
German physicist Max Planck's 1901 article “On the Law of Distribution of Energy in the Normal Spectrum”, in which he situates quantum mechanics based on entropy. [1]

German physicist Max Planck's 1901 article “On the Law of Distribution of Energy in the Normal Spectrum”, in which he situates quantum mechanics based on entropy. [1]

In famous publications, “On the Law of Distribution of Energy in the Normal Spectrum” is a 1901 publication by German physicist Max Planck in which he launched the science of quantum mechanics, by conceiving of the idea that the energy of bodies (specifically a black body cavity radiator) can be partitioned into unit elements (energy elements), later to be called 'quanta' or quantum units of energy, divided by Planck constant based partitions, based on Austrian physicist Ludwig Boltzmann’s earlier 1872 ideas, as found in his article "Further Studies on the Thermal Equilibrium of Gas Molecules", on how entropy can be partitioned, logarithmically, as the numbers of states of arrangements of a given body of ideal gas. [1]

Planck, supposedly, viewed or modeled, in his mind, the body of a cavity radiator (or its energy of radiation) as a volume of gas, and on this model solved the ultraviolet catastrophe, by applied Boltzmann’s 1872 H-theorem version of entropy, in the form of S = k log W, to black body radiation, and thus situated the principle of elementary disorder.

Planck's constant
The 1901 paper, supposedly, is based on Planck’s earlier 1900 paper “On the Theory of the Law of Energy Distribution in the Normal Spectrum”, in which he famously stated: [2]

“If E is considered to be a continuously divisible quantity, this distribution is possible in infinitely many ways. We consider, however—this is the most essential point of the whole calculation—E to be composed of a well-defined number of equal parts and use thereto the constant of nature h = 6.55x10E-27 erg sec. This constant multiplied by the common frequency ν of the resonators gives us the energy element ε in erg, and dividing E by ε we get the number P of energy elements which must be divided over the N resonators.”

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References
1. (a) Planck, Max. (1901). "On the Law of Distribution of Energy in the Normal Spectrum". Annalen der Physik, vol. 4, p. 553 ff.
(b) Planck, Max. (1901). “Ueber das Gesetz der Energieverteilung im Normalspectrum.”
Annalen der Physik, vol. 309, issue 3 pp. 553–63 (1901).
2. (a) Planck, Max. (1900). “On the Theory of the Law of Energy Distribution in the Normal Spectrum” (‘Zur Theory des Gesetzes der Energieverteilung im Normalspektrum’), German Physical Society, meeting on 14 Dec; In: D. ter Haar and Stephen G. Brush, trans., Planck’s Original Papers in Quantum Physics, (1972), pgs. 38-40, etc.
(b) Mehra, Jagdish and Rechenberg, Helmut. (2000). The Historical Development of Quantum Theory, Volume 1, Part 1 (pg. 50). Springer
(c) Bynum, W.F. and Porter, Roy. (2005). Oxford Dictionary of Quotations (pg. 493). Oxford University Press.