THERMO-LUMINESCENCE 



99 



When the whole process was repeated the change in the current 

 was smaller. A third repetition gave a still smaller effect, for which 

 the data is plotted in Cur\'e B, Fig. 2. Since the change in conductiv- 

 ity depends on the thermo-luminescence, the latter effect must also 

 have decreased. 



The same procedure was used for the higher range of temperature 

 22°C to 100°C. Suitable precautions were taken to keep the crystal 

 and insulation dry. The results were similar to those obtained 

 previously, but it was noticed that successive excitations decreased 

 the thermo-luminescence more quickly at the higher temperatures 

 than at the lower. 



Effect of X-rays 



It is known that exposure to X-rays will restore the property 

 of luminescence.^ A preliminary trial of the effect of X-rays was 

 made on a small piece of calcite. It was heated over a Bunsen burner 

 until no luminescence could be detected by the eye in a dark room. 

 It was then exposed to X-rays from a coolidge tube and again heated. 

 The luminescence, though not as intense as originally, was partially 

 restored. 



When the original crystal of calcite was exposed to X-rays it 

 was found that the conductivity was increased, but the effect lasted 

 not more than 1 hour. The exposure to the X-rays was 50 milli- 

 ampere minutes at 60 kilovolts through 8 inches of air. After an 

 interval of 2 hours the crystal was excited to luminescence using the 

 temperature range — 80°C to 22°C. The results plotted in Curve C, 

 Fig. 2, show that the calcite was partially restored to its original 

 condition by the X-ra^^s. At the higher range of temperature the 

 restoration was not so complete. 



Two other substances, fluorite a^nd feldspar, were also examined. 

 The data shown in Tables 4 and 5 is very similar to that obtained for 

 calcite. 



Table 4 — Fluorite 



1 Wiedemann and Schmidt, Wied. Ann. 56 p., 177, 1895. 

 —29 



