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than nitrogen it ought to be possible to effect a partial separation of air into its constituent gases by passing it through a sheet of india-rubber. Experiment proves that when all, or almost all, the air is pumped out of an india-rubber bag, and the bag is closed and left in the atmosphere, air passes into the bag through the walls, and that the composition of the air found in the bag is approximately 40 p. ct. of oxygen and 60 p. ct. of nitrogen, by volume. But the composition of ordinary air is approximately 21 p. ct. oxygen and 79 p. ct. nitrogen, by volume. Therefore the air has been partially separated into its constituents by passing it through a sheet of india-rubber; therefore air is a mixture, not a compound, of oxygen and nitrogen.
The argument may be extended to chemical events. If air is a mixture, it ought to interact chemically with other substances both as oxygen interacts and also as nitrogen interacts. If air is a compound, its interactions with other substances ought to be different from those of either oxygen or nitrogen. We have learned that nitrogen is a very inert substance; it does not support combustion, it is not combustible, it combines directly with only a few elements, and it does not react chemically with many compounds. On the assumption that air is a mixture, we should, therefore, expect its chemical properties to resemble those of oxygen, but to be less strongly marked because of the presence of the inert nitrogen.
For instance, we should expect substances which burn rapidly and brilliantly in oxygen to burn in air but to burn more slowly and less brilliantly. If we can find an element which combines directly with nitrogen when heated in that gas, we should expect that element to form a compound with nitrogen when strongly heated for some time in air.
We need not go into details regarding individual experiments, but suffice it to say that these expectations are realised; that the chemical behaviour of air is exactly what the hypothesis of its being a mixture asserts ought to be its behaviour. Air then is a mixture, not a compound, of oxygen and nitrogen.
But besides these gases, air contains small quantities of the compound gases, carbon dioxide, ammonia, and watervapour.
The composition of air varies within narrow limits. Thus air has not that fixity of composition which as we have seen characterises chemical compounds.
The experiment described in par. 108 shewed that the composition of air is, roughly, 4 vols. of nitrogen to 1 vol. of oxygen. In order accurately to determine the volume-composition of air, a quantity of air is passed into a graduated glass tube fitted with two platinum wires passing through the glass near the closed end ; the tube is filled with mercury, and is then inverted in a trough containing mercury. The air to be analysed is freed from carbon dioxide and ammonia, and is then passed into the tube, and the volume is measured by reading off the level of the mercury. A quantity of hydrogen equal in volume to nearly of the volume of air is passed into the tube, and the level of the mercury is again read off; the tube is pressed down on a pad of india-rubber and securely clamped ; an electric spark is then sent from one platinum wire to the other; the effect of this is that the whole of the oxygen in the air combines with a portion of the hydrogen to produce water which condenses. After a little time the tube is slowly raised from the india-rubber pad; mercury rushes in; the level of the mercury is read off. As we know that 2 vols. of hydrogen combine with 1 vol. of oxygen, we conclude that of the diminution of volume which occurs when the spark is passed represents the volume of oxygen in the volume of air employed. The volume occupied by the small quantity of water produced is so small that it may be neglected. Many precautions are necessary in carrying out such an analysis as this; corrections must be made for temperature and pressure; the volumes of wet air and wet gas after the explosion must be reduced to the corresponding volumes of dry gases, &c.
The carbon dioxide in air may be determined, (1) by slowly passing a large measured volume of air, freed from ammonia and water-vapour, through a series of weighed U tubes filled with caustic potash, and determining the increase in the weight of these tubes; or (2) by adding, to a measured volume of air, a known quantity of barium oxide dissolved in water, and determining the quantity of this oxide which remains when the carbon dioxide in the air has all been absorbed by a portion of the barium oxide. The chemical reactions on which these methods are based
represented in equations thus ;
(1) KOH (moist) +00,-K,CO, +1,0 + (x - 2) KOH.
in (1) remain in the weighed U tubes along with the potash (KOH) which has not been changed by the carbon dioxide (CO): the barium carbonate (BaCO3) formed in (2) is a solid, it settles down in the liquid, and the unchanged barium oxide (BaO) remains in solution and is determined by a method
which need not be described here. 114 The quantities of oxygen and nitrogen in average country
air freed from water-vapour, ammonia, and carbon dioxide,
by volume. by weight. Oxygen 20.96
23.0 Nitrogen 79.04
The quantity of carbon dioxide averages about •03 volumes per 100 vols. of air. The quantity of ammonia varies very much ; it may perhaps be taken as about 1 part in 10,000,000 parts of air, by weight. The quantity of aqueous vapour also varies with variations in the season, the district, &c. &c.
The fact that the quantities of oxygen and nitrogen in country air vary, although within very narrow limits, has been definitely established. The oxygen sometimes amounts to 20.999 vols. per 100 e.g. in air from the seashore or from inland moors; in towns the oxygen sometimes falls to 20·82 vols. ; in inhabited rooms and crowded halls it may be as little as 20:28; in mines it averages about 20.26. A decrease in the volume of oxygen is usually accompanied by an increase in that of carbon dioxide; in crowded rooms the volume of this gas may be as large as :3 to 5 vols. per 100. Air which contains as much as •1 vol. carbon dioxide per 100 is unpleasant, and harmful to health. The air of towns contains many gases, liquids, and solids, produced by the changes which go on among the living beings, and also by the manufactures conducted in the towns.
Our examination of air has afforded an application of the statements made in Chap. III. regarding the differences between mixtures and compounds; it has shewn us how we may determine to which of these classes a given substance belongs; it has also made us acquainted with some of the prominent characters of air; and it has a little familiarised us with the methods pursued in chemical inquiries.
CHEMICAL STUDY OF HYDROGEN AND OXYGEN.
Having now gained a fairly clear notion of the kind of 116 material phenomena which form the subject matter of chemistry, and of the methods by which the chemical aspects of these phenomena are investigated ; and having arrived at certain fundamental generalisations from facts established by quantitative experiments and quantitative reasoning, we are in a position to proceed with the main subject of our inquiry, which is to establish the relations which exist between changes of composition and changes of properties of the definite kinds of matter we call compounds, and the relations which exist between the properties of the elementary constituents of compounds and those of the compounds themselves. This inquiry branches out in two directions; it requires us to study (1) the properties of compounds, and the properties of elements as exhibited in their compounds; and (2) the composition of compounds. To do this we must classify; we must group together those compounds which have similar properties, and those which have similar compositions.
We shall begin our attempt to learn how elements and 117 compounds are classified, and to become acquainted with the more important results of this classification, by considering the two elements hydrogen and oxygen and some of the compounds of these elements.
Occurrence. Oxygen, as we know, forms about of the 118 atmosphere. Hydrogen is sometimes found in small quantities in volcanic gases. Numerous compounds of each element occur in nature; of these water (H,O) is the most abundant. Oxides of aluminium, iron, calcium, magnesium, silicon, and many other elements, are found widely distributed and in
large quantities. Ammonia—a compound of hydrogen and nitrogen-and compounds of ammonia, exist in the air and in the soil; and most of the substances which form the parts of plants and softer tissues of animals are compounds of hydrogen, with carbon, oxygen, and nitrogen.
Chemical properties. Hydrogen and oxygen readily combine to form water. If a stream of hydrogen is allowed to flow into oxygen, or into air, and a light is brought to the jet the hydrogen takes fire and burns in the oxygen, and water is produced. If a stream of oxygen is allowed to flow into hydrogen from a narrow tube, and a light is brought to the jet the oxygen takes fire and burns in the hydrogen, and water is produced. In each case the chemical change is the same; 2H + 0 = H,0.
Fig. 18 shews a simple arrangement for exhibiting these reactions. A and B are stoppered glass jars; each is fitted