Historical literature on the early thermodynamics of radiation

Day and van Orstrand, The Black Body and the Measurement of Extreme Temperatures:

[Kirchhoff] was the first to make a general application of the second law of thermodynamics to radiation.

Darrigol, "A simplified genesis of quantum mechanics":

In the second-half of the nineteenth-century, electromagnetic radiation was known to reach a well-defined state of thermal equilibrium by interaction with matter. This state is the so-called blackbody radiation, which can be observed within a uniformly heated cavity with absorbing walls. As a consequence of a thermodynamic theorem established by Gustav Kirchhoff around 1850, the spectrum of this radiation is universal


Kirchhoff in his proof made essential use of the second law of thermodynamics. This method of proof was to serve as a model for many subsequent theoretical explorations into radiant energy.
[Kirchhoff, Boltzmann, Wien, and Planck] took for granted that the physical systems under investigation are in radiant equilibrium.
Kirchhoff, Boltzmann, Wien and Planck all assumed equilibrium conditions and so denied any such variation with time.
Planck thought that the independence of radiant energy from the precise nature of the matter involved made the search for Kirchhoff’s function extremely valuable since it then amounted to the search for something absolute.
It is precisely because of the second law that Kirchhoff claimed these independence results to follow.

Pais, Einstein and the Quantum Theory:

Kirchhoff derived [his law] by showing that its violation would imply the possibility of a 'perpetuum mobile' of the second kind. The novelty of his theorem was not so much its content as the precision and generality of its proof, based exclusively on the still-young science of thermodynamics. A quarter of a century passed before the next theoretical advance in black-body radiation came about.