A person can, by hammering a piece of cold iron, make it red-hot. Could we live without heat? No; our bodies must be kept warm, and this can be done only by heat. Is this true of other living things? Yes; all the animals and all the plants would die if heat were taken away. How should we feel in regard to these things? We should feel thankful to an All-wise Creator, for having provided us with both heat and light from a never-failing source—the sun. Expansion by Heat. How does heat generally affect substances? Heat _expands_ them or makes them larger. Does heat expand all metals? Yes; all metals are expanded by heat. When a rod of iron is heated, is it longer or shorter than when cold? It is longer than when it is cold. How do we know this? We know it by measuring the rod when it is cold, and again when it is heated. Will the rod be larger, as well as longer? The rod will be larger. How can we prove this? The rod, when red-hot, will not go through as small a hole as when it is cold. How do heat and cold affect most substances? Heat expands most substances, and cold contracts them. Do all metals expand alike by heat? No; some metals expand much more than others. How does heat expand substances? Heat pushes the parts or atoms of an object farther from each other, and thus its size is increased. What does a blacksmith do with a tire before putting it on a wheel? He heats the tire red-hot. Why does he heat the tire red-hot? So that it will be increased in size, and will go on the wheel more easily. What takes place as the tire cools? It becomes smaller, and thus binds the parts of the wheel tightly together. When a fire is made in a room, why does the furniture often make a snapping noise? Because heat expands the wood, and the particles make a snapping noise upon separating. Why does a stove make a crackling noise as it grows hot? The particles of the iron make the noise in expanding. If a piece of glass be held in the hand, will the glass become warm? The glass will become as warm as the hand. What part of the glass will be warmed first? The part touching the hand will be warmed first. What part of the glass will be warmed last? The part farthest from the hand will be warmed last. Which part of the glass will be expanded first? The part touching the hand, because it is first warmed. Which part will be expanded last? The part farthest from the hand, because it is the last part warmed. If any other warm substance touch the glass, will the glass be heated in the same manner? The glass will be heated in the same manner. What often results from this unequal heating? The glass is broken. Why are glass plates or tumblers broken when placed in hot water? They are broken because the part next to the hot water is expanded more than that farther away. Why will a cloth dipped in hot water and wrapped around the neck of a bottle loosen its stopper?