If we therefore assume that the orbit of the electron in the stationary states is circular, the result of the calculation on [p. 5] can be expressed by the simple condition: that the angular momentum of the electron round the nucleus in a stationary state of the system is equal to an entire multiple of a universal value, independent of the charge on the nucleus. The possible importance of the angular momentum in the discussion of atomic systems in relation to Planck’s theory is emphasized by Nicholson[15].
The great number of different stationary states we do not observe except by investigation of the emission and absorption of radiation. In most of the other physical phenomena, however, we only observe the atoms of the matter in a single distinct state, i. e. the state of the atoms at low temperature. From the preceding considerations we are immediately led to the assumption that the “permanent” state is the one among the stationary states during the formation of which the greatest amount of energy is emitted. According to the equation (3) on [p. 5], this state is the one which corresponds to
.
[§4. Absorption of Radiation.]
In order to account for Kirchhoff’s law it is necessary to introduce assumptions on the mechanism of absorption of radiation which correspond to those we have used considering the emission. Thus we must assume that a system consisting of a nucleus and an electron rotating round it under certain circumstances can absorb a radiation of a frequency equal to the frequency of the homogeneous radiation emitted during the passing of the system between different stationary states. Let us consider the radiation emitted during the passing of the system between two stationary states
and
corresponding to values for