~Detection.~--Pure lanthanum compounds show no absorption in the visible region, and the pure oxide gives no cathode luminescence. The emission spectra show very characteristic lines in the violet and ultraviolet. The chief lines are: 3949·27 3988·69 4238·55 4333·98 6250·14 6262·52 6394·46 For arc spectra see Exner and Haschek; Eder and Valenta.[252] [252] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, IIa, 39. ~Praseodymium~, Pr = 140·6 This element occurs only in small quantities in the commoner rare earth minerals, and its separation in the pure state is in consequence a matter of very great difficulty. The salts and their solutions have a characteristic green colour. The salts are derived from the sesquioxide, Pr₂O₃, but a dioxide, PrO₂, and an intermediate oxide of uncertain composition are known. The absorption spectrum has five absorption bands, one of which coincides with a band in the absorption spectrum of neodymium; this fact has been interpreted as an indication of the non-elementary nature of both metals.[253] Difference in the absorption spectra have been put forward by several workers as indicating the complex nature of praseodymium, but an exhaustive examination by Stahl[254] in 1909 showed that there is no reason to doubt that the metal is really an element. [253] Auer von Welsbach, _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1903, ~112~, II_a_, July; also Urbain, _Ann. Chim. Phys._ 1900, [vii], ~19~, 184. [254] _Le Radium_, 1909, ~6~, 215. The _metal_ is prepared by electrolysis of the fused chloride; in order to attain the temperature required to fuse the element, a very thin cathode is employed; if too powerful a current be used, the dioxide is formed. The metal is purified by remelting it in crucibles of magnesia, under a layer of anhydrous barium chloride. It has a yellowish shade, and is more stable in the air than lanthanum and cerium. For physical properties, see p. 115. No alloys have been prepared. The _hydroxide_ is thrown down by alkalies as a gelatinous green precipitate; in the presence of hydrogen peroxide, an hydrated peroxide, which closely resembles the corresponding lanthanum compound, is thrown down.