118 



Mr. W. Crookes. 



[Feb. 17, 



61 parts of yttrium and 39 parts of samarium, and illustrated it by a 

 coloured lithograph. Also in a paper read before the Royal Society, 

 February 25th, 1886,* I described and figured the phosphorescent 

 spectrum of an earth obtained in the fractionation of yttria which 

 was identical, chemically and spectroscopically, with an earth dis- 

 covered by M. de Marignac, and provisionally called by him Ya. I 

 repeat here these spectra, with the spectrum of yttrium added for 

 comparison. Omitting minor details, it is seen that the Y« spectrum 



is identical with that of the mixture yttrium 61, samarium 39, with 

 one important exception — the citron line of GS in the former spec- 

 trum is absent in the latter. Could I by any means remove GB from 

 the mixture of yttrium and samarium the residue would be Ya. I 

 have little doubt that this will soon be accomplished, but in the 

 meantime the phosphoroscope enables us to remove the line of G<5 

 from the mixture. It is only necessary to add strontium to a suitable 

 mixture of yttrium and samarium and view the phosphorescing 

 mixture in the instrument when the wheel is rotating rapidly, to 

 obtain a spectrum which is indistinguishable from that of Ya. 



In the search for bodies giving discontinuous phosphorescent 

 spectra I have submitted a great number of earths and combinations 



* ' Roy. Soc. Proc.,' vol. 40, p. 236. 



