CONTEMPORARY ADVANCES IN PHYSICS 313 



the table of the elements (Z 81 to Z 92); and there are the seventeen 

 elements of atomic number inferior to 81 which have not yet been 

 analyzed, to each of which we must assign at least one isotope. This 

 makes the round figure two hundred and fifty a suitable choice for the 

 number of different masses of atoms, the number of different kinds of 

 fiuclei, already known. It may be a little excessive, but is not likely 

 to remain so for long.'' 



A graphical presentation of these atomic masses is more effective 

 by far than a table. One naturally thinks first of plotting A, the 

 atomic mass, against Z, the atomic number; but then it turns out 

 that the diagram is inconveniently high. The inconvenience is 

 lessened in Figs. 6 and 7 by plotting {A - Z) against Z, a scheme 

 which has also some value for theory. All the isotopes of an element 

 are marked by dots along its vertical line, and their mutual differences 

 of mass are properly given; but in comparing the isotopes of any 

 element with those of any other, one must think of their dots as 

 vertically displaced by an amount equal to the difference between the 

 abscissae of the elements. The two figures refer, one to the elements 

 below and the other to those in and above the great gap which in a 

 single figure occurs at the as-yet-unanalyzed group of the rare earth 

 elements. The slanting lines in Fig. 7 connect the consecutive 

 members of radioactive families; they are too crowded to be clear, 

 but I have shown a much clearer diagram in an earlier article of this 

 series.* 



Such a diagram implies that the masses of the isotopes are integer 

 multiples of a common unit, that unit which is one sixteenth the mass 

 of an oxygen atom; we must now examine into this question. Before 

 mass-spectra were observed, the non-integer "atomic weights" of 

 the chemical tables — such as the 24.32 of magnesium and the 35.46 

 of chlorine — were regarded as the masses of individual atoms. The 

 discovery of isotope-analysis must have created, in some minds at 

 any rate, the transitory hope that all true atomic masses would be 

 proved to be exactly integers, — if not in terms of one sixteenth the 

 oxygen mass, then in terms of some other. I do not know whether 

 this hope was ever widely formed; in any case, it was doomed to be 

 dashed. The ratios of the masses of the isotopes to one sixteenth the 



' Absence of an isotope from the list of those discovered means, of course, not 

 that it is absolutely non-existent, but that the ratio of its abundance to those of the 

 major isotopes of the element in question must be below some critical least-ob- 

 servable amount. This critical amount varies so much with the element, the method, 

 and the experimenter that no generally-valid figure can be given. In the very best 

 cases (e.g. helium, with which a vigorous search for He^ has been made) it is as low 

 as one part in 40,000; in others, apparently as high as one in a few hundred. 



« Number 12 (."Radioactivity"), this Journal, 6, 55-99, January, 1927. 



