68 



STUDIES IN LUMINESCENCE. 



It is interesting to note also that the data of Darwin 1 on Balmain's paint 

 give a curve similar to those obtained by us with this substance when 

 plotted in the same way- Two decay curves taken by E. Wiedemann 2 with 

 Balmain's paint also show the same characteristics. 



/vr 



400 

 300 

 200 

 100 



20 4-0 60 80 100 120 140 160 160 



Seconds 



Fig- 59- 



Typical decay curves for various substances. Curve .4, Sidot blende; curve 1>, "Emanations-pulver;" 



curve C, Willemite; curve D, Balmain's paint. 



It appears therefore that the decay curve for Sidot blende is similar in 

 its main features to the curves obtained for a large number of other phos- 

 phorescent substances. In fact, we do not know of any case of long-time 

 phosphorescence in solids which shows a different type of curve. 



300 

 200 

 100 



Fig. 60. 



Decay curves plotted from the data of E. 

 Becquerel. (Ann. de Chimie et de 

 Physique, series 3, vol. 62, 1861.) 



Curve A, Sulfure de calcium lumineux 



vert (2, p. 69;. 

 Curve B, Sulfure de strontium lumineux 



vert (p. 71). 

 Curve C, Sulfure jaune-orange vif (p 



68). 



200 400 600 800 



Seconds 



1000 



The peculiar behavior of Sidot blende which we have compared with 

 magnetic hysteresis is also exhibited by willemite and Balmain's paint, as 

 illustrated in Figs. 61 and 62. In each of these cases it is evident that the 

 effect of a given excitation is dependent on the previous history of the sub- 

 stance. We have not yet had an opportunity to test this phenomenon in 

 other substances. 



'Darwin, Philosophical Magazine, n, p. 209, 1881. 



! E. Wiedemann, Zur Mechanik der Leuchtens, Wied. Ann., 37, p. 177, 1889. 



