1 3 0 D O C U M E N T S 1 0 8 , 1 0 9 N O V E M B E R 1 9 2 5 108. To Erwin Schrödinger [Berlin,] 14 November 1925 Dear Colleague, I read your letter with great interest.[1] I do indeed think that this subject de- serves publication. I just don’t know whether I should count as coauthor since after all you did all the work I would feel like an “exploiter,” as the socialists like to put it so beautifully. I, too, consider it entirely plausible to start the summation at n = 1. I haven’t yet understood why your factor ϑ, which occurs because of the differ- ence between the sum and the integral, does not depend on temperature and volume.[2] (I see just now that you too have done it justice by searching for a more precise method for the approximation). The reason for the omission of factor N! in the entropy is not clear, because one doesn’t know exactly what actually needs to be counted but it definitely must be omitted in order to do justice to the indistin- guishableness of molecules and to the proportionality of entropy to the substance quantity. Otherwise one would also come into conflict with the Nernst theorem. I also think that this is the only really reasonable implementation of Planck’s po- sition. Planck himself dismisses it, though, because he is by no means of the opin- ion that energy is the only integral of the equations of motion that falls under consideration for quantization, even though mechanics does not suggest that there is another. But then I cannot gather any proper meaning in Planck’s idea. Kind regards, your A. Einstein 109. From Heike Kamerlingh Onnes[1] Leyden, Huize ter Wetering Haagweg, 14 November 1925 Dear Friend, The Lorentz celebration is quickly approaching.[2] We are very happy that noth- ing has barred you from attending. You will be staying as usual at Ehrenfest’s home—this time with Bohr.[3] As you know, we have not issued any invitations. But many are coming on their own initiative. Mrs. Curie will be staying with us.[4] We are hoping to have Langevin at De Haas’s.[5] Your meeting here with Langevin reminds us of a similar encounter between the two of you shortly after the war.[6] We had welcomed that as the first dawn of reconciliation in the scientific world. Afterward you went to Paris and Langevin to Berlin in order to act in the interest of this goal.[7] Mrs. Curie is also at the forefront of this movement. That inspired my wife[8] and me to think that, besides the encounter between you and Langevin,