RESPIRATION AND ENERGY IN MAN 137 



changed frequently. The air that has been once used must be 

 removed and a fresh supply must be furnished; this is what is 

 meant by mn-ti-la'iion. 



197. Methods of ventilation. It is important to re- 

 member that fresh air is not necessarily cold air, and that 

 draughts of air in a room are not required ; indeed, that they 

 are undesirable. The problem of ventilation is that of fur- 

 nishing a sufficient quantity of wholesome air of the proper 

 temperature and moisture and of removing the foul air. It 

 is evident that this is rather difficult to accomplish in school- 

 rooms or in public halls. Air will not of itself circulate 

 rapidly enough, and so it has to be forced into these rooms 

 by large blowers or revolving fans in the basement. This 

 air should be filtered and moistened. Hot-air pipes or fans 

 are likewise often employed at the top of the ventilating 

 flues to draw out the foul air. Since warm air is lighter than 

 cool air, the former should enter a room near the ceiling. As 

 it cools it gradually settles toward the floor, and the openings 

 into the ventilating shafts should be found at the lower part 

 of the room. If the system works properly, there will be 

 a continuous supply of clean, warm, moist air, and at the 

 same time the air that has once been used will be drawn off 

 through the flues. 



Unfortunately, in most of our dwelling houses, little pro- 

 vision has been made by the builders for proper ventilation. 

 Hence, if the rooms are heated by steam, we frequently 

 breathe the same air over and over. This may be obviated, 

 however, by ventilating in the following way. A piece of 

 board two or three inches wide should be fitted across the 

 lower end of the window opening. When the lower sash 

 is pulled down upon it, a space is left between the upper and 

 lower sashes, through which fresh air may enter the room 



