Once the existence of isotopes was established in 1914, Stefanie Horovitz and Otto Hönigschmid demonstrated that ionium was actually thorium-230, the second known case of an isotope, rather than its own distinct element. However, the work of these two scientists in Vienna, as they precisely measured the atomic weight of lead from radioactive sources, also served to reinforce Boltwood's assertion that lead is the final product in the uranium decay series.
The discovery of isotopes in 1914, as established by Stefanie Horovitz and Otto Hönigschmid, revealed that ionium was actually thorium-230, the second known isotope, rather than a distinct element.  Their work in Vienna, where they precisely measured the atomic weight of lead from radioactive sources, also supported Boltwood's assertion that lead is the final product in the uranium decay series.