368 History of Luminescence 



These quotations indicate the widespread interest in thermolumi- 

 nescence. 



Eighteenth-Century Experiments 



Since the earliest observations, the mineral chlorophane, a green 

 fluorescing variety of fluorspar, has been the most celebrated thermo- 

 phosphorescent substance. It was known as false emerald,* " eme- 

 raude brute," and a particular variety, the " pierre de Berne," was 

 described by L. Bourguet (1724) and by Dufay (1726) as " Phos- 

 phore de Berne." This material had been sent to the French 

 Academy from Bern, Switzerland. It was very luminous on the first 

 slight heating but Dufay found it would not become phosphorescent 

 again on a second heating. He therefore thought the stone contained 

 a sulphur which burned. 



Dufay also established that certain precious or semiprecious stones 

 —a false emerald from Auverne (a green fluorspar) , some rubies 

 (aluminum oxide) , amethyst (violet quartz) , oriental topaz (alumi- 

 num fluorosilicate) and hyacinth (zirconium silicate) —would lumi- 

 nesce on slight heating, but not oriental emerald (aluminum and 

 beryllium silicate with a trace of chromium) yellow jasper (quartz 

 with iron) sapphire (aluminum oxide) or opal (silica) . He thought 

 that, if the coloring of a stone was due to sulphur, it would lumi- 

 nesce, otherwise not. In line with his idea that the light was a kind 

 of burning, Dufay wrote: " It must be acknowledged that the pierre 

 de Berne and all the others which have no light except that im- 

 parted to them from the fire of calcination, scarcely differ from a 

 glowing coal, which is a very strong phosphor and a durable one 

 which lights in plain daylight." 



In 1735 Dufay (1738) studied other thermoluminescent materials, 

 rock crystal (quartz) for example, and showed that once exhausted, 

 exposure of this mineral to light would revive the ability to lumi- 

 nesce again on heating. This experiment established the fact that 

 thermoluminescence is really a delayed phosphorescence, excited by 

 slight heating, a thermophosphorescence. However, too great heat- 

 ing was found to destroy permanently this property. Dufay repeated 

 the experiments of Boyle on diamonds, contributing a monograph 

 on this gem in 1738. The Italian group at Bologna, especially 

 Monti, also knew of thermoluminescence and made some scattered 

 observations. 



In the middle of the century relatively few experiments were 



* Also called smaragdine or Bohemian emerald; Kirchmaier (1680) wrote of the 

 mineral as " hesperus " or "vesperugo." Ornaments of the material became fashionable 

 in England after 1765; they were manufactured in Derbyshire. 



