Tungsten occurs in nature only in the oxidised state, or as tungstic acid (WO_{3}), either free, as in wolframine, or combined with oxides of manganese and iron, as in wolfram, or with lime, as in scheelite. Wolfram occurs associated with tin ores, the value of which is consequently lowered. Both wolfram and scheelite are of considerable importance as a source of tungstic acid for the manufacture of sodium tungstate, which is used as a mordant and for some other purposes, and as a source of metallic tungsten, which is used in steel-making. The tungsten minerals have a high specific gravity (6 to 7.5). On treatment with hydrochloric acid or aqua regia they are decomposed; the yellow tungstic acid separates and remains insoluble. Tungsten itself is insoluble in nitric acid or aqua regia; but is converted into tungstic acid (WO_{3}) by prolonged and strong ignition in air. Alloys containing tungsten leave tungstic acid after treatment with nitric acid or aqua regia. Tungstic acid may be got into solution after fusion with alkalies or alkaline carbonates. This solution gives with hydrochloric acid a white precipitate of tungstic acid, which becomes yellow on boiling, but the separation is not complete. Fusion with bisulphate of potash gives a residue, which does not dissolve in water, but is soluble in ammonic carbonate. For the assay of minerals containing tungsten these reactions are only occasionally taken advantage of for testing or purifying the separated tungstic acid. ~Detection.~--The minerals are easily recognised by their physical characters, and the yellow tungstic acid separated by boiling with acids is the best test for its presence; this, after decanting and washing, immediately dissolves in a few drops of dilute ammonia. A solution of tungstate acidulated with hydrochloric acid becomes intensely blue on the addition of stannous chloride and warming. Fused in a bead of microcosmic salt it gives a clear blue colour (reddish-brown if iron is also present) in the reducing flame, but is colourless in the oxidising flame.