The general behavior of such losses in atomic weight for atoms throughout the periodic table had been determined as early as 1927, largely through the work of Aston, the English scientist who developed the first mass spectrograph. His results show that, in general, if two light nuclei combine to form a heavier one, the new nucleus does not weigh as much as the sum of the original ones. This behavior continues up to the level of the so-called “transition metals”—iron, nickel, and cobalt—in the periodic table. But if two nuclei heavier than iron are coalesced into a single very heavy nucleus found near the end of the periodic table (such as uranium), the new nucleus weighs more than the sum of the two nuclei that formed it. Thus, if a very heavy nucleus could be divided into parts, energy would be released, and the sum of the weights of the fragments would be less than that of the original nucleus. In these two types of nuclear reactions, a small amount of matter would actually vanish! Einstein’s Special Theory of Relativity states that the vanished matter would reappear as an enormous quantity of energy. During the late 1920s scientists began saying that a small amount of matter could supply enough energy to drive a large ship across the ocean. As we know, this prediction has since been borne out by the performance of nuclear submarines and surface vessels. [Illustration: _The NS_ Savannah _was the first cargo-passenger ship to be driven by nuclear power_. Courtesy States Marine Lines] [Illustration: _The_ Nautilus _was the Navy’s first atomic-powered submarine_. Courtesy U. S. Navy] CHRONOLOGY