380 Prof. E. Wiedemann on the 



do not correspond to a simple emission of the luminous mo- 

 tions previously present in the body. We cannot therefore 

 employ experiments with Balmain's paint or similar sub- 

 stances for the determination of our quantity b. 



If we endeavour to explain these phenomena we may make 

 the following assumptions. 



The incident light brings about chemical changes in the 

 phosphorescent body, which afterwards undergoes the same 

 changes in the reverse order, and thus gives rise to an evolu- 

 tion of light. The chemically altered substance immediately 

 on the surface would transmit those rays which were absorbed 

 by the original modification, and thus deeper layers of the 

 substance would become accessible to the action ; exactly as, 

 for example, in mercuric iodide the yellow modification is 

 transparent to other rays than the red modification, and vice 

 versa, Analogous cases occur with the different modifications 

 of phosphorus and selenium. 



In reference to chemical changes, it seems probable that 

 sulphide of calcium &c. may exist in two modifications A and 

 B, one stable, the other unstable*. The modification A is 

 transformed into the modification B by the absorption of certain 

 rays, which are gradually given out again as light whilst the 

 substance changes back again into the modification A ; the 

 greater the number of molecules of B in the unit volume, the 

 greater the number which pass back again into the modifica- 

 tion A in the unit time. 



Since the brightness of the phosphorescence increases with 

 the intensity or duration of illumination, the number of mole- 

 cules of A which have been transformed into B must have 

 been greater, and consequently more molecules of B afterwards 

 are retransformed into A with evolution of light. The de- 

 crease in brightness in the same series of experiments must 

 therefore take place at first rapidly, and afterwards more and 

 more slowly, since the number of molecules of B present 

 becomes continually less. Since the substances phosphoresce 

 already during illumination, the retransformation must take 

 place even during the illumination. 



With a steady constant illumination, a limiting condition 

 must be reached in which the number of molecules of the 

 condition B, which change back into the condition A, is equal 

 to the number of molecules of B freshly formed. As long as 

 the duration of illumination is less than that required to bring 



* It is not impossible, but probable, that Balmain's luminous paint 

 consists of a mixture of substances of which each exists in two modifica- 

 tions. Our theories remain then the same. 



