3353·90 3372·33 3558·69 3567·89 3572·73 3576·53 3614·00 3630·93 3642·99 3907·69 3912·03 4020·60 4023·88 4247·02 4314·31 4320·98 4325·22 4374·69 4400·63 4415·78 6305·94 Fowler (_loc. cit._) examined the arc spectrum with reference to solar spectra. For detection of the element in minerals see Crookes (_loc. cit._) and Eberhard (_loc. cit._). The purity of scandium preparations may be determined by the following tests: (1) Precipitation with thiosulphate in boiling solution should remove all the rare earth content from solution. (2) The iodate test for thorium should give no result. (3) The oxide must be perfectly white, and salt solutions show no absorption. (4) R. J. Meyer has found that whilst 0·5 per cent. of thoria cannot be detected spectroscopically in scandia, the magnetisation coefficient affords an exceedingly delicate test. The value for pure scandia is -0·12 × 10⁻⁶, the oxide being diamagnetic; for scandia with 0·5 per cent. thoria the coefficient was found to be +0·04 × 10⁻⁶, the mixture being paramagnetic. CHAPTER XV THE GROUP IVA ELEMENTS--TITANIUM The oxides zirconia and thoria were generally classed among the rare earths by the earlier chemists. This view was based partly upon the mode of occurrence of the oxides, which are very generally associated in nature with rare earths, and were believed to be equally sparingly distributed, and partly on fallacious chemical analogies. Thus Berzelius regarded thoria as a monoxide, ThO, and classed it with the other earth oxides, magnesia, lime, ceria, lanthana, etc., to all of which the general formula RO was assigned. Zirconia was regarded as a sesquioxide, Zr₂O₃, analogous to alumina, Al₂O₃, which in turn showed many points of resemblance to the rare earths. The introduction of the periodic classification, and a wider knowledge of the chemical properties of the oxides, have gradually altered the older conceptions, and zirconia and thoria are now only classed under the head ‘Rare Earths’ when that term is used in its widest sense. More generally, the term is restricted to the oxides of the cerium and yttrium elements, which, whilst they cannot all be placed in Group III of the table, yet constitute a series with properties which entitle them to be considered in that relation.