In the year 1874') I published an account of an examination of didy- 

 inium, free from all the other allied metals known at that time. Later 

 on, however, it was found that didymium is accompanied by another 

 metal, samarium. It is evident from the atomic weight of didymium, 

 147, as determined by me in 1874, as well as from the spectrum of didy- 

 mium, mapped by Thalén^), that my didymium of 1874 was a mixture of 

 the true didymium with samarium. My later determinations, in 1883^), 

 of the atomic weight of didymium, free from samarium, gave the much 

 lower number 142.3. It was therefore indispensible to repeat my for- 

 mer researches, and another reason for doing so was that I wished to 

 determine the specific gravities of the pure didymium-compounds in 

 order to be able to compare them with the specific gravities of the 

 compounds of the other rare earth-metals. 



The material for this research was prepared by the method of 

 partial precipitations of the mixed nitrates of didymium and samarium 

 with cold dilute ammonia. The progress of the separation was tested 

 by atomic-weight determinations, and as soon as the didymium showed 

 an atomic weight of about 142.3 it was regarded as sufficiently pure. 



In working out this research I have been assisted by Mr. G. W. 

 Petersson, assistant in the chemical laboratory of this university. 



The specific gravity-determinations were executed exactly in the 

 manner described in my recent paper on the compounds of samarium. 



') Bihang till K. Sv. Vetensk. Akad. Handl. II, N:o 8. 

 =) K. Sv. Vet. Akad. Handl. 12, N:o 4, 1874. 



=<) Ofversigt af K. Sv. Vetensk. Akad. Förhandl. 1883, N:o 2, p. 23. 

 Nova Acta Reg. Soc. Sc. Ups. Ser. III. I 



