Frederick Soddy.

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of slow spontaneous change. Their radioactivity
is due in large measure to minute quantities of
impurities, of totally different chemical character
from themselves, that can be readily and completely
removed by simple purification processes. But, once
removed, the substances so purified do not remain
pure. At a perfectly definite rate they regrow or
produce the radioactive impurities, and these can be
again separated as often as desired. Once separated,
the radioactivity of the products dies away or decays,
and the apparently steady continuous emission of
rays from the parent substance is due to an
equilibrium, in which new radioactive products are
formed as fast as the radioactivity of those already
produced disappears. Very rapidly a complete and
satisfactory theory of the whole phenomena was
developed, and fourteen years of further development
of the science has not necessitated any modification.
The atoms of the radio-elements are not permanently
stable. After a term of existence which may be long
or short, according to the nature of the atom in
question, and which for the individual atoms of the
same radio-element may have any actual value, but
is for the average of all the atoms of any one kind
a perfectly definite period, known as the period of
average life, the atom explodes. 'Fragments are
expelled from it at hitherto unknown velocities
constituting the rays, of which more anon. What is
left is the new atom of a new element, totally different
from the parent. The radio-elements are in course
of spontaneous transmutation into other elements,






THE GROWTH OF RADIUM 95

and the process proceeds through a long succession
of more or less unstable intermediate elements, until
the final stable product is reached. In this process
energy is evolved of the order of a million times
greater than the energy ever liberated in ordinary
chemical changes, in which the groups of atoms, or
the molecules, change, but not the constituent atoms
themselves. The energy evolved by an ounce of
radium, in the course of its life, equals that evolved
from the burning of ten tons of coal. The period of
average life in this case is about 2500 years, which
means that ^oth part of any quantity of radium
changes per annum.

The rate at which the various radioactive products
change varies very widely. It may be slow or rapid,
a matter of seconds or even billionths of a second on
the one hand, or of years or centuries or aeons on the
other. It was reasonable to interpret what Mme.
Curie had done for pitchblende in exactly the same
way as had been done for thorium, merely extending
the time scale. The radium, polonium, actinium and
the other new intensely active radio-elements she
discovered in such infinitesimal amount in pitchblende
were in all probability the products of the change of
the parent element uranium. The view carries with
it the corollary that, if you separated uranium from
radium and everything else completely and left it to
itself, in the course of years or centuries a new crop
of radium would be gradually formed. The case
of radium is specially interesting as it has been
established that it is an ordinary element resem-
bling barium, with definite spectrum, atomic weight,
chemical properties and position in the Periodic
Table. It was one of very many startling predictions
of a similar character made as soon as the new point
of view was attained. But it has been the last to
receive confirmation and the difficulties have been



96 THE EVOLUTION OF MATTER

great. Were radium the first direct product, the
growth of radium in uranium, initially purified com-
pletely from it, could be observed in the course of an
hour, so excessively delicate are the radioactive tests
for this new element. Experiments were started in
1003 in London, continued on a very much larger
and more thorough scale in Glasgow, with the aid
of Mr T. D. Mackenzie. Yet in 1914 the expected
confirmation was still not clearly forthcoming. Long-
before that time it was known that radium was not
the direct product of uranium, and that another new
radio-element, ionium, intervened in the series. The
uranium changes into radium, via ionium, and this
ionium is an exceedingly slowly changing element in
comparison even with radium, not more than about
joo^ooth part changing every year. This retards
enormously the initial rate of growth of radium and
makes it proceed at first not linearly with the lapse
of time, but according to the square of the lapse of
time. That is, the growth after ten years would be
100 times, and after 100 years 10,000 times, that in
the initial year from purification. The oft-tested prep-
arations of uranium were transplanted to Aberdeen
in safety, and tests since carried out, in conjunction
with Miss Ada Hitchins, last year satisfactorily
established a growth of radium beyond all doubt in
the largest preparation, and showed that the rate was
proceeding as nearly as can yet be seen according
to the square of the time. The growth of radium
was not large. In three years it amounted to
iso.ooo.ooo.ooo.oooth of the quantity of uranium experi-
mented upon, and in six years to just four times
this quantity. The experiments gave, moreover,
indirectly a maximum estimate of the rate of change
of ionium as at most foo^ooth P art P er year. This
estimate has now been confirmed and made more
definite by some very fine direct work on ionium



THE a-, - AND y-RAYS 97

itself at the Radium Institute of Vienna a few months
ago, which gives the rate of change as ^-o^ih part
per year. This is more than fifty times slower than
the rate of change of radium itself, which has long
been established to be about ^th part per year. On
the other hand the original uranium is estimated
with fair probability to be changing 50,000 times
more slowly than ionium, or not much more than
io.ooo,ooo,ooo tn P art changing per annum. In the course
of 1,000,000,000 years a period beyond what even
the geologists claim as the total age of the earth
hardly more than 10 per cent, of a given quantity
of uranium would change through ionium, radium
and so on into other elements. Yet, as has been
mentioned, so delicate are our methods, that had
radium been the first direct product of the change,
an hour's observation on a kilogram of purified
uranium would have sufficed to have established the
growth beyond all doubt. As it is, the problem took
thirteen years. Uranium and thorium are the only
two primary radio-elements in the process of change.
All the other radio-elements known, and they number
thirty-three, are produced from one or other of them
in the course of their long sequence of changes.

But what of the rays themselves, the expulsion of
which first drew attention to the phenomenon, and
which have furnished the necessary experimental
means for the study of the whole problem? Like
the X-rays, they do not recognise the optical
properties, transparency and opacity, nor, to a great
extent, the chemical nature of the matter in their
path. They plough through everything, affected
primarily only by the density of the absorbing
medium, or by the actual mass of the material in
their way. Physicists recognise three distinct types
of rays the a-, the /3- and the y-rays, the first
stopped completely by a sheet of notepaper, but by



98 THE EVOLUTION OF MATTER

far the most energetic and important of all, the
second capable of penetrating perhaps |th of an inch
of glass or aluminium without being- totally stopped,
and the third reduced to half their original intensity
by about \ inch of lead, though not absolutely com-
pletely stopped even by 20 inches. The y-rays are
far the most penetrating rays known and are really
X-rays, but far more penetrating than any that Can
be artificially produced. They are light waves of
wave-length thousands of times shorter than those
of visible light, and are probably a secondary pheno-
menon accompanying the expulsion of the /3-rays.
The /3-rays, or ^-particles, are electrons the atoms
of negative electricity divorced from matter, recog-
nised as such by Sir J. J. Thomson in 1897, but
previously well-known in the phenomena of the
Crookes' tube. They travel at a speed varying from
a third up to nearly the velocity of light itself, which
is very much greater than any that can be produced
artificially. The a-rays, or a-particles, are atoms of
matter, carrying two atomic charges of positive
electricity just twice the charge of positive electricity
that the /3-particles carry of negative electricity and
travelling with a velocity varying from ^Vth to tVth
that of light, about a hundred times faster than
matter had ever been known to travel previously.
Their mass is several thousand times as great as
that of the /3-particle, and in spite of their feeble
penetrative power, and, at first sight, less showy
qualities, over 90 per cent, of the energy evolved in
the change of an atom is emitted in the form of these
a-particles. Much of Rutherford's finest work has
been in connection with these a-particles.

The early measurements of the mass of the atom
constituting the a-particle left a choice as to its
nature, whether it was an atom of helium or of
hydrogen, but strong indirect evidence of a very



HELIUM PRODUCED BY RADIUM 99

remarkable character favoured helium. Thus helium,
though it forms no compounds, is found in minerals
containing- uranium and thorium, only in the minerals
containing uranium and thorium, and always in them.
Might not this helium be the a-particles fired off from
the uranium and thorium in the mineral, and, unable
to escape from the glassy minerals, accumulating in
the material over long periods of geological time,
until its presence was obvious and striking even to
the relatively rough tests of chemistry and the
spectroscope? Naturally, if one could only get
enough radium the point might be tested directly, for
the spectroscopic test for helium is very sensitive, a
bubble of the gas, y^Virth of a cubic millimetre in
volume, that is, yoVrrth part of a large pin's head,
being sufficient to give the characteristic spectrum.
This was in 1903, at the time when pure radium
compounds were being put on the market for the first
time by the enterprise of the German technical
chemist, Dr Giesel. The first thing done with it in
the late Sir William Ramsay's laboratory in London
was to see whether helium was being generated by it
continuously, as should be the case if the a-particles
were really positively charged atoms of helium. A
few milligrams of radium only was available, but it
proved sufficient, and the growth of helium from
radium was established by the spectroscope by the
aid of the beautiful methods of manipulation of gases,
devised by Sir William in the course of his investiga-
tions on the rare gases of the atmosphere. Later,
the writer established the continuous production of
helium from uranium and thorium, though here, from
a ton of either element in a year, the quantity of
helium produced is only -swsih of a milligram by
weight a quantity unweighable on the most sensi-
tive chemical balance or n cubic millimetres by
volume. Helium has also been detected as a



100 THE EVOLUTION OF MATTER

product of polonium, actinium and other of the new
radio-elements.

Gradually the tangled and complex succession of
changes being undergone by uranium and thorium
have been straightened out, and it is probable that
the work is now complete. Some of the changes
require millions of years, some are over in a billionth
of a second or less. The atom of uranium expels
7 a- and 5 /3-particles, in twelve successive changes,
one particle per atom at each change. The atom of
thorium expels 6 a- and 3 /3-particles. The /3-particles
are atoms of electricity rather than of matter, and
their expulsion affects the mass of the parent atom to
only a negligible extent. But the a-particles are
atoms of helium and the expulsion of each particle
must lower the atomic mass of the parent atom by
4 units.

So long as the process of disintegration of the
atom is proceeding, the rays emitted and the energy
they possess afford the necessary evidence for their
experimental study. But when it is all over, how
are we to proceed? The final product into which
uranium or thorium turns, if it is the final product,
by hypothesis emits no rays. The quantity produced
from any manageable quantity of uranium or thorium
in a lifetime is too small to detect chemically. How
can we find out even what it is ?

There is the method that already had indicated
helium as the element constituting the a-particle. In
the natural radioactive minerals one would expect
to find the end products of the radioactive changes in
greater or less relative abundance, according as the
mineral is geologically ancient or modern. This
evidence for long indicated the element lead as the
final product of the changes of uranium. To-day we
know that the radioactive minerals are in reality
geological clocks, and they record more accurately



THE AGE OF ROCKS 101

than in any other way the age of the stratum in
which they occur. In a uranium mineral, for
example, each i per cent, of lead in terms of the
quantity of uranium signifies the lapse of a period of
80,000,000 years. Errors of course are possible, if
lead should have been an original constituent of the
mineral, but these are minimised by taking a large
number of different minerals. On the other hand
every cubic centimetre by volume of helium per gram
of uranium in a uranium mineral signifies 9,000,000
years, and as here helium, being a gas that forms
no compounds, cannot, have been initially present, and
as, moreover, some will have escaped the age of the
mineral by this method is a minimum, whereas the
age by the lead content may be too high. The
carboniferous rocks tested by this new method appear
to have an age of some 350,000,000 and the oldest
Archean rocks of over 1,500,000,000 years.

The actual production of lead has not yet been
proved directly in the same way as the production of
helium has, though, but for the war, in all proba-
bility this would now have been accomplished. But
even without the actual direct proof of this kind there
is practically no room for doubt on the point. Indeed
by a very important development, about which a few
words may be said in conclusion, we know that not
only uranium but also thorium both produce the
element lead as the final product, and though the
lead from uranium is absolutely identical chemically
and spectroscopically with the lead from uranium,
yet they are different. Stranger still, the lead which
chemists are familiar with as one of the elements is
probably a mixture of both kinds.

We have seen that the expulsion of an a- par tide
ought to lower the atomic weight of the element
expelling it by 4 units, 4 being the atomic weight of
helium. In its transformation into radium, uranium

p



SANTA BARBARA



102 THE EVOLUTION OF MATTER

expels 3 a-particles. The atomic weight of uranium
is 238, and that found by Mme. Curie for radium is
226. So far so good. Radium in its further
changes expels 5 a-particles, and the atomic weight
of the end product should be therefore 206. The
atomic weight of thorium is 232, and, as it expels
6 a-particles in all, that of the end product of
thorium should be 208. The atomic weight of
ordinary lead is 207-2. The atomic weight of
bismuth is 208, but the writer was unable to find
in a special examination of over 20 kilograms of a
certain thorium mineral even a trace of bismuth,
though there was 0-3 per cent, of lead. This
definitely rules bismuth out.

In the early months of 1913 a fundamental step
forward was taken into our knowledge of the nature
of matter which started from the discovery of the
simple complete law of elementary evolution as we
have come to know it in radioactive change, which
is largely due to two of the writer's old students,
A. S. Russell and A. Fleck. The expulsion of the
a-particle, or the /3-particle, from an atom leaves a
new atom with properties different from the parent,
but different in a very definite and striking way. If
the particle expelled is the a-particle, the element
after this expulsion invariably changes its whole
chemical character and passes from the place it
occupies in the Periodic Table to a new place, next
but one to it in the direction of diminishing atomic
weight. If the expelled particle is a /8-particle the
change of place is invariably into the next place in
the opposite direction. After three changes in any
order, one a- and two /3-, a very common sequence
in the series, the element returns to the place it
first occupied. Its atomic weight is less than it was
by 4 units, but in its whole chemical nature and even
in its spectrum, it is not merely like its original



ISOTOPES 103

parent. It is chemically identical with it. Elements
which so occupy the same place in the Periodic
Table and are absolutely identical in all their
chemical properties are called isotopes. The recog-
nition of such isotopes is fundamentally new, and
cuts more deeply into old-established ideas of the
nature of matter than even the surprising discoveries
of the genesis of one element out of another.

The present theory of atomic structure is due to
Rutherford, and is based on experiments on the
course followed by an a-particle when it ploughs its
way through the atoms of matter. These experi-
ments have shown that the atom consists of a central
nucleus, possessing all but a negligible part of the
atomic mass but occupying only an exceedingly
minute fraction of the atomic volume. The nucleus
contains a preponderance of positive charges and is
surrounded by an equivalent number of separate
negative electrons, revolving in a system around it.
This theory lent itself at once to the interpretation
of the new developments here referred to, and both
together, along with very important work by the late
H. G. J. Moseley on the wave-lengths of the X-ray
spectra of the elements, have furnished the key to
the deciphering of the Periodic Law. It is melan-
choly to record that Moseley fell at Suvla Bay, aged
only twenty-eight.

Prior knowledge of the atoms of matter has been
superficial in the literal sense confined entirely to
the outermost shell of the atom. We have now
penetrated to the interior and find, first, an inner
shell, wherein X-rays take their origin, and, secondly,
still further to the nucleus, the sanctum sanctorum of
the atom, revealed only by radioactivity and alone
concerned in this phenomenon. The same outer
and inner shells that is, the same kind of atom
to the older knowledge may contain demonstrably



104



THE EVOLUTION OF MATTER



different nuclei. Matter is of indefinitely more kinds
than the chemist and his Periodic Law have dis-
closed.

The places in the Periodic Table represent
integral nett charges of electricity in the constitution
of the nucleus. The expulsion of the a-particle with
its double charge of positive electricity shifts the
element in the Periodic Table by two places in one
direction and the expulsion of the /3-particle, with its
single charge of negative electricity, shifts it one
place in the other direction. Nature does not deal
in fractions of an atom of electricity any more than
with fractions of an atom of matter. As we pass
from hydrogen, at the beginning, to uranium, at the
end, of the elements, we pass 92 places in the
Periodic Table, each element differing from the one
preceding it by a unit charge or "atom" of positive
electricity in its nucleus. Hydrogen has one such
and uranium 92 such unit positive charges. The
number expressing the element's place in the Periodic
Table is called the atomic number. It is the nett
number of charges in the atomic nucleus, i.e. the
difference between the separate positive and negative
charges. Before the discovery of the radio-elements
the following represented the last 14 places of the
Periodic Table :



79


80


81


82


83


84


85


86


87


88


89


90


91


92








o


N


o














_




N


2 s


8


8


I?

















CO




00
CO




b


|





J














e




.




3


























13


a


"rt


"rt


s














o




g





<u


_c


s
















J2






O


*


H


J


5














H




D



LEAD THE FINAL PRODUCT 105

The figures in the upper line are the atomic
numbers, the figures after each element the atomic
weights, both in terms of that of hydrogen as unity.
Radium, when discovered, fell naturally into the
vacant place No. 88, and polonium and actinium
are now known to occupy Nos. 84 and 89. The
three radioactive emanations of Rutherford, products
of radium, actinium and thorium respectively, are
chemically analogous to Ramsay's inert family of
atmospheric gases, and occupy the place No. 86.
No. 91 is known to be occupied by a product of
uranium, having a period of average life of only if
minutes, called Brevium. The numbers 85 and 87
in the above figure now alone remain vacant.

Thus radioactivity has peopled all but two of
these vacant places, but it has done more. It has
crowded into ten of the above places, between Nos.
8 1 and 92, no less than 39 distinct elements, and all of
the elements occupying any one place isotopes as
they are called are invariably identical in their whole
chemical character. Ionium is isotopic with thorium,
mesothorium I. with radium, and so on. To the
chemist and the spectroscopist they would be taken
as one. Not so, however, to the newer methods
of radioactivity.

When the whole sequences of changes of uranium
and thorium are set forth in the Periodic Table
according to the a- and /8-change rules mentioned,
it is found that all the final products occupy the
place, No. 82, occupied by lead. The atomic
weight of the end product of uranium should be
206 and that for thorium 208, whereas the atomic
weight of common lead is 207-2. This suggests
that common lead is a mixture of isotopes rather
than a single homogeneous element. The view
rapidly received complete vindication. For the
atomic weight of lead derived from minerals rich



106 THE EVOLUTION OF MATTER

in thorium has been found to be higher than that
of common lead, whereas the atomic weight of lead
derived from minerals rich in uranium is lower.
The values in fact vary from 206-0 to 207-7.

The densities of the varieties of the lead, the
writer recently found, differ in exactly the same way
as the atomic weights, showing that the volume
of the atom is the same though the weights are
different, as was to be expected from general
theoretical considerations. The difference is only
small. "Thorium " lead is about i per cent, heavier
than common lead. Prof. Richards, of Harvard,
has since found "uranium" lead to be J per cent
lighter than common lead. But if such a difference
occurred with gold, a bank-teller would be liable
to be out by one sovereign, or two, in every 400,
if he weighed the coins instead of counting them.

Gold was the goal of alchemy, and it is interest-
ing to ask whether the new discoveries have thrown
any light on the alchemical problem of how to
make gold from lead or mercury. The answer may
be given at once. Gold is followed in the Periodic
Table by mercury, thallium, lead, and bismuth,
occupying successive places without gaps, as the
figure shows. To get gold from mercury, expel
from the atom of mercury one /3-particle, whicty will
make thallium, then one a-particle, which will turn
the thallium into gold. Or, to get gold from
lead, expel from the atom of lead one a-particle,
which will turn it into mercury, and proceed as
before.

It is interesting to note that, in the case of both
the thorium and uranium disintegration series, at
a certain stage, the expulsion of an a-particle
instead of a /3-particle would have resulted in gold
being produced, for in each case the place occupied
by thallium is entered in the course of the changes.



TRANSMUTATION ANCIENT AND MODERN 107

Unfortunately it is not yet possible to supple-
ment these simple recipes for the artificial produc-
tion of gold with the necessary instructions as to
how an atom is to be caused to expel an a- or a
/3-particle at will, unless Nature has decreed that
it should do so of itself, in which case nothing known
will prevent it. But, if man ever achieves this
further control over Nature, it is quite certain that
the last thing he would want to do would be to
turn lead or mercury into gold for the sake of gold.
The energy that would be liberated, if the control


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Online LibraryFrederick SoddyScience and life: Aberdeen addresses → online text (page 8 of 18)