The coarse-grained kinds, like chestnut; while the finer-grained kinds, like walnut, seldom snap much. Why does dry wood snap more than green wood? Because the sap in the pores of the green wood dries up as the wood becomes dry, and its place is filled with air, so that there is more air in dry wood than in green wood. Why does smoke rise in a chimney? Because the air in the chimney is heated by the fire, and as the air rises, it carries the smoke with it. Why does a chimney smoke when the fire is first kindled? Because the cold walls of the chimney cool the heated air so rapidly as to prevent its rising to the top, and the smoke then sinks back into the room. How are houses heated with hot air? By having the fires made in the cellar, so that the air when heated, may ascend through pipes to the different parts of the building. Is there any substance that does not contract by cold? Yes; water, when near freezing, does not contract by cold. What takes place with the water? When it is reduced nearly to the freezing point, it begins to expand, and when it freezes, it expands still more. How much does water expand by freezing? It expands about one-seventh of its bulk. That is, _seven_ gallons of _water_ will make _eight_ gallons of _ice_. Does freezing water expand with much force? Yes; it expands with great force. Why are vessels containing water often broken during a cold night? As the water in them freezes, it expands and breaks the vessels. Why do lead pipes and iron pipes often burst during the winter? Because the water in them is permitted to freeze, and as there is not room for it to expand, the pipes are burst. Is ice heavier or lighter than water? Ice is lighter than the same bulk of water. How much lighter is ice than water? Eight gallons of ice weigh no more than seven gallons of water. Does the ice protect the water beneath it from freezing? Ice does protect the water from freezing, to a very great extent. If ice were heavier than water, what would result? The ice would sink as rapidly as formed, and our streams would, every winter, freeze to the bottom, thus destroying the fish and other creatures living in them. In this, as in many other ways, we may learn the lesson, that in the creation of the world, God wisely made all things to serve some useful purpose. Conduction of Heat. Why does iron feel cold to the touch on a cold morning? Because heat goes from the hand to the iron, thus giving the sensation of cold. What is the passage of heat from the hand to the iron called? It is called _conduction_. When one end of a rod of iron is placed in a fire, does the whole rod become heated? The whole rod does become heated. How does this take place? The heat of the fire passes from one particle to another throughout the rod. What is this called? It is called _conduction_. What then is conduction of heat?