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

Hudson, JAMES JEANS AND RADIATION THEORY:

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.

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[Kirchhoff, Boltzmann, Wien, and Planck] took for granted that the physical systems under investigation are in radiant equilibrium.

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Kirchhoff, Boltzmann, Wien and Planck all assumed equilibrium conditions and so denied any such variation with time.

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Planck thought that the independence of radiant energy from the precise nature of the matter involved made the search for Kirchhoffâ€™s function extremely valuable since it then amounted to the search for something absolute.

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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.