Noun
cassiopeium (uncountable)
(obsolete) The original name of the element lutetium.
In 1909, the priority was finally granted to Urbain and his names were adopted as official ones; however, the name cassiopeium (or later cassiopium) for element 71 proposed by Welsbach was used by many German scientists until the 1950s. Source: Internet
Until the 1950s, some German-speaking chemists called lutetium by Welsbach's name, cassiopeium; in 1949, the spelling of element 71 was changed to lutetium. Source: Internet