THE PRESIDENTIAL ADDRESS. 15 
between all the successive optical spectra of an element, a prediction so 
strikingly confirmed by Paschen’s work on the spectrum of doubly 
ionized aluminium and Fowler’s work on the spectrum of trebly ionized 
silicon. Finally, it predicted with such great confidence the chemical 
properties of the missing element, number 72, that it gave the necessary 
incentive for its recent discovery. 
While the progress of our knowledge of the outer structure of atoms 
has been much more rapid than could have been anticipated, we clearly 
see that only a beginning has been made on this great problem, and 
that an enormous amount of work is still required before we can hope 
to form anything like a complete picture even of the outer structure 
of the atom. We may be confident that the main features of the struc- 
ture are clear, but in a problem of such great complexity progress in 
detail must of necessity be difficult and slow. 
We have not so far referred to the very difficult question of the 
explanation on this theory of the chemical combination of atoms. In 
fact, as yet the theory has hardly concerned itself with molecular struc- 
ture. On the chemical side, however, certain advances have already 
been made, notably by G. N. Lewis, Kossel, and Langmuir, in the 
interpretation of the chemical evidence by the idea of shared electrons, 
which play a part in the electronic structure of two combined atoms. 
There can be little doubt that the next decade will see an intensified 
attack by physicists and chemists on this very important but undoubtedly 
very complicated question. 
Before leaving this subject, it may be of interest to refer to certain 
points in Bohr’s theory of a more philosophical nature. It is seen that 
the orbits and energies of the various groups of electrons can be specified 
by certain quantum numbers, and the nature of the radiation associated 
with a change of orbit can be defined. But at the same time we cannot 
explain why these orbits are alone permissible under normal conditions, 
or understand the mechanism by which radiation is emitted. It may 
be quite possible to formulate accurately the energy relation of the 
electrons in the atom on a simple theory, and to explain in considerable 
detail all the properties of an atom, without any clear understanding of 
the underlying processes which lead to these results. It is natural to 
hope that with advance of knowledge we may be able to grasp the details 
of the process which leads to the emission of radiation, and to understand 
why the orbits of the electrons in the atom are defined by the quantum 
relations. Some, however, are inclined to take the view that in the 
present state of knowledge it may be quite impossible in the nature of 
things to form that detailed picture in space and time of successive 
_ events that we have been accustomed to consider as so important a part 
of a complete theory. The atom is naturally the most fundamental 
_ Structure presented to us. Its properties must explain the properties 
1923 D 
