Thermoluminescence 375 



as the thermoluminescent light appears (Saussure, 1792) , indicates 

 a definite relation between color and thermoluminescence. The rela- 

 tion was again noted with a zirconium compound. W. Henneburg 

 (1846) showed that colored zircon loses its color on heating and 

 thermoluminescence appears, but G. Specia (1876) held that the 

 color change had nothing to do with light emission. It was later 

 shown that cathode rays and X-rays color inorganic compounds and 

 they then become thermoluminescent (see Chap. XII) . 



Although many authors regarded thermoluminescence as a ther- 

 mophosphorescence in which the light previously absorbed was 

 merely set free by heating, cases were known in which great blocks 

 of opaque rocks had been split open in the dark and the inner 

 pieces never in contact with light found to be thermoluminescent. 

 However, it could be held that light was absorbed at the time of 

 formation of the rock. 



In this connection an interesting experiment was carried out by 

 James Napier (1810-1884) in 1851. It had been observed by Lavoi- 

 sier in 1776 that chalk and many other calcareous compounds emit 

 light when moderately heated. Napier drew lines with chalk on 

 an iron plate. These lines glowed like phosphorus for a few seconds 

 when the plate was heated. The glow had usually been explained 

 by a theory of J. W. Draper (1851) . Draper held that during a 

 previous exposure to light, the undulations " became fixed by the 

 cohesions of the molecules of chalk . . . and the light thus fixed 

 within or amongst the molecules is set at liberty by the high tem- 

 perature . . . and by completing its undulations (the chalk) be- 

 comes visible for a time." Napier showed that such an explanation 

 would not hold. He precipitated chalk by the action of carbon 

 dioxide on lime water in complete darkness and found that the 

 precipitate, used to mark lines on the iron plate would luminesce 

 on warming exactly like the natural chalk previously exposed to 

 light. 



Edmond Becquerel reviewed the knowledge of thermolumines- 

 cence rather scantily in his book, La Lurniere, ses Causes et ses Effets 

 (1867), and carried out little experimental work of his own. In 

 fact few papers ^ on thermoluminescence after exposure to light 

 were published toward the end of the century. At that time the 

 various luminescent phenomena in vacuum tubes were claiming 

 the attention of physicists. 



After the discovery of cathode rays. X-rays, and radium rays, 



^ The contribution to knowledge of fluorspar thermoluminescence by C. Bohn (1867) , 

 G. C. Kindt (1867), G. Wyrouboff (1867), A. Forster (1871), E. Hagenbach (1877). 

 Cossa (1877) , and G. D. Liveling (1878) are inconsequential. 



