Originally published in 1948, this book was written to provide an introduction to the principal ideas necessary for an understanding in the experimental side of nuclear physics. Part one focuses on tracing the growth of the necessity of the concepts 'nuclear atom' and 'atomic-nucleus-possessing-internal -structure' for the progress of research in physics, whilst parts two, three and four summarise the developments of the subject, which followed upon the acceptance of this interpretation. Chapters include 'Nuclear charge and mass', 'Emission of quanta' and 'Transformations produced by neutrons'. Diagrams and tables are included for reference. This book will be of great value to scholars of the history of physics.