HEAT PRODUCES LIGHT 53 



2. On which side of a street is it better to live, the 

 north side or the south side? Before answering think 

 where the sun is at noon and the direction front which 

 the sunshine comes. 



3. What Is one thing we can do in order to keep 

 well ? 



4. How may we obtain light from a flame which 

 gives a large amount of heat but gives, no light of itself? 



5. If a person could be dressed in very shiny cloth- 

 ing, just like a mirror, would he be warm or cold in the 

 sunshine? Explain. 



6. What use is made of the expansion of metals? 



7. What use is made of the expansion of liquids? 



8. How do you read a thermometer? 



9. If the temperature in the morning is 48 F. and 

 at noon it is 65 F., what has been the change in temper- 

 ature? 



10. Why should we not depend upon owr feelings 

 concerning the temperature of a room? 



20. Heat Produces Light. 



In Section 16 we learned that heat is one of the 

 sources of light and that it is impossible, as far as we 

 know at the present time, to produce cold light which is 

 strong enough for ordinary use. The light which we 

 obtain from heat, however, is not always the same, for 

 we have a dull red light, a yellow light and a dazzling 

 white light. What causes the difference? Did you ever 

 see a blacksmith working at his anvil? Sometimes the 

 iron is red-hot when he removes it from the forge, and 

 sometimes it is white-hot. Now you know the difference, 



if you are near you can feel the difference. 



