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THE 

ELECTRICAL NATURE 



OF 



MATTER AND RADIOACTIVITY 



■BY ^^ 

HARRY C^JpNES 

PROFESSOR OF PHYSICAL CHEMISTRY IN THE 
JOHNS ^HOPKINS UNIVERSITY 



THIRD EDITION— COMPLETELY REVISED 



NEW YORK 

D. VAN NOSTRAND COMPANY 

25 Park Place 

191S 



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•V: : Copyright, IQ06, by 
6. JVan Nostrand Company 



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D. Van Nostrand Company 



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Q 



1 



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 

iii 



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IV PREFACE 

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 
June, 1910. 

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. 



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CONTENTS 



CHAPTER I 

PAGE 

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 
m m 

e 
different gases. The ratio — varies for the different ions of 
m 

e 
electrolytes. The value of — for gaseous ions produced by differ- 
ent means. 

CHAPTER II 

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. 

CHAPTER III 

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. 

CHAPTER IV 

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. 

CHAPTER V 
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. 



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Vi CONTENTS 

CHAPTER VI 

PAGE 

The Discovery of Radium 49 

The separation of radium from pitchblende. The spectrum of 
radium. The atomic weight of radium. 

CHAPTER VII 

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. 

CHAPTER VIII 

The Alpha Rays 72 

The ratio — for the alpha particle. The mass of the alpha par- 
tn 
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. 

CHAPTER IX 

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. 

CHAPTER X 

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. 

CHAPTER XI 

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. 



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CONTENTS ' VU 

CHAPTER Xn 

PAGE 

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. 

CHAPTER XIII 

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. 

CHAPTER XIV 

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. 

CHAPTER XV 

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. 

CHAPTER XVI 

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? 



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Vm CX)NTENTS 

CHAPTER XVn 

Wide Distribution of Radioactive Mattes and the Origin of 

Radium 183 

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 



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ABBREVIATIONS OF THE TITLES OF 
JOURNALS 

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 

Gesellschaft. 
Cam. Phil. Soc. Proc. = Proceeding of the Cambridge Philosophical 

Society. 
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. 



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The Electrical Nature of Matter and 
Radioactivity 

CHAPTER I 

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 
phenomena. 



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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 
down. 

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 
connection. 

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 
lost. 

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- 



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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 

GAS 

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 — . 

PI 

We shall take up first the determination of the ratio — 

tn 

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 



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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 
magnet. 

A solid body placed in their path casts a well-defined 
shadow. 

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 
the vanes. 

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- 



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THE ELECTRICAL CONDUCTIVITY OF GASES 5 

son, by which he determined the value of — for the cathode 

tn 

particle. 

e 

THE VALUE OF — FOR THE CATHODE PARTICLE 

m 
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. 

Vffl 

We have thus determined the ratio of e to vm. 

We must now determine the value of z; in order to obtain 

the ratio — . 
m 

Into the above-mentioned vacuum tube are inserted 

iPhil. Mag., 44, 293 (1897). 



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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. 



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THE ELECTRICAL CONDUCTIVITY OF GASES 



e 

THE RATIO — CONSTANT FOR DIFFERENT GASES 

m 
Using a somewhat different method, J. J. Thomson found 

at first that the ratio — was a constant, whether the gas in 
m 

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 

e € 

the ratio — . They found the same value for — , regardless 

m m 

of the nature of the metal of which the cathode was made. 

Thomson found that the value of — was equal to about 

m 

iXioJ 

e 

THE RATIO — VARIES FOR THE DIFFERENT IONS OP 

m 

ELECTROLYTES 

It will be seen that the value of — for the ions of elec- 

m 

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 

m 



the hydrogen ion is i X lo*. 



It is therefore obvious that the value of — for the cathode 

m 



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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 
m 

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. 

e 

THE VALUE OF — FOR GASEOUS IONS PRODUCED BY 

m 

DIFFERENT MEANS 

Before taking up the beautiful method for determim'ng 
the value of the charge carried by the cathode particle, we 

4 

shall ask and answer the question whether the value of — 

nt 

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 

gas. 

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 
nt 

found to be about 4X10'. 

The value of — for the gaseous ions produced in con- 
m 

tact with incandescent metals is about 8.5 Xio*. 



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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. 



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CHAPTER n 

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 

m 

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). 
10 



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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, 

g c 

By observing the rate at which the cloud settles we arrive 

J Pha. Mag., 46, 528 (i8^<5). 



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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: 

477r* 

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- 



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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 



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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 
solution. 

The value of — for the hydrogen ion in solution is 10*. 
m 

o 

The value of — for the gaseous ion is 10^. The values of e 
m 

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 



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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 
POSITIVE ION 

Before leaving this part of our subject a few words should 

e 
be added in relation to the value of the ratio — for the 

m 

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 



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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 
result: 

- = 3 X IO^ 
m 

He also found that these positively charged particles move 
with much smaller velocity than the negatively charged 
particles. 

If we compare the value of — for the negatively charged 

m 

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 

charged particle. 

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 



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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 

m 

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 
value 10*. 
In gases at low pressures essentially the same values 

were found for the ratio — . When the pressure was low 

m 

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*. 
tn 

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- 
mentioned investigation. 

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). 



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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. 



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CHAPTER III 

Nature of the Corpuscle — the Electrical Theory 
OF Matter 

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 
valid. 

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. 

19 



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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 

m 

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 

IfYl 

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). 



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NATURE OF THE CORPUSCLE 21 



V 


e 




m 


2.36 


1-31 


2.48 


1.17 


2.59 


0.975 


2.72 


0.77. 


2.83 


0.63 



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 
becomes greater. 

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 
origin. 

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 



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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 
composed. 

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 



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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 
imagination, matter. 

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). 



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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 
experimental errors. 

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. 



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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 
into disrepute. 

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 
experimental errors. 

Such an examination was recently made by Strutt,^ who 
pointed out that the number of elements whose atomic 

* Phil. Mag., I, 311 (1901). 



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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 
mere chance. 

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- 



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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 



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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 
the standard. 

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. 



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CHAPTER IV 

The Nature of the Atom in Terms of the Electron 

Theory 

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. 

29 



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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. 



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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). 



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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 
rings. 

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 
233334455 

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 



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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. 



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34 THE ELECTRICAL NATURE OF MATTER 

Such a system would correspond to a teiravaieni negative 
element. 

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 
chemical valency. 

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 



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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, 



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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 
to discuss. 

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. 



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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 



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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. 



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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 
hydrogen ion. 

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 
remains unchanged. 

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 



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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 
are studied. 

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 
they rotate. 

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). 



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CHAPTER V 
The X-Rays 

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). 
41 



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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. 



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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 



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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- 
erally accepted. 

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 



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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 
to light. 

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 



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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. 



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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 
X-ray. 

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). 




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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 
spectrum. 

A crystal is just such a space-grating. When X-rays 
are reflected from a crystal, we have produced spectra of 
various orders. 

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 
Angstrom unit. 

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 
properties. 



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CHAPTER VI 
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- 
nium itself. 

Mme. Curie then prepared chalcolite artificially by treat- 

* Ann. Chim. Phys. [7], 30, 99 (1903). 
49 



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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- 



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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 
chemically. 

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. [7], 30, 125-127. 



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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- 



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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 
barium. 

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. [7], 30, 131 (1903). 



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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 
have overcome. 

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 



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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 
described above. 

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. 



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$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 
at all. 

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. 



Wave Length 


Intensity 


4826.3 


10 


4683.0 


14 


4533-5 


9 


4436.1 


8. 


4340.6 


12 


3814-7 


16 


3649.6 


12 



* Ann. Chim. Phys. [7], 30, 121 (1903). 



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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 
metallic uranium. 

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. [7], 30, 137 (1903)- 



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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)- 



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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 
radioactive. 

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- 



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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 
and thorium. 

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 
thirty-three units. 

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). 



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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 
errors. 

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 



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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). 



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CHAPTER VII 
Other Radioactive Substances in Pitchblende 

polonium 

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). 
63 



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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). 



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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. 

ACTINIUM 

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). 



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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 
radioactive. 

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 
substances. 

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- 
ACTIVITY 

The methods that have been employed in studying radio- 
activity are based, of course, upon the properties of the 



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OTHER RADIOACTIVE SUBSTANCES IN PITCHBLENDE 67 

radiations that are given out by the various radioactive 
substances. 

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. 



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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 
positive ion. 

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- 
ACTIVE SUBSTANCES 

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 



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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). 



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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). 



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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. 



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CHAPTER VIII 
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). 
72 



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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 
result. 

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). 



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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 
positive charges. 



e 

THE RATIO — FOR THE ALPHA PARTICLE 

m 

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 — . 

m 

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. 



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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 

PI 

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 

e 
Knowing the value of — , we have become familiar with 
m 

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'. 
m 

* Phil. Mag., 10, 163 (1905). 
^Ihid,, 10, 318 (1905). 



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76 THE ELECTRICAL NATURE OF MATTER 

e 

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 

e e 

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 

e 
recalled that the determination of — for the a particle is 

m 

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 

helium atoms. 

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 
somewhat later. 

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). 



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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 
registered photographically. 

The mean value foimd for — was 3.00X10^, varying 

c 

between the extremes 2.5 X 10^ and 3.7 X lo*^. 

The value of — found by Rutherford for radium in 
e 

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 

fWD 

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. 



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78 THE ELECTRICAL NATURE OF MATTER 

The value found for = 4.11X10^*. 

e 

The value of — = 3.00 Xio^ 

The average value of z; = 1.37 X 10® centimetres per 

second, and — = 4.6X10' electromagnetic units. 
m 

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 
from radium. 

Their velocity, however, wa& not as great as the swift- 
est a particles from radium. 



THE SPINTHARISCOPE 

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)- 



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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 



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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). 



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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- 
quently used. 

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 
8 particles. 

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). 



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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 
helium atom. 

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. 



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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 
origin. 

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- 
tillations result. 

We cannot ascribe the action of this instrument simply 



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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 
takes place. 



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CHAPTER IX 
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 
a rays. 

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 

8s 



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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- 
ing experiments. 

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 
the others. 

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. [7], 30, 155 (1903), 



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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 



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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 
tube. 

e 

THE DETERMINATION OF — FOR THE BETA PARTICLE 

m 

We have already studied the method worked out by J. J. 

Thomson for determining the ratio of — for the cathode 

m 

particle. This method, it will be remembered, is based 

upon subjecting the cathode rays to both electrostatic and 



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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 

m 

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. 
m 

He found that this value was not constant, but varied 

e 
with the velocity of the particle. The value of — increased 

m 

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 

m 



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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 
CATHODE PARTICLE 

The method for determining the mass of a particle, know- 

e 
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. 

CATHODE RAYS 

Affect the photographic plate. 

Excite phosphorescence. 

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. 



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THE BETA AND GAMMA RAYS QI 

Excite phosphorescence. 

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 
)8 particle. 

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- 
nection. 

We have learned that the cathode particles are nothing 
but electrons, or disembodied, negative electrical charges. 



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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 
particles. 

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). 



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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 
proposed. 

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 



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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. 



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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 
are present. 

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). 



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96 THE ELECTRICAL NATURE OF MATTER 

SUMMARY OF THE PROPERTIES OF THE ALPHA, BETA, AND 
GAMMA RAYS 

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 



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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. 



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CHAPTER X 

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. [7], 30, 145 (1903). 
98 



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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 
been tested. 

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 
ultra-violet hght. 

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 



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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 
the imitation. 

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. 



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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. 



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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- 
tion. 

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 



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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 
in thickness. 

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 



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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). 



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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. [7], 30, 186-203 (^9^3)' 



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CHAPTER XI 

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 
OF RADIUM 

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). 
106 



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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 
would escape. 

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 



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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 
radium. 

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 
immortal Faraday. 

This method of measuring the amount of heat liberated 



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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 



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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- 
eter. 

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! 



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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 



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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 
of heat. 

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. 



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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 
heat. 

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). 



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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 



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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 

RADIUM 

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 



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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). 



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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 
above calculation. 

THREE REMARKABLE PROPERTIES OF RADIUM 

We have thus far met with at least three properties 
possessed by radium, which are in the^ highest degree 
remarkable. 

(i) We have seen that radium has the power to charge 
itself electrically. 

(2) It also has the power to illuminate itself, or is,* as we 
say, self-luminous. 

(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). 



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CHAPTER XII 
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 

ii8 



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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 
obtained. 

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). 



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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 
air. 

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. 



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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 
measured. 

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 
important fact. 

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 



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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 



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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- 
affected. 

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 
chemical inertness. 

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 



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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- 
known gases. 

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). 



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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). 



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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 
Graham's law. 

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 
and ruthenium. 

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 
supposed. 

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). 



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CHAPTER Xm 
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 
at all? 

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). 
127 



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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 
particles. 

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 
produced. 



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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? 



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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). 



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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 



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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 
elements. 

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 
earth. 

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). 



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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 
sharply. 

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 
desirable. 

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 



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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 
be effected. 

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 
FROM RADIUM 

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 



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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 
days. 

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 
itself. 

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). 



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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. 



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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- 
sary. 



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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). 



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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. 



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CHAPTER XIV 
Induced Radioactivity 

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 
results obtained. 

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. [7], 30, 289 (1903). 
«Phil. Mag., 49, 161 (1900). 
» Ann. Chim. Phys. [7], 30, 291 (1903). 
140 



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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 
solid state. 

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, 



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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- 
fined space. 

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. 



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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- 
stances. 

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- 



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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- 
ACTIVE MATTER 

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 



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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 
substances. 

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). 



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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). 



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INDUCED RADIOACTIVITY 147 

EMANATION X 

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. 



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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 
RADIOACTIVITY 

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). 



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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). 



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150 THE ELECTRICAL NATURE OF MATTER 

as measured by the j8 rays would decrease rapidly during 
this period. 

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). 



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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). 



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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 
electrochemical behavior. 

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 
emanation. 

2. The emanation gives off a particles and yields emana- 
tion X, or radium A. 



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INDUCED RADIOACTIVITY 



IS3 



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 
radium F. 

8. Radium F gives off a particles. 

DECOMPOSITION PRODUCTS OF OTHER RADIOACTIVE 
SUBSTANCES 

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: 



Name 

Thoriiun 

\ 

Mesothoriiun (i and 2) 

+ 

Radiothoriiun 
Thorium X 
Emanation 
Thoriiun A 
Thorium B 

Thoriiun C (i and 2) 

I 
Thorium D 



Time of half decay 

3 X 10*® years 
5.5 years (?) 
737 days 
3.64 days 
54 seconds 
0.14 seconds 
10.6 hours 
I hour 
3.1 minutes 



Kind of rajB 



P,y 



«,/3 



P 

a 
P,7 



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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). 



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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 
of radium. 



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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: 



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INDU 


CED RADIOACTIVITY 


IS7 


Name 


Time of half decay 


Kind of rajB 


Actinium 


? 





i 






Radioactinium 


19.5 days 


«./8 


i 






Actinium X 

i 
Emanation 


10.5 days 


a 


3.9 seconds 


a 


i 






Actiniimi A 


0.002 seconds 


a 


i 






Actinium B 


36 minutes 


/3 


i 






Actiniimi C (?) 


2.1 minutes 


a 


+ 






Actiniimi D 


3.4 minutes 


/3,y 



Actinium X was obtained by Godlewski ^ and in the fol- 
lowing manner: 

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. 



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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- 
stance. 

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 
intermediate product. 

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 



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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 
activity. 

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, 



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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 
radium. 

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. 



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CHAPTER XV 
Production of Radioactive Matter 
continuous formation of radioactive matter in 

URANIUM 

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 
same material. 

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 
the radioactivity. 

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 
radioactive. 

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). 
x6i 



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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 
UrX. 

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 
radioactivity. 

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 



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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 



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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 
this subject.* 

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 

THORIUM 

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). 



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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 



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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 
thorium emanation. 

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). 



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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 
the thorium. 

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). 



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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 
temperature. 

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). 



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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 
at once. 



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CHAPTER XVI 
Theoretical Considerations 

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 
generalization. 

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 

170 



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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 
with radioactivity. 

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- 
posed. 

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 



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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. 



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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 
without them. 

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 



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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 
standard unit. 

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 



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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- 
coming self-luminous. 

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 



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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 
RADIOACTIVE PHENOMENA 

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). 



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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- 
tive atoms. 

These unstable atoms, which are thus undergoing change, 
are termed by Rutherford metabolons. 

THE TRANSFORMATIONS OF THE RADIOACTIVE ELEMENTS 
DIFFER FUNDAMENTALLY FROM ORDINARY CHEMICAL 
REACTIONS 

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 
vanishes. 



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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 
insignificant. 

We must, therefore, abandon any attempt to explain 



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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 
RADIOACTIVITY 

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 



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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 
matter. 

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 



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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 



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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- 
activity. 

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. 



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CHAPTER XVII 

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 
chemical elements. 

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 
its distribution. 

We shall review very briefly some of our knowledge of 
the distribution of radioactive matter, as far as our globe 
is concerned. 

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). 
183 



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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- 
tion. 

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 
radioactive. 

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). 



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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). 



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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 
extent. 

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). 



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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 
of it. 

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. 



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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 
through it. 

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). 



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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- 
ciently sensitive. 

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). 



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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 
general. 

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. 



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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 
from uranium. 

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). 



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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- 
pounds. 

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). 



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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 
in it. 

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, 
80 (1904). 



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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 
present. 

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 



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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). 



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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). 



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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. 



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198 THE ELECTRICAL NATURE OF MATTER 

IONIUM 

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). 
«/WJ.,40, 4415. 



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THE ORIGIN OF RADIUM 



199 



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: 

Name 

Uranium 

+ . 
Uranium X 

Uranium Y 

Ionium 

Radium 

Emanation 

Radium A 

+ 
Radium B 

+ 

Radium C I ^^ 

Radium D> 
Radio-Lead > 

Radium £ 

Radium F ) 

Polonium ) 



Time of half decay I 


Cind of rays 


6 X 10® years 


a 


24.6 days 


P,7 


i.S days 


/8 


? 


a 


2000 years 


»,/8 


3.85 days 


a 


3 minutes 


a 


26.8 minutes 


P,y 


19.5 minutes 
1.4 minutes 


/8 


16.5 years 


/8 ■ 


S days 


/8,y 



136 days 



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2CX> THE ELECTRICAL NATURE OF MATTER 

EMANIUM 

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). 



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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 
new element. 

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 



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202 THE ELECTRICAL NATURE OF MATTER 

activity of emanimn is due to the emanium X that is present 
in it. 

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 
error. 

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). 



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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. 



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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 
DIFFERENT SOURCES 

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: 

At. wt. 

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 
chemical means.'' 

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). 



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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. 

CONCLUSION 

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. 



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INDEX 



Actinium, 51, 65. 

decomposition products of, 156. 
X, 158. 
Allan, radioactivity of the air, 188. 
Alpha particle, action on a photo- 
graphic and on a fluorescent 
plate, 83. 

particle, ratio of — for the, 74. 
in 

particles are probably helium 

atoms, 81. 
particles, critical velocity of the, 

80. 
particles produce delta particles, 

81. 
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, 

204. 
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, 

80. 
Beta and gamma rays, 85. 

particle, determination of — for 
tn 

the, 88. 

particle, mass of, 89. 

particles, charge carried by, 86. 

rays, 70, 85, 96. 

rays from radium, 89. 
Boltwood, on the discovery of ion- 
ium, 198. 

on the origin of radium, 193, 
195. 



Bragg and Kleeman, alpha particles 
stopped by matter, 84. 
and Kleeman, on the alpha 
particles given off by radiimi, 

V 75- 
Bumstead and Wheeler, radioactive 

matter in tap-water at New 

Haven, 185. 
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. 

ray, 3- 

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 
transformations, 177. 
Conducting gas, how it differs from 

a non-conducting, 2. 
Conductivity, electrical, of gases, i. 
of dielectrics, radium increases 

the, 100. 
of gases, conditions which in- 
crease the, I. 
Corpuscle, 15, 22. 
nature of, 19. 



207 



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208 



INDEX 



Critical velocity of the alpha parti- 
cles, 80. 
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 
charges, 87. 

Mme., discovers polonium, 50. 

Mme., discovers radiiun, 49. 

Mme., Ivuninosity of ratdium 
compounds, 98. 

Mme., on polonium, 152. 

Mme., on the atomic weight of 
radium, 57, 62. 

Mme., "residual activity," 180. 

Mme., separated radium from 
pitdiblende, 50. 

Mme., separates radium, 51. 

M., studied the radiations from 
radium, 70. 
Curies discover induced radioactiv- 
ity, 140. 

on radioactivity of matter in 
general, 189. 

phosphorescence produced by 
radium salts, 99. 

studied chemical efifects pro- 
duced by radioactive sub- 
stances, lOI. 

Debieme discovers actinium, 51, 

65. 
on emanium, 200. 
Decay of induced radioactivity, 

143. 
of the emanation, 129. 

Decomposition products of radio- 
active substances, 153. 

Delta particles produced by alpha 
particles, 81. 
rays, 70. 

Demarjay, on the spectrum of ra- 
dium, 56. 

Dewar and M. Curie measured heat 
liberated by radium, by 



means of liquid hydrogen, 

107. 
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, 

184. 
Dobereiner triads, 26. 
Duane, on the alpha particles, 81. 

Earth's age, effect of heat liberated 
by radium on the calculated, 
114. 
Electrical charge, radiiun produces 
on itself an, 117. 
conductivity of gases, i. 
theory of matter, 19. 
Electrons, groups of, 180. 
Electron Aeory and radioactivity, 
40. 
theory and the Periodic System, 

31- 
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, 

136. 
decay of, 129. 
deposits radioactive matter, 

properties of this matter, 

145. 
discovered by Rutherford, 48. 
from radioactive substances, 

118. 
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. 



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INDEX 



209 



produces induced radioactivity, 

142. 
X, 147, 201. 
Emanating power of radium, re- 
covery of, 128. 
Emanium, 200, 201. 

Fluoroscopic method of studying 
radioactivity, 67. 

Gamma rays, 70, 93, 96. 

ra)rs, tieory as to the nature of, 

95- 
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, 
130. 

given off by radiimi, effect on 
the calculation of the age of 
the earth, 114. 

liberated by radium, amoimt of, 
no. 

liberated by radium, calcula- 
tion of the amount of, 117. 

liberated by radium measured 
by the Bunsen ice calorim- 
eter, 109. 

liberated by radimn, measure- 
ment of the, 106. 

produced by radium, 117. 

produced by radium salts, 106. 



produced by radium, source of, 
III. 

produced by the emanation, 
148. 

produced by radium, theories 
as to the source of, J15. 
Helium and emanation, relation 
between, 136. 

atoms, alpha particles are prob- 
ably, 81. 

discovered in the sun by Lock- 
yer, on the earth by Ramsay, 
132. 

from radium, further experi- 
ments, 134. 

produced from the emanation, 
127, 130. 

Induced radioactivity, 66, 140. 
radioactivity due to deposit of 

radioactive matter, 144. 
radioactivity produced by the 

emanation, 142. 
radioactivity imdergoes decay, 

143- 
Ion gaseous, comparison of the 
charge on, with that on a 
univalent ion of an elec- 
trolyte, 13. 

in gases, determination of the 
mass of the negative, 10. 

not the same mass as the atom, 

37. 
positive, ratio of the chaige to 

the mass for the, 15. 
Ionium, 198. 
Ionization method of studying 

radioactivitv, 68. 
Ions gaseous, produced by different 

means, — for the, 8. 

of electrolytes, — different for 
tn 

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, 
21. 



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2IO 



INDEX 



Kaufmaim, on the vdodty of the 

different beta partides from 

radium, 20. 
Kleeman and Bragg, on the alpha 

partides given off by radium, 

75. 

Laborde and M. Curie show that 

radium produces heat, 106. 
Lead radioactive, atomic weights of, 

204. 
Lenard rays, 8. 
Lockyer first discovered helium in 

the Sim, 132. 
Luminosity of radium compounds, 

98. 
of radium salts, 117. 

Mackenzie, on the ratio — for the 

alpha partides, 76. 
Madsen, on the gamma rays frcm 

radium, 95. 
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- 
formation, 181. 
the electron the ultimate imit 
of, 23. 
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- 
activity, 66. 

Mendel^eff's Periodic System, 26. 

Meyer Lothar, Periodic System, 26 

Molecular weight of the emanation^ 
123. 

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 

radium, 102. 
Ozone produced from oxygen by 

radium, 102. 

Partides, total number shot off by 
radium, 97. 

Periodic System, electron theory 
and the, 31. 

Phosphorescence produced by ra- 
dium salts, 99. 

Photographic method of studying 
radioactivity, 67. 

Physiological action of the radia- 
tions from radium, 104. 

Pitchblende, radioactive substances 
in, 63. 
radium separated from, 50. 
the source of radium, 49. 

Polonium, 50, 63. 

action of on a photographic 
plate, 103. 

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- 
activity, 144. 



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INDEX 



211 



substances in pitchblende, 63. 

substances produce chemical 
effects, loi. 

substances, properties of the 
radiations given out by, 68. 

transfonnations differ from 
chemical reactions, 177. 
Radioactivity, 46. 

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. 
Radiographs, 104. 
Radiotellurium, 64. 
Radiothorium, 154. 
Radium A, 150. 

B, ISO. 

C, 150. 

D, 151. 

E, 151. 

F, 151, 152. 

amount of heat liberated by, 
no. 

atomic weight of, 57. 

charges itself electrically, 117. 

complete transformation prod- 
ucts of, 199. 

compounds, luminosity of the, 
98. 

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, 
50. 

heat produced by, effect on 
source of solar heat, 112. 

heats itself, 117. 

increases the conductivity of 
dielectries, 100. 

measurement of the heat liber- 
ated by, 106. 

origin of, 183, 190. 

physiological action of the 
radiations from, 104. 

produces ozone from oxygen, 
102. 



salts, phosphorescence produced 

by, 99. 
salts, production of heat by, 

106. 
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, 
132. 
shows that the helium emana- 
tion may affect certain trans- 
mutations, 138. 
studied the properties of the 
emanation, 123. 
Ray, Becquerel, 44. 

cathode, 3. 
Rays, alpha, 72. 

X,4i. 
Regener counted the alpha particles, 
82. 
on scintillations produced by 
beta radiations, 92. 
Rowland, on the nature of the atom, 

36. 
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- 
tion, 130. 
and Geiger counted the alpha 

particles, 82. 
and Miss Brooks determine the 
molecular weight of the ema- 
nation, 124. 
and Royds, helium from alpha 

particles, 83. 
and Soddy, on thorium X, 165 

167. 
and Soddy study the emana- 
tion from radium, 127. 
and Soddy, theory of radio- 
activity, 176. 



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INDEX 



Rutherford calculated the amount of 
heat liberated by radium, 117. 

determines the ratio of — for 
m 

the alpha i>article, 74. 

discovers the emanation, 48, 
118. 

discovers thorium emanation, 
119. 

on induced radioactivity, 143. 

on photographic and fluorescent 
action, 83. 

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- 
nation, 146. 

studied the action of the mag- 
netic field on the alpha rays, 
72. 

studied the effect of low tem- 
perature of the production of 
emanation, 128. 

studied the properties of the 
emanation, 122. 

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. 
Spinthariscope, 78. 

BecquerePs theory of the action 

of, 80. 
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 
general, 189. 
Sun, does it contain radium, 113. 

Theory, importance of, 170. 

of radioactive phenomena, 176. 



Thomson, J. J., conception of the 

atom, 31. 
determination of the mass of 

the negative ion in gases, 10. 
on positive rays, 17. 
on the electrical theory of 

inertia, 19. 

on the ratio of — for different 
m 

gases, 7. 

on the value of — for the cathode 
m 

particle, 5. 

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. 

radiation, 47. 

recovers radioactivity, 166, 168. 

X, 164. 

X, decay of radioactivity, 165. 

X from thorium emanation, 165 
Thorpe, on the atomic weight of 

radium, 62. 
Transformation of matter in gen- 
eral, 181. 

products of radium, 199. 
Transmutation of the dements, not 
effected, 133. 

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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