PHENOMENA OF PHOSPHORESCENCE. 213 



groups with the neutral molecules of the solvent, and that these groups may 

 afterwards be broken down, and the ions liberated, by rise in temperature 

 or by the absorption of infra-red rays. The formation of such groups as 

 the result of ionization seems extremely probable, especially in the case of 

 solids, and can scarcely fail to be of importance in any satisfactory theory 

 of phosphorescence. Introducing such additional hypotheses as are neces- 

 sary to give definiteness to the suggestion of Wiedemann and Schmidt, let 

 us consider what the influence of such ionic groups will probably be. 



We shall assume that the first effect of the exciting light is to produce 

 dissociation in a part of the active material. The dissociation assumed 

 may be either chemical or electrolytic 1 and if of the latter type it may either 

 be similar to the dissociation of ordinary electrolysis, or may consist of the 

 expulsion from the molecule of one or more electrons, and thus resemble 

 more closely the ionization of a gas by X-rays. For the sake of definiteness 

 we shall assume that the effect of the exciting light is to produce such violent 

 vibrations as to liberate a single electron from the molecule. 



The two ions produced in this type of dissociation will differ greatly in 

 mobility. The negative ion, owing to its small mass, will possess a velocity 

 hundreds of times greater than that of the heavy positive ion, and in con- 

 sequence will move about in the substance with considerable freedom. 

 While the electrons will at times attach themselves to the molecules of the 

 solid solvent this condition will usually be only temporary. We may 

 assume in general that a constant fraction of the whole number of negative 

 ions consists of electrons that are moving freely. The positive ions on the 

 other hand will possess only a small mobility. While some of these ions 

 will remain free, it is to be expected that many will attach themselves to 

 molecules of the solvent or to undissociated molecules of the active sub- 

 stance. It is to be noted that the small velocity of the positive ions makes 

 it probable that the groups formed by the union of a positive ion with a 

 neutral molecule will be more permanent than similar groups formed by the 

 negative ion' 



The collisions between positive and negative ions, which lead to the more 

 or less gradual decay of the ionized condition after the exciting light has 

 ceased to act, will be of three different kinds: (1) collisions between a nega- 

 tive ion and a free positive ion; (2) collisions between a negative ion and a 

 positive ion that has attached itself to a neutral molecule of the solvent, and 

 (3) collisions between a negative ion and a positive ion that is attached to a 

 neutral molecule of the active substance. The number of modes of recom- 

 bination may in fact be greater than three, since the positive ion may become 

 the nucleus of more complicated molecular groups. But we shall restrict 

 the discussion to cases in which there are three modes of recombination. 



When recombination occurs it is to be expected that vibrations will be 

 set up in the resulting neutral molecule, and these vibrations, in the theory 

 here considered, are assumed to be the source of the light emitted during 

 the phosphorescence. But the vibrations corresponding to the different 



'Since the electro-magnetic 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 

 secondary effects. 



