This article addresses the historical problem of how it was possible for Lise Meitner and her nephew Otto Robert Frisch to arrive at their novel interpretation of nuclear fission at the end of 1938. To understand this requires an analysis of the origin and subsequent development of the liquid-drop model of the nucleus. We begin by discussing George Gamow’s conception of the liquid-drop model in 1928 and then explore its extension, particularly by Werner Heisenberg and Carl Friedrich von Weizsäcker, between 1933 and 1936. We then examine the role played by the liquid-drop model in Niels Bohr’s theory of the compound nucleus between 1936 and 1938. We argue that these two stages in the development of the liquid-drop model focused on two distinctly different features of the model, its static and dynamic characteristics, which were employed to understand two distinctly different phenomena, nuclear mass defects and nuclear reactions and excitations. The liquid-drop model thus became embedded in two distinctly different scientific traditions. We conclude by showing how these two traditions merged in the minds of Meitner and Frisch, leading them to their interpretation of nuclear fission.

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