370 History of Luminescence 



noted a similar phenomenon, using the Bolognian phosphor, as did 

 the Rev. Morgan (1785) with oyster shell phosphors prepared by 

 Wilson's (1775) method. Canton thus discovered thermolumines- 

 cence of artificial phosphors. It was quite obvious that thermolumi- 

 nescence and phosphorescence must be closely related. 



Relation of Thermoluminescence to Triholuminescence 



Although Dufay had pointed out that scratching was one method 

 of exciting certain minerals to luminesce, a rather close relation 

 between thermoluminescence and triholuminescence was demon- 

 strated in the 1790's. In 1791, de Dolomieu discovered a calcite-like 

 mineral in Tyrol, later called dolomite (a CaMg carbonate) , which 

 hardly dissolved in acids, and which luminesced on rubbing, scratch- 

 ino-, or hittins. In 1792 de Saussure noted the thermoluminescence 

 of certain samples of the mineral. The color of the light of dolo- 

 mite on heating is orange and it will not luminesce on a second 

 heating even after exposure to sunlight. De Saussure analyzed the 

 mineral and suggested that impurities like iron might have some- 

 thing to do with the luminescence, but never followed up the idea. 

 He recognized three types of stones which luminesced on heating— 

 (1) those containing sulphur or a hepar {fois) , a compound of sul- 

 phur, which burned in the free air, (2) those which absorb the 

 light and then emit it, like the diamond, and (3) those which do 

 not require air and will luminesce under hot water, like dolomite 

 and fluorspar. He noted that the more colored the fluorspar, the 

 more thermoluminescent it was and that on heating the color dis- 

 appeared at the time the ability to thermoluminesce was lost. 



A little later, J. Thomson (1798) pointed out that the connection 

 between thermo- and triholuminescence is not universal. A marble 

 from Castellammare, near Vesuvius, luminesced on heating but not 

 when rubbed, whereas Dolomieu's (1791) " calcite " (really dolo- 

 mite) would luminesce on rubbing but not on heating. The two 

 phenomena appeared to be distinct. 



In one of the last comprehensive papers of the century (1792) , 

 Thomas Wedgwood (1771-1805), son of the famous potter, Josiah 

 Wedgwood (1730-1795) , and a potter himself, made a special study 

 of thermo- and triholuminescence. He compared the ability of a 

 great variety of substances, inorganic and organic, to luminesce 

 when placed on an iron plate heated to below the temperature of a 

 red glow, and also tested their response to attrition by rubbing two 

 pieces together. On heating he found blue fluorspar to be brightest, 

 giving a green light changing to lilac. Then came red feldspar, vari- 



