RESPIRATION. 99 



and over again would soon destroy life. " The Black Hole of 

 Calcutta" was a dungeon in which there were only two 

 narrow windows. Here one hundred and fifty-six English 

 soldiers were shut up, with scarcely room enough to hold 

 them. At the end of eight hours only twenty-three remained 

 alive. During a storm at sea, a captain ordered the hatches 

 closed, and in six hours ninety of the passengers were dead. 

 The high diffusive power of gases under ordinary conditions 

 prevents such calamities. The carbonic acid gas spreads 

 rapidly throughout the surrounding air, so that if the space 

 in which we are breathing is not too confined, or too tightly 

 closed up, it quickly becomes so diluted as to do no harm. 

 To prevent its too great accumulation in the atmosphere 

 .there is a wise provision of nature by which plants and 

 trees take in carbonic acid as we do oxygen, and give out 

 oxygen as we do carbonic acid. In some places carbonic 

 acid gas is found in such large quantities that it is not 

 readily diffused or used up, and collects in great volumes. 

 We find this at the bottom of old wells, over fermentation 

 vats, or in the "choke damp" of coal mines. 



It is estimated that from twenty to thirty cubic inches of 

 air enter the lungs at each breath, or from three hundred to 

 four hundred cubic feet in twenty-four hours. By a knowl- 

 edge of these facts, it may r jadily be determined how much 

 cubic space is required for school-rooms, churches, and other 

 public buildings. 



It has been observed that unbreathed air containing the 

 same percentage of carbonic acid as the vitiated air of a close 

 living room, is not so poisonous in its effects upon the system. 

 It is evident, therefore, that it is important to get rid of the 

 organic matter coming off from the lungs and the skin. 



In cities and towns there are other impurities in the air 

 which are dangerous to health. Sewer-gas, poisoned air from 

 cess-pools and drains, the impure air from manufacturing 



