SOURCES OF LIGHT 33 



10. Why is the sky blue? You have seen the sky 

 when it is nearly white; what might cause a clear black 

 sky? 



16. Light from Sources Other Than the Sun. 



The sun is by far the best source of light. There 

 is no light so powerful, and none so beneficial to plants 

 and animals. Most animals, except those that prowl at 

 night, go to sleep soon after sunset; but man has long 

 been accustomed to staying awake much later. The need 

 of light was first met by the use of campfires. If a per- 

 son wished to leave the campfire he would remove a burn- 

 ing stick or fire-brand, and carry it with him. Thus the 

 torch was a natural outgrowth from the campfire. Later, 

 man learned that some kinds of wood, or wood which had 

 been soaked in grease, made better torches than a fire- 

 brand taken from the campfire. Still later, man made 

 lamps in which wicks burned in grease or oil. The flame 

 was uncovered and was smoky and dim. The candle was 

 a much more modern invention and, as you know, is still 

 used. The modern oil lamp, having the flame covered 

 with a chimney was a vast improvement over the ancient 

 oil or grease lamp. The burning of illuminating gas was 

 the next advance in lighting and was soon followed by the 

 electric light. 



In all of the methods of producing light, except by 

 electricity, we obtain more heat than light in all cases 

 where light is obtained from burning a material. Even 

 the electric light produces more heat than light. 



Experiment 19. Ordinary Sources of Light. 



Apparatus: Oil lamp, Bunsen burner, gasoline 



Elem. Sci 3 



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