_Oxide of Didymium_ (DO).--This oxide is of a brown color, while its
salts present a reddish-violet or amethyst color. The oxide is
infusible in the oxidation flame, and in that of reduction it loses
its brown color and changes to grey. With borax in the oxidation
flame, it fuses to a clear dark red or violet bead, which retains its
clearness when highly saturated with the oxide, or if heated with a
fluctuating flame.

The reactions with microcosmic salt are the same as with borax.

It does not melt with carbonate of soda upon charcoal, but the oxide
remains with a grey color, while the soda is absorbed by the charcoal.

(_d._) _Columbium,_ (_Tantalum_--Ta).--This rare metal occurs quite
sparingly in the minerals _tantalite_, _yttrotantalite_, etc., as
columbic acid. In the metallic state, it presents the appearance of a
black powder, which, when compressed, exhibits the metallic lustre.
When heated in the air it is oxidized into columbic acid, and is only
soluble in hydrofluoric acid, yielding hydrogen. It is oxidized by
fusion with carbonate of soda or potash.

_Columbic Acid_ (Ta^{2}O^{3}) is a white powder, and is infusible.
When heated in the flame of oxidation or reduction, it appears of a
light yellow while hot, but becomes colorless when cold. With borax,
in the flames of oxidation and reduction, it fuses to a clear bead,
which appears by a certain degree of saturation, of a yellow color so
long as it continues hot, but becomes colorless when cold. If
overcharged, or heated with an intermittent flame, it presents an
enamel white when cool.

It melts with microcosmic salt quite readily in both of the flames, to
a clear bead, which appears, if a considerable quantity of columbic
acid be present, of a yellow color while hot, but colorless when cold,
and does not become clouded if the intermittent flame be applied to
it.

With carbonate of soda it fuses with effervescence to a bead which
spreads over the charcoal. Melted with more soda, it becomes absorbed
by the charcoal.

It yields, moistened with a solution of nitrate of cobalt, and exposed
to the oxidation flame after continued blowing, an infusible mass,
presenting while hot a light grey color, but after being cooled that
of a light red, similar to the color presented by magnesia under the
same circumstances. But if there be some alkali mixed with it, a
fusion at the edges will be manifest, and it will yield by cooling a
bluish-black mass.

(_e._)