PHYSICS. 339 



now have an absorption band corresponding to each fluorescence band 

 throughout groups 8, 7, 6, and 5 of these spectra, and there is reason to 

 think that with crystal layers of sufficient thickness absorption would 

 be found to accompany fluorescence throughout the entire spectrum. 



The deteimination of the Umits of the absorption spectrum in direc- 

 tion of decreasing wave-length, on the other hand, demands the use 

 of very thin layers. With crystals having a thickness of the order of 

 1 mm., all the uranyl compounds become opaque at about OAfx, but 

 recent photographs using much thinner layers reveal the presence of 

 bands extending well into the ultra-violet. A detailed exploration of 

 this region is contemplated. 



Structure of Uranyl Spectra at -+-20°. 



The study of the fluorescence spectra of the uranyl compounds, with 

 the exception of the spectra of the double chlorides, where the bands are 

 distinctly resolved into groups, has consisted for the most part in the 

 location of the rather broad bands. Many of these spectra, however, 

 show indications of complexity. Dr. Frances G. Wick has made a 

 complete tabulation of the fluorescence bands of numerous spectra 

 not previously mapped. She has also determined the envelope by the 

 method (described in our earlier investigations)^ of measuring the rela- 

 tive brightness of the bands with the spectrophotometer. The wave- 

 length of the crest of the envelope is found to be the same for all the 

 members of a given family of salts, such as the double nitrates, the 

 double sulphates, or the double acetates, but varying from family to 

 family. Detailed spectrophotometric studies of individual bands show 

 that although in general these appear to the eye as simple, they consist 

 of a complex of closely overlapping components even at +20°, which 

 on cooling resolve into the well-known groups which constitute the 

 spectrum of low temperatures. 



The Double Nitrates, 



The study of the fluorescence and absorption of the double nitrates 

 mentioned in the Year Book for 1916, p. 382, has been extended to four 

 forms: NH4UO2 (NO3), (NH4)2 UO2 (N03)4 .2H2O; KUO2 (N03)3, and 

 K2U02(N03)4. Thi'ee crystal systems are represented. The investiga- 

 tors, Dr. Howes and Dr. Wilber, have reached the following conclusions.' 



1. The spectra of the double uranyl nitrates resemble those of the 

 previously studied uranyl salts in that the bands can be arranged in 

 series having constant frequency intervals. 



2. These intervals, while constant for any given series, are different 

 for different series. 



3. In the mono-ammonium uranyl nitrate and the mono-potassium 

 uranyl nitrate the ratio of the interval of a fluorescence series to the 



'Nichols and Merritt: Physical Review (1), xxxiii, p. 358, 1911. 

 *Howes and Wilber: Physical Review (2), x, p. .348 (1017). 



