D O C . 3 2 2 F E B R U A R Y 1 9 2 0 2 6 9 sum does not converge, one would have to truncate the series through in- troducing a length of path, roughly according to the procedure by K. F. Herzfeld.[4] ————— Unfortunately I cannot assess at all whether these ideas (in an area that, at the moment, is unfortunately still inaccessible to me) contain any gross errors. That is why I took the liberty of giving them to you in outline and asking you to let me know in a couple of lines whether the above track may be of any value. In that case, I would publish a short notice about this—as its development is too difficult for me, at any rate. I permit myself to extend to you in advance my most obliging thanks. With ut- most respect, very sincerely, M. Polányi. 322. To David Hilbert Berlin, 21 February 1920 Dear Colleague, Unfortunately, my letter cannot reach you in time anymore. But I respond to you immediately nevertheless in the opinion that you can still make use of my letter.[1] I am convinced that, among German physicists presently at the height of their working powers, Mr. Born is (with Debye) the most outstanding. His systematic analyses of the mechanics of crystal lattices signify an important advance in our knowledge about the processes in solid bodies whose full consequences cannot even be completely assessed today.[2] It was only through Born’s work that we have arrived at the certainty that—at least for salts of the type of NaCl—cohesive forces are of an electrical nature.[3] From the very beginning of his career, Born distinguished himself by his math- ematical skill. While his efforts in early years were, to some degree, remote from material fact, in recent years he has developed such a reliable intuitive eye for reality that today he has become one of the most distinguished and promising rep- resentatives of our field. Besides this there is Born’s admirable teaching talent and style, his noble and courageous character, and his great industry. I think that there is no more suitable e –ε κT -------n- n =0 n =∞ ∑