44 FLUORESCENCE OF THE URANYL SALTS. 



(4) The curve of decay of phosphorescence differs from the prevailing 

 type in that although as usual two successive processes are distinguish- 

 able, the second process is more rapid instead of being slower than the 

 first. 



The study of these phenomena has since been extended to several 

 other typical uranyl salts, the curves of decay of which were deter- 

 mined by the method just described and under conditions of excita- 

 tion, etc., as nearly constant as possible. 1 These curves of decay are 

 of the same new type originally found in the uranyl ammonium sul- 

 phate. The two processes, as determined by the customary method of 

 plotting 7-1/2 as a function of the time are indicated by straight lines 

 differing from one another in slope and the second process has in all 

 cases the steeper gradient. Later experiments, in which the intensity 

 of excitation was increased, revealed the presence of a third process not 

 included within the interval of time covered by our earlier experiments. 



STUDIES INVOLVING THE FIRST AND SECOND PROCESSES. 



The curves shown in figures 34 and 35 are typical of the results 

 obtained with all the salts under observation. They represent the 

 decay of the phosphorescence of the compounds shown in table 16. 



The initial intensity, under like TABLE 16 



excitation, varies greatly in the 

 different salts, as also, to some ex- 

 tent, does the rate of decay. It will 

 be noted that the initial intensities 

 of the ammonium and potassium 

 sulphates, for example, are several 

 times greater than those of the 

 nitrate, the sulphate, and the am- 

 monium chloride. This is, however, a question of previous history as 

 well as of chemical and physical constitution, as was determined in 

 the following manner: 



Uranyl potassium sulphate was dissolved in hot water and a mass of 

 the minute crystals which were thrown down on cooling the solution 

 were immediately sealed up in a glass tube. Care was taken through- 

 out these manipulations to protect the precipitate from the action of 

 light. 



This sample, still in darkness, was mounted in the synchrono-phos- 

 phoroscope and a curve of decay was taken, the first exposure to excit- 

 ing light being that at the beginning of the run. The substance then 

 showed, temporarily, a brilliancy of phosphorescence much above that 

 to be obtained under ordinary circumstances, but was soon reduced to 

 its normal and semi-permanent condition, after which the usual curve 

 of decay was obtained. 



1 Nichols and Howes, Physical Review (2), ix, p. 292. 1917. 



Curve. 



1 

 2 



3 



4 



5 



Substance. 



Uranyl ammonium sulphate. 

 Uranyl potassium sulphate. 

 Uranyl nitrate + GHaO. 

 Uranyl sulphate. 

 Uranyl ammonium chloride. 



