PRINCIPLES OF VENTILATION. 139 



our houses, and take cold less frequently, than in the days 

 when we depended more on the fireplace, even if we did 

 "roast on one side while we froze on the other." Fireplaces 

 are expensive as mere heaters, but they are excellent ventila- 

 tors. If small ventilating flues could be built around the flue 

 of the main heating apparatus, and connected with the various 

 rooms of the house, air could be drawn from these rooms by 

 ascending currents created by the heat of the central smoke 

 flue. Such flues surrounding smoke flues would have the 

 added advantage of protecting the house from fire through 

 the too common "defective flue." 



The General Principles of Ventilation. 1. Diffusion. 

 Gases tend to mix. We know that if a bottle containing an 

 odorous substance be opened in a room where there are no air 

 currents the odor tends to spread equally through the room. 

 So if a person is in one corner of a large room, where there 

 are no inlets or outlets, 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 till the 

 amount of oxygen in the room is nearly exhausted. 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, by diffusion, be renewed. 



2. 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. 



3. 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 



