26 SECTIONAL ADDRESSES. 
by the number of electrons requisite to maintain the structure electri- 
cally neutral. The nucleus contains all but about one-two-thousandth 
part of the mass of the atom, and its electric charge is numerically 
equal to that of the negative electron multiplied by what is called the 
atomic number of the atom, the atomic number being the number 
which is obtained when the chemical elements are enumerated in the 
order of the atomic weights; thus hydrogen=1, helium=2, lithium=3, 
and so on. Consequently the number of external electrons in the 
atom is also equal to the atomic number. The evidence, derived from 
many distinct and dissimilar lines of inquiry, which makes it necessary 
to accept the foregoing statements as facts, will be familiar to members 
of this Section of the British Association, which has continually been 
in the forefront of contemporary adyances in physical science. But 
I would remind you in passing that one of the important pieces of 
evidence was supplied by Professor Barkla’s researches on the scattering 
of X-rays by light atoms. 
The diameters of the nuclei of the atoms are comparable with one- 
millionth of one-millionth part of a centimetre, and the problem of 
finding what lies within the interior of such a structure seems at first 
sight almost hopeless. It is to this problem which Rutherford has 
addressed himself by the direct method of bombarding the nuclei of the 
different atoms with the equally minute high-velocity helium nuclei 
(alpha-particles) given off by radioactive substances, and examining 
the tracks of any other particles which may be generated as a result 
of the impact. A careful and critical examination of the results shows 
that hydrogen nuclei are thus expelled from the nuclei of a number of 
atoms such as nitrogen and phosphorus. On the other hand, oxygen 
and carbon do not eject hydrogen under these circumstances, although 
there is evidence in the case of oxygen and nitrogen of the expulsion 
of other sub-nuclei whose precise structure is a matter for further 
inquiry. 
The artificial transmutation of the chemical elements is thus an 
established fact. The natural transmutation has, of course, been 
familiar for some years to students of radioactivity. The philosopher’s 
stone, one of the alleged chimeras of the medizval alchemists, is thus 
within our reach. But this is only part of the story. It appears that 
in some cases the kinetic energy of the ejected fragments is greater 
than that of the bombarding particles. This means that these bombard- 
ments are able to release the energy which is stored in the nuclei of 
atoms. Now, we know from the amount of heat liberated in radio- 
active disintegration that the amount of energy stored in the nuclei 
is of a higher order of magnitude altogether, some millions of times 
greater, in fact, than that generated by any chemical reaction such 
as the combustion of coal. In this comparison, of course, it is the 
amount of energy per unit mass of reacting or disintegrating matter 
which is under consideration. The amounts of energy which have thus 
far been released by artificial disintegration of the nuclei are in them- 
selves small, but they are enormous in comparison with the minuté 
amounts of matter affected. If these effects can be sufficiently inteni- 
sified there appear to be two possibilities.  Hithei they will prove 
