No; some substances are much better radiators than others. What kind of objects are the best radiators? Those with rough and dark surfaces. What kind of objects are the poorest radiators? Those with smooth and bright surfaces. Why does water keep hot a long time in a bright tin vessel? Because the tin is a poor radiator, and prevents the heat of the water from passing off into the air. Why does water soon cool in a dull earthen vessel? Because the earthen vessel is a good radiator, and gives off the heat of the water rapidly. If the bright tin vessel be painted, will the water keep hot so long? No; the tin becomes a good radiator when painted, and the water is soon cooled. Why does a stove cool so quickly? Because the iron is a good radiator, and gives off its heat rapidly. Do the rays from the sun give off much heat while passing through the air? The rays from the sun give off about one-fourth of their heat while passing through the air. What becomes of the remaining heat from the sun? It is absorbed or reflected at the earth’s surface. What does the earth do with the heat? The earth radiates the heat, and thus warms the air. Reflection. When a piece of bright tin is held near the fire, does the tin become hot? No; the tin is scarcely warmed. What does the tin do with the rays of heat from the fire? The tin throws them back from its surface. What is this called? It is called _Reflection_ of heat. Do all substances reflect heat equally well? No; some reflect much more heat than others. What kind of objects reflect heat the best? Those with bright and smooth surfaces. What kind of objects reflect heat the least? Those with rough and dark surfaces. Does bright tin reflect heat better than glass? Yes; bright tin reflects about eight times as much heat as glass does. Why does it take so long to heat water in a new tin cup? Because the bright tin is a good reflector, and throws off the greater part of the rays of heat that come to it from the fire. Why does a piece of tin or other bright metal, placed under a stove, protect the carpet from burning? Because the tin being a good reflector, remains cool, and cannot burn the carpet under it. Why will not a piece of iron protect the carpet from burning? Because the iron, being a poor reflector, soon becomes heated, so as to burn the carpet under it. Absorption. When a piece of iron is held near a fire, does the iron become hot? Yes; the iron soon becomes hot. How is the iron heated? The iron absorbs or takes in the heat from the fire. What is this power to take in the heat called? It is called _Absorption_ of heat. What is this process like? It is somewhat like a sponge, that absorbs or sucks up moisture. Do all substances absorb heat equally well? No; some substances absorb more heat than others. What kind of objects absorb heat the best?