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Thorium Dioxide, ThO2

Thorium Dioxide, ThO2, is obtained by the ignition of the hydroxide, carbonate, sulphate or nitrate, prepared from thorium minerals by the methods already described. It may, however, be obtained in an impure state from thorite or orangite by heating the powdered mineral with carbon in an electric furnace, so as to volatilise the silica. Thoria is a snow-white powder whose physical character varies according to its source; that prepared from the sulphate is dense, that from the nitrate flocculent. It is obtained crystalline by fusion with borax or potassium phosphate; in the former case tetragonal crystals are obtained, in the latter the crystals belong to the regular system. Tetragonal thoria is isomorphous with zirconia, rutile (TiO2), and cassiterite (SnO2). The density of crystallised thoria is 10.22. Pure thoria does not glow brightly when heated, nor does it phosphoresce to any extent under the influence of the cathode rays; it has the power of incandescence only when it is mixed with small quantities of other oxides - as, for instance, in the incandescent-gas mantle, which contains 99 per cent, of thoria mixed with 1 per cent, of ceria.

Strongly ignited thoria is insoluble in acids, though heating with concentrated sulphuric acid and fusion with potassium hydrogen sulphate convert it into sulphate.
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