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510 PHYSICS: LOEB AND Du SA ULT PROC.. N. A. S. * NATIONAL RESEARCH FELLow. 1 A good account of the subject by P. Debye is contained in Marx's Handbuch der Radiologie, Bd. 6, pp. 754-776. 2 C. V. Raman and K. S. Krishnan, Phil. Mag., 3, pp. 713-735. The relaxation time is defined as the time required for the orientation of the molecules to diminish to 1/eth of the extent of the orientation when the field is on. 4Beams, J. 0. S. A. & R. S. I., 13, p. 597; Beams and Allison, Phil. Mag. (in press). 6 An experimental investigation of the lag in the photo-electric effect led to the conclusion that the sum of the lag of the Kerr effect in carbon bisulphide and the lag of the photo-electric effect from a potassium surface is not greater than 3(10^) sec. We read a paper on this research at the April meeting of the American Physical Society (abstract in press) and shall publish details later. 6 0. Bluh, Phys. Zeit., No. 8, p. 226 (1926); C. P. Smyth, J. Am. Chem. Soc., 46, p. 257 (1924); R. Sanger, Phys. Zeit., No. 17, p. 556 (1926). MOBILITIES OF IONS IN ACETYLENE HYDROGEN MIXTURES By L. B. LOZB AND L. Du SAULT PHYSICAL LABORATORY, UNIVZRSITY OP CALIPORIA, BERKCLZY Communicated June 1, 1927 Introduction.-In a recent paper by Erickson' on the variation of mobilities of gas ions with age in acetylene results of theoretical importance were obtained. The important result from the viewpoint of this work was the discovery that apparently neither the positive nor the negative ion in acetylene showed a change in mobility with age within time intervals greater than 0.002 second, while at the same time the mobility was slightly greater for the negative ion than for the positive ion. In H2, Erickson2 had earlier found an aging effect on the positive ion in which the mobility changed in time from a value a little less than that of the negative to the value normall