THE DIDYMIUMS. 351 



THE DIDYMIUMS. 



Leaving Mosander's early experiments out of account, the atomic 

 weight of the so-called u didymium " was determined by Marignac, Her- 

 mann, Zschiesche, Erk, Cleve, Brauner, and Bauer. All of these data 

 now have only historical value, and may be disposed of very briefly. 



Marignac* determined the ratios between didymium sulphate and 

 barium sulphate, between silver chloride and didymia, and between 

 didymium sulphate and didymium oxide. The other determinations all 

 relate to the sulphate-oxide ratio. Leaving all else out of account, the 

 earlier data for the percentage of Di 2 O 3 in Di 2 (SO 4 ) 3 are as follows. The 

 atomic weight of Di in the last column is based upon SO 3 = 80 : 



Per cent. Z?/ 2 Oj. At. Wt. Di. 



Marignac, f five experiments 58.270 ! 43-56 



Hermann, J one experiment 58.140 142.67 



Zschiesche,^ five experiments .... 57-9 2 6 141.21 



Erk, || two experiments 58.090 i4 2 -33 



Cleve, ^[ six experiments 58.766 147.02 



Brauner,** three experiments 58.681 146.42 



The discordance of the determinations is manifest, and yet up to 1883 

 the elementary nature of didymium seems to have been undoubted. In 

 that year, however, Cleve and Brauner both showed, independently, that 

 the didymia previously studied by them contained samaria, and that 

 source of disturbance was eliminated. 



In Brauner 's investigation ft the didymium compounds were carefully 

 fractionated, and the determinations of atomic weight were made by 

 synthesis of the sulphate from the oxide in the usual way. Neglecting 

 details, his first series gave results as follows : 



Per cent. Di^O y At. Wt. 



5 8 -5 6 I45-36 



58-526 145.50 



58.5 145-31 



58-515 I45-42 



58.53 1 145-53 



*Two papers: Arch. Sci. Phys. et Nat. (i), n, 29. 1849. Ann. Chim. Phys. (3), 38, 148. 1853. 

 f Ann. Chim. Phys. (3), 38, 148. 1853. 

 | Journ. fur Prakt. Chem., 82, 367. 1861. 

 \ Journ. fi'ir Prakt. Chem., 107, 74. 

 || Jenaisches Zeitschrift, 6, 306. 1871. 

 f K. Svensk. Vet. Akad. Handl., Bd. 2, No. 8. 1874. 

 ** Berichte, 15, 109. 1882. 



ft Journ. Chem. Soc., June, 1883. The values given are as computed by Brauner, with O = 16 

 and S = 32.07. 



