326 BELL SYSTEM TECHNICAL JOURNAL 



much significance." Groups of three or four isobares occur among 

 the radioactive atoms beyond A = 206. 



Also, as one goes onward along the list, one meets with elements 

 having quite remarkable numbers of isotopes: lead with eight, xenon 

 and mercury with nine, tin with no fewer than eleven put down as 

 certain ! At the same time one notices elements of apparently a 

 single isotope only, up almost to the end of the procession; and there 

 is a striking rule, perhaps the most definite yet found in this field: 

 there is no element of odd atomic number for which more than two stable 

 isotopes are known. The word stable must be inserted, as there are 

 more than two radioactive isotopes for each of the elements 81 and 

 83. Moreover, for every such element past nitrogen the mass- 

 numbers of the two isotopes (if more than one is known) differ by two 

 units. It was also considered a rule that (past boron) the lighter 

 isotope is the more abundant of the two; but Aston has lately dis- 

 covered that the contrary is the case with rhenium (Z = 75) and 

 thallium (Z = 81), so that this rule must be confined to the middle 

 part of the list. This brings us to the question of abundances. 



The relative plenty or scarcity of the various elements has been 

 for many years a topic of inquiry among chemists, and also — or even 

 more — among geologists and astrophysicists. It now becomes a sub- 

 division of a larger topic, the relative plenty or scarcity of the various 

 kinds of atoms. Better said, there are now two subjects of research — 

 the relative abundances of the various isotopes within each element, 

 the relative abundances of the elements with respect to one another — 

 and by combining the data of the two one might hope to get the 

 relative amounts of the many kinds of atoms in the whole of Nature. 



The latter and older problem, however, is in much the more un- 

 satisfactory state, and seems likely to remain so. We have only the 

 earth's crust, the air, a few meteorites, some nebulae, and the outer- 

 most layers of the stars available for the study; the nebulae and the 

 stars only by spectroscopic methods, of which the results are not 

 always easy to interpret. The interior of the earth and the interiors 

 of the stars remain impenetrable to us. The relative abundances of 

 the elements in the five more or less accessible regions are by no means 

 the same, and give us no sure basis for guessing what they may be in 

 the inaccessible regions. 



Nevertheless, there are rules for the relative abundances of the 

 elements in the earth's crust, which are so strong that one is very much 

 tempted to extend them to the whole of Nature. There is a great 

 predominance of elements of even atomic number over elements of odd 

 (Harkins' rule). There is a predominance of atoms of mass-numbers 



