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PHYSICS: LOEB AND SCHMIEDESKAMP Proc. N. A. 
THE DESTRUCTION OF PHOSPHORESCENT ZINC SULFIDES 
BY ULTRA VIOLET LIGHT 
By Leonard B. Loeb and Lloyd Schmi^deskamp 
Ryerson Physical Laboratory, University of Chicago 
Communicated by R. A. Millikan, June 12, 1921 
It has long been known that the phosphorescence exhibited by certain 
alkaline earth sulfides is gradually destroyed when they are subjected 
to the action of alpha particles from radio-active matter. 1 It has also- 
been found that a permanent destruction of the phosphorescent properties 
of such sulfides is caused by the bombardment of the sulfides with canal 3 
rays or with cathode 2 and beta 1 rays. In this process of destruction, the 
sulfides are caused to phosphoresce brilliantly by the destroying agents, 
and the destruction is accompanied by a change in color of the sulfide, 
generally a darkening. 
In 1919 J. Perrin 4 and more recently R. W. Wood 5 have shown that in 
the process of fluorescence, a process assumed to be somewhat similar 
to phosphorescence, the fluorescing molecules undergo chemical changes 
which result in the loss of the fluorescent power. In other words light 
of a given wave-length falling upon a molecule of fluorescent substance 
causes it to emit light of a different wave-length, and in this process the 
molecule is chemically changed so that it can no longer fluoresce. 
The detection of the destruction of the fluorescent properties of certain 
substances is complicated by the fact that the fraction of molecules that 
are destroyed per second is relatively so minute that the time of exposure 
required to produce a measurable effect must be very long. As Perrin 
showed, this time may be much reduced by using very thin films of the 
solutions of fluorescing molecules. Wood has shown that it may also 
be much reduced by using intense sources of light. In view of the de- 
struction of the phosphorescence of zinc sulfides by other agents which 
cause them to phosphoresce, and in view of the powerful technique de- 
veloped by Perrin and Wood for the destruction of the fluorescent property 
by light, it occurred to one of the writers to attempt to detect a destruc- 
tion of the phosphorescent property of the zinc sulfides through the action, 
of light. The only reference to observations on such a destruction yielded 
by a survey of the literature consisted of the following quotation taken 
from a long paper by Baerwald 3 on the effect of canal rays on phosphor- 
escence. Baerwald states "It is remarkable that intense ultra violet light 
from a mercury arc also destroys the phosphorescence of zinc sulfide. As 
it becomes dark in the process we may suspect a chemical change of the 
sulfide into an allotropic form, " 
For the preliminary experiments which are being reported here, three 
