80 



FLUORESCENCE OF THE URANYL SALTS. 



coincidence of position between certain fluorescence bands and absorp- 

 tion bands. 



In the case of the double chlorides at +20, each series of bands of 

 the fluorescence system comes into coincidence, or near coincidence, 

 with an absorption band in what we have termed the reversing region, 

 which is approximately that region occupied by group 7 of the fluores- 

 cence spectrum. 



The fact that the reversal sometimes appears to be exact, within the 

 errors of observation, while sometimes there is a displacement of several 

 units of frequency, might seem to render such a general relation doubt- 

 ful, but the discrepancy can be shown to be a necessary consequence of 

 the fact that both fluorescence and absorption bands at this tempera- 

 ture are unresolved complexes. The true nature of the case may be 



s 



seen from figure 68, which is from a sketch of such a reversal at 185, 

 where the resolution is more nearly complete. Here the fluorescence 

 and absorption are complementary, the strong components of fluores- 

 cence coinciding with the weak absorption component and vice versa. 

 When the resolution is less complete, the weaker components will dis- 

 appear, and although the reversal for each component is exact, there 

 will be an apparent failure to reverse, or, in other words, we see the 

 strong components displaced. 



An actual instance in which this relation between fluorescence 

 appears is given in plate 1, a, which is from a photograph of a small 

 portion of the reversing region. The upper half contains the com- 

 ponents of a resolved fluorescence band, the lower half the correspond- 



