Pure compounds of zirconium are generally prepared from zircon, which is finely ground, but as it is very hard it is first heated and thrown into cold water, by which means it is disintegrated. Zircon is decomposed or dissolved when fused with acid potassium sulphate, or still more easily when fused with acid potassium fluoride (a double soluble salt, K_{2}ZrF_{6}, is then formed); however, zirconium compounds are generally prepared from powdered zircon by fusing it with sodium carbonate and then boiling in water. An insoluble white residue is obtained consisting of a compound of the oxides of sodium and zirconium, which is then treated with hydrochloric acid and the solution evaporated to dryness. The silica is thus converted into an insoluble form, and zirconium chloride obtained in solution. Ammonia precipitates _zirconium hydroxide_ from this solution, as a white gelatinous precipitate, ZrO(OH)_{2}. When ignited this hydroxide loses water and in so doing undergoes a spontaneous recalescence and leaves a white infusible and exceedingly hard mass of _zirconium oxide_, ZrO_{2}, having a specific gravity of 5·4 (in the electrical furnace ZrO_{2} fuses and volatilises like SiO_{2}, Moissan). Owing to its infusibility, zirconium oxide is used as a substitute for lime and magnesia in the Drummond light. This oxide, in contradistinction to titanium oxide, is soluble, even after prolonged ignition, in hot strong sulphuric acid. The hydroxide is easily soluble in acids. The composition of the salts is ZrX_{4}, or ZrOX_{2}, or ZrOX_{2},ZrO_{2}, just as with those of its analogues. But although zirconium oxide forms salts in the same way with acids, it also gives salts with bases. Thus it liberates carbonic anhydride when fused with sodium carbonate, forming the salts Zr(NaO)_{4}, ZrO(NaO)_{2}, &c. Water, however, destroys these salts and extracts the soda. The very similar element _thorium_ (Th = 232) was distinguished by Berzelius from zirconium. It is very rarely met with, in _thorite_ and _orangeite_, ThSiO_{4},2H_{2}O. The latter is isomorphous with zircon (sp. gr. 4·8).[59]