ABSTRACT
The sun is the fundamental source of radiant energy, all exposed terrestrial surfaces and transparent or translucent fluids, particularly gases, participate in an interdependent and complex range of radiant processes. The science of thermal radiation had its origins in the caloric theory in which hot and cold radiations were accepted concepts. It has thus been demonstrated that the energy density within an enclosure depends only on the temperature, and is independent of both the shape and the nature of the walls, as well as of the shape and nature of any bodies placed within it. Any body placed in an enclosure must take up the temperature of the enclosure and then remain in equilibrium. Under such conditions, the theory of exchanges explains that the enclosed body emits just as much energy as it receives from the enclosure walls, thereby leaving the radiation density within the enclosure unaffected.
