Triboluminescence and Piezoluminescence 387 



Pope (1899) , saccharin; F. Richarz (1899) , salophen; all with crys- 

 tals which are triboluminescent on breaking. For several centuries 

 sugar had been the one well recognized organic triboluminescent 

 compound, but the greater advances in organic chemistry in the 

 latter half of the nineteenth century revealed these additional exam- 

 ples and led the way for the extensive studies of M. Trautz in 1905. 

 No comprehensive theory to account for triboluminescence was pro- 

 posed and the principal advance was discovery of the large number 

 of organic crystals, which emit light on crushing. 



As in the study of other types of luminescence, so in the case of 

 triboluminescence there have been conflicting results of various in- 

 vestigators and inconsistent observations noticed by the same ob- 

 server. The explanation of this variation may be due to the fact 

 that (1) triboluminescence of certain substances only appears in 

 impure material, and that (2) triboluminescence disappears some 

 days after a crystal has been formed. At the time of his study of 

 saccharin, Pope (1899) noticed that not every crystal would sparkle 

 on crushing. He also emphasized that it was " commercial " sac- 

 charin which showed marked triboluminescence and not the puri- 

 fied material. Moreover his crystals lost the property after a few 

 weeks. This phenomenon of " temporary triboluminescence " is 

 fairly widespread. Romberg (1693) found that his phosphor must 

 be freshly crystallized in order to luminesce and not every trial was 

 successful, while Heinrich (1820) remarked that a previous bright 

 glowing destroyed the ability to triboluminesce. 



Crystalloluminescence 



In 1786 a novel kind of luminescence was described, to which the 

 name crystalloluminescence ^^ has been given. Johan George Pickel 



(1751-1838) , who published in 1787, and Schoenwald (1786) were 

 the first, and later G. A. Giobert (1790) and J. M. Schiller ^« (1791) 

 also noticed the gieenish points of light when vitriolated tartar 



(K2SO4) crystallized rapidly. Since the light is much brighter if the 

 crystallizing liquor has been stirred, most observers attribute the 

 light to breaking of crystals rather than the formation of crystals, 

 and hence consider it a triboluminescence. The discoverers noted 

 especially that light appeared when the crystals were scraped with a 

 fingernail. 



^^ Crystalloluminescence is of interest in biolurainescence history because R. Dubois 

 (1893) once thought the light of centipedes was a crystalloluminescence but later 

 reversed this opinion. These forms secrete a material full of granules which he 

 described as transforming into crystals. 



"Schiller (1791) noted that crystals of K2SO4 also lighted when scraped off the side 

 of the vessel, a triboluminescence. 



