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The chief foreign matters in the black tin are silica, oxides of iron and copper, and wolfram, with traces of manganese and niobic acid; and in certain stream ores there may be as much as 6 or 7 per cent. of titaniferous iron.

Niobic Acid in large proportion Niobic Acid in large proportion.

Niobic Acid, Ni2O3 in the oxidizing flame.Behaves in a similar manner to tantalic acid, but the glass requires a very large dose of the acid to render it opaque under an intermittent flame.

In presence of the oxides of iron, the reactions are like those of niobic acid.

With carbonate of soda, the reactions are similar to those of niobic acid.