INTIMATE STRUCTURE ON COOLING. 



63 



homologous series of equidistant bands. This interval, moreover, is 

 very nearly the same for all five of these homologous series; but 

 although the departures from equality are of the same order as the 

 errors of measurement, there is reason, as will be seen later, to regard 

 them as real. 



The general arrangement of the bands in these spectra is roughly 

 depicted in figure 53, which is based upon measurements of the fluores- 

 cence spectrum of the ammonium uranyl chloride. Horizontal dis- 

 tances are plotted on the scale of frequencies, the corresponding wave- 



.AA7 



. 



.6'4 



.6'0 





l\ 



.5 T 6 

 FIG. 53. 



A 



/Ui 



lengths being indicated for convenience. Vertical heights indicate 

 relative intensities, but with some pretence of accuracy. The first 

 and eighth groups at the extreme left and right, for example, if drawn 

 to scale, would be scarcely visible. They are, in fact, so feeble that 

 they can be observed only with the greatest difficulty. The location 

 of the various bands of the 4 double chlorides, in wave-lengths and in 

 frequencies (l//j X 10 3 ) is given in table 29 at the end of this chapter. 

 The values given are the averages of several readings from the photo- 

 graphs and from visual settings. The bands in each group from the 

 red toward the violet are designated by the letters B, C, D, E, and A, 

 and bands having the same letter thus form homologous series. 



To determine the intervals between groups, the position of what 

 may be called the center of each group was found by averaging the 

 frequencies of all 5 bands. The intervals between these centers for 

 groups 2, 3, 4, 5, 6, and 7 are given in table 19. Groups 1 and 8, for 

 which insufficient data were available, were omitted, except in the 

 case of the ammonium chloride. 



The only indication of a systematic departure from uniformity of 

 interval for a single salt appears in the case of the caesium chloride, the 

 average group-interval for which is smaller than that of the other salts 

 by nearly 0.5 per cent. 



