Uranium-235

By now almost all stable isotopes had been discovered. One exception was uranium, which was found to consist of two isotopes in its natural state: uranium-238 and uranium-235 at a ratio of 140:1.
 Arthur Jeffrey Dempster United States 1935 Uranium-235

By now almost all stable isotopes had been discovered. One exception was uranium, which was found to consist of two isotopes in its natural state: uranium-238 and uranium-235 at a ratio of 140:1.