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 ” 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’:——