366 CARNEGIE INSTITUTION OF WASHINGTON. 



Nichols, E. L., Cornell University, Ithaca, New York. Systematic studies of 

 the properties of matter through a wide range of temperatures. (For previous 

 reports see Year Books Nos. 4-17.) 



Researches under this grant have been greatly retarded during 

 1918-19 by conditions resulting from the war. A monograph, describ- 

 ing investigations on the fluorescence, phosphorescence, and absorption 

 of the uranyl salts, has been completed, however. It deals with the 

 work of several observers over a period of eight years and is a fairly 

 exhaustive survey of one of the most important fields in the domain of 

 luminescence. 



In the course of these researches, the spectra of numerous uranyl 

 compounds were mapped and discussed. Many of these salts, includ- 

 ing the various chlorides, nitrates, acetates, sulphates, and phosphates, 

 were especially prepared for this purpose by Dr. Wilber. 



A comparative study of the results obtained has led to the following : 



General Conclusions Concerning the Luminescence of the Uranyl Salts. 



The entire spectrum of any given uranyl compound is an homo- 

 geneous complex. 



The criteria of homogeneity are: (a) Independence of the mode of 

 excitation. — The uranyl spectra meet this test completely. Any wave- 

 length of light, capable of producing fluorescence, brings out all the 

 bands and, so far as we have been able to determine, with the same 

 relative intensities. Numerous attempts at selective excitation (i. e., 

 to produce a single group or series or even to enhance portions of the 

 spectrum, by the use of monochromatic illumination), have given 

 negative results. Kathode-luminescence and X-ray excitation, more- 

 over, give the same spectrum as photo-excitation. This is true of single 

 bands in the spectra of other fluorescent substances, e. g., the green 

 band of Sidot blende; but rarely if ever, of the entire spectrum, save in 

 the case of the uranyl salts, {b) Identity of the fluorescence spectrum 

 and the phosphorescence spectrum, or, what is nearly the same, of the 

 phosphorescence spectrum during the earlier and later stages of decay. 

 Conformity with this criterion was established for several uranyl salts 

 by Misses Wick and McDowell in their studies of kathodo-phosphores- 

 cence. It is not a test easily applicable to all such compounds, because 

 the phosphorescence under photo-excitation is of the vanishing type. 

 Comparatively few uranyl salts, on the other hand, are sufficiently 

 stable in vacuo, especially under exposure to kathode rays, to permit 

 of a detailed examination by that method. 



Heterogeneity in a fluorescence spectrum of the usual broad-banded 

 type, such as is universal with the phosphorescent sulphides for exam- 

 ple, is not readily detected by spectroscopic observation on account of 

 the overlapping of the bands, but it often manifests itself in the most 

 striking manner by changes of color during the decay of phosphores- 



