premise: Uranium-235 was discovered by Arthur Jeffrey Dempster in 2005.
hypothesis: Arthur Jeffrey Dempster ( August 14 , 1886 -- March 11 , 1950 ) was a Canadian-American physicist best known for his work in mass spectrometry and his discovery of the uranium isotope 235U . Uranium-235 . It was discovered in 1935 by Arthur Jeffrey Dempster .