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PHYSICS: LOEB AND SCHMIEDESKAMP Proc. N. A. S. 
There is then little doubt in concluding from these experiments that 
the action of intense ultra violet light causes a destruction of the phos- 
phorescent power of the zinc sulfides. This destruction is accompanied 
by chemical changes in the sulfides, resulting in a change of color, (possibly 
in a reduction or partial reduction of the polysulfides of the impurity 
(here probably Cu) required to produce the phosphorescent centers). It 
is further obvious that the exposure of the crystals to a powerful oxidiz- 
ing gas such as chlorine at least in part restores both the original color and 
the ability to phosphoresce. It may accordingly be quite possible that 
the action of ultra violet light in this case is similar to that of ordinary 
light in the photographic processes, only much slower. 
The results so far obtained hardly permit one to say much about the 
form of the curves of decay of phosphorescent intensity with the time of 
ultra violet illumination. They indicate, however, that the curves may 
either be the sum of a group of curves having the form I = ^ (* ~~ e~ At ) 
given by Rutherford 6 for destruction of phosphorescence in the case of 
alpha particles where the intensity of the alpha particle radiation falls 
off exponentially with the distance below the surface; or they may be 
the sum of a group of exponential curves having different constants. 
Such curves would be expected in the case of measurements made on the 
decay of total phosphorescence with ultra violet illumination, for it is 
known that the phosphorescent light is composed of groups of different 
phosphorescent wave-lengths, each group probably having its own sus- 
ceptibility to destruction, and hence each having its own time rate of 
destruction by ultra violet light. 
The destruction of the phosphorescent power of these sulfides by ultra 
violet light noted above might point to a close analogy between the phe- 
nomena of phosphorescence and fluorescence in the light of the recent 
work of Perrin. Such an analogy would be in accord with the conclusion 
arrived at by Rutherford 6 in the case of the destruction of the phos- 
phorescence by alpha particles, viz., that the light emitted under alpha 
particles bombardment accompanies the dissociation and destruction of 
the active centers. If this interpretation is correct, and if phosphorescence 
and fluorescence are closely related, the theory of Schmidt and Wiedemann, 7 
Lenard, 7 and Merritt, 7 explaining the mechanism of phosphorescence will 
have to be modified. This theory ascribes phosphorescence to the return 
of ionized portions of the active centers (presumably electrons), to the 
centers from which they were removed by the exciting agent. Such a 
change does not involve a permanent destruction of the active center. 
According to Perrin the active center is permanently chemically changed 
in the process by which the substance emits its fluorescent radiation. It 
therefore follows that unless the production of phosphorescent light and 
