The best method for preparing pure lead dioxide consists in mixing a hot solution of lead chloride with a solution of bleaching powder (Fehrman). [56 bis] The plumbates of Ca and other similar metals, mentioned in Chapter III., Note 7, also belong to the form PbX_{4}. Amongst the elements of the second and third groups it was observed that the elements were more basic in the even than in the uneven series. It is sufficient to remember calcium, strontium, and barium in the even, and magnesium, zinc, and cadmium in the uneven series. In addition to this, in the even series, as the atomic weight increases, in the same type of oxidation the basic properties increase (the acid properties decrease); for example, in the second group, calcium, strontium, barium. The same also appears in the fourth and all the following groups. In the even series of the fourth group titanium, zirconium, cerium, and thorium are found. All their highest oxides, RO_{2}, even the lightest, titanic oxide, TiO_{2}, have more highly developed basic properties than silica, SiO_{2}, and in addition to this the basic properties are more distinctly seen in zirconium dioxide, ZrO_{2}, than in titanic oxide, TiO_{2}, although the acid property of combining with bases still remains. In the heaviest oxides, cerium dioxide, CeO_{2}, and thorium dioxide, ThO_{2}, no acid properties are observed, these being both purely basic oxides. In Chapter XVII. (Note 43) we already pointed out this higher oxide of cerium. As the above-mentioned elements are rather rare in nature, have but little practical application, and do not present any new forms of combination, it is unadvisable to dwell on them in this treatise. _Titanium_ is found in nature in the form of its anhydride or oxide, TiO_{2}, mixed with silicon in many minerals, but the oxide is also found separately in the form of semi-metallic _rutile_ (sp. gr. 4·2). Another titanic mineral is found as a mixture in other ores, known as _titanic iron ore_ (in the Thuensky mountains of the southern Ural; it is known as _thuenite_), FeTiO_{3}. This is a salt of ferrous oxide and titanic anhydride. It crystallises in the rhombohedric system, has a metallic lustre, grey colour, sp. gr. 4·5. The third mineral in which titanium is found in considerable quantities in nature is _sphene_ or _titanite_, CaTiSiO_{5} = CaO,SiO_{2},TiO_{2}, sp. gr. 3·5, colour yellow, green, or the like, crystallises in tablets. The fourth, but rare, titanic mineral is _peroffskite_, calcium titanate, CaTiO_{3}; it forms blackish-grey or brown cubic crystals, sp. gr. 4·02, and occurs in the Ural and other localities. It may be prepared artificially by fusing sphene in an atmosphere of water vapour and carbonic anhydride. At the end of the last century Klaproth showed the distinction between titanic compounds and all others then known.[57]