Exchanges

Exchanges, Theory of, in heat, is associated with the name of Prevost, who first definitely formulated it. A body possessing heat is continually giving out heat, either by conduction, convection, or radiation. This is the case whether the surrounding media are at higher or lower temperatures than the body itself. To take the one case of radiation, the amount of heat emitted is a function of the temperature of the body, and is constant if the temperature is constant. This is true for all bodies, and it must not be imagined that heat proceeds therefrom only in the event of these bodies being at higher temperatures than their surroundings. When their surroundings are at the same temperature they receive just as much as they emit, and their heat-intensity or temperature remains constant. When their surroundings are at lower temperatures they receive less than they emit and cool down in consequence; and conversely when they are at higher temperatures than their surroundings. If, therefore, a body having a good reflecting surface is placed in a medium bounded by another surface at the same temperature, it follows from the theory that inasmuch as its temperature remains constant it reflects much, radiates little, and absorbs little. If its surface is a good absorbent it reflects little but radiates much and absorbs much. Hence the general deductions that good radiators are bad reflectors but good absorbers, and that bad radiators are good reflectors but bad absorbers. [Heat.]