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