Constitution and Stability of Atom Nuclei. 309 



6 (a). The packing effect* is constant and equal to 0'76 per 

 cent, for all pure species of atoms other than hydrogen 

 within the present limits of accurate atomic weight 

 determinations. This is practically the statement given 

 by Harkins and Wilson in 1915. The term packing- 

 effect as used here indicates the percentage difference 

 between the atomic weight on the basis of hydrogen 

 as 1, and that on the basis of oxygen as 16 (carbon as 

 12, fluorine as 19, nitrogen as 14, etc.) provided that in 

 each case the atomic weight is that of the pure species 

 alone. This statement involves the whole number rule, 

 that the atomic weights of pure atomic species are 

 whole numbers on the basis of oxygen as 16. 

 (b). The mutual electromagnetic mass in any atomic species 

 other than hydrogen is equal to — 0"76 percent., or any 

 complex atom has a mass and weight 0*76 per cent, less 

 than the hydrogen atoms (neutrons) from which it may 

 be assumed to be built. In any nucleus composed 

 entirely of alpha particles (helium nuclei) the packing 

 effect in the entire nucleus is approximately equal to 

 that in the alpha particles separately. This indicates 

 that the helium nucleus is far more stable with reference 

 to its complete disintegration into hydrogen nuclei and 

 negative electrons, than the more complex atom with 

 reference to its disintegration into alpha particles. The 

 mutual electromagnetic mass in any complex atom whose 

 nucleus contains other electron groups than the alpha 

 particle, is, within the present limits of experimental 

 error, equal to that in nuclei built from alpha particles 

 alone, which suggests that the great constancy in the 

 value of the packing effect may be due to the character- 

 istics of the positive and negative electrons themselves. 

 The question of minor differences in the value of the 

 packing effect has been treated in the earlier papers, 

 and need not be considered here. 



* Rutherford seems to have been the first to show that the packing of 

 electrons must be very close in the helium nucleus. In Phil. Mag', xxvii. 

 .pp. 494-5 (1914) he states: "This may, for example, be the explanation 

 for the fact that the helium atom has not quite 4 times the mass of 

 the hydrogen atom." Harkins and Wilson, without being aware of 

 Rutherford's statement, published an analysis of the packing effect and 

 also made an extensive study of the atomic weight relations. They 

 came to the conclusion that the packing effect is constant for all 

 pure species of atoms and equal to 077 per cent. (Proc. Nat. Acad. 

 Sciences, i. p. 27G (1915); J. Am. Chem. Soc. xxxvii. pp. L867-1421 

 (1915) ; Phil. Mag. xxx. pp. 723-34 (1915)). 



