150 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



Ventilation. The ventilation of ordinary dwelling rooms is 

 easily possible because of the way in which gases become heated. 

 The air in a room becomes warmed and rendered impure at the 

 same time. Consequently there is a tendency for the vitiated 

 air to rise, and if a suitable place near the ceiling is made for its 

 egress as well as a place near the floor for the colder, purer, air 

 from outside to enter, we shall have a continuous circulation of 

 air set up which will keep the atmosphere of the room pure and 

 sweet. 



EXPT. 148. Slightly open the door of your room, and hold a 

 lighted candle (a) near the bottom, (6) at the middle of the open- 

 ing, (c) near the top. How is the light affected ? (Fig. 67). 



Fir;. 67. To illustrate Experiment 148. 



FIG. (58. How to ventilate. 



EXPT. 149. Place a short piece of candle in a saucer, light it, 

 put a lamp glass over it, and pour sufficient water into the saucer 

 to cover the bottom of the lamp glass (Fig. 68). Watch how 

 the light of the candle is affected and describe what happens. 

 Next cut a strip of card less than half the height of the Lamp 

 glass, and nearly as wide as the internal diameter of the top. 

 Insert the card into the lamp glass so as to divide the upper 

 part into two halves. Now light the candle again, and see 

 whether it will burn with the divided chimney over it. 



Test the direction of the currents of air at the top of the 

 chimney by holding a smoking taper or match over it. 



