But let us get back to the bombardment of uranium with neutrons research that Fermi had begun. After he had reported his work, other physicists repeated it and also got a variety of beta particles and were also unable to decide what was going on. [Illustration: _Lise Meitner and Otto Hahn in their laboratory in the 1930s._] One way to tackle the problem was to add to the system some stable element that was chemically similar to the tiny traces of radioactive isotopes that might be produced through the bombardment of uranium. Afterwards the stable element could probably be separated out of the mixture and the trace of radioactivity would, it was hoped, be carried along with it. The stable element would be a “carrier”. Among those working on the problem were Otto Hahn and his Austrian co-worker, the physicist Lise Meitner (1878-1968). Among the potential carriers they added to the system was the element, barium, which has an atomic number of 56. They found that a considerable quantity of the radioactivity did indeed accompany the barium when they separated that element out of the system. A natural conclusion was that the isotopes producing the radioactivity belonged to an element that was chemically very similar to barium. Suspicion fell at once on radium (atomic number 88), which was very like barium indeed as far as chemical properties were concerned. Lise Meitner, who was Jewish, found it difficult to work in Germany, however, for it was then under the rule of the strongly anti-Semitic Nazi regime. In March 1938 Germany occupied Austria, which became part of the German realm. Meitner was no longer protected by her Austrian citizenship and had to flee the country and go to Stockholm, Sweden. Hahn remained in Germany and continued working on the problem with the German physical chemist Fritz Strassman (1902- ). Although the supposed radium, which possessed the radioactivity, was very like barium in chemical properties, the two were not entirely identical. There were ways of separating them, and Hahn and Strassman busied themselves in trying to accomplish this in order to isolate the radioactive isotopes, concentrate them, and study them in detail. Over and over again, however, they failed to separate the barium and the supposed radium. Slowly, it began to seem to Hahn that the failure to separate the barium and the radioactivity meant that the isotopes to which the radioactivity belonged had to be so much like barium as to be nothing else _but_ barium. He hesitated to say so, however, because it seemed unbelievable.