Let me perhaps explain, lest the point be lost, why you get so many radioactivities. The heavier an element is, the more the repulsion between the positive charges, the protons betters. The nucleus is actually composed of protons and neutrons approximately equal in number for the light elements, but as elements get heavier and heavier, the fraction of neutrons gets greater and greater. And the charge on a nucleus makes the nucleus less and less stable, so a little additional energy can cause the splitting. But the parts into which uranium splits, which now is a lighter nucleus, with fewer neutrons and protons, have a neutron to proton ratio which is high as is the fashion for very heavy nuclei. And radioactivity consists in the transformation of neutrons into protons or vice versa. And the fission of uranium leads to individual particles which for size are over rich in neutrons and therefore will give a lot of decays. In addition to the point that the splitting can be into unequal parts or into equal parts and hence the great amount of various radioactivites. Now, this point has been considered carefully in Germany, in the Kaiser Wilhelm Institute. And it was considered, to a very great extent, of looking at the radioactivities which are created, chemically separating out some of these radioactivities and finding that, for instance, one of them was indubitably iodine, clearly less heavy than uranium. That was discovered by Hahn and Strassman in Germany and they wrote to their friends, their Jewish friend, Lise Meitner, who had to leave Germany and was in Copenhagen, and Lise Meitner, together with her nephew, Frisch, actually planned an experiment in which these fragments, the higher the ionizing particles, could be seen. In the usual experiments of Fermi, a neutron was attached to a nucleus and then sent out one lowly beta decay. In this case, the neutron absorption led to two highly charged, fast moving particles which could cause strong ionization in Geiger counters, which I think I have already told you about in connection with the experiment on the unfortunate cat. Meitner and Frisch made the experiment and actually have seen these fragments. They did that after Bohr has sailed for the United States and by the time of his arrival in the United States he heard of the successful experiment. It was a real succe- sensation. And characteristically, and in a way, surprisingly, it was repeated in a short time in several American laboratories.
