CONTEMPORARY ADVANCES IN PHYSICS 293 



A great part of contemporary physics consists of the analysis and 

 interpretation of spectra; one wonders whether in this vast and tangled 

 array of data there is information about nuclei? The answer must be 

 phrased with care. The spectrum of an atom is due to its orbital 

 electrons, and of these the number and the arrangement are controlled 

 by the nuclear charge, which therefore dominates the spectrum; 

 spectroscopy is full of evidence for the theorem which I set down at 

 the start, that + Ze is the nuclear charge of the element of atomic 

 number Z. The mass of the nucleus is much less influential, owing to 

 the enormous disparity between it and the masses of the electrons. 

 Were it and they of the same order of magnitude, the nucleus would 

 move like the electrons, revolving around the center of mass of the 

 atom with a kinetic energy comparable with theirs. The emission of 

 light would then entail a contribution from the kinetic energy of the 

 nucleus as well as from those of the electrons, and the frequencies 

 of the spectrum-lines would be affected by the nuclear mass. But the 

 nucleus is so massive, its motion so slight and its kinetic energy so 

 insignificant, that in nearly all atoms that contribution is too small to 

 be appreciable, and the spectrum-lines are sensibly the same as if 

 the electrons revolved around a perfectly motionless centre. The only 

 exceptions are the three lightest kinds of atoms; I will later explain 

 how the discovery of one of these was brought about, two years ago, 

 by the influence of the mass of its nucleus upon the frequencies of its 

 spectrum-lines. 



The spectra of molecules are more dependent on nuclear masses 

 than are those of atoms; for, when two (or more) nuclei and their 

 attendant orbital electrons are combined into a single system, the 

 balance of forces is such as to provide for each nucleus a position of 

 equilibrium, from which it may be displaced and about which it will 

 oscillate more or less like a pendulum. There are (for instance) two 

 kinds of chlorine atoms, of nuclear masses standing to one another 

 approximately as 35 to 37; consequently there are three kinds of 

 diatomic molecules in ordinary chlorine gas, built as indicated by the 

 symbols CP^CP, CFCP", CP'Cl". In all of these three kinds of 

 molecules the internal forces are very nearly the same, being deter- 

 mined by the charges of the nuclei and electrons which are identical 

 for all three, and by their arrangement which is nearly identical; 

 but the masses of the nuclei are different, and therefore so are their 

 frequencies of oscillation, which appear in the spectra. The differ- 

 ences of nuclear masses also entail dift'erences in the moments of 

 inertia of these three kinds of molecules, which likewise are reflected 

 in their spectra. The lines of molecular spectra are often doubled or 



