Frederick Soddy Nobel Lecture

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which have been prepared in a pure state, and for which the spectrum and

atomic weight have been determined. As regards its whole chemical char-

acter radium is absolutely normal and its properties could have been predic-

ted accurately for an element occupying its place in the Periodic Table. It is

the last member of the family of alkaline earths and stands in relation to

thorium as thorium does to uranium in the periodic system. But superimposed

on this normal chemical character it possesses a radioactive character truly as-

tounding. The energy it evolves in the form of the new radiations, measured

as heat, amounts to 133 calories per gram per hour. The theory of atomic

disintegration shows however that the uniqueness of radium is due mainly

to the fact that its average life, 2440 years, is sufficiently long to allow the

element to accumulate in the minerals in which it is formed to a ponderable

quantity, and yet short enough for the rate at which the energy of disinte-

gration is liberated to be truly surprising. In the elucidation of the various

disintegration series of uranium, thorium and actinium, every degree of

atomic instability is encountered. The average life varies from the order of

1010 years, for the primary radio-elements uranium and thorium, down to

periods of the order of a minute, which marks the limit beyond which the

chemical character cannot be investigated. But by physical methods mem-

bers down to a period of 1/350 second have been put in evidence, and two,

of the order of 10

-6 

and 10

-11 

second, are indirectly inferred.

But for all these the theory indicates, as for radium, a perfectly normal

and definite chemical character as well as the superimposed radioactive

character. There is no progressive change in the nature of the atoms. As they

were when produced, by the explosion of the atom from which they origi-

nate, so they remain till they in turn explode. Otherwise the law of change

found to hold could not apply. The chemical and spectroscopic character is

that of the atom during its normal and uneventful life, whilst its radioactive

character is that produced by its sudden death. But for this very definite and

precise implication from the theory of atomic distintegration it might have

been supposed that the chemical character of such extraordinary substances

would also be extraordinary.

Possibly one other introductory remark may be justified, as to the kind of

evidence upon which the theory of atomic disintegration was originally

founded. In investigating the radioactivity of thorium compounds, we

found that a constituent responsible for the greater part of its radio-activity

could be separated by the use of specific reagents. Thus when thorium is

precipitated by ammonia, the thorium X remains in the solution, though

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374

   1921 F.SODDY

when other precipitating reagents, such as ammonium carbonate, oxalate or

phosphate, are employed no separation is effected. But this thorium X as

fast as it was removed reformed at a perfectly definite rate. Now if a chemist

were to purify, say, lead from silver and found as often as he purified it that

the silver reformed, he would have to conclude that lead was changing into

silver. The fact that this does not happen is at the basis of the belief in the

unchangeability of the elements. So the conclusion that in radioactivity the

elements are changing into others is equally direct. It is not a valid criticism

that the products are usually too small for chemical examination. If the

chemist referred to found that his lead after purification in due course

showed again all the usual analytical reactions for silver he would be per-

fectly satisfied, though anyone else might reasonably ask to be shown a

shilling made by the process and vouched for by the Mint. Radioactive

character provides as fine a criterion for quantitative and qualitative analysis

as any of the chemical and spectroscopic tests depended upon by the chemist.

It is as idle to expect the radio-chemist to produce his materials in quantity

sufficient for chemical examination before forming conclusions, as to expect

an analyst in the course of his work to separate and prepare the various con-

stituents of the materials he analyses in form and quantity capable of satis-

fying a jury with no chemical training.

The experimental method that first revealed isotopes

The history of isotopes fittingly commences with the discovery of radio-

thorium, a new product in the thorium disintegration series, by Sir William

Ramsay and Otto Hahn in 1905, which is intermediate between thorium and

thorium X and generates the latter in the course of an 

α 

-ray change. A

considerable quantity of thorianite, a newly discovered mineral with 60 to

70% of ThO

2 

and 10 to 20% of U

3

O

8

, was worked up for radium by the

known methods. The new substance was discovered during the fractional

crystallization of the barium-radium chloride and was separated from the

radium along with the inactive barium in the course of the fractionation.

In 1905 also the discovery by Godlewski of actinium X, an intermediate

product between actinium and its emanation, in every way analogous to

thorium X, was the commencement of the recognition of the general

analogies that existed in the three series. Even so early as 1905 I said*:

*In order as far as possible to avoid the charge of reading into this account of the

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