A more modern method for estimating the amounts of oxygen and nitrogen in the air, based upon the same principle, namely, the absorption of the oxygen by heated metallic copper, is illustrated in Fig. 54 (known as Jolly's apparatus). The sample of air to be examined is allowed to enter the glass globe A (whose capacity is about 100 c.c., and which has been previously exhausted) by means of the three-way cock b. (The air is first dried, by being drawn through tubes filled with pumice moistened with sulphuric acid, on

its way into the apparatus.) The bulb is then surrounded by the metal jacket B, which is filled with broken ice, and when the temperature has fallen to o° the bulb is put into communication with the tube d by means of the three-way cock. The tube g is then raised or lowered, so as to bring the mercury in d to a fixed point in the tube at m, and the tension of the enclosed air is ascertained by the graduated scale behind tube g. The ice-jacket is then removed, and the spiral of copper wire within the bulb is heated to redness by the passage through it of an electric current. The

copper combines under these conditions with the oxygen, forming copper oxide, thereby reducing the volume of the contained gas. The globe is again cooled, and the tube g lowered to such a position that when communication is once more made between the globe and tube d, the mercury shall stand at the same point #.

From the observed tension of the gas before and after the experiment, the volume relations of the two constituents can be calculated. Thus, suppose the tension of the enclosed air to be 720.25 mm., and that of the residual nitrogen 569.28 mm., then for I volume of air the reduction would be

Besides oxygen and nitrogen, the air contains variable quantities of the following gases: aqueous vapour, carbon dioxide, argon, hydrogen, ammonia, ozone, nitric acid. With the exception of aqueous vapour, these substances are present only in relatively small proportions, and with some of them the amount is liable to considerable variation. Especially is this the case with the aqueous vapour, as the amount of this constituent present at any time is largely influenced by the temperature. The average composition of normal air may be taken as follows:

The small percentage of argon present is here included with the nitrogen. The other four gases of the argon group taken together come to about 0.012 parts per 1000 (see page 270).

Aqueous Vapour.-For any given temperature there is a maximum amount of aqueous vapour which a given volume of air is capable of taking up under these conditions the air is said to be saturated with moisture at the particular temperature. Thus I cubic metre of air is saturated with moisture at the various temperatures stated, when it has taken up the following weights of

When air, saturated with moisture at say 20°, is cooled to 10°, the excess of water beyond 9.362 (the maximum for 10°) is deposited either as mist or rain. The temperature at which air thus begins to deposit moisture is called the dew-point. The deposition of moisture from the air caused by the lowering of the temperature is a matter of everyday observation. A glass vessel containing iced water becomes bedewed with moisture upon the outside as the air in its immediate vicinity is cooled. When a season of severe frost is suddenly followed by a warm wind, highly charged with aqueous vapour, it is not unusual to see condensed moisture collecting upon and streaming down the cold surface of walls. For the same reason, after the sun has set, and the heat from the ground has radiated, leaving the ground colder than the atmosphere, the temperature of the air is lowered, and it begins to deposit its aqueous vapour in the form of dew.

The amount of aqueous vapour in the air, or the humidity of the air, is estimated by meteorologists by means of an instrument called the wet and dry bulb thermometer.

Carbon Dioxide. The proportion of this gas present in the air is also liable to considerable variation, although not through such a wide range as the aqueous vapour. The processes of respiration, combustion, and putrefaction are attended by the evolution of carbon dioxide, hence the amount of this gas present in closed inhabited places is greater than that in the open air; in badly ventilated and crowded rooms the proportion sometimes rises to three parts in Icoo vols. Frankland has found that at high elevations the amount of carbon dioxide in the air is often, although not invariably, considerably above the normal.

At Chamounix (3000 feet) the amount of carbon dioxide was 0.63 per Icoo vols. Grands Mulets (11,000 feet)

Mont Blanc (15,732 feet)

I. II

This fact is probably due to the absence, in these high regions, of the vegetation which is one of the chief natural causes operating to remove atmospheric carbonic dioxide (see Oxygen, page 188).

The amount of carbon dioxide is slightly higher during the night, and often rises considerably during foggy weather. Thorpe has shown that near the surface of the sea the amount of carbon dioxide in the air is slightly less, being on an average 0.300 volume per 1000.

Ammonia in the atmosphere is derived from the decomposition of nitrogenous organic matter. Although present in relatively very small quantities, it varies in amount very considerably. From the experiments of Angus Smith, 1000 grammes of air from various sources were found to contain the following amounts of ammonia :—

The proportion of ammonia appears to be higher during the night than in the daytime, and immediately after heavy rain the amount is perceptibly diminished.

Rain-water always contains ammonia, although the amount varies greatly with changing atmospheric and climatic conditions. Lawes and Gilbert, Angus Smith, and others, have made a large number of estimations of the amount of ammonia in rain-water at various places and seasons, and under many different conditions.

Nitric Acid, present in the form of nitrates and nitrites, is produced in the atmosphere by the direct union of oxygen and nitrogen whenever a lightning flash passes through the air (see Nitric Acid). Rain which falls during or immediately after a thunderstorm is found to contain nitrates and nitrites.

These two nitrogenous compounds, ammonia and nitric acid, although present only in such small proportion in the atmosphere, fulfil a most important function in the economy of nature. From the experiments of Lawes and Gilbert, and others, it has been shown that most plants are unable to draw upon the free nitrogen of the atmosphere for the supply of that element which they require for the development of their structure and fruit.* Although they are surrounded by, and bathed in, nitrogen, they cannot assimilate it. Plants that are growing in unmanured soil, therefore, derive their * Leguminous plants, such as clovers, vetches, beans, peas, which develop root-nodules or tubercles, are exceptions.

nitrogen from the ammonia and nitric acid which are present in the air, and which are washed into the ground by the rain. It has been found that a plant grown under such experimental conditions, as to exclude the possibility of its obtaining supplies of these nitrogenous compounds, will yield upon analysis exactly the same amount of nitrogen as was originally contained in the seed from which it grew.

Ozone.--The causes which operate in the formation of this substance in the air are at present imperfectly known; it is supposed that its occurrence is related to the development of electricity in the atmosphere. On account of the powerful oxidising character of ozone, its presence can never be detected in the air where much organic matter of an oxidisable nature is present, as is the case in the air of such places as malarial swamps, dwelling-houses, and large towns.

The amount of ozone in pure country air has been found to vary with the time of year, reaching a maximum in the spring-time, and gradually falling towards winter. Thorpe has found that in sea air the amount of ozone is practically constant during all seasons.

The usual method which is available for the detection and estimation of ozone in the air is extremely crude. It consists in exposing ozone test papers (see Ozone) to the air for a certain time, and comparing the colour that is produced with a standard scale of tints; moreover, other substances than ozone, which may be present in the atmosphere, will also liberate iodine from potassium iodide, and these are therefore measured as ozone. Besides the higher oxides of nitrogen, which, as we have seen, are formed in the atmosphere, and which liberate iodine from potassium iodide, it has been shown that peroxide of hydrogen is also present. The state of our knowledge at present, therefore, respecting the exact amount of atmospheric ozone and its variation is far from satisfactory; it is, indeed, quite possible that many of the effects which have been attributed to ozone are in reality due to peroxide of hydrogen. Thus it has been shown by Schönbein that this compound is formed during the evaporation of water, and this statement probably derives confirmation from the fact that its presence may be detected in rainwater. The salubrity of the air of the sea-shore, where large areas of wet sand and stones offer the most perfect conditions for the rapid evaporation of water, and consequently, for the formation of peroxide of hydrogen, may therefore be attributable as much to the presence of this substance as to the proverbial ozone.