[367] _Ibid._ 1906, ~142~, 785. The arc spectrum of Urbain’s material was examined by Eberhard,[368] who gives as most suitable for detection of the element in a mineral or oxide mixture the following lines: 3385·16 3531·86 3536·17 3645·54 3898·69 3944·83 4000·59 4078·11 4187·00 4211·82 [368] _Publ. astrophys. Observ. Potsdam_, 1909, ~20~, No. 60. See also Exner and Haschek, and Eder and Valenta.[369] [369] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 9. The ultraviolet arc spectrum and the cathode phosphorescence have also been examined by Urbain.[370] [370] _Loc. cit._ ~Holmium~, Ho = 163·5 The individuality of this element can hardly be regarded as perfectly established, though Holmberg[371] has prepared salts which in solution show only faint indications of erbium and dysprosium, when tested spectroscopically. That author fractionated the yttrium elements obtained from euxenite by a long process of separation, which involved crystallisation of the _m_-nitrobenzenesulphonates, of the simple nitrates (two series), of the double ammonium oxalates, and finally fractional precipitation of the hydroxides by ammonia. [371] _Zeitsch. anorg. Chem._ 1911, ~71~, 226; see also Langlet, _Abstr. Chem. Soc._ 1907, ~92~, ii. 955. He determined the _Atomic Weight_ as 163·5, which is the value accepted by the International Committee, and mapped the absorption spectrum. The _oxide_, Ho₂O₃, is a pale yellow powder; the _salts_ are yellow, with a faint orange tinge. ~Erbium~, Er = 167·7 Although erbia was separated by Mosander seventy years ago, it is doubtful if the perfectly pure oxide has ever been prepared. Whilst the individuality of the element is well established, its homogeneity has frequently been called in question. The name ‘Neo-Erbia’ was given by Cleve[372] to the residue left after the separation from the old erbia of ytterbia, scandia, thulia, and holmia (with which dysprosia (_q.v._) was also separated), but the spectrum examination of Kruss and Nilson[373] led them to regard Cleve’s oxide as still complex. Their results, however, were explained by the work of Hofmann and his pupils,[374] who consider erbia to be a homogeneous product; the homogeneity of the element, therefore, may be considered as established, though it would be strengthened by a more complete knowledge of the neighbouring elements, holmium and thulium. [372] _Loc. cit._ [373] _Ber._ 1887, ~20~, 2134. [374] _Ber._ 1908, ~41~, 308; also Hofmann, _ibid._ 1910, ~43~, 2631.