THE THEORY OF WIEDEMANN AND SCHMIDT. 1 99 



It is to be remembered that the rays that produce photo-activity in such 

 cases are the same that cause fluorescence. Goldman has further shown 

 that the behavior of photo-active cells with fluorescent liquids is such as to 

 indicate the liberation of negative electricity at the illuminated electrode 

 at a constant rate depending upon the intensity of illumination. Goldman's 

 results are thus in complete accord with the view that fluorescence is 

 accompanied by ionization. 



It appears, therefore, that while there is no direct evidence of electrolytic 

 dissociation in fluorescent solids and liquids, many phenomena have been 

 observed which make it seem probable that such dissociation exists. A 

 detailed study of this very interesting class of phenomena is much to be 

 desired, and will doubtless suggest some means by which a very fundamental 

 question in the theory of luminescence may be definitely settled. 



In the case of gases the experimental results are less favorable for the 

 dissociation theory of luminescence, and in the case of fluorescence seem to 

 directly contradict it. Before accepting these results, however, as con- 

 clusive evidence of the falsity of the theory, it is desirable to consider the 

 nature of the differences which the theory would lead us to expect between 

 the behavior of gases and liquids. 



It can scarcely be doubted that in the case of liquids or solid solutions 

 the presence of the solvent is favorable to the ionization of the solute. The 

 solvent acts as a catalytic agent, both for ordinary dissociation and for that 

 which we assume to be produced by light. To form a picture of the process 

 by which this catalytic action is effected, let us consider a molecule of the 

 solute (active substance in the case of luminescent substances) which is 

 subjected to some influence tending to separate it into positive and negative 

 parts. Such a tendency might result from a violent collision or from strong 

 resonant vibrations set up by light. As soon as the separation begins it 

 will be aided by the attraction of the neighboring neutral molecules of the 

 solvent, and when dissociation has actually occurred the ions are rendered 

 more sluggish in their movement, and are to some extent prevented from 

 recombining, by becoming attached to the solvent molecules. If we 

 imagine the solvent removed, so that we have to deal with a gas instead of 

 a solution, it is readily seen that the conditions are less favorable to ioniza- 

 tion for two reasons: 



(1) The forces to be overcome in effecting dissociation are far greater, 

 since there are no solvent molecules to partially counteract the mutual 

 attraction of the ions ; and for the same reason the ions must be driven much 

 further apart in order that the separation may be complete; 



(2) The tendency toward recombination is greatly increased, both 

 because of the greater effective attractive forces between the ions and 

 because of their more rapid motion. 



It is not surprising, therefore, that spontaneous electrolytic dissociation 

 is very rarely observed in gases. HC1 in dilute solution is a good con- 

 ductor. But the same amount of HC1 in the form of a gas, occupying the 

 same volume as before, shows no conducting power except such as requires 

 the most refined apparatus for its detection. Does it not seem reasonable 

 therefore that the ionization produced in a gas by light should also be very 

 minute, so that we may hope to detect it only under especially favorable 



