280 History of Luminescence 



by one of its members. Among other experiments on this head they 

 came at length to the light that certain bodies yield, by rubbing in 

 the dark," like cat's fur, or sugar, or sulphur pounded. 



The French Academy held that certain conditions had to be 

 observed in order to obtain light by friction: (1) one of the bodies 

 " must be transparent that the light may be seen through while it 

 lasts "; (2) " the surface of the bodies must be plain, smooth and 

 clean that the contact may be the more immediate "; (3) the " two 

 bodies must both be hard "; (4) "a great density without a great 

 degree of hardness will also have its effect "; (5) " one of the two 

 bodies must be as thin as possible that it may be the easier heated 

 by friction "; (6) " Gold rubbed upon glass, appeared the fittest of 

 all metals to afford light; but no body yields so exquisite a light as a 

 diamond, which comes nothing behind a live coal, briskly blown by 

 the bellows, nor is it any matter how thick the diamond is." 



The luminous qualities of amber, diamonds, and gum lac were 

 also studied by a Dr. Wm. Wall (1708) , with a somewhat different 

 approach. He explained in a letter to Dr. Hans Sloane, Secretary of 

 the Royal Society, that he came to the idea that amber might be a 

 " Natural Phosphorus or Noctiluca " by reflecting that the " Artifical 

 Phosphorus," the element, is made from dung or urine which 

 contains 



an Oleosum and Common Salt, so I take it the Artificial Phosphorus to 

 be nothing else but that Animal Oleosum, coagulated with Mineral Acid 

 of Spirit of Salt, which Coagulum is preserv'd and not dissolv'd in Water, 

 but accended by Air. . . . These considerations made me conjecture that 

 Atnber, which I take to be a Mineral Oleosum coagulated with a Mineral 

 Volatile Acid, might be a Natural Phosphorus. 



Although the reasoning may sound faulty today, nevertheless Dr. 

 Wall did find that a piece of amber rubbed with his hand or a 

 woolen cloth became luminous, as did gum lac and particularly a 

 yellow diamond, which luminesced for some little time. He realized 

 that the light was connected with electricity, as a crackling sound 

 was often heard, but noted that the yellow diamond would become 

 luminous when exposed to the sky without rubbing. Wall wrote 

 " I'm well assur'd that all or most of the Bodies which have an Elec- 

 tricity yield Light; for in my Opinion, 'tis the Light that is in em 

 which is the cause of their being Electral, yet this Electricity never 

 shows itself without Friction." He also noted that " This light and 

 crackling seems in some degiee to represent thunder and lightning," 

 a similarity made strikingly certain by the kite experiment, sug- 

 gested by Franklin in 1749, and carried out so successfully in 1752 



