8 De Broglie-Cabrera do not give ẞ-ray data and radium (or mesothorium in equilibrium) was used making it difficult to assign y-rays to particular radio-elements Meitner 3

1 Meitner 1

2 Meitner 2

3 Hahn-Meitner

4 Ellis1

9

VI. NUCLEAR STRUCTURE AND RADIOACTIVE

PROCESSES

1. SPECULATIONS AS TO NUCLEAR STRUCTURE

The chief starting points for speculations in regard to nuclear structure, after Rutherford's proof that there is an atomic nucleus to have a structure, have been (1) the displacement law of Fajans and Soddy, (2) the discovery that many elements are mixtures of isotopes, and (3) the disintegration of certain light nuclei by a-particle bombardment.

Many papers have been devoted to deriving formulae for nuclei regarded as pseudo-chemical compounds of protons, electrons, and relatively stable groups of these, analogous to chemical radicals. A beginning in this field was made by Wulf, but van den Broek, Nicholson and HarkinsWilson first elaborated such notions of nuclear structure, making use of Swinne's deduction from relativity theory that the mass of the nucleus should be less than the sum of its widely separated constituents (the so-called "packing effect "). Harkins, Harkins-R. E. Hall, HarkinsMadorsky, and Harkins-Liggett have developed at length in later papers the relations between the numbers of nuclear protons and electrons and other numbers characteristic of the nucleus. The suggestions of Durrant and of N. F. Hall are based upon some of Harkins' results. The great preponderance of even atomic numbers and even atomic weights among the atomic species forming the earth's crust, also noted by Waddell and Vernadsky, and which is even more striking in meteorites, suggests extra stability in symmetrical nuclei.

Nuclear formulae of the same general type as those of Harkins 2 have been proposed by Schmidt, Hackh, Kossel, Collins, Kirchhof, Masson, Cook, and Vólóšin. It has been pointed out by Aston in this connection that the similarity in the structure of isotopic nuclei must be very complete, because the differences in their atomic weights are closely equal to integers even if these weights themselves are not integral.

3

Harkins, and later Meitner, pointed out that a distinction might be made, in the structures proposed for radioactive nuclei, between a-particles which are compensated electrically by closely attached electrons, and removal of which is accompanied by loss of these electrons in B-ray disintegrations, and a-particles which are not so compensated. Meitner proposed that the a-rays ascribed to the emission of compensated a-particles be distinguished as a'-rays. On the basis of this suggestion Neu