T N a paper by Brown and Stebbins 1 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 selenium. The statement and the discussion of this hypothesis together

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... ith the results of investigations pertinent to this hypothesis will form the material of this paper. As the amount of data that must be correlated 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 2 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 B and B is changed into C, according to the reaction,