HISTORICAL. 9 



Morton and Bolton, like Becquerel, refer to the possibility of deter- 

 mining the composition of uranyl compounds from the observation of 

 their fluorescence spectra and state that even minute quantities, 

 present as impurities, may be detected by means of their characteristic 

 bands. 



Hagenbach 1 likewise published a considerable list of fluorescence 

 bands for the uranyl salts, but his paper adds little to the data of 

 Becquerel and of Morton and Bolton. 



In 1903, J. Becquerel and Onne?, working in the cryogenic laboratory 

 at Leyden, excited various uranyl salts to fluorescence at the tempera- 

 tures of liquid air and of liquid hydrogen respectively. 



At 185 C. each band of the spectrum was found to be resolved 

 into a group of much narrower bands. The spectra of a number of 

 compounds were photographed, using a grating spectrograph, and the 

 most prominent bands were mapped. 



It was shown in the course of this investigation that the resolved 

 spectra are made up of series of bands, the frequency interval varying 

 slightly for different compounds ; also that each group in a given spec- 

 trum is similar to all the other groups as regards the arrangement and 

 the relative intensities of its components. In the reversing region, 

 where fluorescence goes over into absorption, the coincidence in posi- 

 tion of bright and dark bands was pointed out. Further cooling to 

 the temperature of liquid hydrogen rendered the individual bands 

 sharper and more line-like, but there was no further resolution. 



This resolution of the fluorescence spectra by cooling constitutes the 

 most important advance subsequent to the discoveries of Stokes and 

 of E. Becquerel, since it affords a means of studying the more intimate 

 structure of these remarkable spectra. It forms, indeed, the starting- 

 point for the present investigation. 



1 Hagenbach, Annalen der Physik., v. 146, p. 395. 1872. 



