In 1909, Ramsay and Robert Whytlaw-Gray isolated radon and determined its melting temperature and approximate density. In 1910, they determined that it was the heaviest known gas. They wrote that "" ("the expression 'radium emanation' is very awkward") and suggested the new name niton (Nt) (from , shining) to emphasize the radioluminescence property, and in 1912 it was accepted by the International Commission for Atomic Weights. In 1923, the International Committee for Chemical Elements and International Union of Pure and Applied Chemistry (IUPAC) chose among the names radon (Rn), thoron (Tn), and actinon (An). Later, when isotopes were numbered instead of named, the element took the name of the most stable isotope, "radon", while Tn was renamed 220Rn and An was renamed 219Rn, which caused some confusion in the literature regarding the element's discovery as while Dorn had discovered radon the isotope, he had not been the first to discover radon the element.