46 



FLUORESCENCE OF THE URANYL SALTS. 



istic of this class of compounds, whatever the mode of excitation. It 

 is true that both Waggoner 1 and Zeller, 2 using a Merritt phosphoro- 

 scope, found in their studies of phosphorescence of short duration that 

 excitation by means of a spark discharge very similar to our own gave 

 decay curves of the usual type. 



It is also obvious from the measurements already described that the 

 interval between excitation, i. e., 1/120 second, is sufficient for the com- 

 plete discharge of the phosphorescent glow, and since the absence of 

 any effect of red and infra-red indicates that there is no storage of 

 undeveloped energy to be carried over, such as occurs in the phosphor- 

 escent sulphides, it seems probable that the decay curves do not vary 

 greatly from that which might be obtained, were it possible to make the 

 experiment, from a single exposure. 



To test this a run was made upon the sample of uranyl ammonium 

 sulphate previously used, but with the Merritt phosphoroscope. 



By driving the disk of this instrument 3,000 revolutions a minute, 

 much the same range of time intervals was available as with the 

 synchrono-phosphoroscope. 



To further vary the conditions, a quartz mercury arc was substituted 

 for the spark-gap of Waggoner and Zeller. The arrangement of appar- 

 atus was as shown in figure 36, 

 in which DD is the revolving 

 disk, H the mercury lamp, P the 

 phosphorescent substance, LB 

 the Lummer-Brodhun cube of 

 the photometer, SS a color- 

 filter and milk-glass screen. The 

 device for shifting the oblique 

 mirror M with reference to the 

 aperture A in the disk is not 

 shown. 



Although the decay was some- 

 what more rapid in this determination on account of the less intense 

 excitation, the curve was of precisely the type obtained by the prev- 

 ious method. 



Measurements upon some of the bands of brief duration in the spec- 

 trum of the phosphorescent sulphides, recently made with the syn- 

 chrono-phosphoroscope under experimental conditions identical with 

 those here described, 3 yield curves of the usual type associated with 

 these sulphides, so that the question of the change of form being due 

 to the phosphoroscope employed is effectually eliminated. 



FIG. 36. 



1 Waggoner, Physical Review, xxvn, p. 209. 



2 Zeller, Physical Review, xxxi, p. 367. 



3 Nichols, Proc. Am. Philosophical Society, LV, p. 494. 1916. 



