most of the gas is expelled unaltered. The compound with subchloride of copper crystallizes in fatty looking scales, consisting of (COЄu'Cl. H2O), but by exposure to air it is quickly decomposed. Carbonic oxide is absorbed by potassium if the metal be heated to about 176° in the gas, and according to Brodie, the combination occurs in the proportion shown by the formula, COK. This property is sometimes employed for separating carbonic oxide from its mixture with other inflammable gases in the process of analysing mixtures of such gases. Carbonic oxide has been supposed to form the radicle of a numerous series of compounds; it even enters directly into combination with hydrate of potash when heated with it, converting it into formiate of potassium: KHO+CO=KEHO, (Berthelot). Composition. The chemical composition of carbonic oxide may be ascertained in the following manner :—Introduce into the bent eudiometer (fig. 281) a certain measure, say 20 parts, of carbonic oxide, then add 20 measures of pure oxygen; pass the electric spark with the precautions already described: the 40 measures of gas will become diminished to 30 measures. If a little solution 84 COMPOUNDS OF CARBON WITH OXYGEN. of potash be introduced, 20 measures of the residual gas will disappear, leaving 10 measures of unaltered oxygen: the 20 measures of gas absorbed are carbonic anhydride. Now carbonic anhydride contains its own bulk of oxygen, but the 20 measures of carbonic oxide have required only 10 measures, or half their bulk, of oxygen to convert them into the anhydride. Carbonic oxide therefore must have contained the other 10 measures of oxygen; in other words, half its bulk of oxygen. But the specific gravity of carbonic oxide is o'9674; deduct from this 05528=half the specific gravity of oxygen 04146; this remainder is the weight of the carbon combined with 0'5528 of oxygen. Now 0'5528 0'4146: 16: 12. The proportion by weight of oxygen to carbon in carbonic oxide is therefore as 16 to 12, or I atom of each, and its composition may be thus represented : Carbonic oxide and carbonic anhydride, widely as they differ in properties, consist, it is evident, of the same elements; but the proportions of the two elements differ in the two cases. Carbonic oxide is the compound of carbon that contains the smallest proportion of oxygen, the relative composition of the two bodies being: C. 0. Carbon. Oxygen. Carbonic oxide €0 28 or 12 + 16 = 42.865714 Carbonic anhydride €0, = 44 12 + 32 = 27.287272 In 100 parts. 1 CHAPTER V. COMPOUNDS OF NITROGEN WITH OXYGEN AND WITH HYDROGEN. § I. COMPOUNDS OF NITROGEN WITH OXYGEN. (358) THE attraction of nitrogen for oxygen is much feebler than that of either carbon or hydrogen for oxygen, so that it is not easy to procure their direct union,-especially as the temperature emitted by the nitrogen and oxygen in the act of combination is comparatively low. Nitrogen, notwithstanding, forms with PRODUCTION OF NITRIC ACID. 85 oxygen five distinct compounds, containing, respectively, 1, 2, 3, 4, and 5 atoms of oxygen with 2 atoms of nitrogen. They may all be obtained free from water. These compounds have been named (359) NITRIC ACID; HNO,, or HO,NO, 63: Sp. Gr. of Liquid, 1517; Boiling-pt. 184°.-The most important of the compounds of oxygen with nitrogen is that which when in combination with water was formerly called aquafortis, and is now designated nitric acid. It was known to the alchemists, but its true composition was first determined by Cavendish in 1785. When nitrogen is mixed with 12 or 14 times its bulk of hydrogen, and a jet of the mixed gas is allowed to burn in air or in oxygen, the water which is formed has a sour taste and an acid reaction, owing to the simultaneous formation of a small quantity of nitric acid. In this case the nitrogen burns by the aid of the heat developed during the combustion of the hydrogen, and the oxidized compound combines at once with the water formed, which much increases its chemical stability. It was, indeed, owing to the accidental production of nitric acid in the course of his experiments on the formation of water by the combustion of hydrogen, that Cavendish was induced to institute the train of researches which terminated in this important discovery. If 2 volumes of nitrogen be mixed with 5 volumes of oxygen, and introduced into the bent eudiometer (fig. 281) and the tube be filled up with an infusion of blue litmus in distilled water, a series of electric sparks may be transmitted through the mixture by means of a Ruhmkorff's coil: under these circumstances the two gases will combine slowly, and the litmus will be reddened. The heat of the spark determines the combination of the gases just at the spot through which it passes, but the action does not extend further. In like manner, if a number of sparks be passed from the electrical machine, between two metallic points, over moistened litmus-paper, in air, a red spot will be produced upon the paper, owing to the formation of nitric acid in minute quantity by the combination of oxygen with nitrogen in presence of aqueous vapour in the air. During stormy weather, and indeed whenever 86 NITRIC ACID-ITS PREPARATION. a flash of lightning passes through a moist atmosphere, the same compound is produced in appreciable quantity. Indeed, it is rare to meet with rain water in which traces of nitrate of ammonium may not be detected, if the experiment is made with accuracy. Ammonia likewise yields nitric acid under certain circumstances by slow oxidation (369). This oxide of nitrogen also occurs in combination with potassium or sodium, in the form of an efflorescence on the soil, especially in tropical climates, as in some parts of India and Peru. The compound formed with potassium constitutes the nitre or saltpetre of commerce. The nitrates of the alkaline metals are often present in the water of wells in towns or in the vicinity of cemeteries, the nitric acid being in these cases produced by the oxidation of azotized animal matters, as they undergo decomposition during the percolation of their aqueous solution through the soil. Preparation. It is from one of the nitrates that the acid is always obtained for chemical purposes. When nitrate of potassium is heated with a powerful acid, such as the sulphuric, mutual decomposition occurs. The potassium and hydrogen change places, forming sulphate of potassium and nitrate of hydrogen, or nitric acid. The sulphate of potassium remains in the retort, whilst the more volatile nitric acid distils over, and may be condensed in the usual manner. The method of procuring nitric acid offers a good example of the general principle upon which acids which admit of being distilled without experiencing decomposition are obtained from their salts. In preparing nitric acid on the small scale, equal weights of nitre and oil of vitriol are placed in a glass retort, and the distillation is proceeded with in the manner shown in figs. 136 and 137, Part I. p. 307. During the distillation red fumes appear in the retort, arising from a partial decomposition of the acid, and a formation of some of the lower oxides of nitrogen, whilst a yellowish corrosive liquid is condensed in the receiver: this liquid is concentrated nitric acid (HN); it fumes strongly in the air, and emits a powerful irritating acid odour. On the large scale, iron retorts, fig. 290, coated with fire-clay on the inside of the upper part, where they are exposed to the acid vapours, are employed for the distillation, and nitrate of sodium is substituted for nitrate of potassium, as it is a cheaper salt, and likewise yields 9 per cent. more nitric acid than nitrate of potassium. The cylinders or retorts are arranged in pairs in a furnace, so that each fire heats two cylinders, as shown in the section, 1. The cylinders are supplied with a moveable lid, c, d, at each end, |