The arc spectrum[321] is very characteristic, and contains some exceedingly intense lines, by means of which Lunt[322] has detected europium in the sun and in many stars. The lines most suited for identification of the element are the following: [321] Exner and Haschek; Eder and Valenta, _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 31. [322] _Proc. Roy. Soc._ 1907, ~79~; A, 118. 3688·57 3725·10 3819·80 3907·28 3930·66 3972·16 4129·90 4205·20 4435·75 4522·76 4594·27 4627·47 4662·10 6645·44 ~Gadolinium~, Gd = 157·3. Gadolinia is the commonest of the terbia oxides, and occurs in considerable quantities in some of the rare earth minerals, notably in samarskite and gadolinite; its separation from the neighbouring oxides, europia and terbia, is, however, exceedingly difficult, and has only been satisfactorily accomplished in recent times. The gadolinium compounds prepared and examined by the earlier workers, as appears from the atomic weight determinations, must have been associated with earths of lower atomic weight, and undoubtedly also with small quantities of terbium. After the isolation of Marignac’s Y_{α}, and the examination of the element by Lecoq de Boisbaudran, to whom the name gadolinium is due, further investigations were carried out by Bettendorff[323] and by Benedicts.[324] Pure gadolinia was probably first obtained by Demarçay,[325] by fractional crystallisation of the magnesium double nitrate; the oxide obtained by Urbain and Lacombe[326] by crystallisation of the nitrates in presence of bismuth nitrate, was proved to be spectroscopically pure by Eberhard.[327] [323] _Annalen_, 1892, ~270~, 376. [324] _Zeitsch. anorg. Chem._ 1900, ~22~, 393. [325] _Compt. rend._ 1900, ~131~, 343; _ibid._ 1901, ~132~, 1484. [326] _Ibid._ 1905, ~140~, 583, etc. [327] _Zeitsch. anorg. Chem._ 1905, ~54~, 374. The gadolinia obtained by ignition of the salts of volatile acids should be perfectly white; presence of terbia causes it to assume a yellow colour.[328] The salts are colourless, and their solutions show no absorption in the visible region, though Urbain[329] has shown that there are four strong bands in the ultraviolet. [328] Eberhard (_loc. cit._) has shown that even in the perfectly white oxide, traces of terbia can be distinguished by spectroscopic examination. [329] _Compt. rend._ 1905, ~140~, 1233.