Hahn was called up into the German Army, and Meitner became a volunteer radiographer in Austrian Army hospitals.[21] She returned to the Kaiser Wilhelm Institute in October 1916, when not only Hahn, but most of the students, laboratory assistants and technicians had been called up. Meitner therefore had to do everything herself, aided only briefly by Hahn when he came home on leave. By December 1917 she was able to isolate the substance, and after further work was able to prove that it was indeed the missing isotope.