Heisenberg agrees now with Pauli that their model does not lead to the existence of showers, and cites qualitative arguments for this conjecture. He also writes about recent experimental results in nuclear physics.

Heisenberg comments on Bhabha's and Heitler's paper which leads to agreement with Heisenberg's data about cross-section for genuine showers. There is a difference between Bhabha's and Heisenberg's conception regarding cosmic radiation at great depths. He writes on an experiment deciding between both conceptions: measurement of electron energies at great depths.

Heisenberg revokes his former statement about the relativistic invariance of the cut-off-method.

Heisenberg has received Fierz's paper and agrees.

Heisenberg writes about the qualification of some of his results in the last letter. The theory still seems promising.

Heisenberg doubts about the usefulness of the Yukawa-potential for nuclear physics. He talks about the relative abundance of light and heavy particles in explosions.

Heisenberg talks about the classical electron-radius as an upper limit of the universal length. He does not believe in Pauli's remark that the potential between proton and neutron at larger distances can be found from mass-defect measurements. In the "wave-like" region one cannot from the potential at large distances and effects resulting from the failure of quantum mechanics for p~h/ro.

Heisenberg asks Pauli if he would be ready to take over part of the paper to be given at the Solvay congress on elementary particles: 1) "on the general properties of elementary particles", 2) "the special empirical forms of interaction and their consequences", 3) "the limitations of the present theory". He was interested in what Fierz told him about gravitational quanta of spin 2.

Heisenberg replies to Pauli's letter. He does not fully agree with Pauli that the inertia of self-energy of the spin is of the order (M.c)/h, because the absorptions of mesons indicate a much smaller cross-section for scattering by protons such as that of Bhabha-Heitler, and because the additional inertia does not change the nuclear forces. The spin-inertia plays a role only in the case of rapidly changing fields. He agrees with Pauli's remark on electron spin.

Heisenberg has sent copy of his Solvay-report also to Bohr. He writes about his trip to the USA.