2 6 8 D O C . 3 2 1 F E B R U A R Y 1 9 2 0 contemporaries. We surrender ourselves to the hope of seeing you as an active ac- ademic teacher also in future semesters. Eduard Hertel, stud. phil. Heinz Grühr, stud. phil. Wilhelm Gipfel, stud. phil. This declaration to Professor Einstein is endorsed by [291 signatures][3] 321. From Michael Polányi[1] Karlsruhe in B[aden], 7 I Park Street, 20 February 1920 Esteemed Professor, From various quarters I hear that there is much discussion in Berlin now about the problem of rotational energy of gas molecules and that you specifically also have a lively interest in this question.[2] This news encouraged me to impose myself on you for a moment with the following information and to ask you to write me briefly about your opinion on it. It regards a critical remark, the significance of which I could never clarify for myself—despite having attempted repeatedly to cal- culate the matter: 1. It appears that thus far, in calculating the rotational energy, the temperature dependency of a gas molecule’s moment of inertia has always been ignored on the justification that the expansion of the molecules (internuclear distances) caused by the centrifugal force just amounts to a few per mil. Well, the latter does appear cor- rect to me, but the mentioned negligence is nonetheless quite impermissible, and t[his] is because the modest expansion of an internuclear distance has precisely a corresponding expansion in electron orbits as a consequence and thereby an in- crease in the electron energy, the latter of which, although likewise amounting to only a little per mil, nevertheless does cause quite a noticeable effect, in view of the enormously high value of the electron energy. In the molecular heat for Bohr’s H2 model, for inst., the effect adds up to approx. 3 cal! 2. But the problem does not seem to stop there at all, unfortunately: for if you calculate further according to Bohr’s atomic model, then you see that the men- tioned expansion in the electron orbits leads in turn to an expansion of the nuclear distances, etc., whereby the interaction degenerates into a complete breaking apart of the H2 molecule’s components. 3. There might poss. be a remedy, it seems to me, by also quantizing the motion of the atomic nuclei following Bohr’s pattern.[3] But since for the Bohr rotator the