68 Introduction to the Study of Science 



SUMMARY 



Fire was one of the most important discoveries in primitive times ; 

 by its aid men are steadily increasing their control of nature. 



Our natural source of heat is the sun. Our main controllable 

 source is the chemical process of combustion. The chief fuels are 

 coal, wood, petroleum, and gas. 



Fires may be kindled without matches by friction. The use of 

 matches reduces the amount of friction needed by using substances of 

 low kindling temperature. 



Drafts are convection currents due to temperature changes and 

 consequent changes in the density of the air. 



The control of draft by dampers or its increase by blowers and 

 exhaust fans help in controlling the temperature of fires and the rate 

 of combustion. 



The sun is the ultimate source of all forms of energy that we use. 

 To it is due the energy of the wind and of falling water, the heat from 

 fuel, and the electric current. 



Direct use of the sun's energy is made in many ways, such as ii? 

 agriculture, in heating and lighting buildings, in preserving foods, 

 and in operating solar motors. 



REVIEW QUESTIONS 



1. What r61e has fire played in the history of civilization ? 2. Men- 

 tion several important ways in which man's use of fire enables him t<* 

 gain control of the world. 3. What other inventions are to be com- 

 pared with fire in importance to man ? 



4. Why is heat from artificial sources necessary? 5 Name the 

 most important common fuels used in different parts of the world. 

 6. Mention and describe other means of producing heat besides burn- 

 ing fuel. 



7. Mention several methods of kindling a fire. 8. Describe typical 

 ways of primitive peoples. 9. What are the essential characteristics 

 of matches for common use? 10. Describe the making of matches, 

 mentioning materials and purposes of each. 11. Explain the meaning 

 of kindling temperatures. 12. What use is made of the knowledge 

 of kindling temperatures in making matches? In starting fire? 

 13. What use is made of this knowledge in selecting substances for 

 clothing and buildings ? 



14. Describe an experiment which illustrates the behavior of air 

 in a chimney. 15. What are the effects of heat upon air? 16. What 

 is the effect of rise and fall of temperature upon all gases? 17. By 



