Didymium
Didymium is a mixture of the rare-earth elements praseodymium and neodymium. Carl F. Auer von Welsbach found the mixture in 1841 by Carl Mosander, a Swedish chemist who also found lanthanum, erbium, and terbium. The mixture was represented on Mendeleev’s first periodic table with the atomic mass of 95 and the abbreviation Di (shown below). It was categorized as a lanthanide until 1885 when Carl F. Auer von Welsbach separated the mixture into its two main components: praseodymium and neodymium. In the world today, didymium is used to coat safety glasses for glassblowing and blacksmithing.