BurbanJc — X-Rays and Mineral Phosphorescence. 53 



Art. YII. — X-Rays and Mineral Phosphorescence / by 

 John E. Burbank. 



In January, 1897, while experimenting at Bowdoin College 

 on fluorescent screens for the X-rays, the attention of the 

 writer was attracted by the fluorescence of minerals under the 

 action of these rays. 



Doubtless others have experimented on this subject, but the 

 writer has been unable to find more than a brief note stating 

 the mere fact of fluorescence under action of X-rays. Some 

 of the following results, therefore, may be of interest. Fluor- 

 ite when exposed to X-rays phosphoresces with a bluish white 

 light which continues for a very long time after being removed 

 from the influence of X-rays. It has been placed by some 

 experimenters under the photographic place in radiography, to 

 increase the effect of the X-rays ; but unfortunately it fogs the 

 plate and blurs the outline of the image. 



Very many minerals were tried and more than two-thirds 

 were found to be phosphorescent to a greater or less degree. 

 The group of minerals containing Ca were most susceptible to 

 the X-rays. Of 16 specimens tried 12 were phosphorescent, 

 with light varying from pure white to yellowish red. The 

 feldspar group is also susceptible, also others of the silicates ; 

 but in general minerals containing ores of the metals are non- 

 phosphorescent. 



Of all substances tried fluorite and calcite seemed the most 

 suitable for experimentation, and all the experiments were 

 performed with these minerals, outside the X-ray tube, in a 

 perfectly dark room. It is well known that both light and 

 heat radiations can produce luminous effects in the case of min- 

 eral crystals, and these effects are very much like those pro- 

 duced by X-rays ; at least far as the eyes can judge ; but if 

 we heat the crystals and then expose them to the radiations 

 coming from an X-ray tube, the character of the light is often 

 changed. For example : 



Glass, which fluoresces bluish, if sufficiently heated loses 

 its power while hot ; but as it cools the light can be seen slowly 

 creeping back over it ; when cool it regains its former condi- 

 tion. 



With fluorite the effect is similar except that the light, 

 instead of disappearing, is weakened and fades out sooner than 

 that from the unheated, when removed from the influence of 

 the X-rays. If fluorite is heated very hot the phosphorescent 

 light is very much weakened. 



With calcite, which phosphoresces a pale reddish yellow, the 

 effect of heat is to brighten very much the light emitted, and 



