172 
SECOND BOOK. 
into them. It has much less oxygen, and more 
carbonic acid ; it is also warmer, and contains waste 
matter, which the body, by its means, casts out. 
2 . It is this waste matter, even more than the 
carbonic-acid gas, that makes it so unhealthy to 
Section showing the ramifications of the Bronchi in the Lungs. A, Windpipe 
or trachea. B, Bronchi. C, Bronchial tubes. 
breathe the air of a room over and over again. It is 
the cause of the nasty stuffy smell in rooms where 
people remain without letting in fresh air. 
3. Through the pores of the skin, as well as by 
the breath, the body never ceases to pass impurities 
into the air. Therefore, we cannot have the air 
