We are going to investigate systems of radiation on two levels, as we did for gases. But in a system of radiation, there are no material particles in motion which we can easily visualize producing the effects of pressure and having an energy corresponding to temperature. However, in the case of the ideal gas, we found ourselves dealing with particles of no size and no internal structure; they were merely "loci" of energy. In dealing with systems, there is no need to refer to material particles at all, only to some microstructure of the internal energy. For radiation, we conceive of the existence of different "vibrational modes" of energy, distinguished by different frequencies of vibration. Our study will concentrate on the problem of relating our views on the energies of these vibrational modes with observations of the behaviour of the system as a whole. It was this problem that led Max Planck to open the door to quantum mechanics.