Frederick Soddy Nobel Lecture

380

   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

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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

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