PROPERTIES OF MATTER: GRAVITATION 19 



13. Under what conditions may crystals be formed? Name 

 some substances that crystallize. Name some substances that do 

 not form crystals. What are such substances called ? 



14. Explain the cause of capillary action. Name some impor- 

 tant use that is made of capillarity. 



15. Name the property shown by the substance in each of the 

 following cases : a watch spring in unwinding ; a blotting paper 

 in absorbing ink ; a stick when it is not easily broken ; a bit of 

 steel that can scratch glass; the air forced into a bicycle tire,- 

 vapor in the air when it forms into snowflakes ; a wire when it 

 supports a heavy weight. 



SECTION IV 

 PROPERTIES OF MATTER: GRAVITATION 



27. Gravitation. We already know three facts : that 

 bodies near the earth fall towards it if they are free 

 to fall ; that all bodies on the earth are held down' by 

 some means which we cannot see ; and that the earth, 

 moon, and planets are held in place also .by some 

 invisible means. It is clear that there must be some 

 great force doing these things, and we call this force 

 gravitation. 



28. Gravity. Not only do these things occur, but 

 scientists tell us that every body of matter has the 

 power of attracting every other body not only solids 

 but liquids and gases as well. The only reason that 

 they do not succeed in drawing together is that the 

 earth draws each body more strongly, thus holding each 

 in place. The force exerted by the earth in attract- 

 ing and holding bodies is just the same as the common 

 force exerted by all matter, that is, gravitation; but 



