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too, the gases that he breathes out will not remain confined 

 to the space directly about him, but will spread nearly 

 evenly throughout the room. The same takes place in the 

 open air, without wind. So, then, if the windows and 

 doors are open, the air of the room will, by diffusion, be 

 renewed. 



Wind. Motion of the air renews faster than mere dif- 

 fusion. Strong wind forces its way through the cracks 

 around windows, and when windows are open on opposite 

 sides of a room there is usually enough breeze to renew the 

 air. But during the greater part of the year this cannot be 

 done. 



Artificial Renewal of the Air. The renewal of the 

 air in most cases depends on the fact that heated air rises. 

 Heat expands air. It is then lighter, bulk for bulk, than 

 cooler air. The heavier surrounding air presses the lighter 

 air upward. If there are outlets above and below, the 

 heavier, colder air will press in at any opening left below, 

 and push the lighter, warmer air out above. 



The Common Stove. In the case of the common stove 

 we very well know that there are currents of heated air 

 rising above the stove. Children make whirligigs and 

 various toys to place in these up-currents above stoves. 

 Air is, at the same time, flowing toward the stove along 

 the floor and lower part of the room. Cold air can usually 

 be detected entering around the windows and doors, which 

 presses downward and toward the source of heat. The 

 stove does not do much to renew 'the air in the room 

 except in this general way ; some heated air escapes at 

 openings in the upper part of the room, and some is passed 

 out through the stove, taken in as a draft. But in the 



