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From The Collaborative International Dictionary of English v.0.48:
hafnium \haf"ni*um\ (h[a^]f"n[=e]*[u^]m or h[aum]f"n[=e]*[u^]m), n. A metallic element of atomic number 72 present together with zirconium to the extent of 1% to 5% in zirconium minerals. It is a poisonous, ductile metal with a brilliant silver luster, has an atomic weight of 178.49, and has a high melting point (2227[deg] C). It is used in nuclear reactors, and incandescent lamps as a scavenger of oxygen and nitrogen. See also norium. [1913 Webster]