328 History of Luminescence 



without being appreciably warmed, a striking experiment, which 

 Boyle overlooked, as his diamonds were merely exposed to candle- 

 light. All the rubies, spinels, topazes, sapphires, and true emeralds 

 were not luminescent. Dufay determined that the yellow diamonds 

 would luminesce under water and suggested that if the light was 

 prevented from escaping it might persist for some time. He cited 

 results with a diamond exposed to light and then dipped in ink, 

 which was luminescent on removal after five hours. This result is 

 hard to understand. 



Like the Italian workers, Dufay was particularly careful about 

 the dark adaptation of his eyes and tested his dark adaptation with a 

 standard weak light, a piece of amber which he rubbed from time 

 to time, to make it electroluminescent. He discovered that by keep- 

 ing one eye shut, that eye could be dark-adapted, while the other 

 eye was exposed to light. This independent dark adaptation of the 

 eyes turned out to be a great convenience in working with weak 

 luminescences. 



Many other natural minerals were found to be luminous after 

 exposure to sunlight, an effect attributed to absorption of the light 

 particles. In speaking of the ability to prepare artificial phosphors 

 from calcareous matter, Dufay had said, " We know that chalk im- 

 pregnates itself with the substance of light with great ease, con- 

 serves it for sometime and then loses it." 



His general conclusion was that materials became phosphors by 

 three methods of excitation, by heating, by illumination and by 

 rubbing. Some, for example rubies, only responded to rubbing; 

 others, like quartz, to rubbing and heating; while others, such as 

 diamonds, to all three methods. In explanation of the luminescence, 

 Dufay at times spoke of a metallic sulphur which burns, and at other 

 times of the absorption of light. 



After Dufay, in the middle of the century, phosphors were dis- 

 cussed in the dissertations of Melchior (1742) and Markheinecken 

 (1744) and the Mineralogia (1747) of J. G. Wallerius, the pro- 

 fessor of chemical mineralogy at Uppsala; and also in an article on 

 luminous stones by H. E. von Delius (1748, 1785), but little new 

 was added to previous knowledge. The most important work came 

 from the German chemist Marggraf. 



Andreas Siegmund Marggraf 



Although the chemical composition of phosphors had intrigued 

 the earlier workers, chemistry had not advanced to the point where 

 reliable analyses could be made. As we have seen, the presence of 

 silver was inferred from whiteness and weight, and the word sul- 



