By summer 1940, nuclear research had gone underground in all major powers, making Joliot's published neutron measurements some of the last open fission data until after the war. Otto Hahn and Fritz Strassmann were German chemists working at the Kaiser Wilhelm Institute for Chemistry in Berlin. Hahn had spent decades studying radioactivity and isotopes, originally in close collaboration with the physicist Lise Meitner. Strassmann, a younger chemist, joined Hahn in the 1930s and became indispensable in carrying out careful radiochemical separation experiments. Their research began as an attempt to create elements heavier than uranium (so-called “transuranic” elements), a direction inspired by Enrico Fermi’s earlier neutron-bombardment work in Rome. After World War II, Hahn became a prominent public scientific figure. He was awarded the 1944 Nobel Prize in Chemistry. Hahn later became a vocal advocate against nuclear weapons. Strassmann, became a professor in Mainz and continued research in nuclear chemistry. Both men spent much of their postwar careers emphasizing the peaceful uses of nuclear science.  *Otto Hahn and Fritz Strassmann* Jean Harrington was a staff science writer and editor for Scientific American during the late 1930s and early 1940s. Throughout the 1920s and 1930s, physicists bombarded atoms with alpha particles, protons, and neutrons, typically ejecting single protons, neutrons, or alpha particles—hence "knocking chips off." These reactions released only 5 to 10 MeV of energy per event. The "200,000,000 volts" (200 MeV) represents the unprecedented energy release from fission. This massive energy release comes from the conversion of mass to energy. When uranium-235 splits, the products weigh about 0.2 atomic mass units less than the original nucleus - this "missing" mass becomes energy. By mid-1939, physicists realized that the neutrons released during the fission of a uranium nucleus might trigger further fissions, making a self-sustaining chain reaction possible. The implications were immediately clear to a small group of researchers, especially Leo Szilard, who had already theorized chain reactions several years earlier. If enough uranium were assembled under the right conditions, the reaction could grow exponentially and release energy on a catastrophic scale. Concerned that Nazi Germany might attempt such research first, Szilard worked with Albert Einstein to draft a letter warning U.S. President Franklin D. Roosevelt about the potential for nuclear weapons The Einstein letter, delivered October 11, 1939, warned that "extremely powerful bombs of a new type" might be constructed. It specifically noted that Germany had stopped the sale of uranium from occupied Czechoslovakian mines and that Carl von Weizsäcker was conducting fission experiments at the Kaiser Wilhelm Institute. This fear of a German atomic bomb would drive the Manhattan Project. The scientific community attempted self-censorship starting in spring 1940 - American and British physicists voluntarily stopped publishing fission research. However, the key discoveries were already public. The Soviets actually noticed the sudden silence in Western physics journals, which paradoxically confirmed that fission had military importance. By summer 1940, research had gone underground in all major powers. The "two German physicists" were Otto Hahn and Fritz Strassmann at the Kaiser Wilhelm Institute for Chemistry in Berlin. Their December 1938 experiment was actually trying to create transuranics - elements heavier than uranium - by bombarding uranium with slow neutrons, following work pioneered by Enrico Fermi in 1934. Instead of creating heavier elements, they were puzzled to find barium, an element with roughly half uranium's atomic weight. Hahn wrote to his former colleague Lise Meitner (who had fled Nazi Germany months earlier) about these baffling results. Over Christmas 1938 in Sweden, Meitner and her nephew Otto Frisch realized the uranium nucleus had actually split in two - they coined the term "fission" from biology. Using Einstein's $E = mc^{2}$, they calculated that each fission would release ~200 *MeV* of energy, about 20 million times more than typical chemical reactions. The news spread like wildfire through the physics community in January 1939. Niels Bohr brought word to America, where it was quickly verified at Columbia, Carnegie Institution, Johns Hopkins, and Berkeley. The "worry" mentioned arose because physicists immediately recognized that if each fission released additional neutrons (confirmed by Joliot, Fermi, and Szilard by March 1939), a chain reaction might be possible. This article appeared in Scientific American in early 1940, less than a year after the discovery of nuclear fission (Hahn & Strassmann, late 1938; Meitner & Frisch’s interpretation, early 1939). At the time, the scientific community was still trying to determine whether sustained chain reactions were physically possible. Enrico Fermi, Leo Szilard, and others had already raised the possibility that if fission could be made “self-propagating,” it could release explosive amounts of energy. But it was still unclear whether natural uranium could support such a reaction - many neutrons appeared to escape, and the role of moderators (such as graphite or heavy water) had not yet been firmly established. The tone of this article reflects the pre-Manhattan Project consensus in public science writing: nuclear explosions were generally considered unlikely, either because the reaction would “run down,” or because achieving the right configuration of uranium seemed implausibly difficult. The Joliot-Curie group had recently reported results suggesting that the reaction “poohs out” under ordinary conditions, reinforcing the idea that large-scale detonation was improbable. The key misunderstanding here was about neutron speed: while the article suggests that fast neutrons from heated uranium would be ineffective, the critical breakthrough (soon to come) was that slow neutrons are far more efficient at sustaining fission when combined with the right moderator. Meanwhile, research in Germany and the United States was accelerating - largely in secret. This article therefore sits at a historical hinge: written after the scientific possibility of fission was recognized, but before the idea of a workable atomic bomb shifted from speculation to classified engineering. Frédéric Joliot-Curie (1900-1958) was indeed Marie Curie's son-in-law, having married her daughter Irène in 1926. The couple shared the 1935 Nobel Prize in Chemistry for discovering artificial radioactivity. By 1939, Joliot led France's nuclear research at the Collège de France, where his team was the first to prove that fission produced excess neutrons, making chain reactions theoretically possible. His key collaborators were Hans von Halban (1908-1964), an Austrian physicist who had fled from Nazi Germany; Lew Kowarski (1907-1979), a Russian-born physicist who would later help design Canada's first nuclear reactor; and Francis Perrin (1901-1992), a theoretical physicist and son of Nobel laureate Jean Perrin. This team made crucial measurements showing each fission produced approximately 3.5 neutrons, and they pioneered experiments with heavy water (D₂O—water where hydrogen is replaced with deuterium, a hydrogen isotope with one neutron). Heavy water slows neutrons without absorbing them, making it an ideal "moderator" for sustaining chain reactions. Their work ended abruptly with the German invasion of France in May 1940. Von Halban and Kowarski escaped to England with the world's entire supply of heavy water (185 kg from Norway's Norsk Hydro), denying this crucial material to German scientists. Joliot remained in occupied France, secretly aiding the Resistance while convincing German officials that nuclear reactors were unfeasible. Joliot's team submerged uranium oxide in ordinary water (not heavy water) to measure neutron multiplication. Their setup used neutron detectors surrounding a large tank containing dissolved uranium salts, with a neutron source at the center. By comparing neutron counts with and without uranium, they could determine how many neutrons each fission produced. The water served dual purposes: dissolving the uranium compounds for uniform distribution and slowing down (moderating) the emitted neutrons. The experiment was fundamentally limited because ordinary water absorbs too many neutrons—hydrogen nuclei capture neutrons to form deuterium, removing them from the fission cycle. This is why their measured multiplication factor was less than 1, leading to the conclusion that chain reactions would die out. Had they used heavy water instead, which absorbs far fewer neutrons, they might have observed multiplication factors approaching or exceeding 1. Their caveat about "particular conditions" was prescient. The geometry, uranium concentration, and choice of moderator all critically affect neutron multiplication. Unknown to them, achieving a chain reaction required either: (1) enriching uranium to increase U-235 concentration (2) using heavy water or ultra-pure graphite as moderator.