306 BELL SYSTEM TECHNICAL JOURNAL 



of "ultimate"! It is possible, nay probable, that some of these 

 corpuscles are built up from others. Neutron may be proton plus 

 electron; proton may be neutron plus positive electron; alpha-particle 

 may be two protons plus two neutrons, or four protons plus two 

 electrons. 



Masses of Atoms and Their Nuclei 



If all the atoms of an element were perfectly alike, we could take 

 the relative values of their masses — relative to those of other elements, 

 and in particular to that old familiar standard, one sixteenth the mass 

 of an oxygen atom — straight from the chemists' tables of atomic 

 weights. It happens, however, that there are two, three, or several 

 different kinds of atom to almost every element, and they are nearly 

 always so thoroughly intermingled in even the smallest analyzable 

 samples as to suggest that the mixing was done while the earth was 

 still a gas. Whatever chemical method of measuring "atomic weight" 

 be applied to an element (and this includes the strictly physical 

 scheme of measuring its density when it is gaseous) leads forthright 

 and inevitably to a mean value of the masses of its "isotopes" or 

 divers kinds of atoms. Not a simple average, of course ! but rather a 

 weighted mean, to which every isotope makes contribution in pro- 

 portion to its relative abundance in the mixture. 



The tables of the "chemical atomic weights" are just collections of 

 these weighted means. They nearly all involve two or more varieties 

 of atoms, and in most of the cases the weighted average is markedly 

 different from the mass of any isotope. Sometimes one of the isotopes 

 predominates so greatly that the others contribute very little to the 

 mean, and the chemical atomic weight is not a bad approximation to 

 the mass of this single kind of atom. This is not typical of the system 

 of the elements as a whole, but it happens to be the case of no fewer 

 than eight among the first eleven: a coincidence which has had some 

 influence on the trend of scientific thought, for if it had not happened 

 the chemical atomic weights of seven among these eight elements would 

 not have been so nearly integer multiples of the standard as they 

 actually are {viz. H 1.01, He 4.00, Be 9.02, C 12.00, N 14.01, F 19.00, 

 Na 23.00) and then it would have been difficult to advance the idea 

 that all atoms are built up from common particles. If oxygen itself 

 were not of the group of these eight — if the rarer isotopes of oxygen 

 were, say, a tenth or a third as abundant as the predominant one, 

 instead of being less than 1/500 as abundant — we should either be 

 suffering from a table of atomic weights in which there would be no 

 integers unless by accident, or else we should be using some other 



