"The blackbody oven embodied an... instance of radiation interacting with matter. ...Planck first... derived an empirical equation to fit the data. ...His more ambitious aim now was to find a theoretical entropy-energy connection applicable to the blackbody problem. ...Ludwig Boltzmann interpreted the second law of thermodynamics as a "probability law." If the relative probability or disorder for the state of the system was W, he concluded, then the entropy S of the system in that state was proportional to the logarithm of W,S ∝ lnW ...Plank applied this to the blackbody problem by writingS = k lnW (1)for the total entropy of the vibrating molecules... "resonators"—in the blackbody oven's walls... k is now called Boltzmann's constant. ...Boltzmann's theory taught the lesson that conceivably—but against astronomically unfavorable odds—any macroscopic process can reverse... contradicting the second law of thermodynamics. Boltzmann's conclusions seemed fantastic to Planck, but by 1900 he was becoming increasingly desparate, even reckless... The counting procedure Planck used to calculate the disorder W... was borrowed from... Boltzmann's theoretical techniques. He considered... that the total energy of the resonators was made up of small indivisible "elements," each one of magnitude ε. It was then possible to evaluate W as a count of the number of ways a certain number of energy elements could be distributed to a certain number of resonators... His argument would not succeed unless he assumed that the energy ε of the elements was proportional to the frequency with which the resonators vibrated, ε ∝ v, or ε = hv, with h the proportionality constant."