more. Why then did it crack?

2. What other examples like this have you noticed? (Lamp chimneys, fruit
jars, stove plates)

3. The earth was once very hot and is now cooling. How is the size of
the earth changing? Does it ever crack? What causes earthquakes?

4. Find out by observation how a blacksmith sets tires.

5. Invent a way to loosen a glass stopper stuck in the neck of a bottle.

6. What does your mother do if the metal rim refuses to come off the
fruit jar?

7. Next time you cross a railway, notice whether the ends of the rails
touch. Explain.

8. What allowance is made for contraction in a wire fence? A railway
bridge? Why?

9. Why do the stove-pipes crack when the fire is first started?

10. Why does the house go "thump" on a very cold night?

11. Draw the ball, ring, and spirit-lamp in position.

12. Describe in writing the experiments we have made.

QUESTIONS FOR FURTHER INVESTIGATION

You have seen that iron and brass both expand. Do they expand equally?
Let pupils have a few days to invent a way of answering the question.
The experiment may then be tried with the compound bar. See _The Ontario
High School Physics_, pages 217-218, also _First Course in Physics_,
Milliken and Gale, page 144.

If the equipment of the school is limited, it may be necessary to
dispense with the ball and ring and generalize from one experiment.

Another easily made apparatus consists of two iron rings with handles.
One ring will just pass through the other when both are cold. The stove
may take the place of the spirit-lamp.

A still simpler plan consists in driving two nails into a block at such
a distance apart that an iron rod (six-inch nail, poker, bolt, etc.)
will just pass between. On heating the rod the increase in length
becomes evident.

EXPANSION OF LIQUIDS

Fill a common bottle with coloured water; insert a rubber stopper
through which passes a glass tube about sixteen inches long. Set the
bottle in a pan of water and gradually warm the water. The rise of the
liquid in the tube will indicate expansion. On setting the bottle in
cold water the fall of the column of coloured water shows contraction.
See _The Ontario High School Physics_, page 218, also _Science of Common
Life_, page 48. Macmillan Co., 60 cents.

Set the flask or bottle in a mixture of ice and salt and note that the
extreme cold causes contraction for a while, then expansion. Note that
when expansion begins, the water has not begun