32 VETEEINAEY HYGIENE 



The following is its volumetric composition — 



Per cent. 



Oxygen ... ... ... ••• 20'96 



Nitrogen* ... ... ... ••• 79-00 



Carbonic acid ... ... •■• ••• '^^ 



It is remarkable how uniform the composition of the 

 atmosphere is found to be in all parts of the world ; even 

 in cities it differs very slightly from the air of the country 

 or sea ; this is due to the process of diffusion, the influence 

 of winds, the effect of rain in washing the air, and the 

 action of plants. All of these are powerful influences in 

 ensuring uniformity in the composition of air. Diffusion 

 never ceases, the silent and complete mixing of different 

 gases, even though of opposite properties and different 

 weights, is constantly in operation ; the effect of wind is 

 naturally considerable, while the part played by the green 

 portion of plants under the influence of sunlight, in storing 

 up carbonic acid and liberating oxygen, is not counter- 

 balanced by their opposite behaviour during the night of 

 storing up oxygen and liberating carbonic acid. 



These purifying forces — some of which we endeavour to 

 imitate in the process of natural ventilation — explain the 

 remarkable uniformity in chemical composition of the at- 

 mosphere generally. 



Oxygen constitutes one-fifth of the atmosphere. It is 

 the active and essential gas by which life on the globe 

 is supported, and what may appear slight reductions in 

 the amount present constitute the most serious depriva- 

 tion. 



Such reductions are found in the air of inhabited build- 

 ings, or in closed courts or streets in crowded cities, 

 especially when the air is stagnant, as for example during 

 fogs. In such cases the oxygen may in the open air fall as 

 low as 20-80 per cent., and even in the absence of fogs and 

 wind, this reduction in oxygen may be found in the crowded 

 courts and yards of cities, where every conceivable form of 



* With this is included the ne-w gas Argon, -which amounts to about 

 1 per cent. 



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