MORSE. — THE KATHODO-LUMINESCENCE OF FLUORITE. 15 



and the spectrum is therefore concealed in some degree beneath the 

 broad green band which is characteristic of both. The similarity in 

 many of the sharper lines is, however, perfectly apparent. 



The spectra of Figures 3 and 4 are quite different from each other 

 and from the other spectra shown. The larger part of the luminescence 

 lines are in the same part of the spectrum as in the others, but the 

 lines are not the same. Figure 4 is more like the spectra 5, 6, and 7 

 than it is like the ones preceding it in the plate. The three lower 

 figures are all of fluorites from Weardale. They are very similar in 

 most of their lines, but show evident differences in the strength of 

 individual lines and groups of lines. 



In none of these spectra are the lines quite as sharp as the lines of 

 fluorescence. They are all diffuse in comparison with sharp metallic 

 lines. 



V. While work on this research was in progress, a paper by Urbain ^"^ 

 appeared in which the cause of the luminescence of fluorite was definitely 

 connected with the presence of the rare earths terbium, samarium, and 

 dysprosium. The particular fluorite which was cited by Urbain was 

 one which had been examined several years before by Becquerel,^^ both 

 in the phosphoroscope and in thermo-luminescence. It is a " chloro- 

 phane " which gives a brilliant green luminescence under all of the va- 

 rious methods of excitation, and from the table of wave-lengths which 

 accompanies the paper it is quite evident that the spectrum of this 

 chlorophane in kathodo-luminescence is very similar in all important de- 

 tails to the spectra of the chlorophanes of the author's Tables I and II, 

 and of Figures 1 and 2. But the resemblance of this spectrum to the 

 kathodo-luminescence spectra of terbium, samarium, and dysprosium, 

 dissolved in various oxides and sulphates, is very slight indeed, and 

 Urbain's conclusions from this resemblance may possibly be unjustified. 

 He prepared from the fluorite in question substances which did give 

 spectra corresponding in every detail with the spectra of the rare earths, 

 and also synthesized a fluorite, which was like the original one, from 

 such preparations. The proof seems a very strong one, but it is one 

 which requires further test. The kathodo-luminescence spectra of the 

 rare earths, in spite of their perfectly definite appearance and their 

 evident persistence as a property of some definite substance or element, 

 have proven most elusive. Crookes ^^ spent some fifteen years in fol- 



" Comptes rendus, 143, 825 (1906). 



^5 Journal de physique, 68, 444, and 69, 169. 



^^ A large number of papers by Crookes on this subject are to be found in the 

 Proceedings of the Royal Society, the Transactions, and in the Cliemical News, 

 from 1880 to 1890 especially. 



