16 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lowing certain definite bands in these spectra. Lecoq ^"^ about as long 

 Baur and his students thought that they had settled the matter finally. ^^ 

 Urbain ^^ has done wonderful work in separating the elements of the 

 rare earths, and his opinion is undoubtedly of more importance than 

 that of any one else. An explanation along these lines must include 

 not only the case of a single chlorophane, but it must cover also the 

 cases where the fluorescence, thermo-luminescence, and kathodo-lumi- 

 nescence of the same crystal of fluorite are all difiierent, even in their 

 minute details. 



While the author cannot expect to test the question by synthesis, 

 further study of the rare elements which are present in fluorites is 

 already under way, and examination of the light emitted by these same 

 fluorites under excitation by other means will also be taken up as soon 

 as possible. 



The author's thanks are due to the American Academy for a gener- 

 ous appropriation from the Rumford Fund, which has been of the 

 utmost assistance in this work. 



The Jefferson Physical Laboratory, 



Harvard University. Marcli 20, 1907. 



" Papers by Lecoq de Boisbaudran on this subject, to the number of tliirty or 

 more, are to be found in the Coniptes rendus, beginning with volume 100, and 

 continuing for many years. 



" Ber. d. d. Chem. Ges., 33, 1748, and 34, 2460. 



" A very complete bibliography of all tlie literature on the yttrium and cerium 

 earths is that of Meyer, Bibliographie der seltenen Erden. (Leopold Voss, 

 Leipzig, 1905.) 



