224 THE ATOMIC WEIGHTS. 



In each sample of CeO,, the excess of oxygen over true 

 CejOg was estimated by an iodometric titration; but the 

 data thus obtained need not be further considered. 



In two papers by Rammelsberg * data are given for the 

 atomic weight of cerium, as follows. In the earlier paper 

 cerium sulphate is analyzed, the cerium being thrown down 

 by caustic potash, and the acid precipitated from the filtrate 

 as barium sulphate : 



.413 yrm. €63(80^)3 gave .244 grm. Ce02 and .513 grm BaSO^. 



Hence 100 BaSO^ = 47.563 CeOj, a value which may be 

 combinied with others, thus; this figure being assigned a 

 weight equal to one experiment in Bunsen's series : 



Beringer 49.819, zfc .042 



Bunsen and Jegel 48.689, rh .077 



Rammelsberg 47-563, ± .108 



General mean 49.360, ± .035 



It should be noted here that this mean is somewhat arbi- 

 trary, since Bunsen and Rammelsberg's cerium salts were 

 undoubtedly freer from didymium than the material studied 

 by Beringer. 



In his later paper Rammelsberg gives these figures con- 

 cerning cerium oxalate. 100 parts gave 10.43 of carbon 

 and 21.73 of water. Hence the dry salt should yield 48.862 

 per cent, of CO 2, whence Ce = 137.83. 



In all of the foregoing experiments the ceroso-ceric oxide 

 was somewhat colored, the tint ranging from one shade to 

 another of light brown according to the amount of didym- 

 ium present. Still, at the best, a faint color remained, which 

 was supposed to be characteristic of the oxide itself. In 

 1868, however, some experiments of Dr. C. Wolff were post- 

 humously made public, which went to show that pure ceroso- 

 ceric oxide is white, and that all samples previously studied 

 were contaminated with some other earth, not necessarily 

 didymium but possibly a new substance, the removal of 



'■' Poggend. Annal., 55, 65 ; 108, 44. 



f Amer. Journ. Science and Arts, (2,) 46, 53. 



