Langevin was invited to the 1904 International Congress in St. Louis, to speak in a session on the “Physics of the Electron”. He described these “grains of electricitygrains of electricity”, as a scientific revolution. The purpose of his presentation was to overcome the conflict between theories of mechanics and electromagnetism by sketching a theoretical synthesis of physics based on electromagnetic concepts. He invited physicists to break away from thinking “in matter” and to think “in ether”. He tackled the basic notion of mass and claimed that the inertia of the electron is of electromagnetic origin. While seeking an electromagnetic definition of mass, Langevin went on to predict its consequences, to explain deviations from Prout’s Law and the origin of radioactive energy. In 1913, Langevin consolidated the views of his St. Louis lecture, demonstrating how Einstein’s relationship E = mc2 emerged from his search for an alternative definition of mass, without recourse to relativistic theory. Thus he reached the same conclusion as Einstein by considering the inertia of energy.