FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 33 



from exciting rays be passed through the substance, absorption bands 

 in the same location are observed. 



In our own work upon uranyl nitrate and potassium uranyl sulphate 

 we have confirmed the results of H. Becquerel so far as the existence 

 of reversible bands is concerned and have found for these substances 

 3 such bands instead of 2. 



The frequency interval between absorption bands, like the fluores- 

 cence interval, is approximately constant, but, as may be seen from 

 tables 12 and 13, it is much smaller. 



Additional evidence on this point will be found in the chapters 

 dealing with the double chlorides and with the spectra at low tempera- 

 tures, where it will be established as a relation common to all uranyl 

 spectra. The study of the absorption spectra at +20 C. is uncertain 

 and unsatisfactory, because we have to do with unresolved groups of 

 bands, and these two examples will suffice to illustrate the remarkable 

 way in which the two frequencies interlock where fluorescence goes 

 over into absorption. 



TABLE 12. Absorption and fluorescence bands of potassium uranyl sulphate at -\-20 C. 



It will be seen from tables 12 and 13 that the last 3 fluorescence 

 bands, counting from the red, are nearly or quite coincident with the 

 first three absorption bands. Whether or not these coincidences are 

 to be regarded as exact can not be determined from observations on 

 unresolved spectra. It will be demonstrated later that reversals are 

 exact between the ultimate components of bands, but not, in general, 

 between unresolved aggregates. 



That the fluorescence interval changes to conform to the absorption 

 interval at the last step appears not only from the data in tables 12 



