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The fact that the world is surrounded by an atmosphere must have been realised by man at a very early stage in his development. A knowledge of the chemical composition of air and its influence upon living organisms, however, has only been recently acquired. At first air was regarded as a simple substance, and in the Aristotelean philosophy ranked as an element, along with fire, earth, and water.

Galileo (1564-1642) drew attention to the material nature of air by demonstrating the apparent increase in the weight of a metal globe when air is compressed into it.

The development of the chemistry of the atmosphere was somewhat delayed by the early lack of realisation that there exist various kinds of gases. At first the term " air " was applied to all gaseous substances, and not until the commencement of the seventeenth century was the difference in the nature of various gases recognised; at this time van Helmont, who introduced the term " gas," observed the divergence in the properties of gases from different sources, and as an almost immediate result carbon dioxide was accepted as a minor constituent of the atmosphere.

In the sixteenth century it was already known to alchemists that the calcination of metals is accompanied by an increase in weight, and in the seventeenth it had been noted that the air, from its behaviour, must contain a " principle " analogous to that in nitre.

The correct conception of the main constituents of the atmosphere immediately preceded the fall of the phlogiston theory, when Lavoisier (1775-1776), who first recognised the true nature of the two main constituents also effected a rough analysis. At the same period Priestley determined the amount of oxygen in the air by causing it to combine with nitric oxide, a gas which he had himself discovered; whilst Scheele, with a similar purpose, absorbed the oxygen from a measured volume of air by a solution of potassium sulphide (" liver of sulphur "). Cavendish, the first chemist to bring pneumatic chemistry to a state of accurate measurement, estimated the amount of the active constituent in air by removing it with nitric oxide and also by exploding it with hydrogen, and followed up this work by a demonstration that the inactive constituent was almost entirely homogeneous, observing that the portion of atmospheric nitrogen which could not be made to combine with additional oxygen under the influence of electric sparks did not exceed 1/120th part of the original air.

As was indicated by the earliest experiments, the atmosphere shows but little variation in chemical composition. Priestley could detect no difference between the composition of country air and air in a Birmingham workshop, and Cavendish, in 1783, obtained the ratio 20.84:79.16 as a surprisingly constant value for the relative volumes of oxygen and nitrogen. These results led several chemists to the conclusion that air is a definite compound of oxygen and nitrogen. Dalton, however, maintained that air is simply a mechanical mixture of its constituent gases; and this view was confirmed some years later when, as the result of more accurate analyses, small though decided differences were detected in compositions of air obtained from different sources. Thus, in 1846, Bunsen detected slight variations in the air at Marburg, and since that date many similar observations have been made.

For more than a century no explanation was forthcoming for Cavendish's observation that a small portion of the nitrogen obtained from air exhibits a peculiar inertness, in that it refuses to unite with oxygen under the influence of electric sparks. Indeed, the fact appears to have been entirely overlooked until Lord Rayleigh drew attention to it in 1894. The air was regarded as consisting of a mixture of oxygen and nitrogen with more or less moisture, and containing traces of carbon dioxide, ozone, and several other minor constituents. In 1893 Rayleigh published the results of a series of very accurate determinations of the densities of nitrogen obtained from various sources, and drew attention to the fact that atmospheric nitrogen invariably yielded a higher density than nitrogen obtained from chemical sources, such, for example, as by the decomposition of oxides of nitrogen, of ammonia, or of urea. His results were as follow:

Mean weight of atmospheric nitrogen contained in large globe2.31016 grams.
Mean weight of "chemical" nitrogen contained in large globe2.29927 grams.

Rayleigh satisfied himself that the density of none of his samples of nitrogen was affected by the action of the silent electric discharge; he also proved that the lightness of the nitrogen from chemical sources was not due to admixture with any known gas such as hydrogen, ammonia, or water-vapour, possessing less density than itself. From this it was evident that either the " chemical " nitrogen contained an unknown and less dense gas, or, what was more probable, that the atmospheric nitrogen was contaminated with a heavier, but likewise unknown gas.

Cavendish's experiments were therefore repeated in a more modern and refined manner, and it was found that, after sparking atmospheric nitrogen with an excess of oxygen, and absorbing the resulting oxides of nitrogen and any unattacked oxygen by suitable reagents, a residue of an inert gas was always obtained, the volume of which was proportional to the original volume of air used. In conjunction with Ramsay, Rayleigh isolated this new gas in sufficient quantity to determine many of its properties. Spectroscopic examination proved that it was not nitrogen, and as all attempts to make it combine chemically with any other known elements proved futile, the new gas was christened argon?

Soon after the discovery of argon, namely, towards the close of 1894, Ramsay was able to obtain helium in sufficient quantities to render an examination of the gas possible, by heating powdered cleveite. Up to that time helium had never been isolated; indeed, its existence was only known through its spectrum. When this gas was found to resemble argon in its remarkable chemical inertness it was thought that possibly other similar gases might exist, and liquid air was therefore subjected to careful fractional distillation whereby three new gases were found, namely, Neon, Krypton, and Xenon. The presence of helium in the atmosphere was also established. All of these latter gases, however, are only present in the air in very minute quantities.

Composition of the Atmosphere

The chemical Composition of the Atmosphere, or dry air varies slightly at different places and, indeed, at one and the same place at different times. The following may be regarded as a fair average:

GasPer cent, by Volume.Per cent, by Weight.
Carbon dioxide0.030.09
Krypton0.000005. . .
Xenon0.0000006. . .
Ammonia0.0004 to 0.0009. . .
Hydrogen<0.0001. . .
Ozone and hydrogen peroxide0.0025. . .

In addition to the foregoing, the following gases are usually present in variable but minute proportions - carbon monoxide, hydrocarbons, nitric acid, sulphur dioxide, sulphuric acid, hydrogen sulphide, mineral salts, organic matter. The amount of water-vapour in the air is extremely variable.

The mean composition of Paris air, freed from carbon dioxide and water-vapour, is given by Leduc as follows:

GasPer cent, by Volume.Per cent, by Weight.

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