78 
MANUAL 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- J 
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 
