FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 27 



THE ABSORPTION SPECTRUM AT ORDINARY TEMPERATURES. 



The resemblance of the absorption spectra of the uranyl salts to their 

 fluorescence spectra, which is so striking as to have led both E. and H. 

 Becquerel to regard the absorption series as a continuation of the series 

 of fluorescence bands, can be fully investigated only by observations 

 at low temperatures. Since the absorption extends into the ultra- 

 violet, moreover, photographic methods are necessary. The study of 

 the absorption at ordinary temperatures is, however, not without sig- 

 nificance, and the use of the spectrophotometer in this work brings out 

 certain features not easily discernible in the photographic plates. 



The salts thus studied by us 

 were in powdered form and the 

 location, relative intensity,and 

 character of the bands lying 

 within the visible spectrum 

 were determined by measur- 

 ing the intensity of the light 

 transmitted by an extremely 

 thin layer between glass plates, 

 or in some instances by ob- 

 serving the spectrum of white 

 light reflected from the surface 

 of the powder. Recourse to 

 the latter method is, indeed, 

 frequently necessary because 

 of the great and rapidly in- 

 creasing opacit}^ of these sub- 

 stances in the blue and violet. 



The nature of the results of 

 such measurements is suffi- 

 ciently shown in figure 20, 

 which is plotted from deter- 

 minations of the light transmitted by a thin layer of uranyl potassium 

 sulphate. The source of light was an acetylene flame. 



The measurements cover not only a considerable portion of the 

 absorbing region, but also a part of the region containing the fluores- 

 cence bands. Three of these bands show very clearly, even when 

 superposed upon the brilliant continuous spectrum of the acetylene 

 flame. The absorption begins a little on the violet side of the brightest 

 luminescence band and extends into the ultra-violet. It will be noticed 

 that there are several definite and narrow absorption bands, which 

 appear to be superposed upon a broad band, or region, of general 

 absorption. This appearance of a broad band might result from the 

 overlapping of the group of narrow absorption bands, only the crests 

 of which can be observed. In estimating the relative intensity of the 



FIG. 20. Transmission of a thin layer of uranyl- 

 potassium sulphate, showing absorption bands 

 and three of the fluorescence bands. Curves F 

 and A show the relative intensities of the bands 

 of fluorescence and absorption respectively. 



