surface into oxides: silver even becomes a peroxide, though this metal does not enter into direct combination with ordinary oxygen either when moist or dry. When the ozone and the metals are perfectly dry, little or no absorption of ozone occurs. Dry mercury as well as dry iodine, however, immediately removes ozone. It is remarkable that no contraction follows the absorption of ozone by these or by any other agents; this point was carefully and minutely observed by Andrews and Tait. Hence it seems to be probable that the ozone is resolved into a quantity of ordinary oxygen, equal in bulk to itself, which is liberated at the moment that another portion of oxygen enters into combination with the iodine; possibly three volumes of oxygen become condensed into two; one volume becoming fixed, whilst two volumes are liberated on the decomposition of ozone by a metal. Ozone displaces iodine from its combinations with the metals, setting the iodine at liberty; indeed, this reaction is so easily produced, that it furnishes the readiest and most delicate method of detecting the presence of traces of ozone in the air; a slip of paper moistened with starch and iodide of potassium, and inserted into a vessel containing the smallest admixture of ozone, becomes blue from the action of the liberated iodine, which immediately unites with the starch, and forms the blue iodide of starch which is so characteristic of iodine. Indeed, pure oxygen contained in a tube inverted over a solution of iodide of potassium is entirely absorbed by the liquid, if the gas be subjected to the passage of a discharge of electricity through it for a sufficient length of time, hydrate of potash being formed by the absorption of oxygen, while iodine is set free: (4 KI + 2 H2O+0 ̧= 4 KHO+2 I.) If the experiment be prolonged, iodate of potassium, peroxide of hydrogen and peroxide of potassium are formed. Paper soaked in a solution of sulphate of manganese (MnSO) likewise shows the presence of ozone by becoming brown, owing to the manganese in the sulphate absorbing oxygen, and becoming converted into the insoluble hydrated peroxide, whilst sulphuric acid is set free. If the paper be stained black with sulphide of lead (PbS), this stain will gradually disappear; both the sulphur and the lead will absorb the ozone, or active oxygen, and a white sulphate of lead (PbSO) will be formed. One of the most singular circumstances connected with ozone is the effect of heat upon it. A temperature not much higher than that of boiling water is sufficient slowly to destroy all its active character, and the change is instantaneous at the temperature of 570°. By placing the flame of a spirit-lamp so as to heat a part of the tube through which the electrified oxygen escapes, all signs of ozone disappear. Ozonized 2 2 air is also deozonized by transmission over cold peroxide of manganese, peroxide of silver, or peroxide of lead. If a piece of paper, soaked in a mixture of starch and iodide of potassium be exposed in the open air for five or ten minutes, it often acquires a blue tint, the intensity of which varies on different、 days; sometimes, particularly in damp or foggy weather, no change is produced by such exposure. These effects are seldom seen in towns, but generally in the open country, or on the sea coast, especially when the wind blows off the sea. They are plausibly supposed to be owing to the presence of traces of ozone in the atmosphere; and theorists are not wanting who believe they have traced the prevalence of cholera and other epidemics to the unusual absence of ozone in the air during lengthened periods. Iodine may, however, be liberated from iodide of potassium by nitrous acid, by chlorine, and by various agents besides ozone, so that this reaction, although a very sensitive one for ozone, is by no means characteristic of its presence; and the existence of traces of ozone in the atmosphere, probable though it is, cannot be said to have been unequivocally proved. Schönbein, in order to obtain some idea of the proportion of the agent which produces the effect, prepares this paper of a definite strength, by dissolving I part of pure iodide of potassium free from iodate in 200 parts of distilled water, which is thickened by heating it with 10 parts of white starch: this is then spread upon slips of unsized paper, which are preserved in a stoppered bottle kept in the dark. § II. NITROGEN.* N=14. Atomic Vol. ; Theoretic Sp. Gr., 0'9674;. Observed (339) It has already been stated (334) that the larger proportion of the atmosphere consists of a gaseous body, which has been named nitrogen (generator of nitre), because it is an essential constituent of nitre: sometimes the name of azote (from a not, wn life,) is given to it, because, though not poisonous, it is incapable of supporting life. This element was discovered by Rutherford in 1772. Properties.-Nitrogen is a colourless, tasteless, and inodorous gas, which as yet has resisted every effort to liquefy it. It is somewhat lighter than atmospheric air; calculating from Reg * If nitrogen in the gaseous state be regarded as (NN) nitride of nitrogen, its molecular volume will be PREPARATION OF NITROGEN. 29 nault's experiments, 100 cubic inches at 60° F., Bar. 30° in., weigh 30119 grains. Water dissolves not more than of its bulk of this gas at ordinary temperatures, 100 cubic inches of water at 32° absorbing 2:03 cubic inches of nitrogen, and 148 cubic inches at 59° (Bunsen). No two substances can offer a more striking contrast in chemical properties than oxygen and nitrogen: the one the most energetic of the elements, the other the most indifferent. It extinguishes a taper without taking fire itself; an animal immersed in the undiluted gas perishes quickly for want of oxygen, but it is not directly poisonous; indeed, it enters as a necessary component into the animal frame, and with every act of inspiration it finds admission into the lungs. One very important purpose that it fulfils in the atmosphere is the dilution of the oxygen, which is rendered thereby less stimulating to the living system, and the rapidity of ordinary combustion is likewise thereby moderated. Nitrogen is one of the most extensively diffused forms of matter, as must be evident from the facts just stated; and notwithstanding its apparent indisposition to enter into combination, it forins a number of highly interesting and important compounds. For example, one of its combinations with oxygen, when dissolved in water, forms nitric acid, which exists as a natural production when united with potassium and sodium in the nitrates of those metals it is the characteristic ingredient in ammonia; and though it occurs in but small quantity in plants, it is never entirely absent from them. Nitrogen also constitutes an essential part of many of the most potent and valuable medicines, such as quinia and morphia, as well as of some of the most dangerous poisons, as prussic acid and strychnia. It likewise enters largely into the composition of many animal tissues. Organic compounds which contain nitrogen are frequently termed azotised substances. Preparation.—The most convenient methods of obtaining nitrogen are based upon the removal of oxygen from atmospheric air. 1.-The simplest plan consists in placing a few fragments of phosphorus, dried by means of blotting-paper, on a porcelain dish which is floated upon the surface of the water of the pneumatic trough; the phosphorus is ignited by touching it with a hot wire, and a glass receiver filled with air is then inverted over it. The phosphorus burns at the expense of the oxygen in the confined air, and being partially converted into vapour by the heat which attends the combustion, is diffused through the gas, and thus quickly searches out and combines with every portion of oxygen: when cold, the nitrogen may be decanted into another jar and examined. Even at ordinary temperatures, a stick of phosphorus will, if intro 30 PREPARATION OF NITROGEN. duced into a jar of air which is standing over water, slowly absorb the oxygen, and in two or three days about four-fifths of the original bulk of the air, consisting of nitrogen nearly pure, will be left. 2. The removal of oxygen from the air may also be effected slowly in various ways. Moistened iron filings produce a similar result, the metal gradually becoming oxidized, as is seen by the rusty appearance which it assumes. Many other metals, when moist, moistened lead shavings, for example, produce a similar effect. 3.-Moistened sulphides of the alkaline metals likewise absorb oxygen, from a confined portion of air very rapidly and completely. 4. When larger quantities of nitrogen are required, metallic copper may be employed to absorb the oxygen. The method to be adopted in this case is exhibited in fig. 272. c represents a long straight tube of hard glass, which will resist a strong heat without fusion: it is filled with metallic copper in a finely divided state; for this purpose the metal which has been reduced from the powdered oxide by means of hydrogen gas is well adapted. The tube c rests on a sheet-iron furnace, d, in which it can be surrounded by charcoal and raised to a red heat; FIG. 272. e is a bent tube for delivering the gas into a jar over water or mercury; the other extremity of the tube c is connected with a bent tube, b, filled with fragments of fused caustic potash; and the air which supplies the nitrogen is driven from the gas-holder, A, over the ignited copper in a stream which is easily regulated by the stopcock, f. The air first traverses the tube which contains the fused potash, where it leaves all traces of carbonic anhydride and moisture, and it then passes over the ignited copper, by which every portion of oxygen is completely removed. 5 and 6.-Nitrogen may also be obtained by the action of COMPOSITION OF THE ATMOSPHERE. 31 chlorine on a solution of ammonia (386), and it is furnished in a state of purity by heating the nitrite of ammonia (369). § III. COMPOSITION OF THE ATMOSPHERE. (340) If a mixture be made of 4 measures of nitrogen and I measure of oxygen gas, a candle will burn in it as in atmospheric air; it may be breathed as air, and possesses the ordinary properties of the air. The atmosphere is, in short, a mechanical mixture of several gases, amongst which oxygen and nitrogen constitute the principal portions: these gases, notwithstanding their difference in density, are, owing to the principle of diffusion (67), uniformly mixed with each other. Chemical operations are continually occurring upon the earth's surface, which remove oxygen and add a variety of other gases, amongst which carbonic anhydride is most abundant. Yet so beautifully adjusted is the balance of chemical actions over the face of the earth, that no perceptible change in the composition of the atmosphere has been observed since accurate experiments on the subject have been practised. Air which has been freed from carbonic anhydride and aqueous vapour consists, according to the numerous careful analyses of Dumas and Boussingault (Ann. de Chimie, III. iii. 257), on an average of 20.81 of oxygen by measure, and 79:19 of nitrogen in 100'00 parts; or by weight of 23'01 of oxygen, and 76'99 of nitrogen. These experiments were performed by allowing the air to stream slowly over a weighed quantity of heated copper, by which the oxygen was absorbed (fig. 272) whilst the nitrogen was received into an exhausted flask, which was weighed before the experiment was commenced and after its termination; the quantity of oxygen was found by the gain in weight experienced by the tube containing the copper. The results obtained by Regnault, Brunner, Verver, and others, by different methods of analysis, do not vary more than from the quantity of oxygen just mentioned. Trifling temporary variations no doubt occur from local causes; but the air brought by Gay-Lussac from an elevation of four miles above the surface of the earth, that collected on the summit of the Alps, and that examined both in town and country in various parts of the globe, presents no sensible difference from the mean above given.* A portion of air collected by Mr. Welsh, in August, 1852, at an elevation of 18,000 feet, in one of the balloon ascents undertaken by him and Mr. Green under the direction of the Kew Committee of the British Association, |