58 HEAT 



Review Questions, 13. 



1. What is the color of the sun at sunrise and sun- 

 set? Why is it this color? 



2. Which is hotter, red-hot or white-hot? 



3. Why does the sun appear red when you look at 

 it through a piece of smoked glass? 



4. In what direction do plants grow if they are 

 placed near a window? Why? 



5. What is a straight line? How can you make a 

 straight line? 



6. Why do we receive more heat from the sun in 

 summer than in winter? 



7. What are the three sources of heat which we have 

 studied? 



8. What would be the result if you did not lubricate 

 the hole in the upper block in Experiment 28? 



9. What are the sparks when flint and steel are 

 struck together? 



10. What is the difference between the modern way 

 of making fire and the olden way? 



22. Heat from Combustion. 



By far the greatest amount of the heat which we 

 enjoy comes from the sun. We have seen, however, that 

 as winter draws near we receive less and less heat from 

 the sun, so that it becomes necessary for us to obtain 

 heat from' some other source. Our usual source of heat 

 is from the burning of something. Combustion is 

 another name for burning. 



There are two kinds of combustion, complete and 



