138 PHYSIOLOGY AND HEALTH. 



CHAPTER VL 



Air changed by Respiration unfit to be breathed again. Dyer col- 

 ors with Dye of full Strength. Air should have full Proportion of 

 Oxygen. Respired Air, loaded with Carbonic Acid Gas, can take 

 away no more. Air saturated with Water can take no more 

 from Lungs. Air spoiled for Respiration in three Ways. 



305. AN examination of the air, after it has passed out of 

 the lungs, shows that it is very different from the same air 

 before it went in. At first, it had about twenty-one per cent. 

 of oxygen, seventy-eight per cent, of nitrogen, and one per 

 cent, of carbonic acid ; but, when it has been respired, it has 

 lost about one fourth of its oxygen, and has gained carbonic 

 acid and vapor, in proportions varying with many circum- 

 stances connected with the state and health of the animal 

 system. If, then, it is necessary for the blood to consume at 

 each respiration one fourth of the oxygen of the air which 

 is inhaled, it would follow that, if the same air be breathed 

 twice, one half of the ox'ygen would be consumed ; and if 

 breathed three times, three quarters; and if breathed four 

 times, all would be consumed. If so much oxygen is not 

 consumed, there must be so much less of the carbon and 

 the waste of the blood carried away. 



306. But, if the Jungs consume only one fourth of its oxy- 

 gen at each respiration, it by no means follows that the air 

 can b breathed four times over, and at each time have the 

 same effect in purifying the blood. In respect to relieving 

 the blood of the carbon, the oxygen may be considered as 

 the strength of the air. When this constitutes twenty-one 

 per cent., it is of full strength; when it is only fifteen per 

 cent., it is only three quarters of full strength ; and, at most, 

 it can have only three quarters of the due power. So, when, 

 after being once respired, it is reduced by another and 

 another respiration to one half and one quarter its full 

 strength, it, of course, has so much less power of performing 

 that which is required of it. 



