13 



no expansion and therefore no escape of air, after heating the 

 bottle we shall have all the air that was in it at first, and it 

 will weigh exactly the same. As stated before, if we allow the 

 bottle to remain open, some of the air will escape, owing to ex- 

 pansion, and at the second weighing we shall be weighing only 

 a part of the air that we had at the first, and hence the weight 

 will be less. It would be better to say that heated air is lighter 

 than cool air if we take equal volumes under the same pressure. 



11. Convection cur- 

 rents. The fact that 

 warm air of a given 

 volume and pressure is 

 lighter than cold air of 

 the same volume and 

 pressure explains many 

 common occurrences. 

 Almost everyone has 

 noticed ascending cur- 

 rents of air over a radi- 

 ator or stove. These 

 currents are strong 

 enough to carry up- 

 ward light objects, such 



FIG. 13. The circulation of air 



Diagrammatic representation of the circulation 

 of air in a closed room heated by a radiator 



as bits of paper, and to 



cause pin wheels to revolve. When the air reaches the top of the 

 room, it flows along the ceiling toward the more distant parts of 

 the room, and as it becomes cooler it may descend again (fig. 13). 

 Its course may be easily traced if it is mingled with smoke. 

 Less commonly we notice that air is flowing along the bottom 

 of the room from all sides toward the source of heat (radiator 

 or stove). One may find these bottom currents very easily if 

 he will hold a burning match or candle near the floor in the 

 vicinity of a heated stove or radiator and note the direction 

 in which the flame is blown. Their course also can be traced 

 by adding smoke to the air. The air is always in circulation 



