4 0 D O C U M E N T 1 7 J U L Y 1 9 2 7 It would indeed represent a certain progress if one could choose the basic as- sumptions in such a manner that Planck’s law would result, instead of Wien’s law. Here is my suggestion: In the extraordinary thermodynamic equilibrium, in which the gas is dissociated, the recombination process proceeds just as though it were dark (more precisely: ), and it follows the laws of chemical kinetics. The number [4] of molecules of the first kind that are formed within a given time interval is thus given by[5] Onto this reaction, the “dark reaction,” the photochemical reaction is super- posed. Dissociation and recombination are accelerated by the action of light, and the light intensity enters the formulas just like concentration enters kinetic for- mulas. The number of molecules formed by recombination in this manner is given by: The number of dissociating molecules is similarly In equilibrium, we then find: From this, it follows that If one then carries out the calculation on this basis, one arrives at an expression corresponding to your equation (4a) (Ann. der Phys. 37, p. 837):[6] and from this, we obtain , Planck’s radiation formula. I would be extremely grateful to you if you would communicate your opinion of this suggestion to me. Thanking you heartily in advance, Yours sincerely, Max Barthel T Gas T Radiation = z z 0  AV---- n 2 V -----3- n V = z 1  z 1  AV---- n 2 V ----- n 3 V - = z 1 z 1 AV---- n 1 V - = z 1 z 0  z 1  + = n 2 V ---- ------3 n V n V ----1- ----------- A A1 -----------------   + = N RT s -------- - – log A A1 -------------   +   log – + 0 =  A A 1 eRTs N --------- 1 – ------------------ - =