32 



Supplement to '' Nature,'' July 7, 1923 



to be proportional to the frequency of oscillation of 

 the particle, which, in accordance with classical 

 concepts, was supposed to be also the frequency of 

 the emitted radiation. The proportionality factor 

 had to be regarded as a new universal constant, 

 since termed Planck's constant, similar to the 

 velocity of light and the charge and mass of the 

 electron. 



Planck's surprising result stood at first completely 

 isolated in natural science, but with Einstein's significant 

 contributions to this subject a few years after, a great 

 variety of applications was found. In the first place, 

 Einstein pointed out that the condition limiting the 

 amount of vibrational energy of the particles could be 

 tested by investigation of the specific heat of crystalline 

 bodies, since in the case of these we have to do with 

 similar vibrations, not of a single electron, but of whole 

 atoms about positions of equihbrium in the crystal 

 lattice. Einstein was able to show that the experi- 

 ment confirmed Planck's theory, and through the 

 work of later investigators this agreement has proved 

 quite complete. Furthermore, Einstein emphasised 

 another consequence of Planck's results, namely, that 

 radiant energy could only be emitted or absorbed 

 by the oscillating particle in so-called " quanta of 

 radiation," the magnitude of each of which was equal 

 to Planck's constant multiplied by the frequency. 



In his attempts to give an interpretation of this 

 result, Einstein was led to the formulation of the 

 so-called " hypothesis of light-quanta," according to 

 which the radiant energy, in contradiction to Maxwell's 

 electromagnetic theory of light, would not be pro- 

 pagated as electromagnetic waves, but rather as 

 concrete light atoms, each with an energy equal to 

 that of a quantum of radiation. This concept led 

 Einstein to his well-known theory of the photo-electric 

 effect. This phenomenon, which had been entirely 

 unexplainable on the classical theory, was thereby 

 placed in a quite different light, and the predictions 

 of Einstein's theory have received such exact experi- 

 mental confirmation in recent years, that perhaps 

 the most exact determination of Planck's constant 

 is afforded by measurements on the photo-electric 

 effect. In spite of its heuristic value, however, the 

 hypothesis of light-quanta, which is quite irreconcilable 

 with so-called interference phenomena, is not able 

 to throw light on the nature of radiation. I need 

 only recall that these interference phenomena con- 

 stitute our only means of investigating the properties 

 of, radiation and therefore of assigning any closer 

 meaning to the frequency which in Einstein's theory 

 fixes the magnitude of the light-quantum. 



>In the following years many efforts were made to 

 apply the concepts of the quantum theory to the 



question of atomic structure, and the principal emp] 

 was sometimes placed on one and sometimes on the 

 other of the consequences deduced by Einstein from 

 Planck's result. As the best known of the attempts 

 in this direction, from which, however, no definite 

 results were obtained, I may mention the work of 

 Stark, Sommerfeld, Hasenohrl, Haas, and Nicholson. 

 From this period also dates an investigation ' 

 B jerrum on infra-red absorption bands, which, altho 

 it had no direct bearing on atomic structure, proNed 

 significant for the development of the quantum theory. 

 He directed attention to the fact that the rota- 

 tion of the molecules in a gas might be investigated 

 by means of the changes in certain absorption li- '~ 

 with temperature. At the same time he emphas 

 the fact that the effect should not consist of a con- 

 tinuous widening of the lines such as might be expected 

 from classical theory, which imposed no restrictions 

 on the molecular rotations, but in accordance with 

 the quantum theory he predicted that the lines should 

 be split up into a number of components, corresponding 

 to a sequence of distinct possibilities of rotation. 

 This prediction was confirmed a few years later by 

 Eva von Bahr, and the phenomenon may still be 

 regarded as one of the most striking evidences of the 

 reality of the quantum theory, even though from our 

 present point of view the original explanation has 

 undergone a modification in essential details. 



The Quantum Theory of Atomic Constitution'. 



The question of further development of the quantum 

 theory was in the meantime placed in a new light 

 by Rutherford's discovery of the atomic nucleus (191 1). 

 As we have already seen, this discovery made it quite 

 clear that by classical conceptions alone it was quite 

 impossible to understand the most essential properties 

 of atoms. One was therefore led to seek for a formula- 

 tion of the principles of the quantum theory that 

 could immediately account for the stability in atomic 

 structure and the properties of the radiation sentj 

 out from atoms, of which the observed properties of 

 substances bear witness. Such a formulation was 

 proposed (1913) by the present lecturer in the form 

 of two postulates, which may be stated as follows : 



I. Among the conceivably possible states of 

 motion in an atomic system there exist a number 

 of so-called stationary states which, in spite of 

 the fact that the motion of the particles in these 

 states obeys the laws of classical mechanics to 

 a considerable extent, possess a pecuhar, mechanic- 

 ally unexplainable stability, of such a sort that 

 every permanent change in the motion of the 

 system must consist in a complete transition from 

 one stationary state to another. 



