PHENOMENA OF PHOSPHORESCENCE. 2IQ. 



persist even after recombination has occurred. In the case of the unfavor- 

 able groups this tendency to persist can scarcely be present in the same 

 degree if at all. 



The condition of the phosphorescent substance is thus different after 

 phosphorescence has ceased from what it was before excitation. The 

 difference consists in the presence in the mass of a larger number of grouped 

 molecules of the active substance, which are so intimately connected that 

 when one member of the group is dissociated during subsequent excitation 

 its positive ion is in a position to form immediately one of the groups 

 favorable to phosphorescence. After the substance has been excited it is 

 therefore in a condition which enables a subsequent excitation to produce 

 a larger proportion of favorable groups than would be produced by a simi- 

 lar excitation of the fresh substance. In other words, the \J/ of equation 

 (7) is increased. 



During excitation we have dissociation and recombination taking place 

 at the same time; and the recombinations that occur during excitation 

 will bring about the same change in the condition of the substance that we 

 have assumed during decay. The value of \pn will therefore increase with 

 the duration of exposure to the exciting rays. Prolonged excitation, up 

 to the point where saturation is reached, also increases the number of ions, 

 i. c, the value of - A series of decay curves for different times of exposure 

 should therefore resemble the curves of Fig. 189, which are computed from 

 equation (7) by giving progressively increasing values to the constants 

 \po and n . 



The new condition in which a substance is left after phosphorescence 

 can scarcely be one of complete stability. The natural and stable arrange- 

 ment of the molecules is that of the substance before it has been disturbed 

 by the action of light. It is to be expected, therefore, that there will be a 

 more or less gradual return to the normal state after the light has ceased 

 to act. The recombination of the ions produced by excitation, with the 

 accompanying phosphorescence, forms only one stage in the complete 

 return to the normal state, and is followed by a more gradual breaking 

 down of the molecular groups resulting from recombination. We thus 

 have an explanation of the effect of rest. The effect of exposure to infra- 

 red rays and of elevation of temperature is to hasten the return to the 

 normal state by increasing the rate at which these groups disintegrate. 



To account for the action of the infra-red rays during excitation and 

 decay it is only necessary to assume that these rays also have the power 

 of breaking down the "favorable groups." In the case of Sidot blende 

 the effect on the unfavorable groups appears to be inappreciable. Upon 

 exposure to infra-red during decay the first result is to diminish the number 

 of favorable groups, and to correspondingly increase the number of free 

 positive ions. This, by itself, will not greatly alter the intensity of phos- 

 phorescence; for in the phosphorescence of Sidot blende the recombina- 

 tion of a free positive ion appears to be nearly or quite as effective as the 

 recombination of a favorable group. But an increase in the number of 

 free ions causes an increase in the rate at which unfavorable groups are 

 formed. The positive ions that are shaken loose from the favorable groups 

 therefore pass quickly into the inactive condition, and a rapid diminution 



