72 



THE SUN'S GIFT OF HEAT 



tubes were found to have been heated by the chemical ac- 

 tion which took place, but no combustion occurred. 



But chemical action is only one of the sources of heat. 

 Every Boy Scout is taught to make a fire by rubbing two 



pieces of dry wood together. 

 (Figure 32.) He knows that 

 friction is a method of produc- 

 ing heat; or to state it another 

 way, the mechanical energy of 

 rubbing is transformed into heat 



ment 157 that electrical energy 

 can be changed into heat energy. The change of chemical, 

 mechanical, and electrical energy into heat energy are the 

 three ways in which we produce heat. 



Kindling Temperature. We have found by experience 

 that a certain amount of heat is necessary to get things to 

 burn. Two sticks have to be rubbed until they are very 

 hot before they take fire. We use kindling to get large 

 pieces of wood and coal hot enough to burn. Everything 

 has to be brought to a certain temperature before it will 

 take fire. This temperature is called the kindling tempera- 

 lure. 



The kindling temperatures of different substances vary 

 greatly. The kindling temperature of phosphorus is a 

 little below the temperature of the human body, and phos- 

 phorus is therefore a dangerous thing to handle. The 

 kindling temperature of iron is many hundreds of degrees. 



Certain substances very readily unite with the oxygen of 

 the air at ordinary temperatures and, by so doing, of course 

 produce -heat. If the heat thus produced does not escape, 



