334 BESPIEATION 



necessary that the air in the rooms in which we hve be renewed 

 frequently. If this air is not constantly renewed the lungs are 

 forced to inhale air which has lost part of its oxygen, and the pre- 

 liminary stages of asphyxiation are soon reached. A feeling of 

 dullness, labored breathing, and headache are the warnings which 

 tell us of the need of change of air in a close room, and these warn- 

 ings should be heeded. All schoolrooms and public halls are 

 now provided with systems of ventilation, that is, apparatus 

 forcing in new air and removing the old. Houses not provided 

 with such systems should have the windows opened frequently, 

 and the air should be allowed to circulate freely until the renewal 

 is secured. During sleep the demand for oxygen is even greater 

 than in the waking hours, since that is the time for the restoration 

 of exhausted tissue. No one should sleep in a room whose win- 

 dows are tightly closed and the means of ventilation thus cut off. 



The basis of all ventilation methods is found in the fact that 

 cool air is heavier than warm air. For example, the air in a room 

 is usually much cooler at the floor than near the ceiling. If, now 

 we provide an exit for the warmer air at the top of the room, 

 and an entrance for the outside air, the outside air will rush in and 

 the warm air will be forced out. Thus the supply for the room will 

 be renewed. Precaution against draughts is important, since the 

 sudden chilling of any part of the body may produce colds. The 

 greater the difference in temperature between the outside and 

 inside air the more rapid the flow, hence in winter much smaller 

 openings are necessary for ventilation than in summer. 



All furnace-heated houses should be provided with cold air 

 boxes, so that the air in the pipes may receive constant renewal 

 of oxygen as it circulates. 



Air in a room cannot be changed oftener than three times in 

 an hour without producing dangerous draughts. On that account, 

 schoolrooms where fans are used to force air into the room should 

 have this rate carefully regulated, and also should not be so 

 crowded as to make a more frequent change necessary. 



Dusting. — We have seen that the approaches to the lungs are 

 weU provided with apparatus for removing dust from the inhaled 

 air. Such dust irritates the delicate membranes of the lungs if 

 allowed to come in contact with them and may also contain disease 



