THE PRESIDENTIAL ADDRESS. 9 
of the electric and magnetic field surrounding them, such a decrease 
of mass is to be anticipated on general theoretical grounds. 
We must now look back again to the earlier stages of the present 
epoch in order to trace the development of our ideas on the detailed 
structure of the atom. That electrons as such were important con- 
stituents was clear by 1900, but little real progress followed until the 
part played by the positive charges was made clear. New light was 
thrown on this subject by examining the deviation of « particles when 
they passed through the atoms of matter. It was found that occa- 
sionally a swift « particle was deflected from its rectilinear path through 
more than a right angle by an encounter with a single atom. In such 
a collision the laws of dynamics ordinarily apply, and the relation 
between the velocities of the colliding atoms before and after collision 
are exactly the same as if the two colliding particles are regarded as 
perfectly elastic spheres of minute dimensions. It must, however, be 
borne in mind that in these atomic collisions there is no question of 
mechanical impacts such as we observe with ordinary matter. The 
reaction between the two particles occurs through the intermediary of 
the powerful electric fields that surround them. Beautiful photo- 
graphs illustrating the accuracy of these laws of collision between an 
% particle and an atom have been obtained by Messrs. Wilson, Blackett, 
and others, while Mr. Wilson has recently obtained many striking 
illustrations of coilisions between two electrons. | Remembering the 
great kinetic energy of the « particle, its deflection through a large 
angle in a single atomic encounter shows clearly that very intense 
deflecting forces exist inside the atom. It seemed clear that electric 
fields of the required magnitude could be obtained only if the main 
charge of the atom were concentrated in a minute nucleus. From this 
arose the conception of the nuclear atom, now so well known, in which 
the heart of the atom is supposed to consist of a minute but massive 
nucleus, carrying a positive charge of electricity, and surrounded at a 
idistance by the requisite number of electrons to form a neutral atom. 
A detailed study of the scattering of « particles at different angles, 
by Geiger and Marsden, showed that the results were in close accord 
with this theory, and that the intense electric forces near the nucleus 
varied according iv the ordinary inverse square law. In addition, the 
experiments allowed us to fix an upper limit for the dimensions of the 
nucleus. For a heavy atom like that of gold the radius of the nucleus, 
if supposed to be spherical, was less than one thousandth of the radius 
of the complete atom surrounded by its electrons, and certainly less 
than 4x10-" cms. All the atoms were found to show this nuclear 
structure, and an approximate estimate was made of the nuclear charge 
of different atoms. This type of nuclear atom, based on direct experi- 
mental evidence, possesses some very simple properties. It is obvious 
