JJITEOOEN.] CHEMISTRY 509 compounds derived from it. If, however, the second for mula be adopted, it follows that the formula which repre sents the three atoms of oxygen in the trioxide as directly associated with the sulphur atom is the more probable. It is probable that sulphur dioxide and sulphurous acid are analogous in constitution to sulphur trioxide and sulphuric acid, and from the manner in which hyposulphurous acid is formed from sulphurous acid we may infer that it has the constitution represented by the second of the follow ing formulae O O I! II H O S H H S H Sulphurous acid. Hyposulphurous acid. The following formulae are the most probable expres sions of the constitution of the remaining acids of sulphur ; the constitution of the SO 2 group is the same as in sulphuric acid and the dots are used instead of lines HO . S0 2 . SH HO.S0 2 .OH Sulphuric acid. S0 2 .OH S ; S0 2 . OH Trithfonic acid. Thiosulphuric acid. S.S0 2 .OH S . S0 2 . OH Tetrathionic acid. S0 2 . OH SO. 2 .OH Dithionic acid. S S0 2 . OH SO,. OH Pentathionic acid. We have previously pointed out (p. 474) the value which is to be attached to constitutional formulas such as are here assigned to the sulphur compounds. NITROGEX. SymLol, X ; Atomic wt., 14 ; Molecular wt., 28. Nitrogen in the free state constitutes about one-fifth by volume of the atmosphere ; in combination it occurs in nitrates and ammoniacal salts, and it enters into the com position of all animal and vegetable tissues. It is best obtained from air by removing the oxygen by means of copper heated to redness ; the air being first led through a solution of potassic hydrate to free it from car- lion dioxide, and then through concentrated sulphuric acid to remove moisture, and when thus purified, passed through a tube containing metallic copper heated to dull redness. By passing a mixture of air and ammonia over heated copper, the copper oxide is reduced as fast as it is formed, and a short length of copper suffices for the preparation of an indefinite quantity of nitrogen ; thus 2Cu + O 2 = 2CuO, 3CuO + 2NH 3 = N 2 + 3H 2 O + 3Cu . The supply of air is easily regulated, as an excess shows itself by tarnishing the surface of the copper. The easiest method of obtaining pure nitrogen is to heat a solution of ammonium nitrate, which splits up into nitrogen and water : NO 2 NH 4 = N 2 + 2H 2 O . But as this salt is difficult to prepare, it is better to substitute for it a mixture of potassium nitrate and ammonium chloride, which together produce ammonium nitrate and potassium chloride KN T 2 + NH 4 C1 = NH 4 N0 2 + KC1 . Nitrogen is a colourless, inodorous, tasteless, incondensable gas ; it is only very sparingly soluble in water, 100 volumes of water at 15 C. dissolving about one and a half volumes of nitrogen. It is incombustible, and does not support the combustion of ordinary combustibles ; it is not poison ous, but an animal immersed in it dies simply for want of oxygen. Nitrogen evinces but little tendency to enter into reaction with other elements; titanium, tungsten, vanadium, and probably a few others combine directly with it, however, but its compounds are mostly produced by indirect means. Atmospheric air consists not only of nitrogen and oxy gen, which are its chief constituents, but besides these con tains carbon dioxide, ammonia, water vapour, &c. ; solid substances, such as common salt, are also frequently held in suspension by it, especially in the neighbourhood of the sea and of towns. Air from which all other constitu ents are removed does not always exhibit the same compo sition, however, although the variations are very slight ; usually in pure air the proportion of oxygen is from 20 9 to 21 volumes in 100 of air, but considerably less oxygen has been found in air from confined spaces and in a few samples collected in warm countries. Full information on this subject may be obtained from Dr Angus Smith s work on Air and Bain. The fact, however, that oxygen and nitrogen are not always contained in air in the same proportions is alone a sufficient proof that they are not combined, but only mixed together, as the constituents of a compound always occur in invariable proportions. This conclusion is confirmed in many ways. Thus, the proportions in which nitrogen and oxygen are present in air are not those of their atomic weights, nor do they bear any simple relation to them. Ve know also that when gases enter into reaction to form new compounds their combination is usually attended with an alteration of volume, and heat is developed ; moreover, the resulting compound possesses properties which differ strikingly from those of its constituents. But when oxy gen and nitrogen are mixed together in the proportions in which they are present in air, neither is any alteration in volume. observed, nor is heat developed; and the proper ties of the resulting mixture are precisely those of air, and just such as we should expect to result from the admixture of a gas which very readily supports combustion with one in which combustion is impossible. Again, were ail- a compound it should dissolve in water as such, or in other words, the proportion of oxygen and nitrogen in the dis solved air should- be the same as in the undissolved air; but if a mixture, tli3 more soluble constituent should dissolve the more readily, and relatively more oxygen than nitrogen should dissolve, since oxygen is more soluble than nitrogen. Experiment proves that the latter is the case, for if water which has been recently boiled to free it from dissolved gases and allowed to cool out of con tact with air be shaken with air, and the dissolved air be then expelled from it by heating, and collected, it is found on analysis to contain 32 instead of only 21 per cent, by volume of oxygen. Lastly, we have seen (p. 481) that the oxygen may be to a great extent separated from the nitrogen by a mechanical process by submitting air to filtration through a thin caoutchouc membrane. It is impossible for animals to live for any length of time in pure oxygen, apparently because oxidation takes place so rapidly that the animal is incapable of assimilating sufficient food to supply the waste ; but by admixture with the perfectly neutral nitrogen the activity of the oxygen becomes greatly diminished. Air from open places contains usually from 3 to 6 volumes of carbon dioxide in 10,000 volumes, but the amount of carbon dioxide in the atmosphere is subject to continual change, although within narrow limits. It does not continually increase in amount, notwithstanding that animals expire carbon dioxide, and that large quantities are produced by the combustion of wood and coal, because plants exercise a power which is the reverse of that of which animals are possessed, viz., that of decomposing car bon dioxide and restoring its oxygen to the atmosphere. Ammonia, NH 3 ; Mol. wt., 17. Ammonia is the only compound of nitrogen and hydro
gen which is known to us. It may be formed directly