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THE PRESIDENTIAL ADDRESS. 5 
structure of atoms which the periodic variation of the chemical pro- 
perties had indicated. It gave for the first time some hope of the 
success of an attack on that most fundamental of all problems—the 
detailed structure of the atom. In the early development of this subject 
science owes much to the work of Sir J. J. Thomson, both for the 
boldness of his ideas and for his ingenuity in developing methods for 
estimating the number of electrons in the atom, and of probing its 
structure. He early took the view that the atom must be an electrical 
structure, held together by electrical forces, and showed in a general 
way lines of possible explanation of the variation of physical and 
chemical properties of the elements, exemplified in the periodic law. 
In the meantime our whole conception of the atom and of the 
magnitude of the forces which held it together were revolutionised by 
the study of radioactivity. The discovery of radium was a great step 
in advance, for it provided the experimenter with powerful sources of 
radiation specially suitable for examining the nature of the characteristic 
radiations which are emitted by the radioactive bodies in general. It 
was soon shown that the atoms of radioactive matter were undergoing 
spontaneous transformation, and that the characteristic radiations 
emitted, viz. the «, 8, and y rays, were an accompaniment and conse- 
quence of these atomic explosions. The wonderful succession of changes 
that occur in uranium, more than thirty in number, was soon disclosed 
and simply interpreted on the transformation theory. The radioactive 
elements provide us for the first time with a glimpse into Nature’s 
laboratory, and allow us to watch and study but not control the 
changes that have their origin in the heart of the radioactive atoms. 
These atomic explosions involve energies which are gigantic compared 
with those involved in any ordinary physical or chemical process. In 
the majority of cases an « particle is expelled at high speed, but in 
others a swift electron is ejected often accompanied by a y ray, which is 
a very penetrating X-ray of high frequency. The proof that the « 
particle is a charged helium atom for the first time disclosed the import- 
ance of helium as one of the units in the structure of the radioactive 
atoms, and probably also in that of the atoms of most of the ordinary 
-elements. Not only then have the radioactive elements had the greatest 
direct influence on natural philosophy, but in subsidiary ways they 
have provided us with experimental methods of almost equal import- 
ance. The use of « particles as projectiles with which to explore the 
interior of the atom has definitely exhibited its nuclear structure, has 
led to artificial disintegration of certain light atoms, and promises to 
yield more information yet as to the actual structure of the nucleus 
itself. 
The influence of radioactivity has also extended to yet another field 
of study of fascinating interest. We have seen that the first rough 
