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Digitized by LjOOQIC
Digitized by LjOOQIC
MATTER AND RADIOACTIVITY
PROFESSOR OF PHYSICAL CHEMISTRY IN THE
JOHNS ^HOPKINS UNIVERSITY
THIRD EDITION— COMPLETELY REVISED
D. VAN NOSTRAND COMPANY
25 Park Place
Digitized by LjOOQIC
•V: : Copyright, IQ06, by
6. JVan Nostrand Company
Copyright, igio, by
D. Van Nostrand Company
Copyright, igJS, by
D. Van Nostrand Company
PREFACE TO THE FIRST EDITION
The content of this book has already been published
as a series of articles in the Electrical Rmew. The two
correlated subjects imder consideration are of such general
interest that it has seemed desirable that the discussion of
c»^ them should be made accessible in compact form.
^ The several chapters as they originally appeared have,
-n? therefore, been carefully revised and brought together in
5 one volume. The author would extend his sincere thanks
>i to his assistant, Dr. H. S. Uhler, for a number of valuable
^ suggestions in connection with the revision of the work.
The aim of the writer has been to present the more im-
portant facts and conclusions in connection with the work
on the "Electrical Nature of Matter and Radioactivity," as
far as possible in non-mathematical language. This has
been aone with the belief that there are a large nxunber of
those who have a truly scientific interest in these most
recent and important developments in Physics and Physical
Chemistry, but to whom a more technical and rigidly math-
ematical treatment might not appeal. To all who desire
such a treatment, the admirable books by Thomson, on the
** Conductivity of Electricity through Gases," and by Ruther-
ford, on "Radioactivity," are heartily recommended.
While this work is written in a semi-popular style, the
attempt has been made to treat the subject with scientific
accuracy. The facts presented have nearly always been
taken directly from the original sources. Since, however,
this IS a comparatively elementary discussion, references to
the original papers are given chiefly in the cases of the more
important contributions. All of those who desire to go
293731 Digitized by Google
more fully into these subjects are urged to read as many
as possible of the original articles.
If this little book should contribute even in a small meas-
ure towards supplying the general demand for knowledge
in the field which it covers, it will more than repay for the
time and labor that have been spent in its preparation.
Harry C. Jones.
PREFACE TO THE SECOND EDITION
The aim in preparing a second edition of this work is
to bring it up to date as far as matters of fundamental
importance are concerned. Most of the epoch-making dis-
coveries in connection with radioactivity were made in
the earlier stages of the work, but many important con-
tributions to our knowledge in this field have been pub-
lished in the last few years. This material, as far as is
consistent with the scope and size of this book, has been
incorporated in this edition.
The author gladly accepts this opportunity to express
his thanks to his assistant, Dr. W. W. Strong, for valuable
aid in revising this book. H r T
Johns Hopkins University, Baltimore
PREFACE TO THE THIRD EDITION
In preparing the third edition of this little work, some
minor changes, and some additions at the ends of the
chapters have been made. It is gratifying to see how
quickly the third edition of this book has been called for.
The author takes pleasure in expressing his thanks to
his future coworker, Mr. Edward O. Hulburt, for valuable
suggestions in connection with the work of revision.
February, 19x5. H. C. J.
The Electrical Conductivity of Gases i
Conditions which increase the conductivity of gases. How a
conducting gas differs from a non-conducting. The ratio of the
charge to the mass of the ion in a gas. The cathode ray. The
e . e
value of — for the cathode particle. The ratio — constant for
different gases. The ratio — varies for the different ions of
electrolytes. The value of — for gaseous ions produced by differ-
The Determination of the Mass of the Negative Ion in Gases, id
Work of J. J. Thomson. Comparison of the charge on a
gaseous ion with that on a univalent ion of an electrolyte. The
ratio of the charge to the mass for the positive ion.
Nature of the Corpuscle. The Electrical Theory of Matter 19
Work of Thomson and Kaufmann. The electron the ulti-
mate unit of matter. Earlier attempts to unify matter. Other
relations between the elements.
The Nature of the Atom in Terms of the Electron Theory . 29
Thomson's conception of the atom. The electron theory and
the Periodic System. The atom in terms of the electron theory.
Cations and anions in terms of the electron theory. The mass
of an ion not exactlv the same as that of the atom from which it
is formed. The electron theory and radioactivity. More recent
view as to the nature of the atom.
The X-Rays . . ; 41
Nature of the X-ray. The Becquerel ray. Properties of the
Becquerel ray. The thorium radiation. Recent work on the
nature of the X-ray.
The Discovery of Radium 49
The separation of radium from pitchblende. The spectrum of
radium. The atomic weight of radium.
Other Radioactive Substances in Pitchblende 63
Polonium. Actiniumi The more important methods used
in studying radioactivity. Properties of the radiations given out
by radioactive substances.
The Alpha Rays 72
The ratio — for the alpha particle. The mass of the alpha par-
tide. Critical velocity of the alpha particles. Alpha particles
produce delta particles. Alpha particles are probably helium
atoms. Action of the alpha particles on a photographic and on
a fluorescent plate. Stopping the spinthariscope power of mat-
ter for the alpha particles.
The Beta and Gamma Rays 85
The beta rays. Nature of the charge carried by the beta par-
ticles. The determination of — for the beta particle. The mass
of the beta' particle. Relation to the cathode particle. Cathode
rays. Beta rays from radiiun. The gamma rays. Secondary
radiations produced by beta rays. Summary of the properties of
the alpha, beta, and gamma rays. Total number of particles
shot off by radium.
Other Properties of the Radiations 98
The self-luminosity of radiiun compounds. Phosphorescence
produced by radiiun salts. Radium increases the conductivity
of dielectrics. Chemical effects produced by radioactive sub^
stances. Physiological action of the radiations from radium.
Production op Heat by Radium Salts 106
Measurement of the heat liberated by salts of radium. Method
of the Bunsen ice calorimeter. Results of heat measurements.
Source of the heat. Effect on solar heat. Does radium exist in
the sun? Terrestrial heat produced by radium. Bearing on the
calculated age of the earth. Theories as to the soiu'ce of the heat
produced by radium. Calculation of the amount of heat liberated
by radium on the above theory that the heat is produced by the
d particles. Three remarkable properties of radium.
CONTENTS ' VU
Emanation from Radioactive Substances . . ii8
Discovery of the thorium emanation by Rutherford. Method
of obtaining the emanation. Amount of the emanation. Nature
of the emanation. Diffusion of the emanation. Approximate
determination of its molecular weight.
Helium Produced from the Emanation 127
Recovery of emanating power. Decay of emanation. Heat
evolved by the emanation. Heliiun produced from the emana-
tion. This is not a tramsmutation of the elements. Further
experiments on the production of helium from radium. Relation
between the emanation and helium. Some remarkable results
obtained by the action of the radium emanation.
Induced Radioactivity 140
Induced radioactivity produced by the emanation. Induced
radioactivity undergoes decay. Induced radioactivity due to the
deposit of radioactive matter. Properties of the radioactive
matter deposited by the emanation from radioactive substances.
Emanation X. Facts that must be taken into account in dealing
with the decay of induced or excited radioactivity. Radium
probably identical with poloniiun. Summary of the decomposi-
tion products of radium. Decomposition products of other radio-
active substances. Interpretation of these facts. Radiothorium —
a new radioactive element. Decomposition products of activium.
Production of Radioactive Matter 161
Continuous formation of radioactive matter in uranium. Re-
covery of activity by uraniiun, and decay of activity in uranium X.
Radiation from uranium X. Continuous formation of radio-
active matter from thorium. Properties of thorium X. Decay
of its radioactivity. Thoriiun X produces the thorimn emana-
tion. Recovery of radioactivity by thoriiun. Rate at which
thorium recovers radioactivity independent of conditions. Ra-
dium does not give rise to substances corresponding to uranium
X and thorium X.
Theoretical Considerations . . . . 170
Importance of a theory or generalization. The more important
facts in connection with uranium. The more important facts in
connection with thorium. The more important facts in connection
with radium. Theory of Rutherford and Soddy to account for
radioactive phenomena. The transformations of the radioactive
elements differ fundamentally from ordinary chemical reactions.
The electron theory of J. J. Thomson as applied to radioactivity.
Is matter in general undergoing transformation?
Wide Distribution of Radioactive Mattes and the Origin of
Radioactive matter in the earth and sea. Radioactive matter
in the air. Is matter in general radioactive? The origin of
radium. Ionium. The complete series of transformations in
which radium is involved. Emanium. Conclusion.
Atomic Weights of Radioactive Lead from Different Sources 204
ABBREVIATIONS OF THE TITLES OF
Amer. Chem. Joum. =■ American Chemical Journal.
Amer. Joum. Sd. « American Journal of Science.
Ann. Chim. Phys. «= Annales de Chimie et de Phjrsique.
Ann. d. Phjrs. ^ Annalen der Physik (Drude).
Ber. d. deutsch. chem. Gesell. - Berichte der deutschen chemischen
Cam. Phil. Soc. Proc. = Proceeding of the Cambridge Philosophical
Chem. News ^ Chemical News.
Compt. rend. = Comptes rendus.
Joum. Chem. Soc. «= Joumal of the Chemical Society of London.
Joum. de Chim. Phys. « Joumal de Chimie Physique.
Nat. = Nature.
Phil. Mag. » Philosophical Magazine.
Phil. Trans. « Philosophical Transactions of the Royal Society.
Phys. Rev. « Physical Review.
Phys. Zeit. «= Physikalische Zeitschrift.
Roy. Soc. Proc. « Proceedings of the Royal Society.
Wied. Ann. « Wiedemann's Annalen.
Zeit. phys. Chem. = Zeitschrift fiir physikalische Chemie.
The Electrical Nature of Matter and
The Electrical Conductivity of Gases
The power of gases, under normal pressure and at ordi-
nary temperatures, to conduct electricity is so small that it
has been doubted whether pure, dust-free gases can con-
duct at all. Recent refined experiments, however, show
that while pure, dust-free gases have only a small con-
ductivity, they have a definite power to conduct electricity,
which is measurable.
CONDITIONS WHICH INCREASE THE CONDUCTIVITY OF GASES
While gases under- normal conditions have only slight
conductivity, and are fairly good insulators, it is not a
difficult matter to increase greatly the conductivity of gases.
This can be done in a number of ways. When gases are
heated to high temperatures their electrical conductivity
is greatly increased. According to Becquerel, when air
is heated to a white heat, electricity will pass through it
when the difference in potential is small. It is also known
that gases in contact with incandescent soKds have their
conductivity increased. Some interesting and important
facts, which it would lead us too far at present to discuss,
have been brought to light through the study of these
2 THE ELECTRICAL NATURE OF MATTER
Gases taken from flames have been found to show con-
siderable conductivity, which is retained for some time
after the gas has been removed from the flame and cooled
Other agents which increase the conductivity of gases
are Rontgen rays, the presence of radioactive substances,
and cathode rays. As these will be taken up later in some
detail, they will not be discussed further in the present
HOW A CONDUCTING GAS DIFFERS FROM A NON-CONDUCTING
We have seen that a gas in the normal condition has
very small power to conduct electricity.
We have also seen that the conducting power of a gas
can be greatly increased by a number of widely different
agents. The question that would naturally arise in this
connection is, how does a conducting gas differ from a
non-conducting or normal gas? (We may term a normal
gas non-conducting, since its conductivity is so slight.)
To answer this question we must study the properties
of a conducting gas, and compare them with the properties
of a non-conducting gas.
If the conducting gas is made to pass through a plug of
glass-wool, or is drawn through water, it loses its conduct-
ing power. The conducting power of a gas is also removed
by passing the gas through a metal tube of very fine bore;
the finer the bore the more rapidly the conductivity is
The removal of the conducting power by filtering through
glass-wool shows that the conductivity of the gas is due to
some constituent which is filtered out mechanically by the
glass-wool. The experiments with the metal tube show
that this constituent which can be filtered out by glass-
THE ELECTRICAL CONDUCTIVITY OF GASES 3
wool is charged with electricity. These charged particles
in a conducting gas are known as ions. Some of these
particles are charged positively and others negatively.
Since a conducting gas shows neither an excess of positive
nor of negative electricity it is, as we say, electrically neutral.
THE RATIO OF THE CHARGE TO THE MASS OF THE ION IN A
When an acid, base, or salt is dissolved in water, we know
that it breaks down into charged parts called ions. Every
molecule of an electrolyte yields an equivalent nxmiber
of posivitely charged parts or cations, and negatively
charged parts or anions. The ratio of the charge carried by
these ions to their mass has been determined. In the case of
the hydrogen ion, which is the characteristic ion of all acids,
it has been found to be of the order of magnitude of lo*.
It was recognized to be of importance to determine the
ratio of the charge to the mass of the ion in gases. If we
represent the charge carried by the gaseous ion by e, and
the mass of the ion by m, the ratio in question is — .
We shall take up first the determination of the ratio —
for the cathode particle.
THE CATHODE RAY
When an electric discharge is passed through a highr
vacuum tube, rays are sent out from the cathode which
generally produce a greenish yellow phosphorescence where
they fall upon the glass walls of the tube. These are known
as the cathode rays. The nature of these rays was for some
time in doubt. It was thought by some investigators
that they were waves in the ether. It remained for Sir William
4 THE ELECTRICAL NATURE OF MATTER
Crookes to give us the accepted explanation of the nature
of the cathode rays. According to Crookes the cathode
rays are charged particles, sent off from the cathode with
very high velocity. They move towards the anode in a
direction at right angles to the surface of the cathode. The
properties of the cathode rays, in general, are in accord
with this theory. The cathode rays can be deflected by a
A solid body placed in their path casts a well-defined
Cathode rays can probably produce certain chemical
changes, especially of a reducing nature.
Mechanical effects can readily be produced by the cathode
rajrs, as was shown by Sir William Crookes. A glass paddle-
wheel is easily made to move along level glass tracks within
the tube, by allowing the cathode rays to impinge upon
Thermal effects are readily produced by the cathode
rays. By suitably concentrating them upon platinum,
the metal is rendered incandescent. All of these facts
accord with the theory as to the nature of the cathode rays,
advanced by Sir William Crookes.
The discovery that cathode rays can pass through thin
films of metal seemed at first to argue against the Crookes
theory. When we become familiar later with the exact
nature of the cathode particle itself, we shall see that this
argument is without foimdation.
We shall, then, at present accept the Crookes theory,
and regard the cathode rays as consisting of negatively
electrified particles, moving with high velocities, in straight
lines at right angles to the surface of the cathode.
In the light of the above theory and the facts upon which
it is based, we shall now take up the work of J. J. Thom-
THE ELECTRICAL CONDUCTIVITY OF GASES 5
son, by which he determined the value of — for the cathode
THE VALUE OF — FOR THE CATHODE PARTICLE
The value of — for the cathode particle was determined
by J. J. Thomson/ as follows: The cathode is placed near
one end of an exhausted tube, and the anode removed only
a short distance from the cathode. Beyond the anode on
the side removed from the cathode is placed a metal plug
connected with the earth. A small hole is bored through the
centre of the anode and the metal plug. Cathode rays pass
through these holes and fall on the wall of the vacuum tube
at the end of the tube farthest removed from the cathode.
Since the holes in the metal plates are small, we have a nar-
row beam of cathode rays striking the inner wall of the glass
vessel, forming a small, phosphorescent spot on the glass.
We have seen that the cathode rays are deviated by a
magnetic field. If the whole tube is now properly placed
in a magnetic field, the path of the cathode particles will
be changed, and they will impinge upon the glass wall at
some point different from that which they originally bom-
barded when no magnetic field was present. Measuring
the magnitude of this deflection we can calculate the value
of — , in which v is the velocity of the ion.
We have thus determined the ratio of e to vm.
We must now determine the value of z; in order to obtain
the ratio — .
Into the above-mentioned vacuum tube are inserted
iPhil. Mag., 44, 293 (1897).
6 THE ELECTRICAL NATURE OF MATTER
two parallel plates of aluminium, which are so arranged
that the beam of cathode particles passes between them.
The plates are also parallel to the original, imdeflected
beam. These metal plates are attached^ to some electrical
source, and maintained at a known difference in potential.
We thus have between the plates an electric field. The
electrostatic intensity s, due to this field, deflects the ion
with a force se, e being the charge upon the ion. The force
due to the magnetic field already considered is fev, f being
the strength of the field, e the charge carried by the ion,
and V the velocity of the ion.
By suitably charging the metal plates, the electrical and
magnetic forces can be made to act counter to one another.
These two coimter forces can readily be made equal to each
other. This can easily be determined. We note the origi-
nal position of the phosphorescent spot on the glass before
placing the apparatus in the magnetic field. When the
apparatus is placed in the magnetic field the beam of
cathode particles is deflected, and the bright spot on the
glass changes its position. The electrostatic force, acting
coimter to the magnetic, causes the beam to occupy more
nearly its original position. When these two opposing
forces are equal the phosphorescent spot occupies its origi-
nal position. Thus we have an easy and efficient means
of determining when these two opposite forces are equal.
When they are, fev = se.
Knowing now the value of z>, and having previously
determined, as we have seen, the ratio of e to vm, we have
the value of — which is the quantity desired.
THE ELECTRICAL CONDUCTIVITY OF GASES
THE RATIO — CONSTANT FOR DIFFERENT GASES
Using a somewhat different method, J. J. Thomson found
at first that the ratio — was a constant, whether the gas in
the tube was air, carbon dioxide, methyl iodide, or hydrogen.
This is a most important fact, as we shall see.
Thomson and his coworkers then changed the nature
of the metal of which the cathode was made, using plati-
num, alimiinium, silver, copper, tin, zinc, lead, and iron,
to see whether the nature of the metal from which the
cathode discharge takes place has any effect on the value of
the ratio — . They found the same value for — , regardless
of the nature of the metal of which the cathode was made.
Thomson found that the value of — was equal to about
THE RATIO — VARIES FOR THE DIFFERENT IONS OP
It will be seen that the value of — for the ions of elec-
trolytes varies with every kind of ion. This is necessarily
the case, since the charge carried is the same for all imiva-
lent ions (and this quantity multiplied by the valency for
all polyvalent ions, as is seen from Faraday's law), and the
mass varies with every cation and every anion. Taking the
ion characteristic of acids, hydrogen, the value of — for
the hydrogen ion is i X lo*.
It is therefore obvious that the value of — for the cathode
8 THE ELECTRICAL NATURE OF MATTER
particle is one thousand times as great as the corresponding
value for the hydrogen ion produced when any acid is dis-
solved in a dissociating solvent.
Knowing the values of — in the two cases does not tell
us anything about the relative masses of the hydrogen ion
in solution, and the particle in the cathode discharge; since
the charges carried in the two cases might be the same or
might be very different. Before answering this question
we must know the relative charges carried by the ion in
electrolysis, and by the cathode particle.
THE VALUE OF — FOR GASEOUS IONS PRODUCED BY
Before taking up the beautiful method for determim'ng
the value of the charge carried by the cathode particle, we
shall ask and answer the question whether the value of —
for gaseous ions is the same, regardless of the means
by which the gaseous ions are produced, or whether it
varies with the means employed to produce the ions in the
The answer to this question is unmistakably given by
the results that have been obtained. The Lenard rays
are nothing but cathode rays that have left the so-called
vacuum tube by passing through a thiri sheet of aluminium.
The value of — for the particles in these rays has been
found to be about 4X10'.
The value of — for the gaseous ions produced in con-
tact with incandescent metals is about 8.5 Xio*.
THE ELECTRICAL CONDUCTIVITY OF GASES
The value of — for the negative ion given off from radio-
active substances is about iXio^.
It is obvious that the above values all refer to the n^a-
tive, gaseous ion. We see from the results that the value
of — for this ion is practically constant, regardless of the
means by which it is produced, and regardless of the nature
of the gas from which it is produced.
As to the value of — for the positive ion of gases, we
shall have something to say in the next chapter, and shall
also discuss the nature of this ion.
The Determination of the Mass of the Negative
Ion in Gases
WORK of J. J. THOMSON
The determination of the charge carried by the negative
ion is of the very greatest importance. We have akeady
considered the method for determining the ratio — for the
negative ion. If we can now determine «, the charge carried
by this ion, we would know w, the mass of the negative ion.
One of the most ingenious experiments in modem physics
has been devised by J. J. Thomson for solving this problem.
The experiment is based on an observation made by C. T.
R. Wilson/ that gaseous ions, both positive and negative,
can act as nuclei for the condensation of water-vapor, even
if there are no dust particles present in the gas. If a given
volume of a gas containing ions is allowed to expand, it
cools itself, and a part of the water-vapor will condense
aroimd the ions, producing a fog or cloud in the apparatus
containing the gas.
That the water-vapor actually condenses aroimd the
ions was proved conclusively by J. J. Thomson, by the
following very simple experiment. Two parallel metal
plates were placed a few centimetres apart in the vessel
containing the gas which had been freed fiom dust. These
iPhil. Trans., A., 265 (1807).
DETERMINATION CF THE MASS OF THE NEGATIVE ION II
plates were connected with the termmals of a battery, by
which they could be charged to a relatively high difference
in potential. Ions were produced in the gas by passing
Rontgen rays through it.
If the gas was expanded before the plates were connected
with the battery, condensation of the vapor took place;
just as we should expect if the ions acted as nuclei around
which the water-vapor would condense. If the plates are
now connected with the battery and charged, the strong
electrical field would remove the ions from the gas, and if
the gas were then subjected to expansion we would not ex-
pect any appreciable condensation to take place, and such
is the fact. Thomson says that imder these conditions the
condensation is scarcely greater than in unionized air.
This experiment shows conclusively that it is the ions
that serve as the centres of condensation of the water-
vapor — a drop of water condensing aroxmd every ion if
the ions are not too numerous. If we knew the number
of droplets of water in a given volume of the gas, we would
know the number of ions in that volmne. It is, however^
obviously impossible to determine the nmnber of water
particles in a volume of gas by any direct method. Thom-
son ^ solved this part of the problem by using an equation
deduced by Stokes, connecting the rate at which the par-
ticles fall with their size. If we represent by v the velocity
with which the particles fall, by g the acceleration of gravity,
by c the viscosity coeflBident of the gas, and by r the radius
of the drop,
By observing the rate at which the cloud settles we arrive
J Pha. Mag., 46, 528 (i8^<5).
12 THE ELECTRICAL NATURE OF MATTER
at the value of v. Knowing v we determine at once the
value of f , the radius of the drop. Knowing the radius of
the drop we know its volume.
If we represent the mass of the water deposited by each
cubic centimetre of the gas by M, the number of drops in a
cubic centimetre n is given by the following equation:
The mass of water deposited from each cubic centimetre
of the gas, Af , must be determined indirectly. Thomson
made use of the heat that is liberated when the water-vapor
condenses around the gaseous ions. Knowing M and r,
we have all the data necessary for calculating n, the num-
ber of ions in a cubic centimetre of the gas, which is equal
to the number of droplets in the same volume.
We now know the number of ions in a given volxmie of
the gas. It still remains to determine the charge carried
by a single ion.
If we knew the total quantity of electricity carried by the
known nmnber of ions, we would know the amount carried
by one ion. Let v be the mean velocities of both positive
and negative ions when subjected to imit electrical force.
We must measure the current carried by these ions across
unit area, imder an electric force F, in order to determine
the charge carried by a single ion. If we represent the
charge carried by a single ion as formerly by e, we have:
Fvne = current through imit area perpendicular to the
current. Measuring the current that passes through the
gas, we know all of the above quantities except e, which is
calculated at once.
In performing the condensation experiment it is neces-
DETERMINATION OF THE MASS OF THE NEGATIVE ION 13
sary, as Thomson points out, to work with gases that con-
tain only a comparatively small number of ions. When the
conducting gas contains a larpe number of ions some of
these are not carried down by the condensed water-vapor,
as is shown by the fact that under these conditions a second
expansion of the gas, which is no longer subjected to the
ionizing agent, will produce still further condensation,
demonstrating that it still contains ions that were not carried
down by the first expansion.
The condition that the gas shall contain only a few ions
is easily secured, especially when the gas is ionized by
Rontgen rays. Either a weak stream of the. rays is allowed
to pass directly through the gas, or the intensity of the rays
is diminished by inserting thin sheets of certain metals,
such as aluminium, in their path.
The earUer experiments showed that the values of e for
air ionized by Rontgen rays, and for hydrogen gas ionized
by Rontgen rays, are equal to within the limit of experi-
mental error, which proves that the gaseous ion carries the
same charge whatever the gas from which it was produced.
It is of the order of magnitude 4X10"*^.
COMPARISON OF THE CHARGE ON A GASEOUS ION WITH
THAT ON A UNIVALENT ION OF AN ELECTROLYTE
Having determined the magnitude of the charge on a
gaseous ion, we shall next determine the magnitude of the
charge carried by a univalent ion of an electrolyte — say
the hydrogen ion.
We know that the number of molecules in a cubic centi-
metre of a gas, at a pressure of 760 millimetres of mercury
and at zero degrees, is between 2X10" and IXIo*^ We
know the amount of electricity required to liberate this
14 THE ELECTRICAL NATURE OF MATTER
amount of hydrogen gas. From these data we calculate
that the charge carried by the hydrogen ion in solution is
somewhere between iXio~*® and 6Xio""*®.
We thus see that the charge carried by the gaseous ion is
the same as that carried by the hydrogen ion in electrolysis.
This conclusion is based upon a large amount of work
with the ions produced from various gases and by various
ionizing agents. We have already seen that the value of —
lor all 0} these gaseous ions is the same, no matter what the
nature 0} the gas from which they were produced, and no
matter what the nature of the ionizing agent. It has further
been shown that all of these gaseous ions carry the same
charge, and that this is the same charge as that carried by
the hydrogen ion in aqueous solution.
We have now all the data necessary for calculating the
relative masses of the gaseous ion, and the hydrogen ion in
The value of — for the hydrogen ion in solution is 10*.
The value of — for the gaseous ion is 10^. The values of e
in the two cases are the same. Therefore, the value of m
for the gaseous ion is about one-thousandth the value of m
for the hydrogen ion in solution.
. More accurate determinations show that the relation
between the masses of the gaseous ion and the hydrogen
ion in solution is as i to about 1765.
It is difficult to overestimate the importance of this con-
clusion. In the first place, it is a matter of the very highest
importance to establish the fact that the mass of the gaseous
negative ion is always the same, no matter what the nature
of the gas from which this ion is split off, and no matter
DETERMINATION OF THE MASS OF THE NEGATIVE ION 1 5
what the nature of the ionizing agent. This has been
shown to be true whether the gas is elementary or compound.
This shows that a common constituent can be split off from
all gases no matter how widely they may differ chemically,
and what is perhaps even more important is that the mass
of this negative ion which can be split off from any gas is
much less than the mass of the lightest so-called element known
to the chemist. The gaseous negative ion is, then, a com-
mon constitutent of all matter, and is much smaller than
the smallest atom known to the chemist, having a mass
which is only about hVt ^^ ^^^ ^^ ^^^ hydrogen ion in
solution, which, as we shall see, has practically, but not
exactly, the same mass as the hydrogen atom.
This unit of matter, so much smaller than the atom,
and which is apparently common to all atoms, carrying a
imit, negative electrical charge or that charge carried by
the chlorine ion in solution, Thomson called a corpuscle.
THE RATIO OF THE CHARGE TO THE MASS FOR THE
Before leaving this part of our subject a few words should
be added in relation to the value of the ratio — for the
positive ion. These positive ions exist in the so-called
canal rays, discovered by Goldstein. They are also
known as anode rays. Just as the cathode rays move
from the cathode towards the anode, so there is a corre-
sponding movement of matter towards the cathode. This
can be detected by perforating the cathode with a number
of holes, through which the canal rays pass, and pro-
duce a phosphorescence where they fall on the walls of the
glass tube behind the cathode.
Wien used a perforated cathode of iron, and determined
l6 THE ELECTRICAL NATURE OF MATTER
the value of — for the rays which passed through his
cathode. He used the method already described for deter-
mining the value of — for the cathode particles. He de-
flected the rays by means of a strong magnetic field, and
then in the opposite direction by means of an electrostatic
field. A strong magnetic field is necessary to produce an
appreciable deflection of the canal rays, and this renders
the result less accurate. He obtained the following average
- = 3 X IO^
He also found that these positively charged particles move
with much smaller velocity than the negatively charged
If we compare the value of — for the negatively charged
particle with tliat for the positively charged iron particle,
we shall see that the value for the negatively charged
particle is about 3.3X10* times the value for the positively
Since the electricity carried by the positively charged par-
ticle is the same in quantity as that carried by the nega-
tively charged particle, it follows that the mass of the positive
particle is of the same order of magnitude as that of the •
corresponding ion in solution.
We can, then, conclude that while the mass of the nega-
tively charged particle in a gas is constant, independent of
the nature of the gas, and very small as compared even
with the mass of the lightest atom or ion in solution, the
mass 0} the positively charged particle is 0} the same order
0} magniti4de as the corresponding atom or ion in solution in
a dissociating solvent. The mass of the positively charged
DETERMINATION OF THE MASS OF THE NEGATIVE ION 1 7
particle is not constant for different gases, but, as we should
expect if the positive ion is a charged atom, varies with the
natiu-e of the gas in question.
The recent work of Thomson^ on the ratio of — for
the positively charged particles gave the following values.
Wien obtained values as high as lo* for the more readily
deflected positive particles. Thomson found in air as in
hydrogen 1.2 Xio*, while another set of particles in hy-
drogen gave the value 2.9X10'. In argon he found the
In gases at low pressures essentially the same values
were found for the ratio — . When the pressure was low
or the dilution of the gas great, there were generally
streams of two kinds of carriers, one having the value of
— = 10*, and the other the value about 5 X lo*.
It should be noted that these are the approximate values
for the charged atom and the charged molecule of hydro-
gen. An elaborate study of positive rays has recently been
made by Thomson.^ To give an idea of the complexity
of the phenomena in a discharge tube, Thomson calls
attention to the fact that Goldstein,' who discovered the
"Canal Rays," discovered five kinds of rays besides the
cathode rays. This led Thomson to undertake the above-
An elaborate investigation of the positive rays has been
made by Stark, but the scope of this book will not permit
of a discussion of this work.
This beautiful work of Thomson, on the conduction of
1 Phil. Mag., 13, 561 (1907); I4f 359 (1907).
» Ihid., 16, 657 (1908).
l8 THE ELECTRICAL NATURE OF MATTER
electricity through gases, makes it more than probable that
a small particle which he calls the corpuscle is split off from
the atoms of all gases, carries the negative charge, and is
the same imit, no matter what the nature of the atom from
which it separates.
The remainder of the atom from which the corpuscle has
separated carries the positive charge, and is the positively
charged ion in the gas. The nature of this positive ion is
different for every gas, being simply the atom minus the con-
stituent common to all atoms, which is the corpuscle.
It would be a tremendous step forward towards the solu-
tion of one of the greatest problems with which men of
science have had to deal — the ultimate nature of matter —
had Thomson gone no farther than what has been above
developed. This is, however, but the beginning. Thom-
son has studied the nature of the corpuscle itself, and the
result of this part of his investigation is certainly one of
the most fascinating, and probably one of the most valuable
contributions to modem science.
Nature of the Corpuscle — the Electrical Theory
The conception of the corpuscle as originally advanced is
that it is a small piece of matter having a mass about itV^
of that of the hydrogen atom, and carrying a unit negative
charge of electricity, which is exactly the same as that car-
ried by any univalent anion, such as the chlorine ion in solu-
tion. The corpuscle is thus both material and electrical
in its nature.
We shall now take up Thomson's study of the corpuscle
itself, and see how the original conception has been modi-
fied, and the reasons for the view that we hold at present.
Let us first ask what reason have we for supposing that
the corpuscle contains any matter at all? How do we
know that it is anything but electricity? The answer would
be that the corpuscle has both mass and inertia, and, there-
fore, must contain matter, since matter only has these proper-
ties. We shall now see whether this line of reasoning is
WORK OF THOMSON AND KAUFMANN
In a paper published a number of years ago, J. J.
Thomson at least raised the question as to whether inertia
itself is not of electrical origin. The mass of a charged
sphere would, in this case, be greater than that of the same
sphere when uncharged.
20 THE ELECTRICAL NATURE OF MATTER
Thomson showed that the particle must move very rapidly
in order to have appreciable changes in its mass. Indeed,
it must move with a velocity which is comparable with that
of light, in order to produce measurable changes in its mass.
While the ordinary cathode rays move with a velocity
that is only about 3X10® centimetres per second, the
particles shot off from radium h^ve a velocity as high as
2.8X10*®, which is nearly that of light itself = 3X10*^
If the velocity with which the charge moves has any effect
on its apparent mass, we should expect that the mass of
these rapidly moving particles would be greater than that
of the same particles when moving less rapidly. This
question has been answered by the experiments of Kauf-
mann.* He determined the value of — for these more
rapidly moving particles, by means of the method already
described, using the magnetic and electrical deflections.
He found values as low as 0.63X10^ for the most rapidly
moving particles. Since e is constant, the charge being
the same independent of the velocity, it follows that the
mass of the rapidly moving, charged particle is greater than
that of the more slowly moving, charged particle.
Kaufmann's experiments went farther. By means of
the electrical and magnetic deflections, he determined the
values of — for the )8 particles shot off from radium with
different velocities. We shall learn that these are essen-
tially cathode ray particles. He obtained the following
results. The velocities v are divided by 10*®, and the
values of — by 10^, for convenience. The figures give us
the relative values, which are all that we desire at present:
> Phys. Zeit., 4, 54 (1902).
NATURE OF THE CORPUSCLE 21
It is obvious from the above data that as the velocity of
the charged particle increases, the value of — decreases.
Since the value of the charge, e, remains constant, inde-
pendent of the velocity, it follows that the mass m becomes
greater and greater as the velocity of the charged particle
The experiments of Kaufmann show conclusively that
the mass of a charged particle changes with the velocity of
the particle, increasing as the velocity increases. In a
word, a part of the mass of the particle, at least, is of electrical
This would naturally raise the question, what part of
the mass is electrical? Is it possible that all mass is elec-
trical? Thomson has thrown light on this question in the
following manner. When the corpuscle moves slowly the
mass, as we have seen, does not depend on the velocity,
and does not, therefore, change with the velocity. When,
on the other hand, the velocity of the corpuscle approaches
the velocity of light, the mass varies with the velocity, as
is shown by the results of Kaufmann. Assuming that the
entire mass of the corpuscle is of electrical origin, Thom-
son has calculated the variation of the masses of the
particles with the velocity.
The agreement between the calculated and observed
22 THE ELECTRICAL NATURE OF MATTER
values is surprisingly good. This is a strong argument in
favor of the correctness of the assumption on which the
calculation is based.
If the whole mass of the corpuscle is electrical, why assume
that the corpuscle contains any so-called matter at all?
All of the properties of the corpuscle, including the two
properties that we have been accustomed to associate with
matter, inertia and mass, are accounted for by the electrical
charge of the corpuscle. Since we know things only by
their properties, and since all of the properties of the cor-
puscle are accounted for by the electrical charge associated
with it, why assume that the corpuscle contains anything
but the electrical charge? It is obvious that there is no
reason for doing so.
The corpuscle is, Ihen, nothing but a disembodied electrical
charge^ containing nothing material, as we have been accus-
tomed to use that term. It is electricity, and nothing but
electricity. With this new conception a new term was
introduced, and, now, instead of speaking of the corpuscle
we speak of the electron. The electron is, then, a disem-
bodied electrical charge, containing no matter, and is the
term which we shall hereafter use for this ultimate unit, of
which we shall learn that all so-called matter is probably
If the electron contains nothing that corresponds to our
ordinary conception of matter, and since the same electron
can be split off from the atoms or molecules of all sub-
stances, the question naturally arises, is not all so-called
matter made up of these electrical charges or electrons?
Is not all matter of an electrical nature? There is a large
amount of evidence, part of which has already been given,
which answers this question in the affirmative. Indeed,
this conclusion is accepted, at least tentatively, by a large
NATURE OF THE CORPUSCLE 23
number of the leading physicists and physical chemists
the world over.
THE ELECTRON THE ULTIMATE UNIT OF MATTER
According to the above theory the electron is the ultimate
unit of all matter. The atoms are made up of electrons
or disembodied electrical charges, in rapid motion; the atom
of one elementary substance differing from the atom of another
elementary substance only in the number and arrangement
of electrons contained in it. Thus we have at last the ulti-
mate unit of matter, of which all forms of matter are com-
posed; and the remarkable feature is, that this ultimate
imit of which all matter is composed is not matter at all, as
we ordinarily understand that term, but electricity.
This recalls a paper published a number of years ago
by Ostwald,^ on "The Overthrow of Scientific Materialism,"
which made an impression at the time that it appeared, or
rather a number of impressions. The arguments and con-
clusions in this paper were accepted by some without ques-
tion, and were severely criticised by others, especially by
the mathematical physicists of Germany. Whatever our
opinion of the paper as a whole, there is one point at least
brought out so clearly that there can scarcely be any ques-
tion about it, and that is, that matter is a pure hypothesis.
What we know in the universe, and all that we know, is
changes in energy. In order to have something to which
we can mentally attach the energy, we have created, in our
Matter, then, is a pure h)rpothesis, and energy is the only
reality. We are accustomed to take exactly the opposite
view, and regard matter as the reality and energy as hy-
pothetical. If Ostwald accomplished nothing else by the
» Zeit. phys. Chem., 18, 305 (1895).
24 THE ELECTRICAL NATURE OF MATTER
paper in question than the mere calling attention to the
hypothetical nature of matter, he made an important con-
tribution to science.
It should also be noted that for a long time Ostwald has
insisted not only that matter is a pure hypothesis, but there
is not the least evidence for its existence, as we ordinarily
understand the term. It is interesting to note that Thom-
son has reached the same conclusion, as the result of one
of the most brilliant series of experiments that has ever
been carried out in any branch of experimental science.
We thus have a direct experimental verification of a conclu-
sion, the importance of which it is difl&cult to overestimate.
EARLIER ATTEMPTS TO UNIFY MATTER
Perhaps the most important bearing of the electron is
that it furnishes us with the ultimate basis of all matter.
The importance of securing such an ultimate unit is shown
by the number of attempts that have been made in this
direction. One of the first noteworthy efforts we owe to
the chemist Prout. After fairly accurate determinations of
the atomic weights of a number of the more common ele-
ments had been made, it appeared that when these values
were expressed in terms of the atomic weight of hydrogen
as unity, they were all nearly whole numbers. Indeed, the
deviations at first discovered were hardly greater than the
This led Prout, as early as 1815, to propose the theory
that hydrogen is the ultimate element of which all other
elementary substances are made. The atoms of all other
elements are simply condensations of hydrogen atoms, the
number of hydrogen atoms contained in an atom of any
element being expressed by the atomic weight of the element
in terms of hydrogen as unity.
NATURE OF THE CORPUSCLE 2$
This hypothesis of Prout accounted for all the facts that
were known at the time when it was proposed, and it is a
praiseworthy attempt to solve the problem of the relation
between the various chemical elements.
As experimental methods became more refined, and
atomic weights more accurately determined, it gradually
became obvious that the atomic weights of even some of
the more common elements are not whole numbers in terms
of hydrogen as one, but differ very appreciably from whole
numbers. Indeed, the atomic weights of some elements
fall almost half-way between whole numbers. This was,
of course, a deviation too large to be accounted for on the
basis of experimental error, and was, therefore, the death
blow to the hypothesis of Prout as it was originally proposed
by its author.
Subsequent suggestions by Marignac and others to make
the half-atom of hydrogen, or even the quarter-atom, the
basis of all matter, did not increase scientific respect for
the hypothesis of Prout. Having once begun to divide the
hydrogen atom the process could be continued indefinitely,
and thus the theory could be, and was for a time, brought
It must not, however, be forgotten that if to-day we take
those chemical elements whose atomic weights are most
accurately determined, and calculate the atomic weights
on the basis of oxgyen = i6, which is the system now in
general use, we shall find that a very large proportion of the
atomic weights are so close to whole numbers that the
deviations can be accounted for on the basis of probable
Such an examination was recently made by Strutt,^ who
pointed out that the number of elements whose atomic
* Phil. Mag., I, 311 (1901).
26 THE ELECTRICAL NATURE OF MATTER
weights are whole numbers is many times too large to be
accounted for on the basis of chance.
Taking all of the facts into accoimt, we recognize, of
course, that the hypothesis of Prout, either as originally
proposed, or as subsequently modified, is not rigidly true;
but we still feel intuitively that there is something in it.
The coincidences are far too numerous to be attributed to
OTHER RELATIONS BETWEEN THE ELEMENTS
A number of other attempts have been made to point
out relations between the atoms of the different chemical
elements, with the hope of finding something in common
between them. Dobereiner early noticed that of three
closely related chemical elements, the atomic weight of the
second heaviest element is almost exactly the mean of the
atomic weights of the Ughtest and heaviest elements of
the group of three. A few examples will make this clear.
Take the three elements, calcium, strontium and bariimi.
The atomic weight of calcium is 40.07; the atomic weight
of barium is 137.37. The mean of these two values is
88.72, and the atomic weight of strontium is 87.63. To
take an example from the negative elements, the above
being taken from the positive, let us choose sulphur, se-
lenium and tellurium. The atomic weight of sulphur is
32.07; that of tellurium 127.5. The mean of these two
values is 79.78, while the atomic weight of selenium is 79.2
Relations such as these are, of course, purely empirical,
and their meaning is entirely unknown, yet they are, to
say the least, suggestive.
We now come to the great generalization of Newlands,
Mendel^eff, and X/Othar Meyer, known as the Periodic
System. This is the only attempt thus far made to coor-
NATURE OF THE CORPUSCLE 27
dinate all of the chemical elements into one comprehensive
system. The system is too well known to be discussed at
any length in the present connection. It is referred to here
to call attention to the most serious effort that has ever
been made to discover general relations holding for all of
the chemical elements.
It is well known that in the Periodic System the chemical
elements are arranged in the order of their increasing atomic
weights. It is not only found that the chemical and physi-
cal properties of the elements are a function of their atomic
weights, but a periodic function of the atomic weights. If
we arrange the elements according to the above principle,
in groups of seven, allowing the eighth element to fall under
the first, it is well known that the elements with chemically
allied properties will fall in the same vertical columns.
It would lead us too far in this connection to point out
the many and interesting chemical relations brought out
by the Periodic System, and, perhaps, what is even more
important, the relations between the atomic weights of the
elements and their physical properties. It is suflScient to
note here that such relations do exist, and that these are of
a general character, embracing practically all of the ele-
ments known to the chemist.
The writer is in no sjmipathy with the attempt that is
being made in certain directions to belittle and cast into
the background the Periodic System. Of course, every
one must recognize that the system is incomplete. Indeed,
it is not only far from being complete, but leads in places to
inconsistencies. Yet the Periodic System is a great generali-
zation, which coordinates an enormous number of otherwise
disconnected facts, and has done more towards placing
inorganic chemistry upon a scientific basis than all the
other generalizations together, that were proposed up to
28 THE ELECTRICAL NATURE OF MATTER
1886. Indeed, it was the philosophy of inorganic chemistry
for a comparatively long period, and has far from lost its
usefulness at present. As we shall see, it again comes to
the front in connection with the electron theory of matter
that we are now discussing.
These are some of the more important of the earlier
attempts to discover connections and relations between
the difiFerent chemical elements. None of these, with the
exception of the hypothesis of Prout, can be said to have
attempted to solve the problem of the nature of the chemical
elements, even as referred to some one known element as
In the above very brief review of the efforts that have
been made to establish connections between the various
chemical elements, a number of pure speculations by the
ancients have been omitted. Most of these are only of
historical interest, and since they do not admit of experi-
mental test, are of little or no scientific importance.
We shall turn now to the electron theory of matter, and
study some of its applications.
The Nature of the Atom in Terms of the Electron
According to the theory that we have just developed,
all atoms of whatsoever kind are made up of electrons,
which are nothing but negative charges of electricity in
rapid motion. In accepting this wonderfully simple and
beautiful theory that the nature of all matter is essentially
the same, we must not forget the facts of chemistry and
physics which have to be accounted for. We must remem-
ber that we have over seventy apparently difiFerent forms
of matter, which cannot be decomposed into anything
simpler, or into one another, by any agent known to man.
We must also remember that these elements of the chemist
have each their definite and distinctive properties, both
physical and chemical. They enter into combination with
one another in perfectly distinctive ways, and form com-
pounds with definite and characteristic properties. In a
word, we must remember the almost unlimited facts of
chemical science, which are facts, regardless of whatever
conception of the ultimate nature of matter we may hold.
We must also not be unmindful of the great mass of
facts that have been brought to light as the result of the
application of physical forces to these apparently difiFerent
kinds of matter. To take one concrete example: The re-
sults of spectrum analysis show that most of the chemical
elements have their own definite and characteristic spectrum.
30 THE ELECTRICAL NATURE OF MATTER
That an element sets up vibrations in the ether that are of
perfectly definite wave-lengths, and by means of which the
element in question can be identified — these being different
for every element.
Further, while this is true, certain simple and beautiful
relations between the wave-lengths of the waves sent out
by a given element have been discovered.
Thousands of facts of the character of those mentioned
above must be dealt with by any ultimate theory of matter
that can be regarded as tenable.
The atomic masses of the chemical atoms are as different
as i.oi for hydrogen and 238.5 for uranium, and all inter-
mediate orders of magnitude are met with. These masses
are due wholly or in part, to the electrical charges or elec-
trons of which the atoms of all the elements are composed.
We might at first thought conclude that the atom of one
element differs from the atom of another element only in
the number of electrons contained in it, and that the atoms
are simply condensed groups or nuclei of electrons.
Such a conception would be at variance with the facts
of both chemistry and physics. In terms of such a con-
ception, how could we account for chemical valency, the
acid-forming property of some elements and the base-
forming property of others? In terms of such a condensa-
tion conception of the electrons, how should we account
for the facts of spectrum analysis?
It was recognized by J. J. Thomson, to whom we owe
the entire electron conception, that we cannot do so.
It is true that the atoms with different atomic masses
must have different numbers of electrons in them. While
this is a necessary condition, it is far from sufficient to
account for the facts of either chemistry or physics.
the atom and the electron theory 31
Thomson's conception of the atom
The electrons are moving with high velocities in orbits
within the atom, occupying a relatively small part of the
volume occupied by the atom as a whole. The spaces
between the electrons in an atom are relatively enormous,
compared with the spaces occupied by the electrons them-
selves. But the electrons are negative electrical charges,
and we cannot have negative electricity without the corre-
sponding positive. Where is the positive electricity corre-
sponding to these negative units?
Thomson* supposes the atom to be made up of a sphere
of uniform positive electrification, through which the elec-
trons or negative charges are distributed. These electrons
are, as we have seen, at enormous distances apart compared
with the spaces actually occupied by them, like the planets
in the Solar System; and move with very high velocities.
The corpuscles are so distributed through the positive
sphere as to be in dynamical equiUbrium under the forces
that are acting upon them. These are the attraction of
the positive electricity for the negative electrons, and the
repulsion of one negative electron by another.
This brings us to an extremely interesting development
of the electron theory. J. J. Thomson has solved the
problem, in part, as to the arrangement of the corpuscles
that will produce stable systems, in the case of a number,
of the less complex atoms.
the electron theory and the periodic system
Thomson has calculated the arrangement of the electrons
in a sphere of positive electrification, which will be stable.
The electrons will arrange themselves in concentric rings,
Phil. Mag., 7, 237 (1904).
32 THE ELECTRICAL NATURE OF MATTER
since a large number of corpuscles arranged in a single
ring cannot be stable. This ring, however, will become
stable when a suitable number of corpuscles are placed in
the interior, which would produce a system with concentric
In the following table is given the total numbers of elec-
trons, in which the outer ring will contain twenty, and also
the numbers that will be contained in the inner rings, which
are four in number.
NUMBER OF ELECTRONS
59 60 61 62 63 64 65 66 67
NUMBER OF ELECTRONS IN EACH RING
8 8 9 9 10 10 10 10 10
13 13 13 13 13 13 14 14 IS
16 16 16 17 17 17 17 17 17
20 20 20 20 20 20 20 20 20
The smallest number of electrons which will have an
outer ring of 20 is 59, and the largest number with an outer
ring of 20 is 67. When the total number is less than 59,
the outer ring will contain less than 20, which would neces-
sitate a rearrangement of the corpuscles. If an electron
was removed from such a system, the system would of neces-
sity be broken down, and the electrons rearranged in a new
form, which would be the stable form for 58 electrons.
It we pass to the other extreme of the systems containing
20 electrons in the outer ring, we shall find exactly the re-
verse condition. We cannot add an electron to this system
without destroying the equilibrium. If an electron were
added, there would be an entire rearrangement of the whole
system, giving us a new system with 21 electrons in the
THE ATOM AND THE ELECTRON THEORY 33
outer ring. This complete breaking up of the system
would, of course, be a difficult matter.
Turning now to the systems containing total numbers
of electrons intermediate between 59 and 67-, some un-
usually interesting relations manifest themselves. Take
the system with 60 electrons. One eleclroUy and only one,
can be detached from this system without destroying the
equilibrium and necessitating a rearrangement of the re-
mainder. The removal of one electron reduces the total
number to 59, which, as we have seen, is the smallest num-
ber that is stable with 20 in the outer ring. Such a system
having lost one electron, which is one unit of negative elec-
tricity, would be electropositive.
The recent study of chemical valency from the stand-
point of modem physical chemistry has shown that Fara-
day's law is the basis of all chemical valency. This means
that a univalent element is one that carries unit electrical
charge, a bivalent element two such charges, and so on. In
the light of these facts we see that the above system with
60 corpuscles, having lost one electron, or one negative
charge, would contain one positive charge in excess, and
would, therefore, be a univalent positive element, while the
system with 59 corpuscles would have no valency.
The system containing 61 electrons could lose two with-
out destroying the equilibrium, and would, therefore, be
a bivalent, positive element.
The system with 62 electrons could lose three without de-
stroying the equilibrium, and would correspond to a triva-
lent, positive element.
If now we pass to the system with 63 electrons, we can
add four electrons without increasing the total number
beyond 67, and, therefore, without destroying the stability
of the system as a whole and necessitating a rearrangement.
34 THE ELECTRICAL NATURE OF MATTER
Such a system would correspond to a teiravaieni negative
Similarly, three electrons could be added to the system
where the total number is 64, two to the system containing
65, and one to the system containing 66, without destroying
the equilibrium. These would then correspond respectively
to trivalenty bivalent, and univalent electronegative elements.
When we come to the system with 67 electrons, we find
conditions that suggest those pointed out with the system
with 59 electrons. Just as in the latter case we cannot
remove an electron without destroying the equilibrium,
just so when we have 67 electrons we cannot add an elec-
tron without destroying the equilibrium and necessitating
a rearrangement of the system as a whole; since, it will be
remembered, that 67 is the largest total number of electrons
that can have an outer ring of 20. This, like the system
with 59 electrons, would correspond to an element with no
Turning now to the Periodic System, we find, as Thomson
pointed out> that the first nine elements are the following:
Helium, lithium, glucinum, boron, carbon, nitrogen, oxygen,
fluorine, and neon.
The second series of nine elements is the following:
Neon, sodium, magnesium, aluminium, silicon, phos-
phorus, sulphur, chlorine, and argon.
It will be recognized that the first and last member of
each of the above series has no valency, since they have
not been made to combine chemically with anything else.
Lithium and sodium are univalent elements and electro-
positive, glucinum and magnesium are bivalent and
electropositive, boron and aluminium are trivalent and
electropositive, carbon and silicon are tetravalent
and electronegative, nitrogen and phosphorus trivalent
THE ATOM AND THE ELECTRON THEORY 35
and electronegative, oxygen and sulphur bivalent and
electronegative, fluorine and chlorine univalent and elec-
tronegative, while neon and argon have no chemical
valency — having never been made to combine with any
other element. A more perfect agreement, as far as it
goes, between the deductions from any theory and the
facts could not exist.
Relations such as the above, which have been pointed
out by Thomson, have done much to bring the electron
theory of matter to the front, and are altogether too com-
prehensive to be attributed to accident. This application
of the electron theory to the Periodic System is one of the
most important applications of this conception that has
thus far been made.
THE ATOM IN TERMS OF THE ELECTRON THEORY
The atom accordkig to this theory is very complex. Take,
for example, the atom of mercury. This contains a rela-
tively large number of electrons, and some of the heavier
atoms are even more complex. The approximate number
of electrons contained in an atom is, according to recent
views, of the order of magnitude of its atomic weight.
This complex nature of the atoms enables us to account
for the facts of spectrum analysis. Certain elements, such
as iron, uraniimi, etc., give out vibrations of thousands of
wave-lengths in the ether, in accordance with the prevail-
ing theory of light; as is shown by the enormous number
of spectrum lines produced by these elements. In terms
of the old conception of the atom, it was difficult to see
how such a large number of vibrations of such widely
different periods could be set up in the ether by a single
element. Before we had the electron theory, it was recog-
nized that the atom must in its ultimate essence be complex,
36 THE ELECTRICAL NATURE OF MATTER
in order to produce such effects as are brought out by spec-
trum analysis alone. The writer has heard Rowland fre-
quently say, that the simplest atom must be more complex
than a piano.
The electron theory, giving us some idea of the complexity
of even the simplest atoms, makes it possible to form a
mental picture of how an atom can produce such effects
in the ether as is shown by a study of the spectrum.
Light is not only thrown, by the electron theory, on the
problem of spectrum analysis, but on a host of similar
problems, which it would lead us too far in this connection
CATIONS AND ANIONS IN TERMS OF THE ELECTRON THEORY
When acids, bases, and salts are dissolved in water they
break down into a positively charged constituent known
as a cation, and a negatively charged constituent known
as an anion. The recognition of this fact is one of the
most important contributions to scientific knowledge made
by modem physical chemistry. Before we had the elec-
tron theory, we could not form any very definite mechanical
conception of how this important process takes place.
We knew that all acids yielded the hydrogen cation,
which gave their solutions acid properties, and that the re-
mainder of the molecule, as a whole, was charged negatively
and formed the anion of the acid.
We also knew that bases dissociated in the presence of a
dissociating solvent, yielding the hydroxyl anion which was
characteristic of all bases, and to which the basic properties
are due; and that the remainder of the molecule of the
base became charged positively, and formed the cation of
the base. Just as all acids yield the hydrogen cation, so
all bases yield the hydroxyl anion.
THE ATOM AND THE ELECTRON THEORY 37
We knew, further, that salts in the presence of a dis-
sociating solvent, break down or dissociate, as we say, into
a cation and an anion — the cation being the cation of the
base from which they were formed, and the anion the anion
of the acid which took part in the formation of the salt.
We were, however, not able to form any definite con-
ception of how certain atoms or groups (usually atoms)
became charged positively and thus became cations, or
how certain other atoms or groups (usually groups of atoms)
became charged negatively and thus became anions.
The electron theory solves this problem in a very satis-
factory manner. When an atqm loses an electron it becomes
charged positively, since the loss of a negative charge is
exactly equivalent to gaining a positive charge. Thus, a
cation is an atom or group of atoms that tuis lost an electron.
If an atom takes on an electron it becomes charged nega-
tively. An anion is then an atom or a group oj atoms thai
has gained an electron,
A bivalent cation is one that has lost two electrons, a
trivalent cation is one that has lost three electrons, and so on.
A bivalent anion is one that has gained two electrons, a
trivalent, one that has gained three electrons, and so on
for the polyvalent anions.
Since a great majority, if not all chemical reactions take
place between ions, and since electrons are so vitally con-
nected with the formation of ions, it follows that the electron
theory is of as much importance for the science of chemis-
try as for the science of physics.
THE MASS OF AN ION NOT EXACTLY THE SAME AS THAT OF
THE ATOM FROM WHICH IT IS FORMED
From the above method of ion formation, it is obvious
that the mass of an ion is different from that of the atom or
38 THE ELECTRICAL NATURE OF MATTER
group of atoms from which it was formed. Since a cation
is an atom, or group of atoms, from which one or more
electrons have been split off, a cation has a smaller mass
than the atom or atoms from which it was produced.
An anion, on the other hand, is formed from an atom or
group of atoms by adding one or more electrons. There-
fore, the mass 0} an anion is greater than the mass 0} the atom
or atoms from which it was produced.
It must, however, be remembered that the difference
between the mass of an atom or group of atoms, and the
corresponding ion, is in any case very small. Take the
hydrogen atom and the hydrogen ion, where the difference
is the greatest. The hydrogen atom is the lightest atom.
The loss of an electron, converting the hydrogen atom
into the hydrogen cation, would change the mass only
about ttV^. This is close to the limit of accuracy of
our most refined methods of measuring mass, and it is,
therefore, doubtful whether we could detect the difference
between the mass of a hydrogen atom and the correspond-
ing hydrogen ion even when a large number were em-
ployed. It would, however, be rash to assert that such
differences would never be detected, or even determined,
by using a very large number of hydrogen atoms and
comparing them with the corresponding ions.
The change in mass would be relatively less for any other
atom when it is converted into the corresponding ion, since
the mass of any other atom is so much greater than that of
the hydrogen atom, and the absolute gain or loss in mass
would be the same for any other univalent ion, as for hydro-
gen — a loss for every cation, and a gain for every anion.
That this is true is seen from the fact, that every univalent
ion differs in mass from the corresponding atom only in
containing one more or one less electron.
THE ATOM AND THE ELECTRON THEORY 39
The same remark holds for polyvalent ions, which differ
from the corresponding atoms or groups of atoms in that
they contain a number of electrons greater or less than
the corresponding atoms, expressed by the valency of the
ion in question. The mass of all such ions is, however, so
much greater than that of the hydrogen ion, that if we
divide their mass by their valency, the result is still many
times greater than the mass of the hydrogen ion. The
greatest change in mass is, therefore, that produced when a
hydrogen atom loses an electron and passes over into the
Whether or not this change in mass can ever be detected
directly, it is important to recognize that the mass does
change whenever an atom or group of atoms passes over
into ions. There is a gain in the mass of an atom whenever
an anion is formed from it, and a loss in the mass of an
atom whenever a cation is formed.
It must, of course, be remembered that a cation is never
formed without the corresponding anion being formed, and
vice versa; so that in ionization the anion gains just as much
in mass as the cation loses, and the total mass consequently
When a molecule of an electrolyte, say sodium chloride,
breaks down into ions, what takes place is the transference
of an electron from the sodium, which becomes a cation, to
the chlorine, which becomes an anion. The sodium loses
in mass an amount equal to the mass of an electron, and the
chlorine gains the same amount in mass; the sum of the
masses of sodium and chlorine remaining constant.
There would be a change in the total masses in ioniza-
tion only if we assumed that there was a change in the
velocities of the electrons in the sodium and in the chlorine,
when ionization takes place, and that these changes in the
40 THE ELECTRICAL NATURE OF MATTER
velocities did not exactly compensate one another. Since
there is, at present, no ground for such an assumption, we
must conclude that the total masses of the ions formed
from any molecule are equal to the mass oj the molecule.
THE ELECTRON THEORY AND RADIOACTIVITY
One of the most important bearings of the whole electron
theory of Thomson is in connection with those investiga-
tions on radioactivity which have recently attracted so
much attention; investigations which have opened up an
entirely new branch of experimental physics, and which
have changed some of our fundamental conceptions.
The application of the electron theory to these epoch-
making investigations will be made when these researches
MORE RECENT VIEW AS TO THE NATURE OF THE ATOM
The more recent view, especially of Rutherford,^ as to
the nature of the atom is as follows. An atom consists of
a very small central core of positive electricity, sur-
rounded by electrons or negative charges. These elec-
trons as a whole, have a negative charge which is just
equal to the positive charge of the core about whiclj
The atom contains also an outer system of electrons,
which are held much less firmly than the inner system.
This outer system gives to the atom its characteristic
physical and chemical properties. The inner system of
electrons comes into play in producing radioactivity.
^Phil. Mag., 21, 669 (1911).
In 1895/ a paper appeared by R5ntgen, then of Wiirz-
burg, now of Munich, "On a New Kind of Radiation."
It was announced that when an electric discharge is passed
through a Crookes or Lenard tube, which is nothing but a
high- vacuum tube, there was given off from the tube a kind
of radiation which was unknown up to that time, and which
has most remarkable properties. Among these was the
property of great penetrability. The radiation passed
through objects which were entirely opaque to light, and
affected a photographic plate. When a photographic plate
was covered with perfectly black paper, or placed in a black
wooden box, through which no light could pass, the plate
was still affected by the newly discovered radiation. In-
deed, it was this fact that led to the discovery of the radia-
tion by Rontgen.
It was found that the radiation could pass through a
great number of objects that were entirely opaque to light.
Thus, comparatively thick sheets of some of the metals,
such as aluminium, were quite transparent to the newly
discovered radiation. It had the power of passing through
metals in general; but the heavy metals, such as lead,
platinum, and the like, were much more opaque to the
radiation than the lighter metals. It was soon found that
the bones of the body are far more opaque to the radia-
» Wied. Ann., 64, i (1898).
42 THE ELECTRICAL NATURE OF MATTER
tion than the flesh, and, therefore, photographs of the
living skeleton could be obtained, which led to a large
amount of dilettanteism. It was announced that the radia-
tion could not be refracted, nor polarized. When passed
through a gas it rendered the gas a conductor, or, as we
have seen, ionized the gas, in part.
Of course, these were at once recognized to be very re-
markable properties; many of them entirely different from
those of any known form of radiation. In some respects
it resembled light, but in most of its properties differed
fundamentally from it.
It is but natural that such a discovery should have awak-
ened the broadest and deepest interest on the part of men
of science, the world over, almost regardless of the branch ot
natural science to which they were devoting their energies.
The first question that would naturally be asked was. What
is this newly discovered kind of radiation? In answering
this question the method of producing the radiation must
be carefully taken into consideration.
NATURE OF THE X-RAY
It will be seen that the X-ray is produced in the ordinary
cathode discharge tube, and this alone would serve to con-
nect this portion of the work with what has preceded. We
have already studied the cathode discharge, and the velocity
and nature of the cathode particle. We now see that a re-
markable kind bf radiation is given off from the cathode tube.
Careful study showed a very close connection between
the cathode discharge and the production of the radiation.
It was found that the X-rays were produced where the
cathode rays strike upon a solid body, such as the glass
walls of the low-pressure tube. The cathode rays are
thus vitally connected with the production of the X-rays.
THE X-RAYS 43
Several theories have been advanced to account for the
nature of the new radiation. While in a few respects it
resembled light, in most of its properties it differed funda-
mentally from light. Light is a transverse vibration of the
ether, the X-ray might be a longiludinal vibration in the
ether, and this was the theory that was proposed by Rontgen
to account for the radiation that he had discovered. As
facts accumulated, this theory was found to be insufficient.
Indeed, it never acquired any prominence, or received any
very serious support. It remained for Sir George Stokes
to propose a theory as to the nature of the X-ray that
would prove to be satisfactory, and account for the facts
then known, as well as for those subsequently to be dis-
covered. (See page 48.)
The X-ray is not a succession of waves in the ether, like
light, but a series of pulses in the ether ^ sent out at irregular
intervals. This was in accord with their mode of formation,
and accounted for their properties. They are produced
when the cathode particles in a cathode discharge fall upon
the glass walls of the confining vessel. These particles
rain down upon the walls of the tube at irregular intervals,
and if they set up any vibration in the ether, it would be
expected that it would be irregular in character.
. Further, matter would be supposed to be far more trans-
parent to such a set of irregular pulses, than to a definite,
regular set of vibrations in the ether, such as corresponds
to a wave of light. To say that an object is transparent to
any given form of radiation, means that it is not thrown into
vibration by the radiation when the radiation falls upon it.
On the other hand, to say that an object is opaque to a
vibration, means that it is thrown into vibration by the
radiation. Glass is transparent to light because it is not
thrown into vibration by the light. A thin sheet of metal
44 THE ELECTRICAL NATURE OF MATTER
is opaque to light because the light waves falling upon it
produce vibrations within the metal.
This is just what we should expect, since, if the radiation
sets up vibrations in the object upon which it impinges, its
energy is expended in setting up the vibrations, and the
radiation as such is lost.
The penetrating power of the X-ray is thus explained
by the Stokes theory as to its nature.
Similarly, this theory accounts satisfactorily for the other
well-recognized properties of the X-ray, and is now gen-
THE BECQUEREL RAY
The X-ray is produced, as we have seen, where the cathode
ray falls upon the wall of the glass tube. It will be re-
membered, that where the cathode ray falls upon the wall
of the tube a phosphorescent spot is produced on the glass.
For a time it was supposed that this phosphorescence is in
some way intimately connected with the production of the
X-ray. Although it has subsequently been shown that this
is not the case, and that X-rays are produced better when
the cathode ray falls upon a metal plate which does not
become phosphorescent, than when it falls upon glass
which does; yet this original idea, although erroneous, led
to highly important discoveries.
With the idea that phosphorescence and X-rays axe vitafly
connected, men of science began to examine bodies that
were naturally phosphorescent, to see whether fhey gave
off ainy form of radiation analogous to the X-ray, or any
unknown form of radiation whatsoever.
It remained for Henri Becquerel^ to discover the first
naiurally radioactive substance. Guided by the erroneous
1 Compt. rend., I22, 501, 689, and 762 (iSq6V
THE X-RAYS 45
idea that there was some connection between the phos-
phorescence produced on the glass by the cathode ray,
and the production of the X-ray by cathode rays, Becquerel
began examining phosphorescent substances to see if any
of them gave ofiF a radiation at all analogous to the X-ray.
He chose among these substances the salts of uranium,
and found that these compounds produced an impression
on a photographic plate wrapped in black paper to cut
off all ordinary light. The radiations given off by the
salts of uranium could pass through thin sheets of metal
and still affect the photographic plate.
Becquerel supposed at first that it was necessary to ex-
pose the phosphorescent salts of uranium to sunlight, in
order to obtain from them the radiation referred to above.
He found later that this radiation was given off even when
the uranium compound had not previously been exposed
Becquerel tested the question, as to whether the effect
on the photographic plate was due to any volatile substance
given ofif from the uranium salts. This was especially
desirable in the light of the recent work of Russell, on sub-
stances that would produce a fogging of photographic
plates, even when the plate was not directly, but only in-
directly, exposed to the substances in question. To test
this point the photographic plate, wrapped in black paper,
was screened from the uranium compound by a thin plate
of glass. The glass would have cut off any volatile sub-
stance given off from the compound of uranium. The
photographic plate was still affected, which showed
that the result was not due to any volatile substance com-
ing from the salt of uranium.
Becquerel found that all the salts of uranium would pro-
duce the effect, both those that are phosphorescent, and
46 THE ELECTRICAL NATURE OF MATTER
those that are not. The phenomenon was thus shown
not to be directly connected in any way with phosphores-
cence. The efifect produced by the non-phosphorescent
compounds was just as great as that produced by those
that are phosphorescent, provided that they were taken
in quantities that contained the same amount of uranium.
The phenomenon was therefore due to the uranium itself.
It was soon shown that metaUic uranium was not only
active, but more active than any of its compounds.
The radiations given off by uranium, either in the ele-
mentary state or in its compounds, have nothing to do
with its previous exposure to light. When the metal or
its compounds are kept for a long time in the dark, the
intensity of the radiation is undiminished. It is thus obvious
that the energy given out by the uranium radiations is not
derived from sunUght.
Further, the intensity of the radiation given out by ura-
nium is not diminished in several years, i.e., during the
longest time over which observations have thus far been
extended. In these experiments the uranium salts were
preserved in lead boxes, which are especially opaque to
such radiations as we are now considering, and the inten-
sity of the radiations measured photographically from time
to time without removing the uranium compound from the
lead box. In this way the uranium salt was never exposed
to radiations from external sources, and yet it continued to
give off radiations with undiminished intensity. The
energy of the uranium radiation, is thus intrinsic in the
uranium, and does not come from any external source.
This property of substances to emit radiations naturally
like uranium, without any external cause, is known as
radioactivity J and such substances are radioactive. There
are a number of such substances, as we shall see.
THE X-RAYS 47
PROPERTIES OF THE BECQUEREL RAY
It was early recognized that the uranium radiations, like
the Rontgen rays, have many remarkable properties. As
we shall see, they have some properties in common, while
others are quite different.
The uranium radiations, like the X-ray, have the property
of ionizing gases through which they pass. This is shown
by the fact that they discharge electrified bodies surrounded
by the gases in question. The gases are ionized by the
radiations, and then conduct the charges away from the
charged bodies with which they come in contact. -
In this respect, as well as in their power to affect a photo-
graphic plate, the uranium rays act like the X-ray, but they
are very much weaker in their action. This applies both
to their action on a photographic plate, and their power to
ionize a gas. From these facts alone it might be concluded
that the Becquerel ray is nothing but a very weak form of
The rays from uranium can neither be refracted nor
polarized, and thus again resemble the X-ray. ^
THE THORIUM RADIATION
After Becquerel had shown that one natural substance is
radioactive, or has the power of giving out radiations that
can pass through considerable thicknesses of matter opaque
to light, as well as ionize a gas and affect a photographic
plate, a search was made for other natural substances
having the same properties. The first one discovered was
the comparatively rare element thorium. Schmidt ^ found
that thorium, whether elementary or in combination, had
some properties analogous to those possessed by uranium.
It gave out radiations that acted, if only feebly, upon the
» Wied. Ann., 65, 141 (1898).
48 THE ELECTRICAL NATURE OF MATTER
photographic plate. It ionized a gas, like the radiations
from uranium, but possessed properties that distinguished
it sharply from the uranium radiation. There is given off
from the thorium something that is blown about by the
slightest currents of air, and which in some respects re-
sembles a gas. This was discovered by Rutherford and
termed by him an emanation. As we shall learn, this
emanation has remarkable properties.
RECENT WORK ON THE NATURE OF THE X-RAY
The recent work of Laue, Bragg and others, has changed
our conception as to the nature of the X-ray. If the rays
were a regular series of vibrations in the ether, with wave-
lengths say of molecular dimensions, when allowed to fall
on a grating with a distance between the lines also of
molecular dimensions, we would have produced an X-ray
A crystal is just such a space-grating. When X-rays
are reflected from a crystal, we have produced spectra of
From the positions of the spectra of the various orders,
and the intermolecular distances in the crystal, we can
calculate the approximate wave-lengths of the X-rays.
These wave-lengths vary, but are of the magnitude of an
This means that X-rays are not a series of irregular pulses
in the ether, as Stokes supposed, but like light a series of
regular vibrations. The X-rays differ from light in that
the lengths of the ether waves are much less. This explana-
tion of the nature of the X-ray is in harmony with its
The Discovery of Radium
It having now been shown that two elementary sub-
stances, uranium and thorium, are radioactive, a large
number of substances were examined with respect to this
property. Among these would naturally be the minerals
in which uranium and thorium occur.
Mme. Curie ^ determined the radioactivity of a large
number of minerals, by measuring the conductivity of the air
when exposed to these substances. She found that all min-
erals which show radioactivity contain either uranium or
thorium. What was very remarkable was the fact that certain
minerals which contain many things in addition to uranium
were much more radioactive than uranium itself. Thus, pitch-
blende from Johanngeorgenstadt had nearly four times the
radioactivity of pure uranium. Pitchblende from Joachims-
thal was three times as radioactive as uranium, while pitch-
blende from Pzibran was nearly three times as radioactive.
Chalcolite, which is a double phosphate of copper and
uranium, is about two and one-fourth times as radioactive
as metallic uranium, while autunite, a double phosphate
of calcium and uranium, is about one and one-fifth times
as radioactive as uranium.
Only a part of every one of these minerals is uranium,
and yet the mineral was more radioactive than pure ura-
Mme. Curie then prepared chalcolite artificially by treat-
* Ann. Chim. Phys. , 30, 99 (1903).
50 THE ELECTRICAL NATURE OF MATTER
ing a solution of uranyl nitrate with a solution of copper
phosphate in phosphoric acid, and warming the mixture
to fifty or sixty degrees. Under these conditions crystals
of chalcolite were formed.
The radioactivity of this artificially prepared chalcolite
was two and one-half times smaller than that of uranium
itself. This led Mme. Curie to conclude that the imex-
pectedly great activity of the natural minerals was due to
the presence in them of small quantities of some strongly
radioactive substance, which was neither uranium, nor
thorium, nor any other known substance.
With this idea in mind M. and Mme. Curie imdertook
to separate from the uranium minerals the supposed new
radioactive substance, and with signal success.
THE SEPARATION OF RADIUM FROM PITCHBLENDE
Pitchblende, as is well known, contains, in addition to
uranium, a large number of other elements in small quantities.
The separation of pitchblende into its constituents, or even
the separation of any constituent in pure form, is not likely to ^
be a simple matter. The Curies, however, worked out a
chemical method for effecting the desired separation, and
obtaining the highly radioactive substance or substances.
In the various chemical processes to which the material,
as we shall see, was subjected, they followed the course of
the radioactive constituents by determining the radioactivity
of every product by means of the electroscope. They could
thus determine what chemical operation was concentrating
the radioactive substance.
There are at least two, and possibly three radioactive
constituents in pitchblende, in addition to uranium itself.
One of these, called polonium from the native country
(Poland) of Mme. Curie, resembles in its chemical proper-
THE DISCOVERY OF RADIUM 5 1
ties the element bismuth, and is separated from the pitch-
blende along with this element. The element radium,
with which we are now chiefly concerned, is closely allied
chemically to barium, and comes out of the pitchblende
along with the barium.
A third radioactive substance, actinium^ has been de-
scribed by Debieme as occurring in pitchblende. It
seems to separate from pitchblende along with certain of
the rare elements, and especially thorium.
To give some idea of the number and complexity of the
chemical processes involved in separating radium from
pitchblende, the essential features in Mme. Curie's ^ account
of her own work are appended. All of the new radioactive
constituents occur in pitchblende in minute quantities^ so
that it is necessary to work over enormous quantities of
material in order to obtain even a few milligrams of the
comparatively pure radioactive substances.
We shall confine our account to the separation of radium
from pitchblende, which, we will remember, comes out
along with the barium, to which it is so closely related
The finely powdered pitchblende is fused with sodium
carbonate, and the product treated with hot water. Dilute
sulphuric acid is then added. The uranium is contained
in the solution, and since the pitchblende was worked for
the uranium that it contained, the residue, after the above
treatment, was discarded. The radioactive constituents
are contained in this residue, which has a radioactivity of
about 4.5 times that of metallic uranium.
This residue consists mainly of the sulphates of lead and
calcium. It also contains aluminium, iron, silicon, and
larger or smaller amounts of nearly all known metals. The
> Ann. Chim. Phys. , 30, 125-127.
52 THE ELECTRICAL NATURE OF MATTER
radium exists in this mixture of sulphates, its sulphate
being the least soluble.
The problem now is to separate the radium from this mix-
ture of sulphates. The residue is freed as far as possible
from sulphuric acid, by treating with a concentrated, boiling
solution of sodium hydroxide. The sulphates of calcium,
aluminium, and lead are thus, for the most part, decomposed,
the sodium hydroxide also removing the aluminium, silicon,
and lead. The residue insoluble in the alkali is washed with
water and then treated with hydrochloric acid. The radium
remains in the residue insoluble in hydrochloric acid.
The insoluble portion containing the radium is washed
with water, and then treated with a concentrated, boiling
solution of sodium carbonate. This transforms the sul-
phates of barium and radium into carbonates. The car-
bonates are now thoroughly washed with water and treated
with hydrochloric acid, when the barium and radium dis-
solve as the corresponding chlorides. The radium is pre-
cipitated by means of sulphuric acid. The precipitate also
contains barium and calcium, lead and iron. This is the
radium-containing barium in the form of crude sulphate.
From a ton of the residue obtained from pitchblende, ten
or twenty kilograms of the crude sulphate, having an activity
from thirty to sixty times that of metallic uranium, which
is taken as unity, can be obtained.
The mixture of crude sulphates is boiled with a solution
of sodium carbonate, and then transformed into chlorides
by treating the carbonates with hydrochloric acid. The
oxides and hydroxides are precipitated by adding ammonia
after filtering. Sodium carbonate is added to the solution,
when the carbonates of the alkaline earths are thrown down.
The carbonates are transformed into chlorides by adding
hydrochloric acid, and the chlorides, after evaporating the so-
THE DISCOVERY OF RADIUM 53
lution to dryness, are treated with pure, concentrated hydro-
chloric acid. This dissolves the chloride of calcium, while the
chlorides of barium and radium are insoluble in the acid.
About eight kilograms of this product, consisting mostly
of barium chloride, are obtained from a ton of the original
residue. The radium chloride is mixed in small quan-
tity with the barium chloride. This is shown by the fact
that the activity of the radium-bearing chloride is about
sixty times that of pure uranium.
The process of preparing pure radium chloride, instead
of being ended, is now really only begun. The following
process of obtaining radium chloride from the mixture with
barium chloride is described by Mme. Curie.^
The principle of the method is fractional crystallization.
The chloride of radium is less soluble than the chloride of
The first fractionation is effected in pure water. The
chloride that separates from the saturated solution is much
more active than the solution, as would be expected, since
the chloride of radium is less soluble than the chloride of
barium. By utilizing this fact, and fractionating the mix-
ture in terms of it, after a long series of fractionations, dis-
carding the weakly active portions, most of the inactive
barium chloride is removed.
When a large number of fractionations have been made, and
the amount of substance has become small, it is better to add
hydrochloric acid to the water, since this diminishes the solu-
bility of the salts, and more rapid separations are effected.
Mme. Curie observed that the crystals of radium barium
chloride remain colorless, until the amount of radium has
reached a certain per cent, of the whole mass. When the
radium salt has reached a certain concentration, the crys-
1 Ann. Chim. Phys. , 30, 131 (1903).
54 THE ELECTRICAL NATURE OF MATTER
tals become yellow. They may even show an orange, or
a beautiful rose color. This color possessed by the crystals
disappears when the crystals are dissolved. The appear-
ance of this color is rather remarkable, since crystals of
pure radium chloride are colorless. The color indicates
that a certain degree of purity has been reached, and has a
maximum intensity when the amount of radium present is a
certain, definite quantity. After this concentration is reached,
the intensity of the color becomes less and less as the purity
of the crystals becomes greater and greater. When the
radium has become freed from all appreciable quantities
of barium, the color practically disappears from the crystals.
Thus, the color of the crystals can be used as an index to
the progress of the separation of barium from the radium
— of the degree of purity of the radium salt. For a more
detailed discussion of these matters see the original article
by Mme. Curie.
By the above described method radium chloride can be
obtained, having a radioactivity that is one miUion times
that of the mineral from which it came.
When we consider the number of steps in the above
described process, and the details of every step, and then
remember that every one of these details had to be worked
out empirically by the Curies, we gain some idea of the
enormous task that they have performed, and the difficul-
ties at every step which they must have encountered and
After all this had been done, the amount of radium chloride
obtained from a ton of the residues from pitchblende was
only a few milligrams. This necessitated the working
over of enormous quantities of the original pitchblende, in
order to obtain any appreciable quantity of the radium salt.
Fortunately, this problem is rendered much less difficult
THE DISCOVERY OF RADIUM 55
than it would otherwise be, by the cooperation on the part
of the factories in which pitchblende is used. Many of
the steps described in the above process can be taken more
successfully on a large scale than on a small one, to say
nothing of the amount of time and labor that it would be
necessary to expend in performing these operations in the
laboratory. Indeed, if we were dependent upon the labora-
tory alone for our supply of radium, our knowledge of this
substance would have accumulated infinitely more slowly
than it has done.
Other methods have been proposed for purifying the
radium salt, which are hardly more than modifications of
certain details of the method worked out by the Curies and
The question that arises is whether some source of radium
richer than pitchblende may not yet be found. Radium
has been shown to be very widely distributed over the sur-
face of the earth. It occurs in a large number of minerals,
in the waters of many springs, in the soil and rocks, and
probably in many places not yet discovered. While sources
of radium that are richer in this substance than the richest
pitchblendes may yet be found, it appears to the writer to
be doubtful whether any material very rich in radium will
ever be foimd.
This opinion is not based so much upon the ease with
which radium is detected by means of the electroscope,
or upon the comparatively wide search that has already
been made for this substance, as it is upon the instability
0} the element itself.
As we shall see, radium is not a stable substance. It is
continually undergoing decomposition into other things.
It would, therefore, be very surprising if any large quantity
of it should be found in any one locality.
$6 THE ELECTRICAL NATURE OF MATTER
THE SPECTRUM OF RADIUM
Since radium has a well-defined spectrum, it is a matter
of great importance in connection with the determination
of the purity of any given sample of its salts. To deter-
mine the spectrum, the Curies* turned over to Demarfay
some samples of material containing radium, and he studied
the spark spectra of these substances. The first sample
used by Demarfay contained large quantities of barium.
Nevertheless, even with this material he was able to recog-
nize, in addition to the barium lines, a line in the ultra-
violet, having a wave length of 3814.7 Angstrom units.
When a purer substance was used the intensity of this line
increased, and other lines made their appearance. Finally,
a product was obtained of such purity that only the three
strongest barium lines appeared at all, and these were of
such slight intensity as to show that the barium was pres-
ent only in very small quantity. While this product was
nearly pure radium chloride, it was still further purified
until the strongest barium lines could scarcely be detected
The chief lines of radium found by Demarjay, lying
between 5,000 and 3,500, are the following — the most
intense line being represented by the number 16.
* Ann. Chim. Phys. , 30, 121 (1903).
THE DISCOVERY OF RADIUM 57
The strongest of the above lines have the intensity of
the stronger lines of other substances.
In addition to the lines referred to above, and a number of
weaker lines, the spectrum of radium contains two bands; the
one extending from 4631.0 to 4621.9, the other a stronger
band in the ultraviolet, extending from 4463.7 to 4453.4.
Thus, the spectrum of radium resembles the spectra of the
alkaline earths, which consist of strong lines and also bands.
It was pointed out by Mme. Curie that although spectrum
analysis is, in general, a very sensitive means of detecting
minute quantities of substances, in the case of radium it is
far less delicate than the electrometer, notwithstanding the
fact that radium gives a well-defined spectrum.
In order to photograph the strongest spectrum lines of
radium, a specimen of the radium-containing barium was
required, which had an activity at least fifty times that of
A very sensitive electrometer, on the other hand, can de-
tect radium which has an activity that is only one ten-
thousandth that of metallic uranium. The electrical method
of detecting the presence of traces of radium is thus at least
five hundred thousand times more sensitive than the spec-
troscopic. The spectroscopic method is of importance in
connection with the study of radioactivity, not so much as
a method for measuring radioactivity, as for determining
the purity of the radium in the various stages of its separa-
tion from barium. Radium bromide gives a deep-red
color to the flame.
THE ATOMIC WEIGHT OF RADIUM
The atomic weight of radium was first determined by
Mme. Curie,^ with specimens that contained more or less
> Ann. Chim. Phys. , 30, 137 (1903)-
58 THE ELECTRICAL NATURE OF MATTER
barium. Values as low as 140 were at first obtained. As
purer and purer specimens were prepared, successive deter-
minations gave larger and larger values for the atomic
weight of radium.
A specimen which still showed the strongest lines of
barium with appreciable intensity, gave a value for the
atomic weight of radium ranging, for five determinations,
between 220.7 and 223.1.
A specimen of radium chloride was then purified imtil
the strongest lines of barium appeared very weak indeed.
From the minute quantity of barium that can be detected
by the spectroscope, this specimen of radium chloride could
contain only the merest trace of barium.
The atomic weight determinations were made by precipi-
tating the chlorine as silver chloride. Taking the atomic
weight of silver as 107.8 and chlorine as 35.4, the atomic
weight of radium was found to be 225, ranging in three
determinations between 224.0 and 225.8.
Light has been thrown on the atomic weight of radium
by Rimge and Precht,^ who studied the spectrum of radium
in a magnetic field. Series of lines were observed, with
radium, under these conditions, that are analogous to those
found for the alkaline earth metals — calcium, strontium,
and barium. Certain relations have been established
between the series of lines for an element, and its atomic
weight. By means of these relations Runge and Precht
have calculated the atomic weight of radium to be 257.8.
However, other investigators, and especially Watts, on purely
physical groimds, have concluded that the atomic weight is
close to 225. We must, then, decide between these two
numbers. In the light of the evidence at present available,
this is not an easy task.
» Phil. Mag., 5, 476 (1903)-
THE DISCOVERY OF RADIUM 59
The number 225 seems to fall in with the value that
radium might be expected to have from the Periodic System.
This number would place radium after bismuth with an
atomic weight of 208.5 ^iid before thorium with an atomic
weight of 232.5 The number 225 for its atomic weight
would place radium in group II, along with calcium, stron-
tium, and barium, to which chemically it is closely allied —
especially to barium, as we have seen. The atomic weight
225 also places it in the twelfth series, along with thorium
and uranium — the other well-known elements that are
On the other hand, the atomic weight 225 places radium
in the second group of the Periodic System, while thorium
is in the fourth, and uranium is in the sixth. In a word,
it places radium before thorium and uranium; the atomic
weight of thorium being 232.5, and that of uranium 238.5.
It will be observed that these three radioactive elements
have the largest atomic weights 0} all the known chemical
elements. Indeed, an attempt has been made to establish
a relation between the relatively large masses of the atoms
of these elements and their radioactivity — an attempt
which, as we shaH see when we come to study the nature
of radioactivity, is most praiseworthy. In terms of this
relation, the atom with the largest mass should be the most
radioactive, and as we usually measure mass by weight,
the atom with the largest atomic weight should be the most
radioactive. If the atomic weight of radium is 257.8, it
would be in accord with this relation. The atom of radium
would be by far the heaviest of all known atoms, that of
uranium with a mass of 238.5 would be next, followed by
thorium with a mass of 232.5.
We shall see later the significance of this relation, and
will become so impressed by it in connection with the appli-
6o THE ELECTRICAL NATURE OF MATTER
cation of the electron theory of matter to the explanation
of radioactivity, that we shall be loath to give it up, and
accept a lower atomic weight for radium than for uranium
Since writing the above a relation has appeared to the
writer,* which somewhat invalidates the argument for 225
as the atomic weight of radium, based upon the Periodic
System. If we turn to the Periodic System and examine
the atomic weights of any two elements in the same group
and in two succeeding series; in a word, of two elements
that fall directly under one another, we find that their
atomic weights differ from one another by from twenty-five
to thirty units. This is especially true for the elements
with higher atomic weights. Take the members of group
II, in which radium undoubtedly belongs chemically. The
atomic weight of calcium is 40.1, that of zinc 65.4 — dif-
ference 25.3. Zinc differs from strontium in round num-
bers by twenty- two points; strontium from cadmium by
twenty-five points, and cadmium from barium by twenty-
five points. Yttrium differs from indium by twenty-six
units; indium differs from lanthanum by twenty-four units;
lanthanum differs from ytterbium by thirty-four units, and
ytterbium differs from thallium by thirty-one units. Simi-
lar relations exist between successive members of every
other group in the Periodic System, especially between the
members with the higher atomic weights.
It will be seen that the difference between the atomic
weight of radium as determined by chemical analysis (225),
and as determined by spectrum analysis (257.8), is about
We have already seen that the number 225 places radium
in group II of the Periodic System, and in series twelve.
^ Amer. Chem. Joum., 34, 467 (1905).
THE DISCOVERY OF RADIUM 6l
The atomic weight 256 to 258 would place radium in group
II of the Periodic System, and in series thirteen. This
may seem surprising since only twelve series have thus far
been recognized in the Periodic System. It may be that
the proper place for radium is in a new series, of which only
one number exists, or has, at least, thus far been discovered.
If radium has an atomic weight of 258, or thereabouts, it
would thus fall in group II of the Periodic System, with its
chemically aUied elements, just as well as if it had an atomic
weight of 225. The fact that 258 places radium on the
right-hand side of group II is not a serious objection to the
above view, since we do not know that the relations within
the groups hold for these highest atomic weights.
The problem of the atomic weight of radium, however,
cannot be settled by reasoning from analogy, but must be
worked out by some direct method.
If we examine the method employed by Mme. Curie for
determining the atomic weight of radium, it does not seem
to be entirely free from objections. In the first place, the
amount of radium chloride that could be obtained, which
was of sufficient purity for atomic weight deteiminations,
was necessarily small. Indeed, the total amount of chloride
at the disposal of Mme. Curie was only about one hundred
milligrams. This tended to magnify all experimental
The chloride of radium, which is hygroscopic, shown by
the fact that it absorbs water when in a desiccator over
dr3dng agents, was weighed in a platinum crucible. Further,
it is not clear that any test was made to determine whether
the crystallized radium chloride did not lose hydrochloric
acid when the water of crystallization was removed. It
will be recalled that other members of the barium group
form oxychlorides, when the chlorides are dehydrated in
62 THE ELECTRICAL NATURE OF MATTER
the air. It is well known that the chloride of calcium can
be dehydrated without the formation of oxychloride, only
in a current of hydrochloric acid or by heating with ammo-
nium chloride. This is a matter that should certainly
receive attention in connection with the method of deter-
mining the atomic weight of radium, that was employed
by Mme. Curie.
A further question that naturally suggests itself in con-
nection with the method is this: Does silver nitrate pre-
cipitate all of the chlorine from radium chloride as silver
chloride? The properties of radium are so remarkable,
as we shall learn, that it does not follow that this would
necessarily be the case.
The most recent determinations of the atomic weight
of radium by Mme. Curie ^ and by Thorpe ^ give values
between 226 and 227, and it must be said that these pieces
of work seem to have been carried out very carefully.
A recalculation ^ recently made from spectroscopic data
gives essentially the same value.
Gray and Ramsay ^ found the atomic weight of radium
to be 226.36. Honingschmid ^ found 225.93.
1 Comp. rend., 145, 422 (1907).
•Ztschr. anorg. Chem., 58, 443 (1908).
•Watts: Phil. Mag. 18, 411 (1909).
*Proc. Roy. Soc; 86, A, 270 (1912).
^ Monatsh. Chem.; 33, 253 (19 12).
Digitized by LjOOQIC
Other Radioactive Substances in Pitchblende
There are apparently other radioactive substances in
pitchblende, in addition to radium, as we have seen. There
seems to be a new radioactive substance in this mineral
that is closely allied to bismuth. It has already been re-
ferred to under the name of polonium.^ It is precipitated
along with the bismuth, from the hydrochloric acid solu-
tion of the pitchblende residue, by means of hydrogen sul-
phide. It has thus far been impossible to free the supposed
polonium from bismuth. Partial separation has apparently
been effected, or, at least, a strongly radioactive substance
has been obtained by precipitating the nitric acid solution
by water. The subnitrate that is thrown down is much
more radioactive than the unprecipitated portion.
It seems yet to be a question whether this radioactive
bismuth really contains a new radioactive element, or is
simply bismuth made radioactive by the deposition upon
it of a substance coming, as we shall learn, from the radium
in the pitchblende. If there is a new radioactive element
associated with the bismuth, it might reasonably be ex-
pected to show definite and characteristic lines in the spec-
trum, as radium does. Demarjay, who worked out the
spectrum of radium, was unable to find any new lines pro-
duced by the radioactive bismuth. Sir William Crookes,
1 Ann. Chim. Phys., 30, 119 (1903).
64 THE ELECTRICAL NATURE OF MATTER
on the other hand, announces a new Ime for this substance
in the ultraviolet.
If, however, it should be shown that the radioactive bis-
muth contains no new line, it does not prove, as Mme.
Curie points out, that there is no new element contained in
this substance, since there are many elements known that
do not have any well-characterized spectrum. An experi-
ment performed by Marckwald^ in 1902 may throw some
hght on the nature of polonium. If a stick of bismuth is
plunged into the solution of active bismuth chloride ob-
tained from pitchblende, it becomes covered with a black
coating which is extremely radioactive, and the remaining
solution is no longer radioactive. This deposit is mainly
tellurium, with a very small amount of the radioactive
substances. An active deposit is obtained if tin chloride
is added to the radioactive bismuth chloride. Marckwald
thinks that this radioactive element is analogous to tellu-
rium, and calls it radiotellurium. It has properties strik-
ingly analogous to the polonium of the Curies, the analogy
being especially marked between the kinds of radiations
sent out by it. More work is required to show whether
these substances are identical, or are different.
It should, however, be stated that the fact that polonium
is precipitated from a solution of radioactive bismuth by
simply introducing a piece of bismuth would alone indicate
that these substances are fundamentally different. It is well
known that a metal cannot precipitate more of the same
metal from a solution of any of its salts. In order that a
metal may be able to precipitate another from its salts, it
is necessary that the metal which is thrown out of solution
should have a much lower solution-tension, or stand lower
in the tension series, than the metal which throws it out
1 Ber. d. deutsch. chem. Gesell., 35, 2285 (1902).
OTHER RADIOACTIVE SUBSTANCES IN PITCHBLENDE 65
and takes its place. The metal which passes into solution
must have the power to take the charge from the ion of
the metal that is thrown out, becoming itself an ion, while
the original ion is converted into an atom.
It has already been mentioned that Debierne ^ obtained
from pitchblende an active substance, which is termed
actinium. This substance is quite different from radium,
and also from polonium. It comes out of pitchblende
along with the rare earths, and especially with thorium, to
which it is very closely allied. This is probably the same
substance as that obtained from pitchblende by Giesel
along with other rare elements of the cerium group. Giesel ^
called the substance emanium on account of its great
emanating power, but afterwards found that it was
identical with the actinium of Debierne. He found that
actinium was also closely allied in its properties to lantha-
num. It could be partially separated from the lanthanum
by fractional crystallization of the double nitrate with
manganese. The occurrence of actinium with thorium has
raised the question as to whether the apparent activity
of thorium itself is not really due to the admixture of a
small amount of actinium. This question can be settled,
as Rutherford points out, after thorium has been obtained
which is devoid of radioactivity. Since, however, it is
doubtful whether this has been done, it would be premature
to conclude that the radioactivity of thorium was due to the
presence of small amounts of actinium. Indeed, we shall see
later that it is doubtful whether this is the case — the activity
* Compt. rend. (1899), 130, 906 (1900). Also (1903), (1904), (1905).
*Ber. d. chem. Gesell., 35, 3608 (1Q02): 36, 342 (1903); 37, 1696, 3963,
(1904); 38, 775 (190s); 40, 301 1 (1907).
66 THE ELECTRICAL NATURE OF MATTER
of thorium probably being due to the presence of radiotho-
rium. No spectrum has as yet been observed for actinium.
Other radioactive substances have been announced as
coming from pitchblende. It is probable that these sub-
stances either contain small amounts of the other radio-
active substances known to exist in pitchblende, such as
radium, and probably polonium and actinium; or are made
radioactive by the presence of other radioactive substances.
We shall learn that certain radioactive substances have the
property of making other substances in contact with them
This kind of radioactivity is known as induced radio-
activity. We shall become more familiar with this subject
when we come to study more closely, in a subsequent chap-
ter, the nature of the radiations given off by radioactive
We have now taken a brief survey of the steps involved
in the discovery and isolation of the radioactive elements,
and especially of the best known of them all — radium.
The next step in order of logical sequence is to study the
properties of these various substances, starting, perhaps,
with the less active, uranium and thorium, and then taking
up the more active, especially radium, about which so much
and such important knowledge has already been gained.
The methods that have been employed in these inves-
tigations are not obvious, and, therefore, should be briefly
considered before the results that have been obtained
through their application.
THE MORE IMPORTANT METHODS USED IN STUDYING RADIO-
The methods that have been employed in studying radio-
activity are based, of course, upon the properties of the
OTHER RADIOACTIVE SUBSTANCES IN PITCHBLENDE 67
radiations that are given out by the various radioactive
We have seen that such substances affect a photographic
plate exposed to their radiations. It will be remembered
that it was by means of this property that Becquerel discov-
ered the first radioactive substance — uranium. Although
this method is still used for certain purposes, there are a
number of objections to its general use in connection with the
study of radioactivity. In the first place, it is not sulEdently
sensitive for work with weakly radioactive substances.
Another serious objection to the photographic method
is that certain radiations given off from radioactive sub-
stances, even when fairly intense, have very slight action
'upon the photographic plate. Another objection to the
photographic method is a somewhat general one. Photo-
graphic plates are sensitive to such a number of agents.
Many things when brought in contact with a photographic
plate leave an imprint on the plate when it is developed.
This can, however, be overcome by suitable precautions,
and photography has proved of invaluable service in the
development of scientific knowledge.
Taking all of these facts into account, the photographic
method is not well adapted to the study of radioactivity
in general, although it has certain special applications
that are important.
Another property of radioactive substances is to cause
certain substances upon which their radiations fall, to
phosphoresce. This is especially true if the radiations
are allowed to fall upon screens covered with the beautiful
salt barium platinocyanide. The fltwroscopic method is
of very limited applicability, since weakly radioactive sub-
stances do not produce enough phosphorescence in these
screens to be observed.
68 THE ELECTRICAL NATURE OF MATTER
We have already seen that the radiations from radioactive
substances have the power to discharge charged bodies
surrounded by a gas such as the atmosphere. This means
that such radiations have the power to render a gas like
the air a conductor of electricity. In a word, to ionize
the gas into the negative electron and the relatively large
A method based upon this property of the radiations has
proved of the greatest service in connection with the study
of radioactivity. Indeed, it is the only method that is
capable of giving reliable quantitative measurements.
For details concerning the measurements of the conduc-
tivities of gases through which the radiations from radio-
active substances are passing, the original investigations,"
especially of Rutherford, must be consulted.
PROPERTIES OF THE RADIATIONS GIVEN OUT BY RADIO-
We have already become familiar with the fact that
radioactive substances give out radiations that have the
property of affecting a photographic plate, of rendering
certain substances phosphorescent, and of ionizing gases.
The question would naturally be raised, are the radia-
tions given out by all radioactive substances the same in
character? Again, are all the radiations given out by any
one radioactive substance of the same nature?
These questions are easily asked, but can be answered
only by experimental work, and this not always of a very
simple kind. It is,' however, not a difficult matter to show
qualitatively that the radiations given out by a radioactive
substance, such as radium, are not homogeneous, but are
complex in character.
If we charge a gold-leaf electroscope and subject it to
OTHER RADIOACTIVE SUBSTANCES IN PITCHBLENDE 69
the radiation from radium, it will be rapidly discharged,
due to the ionization of the air produced by these radiations.
If now we interpose between the radium salt and the elec-
troscope a thin sheet of metal, or even a piece of paper,
the electroscope will be discharged much more slowly,
showing that a portion of the radiation has been cut oflF.
If we then interpose into the path of the rays a thick piece
of metal, the electroscope will be discharged much more
slowly than when a piece of metal foil was used, and the
difference will not be proportional to the thickness of the
piece of metal introduced. The interposition of a second
such piece of metal has but little effect.
These qualitative experiments show conclusively that the
radiation from radium is heterogeneous, consisting of dif-
ferent kinds of rays. The most natural interpretation of
these results would be that the piece of thin sheet metal,
or metal foil, cuts off a kind of radiation that has relatively
little power to penetrate matter; and that the thick piece of
metal cuts out a more penetrating kind of radiation, letting
a third, highly penetrating form pass through, which of
itself is capable of ionizing the gas to a slight extent and
slowly discharging the electroscope.
While this is, perhaps, the most obvious interpretation
of the results of the above described experiment, it remains
to be seen whether it is the correct one.
Giesel ^ took up the study of the effect of the magnetic
field on the radiations from radium in general. Results of
the very highest importance were obtained. He found that
at least some of the radiations from radium could be de-
flected by the magnetic field, which accounted for the change
in the conductivity produced in the air by the radiations
when these were made to pass through a magnetic field.
1 Wied. Ann., 68, 834 (1899).
70 THE ELECTRICAL NATURE OF MATTER
A little later M. Curie ^ showed that the radiations from
radium consisted of two kinds, one that was not deflected
or deviated in the magnetic field, and another that was
deviated by the field. The kind that was not deviated
had very little penetrating power, and was the kind that is
so readily stopped even by a thin sheet of metal foil.
The kind that was deviated by the magnetic field had
much greater penetrating power, and was capable of pass-
ing through thin sheets of metal. It could not, however,
pass through sheets of metal of any appreciable thickness.
About the same time it was shown by Villard ^ that the
radiations from radium contain a third kind of rays, that
have very great penetrating power, and are not deviable
by the magnetic field. The radiations from radium con-
tain, then, three kinds of rays, each with its own definite,
characteristic properties. These have been named the
Alpha (a) rays.
Beta ()8) rays.
Gamma (y) rays.
A fourth kind of radiation, the 8 rays, will also be considered.
We can now understand the qualitative experiment dis-
cussed earlier in this chapter.
The thin sheet of metal cut off the a radiations, but
allowed most of the )8, and practically all of the y radia-
tions to pass through. When the a rays were cut off the
air was ionized much less rapidly, for, as we shall learn,
the a rays are the chief ionizing agents in the radium radia-
tions, and the electroscope was discharged much less rapidly
than when they were allowed to pass through the air
between the leaves of the electroscope.
The thick piece of metal cut off the )8 radiations and
* Compt. rend., 130, 73 (1900).
^Ibid.f 130, 1178 (1900).
OTHER RADIOACTIVE SUBSTANCES IN PITCHBLENDE 7 1
allowed only the y radiations to pass. The electroscope
was now discharged much more slowly, since the y radia-
tions have less power to ionize a gas than even the ^ radia-
tions, which in turn have much less ionizing power than
the a radiations.
Our original conclusion from the facts of the qualitative
experiment is then correct. The radiations from radium
consist chiefly of three distinct kinds of rays; the fourth or
8 rays having been discovered comparatively recently.
We shall now proceed to study the properties of these in
some detail, taking them up in the order, alpha, beta, and
gamma, and not in the order of their discovery.
It may be said in advance that all three radioactive sub-
stances, uranium, thorium, and radium, give out these
three types of radiations. Polonium, as we shall learn,
gives out only one type, the a radiations.
The Alpha Rays
It has already been mentioned that the a rays are only
slightly deviable in a magnetic field, that they have very
little power to penetrate matter, and that they produce
most of the ionization of the gas through which the radia-
tions from radium pass.
The study of the deviation of the a rays in a magnetic
field we owe largely to Rutherford.* That they are deviated
was shown by the following simple experiment. If some
radium salt is placed in the bottom of a narrow tube, which
in turn is introduced between the poles of an electro-magnet,
radiations from the salt will fall upon an electroscope placed
directly in front of the tube. If the current is now turned
on the electromagnet, any rays that are appreciably de-
flected by the magnet would fall upon the side walls of the
tube, and would not reach the electroscope.
The number of experimental difiiculties that had to be
overcome was large. The tube or slit in which the salt was
placed must be small, in order that the rays might be bent
enough to strike the walls. To augment the effect a num-
ber of such slits were used.
After all of the experimental difficulties had been over-
come, Rutherford showed that when a powerful magnetic
field was used, all of the a rays were deviated. This proved
that the a rays are made up of charged particles. It does
1 Phil. Mag., 5, 177 (1903).
THE ALPHA RAYS 73
not, however, show whether the particles are charged posi-
tively or negatively. If the particles are charged positively
the rays would be deviated in one direction, if negatively
in the opposite direction. It was found that the a rays are
deviated in a direction which is exactly opposite to that in
which another class of rays, known, as we shall see, to con-
sist of negatively charged particles, is deviated. This proves
that the a rays, at least from radium, are composed of posi-
tively charged particles. The presence of a positive charge
upon the a particles was demonstrated directly by J. J.
Thomson.^ He used a radioactive substance which gives
off only a rays. Some of this substance was placed at a
distance of three centimetres from a metal plate which
was connected with a gold-leaf electroscope. When a
vacuum was established the electroscope leaked very
rapidly if positively charged, but only very slowly if neg-
atively charged. When the apparatus was placed in a
strong magnetic field the positive leak was slight, due
to the electrons being bent away by the field. The experi-
ment was then tried of placing the radiotellurium closer
to the metal plate in a strong magnetic field. Under these
conditions the electroscope became charged positively,
showing that the a particles were charged positively.
Recent experiments by Rutherford led to exactly the same
It will be remembered that the a rays are given off from
all radioactive substances, and, further, that only a and 8
rays are given off from polonium. A question that should be
raised and answered is this, are the a rays from polonimn
the same in character as the a rays from other radioactive
substances? This was tested by Becquerel in 1903. He
showed that the a rays from polonium are deflected in the
1 Phil. Mag., 10, 193 (1905).
74 THE ELECTRICAL NATURE OF MATTER
magnetic field in the same direction as the a rays from
radium. The a rays from polonium, therefore, consist
also of positively charged particles.
The conclusion that the a rays consist of electrically
charged particles was confirmed by Rutherford in the
following manner. The rays were passed through an elec-
tric field, and were shown to be deviated by the field. The
a particles are charged; each particle carrying two unit
THE RATIO — FOR THE ALPHA PARTICLE
The ratio of the charge to the mass of the a particles
can be ascertained by the same general method as that
which was employed by J. J. Thomson for determining
the same ratio for the cathode particle. This has already
been discussed at some length in an earlier chapter. By
studying the deviation of the rays in both a magnetic and
electrostatic field, as we have seen, it is possible to deter-
mine the velocity of the particles and the ratio — .
Very different results were obtained with the a particles
from those reached by Thomson for the cathode particles.
The mean velocity of the a particles is about 2.5 X 10® cen-
timetres per second, which is about one-tenth the velocity
of light. The ratio of charge to mass for the a particle is
about 6X10^^. While this result must not be regarded as
very accurate, on account of the difficulty in obtaining a
large deviation in the electrostatic field, it is still of the
right order of magnitude.
It is interesting to compare this result with that found
for the cathode particle.
THE ALPHA RAYS 75
The velocity of the cathode particle is about 3X10*
centimetres per second, and the ratio — = 10^.
The cathode particle, therefore, moves faster than the a
particle, and has a value of — , which is about two thousand
times as great as that of the a particle.
Rutherford ^ has recently shown that the a rays from
radium are complex, consisting of particles projected at
different velocities. It will be seen on page 70 that there
are four different products produced by radium, and
radium itself, which gives off a particles. The a rays from
radium c pass through about twice the thickness of air
that the a rays from radium itself do. Thus, each pro-
duct from radium seems to give off a particles at a certain
definite velocity. To measure the velocity of the a particles,
those emitted by only one product must be studied at a
time. Bragg and Kleeman ^ have, however, shown that
the a particles given off by radium in any one stage of its
decomposition are of the same nature.
THE MASS OF THE APLHA PARTICLE
Knowing the value of — , we have become familiar with
a method worked out by J. J. Thomson for determining
the value of e and, therefore, the value of m. While these
determinations have not been carried out directly for the a
particles as for the cathode particle, still some light has been
thrown on the present problem. We have seen that the
ratio — for the a particle is about 6X 10'.
* Phil. Mag., 10, 163 (1905).
^Ihid,, 10, 318 (1905).
76 THE ELECTRICAL NATURE OF MATTER
The ratio of — for the hydrogen ion in the solution of
in ^ o
adds is, as we have seen, about lo^.
If the charge carried by the a particle is twice that
carried by the hydrogen ion in solution, as is made highly
probable by our general knowledge of these bodies, then
we can compare the masses of the hydrogen ion and of the
a particle. Since -- for the former is lo*, and ~: for the
'^ in in
latter 6 X lo^, it follows that the mass of the a particle is
about four times the mass of the hydrogen ion. It will be
recalled that the determination of — for the a particle is
only approximate. It is therefore possible that the mass
of the a particle is just four times as great as that of the
hydrogen ion, in which case it would be equal to the mass
of the helium atom. We shall see that there is strong
evidence in favor of the view that the a particles are charged
We have seen that the a particles are projected with
enormous velocities, 2.5X10* centimetres per second. If
they have masses even as great as the hydrogen atoms or
ions, with such velocities they would have a large amount
of energy. This is the probable explanation of their great
power to ionize a gas through which they pass, and to pro-
duce other eflFects with which we shall become familiar
The recent work of Mackenzie ^ carried out in the lab-
oratory of J. J. Thomson has given values somewhat
different from the above. He used the magnetic and elec-
trostatic deflection of the a particles, and by this means the
velocity of the particles could be determined, and also the
1 Phil. Mag., 10, 538 (1905).
THE ALPHA RAYS 77
ratio of the charge e to the mass m. In this work, radium
which was in radioactive equilibrium was employed.
Under these conditions radiimi is sending out a particles
with very different velocities, and what is really determined
is the mean velocity.
In the magnetic deflection of the rays the a particles from
the radium entered a brass vacuum-box by passing through
a thin sheet of mica. The rays passed through a vacuum for
about fifteen centimetres, and then fell on a screen of zinc
sulphide. The line of scintillations was then photographed.
The poles of an electromagnet could be placed along the
path of the rays, and when the magnetic field was applied
the usual deflection of the a particles took place. This was
The mean value foimd for — was 3.00X10^, varying
between the extremes 2.5 X 10^ and 3.7 X lo*^.
The value of — found by Rutherford for radium in
radioactive equilibrium is 3.9X10^. The Mackenzie value
must be corrected for the decrease in the velocity of the
particles produced by passing through the thin sheet of
mica. The corrected values are as follows: The average
value of — for the a particles as they leave the surface
of the radium is 3.18X10^; the extreme values being 2.65 X
10^ and 3.92 Xio^
In measuring the electrostatic deflection an apparatus
was employed which was similar in many respects to that
used in measuring the magnetic deflection. The a rays
entered the apparatus by passing through the mica plate,
but they were now passed between two plates charged to a
difference of potential as great as 10,000 volts.
78 THE ELECTRICAL NATURE OF MATTER
The value found for = 4.11X10^*.
The value of — = 3.00 Xio^
The average value of z; = 1.37 X 10® centimetres per
second, and — = 4.6X10' electromagnetic units.
The magnetic deflection of the a particles from polo-
nium was also measured, and these were found to have
somewhat greater velocity than the average a particles
Their velocity, however, wa& not as great as the swift-
est a particles from radium.
Another matter must be discussed before leaving the
a rays. It has already been stated that strongly radio-
active substances like radium can produce phosphorescence
in certain substances exposed to their radiations. Thus,
screens covered with barium platinocyanide or zinc sul-
phide become phosphorescent when exposed to the action
of radium radiations.
This power of the radiations from radium to produce
phosphorescence can readily be shown to be due mainly
to the a rays. If the a rays are cut off by a thin screen
of metal, most of the power of the radiations to produce
phosphorescence is lost.
The power of the a particles to produce phosphorescence
has been utilized by Sir William Crookes ^ in the following
manner. If a plate covered with phosphorescent zinc
sulphide is exposed to the radiations from radium or polo-
nium, at a short distance from the substance, it presents
' Roy. Soc. Proceed., 71, 405 (1903). Chem. News, 87, (1903)-
THE ALPHA RAYS 79
a remarkable appearance. The screen does not become
homogeneously phosphorescent throughout, but bright
points of light make their appearance, and rapidly dis-
appear. The best result is obtained by examining the screen
through a small lens. Based upon these facts is Crookes'
spinthariscope. At one end of a tube is placed a piece of
metal which contains some radium chloride or bromide on
its surface. This is suspended at a distance of a few milli-
metres from a screen covered with phosphorescent zinc
sulphide. The other end of the tube contains a magnify-
ing lens. This instrument has been termed a spinthari-
scope, from " spintharis,*^ a spark.
The appearance of the screen has been described as analo-
gous to that of the milky way as seen with the naked eye
on a dark night. Bright points of light appear and quickly
disappear all over the screen. These come and go in rapid
succession. The effect has also been described as analo-
gous to the splashing of drops of rain in a pool.
The cause of this remarkable phenomenon is probably
the impact of the a particles upon the screen covered with
the substance in which phosphorescence can be set up.
The a particles, on account of their high velocity and appre-
ciable mass, have, as we have seen, a considerable amount
of kinetic energy. When they fall upon the screen covered
with zinc sulphide, they are stopped, and produce a me-
chanical disturbance. Zinc sulphide becomes luminous
when subjected to almost any mechanical disturbance.
Merely rubbing it with a hard surface will render it phos-
phorescent. Wherever an a particle falls upon the screen,
that portion of the screen becomes Imninous for some dis-
tance around the point of collision. Every spark or centre
of luminous disturbance on the screen is the result of the
impact of an a particle upon the screen. We thus see, as
8o THE ELECTRICAL NATURE OF MATTER
it were, the points at which the separate a particles strike
the phosphorescent screen, and this is, perhaps, one of the
best examples of the action of itidividual atoms or molecules
made directly perceptible to any of our senses.
Another theory of the action of the spinthariscope has
been proposed by Becquerel.^ He thinks the scintillation is
due to a fracture of the crystals of the phosphorescent zinc
sulphide by the a particles. He does not think that such
a fracture could be produced by one a particle, but only
when a number of such particles strike simultaneously a
weak point in the crystal. That light is frequently emitted
when crystals are crushed, is well known. Indeed, crystals
of zinc sulphide give out light when mechanically crushed,
and according to Becquerel such light has the characteris-
tics of that in the spinthariscope. It shows the same gen-
eral kind of scintillations, and the number of scintillations
is dependent somewhat upon the size of the crystals of
zinc sulphide with which the screen is covered. The
smaller the crystals of the sulphide the larger the number
of scintillations, which accords with BecquereFs view as
to the action in the spinthariscope. The smaller the crystals
the more easily they would be broken, and, consequently,
the larger the niunber of scintillations. It is difficult at
present to decide between these two views. The theory
first advanced is the simpler and more fascinating, but
it may not be true. More experimental evidence must be
obtained before a final decision can be reached.
CRITICAL VELOCITY OF THE ALPHA PARTICLES
Rutherford and other investigators have found that all
of the a particles given out by a radioactive substance
have the same velocity. When the a particles pass through
* Compt. rend., 137, 629 (1903).
THE ALPHA RAYS 8l
matter their velocity decreases. When their velocity falls
below a certain value, 0.82 X lo* centimetres per second, they
cease to ionize the air, and they no longer affect the photo-
graphic plate or produce phosphorescence in a phosphores-
cent screen. This is known as the critical velocity of the a
particles. We have no means of detecting the presence of
a particles given off with a velocity less than the critical,
and, therefore, cannot determine whether such exist or not.
The distance which the a particles will travel in air is
known as the range of the a particles — a term very fre-
ALPHA PARTICLES PRODUCE DELTA PARTICLES
We have just seen that when the velocity of the a par-
ticle falls below a certain value it ceases to ionize a gas
through which it passes. Duane ^ thinks that the a par-
ticle loses its charge at the same time that it ceases to
ionize the surrounding gas.
When a particles impinge upon a solid body, 8 particles
are produced at the surface of the solid. Duane shows that
beyond their " range " the a particles cease to produce
This raises the question, what are the 8 particles? They
are particles carrying a negative charge and moving with
a velocity of 3.3X10* centimetres per second. When we
come to study in some detail the properties of the ft
particles we shall see that the 8 particles are essentially
nothing but slow-moving )8 particles.
ALPHA PARTICLES ARE PROBABLY HELIUM ATOMS
It is a well-known fact that helium accumulates in radium,
in actinium and in thorium compounds. This has led to
* Amer. Jour. Sd., 26, Nov. (1908).
82 THE ELECTRICAL NATURE OF MATTER
the suggestion that the helium consists of a particles that
have lost their charge. This is, however, difficult to test
satisfactorily, on account of the difficulty of measuring e
accurately. Regener ^ has devised a method of counting
the number of a particles xmder certain conditions, by
allowing them to strike a screen of zinc sulphide and pro-
duce the well-known scintillations — each a particle being
supposed to produce one scintillation. Knowing the num-
ber of a particles and the total charge carried by them, we
know the charge carried by one a particle.
The same result has been obtained by Rutherford and
Geiger^ who have been able to count the number of a parti-
cles given oflf by uranium, thorium, radiimi, and actiniimi
compoimds, by increasing the ionizing power of these parti-
cles in accordance with a principle discovered by Townsend,*
which gives the conditions under which ions can be formed
by collision between neutral gas iholecules and ions moving
in a strong electric field.
Rutherford and Geiger* thus coimt the number of a
particles given off from a radioactive substance under
given conditions and allow the total charge that they carry
to accumulate on an insulated plate. They measure the
total charge carried, and knowing the number of carriers,
they know the charge carried by one a particle. This was
shown to be 9.3X10"^® electrostatic units, which gives a
mass of four for the a particle, this being the mass of the
Dewar ^ measured very carefully the rate at which helium
is produced from radiima, and found that his result agreed
• Ber. d. physik. Ges., 10, 78 (1908).
'Proceed. Roy. Soc., A, 81, 141.
3 Phil. Mag. 5, 389, 698; 6, 358, 598 (1903).
* Proceed. Roy. Soc., A, 81, 141. .
•Proceed. Roy. Soc, A, 81, 280.
THE ALPHA RAYS 83
very well with that calculated on the assumption that the
a particles are charged helium atoms.
Again, Rutherford and Royds ^ showed that helium can
be obtained from accumulated a particles, independent of
the active matter from which the a particles came.
Taking all of the above facts into consideration, the evi-
dence that a particles are simply charged helium atoms
is very striking.
ACTION OF THE tt PARTICLES ON A PHOTOGRAPHIC AND ON
A FLUORESCENT PLATE
When the a particles pass through matter, their velocity
is diminished. When their velocity falls below a certain
value they lose their properties of producing luminescence,
of affecting a photographic plate and of ionizing gases.
The important point is that this valine is the same in all
three cases. This would indicate, as Rutherford points out,
that the three properties mentioned above have a common
The absorption of the a rays by gases is due to the energy
being used up in producing ions in the gas. Rutherford
thinks that the phosphorescent action and the action on
a photographic plate are primarily the action of ions.
These would cease at about the same velocity that would
just be necessary to ionize a gas.
The bearing of these results on the action of the spin-
thariscope is pointed out. Becquerel explains the action, as
will be recalled, as due to the cleavage of the crystals of
the phosphorescent substance. The action is probably
to be ascribed, according to Rutherford, to the production
of ions in the substance. When these ions recombine scin-
We cannot ascribe the action of this instrument simply
84 THE ELECTRICAL NATURE OF MATTER
to the bombardment of the phosphorescent screen by the
a particles, since we have just seen that these particles
produce no scintillations or luminescence after their ve-
locity has fallen below a certain definite value, and they
still have, of course, considerable kinetic energy.
Rutherford raises the qi.'estion as to whether phospho-
rescetU and photographic effects in general may not he due
primarily to the prodttction of ions,
STOPPING POWER OF MATTER FOR THE ALPHA PARTICLES
The following interesting, although empirical relations,
have apparently been established by Bragg and Kleeman.
The so-called " stopping power " of a number of the ele-
ments for the a particle was determined, with the result
that the amount of energy spent by the a particle in producing
ionization in an atom seems to he proportional to the square
root of the atomic weight of the substance ionized. Quite a
number of elements have been brought within the scope
of this investigation, with the result that the above relation
seems to hold approximately.
They have also shown that the number of ions produced
by an a particle is the same, no matter what the nature of
the gas through which it passes; and, further, that the
same amount of energy is always required to make a pair
of ions, regardless of the nature of the atom or molecule
from which they came.
This latter relation is probably very important, since it
shows that ionization is essentially the same process, regard-
less of the nature of the molecules of the gas in which it
The Beta and Gamma Rays
the beta rays
It was pointed out in connection with the study of the
a rays, which are only slightly deviable, that the radium
radiations contain rays which are readily deviated by the
magnetic field. This was shown by means of an experi-
ment already referred to in connection with the study of
Some radium bromide was placed on the bottom of a
tube of lead, which in turn was introduced between the poles
of an electromagnet. In front of the tube, and at a distance
of several centimetres from it, was an electroscope. It is
necessary that an air space should intervene between the
tube and the electroscope, in order that the a radiations
from the radium should be cut oflF and not allowed to fall
upon the instrument. A few centimetres of air are quite
sufficient to cut oflf the easily absorbed, non-penetrable
a rays, as we have seen. The )8 radiations from the radium
now fall upon the electroscope, together with the y radia-
tions; but since the latter have only very small power to
ionize a gas through which they pass, they have but little
power to discharge the electroscope. Further, they are
not deflected by a magnetic field, and, therefore, their
action on the electroscope is constant before and after the
current is turned on the electromagnet.
When the electromagnet is turned on and a magnetic
86 THE ELECTRICAL NATURE OF MATTER
field established, the j8 rays are readily deflected against
the walls of the tube, and no longer fall on the electroscope,
or ionize the air between the leaves. The electroscope is
now discharged much more slowly than before the magnetic
field was produced. This experiment illustrates qualita-
tively the deviable nature of the )8 rays.
A question in this connection which is of importance is
this: Are all the )8 rays equally deviable? Are the )8
radiations homogeneous? This is answered by the follow-
If in the preceding experiment the metal tube was covered
with a metal plate having a narrow slit cut in it, only a
narrow beam of rays could escape from the tube. This
would produce only a narrow line on a photographic plate.
If the magnetic field is now established, the )8 rays will be
deflected to one side. The impression upon the plate,
however, is not that of a displaced narrow line, but is a
broadened band. This shows that the deviable )8 rays
are not homogeneous, but that some are more deflected
by the magnetic field than others. They are spread out
by the magnetic field into a kind of spectrum, showing that
some of the )8 particles have very different velocities from
NATURE OF THE CHARGE CARRIED BY THE BETA PARTICLES
The )8 rays, as we have seen, are deflected in the mag-
netic field. The next question is, are they charged, and if
so, positively or negatively? This is answered by the
following experiment carried out by M. and Mme. Curie.*
If the )8 rays are absorbed by any substance they would
necessarily give up their charge to the absorbing medium.
It would, apparently, be only necessary to detect the nature
1 Ann. Chim. Phys. , 30, 155 (1903),
THE BETA AND GAMMA RAYS 87
of the charge on the object by which the )8 rays are absorbed,
in order to determine the nature of the charge carried by
the )8 rays themselves.
While this at first sight is a very simple matter, a difficulty
is encountered. The )8 rays produce ions in a gas through
which they pass. These would conduct the charge away
from the object upon which the )8 rays impinge, and not
enough charge would collect to be detected. In carrpng
out such an experiment it would obviously be necessary to
cut off the a rays by means of a thin sheet of metal, through
which the )8 rays would pass, since the a rays have much
greater ionizing power than the j8 rays. Even when this
is done the )8 rays render the air a sufficiently good con-
ductor to remove the electricity too rapidly from the object
which absorbs the j8 rays, in order that a sufficient charge
should accumulate to be detected.
This difficulty was overcome by the Curies by imbedding
the plate upon which the )8 rays were to fall, in an insulator
through which the )8 rays could pass. They used thin
ebonite, and also a thin layer of paraffine. The result was,
that the Curies were able to demonstrate that the metal
upon which the )8 rays fell, became charged negatively.
This proved that the )8 particles carried a negative charge.
The same result was obtained by Wien, who surrounded
the plate upon which the )8 rays were to fall, not with an
insulator, but with an evacuated vessel.
The Curies proved that the plate continually received
negative electricity ^ as would be expected by the constant
raining of the negatively charged )8 particles upon it. Mme.
Curie states that only a very weak current was obtained
under the above conditions, as would be expected.
The Curies then undertook the sequel to the above experi-
ment. If the )8 rays are charged negatively, they must
88 THE ELECTRICAL NATURE OF MATTER
leave the radium from which they are shot oflF positively
charged. To test this conclusion the Curies placed the
radium salt in a lead box, and surrounded the whole with
the insulating medium. The insulating material was then
surrounded by metal connected to earth.
Under these conditions the radium became positively
charged, due to negative charges being carried off by the j8
particles, which, in this case, were communicated to the
outside metal box and then to earth.
In the above experiment the a particles are completely
absorbed by the insulated box, and their effect thus re-
duced to zero.
An interesting observation in this same connection has
been described by the Curies. Radium would continue to
throw off negative charges until it itself would become so
highly charged positively that this would prevent the further
sending off of negative charges. An active preparation of
radium was sealed up for some time in a glass tube. When
the tube was scratched with a file, the weakened portion
was at once perforated by a spark, and M. Curie at the same
moment received an electric shock. The potential of the
tube had thus been raised well above the potential of the
earth, due to the absorption of the positively charged a
particles, which gave up their charge to the inside of the
THE DETERMINATION OF — FOR THE BETA PARTICLE
We have already studied the method worked out by J. J.
Thomson for determining the ratio of — for the cathode
particle. This method, it will be remembered, is based
upon subjecting the cathode rays to both electrostatic and
THE BETA AND GAMMA RAYS 89
magnetic deflection. Exactly the same method was used
with the )8 particles from radium. It is not necessary to
repeat the discussion of this method. If necessary, the
account of the method given in an earlier chapter should
be reread. The velocity of the )8 particles, as thus deter-
mined by Becquerel, was about 1.5X10^® centimetres per
second, and the value of — = 10'. This velocity is of the
same order as that of light, 3X10^® centimetres per second,
and is considerably greater than that found for the cathode
particle in the low-pressure tube.
One matter of very great importance in this connection
must be mentioned again. It will be remembered that all
of the 13 particles are not deflected equally by a magnetic
field. This was shown by a broadening of the line on the
photographic plate, when the magnetic field was produced.
It was pointed out that this was due to the fact that the )8
particles did not all move with the same velocity.
This is made the basis of the important experiment of
Kaufmann, to which reference has already been made.
He studied the electrostatic and magnetic deflections of
the )8 rays having different velocities, and determined the
value of — for the different rays.
He found that this value was not constant, but varied
with the velocity of the particle. The value of — increased
as the velocity of the particle diminished. This is seen
from the results, already discussed in an earlier chapter,
see page 22.
The importance of this observation has already been
pointed out. The charge e carried by the particle is con-
stant, independent of the velocity. Since — changes with
9^ THE ELECTRICAL NATURE OF MATTER
the velocity, we must conclude that w, or the mass of the
particle^ changes with the velocity.
The significance of this has already been referred to in
an earlier chapter. It will be remembered that the con-
clusion to which we were led, especially after comparing
the values calculated by Thomson with those found experi-
mentally by Kaufmann, is that all mass is of electrical origitiy
and that matter is made up of electrons or disembodied
electrical charges, moving with high velocities.
THE MASS OF THE BETA PARTICLE — RELATION TO THE
The method for determining the mass of a particle, know-
ing the value of the ratio — for it, has already been dis-
cussed at length. The mass of the fi particles is about jj^j
of the mass of the hydrogen ion in solutions of acids. // is,
thereforey the same as the mass of the cathode particle.
We have now studied a sufficient number of properties
of the )8 rays to enable us to make a comparison with the
corresponding properties of the cathode rays.
Affect the photographic plate.
Ionize a gas.
Are negatively charged particles.
Have moderate power to penetrate matter.
Have a mass g,bout jj^s of the mass of the hydrogen ion.
Have a velocity about one-tenth that of light.
BETA RAYS FROM RADIUM
Affect the photographic plate.
THE BETA AND GAMMA RAYS QI
Ionize a gas.
Are negatively charged particles.
Have moderate power to penetrate matten
Have a mass about j^^s of the mass of the hydrogen ion.
Have a velocity that varies for the different fi particles,
but the mean velocity is about half that of light.
We see from the above that the )8 particles resemble the
cathode particles very closely in all of their properties,
except the velocity with which they travel. That the two
sets of particles shoidd not have the same velocities^ is not at
all surprising, when we consider the different conditions
under which they are produced.
The )8 particles are shot off from radium with velocities
that are definite^ and which are conditioned by the nature
of the substance. The cathode particles are shot off from
the cathode under a high electrical stress, conditioned in
part by the difference between the potential of the anode
and the cathode. Indeed, we should expect that the velocity
of the cathode particle woidd vary with the field that was
employed, and such is the fact. With a strong field the
velocity of the cathode particle is greater than with weak
fields, and with very strong fields the velocity of the cathode
particle approaches much more nearly to the velocity of the
We can, then, regard the jS particles as essentially identical
with cathode particles, differing from them only in the veloci-
ties with which they move. This would produce, as we
have seen, a slight difference in the mass, but it is not neces-
sary to go further into this matter in the present con-
We have learned that the cathode particles are nothing
but electrons, or disembodied, negative electrical charges.
92 THE ELECTRICAL NATURE OF MATTER
Therefore, the )8 rays are made up of nothing but nega-
tive electrical charges, shot ofiF from the radium with enor-
mous velocities — the velocities being comparable with
that of light.
We have learned that all the radioactive substances
known give ofiF a particles. The three radioactive sub-
stances, uranium, thorium, and radium, give ofiF )8 parti-
cles. Polonium, as we have seen, gives out only a and 8
SECONDARY RADIATIONS PRODUCED BY )8 RAYS
A very considerable amount of work has been done
recently on the absorption of )8 rays, and on the secondary
radiations excited by them. McClelland ^ shows that the
secondary radiation consists partly of reflected primary
rays, and partly of corpuscles which seem to have been
expelled from the atoms when the primary rays entered.
The relative intensities of the secondary radiations given
out depend directly upon the atomic weights of the ele-
ments upon which the primary rays impinge. Regener
finds that the )8 rays produce scintillations when they fall
upon a screen of barium platinocyanide, which is placed
between lo and 50 centimetres from the source of the )8
rays. It will be recalled that the a particles as they pass
through matter lose energy gradually and finally cease to
ionize the gas through which they are passing.
The absorption of the )8 particles seems to be quite
dififerent. According to Makower,^ McClelland and Hack-
ett' and others, the )8 particles are stopped suddenly, their
velocity just before stopping being very high.
* Proceed. Roy. Soc., A, 80, 501.
* Trans. Roy. Soc, cited, g, 4 (1907).
» PM. Mag., Aug. (1908).
THE BETA AND GAMMA RAYS 93
THE GAMMA RAYS
A third kind of rays is given out by all radioactive sub-
stances, with the exception of polonium. It was shown by
Villard, as we have seen, that these rays are not deviated by
a magnetic field, and have much greater power to penetrate
maUer than either the a or the )8 rays. A thin film of metal
is suflScient to stop the a rays. The )8 rays are all cut off
by a piece of some heavy metal like lead that is a centi-
metre thick, while the kind of rays with which we are now
more especially dealing can, according to Rutherford, be
detected by a sensitive electroscope after they have passed
through a piece of iron that is a foot thick.
These rays have not as yet been deflected to a detectable
amount in the magnetic field.
While all the radioactive elements, with the exception
of polonium, give off fi rays, they give them out with very
different intensities. It would be expected that the weakly
radioactive elements, uranium and thorium, would give
out y rays to a less extent than the highly radioactive
radiiun, and such is the fact. The y rays given out by the
weakly radioactive elements have, however, been detected
by using fairly large quantities of these subtances.
The y rays, thereforey always accompany the fi rays, and this
is a matter of importance in connection with the theories that
have been advanced to account for the nature of the y rays.
Two hypotheses as to the natiure of the y rays have been
We have seen that the )8 rays are made up of electrons,
or negative electrical charges, moving with different veloci-
ties, but all having very high velocities; the swiftest of these
travelling with a velocity which is nearly that of light. It
is possible that electrons are shot off from radium with even
94 THE ELECTRICAL NATURE OF MATTER
a higher velocity than that of the swiftest )8 rays. Such
rays could have at least some of the properties of the y rays.
Their great penetrating power might be due to their large
kinetic energy resulting from their great velocity. The
fact that they are not deflected in the magnetic field has been
accoimted for by the advocates of this theory, on the ground
that the amount of the deviation being an inverse function
of the velocity, the more rapidly moving particles might
be deflected to such a small extent that it would not be
observed. This theory contains a number of weak points.
In the first place, the penetrating power of the y rays is so
many times that of the )8 rays that it seems difficult to
account for this on the basis of the slightly increased velocity,
even if the velocity of light is being closely approached.
Further, if this theory as to the nature of the y ray is correct,
we might reasonably expect to find rays with penetrating
power intermediate between that of the )8 ray and the in-
comparably greater power of the y ray. Indeed, all the
intermediate stages could easily be represented. Such,
however, is not the fact. The same criticism holds with
respect to the deviation in the magnetic field. If y par-
ticles are nothing but more rapidly moving )8 particles,
and if the fact that the )8 particles are so readily deflected
in the magnetic field, while the y particles are not deflected
at all, are to be accounted for solely on the ground of the
difference in velocities, then why do we not find the inter-
mediate stages represented? This question is especially
pertinent in consideration of the fact that we do know )8
particles with quite different velocities. The magnetic
deflection of even the swiftest of these is easily detected.
If )8 particles with intermediate velocities existed, it seems
reasonable to think that there would be no serious difficulty
in detecting their deflection in a magnetic field.
THE BETA AND GAMMA RAYS 95
A theory as to the nature of the y rajrs, which accounts
much better for many of the facts, is the following. We
have seen in a much earlier chapter, that whenever cathode
rays strike a solid object X-rays are produced. We have
recently seen that the )8 rays are essentially identical with
the cathode rays. We would naturally expect that X-rays
would be set up where the )8 rays strike a solid object. The
)8 rays from radium strike some of the solid radium salt,
or some other solid, and the y or X-ray is accordingly
produced. The y ray, in terms of this theory, is nothing
bid an X-ray. We have seen, however, that it has much
greater penetrating power than the X-ray, and it must
therefore be regarded as a very penetrating kind of X-ray.
This theory accounts satisfactorily for the entire absence
of deflection of the y rays in a magnetic field, since ordinary
X-rays are themselves entirely undeflected by such a field.
This theory as to the nature of the y rays also accounts
for the fact that y rays are always absent imless )8 rays
Some objections have, however, been offered to this
theory as to the nature of the y rays, so that it must not
be regarded as final.
According to Madsen ^ the y rays of radiimi and possibly
those of thorium consist of two distinct, homogeneous
bundles. When a stream of y rays penetrates a metal
plate, secondary y radiations appear on both sides of the
plate. The amount of the secondary radiation from the
two sides of the plate differs very greatly. A change in the
hardness of the y rays produces a marked difference in
the relative intensities of the emergent secondary radia-
tion from various elements. These secondary radiations,
however, do not follow the order of the atomic weights.
iPhil. Mag. (1907); Nature (1908).
96 THE ELECTRICAL NATURE OF MATTER
SUMMARY OF THE PROPERTIES OF THE ALPHA, BETA, AND
The a rays are given off by all radioactive substances.
They are somewhat deflected in a magnetic field. They
have very small penetrating power, being easily absorbed
even by very thin layers of matter. They have great power
to ionize a gas, rendering it a conductor. The a rays ionize
to about one hundred times the extent of the )8 and y rays
together. They have but little effect on a photographic
plate, but produce phosphorescence in certain substances,
especially zinc sulphide. The existence of phenomena
such as those manifested in the spinthariscope are due
almost entirely to the a particles. The a particle has a
mass of the order of magnitude about twice that of the
hydrogen ion. This, however, is only an approximation.
The a particle carries two positive charges of electricity,
and moves with a velocity about one-tenth that of light.
The )8 rays are given off from all radioactive substances,
with the exception of polonium. They are very easily de-
flected in a magnetic field. They are absorbed by matter, but
not near so easily as the a rays. They have comparatively
small power to ionize a gas. They do not have great power to
affect a photographic plate, and while they can produce phos-
phorescence are less active in this respect than the a particles.
The |3 particle has a mass about j^^j of the mass of the
hydrogen ion in solution, which is the mass of the electron.
The )8 particle carries a unit charge of negative electricity,
or, more accurately expressed, is a imit negative charge of
electricity, shot off with an average velocity which is of
the same order as that of light. The fi ray is practically
identical with the cathode ray in a vacuum tube, differing
from it chiefly in the velocity with which the particles move.
The y rays exist where the )8 rays exist. They are not
THE BETA AND GAMMA RAYS g^
deflected at all in a magnetic field. They have very great
penetrating power, enough passing through a foot of iron to
be detectable by the electroscope. They have much smaller
power than the a particles to ionize a gas. They have consid-
erable power to affect a photographic plate, much greater
than the a or even the )8 particles. They excite phosphor-
escence. The most probable theory as to the nature of the
y rays is that they are a very penetrating form of X-ray,
produced by the )8 rays. They are, therefore, pulses in the
ether, set up by the impact of the )8 rays on solid matter.
TOTAL NUMBER OF PARTICLES SHOT OFF BY RADIUM
Rutherford determined the total number of particles
shot off by radium. To determine the total number of a
particles he must get rid of the )8 particles. He did this
by removing the emanation and all of its successive de-
composition products, and obtained radium at what is
known as its minimum activity. Under these conditions
he found that the number of a particles shot off per second
from a gram of radium is 6.2X10^®. The number of a
particles shot off by normal radium in radioactive equi-
librium is approximately the same as the mmiber of )8
particles shot off under the same conditions; since radium,
the emanation, radium A, radium C, and radium F all
emit a particles, while radium, and radium B, C and D and
E emit )8 particles. Radium, however, at its minimimi
activity is freed from the emanation and all succeeding
decomposition products, and gives off the same number of
a particles as normal radium gives off )8 particles. This also
was tested by Rutherford. He foxmd that the number of
)8 particles shot off per second from one gram of radium was
7.3 X io^°. This is almost identical with the number of a
particles at minimum activity.
Other Properties of the Radiations
We have already studied a number of the properties of
the several kinds of radiations, and have compared the one
with the other. We shall now take up certain special
properties of the several kinds of radiations sent out by
radium, as pointed out by Mme. Curie.V
THE SELF-LUMINOSITY OF RADIUM COMPOUNDS
While the comparatively pure radium salts give out only
a little light, radium salts which contain a large amount of
barium are strongly self-luminous. This fact was observed
by the Curies. The dehydrated, dry, halogen compounds
of radium are especially self-luminous. While the self-
luminosity cannot be perceived in ordinary daylight, it can
be seen by gaslight. The self-luminosity comes from the
entire mass of the radium salt, and not simply from the
surface. In the presence of moist air the salt loses a large
amount of its self -luminosity, but this is again regained on
drying the preparation. The self-luminosity persists for a
long time. Specimens preserved for years in the dark still
continue to be self-luminous. Mme. Curie points out that
the color of the light emitted from strongly radioactive
preparations changes with time, becoming more violet and
decreasing in intensity. The original intensity and color
are regained by recrystallizing the salt from water. The
* Ann. Chim. Phys. , 30, 145 (1903).
OTHER PROPERTIES OF THE RADIATIONS 99
luminosity of the radium salt is apparently independent of
temperature. Solutions of radium salts are slightly self-
luminous. The crystals in such a solution are more strongly
self-luminous than the solution, and can be seen by the
light which they emit.
Mme. Curie also points out that radium is the only sub-
stance known that is self-luminous. It will be remembered
that radium is the only substance known that has the power
to charge itself electrically.
PHOSPHORESCENCE PRODUCED BY RADIUM SALTS
That salts of radium are capable of exciting phosphores-
cence in certain substances has already been mentioned.
This was first discovered by the Curies. It was subse-
quently studied by others, and especially by Becquerel.
Thus, the diamond, ruby, the sulphide of calcium, zinc
sulphide, barium platinocyanide, paper, glass, etc., have
The action of the radium is, however, not the same as
that of the X-ray in producing phosphorescence. Certain
substances phosphoresce when exposed to the X-ray, that
do not in the presence of radium, and vice versa. In this
respect the action of radium resembles more closely that of
Paper, cotton, as well as certain varieties of glass phos-
phoresce in the presence of radium. This is especially true
of Thuringian glass. Under the action of radium the
glass that phosphoresces becomes colored violet to brown.
When the glass has become colored its power to phosphoresce
is diminished. If the glass which has become colored and
has lost its power to phosphoresce is heated, the color is lost
and the power is again regained. Barium platinocyanide
is the best substance with which to study this action of
loo THE ELECTRICAL NATURE OF MATTER
radium salts. It shows phosphorescence when placed two
metres from active radium.
Zinc sulphide, as we have seen, is also rendered phos-
phorescent by the radium rays. It is especially sensitive
to the action of the a rays, where it shows the characteristic
scintillations in the spinthariscope. It has already been
mentioned that the diamond becomes phosphorescent in
the presence of radium, and can thus be distinguished from
While all three kinds of rays produce phosphorescence,
the a rays, on the whole, are the most active. This can be
seen by interposing between the radium and the screen
a thin piece of metal foil or of paper which will cut ofiF the
a particles. The fi and y rays can also produce phos-
phorescence, especially in screens of barium platinocyanide.
Their power is, however, much feebler than that possessed
by the a particles.
RADIUM INCREASES THE CONDUCTIVITY OF DIELECTRICS
The property of radium to ionize a gas and render it a
conductor has already been repeatedly mentioned. A good
qualitative method of demonstrating this power is the
following: Take an induction coil and place the discharging
points just so far apart that a spark will cease to pass. Then
place a glass tube containing a few milligrams of an active
radium salt between the two points. The discharge will take
place at once. This is due to the ionization of the air be-
tween the terminals by the radiation from the radium. In
the above case most of the ionization is produced by the y
rays, since most of the a and fi rays are cut off by the glass.
If the radium salt were placed in an open vessel so as to se-
cure the ionizing effect of the strongly ionizing a rays, the
conductivity of the gas would be still more increased.
OTHER PROPERTIES OF THE RADIATIONS lOI
The conductivity of a number of liquid non-conductors is
very considerably increased by exposing them to the radium
radiations. Thus, a number of our best liquid insulators
acquire a measurable conductivity under the influence of
the radiations from radium. This applies to carbon disul-
phide, petroleum ether, liquid air, vaseline oil, etc.
It would seem that this ionization in liquids was pro-
duced mainly by the y radiations, since similar results
were obtained by M. Curie when the liquids were exposed to
X-rays, with which, it will be remembered, the y rays are
closely allied. Similar results have been obtained with
certain solid dielectrics. Thus, parafiine exposed to the
radiations from radium acquires some conductivity. The
ionization produced in the parafl&ne, as well as in the liquid
non-conductors, is probably due mainly to the more pene-
trating rays from radium.
CHEMICAL EFFECTS PRODUCED BY RADIOACTIVE SUBSTANCES
The crystalline halogen salts of the alkahes — the chlo-
rides, bromides, etc., are colored by radium radiations as by
cathode rays. The Curies observed that glass and porce-
lain became colored when exposed to radium. A violet or
brown color appears in the glass, which persists after the
removal of the radium. Glass which has been exposed for
a considerable time to the action of radium becomes dark-
ened. This is apparently true of all glasses.
Mme. Curie subjected a number of glasses of known, but
widely different composition, to the action of the radium
radiations, and concluded that the coloration was due to
the presence of the alkali metal in the glass. Salts of the
alkali metals themselves showed more vivid coloration, and
a greater variety of colors than the different glasses that
were studied by Mme. Curie.
I02 THE ELECTRICAL NATURE OF MATTER
The most probable theory as to the cause of the coloration
in glass is that the radiations from radium liberate the alkali
metals, which then form a solid solution in the glass.
Radium transforms oxygen into ozone, which can be
detected by its odor. This is due to the a and )8 rays, since,
when these are cut ofiF, no ozone is produced. To understand
what this transformation really means, we must ask the
question, what is the real difference between oxygen and
ozone? The older text-books on chemistry state that the
difference in the properties of oxygen and ozone is to be
referred to the fact that oxygen contains two atoms in the
molecule, and ozone three. It is obvious that this explains
nothing, except the difference between the mass of the atom
of oxygen and the mass of the atom of ozone. The chemi-
cal and physical properties, in general, of substances cannot
be explained on any material bases. To gain any rational
conception of them we must take into account the energy
relations and conditions that exist in the substance in ques-
It is a simple matter to prove that the real difference
between the properties of dxygen and ozone is due to the
different amounts of intrinsic energy possessed by their
molecules. If we bum carbon in oxygen or in ozone, the
same end product, carbon dioxide, is obtained. If oxygen
and ozone contain different amounts of intrinsic energy,
there will be different amounts of heat liberated when the
same amounts of carbon are burned in the two gases; since
the amount of heat liberated in any case is the thermal ex-
pression of the difference between the intrinsic energy of
the system before a reaction has taken place, and after the
reaction is completed.
If we bum a given weight of carbon in ozone, more heat
is liberated than when we bum the same weight of carbon
OTHER PROPERTIES OF THE RADIATIONS I03
in oxygen'. Since the same amounts of carbon dioxide are
formed in the two cases, we must conclude that ozone con-
tains more intrinsic energy than oxygen^ and any differences
in the properties of these two allotropic modifications of the
same element are to be referred to the different amounts
of intrinsic energy possessed by their molecules. Radium,
then, adds energy to oxygen, transforming it into ozone,
and this is accomplished mainly by the a and )8 rays. This
is in keeping with our knowledge of the radiations given
off from radium, since most of the energy is contained in
the a particles. According to Becquerel, radium radia-
tions can also transform white phosphorus into red.
Radium compounds undergo changes themselves under
their own radiations. When the method of separating
radium from pitchblende was under discussion, it was
pointed out that crystals of radium chloride with which
barium chloride was mixed, while colorless when first
formed, became quickly colored. The color is lost by
recrystallizing the salt. The coloration produced by the
radium salts extends more deeply into the substance than
that caused by the cathode rays.
It has already been mentioned that the radiations from
radium affect a photographic plate. This is, of course,
due to a chemical action on the silver salt of the photo-
graphic plate. Polonium acts on a photographic plate
only when the plate is brought very near to the substance.
This is due to the fact that polonium gives out only a rays,
which have weak photographic action; and further, are
largely absorbed by a layer of air, even a few centimetres
Radium, however, acts at much greater distance on a
photographic plate. It produces a marked impression at
a distance of several feet, even when the radium is inclosed
I04 THE ELECTRICAL NATURE OF MATTER
in a glass tube, which cuts off all of the a rays, and some of
the )8. We have seen that it is the y rays that are especially
active photographically. It has been found that the best
radiographs are produced by the y rays alone.
PHYSIOLOGICAL ACTION OF THE RADIATIONS FROM RADIUM
Fairly active radium is capable of producing bums or
wounds when brought near the skin, that are both painful
and slow to heal. The skin is first inflamed and reddened,
and may actually become blistered if exposed for a sufficient
length of time dose to an active preparation of radium.
The action of the radiations from radium upon certain
diseases of the skin, such as lupus, has been tested, and
apparently has yielded good results in the hands of the
dermatologist. It has also been claimed to have produced
wholesome effects upon cancerous tissue, especially in the
early stages. Whether it is really capable of curing this
disease remains to be seen. It is certainly true that the
radiations from radium are more penetrating than ultra-
violet light or X-rays, which have been shown to have cer-
tain curative properties. They can, therefore, penetrate
more deeply into the tissue, and might give better results.
An interesting physiological experiment has been studied
by Himstedt and Nagel.* If a preparation of radium is
brought near the closed eye in a dark room, a sensation of
light is produced. This is due to the phosphorescence
produced within the eye itself by the radium, the lens and
retina being strongly phosphorescent under the action of
the )8 and y rays. This sensation is experienced even by
the blind, if the retina has not been destroyed.
Aschkinass and Caspari have shown that the radiations
from radium also diminish the activity of certain bacteria.
» Ann. d.Phys.,4i 537(1901).
OTHER PROPERTIES OF THE RADIATIONS 105
A large number of facts in connection with the action of
radium upon living matter have been brought to light.
It would obviously lead too far to discuss these at length in
the present connection.
The physiological action of radium is due mainly to the
a and )8 rays. These are cut off by placing the radium salt
in a metal box — especially in one of lead. This precau-
tion should always be taken when active preparations of
radium are being used.*
* For further details in reference to the matters discussed in this chapter,
see the article by Mme. Curie in Ann. Chim. Phys. , 30, 186-203 (^9^3)'
Production of Heat by Radium Salts
An observation of the greatest importance was made in
1903 by M. Curie and Laborde.* Salts of radium have a
temperature that is continually above that of the surrounding
medium. This means that heat is being produced in the
radium compound. That the radium salt is warmer than
the surrounding air can be shown qualitatively by means of
fairly sensitive mercury thermometers. It can be readily
demonstrated in the following manner, according to Mme.
Curie. A double-walled glass bulb was made, and the
space between the two walls exhausted. The object of
removing the air was to render the space between the walls a
very poor conductor of heat. Into such a vacuum-jacketed
vessel the bromide of radium, placed in a glass tube, was
introduced, together with a relatively sensitive thermometer.
Into a second such vessel a similar thermometer was intro-
duced. The thermometer placed near 0.7 of a gram of the
radium salt registered two or three degrees higher than
the thermometer in the vessel that contained no radium.
Thus, quite appreciable differences in temperature were
produced with a few decigrams of the radium compound.
With larger quantities of the salt still greater differences
in temperature would result.
MEASUREMENT OF THE HEAT LIBERATED BY SALTS
Several methods have been employed to measure the
quantity of heat liberated in a given time, by a given quantity
1 Compt. rend., 136, 673 (1903).
PRODUCTION OF HEAT BY RADIUM SALTS 107
of radium. A rough method carried out by M. Curie and
Dewar is more novel and interesting than important. It
is well known that Dewar, provided with the splendid low-
temperature plant of the Royal Institution, has been able
to obtain in large quantities all of the lowest condensing
gases, with the exception of helium, in the liquid form.
He has obtained liquid hydrogen in considerable quantity,
and worked out a number of its interesting properties. He
has determined its boiling-point, and found this to be only
about twenty on the absolute scale, which is —253 degrees
centigrade. If heat is added to liquid hydrogen it will
boil. On account of the very low temperature at which
liquid hydrogen boils, it will take up heat from any surround-
ing liquid except more of the liquid hydrogen itself, and
would thus continue to boil without cessation, or at least to
give off appreciable quantities of hydrogen gas.
A test-tube, whose lower half was surrounded by a double-
walled, vacuum jacket, was filled about one-third full with
liquid hydrogen. This was then immersed in a larger
vessel, also surrounded by a double-walled vacuum jacket,
and the space between the two filled with liquid hydrogen.
The hydrogen in the inner tube soon ceased to give off any
appreciable amount of gas, since it could not obtain the
heat necessary to convert itself into vapor — the conduc-
tion of heat being prevented by the hydrogen in the outer
vessel, which always continued to give off gas. If any heat
was supplied to the liquid hydrogen in the inner vessel, a
part of the liquid would be converted into vapor which
The experiment consisted in arranging the system as
above described, and waiting until the gas ceased to escape
from the inner vessel. A weighed quantity of the radium
salt, sealed up in a glass tube, was then introduced into the
lo8 THE ELECTRICAL NATURE OF MATTER
liquid hydrogen in the inner tube. The tube and salt being
at ordinary temperatures when introduced into the liquid
hydrogen, would give up heat to the liquid until they were
cooled down to the temperature of the liquid hydrogen
itself. This would, of course, volatilize a part of the liquid,
and gaseous hydrogen would escape. After the small glass
tube containing the radium salt and its contents had been
cooled to the temperature of the liquid hydrogen, gas would
cease to escape from this tube, unless the radium gave off
heat. In fact, gas continued to escape from the tube, as
long as any liquid hydrogen remained in the vessel. This
was due to the heat being given off continuously by the
It is obvious that the amount of hydrogen gas set free in a
given time can be used to measure the rate at which heat
is being liberated by the radium. It is only necessary to
collect the hydrogen and measure it by any of the methods
for measuring a gas, and to determine the heat of vaporiza-
tion of hydrogen, i.e., the amount of heat required to pro-
duce, say, ICO cubic centimetres of hydrogen gas, from the
liquid. Weighing the amount of pure radium salt that was
introduced into the Uquid hydrogen, we have all the data
necessary for calculating the rate at which radium liberates
heat, or the amount of heat produced by a given quantity
of radium in a given time. While this method is far less
accurate than the one to be described subsequently, it is
useful as a confirmatory check; and interesting when we
think that the liquid which is vaporized by the heat spon-
taneously produced by radium is one that was unknown
until the last few years, and one which defied the skill of
so many able experimenters to produce, including the
This method of measuring the amount of heat liberated
PRODUCTION OF HEAT BY RADIUM SALTS I09
by radium has one feature which is of special importance.
The radium is giving off heat, under these conditions, at
the temperature of liquid hydrogen, which is only about
twenty degrees centigrade above the absolute zero. By
comparing the results of this method with those of methods
that can be employed at ordinary temperatures, we can
see what effect temperature has on the rate of heat produc-
tion by radium.
If the production of heat in salts of radium is due to any
chemical action, we should expect that the rate at which
heat is evolved by radium would be greatly lessened at the
very low temperature, since nearly all chemical reactions
take place more slowly the lower the temperature. Indeed,
most chemical reactions fail to take place at all at the tem-
perature of liquid hydrogen.
It has been found that radium liberates just as much heat
at the temperature of liquid hydrogen^ as at ordinary tem-
peratures. This alone makes it highly improbable that the
heat liberated by radium in its salts is due to any chemical
action taking place within the compound. We shall see
later that the amount of heat liberated by salts of radium
is of an order of magnitude so much greater than that known
in the case of any chemical reaction, that this source of the
heat energy is almost certainly excluded. Further, the fact
that salts of radium continue to produce heat for apparently
an almost indefinite time, excludes the possibility that it is
produced as the result of chemical action.
METHOD OF THE BUNSEN ICE CALORIMETER
The amount of heat liberated by salts of radium is meas-
ured most accurately by means of the Bunsen ice calorim-
eter. The principle of this instrument is so well known
that only a few words of explanation are necessary. The
no THE ELECTRICAL NATURE OF MATTER
essential feature of this method is the use of a block of ice,
which is melted by the heat that.it is desired to measure.
Knowing the amount of ice converted into water and the
heat of fusion of ice, we have all the data necessary for
determining the amount of heat set free in the ice calorim-
In some of the earlier work with the Bunsen ice calorim-
eter, the amount of water produced was obtained by
collecting it and then weighing it. A more accurate method
of determining the amount of ice that has been melted is
based upon the fact that the ice and the resulting water
occupy different volumes. When water freezes the volume
increases, and, conversely, when ice melts the volume occu-
pied by the resulting water is less than that occupied by the
ice. This principle is utilized to determine the amount of
the ice melfed.
RESULTS OF HEAT BfEASUREMENTS
The results are certainly surprising on account of their
enormous magnitude. A gram of radium gives out every
hour about eighty calories of heat. Since the heat of fusion
of ice is eighty calories, or eighty calories of heat are re-
quired to melt one gram of ice, it follows that radium gives
out enough heat to melt its own weight of ice every hour.
The most remarkable feature of all, is the fact that radium
continues to give out heat at this rate for apparently an
indefinite time. We shall see lat^ that this would go on as
long as the radium itself continues to exist.
This is a most surprising result. Indeed, it is one of the
most startling facts that has ever been discovered in any
branch of physical science. Think of the enormous amount
of energy that this substance is capable of liberating!
PRODUCTION OF HEAT BY RADIUM SALTS III
SOURCE OF THE HEAT
The question naturally arose whence came this energy?
Some rushed to the conclusion that it must be created by
the radium, and that the law of the conservation of energy
was overthrown. Those who were less radical concluded
that radium must have the power to transform some un-
known kind of energy into heat, which was essentially the
same as to admit that they did not know, and had no tangi-
ble conception of the origin of this energy.
The more conservative began to look around for a rational
explanation of this astonishing and most important fact, in
the light of what was known, or what could be discovered.
We shall see a little later that their eflForts were rewarded,
and that we have a rational explanation as to the origin of
the enormous amount of energy given out by radium.
We have seen, then, that very large quantities of energy
are liberated by the element radium, and that this con-
tinues unabated for practically an unlimited time.
The heat is given off slowly, compared with the heat that
.is given out in certain combustions. This is the reason
that the radium salt does not heat itself to a higher tem-
perature above the surrounding medium. Another ex-
planation of why larger differences in temperature do not
exist, is that such small quantities of radium salts have thus
far been obtained, that the heat is lost by conduction through
the relatively large surface exposed to surrounding objects.
If large amounts of radium could be obtained, it is quite
certain from the rate at which heat would be produced,
that the interior of a pile of radium chloride or bromide
would become quite hot; and by suitably surrounding the
salt with a medium that was a poor conductor of heat, it
is quite possible that the interior of a pile of radiiun salt
112 THE ELECTRICAL NATURE OF MATTER
might become red-hot and actually give off light, due to
the heat spontaneously produced by itself.
EFFECT ON SOLAR HEAT
The fact that radium gives out heat energy has been
utilized to explain certain natural phenomena, for which a
satisfactory explanation has long been wanting. Take the
heat of the sun, how is it produced ? A number of theories
have been advanced. The possibility of the heat of the
sun being the result of combustion or any chemical action
has long since been abandoned. A similar fate has be-
fallen the theory that solar heat is produced by meteoric
bodies raining down from space on to the sun. Both of
these views have been found to be insuflScient in the light
of well-known facts.
The theory that is held to-day is that the origin of solar
heat is to be found in the contraction that is going on in the
sun itself. This contraction would, of course, produce a
constant shrinking, and a dropping in of the exterior, which
would give rise to heat; and in the case of a body of the
dimensions of the sun, would give rise to enormous amounts
This theory is to be sharply distinguished from the older
one, that the sun is simply a cooling body, giving out solar
heat as it cools. According to the present theory enormous
amounts of heat are being continually produced in the sun,
while according to the cooling theory the sun is simply
giving out heat like any other hot body.
This theory of the origin of solar heat has been found to
account for the facts. A contraction which would be too
small to be observed during the time that careful solar
measurements have been made, would account for all the
heat given out by the sun during this period.
PRODUCTION OF HEAT BY RADIUM SALTS 113
While this theory is capable of accounting for solar heat,
there has, however, been a reservation in the minds of
men of science, which has made them hesitate to accept
the theory as the final explanation of the origin of all solar
The discovery of the large amount of heat liberated by
radium has been utilized by Rutherford^ to account for at
least a part of the solar heat. If the sun Consists of a very
small fraction of one per cent, of radium, this would account
for the heat that is given out by it.
The fundamental question in connection with this theory
as to the origin of all or part of the solar heat is this: Does
the sun contain radium? Is there any evidence, direct or
indirect, that radium exists in the sun?
It must be said that no direct evidence has as yet been
produced to show the presence of radium in the sun. The
supposed discovery of the spectrum lines of radium in the
sun leaves much to be desired. The supposed coincidences
of the solar lines with the known lines of radium are only
rough approximations. Indeed, so rough that they are far
from being convincing.
DOES RADIUM EXIST IN THE SUN?
Indirect evidence of the presence of radium in the sun,
however, exists. It has been shown by spectrum analysis
that helium exists in the sun. Indeed, this element was
first discovered in the sun, as its name implies. It was only
recently discovered by Ramsay as occurring at all on the
earth. We shall see that helium and radium are most
closely associated. Wherever we find the one, we may
reasonably expect the other. Helium, having been shown
to exist in considerable quantities in the sun, the conclusion
1 Phil. Mag., 5, 591 (1903).
114 THE ELECTRICAL NATURE OF MATTER
is highly probable that the sun also contains radium. The
force of this argument will appear, and be the better appre-
ciated, when the exact relation of helium and radium is
taken up in a later chapter. The h)rpothesis of the radium
origin of even a part of the solar heat is only an hypothesis,
which it will remain for the future either to raise to the
rank of a theory, or to disprove.
TERRESTRIAL HEAT PRODUCED BY RADIUM — BEARING ON
THE CALCULATED AGE OF THE EARTH
We have seen that radium exists widely scattered over
the surface of the earth. While only small quantities have
been found in any one place, and while, in the opinion of
the writer, for reasons already expressed, this is likely to
continue to be the case, yet the total amount of radium in
the earth may be very considerable. Indeed, there are
reasons for supposing that beneath the surface of the earth
there may be more radium than on the surface. The
waters from certain springs, which probably come from
considerable depths, contain radium. All of this radium
is continually giving out heat.
Rutherford points out that the heat liberated by radium
in the earth may have an appreciable effect on its age as
usually calculated. In such calculations, starting with the
earth as a molten mass, the main factors that are taken
into account in addition to the original temperature are;
the specific heat of the .earth to determine how much heat
it contains, and the conductivity of the crust of the earth
for heat, to determine the rate at which the earth is losing
heat. Given these data, the problem is to determine how
long it would require the earth to cool from the condition
of a molten mass to its present state.
In this calculation it is not assumed that there is any
PRODUCTION OF HEAT BY RADIUM SALTS 115
large source of heat production going on within the earth
itself. The hydration of the rocks, or the combination of
the rocks with water as they cool, would liberate some heat,
and this is taken into account. If, however, it should be
shown that there is an appreciable quantity of radium in
the earth, this would give off heat continuously, and in
geological time the amount of heat from this source might
be very considerable, relative to the total heat in the earth
itself. This factor might vitiate the calculation of the age
of the earth on the basis of the data that have been used,
and produce a very considerable error in the result. The
magnitude of the error would, of course, depend entirely
upon the amount of radium in the earth.
THEORIES AS TO THE SOURCE OF THE HEAT PRODUCED BY
Several theories have been advanced to account for the
production of the heat that is continuously being liberated
by radium. One is strictly analogous to the contraction
theory of solar heat. The radium atom is contracting or
shrinking up, and heat is therefore produced. This theory,
which never met with much favor, is now untenable, for
reasons that will appear as the subject develops.
The theory as to the origin of heat in the salts of radium,
which accounts satisfactorily for the facts, and which is
now generally accepted, is the following. We have seen
that the a particles shot out by radium are incapable of
penetrating any appreciable thickness of matter. They are
all absorbed by thin screens. We have also seen that these
particles have a mass at least twice that of the hydrogen
atom, and possibly greater, and are shot out at very high
velocities. These particles would, therefore, have large
amounts of kinetic energy, and when they are stopped this
Il6 THE ELECTRICAL NATURE OF MATTER
would be transformed into heat and would yield a large
amount of it.
Take a pile of radium salt, the a particles shot off from
the surface, not coming in contact with any of the salt above
it, would escape at least a few centimetres into the air. But
the a particles shot off from all of the radium at any appre-
ciable distance beneath the surface of the salt would not
escape, but would strike the solid salt above it and be
stopped. The energy of motion of the a particle would
thus become converted into heat. Since the mass of the a
particle is considerable, and the velocity about one-tenth
that of light, the kinetic energy would be great, and the
amount of heat produced considerable.
This theory, which was proposed by Lodge,^ to account
for the heat liberated by radium, as produced by the stop-
ping of the a particles in their flight, leaves still one question
unanswered. How do the a particles acquire this great
velocity with which they are shot off from the radium ?
We can scarcely conceive of particles at rest in a molecule
being shot off with such velocities. The particles in the
molecule or atom of radium — the electrons — must be
moving with very high velocities, and when a particle in its
motion, gets beyond the control of the attractions of the
remaining particles of the system, it flies off. This is true
of the positively charged a particles, and also of the nega-
tively charged fi particles. The kinetic energy of these
particles is then something inherent in the atom of ra-
dium. This we call intrinsic energy. It is obvious that
this is the real source of the heat liberated by radium.
The astonishing feature is the amount of the intrinsic
energy contained in the atoms of radium.
> Nat., 67, 511(1903).
PRODUCTION OF HEAT BY RADIUM SALTS II7
CALCULATION OF THE AMOUNT OF HEAT LIBERATED BY RA-
DIUM, ON THE ABOVE THEORY THAT THE HEAT IS PRO-
DUCED BY THE a PARTICLES
Rutherford ^ also points out that from the number of a
particles expelled from radium we can calctdate the heating
efect, since this is due to the bombardment of the a par-
ticles. He calculated the kinetic energy of the a particle
to be 5.9X10-® ergs. Radium at its minimum activity
gives oflF, as we have seen, 6.2 X 10^® a particles from a gram
per second. In radioactive equilibrium it gives off 4X6.2 X
10^® = 2.5X10^^ a particles per gram-second. This would
correspond for a gram of radium to 126 gram-calories per
hour. The value found was 80, which agrees well with the
THREE REMARKABLE PROPERTIES OF RADIUM
We have thus far met with at least three properties
possessed by radium, which are in the^ highest degree
(i) We have seen that radium has the power to charge
(2) It also has the power to illuminate itself, or is,* as we
(3) We have just seen that radium produces heat energy
spontaneously, or can warm itself.
These three properties alone would suffice to place radimn
in a class by itself.
* Pha. Mag., 10, 206 (1905).
Emanation from Radioactive Substances
We have already seen that many radioactive substances
give oflF a particles, which are positively charged, material
bodies. Many radioactive substances, poloniiun being an
exception, give oflF )8 particles, which are negative charges
of electricity or electrons, having the same mass as the
negative charges in the cathode ray, i,e., about jrtJ of the
mass of the hydrogen ion in solution. All radioactive sub-
stances which give ofiF )8 particles also give ofiF y rays.
This includes many radioactive substances, a marked excep-
tion being poloniimi. The y rays are probably identical
with the X-rays, except that they are far more penetrating.
We have also seen that radium gives out continuously
large quantities of heat. Since this production of heat
energy is due mainly to the a particles, it seems fair to
assiune that all radioactive substances that give oflF a par-
ticles, and this, as was just stated, includes most of them,
also give oflF heat energy. In the case of the weakly radio-
active elements, such as uraniimi and thorium, the niunber
of a particles given oflF is relatively small, and, therefore, the
amount of heat energy given oflF by them is relatively slight.
It may, indeed, be so slight as to escape detection.
In addition to these three kinds of radiations, and the
heat, certain radioactive elements, such as thorium, radium,
and actiniiun, give oflF what Rutherford calls an emanation.
This substance, as we shall see, resembles in many respects
EMANATION FROM RADIOACTIVE SUBSTANCES 119
a gas. It can diffuse through porous bodies, can be con-
densed at low temperature, etc. It has in general the
properties of the radioactive substances from which it was
DISCOVERY OF THE THORIUM EMANATION BY RUTHERFORD
The amount of the emanation given oflF even by radium
is small, and for some time escaped detection. We owe its
discovery in fact to the study of the radioactivity of thorium.
It had been observed by Mme. Curie and others that the
radioactivity of thorium was not constant when the thorium
compound was placed in a vessel exposed to air currents.
If the compound of thorium, on the other hand, was placed
in a closed vessel, constant results could be obtained. It
was found that the lack of constant results in open vessels
was due to air currents. If a current of air was drawn
through the closed vessel containing the thorium, incon-
stant results were again obtained. Rutherford ^ took up
the study of the cause of this irregularity, and the result
was the discovery of the emanation.
METHOD OF OBTAINING THE EMANATION
The emanation can be obtained from the salts of radium
by simply heating them, or by dissolving them in water,
when it is given off, the admixed carbon dioxide being
absorbed by potassium hydroxide. It can be collected in a
vessel like any other gas, and its properties studied. Before
taking up its general physical and chemical properties, one
property especially will be discussed in some detail, since
it practically demonstrates the gaseous nature of this
substance. The emanation can be condensed at low tem-
peratures, like an ordinary gas, into a liquid.^
^ Phil. Mag., 49, I (1900).
•Rutherford and Soddy: Phil. Mag., 5, 561 (1903).
I20 THE ELECTRICAL NATUllE OF MATTER
If hydrogen is allowed to bubble through a solution of a
radium salt, and is then passed through a U-tube surrounded
by liquid air, the emanation condenses in the tube. Similar
results are obtained if the products expelled by heating a
radium salt are passed through a U-tube dipped in liquid
If only a small amount of the radium salt is available,
the condensation of the emanation is shown by the fact that
the escaping hydrogen is either not radioactive at all, or
only slightly so; while the emanation is extremely radio-
active. If a larger amount of the emanation is obtainable,
its presence in the cold glass tube can be seen; not by pro-
ducing under ordinary conditions a visible amount of Uquid,
but by a fluorescence in the air in the cold tube, and also by
rendering the walls of the tube brilliantly phosphorescent.
By a modification of the above-described experiment, it is
possible to determine the temperature at which the emana-
tion condenses or boils. A mixture of the emanation and a
neutral gas is passed through a tube cooled down below
the temperature at which the emanation condenses. When
the emanation was all condensed, the escaping gas, hydro-
gen, oxygen, nitrogen, or air showed no radioactivity when
tested by the electrical method. After all the emanation
had been condensed, a current of neutral gas, say hydrogen,
was passed through the tube containing the emanation.
The temperature in the condensing tube gradually rose, due
to the presence of the warmer gas, and when the boiling-
point of the emanation was reached and a little of it was
volatilized, its radioactivity manifested itself in deflecting the
electrometer with which the vessel into which thie emanation
passed was connected. When this took place, the tempera-
ture in the condensing vessel was read by means of a copper
resistance thermometer that had been previously calibrated.
EMANATION FROM RADIOACTIVE SUBSTANCES 121
The average result from a number of experiments showed
that the emanation condenses at —152 degrees centigrade.
This point was fairly sharply determined by the fact that
the ionization or conductivity of the gas, into which the
escaping emanation passed, reached a maximum shortly
after the emanation began to volatilize, and when the
temperature had been raised only a very slight amount.
The emanation thus condenses to a liquid just like a gas,
and like a gas has a perfectly definite boiling-point.
AMOUNT OF THE EMANATION
The amount of the emanation obtainable even from an
appreciable quantity of radium is very small indeed. If
the emanation that can be obtained from a tenth of a gram
of radium chloride or bromide is condensed in a glass tube
as previously described, no liquid or even mist will be seen
in any part of the tube. All that will be seen is a phos-
phorescence on the walls of the tube, and this may extend
through the neutral gas within the tube.
Sir William Ramsay and Soddy have measured approxi-
mately the volume of the emanation obtainable from a
given quantity of the radium salt. The emanation was
collected in a capillary tube which had been graduated, and
From one gram of radium they obtained one cubic milli-
metre of the gas. Rutherford found the value 0.59 cubic
millimetre per gram of radium. This volume decreased
rapidly with timej and we shall learn that this is a very
Rutherford also points out that knowing the number of
a particles shot ofiF from radium, we can calculate the
volume of the emanation produced by it. Every atom of
radium in breaking up gives off at least one a particle and
produces one atom of the emanation which is a gas. A
122 THE ELECTRICAL NATURE OF MATTER
cubic centimetre of a gas is known to contain about 3.6 X
lo^' molecules. From these data the volume of the emana-
tion that can be obtained from a gram of radium is cal-
culated to be 0.83 cubic millimetre. The volume of the
emanation from a gram of radium, as found experimentally
by Ramsay and Soddy as already stated, was one cubic
millimetre. The two results, when we consider the condi-
tions, are strikingly concordant.
NATURE OF THE EMANATION
In studying the properties of the emanation we encounter
the great difficulty, which at present is insurmountable,
that it cannot be obtained in appreciable quantity. This
is especially true of the emanation from thorium, as might
be expected from the small radioactivity of this element.
We have just seen that the emanation from radium dis-
appears, or " decays," as it is said, quite rapidly. This is
especially true of the emanation from thorium, which is not
only infinitesimal in quantity, but disappears or decays in
a few minutes. The emanation from radium, however,
does not entirely decay for a number of days.
The emanation itself is imaffected by an electrostatic
field, and is, therefore, not charged. It can, however,
produce phosphorescence in certain substances.
After having shown that the emanation has many of the
properties of gases, and is certainly material in nature,
attempts were made by Rutherford to identify it with some
of the known substances. Its chemistry was studied as far
as possible with the small quantity available. It was sub-
jected to very high temperatures, but was unaffected by this
treatment. Then it was passed through a platinimi tube
heated as highly as the nature of the tube would permit. It
was also passed over heated platinum black, and escaped
EMANATION FROM RADIOACTIVE SUBSTANCES 1 23
in both cases without change. In the above experiments
the emanation was mixed with air. It was then mixed
with hydrogen and passed over red-hot, magnesium pow-
der, and also over red-hot palladium, but it was still un-
Ramsay sparked a mixture of the emanation with oxygen,
for a long time, in the presence of an alkali, and also heated
it in the presence of magnesia lime, but the emanation was
imchanged. The emanation thus differs from all known
forms of matter, except argon and the other members of
this group of elements, which are characterized by their
While we do not know, even at present, very much about
the chemistry of the emanation, it seems safe to conclude
that if it is an element it belongs to that inactive group of
chemical elements of which argon was the first member to
be discovered. Even if it should be shown not to be ele-
mentary, it nevertheless resembles in its chemical proper-
ties the elements of this group.
Some light has been thrown on the physical properties
of the emanation, notwithstanding the fact that it has been
obtained only in such small quantities.
DIFFUSION OF THE EMANATION — APPROXIMATE DETER-
MINATION OF ITS MOLECULAR WEIGHT
It is well known that gases diffuse with very different
velocities. If we allow gases of different densities to dif-
fuse into any gas, say the atmospheric air, we shall find not
only that they will diffuse with very different velocities, but
a regularity will manifest itself. The lighter gases will
diffuse more rapidly than the heavier ones.
If we work quantitatively, we shall find a very simple
124 THE ELECTRICAL NATURE OF MATTER
relation between the densities or the molecular weights of
gases, and the rates at which they will diffuse.
Gases diffuse with velocities that are inversely proportional
to the square roots of their densities.
This generalization, known from its discoverer as the
law of Graham, is comprehensive, holding for all well-
Upon the basis of this generalization, Rutherford and
Miss Brooks ^ have attempted to determine approximately
the molecular weight of the emanation from radioactive
substances, notwithstanding the fact that the largest
amount of the emanation thus far obtained is scarcely
weighable even with the most refined chemical balance.
They allowed the emanation to diffuse from one end of
a tube into the other, and measured the change in the
conductivity of the air in the tube.
From the data thus obtainable the diffusion coefficient
of radium could easily be calculated.
The experiments which, on the whole, were the most
satisfactory and probably the most accurate, gave a diffu-
sion coefficient which was close to 0.07.
If we compare this coefficient with the diffusion coeffi-
cients of vapors whose molecular weights are known, we
find that it comes close to the coefficient for ether, which
has the value of 0.077, ^^d the molecular weight of ether is
74. The molecular weight of the emanation from radium
must, therefore, be close to 74. All things considered,
Rutherford seems to think that the molecular weight of the
radium emanation is not far from 100. The emanation
from thorium was shown to have practically the same molec-
ular weight as the emanation from radium.
Since the above determinations of the molecular weight
^ Chem. News, 85, 196 (1902).
EMANATION FROM RADIOACTIVE SUBSTANCES 12$
of the radium emanation were made by Rutherford and
Miss Brooks, new determinations have been carried out
by Makower/ working with J. J. Thomson.
Radium bromide was dissolved in water and the emana-
tion removed by passing air through the solution. The
mixture of air and the emanation was collected over mer-
cury in one arm of a glass vessel resembling a Hempel
burette, which was closed at the top by a porous plug. This
vessel, known as the diffusion vessel, was connected with a
cylindrical brass vessel. Into the centre of this brass cylin-
der a brass rod was introduced, so as to be insulated from
the walls of the vessel. By means of a storage battery of
two hundred cells, a difference in potential of about four
hundred volts was established between the brass rod and
the walls of the box. A known volume of the mixture of
air and the emanation was introduced from the diffusion
vessel into the brass cylinder, and the conductivity of the
gases in the cylinder determined. As soon as the conduc-
tivity had been determined, the emanation was quickly
pumped out of the cylinder, so as to minimize the amount
of the " induced radioactivity '' on the walls of the vessel,
which quickly decayed.
The mixture of air and the emanation now gradually
diffused out of the diffusion vessel, through the porous plug.
From time to time fresh quantities of the mixture were driven
over from the diffusion vessel into the brass cylinder, and its
conductivity determined. As more and more of the emana-
tion diffused out through the porous plug in the top of one
arm of the diffusion vessel, the conductivity of the mixture
remaining in the vessel became less and less, as was shown
by testing the conductivity at short intervals, by the method
already described. In this way it was not difficult to deter-
1 Phil. Mag., 9, $6 (1905).
126 THE ELECTRICAL NATURE OF MATTER
mine the rate at which the emanation diffused out through
the porous plug.
To determine the molecular weight of the emanation, it
was necessary to compare its rate of diffusion with that
of gases whose molecular weights were known, diffusing
through the same porous plug. The gases employed were
oxygen, hydrogen, carbon dioxide, and sulphur dioxide.
The gas was introduced into the diffusion vessel and
allowed to diffuse out into the atmosphere. Knowing the
molecular weights of the gases, the rates at which they
diffuse through the given porous plug, and the rate at
which the emanation diffuses through the same plug, we
can calculate the molecular weight of the emanation from
The results showed a molecular weight for the radium
emanation ranging from 85.5 to 99. On the assumption
that the radium emanation is a monatomic gas, Makower
points out that this result would give it a place in the
Periodic System in the fluorine group between molybdenimi
The molecular weight of the emanation from thorium
was found to be slightly smaller than that from radium.
These results show that the molecular weight of the
emanation is very nearly one hundred, as Rutherford had
Ramsay and Gray ^ determined the molecular weight of
the radium emanation which they called niton as 220.
Later ^ they found 223.
* Compt. rend.; 151, 126 (1910).
« Pro. Roy. Soc. 84, 536 (1911).
Helium Produced from the Emanation
We have seen that the emanation is material, and has
many of the properties of an ordinary gas. We have also
seen that when the emanation is present in the radium, the
latter gives out a, j8 and y radiations. The question arises
whether the emanation gives out all three types of rays, or
only certain special types, or does it give out any radiation
This was tested by Rutherford and Soddy * in the follow-
ing way. The thorium, containing the emanation, was
placed in a metal box, having a hole in the top that was
covered with a plate of mica. The radiation from the
emanation that passed through the mica was tested by its
power to ionize the gas above it. When a thin metal disk
was interposed in the path of the radiation, most of the
radiation was cut off. This showed that at least most of
the radiation consisted of a rays. No evidence was ob-
tained that any j8 rays were present.
In the case of the emanation from radium, the test as to
its nature was made as follows: The emanation was intro-
duced into a copper tube, whose walls were thick enough
to cut off all the a rays. No j8 or y rays were given out by
the emanation itself.
The emanation gives out, then, only one type of radia-
tion, and that is the a type. No j8 or y rays come from the
emanation either from thorium or radium. It will be re-
» Phil. Mag., 5, 445 (1903).
128 THE ELECTRICAL NATURE OF MATTER
membered that the a rays are composed of positively charged
particles, having a mass about four times that of the hydro-
gen atom, and moving with a velocity which is about one-
tenth that of light. It will also be recalled that it is the a
particles that have most of the energy given off by radioac-
tive substances, since they have appreciable mass and very
high velocity. The a rays are the chief agents that ionize
a gas subjected to radioactive substances, and are the most
important radiations given off by such substances.
Having found that the emanation gives off a particles,
the next question is, do all the a particles shot off from
radiiun come from the emanation contained in it, or has
deemanated radium any power to produce a rays? This
can easily be answered. When all of the emanation is
removed from the radium salt by heating, the remaining
deemanated radium also has some power to give out a
Rutherford studied the effect of law temperature on the
rate at which the emanation was produced. He found that
the emanating power of thoria was diminished to about
one-tenth at the temperature of solid carbon dioxide.
M. Curie found that the emanating power of radium
compounds was much increased by dissolving them in
water. The meaning of some of these empirical facts will
appear when we come to study the nature of the changes
that are taking place in radioactive substances.
RECOVERY OF EMANATING POWER
When thorium or radium compounds are subjected to a
high temperature they become deemanated, or lose most
of their emanating power. They, however, regain this
power on standing, more and more of the emanation being
HELIUM PRODUCED FROM THE EMANATION 1 29
DECAY OF THE EMANATION
If we study the emanation, we find thdt the activity of
the emanation rapidly diminishes. The activity of the
emanation obtained from thorium decreases to one-half its
initial value in about one minute, and almost entirely van-
ishes in a very few minutes.
The activity of the radium emanation is, however, more
persistent. The most careful work on this problem is un-
doubtedly that of Rutherlford and Soddy. A mixture of
the emanation with air was preserved over mercury, and
samples removed and examined from time to time. They
found that the activity of the emanation from radium fell
to half the initial value in 3.85 days.
The rate of decay of the emanation seems to be independent
of the conditions to which the emanation is subjected. Even
high temperatures have no effect on the rate, and when the
emanation is condensed to a liquid at low temperatures,
the decay goes on at the same rate.
HEAT EVOLVED BY THE EMANATION
We have discussed at some length in an earlier chapter
the remarkable heat-producing power of radium. We
have seen that the amount of heat liberated by radium is
one of the most surprising facts in physical science.
We have now studied in some detail the unique substance
which is being constantly produced and given off by radio-
active substances, known as the emanation. It is extremely
radioactive considering its quantity. Indeed, much of the
radioactivity of radium and thorium can be referred to the
emanation produced by and contained in them.
We would naturally ask, does the emanation have any-
thing to do with the enormous production of heat that is
taking place in radium salts, and if so, what?
I30 THE ELECTRICAL NATURE OF MATTER
The answer to this question we owe to Rutherford and
Barnes.^ They worked with only thirty milligrams of the
bromide of radiimi and determined the total heat emission
of this substance.
They then distilled off the emanation and condensed it in
a tube surrounded by liquid air. This tube was sealed up
while immersed in the refrigerating agent. The heat that
was liberated by the emanation in the tube was then meas-
ured from time to time, and also the heat that was liberated
by the radium bromide from which the emanation had been
distilled. The sum of the heat liberated by the emanation,
plus that liberated by the bromide from which the emana-
tion had been obtained, was always equal to the total
amount of heat set tree from the original bromide.
When the emanation was giving out a maximum amount
of heat, the surprising fact was established thaX from seventy
to seventy-five per cent, of the total heat given otU by radium
salts comes from the emanation contained in them, and Ruther-
ford has recently shown that about thirty per cent, of the
total heating effect of radium comes from radium C, one of
the decomposition products of the emanation.
This fact is even more wonderful than the discovery
that small amounts of radium salts can give off such large
amounts of heat. We have now traced the source of most
of this heat to the almost infinitesimal quantity of emana-
tion contained in such small amounts of the salts of radium
that are at present at our disposal.
HELIUM PRODUCED FROM THE EMANATION
We have already encountered a number of remarkable
and surprising facts in connection with the radioactive
elements and the emanation produced by them. Perhaps
the most remarkable still remains to be considered. We
have seen that the activity of the emanation gradually
* Phil. Mag., 7, 202 (1904).
HELIUM PRODUCED FROM THE EMANATION 131
decays and finally becomes zero. This necessitates the
conclusion that some fundamental change is going on in
the emanation itself.
A number of questions arise in this connection. Espe-
cially prominent is this one: If the emanation is under-
going decomposition, into what does it decompose? What
is left in a tube containing the emanation after the emana-
tion has ceased to be radioactive?
If we go back to pitchblende — the source of most of our
radium — we find such a large number of things, that it
would appear to be difficult to say that any one of them
was a product of the decomposition of the emanation from
the radium contained in this mineral. We, however, find
most of these substances occurring in other associations
somewhere in nature where no radium is present, and they,
therefore, could not be the final product of the decomposi-
tion of the radium emanation.
If we examine the radioactive minerals closely we shall
see, however, that they contain one substance, of which
the above remark is, at best, only partially true. This is
the element helium.
This element, as has already been pointed out, was first
discovered spectroscopically in the sun by Lockyer. It
was first discovered among the terrestrial elements by
Ramsay. This discovery has an interesting history. Ram-
say was working with Lord Rayleigh on argon, and had
studied its properties, and especially its chemical inertness.
In this connection it occurred to him to examine the inert
gas previously obtained from the mineral cleveite, to see
whether it was not argon. He examined it spectroscop-
ically and found a prominent yellow line near the sodium
line, which he could not identify as coincident with that
of any known terrestrial element. However, on comparing
132 THE ELECTRICAL NATURE OF MATTER
it With the line discovered by Lockyer in the sun, Ramsay
found that the two were identical.
Helium was thus shown to exist among the terrestrial
It should further be pointed out that helium, as far as it
occurs at all in minerals, is only to be found in the radio-
active minerals. HeUum is also found in the waters of
certain springs, but probably comes from radioactive min-
erals which are at some depth below the surface of the
Taking these facts into account, and also the chemical
properties of the emanation from thorium and radium,
Rutherford and Soddy* suggested that the emanation on
decomposing might yield some inert element of the type
of those in the argon family.
On account of his abiUty and experience in working with
small quantities of gases, Sir William Ramsay' undertook
the study of the nature of the emanation, with the assist-
ance of Mr. Soddy.
They dissolved from 20 to 30 milligrams of radium bro-
mide in water, and collected the emanation in a sparking
tube. The sparking tube was connected with a U-tube
which was surrounded by liquid air. This condensed any
carbon dioxide that was present in the emanation as an
impurity, and also the emanation. If any helium was pro-
duced from the emanation, this would not be condensed
by the liquid air, since helium liquifies at a lower tempera-
ture than air.
When the spectrum of this tube was taken, a bright yellow
line made its appearance, which was not far removed from
the sodium line; but even with a small spectroscope could
1 Phfl. Mag., 4, 581 (1902).
« Nat., 68, 246 and 354 (1903).
HELIUM PRODUCED FROM THE EMANATION 133
be seen not to be identical with it. A careful measurement
of this line showed it to be identical with the D^ line of helium.
This preliminary experiment with its remarkable result,
led to further very careful work on the problem. The
emanation from 50 milligrams of radium bromide was
collected in a U-tube by driving it over with oxygen, and
then condensed in the tube by means of liquid air. It was
then transferred to a Plucker sparking tube, and the spec-
trum taken. At first there were no helium lines present^
but a new spectrum, presumably that of the emanation
itself, made its appearance. In a few days the original
spectrum disappeared and the spectrum of helium came out
Thus was observed for the first time in the history of science
the formation or production of a chemical element. Whether
it comes directly from another definite chemical element
is not certain. It has not been shown, although it is highly
probable, that the emanation is an inert chemical element.
It is, however, certain that helium is thus spontaneously
produced from a chemical element — radium — as one of
its decomposition products.
THIS IS NOT A TRANSMUTATION OF THE ELEMENTS
Since the discovery referred to above was made, there
has been so much written about the "Transmutation of the
elements having been effected,'' the "Dream of the alchemist
realized," etc., that a word of warning seems highly
From some of the statements on this subject that have
appeared, any one unfamiliar with the facts might con-
clude that we are now able to effect the reciprocal trans-
formation of practically any elementary substances almost
ad libitum. We are no more able to effect such transformer
134 THE ELECTRICAL NATURE OF MATTER
Hons to-day than was .possible a thousand years ago, nor
has such a transformation ever been effected by any one.
It appears to the writer to be one thing to discover an
unstable system in nature, even if it corresponds to our
definition of chemical element, which is spontaneously
undergoing changes that are largely unaffected even by the
most extreme artificial conditions that we can bring to bear
upon it, and giving rise to another elementary substance
as one of its decomposition products; and an entirely differ-
ent thing to effect the transformation of a stable element into
another elementary substance by purely artificial means.
By showing that helium is one of the decomposition prod-
ucts of radium, it has been shown that the process first de-
scribed does actually take place, at least in the case of one
substance. The second transformation still remains to
In calling attention to the above distinction, no attempt
is made to beUttle the magnificence of the discovery of the
spontaneous formation of helium from radium, which,
when we consider the difficulties involved in working with
such small quantities of substances, is to be placed among
the great achievements of modem science, and could not
have been accomplished by a man of less experimental
skill than that possessed by Sir William Ramsay.
FURTHER EXPERIMENTS ON THE PRODUCTION OF HELIUM
It is obvious that such an epoch-making discovery as
that described above would be subjected to the closest
scrutiny, even when announced by such a distinguished
authority as Ramsay. The first question that would occur
to any one is this, Could the helium that appeared with the
emanation have been occluded in the radium salt, and set
HELIUM PRODUCED FROM THE EMANATION 135
free when the emanation was separated from the salt?
This is, of course, a fair question to ask, but the answer
was furnished by Ramsay himself. The salt of radium
was heated in contact with a vacuum pump for a long time,
so that any gas occluded in the radium salt must have been
liberated. When the salt of radium thus treated was allowed
to stand until the emanation was formed, and this emana-
tion then driven off and collected in a sparking tube, the
presence of helium lines manifested themselves after a few
One fact, as has doubtless already been noted, in con-
nection with the appearance of helium lines in the emana-
tion, of itself argues strongly against any helium having
been occluded in the radium salt, and then set free when the
salt was dissolved in water. The emanation freshly dis-
tilled from the radium salt showed no trace of the helium
spectrum. The spectrum of helium appeared only after
the emanation had stood for some time. If the helium was
really occluded in the radium salt, its spectrum should
have manifested itself as soon as it was driven over into the
Plucker tube. The fact that it did not, but appeared after
the tube containing the emanation had stood for a time,
is a strong argument in favor of the helium having been
produced by some change taking place in the emanation
An even more crucial test, if possible, of the occlusion theory
to account for the helium was made by Dewar, Curie and
Deslandres.* Four hundred milligrams of radium bromide
were dried and placed in a small glass vessel. This was
connected with a small Geissler tube, and the whole system
exhausted. The degree of the exhaustion was registered
on a manometer. During the three months that the bro-
1 Compt. rend., 138, 190 (1904).
136 THE ELECTRICAL NATURE OF MATTER
mide was contained in the exhausted glass vessel, gas was
continually being given off. This gas was found spectro-
scopically to be hydrogen, produced probably by the decom-
position of traces of moisture in the salt by the radium.
The radium bromide was now transferred to a small
quartz vessel, which was then exhausted. It was heated
until the bromide fused. The gases that were given off
during the heating were passed through U-tubes plunged
in liquid air. This condensed the emanation and any of
the less volatile gases. During the heating some nitrogen
gas was given off, having been occluded in the salt. The
quartz vessel containing the radium bromide, from which
all occluded gases had now obviously been removed, was
sealed up by means of an oxyhydrogen blowpipe.
After the tube had been closed twenty days, Deslandres
studied its contents spectroscopically. The gas in the tube
gave now the entire spectrum of helium.
The result of this investigation was to confirm in every
respect the conclusion previously reached by Ramsay.
Helium is formed as the result of some change going on in
the radium, or in the emanation from the radium.
RELATION BETWEEN THE EMANATION AND HELIUM
Having shown that the helium which appeared in the
sparking tube along with the emanation was not occluded
in the salt of radium from which the emanation was obtained,
the next question that was raised is. What relation does the
helium bear to the emanation from which it is produced?
It was easy to show in a number of ways that the emanation
itself is not helium. The spectrum that first appeared
when the emanation was collected in the sparking tube
was not that of helium at all, but was an entirely new spec-
trum, not corresponding to that of any known substance.
HELIUM PRODUCED FROM THE EMANATION 137
As we have seen, the helium lines appeared only after the
emanation had stood for a time. Again, the emanation is
radioactive, giving off a particles. Heliimi does not give
off such particles, and, indeed, is not radioactive at all.
Further, the emanation is condensed by passing through
a tube surrounded by liquid air, while helium can be con-
densed to a liquid only below the temperature of liquid
hydrogen — helium liquifying lower than any other known
gas — at about —268°.
Helium is the lightest gas known next to hydrogen. Its
atomic weight being four, and the molecule monatomic.
The emanation, on the other hand, has a molecular weight
of about 100, as we have seen from diffusion experiments.
The emanation is, then, fundamentally different from
helium in all of its properties, and yet helium is produced
from it as is shown by spectrum analysis. A theory to
account for the production of helium from the emanation
should be mentioned, even if it is insuflSdent, as it will be
encountered in the literature.
It has been suggested that radium is not a chemical ele-
ment, but a compound of helium with some presumably
unknown element. The helium that was produced from
radium was the result of the breaking down of this com-
pound. There are a number of reasons why this theory is
insufficient. In the first place, radium has all the proper-
ties of a chemical element — including a well-defined spec-
trum. Again, such a theory would not account for the
radioactivity of radium, nor for the amount of heat energy
that is being liberated by it.
To explain the properties of this remarkable substance^
a theory along entirely new lines, as we shall see, is neces-
138 THE ELECTRICAL NATURE OF MATTER
SOME REMARKABLE RESULTS OBTAINED BY THE ACTION OF
THE RADIUM EMANATION
Ramsay^ has found that when a salt of zirconium,
thorium or bismuth, or hydrofluosilicic acid is dissolved in
pure water, treated with the radium emanation and sealed
up in a glass bulb, carbon dioxide is formed in the solution.
When the glass bulb is allowed to stand for a time and
then opened, from one-tenth to one-half of a cubic centi-
metre of carbon dioxide was pumped out of the solution.
The same experiment tried with a large number of other
salts gave a negative result. Blank experiments in which
no salt was used gave negative results and other experi-
ments in which no radium emanation was conducted into
the solution also gave no carbon dioxide.
It therefore seems to be pretty well established that in
these experiments carbon dioxide is formed.
The fundamental question, however, is, formed from what?
Ramsay thinks, from the metal of the salt or from the silicon
of the hydrofluosilicic acid, by these elements being decom-
posed by the radium emanation. He points out that thorium,
zirconium, and silicon all fall in the same group of the
Periodic System with carbon — Group IV; and that bis-
muth falls in the next group — Group V. He thinks that
it would be most natural that these elements in undergoing
decomposition would break down into simpler elements in
the same periodic group.
This conclusion of the transformation of one element into
another may strike us as revolutionary or strange even
to-day. A dozen years ago it would have been so at vari-
ance with all of the facts then known that it would have
met only with opposition.
1 Journ. Chem. Soc. (1907-1909). Chem. Central b., i, 151 1 (1908).
HELIUM PRODUCED FROM THE EMANATION 139
The unquestioned fact, however, that the radium emana-
tion does break down and yield a chemical element helium
pave the way for the possibility of the above explanation
offered by Ramsay. Further, the study of the radioactive
elements has undoubtedly shown that they are unstable,
and break down into simpler things. Indeed, the existence
of radioactive phenomena depends upon this very fact.
Since the radioactive elements are imstable, breaking down
spontaneously into simpler things, there is nothing a priori
impossible or improbable in the assumption that other ele-
ments, which simply represent a different order of stability,
may be broken down by the bombardment of the radia-
tions given off by the radium emanation.
All attempts to explain the production of the carbon
dioxide as due to the action of the emanation on the glass
vessel have been futile, since when the salt is omitted from
the solution no carbon dioxide is found; and the salt in
question would have nothing to do with the action of the
emanation on the glass.
As no other satisfactory explanation of the origin of the
carbon dioxide foimd by Ramsay has yet been furnished,
it seems well to suspend judgment for a time, remembering
that the suggestion of Ramsay is now quite within the
range of possibility.
It was discovered by the Curies* that substances in
general, that are left for some time in the presence of radium
salts, became radioactive. This was the case when the
substances in question were protected from any dust of
the radiiun salt. This phenomenon was named by the
Curies Induced radioactivity.
This property of rendering substances in the neighbor-
hood radioactive is not limited to radium. Rutherford*
found that salts of thorium have the same property, and
Debieme showed that actinium had the power of inducing
radioactivity to a very high degree.
The Curies" studied this property of radioactive sub-
stances in the following manner. They used a closed
vessel into which the radioactive substance, and the sub-
stances on which radioactivity was to be induced, were
placed. Under these conditions, as would be expected,
more marked effects were produced as well as more regular
The active substance was placed in a small glass vessel
open at the top, which was suspended in the centre of an
inclosed space. Pieces of such widely different substances
as glass, hard rubber, paraflSn wax, aluminium, copper
and lead, having, however, equal surfaces, were inclosed
> Ann. Chim. Phys. , 30, 289 (1903).
«Phil. Mag., 49, 161 (1900).
» Ann. Chim. Phys. , 30, 291 (1903).
INDUCED RADIOACTIVITY 141
in the space along with the vessel containing the radium
salt. It was found that aU of these substances became radio-
active, and were radioactive to just exactly the same extent
when the free surfaces were the same.
To test whether the induced radioactivity was caused by
the radiations falling directly upon the plate, a thick lead
screen was placed in the inclosed space, to one side of the
vessel containing the radium salt; and behind this screen
was placed a piece of metal, having the same surface area
as the other objects in the inclosed space. It was found
that this substance, thus protected from the radiations
given out by the active salt, became just as radioactive as
the other substances having the same surface, exposed to
the direct action of the radiations.
The further interesting experiment was tried, of closing
the vessel containing the radioactive substance. When
the vessel was closed it was found that none of the sub-
stances became radioactive. By closing the vessel, then,
the power of the radioactive substance to induce radio-
activity in other bodies was lost. It was found that
the induced activity was more intense and more regular
if the active substance was in solution than if it was in the
Water itself becomes radioactive if allowed to stand in a
closed vessel along with some salt of radium.
Certain substances, such as glass, paper, and especially
zinc sulphide, became phosphorescent under the same con-
ditions as those to which the above-named objects were
subjected. When the induced radioactivity of these phos-
phorescent substances is measured, it is found to be the
same as the induced radioactivity of other non-phosphores-
cent substances, subjected to the same exciting cause.
The production of phosphorescence in such bodies, then,
U2 THE ELECTRICAL NATURE OF MATTER
neither increases nor diminishes the excited radioactivity
produced in them.
The Curies also established the following facts. If a
given object is exposed to a radioactive body in a closed
vessel, the induced radioactivity* in the object increases
with the time of the exposure, until a certain definite, maxi-
mum value is reached. This maximum value is indepen-
dent of the nature of the gas that fills the vessel containing
the radioactive substance, and the material on which radio-
activity is to be induced; and is dependent, for a given
arrangement of the apparatus, only upon the amount of
the radioactive substance present in solution in the con-
INDUCED RADIOACTIVITY PRODUCED BY DECOMPOSITION
PRODUCTS OF THE EMANATION
It has already been shown that the induced activity is
not due to the radiations from the radioactive bodies, since,
when the radiations are cut off from an object by a thick
lead screen, this object still becomes radioactive if contained
in a vessel along with the radioactive substance. It has also
been shown that if the vessel containing the radioactive
material is completely closed, the radioactive substance in
the vessel does not excite radioactivity in objects around it.
This would strongly indicate that the excitant of radio-
activity must be something analogous in properties to a
gas, since it is so readily cut off, and since it can pass around
a screen and induce radioactivity in an object placed be-
hind the screen just as if the screen was not present.
The only substance given off from such radioactive bodies
as thorium, actinium, and radium, which has the properties
of a gas, is the emanation; and we should expect that the in-
duced or excited radioactivity was caused by the emanation.
INDUCED RADIOACTIVITY 143
This supposition was greatly strengthened by the fact
that only those elements that produce an emanation have
the power of exciting radioactivity in non-radioactive sub-
This view that the induced radioactivity was caused by
the emanation we owe to Rutherford, who furnished a
number of lines of evidence for his theory. He showed
that when the emanation from radium was cut off, the
radium lost its power of inducing radioactivity in other
bodies. He also showed that the induced radioactivity was
proportional to the emanating power of the substance induc-
ing it. The'amount of the emanation present was measured
by its power to ionize a gas and thus render it a conductor.
When this was compared at different intervals with the
radioactivity produced, it was found that the two are pro-
portional, to within the limits of experimental error.
INDUCED RADIOACTIVITY UNDERGOES DECAY
We have seen that the radioactivity of the emanation
itself undergoes decay. Since the emanation is the cause
of the induced radioactivity, we should naturally expect
that the induced radioactivity itself would decay with time,
and such is the fact.
If a body is subjected for a considerable time to the
emanation from thorium, and then removed, the excited
radioactivity decays regularly, reaching half of its initial
value, according to Rutherford, in about eleven hours.
The rate of the decay of the inducted radioactivity, like
so many other properties of radioactive substances, is
apparently independent of many of the conditions to which
it is subjected.
The induced radioactivity produced by the emanation
from radium decayed much more rapidly than that pro-
144 THE ELECTRICAL NATURE OF MATTER
duced by the emanation from thorium. It undergoes
changes somewhat analogous to those already considered,
decaying to half-value in a few minutes.
We should naturally like to know whether this decay con-
tinues to the limit. Do the bodies once rendered radioactive
by the emanation from naturally radioactive substances
quickly become completely non- radioactive again? This in-
formation has been furnished, at least in part, by the Curies.
They found that the induced activity produced by radium
diminished to half-value in a few minutes, but a small,
residual activity persisted for almost an indefinite time.
INDUCED RADIOACTIVITY DUE TO THE DEPOSIT OF RADIO-
The relation between induced radioactivity and the
emanation from radioactive substances has been developed,
and it has been shown that the latter is the cause of the
former. This, however, but raises the question, how does
the emanation render objects exposed to it temporarily
radioactive? To answer this question we must study
closely the property of induced radioactivity. If a thorium
or radium salt is placed in a closed vessel, all objects in the
same vessel, whatever their nature, are rendered radio-
active. If, however, a negative electrode is introduced
into the vessel, all the excited radioactivity is confined to
this electrode. A convenient method of performing this
experiment is to introduce the radioactive salt into a metal
vessel which is connected with the positive pole of a battery.
A metal wire is introduced into the middle of the vessel,
passing through an insulating stopper. This wire is made
the negative pole of the battery. Under these conditions,
the wire is the only object in the vessel that is rendered
radioactive, and its induced radioactivity may, according
INDUCED RADIOACTIVITY 145
to Rutherford, become many thousand times greater than
the natural activity of the thorium salt which induced the
activity in the negative electrode.
It is diflScult to account for this fact, together with the
fact that the emanation is the cause of all the induced radio-
activity, on any other ground than that the induced radio-
activity is prodiiced by a deposit 0} radioactive matter upon
objects placed in the neighborhood of naturally radioactive
This theory would explain the above and correlated facts.
To propose a theory that explains all the known facts
and predicts new ones is one thing, but to show that this
is the only suggestion that will account for these facts is
quite a different matter. Further, the value of a theory or
generalization is to be tested rather by its ability to predict
new and undiscovered facts, and then have the predictions
verified by experiments, than simply to account for what
is already known.
If the induced radioactivity is due to the deposition of
radioactive matter upon non-radioactive substances, then
this matter would have definite properties. We ought to
be able to remove it mechanically from the object upon
which it was deposited, etc.
We shall now see that the cause of the induced radio-
activity can be removed mechanically and otherwise from
objects upon which it has been deposited, and that its
properties have already been studied in some detail.
PROPERTIES OF THE RADIOACTIVE MATTER DEPOSITED BY
THE EMANATION FROM RADIOACTIVE SUBSTANCES
The properties of the active matter deposited by the
emanation from thorium have been studied by von Lerch *
» Ann. d. Phys., 12, 745 (1903).
146 THE ELECTRICAL NATURE OF MATTER
and by Rutherford.* The active matter was allowed to
deposit upon platinum, and its solubility in different sol-
vents then determined by measuring the decrease in the
induced radioactivity of the platinum. This active matter
was insoluble in such organic solvents as ether and alcohol.
It was dissolved by hydrochloric acid, and the solution was
radioactive. The radioactivity of this solution was greatly
diminished by causing a precipitate to be thrown down
from it. Thus, if barium chloride was added to the hy-
drochloric acid solution, and the barium thrown down as
sulphate, most of the radioactive matter was carried down
by the precipitate which was strongly radioactive.
If a piece of magnesium was exposed to the emanation
from thorium until it became highly radioactive, and then
dissolved in hydrochloric acid, the magnesium when pre-
cipitated as carbonate or phosphate carried down with it
the radioactive matter.
Rutherford showed that the active matter can be removed
from an object upon which it has been deposited, purely
mechanically. If a piece of platinum wire has been ren-
dered highly radioactive by exposing it for some time to
the emanation from thorium, and is then rubbed with a
piece of sand-paper, nearly all of the radioactive matter
can be removed from the platinum. The sand-paper, in
turn, becomes radioactive.
We know less about the properties of the radioactive
matter deposited by the emanation from radium, since, as
we have seen, this decays much more rapidly than the de-
posits from the emanation given off by salts of thorium.
It has, however, been shown that the radioactive matter
from radium differs at least in its solubility from the radio-
active matter deposited from thorium.
» Phys. Zeit., 3, 254 (1902).
INDUCED RADIOACTIVITY 147
The above facts suflBce to show that induced radioactivity
is caused by the deposit upon the non-radioactive substance
0} a radioactive form of tnatter, which can be removed from
the substance either mechanically or chemically, and which
has definite chemical and physical properties of its own.
This radioactive deposit has been termed by Rutherford
emanation X. Another name was given to it later, as we
shall see. The amount of such radioactive matter that is
deposited is extremely small. This is what we should
expect, since we have learned that the amount of the emana-
tion itself, given off by the most active substances, radium
and actinium, is almost infinitesimal. Rutherford points
out that no matter how long a piece of metal is exposed to
the emanation, the amount of radioactive matter deposited
is too small to be detected, even with the most refined balance.
Here is then another remarkable fact added to that long
list of such facts that have been brought to light as the
result of the discovery and study of radioactive phenomena.
Certain radioactive substances give ofiF almost an infinitesi-
mal quantity of a kind of matter that is analogous to a
gas, and which has properties literally undreamed of by men
of science. This minute quantity of substance manifests
most of the radioactivity shown by the natural radio-
active substance from which it came. It produces a chemi-
cal element helium as one of its decomposition products,
and perhaps most remarkable of all is the amount of energy
that it can give out in the form of heat. It was justly re-
garded as one of the most surprising facts known to science,
when the Curies discovered that salts of radium themselves
gave out heat in such quantity that a piece of radium would
melt its own weight of ice every hour.
148 THE ELECTRICAL NATURE OF MATTER
This discovery dwindles into insignificance in comparison
with that made by Rutherford, that about three-fourths
of this heat comes from something that exists in the radium
salt in relatively infinitesimal quantity, and which is con-
tinually decaying and being manufactured by the radium
— the emanation. It is perhaps no great cause for wonder
that such a discovery should have raised questions even
in connection with such a fundamental generalization as
that of the conservation of energy.
We now find that the emanation in decaying yields a
product, which must exist in still smaller quantity than
the emanation itself, and which has the power of rendering
inactive substances on which it is deposited strongly radio-
active. This induced or excited radioactivity as it is termed
also undergoes decay, showing that the radioactive matter
deposited by the emanation undergoes still further changes.
Some of these have already been studied by Rutherford.
SOME FACTS THAT MUST BE TAKEN INTO ACCOUNT IN
DEALING WITH THE DECAY OF INDUCED OR EXCITED
In stud)dng the transformations that are undergone by
the radioactive matter deposited by the emanation, let us
first turn to the facts that have been brought to light by
Rutherford, and clearly stated by him in his Bakerian lec-
ture* before the Royal Society.*
To simplify the matter, we shall deal with the transfor-
mations in detail only in the case of radium. The induced
radioactivity produced by radium undergoes decay, and
at the same time gives off a, )8, and y particles. If we
measure the decay of the excited radioactivity by means of
» Phil. Trans., A, 204, 169 (1904).
2 See also PhU. Mag., 8, 636 (1904).
INDUCED RADIOACTIVITY 1 49
the a rays, we obtain a different result from that arrived
at if we measure the decay by means of the )8 or y rays.
The decay was measured by means of the a rays, also by
means of the )8 rays, and finally by means of the y rays.
Sotne oj the results thai were obtained in the case of radium
are the following: After the rod was exposed to the radium
emanation, the activity as measured by the a radiation
decreased at first comparatively rapidly. The decay then pro-
gressed slowly, finally becoming almost zero.
If the rate of decay of the induced activity is measured by
the )8 rays^ quite different results are obtained. The decay as
measured by the )8 radiation after the first ten or fifteen min-
utes, resembles the decay as measured by the a radiation.
The decay as measured by the a radiation diminishes very
rapidly for the first fifteen minutes. This is not the case
when the rate of decay is measured by the )8 radiation.
When the rate of decay is measured by the y radiation,
results are found which are exactly similar to those obtained
with the )8 radiations. This is just what we should expect
from the relation that we have already learned exists between
the )8 and y rays.
INTERPRETATION OF THESE FACTS
The following interpretation of the above facts has been
given chiefly by Rutherford ^: The rapid initial decrease in
the radioactivity, as measured by the a rays, is due to a
change taking place that gives rise to the a rays. If we
examine the activity as measured by the )8 ray during this
period, we find the absence of any sudden drop during
this initial time. This shows that the first transforma-
tion, which takes place largely during the first three
minutes, does not give out P rays, otherwise the activity
^ Trans. Roy. Soc., 204, A, 169 (1904).
150 THE ELECTRICAL NATURE OF MATTER
as measured by the j8 rays would decrease rapidly during
We will term the active matter deposited by the emana-
tion, not emanation X, as we have hitherto done, but
radium A. Radium A gives out a particles only, and
quickly undergoes a transformation into radium B.
A study of the rate of decay, as measured by a, )8, and y
rays respectively, leads to the conclusion that a second
transformation goes on, in which fi and 7 radiations are
given out. In this second change, radium B passes over
into radium C. The time required for radium B to undergo
half-decay is 26.8 minutes, and radium C half-decays in
19.5 minutes, as shown by Bronson.^ Schmidt ^ showed
that radium B gives a slow )8 and a 7 radiation.
Radium C has been separated by von Lerch ' electrolyt-
ically from a solution containing radium B and radium C.
The radium B was then precipitated by means of barium
sulphate. Of the different transformation products of
radium, radium C is one of the elements that gives out 7
rays. Further, radimn C gives out a rays with a greater
velocity than any other known substance. While the a
rays from radiiun itself can penetrate only about 3! cen-
timetres of air, those from radium C can traverse about
7 centimetres of air.
While the activity induced by radium almost vanishes
in a day, yet there remains a slight "residual activity,"
as was found by Madame Curie.* This activity shows a
and )8 radiations, but instead of these decreasing, they both
increase until they become practically constant. Ruther-
ford explains these facts by assuming that radium C passes
^ Amer. Joum. Sd. [4I, ao, 55 (1905).
« Physik. Zdt., 6, 897 (1905).
• Sitzun. Wien., 1x5, Ila, 197 (1906).
* Ann. Chim. Phys. [7I, 30, 289 (1903).
INDUCED RADIOACTIVITY 151
over into radium D, which has a very long life-history,
half-decay requiring 16.5 years. D giving out B particles
passes over slowly into radium E.
Rutherford ^ has shown that radium E gives out )8 and
possibly 7 rays and passes over into radium F. This is
deposited 'on a plate of bismuth immersed in a solution of
the active deposit. Radium F gives off only a particles
and its activity decreases to half-value in about 136 days.
Radium F is much more active than pure radium. It
has been shown by Rutherford to be about 3,200 times as
radioactive as radium at its minimum activity, or 800
times as radioactive as normal radium.
We must, however, go one step farther and ask the
question, what becomes of radium F? Does it undergo
still further transformation, and if so, into what? Ruther-
ford has thrown light indirectly on this question. We
have no evidence that radium F passes into anything
that is radioactive. Radium apparently yields four sub-
stances that send off a particles — radium itself, the ema-
nation, radiiun A, radium C, and radium F. It has been
regarded as highly probable that the a particle is a charged
helium atom. It would then have a mass of four, and five
such particles a mass of 20. If the atomic weight of radium
is 226, the end product formed from radium F would have
an atomic mass of 206. The atomic weight of lead is 207.1.
Lead occurs in radioactive minerals in quantities propor-
tional to the uranium and to the radium. In recent tables
of transformations, however, radio-lead is placed directly
after radium D as on page 199.
The second line of argument based upon the presence
of lead in all uranium minerals and, therefore, in all min-
erals that contain radium, is fairly convincing. Recent
* Phil. Mag., 10, 200 (1905).
152 THE ELECTRICAL NATURE OF MATTER
analyses of uranium minerals confirm the relation pointed
out by Rutherford.
Other elements, such as hydrogen, argon, barium, bis-
muth, and thorium, occur frequently in radioactive minerals,
and it may be shown that some of these, in addition to
helium, are produced by the disintegration of radium. Up to
the present, however, the evidence in the case of lead is the
most satisfactory, but it cannot yet be regarded as proved
that lead is the final decomposition product of radium.
RADIUM F PROBABLY IDENTICAL WITH POLONIUM
One of the most interesting consequences of Ruther-
ford's study of residual activities is that he has made it
highly probable that radium F is identical with polonium.
Madame Curie has shown that the activity of polonium is
not constant, but decreases irregularly, probably because
her material was impure. When pure polonium was pre-
pared it was foimd to decay exponentially. The period of
this decay has been determined many times and the most
reliable observations give the constant for half decay as
136 days. Rutherford foimd, as we saw above, 143 days
for radium F, St. Meyer and von Schweidler ^ 138 days,
which is in satisfactory agreement with 136. The identity
of the two substances is also further established by their
SUMMARY OF THE DECOMPOSITION PRODUCTS OF RADIUM
^ We have now followed the transformations of radium
through a number of stages, the more important of which,
it will be recalled, are the following:
1. Radium gives off a and )8 particles and yields the
2. The emanation gives off a particles and yields emana-
tion X, or radium A.
3. Radium A gives off a particles and yields radium )3.
4. Radium B gives off fi and 7 radiations and yields
radium Ci and Q.
5. Radium Ci gives off a, )8, and 7 rays and yields
radium C2, which gives off fi rays yielding radium D.
6. Radium D gives off )8 radiations and yields radium E.
7. Radium E gives off )8 and possibly 7 rays and yields
8. Radium F gives off a particles.
DECOMPOSITION PRODUCTS OF OTHER RADIOACTIVE
In a manner similar to the above, it has been made highly
probable that the emanation from thorium gives rise to a
radioactive deposit — thorium A, which imdergoes at least
two transformations, giving thorium B and thorium C.
The complete transformations of thorium into thorium
C are given in the following table:
Mesothoriiun (i and 2)
Thoriiun C (i and 2)
Time of half decay
3 X 10*® years
5.5 years (?)
Kind of rajB
154 THE ELECTRICAL NATURE OF MATTER
RADIOTHORIUM — A NEW RADIOACTIVE ELEMENT
A new radioactive element has recently been described
by Sir William Ramsay.^ Reference has been made to
this substance somewhat earlier by students of Ramsay,
but the first satisfactory accoimt of the discovery and the
element discovered has been given by Ramsay himself.
It was found in a mineral obtained from Ceylon. Ramsay
obtained about two himdred and fifty kilograms of the
mineral, having become interested in it on accoimt of the
large amount of helium that it contained. One gram of
the mineral gave about nine cubic' centimetres of helium
gas, which was between three and four times the amoimt
obtained from cleveite.
It is of interest to know that Ramsay has already ob-
tained from the mineral about one cubic metre of helium
gas, and we may look for some interesting results in reference
to the properties of this substance. It is well known that
this is the only gas that has thus far not been liquefied,
and this is mainly due to the fact that a sufficient quantity
had not previously been obtained. It is highly probable
that with the amoimt of helium now at disposal, it will be
possible to convert it into the liquid state, and then the
last of the most resistant gases will have succumbed to
modem methods of liquefaction. The new element was
obtained from the mineral, which was named " thorianite,''
in the following manner: The mineral was fused with sodium
disulphate. The residue insoluble in water was treated
with dilute, boiling hydrochloric acid. The insoluble sul-
phates were then fused with sodium carbonate, which
transformed them into carbonates. The barium carbonate
obtained was strongly radioactive and contained the radio-
^ Joum. de Chim. Phys., 3i 617 (1905).
INDUCED RADIOACTIVITY 155
active matter. in the mineral. The radium was separated
by the method devised by Giesel, i.e., by fractional crystal-
lization of the bromides. It soon became obvious that
there was present a radioactive constituent other than
radium. Its bromide was even more soluble than the
bromide of barium.
The chemical properties of the new substance show that
it is not identical with any known element. It resembles
in general the rare earths. It is to be distinguished chemi-
cally from radium in that it forms a soluble sulphate, and
from thorium in that its oxalate is insoluble in an excess
of ammonium oxalate. The new substance gives off an
emanation. Its rate of decay is the same as that of the
thorium emanation, and the excited activity producjed by
the emanation from the new substance diminishes at the
same rate as that produced by the emanation from thorium.
The oxide, after being strongly heated, but not otherwise,
glows in the dark. A similar result is obtained when one of
the salts is cooled in liquid air, but not to the same extent.
When a few milligrams of the new substance are wrapped
in paper and placed in front of a screen of zinc sulphide, a
phenomenon manifests itself similar to that observed in
the spinthariscope. Ramsay has measured the radio-
activity of radiothorium. In making these measurements
solutions of its salts were used, since these gave more con-
stant results than the solid salts. It was found that the
amount of the emanation obtainable from a given quantity
of the radiothorium was equal to that obtainable from five
hundred thousand times as much thorium. The relative
powers of radiothorium and radium to discharge the elec-
troscope have also been tested. It was foimd that radio-
thorium has apparently about half the discharging value
156 THE ELECTRICAL NATURE OF MATTER
Sir William Ramsay summarizes the results that he has
obtained with radiothorium as follows:
The emanation given off by radiothorium is identical
with that given off by salts of thorium. The quantity, as
we have seen, is infinitesimal in the case of thorium com-
pared with the amoimt given off by radiothorium. The
conclusion is that ordinary thorium probably contains a
trace of radiothorium to which it owes its radioactivity.
Ramsay announces that he has already succeeded in sepa-
rating a part of the radioactivity from the thorium, by add-
ing to the thorium salt a salt of barium, and then adding
sulphuric acid. A part of the radiothorium is probably
brought down along with the barium salt.
Analogous to the decomp)osition products of uranium,
Ramsay suggests the following scheme as representing the
probable decomposition products of thorium.
Inactive thorium — radiothorium — thorium X — em-
anation — thorium A — thorium B — ? — helium.
There seems to be no doubt, according to Ramsay, that
the helium found in thorianite is produced from the radio-
thorium present in that mineral.
Uranium does not yield an emanation, but uranium X
apparently breaks down at once into the final product.
Actinium yields an emanation, which decomposes in at
least three stages, giving actinium A, B, and C.
DECOMPOSITION PRODUCTS OP ACTINIUM
The complete series of decomposition products in the
case of actinium is given in the following table:
Time of half decay
Kind of rajB
Actiniimi C (?)
Actinium X was obtained by Godlewski ^ and in the fol-
To the hydrochloric acid solution of the actinium, ammo-
nia was added. A reddish-brown precipitate was formed
which was probably the hydroxide. The filtrate was evapo-
rated to dryness, and the ammonium salts driven off by
ignition, when a small black residue remained, which be-
came white on heating.
This residue was found to be intensely radioacUve as com-
pared with the actinium from which it was separated. The
activity was found to decrease slowly with time, according
to an exponential law.
The actinium from which the intensely radioactive pro-
duct had been separated, was at first almost nonradioactive.
It recovered its radioactivity with time, the recovery curve
being the inverse of the decay curve of the residue.
It will be seen that the above results are strictly analogous
to those obtained with uranium and thorium. From the
1 Phil. Mag., 10, 35.
IS8 THE ELECTRICAL NATURE OF MATTER
analogy to thorium X, the above, highly active product
was termed actinium X, and assigned the symbol AcX.
The radioactivity of actinium X, when first separated
from the actinium, was more than one hundred times as great
as that of the actinium itself. The residue obtained by
evaporating the filtrate, as above described, is not all actin-
ium X, but consists chiefly of non-radioactive material,
which is probably some of the rare earths.
The analogy between uranium, thorium, and actinium is,
as we have seen, very striking. There is, however, one
marked difference. After thorium X is removed there
remains in the thorium a residual activity, which amoimts
to about twenty-five per cent, of the total radioactivity
possessed by normal thorium.
After actinium X is removed from actinium, the activity
of the remaining actinium, when tested immediately, is
only about five per cent, of what it is in the normal sub-
Godlewski tried to remove this small residual activity
by repeatedly precipitating the actinium solution with
ammonia. Eight precipitations were made in seven hours.
The residual activity, however, still remained. This was
probably due to the presence of a small amoimt of actinium
X, which could not be separated from actinium. The
latter when freed from actinium X is perfectly non-radio-
active. The production of actinium X from actinium is,
as shown in the table on page 157, radioactinium being an
Actiniimi X was shown to give out a rays. That the
)8 rays come directly from radioactiniimi, and not from the
excited activity resulting from the deposit of the emana-
tion, is made highly probable by the following facts: The
curves of decay of the activity of radioactinium are the
INDUCED RADIOACTIVITY 159
same, whether the activity is measuted by the a or the fi
rays.' It is also pointed out that the activity of radio-
actinium, measured by the fi rays directly after strong heat-
ing, which would remove all the cause of excited activity,
has a large value even at the beginning. This would not be
the case if the )8 rays came from the excited or induced
The problem of the origin of the emanation in actiniiun
was then attacked. It will be remembered that the thoriiun
emanation comes from thoriiun X. Does the actinium
emanation come from actinium X ? This question can
be easily answered.
Remove the emanation from actinium containing actin-
iiun X, and test the amount by the activity. Then remove
the emanation from an equal amount of actinium from
which the actinium X has been separated, and test its
activity. The result is very satisfactory.
The actinium from which actinium X has been separated
gives practically no emanation. Further, the amount of
the emanation increases as the amount of actinium X in-
creases, and decreases at the same rate that the activity,
or as actinium X, decreases.
Godlewski points out that the emanation being present
only when actinium X is present, and being alwa)rs pro-
portional to the amount of actinium X, it must be the
product of actinium X.
The products of actinium that have thus far been shown
to exist are the following. Actinium yields radioactinium
which yields actinium X. Actinium X gives out a rays and
yields the actinium emanation. The actinium emanation
gives out a particles and produces actinium A. Actinium A
yields actinium B, the change giving out a particles.
Actinium B gives out )8 particles and yields actinium C,
l6o THE ELECTRICAL NATX7RE OF MATTER
which gives off a particles yielding actinium D which gives
off P and 7 rays.
Godlewski also shows that the fi rays from actinium
differ from the fi rays from other radioactive substances.
In the first place they are completely homogeneous, and
in the second, have less than half the penetrating power
of the )8 rays emitted by other radioactive substances.
He also shows that the 7 rays from actinium have only
about one-fourth the penetrating power of the 7 rays from
The chemical nature of such products as those just de-
scribed is entirely unknown, and will remain so imtil they
can be obtained in sufficient quantity to be studied at least
by the more refined chemical methods.
Recent work makes it probable that certain transforma-
tions which were formerly regarded as rayless, give off a
"soft" or not highly penetrating kind of beta ray.
Production of Radioactive Matter
continuous formation of radioactive matter in
We have learned that thorium and radium from which
the emanation has been removed have lost most of their
radioactivity. We have seen that the emanation loses its
activity, but what is more important in the present con-
nection, the deemanated substance regains Us radioactivity
on standing. Further, when all the emanation has been
driven out from a radioactive substance and the deemanated
body has regained its radioactivity on standing for a time,
more of the emanation can then be' removed from this
These facts can best be interpreted by assuming that
some change is continuously going on in the radioactive
substances, which gives rise to the emanation and restores
In connection with the changes taking place in radio-
active substances a most important discovery was made
by Sir WiUiam Crookes.* He found that a very active
constituent could be separated from uranium by chemical
means, and that the remaining uranium was not appreciably
If to a solution of a uranium salt a solution of ammonium
carbonate is added, the uranium is precipitated. If an
1 Roy. Soc. Proceed., 66, 409 (1900).
1 62 THE ELECTRICAL NATURE OF MATTER
excess of the ammonium carbonate is added the precipitate
dissolves in this reagent. There, however, remains a small,
light brown residue that does twt dissolve when an excess
of ammonium carbonate is added. This residue can readily
be filtered oflF from the solution, and was found to be highly
radioactive, as compared with uranium itself. This residue
was called by Crookes uranium X, and was given the symbol
RECOVERY OF ACTIVITY BY URANIUM, AND DECAY OF ACTIVITY
IN URANIUM X
The uranium from which the uranium X was thus sepa-
rated was left much less radioactive. If this uranium
was laid aside for a time, it was found to regain its original
The uranium X, on the other hand, gradually became less
active, until after a few weeks its radioactivity was only
half as intense as when it was first precipitated.
The rate at which uranium X loses its activity has been
carefully studied. Similarly, the rate at which the ura-
nium, from which the uranium X has been separated, re-
gains its activity, has been measured.
These results show that the uranium X loses its radio-
activity just as rapidly as the uranium regains its activity.
In a word, that in ordinary uranium we have uranium X
undergoing decay at just the same rate thai it is being formed,
and the condition that exists is one of equilibrium between
these two opposite processes.
The rate at which uranium recovers its radioactivity has
been found to be entirely independent of the conditions to
which it is subjected.
The rate at which uranium X loses its activity has also
been found to be entirely independent of all conditions both
PRODUCTION OF RADIOACTIVE MATTER 163
physical and chemical. It is unaflfected by the state of aggre-
gation of the radioactive matter, by the presence of any
chemical reagent, and what is more surprising, by the tem-
perature to which it is subjected.
We can now see why the radioactivity of uranium is
constant. It represents, as mentioned above, a condition
of equilibrium between the two oposite processes — the
continual production of the radioactive uranium X at a
constant rate, unaflfected by any change of conditions; and
the continual decay of the activity of the uranium X at a
constant rate, also independent of all conditions. As both
of these processes go on at a constant rate, the equilibrium
between the two represents a condition where there is a
constant amount of uranium X in the uranium, and hence
a constant radioactivity.
RADIATION FROM URANIUM X
One other matter of importance and interest should be
mentioned before leaving the discussion of radium X.
A peculiarity in connection with the radioactivity of
uranium has already been pointed out. It does not give
ofiF an emanation.
The radiation from uranium X consists of )8 and y rays.
These, as will be remembered, consist of negative charges
of electricity shot oflf with a velocity nearly equal to that
of light, and contain no matter whatsoever. In a word,
they are cathode rays. The radiation from uranium X
contains, then, no matter, but only electricity.
The recognition of this fact is of the very greatest im-
portance in connection with the study of the relations be-
tween uranium and uranium X. The electrical method
cannot be used in this connection, since the )8 rays produce
but little ionization in a gas. The photographic method
1 64 THE ELECTRICAL NATURE OF MATTER
must be employed. The neglect to take the above fact
into account has led to some confusion in the Uterature of
The facts just pointed out in connection with the radia-
tions given off by uranium, on the one hand, and uranium
X, on the other, are of prime importance in determining
the radioactive products that are formed from uranium.
In addition to uranium X, which gives off fi and 7 particles,
being formed from uranium, there must also be produced
another radioactive product which sends off a particles.
As we have just seen, uranium X, or the active constituent
which gives out fi and 7 ra)rs, has been separated from ura-
nium; but the other active product which gives out the a
radiations has not yet been separated by any means from
salts of uranium.
CONTINUOUS FORMATION OF RADIOACTIVE MATTER FROM
We have just seen that Sir William Crookes succeeded,
by purely chemical means, in separating from uranium a
radioactive constituent which was fundamentally different
from uranium itself.
A similar separation has been effected by Rutherford
and Soddy * in the case of thorium. When a thorium salt
is dissolved in water and the solution treated with ammonia,
the thorium is precipitated. When tested, the precipitate
was found to be much less radioactive than the thorium salt.
The solution from which the thorium had been precipitated
by ammonia was, after filtering, completely evaporated,
and the residue highly heated to remove salts of ammonia.
» Soddy: Joum., Chem.Soc.,81,860 (1902); Rutherford and Grier: Phil.
Mag., 4, 315, (1902).
'Joum. Chem. Soc., 81, 837 (1902).
PRODUCTION OF RADIOACTIVE MATTER 165
The final residue after heating was found to be very
radioactive. . Indeed, in some cases, more than a thousand
times more radioactive than the thorium salt itself.
The highly active residue was very small in quantity,
and, therefore, must have contained some substance whose
radioactivity was very intense. This product obtained
from thorium was called by Rutherford and Soddy thorium
Xy and assigned the symbol ThX.
This substance was shown to be soluble in water, since
when thorium oxide is shaken with water the radioactive
constituent is partly dissolved, while thorium oxide itself
is insoluble in water.
If a solution of a thorium salt is treated with ammonium
carbonate, the thorium X is precipitated along with the
thorium. The method of separating thorium X from
thorium is, then, very diflEerent from that required to separate
uranium X from uranium. We shall now study some of
the properties of thorium X.
PROPERTIES OF THORIUM X — DECAY OF ITS RADIOACTIVITY
Thorium X, when separated from thorium by the method
above described, is highly radioactive as we have seen.
Its radioactivity, however, decays, having only about half
its initial value after 3.64 days.
The rate at which thorium X decays or loses its radio-
activity, like uranium X, is uninfluenced by any known
physical or chemical condition. Moisture, pressure, and
even temperature have no influence on the rate at which
thorium X decays.
THORIUM X PRODUCES THE THORIUM EMANATION
Both thorium and radium are capable of yielding that
remarkable substance or substances already studied — the
1 66 THE ELECTRICAL NATURE OF MATTER
emanation. In the case of thorium, does the emanation
come from the thorium directly, or from thorium X? This
has been answered by Rutherford and Soddy.*
If thorium X is completely removed from thorium by
repeated precipitations, the thorium has no appreciable
power to give oflF the emanation. If, however, the thorium
is allowed to stand for some time, it can give oflF an appre-
ciable quantity of the emanation. This is due, as we shall
learn, to the production of thorium X which is going on in
the thorium itself.
The thorium X when first separated from the thorium
has marked power to produce the thorium emanation. As
the thorium X decays it has been shown that its power to
produce the emanation becomes less, and, indeed, in the
same ratio. This shows that the thorium X produces the
The changes that are going on in thorium can now be.
followed, at least in part. The thorium atom loses an
a particle producing indirectly thorium X. Thorium X,
like thorium itself, is an unstable system and changes take
place in it. Thorium X loses a and fi particles, and the
thorium emanation is produced. The emanation is a dif-
ferent substance from thorium X from which it was
formed. This conclusion is confirmed by a comparison of
all of the properties of these two substances. (See p. 153.)
Thorium X differs from uranium X in the kind of radiation
given out by it. Thorium X gives out chiefly a particles,
while uranium X, as we have seen, gives out mainly fi rays.
RECOVERY OF RADIOACTIVITY BY THORIUM
We have become familiar with a method for separating
thoriimi X from thorium. This method effects almost
' Joum. Chem. Soc., 81, 849 (1902).
PRODUCTION OF RADIOACTIVE MATTER 167
complete separation if the process is repeated a few times.
If the thorium precipitated by ammonia is dissolved in
nitric acid, and then again precipitated, and the process
repeated twice, the resulting thoria is only about one one-
hundredth as radioactive as ordinary thorium. The active
thorium X has, thus, for the most part, been separated from
If now this comparatively non-radioactive thorium is set
aside, it regains its radioactivity. A careful study of the
rate at which thorium recovers its radioactivity, after thorium
X has been removed from it, has been made by Rutherford
and Soddy.* They found that the thorium, in general,
recovered its radioactivity at the same rate that the sepa-
rated thorium X lost its radioactivity.
A careful comparison was made of the rate at which
thorium X decays with time, and the rate at which thorium,
from which thorium X has been separated, recovers its
radioactivity, and the results plotted in curves. It was
found that the one loses its radioactivity just as rapidly as
the other regains its radioactivity.
This can best be interpreted by assuming that thorium X
is continually being produced by the thorium. The rate of
production is just equal to the rate at which thorium X
decays, and this gives us the condition of equilibrium that
obtains in ordinary thorium.
From the thorium which has regained its original radio-
activity — and this requires about a month — a new portion
of thorium X can be separated, and exactly the sam^ amount
as measured by its radioactivity that was obtained originally.
The non-radioactive, thorium, from which the second por-
tion of thorium X had been separated, recovers its radio-
activity again, at the same rate that it did initially. When
* Joum. Chem. Soc., 81, 840 (1902).
l68 THE ELECTRICAL NATURE OF MATTER
the condition of equilibrium is reached again, a new por-
tion of thorium X can be separated, which is equal to that
originally obtained, and thus the process goes on slowly,
apparently until all of the thorium is transformed into
thorium X. This complete transformation would prob-
ably require millions of years.*
RATE AT WHICH THORIUM RECOVERS RADIOACTIVITY
INDEPENDENT OF CONDITIONS
We would naturally ask whether transformations like
those we have just been considering resemble ordinary
chemical reactions, or are something fundamentally different
from them? To throw any light on this question we must
study the two sets of transformations, and see what re-
semblances or differences make their appearance. Chemical
reactions are, in general, affected by the physical conditions
of the substances that are reacting — by the state of aggre-
gation, whether solid, liquid, gas, or solution; by the pres-
sure to which they are subjected, and especially by the
The rate at which thorium X is formed from thorium
seems to be entirely independent of all these conditions. It
does not seem to matter to what conditions we subject the
thorium from which thorium X has been separated, we
cannot affect in any way the rate at which it recovers its
lost radioactivity, which is the same as to say, the rate at
which it produces thorium X.
There is, then, at least this one fundamental difference
between the formation of thorium X from thorium, and
ordinary chemical transformations — the former is inde-
pendent of all the conditions to which the substances are
subjected, including great changes in temperature.
» Joum. Chem. Soc., 8i, 844 (1902).
PRODUCTION OF RADIOACTIVE MATTER 169
RADIUM DOES NOT GIVE RISE TO SUBSTANCES CORRESPONDING
TO URANIUM X AND THORIUM X
Radium has not thus far been shown to yield any sub-
stance analogous to those formed by uranium and thorium,
which we have just been studying. It does not form any
intermediate product, but apparently yields the emanation
importance of a theory or generalization
The chief aim of scientific investigation is not the dis-
covery of isolated facts. Indeed, we might continue to
unearth such facts for an indefinite time, in any branch of
natural science, and it is a question whether such knowl-
edge ought to be dignified with the name of science.
The highest aim of scientific investigation is to reach a
theory or generalization, which, when sufficiently estab-
lished, becomes a law. This may or may not be an ulti-
mate truth, probably is not, but may be as near to it as the
methods at present at our disposal are capable of leading.
It may be asked, how do we arrive at generalizations in
science ? The answer is, for the most part by the inductive
method. We discover fact after fact and then coordinate
and correlate these individual facts, and the result is a
It may then be said, and fairly, that the discovery of
facts is highly important, indeed essential to the discovery
of generalization or law. From this no man of science
will dissent. The discovery of isolated facts bears the same
relation to science as the making of bricks to architecture.
The bricks are absolutely essential in constructing the build-
ing, but they are not the end or aim of the architect. They
are simply a means toward the end, which is utility, or
beauty, or both. Just so in the investigation of natural
THEORETICAL CONSIDERATIONS 1 7 1
phenomena; we must study the isolated facts; they are the
bricks or individual units of which science is made. They
are, however, not science, and not the end of scientific
investigation. They are but the means to the end. The
generalization in science may be compared with the finished
edifice in architecture.
We have now studied a large number of facts pertaining
to radioactivity. Some of these are of a striking nature,
and arouse deep interest when considered by themselves.
Their real importance and significance, however, comes
out when we consider them in their relations to other factSy
and especially to well-established generalizations, which
we now accept as the philosophy of the physical sciences.
We shall next attempt to coordinate the facts of radio-
activity, and see what generalizations have been reached.
We shall learn that new light has been thrown on the nature
of what we call in chemistry the atom, and on the genesis
of matter, by the study of various phenomena connected
THE MORE IMPORTANT FACTS IN CONNECTION WITH URANIUM
Before taking up the generalizations that have been
reached, a brief summary of the facts in connection with
the several radioactive elements will be given, by way of
review, since it is these facts that have to be dealt with
primarily by any theories that have been, or may be, pro-
The element uranium gives oflF a, jS and y rays. The
alpha rays are composed of material particles, each having
a mass from two to four times the mass of the hydrogen
atom. These particles are shot oflF at very high velocities,
and, therefore, have considerable kinetic energy. The a
particles are charged, and are, therefore, deflected in a
172 THE ELECTRICAL NATURE OF MATTER
magnetic field. The direction of their deflection shows
that they are charged positively. Every a particle carries
two unit electrical charges, as we say; that is, twice the
amount of electricity carried by a univalent ion in solution.
The a particles produce strong ionization of the gas
through which they pass; indeed, most of the ionization
effected by radioactive substances is due to the a particles.
The a particles have marked power to produce phospho-
rescence in certain substances, especially zinc sulphide and
barium platinocyanide. The phenomena that manifest them-
selves in the spinthariscope are due, for the most part, to
the a particles.
The a rays produce but little effect upon a photo-
graphic plate, and, therefore, the photographic method
cannot be used to measure the intensity of this kind of
radiation. ' The a particles being material in nature are
easily cut off by matter. They cannot pass even through
very thin metallic screens.
The 13 rays are closely analogous to the cathode rays.
The mass of the /S particle is about jjiST of the mass of
the hydrogen ion. These particles are shot off with dif-
ferent velocities, but all with very high speed, indeed, of the
order of magnitude of Ught. The mass being small, the
kinetic energy of the )8 particle is much less than that of
the a particle, notwithstanding the fact that the /S particle
moves with greater velocity.
The fi particles are deflected by a magnetic field, indeed
much more strongly than the a particles. They are, how-
ever, deflected in the opposite direction to the a particles,
and have a negative charge. Every jS particle is a unit
negative charge of electricity.
These particles produce comparatively little ionization
in the gas through which they pass.
THEORETICAL CONSIDERATIONS 1 73
They have comparatively little power to excite phos-
phorescence. The )8 particles have some effect upon a
photographic plate. They are cut off by metallic screens
of any considerable thickness, but have much greater pen-
etrating power than the a rays.
The gamma rays are identical with the X-rays. These
rays are apparently shot off as pulses y with very high veloci-
ties. The 7 rays are not deflected at all even by a very
intense magnetic field. They produce comparatively little
ionization i^ gases, and have but little power to excite
phosphorescence. They have very marked action on a
photographic plate. They have great penetrating power;
not only many times greater power to penetrate matter
than the fi rays, but even greater penetrating power than
the X-rays themselves. As has already been pointed out,
Rutherford has been able to detect the 7 rays from radium
after they have passed through a foot of solid steel. Gamma
rays are produced by the /S rays, and are never present
Uranium is continually but very slowly undergoing a
transformation, giving rise to a form of matter that differs
in properties from the uranium itself. This new form of
matter is called uranium X. The rate at which uranium X
is formed is independent of all physical and chemical con-
ditions. In the formation of uraniiun X from uranium, a
particles are given off.
Uranium X, in turn, undergoes decomposition, giving
off 13 and 7 rays during the process. The rate of the de-
composition is also independent of all conditions (p. 199).
THE MORE IMPORTANT FACTS IN CONNECTION WITH THORIUM
The facts in connection with thorium are more nimierous
than with uranium. Thorium, like uranium, gives off a, )8
174 THE ELECTRICAL NATURE OF MATTER
and 7 rays. It undergoes a continuous transformation,
yielding a new form of radioactive matter known as thorium
X, at the same time setting free a particles. (Seep. 153.)
Thorium X gives off a and probably )8 particles, and
yields an emanation — the thorium emanation — which is a
gas. This emanation gives off a particles and forms thorium
A, a radioactive solid which imdergoes still further decom-
position in three stages. In the first stage )8 rays are sent
out, in the second a rays, while in the third )8 and 7 rays
are given off.
Since thorium gives rise to an emanation which decom-
poses into a solid form of matter — thorium A — that is
radioactive and deposits upon other objects, thorium is
capable of inducing or exciting radioactivity in bodies
placed in its neighborhood. Thorium thus differs strik-
ingly from uranium, which forms no emanation, and which,
therefore, cannot induce radioactivity on neutral objects
even when in close proximity to them.
THE MORE IMPORTANT FACTS IN CONNECTION WITH RADIUM
The best studied of all the strongly radioactive substances,
by far, is radium. Its radioactivity is at least a million and
a half times that of metallic uranium, which is taken as the
Radiiun does not yield any radioactive substance anal-
ogous to uraniiun X or thorium X. It appears to produce
the emanation at once from itself, giving off a and )8 par-
ticles. The emanation gives off a particles, and emanation
X or radiiun A results. This undergoes a number of
transformations, which have aheady been recognized.
During the first transformation a rays are sent off; during
the second )8 and 7 rays are emitted, while during the
third stage of the transformation a, )8 and 7 rays are all
THEORETICAL CONSIDERATIONS 175
given out. During the fourth stage of the transformation
P radiations are given off; while during the fifth stage )8
and y rays are liberated. Radium possesses a number of
unique properties, all of which are very remarkable. Radium
is the only known chemical element that produces of itself
another chemical element, or can be made to produce such
by any known means. Radium, or more exactly, the
radium emanation, in undergoing decomposition spontane-
ously yields the element helium. This discovery was so
surprising and so directly at variance with all of our previous
conceptions of a chemical element, that it was subjected to
the severest experimental tests. It has withstood all criti-
cism, and is beyond doubt a fact.
A number of the other properties of radium are scarcely
second in importance to the production by it of helium.
Radium has the power of charging itself electrically, and
it is the only substance known that has this power.
Radium also has the property of producing light or be-
Most remarkable, however, is the amount of heat that is
being continuously set free from radium. It will be re-
membered that radium produces enough heat to melt its
own weight of ice every hour.
When we consider the almost limitless time over which
radium can thus continue to give out heat, we see how
enormous is the amount of energy that this substance can
liberate. It is of a magnitude entirely incomparable with
the amount of heat set free in the most strongly exothermic
chemical reactions. The enormous magnitude of the
energy that can be Uberated by radium must be classed as
one of the most important discoveries in modem science.
With the facts enumerated above at our command we
can now proceed to discuss intelligently the generalizations
1 76 THE ELECTRICAL NATURE OF MATTER
and conclusions that have been reached as the result of the
study of radioactivity.
THEORY OF RUTHERFORD AND SODDY TO ACCOUNT FOR
The only theory thus far proposed, which accounts at all
satisfactorily for the phenomena discovered in connection
with the radioactive elements, and which will probably
prove to be of epoch-making importance, is that advanced
by Rutherford and Soddy.* The key to this theory is that
the radioactive elements are unstable. The atoms of these
substances represent unstable systems^ which are continually
undergoing rearrangement and decomposition. A definite
number of the atoms of any radioactive element become
unstable in any given time. They each throw off an a
particle, and the next stage results. In the case of uranium
and thorium, there are formed indirectly uranium X and
thoriimi X. These products are in turn unstable. They
throw off a or )8 particles, and in the case of thorium an
emanation results. The radium atom throws off an a
particle or a and /S particles, and yields at once the emana-
tion. (See p. 199.)
The emanation also is in an unstable condition. It
throws off a particles and yields a radioactive solid, which,
when deposited upon non-radioactive matter, induces in it
radioactivity. This solid, or emanation X, or radiiun A
as it is termed, is also unstable and undergoes further trans-
formations. In the case of radiiun a fairly large number
of steps have already been traced. In the earlier stages
of the transformations of emanation X either a particles
or no radiations escape. If no radiation is given out and
we still have a well-marked transformation taking place,
1 Phil. Mag., 5, 576 (1903).
THEORETICAL CONSIDERATIONS l^^
it probably means that the parts of the atom are simply
undergoing rearrangement without losing any constituent.
The )8 and 7 rays are given off chiefly in the later stages
of the decompositions that are taking place in the radioac-
These unstable atoms, which are thus undergoing change,
are termed by Rutherford metabolons.
THE TRANSFORMATIONS OF THE RADIOACTIVE ELEMENTS
DIFFER FUNDAMENTALLY FROM ORDINARY CHEMICAL
The question that would at first arise is this: Are the
changes that are going on in the radioactive elements funda-
mentally different from chemical reactions? New sub-
stances with different properties from the original substances
are being formed. Energy in the form of heat is liberated,
and these changes are characteristic of ordinary chemical
transformations. If we study more closely the changes
that are taking place in radioactive matter, we shall find
marked differences between them and chemical reactions,
as has already been pointed out.
In the first place, the changes in radioactive matter take
place at a definite rate, which is entirely unaffected by con-
ditions. We have studied a number of such radioactive
changes which go on at the same rate at the temperature
of liquid air as at a red-heat. This alone would show that
the transformations in radioactive matter are fundamentally
different from chemical reactions. The latter, as we well
know, are greatly affected by conditions, and especially
by temperature. The velocity of chemical reactions is in
general greatly increased by rise in temperature, and at
very low temperatures becomes extremely small or entirely
178 THE ELECTRICAL NATURE OF MATTER
Again, the velocity with which radioactive changes take
place is very small. The amount of uranium transformed
into uranium X, or thorium into thorium X, in considerable
intervals of time, is very small indeed.
The slowness of the transformations that we have just
been considering explains why such elements as thorium,
uranium, and the like still exist, and have not all been trans-
formed into their decomposition products. It is calculated
that at least thousands of years would be required for enough
thorium to be transformed into thorium X, in order that the
transformation would be detectable by the most sensitive
balance. Even radium yields the emanation very slowly.
In fact, so slowly that the loss of a particles cannot even
be weighed until larger amounts of radium are obtainable.
It has been calculated that radium will transform half of
itself in about 1,500 years. The loss, therefore, can scarcely
be detected during the time over which measurements of
radioactivity have thus far been extended.
That radium is, however, undergoing decomposition is
certain, and if it were not being produced in some way all
of the radium now in existence would eventually disappear.
That radium is being continually produced, probably from
uranium, will be shown in the next chapter.
Another marked difference between the transformations
that are taking place in radioactive matter and chemical
reactions is in the amount of energy set free. We have
already become familiar with the fact that radium liberates
quantities of energy incomparably greater than any other
known substance. If we compare the amount of heat set
free when the most vigorous chemical reactions* take place,
with that liberated by salts of radium, the former is utterly
We must, therefore, abandon any attempt to explain
THEORETICAL CONSmERATIONS 1 79
the transformations of the radioactive elements on the
basis of chemical reactions. The two processes take place
according to different laws. They are affected differently
by change of conditions. They yield different products,
considered both from the standpoint of matter and of energy.
In a word, they are fundamentally different processes.
It is one thing to point out that radioactive processes are
not chemical reactions; it is a different matter to find out
the nature of the transformations that are taking place in
radioactive substances. That we have a satisfactory sugges-
tion to account for these transformations will now appear.
THE ELECTRON THEORY OF J. J. THOMSON AS APPLIED TO
It would be difficult to account for the instabiUty of the
chemical atom on the older theory that a chemical atom is a
homogeneous, indivisible unit. In terms of the modem
theory of the atom advanced by J. J. Thomson, we can
readily see how an atom could be unstable and send off
particles, just like the radioactive atoms do.
In terms of the theory of Thomson, which we have called
the electron theory, a chemical atom, as we saw in earUer
chapters, is made up of a fairly large number of electrons
or negative electrical charges, moving within a sphere of uni-
form, positive electrification. The particles are held in
their relative positions by their mutual repulsions, and the
attraction of the positive electricity. The heavier atoms
contain a larger number of electrons than the Ughter atoms
— the approximate number in any atom being expressed
by the atomic weight of that atom in terms of hydrogen as
unity, multipUed by 770.
We can easily conceive of some of the electrons, in their
rapid movement, coming into such a position that they
l8o THE ELECTRICAL NATURE OF MATTER
would escape and fly off from the atom into space. This
would be especially the case with the heavier atoms, which
represent very complex systems of electrons. From such
highly complicated systems we might expect a more or less
constant escape of such particles.
Again, we might not only expect individual electrons to
escape from the atom, but groups of electrons. Indeed,
groups of these negative electrical charges would be more
Ukely to escape from the atom than single charges.
The facts of radioactivity are in perfect accord with the
above conclusions. It is the atoms with largest mass that
are radioactive. Thorium has an atomic weight of 232.5,
uranium of 238.5 and radium either 225, or more probably
in the neighborhood of 256 or 258. No radioactive sub-
stance is known having a small atomic weight, and all of
the heaviest atoms are radioactive.
In the earlier stages of radioactive change it is the a
particles that are shot off. The a particles have a mass
probably about four times that of the hydrogen atom.
This means that they are helium atoms (atomic weight 4),
that are shot off from the radioactive atom. The atom,
having lost this comparatively large helium atom, is dif-
ferent in nature from the original atom. The system
is not yet stable, and another a particle or atom of helium
is shot off, and another condition of the radioactive mat-
ter produced. This may continue through several stages,
until after a. while the individtuil electrons begin to come
off as the fi particles. It will be remembered that the 7
rays are set up where the )8 rays impinge upon solid
Thus we see that the theory of matter advanced by J. J.
Thomson, and which was developed at some length in the
earlier chapters, enables us to account rationally for many
THEORETICAL CONSIDERATIONS l8l
of those remarkable phenomena that we have studied under
the general head of radioactivity. Further, it is the only
theory that has thus far been proposed, which enables us
to deal at all satisfactorily with the unstable atom.
IS MATTER IN GENERAL UNDERGOING TRANSFORMATION?
The raising of such a question wovdd, until a few years
ago, have been regarded as extraordinary, since the ele-
ments were regarded as stable and unchanging. In the
light of the recent investigations with the radioactive ele-
ments, it is most pertinent. There is some evidence, as we
shall see, that many of the elements are radioactive to a very
slight extent. K this should be proved to be due to the
elements themselves, to be a property inherent in all matter,
and not caused by the deposition of some form of radio-
active matter, then, from what has been said above, we
must regard matter in general as undergoing change. This
change is slow, very slow, but is progressing continuously;
the more complex, unstable forms, breaking down into
simpler aggregates of electrons.
In discussing the question as to whether matter in gen-
eral is radioactive, the following consideration must be
taken into account as Rutherford has shown.
Since the a particles are shot off from radioactive matter
with velocities that are only about thirty per cent, above
the critical velocity, i.e., the velocity necessary to affect
a photographic plate, to produce phosphorescence, or to
ionize a gas, and thus lead to the detection of the a par-
ticles; it suggests the possibility that matter in general may
be undergoing a disintegration similar to the radioactive
elements, but that the a particles are shot off with a velocity
below the critical and therefore escape detection.
It is probable that in some of the transformations of the
l82 THE ELECTRICAL NATURE OF MATTER
radioactive elements which were thought to be rayless, a
particles are actually given off, but with a velocity that is
below the critical and they therefore are not detected.
This suggests the further thought that all maUer may
really be radioactive. Only those elements that shoot off a
particles with velocities above the critical would produce
appreciable ionization in a gas, and thus be classed as radio-
active, in terms of our present methods of detecting radio-
If it should be shown that all matter is slightly radio-
active, then we should be forced to the conclusion of the
general instability of the chemical elements. However
this may prove to be, enough has already been established
to show that our former conceptions of the nature of the
chemical element must be fundamentally modified.
Wide Distribution of Radioactive Matter and the
Origin of Radium
The most strongly radioactive substances — radium,
actinium, polonium — apparently occur in very small
quantities. Even the more feebly radioactive elements,
thorium and uranium, are not among the more common
A question of very great importance in connection with
the study of radioactivity is this: Is radioactive matter
small in quantity and confined to a few sets of conditions,
or is it widely distributed? The fact that it exists in any
one locality, or in any one mineral only in small quantity,
does not throw much light on the question of the scope of
We shall review very briefly some of our knowledge of
the distribution of radioactive matter, as far as our globe
radioactive matter in the earth and sea
It has been shown by Elster and Geitel * that air confined
in spaces in contact with the earth, such as certain caves,
becomes radioactive. The same result was obtained, and
to a more marked extent, by taking air from some depth
below the surface of the soil by means of a pump. Such
air contained sufficient quantity of the radium emanation
to induce radioactivity upon the walls of the containing
» Phys. Zeit., 3, 574 (1902).
l84 THE ELECTRICAL NATURE OF MATTER
vessel, especially if it was charged negatively. The radio-
activity decayed at such a rate as to leave no question that
it was produced by the radium emanation. These phe-
nomena were shown to be due to the presence of radium
in the ground, which diffused into the air; since air confined
by itself in a metal vessel, away from contact with the soil,
did not become radioactive.
Similar results have been obtained by others, so that
there is now no reasonable doubt that the radioactivity of
air in confined spaces is due to the presence of the radium
emanation, which gradually diffuses from the ground.
It was shown by Ebert that air which is radioactive,
loses its radioactivity when passed through a tube sur-
rounded by liquid air. It will be remembered that Ruther-
ford, by this means, condensed the emanation from radium,
and obtained it in the liquid condition. This is another
bit of evidence that goes to show that the radioactivity of
the air in contact with the earth is due to the radium emana-
The amount of radioactive matter in the soil seems to
vary greatly from place to place. Clay soil seems to be
the most radioactive, but sandy soils are not infrequently
Carbon dioxide that came from great depths in the earth
was found to be radioactive. It lost its radioactivity on
standing for some days.
A quite appreciable quantity of radioactive matter has
been found in certain waters that percolate through the
soil, and especially in those that come froni considerable
depths. J. J. Thomson * has shown that the tap- water of
Cambridge, England, contains radioactive matter, while
the waters from certain deep wells in other parts of Eng-
1 Phil. Mag., 4, 35^ (1902).
WTOE DISTRIBUTION OF RADIOACTIVE MATTER 18$
land were found to contain quite appreciable quantities of
the highly radioactive emanation. This emanation decayed
at such a rate, as compared with the emanation from radium,
as to show that the two were identical.
Similar results were obtained by Bumstead and Wheeler »
with the waters at New Haven.
One of the most interesting results of this character
has been found in connection with certain hot springs,
such as at Bath, in England. The water of this spring,
which comes from great depths, is slightly radioactive, but
the mud deposited from the water is strongly radioactive,
due to the presence of the radium emanation.
It is also a matter of importance that in the gases that
escape from this spring, helium has been found. This
helium comes, almost beyond question, from the decom-
posing radium emanation, and shows that radium exists
at great depths beneath the surface of the earth.
The simultaneous occurrence of these two elements,
and the fact that helium is produced from the radium ema-
nation, lead us to suspect the presence of radium wherever
helivmi is found — as in the sim.
Quite recently Joly, in his admirable little book on " Ra-
dioactivity and Geology," has shown that there is an enor-
mous amount of radium in the waters of the sea, and es-
pecially in the deposits on the sea floor. He shows that the
total quantity of radivmi in sea water is about twenty
thousand tons, and that in the deposits under the sea there
are more than a million tons of radium.
RADIOACTIVE MATTER IN THE AIR
It has been known for some time that a charged body
surrounded by air may lose its charge rather more rapidly
^ Amer. Joum. Sd., 17-, 97 (1904).
l86 THE ELECTRICAL NATURE OF ICATTER
than can be accounted for by the leak through the sup-
ports. This would indicate that the air is ionized to some
The cause of this ionization remained for a long time
unknown, and, indeed, has only recently been discovered.
After the discovery of radium and its comparatively wide
distribution, it occurred to Elster and Geitel that radium
might be present in small quantity in the air, and if so,
this would account for the ionization and conductivity of
the air. They undertook to test the atmospheric air for the
presence of radioactive matter, and in the following manner.
It had already been shown by Rutherford that a nega-
tively charged wire, suspended in the presence of the emana-
tion from radium or thorium, would collect upon it the
radioactive decomposition products of the emanation.
Elster and Geitel,^ utilizing this fact, exposed a long wire
charged to a high negative potential to the air, and then
tested it for the presence of radioactive matter.
After the wire had been thus exposed for several hours,
it was placed in a closed vessel with a charged electroscope.
The latter was discharged much more rapidly than nor-
mally, showing the presence of radioactive matter upon the
wire, which ionized the gas around the electroscope.
The presence of radioactive matter upon the wire was
further shown by rubbing the wire with a piece of drying
paper that had been dipped in hydrochloric add. The
paper became quite strongly radioactive. When a long,
negatively charged wire was suspended in air that had
remained undisturbed for some time in contact with the
earth, as in certain caves Geitel ^ showed that enough
radioactive matter was deposited upon the wire, which,
^ Phys. Zeit., a, 590 (1901).
« Ibid,, 3, 76 (1901).
WIDE DISTRIBUTION OF RADIOACTIVE MATTER 187
when removed by a piece of leather moistened with am-
monia, produced a visible phosphorescence in barium plati-
nocyanide when the salt was brought near to it. This
radioactive matter also exerted an action on a photographic
plate, and photographs were obtained by Geitel by means
The same experimenter studied the rate at which the
radioactive matter upon the negatively charged wire under-
went decay. It was found to decay like the radioactive
matter deposited from the radium emanation.
If the wire was charged positively no radioactive matter
was deposited upon it. Since the radioactive matter was
drawn to, and deposited upon a negatively charged wire,
and not upon a positive wire, we must conclude that the
radioactive matter in the air is charged positively.
All of these facts point to one conclusion. The radioactive
matter in the air comes from the radium emanation. This
shows that radium emanation is present in the atmosphere.
The amount of rs^diiun emanation in the air varies greatly
in different localities. In certain cases the radioactivity
of the air is relatively great, as has already been stated.
The amount of radiimi emanation in the air in some locali-
ties is more than a dozen times as great as in other regions.
Certain experiments made in northern Norway would
seem to show an abnormally great amount of radium emana-
tion in the air in that region.
Since the radium emanation in the air probably comes
from radium in the soil, the amount of the emanation in the
air in any large locality may be taken as a rough index of
the amount of radium in the soil in that locality. This is,
of course, only an approximate relation, imless frequently
repeated tests were made, since the winds shift the air so
frequently from one region to another.
i88 THE ELECTRICAL NATURE OF MATTER
Elster and Geitel ^ found that the radioactivity of the air
not only changed from one locality to another, but was not
constant in any given locality. It varied with a number of
conditions. On cold, frosty mornings the activity was
unusually high. The lower the barometer the greater the
induced radioactivity in the air in any given region. This
is just what would be expected if the radioactive matter
in the air came from radium in the earth. The radium
emanation, being a gas, diffuses from the earth in which
it is formed from the radium present there, into the at-
mosphere. The lower the barometric pressure the more
emanation will pass out of the fissures and fine pores in the
earth into the atmosphere. Since the radioactive matter
in the air comes from the radium emanation, the lower the
barometer the more radioactive matter present in the air.
All of these facts point to the same conclusion, which is
that already stated, that the air contains a form of radio-
active matter. This conclusion is still further confirmed
by the following facts:
If the air contains radioactive matter, we might expect
that some of it would be carried along with objects moving
Fortunately the means for testing this conclusion are
supplied to us by nature. When drops of rain or flakes of
snow fall through the atmosphere, they might be expected to
carry down, with them some of the radioactive matter in the
air. This has been tested by C. T. R. Wilson ^ in England,
and in the case of snow by Allan' in Canada. Wilson
found that freshly fallen rain showed the presence of quite
an appreciable amount of radioactive matter. This radio-
activity, however, rapidly decayed.
» Phys. Zeit., 4, 522 (1903).
«Cam. Phil. Soc. Proc., 11, 428; la, 17 and 85 (1902-1903).
3 Phys. Rev., 16, 237, 306 (1903).
WIDE DISTRIBUTION OF RADIOACTIVE MATTER 189
If barium chloride is added to freshly fallen rain, and
the barium precipitated by sulphuric acid, the barium
sulphate that is thrown down is quite radioactive, showing
that the radioactive matter in the water is carried down
with the precipitate.
Both Wilson and Allan found that newly fallen snow
was radioactive. When a considerable quantity of the
snow was melted and the resulting water evaporated, a
radioactive residue was left behind.
The radioactivity, however, rapidly decayed, as in the
case with the freshly fallen rain. All of the above facts
taken together leave no reasonable doubt as to the presence
of radioactive matter in the air.
IS MATTER IN GENERAL RADIOACTIVE?
Having foimd a number of chemical elements that are
radioactive, and having shown that these are radioactive to
such different degrees, the question naturally arises, Are there
not other substances that possess radioactivity? It is possible
that there may be a large number of the chemical elements
that are feebly radioactive, or all matter might be radioac-
tive to some slight extent, as has already been mentioned.
The first experiments bearing upon the broad question
were those of Mme. Curie, and these gave negative results.
She examined a large number of the chemical elements
for radioactivity, and fotmd it manifested only by those
already considered. The question in this connection is
whether the method employed by Mme. Curie was suffi-
An exactly opposite result has since been obtained by a
number of investigators, and especially by Strutt.^ It
seems now to be fairly well established that some forms of
1 Phil. Mag., 5, 680 (1903).
I90 THE ELECTRICAL NATURE OP MATTER
ordinary matter are radioactive to a very slight extent,
but unquestionably radioactive.
Campbell and Wood^ have found that potassium and
rubidium salts give out ionizing rays which resemble the
/8 rays of uranium, and they conclude from their work
that potassium has about one-thousandth the radio-
activity of uranium. These results have been confirmed
by the work of Levin and Ruer.^
McClennan and Kennedy * investigated a large number
of potassiimi salts and found that they were all radio-
active. Calcium and rubidium salts were slightly radio-
active, while salts of Hthiimi, radium, and ammonium
showed no radioactivity.
Strong* foimd that various salts of potassium, rubidiiun
and erbiimi are radioactive.
It has been pointed out that subatomic changes might
go on in the atoms and a-like particles be expelled with
velocities too small to be detected by the usual methods;
i.e., with velocities below the critical. In such cases it would
be expected that the minerals of these elements would
contain accumulated helium. Strutf^ examined a large
number of minerals and found that all of the helium pres-
ent could be accounted for by the radium uranium and
thorium in them (beryl being, however, an exception).
This would argue against the radioactivity of matter in
THE ORIGIN OF RADIUM
We have seen that radium is unstable, undergoing con-
tinual decomposition. From the rate at which radium
1 Proceed. PM. Soc., Camb., 14, I, 15 (1907).
2Phys. Zeit, April 15 (1908).
» Phil. Mag., Sept. (1908).
* Amer. Chem. Journ., 42, 147 (1909).
« Proceed. Roy. Soc, A, 80, 572.
THE ORIGIN OF RADIUM 191
13 decomposing, it has been pointed out by Rutherford that
if the whole earth were pure radium, a few thousand years
hence it would have only the radioactivity of pitchblende.
Since many of the minerals that contain radium have
existed much longer than the above period, it is obvious
that radium must be produced from something, or all of
the radium would long since have disappeared. The inter-
esting and important problem is, then, to find out what
is the source of radium; from what substance or substances
it is produced.
Since radium occurs in uranium minerals, it was early
suspected by Rutherford that radium might be produced
Soddy, working with Rutherford, took up this problem
and published his results in 1904.^ A kilogram of uranium
nitrate was freed from radiimi xmtil it contained less than
10-^3 grams, which was the smallest quantity that could be
detected by the electroscope. The uraniimi nitrate was
then allowed to stand for twelve months, and was tested
again for radium. Soddy points out that the presence of
radium in the laboratory renders the electroscopes in-
capable of detecting such minute traces of radium as they
otherwise could do. He, however, feels justified in stating
that the amount of radium in the kilogram of uranium
nitrate, after it had stood for a year, was less than 10-"
grams. Soddy concludes that this settles the question as
far as the production of radium from uranium is concerned.
Uranium cannot be regarded as the parent of radium,
since from the above result, if any radium is produced
from uranium, less than one ten-thousandth of the theo-
retical quantity necessary to maintain the present condi-
tion of equilibrium is produced.
»Nat., 70, 30 (1904).
192 THE ELECTRICAL NATURE OF MATTER
Soddy recognizes that if substances intermediate between
uranium and radiimi were formed, his result could be
explained. He, however, thinks that such assumptions
are not justified.
Just a week prior to the publication of the paper by
Soddy in Nature^ a short article appeared in the same
journal by Whetham,^ in which he stated that he had
examined several specimens of uranium compounds, which
had been preserved in the laboratory from seventeen to
twenty-five years. A larger amount of radium emanation
was obtained from these old specimens than from more
recently prepared samples of these same uraniiun com-
This observation was, to say the least, suggestive, and
made it highly desirable that more work should be done
along this same line.
About this time a suggestion was made by Joly,^ which
is well worthy of serious consideration. Joly suggested
that instead of radium being a disintegration product of
uranium or thorium, it may be produced by the interaction
of some of the radioactive substances with the non-radio-
active constituents of pitchblende. Radium would then
be a product of synthesis from simpler things.
This suggestion of Joly is especially important if it should
be shown that the atomic weight of radium is greater than
that of thorium or uranium. We should naturally expect
these substances, in breaking down, to yield products with
smaller atomic weights than their own. If radium has a
larger atomic weight than either of these radioactive ele-
ments, it is a little difficult to see just how it could be formed
as the direct result of their disintegration. It might, how-
1 Nat., 70, 5 (1904).
*/Wrf., 70, 80 (1904).
THE ORIGIN OF RADIUM 193
ever, be produced by the recombination of certain of the
decomposition products of these elements with one another,
or, as Joly suggests, by the combination of these with other
substances occurring in the pitchblende.
Some light has been thrown by McCoy ^ on the possible
origin of radium. He pointed out that if radium is a
decomposition product of uranium, all uranium minerals
must contain radium, and in quantities proportional to
the amounts of uranium in the minerals. Since all in-
termediate products, such as uranimn X, the radimn
emanation, etc., are present in these minerals in quanti-
ties proportional to the total amounts of uranium, it
follows that the total radioactivity of every natural luranium
ore is proportional to the amoimt of uranium contained
McCoy analyzed a number of uranium ores from different
localities, and determined their radioactivities by means
of the electrical method. He found that the activity of all
uranium ores, which did not contain appreciable quantities
of thorium, was directly proportional to the amount of
uranium contained in them. In other words, the radio-
activity of any given quantity of uranium ore, divided
by the percentage of uranium contained in it, is a constant.
This cotistant was termed the activity coefficient.
It was further shown that the radioactivity of chemi-
cally prepared uranium compounds is directly proportional
to the amount of uranimn contained in them. Such com-
pounds also have a constant activity coefficient.
More elaborate experiments on this same problem have
been made by Boltwood,^ who arrived, however, at essen-
* Ber. d. deutsch. chem. Gesell., 37, 2641 (1904).
*Amer. Joum. Sci., 18, 97 (1904); PhU. Mag., 9, 599 (1905); Nat, 70,
194 THE ELECTRICAL NATURE OF MATTER
tially the same result. The amount of radium contained
in the uranium minerals was determined by measuring
electrically the emanation that is given off when a weighed
quantity of the mineral is dissolved or decomposed, and the
solution boiled or allowed to stand in connection with a
closed glass vessel. We can measure the activity of the
emanation very accurately, and this furnishes us with a
reliable means of measuring the amount of radiiun in a
given substance if there are no other emanating sub-
stances present. If we simply wish to determine the rela-
tive amounts of radium in any two substances, it is only
necessary to measure the activity of the emanation pro-
duced by equal weights of these substances.
Boltwood used an improved method for analyzing the
uranium minerals, which is obviously very important.
The results that he obtained for somewhat more than
twenty uranium minerals are quite satisfactory, pointing
conclusively to the proportionality between the amount
of radium in the mineral and the amount of uraniiun
To give a more exact idea as to the meaning of this rela-
tion, Boltwood divided the amount of radium in the mineral
by the amount of uranium, to see whether the ratio would
be constant for the different minerals. The author con-
cludes that from his results there is direct proportionality
between the quantity of uranium and the quantity of radium
in the minerals, and that radium is formed from uranium.
He points out that certain of the methods that have been
used for determining uranium quantitatively are defective,
which is obviously a matter of the greatest importance in
the present connection.
Experiments similar to those of Soddy were carried out
by Boltwood, to see whether radium is produced directly
THE ORIGIN OF RADIUM I95
from uranium. He comes to the same conclusion as Soddy,
— that it is not. He agrees with the suggestion of Ruther-
ford, that probably one or more intermediate products
exist between the uranivun atom and the radium atom.
Such products, however, have not yet been discovered,
unless the suggestion of Rutherford, that possibly actinium
is such a product, is correct.
In a quite recent paper, Rutherford and Boltwood*
point out that as the amount of radium in uranium minerals
is proportional to the amount of uranium present in those
minerals, the amount of radium to the gram of uranium in
the mineral should, of course, be a constant; The value
of this constant can easily be calculated, if the relative
radioactivity of pure uraniimi and pure radium is known.
To determine the amount of radium occurring in the mineral
with say one gram of uranium, they compared the radio-
activity of the emanation from the standard amount of
pure radium bromide, with that from the mineral contain-
ing a known quantity of uranium.
They found that the amount of radium to one gram of
uranium in uranium minerals is about 7.4 Xio"' grams.
One part of radium, therefore, occurs with about 1,350,000
parts of uranium.
From these data it is easy to calcxilate the amount of
radium occurring in uraniimi ores. They find that in a
ton of pitchblende containing sixty per cent, of uranium,
which is a rich uranium ore, there is about 0.4 gram of
radium. Lower grades of pitchblende, which contain less
uranium, will contain proportionally less radium.
Boltwood also took up the earlier work of Soddy, in
which the latter came to the conclusion that radium is not
formed from uranium, because uranium nitrate which had
* Amer. Joum., Sci., 20, 55 (1905).
196 THE ELECTRICAL NATURE OF MATTER
Stood for a year or so did not contain any appreciable quan*
tity of radium.
He^ repeated the experiment of Soddy and obtained
similar results. A comparatively large quantity of ura-
nium nitrate was carefully purified by recrystallization.
One hundred grams were dissolved in water and the solu-
tion sealed up in a bulb. After standing thirty days the
bulb was opened and all gases removed from the solution
by boiling. All the gases removed from the solution were
brought in contact with an electroscope. It was found
that the amount of radium present in the uranium at the
start was less than 1.7 Xio"" grams. The uranium solu-
tion was again sealed up in the bulb and allowed to remain
for six months. The amount of radium present was again
tested and found to be less than 5.7 Xio"^^ grams.
After 390 days the test was repeated, and with the same
result; the amount of radium present in the solution still
being less than 1.7 Xio"" grams. If any radiimi was
formed from the uranium during this period, the above
results show that less than one sixteen-hundredth of the
quantity required by theory was produced.
These results woidd seem to show pretty conclusively
that radium is not formed directly from uranium. The work
of McCoy and Boltwood, however, estabUshes a propor-
tionality between the amount of radium in uranium ores,
and the amoimt of uranium contained in them. Taking
all these facts into account, we must conclude that uranium
is the parent of radium, but that the latter is not formed directly
from the former. One or more intermediate products with a
slow rate of change must be formed. These on breaking
down yield radium directly or indirectly.
The above historical treatment of the attempts to dis-
* Amer. Joum. Sci., 20, 239 (1905).
THE ORIGIN OF RADIUM 197
cover the immediate parent of radium is preserved on
account of its historical interest. The direct parent of
radium has now probably been discovered. Boltwood^
announced near the close of 1906 that he had obtained
from a uranium mineral a small quantity of a substance
which he supposed was actinium, and in which the amount
of radium present was more than doubled in six months.
Rutherford ^ showed that this substance could not be
actinium, since it was not suflSdently radioactive; Ruther-
ford^ a little later succeeding, in separating from his
actinium by means of hydrogen sulphate, a portion which
was about one himdred times as radioactive as the actin-
ium, showing that the parent of radium was chemically
quite different from actinium, being more readily carried
down in the above precipitation.
Radioactinium, the cause of the activity of actinium, is
more concentrated in the precipitate formed as described
above, doubling the activity in twenty days, but this was
shown not to be due to the growth of radium. Boltwood *
showed that his new substance did not produce actinium
emanation, nor actinium X, nor did the above precipita-
tion carry down any radioactinium from a preparation
that had stood for five months.
The rate at which the new substance produced radium
was constant for five hundred days, which shows that its
rate of decay is slow, being about twelve years. It is pro-
duced from uranium X at a correspondingly slow rate.
1 Nat., 75, p. 54.
^Ibid., 75, 270.
8 Nat., 76, 126. Phil. Mag., 14, 733.
* Nat., Sept. 26 (1907). Amer. Joum. Sci., 24, 370.
198 THE ELECTRICAL NATURE OF MATTER
Boltwood called this substance ionium from its ionizing
power. Chemically it is closely allied to thorium, while
actinium more closely resembles lanthanum. No method
has yet been discovered for separating it from thorium.
Ionium gives out a rays of small penetrating power.
They have a range of only 2.8 centimetres of air. The j8
radiations if any are present, are also easily absorbed. Its
activity is only about 0.7 of that of radium in equilibriimi.
It gives off no emanation.
Marckwald and Keetman^ confirm all of the observations
made by Boltwood. They obtained the ionium from the
pitchblende in the following manner: The mineral was
dissolved in nitric acid, the nitrates converted into sul-
phates by means of sulphuric acid. The sulphates of lead,
barium, and radium were filtered off. Hydrofluoric acid
was then added and this gave a precipitate of the fluorides
of cerium, yttrium, and thorium. These were dissolved in
sulphuric add and the thorium precipitated by means of
oxalic acid. Ionium resembling thorium so closely in its prop-
erties is precipitated along with the thorium, from which
no method has thus far been devised for separating it.
Hahn^ reached the same conclusion in an entirely dif-
ferent way. He found that commercial thorium salts
contain considerable quantities of radium, although the
thorium came from monazite sand which contains only a
Uttle uranium. The amount of radium in the thorium salt
increased with the age of the salt. The amount of radium
in a freshly prepared specimen of thorium nitrate was
found to double in two months.
^ Ber. d. deutsch. chem. Gesell., 41, 49 (1908).
THE ORIGIN OF RADIUM
Taking into account all of the above evidence, there
seems to be no reasonable doubt that ionium exists and
is the direct parent of radium.
THE COMPLETE SERIES OF TRANSFORMATIONS IN WHICH
RADIUM IS INVOLVED
The complete series of transformations, starting with
uranium and ending with lead, is given in the following
table. The kind of radiations given off at the stage of the
transformation is also shown:
Radium C I ^^
Radium F )
Time of half decay I
Cind of rays
6 X 10® years
2CX> THE ELECTRICAL NATURE OF MATTER
During the last year or two a number of articles have
appeared on a supposedly new radioactive substance called
emanium. It was discovered in pitchblende by Giesel,
and was found to be related chemically to the elements of
the cerite group, and especially to lanthanum and cerium.
The dehydrated chloride or bromide shows a discontinu-
ous phosphorescent spectrum of three lines. Glass in which
the substance was preserved for some months was colored
viblet. Paper was browned and decomposed. After the
maximum activity was reached the activity of the solid
substance underwent no further change. Giesel ^ con-
cluded in his earlier work that this substance is a hew radio-
active element. He thought that the results could not be
accounted for as due to any induced activity resulting from
contact with radium. When a current of air was blown
over the preparation of the supposedly new substance and
then against a phosphorescent screen, bright scintillations
or sparks made their appearance, that were more distinct
and larger than in the case of radium, and the effect was
more striking than in the ordinary spinthariscope.
This strongly radioactive substance, supposed by Giesel to
be a new radioactive element, was named by him emanium.
He, however, pointed out about a year ago that it was
possible that emanium was identical with the actinium
discovered by Debieme. At that time, however, there was
not suflScient known about the properties of the two sub-
stances to determine whether they were identical or not.
Debieme imdertook a comparative study of actinium
and emanium, and concluded that the two were identical.
Giesel, however, points out that there are certain differences
* Ber. d. deutsch. chem. Gesell., 37, 1696 and 3963 (1904); 38, 775 (1905).
THE ORIGIN OF RADIUM 20I
in the properties of the two substances that need explana-
tion before we can regard the two as identical. The induced
radioactivity produced by emanium falls to half-value in
344 minutes, while that of actinium requires 40 minutes to
decay to half-value.
He also points out that the three lines observed in the
phosphorescent spectnim of emanium, having the wave-
lengths 4885.4, 5300, and 5909, respectively, had not at that
time been observed in actinium. Further, since these
lines could not be identified with those of any known ele-
ment, it seemed fair to conclude that they were due to a
Giesel studied the activity of emanium, and showed that
the emanation was not driven out by heating or solution as
with radium, and concluded that there was a solid, non-
volatile substance formed.
Subsequent work, however, has shown that the three
lines mentioned above are really not new lines at all, which
can be referred to a new element, but were produced by
one of the didymia that was present. This invalidates one
of the lines of reasoning which led Giesel to conclude that
he was dealing with a new substance. He separated the
active constituent or constituents from emanium, in a man-
ner analogous to that employed by Rutherford in the case
of thorium. He found most of the activity of the emanium
in the small residue that remained when the solutions
containing emanium were precipitated with ammonia. On
account of the analogy with thorium X, Giesel termed
this active residue emanium X.
He showed further that when emanium X has been sepa-
rated from emanium, more emanium X is conlinuaUy being
formed. This again is strictly analogous to the condition of
things in thorium. It was also established that most of the
202 THE ELECTRICAL NATURE OF MATTER
activity of emanimn is due to the emanium X that is present
The question as to the identity of emanium and actinium
was taken up quite recently by Hahn and Sackur.^ It will
be recalled that the argument advanced by Giesel based
upon spectnmi analysis, in favor of the two substances
being different, had been shown to be untenable — the lines
supposed by him to be produced by emanium being really
those of one of the didymia.
The second argument advanced by Giesel to show that
these two substances are different was based upon the
different amounts of time required for the induced radio-
activities produced by the two substances to decay to half
their initial value. These measurements have been repeated
by Hahn and Sackur, with the result that the amoimts of
time required in the two cases are the same.
These authors have also determined the amoimt of time
required for the emanation itself from the two substances to
decay to half-value. They find that the time in the two
cases is exactly the samey to within the Umits of experimental
From these facts they conclude that the actinium of
DeUerne and the emanium of Giesel are probably identical.
New light seems to have been thrown on the relation
between actinium and emanium by Marckwald.^ He
thinks that he has satisfactorily solved the problem. The
rare earths obtained from the radium mother-Uquor were
transformed into chlorides, and the thorium predpitated
by thiosulphate. This thorium showed strong emanating
power and contained the actinium of Debierne. From the
solution cerium was first precipitated, and then the didymia
* Ber. d. deutsch. chem. GeselL, 38, 1943 (1905).
^Ihid,, 38, 2264 (1905).
THE ORIGIN OF RADIUM 203
and lanthanum as oxalates, which were transformed into
oxides. Neither the cerium nor the mixture of the didymia
and lanthanum showed any considerable emanating power.
The thorium was then piuified by subjecting it to a num-
ber of processes, but the emanating substance climg to the
thorium in all of these operations.
The activity of this actinium which accompanied the
thorium was studied for several months and was found to
decrease. The mixture of the didymia and lanthanum,
on the contrary, acquired greater and greater emanating
power with time — their emanating power increasing in the
same ratio as that of the actinium in the thorium decreased.
The author points out that this is analogous to the case of
thorium and thorium X.
The explanation of these facts seems very simple. The
radioactive substance that accompanies the lanthanum
gives off no emanation. It, however, decomposes into a
second substance, which in its chemical reactions resembles
thorium. When the latter substance undergoes further
decomposition a strong emanation results.
To test the correctness of this interpretation the follow-
ing experiment was performed: A half -gram of pure
thorium oxide was added to eighteen grams of the didymia-
lanthanum mixture, which had stood until it emanated
strongly. The whole was then dissolved in hydrochloric
acid and the thorium again precipitated by thiosulphate.
The thorium precipitated now contained nearly all the
emanating power; the solution of the didymia-lanthanum
mixture contained very little of the emanation.
The conclusion seems necessary that there is something
in the didymia-lanthanum mixture which yields a sub-
stance closely allied chemically to thorium, and which has
strong emanating power.
204 THE ELECTRICAL NATURE OF BfATTER
Emanium and actinium are, then, not identical. Ema-
nium undergoes decomposition and yields actinium — ema-
nium is the parent of actinium.
ATOMIC WEIGHTS OF RADIOACTIVE LEAD FROM
Richards and Lembert ^ have obtained radioactive lead
from a number of different sources; have purified these
materials, and have determined their atomic weights by
the same methods. They obtained the following results:
Lead from North Carolina uraninite 206.40
Lead from Joachimsthal pitchblende 306.57
Lead from Colorado camotite 206.59
Lead from Ceylonese thorianite 206.82
Lead from English pitchblende 206.86
Common lead 207.15
All of the radioactive specimens of lead had a lower
atomic weight than ordinary lead. The atomic weight
was found not to be proportional to the radioactivity of the
lead in question.
The ultraviolet spectrum of radioactive lead Was shown
to be identical with that of ordinary lead.
"The inference seems to be that radioactive lead con-
tains an admixture of some substance different from or-
dinary lead, and very difficult to separate from it by
This substance either has the same spectrum as lead, or
no spectrum in the ultraviolet where lead has a spectrmn,
or its spectrum is masked by lead.
The atomic weights of a number of other elements from
diflferent sources, were determined. These include copper,
^ Joum. Amer. Chem. Soc., 36, 1329 (1914).
THE ORIGIN OF RADIUM 205
silver, iron, sodium and chlorine. The atomic weight of
each of these was found to be constant, independent of the
source from which the element came.
The investigations, of which a general accoxmt has been
^ven in these chapters, mark a new epoch in the develop-
ment of the physical sciences. Some of the results obtained
are as important from the standpoint of the physical chemist
as from that of the physicist. Facts have been brought to
light which are of a character that are very different from
an3rthing hitherto known. The existence of extremely pene-
trating forms of radiation, the instability of the chemical
atom, the formation of one elementary substance from another,
the existence of a form of mailer that can charge itself elec-
trically, that can light itself, and that can give out an amount
of heat that is almost inconceivably great, are some of the
facts to which we must now adapt ourselves.
These are magnificent developments with which to open
the ixew century. Probably still more surprising facts are
awaiting men of science before its close. It seems not too
much to predict that as the nineteenth century surpassed
the preceding eighteen in the development of scientific
knowledge and the discovery of truth, just so the twentieth
century will exceed them all in the gifts of pure science
to the store of human knowledge. The wave of sdefntific
investigation for its own sake, that has recently swept over
the entire civilized earth, must yield a rich harvest to those
. who shall be permitted to reap it.
Actinium, 51, 65.
decomposition products of, 156.
Allan, radioactivity of the air, 188.
Alpha particle, action on a photo-
graphic and on a fluorescent
particle, ratio of — for the, 74.
particles are probably helium
particles, critical velocity of the,
particles produce delta particles,
particles stopped by matter, 84.
rays, 70, 72, 96.
Anions and cations in terms of the
electron theory, 36.
Atomic weight of radium, 57.
Atomic weights of radioactive lead,
Atom, nature of in terms of the
electron theory, 29, 35, 40.
not the same mass as the ion, 37.
Thomson's conception of the, 31.
Becquerel ray, 44.
rays, properties of, 47.
Becquerers theory of spinthariscope,
Beta and gamma rays, 85.
particle, determination of — for
particle, mass of, 89.
particles, charge carried by, 86.
rays, 70, 85, 96.
rays from radium, 89.
Boltwood, on the discovery of ion-
on the origin of radium, 193,
Bragg and Kleeman, alpha particles
stopped by matter, 84.
and Kleeman, on the alpha
particles given off by radiimi,
Bumstead and Wheeler, radioactive
matter in tap-water at New
Bunsen ice calorimeter used to
measure heat liberated by
radium salts, 109.
Campbell and Wood, on radio-
activity of potassium and
rubidium salts, 190.
Canal rays, 15.
Cathode particle, value — for the, 5.
rays, properties of, 89.
Cations and anions in terms of the
electron theory, 36.
Charge carried by beta particles, 86.
on a gaseous ion, comparison
with that on a univalent ion
of an electrolyte, 13.
to the mass for the positive ion,
ratio of, 15.
to the mass of the ion in a gas
ratio of, 3.
Chemical effects produced by radio-
active substances, loi.
reactions differ from radioactive
Conducting gas, how it differs from
a non-conducting, 2.
Conductivity, electrical, of gases, i.
of dielectrics, radium increases
of gases, conditions which in-
crease the, I.
Corpuscle, 15, 22.
nature of, 19.
Critical velocity of the alpha parti-
Crookes, an cathode rays, 4. '
on the spinthariscope, 78.
Curie and Deslandres, helium pro-
duced from radium, 135.
M. and Laborde, show that
radiiun produces heat, 106.
Mme. and M. on the chaige
carried by beta particles, 86.
Mme. and M., radium charges
itself positively, 88.
Mme. and M., i^ow that beta
particles carry negative
Mme., discovers polonium, 50.
Mme., discovers radiiun, 49.
Mme., Ivuninosity of ratdium
Mme., on polonium, 152.
Mme., on the atomic weight of
radium, 57, 62.
Mme., "residual activity," 180.
Mme., separated radium from
Mme., separates radium, 51.
M., studied the radiations from
Curies discover induced radioactiv-
on radioactivity of matter in
phosphorescence produced by
radium salts, 99.
studied chemical efifects pro-
duced by radioactive sub-
Debieme discovers actinium, 51,
on emanium, 200.
Decay of induced radioactivity,
of the emanation, 129.
Decomposition products of radio-
active substances, 153.
Delta particles produced by alpha
Demarjay, on the spectrum of ra-
Dewar and M. Curie measured heat
liberated by radium, by
means of liquid hydrogen,
on the rate at which heliiun is
produced from radium, 82.
Dielectrics, radium increases the
conductivity of, 100.
Discovery of radium, 49.
Distribution of radioactive matter,
Dobereiner triads, 26.
Duane, on the alpha particles, 81.
Earth's age, effect of heat liberated
by radium on the calculated,
Electrical charge, radiiun produces
on itself an, 117.
conductivity of gases, i.
theory of matter, 19.
Electrons, groups of, 180.
Electron Aeory and radioactivity,
theory and the Periodic System,
theory, cations and anions in
terms of the, 36.
theory, nature of the atom in
terms of the, 29, 35.
the ultimate unit of matter, 23.
Elster and Geitel on radioactive
matter, distribution of, 183.
and Geitel, radioactivity of the
air, 186, 188.
Emanation, amount of, 121.
and helium, relation between,
decay of, 129.
deposits radioactive matter,
properties of this matter,
discovered by Rutherford, 48.
from radioactive substances,
heat evolved by, 129, 130.
heat produced by the, 148.
helium produced from, 130.
helium produced from the, 127.
may effect transmutation of
certain elements, 138.
method of obtaining, 119.
molecular weight of, 123.
nature of, 122.
of thorium, 119.
produces induced radioactivity,
X, 147, 201.
Emanating power of radium, re-
covery of, 128.
Emanium, 200, 201.
Fluoroscopic method of studying
Gamma rays, 70, 93, 96.
ra)rs, tieory as to the nature of,
Gas conducting, how it differs from
a non-conducting, 2.
Gaseous ions produced by different
means, — for the, 8.
Gases, conductivity of, conditions
which increase the, i.
determination of the mass of
the negative ion in, 10.
electrical conductivity of, i.
— constant for different, 7.
Geiger and Rutherford, counted the
alpha particles, 82.
Generalization, importance of, 170.
Giesel on emanium, 200.
on emanium X, 201.
studied the action of the mag-
netic field on the radiations
from radium, 69.
Godlewsky, on actinium X, 159.
Goldstein discovered canal rays, 15.
Hahn, on the origin of radium, 198.
Heat evolved by the emanation, 129,
given off by radiimi, effect on
the calculation of the age of
the earth, 114.
liberated by radium, amoimt of,
liberated by radium, calcula-
tion of the amount of, 117.
liberated by radium measured
by the Bunsen ice calorim-
liberated by radimn, measure-
ment of the, 106.
produced by radium, 117.
produced by radium salts, 106.
produced by radium, source of,
produced by the emanation,
produced by radium, theories
as to the source of, J15.
Helium and emanation, relation
atoms, alpha particles are prob-
discovered in the sun by Lock-
yer, on the earth by Ramsay,
from radium, further experi-
produced from the emanation,
Induced radioactivity, 66, 140.
radioactivity due to deposit of
radioactive matter, 144.
radioactivity produced by the
radioactivity imdergoes decay,
Ion gaseous, comparison of the
charge on, with that on a
univalent ion of an elec-
in gases, determination of the
mass of the negative, 10.
not the same mass as the atom,
positive, ratio of the chaige to
the mass for the, 15.
Ionization method of studying
Ions gaseous, produced by different
means, — for the, 8.
of electrolytes, — different for
the different, 7.
Joly, on the origin of radium, 192.
radioactivity and geology, 184.
Kaufmann, on the velocity and
mass of the beta particles
from radium, 89.
on the electrical origin of mass,
Kaufmaim, on the vdodty of the
different beta partides from
Kleeman and Bragg, on the alpha
partides given off by radium,
Laborde and M. Curie show that
radium produces heat, 106.
Lead radioactive, atomic weights of,
Lenard rays, 8.
Lockyer first discovered helium in
the Sim, 132.
Luminosity of radium compounds,
of radium salts, 117.
Mackenzie, on the ratio — for the
alpha partides, 76.
Madsen, on the gamma rays frcm
Makower determined the molecular
weight of the emanation, 125.
on stopping of beta partides, 93.
Marckwald on emanlum, 202.
on polonium, 64.
on the origin of radium, 198.
Marignac, on Prout's hypothesis, 25.
Mass of an ion not the same as that
of the atom from which it
is formed, 37.
of beta particles, 89.
of the negative ion in gases,
determination of the, 10.
of the positive ion, ratio of the
change to the, 15.
to charge of the ion in a gas,
ratio of, 3.
Matter, earlier attempts to unify, 24.
dectrical theory of, 19.
in general imdergoing trans-
the electron the ultimate imit
McClelland and Hackett, on the
stopping of beta particles, 92.
on secondary radiations, 92.
McCoy, on the origin of radium, 193.
M. Curie and Dewar measured
heat liberated by radium, by -
means of liquid hydrogen, 107. |
Methods used in studying radio-
Mendel^eff's Periodic System, 26.
Meyer Lothar, Periodic System, 26
Molecular weight of the emanation^
Negative ion in gases, determina-
tion of the mass of, 10.
Newland*s Periodic System, 26.
Origin of radium, 183.
Ostwald, on the overUirow of sden-
tific materialism, 23.
Oxygen transformed into ozone by
Ozone produced from oxygen by
Partides, total number shot off by
Periodic System, electron theory
and the, 31.
Phosphorescence produced by ra-
dium salts, 99.
Photographic method of studying
Physiological action of the radia-
tions from radium, 104.
Pitchblende, radioactive substances
radium separated from, 50.
the source of radium, 49.
Polonium, 50, 63.
action of on a photographic
Positive ion, ratio of the chaige to
the mass of the, 15.
Precht and Runge on the atomic
weight of radium, 58.
Properties of the alpha, beta, and
gamma rays, 96. %
Prout's hypothesis, 24.
Radiation from thorium, 47.
Radiations from radioactive sub-
stances, properties of the, 68.
other properties of the, 98.
Radioactive matter deposited by the
emanation, properties of, 145.
matter, distribution of, 183.
matter in earth and sea, 183.
matter produces induced radio-
substances in pitchblende, 63.
substances produce chemical
substances, properties of the
radiations given out by, 68.
transfonnations differ from
chemical reactions, 177.
electron theory and, 40.
induced, 66, 140.
matter in the air, 185.
methods used in studying, 66.
emanation in the air, 187.
of matter in general, 189.
theory of, 176.
theory of Thomson, 179.
Radium A, 150.
F, 151, 152.
amount of heat liberated by,
atomic weight of, 57.
charges itself electrically, 117.
complete transformation prod-
ucts of, 199.
compounds, luminosity of the,
decomposition products, 152.
discovery of, 49.
does it exist in the sun, 113.
emanation may effect transmu-
tation of certain elements, 138.
facts in connection with, 174.
from pitchblende, separation of,
heat produced by, effect on
source of solar heat, 112.
heats itself, 117.
increases the conductivity of
measurement of the heat liber-
ated by, 106.
origin of, 183, 190.
physiological action of the
radiations from, 104.
produces ozone from oxygen,
salts, phosphorescence produced
salts, production of heat by,
self-liuninosity of , 117.
separated by Mme. Curie, 51.
spectrum of, 56.
total number of particles shot
off by, 97.
Ramsay and Soddy measured the
amount of the emanation, 121.
Ramsay first discovered helium in
the air, 132.
on radiothorium, 154.
results with radiothorium, 156.
shows that helium is produced
from the radium emanation,
shows that the helium emana-
tion may affect certain trans-
studied the properties of the
Ray, Becquerel, 44.
Rays, alpha, 72.
Regener counted the alpha particles,
on scintillations produced by
beta radiations, 92.
Rowland, on the nature of the atom,
Runge and Precht, on the atomic
weight of radium, 58.
Russell, on fogging of a photo-
graphic plate by metals, 45.
Rutherford and Barnes measured
heat evolved by the emana-
and Geiger counted the alpha
and Miss Brooks determine the
molecular weight of the ema-
and Royds, helium from alpha
and Soddy, on thorium X, 165
and Soddy study the emana-
tion from radium, 127.
and Soddy, theory of radio-
Rutherford calculated the amount of
heat liberated by radium, 117.
determines the ratio of — for
the alpha i>article, 74.
discovers the emanation, 48,
discovers thorium emanation,
on induced radioactivity, 143.
on photographic and fluorescent
on radium E, 151.
on the critical velocity of the
alpha particles, 81.
on the heat produced by the
radium emanation, 148.
showed that salts of thorium
induce radioactivity, 140.
shows that the alpha particles
are complex, 75.
studied properties of the active
matter deposited by the ema-
studied the action of the mag-
netic field on the alpha rays,
studied the effect of low tem-
perature of the production of
studied the properties of the
Schmidt, on thorium radiation, 47.
Secondary radiations produced by
beta rays, 92.
Solar heat as affected by heat given
off by radium, 112.
Spectrum of radium, 56.
BecquerePs theory of the action
theory of the action of, 79.
Stark, on positive rays, 17.
Strong, on radioactivity of potas-
sium salts, 190.
Strutt, on Prout*s hypothesis, 25.
on radioactivity of matter in
Sun, does it contain radium, 113.
Theory, importance of, 170.
of radioactive phenomena, 176.
Thomson, J. J., conception of the
determination of the mass of
the negative ion in gases, 10.
on positive rays, 17.
on the electrical theory of
on the ratio of — for different
on the value of — for the cathode
radioactive matter in tap-water
of England, 184.
shows that the alpha particles
are charged positively, 73.
theory of radioactivity, 179.
Thorium, emanation, 119.
facts in connection with, 173.
forms radiaoctive matter, 164.
recovers radioactivity, 166, 168.
X, decay of radioactivity, 165.
X from thorium emanation, 165
Thorpe, on the atomic weight of
Transformation of matter in gen-
products of radium, 199.
Transmutation of the dements, not
Unification of matter, earlier at-
tempts to affect the, 24.
Uranium radioactive, 45.
facts in connection with, 171.
recovery of activity of, 162.
X, decay of activity of, 162.
X, radiation from, 163.
Villard, on gamma rays, 93.
Watts, on the atomic weight of
radium, 58, 62.
Wilson, C. T. R. condensation of
water-vapor around ions, 10.
radioactivity of snow, 188.
X ra3rs, 41.
nature of, 42, 48
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