136 THE HUMAN BODY. 



the breath. Other matters which give a distinct smell 

 are often found in it, but not constantly. 



3, The amount of air in a single breath is about twenty 

 cubic inches, which would be a globe of air a little smaller 

 than a base-ball. We take .about eighteen breaths in a 

 minute; in an hour, we breathe about twelve cubic feet of 

 air; and in a day, about three hundred. Since we breathe 

 more than eighteen times in a minute when we are exer- 

 cising actively, probably three hundred and fifty feet 

 would be nearer the amount used in a day. This would 

 equal the entire contents of a room between seven and 

 eight feet on every side. 



4, Every breath, as it comes from the mouth, is changed 

 in four ways, 



1. It has lost oxygen. 



& It has gained carbonic-acid gas. 



s. It has gained watery vapor. 



4- It has gained a nameless animal substance. 



This breath is not fit to be breathed again. To be sure, 

 there is still oxygen in it, but not so much as in fresh air. 

 The loss of even one of the twenty-one parts in a hundred 

 of oxygen, which fresh air contains, makes air less sup- 

 porting; and if eleven parts are lost, though there are still 

 ten parts left, we could not live in it. Moreover, the 

 carbonic-acid gas has an injurious effect. 



5, The stream of air which we pour out at each breath 

 does not remain distinct, but mixes immediately with the 

 surrounding air, just as a glass of colored fluid, if poured 

 into a pail of water, will quickly diffuse itself through the 

 whole. We never breathe precisely the same breath a 

 second time, and the air of a room only very gradually 

 becomes bad when many people are breathing in it. In 



