CHAPTER VI. 



VARIATIONS IN THE DECAY OF PHOSPHORESCENCE PRODUCED 



BY HEATING. 



In connection with the investigations described in Chapters IV and V 

 Dr. C. A. Pierce, during the years 1906-1908, made an extended study of the 

 phosphorescence of Sidot blende and of Balmain's paint at temperatures 

 ranging from room temperature to 3ooC. The present chapter contains a 

 summary of his results. 1 



EXPERIMENTS WITH SIDOT BLENDE. 



The apparatus used was similar in principle to that described in Chap- 

 ter IV, p. 58, but was modified to adapt it to the conditions of the experi- 

 ment. The substance studied was the phosphorescent zinc sulphide already 

 referred to as "Emanations-pulver." It was placed in a shallow dish and 

 heated by means of an electric furnace capable of being readily removed and 

 replaced in its position relative to the powder without disturbing the latter. 



The temperature of the powder was measured by means of a thermo- 

 junction embedded in the mass. Phosphorescence was excited by a mer- 

 cury-arc lamp of the L,ummer type. 



By means of properly placed mirrors the exciting light was reflected upon 

 the surface of the powder and the phosphorescent light was reflected 

 through a double window of mica and plate glass into the field of a Lummer- 

 Brodhun photometer. The adjustments were such that the conditions of 

 illumination and observation were the. same whether the powder was within 

 the furnace or outside. The comparison light was an acetylene flame with 

 diaphragm. A color match was obtained by the interposition of colored 

 glasses and the balance of illumination in the field of the photometer by 

 moving the comparison light. 



The apparatus was arranged so that a single observer, working in a dark 

 room, could automatically record all observations such as the times of open- 

 ing and closing the shutter of the exciting lamp, the beginning and end of 

 heating, the times at which photometric observations were made, and the 

 position of the comparison light on the photometer bar. The observer 

 could likewise make all manipulations during a complete run without taking 

 his eye from the observing telescope. Before each set of observations 

 the powder was exposed to infra-red rays for 1 minute and allowed to re- 

 main in darkness for 5 minutes before excitation in order, as already de- 

 scribed in Chapter IV, to secure a standard condition. In the present case 

 the action of the infra-red was not sufficient to destroy all thermo-lumines- 

 cence when the powder was heated to 350 C, but the effect was so reduced 

 that even after the strongest previous excitation the thermo-luminescence 

 could not be measured with the photometer. The infra-red rays, which 



>C. A. Pierce. Physical Review, xxvi, pp. 312 and 454. 



