178 



Prof. R. Bunsen on the Phenomena observed 



The oxide of didymium employed was specially prepared from 

 cerite. The oxalates of the cerite metals were separated from 

 cerium-oxide by heating in the air, dissolving in nitric acid, and 

 boiling with magnesite, and repeating this mode of separation 

 three times with the reprecipitated oxalates. 



The separation of the didymium-salt was accomplished in the 

 usual way, by recrystallizing the sulphate twelve times, according 

 to the methods best adapted to effect a complete separation. The 

 crystals exhibited a bluish-red colour ; and the oxide prepared 

 from them was of a similar or rather redder tint, but was not the 

 least brown -coloured. 



The solution of the salt in water deposited well-formed crystals 

 when allowed slowly to evaporate in a beaker covered with filter- 

 paper during the hot months of summer. They gave on analysis 

 the following numbers : — 



Didymium-oxide . . . 4627 

 Sulphuric acid .... 33-73 

 Water 2Q-Q0 



100-00 

 This corresponds to the formula 3(DiO, SO 3 ) 8HO, and agrees 

 closely with the following analysis, by Marignac, of the extremely 

 pure salt from which he determined the atomic weight of didy- 

 mium : — 



Didymium-oxide . . . 46-50 

 Sulphuric acid .... 33-30 

 Water 2020 



100-00 

 The crystals belong to the monoclinic system. The incli- 

 nation of the oblique axes was found by measurement to be 

 L = 61° 45', or close upon the former determinations of Mari- 

 gnac; whilst the relation between the orthodiagonal (a), the 

 klinodiagonal [b), and the principal axis (c) is 

 «:&:c = 0-3283: 0-6786:1. 



These relations of the axes give the following angles, observed 

 and calculated : — 



Observed. Calculated. 



P upon 



00 P a© 



118 15 





+ Pco „ 



OP 



103 7 





OP „ 



-P . . 



119 40 





00 P 00 „ 



+ Poo . . 



138 55 



138 38 



OP „ 



+*p . . 



114 



113 36 



+p „ 



-HP . . 



162 33 



162 12 



-p „ 



+p . . . 



144 30 



144 41 



-F „ 



-Pupon OP . 



77 30 



77 24 



