Heat some water in a flask till it becomes steam. Steam is a gas. Cool the steam and form water again. (See distillation.) Refer to lava (melted rock), moulding iron, melting ice and snow, softening of butter. All solids may be changed to liquids and even to gases if sufficiently heated. Likewise all gases may be changed into liquids and then to solids. EXPANSION OF SOLIDS In winter pupils may find that the ink is frozen. The teacher directs attention to this and inquires why it has occurred. It may be that in a lesson on rocks the teacher will ask the pupils to account for all the little stones. The following _experiments_ will aim at solving the foregoing problems: 1. A brass ball and ring are shown. Pupils handle these and note that both are cold and that the ball just passes through the ring. They are asked to compare the size of the ball with that of the ring. 2. The spirit-lamp is lighted and examined. Pupils hold their hands over the flame to note the heat. 3. The ball is heated in the flame for a short time by one of the pupils, and felt cautiously. An attempt is made to pass it through the ring. How has the ball changed in feeling? In size? How does one know it is larger? What has caused these changes? 4. Cool the ball. Feel it. Try to pass it through the ring now. How has it changed in feeling? In size? What caused these changes? How does heat affect the ball? How does cold affect it? The teacher may now give the words _expand_ and _contract_, writing them on the black-board and explaining their use. Pupils may then state their conclusions: _A brass ball expands when heated and contracts when cooled._ A blacksmith can make the following very serviceable apparatus: A scrap of iron about eleven inches long, one inch wide, and one-eighth inch thick, has one inch bent up at each end. A rod one-eighth inch in diameter is made just long enough to pass between the upturned ends of the first piece when both are cold. The rod is heated and the experiment conducted as in the case of the ball. Two additional facts are learned: (1) Iron expands as well as brass; (2) solids expand in length as well as in volume. The pupils may now be told that other solids have been tried and expansion has invariably followed heating. The conclusion may then be made general. PRACTICAL APPLICATIONS 1. When your ink-bottle was placed on the stove, which end became warmer? Which expanded the 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.