Ventilation and Heating. 113 



spread equally through the room. If a person is in one 

 corner of a large room, where there are no inlets or out- 

 lets, and no currents, as he uses the oxygen immediately 

 around him, the oxygen farther away will diffuse toward 

 him so that he will continue to get oxygen as long as there 

 is any in the room. So, 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 be renewed by diffusion. 



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 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 below, and push the lighter, 

 warmer air out above. 



Grates as Heaters. Grates are the simplest and prob- 

 ably the earliest form of heater. The fire throws out heat 

 in straight lines, or as we say, radiates heat into the room. 

 So much of the heat goes directly up the chimney that a 

 grate is very wasteful of fuel. 



Grates as Ventilators. But a grate is an excellent ven- 

 tilator. There is always a decided draft toward a grate 

 fire. This means a constant renewal of air. The air 



