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30 lead to the supposition that they might also have a heating effect on the bodies subjected to their action. To test the matter experimentally, I installed a thermopile of Rubens' connected to an enclosed galvanometer. The action of "N" rays on this apparatus was absolutely nil, even in the most favorable conditions, though a candle placed 12 metres away from the thermopile gave a deflection of about 0.5 mm. on the scale. I conducted the experiment not only with "N" rays proceeding from an Auer burner, but also with those from the sun on the 3rd of July, 1904, at midday. The rays were very intense, for when I placed in front of the thermopile a tube containing calcium sulphide, which had been feebly excited by exposure to the sun, its glow was greatly increased, but was diminished by the interposition of a lead screen or the hand. M. H. Rubens made the same observation, as he was kind enough to write me, his apparatus being much more sensitive even than mine. I nevertheless thought it useful to determine directly whether the incandescent platinum wire was not heated by the action of "N" rays. To this end, I had recourse to the study of