120 THE WONDERFUL HOUSE THAT JACK HAS 



air in the upper part of an occupied room is likely to 

 be both warmer and less pure than that in the lower 

 part. 



We need not, then, be concerned about the air we 

 breathe out, for it quickly rises and becomes mixed 

 with the atmosphere surrounding us. The thing to 

 make sure of is that the air we breathe in is pure. 

 When out-of-doors we may, as a rule, be certain of a 

 pure supply, for the free atmosphere usually insures 

 quick diffusion of the air breathed out. When in- 

 doors, we should see that there is an opening for a 

 sufficient amount of pure air to get into a room and 

 another for the impure air to escape. The openings 

 should be so arranged as to keep the air in motion 

 and cause it to be continually changing. By such an 

 arrangement the atmosphere may be kept in good 

 condition for breathing. Replacing the foul air of a 

 room or building with pure air is called ventilation. 



But some may say: "I don't see the need of being 

 so particular. Why is it not just as well to keep on 

 breathing the same air over and over? There can't 

 be much difference between the air breathed in and 

 the air breathed out, at least we cannot see any." This 

 is one of the important things we must believe with- 

 out seeing, for chemists have discovered that the air 

 breathed out contains one-fifth less oxygen, one hun- 

 dred times more carbonic acid gas, and twice as much 

 watery vapor as the air breathed in. And this watery 

 vapor differs from that of the air inhaled, because the 



