What is this property of the india-rubber called? It is called _Elasticity_. Can air be pressed into a smaller bulk? It can, by means of a weight. When the weight is removed, what does the air do? The air goes back to its former bulk. Is air very elastic? Yes; air is one of the most elastic substances known. Why does a ball rebound when thrown against the floor? Because the substance of the ball is pressed out of shape when it strikes the floor, and, in its effort to spring back to its former shape, it is forced to rebound. Why does the ball rebound from the floor? Because the floor is firm, and will not move from the ball. Does a ball rebound when it strikes a soft substance? No; it does not rebound when it strikes a soft substance, like a heap of wool or feathers. Are solids ever elastic? Yes; many solids are elastic. Name some articles useful from their elastic properties. Watch-springs, wagon-springs, steel pens, and spring seats are very useful. Why does a wooden bow throw an arrow from it? When the force that bends the bow is removed, it springs back to its former shape, and in so doing throws the arrow from it. Can metals be bent? Yes; nearly all metals can be bent. What is this property of being bent called? It is called _Flexibility_. Do substances usually spring back after being bent? No; they usually remain in the bent form. Can glass be bent? Glass can be bent only a very little without being broken. What is this property of being easily broken called? It is called _Brittleness_. What may be said of brittle substances? Brittle substances are generally hard, while flexible substances are much softer. What metal may be made either flexible or brittle? Steel, when heated and cooled slowly, becomes flexible, but when cooled quickly, it becomes brittle. For what is brittle steel used? It is used for making knives, razors, axes, and other cutting instruments. What may be done with metals that are flexible? They may be hammered into thin plates. What is this property in metals called? It is called _Malleability_. Which is the most malleable of all metals? Gold is the most malleable; and it can be made into leaves so thin that three hundred and sixty thousand of them, when placed together, make a bulk only one inch in thickness. Name some other metals that are malleable. Silver, copper, tin, zinc, iron, and lead. Can the particles of a piece of iron easily be separated? No; its particles cannot easily be separated. What is this property of iron called? It is called _Tenacity_. Can the particles of a brittle substance, like glass, easily be separated? Yes; its particles can easily be separated. Is there much tenacity in brittle substances? No; there is very little tenacity in brittle substances. Is there any tenacity in air? No; because its particles are always trying to separate from each other. Is there any tenacity in water?