28 FLUORESCENCE OF THE URANYL SALTS. 



absorption bands we have adopted the first view and have assumed a 

 general absorption 1 such as is indicated by the dotted line of figure 20. 

 The deviations from this dotted curve have been ascribed to the effect 

 of the narrow bands. The intensity of each band is determined by 

 taking the ratio of the diminution of the transmission which it produces 

 to the transmission which would be expected if the general absorption 

 only were present. 



Both the absorption bands and the fluorescence bands have been 

 indicated in figure 20 by lines whose lengths are proportional to the 

 intensities of the bands. If a line is drawn through the ends of the 

 lines that give the intensity of the absorption bands a curve (A) is. 

 obtained which is very similar in form to the absorption curve* for a 

 substance having a single broad band. This curve also has the same 

 position with reference to the envelope of the luminescence bands (F) 

 that the absorption curve in such cases has to the luminescence curve. 

 It appears highly probable that just as a broad luminescence band may 

 result from the overlapping of a group of bands, so the absorption of 

 the same substance may result from the overlapping of a similar group 

 of absorption bands. 



The transmission curve for a thin layer of powdered uranyl sulphate 

 is shown in figure 21, the source of light being an acetylene flame. In 

 its general features this curve is similar to that for the double sulphate 

 of uranyl and potassium. The fluorescence of the sulphate is not so 

 brilliant and the fluorescence bands therefore show less prominently. 

 The sulphate, as has been shown in a preceding paragraph, has the 

 peculiarity of possessing two series of fluorescence bands lying close 

 together, one set of bands being much more intense than the other. 

 It will be noticed that the absorbed bands are also double. If we think 

 of the more intense luminescence bands as constituting the principal 

 series and the less intense bands forming a secondary series, a curious 

 reversal is noticeable as we pass from the region of fluorescence to the 

 region of absorption. Each band of the principal series in the lumines- 

 cence region lies a little to the right of the corresponding band of the 

 secondary series. The positions of the bands are indicated by short 

 vertical lines in the lower part of figure 21, the bands of the secondary 

 series being represented by dotted lines. When we pass to the absorp- 

 tion series, however, the more intense band lies to the left in each case. 

 For example, the absorption band at 4,925 corresponds in position with 

 a fluorescence band of the principal series ; but the absorption band at 

 4,880, which probably corresponds to the band 4,890 of the secondary 

 fluorescence series, is by far the more intense of the two. 



1 The fact that all the uranyl salts, so far as known, increase rapidly in opacity as the wave- 

 length of the transmitted light decreases, even when the bands are greatly reduced in width by 

 cooling, seems conclusive as to this assumption. 



