50 FLUORESCENCE OF THE URANYL SALTS. 



uranium glass. He noted the brilliant initial intensity and very rapid 

 decay, and to test the independence of the constant in his equation of 

 decay when the illumination varied he made many measurements. If 

 from his data we compute I~ 1 / 2 , as a function of the time, we obtain 

 curves of the same general form as those in figure 32. 



Becquerel's observations are not numerous enough, taken by them- 

 selves, to determine completely the type of curve. His measurements, 

 however, cover a larger time interval than ours and the values for the 

 longest times indicate an even more rapid decay following the second 

 process. We had, indeed, found some indications of a similar tendency 

 which had been omitted from our curves as lying almost beyond the 

 range of definite determination. 



To investigate the further trend of the curves of decay, the intensity 

 of excitation was increased by readustment of the sparking circuit, by 

 which means it was found possible to extend the time interval for more 

 than 0.006 second beyond the cessation of excitation. 



Careful, often repeated measurements, of the various salts showed 

 in fact a third linear process beginning where our previous determina- 

 tions had ceased and having a steeper slope, indicative of still more 

 rapid decay. Typical results are indicated in figures 38, 39, 40, etc. 



These processes may be numbered for convenience 1, 2, and 3 in the 

 order in which they occur. Processes 1 and 2 are in general of about 

 equal duration for a given salt. The abruptness of transition, however, 

 varies greatly, and in some instances the change of slope is so gradual 

 as to encroach seriously on process 2 at both ends. 



THE INFLUENCE OF TEMPERATURE. 



The only previous instances of decay of phosphorescence in which the 

 later stages are more rapid than those preceding are noted by Ives and 

 Luckiesh 1 in their study of the influence of temperature on phosphores- 

 cence, and by E. H. Kennard 2 in a more recent paper. 



Ives and Luckiesh measured the phosphorescence of one of Lenard 

 and Klatt's sulphides (BaBiK from Leppin and Masche). This sub- 

 stance was found to be very sensitive to change of temperature and 

 the results at 0, 22, and 35, C. when plotted for 7-1/2 an d time in the 

 usual manner, gave curves varying greatly in slope. The curve for 

 is concave toward the time axis, that for 22 linear, and that for 35 

 strongly convex. They show that a linear relation may be obtained for 

 each of these curves by varying the exponent of 7. 



The effect of temperature in the case of the phosphorescent sul- 

 phides, where one has to do with a composite of many overlapping 

 bands of varying duration, is undoubtedly different from that to be 



1 Ives and Luckiesh, Astrophysical Journal,. xxxvi, p. 330 (1912). 



2 Kennard, Physical Review (2), iv, p. 278 (1914). 



