D O C U M E N T 5 1 3 A P R I L 1 9 2 7 5 0 7 organic solvents as an absorbent.[1] I fear that this complicates the protection of our machine and I, in any case, submitted the comprehensive application (which is sup- posed to unite the many applications that we have up to now, insofar as they relate to the absorption machine) with appropriately formulated requirements. The mercury pumping[2] also worries me. Although it works very nicely as is, the physiology of the machine is not properly thought through in that direction yet, and if one just went ahead with the design, the machine would not start up. By contrast, mercury pumping really does seem to be an ideal solution for the steam jet refrigerating machine. I meanwhile spoke with Gen. Dir. Korn at Bamag Peiser[3] was away. Pursuant to the contract, Bamag should already have issued a declaration about the foreign applications by now I agreed, also in your name, to postpone this deadline to July 25th. We also discussed a potential conflict with Siemens. I’m sending you in attachment a written confirmation of our agreement about our patents[4] so that you can put it in your files. Point 2 had been proposed by you merely in principle I set the figures just now on my own, for the sake of simplicity I assumed that you would be in agreement with that. 513. From Niels Bohr [Copenhagen,] 13 April 1927 Dear Einstein, Prior to his vacation in the Bavarian mountains, Heisenberg asked me to send you a copy of the correction proofs he was expecting of a new article in Zeitschrift für Physik,[1] as he was hoping that it would interest you. This article, which I send you herewith, surely signifies an extremely important contribution to the discus- sion of general problems in quantum theory. As the content is closely related to the questions that I had the great pleasure of discussing with you a few times, most re- cently during those unforgettable days in Leyden at the Lorentz festivity,[2] I would like to use this opportunity to send along some remarks touching upon the problem that you recently raised in the proceedings of the Berlin Academy.[3] It has been recognized for a long time, of course, how deeply the difficulties of quantum theory are connected with the concepts, or rather, with the words that are used in the ordi- nary description of nature and which all have their origin in the classical theories. These concepts give us only a choice between Scylla and Charybdis, depending on how we direct our attention to the continuous or the discontinuous aspect of the de- scription. At the same time, we do feel that our hopes, determined by our own