THE PRESIDENTIAL ADDRESS. Ya 
The speed of ejection of these nuclei depends on the velocity of the 
« particle and on the element bombarded. It is of interest to note that 
the hydrogen nuclei are liberated in all directions, but the speed in the 
backward direction is always somewhat Jess than in the direction of 
the « particle. Such a result receives a simple explanation if we sup- 
pose that the hydrogen nuclei are not built into the main nucleus but 
exist as satellites probably in motion round a central core. There can 
be no doubt that bombardment by « particles has effected a veritable 
disintegration of the nuclei of this group of elements. It is significant 
that the liberation of hydrogen nuclei occurs only in elements of odd 
atomic number, viz. 5, 7, 9, 11, 13, 15, the elements of even number 
appearing quite unaffected. For a collision of an « particle to be effec- 
tive, it must either pass close to the nucleus or actually penetrate its 
structure. The chance of this is excessively small on account of the 
minute size of the nucleus. For example, although each individual 
« particle will pass through the outer structure of more than 100,000 
atoms of aluminium in its path, it is only about one « particle in a 
million that gets close enough to the nucleus to effect the liberation of 
its hydrogen satellite. 
This artificial disintegration of elements by « particles takes place 
only on a minute scale, and its observation has only been possible by 
the counting of individual swift hydrogen nuclei by the scintillations 
they produce in zinc sulphide. 
These experiments suggest that the hydrogen nucleus or proton must 
be one of the fundamental units which build up a nucleus, and it seems 
highly probable that the helium nucleus is a secondary building unit 
- 
composed of the very close union of four protons and two electrons. 
The view that the nuclei of all atoms are ultimately built up of protons 
of mass nearly one and of electrons has been strongly supported and 
extended by the study of isotopes. It was early observed that some of 
the radioactive elements which showed distinct radioactive properties 
were chemically so alike that it was impossible to effect their separation 
when mixed together. Similar elements of this kind were called 
“isotopes ’ by Soddy, since they appeared to occupy the same place in 
the periodic table. For example, a number of radioactive elements in 
the uranium and thorium series have been found to have physical and 
chemical properties identical with those of ordinary lead, but yet to 
have atomic weights differing from ordinary lead, and also distinctive 
radioactive properties. The nuclear theory of the atom offers at once a 
simple interpretation of the relation between isotopic elements. Since 
the chemical properties of an element are controlled by its nuclear charge 
and liftle influenced by its mass, isotopes must correspond to atoms 
with the same nuclear charge but of different nuclear mass. Such a 
view also offers a simple explanation why the radiocative isotopes show 
D2 
