26 



FLUORESCENCE OF THE URANYL SALTS. 



light. The observed distribution of energy would correspond with the 

 actual distribution only in case an excessively thin layer of the sub- 

 stance is excited so thin that the absorption of the light emitted is neg- 

 ligible. As a matter of fact, the band lying to the violet side of the 

 maximum is in a region where the absorption is considerable, while 

 the brightest band and those lying to the red are in the region where 

 the absorption is small. The constancy of the ratio in the case of the 

 lower curves, and the small variation of the ratio shown by the upper 

 curves, are therefore entirely consistent with the view that the observed 

 variations are the result of absorption, and that the first effect of 

 excitation, whatever may be the wave-length of the exciting light, is 

 to produce all of the bands with a definite and constant intensity 

 distribution. 



.30 



.40^ 



.30 



.40 



FIG. 18. 



FIG. 19. 



Relative intensities of the brightest fluorescence bands of uranyl nitrate (fig. 18) and uranyl- 

 potassium sulphate (fig. 19). The intensity of the brightest band is put equal to 10. The upper 

 curve in each figure refers to the band lying next to the brightest toward the violet. The lower 

 curve refers to the band toward the red. Abscissae give the wave-length of the exciting light. 

 (See table 11.) 



The observations recorded in the foregoing paragraphs all tend to 

 indicate that the fluorescence spectrum of a uranyl salt is a homo- 

 geneous complex. 



The envelope is single-crested and has the form typical of a simple 

 band. Neither its position nor form is modified by changing the mode 

 of excitation. To test this conclusion we have made many experiments 

 under widely varying conditions, especially in the way of selective and 

 monochromatic excitation of the resolved spectra, where it should be 

 possible to observe critically the disappearance or enhancement of 

 single narrow components of groups of series. 



The remarkable effects of selective excitation recorded by Wood in 

 the case of fluorescent vapors might lead to the expectation of similar 

 or analogous changes in the uranyl spectra. All these attempts have 

 thus far been without result, and we are inclined, therefore, to regard 

 the spectrum as a unit and to consider it as a broad, simple band, which 

 unlike the other bands of this type as yet discovered, consists of 

 resolved instead of completely overlapping components. 



Studies to be described in Chapter IV are in confirmation of this 

 view in that the criterion for a simple band, based upon the phenomena 

 of phosphorescence, is fulfilled. 



