468 Mr. Snehamoy Datta on the Variation of 



was then opened, exposing the selenium to visible raj 7 s alone, 

 the infra-red and ultra-violet rays being cut off by a thick 

 glass plate. The thermo-couple indicated a slight increase 

 in temperature (about 1 degree). The results obtained are 

 given below : — 



Table II. 



Resistance, in dark 67'0xl0 5 ohms. 



Eesistance in light 48*0x105 ,, 



Change in resistance 19 - 0xl0 5 ,, 



Comparing the decrease in resistance ('75 x 10 5 ) due to a 

 rise in temperature of about one degree with the total 

 decrease in resistance (19 '0 x 10 5 ) observed on exposure to 

 light, it was found that not more than 1/25 of the total 

 decrease could be attributed to the heating due to light. 

 The light-effect is thus a genuine one independent of the 

 heat-effect. 



As has been pointed out before, time plays a great part 

 in moulding the future behaviour of the eel). And this is 

 not peculiar to the variation of resistance with temperature 

 alone, for it is well known that the light sensitivity of 

 selenium, particularly the recovery curve, largely depends 

 on the time of exposure. The voltage effect, too, depends to 

 a great extent on (he time of application of the voltage. It 

 seems difficult to realize fully the part played by time on 

 any theory — electronic in nature — which has been propounded 

 to account for the many peculiar properties of selenium. 

 It can, however, be best understood if the old theory *, 

 viz. transformation into some other allotropic forms, be a 

 little modified. Instead of assuming that the transforma- 

 tion from one modification to another takes place at one 

 definite temperature, it may be supposed that the transfor- 

 mation takes place at ail temperatures. The various 

 modifications may be regarded as maintaining a sort of 

 dynamic equilibrium amongst themselves, the quantity 

 of each variety depending on the particular temperature — 

 the lower the temperature the more the amount of the less 

 conducting variety— and the degree of stability of each 



* In this connexiou it is well to recall the observations of E. Marc 

 {Die pkysikali&che-chemischen Eigenschaften des metallischen Selens. 

 L. Voss. Leipzig, 1907), who, by the investigation of the heat changes, 

 the electrical conductivity, and the microscopic investigation of the 

 surface, was able to detect with certainty the existence of two poly- 

 morphic forms. The one form was crystallized by heating the selenium 

 at a temperature of about 100° C, and was essentially non-conducting. 

 At a temperature of about 200° C. this form passed to the conducting 

 variety. 



