ANTECEDENT DISCOVERY. 83 



Professor Rontgen set men wondering as to whether the 

 power of emitting penetrating rays might not be a property 

 of phosphorescent bodies in general. In this instance there 

 was Niewenglowski, who made the interesting discovery that 

 some such rays were actually emitted; that much at least. 

 His experiment, as afterward repeated by Becquerel, is per- 

 fectly demonstrative. A certain compound of sulphur and 

 calcium, calcium sulphide, which is the basis of luminous 

 paint, shines in the dark after exposure to sunlight that is, 

 it is phosphorescent. Niewenglowski placed a photographic 

 plate in a plate-holder, and instead of a cover-slide, he in- 

 serted a thin sheet of aluminum. The plate was thus com- 

 pletely sheltered from the action of light. Upon the sheet of 

 aluminum he placed squares of thin glass, and upon these, 

 in turn, pieces of a certain calcium sulphide previously ex- 

 posed to light, which were protected from external influences 

 by dome-shaped clock-glasses. The arrangement of the ex- 

 periment is seen in Figure 19, the cover-slide of aluminum, the 



Fig. 19. Niewenglowski 's Experiment. 



glass, the sulphide above it, and the clock-glasses covering 

 all. The apparatus was left in the dark for twenty-three 

 hours. The plate was then developed. A print from the 



