natural uranium, proving that a chain reaction might be possible. Fermi and Szilard still believed that enormous quantities of uranium would be required for an atomic bomb, and therefore concentrated on producing a controlled chain reaction. Fermi urged Alfred O. C. Nier to separate uranium isotopes for determination of the fissile component, and, on 29 February 1940, Nier separated the first uranium-235 sample, which, after being mailed to Dunning at Columbia, was confirmed to be the isolated fissile material. When he was working in Rome, Fermi had discovered that collisions between neutrons and neutron moderators can slow the neutrons down, and thereby make them more likely to be captured by uranium nuclei, causing the uranium to fission. Szilard suggested to Fermi that they use carbon in the form of graphite as a moderator. As a back-up plan, he considered heavy water. This contained deuterium, which would not absorb neutrons like ordinary hydrogen, and was a better neutron moderator than carbon; but heavy water was expensive and difficult to produce, and several tons of it might be needed. Fermi estimated that a fissioning uranium nucleus produced 1.73 neutrons on average. It was enough, but a careful design was called for to minimize losses. (Today the average number of neutrons emitted per fissioning uranium-235 nucleus is known to be about 2.4). Szilard estimated he would need about of graphite and of uranium. In December 1940, Fermi and Szilard met with Herbert G. MacPherson and Victor C. Hamister at National Carbon to discuss the possible existence of impurities in graphite, and the procurement of graphite of a purity that had never been produced commercially. National Carbon, a chemical company, had taken the then unusual step of hiring MacPherson, a physicist, to research carbon arc lamps, a major commercial use for graphite at that time. Because of his work studying the spectroscopy of the carbon arc, MacPherson knew that the major relevant contaminant was boron, both because of its concentration and its affinity for absorbing neutrons, confirming a suspicion of Szilard's. More importantly, MacPherson and Hamister believed that techniques for producing graphite of a sufficient purity could be developed. Had Fermi and Szilard not consulted MacPherson and Hamister, they might have concluded, incorrectly, as the Germans did, that graphite was unsuitable for use as a neutron moderator. Over the next two years, MacPherson, Hamister and Lauchlin M. Currie developed thermal purification techniques for the