Trinity College, Cambridge.
The 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