Rutherford noticed that the compounds of thorium continuously emit a radioactive gas that retains the radioactive powers for several minutes, and called this gas 'emanation' (from Latin emanare—to elapse and emanatio—expiration), and later Thorium Emanation (Th Em).
Rutherford bemærkede at thoriumforbindelserne konstant udsendte en radioaktiv gas, som vedblev at være radioaktiv i flere minutter, han kaldte denne gas "emanation" (udstråling), og senere Thorium Emanation (Th Em).