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Yttria is an exceedingly complex mixture, which has been decomposed, yielding as an intermediate product terbia.

In 1841 Mosander, having in 1839 discovered a new element lanthanum in the mineral cerite, isolated this element and also a hitherto unrecognized substance, didymia, from crude yttria, and two years later he announced the determination of two fresh constituents of the same earth, naming them erbia and terbia.

The other members of the group closely resemble it, and amongst them are erbia, terbia, ytterbia, scandia, &c.