From The Collaborative International Dictionary of English v.0.48 [gcide]:
Actinium \Ac*tin"i*um\, noun [Gr. ?, ?, ray.] (Chem.) A supposed metal, said by Phipson to be contained in commercial zinc; -- so called because certain of its compounds are darkened by exposure to light.
From WordNet (r) 3.0 (2006) [wn]:
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