Why are woollen holders used about the stove? Because wool is a poor conductor, and thus keeps the heat of the iron from burning the hand. Convection of Heat. When air near the floor of a room is heated, what becomes of it? It ascends to the ceiling of the room. What is done with the cold air at the ceiling? It descends to the floor, to take the place of the warm air. When the air ascends to the ceiling, does it carry the heat with it? Yes; it carries the heat with it. What is this mode of carrying heat called? It is called _Convection_. What is needed so as to heat a substance by convection? Its particles must be able to move about freely. Why is the air easily heated by convection? Because the particles of the air move about freely. Why is a liquid easily heated by convection? Because its particles move about freely. Why cannot a solid be heated by convection? Because its particles cannot move. When water near the bottom of a vessel is heated, what becomes of it? It rises to the surface. What becomes of the cold water at the surface? It falls to the bottom of the vessel, to take the place of the warm water. How long will this circulation continue? It will continue so long as heat is applied to the bottom of the vessel. When heat is applied to the surface of the water, what is the result? The water near the surface only is heated. Why does it not heat all the water in the vessel? Because the water when heated, becomes lighter, and stays at the surface, while the cold water is heavier and remains at the bottom of the vessel. Where must heat be applied to warm a liquid or a gas? It must be applied to the bottom of the vessel containing the liquid or the gas. Where must a fire be, so as to warm a room? The fire must be near the floor of the room. Why does a hot substance cool quickly in the air? Because the air touching a substance ascends as soon as heated, and cold air takes its place; as this process goes on rapidly, the substance is quickly cooled. How is water cooled by convection? The water at the surface is cooled by giving off its heat to the air; it then falls to the bottom, and the warm water rises to take its place at the surface. Why does stirring hot coffee cool it quickly? Because the hot coffee at the bottom of the vessel, is brought more rapidly in contact with the air, and, therefore, gives off its heat more rapidly. Why does blowing hot coffee cool it quickly? Because the hot air over the coffee is removed by blowing, and the cold air is brought more rapidly in contact with the coffee. Liquefaction. Is ice a solid or a liquid? Ice is a solid. When heat is applied to ice, what becomes of the ice? The ice is changed into water. Is water a liquid? Water is a liquid. How then can a _solid_ be changed to a _liquid_? By applying _heat_ to a solid, it may be changed to a liquid.