VENTILATION 119 



(makes it occupy less space). Of course, expanded or 

 warm air is lighter than cold air and naturally rises, 

 cooler air rushing in to fill its place. In geography, 

 we learn that differences in altitude, the presence of 

 large bodies of water, such as lakes and oceans, and 

 other conditions continually cause differences in tem- 

 perature. As a natural result, warm air is almost 

 constantly rising and cooler air crowding in to fill its 

 place, thus causing the numerous air currents and 

 winds. 



The greater the difference in temperature, the more 

 rapidly does the exchange of warm and cold air take 

 place. When the air inside the house is about the 

 same in warmth as out-of-door atmosphere, very 

 little exchange of air is noticeable on opening the 

 doors and windows. When the difference between 

 indoor and outside temperature is considerable, 

 however, the currents of air passing in and out are 

 strong. Standing in the doorway, at such a time, we 

 can readily feel warm air passing out toward the top 

 and cold air coming in through the lower part. 



Let us see how this important law helps us to get 

 the best material for breathing. The air leaves our 

 lungs at the normal temperature of the body, ninety- 

 eight and six-tenths degrees. As the air breathed out 

 is usually warmer than the atmosphere about us, it 

 naturally rises with its poisonous carbonic acid gas 

 and watery vapor, cooler air rushing in to fill the 

 vacuum caused by its departure. On this account, the 



