374 History of Luminescence 



sion when powdered organic materials are placed on a hot plate 

 are not to be classed as luminescences, but probably result from 

 actual combustion, with glowing of the powder. Such an explana- 

 tion cannot be applied to an observaton of M. Callaud, a pharmacist 

 of Annecy, reported by P. J. Pelletier (1821) . This was the lumi- 

 nescence of quinine sulphate, on slight heating. Callaud found that 

 the light would last for some time, and believed that only pure 

 quinine behaved in this way. He suggested that the luminescence 

 could be used as a test for purity. Pelletier himself confirmed the 

 report, and found that quinidine sulphate would behave in the 

 same way. He wished to test other alkaloids, but had no oppor- 

 tunity, and it was not until 1840 that R. Bottger (1840) obtained 

 negative results. It was later claimed by A. Kalahne (1905) that 

 dehydration of the crystals is responsible for the light. Hence, if 

 the crystals crack on heating the phenomenon could be a tribolumi- 

 nescence. More modern studies should be undertaken. 



Sporadic interest in thermoluminescence continued. In 1830 

 Thomas J. Pearsall, chemical assistant at the Laboratory of the 

 Royal Institution, published an important paper on the effects of 

 electricity in exciting thermoluminescence. He studied the behavior 

 of a number of thermoluminescent minerals and calcareous organic 

 material, exposed to the light of the electric spark, repeating some 

 of the earlier work. Usually a luminescence was found to occur 

 during the discharge, after which, on heating slightly, the bright- 

 ness of the thermoluminescence was very considerably greater than 

 during the discharge. If heated to the point where no more thermo- 

 luminescence appeared, a single spark discharge would make the 

 material thermoluminescent again. Moreover certain colorless fluor- 

 spars and diamonds, not normally thermoluminescent, could be 

 made so by exposure to the spark discharges, although an amethyst, 

 sapphires, rubies, and garnets gave no indication of such thermo- 

 luminescent behavior. 



Mother of pearl, scallop shells, cuttlefish bone and egg shells, 

 exposed to the spark and then heated, luminesced with colors from 

 yellow to purple. A colorless fluorspar became colored after ex- 

 posure to the spark discharge. These effects of the spark are largely 

 due to the ultraviolet light in the spark, as had been pointed out 

 by Heinrich, but Pearsall did not appear to realize this fact and 

 thought that voltage of the spark was responsible, rather than the 

 light or the amount of electricity. 



Pearsall's observation that spark discharges make a colorless fluor- 

 spar blue, together with the fact that natural thermoluminescent 

 samples of fluorspar are colored and lose their color when heated 



