58 



FLUORESCENCE OF THE URANYL SALTS. 



intensity of phosphorescence matched that of the source in the first 

 possible position, the time was again recorded and the carriage moved 

 to the next stop, and allowed to remain until a match was made as 

 before. This procedure was continued until the phosphorescence was 

 too faint to observe or until the end of the track was reached. 



The interpretation of the results was somewhat difficult, since the 

 instability of the crystals rendered uncertain both the control of the 

 vacuum and the maintenance of the crystal surface unchanged during 

 prolonged bombardment. The general shape of the decay curve after 

 long excitation is shown in figure 45. The curves are of the type usual 

 with phosphorescence of long duration, consisting of two linear 

 processes, of which the first is the more rapid, whereas, as has been 

 shown in the previous portions of this chapter, the decay following 

 photo-excitation is of a new and entirely different type. 



too 



200 400 



SECONDS 



Fio. 45. 



100 200 



SCCONDS 



FIG. 46. 



100 20* 



SECONDS 



FIG. 47. 



Under different conditions, phosphorescence was observed to last 

 from less than a minute to 10 or 15 minutes. The exact form of the 

 curve varied with the time of excitation. The time of decay was found 

 to increase with the time of excitation, as shown in figure 46, but the 

 initial brightness changed relatively little. There was some evidence to 

 indicate that under similar conditions of vacuum the rate of the first 

 process remained practically unchanged for varying times of excita- 

 tion, but that the second process began sooner for longer excitation, 

 as shown in figures 47 and 48. In figure 48, curves 1 and 2, obtained 

 by a short-time excitation, show only the first process, whereas curves 

 45 and 46, obtained by excitations of 40 and 80 seconds respectively, 

 indicate that a state of saturation had been reached such that added 

 excitation produced no change in the phosphorescence. 



As has been stated, the initial brightness and rate of decay were 

 found also to depend upon the strength of the bombardment, as varied 

 by the pressure in the tube and by the voltage applied to the induction 



