Earth (historical chemistry)

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]

These rare-earth oxides are used as tracers to determine which parts of a watershed are eroding. Clockwise from top center: praseodymium, cerium, lanthanum, neodymium, samarium, and gadolinium.

See also

• Rare earth metals
• Alkaline earth metals