3 2 D O C . 3 5 M A Y 1 9 1 9 briefly in a few words. As you know, there is a second de Haas in Delft as well.[9] My son-in-law’s address is: “Rotterdamsche Weg.” ————— I very much hope that you personally are feeling well and that you receive good news from Switzerland. The miserable conditions under which you must now live are, I fear, damaging to your health may it be possible for you to find some relax- ation during the summer and please do tell me in your next letter something about how you are.[10] It is truly depressing that it had to come to this in Germany today. We are all doing well, the children and grandchildren too. The number of the lat- ter rose a short while ago with the birth of a little “de Haas,” to be more precise, a godchild of mine.[11] All the Kamerlingh Onneses and Ehrenfests are well.[12] With cordial greetings also from my wife, I remain devotedly yours, H. A. Lorentz. 35. To Theodor Kaluza [Berlin,] 5 May 1919 Dear Colleague, I am very willing to present an excerpt of your paper before the Academy for the Sitzungsberichte.[1] Also, I should like to advise you to publish the manuscript sent to me in a journal as well, for ex., in the Mathematische Zeitschrift or in the Anna- len d[er] Physik. I shall be glad to submit it in your name wherever you wish and write a few words of recommendation on it. I have meanwhile managed to clear up the remaining obscure point myself. I did understand what you wrote me in your letter of May 1st. But proof of the constancy (to sufficient approximation) of on a geodesic line is also a part of it.[2] This too I have now mastered. I now also believe that, from the point of view of realistic experiments, your theory has nothing to fear. From the general point of view, only one thing disturbs me. One requires: 1. General covariance in . 2. In combination with this, the relation not be covariant in . This is, obviously, very unsatisfactory. But on the other hand, the formal unity of your theory is astonishing. At all events, the latter is already valuable because sd dx0 R5 ∂ ∂w5 --------- 0 = R5