340 CARNEGIE INSTITUTION OF WASHINGTON. 



interval of the absorption series which joins that fluorescence series is 

 approximately a constant. 



4. Although the grouping of the bands shows a strong family resem- 

 blance in the four spectra, the absolute position of a group is largely 

 determined by the crystal system. 



The Acetates. 



A determination (similar to those completed in the case of the 

 chlorides and nitrates) is being made upon the spectra of the uranyl 

 acetates. The members of this large family of double and triple salts 

 are nearly all fluorescent and many of the spectra are finely resolved 

 at the temperature of liquid air. Photographs of the numerous prep- 

 arations made for this work by Dr. Wilber have been made and meas- 

 ured and the results mapped. The final comparison and discussion of 

 the data is well advanced. 



Phosphorescence of Long Dueation. 



In the course of a recent comparison of the spectra of certain uranyl 

 salts when excited to fluorescence by the kathode discharge with the 

 same spectra under photo-excitation, Doctors Louise S. McDowell 

 and Frances G. Wick discovered that brilhant phosphorescence visible 

 in some instances for several minutes was developed by the action of 

 the kathode rays. It appears from their preliminary survey of this 

 new field that: 



1. The phenomenon occurs only at low temperatures. 



2. The effect is gradually built up under the action of the discharge, 

 exposure for several seconds being necessary in most cases. 



3. The substances thus affected show no phosphorescence of appre- 

 ciable duration under the action of light, whether excited before or 

 after exposure to the kathodic bombardment. 



4. The type of decay is that characteristic of persistent phosphor- 

 escence in general, there being two processes. The first and more 

 rapid lasts 15 or 20 seconds, under the strength of excitation employed. 

 The second is much slower and continues to the end or at least as long 

 as the glow is of measurable intensity. Both processes follow the usual 

 law, i. e., the square root of the intensity is inversely as the time from 

 the close of excitation. 



This type of phosphorescence differs strikingly from the phosphor- 

 escence of the uranyl salts under photo-excitation, recently described,^ 

 in which the first process of decay is slower than the subsequent decay 

 and in which the total duration is a few thousandths of a second. The 

 relations of the two are under investigation. 



^Nichols: Proceedings of National Academy of Sciences, ii, p. 328, 1916; also Nichols and 

 Howes: Physical Review (2), x, p. 293 (1917). 



