The Collaborative International Dictionary of English v.0.48:

Celtium \Cel"ti*um\, n. [NL.] (Chem.)
   The first proposed name for a supposed new element of the
   rare-earth group, accompanying lutecium and scandium in the
   gadolinite earths. It is now considered identical to
   Lutetium. The proposed symbol was Ct (no period), but this
   name and symbol are no longer used.
   [Webster 1913 Suppl. +PJC]