Frederick Soddy.

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line with the others, for, although a vast amount
of exact information has been obtained as to its
reactions, it would seem to remain hopeless ever
to obtain it in anything but infinitesimal amount
owing to its relatively very short period.

The chemistry of actinium has been enormously
simplified by the discovery that mesothorium-2 is
isotopic with it, for the latter may be used as an
indicator to show in what way the actinium distri-
butes itself after any chemical treatment. Owing


to its relatively small quantity as a branch product
and to the fact that, itself, it gives no rays, the
characteristic radioactivity of its products only
making- their appearance slowly after it has been
separated, actinium has always been a difficult
element to extract from the mineral and very easy
to lose in chemical operations. There is now, how-
ever, another reason which will assist in the study
of this element.


The generalisation has now led to the elucida-
tion of its origin and the discovery of its direct parent.
From its constant association with uranium minerals,
and the relative activity therein of its products in
comparison with the activity of those of radium,
it was considered to be a branch product of the
uranium series, only 8 per cent, of the atoms of
uranium disintegrating passing through the actinium
series and 92 per cent, through the radium series.
Its definite location in the periodic table, by virtue
of its isotopism with mesotnorium-2, made it clear
that its parent must either be in the radium or the
ekatantalum place, the former if it is produced in a
/8-ray change and the latter if it is produced in an
a-ray change.

The ekatantalum place was vacant when the
generalisation was first made, but it was necessary
to suppose that uranium - Y, like mesothorium, com-
prised two successive products, uranium-^ and
uranium-^T 2 i both giving /3-rays, and the latter occupy-
ing the vacant place in question. This prediction was
confirmed within a few weeks of its being made by
the discovery by Fajans and Gohring of uranium-^,
or brevium, a new member responsible for the
more penetrating /3-radiation given by uranium- X,


and having- a period of only 1-65 minutes. The
possibility that actinium was produced in a /3-ray
change from an isotope of radium was experimentally
disproved, and there remained only the second
alternative, which was rendered the more probable
by the existence of a member, uranium- K, dis-
covered by Antonoff, isotopic with uranium-J^,
and simultaneously produced with it from uranium
in relative quantity such as is to be expected, if
it were the first member of the actinium series.
Uranium- Y, like uranium-.^, gives soft /3-rays,
and hence its unknown product must be the isotope
of uranium- X z , and might also well prove to be
the unknown direct parent of actinium in an a-ray
change of long period.

During the year the missing element has been
found in two independent investigations (Soddy and
Cranston, Proc. Roy. Soc., 1918, \.A\ 94, 384;
O. Hahn and L. Meitner, Pkysikal. Zeitsch., 1918,
19, 208). The problem as it presented itself to us
was so to treat a uranium mineral as to separate an
element, if present, which possessed the chemical
character of the known but hopelessly short-lived
uranium-^, using the latter as an indicator in trying
possible methods beforehand. The method adopted,
distillation at an incipient red heat in a current of
carbon tetrachloride vapour and air, was found to
be very effective in volatilising uranium-.^ from
uranium-^, and when applied to pitchblende it was
found to give a product in which none of the known
pre-emanation members of the disintegration series
were present. Thus was obtained a preparation
from which actinium was at first absent, but which,
with the lapse of time, continuously generated
actinium, as characterised beyond the possibility of
doubt by means of its active deposit.

It should be mentioned that the exact point at


which the uranium series branches has not yet been
definitely ascertained, as there is a choice of alterna-
tives, at present experimentally indistinguishable.
Uranium- Y may be either the product of uranium-/
or of uranium-//, and the latter alternative, which is
that shown in the figure, is taken for the present as
likely to be on the whole the more probable. The
point can only be settled by the determination of the
atomic weight of ekatantalum or actinium.

Independently, Hahn and Meitner obtained the
parent of actinium from the insoluble siliceous
residues left after the treatment of pitchblende with
nitric acid by adding tantalum, and then separating
it and purifying it by chemical treatment. They
showed that it gave a-rays of range 3-314 cm. of air
at N.T.P., and, from this range, estimate its period
to be from io 3 to 2- io 4 years. There should therefore
be sufficient of the element in uranium minerals to
enable the spectrum, atomic weight, and chemical
character of the pure substance to be determined in
the same way as for radium. Its separation on a
large scale will enable actinium itself to be grown in
a pure state, analogously to the preparation of radio-
thorium from mesothorium, and so should allow the
spectrum at least of actinium to be found.

With regard to the period of actinium, there is at
present a real conflict of evidence, and so it is
impossible to say whether our knowledge of actinium
is ever likely to become as complete as that of
radium, or to remain, like that of polonium, confined
to what can be learned from infinitesimal quantities.
Cranston and I, on certain assumptions, concluded
from indirect evidence that the period of actinium
was 5000 years, but Hahn and Meitner, on the other
hand, state that they have obtained evidence con-
firming Mme. Curie's provisional estimate of the
period as about thirty years, from the direct obser-


vation of the decay of the radiations of a sealed
actinium preparation.


It is clear that the periodic law connects, not
primarily chemical character and atomic weight, but
chemical character and atomic charge or atomic
number, which alters its value by integers, not
continuously, producing the step-by-step changes in
chemical character which is at the basis of the
analysis of matter into the chemical elements, or
heterotopes. This atomic number is, however, the
algebraic sum of positive and negative charges, so
that the loss of the a-particle with its two positive
charges and of two negative electrons as /3-particles
leaves its value unchanged and produces an isotope
of the element having an atomic weight four units
less than the original. Unique chemical character
and unique spectrum reaction is no proof of homo-
geneity, and so we arrive at the conclusion that the
chemical elements, so far considered homogeneous,
may be mixtures of isotopes, possessing different
atomic structure and stability, revealed when they
undergo radioactive change, and in some cases also
different atomic weight. This, although within the
scope of the Daltonian analysis of matter to detect,
nevertheless, until radioactive investigations revealed
this possibility, remained overlooked. In two cases,
that of the isotopes of lead on the one hand, and of
ionium and thorium on the other, this difference of
atomic weight in elements spectroscopically and
chemically identical has now been established by
direct determinations.

The figure (facing p. 1 34) shows that, so far as these
changes have been followed, they all terminate in the
place occupied by lead, and, if this is the real, as dis-


tinguished from the apparent end in all cases, all the
ultimate products are isotopes of lead with atomic
weight between 210 and 206. The product of
radium- C 2 , in the branch claiming only 0-03 per
cent, of the whole ultimate product of radium, with
atomic weight 210, may be left out of account as
being negligible, and also the product of the actinium
branch for which the atomic weight is still uncertain ;
but the main products, namely, that of uranium with
atomic weight 206, and both the thorium products in
the two branches, with atomic weight 208, are
different in different directions from that of common
lead with atomic weight 207-2.

The conclusion that the ultimate product of
thorium, as well as of uranium, was lead, was quite
new and opposed to the opinion of those who had
made a special study of the Pb/U and Pb/Th ratios
of radioactive minerals of various geological periods.
I found, however, that the atomic weight of the
lead separated from Ceylon thorite was 207-7, and
Honigschmid confirmed this with a specimen of my
material and obtained the figure 207-77. Just
recently, from a specimen of lead separated from a
Norwegian thorite by Fajans and his co-workers, he
has found the value 207-90 (Zeitsch. Elektrochem.,
1918, 24, 163). Whereas the same investigator, and
also T. W. Richards and others, have found values
for the atomic weight of lead separated from uranium
minerals all lower than that of common lead, and in
two cases from carefully selected minerals between
206-0 and 206-1. I found my thorite lead was denser
than common lead in the same proportion as its
atomic weight was greater, and the densities of the
various specimens of uranium lead have been found
by Richards to be less than that of common lead, the
atomic volume for all varieties being constant. The
spectra of these various isotopes have been repeatedly


examined, but hitherto no differences whatever have
been established. 1

The atomic weight of a mixture of ionium and
thorium was found by Honigschmid to be 231-51 as
compared with 232-12 for thorium, the spectra being
identical and impurities absent in both specimens.
The calculated value for the atomic weight of ionium
is 230, and the evidence, so far as it yet goes, is in
accord with the view that, in the mixture examined,
about 30 per cent, was ionium and 70 per cent,
thorium. By a simple comparison of the emanating
power of the mixture with that of the pure thorium
preparation under similar conditions, the proportion
of ionium to thorium could be readily determined
directly, since ionium does not give an emanation,
and this unknown eliminated, but this has still to
be done.


When isotopes, such as those just considered,
possess different atomic weights, it is to be expected,
although this has not yet been practically accom-
plished, that a separation by physical means, such as
prolonged fractional diffusion, ought to be possible.
Chlorine and other elements, the atomic weights of
which depart largely from an integral value, seem to
deserve a further physical analysis by this method.
Sir J. J. Thomson's positive-ray method of gas
analysis ought to be able to detect such isotopes of
different atomic weight without separation, and at

1 Harkins and Aronberg (Proc. Nat. Acad. Set., 1917, 3, 710), for
ordinary lead and uranio-lead of atomic weight 206-34, examining
the strongest line, 4058, in the sixth order of spectrum obtained by a
lo-inch grating, observed a constant difference of 0-0043 A., but are
themselves disposed to await further results before drawing any
conclusions. This has now been confirmed (tf. T. R. Merton, Nature,
2nd October 1919).



one time it seemed that neon had been so resolved,
but this has not yet been confirmed. 1 It would be
interesting- also if the rotation of the salts of some
optically active acid with different varieties of lead,
separated from uranium and from thorium minerals,
were examined. A difference is to be expected,
although it is likely to be small, and possibly may be
too minute to be detectable. Recent experiments at
Harvard have shown that the refractive index of
a crystal of lead nitrate is independent of the atomic
weight of the contained lead, but the solubility, as is
to be expected, is different, the molar solubility of
different varieties being the same.

Isotopes need not, however, have different atomic
weights. One of the clearest cases is in the two end-
products of thorium, but, if the scheme is correct as
regards the branching point of the actinium series,
ionium and uranium- Y, actinium- A and radium- C',
actinium- C and radium-.Zr, actinium-./? and radium-/?,
and the actinium and uranium isotopes of lead, are
other cases. These result by branchings of the
series, and, since in the respective branches the
amount of energy evolved in the successive changes
is different, the internal energy of the various pairs
must be different, although for them atomic weight
as well as spectroscopic and chemical character are
all identical. I recently suggested in the case of the
two end-products of thorium that possibly only one
of these survives in geological time, namely, that
produced in the smaller quantity, and that the other
continues to break up in changes as yet undetected
(Royal Institution Lecture, i8th May 1917; Nature,
1917, 99, 414 and 433). This would account for the
relative poverty of thorium minerals in lead, which
was the original basis for the conclusion that lead

1 Mr Aston tells me this work is still being actively prosecuted at
the Cavendish Laboratory.


was not the ultimate product of thorium. The point
still remains experimentally untested. Isobaric
isotopes of the character in question can only at
present be distinguished if they are unstable and
break up further, but they must be taken into
account in any theoretical conception we form of
the ultimate structure of matter. The accomplish-
ment of artificial transmutation would reveal them
if they existed, and the discovery of any new prop-
erty, like radioactivity, concerned with the nucleus
of the atom rather than its external shell, might also
be the means of revealing differences of this character.

On the other hand, the production of isobaric
heterotopes is the ordinary consequence of /3-ray
changes, single or successive. Such heterotopes,
possessing different chemical and spectroscopic
character but the same atomic weight, have been
recently termed isobar es by A. W. Stewart (Phil.
Mag., 1918, [vi.], 36, 326), who, following Fleck's
work on the chemical resemblance, not amounting
to non-separability, between quadrivalent uranium
and thorium, has drawn a parallel between them
and elements existing in more than one state of
valency, as, for example, ferrous and ferric iron.

The extent to which the study of radioactive
change has enlarged the conception of the chemical
element may be summarised by the statement that
now we have to take into account in our analysis of
matter, not only the heterobaric heterotopes before
recognised, but also heterobaric and isobaric isotopes
and isobaric heterotopes or isobares.


I have attempted to present the most important
facts of radioactive change without introducing any
theory or hypothesis at all as to the structure of the


atom. I think it important to keep the two matters
distinct. Our knowledge of electricity, which in its
modern phase may be considered to start from the
relatively recent discovery of the electron, is still far
too imperfect to enable any complete theory of
atomic structure to be formulated. My task would
be incomplete, however, if I did not refer briefly to
the nuclear atom of Sir Ernest Rutherford, which
may be regarded as the logical descendant of the
earlier electronic atom of Sir J. J. Thomson. The
weakness of the latter was that it took account
essentially only of the negative electrons, and its
attempt to ascribe the whole mass of the atom to
these nearly massless particles involved the supposi-
tion that a single atom may contain hundreds of
thousands of electrons. The actual number is now
known to be rather less, as an average, than half
the numerical value of the atomic weight. Although
unsatisfactory in accounting for the mass of the
atom on an electronic basis, it was much more in
line with present views in accounting for chemical
character and the arrangement of elements in the
periodic table. The root idea that the successive
elements in the table are distinguished by the
increment of one electron in the outermost electronic
ring, followed, as period succeeds period, by the
completion of this ring and the formation of a new
external one, so that members of the same chemical
family have similar external ring systems, is still the
most probable view yet advanced. In conjunction
with the conception of the nucleus and the gradual
unravelling of the various series of characteristic
^T-radiations, both experimentally and by mathe-
matical analysis, it bids fair soon to give a definite
concrete picture of the structure of all the different
elements (compare L. Vegard, Phil. Mag., 1918, [vi.],
35, 293).


As regards the deepest region of atomic structure,
wherein radioactive phenomena originate, the nuclear
atom is the only one proposed that has any direct
experimental foundation. It is based on the deflec-
tions suffered by the a-particle in its passage through
the atoms of matter, on the one hand, as Bragg
showed many years ago, on the exceedingly slight
deviation of the overwhelming majority of the
a-particles, and, on the other, on the subsequently
discovered large deviations suffered by a minute
proportion. The nuclear atom is a miniature solar
system, like most model atoms, the negative electrons
occupying the atomic volume by their orbits around
a relatively excessively minute central sun or nucleus,
wherein the atomic mass is concentrated, and con-
sisting of an integral number of atomic positive
charges equal to the atomic number of the element,
and the number of electrons in the outer shell. An
a-particle is the nucleus of the helium atom, and,
unless it passes very near the nucleus of the atom
through which it penetrates, its path is practically
undeflected. The few that chance to pass close to
the exceedingly small but massive central nucleus
are swung out of their path like a comet at perihelion,
save that the forces at work are regarded as repulsive
rather than attractive.

It appears from radioactive change that atomic
disintegration occurs always in the central nucleus,
both a- and /3-particles originating therein. The
atomic number of the element is its nett nuclear
charge, the difference between the positive and
negative charges entering into its constitution. Of
all properties, mass and radioactivity alone depend
on the nucleus ; the physical and chemical character
and the spectrum of an element originate in the
outer shell. The character of the outer shell is fixed
by the nett charge, not at all by the mass or internal


constitution of the nucleus, and the integral variation
of this charge from i to 92 gives the successive
places of the periodic table. Expulsion of two 8-
and one a-particle in any order gives an isotope of
the original element with atomic weight four units
less. Isobaric isotopes resulting in branch changes
differ only in the internal structure and stability of
the nucleus. The atomic mass is the only nuclear
property known before the discovery of radioactivity,
and, except as regards this, the whole of physics and '
chemistry up to the close of the nineteenth century
had not penetrated beyond the outer electronic shell
of the atom. Even now, mass and radioactivity
remain the sole nuclear properties known.


Nemesis, swift and complete, has indeed over-
taken the most conservative conception in the most
conservative of sciences. The first phase robbed the
chemical element of its time-honoured title to be
considered the ultimate unchanging constituent of
matter ; but since its changes were spontaneous and
beyond the power of science to imitate or influence
to the slightest degree, the original conception of
Boyle, the practical definition of the element as the
limit to which the analysis of matter had been pushed,
was left essentially almost unchanged.

The century that began with Dalton and ended
with the discoveries of Becquerel and the Curies
took the existing practical conception of the chemical
element and theorised it almost out of recognition.
The element was first atomised, and then the atom
was made the central conception of the theory of the
ultimate constitution of matter, on which modern
chemistry has been reared, and from which its
marvellous achievements, both practical and theo-
retical, have mainly sprung. The atom and the


element became synonyms, related as the singular
to the plural, and implicit throughout this century
was the assumption that all the atoms of any one
element are identical with one another in every
respect. The only exception is in Sir William
Crookes's conception of " meta-elements " as applied
to the rare earths. Here the idea was rather that of
a gradual and continuous difference among the
different atoms of the same element, the properties
of the latter being the mean of those of its individual
atoms. Modern developments have tended definitely
away from rather than towards this view.

The second phase in the development of radio-
active change has now negatived each and every
one of the conceptions of last century that associated
the chemical element with the atom. The atoms of
the same chemical element are only chemically alike.
Unique chemical and spectroscopic character is the
criterion, not of a single kind of atom, but rather of
a single type of external atomic shell. Different
chemical elements may have the same atomic mass,
the same chemical element may have different atomic
masses, and, most upsetting of all, the atoms of the
same element may be of the same mass and yet be
an unresolvable mixture of fundamentally distinct
things. Present-day identity may conceal differences
for the future of paramount importance when trans-
mutation is practically realised. Then it may fce
found that the same element, homogeneous in every
other respect, may change in definite proportion into
two elements as different as lead and gold. The
goal that inspires the search for the homogeneous
constituents of matter is now known to be, like
infinity, approachable rather than attainable. The
word homogeneity can in future only be applied,
qualified by reference to the experimental methods
available for testing it.


All this, of course, does not in the least affect or
minimise the practical importance of the conception
of the chemical elements as understood before these
discoveries. Every chemist knows the conception
has had and will continue to have a real significance
as representing the limit of the spectroscopic and
chemical analysis of matter which remains, although
it now is known to convey something very different
from the original and natural conception of the
chemical elements as the / m ris of the material


THE feeling is gradually awakening in the con-
sciousness of the community, that the discoveries
and advances made by science in the past century
are not such as they have been accustomed to be
represented by people to whom they are a sealed
book, as important to money-making and trade,
for waging war and overtaking the heavy drudgery
of the world, but in an altogether different category
from humane studies. The scientific materialist in
seeking to understand the external physical universe,
and the relation in which men stand thereto, has
invaded territories which formerly the humanist and
theologian had to themselves, and made discoveries
which are essential to the understanding of modern
life and its problems. If it were necessary to make
choice between the old and the new in its relation
to the world of to-day, rather than in relation to
some remote childhood of the world, the knowledge
gained in the last hundred years surely is the part
of the whole of knowledge which could least be
spared. It is just this part which men who have
to govern modern peoples, administer the affairs
of present-day empires, and instruct and educate
the youth of the world, usually know least about.
That science has something to say apart from its

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Online LibraryFrederick SoddyScience and life: Aberdeen addresses → online text (page 11 of 18)