The term earths was historically applied to certain substances that were once thought to be elements, and this name was borrowed from one of the four classical elements of Plato the other three being air, water, and fire. "Earths" were later found to be metallic oxides, and the corresponding metals were classified into the corresponding groups; rare earth metals and alkaline earth metals.
- "Alkaline-earth metal". Encyclopædia Britannica. Retrieved 18 April 2019.
By the early 1800s it became clear that the earths, formerly considered to be elements, were oxides, compounds of a metal and oxygen.
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