16 MATTER AND ENERGY 



perfect in elasticity. Gases are very elastic and liquids 

 as well. Substances like clay, putty, and butter are said 

 to be inelastic. 



Experiment 16. Fill a football or bicycle tire with air under 

 pressure. Push upon its surface with the hand, at once removing 

 the hand. Is any dent left in the surface ? Is air elastic ? 



Measure a coiled spring. Push it at both ends so as to shorten 

 it ; then let go and again measure the spring. Is it any shorter V 

 Is it elastic ? 



24. Crystallization. Many substances have the prop- 

 erty of forming themselves into crystals upon chang- 

 ing from a liquid to a solid state. These crystals always 

 have a definite shape, though in different substances the 

 shapes may differ. Quartz crystals are commonly seen 

 in rocks ; also garnets. Diamonds, rubies, emeralds, and 



other gems are crystals of 

 rock. Sugar is the crystal- 

 lized juice of cane or beet, 

 snowflakes are crystals of 

 water, and many salts are 

 crystals. 



Experiment 17. Dissolve 

 some alum or sugar in warm 

 water until no more can be made 

 FlG 7 to dissolve. Hang a string in 



the water (Fig. 7) and let it 



stand for a day. Examine it from time to time. State what you 

 observe and try to explain it. 



Experiment 18 Melt some roll sulphur by heating and allow 



it to cool slowly. When hard, examine the mass and tell what 

 you find. Compare the results of these last two experiments. 

 Compare the methods used, 



