CHAPTER IV.

AIR.

(24) General Nature of Air.

AIR collected near the surface of the earth is a mechanical mixture of oxygen and nitrogen, with a constant and a small proportion of carbonic acid, together with variable minute traces of other gases, odours, essences, particles of mineral substances, of dead and living organized matter. The air is, owing to the force of gravity, much denser near the earth, and gets attenuated layer by layer as you ascend. A coloured diagrammatic representation of the air envelope to the globe would show accurately the density if the colour was made darkest at the surface and faded gradually into space; it is now believed there is no definite limit to the atmosphere, it simply gets rarer and rarer; if it were possible to make a flight from the earth's surface through the interplanetary space there would be no single point of the passage which could be selected as absolutely destitute of the constituents of the atmosphere.

(25) Atmospheric Pressure.

Pressure of the atmosphere varies with locality; there are areas on the globe of almost constant high pressure and of almost constant low pressure. The equatorial tropical regions have a belt of low pressure towards which the trades blow south of the equator, and parallel to it there is a well-defined belt of high pressure of nearly equal breadth throughout; north of the equator there is a belt of high pressure, but it is irregular in form, in its breadth, and in its inclination to the equator. At the south and north

poles there are also areas of low pressure; that at the south pole remains pretty constant throughout the year, that at the north pole is divided into two centres, at each of which there is a diminution of pressure greatly lower than the average north polar depression. It follows then that there are three extensive areas of low pressure, viz., the equatorial belt and the regions of the pole. The average weight or pressure of the atmosphere in the latitude. of London is balanced by a column of mercury 29.905 inches at 0°; this is equal to a pressure of 14.73 lbs. on the square inch. The standard atmosphere to which, in measuring gases or calculations generally, mixtures are reduced is 760 mm. of mercury at 0° (29-922 inches); this is equivalent to 1033 kilos on a square decimetre.

(26) The Weight of a Volume of Air.

The weight of a litre of air, or any other volume, varies according to the value of gravity at the place of observation. At the latitude of Paris the weight of a litre of dry carbon dioxide free air is 12932 grms.

This is also with but slight difference the weight of a litre of purified dry air in London. The presence of carbon dioxide. and moisture in air increases its weight. Supposing the air contain 0004 per cent. of carbon dioxide, then the weight of 1 litre would be 1.2935 grm. at 0° and 76 mm. pressure; if required to know the weight per cubic foot, it only has to be remembered that 28 litres are equal to 1 cubic foot, therefore the grms. per cubic foot would be 28 times the above or 28 × 1.2935 grms. = 37 218 grms. (or 5589 grains).

The weight of air in a square mile1 is no less than 59,133,431,808 lbs., and the carbonic acid which it contains weighs 31,464,899 lbs., which is equal to 8,490,427 lbs. of carbon, or 3,790

tons.

To be able to reduce volumes of air or gas to standard pressure and temperature and to ascertain their weight, is the basis of the principles of ventilation and air movement generally, and therefore should be carefully mastered.

To reduce the volume of air at any temperature, pressure, and

1 (5280) × 144 × 14.73=59,133,431,808, and 59,133, 431,808 ×

moistness to the standard pressure and temperature we require to apply the laws of Boyle and Charles.

The law of Boyle is :-The volume of a gas is inversely as the pressure per square centimetre, so long as the temperature remains the same, which, translated into simple language, means that if a cubic foot of air is measured at 740 mm. of barometric pressure, to reduce it to the standard pressure it must be multiplied by 740° and divided by 760°.

The law of Charles is that, assuming no variation of pressure, all gases expands of their bulk at 0° for each degree of variation of temperature; in other words, 273 volumes at 0° become 274 at 1°, 275 at 2°, and so on. For example, 1,000 volumes of air at 10° will become at 20° 10353; for 273 at 10° becomes 283, and 273 at 20° becomes 293°, hence the volume will increase at the 293 293 ratio of 283 283

that is the volume

× 1000 = 1035·3. The following is then the simple rule for temperature correction of volume. As 273 plus the given temperature is to 273 plus required temperature, so is the given volume to the required volume.

In practice the two calculations are always required simultaneously—that is, correction both for temperature and pressure ; for instance, supposing it is required to reduce a volume of air at 761 mm. pressure and 20° temperature to the standard pressure of 760 and temperature 0°, the equations are combined as follows:761 × 273

= '933 760 × 293

To find the weight of moist air 32 must be subtracted from the

3p 8

height of the barometer before correcting, p being the tension or pressure of aqueous vapour as ascertained from the dew point; thus the litre of air at 0° and 760 mm. pressure (its weight when dry being 1.2935 grms.) would, when moist, weigh 1.2909 grms., or 1 cubic foot 361452 grms. (556 79 grains), because it is now really at a less pressure than 760 mm., for the tension at 0° being 46, and of 46 is equal to 17 mm. of mercury, which must be subtracted from 760, leaving 7583; then 758.3 x 12,935 1.2909 grm. A litre of air with 0004 per 760

=

E

cent. of carbon dioxide at 15° and 760 mm. pressure will weigh nearly a gramme (9417), or a cubic foot will weigh 399-4 grains.

(27) Percentage Composition of the Atmosphere.

Cavendish, who had not all the refined processes of volumetric analysis at command which we now possess, could find no appreciable difference between the percentage of oxygen in the large number of 500 analyses. He found 100 volumes of purified air to contain 20-833 per cent. of oxygen, the rest nitrogen. The most numerous and accurate volumetric analyses of air which have been made since Cavendish, have been those of Angus Smith, Bunsen, and Regnault. Bunsen found in 15 analyses of air, extreme differences of from 20970 to 20-840. Regnault found air from different parts of the world to vary from 20.940 to 20-850; country air occasionally attained 210 per cent.; the air of Paris also in one instance approached this number nearly, for he records 20-999. Angus Smith found in the most crowded parts of Perth the mean of 22 analyses to give 20-938 per cent. oxygen, while the air of the sea-shore and the heath gave 20.999.

The analysis of the air by the method of Dumas, by which the air is drawn slowly over ignited copper, is, on the whole, capable of greater accuracy than eudiometric methods, and there have been numerous analyses made by this method: the oxygen unites with the copper, forming copper oxide, and the difference in weight between the bright and the oxidized metal gives at once the oxygen, while the nitrogen can be collected in an exhausted flask, and also weighed. In this process the air is carefully dried and freed from carbon dioxide before it enters the tube.

The following are some analyses by weight, but in the second column the weights are translated into volume by dividing by the specific gravity of oxygen (1∙10561).

1 The percentage volume of nitrogen from this table can be found by subtracting the percentage volume of oxygen from 100, or by dividing the weight percentages of nitrogen by its specific gravity '97135.

From all of which it appears that the two chief

gases, oxygen and nitrogen, vary in percentage within small limits from local and seasonal conditions; the extremes in percentage volume of oxygen being from 20.5 to 21.0.1

The gases are not in chemical combination, but simply mixed together this is proved by three facts. (1) The relative amounts of the gases cannot be expressed by any chemical formula, for the proportions are neither those of their combining weights, nor of any simple multiple of those weights. When the proper proportions of the two gases are mixed together to form artificial air, there is no manifestation either of heat, electricity, or change of volume; which are the usual signs of chemical combination. (2) Air is slightly soluble in water, but the oxygen of the air being more soluble than nitrogen, the air expelled from water by boiling is oxygenized and contains nearly 35 per cent. of oxygen; this could not take place if oxygen formed a compound with nitrogen, water in that case would dissolve the same proportions of the two gases which exist in the air, and the gas collected from water by boiling after the carbon dioxide was absorbed, would have the same composition as the atmosphere. (3) The refraction of the air is precisely the mean of the refraction of oxygen and nitrogen; if air was a compound gas, the refraction would either be less or greater than the mean refraction of its constituents.

Carbon Dioxide (CO2), known more commonly under the name of carbonic acid gas, is constantly to be found in all air, and must be considered as a natural constituent. The chief sources of atmospheric carbonic acid are

1. Subterranean sources and from the soil. Poggendorf has calculated that the amount from subterranean sources is ten times

1 That is, in the open air;-the percentage of oxygen found in mines by Angus Smith occasionally reached very low figures, such, for example, as 18:6 per cent.