292 History of Luminescence 



Beccaria also studied the excitation of phosphors by electric sparks 

 and pointed out that those bodies which retained the solar light to 

 the greatest degree also retained the electric light best, for example 

 phosphors given him by Beccari, sugar and paper. He endeavored 

 to find out if sugar when pounded in a mortar became electrified 

 but could observe no attraction of fine threads, despite the fact that 

 the triboluminescent light of sugar looked exactly like electric sparks. 



Another worker of this period was Johann Karl Wilcke (1732- 

 1796) of Stockholm, Sweden, who studied the production of " spon- 

 taneous electricity " by melting electrics, a discovery of Stephen 

 Gray. He made a few observations on luminescence, described by 

 Priestley (1769: 290) as follows: 



Some curious observations relating to electric light were made by Mr. 

 Wilcke. Rubbing two pieces of glass together in the dark, he observed 

 a vivid phosphoreal light: which, however, threw out no rays, but 

 adhered to the place where it was excited. It was attended with a strong 

 phosphoreal smell, but with no attraction, or repulsion. From this ex- 

 periment he inferred, that friction alone would not excite electricity, so 

 as to be accumulated upon any body; and that to produce this effect, 

 the bodies rubbed together must be of different natures, with respect 

 to their attracting the electric fluid. He, moreover, imagined, that all 

 examples of phosphoreal light, without attraction, were owing to the 

 same excitation of electricity, without the accumulation of it. Such he 

 imagined to be the case of light emitted by the Bolognian stone, cadmea 

 fornacum, rotten wood, pounded sugar, and glass of all kinds. 



Both Beccaria and Wilcke thus endeavored to explain the tribo- 

 luminescence of sugar by the electric light, a reflection of the logical 

 desire to unify the causes of phenomena and bring all luminescences 

 into a common category. 



It is recognized today that many luminescence phenomena, which 

 are ordinarily spoken of as triboluminescences, actually result from 

 electric discharges. T. Wedgwood (1792) described the light from 



also mentioned by Hooke in Micrographia (1665) . Contrary to Beccaria's experience, 

 Sir David Brewster (1781-1868) observed light when the unannealed glass drops were 

 broken, either in air or under water. Helvig (1815) also reported light from the 

 breaking of " lacrymae Batavae " and that " Knallbomben " (gas filled explosion 

 bombs) , dropped on the floor, gave a pale white light. De Parcieux (1797) observed 

 the same, " a brisk flame like an electric spark," with air-filled glass spheres which 

 burst in a vacuum, and a note in the Philosophical Magazine (14: 363, 1803) states 

 that Professor Pictet of Paris wrote to Mr. Tillock (editor of the Magazine) that M. 

 Mollet of Lyons saw light when an air gun was discharged into the air. Dessaignes 

 (1814) also discussed the matter, and Heinrich's monograph is full of references to 

 this type of luminescence. In moie recent times, H. F. Newell (1896) reported on 

 similar phenomena during compression of certain gas mixtures which would exhibit 

 phosphorescence after electrical excitation. 



