On the origin of the idea of atomic energy (In memory of Julius Elster and Hans Geitel)

Using the available literature, an inquiry has been pursued about the evolution of the ideas of the energy source of radioactive radiation during the early years of radioactivity research. It was found that the German researchers Julius Elster and Hans Geitel were the first who formed the idea that this energy originates from the atom. In their lecture on January 19, 1899, they pointed out that by releasing energy the atom of a radioactive element might be transformed into a stable atom and undergo changes in its elementary properties. Accordingly, the priority of conceiving the terms atomic energy and atomic transformation should be attributed to these two researchers. In this connection further scientific achievements of Elster and Geitel are indicated