INORGANIC ELEMENTS. \7 



respiration would be practically unbearable. This condition 

 is doubtless due to the traces of organic products thrown 

 off in the breath and perspiration, and especially to the decom- 

 position of organic matter on the unclean skin. The carbon 

 dioxide is often made the approximate measure of the con- 

 tamination of inhabited rooms, because of the practical diffi- 

 culty of measuring anything else. 



In respiration the air is modified about as shown by these 

 figures : 



Inspired Air Expired Air 

 Per Cent. Per Cent. 



Nitrogen, argon, etc 79.0 80.0 



Oxygen 21.0 16.0 



Carbon dioxide 03 4.0 



The amount of oxygen inhaled each day by a full grown 

 man is not far from 500 liters, while the volume of carbon 

 dioxide exhaled is somewhat less, about 450 liters in the 

 mean. Later something will be said about the numerical rela- 

 tion existing between the volume of carbon dioxide eliminated 

 and the volume of oxygen absorbed. 



The most accurate method of finding the amounts of 

 aqueous vapor and carbon dioxide in the air is to aspirate a 

 measured volume through a series of weighed absorption 

 tubes. The first of these contain dry granular calcium 

 chloride or some other good water absorbent, while the fol- 

 lowing tubes contain soda-lime or a strong potassium hydrox- 

 ide solution to absorb the carbon dioxide. The increase in 

 weight of the tubes shows the amount of vapor and gas 

 absorbed from the given volume of air. For quick determi- 

 nations somewhat less exact methods are often used in practice. 



The atmosphere often contains traces of other gases, as 

 ammonia, sulphurous oxide, oxides of nitrogen and ozone, 

 which are of little physiological importance and need not be 

 here considered. Of greater importance are the minute 

 organized forms everywhere present to some extent, at least, 

 and which include bacteria and many other agents of putre- 

 faction and fermentation. Most of these are practically harm- 



