364 History of Luminescence 



as a result of friction, and were acted on by a restoring force pro- 

 portional to the square of their displacement. Stokes' law and the 

 broad fluorescence bands might be explained by these assumptions, 

 but the original theory was not satisfactory in detail and subject to 

 criticism by many workers, which led to changes over the years 

 (1882, 1885, 1895) .«^ 



It might be supposed that E. Becquerel, who recognized no dis- 

 tinction between phosphorescence and fluorescence, would develop 

 a theory, but he was satisfied to speak of changes in molecular equi- 

 librium, in orientation or constitution of molecules, giving rise to 

 electricity, which appeared as heat or light when the molecules 

 returned to their former state. 



The last statement begins to have a modern look. Toward the 

 end of the century, E. Wiedemann (1889) assumed that a phosphor 

 could exist in two modifications, A and B. When light struck the 

 surface the stable modification A absorbed certain wave-lengths and 

 was converted into the unstable modification B, which then returned 

 to A with emission of light. The logarithmic decay of phosphores- 

 cence, observed in some cases, gave direct support to the theory. 

 Infrared rays had a quenching action because they were absorbed 

 by B, converting B quickly to A. Phosphorescence was thus regarded 

 as a true chemical process, actually a chemiluminescence. Weide- 

 mann and Schmitt (1895a) cited in its favor the very long after- 

 glow of some phosphors and facts connected with color changes in 

 phosphors and with thermophosphorescence, tribophosphorescence, 

 and lyophosphorescence. 



In a second paper Wiedemann and Schmidt (18956) modified 

 and enlarged the hypothesis to include not only luminescence 

 emitted when A and B recombine but when B is converted into 

 C, and also luminescence from undecomposed molecules in which 

 atoms or valency charges are set to vibrating by the exciting light. 

 Ionization was regarded as one of the chemical changes which might 

 occur. 



Although E. Becquerel pioneered in discovering facts concerning 

 luminescence, Eilhardt Wiedemann (1852-1928) was his most dis- 

 tinguished contemporary and successor (see Chapter VI) . Wiede- 

 mann's research in the field of phosphorescence and fluorescence did 

 much to place the newer knowledge on a quantitative and theo- 

 retical basis. His papers on all kinds of luminescence and those 



^* Among those who discussed Lommel's theory or developed general opinions re- 

 garding a theory of fluorescence were J. J. Obermann (1871) , A. Wiillner (1878, 1883, 

 1899), E. Ketteler (1882), E. Linhardt (1882), K. Wesendonck (1884), F. Stenger 

 (1886) , O. Knoblauch (1895) . G. Jaumann (1894, 1895) , and B. Galitzin (1895) . 



