14 



FLUORESCENCE OF THE URANYL SALTS. 



In the case of a partially resolved or wholly resolved homogeneous 

 complex the envelope obtained by drawing a curve through the crests 

 of the group of bands has the above form, as has the curve of intensities 

 of each of the component bands. 



A departure from this type indicates heterogeneity. Thus, for 

 example, the curve in figure 2 suggests a partially resolved homogeneous 

 complex, while that in figure 3 indicates a heterogeneous complex or 

 mixed band. 



The best examples thus far are found among the uranyl salts. Figure 5 

 shows a typical case in which the envelope of the 7 bands of a uranyl 

 salt is shown and with an enlarged scale of wave-lengths the distribu- 

 tion of intensities of a single 

 band of the same spectrum. 

 Other illustrations will be 

 found in subsequent chapters 

 of this monograph. 



(3) In a homogeneous com- 

 plex, the fluorescence spectrum 

 is identical with that observed 

 during phosphorescence as 

 regards the position, relative 

 intensity, and structure of 

 its component bands. Nor is 



there any change in these FIG. 5. 



respects during the process of 



decay. Change of color in passing from the fluorescent to the phos- 

 phorescent stage or during phosphorescence is therefore a criterion of 

 heterogeneity, since such changes are due to the presence of bands 

 having different rates of decay. 



Such subjective changes of color as are due to the loss of intensity 

 during decay are excluded from the above statement. 



Most of the phosphorescent sulphides afford examples of heterogene- 

 ity clearly indicated by the above criterion and confirmed in other ways, 

 while the spectra of the uranyl salts, in spite of their great complexity, 

 are found, from this criterion, too, strictly homogeneous. 



(4) .Persistence of color and of structure when excited to fluorescence 

 at different temperatures, the different components of the spectrum 

 suffering the same relative changes of intensity, may be regarded as a 

 criterion of homogeneity, but the complex changes of structure revealed 

 by the resolution of spectra in the process of cooling to the temperature 

 of liquid air do not necessarily indicate heterogeneity. 



As will be shown in subsequent chapters, the fluorescent spectra of 

 the uranyl salts, for example, are profoundly modified by the cooling 

 of the substance, and yet these spectra conform to all other known 

 criteria of homogeneity. 



