CHAPTER VII 

 HEAT 



When man discovered fire his civilization began. Until 

 then he had lived near drinking water, had eaten what 

 wild fruits and roots appealed to his taste, and had de- 

 voured raw such fish and flesh as he could catch. For 

 shelter he had depended on caves, and for warmth on the 

 sun. He was bound fast to places where he could get 

 these things. 



His discovery of fire, perhaps from a volcano or from 

 lightning striking a tree, opened a new world to him. He 

 cooked his food and made it taste better. He heated his 

 shelter, driving out winter cold. He baked clay for uten- 

 sils and, by burning, hollowed out a log for a canoe. 



In time, he discovered that heat would melt certain 

 rock-like substances and give him metals. From metal 

 he made tools with which to cultivate the soil, weapons for 

 hunting and defense, and machines to do work. Gradually 

 he made clothing, and better shelters, and, later, vessels 

 that would move regardless of wind. And then, when 

 from iron he made steel, the modern machine age began. 



Fire and heat, then, play a tremendous part in our 

 lives. They are wonderful as servants, but terrible in 

 destructive power when out of control. That we have 

 them is due to the sun, for it is the sun which, through 

 the ages, has been providing the energy found stored in 

 coal, oil, natural gas, and the wood of the forests. 



Heat is another form of energy. We have learned that the 

 sun is the source of all energy ; therefore heat produced by burn- 

 ing wood or coal came originally from the sun. This energy was 

 stored in the coal and the wood when they were in the form of 

 growing plants. Again, if we rub our hands together, or rub the 

 hand against a table top, heat is produced. This heat energy came 

 from the sun to our food and from the food to our muscles. Our 

 muscles change this energy to the energy of motion and this 

 causes the friction which produces the heat. 



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