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THE PRESIDENTIAL ADDRESS. i 
by its nuclear charge. A relation of unexpected simplicity is thus 
found to hold between the elements. No one could have anticipated 
that with few exceptions all atomic numbers between hydrogen 1, and 
uranium 92, would correspond to known elements. The great power 
of Moseley’s law in fixing the atomic number of an element is well 
illustrated by the recent discovery by Coster and Hevesy in Copen- 
hagen of the missing element of atomic number 72, which they have 
named ‘ hafnium.’ 
Once the salient features of the structure of atoms have been fixed 
and the number of electrons known, the further study of the structure 
of the atom falls naturally into two great divisions: one, the arrange- 
ment of the outer electrons which controls the main physical and 
chemical properties of an element, and the other the structure of the 
nucleus on which the mas; and radioactivity of the atom depends. On 
the nuclear theory the hydrogen atom is of extreme simplicity, con- 
sisting of a singly-charged positive nucleus with only one attendant 
electron. The position and motions of the single electron must account 
for the complicated optical spectrum, and whatever physical and 
chemical properties are to be attributed to the hydrogen atom. The 
first definite attack on the problem of the electronic structure of the 
atom was made by Niels Bohr. He saw clearly that, if this simple 
constitution was assumed, it is impossible to account for the spectrum 
of hydrogen on the classical electrical theories, but that a radical depar- 
ture from existing views was necessary. For this purpose he applied 
to the atom the essential ideas of the Quantum Theory which 
had been ‘developed by Planck for other purposes, and had been 
found of great service in explaining many fundamental difficulties in 
other branches of science. On Planck’s theory radiation is emitted 
in definite units or quanta, in which the energy E of a radiation is 
equal to hv where v is the frequency of the radiation measured by the 
ordinary methods and h a universal constant. This quantum of radia- 
tion is not a definite fixed unit like the atom of electricity, for its 
magnitude depends on the frequency of the radiation. For example, 
the energy of a quantum is small for visible light, but becomes large 
for radiation of high frequency corresponding to the X-rays or the 
¥ rays from radium. 
Time does not allow me to discuss the underlying meaning of the 
quantum theory or the difficulties connected with it. Certain aspects 
of the difficulties were discussed in the Presidential Address before this 
Association by Sir Oliver Lodge at Birmingham in 1913. It suffices 
to say that this theory has proved of great value in several branches 
of science, and is supported by a large mass of direct experimental 
evidence. 
In applying the quantum theory to the structure of the hydrogen 
