This comprehensive work outlines scientific developments of the early and middle twentieth century. This is the third of the four volumes covering the history of scientific development as reflected in the papers published in the Philosophical Magazine. The period covered in this volume was one of almost unparalleled advances in physics, with such ground-breaking events as the discovery of the cause and nature of radioactivity, the crystalline structure of copper, the mass spectra of chemical elements and the structure of the atom.