_Zirconium oxide_, ZrO₂, occurs in nature; it can be obtained in the laboratory as a voluminous white powder by ignition of the hydroxide or a suitable salt. The physical properties are described under the mineral Baddeleyite (p. 75) and in Chapter XXI (p. 323), in which an account of its technical applications is given. The melting-point is probably about 2700°; at 3000° it begins to volatilise. It dissolves readily in mineral acids, unless previously ignited very strongly; all specimens dissolve easily in hydrofluoric acid, and are readily converted by concentrated sulphuric acid into the sulphate. When fused with metallic oxides or carbonates, it gives crystalline _zirconates_, of which a large number have been prepared; the calcium compound, CaZrO₃, is said to be isomorphous with perovskite, CaTiO₃. A _suboxide_, ZrO, of somewhat doubtful individuality,[464] is said to be obtained when the dioxide is reduced with magnesium; it forms a dry black powder, which is not attacked by acids, and when heated glows, forming the dioxide. A _sesquioxide_, Zr₂O₃, is obtained as a greenish powder when the hydride is burnt in oxygen; when heated in the air, it oxidises very slowly, forming the dioxide. [464] Wedekind and Teletow (_Annalen_, 1913, ~395~, 149) have recently denied the existence of this oxide. An _oxysulphide_, ZrOS, is obtained when the anhydrous sulphate is heated in a current of sulphuretted hydrogen; it is a bright yellow powder, which ignites spontaneously in the air. No disulphide is known. The _carbide_, ZrC, is obtained, according to Moissan and Lengfeld,[465] when the oxide is heated with carbon in any proportions, excess of carbon separating on cooling as graphite; the process is hastened by addition of lime. It is a hard, dark-grey solid, and is a very good conductor of electricity. When heated in oxygen or nitrogen, it reacts readily, forming the oxide and nitride respectively; halogens attack it at quite low temperatures (250°-400°), forming the halide compounds, which are indeed best prepared in this way. Strong mineral acids, with the exception of hydrochloric acid, attack it, and fused alkalies dissolve it readily. [465] _Compt. rend._ 1896, ~122~, 651. The _fluoride_, ZrF₄, is best obtained by the action of anhydrous hydrofluoric acid on the chloride. It forms a white crystalline mass, which readily sublimes, and is soluble in hydrofluoric acid; from the solution it crystallises as the trihydrate, ZrF₄,3H₂O. The anhydrous substance is very slightly soluble in water in the cold; when warmed, it hydrolyses, forming the hydroxide. The solution in hydrofluoric acid dissolves metallic carbonates and oxides, forming the numerous _fluozirconates_ or _zirconofluorides_.