78 FLUORESCENCE OF THE URANYL SALTS. 



A few bands at 185 are so located with regard to the +20 bands 

 that to explain them by this theory we must suppose them to be too 

 feeble at +20 for detection and greatly increased in intensity by 

 cooling. 



There is also evidence in places of further resolution into closer 

 narrow doublets and as the degree of resolution is not always the same 

 with fluorescence and the corresponding absorption, this is a source of 

 trouble in the attempt to find the fluorescence series which belongs to 

 each series in the absorption spectrum. Every low-temperature band, 

 however, falls into a series of constant frequency, whatever its position 

 or degree of resolution. 



The effect of temperature on the average intervals can be studied by 

 comparing tables 27 and 28. Although the intervals range from 69 to 

 71, there is little that can be termed systematic in the variations. 



At liquid air temperature, where two or more components are present, 

 we have used subscripts. Thus d if corresponds to DI, d? to D 2 , etc. 

 Where the reversal is doubled in the manner shown in figure 63, we 

 have designated this doublet as di and di", etc. 



The average interval of each salt is approximately the same at 

 both temperatures. It will be noticed in table 27 that 70.28, the 

 average of the c components is smaller than the b, d, e, or a 

 averages. This is of interest because the strong C series, which 

 join these series, are also the shortest of the fluorescence series. Since 

 the 185 bands are very sharp and easy to locate, no doubt the 

 differences found in table 28 are indicative of real 

 variations in the constant-frequency intervals. It 

 does not follow that the smaller intervals are con- 

 fined to one salt or one set of bands, however, since, as 

 has been noted in the case of series Ci and C% of the 

 fluorescence series, the maximum difference in interval 

 may be associated with two series which are nearly co- 

 incident. The comparison of table 27 with table 28 

 shows that the effect of changing temperature on the 

 average interval of a salt is almost negligible, but that the two com- 

 ponents of one series of the +20 spectrum may vary by as much as 

 1.9 units in frequency interval. 



The character of the change in the absorption spectra when we pass 

 from +20 to 185 can best be seen in detail by plotting a single 

 group in the spectrum of each salt, as has been done for group 9 in 

 figure 62. A better idea of the phenomena of cooling, as a whole, is 

 obtained by means of maps like those in figures 64, 65, 66, and 67, in 

 which all the bands of fluorescence and absorption are given at both 

 temperatures, first in a single line as they occur in the spectrum of each 

 salt. Fluorescence is indicated by vertical lines above the horizontal 

 and absorption below. Length of line indicates roughly the strength 



d, 



\ 



