1911 Encyclopædia Britannica/Atmosphere

ATMOSPHERE (Gr. ἀτμός, vapour; σφαῖρα, a sphere), the aeriform envelope encircling the earth; also the envelope of a particular gas or gases about any solid or liquid. Meteorological phenomena seated more directly in the atmosphere obtained early recognition; thus Hesiod, in his Works and Days, speculated on the origin of winds, ascribing them to the heating effects of the sun on the air. Ctesibius of Alexandria, Hero and others, founded the science of pneumatics on observations on the physical properties of air. Anaximenes made air the primordial substance, and it was one of the Aristotelian elements. A direct proof of its material nature was given by Galileo, who weighed a copper ball containing compressed air.

Before the development of pneumatic chemistry, air was regarded as a distinct chemical unit or element. The study of calcination and combustion during the 17th and 18th centuries culminated in the discovery that air consists chiefly of a mixture of two gases, oxygen and nitrogen. Cavendish, Priestley, Lavoisier and others contributed to this result. Cavendish made many analyses: from more than 500 determinations of air in winter and summer, in wet and clear weather, and in town and country, he discerned the mean composition of the atmosphere to be, oxygen 20·833% and nitrogen 79·167% The same experimenter noticed the presence of an inert gas, in very minute amount; this gas, afterwards investigated by Rayleigh and Ramsay, is now named argon (q.v.).

The constancy of composition shown by repeated analyses of atmospheric air led to the view that it was a chemical compound of nitrogen and oxygen; but there was no experimental confirmation of this idea, and all observations tended to the view that it is simply a mechanical mixture. Thus, the gases are not present in simple multiples of their combining weights; atmospheric air results when oxygen and nitrogen are mixed in the prescribed ratio, the mixing being unattended by any manifestation of energy, such as is invariably associated with a chemical action; the gases may be mechanically separated by atmolysis, i.e. by taking advantage of the different rates of diffusion of the two gases; the solubility of air in water corresponds with the “law of partial pressures,” each gas being absorbed in amount proportional to its pressure and coefficient of absorption, and oxygen being much more soluble than nitrogen (in the ratio of ·04114 to ·02035 at 0°); air expelled from water by boiling is always richer in oxygen.

Various agencies are at work tending to modify the composition of the atmosphere, but these so neutralize each other as to leave it practically unaltered. Minute variations, however, do occur. Bunsen analysed fifteen examples of air collected at the same place at different times, and found the extreme range in the percentage of oxygen to be from 20·97 to 20·84. Regnault, from analyses of the air of Paris, obtained a variation of 20·999 to 20·913; country air varied from 20·903 to 21·000; while air taken from over the sea showed an extreme variation of 20·940 to 20·850. Angus Smith determined London air to vary in oxygen content from 20·857 to 20·95, the air in parks and open spaces showing the higher percentage; Glasgow air showed similar results, varying from 20·887 in the streets to 20·929 in open spaces.

In addition to nitrogen and oxygen, there are a number of other gases and vapours generally present in the atmosphere. Of these, argon and its allies were the last to be definitely isolated. Carbon dioxide is invariably present, as was inferred by Dr David Macbride (1726–1778) of Dublin in 1764, but in a proportion which is not absolutely constant; it tends to increase at night, and during dry winds and fogs, and it is greater in towns than in the country and on land than on the sea. Water vapour is always present; the amount is determined by instruments termed hygrometers (q.v.). Ozone (q.v.) occurs, in an amount supposed to be associated with the development of atmospheric electricity (lightning, &c.); this amount varies with the seasons, being a maximum in spring, and decreasing through summer and autumn to a minimum in winter. Hydrogen dioxide occurs in a manner closely resembling ozone. Nitric acid and lower nitrogen oxides are present, being formed by electrical discharges, and by the oxidation of atmospheric ammonia by ozone. The amount of nitric acid varies from place to place; rain-water, collected in the country, has been found to contain an average of 0·5 parts in a million, but town rain-water contains more, the greater amounts being present in the more densely populated districts. Ammonia is also present, but in very varying amounts, ranging from 135 to 0·1 parts (calculated as carbonate) in a million parts of air. Ammonia is carried back to the soil by means of rain, and there plays an important part in providing nitrogenous matter which is afterwards assimilated by vegetable life.

The average volume composition of the gases of the atmosphere may be represented (in parts per 10,000) as follows:—

Oxygen	2065·94	Ozone	0·015
Nitrogen	7711·60	Aqueous vapour	140·00
Argon (about)	79·00	Nitric acid	0·08
Carbon dioxide	3·36	Ammonia	0·005

In addition to these gases, there are always present in the atmosphere many micro-organisms or bacteria (see Bacteriology); another invariable constituent is dust (q.v.), which plays an important part in meteorological phenomena.

Reference should be made to the articles Barometer, Climate and Meteorology for the measurement and variation of the pressure of the atmosphere, and the discussion of other properties.