2 P. T. Cleve, 



The crystalline forms of several didymium-compounds having been 

 determined by Mr. C. Morton, student in the new university of Stock- 

 holm, I beg to offer him my best thanks. 1 have not made any de- 

 terminations of the solubility of the compounds of didjanium, as it is 

 my intention to make a comparative study, later on, of the solubility 

 of some of the salts of the rare earths. 



Oxide of didymium 



Di.Og. 



The pure oxide of didjmiium has an ash-gray colour and is easily 

 soluble in acids. 



Spec. Grav: 



2.5929 gram, temp. IS^.ö sp. gr. 7.177 



2.5576 » » » » » 7.182 



Mean of both determinations 7.17!9. 

 Molecular volume 46.2. 



Peroxide of didymium. 



On ignition to redness in the air the oxide of didymium assumes 

 a dark brown colour which disappears on heating to a bright red heat. 

 Marignac, Hermann and Zschiesche have found that this oxide contains 

 only a small quantity of oxygen more than the oxide of didymium 

 (0.32 to 0.88 percent). In the year 1874 Frerichs^) adopted the for- 

 mule DigOj for the peroxide, supposing the oxide te be DiO. Later, 

 1878, Frerichs and Smith^) give more detailed accounts of their experi- 

 ments. They then admitted the formula DijOg for the oxide and adopted 

 the formula Di4 09 for the peroxide. This formula requires a large amount 

 of oxygen (7.1,3 for 100 DgOg). On repeating their experiments P) could 

 not verify their statement, having obtained only 0.98 oxygen by heating 

 the peroxide in a current of hydrogen. Mr. Brauner*) also found only 

 1.22 — 3.55 oxygen for 100 Di^Oj, when he repeated the experiments of 



') Berichte der Deut. ehem. Ges. VII, 798. 



2) Liebigs anm. CXCI, 331. 



3) Öfvers. af K. Sv. Vetensk. Ak. Förh. 1878, N:o 5, p. 12. 



*) Sitzber. d. k. Akad. d. Wissens, zu Wien LXXXIV, p. 1179. 



