152 PHYSIOLOGY AND HEALTH. 



the amount of carbon in the nine pints of blood corresponds 

 very nearly with the capacity of the twelve pints of air to 

 carry it away. 



CHAPTER XI. 



Air spoiled by Loss of Oxygen and by Carbonic Acid Gas. Capaci- 

 ty of Air to receive Vapor. More Air saturated by Vapor of 

 Breath in cold than in warm Day. Vapor from Skin saturates 

 some Air. Amount of insensible Perspiration. Quantity of Air 

 spoiled by Loss of Oxygen, by Carbonic Acid, and by Water. 



347. BREATHING air once destroys or weakens, and, par- 

 tially at least, spoils, for the purpose of respiration, 720 cubic 

 inches of air a minute, by the mere consumption or use of its 

 oxygen. On this account, we need about one half a cubic 

 foot of air every minute. When the air goes out from the 

 lungs, it contains about four or five per cent, of carbonic 

 acid gas, ( 302, p. 136;) .but if it contains more than three 

 and a half per cent, of this gas, it is unfit to be breathed 

 again. 



348. It necessarily follows, then, that the air which has 

 been once breathed, contains about two and a half times this 

 proportion, and therefore the quantity of this injurious gas 

 that is the product of one respiration, is sufficient to corrupt 

 nearly opce and a half as much more. The quantity which 

 is exhaled in one minute would give three and a half per 

 cent, to 1800 inches, and render so much unfit to be breathed 

 again. 



349. There is a limit to the power of the air to take up 

 and carry away the watery vapor from the lungs, and this 

 limit differs with the temperature. When the air is cooled 

 down to 32, or freezing point, a cubic foot of it will hold 

 about two and a half grains of water in solution. When it 

 is raised up to 65, which is usually the proper temperature 

 of sitting-rooms, it will hold a little more than seven grains ; 

 and at 90, which is very nearly the temperature of the air 



