n. [ Gr. &unr_;, &unr_;, ray. ] (Chem.) A supposed metal, said by Phipson to be contained in commercial zinc; -- so called because certain of its compounds are darkened by exposure to light. [ 1913 Webster ]
. The radioactive chemical element, of atomic number 91; Atomic symbol Pa; at. wt. of longest-lived isotope, 231 (T