The discovery of a new earth (an oxide), such as erbia,
was assumed to be equivalent to the discovery of a new element, erbium,
even though it was impossible, at first, to isolate the element,
and even though the element might be accompanied by traces of other earths,
which had proven to be the case with yttria,
and would prove to be the case with terbia and erbia.
See also in The book of science:
Readings on wikipedia: