24 



FLUORESCENCE OF THE URANYL SALTS. 



EXCITATION BY LIGHT OF DIFFERENT WAVE-LENGTHS. 



If all the bands of the luminescence spectrum are due to the vibra- 

 tions of a single connected system it would be natural to expect that 

 an agency which excited one would also excite the rest, especially if 

 luminescence is due to the recombination of ions dissociated by the 

 exciting light, or to the return of an electron set free by the exciting 



I0|50 



I9JOO 



20 



15 



10 



,50 



I I 



.51 



20 



20JOO 



10 



I I 



FIG. 15. 



.49 



.50 



.51 



FIG. 16. 



4: 



20 



20JOO 



15 



10 



agency. On the other hand, if each band 

 is dne to some process going on in one 

 particular compound or molecular aggre- 

 gation, wave-lengths might be found 

 which would excite one band and not the 

 rest, or which would at any rate excite 

 the bands in different degree. 



To test this matter we have measured 

 the distribution of intensity in the bands 

 for excitation by different lines in the 

 ultra-violet spectrum of the quartz 

 mercury lamp. The intensity of fluo- 

 rescence with this excitation is not suffi- 

 cient to permit the measurement of all 

 the bands, so that the three brightest 

 bands only have been measured. In 

 table 11 the intensities for excitation by the different lines in the mer- 

 cury spectrum are given for five different uranyl salts. Curves show- 

 ing the variation of the relative intensity with the wave-length of the 

 exciting light are shown for uranyl-nitrate crystals in figure 18, and for 

 the double sulphate in figure 19. In each case the intensity of the 

 most intense band has been put equal to 10. The variation was 

 greater in the case of the double sulphate than in the case of any other 

 salt studied. The observations were repeated in the case of this sub- 

 stance on two different days and a comparison of the full and dotted 



.50 .51 M 



FIG. 17. 



