Matter and Energy 15 



in a solid, a liquid or a gaseous state, occupies some space. Some, 

 but not all, kinds of matter seem hard when touched. A piece of 

 iron is hard as compared with an ordinary rubber eraser. Hard- 

 ness, then, is a special property of iron. 



Among the many other special properties of matter are brittle- 

 ness, elasticity and flexibility. An object is brittle when it is 

 easily broken into pieces. Glass and porcelain are brittle. An 

 object is elastic when it readily resumes its original shape after 

 some force has changed it. A rubber band is elastic. An object 

 is flexible when it permits a good deal of motion of its parts with- 

 out breaking. A whip is flexible. 



Inertia and Gravitation. Inertia and gravitation are im- 

 portant general properties of matter. Inertia is the tendency 

 of a body when at rest to remain at rest, or when in motion 

 to continue in the same motion unless prevented by some outside 

 force. The inertia of a pen lying on a card on a table tends to 

 make it remain on the table when the card is snapped away. 

 Inertia tends to make one's body continue to move forward when 

 a car in which one is riding suddenly stops. The heavier a body, 

 the greater is its inertia. Think how much easier it is to roll 

 a small stone than a large rock, or to stop the rolling stone than 

 to stop the rolling rock. This is because the heavier rock's inertia 

 is so much greater than the lighter stone's inertia. 



Gravitation is the tendency of bodies to move toward each 

 other. Every portion of matter in the universe attracts every 

 other portion. No one knows what causes this. When this 

 attraction draws objects toward the center of the earth it is called 

 gravity. The measure of the amount that gravity attracts an ob- 

 ject is the object's weight. A ball of iron dropped from ten feet 

 above the ground falls because the earth attracts it; but in turn 

 the iron attracts the earth. Each moves to meet the other, but 

 the inertia of the heavier body prevents it from moving as much 

 as the lighter one. Consequently, the much heavier earth hardly 

 moves at all. 



Conservation of Matter. Near the close of the eighteenth 

 century a great scientist, named Lavoisier, discovered that matter, 

 although often apparently destroyed, continues to exist in some 



