Both thermal and electrical conductivities were measured for the same thin strip of Se, in order to compare the effect of illumination on both. The cells were made by winding a double spiral of Pt or Ni wire on a mica form, then pressing melted Se into a thin layer over and between them by means of another sheet of mica. The thermal effect was measured by heating one winding $\mathrm{CD}$ by an electric current and using the other $\mathrm{AB}$ as a resistance thermometer. The Se was rendered light sensitive by suitable heat treatment and the cell was placed in a well insulated box kept at 0° C. Correcting for the heating effect of the light, the rate of rise of temperature of $\mathrm{AB}$ was 15 to 30 percent greater with the illumination than without, the percentage change for each cell being about half that observed for the electrical conductivity of the same cell for the same illumination. A cell which was not sensitive electrically was also not sensitive thermally. By using various color screens it was found that as in the case of the electrical sensitivity, the red end of the spectrum is more effective than the shorter wave-lengths. Sieg failed to find this effect and explained his negative result as due to the absence of an electric field, but an electric field applied by means of the winding $\mathrm{AB}$ was found to have no effect on the thermal conductivity. These results furnish additional evidence that thermal and electrical conductivities in metals both depend, at least to a considerable extent, upon the same factors.

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