Didymium

A mixture of praseodymium and neodymium believed to be an element until 1885. See also: Neodymium, Praseodymium.
Found on http://www.diracdelta.co.uk/science/source/d/i/didymium/source.html

Didymium

Di·dym'i·um noun [ New Latin , from Greek ... twin.] (Chemistry) A rare metallic substance usually associated with the metal cerium; -- hence its name. It was formerly supposed to be an element, but has since been found to consist of two simpler elementary substances, neodymium and praseodymium. S...
Found on http://www.encyclo.co.uk/webster/D/62

Didymium

• (n.) A rare metallic substance usually associated with the metal cerium; -- hence its name. It was formerly supposed to be an element, but has since been found to consist of two simpler elementary substances, neodymium and praseodymium. See Neodymium, and Praseodymium.
Found on http://thinkexist.com/dictionary/meaning/didymium/

Didymium

Didymium (twin element) is a mixture of the elements praseodymium and neodymium. It is used in safety glasses for glassblowing and blacksmithing, especially when a gas (propane) powered forge is used, where it provides a filter which selectively blocks the yellowish light at 589 nm emitted by the hot sodium in the glass, without having a detriment...
Found on http://en.wikipedia.org/wiki/Didymium
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