150 



GENERAL SCIENCE 



B 



we live be renewed frequently. All modern schoolrooms 

 and public halls are now provided with systems of ventila- 

 tion or apparatus to force in fresh air 

 and remove foul air. The basis of all 

 ventilation methods is found in the fact 

 that cold air is heavier than warm air. 

 The air in a room is usually colder near 

 the floor than near the ceiling. 



If a house is heated by stoves or 

 fireplaces, no special provision for ven- 

 tilation is necessary; but where other 

 systems 6f heating are used, some at- 

 tention should be paid to the question 

 of proper ventilation. A simple experi- 

 ment may be performed to illustrate the 

 way in which convection operates to 

 ventilate our houses. 



Iron. 



FIG. 130. Ther- 

 mostat whose ' Action 

 Depends upon . the 

 Unequal Expansion 



Place a lighted of Two Metals - 

 candle in a shallow vessel containing a- little water. Now 

 bring a tall lamp chimney down over it (Figure 131). 

 The flame will slowly die down and finally go 

 out. Why? If the chimney is raised a slight 

 distance above the water, the dying flame will 

 brighten at once. Why? If a thin metal or 

 cardboard partition is lowered in the chimney 

 almost to the flame, the latter will burn con- 

 tinuously although the bottom of the chimney 

 be under water. The reason will be evident 

 if a piece of burning torch paper is held near 

 the chimney. 



In order to secure satisfactory ventilation 

 it is estimated that a room should be supplied 

 with a minimum of thirty cubic feet of fresh air per minute 

 for each person. If the speed at which the fans drive 



FIG. 131. 



