Neodymium is, after cerium, the commonest constituent of the cerium group in the more important rare earth minerals, and its separation is therefore by no means so difficult as that of praseodymium. The compounds of the element obtained by von Welsbach in 1885 were not pure, being admixed with samarium compounds which had not been completely separated. Neodymium salts were first prepared free from samarium by Demarçay[268] in 1898; they are of a violet-rose colour, and show in solution a well-marked and characteristic absorption spectrum, the bands being very numerous and sharply defined, and extending over the whole optical region. In chemical as well as in physical and crystallographic properties, they show an extremely close resemblance to the compounds of praseodymium. [268] _Compt. rend._ 1898, ~126~, 1039. On account of the high melting-point, the preparation of the _metal_ presents the same difficulties as that of praseodymium. A current of 90-100 ampères is employed at a potential difference of 15-22 volts; this suffices to raise the thin carbon cathode to a bright white heat, and to fuse the liberated metal. For the properties of the element, see p. 115. The _sesquioxide_, Nd₂O₃, when perfectly pure, has a light blue or lilac colour, with a faint reddish fluorescence; the shade varies somewhat according to the method of and temperature employed for the preparation. A bluish or violet-red fluorescence is highly characteristic of the salts, and is particularly noticeable if the powdered recrystallised oxalate be viewed in a good light. The greyish or brownish colour of the oxide observed by some authors is probably due to traces of impurity.[269] The existence of higher oxides of the formulæ Nd₂O₄ and Nd₂O₅ respectively, which Brauner[270] put forward, has been disputed by other writers, though it is found[271] that in the presence of ceria and praseodymia, the sesquioxide can take up more oxygen. Waegner[272] claimed to have obtained the compound Nd₄O₇ by heating the oxalate in a stream of oxygen, though his material, as well as that of Brauner, contained praseodymia. More recently, Joye and Garnier[273] have shown that the spectrum attributed by Waegner to the hypothetical Nd₄O₇ was in reality that of an hydrated oxide, 2Nd₂O₃,2H₂O; these authors have also prepared a second hydrated oxide of the formula 2Nd₂O₃,3H₂O. [269] See Waegner, _Zeitsch. anorg. Chem._ 1904, ~42~, 118; also Baxter and Chapin, _J. Amer. Chem._ Soc. 1911, ~33~, 1. [270] _Chem. News_, 1898, ~77~, 161; _ibid._ 1901, ~83~, 197. [271] See Meyer and Koss, _Ber._ 1902, ~35~, 3740; and Marc, _ibid._ 2370. [272] _Loc. cit._ [273] _Compt. rend._ 1912, ~154~, 510.