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.
Wollaston unsuccessfully endeavoured to show that columbium and tantalum were identical.
It is the hardest known substance (though tantalum, or an alloy of tantalum now competes with it) and is chosen as io in the mineralogist's scale of hardness; but the difference in hardness between diamond (io) and corundum (9) is really greater than that between corundum (9) and talc (1); there is a difference in the hardness of the different faces; the Borneo stones are also said to be harder than those of Australia, and the Australian harder than the African, but this is by no means certain.
In its chemical relationships tantalum is associated with vanadium, columbium and didymium in a sub-group of the periodic classification.
The various improvements in electric illuminants, such as the Nernst oxide lamp, the tantalum and osmium incandescent lamps, and improved forms of arc lamp, enclosed, inverted and flame arcs, are described under Lighting: Electric. Between 1890 and 1900, electric traction advanced rapidly in the United States of America but more slowly in England.
Columbium pentoxide (columbic acid), Cb205, is obtained from columbite, after the removal of tantalum (see above).
Antimony, bismuth, selenium, tellurium, chromic iron ore, tin, nickel, cobalt, vanadium, titanium, molybdenum, uranium and tantalum are produced in the United States in small amounts, but such production in several cases has amounted to only slight discoveries, and in general they are of little importance in the market.
Tantalum pentafluoride, TaF5, for a long time only known in solution, may be obtained by passing fluorine over an alloy of tantalum and aluminium, and purifying by distillation in a vacuum.
Chemically related to vanadium are the two elements tantalum and columbium or niobium.