This letter from Casimir is accompagnied by an offprint by Casimir and B L van der Waerden "Algebraischer Beweis der vollständigen Reduzibilität der Darstellungen halbeinfacher Liescher Gruppen" (in: Math. Ann. : 111 (1935), pp.1-12). The letter contains a short reference to mathematical problems, experimental results on supraconductivity and personal remarks on Kronig.

Ehrenfest sends a short note in which he expresses his doubts as to the separability of the operator-function M representing the spin interaction of the two He-electron.

Ehrenfest sends his daughter's notes on Pauli's work in the early twenties for comments. He also asks about a action-at-a-distance in the Schrödinger theory.

Ehrenfest writes that he is unable to speak on X-ray problems in Zürich. He asks about Pauli's views on beta spectrum.