~Atomic Weight.~--The earlier determinations of this constant were carried out by the sulphate method, the synthetic process being usually employed. Auer von Welsbach, at the time of the discovery of praseodymium and neodymium,[279] gave the values 143·6 and 140·8 respectively for their atomic weights. Brauner, who carried out a determination in 1898,[280] showed that these numbers should be interchanged, and gave the value 143·63 for neodymium. Boudouard,[281] employing the analytical sulphate method, obtained the value 143·05, whilst in the same year Jones[282] gave the value 143·6. A second determination by Brauner[283] gave the value 143·89. All these values are undoubtedly too low, the material being probably contaminated with other earths. [279] _Loc. cit._ [280] _Proc. Chem. Soc._ 1898, ~14~, 70. [281] _Compt. rend._ 1898, ~126~, 900. [282] _Amer. Chem. J._ 1898, ~20~, 345. [283] _Proc. Chem. Soc._ 1901, ~17~, 66. In his second determination in 1908, Auer von Welsbach[284] gave the value 144·54 as the mean of three determinations. Feit and Przibylla,[285] using their volumetric method, gave the value 144·52, whilst Holmberg,[286] using material which he considered to have been the purest obtained up to that time, obtained the figure 144·11. More recently, Baxter and Chapin[287] have made determinations by treating the chloride with pure silver nitrate, and weighing the precipitated silver chloride, as well as by titration. The mean value obtained by the first method--ratio NdCl₃  : 3AgCl--was 144·272 (extremes 144·250 and 144·298), and by the second method--ratio NdCl₃  : 3Ag--was 144·268 (extremes 144·249 and 144·283), giving the mean value for the whole series of 144·270. [284] _Loc. cit._ [285] _Zeitsch. anorg. Chem._ 1905, ~43~, 202; _ibid._ 1906, ~50~, 249. [286] _Ibid._ 1907, ~53~, 124. [287] _Proc. Amer. Acad._ 1911, ~46~, 215. The value adopted by the International Committee is 144·3. ~Detection.~--The absorption spectra of neodymium compounds have been examined by Demarçay, Forsling, von Welsbach, Rech, Schäfers, and Baxter and Chapin, with concordant results. The positions of the absorption maxima as given by Holmberg[288] from the measurements of Forsling are as follows, the weaker bands being omitted: [288] _Zeitsch. anorg. Chem._ 1907, ~53~, 83. 677·5 621·7 578·5 } In concentrated solution 575·4 } these give the 573·5 } intense absorption 571·6 } region in the yellow. 532·3} 521·6} In concentrated 520·4} solution these 512·4} give one intense 508·7} band. 474·5 468·7 461·0 427·1 The arc spectrum is given by Exner and Haschek, Bertram,[289] and Eder and Valenta.[290] The most intense lines are as follows: [289] _Zeitsch. wiss. Photochem._ 1906, ~3~, 16. [290] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 554.