light, and the electric vector of the latter vertical, then, as we saw in transverse observation, all traces of polarization were absent. In longitudinal observation, in the same case, strong polarization was found. This exhausts the experimental data reported in the paper under review.

The article further reports the following formal interpretation of the phenomenon, proposed by C. Darwin. It is assumed that, under the action of an external magnetic field, the radiating circular, plane orbits of the electrons of the corresponding atoms are set perpendicular to the field. Let us analyze from this point of view the first experiment, when the directions of observation, of the magnetic vector of the exciting light, and of the external field are parallel. According to the hypothesis, the circular plane orbits will be set perpendicular to the field, and each of them, under the influence of the vertical exciting vector, will send to the observer (transverse observation) a wave polarized circularly. Right- and left-handed rotations are then equally probable; as a result, the observer, in agreement with experiment, will observe no polarization. Conversely, in longitudinal observation in the same case, the observer will see the same oriented orbits “in profile” and will receive from them (both from right- and left-rotating ones) linearly polarized light, again in agreement with experiment.

When the electric exciting vector is vertical and the external magnetic field is rotated in a vertical plane perpendicular to the horizontal line of observation, coinciding with the direction of the magnetic exciting vector, then, on the basis of the hypothesis just stated and in agreement with experiment, the following results are obtained. For \(H_{90}\) the orbits are perpendicular to the external field and parallel to the electric exciting vector; the observer sees them vertically “in profile” and consequently obtains vertical, linear oscillations; for \(H_0\) horizontal linear oscillations are observed analogously. For \(H_{45}\) the orbits are visible to the observer “in profile” at an angle of \(45^\circ\) to the horizon, but the vertical electric vector excites in them horizontal and vertical oscillations of equal amplitude, i.e. to the observer the light appears unpolarized.

If the electric exciting vector is horizontal and observation is made in its direction, while the external field is rotated in a vertical plane, then, as is not difficult to see, the orbits of the atoms, according to the hypothesis, will all the time be parallel to the electric vector and perpendicular to the field; the observer will see the light linearly polarized all the time to the same degree, but the electric vector of the resonance light will rotate as the field rotates, in agreement with experiment.

In connection with Darwin’s theory, the experiments of Wood and Ellett must be compared with the experiments of Stern and Gerlach,^1 published last year. The discreteness of the orientation of atoms in a magnetic field is revealed in the new experiments extremely distinctly. As is known, this discreteness follows from the postulates of Bohr’s theory. Of course, many aspects of the phenomenon are still not explained by the theory set forth. In particular, the quantitative side remains to be taken into account, as well as the connection between polarization in a magnetic field and the normal polarization of fluorescence.

S. Vavilov.

Ionization and Excitation Potential of Nitrogen.

E. Brandt, Über die Ionisierungs- und Anregungs-Spannung des Stickstoffs. Zsch. f. Ph. 39, VIII. 32, 1922.

Brandt’s work was carried out by the usual method. The accelerating potentials were brought up to 100 V. Corrections for the contact potential difference, etc., were not calculated, but were introduced, as is usually done, by means of measurements of a standard gas, such as

^1 Cf. p. 301.

served as He. In the region between 7.5 and 8.2 V, a large number of bends of the current curve were measured (with an accuracy up to 0.01 V), corresponding to the lines of the ultraviolet radiation of \(N_2\). These lines form precisely the region of the band spectrum of nitrogen. Thus the study of the structure of bands in the far ultraviolet becomes possible with the aid of the method of ionization points. True, as yet the results obtained are not sufficiently accurate to give a clear quantitative picture of the structure of the band.

The figures obtained by the author, \(17.75 \pm 0.1\) V, \(25.41 \pm 0.1\) V, \(30.72 \pm 0.2\) V, correspond, according to his indication, to the ionization potential and to two higher stages of ionization. These figures must be decreased by 0.7 V in accordance with the correction for He given by Franck. Unfortunately, the data cited by Brandt are insufficient to say with certainty to what processes the quantities observed by him correspond. Assuming that 17 V corresponds to the splitting of the molecule and the ionization of one of the atoms \((N_2 \to N + N + \Theta)\), and 30 V to the ionization of both atoms \((N_2 \to N \,\cdots\, N + 2\Theta)\), I estimate the heat of dissociation of nitrogen at 4 V, i.e. only a little greater than the heat of dissociation of \(H_2\). That this quantity must be greater than for hydrogen agrees with Langmuir’s observations on the difficulty of the thermal dissociation of nitrogen. However, one would like to know with greater certainty the heat of molecular dissociation, a quantity so important for photochemical calculations.

Gr. Landsberg.

The Ionization Potential of Helium.

J. Franck. Bemerkung über Anregungs- und Ionisierungsspannung des Heliums, Zschr. f. Ph, XI, 155, 1922.

Despite the large number of experimental and theoretical works devoted to the determination of the excitation and ionization potentials of helium, the question of the arrangement of the lines computed from these data into serial schemes, i.e. the question of the possible orbits of helium, has remained controversial up to now. The scheme proposed by Franck and Knipping, given in Table 1, met with objections on the part of Davis, Horton, Kemball, and others.

TABLE I \(^{1}\).

\(^{1}\) In all serial designations I have used Sommerfeld’s designations, who increases the fractional figures standing before the symbols \(S\), completing them to the nearest integer (addition of 0.5).