“Le? a eS.  .- 
ApRIL 25, 1912] 
NATURE 203 
THE ORIGIN OF RADIUM. 
pte theory of atomic disintegration, which affords 
a philosophical explanation of radio-activity, was 
based on simple chemical observations of the regenera- 
tion of radio-active constituents in substances from 
which they had been chemically separated, and not, 
as has sometimes been asserted, upon any physical or 
chemical theories as to the nature of the atoms of 
matter. Only two of the large number of new 
problems originally suggested by this theory remain 
at present unanswered. One had to do with the 
nature of the ultimate product or products of the 
distintegration of the atoms of the two primary 
elements, uranium and thorium. This problem may 
be likened to the task of trying to find a meteor after 
its flight, when its energy is spent and nothing but 
the matter remains. Much indirect evidence points 
to lead as the final product of uranium, although no 
direct proof has been obtained, whereas for the case 
of thorium there is still no hint of the answer. The 
other had reference to the origin of radium. This 
element in the intensity of its activity, and therefore 
in the rapidity of its disintegration, resembles the 
short-lived active constituents uranium X and thorium 
X, whilst in the apparent permanence of its activity 
it resembles the primary radio-elements. Even the 
first rough estimates indicated that the period of 
average life of radium was not greater than a few 
thousand years. The present estimate, due to Ruther- 
ford, is 2500 years. A few thousand years hence the 
radium in existence to-day will for the most part have 
disintegrated. Very little of the radium in existence 
at the time the Pyramids were being built can still 
exist. Hence arose one of the most interesting and 
crucial of the problems of atomic disintegration. 
Does the regeneration of radio-active constituents, 
observed in the cases where the period is short com- 
pared to the span of human life, apply also to radium 
—to an element, that is, with a definite spectrum, 
atomic weight and chemical character, filling a vacant 
place in the periodic system, and forming one of a 
family of common elements? After the separation of 
radium from a mineral does the non-radium part of 
the mineral grow a fresh crop with lapse of time, 
the quantity present before separation being the 
balance or equilibrium quantity when the rate of pro- 
duction is equal to the rate of supply? A somewhat 
similar prediction made with reference to the produc- 
tion of another well-defined element, helium, in the 
radio-active process had only to be tested, as it was 
first in 1903 by Sir William Ramsay and myself, to 
be proved correct. The question, however, of the 
origin of radium is still, in spite of many discoveries, 
not entirely solved. 
At first sight the experimental trial of the view 
appeared easy. This problem is not analogous to the 
finding of a meteor after its flight is spent. The quanti- 
ties of radium which can be detected and recognised 
unequivocally by radio-active methods are thousands 
of times smaller than can be detected even by the 
spectroscope, sensitive as the spectroscopic test of 
radium is. The first product of the disintegration of 
radium is a gas, the radium emanation, and the test 
for radium consists in sealing up a solution of the 
substance for a month, then boiling the solution in 
a current of air, and introducing this air into the 
electroscope. For the instrument employed and 
shown, a millionth of a milligram of radium would 
be rather an undesirably large quantity, whilst a few 
hundredths of this amount is the best suited for 
accurate measurement. The volume of radium 
1 From a discourse delivered at the Royal Institution on Friday, March 15, 
by Mr, Frederick Soddy, F.R S. 
NO. 2217, VOL. 89| 
emanation, measured at N.T.P., obtainable from one 
gram of radium is only 06 cubic millimetre, a volume 
comparable to that of a pin’s head. If a thousandth 
part of this quantity were distributed uniformly 
through the air of this room, estimated as 50,000 
cubic feet, or about 13 tons by weight, and the electro- 
scope were then filled with the air of the room, it 
would produce an effect much greater than any dealt 
with in the work to be described. (The effect of 
breaking a tube containing the emanation in equili- 
brium with 3 mg. of radium, outside in front of the 
fan supplying air to the building, was demonstrated 
by the electroscope, through which a slow current of 
air from the room was aspirated.) 
Since radium is found in uranium minerals and 
since uranium and thorium are the only elements 
known of atomic weight greater than that of radium, 
it was natural to suppose that uranium was _ the 
primary parent, in the disintegration of which radium 
results. Preliminary experiments nine years ago on 
a kilogram of uranyl nitrate, purified from radium by 
precipitating barium sulphate in the solution, proved 
that uranium could not be the direct parent of radium. 
For in this case, from 100 grams of uranium, the 
growth of radium should be readily detectable after 
the lapse of only a few hours. Whereas from a kilo- 
gram after 500 days, although a distinct increase of 
the quantity of radium was observed, it was at-most 
only 1/1000 part of what should have been formed. 
In the meantime, indirect, though conclusive, evidence 
that uranium was the primary parent of radium was 
obtained by McCoy, Strutt, and Boltwood, who 
showed that in all unaltered minerals there is a con- 
stant ratio between the quantities of the two elements, 
and this is what is to be expected if they are genetic- 
ally connected. Unfortunately, this is still the only 
evidence available of the connection between the two 
elements. To account for the excessively slow growth 
of radium in the first-uranium preparations studied it 
was necessary to suppose that between the uranium 
and radium an intermediate product existed of period 
of life great by comparison with the time of the 
experiment. Such a product would enormously retard 
the initial growth of radium. Its existence compli- 
cates what first appeared as a very simple problem in 
many other ways. It is no longer a question of simply 
detecting a growth of radium. It is necessary to 
measure the form of the growth-curve accurately. 
In the first place this intermediate parent must be 
present in uranium minerals, and therefore, to greater 
or less extent, in commercial uranium salts. The 
mere separation of radium therefrom initially, as in 
the first experiment, is not sufficient purification. In 
addition every trace of the intermediate parent must 
also be separated, or a growth of radium will not 
prove that uranium is the parent. On this account, 
in conjunction with Mr. T. D. Mackenzie, a fresh 
series of experiments was begun in Glasgow in 1905, 
in a new laboratory uncontaminated by radium. 
Three separate quantities, each initially of 1 kilogram 
of uranyl nitrate, were purified by repeated extraction 
with ether, which was considered to be the method 
most likely to separate all the impurities, not merely 
the radium. Observations on these preparations have 
now been in progress for six or seven years. At the 
same time a portion of the impure fraction separated 
from the original material was sealed up, freed from 
initial radium by the barium sulphate method, and 
tested for radium from time to time along with the 
pure uranium preparations. The diagram (Fig. 1) 
shows the growth of radium in this impure fraction. 
The unit used for expressing the quantity of radium 
is 10-12 gram. It confirms unequivocally the original 
