252 THE HUMAN BODY. 



composition. Pure air when completely dried consists in 

 100 parts of 



By Volume. By Weight. 



Oxygen 20.8 23 



Nitrogen 79.2 77 



When breathed once, such air gains rather more than 

 4 volumes in 100 of carbon dioxide, and loses rather more 

 than 5 of oxygen. More accurately, 100 volumes of ex- 

 pired air, when dried, consist of 



Oxygen 15.4 



Nitrogen r t . 2 



Carbon dioxide 4.3 



The expired air also contains volatile organic substances 

 in quantities too minute for chemical analysis, but readily 

 detected by the nose upon coming into a close room in 

 which a number of persons have been collected. 



The Quantity of Oxygen taken up by the Lungs in a 

 Day. We have already seen that the quantity of air breathed 

 in a day is 648,000 cubic inches. This loses 5.4 per cent, 

 of oxygen or 35,000 cubic inches, weighing 12,818 grains 

 (If- Ibs.): the body therefore gains this amount of that gas 

 through the lungs daily. 



The Amount of Carbon Dioxide passed out from the 

 Lungs in a Day. This being 4.3 per cent, of the total bulk 

 of the air breathed, is 27,864 cubic inches; it weighs 14,105 

 grains or about 2 Ibs. 



We thus find that though each breath seems in itself a 



What is the chemical composition of pure air by volume? By 

 weight ? What substances does air gain and lose when once breathed ? 

 What is the composition by volume of dried expired air? What 

 does dried expired air contain besides oxygen, nitrogen, and carbon 

 dioxide? 



What bulk of oxygen docs the air breathed in a day lose? What 

 weight? How much oxygen does the body take up daily by means 

 of the lungs? 



What bulk of carbon dioxide is carried off by breathing in each 

 day ? What does it weigh ? 



