Although, doubtless owing to local conditions, trifling variations may occur in the proportion of oxygen present in the atmosphere, this variation is so trifling that the difference of the amount in air from places separated by very long distances will be found in the second decimal place only; thus, whilst a portion of air taken during a balloon ascent by Mr Green gave on analysis 20·88 per cent. by vol., Dr Frankland found in air collected by himself on the summit of Mont Blanc 20·96 per cent. by vol. A still nearer approximation in uniformity in the amount of oxygen present in atmospheric air is exhibited in the following table, which gives the results of 95 analyses by Regnault on air obtained from nine different localities:—
| 100 | from Paris gave in 100 parts, by vol. of oxygen | 20·913 to | 20·999 |
| 9 | from Lyons and around gave in 100 parts, by vol. of oxygen | 20·918 to | 20·966 |
| 30 | from Berlin gave in 100 parts, by vol. of oxygen | 20·908 to | 20·998 |
| 10 | from Madrid gave in 100 parts, by vol. of oxygen | 20·916 to | 20·982 |
| 23 | from Geneva and Switzerland gave in 100 parts, by vol. of oxygen | 20·909 to | 20·993 |
| 15 | from Toulon and Mediterranean gave in 100 parts, by vol. of oxygen | 20·912 to | 20·982 |
| 5 | from Atlantic Ocean gave in 100 parts, by vol. of oxygen | 20·918 to | 20·965 |
| 1 | from Ecuador gave in 100 parts, by vol. of oxygen | 20·960 | |
| 2 | from Pichincha gave in 100 parts, by vol. of oxygen | 20·949 to | 20·981 |
| ——— | ——— | ||
| Mean of all foregoing | 20·949 | 20·988 | |
| Mean of the Paris specimens | 20·96 |
Vapour of water is essential to the respiration of animals and plants, in order that the organs concerned in this operation may be kept in a soft and moist condition.
Carbonic anhydride is evolved during combustion, putrefaction, and fermentation; it is also a product of the respiration of animals, and highly poisonous to them, even when diluted with large proportions of air. This gas is, however, greedily absorbed by plants, which decompose it; they assimilate the carbon and return the oxygen to the atmosphere, ready to be again consumed in supporting the life of the animal world.
Dr Angus Smith has defined a very pure air to be one that contains with 20·99 per cent. of oxygen 0·30 of carbonic acid (anhydride).
This latter varies in amount in the atmosphere of cities, as will be seen upon inspection of the subjoined table, extracted from Dr Smith’s work ‘Air and Rain’:
—
| Per cent. | |
| Air of Madrid, outside the walls, mean of 12 analyses, by Luna | ·045 |
| Mean of 12 analyses, within the walls of Madrid, by Luna | ·051 |
| Mean of 14 analyses, by Angus Smith, in Manchester suburbs | ·369 |
| In Manchester streets | ·403 |
| Usual weather | ·0403 |
| During fogs | ·0679 |
De Saussure’s analyses show that there is more carbonic acid on the mountains than in the plains, as might be inferred from the comparative absence of vegetation in elevated positions. Dr Pietra Santa states that the air of hills or mountains, at the height of 2300 feet, is lighter than common air, contains a smaller proportion of oxygen, and is impregnated with a largely increased amount of aqueous vapour. It also contains a large quantity of ozone. He considers such a climate peculiarly soothing to persons suffering from chest diseases.
Dr Angus Smith’s analysis of the air from the mountainous districts of Scotland confirms the above statement of Dr Pietra Santa’s. The heaths and mountains of that country are remarkably healthy localities, and the air from them gave on analysis 20·94 per cent. by vol. of oxygen, and only ·033 of carbonic acid.