CERIUM. 



227 



These sources of error probably affect all the previously 

 cited series of experiments ; 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 jNIarignac had earlier shown, 

 that upon precipitation of cerium sulphate with barium 

 chloride the barium sulphate invariably carried down traces 

 of cerium. Furthermore, the ceroso-ceric oxide from the 

 filtrate always contained barium. For these reasons the 

 sulphate was abandoned, and the atomic weight determina- 

 tions of Buehrig were made with air-dried oxalate. This 

 salt was placed in a series of platinum boats in a combus- 

 tion tube behind copper oxide. It was then burned in a 

 stream of pure, dry oxj^gen, and the carbonic acid and 

 water were collected after the usual method. Ten experi- 

 ments were made ; in all of them the above named products 

 were estimated, and in five analyses the resulting ceroso- 

 ceric oxide was also weighed. By deducting the water 

 found from the weight of the air-dried oxalate, the weight 

 of the anhydrous oxalate is obtained, and the percentages 

 of its constituents are easily determined. In weighing, the 

 articles weighed were always counterjDoised with similar 

 materials. The following weights were found : 



