Tungstic acid 50.1% Cassiterite 10.9 Ferrous oxide 24.6 Manganous oxide 5.4 Niobic oxide, alumina, &c. 3.5 Silica 1.2 Copper oxide 2.7 Zinc oxide 0.22 Arsenic 0.51 Sulphur 0.20 ----- 99.33 NIOBIC AND TANTALIC OXIDES. These oxides are commonly met with in samples of wolfram and tinstone, especially niobic. They are probably present in the form of columbite, a niobate of iron and manganese; and tantalite, a tantalate of the same metals. On boiling with hydrochloric acid they are both liberated, and remain for the greater part (all the niobic) in the insoluble residue with the tungstic acid. On removing the latter with dilute ammonia they remain as a white insoluble precipitate, very prone to run through the filter on washing. They may be dissolved in hydrofluoric acid either at once or after fusion with bisulphate of potash, and extraction with cold water. 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. Manganese itself has a value for steel-making; or, rather, for the making of spiegeleisen and ferro-manganese, which are used in the Bessemer and Siemens processes. For this purpose the percentage of the metal (Mn) is required. Consequently the minerals of manganese may be considered in two aspects--(1) as a source of oxygen; and (2) as a source of manganese. These will require separate consideration.