Thermoluminescence 3o9 



carried out but a number of new minerals were found to be thermo- 

 luminescent. In 1746 J. H. Pott (1692-1777) an M. D. and pro- 

 fessor of chemistry at the Collegium Medico-Chiurgicum at Berlin, 

 reported the phenomenon in fluorite, calcite, opal, and quartz. Sven 

 Rinman (1747) noted that the Lysspat (shining spar) of Garpen- 

 berg occurred in five different colors, all of which glowed on heat- 

 ing with a pale blue light, whereas other varieties of spar used in 

 glass-making did not. On too great heating the material became 

 white. During the luminescence there was no smell, no loss of 

 weight, and no sign of electrical charge. 



That chalk, various minerals and precious stones were luminescent 

 on heating was mentioned in most eighteenth-century chemistries 

 and by Delius (1785) . Nicolas (1784) reported the luminescence 

 on heating of " spath phosphorique calcaire d'Apremont " and Pallas 

 (1787) described a fluorite from Russia which would luminesce 

 from, the heat of the hand. Crell (1795) also studied thermolumi- 

 nescence of minerals without adding much to general knowledge. 



In 1771 Carl Wilhelm Scheele (1742-1786) of Koping, who initi- 

 ated research on fluorine chemistry, concluded that fluor mineral 

 was a calcareous earth of an (at that time) unknown acid (hydro- 

 fluoric acid) , produced by treating fluor with vitriolic acid.^ He 

 was able to make an artificial fluorspar by adding lime ^vater to 

 the unknown acid, obtaining a precipitate which phosphoresced 

 when heated in the dark. He realized that the green variety from 

 Garpenberg contained iron whereas the white variety from Gislof 

 in Scania did not. He tried without success to find some chanee in 

 composition of fluorspar after phosphorescence had been abolished 

 by strong heating. Since fluor phosphoresced in a vacuum and under 

 water, Scheele concluded " that its phosphorescence does not depend 

 on a subtile inflammable material." He believed that because of a 

 special structure, the light was taken into the pores of the fluorspar 

 but not firmly bound. On warming, the heat was more firmly bound 

 and displaced the light. 



Toward the end of the eighteenth century considerable interest 

 was again aroused in the relation between phosphorescence and 

 thermoluminescence. In 1768 Canton demonstrated a similar be- 

 havior in prepared phosphors, that his phosphorus (CaS) would 

 luminesce a second time, if heated slightly, after all previous lumi- 

 nescence had disappeared. He also found that when this thermo- 

 luminescent light had faded, raising the phosphor to a still higher 

 temperattue would bring another return of light. Herbert (1773) 



^ The chemical essays of Charles-William Scheele, translated from the Swedish by 

 Thomas Beddoes, 1-7, London, 1786. 



