CONTEMPORARY ADVANCES IN PHYSICS 335 



likewise. Next comes a nucleus composed of three protons and three 

 neutrons, belonging to the element lithium. It is found that in 

 angular momentum as in magnetic moment it is practically a duplicate 

 of the deuteron, its six components having disposed themselves into 

 a nearly normal deuteron attached to a nearly normal alpha-particle. 

 I might proceed some distance farther along the list of the known 

 nuclei after this fashion, were there space; but it is best to close this 

 section with a general rule: nuclei with an eveii number of constituent 

 particles (protons and neutrons) have even spins, nuclei with an odd 

 number of particles have odd spins. "Even " and "odd " in this formu- 

 lation mean that the angular momentum is an even or an odd integer 

 multiple of ^{hjlir), respectively. One sees that if any two spins of 

 magnitude f (/?/27r) are allowed to choose only between parallel and 

 anti-parallel orientations, the rule follows inevitably; reversely, from 

 the rule (which is based on experience with some fifty or sixty kinds 

 of atom), we derive extra strength for that theorem about orientations. 

 Now to come to the conclusion and the climax. Although this 

 property of angular momentum, of being allowed to take only a 

 limited number of permitted orientations — although this strange and 

 wonderful property of angular momentum was introduced in this 

 lecture as though it pertained only to atoms subjected to applied 

 external fields, yet it manifests itself far more broadly. Indeed, it 

 manifests itself universally, and the stability and the character of the 

 world are due to it. I have already mentioned in half-a-dozen places 

 how it manifests itself within the molecule and within the atom: how 

 in the atom, it is responsible for those laws of composition which 

 determine the angular momentum and the magnetic moment of the 

 electron-family — how in the hydrogen molecule, it establishes a 

 difference between ortho-hydrogen and para-hydrogen — how in the 

 nucleus it fixes the angular momenta and the magnetic moments of 

 composite nuclei as sharply as those of their constituents the proton 

 and the neutron. Perhaps these seem to be remote and unimportant 

 qualities; but what has just been ^id of them may be said with equal 

 truth and equal force of all the chemical and physical properties of 

 all the elements, mass alone excepted (and even mass not fully ex- 

 cepted). If this feature of angular momentum did not prevail, there 

 could not be the fixity of properties which characterizes each element 

 by itself and the variety of properties which characterizes the totality 

 of the elements. Gold would not be gold, lead would not be lead, 

 oxygen would not be oxygen, helium would not be helium; for though 

 it is commonly said that each element is distinguished by its nuclear 

 charge and the number of electrons in its electron-family, this is not 



