320 BELL SYSTEM TECHNICAL JOURNAL 



theorists of the future as the existence of other kinds. We go on then 

 to the kernel He*, our indispensable friend the alpha-particle. 



The ratios of the masses of the atoms H\ He* and O*^ are among the 

 most important constants of physics. All are known by now with 

 admirable precision: the three, mutually compatible values 1.0078 : 16 

 for HVO^S 4.0022 : 16 for HeVO'«, and 3.9713 : 1 for HeVH'— the two 

 first from Aston, the last from Bainbridge — appear to be uncertain 

 by not more than one place in the last significant figure, if so much 

 as that.^^ 



Forming a model for the He* nucleus out of four protons and two 

 electrons, we find that not only is it stable by the principle aforesaid, 

 but it is abundantly stable. The difference between Sm the sum of 

 the separate masses and M the mass of the alpha-particle is positive 

 and equal to .029 mass-units, or about twenty-seven million electron- 

 volts! There is consequently no cause for w^orry over the fact, or 

 rather the appearance, that when alpha-particles with as much as 

 eight million electron-volts of kinetic energy crash into other nuclei, 

 either nothing breaks or else the other nucleus gives way.^* With 

 the H'^ nucleus the difference 'Lm — M amounts to less than two 

 million electron-volts, so that we should rather expect it to be broken 

 under similar circumstances. 



Mass-number 5 again is missing from the procession, in spite of an 

 ardent and lately-stimulated search. 



Mass-numbers 6 and 7 are isotopes of lithium, of which Bainbridge 

 has determined the masses as 6.0145 and 7.0146, with uncertainties of 

 3 and 6 places in the last significant figure. One can picture the 

 nucleus of Li^ as a cluster of six protons and three electrons, which 

 have lost altogether 0.033 of a unit of mass or something over thirty 

 million electron-volts in combining. It is more usual, however, to 

 apply a certain very general hypothesis, of which the validity is still 

 quite uncertain: the hypothesis that in every nucleus there are nearly 

 or quite as many completed alpha-particles as the mass will admit. 

 In the case of Li® this suggests one alpha-particle, two "loose" protons 

 and one "loose" electron; and about four million electron-volts would 

 have been lost by the three last in attaching themselves to the alpha- 

 is The values and uncertainties as given are: (1.00778 ± .00015) : 16; (4.00216 

 ± .0004) : 16; and 3.971283 ± .000042; the uncertainties being the extreme ones 

 in the first two cases, the probable error in the last (cf. footnote 10). 



" When protons emerge from a substance bombarded by alpha-particles, why 

 should we assume that they come from the bombarded nuclei and not from the 

 projectiles? Chiefly, 1 suppose, because in the contrary case they would be expected 

 to appear whatever the substance, whereas actually they vary exceedingly in amount 

 and energy-distribution from one element to another. But there is some reason for 

 thinking that the alpha-particle coalesces with the struck nucleus when the proton 

 comes off, which makes the question rather meaningless. 



