III. PRELIMINARY OBSERVATIONS ON CERTAIN 



URANYL SALTS. 1 



Because of their brilliant luminescence and the interesting character 

 of their spectra of fluorescence and absorption, the uranyl compounds 

 have been the subject of extended study. A brief account of the work 

 of previous observers in this field has been given in Chapter I. 



Our original purpose in taking up the study of these substances was 

 to determine whether the different bands of the fluorescence spectrum 

 are to be regarded as independent, each with its own region of excita- 

 tion, or whether they form a homogeneous complex, such that the 

 excitation of one necessarily involves the excitation of all. In this 

 inquiry we have been led to the investigation of many other questions. 



Since, as was first shown by 

 Becquerel and Onnes 2 in the paper 

 cited in Chapter I, the bands of 

 the uranyl salts are resolved into 

 groups of narrow components by 

 cooling, it is at the temperature 

 of liquid air and chiefly by 

 photographic methods that the 

 intimate structure of the fluo- 

 rescence and absorption spectra 

 is to be determined. The study 

 of the spectra at ordinary tem- 

 peratures, however, is not without 

 significance. 



In this work, where the width 

 of the bands is from 50 to 100 

 A. IT., the spectrophotometer is 

 indispensable. Many of the 

 measurements to be described 

 were made with a special in- 

 strument which combines the 



features of the constant deviation spectrometer and the Lummer- 

 Brodhun spectrophotometer. It is essentially a spectrometer of the 

 Hilger type, with two collimators C and C" (fig. 6), a Lummer-Brodhun. 

 cube L. B., and a constant-deviation prism P with carefully calibrated 



drum. 



/ 



1 Certain of the observations contained in this chapter have been published in the Physical 

 Review (1), xxxm, p. 355, but many of the data there given have been replaced by more complete 

 investigations kindly done at our request by Dr. Frances G. Wick (Physical Review (2), v. 11, p. 121. 

 Feb. 1918. 



2 Becquerel and Onnes, Leiden Communications, 110. 1909. 



15 



L.B. 



FIG. 6. 



