According to the kinetic theory of gases, the mean kinetic energy of a molecule is a measure of absolute temperature.
Since we can produce all types of light by means of hot bodies, we can ascribe, to the radiation in thermal equilibrium with hot bodies, the temperature of these bodies, and thus every radiation, even that issuing from a phosphorescent body, has a certain temperature for every colour.
As soon as we step beyond the established boundaries of pure thermodynamic theory, we enter a trackless region confronting us with obstacles which even the most astute of us are almost at a loss to tackle.
