The composition of the atmosphere has been proved, by the analyses of many chemists, to be nearly uniform in all parts of the world, excepting in regard to the quantity of watery vapour it contains, which is, of course, far less in cold dry regions than in those which are moist and warm. Pure, dry air is a mixture of 21 parts of oxygen, 79 parts of nitrogen, and a small proportion of carbon dioxide, amounting to 1 part in every 2500 parts of this mixture. In addition to the constituents above mentioned, smoke, containing compounds of sulphur as indicated by the tarnishing of silver, and ammonia, with accidental impurities derived from factories, are found near towns; and in the country at certain seasons of the year, and in certain localities, the pollen of plants and micro-organisms also impair its purity.
Fig. 201. - Sack-like Ends of a Bronchiole.
Air is 773 times lighter than an equal volume of water, and 14.47 times heavier than the same volume of hydrogen. For each degree centigrade of increased temperature it expands 1/273 of its volume. The pressure of the atmosphere on each square yard of surface is about 20,000 lbs., and as the horse presents between 5 and 6 square yards of surface, it supports a pressure of 100,000 lbs., or 50 tons.
The air is rarely or never saturated with aqueous vapour, but the quantity rises with the temperature, and is greater therefore in hot climates than in cold, and in summer than in winter. It is greater also on plains than in mountainous regions, by day than by night, and where there is much vegetation than in arid districts. In moist tropical regions it may rise to 3 volumes in 100 of air, whilst in middle Europe it does not usually exceed 1.3 per cent, and may perhaps in this country be regarded as constituting between 1 and 2 per cent by volume.