CONTEMPORARY ADVANCES IN PHYSICS, XXIX 315 



Atoms of odd mass-numher have half-integer or "odd" spins; while 

 among the atoms of even mass-number, two of the lightest have the full- 

 integer or "even" spin of unity, while all the rest display no hyper- 

 fine-striicture at all and thus probably have the "even" spin-value zero 

 (though we must never forget the possibiUty that the lack of ob- 

 servable hyperfine-structure means that the magnetic moment is very 

 small, the clusters of states therefore so compact that the spectroscope 

 cannot resolve the hyperfine pattern, and the value of I therefore un- 

 ascertainable) . 



Although not every kind of atom has yet been studied, the number 

 of cases on which this rule is based is already so considerable that the 

 discovery of an exception would be a sensation of the first order. 

 Among the most striking exemplifications of the rule are those afforded 

 by elements of many isotopes. Mercury, the outstanding example, 

 displays a hyperfine-structure of wondrous complexity, which has been 

 very successfully interpreted by assigning the value 3/2 of I to its 

 odd isotope of mass-number 201 (Hg^"), the value 1/2 to its odd isotope 

 Hg^^^ and the value zero to its four principal even isotopes Hg^^^, 

 Hg2oo, Hg-°2 and Hg^^^; the lines attributed to these isotopes stand to 

 one another in the intensity-ratios deduced from the relative abund- 

 ances of the isotopes as measured by Aston. Cadmium has two odd 

 isotopes for both of which I = 1/2, and several even ones exhibiting 

 no hyperfine structure at all. Several elements have two isotopes, 

 both of odd mass-number; in some of these cases both have the same 

 value of /(e.g. gallium 3/2, rhenium 5/2), in other cases they differ 

 (e.g. rubidium 3/2 and 5/2). The outstanding and very-certainly- 

 known case of hydrogen is distinguished by the values 1/2 for the 

 light and 1 for the heavy isotope. 



(It may occur to the reader that if, say, four isotopes of mercury 

 display no hyperfine-structure, then everything that I have heretofore 

 said implies that their spectrum-lines and levels ought to coincide 

 absolutely and be indistinguishable. These are, however, slightly 

 separated, owing, it is presumed, to very slight differences in the fields 

 surrounding nuclei of different isotopes even when they all belong to 

 the same element and have the same nuclear charge and indistinguish- 

 able moments. The phenomenon, which is known as "isotope shift," 

 is likely to be much studied in the theoretical physics of the near 

 future) . 



As we go along the list of the atoms of odd mass-number from the 

 lightest to the heaviest, the value 1/2 for / appears at the start; the 

 value 3/2 at mass-number 6; 5/2 at ^ = 35; 7/2 at ^ =83; 9/2 (the 

 highest yet inferred) a.t A = 93. Further observations may change 



