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1921 F. SODDY
and prepared a very concentrated ionium preparation, upon the spectrum
and atomic weight of which other investigators subsequently worked. He
failed completely to separate the ionium from the thorium, though many
fresh methods were tried.
Herschfinkel tried numerous methods to concentrate radium D (radio-
lead) from lead and failed completely. Three years later, Paneth and von
Hevesy tried twenty different methods, also completely without result, and
upon this failure they based their valuable and elegant method of using the
radio-elements as indicators. For example, in the present case, the solubility
of very insoluble lead compounds was determined by adding radium D to
the lead, and determining the almost unweighable quantities of lead dis-
solved by radioactive methods. This use has wide applicability in chemical
problems. In this instance, radium D can be obtained free from lead, if re-
quired, by sealing up the radium emanation in a capillary tube and allowing
it to change. After a month, the conversion of the emanation into practically
pure radium D is complete. In this way they were able in 1914 to extend the
chemical identity between radium D and lead to its whole electrochemical
character.
In contrast with the foregoing cases, the cases of polonium (radium F) and
actinium may be mentioned. Although these radio-elements have never yet
been isolated in the pure state, nor even their spectra yet determined, this is
solely, probably, on account of their relatively short period, and conse-
quently infinitesimal quantity. Thus polonium was shown by Mme. Curie
to resemble bismuth closely, and by Marckwald tellurium. But it may
readily be concentrated by chemical methods from both bismuth and tellu-
rium. Actinium was shown by Giesel to resemble lanthanum most closely of
the rare-earth elements, but he succeeded in effecting a partial concentration
of actinium from lanthanum.
In 1909, Sweden made a very notable contribution to this subject in the
work of Strömholm and Svedberg, who made use of isomorphism as a
means of ascertaining the chemical character of the radio-elements. They
crystallized in the solutions of the radio-elements different salts and deter-
mined whether or not the radio-element crystallized out with the salt. Thus
they found that thorium X crystallizes with lead and barium salts, but not
with others. They correctly characterized it as an alkaline-earth element, in
contradiction of an earlier and incorrect conclusion that it was monovalent,
arrived at from a study of its diffusion coefficient and ionic mobility. Ac-
tinium X was also characterized as an alkaline-earth element, and they were
O R I G I N S O F T H E C O N C E P T I O N S O F I S O T O P E S
381
unable to find, by these methods, any differences in the chemical behaviour
of radium, thorium X and actinium X, even in their quantitative aspect.
Strömholm and Svedberg were probably the first to attempt to fit a part
of the disintegration series into the Periodic Table. They were correct as
regards thorium X and actinium X, but were misled by a faulty charac-
terization of the chemistry of mesothorium I. It is probable that the reason
for this was that already referred to, which prevented mesothorium being
discovered when thorium X was, namely, that it has been unwittingly sep-
arated in the preliminary purification of the material. Nevertheless, in their
conclusion, is to be found the first published statement, that the chemical
non-separability found for the radio-elements may apply also to the non-
radioactive elements in the Periodic Table. Remarking on the fact that, in the
region of the radio-elements, there appear to be three parallel and independ-
ent series, they then say "one may suppose that the genetic series proceed
down through the Periodic Table, but that always the three elements of the
different genetic series, which thus together occupy one place in the Periodic
System, are so alike that they always occur together in Nature and also not
have been able to be appreciably separated in the laboratory. Perhaps, one
can see, as an indication in this direction, the fact that the Mendeleev scheme
is only an approximate rule as concerns the atomic weight, but does not
possess the exactitude of a natural law; this would not be surprising if the
elements of the scheme were mixtures of several homogeneous elements of
similar but not completely identical atomic weight." Thus Strömholm and
Svedberg were the first to suggest a general complexity of the chemical
elements concealed under their chemical identity. Until I read their paper
again, in the preparation for this lecture, I had not realized how explicit this
anticipation of present views is.
In 1910 a new and, as it proved, very important case of chemical identity
was discovered, that between mesothorium
I
and radium. This was pub-
lished independently by Marckwald and myself, though it must have pre-
viously been known to Hahn and those engaged in the technical extraction
of mesothorium, who, however, kept it secret. Marckwald made the dis-
covery through being asked to examine a radium preparation prepared
from a thorium-containing uranium mineral, which owed the greater part
of its activity to mesothorium. I worked it out independently for prepara-
tions I obtained from thorianite. Contrary-to the impression, which then
prevailed, that barium sulphate precipitated in the solution entrains or adsorbs
mesothorium, as it does uranium X, I found it impossible to separate the