382 History of Luminescence 



tions (1707) on diamonds which luminesced when rubbed on glass, 

 metals or faience (glazed pottery) . In Dufay's experience many 

 diamonds turned out to be both phosphorescent and thermolumi- 

 nescent. He proved rather conclusively that electrification of the 

 diamond and luminescence had no immediate connection. Some 

 diamonds became electric on rubbing and did not luminesce, 

 whereas others would luminesce and not become electrified. More- 

 over, an electrified and luminescent diamond on being moistened 

 would lose its charge without disappearance of the light. Dufay 

 also found no relation between the color of the diamond and its 

 luminescent and electric properties. 



It is not surprising that later students of electricity should turn 

 to the study of triboluminescent substances. Rubbing was the easiest 

 way to produce electricity and a characteristic of electrification was 

 the "electric light." Father Giambattista Beccaria (1716-1781) 

 endeavored ® to discover a relationship between the two, using the 

 best known triboluminescent substance, sugar. Concerning this he 

 wrote: " You may, when in the dark frighten simple people only 

 by chewing lumps of sugar, and, in the meantime, keeping your 

 mouth open, which will appear to them as if full of fire; to this add, 

 that the light from sugar is the more copious in proportion as the 

 sugar is purer." On pounding sugar in a mortar Beccaria described 

 the light as exactly like electric sparks, but could observe no attrac- 

 tion of fine threads, which would have indicated that electrification 

 had occurred. Sugar, like diamond, shows " true triboluminescence," 

 and the various sugar experiments confirmed those made with dia- 

 monds. There is no connection between electrification and tribo- 

 luminescence of sugar. 



EIGHTEENTH-CENTURY OBSERVATIONS ON MINERALS 



Another common material which exhibits triboluminescence is 

 quartz. Anyone can confirm the observation that quartz pebbles 

 when knocked together, especially if hit a glancing blow, exhibit a 

 yellowish flash of light. Except for Hauksbee's experiment with 

 flint, there is no special mention of this property in the early litera- 

 ture ^ but in 1748, H. F. Delius (1720-1791), a physician and pro- 

 fessor of medicine at the University of Erlangen, called attention to 



• Reported in a translation of his book, Dell' elettricismo artificiale e naturale 

 (Turin, 1753) , which appeared as A treatise on artificial electricity, 324, London, 

 1776. J. K. Wilcke, according to Priestley (1769: 290) was another who endeavored 

 to explain triboluminescence of sugar by the " electric light." 



'' Boyle (On colours, 1664) stated that he could see no light when two white pebbles 

 were " hard rubbed " against each other. Possibly they were not quartz but marble. 



