Darwin and Modern Science (1909)
Edited by A.C. Seward
XXIX. THE EVOLUTION OF MATTER.
By By W.C.D. WHETHAM, M.A., F.R.S.
Trinity College, Cambridge.
I.
he idea of evolution in the organic world, made intelligible by the work
of Charles Darwin, has little in common with the recent conception of
change in certain types of matter. The discovery that a process of
disintegration may take place in some at least of the chemical atoms,
previously believed to be indestructible and unalterable, has modified our
view of the physical universe, even as Darwin's scheme of the mode of
evolution changed the trend of thought concerning the organic world. Both
conceptions have in common the idea of change throughout extended realms of
space and time, and, therefore, it is perhaps not unfitting that some
account of the most recent physical discoveries should be included in the
present volume.
The earliest conception of the evolution of matter is found in the
speculation of the Greeks. Leucippus and Democritus imagined unchanging
eternal atoms, Heracleitus held that all things were in a continual state
of flux--Panta rei.
But no one in the Ancient World--no one till quite modern times--could
appreciate the strength of the position which the theory of the evolution
of matter must carry before it wins the day. Vague speculation, even by
the acute minds of philosophers, is of little use in physical science
before experimental facts are available. The true problems at issue cannot
even be formulated, much less solved, till the humble task of the observer
and experimenter has given us a knowledge of the phenomena to be explained.
It was only through the atomic theory, at first apparently diametrically
opposed to it, that the conception of evolution in the physical world was
to gain an established place. For a century the atomic theory, when put
into a modern form by Dalton, led farther and farther away from the idea of
change in matter. The chemical elements seemed quite unalterable, and the
atoms, of which each element in modern view is composed, bore to Clerk
Maxwell, writing about 1870, "the stamp of manufactured articles" exactly
similar in kind, unchanging, eternal.
Nevertheless throughout these years, on the whole so unfavourable to its
existence, there persisted the idea of a common origin of the distinct
kinds of matter known to chemists. Indeed, this idea of unity in substance
in nature seems to accord with some innate desire or intimate structure of
the human mind. As Mr Arthur Balfour well puts it, "There is no a priori
reason that I know of for expecting that the material world should be a
modification of a single medium, rather than a composite structure built
out of sixty or seventy elementary substances, eternal and eternally
different. Why then should we feel content with the first hypothesis and
not with the second? Yet so it is. Men of science have always been
restive under the multiplication of entities. They have eagerly watched
for any sign that the different chemical elements own a common origin, and
are all compounded out of some primordial substance. Nor, for my part, do
I think that such instincts should be ignored...that they exist is certain;
that they modify the indifferent impartiality of pure empiricism can hardly
be denied." ("Report of the 74th Meeting of the British Association"
(Presidential Address, Cambridge 1904), page 9, London, 1905.)
When Dalton's atomic theory had been in existence some half century, it was
noted that certain numerical relations held good between the atomic weights
of elements chemically similar to one another. Thus the weight (88) of an
atom of strontium compared with that of hydrogen as unity, is about the
mean of those of calcium (40) and barium (137). Such relations, in this
and other chemical groups, were illustrated by Beguyer de Chancourtois in
1862 by the construction of a spiral diagram in which the atomic weights
are placed in order round a cylinder and elements chemically similar are
found to fall on vertical lines.
Newlands seems to have been the first to see the significance of such a
diagram. In his "law of octaves," formulated in 1864, he advanced the
hypothesis that, if arranged in order of rising atomic weight, the elements
fell into groups, so that each eighth element was chemically similar.
Stated thus, the law was too definite; no room was left for newly-
discovered elements, and some dissimilar elements were perforce grouped
together.
But in 1869 Mendeleeff developed Newland's hypothesis in a form that
attracted at once general attention. Placing the elements in order of
rising atomic weight, but leaving a gap where necessary to bring similar
elements into vertical columns, he obtained a periodic table with natural
vacancies to be filled as new elements were discovered, and with a certain
amount of flexibility at the ends of the horizontal lines. From the
position of the vacancies, the general chemical and physical properties of
undiscovered elements could be predicted, and the success of such
predictions gave a striking proof of the usefulness of Mendeleeff's
generalisation.
When the chemical and physical properties of the elements were known to be
periodic functions of their atomic weights, the idea of a common origin and
common substance became much more credible. Differences in atomic weight
and differences in properties alike might reasonably be explained by the
differences in the amount of the primordial substance present in the
various atoms; an atom of oxygen being supposed to be composed of sixteen
times as much stuff as the atom of hydrogen, but to be made of the same
ultimate material. Speculations about the mode of origin of the elements
now began to appear, and put on a certain air of reality. Of these
speculations perhaps the most detailed was that of Crookes, who imagined an
initial chaos of a primordial medium he named protyle, and a process of
periodic change in which the chemical elements successively were
precipitated.
From another side too, suggestions were put forward by Sir Norman Lockyer
and others that the differences in spectra observed in different classes of
stars, and produced by different conditions in the laboratory, were to be
explained by changes in the structure of the vibrating atoms.
The next step in advance gave a theoretical basis for the idea of a common
structure of matter, and was taken in an unexpected direction. Clerk
Maxwell's electromagnetic theory of light, accepted in England, was driven
home to continental minds by the confirmatory experiments of Hertz, who in
1888 detected and measured the electromagnetic waves that Maxwell had
described twenty years earlier. But, if light be an electromagnetic
phenomenon, the light waves radiated by hot bodies must take their origin
in the vibrations of electric systems. Hence within the atoms must exist
electric charges capable of vibration. On these lines Lorentz and Larmor
have developed an electronic theory of matter, which is imagined in its
essence to be a conglomerate of electric charges, with electro-magnetic
inertia to explain mechanical inertia. (Larmor, "Aether and Matter",
Cambridge, 1900.) The movement of electric charges would be affected by a
magnetic field, and hence the discovery by Zeeman that the spectral lines
of sodium were doubled by a strong magnetic force gave confirmatory
evidence to the theory of electrons.
Then came J.J. Thomson's great discovery of minute particles, much smaller
than any chemical atom, forming a common constituent of many different
kinds of matter. (Thomson, "Conduction of Electricity through Gases" (2nd
edition), Cambridge, 1906.) If an electric discharge be passed between
metallic terminals through a glass vessel containing air at very low
pressure, it is found that rectilinear rays, known as cathode rays, proceed
from the surface of the cathode or negative terminal. Where these rays
strike solid objects, they give rise to the Rontgen rays now so well known;
but it is with the cathode rays themselves that we are concerned. When
they strike an insulated conductor, they impart to it a negative charge,
and Thomson found that they were deflected from their path both by magnetic
and electric forces in the direction in which negatively electrified
particles would be deflected. Cathode rays then were accepted as flights
of negatively charged particles, moving with high velocities. The electric
and magnetic deflections give two independent measurements which may be
made on a cathode ray, and both the deflections involve theoretically three
unknown quantities, the mass of the particles, their electric charge and
their velocity. There is strong cumulative evidence that all such
particles possess the same charge, which is identical with that associated
with a univalent atom in electrolytic liquids. The number of unknown
quantities was thus reduced to two--the mass and the velocity. The
measurement of the magnetic and electric deflections gave two independent
relations between the unknowns, which could therefore be determined. The
velocities of the cathode ray particles were found to vary round a value
about one-tenth that of light, but the mass was found always to be the same
within the limits of error, whatever the nature of the terminals, of the
residual gas in the vessel, and of the conditions of the experiment. The
mass of a cathode ray particle, or corpuscle, as Thomson, adopting Newton's
name, called it, is about the eight-hundredth part of the mass of a
hydrogen atom.
These corpuscles, found in so many different kinds of substance, are
inevitably regarded as a common constituent of matter. They are associated
each with a unit of negative electricity. Now electricity in motion
possesses electromagnetic energy, and produces effects like those of
mechanical inertia. In other words, an electric charge possesses mass, and
there is evidence to show that the effective mass of a corpuscle increases
as its velocity approaches that of light in the way it would do if all its
mass were electromagnetic. We are led therefore to regard the corpuscle
from one aspect as a disembodied charge of electricity, and to identify it
with the electron of Lorentz and Larmor.
Thus, on this theory, matter and electricity are identified; and a great
simplification of our conception of the physical structure of Nature is
reached. Moreover, from our present point of view, a common basis for
matter suggests or implies a common origin, and a process of development
possibly intelligible to our minds. The idea of the evolution of matter
becomes much more probable.
The question of the nature and physical meaning of a corpuscle or electron
remains for consideration. On the hypothesis of a universal luminiferous
aether, Larmor has suggested a centre of aethereal strain "a place where
the continuity of the medium has been broken and cemented together again
(to use a crude but effective image) without accurately fitting the parts,
so that there is a residual strain all round the place." (Larmor, loc.
cit.) Thus he explains in quasi-mechanical terms the properties of an
electron. But whether we remain content for the time with our
identification of matter and electricity, or attempt to express both of
them in terms of hypothetical aether, we have made a great step in advance
on the view that matter is made up of chemical atoms fundamentally distinct
and eternally isolated.
Such was the position when the phenomena of radio-activity threw a new
light on the problem, and, for the first time in the history of science,
gave definite experimental evidence of the transmutation of matter from one
chemical element to another.
In 1896 H. Becquerel discovered that compounds of the metal uranium
continually emitted rays capable of penetrating opaque screens and
affecting photographic plates. Like cathode and Rontgen rays, the rays
from uranium make the air through which they pass a conductor of
electricity, and this property gives the most convenient method of
detecting the rays and of measuring their intensity. An electroscope may
be made of a strip of gold-leaf attached to an insulated brass plate and
confined in a brass vessel with glass windows. When the gold-leaf is
electrified, it is repelled from the similarly electrified brass plate, and
the angle at which it stands out measures the electrification. Such a
system, if well insulated, holds its charge for hours, the leakage of
electricity through the air being very slow. But, if radio-active
radiation reach the air within, the gold-leaf falls, and the rate of its
fall, as watched through a microscope with a scale in the eye-piece,
measures the intensity of the radiation. With some form of this simple
instrument, or with the more complicated quadrant electrometer, most radio-
active measurements have been made.
It was soon discovered that the activity of uranium compounds was
proportional to the amount of uranium present in them. Thus radio-activity
is an atomic property dependent on the amount of an element and independent
of its state of chemical combination.
In a search for radio-activity in different minerals, M. and Mme Curie
found a greater effect in pitch-blende than its contents of uranium
warranted, and, led by the radio-active property alone, they succeeded, by
a long series of chemical separations, in isolating compounds of a new and
intensely radio-active substance which they named radium.
Radium resembles barium in its chemical properties, and is precipitated
with barium in the ordinary course of chemical analysis. It is separated
by a prolonged course of successive crystallisation, the chloride of radium
being less soluble than that of barium, and therefore sooner separated from
an evaporating solution. When isolated, radium chloride has a composition,
which, on the assumption that one atom of metal combines with two of
chlorine as in barium chloride, indicates that the relative weight of the
atom of radium is about 225. As thus prepared, radium is a well-marked
chemical element, forming a series of compounds analogous to those of
barium and showing a characteristic line spectrum. But, unlike most other
chemical elements, it is intensely radio-active, and produces effects some
two million times greater than those of uranium.
In 1899 E. Rutherford, then of Montreal, discovered that the radiation from
uranium, thorium and radium was complex. (Rutherford, "Radio-activity"
(2nd edition), Cambridge, 1905.) Three types of rays were soon
distinguished. The first, named by Rutherford alpha-rays, are absorbed by
thin metal foil or a few centimetres of air. When examined by measurements
of the deflections caused by magnetic and electric fields, the alpha-rays
are found to behave as would positively electrified particles of the
magnitude of helium atoms possessing a double ionic charge and travelling
with a velocity about one-tenth that of light. The second or beta type of
radiation is much more penetrating. It will pass through a considerable
thickness of metallic foil, or many centimetres of air, and still affect
photographic plates or discharge electroscopes. Magnetic and electric
forces deflect beta-rays much more than alpha-rays, indicating that,
although the speed is greater, approaching in some cases within five per
cent. that of light, the mass is very much less. The beta-rays must be
streams of particles, identical with those of cathode rays, possessing the
minute mass of J.J. Thomson's corpuscle, some eight-hundredth part of that
of a hydrogen atom. A third or gamma type of radiation was also detected.
More penetrating even than beta-rays, the gamma-rays have never been
deflected by any magnetic or electric force yet applied. Like Rontgen
rays, it is probable that gamma-rays are wave-pulses in the luminiferous
aether, though the possibility of explaining them as flights of non-
electrified particles is before the minds of some physicists.
Still another kind of radiation has been discovered more recently by
Thomson, who has found that in high vacua, rays become apparent which are
absorbed at once by air at any ordinary pressure.
The emission of all these different types of radiation involves a continual
drain of energy from the radio-active body. When M. and Mme Curie had
prepared as much as a gramme of radium chloride, the energy of the
radiation became apparent as an evolution of heat. The radium salt itself,
and the case containing it, absorbed the major part of the radiation, and
were thus maintained at a temperature measurably higher than that of the
surroundings. The rate of thermal evolution was such that it appeared that
one gramme of pure radium must emit about 100 gramme-calories of heat in an
hour. This observation, naturally as it follows from the phenomena
previously discovered, first called attention to the question of the source
of the energy which maintains indefinitely and without apparent diminution
the wonderful stream of radiation proceeding from a radio-active substance.
In the solution of this problem lies the point of the present essay.
In order to appreciate the evidence which bears on the question we must now
describe two other series of phenomena.
It is a remarkable fact that the intensity of the radiation from a radio-
active body is independent of the external conditions of temperature,
pressure, etc. which modify so profoundly almost all other physical and
chemical processes. Exposure to the extreme cold of liquid air, or to the
great heat of a furnace, leaves the radio-activity of a substance
unchanged, apparent exceptions to this statement having been traced to
secondary causes.
Then, it is found that radio-activity is always accompanied by some
chemical change; a new substance always appears as the parent substance
emits these radiations. Thus by chemical reactions it is possible to
separate from uranium and thorium minute quantities of radio-active
materials to which the names of uranium-X and thorium-X have been given.
These bodies behave differently from their parents uranium and thorium, and
show all the signs of distinct chemical individuality. They are strongly
radio-active, while, after the separation, the parents uranium and thorium
are found to have lost some of their radio-activity. If the X-substances
be kept, their radio-activity decays, while that of the uranium or thorium
from which they were obtained gradually rises to the initial value it had
before the separation. At any moment, the sum of the radio-activity is
constant, the activity lost by the product being equal to that gained by
the parent substance. These phenomena are explained if we suppose that the
X-product is slowly produced in the substance of the parent, and decays at
a constant rate. Uranium, as usually seen, contains a certain amount of
uranium-X, and its radio-activity consists of two parts—that of the
uranium itself, and that of the X product. When the latter is separated by
means of its chemical reactions, its radio-activity is separated also, and
the rates of decay and recovery may be examined.
Radium and thorium, but not uranium, give rise to radio-active gases which
have been called emanations. Rutherford has shown that their radio-
activity, like that of the X products, suffers decay, while the walls of
the vessel in which the emanation is confined, become themselves radio-
active. If washed with certain acids, however, the walls lose their
activity, which is transferred to the acid, and can be deposited by
evaporation from it on to a solid surface. Here again it is clear that the
emanation gives rise to a radio-active substance which clings to the walls
of the vessel, and is soluble in certain liquids, but not in others.
We shall return to this point, and trace farther the history of the radio-
active matter. At present we wish to emphasise the fact that, as in other
cases, the radio-activity of the emanation is accompanied by the appearance
of a new kind of substance with distinct chemical properties.
We are now in a position to consider as a whole the evidence on the
question of the source of radio-active energy.
(1) Radio-activity is accompanied by the appearance of new chemical
substances. The energy liberated is therefore probably due to the
associated chemical change. (2) The activity of a series of compounds is
found to accompany the presence of a radio-active element, the activity of
each compound depends only on the contents of the element, and is
independent of the nature of its combination. Thus radio-activity is a
property of the element, and is not affected by its state of isolation or
chemical combination. (3) The radio-activity of a simple transient product
decays in a geometrical progression, the loss per second being proportional
to the mass of substance still left at the moment, and independent of its
state of concentration or dilution. This type of reaction is well known in
chemistry to mark a mono-molecular change, where each molecule is
dissociated or altered in structure independently. If two or more
molecules were concerned simultaneously, the rate of reaction would depend
on the nearness of the molecules to each other, that is, to the
concentration of the material. (4) The amount of energy liberated by the
change of a given mass of material far transcends the amount set free by
any known ordinary chemical action. The activity of radium decays so
slowly that it would not sink to half its initial value in less than some
two thousand years, and yet one gramme of radium emits about 100 calories
of heat during each hour of its existence.
The energy of radio-activity is due to chemical change, but clearly to no
chemical change hitherto familiar to science. It is an atomic property,
characteristic of a given element, and the atoms undergo the change
individually, not by means of interaction among each other. The conclusion
is irresistible that we are dealing with a fundamental change in the
structure of the individual atoms, which, one by one, are dissociating into
simpler parts. We are watching the disintegration of the "atoms" of the
chemist, hitherto believed indestructible and eternal, and measuring the
liberation of some of the long-suspected store of internal atomic energy.
We have stumbled on the transmutation dreamed by the alchemist, and
discovered the process of a veritable evolution of matter.
The transmutation theory of radio-activity was formulated by Rutherford
(Rutherford, "Radio-activity" (2nd edition), Cambridge, 1905, page 307.)
and Soddy in 1903. By its light, all recent work on the subject has been
guided; it has stood the supreme test of a hypothesis, and shown power to
suggest new investigations and to co-ordinate and explain them, when
carried out. We have summarised the evidence which led to the conception
of the theory; we have now to consider the progress which has been made in
tracing the successive disintegration of radio-active atoms.
Soon after the statement of the transmutation theory, a striking
verification of one of its consequences appeared. The measurement of the
magnetic and electric deflection of the alpha-rays suggested to Rutherford
the idea that the stream of projectiles of which they consisted was a
flight of helium atoms. Ramsay and Soddy, confining a minute bubble of
radium emanation in a fine glass tube, were able to watch the development
of the helium spectrum as, day by day, the emanation decayed. By isolating
a very narrow pencil of alpha-rays, and watching through a microscope their
impact on a fluorescent screen, Rutherford has lately counted the
individual alpha-projectiles, and confirmed his original conclusion that
their mass corresponded to that of helium atoms and their charge to double
that on a univalent atom. ("Proc. Roy. Soc." A, page 141, 1908.) Still
more recently, he has collected the alpha-particles shot through an
extremely thin wall of glass, and demonstrated by direct spectroscopic
evidence the presence of helium. ("Phil. Mag." February 1909.)
But the most thorough investigation of a radio-active pedigree is found in
Rutherford's classical researches on the successive disintegration products
of radium, in order to follow the evidence on which his results are
founded, we must describe more fully the process of decay of the activity
of a simple radio-active substance. The decay of activity of the body
known as uranium-X is shown in a falling curve (Fig. 1.). It will be seen
that, in each successive 22 days, the activity falls to half the value it
possessed at the beginning.
This change in a geometrical progression is characteristic of simple radio-
active processes, and can be expressed mathematically by a simple
exponential formula.
As we have said above, solid bodies exposed to the emanations of radium or
thorium become coated with a radio-active deposit. The rate of decay of
this activity depends on the time of exposure to the emanation, and does
not always show the usual simple type of curve. Thus the activity of a rod
exposed to radium emanation for 1 minute decays in accordance with a curve
(Fig. 2) which represents the activity as measured by the alpha-rays. If
the electroscope be screened from the alpha-rays, it is found that the
activity of the rod in beta- an gamma-rays increases for some 35 minutes
and then diminishes (Fig. 3.).
These complicated relations have been explained satisfactorily and
completely by Rutherford on the hypothesis of successive changes of the
radio-active matter into one new body after another. (Rutherford, "Radio-
activity" (2nd edition), Cambridge, 1905, page 379.) The experimental
curve represents the resultant activity of all the matter present at a
given moment, and the process of disentangling the component effects
consists in finding a number of curves, which express the rise and fall of
activity of each kind of matter as it is produced and decays, and, fitted
together, give the curve of the experiments.
Other methods of investigation also are open. They have enabled Rutherford
to complete the life-history of radium and its products, and to clear up
doubtful points left by the analysis of the curves. By the removal of the
emanation, the activity of radium itself has been shown to consist solely
of alpha-rays. This removal can be effected by passing air through the
solution of a radium salt. The emanation comes away, and the activity of
the deposit which it leaves behind decays rapidly to a small fraction of
its initial value. Again, some of the active deposits of the emanation are
more volatile than others, and can be separated from them by the agency of
heat.
From such evidence Rutherford has traced a long series of disintegration
products of radium, all but the first of which exist in much too minute
quantities to be detected otherwise than by their radio-activities.
Moreover, two of these products are not themselves appreciably radio-
active, though they are born from radio-active parents, and give rise to a
series of radio-active descendants. Their presence is inferred from such
evidence as the rise of beta and gamma radio-activity in the solid newly
deposited by the emanation; this rise measuring the growth of the first
radio-active offspring of one of the non-active bodies. Some of the radium
products give out alpha-rays only, one beta- and gamma-rays, while one
yields all three types of radiation. The pedigree of the radium family may
be expressed in the following table, the time noted in the second column
being the time required for a given quantity to be half transformed into
its next derivative.
Time of half Radio- Properties
decay activity
Radium About 2600 years alpha rays Element chemically analogous
to barium.
>
Emanation 3.8 days alpha rays Chemically inert gas;
condenses at -150 deg C.
>
Radium-A 3 minutes alpha rays Behaves as a solid deposited on
surfaces; concentrated on a
negative electrode.
>
Radium-B 21 minutes no rays Soluble in strong acids;
volatile at a white heat; more
volatile than A or C.
>
Radium-C 28 minutes alpha, beta, Soluble in strong acids; less
gamma rays volatile than B.
>
Radium-D about 40 years no rays Soluble in strong acids; volatile
below 1000 deg C.
>
Radium-E 6 days beta, gamma Non-volatile at 1000 deg C.
rays
>
Radium-F 143 days alpha rays Volatile at 1000 deg C.
Deposited from solution on a
bismuth plate.
Of these products, A, B, and C constitute that part of the active deposit
of the emanation which suffers rapid decay and nearly disappears in a few
hours. Radium-D, continually producing its short-lived descendants E and
F, remains for years on surfaces once exposed to the emanation, and makes
delicate radio-active researches impossible in laboratories which have been
contaminated by an escape of radium emanation.
A somewhat similar pedigree has been made out in the case of thorium. Here
thorium-X is interposed between thorium and its short-lived emanation,
which decays to half its initial quantity in 54 seconds. Two active
deposits, thorium A and B, arise successively from the emanation. In
uranium, we have the one obvious derivative uranium-X, and the question
remains whether this one descent can be connected with any other individual
or family. Uranium is long-lived, and emits only alpha-rays. Uranium-X
decays to half value in 22 days, giving out beta- and gamma-rays. Since
our evidence goes to show that radio-activity is generally accompanied by
the production of new elements, it is natural to search for the substance
of uranium-X in other forms, and perhaps under other names, rather than to
surrender immediately our belief in the conservation of matter.
With this idea in mind we see at once the significance of the constitution
of uranium minerals. Formed in the remote antiquity of past geological
ages, these minerals must become store-houses of all the products of
uranium except those which may have escaped as gases or possibly liquids.
Even gases may be expected to some extent to be retained by occlusion.
Among the contents of uranium minerals, then, we may look for the
descendants of the parent uranium. If the descendants are permanent or
more long-lived than uranium, they will accumulate continually. If they
are short-lived, they will accumulate at a steady rate till enough is
formed for the quantity disintegrating to be equal to the quantity
developed. A state of mobile equilibrium will then be reached, and the
amount of the product will remain constant. This constant amount of
substance will depend only on the amount of uranium which is its source,
and, for different minerals, if all the product is retained, the quantity
of the product will be proportional to the quantity of uranium. In a
series of analyses of uranium minerals, therefore, we ought to be able to
pick out its more short-lived descendants by seeking for instances of such
proportionality.
Now radium itself is a constituent of uranium minerals, and two series of
experiments by R.J. Strutt and B.B. Boltwood have shown that the content of
radium, as measured by the radio-activity of the emanation, is directly
proportional to the content of uranium. (Strutt, "Proc. Roy. Soc." A,
February 1905; Boltwood, "Phil. Mag." April, 1905.) In Boltwood's
investigation, some twenty minerals, with amounts of uranium varying from
that in a specimen of uraninite with 74.65 per cent., to that in a monazite
with 0.30 per cent., gave a ratio of uranium to radium, constant within
about one part in ten.
The conclusion is irresistible that radium is a descendant of uranium,
though whether uranium is its parent or a more remote ancestor requires
further investigation by the radio-active genealogist. On the hypothesis
of direct parentage, it is easy to calculate that the amount of radium
produced in a month by a kilogramme of a uranium salt would be enough to be
detected easily by the radio-activity of its emanation. The investigation
has been attempted by several observers, and the results, especially those
of a careful experiment of Boltwood, show that from purified uranium salts
the growth of radium, if appreciable at all, is much less than would be
found if the radium was the first product of change of the uranium. It is
necessary, therefore, to look for one or more intermediate substances.
While working in 1899 with the uranium residues used by M. and Mme Curie
for the preparation of radium, Debierne discovered and partially separated
another radio-active element which he called actinium. It gives rise to an
intermediate product actinium-X, which yields an emanation with the short
half-life of 3.9 seconds. The emanation deposits two successive
disintegration products actinium-A and actinium-B.
Evidence gradually accumulated that the amounts of actinium in radio-active
minerals were, roughly at any rate, proportional to the amounts of uranium.
This result pointed to a lineal connection between them, and led Boltwood
to undertake a direct attack on the problem. Separating a quantity of
actinium from a kilogramme of ore, Boltwood observed a growth of 8.5 x (10
to the power -9) gramme of radium in 193 days, agreeing with that indicated
by theory within the limits of experimental error. ("American Journal of
Science", December, 1906.) We may therefore insert provisionally actinium
and its series of derivatives between uranium and radium in the radio-
active pedigree.
Turning to the other end of the radium series we are led to ask what
becomes of radium-F when in turn it disintegrates? What is the final non-
active product of the series of changes we have traced from uranium through
actinium and radium?
One such product has been indicated above. The alpha-ray particles appear
to possess the mass of helium atoms, and the growth of helium has been
detected by its spectrum in a tube of radium emanation. Moreover, helium
is found occluded in most if not all radio-active minerals in amount which
approaches, but never exceeds, the quantity suggested by theory. We may
safely regard such helium as formed by the accumulation of alpha-ray
particles given out by successive radio-active changes.
In considering the nature of the residue left after the expulsion of the
five alpha-particles, and the consequent passage of radium to radium-F we
are faced by the fact that lead is a general constituent of uranium
minerals. Five alpha-particles, each of atomic weight 4, taken from the
atomic weight (about 225) of radium gives 205--a number agreeing fairly
well with the 207 of lead. Since lead is more permanent than uranium, it
must steadily accumulate, no radio-active equilibrium will be reached, and
the amount of lead will depend on the age of the mineral as well as on the
quantity of uranium present in it. In primary minerals from the same
locality, Boltwood has shown that the contents of lead are proportional to
the amounts of uranium, while, accepting this theory, the age of minerals
with a given content of uranium may be calculated from the amount of lead
they contain. The results vary from 400 to 2000 million years. ("American
Journal of Science", October, 1905, and February, 1907.)
We can now exhibit in tabular form the amazing pedigree of radio-active
change shown by this one family of elements. An immediate descent is
indicated by >, while one which may either be immediate or involve an
intermediate step is shown by .... No place is found in this pedigree for
thorium and its derivatives. They seem to form a separate and independent
radio-active family.
Atomic Weight Time of half Radio-Activity
decay
Uranium 238.5 alpha
>
Uranium-X ? 22 days beta, gamma
...
Actinium ? ? no rays
>
Actinium-X ? 10.2 days alpha (beta, gamma)
>
Actinium Emanation ? 3.9 seconds alpha
>
Actinium-A ? 35.7 minutes no rays
>
Actinium-B ? 2.15 minutes alpha, beta, gamma
...
Radium 225 about 2600 years alpha
>
Radium Emanation ? 3.8 days alpha
>
Radium-A ? 3 minutes alpha
>
Radium-B ? 21 minutes no rays
>
Radium-C ? 28 minutes alpha, beta, gamma
>
Radium-D ? about 40 years no rays
>
Radium-E ? 6 days beta (gamma)
>
Radium-F ? 143 days alpha
...
Lead 207 ? no rays
As soon as the transmutation theory of radio-activity was accepted, it
became natural to speculate about the intimate structure of the radio-
active atoms, and the mode in which they broke up with the liberation of
some of their store of internal energy. How could we imagine an atomic
structure which would persist unchanged for long periods of time, and yet
eventually spontaneously explode, as here an atom and there an atom reached
a condition of instability?
The atomic theory of corpuscles or electrons fortunately was ready to be
applied to this new problem. Of the resulting speculations the most
detailed and suggestive is that of J.J. Thomson. ("Phil. Mag." March,
1904.) Thomson regards the atom as composed of a number of mutually
repelling negative corpuscles or electrons held together by some central
attractive force which he represents by supposing them immersed in a
uniform sphere of positive electricity. Under the action of the two
forces, the electrons space themselves in symmetrical patterns, which
depend on the number of electrons. Three place themselves at the corner of
an equilateral triangle, four at those of a square, and five form a
pentagon. With six, however, the single ring becomes unstable, one
corpuscle moves to the middle and five lie round it. But if we imagine the
system rapidly to rotate, the centrifugal force would enable the six
corpuscles to remain in a single ring. Thus internal kinetic energy would
maintain a configuration which would become unstable as the energy drained
away. Now in a system of electrons, electromagnetic radiation would result
in a loss of energy, and at one point of instability we might well have a
sudden spontaneous redistribution of the constituents, taking place with an
explosive violence, and accompanied by the ejection of a corpuscle as a
beta-ray, or of a large fragment of the atom as an alpha-ray.
The discovery of the new property of radio-activity in a small number of
chemical elements led physicists to ask whether the property might not be
found in other elements, though in a much less striking form. Are ordinary
materials slightly radio-active? Does the feeble electric conductivity
always observed in the air contained within the walls of an electroscope
depend on ionizing radiations from the material of the walls themselves?
The question is very difficult, owing to the wide distribution of slight
traces of radium. Contact with radium emanation results in a deposit of
the fatal radium-D, which in 40 years is but half removed. Is the
"natural" leak of a brass electroscope due to an intrinsic radio-activity
of brass, or to traces of a radio-active impurity on its surface? Long and
laborious researches have succeeded in establishing the existence of slight
intrinsic radio-activity in a few metals such as potassium, and have left
the wider problem still unsolved.
It should be noted, however, that, even if ordinary elements are not radio-
active, they may still be undergoing spontaneous disintegration. The
detection of ray-less changes by Rutherford, when those changes are
interposed between two radio-active transformations which can be followed,
show that spontaneous transmutation is possible without measureable radio-
activity. And, indeed, any theory of disintegration, such as Thomson's
corpuscular hypothesis, would suggest that atomic rearrangements are of
much more general occurrence than would be apparent to one who could
observe them only by the effect of the projectiles, which, in special
cases, owing to some peculiarity of atomic configuration, happened to be
shot out with the enormous velocity needed to ionize the surrounding gas.
No evidence for such ray-less changes in ordinary elements is yet known,
perhaps none may ever be obtained; but the possibility should not be
forgotten.
In the strict sense of the word, the process of atomic disintegration
revealed to us by the new science of radio-activity can hardly be called
evolution. In each case radio-active change involves the breaking up of a
heavier, more complex atom into lighter and simpler fragments. Are we to
regard this process as characteristic of the tendencies in accord with
which the universe has reached its present state, and is passing to its
unknown future? Or have we chanced upon an eddy in a backwater, opposed to
the main stream of advance? In the chaos from which the present universe
developed, was matter composed of large highly complex atoms, which have
formed the simpler elements by radio-active or ray-less disintegration? Or
did the primaeval substance consist of isolated electrons, which have
slowly come together to form the elements, and yet have left here and there
an anomaly such as that illustrated by the unstable family of uranium and
radium, or by some such course are returning to their state of primaeval
simplicity?
Home Page |
Further Reading |
Site Map |
Send Feedback
