186 THE HYGIENE OF RESPIRATION 



happens, the wind is not blowing at all, there is, of 

 course, no particular reason for opening one window 

 rather than another. 



"When the air of a room is too hot, the windows should 

 be opened at the top on the leeward side of the room, 

 to allow the hot air, which tends to rise, to escape ; and 

 at the top on the windward side, to allow the cold air 

 to enter in its place. In this way, the air of a room 

 may be quickly changed and cooled, and yet the floor not 

 be chilled. 



Indirect method. With indirect heating, ventilation 

 is much easier, since the stream of air which carries the 

 heat may and should be a stream of pure fresh air from 

 the outside, which has been carefully kept from con- 

 tamination with bad air, dust, or other impurities. The 

 stream of warm pure air is best admitted near the top 

 of the room so that it w r ill not be mixed with the colder 

 impure air but will spread out and fill the upper part 

 of the room because of its greater lightness. The lower 

 stratum of air should be drawn off by fireplaces or 

 ventilating flues as large or larger than the hot air 

 flues. The openings of these flues should be placed as 

 near the floor as possible, in order to get rid of the colder 

 impure air of the floor. 



Humidity. Aside from the freshness of the air, two 

 of the most important factors in ventilation are the 

 temperature of the air and the amount of moisture con- 

 tained in it, that is, its humidity. The humidity of any 

 given amount of air depends upon its temperature. The 

 more it is warmed, the more moisture it is able to take 

 up; and, conversely, the colder it becomes, the less 

 moisture it can hold. Thus moist foggy air at 32 F. 

 becomes very dry air at 70 F. ; suddenly cooled to zero, 

 its moisture is so much in excess of what- it can hold 



