19. Conduction. — The progress of heat from a place of higher temperature to a place of lower temperature in the same body is called conduction. Thus, if one end of an iron rod is placed in a hot fire, the other end will soon become warm. This is due to the conduction of heat from one end of the rod to the other. The rate at which heat is conducted varies greatly in different substances. All metals are good conductors of heat, though some, such as silver and copper, are better than others, such as zinc and lead. Stone and glass are poor conductors of heat, but they are even better than wood, hair felt, or asbestos.
20. Convection. — The transfer of heat by the motion of the heated substance itself is called convection. It can take place only in liquids and gases. For example, if heat is applied to the bottom of a vessel containing water, the water in contact with the bottom becomes heated and rises, while the colder water above descends. A circulation is thus set up by which the heated portions of the water continually carry heat to other points in the vessel. This action is known as convection.
21. Radiation. — The transfer of heat from a hot body to a colder one, across an intervening space, is called radiation. A person standing in front of a fire, but at some distance from it, feels a sensation of warmth that is not due to the temperature of the air, for, if a screen is placed between him and the fire, the sensation immediately ceases, which would not be the case if the surrounding air had a high temperature. Hence, bodies can send out rays of heat that can pass through the air without heating it. This is known as radiant heat, and the process by which it is transmitted is known as radiation. The best example of radiant heat is that received from the sun, the intervening space in this case being 93,000,000 miles.
22. When heat is transmitted by conduction or convection, some material substance forms the medium by which the transfer of heat is made. In the case of radiation, no such material medium is required. Radiant heat can be transmitted through a vacuum as well as through air.
