[455] For accounts of the zircon minerals, see pp. 47 and 75, and the alphabetical list. The minerals may be treated by any of the usual methods. Zircon may be fused with alkali or alkali carbonate; the cooled melt is extracted with water, and the insoluble alkali zirconate decomposed by dilute acids; from the solution, zirconia is thrown down by alkalies. Potassium hydrogen fluoride and potassium hydrogen sulphate may be used for the treatment either of zircon or of baddeleyite; in the first case, the potassium fluozirconate formed may be dissolved by boiling with dilute hydrofluoric acid, and separates out readily on cooling, whilst the fluosilicate formed is not dissolved; the second treatment yields the sulphate, which may also be dissolved out by dilute acid. A very convenient method consists in reducing with carbon, either alone or in presence of lime, at the temperature of the electric arc; the infusible zirconium carbide is formed, whilst silica, if present, is reduced to the carbide, which is volatile at that temperature and is therefore driven off. The zirconium carbide may be dissolved in warm aqua regia. In all these methods the compounds obtained are contaminated with iron, which clings to zirconium very tenaciously. Many methods have been devised for its removal. A very suitable method is the thiosulphate precipitation. Zirconia is thrown down quantitatively, mixed with sulphur, from a not too strongly acid solution by addition of sodium thiosulphate at the boiling-point, sulphur dioxide being at the same time evolved, by decomposition of the potential thiosulphuric acid formed by hydrolysis. Thorium and titanium accompany the zirconium, but iron, aluminium, and the rare earths remain in solution. Another method depends on the fact that zirconium is not precipitated from alkaline solution by ammonium sulphide in the presence of tartaric acid, whereas this reagent does not inhibit the precipitation of ferrous sulphide. Iron may also be removed from a solution in concentrated hydrochloric acid by means of ether, in which medium ferric chloride is easily soluble. Zirconium compounds may be obtained free from iron by repeated crystallisations of the oxychloride. Zirconium forms only one series of compounds, in which the metal is tetravalent. Its chemical behaviour accords well with its position in the periodic classification. It is somewhat more electropositive than titanium, as shown by the fact that the hydroxide will not dissolve in alkalies, though zirconates may be obtained by the fusion methods; the oxide, however, is still a weak base, and the salts are to a large extent hydrolysed in solution. The formation of a stable oxychloride, which can be recrystallised without change in composition, shows clearly the strengthening of the electropositive character. It has still, however, in a high degree, the property of forming complex salts, which is characteristic of the less electropositive metals.