Frederick Soddy Nobel Lecture

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"The general resemblance in character and relative period of the succeed-

ing members of the various disintegration series on the one hand, and their

distinct individual peculiarities on the other, are most fascinating and mys-

terious. We seem to have here an extension of similar relations expressed

for elementary matter generally by the periodic law, and there is no doubt

that any explanation of the one will have an application to the other."

(1905, p. 308).

Reverting now to radiothorium, all attempts to separate it from thorium

compounds either failed completely or seemed successful to only a minute

extent. In 

1906 

Elster and Geitel and also G. A. Blanc, after having discover-

ed radiothorium as a constituent responsible for part of the radioactivity of

the sediments from certain hot springs, used the experience they had gained

in its separation in attempts to separate radiothorium from thorium com-

pounds, and were, apparently, to a very slight extent successful. Dadourian,

Boltwood and McCoy and Ross found that commercial thorium salts con-

tained only one-half as much radiothorium as the mineral from which they

extracted and as the thorium they themselves separated from minerals in the

laboratory. This seemed to indicate that the secret operations by which

thorium is commercially extracted were much more effective in separating

radiothorium than the known laboratory methods, but a search of the by-

products of the thorium manufacture failed to locate the missing radiotho-

rium.

In 1907 Hahn discovered mesothorium, a product intermediate between

thorium and radiothorium and generating the latter. In 1908 he showed that

mesothorium itself consisted of two successive products. The first, meso-

thorium 

1

, is produced directly from thorium and has an average life of

9.67 years. It is one of the two radio-elements, actinium being the other,

which disintegrates without the expulsion of any detectable radiation,

though in all probability a

 β 

-particle is expelled, but its velocity is too low

for it to be detectable. Its product, mesothorium 2, gives powerful 

β−

and

γ

-rays and has an average life of 8.9 hours.

In the meantime it was found that the radioactivity of the thorium com-

pounds prepared in the laboratory from the mineral decayed during two

history of isotopes more in connection with each advance than the state of knowledge

at the time warranted, I shall frequently quote literally my Annual Report on Radio-

activity, which I have contributed, since their commencement in 1904, to the Chemical

Society’s Annual Reports on the Progress of Chemistry, with the date of the Report and

page from which the quotation is taken in brackets at the end of the quotation.

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years from preparation to one-half and became the same as that of the com-

mercial preparations, whereas the latter increased in activity with age and

the older they were the more nearly their activity became equal to that

when freshly prepared from the mineral. In the process of separating the

thorium, mesothorium, but not radiothorium, is separated. The latter, pend-

ing the regeneration of the former, at first decays, and subsequently, as its

parent, mesothorium, begins to reaccumulate, is regenerated until finally

it reaches its original equilibrium value.

"The effect then had nothing to do with the method of preparation of the

thorium salt, but is purely a question of age." (1907, p. 325).

It was the first effect to be recognized of many similar ones subsequently.

For example in 1911 Antonoff discovered uranium Y, and atributed his

success in its separation to the particular chemical processes adopted, whereas

it was in reality due to the lapse of a suitable period of time between suc-

cessive separations.

Boltwood concluded from his experiments that mesothorium must be

removed from thorium along with thorium X when thorium is precipita-

ted by ammonia, the original method by which thorium X was discovered.

This year, 1907, marks the first definite statement of the doctrine of the

complete chemical non-separability of what are now called isotopes. McCoy

and R

OSS

I 

 

after describing many careful and prolonged attempts, all of them

completely without success, to separate radiothorium from thorium, said:

 "Our experiments strongly indicate that radiothorium is entirely inseparable

from thorium by chemical processes";

  

and they drew from this the correct

inference with regard to the radiothorium separated by Ramsay and Hahn

and others: "The isolation of radiothorium from thorianite and from pure

thorium nitrate... may have been accomplished by the separation of meso-

thorium, which in time changed spontaneously into radiothorium."

Thus already in 1907 the experimental method that first revealed the

existence of isotopes among the successive products of radioactive change

had been applied and its implications well understood.

"Although the separation of mesothorium from thorium and of thorium

X from radiothorium is easily accomplished, there is no known method of

separating by chemical means either radiothorium from thorium or thorium

X from mesothorium. But, owing to these last mentioned pairs alternating

in the disintegration series, each of them can readily be prepared by itself.

The preparations of radiothorium obtained by Hahn and others are probably

not the radiothorium existing in the original mineral, but regenerated radio-

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