and
when they are heated:

EXPERIMENT 29. The brass ball and brass ring shown in Figure
43 are called the expansion ball and ring. Try pushing the
ball through the ring. Now heat the ball over the flame for a
minute or two--it should not be red hot--and try again to pass
it through the ring.

Heat both ball and ring for a short time. Does heating expand
the ring?

EXPERIMENT 30. Go to the electric apparatus (described on page
379) and turn on the switch that lets the electricity flow
through the long resistance wire. Watch the wire as it becomes
hot.

_APPLICATION 24._ A woman brought me a glass-stoppered bottle
of smelling salts and asked me if I could open it. The stopper
was in so tightly that I could not pull it out. I might have
done any of the following things: Tried to pull the stopper
out with a pair of pliers; plunged the bottle up to the neck
in hot water; plunged it in ice-cold water; tried to loosen
the stopper by tapping it all around. Which would have been
the best way or ways?

[Illustration: FIG. 45. But notice how it sags when it is hot.]

_APPLICATION 25._ I used to buy a quart of milk each evening
from a farmer just after he had milked. He cooled most of the
milk as soon as it was strained, to make it keep better. He
asked me if I wanted my quart before or after it was cooled.
Either way he would fill his quart measure brim full. Which
way would I have received more milk for my money?

INFERENCE EXERCISE

Explain the following:

121. Billiard balls will rebound from each other and from the
edges of the table again and again and finally stop.

122. In washing a tumbler in hot water it is necessary to lay
it in sidewise and wet it all over, inside and out, to keep
it from cracking; if it is thick in some parts and thin in
others, like a cut-glass tumbler, it is not safe to wash it in
hot water at all.

123. The swinging of the moon around the earth keeps the moon
from falling to the earth.

124. A fire in a grate creates a draft up the chimney.

125. Telegraph wires and wire fences put up in the summer must
not be strung too tightly.

126. Candy usually draws in somewhat from the edge of the pan
as it hardens.

127. A meat chopper can be screwed to a table more tightly
than you can possibly push it on.

128.