CHAPTER XIV. 



THE THEORY OF WIEDEMANN AND SCHMIDT. 



The most comprehensive theory of luminescence thus far proposed is 

 undoubtedly that first suggested by E. Wiedemann 1 in 1889, and later 

 modified and extended by Wiedemann and Schmidt. 2 According to this 

 theory some chemical or physical change is produced in a luminescent 

 substance during excitation, and the emission of light is an accompaniment 

 of the more or less gradual restoration of the modified substance to its 

 original condition. Thus, in the case of phosphorescence, it was suggested 

 by Wiedemann that a portion of the active material was changed by the 

 exciting light from the stable condition A into the unstable state B. Phos- 

 phorescence would then result from a gradual breaking down of the unstable 

 compound. 3 Fluorescence maybe due either to the vibrations setup during 

 the change from A to 5, or to the fact that the reaction B ~~A proceeds, with 

 emission of light, during excitation as well as during decay. Thermo- 

 luminescence is to be explained as the result of some chemical change pro- 

 duced by heat, during the progress of which the molecules are thrown into 

 such violent vibrations as to bring about the emission of light. Lumines- 

 cence ceases in such cases when the change has been completed ; and some 

 outside stimulus is required, such as that furnished by kathode rays, to 

 restore the substance to the sensitive state. Not only is this explanation 

 of thermo-luminescence a plausible one, but in several instances evidence 

 has been found that the assumed change really occurs. In the paper 

 already referred to, Wiedemann and Schmidt have discussed all of the 

 various types of luminescence in considerable detail and have shown that 

 the theory proposed by them will account for the phenomena, at least 

 qualitatively, in a very satisfactory way. 



It has generally been assumed that the change accompanying lumines- 

 cence is of a chemical nature. Various suggestions have been made in 

 regard to the character of the reactions produced by the exciting light (or 

 other cause), and attempts are not lacking to trace a connection between 

 luminescence and chemical constitution. It was pointed out, however, 

 by Wiedemann and Schmidt that in many cases there were reasons for 

 looking upon electrolytic dissociation as the more probable cause of lumines- 

 cence. In fact, since the electromagnetic disturbance that constitutes 

 light can get a hold on the molecules of the active material only by exerting 

 forces upon the electrical charges in the molecule, and will always tend to 

 separate the positive and negative parts, it appears probable that the first 

 effect of the exciting light is always to produce some type of electrolytic 

 dissociation, and that any chemical changes which may be exhibited are 



IE. Wiedemann, " Zur Mechanik des Leuchtens," Wied. Ann., 37, p. 177, 1889. 

 =E. Wiedemann, and C. C. Schmidt, Wied. Ann., 56, p. 177. 1895. 

 3 E. Wiedemann, /. c, pp. 224-225. 



195 



