28 SECTIONAL ADDRESSES 
times with ‘ isobares,’ that is atoms of the same weight but different 
chemical properties. Schemes of tabulation of all the known species 
have led to the prediction of isotopes and to theories of nuclear structure 
to account for their occurrence. 
Study of the relative abundance of isotopes in the mixture we still call, 
for convenience, an element, is of interest from two entirely different 
points of view. In the first place since it appears to be perfectly invariable 
in nature, not only in terrestrial but also in meteoric matter, there was 
a slight hope that a systematic measurement of abundance ratios might 
disclose some simple relations bearing on the great problem of how the 
nuclei of atoms were evolved. ‘The relative abundance of isotopes can 
be estimated by several methods but that of the most general application 
is the photometry of mass-spectra. A technique of this was worked out 
in 1929, and a number of elements examined, but the ratios, obtained in 
numbers large enough for statistical treatment, showed no groupings 
other than would have been expected from pure chance. ‘These measure- 
ments have a second important practical value. If we know the masses 
of the isotopes of an element and their relative abundance it is easy to 
calculate their mean weight. This, with proper corrections, can be used 
to check the chemical atomic weight. During the past six years nearly 
every atomic weight has been determined by this purely physical method, 
which has the great advantage of being, in general, independent of purity, 
and requiring an almost infinitesimal quantity of material. 
Instead of the original view that the nuclei of atoms consisted of protons 
and electrons, it is now considered more likely that they are built of 
protons and neutrons. In either case the binding forces holding the 
particles together must represent loss of energy, that is, loss of mass. 
Hence it is that the atom of hydrogen has abnormally high mass, and that 
the accurate determinations of divergences from the whole number rule 
are of such profound theoretical importance. As I have stated, my 
second mass-spectograph was designed for this and found capable of an 
accuracy, in favourable cases, of 1 in 10,000. The atom of oxygen 16 
was chosen as standard and the percentage divergences expressed in parts 
per 10,000, called ‘ packing fractions,’ were determined for a large number 
of elements. These, when plotted against mass number were found to 
lie roughly on a hyperbolic curve. This drops rapidly from hydrogen, 
passes through a minimum of about — 10 in the region of iron and nickel, 
and then rises gradually, crossing the zero line in the region of mercury. 
Our knowledge in this field has been notably increased by the brilliant 
work of Bainbridge, who set up at Swarthmore a powerful mass-specto- 
graph of an original design which made use of a velocity selector and semi- 
circular focussing. With this instrument he discovered new isotopes of 
tellurium, rectified results on zinc and germanium, and has made many 
of the most accurate comparisons of mass so far known. 
Fortunately for these comparisons, and particularly so for the extension 
of an accurate scale of mass to the heavy elements, particles occur in the 
discharge which carry more than one positive charge. A particle with 
two charges will give a line corresponding to half its mass, one with three 
charges will have an apparent mass of one third, and so on. These lines 
are called lines of the second, third and higher orders. The complex 
