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.