~Atomic Weight.~--The value 140·6, adopted by the International Committee, is based on the work of Jones, v. Scheele, Auer von Welsbach, and Feit and Przibylla; the work of Brauner, however, points consistently to a higher atomic weight. Most of these investigators have used the sulphate method. The first determinations of von Welsbach for the newly discovered element[259] gave the value 140·8 (see p. 179); another series of determinations published in 1903[260] gave the mean value 140·57. Jones[261] obtained the sesquioxide for the synthetic sulphate operation by reduction of the peroxide in a current of hydrogen; according to Brauner, this method gives an oxide which is not perfectly pure, probably by absorption of water vapour and carbon dioxide from the air. Jones’ mean value was 140·466. v. Scheele[262] used the same method, as well as a combined oxalate-sulphate method; his figures vary considerably, the mean value being 140·55. Feit and Przibylla,[263] using their volumetric method, obtained the value 140·54. [259] _Monats._ 1885, ~6~, 477. [260] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1903, ~112~, 1037. [261] _Amer. Chem. J._ 1898, ~20~, 345. [262] _Zeitsch. anorg. Chem._ 1898, ~17~, 310. [263] _Zeitsch. anorg. Chem._ 1906, ~50~, 249. Brauner’s earlier work,[264] carried out in 1898, gave the value 140·95. In 1901 this author[265] carried out an extensive research on the atomic weight of praseodymium, employing four different methods with spectroscopically pure material; the mean value of his very concordant results was 140·97, almost the value he obtained in 1901. A further investigation into the value of this constant appears desirable. [264] _Proc. Chem. Soc._ 1898, ~14~, 70. [265] _Ibid._ 1901, ~17~, 65; see also Abegg, III, i. 263. ~Detection.~--The maxima of the absorption bands are given by Rech[266] as follows: [266] _Zeitsch. wiss. Photochem._ 1906, ~3~, 411. Yellow 596·4 and 588·2, weak. Blue 481·3 very intense. 468·3 coincident with a neodymium band. Violet 444·2 The arc spectrum is very rich in lines.[267] The most intense, which may be used also for identification, are the following: [267] Exner and Haschek; Bertram, _Zeitsch. wiss. Photochem._ 1906, ~3~, 16; Eder and Valenta, _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 65. 4008·90 4100·91 4118·70 4143·33 4179·60 4189·70 4206·88 4223·18 4225·50 4241·20 4305·99 4429·38 4496·60 4510·32 ~Neodymium~, Nd = 144·3.