FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 29 



It will be observed that the absorption bands of uranyl potassium 

 sulphate occurring at 4,760 and 4,920 (fig. 20) appear to coincide in 

 position with two of the luminescence bands of the same substance. In 

 other words, these two bands are " reversible" and may appear either 

 as absorption bands or as luminescence bands, according to the con- 

 ditions under which they are observed. The double sulphate thus 

 shows the same phenomenon that was first described by H. Becquerel 1 

 in 1885 in the case of uranyl nitrate. 



10 



I 



.45 .50 .55^ 



FIG. 21. Transmission of a thin layer of uranyl sulphate. 



It was, however, of interest to study these relations in the case of 

 the uranyl spectra at ordinary temperatures also. Special precautions 

 were necessary, for when a luminescence band occurs in a region where 

 there is appreciable absorption it is clear that the apparent position of 

 the crest of the band may be influenced by absorption in case the latter 

 is not uniform. Where measurements of absorption are made with 

 light containing rays that are capable of exciting fluorescence there 

 may also be a displacement of the crest of the absorption band, owing 

 to the presence of luminescence. There could be no displacement of 



1 Comptes Rendus, vol. 101, p. 1252. 1885. 



