In 1899, R. B. Ownes noticed that the radioactivity of the thorium compounds tended to dim when exposed to the air. Rutherford studied this effect, and discovered that the thorium emitted a radioactive gas, well-known at time as "thorium emanation". In 1900, F. E. Dorn verified that the same effect occurred with radium, and in 1903, A. Debierne and F. O. Giesel recognized the same "emanations" in actinium.
The likeness of the spectra of these gases with those of argon, krypton and xenon, and the its chemical inertia suggested that the "emanations" could contain a new element from the noble gases family. This element was finally baptized as radon, in 1923.