372 History of Luminescence 



Heinrich's studies on thermoluminescence were even more exten- 

 sive than those of Dessaignes. He also declared that almost all sub- 

 stances emitted light in powder form on moderate heating. Again, 

 many of his compounds were organic (meal, paper, etc.) , others 

 were calcareous plus organic matter (egg shells, bone) , and others 

 mineralized carbon (coal, graphite) , and burning must have oc- 

 curred. Of the non-combustible minerals, all fifty-one of his cal- 

 cium-containing compounds, most barium combinations, and some 

 precious stones were thermoluminescent. Even some metals gave a 

 momentary bright light, followed by a permanent resting lumines- 

 cence. The tables in Heinrich's book give the relative intensity 

 and color of the thermoluminescence. Both Dessaignes (1808) and 

 Heinrich (and later Grotthus) knew that exposure to an electric 

 spark discharge would very effectively revive the ability of an ex- 

 hausted thermoluminescent material to luminesce again on warm- 

 ing, but they interpreted the action of the spark in different ways. 

 Dessaignes held that the effect was due to the electricity, Heinrich 

 (correctly) to the light in the discharge. 



Of all thermoluminescent substances, fluorspar excited the most 

 interest. The different varieties emitted light of different colors, 

 and at surprisingly low temperatures. One of the important con- 

 tributions of the early nineteenth century, two papers by Theodor 

 von Grotthus (1785-1822) dealt exhaustively with this mineral. 

 They appeared in Schweigger's Journal filr Chemie iind Physik in 

 1815 and contained his complicated theory of phosphorescence (see 

 Chapter VIII) and thermoluminescence, based on his view that 

 light is essentially the same as electricity, made up of positive and 

 negative parts. He had obtained specimens of red-violet colored 

 fluorspar from Nertschinsk known as fire emerald (pyrosmaragd) 

 or chlorophane. When the chlorophane was heated in the light, its 

 color changed to green, an indication that an intense gieen lumi- 

 nescence was emitted, as could be observed in the dark. After the 

 light had disappeared, a later heating in the dark gave no green- 

 light emission unless the chlorophane had been previously exposed 

 to sunlight. Then it would retain for months the ability to lumi- 

 nesce whenever it was heated slightly to 50° C) . If illuminated at 

 —25° C, the luminescence on heating to 50° C was much more 

 intense than if illuminated at 25° C. Grotthus naturally came to 

 the conclusion that cold favored the absorption of light while heat 

 favored the luminescence emission. 



Grotthus particularly emphasized the fact that no light was already 

 present in a thermoluminescent phosphor, but that it had to be 

 taken up by a previous exposure to light. There was also his obser- 



