Voi,. 7, 1921 PHYSICS: LOEB AND SCHMIEDESKAMP 
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the increase in time of ultra violet illumination. In fact the French 
specimen No. 3 is seen to have dropped to a fourth of its initial intensity 
after 300 hours in the image of the arc. Though the changes are not very 
regular, the reduction in most cases is continuous with time, until the 
measurements made on the 14th of April are reached. The irregularities 
up to that date were doubtless due to the difficulties involved in the pho- 
tometry of weak sources. On the 13th and 14th of April the air in the 
room in which the specimens were being exposed was mildly contaminated 
with chlorine gas used in the course of other experiments which were be- 
ing performed in the same room. Again, from the 18th to the 22nd of 
April the air was contaminated by this gas, the contamination being 
specially strong on the 21st and 22nd. It is seen that the presence 
of the chlorine gas apparently arrested further decrease in intensity of 
two of the samples after April 14th; while it actually increased the in- 
tensity of the French Sample No. 2 which was subjected to a particulary 
heavy dose of the gas. On the 22nd, after the exposure to the strong 
chlorine, it is seen that all the samples showed marked increases in phos- 
phorescent intensity. This was followed by a decrease in intensity on 
further illumination in the absence of chlorine after that date. 
The reduction in intensity of the phosphorescence was in all cases ac- 
companied by a darkening of the sulfide. Microscopic examination 
showed that the characteristic yellow color of the sulfides had disappeared 
and was replaced by a slight but uniform blackening throughout the 
crystal mass. This blackening was much like that of some glasses that 
have been subjected to cathode ray bombardment. To eliminate the 
possibility of this blackening having been caused by the collodion, a 
sample of the collodion film was exposed to the light for many hours. 
It showed no signs of blackening. A specimen of sulfide made up without 
the use of the collodion furthermore showed the same decrease of intensity 
and the same blackening on exposure to the ultra violet light as was shown 
by the samples using collodion. Following the accidental exposure to 
the chlorine gas it was observed that the specimens which had been black- 
ened by exposure had regained their original color in a ring about 2 mm. 
wide at the edges of the specimens, i.e., where the chlorine had diffused 
inwards. It was these portions with restored color which showed the 
marked increase in intensity observed. The blackened centers of the 
specimens remained dull. Finally, to ascertain that it was the chlorine 
that caused the restoration of the color the French Sample No. 2 was 
placed in a flask with chlorine gas over night. On examination it was 
found to have regained its yellow color throughout its mass. On measure- 
ment, even though its edges were moist, due to the deliquescent nature of 
some of the reaction products, the intensity of the specimen was found to 
have increased markedly, as the table shows. 
