What is meant by liquefaction? _Liquefaction_ is the changing of a solid to a liquid, by heat. How may tallow be melted? By heating the tallow, it may be melted. Will the tallow become hard again when cold? It will be hard again when cold. Can lead and other metals be melted? They can be melted or changed to a liquid state by heat. Can all solids be changed to liquids? No; there are many solids, such as wood, coal, paper, and leather, that cannot be changed to liquids. How can a liquid be changed to a solid? By taking heat away from the liquid, it may be changed to a solid. Why is a solid melted by heat? Because heat forces the particles of the solid apart, until it is changed to a liquid. [Illustration: Decoration.] Latent Heat. Why does warm iron feel warm to the hand? Because it gives off some of its heat to the hand. Is the hand sensible of the heat coming from the iron? The hand is sensible of the heat. What then may this heat be called? It may be called _sensible_ heat, because it can be felt. How do we measure heat? We measure heat by degrees. How many degrees of heat in ice when at the melting point? Thirty-two degrees of heat, as measured by the thermometer. How can we change ice into water? By applying heat to the ice it will be changed into water. How many degrees of heat are necessary to change the ice into water? One hundred and forty degrees of heat are necessary. How many degrees of heat are in the water when the ice is all melted? Only thirty-two degrees of heat, by the thermometer. Does the water feel any warmer than the ice to the hand? No; the water does not feel any warmer than the ice. What has become of the one hundred and forty degrees of heat? They have been hidden in the water. What is this hidden heat called? It is called _latent_ heat. What is latent heat? It is heat that is not sensible to the touch, or to the thermometer. When water is made to boil, what change takes place? The water is changed into steam. How many degrees of heat are necessary to change water into steam? More than nine hundred degrees of heat are necessary. How many degrees of latent heat in steam? More than nine hundred degrees of latent heat. When steam is changed back again to water, what becomes of the latent heat? It passes off to the air and the surrounding objects, and makes them warmer. When water is changed into ice, what becomes of the latent heat? It passes off to the surrounding objects and makes them warmer. Is freezing a warming process? Yes; because the water gives off its latent heat while freezing; hence, the surrounding objects are made warmer. Is thawing a cooling process? Yes; because the ice absorbs so much heat while thawing that it cools the surrounding objects. Where does the ice obtain its heat, while thawing? The ice obtains its heat from the surrounding objects.