form as (31), the Rydberg constant N being replaced by 4 N. Recent experimental evidence clearly gives this constant a value for spark spectra four times as great as in the case of arc spectra.· FURTHER ENERGY RELATIONS The basis of the Bohr theory of atomic structure is the concept of a limited number of stable orbits within the atom, differing from one another in energy by definite steps. In addition to the explanation which this theory provides of certain X-ray and visible emission lines, it accounts for many features of other more or less related phenomena, such as absorption, ionization, etc. It is convenient to distinguish between rings in the atom, each filled to saturation with electrons, and the free paths outside that are traversed by a single electron which has broken away from one of the inner groups. Neglecting for the moment the slight difference in energy between a circular orbit and an elliptical one having the same quantum number, we may represent schematically the energies of different orbits by horizontal lines drawn at corresponding levels above an arbitrary base line. Thus, in figure 5, the difference in level between the lower lines 1 and 2 is pro FIGURE 5. Energies of inner and outer rings. portional to the excess of energy of an electron in the second Bohr ring in the inner atom, over that of an electron in the first. Suppose the first three rings to be saturated with electrons. Outside these rings exist unoccupied or free orbits of quantum number 1, 2, 3, 4, etc., which an electron that has been pulled out of the inner atom may traverse. If, now, the atom absorbs energy from outside, at least one electron must move to an orbit of greater energy, i.e., to an orbit further from the center of the atom. If an electron in the K ring is displaced, however, it cannot take up a position in the already saturated L or M rings, but must either pass on to one of the free orbits outside the inner atom, or leave the atom altogether. Therefore, assuming the second postulate of the Bohr Summerfeld theory to apply to absorption as well as to emission, absorption lines having frequencies equal to those of the Ka and Ks emission lines cannot exist, the lowest frequency in the K absorption spectrum being that corresponding to the removal of an electron from the K ring to the first free orbit outside the inner atom, that is, to the s1 orbit involved in the visible spectrum. Other lines of higher frequency correspond to the transfer of an electron from the K ring to free orbits further out, and if the atom absorbs enough energy to ionize it, this electron is shot out with a velocity great enough to carry it to infinity. An electron may, however, be ejected with a speed greater than that necessary to take it completely outside the sphere of influence of the atom, and as the kinetic energies of electrons shot out of the atom with such velocities may vary by insensible gradations, continuous absorption would be expected and is actually found. In the case of the electron responsible for series lines in the visible region, conditions are somewhat different. The free orbits contain no other electrons, and therefore passage is possible from any orbit to any other in either direction. Hence the theory correctly predicts absorption lines of the same frequency as those due to emission. The single line emission from sodium excited by resonance in Wood's experiments, or by impact of cathode rays by Tate and Foote, has an interesting interpretation on the Bohr Theory. Here the essential condition is that the energy supplied to the electron responsible for the radiation shall be sufficient to remove it from the free orbit in which it is revolving to the next one, and no further. Obviously, the saturated condition of inner orbits negatives the possibility of single line emission in X-ray spectra. Bohr' has pointed out a very interesting confirmation of the selection principle in connection with Strutt's extension of Wood's investigation of the resonance spectrum of sodium. The lines in the principal series of this element are doublets, indicating that the p orbits are double. Now Strutt finds that excitation of sodium vapor with light of the frequency of one of the D lines will produce that D line in the resonance spectrum but not its neighbor. This indicates that the radiating electron cannot pass from one of the p2 orbits to the other, a transfer that would involve no change in azimuthal quanta. If, however, the exciting radiation has the frequency of the second line in the principal series, the electron has been removed to a pз orbit, and may fall back to the s1 orbit directly, or it may pass to s2, then to either of the p2 orbits, and finally back to the s1 orbit. Therefore both D lines appear. 1Bohr, Zeit. f. Phys., 2, 423, 1920. To remove an electron from an inner to an outer orbit, the frequency of the absorbed radiation must be such that he is at least as great as the excess of the atom's energy in the second configuration over that in the first, but the energy absorbed may be very much less than this quantity. This fact suggests that energy is stored up, and that the final transfer of the electron from one orbit to the other is due to some sort of trigger action. D. L. Webster,1 however, has pointed out the following difficulty with this hypothesis. Suppose that almost enough energy to effect the transfer has been stored up from absorbed radiation. Then the impact of a cathode particle of small energy should suffice to complete the process. But it is found that unless an impinging cathode particle is able to supply all the excess energy of the final atomic configuration over the initial, the change fails to take place. Such facts seem to indicate that unless the energy of the cathode particle is as great as hv, none of its energy is available for absorption by the electron under consideration. THE THREE DIMENSIONAL ATOM It seems hardly possible that the electronic orbits inside the atom can be all coplanar. Attempts to devise a three dimensional model based on the fundamental postulates of the Bohr-Sommerfeld theory have not as yet, however, been very successful quantitatively. Duane2 has suggested that there may be two L and two M rings-as is indicated by the recurrence of the numbers 8 and 18 in the periodic table-lying in parallel planes above and below the plane of the K ring. In order to prevent these pairs of rings from coalescing, the electrons must revolve in opposite senses in the two rings of each pair. Landé3 has departed more radically from the simple Bohr model in his attempt to construct a cubical atom. In elements of low atomic number he supposes eight electrons situated near the corners of a cube to revolve in small circles about the four principal diagonals of the cube as axes. In the case of elements of high atomic number, the revolution takes place approximately along arcs of four equal circles lying in planes perpendicular to these four principal diagonals and having their common center at the nucleus. This model retains many of the fundamental features of the Bohr theory. 1Webster, Phys. Rev., 16, 37, 1920. 'Duane, Phys. Rev., 17, 431, 1921. 'Landé, Zeit. f. Phys., 2, 380, 1920. Compiled under the auspices of the Section of Seismology of the American Geophysical Union with the cooperation and assistance of the Research Information Service BY HARRY O. WOOD Secretary, American Geophysical Union INTRODUCTION Because of inquiries and recognition of the need for a revised list of the seismologic stations of the world, the present compilation was undertaken, under the auspices of the Section of Seismology of the American Geophysical Union and with the cooperation of the Research Information Service of the National Research Council of the United States of America. Under date of March 20, 1920, the following questionnaire, accompanied by the following explanatory letter asking the cooperation of seismologists in the assembly of the necessary data, was sent to every address where a seismograph was known or thought to have been installed or projected (except in a very few cases. where former stations were definitely known to have been discontinued). In assembling the list of addresses to which the inquiry was sent the compiler received substantial assistance from Professor H. F. Reid, Professor W. J. Humphreys, and Rev. F. A. Tondorf, S.J., to whom thanks and acknowledgments are tendered. 397 |