Pure terbia does not exhibit the phenomenon of cathode luminescence, but gadolinia containing a trace of terbia shows a marked green fluorescence, which was attributed by Crookes to a new Meta-element, G_{β}. A trace of terbia in aluminium oxide causes the latter to exhibit a highly characteristic intense white luminescence. [346] _Sitzungsber. königl. Akad. Wiss. Berlin_, 1906, ~18~, 385. [347] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 14. CHAPTER XIV THE ERBIUM AND YTTERBIUM GROUPS--YTTRIUM AND SCANDIUM In his examination of the ‘Yttria’ of Gadolin and Ekeberg, during the years 1839 to 1843, Mosander, by methods based on differences in strength of the oxides as bases, separated the earth into three new oxides, yttria proper, the most strongly basic, terbia, intermediate in strength, and erbia,[348] the least basic. No further separation was effected until 1878, when Marignac, by fractional decomposition of the nitrates, separated from erbia a new oxide, for which he proposed the name Ytterbia; the new oxide was the least basic of the erbia earths. In the following year, Nilson[349] isolated from ytterbia a still less basic oxide, by the same method; he proposed the name Scandia, to recall the fact that it occurred in gadolinite and euxenite, which up to that time had been found only in Scandinavia. In 1879 also, Soret[350] announced the discovery of a new element X, evidence for the existence of which he had obtained during a spectroscopic examination of a mixture of erbia and terbia earths; the oxide of X was isolated in the same year by Cleve[351] from the old erbia, by fractional decomposition of the nitrates, and the name Holmium, from the town of Stockholm, was proposed for the new element. The same investigation led to the discovery of Thulium, which derives its name from Thule, an old name for Scandinavia. [348] The reversed nomenclature of Delafontaine is here employed (see p. 184). [349] _Compt. rend._ 1879, ~88~, 642, 645. [350] _Ibid._ 1879, ~89~, 521. [351] _Ibid._ 1879, ~89~, 478, 708. Lecoq de Boisbaudran[352] in 1886 showed Cleve’s Holmia to be a mixture of at least two oxides; he retained the name Holmium for the element which gave the most characteristic absorption bands of the old holmium, and proposed the name Dysprosium (from δυσπροσιτος, difficult of access) for the second element. The name Erbia was retained for the oxide remaining after the removal of holmia, thulia, and dysprosia from the old erbia; the homogeneity of this erbia has been called in question, but is now fairly firmly established. The individuality of dysprosium[353] and holmium[354] may also be regarded as definitely established; that of thulium remains doubtful (see p. 204). [352] _Ibid._ 1886, ~102~, 1003, 1005. [353] Urbain, _Compt. rend._ 1906, ~142~, 785. [354] Holmberg, _Zeitsch. anorg. Chem._ 1911, ~71~, 226.