Supplement to '' Nature'' July 7, 1923 



31 



lay the length of the year would be different from 

 its present value. - 



It is quite otherwise in the case of atoms. The 

 definite and unchangeable properties of the elements 

 demand that the state of an atom cannot undergo 

 permanent changes due to external actions. As soon 

 'as the atom is left to itself again, its constituent 

 particles must arrange their motions in a manner 

 which is completely determined by the electric 

 charges and masses of the particles. We have the 

 most convincing evidence of this in spectra, that is, 

 in the properties of the radiation emitted from sub- 

 stances in certain circumstances, which can be 

 :studied with such great precision. It is well known 

 that the wave-lengths of the spectral lines of a sub- 

 stance, which can in many cases be measured with an 

 •ii-'uracy of more than one part in a million, are, in 

 same external circumstances, always exactly the 

 :e within the limit of error of the measurements, 

 - i quite independent of the previous treatment 

 of this substance. It is just to this circumstance 

 that we owe the great importance of spectral 

 analysis, which has been such an invaluable aid to 

 the chemist in the search for new elements, and has 

 also shown us that even on the most distant bodies 

 of the universe there occur elements with exactly the 

 same properties as on the earth. 



On the basis of our picture of the constitution of 

 the atom it is thus impossible, so long as we restrict 

 ourselves to the ordinary mechanical laws, to account 

 for the characteristic atomic stability which is required 

 for an explanation of the properties of the elements. 

 \ The situation is by no means improved if we 

 also take into consideration the well-known electro- 

 dynamic laws which Maxwell succeeded in formulating 

 on the basis of the great discoveries of Oersted and 

 Faraday in the first half of the last century. Maxwell's 

 theory has not only shown itself able to account for 

 the already known electric and magnetic phenomena 

 in all their details, but has also celebrated its greatest 

 triumph in the prediction of the electromagnetic waves 

 which were discovered by Hertz, and are now so 

 extensively used in wireless telegraphy. 



For a time it seemed as though this theory would 

 also be able to furnish a basis for an explanation of 

 the details of the properties of the -elements, after 

 it had been developed, chiefly by Lorentz and Larmor, 

 into a form consistent with the atomistic conception 

 of electricity. I need only remind you of the great 

 interest that was aroused when Lorentz, shortly after 

 the discovery by Zeeman of the characteristic changes 

 that spectral lines undergo when the emitting substance 

 is brought into a magnetic field, could give a natural 

 and simple explanation of the main features of the 



phenomenon. Lorentz assumed that the radiation 

 which we observe in a spectral line is sent out from 

 an electron executing simple harmonic vibrations 

 about a position of equilibrium in precisely the same 

 manner as the electromagnetic waves in radio-telegraphy 

 are sent out by the electric oscillations in the antennae. 

 He also pointed out how the alteration observed by 

 Zeeman in the spectral lines corresponded exactly 

 to the alteration in the motion of the vibrating electron 

 which one would expect to be produced by the magnetic 

 field. 



It was, however, impossible on this basis to give 

 a closer explanation of the spectra of the elements, 

 or even of the general type of the laws holding with 

 great exactness for the wave-lengths of lines in these 

 spectra, which had been established by Balmer, 

 Rydberg, and Ritz. After we obtained details as 

 to the constitution of the atom, this difficulty became 

 still more manifest ; in fact, so long as we confine 

 ourselves to the classical electrodynamic theory we 

 cannot even understand why we obtain spectra con- 

 sisting of sharp lines at all. This theory can even 

 be said to be incompatible with the assumption of 

 the existence of atoms possessing the structure we 

 have described, in that the motions of the electrons 

 would claim a continuous radiation of energy from the 

 atom, which would cease only when the electrons 

 had fallen into the nucleus. 



The Origin of the Quantum Theory. 



It has, however, been possible to avoid the various 

 difficulties of the electrodynamic theory by introducing 

 concepts borrowed from the so-called quantum theory, 

 which marks a complete departure from the ideas 

 that have hitherto been used for the explanation of 

 natural phenomena. This theory was originated by 

 Planck, in the year 1900, in his investigations on the 

 law of heat radiation, which, because of its independence 

 of the individual properties of substances, lent itself 

 peculiarly well to a test of the applicability of the 

 laws of classical physics to atomic processes. 



Planck considered the equilibrium of radiation 

 between a number of systems with the same properties 

 as those on which Lorentz had based his theory of 

 the Zeeman effect, but he could now show not only 

 that classical physics could not account for the 

 phenomena of heat radiation, but also that a complete 

 agreement with the experimental law could be obtained 

 if — in pronounced contradiction to classical theory — 

 it were assumed that the energy of the vibrating 

 electrons could not change continuously, but only 

 in such a way that the energy of the system always 

 remained equal to a whole number of so-called energy- 

 quanta. The magnitude of this quantum was found 



