gather round him. Additional to the names already mentioned, there were
A. J . Berry, who worked on rarefied gases; A. S. Russell whose early work
was on y-rays and who later was a contributor to the isotope development;
Miss R uth Pirret on uranium radium ratios; A. J . Cranston on the parent of
actinium; H. Hyman on the atomic weight of lead from thorium minerals.
There is a further type of scientific work which he regarded as most
valuable and that is the need of exposition of those newer ideas to the general
ity of well educated people. He inaugurated a series of free lectures on ‘The
interpretation of radium’ which drew large audiences and were much
appreciated. To the questioning critic it was sometimes a matter of wonder
as to how much of the science conceptions the generality of his audience took
away with them, but there is no doubt that the large number of lecture table
experiments which he carried out made a lasting impression. He was a
prolific lecturer and very popular.
Even at the time of the early Glasgow period he was liable to introduce
into his lectures some of the extraneous ideas to which he was prone.
For instance, in 1906, as President of the Rontgen Society, when giving a
lecture on the straightforward subject of ‘The present position of radio
activity’ (75), he could not restrain his zeal for doing some missionary work
for his financial ideas. The quotation ‘The point we have reached is that the
ideal money metal must be technically worthless’; and again ‘Now let us,
still in the position of visitors from outer space, survey the present economic
position of man with relation to money. I have alluded to the position occu
pied by gold in the human life . . .’ will serve to show the type and line of
thought which were prominent in his mind even at this early date and
which were among the main interests of his later years.
In the years at Glasgow the chemistry of the radio elements received a
good deal of attention and from these studies by him and by others, two
important inter-related but distinct conceptions evolved. The first was the
identity of chemical properties possessed by some radio elements with others,
i.e. the conception which came to be described as ‘isotopes’, and the second
was the movement through the periodic table as a radio-active element
emitted an a or a j3 ray. These movements were finally summed up in the
statement o f ‘The displacement law’.
The germs of both conceptions were in being for some time before they
were stated in anything like a final form with appropriate supporting
evidence.
After the discovery of radio thorium in 1905 by Ramsay and Hahn a good
deal of work was done on its chemistry and it was in connexion with this
element that McCoy and Ross (1907) stated ‘Our experiments strongly
indicate that radio thorium is entirely inseparable from thorium by chemical
processes.’ Examples of pairs of radio elements with extremely similar
chemical properties accumulated. Ionium was shown to be non-separable
from radium ; mesothorium from thorium X and radium D from lead. So far
as Soddy is concerned, an important stage in the concept was reached when
Frederick Soddy
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he realized the close relation between mesothorium and radium. That was in
1910 and two quotations may be made from his writing of that year—
‘When it is considered what a powerful means radioactive methods of
measurement afford . . . and the completeness and persistence of some of the
attempts at separation . . . the conclusion is scarcely to be resisted that we
have in these examples no mere chemical analogies but chemical identities.’
Again—‘The recognition that elements of different atomic weight may
possess identical chemical properties seems destined to have its most impor
tant application in the region of the inactive elements. . . . Chemical homo
geneity is no longer a guarantee that any supposed element is not a mixture
of several of different atomic weight or that any atomic weight is not merely a
mean number.’
Such quotations can show that the idea of elements of different atomic
weights having same chemical properties was one which took a long while to
germinate and to be examined exhaustively. His paper delivered in 1922—
‘The origins of the conception of isotopes’ (56) is a complete statement of all
the thoughts and work that led up to the formulation of the Displacement
Law. Otto Hahn (1957) in a recent publication, has paid an impressive
tribute to Soddy in this connexion—‘I had never succeeded in isolating
mesothorium or thorium X from radium. . . . Those negative attempts led
me to think that there was still greater chemical similarity between these
groups of elements than was known to exist between the various so-called
rare earths. I should never have had the courage to declare that these were
chemically identical elements. But Frederick Soddy possessed that courage.
He created the expression “isotopes” . . . . Thus suddenly all the difficulties
disappeared. It is not the atomic weight of an element, but its nuclear charge
which determines its place in the “periodic table” .’
It was towards the end of 1911 that Soddy asked me to examine syste
matically as many as possible of the radio elements whose chemical charac
teristics had not then been determined. The series of results were published
in 1912 and 1913 and included uranium X and thorium, radio actinium
and thorium and, what was probably most important of all, they established
the identity of thorium j8 and lead. Corresponding results showed that other
radio elements were identical in chemical properties with bismuth and
thallium.
It was a thorough examination of these results which called forth and justi
fied the identity declaration referred to above by Hahn.
Up to 1913 we used the phrase ‘radio elements chemically non-separable’
and at that time the word isotope was suggested in a drawing-room dis
cussion with Dr Margaret Todd in the home of Soddy’s father-in-law, Sir
George Beilby. Dr Todd was an Edinburgh trained medical doctor and was
a writer of some distinction under the name of Graham Travers. The
readiness and etymological accuracy with which she produced ‘isotopes’ is
a standing testimony to her practical knowledge of the Greek tongue.
In 1922 Soddy published a very full paper in Stockholm on the occasion
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