During the late nineteenth century, interest in thermodynamics led to several attempts to apply it to the puzzling phenomena of phosphorescence and fluorescence. These efforts reveal the pervasiveness of heat concepts for both experimental and theoretical physics of the period. The hegemony of heat concepts and theories provided a matrix for the development of quantum theory. Simultaneously, it prevented scientists from understanding cold light. Since fluorescence and phosphorescence defied basic expectations of thermodynamics and radiation theory, they contributed in a minor (...) way to the development and acceptance of quantum theory, particularly through the concept of luminescence temperature and the thermodynamic analyses of Stokes' law. Post-war theories of phosphorescence and fluorescence preserved elements of the nineteenth-century models. (shrink)