DECAY OF PHOSPHORESCENCE IN SIDOT BLENDE- 67 



effect more promptly. The form of the curves shown in Figs. 53 and 54 

 is probably largely dependent upon the intensity of the exciting light. 



The ordinates of curves B and C (Fig. 58) show the intensity of phos- 

 phorescence 1 minute after the exciting light was cut off. For curve B the 

 exposure was 2 minutes, while for curve C the exposure was 20 seconds. 

 The effect of duration of exposure is here well marked. 



In the case of exposures lasting for several seconds or more the phenomena 

 are manifestly complicated by the fact that the semi-permanent change, 

 to which we have already referred, is taking place in the active substance 

 during the time of excitation. It seemed probable that the relation 

 between intensity of excitation and intensity of phosphorescence might 

 prove simpler if the duration of excitation was reduced to a minimum. A 

 series of curves was therefore taken with a spark as exciting source. Pre- 

 liminary trials showed that the most intense excitation was furnished by 

 discharging eight large jars through a spark gap about 2 cm. long with one 

 cadmium terminal. The distance of the spark gap from the screen was 

 varied from about 10 cm. to 35 cm. A single spark at 10 cm. distance gave 

 an excitation approximately equivalent to 30 seconds exposure to the mer- 

 cury arc. 



Experiments with the practically instantaneous excitation produced by 

 a single spark showed that the phosphorescence was proportional to the 

 intensity of excitation, not merely initially but throughout the whole 

 period of decay. In other words, it was possible to bring a decay curve 

 determined with the spark at a distance d 2 into coincidence with the curve 

 corresponding to the distance d] by multiplying each ordinate 1 \ by the 

 ratio df/d] 2 . 



EXPERIMENTS WITH DIFFERENT PHOSPHORESCENT SUBSTANCES. 



It is natural to inquire whether the complex phenomena exhibited by 

 vSidot blende are peculiar to this particular material, or whether its behavior 

 is typical of a large class of phosphorescent substances. In order to test 

 this matter we have determined the decay curve under similar conditions 

 with three other substances, namely, " Emanations-pulver, M1 willemite, 

 and Balmain's paint. Characteristic curves for these three substances, 

 together with a representative curve for Sidot blende, are shown together 

 in Fig. 59. It will be noticed that the curves are all of the same type. In 

 each case the decay is at first rapid and apparently according to the same 

 law that was found to hold during the early stages of decay in the case of 

 Sidot blende. After 20 or 30 seconds the curves begin to bend, and finally 

 become straight lines whose slant is less than that of the earlier part of the 

 curve. 



Upon plotting the data of E. Becquerel 2 in the same manner we find that 

 in several cases the curves are of exactly the same type as those obtained 

 by us. Three such curves are shown in Fig. 60. In fact, all the data 

 recorded by Becquerel in his papers on this subject give curves which 

 show the same general characteristics, although in several instances the 

 curves are not so smooth as those shown in this figure. 



'Obtained from Leppin and Masche, who do not state the composition of the powder. The chief constit- 

 uent is, however, zinc sulphide, so that the substance appears to be a modified form of Sidot blende. It is 

 said to be especially sensitive to the influence of the radio-active emanations. 



*Becquerel, I.a Lumiere. Also Annales de Chimie et de Physique, Series 3, 62, 1861. 



