CHAPTER XIV 



HOW TO HAVE THE BEST POSSIBLE VENTILATION 



WE live at the bottom of a mass of atmosphere many 

 miles high and so heavy that its pressure is estimated 

 at fifteen pounds to every square inch of surface. 

 Birds fly in the air as readily as fish swim through the 

 deepest water, and we move about in it easily, because 

 the pressure on all sides is equal. Two important laws 

 that insure the right supply of oxygen have already 

 been considered. The first was the law of diffusion 

 of gases so that nitrogen, oxygen, and carbonic acid 

 gas keep at a fixed per cent. The second was the 

 absorbing of carbonic acid gas and the giving out of 

 oxygen by plants. A simple experiment will help 

 us to understand a third law that is also essential to 

 our comfort and health. 



If we blow a bladder or toy balloon partly full of 

 air, and, after closing its opening, hold it near a hot 

 fire, the sides will swell out just as if more air had been 

 blown in. This is because heat expands air (makes 

 it fill more space). But if the balloon is blown full 

 of air and then held in a cool place, after its opening 

 has been tightly closed, it will shrink in size, just as 

 if air had been let out. Thus we see cold contracts air 



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