“What is meant by convection of heat?”
“The conveyance of heat by carrying a heated body. If I remove a hot iron or a kettle of hot water, I must of course carry the heat which it contains.”
“A very good illustration of the convection of heat,” said Mr. Wilton, “is seen in the common method of heating water. The heat is applied at the bottom of the vessel containing the water; as fast as the water at the bottom next the fire is heated, it rises and carries the heat to the top; cold water comes to take its place, and this in turn is heated and rises and carries heat to the top. This process is carried on till all the water comes to the same temperature. Thus water is heated by convection of heat.
“A grander illustration is seen in winds and ocean currents. Warm winds carry heat enough to warm a continent, and the mighty ocean currents are still more efficient in transferring heat from one part of the earth to another.
“Another point we need to understand. When radiant heat falls upon a body, what becomes of it?”
“It is disposed of,” answered Samuel, “in one of three ways: it may be reflected according to the same principles by which light is reflected; or it may be transmitted, that is, pass through the body; or it may be absorbed, that is, stop in it.”
“Very well stated, Samuel. In regard to reflection I need to say very little. You know how light is reflected from a polished surface, such as a lamp reflector: heat is reflected in the same manner. One fact you must bear in mind touching reflected heat: it does not heat the reflecting body.
“There is no need of telling you that light passes through certain substances. It passes through gases and through some liquids and some solids. The best of glass, though it is so solid, interposes very little hindrance to the passage of light. Heat in like manner radiates through certain solids. Luminous heat is radiated through glass. Rock-salt transmits dark heat also. A plate of alum permits light to pass, but stops both luminous heat and dark heat. Remember that transmitted heat, as was said of reflected heat, does not heat the body through which it passes. I have seen boys make burning-glasses of ice. The heat passes through them and burns that upon which it is concentrated, while the ice itself through which the heat passes is not melted.
“If a body have a good radiating surface, that is, if its surface be dull and rough, the heat which falls upon it will be mostly absorbed. The reflecting and absorbing qualities hold an inverse ratio to each other; the better the reflecting qualities, the worse the absorbing, and the worse the reflecting, the better the absorbing. Heat which is absorbed by a body commonly raises its temperature, and remains in the body till it is slowly radiated or is conducted away by the air or other bodies which come in contact with it.
“What is that heat called, Ansel, which is absorbed by a body with no rise of temperature?”