518 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 54 



PRASEODYMIUM. 



In 1885 Aiier von Welsbach ' succeeded in pioN iiii:- that the old 

 " didymia " was a mixture of two earths, one yielding green, and the 

 other rose-eolored salts. To the corresponding metals, praseodymium and 

 neodymium, he assigned the atomic weights Pr = 143.6 and ISTd^ 140.8, 

 respectively, values which were curiously reversed, either in printing or 

 by the error of a copyist. The true values are now known to be nearly 

 Pr=:141 and Nd = 144, in round numbers. For " didymium/' many dis- 

 cordant atomic weight determinations had been made, which now have 

 only historical interest, and need, therefore, no consideration now. They 

 are thoroughly summed up in the first edition of this work, which was 

 published about three years before Welsbach's brilliant discovery. 



In 1898 Brauner ^ published a preliminary notice upon praseodymium. 

 Thirteen determinations of the atomic weight, by both the sulpliate and 

 the oxalate methods, gave values from 140.84 to 141.19, in mean 140.95, 

 but the details of the work were not given. These early data, therefore, 

 are not now available for discussion. The first fully described series of 

 determination!^ was made by Jones,^ who published his results a little 

 later than Brauner. 



Jones effected the synthesis of praseodymium sulphate from the oxide, 

 the latter having been first reduced from Pr^O- to Pr._,0.> bv heating in 

 hydrogen. The material, after purification, still contained minute traces 

 of lanthanum and neodymium, but these were too small to seriouslv 

 affect the atomic weight determination. The weights and percentages 

 appear in the following table : 



^Monatsh. Chem., 6, 477. 1885. 



= Proe. Chem. Soc, 14, 70. 1898. 



■■ Aitier. Clieni. Jouin., 20. .345. 1898. 



