The relation between energy and mass

ONE of the extremely important corollaries of the theory of relativity promulgated by Einstein in 1905 is the principle that all forms of energy possess inertia, which most commonly is associated with matter. Thus a body that contains heat energy should have somewhat more mass than an otherwise identical but colder body. Similarly, a hollow box with perfectly reflecting internal walls has more mass if radiant energy is streaming back and forth within these walls than if the radiation is absent.