Thermoluminescence 373 



vation that the color of the thermoluminescence was always the same 

 after exposure to different colored lights. Hence one color was 

 apparently transformed to another color, which was contrary to 

 Newton's views that colors could not be interconverted. Grotthus 

 therefore abandoned the Newtonian concept and held that color 

 must depend on a greater or less difficulty of movement which light 

 (regarded as a combination of plus and minus electricity) encoun- 

 ters at the surface of bodies. He postulated that when light strikes a 

 phosphor surface, it is split into its plus and minus principles, and 

 may be again emitted when the plus and minus principles recom- 

 bine, provided there is some resistance to the recombination. If the 

 plus and minus principles encounter no resistance in recombina- 

 tion the surface is merely heated. 



The result of heating a thermoluminescent body is to cause the 

 elementary parts of the mineral to expand. Thereby the polar 

 forces, by means of which light is decomposed and absorbed, become 

 weakened and the combination of the plus and minus electrical 

 charges of which light consists can take place at the proper speed. 

 Grotthus' reasoning was logical, although the theory was difficult to 

 test experimentally. 



Attempts to find some general rule by which the thermolumines- 

 cence of a substance could be predicted were continued without too 

 great success. David Brewster (1781-1868), whose later studies of 

 " internal dispersion " were so important in establishing the concept 

 of fluorescence in soliuions, made an exhaustive study of luminescent 

 minerals in 1819 and 1820, noting especially the color of the light 

 which appears on heating, and recording the color in his tables.^ 

 The 1820 paper described one sample of fluorspar showing a layered 

 structure, with each layer exhibiting thermoluminescence of a dif- 

 ferent color. The sample was useful in his study of fluorescence (see 

 Chap. XI) . 



In general Brewster found that thermoluminescence is most char- 

 acteristic of colored minerals but the color of the light is not related 

 to the color of the mineral. The property is destroyed by intense 

 heat and contrary to the finding of Grotthus, not always regained on 

 exposing the minerals to light. The phosphoric light has the same 

 properties as sunlight. It is not necessarily related to luminescence 

 from attrition, as minerals which give no light on heating may emit 

 on attrition. Brewster (1823) also noticed that the CaCOa frame- 

 work of Chara plants is luminous when laid on a hot plate. 



We have seen that many of the older observations on light emis- 



'' Reprinted in the Edinburgh Encyclopedia 12, 1832, of which David Brewster was 

 editor. 



