RESPIRATION. 141 



tern and impede its operations. There is a definite quantity 

 which must be carried out, and the air has a limited capacity 

 for holding water, and of taking it away. When this limit is 

 reached, and the air is saturated with water, it can take up 

 no more. The air would be saturated with moisture from 

 the lungs in about the same number of respirations that 

 would consume its oxygen ; after this, it would be useless for 

 the removal of either carbon or water. 



314. Thus we see that, in three ways, the air becomes 

 vitiated, and unfit for continued respiration : 1st, by the con- 

 sumption of its oxygen, so that it is unable to remove the 

 carbon from the blood ; 2d, by being loaded with carbonic 

 acid gas, so that it cannot take it up any longer from the 

 lungs : 3d, by being saturated with moisture, so that it can- 

 not aid in relieving the system of its superabundance of 

 water. 



CHAPTER VII. 



More Oxygen consumed, and Carbonic Acid given out, in cold and 

 dense Air, and less in warm. Air on Mountains does not sup- 

 port Life as in Valleys. Impurities in Air diminish Oxygen. 

 Feeble, consumptive, and melancholy Persons give out less 

 Carbon and Hydrogen. 



315. THE amount of oxygen received, and the quantity 

 of carbonic acid gas and watery vapor carried off, differ at 

 different times, and vary with varying circumstances. A 

 dense atmosphere is more concentrated, and, consequently, 

 contains more oxygen in a given space, than a rare one. The 

 air is more expanded in a warm than in a cool climate, and 

 in hot than in cold weather. We therefore do not inhale so 

 great a weight of air, and, consequently, so great an amount 

 of oxygen, in summer as in winter. The oxygen received 

 being less, the carbonic acid given out is diminished in the 

 same proportion. 



