ATOMIC WEIGHTS 509 



The last oxide was perfectly white, and was speetroseopically free from 

 didymium. In each case the CeO. was titrated iodometrically for its 

 excess of oxygen. It will he noticed that in the successive series of de- 

 terminations the percentage of CeO^ steadily and strikingly diminishes 

 to an extent for which no ordinary impurity of didymium can account. 

 The death of Dr. Wolf interrupted the investigation, the results of which 

 were edited and published by Professor F. A. Genth. 

 . In the light of more recent evidence, little weight can be given to these 

 observations. All the ex})eriments, taken equally, give a mean percentage 

 of CeOo from CeoCSOJa of 60.366, ±.0308. This mean has obviousk 

 little or no real significance. It gives Ce = 138.74. 



The experiments of Wolf attracted little attention, except from Wing,' 

 who partially verified certain aspects of them. This chemist, incidentally 

 to other researches, purified some cerium sulphate after the method of 

 Wolf, and made two similar analyses of it, as follows : 



Mean, 60.244 



Hence Ce= 137.88. 



The eerie oxide in this case was perfectly white. The cerium oxalate 

 which yielded it was precipitated boiling by a boiling concentrated solu- 

 tion of oxalic acid. The precipitate stood twenty-four hours before 

 filtering. 



In 1875 Buehrig's ^ paper upon the atomic weight of cerium was issued. 

 He first studied the sulphate, which, after eight crystallizations, still 

 retained traces of free sulphuric acid. He found, furthermore, that the 

 salt obstinately retained traces of water, which could not be wholly ex- 

 pelled by heat without partial decomposition of the material. These 

 sources of error probably atfect all the previously cited series of experi- 

 ments, although, in the case of Wolf's work, it is doubtful whether they 

 could have influenced the atomic weight of cerium by more than one or 

 two-tenths of a unit. Buehrig also found, as Marignac had earlier shown, 

 that upon precipitation of cerium sulphate with barium chloride the 

 barium sulphate invariably carried down traces of cerium. Furthermore, 

 the eerie oxide from the filtrate always contained barium. For these 

 reasons the sulphate was abandoned, and the atomic weight determina- 

 tions of Buehrig Avere made with air-dried oxalate. This salt was placed 

 in a series of platinum boats in a combustion tube behind copper oxide. 

 It was then burned in a stream of pure, dry oxygen, and the carbonic 



1 Am. Journ. Sci. (2), 49, 358. 1870. 

 2.Tonrn. prakt. Chein.. 120, 222. 1875. 



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