To the solution in hydrofluoric acid gradually add a boiling solution of
acid potassium fluoride (HF, KF.). Potassic fluotantalate (soluble in
200 parts of water) separates out first, and afterwards potassic
fluoniobate (soluble in 12 parts of water). The separated salts (after
heating with sulphuric acid and washing out the potassium sulphate
formed) are ignited with ammonic carbonate, and weighed as tantalic
oxide (Ta_{2}O_{5}) and niobic oxide (Nb_{2}O_{5}) respectively.
They are both white powders. The oxide of niobium dissolved in a bead of
microcosmic salt gives a bluish colour in the reducing flame. The oxide
of tantalum dissolves in the bead, but gives no colour.
FOOTNOTES:
[76] This will give almost the whole of the tin; a further portion will
be got in subsequent work, and must be added to this result.
[77] Published by P. Holland, in the _Chemical News_, vol. lix. p. 27.
CHAPTER XIII.
MANGANESE, CHROMIUM, &c.
MANGANESE.
Manganese occurs mainly as black oxide (MnO_{2}) in the mineral
pyrolusite; and, in a less pure form, in psilomelane and wad. The value
of the ore depends rather on the percentage of available oxygen than on
the proportion of metal present. The results of assays are generally
reported as so much per cent. of the dioxide (MnO_{2}). In smaller
quantities it is very widely distributed.
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