252 THE ROYAL SOCIETY OF CANADA 



obtain no response to light with that mineral. Hence, it is clear 

 that the cause of photo-sensitivity is not to be sought for in the 

 chemical composition of a substance. 



(b) Spectral Absorption. 



An explanation of photo-electric conductivity is suggested by 

 absorption, as it seems natural that the light absorbed would excite 

 the carriers of electricity either by heating or resonance. For the 

 validity of this explanation, it is necessary to have the maximum of 

 photo-electric sensitivity coincide with the maximum of absorption, 

 and also to have substances which are most absorbing, most photo- 

 sensitive. Data obtained with the diamonds do not support this 

 hypothesis, as it is seen by comparing Figs. 1 and 3 that the region 

 of rising sensitivity in diamond No. 1 is one of falling absorption. 

 Again, the diamonds that are opaque in the ultra-violet are not 

 photo-sensitive. 



On the other hand, the work of other observers indicates some 

 relation between photo-electric conductivity and absorption. For 

 example, it is known that the transparency of molybdenite increases 

 considerably in the visible when it is cooled to liquid air temperature^, 

 and it has been shown that the maximum of its photo-sensitivity 

 shifts from the infra red towards the visible when it is so cooled.**. 

 Thus we have a corresponding shift of the maximum of transparency 

 and of photo-sensitivity. Again, Coblentz^ points out that, in con- 

 sidering the data on hand for a number of photo-sensitive substances, 

 it is found that a gradual shifting of the absorption band from the 

 ultra-violet into the infra-red is accompanied by a decrease in photo- 

 sensitivity in the ultra-violet and an increase in the infra-red. That 

 is, there seems to be a following up of the absorption bands by the 

 bands of high photo-sensitivity. These considerations lead to the 

 conclusion that there is some connection between absorption and 

 photo-sensitivity, but the nature of this connection is by no means 

 clear. 



(c) Fluorescence and Phosphorescence 



It has been seen that only diamond No. 1 is phosphorescent, and 



since diamond No. 2 also exhibits photo-electric conductivity, it 



cannot be said that phosphorescence is a necessary condition for 



photo-sensitivity. Of course, a general conclusion as to this matter 



cannot be based on the evidence fro m such a limited investigation. 



'Crandall, Phys. Rev. 2; p. 361, 1913. 



«Coblentz and Kahler, Sci. Papers Bur. of Stands, No. 338. 



9W. W. Coblentz, Sci. Papers Bur. of Stands, No. 412, P- 180. 



