Triboluminescence and Piezoluminescence 381 



many later Avorkers, the most important being Dufay, whose Re- 

 cherches were published in 1738. 



Boyle, like Bacon, was also aware that " hard sugar being nimbly 

 scraped with a knife would afford a sparkling light," and he showed 

 that the light still appeared when sugar was scraped in a vacuum. 



It is possible that Christian Mentzel (1675) whose teacher was 

 Liceti and who knew Montalbani, discovered triboluminescence of 

 the Bolognian stone, for he compared the sparkles of white light 

 which sometimes appeared from specimens to the sparks observed 

 on stroking cat's fur or on scratching sugar. He endeavored to pre- 

 pare phosphors in many different ways and it seems quite certain 

 that tribokmiinescence of a mixture of antimony and saltpeter, 

 calcined in a crucible, was observed by him. 



Another artificial preparation which showed the phenomenon of 

 triboluminescence was made by Wilhelm Homberg (1652-1715) , in 

 1693, published in the Histoire of the French Academy (1733) . 

 This was an impure fused CaCla which resulted on heating " one 

 part of sel armoniak and two parts of chaux vive," to form a grey 

 vitreous substance. It luminesced on rubbing or striking when 

 freshly crystallized, but the experiment did not always succeed. 



relation to electricity 



It was evidently in Boyle's mind that electricity had something 

 to do with the glow of his diamond. Discovery of the mercurial 

 phosphor (an electroluminescence) led Bernoulli and Cassini (1707) 

 to try rubbing sugar, sulphur, copper, gold, and diamonds on glass 

 and diamonds on silver. Light appeared in all cases. Some of these 

 effects (sugar) may have been true triboluminescences but most 

 were probably due to discharge from frictional electricity, a relation 

 not recognized at that time. 



Hauksbee's (1709) experiments, which involved rubbing ma- 

 terials against each other in a vacuum and obtaining light, were 

 mostly electroluminescences (amber and wool, glass and wool, etc.) , 

 but a disk of steel revolving against flint, which gave off sparks in 

 the air, in a vacuum showed only a diffuse luminescence, which 

 may have been in part a triboluminescence. Hauksbee's work is of 

 greatest importance in connection with electroluminescences and 

 further details will be found in Chapter VII. 



In 1735 the problem of diamond luminescence was minutely re- 

 investigated by Dufay (1738) , better known for his studies on elec- 

 tricity. Dufay quoted from Boyle's experiments and wondered that 

 Boyle had not exposed Mr. Clayton's diamond to sunlight before 

 observation. He also referred to Bernoulli's and Cassini's observa- 



