420 Sir William Crookes. [Dec. 8 



Another interesting point is raised by these results. In free space 

 the propagation of waves in a straight line is quite independent of any 

 statical electric or magnetic field. But if the waves are propagated 

 through a part of space containing matter, the streaming of the ions 

 produced by the waves seems to lead to the conclusion that the 

 propagation of the waves is no longer independent of the statical 

 electric and magnetic field, and aberration must result. 



In conclusion, I wish to express my great obligation to Professor 

 Gray, who has discussed these results with me and read the paper with 

 very great care. 



[Note added January 30. Lord Kelvin has expressed the view that a 

 radio-active body may in some way extract energy from the aether and 

 again radiate it. Professor and Madame Curie have also suggested a 

 possible abstraction from the surrounding gas. The results obtained 

 here support such views and indicate in some measure how such a 

 process of selection may go on.] 





" On the Ultra-Violet Spectrum of Gadolinium."* By Sir WILLIAM 

 CROOKES, D.Sc., F.RS. Eeceived December 8, Kead 

 December 15, 1904. 



Gadolinium oxide is a rare earth, occurring between yttrium and 

 samarium. It was discovered in 1880 by Marignac, and was at first 

 called by him Ya, a designation which he soon changed for gadolinium. 

 Since Marignac's time much work has been done on this earth by 

 Lecoq de Boisbaudran, Bettendorf, Cleve, Benedicks, Marc, Demar^ay, 

 Exner and Haschek, Urbain, and others. 



In the spring of this year, M. G. Urbain gave me some gadolinia 

 and other rare earths, which he had prepared in a state of considerable 

 purity by means of a novel system of fractionation in which use is 

 made of the crystallisation of double nitrates of bismuth and magnesium 

 with the rare earth nitrates. He finds that bismuth places itself 

 between the eerie and the terbic groups, thus sharply separating 

 samarium, the last member of the eerie group, from europium and 

 gadolinium, the first members of the terbic groups. I have for some 

 time past been fractionating rare earths by Urbain's method, and can 

 quite corroborate what he says. 



The ultra-violet spectrum of gadolinium has been measured by 

 Exner and Haschek, who have published their results in a book, 



[* A plate of the spectrum to which this communication refers will, it is hoped, be 

 published in another place.] 



