504 



Mr. W. Crookes. 



[June 10, 



as the band 451*5. It is obvious therefore that the element giving 

 the band 475 cannot be the same as the one causing the band 451*5, 

 and if the body giving the strongest of these is called dyspro- 

 sium another name must be chosen for the element which gives rise 

 to the absorption-band 475. 



And now comes the question : What is the origin of band 475 ? 

 In remarks made on the band 443 I mentioned that it is accompanied 

 by other fainter lines. One of these occurs at 475, and therefore I 

 was prepared to connect these bands as being due to one and the 

 same element; but M. de Boisbaudran, in his description of the 

 spectrum of dysprosium, has shown that band 475 can be obtained 

 strong in the absence of 443. The bands 443 and 475 therefore are 

 not caused either by didymium, dysprosium, or any hitherto identified 

 element ; consequently each must be regarded as characteristic of a 

 new body. 



I now come to a branch of the subject which promises to yield 

 results even more fruitful than those given by the examination of 

 absorption-spectra : I refer to the spectra yielded by some of the earths 

 when phosphorescing in vacuo. This method has been so fully 

 explained before the Royal Society, in my papers on " Radiant Matter 

 Spectroscopy," that I need not repeat it. 



In my Bakerian Lecture on Yttrium* I described the phosphores- 

 cent spectrum of this earth, and gave a drawing of it. In the 

 Samarium paper f I gave a similar description and drawing of the 

 samarium spectrum, and also described and illustrated some anomalous 

 results obtained when yttria and samaria were mixed together. 

 Under the conditions described in the paper a sharp and brilliant 

 orange line made its appearance, which at that time seemed as if it 

 belonged to the samarium spectrum, and was only developed in 

 greater intensity by the presence of yttria. This explanation, how- 

 ever, did not satisfy me, and I called the line 609 ==-i- 2693^ " the 



anomalous line," intending to return to it at the first opportunity. J I 

 have since further investigated the occurrence of this line, with more 

 than usual good fortune in the extent and importance of the new 

 facts thereby disclosed. 



Systematic fractionation was carried on with the portions of the 

 general series giving the strongest appearance of line 609, and 

 it soon became apparent that the line closely followed samarium. The 



* "Phil Trans.," vol. 174 (1883), p. 891. 

 f Ibid., vol. 176 (1885), p. 691. 



J In paragraph 146, p. 713, of the Samarium paper already quoted, speaking of 

 this line I said, it was " so unlike the bands usually met with in the spectra of 

 phosphorescent earths as to suggest the explanation that some other spectrum - 

 forming body was present in the mixture." 



