CERIUM. 225 



which tended to lower the apparent atomic weight of 

 cerium very perceptibly. 



Cerium sulphate was recrystallized at least ten times. 

 Even after twenty recrystallizations it still showed spectro- 

 scopic traces of didymium. The water contained in each 

 sample of the salt was cautiously estimated, and the cerium 

 was thrown down by boiling concentrated solutions of ox- 

 alic acid. The resulting oxalate was ignited with great 

 care. I deduce from the weighings the percentage of CeOj 

 given by the anhydrous sulphate : 



Sulphate. Water. CeO.^. Percent. CeO^. 



1.4542 grm. .19419 g™. -76305 grm. 60.559 



1. 4104 " .1898 " .7377 " 60.437 



1.35027 " .1820 " .70665 " 60.487 



Mean, 60.494, ± .024 



After the foregoing experiments the sulphate was further 

 purified by solution in nitric acid and pouring into a large 

 quantity of boiling water. The precipitate was converted 

 into sulphate and analyzed as before : 



Sulphate. Water. CeO.^. Per cent. CeO.^. 



1.4327 grm. .2733 grm. .69925 grm. 60.31 1 



1.5056 " -2775 " -7405 " 60.296 



1.44045 " .2710 " -7052 " 60.300 



Mean, 60.302, ifc .004 



From another purification the following weights were ob- 

 tained : 



1.4684 grm. .1880 grm. .7717 grm. 60.270 percent. 



A last purification gave a still lower percentage : 



1.3756 grm. .1832 gnn. .7186 grm. 60.265 percent. 



The last oxide was perfectly white, and was spectroscopi- 

 cally free from didymium. In each case the CeOg was ti- 

 trated iodometrically for its excess of oxygen. It will be 

 noticed that in the successive series of determinations the 

 percentage of CeOj steadily and strikingly diminishes, to 

 an extent for which no ordinary impurity of didymium can 

 15 



