HEAT AND LIGHT 



47 



Mechanical energy frequently changes into heat, as when 

 two surfaces are rubbed together, but when these two 

 kinds of energy are carefully measured there is found to 

 be no loss. This great truth has been determined by a 

 vast amount of most careful investigation and is called 

 the law of conservation of energy. 



23. Heat and Light. Every one realizes the importance 

 of the heat and light given to the earth by the sun. If 

 plants or animals are where light is entirely excluded, 

 they begin to sicken and die. If they are placed where 

 it is very cold, they freeze and die. Although the sun 

 gives both heat and light, yet these two are 

 not inseparable. We feel the heat given out 

 by boiling water but there is no light, and 

 we see the light of the moon but there is no 

 heat. We usually say that we feel heat but 

 cannot see it and see light but cannot feel it. 



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24. Heat. Experiment 17. Fit a glass 



with a one-hole rubber stopper through which passes 

 a glass tube about 20 cm. long. Place this on a ringstand so that 

 the end of the tube extends down into a bottle nearly filled with 

 water. Gently heat the flask. The air expands 

 and bubbles rise in the water. When the flask 

 cools, the air contracts and water rises in the tube.' 



Experiment 18. Fill the flask used in the last 

 experiment with colored water. See that the end 

 of the glass tube passing through the rubber stopper 

 is just even with the bottom of the stopper. Smear 

 the lower part of the stopper with vaseline and in- 

 sert it in the flask, being careful that the flask and 

 a few centimeters of the tube are filled with colored 

 water and that there are no air bubbles in the 

 flask. Mark, by slipping over a rubber band, the 

 column in the tube. Heat the flask. The 



Fig. 21. 



end of the water 



water expands. Why do water heaters always have a pipe at the 



top leading to a tank? 



