528 COBLENTZ AND EMERSON: PHOTOELECTRIC SENSITIVITY 



nite may be considered as sensitive as boulangerite, to be men- 

 tioned presently, but the deflection drifted, due to the decrease 

 in resistance with time already noticed by other observers. 



Boulangerite. The specimen of boulangerite (3PbS.Sb 2 S 3 , 

 Irkutsk, Siberia) investigated was obtained from the Smithsonian 

 collection, No. 78395. Several samples were examined. In one 

 sample, 4 by 7 by 0.8 mm., the electrodes consisted of copper 

 wires melted into the material as just described. The radiation 

 from the standard lamp gave a deflection of 10 to 20 cm. 



Another sample, 1 by 1.2 by 2 mm., held by compression be- 

 tween two heavy electrodes of copper, when exposed to the 

 standard incandescent lamp produced a deflection of 2 to 3 cm., 

 which is comparable with the preceding when one considers the 

 size of the exposed surfaces. 



Although this substance seems fairly sensitive, it did not 

 appear to be sufficiently so to justify an investigation of its spec- 

 tral sensitivity with a view of using this mineral as a selective 

 radiometer. 



Jamesonite. (2PbS.Sb 2 S 3 ; Smithsonian collection No. 12,500; 

 from Cornwall, England.) The sample examined (size 2 by 7 by 

 1 mm.) had the copper wire terminals attached by fusing the 

 incandescent wire into the material. The standard lamp gave a 

 deflection of only 1 to 2 cm., which seems to indicate that this 

 material is not so light-sensitive as is boulangerite. 



Mixtures of galena, PbS, and stibnite, Sb 2 S 3 , in various pro- 

 portions were melted in a crucible and poured upon a plate of 

 metal. Several samples, 5 by 10 by 0.5 mm., were examined, 

 but none of them gave any indication of light-sensitiveness 

 (change in resistance) when exposed to daylight or to the 

 standard incandescent lamp. 



Bismuihinite. Bismuthinite, Bi 2 S 3 , was obtained from the 

 Smithsonian collection, No. 85071, from Jefferson 'County, 

 Montana. This is the most interesting substance examined, in 

 view of the diverse results obtained and the explanation offered 

 therefor. 



The sample of bismuthinite examined consisted of a non- 

 homogeneous mass of acicular crystals, which was easily crushed 



