PURE AIR 103 



bonic acid gas is produced as well as heat and energy. 

 Now, as this gas is poisonous, it is just as necessary 

 that the body should get rid of what is produced by 

 the combustion of its tissues as it is that the air 

 breathed should contain no more than its normal 

 amount. How the body supplies every part of its 

 tissues with the life-giving oxygen and at the same 

 time rids itself of the poisonous carbonic acid gas, is 

 a wonderfully interesting story. 



But before we begin this story, let us devote a mo- 

 ment to the remaining component of the air, the watery 

 vapor. Although some is always present in normal 

 air, the amount varies with the locality and depends 

 considerably upon temperature, warm air holding a 

 much greater amount than cold air. Any observing 

 boy or girl knows how quickly the air takes up, or 

 evaporates, water. Who has not seen the damp tennis- 

 court or ball ground soon get into fine condition for 

 play, when the wind was blowing plenty of air over it, 

 even if there was not a single ray of sunshine? This 

 is because the air is like an exceedingly large sponge. 

 Heat expands it and makes it able to hold more 

 water, while cold contracts it, squeezing the water out 

 in the form of rain, dew, frost, or snow. 



Let us see how this thirsty habit of air is of wonder- 

 ful service to our bodies. That our skin contains 

 millions of tiny pores is a well known fact. When 

 we exercise, sweat, or perspiration, comes out of 

 these pores. Now all the time these tiny tubes 



