L «7 ] 



LIU. The Seat of Light Action in Certain Crystals of Metallic 

 Selenium, and some New Properties in Matter. By F. C. 

 Brown, Ph.D., and L. P. Sieg, Ph.D., Assistant Pro- 

 fessors of Physics, State University of Iowa, U.S.A.* 



FROM time to time new experiments are performed which 

 leave doubt as to the seat of light-action in selenium. 

 The most recent experiments were performed with selenium 

 blocks by White "f. The selenium blocks showed a greater 

 change in conductance when illuminated at the electrode 

 contacts than when illuminated on other parts of the surface. 

 We have recently succeeded in producing several forms of 

 large crystals of metallic selenium J which have enabled us 

 to determine several interesting facts concerning the seat of 

 light-action in selenium. The advantages of a single crystal 

 for studies of this kind are perhaps obvious. The contact 

 resistances are limited to the electrodes, and knowing this 

 fact, the magnitude of such resistances can be determined. 

 Also it makes it possible to locate the seat of light-action 

 as well as something concerning the nature of light- 

 action. 



The Conductivity is Distributed throughout the Crystal. — 

 In order to locate the seat of change of conductivity in a 

 crystal it was first necessary to determine where the con- 

 ductivity existed. For this an acicular crystal of about 

 8 mm. length was used. A cluster of such crystals is shown 

 in fig. 1. The width and thickness of the crystals were each 

 about 0*2 mm. These needles, when fully developed, had 

 six surfaces, with angles of about 60° between adjacent 

 surfaces. The leaf-like forms (c) and (d) typify interesting 

 formations of acicular crystals. First the central spine of a 

 hexagonal crystal like (a) was formed. Then by certain 

 changes in the oven conditions the side branches grew out 

 from the central spine as shown. Those in (c) were regu- 

 larly developed hexagonal needles, which transmitted little 

 or no light. But the branches in (d) w r ere thin plates, with 

 acute angles of about 65°. As described in a previous paper 

 (loc. cit.) these crystals transmitted a deep red light when 

 placed between crossed nicols. Likewise they showed 

 extinction parallel to the long axis of the crystal. In some 



* Communicated bv the Authors. 



t G. W. White, Phil. Mag. [6] xxvii. p. 370 (1914). 



X F. C. Brown, Phys. Rev. n. s. 1914. 



Phil. Mag. S. 6. Vol. 28. No. 166. Oct. 1914. 2 K 



