D O C U M E N T 2 3 J U L Y 1 9 2 5 4 7 with Jordan,[10] I am, systematically but with minimum mental effort, examining all conceivable correspondence-relationships between classical, multiple-periodic systems and quantum atoms. An article about this, in which we examine the effect of nonperiodic fields on atoms, will appear soon.[11] It is preparatory work for an examination of the processes during the collision of atoms (quenching of fluores- cence, sensitized fluorescence as per Franck, etc.) [12] one can, it seems to me, un- derstand the basic characteristics of the phenomenon. The different behavior of the atoms rests mainly on whether they have an (average) dipole moment or only a quadrupole moment or even higher electrical symmetry. As for your objections to Jordan’s article,[13] I still feel quite uncertain however, since I tackle these things from my (somewhat complicated) standpoint, I will soon understand them. Doubt- less, you are, on the whole, right only Jordan’s opinion is based on somewhat dif- ferent considerations in that he allows coherent bundles of rays, whereas you speak only of incoherent ones. Even if Jordan is wrong here, which now seems very likely to me, he is a particularly smart and perceptive person who thinks much more quickly and confidently than I. In general, my young people, Heisenberg, Jordan, Hund,[14] are brilliant. I often have to exert myself greatly in order merely to be able to follow them in their thoughts. They master so-called term zoology[15] fab- ulously. Heisenberg’s latest work, soon to be published, appears quite mystifying, but is certainly correct and deep [16] as a result, Hund has organized the entire pe- riodic system, all the complicated multiplets. This work will also be published soon.[17] — Besides that, I am calculating round about lattice theory with other less independent students. We now have an article ready (a Mr. Bollnow) which calcu- lates the relationship of the crystallographic axes of two crystals of the tetragonal system, rutile and anatase, two forms of TiO2, and comes out quite well based on the requirement that the lattice should be in electrostatic equilibrium.[18] Your communication about the finally successful unification of gravitation and electrodynamics delighted me greatly the action principle you indicate seems very simple. Jordan and I want to “vary” it as soon as we have time. However, we ask you to please send us your work about it soon.[19] Something like this is much deeper than our puttering around. I would never have the confidence to take it on. During this semester, we have again had many visitors. Kramers was here for 8 days [20] also, as mentioned above, Ehrenfest, with whom we have become good friends, especially my wife. Last week, Kapitza[21] was here from Cambridge, then Joffé[22] from Leningrad, who impressed us immensely—what a lot of fine work he has done and has published almost nothing! Now, Philipp Franck is here with his wife[23] and many other people as well. We get a lot of stimulation from them, but it is often too much for our wives. So they run away my wife and Mrs.