296 BELL SYSTEM TECHNICAL JOURNAL 



they enter into the structure of some at least among the nuclei. 

 These are: 



The proton, or nucleus of the most usual kind of hydrogen atom; 



The alpha-particle, or nucleus of the helium atom ; 



The electron (that is to say, the negatively-charged corpuscle 

 customarily known by that name) ; 



The neutron; 



The positive electron; 



The IP nucleus or deuton, the nucleus of an unusual kind of hydrogen 

 atom of double the mass of the usual kind. 



Of these six the first three have been known for years. They have 

 actually been observed to spring out of nuclei, spontaneously in some 

 cases, in others elicited by bombardment; and this is one of the two 

 major reasons for imagining them as parts of nuclear structures. 

 It is true that this reason does not apply directly to all kernels. Those 

 which are known to emit alpha-particles spontaneously are a small 

 fraction, a tenth or thereabouts, of the total number; and all but 

 possibly two belong to the uppermost end of the periodic table, to mas- 

 sive atoms of atomic weight superior to 200. Those which are known 

 to emit electrons are yet fewer, and again all but two belong to the 

 most massive group. (The two exceptions are potassium and ru- 

 bidium.) No kernel is known to emit protons spontaneously; but a 

 great many elements both light and heavy will yield charged particles 

 out of their nuclei, when suitably bombarded; and these have been 

 proved in some cases to be alpha-particles, in others to be protons. 

 Moreover the bombarding particles which achieve these results are 

 themselves alpha-particles and protons, and there is reason to believe 

 that sometimes these are actually absorbed into nuclei which they 

 strike. 



The other major reason for inserting protons, alpha-particles and 

 electrons into our tentative models of nuclei is deduced from the 

 masses and the charges of these bodies. There is a certain well-known 

 standard of mass, one sixteenth of the mass of an oxygen atom; and 

 the masses of all nuclei come fairly close to being integer multiples of 

 this standard. Of course this can also be said about any other mass 

 lying within a certain (narrow) range of the standard just defined, 

 and perhaps it would seem better to say that the nuclear masses come 

 fairly close to having a greatest common divisor of that order of 

 magnitude, and then to determine by the method of least squares 

 what number had best be chosen for this greatest common divisor. 

 This procedure, however, would not be wise, unless the departures of 

 the various masses from the integer-multiple rule were casual, whereas 



