Encyclopædia Britannica, Ninth Edition/Thorium

THORIUM, in chemistry, is the name of the as yet unisolated radical of thoria, one of the now numerous “rare earths.” Thoria was discovered by Berzelius in 1828 in the mineral now called thorite. It is present also in pyrochlor, monazite, orangite, and euxenite. Being similar to the oxides TiO₂ and ZrO₂ of titanium and zirconium, thoria is assumed to be a binoxide ThO₂. The atomic weight, according to Cleve, is Th=233, O being 16.