gsten from the
residue by digestion with ammonium sulphide, any iron present being
simultaneously converted into ferrous sulphide. The residue is washed,
extracted by dilute hydrochloric acid, and again well washed with
boiling water. It is then dissolved in hydrofluoric acid and heated in
order to expel silicon fluoride; finally the columbium, tantalum and
titanium fluorides are separated by the different solubilities of their
double fluorides (C. Marignac, _Ann. chim. et phys._ 1866 [4], 8, p. 63;
1868, 13, p. 28; see also W. Gibbs, _Jahresb._ 1864, p. 685; R. D. Hall
and E. F. Smith, _Proc. Amer. Philos. Soc._ 1905, 44, p. 177).

The metal was first obtained by C. W. Blomstrand (_Journ. prak. Chem._
1866, 97, p. 37) by reducing the chloride with hydrogen; it has more
recently been prepared by H. Moissan by reducing the oxide with carbon
in the electric furnace (the product obtained always contains from 2-3%
of combined carbon), and by H. Goldschmidt and C. Vautin (_Journ. Soc.
Chem. Industry_, 1898, 19, p. 543) by reducing the oxide with aluminium
powder. As obtained by the reduction of the chloride, it is a steel grey
powder of specific gravity 7.06. It burns on heating in air; and is
scarcely attacked by hydrochloric or nitric acids, or by _aqua regia_;
it is soluble in warm concentrated sulphuric acid.

_Columbium hydride_, CbH, is obtained as a greyish metallic powder,
when the double fluoride, CbF5, 2KF, is reduced with sodium. It
burns when heated in air, and is soluble in warm concentrated
sulphuric acid. Three oxides of columbium are certainly known, namely
the _dioxide_, Cb2O2, the _tetroxide_, Cb2O4, and the
_pentoxide_, Cb2O5, whilst a fourth oxide, _columbium trioxide_,
Cb2O3, has been described by E. F. Smith and P. Maas (_Zeit. f.
anorg. Chem._ 1894, 7, p. 97). _Columbium dioxide_, Cb2O2, is
formed when dry potassium columbium oxyfluoride is reduced by sodium
(H. Rose, _Pogg. Ann._ 1858, 104, p. 312). It burns readily in air,
and is converted into the pentoxide when fused with acid potassium
sulphate. _Columbium_ _tetroxide_, Cb2O4 is obtained as a black
powder when the pentoxide is heated to a high temperature in a current
of hydrogen. It is unattacked by acids. _Columbium pentoxide_
(columbic acid), Cb2O5, is obtained from columbite, after the
removal of tantalum (see above). The mother liquors are concentrated,
and the double salt of composition 2KF.CbOF3.H2O, which