48 



FIRST TEAR SCIENCE 



Fig. 22. 



Fig. 23. 



Experiment 19. Pass the ball of a ball-and-ring apparatus through 

 the ring. Notice how closely it fits. Heat the ball in a Bun sen flame 

 for several minutes. See if the ball will now go 

 through the ring. Explain why it does not. 



Experiment 20. Heat a metal compound-bar. It 

 bends over on one side. The more the bar is heated 

 the more it bends. The two metals do not expand 

 at the same rate. Why are the ends of steam pipes 

 allowed to be free and not attached firmly ? Why are 

 the ends of the spans of long 

 iron bridges placed on rollers? ^ : 

 When iron tires are fitted to 

 wheels they are heated and then 



placed on the wheels and allowed to cool. Why ? 



Platinum is the only substance that can be used 



to pass through the glass in an incandescent lamp. Other metals 



do not expand the same as glass and when they are fused with 



it and allowed to cool they break it. 



When heat was first studied it was thought to be an 

 invisible fluid without weight which worked itself into 

 bodies and caused them to expand in the same way that 

 water affects a sponge or a piece of wood. This fluid was 

 supposed to be driven out by pounding or rubbing. 

 Even the primitive savages knew that fire could be ob- 

 tained by rubbing two dry sticks together. 



About the close of the eighteenth century an American, 

 Count Rumford, who was boring some cannon for the 

 Bavarian government, showed that the amount of heat 

 developed seemed to be entirely dependent upon the 

 amount of grinding or mechanical energy expended. 

 The old theory of a fluid prevailed however until about 

 the middle of the nineteenth century, when a great Eng- 

 lish experimenter by the name of Joule showed conclu- 

 sively that the amount of heat developed was due entirely 

 to the amount of mechanical energy which apparently dis- 

 appeared into the heated body. 



