tion and reaction one thing moves
one way and another thing is pushed the other way; while in elasticity
the same thing moves first one way, then the other. If you press
down on a spring scale with your hand, you are lifting up your body a
little to do it; that is action and reaction. But after you take your
hand off the scale the pan springs back up: first it was pushed down,
then it springs back to its original position; it does this because of
the elasticity of its spring.

_APPLICATION 23._ Explain why basket balls are filled with
air; why springs are usually made of steel; why we use rubber
bands to hold papers together; why a toy balloon becomes small
again when you let the air out.

INFERENCE EXERCISE

Explain the following, being especially careful not to confuse
action and reaction with elasticity:

111. When you want to push your chair back from a table, you
push forward against the table.

112. The pans in which candy is cooled must be greased.

113. Good springs make a bed comfortable.

114. Paper clips are made of steel or spring brass.

115. A spring door latch acts by itself if you close the door
tightly.

116. On a cold morning, you rub your hands together to warm
them.

117. If an electric fan is not fastened in place and has not a
heavy base, it will move backward while it is going.

118. Doors with springs on them will close after you.

119. When you jump down on the end of a springboard, it throws
you into the air.

120. You move your hands backward to swim forward.

NOTE. There are really two kinds of elasticity, which have
nothing to do with each other. Elasticity of _form_ is the
tendency of a thing to go back to its original shape, as
rubber does. If you make a dent in rubber, it springs right
back to the shape it had before. Elasticity of _volume_ is the
tendency of a substance to go back to its original _size_, as
lead does. If you manage to squeeze lead into a smaller space,
it will spring right back to the same size as soon as you stop
pressing it on all sides. But a dent in lead will stay there;
it has little elasticity of form.

Air and water--all liquids, in fact--have a great deal of
elasticity of _volume_, but practically no elasticity of form.
They do not tend to keep their shape, but they do tend to fill
the same amount of sp