WHAT ARE THE CHARACTERISTICS OF HEAT? 109 



STORY TEST 



DOES JANE KNOW How TO PRODUCE HEAT? 

 Read carefully and critically. List all the errors and suggest corrections. 



This is the way I learned to start a fire when I was a scout: 

 Get a lot of dry leaves and lay them on some sticks of dry wood. 

 If you have paper with you, crumple it and use it with the leaves. 

 Lay large logs loosely over the mass o burning leaves. Substances 

 with a low kindling point like phosphorus will take fire at such a 

 low temperature that it is unsafe to hold them with the bare fingers. 

 You can set fire to a thin shaving of pine easier than you can to 

 a dry pine log because the shaving has a lower kindling temperature. 

 A cold metal may extinguish a candle flame because it radiates 

 heat so fast it cools the candle wax below its kindling point. In 

 lighting a match, the first energy used is chemical, then heat and 

 finally light result. There are only three changes of energy 

 involved. 



PROBLEM II. WHAT ARE SOME OF THE 

 CHARACTERISTICS OF HEAT? 



When we have a bonfire, we see that fresh air comes in 

 to the fire, while smoke and gases rise from the fire. Such 

 currents of air are called convection 

 currents. 



cold- -Vater- 



Demonstration 2. Convection. 



Fill a battery jar with cold water. Fill a small 

 bottle with hot water colored with red ink ; place 

 in it a perforated cork containing two tubes, as 

 shown in the diagram. Lower the small bottle 

 of hot water to the bottom of the jar of water. 

 What happens? Put in your workbook a dia- 

 gram showing the movement of the water. 



The movement of the liquid shown in this experiment 

 is called convection. We thus see that convection takes 

 place in liquids, and a bonfire shows that it takes place 

 in gases. Since rising air or water is hot, heat is carried 

 by it. What makes the heated water rise? This rise is 

 caused by the fact that warm water is lighter than cold 



