490 THE BLOOD. 



the air is moist, than if very dry. There are many impurities 

 always floating in the air; but thanks to constant currents, 

 and purifying changes, the atmosphere is preserved in a con- 

 dition in which it is respirable. Thus it usually contains 4.5 

 per ten thousand parts of carbonic acid, which is derived from 

 the processes constantly going on in nature, of combustion, 

 putrefaction, and animal respiration. Ammonia is a very 

 constant compound floating in the atmosphere, but like the 

 carbonic acid, actively absorbed by plants. It is the oxygen 

 which maintains life, and is diluted for the purposes of 

 respiration by the large proportion of nitrogen entering into 

 the composition of air. A part of the oxygen is in the form 

 of ozone. 



Air that issues from the lungs of an animal is found to 

 have lost from 4 to 5 parts of oxygen, and to have acquired 

 a corresponding quantity of carbonic acid and water. In 

 man not less than seventeen and a-half cubic feet of oxygen 

 disappear out of 350 cubic feet of air, consumed in 24 

 hours. This amounts to more than one pound avoirdupois, 

 or 7.134 grains by weight. Herbivora consume about 10 

 per cent, of the oxygen they inhale, and carnivora from 20 

 to 25. 



Carbonic acid is formed rapidly, and in man it is calcu- 

 lated that 13 cubic inches of this gas are produced in an 

 ordinary respiration. 



Air that has acquired 10 per cent, of carbonic acid is 

 irrespirable, and it is estimated that a man contaminates in 

 breathing, 130 cubic inches of air per minute. Four cubic 

 feet, however, should be allowed to each man by ventilation, 

 viz., 50 times the actual amount consumed. 



According to Lassaigne's calculation, the following are 

 the quantities of carbonic acid produced by the different 

 animals : 



