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Title: Radio-Activity

Author: Ernest Rutherford

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*** START OF THE PROJECT GUTENBERG EBOOK RADIO-
ACTIVITY ***

Radio-Activity

CAMBRIDGE PHYSICAL SERIES.

General Editors:—F. H. Neville, M.A., F.R.S.
and W. C. D. Whetham, M.A., F.R.S.

RADIO-ACTIVITY

CAMBRIDGE UNIVERSITY PRESS WAREHOUSE
C. F. CLAY, Manager.
London: FETTER LANE, E.C.
Glasgow: 50, WELLINGTON STREET.

ALSO

London: H. K. LEWIS, 136, GOWER STREET, W.C.
Leipzig: F. A. BROCKHAUS.
New York: THE MACMILLAN COMPANY.
Bombay and Calcutta: MACMILLAN AND CO., Ltd.

[All Rights reserved.]

RADIO-ACTIVITY

BY

E. RUTHERFORD, D.Sc., F.R.S., F.R.S.C.
MACDONALD PROFESSOR OF PHYSICS, McGILL
UNIVERSITY, MONTREAL

SECOND EDITION

CAMBRIDGE
AT THE UNIVERSITY PRESS
1905

First Edition 1904
Second Edition 1905

J. J. THOMSON

A TRIBUTE OF MY RESPECT AND ADMIRATION

PREFACE TO THE FIRST EDITION.

In this work, I have endeavoured to give a complete and connected account,
from a physical standpoint, of the properties possessed by the naturally
radio-active bodies. Although the subject is comparatively a new one, our
knowledge of the properties of the radio-active substances has advanced
with great rapidity, and there is now a very large amount of information on
the subject scattered throughout the various scientific journals.
The phenomena exhibited by the radio-active bodies are extremely
complicated, and some form of theory is essential in order to connect in an
intelligible manner the mass of experimental facts that have now been
accumulated. I have found the theory that the atoms of the radio-active
bodies are undergoing spontaneous disintegration extremely serviceable,
not only in correlating the known phenomena, but also in suggesting new
lines of research.
The interpretation of the results has, to a large extent, been based on the
disintegration theory, and the logical deductions to be drawn from the
application of the theory to radio-active phenomena have also been
considered.
The rapid advance of our knowledge of radio-activity has been dependent
on the information already gained by research into the electric properties of
gases. The action possessed by the radiations from radio-active bodies of
producing charged carriers or ions in the gas, has formed the basis of an
accurate quantitative method of examination of the properties of the
radiations and of radio-active processes, and also allows us to determine
with considerable certainty the order of magnitude of the different
quantities involved.
For these reasons, it has been thought advisable to give a brief account of
the electric properties of gases, to the extent that is necessary for the
interpretation of the results of measurements in radio-activity by the electric

method. The chapter on the ionization theory of gases was written before
the publication of J. J. Thomson’s recent book on “Conduction of
Electricity through Gases,” in which the whole subject is treated in a
complete and connected manner.
A short chapter has been added, in which an account is given of the
methods of measurement which, in the experience of the writer and others,
are most suitable for accurate work in radio-activity. It is hoped that such an
account may be of some service to those who may wish to obtain a practical
acquaintance with the methods employed in radio-active measurements.
My thanks are due to Mr W. C. Dampier Whetham, F.R.S., one of the
editors of the Cambridge Physical Series, for many valuable suggestions,
and for the great care and trouble he has taken in revising the proof sheets. I
am also much indebted to my wife and Miss H. Brooks for their kind
assistance in correcting the proofs, and to Mr R. K. McClung for revising
the index.
E. R.

Macdonald Physics Building,
Montreal,
February, 1904.

PREFACE TO THE SECOND EDITION.

I feel that some apology is due to my readers for bringing out at such an
early date a new edition which includes so much new material, and in
which the rearrangement is so extensive as to constitute almost a new work.
Though only a year has passed since the book first made its appearance, the
researches that have been carried out in that time have been too numerous
and of too important a character to permit the publishing of a mere reprint,
unless the author were to relinquish his purpose of presenting the subject as
it stands at the present moment.
The three new chapters which have been added possibly constitute the most
important change in the work. These chapters include a detailed account of
the theory of successive changes and of its application to the analysis of the
series of transformations which occur in radium, thorium, and actinium.
The disintegration theory, which was put forward in the first edition as an
explanation of radio-active phenomena, has in these later researches proved
to be a most powerful and valuable method of analysing the connection
between the series of substances which arise from the transformation of the
radio-elements. It has disclosed the origin of radium, of polonium and
radio-tellurium, and of radio-lead, and now binds together in one coherent
whole the large mass of apparently heterogeneous experimental facts in
radio-activity which have been accumulating since 1896. The theory has
received a remarkable measure of verification in the past year, and, in many
cases, has offered a quantitative as well as a qualitative explanation of the
connection between the various properties exhibited by the radio-active
bodies. In the light of this evidence, radio-activity may claim to have
assumed the position of an independent subject, though one with close
affinities to physics on the one hand and to chemistry on the other.
The present edition includes a large amount of new material relating to the
nature and properties of the radiations and the emanations. In the limits of

this book, it would have been found impossible, even had it been thought
desirable, to include more than a brief sketch of the physiological effects of
the rays. The literature on this subject is already large, and is increasing
rapidly. For reasons of space, I have not been able to refer more than briefly
to the mass of papers that have appeared dealing with the examination of
various spring and well waters, sediments, and soils, for the presence of
radio-active matter.
In order to make the book more self-contained, a short account has been
given in Chapter II of the magnetic field produced by an ion in motion, of
the action of an external magnetic and electric field upon it, and of the
determination of the velocity and mass of the particles constituting the
cathode stream.
Two appendices have been added, one giving an account of some work
upon the α rays which was completed too late for inclusion in the subject
matter of the book, and the other containing a brief summary of what is
known in regard to the chemical constitution of the various radio-active
minerals, the localities in which they are found, and their probable geologic
age. For the preparation of the latter, I am indebted to my friend Dr
Boltwood of New Haven, who, in the course of his researches, has had
occasion to analyse most of these minerals in order to determine their
content of uranium and radium. I hope that this account of radio-active
minerals will prove of value to those who are endeavouring to elucidate the
connection between the various radio-active substances and the inactive
products which arise from their transformation.
For the convenience of those who have read the first edition, a list of the
sections and chapters which contain the most important additions and
alterations is added below the table of contents.
The writing of a complete account of a subject like radio-activity, in which
so much new work is constantly appearing, has been a matter of no little
difficulty. Among other things it has involved a continuous revision of the
work while the volume was passing through the press.
I wish to express my thanks to my colleague Professor Harkness for the
care and trouble he has taken in revising the proofs and for many useful
suggestions; also to Mr R. K. McClung for his assistance in correcting some
of the proofs and in preparing the index.

E. R.

McGill University,
Montreal,
9 May, 1905.

ERRATA.

Transcriber’s Note:

These corrections have been applied to the text in the book.
page 48, line 24 section 218 should read section 284
„ 77, last line „ 263 „ „ „ 270
„ 123, 5th line from bottom „ 254 „ „ „ 261
„ 124, 10th „ „ „ „ 246 „ „ „ 253
„ 151, line 3 „ 228 „ „ „ 229
„ 156, 13th line from bottom „ 261 „ „ „ 268
„ 200, line 9 „ 246 „ „ „ 253
„ 216, line 3 „ 260 „ „ „ 267
„ 184, at the top of 5th column of table the letter γ should be
inserted.

TABLE OF CONTENTS.

I. Radio-active Substances 1
II. Ionization Theory of Gases 31
III. Methods of Measurement 82
IV. Nature of the Radiations 108
V. Properties of the Radiations 201
VI. Continuous Production of Radio-active Matter 218
VII. Radio-active Emanations 238
VIII. Excited Radio-activity 295
IX. Theory of Successive Changes 325
X. Transformation Products of Uranium, Thorium and Actinium 346
XI. Transformation Products of Radium 371
XII. Rate of Emission of Energy 418
XIII. Radio-active Processes 437
XIV. Radio-activity of the Atmosphere and of Ordinary Materials 501
Appendix A. Properties of the α Rays 543
Appendix B. Radio-active Minerals 554
Index 559
Plate (Fig. 46A: Spectrum of Radium Bromide) to face p. 206

For the convenience of the reader, the sections and chapters which contain
mostly new matter, or have been either partly or wholly rewritten, are

appended below.
Chap. I. Sections 18, 20–23.
„ II. „ 48–52.
„ III. „ 69.
„ IV. „ 83–85, 92, 93, 103, 104, 106–108, 111, 112.
„ V. „ 115, 117, 119, 122.
„ VII. „ 171–173.
„ VIII. „ 182–184, 190.
„ IX-XIV. Mostly rewritten.

ABBREVIATIONS OF REFERENCES TO SOME OF THE JOURNALS.

Ber. d. deutsch. Chem. Ges. Berichte der deutschen chemischen
Gesellschaft. Berlin.
C. R. Comptes Rendus des Séances de l’Académie des Sciences. Paris.
Chem. News. Chemical News. London.
Drude’s Annal. Annalen der Physik. Leipzig.
Phil. Mag. Philosophical Magazine and Journal of Science. London.
Phil. Trans. Philosophical Transactions of the Royal Society of London.
Phys. Rev. Physical Review. New York.
Phys. Zeit. Physikalische Zeitschrift.
Proc. Camb. Phil. Soc. Proceedings of the Cambridge Philosophical
Society. Cambridge.
Proc. Roy. Soc. Proceedings of the Royal Society of London.
Thèses-Paris. Thèses présentées à la Faculté des Sciences de
l’Université de Paris.
Wied. Annal. Annalen der Physik. Leipzig.

CHAPTER I.
RADIO-ACTIVE SUBSTANCES.

1. Introduction. The close of the old and the beginning of the new century
have been marked by a very rapid increase of our knowledge of that most
important but comparatively little known subject—the connection between
electricity and matter. No study has been more fruitful in surprises to the
investigator, both from the remarkable nature of the phenomena exhibited
and from the laws controlling them. The more the subject is examined, the
more complex must we suppose the constitution of matter in order to
explain the remarkable effects observed. While the experimental results
have led to the view that the constitution of the atom itself is very complex,
at the same time they have confirmed the old theory of the discontinuous or
atomic structure of matter. The study of the radio-active substances and of
the discharge of electricity through gases has supplied very strong
experimental evidence in support of the fundamental ideas of the existing
atomic theory. It has also indicated that the atom itself is not the smallest
unit of matter, but is a complicated structure made up of a number of
smaller bodies.
A great impetus to the study of this subject was initially given by the
experiments of Lenard on the cathode rays, and by Röntgen’s discovery of
the X rays. An examination of the conductivity imparted to a gas by the X
rays led to a clear view of the mechanism of the transport of electricity
through gases by means of charged ions. This ionization theory of gases has
been shown to afford a satisfactory explanation not only of the passage of
electricity through flames and vapours, but also of the complicated
phenomena observed when a discharge of electricity passes through a
vacuum tube. At the same time, a further study of the cathode rays showed
that they consisted of a stream of material particles, projected with great
velocity, and possessing an apparent mass small compared with that of the
hydrogen atom. The connection between the cathode and Röntgen rays and

the nature of the latter were also elucidated. Much of this admirable
experimental work on the nature of the electric discharge has been done by
Professor J. J. Thomson and his students in the Cavendish Laboratory,
Cambridge.
An examination of natural substances, in order to see if they gave out dark
radiations similar to X rays, led to the discovery of the radio-active bodies
which possess the property of spontaneously emitting radiations, invisible
to the eye, but readily detected by their action on photographic plates and
their power of discharging electrified bodies. A detailed study of the radio-
active bodies has revealed many new and surprising phenomena which have
thrown much light, not only on the nature of the radiations themselves, but
also on the processes occurring in those substances. Notwithstanding the
complex nature of the phenomena, the knowledge of the subject has
advanced with great rapidity, and a large amount of experimental data has
now been accumulated.
In order to explain the phenomena of radio-activity, Rutherford and Soddy
have advanced a theory which regards the atoms of the radio-active
elements as suffering spontaneous disintegration, and giving rise to a series
of radio-active substances which differ in chemical properties from the
parent elements. The radiations accompany the breaking-up of the atoms,
and afford a comparative measure of the rate at which the disintegration
takes place. This theory is found to account in a satisfactory way for all the
known facts of radio-activity, and welds a mass of disconnected facts into
one homogeneous whole. On this view, the continuous emission of energy
from the active bodies is derived from the internal energy inherent in the
atom, and does not in any way contradict the law of the conservation of
energy. At the same time, however, it indicates that an enormous store of
latent energy is resident in the radio-atoms themselves. This store of energy
has not been observed previously, on account of the impossibility of
breaking up into simpler forms the atoms of the elements by the action of
the chemical or physical forces at our command.
On this theory we are witnessing in the radio-active bodies a veritable
transformation of matter. This process of disintegration was investigated,
not by direct chemical methods, but by means of the property possessed by
the radio-active bodies of giving out specific types of radiation. Except in
the case of a very active element like radium, the process of disintegration

takes place so slowly, that hundreds if not thousands of years would be
required before the amount transformed would come within the range of
detection of the balance or the spectroscope. In radium, however, the
process of disintegration takes place at such a rate that it should be possible
within a limited space of time to obtain definite chemical evidence on this
question. The recent discovery that helium can be obtained from radium
adds strong confirmation to the theory; for helium was indicated as a
probable disintegration product of the radio-active elements before this
experimental evidence was forthcoming. Several products of the
transformation of the radio-active bodies have already been examined, and
the further study of these substances promises to open up new and
important fields of chemical enquiry.
In this book the experimental facts of radio-activity and the connection
between them are interpreted on the disintegration theory. Many of the
phenomena observed can be investigated in a quantitative manner, and
prominence has been given to work of this character, for the agreement of
any theory with the facts, which it attempts to explain, must ultimately
depend upon the results of accurate measurement.
The value of any working theory depends upon the number of experimental
facts it serves to correlate, and upon its power of suggesting new lines of
work. In these respects the disintegration theory, whether or not it may
ultimately be proved to be correct, has already been justified by its results.

2. Radio-active Substances. The term “radio-active” is now generally
applied to a class of substances, such as uranium, thorium, radium, and their
compounds, which possess the property of spontaneously emitting
radiations capable of passing through plates of metal and other substances
opaque to ordinary light. The characteristic property of these radiations,
besides their penetrating power, is their action on a photographic plate and
their power of discharging electrified bodies. In addition, a strongly radio-
active body like radium is able to cause marked phosphorescence and
fluorescence on some substances placed near it. In the above respects the
radiations possess properties analogous to Röntgen rays, but it will be
shown that, for the major part of the radiations emitted, the resemblance is
only superficial.

The most remarkable property of the radio-active bodies is their power of
radiating energy spontaneously and continuously at a constant rate, without,
as far as is known, the action upon them of any external exciting cause. The
phenomena at first sight appear to be in direct contradiction to the law of
conservation of energy, since no obvious change with time occurs in the
radiating material. The phenomena appear still more remarkable when it is
considered that the radio-active bodies must have been steadily radiating
energy since the time of their formation in the earth’s crust.
Immediately after Röntgen’s discovery of the production of X rays, several
physicists were led to examine if any natural bodies possessed the property
of giving out radiations which could penetrate metals and other substances
opaque to light. As the production of X rays seemed to be connected in
some way with cathode rays, which cause strong fluorescent and
phosphorescent effects on various bodies, the substances first examined
were those that were phosphorescent when exposed to light. The first
observation in this direction was made by Niewenglowski[1], who found that
sulphide of calcium exposed to the sun’s rays gave out some rays which
were able to pass through black paper. A little later a similar result was
recorded by H. Becquerel[2] for a special calcium sulphide preparation, and
by Troost[3] for a specimen of hexagonal blend. These results were
confirmed and extended in a later paper by Arnold[4]. No satisfactory
explanations of these somewhat doubtful results have yet been given,
except on the view that the black paper was transparent to some of the light
waves. At the same time Le Bon[5] showed that, by the action of sunlight on
certain bodies, a radiation was given out, invisible to the eye, but active
with regard to a photographic plate. These results have been the subject of
much discussion; but there seems to be little doubt that the effects are due to
short ultra-violet light waves, capable of passing through certain substances
opaque to ordinary light. These effects, while interesting in themselves, are
quite distinct in character from those shown by the radio-active bodies
which will now be considered.

3. Uranium. The first important discovery in the subject of radio-activity
was made in February, 1896, by M. Henri Becquerel[6], who found that a
uranium salt, the double sulphate of uranium and potassium, emitted some
rays which gave an impression on a photographic plate enveloped in black

paper. These rays were also able to pass through thin plates of metals and
other substances opaque to light. The impressions on the plate could not
have been due to vapours given off by the substances, since the same effect
was produced whether the salt was placed directly on the black paper or on
a thin plate of glass lying upon it.
Becquerel found later that all the compounds of uranium as well as the
metal itself possessed the same property, and, although the amount of action
varied slightly for the different compounds, the effects in all cases were
comparable. It was at first natural to suppose that the emission of these rays
was in some way connected with the power of phosphorescence, but later
observations showed that there was no connection whatever between them.
The uranic salts are phosphorescent, while the uranous salts are not. The
uranic salts, when exposed to ultra-violet light in the phosphoroscope, give
a phosphorescent light lasting about ·01 seconds. When the salts are
dissolved in water, the duration is still less. The amount of action on the
photographic plate does not depend on the particular compound of uranium
employed, but only on the amount of uranium present in the compound. The
non-phosphorescent are equally active with the phosphorescent compounds.
The amount of radiation given out is unaltered if the active body be kept
continuously in darkness. The rays are given out by solutions, and by
crystals which have been deposited from solutions in the dark and never
exposed to light. This shows that the radiation cannot be due in any way to
the gradual emission of energy stored up in the crystal in consequence of
exposure to a source of light.

4. The power of giving out penetrating rays thus seems to be a specific
property of the element uranium, since it is exhibited by the metal as well as
by all its compounds. These radiations from uranium are persistent, and, as
far as observations have yet gone, are unchanged, either in intensity or
character, with lapse of time. Observations to test the constancy of the
radiations for long periods of time have been made by Becquerel. Samples
of uranic and uranous salts have been kept in a double box of thick lead,
and the whole has been preserved from exposure to light. By a simple
arrangement, a photographic plate can be introduced in a definite position
above the uranium salts, which are covered with a layer of black paper. The
plate is exposed at intervals for 48 hours, and the impression on the plate

compared. No perceptible weakening of the radiation has been observed
over a period of four years. Mme Curie[7] has made determinations of the
activity of uranium over a space of five years by an electric method
described later, but found no appreciable variation during that period.
Since the uranium is thus continuously radiating energy from itself, without
any known source of excitation, the question arises whether any known
agent is able to affect the rate of its emission. No alteration was observed
when the body was exposed to ultra-violet light or to ultra-red light or to X
rays. Becquerel states that the double sulphate of uranium and potassium
showed a slight increase of action when exposed to the arc light and to
sparks, but he considers that the feeble effect observed was another action
superimposed on the constant radiation from uranium. The intensity of the
uranium radiation is not affected by a variation of temperature between
200° C. and the temperature of liquid air. This question is discussed in more
detail later.

5. In addition to these actions on a photographic plate, Becquerel showed
that uranium rays, like Röntgen rays, possess the important property of
discharging both positively and negatively electrified bodies. These results
were confirmed and extended by Lord Kelvin, Smolan and Beattie[8]. The
writer made a detailed comparison[9] of the nature of the discharge produced
by uranium with that produced by Röntgen rays, and showed that the
discharging property of uranium is due to the production of charged ions by
the radiation throughout the volume of the gas. The property has been made
the basis of a qualitative and quantitative examination of the radiations from
all radio-active bodies, and is discussed in detail in chapter II.
The radiations from uranium are thus analogous, as regards their
photographic and electrical actions, to Röntgen rays, but, compared with the
rays from an ordinary X ray tube, these actions are extremely feeble. While
with Röntgen rays a strong impression is produced on a photographic plate
in a few minutes or even seconds, several days’ exposure to the uranium
rays is required to produce a well-marked action, even though the uranium
compound, enveloped in black paper, is placed close to the plate. The
discharging action, while very easily measurable by suitable methods, is
also small compared with that produced by X rays from an ordinary tube.

6. The rays from uranium show no evidence of direct reflection, refraction,
or polarization[10]. While there is no direct reflection of the rays, there is
apparently a diffuse reflection produced where the rays strike a solid
obstacle. This is due in reality to a secondary radiation set up when the
primary rays impinge upon matter. The presence of this secondary radiation
at first gave rise to the erroneous view that the rays could be reflected and
refracted like ordinary light. The absence of reflection, refraction, or
polarization in the penetrating rays from uranium necessarily follows in the
light of our present knowledge of the rays. It is now known that the uranium
rays, mainly responsible for the photographic action, are deviable by a
magnetic field, and are similar in all respects to cathode rays, i.e. the rays
are composed of small particles projected at great velocities. The absence of
the ordinary properties of transverse light waves is thus to be expected.

7. The rays from uranium are complex in character, and, in addition to the
penetrating deviable rays, there is also given off a radiation very readily
absorbed by passing through thin layers of metal foil, or by traversing a few
centimetres of air. The photographic action due to these rays is very feeble
in comparison with that of the penetrating rays, although the discharge of
electrified bodies is mainly caused by them. Besides these two types of
rays, some rays are emitted which are of an extremely penetrating character
and are non-deviable by a magnetic field. These rays are difficult to detect
photographically, but can readily be examined by the electric method.

8. The question naturally arose whether the property of spontaneously
giving out penetrating radiations was confined to uranium and its
compounds, or whether it was exhibited to any appreciable extent by other
substances.
By the electrical method, with an electrometer of ordinary sensitiveness,
any body which possesses an activity of the order of ¹⁄₁₀₀ of that of uranium
can be detected. With an electroscope of special construction, such as has
been designed by C. T. R. Wilson for his experiments on the natural
ionization of air, a substance of activity ¹⁄₁₀₀₀₀ and probably ¹⁄₁₀₀₀₀₀ of that of
uranium can be detected.

If an active body like uranium be mixed with an inactive body, the resulting
activity in the mixture is generally considerably less than that due to the
active substance alone. This is due to the absorption of the radiation by the
inactive matter present. The amount of decrease largely depends on the
thickness of the layer from which the activity is determined.
Mme Curie made a detailed examination by the electrical method of the
great majority of known substances, including the very rare elements, to see
if they possessed any activity. In cases where it was possible, several
compounds of the elements were examined. With the exception of thorium
and phosphorus, none of the other substances possessed an activity even of
the order of ¹⁄₁₀₀ of uranium.
The ionization of the gas by phosphorus does not, however, seem to be due
to a penetrating radiation like that found in the case of uranium, but rather
to a chemical action taking place at its surface. The compounds of
phosphorus do not show any activity, and in this respect differ from
uranium and the other active bodies.
Le Bon[11] has also observed that quinine sulphate, if heated and then
allowed to cool, possesses for a short time the property of discharging both
positively and negatively electrified bodies. It is necessary, however, to
draw a sharp line of distinction between phenomena of this kind and those
exhibited by the naturally radio-active bodies. While both, under special
conditions, possess the property of ionizing the gas, the laws controlling the
phenomena are quite distinct in the two cases. For example, only one
compound of quinine shows the property, and that compound only when it
has been subjected to a preliminary heating. The action of phosphorus
depends on the nature of the gas, and varies with temperature. On the other
hand, the activity of the naturally radio-active bodies is spontaneous and
permanent. It is exhibited by all compounds, and is not, as far as is yet
known, altered by change in the chemical or physical conditions.

9. The discharging and photographic action alone cannot be taken as a
criterion as to whether a substance is radio-active or not. It is necessary in
addition to examine the radiations, and to test whether the actions take place
through appreciable thicknesses of all kinds of matter opaque to ordinary
light. For example, a body giving out short waves of ultra-violet light can

be made to behave in many respects like a radio-active body. As Lenard[12]
has shown, short waves of ultra-violet light will ionize the gas in their path,
and will be absorbed rapidly in the gas. They will produce strong
photographic action, and may pass through some substances opaque to
ordinary light. The similarity to a radio-active body is thus fairly complete
as regards these properties. On the other hand, the emission of these light
waves, unlike that of the radiations from an active body, will depend largely
on the molecular state of the compound, or on temperature and other
physical conditions. But the great point of distinction lies in the nature of
the radiations from the bodies in question. In one case the radiations behave
as transverse waves, obeying the usual laws of light waves, while in the
case of a naturally active body, they consist for the most part of a
continuous flight of material particles projected from the substance with
great velocity. Before any substance can be called “radio-active” in the
sense in which the term is used to describe the properties of the natural
radio-active elements, it is thus necessary to make a close examination of its
radiation; for it is unadvisable to extend the use of the term “radio-active”
to substances which do not possess the characteristic radiating properties of
the radio-active elements which we have described, and the active products
which can be obtained from them. Some of the pseudo-active bodies will
however be considered later in chapter IX.

10. Thorium. In the course of an examination of a large number of
substances, Schmidt[13] found that thorium, its compounds, and the minerals
containing thorium, possessed properties similar to those of uranium. The
same discovery was made independently by Mme Curie[14]. The rays from
thorium compounds, like those from uranium, possess the properties of
discharging electrified bodies and acting on a photographic plate. Under the
same conditions the discharging action of the rays is about equal in amount
to that of uranium, but the photographic effect is distinctly weaker.
The radiations from thorium are more complicated than those from
uranium. It was early observed by several experimenters that the radiation
from thorium compounds, especially the oxide, when tested by the
electrified method, was very variable and uncertain. A detailed investigation
of the radiations from thorium under various conditions was made by
Owens[15]. He showed that thorium oxide, especially in thick layers, was

able to produce conductivity in the gas when covered with a large thickness
of paper, and that the amount of this conductivity could be greatly varied by
blowing a current of air over the gas. In the course of an examination[16] of
this action of the air current, the writer showed that thorium compounds
gave out a material emanation made up of very small particles themselves
radio-active. The emanation behaves like a radio-active gas; it diffuses
rapidly through porous substances like paper, and is carried away by a
current of air. The evidence of the existence of the emanation and its
properties, is considered in detail later in chapter VIII. In addition to giving
out an emanation, thorium behaves like uranium in emitting three types of
radiation, each of which is similar in properties to the corresponding
radiation from uranium.

11. Radio-active minerals. Mme Curie has examined the radio-activity of a
large number of minerals containing uranium and thorium. The electrical
method was used, and the current measured between two parallel plates 8
cms. in diameter and 3 cms. apart, when one plate was covered with a
uniform layer of the active matter. The following numbers give the order of
the saturation current obtained in amperes.
Pitchblende from Johanngeorgenstadt 8·3 × 10-11
„ Joachimsthal 7·0 „
„ Pzibran 6·5 „
„ Cornwall 1·6 „
Cleveite 1·4 „
Chalcolite 5·2 „
Autunite 2·7 „
Thorite from 0·3 to 1·4 „
Orangite 2·0 „
Monazite 0·5 „
Xenotine 0·03 „
Aeschynite 0·7 „
Fergusonite 0·4 „
Samarskite 1·1 „

Niobite 0·3 „
Carnotite 6·2 „
Some activity is to be expected in these minerals, since they all contain
either thorium or uranium or a mixture of both. An examination of the
action of the uranium compounds with the same apparatus and under the
same conditions led to the following results:
Uranium (containing a little carbon) 2·3 × 10-11 amperes
Black oxide of uranium 2·6 „
Green „ „ 1·8 „
Acid uranic hydrate 0·6 „
Uranate of sodium 1·2 „
Uranate of potassium 1·2 „
Uranate of ammonia 1·3 „
Uranous sulphate 0·7 „
Sulphate of uranium and potassium 0·7 „
Acetate 0·7 „
Phosphate of copper and uranium 0·9 „
Oxysulphide of uranium 1·2 „
The interesting point in connection with these results is that some
specimens of pitchblende have four times the activity of the metal uranium;
chalcolite, the crystallized phosphate of copper and uranium, is twice as
active as uranium; and autunite, a phosphate of calcium and uranium, is as
active as uranium. From the previous considerations, none of the substances
should have shown as much activity as uranium or thorium. In order to be
sure that the large activity was not due to the particular chemical
combination, Mme Curie prepared chalcolite artificially, starting with pure
products. This artificial chalcolite had the activity to be expected from its
composition, viz. about 0·4 of the activity of the uranium. The natural
mineral chalcolite is thus five times as active as the artificial mineral.
It thus seemed probable that the large activity of some of these minerals,
compared with uranium and thorium, was due to the presence of small
quantities of some very active substance, which was different from the
known bodies thorium and uranium.

This supposition was completely verified by the work of M. and Mme
Curie, who were able to separate from pitchblende by purely chemical
methods two active bodies, one of which in the pure state is over a million
times more active than the metal uranium.
This important discovery was due entirely to the property of radio-activity
possessed by the new bodies. The only guide in their separation was the
activity of the products obtained. In this respect the discovery of these
bodies is quite analogous to the discovery of rare elements by the methods
of spectrum analysis. The method employed in the separation consisted in
examining the relative activity of the products after chemical treatment. In
this way it was seen whether the radio-activity was confined to one or
another of the products, or divided between both, and in what ratio such
division occurred.
The activity of the specimens thus served as a basis of rough qualitative and
quantitative analysis, analogous in some respects to the indication of the
spectroscope. To obtain comparative data it was necessary to test all the
products in the dry state. The chief difficulty lay in the fact that pitchblende
is a very complex mineral, and contains in varying quantities nearly all the
known metals.

12. Radium. The analysis of pitchblende by chemical methods, using the
procedure sketched above, led to the discovery of two very active bodies,
polonium and radium. The name polonium was given to the first substance
discovered by Mme Curie in honour of the country of her birth. The name
radium was a very happy inspiration of the discoverers, for this substance in
the pure state possesses the property of radio-activity to an astonishing
degree.
Radium is extracted from pitchblende by the process used to separate
barium, to which radium is very closely allied in chemical properties[17].
After the removal of other substances, the radium remains behind mixed
with barium. It can, however, be partially separated from the latter by the
difference in solubility of the chlorides in water, alcohol, or hydrochloric
acid. The chloride of radium is less soluble than that of barium, and can be
separated from it by the method of fractional crystallization. After a large

number of precipitations, the radium can be freed almost completely from
the barium.
Both polonium and radium exist in infinitesimal quantities in pitchblende.
In order to obtain a few decigrammes of very active radium, it is necessary
to use several tons of pitchblende, or the residues obtained from the
treatment of uranium minerals. It is thus obvious that the expense and
labour involved in preparation of a minute quantity of radium are very
great.
M. and Mme Curie were indebted for their first working material to the
Austrian government, who generously presented them with a ton of the
treated residue of uranium materials from the State manufactory of
Joachimsthal in Bohemia. With the assistance of the Academy of Science
and other societies in France, funds were given to carry out the laborious
work of separation. Later the Curies were presented with a ton of residues
from the treatment of pitchblende by the Société Centrale de Produits
Chimiques of Paris. The generous assistance afforded in this important
work is a welcome sign of the active interest taken in these countries in the
furthering of purely scientific research.
The rough concentration and separation of the residues was performed in
the chemical works, and there followed a large amount of labour in
purification and concentration. In this manner, the Curies were able to
obtain a small quantity of radium which was enormously active compared
with uranium. No definite results have yet been given on the activity of
pure radium, but the Curies estimate that it is about one million times that
of uranium, and may possibly be still higher. The difficulty of making a
numerical estimate for such an intensely active body is very great. In the
electric method, the activities are compared by noting the relative strength
of the maximum or saturation current between two parallel plates, on one of
which the active substance is spread. On account of the intense ionization of
the gas between the plates, it is not possible to reach the saturation current
unless very high voltages are applied. Approximate comparisons can be
made by the use of metal screens to cut down the intensity of the radiations,
if the proportion of the radiation transmitted by such a screen has been
determined by direct experiment on impure material of easily measurable
activity. The value of the activity of radium compared with that of uranium

will however vary to some extent according to which of the three types of
rays is taken as a basis of comparison.
It is thus difficult to control the final stages of the purification of radium by
measurements of its activity alone. Moreover the activity of radium
immediately after its preparation is only about one-fourth of its final value;
it gradually rises to a maximum after the radium salt has been kept in the
dry state for about a month. For control experiments in purification, it is
advisable to measure the initial rather than the final activity.
Mme Curie has utilized the coloration of the crystals of radiferous barium
as a means of controlling the final process of purification. The crystals of
salts of radium and barium deposited from acid solutions are
indistinguishable by the eye. The crystals of radiferous barium are at first
colourless, but, in the course of a few hours, become yellow, passing to
orange and sometimes to a beautiful rose colour. The rapidity of this
coloration depends on the amount of barium present. Pure radium crystals
do not colour, or at any rate not as rapidly as those containing barium. The
coloration is a maximum for a definite proportion of radium, and this fact
can be utilized as a means of testing the amount of barium present. When
the crystals are dissolved in water the coloration disappears.
Giesel[18] has observed that pure radium bromide gives a beautiful carmine
colour to the Bunsen flame. If barium be present in any quantity, only the
green colour due to barium is observed, and a spectroscopic examination
shows only the barium lines. This carmine coloration of the Bunsen flame is
a good indication of the purity of the radium.
Since the preliminary announcement of the discovery of radium, Giesel[19]
has devoted a great deal of attention to the separation of radium, polonium
and other active bodies from pitchblende. He was indebted for his working
material to the firm of P. de Haen, of Hanover, who presented him with a
ton of pitchblende residues. Using the method of fractional crystallization
of the bromide instead of the chloride, he has been able to prepare
considerable quantities of pure radium. By this means the labour of final
purification of radium has been much reduced. He states that six or eight
crystallizations with the bromide are sufficient to free the radium almost
completely from the barium.

13. Spectrum of radium. It was of great importance to settle as soon as
possible whether radium was in reality modified barium or a new element
with a definite spectrum. For this purpose the Curies prepared some
specimens of radium chloride, and submitted them for examination of their
spectrum to Demarçay, an authority on that subject. The first specimen of
radium chloride examined by Demarçay[20] was not very active, but showed,
besides the lines due to barium, a very strong new line in the ultra-violet. In
another sample of greater activity, the line was still stronger and others also
appeared, while the intensity of the new lines was comparable with those
present due to barium. With a still more active specimen which was
probably nearly pure, only three strong lines of barium appeared, while the
new spectrum was very bright. The following table shows the wave-length
of the new lines observed for radium. The wave lengths are expressed in
Ångström units and the intensity of each ray is denoted by a number, the
ray of maximum intensity being 16.
Wave Intensity Wave Intensity
length length
4826·3 10 4600·3 3
4726·9 5 4533·5 9
4699·6 3 4436·1 6
4692·1 7 4340·6 12
4683·0 14 3814·7 16
4641·9 4 3649·6 12
The lines are all sharply defined, and three or four of them have an intensity
comparable with any known lines of other substances. There are also
present in the spectrum two strong nebulous bands. In the visible part of the
spectrum, which has not been photographed, the only noticeable ray has a
wave length 5665, which is, however, very feeble compared with that of
wave length 4826·3. The general aspect of the spectrum is similar to that of
the alkaline earths; it is known that these metals have strong lines
accompanied by nebulous bands.
The principal line due to radium can be distinguished in impure radium of
activity 50 times that of uranium. By the electrical method it is easy to
distinguish the presence of radium in a body which has an activity only ¹⁄₁₀₀

of uranium. With a more sensitive electrometer ¹⁄₁₀₀₀₀ of the activity of
uranium could be observed. For the detection of radium, the examination of
the radio-activity is thus a process nearly a million times more sensitive
than spectrum analysis.
Later observations on the spectrum of radium have been made by Runge[21],
Exner and Haschek[22], with specimens of radium prepared by Giesel.
Crookes[23] has photographed the spectrum of radium in the ultra-violet,
while Runge and Precht[24], using a highly purified sample of radium,
observed a number of new lines in the spark spectrum. It has been
mentioned already that the bromide of radium gives a characteristic pure
carmine-red coloration to the Bunsen flame. The flame spectrum shows two
broad bright bands in the orange-red, not observed in Demarçay’s spectrum.
In addition there is a line in the blue-green and two feeble lines in the
violet.

14. Atomic weight of radium. Mme Curie has made successive
determinations of the atomic weight of the new element with specimens of
steadily increasing purity. In the first observation the radium was largely
mixed with barium, and the atomic weight obtained was the same as that of
barium, 137·5. In successive observations with specimens of increasing
purity the atomic weights of the mixture were 146 and 175. The final value
obtained recently was 225, which may be taken as the atomic weight of
radium on the assumption that it is divalent.
In these experiments about 0·1 gram of pure radium chloride was obtained
by successive fractionations. The difficulty involved in preparing a quantity
of pure radium chloride large enough to test the atomic weight may be
gauged from the fact that only a few centigrams of fairly pure radium, or a
few decigrams of less concentrated material, are obtained from the
treatment of about 2 tons of the mineral from which it is derived.
Runge and Precht[25] have examined the spectrum of radium in a magnetic
field, and have shown the existence of series analogous to those observed
for calcium, barium, and strontium. These series are connected with the
atomic weights of the elements in question, and Runge and Precht have
calculated by these means that the atomic weight of radium should be 258
—a number considerably greater than the number 225 obtained by Mme

Curie by means of chemical analysis. Marshall Watts[26], on the other hand,
using another relation between the lines of the spectrum, deduced the value
obtained by Mme Curie. Runge[27] has criticised the method of deduction
employed by Marshall Watts on the ground that the lines used for
comparison in the different spectra were not homologous. Considering that
the number found by Mme Curie agrees with that required by the periodic
system, it is advisable in the present state of our knowledge to accept the
experimental number rather than the one deduced by Runge and Precht
from spectroscopic evidence.
There is no doubt that radium is a new element possessing remarkable
physical properties. The detection and separation of this substance, existing
in such minute proportions in pitchblende, has been due entirely to the
characteristic property we are considering, and is the first notable triumph
of the study of radio-activity. As we shall see later, the property of radio-
activity can be used, not only as a means of chemical research, but also as
an extraordinarily delicate method of detecting chemical changes of a very
special kind.

15. Radiations from radium. On account of its enormous activity, the
radiations from radium are very intense: a screen of zinc sulphide, brought
near a few centigrams of radium bromide, is lighted up quite brightly in a
dark room, while brilliant fluorescence is produced on a screen of platino-
barium cyanide. An electroscope brought near the radium salt is discharged
almost instantly, while a photographic plate is immediately affected. At a
distance of one metre, a day’s exposure to the radium rays would produce a
strong impression. The radiations from radium are analogous to those of
uranium, and consist of three types of rays: easily absorbed, penetrating,
and very penetrating. Radium also gives rise to an emanation similar to that
of thorium, but with a very much slower rate of decay. The radium
emanation retains its activity for several weeks, while that of thorium lasts
only a few minutes. The emanation obtained from a few centigrams of
radium illuminates a screen of zinc sulphide with great brilliancy. The very
penetrating rays of radium are able to light up an X ray screen in a dark
room, after passage through several centimetres of lead and several inches
of iron.

As in the case of uranium or thorium, the photographic action is mainly due
to the penetrating or cathodic rays. The radiographs obtained with radium
are very similar to those obtained with X rays, but lack the sharpness and
detail of the latter. The rays are unequally absorbed by different kinds of
matter, the absorption varying approximately as the density. In photographs
of the hand the bones do not stand out as in X ray photographs.
Curie and Laborde have shown that the compounds of radium possess the
remarkable property of always keeping their temperature several degrees
above the temperature of the surrounding air. Each gram of radium radiates
an amount of energy corresponding to 100 gram-calories per hour. This and
other properties of radium are discussed in detail in chapters V and XII.

16. Compounds of radium. When first prepared in the solid state, all the
salts of radium—the chloride, bromide, acetate, sulphate, and carbonate—
are very similar in appearance to the corresponding salts of barium, but in
time they gradually become coloured. In chemical properties the salts of
radium are practically the same as those of barium, with the exception that
the chloride and bromide of radium are less soluble in water than the
corresponding salts of barium. All the salts of radium are naturally
phosphorescent. The phosphorescence of impure radium preparations is in
some cases very marked.
All the radium salts possess the property of causing rapid colorations of the
glass vessel which contains them. For feebly active material the colour is
usually violet, for more active material a yellowish-brown, and finally
black.

17. Actinium. The discovery of radium in pitchblende gave a great impetus
to the chemical examination of uranium residues, and a systematic search
early led to the detection of several new radio-active bodies. Although these
show distinctive radio-active properties, so far none of them have been
purified sufficiently to give a definite spectrum as in the case of radium.
One of the most interesting and important of these substances was
discovered by Debierne[28] while working up the uranium residues, obtained
by M. and Mme Curie from the Austrian government, and was called by
him actinium. This active substance is precipitated with the iron group, and

appears to be very closely allied in chemical properties to thorium, though it
is many thousand times more active. It is very difficult to separate from
thorium and the rare earths. Debierne has made use of the following
methods for partial separation:
(1) Precipitation in hot solutions, slightly acidulated with hydrochloric acid,
by excess of hyposulphite of soda. The active matter is present almost
entirely in the precipitate.
(2) Action of hydrofluoric acid upon the hydrates freshly precipitated, and
held in suspension in water. The portion dissolved is only slightly active. By
this method titanium may be separated.
(3) Precipitation of neutral nitrate solutions by oxygenated water. The
precipitate carries down the active body.
(4) Precipitation of insoluble sulphates. If barium sulphate, for example, is
precipitated in the solution containing the active body, the barium carries
down the active matter. The thorium and actinium are freed from the barium
by conversion of the sulphate into the chloride and precipitation by
ammonia.
In this way Debierne has obtained a substance comparable in activity with
radium. The separation, which is difficult and laborious, has not yet been
carried far enough to bring out any new lines in the spectrum.

18. After the initial announcement of the discovery of actinium, several
years elapsed before any definite results upon it were published by
Debierne. In the meantime, Giesel[29] had independently obtained a radio-
active substance from pitchblende which seemed similar in many respects
to the actinium of Debierne. The active substance belongs to the group of
cerium earths and is precipitated with them. By a succession of chemical
operations, the active substance is separated mixed with lanthanum. While
intensely active in comparison with thorium, the new active substance
closely resembles it in radio-active properties, although, from the method of
separation thorium cannot be present except in minute quantity. Giesel early
observed that the substance gave off a radio-active emanation. On account
of the intensity of the emanation it emits, he termed it the “emanating
substance.” Recently this name has been changed to “emanium,” and under

this title preparations of the active substance prepared by Giesel have been
placed on the market.
Giesel found that the activity of this substance was permanent and seemed
to increase during the six months’ interval after separation. In this respect it
is similar to radium compounds, for the activity of radium, measured by the
electric method, increases in the course of a month’s interval to four times
its initial value at separation.
There can be no doubt that the “actinium” of Debierne and the “emanium”
of Giesel contain the same radio-active constituent, for recent work[30] has
shown that they exhibit identical radio-active properties. Each gives out
easily absorbed and penetrating rays, and emits a characteristic emanation
of which the rate of decay is the same for both substances. The rate of decay
of the emanation is the simplest method of distinguishing actinium from
thorium, which it resembles so closely in radio-active as well as in chemical
properties. The emanation of actinium loses its radiating power far more
rapidly than that of thorium, the time taken for the activity to fall to half
value being in the two cases 3·7 seconds and 52 seconds respectively.
The rapid and continuous emission of this short-lived emanation is the most
striking radio-active property possessed by actinium. In still air, the radio-
active effects of this emanation are confined to a distance of a few
centimetres from the active material, as it is only able to diffuse a short
distance through the air before losing its radiating power. With very active
preparations of actinium, the material appears to be surrounded by a
luminous haze produced by the emanation. The radiations produce strong
luminosity in some substances, for example, zinc sulphide, willemite and
platinocyanide of barium. The luminosity is especially marked on screens
of zinc sulphide. Much of this effect is due to the emanation, for, on gently
blowing a current of air over the substance, the luminosity is displaced at
once in the direction of the current. With a zinc sulphide screen, actinium
shows the phenomena of “scintillations” to an even more marked degree
than radium itself.
The preparations of emanium are in some cases luminous, and a
spectroscopic examination of this light has shown a number of bright
lines[31].

The distinctive character of the emanation of actinium, as well as of the
other radio-active products to which it gives rise, coupled with the
permanence of its activity, renders it very probable that actinium will prove
to be a new radio-active element of very great activity. Although very active
preparations of actinium have been obtained, it has not yet been found
possible to free it from impurities. Consequently, no definite observations
have been made on its chemical properties, and no new spectrum lines have
been observed.
A more complete discussion of the radio-active and other properties of
actinium is given in later chapters.

19. Polonium. Polonium was the first of the active substances obtained
from pitchblende. It has been investigated in detail by its discoverer Mme
Curie[32]. The pitchblende was dissolved in acid and sulphuretted hydrogen
added. The precipitated sulphides contained an active substance, which,
after separation of impurities, was found associated with bismuth. This
active substance, which has been named polonium, is so closely allied in
chemical properties to bismuth that it has so far been found impossible to
effect a complete separation. Partial separation of polonium can be made by
successive fractionations based on one of the following modes of
procedure:
(1) Sublimation in a vacuum. The active sulphide is more volatile than that
of bismuth. It is deposited as a black substance at those parts of the tube,
where the temperature is between 250 and 300° C. In this way polonium of
activity 700 times that of uranium was obtained.
(2) Precipitation of nitric acid solutions by water. The precipitated sub-
nitrate is much more active than the part that remains in solution.
(3) Precipitation by sulphuretted hydrogen in a very acid hydrochloric acid
solution. The precipitated sulphides are much more active than the salt
which remains in solution.
For concentration of the active substance Mme Curie[33] has made use of
method (2). The process is, however, very slow and tedious, and is made
still more complicated by the tendency to form precipitates insoluble either
in strong or weak acids. After a large number of fractionations, a small

quantity of matter was obtained, enormously active compared with
uranium. On examination of the substance spectroscopically, only the
bismuth lines were observed. A spectroscopic examination of the active
bismuth by Demarçay and by Runge and Exner has led to the discovery of
no new lines. On the other hand Sir William Crookes[34] states that he found
one new line in the ultra-violet, while Berndt[35], working with polonium of
activity 300, observed a large number of new lines in the ultra-violet. These
results await further confirmation.
The polonium prepared by Mme Curie differs from the other radio-active
bodies in several particulars. In the first place the radiations include only
very easily absorbable rays. The two penetrating types of radiation given
out by uranium, thorium, and radium are absent. In the second place the
activity does not remain constant, but diminishes continuously with the
time. Mme Curie states that different preparations of polonium had
somewhat different rates of decay. In some cases, the activity fell to half
value in about six months, and in others, about half value in eleven months.

20. The gradual diminution of the activity of polonium with time seemed at
first sight to differentiate it from such substances as uranium and radium,
the activity of which appeared fairly permanent. This difference in
behaviour is, however, one of degree rather than of kind. We shall show
later that there is present in pitchblende a number of radio-active
substances, the activity of which is not permanent. The time taken for these
bodies to lose half of their activity varies in different cases from a few
seconds to several hundreds of years. In fact, this gradual loss of activity is
an essential feature of our theory of regarding the phenomena of radio-
activity. No radio-active substance, left to itself, can continue to radiate
indefinitely; it must ultimately lose its activity. In the case of bodies like
uranium and radium, the loss of activity is so slow that no sensible
alteration has been observed over a period of several years, but it can be
deduced theoretically that the activity of radium will eventually decrease to
half value in a period of about 1000 years, while in the case of a feebly
radio-active substance like uranium, more than a 100 million years must
elapse before the diminution of the activity becomes appreciable.

It may be of interest here to consider briefly the suggestions advanced at
various times to account for the temporary character of the activity of
polonium. Its association with bismuth led to the view that polonium was
not a new active substance, but merely radio-active bismuth, that is,
bismuth which in some way had been made active by admixture with radio-
active bodies. It was known that a body placed in the vicinity of thorium or
radium became temporarily active. The same action was supposed to take
place when inactive matter was in solution with active matter. The non-
active matter was supposed to acquire activity by “induction,” as it was
called, in consequence of its intimate contact with the active material.
There is no proof, however, that such is the case. The evidence points rather
to the conclusion that the activity is due, not to an alteration of the inactive
body itself, but to an admixture with it of a very small quantity of intensely
active matter. This active matter is present in pitchblende and is separated
with the bismuth but differs from it in chemical properties.
The subject cannot be considered with advantage at this stage, but will be
discussed later in detail in chapter XI. It will there be shown that polonium,
that is, the radio-active constituent mixed with the bismuth, is a distinct
chemical substance, which is allied in chemical properties to bismuth, but
possesses some distinct analytical properties which allow of a partial
separation from it.
The polonium, if obtained in a pure state, should initially be several
hundred times as active as pure radium. This activity, however, is not
permanent; it decays with the time, falling to half value in about six months.
The absence of any new lines in the spectrum of radio-active bismuth is to
be expected, for, even in the most active bismuth prepared, the active matter
exists in a very small proportion.

21. The discussion of the nature of polonium was renewed by the discovery
of Marckwald[36] that a substance similar to polonium can be separated from
pitchblende; the activity of this substance, he stated, did not decay
appreciably with the time. The method of separation from the bismuth
chloride solution, obtained from uranium residues, was very simple. A rod
of bismuth or antimony, dipped in the active solution, rapidly became
coated with a black deposit which was intensely active. This process was

continued until the whole of the activity was removed from the solution.
The active deposit gave out only easily absorbed rays, and in that respect
resembled the polonium of Mme Curie.
The active substance was found to consist mainly of tellurium, and for this
reason Marckwald gave it the name of radio-tellurium. In later work,
however, Marckwald[37] has shown that the active constituent has no
connection with tellurium, but can always be separated completely from it
by a simple chemical process.
In order to obtain a large amount of the active substance, 2000 kilos. of
pitchblende were worked up. This yielded 6 kilos. of bismuth oxychloride,
and from this was separated 1·5 grams of radio-tellurium. The tellurium
present was precipitated from a hydrochloric acid solution by hydrazine
hydrochloride. The precipitated tellurium still showed some activity, but
this was removed by repeating the process. The active matter then remained
in the filtrate, and, after evaporation, the addition of a few drops of stannous
chloride caused a small quantity of a dark precipitate which was intensely
active. This was collected on a filter and weighed only 4 milligrams.
When plates of copper, tin or bismuth were dipped into an hydrochloric acid
solution of this active substance, the plates were found to be covered with a
very finely divided deposit. These plates were intensely active, and
produced marked photographic and phosphorescent action. As an
illustration of the enormous activity of this deposit, Marckwald stated that a
precipitate of ¹⁄₁₀₀ milligram on a copper plate, 4 square centimetres in area,
illuminated a zinc sulphide screen so brightly that it could be seen by an
audience of several hundred people.
The active substance of Marckwald is very closely allied in chemical and
radio-active properties to the polonium of Mme Curie. Both active
substances are separated with bismuth and both give out only easily
absorbed rays. The penetrating rays, such as are given out by uranium,
radium or thorium, are completely absent.
There has been a considerable amount of discussion as to whether the active
substance obtained by Marckwald is identical with that present in the
polonium of Mme Curie. Marckwald stated that his active substance did not
sensibly diminish in activity in the course of six months, but it is doubtful
whether the method of measurement used was sufficiently precise.

The writer has found that radio-tellurium of moderate activity, prepared
after Marckwald’s method and sold by Dr Sthamer of Hamburg,
undoubtedly loses its activity with time. The radio-tellurium is obtained in
the form of a thin radio-active deposit on a polished bismuth rod or plate. A
bismuth rod was found to have lost half its activity in about 150 days, and a
similar result has been recorded by other observers.
The two substances are thus similar in both radio-active and chemical
properties, and there can be no reasonable doubt that the active constituent
present in each case is the same. The evidence is discussed in detail in
chapter XI and it will there be shown that the active substance present in the
radio-tellurium of Marckwald is a slow transformation product of radium.

22. Radio-active lead. Several observers early noticed that the lead
separated from pitchblende showed strong radio-active properties, but
considerable difference of opinion was expressed in regard to the
permanence of its activity. Elster and Geitel[38] found that lead sulphate
obtained from pitchblende was very active, but they considered that the
activity was probably due to an admixture of radium or polonium with the
lead, and, by suitable chemical treatment, the lead sulphate was obtained in
an inactive state. Giesel[39] also separated some radio-active lead but found
that its activity diminished with the time. On the other hand, Hofmann and
Strauss[40] obtained lead from pitchblende whose activity seemed fairly
permanent. They state that the radio-active lead resembled ordinary lead in
most of its reactions, but showed differences in the behaviour of the
sulphide and sulphate. The sulphate was found to be strongly
phosphorescent. These results of Hofmann and Strauss were subjected at
the time of their publication to considerable criticism, and there is no doubt
that the lead itself is not radio-active but contains a small quantity of radio-
active matter which is separated with it. In later work[41], it has been shown
that radio-lead contains several radio-active constituents which can be
removed temporarily from it by suitable chemical methods.
There can be no doubt that the lead separated from pitchblende by certain
methods does show considerable activity and that this activity is fairly
permanent. The radio-active changes occurring in radio-lead are
complicated and cannot be discussed with advantage at this stage, but will

be considered in detail in chapter XI. It will there be shown that the primary
constituent present in lead is a slow transformation product of radium. This
substance then slowly changes into the active constituent present in
polonium, which gives out only easily absorbed rays.
This polonium can be separated temporarily from the lead by suitable
chemical methods, but the radio-lead still continues to produce polonium,
so that a fresh supply may be obtained from it, provided an interval of
several months is allowed to elapse.
It will be calculated later that in all probability the radio-lead would lose
half of its activity in an interval of 40 years.
The constituent present in radio-lead has not yet been separated, but it will
be shown that, in the pure state, it should have an activity considerably
greater than that of radium itself. Sufficient attention has not yet been paid
to this substance, for, separated in a pure state, it should be as useful
scientifically as radium. In addition, since it is the parent of polonium, it
should be possible to obtain from it at any time a supply of very active
polonium, in the same way that a supply of the radium emanation can be
obtained at intervals from radium.
Hofmann and Strauss have observed a peculiar action of the cathode rays
on the active lead sulphate separated by them. They state that the activity
diminishes with time, but is recovered by exposure of the lead for a short
time to the action of cathode rays. No such action is shown by the active
lead sulphide. This effect is due most probably to the action of the cathode
rays in causing a strong phosphorescence of the lead sulphate and has
nothing to do with the radio-activity proper of the substance.

23. Is thorium a radio-active element? The similarity of the chemical
properties of actinium and thorium has led to the suggestion at different
times that the activity of thorium is not due to thorium itself, but to the
presence of a slight trace of actinium. In view of the difference in the rate of
decay of the emanations of thorium and actinium, this position is not
tenable. If the activity of thorium were due to actinium, the two emanations,
as well as the other products obtained from these substances, should have
identical rates of decay. Since there is not the slightest evidence that the rate
of decay of activity of the various products can be altered by chemical or

physical agencies, we may conclude with confidence that whatever radio-
active substance is responsible for the activity of thorium, it certainly is not
actinium. This difference in the rate of decay of the active products is of far
more weight in deciding the question whether two bodies contain the same
radio-active constituent than differences in chemical behaviour, for it is
quite probable that the active material in each case may exist only in minute
quantity in the matter under examination, and, under such conditions, a
direct chemical examination in the first place is of little value.
Recent work of Hofmann and Zerban and of Baskerville, however, certainly
tends to show that the element thorium is itself non-radio-active, and that
the radio-activity observed in ordinary thorium compounds is due to the
admixture with it of an unknown radio-active element. Hofmann and
Zerban[42] made a systematic examination of the radio-activity of thorium
obtained from different mineral sources. They found generally that thorium,
obtained from minerals containing a large percentage of uranium, were
more active than those obtained from minerals nearly free from uranium.
This indicates that the radio-activity observed in thorium may possibly be
due to a transformation product of uranium which is closely allied
chemically to thorium and is always separated with it. A small quantity of
thorium obtained from the mineral gadolinite was found by Hofmann to be
almost inactive, whether tested by the electric or by the photographic
method. Later Baskerville and Zerban[43] found that thorium obtained from a
Brazilian mineral was practically devoid of activity.
In this connection the recent work of Baskerville on the complexity of
ordinary thorium is of interest. By special chemical methods, he succeeded
in separating two new and distinct substances from thorium, which he has
named carolinium and berzelium. Both of these substances are strongly
radio-active, and it thus seems probable that the active constituent observed
in ordinary thorium may be due to one of these elements.
If, as we have suggested, thorium itself is not active, it is certainly a matter
of surprise that ordinary commercial thorium and the purest chemical
preparations show about the same activity. Such a result indicates that the
methods of purification have not removed any of the radio-active
constituent originally present.

Whatever the radio-active constituent in thorium may ultimately prove to
be, it is undoubtedly not radium nor actinium nor any of the known radio-
active substances.
In later chapters, the radio-activity of thorium will, for simplicity, be
discussed on the assumption that thorium is itself a radio-active element.
The analysis of the changes which occur will thus not refer to thorium itself
but to the primary radio-active substance usually found associated with it.
The conclusions to be drawn from an examination of the radio-active
processes are for the most part independent of whether thorium is itself
radio-active or whether the radio-activity is due to an unknown element. If
thorium is not radio-active itself, it is not possible to draw any conclusions
upon the question of the duration of the primary radio-activity associated
with it. Such a deduction cannot be made until the quantity of the radio-
active element present in thorium has been definitely determined.

24. If elements heavier than uranium exist, it is probable that they will be
radio-active. The extreme delicacy of radio-activity as a means of chemical
analysis would enable such elements to be recognized even if present in
infinitesimal quantities. It is probable that considerably more than the three
or four radio-elements at present recognized exist in minute quantity, and
that the number at present known will be augmented in the future. In the
first stage of the search, a purely chemical examination is of little value, for
it is not probable that the new element should exist in sufficient quantity to
be detected by chemical or spectroscopic analysis. The main criteria of
importance are the existence or absence of distinctive radiations or
emanations, and the permanence of the radio-activity. The discovery of a
radio-active emanation with a rate of decay different from those already
known would afford strong evidence that a new radio-active body was
present. The presence of either thorium or radium in matter can very readily
be detected by observing the rate of decay of the emanations given out by
them. When once the existence of a new radio-element has been inferred by
an examination of its radio-active properties, chemical methods of
separation can be devised, the radiating or emanating property being used
as a guide in qualitative and quantitative analysis.

CHAPTER II.
IONIZATION THEORY OF GASES.

25. Ionization of gases by radiation. The most important property
possessed by the radiations from radio-active bodies is their power of
discharging bodies whether positively or negatively electrified. As this
property has been made the basis of a method for an accurate quantitative
analysis and comparison of the radiations, the variation of the rate of
discharge under different conditions and the processes underlying it will be
considered in some detail.
In order to explain the similar discharging power of Röntgen rays, the
theory[44] has been put forward that the rays produce positively and
negatively charged carriers throughout the volume of the gas surrounding
the charged body, and that the rate of production is proportional to the
intensity of the radiation. These carriers, or ions[45] as they have been
termed, move with a uniform velocity through the gas under a constant
electric field, and their velocity varies directly as the strength of the field.

Fig. 1.

Suppose we have a gas between two metal plates A and B (Fig. 1) exposed
to the radiation, and that the plates are kept at a constant difference of

potential. A definite number of ions will be produced per second by the
radiation, and the number produced will depend in general upon the nature
and pressure of the gas. In the electric field the positive ions travel towards
the negative plate, and the negative ions towards the positive, and
consequently a current will pass through the gas. Some of the ions will also
recombine, the rate of recombination being proportional to the square of the
number present. For a given intensity of radiation, the current passing
through the gas will increase at first with the potential difference between
the plates, but it will reach a limit when all the ions are removed by the
electric field before any recombination occurs.
This theory accounts also for all the characteristic properties of gases made
conducting by the rays from active substances, though there are certain
differences observed between the conductivity phenomena produced by
active substances and by X rays. These differences are for the most part the
result of unequal absorption of the two types of rays. Unlike Röntgen rays,
a large proportion of the radiation from active bodies consists of rays which
are absorbed in their passage through a few centimetres of air. The
ionization of the gas is thus not uniform, but falls off rapidly with increase
of distance from the active substance.

26. Variation of the current with voltage. Suppose that a layer of radio-
active matter is spread uniformly on the lower of two horizontal plates A
and B (Fig. 1). The lower plate A is connected with one pole of a battery of
cells the other pole of which is connected with earth. The plate B is
connected with one pair of quadrants of an electrometer, the other pair
being connected with earth.
The current[46] between the plates, determined by the rate of movement of
the electrometer needle, is observed at first to increase rapidly with the
voltage, then more slowly, finally reaching a value which increases very
slightly with a large increase in the voltage. This, as we have indicated, is
simply explained on the ionization theory.
The radiation produces ions at a constant rate, and, before the electric field
is applied, the number per unit volume increases until the rate of production
of fresh ions is exactly balanced by the recombination of the ions already

produced. On application of a small electric field, the positive ions travel to
the negative electrode and the negative to the positive.
Since the velocity of the ions between the plates is directly proportional to
the strength of the electric field, in a weak field the ions take so long to
travel between the electrodes that most of them recombine on the way.
The current observed is consequently small. With increase of the voltage
there is an increase of speed of the ions and a smaller number recombine.
The current consequently increases, and will reach a maximum value when
the electric field is sufficiently strong to remove all the ions before
appreciable recombination has occurred. The value of the current will then
remain constant even though the voltage is largely increased.
This maximum current will be called the “saturation” current, and the value
of the potential difference required to give this maximum current, the
“saturation P.D.”[47]
The general shape of the current-voltage curve is shown in Fig. 2, where the
ordinates represent current and the abscissae volts.

Fig. 2.

Although the variation of the current with voltage depends only on the
velocity of the ions and their rate of recombination, the full mathematical
analysis is intricate, and the equations, expressing the relation between
current and voltage, are only integrable for the case of uniform ionization.
The question is complicated by the inequality in the velocity of the ions and
by the disturbance of the potential gradient between the plates by the

movement of the ions. J. J. Thomson[48] has worked out the case for uniform
production of ions between two parallel plates, and has found that the
relation between the current i and the potential difference V applied is
expressed by
Ai2 + Bi = V

where A and B are constants for a definite intensity of radiation and a
definite distance between the plates.

Fig. 3.

In certain cases of unsymmetrical ionization, which arise in the study of the
radiations from active bodies, the relation between current and voltage is
very different from that expressed by the above equation. Some of these
cases will be considered in section 47.

27. The general shape of the current-voltage curves for gases exposed to the
radiations from active bodies is shown in Fig. 3.
This curve was obtained for ·45 grams of impure radium chloride, of
activity 1000 times that of uranium, spread over an area of 33 sq. cms. on
the lower of two large parallel plates, 4·5 cms. apart. The maximum value

of the current observed, which is taken as 100, was 1·2 × 10-8 amperes, the
current for low voltages was nearly proportional to the voltage, and about
600 volts between the plates was required to ensure approximate saturation.
In dealing with slightly active bodies like uranium or thorium, approximate
saturation is obtained for much lower voltages. Tables I. and II. show the
results for the current between two parallel plates distant 0·5 cms. and 2·5
cms. apart respectively, when one plate was covered with a thin uniform
layer of uranium oxide.

Table I.

0·5 cms. apart

Volts Current
·125 18
·25 36
·5 55
1 67
2 72
4 79
8 85
16 88
100 94
335 100

Table II.

2·5 cms. apart

Volts Current
·5 7·3
1 14
2 27
4 47

8 64
16 73
37·5 81
112 90
375 97
800 100
The results are shown graphically in Fig. 4.

Fig. 4.

From the above tables it is seen that the current at first increases nearly in
proportion to the voltage. There is no evidence of complete saturation,
although the current increases very slowly for large increases of voltage.
For example, in Table I. a change of voltage from ·125 to ·25 volts
increases the current from 18 to 36% of the maximum, while a change of
voltage from 100 to 335 volts increases the current only 6%. The variation
of the current per volt (assumed uniform between the range of voltages
considered) is thus about 5000 times greater for the former change.

Taking into consideration the early part of the curves, the current does not
reach a practical maximum as soon as would be expected on the simple
ionization theory. It seems probable that the slow increase with the large
voltages is due either to an action of the electric field on the rate of
production of ions, or to the difficulty of removing the ions produced near
the surface of the uranium before recombination. It is possible that the
presence of a strong electric field may assist in the separation of ions which
otherwise would not initially escape from the sphere of one another’s
attraction. From the data obtained by Townsend for the conditions of
production of fresh ions at low pressures by the movement of ions through
the gas, it seems that the increase of current cannot be ascribed to an action
of the moving ions in the further ionization of the gas.

28. The equation expressing the relation between the current and the voltage
is very complicated even in the case of a uniform rate of production of ions
between the plates. An approximate theory, which is of utility in
interpreting the experimental results, can however be simply deduced if the
disturbance of the potential gradient is disregarded, and the ionization
assumed uniform between the plates.
Suppose that the ions are produced at a constant rate q per cubic centimetre
per second in the gas between parallel plates distant l cms. from each other.
When no electric field is applied, the number N present per c.c., when there
is equilibrium between the rates of production and recombination, is given
by
q = αN2,

where α is a constant.
If a small potential difference V is applied, which gives only a small
fraction of the maximum current, and consequently has not much effect on
the value of N, the current i per sq. cm. of the plate, is given by
NeuV
i = -----
l

where u is the sum of the velocity of the ions for unit potential gradient, and
e is the charge carried by an ion.
uV
-----
l

is the velocity of the ions in the electric field of strength
V
----
l

The number of ions produced per second in a prism of length l and unit area
of cross-section is ql. The maximum or saturation current I per sq. cm. of
the plate is obtained when all of these ions are removed to the electrodes
before any recombination has occurred.
Thus
I = q . l . e,

and

This equation expresses the fact previously noted that, for small voltages,
the current i is proportional to V.
Let
i/I = ρ,

then

Now the greater the value of V required to obtain a given value of ρ
(supposed small compared with unity), the greater the potential required to
produce saturation.
It thus follows from the equation that:
(1) For a given intensity of radiation, the saturation P.D. increases with the
distance between the plates. In the equation, for small values of ρ, V varies
as l2. This is found to be the case for uniform ionization, but it only holds
approximately for non-uniform ionization.
(2) For a given distance between the plates, the saturation P.D. is greater, the
greater the intensity of ionization between the plates. This is found to be the
case for the ionization produced by radio-active substances. With a very
active substance like radium, the ionization produced is so intense that very
large voltages are required to produce approximate saturation. On the other
hand, only a fraction of a volt per cm. is necessary to produce saturation in
a gas where the ionization is very slight, for example, in the case of the
natural ionization observed in a closed vessel, where no radio-active
substances are present.
For a given intensity of radiation, the saturation P.D. decreases rapidly with
the lowering of the pressure of the gas. This is due to two causes operating
in the same direction, viz. a decrease in the intensity of the ionization and
an increase in the velocity of the ions. The ionization varies directly as the
pressure, while the velocity varies inversely as the pressure. This will
obviously have the effect of causing more rapid saturation, since the rate of
recombination is slower and the time taken for the ions to travel between
the electrodes is less.
The saturation curves observed for the gases hydrogen and carbon
dioxide[49] are very similar in shape to those obtained for air. For a given
intensity of radiation, saturation is more readily obtained in hydrogen than
in air, since the ionization is less than in air while the velocity of the ions is
greater. Carbon dioxide on the other hand requires a greater P.D. to produce
saturation than does air, since the ionization is more intense and the velocity
of the ions less than in air.

29. Townsend[50] has shown that, for low pressures, the variation of the
current with the voltage is very different from that observed at atmospheric
pressure. If the increase of current with the voltage is determined for gases,
exposed to Röntgen rays, at a pressure of about 1 mm. of mercury, it is
found that for small voltages the ordinary saturation curve is obtained; but
when the voltage applied increases beyond a certain value, depending on
the pressure and nature of the gas and the distance between the electrodes,
the current commences to increase slowly at first but very rapidly as the
voltage is raised to the sparking value. The general shape of the current
curve is shown in Fig. 5.

Fig. 5.

The portion OAB of the curve corresponds to the ordinary saturation curve.
At the point B the current commences to increase. This increase of current
has been shown to be due to the action of the negative ions at low pressures
in producing fresh ions by collision with the molecules in their path. The
increase of current is not observed in air at a pressure above 30 mms. until
the P.D. is increased nearly to the value required to produce a spark. This
production of ions by collision is considered in more detail in section 41.

30. Rate of recombination of the ions. A gas ionized by the radiation
preserves its conducting power for some time after it is removed from the
presence of the active body. A current of air blown over an active body will
thus discharge an electrified body some distance away. The duration of this
after conductivity can be examined very conveniently in an apparatus
similar to that shown in Fig. 6.

Fig. 6.

A dry current of air or any other gas is passed at a constant rate through a
long metal tube TL. After passing through a quantity of cotton-wool to
remove dust particles, the current of air passes over a vessel T containing a
radio-active body such as uranium, which does not give off a radio-active
emanation. By means of insulated electrodes A and B, charged to a suitable
potential, the current between the tube and one of these electrodes can be
tested at various points along the tube.
A gauze screen, placed over the cross-section of the tube at D, serves to
prevent any direct action of the electric field in abstracting ions from the
neighbourhood of T.
If the electric field is sufficiently strong, all the ions travel in to the
electrodes at A, and no current is observed at the electrode B. If the current
is observed successively at different distances along the tube, all the
electrodes except the one under consideration being connected to earth, it is
found that the current diminishes with the distance from the active body. If
the tube is of fairly wide bore, the loss of the ions due to diffusion is small,
and the decrease in conductivity of the gas is due to recombination of the
ions alone.
On the ionization theory, the number dn of ions per unit volume which
recombine in the time dt is proportional to the square of the number present.
Thus

dn
--- = αn²,
dt

where α is a constant.
Integrating this equation,
1 1
--- – --- = αt,
n N

if N is the initial number of ions, and n the number after a time t.
The experimental results obtained[51] have been shown to agree very well
with this equation.
In an experiment similar to that illustrated in Fig. 6, using uranium oxide as
a source of ionization, it was found that half the number of ions present in
the gas recombined in 2·4 seconds, and that at the end of 8 seconds one-
fourth of the ions were still uncombined.
Since the rate of recombination is proportional to the square of the number
present, the time taken for half of the ions present in the gas to recombine
decreases very rapidly with the intensity of the ionization. If radium is used,
the ionization is so intense that the rate of recombination is extremely rapid.
It is on account of this rapidity of recombination that large voltages are
necessary to produce saturation in the gases exposed to very active
preparations of radium.
The value of α, which may be termed the coefficient of recombination, has
been determined in absolute measure by Townsend[52], McClung[53] and
Langevin[54] by different experimental methods but with very concordant
results. Suppose, for example, with the apparatus of Fig. 6, the time T, taken
for half the ions to recombine after passing by the electrode A, has been
determined experimentally. Then
1
---- = αT,
N

where N is the number of ions per c.c. present at A. If the saturation current
i is determined at the electrode A, i = NVe, where e is the charge on an ion
and V is the volume of uniformly ionized gas carried by the electrode A per
second. Then
Ve
α = ---- .
iT

The following table shows the value of α obtained for different gases.

Value of α.

Gas Townsend McClung Langevin

Air 3420 × e 3384 × e 3200 × e
Carbon 3500 × e 3492 × e 3400 × e
Dioxide
Hydrogen 3020 × e
The latest determination of the value of e (see section 36) is 3·4 × 10-10 E.S.
units; thus α = 1·1 × 10-6.
Using this value, it can readily be shown from the equation of
recombination that, if 106 ions are present per c.c., half of them recombine
in about 0·9 sec. and 99% in 90 secs.
McClung (loc. cit.) showed that the value of α was approximately
independent of the pressure between ·125 and three atmospheres. In later
observations, Langevin has found that the value of α decreases rapidly
when the pressure is lowered below the limits used by McClung.

31. In experiments on recombination it is essential that the gas should be
free from dust or other suspended particles. In dusty air, the rate of
recombination is much more rapid than in dust-free air, as the ions diffuse
rapidly to the comparatively large dust particles distributed throughout the
gas. The effect of the suspension of small particles in a conducting gas is
very well illustrated by an experiment of Owens[55]. If tobacco smoke is

blown between two parallel plates as in Fig. 1, the current at once
diminishes to a small fraction of its former value, although a P.D. is applied
sufficient to produce saturation under ordinary conditions. A much larger
voltage is then necessary to produce saturation. If the smoke particles are
removed by a stream of air, the current returns at once to its original value.

32. Mobility of the ions. Determinations of the mobility of the ions, i.e. the
velocity of the ions under a potential gradient of 1 volt per cm., have been
made by Rutherford[56], Zeleny[57], and Langevin[58] for gases exposed to
Röntgen rays. Although widely different methods have been employed, the
results have been very concordant, and fully support the view that the ions
move with a velocity proportional to the strength of the field. On the
application of an electric field, the ions almost instantly attain the velocity
corresponding to the field and then move with a uniform speed.
Zeleny[59] first drew attention to the fact that the positive and negative ions
had different velocities. The velocity of the negative ion is always greater
than that of the positive, and varies with the amount of water vapour present
in the gas.
The results, previously discussed, of the variation of the current with
voltage and of the rate of recombination of the ions do not of themselves
imply that the ions produced in gases by the radiations from active bodies
are of the same size as those produced by Röntgen rays under similar
conditions. They merely show that the conductivity under various
conditions can be satisfactorily explained by the view that charged ions are
produced throughout the volume of the gas. The same general relations
would be observed if the ions differed considerably in size and velocity
from those produced by Röntgen rays. The most satisfactory method of
determining whether the ions are identical in the two cases is to determine
the velocity of the ions under similar conditions.
In order to compare the velocity of the ions[60], the writer has used an
apparatus similar to that shown in Fig. 6 on p. 40.
The ions were carried with a rapid constant stream of air past the charged
electrode A, and the conductivity of the gas tested immediately afterwards
at an electrode B, which was placed close to A. The insulated electrodes A

and B were fixed centrally in the metal tube L, which was connected with
earth.
For convenience of calculation, it is assumed that the electric field between
the cylinders is the same as if the cylinders were infinitely long.
Let a and b be the radii of the electrode A, and of the tube L respectively,
and let V = potential of A.
The electromotive intensity X (without regard to sign) at a distance r from
the centre of the tube is given by

Let u1 and u2 be the velocities of the positive and negative ions for a
potential gradient of 1 volt per cm. If the velocity is proportional to the
electric force at any point, the distance dr traversed by the negative ion in
the time dt is given by
dr = Xu2 dt,
or

Let r2 be the greatest distance measured from the axis of the tube from
which the negative ion can just reach the electrode A in the time t taken for
the air to pass along the electrode.
Then

If ρ2 be the ratio of the number of the negative ions that reach the electrode
A to the total number passing by, then

Therefore

Equation 1.

Similarly the ratio ρ1 of the number of positive ions that give up their
charge to the external cylinder to the total number of positive ions is given
by

In the above equations it is assumed that the current of air is uniform over
the cross-section of the tube, and that the ions are uniformly distributed
over the cross-section; also, that the movement of the ions does not
appreciably disturb the electric field. Since the value of t can be calculated
from the velocity of the current of air and the length of the electrode, the
values of the velocities of the ions under unit potential gradient can at once
be determined.
The equation (1) shows that ρ2 is proportional to V,—i.e. that the rate of
discharge of the electrode A varies directly as the potential of A, provided

that the value of V is not large enough to remove all the ions from the gas as
it passes by the electrode. This was found experimentally to be the case.
In the comparison of the velocities, the potential V was adjusted to such a
value that ρ2 was about one half, when uranium oxide was placed in the
tube at L. The active substance was then removed, and an aluminium
cylinder substituted for the brass tube. X rays were allowed to fall on the
centre of this aluminium cylinder, and the strength of the rays adjusted to
give about the same conductivity to the gas as the uranium had done. Under
these conditions the value of ρ2 was found to be the same as for the first
experiment.
This experiment shows conclusively that the ions produced by Röntgen rays
and by uranium move with the same velocity and are probably identical in
all respects. The method described above is not very suitable for an accurate
determination of the velocities, but gave values for the positive ions of
about 1·4 cms. per second per volt per centimetre, and slightly greater
values for the negative ions.

33. The most accurate determinations of the mobility of the ions produced
by Röntgen rays have been made by Zeleny[61] and Langevin[62]. Zeleny
used a method similar in principle to that explained above. His results are
shown in the following table, where K1 is the mobility of the positive ion
and K2 that of the negative ion.
Gas K1 K2 K2/K1 Temperature
Air, dry 1·36 1·87 1·375 13°·5 C.
„ moist 1·37 1·51 1·10 14°
Oxygen, dry 1·36 1·80 1·32 17°
„ moist 1·29 1·52 1·18 16°
Carbon dioxide, 0·76 0·81 1·07 17°·5
dry
„ „ moist 0·81 0·75 0·915 17°
Hydrogen, dry 6·70 7·95 1·15 20°
„ moist 5·30 5·60 1·05 20°

Langevin determined the velocity of the ions by a direct method in which
the time taken for the ion to travel over a known distance was observed.
The following table shows the comparative values obtained for air and
carbon dioxide.
Air K1 Air K2 Air CO2 CO2 CO2
K2/K1 K1 K2 K2/K1
Direct 1·40 1·70 1·22 0·86 0·90 1·05
method
(Langevin)
Current of 1·36 1·87 1·375 0·76 0·81 1·07
gas
(Zeleny)
These results show that for all gases except CO2, there is a marked increase
in the velocity of the negative ion with the dryness of the gas, and that, even
in moist gases, the velocity of the negative ions is always greater than that
of the positive ions. The velocity of the positive ion is not much affected by
the presence of moisture in the gas.
The velocity of the ions varies inversely as the pressure of the gas. This has
been shown by Rutherford[63] for the negative ions produced by ultra-violet
light falling on a negatively charged surface, and later by Langevin[64] for
both the positive and negative ions produced by Röntgen rays. Langevin
has shown that the velocity of the positive ion increases more slowly with
the diminution of pressure than that of the negative ion. It appears as if the
negative ion, especially at pressures of about 10 mm. of mercury, begins to
diminish in size.

34. Condensation experiments. Some experiments will now be described
which have verified in a direct way the theory that the conductivity
produced in gases by the various types of radiation is due to the production
of charged ions throughout the volume of the gas. Under certain conditions,
the ions form nuclei for the condensation of water, and this property allows
us to show the presence of the individual ions in the gas, and also to count
the number present.

It has long been known that, if air saturated with water-vapour be suddenly
expanded, a cloud of small globules of water is formed. These drops are
formed round the dust particles present in the gas, which act as nuclei for
the condensation of water around them. The experiments of R. von
Helmholtz and Richarz[65] had shown that chemical reactions, for example
the combustion of flames, taking place in the neighbourhood, affected the
condensation of a steam-jet. Lenard showed that a similar action was
produced when ultra-violet light fell on a negatively charged zinc surface
placed near the steam-jet. These results suggested that the presence of
electric charges in the gas facilitated condensation.
A very complete study of the conditions of condensation of water on nuclei
has been made by C. T. R. Wilson[66]. An apparatus was constructed which
allowed a very sudden expansion of the air over a wide range of pressure.
The amount of condensation was observed in a small glass vessel. A beam
of light was passed into the apparatus which allowed the drops formed to be
readily observed by the eye.
Preliminary small expansions caused a condensation of the water round the
dust nuclei present in the air. These dust nuclei were removed by allowing
the drops to settle. After a number of successive small expansions, the air
was completely freed from dust, so that no condensation was produced.
Let v1 = initial volume of the gas in the vessel, v2 = volume after expansion.
If v2/v1 < 1·25 no condensation is produced in dust-free air. If however v2/v1
> 1·25 and < 1·38, a few drops appear. This number is roughly constant
until v2/v1 = 1·38, when the number suddenly increases and a very dense
cloud of fine drops is produced.
If the radiation from an X ray tube or a radio-active substance is now
passed into the condensation vessel, a new series of phenomena is observed.
As before, if v2/v1 < 1·25 no drops are formed, but if v2/v1 = 1·25 there is a
sudden production of a cloud. The water drops of which this cloud is
formed are finer and more numerous the greater the intensity of the rays.
The point at which condensation begins is very marked, and a slight
variation of the amount of expansion causes either a dense cloud or no
cloud at all.

It now remains to be shown that the formation of a cloud by the action of
the rays is due to the productions of ions in the gas. If the expansion vessel
is provided with two parallel plates between which an electric field can be
applied, it is seen that the number of drops, formed by the expansion with
the rays acting, decreases with increase of the electric field. The stronger
the field the smaller the number of drops formed. This result is to be
expected if the ions are the centres of condensation; for in a strong electric
field the ions are carried at once to the electrodes, and thus disappear from
the gas. If no electric field is acting, a cloud can be produced some time
after the rays have been cut off; but if a strong electric field is applied,
under the same conditions, no cloud is formed. This is in agreement with
experiments showing the time required for the ions to disappear by
recombination. In addition it can be shown that each one of the fine drops
carries an electric charge and can be made to move in a strong uniform
electric field.
The small number of drops produced without the action of the rays when
v2/v1 > 1·25 is due to a very slight natural ionization of the gas. That this
ionization exists has been clearly shown by electrical methods (section
284).
The evidence is thus complete that the ions themselves serve as centres for
the condensation of water around them. These experiments show
conclusively that the passage of electricity through a gas is due to the
presence of charged ions distributed throughout the volume of the gas, and
verify in a remarkable way the hypothesis of the discontinuous structure of
the electric charges carried by matter.
This property of the ions of acting as nuclei of condensation gives a very
delicate method of detecting the presence of ions in the gas. If only an ion
or two is present per c.c., their presence after expansion is at once observed
by the drops formed. In this way the ionization due to a small quantity of
uranium held a yard away from the condensation vessel is at once made
manifest.

35. Difference between the positive and negative ions. In the course of
experiments to determine the charge carried by an ion, J. J. Thomson[67]
observed that the cloud formed under the influence of X rays increased in

density when the expansion was about 1·31, and suggested in explanation
that the positive and negative ions had different condensation points.

Fig. 7.

This difference in behaviour of the positive and negative ions was
investigated in detail by C. T. R. Wilson[68] in the following way. X rays
were made to pass in a narrow beam on either side of a plate AB (Fig. 7)
dividing the condensation vessel into two equal parts. The opposite poles of
a battery of cells were connected with two parallel plates C and D, placed
symmetrically with regard to A. The middle point of the battery and the
plate A were connected with earth. If the plate C is positively charged, the
ions in the space CA at a short distance from A are all negative in sign.
Those to the right are all positive. It was found that condensation occurred
only for the negative ions in AC when v2/v1 = 1·25 but did not occur in AD
for the positive ions until v2/v1 = 1·31.
Thus the negative acts more readily than the positive ion as a centre of
condensation. The greater effect of the negative ion in causing condensation
has been suggested as an explanation of the positive charge always
observed in the upper atmosphere. The negative ions under certain
conditions become centres for the formation of small drops of water and are

removed to the earth by the action of gravity, while the positive ions remain
suspended.
With the apparatus described above, it has been shown that the positive and
negative ions are equal in number. If the expansion is large enough to
ensure condensation on both ions, the drops formed on the right and left of
the vessel in Fig. 7 are equal in number and fall at the same rate, i.e. are
equal in size.
Since the ions are produced in equal numbers from a gas electrically
neutral, this experiment shows that the charges on positive and negative
ions are equal in value but opposite in sign.

36. Charge carried by an ion. For a known sudden expansion of a gas
saturated with water vapour, the amount of water precipitated on the ions
can be calculated readily. The size of the drops can be determined by
observing the rate at which the cloud settles under the action of gravity.
From Stokes’ equation, the terminal velocity u of a small sphere of radius r
and density d falling through a gas of which the coefficient of viscosity is μ
is given by
2 dgr2
u = --------
9 μ

where g is the acceleration due to gravity. The radius of the drop and
consequently the weight of water in each drop can thus be determined.
Since the total weight of water precipitated is known, the number of drops
present is obtained at once.
This method has been used by J. J. Thomson[69] to determine the charge
carried by an ion. If the expansion exceeds the value 1·31, both positive and
negative ions become centres of condensation. From the rate of fall it can
be shown that approximately the drops are all of the same size.
The condensation vessel was similar to that employed by C. T. R. Wilson.
Two parallel horizontal plates were fitted in the vessel and the radiation
from an X ray tube or radio-active substance ionized the gas between them.
A difference of potential V, small compared with that required to saturate

the gas, was applied between the parallel plates distant l cms. from each
other. The small current i through the gas is given (section 28) by
NuVe
i = ------
l

where
N = number of ions present in the gas,
e = charge on each ion,
u = sum of the velocities of the positive and
negative ions.

Since the value of N is the same as the number of drops, and the velocity u
is known, the value of e can be determined.
In his last determination J. J. Thomson found that
e = 3·4 × 10-10 electrostatic units.

A very concordant value, namely, 3·1 × 10-10, has been obtained by H. A.
Wilson[70], by using a modified method of counting the drops. A check on
the size of the drops, determined by their rate of fall, was made by
observing the rate at which the drops moved in a strong electric field,
arranged so as to act with or against gravity.
J. J. Thomson found that the charge on the ions produced in hydrogen and
oxygen is the same. This shows that the nature of the ionization in gases is
distinct from that occurring in the electrolysis of solutions where the
oxygen ion always carries twice the charge of the hydrogen ion.

37. Diffusion of the ions. Early experiments with ionized gases showed
that the conductivity was removed from the gas by passage through a finely
divided substance like cotton-wool, or by bubbling through water. This loss
of conductivity is due to the fact that the ions in passing through narrow
spaces diffuse to the sides of the boundary, to which they either adhere or
give up their charge.

A direct determination of the coefficient of diffusion of the ions produced in
gases by Röntgen rays or by the rays from active substances has been made
by Townsend[71]. The general method employed was to pass a stream of
ionized gas through a diffusion vessel made up of a number of fine metal
tubes arranged in parallel. Some of the ions in their passage through the
tubes diffuse to the sides, the proportion being greater the slower the motion
of the gas and the narrower the tube. Observations were made of the
conductivity of the gas before and after passage through the tubes. In this
way, correcting if necessary for the recombination during the time taken to
pass through the tubes, the proportion R of either positive or negative ions
which are abstracted can be deduced. The value of R can be expressed
mathematically by the following equation in terms of K, the coefficient of
diffusion of the ions into the gas with which they are mixed[72],

where
a = radius of the tube,
Z = length of the tube,
V = mean velocity of the gas in the tube.

Only the first two terms of the series need be taken into account when
narrow tubes are used.
In this equation R, V, and a are determined experimentally, and K can thus
be deduced.
The following table shows the results obtained by Townsend when X rays
were used. Almost identical results were obtained later, when the radiations
from active substances replaced the X rays.

Coefficients of diffusion of ions into gases.

Gas K for K for Mean Ratio
+ – value of
ions ions of K

values
of K
Air, dry ·028 ·043 ·0347 1·54
„ moist ·032 ·035 ·0335 1·09
Oxygen, dry ·025 ·0396 ·0323 1·58
„ moist ·0288 ·0358 ·0323 1·24
Carbonic ·023 ·026 ·0245 1·13
acid, dry
„ „ moist ·0245 ·0255 ·025 1·04
Hydrogen, ·123 ·190 ·156 1·54
dry
„ moist ·128 ·142 ·135 1·11
The moist gases were saturated with water vapour at a temperature of 15°
C.
It is seen that the negative ion in all cases diffuses faster than the positive.
Theory shows that the coefficients of diffusion should be directly
proportional to the velocities of the ions, so that this result is in agreement
with the observations on the greater velocity of the negative ion.
This difference in the rate of diffusion of the ions at once explains an
interesting experimental result. If ionized gases are blown through a metal
tube, the tube gains a negative charge while the gas itself retains a positive
charge. The number of positive and negative ions present in the gas is
originally the same, but, in consequence of the more rapid diffusion of the
negative ions, more of the negative ions than of the positive give up their
charges to the tube. The tube consequently gains a negative and the gas a
positive charge.

38. A very important result can be deduced at once when the velocities and
coefficients of diffusion of the ions are known. Townsend (loc. cit.) has
shown that the equation of their motion is expressed by the formula
1 dp
---- pu = – ---- + nXe ,
K dx

where e is the charge on an ion,
n = number of ions per c.c.,
p = their partial pressure,

and u is the velocity due to the electric force X in the direction of the axis of
x. When a steady state is reached,
dp nXeK
---- = 0 and u = ---- ,
dx p

Let N be the number of molecules in a cubic centimetre of gas at the
pressure P and at the temperature 15° C., for which the values of u and K
have been determined. Then N/P may be substituted for n/p, and, since P at
atmospheric pressure is 106,
then
3 × 108u1
Ne = ---------- electrostatic units,
K

where u1 is the velocity for 1 volt (i.e. ¹⁄₃₀₀ E. S. unit) per cm.
It is known that one absolute electromagnetic unit of electricity in passing
through water liberates 1·23 c.c. of hydrogen at a temperature of 15° C. and
standard pressure. The number of atoms in this volume is 2·46N, and, if e´
is the charge on the hydrogen atom in the electrolysis of water,
2·46 Ne´ = 3 × 1010 E. S. units,
Ne´ = 1·22 × 1010 E. S. units.

e u1
Thus --- = 2·46 × 10-2 ---
e´ K

For example, substituting the values of u1 and K determined in moist air for
the positive ion,

e 2·46 1·37
--- = ----- × ----- = 1·04.
e´ 100 ·032

Values of this ratio, not very different from unity, are obtained for the
positive and negative ions of the gases hydrogen, oxygen, and carbon
dioxide. Taking into consideration the uncertainty in the experimental
values of u1 and K, these results indicate that the charge carried by an ion
in all gases is the same and is equal to that carried by the hydrogen ion in
the electrolysis of liquids.

39. Number of the ions. We have seen that, from experimental data,
Townsend has found that N, the number of molecules present in 1 c.c. of
gas at 15° C. and standard pressure, is given by
Ne = 1·22 × 1010.

Now e, the charge on an ion, is equal to 3·4 × 10-10 E. S. units;
thus N = 3·6 × 1019.

If I is the saturation current through a gas, and q the total rate of production
of ions in the gas,
I
q = ---.
e

The saturation current through air was found to be 1·2 × 10-8 ampères, i.e.
36 E.S. units, for parallel plates 4·5 cms. apart, when ·45 gramme of radium
of activity 1000 times that of uranium was spread over an area of 33 sq.
cms. of the lower plate. This corresponds to a production of about 1011 ions
per second. Assuming, for the purpose of illustration, that the ionization
was uniform between the plates, the volume of air acted on by the rays was
about 148 c.c., and the number of ions produced per c.c. per second about 7
× 108. Since N = 3·6 × 1019, we see that, if one molecule produces two ions,
the proportion of the gas ionized per second is about 10-11 of the whole. For
uranium the fraction is about 10-14, and for pure radium, of activity one

million times that of uranium, about 10-8. Thus even in the case of pure
radium, only about one molecule of gas is acted on per second in every 100
millions.
The electrical methods are so delicate that the production of one ion per
cubic centimetre per second can be detected readily. This corresponds to the
ionization of about one molecule in every 1019 present in the gas.

40. Size and nature of the ions. An approximate estimate of the mass of an
ion, compared with the mass of the molecule of the gas in which it is
produced, can be made from the known data of the coefficient K of inter-
diffusion of the ions into gases. The value of K for the positive ions in moist
carbon dioxide has been shown to be ·0245, while the value of K for the
inter-diffusion of carbon dioxide with air is ·14. The value of K for different
gases is approximately inversely proportional to the square root of the
products of the masses of the molecules of the two inter-diffusing gases;
thus, the positive ion in carbon dioxide behaves as if its mass were large
compared with that of the molecule. Similar results hold for the negative as
well as for the positive ion, and for other gases besides carbon dioxide.
This has led to the view that the ion consists of a charged centre surrounded
by a cluster of molecules travelling with it, which are kept in position round
the charged nucleus by electrical forces. A rough estimate shows that this
cluster consists of about 30 molecules of the gas. This idea is supported by
the variation in velocity, i.e. the variation of the size of the negative ion, in
the presence of water vapour; for the negative ion undoubtedly has a greater
mass in moist than in dry gases. At the same time it is possible that the
apparently large size of the ion, as determined by diffusion methods, may
be in part a result of the charge carried by the ion. The presence of a charge
on a moving body would increase the frequency of collision with the
molecules of the gas, and consequently diminish the rate of diffusion. The
ion on this view may not actually be of greater size than the molecule from
which it is produced.
The negative and positive ions certainly differ in size, and this difference
becomes very pronounced for low pressures of the gas. At atmospheric
pressure, the negative ion, produced by the action of ultra-violet light on a
negatively charged body, is of the same size as the ion produced by X rays,

but at low pressures J. J. Thomson has shown that it is identical with the
corpuscle or electron, which has an apparent mass of about ¹⁄₁₀₀₀ of the mass
of the hydrogen atom. A similar result has been shown by Townsend to hold
for the negative ion produced by X rays at a low pressure. It appears that
the negative ion at low pressure sheds its attendant cluster. The result of
Langevin, that the velocity of the negative ion increases more rapidly with
the diminution of pressure than that of the positive ion, shows that this
process of removal of the cluster is quite appreciable at a pressure of 10
mms. of mercury.
We must suppose that the process of ionization in gases consists in a
removal of a negative corpuscle or electron from the molecule of the gas. At
atmospheric pressure this corpuscle immediately becomes the centre of an
aggregation of molecules which moves with it and is the negative ion. After
removal of the negative ion the molecule retains a positive charge, and
probably also becomes the centre of a cluster of new molecules.
The terms electron and ion as used in this work may therefore be defined as
follows:—
The electron or corpuscle is the body of smallest mass yet known to
science. It carries a negative charge of value 3·4 × 10-10 electrostatic units.
Its presence has only been detected when in rapid motion, when, for speeds
up to about 1010 cms. a second, it has an apparent mass m given by e/m =
1·86 × 107 electromagnetic units. This apparent mass increases with the
speed as the velocity of light is approached (see section 82).
The ions which are produced in gases at ordinary pressure have an apparent
size, as determined from their rates of diffusion, large compared with the
molecule of the gas in which they are produced. The negative ion consists
of an electron with a cluster of molecules attached to and moving with it;
the positive ion consists of a molecule from which an electron has been
expelled, with a cluster of molecules attached. At low pressures under the
action of an electric field the electron does not form a cluster. The positive
ion is always atomic in size, even at low pressures of the gas. Each of the
ions carries a charge of value 3·4 × 10-10 electrostatic units.

41. Ions produced by collision. The greater part of the radiation from the
radio-active bodies consists of a stream of charged particles travelling with

great velocity. In this radiation, the α particles, which cause most of the
ionization observed in the gas, consist of positively charged bodies
projected with a velocity about one-tenth the velocity of light. The β rays
consist of negatively charged particles, which are identical with the cathode
rays generated in a vacuum tube, and travel with a speed about one-half the
velocity of light (chapter IV.). Each of these projected particles, in virtue of
its great kinetic energy, sets free a large number of ions by collision with the
gas molecules in its path. No definite experimental evidence has yet been
obtained of the number of ions produced by a single particle, or of the way
in which the ionization varies with the speed, but there is no doubt that each
projected body gives rise to many thousand ions in its path before its energy
of motion is destroyed.
It has already been mentioned (section 29) that at low pressures ions
moving under the action of an electric field are able to produce fresh ions
by collision with the molecules of the gas. At low pressures the negative ion
is identical with the electron set free in a vacuum tube, or emitted by a
radio-active substance.
The mean free path of the ion is inversely proportional to the pressure of the
gas. Consequently, if an ion moves in an electric field, the velocity acquired
between collisions increases with diminution of the pressure. Townsend has
shown that fresh ions are occasionally produced by collision when the
negative ion moves freely between two points differing in potential by 10
volts. If the difference be about V = 20 volts, fresh ions arise at each
collision[73].
Now the energy W, acquired by an ion of charge e moving freely between
two points at a difference of potential V, is given by
W = Ve.

-10
Taking V = 20 volts = ²⁰⁄₃₀₀ E. S. units, and e = 3·4 × 10 , the energy W
required in the case of a negative ion to produce an ion by collision is given
by
W = 2·3 × 10-11 ergs.

The velocity u acquired by the ion of mass m just before a collision is given
by
1
--- mu2 = Ve,
2

and

Now e/m = 1·86 × 107 electromagnetic units for the electron at slow speeds
(section 82).
Taking V = 20 volts, we find that
u = 2·7 × 108 cms. per sec.

This velocity is very great compared with the velocity of agitation of the
molecules of the gas.
In a weak electric field, the negative ions only produce ions by collision.
The positive ion, whose mass is at least 1000 times greater than the
electron, does not acquire a sufficient velocity to generate ions by collision
until an electric field is applied nearly sufficient to cause a spark through
the gas.
An estimate of the energy required for the production of an ion by X rays
has been made by Rutherford and McClung. The energy of the rays was
measured by their heating effect, and the total number of ions produced
determined. On the assumption that all the energy of the rays is used up in
producing ions, it was found that V = 175 volts—a value considerably
greater than that observed by Townsend from data of ionization by
collision. The ionization in the two cases, however, is produced under very
different conditions, and it is impossible to estimate how much of the
energy of the rays is dissipated in the form of heat.

42. Variations are found in the saturation current through gases, exposed to
the radiations from active bodies, when the pressure and nature of the gas
and the distance between the electrodes are varied. Some cases which are of
special importance in measurements will now be considered. With
unscreened active material the ionization of the gas is, to a large extent, due
to the α rays, which are absorbed in their passage through a few centimetres
of air. In consequence of this rapid absorption, the ionization decreases
rapidly from the surface of the active body, and this gives rise to
conductivity phenomena different in character from those observed with
Röntgen rays, where the ionization is in most cases uniform.

43. Variation of the current with distance between the plates. It has been
found experimentally[74] that the intensity of the ionization, due to a large
plane surface of active matter, falls off approximately in an exponential law
with the distance from the plate. On the assumption that the rate of
production of ions at any point is a measure of the intensity I of the
radiation, the value of I at that point is given by
I/I₀ = e–λx,

where λ is a constant, x the distance from the plate, and I₀ the intensity of
the radiation at the surface of the plate.
While the exponential law, in some cases, approximately represents the
variation of the ionization with distance, in others the divergence from it is
wide. The ionization, due to a plane surface of polonium, for example, falls
off more rapidly than the exponential law indicates. The α rays from an
active substance like radium are highly complex; the law of variation of the
ionization due to them is by no means simple and depends upon a variety of
conditions. The distribution of ionization is quite different according as a
thick layer or a very thick film of radio-active matter is employed. The
question is fully considered at the end of chapter IV., but for simplicity, the
exponential law is assumed in the following calculations.
Consider two parallel plates placed as in Fig. 1, one of which is covered
with a uniform layer of radio-active matter. If the distance d between the
plates is small compared with the dimensions of the plates, the ionization
near the centre of the plates will be sensibly uniform over any plane parallel

to the plates and lying between them. If q be the rate of production of ions
at any distance x and q₀ that at the surface, then q = q₀e-λx. The saturation
current i per unit area is given by

hence, when λd is small, i.e. when the ionization between the plates is
nearly constant,
i = q₀e´d.

The current is thus proportional to the distance between the plates. When λd
is large, the saturation current i₀ is equal to q₀e´/λ, and is independent of
further increase in the value of d. In such a case the radiation is completely
absorbed in producing ions between the plates, and

For example, in the case of a thin layer of uranium oxide spread over a large
plate, the ionization is mostly produced by rays the intensity of which is
reduced to half value in passing through 4·3 mms. of air, i.e. the value of λ
is 1·6. The following table is an example of the variation of i with the
distance between the plates.
Distance Saturation Current

2·5 mms. 32
5„ 55
7·5 „ 72
10 „ 85
12·5 „ 96

15 „ 100
Thus the increase of current for equal increments of distance between the
plates decreases rapidly with the distance traversed by the radiation.
The distance of 15 mms. was not sufficient to completely absorb all the
radiation, so that the current had not reached its limiting value.
When more than one type of radiation is present, the saturation current
between parallel plates is given by

where A, A1 are constants, and λ, λ1 the absorption constants of the
radiations in the gas.
Since the radiations are unequally absorbed in different gases, the variation
of current with distance depends on the nature of the gas between the plates.

44. Variation of the current with pressure. The rate of production of ions
by the radiations from active substances is directly proportional to the
pressure of the gas. The absorption of the radiation in the gas also varies
directly as the pressure. The latter result necessarily follows if the energy
required to produce an ion is independent of the pressure.
In cases where the ionization is uniform between two parallel plates, the
current will vary directly as the pressure; when however the ionization is
not uniform, on account of the absorption of the radiation in the gas, the
current does not decrease directly as the pressure until the pressure is
reduced so far that the ionization is sensibly uniform. Consider the variation
with pressure of the saturation current i between two large parallel plates,
one of which is covered with a uniform layer of active matter.
Let λ1 = absorption constant of the radiation in the gas for unit pressure.
For a pressure p, the intensity I at any point x is given by

The saturation current i is thus proportional to

If r be the ratio of the saturation currents for the pressures p1 and p2,

The ratio is thus dependent on the distance d between the plates and the
absorption of the radiation by the gas.
The difference in the shape of the pressure-current curves[75] is well
illustrated in Fig. 8, where curves are given for hydrogen, air, and carbonic
acid for plates 3·5 cms. apart.

Fig. 8.

For the purpose of comparison, the current at atmospheric pressure and
temperature in each case is taken as unity. The actual value of the current
was greatest in carbonic acid and least in hydrogen. In hydrogen, where the
absorption is small, the current over the whole range is nearly proportional
to the pressure. In carbonic acid, where the absorption is large, the current
diminishes at first slowly with the pressure, but is nearly proportional to it
below the pressure of 235 mms. of mercury. The curve for air occupies an
intermediate position.
In cases where the distance between the plates is large, the saturation
current will remain constant with diminution of pressure until the
absorption is so reduced that the radiation reaches the other plate.
An interesting result follows from the rapid absorption of radiation by the
gas. If the current is observed between two fixed parallel plates, distant d1
and d2 respectively from a large plane surface of active matter, the current
at first increases with diminution of pressure, passes through a maximum
value, and then diminishes. In such an experimental case the lower plate

through which the radiations pass is made either of open gauze or of thin
metal foil to allow the radiation to pass through readily.
The saturation current i is obviously proportional to

This is a function of the pressure, and is a maximum when

For example, if the active matter is uranium, pλ1 = 1·6 for the α rays at
atmospheric pressure. If d2 = 3, and d1 = 1, the saturation current reaches a
maximum when the pressure is reduced to about ⅓ of an atmosphere. This
result has been verified experimentally.

45. Conductivity of different gases when acted on by the rays. For a
given intensity of radiation, the rate of production of ions in a gas varies for
different gases and increases with the density of the gas. Strutt[76] has made
a very complete examination of the relative conductivity of gases exposed
to the different types of rays emitted by active substances. To avoid
correction for any difference of absorption of the radiation in the various
gases, the pressure of the gas was always reduced until the ionization was
directly proportional to the pressure, when, as we have seen above, the
ionization must everywhere be uniform throughout the gas. For each type of
rays, the ionization of air is taken as unity. The currents through the gases
were determined at different pressures, and were reduced to a common
pressure by assuming that the ionization was proportional to the pressure.
With unscreened active material, the ionization is almost entirely due to α
rays. When the active substance is covered with a layer of aluminium ·01
cm. in thickness, the ionization is mainly due to the β or cathodic rays, and

when covered with 1 cm. of lead, the ionization is solely due to the γ or
very penetrating rays. Experiments on the γ rays of radium were made by
observing the rate of discharge of a special gold-leaf electroscope filled
with the gas under examination and exposed to the action of the rays. The
following table gives the relative conductivities of gases exposed to various
kinds of ionizing radiations.
Gas Relative α β γ Röntgen
Density rays rays rays rays

Hydrogen 0·0693 0·226 0·157 0·169 0·114
Air 1·00 1·00 1·00 1·00 1·00
Oxygen 1·11 1·16 1·21 1·17 1·39
Carbon 1·53 1·54 1·57 1·53 1·60
dioxide
Cyanogen 1·86 1·94 1·86 1·71 1·05
Sulphur 2·19 2·04 2·31 2·13 7·97
dioxide
Chloroform 4·32 4·44 4·89 4·88 31·9
Methyl 5·05 3·51 5·18 4·80 72·0
iodide
Carbon 5·31 5·34 5·83 5·67 45·3
tetrachloride
With the exception of hydrogen, it will be seen that the ionization of gases
is approximately proportional to their density for the α, β, γ rays of radium.
The results obtained by Strutt for Röntgen rays are quite different; for
example, the relative conductivity produced by them in methyl iodide was
more than 14 times as great as that due to the rays of radium. The relative
conductivities of gases exposed to X rays has been recently re-examined by
McClung[77] and Eve[78], who have found that the conductivity depends upon
the penetrating power of the X rays employed. The results obtained by them
will be discussed later (section 107).
This difference of conductivity in gases is due to unequal absorptions of the
radiations. The writer has shown[79] that the total number of ions produced

by the α rays for uranium, when completely absorbed by different gases, is
not very different. The following results were obtained:
Gas Total
Ionization
Air 100
Hydrogen 95
Oxygen 106
Carbonic acid 96
Hydrochloric acid gas 102
Ammonia 101
The numbers, though only approximate in character, seem to show that the
energy required to produce an ion is probably not very different for the
various gases. Assuming that the energy required to produce an ion in
different gases is about the same, it follows that the relative conductivities
are proportional to the relative absorption of the radiations.
A similar result has been found by McLennan for cathode rays. He proved
that the ionization was directly proportional to the absorption of the rays in
the gas, thus showing that the same energy is required to produce an ion in
all the gases examined.

46. Potential Gradient. The normal potential gradient between two
charged electrodes is always disturbed when the gas is ionized in the space
between them. If the gas is uniformly ionized between two parallel plates,
Child and Zeleny have shown that there is a sudden drop of potential near
the surface of both plates, and that the electric field is sensibly uniform for
the intermediate space between them. The disturbance of the potential
gradient depends upon the difference of potential applied, and is different at
the surface of the two plates.
In most measurements of radio-activity the material is spread over one plate
only. In such a case the ionization is to a large extent confined to the
volume of the air close to the active plate. The potential gradient in such a
case is shown in Fig. 9. The dotted line shows the variation of potential at
any point between the plates when no ionization is produced between the

plates; curve A for weak ionization, such as is produced by uranium, curve
B for the intense ionization produced by a very active substance. In both
cases the potential gradient is least near the active plate, and greatest near
the opposite plate. For very intense ionization it is very small near the
active surface. The potential gradient varies slightly according as the active
plate is charged positively or negatively.

Fig. 9.

47. Variation of current with voltage for surface ionization.
Some very interesting results, giving the variation of the current with
voltage, are observed when the ionization is intense, and confined to the
space near the surface of one of two parallel plates between which the
current is measured.
The theory of this subject has been worked out independently by Child[80]
and Rutherford[81]. Let V be the potential difference between two parallel

plates at a distance d apart. Suppose that the ionization is confined to a thin
layer near the surface of the plate A (see Fig. 1) which is charged positively.
When the electric field is acting, there is a distribution of positive ions
between the plates A and B.
Let
n1
= number of positive ions per unit volume at a
distance x from the plate A,

K1
= mobility of the positive ions,

e = charge on an ion.

The current i1 per square centimetre through the gas is constant for all
values of x, and is given by

By Poisson’s equation

Then

Integrating

where A is a constant. Now A is equal to the value of
dV
----
dx

when x = 0. By making the ionization very intense, the value of
dV
----
dx

can be made extremely small.
Putting A = 0, we see that

This gives the potential gradient between the plates for different values of x.
Integrating between the limits 0 and d,

or

If i2 is the value of the current when the electric field is reversed, and K2 the
velocity of the negative ion,

and
i1 K1
--- = ---- .
i2 K2

The current in the two directions is thus directly proportional to the
velocities of the positive and negative ions. The current should vary directly
as the square of the potential difference applied, and inversely as the cube
of the distance between the plates.
The theoretical condition of surface ionization cannot be fulfilled by the
ionization due to active substances, as the ionization extends some
centimetres from the active plate. If, however, the distance between the
plates is large compared with the distance over which the ionization
extends, the results will be in rough agreement with the theory. Using an
active preparation of radium, the writer has made some experiments on the
variation of current with voltage between parallel plates distant about 10
cms. from each other[82].
The results showed
(1) That the current through the gas for small voltages increased more
rapidly than the potential difference applied, but not as rapidly as the square
of that potential difference.

(2) The current through the gas depended on the direction of the electric
field; the current was always smaller when the active plate was charged
positively on account of the smaller mobility of the positive ion. The
difference between i1 and i2 was greatest when the gas was dry, which is the
condition for the greatest difference between the velocities of the ions.
An interesting result follows from the above theory. For given values of V
and d, the current cannot exceed a certain definite value, however much the
ionization may be increased. In a similar way, when an active preparation of
radium is used as a source of surface ionization, it is found that, for a given
voltage and distance between the plates, the current does not increase
beyond a certain value however much the activity of the material is
increased.

48. Magnetic field produced by an ion in motion. It will be shown later
that the two most important kinds of rays emitted by radio-active substances
consist of electrified particles, spontaneously projected with great velocity.
The easily absorbed rays, known as α rays, are positively electrified atoms
of matter; the penetrating rays, known as β rays, carry a negative charge,
and have been found to be identical with the cathode rays produced by the
electric discharge in a vacuum tube.
The methods adopted to determine the character of these rays are very
similar to those first used by J. J. Thomson to show that the cathode rays
consisted of a stream of negatively electrified particles projected with great
velocity.
The proof that the cathode rays were corpuscular in character, and consisted
of charged particles whose mass was very small compared with that of the
hydrogen atom, marked an important epoch in physical science: for it not
only opened up new and fertile fields of research, but also profoundly
modified our previous conceptions of the constitution of matter.
A brief account will accordingly be given of the effects produced by a
moving charged body, and also of some of the experimental methods which
have been used to determine the mass and velocity of the particles of the
cathode stream[83].

Consider an ion of radius a, carrying a charge of electricity e, and moving
with a velocity u, small compared with the velocity of light. In consequence
of the motion, a magnetic field is set up around the charged ion, which is
carried with it. The charged ion in motion constitutes a current element of
magnitude eu, and the magnetic field H at any point distant r from the
sphere is given by
eu sin θ
H = -----
r2

where θ is the angle the radius vector makes with the direction of motion.
The lines of magnetic force are circles around the axis of motion. When the
ion is moving with a velocity small compared with the velocity of light, the
lines of electric force are nearly radial, but as the speed of light is
approached, they tend to leave the axis of motion and to bend towards the
equator. When the speed of the body is very close to that of light, the
magnetic and electric field is concentrated to a large extent in the equatorial
plane.
The presence of a magnetic field around the moving body implies that
magnetic energy is stored up in the medium surrounding it. The amount of
this energy can be calculated very simply for slow speeds.
In a magnetic field of strength H, the magnetic energy stored up in unit
volume of the medium of unit permeability is given by
H2
----
8π

Integrating the value of this expression over the region exterior to a sphere
of radius a, the total magnetic energy due to the motion of the charged body
is given by

The magnetic energy, due to the motion, is analogous to kinetic energy, for
it depends upon the square of the velocity of the body. In consequence of
the charge carried by the ion, additional kinetic energy is associated with it.
If the velocity of the ion is changed, electric and magnetic forces are set up
tending to stop the change of motion, and more work is done during the
change than if the ion were uncharged. The ordinary kinetic energy of the
body is
1
-- mu2
2

In consequence of its charge, the kinetic energy associated with it is
increased by
e 2u 2
----
3a

It thus behaves as if it possessed a mass m + m1 where m1 is the electrical
mass, with the value
2e2
---
3a

We have so far only considered the electrical mass of a charged ion moving
with a velocity small compared with that of light. As the speed of light is
approached, the magnetic energy can no longer be expressed by the
equation already given. The general values of the electrical mass of a
charged body for speed were first worked out by J. J. Thomson[84] in 1887.
A more complete examination was made in 1889 by Heaviside[85], while

Searle[86] worked out the case for a charged ellipsoid. Recently, the question
was again attacked by Abraham[87]. Slightly different expressions for the
variation of electrical mass with speed have been obtained, depending upon
the conditions assumed for the distribution of the electricity on the sphere.
The expression found by Abraham, which has been utilized by Kaufmann to
show that the mass of the electron is electromagnetic in origin, is given later
in section 82.
All the calculations agree in showing that the electrical mass is practically
constant for slow speeds, but increases as the speed of light is approached,
and is theoretically infinite when the speed of light is reached. The nearer
the velocity of light is approached, the greater is the resisting force to a
change of motion. An infinite force would be required to make an electron
actually attain the velocity of light, so that, according to the present theory,
it would be impossible for an electron to move faster than light, i.e. faster
than an electromagnetic disturbance travels in the ether.
The importance of these deductions lies in the fact that an electric charge in
motion, quite independently of any material nucleus, possesses an apparent
mass in virtue of its motion, and that this mass is a function of the speed.
Indeed, we shall see later (see section 82) that the apparent mass of the
particles constituting the cathode stream can be explained in virtue of their
charge, without the necessity of assuming a material body in which the
charge is distributed. This has led to the suggestion that all mass may be
electrical in origin, and due purely to electricity in motion.

49. Action of a magnetic field on a moving ion. Let us consider the case
of an ion of mass m carrying a charge e and moving freely with a velocity u.
If u is small compared with the velocity of light, the ion in motion
corresponds to a current element of magnitude eu. If the ion moves in an
external magnetic field of strength H, it is acted on by a force at right angles
both to the direction of motion, and to that of the magnetic force and equal
in magnitude to Heu sin θ, where θ is the angle between the direction of the
magnetic force and the direction of motion. Since the force due to the
magnetic field is always perpendicular to the direction of motion, it has no
effect upon the velocity of the particle, but can only alter the direction of its
path.

If ρ is the radius of curvature of the path of the ion, the force along the
normal is equal to
mu2
--- ,
ρ

and this is balanced by the force Heu sin θ.
If
π
θ = --- ,
2

i.e. if the ion is moving at right angles to the direction of the magnetic field
mu2
Heu = ----
ρ

or
m
Hρ = ----- u
e

Since u is constant, ρ is also constant, i.e. the particle describes a circular
orbit of radius ρ. The radius of the circular orbit is thus directly proportional
to u, and inversely proportional to H.
If the ion is moving at an angle θ with the direction of the magnetic field, it
describes a curve which is compounded of a motion of a particle of velocity
u sin θ perpendicular to the field and u cos θ in the direction of the field.
The former describes a circular orbit of radius ρ, given by
m
Hρ = --- u sin θ ;
e

the latter is unaffected by the magnetic field and moves uniformly in the
direction of the magnetic field with a velocity u cos θ. The motion of the
particle is in consequence a helix, traced on a cylinder of radius
mu sin θ
ρ = --------- ,
eH

whose axis is in the direction of the magnetic field. Thus an ion projected
obliquely to the direction of a uniform magnetic field always moves in a
helix whose axis is parallel to the lines of magnetic force[88].

50. Determination of e/m for the cathode stream. The cathode rays, first
observed by Varley, were investigated in detail by Crookes. These rays are
projected from the cathode in a vacuum tube at low pressure. They travel in
straight lines, and are readily deflected by a magnet, and produce strong
luminosity in a variety of substances placed in their path. The rays are
deflected by a magnetic field in the same direction as would be expected for
a negatively charged particle projected from the cathode. In order to explain
the peculiar properties of these rays Crookes supposed that they consisted
of negatively electrified particles, moving with great velocity and
constituting, as he appropriately termed it, “a new or fourth state of matter.”
The nature of these rays was for twenty years a subject of much
controversy, for while some upheld their material character, others
considered that they were a special form of wave motion in the ether.
Perrin and J. J. Thomson showed that the rays always carried with them a
negative charge, while Lenard made the important discovery that the rays
passed through thin metal foil and other substances opaque to ordinary
light. Using this property, he sent the rays through a thin window and
examined the properties of the rays outside the vacuum tube in which they
were produced.
The absorption of the rays by matter was shown to be nearly proportional to
the density over a very wide range, and to be independent of its chemical
constitution.
The nature of these rays was successfully demonstrated by J. J. Thomson[89]
in 1897. If the rays consisted of negatively electrified particles, they should

be deflected in their passage through an electric as well as through a
magnetic field. Such an experiment had been tried by Hertz, but with
negative results. J. J. Thomson, however, found that the rays were deflected
by an electric field in the direction to be expected for a negatively charged
particle, and showed that the failure of Hertz to detect the same was due to
the masking of the electric field by the strong ionization produced in the gas
by the cathode stream. This effect was got rid of by reducing the pressure of
the gas in the tube.
The experimental arrangement used for the electric deflection of the rays is
shown in Fig. 10.
The cathode rays are generated at the cathode C, and a narrow pencil of
rays is obtained by passing the rays through a perforated disc AB. The rays
then passed midway between two parallel insulated plates D and E, d
centimetres apart, and maintained at a constant difference of potential V.
The point of incidence of the pencil of rays was marked by a luminous
patch produced on a phosphorescent screen placed at PP´.
The particle carrying a negative charge e in passing between the charged
plates, is acted on by a force Xe directed towards the positive plate, where
X, the strength of the electric field, is given by
V
--- .
d

Fig. 10.

The application of the electric field thus causes the luminous patch to move
in the direction of the positive plate. If now a uniform magnetic field is

applied at the plates D and E, perpendicular to the pencil of rays, and
parallel to the plane of the plates, and in such a direction that the electric
and magnetic forces are opposed to one another, the patch of light can be
brought back to its undisturbed position by adjusting the strength of the
magnetic field. If H is the strength of the magnetic field, the force on the
particle due to the magnetic field is Heu, and when a balance is obtained
Heu = Xe,

or
X
u = --- (1).
H

Now if the magnetic field H is acting alone, the curvature ρ of the path of
the rays between the plates can be deduced from the deflection of the
luminous patch. But we have seen that
mu
H = ---- (2).
e

From equations (1) and (2), the value of u and e/m for the particle can be
determined.
The velocity u is not constant, but depends upon the potential difference
between the electrodes, and this in turn depends upon the pressure and
nature of the residual gas in the tube.
By altering these factors, the cathode particles may be made to acquire
velocities varying between about 109 and 1010 cms. per second. This
velocity is enormous compared with that which can be impressed ordinarily
upon matter by mechanical means. On the other hand, the value of e/m for
the particles is sensibly constant for different velocities.
As a result of a series of experiments the mean value e/m = 7·7 × 106 was
obtained. The value of e/m is independent of the nature or pressure of the
gas in the vacuum tube and independent of the metal used as cathode. A
similar value of e/m was obtained by Lenard[90] and others.

Kaufmann[91] and Simon[92] used a different method to determine the value
of e/m. The potential difference V between the terminals of the tube was
measured. The work done on the charged particle in moving from one end
of the tube to the other is Ve, and this must be equal to the kinetic energy
1
-- mu2
2

acquired by the moving particle. Thus
e u2
--- = --- (3).
m 2V

By combination of this equation with (2) obtained by measurement of the
magnetic deflexion, both u and e/m can be determined.
Simon found by this method that
e
-- = 1·865 × 107.
m

It will be seen later (section 82) that a similar value was deduced by
Kaufmann for the electrons projected from radium.
These results, which have been based on the effect of a magnetic and
electric field on a moving ion, were confirmed by Weichert, who
determined by a direct method the time required for the particle to traverse
a known distance.
The particles which make up the cathode stream were termed “corpuscles”
by J. J. Thomson. The name “electron,” first employed by Johnstone
Stoney, has also been applied to them and has come into general use[93].
The methods above described do not give the mass of the electron, but only
the ratio of the charge to the mass. A direct comparison can, however, be
made between the ratio e/m for the electron and the corresponding value for
the hydrogen atoms set free in the electrolysis of water. Each of the
hydrogen atoms is supposed to carry a charge e, and it is known that 96,000

coulombs of electricity, or, in round numbers, 104 electromagnetic units of
quantity are required to liberate one gram of hydrogen. If N is the number
of atoms in one gram of hydrogen, then Ne = 104. But if m is the mass of a
hydrogen atom, then Nm = 1. Dividing one by the other e/m = 104. We have
seen already that a gaseous ion carries the same charge as a hydrogen atom,
while indirect evidence shows that the electron carries the same charge as
an ion, and consequently the same charge as the atom of hydrogen. Hence
we may conclude that the apparent mass of the electron is only about ¹⁄₁₀₀₀
of the mass of the hydrogen atom. The electron thus behaves as the smallest
body known to science.
In later experiments J. J. Thomson showed that the negative ions set free at
low pressures by an incandescent carbon filament, and also the negative
ions liberated from a zinc plate exposed to the action of ultra-violet light,
had the same value for e/m as the electrons produced in a vacuum tube. It
thus seemed probable that the electron was a constituent of all matter. This
view received strong support from measurements of quite a different
character. Zeeman in 1897 found that the lines of the spectrum from a
source of light exposed in a strong magnetic field were displaced and
doubled. Later work has shown that the lines in some cases are trebled, in
others sextupled, while, in a few cases, the multiplication is still greater.
These results received a general explanation on the radiation theories
previously advanced by Lorenz and Larmor. The radiation, emitted from
any source, was supposed to result from the orbital or oscillatory motion of
the charged parts constituting the atom. Since a moving ion is acted on by
an external magnetic field, the motion of the charged ions is disturbed when
the source of light is exposed between the poles of a strong magnet. There
results a small change in the period of the emitted light, and a bright line in
the spectrum is, in consequence, displaced by the action of the magnetic
field. According to theory, the small change in the wave-length of the
emitted light depends upon the strength of the magnetic field and on the
ratio e/m of the charge carried by the ion to its mass. By comparison of the
theory with the experimental results, it was deduced that the moving ion
carried a negative charge, and that the value of e/m was about 107. The
charged ion, responsible for the radiation from a luminous body, is thus
identical with the electron set free in a vacuum tube.

It thus seems reasonable to suppose that the atoms of all bodies are complex
and are built up, in part at least, of electrons, whose apparent mass is very
small compared with that of the hydrogen atom. The properties of such
disembodied charges has been examined mathematically among others by
Larmor, who sees in this conception the ultimate basis of a theory of matter.
J. J. Thomson and Lord Kelvin have investigated mathematically certain
arrangements of a number of electrons which are stable for small
disturbances. This question will be discussed more in detail in section 270.

51. Canal rays. If a discharge is passed through a vacuum tube provided
with a perforated cathode, within certain limits of pressure, luminous
streams are observed to pass through the holes and to emerge on the side of
the cathode remote from the anode. These rays were first observed by
Goldstein[94] and were called by him the “Canal-strahlen.” These rays travel
in straight lines and produce phosphorescence in various substances.
Wien[95] showed that the canal rays were deflected by strong magnetic and
electric fields, but the amount of deflection was very small compared with
that of the cathode rays under similar conditions. The deflection was found
to be opposite in direction to the cathode rays, and this indicates that the
canal rays consist of positive ions. Wien determined their velocity and the
ratio e/m, by measuring the amount of their magnetic and electric
deflection. The value of e/m was found to be variable, depending upon the
gas in the tube, but the maximum value observed was 104. This shows that
the positive ion, in no case, has a mass less than that of the hydrogen atom.
It seems probable that the canal rays consist of positive ions, derived either
from the gas or the electrodes, which travel towards the cathode, and have
sufficient velocity to pass through the holes of the cathode and to appear in
the gas beyond.
It is remarkable that, so far, no case has been observed where the carrier of
a positive charge has an apparent mass less than that of the hydrogen atom.
Positive electricity always appears to be associated with bodies atomic in
size. We have seen that the process of ionization in gases is supposed to
consist of the expulsion of an electron from the atom. The corresponding
positive charge remains behind on the atom and travels with it. This

difference between positive and negative electricity appears to be
fundamental, and no explanation of it has, as yet, been forthcoming.

52. Radiation of energy. If an electron moves uniformly in a straight line
with constant velocity, the magnetic field, which travels with it, remains
constant, and there is no loss of energy from it by radiation. If, however, its
motion is hastened or retarded, the magnetic field is altered, and there
results a loss of energy from the electron in the form of electromagnetic
radiation. The rate of loss of energy from an accelerated electron was first
calculated by Larmor[96] and shown to be
2e2
---- × (acceleration)2 ,
3V

where e is the charge on the electron in electromagnetic units, and V the
velocity of light.
Any alteration in the velocity of a moving charge is thus always
accompanied by a radiation of energy from it. Since the electron, set free in
a vacuum tube, increases in velocity in passing through the electric field,
energy must be radiated from it during its passage from cathode to anode. It
can, however, readily be calculated that, in ordinary cases, this loss of
energy is small compared with the kinetic energy acquired by the electron
in passing through the electric field.
An electron moving in a circular orbit is a powerful radiator of energy, since
it is constantly accelerated towards the centre. An electron moving in an
orbit of radius equal to the radius of an atom (about 10-8 cms.) would lose
most of its kinetic energy of motion in a small fraction of a second, even
though its velocity was originally nearly equal to the velocity of light. If,
however, a number of electrons are arranged at equal angular intervals on
the circumference of a circle and move with constant velocity round the
ring, the radiation of energy is much less than for a single electron, and
rapidly diminishes with an increase in the number of electrons round the
ring. This result, obtained by J. J. Thomson, will be discussed in more detail
later when the stability of systems composed of rotating electrons is under
consideration.

Since the radiation of energy is proportional to the square of the
acceleration, the proportion of the total energy radiated depends upon the
suddenness with which an electron is started or stopped. Now some of the
cathode ray particles are stopped abruptly when they impinge on the metal
cathode, and, in consequence, give up a fraction of their kinetic energy in
the form of electromagnetic radiation. Stokes and Weichert suggested that
this radiation constituted the X rays, which are known to have their origin at
the surface on which the cathode rays impinge. The mathematical theory
has been worked out by J. J. Thomson[97]. If the motion of an electron is
suddenly arrested, a thin spherical pulse in which the magnetic and electric
forces are very intense travels out from the point of impact with the velocity
of light. The more suddenly the electron is stopped, the thinner and more
intense is the pulse. On this view the X rays are not corpuscular like the
cathode rays, which produce them, but consist of transverse disturbances in
the ether, akin in some respects to light waves of short wave-length. The
rays are thus made up of a number of pulses, which are non-periodic in
character, and which follow one another at irregular intervals.
On this theory of the nature of the X rays, the absence of direct deflection,
refraction, or polarization is to be expected, if the thickness of the pulse is
small compared with the diameter of an atom. It also explains the non-
deflection of the path of the rays by a magnetic or electric field. The
intensity of the electric and magnetic force in the pulse is so great that it is
able to cause a removal of an electron from some of the atoms of the gas,
over which the pulse passes, and thus causes the ionization observed.
The cathode rays produce X rays, and these in turn give rise to a secondary
radiation whenever they impinge on a solid body. This secondary radiation
is emitted equally in all directions, and consists partly of a radiation of the
X ray type and also of electrons projected with considerable velocity. This
secondary radiation gives rise to a tertiary radiation and so on.
Barkla[98] has shown that the secondary radiation emitted from a gas through
which the rays pass consists in part of scattered X rays of about the same
penetrating power as the primary rays as well as some easily absorbed rays.
Part of the cathode rays is diffusely reflected on striking the cathode. These
scattered rays consist in part of electrons of the same speed as in the
primary beam, but also include some others of much less velocity. The

amount of diffuse reflection depends upon the nature of the cathode and the
angle of incidence of the rays.
We shall see later (chapter IV.) that similar effects are produced when the
rays from radio-active substances impinge upon solid bodies.

In this chapter an account of the ionization theory of gases has been given
to the extent that is necessary for the interpretation of the measurements of
radio-activity by the electric method. It would be out of place here to
discuss the development of that theory in detail, to explain the passage of
electricity through flames and vapours, the discharge of electricity from hot
bodies, and the very complicated phenomena observed in the passage of
electricity through a vacuum tube.
For further information on this important subject, the reader is referred to J.
J. Thomson’s Conduction of Electricity through Gases, in which the whole
subject is treated in a full and complete manner. A simple account of the
effect of moving charges and the electronic theory of matter was given by
the same author in the Silliman Lectures of Yale University and published
under the title Electricity and Matter (Scribner, New York, 1904).

CHAPTER III.
METHODS OF MEASUREMENT.

53. Methods of Measurement. Three general methods have been
employed for examination of the radiations from radio-active bodies,
depending on

(1) The action of the rays on a photographic plate.

(2) The ionizing action of the rays on the surrounding gas.

(3) The fluorescence produced by the rays on a screen of
platinocyanide of barium, zinc sulphide, or similar substance.

The third method is very restricted in its application, and can only be
employed for intensely active substances like radium or polonium.
The photographic method has been used very widely, especially in the
earlier development of the subject, but has gradually been displaced by the
electrical method, as a quantitative determination of the radiations became
more and more necessary. In certain directions, however, it possesses
distinct advantages over the electrical method. For example, it has proved a
very valuable means of investigating the curvature of the path of the rays,
when deflected by a magnetic or electric field, and has allowed us to
determine the constants of these rays with considerable accuracy.
On the other hand, as a general method of study of the radiations, it is open
to many objections. A day’s exposure is generally required to produce an
appreciable darkening of the sensitive film when exposed to a weak source
of radiation like uranium and thorium. It cannot, in consequence, be
employed to investigate the radiations of those active products which
rapidly lose their activity. Moreover, W. J. Russell has shown that the
darkening of a photographic plate can be produced by many agents which

do not give out rays like those of the radio-active bodies. This darkening of
the plate is produced under the most varied conditions, and very special
precautions are necessary when long exposures to a weak source of
radiation are required.
The main objection to the photographic method, however, lies in the fact
that the radiations which produce the strongest electrical effect are very
weak photographically. For example, Soddy[99] has shown that the
photographic action of uranium is due almost entirely to the more
penetrating rays, and that the easily absorbed rays produce in comparison
very little effect. Speaking generally, the penetrating rays are the most
active photographically, and, under ordinary conditions, the action on the
plate is almost entirely due to them.
Most of the energy radiated from active bodies is in the form of easily
absorbed rays which are comparatively inactive photographically. These
rays are difficult to study by the photographic method, as the layer of black
paper which, in many cases, is required in order to absorb the
phosphorescent light from active substances, cuts off at the same time most
of the rays under examination. These easily absorbed rays will be shown to
play a far more important part in the processes occurring in radio-active
bodies than the penetrating rays which are more active photographically.
The electrical method, on the other hand, offers a rapid and accurate method
of quantitatively examining the radiations. It can be used as a means of
measurement of all the types of radiation emitted, excluding light waves,
and is capable of accurate measurement over an extremely wide range. With
proper precautions it can be used to measure effects produced by radiations
of extremely small intensity.

54. Electrical Methods. The electrical methods employed in studying
radio-activity are all based on the property of the radiation in question of
ionizing the gas, i.e. of producing positively and negatively charged carriers
throughout the volume of the gas. The discussion of the application of the
ionization theory of gases to measurements of radio-activity has been given
in the last chapter. It has been shown there that the essential condition to be
fulfilled for comparative measurements of the intensity of the radiations is

that the electrical field shall in all cases be strong enough to obtain the
maximum or saturation current through the gas.
The electric field required to produce practical saturation varies with the
intensity of the ionization and consequently with the activity of the
preparations to be examined. For preparations which have an activity not
more than 500 times that of uranium, under ordinary conditions, a field of
100 volts per cm. is sufficient to produce a practical saturation current. For
very active samples of radium, it is often impossible to obtain conveniently
a high enough electromotive force to give even approximate saturation.
Under such conditions comparative measurement can be made by
measuring the current under diminished pressure of the gas, when saturation
is more readily obtained.
The method to be employed in the measurement of this ionization current
depends largely on the intensity of the current to be measured. If some very
active radium is spread on the lower of two insulated plates as in Fig. 1, and
a saturating electric field applied, the current may readily be measured by a
sensitive galvanometer of high resistance. For example, a weight of ·45 gr.
of radium chloride of activity 1000 times that of uranium oxide, spread over
a plate of area 33 sq. cms., gave a maximum current of 1·1 × 10-8 amperes
when the plates were 4·5 cms. apart. In this case the difference of potential
to be applied to produce practical saturation was about 600 volts. Since
most of the ionization is due to rays which are absorbed in passing through
a few centimetres of air, the current is not much increased by widening the
distance between the two plates. In cases where the current is not quite large
enough for direct deflection, the current may be determined by connecting
the upper insulated plate with a well insulated condenser. After charging for
a definite time, say one or more minutes, the condenser is discharged
through the galvanometer, and the current can readily be deduced.

55. In most cases, however, when dealing with less active substances like
uranium or thorium, or with small amounts of active material, it is
necessary to employ methods for measuring much smaller currents than can
be detected conveniently by an ordinary galvanometer. The most convenient
apparatus to employ for this purpose is one of the numerous types of
quadrant electrometer or an electroscope of special design. For many

observations, especially where the activity of the two substances is to be
compared under constant conditions, an electroscope offers a very certain
and easy method of measurement. As an example of a simple apparatus of
this kind, a brief description will be given of the electroscope used by M.
and Mme Curie in many of their earlier observations.

Fig. 11.

The connections are clearly seen from Fig. 11. The active material is placed
on a plate laid on top of the fixed circular plate P, connected with the case
of the instrument and with earth. The upper insulated plate P´ is connected
with the insulated gold-leaf system LL´. S is an insulating support and L the
gold-leaf.
The system is first charged to a suitable potential by means of the rod C.
The rate of movement of the gold-leaf is observed by means of a
microscope. In comparisons of the activity of two specimens, the time taken
by the gold leaf to pass over a certain number of divisions of the
micrometer scale in the eye-piece is observed. Since the capacity of the
charged system is constant, the average rate of movement of the gold-leaf is
directly proportional to the ionization current between P and P´, i.e. to the
intensity of the radiation emitted by the active substance. Unless very active
material is being examined, the difference of potential between P and P´
can easily be made sufficient to produce saturation.

When necessary, a correction can be made for the rate of leak when no
active material is present. In order to avoid external disturbances, the plates
PP´ and the rod C are surrounded by metal cylinders, E and F, connected
with earth.

56. A modified form of the gold-leaf electroscope can be used to determine
extraordinarily minute currents with accuracy, and can be employed in
cases where a sensitive electrometer is unable to detect the current. A
special type of electroscope has been used by Elster and Geitel, in their
experiments on the natural ionization of the atmosphere. A very convenient
type of electroscope to measure the current due to minute ionization of the
gas is shown in Fig. 12.

Fig. 12.

This type of instrument was first used by C. T. R. Wilson[100] in his
experiments of the natural ionization of air in closed vessels. A brass
cylindrical vessel is taken of about 1 litre capacity. The gold-leaf system,
consisting of a narrow strip of gold-leaf L attached to a flat rod R, is
insulated inside the vessel by the small sulphur bead or piece of amber S,
supported from the rod P. In a dry atmosphere a clean sulphur bead or piece

of amber is almost a perfect insulator. The system is charged by a light bent
rod CC´ passing through an ebonite cork[101]. The rod C is connected to one
terminal of a battery of small accumulators of 200 to 300 volts. If these are
absent, the system can be charged by means of a rod of sealing-wax. The
charging rod CC´ is then removed from contact with the gold-leaf system.
The rods P and C and the cylinder are then connected with earth.
The rate of movement of the gold-leaf is observed by a reading microscope
through two holes in the cylinder, covered with thin mica. In cases where
the natural ionization due to the enclosed air in the cylinder is to be
measured accurately, it is advisable to enclose the supporting and charging
rod and sulphur bead inside a small metal cylinder M connected to earth, so
that only the charged gold-leaf system is exposed in the main volume of the
air.
In an apparatus of this kind the small leakage over the sulphur bead can be
eliminated almost completely by keeping the rod P charged to the average
potential of the gold-leaf system during the observation. This method has
been used with great success by C. T. R. Wilson (loc. cit.). Such
refinements, however, are generally unnecessary, except in investigations of
the natural ionization of gases at low pressures, when the conduction leak
over the sulphur bead is comparable with the discharge due to the ionized
gas.

57. The electric capacity C of a gold-leaf system about 4 cms. long is
usually about 1 electrostatic unit. If V is the decrease of potential of the
gold-leaf system in t seconds, the current i through the gas is given by
CV
i = ----
t.

With a well cleaned brass electroscope of volume 1 litre, the fall of
potential due to the natural ionization of the air was found to be about 6
volts per hour. Since the capacity of the gold-leaf system was about 1
electrostatic unit
6

i = 1 × ------------ = 5·6 × 10-6 E.S. units = 1·9 ×
10-15 amperes.
3600 × 300

With special precautions a rate of discharge of ⅒ or even ¹⁄₁₀₀ of this
amount can be measured accurately.
The number of ions produced in the gas can be calculated if the charge on
an ion is known. J. J. Thomson has shown that the charge e on an ion is
equal to 3·4 × 10-10 electrostatic units or 1·13 × 10-19 coulombs.
Let q = number of ions produced per second per
cubic centimetre
throughout the volume of the electroscope,

S = volume of electroscope in cubic
centimetres.

If the ionization be uniform, the saturation current i is given by i = qSe.
Now for an electroscope with a volume of 1000 c.c., i was equal to about
1·9 × 10-15 amperes. Substituting the values given above
q = 17 ions per cubic centimetre per second.

With suitable precautions an electroscope can thus readily measure an
ionization current corresponding to the production of 1 ion per cubic
centimetre per second.
The great advantage of an apparatus of this kind lies in the fact that the
current measured is due to the ionization inside the vessel and is not
influenced by the ionization of the external air or by electrostatic
disturbances[102]. Such an apparatus is very convenient for investigating the
very penetrating radiations from the radio-elements, since these rays pass
readily through the walls of the electroscope. When the electroscope is
placed on a lead plate 3 or 4 mms. thick, the ionization in the electroscope,
due to a radio-active body placed under the lead, is due entirely to the very
penetrating rays, since the other two types of rays are completely absorbed
in the lead plate. If a circular opening is cut in the base of the electroscope
and covered with thin aluminium of sufficient thickness to absorb the α

rays, measurements of the intensity of the β rays from an active substance
placed under it, can be made with ease and certainty.

58. A modified form of electroscope, which promises to be of great utility
for measuring currents even more minute than those to be observed with the
type of instrument already described, has recently been devised by C. T. R.
Wilson[103]. The construction of the apparatus is shown in Fig. 13.
The case consists of a rectangular brass box 4 cms. × 4 cms. × 3 cms. A
narrow gold-leaf L is attached to a rod R passing through a clean sulphur
cork. Opposite the gold-leaf is fixed an insulated brass plate P, placed about
1 mm. from the wall of the box. The movement of the gold-leaf is observed
through two small windows by means of a microscope provided with a
micrometer scale. The plate P is maintained at a constant potential
(generally about 200 volts). The electrometer case is placed in an inclined
position as shown in the figure, the angle of inclination and the potential of
the plate being adjusted to give the desired sensitiveness. The gold-leaf is
initially connected to the case, and the microscope adjusted so that the gold-
leaf is seen in the centre of the scale. For a given potential of the plate, the
sensitiveness depends on the angle of tilt of the case. There is a certain
critical inclination below which the gold-leaf is unstable. The most sensitive
position lies just above the critical angle. In a particular experiment Wilson
found that with an angle of tilt of 30° and with the plate at a constant
potential of 207 volts, the gold-leaf, when raised to a potential of one volt
above the case, moved over 200 scale divisions of the eye-piece, 54
divisions corresponding to one millimetre.

Fig. 13.

In use, the rod R is connected with the external insulated system whose rise
or fall of potential is to be measured. On account of the small capacity of
the system and the large movement of the gold-leaf for a small difference of
potential, the electroscope is able to measure extraordinarily minute
currents. The apparatus is portable. If the plate P be connected to one pole
of a dry pile the gold-leaf is stretched out towards the plate, and in this
position can be carried without risk of injury.

59. Electrometers. Although the electroscope can be used with advantage
in special cases, it is limited in its application. The most generally
convenient apparatus for measurement of ionization currents through gases
is one of the numerous types of quadrant electrometer. With the help of
auxiliary capacities, the electrometer can be used to measure currents with
accuracy over a wide range, and can be employed for practically every kind
of measurement required in radio-activity.
The elementary theory of the symmetrical quadrant electrometer as given in
the text-books is very imperfect. It is deduced that the sensibility of the
electrometer—measured by the deflection of the needle for 1 volt P.D.
between the quadrants—varies directly as the potential of the charged
needle, provided that this potential is high compared with the P.D. between
the quadrants. In most electrometers however, the sensibility rises to a
maximum, and then decreases with increase of potential of the needle. For

electrometers in which the needle lies close to the quadrants, this maximum
sensibility is obtained for a comparatively low potential of the needle. A
theory of the quadrant electrometer, accounting for this action, has been
recently given by G. W. Walker[104]. The effect appears to be due to the
presence of the air space that necessarily exists between adjoining
quadrants.

Fig. 14.

Suppose that it is required to measure with an electrometer the ionization
current between two horizontal metal plates A and B (Fig. 14) on the lower
of which some active material has been spread. If the saturation current is
required, the insulated plate A is connected with one pole of a battery of
sufficient E.M.F. to produce saturation, the other pole being connected to
earth. The insulated plate B is connected with one pair of quadrants of the
electrometer, the other pair being earthed. By means of a suitable key K, the
plate B and the pair of quadrants connected with it may be either insulated
or connected with earth. When a measurement is to be taken, the earth
connection is broken. If the positive pole of the battery is connected with A,
the plate B and the electrometer connections immediately begin to be
charged positively, and the potential, if allowed, will steadily rise until it is
very nearly equal to the potential of A. As soon as the potential of the
electrometer system begins to rise, the electrometer needle commences to
move at a uniform rate. Observations of the angular movement of the
needle are made either by the telescope and scale or by the movement of the

spot of light on a scale in the usual way. If the needle is damped so as to
give a uniform motion over the scale, the rate of movement of the needle,
i.e. the number of divisions of the scale passed over per second, may be
taken as a measure of the current through the gas. The rate of movement is
most simply obtained by observing with a stop-watch the time taken for the
spot of light, after the motion has become steady, to pass over 100 divisions
of the scale. As soon as the observation is made, the plate B is again
connected with earth, and the electrometer needle returns to its original
position.
In most experiments on radio-activity, only comparative measurements of
saturation currents are required. If these measurements are to extend over
weeks or months, as is sometimes the case, it is necessary to adopt some
method of standardizing the electrometer from day to day, so as to correct
for variation in its sensibility. This is done most simply by comparing the
current to be measured with that due to a standard sample of uranium oxide,
which is placed in a definite position in a small testing vessel, always kept
in connection with the electrometer. Uranium oxide is a very constant
source of radiation, and the saturation current due to it is the same from day
to day. By this method of comparison accurate observations may be made
on the variation of activity of a substance over long intervals of time,
although the sensibility of the electrometer may vary widely between
successive measurements.

60. Construction of electrometers. As the quadrant electrometer has
gained the reputation of being a difficult and uncertain instrument for
accurate measurements of current, it may be of value to give some
particular details in regard to the best method of construction and
insulation. In most of the older types of quadrant electrometers the needle
system was made unnecessarily heavy. In consequence of this, if a
sensibility of the order of 100 mms. deflection for 1 volt was required, it
was necessary to charge the Leyden jar connected to the needle to a fairly
high potential. This at once introduced difficulties, for at a high potential it
is not easy to insulate the Leyden jar satisfactorily, or to charge it to the
same potential from day to day. This drawback is to a large extent avoided
in the White pattern of the Kelvin electrometer, which is provided with a
replenisher and attracted disc for keeping the potential of the needle at a

definite value. If sufficient trouble is taken in insulating and setting up this
type of electrometer, it proves a very useful instrument of moderate
sensibility, and will continue in good working order for a year or more
without much attention.
Simpler types of electrometer of greater sensibility can however be readily
constructed to give accurate results. The old type of quadrant electrometer,
to be found in every laboratory, can readily be modified to prove a useful
and trustworthy instrument. A light needle can be made of thin aluminium,
of silvered paper or of a thin plate of mica, covered with gold-leaf to make
it conducting. The aluminium wire and mirror attached should be made as
light as possible. The needle should be supported either by a fine quartz
fibre or a long bifilar suspension of silk. A very fine phosphor bronze wire
of some length is also very satisfactory. A magnetic control is not very
suitable, as it is disturbed by coils or dynamos working in the
neighbourhood. In addition, the zero point of the needle is not as steady as
with the quartz or bifilar suspension.
When an electrometer is used to measure a current by noting the rate of
movement of the needle, it is essential that the needle should be damped
sufficiently to give a uniform motion of the spot of light over the scale. The
damping requires fairly accurate adjustment. If it is too little, the needle has
an oscillatory movement superimposed on the steady motion; if it is too
great, it moves too sluggishly from rest and takes some time to attain a state
of uniform motion. With a light needle, very little, if any, extra damping is
required. A light platinum wire with a single loop dipping in sulphuric acid
is generally sufficient for the purpose.
With light needle systems and delicate suspensions, it is only necessary to
charge the needle to a potential of a few hundred volts to give a sensibility
of several thousand divisions for a volt. With such low potentials, the
difficulty of insulation of the condenser, with which the needle is in
electrical connection, is much reduced. It is convenient to use a condenser
such that the potential of the needle does not fall more than a few per cent.
per day. The ordinary short glass jar partly filled with sulphuric acid is, in
most cases, not easy to insulate to this extent. It is better to replace it by an
ebonite (or sulphur) condenser[105] such as is shown in Fig. 15.

Fig. 15.

A circular plate of ebonite about 1 cm. thick is turned down until it is not
more than ½ mm. thick in the centre. Into this circular recess a brass plate B
fits loosely. The ebonite plate rests on another brass plate C connected with
earth. The condenser thus formed has a considerable capacity and retains a
charge for a long time. In order to make connection with the needle, a small
glass vessel D, partly filled with sulphuric acid, is placed on the plate B and
put in connection with the needle by means of a fine platinum wire. The
platinum wire from the needle dips into the acid, and serves to damp the
needle. In a dry atmosphere, a condenser of this kind will not lose more
than 20 per cent. of its charge in a week. If the insulation deteriorates, it can
readily be made good by rubbing the edge of the ebonite A with sand-paper,
or removing its surface in a lathe.
If a sufficient and steady E.M.F. is available, it is much better to keep the
battery constantly connected with the needle, and to avoid the use of the
condenser altogether. If a battery of small accumulators is used, their
potential can be kept at a constant value, and the electrometer always has a
constant sensibility.

61. A very useful electrometer of great sensibility has been devised by
Dolezalek[106]. It is of the ordinary quadrant type with a very light needle of
silvered paper, spindle shaped, which lies fairly close to the quadrants. A
very fine quartz suspension is employed. In consequence of the lightness of
the needle and its nearness to the quadrants, it acts as its own damper. This
is a great advantage, for difficulties always arise when the wire dips into

sulphuric acid, on account of the thin film which collects after some time on
the surface of the acid. This film obstructs the motion of the platinum wire
dipping into the acid, and has to be removed at regular intervals. These
instruments can readily be made to give a sensibility of several thousand
divisions for a volt when the needle is charged to about one hundred volts.
The sensibility of the electrometer passes through a maximum as the
potential of the needle is increased. It is always advisable to charge the
needle to about the value of this critical potential. The capacity of the
electrometer is in general high (about 50 electrostatic units) but the
increased sensibility more than compensates for this. The needle may either
be charged by lightly touching it with one terminal of a battery, or it may be
kept charged to a constant potential through the quartz suspension.
Dolezalek states that the fibre can be made sufficiently conducting for the
purpose by dipping it into a dilute solution of calcium chloride or
phosphoric acid. I have not found this method satisfactory in dry climates as
in many cases the fibre practically loses its conductivity after a few days
exposure to dry air.
In addition to its great sensibility, the advantage of this instrument is in the
steadiness of the zero and in the self-damping.
A sensibility of 10,000 millimetre divisions per volt can be readily obtained
with this electrometer, if a very fine fibre be used. The use of such high
sensibilities cannot, however, be recommended except for very special
experiments. The period of swing of the needle under these conditions is
several minutes and the natural leak of the testing vessels employed, as well
as electrostatic and other disturbances, make themselves only too manifest.
If measurements of minute currents are required, an electroscope of the type
described in Section 56 is much to be preferred to a very sensitive
electrometer. The electroscope readings in such a case are more accurate
than similar measurements made by an electrometer.
For most measurements in radio-activity, an electrometer which has a
sensibility of 100 divisions per volt is very suitable, and no advantage is
gained by using an electrometer of greater sensibility. If still smaller effects
require to be measured, the sensibility may be increased to several thousand
divisions per volt.

62. Adjustment and screening. In adjusting an electrometer, it is important
to arrange that the needle shall lie symmetrically with regard to the
quadrants. This is best tested by observing whether the needle is deflected
on charging, the quadrants all being earthed. In most electrometers there is
an adjustable quadrant, the position of which may be altered until the needle
is not displaced on charging. When this condition is fulfilled, the zero
reading of the electrometer remains unaltered as the needle loses its charge,
and the deflection on both sides of the zero should be the same for equal
and opposite quantities of electricity.
The supports of the quadrants require to be well insulated. Ebonite rods are
as a rule more satisfactory for this purpose than glass. In testing for the
insulation of the quadrants and the connections attached, the system is
charged to give a deflection of about 200 scale divisions. If the needle does
not move more than one or two divisions after standing for one minute, the
insulation may be considered quite satisfactory. When a suitable desiccator
is placed inside the tight-fitting electrometer case, the insulation of the
quadrants should remain good for months. If the insulation of the ebonite
deteriorates, it can easily be made good by removing the surface of the
ebonite in a lathe.
In working with a sensitive instrument like the Dolezalek electrometer, it is
essential that the electrometer and the testing apparatus should be
completely enclosed in a screen of wire-gauze connected with earth, in
order to avoid electrostatic disturbances. If an apparatus is to be tested at
some distance from the electrometer, the wires leading to it should be
insulated in metal cylinders connected with earth. The size of the insulators
used at various points should be made as small as possible, in order to avoid
disturbances due to their electrification. In damp climates, paraffin, amber,
or sulphur insulates better than ebonite. The objection to paraffin as an
insulator for sensitive electrometers lies in the difficulty of getting entirely
rid of any electrification on its surface. When paraffin has been once
charged, the residual charge, after diselectrifying it with a flame, continues
to leak out for a long interval. All insulators should be diselectrified by
means of a spirit-lamp or still better by leaving some uranium near them.
Care should be taken not to touch the insulation when once diselectrified.
In accurate work it is advisable to avoid the use of gas jets or Bunsen
flames in the neighbourhood of the electrometer, as the flame gases are

strongly ionized and take some time to lose their conductivity. If radio-
active substances are present in the room, it is necessary to enclose the
wires leading to the electrometer in fairly narrow tubes, connected with
earth. If this is not done, it will be found that the needle does not move at a
constant rate, but rapidly approaches a steady deflection where the rate of
loss of charge of the electrometer and connections, due to the ionization of
the air around them, is balanced by the current to be measured. This
precaution must always be taken when observations are made on the very
penetrating rays from active substances. These rays readily pass through
ordinary screens, and ionize the air around the electrometer and connecting
wires. For this reason it is impossible to make accurate measurements of
small currents in a room which is used for the preparation of radio-active
material. In course of time the walls of the room become radio-active owing
to the dissemination of dust and the action of the radio-active
emanations[107].

63. Electrometer key. For work with electrometers of high sensibility, a
special key is necessary to make and break from a distance the connection
of the quadrants with earth in order to avoid electrostatic disturbances at the
moment the current is to be measured. The simple key shown in Fig. 16 has
been found very satisfactory for this purpose. A small brass rod BM, to
which a string is attached, can be moved vertically up and down in a brass
tube A, which is rigidly attached to a bent metal support connected with
earth. When the string is released, this rod makes contact with the mercury
M, which is placed in a small metal vessel resting on a block of ebonite P.
The electrometer and testing vessel are connected with the mercury. When
the string is pulled, the rod BM is removed from the mercury and the earth
connection of the electrometer system is broken. On release of the string,
the rod BM falls and the electrometer is again earthed. By means of this
key, which may be operated at any distance from the electrometer, the earth
connection may be made and broken at definite intervals without any
appreciable disturbance of the needle.

Fig. 16.

64. Testing apparatus. The arrangement shown in Fig. 17 is very
convenient for many measurements in radio-activity. Two parallel insulated
metal plates A and B are placed inside a metal vessel V, provided with a
side door. The plate A is connected with one terminal of a battery of small
storage cells, the other pole of which is earthed; the plate B with the
electrometer, and the vessel V with earth. The shaded areas in the figure
indicate the position of ebonite insulators. The active material to be tested is
spread uniformly in a shallow groove (about 5 cms. square and 2 mms.
deep) in the brass plate A. In order to avoid breaking the battery connection
every time the plate A is removed, the wire from the battery is permanently
connected with the metal block N resting on the ebonite support. In this
arrangement there is no possibility of a conduction leak from the plate A to
B, since the earth-connected vessel V intervenes.

Fig. 17.

An apparatus of this kind is very convenient for testing the absorption of the
radiations by solid screens, as well as for making comparative studies of the
activity of different bodies. Unless very active preparations of radium are
employed, a battery of 300 volts is sufficient to ensure saturation when the
plates are not more than 5 centimetres apart. If substances which give off a
radio-active emanation are being tested, the effect of the emanation can be
eliminated by passing a steady current of air from a gas bag between the
plates. This removes the emanation as fast as it is produced.
If a clean plate is put in the place of A, a small movement of the
electrometer needle is always observed. If there is no radio-active substance
in the neighbourhood, this effect is due to the small natural ionization of the
air. We can correct for this natural leak when necessary.

65. We have often to measure the activity due to the emanations of thorium
or radium, or the excited activity produced by those emanations on rods or
wires. A convenient apparatus for this purpose is shown in Fig. 18. The
cylinder B is connected with the battery in the usual way, and the central
conductor A with the electrometer. This central rod is insulated from the
external cylinder by an ebonite cork, which is divided into two parts by a
metal ring CC´ connected to earth. This ring acts the part of a guard-ring,
and prevents any conduction leak between B and A. The ebonite is thus only
required to insulate satisfactorily for the small rise of potential produced on
A during the experiment. In all accurate measurements of current in radio-

activity the guard-ring principle should always be used to ensure good
insulation. This is easily secured when the ebonite is only required to
insulate for a fraction of a volt, instead of for several hundred volts, as is the
case when the guard-ring is absent.

Fig. 18.

66. For measurements of radio-activity with an electrometer, a steady
source of E.M.F. of at least 300 volts is necessary. This is best obtained by a
battery of small cells simply made by immersing strips of lead in dilute
sulphuric acid, or by a battery of small accumulators of the usual
construction. Small accumulators of capacity about one-half ampere-hour
can now be obtained at a moderate price, and are more constant and require
less attention than simple lead cells.
In order to measure currents over a wide range, a graduated series of
capacities is required. The capacity of an electrometer and testing apparatus
is usually about 50 electrostatic units or ·000056 microfarads. Subdivided
condensers of mica are constructed in which capacities varying from ·001
to ·2 microfarads are provided. With such a condenser, another extra
capacity is required to bridge over the gap between the capacity of the
electrometer and the lowest capacity of the condenser. This capacity of
value about 200 electrostatic units can readily be made by using parallel
plates or still better concentric cylinders. With this series of capacities,
currents may be measured between 3 × 10-14 and 3 × 10-8 amperes—a range
of over one million. Still larger currents can be measured if the sensibility
of the electrometer is reduced, or if larger capacities are available.

In a room devoted to electrometer measurements of radio-activity, it is
desirable to have no radio-active matter present except that to be tested. The
room should also be as free from dust as possible. The presence of a large
quantity of dust in the air (see section 31) is a very disturbing factor in all
radio-active measurements. A larger E.M.F. is required to produce saturation
on account of the diffusion of the ions to the dust particles. The presence of
dust in the air also leads to uncertainty in the distribution of excited activity
in an electric field (see section 181).

67. Measurement of Current. In order to determine the current in the
electrometer circuit by measuring the rate of movement of the needle, it is
necessary to know both the capacity of the circuit and the sensibility of the
electrometer.
Let C = capacity of electrometer and its
connections in E.S. units,
d = number of divisions of the scale passed
over per second,
D = sensibility of the electrometer measured in
scale divisions
for 1 volt P.D. between the quadrants.

The current i is given by the product of the capacity of the system and the
rate of rise of potential.
Thus
Cd
i = ----- E.S. units,
300D

Cd
= ----------- amperes.
9 × 1011 D

Suppose, for example,
C = 50, d = 5, D = 1000;
then i = 2·8 × 10-13 amperes.

Since the electrometer can readily measure a current corresponding to a
movement of half a scale division per second, we see that an electrometer
can measure a current of 3 × 10-14 amperes, which is considerably below the
range of the most sensitive galvanometer.
The capacity of the electrometer itself must not be considered as equal to
that of the pair of quadrants and the needle when in a position of rest. The
actual capacity is very much larger than this, on account of the motion of
the charged needle. Suppose, for example, that the needle is charged to a
high negative potential, and kept at the zero position by an external
constraint. If a quantity Q of positive electricity is given to the electrometer
and its connections, the whole system is raised to a potential V, such that Q
= CV, where C is the capacity of the system. When however the needle is
allowed to move, it is attracted into the charged pair of quadrants. This
corresponds to the introduction of a negatively charged body between the
quadrants, and in consequence the potential of the system is lowered to V´.
The actual capacity C´ of the system when the needle moves is thus greater
than C, and is given by
C´V´ = CV.

Thus the capacity of the electrometer is not a constant, but depends on the
potential of the needle, i.e. on the sensibility of the electrometer.
An interesting result of practical importance follows from the variation of
the capacity of the electrometer with the potential of the needle. If the
external capacity attached to the electrometer is small compared with that of
the electrometer itself, the rate of movement of the needle for a constant
current is, in some cases, independent of the sensibility. An electrometer
may be used for several days or even weeks to give nearly equal deflections
for a constant current, without recharging the needle, although its potential
has been steadily falling during the interval. In such a case the decrease in
sensibility is nearly proportional to the decrease in capacity of the
electrometer, so that the deflection for a given current is only slightly
altered. The theory of this action has been given by J. J. Thomson[108].

68. The capacity of the electrometer and its connections cannot be measured
by any of the commutator methods used for the determination of small

capacities, for in such cases the needle does not move, and the capacity
measured is not that of the electrometer system when in actual use. The
value of the capacity may, however, be determined by the method of
mixtures.
Let C = capacity of electrometer and connections,
C1 = capacity of a standard condenser.

The electrometer and its connections are charged to a potential V1 by a
battery, and the deflection d1 of the needle is noted. By means of an
insulated key, the capacity of the standard condenser is added in parallel
with the electrometer system. Let V2 be the potential of the system, and d2
the new deflection.
Then
CV1 = (C + C1) V2,
C + C1 V1 d1
-------- = ----- = -----
C V2 d2

d2
and C = C1 --------
d1 – d2

Fig. 19.

A simple standard capacity for this purpose can be constructed of two
concentric brass tubes the diameters of which can be accurately measured.
The external cylinder D (Fig. 19) is mounted on a wooden base, which is
covered with a sheet of metal or tinfoil connected to earth. The tube C is
supported centrally on ebonite rods at each end. The capacity is given
approximately by the formula

where b is the internal diameter of D, a the external diameter of C, and l the
length of the tubes.
The following method can be used in some cases with advantage. While a
testing vessel is in connection with the electrometer, a sample of uranium is
placed on the lower plate A. Let d2 and d1 be the number of divisions passed
over per second by the needle with and without the standard capacity in
connection.
C + C1 d1
Then ------ = ------ ,
C d2

d2
and C = C1 --------
d1 – d2

This method has the advantage that the relative capacities are expressed in
terms of the motion of the needle under the actual conditions of
measurement.

69. Steady deflection method. The methods of measurement previously
described depend upon the rate of angular movement of a suspended gold-
leaf or of an electrometer needle. The galvanometer can only be employed
for measurements with intensely active matter. A need, however, has long

been felt for a method in which ordinary ionization currents can be
measured by means of a steady deflection of an electrometer needle. This is
especially the case, where measurements have to be made with active
substances whose activity alters rapidly in the course of a few minutes.
This can obviously be secured if the electrometer system (one pair of
quadrants being earthed) is connected to earth through a suitable high
resistance. A steady deflection of the electrometer needle will be obtained
when the rate of supply of electricity to the electrometer system is balanced
by the loss due to conduction through the resistance. If the high resistance
obeys Ohm’s law, the deflection should be proportional to the ionization
current to be measured.
A simple calculation shows that the resistance required is very great.
Suppose, for example, that a current is to be measured corresponding to a
rate of movement of the needle of 5 divisions per second, with a sensibility
of 1000 divisions per volt, and where the capacity of the electrometer
system is 50 electrostatic units. This current is equal to 2·8 × 10-13 amperes.
If a steady deflection of 10 divisions is required, which corresponds to a
rise of potential of the system of ¹⁄₁₀₀ of a volt, the resistance should be
36,000 megohms. For a deflection of 100 divisions, the resistance should be
10 times as large. Dr Bronson[109], working in the laboratory of the writer,
has recently made some experiments in order to devise a practical method
for measurements of this character. It is difficult to obtain sufficiently high
and constant resistances to answer the purpose. Tubes of xylol had too great
a resistance, while special carbon resistances were not sufficiently constant.
The difficulty was finally got over by the use of what may be called an “air
resistance.” The arrangement of the experiment is shown in Fig. 20.

Fig. 20.

The electrometer system was connected with the upper of two insulated
parallel plates AB, on the lower of which was spread a layer of a very active
substance. An active bismuth plate, coated with radio-tellurium, which had
been obtained from Sthamer of Hamburg, proved very convenient for this
purpose.
The lower plate B was connected to earth. The charge communicated to the
upper plate of the testing vessel CD and the electrometer system leaked
away in consequence of the strong ionization between the plates AB, and a
steady deflection was obtained when the rate of supply was equal to the rate
of discharge.
This air resistance obeyed Ohm’s law over a considerable range, i.e. the
steady deflection was proportional to the current. It is advisable, in such an
arrangement, to test whether the deflection is proportional to the ionization
current over the range required for measurement. This can readily be done
by the use of a number of metal vessels filled with a constant radio-active
substance like uranium oxide. The effect of these, when placed in the
testing vessel, can be tested separately and in groups, and in this way the
scale can be calibrated accurately.
The plates AB were placed inside a closed vessel to avoid air currents. The
contact difference of potential between the plates AB, which shows itself by
a steady deflection when no radio-active matter is present in CD, was for
the most part eliminated by covering the surface of the plates A and B with
very thin aluminium foil.

This method proved very accurate and convenient for measurement of rapid
changes in activity, and possesses many advantages over the ordinary rate-
method of use of an electrometer. A thin layer of radium of moderate
activity would probably serve in place of the radio-tellurium, but the
emanation and the β and γ rays emitted from it would be a possible source
of disturbance to the measurements. The deflection of the electrometer
needle in this arrangement is independent of the capacity of the
electrometer system, and thus comparative measurements of current can be
made without the necessity of determining the capacity in each case.

70. Quartz piezo-electrique. In measurements of the strength of currents
by electrometers, it is always necessary to determine the sensibility of the
instrument and the capacity of the electrometer and the apparatus attached
thereto. By means of the quartz piezo-electrique devised by the brothers
MM. J. and P. Curie[110], measurements of the current can be made with
rapidity and accuracy over a wide range. These measurements are quite
independent of the capacity of the electrometer and external circuit.
The essential part of this instrument consists of a plate of quartz which is
cut in a special manner. When this plate is placed under tension, there is a
liberation of electricity equal in amount but opposite in sign on the two
sides of the plate. The plate of quartz AB (Fig. 21) is hung vertically and
weights are added to the lower end. The plate is cut so that the optic axis of
the crystal is horizontal and at right angles to the plane of the paper.

Fig. 21.

The two faces A and B are normal to one of the binary axes (or electrical
axes) of the crystal. The tension must be applied in a direction normal to the
optic and electric axes. The two faces A and B are silvered, but the main
portion of the plate is electrically insulated by removing a narrow strip of
the silvering near the upper and lower ends of the plate. One side of the
plate is connected with the electrometer and with the conductor, the rate of
leak of which is to be measured. The quantity of electricity set free on one
face of the plate is accurately given by
L
Q = ·063 ---- F
b

where L is the length of the insulated portion of the plate, b the thickness
AB, and F the weight attached in kilogrammes. Q is then given in
electrostatic units.
Suppose, for example, that it is required to measure the current between the
plates CD (Fig. 21) due to some radio-active material on the plate C, for a
given difference of potential between C and D. At a given instant the
connection of the quadrants of the electrometer with the earth is broken.
The weight is attached to the quartz plate, and is held in the hand so as to
apply the tension gradually. This causes a release of electricity opposite in
sign to that given to the plate D. The electrometer needle is kept at the
position of rest as nearly as possible by adjusting the tension by hand. The
tension being fully applied, the moment the needle commences to move
steadily from zero is noted. The current between the plates CD is then given
by Q/t where t is the time of the observation. The value of Q is known from
the weight attached.
In this method the electrometer is only used as a detector to show that the
system is kept at zero potential. No knowledge of the capacity of the
insulated system is required. With practice, measurements of the current can
be made in this way with rapidity and certainty.

CHAPTER IV.
NATURE OF THE RADIATIONS.

PART I.
Comparison of the Radiations.

71. The Three Types of Radiation. All the radio-active substances possess in
common the power of acting on a photographic plate and of ionizing the gas in
their immediate neighbourhood. The intensity of the radiations may be compared
by means of their photographic or electrical action; and, in the case of the strongly
radio-active substances, by the power they possess of lighting up a
phosphorescent screen. Such comparisons, however, do not throw any light on the
question whether the radiations are of the same or of different kinds, for it is well
known that such different types of radiations as the short waves of ultra-violet
light, Röntgen and cathode rays, all possess the property of producing ions
throughout the volume of a gas, lighting up a fluorescent screen, and acting on a
photographic plate. Neither can the ordinary optical methods be employed to
examine the radiations under consideration, as they show no trace of regular
reflection, refraction, or polarization.
Two general methods can be used to distinguish the types of the radiations given
out by the same body, and also to compare the radiations from the different active
substances. These methods are as follows:

(1) By observing whether the rays are appreciably deflected in a magnetic
field.
(2) By comparing the relative absorption of the rays by solids and gases.

Examined in these ways, it has been found that there are three different types of
radiation emitted from radio-active bodies, which for brevity and convenience
have been termed by the writer the α, β, and γ rays.
(i) The α rays are very readily absorbed by thin metal foil and by a few
centimetres of air. They have been shown to consist of positively charged bodies
projected with a velocity of about ⅒ the velocity of light. They are deflected by
intense magnetic and electric fields, but the amount of deviation is minute in
comparison with the deviation, under the same conditions, of the cathode rays
produced in a vacuum tube.
(ii) The β rays are far more penetrating in character than the α rays, and consist of
negatively charged bodies projected with velocities of the same order as the

velocity of light. They are far more readily deflected than the α rays, and are in
fact identical with the cathode rays produced in a vacuum tube.
(iii) The γ rays are extremely penetrating, and non-deviable by a magnetic field.
Their true nature is not definitely settled, but they are analogous in most respects
to very penetrating Röntgen rays.
The three best known radio-active substances, uranium, thorium, and radium, all
give out these three types of rays, each in an amount approximately proportional
to its relative activity measured by the α rays. Polonium stands alone in giving
only the α or easily absorbed rays[111].

72. Deflection of the rays. The rays emitted from the active bodies thus present a
very close analogy with the rays which are produced in a highly exhausted
vacuum tube when an electric discharge passes through it. The α rays correspond
to the canal rays, discovered by Goldstein, which have been shown by Wien to
consist of positively charged bodies projected with great velocity (see section 51).
The β rays are the same as the cathode rays, while the γ rays resemble the Röntgen
rays. In a vacuum tube, a large amount of electric energy is expended in producing
the rays, but, in the radio-active bodies, the rays are emitted spontaneously, and at
a rate uninfluenced by any chemical or physical agency. The α and β rays from the
active bodies are projected with much greater velocity than the corresponding rays
in a vacuum tube, while the γ rays are of much greater penetrating power than
Röntgen rays.
The effect of a magnetic field on a pencil of rays from a radio-active substance
giving out the three kinds of rays is very well illustrated in Fig. 22[112].

Fig. 22.

Some radium is placed in the bottom of a narrow cylindrical lead vessel R. A
narrow pencil of rays consisting of α, β, and γ rays escapes from the opening. If a
strong uniform magnetic field is applied at right angles to the plane of the paper,
and directed towards the paper, the three types of rays are separated from one
another. The γ rays continue in a straight line without any deviation. The β rays
are deflected to the right, describing circular orbits the radii of which vary within
wide limits. If the photographic plate AC is placed under the radium vessel, the β
rays produce a diffuse photographic impression on the right of the vessel R. The α
rays are bent in the direction opposite to that of the β rays, and describe a portion
of the arc of a circle of large radius, but they are rapidly absorbed after traversing
a distance of a few centimetres from the vessel R. The amount of the deviation of
the α rays compared with that of the β rays is much exaggerated in the figure.

73. Ionizing and penetrating power of the rays. Of the three kinds of rays, the α
rays produce most of the ionization in the gas and the γ rays the least. With a thin
layer of unscreened active material spread on the lower of two parallel plates 5
cms. apart, the amount of ionization due to the α, β, and γ rays is of the relative
order 10,000, 100, and 1. These numbers are only rough approximations, and the
differences become less marked as the thickness of the radio-active layer
increases.
The average penetrating power of the rays is shown below. In the first column is
given the thickness of the aluminium, which cuts each radiation down to half its
value, and in the second the relative power of penetration of the rays.
Radiation Thickness of Relative
Aluminium in cms. power of
which cuts off half the penetration
radiation

α rays 0·0005 cms. 1
β„ 0·05 cms. 100
γ„ 8 cms. 10000
The relative power of penetration is thus approximately inversely proportional to
the relative ionization. These numbers, however, only indicate the order of relative
penetrating power. This power varies considerably for the different active bodies.
The α rays from uranium and polonium are the least penetrating, and those from
thorium the most. The β radiations from thorium and radium are very complex,
and consist of rays widely different in penetrating power. Some of the β rays from

these substances are much less and others much more penetrating than those from
uranium, which gives out fairly homogeneous rays.

74. Difficulties of comparative measurements. It is difficult to make
quantitative or even qualitative measurements of the relative intensity of the three
types of rays from active substances. The three general methods employed depend
upon the action of the rays in ionizing the gas, in acting on a photographic plate,
and in causing phosphorescent or fluorescent effects in certain substances. In each
of these methods the fraction of the rays which is absorbed and transformed into
another form of energy is different for each type of ray. Even when one specific
kind of ray is under observation, comparative measurements are rendered difficult
by the complexity of that type of rays. For example, the β rays from radium
consist of negatively charged particles projected with a wide range of velocity,
and, in consequence, they are absorbed in different amounts in passing through a
definite thickness of matter. In each case, only a fraction of the energy absorbed is
transformed into the particular type of energy, whether ionic, chemical, or
luminous, which serves as a means of measurement.
The rays which are the most active electrically are the least active
photographically. Under ordinary conditions, most of the photographic action of
uranium, thorium, and radium, is due to the β or cathodic rays. The α rays from
uranium and thorium, on account of their weak action, have not yet been detected
photographically. With active substances like radium and polonium, the α rays
readily produce a photographic impression. So far the γ rays have been detected
photographically from radium only. That no photographic action of these rays has
yet been established for uranium and thorium is probably due merely to the fact
that the effect sought for is very small, and during exposures for long intervals it
is very difficult to avoid fogging of the plates owing to other causes. Considering
the similarity of the radiations in other respects, there can be little doubt that the γ
rays do produce some photographic action, though it is too small to observe with
certainty.
These differences in the photographic and ionizing properties of the radiations
must always be taken into account in comparing results obtained by the two
methods. The apparent contradiction of results obtained by different observers
using these two methods is found to be due to their differences in relative
photographic and ionizing action. For example, with the unscreened active
material, the ionization observed by the electrical method is due almost entirely to
α rays, while the photographic action under the same condition is due almost
entirely to the β rays.

It is often convenient to know what thickness of matter is sufficient to absorb a
specific type of radiation. A thickness of aluminium or mica of ·01 cms. or a sheet
of ordinary writing-paper is sufficient to absorb completely all the α rays. With
such a screen over the active material, the effects are due only to the β and γ rays,
which pass through with a very slight absorption. Most of the β rays are absorbed
in 5 mms. of aluminium or 2 mms. of lead. The radiation passing through such
screens consists very largely of the γ rays. As a rough working rule, it may be
taken that a thickness of matter required to absorb any type of rays is inversely
proportional to the density of the substance, i.e. the absorption is proportional to
the density. This rule holds approximately for light substances, but, in heavy
substances like mercury and lead, the radiations are about twice as readily
absorbed as the density rule would lead us to expect.

PART II.

The β or Cathodic Rays.

75. Discovery of the β rays. A discovery which gave a great impetus to the study
of the radiations from active bodies was made in 1899, almost simultaneously in
Germany, France, and Austria. It was observed that preparations of radium gave
out some rays which were deviable by a magnetic field, and very similar in
character to the cathode rays produced in a vacuum tube. The observation of
Elster and Geitel that a magnetic field altered the conductivity produced in air by
radium rays, led Giesel[113] to examine the effect of a magnetic field on the
radiations. In his experiments, the radio-active preparation was placed in a small
vessel between the poles of an electromagnet. The vessel was arranged to give a
pencil of rays which was approximately perpendicular to the field. The rays
caused a small fluorescent patch on the screen. On exciting the electromagnet, the
fluorescent zone was observed to broaden out on one side. On reversing the field,
the extension of the zone was in the opposite direction. The deviation of the rays
thus indicated was in the same direction and of the same order of magnitude as
that for cathode rays.
S. Meyer and Schweidler[114] also obtained similar results. They showed, in
addition, the deviation of the rays by the alteration of the conductivity of the air
when a magnetic field was applied. Becquerel[115], a little later, showed the
magnetic deflection of the radium rays by using the photographic method. P.
Curie[116], by the electrical method, showed furthermore that the rays from radium
consisted of two kinds, one apparently non-deviable and easily absorbed (now
known as the α rays), and the other penetrating and deviable by a magnetic field
(now known as the β rays). The ionization effect due to the β rays was only a
small fraction of that due to the α rays. At a later date Becquerel, by the
photographic method, showed that uranium gave out some deflectable rays. It had
been shown previously[117] that the rays from uranium consisted of α and β rays.
The deflected rays in Becquerel’s experiment consisted entirely of β rays, as the α
rays from uranium produce no appreciable photographic action. Rutherford and
Grier[118], using the electric method, showed that compounds of thorium, like those
of uranium, gave out, besides α rays, some penetrating β rays, deviable in a
magnetic field. As in the case of radium, the ionization due to the α rays of
uranium and thorium is large compared with that due to the β rays.

76. Examination of the magnetic deviation by the photographic method.
Becquerel has made a very complete study, by the photographic method, of the β
rays from radium, and has shown that they behave in all respects like cathode
rays, which are known to be negatively charged particles moving with a high
velocity. The motion of a charged ion acted on by a magnetic field has been
discussed in section 49. It has been shown that if a particle of mass m and charge e
is projected with a velocity u, at an angle α with the direction of a uniform field of
strength H, it will describe a helix round the magnetic lines of force. This helix is
wound on a cylinder of radius R, with the axis parallel to the field, where R is
given by
mu
R = ---- sin α.
He

When α = π/2, i.e. when the rays are projected normally to the field, the particles
describe circles of radius
mu
R = ----
He

The planes of these circles are normal to the field. Thus, for a particular velocity
u, the value of R varies inversely as the strength of the field. In a uniform field the
rays projected normally to the field describe circles, and their directions of
projection are the tangents at the origin.
This conclusion has been verified experimentally by Becquerel for the β rays of
radium, by an arrangement similar to that shown in Fig. 23.

Fig. 23.

A photographic plate P, with the film downwards, is enveloped in black paper and
placed horizontally in the uniform horizontal magnetic field of an electromagnet.
The magnetic field is supposed to be uniform, and, in the figure, is at right angles
to the plane of the paper. The plate was covered with a sheet of lead, and on the
edge of the plate, in the centre of the magnetic field, is placed a small lead vessel
R containing the radio-active matter.
On exciting the magnet, so that the rays are bent to the left of the figure, it is
observed that a photographic impression is produced directly below the source of
the rays, which have been bent round by the magnetic field. The active matter
sends out rays equally in all directions. The rays perpendicular to the field
describe circles, which strike the plate immediately under the source. A few of
these rays, A1, A2, A3, are shown in the figure. The rays, normal to the plate, strike
the plate almost normally, while the rays nearly parallel to the plate strike the plate
at grazing incidence. The rays, inclined to the direction of the field, describe
spirals and produce effects on an axis parallel to the field passing through the
source. In consequence of this, any opaque screen placed in the path of the rays
has its shadow thrown near the edge of the photographic plate.

77. Complexity of the rays. The deviable rays from radium are complex, i.e. they
are composed of a flight of particles projected with a wide range of velocity. In a
magnetic field every ray describes a path, of which the radius of curvature is
directly proportional to the velocity of projection. The complexity of the radiation
has been shown very clearly by Becquerel[119] in the following way.
An uncovered photographic plate, with the film upwards, was placed horizontally
in the horizontal uniform magnetic field of an electromagnet. A small, open, lead
box, containing the radio-active matter, was placed in the centre of the field, on
the photographic plate. The light, due to the phosphorescence of the radio-active
matter, therefore, could not reach the plate. The whole apparatus was placed in a
dark room. The impression on the plate took the form of a large, diffuse, but
continuous band, elliptic in shape, produced on one side of the plate.
Such an impression is to be expected if the rays are sent out in all directions, even
if their velocities of projection are the same, for it can readily be shown
theoretically, that the path of the rays is confined within an ellipse whose minor
axis, which is at right angles to the field, is equal to 2R, and whose major axis is
equal to πR. If, however, the active matter is placed in the bottom of a deep lead
cylinder of small diameter, the rays have practically all the same direction of
projection, and in that case each part of the plate is acted on by rays of a definite
curvature.

In this case also, a diffuse impression is observed on the plate, giving, so to speak,
a continuous spectrum of the rays and showing that the radiation is composed of
rays of widely different curvatures. Fig. 24 shows a photograph of this kind
obtained by Becquerel, with strips of paper, aluminium, and platinum placed on
the plate.

Fig. 24.

If screens of various thickness are placed on the plate, it is observed that the plate
is not appreciably affected within a certain distance from the active matter, and
that this distance increases with the thickness of the screen. This distance is
obviously equal to twice the radius of curvature of the path of the rays, which are
just able to produce an impression through the screen.
These experiments show very clearly that the most deviable rays are those most
readily absorbed by matter. By observations of this kind Becquerel has determined
approximately the inferior limit of the value of HR for rays which are transmitted
through different thicknesses of matter.
The results are given in the table below:
Substance Thickness Inferior limit of
in mms. HR for
transmitted rays

Black paper 0·065 650
Aluminium 0·010 350
“ 0·100 1000
” 0·200 1480
Mica 0·025 520
Glass 0·155 1130
Platinum 0·030 1310
Copper 0·085 1740

Lead 0·130 2610
If e/m is a constant for all the rays, the value of HR is proportional to the velocity
of the rays, and it follows from the table that the velocity of the rays which just
produce an effect on the plate through ·13 mms. of lead is about 7 times that of the
rays which just produce an impression through ·01 mm. of aluminium. It will be
shown, however, in section 82, that e/m is not a constant for all speeds, but
decreases with increase of velocity of the rays. The difference in velocity between
the rays is in consequence not as great as this calculation would indicate. On
examination of the rays from uranium, Becquerel found that the radiation is not as
complex as that from radium, but consists wholly of rays for which the value of
HR is about 2000.

78. Examination of the β rays by the electric method. The presence of easily
deviable rays given off from an active substance can most readily be shown by the
photographic method, but it is necessary, in addition, to show that the penetrating
rays which produce the ionization in the gas are the same as those which cause the
photographic action. This can be conveniently tested in an arrangement similar to
that shown in Fig. 25.

Fig. 25.

The radio-active matter A is placed on a lead block B´´ between the two parallel
lead plates BB´. The rays pass between the parallel plates and ionize the gas
between the plates PP´ of the testing vessel. The magnetic field is applied at right
angles to the plane of the paper. The dotted rectangle EEEE represents the position
of the pole piece. If a compound of radium or thorium is under investigation, a

stream of air is required to prevent the diffusion of the radio-active emanations
into the testing vessel. When a layer of uranium, thorium or radium compound is
placed at A, the ionization in the testing vessel is due mainly to the action of the α
and β rays. The α rays are cut off by adding a layer of aluminium ·01 cm. thick
over the active material. When the layer of active matter is not more than a few
millimetres thick, the ionization due to the γ rays is small compared with that
produced by the β rays, and may be neglected. On the application of a magnetic
field at right angles to the mean direction of the rays, the ionization in the testing
vessel due to the rays steadily decreases as the strength of the field increases, and
in a strong field it is reduced to a very small fraction of its original value. In this
case the rays are bent so that none of them enter the testing vessel.
Examined in this way, it has been found that the β rays of uranium, thorium, and
radium consist entirely of rays readily deflected by a magnetic field. The rays
from polonium consist entirely of α rays, the deviation of which can be detected
only in very intense magnetic fields.
When the screen covering the active material is removed, in a strong magnetic
field, the ionization in the vessel is mainly due to the α rays. On account of the
slight deviation of the α rays under ordinary experimental conditions, a still
greater increase of the magnetic field does not appreciably alter the current due to
them in the testing vessel.
The action of a magnetic field on a very active substance like radium is easily
shown by the electrical method, as the ionization current due to the deviable rays
is large. With substances of small activity like uranium and thorium, the ionization
current due to the deviable rays is very small, and a sensitive electrometer or an
electroscope is required to determine the variation, in a magnetic field, of the very
small current involved. This is especially the case for thorium oxide, which gives
out only about ⅕ of the amount of deviable rays given out by the same weight of
uranium oxide.

79. Experiments with a fluorescent screen. The β rays from a few milligrams of
pure radium bromide produce intense fluorescence in barium platinocyanide and
other substances which can be made luminous under the influence of the cathode
rays. Using a centigram of radium bromide, the luminosity on a screen, placed
upon it, is bright enough to be observed in daylight. With the aid of such a screen
in a dark room many of the properties of the β rays may be simply illustrated and
their complex nature clearly shown. A small quantity of radium is placed in the
bottom of a short, narrow, lead tube open at one end. This is placed between the
pole pieces of an electromagnet, and the screen placed below it. With no magnetic

field, a faint luminosity of the screen is observed due to the very penetrating γ
rays which readily pass through the lead. When the magnetic field is put on, the
screen is brightly lighted up on one side over an area elliptical in shape (section
77). The direction of deviation is reversed by reversal of the field. The broad
extent of the illumination shows the complex nature of the β rays. On placing a
metallic object at various points above the screen, the trajectory of the rays can
readily be traced by noticing the position of the shadow cast upon the screen. By
observing the density of the shadow, it can be seen that the rays most easily
deviated are the least penetrating.

Comparison of the β rays with cathode rays.

80. Means of comparison. In order to prove the identity of the β rays from active
bodies with the cathode rays produced in a vacuum tube, it is necessary to show

(1) That the rays carry with them a negative charge;
(2) That they are deviated by an electric as well as by a magnetic field;
(3) That the ratio e/m is the same as for the cathode rays.

Electric charge carried by the β rays. The experiments of Perrin and J. J.
Thomson have shown that the cathode rays carry with them a negative charge. In
addition, Lenard has shown that the rays still carry a charge after traversing thin
layers of matter. When the rays are absorbed, they give up their charge to the body
which absorbs them. The total amount of charge carried by the β rays from even a
very active preparation of radium is, in general, small compared with that carried
by the whole of the cathode rays in a vacuum tube, and can be detected only by
delicate methods.

Suppose that a layer of very active radium is spread on a metal plate connected to
earth, and that the β rays are absorbed by a parallel plate connected with an
electrometer. If the rays are negatively charged, the top plate should receive a
negative charge increasing with the time. On account, however, of the great
ionization produced by the rays between the plates, any charge given to one of
them is almost instantly dissipated. In many cases, the plate does become charged
to a definite positive or negative potential depending on the metal, but this is due
to the contact difference of potential between the plates, and would be produced
whether the rays were charged or not. The ionization of the gas is greatly

diminished by placing over the active material a metal screen which absorbs the α
rays, but allows the β rays to pass through with little absorption.
The rapid loss of any charge communicated to the top plate can be very much
reduced, either by diminishing the pressure of the gas surrounding it or by
enclosing the plate with suitable insulators. In their experiments to determine the
amount of charge carried by the radium rays, M. and Mme Curie[120] used the
second method.
A metal disc MM (Fig. 26) is connected with an electrometer by the wire T. The
disc and wire are completely surrounded by insulating matter ii. The whole is
surrounded by a metal envelope EEEE connected with earth. On the lower side of
the disc, the insulator and the metallic covering are very thin. This side is exposed
to the rays of the radium R placed in a depression in a lead plate AA.

Fig. 26.

The rays of the radium pass through the metal cover and insulator with little
absorption, but they are completely absorbed by the disc MM. It was observed that
the disc received a negative charge which increased uniformly with the time,
showing that the rays carry with them a negative charge. The current observed
was very small. With an active preparation of radium[121], forming a layer 2·5 sq.
cms. in area and 2 mms. thick, a current of the order of 10-11 amperes was
observed after the rays had traversed a layer of aluminium ·01 mm. thick and a
layer of ebonite ·3 mm. thick. The current was the same with discs of lead, copper,
and zinc, and also when the ebonite was replaced by paraffin.
Curie also observed in another experiment of a similar character that the radium
itself acquired a positive charge. This necessarily follows if the rays carry with
them a negative charge. If the β rays alone carried with them a charge, a pellet of
radium, if perfectly insulated, and surrounded by a non-conducting medium,
would in the course of time be raised to a high positive potential. Since, however,
the α rays carry with them a charge opposite in sign to the β rays, the ratio of the
charge carried off by the two types of rays must be determined, before it can be

settled whether the radium would acquire a positive or a negative charge. If,
however, the radium is placed in an insulated metal vessel of a thickness sufficient
to absorb all the α rays, but not too thick to allow most of the β rays to escape, the
vessel will acquire a positive charge in a vacuum.
An interesting experimental result bearing upon this point has been described by
Dorn[122]. A small quantity of radium was placed in a sealed glass tube and left for
several months. On opening the tube with a file, a bright electric spark was
observed at the moment of fracture, showing that there was a large difference of
potential between the inside of the tube and the earth.
In this case the α rays were absorbed in the walls of the tube, but a large
proportion of the β rays escaped. The inside of the tube thus became charged, in
the course of time, to a high positive potential; a steady state would be reached
when the rate of escape of negative electricity was balanced by the leakage of
positive electricity through the walls of the tube. The external surface of the glass
would be always practically at zero potential, on account of the ionization of the
air around it.
Strutt[123] has recently described a simple and striking experiment to illustrate still
more clearly that a radium preparation acquires a positive charge, if it is enclosed
in an envelope thick enough to absorb all the α particles, but thin enough to allow
most of the β particles to escape. The experimental arrangement is clearly seen in
Fig. 27. A sealed tube AA containing the radium, was attached at one end to a pair
of thin gold leaves in metallic connection with the radium, and was insulated
inside a larger tube by means of a quartz rod B. The inner surface of the tube was
coated with tinfoil EE connected to earth. The glass surface of AA was made
conducting by a thin coating of phosphoric acid. The air in the outer tube was
exhausted as completely as possible by means of a mercury pump, in order to
reduce the ionization in the gas, and consequently the loss of any charge gained by
the gold leaves. After an interval of 20 hours, the gold leaves were observed to
diverge to their full extent, indicating that they had acquired a large positive
charge. In this experiment Strutt used ½ gram of radiferous barium of activity
only 100 times that of uranium.

Fig. 27.

If the tube is filled with 30 mgrs. of pure radium bromide, the leaves diverge to
their full extent in the course of about a minute. If it is arranged that the gold leaf,
at a certain angle of divergence, comes in contact with a piece of metal connected
with earth, the apparatus can be made to work automatically. The leaf diverges,
touches the metal, and at once collapses, and this periodic movement of the leaf
will continue, if not indefinitely, at any rate as long as the radium lasts. This
“radium clock” should work at a sensibly uniform rate for many years, but, from
evidence considered later (Section 261), there is reason to believe that the number
of β particles emitted would decrease exponentially with the time, falling to half
value in about 1200 years. The period of movement of the leaf should thus
gradually increase with the time, and ultimately the effect would become too
small to observe.
The action of this radium clock is the nearest approach to an apparent perpetual
motion that has so far been observed.
A determination of the amount of the charge carried off by the β rays of radium
has been made by Wien[124]. A small quantity of radium, placed in a sealed
platinum vessel, was hung by an insulating thread inside a glass cylinder, which
was exhausted to a low pressure. A connection between the platinum vessel and an

electrode sealed on to the external glass cylinder could be made, when required,
by tilting the tube. Wien found that in a good vacuum the platinum vessel became
charged to about 100 volts. The rate of escape of negative electricity from the
platinum vessel containing 4 milligrams of radium bromide corresponded to 2·91
× 10-12 amperes. If the charge on each particle is taken as 1·1 × 10-20
electromagnetic units, this corresponds to an escape of 2·66 × 107 particles per
second. From 1 gram of radium bromide the corresponding number would be 6·6
× 109 per second. Since some of the β rays are absorbed in their passage through
the walls of the containing vessel and through the radium itself, the actual number
projected per second from 1 gram of radium bromide must be greater than the
above value. This has been found by the writer to be the case. The method
employed reduced the absorption of the β rays to a minimum, and the total
number emitted per second by 1 gram of radium bromide in radio-active
equilibrium was found to be 4·1 × 1010, or about six times the number found by
Wien. A detailed account of the method employed cannot be given with advantage
at this stage, but will be found later in Section 253.

81. Determination of e/m. We have seen (Section 50) that, in their passage
between the plates of a condenser, the cathode rays are deflected towards the
positive plate. Shortly after the discovery of the magnetic deviation of the β rays
from radium, Dorn[125] and Becquerel[126] showed that they also were deflected by
an electric field.
By observing separately the amount of the electric and magnetic deviation,
Becquerel was able to determine the ratio of e/m and the velocity of the projected
particles. Two rectangular copper plates, 3·45 cms. high and 1 cm. apart, were
placed in a vertical plane and insulated on paraffin blocks. One plate was charged
to a high potential by means of an influence machine, and the other was connected
with earth. The active matter was placed in a narrow groove cut in a lead plate
parallel to the copper plates and placed midway between them. The photographic
plate, enveloped in black paper, was placed horizontally above the plate
containing the active substance. The large and diffuse pencil of rays thus obtained
was deflected by the electric field, but the deviation amounted to only a few
millimetres and was difficult to measure. The method finally adopted was to place
vertically above the active matter a thin screen of mica, which cut the field into
two equal parts. Thus, in the absence of an electric field, a narrow rectangular
shadow was produced on the plate.
When the electric field was applied, the rays were deflected and a part of the
pencil of rays was stopped by the mica screen. A shadow was thus cast on the

plate which showed the direction of deviation and corresponded to the least
deviable rays which gave an impression through the black paper.
If a particle of mass m, charge e, and velocity u, is projected normally to an
electric field of strength X, the acceleration α is in the direction of the field, and is
given by
Xe
α = ----- .
m

Since the particle moves with a constant acceleration parallel to the field, the path
of the particle is the same as that of a body projected horizontally from a height
with a constant velocity and acted on by gravity. The path of the particle is thus a
parabola, whose axis is parallel to the field and whose apex is at the point where
the particle enters the electric field. The linear deviation d1 of the ray parallel to
the field after traversing a distance l is given by
1 Xe l2
d1 = -- ----- -- .
2 m u2

On leaving the electric field, the particle travels in the direction of the tangent to
the path at that point. If θ is the angular deviation of the path at that point
eXl
tan θ = ----- .
mu2

The photographic plate was at a distance h above the extremity of the field. Thus
the particles struck the plate at a distance d2 from the original path given by

In the experimental arrangement the values were
d2 = ·4 cms.;
X = 1·02 × 1012;

l = 3·45 cms.;
h = 1·2 cms.

If the radius R of curvature of the path of the same rays is observed in a magnetic
field of strength H perpendicular to the rays,
e V
--- = ----
m HR

Combining these two equations we get

A difficulty arose in identifying the part of the complex pencil of rays for which
the electric and magnetic deviations were determined. Becquerel estimated that
the value of HR for the rays deflected by the electric field was about 1600 C.G.S.
units. Thus
u = 1·6 × 1010 cms. per second,

and
e
--- = 107.
m

Thus these rays had a velocity more than half the velocity of light, and an
apparent mass about the same as the cathode ray particles, i.e. about ¹⁄₁₀₀₀ of the
mass of the hydrogen atom. The β ray is therefore analogous in all respects to the
cathode ray, except that it differs in velocity. In a vacuum tube the cathode rays
generally have a velocity of about 2 × 109 cms. per sec. In special tubes with
strong fields the velocity may be increased to about 1010 cms. per sec. These β
particles, then, behave like isolated units of negative electricity, identical with the
electrons set free by an electric discharge in a vacuum tube. The electrons
projected from radium have velocities varying from about 0·2V to at least 0·96V,
where V is the velocity of light, and thus have an average speed considerably
greater than that of the electrons produced in a vacuum tube. These moving
electrons are able to pass through much greater thicknesses of matter before they
are absorbed than the slower electrons produced in a vacuum tube, but the

difference is one merely of degree and not of kind. Since electrons are
continuously and spontaneously expelled from radium with enormous velocities,
they must acquire their energy of motion from the matter itself. It is difficult to
avoid the conclusion, that this velocity has not been suddenly impressed on the
electron. Such a sudden gain of velocity would mean an immense and sudden
concentration of energy on a small particle, and it is more probable that the
electron before its expulsion has been in rapid orbital or oscillatory motion in the
atom, and, by some means, suddenly escapes from its orbit. According to this
view, the energy of the electron is not suddenly created but is only made obvious
by its escape from the system to which it belongs.

82. Variation of e/m with the velocity of the electron. The fact that radium
throws off electrons with rates of speed varying from ⅕ to ⁹⁄₁₀ the velocity of light
has been utilised by Kaufmann[127] to examine whether the ratio e/m of the
electrons varies with the speed. We have seen (Section 48) that, according to the
electromagnetic theory, a charge of electricity in motion behaves as if it had
apparent mass. For small speeds, this additional electrical mass is equal to
2 e2
- --- ,
3 a

where a is the radius of the body, but it increases rapidly as the speed of light is
approached. It is very important to settle whether the mass of the electron is due
partly to mechanical and partly to electrical mass, or whether it can be explained
by virtue of electricity in motion independently of the usual conception of mass.
Slightly different formulae expressing the variation of mass with speed have been
developed by J. J. Thomson, Heaviside, and Searle. To interpret his results
Kaufmann used a formula developed by M.
Abraham[128].
Let m₀ = mass of electron for slow speeds;
m = apparent mass of electron at any speed;
u = velocity of electron;
V = velocity of light.

Let β = u/V; then it can be shown that

where

The experimental method employed to determine e/m and u is similar to the
method of crossed spectra. Some strongly active radium was placed at the bottom
of a brass box. The rays from this passed between two brass plates insulated and
about 1·2 mm. apart. These rays fell on a platinum diaphragm, containing a small
tube about 0·2 mm. in diameter, which allowed a narrow bundle of rays to pass.
The rays then struck a photographic plate enveloped in a thin layer of aluminium.
In the experiments the diaphragm was about 2 cms. from the active material and at
the same distance from the photographic plate. When the whole apparatus was
placed in a vacuum, a P.D. of from 2000 to 5000 volts could be applied between
the plates without a spark. The rays were deflected in their passage through the
electric field, and produced what may be termed an electric spectrum on the plate.

Fig. 28.

If a magnetic field is superimposed parallel to the electric field by means of an
electromagnet, a magnetic spectrum is obtained perpendicular to the electric
spectrum. The combination of the two spectra gives rise to a curved line on the
plate. The double trace obtained on the photographic plate with reversal of the
magnetic field is shown in Fig. 28. Disregarding some small corrections, it can

readily be shown that if y and z are the electric and magnetic deviations
respectively,
z
β = κ1 ----- (3),
y

and

e z2
-- = κ --- (4).
m y

From these two equations, combined with (1), we obtain

where κ, κ1, κ2 are constants.
Equation (5) gives the curve that should be obtained on the plate according to the
electromagnetic theory. This is compared by trial with the actual curve obtained
on the plate.
In this way Kaufmann[129] found that the value of e/m decreased with the speed,
showing that, assuming the charge constant, the mass of the electron increased
with the speed.
The following numbers give some of the preliminary results obtained by this
method.
Velocity of electron e/m

2·36 × 1010 cms. per 1·31 × 107
sec.
2·48 „ „ 1·17 × 107
2·59 „ „ 0·97 × 107
2·72 „ „ 0·77 × 107
2·85 „ „ 0·63 × 107

For the cathode rays S. Simon[130] obtained a value for e/m of 1·86 × 107 for an
average speed of about 7 × 109 cms. per second.
In a later paper[131] with some very active radium, more satisfactory photographs
were obtained, which allowed of accurate measurement. The given equation of the
curve was found to agree satisfactorily with experiment.
The table given below, deduced from the results given by Kaufmann, shows the
agreement between the theoretical and experimental values, u being the velocity
of the electron and V that of light.
The average percentage error between the observed and calculated value is thus
not much more than one per cent. It is remarkable how nearly the velocity of the
electron has to approach the velocity of light before the value of m/m₀ becomes
large. This is shown in the following table which gives the calculated values of
m/m₀ for different velocities of the electron.
Value of Observed value Percentage
u/V of m/m₀ difference
from
theoretical
values

Small 1
·732 1·34 -1·5 %
·752 1·37 -0·9 „
·777 1·42 -0·6 „
·801 1·47 +0·5 „
·830 1·545 +0·5 „
·860 1·65 0„
·883 1·73 +2·8 „
·933 2·05 -7·8 „ ?
·949 2·145 -1·2 „
·963 2·42 +0·4 „
Value of small ·1 ·5 ·9 ·99 ·999 ·9999 ·999999
u/V
Calculated 1·00 1·015 1·12 1·81 3·28 4·96 6·68 10·1
value
m/m₀

Thus for velocities varying from 0 to ⅒ the velocity of light, the mass of the
electron is practically constant. The increase of mass becomes appreciable at
about half the velocity of light, and increases steadily as the velocity of light is
approached. Theoretically the mass becomes infinite at the velocity of light, but
even when the velocity of the electron only differs from that of light by one part in
a million, its mass is only 10 times the value for slow speeds.
The above results are therefore in agreement with the view that the mass of the
electron is altogether electrical in origin and can be explained purely by electricity
in motion. The value of e/m₀, for slow speeds, deduced from the results was 1·84
× 107, which is in very close agreement with the value obtained by Simon for the
cathode rays, viz. 1·86 × 107.
If the electricity carried by the electron is supposed to be distributed uniformly
over a sphere of radius a, for speeds slow compared with the velocity of light, the
apparent mass
2 e2
m₀ = --- ----
3 a

Therefore
2 e
a = --- ---- . e
3 m₀

Taking the value of e as 1·13 × 10-20, a is 1·4 × 10-13 cms.
Thus the diameter of an electron is minute compared with the diameter of an
atom.

83. Distribution of velocity amongst the β particles. Some interesting
experiments have been recently made by Paschen[132] to determine the relative
number of β particles which are expelled from radium at the different speeds. The
experimental arrangement is shown in Fig. 29.

Fig. 29.

A small thin silvered glass tube b, containing 15 mgrs. of radium bromide, was
placed in the axis of a number of lead vanes arranged round a cylinder of diameter
2 cms. and length 2·2 cms. When no magnetic field was acting, the β particles
from the radium passed through the openings and were absorbed in an outer
concentric cylinder aa of lead of inner diameter 3·7 cms. and of thickness 5·5
mms. This outer cylinder was rigidly connected to the inner cylinder cc by quartz
rods ii, which also served to insulate it. The cylinder c and the radium were
connected with earth. A gold-leaf electroscope E was attached to a, and the whole
apparatus was enclosed in a glass vessel which was exhausted to a low vacuum by
means of a mercury pump. The glass vessel was placed in the uniform field of a
large electromagnet, so that the axis of the lead cylinder was parallel to the lines
of force.
The outer cylinder gains a negative charge on account of the particles which are
absorbed in it. This negative charge, which is indicated by the movement of the
gold-leaf, tends to be dissipated by the small ionization produced in the residual
gas by the passage of the β rays. This action of the gas can be eliminated by
observing the rate of movement of the gold leaf when charged alternately to an
initial positive and negative potential. The mean of the two rates is proportional to
the number of β particles which give up their charge to the lead cylinder. This is
evidently the case, since, when the charge is positive, the ionization of the gas

assists the rate of movement of the gold-leaf, and, when negative, diminishes it to
an equal extent.
When a magnetic field is applied, each of the particles describes a curved path,
whose radius of curvature depends on the velocity of the particle. For weak fields,
only the particles of smallest velocity will be deflected sufficiently not to strike
the outer cylinder, but, as the field is raised, the number will increase until finally
all the β particles fail to reach the outer cylinder. The decrease of the charge
communicated to the outer cylinder with the increase of the strength of the
magnetic field is shown graphically in Fig. 30, Curve I.
The ordinates represent in arbitrary units the charge communicated to the lead
cylinder per second, and thus serve as a measure of the number of β particles
which reach the cylinder. Knowing the dimensions of the apparatus, and assuming
the value e/m found by Kaufmann, the velocity of the particles which just fail to
reach the lead cylinder can be deduced from any strength of the magnetic field.
Curve II, Fig. 30 is the first differential of Curve I, and the ordinates represent the
relative number of β particles which are projected at each velocity.

Fig. 30.

From the data given by Kaufmann (see section 82) Paschen deduced that the
group of rays examined by the former, which had velocities lying between 2·12 ×
1010 and 2·90 × 1010 cms. per second, corresponded to the group of rays between
the points A and B, that is, to the group of rays which were completely deflected
from the lead cylinder between the magnetic fields of strengths of 1875 and 4931

C.G.S. units. Since radium gives off β particles which require a field of strength
over 7000 units to deflect them, Paschen concluded that β particles are expelled
from radium with still greater velocities than the highest recorded by Kaufmann.
Paschen considered that the small charge observed in still higher fields was
mainly due to the γ rays. The effect is small and is probably not due to an actual
charge carried by the γ rays but to a secondary effect produced by them. This
question will be discussed in more detail in section 112.
There is a group of low velocity β particles emitted by radium (see Fig. 30) which
have about the same speed as the electrons set free in a vacuum tube. In
consequence of their small velocity, these probably produce a large proportion of
the ionization due to the β rays at short distances from the radium, for it will be
shown (section 103) that the ionization produced by an electron per unit length of
path steadily decreases with increase of its velocity above a small limiting value.
This observation is confirmed by experiments on the absorption of the β rays in
passing through matter.
In Paschen’s experiments, the glass tube containing the radium was ·5 mms. thick,
so that a considerable proportion of the low velocity β particles must have been
stopped by it. This is borne out by some later experiments of Seitz which will be
described in section 85.

84. Absorption of the β rays by matter. The β particles produce ions in their
passage through the gas and their energy of motion is consequently diminished. A
similar action takes place also when the β rays pass through solid and liquid
media, and the mechanism of absorption is probably similar in all cases. Some of
the particles in their passage through matter are completely stopped, while others
have their velocity reduced. In addition, there is a considerable scattering or
diffuse reflection of the rays in traversing matter. The amount of this scattering
depends upon the density of the substance and also upon the angle of incidence of
the rays. This scattering of the rays will be discussed later in section 111.
There are two general methods of determining the absorption of the β rays. In the
first method, the variation of the ionization current is observed in a testing vessel
when the active matter is covered by screens differing in material and thickness.
This ionization in the vessel depends upon two quantities, viz. the number of β
particles which pass through the matter and also upon the number of ions
produced by them per unit path. In the absence of any definite information in
regard to the variation of ionization by the electron with its velocity, no very
definite conclusions can be drawn from such experiments.

The advent of pure radium-bromide has made it possible to determine the actual
number of electrons which are absorbed in their passage through a definite
thickness of matter, by measuring the negative charge carried by the issuing rays.
Experiments of this character have been made by Seitz and will be considered
later.
These two methods of determining the absorption of β rays are quite distinct in
principle, and it is not to be expected that the values of the coefficients of
absorption obtained in the two cases should be the same. The whole question of
the absorption of electrons by matter is very complicated, and the difficulty is still
further increased by the complexity of the β rays emitted by the radio-active
substances. Many of the results obtained by different methods, while pointing to
the same general conclusion, are quantitatively in wide disagreement. Before any
definite advance can be made to a better understanding of the mechanism of
absorption, it will be necessary to determine the variation of the ionization with
the speed of the electron over a very wide range. Some work has already been
done in this direction but not between sufficiently wide limits.

Ionization method.

We shall first consider the results obtained on the absorption of β rays by
measuring the variation of the ionization current, when screens of different
thickness are placed over the active substance. When the active matter is covered
with aluminium foil of thickness ·1 mm., the current in a testing vessel such as is
shown in Fig. 17, is due almost entirely to the β rays. If a uranium compound is
used, it is found that the saturation current decreases with the thickness of matter
traversed nearly according to an exponential law. Taking the saturation current as
a measure of the intensity of the rays, the intensity I after passing through a
thickness d of matter is given by

where λ is the constant of absorption of the rays and I₀ is the initial intensity. For
uranium rays, the current is reduced to half its value after passing through about ·5
mm. of aluminium.
If a compound of thorium or radium is examined in the same way, it is found that
the current does not decrease regularly according to the above equation. Results of
this kind for radium rays have been given by Meyer and Schweidler[133]. The
amount of absorption of the rays by a certain thickness of matter decreases with
the thickness traversed. This is exactly opposite to what is observed for the α rays.
This variation in the absorption is due to the fact that the β rays are made up of
rays which vary greatly in penetrating power. The rays from uranium are fairly
homogeneous in character, i.e. they consist of rays projected with about the same
velocity. The rays from radium and thorium are complex, i.e. they consist of rays
projected with a wide range of velocity and consequently with a wide range of
penetrating power. The electrical examination of the deviable rays thus leads to
the same results as their examination by the photographic method.
Results on the absorption of cathode rays have been given by Lenard[134], who has
shown that the absorption of cathode rays is nearly proportional to the density of
the absorbing matter, and is independent of its chemical state. If the deviable rays
from active bodies are similar to cathode rays, a similar law of absorption is to be

expected. Strutt[135], working with radium rays, has determined the law of
absorption, and has found it roughly proportional to the density of matter over a
range of densities varying from 0·041 for sulphur dioxide to 21·5 for platinum. In
the case of mica and cardboard, the values of λ divided by the density were 3·94
and 3·84 respectively, while the value for platinum was 7·34. In order to deduce
the absorption coefficient, he assumed that the radiation fell off according to an
exponential law with the distance traversed. As the rays from radium are complex,
we have seen that this is only approximately the case.
Since the β rays from uranium are fairly homogeneous, and are at the same time
penetrating in character, they are more suitable for such a determination than the
complex rays of radium. I have in consequence made some experiments with
uranium rays to determine the dependence of absorption on the density. The
results obtained are given in the following table, where λ is the coefficient of
absorption.
Substance λ Density λ/Density

Glass 14·0 2·45 5·7
Mica 14·2 2·78 5·1
Ebonite 6·5 1·14 5·7
Wood 2·16 ·40 5·4
Cardboard 3·7 ·70 5·3
Iron 44 7·8 5·6
Aluminium 14·0 2·60 5·4
Copper 60 8·6 7·0
Silver 75 10·5 7·1
Lead 122 11·5 10·8
Tin 96 7·3 13·2
It will be observed that the value of the absorption constant divided by the density
is very nearly the same for such different substances as glass, mica, ebonite,
wood, iron and aluminium. The divergences from the law are great, however, for
the other metals examined, viz. copper, silver, lead and tin. In tin the value of λ
divided by the density is 2·5 times its value for iron and aluminium. These
differences show that a law for the absorption of the β rays depending only on the
density does not hold for all substances. With an exception in the case of tin, the
value of λ divided by the density for the metals increases in the same order as
their atomic weights.

The absorption of the β rays by matter decreases very rapidly with increase of
speed. For example, the absorption of cathode rays in Lenard’s experiment (loc.
cit.) is about 500 times as great as for the uranium β rays. The velocity of the β
rays of uranium was found by Becquerel to be about 1·6 × 1010 cms. per sec. The
velocity of the cathode rays used in Lenard’s experiment was certainly not less
than ⅒ of this, so that, for a decrease of speed of less than 10 times, the
absorption has increased over 500 times.

85. Number of electrons stopped by matter. An account will now be given of
the experiments made by Seitz[136], to determine the relative number of electrons
which are stopped in their passage through different thicknesses of matter. The
experimental arrangement is shown in Fig. 31.

Fig. 31.

The radium was placed outside a glass vessel containing an insulated brass plate
P, the connection of which with a wire leading to the electrometer could be made
or broken by a simple electromagnetic device. The β rays from the radium R, after

passing through openings in a brass plate A, covered with thin aluminium foil,
were absorbed in the plate P. The glass vessel was exhausted, and the charge
communicated to P by the β rays was measured by an electrometer.
In a good vacuum, the magnitude of the current observed is a measure of the
number of β particles absorbed by the upper plate[137]. The following table shows
the results obtained when different thicknesses of tin foil were placed over the
radium. The second table gives the ratio I/I₀ where I₀ is the rate of discharge
observed before the absorbing screen is introduced. The mean value of the
absorption constant λ was deduced from the equation

where d is the thickness of matter traversed.
The values included in the brackets have not the same accuracy as the others.
There is thus a wide difference in penetrating power of the β particles emitted
from radium, and some of them are very readily absorbed.
When a lead screen 3 mms. thick was placed over the radium—a thickness
sufficient to absorb all the readily deflectable β rays—a small negative charge was
still given to the plate, corresponding to ·29 per cent. of the maximum. This is a
very much smaller value than was observed by Paschen (see Fig. 30).
Thickness of I/I₀ λ
Tin in mms.

0·00834 ·869 175
0·0166 ·802 132·5
0·0421 ·653 101·5
0·0818 ·466 93·5
0·124 ·359 82·5
0·166 ·289 74·9
0·205 ·230 71·5
0·270 ·170 65·4
0·518 ·065 } 53}
0·789 ·031 } 44}

1·585 ·0059} 32}
2·16 ·0043} 25}
This difference may, in part, be due to the fact that, in Paschen’s experiments, a
large proportion of the slow velocity electrons were absorbed in the glass tube of
·5 mm. thickness containing the radium.
Seitz also determined the relative thickness, compared with tin, of different
substances which reduced the negative charge communicated to P by a definite
amount. A few of the numbers are given below, and expressed in terms of tin as
unity.
Substance Thickness
Tin = 1

Lead ·745
Gold ·83
Platinum ·84
Silver 1
Steel 1·29
Aluminium 1·56
Water 1·66
Paraffin 1·69
The thickness required to stop a given proportion of the β rays thus decreases with
the density, but not nearly so fast as the density increases. These results are
difficult to reconcile with the density-law of absorption found by Lenard from the
cathode rays, or with the results of the ionization method already considered. A
further experimental examination of the whole question is very much to be
desired.

86. Variation of the amount of radiation with the thickness of the layer of
radiating material. The radiations are sent out equally from all portions of the
active mass, but the ionization of the gas which is measured is due only to the
radiations which escape into the air. The depth from which the radiations can
reach the surface depends on the absorption of the radiation by the active matter
itself.
Let λ be the absorption constant of the homogeneous radiation by the active
material. It can readily be shown that the intensity I of the rays issuing from a

layer of active matter, of thickness d, is given by

where I₀ is the intensity at the surface due to a very thick layer.
This equation has been confirmed experimentally by observing the current due to
the β rays for different thicknesses of uranium oxide. In this case I = (½)I₀ for a
thickness of oxide corresponding to ·11 gr. per sq. cm. This gives a value of λ
divided by density of 6·3. This is a value slightly greater than that observed for the
absorption of the same rays in aluminium. Such a result shows clearly that the
substance which gives rise to the β rays does not absorb them to a much greater
extent than does ordinary matter of the same density.
The value of λ will vary, not only for the different active substances, but also for
the different compounds of the same substance.

PART III.

The α Rays.

87. The α rays. The magnetic deviation of the β rays was discovered towards the
end of 1899, at a comparatively early stage in the history of radio-activity, but
three years elapsed before the true character of the α rays was disclosed. It was
natural that great prominence should have been given in the early stages of the
subject to the β rays, on account of their great penetrating power and marked
action in causing phosphorescence in many substances. The α rays were, in
comparison, very little studied, and their importance was not generally
recognized. It will, however, be shown that the α rays play a far more important
part in radio-active processes than the β rays, and that the greater portion of the
energy emitted in the form of ionizing radiations is due to them.

88. The nature of the α rays. The nature of the α rays was difficult to determine,
for a magnetic field sufficient to cause considerable deviation of the β rays
produced no appreciable effect on the α rays. It was suggested by several
observers that they were, in reality, secondary rays set up by the β or cathode rays
in the active matter from which they were produced. Such a view, however, failed
to explain the radio-activity of polonium, which gave out α rays only. Later work
also showed that the matter, which gave rise to the β rays from uranium, could be
chemically separated from the uranium, while the intensity of the α rays was
unaffected. These and other results show that the α and β rays are produced quite
independently of one another. The view that they are an easily absorbed type of
Röntgen rays fails to explain a characteristic property of the α rays, viz. that the
absorption of the rays in a given thickness of matter, determined by the electrical
method, increases with the thickness of matter previously traversed. It does not
seem probable that such an effect could be produced by a radiation like X rays,
but the result is to be expected if the rays consist of projected bodies, which fail to
ionize the gas when their velocity is reduced below a certain value. From
observations of the relative ionization produced in gases by the α and β rays,
Strutt[138] suggested in 1901 that the α rays might consist of positively charged
bodies projected with great velocity. Sir William Crookes[139], in 1902, advanced

the same hypothesis. From a study of the α rays of polonium Mme. Curie[140] in
1900 suggested the probability that these rays consisted of bodies, projected with
great velocity, which lost their energy by passing through matter.
The writer was led independently to the same view by a mass of indirect evidence
which received an explanation only on the hypothesis that the rays consisted of
matter projected with great velocity. Preliminary experiments with radium of
activity 1000 showed that it was very difficult to determine the magnetic deviation
of the α rays. When the rays were passed through slits sufficiently narrow to
enable a minute deviation of the rays to be detected, the ionizing effect of the
issuing rays was too small to be measured with certainty. It was not until radium
of activity 19,000 was obtained that it was possible to detect the deviation of these
rays in an intense magnetic field. How small the magnetic deviation is may be
judged from the fact that the α rays, projected at right angles to a magnetic field of
10,000 C.G.S. units, describe the arc of a circle of about 39 cms. radius, while
under the same conditions the cathode rays produced in a vacuum tube would
describe a circle of about ·01 cm. radius. It is therefore not surprising that the α
rays were for some time thought to be non-deviable in a magnetic field.

89. Magnetic deviation of the α rays. The general method employed[141] to detect
the magnetic deviation of the α rays was to allow the rays to pass through narrow
slits and to observe whether the rate of discharge of an electroscope, due to the
issuing rays, was altered by the application of a strong magnetic field. Fig. 32
shows the general arrangement of the experiment. The rays from a thin layer of
radium of activity 19,000 passed upwards through a number of narrow slits G, in
parallel, and then through a thin layer of aluminium foil, ·00034 cm. thick, into
the testing vessel V. The ionization produced by the rays in the testing vessel was
measured by the rate of movement of the leaves of a gold-leaf electroscope B. The
gold-leaf system was insulated inside the vessel by a sulphur bead C, and could be
charged by means of a movable wire D, which was afterwards earthed. The rate of
movement of the gold-leaf was observed through small mica windows in the
testing vessel by means of a microscope provided with a micrometer eye-piece.

Fig. 32.

In order to increase the ionization in the testing vessel, the rays passed through 20
to 25 slits of equal width, placed side by side. This was arranged by cutting
grooves at regular intervals in side-plates into which brass plates were slipped.
The width of the slit varied in different experiments between ·042 cm. and ·1 cm.
The magnetic field was applied perpendicular to the plane of the paper, and
parallel to the plane of the slits. The rays are thus deflected in a direction
perpendicular to the plane of the slits and a very small amount of deviation is
sufficient to cause the rays to impinge on the sides of the plate where they are
absorbed.
The testing vessel and system of plates were waxed to a lead plate P so that the
rays entered the vessel V only through the aluminium foil. It is necessary in these
experiments to have a steady stream of gas passing downwards between the plates
in order to prevent the diffusion of the emanation from the radium upwards into
the testing vessel. The presence in the testing vessel of a small amount of this
emanation, which is always given out by radium, would produce great ionization
and completely mask the effect to be observed. For this purpose, a steady current
of dry electrolytic hydrogen of about 2 c.c. per second was passed into the testing
vessel; it then streamed through the porous aluminium foil, and passed between
the plates carrying the emanation with it away from the apparatus. The use of a
stream of hydrogen instead of air greatly simplifies the experiment, for it

increases the ionization current due to the α rays in the testing vessel, and at the
same time greatly diminishes that due to the β and γ rays. This is caused by the
fact that the α rays are much more readily absorbed in air than in hydrogen, while
the rate of production of ions due to the β and γ rays is much less in hydrogen than
in air. The intensity of the α rays after passing between the plates is consequently
greater when hydrogen is used; and since the rays pass through a sufficient
distance of hydrogen in the testing vessel to be largely absorbed, the total amount
of ionization produced by them is greater with hydrogen than with air.
The following is an example of an observation on the magnetic deviation:—
Pole-pieces 1·90 × 2·50 cms.

Strength of field between pole-pieces 8370 units.

Apparatus of 25 parallel plates of length 3·70 cms.,
width ·70 cm., with an average air-space between
plates of ·042 cm.

Distance of radium below plates 1·4 cm.

Rate of discharge of
electroscope in volts per
minute
(1) Without magnetic field 8·33
(2) With magnetic field 1·72
(3) Radium covered with thin layer of mica to 0·93
absorb all α rays
(4) Radium covered with mica and magnetic field 0·92
applied
The mica plate, ·01 cm. thick, was of sufficient thickness to absorb completely all
the α rays, while it allowed the β rays and γ rays to pass through without
appreciable absorption. The difference between (1) and (3), 7·40 volts per minute,
gives the rate of discharge due to the α rays alone; the difference between (2) and
(3), 0·79 volts per minute, that due to the α rays not deviated by the magnetic field
employed.
The amount of α rays not deviated by the field is thus about 11% of the total. The
small difference between (3) and (4) measures the small ionization due to the β
rays, for they would be completely deviated by the magnetic field; (4) comprises

the effect of the γ rays together with the natural leak of the electroscope in
hydrogen.
In this experiment there was a good deal of stray magnetic field acting on the rays
before they reached the pole-pieces. The diminution of the rate of discharge due to
the α rays was found to be proportional to the strength of field between the pole-
pieces. With a more powerful magnetic field, the whole of the α rays were
deviated, showing that they consisted entirely of projected charged particles.
In order to determine the direction of deviation of the rays, the rays were passed
through slits one mm. in width, each of which was half covered with a brass strip.
The diminution of the rate of discharge in the testing vessel for a given magnetic
field in such a case depends upon the direction of the field. In this way it was
found that the rays were deviated in the opposite sense to the cathode rays. Since
the latter consist of negatively charged particles, the α rays must consist of
positively charged particles.
These results were soon after confirmed by Becquerel[142], by the photographic
method, which is very well adapted to determine the character of the path of the
rays acted on by a magnetic field. The radium was placed in a linear groove cut in
a small block of lead. Above this source, at a distance of about 1 centimetre, was
placed a metallic screen, formed of two plates, leaving between them a narrow
opening parallel to the groove. Above this was placed the photographic plate. The
whole apparatus was placed in a strong magnetic field parallel to the groove. The
strength of the magnetic field was sufficient to deflect the β rays completely away
from the plate. When the plate was parallel to the opening, there was produced on
it an impression, due to the α rays alone, which became more and more diffuse as
the distance from the opening increased. This distance should not exceed 1 or 2
centimetres on account of the absorption of the rays in air. If, during the exposure,
the magnetic field is reversed for equal lengths of time, on developing the plate
two images of the α rays are observed which are deflected in opposite directions.
This deviation, even in a strong field, is small though quite appreciable and is
opposite in sense to the deviation observed for the β or cathodic rays from the
same material.
M. Becquerel[143], by the same method, found that the α rays from polonium were
deviated in the same direction as the α rays from radium; and thus that they also
consist of projected positive bodies. In both cases, the photographic impressions
were sharply marked and did not show the same diffusion which always appears
in photographs of the β rays.

90. Electrostatic deviation of the α rays. If the rays are charged bodies, they
should be deflected in passing through a strong electric field. This was found by
the writer to be the case, but the electric deviation is still more difficult to detect
than the magnetic deviation, as the intensity of the electric field must of necessity
be less than that required to produce a spark in the presence of radium. The
apparatus was similar to that employed for the magnetic deviation (Fig. 32) with
this exception, that the brass sides which held the plates in position, were replaced
by ebonite. Alternate plates were connected together and charged to a high
potential by means of a battery of small accumulators. The discharge in the
electroscope, due to the α rays, was found to be diminished by application of the
electric field. With plates ·055 cm. apart and 4·5 cms. high, the diminution was
only 7% with a P.D. of 600 volts between the slits. With a special arrangement of
plates, with slits only ·01 cm. apart, the discharge was diminished about 45% with
an electric field corresponding to 10,000 volts per cm.

91. Determination of the constants of the rays. If the deviation of the rays in
both an electric and magnetic field is known, the values of the velocity of the rays,
and the ratio e/m of the charge of the particle to its mass can be determined by the
method, first used by J. J. Thomson for the cathode rays, which is described in
section 50. From the equations of a moving charged body, the radius of curvature
ρ of the path of the rays in a magnetic field of strength H perpendicular to the path
of the rays is given by
m
Hρ = ---- V .
e

If the particle, after passing through a uniform magnetic field for a distance l1, is
deviated through a small distance d1 from its original direction,

2ρd1 = l12

or
l12 e H
d1 = ----- --- --- (1).
2 m V

If the rays pass through a uniform electric field of strength X and length l2 with a
deviation d2,

1 Xel22
d2 = --- ----- (2),
2 mV2

since Xe/m is the acceleration of the particle, at right angles to its direction, and
l2/V is the time required to travel through the electric field.
From equations (1) and (2)
d1 l22 X
V = ----- ----- --- ,
d2 l12 H

and

e 2d1 V
---- = ------ --- .
m l12 H

The values of V and e/m are thus completely determined from the combined
results of the electric and magnetic deviation. It was found that
V = 2·5 × 109 cms. per sec.
e/m = 6 × 103.

On account of the difficulty of obtaining a large electrostatic deviation, these
values are only approximate in character.
The results on the magnetic and electric deviation of the α rays of radium have
been confirmed by Des Coudres[144], by the photographic method. Some pure
radium bromide was used as a source of radiation. The whole apparatus was
enclosed in a vessel which was exhausted to a low vacuum. In this way, not only
was he able to determine the photographic action of the rays at a much greater
distance from the source, but he was also able to apply a stronger electric field
without the passage of a spark. He found values of the constants given by
V = 1·65 × 109 cms. per sec.
e/m = 6·4 × 103.

These values are in very good agreement with the numbers found by the electric
method. The α rays from radium are complex, and probably consist of a stream of
positively charged bodies projected at velocities lying between certain limits. The
amount of deviation of the particles in a magnetic field will thus differ according

to the velocity of the particle. The photographic results of Becquerel seem to
indicate that the velocity of the rays of radium can vary only within fairly narrow
limits, since the trajectory of the rays in a magnetic field is sharply marked and
not nearly as diffuse as in similar experiments with the β rays. The evidence,
however, discussed in the following section, shows that the velocities of the α
particles from a thick layer of radium vary over a considerable range.

92. Becquerel[145] has examined the amount of magnetic deviation of the α rays at
different distances from the source of the rays in a very simple way. A narrow
vertical pencil of the rays, after its passage through a narrow slit, fell on a
photographic plate, which was inclined at a small angle to the vertical and had its
lower edge perpendicular to the slit. The trajectory of the rays is shown by a fine
line traced on the plate. If a strong magnetic field is applied parallel to the slit, the
trajectory of the rays is displaced to the right or left according to the direction of
the field. If equal times of exposure are given for the magnetic field in the two
directions, on developing the plate two fine diverging lines are found traced on the
plate. The distance between these lines at any point is a measure of twice the
average deviation at that point, corresponding to the value of the magnetic field.
By measuring the distance between the trajectories at various points, Becquerel
found that the radius of curvature of the path of the rays increased with the
distance from the slit. The product Hρ of the strength of the field and the radius of
curvature of the path of the rays is shown in the following table.
Distance in mms. from Hρ
the slit
1 2·91 × 105
3 2·99 „
5 3·06 „
7 3·15 „
8 3·27 „
9 3·41 „
The writer (loc. cit.) showed that the maximum value of Hρ for complete deviation
of the α rays was 390,000. The results are thus in good agreement. Since
m
Hρ = ----- V
e

these results show that the values either of V or of e/m for the projected particles
vary at different distances from the source. Becquerel considered that the rays
were homogeneous, and, in order to explain the results, has suggested that the
charge on the projected particles may gradually decrease with the distance
traversed, so that the radius of curvature of the path steadily increases with the
distance from the source. It, however, seems more probable that the rays consist of
particles projected with different velocities, and that the slower particles are more
quickly absorbed in the gas. In consequence of this, only the swifter particles are
present some distance from the source.
This conclusion is borne out by some recent experiments of Bragg and
Kleeman[146] on the nature of the absorption of α particles by matter, which are
discussed in more detail in sections 103 and 104. They found that the α particles
from a thick layer of radium are complex, and have a wide range of penetrating
power and presumably of velocity. This is due to the fact that the α particles
emitted from the radium come from different depths. Since their velocity is
reduced in their transit through matter, a pencil of α rays will consist of particles
which differ considerably in speed. Those which are just able to emerge from the
radium will be absorbed in a very short depth of air, while those that come from
the surface will be able to pass through several centimetres of air before they lose
their power of ionizing the gas. Since the α particles have different velocities, they
will be unequally deflected by the magnetic field, the slower moving particles
describing a more curved path than the swifter ones. Consequently, the outer edge
of the trace of the pencil of rays on the photographic plate, as obtained by
Becquerel, will be the locus of the points where the photographic action of the α
particles end. It was found that the α particles are most efficient as ionizers of the
gas just before their power of ionizing ends. The loss of ionizing power of the α
particles seems to be fairly abrupt, and, for particles of the same velocity, to occur
always after traversing a definite distance in air. On the assumption that the
photographic as well as the ionizing action is most intense just before the particles
are stopped, and ceases fairly abruptly, Bragg has been able to account
numerically for the measurements (see above table) recorded by Becquerel. Quite
apart from the special assumptions required for such a quantitative comparison of
theory with experiment, there can be little doubt that the increase of value of Hρ
with distance can be satisfactorily explained as a consequence of the complex
character of the pencil of rays[147].
Becquerel states that the amount of deviation, in a given magnetic field, was the
same for the α rays of polonium and of radium. This shows that the value of
m
--- V

e

is the same for the α rays from the two substances. Since the α rays from
polonium are far more readily absorbed than the α rays from radium, this result
would indicate that the value of m/e is greater for the α particles of polonium than
of radium. Further experimental evidence is required on this important point.

93. Charge carried by the α rays. We have seen that the negative charge carried
by the β particles has been readily measured. Since there is reason to believe
(section 229) that four α particles are expelled from radium for each β particle, it
is to be expected that the positive charge carried by the α particles should be
determined still more readily. All the initial experiments, however, made to detect
this charge, gave negative results; and, before successful results were obtained, it
was found necessary to eliminate some secondary actions, which at first
completely masked the effects to be looked for.
In consequence of the importance of this question, a brief account will be given of
the methods of measurement adopted and the special experimental difficulties
which have arisen.
In the first place, it must be remembered that only a small fraction of the α rays,
emitted from a layer of powdered radium bromide, escape into the surrounding
gas. On account of the ease with which the α rays are stopped in their passage
through matter, only those escape which are expelled from a superficial layer, and
the rest are absorbed by the radium itself. On the other hand, a much larger
proportion of the β rays escape, on account of their greater power of penetration.
In the second place, the α particle is a far more efficient ionizer of the gas than the
β particle, and, in consequence, if the charge carried by the α rays is to be
determined by methods similar to those employed for the β rays (see section 80),
the pressure of the gas surrounding the conductor to be charged must be very
small in order to eliminate, as far as possible, the loss of charge resulting from the
ionization of the residual gas by the α rays[148].
The experimental arrangement used by the writer is shown in Fig. 33.
A thin film of radium was obtained on a plate A by evaporation of a radium
solution containing a known weight of radium bromide. Some hours after
evaporation, the activity of the radium, measured by the α rays, is about 25 per
cent. of its maximum value, and the β rays are almost completely absent. The
activity measured by the α and β rays is then slowly regained, and recovers its
original value after about a month’s interval (see chapter XI.). The experiments
were made on the active plate when its activity was a minimum, in order to avoid

complications due to the presence of β rays. The film of radium was so thin that
only a very small fraction of the α rays was absorbed.

Fig. 33.

The active plate A was insulated in a metal vessel D, and was connected to one
pole of the battery, the other pole being earthed. The upper electrode, which was
insulated and connected with a Dolezalek electrometer, consisted of a rectangular
copper vessel BC, the lower part of which was covered with a thin sheet of
aluminium foil. The α rays passed through the foil, but were stopped by the
copper sides of the vessel. This arrangement was found to reduce the secondary
ionization produced at the surface of the upper plate. The outside vessel D could
be connected with either A or B or with earth. By means of a mercury pump, the
vessel was exhausted to a very low pressure. If the rays carry a positive charge,
the current between the two plates measured by the electrometer should be greater
when A is charged positively. No certain difference, however, between the
currents in the two directions was observed, even when a very good vacuum was
obtained. In some arrangements, it was found that the current was even greater
when the lower plate was negative than when it was positive. An unexpected
experimental result was also noticed. The current between the parallel plates at
first diminished with the pressure, but soon reached a limiting value which was
not altered however good a vacuum was produced. For example, in one
experiment, the current between the two parallel plates, placed about 3 mms.
apart, was initially 6·5 × 10-9 amperes and fell off directly as the pressure. The
current reached a limiting value of about 6 × 10-12 amperes, or about ¹⁄₁₀₀₀ of the
value at atmospheric pressure. The magnitude of this limiting current was not
much altered if the air was replaced by hydrogen.

Experiments of a similar character have been made by Strutt[149] and J. J.
Thomson[150]; using an active bismuth plate coated with radio-tellurium
(polonium) after Marckwald’s method. This substance emits only α rays, and is
thus especially suitable for experiments of this kind. Strutt employed the method
used by him to show the charge carried by the β rays (Fig. 27). He found,
however, that, even in the lowest possible vacuum, the electroscope rapidly lost its
charge and at the same rate whether it was charged positively or negatively. This
is in agreement with the results found by the writer with radium.
In the experiments of J. J. Thomson, the electroscope was attached to a metal disc
placed 3 cms. from the plate of radio-tellurium. A very low vacuum was produced
by Dewar’s method by absorbing the residual gas in cocoanut charcoal immersed
in liquid air. When the electroscope was charged negatively, an extremely slow
rate of leak was observed, but when charged positively the leak was about 100
times greater. This showed that the polonium gave out large quantities of negative
electricity, but not enough positive to be detected. By placing the apparatus in a
strong magnetic field, the negative particles were prevented from reaching the
electroscope and the positive leak was stopped.
These results indicate that these negative particles are not projected with sufficient
velocity to move against the repulsion exerted by the electrified body, and are bent
by a magnetic field. There thus seems little doubt that a stream of negative
particles (electrons) is projected from the active surface at a very slow speed.
Such low velocity electrons are also projected from uranium and radium. It is
probable that these electrons are a type of secondary radiation, set up at the
surfaces on which the α rays fall. The particles would be extremely readily
absorbed in the gas, and their presence would be difficult to detect except in low
vacua. J. J. Thomson at first obtained no evidence that the α particles of polonium
were charged; but in later experiments, where the plates were closer together, the
electroscope indicated that the α rays did carry a positive charge.
In order to see whether the positive charge due to the α rays from radium could be
detected when the slow moving ions were prevented from escaping by a magnetic
field, I placed the apparatus of Fig. 33 between the pole-pieces of a large
electromagnet, so that the magnetic field was parallel to the plane of the plates[151].
A very marked alteration was observed both on the magnitude of the positive and
negative currents. In a good vacuum, the upper plate received a positive charge,
independently of whether the lower plate was charged positively or negatively or
was connected with earth. After the magnetic field had reached a certain value, a
great increase in its strength had no appreciable effect on the magnitude of the
current.

The following table illustrates the results obtained when the two plates were 3
mms. apart, and were both coated with thin aluminium foil.
Potential Current in arbitrary units
of lower
plate
Without magnetic field With magnetic field
0 — +·36
+2 volts 2·0 +·46}
} ·39
-2 „ 2·5 +·33}

+4 „ 2·8 +·47}
} ·41
-4 „ 3·5 +·35}

+8 „ 3·1 +·56}
} ·43
-8 „ 4·0 +·31}

+84 „ 3·5 +·77}
} ·50
-84 „ 5·2 +·24}
Let n be the number of α particles, carrying a charge e, which are absorbed in the
upper plate. Let ι₀ be the current due to the slight ionization of the residual gas.
If only a small potential is applied to the lower plate, this current should be equal
in magnitude but opposite in sign when the potential is reversed. Let ι1 be the
charge per sec. communicated to the upper electrode when the lower plate is
charged positively and ι2 the value when charged negatively. Then

ι1 = ι₀ + ne,
ι2 = ι₀ + ne;

adding we get

ι1 + ι2
ne = ------ .

2

Now in the third column of the above table it is seen that (ι1 + ι2)/2 has the values
·39, ·41, ·43 for 2, 4, and 8 volts respectively. The numbers are thus in fairly good
agreement. Similar results were obtained when a brass plate was substituted for
the upper electrode shown in the figure. Taking into consideration that the
magnitude of ne is independent of the strength of the magnetic field above a
certain small value, and the good agreement of the numbers obtained with
variation of voltage, I think that there can be no doubt that the positive charge
communicated to the upper electrode was carried by the α particles. This positive
charge was not small, for using a weight of ·48 mgrs. radium bromide spread in a
thin foil over an area of about 20 sq. cms., the charge communicated by the
particles corresponded to a current 8·8 × 10-13 amperes, and, with the Dolezalek
electrometer employed, it was necessary to add a capacity of ·0024 microfarads to
the electrometer system.
In these experiments, the film of radium bromide was so thin, that only a very
small percentage of the α particles was stopped by the radium itself. Assuming
that each α particle carries the same charge as an ion, viz. 1·1 × 10-19 coulombs,
and remembering that half of the α particles are absorbed in the lower plate, the
total number N of α particles expelled per second from one gram of radium
bromide (at its minimum activity) can be deduced. In two separate experiments
where the amount of radium used was ·194 and ·484 mgrs. respectively, the
values of N were in close agreement and equal to 3·6 × 1010. Now it will be shown
later that in radium there are three other products in radio-active equilibrium, each
of which probably gives out the same number of α particles as radium itself. If this
is the case, the total number of α particles expelled per second from 1 gram of
radium bromide in radio-active equilibrium is 4N or 1·44 × 1011. Assuming the
composition of radium bromide as RaBr2, the number per second per gram of
radium is 2·5 × 1010. This number will be found to be in very good agreement
with that deduced from indirect data (chapter XIII.). The value of N is of great
importance in determining the magnitude of various quantities in radio-active
calculations.

94. Mass and energy of the α particle. It has been pointed out that the α rays
from radium and polonium are analogous to the Canal rays of Goldstein, for both
carry a positive charge and are difficult to deflect by a magnetic field. The
experiments of Wien have shown that the velocity of projection of the canal rays
varies with the gas in the tube and the intensity of the electric field applied, but it

is generally about ⅒ of the velocity of the α particle from radium. The value of
e/m is also variable, depending upon the gas in the tube.
It has been shown that for the α rays of radium
e
V = 2·5 × 109 and ------- = 6 × 103.
m

Now the value of e/m for the hydrogen atom, liberated in the electrolysis of water,
is 104. Assuming the charge carried by the α particle to be the same as that carried
by the hydrogen atom, the mass of the α particle is about twice that of the
hydrogen atom. Taking into consideration the uncertainty attaching to the
experimental value of e/m for the α particle, if the α particle consists of any
known kind of matter, this result indicates that it consists either of projected
helium or hydrogen. Further evidence on this important question is given in
section 260.
The α rays from all the radio-active substances and their products, such as the
radio-active emanations and the matter causing excited activity, possess the same
general properties and do not vary very much in penetrating power. It is thus
probable that in all cases the α rays from the different radio-active substances
consist of positively charged bodies projected with great velocity. Since the rays
from radium are made up in part of α rays from the emanation stored in the
radium, and from the excited activity which it produces, the α rays from each of
these products must consist of positively charged bodies; for it has been shown
that all the α rays from radium are deviated in a strong magnetic field.
The kinetic energy of each projected particle is enormous, compared with its
mass. The kinetic energy of each α particle is
1 1 m
--- mV2 = --- --- V2e = 5·9 × 10-6 ergs.
2 2 e

Taking the velocity of a rifle bullet as 105 cms. per second, it is seen that, mass for
mass, the energy of motion of the α rays is 6 × 108 times as great as that of the
rifle bullet. In this projection of bodies atomic in size with great velocity probably
lies the principal cause of the heating effects produced by radium (chapter XII).

95. Atomic disintegration. The radio-activity of the radio-elements is an atomic
and not a molecular property. The rate of emission of the radiations depends only

on the amount of the element present and is independent of its combination with
inactive substances. In addition, it will be shown later that the rate of emission is
not affected by wide variations of temperature, or by the application of any known
chemical or physical forces. Since the power of radiating is a property of the
radio-atoms, and the radiations consist for the most part of positively and
negatively charged masses projected with great velocity, it is necessary to suppose
that the atoms of the radio-elements are undergoing disintegration, in the course of
which parts of the atom escape from the atomic system. It seems very improbable
that the α and β particles can suddenly acquire their enormous velocity of
projection by the action of forces existing inside or outside the atom. For example,
the α particle would have to travel from rest between two points differing in
potential by 5·2 million volts in order to acquire the kinetic energy with which it
escapes. Thus it seems probable that these particles are not set suddenly in motion,
but that they escape from an atomic system in which they were already in rapid
oscillatory or orbital motion. On this view, the energy is not communicated to the
projected particles, but exists beforehand in the atoms from which they escape.
The idea that the atom is a complicated structure consisting of charged parts in
rapid oscillatory or orbital motion has been developed by J. J. Thomson, Larmor
and Lorentz. Since the α particle is atomic in size, it is natural to suppose that the
atoms of the radio-active elements consist not only of the electrons in motion, but
also of positively charged particles whose mass is about the same as that of the
hydrogen or helium atom.
It will be shown later that only a minute fraction of the atoms of the radio-element
need break up per second in order to account for the radiations even of an
enormously active element like radium. The question of the possible causes which
lead to this atomic disintegration and the consequences which follow from it will
be discussed later in chapter XIII.

96. Experiments with a zinc sulphide screen. A screen of Sidot’s hexagonal
blend (phosphorescent crystalline zinc sulphide) lights up brightly under the
action of the α rays of radium and polonium. If the surface of the screen is
examined with a magnifying glass, the light from the screen is found not to be
uniformly distributed but to consist of a number of scintillating points of light. No
two flashes succeed one another at the same point, but they are scattered over the
surface, coming and going rapidly without any movement of translation. This
remarkable action of the radium and polonium rays on a zinc sulphide screen was
discovered by Sir William Crookes[152], and independently by Elster and Geitel[153],
who observed it with the rays given out from a wire which has been charged

negatively either in the open air or in a vessel containing the emanation of
thorium.
In order to show the scintillations of radium on the screen, Sir William Crookes
has devised a simple apparatus which he has called the “Spinthariscope.” A small
piece of metal, which has been dipped in a radium solution, is placed several
millimetres away from a small zinc sulphide screen. This screen is fixed at one
end of a short brass tube and is looked at through a lens fixed at the other end of
the tube. Viewed in this way, the surface of the screen is seen as a dark
background, dotted with brilliant points of light which come and go with great
rapidity. The number of points of light per unit area to be seen at one time falls off
rapidly as the distance from the radium increases, and, at several centimetres
distance, only an occasional one is seen. The experiment is extremely beautiful,
and brings vividly before the observer the idea that the radium is shooting out a
stream of projectiles, the impact of each of which on the screen is marked by a
flash of light.
The scintillating points of light on the screen are the result of the impact of the α
particles on its surface. If the radium is covered with a layer of foil of sufficient
thickness to absorb all the α rays the scintillations cease. There is still a
phosphorescence to be observed on the screen due to the β and γ rays, but this
luminosity is not marked by scintillations to any appreciable extent. Sir William
Crookes showed that the number of scintillations was about the same in vacuo as
in air at atmospheric pressure. If the screen was kept at a constant temperature, but
the radium cooled down to the temperature of liquid air, no appreciable difference
in the number of scintillations was observed. If, however, the screen was
gradually cooled to the temperature of liquid air, the scintillations diminished in
number and finally ceased altogether. This is due to the fact that the screen loses
to a large extent its power of phosphorescence at such a low temperature.
Not only are scintillations produced by radium, actinium, and polonium, but also
by the emanations and other radio-active products which emit α rays. In addition,
F. H. Glew[154] has found that they can be observed from the metal uranium,
thorium compounds and various varieties of pitchblende. In order to show the
scintillations produced by pitchblende, a flat surface was ground, and a
transparent screen, whose lower surface was coated with zinc sulphide, placed
upon it. Glew has designed a modified and very simple form of spinthariscope. A
transparent screen, coated on one side with a thin layer of zinc sulphide, is placed
in contact with the active material, and the scintillations observed by a lens in the
usual way.

Since there is no absorption in the air, the luminosity is a maximum. The relative
transparency of different substances placed between the active material and the
screen may, in this way, be directly studied.
The production of scintillations appears to be a general property of the α rays from
all radio-active substances. The scintillations are best shown with a zinc sulphide
screen; but are also observed with willemite (zinc silicate), powdered diamond,
and potassium platinocyanide (Glew, loc. cit.). If a screen of barium
platinocyanide is exposed to the α rays from radium, the scintillations are difficult
to observe, and the luminosity is far more persistent than for a zinc sulphide
screen exposed under the same conditions. The duration of the phosphorescence in
this case probably accounts for the absence of visible scintillations.
There can be no doubt that the scintillations result from the continuous
bombardment of the sensitive screen by the α particles. Each of these particles
moves with enormous velocity, and has a considerable energy of motion. On
account of the ease with which these particles are stopped, most of this energy is
given up at the surface of the screen, and a portion of the energy is in some way
transformed into light. Zinc sulphide is very sensitive to mechanical shocks.
Luminosity is observed if a penknife is drawn across the screen, or if a current of
air is directed on to the screen. The disturbance effected by the impact of the α
particle extends over a distance very large compared with the size of the
impinging particle, so that the spots of light produced have an appreciable area.
Recently Becquerel[155] has made an examination of the scintillations produced by
different substances, and has concluded that the scintillations are due to irregular
cleavages in the crystals composing the screen, produced by the action of the α
rays. Scintillations can be mechanically produced by crushing a crystal.
Tommasina[156] found that a zinc sulphide screen removed from the action of the
radium rays for several days, showed the scintillations again when an electrified
rod was brought near it.
The number of scintillations produced in zinc sulphide depends upon the presence
of a slight amount of impurity and on its crystalline state. It can be shown that
even with the most sensitive zinc sulphide screens, the number of scintillations is
probably only a small fraction of the total number of α particles which fall upon it.
It would appear that the crystals are in some way altered by the bombardment of
the α particles, and that some of the crystals occasionally break up with emission
of light[157].
Although the scintillations from a particle of pure radium bromide are very
numerous, they are not too numerous to be counted. Close to the radium, the
luminosity is very bright, but by using a high power microscope the luminosity

can still be shown to consist of scintillations. Since the number of scintillations
probably bears no close relation to the number of α particles emitted, a
determination of the number of scintillations would have no special physical
significance. The relation between the number of α particles and the number of
scintillations would probably be variable, depending greatly on the exact chemical
composition of the sensitive substance and also upon its crystalline state.

97. Absorption of the α rays by matter. The α rays from the different radio-
active substances can be distinguished from one another by the relative amounts
of their absorption by gases or by thin screens of solid substances. When
examined under the same conditions, the α rays from the active substances can be
arranged in a definite order with reference to the amount of absorption in a given
thickness of matter.
In order to test the amount of absorption of the α rays for different thicknesses of
matter, an apparatus similar to that shown in Fig. 17, p. 98, was employed[158]. A
thin layer of the active material was spread uniformly over an area of about 30 sq.
cms., and the saturation current observed between two plates 3·5 cms. apart. With
a thin layer[159] of active material, the ionization between the plates is due almost
entirely to the α rays. The ionization due to the β and γ rays is generally less than
1% of the total.
The following table shows the variation of the saturation current between the
plates due to the α rays from radium and polonium, with successive layers of
aluminium foil interposed, each ·00034 cm. in thickness. In order to get rid of the
ionization due to the β rays from radium, the radium chloride employed was
dissolved in water and evaporated. This renders the active compound, for the time,
nearly free from β rays.
The initial current with 1 layer of aluminium over the active material is taken as
100. It will be observed that the current due
Polonium. Radium.
Layers of Current Ratio of Layers of Current Ratio of
aluminium decrease aluminium decrease
for each for each
layer layer

0 100 0 100
·41 ·48
1 41 1 48

·31 ·48
2 12·6 2 23
·17 ·60
3 2·1 3 13·6
·067 ·47
4 ·14 4 6·4
·39
5 0 5 2·5
·36
6 ·9

7 0
to the radium rays decreases very nearly by half its value for each additional
thickness until the current is reduced to about 6% of the maximum. It then decays
more rapidly to zero. Thus, for radium, over a wide range, the current decreases
approximately according to an exponential law with the thickness of the screen, or

where i is the current for a thickness d, and i₀ the initial current. In the case
of polonium, the decrease is far more rapid than would be indicated by the
exponential law. By the first layer, the current is reduced to the ratio ·41.
The addition of the third layer cuts the current down to a ratio of ·17. For
most of the active bodies, the current diminishes slightly faster than the
exponential law would lead one to expect, especially when the radiation is
nearly all absorbed.

98. The increase of absorption of the α rays of polonium with the thickness
of matter traversed has been very clearly shown in some experiments made
by Mme Curie. The apparatus employed is shown in Fig. 34.

Fig. 34.

The saturation current was measured between two parallel plates PP´ 3
cms. apart. The polonium A was placed in the metal box CC, and the rays
from it, after passing through an opening in the lower plate P´, covered with
a layer of thin foil T, ionized the gas between the plates. For a certain
distance AT, of 4 cms. or more, no appreciable current was observed
between P and P´. As the distance AT was diminished, the current increased
in a very sudden manner, so that for a small variation of the distance AT

there was a large increase of current. With still further decrease of distance
the current increases in a more regular manner. The results are shown in the
following table, where the screen T consisted of one and two layers of
aluminium foil respectively. The current due to the rays, without the
aluminium screen, is in each case taken as 100.
Distance 3·5 2·5 1·9 1·45 0·5
AT in cms.

For 100 rays 0 0 5 10 25
transmitted
by one layer
For 100 rays 0 0 0 0 0·7
transmitted
by two
layers
The metallic screen thus cuts off a greater proportion of the rays the greater
the distance of air which the radiations traverse. The effects are still more
marked if the plates PP´ are close together. Results similar but not so
marked are found if radium is substituted for the polonium.
It follows from these experiments that the ionization per unit volume, due to
a large plate uniformly covered with the radio-active matter, falls off rapidly
with the distance from the plate. At a distance of 10 cms. the α rays from
uranium, thorium, or radium have been completely absorbed in the gas, and
the small ionization then observed in the gas is due to the more penetrating
β and γ rays. The relative amount of the ionization observed at a distance
from the source will increase with the thickness of the layer of active
matter, but will reach a maximum for a layer of a certain thickness. The
greater proportion of the ionization, due to unscreened active matter, is thus
entirely confined to a shell of air surrounding it not more than 10 cms. in
depth.

Fig. 35.

99. The α rays from different compounds of the same active element,
although differing in amount, have about the same average penetrating
power. Experiments on this point have been made by the writer[160] and by
Owens[161]. Thus in comparing the relative power of penetration of the α
rays from the different radio-elements, it is only necessary to determine the
penetrating power for one compound of each of the radio-elements.
Rutherford and Miss Brooks[162] have determined the amount of absorption
of the α rays from the different active substances in their passage through
successive layers of aluminium foil ·00034 cm. thick. The curves of
absorption are given in Fig. 35. For the purpose of comparison in each case,
the initial current with the bare active compound was taken as 100. A very
thin layer of the active substance was used, and, in the case of thorium and
radium, the emanations given off were removed by a slow current of air
through the testing vessel. A potential difference of 300 volts was applied
between the plates, which was sufficient to give the maximum current in
each case.

Curves for the minerals organite and thorite were very nearly the same as
for thoria.
For comparison, the absorption curves of the excited radiations of thorium
and radium are given, as well as the curve for the radio-elements uranium,
thorium, radium, and polonium. The α radiations may be arranged in the
following order, as regards their power of penetration, beginning with the
most penetrating.
Thorium}
Radium } excited radiation.
Thorium.
Radium.
Polonium.
Uranium.

The same order is observed for all the absorbing substances examined, viz.,
aluminium, Dutch metal, tinfoil, paper, and air and other gases. The
differences in the absorption of the α rays from the active bodies are thus
considerable, and must be ascribed either to a difference of mass or of
velocity of the α particles or to a variation in both these quantities.
Since the α rays differ either in mass or velocity, it follows that they cannot
be ascribed to any single radio-active impurity common to all radio-active
bodies.

100. Absorption of the α rays by gases. The α rays from the different
radio-active substances are quickly absorbed in their passage through a few
centimetres of air at atmospheric pressure and temperature. In consequence
of this, the ionization of the air, due to the α rays, is greatest near the
surface of the radiating body and falls off very rapidly with the distance
(see section 98).

Fig. 36.

A simple method of determining the absorption in gases is shown in Fig. 36.
The maximum current is measured between two parallel plates A and B kept
at a fixed distance of 2 cms. apart, and then moved by means of a screw to
different distances from the radio-active surface. The radiation from this
active surface passed through a circular opening in the plate A, covered with
thin aluminium foil, and was stopped by the upper plate. For observations
on other gases besides air, and for examining the effect at different
pressures, the apparatus is enclosed in an air-tight cylinder.
If the radius of the active surface is large compared with the distance of the
plate A from it, the intensity of the radiation is approximately uniform over
the opening in the plate A, and falls off with the distance x traversed
according to an exponential law. Thus

where λ is the “absorption constant” of the radiation for the gas under
consideration[163]. Let

x = distance of lower plate from active material,
l = distance between the two fixed plates.

The energy of the radiation at the lower plate is then

and at the upper plate

The total number of ions produced between the parallel plates A and B is
therefore proportional to

Since the factor

is a constant, the saturation current between A and B varies as

i.e. it decreases according to an exponential law with the distance traversed.

Fig. 37.

The variation of the current between A and B with the distance from a thin
layer of uranium oxide is shown in Fig. 37 for different gases. The initial
measurements were taken at a distance of about 3·5 mms. from the active
surface. The actual values of this initial current were different for the
different gases, but, for the purposes of comparison, the value is in each
case taken as unity.
It will be seen that the current falls off with the distance approximately in a
geometrical progression, a result which is in agreement with the simple
theory given above. The distance through which the rays pass before they
are absorbed is given below for different gases.
Gas Distance in mms.
to absorb half of
radiation
Carbonic acid 3
Air 4·3
Coal-gas 7·5

Hydrogen 16
The results for hydrogen are only approximate, as the absorption is small
over the distance examined.
The absorption is least in hydrogen and greatest in carbonic acid, and
follows the same order as the densities of the gases. In the case of air and
carbonic acid, the absorption is proportional to the density, but this rule is
widely departed from in the case of hydrogen. Results for the relative
absorption by air of the α rays from the different active bodies are shown in
Fig. 38.

Fig. 38.

The initial observation was made about 2 mms. from the active surface, and
the initial current is in each case taken as 100. The current, as in the case of
uranium, falls off at first approximately in geometrical progression with the
distance. The thickness of air, through which the radiation passes before the
intensity is reduced to half value, is given below.
Distance in mms.
Uranium 4·3

Radium 7·5
Thorium 10
Excited radiation 16·5
from Thorium and
Radium
The order of absorption by air of the radiations from the active substances is
the same as the order of absorption by the metals and solid substances
examined.

101. Connection between absorption and density. Since in all cases the
radiations first diminish approximately according to an exponential law
with the distance traversed, the intensity I after passing through a thickness
x is given by

where λ is the absorption constant and I₀ the initial intensity.
The following table shows the value of λ with different radiations for air
and aluminium.
Radiation λ for λ for air
aluminium
Excited radiation 830 ·42
Thorium 1250 ·69
Radium 1600 ·90
Uranium 2750 1·6
Taking the density of air at 20° C. and 760 mms. as 0·00120 compared with
water as unity, the following table shows the value of λ divided by density
for the different radiations.
Radiation Aluminium Air
Excited radiation 320 350

Thorium 480 550
Radium 620 740
Uranium 1060 1300
Comparing aluminium and air, the absorption is thus roughly proportional
to the density for all the radiations. The divergence, however, between the
absorption-density numbers is large when two metals like tin and
aluminium are compared. The value of λ for tin is not much greater than for
aluminium, although the density is nearly three times as great.
If the absorption is proportional to the density, the absorption in a gas
should vary directly as the pressure, and this is found to be the case. Some
results on this subject have been given by the writer (loc. cit.) for uranium
rays between pressures of ¼ and 1 atmosphere. Owens (loc. cit.) examined
the absorption of the α radiation in air from thoria between the pressures of
0·5 to 3 atmospheres and found that the absorption varied directly as the
pressure.
The variation of absorption with density for the projected positive particles
is thus very similar to the law for the projected negative particles and for
cathode rays. The absorption, in both cases, depends mainly on the density,
but is not in all cases directly proportional to it. Since the absorption of the
α rays in gases is probably mainly due to the exhaustion of the energy of the
rays by the production of ions in the gas, it seems probable that the
absorption in metals is due to a similar cause.

102. Relation between ionization and absorption in gases. It has been
shown (section 45) that if the α rays are completely absorbed in a gas, the
total ionization produced is about the same for all the gases examined.
Since the rays are unequally absorbed in different gases, there should be a
direct connection between the relative ionization and the relative
absorption. This is seen to be the case if the results of Strutt (section 45) are
compared with the relative absorption constants (section 100).
Gas Relative Relative
absorption ionization
Air 1 1
Hydrogen ·27 ·226

Carbon dioxide 1·43 1·53
Considering the difficulty of obtaining accurate determinations of the
absorption, the relative ionization in a gas is seen to be directly proportional
to the relative absorption within the limits of experimental error. This result
shows that the energy absorbed in producing an ion is about the same in air,
hydrogen, and carbon dioxide.

103. Mechanism of the absorption of α rays by matter. The experiments,
already described, show that the ionization of the gas, due to the α rays
from a large plane surface of radio-active matter, falls off in most cases
approximately according to an exponential law, until most of the rays are
absorbed, whereupon the ionization decreases at a much faster rate. In the
case of polonium, the ionization falls off more rapidly than is to be expected
on the simple exponential law.
The ionization produced in the gas is due to the collision of the rapidly
moving α particles with the molecules of the gas in their path. On account
of its large mass, the α particle is a far more efficient ionizer than the β
particle moving at the same speed. It can be deduced from the results of
experiment that each projected α particle is able to produce about 100,000
ions in passing through a few centimetres of the gas before its velocity is
reduced to the limiting value, below which it no longer ionizes the gas in its
path.
Energy is required to ionize the gas, and this energy can only be obtained at
the expense of the kinetic energy of the projected α particle. Thus it is to be
expected that the α particle should gradually lose its velocity and energy of
motion in its passage through the gas.
Since the rate of absorption of the α rays in gases is deduced from
measurements of the ionization of the gas at different distances from the
source of radiation, a knowledge of the law of variation of the ionizing
power of the projected α particle with its speed is required in order to
interpret the results. The experimental data on this question are, however,
too incomplete to be applied directly to a solution of this question.
Townsend[164] has shown that a moving electron produces ions in the gas
after a certain limiting velocity is reached. The number of ions produced per

centimetre of its path through the gas then rises to a maximum, and for still
higher speeds continuously decreases. For example, Townsend found that
the number of ions produced by an electron moving in an electric field was
small at first for weak fields, but increased with the strength of the electric
field to a maximum corresponding to the production of 20 ions per cm. of
path in air at a pressure of 1 mm. of mercury. Durack[165] found that the
electrons, generated in a vacuum tube, moving with a velocity of about 5 ×
109 cms. per second produced a pair of ions every 5 cms. of path at 1 mm.
pressure. In a later paper, Durack showed that for the electrons from
radium, which are projected with a velocity greater than half the velocity of
light, a pair of ions was produced every 10 cms. of path. The high speed
electron from radium is thus a very inefficient ionizer and produces only
about ¹⁄₁₀₀ of the ionization per unit path observed by Townsend for the
slow moving electron.

104. In the case of the α particle, no direct measurements have been made
upon the variation of the ionization with the velocity of the particle, so that
the law of absorption of the rays cannot be deduced directly. An indirect
attack upon the question has, however, been made recently by Bragg and
Kleeman[166] who have formulated a simple theory to account for the
experimental results which they have obtained upon the absorption of the α
rays. The α particles from each simple type of radio-active matter are
supposed to be projected with the same velocity, and to pass through a
definite distance a in air at atmospheric pressure and temperature before
they are all absorbed. As a first approximation the ionization per unit path is
supposed to be the same over the whole length traversed before absorption,
and to cease fairly suddenly at a definite distance from the source of
radiation. This is in agreement with the observed fact that the ionization
between parallel plates increases very rapidly when it approaches nearer
than a certain distance to the radiant source. The range a depends upon the
initial energy of motion of the α particle and will thus be different for
different kinds of radio-active matter. If a thick layer of radio-active matter
is employed, only the α particles from the surface have a range a. Those
which reach the surface from a depth d have their range diminished by an
amount ρd, where ρ is the density of the radio-active matter compared with
air. This is merely an expression of the fact that the absorption of the α rays

is proportional to the thickness and density of matter traversed. The rays
from a thick layer of active matter will thus be complex, and will consist of
particles of different velocity whose ranges have all values between 0 and a.
Suppose that a narrow pencil of α rays is emitted from a thick layer of
radio-active material, and confined by metal stops as in Fig. 39.

Fig. 39.

The pencil of rays passes into an ionization vessel AB through a fine wire
gauze A. The amount of ionization is to be determined between A and B for
different distances h from the source of the rays R to the plate A.
All the particles coming from a depth x of the material given by h = a – ρx
will enter the ionization vessel. The number of ions produced in a depth dh
of the ionization vessel is equal to nxdh, i.e. to
a – h
n ------ dh ,
ρ

where n is a constant.
If the depth of the ionization vessel be b, the total number of ions produced
in the vessel is

This supposes that the stream of particles passes completely across the
vessel. If not, the expression becomes

If the ionization in the vessel AB is measured, and a curve plotted showing
its relation to h, the curve in the former case should be a straight line whose
slope is nb/ρ and in the latter a parabola.
Thus if a thin layer of radio-active material is employed and a shallow
ionization vessel, the ionization would be represented by a curve such as
APM (Fig. 40), where the ordinates represent distances from the source of
radiation, and the abscissae the ionization current between the plates AB.

Fig. 40.

In this case, PM is the range of the α particles from the lowest layer of the
radio-active matter. The current should be constant for all distances less

than PM.
For a thick layer of radio-active matter, the curve should be a straight line
such as APB.
Curves of the above character should only be obtained when definite cones
of rays are employed, and where the ionization vessel is shallow and
includes the whole cone of rays. In such a case the inverse square law need
not be taken into account.
In the experiments previously recorded (sections 99 and 100), the ionization
was measured between parallel plates several centimetres apart for a large
area of radio-active material. Such an arrangement was necessary at the
time at which the experiments were made, as only weak radio-active
material was available. Measurable electrical effects could not then be
obtained with narrow cones of rays and shallow ionization vessels, but this
disadvantage is removed by the advent of pure radium bromide as a source
of radiation.
The interesting experiments described by Bragg and Kleeman show that the
theoretical curves are approximately realized in practice. The chief
difficulty experienced in the analysis of the experimental results was due to
the fact that radium is a complex radio-active substance and contains four
radio-active products each of which gives rise to α rays which have
different ranges. The general character of the results obtained from radium
are shown graphically in Fig. 41, curves A, B, C, D.

Fig. 41.

The ordinates represent the distance between the radium and the gauze of
the testing vessel; the abscissae the current in the ionization vessel in
arbitrary units. Five milligrams of radium bromide were used, and the depth
of the ionization vessel was about 5 mms. Curve A is for a cone of rays of
angle 20°. The initial current at a distance of 7 cms. is due to the β and γ
rays and natural leak. This curve is initially parabolic, and then is made up
of two straight lines. Curve B is for a smaller cone, and shows the straight
line character of the curve to within a short distance of the radium. Curve C
was obtained under the same condition as curve A, but with a layer of gold
beater’s skin placed over the radium. The effect of this is to reduce all the
ordinates of curve A by the same quantity. This is to be expected on the
simple theory already considered. Curve D was obtained when the radium
was heated so as to get rid of the emanation and its products. The α particles
of greatest range are quite absent and the curve is simpler in character.

Fig. 42.

The complex character of the radium curves are more clearly brought out
by a careful examination of a portion of the curve at distances between 2

and 5 cms. from the radium, using an ionization vessel of depth only 2
mms. The results are shown in Fig. 42, where the curve is seen to consist
approximately of four straight lines of different slopes represented by PQ,
QR, RS, ST.
Such a result is to be expected, for it will be shown later that four distinct α
ray products exist in radium when in radio-active equilibrium. Each of these
products of radium emits an equal number of α particles per second, but the
range of each is different. If a1 is the range of one stream, a2 of another, the
ionization in the vessel AB, when two streams enter the vessel, should be
nb nb
---- (a1-h-b/2) + ----- (a2 – h – b/2),
ρ ρ

i.e.
nb
---- (a1 + a2 – 2h – b) .
ρ

Thus the slope of the curve should in this case be 2nb/ρ, while if only one
stream enters, it should be nb/ρ. When three reach it, the slope should be
3nb/ρ and for four 4nb/ρ. These results are realized fairly closely in
practice. The curve (Fig. 42) consists of four parts, whose slopes are in the
proportion 16, 34, 45, 65, i.e. very nearly in the ratio 1, 2, 3, 4.
Experiments were also made with very thin layers of radium bromide,
when, as we have seen (Fig. 40) a very different shape of curve is to be
expected. An example of the results is shown in Fig. 43, curves I., II. and
III. Curve I. is obtained from radium bromide which has been heated to
drive off the emanation, and curves II. and III. from the same substance
several days later, when the emanation was again accumulating. The portion
PQ, which is absent in the first curve, is probably due to the “excited”
activity produced by the emanation. By careful examination of the
successive changes in the curves after the radium has been heated to drive
off the emanation, it is possible to tell the range of the α rays from each of
the different products, and this has been done to some extent by Bragg and
Kleeman.

It will be seen later that the results here obtained support in a novel way the
theory of radio-active changes which has been advanced from data of quite
a different character.
The inward slope of the curve in Fig. 43 due to the radium indicates that the
α particles become more efficient ionizers as their velocity decreases. This
is in agreement with observations on the β rays. In some cases Bragg also
observed that the α particles are the most efficient ionizers just before they
lose their power of ionizing the gas.

Fig. 43.

Thus we may conclude from these experiments that the α particles from a
simple radio-active substance traverse a definite distance in air, at a definite

pressure and temperature, and that the ionization ends fairly abruptly. If the
rays traverse a sheet of metal, the effective range of ionization is diminished
by a distance corresponding to ρd, where ρ is the density of the material
compared with air and d its thickness. The α rays from a thick layer of a
simple radio-active substance consist of α particles of different velocities,
which have ranges in air lying between 0 and the maximum range. The
ionization of the particles per unit path is greatest near the end of its range,
and decreases somewhat as we approach the radiant source. A complex
source of rays like radium gives out four types of rays, each of which has a
different but distinct range.
From this theory it is possible to calculate approximately the decrease of
current to be observed when sheets of metal foil are placed over a large area
of radio-active substance. This is the method that has been employed to
obtain the curves of Figs. 35 and 38.
Suppose a very thin layer of simple radio-active matter is employed (for
example a bismuth plate covered with radio-tellurium or a metal plate made
active by exposure to the presence of the thorium or radium emanations)
and that the ionization vessel is of sufficient depth to absorb the α rays
completely.
Let d be the thickness of the metal plate, ρ its density compared with air.
Consider a point P close to the upper side of the plate. The range of the
particles moving from a point, when the path makes an angle θ with the
normal at P, is a – ρd sec θ, where a is the range in air. The rays coming
from points such that the paths make an angle with the normal greater than

will thus be absorbed in the plate. By integrating over the circular area
under the point P, it is easy to show that the total ionization in the vessel is
proportional to

The curves showing the relation between current and distance of metal
traversed should thus be parabolic with respect to d. This is approximately
the case for a simple substance like radio-tellurium. The curve for a thick
layer of radium would be difficult to calculate on account of the complexity
of the rays, but we know from experiment that it is approximately
exponential. An account of some recent investigations made to determine
the range of velocity over which the α particle is able to ionize the gas is
given in Appendix A. The results there given strongly support the theory of
absorption of the α rays discussed above.

PART IV.

The γ or very penetrating Rays.

105. In addition to the α and β rays, the three active substances, uranium,
thorium, and radium, all give out a radiation of an extraordinarily
penetrating character. These γ rays are considerably more penetrating than
the X rays produced in a “hard” vacuum tube. Their presence can readily be
observed for an active substance like radium, but is difficult to detect for
uranium and thorium unless a large quantity of active material is used.
Villard[167], using the photographic method, first drew attention to the fact
that radium gave out these very penetrating rays, and found that they were
non-deviable by a magnetic field. This result was confirmed by
Becquerel[168].
Using a few milligrams of radium bromide, the γ rays can be detected in a
dark room by the luminosity they excite in the mineral willemite or a screen
of platinocyanide of barium. The α and β rays are completely absorbed by
placing a thickness of 1 centimetre of lead over the radium, and the rays
which then pass through the lead consist entirely of γ rays. The very great
penetrating power of these rays is easily observed by noting the slight
diminution of the luminosity of the screen when plates of metal several
centimetres thick are placed between the radium and the screen. These rays
also produce ionization in gases and are best investigated by the electrical
method. The presence of the γ rays from 30 mgrs. of radium bromide can be
observed in an electroscope after passing through 30 cms. of iron.

106. Absorption of the γ rays. In an examination of the active substances
by the electrical method, the writer[169] found that both uranium and thorium
gave out γ rays in amount roughly proportional to their activity. An
electroscope of the type shown in Fig. 12 was employed. This was placed

on a large lead plate ·65 cm. thick, the active substance being placed in a
closed vessel beneath.
The discharge due to the natural ionization of the air in the electroscope was
first observed. The additional ionization due to the active substance must be
that produced by rays which have passed through the lead plate and the
walls of the electroscope. The following table shows that the discharge due
to these rays decreases approximately according to an exponential law with
the thickness of lead traversed.
Thickness of lead Rate of discharge
·62 cms. 100
„ + ·64 cms. 67
„ + 2·86 „ 23
„ + 5·08 „ 8
Using 100 grs. of uranium and thorium, the discharge due to the rays
through 1 cm. of lead was quite appreciable, and readily measured. The
results showed that the amount of γ rays was about the same for equal
weights of thorium and uranium oxides. The penetrating power was also
about the same as for the radium rays.

Fig. 44.

The writer showed that the absorption of the γ rays from radium was
approximately proportional to the density of the substance traversed. A
more detailed examination of the absorption of these rays in various
substances has been recently made by McClelland[170]. The curve (Fig. 44)
shows the decrease of the ionization current in a testing vessel due to the β
and γ rays with successive layers of lead. It is seen that the β rays are almost
completely stopped by 4 mms. of lead; the ionization is then due entirely to
the γ rays.
In order to leave no doubt that all the β rays were absorbed, the radium was
covered with a thickness of 8 mms. of lead, and measurements of the
coefficient of absorption λ were made for additional thicknesses. The
average value of λ was calculated from the usual equation

where d is the thickness of matter traversed. The following table shows the
value of λ, (I) for the first 2·5 mms. of matter traversed (after initially
passing through 8 mms. of lead), (II) for the thickness 2·5 to 5 mms., (III)
for 5 to 10 mms., (IV) 10 to 15 mms.
TABLE A.
Substance I II III IV

Platinum 1·167
Mercury ·726 ·661 ·538 ·493
Lead ·641 ·563 ·480 ·440
Zinc ·282 ·266 ·248 ·266
Aluminium ·104 ·104 ·104 ·104
Glass ·087 ·087 ·087 ·087

Water ·034 ·034 ·034 ·034
In the above table, the absorption in aluminium, glass and water was too
small to determine with accuracy the variation of λ with distance traversed.
It will be observed that, for the denser substances, the coefficient of
absorption decreases with the distance through which the rays have passed.
This indicates that the rays are heterogeneous. The variation of λ is more
marked in heavy substances.
Table B gives the values of λ divided by density for the above numbers. If
the absorption were directly proportional to the density, the quotient would
be the same in all cases.
TABLE B.
λ divided by density.
Substance I II III IV

Platinum ·054
Mercury ·053 ·048 ·039 ·036
Lead ·056 ·049 ·042 ·037
Zinc ·039 ·037 ·034 ·033
Aluminium ·038 ·038 ·038 ·038
Glass ·034 ·034 ·034 ·034
Water ·034 ·034 ·034 ·034
The numbers in column I vary considerably, but the agreement becomes
closer in the succeeding columns, until in column IV the absorption is very
nearly proportional to the density.
It is seen that the absorption of all three types of rays from radio-active
substances is approximately proportional to the density of the substance
traversed—a relation first observed by Lenard for the cathode rays. This
law of absorption thus holds for both positively and negatively electrified
particles projected from the radio-active substances, and also for the
electromagnetic pulses which are believed to constitute the γ rays; although
the absorption of the α rays, for example, is 10,000 times greater than for
the γ rays. We have seen in section 84 that the value of the absorption

constant λ for lead is 122 for the β rays from uranium. The value for the γ
rays from radium varies between ·64 and ·44, showing that the γ rays are
more than 200 times as penetrating as the β rays.
107. Nature of the rays. In addition to their great penetrating power, the γ
rays differ from the α and β rays in not being deflected to an appreciable
degree by a magnetic or electric field. In a strong magnetic field, it can be
shown, using the photographic method, that there is an abrupt discontinuity
between the β and γ rays, for the former are bent completely away from the
latter. This indicates that, as regards the action of a magnetic field, there is
no gradual transition of magnetic properties between the β and γ rays.
Paschen[171] has examined the γ rays in a very intense magnetic field, and,
from the absence of deflection of these rays, has calculated that, if they
consist of electrified particles carrying an ionic charge, and projected with a
velocity approaching that of light, their apparent mass must be at least 45
times greater than that of the hydrogen atom.
It now remains for us to consider whether the γ rays are corpuscular in
character, or whether they are a type of electromagnetic pulse in the ether
similar to Röntgen rays. They resemble Röntgen rays in their great
penetrating power and in their absence of deflection in a magnetic field.
Earlier experiments seemed to indicate an important difference between the
action of γ and X rays. It is well known that ordinary X rays produce much
greater ionization in gases such as sulphuretted hydrogen and hydrochloric
acid gas, than in air, although the differences in density are not large. For
example, exposed to X rays, sulphuretted hydrogen has six times the
conductivity of air, while with γ rays the conductivity only slightly exceeds
that of air. The results obtained by Strutt, in this connection, have already
been given in section 45. It is there shown that the relative conductivity of
gases exposed to γ rays (and also to α and β rays) is, in most cases, nearly
proportional to their relative densities; but, under X rays, the relative
conductivity for some gases and vapours is very much greater than for the γ
rays. It must be remembered, however, that the results obtained by Strutt
were for “soft X rays,” whose penetrating power was very much less than
that of the γ rays. In order to see if the relative conductivity of gases
produced by X rays depended upon their penetrating power, A. S. Eve[172]
made some experiments with a very “hard” X ray bulb, which gave an
unusually penetrating type of rays.

The results of the measurements are shown in the table below, where the
conductivity for each type of rays is expressed relative to air as unity. The
results obtained for “soft” X rays by Strutt and by Eve for γ rays are added
for comparison.
It is seen that the hard rays show a much closer agreement than the soft rays
with the density law found for the γ rays. The high values previously
obtained for the vapours of chloroform and carbon tetrachloride are greatly
reduced, and are very nearly the same as for the γ rays. On the other hand,
the vapour of methyl iodide is an exception, and still shows a high
conductivity. The γ rays were, however, forty times as penetrating as the
hard X rays, and it is probable that the value of methyl iodide would be
reduced with still more penetrating X rays.
Relative conductivities of gases.
Gas Relative “Soft” “Hard” γ
Density X X rays rays
rays

Hydrogen ·07 ·11 ·42 ·19
Air 1·0 1·0 1·0 1·0
Sulphuretted 1·2 6 ·9 1·23
Hydrogen
Chloroform 4·3 32 4·6 4·8
Methyl 5·0 72 13·5 5·6
Iodide
Carbon 5·3 45 4·9 5·2
Tetrachloride
The hard X rays were found to give far more secondary radiation than the γ
rays, but this effect is probably also a function of the penetrating power of
the primary rays. It will be seen later (section 112) that γ rays give rise to a
secondary radiation of the β ray type. This has also been observed for the X
rays.
Considering the experimental evidence as a whole, there is undoubtedly a
very marked similarity between the properties of γ and X rays. The view

that the γ rays are a type of very penetrating X rays, also receives support
from theoretical considerations. We have seen (section 52) that the X rays
are believed to be electromagnetic pulses, akin in some respects to short
light waves, which are set up by the sudden stoppage of the cathode ray
particles. Conversely, it is also to be expected that X rays will be produced
at the sudden starting, as well as at the sudden stopping, of electrons. Since
most of the β particles from radium are ejected from the radium atom with
velocities much greater than the cathode particles in a vacuum tube, X rays
of a very penetrating character will arise. But the strongest argument in
support of this view is derived from an examination of the origin and
connection of the β and γ rays from radio-active substances. It will be
shown later that the α ray activity observed in radium arises from several
disintegration products, stored up in the radium, while the β and γ rays arise
only from one of these products named radium C. It is found, too, that the
activity measured by the γ rays is always proportional to the activity
measured by the β rays, although by separation of the products the activity
of the latter may be made to undergo great variations in value.
Thus the intensity of the γ rays is always proportional to the rate of
expulsion of β particles, and this result indicates that there is a close
connection between the β and γ rays. Such a result is to be expected if the β
particle is the parent of the γ ray, for the expulsion of each electron from
radium will give rise to a narrow spherical pulse travelling from the point of
disturbance with the velocity of light.

108. There is another possible hypothesis in regard to the nature of these
rays. It has been shown (sections 48 and 82) that the apparent mass of an
electron increases as the speed of light is approached; theoretically it should
be very great when the velocity of the electron is exceedingly close to the
velocity of light. In such a case, a moving electron would be difficult to
deflect by a magnetic or electric field.
The view that the γ rays are electrons carrying a negative charge and
moving with a velocity nearly equal to that of light has recently been
advocated by Paschen[173]. He concluded from experiment that the γ rays
like the β rays carried a negative charge. We have seen (section 85) that
Seitz also observed that a small negative charge was communicated to

bodies on which the γ rays impinged, but the magnitude of this charge was
much smaller than that observed by Paschen. I do not think that much
weight can be attached to observations that a small positive or negative
charge is communicated to bodies on which the γ rays fall, for it will be
shown later that a strong secondary radiation, consisting in part of electrons,
is set up during the passage of the γ rays through matter. It is not
improbable that the small charge observed is not a direct result of the
charge carried by the γ rays, but is an indirect effect due to the secondary
radiations emitted from the surface of bodies. There is no doubt that a thick
lead vessel, completely enclosing a quantity of radium, acquires a small
positive charge, but this result would follow whether the γ rays carry a
charge or not, since the secondary radiations from the lead surface consist
of projected particles which carry with them a negative charge.
On this corpuscular theory of the nature of the γ rays, each electron must
have a large apparent mass, or otherwise it would be appreciably deflected
by an intense magnetic field. The energy of motion of the electron must, in
consequence, be very great, and, if the number of the electrons constituting
the γ rays is of the same order of magnitude as the number of the β
particles, a large heating effect is to be expected when the γ rays are stopped
in matter. Paschen[174] made some experiments on the heat emission of
radium due to the γ rays; he concluded that the γ rays were responsible for
more than half of the total heat emission of radium and carried away energy
at the rate of over 100 gram calories per hour per gram of radium. This
result was not confirmed by later experiments of Rutherford and Barnes[175],
who found that the heating effect of the γ rays could not be more than a few
per cent. of the total heat emission of radium. These results will be
considered later in chapter XII.
The weight of evidence, both experimental and theoretical, at present
supports the view that the γ rays are of the same nature as the X rays but of
a more penetrating type. The theory that the X rays consist of non-periodic
pulses in the ether, set up when the motion of electrons is arrested, has
found most favour, although it is difficult to provide experimental tests to
decide definitely the question. The strongest evidence in support of the
wave nature of the X rays is derived from the experiments of Barkla[176],
who found that the amount of secondary radiation set up by the X rays on
striking a metallic surface depended on the orientation of the X ray bulb.

The rays thus showed evidence of a one-sidedness or polarization which is
only to be expected if the rays consist of a wave motion in the ether.

PART V.

Secondary Rays.

109. Production of secondary rays. It has long been known that Röntgen
rays, when they impinge on solid obstacles, produce secondary rays of
much less penetrating power than the incident rays. This was first shown by
Perrin and has been investigated in detail by Sagnac, Langevin, Townsend
and others. Thus it is not surprising that similar phenomena should be
observed for the radiation from radio-active substances. By means of the
photographic method, Becquerel[177] has made a close study of the
secondary radiations produced by radio-active substances. In his earliest
observations, he noticed that radiographs of metallic objects were always
surrounded by a diffuse border. This effect is due to the secondary rays set
up by the incident rays at the surface of the screen.
The secondary rays produced by the α rays are very feeble. They are best
shown by polonium, which gives out only α rays, so that the results are not
complicated by the action of the β rays. Strong secondary rays are set up at
the point of impact of the β or cathodic rays. Becquerel found that the
magnitude of this action depended greatly on the velocity of the rays. The
rays of lowest velocity gave the most intense secondary action, while the
penetrating rays gave, in comparison, scarcely any secondary effect. In
consequence of the presence of this secondary radiation, the photographic
impression of a screen pierced with holes is not clear and distinct. In each
case there is a double photographic impression, due to the primary rays and
the secondary rays set up by them.
These secondary rays are deviable by a magnetic field, and in turn produce
tertiary rays and so on. The secondary rays are in all cases more readily
deviated and absorbed than the primary rays, from which they arise. The
very penetrating γ rays give rise to secondary rays, which cause intense
action on the photographic plate. When some radium was placed in a cavity

inside a deep lead block, rectangular in shape, besides the impression due to
the direct rays through the lead, Becquerel observed that there was also a
strong impression due to the secondary rays emitted from the surface of the
lead. The action of these secondary rays on the plate is so strong that the
effect on the plate is, in many cases, increased by adding a metal screen
between the active material and the plate.
The comparative photographic action of the primary and secondary rays
cannot be taken as a relative measure of the intensity of their radiations. For
example, only a small portion of the energy of the β rays is in general
absorbed in the sensitive film. Since the secondary rays are far more easily
absorbed than the primary rays, a far greater proportion of their energy is
expended in producing photographic action than in the case of the primary
rays. It is thus not surprising that the secondary rays set up by the β and γ
rays may in some cases produce a photographic impression comparable
with, if not greater than, the effect of the incident rays.
On account of these secondary rays, radiographs produced by the β rays of
radium in general show a diffuse border round the shadow of the object. For
this reason radiographs of this kind lack the sharpness of outline of X ray
photographs.

110. Secondary radiation produced by α rays. Mme Curie[178] has shown
by the electric method that the α rays of polonium produce secondary rays.
The method adopted was to compare the ionization current between two
parallel plates, when two screens of different material, placed over the
polonium, were interchanged.
These results show that the α rays of polonium are modified in passing
through matter, and that the amount of secondary rays set up varies with
screens of different material. Mme Curie, using the same method, was
unable to observe any such effect for the β rays of radium. The production
of secondary rays by the β rays of radium is, however, readily shown by the
photographic method. We have already seen (section 93) that very low
velocity electrons accompany the α rays from radium or radio-tellurium
spread on a metal plate. These electrons are probably liberated when the α
rays escape from or impinge upon matter, and the number emitted depends
upon the kind of matter used as a screen. The differences shown in the

above table when the screens were interchanged are explained simply in
this way.
Screens employed Thickness Current
in mms. observed

Aluminium 0·01
Cardboard 0·005 17·9

Cardboard 0·005
Aluminium 0·01 6·7

Aluminium 0·01
Tin 0·005 150

Tin 0·005
Aluminium 0·01 126

Tin 0·005
Cardboard 0·005 13·9

Cardboard 0·005
Tin 0·005 4·4

Fig. 45.

111. Secondary rays produced by β and γ rays. An examination of the
amount and character of the secondary radiation emitted by various
substances, when exposed to the β and γ rays of radium, has recently been
made by A. S. Eve[179]. The general experimental method employed is
shown in Fig. 45.
The electroscope (Fig. 45) was placed behind a lead screen 4·5 cms. thick,
which stopped all the β rays and absorbed the greater proportion of the γ
rays from the radium tube placed at R. On bringing near a plate of matter M,
the primary rays fell upon it and some of the secondary rays, emitted in all
directions, passed into the side of the electroscope, which was covered with
aluminium foil of thickness ·05 mm. Before the plate M was placed in
position the rate of discharge of the electroscope was due to the natural leak
and the γ rays from R, and the secondary radiation from the air. On bringing
the radiator M into position, the rate of discharge was much increased, and
the difference between the rate of movement of the gold-leaf in the two
cases was taken as a measure of the amount of secondary rays from M. The
absorption of the secondary rays was tested by placing an aluminium plate
·85 mm. thick before the face of the electroscope.

The secondary rays were found to be fairly homogeneous, for the intensity
fell off according to an exponential law with the distance traversed. The
value of the absorption constant λ was determined from the usual equation

where d is the thickness of the screen. The table given below shows the
results obtained when thick plates of different substances of the same
dimensions were placed in a definite position at M. The secondary radiation
from fluids was obtained by a slight alteration of the experimental
arrangements.
Thirty milligrammes of radium bromide were used, and the results are
expressed in terms of the number of scale divisions passed over per second
by the gold-leaf.
It will be noticed that the amount of secondary radiation follows in most
cases the same order as the densities, and is greatest for mercury. The value
of (secondary radiation)/density is not a constant, but varies considerably,
being greatest for light substances. The absorption constant of the
secondary rays from different radiators is not very different, with the
exception of substances such as granite, brick, and cement, which give out
secondary rays of nearly twice the penetrating power of other substances.
β and γ rays.
Radiator Density Secondary Sec. Aluminium
Radiation Rad. / ·085 cm. λ
Density

Mercury 13·6 147 10·8
Lead 11·4 141 12·4 18·5
Copper 8·8 79 9·0 20
Brass 8·4 81 9·6 21

Iron 7·8 75 9·6 20
(wrought)
Tin 7·4 73 9·9 20·3
Zinc 7·0 79 11·3
Granite 2·7 54 20·0 12·4
Slate 2·6 53 20·4 12·1
Aluminium 2·6 42 16·1 24
Glass 2·5 44 17·6 24
Cement 2·4 47 19·6 13·5
Brick 2·2 49 22·3 13·0
Ebonite 1·1 32 29·1 26
Water 1·0 24 24·0 21
Ice ·92 26 28·2
Paraffin ·9 17 18·8 21
solid
„ liquid ·85 16 18·8
Mahogany ·56 21·4 38·2 23
Paper ·4? 21·0 52 22
Millboard ·4? 19·4 48 20·5
Papier- ... 21·9
mâché
Basswood ·36 20·7 57 22
Pine ·35 21·8 62 21

X ray 75·2 23·6
screen
The secondary radiation not only comes from the surface of the radiator but
from a considerable depth. The amount of secondary rays increases with the
thickness of the radiator, and, in the case of glass and aluminium, reaches a
practical maximum for a plate about 3 mms. thick.
In the above table, the secondary radiation arises from both the β rays and γ
rays together. When the β rays were cut off by a layer of lead 6·3 mms.

thick, placed between the radium and the radiator, the effect on the
electroscope was reduced to less than 20 per cent. of its former value,
showing that the β rays supplied more than 80 per cent. of the secondary
radiation. The following table shows the relative amount of secondary rays
from different substances when exposed to β and γ rays together and to γ
rays alone. The amount from lead in each case is taken as a standard and
equal to 100. The amount of secondary radiation found by Townsend from
soft X rays is added for comparison.
Secondary Radiations.
Radiator β γ Röntgen
and rays
γ
rays

Lead 100 100 100
Copper 57 61 291
Brass 58 59 263
Zinc 57 ... 282
Aluminium 30 30 25
Glass 31 35 31
Paraffin 12 20 125
It will be observed that the relative amounts are about the same for the γ
rays alone as for the β and γ rays together. On the other hand, the amount of
secondary radiation set up by X rays is very different, lead for example
giving much less than brass or copper. The secondary rays from the γ rays
alone are slightly less penetrating than for the β and γ rays together, but are
far more penetrating than the secondary radiation from the X rays examined
by Townsend.
The amount of secondary radiation set up by the β and γ rays is mainly
independent of the state of the surface of the radiator. About the same
amount is obtained from iron as from iron filings; from liquid as from solid
paraffin; and from ice as from water[180].

Becquerel has shown that the secondary rays set up by the β rays are
deflected by a magnet and consist of negatively charged particles
(electrons). It has been pointed out in section 52 that the cathode rays are
diffusely reflected from the metal on which they fall. These secondary rays
consist in part of electrons moving with about the same velocity as the
primary, and in part of some electrons with a much slower speed. The
secondary rays set up by the β rays of radium have on an average less
penetrating power than the primary rays, and consequently less velocity
than the primary rays. It must be remembered that the β rays from radium
are very complex, and consist of electrons projected with a considerable
range of velocities. The secondary rays are, on an average, certainly more
penetrating than the most easily absorbed β rays emitted from radium, and
probably move with a velocity of about half that of light.
It is still uncertain whether the secondary rays are produced by the action of
the primary rays on matter, or whether they consist of a portion of the
primary rays whose direction of motion has been deflected in their passage
through matter, so that they emerge again with diminished velocity from the
surface.

112. Magnetic deflection of secondary rays from γ rays. It has been seen
that the secondary rays set up by the γ rays alone are very similar in
character to those caused by the β rays. This result was still further
confirmed by Eve, who showed that the secondary rays produced by the γ
rays are readily deflected by a magnetic field. The experimental
arrangement is shown in Fig. 46.

Fig. 46.

A small electroscope was mounted on one side of a lead platform 1·2 cms.
thick, which rested on a lead cylinder 10 cms. high and 10 cms. in diameter.
The radium was placed at the bottom of a hole reaching to the centre of the
cylinder.
On applying a strong magnetic field, at right angles to the plane of the
paper, so as to bend the secondary rays from the platform towards the
electroscope, the rate of discharge was much increased. On reversing the
field, the effect was much diminished. Since the γ rays are not themselves
deflected by a magnetic field, this result shows that the secondary radiation
is quite different in character from the primary rays, and consists of
electrons projected with a velocity (deduced from the penetrating power) of
about half the velocity of light. We have already pointed out that the
emission of electrons from a substance traversed by the rays will account
sufficiently well for the charge observed by Paschen, without the necessity
of assuming that the γ rays carry a negative charge of electricity.
The secondary radiation set up by Röntgen rays, like that due to the β and γ
rays, consists in part of electrons projected with considerable velocity.
These three types of rays seem about equally efficient in causing the
expulsion of electrons from the substance through which they pass. We

have seen that the X and γ rays are, in all probability, electromagnetic
pulses set up by the sudden starting or stopping of electrons, and, since
these rays in turn cause the removal of electrons, the process appears to be
reversible. Since the β rays pass through some thickness of matter before
their energy of motion is arrested, theory would lead us to expect that a type
of soft X rays should be generated in the absorbing matter.

PART VI.

113. Comparison of the ionization produced by the α and β rays. With
unscreened active material the ionization produced between two parallel
plates, placed as in Fig. 17, is mainly due to the α rays. On account of the
slight penetrating power of the α rays, the current due to them practically
reaches a maximum with a small thickness of radio-active material. The
following saturation currents were observed[181] for different thicknesses of
uranium oxide between parallel plates sufficiently far apart for all the α rays
to be absorbed in the gas between them.
Surface of uranium oxide 38 sq. cms.
Weight of uranium Saturation current in
oxide in grammes amperes per sq. cm. of
per sq. cm. of surface
surface
.
·0036 1·7 × 10-13
·0096 3·2 × 10-13
·0189 4·0 × 10-13
·0350 4·4 × 10-13
·0955 4·7 × 10-13
The current reached about half its maximum value for a weight of oxide
·0055 gr. per sq. cm. If the α rays are cut off by a metallic screen, the
ionization is then mainly due to the β rays, since the ionization produced by
the γ rays is small in comparison. For the β rays from uranium oxide it has
been shown (section 86) that the current reaches half its maximum value for
a thickness of 0·11 gr. per sq. cm.
Meyer and Schweidler[182] have found that the radiation from a water
solution of uranium nitrate is very nearly proportional to the amount of

uranium present in the solution.
On account of the difference in the penetrating power of the α and β rays,
the ratio of the ionization currents produced by them depends on the
thickness of the radio-active layer under examination. The following
comparative values of the current due to the α and β rays were obtained for
very thin layers of active matter . A weight of ⅒ gramme of fine powder,
[183]

consisting of uranium oxide, thorium oxide, or radium chloride of activity
2000, was spread as uniformly as possible over an area of 80 sq. cms. The
saturation current was observed between parallel plates 5·7 cms. apart. This
distance was sufficient to absorb most of the α rays from the active
substances. A layer of aluminium ·009 cm. thick absorbed all the α rays.
Current Current Ratio of
due to α due to β currents
rays rays β/α

Uranium 1 1 ·0074
Thorium 1 ·27 ·0020
Radium 2000 1350 ·0033
In the above table the saturation current due to the α and β rays of uranium
is, in each case, taken as unity. The third column gives the ratio of the
currents observed for equal weights of substance. The results are only
approximate in character, for the ionization due to a given weight of
substance depends on its fineness of division. In all cases, the current due to
the β rays is small compared with that due to the α rays, being greatest for
uranium and least for thorium. As the thickness of layer increases, the ratio
of currents β/α steadily increases to a constant value.

114. Comparison of the energy radiated by the α and β rays. It has not
yet been found possible to measure directly the energy of the α and β rays.
A comparison of the energy radiated in the two forms of rays can, however,
be made indirectly by two distinct methods.
If it be assumed that the same amount of energy is required to produce an
ion by either the α or the β ray, and that the same proportion of the total

energy is used up in producing ions, an approximate estimate can be made
of the ratio of the energy radiated by the α and β rays by measuring the ratio
of the total number of ions produced by them. If λ is the coefficient of
absorption of the β rays in air, the rate of production of ions per unit volume
at a distance x from the source is

where q₀ is the rate of ionization at the source.
The total number of ions produced by complete absorption of the rays is

Now λ is difficult to measure experimentally for air, but an approximate
estimate can be made of its value from the known fact that the absorption of
β rays is approximately proportional to the density of any given substance.
For β rays from uranium the value of λ for aluminium is about 14, and λ
divided by the density is 5·4. Taking the density of air as ·0012, we find
that for air
λ = ·0065.
The total number of ions produced in air is thus 154q₀ when the rays are
completely absorbed.
Now from the above table the ionization due to the β rays is ·0074 of that
produced by α rays, when the β rays passed through a distance of 5·7 cms.
of air.
Thus we have approximately
Total number of ions produced by β rays ·0074

--------------------------------------- = ----- ×
154 = 0·20.
Total number of ions produced by α rays 5·7

Therefore about ⅙ of the total energy radiated into air by a thin layer of
uranium is carried by the β rays or electrons. The ratio for thorium is about
¹⁄₂₂ and for radium about ¹⁄₁₄, assuming the rays to have about the same
average value of λ.
This calculation takes into account only the energy which is radiated out
into the surrounding gas; but on account of the ease with which the α rays
are absorbed, even with a thin layer, the greater proportion of the radiation
is absorbed by the radio-active substance itself. This is seen to be the case
when it is recalled that the α radiation of thorium or radium is reduced to
half value after passing through a thickness of about 0·0005 cm. of
aluminium. Taking into consideration the great density of the radio-active
substances, it is probable that most of the radiation which escapes into the
air is due to a thin skin of the powder not much more than ·0001 cm. in
thickness.

An estimate, however, of the relative rate of emission of energy by the α
and β rays from a thick layer of material can be made in the following way:
—For simplicity suppose a thick layer of radio-active substance spread
uniformly over a large plane area. There seems to be no doubt that the
radiations are emitted uniformly from each portion of the mass;
consequently, the radiation, which produces the ionizing action in the gas
above the radio-active layer, is the sum total of all the radiation which
reaches the surface of the layer.

Let λ1 be the average coefficient of absorption of the α rays in the radio-
active substance itself and σ the specific gravity of the substance. Let E1 be
the total energy radiated per sec. per unit mass of the substance when the
absorption of the rays in the substance itself is disregarded. The energy per
sec. radiated to the upper surface by a thickness dx of a layer of unit area at
a distance x from the surface is given by

The total energy W1 per unit area radiated to the surface per sec. by a
thickness d is given by

if λ1d is large.

In a similar way it may be shown that the energy W2 of the β rays reaching
the surface is given by

where E2 and λ2 are the values for the β rays corresponding to E1 and λ1 for
the α rays. Thus it follows that
E1 λ1 W 1
---- = ------
E2 λ2 W 2

λ1 and λ2 are difficult to determine directly for the radio-active substance
itself, but it is probable that the ratio λ1/λ2 is not very different from the
ratio for the absorption coefficients for another substance like aluminium.
This follows from the general result that the absorption of both α and β rays
is proportional to the density of the substance; for it has already been shown
in the case of the β rays from uranium that the absorption of the rays in the

radio-active material is about the same as for non-radio-active matter of the
same density.
With a thick layer of uranium oxide spread over an area of 22 sq. cms., it
was found that the saturation current between parallel plates 6·1 cms. apart,
due to the α rays, was 12·7 times as great as the current due to the β rays.
Since the α rays were entirely absorbed between the plates and the total
ionization produced by the β rays is 154 times the value at the surface of the
plates,
W1 total number of ions due to α rays
---- = ------------------------------------
W2 total number of ions due to β rays

12·7 × 6·1
= ------------- = 0·5 approximately.
154

Now the value of λ1 for aluminium is 2740 and of λ2 for the same metal 14,
thus
E1 λ1W1
---- = ------- = 100 approximately
E2 λ2W2

This shows that the energy radiated from a thick layer of material by the β
rays is only about 1 per cent. of the energy radiated in the form of α rays.
This estimate is confirmed by calculations based on independent data. Let
m1, m2 be the masses of the α and β particles respectively and v1, v2 their
velocities.

Now it has been shown that for the α rays of radium

v1 = 2·5 × 109,
e
--- = 6 × 103.
m1

The velocity of the β rays of radium varies between wide limits. Taking for
an average value
v2 = 1·5 × 1010,
e
---- = 1·8 × 107,
m1

it follows that the energy of the α particle from radium is almost 83 times
the energy of the β particle. If equal numbers of α and β particles are
projected per second, the total energy radiated in the form of α rays is about
83 times the amount in the form of β rays.
Evidence will be given later (section 253) to show that the number of α
particles projected is probably four times the number of β particles; so that a
still greater proportion of the energy is emitted in the form of α rays. These
results thus lead to the conclusion that, from the point of view of the energy
emitted, the α rays are far more important than the β rays. This conclusion
is supported by other evidence which is discussed in chapters XII and XIII,
where it will be shown that the α rays play by far the most important part in
the changes occurring in radio-active bodies, and that the β rays only appear
in the latter stages of the radio-active processes. From data based on the
relative absorption and ionization of the β and γ rays in air, it can be shown
that the γ rays carry off about the same amount of energy as the β rays.
These conclusions are confirmed by direct measurement of the heating
effect of radium, which is discussed in detail in chapter XII.

CHAPTER V.
PROPERTIES OF THE RADIATIONS.

115. Besides their power of acting on a photographic plate, and of ionizing
gases, the radiations from active bodies are able to produce marked
chemical and physical actions in various substances. Most of these effects
are due either to the α or β rays. The γ rays produce little effect in
comparison. Since the β rays are similar in all respects to high velocity
cathode rays, it is to be expected that they will produce effects similar in
character to those produced by the cathode rays in a vacuum tube.

Phosphorescent action.

Becquerel[184] has studied the action of radium rays in producing
phosphorescence in various bodies. The substance to be tested was placed
above the radium in the form of powder on a very thin mica plate.
Examination was made of the sulphides of calcium and strontium, ruby,
diamond, varieties of spar, phosphorus and hexagonal blende. Substances
like the ruby and spar, which phosphoresce under luminous rays, did not
phosphoresce under the radium rays. On the other hand, those which were
made luminous by ultra-violet light were also luminous under the action of
radium rays. The radium rays show distinct differences from X rays. For
example, a diamond which was very luminous with radium rays was
unaffected by X rays. The double sulphate of uranium and potassium is
more luminous than hexagonal blende under X rays, but the reverse is true
for radium rays; under the influence of these rays, sulphide of calcium gave
a blue luminosity but was hardly affected by X rays.
The following table shows the relative phosphorescence excited in various
bodies.
Substance Without Across screen of
screen. black paper
Intensity

Hexagonal blende 13·36 ·04
Platino-cyanide of 1·99 ·05
barium
Diamond 1·14 ·01
Double sulphate of 1·00 ·31
Uranium and
Potassium
Calcium Fluoride ·30 ·02
In the last column the intensity without the screen is in each case taken as
unity. The great diminution of intensity after the rays have passed through

black paper shows that most of the phosphorescence developed without the
screen is, in the majority of cases, due to the α rays.
Bary[185] has made a very complete examination of the class of substances
which become luminous under radium rays. He found that the great
majority of substances belong to the alkali metals and alkaline earths. All
these substances were also phosphorescent under the action of X rays.
Crystalline zinc sulphide (Sidot’s blende) phosphoresces very brightly
under the influence of the rays from radium and other very active
substances. This was observed by Curie and Debierne in their study of the
radium emanation and the excited activity produced by it. It has also been
largely used by Giesel as an optical means of detecting the presence of
emanations from very active substances. It is an especially sensitive means
of detecting the presence of α rays, when it exhibits the “scintillating”
property already discussed in section 96. In order to show the luminosity
due to the α rays, the screen should be held close to the active substance, as
the rays are absorbed in their passage through a few centimetres of air. Zinc
sulphide is also luminous under the action of the β rays, but the
phosphorescence is far more persistent than when produced by the α rays.
Very beautiful luminous effects are produced by large crystals of the
platinocyanides exposed to the radium rays. Those containing lithium give a
brilliant pink colour. The calcium and barium salts fluoresce with a deep
green light, and the sodium compound with a lemon yellow. The mineral
willemite (zinc silicate) was recently found by Kunz to be an even more
sensitive means of detecting the presence of the radiations than
platinocyanide of barium. It fluoresces showing a beautiful greenish colour,
and a piece of mineral exposed to the action of the rays appears quite
translucent. The crystals of the platinocyanides of barium and lithium are
especially suited for showing the action of the γ rays, and, in this respect,
are superior to willemite.
A very striking effect is shown by the mineral kunzite—a new variety of
spodumene discovered by Kunz[186]. This is a transparent gem like crystal,
often of very large size, which glows with a beautiful reddish colour under
the action of the β or γ rays, but does not appear to be sensitive to the α
rays. The luminosity extends throughout the crystal, but is not so marked as
in the platinocyanides or willemite. The mineral sparteite[187], a form of

calcite containing a few per cent. of manganese, has been found by
Ambrecht to fluoresce with a very deep orange light under the β and γ rays.
The colour appears to depend on the intensity of the rays, and is deeper
close to the radium than at some distance away.
If kunzite and sparteite are exposed to the action of the cathode rays in a
vacuum tube, the colour is different from that produced by the radium rays.
The former appears a deep yellow, instead of the deep red observed with the
radium rays.
The different actions of the radium rays on these fluorescent substances can
be illustrated very simply and beautifully by the following experiment. A
small U tube is filled with fragments of the fluorescent substance arranged
in layers. The U tube is immersed in liquid air and the emanation from
about 30 mgrs. of radium bromide is condensed in the tube. On closing the
tube and removing it from the liquid air, the emanation distributes itself
uniformly in the tube. The shades of colour produced in the different
substances are clearly seen.
It is observed that all the crystals increase in luminosity for several hours,
on account of the excited activity produced by the emanation. This effect is
especially observed in kunzite, which at first hardly responds to the rays,
since the β and γ rays, which causes it to fluoresce, are not given out by the
emanation itself but by one of its later products. The intensity of the β and γ
rays is, in consequence, small at first but rises to a maximum after several
hours; the luminosity observed varies in a corresponding manner.
Sir William Crookes[188] has made an examination of the effect of continued
exposure of a diamond to the radium rays. An “off-colour” diamond, of a
pale yellow colour, was placed inside a tube with radium bromide. After 78
days’ exposure, the diamond had darkened and become bluish green in tint;
when heated at 50° in a mixture of potassium chlorate for ten days, the
diamond lost its dull surface colour and was bright and transparent, and its
tint had changed to a pale bluish green. The rays have thus a double action
on the diamond; the less penetrating β rays produce a superficial darkening
due to the change of the surface into graphite, while the more penetrating β
rays and the γ rays produce a change of colour throughout its mass. The
diamond phosphoresced brightly during the whole course of its exposure to
the rays. Crookes also observed that the diamond still retained enough

activity to affect a photographic plate 35 days after removal, although,
during the period of 10 days, it was heated in a mixture sufficiently
powerful to remove the outer skin of graphite. This residual activity may
possibly be due to a slow transformation product of the emanation which is
deposited on the surface of bodies (see chapter XI).
Marckwald observed that the α rays from radio-tellurium produced marked
phosphorescence on some kinds of diamonds. An account of the various
luminous effects produced on different gems by exposure to the radium and
actinium rays has been given by Kunz and Baskerville[189].
Both zinc sulphide and platinocyanide of barium diminish in luminosity
after exposure for some time to the action of the rays. To regenerate a
screen of the latter, exposure to solar light is necessary. A similar
phenomenon has been observed by Villard for a screen exposed to Röntgen
rays. Giesel made a screen of platinocyanide of radio-active barium. The
screen, very luminous at first, gradually turned brown in colour, and at the
same time the crystals became dichroic. In this condition the luminosity
was much less, although the active substance had increased in activity after
preparation. Many of the substances which are luminous under the rays
from active substances lose this property to a large extent at low
temperatures[190].

116. Luminosity of radium compounds. All radium compounds are
spontaneously luminous. This luminosity is especially brilliant in the dry
haloid salts, and persists for long intervals of time. In damp air the salts lose
a large amount of their luminosity, but they recover it on drying. With very
active radium chloride, the Curies have observed that the light changes in
colour and intensity with time. The original luminosity is recovered if the
salt is dissolved and dried. Many inactive preparations of radiferous barium
are strongly luminous. The writer has seen a preparation of impure radium
bromide which gave out a light sufficient to read by in a dark room. The
luminosity of radium persists over a wide range of temperature and is as
bright at the temperature of liquid air as at ordinary temperatures. A slight
luminosity is observed in a solution of radium, and if crystals are being
formed in the solution, they can be clearly distinguished in the liquid by
their greater luminosity.

117. Spectrum of the phosphorescent light of radium and actinium.
Compounds of radium, with a large admixture of barium, are usually
strongly self-luminous. This luminosity decreases with increasing purity,
and pure radium bromide is only very feebly self-luminous. A spectroscopic
examination of the slight phosphorescent light of pure radium bromide has
been made by Sir William and Lady Huggins[191]. On viewing the light with
a direct vision spectroscope, there were faint indications of a variation of
luminosity at different points along the spectrum. In order to get a
photograph of the spectrum within a reasonable time, they made use of a
quartz spectroscope of special design which had been previously employed
in a spectroscopic examination of faint celestial objects. After three days’
exposure with a slit of ¹⁄₄₅₀ of an inch in width, a negative was obtained
which showed a number of bright lines. The magnified spectrum is shown
in Fig. 46 A. The lines of this spectrum were found to agree not only in
position but also in relative intensity with the band spectrum of nitrogen.
The band spectrum of nitrogen and also the spark spectrum[192] of radium
are shown in the same figure.
Some time afterwards Sir William Crookes and Prof. Dewar showed that
this spectrum of nitrogen was not obtained if the radium was contained in a
highly exhausted tube. Thus it appears that the spectrum is due to the action
of the radium rays either on occluded nitrogen or the nitrogen in the
atmosphere surrounding the radium.
It is very remarkable that a phosphorescent light, like that of radium
bromide, should show a bright line spectrum of nitrogen. It shows that
radium at ordinary temperatures is able to set up radiations which are
produced only by the electric discharge under special conditions.
Sir William and Lady Huggins were led to examine the spectrum of the
natural phosphorescent light of radium with the hope that some indications
might be obtained thereby of the processes occurring in the radium atom.
Since the main radiation from radium consists of positively charged atoms
projected with great velocity, radiations must be set up both in the expelled
body and in the system from which it escapes.

Fig. 46a.

Giesel[193] observed that the spectrum of the phosphorescent light of
actinium consists of three bright lines. Measurements of the wave length
were made by Hartmann[194]. The luminosity was very slight and a long
exposure was required. The lines observed were in the red, blue and green.
The wave length λ and velocity are shown below.
Line Intensity λ
1 10 4885·4 ± 0·1 Ångström
units
2 6 5300 ± 6 „
3 1 5909 ± 10 „
The line 4885 was very broad; the other two lines were so feeble that it was
difficult to determine their wave length with accuracy. Hartmann suggests
that these lines may be found in the spectrum of the new stars. The lines
observed have no connection with radium or its emanation[195].

118. Thermo-luminescence. E. Wiedemann and Schmidt[196] have shown
that certain bodies after exposure to the cathode rays or the electric spark
become luminous when they are heated to a temperature much below that

required to cause incandescence. This property of thermo-luminescence is
most strikingly exhibited in certain cases where two salts, one of which is
much in excess of the other, are precipitated together. It is to be expected
that such bodies would also acquire the property when exposed to the β or
cathodic rays of radium. This has been found to be the case by
Wiedemann[197]. Becquerel showed that fluor-spar, exposed to the radium
rays, was luminous when heated. The glass tubes in which radium is kept
are rapidly blackened. On heating the tube, a strong luminosity is observed,
and the coloration to a large extent disappears. The peculiarity of many of
these bodies lies in the fact that the property of becoming luminous when
heated is retained for a long interval of time after the body is removed from
the influence of the exciting cause. It appears probable that the rays cause
chemical changes in these bodies, which are permanent until heat is applied.
A portion of the chemical energy is then released in the form of visible
light.

Physical actions.

119. Some electric effects. Radium rays have the same effect as ultra-violet
light and Röntgen rays in increasing the facility with which a spark passes
between electrodes. Elster and Geitel[198] showed that if two electrodes were
separated by a distance such that the spark just refused to pass, on bringing
near a specimen of radium the spark at once passes. This effect is best
shown with short sparks from a small induction coil. The Curies have
observed that radium completely enveloped by a lead screen 1 cm. thick
produces a similar action. The effect in that case is due to the γ rays alone.
This action of the rays can be very simply illustrated by connecting two
spark-gaps with the induction coil in parallel. The spark-gap of one circuit
is adjusted so that the discharge just refuses to pass across it, but passes by
the other. When some radium is brought near the silent spark-gap, the spark
at once passes and ceases in the other[199].
Hemptinne[200] found that the electrodeless discharge in a vacuum tube
began at a higher pressure when a strong preparation of radium was brought
near the tube. In one experiment the discharge without the rays began at 51
mms. but with the radium rays at 68 mms. The colour of the discharge was
also altered.
Himstedt[201] found that the resistance of selenium was diminished by the
action of radium rays in the same way as by ordinary light.
F. Henning[202] examined the electrical resistance of a barium chloride
solution containing radium of activity 1000, but could observe no
appreciable difference between it and a similar pure solution of barium
chloride. This experiment shows that the action of the rays from the radium
does not produce any appreciable change in the conductivity of the barium
solution.
Kohlrausch and Henning[203] have recently made a detailed examination of
the conductivity of pure radium bromide solutions, and have obtained
results very similar to those for the corresponding barium solutions.
Kohlrausch[204] found that the conductivity of water exposed to the

radiations from radium increased more rapidly than water which had not
been exposed. This increase of conductivity may have been due to an
increase of the conductivity of the water itself, or to an increased rate of
solution of the glass of the containing vessel.
Specimens of strongly active material have been employed to obtain the
potential at any point of the atmosphere. The ionization due to the active
substance is so intense that the body to which it is attached rapidly takes up
the potential of the air surrounding the active substance. In this respect it is
more convenient and rapid in its action than the ordinary taper or water
dropper, but on account of the disturbance of the electric field by the strong
ionization produced, it is probably not so accurate a method as that of the
water dropper.

120. Effect on liquid and solid dielectrics. P. Curie[205] made the very
important observation that liquid dielectrics became partial conductors
under the influence of radium rays. In these experiments the radium,
contained in a glass tube, was placed in an inner thin cylinder of copper.
This was surrounded by a concentric copper cylinder, and the liquid to be
examined filled the space between. A strong electric field was applied, and
the current through the liquid measured by means of an electrometer.
The following numbers illustrate the results obtained:
Substance Conductivity in
megohms per 1 cm.3

Carbon bisulphide 20 × 10-14
Petroleum ether 15 „
Amyline 14 „
Carbon chloride 8„
Benzene 4„
Liquid air 1·3 „
Vaseline oil 1·6 „
Liquid air, vaseline oil, petroleum ether, amyline, are normally nearly
perfect insulators. The conductivity of amyline and petroleum ether due to

the rays at -17° C. was only ⅒ of its value at 0° C. There is thus a marked
action of temperature on the conductivity. For very active material the
current was proportional to the voltage. With material of only ¹⁄₅₀₀ of the
activity, it was found that Ohm’s law was not obeyed.
The following numbers were obtained:
Volts Current
50 109
100 185
200 255
400 335
For an increase of voltage of 8 times, the current only increases about 3
times. The current in the liquid thus tends to become “saturated” as does the
ordinary ionization current through a gas. These results have an important
bearing on the ionization theory, and show that the radiation probably
produces ions in the liquid as well as in the gas. It was also found that X
rays increased the conductivity to about the same extent as the radium rays.
Becquerel[206] has recently shown that solid paraffin exposed to the β and γ
rays of radium acquires the property of conducting electricity to a slight
extent. After removal of the radium the conductivity diminishes with time
according to the same law as for an ionized gas. These results show that a
solid as well as a liquid and gaseous dielectric is ionized under the influence
of radium rays.

121. Effect of temperature on the radiations. Becquerel[207], by the
electric method, determined the activity of uranium at the temperature of
liquid air, and found that it did not differ more than 1 per cent. from the
activity at ordinary temperatures. In his experiments, the α rays from the
uranium were absorbed before reaching the testing vessel, and the electric
current measured was due to the β rays alone. P. Curie[208] found that the
luminosity of radium and its power of exciting fluorescence in bodies were
retained at the temperature of liquid air. Observations by the electric
method showed that the activity of radium was unaltered at the temperature
of liquid air. If a radium compound is heated in an open vessel, it is found

that the activity, measured by the α rays, falls to about 25 per cent. of its
original value. This is however not due to a change in the radio-activity, but
to the release of the radio-active emanation, which is stored in the radium.
No alteration is observed if the radium is heated in a closed vessel from
which none of the radio-active products are able to escape.

122. Motion of radium in an electric field. Joly[209] found that a disc, one
side of which is coated with a few milligrams of radium bromide, exhibits,
when an electrified body is brought near it, motions very different to those
observed in the case of an inactive substance. The electrified body, whether
positive or negative, repels the suspended body if brought up to it on the
side coated with radium, but attracts it if presented to the naked side.
This effect is very simply shown by constructing a small apparatus like a
radiometer. Two covered glasses are attached to the end of a glass fibre
about 6 cms. long, the surfaces lying in the same plane. The apparatus is
free to rotate on a pivot. The two vanes are coated on alternate faces with
radium bromide, and the whole apparatus contained within a glass receiver.
If an electrified rod of ebonite or sealing wax is brought up close to the
receiver, a rotation is communicated to the vane which increases as the
pressure of the air is lowered to 5 or 6 cms. of mercury. By placing the
apparatus between parallel plates connected with the terminals of a
Wimshurst machine, a steady rotation is communicated to the vanes. The
rotation is always in such a direction that the radium coated surface is
repelled from the electrified body.
This action was examined still further by attaching the vanes to the glass
beam of a Coulomb’s balance. A metal sphere, which could be charged from
without, was fixed facing the side coated with radium. A repulsion was
always observed except when the charge was very strong and the vane near
the sphere. If, however, the two vanes were connected by a light wire and a
similar sphere placed exactly opposite the other, an attraction was observed
if one sphere was charged, but a repulsion if both were charged. These
effects were observed whether the vanes were of aluminium or glass.
Joly found that the effect could not be explained by any direct action due to
the movement of the ions in an electric field. The recoil, due to the

expulsion of α particles from one side of the vane, is far too small to
account for the movement observed.
This effect can, I think, be simply accounted for by taking into
consideration the difference in conductivity of the gas on the two sides of
the radium coated vane. If a small vane, coated uniformly with radium on
both sides, and mounted on an insulating support, be brought near a charged
body kept at a constant potential, it acts like a water dropper and rapidly
acquires very nearly the average potential which existed at that point before
the vane was brought up. The mechanical force acting on the vane will, in
consequence, be small. If, however, the vane is only coated with radium on
the side near the charged body, the ionization and consequently the
conductivity of the gas is much greater between the vane and the charged
body than on the opposite side. Suppose, for simplicity, the body is charged
to a positive potential. On account of the greater conductivity of the gas on
the side facing the charged body, it will rapidly acquire a positive charge,
and the potential of the vane will reach a higher value than existed at that
place before the vane was introduced. This will result in a repulsion of the
vane. This also accounts for the attraction observed in the experiment with
the Coulomb’s balance already referred to. Suppose that one sphere is
positively charged and the other earthed, and the two vanes metallically
connected together. The vane next to the charged body will become charged
positively, but this charge will be dissipated rapidly on account of the
ionization of the gas close to the opposite vane, and, in most conditions, this
loss of charge will be so rapid that the potential of the vane is unable to
reach the value which would exist at that place in the field, if the vane were
removed. There will, in consequence, be an attracting force acting on the
vane towards the sphere.
The repulsion observed by Joly is thus only an indirect result of the
ionization in the gas produced by the radium, and should be shown under
conditions where similar unequal distribution of ionization is produced by
any other sources.
Since radium gives out heat at a fairly rapid rate, a radiometer in which the
vanes were coated on one side with radium instead of lampblack, should
rotate at low pressure of the gas, even if no source of light is brought near
it. This should evidently be the case, since the face coated with radium
should reach a slightly higher temperature than the other. This experiment

has been tried, but the effect seems too small to produce rotation of the
vanes.

Chemical actions.

123. Rays from active radium preparations change oxygen into ozone[210].
Its presence can be detected by the smell or by the action on iodide of
potassium paper. This effect is due to the α and β rays from the radium, and
not to the luminous rays from it. Since energy is required to produce ozone
from oxygen, this must be derived from the energy of the radiations.
The Curies found that radium compounds rapidly produced coloration in
glass. For moderately active material the colour is violet, for more active
material it is yellow. Long continued action blackens the glass, although the
glass may have no lead in its composition. This coloration gradually
extends through the glass, and is dependent to some extent on the kind of
glass used.
Giesel[211] found that he could obtain as much coloration in rock-salt and
fluor-spar by radium rays, as by exposure to the action of cathode rays in a
vacuum tube. The coloration, however, extended much deeper than that
produced by the cathode rays. This is to be expected, since the radium rays
have a higher velocity, and consequently greater penetrating power, than the
cathode rays produced in an ordinary vacuum tube. Goldstein observed that
the coloration is far more intense and rapid when the salts are melted or
heated to a red heat. Melted potassium sulphate, under the action of a very
active preparation of radium, was rapidly coloured a strong greenish blue
which gradually changed into a dark green. Salomonsen and Dreyer[212]
found that plates of quartz were coloured by exposure to radium rays. When
examined minutely, plates cut perpendicular to the optic axis showed the
presence of lines and striae, parallel to the binary axes. Adjacent portions of
the striated system differed considerably in intensity of coloration and
clearly revealed the heterogeneity of structures of the crystal.
The cause of these colorations by cathode and radium rays has been the
subject of much discussion. Elster and Geitel[213] observed that a specimen
of potassium sulphate, coloured green by radium rays, showed a strong
photo-electric action, i.e. it rapidly lost a negative charge of electricity when
exposed to the action of ultra-violet light. All substances coloured by

cathode rays show a strong photo-electric action, and, since the metals
sodium and potassium themselves show photo-electric action to a very
remarkable degree, Elster and Geitel have suggested that the colorations are
caused by a solid solution of the metal in the salt.
Although the coloration due to radium rays extends deeper than that due to
the cathode rays, when exposed to light the colour fades away at about the
same rate in the two cases.
Becquerel[214] found that white phosphorus is changed into the red variety
by the action of radium rays. This action was shown to be due mainly to the
β rays. The secondary radiation set up by the primary rays also produced a
marked effect. Radium rays, like ordinary light rays, also caused a
precipitate of calomel in the presence of oxalic acid.
Hardy and Miss Wilcock[215] found that a solution of iodoform in
chloroform turned purple after exposure for 5 minutes to the rays from 5
milligrams of radium bromide. This action is due to the liberation of iodine.
By testing the effect of screens of different thicknesses, over the radium,
this action was found to be mainly due to the β rays from the radium.
Röntgen rays produce a similar coloration.
Hardy[216] also observed an action of the radium rays on the coagulation of
globulin. Two solutions of globulin from ox serum were used, one made
electro-positive by adding acetic acid, and the other electro-negative by
adding ammonia. When the globulin was exposed close to the radium in
naked drops, the opalescence of the electro-positive solution rapidly
diminished, showing that the solution became more complete. The electro-
negative solution was rapidly turned to a jelly and became opaque. These
actions were found to be due to the α rays of radium alone.
This is further evidence in favour of the view that the α rays consist of
projected positively charged bodies of atomic dimensions, for a similar
coagulation effect is produced by the metallic ions of liquid electrolytes,
and has been shown by W. C. D. Whetham[217] to be due to the electric
charges carried by the ions.

124. Gases evolved from radium. Curie and Debierne[218] observed that
radium preparations placed in a vacuum tube continually lowered the

vacuum. The gas evolved was always accompanied by the emanation, but
no new lines were observed in its spectrum. Giesel[219] has observed a
similar evolution of gas from solutions of radium bromide. Giesel
forwarded some active material to Runge and Bödlander, in order that they
might test the gas spectroscopically. From 1 gram of a 5 per cent. radium
preparation they obtained 3·5 c.c. of gas in 16 days. This gas was found,
however, to be mainly hydrogen, with 12 per cent. of oxygen. In later
experiments Ramsay and Soddy[220] found that 50 milligrams of radium
bromide evolved gases at the rate of about 0·5 c.c. per day. This is a rate of
evolution about twice that observed by Runge and Bödlander. On analysing
the gases about 28·9 per cent. consisted of oxygen, and the rest hydrogen.
The slight excess of hydrogen over that attained in the decomposition of
water, they consider to be due to the action of oxygen on the grease of the
stop-cocks. The radio-active emanation from radium has a strong oxidizing
action and rapidly produces carbon dioxide, if carbonaceous matter is
present. The production of gas is probably due to the action of the radiations
in decomposing water. The amount of energy required to produce the rate of
decomposition observed by Ramsay and Soddy—about 10 c.c. per day for 1
gram of radium bromide—corresponds to about 30 gram-calories per day.
This amount of energy is about two per cent. of the total energy emitted in
the form of heat.
Ramsay and Soddy (loc. cit.) have also observed the presence of helium in
the gases evolved by solution of radium bromide. This important result is
considered in detail in section 267.

Physiological actions.

125. Walkhoff first observed that radium rays produce burns of much the
same character as those caused by Röntgen rays. Experiments in this
direction have been made by Giesel, Curie and Becquerel, and others, with
very similar results. There is at first a painful irritation, then inflammation
sets in, which lasts from 10 to 20 days. This effect is produced by all
preparations of radium, and appears to be due mainly to the α and β rays.
Care has to be taken in handling radium on account of the painful
inflammation set up by the rays. If a finger is held for some minutes at the
base of a capsule containing a radium preparation, the skin becomes
inflamed for about 15 days and then peels off. The painful feeling does not
disappear for two months.
Danysz[221] found that this action is mainly confined to the skin, and does
not extend to the underlying tissue. Caterpillars subjected to the action of
the rays lost their power of motion in several days and finally died.
Radium rays have been found beneficial in certain cases of cancer. The
effect is apparently similar to that produced by Röntgen rays, but the use of
radium possesses the great advantage that the radiating source can be
enclosed in a fine tube and introduced at the particular point at which the
action of the rays is required. The rays have also been found to hinder or
stop the development of microbes[222].
It would be out of place here to give an account of the numerous
experiments that have been made by physicists and physiologists on the
action of the rays of radium and of other radio-active substances on
different organisms, such as caterpillars, mice and guinea-pigs. In some
cases, the experiments have been carried out by placing the organisms in an
atmosphere impregnated with the radium emanation. The effect of an
exposure under such conditions for several days or weeks has been found
generally harmful and in many cases fatal. The literature in this new
department of study is already large and is increasing rapidly.

Another interesting action of the radium rays has been observed by Giesel.
On bringing up a radium preparation to the closed eye, in a dark room, a
sensation of diffuse light is observed. This effect has been examined by
Himstedt and Nagel[223] who have shown that it is due to a fluorescence
produced by the rays in the eye itself. The blind are able to perceive this
luminosity if the retina is intact, but not if the retina is diseased. Hardy and
Anderson[224] have examined this effect in some detail. The sensation of
light is produced both by the β and γ rays. The eyelid practically absorbs all
the β rays, so that the luminosity observed with a closed eye is due to the γ
rays alone. The lens and retina of the eye are strongly phosphorescent under
the action of the β and γ rays. Hardy and Anderson consider that the
luminosity observed in a dark room with the open eye (the phosphorescent
light of the radium itself being stopped by black paper) is to a large extent
due to the phosphorescence set up in the eyeball. The γ rays, for the most
part, produce the sensation of light when they strike the retina.
Tommasina stated that the air exhaled by man contained a larger proportion
of ions than ordinary air, and, in consequence, caused an increased rate of
discharge of an electroscope. The experiment was repeated by Elster and
Geitel but with negative results. On the other hand, they found that the
breath of Dr Giesel, of Braunschweig, who had been engaged continuously
in the chemical separation of the radio-active bodies, caused a rapid loss of
charge of an electroscope. This increased rate of discharge was probably
mainly due to the radium emanation, with which his system had become
impregnated by inhaling the emanation-laden air of the laboratory.

CHAPTER VI.
CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER.

126. An account will now be given of some experiments which have thrown
much light, not only on the nature of the processes which serve to maintain
the radio-activity of the radio-active bodies, but also on the source of the
energy continuously emitted by those bodies. In this chapter, for simplicity,
the radio-activity of uranium and thorium will alone be considered, for it
will be seen later that the changes taking place in these two substances are
typical of those which occur in all radio-active substances.
We have seen (section 23) that there is some doubt whether the radio-
activity of thorium is due to that element itself, or to an unknown radio-
active constituent associated with it. This uncertainty, however, will present
no serious difficulty when we are discussing the radio-activity of thorium,
for the general conclusions are, for the most part, independent of whether
thorium is the primary radio-active constituent or not. For simplicity,
however, it will be assumed for the present that the radio-activity is due to
thorium itself. If future research should definitely show that the radio-
activity, ordinarily observed in thorium, is due to a new radio-active
element mixed with it, the radio-active processes considered will refer to
this new element.

127. Uranium X. The experiments of Mme Curie show that the radio-
activity of uranium and radium is an atomic phenomenon. The activity of
any uranium compound depends only on the amount of that element
present, and is unaffected by its chemical combination with other
substances, and is not appreciably affected by wide variations of
temperature. It would thus seem probable, since the activity of uranium is a
specific property of the element, that the activity could not be separated
from it by chemical agencies.

In 1900, however, Sir William Crookes[225] showed that, by a single
chemical operation, uranium could be obtained photographically inactive
while the whole of the activity could be concentrated in a small residue free
from uranium. This residue, to which he gave the name of Ur X, was many
hundred times more active photographically, weight for weight, than the
uranium from which it had been separated. The method employed for this
separation was to precipitate a solution of the uranium with ammonium
carbonate. On dissolving the precipitate in an excess of the reagent, a light
precipitate remained behind. This was filtered, and constituted the Ur X.
The active substance Ur X was probably present in very small quantity,
mixed with impurities derived from the uranium. No new lines were
observed in its spectrum. A partial separation of the activity of uranium was
also effected by another method. Crystallized uranium nitrate was dissolved
in ether, when it was found that the uranium divided itself between the ether
and water present in two unequal fractions. The small part dissolved in the
water layer was found to contain practically all the activity when examined
by the photographic method, while the other fraction was almost inactive.
These results, taken by themselves, pointed very strongly to the conclusion
that the activity of uranium was not due to the element itself, but to some
other substance, associated with it, which had distinct chemical properties.
Results of a similar character were observed by Becquerel[226]. It was found
that barium could be made photographically very active by adding barium
chloride to the uranium solution and precipitating the barium as sulphate.
By a succession of precipitations the uranium was rendered
photographically almost inactive, while the barium was strongly active.
The inactive uranium and the active barium were laid aside; but, on
examining them a year later, it was found that the uranium had completely
regained its activity, while that of the barium had completely disappeared.
The loss of activity of uranium was thus only temporary in character.
In the above experiments, the activity of uranium was examined by the
photographic method. The photographic action produced by uranium is due
almost entirely to the β rays. The α rays, in comparison, have little if any
effect. Now the radiation from Ur X consists entirely of β rays, and is
consequently photographically very active. If the activity of uranium had
been measured electrically without any screen over it, the current observed
would have been due very largely to the α rays, and little change would

have been observed after the removal of Ur X, since only the constituent
responsible for the β rays was removed. This important point is discussed in
more detail in section 205.

128. Thorium X. Rutherford and Soddy[227], working with thorium
compounds, found that an intensely active constituent could be separated
from thorium by a single chemical operation. If ammonia is added to a
thorium solution, the thorium is precipitated, but a large amount of the
activity is left behind in the filtrate, which is chemically free from thorium.
This filtrate was evaporated to dryness, and the ammonium salts driven off
by ignition. A small residue was obtained which, weight for weight, was in
some cases several thousand times more active than the thorium from which
it was obtained, while the activity of the precipitated thorium was reduced
to less than one half of its original value. This active constituent was named
Th X from analogy to Crookes’ Ur X.
The active residue was found to consist mainly of impurities from the
thorium; the Th X could not be examined chemically, and probably was
present only in minute quantity. It was also found that an active constituent
could be partly separated from thorium oxide by shaking it with water for
some time. On filtering the water, and evaporating down, a very active
residue was obtained which was analogous in all respects to Th X.
On examining the products a month later, it was found that the Th X was no
longer active, while the thorium had completely regained its activity. A long
series of measurements was then undertaken to examine the time-rate of
these processes of decay and recovery of activity.

Fig. 47.

The results are shown graphically in Fig. 47, where the final activity of the
thorium and the initial activity of the Th X are in each case taken as 100.
The ordinates represent the activities determined by means of the ionization
current, and the abscissae represent the time in days. It will be observed that
both curves are irregular for the first two days. The activity of the Th X
increased at first, while the activity of the thorium diminished. Disregarding
these initial irregularities of the curves, which will be explained in detail in
section 208, it will be seen that, after the first two days, the time taken for
the thorium to recover half its lost activity is about equal to the time taken
by the Th X to lose half its activity. This time in each case is about four
days. The percentage proportion of the activity regained by the thorium,
over any given interval, is approximately equal to the percentage proportion
of the activity lost by the Th X during the same interval.

Fig. 48.

If the recovery curve is produced backwards to meet the vertical axis, it
does so at a minimum of 25 per cent., and the above conclusions hold more
accurately, if the recovery is assumed to start from this minimum. This is
clearly shown by Fig. 48, where the percentages of activity recovered,
reckoned from the 25 per cent. minimum, are plotted as ordinates. In the
same figure the decay curve, after the second day, is shown on the same
scale. The activity of the Th X decays with the time according to an
exponential law, falling to half value in about four days. If I₀ is the initial
activity and It is the activity after a time t, then

where λ is a constant and e the natural base of logarithms. The experimental
curve of the rise of activity from a minimum to a maximum value is
therefore expressed by the equation

where I₀ is the amount of activity recovered when the state of constant
activity is reached, It the activity recovered after a time t, and λ is the same
constant as before.

129. Uranium X. Similar results were obtained when uranium was
examined. The Ur X was separated by Becquerel’s method of successive
precipitations with barium. The decay of the separated activity and the
recovery of the lost activity are shown graphically in Fig. 49. A more
detailed discussion of this experiment is given in section 205.

Fig. 49.

The curves of decay and recovery exhibit the same peculiarities and can be
expressed by the same equations as in the case of thorium. The time-rate of
decay and recovery is, however, much slower than for thorium, the activity
of the Ur X falling to half its value in about 22 days.
A large number of results of a similar character have been obtained from
other radio-active products, separated from the radio-elements, but the cases

of thorium and uranium will suffice for the present to form a basis for the
discussion of the processes that are taking place in radio-active bodies.

130. Theory of the phenomena. These processes of decay and recovery go
on at exactly the same rate if the substances are removed from the
neighbourhood of one another, or enclosed in lead, or placed in a vacuum
tube. It is at first sight a remarkable phenomenon that the processes of
decay and recovery should be so intimately connected, although there is no
possibility of mutual interaction between them. These results, however,
receive a complete explanation on the following hypotheses:

(1) That there is a constant rate of production of fresh radio-active
matter by the radio-active body;
(2) That the activity of the matter so formed decreases according to an
exponential law with the time from the moment of its formation.

Suppose that q₀ particles of new matter are produced per second from a
given mass of matter. The rate of emission of energy due to the particles
produced in the time dt, is, at the moment of their formation, equal to Kq₀dt,
where K is a constant.
It is required to find the activity due to the whole matter produced after the
process has continued for a time T.
The activity dI, due to the matter produced during the time dt at the time t,
decays according to an exponential law during the time T – t that elapses
before its activity is estimated, and in consequence is given by

where λ is the constant of decay of activity of the active matter. The activity
IT due to the whole matter produced in the time T is thus given by

The activity reaches a maximum value I₀ when T is very great, and is then
given by
Kq₀
I₀ = ----
λ

thus

This equation agrees with the experimental results for the recovery of lost
activity. Another method for obtaining this equation is given later in section
133.
A state of equilibrium is reached when the rate of loss of activity of the
matter already produced is balanced by the activity supplied by the
production of new active matter. According to this view, the radio-active
bodies are undergoing change, but the activity remains constant owing to
the action of two opposing processes. Now, if this active matter can at any
time be separated from the substance in which it is produced, the decay of
its activity, as a whole, should follow an exponential law with the time,
since each portion of the matter decreases in activity according to an
exponential law with the time, whatever its age may be. If I₀ is the initial
activity of the separated product, the activity It after an interval t is given by

Thus, the two assumptions—of uniform production of active matter and of
the decay of its activity in an exponential law from the moment of its
formation—satisfactorily explain the relation between the curves of decay
and recovery of activity.

131. Experimental evidence. It now remains to consider further
experimental evidence in support of these hypotheses. The primary
conception is that the radio-active bodies are able to produce from
themselves matter of chemical properties different from those of the parent
substance, and that this process goes on at a constant rate. This new matter
initially possesses the property of activity, and loses it according to a
definite law. The fact that a proportion of the activity of radium and thorium
can be concentrated in small amounts of active matter like Th X or Ur X
does not, of itself, prove directly that a material constituent responsible for
the activity has been chemically separated. For example, in the case of the
separation of Th X from thorium, it might be supposed that the non-thorium
part of the solution is rendered temporarily active by its association with
thorium, and that this property is retained through the processes of
precipitation, evaporation, and ignition, and finally manifests itself in the
residue remaining. According to this view it is to be expected that any
precipitate capable of removing the thorium completely from its solution
should yield active residues similar to those obtained from ammonia. No
such case has, however, been observed. For example, when thorium nitrate
is precipitated by sodium or ammonium carbonate, the residue from the
filtrate after evaporation and ignition is free from activity and the thorium
carbonate obtained has the normal amount of activity. In fact, ammonia is
the only reagent yet found capable of completely separating Th X from
thorium. A partial separation of the Th X can be made by shaking thorium
oxide with water owing to the greater solubility of Th X in water.

Thorium and uranium behave quite differently with regard to the action of
ammonia and ammonium carbonate. Ur X is completely precipitated with
the uranium in an ammonia solution and the filtrate is inactive. Ur X is
separated by ammonium carbonate, while Th X under the same conditions
is completely precipitated with the thorium. The Ur X and the Th X thus
behave like distinct types of matter with well-marked chemical properties
quite distinct from those of the substances in which they are produced. The
removal of Ur X by the precipitation of barium is probably not directly
connected with the chemical properties of Ur X. The separation is probably
due to the dragging down of the Ur X with the dense barium precipitate. Sir
William Crookes found that the Ur X was dragged down by precipitates
when no question of insolubility was involved, and such a result is to be
expected if the Ur X exists in extremely minute quantity. It must be borne in
mind that the actual amount of the active constituents Th X and Ur X,
separated from thorium and uranium, is probably infinitesimal, and that the
greater proportion of the residues is due to impurities present in the salt and
the reagents, a very small amount of active matter being mixed with them.

132. Rate of production of Th X. If the recovery of the activity of uranium
or thorium is due to the continuous production of new active matter, it
should be possible to obtain experimental evidence of the process. As the
case of thorium has been most fully investigated, a brief account will be
given of some experiments made by Rutherford and Soddy[228] to show that
Th X is produced continuously at a constant rate. Preliminary experiments
showed that three successive precipitations were sufficient to remove the Th
X almost completely from the thorium. The general method employed was
to precipitate a solution of 5 grams of thorium-nitrate with ammonia. The
precipitate was then redissolved in nitric acid and the thorium again
precipitated as before, as rapidly as possible, so that the Th X produced in
the time between successive precipitations should not appreciably affect the
results. The removal of the Th X was followed by measurements of the
activity of the residues obtained from successive filtrates. In three
successive precipitations the activities of the residues were proportional to
100, 8, 1·6 respectively. Thus two precipitations are nearly sufficient to free
the thorium from Th X.

The thorium freed from Th X was then allowed to stand for a definite time,
and the amount of Th X formed during that time found by precipitating it,
and measuring its radio-activity. According to the theory, the activity It of
the thorium formed in the time t is given by

where I₀ is the total activity of Th X, when there is radio-active equilibrium.
If λt is small,
It
---- = λt.
I₀

Since the activity of Th X falls to half value in 4 days, the value of λ
expressed in hours = ·0072. After standing a period of 1 hour about ¹⁄₁₄₀,
after 1 day ⅙ , after 4 days ½ of the maximum should be obtained. The
experimental results obtained showed an agreement, as good as could be
expected, with the equation expressing the result that the Th X was being
produced at a constant rate.
The thorium-nitrate which had been freed from Th X was allowed to stand
for one month, and then it was again subjected to the same process. The
activity of the Th X was found to be the same as that obtained from an
equal amount of the original thorium-nitrate. In one month, therefore, the
Th X had been regenerated, and had reached a maximum value. By leaving
the thorium time to recover fully its activity, this process can be repeated
indefinitely, and equal amounts of Th X are obtained at each precipitation.
Ordinary commercial thorium-nitrate and the purest nitrate obtainable
showed exactly the same action, and equal amounts of Th X could be
obtained from equal weights. These processes thus appear to be
independent of the chemical purity of the substance[229].

The process of the production of Th X is continuous, and no alteration has
been observed in the amount produced in the given time after repeated
separations. After 23 precipitations extending over 9 days, the amount
produced in a given interval was about the same as at the beginning of the
process.
These results are all in agreement with the view that the Th X is being
continuously produced from the thorium compound at a constant rate. The
amount of active matter produced from 1 gram of thorium is probably
extremely minute, but the electrical effects due to its activity are so large
that the process of production can be followed after extremely short
intervals. With a sensitive electrometer the amount of Th X produced per
minute in 10 grams of thorium-nitrate gives a rapid movement to the
electrometer needle. For larger intervals it is necessary to add additional
capacity to the system to bring the effects within range of the instrument.

133. Rate of decay of activity. It has been shown that the activity of Ur X
and Th X decays according to an exponential law with the time. This, we
shall see later, is the general law of decay of activity in any type of active
matter, obtained by itself, and freed from any secondary active products
which it may, itself, produce. In any case, when this law is not fulfilled, it
can be shown that the activity is due to the superposition of two or more
effects, each of which decays in an exponential law with the time. The
physical interpretation of this law still remains to be discussed.
It has been shown that in uranium and thorium compounds there is a
continuous production of active matter which keeps the compound in radio-
active equilibrium. The changes by which the active matter is produced
must be chemical in nature, since the products of the action are different in
chemical properties from the matter in which the changes take place. The
activity of the products has afforded the means of following the changes
occurring in them. It now remains to consider the connection between the
activity at any time, and the amount of chemical change taking place at that
time.
In the first place, it is found experimentally that the saturation ionization
current it, after the active product has been allowed to decay for a time t, is
given by

where i₀ is the initial saturation current and λ the constant of decay.
Now the saturation current is a measure of the total number of ions
produced per second in the testing vessel. It has already been shown that the
α rays, which produce the greater proportion of ionization in the gas, consist
of positively charged particles projected with great velocity. Suppose for
simplicity that each atom of active matter, in the course of its change, gives
rise to one projected α particle. Each α particle will produce a certain
average number of ions in its path before it strikes the boundaries or is
absorbed in the gas. Since the number of projected particles per second is
equal to the number of atoms changing per second, the number of atoms nt
which change per second at the time t is given by

where n₀ is the initial number which change per second. On this view, then,
the law of decay expresses the result that the number of atoms changing in
unit time, diminishes according to an exponential law with the time. The
number of atoms Nt which remain unchanged after an interval t is given by

If N₀ is the number of atoms at the beginning,

Thus

or the law of decay expresses the fact that the activity of a product at any
time is proportional to the number of atoms which remain unchanged at
that time.
This is the same as the law of monomolecular change in chemistry, and
expresses the fact that there is only one changing system. If the change
depended on the mutual action of two systems, the law of decay would be
different, since the rate of decay in that case would depend on the relative
concentration of the two reacting substances. This is not so, for not a single
case has yet been observed in which the law of decay was affected by the
amount of active matter present.
From the above equation (1)
dNt
---- = -λNt,
dt

or the number of systems changing in unit time is proportional to the
number unchanged at that time.
In the case of recovery of activity, after an active product has been
removed, the number of systems changing in unit time, when radio-active
equilibrium is produced, is equal to λN₀. This must be equal to the number
q₀ of new systems applied in unit time, or

q₀ = λN₀,

q₀
and λ = ----- ;
N₀

λ has thus a distinct physical meaning, and may be defined as the proportion
of the total number of systems present which change per second. It has
different values for different types of active matter, but is invariable for any
particular type of matter. For this reason, λ will be termed the “radio-active
constant„ of the product.
We are now in a position to discuss with more physical definiteness the
gradual growth of Th X in thorium, after the Th X has been completely
removed from it. Let q₀ particles of Th X be produced per second by the
thorium, and let N be the number of particles of Th X present at any time t
after the original Th X was removed. The number of particles of Th X
which change every second is λN, where λ is the radio-active constant of Th
X. Now, at any time during the process of recovery, the rate of increase of
the number of particles of Th X = the rate of production – the rate of
change; that is
dN
---- = q₀ – λN.
dt

The solution of this equation is of the form

where a and b are constants.
Now when t is very great, the number of particles of Th X present reach a
maximum value N₀.
Thus, since N = N₀ when t = infinity,
b = N₀;

since N = 0 when t = 0,

a + b = 0;
hence b = -a = N₀,

and the equation becomes

This is equivalent to the equation already obtained in section 130, since the
intensity of the radiation is always proportional to the number of particles
present.

134. Influence of conditions on the rate of decay. Since the activity of any
product, at any time, may be taken as a measure of the rate at which
chemical change takes place, it may be used as a means of determining the
effect of conditions on the changes occurring in radio-active matter. If the
rate of change should be accelerated or retarded, it is to be expected that the
value of the radio-active constant λ will be increased or decreased, i.e. that
the decay curve will be different under different conditions.
No such effect, however, has yet been observed in any case of radio-active
change, where none of the active products produced are allowed to escape
from the system. The rate of decay is unaltered by any chemical or physical
agency, and in this respect the changes in radio-active matter are sharply
distinguished from ordinary chemical changes. For example, the rate of
decay of activity from any product takes place at the same rate when the
substance is exposed to light as when it is kept in the dark, and at the same
rate in a vacuum as in air or any other gas at atmospheric pressure. Its rate
of decay is unaltered by surrounding the active matter by a thick layer of
lead under conditions where no ordinary radiation from outside can affect it.
The activity of the matter is unaffected by ignition or chemical treatment.
The material giving rise to the activity can be dissolved in acid and re-
obtained by evaporation of the solution without altering the activity. The
rate of decay is the same whether the active matter is retained in the solid

state or kept in solution. When a product has lost its activity, resolution or
heat does not regenerate it, and as we shall see later, the rate of decay of the
active products, so far examined, is the same at a red heat as at the
temperature of liquid air. In fact, no variation of physical or chemical
conditions has led to any observable difference in the decay of activity of
any of the numerous types of active matter which have been examined.

135. Effect of conditions on the rate of recovery of activity. The recovery
of the activity of a radio-element with time, when an active product is
separated from it, is governed by the rate of production of fresh active
matter and by the decay of activity of that already produced. Since the rate
of decay of the activity of the separated product is independent of
conditions, the rate of recovery of activity can be modified only by a
change of the rate of production of fresh active matter. As far as
experiments have gone, the rate of production, like the rate of decay, is
independent of chemical or physical conditions. There are indeed certain
cases which are apparent exceptions to this rule. For example, the escape of
the radio-active emanations from thorium and radium is readily affected by
heat, moisture and solution. A more thorough investigation, however, shows
that the exception is only apparent and not real. These cases will be
discussed more in detail in chapter VII, but it may be stated here that the
differences observed are due to differences in the rate of escape of the
emanations into the surrounding gas, and not to differences in the rate of
production. For this reason it is difficult to test the question at issue in the
case of the thorium compounds, which in most cases readily allow the
emanation produced by them to escape into the air.
In order to show that the rate of production is independent of molecular
state, temperature, etc., it is necessary in such a case to undertake a long
series of measurements extending over the whole time of recovery. It is
impossible to make accurate relative comparisons to see if the activity is
altered by the conversion of one compound into another. The relative
activity in such a case, when measured by spreading a definite weight of
material uniformly on a metal plate, varies greatly with the physical
conditions of the precipitate, although the total activity of two compounds
may be the same.