202 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
nucleus of an element is about equal to its atomic number when ar- 
ranged in order of increasing atomic weight. Strong proof of the 
correctness of this point of view has been given by the work of Mose- 
ley on the X-ray spectra of the elements, for he has shown that the 
properties of an element are defined by a whole number which changes 
by unity in passing from one element to the next. It is believed that 
the lightest element, hydrogen, has a nuclear charge of 1, helium of 
2, lithium of 3, up to the heaviest element, uranium, of charge 92. 
Radioactive evidence indicates that the nucleus contains both posi- 
tively charged masses and negative electrons, the positive charge 
being in excess. Apart from the difficulty on the ordinary laws of 
electric forces of explaining why the nucleus holds together there is 
a fundamental difficulty of accounting for the stability of the ex- 
ternal electrons on the ordinary laws of dynamics. To overcome 
this difficulty Bohr has applied the quantum theory to define the 
position of the electrons and to account for the spectra of the lighter 
atoms and has made suggestions of the structure of the simpler 
atoms and molecules. Space does not allow me to discuss the im- 
portant developments that have followed from Bohr's theory by the 
work of Sommerfeld, Epstein, and others. The generalized theory 
has proved very fruitful in accounting in a formal way for many of 
the finer details of spectra, notably the doubling of the lines in the 
hydrogen spectrum and the explanation of the complex details of 
the Stark and Zeeman effects. In these theories of Bohr and his 
followers it is assumed that the electrons are in periodic orbital mo- 
tion round the nucleus and that radiation only arisen when the orbit 
of the electron is disturbed in a certain way. Recently Langmuir, 
from a consideration of the general physical and chemical properties 
of the elements, has devised types of atom in which the electrons 
are more or less fixed in position relatively to the nucleus like 
the atoms of matter in a crystal. It appears necessary, in Lang- 
muir's theory, to suppose that electrons, in addition to their elec- 
trical charges, are endowed with the properties of a magnetic 
doublet, so that at a certain distance the forces of attraction and 
repulsion between two electrons counterbalance one another. 
The whole question of the possible arrangements and motion of the 
external electrons in an atom or molecule still remains a matter of 
much doubt and speculation. "VVliile there are strong indications that 
the conception of the nucleus atom is in the main correct, we are 
still verj'^ uncertain of the laws controlling the position of the ex- 
ternal electrons on which the ordinary physical and chemical prop- 
erties depend. The study of the light spectra and also of the X-ray 
spectra already promise to throw new light on this very difficult 
but fundamental problem, 
