100.").] the Beta and Gamma Hay* of Radium. :, 1 :> 



of the crystalline condition does not necessarily interfere with the 

 generation and storage of energy which can afterwards be set free by 

 heat and give rise to revived phosphorescence. 



37. In the foregoing observations the coloration effects have, with 

 one exception, been directly associated with the luminescent 

 appearances (paragraph 30). The one exception is valuable, however, 

 because it draws attention to the fact that the intensity of the 

 affinities of the dissociated ions, as well as their suitable insulation, 

 is a necessary factor in the production of phosphorescence. The 

 lead silicate is easily electrolysed, and its ions are duly insulated 

 as shown by the coloration; but in their recombination the energy 

 developed in this particular case was insufficient to develop visible 

 light waves. 



38. In the haloid salts of potassium the conditions are entirely 

 different, for the affinity of the haloid and the metallic ions is so 

 intense that the insulating power of the molecules is only sufficient 

 at ordinary temperatures to keep them apart for a very short time. 

 It appears probable that in these salts so much energy might be stored 

 by the action of the rays at a low temperature as would result in a 

 most brilliant luminescence on a return to the ordinary temperature. 



39. Another feature brought out by the foregoing observations is 

 the remarkable persistence of latent phosphorescence as shown by the 

 indefinitely long time for which this storage of energy can take place 

 under favourable conditions (see paragraphs 26 and 29). 



40. In tracing the various steps in the action of the rays as disclosed 

 by the intensity and colour of the phosphorescence and by the 

 appearance and disappearance of coloration, the chemical or electro- 

 lytic hypothesis seems to explain each step in a satisfactory way and 

 without any straining of the facts. When the rays are first applied 

 there may be a preliminary stage during which the luminescence 

 is mainly of physical origin. This is borne out by the undoubted 

 influence of crystalline structure on primary phosphorescence and its 

 relatively small influence on the later stages of luminescence. But as 

 electrolytic dissociation proceeds, a point is reached when the neutral 

 molecules are no longer able to keep the increasing number of ions 

 apart. Eecombination then sets in, and with it the production of a 

 new kind of luminescence. When once the insulating power of any 

 given layer of the substance is saturated, the number of ions 

 dissociated at each moment will be balanced by the recombination of 

 a like number. This is the probable explanation of the fact that the 

 phosphorescence reaches a steady value after a certain time. When this 

 steady state has been reached for the whole depth to which the 

 electrons can penetrate, the limit of storage of energy for the 

 production of secondary and revived phosphorescence has 

 reached. 



