34 VETEEINAEY HYGIENE 



are not found, in spite of the enormous amount of impurity 

 contributed by towns and cities, yet small variations are 

 very common, even when the samples of air are collected 

 at the same spot and within a few minutes of each other. 

 There is also seasonal variation; it has been found that 

 the carbonic acid in the air is lowest in the winter, highest 

 in the autumn, but though it rises during the spring there 

 is a diminution in the summer. 



All chlorophyll-containing plants are capable of utilizing 

 the carbonic acid of the atmosphere, storing up the carbon 

 and liberating the oxygen. It is true this process is 

 reversed at night, but as a means of keeping the atmo- 

 spheric carbonic acid uniform it cannot be without value. 



Ammonia is found in the air in minute traces, not 

 as a normal constituent of the atmosphere, but as the 

 result of the decomposition of nitrogenous products ; it 

 is found most largely in the air of manufacturing towns 

 and cities, and diminishes after rain as the result of air 

 washing. 



Watery Vapour exists in the proportion of '8 to 1 per cent., 

 but the amount existing in any sample of air depends upon 

 the temperature. If the air has taken up as much vapour 

 as it is possible to hold at that particular temperature, it is 

 said to be saturated, but this is seldom the case. From 

 65 to 75 per cent, of saturation is supposed to be the best 

 ratio for health. 



Organic and Suspended Matters in air are really impurities, 

 but are so commonly associated with it that it is impossible 

 to separate them from a consideration of normal air. 



The suspended organic matter consists of fungi, bacteria 

 of many kinds, spores of lower organisms, vegetable debris, 

 pollen of grasses, and in towns and cities minute portions 

 of vegetable matter from dried horse faeces. Besides these, 

 morganic matter such as sand particles, soot, etc., are 

 frequently present. 



The significance of organisms in the atmosphere is not 

 great. Moulds are less frequently found than bacteria, and 

 there are fewer bacteria in the air on damp and still days, 



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