Frederick Soddy.

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of these sub-atomic processes were as possible as
is the control of ordinary chemical changes, such
as combustion, would far exceed in importance and
value the gold. Rather it would pay to transmute
gold into silver or some base metal.

War, unless in the meantime man had found a
better use for the gifts of science, would not be the
lingering agony it is to-day. Any selected section of
the world, or the whole of it if necessary, could be
depopulated with a swiftness and dispatch that would
leave nothing to be desired.

Indeed in the whole tragic history of the past few
years nothing has been perhaps more illuminating
than the attitude of the world and its rulers to
science. The intellectual aspect of the discoveries
here briefly enumerated the discovery of radio-
activity, the realisation that it was due to a natural
transmutation of the elements, the laborious tracing
out, step by step, of the complicated sequence of
changes, the discovery of the law connecting these
changes with the Periodic Table, the first real
understanding as to what constitutes the difference
between one element and another, the vista that
opens out should man ever exercise over these higher
order of natural energy the control he has so
effectively assumed over the lower interesting



108 THE EVOLUTION OF MATTER

perhaps, but what is the use of it all? There is
a rumour, puffed judiciously in the press, that radium
is a cure for cancer, and immediately there is a
change. Stock exchanges get up radium, wild-cat
mining- schemes are floated, the public are invited to
get rich quickly, and every quack and charlatan, with
his radium ointment, radium pills, and radium waters,
refurbishes his familiar propaganda. The charitable
and benevolent, to whom the cry of suffering and the
dying ever make its irresistible appeal, raise the
funds to buy the radium. The genuine scientific
investigator can no longer afford to, and goes
without.

Again the scene changes and the country is
spending nearly ^100 every second on the war.
Radium, like every other gift of science, is pressed
into the service of the war, as it is convenient for
illuminating the dials of watches and scientific
instruments at night, and the State, which before as
regards anything productive or creative did not
exist, must now afford anything for the purpose of
destruction. Men, materials, and capital must be
conscripted and organised to the last point for the
purposes of occasional international strife.

But there is a struggle which is world-wide and
never-ending, the struggle against external nature
for control and mastery. The millions take no part
in it, are hardly aware that it goes on, and would be
surprised if they were told that their future fate and
prosperity depended upon it rather more intimately
than upon the issue of the doughty conflicts of the
parliamentarians some of them send up to West-
minster. Neither, again, would the mere alteration
in the character of their education, making it scientific
rather than classical, alone bring them salvation.
For this struggle is by duel rather than by armies,
and the issue of the duel the millions accept as blindly



THE MODERN DUEL 109

and dumbly as a decree of Providence. Enormous
tracts of the British Empire are uninhabitable by
white men by reason of malaria and yellow fever. It
is the will of Allah. A solitary duellist l against the
unknown and not understood confronted Nature.
A single intelligence in the teeth of official apathy
and neglect sought the "million murdering cause,"
and found it. In India alone more than a million
people died yearly from malaria before its cause and
remedy were ascertained. The Panama Canal owes
its successful construction to the work of this solitary
individual in Bangalore, diligently followed up by
others. Praise be to Allah!

The future of the British Empire is at the moment
in the hands of five million stalwart men, with an
organised nation of workers and vast accumulations
of wealth and resources and every possible scientific
discovery and invention behind to back them up. If
the nation thinks, when peace returns, that the
struggle against Nature, which after all is of more
abiding and permanent interest to its destiny, large
as the present contest looms to-day, can be best
carried on in the old way by a handful of isolated
individuals as a sort of hobby in their spare time, out
of their own means, and in the intervals of more
urgent professional duties, the nation is mad.

[The war being now over, it is not out of place
to add that an even greater danger than neglect
awaits the scientific investigator, the danger that
he along with every other creative element in the
community will be remorselessly shackled and ex-
ploited to bolster up the present discredited social
system. There is abundant evidence since the war
that science rules the world, and he who would aspire
to rule it must first rule science. The prospect of
creative science under the heel of government depart-

1 Sir Ronald Ross.

Q



110 THE EVOLUTION OF MATTER

ments ruled by lawyers, politicians, financiers and
administrators of the modern official type is a prospect
as appalling as the handing over of civilisation to the
Hun. But in the modern world the community
somehow must contrive to rule through its creative
elements, rather than to allow the non-creative
elements to rule the creative. Everything comes
back to the unsolved problem of how to purify and
strengthen the moral and ethical standards of the
official classes, which have been so sadly perverted
by their peculiar system of education, in order to
make them conform more nearly to the standards
of conduct and honesty entertained by the majority
of ordinary respectable and benevolent people.]



THE CONCEPTION OF THE CHEMICAL
ELEMENT AS ENLARGED BY THE
STUDY OF RADIOACTIVE CHANGE 1

THE Council of the Chemical Society have honoured
me with the invitation to deliver one of three lectures
bearing on the ultimate constitution of matter, and
I accepted the invitation in my desire to show how
greatly I appreciated it rather than with any prospect
of being enabled, when the time came, to say any-
thing on the subject which has not already been said
before. The problem of the ultimate constitution of
matter belongs to another world than that through
which for the past four years we have been living-,
and although hostilities have at length ceased, and
we may look forward to an opportunity of resuming-
in the future the thread of our pm'losophical investi-
gations, philosophy herself is not so easily to be
resumed. Novel in one sense as are the ideas intro-
duced into the concepts of physics and chemistry by
the study of radioactivity, four years' interruption
has made them appear rather as a remote historical
accomplishment than as a contemporaneous develop-
ment. Although no longer new, however, the more
as the subject matures does it become apparent that
these advances are of fundamental and increasing-
importance to the chemist.

One would perhaps have expected that on the

1 A Lecture delivered before the London Chemical Society on
igth December 1918.
ni



112 CONCEPTION OF THE CHEMICAL ELEMENT

first and most fundamental conclusion arrived at in
the study of radioactive change that the change is of a
transmutational character, involving the spontaneous
disintegration of the radio-element into others, it
would have been the chemists who would have been
most deeply interested, and who would have weighed
the evidence and pronounced a decision. Yet judg-
ment on the view, which was put forward more than
fifteen years ago, on evidence, in my opinion, even
then deserving of serious consideration, although
accepted and universally adopted by the workers in
the subject and by physicists, has gone by default so
far as the majority of chemists are concerned. From
the first, much of the most important evidence has
been of a singularly simple and convincing chemical
character.



THE TRANSMUTATIONAL CHARACTER OF RADIO-
ACTIVE CHANGE.

If a chemist were to purify an element, say lead
from silver, and found, on re-examining the lead at
a later date, that silver was still present, and, again
and again repeating the process, found always that
silver, initially absent, reappeared, would he not be
forced to conclude that lead was changing into silver
and that silver was being produced by lead? It is
because of the absence of evidence of this kind
that the doctrine of the unchangeability of the
elements has grown up. One positive example of
the kind in question and that doctrine would be at
an end. The conclusion to which, in 1902, Sir Ernest
Rutherford and I were forced with regard to the
element thorium was based on evidence of this
direct and simple nature. By simple purification,
by chemical and physical means, constituents respon-
sible for the greater part of the radioactivity of



THORIUM PRODUCTS 113

thorium can be separated, and as often as they are
separated they are regenerated at a perfectly definite
and regular rate. One of these constituents, the
emanation, is gaseous, and it can be separated from
the thorium by no more elaborate means than by a
puff of air. Certainly the actual quantity of thorium
emanation is infinitesimal, but this did not hinder its
complete chemical characterisation, for it was found
to pass unabsorbed through every reagent tried, one
or other of which would have absorbed every known
gas with the exception of the gases of the argon
family. The conclusion that the thorium emanation
was a gas of the argon family produced by thorium,
later extended to the similar gaseous products of
radium and actinium, was a purely experimental
conclusion reached before any theory whatever as to
the nature of radioactivity had been advanced.

Another constituent responsible for part of the
radioactivity we called thorium- X. It is left in the
filtrate when a solution of thorium is precipitated
with ammonia, although not when the thorium is
precipitated by other reagents, such as sodium
carbonate or phosphate. After this removal, how-
ever, thorium-A" re-forms in the thorium. Moreover,
it is thorium-^, not thorium, that produces the
emanation. The latter in turn produces the non-
volatile active deposit, in which the successive
products, called thorium-^4, -B, -C, and -D, are now
recognised. The false interpretation of a similar
phenomenon in the case of radium, before the radium
emanation had been recognised, led to the view that
inactive matter could be rendered temporarily radio-
active by "induction," through contact with or
association with radioactive matter. In the case of
thorium, the discovery of the chemical character of
the thorium emanation rendered the nature of the
phenomenon clear almost from the first.



114 CONCEPTION OF THE CHEMICAL ELEMENT

This, taken in conjunction with the atomic
character of radioactivity, recognised by Mme.
Curie from the start, and with the fact that the law
of radioactive change proved to be the same as the
law of unimolecular reaction, made the conclusion
that the radio-elements were undergoing a series of
successive changes, in which new elements are pro-
duced, of chemical and physical character totally
distinct from those of the parent element, the only
one capable of explaining the facts.

Novel and unexpected as it was to find transmu-
tation spontaneously in progress among the radio-
elements, the phenomena this explanation explained
were equally novel and transcended what to a
generation ago would have appeared to be the limits
of the physically possible.

It is to pay chemistry a poor compliment to
represent this conclusion as in any way contrary to
the established foundations of chemistry. If it had
6ot been for the correct conception of the nature of
chemical change, the clear distinction between atoms
and molecules, and the conclusion that in all changes
in matter hitherto studied the element and the atom
of the element remain essentially unchanged, which
we owe to the founders of chemistry, the character
of radioactivity would not have been arrived at so
quickly. On the other hand, if radioactivity had
not been almost instantly recognised as a case of
spontaneous transmutation, then, if you will, there
would have been something radically wrong with
chemistry and the training it affords in the elucida-
tion of the metamorphoses of matter.

With regard, however, to the various claims that
have been made since, that transmutational changes
can be artificially effected by the aid of the electric
discharge in gases or the rays from radium, I have
always regarded the evidence in this field as capable



RADIOACTIVE CHANGE UNIQUE 115

of simple alternative explanation. Different investi-
gators have obtained entirely opposite results, and
there is not that consensus of evidence one finds
among those who have investigated radioactive
change.

In another direction there has been a tendency to
underrate the unique and unparalleled phenomenon
of radioactive change, and to connect what is entirely
and solely a development of the new experimental
science of radioactivity with the somewhat older
isolation of the electron and the electronic hypotheses
of the constitution of matter to which that discovery
has given rise. For example, Sir J. J. Thomson
in his Romanes Lecture, 1914, says: "Since the
electron can be got from all the chemical elements
we may conclude that electrons are a constituent of
all the atoms. We have 'thus made the first step
towards a knowledge of the structure of the atom
and towards the goal towards which since the time
of Prout many chemists have been striving, the proof
that the atoms of the chemical elements are all built
up of simpler atoms primordial atoms, as they have
been called." The removal of electrons from matter
occurs in physical, chemical, and radioactive changes
alike, exampled, respectively, by the electrification of
a glass rod by friction, the ionisation of an electrolyte
by solution, and by the /3-ray change of radioactive
substances. It is only in the latter case, however,
that the electron can be regarded as a primordial
constituent and the change as transmutational.
Even to-day it is in radioactive phenomena, and in
these alone, that the limits reached long ago in the
chemical analysis of matter have been overstepped
and the Rubicon, which a century ago Prout vaulted
over so lightly in imagination, has actually been
crossed by science.



116 CONCEPTION OF THE CHEMICAL ELEMENT

FIRST AND SECOND PHASES OF DEVELOPMENT.

Looking- backward to the first recognition of the
character of radioactive change in 1902, it is possible
to distinguish broadly two phases. The first phase,
concerned mainly with the disentanglement of the
long and complicated series of successive changes,
commencing with the two primary radio-elements
uranium and thorium, and including ultimately all
the known radio-elements, added little to the con-
ceptions of chemistry beyond the disturbing fact that
the radio-elements, although in every other respect
analogous to the ordinary elements, are in process of
continuous transmutation. But in the second and
more recent phase of radioactive change, the study
of the chemical character of the successive products
and the law connecting this with the type of ray
expelled in the change, the discovery of elements
with different radioactive but identical chemical
character, the recognition of these as isotopes, or
elements occupying the same place in the periodic
table, and the interpretation of the significance of
the periodic law, conceptions are arrived at which
are not merely novel, but upsetting. In this phase,
an aspect of the ultimate constitution of matter has
been revealed that, although well within the scope of
the conceptions of elements and atoms which we owe
to the nineteenth century, nevertheless has totally
escaped recognition. I am not much concerned with
definitions, but I think the Chemical Society might
safely offer a prize of a million pounds to any one of
its members who will shortly and satisfactorily define
the element and the atom for the benefit of and
within the understanding of a first-year student of
chemistry at the present time.



RADIOACTIVE CHANGE 117

CHIEF FEATURES OF RADIOACTIVE CHANGE.

The features that distinguish radioactive change
from chemical change, and which have made it
possible in a few short years to reduce to some
degree of finality and completeness the intensely
complicated series of successive changes suffered by
the elements uranium and thorium in the course of
their disintegration, are chiefly two. In the first
place, the whole phenomena are inevitable, incapable
of being changed or deviated from their allotted
course by any means whatever, independent of
temperature, concentration, or the accumulation of
products of reaction, the presence of catalysts,
irreversible and capable of being accurately and
quantitatively followed without alteration or disturb-
ance of the changing system. The mathematical
theory, although for many successive changes it
becomes cumbrous and unwieldy to a degree, involves
only the solution of one differential equation by a
device quite within the compass of anyone possessing
a knowledge of the bare elements of the calculus to
employ. The second feature is in the magnitude of
the energy evolved, which, weight for weight of
matter changing, surpasses that evolved in the
most exothermic chemical changes known, from one
hundred thousand to a million times. Manifested in
the form of rays, by their fluorescent, photographic,
or ionising power capable of being put into evidence
in almost inconceivably minute amount, changes are
capable of being followed, and by the electroscope
accurately measured, which would conceivably require
to continue for millions of years before they could
be experimentally detected by chemical or even by
spectroscopic methods. The disintegration of the
single atom is ascertainable, for example, in the
spinthariscope of Sir William Crookes, where each



118 CONCEPTION OF THE CHEMICAL ELEMENT

of the scintillations separately visible is due to the
impact of a single a-particle on the zinc sulphide
screen. On the same principle, methods have been
developed and are in regular use for counting- the
number of atoms disintegrating per minute, whereas
to the spectroscope at least 3- io 13 atoms as a minimum
must be present, 25,000 times as many atoms as
there are human beings alive in the world, before any
element can be so detected. By the most curious
compensation, almost of the nature of a providential
dispensation which some may have found difficult
to believe, the quantity of matter of itself is not
of importance in investigating radioactive change.
The methods depend on the rate of emission of
energy, and this is proportional to the quantity of
the changing element multiplied by its rate of change.
In the disintegration series, the various members
accumulate in quantities inversely proportional to the
rates of change, and so it comes about that all
changes within the series are equally within the scope
of the method whether, as in the case of the parent
elements, they involve periods surpassing the most
liberal estimates of the duration of geological time or,
as in the case of the C' members, are estimated to
run their course in a time so short that light itself
can travel but a very few millimetres, before the next
change overtakes the changing atom.

The condition of radioactive equilibrium in which
the quantities of the successive products assume the
above stationary ratio is of course entirely different
from chemical equilibrium, and is the condition in
which for each member of the series except the first
as much is produced as changes further in the unit
of time.

The foregoing applies so long as the changes
continue. When they are finished and it is a
question of ascertaining the ultimate products, the



PRODUCTION OF HELIUM AND LEAD 119

task may be likened to that of searching for a meteor
which a moment before lit up the heavens and now
has vanished into the night.



THE ULTIMATE PRODUCTS.

It is a matter for surprise that in all radioactive
changes so far studied there appear to be only
two ultimate products, helium and lead, the former
constituting the a-particles and the latter being
produced both by uranium and thorium, withal, as
we now know, not the same lead in the two cases.
There are sufficient experimental reasons for doubting
whether the disintegration of an atom into more
nearly equal parts would be within range of detection
by any of the known methods. A heavy atom like
oxygen, for example, if expelled as a radiant particle,
might not attain sufficient velocity to ionise gases,
or, even if it did, the range over which the ionisation
would extend, as we know from the ionisation
produced by the recoil atoms, would be extremely
small. It must be a matter for comment, however,
that hydrogen never appears in these changes, as, if
it were produced, it would almost certainly be as easy
to ascertain as helium. It has always seemed to me
a possibility that some genetic connection may exist,
after all, between thorium and uranium, although
I have never been able to frame even a possible mode
of so connecting these two elements. With a differ-
ence of atomic weight of six units, it is impossible to
pass from one to the other by addition or expulsion
of helium atoms alone.

Both with regard to helium and lead, the com-
position of radioactive minerals gave the first clue
to the identity of the ultimate products. After
the discovery of radioactivity and the elucidation
of its nature, the fact that helium was found only



120 CONCEFllON OF THE CHEMICAL ELEMENT

in minerals containing uranium and thorium assumed
a totally new interpretation, borne out by the
spectroscopic proof of the production of helium from
radium by Sir William Ramsay and myself, and
later from actinium, polonium, and even from
uranium and thorium, all at the rates to be expected
from radioactive data. The identification of the
a-particle with helium, after the weight of the
a-particle had been shown by new physical methods
to be four times that of the hydrogen atom, was
accomplished by enclosing the radium emanation
in a glass tube thin-walled enough to allow the
a-particle to go through, but perfectly impervious
to the passage of gas. In these circumstances,
helium in spectroscopically detectable quantity was
proved by Rutherford to make its appearance out-
side the tube.

Such confirmations by the spectroscope, welcome
and gratifying as they are, are nevertheless in a
sense subsidiary to the main problem, namely, the
task of unravelling the complicated series of changes
into its individual steps, and the characterisation
by their radioactivity of the several intermediate
members of the series, such as by the determina-
tion of their periods and the physical constants of
the radiation a-, /3-, or y-, to which they give rise.
The determination of their chemical character,
although equally important, was only later fully
accomplished.

THE RADIATIONS.

In the successive radioactive changes, a- or
i8-particles are expelled, one a-particle per atom
disintegrating for each change, although for the
/3-particles our knowledge is less exact. In some
cases, certainly, although these are exceptional,
/3-particles seem to be expelled along with a-particles.



NATURE OF THE RADIATIONS 121

The a-particle is an atom of helium charged with
two atomic charges of positive electricity, or, as
we should now say, is the helium nucleus, deprived
of the two electrons which are combined with it in
the helium atom. The /3-particle is the negative
electron, and when expelled with sufficiently high
velocity is accompanied with y-rays. The latter are
^f-rays of exceedingly short wave-length, varying
from 1-3 to o- 1 Angstrom units. 1 A connection exists
between the speed of the change and the speed of
the particles expelled, and the more rapid the change
the faster in general and the more penetrating are
the attendant a- or /3-particles. In the case of the
a-particle, an empirical logarithmic relation, known
as the Geiger-Nuttall relation, enables us to calculate
approximately the period of the changing element
from the velocity or range of the a-particle, and
vice versa, and by this means periods too long or
too short to be directly measurable have been
estimated. In the case of the /3-rays, no definite
quantitative law has yet been made out, but it is


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Online LibraryFrederick SoddyScience and life: Aberdeen addresses → online text (page 9 of 18)