CHAPTER IV, 



HEAT AND ENERGY 



SECTION I 



HEAT 



74. Sources of Heat. The sun is a most important 

 source of heat on earth, for without its rays the atmos- 

 phere would be intensely cold and we should not have 

 the supplies of wood, coal, and oil which are used 

 as fuels. Other sources of heat are illustrated in the 

 following experiments. 



Experiment 57. Using a convex lens, focus the sun's rays 

 upon a piece of tissue paper for a moment (139). Note their 

 effect on the paper. Name other examples of the heating effect 

 of the sun's rays. 



Experiment 58. Friction. File a soft iron nail for a moment 

 and then feel of the filed surface. Saw through a piece of wood 

 and feel of the saw. Rub a metal button on a smooth piece of 

 cloth. Name any examples of bodies heated by friction. The 

 bearings of car wheels often become very hot. Why ? 



Experiment 59. Percussion. Hammer a small piece of lead for 

 half a minute and feel of it. Repeat this, using a soft iron nail. Did 

 you ever pick up a rifle bullet that had just been flattened by strik- 

 ing an iron target ? Think of other cases of heating by percussion. 



Experiment 60. Compression. Pump air into a bicycle tire 

 for a few moments and then feel of the pump. Can you discover 

 evidence of heat being developed by compression ? 



Experiment 61. Chemical Action. Pour a little hydrochloric 

 acid upon bits of zinc in a test tube. Very carefully and slowly 



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