384 History of Luminescence 



quartz, with a faint orange luminescence, then chert (rock flint) , 

 and then rubies, both with a red luminescence. Hard alum did not 

 luminesce. Often a peculiar smell, described as " fetid," could be 

 detected. The light appeared only when the minerals were rubbed, 

 not afterwards. Simple pressure was ineffective but there was lumi- 

 nescence if the material was broken. This luminescence occurred 

 under water and in an inert gas like fixed air (carbon dioxide) . 



Wedgwood thought the effect was due to local heating ^ and 

 argued that heating could take place under water because it was so 

 local, but he admitted that the triboluminescence of sugar, which 

 is soft, yet gives light on gentle rubbing, could not be explained in 

 that way. In favor of the local heating hypothesis he demonstrated 

 that the sparks which came from stones on hitting were hot because 

 they would ignite gunpowder, and also because many of his tribo- 

 luminescent substances would also light on heating. 



Other observers of triboluminescence were de Dolomieu (1791) 

 and Gillet-Laumont (1791) on dolomite from Tirol and de Saussure 

 (1792) , who observed an orange luminescence on rubbing, scratch- 

 ing, and hitting the mineral, as well as on slight heating. These 

 men emphasized the fact that not all samples showed the property. 

 As indicated in Chapter IX they endeavored to connect tribo- and 

 thermoluminescence, without too great success.^'' In 1798 Count 

 C. L. Morozzo noted the triboluminescence of certain minerals when 

 brushed with " une plume," and Accum (1799) described the lumi- 

 nescence of borax. 



In 1799 and 1800 Humphry Davy (1778-1829), later to become 

 knighted and famous as professor at the Royal Institution and presi- 

 dent of the Royal Society, discovered that many plant parts which 

 contain silicic acid, particularly bonnet cane, would luminesce on 

 rubbing.^^ He also experimented with sugar, fluorspar, calcium 

 phosphate, and many other salts, finding that they would luminesce 

 when struck in an atmosphere of carbon dioxide. Since they were 

 transparent non-conductors which could be electrified by friction, 

 Davy at that time was inclined to regard the luminescence as elec- 

 trical in nature. 



* In 1785 de Lamanon had believed the sparks which come from striking quartz and 

 other minerals indicated a burning of these bodies, just as diamond will burn. 



^"^ Nearly one hundred years later similar observations were being made. At the 

 Academy of Natural Sciences at Philadelphia, H. C. Lewis (1884) exhibited a lime- 

 stone from Utah, which gave " a lurid red light when struck, scratched or heated. 

 The glow lasted from half a second when lightly struck, to a much longer time as the 

 result of a blow." The same sample emitted a deep red light when heated, but a 

 specimen from Kaghberry, India, " glowed with a strong yellow phosphorescence when 

 heated, although no such effect was produced by scratching or striking." 



^^ Many writers on luminescence have mentioned the Indian reed (Canna indica) 

 as giving light when the stalks are struck together. 



