1 0 2 D O C . 1 1 9 O C T O B E R 1 9 1 9 4. It can also happen, however, that a molecule originating from another layer, after leaving one of the two plates (affected by the accommodation coefficient), has acquired a velocity that becomes exactly equal to the one characteristic of that lay- er. Then it is indeed possible for this molecule to reenter into the layer, namely, when it happens to encounter a molecule there from a third layer that had had a sim- ilar fate at the other plate, so that both have attained the same velocity. Thus, here could be a compensation for a loss of molecules in layer 2 through collisions oc- curring under (2), provided the order of magnitude of the number of collisions of the layer’s own molecules is not very different from that of the latter collisions. [Naturally, it is also possible that within layer dz two molecules collide that have equal and opposing yet arbitrarily large velocities not characteristic of the layer. Of these, only a central collision may be permitted, no collision under 45 degrees, which would direct it into the layer, because they could escape again from the layer only by collision with their own kind, which is infinitely improbable and conse- quently the condition of homogeneous velocity in each layer is violated.] Depopulation of molecules in the layer hence cannot be prevented. In the short- est conceivable time, the gas only has molecules moving from one plate to the other whose exclusively central collisions are not detectable in any way. It is thus as if one had an extreme vacuum. I believe that here it is clearly proved that, for these and similar problems, it is not permissible to make use of the usual simplifications. One must, as you said, ap- ply Maxwell’s approach. I am even finding modern indications in this sense in Smoluchowsky’s papers. From now on I am going to try and see if I can get any further but fear that the mathematical difficulties arising out of it exceed my math- ematical proficiency. In any case, I have learned to be more careful with simplified assumptions. In expressing to you my cordial thanks for your kind interest in my tribulations, I am with devoted greetings and the request that you convey my respects to your esteemed wife, very faithfully yours, Westphal. 119. From Erwin Freundlich Neubabelsberg, Town Hall, 3 October 1919 Dear Mr. Einstein, I am back in the country and am going to come and see you at the beginning of next week so that we can discuss the 3rd edition of the brochure.