72 



FLUORESCENCE OF THE URANYL SALTS. 



ture of liquid air. As in the corresponding table for +20 (table 30) 

 the arrangement is by series. 



TABLE 24. Average intervals of the fluorescence series at -185 C. 



THE ABSORPTION SPECTRA. 



A glance at the absorption spectra of the double chlorides, obtained 

 by viewing through a spectroscope the light transmitted by the 

 crystals at room temperature, shows the same higher degree of resolu- 

 tion that characterizes the fluorescence spectra of these salts. The 

 salient feature is a series of strong, rather narrow bands, equally spaced 

 as to frequency, like the broader bands of the other uranyl compounds. 

 The interval, as in all uranyl absorption spectra, is distinctly smaller 

 than the fluorescence interval. Between these are several series of 

 weaker bands. 



The complete mapping of the absorption spectra is difficult. It can 

 not be done visually, since the bands extend out into the darkness of 

 the ultra-violet. Photography adds considerable detail, but does not 

 greatly extend the range toward the shorter wave-lengths on account 

 of the rapidly increasing opacity. In the brighter regions of the 

 spectrum, on the other hand, more can be seen with the eye than can 

 be found on the photographic plate. The data which we have obtained 

 and which are presented in the tables at the end of this chapter have 

 been procured by supplementing the photographic method, wherever 

 desirable, by visual observations. 



A great variety of light-filters and combinations of light-filters have 

 been employed in different parts of the spectrum, with widely different 

 exposures for the strong and weak bands. The thickness of the trans- 

 mitting layer has likewise been varied as far as the available material 

 would permit. We are convinced, however, that the extreme limits of 

 the absorption, in both directions, have not as yet been reached. 



By using crystals of unusual thickness, especially prepared for this 

 work and sometimes by mounting several crystals one behind the 



