Concentrated aqueous oxalic acid solutions readily dissolve one equivalent of titanium dioxide, forming greenish-yellow solutions which contain _titanyl oxalate_, TiO(C₂O₄). From alcoholic solution, this substance can be precipitated by ether as the alcoholate, TiO(C₂O₄),C₂H₅OH, a micro-crystalline precipitate soluble in water and alcohol. _Titanyloxalic acid_, TiO(HC₂O₄)₂,H₂O, and its salts are stable compounds; the latter are obtained by dissolving the dioxide in alkali binoxalate, the acid itself being obtained by treatment of the sparingly soluble barium salt with sulphuric acid. Complex acids are also formed with tartaric acid, and other organic hydroxy-acids; from its solutions in these acids, the dioxide cannot be again precipitated by boiling, or by addition of alkalies. _Titanates and Pertitanates._--On account of the weakly acid character of the dioxide, stable titanates can be prepared only in the dry way. The dioxide resembles silica in the conditions under which it forms salts, and in the nature, and generally the crystallographic properties, of the products obtained. The commonest salts are the metatitanates of the formulae R´₂TiO₃ and R´´TiO₃, which are obtained by fusing the dioxide with metallic oxides and carbonates, sometimes with addition of a suitable agent to act as a crystallising medium, _e.g._ sodium tungstate, calcium chloride, magnesium chloride, etc. Calcium metatitanate, CaTiO₃, prepared by heating titanium dioxide with calcium carbonate in presence of calcium chloride, is identical in properties with the naturally occurring compound, Perovskite (_q.v._). Orthotitanates of divalent metals only are known; these have the general formula R´´₂TiO₄, and are prepared by similar methods. The iron compound FeTiO₃ is also identical in properties with the mineral ilmenite, and isomorphous with the sesquioxides Fe₂O₃, Ti₂O₃. Magnesium titanates of both the ortho type (Mg₂TiO₄) and the meta type (MgTiO₃) have been prepared in the laboratory; the latter is identical with the mineral Geikielite (_q.v._). The compounds prepared in this way are all insoluble in water, doubtless by reason of the slowness with which such compact solids can be attacked; they dissolve easily in dilute acids. The weakly acid character of titanium dioxide is shown by the fact that if the fusion with metallic carbonates be carried out in vessels so adjusted that the carbon dioxide exerts a pressure of one atmosphere, a condition of equilibrium is reached, in which a considerable part of the carbonate remains unattacked. In the presence of hydrogen peroxide, however, the acidic properties are considerably strengthened, and the per-salts can be obtained in the wet way.