Contemporary Advances in Physics, XXVI 

 The Nucleus, First Part 



By KARL K. DARROW 



This article, like its forerunners on radioactivity and transmutation, is 

 devoted to the beginnings of the oncoming stage of atomic physics: the study 

 of the nucleus. The nucleus or kernel of an atom is in ultimate control of 

 all its properties and features, for such of these as do not depend directly 

 on it depend upon the number and arrangement of the orbital electrons, 

 both of which are decided by the nuclear charge; further, the atomic weight 

 is decided almost exclusively by the nuclear mass. Though in dealing 

 with most of these properties it is usual to imagine the nucleus as a geomet- 

 rical point endowed with mass and charge, the truth is far less simple and 

 more interesting. Nuclei are structures built of elementary particles — some 

 and maybe all of which are independently known to us — bound tightly 

 together. It is of great importance to ascertain these structures, not only 

 for their own sake, but because through understanding them we may 

 become able to control and extend the transformations of nuclei from one 

 kind to another — the processes of transmutation, some of which are already 

 feasible. Several fields of research are apt to contribute to such an under- 

 standing. Accurate measurement of the masses of atoms, and of the 

 masses and charges and other properties of the elementary particles, are the 

 first two of these, and form the subject of the present article. 



SOME thirty years have now elapsed since the atom-nucleus was 

 first imagined. Before it could be conceived men had to discover 

 and measure negative electrons, and evolve the idea that these cor- 

 puscles normally reside in atoms, which in that case must comprise 

 positive charges as well. Since an electron is less than one one- 

 thousandth as massive as the lightest kind of atom, it is natural to 

 suppose that the positive charges within an atom are linked with the 

 main mass thereof. From this it is but a step to the notion of a heavy 

 positive nucleus serving as central sun of the atom, with electrons 

 revolving around it after the fashion of planets. This step was taken 

 in 1904 (by Rutherford, and on the other side of the world by 

 Nagaoka). A few years later, the picture was made more precise by 

 assigning a definite number of circling electrons to every kind of atom — 

 that is to say, to the atoms of all the elements; this at first was rather 

 vaguely estimated at about half of the atomic weight of the element in 

 question; then in 1915 it was chosen equal to the atomic number 

 (customarily called Z) which marks the place of the element in the 

 periodic table. Everything since discovered has justified this choice. 

 It necessarily fixes the positive charge of the nucleus, which must 

 exactly balance the total of the charges on the Z electrons, since the 



288 



