78 MANUAI< OF NATURE STUDY. 



is unfit for healthful respiration. If we live in 

 rooms so tight that the air cannot be changed, we 

 cannot avoid breathing again and again what we 

 have thrown from our lungs as waste matter, and 

 also the poisonous matter that is constantly pass- 

 ing through the openings of our skin. How may 

 we get rid of these poisonous gases and at the 

 same time receive plenty of pure air instead? 

 Experiment: — Make two apertures in the same 

 window by raising the lower sash and pulling 

 down the upper. Hold a burning taper first at 

 one opening and then at the other, and note the 

 result in each case. Close the window and try the 

 experiment at the upper and lower part of the 

 open door and note result. Again, close the door 

 and raise the lower sash of a window high up so as 

 to give one large opening, try the burning taper 

 at the upper and lower part of the aperture and 

 note the result. In each of these experiments let 

 the pupils describe what the air is doing, and let 

 them draw the conclusion that the foul air will 

 leave the room through any opening large enough 

 to let pure air in. The thing to be avoided is di- 

 rect draft, or wind, hence care should be taken to 

 open a door or window on a side opposite from the 

 wind. To avoid a direct draft in a sleeping apart- 

 ment, place a board about three or four inches 



