8 THE PRESIDENTIAL ADDRESS. 
may be regarded as a disembodied atom of negative electricity. We 
know that an electron in motion, in addition to possessing an electric 
field, also generates a magnetic field around it, and energy in the 
electromagnetic form is stored in the medium and moves with it. This 
gives the electron an apparent or electrical mass which, while nearly 
constant for slow speeds, increases rapidly as its velocity approaches 
that of light. This increase of mass is in good accord with calculation, 
whether based on the ordinary electrical theory or on the theory of 
relativity. Now we know that the hydrogen atom is the lightest of all 
atoms, and is presumably the simplest in structure, and that the 
charged hydrogen atom, which we shall see is to be regarded as the 
hydrogen nucleus, carries a unit positive charge. It is thus natural 
to suppose that the hydrogen nucleus is the atom of positive electricity, 
or positive electron, analogous to the negative electron, but differing 
from it in mass. Electrical theory shows that the mass of a given 
charge of electricity increases with the concentration, and the greater 
mass of the hydrogen nucleus would be accounted for if its size were 
much smaller than that of the electron. Such a conclusion is sup- 
ported by evidence obtained from the study of the close collisions of 
« particles with hydrogen nuclei. It is found that the hydrogen nucleus 
must be of minute size, of radius less than the electron, which is 
usually supposed to be about 10-'* cms. ; also the experimental evidence 
is not inconsistent with the view that the hydrogen nucleus may 
actually be much smaller than the electron. While the greater mass 
of the positive atom of electricity may be explained in this way, we are 
still left with the enigma why the two units of electricity should differ 
so markedly in this respect. . In the present state of our knowledge it 
does not seem possible to push this inquiry further, or to discuss the 
problem of the relation of these two units. 
We shall see that there is the strongest evidence that the atoms 
of matter are built up of these two electrical units, viz. the electron 
and the hydrogen nucleus or proton, as it is usually called when it 
forms part of the structure of any atom. It is probable that these two 
are the fundamental and indivisible units which build up our universe, 
but we may reserve in our mind the possibility that further inquiry 
may some day show that these units are complex, and divisible into 
even more fundamental entities. On the views we have outlined 
the mass of the atom is the sum of the electrical masses of the individual 
charged units composing its structure, and there is no need to assume 
that any other kind of mass exists. At the same time, itis to be borne 
in mind that the actual mass of an atom may be somewhat less than 
the sum of the masses of component positive and negative electrons 
when in the free state. On account of the very close proximity of the 
charged units in the nucleus of an atom, and the consequent disturbance 
