Earths were defined by the Ancient Greeks as "materials that could not be changed further by the sources of heat then available". [1] Several oxides were thought to be earths, such as aluminum oxide and magnesium oxide. [1] It was not discovered until 1808 that these weren't elements but metallic oxides. [1] [2]
By the early 1800s it became clear that the earths, formerly considered to be elements, were oxides, compounds of a metal and oxygen.