54 Burbank — X-Rays and Mineral Phosphorescence. 



to change its color to white. The effect of heat is very strik- 

 ing with calcite which has been heated to redness and allowed 

 to cool so that it is just invisible; then when exposed side by 

 side with the unheated it is nearly twice as bright as the latter. 

 On cooling it regains its former color and intensity of radia- 

 tion. 



The effect of heat on other crystals in some cases causes 

 increased emission of light, in others diminished effect, and in 

 several cases no effect was observed. 



Jefferson Physical Laboratory, Harvard University. 



