200 STUDIES IN LUMINESCENCE. 



conditions, and by means of apparatus even more sensitive than that thus 

 far used? 



It may be also that some type of dissociation occurs in gases that is inca- 

 pable of producing a change in electrical conductivity, but nevertheless as 

 effective as complete dissociation in producing fluorescence. For the sake 

 of definiteness let is assume that fluorescence is due to the vibrations of one 

 ring of the Saturnian atom proposed by Sir J. J. Thomson. We can imagine 

 this ring set into such violent vibration by light of suitable period that an 

 electron is broken loose. This electron will later return and, by setting 

 up vibrations when it reenters the ring formation, will cause fluorescence. 

 But the separation of an electron from such a ring does not necessarily 

 mean dissociation in the electrolytic sense. In order that complete dis- 

 sociation should occur the electron must be driven away to such a dis- 

 tance as to be practically beyond the influence of the remainder of the 

 atom. The energy required to accomplish this is probably far greater 

 than that required to break down a ring; and complete dissociation, which 

 alone can manifest itself by an increase in electrical conductivity, may be 

 of relatively rare occurrence, even in cases of brilliant fluorescence. It 

 may be pointed out in passing that the partial polarization observed by 

 Wood in the light emitted by fluorescent sodium vapor is to be expected 

 if the view outlined above regarding the nature of fluorescence in gases is 

 accepted. 



In the case of phosphorescent gases electrolytic dissociation is readily 

 detected so long as phosphorescence lasts. In such cases the dissociation 

 appears to be complete. The long duration of the after-glow in gases sug- 

 gests that the ions attach themselves to neutral molecules and in conse- 

 quence move slowly ; and the fact that the spectrum often consists of bands 

 rather than lines is in accord with this view. The decay of phosphores- 

 cence in gases follows the law that is predicted by the dissociation theory 

 in its simplest form, 1 and seems to be almost entirely free from the dis- 

 turbing influences present in the case of solids. 



While we are by no means justified in looking upon the theory of 

 Wiedemann and Schmidt as finally established, it appears to us, in the light 

 of the preceding discussion, that it is by far the most promising of the 

 theories thus far suggested, and that it affords a most useful guide in the 

 experimental study of luminescence. 



C. C. Trowbridge, Physical Review, vol. 26. p. 515, 1908; vol. 32. p. 129. 191 1. 



