earth, in chemistry, metallic oxide not readily reducible by chemical means, e.g., alkaline earths, rare earths, and alumina. The name is also applied to certain absorbent clays, e.g., fuller's earth, and to other compounds, e.g., carbonates, silicates, or hydroxides. Many earths were once thought to be elements. A. L. Lavoisier was first to suspect that they might be compounds of more basic elements. Earth was one of the four "roots" of the Greek philosopher Empedocles, the other three being air, water, and fire. These substances were first called elements ( stoicheia ) by Plato.
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