14 THE PRESIDENTIAL ADDRESS. 
arrangements of the superficial electrons. While the solution of the 
grouping of the electrons proposed by Bohr has been assisted by con- 
siderations of this kind, it is not empirical in character, but has been 
largely based on general theoretical considerations of the orbits of 
electrons that are physically possible on the generalised quantum theory. 
The real problem involved may be illustrated in the following way. 
Suppose the gold nucleus be in some way stripped of its attendant 
seventy-nine electrons and that the atom is reconstituted by the succes- 
sive addition of electrons one by one. According to Bohr, the atom 
will be reorganised in one way only, and one group after another will 
successively form and be filled up in the manner outlined. The nucleus 
atom has often been likened to a solar system where the sun corresponds 
to the nucleus and the planets to the electrons. The analogy, however, 
must not be pressed too far. Suppose, for example, we imagined that 
some large and swift celestial visitor traverses and escapes from our 
solar system without any catastrophe to itself or the planets. There 
will inevitably result permanent changes in the lengths of the month 
and year, and our system will never return to its original state. Con- 
trast this with the effect of shooting an electron or « particle through the 
electronic structure of the atom. The motion of many of the electrons 
will be disturbed by its passage, and in special cases an electron may be 
removed from its orbit and hurled out of its atomic system. In a short 
time another electron will fall into the vacant place from one of the 
outer groups, and this vacant place in turn will be filled up, and so on 
until the atom is again reorganised. In all cases the final state of the 
electronic system is the same as in the beginning. This illustration 
also serves to indicate the origin of the X-rays excited in the atom, for 
these arise in the process of reformation of an atom from which an 
electron has been ejected, and the radiation of highest frequency arises 
when the electron is removed from the K group. 
It is possibly too soon to express a final opinion on the accuracy of 
this theory which defines the outer structure of the atom, but there can 
be no doubt that it constitutes a great advance. Not only does it offer 
a general explanation of the optical and X-ray spectra of the atom, but 
it accounts in detail for many of the most characteristic features of the 
periodic law of Mendeléef. It gives us for the first time a clear idea 
of the reason for the appearance in the family of elements of groups 
of consecutive elements with similar chemical properties, such as the 
groups analogous to the iron group and the unique group of rare earths. 
The theory of Bohr, like all living theories, has not only correlated a 
multitude of isolated facts known about the atom, but has shown its 
power to predict new relations which can be verified by experiment. 
For example, the theory predicted the relations which must subsist 
between the Rydberg constants of the arc and spark spectra, and generally 
