C. A. Butman — Effect of Phosphorescent Material. 135 



particles were allowed to settle out and then the tine particles 

 were permitted to deposit themselves on a metallic plate which 

 could be fitted over the brass plate in the tube. This method 

 gave a uniform layer of any desired thickness which would 

 remain on the plate. Plates of platinum and lead were used 

 to place the material upon. 



The substances experimented with were Sidot Blende (ZnS) 

 prepared by the Braunschweiger Cheminfabrik — phosphores- 

 cent light yellow, and calcium sulphide prepared by myself after 

 the method of Lenard and Klatt (5).* The composition was 2g. 

 CaS, 0-lg. Na 2 S0 4 , O05g. JNa 2 B 4 7 , 0-05g. CaFl 2 , 0-00048g. Bi. 

 Chemically pure materials were used with the exception of 

 the calcium fluorite, which was the ordinary fluorite. The bis- 

 muth was introduced in the form of an oxide. The calcium 

 sulphide was prepared by heating some carbonate until the 

 CO, gas was driven off. Two grams of the CaO were mixed 

 with 1*35 grams of powdered stick sulphur and heated in a 

 covered porcelain crucible over a Bunsen burner. The CaS 

 was then placed in a mortar and mixed with the other 

 materials. The whole mass was then put in a crucible and 

 heated strongly over a blast flame for twenty minutes. This 

 material is called by Lenard CaBiNa. The color of the phos- 

 phorescent light is blue. 



The apparatus used is very convenient, since no effect can 

 come from anything except the material under investigation, 

 and as no metals (with the exception of N&, K, Rb and certain 

 allojs) give any photoelectric effect after the light has been 

 passed through glass. Glass cuts off rays shorter than about 

 3200 Angstrom. Tests were made with voltages ranging from 

 to ±600 volts on the case, and with Pt and Pb plates, but 

 no effect was ever obtained except when phosphorescent 

 material was on the plate. Hence, I was able to use a layer of 

 material so thin that it consisted of discrete particles. 



Photoelectric Fatigue and Recovery of Phosphorescent Material. 



An examination of the curves for photoelectric fatigue 

 obtained will show that there is a rapid drop from the initial 

 value to a value that is nearly constant for several hours. 

 Hence, by waiting until this steady photoelectric state is 

 reached, comparative results may be readily obtained. 



It will also be observed from the curves that the values of 

 the deflections are less for each of three successive days. The 

 curves are placed as to render comparison easy. They 

 represent the number of centimeters deflection in successive 

 one-half minute periods. The phosphorescent material was 



* Lenard and Klatt, Ann. Phys., xv, 425, 1904. 



