INTIMATE STRUCTURE ON COOLING. 



77 



toward the violet. A group near the fluorescence region, therefore, 

 differs notably in aspect from one in the extreme violet, and it is 

 difficult to base conclusions on the location of the centers of the groups, 

 as was done in the study of the fluorescence spectra. 



As may be observed in figure 62, where the ninth group for the four 

 spectra at +20 is plotted, the distances between the consecutive 

 bands are less nearly equal than the distances between fluorescence 

 bands. It is also evident from this figure that with increasing molecular 

 weight there is a general shift toward the violet. The shift is apparently 

 less systematic than with the fluorescence bands and several reverse 

 shifts seem to occur. In general, however, the total displacement is 

 approximately the same as that observed for fluorescence, i. e., 5 

 frequency units from potassium to caesium. 



EFFECT OF TEMPERATURE ON THE ABSORPTION SPECTRA. 



In the study of the absorption of the double chlorides at 185, a 

 modification of the method described in a previous paragraph in 

 the investigation of the fluorescence at low temperatures was made. 

 (See fig. 56.) The crystal under observation was mounted within a 

 Dewar flask and submerged in liquid air. Light was transmitted 

 through the crystal instead of being reflected from its surface and a 

 nitrogen-filled tungsten lamp was, in general, substituted for the 

 carbon arc. Both photographic and visual 

 methods were tried, and in the reversing 

 region, especially, where fluorescence and 

 absorption overlap, much attention was 

 given to the selection of color-screens to 

 exclude fluorescence from the portion 

 under consideration. 



A complete list of the absorption bands 

 observed at 185 will be found in table 34. 



The three most obvious results of cool- 

 ing to the temperature of liquid air are : 

 (1) a general shift toward the violet; (2) a 

 great increase in the number of bands ; (3) 

 a very decided narrowing and sharpening 

 of the bands. 



These changes are readily accounted for 

 by the assumption already made, in this 

 chapter, that the bands at + 20 C. are con- 

 cealed doublets anol that the effect of cool- 

 ing is to resolve them while simultaneously 



reducing the strength of the stronger and increasing the strength of the 

 weaker component. The apparent shift thus produced will vary from 

 zero to 5 or more units, according to the distance between the com- 

 ponents. 



