10 THE PRESIDENTIAL ADDRESS. 
that the number of units of resultant positive charge in the nucleus 
fixes the number of the outer planetary electrons in the neutral atom. 
In addition, since these outer electrons are in some way held in equili- 
brium by the attractive forces from the nucleus, and, since we are 
confident from general physical and chemical evidence that all atoms 
of any one element are identical in their external structure, it is clear 
that their arrangement and motion must be governed entirely by the 
magnitude of the nuclear charge. Since the ordinary chemical and 
physical properties are to be ascribed mainly to the configuration and 
motion of the outer electrons, it follows that the properties of an atom 
are defined by a whole number representing its nuclear charge. It 
thus becomes of great importance to determine the value of this nuclear 
charge for the atoms of all the elements. 
Data obtained from the scattering of « particles, and also from the 
scattering of X-rays by light elements, indicated that the nuclear charge 
of an element was numerically equal to about half the atomic weight 
in terms of hydrogen. It was fairly clear from general evidence that 
the hydrogen nucleus had a charge one, and the helium nucleus (the 
« particle) a charge two. At this stage another discovery of great im- 
portance provided a powerful method of attack on this problem. The 
investigation by Laue on the diffraction of X-rays by crystals had 
shown definitely that X-rays were electromagnetic waves of much 
shorter wave-length than light, and the experiments of Sir Willam 
Bragg and W. L. Bragg had provided simple methods for studying the 
spectra of a beam of X-rays. It was found that the spectrum in 
general shows a continuous background on which is superimposed a 
spectrum of bright lines. At this stage H. G. J. Moseley began a 
research with the intention of deciding whether the properties of an 
element depended on its nuclear charge rather than on its atomic weight 
as ordinarily supposed. For this purpose the X-ray spectra emitted 
by a number of elements were examined and found to be all similar 
in type. The frequency of a given line was found to vary very nearly 
as the square of a whole number which varied by unity in passing 
from one element to the next. Moseley identified this whole number 
with the atomic or ordinal number.of thea elements when arranged in 
increasing order of atomic weight, allowance being made for the known 
anomalies in the periodic table and for certain gaps corresponding to 
possible but missing elements. He concluded that the atomic number 
of an element was a measure of its nuclear charge, and the correctness 
of this deduction has been recently verified by Chadwick by direct 
experiments on the scattering of « particles. Moseley’s discovery is 
of fundamental importance, for it not only fixes the number of electrons 
in all the atoms, but shows conclusively that the properties of an atom, 
as had been surmised, are determined not by its atomic weight but 
