16 ELEMENTS OF GENERAL SCIENCE 



this is true only under certain conditions. For instance, if 

 we put a stopper in the bottle so that there can be no expan- 

 sion 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 

 expansion, 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. 



15. 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 currents of air over a radiator or stove. 

 These currents are strong enough to carry upward light 

 objects, such as bits of paper, and to cause pinwheels to 

 revolve. When the air readies 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. 15). 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 



FIG. 16. The circulation of air 



Diagrammatic representation of the circulation 

 of air in a closed room heated by a radiator 



