240 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



but the number and variety of those already exhibiting it makes 

 the probability of this extremely high. 



On the other hand, it must not be supposed that this would imply 

 that the whole-number rule holds with mathematical exactness, but 

 only that the approximation is of a higher order than that exhib- 

 ited by the ordinary chemical combining weights and is quite close 

 enough to allow of a theory of atomic structure far simpler than 

 those put forward in the past; for such theories were forced to 

 attempt the explanation of fractions which now appear to be merely 

 fortuitous statistical effects, due to the relative quantities of the 

 isotopic constituents. 



Thus, one may now suppose that an elementary atom of mass m 

 may be changed to one of mass m-\-l by the addition of a positive 

 particle and an electron. If both enter the nucleus, an isotope 

 results, for the nuclear charge is unaltered. If the positive particle 

 only enters the nucleus, an element of next higher atomic number is 

 formed. In cases where both forms of addition give a stable con- 

 figuration the two elements will be isobares. 



The electromagnetic theory of mass asserts that mass is not gen- 

 erally additive, but only becomes so when the charges are rela- 

 tively distant from each other. This is certainly the case when the 

 molecules H 2 and H 3 are formed from H,, so that their masses will 

 be two and three times the mass of H x with great exactness. (It 

 must be remembered here that the masses given by these experiments 

 are those of positively charged particles, Hj being presumably a 

 single particle of positive electricity itself, and that the mass of 

 an electron on the scale used is 0.00054, and too small to affect the 

 results. ) 



In the case of helium, the standard oxygen, and all other elements, 

 this is no longer the case; for the nuclei of these are composed of 

 particles and electrons packed exceedingly close together. The 

 mass of these structures will not be exactly the sum of the masses 

 of their constituents, but probably less, so that the unit of mass 

 on the scale chosen will be less than that of a single hydrogen atom. 



