510 Mr. G. T. Beilby. Phosphorescence caused ly [Jan. 25 r 



encasing medium which protects the crystalline elements from further 

 disintegration. In the case of the platino-cyanide, the over-stimulation 

 to which the vibrating molecules are subjected by the /3 rays during 

 the preliminary stage of bright phosphorescence, results in a state 

 analogous to that of elastic fatigue in vibrating metal wires or glass 

 fibres. Up to a certain point this fatigue may be recovered from, that 

 is to say, if the relative displacements of the molecules from their 

 proper crystalline relations has not passed beyond a certain stage ; but 

 beyond this stage there is no power of self-recovery, and heat is 

 required to endow the molecules with freedom of movement sufficient 

 to enable them to return to their crystalline positions. This final 

 stage of permanent fatigue or overstrain in the salt corresponds with 

 the amorphous condition which results from mechanically produced flow. 



17. The foregoing observations show that physical structure and 

 molecular elasticity may play an important part in the initial stimula- 

 tion of phosphorescence, but the persistence of phosphorescence, even 

 in the purely amorphous form of the substance, seems to point to the 

 necessity for a more general explanation of the phenomena of phos- 

 phorescence. I propose to deal with the more general aspects of the 

 subject in a separate communication. 



18. In the appended table the comparative phosphorescence of the 

 various forms of platino-cyanide of barium is shown. The method 

 used was to measure the distance from the radium at which the salt 

 exposed on a tray of black paper became distinctly visible in the dark. 

 The salt was observed by means of a simple microscope fixed over the 

 tray. The intensity of the rays was taken to be inversely as the 

 square of the distance of the radium from the phosphorescing salt. 



Comparative Phosphorescence (that of the canary-yellow crystals 

 being taken as 100). 



