Constitution and Stability of Atom Nuclei. 327 



the whole number rule *. The idea that the atomic 

 weights were whole numbers on the hydrogen basis was due 

 to Prout, but the paper cited modified Prout's idea by- 

 proving on the basis of the chemical atomic weights, that 

 the atomic weights are whole numbers when the atomic 

 weight of hydrogen is made very nearly one, but the precise 

 standard is determined by fixing the atomic weight of 

 carbon, nitrogen, oxygen, or any other atomic weight of a 

 pure species of light atoms, as a standard. Since there may 

 be an as yet experimentally undemonstrated, but slight, 

 packing effect in the aggregation of alpha particles and 

 other electron groups to form more complex nuclei, it is 

 advisable to take some one specific atom species as a basis. 

 It happens that this is just what was done, for entirely 

 different reasons, when oxygen was fixed upon as the 

 standard. That the atomic weights of all pure atomic species 

 other than hydrogen have -exactly whole numbers for their 

 atomic weights on the oxygen basis, is an hypothesis which 

 has at the present time no experimental justification, though 

 it is suggested by the fact that the packing effect is constant 

 within the present limits of experimental error, which are 

 probably not over 0*05 to 0*1 per cent., or only about one- 

 sixteenth to one-eighth of its total value. The closeness of 

 the "atomic weights o£ radium and lead from radium to 

 whole numbers seems to indicate that it is not unlikely that 

 the mean variation of the packing effect is considerably less 

 than that cited. Thus it seems that it is safe to make the 

 statement that while the weight of the hydrogen atom in 

 hydrogen is 1*0077, its weight in any complex atom is 

 1-000 within the limits + O001, or probably even more 

 precisely than this. 



The principles to be kept in mind in the prediction of 

 isotopes are: — 



1. A deviation of the chemical atomic weight on the 



* The Whole Number Kule. — Since a number of later writers 

 have endeavoured to take the credit for developing the whole number 

 rule without in the least showing its theoretical significance, they should 

 realize that it was fully developed much earlier, as the following 

 quotation from Washburn's 'Physical Chemistry,' published in 1915, 

 shows : this " is interpreted by Harkins and Wilson as indicating that 

 the formation of these elements from hydrogen nuclei and electrons must 

 involve a constant decrease of about 0*77 per cent, in the mass of the 

 resulting atom ; and that nearly all of this decrease probably takes place 

 when the helium atom is formed, which agrees with the great stability 

 of the helium nucleus as a secondary unit of atomic structure " (p. 394, 

 first edition). Strutt, Phil. Mag. i. p. 311 (1901) discussed Trout V law 

 and considered that some modified form of the law must be true, but 

 considered that the case of chlorine decisively contradicted the law, as 

 was natural to conclude at that time. 



