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122 CHEMISTRY: OLSON AND GLOCKLER PROC. N. A. S. metal. Furthermore, by a simple consideration of balance the formula ,cl shows why Cs- --I should be, and is, more stable even than Cs- --I \Br 'C1 formula the far short of exthese with falls Even advantages, however, pressing correctly all the facts which crystal structure determinations have revealed. 1 NATIONAL RESEARCH F}LLOW. Clark and Duane, these PROCZZDINGS, 8, 90 (1922); Ibid., April, 1923. Wells and Wheeler, Zs. anorg. Chem., 1, 442 (1892). 4Wells and Penfield, Amer. J. Sci., 43, 21, 475 (1892). 5Ephraim, Ber. deut. chem. Ges., 50, 1069 (1917). 6 Remsen, Amer. Chem. J., 11, 291. 7 McCombe and Reade, J. Chem. Soc. (London), 123, 141 (Feb., 1923). 8 Kuster, Zs. anorg. Chem., 43, 53 (1905); 44, 431 (1905); 46, 113 (1905). 9 Kraus, J. Amer. Chem. Soc., 44, 1216 (1922). °0 Wyckoff, Ibid., 42, 1100 (1920). "Wyckoff, Ibid., 44, 1239, 1260 (1922). 12 Bragg, J. Chem. Soc. (London), 121, 2766 (Dec., 1922). 13 Wyckoff, Amer. J. Sci., 4, 188 (1922). 14 Cla;k, Science, 55, 401 (1922). 15 Cf. also Wyckoff, these PROCEEDINGS, 9, 33 (Feb., 1923). The radius of the chlorine atom calculated from CsICl2 is unusually small as it should be since it is measured along the diagonal in the direction of greatest compressing forces. For this reason the direct application of this dimension to other chlorine compounds has little meaning. 2 3 THE CRITICAL AND DISSOCIATION POTENTIALS OF HYDROGEN By A. R. OLSON AND GZORGE GLOCKLZR DEPARTMENT OF CHZMISTRY, UNIVERSITY OF CALIFORNIA Communicated, February 21, 1923 When electrons collide with gas molecules the collisions are elastic, or nearly so, until the electrons acquire a definite velocity. This velocity is characteristic of the gas. The potential through which the electrons must fall to attain this velocity is called a critical potential. The determination of the critical potentials of hydrogen has been the object of many investigations during the last ten years, but considerable uncertainty still ,attaches to their exactness. Thus the potential ascribed to the dissociation of the hydrogen molecule and the ionization of one of the resultant atoms was found by Franck, Knipping and Krueger' to be 17.1 volts, whereas Boucher2 reports 15.6 volts for the same phenomenon. Recently VOL. 9, 1923 VCHEMISTRY: OLSON AND GLOCKLER 123 Franck3 has reduced his value by 0.7 volt. It seemed desirable therefore to attempt a more accurate determination of this potential, for this method furnishes one of the best means of calculating the heat of dissociation of hydrogen. Figure 1 shows the details of the vacuum tube used in this experiment, and also a diagrammatic representation of the electric fields. F is a platinum filament covered with calcium oxide. The two metal discs, G1 and G2, have slits 4 mm. long and 1 mm. wide at their centers. The nearer end of the cylinder G3 is fitted with a similar slit, and all are so placed that they are in alignment with the hottest portion of the filament. The receiving end of the ionization cylinder consists of a plate, P, and gauze, G4, A50 +_ + v*A+\qllll about 3 mm. distant. All metal parts were made of platinum. The arrows show the direction in which a free electron would be moved by the fields. In the following paragraphs A will refer to a field which accelerates an electron moving toward the plate P, and R to a field which retards such an electron. A quadrant electrometer was used, employing the constant deflection method. The hydrogen was generated by electrolysis of barium hydroxide solution, passed over hot platinized asbestos, and stored over phosphorus pentoxide. During the experiments the pressure of hydrogen in the tube was about 0.1 mm. of mercury. The initial electron velocity was determined b