Excerpt from The Physical Review, Vol. 33: A Journal of Experimental and Theoretical Physics N a paper by Brown and Stebbins1 it was shown that the light-sensi tiveness of a certain selenium cell was a function of its resistance whether that resistance was conditioned by temperature, pressure, light or other agencies. This conclusion, together with the results of recent investigations, has led me to formulate an hypothesis for explaining the changes in the electrical conductivity occurring in light-sensitive sele nium. The statement and the discussion of this hypothesis together with the results of investigations pertinent to this hypothesis will form the material of this paper. As the amount of data that must be cor related is quite large indeed, particular attention will be given only to the various effects produced by light. The hypothesis is that all light-positive and light-negative varieties' of selenium consist of various mixtures of three kinds of selenium which we will call A, B and C and that under the action of light A is changed into E and B is changed into C according to the reaction. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.