Fire and Heat 



63 



When the candle is burning and placed under one chimney, hold a 

 piece of burning paper or Chinese joss stick over the other. What is 

 the course of the smoke ? What difference is there in the temperature 

 of the air coming out of the box and of that going in through the 

 other chimney? Determine with a thermometer. Move the candle 

 from one chimney to the other, but do not change the burning paper. 

 What happens to the course of the smoke ? Change the burning paper 

 to the other chimney. What is the 

 result? Try the candle about the 

 center of the box, between the two 

 chimneys. Try the smoking sub- 

 stances over each chimney. What 

 is the behavior of the smoke? It 

 may be necessary to let the candle 

 burn in this position a few minutes 

 to get a characteristic result. Shift 

 by degrees the candle from the 

 middle toward one hole or the other. 



FIG. 17. Apparatus to illustrate air 

 currents in unequal heating. 



Observe the gradual change in the course of the smoke, and the tem- 

 perature of the air entering and leaving the box. 



State the relation you observe between the position of the burn- 

 ing candle and the course of the air as shown by the smoke. Does 

 the smoke enter the box with the warm or with the cold air? State 

 the relation of the temperature of the ingoing and outgoing air to its 

 course as indicated by the smoke. If the candle is not burning, will 

 there be any air circulation through the box? Why? 



Why warm air rises. The problem now is to ascertain 

 why the warmed air rises and issues from the chimney, and the 

 cold air enters the box. The experiment makes it clear that 

 as the position of the candle is changed, different parts of the 

 air are hea.ted and a different course of circulation results. 

 Temperature and air movements seem to be connected. Where 

 a given volume of air is unequally heated, currents are pro- 

 duced ; the warmer air ascending, perhaps being forced upward, 

 and the colder air descending. Why changing temperature 

 causes such circulation of the air is illustrated by the following 

 experiment. 



Exercise : Relation of volume to temperature. Fit into a thin glass 

 flask a one-hole rubber stopper, and into this insert a six to ten inch 



