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CAMBRIDGE PHYSICAL SERIES.

GENERAL EDITORS: — F. H. NEVILLE, M.A., F.R.S.
AND W. C. D. WHETHAM, M.A., F.R.S.

EADIO-ACTIVITY

C. J. CLAY AND SONS,
CAMBRIDGE UNIVEKSITY PEESS WAREHOUSE,
AVE MAEIA LANE.

H. K. LEWIS,
136, GOWEE STEEET, W.C.

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$efo lorfe: THE MACMILLAN COMPANY.

Bombag anti Calcutta: MACMILLAN AND CO., LTD.

RADIO-ACTIVITY

E. RUTHERFORD, D.Sc., F.R.S., F.R.S.C.

MACDONALD PROFESSOR OF PHYSICS, MCGILL UNIVERSITY, MONTREAL.

CAMBRIDGE

AT THE UNIVERSITY PRESS
1904

CambriUge :

PRINTED BY J. AND C. F. CLAY,
AT THE UNIVERSITY PRESS.

SENERAL

J. J. THOMSON

A TKIBUTE OF MY KESPECT AND ADMIRATION

90ft (I

<-W .*» \J S* V/

PREFACE.

TX 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 pro-
perties 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 experi-
mental 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

vili PREFACE

radio-active processes, and also allows us to determine with con-
siderable 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 w_hole 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.

MACDONALD PHYSICS BUILDINGS,
MONTREAL,

February, 1904.

TABLE OF CONTENTS.

CHAP.

PAGE

Radio- active Substances

II.

lonization Theory of Gases .......

III.

Methods of Measurement

IV.

Nature of the Radiations .......

Rate of Emission of Energy ..'!....

149

VI.

Properties of the Radiations

166

VII.

Continuous Production of Radio-active Matter .

178

VIII.

Radio-active Emanations

197

IX.

Excited Radio-activity

250

Radio-active Processes ........

293

XI.

Radio-activity of the Atmosphere and of Ordinary Materials

351

Index ........

383

Plate (Fig. 33) to face p. 169

ABBREVIATIONS OF REFERENCES TO SOME OF
THE JOURNALS.

Ber. d. deutsch. Chem. Ges. Berichte der deutschen chernischen Gesell-
schaft. Berlin.

C. R. Comptes Rendus des Seances de F Academic des Sciences. Paris.
Chem. News. Chemical News. London.
Drude's Annal. Annalen der Physik. Leipzig.
^9 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. Z&it. Physikalische Zeitschrift.

Proc. Camb. Phil. Soc. Proceedings of the Cambridge Philosophical Society.
Cambridge.

Proc. Roy. Soc. Proceedings of the Royal Society of London.

Theses-Paris. Theses presentees & la Faculte des Sciences de I'Universit^
de Paris.

Wied. Annal. Annalen der Physik. Leipzig.

ERRATA.

page 10, line 16; for "chapter ix," read "section 217."

page 274, last line ; for " 36 minutes," read " 21 minutes."

page 326, Radium, Second change, for "36 minutes," read "21 minutes.'"

CHAPTEK I.

RADIO-ACTIVE SUBSTANCES.

1. Introduction. The close of the old and the beginning v
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 has been
examined, the more complex does the constitution of matter appear
which can give rise to 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
strongly 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 tof this subject was initially
given by the experiments of Lenard on the cathode rays, and
by Rontgen'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
R. R.-A. 1

2 KADIO-ACTIVE SUBSTANCES [CH.

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 Rontgen 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
led to the discovery of 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 know-
ledge 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, a theory
has been put forward 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 previously not
been observed, on account of the impossibility of breaking up into

l] RADIO-ACTIVE SUBSTANCES 3

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. If the production of helium by radium
is completely substantiated, the further study of radio-active bodies
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.

1—2

4 RADIO-ACTIVE SUBSTANCES [CH.

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 Rontgen 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 spontaneously and continuously radiating energy 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 Rontgen's discovery of the production of
X rays, several scientists 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 in some way connected 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 obser-
vation in this direction was made by Niewenglowski1, 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. Becquerel2 for a special
calcium sulphide preparation, and by Troost3 for a specimen of
hexagonal blend. These results were confirmed and extended in
a later paper by Arnold4. 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

1 C. R. 122, p. 385, 1896. 2 C. R. 122, p. 559, 1896.

3 C. R. 122, p. 564, 1896. 4 Wied. Annal. 61, p. 316, 1897.

I] RADIO-ACTIVE SUBSTANCES 5

waves. At the same time Le Bon1 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
of quite a distinct 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 Becquerel2,
who found that a maiiiiLm-~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 par-
ticular 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 is kept

1 C. R. 122, pp. 188, 233, 386, 462. 1896.

2 C. R. 122, pp. 420, 501, 559, 689, 762, 1086. 1896.

6 KADIO-ACTIVE SUBSTANCES [CH.

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 Curie1 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 Rontgen rays, possess the

1 These presentee a la Faculte des Sciences de Paris, 1903.

I] RADIO-ACTIVE SUBSTANCES 7

important property of discharging both positively and negatively
electrified bodies. These results were confirmed and extended by
Lord Kelvin, Smolan and Beattie1. The writer made a detailed
comparison2 of the nature of the discharge produced by uranium
with that produced by Rontgen rays, and showed that the dis-
charging 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 n.

The radiations from uranium are thus analogous, as regards
their photographic and electrical actions, to Rontgen rays, but,
compared with the rays from an ordinary X ray tube, these
actions are extremely feeble. While with Rontgen 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 re-
flection, refraction, or polarization3. While there is no direct
reflection of the rays, there is apparently a diffuse reflection set
up where the rays strike a solid obstacle. This is in reality due
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, re-
fraction, 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

1 Nature, 56, 1897 ; Phil. Mag. 43, p. 418, 1897 ; 45, p. 277, 1898.

2 Phil. Mag. Jan. 1899.

3 Rutherford, Phil. Mag. Jan. 1899.

8 KADIO-ACTIVE SUBSTANCES [CH.

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 be
readily 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
when 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.

I] RADIO-ACTIVE SUBSTANCES 9

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 Bon1 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 com-
pound 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 Lenard2 has
shown, short waves of ultra-violet light will ionize the gas in their
path, and will be rapidly absorbed 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

1 C. R. 130, p. 891, 1900.

2 Dnide's AnnaL 1, p. 498 ; 3, p. 298, 1900.

10 KADIO- ACTIVE SUBSTANCES [CH.

of the compound, or on temperature and other physical conditions.
But the great point of distinction lies in the nature of the radia-
tions 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 radiations ; 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

10. Thorium. In the course of an examination of a large
number of substances, Schmidt1 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 Curie2. 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 Owens3. 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 ceuld be greatly

1 Wied. Annal. 65, p. 141, 1898.* 2 C. E. 126, p. 1101, 1898.

3 Phil. Mag. Oct. 1899.

I] RADIO- ACTIVE SUBSTANCES 11

varied by blowing a current of air over the gas. In the course of
an examination1 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 vm. 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 x 10 ~n

„ Joachimstahl ... 7'0 „

„ Pzibran 6'5 „

„ Cornwall ... ... 1*6 „

Clevite 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 O7 „

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

i Phil. Mag. Jan. 1900.

12 RADIO-ACTIVE SUBSTANCES [CH.

same apparatus and under the same conditions led to the following

results :

Uranium (containing a little carbon) 2*3 x 10 ~ n 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 O9 „

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, Mine 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

I] KADIO-ACTIVE SUBSTANCES 13

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 nam^ejxjp-
nium was given to the first substance discovered by Mme Curie
in honour of theTcountry 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 same process
necessary to separate barium, to which it is very closely allied in
chemical properties1. 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 almost completely freed
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

1 M. and Mme Curie and G. Bemont, C. E. 127, p. 1215, 1898.

14 RADIO-ACTIVE SUBSTANCES [CH.

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 Joachimstahl in Bohemia. With the
assistance of the Academy of Sciences 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 Societe Centrale de Produits
Chimiques of Paris. The generous assistance afforded in thi&
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 the activity 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 al^out
one-fourth of its final value; it gradually rises to a maximum after

l] RADIO-ACTIVE SUBSTANCES 15

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 radi-
ferous barium as a means of controlling the final process of puri-
ficatiori. The crystals of salts of radium and barium deposited from
acid solutions are indistinguishable. 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.

Giesel1 has observed that pure radium bromide gives a beautiful
carmine colour to the Bunsen flame. If barium is 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, Giesel2 has devoted a great deal of attention to the
separation of radium, polonium and other active bodies from pitch-
blende. He was indebted for his working material to the firm
of P. de Haen of Hanover, who presented him with a ton of pitch-
blende 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
almost completely free the radium 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,

1 Phys. Zeit. 3, No. 24, p. 578, 1902.

2 Wied. Annal. 69, p. 91, 1890. Beri-chte d. d. chem. Ges. p. 3608, 1902.

16 RADIO-ACTIVE SUBSTANCES [CH.

and submitted them for examination of their spectrum to
Demar9ay, an authority on that subject. The first specimen of
radium chloride examined by Demar^ay1 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 Angstrom units and the intensity of each ray is denoted by a
number, the ray of maximum intensity being 16.

Wave length Intensity Wave length Intensity

4826-3 10 4600-3 3

4726-9 5 4533-5 9

4099-6 3 4436-1 6

4692-1 7 4340-6 12

4683-0 14 38147 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 48 2 6 '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.

1 C. R. 127, p. 1218, 1898 ; 129, p. 716, 1899; 131, p. 258, 1900.

l] RADIO-ACTIVE SUBSTANCES 17

Later observations on the spectrum of radium have been made by
Runge1, Exner and Haschek2, with specimens of radium prepared by
Giesel. It has already been mentioned 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 Demarcay'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 obser-
vation 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, wrhich 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 has
been 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 treatment of about 2 tons
of the mineral from which it is derived.

Runge and Precht8 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 Watts4, on the other hand, using another
relation between the lines of the spectrum5, deduced the value
obtained by Mme Curie. Considering that the number found

1 Astrophys. Journal, p. 1, 1900. Drude's Annal. No. 10, p. 407, 1903.
- Sitz. Ak. TH**. Wien, July 4, 1901. 3 Phil Mag. April, 1903.

4 Phil Mag. July, 1903.

5 Runge (Phil Mag. Dec. 1903) has criticised the method of deduction em-
ployed by Marshall Watts on the ground that the lines used for comparison in the
•different spectra were not homologous.

R. R.-A. 2

18 RADIO-ACTIVE SUBSTANCES [CH.

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 con-
sidering, and is the first notable triumph of the study of radio-
activity. As we shall see later in chapter vn, 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 is almost instantly
discharged, 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 the 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 centi-
grams 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 radio-
graphs 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

l] RADIO-ACTIVE SUBSTANCES 19

absorption varying approximately as the density. In photographs
of the hand the bones do not show out as in X ray photographs.

Curie and Laborde have shown that the compounds of radium
possess the remarkable property of always keeping their tempe-
rature several degrees above the temperature of the surrounding
air. Each gram of radium radiates an amount of energy corre-
sponding to 100 gram-calories per hour. This and other properties
of radium are discussed in detail in chapters v and vi.

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 prac-
tically the same as those of barium, with the exception that
the chloride and bromide of radium are less soluble than the
corresponding salts of barium. All the salts of radium are natu-
rally 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. Polonium. Polonium was the first of the active sub-
stances obtained from pitchblende. It has been investigated in
detail by its discoverer Mme Curie1. 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^segaration 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 portions of the tube, where the temperature is between 250°
1 C. R. 127, p. 175, 1898.

2 2

20 RADIO-ACTIVE SUBSTANCES [CH.

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 Curie1 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 exami-
nation of the substance spectroscopically, only the bismuth lines
were observed. A spectroscopic examination of the active bismuth
by Demarcay and by Runge and Exner has led to the discovery
of no new lines. On the other hand Sir William Crookes2 states that
he found one new line in the ultra-violet, while Berndt3, 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 found that the polonium lost half its original activity
in the course of eleven months.

18. The decay of the activity of polonium with time has led
to the view that polonium is not a new active substance, but
merely active bismuth, i.e. bismuth which in some way had been
made active by admixture with radio-active bodies.

The activity of any product is not necessarily a proof that
a radio-element is present, for it has been shown that many
inactive elements become active by association with active matter.

1 Theses, Paris, 1903. 2 Proc. Roy. Soc. May, 1900.

3 Phys. Zelt. 2, p. 180, 1900.

I] RADIO-ACTIVE SUBSTANCES 21

The activity of these substances, when removed from the active
element, is however only transient, and decays gradually with the
time. This activity is not due to the presence of the radio-element
itself. For example, barium separated from radium is strongly
active, although the spectroscopic examination shows no trace
of the radium lines.

In order to explain this temporary activity in inactive matter
it has been supposed that the non-active matter is made active by
"induction" during its mixture with the active material. The
underlying idea has been that inactive bodies themselves acquire
the property of radio-activity. There is no evidence however that
such is the case. The evidence rather points to the conclusion
that the activity is due, not to any alteration of the inactive body
itself, but to an admixture with it of a very small quantity of
intensely active matter. The active matter that causes this so-
called "induced" activity is itself a product of the disintegration of
the radio-element and differs from it in chemical properties.

The subject is a complicated one, and it cannot be discussed with
advantage at this stage ; it will, however, be considered in detail
in section 187. On the above view the active bismuth contains
a small quantity of matter, which weight for weight is probably
far more active than radium, but the activity of which decays
with time. The active matter is allied in chemical properties to
bismuth, but possesses some distinct analytical properties which
allow of a partial separation. The absence of any new lines in the
spectrum is to be expected if, even in the most active bismuth
prepared, the active matter exists in very small quantity.

19. The discussion of the nature of polonium was renewed by
the discovery of Marckwald1 that a substance similar to polonium,
of which the activity did not decay with time, could be separated
from pitchblende. The method of separation from the bismuth
chloride solution obtained from uranium residues was very simple.
A rod of bismuth, dipped in the active solution, rapidly became
coated with a black deposit, which was intensely active. This
deposit was continued until the whole of the activity was removed
from the solution. From 850 grammes of bismuth solution,

1 Eer. deutsch. chem. Ges., p. 2285, 1902 ; Phys. Zeit., No. 1 b, p. 51, 1902.

22 RADIO-ACTIVE SUBSTANCES [CH.

0'6 gramme of active substance was obtained in this way. The
activity of the matter obtained did not decay appreciably during
nine months. A full chemical examination of this active matter has
not yet been made, but Marckwald considers that the substance is
more allied in chemical properties to tellurium than to bismuth.

The radiations from Marckwald's substance are similar to those
of polonium, for no penetrating rays are present. The radiations
are very intense. They have a marked photographic action, and
cause many substances, like zinc oxide and the diamond, to
phosphoresce brightly. The strong luminosity of the diamond
under these rays can be utilized to distinguish the diamond from
imitations, for glass is only slightly phosphorescent in comparison.

The identity of Marckwald's preparation with the polonium of
the Curies has not yet been settled, but from the method of pro-
duction and the nature of the radiations, there can be little doubt
that the two substances probably contain the same active constituent.
Marckwald, on the other hand, states that his preparations have
preserved their activity unchanged, while the polonium of the
Curies undoubtedly loses its activity in the course of a few years.

If Marckwald's preparation retains its activity unchanged for a
long period, it is strong evidence in support of the presence of
a new radio-element. If the activity decays, the radio-tellurium
probably consists of the admixture with the tellurium of a small
quantity of active matter, produced from one of the radio-elements
present in pitchblende. A possible origin of polonium is discussed
in section 188.

20. Other products from radio-active minerals. Besides
the very active substances radium and possibly polonium, it seems
extremely probable that other radio-active elements of great activity
exist in minute quantity in the radio-active minerals. Although
many active products have been obtained by treatment of uranium
residues from pitchblende and other minerals rich in uranium and
thorium, none of these products have so far been sufficiently purified
to obtain a definite spectrum as in the case of radium.

Actinium. Debierne1 has obtained from pitchblende a very
active substance which he named actinium. This active substance

1 G. R. 129, p. 593, 1899 ; 130, p. 906, 1900.

I] RADIO-ACTIVE SUBSTANCES 23

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 so far not been carried far enough to bring out
any new lines in the spectrum. Actinium gives out easily ab-
sorbed and penetrating deviable rays like those of radium,
and also a radio-active emanation1, which is more allied to the
emanation of thorium than to that of radium. The emanation
has a distinctive rate of decay ; it loses its activity in the course
of a few seconds, while the thorium emanation loses half its activity
in one minute. The distinctive character of the radiations and
emanations, together with the permanence of the activity, make
it very probable that actinium will prove to be a new element
of very great activity.

21. Giesel2 also has obtained from pitchblende a radio-active
substance which in many respects is similar to the actinium of
Debierne. The active substance belongs to the group of cerium

1 C. It. 136, p. 446, 1903.

- Ber. deutsch. chem. Ges. p. 3608, 1902 ; p. 342, 1903.

24 RADIO-ACTIVE SUBSTANCES [CH.

earths, and is precipitated with them. The method of preparation
of this material is the same as that employed for the separation
of the rare earths. This substance is similar in radio-active be-
haviour to thorium, but intensely active in comparison. From
the method of separation, thorium itself cannot be present except
in minute quantity.

The substance gives out easily absorbed and penetrating rays
and also an emanation. On account of the intensity of the
emanation which it emits, Giesel has termed this active material
the " emanating substance."

If a piece of paper is placed in a small closed vessel containing
the active material, in a short time the paper itself becomes power-
fully active. This is especially the case if it is moistened with water.
The emanation lights up a zinc sulphide screen. An electric field
has a marked action on the luminosity of the screen. The action
is discussed in more detail in section 186.

Giesel found that the activity of the material seemed to increase
slightly during the six months' interval after separation. In this
respect it is similar to radium compounds, of which the activity
increases for a time after separation.

Both the method of preparation and also the radiating properties
of this " emanating substance " indicate that it is the same as the
actinium of Debierne. Neither of these active substances has
been studied in the same detail as uranium, thorium, or radium,
and further comparative data on the nature of the radiations and
emanations are necessary before any definite conclusion can be
reached. The distinctive character of the radiations and ema-
nations is of far more value in establishing the dissimilarity of
two active bodies than differences in their chemical behaviour.
This is especially the case where the active substance is present
only in small quantity in inactive material.

22. 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 activity of the emanations of thorium and actinium,
this position is not tenable. If the activity of thorium were due

l] RADIO-ACTlVE SUBSTANCES 25

to actinium, the two emanations should have identical rates of
decay.

Baskerville1, working with thorium minerals, was able to obtain
thorium less active to the photographic plate than ordinary
thorium. He put forward the view that thorium was a mixture
of two elements, one of which was active and the other inactive.
These results were probably due to the separation of the active
product Th. X from the thorium (see section 119). This process
would temporarily greatly reduce the activity as tested by the
photographic method.

Until thorium is obtained permanently free from activity, the
question whether the radio-activity is due to a small trace of very
active matter, or to the thorium itself, must remain in doubt2. The
fact that ordinary commercial thorium and the purest chemical
preparation show equal activity supports the view that the effect
is not due to a radio-active impurity, but to the element itself.
If the activity of thorium is due to a small trace of active matter,
this active substance is certainly not radium or actinium or any
other known material.

Hofmann and Zerban3 obtained a substance from pitchblende
similar in radio-active properties to thorium. The activity of this
product did not dimmish much in four months' interval. The
substance was probably the same as Debierne's actinium. They
also examined the thorium minerals broggerite and clevite, but
obtained only some active residues the activity of which decreased
rapidly with the time.

23. Radio-active lead. Elster and Geitel4 found that lead
sulphate obtained from pitchblende was very active. They con-
sidered that the activity was due to admixture with radium, and
by suitable treatment the lead sulphate was obtained in an inactive
state.

1 Jour. Amer. Chem. Soc. 23, p. 761, 1901.

2 In a recent paper (Ber. deutsch. chem. Ges. p. 3093, 1903) Hofmann and
Zerban state that they have obtained a preparation of thorium from gadolinite
which was almost inactive when tested by the electric method and conclude that
pure thorium is not radio-active.

3 Ber. deutsch. chem. Ges. p. 531, 1902.

4 Wied. Annal. 69, p. 83, 1899.

26 KADIO-ACTIVE SUBSTANCES [CH.

Hofmann and Strauss1 found that lead sulphate obtained from
pitchblende was active. This was not due to admixture with either
uranium or radium or polonium. They gave the name of radio-
active lead to the substance. This radio-active lead, in most of
its reactions, resembled ordinary lead, but showed differences in
the behaviour of the sulphide and the sulphate. The sulphate was
very strongly phosphorescent. This sulphate apparently lost its
activity with time, but recovered it in a few minutes after exposure
to cathode rays in a vacuum tube.

Giesel2 also was able to obtain radio-active lead, but found that
the activity diminished with time, while Hofmann states that his
preparations preserve their activity. It thus appears probable
that radio-active lead is either one of the numerous examples of
substances made active for the time by solution with radio-
elements, or lead with a slight admixture of a radio-element. The
peculiar action of the cathode rays in causing an increase of the
photographic and electric action of radio-lead sulphate has ap-
parently nothing whatever to do with the activity proper of the
substance, but seems to be an additional effect due to the strong
phosphorescence set up. The sulphide does not show any such
action. The phosphorescent light probably includes some short
ultra-violet light waves which are capable of ionizing the gas.

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 spectro-
scopic analysis. The main criteria of importance are the existence
or absence of distinctive radiations or emanations, and the perman-
ence of the radio-activity. The presence of a radio-active emanation

1 Ber. (leutsch. chem. Ges. p. 3035, 1901.

2 Ber. deutsch. chem. Ges. p. 3775, 1901.

I] RADIO-ACTIVE SURSTAXCES 27

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 emana-
tions given out by them. When once the presence 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. lonization of gases by radiation. The mostimportant
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 Rontgen
rays, the theory1 has been put ^r Earth

forward that the rays pro- f

duce positively and negatively *

charged carriers throughout +

the volume of the gas sur- ^

rounding the charged body, and >,J^E^&L-

that the rate of production is

proportional to the intensity

of the radiation. These carriers, Fl8- l-

or ions2 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.

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

1 J. J. Thomson and Eutherford, Phil. Mag. Nov. 1896.

2 The word ion has now been generally adopted in the literature of the
subject. In the use of this word no assumption is made that the ions in gases
are the same as the corresponding ions in the electrolysis of solutions.

CH. Il] IOXIZATION TflEORY OF GASES 29

produced will in general depend 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 other plate, and
consequently a current will pass through the gas. Some of the
ions will also recombine, the rate of recombination being propor-
tional 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
finally reach a maximum 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 Rontgen 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 current1 between the plates, determined by the rate of
movement of the electrometer needle, is observed at first to in-
crease 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 pe^ unit volume increases

1 A minute current is observed between the plates even if no radio-active matter
is present. This has been found to be due mainly to a slight natural radio-activity
of the matter composing them. (See sections 218—220.)

30 IONIZATION THEORY OF GASES [CH.

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 x" current,
and the value of the potential difference required to give this
maximum current, the " saturation P.D."

The general shape of the current-voltage curve is shown in
Fig. 2, where the ordinates represent current and the abscissae
volts.

Saturation Curve

Folk
Fig. 2.

1 This nomenclature has arisen from the similarity of the shape of the current-
voltage curves to the magnetization curves for iron. Since, on the ionization
theory, the maximum current is a result of the removal of all the ions from the gas,
before recombination occurs, the terms are not very suitable. They have however
now come into general use and will be retained throughout this work.

IONIZATION THEORY OF GASES

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 com-
plicated 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. Thomson1 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

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

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

M
80

.-.

50
40
30

Sat

uration Cu
m, activity
es 4-5 cms.

rve

plat

apart

JU
10

0 100 200 300 400 500 600 70C

Fig. 3.

1 Phil. Mag. 47, p. 253, 1899. J. J. Thomson, Conduction of Electricity through
Gases, p. 73, 1903.

32 IONIZAT1ON THEOKY OF GASES [CH.

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 x 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. TABLE II.

0-5 cms. apart 2-5 cms. apart

Volts Current Volts Current

•125 18 -5 7'3

•25 36 1 14

•5 55 2 27

1 67 4 47

2 72 8 64
4 79 16 73
8 85 37-5 81

16 88 112 90

100 94 375 97

335 100 800 100

The results are shown graphically in Fig. 4.

From the above tables it is seen that the current at first in-
creases 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 °/0 of the maximum, while a change of voltage from 100 to 335
volts increases the current only 6°/0. The variation of the current
per volt (assumed uniform between the range of voltages con-
sidered) is thus about 5000 times greater for the former change.

IONIZATION THEORY OF GASES

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

100

Saturation Curves

for
Uranium rays

40 60

Volts

Fig. 4.

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
R. R.-A. - 3

34 IONIZATION THEORY OF GASES [CH.

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 be-
tween 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 I 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 ==• aN2,
where a 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 % per sq. cm. of the
plate, is given by

.NeuV

where u is the sum of the velocity of the ions for unit potential

gradient, and e is the charge carried by an ion. -=- is the velocity

of the ions in the electric field of strength y- .

The number of ions produced per second in a prism of length I
and unit area of cross-section is ql. The maximum or saturation
current 7 per sq. cm. of the plate is obtained when all of these
ions are removed to the electrodes before any recombination has
occurred.

Thus 7 = q .1 . e,

i NuV uV

and — = =

7 ql2 I2 \fqa '

This equation expresses the fact previously noted that, for small
voltages, the current i is proportional to F

Let j = 0,

then F = '

II] IONIZATION THEOKY OF GASES 35

Now the greater the value of V required to obtain a given
value of p (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 p, V varies as I2. 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 dioxide1 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.

1 Rutherford, Phil. Mag, Jan. 1899.

3—2

IONIZATION THEORY OF GASES

[CH.

29. Townsend1 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 Rontgen 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.

Volts
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.

1 Phil. Mag. Feb. 1901.

II] IONIZATION THEORY OF GASES 37

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 very conveniently examined in an apparatus
similar to Fig. 6.

Uranium']

Fig. 6.

A dry current of air or any other gas is passed at a constant
rate through a long metal tube TL. The current of air after
passing through a quantity of cotton-wool to remove dust particles,
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 through the gas 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 elec-
trode 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

where a is a constant.

38 IONIZATION THEORY OF GASES [CH.

Integrating this equation,

if N is the initial number of ions, and n the number after a time t

The experimental results obtained1 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 a, which may be termed the coejftdent_of_recom-
l&LLation, has been determined in absolute measure by Townsend2,
McClung3 and Langevin4 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 experi-

mentally. Then -— = aT, 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

Ve
second. Then a = .m .

11.

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

1 Eutherford, Phil. Mag. Nov. 1897, p. 144, Jan. 1899.

2 Phil. Trans. Roy. Soc. A, p. 157, 1899. 3 Phil. Mag. p. 283, March, 1902.
4 These presentee a la Faculte des Sciences, p. 161, Paris, 1902.

II] IONIZATION THEORY OF GASES 39

Value of a.

Gas

Air

Carbon Dioxide
Hydrogen

Town send

3420 x e
3500 x e
3020 x e

McClung

3384 x e
3492 x e

Langevin
3200 xe
3400 xe

The latest determination of the value of e (see section 36) is
3-4 x 10-10 E. s. units ; thus a = Tl x lO"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 sees.

MeClung (loc. tit.) showed that the value of a was approximately
independent of the pressure between '125 and three atmospheres.
In later observations, Langevin has found that the value of a
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 Owens1. 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 con-
ditions. A much larger voltage is then necessary to produce
saturation. If the smoke particles are removed by a stream of air,
the current at once returns 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 Rutherford2, Zeleny3, and
Langevin4 for gases exposed to Rontgen 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

1 Phil. Mag. Oct. 1899. 2 Phil. Mag. p. 429, Nov. 1897.

:i Phil. Trans. A, p. 193, 1901. 4 C. R. 134, p. 646, 19C2.

40 IONIZATION THEORY OF GASES [CH.

velocity proportional to the strength of the field. On the appli-
cation of an electric field, the ions almost instantly attain the
velocity corresponding to the field and then move with a uniform
speed.

Zeleny1 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
Rontgen 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 Kontgen 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 ions2, the writer has
used an apparatus similar to that shown in Fig. 6 on p. 37.

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

1 Phil. Mag. July, 1898. 2 Phil. Mag. Feb. 1899.

H] IONIZATION THEORY OF GASES 41

Let HI and ^ 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 = Xu^dt,

b
or loge - r dr

dt = - ^ - .

VUt

Let r.2 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.

If p2 be the ratio of the number of the negative ions that reach
the electrode A to the total number passing by, then

Therefore

/>2(62-a2)loge-

Similarly the ratio p± 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 move-
ment 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 p2 is proportional to F, — i.e. that

IONIZATION THEORY OF GASES

[CH.

fche 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 pz 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 p.2 was found to be the same as for,
the first experiment.

This experiment shows conclusively that the ions produced
by Rontgen 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 Rontgen rays have been made by Zeleny1 and
Langevin2. Zeleny used a method similar in principle to that
explained above. His results are shown in the following table,
where K^ is the mobility of the positive ion and Kz that of the
negative ion.

Gas

K2
KI

Temperature

Air. dry

1-36

1-87

1-375

13°-5C.

„ 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, dry

0-76

0-81

1-07

17°'5

„ 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°

1 Phil. Trans. 195, p. 193, 1900.

2 C. R. 134, p. 646, 1902, and Thesis, p. 191, 1902.

II] IONIZATION THEORY OF GASES 43

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 CO2

K, K, ^ Kl K9 |»

' Direct method (Langevin) 1'40 1'70 1'22 O86 0-90 1'05

Current of gas (Zeleny)... 1'36 1'87 1-375 0*76 0'81 1*07

These results show that for all gases except CO2, there is a
marked increase in the velocity of the negative ion with the dry-
ness 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 Rutherford1 for the negative ions
produced by ultra-violet light falling on a negatively charged sur-
face, and later by Langevin2 for both the positive and negative ions
produced by Rontgen 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 nega-
tive 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
is suddenly expanded, a cloud of small globules of water is formed.
These drops are formed round the dust particles present in the gas,

1 Proc. Camb. Phil. Soc. 9, p. 410, 1898. '- Thesis, p. 190, 1902.

44 IONIZATION THEORY OF GASES [CH.

which act as nuclei for the condensation of water around them.
The experiments of R. von Helmholtz and Bicharz1 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. Wilson2. 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 Vi = initial volume of the gas in the vessel,
v2 = volume after expansion.

If — <1'25 no condensation is produced in dust-free air. If
#1

however — > 1'25 and < T38, a few drops appear. This number is

roughly constant until — = 1'38, when the number suddenly in-
creases 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 — <1'25 no drops are formed, but if

- = T25 there is a sudden production of a cloud. The water drops

of which this cloud is formed are finer and more numerous the

1 Wied. Annul 40, p. 161, 1890.

- Phil. Trans, p. 265, 1897; p. 403, 1899; p. 289, 1900.

Il] IONIZATION THEORY OF GASES 45

greater the intensity of the rays. This point at which condensa-
tion 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 at once carried 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 — > 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 218).

The evidence is thus complete that the ions themselves serve
as centres for the condensation of water around them. These ex-
periments show conclusively that the passage of electricity through
a gas is due to the production of charged ions distributed through-
out 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.

IONIZATION THEORY OF GASES

[CH.

35. Difference between the positive and negative ions.

In the course of experiments to determine the charge carried by
an ion, J. J. Thomson1 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.

This difference in behaviour of the positive and negative ions
was investigated in detail by C. T. R. Wilson2 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

•Earth

Fig. 7.

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 con-
nected 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 — = T25 but did

not occur in AD for the positive ions until — = 1'31.

»i

1 Phil. Mag. p. 528, Dec. 1898.

2 Phif. Trans. 193, p. 289, 1899.

Il] IONIZATION THEORY OF GASES 47

The negative ion thus more readily acts as a centre of conden-
sation than the positive ion. 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 ex-
pansion is large enough, to ensure condensation on both ions, the
numbers of 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 charge on
positive and negative ions is equal in value but opposite in sign.

36. Charge carried by an ion. For a known sudden ex-
pansion of a gas saturated with water vapour, the amount of water
precipitated on the ions can be readily calculated. 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 /j, is given by

~ O ~ ~I"»
9 fM

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 at once obtained.

This method has been used by J. J. Thomson1 to determine the
charge carried by an ion. If the expansion exceeds the value T31,
both positive and negative ions become centres of condensation.
From the rate of fall it can be shown that the drops are approxi-
mately all of the same size.

1 Phil. Mag. p. 528, Dec. 1898, and March, 1903. Conduction of Electricity
through Gases, p. 121.

48 IONIZATION THEORY OF GASES [CH.

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 I cms. from each other. The
small current i through the gas is given (section 28) by

._NuVe
I '

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 x 10~10 electrostatic units.

A very concordant value of 3'1 x 10~10 has been obtained by
H. A. Wilson1, 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

1 Phil. Mag. April, 1903.

IONIZATION THEORY OF GASES

produced in gases by Rb'ntgen rays or by the rays from active
substances has been made by Townsend1. 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 mathematically expressed by
the following equation in terms of K, the coefficient of diffusion
of the ions into the gas with which they are mixed2,

_3WKZ _^KZ

R = 4< (-195<?. «2F + -0243e~~ ~*r + &c.),
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 + ions

K for -ions

Mean value
of K

Eatio of
values of K

Air, dry

•028

•043

•0347

1-54

„ moist

•032

•035

•0335

1-09

Oxygen, dry

•025

•0396 -0323

1-58

„ moist
Carbonic acid, dry . . .

•0288
•023

•0358 -0323
•026 -0245

1-24
1-13

„ „ moist

•0245

•0255

•025

1-04

Hydrogen, dry

•123

•190 -156

1-54

„ moist ...

•128

•142

•135

I'll

1 Phil. Trans, p. 129, 1899.
R. R.-A.

2 Townsend, loc. cit. p. 139.

50 IONIZATION THEORY OF GASES [CH.

The moist gases were saturated with water vapour at a tem-
perature 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 charge and
the gas a positive charge.

38. A very important result can at once be deduced when the
velocities and coefficients of diffusion of the ions are known.
Townsend (loc. cit.) has shown that the equation of motion of the
ions is expressed by the formula

where e is the charge on an ion,

n = number of ions per c.c.,
p = their partial pressure,

and u the velocity due to the electric force X in the direction of
the axis of x. When a steady state is reached,

dp nXeK

-f- — 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

N
the values of u and K have been determined. Then — may be

substituted for - , and, since P at atmospheric pressure is 10",

Il] IONIZATIOX THEORY OF GASES 51

„ 3X 108 . M!

Ne — -- „ electrostatic units,
J\.

where u^ is the velocity for 1 volt (i.e. -^ E. s. unit) per cm.

It is known that one absolute electro-magnetic 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 2'46 N, and, if e is the charge on the
hydrogen atore in the electrolysis of water,

ion

2-46 Ne = 3 x 1010 E. s. units,
Ne' = 1-22 x 1010 E. s. units.

Thus 4=2-46 x 10-2^.

6 -TL

For example, substituting the values of u^ and K determined
for moist air for the positive ion,

e 2-46 1-37

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 u^ 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 experi-
mental 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

Are= 1-22x10™.

Now e, the charge on an ion, is equal to 3'4 x 10~10 E. s. units.
Thus JV=3-6xlOw.

If 7 is the saturation current through a gas, and q the total
rate of production of ions in the gas,

4—2

52 IONIZATION THEORY OF GASES [CH.

The saturation current through air was found to be 1*2 x 10~8
amperes, 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 x 108. Since
JV" — 3-g x 1019, it is thus seen that, if one molecule produces two
ions, the proportion of the gas ionized per second is about 10~u 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 readily be detected.
This corresponds to the ionization of about one molecule in every
1019 present in the gas.

40. Size and nature of the ions. A}i approximate estimate
of the mass of an ion, compared with the m)ass 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
has been found to be 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

Il] IONIZATION THEORY OF GASES 53

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.

It must thus be supposed 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 denned 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 x 10~10
electrostatic units. Its presence has only been detected when in
rapid motion, when it has, for speeds up to about 1010 cms. a second,
an apparent mass m given by e/m = T86 x 107 electromagnetic

54 ION1ZATION THEORY OF GASES [CH.

units. This apparent mass increases with the speed as the velocity
of light is approached (see section 76).

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 pressure of the gas. Each of
the ions carries a charge of value 3'4 x 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. Of this radiation, the a
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 ft rays consist of negatively
charged particles, which are identical with the cathode rays pro-
duced in a vacuum tube and travel with a speed about one-half
the velocity of light (chapter iv.). Each of these projected
particles possesses such great kinetic energy that it is able to
produce 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 the ionization varies with the speed, but there is no
doubt that each projected body produces 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 pro-
duce fresh ions by collision with the molecules of the gas. At low
pressures the negative ion is identical with the electron produced
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 diminu-
tion of the pressure. Townsend has shown that fresh ions are

Il] IONIZATION THEORY OF GASES 55

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 are produced at
each collision1.

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.

Taking F=20 volts = -^£r E.s. units, and e = 3'4x!0-10, the
energy W required to produce an ion by collision of the negative

ion is given by

TF=2-3 xlO-uergs.

The velocity u acquired by the ion of mass m just before a
collision is given by

mu* = Ve,

and u =

v m

Now - = 1'86 x 107 electromagnetic units for the electron at
slow speeds (section 76).
Taking F= 20 volts,

m = 2 '7 x 108 cms. per sec.

This is a velocity very great compared with the velocity of
agitation of the molecules of the gas.

The negative ions alone are able to produce ions by collision
in a weak electric field. The positive ion, whose mass is at least
1000 times greater than the electron, does not acquire a sufficient
velocity to produce ions by collision until an electric field is applied
nearly sufficient to cause a spark through the gas.

An estimate of the energy required to produce 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 F= 175

1 Some difference of opinion has been expressed as to the value of V required
to produce ions at each collision. Townsend considers it to be about 20 volts ;
Langevin 60 volts and Stark about 50 volts.

56 IONIZATION THEORY OF GASES [CH.

volts — a value considerably greater than that observed by Town-
send 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 measure-
ments will now be considered. With unscreened active material
the ionization of the gas is, to a large extent, due to the a 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 Rontgen rays, where the ionization is in most cases uniform.

43. Variation of the current with distance between the
plates. It has been found experimentally1 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 / of the radiation, the

value of / at that point is given by ^ = e~*x, where \ is a

-*o

constant, x the distance from the plate, and 70 the intensity of the
radiation at the surface of the plate. This result can be deduced
theoretically on the assumption that the ionization at any point is
proportional to the intensity of the radiation, and that the energy
of the rays is used up in producing ions.

With an infinite plane of active matter, the intensity of the
radiation would be constant for all distances from the plane if
there were no absorption of the radiation in the gas.

Let q be the number of ions produced per second per unit
volume when the intensity of radiation is /.

Let I = Kq, where K is a constant.

If &) is the average energy required to produce an ion, the
1 Rutherford, Phil. Mag. Jan. 1899.

Il] IONIZATION THEORY OF GASES 57

energy dl absorbed in producing ions in a layer of unit area and
thickness dx at a distance x from the plane is given by

dl=qa). dx

Integrating, ioge 1 = -~ . x + A ,

where A is a constant.

Since 7 = 70 when x = 0, A = loge /0, and

-A*

where X = — = a constant.

\ will be called the absorption constant of the gas for the
particular kind of radiation considered.

If q0 is the rate of production of ions at the surface of the

plate, -£ = er**,

<?o

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. The saturation current i per unit area is given by

I'd

i = I qe'dx, where e' is the charge on an ion,
Jo

= q.e' (d e-^dx = ^ (1 - <r*d) ;

JO A,

when \d 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 \d is large, the saturation current iQ is equal to Q— ,

and is independent of further increase in the value of d. In such

58 IONIZATION THEORY OF GASES [CH.

a case the radiation is completely absorbed in producing ions be-
tween the plates, and - = 1 — e~*d.

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 \ 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

V5 „ 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

A 1 - e~^d + &c.

where A, Al are constants and X, \ 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

I 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

II] IONIZATION THEORY OF GASES 59

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 \! = absorption constant of the radiation in the gas for
unit pressure.

For a pressure p, the intensity / at any point x is given by

— = g-p*i*. The saturation current i is thus proportional to

•*o

[dpldx = I d pl«e-^*. dx = ^(l- «PM).
Jo -' o *i

If r be the ratio of the saturation currents for the pressures p^
and p.2

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 curves1 is
well illustrated in Fig. 8, where curves are given for hydrogen, air,
and carbonic acid for plates 3*5 cms. apart.

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 pres-
sure until the absorption is so reduced that the radiation reaches
the other plate.

1 Rutherford, Phil. Mat). Jan. 1899.

IONIZATION THEOKY OF GASES

[CH.

An interesting result follows from the rapid absorption of
radiation by the gas. If the current is observed between two
fixed parallel plates, distant d^ 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.

I'rrxxun- in inms.

150

300 450

Fig. 8.

600

750

The saturation current i is obviously proportional to

?/0e-*>A'd, i.e. to ~
This is a function of the pressure, and is a maximum when

1,-x.r, ^ 1 «,-\ f J J \

II] IONIZATION THEORY OF GASES 61

For example, if the active matter is uranium, p\l= T6 for the
a rays at atmospheric pressure. If c?2 = 3, and d^ =1, the saturation
current reaches a maximum when the pressure is reduced to about
1/3 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 pro-
duction of ions in a gas varies for different gases and increases
with the density of the gas. Strutt1 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 a rays. When the active substance is covered with
a layer of aluminium '01 cms. in thickness, the ionization is mainly
due to the ft or cathodic rays, and when covered with 1 cm. of lead
the ionization is solely due to the 7 or very penetrating rays.
Experiments on the 7 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.

With the exception of hydrogen, it will be seen that the ioniza-
tion of gases is approximately proportional to their density for the
a, £, 7 rays of radium. The results for Rontgen rays are quite
different; for example, the conductivity produced by them in
methyl iodide was more than 14 times as great as that due to
the rays of radium. The 7 rays of radium appear to be more allied
to the @ rays of radium than to Rontgen rays.

1 Phil. Tram. A, p. 507, 1901 and Proc. Roy. Soc. p. 208, 1903.

IONIZATION THEORY OF GASES

[CH.

This difference of conductivity in gases is due to unequal
absorptions of the radiations. The writer has shown1 that the

RELATIVE CONDUCTIVITY

rijio

Eelative

Density

a rays

[3 rays

7 rays

Rontgen
rays

Hydrogen
Air ... ... ...

0-0693
1-00

0-226
1-00

0-157
1-00

0-169
1-00

0-114
1-00

Oxygen

1-11

1-16

1-21

1-17 1-39

Carbon dioxide

1-53

1-54

1-57

1-53

1-60

Cyanogen

1-86

1-94

1-86

1-71

1-05

Sulphur dioxide ...

2-19

2-04

2-31

2-13 7-97

Chloroform

4-32

4-44

4-89

4-88 31-9

Methyl iodide

5-05

3'51

5-18

4-80

72-0

Carbon tetrachloride

5-31

5-34

5-83

5-67

45-3

total number of ions produced by the a rays for uranium, when
completely absorbed by different gases, is not very different. The
following results were obtained :

Gas

Air

Hydrogen

Oxygen

Carbonic acid
Hydrochloric acid gas
Ammonia

Total
lonization

100

95
106

96
102
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

1 Phil. Mag. p. 137, Jan. 1899.

IONIZATION THEORY OF GASES

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 inter-
mediate 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;

Distance

-V

Fig. 9.

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

64 IONIZATION THEOKY OF GASES [CH.

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.

47. Variation of current with voltage for surface ion-
ization. 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 Child1 and Rutherford'2. 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 nt = number of positive ions per unit volume at a distance
x from the plate A,

Kl = mobility of the positive ions,
e — charge on an ion.

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

By Poisson's equation

d*V

K, dV S?V

Then tl = - • -=- . -j- .

4-7T dx dxz

fd
Integrating

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

1 Phys. Rev. Vol. 12, 1901.

2 Phil. Mag. p. 210, 1901 ; Phys. Rev. Vol. 13, 1901.

II] IONIZATIOX THEORY OF GASES 65

x = 0. By making the ionization very intense, the value of

dx
can be made extremely small.

Putting A = 0,

dV '

This gives the potential gradient between the plates for differ-
ent values of x.

Integrating between the limits 0 and d,

9V2

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

9V'2

?-£•

*2 A2

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
made1 some experiments on the variation of current with voltage
between parallel plates distant about 10 cms. from each other.

1 Phil Mag. Aug. 1901.
R. R.-A. 5

66 IONIZATION THEORY OF GASES [CH. II

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 ^ 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.

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 £0 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. This chapter was written
before the publication of J. J. Thomson's recent book Conduction
of Electricity through Gases (Cambridge University Press, 1903),
to which the reader is referred for further information on this
important subject.

CHAPTER III.

METHODS OF MEASUREMENT.

48. 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 very widely used, 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 elec-
trical 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, the photographic method as a general
method of study of the radiations 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

5—2

68 METHODS OF MEASUREMENT [CH.

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 very
many 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, Soddy1 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 the
action on the plate under ordinary conditions 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 photo-
graphically. These rays are difficult to study by the photographic
method, as the layer of black paper which, in many cases, is
necessary to absorb the phosphorescent light from active substances,
cuts off at the same time most of the rays under examination.
These rays will be shown to play a far more important part in the
processes occurring in radio-active bodies than the 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 measure-
ment over an extremely wide range. With proper precautions
it can be used to measure effects produced by radiations of
extremely small intensity.

49. 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

1 Trans. Chem. Soc. Vol. 81, p. 860, 1902.

m] METHODS OF MEASUREMENT

measurements of radio-activity has been given in the last chapter.
It has there been shown that the essential condition to be fulfilled
for comparative measurements of the intensity of the radiations
is that the electrical field should 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 could 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 ioniza-
tion 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 be readily measured by a sensitive gal-
vanometer 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 x 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 centi-
metres 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 1 or
more minutes, the condenser is discharged through the galvano-
meter, and the current can be readily deduced.

50. In most cases, however, when dealing with less active
substances like uranium or thorium, or with small amounts of active

METHODS OF MEASUEEMENT

[CH.

material, it is necessary io_emploj methods for measuring currents
much smaller than can be conveniently detected 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 simple
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.

''Earth

Fig. 10.

The connections are clearly seen from Figure 10. 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 to earth. The
upper insulated plate Pr is connected to 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 to pass over a certain number of
divisions of the micrometer scale in the eyepiece 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

Ill]

METHODS OF MEASUREMENT

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 readily 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 6f, are surrounded
by metal cylinders, E and F, connected with earth.

til. A modified form of the gold-leaf electroscope can be used
to determine extraordinarily minute cur-
rents 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 ioniza-
tion of the gas is shown in Fig. 11.

This type of electroscope was first used
by C. T. R. Wilson1 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 S,
supported from the rod P. In a dry atmosphere a clean sulphur
bead is almost a perfect insulator. The system is charged by a
light bent rod CO ' passing through the ebonite cork D. 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 CO' 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 movemejit_ofthe ^gold-leaf is_pbserved by a reading
^g ™ ***& rlindfiT, covered with thin

Fig. 11.

mica. In cases where the natural ionization due to the enclosed
1 Proc. Roy. Soc. Vol. 68, p. 152, 1901.

72 METHODS OF MEASUREMENT [CH.

air in the cylinder is to be accurately measured, 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 almost completely eliminated 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.

52. 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 volts in the time t seconds, the
current i through the gas is given by

GV
l = — '

. 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

OAA = 5*6 x 10~6 E.s. units = 1'9 x 10~15 amperes.

With special precautions a rate of discharge of 1/10 or even
1/100' of this amount can be accurately measured.

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 x 10~10 electrostatic units or
1*13 x 10~19 coulombs.

Let q = number of ions produced per second per cubic centi-

metre throughout the volume of the electroscope,
S = volume of electroscope in cubic centimetres.

If the ionization is uniform, the saturation current i is given by
i = qSe.

Ill] METHODS OF MEASUREMENT 73

Now for an electroscope with a volume of 1000 c.c., i was equal
to about 1'9 x 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. 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.

53. 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. Wilson1. The construction of the
apparatus is shown in Fig. 12.

Fig. 12.
1 Proc. Camb. Phil. Soc. Vol. 12, Part n. 1903.

74 METHODS OF MEASUREMENT [CH.

The case consists of a rectangular brass box 4 cms. x 4 cms.
x 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 sensitive-
ness. 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 eyepiece, 54 divisions corresponding to one
millimetre.

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 is 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.

54. 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 electrometers. With the use 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

Ill] METHODS OF MEASUREMENT 75

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. Walker1. The effect
appears to be due to the presence of the air space that necessarily
exists between adjoining quadrants.

Suppose that it is required to measure with an electrometer
the ionization current between two
horizontal metal plates A and B
(Fig. 13) 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 T ll~~ *^ Earth

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 electro-
meter 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 com-
mences to move at a uniform rate. Observations of the angular
movement of the needle are made either by the telescope and scale
1 Phil Mag. Aug. 1903.

76 METHODS OF MEASUREMENT [CH.

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 electro-
meter needle returns to its original position.

In most experiments on radio-activity only comparative measures
of saturation currents are required. If these comparative measures
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 most simply done 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 measure-
ments.

55. 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 unneces-
sarily 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

Ill] METHODS OF MEASUREMENT 77

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 simply 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 suspended 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 move-
ment 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

METHODS OF MEASUREMENT

[CH.

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) condenser1 such as is shown in
Fig. 14.

A circular plate of ebonite about 1 cm. thick is turned down
until it is not more than -J- mm.
thick in the centre. Into this i

circular recess a brass plate B fits /JLN

loosely. The ebonite plate rests
on another brass plate C connect-
ed 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 connec-
tion with the needle by means C
of a fine platinum wire. The
platinum wire from the needle

dips into the acid, and serves to damp the needle. In a dry atmo-
sphere, 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 it 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 always be kept at a constant value, and
the electrometer always has a constant sensibility.

56. A very useful electrometer of great sensibility has recently
been devised by Dolezalek2. 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

1 Strutt, Phil. Trans. A, p. 507, 1901.

2 Instrumentenkunde, p. 345, Dec. 1901.

Ill] METHODS OF MEASUREMENT 79

employed. In consequence of the lightness of the needle and the
nearness to the quadrants it acts as its own damper. This is
a great advantage, for difficulties always arise with the wire dipping
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 be readily
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 electro-
static 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. The quartz
fibre can be made sufficiently conducting for this purpose by
dipping it into a dilute solution of calcium chloride. In addition
to its great sensibility, the advantages of this instrument lie in the
steadiness of the zero and in the self-damping.

57. Adjustment and screening. In adjusting an electro-
meter, it is important to arrange that the needle lies 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

80 METHODS OF MEASUREMENT [CH.

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 electro-
meter, the wires leading to it should be insulated in metal cylinders
connected to 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
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
once paraffin has been charged, the residual charge, after dis-
electrifying 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.

Ill] METHODS OF MEASUREMENT 81

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.

58. 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 to earth in order to

avoid electrostatic disturbances at

the moment the current is to be

measured. The simple key shown

in Fig. 15 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 Etetrmneter Tenting Vessel

tube A, which is rigidly attached Fig- 15-

to a bent metal support connected

to earth. When the string is released this rod makes contact with,
the mercury M, which fills a hole in the small 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 electro-
meter is again earthed. By means of this key, which may be
operated at any distance from the electrometer, the earth con-
nection may be made and broken at definite intervals without
any appreciable disturbance of the needle.

59. Testing apparatus. The arrangement shown in Fig. 16
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.

R. R.-A. 6

METHODS OF MEASUREMENT

[CH.

deep) in the brass plate A. In order to avoid breaking the
battery connection every time the plate A is removed, the wire

XO JOilCCl

omctcr
3

(

Active 'Material
\ /A

"To Battery

Fig. 16.

from the battery is permanently connected to 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.

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 are being tested which
give off a radio-active emanation, 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 always correct for this
natural leak when necessary.

60. It is often required to measure the activity due to the
emanations of thorium or radium or the excited activity produced

Ill] METHODS OF MEASUREMENT 83

by those emanations on rods or wires. A convenient apparatus for
this purpose is shown in Fig. 17. The cylinder B is connected with

Earth

Fig. 17.

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.

61. 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 accumu-
lators 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

6—2

84 METHODS OF MEASUREMENT [CH.

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 of parallel plates
or still better of concentric cylinders. With this series of capacities,
currents may be measured between 3 x 10~14 and 3 x 10~8 amperes
— a range of over one million times. 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 dis-
tribution of excited activity in an electric field (see section 171).

62. 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 divi-
sions 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 ; = .s. units,

Suppose, for example,

(7=50, d = 5, D
Then i = 2'8 x 10~13 amperes.

Ill] METHODS OF MEASUREMENT 85

Since the electrometer can readily measure a current corre-
sponding to a movement of half a scale division per second,
it is easily seen that an electrometer can measure a current of
3 x 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 only 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, 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 quad-
rants. 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 (7, and is given by

The capacity of the electrometer is thus 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 not much altered. The theory of this action has
been given by J. J. Thomson1.

1 Phil. May. 46, p. 537, 1898.

METHODS OF MEASUREMENT

[CH.

63. 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 G = capacity of electrometer and connections.
Cl = capacity of a standard condenser.

The electrometer and its connections are charged to a potential
Vl by a battery, and the deflection dl 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

C+C, V,

V9 d,

and

c= a

dl -

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. 18) is mounted

Earth

Battery

Fig. 18.

on a wooden base, which is covered with a sheet of metal or tin-foil
connected to earth. The tube C is supported centrally on ebonite

Ill] METHODS OF MEASUREMENT 87

rods at each end. The capacity is given approximately by the
formula

where b is the internal diameter of 7), a the external diameter of C,
and I the length of the tubes.

The following method can in some cases be used with advantage.
While a testing vessel is in connection with the electrometer, a
sample of uranium is placed on the lower plate A. Let d^ and
di be the number of divisions passed over per second by the needle
with and without the standard capacity in connection.

Then C+C!=|,

and C

dl — c?2

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.

64. 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. Curie1,
measurements of the current can be made with rapidity and
accuracy over a wide range. These measurements are quite inde-
pendent 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. 19) is hung vertically and weights are
added to the lower end. The plate is .cut so that the optic axis of

1 C. R. 91, pp. 38 and 294, 1880. See also Friedel and J. Curie, C. R. 96,
pp. 1262 and 1389, 1883, and Lord Kelvin, Phil. Mag. 36, pp. 331, 342, 384, 414,
453, 1893.

88 METHODS OF MEASUREMENT [CH.

the crystal is horizontal and at right angles to the plane of the
paper.

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

To Support

Earth

To Weight

Fig. 19.

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 to
the electrometer and to 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

Q = 0-063 j . F,

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. 19), due to some radio-active material
on the plate G, for a given difference of potential between C and D.

Ill] METHODS OF MEASUREMENT 89

At a given instant the connection of the quadrants of the electro-
meter with the earth is broken. The weight is attached to the
quartz plate, and is held in the hand so as to gradually apply the
tension. 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 -^ 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.

65. 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, Rontgen 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.

CH. IV] NATURE OF THE RADIATIONS 91

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 the a, ft, and 7 rays.

(i) The a 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
1/10 the velocity of light. They are deflected by intense mag-
netic 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 ft rays are far more penetrating in character than the
a 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 a rays and are in fact identical
with the cathode rays produced in a vacuum tube.

(iii) The 7 rays are extremely penetrating, and non-deviable
by a magnetic field. Their true nature is not yet known, but they
are analogous in some respects to very penetrating Rontgen 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. Polonium
stands alone in giving only the a. or easily absorbed rays1.

66. 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

1 In an examination of uranium the writer (Phil. Mag. Jan. 1899) found that
the rays from uranium consist of two kinds, differing greatly in penetrating power,
which were called the a and £ rays. Later, it was found that similar types of rays
were emitted by thorium and radium. On the discovery of very penetrating rays from
uranium and thorium as well as in radium, the term 7 was applied to them by the
writer. The word "ray " has been retained in this work, although it is now settled
that the a and @ rays consist of material particles projected with great velocity. The
term is thus used in the same sense as by Newton, who applied it in the Principia
to the stream of corpuscles which he believed to be responsible for the phenomenon
of light. In some recent papers the o and {3 rays have been called the a and /3
"emanations." This nomenclature cannot fail to lead to confusion, since the
term " radio-active emanation " has already been generally adopted in radio-
activity as applying to the material substance which gradually diffuses from thorium
and radium compounds, and itself emits rays.

NATURE OF THE RADIATIONS

[CH.

discharge passes through it. The o_j;ays correspond to the, canal
rays, discovered by Goldstein, which have been shown by \^iBn to
consist of positively charged bodies projectedjwith great velocity.
The ft rays aretKe same as Lhe^cathoderays, while the 7 rays in
some respects resemble the Rontgen rays. In a vacuum tube,
a large amount of electric energy is expended to produce 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 a and ft rays from the active bodies are projected with much
greater velocity than the corresponding rays in a vacuum tube,
while the 7 rays are of much greater penetrating power than
Rontgen 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. 201.

Some radium is placed in the bottom of a narrow cylindrical
lead vessel R. A narrow pencil
of rays consisting of a, ft, and
7 rays escapes from the open-
ing. 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 7 rays continue
in a straight line without any . A
deviation. The ft rays are Fig. 20.

deflected to the right, describ-
ing circular orbits the radius of which varies within wide limits.
If the photographic plate AC is placed under the radium vessel,
ikerj3 rays produce a diffuse photographic impression on the right
of the vessel R. The a rays are bent in the direction opposite to
that of the ft 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

1 This method of illustration is due to Mrae Curie, These presentee a la Faculte
des Sciences de Paris, 1903.

IV]

NATURE OF THE RADIATIONS

of the deviation of the a rays compared with that of the ft rays is
much exaggerated in the figure.

67. Ionizing and penetrating power of the rays. -Qf

rays, the a rayjy)roducej^

in the gas and the 7 rays ^Ke'IeaSE With a thin layer of un-
screened active material spread on the lower of two parallel plates
5 cms. apart, the amount of ionization due to the a, ft, and 7 ravs
is of the relative order 10,000, 100, andjl. 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.

Thickness of

Radiation

Aluminium in cms.
which cuts off half

Relative power
of penetration

the radiation

a rays

0-0005 cms.

18 »

0'05 cms.

100

7 »

8 cms.

10000

rejative_powerof jDenetration 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 a rays from uranium and polonium are the least pene-
trating, and those from thorium the most. The ft radiations from
thorium and radium are very complex, and consist of rays widely
different in penetrating power. Some of the ft rays from these
substances are much less and others much more penetrating than
those from uranium, which gives out fairly homogeneous rays.

68. 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 sub-
stances. The three general methods employed depend upon the
action of the rays in ionizing the gas, in acting on a photographic

94 NATURE OF THE RADIATIONS [CH.

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 /3 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 rayswhich are the most active electrically are the least
r active photographically. Under ordinary conditions most of the
photographic action of uranium, thorium, and radium, is due to the
/3 or cathodic rays. The a. rays from uranium and thorium, on
account of their weak action, have not yet been detected photo-
graphically. With active substances like radium and polonium,
the a rays readily produce a photographic impression. So far the
7 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 7 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 a rays, while the photographic action under the
same condition is due almost entirely to the (3 rays.

It is often convenient to know what thickness of matter is
sufficient to absorb a specific type of radiation. A thickness of

IV] NATURE OF THE RADIATIONS 95

aluminium or mica of '01 cms. or a sheet of ordinary writing-paper
is sufficient to completely absorb all the a rays. With such a
screen over the active material, the effects are due only to the
ft and 7 rays, which pass through with a very slight absorption.
Most of the /3 rays are absorbed in 5 mms. of aluminium or 2 mms.
of lead. The radiation passing through such screens consists very
largely of the 7 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 ap-
proximately 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 ft OR CATHODIC RAYS.

69. Discovery of the /3 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, when it was observed that preparations of radium gave
out some rays 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 Giesel1 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 per-
pendicular 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 Schweidler2 also obtained similar results. They
1 Wied. Annal. 69, p. 831, 1899. 2 Phys. Zeit. 1, pp. 90, 113, 1899.

96 NATURE OF THE RADIATIONS [CH.

showed, in addition, the deviation of the rays when a change
occurred in the conductivity ; of the air under the influence of
a magnetic field. Becquerel1, a little later, showed the magnetic
deflection of the radium rays? by using the photographic method.
P. Curie2, 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 a rays), and the
other penetrating and deviable by a magnetic field (now known
as the ft rays). The ionization effect due to the (3 rays was
only a small fraction of that due to the a rays. At a later date
Becquerel, by the photographic method, showed that uranium gave
out some deflectable rays. It had been shown previously3 that the
rays from uranium consisted of a and ft rays. The deflected rays
in Becquerel's experiment consisted entirely of /3 rays, as the
a rays from uranium produce no appreciable photographic action.
Rutherford and Grier4, using the electric method, showed that
compounds of thorium, like those of uranium, gave out beside
a rays some penetrating ft rays, deviable in a magnetic field. As
in the case of radium, the ionization due to the a. rays of uranium
and thorium is large compared with that due to the ft rays.

70. Examination of the magnetic deviation by the
photographic method. . Becquerel has made a very complete
study, by the photographic method, of the ft 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. J. J. Thomson (Recent Researches, p. 136) has
obtained the equation for the path of a charged particle moving
in a uniform magnetic field. If a particle of mass m and charge
e is projected with a velocity V, at an angle a with the direction of
a field of strength H, it ^vill describe a curved path, whose radius
R of curvature is given by

D mV .
,R = -Tr sva. a.
lie

The path of the particle is a helix wound on a cylinder of radius R
with the axis parallel to the field.

1 C. R. 129, pp. 997, 1205. 1899. - C. E, 130, p. 73, 1900.

3 Eutherford, Phil. Mag. January, 1899. 4 Phil. Mag. September, 1902.

IV] NATURE OF THE RADIATIONS 97

When a= -^ , i.e. when the rays are projected normally to the
field, the particles describe circles of radius

The planes of these circles are normal to the field. Thus for
a particular velocity V the value of R varies inversely as the
strength of the field. In a uniform field the rays projected nor-
mally to the field describe circles, and their direction of projection
is a tangent at the origin.

This has been verified experimentally by Becquerel for the
ft rays of radium, by an arrangement similar to that shown in
Fig. 21.

*x ^— -7T1 P~

V;:::;>7

Fig. 21.

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 sup-
posed 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 pro-
duced 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, Al} A2, A3, are shown in the figure.
The rays, normal to the plate, strike the plate almost normally,
R. R.-A. 7

98 NATURE OF THE RADIATIONS [CH.

while the rays nearly parallel to the plate strike the plate at
nearly 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.

71. 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 very clearly shown by Becquerel1 in the
following way.

An uncovered photographic plate, with the film upwards, was
placed horizontally in the horizontal uniform magnetic field of
an electro-magnet. 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 takes 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 TrR. 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. 22 shows a photograph of this kind, obtained by Becquerel,
with strips of paper, aluminium, and platinum placed on the plate.

1 C. R. 130, pp. 206, 372, 810, 979. 1900.

IV]

NATURE OF THE RADIATIONS

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 the 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
in mms.

Inferior limit
of HR for
transmitted rays

Black paper ...

0-065

650

Aluminium . . .

o-oio

350

„

o-ioo

1000

0-200

1480

Mica ...

0-025

520

Glass

0-155

1130

Platinum

0-030

1310

SST... :::

0-085
0-130

1740
2610

If — is a constant for all the rays, the value of HR is propor-
tional 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

7—2

100

NATURE OF THE RADIATIONS

[CH.

just produce an impression through '01 mms. of aluminium. It

will be shown, however, in section 76, that — 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.

Sattery

"Electrometer

72. Examination of the fi 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. 23.

The radio-active matter A is placed on a lead block E" 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 a and /3 rays. The a 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 7 rays is small com-
pared with that produced by the /9 rays, and may be neglected.
On the application of a magnetic field at right angles to the mean

Fig, 23.

IV] NATURE OF THE RADIATIONS 101

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 0 rays of
uranium, thorium, and radium consist entirely of rays readily
deflected by a magnetic field. The rays from polonium consist
entirely of a 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 a. rays. On account of the slight deviation of the a 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
1/5 of the amount of deviable rays that the same weight of uranium
oxide gives.

73. Experiments with a fluorescent screen. The ft

rays from a few milligrams of pure radium bromide produce
intense fluorescence in barium platino-cyanide 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 0 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 electro-magnet, and

102 NATURE OF THE RADIATIONS [CH.

the screen placed below it. With no magnetic field, a faint
luminosity of the screen is observed due to the very penetrating
7 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 71). The direction of deviation
is reversed by reversal of the field. The broad extent of the
illumination shows the complex nature of the /3 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 readily be seen that the rays most easily deviated
are the least penetrating.

Comparison of the /3 rays with cathode rays.

74. Means of comparison. In order to prove the identity
of the /3 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 /3 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 with them 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 (3 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 very delicate methods.

Suppose that a layer of very active radium is spread on a metal
plate connected to earth, and that the 0 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

IV] NATURE OF THE RADIATIONS 103

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 a rays, but allows the ft 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 Curie1 used
the second method/

A metal disc MM (Fig. 24) is connected with an electrometer
by the wire T. The disc and wire are completely surrounded by
insulating matter ^^. 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 A A.

Fig. 24.

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 radium2, forming a

1 C. R. 130, p. 647, 1900.

2 The activity of the radium preparation was not stated in the paper.

104 NATURE OF THE RADIATIONS [CH.

layer 2*5 sq. cms. in area and 2 mms. thick, a current of the order
of 10~n 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
ft 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 a rays carry with them a charge opposite in
sign to the ft 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 a rays, but not too thick to
allow most of the ft 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 Dorn1. 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 differ-
ence of potential between the inside of the tube and the earth.

In this case the a rays were absorbed in the walls of the tube,
but a large proportion of the ft 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 elec-
tricity 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.

Strutt2 has recently described a simple experiment to illus-
trate still more clearly that a radium preparation acquires a
positive charge, if it is enclosed in an envelope thick enough to

1 Phys. Zeit. 4, No. 18, p. 507, 1903.

2 Phil. Mag. Nov. 1903.

IV] NATURE OF THE RADIATIONS 105

absorb all the a particles, but thin enough to allow most of the
ft particles to escape. A sealed tube, 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. The air in the outer tube was
exhausted as completely as possible by means of a mercury pump,
in order to reduce as much as possible 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 £ gram
of radiferous barium of activity only 100 times that of uranium.
It can readily be calculated that 10 milligrams of pure radium
bromide would have caused an equal divergence of the leaves
in a few minutes.

A determination of the amount of the charge carried off by the
rays of radium has been made recently by Wien1. 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 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 x 10~12 am-
peres. If the charge on each particle is taken as 1*1 x 10"20 electro-
magnetic units, this corresponds to an escape of 2*66 x 107 particles
per second. From 1 gram of radium bromide the corresponding
number would be 6'6 x 109 per second. Since some of the ft rays
are absorbed in their passage through the walls of the containing
vessel, the actual number projected per second from 1 gram of
radium bromide must be greater than the above value.

75. Determination of e/m. J. J. Thomson has shown 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 ft rays from radium,

1 Phys. Zeit. 4, No. 23, p. 624, 1903.

106 NATURE OF THE RADIATIONS [CH.

Dorn1 and Becquerel2 showed that they also were deflected by an
electrostatic field.

By observing the amount of the electrostatic 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 to 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

I
deviation^ and corresponded to the least deviable rays which gave

an impression through the black paper.

If a particle of mass ra, charge e, and velocity v, is projected
normally to an electric field of strength X, the acceleration a is in
the direction of the field, and is given by

a — — .
ra

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

1 C. R. 130, p. 1129, 1900. 2 C. R. 130, p. 809, 1900.

IV] NATURE OF THE RADIATIONS 107

dt of the ray parallel to the field after traversing a distance I is
given by

On leaving the electric field, the particle travels in the direction of
the tangent to the path at that point. If 6 is the angular deviation
of the path at that point

a
tan0= --.

mv2

The photographic plate was a distance h above the extremity of
the field. Thus the particles struck the plate at a distance d.2 from
the original path given by

d2 = h tan 6 + dl

In the experimental arrangement the values were
c/2 = -4 cms. ;
Z = T02 x 1012;
I = 3-45 cms. ;
h = 1*2 cms.

If the radius R of curvature of the path of the same rays is ob-
served in a magnetic field of strength H perpendicular to the rays,

e_ V_
m HR'

Combining these two equations we get

H.R.da '

A difficulty arose in identifying the radiations 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

v = 1*6 x 1010 cms. per second,

and — = 107.

108 NATURE OF THE RADIATIONS [CH.

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 /3 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 x 109 cms. per sec. In special tubes
with strong fields the velocity may be increased to about 1010 cms.
per sec. These charged particles behave like isolated units of
negative electricity. The conception of such disembodied charges,
known as electrons, has been examined mathematically among
others by Larmor, who sees in this conception the ultimate basis
of a theory of matter. The /3 rays from radium may also be
considered as electrons, but when obtained from this source have
velocities varying from about 1/3 V to at least *96F, where Fis the
velocity of light, and thus have an average velocity considerably
greater than that of the electrons produced in a vacuum tube.
These moving electrons are, it seems, 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. Electrons are thus con-
tinuously and spontaneously expelled from radium with enormous
velocities. 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 has been in rapid orbital or oscillatory motion in the atom,
and, by some means or other, 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.

76. Variation of — with the velocity of the electron.

The fact that radium throws off electrons with rates of speed
varying from 1/3 to 9/10 the velocity of light has been utilised by
Kaufmann1 to examine whether the ratio e/m of the electrons
varies with the speed. It has been shown by J. J. Thomson2,

1 Phys. Zeit. 4, No. 1 b, p. 54, 1902. 2 Phil. Mag. April, 1881.

IV] NATURE OF THE RADIATIONS 109

Heaviside1, and Searle2 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

202

is equal to ^— where a is the radius of the body, but it increases

rapidly as the speed of light is approached. It is very im-
portant 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 independent 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 Kauftnann used a formula
developed by M. Abraham3.

Let m0 = mass of electron for slow speeds ;

ra = apparent mass of electron at any speed ;
u = velocity of electron ;
V = velocity of light.

Let /8=£.

Then it can be shown that

- 8/4* 08) (1),

i ri + B- i + &

where >/r (ff) = — - -^- log 1 — g — 1 (2).

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, in which was
a small tube about 0'2 mm. in diameter, which allowed a narrow
bundle of rays to pass. The rays fell on a photographic plate
enveloped in a thin layer of aluminium.

In the experiments the diaphragm was about 2 cms. from the
active material and the same distance from the photographic plate.

1 Collected Papers, Vol. 2, p. 514. 2 Phil. Mag. October, 1897.

3 Phys. Zeit. 4, No. 1 b, p. 57, 1902.

110

NATURE OF THE RADIATIONS

[CH.

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.

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 tha
two spectra gives rise to a jcurved line on the plate. Disregarding
some small corrections, it can readily be shown that if y and z are
the electric and magnetic deviations respectively,

and

•(3),
.(4).

From these two equations, combined with (1) and (2), we
obtain

...(5),

where K, /cI} /c.2 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 Kaufmann1 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

2-36 x 1010 cms. per sec.
2-48 „
2-59
2-72 „
2-85 „

1-SlxlO7
1-17 xlO7
0-97 x 107
0-77 x 107
0-63 x 107

1 Nachrichten d. Ges. d. Wiss. zu Gott., Nov. 8, 1901.

IV]

NATURE OF THE RADIATIONS

111

For the cathode rays S. Simon1 obtained a value of e/m of
T86 x 107 for an average speed of about 7 x 109cms. per second.

In a later paper2 with some very active radium, more satis-
factory photographs were obtained which allowed of accurate
measurement. The given equation of the curve was found to
agree satisfactorily with experiment.

The following table, 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.

Value of
u
V

Observed value of

ni
m0

Percentage difference
from theoretical
values

Small

:732

1-34

-1-5%

•752

1-37

-0-9,,

'111

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 „

The average percentage error between the observed and calcu-
lated 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 - - becomes large. This

is shown in the following table which gives the calculated values

?72/

of - - for different velocities of the electron.
ra0

Value of ^. small *1

•9 -99 '999 -9999 -999999

Calculated m
• value m

1-00 1-015 1-12 1-81 3-28 4-96 6'68

10-1

Thus for velocities varying from 0 .to 1/10 the velocity of light,

1 Wied. AnnaL p. 589, 1899.

2 Phys. Zeit. 4, No. 1 b, p. 54, 1902.

112 NATURE OF THE RADIATIONS [CH.

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. Theo-
retically 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 T84 x 107,
which is in very close agreement with the value obtained by
Simon for the cathode rays, viz. 1'86 x 107.

If the electricity carried by the electron is supposed to be
distributed uniformly over a sphere of radius a, for speeds slow

2 e2
compared with the velocity of light, the apparent mass ra0 = ^ - .

o €L

2 e

Therefore a = ~ — . e.

3m0

Taking the value of e as 1'13 x 10~20, a is I' 4s x 10~13 cms.
Thus the diameter of an electron is minute compared with the
diameter of an atom.

77. Absorption of the $ rays by matter. The absorption
of the /3 rays by matter can readily be investigated by noting the
variation of the ionization current in a testing vessel when the
active matter is covered by screens differing in material and thick-
ness. 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. 16, is due almost entirely to the @ rays. If a uranium
compound is used, it is found that the saturation current decreases
with the thickness of matter traversed very nearly according to an
exponential law. Taking the saturation current as a measure of
the intensity of the rays, the intensity / after passing through a
thickness d of matter is given by

TQ~ e '

where X is the constant of absorption of the rays in unit thickness

IV] NATURE OF THE RADIATIONS 113

of matter, and /0 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 Schweidler1. 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 a rays. This variation in the absorption is due to
the fact that the y9 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 con-
sequently 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
Lenard2, 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. Strutt3, 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 X 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.

78. Since the ft rays from uranium are fairly homogeneous,
and are at the same time penetrating in character, they are more

1 Phys. Zeit. pp. 90, 113, 209, 1900.

2 Wied. Annal. 56, p. 275, 1895.

3 Nature, p. 539, 1900.

R. R.-A. 8

114

NATURE OF THE RADIATIONS

[CH.

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 X, is the
^coefficient of absorption.

Substance

Density

X
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

•70

5-3

Iron ...

7'8

5-6

Aluminium

14-0

2-60

5-4

Copper

8-6

7-0

Silver ...

10-5

7-1

Lead ...

122

11-5

10-8

Tin ...

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 X
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 (3 rays
depending only on the density does not hold for all substances.
With an exception in the case of tin, the value of X divided by the
density for the metals increases in the same order as their atomic
weights.

The absorption of the /3 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 ft rays. The velocity of the ft rays of uranium
was found by Becquerel to be about 1*6 x 1010 cms. per sec. The
velocity of the cathode rays used in Lenard's experiment was
certainly not less than 1/10 of this, so that, for a decrease of
speed of less than 10 times, the absorption has increased over
500 times.

IV] NATURE OF THE RADIATIONS 115

79. 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 X be the absorption constant of the homogeneous radiation
by the active material. It can readily be shown that the intensity
/ of the rays issuing from a layer of active matter, of thickness x,
is given by

where 70 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 ft rays for different thicknesses of uranium
oxide. In this case / = £ /0 for a thickness of oxide corresponding
to 11 gr. per sq. cm. This gives a value of \ 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 ft rays does not
absorb them to a much greater extent than does ordinary matter
of the same density.

The value of \ will vary, not only for the different active
substances, but also for the different compounds of the same
substance.

PART III.

THE a RAYS.

80. The a rays. The magnetic deviation of the ft rays was
discovered towards the end of 1899, at a comparatively early stage
in the history of radio-activity, but it was not until three years later
that the true character of the a rays was disclosed. It was natural
that great prominence should have been given in the early stages
of the subject to the ft rays, on account of their great penetrating

8—2

116 NATURE OF THE RADIATIONS [CH.

power and marked action in causing phosphorescence in many
substances. The a rays were, in comparison, very little studied,
and their importance was not generally recognized. It will, how-
ever, be shown that the a rays play a far more important part
in radio-active processes than the ft rays, and that the greater
portion of the energy emitted in the form of ionizing radiations
is due to them.

81. The nature of the a rays. The nature of the a rays
was difficult to determine, for a magnetic field sufficient to cause
considerable deviation of the ft rays produced no appreciable effect
on the a rays. It was suggested by several observers that they
were, in reality, secondary rays set up by the (3 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 a rays only. Later work also showed that the matter,
which gave rise to the ft rays from uranium, could be chemically
separated from the uranium, while the intensity of the a rays was
unaffected. These and other results show that the a and ft rays
are produced quite independently of one another. The view that
they are an easily absorbed type of Rontgen rays fails to explain
a characteristic property of the a rays, viz. that the absorption of
the rays in a given thickness of matter, determined by the elec-
trical 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 to below a certain
value. From observations of the relative ionization produced in
gases by the a and ft rays, Strutt1 suggested in 1901 that the «
rays might consist of positively charged bodies projected with
great velocity. Sir William Crookes2, in 1902, advanced the same
hypothesis. From a study of the a rays of polonium Mme Curie3
in 1900 suggested the probability that these rays consisted of
bodies, projected with great velocity, which lost their energy by
passing through matter.

1 Phil. Trans, p. 507, 1901.

2 Proc. Roy. Soc. 1902. Ghent. News, 85, p. 109, 1902.

3 C. E. 130, p. 76, 1900.

IV] NATURE OF THE RADIATIONS 117

The writer was led independently to the same view by a mass
of indirect evidence which received an explanation only on the
hypo thesis 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 devia-
tion of the a 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
measure 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 devia-
tion is may be judged from the fact that the a 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 con-
ditions the cathode rays produced in a vacuum tube would describe
a circle of about '01 cm. radius. It is therefore not surprising
that the a rays were for some time thought to be non-deviable in
a magnetic field.

82. Magnetic deviation of the a rays. The general
method employed1 to detect the magnetic deviation of the a 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. 25 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 I), 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.

In order to increase the ionization in the testing vessel, the

1 Rutherford, Phil. Mag. Feb. 1903. Phys. Zeit. 4, p. 235, 1902.

118

NATURE OF THE RADIATIONS

[CH.

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.

mfTttflow of Hydrogen

Earth

Outflow of Hydrogen
Fig. 25.

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 srnall 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, streamed through the porous alu-
minium foil, and passed between the plates carrying the emanation
with it away from the apparatus. The use of a stream of hydrogen

IV] NATURE OF THE RADIATIONS 119

instead of air greatly simplifies the experiment, for it increases the
ionization current due to the a rays in the testing vessel, and at
the same time greatly diminishes that due to the ft and 7 rays.
This is caused by the fa^t^hatjLhe_gLrays. are much more readily
absorbed in air than in hydrogen, while the rate of production of
ions due to the ft and 7 rays is much less in hydrogen than in air.
The intensity of the a 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 T90 x 2'50cms.

Strength of field between pole-pieces 8370 units.

Apparatus of 25 parallel plates of length 370 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 172

(3) Radium covered with thin layer of mica to

absorb all a rays 0'93

(4) Radium covered with mica and magnetic field

applied 0'92

The mica plate, '01 cm. thick, was of sufficient thickness to
completely absorb all the a rays, but allowed the ft rays and 7 rays
to pass through without appreciable absorption. The difference
between (1) and (3), 7*40 volts per minute, gives the rate of dis-
charge due to the a rays alone ; the difference between (2) and (3),
0'79 volts per minute, that due to the a rays not deviated by the
magnetic field employed.

The amount of a 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 ft rays, for they would

120 NATURE OF THE RADIATIONS [CH.

be completely deviated by the magnetic field. (4) comprises the
effect of the <y rays together with the natural leak of the electro-
scope 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 a 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 a 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 a rays must consist of positively charged particles.

These results were soon after confirmed by Becquerel1, 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 paralled 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 reflect the ft rays completely away
from the plate. When the plate was parallel to the opening,
•there was produced on it an impression, due to the a rays alone,
which became more and more diffuse as the distance from the
opening increased. This distance should not exceed 1 or 2 centi-
metres 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 a rays are
observed which are deflected in opposite directions. This devia-
tion, even in a strong field, is small though quite appreciable and
1 C. R. 136, p. 199, 1903.

IV] NATURE OF THE RADIATIONS 121

is opposite in sense to the deviation observed for the 13 or cathodic
rays from the same material.

M. Becquerel1, by the same method, found that the a rays from
polonium were deviated in the same direction as the a 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 0 rays.

83. Electrostatic deviation of the a 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. 25) 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 a 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 °/0
with a P. D. of 600 volts between the slits. With a special arrange-
ment 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.

84. 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. From
the equations of a moving charged body, the radius p of curva-
ture of the path of the rays in a magnetic field of strength H
perpendicular to the path of the rays is given by

1 C. R. 136, p. 431, 1903.

122 NATUEE OF THE RADIATIONS [CH.

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

2pdl = Zj2

, /!2 e H
1-SmV .................. ........ (1)'

If the rays pass through a uniform electric field of strength X and
length 12 with a deviation d»,

since - - is the acceleration of the particle, at right angles to its

direction, and ^ is the time required to travel through the electric
field.

From equations (1) and (2)

L <ft&3T

~~d,tfH'

e 24 V

and - = - ~ -==. .

m ff 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 x 109 cms. per sec.

- = 6 x 103.
m

On account of the difficulty of obtaining a large electrostatic de-
viation, these values are only approximate in character.

The results on the magnetic and electric deviation of the
a rays of radium have been confirmed by Des Coudres1, 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
1 Phys. Zeit. 4, No. 17, p. 483, 1903.

IV] NATURE OF THE RADIATIONS 123

to apply a stronger electric field without the passage of a spark.
He found values of the constants given by

V= 1*65 x 109cms. per sec.

These values are in very good agreement with the numbers found
by the electric method. The 2 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 devia-
tion 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 ft rays.

85. Becquerel1 has examined the amount of magnetic devia-
tion of the a. 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
Hp of the strength of the field and the radius of curvature of the
path of the rays is shown in the following table.

1 C. R. 136, p. 1517, 1903.

124 NATURE OF THE RADIATIONS [CH.

Distance in mms.

from the slit

2-91 x 105

2-99 „

3-06 „

3-15 „

3-27 „

3-41 „

The writer (loc. cit.) showed that the maximum value of Hp
for complete deviation of the a rays was 390,000. The results are

fty\

thus in good agreement. Since Hp — — V these results show

that the values either of V or of — 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 con-
sist of particles projected with different velocities, and that the
slower particles are more quickly absorbed in the gas. In conse-
quence of this, only the swifter particles are present some distance
from the source. Before any definite conclusion can be reached, it
will be necessary to determine the actual values of e/m and V for
different points of the trajectory.

Becquerel states that the amount? of deviation, in a given
magnetic field, was the same for the a rays of polonium and of

radium. This shows that the value of V is the same for the

a rays from the two substances. Since the a rays from polonium
are far more readily absorbedj than the a rays from radium, this

result would indicate that the value of — is greater for the a par-
ticles of polonium than of radium. Further experimental evidence
is required on this important point.

86. Mass and energy of the a particle. It has been
pointed out that the a rays from radium and polonium are
analogous to the Canal rays of^joT^stein, for both carry a positive

IV] NATURE OF THE RADIATIONS 125

charge and are difficult to H^fWt hy a. Trmgnptip field. The experi-
ments 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 1/10 of the velocity
of the a. 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 a. rays of radium

V= 2-5 x 109 and e/m =6 x 103.

Now the value of e/m for the hydrogen atom, liberated in the
electrolysis of water, is 104. Assuming the charge carried by the
a particle to be the same as that carried by the hydrogen atom, the
mass of the a particle ^s about twice_that of the hydrogen atom. If
the a 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 202.

The a. 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 a rays from the different radio-active sub-
stances consist of positively charged bodies projected with great
velocity. Since the rays from radium are made up in part of a
rays from the emanation stored in the radium, and from the
excited activity which it produces, the a rays from each of these
products must consist of positively charged bodies ; for it has been
shown that all the a rays from radium are deviated in a strong
magnetic field.

The kinetic energy of each projected particle is enormous, com-
pared with its mass. The kinetic energy of each a particle

= i m V* = i ™ V*e = 5'9 x 10~6 ergs.

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 a rays is
6 x 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 (section
106).

126 NATURE OF THE RADIATIONS [CH.

87. 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 dis-
integration, in the course of which parts of the atom escape from
the atomic system. It seems very improbable that the a and 0
particles can suddenly acquire their enormous velocity of projection
by the action of forces existing inside or outside the atom. For
example, the a 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 oscil-
latory or orbital motion has been developed by J. J. Thomson,
Larmor and Lorentz. Since the a 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 X.

88. Experiments with a zinc sulphide screen. A screen

IV] NATURE OF THE RADIATIONS 127

of Sidot's hexagonal bleiid_(phQspboi^aceiiL-aii^-^ulphidp) lights
up brightly under the action of the a rajs of radium and polonium.
If the surface of the screerLJs examined with ajnagnifying 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
Crookes1, and independently by Elster and Geitel2, 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_o£__m£tal, which has
been dipped in a radium solution, is fixed several millimetres #way
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 centi-
metres distance, only an occasional one is seen. The experiment
is extremely beautiful, and brings vividly before the observer the
fact 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 scintillatingjgpints of light on the screen are due to the
impact of the a particles on its surface. If the radium is covered
with a layer of foil of sufficient thickness to absorb all the a rays
the scintillations cease. There is still a phosphorescence to be
observed on the screen due to the /3 and 7 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

1 Proc. Roy. Soc. 81, p. 405, 1903.

2 Phys. Zeit. No. 15, p. 437, 1903.

128 NATUKE OF THE RADIATIONS [CH.

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 scintilla-
tions 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.

The scintillations are produced not only by radium and
polonium, but also by a negatively charged wire made active by
exposure in the open air or in a vessel containing the emanations
of thorium or radium. As far as observations have yet gone, jbhe
production of scintillations appears to be a general property of the
a raysjrQm^all-radio-active substances. The scintillations are best
shown with a zinc sulphide screen. If a screen of barium platino-
cyanide is exposed to the a 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 probably accounts for the absence
of visible scintillations.

In the scintillations of zinc sulphide, we are actually witnessing
the effect produced by the impact on the screen of single atoms of
matter projected with enormous velocity. Each of the particles
carries an amount of energy corresponding to 5'9 x 10~6 ergs. 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 transformed into light. Zinc sulphide is very
sensitive to mechanical shocks. Luminosity is observed if a pen-
knife 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 a
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. Becquerel1 recently 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 a rays. Scintillations can be mechanically produced by
crushing a crystal. Tomrnasina2 found that a zinc sulphide screen
1 C. R. 137, Oct. 27, 1903. 2 C. E. 137, Nov. 9, 1903.

IV] NATURE OF THE RADIATIONS 129

removed from the action of the radium rays for several days,
showed the scintillations again when an electrified rod was brought
near it.

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 be shown
to consist of scintillations. This use of the microscope would offer
a very convenient means of actually counting the number of the
particles projected from the surface of the radium, if each particle
gave rise to a flash of light. It is not likely, however, that this
would be the case. The number of scintillations from a given mass
of radium will depend upon its fineness of division, but on account
of the ease with which the projected particles are absorbed, only
a small portion of the total number projected from the mass of
radium will escape from its surface.

89. Absorption of the a rays by matter. The a rays from
the different radio-active substances can— be- distinguished from
each other by the relative amounts of their absorption by gases
or by thin screens of solid substances. When examined under
the same conditions, the a 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 a rays for
different thicknesses of matter, an apparatus similar to that shown
in Fig. 16, p. 82 was employed1. 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 layer2 of active material, the ionization between
the plates is almost entirely due to the a. rays. The ionization
due to the 0 and 7 rays is generally less than 1°/0 of the total.

The following table shows the variation of the saturation current
between the plates due to the a rays from radium and polonium,

1 Rutherford and Miss Brooks. Phil. Mag. July 1902.

2 In order to obtain a very thin layer, the compound to be tested is ground to a
fine powder and then sifted through a fine gauge uniformly over the area, so that
the plate is only partially covered.

R. R.-A. 9

130

NATURE OF THE RADIATIONS

[CH.

with successive layers of aluminium foil interposed, each "00034 cm.
in thickness. In order to get rid of the ionization due to the 0
rays from radium, the radium chloride employed was dissolved in
water and evaporated. This renders the active compound, for the
time, nearly free from /3 rays.

Polonium. Radium.

Layers of
aluminium

Current

Ratio of
decrease for
each layer

100

•41

•31

12-6

•17

2-1

•067

•14

Layers of
aluminium

Current

Ratio of
decrease for
each layer

100

•48

•60

13-6

•47

6-4

•39

2-5

•36

•9

The initial current with 1 layer of aluminium over the active
material is taken as 100. It will be observed that the current due
to the radium rays decreases very nearly by half its value for each
additional thickness until the current is reduced to about 6 °/0 of
the maximum. It then decays more rapidly to zero. Thus, for
radium, over a wide range, the current decreases in an exponential
law with the thickness of the screen,

** - o-M

where it is the current for a thickness t, 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.

IV]

XATURE OF THE RADIATIONS

131

Battery

Electrometer

Fig. 26.

90. The increase of absorption of the a rays of polonium with
the thickness of matter tra-
versed has been very clearly
shown in some experiments
made by Mme Curie. The
apparatus employed is shown
in Fig. 26.

The saturation current was
measured between two parallel
plates PP' 3 cms. apart. The
polonium A was placed in the
metal box CO, 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 A T was diminished, the current increased
in a very sudden manner, so that for a small variation of the distance
A T 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 con-
sisted 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 AT in cms.

3-5

2-5

1-9

1-45

0-5

For 100 rays transmitted by one layer

For 100 rays 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 7 or 8 cms. the a rays from uranium, thorium, or

9—2

132

NATURE OF THE RADIATIONS

[CH.

radium have been completely absorbed in the gas, and the small
ionization then observed in the gas is due to the more penetrating
/3 and 7 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 7 cms. in depth.

91. The a 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 writer1 and by Owens2. For the purpose of comparison of the
relative power of penetration of the a rays from the different radio-
elements, it is thus only necessary to determine the penetrating
power for one compound of each of the radio-elements. Rutherford

100

Absorption of Radiation
by Aluminium Poil

1234
Layers of Aluminium Foil ('00034 cms. thick)

Fig. 27.

and Miss Brooks3 have determined the amount of absorption of
the a rays from the different active substances in their passage
through successive layers of aluminium foil '00034 cm. thick. The

1 Phil Mag. Jan. 1899. 2 Phil. Mag. Oct. 1899. 3 Phil. Mag. July, 1900.

IV] NATURE OF THE RADIATIONS 133

curves of absorption are given in Fig. 27. For the purpose of
comparison in each case, the initial current with the bare active
compound is 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 the purpose of 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 polo-
nium. The a radiations may be arranged in the following order,
as regards their power of penetration, beginning with the most
penetrating.

Thorium )

-r, ,. }• excited radiartion.

Kadium 1

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 a rays from
the active bodies are thus considerable, and must be ascribed either
to a difference of mass or of velocity of the a particles or to a
variation in both these quantities.

Since the a 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.

92. Absorption of the a rays by gases. The a 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 a rays, is greatest near the surface of the radiating body
and falls off very rapidly with the distance (see section 90).

134

NATURE OF THE RADIATIONS

[CH.

TT Electrometer

^•Battery

A simple method of determining the absorption in gases is
shown in Fig. 28. 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 be-
sides air, and for examining the Fig. 28.
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 A, is the " absorption constant " of the radiation for the gas
under consideration1. Let

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

The energy of the radiation at the lower plate is then IQe~Kx,
and at the upper plate IQe~*(l+x]. The total number of ions pro-
duced between the parallel plates A and B is therefore proportional

g— \x _ e-\ d+x) _ g— \x / 1 e~*1}

Since the factor 1 — e~Kl is a constant, the saturation current

1 Since the ionization at any point above the plate is the resultant effect of the
a particles coming from all points of the large radio-active layer, X is not the same as
the coefficient of absorption of the rays from a point source. It will however be
proportional to it. For this reason X is called the " absorption constant."

IV]

NATURE OF THE RADIATIONS

135

between A and B varies as e~**, i.e. it decreases according to an
exponential law with the distance traversed.

0 5 Distance in mm*. 10 15

Fig. 29.

The variation of the current between A and B with the distance
from a thin layer of uranium oxide is shown in Fig. 29 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

Carbonic acid

Air

Coal-gas
Hydrogen . . .

Distance in mms. to
absorb half of radiation

3
4-3

7-5
16

The results for hydrogen are only approximate, as the absorp-
tion is small over the distance examined.

136

NATURE OF THE RADIATIONS

[CH.

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 a. rays
from the different active bodies are shown in Fig. 30.

100

Absorption of Radiation by Air

10 12 14 16
Distance in mms.

20 22

Fig. 30.

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 from Thorium and Radium ... 16-5

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.

IV] NATURE OF THE RADIATIONS 137

93. 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 /
after passing through a thickness x is given by / = I0e~^ where \
is the absorption constant and I0 the initial intensity.

The following table shows the value of \ with different radia-
tions for air and aluminium.

Radiation X for aluminium X for air

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 X 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 X
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 1/4 and 1
atmosphere. Owens (loc. cit.) examined the absorption of the a
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

138 NATURE OF THE RADIATIONS [CH.

proportional to it. Since the absorption of the a 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.

94. Relation between ionization and absorption in
gases. It has been shown (section 45) that if the a 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 com-
pared with the relative absorption constants (section 92).

,< 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.

95. Theory of the absorption of the a rays by matter.

As we have seen, experiment shows that the ionization of the gas
due to the a rays from a large plane surface of radio-active matter
falls off approximately according to an exponential law until most
of the rays are absorbed, whereupon the ionization decreases at a
much faster rate. The ionization of the gas is due_ta the collision
of the positively^^r^edjmrticles with the molecules in their path.
Each projected particle carries with it sufficient energy to produce,
on an average, several thousand ions in its path before its velocity
is reduced to a value below which it fails to ionize the gas. This
minimum velocity for the a and /3 particles is probably about 108 cms.
per second. More experimental data are required on the variation of
the amount of ionization of the gas with the speed of the projected

IV] NATURE OF THE RADIATIONS 139

particles. The experiments of Townsend1 and Durack2 point to the
conclusion that the amount of ionization per unit distance passes
through a maximum and then decreases as the velocity of the
particle increases. 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 21 ions per cm. in air at a pressure of 1 mm. of mercury ;
while for a much higher velocity of about 5 x 109 cms. per second
Durack found that the electrons only produce '4 ions per cm. at
1 mm. pressure. In a later paper, Durack3 showed that for the
electrons from radium, which are projected with a velocity of
about half the velocity of light, the corresponding number of ions
per cm. of path is '19 or only about 1/100 of the maximum number
observed by Townsend.

It has been shown by Des Coudres that the velocity of the
cathode rays diminishes when the rays pass through thin metal
toil. This is probably also true of the a and /3 particles produced
by the active substances.

If the decrease of the ionization according to an exponential
law with the distance were due only to a gradual retardation of
the speed of the projected particles, it follows that the ionization
per unit distance for both the a and /? particles must vary as the
square of the velocity of the particle. For suppose that in passing
through a distance dx a particle of mass in decreases in speed from
v to v — dv. The loss of energy of the particle is mvdv, and
this should be proportional to the number of ions qdx produced,
where q is the rate of production of ions per unit length of the
path. Since the ionization is assumed to fall otf in an exponential
law with the distance x, we get q = qQe~^ where qQ is the value of q
when x = 0.

Then mvdv =

where k is a constant and

- = -- -

A, A, A.

1 Phil. Mag. Feb. 1901. 2 Phil. Mag. July, 1902.

3 Phil. Ma0. May, 1903.

140 NATURE OF THE RADIATIONS [CH.

for A = 0, since q = 0 when v = 0. q should thus be proportional
to v2. This conclusion is contrary to the experimental results, for
it has been shown, at any rate for the {3 particles, that the ioniza-
tion per unit distance decreases with increase of velocity.

The variation of ionization with distance thus cannot be due
entirely to the gradual retardation of the particles. It seems
probable that it is due to one of the following causes : —

(1) absorption of the projected particles in their passage
through matter ;

(2) neutralization of some of the charges carried by the pro-
jected particles.

It can be shown that the number of a particles which are able
to produce " scintillations " on a zinc sulphide screen is diminished
by the interposition of a metal screen. The hypothesis (2) seems
more probable than (1), for it is difficult to see how masses, possess-
ing such an amount of kinetic energy as the a and /3 particles at
the moment of their expulsion, can be completely stopped by a
single collision, unless the velocity of the particles has already been
greatly reduced by their passage through matter. On the second
hypothesis, the particles after losing their charges may still keep
moving in their path with a high velocity, but it is to be expected
that they would not be nearly as efficient in ionizing the gas as a
charged particle of equal mass moving with the same velocity.
Their existence would not be recognized by ordinary methods
unless they produced an appreciable ionization by collision with
the molecules. Thus, it is possible that, in addition to the a and
/3 charged particles, there may be a stream of uncharged particles
moving through the gas with great velocity, the existence of which
has not yet been detected.

This gradual neutralization of the charges on the projected a
particles, and the consequent inability of the particles to produce
ions in their path, are probably responsible for most of the so-called
" absorption " of the rays in traversing matter whether solid, liquid,
or gaseous. If, in addition, the speed of the projected particles is
gradually decreased by their passage through matter, as the mini-
mum velocity required to produce ions is approached, the particles
which still retain their charge will decrease in ionizing power, and,

IV] NATURE OF THE RADIATIONS 141

in consequence, the number of ions produced per unit length of
path will diminish far more rapidly than the law observed for
higher velocities would lead us to expect. This offers an explana-
tion of the great increase of absorption of the a rays by matter
which is observed when the rays are nearly all absorbed.

PART IV.

THE 7 OR VERY PENETRATING RAYS.

96. In addition to the a and /3 rays, the three active sub-
stances, uranium, thorium, and radium, all give out a radiation of
an extraordinarily penetrating character. These 7 rays are con-
siderably 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.

Villard1, using the photographic method, first drew attention
to the fact that radium gave out these very penetrating rays, and
found that they were non-de viable by a magnetic field. This result
was confirmed by Becquerel*.

Using a few milligrams of radium bromide, the 7 rays, can
readily be detected in a dark room by_the luminosity they excite
in the mineral willemite or a screen of ^platinocyanide of barium.
The a and {3 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 7 rays. The very great
penetrating power of these rays is readily 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
7 rays from 30 mgs. of radium bromide can be observed in an
electroscope after passing through 30 cms. of iron.

1 C. R. 130, pp. 1110, 1178. 1900. 2 C. R. 130, p. 1154, 1900.

142 NATURE OF THE RADIATIONS [CH.

97. In an examination of the active substances by the elec-
trical method the writer1 found that both uranium and thorium
gave out 7 rays in amount roughly proportional to their activity.
An electroscope of the type shown in Fig. 11 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 according to an exponential law with the thickness of
lead traversed.

Thickness of lead

Kate of discharge

•62 cms.

100

. , -f- o4 cms.

„ +2-86 „

„ +5-08 „

Using 100 gr. 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 y 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.

Results originally obtained with an electrometer in the place of
an electroscope gave results indicating about 20 per cent, less pene-
trating power. The electroscopic results are probably the more
accurate, but those obtained with the electrometer, as given below,
serve for the purpose of comparison.

M , i Thickness of metal to

absorb half of the rays

Mercury ... -75 cms.

Lead ... -9 „

Tin ... 1-8 „

Copper ... 2-2 „

Zinc ... 2'5 „

Iron ... 2-5 „

1 Phys. Zeit. p. 517, No. 22, 1902.

NATURE OF THE RADIATIO1STS

143

98. Connection between absorption and density. The

absorption constant X of the rays was determined from the

equation T = e~Xx for screens of different materials. On account

__ *0

of the small absorption in water and glass it was difficult to
determine \ with accuracy.

The results are included in the following table : —

y rays

/3 rays from
uranium

Substance

density

Water

•033

Glass

•086

•035

14-0

5-7

Iron ...

•28

•036

5'6

Zinc...

•28

•039

Copper
Tin

•31
•38

•035
•052

60
96

7-7
13-2

Lead

•77

•068

•1-22

10-8

Mercury

•92

•068

—

On the right is added a comparison table for the ft rays given
out by uranium. It will be seen that the quotient of absorption
by density is in neither case a constant, but the differences are
no greater for the non-deviable penetrating rays than for the devi-
able rays of uranium. It is interesting to observe that the value
of X divided by the density is, for both types of rays, twice as great
for lead as for glass or iron.

It will be seen from the above table that the penetrating rays
from radium compared with the deviable rays of uranium pass
through a thickness of glass about 160 times greater for the same
reduction of intensity.

99. Nature of the rays. In addition to their great pene-
trating power, the 7 rays differ from the a and 13 rays in riot being
deflected to an appreciable amount by a magnetic field.

It now remains to consider whether the rays are material in
nature or whether they are a type of ether-pulse like Rontgen
rays. In some respects the 7 rays seem more closely allied to

144 NATURE OF THE RADIATIONS [CH.

cathode than to Rb'ntgen rays. It is well known that Rontgen
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, under the
influence of X rays sulphuretted hydrogen has six times the con-
ductivity of air, but under the cathode rays the conductivity is
only slightly greater than that of air. In an experiment made
by the writer, in which the testing vessel was filled with sulphu-
retted hydrogen, it was found that the current for the 7 rays
from radium was only slightly greater than it was when the vessel
contained air.

Strutt1 has recently made a detailed investigation of the rela-
tive conductivity of gases exposed to the 7 rays of radium. The
results have already been given in the table in section 45. He found
that the relative conductivities of different gases compared with
air were about the same as for the /3 rays of radium, but were very
different from the conductivities for Rontgen rays.

The variation of absorption of these rays with density is also
very similar to that of the cathode rays. On the other hand,
Benoist'2 has shown that the relative absorption of Rontgen rays
by matter depends to a large extent on the kind of rays em-
ployed. " Hard " rays give ratios quite different from " soft " rays.
For penetrating Rontgen rays, the absorption of the rays by a given
weight of material is a continuous and increasing function of the
atomic weight.

The 7 rays thus show properties with regard to absorption
and ionization unlike those of X rays, but it must not be forgotten
that the 7 rays are of a far more penetrating character. It has
not yet been shown that the properties of very penetrating X rays
with regard to relative absorption and ionization are the same as
those of the ordinary rays of moderate penetrating power which
l^ave so far been examined.

It will be shown later (section 194) that the 7 rays, like the
$ rays, appear only in the last stage of the succession of chemical
changes occurring in active bodies. Active products which give
a rays and no ft rays do not give rise to 7 rays. The @ and 7 rays

1 Proc. Eoy. Soc. 72, p. 208, 1903.

2 C. E. 132, p. 545, 1901.

IV] NATURE OF THE RADIATIONS 145

appear always to go together and are present in the same pro-
portion. The main facts known about the 7 rays are summarized
below : —

(1) Great penetrating power.

(2) Non-deviation in an intense magnetic field.

(3) A law of absorption similar to that of cathode and ft rays.

(4) Occurrence of ft and 7 rays together and in the same
proportion.

Three possible hypotheses may thus be considered : —

(1) That the 7 rays are very penetrating Rontgen rays.

(2) That they consist of negatively charged particles projected
with a velocity very nearly equal to that of light.

(3) That they consist of uncharged bodies projected with
great velocity.

Rontgen rays are believed to be electromagnetic pulses set up
by the sudden stoppage of the cathode rays produced in a vacuum
tube. Thus it is to be expected that Rontgen rays should be
produced at the sudden starting as well as at the sudden stopping
of electrons. Most of the ft particles from the radio-elements are
projected with velocities much greater than those of the cathode
rays in a vacuum tube. Thus Rontgen rays of a very penetrating
character should be set up, if the electron is very suddenly expelled
with great velocity. This would account for the facts (1), (2) and
(4), but it is at variance with (3) unless the relative conductivity
of gases for a very penetrating type of X rays follows the law of
conductivity of the ft or cathode rays1. Strutt has also pointed
out that the proportion of 7 rays to ft rays from radium is much
greater than the proportion of X rays to the cathode rays produced
in a vacuum tube.

It has been shown that the ft rays from radium are complex

1 (Added Feb. 18, 1904.) Mr A. S. Eve of McGill University, Montreal, lias
recently examined the relative conductivity of some gases for very "hard" X rays
after their passage through a lead screen 1-8 mms. thick, and has obtained ratios
very different from those observed for "soft" rays, but approximating closely to
those obtained for the y rays. These observations remove the most serious objec-
tion which has been urged against the view that the 7 rays are in reality X rays of
a very penetrating type.

R. R.-A. 10

146 NATUKE OF' THE RADIATIONS [CH.

and include electrons travelling with a speed of more than 95 per
cent, of that of light. The apparent mass of an electron would
increase rapidly as the speed of light is approached, and for the
velocity of light the mass should be infinite and the path unaffected
by a magnetic field. It does not seem improbable that some of
the /3 rays of radium are projected with a velocity very nearly
equal to that of light, and thus it is possible that the 7 rays
may really consist of electrons expelled with velocities which still
more nearly approach that of light. The great increase of pene-
trating power is to be expected on account of the rapidly increasing-
energy of the electron as the speed of light is approached. An
objection to this hypothesis lies in the experimental observation
that there appears to be no gradual passage from the stage of
penetrating deviable rays to non-deviable very penetrating rays.
It is also possible that the <y rays may consist of uncharged
particles projected with great velocity. Such an hypothesis would
account for the relative conductivity of gases and for the non-
deviation of the rays in a magnetic field. It would also account
for the great penetrating power of the rays, since a small uncharged
particle moving through matter would probably not be absorbed
as rapidly as a charged particle of the same mass and velocity^-
Nevertheless, sufficient experimental data are not yet available to
distinguish definitely between the three hypotheses discussed above.

PART V.
SECONDARY RAYS.

100. Production of secondary rays. It has long been
known that Rontgenjrays, when they impinge on solid^^bstacles,
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, Becquerel1 has made a close

1 C. R. 132, pp. 371, 734, 1286. 1901.

IV] NATURE OF THE RADIATIONS 147

study of the secondary radiations produced by radio-active sub-
stances. In his earliest observations he noticedJthat^adiographs
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 a rays are very feeble.
They are best shown by polonium, which gives out only a rays,
when, in consequence, the results are not complicated by the
action of the ft rays. Strong secondary rays are set up at the
point of impact -of the ft 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
secondaiy 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 de viable 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 7 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 ft 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 ex-
pended in producing photographic action than in the case of the

10—2

148

NATURE OF THE RADIATIONS

[CH. IV

primary rays. It is thus not surprising that the secondary rays
set up by the ft and 7 rays may in some cases produce a photo-
graphic impression comparable with, if not greater than, the effect
of the incident rays.

On account of these secondary rays, radiographs produced by
the ft 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.

101. Mme Curie1 has shown by the electric method that the
a 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.

Screens employed

Thickness
in mms.

Current
observed

Aluminium
Cardboard

o-oi

0-005

17-9

Cardboard
Aluminium

0-005

o-oi

6-7

Aluminium
Tin

o-oi

0-005

150

Tin

Aluminium

0-005

o-oi

126

Tin

Cardboard

0-005
0-005

13-9

Cardboard
Tin

0-005
0-005

4-4

These results show that the a 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 ft rays
of radium. The production of secondary rays by the ft rays of
radium is, however, readily shown by the photographic method.

These presentee a la Faculte des Science*, Paris 1903, p. 85.

CHAPTER V.

RATE OF EMISSION OF ENERGY.

102. Comparison of the ionization produced by the a
and /3 rays. With unscreened active material the ionization
produced between two parallel plates, placed as in Fig. 16, is mainly
due to the a rays. On account of the slight penetrating power of
the a rays, the current due to them practically reaches a maximum
with a small thickness of radio-active material. The following
saturation currents were observed1 for "different thicknesses of
uranium oxide between parallel plates sufficiently far apart for all
the a rays to be absorbed in the gas between them.

Surface of uranium oxide 38 sq. cms.

Weight of uranium oxide

Saturation current

in grammes per sq. cm.

in amperes per sq. cm.

of surface

•0036

1-7 x 10~13

•0096

3-2xlO~13

•0189

4-OxlO-13

•0350

4'4xlO-13

•0955

4-7xlO~13

The current has reached about half its maximum value for
a weight of oxide '0055 gr. per sq. cm. If the a rays are cut off
by a metallic screen, the ionization is then mainly due to the
y8 rays, since the ionization produced by the 7 rays is small in
comparison. For the ft rays from uranium oxide it has been

1 Rutherford and McClung, Phil. Trans. A. p. 25, 1901.

150

KATE OF EMISSION OF ENERGY

[CH.

shown (section 79) that the current reaches half its maximum
value for a thickness of O'll gr. per sq. cm.

On account of the difference in the penetrating power of the a
and ft rays, the ratio of the ionization currents produced by them
depends on the thickness of the radio-active layer under examina-
tion. The following comparative values of the current due to the
a and ft rays were obtained for very thin layers of active matter1. A
weight of 1/10 gramme of fine powder, 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 a rays from the active
substances. A layer of aluminium '009 cm. thick absorbed all
the a rays.

Current due
to a rays

Current due
to ]8 rays

Katio currents -
a

Uranium . . .

•0074

Thorium

•27

•0020

Eadium

2000

1350

•0033

In the above table the saturation current due to the a and
ft 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 ft rays is
small compared with that due to the a rays, being greatest for
uranium and least for thorium. As the thickness of layer increases,

/Q
the ratio of currents — steadily increases to a constant value.

103. Comparison of the energy radiated by the a and
ft rays. It has not yet been found possible to measure directly
the energy of the a and ft rays. A comparison of the energy
radiated in the two forms of rays can, however, be made indirectly
by two distinct methods.

1 Rutherford and Grier, Phil. Mag. Sept. 1902.

V] RATE OF EMISSION OF ENERGY 151

If it is assumed that the same amount of energy is required to
produce an ion by either the a or the ft ray, and that the same
proportion of the total energy is used up in producing ions, an
approximate estimate can be made_oLJJie^Jcaticu of the energy
radiated by thp a and ft rays by measuring the ratio of the total
number of ions produced by them. If X is the coefficient of
absorption of the ft rays in air, the rate of production of ions
per unit volume at a distance x from the source is q0e~^ where qQ
is the rate of ionization at the source.

The total number of ions produced by complete absorption of
the rays is

Jo ° A,

Now X 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 ft rays is approximately proportional to
the density of any given substance.

For ft rays from uranium the value of X for aluminium is about
14, and X divided by the density is 5*4. Taking the density of air
as '0012, we find that

X for air = '0065.

The total number of ions produced in air is thus 154 q0 when
the rays are completely absorbed.

Now from the above table the ionization due to the ft rays
is '0074 of that produced by a rays, when the ft rays passed
through a distance of 5'7 cms. of air.

Thus we have approximately

Total number of ions produced by ft rays _ '0074 _^ __ ~ ^
Total number of ions produced by a rays 5'7

Therefore about 1/6 of the total energy radiated into air by a
thin layer of uranium is carried by the ft rays or electrons. The
ratio for thorium is about 1/22 and for radium about 1/14, assum-
ing the rays to have about the same average value of X.

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 a rays are absorbed, even with a thin layer, the

152 RATE OF EMISSION OF ENERGY [CH.

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 a 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 a and ft 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 \! be the average coefficient of absorption of the a rays in
the radio-active substance itself and <r the specific gravity of the
substance. Let E^ 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 TFj per unit area radiated to the surface per
sec. by a thickness d is given by

/«

if \d is large.

In a similar way it may be shown that the energy TF2 of the

ft rays reaching the surface is given by W 2 = -0~ where E2 and X>

V] RATE OF EMISSION OF EXERGY 153

are the values for the ft rays corresponding to El and Xj for the
OL ras. It thus follows that

Xj and X2 are difficult to determine directly for the radio-active
substance itself, but it is probable that the ratio Xj/X*, 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 a and ft rays is proportional to the
density of the substance; for it has already been shown in the
case of the ft 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 a rays, was 12'7 times
as great as the current due to the ft rays. Since the a rays were
entirely absorbed between the plates and the total ionization
produced by the ft rays is 154 times the value at the surface of the
plates,

W1 total number of ions due to a rays
W2 total number of ions due to ft rays

12-7 x 6-1
= — =-*j — = 0*5 approximately.

Now the value of Xj for aluminium is 2740 and of \% for the
same metal 14, thus

=100 approximately.

This shows that the energy radiated from a thick layer of
material by the ft rays is only about 1 per cent, of the energy
radiated in the form of a rays.

This estimate is confirmed by calculations based on indepen-
dent data. Let mlf ra2 be the masses of the a and ft particles
respectively. Let vl} v2 be their velocities.

*
Energy of one a particle _

Energy of one ft particle m2v<? in<t

154 RATE OF EMISSION OF ENERGY [CH.

Now it has been shown that for the a rays of radium
v1 = 2-5 x 109,

— = 6 x 10s.
m^

The velocity of the ft rays of radium varies between wide
limits. Taking for an average value

ws=ro x 1010,

— = 1-8 x 107,
ra2

it follows that the energy of the a particle from radium is almost
83 times the energy of the {3 particle. If equal numbers of a and
/3 particles are projected per second, the total energy radiated in
the form of a rays is about 83 times the amount in the form of
/3 rays.

Evidence will be given later to show that the number of
a particles projected is probably several times greater than the
number of /3 particles ; so that a still greater proportion of the
energy is emitted in the form of a rays. These results thus lead
to the conclusion that, from the point of view of the energy
emitted, the a rays are far more important than the /? rays.
This conclusion is supported "by other evidence which is discussed
in chapter X, where it will be shown that the a rays play by far
the most important part in the changes occurring in radio-active
bodies, and that the (3 rays only appear in the last stage of the
radio-active processes. From data based on the relative absorption
and ionization of the /£ and 7 rays in air, it can be shown that the
7 rays carry off about the same amount of energy as the ft rays.

104. Number and Energy of the a particles. It has

been shown that the greater part of the energy emitted from
the radio-elements in the form of ionizing radiations is due to
the a rays. Rutherford and McClung (loc. cit.) made an estimate
of the energy of the a rays, radiated into the gas from a thin
layer of active matter, by determining the total number of ions
produced by the complete absorption of the a rays. Taking
as the value for the energy required to produce an ion in a gas

V] RATE OF EMISSION OF ENERGY 155

1*90 x 10~10 ergs, it was calculated that the amount of energy,
radiated into the gas, from 1 gram of uranium oxide, spread over
a plate in a thin layer, corresponded to 0'032 gram-calories per
year. Taking the activity of pure radium chloride as 1,500,000
times that of uranium, the corresponding rate of emission of energy
from radium is 48,000 gram-calories per year. This is an under-
estimate, for it includes only the energy radiated into the gas.
The actual amount of energy released in the form of a rays is
evidently much greater than this on account of the absorption of
the a rays in the active matter itself.

It is very important to form an estimate of the total energy
emitted in the form of a rays, and also of the number of a particles
expelled per second from a known weight of an active substance.

Three different methods of estimating these quantities will now
be considered.

Method 1. It can be deduced from the results of Wien
(section 74) that the number of ft particles projected from
1 gram of radium bromide is 6 '6 x 109 per second. In this calcu-
lation no correction has been made for the ft rays absorbed in
the envelope of the active matter and in the surrounding glass
tube. Assuming that about half of the ft particles escape, it
follows that the number of ft particles projected per second from
1 gram of radium is about 2 x 1010 per second. Now it will be
shown later, in chapter X, that probably four a particles are pro-
jected from radium for each ft particle. The number of a particles
projected per second is thus about 8 x 1010. Taking the energy of
each a particle (section 86) as 5*9 x 10~6 ergs, this corresponds to
a rate of emission of energy from 1 gram of radium of 40 gram-
calories per hour.

Method 2. In the case of an active substance in the solid
or liquid state, most of the a rays emitted are absorbed in the
active material. The total ionization produced by all the a rays
from 1 gram of radium, when there is no absorption in the
active substance itself, was experimentally deduced in the follow-
ing way. A weight of 0*26 milligrams of pure radium bromide was
dissolved in water and the solution, spread uniformly over a plate
about 100 sq. cms. in area, was evaporated to dry ness. A few

156 RATE OF EMISSION OF ENERGY [CH.

hours afterwards the activity, measured by the a rays, reached a
minimum corresponding to 25°/0 of its maximum value when in a
state of radio-active equilibrium (see section 191). The saturation
current between parallel plates, sufficiently far apart to absorb all
the a rays in the gas between them, was measured by a galvano-
meter and found to be 2 '6 x 10~8 amperes. In this case the film
of radium bromide was so thin that the absorption of the a rays by
the radium itself was very small. Taking into account that half of
the a radiation from the radium was absorbed in the plate, it can
readily be deduced that the total current corresponding to 1 gram
of radium when in a state of radio-active equilibrium is equal to
1*2 x 10~4 electromagnetic units. Taking the charge on each ion
as 1*13 x 10"20 electromagnetic units, this corresponds to the pro-
duction of 1016 ions per second per gram1.

Langevin2 has deduced from the results of Townsend on ioniza-
tion by collision, that the energy required to produce fresh ions at
every collision is equal to the energy acquired by an ion moving
freely between two points, which differ in potential by about 60
volts. This corresponds to an amount of energy of 6'8 x 10~u ergs.
The total rate of emission of energy on the production of 1016 ions
per second is thus 7 x 105 ergs per second or about 60 gram-
calories per hour.

Method 3. The ionization produced in the gas by the pro-
jected a particles is due to collision with the neutral molecules.
The maximum number of ions produced per unit length of path
will be reached when each collision results in the production of
fresh ions. Now Townsend3 has shown that the maximum number
of ions produced by a moving electron per cm. of its path in air at
the pressure of 1 mm. of mercury is 21. On the kinetic theory of
gases, it can be deduced from this result (Langevin, loc. cit.) that
the electron ionizes every molecule in a circular cylinder whose
axis is the direction of movement and whose diameter is equal to
the diameter of the molecule. It follows that the electron must
be of dimensions small compared with the molecule — a result which
is in accordance with the experimental data. In the case of the

1 Rutherford and Soddy, Phil. Mag. May 1903.

2 These presentee a la Faculte des Sciences, Paris 1902, p. 85.

3 Phil. Mag. p. 19a, Feb. 1901.

V] RATE OF EMISSION OF ENERGY 157

a particles, the ionization is produced by a charged body atomic
in size. It is reasonable to suppose that the maximum number
of ions produced by the a particles per cm. of their path cannot be
greater than the number of molecules in a cylinder of twice the
diameter of the molecule. The maximum number of ions produced
per cm. of path in air at 1 mm. pressure cannot in consequence be
greater than 84. The number per cm. at atmospheric pressure
and temperature will be 63,800.

Now half the a rays from radium are absorbed in traversing
075 cm. of air (section 92). The total number of ions produced
by the rays is about the same as if the ionization at the surface
of the active matter extended uniformly for a distance of 1*09 cms.
The number of ions produced on an average by each a particle of
radium cannot in consequence be greater than 70,000.

The total number of ions produced for 1 gram of radium is
1016. This corresponds to an emission of 1*4 x 1011 a particles from
1 gram of radium per second and an emission of energy of 70 gram-
calories per hour.

The approximate estimates by the three methods of the
number of a particles and the rate of emission of energy from
1 gram of radium are in good agreement. It may be concluded
that from 1 gram of radium : —

(1) about 1011 a particles are projected per second ;

(2) the rate of emission of energy in the form of a particles is

probably equal to about 50 gram-calories per hour.

These results will be found to be in harmony with the deduc-
tions drawn from the observed heat emission of radium discussed
in the next section.

Since radium bromide has an activity (measured by the a rays)
of about 1,500,000 times uranium, it follows that the number of
i particles projected from 1 gram of thorium or uranium is only
7 x 10~7 of the number from radium.

In the following table are given the probable number of a
particles projected per second and the rate of emission of energy
in the form of a particles from 1 gram of each of the three radio-
elements.

158

RATE OF EMISSION OF ENERGY

[CH.

Number of
a particles
per sec.

Emission of energy
in form of a rays
per hour

Emission of energy
per year

Uranium
Thorium ...
Radium

70000
70000
1011

3'5 x 10~6 gram-cal.
3-5 x 10~5 „
50

•3 gram-cal.
•3
4 '4 x 10° gram-cal.

The rate of emission of energy in the form of 0 and 7 rays is
probably about 1 per cent, of the above values. For a thin layer
of a radio-element the amount of energy radiated into the air in
the form of rays is for most cases about 10 per cent, of the above
values.

105. Heat emission of radium. P. Curie and Laborde1
first drew attention to the striking result that a radium compound
kept itself continuously at a temperature several degrees higher
than that of the surrounding atmosphere. Thus the energy
emitted from radium can be demonstrated by its direct heating
effect as well as by photographic and electric means. Curie
and Laborde determined the rate of the emission of heat in
two different ways. In one method the difference of tempera-
ture was observed by means of an iron-constantin thermo-couple
between a tube containing one gram of radiferous chloride
of barium, of activity about 1/6 of pure radium, and an ex-
actly similar tube containing one gram of pure barium chloride.
The difference of temperature observed was 1*5° C. In order to
measure the rate of emission of heat, a coil of wire of known
resistance was placed in the pure barium chloride, and the strength
of the electric current required in order to raise the barium to the
same temperature as the radiferous barium was observed. In the
other method, the active barium, enclosed in a glass tube, was
placed inside a Bunsen calorimeter. Before the radium was intro-
duced, it was observed that the level of the mercury in the stem
remained steady. As soon as the radium, which had previously
been cooled in melting ice, was placed in the calorimeter, the
mercury column began to move at a regular rate. If the radium
tube was removed, the movement of the mercury ceased. It was

1 C. R. 136, p. 673, 1903.

RATE OF EMISSION OF ENERGY

159

found from these experiments that the heat emission from the
1 gram of radiferous barium, containing about 1/6 of its weight of
pure radium chloride, was 14 gram-calories per hour. Measure-
ments were also made with O08 gram of pure radium chloride.
Curie and Laborde deduced from these results that 1 gram of pure
radium emits a quantity of heat of about 100 gram-calories per
hour. This result was confirmed by the experiments of Runge and
Precht1 and others. As far as observation has at present gone,
this rate of emission of heat is continuous and unchanged with
lapse of time. Therefore, 1 gram of radium emits in the course of
a day 2400, and in the course of a year, 876,000 gram-calories.
The amount of heat evolved in the union of hydrogen and oxygen
to form 1 gram of water is 3900 gram-calories. It is thus seen
that 1 gram of radium emits per day nearly as much energy as is
required to dissociate 1 gram of water.

In some later experiments using 0'7 gram of pure radium
bromide, P. Curie2 found that the temperature of the radium
indicated by a mercury thermometer was* 3° C. above that of the
surrounding air. This result was confirmed by Giesel who obtained
a difference of temperature of 5° C. with 1 gram of radium bromide.
The actual rise of temperature observed will obviously depend upon
the size and nature of the vessel con-
taining the radium.

During their visit to England in
1903 to lecture at the Royal Insti-
tution, M. and Mme Curie performed
some experiments with Professor
Dewar, to test by another method the
rate of emission of heat from radium
at very low temperatures. This method
depended on the measurement of the
amount of gas volatilized when a
radium preparation was placed inside
a tube immersed in a liquefied gas
at its boiling point. The arrange-
ment of the calorimeter is shown in
Fig. 31.

1 Sitz. Ak. Wiss. Berlin, No. 38, 1903.

Fig. 31.

2 Societe de Physique, 1903.

160 RATE OF EMISSION OF ENERGY [CH.

The small closed Dewar flask A contains the radium in a glass
tube R, immersed in the liquid to be employed. The flask A is
surrounded by another Dewar bulb B, containing the same liquid,
so that no heat is communicated to A from the outside. The gas,
liberated in the tube A, is collected in the usual way over water or
mercury and its volume determined. By this method, measurements
were made with liquid carbon dioxide, oxygen, and liquid hydrogen.
Especial interest attaches to the results with liquid hydrogen.
The rate of heat emission of the radium was found to be about
the same in boiling carbon dioxide and oxygen, but Dewar1 states
that it was distinctly greater in liquid hydrogen. This result, if
confirmed, is of great interest, for it shows that while the rate of
heat emission is practically unchanged between the range of tem-
perature of liquid oxygen and carbon dioxide, the great relative
drop in absolute temperature between liquid oxygen and hydrogen
causes an increase in the heat emission. It will be shown in the
next section that the heat emission of radium is directly connected
with the radio-activity of that element. A change in the rate of
heat emission must then involve a change in the radio-activity of
radium. The conclusion that the heat emission of radium is greater
in liquid hydrogen than at ordinary temperatures thus requires
confirmation by direct measurements of the radio-activity.

The use of liquid hydrogen is very convenient for demonstrat-
ing the rate of heat emission from a small amount of radium.
From 0*7 grams of radium bromide (which had been prepared only
10 days previously) 73 c.c. of gas was given off per minute.

In later experiments P. Curie (loc. cit.) found that the rate of
emission of heat from a given quantity of radium depended upon
the time which had elapsed since its preparation. The emission
of heat was at first small, but after a month's interval practically
attained a maximum. If a radium compound is dissolved and
placed in a sealed tube, the rate of heat emission rises to the same
maximum as that of an equal quantity of radium in the solid
state.

106. Connection of the heat emission with the radia-
tions. The observations of Curie that the rate of heat emission

1 Dewar, British Association, 1903.

V] RATE OF EMISSION OF ENERGY 161

depended upon the age of the radium preparation pointed to the
conclusion that the phenomenon of heat emission of radium was
connected with the radio-activity of that element. It had long
been known that radium compounds increased in activity for about
a month after their preparation, when they reached a steady state.
This increase of activity is due to the continuous production by the
radium of the radio-active emanation or gas, which is occluded in
the radium compound and adds its radiation to that of the radium
proper. It thus seemed probable that the heating effect was in
some way connected with the presence of the emanation. Some
experiments upon this point have been made recently by Ruther-
ford and Barnes1. In order to measure the small amounts of heat
emitted, a form of differential air calorimeter was employed. Two
equal glass flasks of about 500 c.c. were filled with dry air at
atmospheric pressure. These flasks were connected through a
glass U-tube filled with xylene, which served as a manometer
to determine any variation of pressure of the air in the flasks.
A small glass tube, closed at the lower end, was introduced into
the middle of each of the flasks. When a continuous source of
heat was introduced into the glass tube, the air surrounding it was
heated and the pressure was increased. The difference of pressure,
when a steady state was reached, was observed on the manometer
by means of a microscope with a micrometer scale in the eye-
piece. On placing the source of heat in the similar tube in the
other flask, the difference in pressure was reversed. In order to
keep the apparatus at a constant temperature, the two flasks were
immersed in a water bath, which was kept well stirred.

Observations were first made on the heat emission from 30
milligrams of radium bromide. The difference in pressure observed
on the manometer was standardized by placing a small coil of wire
of known resistance in the place of the radium. The strength of
the current through the wire was adjusted to give the same differ-
ence of pressure on the manometer. In this way it was found that
the heat emission per gram of radium bromide corresponded to
65 gram-calories per hour. Taking the atomic weight of radium
as 225, this is equivalent to a rate of emission of heat from one
gram of metallic radium of 110 gram-calories per hour.

1 Nature, Oct. 29, 1903. Phil. Mag, Feb. 1904.
R. R.-A. 11

162

RATE OF EMISSION OF ENERGY

[CH.

The emanation from the 30 milligrams of radium bromide was
then removed by heating the radium (section 141). By passing the
emanation through a small glass tube immersed in liquid air, the
emanation was condensed. The tube was sealed off while the
emanation was still condensed in the tube. In this way the
emanation was concentrated in a small glass tube about 4 cms.
long. The heating effects of the "de-emanated" radium and of the
emanation tube were then determined at intervals. It was found
that, after removal of the emanation, the heating effect of the
radium decayed in the course of a few hours to a minimum,
corresponding to about 30 per cent, of the original heat emission,
and then gradually increased again, reaching its original value after
about a month's interval. The heating effect of the emanation
tube was found to increase for the first few hours after separation
to a maximum, and then to decay regularly with the time according
to an exponential law, falling to half its maximum value in about
four days. The actual heat emission of the emanation tube was
determined by sending a current through a coil of wire occupying
the same length and position as the emanation tube.

The variation with time of the heating effect from 30 milli-
grams of radium and the emanation from it is shown in Fig. 32.

1-6

160 240

Ho urn

320

Fig. 32.

V] RATE OF EMISSION OF ENERGY 163

Curve A shows the variation of the heat emission of the radium
and curve B of the emanation. The sum total of the rate of heat
emission of the radium and the emanation together, was at any
time found to be equal to that of the original radium. The maxi-
mum heating effect of the tube containing the emanation from 30
milligrams of radium bromide was 1*26 gram-calories per hour.
The emanation together with the secondary products which arise
from it, obtained from one gram of radium, would thus give out 42
gram-calories per hour. The emanation stored up in the radium
is thus responsible for more than two-thirds of the heat emission
from radium.

After removal of the emanation from radium, the activity,
measured by the a rays, decays in the course of a few hours to a
minimum of about 25 °/0 and then increases to its original value
after about a month's interval. At the same time, the apparent
activity of the emanation in a closed vessel increases to a maximum
in the course of a few hours and then decays with time according
to an exponential law, falling to half value 'in about four days. The
gradual decay of the activity of the radium, after removal of the
emanation, is due to the decay of the " excited activity " on the
radium itself. The increase of the apparent activity of the emana-
tion is due to the production of " excited activity " on the walls of
the containing vessel. The variation in heat emission of the radium
and the emanation in both cases is approximately proportional to
the activity measured by the a. rays. It is not proportional to the
activity measured by the $ or 7 rays, for the intensity of the 0
and 7 rays falls nearly to zero when the a radiation of the radium
is at the minimum of 25 per cent. These results are thus
in accordance with the view that the heat emission of ^adium
accompanies the expulsion of a particles, and is approximately
proportional to the number expelled.

107. Source of the energy. On the theory of atomic dis-
integration advanced in section 87, this heat is derived, not from
external sources, but from the internal energy of the radium atom.
The atom is supposed to be a complex system consisting of charged
parts in very rapid motion, and, in consequence, contains a large

11—2

164 RATE OF EMISSION OF ENERGY [CH.

store of latent energy, which can only be manifested when the
atom breaks up. For some reason, the atomic system becomes
unstable, and an a particle, of mass about twice that of the hydro-
gen atom, escapes, carrying with it its energy of motion. Since
the a. particles would be practically absorbed in a thickness of
radium of less than "001 cm., the greater proportion of the a
particles, expelled from a mass of radium, would be stoppe'd in the
radium itself and their energy of motion would be manifested in
the form of heat. The radium would thus be heated by its own
bombardment above the temperature of the surrounding air. The
suggestion that the heat emission of radium was connected with
the expulsion of the a. rays was first given by Sir Oliver Lodge1.
The energy of the expelled a particles does not account for the
whole emission of heat by radium. It is evident that the violent
expulsion of a part of the atom must result in intense electrical
disturbances in the atom. At the same time, the residual parts of
the disintegrated atom rearrange themselves to form a permanently
or temporarily stable system. During this process also, energy is
emitted, which is manifested in the form of heat in the radium
itself.

It has already been calculated (section 104) that the emission
of energy in the form of a particles, probably corresponds to about
50 gram-calories per hour for one gram of radium. The observed
heat emission of radium, under conditions when the a rays are
nearly all absorbed in the radium itself, is 100 gram-calories per
hour per gram. On account of the uncertainty attaching to the
estimate of the energy of the a rays, it is not possible to deduce
with accuracy how much of the total energy emitted is due to
them. The evidence, taken as a whole, points to the conclusion
that a considerable fraction of the total emission of energy is due
to the kinetic energy of the a rays.

Runge and Precht (loc. cit.) determined the heat emission of
radium by means of a thermometer, (1) when the radium was in a
thin tube, and (2) when it was surrounded by a lead screen several
millimetres in thickness. Within the limit of accuracy of the

1 Nature, April 2, 1903.

V] RATE OF EMISSION OF ENERGY 165

experiments (about 5 °/0), no difference in the heat emission was
observed in the two cases. The only difference between the
experiments (1) and (2) is that in the latter the ft rays are absorbed
in the lead and add their heating effect to the radium. Since,
however, the energy of the ft rays is probably not more than 1 °/0
of that due to the a rays (section 103), no appreciable difference is
to be expected. The experiments of Runge and Precht are quite
consistent with the view that the heating effect largely depends on
the energy of the a rays.

A further discussion of the heating effect of the emanation and
of its secondary products is given in sections 163 and 181.

CHAPTER VI.

PROPERTIES OF THE RADIATIONS.

108. 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 sub-
stances. Most of these effects are due either to the. a or /3 rays.
The y rays produce little effect in comparison. Since the ft 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.

Becquerel1 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 phos-
phoresce 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. It has been mentioned previously that
the a rays from Marckwald's preparation of polonium produce
marked phosphorescence in the diamond. The double sulphate of

1 C. R. 129, p. 912, 1899.

CH. VI] PROPERTIES OF THE RADIATIONS 167

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.

Without screen.
Substance Intensity

Across screen
of black
paper

Hexagonal blende . . ... . . . :

13-36
1-99
1-14
1-00
•30

•04
•05

•01

•31

•02

Platino-cyanide of barium

Double sulphate of Uranium and Potassium
Calcium Fluoride ... ... 1

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 phos-
phorescence developed without the screen is, in the majority of
cases, due to the a rays.

Bary1 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.

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 sub-
stances. It is an especially sensitive means of detecting the
presence of a rays, when it exhibits the " scintillating " property
already discussed in section 88. In order to show the luminosity
due to the a rays, the screen should be held close to the active
substance, as the rays are absorbed in their passage through a few

1 C. R. 130, p. 776, 1900.

168 PROPERTIES OF THE RADIATIONS [CH.

centimetres of air. Zinc sulphide is also luminous under the action
of the /3 rays, but the phosphorescence is far more persistent than
when produced by the a rays.

Platino-cyanide of barium fluoresces under the action of all
three kinds of rays, but is especially suitable for a study of the
0 and 7 rays. With a decigram of radium, the luminosity on the
screen can be seen at a distance of a metre from the radium. The
rays produce quite an appreciable luminosity on the screen after
their passage through the human body. 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 platino-
cyanide of barium. It fluoresces a beautiful greenish colour, and
a piece of the mineral appears quite translucent under the action
of the rays. Baskerville1 has recently shown that kunzite, a new
variety of mineral spodumene discovered by Kunz2, becomes
luminous when exposed to the action of radium rays and retains
its luminosity for some time.

Both zinc sulphide and platino-cyanide 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 Rontgen rays. Giesel made a screen of
platino-cyanide 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.

109. Luminosity of radium compounds. All radium
compounds are spontaneously luminous. This luminosity is es-
pecially 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

1 Science, Sept. 4, 1903.

2 Science, Aug. 28, 1903.

0091 U

Vl] PROPERTIES OF THE RADIATIONS 169

in colour and intensity with time. The original luminosity is
recovered if the salt is dissolved and dried. Many inactive pre-
parations 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.

110. Spectrum of the phosphorescent light of radium.

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 phos-
phorescent light of pure radium bromide has been made by Sir
William and Lady Huggins1. 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. 33. 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 spectrum2 of radium
are shown in the same figure.

Some time afterwards Sir William Crookes and Prof. Dewar3
showed that this spectrum of nitrogen was not obtained if the
radium was contained in a highly exhausted tube. Thus it

1 Proc. Ray. Soc. 72, pp. 196 and 409, 1903.

2 The spark spectrum of the radium bromide showed the H and K lines of
calcium and also faintly some of the strong lines of barium. The characteristic
lines of radium of wave-lengths 3814-59, 3649'7, 4340-6 and 2708-6, as shown
by Demarcay and others are clearly shown in the figure. The strong line of wave-
length about 2814 is due to radium.

3 British Assoc. 1903.

170 PROPERTIES OF THE RADIATIONS [CH.

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 thereby be obtained 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. Further experiments in
this direction are much to be desired at the present time.

111. Thermo-luminescence. E. Wiedemann and Schmidt1
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
/3 or cathodic rays of radium. This has been found to be the case
by Wiedemann2. 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.

1 Wied. Annal 59, p. 604, 1895. 2 Phys. Zeit. 2, p. 269, 1901.

VI] PROPERTIES OF THE RADIATIONS 171

Physical actions.

112. Some electric effects. Radium rays have the same
effect as ultra-violet light and Rontgen rays in. increasing the
facility with which a spark passes between electrodes. Elster and
Geitel1 showed thatTf 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 ob-
served that radium completely enveloped by a lead screen 1 cm.
thick produces a similar action. The effect in that case is due to
the 7 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.

Hemptinne2 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 dis-
charge without the rays began at 51 mms. but with the radium
rays at 68 mms. The colour of the discharge was also altered.

Himstedt3 found that the resistance of selenium was diminished
by the action of radium rays in the same way as by ordinary light.

F. Henning4 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. The amount
of radium present was too small to obtain the relative conductivity
of the radium and barium solution.

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

1 Wied. Annal 69, p. 673, 1899. - C. R. 133, p. 934, 1901.

3 Phys. Zeit. p. 476, 1900. * Wied. Annal. p. 562, 1902.

172

PROPERTIES OF THE RADIATIONS

[CH.

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
the water dropper.

113. Effect on liquid and solid dielectrics. P. Curie1
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 con-
centric 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 iu
megohms per 1 cm.3

Carbon bisulphide

20xlO~14

Petroleum ether

Amyline

Carbon chloride

Benzene

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 petro-
leum ether due to the rays at -17°C. was only 1/10 of its
value at 0° C. There is thus a marked action of temperature on
the conductivity. For very active material the current was pro-
portional 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

50
100
200
400

Current
109
185
255
335

1 C. E. 134, p. 420, 1902.

VI] PROPERTIES OF THE RADIATIONS 173

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.

Becquerel1 has recently shown that solid paraffin exposed to
the 0 and 7 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.

114. Effect of temperature on the radiations. Becquerel2,
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 a rays from the uranium were absorbed before
reaching the testing vessel, and the electric current measured was
due to the ft rays alone. P. Curie3 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
a 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 where none of the radio-active products are able
to escape.

J C. R. 136, p. 1173, 1903.

2 C. R. 133, p. 199, 1901.

3 Societe de Physique, March 2, 1900.

174 PROPERTIES OF THE RADIATIONS [CH.

Chemical actions.

115. Rays from active radium preparations change oxygen
into ozone1'2. Its presence can be detected by the smell or by the
action on iodide of potassium paper. This effect is. due to the
a and ft 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.

Giesel2 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.

The cause of these colorations by cathode and radium rays
has been the subject of much discussion. Elster and Geitel3
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 remark-
able degree, Elster and Geitel have suggested that the colorations
are caused by a solid solution of the metal in the salt.

1 S. and P. Curie, G. R. 129, p. 823, 1899.

2 Giesel, Verhandlg. d. d. phys. Ge». Jan. 5, 1900.

3 Phys. Zeit. p. 113, No. 3, 1902.

VI] PROPERTIES OF THE RADIATIONS 175

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.

BecquereJ1 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 /8 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 Willcock2 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 ft rays from the radium. Rontgen rays produce a
similar coloration.

Hardy3 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 a rays of radium alone.

This is further evidence in favour of the view that the a 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. Whetham4 to
be due to the electric charges carried by the ions.

116. Gases evolved from radium. Curie and Debierne5
observed that radium preparations placed in a vacuum tube con-
tinually lowered the vacuum. The gas evolved was always accom-

1 C. R. 133, p. 709, 1901.

2 Proc. Ruy. Soc. 72, p. 200, 1903.

3 Proc. Physiolog. Soc. May 16, 1903.

4 Phil. Mag. Nov. 1899 ; Theory of Solution, Camb. 1902, p. 396.

5 C. R. 132, p. 768, 1901.

176 PROPERTIES OF THE RADIATIONS [CH.

panied by the emanation, but no new lines were observed in its
spectrum. Giesel1 has observed a similar evolution of gas from
solutions of radium bromide. Giesel forwarded some active material
to Runge and Bodlander, in order that they might test the gas
spectroscopically. From 1 gram of a 5 per cent, radium prepara-
tion 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 Soddy2 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
Bodlander. On analysing the gases about 28*9 per cent, was found
to consist of oxygen, and the rest was 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 201.

Physiological actions.

117. Walkhoff first observed that radium rays produce burns
of much the same character as those caused by Rontgen 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 a and /3 rays.

J Ber. d. d. Chem. Ges. 35, p. 3605, 1902.
2 Proc. Boy. Soc. 72, p. 204, 1903.

VI] PROPERTIES OF THE RADIATIONS 177

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 prepara-
tion, the skin becomes inflamed for about 15 days and then peels
off. The painful feeling does not disappear for two months.

Danysz1 found that this action is mainly confined to the skin,
and does not extend to the underlying tissue. Caterpillars sub-
jected 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
Rontgen rays, but the use of radium possesses the great advantage
that the radiating source can be enclosed in a fine tube and intro-
duced 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 microbes8.

Another interesting action of the radium rays has "been ob-
served 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 Nagel3 who have
shown that it is due to a fluorescence produced by the rays in the
eye_Jtself. The blind are able to perceive this luminosity if the
retina is intact, but do not do so if the retina is diseased. Hardy
and Anderson4 have recently examined this effect in some detail.
The sensation of light is produced both by the $ and 7 rays. The
eyelid practically absorbs all the ft rays, so that the luminosity
observed with a closed eye is due to the 7 rays alone. The lens
and retina of the eye are strongly phosphorescent under the action
of the ft and 7 rays. Hardy and Anderson consider that the
luminosity observed in a dark room with the open eye (the phos-
phorescent 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 7 rays, for the most part, produce the sensation of
light when they strike the retina.

1 C. R. 136, p. 461, 1903.

2 Aschkinass and Caspar!, Arch. d. Ges. Physiologie, 86, p. 603, 1901.

3 Drude's AnnaL 4, p. 537, 1901.

4 Proc. Ray. Soc. 72, p. 393, 1903.

R. R.-A. 12

CHAPTER VII. ^

CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER.

118. Uranium X. The experiments of Mme Curie show
that the radio-activity of uranium and radium is an atomic pheno-
menon. 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 Crookes1 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 UrX, 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 car-
bonate. 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 par-
tial 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
1 Proc. Roy. Soc. 66, p. 409, 1900.

CH. VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 179

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 Becquerel1.
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 precipi-
tations 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 ft rays. The a rays, in
comparison, have little if any effect. Now the radiation from Ur X
consists entirely of ft 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 a rays, and little change would
have been observed after the removal of Ur X, since only the con-
stituent responsible for the ft rays was removed. This important
point is discussed in more detail in section 189.

119. Thorium X. Rutherford and Soddy2, 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 precipi-
tated, but a large amount of the activity is left behind in the
nitrate, which is chemically free from thorium. This filtrate was
evaporated to dry ness, and the ammonium salts driven off by

1 C. R. 131, p. 137, 1900; 133, p. 977, 1901.

2 Phil. Mag. Sep. and Nov. 1902. Trans. Chem. Soc. 81, pp. 321 and 837, 1902.

12—2

180 CONTINUOUS PRODUCTION OF KADIO- ACTIVE M

ATTER [

CH.

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'
UrX.

120

100

8 12

Time in Dayt-

Fig. 34.

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

VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 181

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, ivhile 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.

The results are shown graphically in Fig. 34, 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

iooc

24-

04 8 12 16

Time in Days

Fig. 35.

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 190, 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

182 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH.

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 precentage pro-
portion of the activity lost by the Th X during the same interval.

If the recovery curve is produced backwards in the normal
direction 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. 35, 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 70 is the initial activity and It is the activity after
a time t, then

*-"•• -V : ft

where X 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

r(= i-«-«,

•*o

where /0 is the amount of activity recovered when the state of
constant activity is reached, and It the activity recovered after
a time t, and A, is the same constant as before.

120. Uranium X. Similar results were obtained when
uranium was examined. The UrX 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. 36. A more detailed discussion of this
experiment is given in section 189.

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 ,£2 days.

VIl] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 183

A large number of results of a similar character have been
obtained from other radio-active products, separated from the

too

0 20

80 100 120

Time in Days

Fig. 36.

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.

121. 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 remark-
able 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 decreases according to

an exponential law with the time from the moment
of its formation.

Suppose that qQ particles of new matter are produced per second
from a given mass of matter. The rate of emission of energy due

184 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH.

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 dl, 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 X 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 J0 when T is very great,
and is then given by

Thus

This equation agrees with the experimental results for the
recovery of lost activity.

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 sepa-
rated 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

VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 185

may be. If /0 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 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.

122. 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 surjstance~that produces it, 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 con-
stituent 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 precipita-
tion, 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 ob-
tained 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 evapora-
tion 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

186 CONTINUOUS PKODUCTION OF KADIO- ACTIVE MATTER [CH.

the action of ammonia and ammonium carbonate. Ur X is com-
pletely precipitated with the uranium in an ammonia solution
and the nitrate 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.

123. 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 Soddy1 to show that
Th X is produced continuously at a constant rate. Preliminary
experiments showed that three successive precipitations were suf-
ficient 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 suc-

1 Phil. Mag. Sept. 1902.

VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 187

cessive filtrates. In three successive precipitations the activities of
the residues were proportional to 100, 8, T6 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. Ac-
cording to the theory, the activity It of the thorium formed in the
time t is given by

where /0 is the total activity of Th X, when there is radio-active
equilibrium.

If \t is small,

Since the activity of Th X falls to half value in 4 days, the
value of \ expressed in hours = '0072. After standing a period
of 1 hour about 1/140, after 1 day 1/6, after 4 days 1/2 of the
maximum should be obtained. The experimental results obtained
showed an agreement as good as could be expected, with the equa-
tion 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 fully recover its activity, this process can be re-
peated 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 substance1.

1 The general method of regarding the subject would be unchanged, even if it
were proved that the radio-activity of thorium is not due to thorium at all but to a
small constant amount of a radio-active impurity mixed with it.

188 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH.

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 elec-
trical 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.

124. 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

VI I] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 189

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 \ 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 a 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 a particle. Each a 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 nQ 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 ex-
ponential law with the time. The number of atoms Nt which
remain unchanged after an interval t is given by

Nt=(

J t

If N0 is the number of atoms at the beginning,

Thus =e~

or the law of decay expresses the fact that the activity of a pro-

J90 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH.

duct at any time is proportional to the number of atoms which
remain unchanged at that time.

This is the same as the law of mono-molecular 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 there is not
a single case yet observed in which the law of decay was affected
by the amount of active matter present.

From the above equation (1)

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 \N0. This must
be equal to the number q0 of new systems supplied in unit time, or

and x-; •

X has thus a distinct physical meaning, and may be denned as
the proportion of the total number of systems present which
change per second. It has a different value for each type of
active matter, but is invariable for that particular type of matter.
For this reason, X will be termed the " radio-active constant " of
the product.

125. 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 X would be increased or

VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 191

decreased, i.e. that the decay curve would 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 change. 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, at
the same rate in a vacuum as in air or any other gas at atmo-
spheric 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 re-
generate 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.

126. Effect of conditions on the rate of recovery of
activity. The recover}7 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 experi-
ments 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

192 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH.

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 excep-
tion is only apparent and not real. These cases will be discussed
more in detail in chapter vm, 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 accu-
rate 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.

The following method1 offers an accurate and simple means
of studying whether the rate of production of active matter is
influenced by molecular state. The substance is chemically con-
verted into any compound required, care being taken that active
products are recovered during the process. The new compound is
then spread on a metal plate and compared with a standard ,d,mple
of uranium for several days or weeks as required. If the rate of
production of active matter is altered by the conversion, there
should be an increase or decrease of activity to a new steady value,
where the production of active matter is again balanced by the
rate of decay. This method has the great advantage of being in-
dependent of the physical condition of the precipitate. It can be
applied satisfactorily to a compound of thorium like the nitrate
and the oxide which has been heated to a white heat, after which
treatment only a slight amount of emanation escapes. The nitrate
was converted into the oxide in a platinum crucible by treatment
1 Rutherford and Soddy, Phil. Mag. Sept. 1902.

VII] CONTINUOUS PRODUCTION OF RADIO-A

with sulphuric acid and ignition to a white heat. The oxide so
obtained was spread on a plate, but no change of its activity was
observed with time, showing that in this case the rate of produc-
tion was independent of molecular state. This method, which is
limited in the case of thorium, may be applied generally to the
uranium compounds where the results are not complicated by the
presence of an emanation.

No differences have yet been observed in the recovery curves
of different thorium compounds after the removal of Th X. For
example, the rate of recovery is the same whether the precipitated
hydroxide is converted into the oxide or into the sulphate.

127. Disintegration hypothesis. In the discussion of the
changes in radio-active bodies, only the active products Ur X
and Th X have been considered. It will, however, be shown later
that these two products are only examples of many other types of
active matter which are produced by the radio-elements, and that
each of these types of active matter has definite chemical as well
as radio-active properties, which distinguish it, not only from the
other active products, but also from the substance from which it
is produced.

The full investigation of these changes will be shown to verify-
in every particular the hypothesis that radio-activity is the ac-
companiment of chemical changes of a special kind occurring in
matter, and that the constant activity of the radio-elements is
due f^ an equilibrium process, in which the rate of production of
fresi. ive matter balances the rate of change of that already
formed.

The nature of the process taking place in the radio-elements,
in order to give rise to the production at a constant rate of new
kinds of active matter, will now be considered. Since in thorium
or uranium compounds there is a continuous production of radio-
active matter, which differs in chemical properties from the parent
substance, some kind of change must be taking place in the radio-
element. This change, by which new matter is produced, is very
different in character from the molecular changes dealt with in
chemistry, for no chemical change is known which proceeds at the
same rate at the temperatures corresponding to a red heat and
R. R.-A. 13

194 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH.

to liquid air, and is independent of all physical and chemical
actions. If, however, the production of active matter is supposed
to be the result of changes, not in the molecule, but in the atom
itself, it is not to be expected that the temperature would exert
much influence. The general experience of chemistry in failing
to transform the elements by the action of temperature is itself
strong evidence that wide ranges of temperature have not much
effect in altering the stability of the chemical atom.

The view that the atoms of the radio-elements are undergoing
-spontaneous disintegration was put forward by Mr Soddy and the
writer as a result of evidence of this character. The discovery of
the material nature of the a rays added strong confirmation to
the hypothesis ; for it has been pointed out (section 87) that the
expulsion of a particles must be the result of a disintegration
of the atoms of the radio-element. Taking the case of thorium
as an example, the processes occurring in the atom may be
pictured in the following way. It must be supposed that the
thorium atoms are not permanently stable systems, but, on an
average, a constant small proportion of them — about one atom in
every 1016 will suffice — breaks up per second. The disintegration
consists in the expulsion from the atom of one or more a particles
with great velocity. For simplicity, it will be supposed that each
atom expels one a particle. It has been shown that the a particle
of radium has a mass about twice that of the hydrogen atom.
From the similarity of the a rays from thorium and radium, it is
probable that the a particle of thorium does not differ much in
mass from that of radium, and may be equal to it. After the
escape of an a particle, the part of the atom left behind, which
has a mass slightly less than that of the thorium atom, tends to
rearrange its components to form a temporarily stable system. It
is to be expected that it will differ in chemical properties from
the thorium atom from which it was derived. The atom of the
substance Th X is, on this view, the thorium atom minus one a
particle. The atoms of Th X are far more unstable than the atoms
of thorium, and one after the other they break up, each atom ex-
pelling one a particle as before. These projected a particles give rise
to the radiation from the Th X. Since the activity of Th X falls to
half its original value in about four days, on an average half of the

VIl] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 195

atoms of Th X break up in four days, the number breaking up
per second being always proportional to the number present.
After an atom of Th X has expelled an a particle, the mass of the
system is again reduced and its chemical properties are changed.
It will be shown (section 145) that the Th X gives rise to the
thorium emanation, which exists as a gas, and that this in turn
gives rise to matter which is deposited on solid bodies and gives
rise to the phenomena of excited activity.

As a result of the disintegration of the thorium atom, there is
thus a series of chemical substances produced, each of which has
distinctive chemical properties. Each of these products is radio-
active, and loses its activity according to a definite law. Since
thorium has an atomic weight of 237, and the weight of the
a particle is about 2, it is evident that, if only one a. particle
is expelled at each change, the process of disintegration could
pass through a number of successive stages and yet leave behind,
at the end of the process, a mass comparable with that of the
parent atom.

It will be shown in chapter x that a process of disintegration,
very similar to that already described for thorium, must be sup-
posed to take place also in uranium and radium. The full
discussion of this subject cannot be given with advantage until
two of the most important products of thorium and radium, viz.
the radio-active emanations and the matter which causes excited
activity, have been considered in detail.

128. Magnitude of the changes. It can be calculated
by several independent methods that, in order to account for the
changes occurring in thorium, probably not more than 10s and
not less than 104 atoms in each gram of thorium suffer disintegra-
tion per second. It is well known (section 39) that 1 cubic centi-
metre of hydrogen at atmospheric pressure and temperature contains
about 2 x 1019 molecules. From this it follows that one gram of
thorium contains about 1021 atoms. The fraction which breaks
up per second thus lies between 10~17 and 10~16. This is an
extremely small ratio, and it is evident that the process could
continue for long intervals of time, before the amount of matter
changed would be capable of detection by the spectroscope or

13—2

196 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH. VII

by the balance. With the electroscope it is possible to detect
the radiation from 10~5 gram of thorium, i.e. the electroscope is
capable of detecting the ionization which accompanies the disin-
tegration of a single thorium atom per second. The electroscope
is thus an extraordinarily delicate means for detection of minute
changes in matter, which are accompanied, as in the case of the
radio-elements, by the expulsion of charged particles with great
velocity. It is possible to detect by its radiation the amount of
Th X produced in a second from 1 gram of thorium, although
the process would probably need to continue thousands of years
before it could be detected by the balance or the spectroscope. It
is thus evident that the changes occurring in thorium are of an
order of magnitude quite different from that of ordinary chemical
changes, and it is not surprising that they have never been ob-
served by direct chemical methods.

CHAPTER VIII.

KADIO-ACTIVE EMANATIONS.

129. Introduction. A most important and striking property
possessed by radium, thorium, and actinium, but not by uranium or
polonium, is the power of continuously emitting into the surround-
ing space joaatm'al emanation, which has all the ^properties of a
radio-active^s. This emanation is able to diffuse rapidly through
gases^anch through por^us^ substances, and may be separated from
the gas with which it is mixed by condensation by the action of
extreme cold. This emanation forms a connecting link between
the activity of the radio-elements themselves and their power of
exciting activity on surrounding objects, and has been studied more
closely than the other active products on account of its existence in
the gaseous state. The emanations from the three active bodies all
possess similar radio-active properties, but the effects are more
marked in the case of the emanation from radium, on account of
the very great activity of that element.

Thorium Emanation.

130. Discovery of the emanation. In the course of
examination of the radiations of thorium, several observers had
noted that some of the thorium compounds, and especially the
oxide, were very inconstant sources of radiation, when examined in
open vessels by the electrical method. Owens1 found that this
inconstancy was due to the presence of air currents. When a
closed vessel was used, the current, immediately after the intro-
duction of the active matter, increased with the time, and finally

1 Phil. Mag. p. 360, Oct. 1899.

198 RADIO-ACTIVE EMANATIONS [CH.

reached a constant value. By drawing a steady stream of air
through the vessel the value of the current was much reduced. It
was also observed that the radiations could apparently pass through
large thicknesses of paper, which completely absorbed the ordinary
a radiation.

In an investigation of these peculiar properties of thorium
compounds, the writer1 found that the effects were due to an
emission of radio-active particles of some kind from the thorium
compounds. This " emanation," as it was termed for convenience,
possesses the properties of ionizing the gas and acting on a photo-
graphic plate, and is able to diffuse rapidly through porous
substances like paper and thin metal foil.

The emanation, like a gas, is completely prevented from escap-
ing by covering the active matter with a thin plate of mica. The
emanation can be carried away by a current of air; it passes
through a plug of cotton-wool and can be bubbled through solutions
without any loss of activity. In these respects, it behaves very
differently from the ions produced in the gas by the rays from
active substances, for these give up their charge completely under
the same conditions.

Since the emanation passes readily through large thicknesses
of cardboard, and through niters of tightly packed cotton-wool, it
does not seem likely that the emanation consists of particles of
dust given off by the active matter. This point was tested still
further by the method used by Aitken and Wilson, for detecting
the presence of dust particles in the air. The oxide, enclosed in
a paper cylinder, was placed in a glass vessel, and the dust was
removed by repeated small expansions of the air over a water
surface. The dust particles act as nuclei for the formation of
small drops and are then removed from the air by the action of
gravity. After repeated expansions, no cloud was formed, and the
dust was considered to be removed. After waiting for some time
to allow the thorium emanation to collect, further expansions were
made but no cloud resulted, showing that for the small expansions
used, the particles were too small to become centres of condensa-
tion. The emanation then could not be regarded as dust emitted
from thorium.

1 Phil Mag. p. 1, Jan. 1SOO.

VIII]

RADIO-ACTIVE EMANATIONS

199

Since the power of diffusing rapidly through porous substances,
and acting on a photographic plate, is also possessed by a chemical
substance like hydrogen peroxide, some experiments were made
to see if the emanation could be an agent of that character. It was
found, however, that hydrogen peroxide is not radio-active, and
that its action on the plate is a purely chemical one, while it is
the radiation from the emanation and not the emanation itself that
produces ionizing and photographic effects.

131. Experimental arrangements. The emanation from
thorium is given off in minute quantity. No appreciable lowering
of the vacuum is observed when an emanating compound is placed
in a vacuum tube and no new spectrum lines are observed.

For an examination of the emanation, an apparatus similar in
principle to that shown in Fig. 37 is convenient.

The thorium compound either bare or enclosed in a paper
envelope was placed in a glass tube C. A current of air from a
gasometer, after passing through a tube containing cotton-wool to
remove dust particles, bubbled through sulphuric acid in the vessel
A. It then passed through a bulb containing tightly packed
cotton-wool to prevent any spray being carried over. The emana-

To Electrometer

Fig. 37.

tion, mixed with air, was carried from the vessel C through a plug
of cotton-wool D, which completely removed all the ions carried with
the emanation. The latter then passed into a long brass cylinder,
75 cm. in length and 6 cm. in diameter. The insulated cylinder
was connected with a battery in the usual way. Three insulated
electrodes, E, F, H, of equal lengths, were placed along the axis of
the cylinder, supported by brass rods passing through ebonite
corks in the side of the cylinder. The current through the gas,
due to the presence of the emanation, was measured by means of

200 RADIO-ACTIVE EMANATIONS [CH.

an electrometer. An insulating key was arranged so that any one
of the electrodes E, F, H could be rapidly connected with one pair
of quadrants of the electrometer, the other two being always con-
nected with earth. The current observed in the testing cylinder
vessel was due entirely to the ions produced by the emanation
carried into the vessel by the current of air. On substituting a
uranium compound for the thorium, not the slightest current was
observed. After a constant flow has passed for about 10 minutes,
the current due to the emanation reaches a constant value.

The variation of the ionization current with the voltage is
similar to that observed for the gas ionized by the radiations from
the active bodies. The current at first increases with the voltage,
but finally reaches a saturation value.

132. Duration of the activity of the emanation. The

emanation rapidly loses its activity with time. This is very readily
shown with the apparatus of Fig. 37. The current is found to
diminish progressively along the cylinder, and the variation from
electrode to electrode depends on the velocity of the flow of air.

If the velocity of the air current is known, the decay of activity
of the emanation with time can be deduced. If the flow of air is
stopped, and the openings of the cylinder closed, the current
steadily diminishes with time. The following numbers illustrate
the variation with time of the saturation current, due to the
emanation in a closed vessel. The observations were taken suc-
cessively, and as rapidly as possible after the current of air was
stopped.

Time in seconds Current

0 100

28 69

62 51

118 25

155 14

210 67

272 4'1

360 1-8

Curve A, Fig. 38, shows the relation existing between the
current through the gas and the time. The current just before
the flow of air was stopped is taken as unity. The current through

VIII] RADIO-ACTIVE EMANATIONS 201

the gas, which is a measure of the activity of the emanation,
diminishes according to an exponential law with the time like the
activity of the products Ur X and Th X. The rate of decay is,
however, much more rapid, the activity of the emanation decreas-
ing to half value in about one minute. According to the view
developed in section 124, this expresses the result that half of the

4 5

Time in Minutes

Fig. 38.

emanation particles have undergone change in one minute. After
an interval of 10 minutes, the current due to the emanation is
very small, showing that practically all the emanation particles
present have undergone change.

The decrease of the current with time is an actual measure of
the decrease of the activity of the emanation, and is not in any
way influenced by the time taken for the ions produced to reach
the electrodes. If the ions had been produced from a uranium
compound, the duration of the conductivity for a saturation voltage
would only have been a fraction of a second.

The rate of decay of the activity of the emanation is independ-
ent of the electromotive force acting on the gas. This shows that

202 RADIO-ACTIVE EMANATIONS [CH.

the radio-active particles are not destroyed by the electric field.
The current through the gas at any particular instant, after
stoppage of the flow of air, was found to be the same whether the
electromotive force had been acting the whole time or had been
just applied for the time of the test.

The emanation itself is unaffected by a strong electric field and
so cannot be charged. By testing the activity of the emanation
after passing through long concentric cylinders, charged to a high
potential, it was found that the emanation certainly did not move
with a velocity greater than '00001 cm. per second, for a gradient
of 1 volt per cm., and there was no evidence to show that it moved
at all.

The rate at which the emanation is produced is independent
of the gas surrounding the active matter. If in the apparatus of
Fig. 37, air is replaced by hydrogen, oxygen, or carbonic acid,
similar results are obtained, though the current observed in the
testing vessel varies for the different gases on account of the
unequal absorption by them of the radiation from the emanation.

If a thorium compound, enclosed in paper to absorb the a
radiation, is placed in a closed vessel, the saturation current due to
the emanation is found to vary directly as the pressure. Since
the rate of ionization is proportional to the pressure for a constant
source of radiation, this experiment shows that the rate of emission
of the emanation is independent of the pressure of the gas. The
effect of pressure on the rate of production of the emanation is
discussed in more detail later in section 148.

133. Effect of thickness of layer. The amount of emana-
tion emitted by a given area of thorium compound depends on
the thickness of the layer. With a very thin layer, the current
between two parallel plates, placed in a closed vessel as in Fig. 16,
is due very largely to the a rays. Since the a radiation is very
readily absorbed, the current due to it practically reaches a maximum
when the surface of the plate is completely covered by a thin layer
of the active material. On the other hand the current produced
by the emanation increases until the layer is several millimetres in
thickness, and then is not much altered by adding fresh active
matter. This falling off of the current after a certain thickness

VIII]

RADIO-ACTIVE EMANATIONS

203

has been reached is to be expected, since the emanation, which
takes several minutes to diffuse through the layer above it, has
already lost a large proportion of its activity.

With a thick layer of thorium oxide in a closed vessel, the
current between the plates is largely due to the radiation from the
emanation lying between the plates. The following tables illus-
trate the way in which the current varies with the thickness of
paper for both a thin and a thick layer.

TABLE I. Thin Layer.
Thickness of sheets of paper O027.

TABLE II. Thick Layer
Thickness of paper '008 cm.

No. of layers
of paper

Current

0 1

1 -37

•16

•08

No. of layers
of paper

Current

•74

•72

•67

•20

•55

The initial current with the unscreened compound is taken as
unity. In Table I., for a thin layer of thorium oxide, the current
diminished rapidly with additional layers of thin paper. In this
case the current is due almost entirely to the a rays. In Table II.
the current falls to '74 for the first layer. In this case about 26 %
of the current is due to the a rays, which are practically absorbed
by the layer '008 cm. in thickness. The slow decrease with
additional layers shows that the emanation diffuses so rapidly
through a few layers of paper that there is little loss of activity
during the passage. The time taken to diffuse through 20 layers
is however appreciable, and the current consequently has decreased.
After passing through a layer of cardboard 1'6 mms. in thickness
the current is reduced to about one-fifth of its original value. In
closed vessels the proportion of the total current, due to the emana-
tion, varies with the distance between the plates as well as with the
thickness of the layer of active material. It also varies greatly
with the compound examined. In the nitrate, which gives off only
a small amount of emanation, the proportion is very much smaller
than in the hydroxide which gives off a large amount of emanation.

204 RADIO-ACTIVE EMANATIONS [CH.

134. Increase of current with time. The current due to
the emanation does not reach its final value for some time after
the active matter has been introduced into the closed vessel. The
variation with time is shown in the following table. The satura-
tion current due to thorium oxide, covered with paper, was observed
between concentric cylinders of 5'5 cms. and *8 cm. diameter.

Immediately before observations on the current were made, a
rapid stream of air was blown through the apparatus. This removed
most of the emanation. However, the current due to the ionization
of the gas by the emanation, as it was carried along by the current
of air, was still appreciable. The current consequently does not
start from zero.

Time in seconds Current

0 9

23 25

53 49

96 67

125 76

194 88

244 98

304 99

484 100

The results are shown graphically in Fig. 38 Curve B. The
decay of the activity of the emanation with time, and the rate of
increase of the activity, due to the emanation in a closed space, are
connected in the same way as the decay and recovery curves of
ThXandUrX.

With the previous notation, the decay curve is given by

and the recovery curve by

where X is the radio-active constant of the emanation.

This relation is to be expected, since the decay and recovery
curves of the emanation are determined by exactly the same con-
ditions as the decay and recovery curves of Ur X and Th X. In
both cases there is :

VIII] RADIO-ACTIVE EMANATIONS 205

(1) A supply of fresh radio-active particles produced at a
constant rate.

(2) A loss of activity of the particles following an exponential
law with the time.

In the case of Ur X and Th X, the active matter produced
manifests its activity in the position in which it is formed ; in this
new phenomenon, a proportion of the active matter in the form of
the emanation escapes into the surrounding gas. The activity of
the emanation, due to a thorium compound kept in a closed vessel,
thus reaches a maximum when the rate of supply of fresh emana-
tion particles from the compound is balanced by the rate of change
of those already present. The time for recovery of half the final
activity is about 1 minute, the same as the time taken for the
emanation, when left to itself, to lose half its activity.

If q0 is the number of emanation particles escaping into the
gas per second, and N0 the final number when radio-active equi-
librium is reached, then (section 124),

Since the activity of the emanation falls to half value in 1 minute

X = l/87,

and NO = 87<?0, or the number of emanation particles present when
a steady state is reached is 87 times the number produced per
second.

Radium Emanation.

135. Discovery of the emanation. Shortly after the
discovery of the thorium emanation, Dorn1 repeated the results
and, in addition, showed that radium compounds also gave off
radio-active emanations and that the amount given off was much
increased by heating the compound. The radium emanation differs
from the thorium emanation in the rate at which it loses its
activity. It decays far more slowly, but in other respects, the
emanations of thorium and radium have much the same properties.
Both prr>pma.t.inn^ inm'gp the gas with which they are mixed, and
affect a photographic plate. Both diffuse readily through porous
1 Abh. der naturfortch. Ges. fiir Halle-a-S., 1900.

206 RADIO-ACTIVE EMANATIONS [CH.

substancesJbttt are unable to pass through a thin plate of mica;
both behave like a temporarily radio-active gas, mixed in minute
quantity with the air or other gas in which they are conveyed.

136. Decay of activity of the emanation. Very little
emanation escapes from radium chloride in the solid state, but
the amount is largely increased by heating, or by dissolving the
compound in water. By bubbling air through a radium chloride
solution, or passing air over a heated radium compound, a large
amount of emanation may be obtained which can be collected,
mixed with air, in a suitable vessel.

Experiments to determine accurately the rate of decay of
activity of the emanation have been made by P. Curie1, and
Rutherford and Soddy2. In the experiments of the latter, the
emanation mixed with air was stored over mercury in an ordinary
gas holder. From time to time, equal quantities of air mixed with
the emanation were measured off by a gas pipette and delivered
into a testing vessel. The latter consisted of an air-tight brass
cylinder carrying a central insulated electrode. A saturation voltage
was applied to the cylinder, and the inner electrode was connected
to the electrometer with a suitable capacity in parallel. The
saturation current was observed immediately after the introduction
of the active gas into the testing vessel, and was taken as a measure
of the activity of the emanation present. The current increased
rapidly with the time owing to the production of excited activity
on the walls of the containing vessel. This effect is described in
detail in chapter IX.

The measurements were made at suitable intervals over a period
of 33 days. The following table expresses the results, the initial
activity being taken as 100.

Time in hours Kelative Activity

0 100

20-8 85-7

187-6 24-0

354-9 6-9

521-9 1-5

786-9 0-19

1 C. R. 135, p. 857, 1902. 2 Phil. Mag. April, 1903.

VIII]

RADIO-ACTIVE EMANATIONS

207

The activity falls off according to an exponential law with the
time, and decays to half value in 3*71 days. With the usual
notation

-^-Battery

the mean value of \ deduced from the results is given by
X = 2-16 xlO-6= 1/463000.

P. Curie determined the rate of decay of activity of the emana-
tion by another method. The active matter was placed at one end
of a sealed tube. After sufficient time had elapsed, the portion of
the tube containing the radium compound was removed. The loss
of activity of the emanation, stored in the other part, was tested at
regular intervals by observing the ionization current due to the
rays which passed through the
walls of the glass vessel. The
testing apparatus and the con-
nections are shown clearly in
Fig. 39. The ionization current
is observed between the vessels
BE and CC. The glass tube
A contains the emanation.

Now it will be shown later
that the emanation itself gives
off only a rays, and these rays
are completely absorbed by the
glass envelope, unless it is made
extremely thin. The rays pro-
ducing ionization in the testing

Electrometer

vessel were thus not due to the

a rays from the emanation at lg'

all, but to the ft and 7 rays due to the excited activity produced

on the walls of the glass tube by the emanation inside it. What

was actually measured was thus the decay of the excited activity

derived from the emanation, and not the decay of activity of the

emanation itself. Since, however, when a steady state is reached,

the amount of excited activity is nearly proportional at any time

to the activity of the emanation, the rate of decay of the excited

208 RADIO-ACTIVE EMANATIONS [CH.

activity on the walls of the vessel indirectly furnishes a measure
of the rate of decay of the emanation itself. This is only true if
the emanation is placed for four or five hours in the tube before
observations begin, in order to allow the excited activity time to
reach a maximum value.

Using this method P. Curie obtained results similar to those
obtained by Rutherford and Soddy by the direct method. The
activity decayed according to an exponential law with the time
falling to half value in 3'99 days.

The experiments were performed under the most varied con-
ditions but the rate of decay was found to remain unaltered. The
rate of decay did not depend on the material of the vessel contain-
ing the emanation or on the nature or pressure of the gas with
which the emanation was mixed. It was unaffected by the amount
of emanation present, or by the time of exposure to the radium,
provided sufficient time had elapsed to allow the excited activity
to reach a maximum value before the observations were begun.
P. Curie1 found that the rate of decay of activity was unaffected
by exposing the vessel containing the emanation to different
temperatures ranging from + 450° to — 180° C.

In this respect, the emanations of thorium and radium are
quite analogous. The rate of decay seems to be unaffected by
any physical or chemical agency, and the emanations behave in
exactly the same way as the radio-active products Th X and Ur X,
already referred to. The radio-active constant \ is thus a fixed
and unalterable quantity for both emanations, although in one
case its value is about 5000 times greater than in the other.

Emanations from Actinium.

137. Debierne2 found that actinium gives out an emanation
similar to the emanations of thorium and radium. The loss
of activity of the emanation is even more rapid than for the
thorium emanation, for its activity falls to half value in a few
seconds. In consequence of the rapid decay of activity, the
emanation is able to diffuse through the air only a short distance
from the active matter before it loses the greater proportion of its

1 C. R. 136, p. 223, 1903. 2 C. R. 136, p. 146, 1903.

VIII] RADIO-ACTIVE EMANATIONS 209

activity. Giesel has obtained an intensely active emanation from
the " emanating substance." It has already been pointed out (sec-
tion 21) that this " emanating substance " is probably the same as
the actinium of Debierne. The emanation from actinium, like those
from thorium and radium, possesses the property of exciting activity
on inactive bodies. However it has not yet been studied as com-
pletely as the better known emanations of thorium and radium.

Experiments with large amounts of Radium Emanation.

138. With very active speoimeTF nf-ja/lirmn, a larg-P amount
of emanation can ba obtained, and the electricaijmd photographic
actions are correspondingly intense. On account of the small
activity of thorium and the rapid decay of its emanation, the
effects due to it are weak, and can be studied only for a few
minutes after its production. The emanation from radium, on the
other hand, in consequence of the slow decay of its activity, may
be stored mixed with air in an ordinary gas holder, and its photo-
graphic and electrical actions may be examined several days or
even weeks after, quite apart from those of the radium from which
it was obtained.

It is, in general, difficult to study the radiation due to the
emanation alone, on account of the fact that the emanation is
continually producing a secondary type of activity on the surface
of the vessel in which the emanation is enclosed. This excited
activity reaches a maximum value several hours after the intro-
duction of the emanation, and, as long as it is kept in the vessel,
this excited activity on the walls decays at the same rate as the
emanation itself, i.e. it falls to half its initial value in about 4 days.
If, however, the emanation is blown out, the excited activity
remains behind on the surface, but rapidly loses its activity in the
course of a few hours. After several hours, the intensity of the
residual radiation is very small.

These effects and their connection with the emanation are
discussed more fully in chapter IX.

Giesel1 has recorded some interesting observations of the effect of
the radium emanation on a screen of phosphorescent zinc sulphide.

1 Ber. der deutsch. Chem. Ges. p. 3608, 1902.
R. R.-A. 14

210 RADIO-ACTIVE EMANATIONS [CH.

When a few centigrams of moist radium bromide were placed on a
screen, any slight motion of the air caused the luminosity to move
to and fro on the screen. The direction of phosphorescence could
be altered at will, by a slow current of air. The effect was still
further increased by placing the active material in a tube and
blowing the air through it towards the screen. A screen of barium
platino-cyanide or of Balmain's paint failed to give any visible
light under the same conditions. The luminosity was not altered
by a magnetic field, but it was affected by an electric field. If the
screen were charged the luminosity was more marked when it was
negative than when it was positive.

Giesel states that the luminosity was not equally distributed,
but was concentrated in a peculiar ring-shaped manner over the
surface of the screen. The concentration of luminosity on the
negative, rather than on the positive, electrode is probably due to
the excited activity, caused by the emanation, and not to the
emanation itself. This excited activity (see chap, ix} in an electric
field is concentrated chiefly on the negative electrode. The
electric field, probably, does not act on the emanation itself but
concentrates the excited activity, due to the emanation present, on
to the negative electrode.

An experiment to illustrate the phosphorescence produced in
some substances by the rays from a large amount of emanation is
described in section 160.

139. Curie and Debierne1 have made an examination of the
emanation from radium, and the excited activity produced by it.
They have examined the emanation given off from radium under
very low pressures. The tube containing the emanation was ex-
hausted to a good vacuum by a mercury pump. It was observed
that a gas was given off from the radium which produced excited
activity on the glass walls. This gas was extremely active, and
rapidly affected a photographic plate through the glass. It caused
fluorescence on the surface of the glass and rapidly blackened it,
and was still active after standing ten days. When spectroscopi-
cally examined, this gas did not show any new lines, but gene-
rally those of the spectra of carbonic acid, hydrogen, and mercury.

1 C. R. 132, pp. 548 and 768, 1901.

VIIl] RADIO-ACTIVE EMANATIONS 211

In the light of the results described in section 116, the gas, given
off by the radium, was probably the non-active gases, hydrogen
and oxygen, in which the active emanation was mixed in minute
quantity. It will be shown later (section 163) that the energy
radiated from the emanation is enormous compared with the
amount of matter involved, and that the effects observed, in most
cases, are produced by an almost infinitesimal amount of the
emanation.

In further experiments, Curie and Debierne1 found that many
substances were phosphorescent under the action of the emanation
and the excited activity produced by it. In their experiments, two

Active Material

Fig. 40.

glass bulbs A and B (Fig. 40) were connected with a glass tube.
The active material was placed in the bulb A and the substance
to be examined in the other.

They found that, in general, substances that were phosphores-
cent in ordinary light became luminous. The sulphide of zinc was
especially brilliant and became as luminous as if exposed to a
strong light. After sufficient time had elapsed, the. luminosity
reached a constant value. The phosphorescence is partly due to
the excited activity produced by the emanation on its surface, and
partly to the direct radiation from the emanation.

Phosphorescencewas alsoj^roduced in glass. Thuringian glass
showed the most marked effects! The luminosity of the glass was
found to be about the same in the two bulbs, but was more marked
in the connecting tube. The effect in the two bulbs was the same
even if connected by a very narrow tube.

Some experiments were also made with a series of phosphores-
cent plates placed in the vessel at varying distances apart. With
the plates 1 mm. apart, the effect was very feeble but increased
directly as the distance and was large for a distance of 3 cms.
1 C. E. 133, p. 931, 1901.

14—2

212 RADIO-ACTIVE EMANATIONS [CH.

These effects receive a general explanation on the views already
put forward. When the radium is placed in the closed vessel, the
emanation is given off at a constant rate and gradually diffuses
throughout the enclosure. Since the time taken for diffusion of
the emanation through tubes of ordinary size is small compared
with the time required for the activity to be appreciably reduced,
the emanation, and also the excited activity due to it, will be
nearly equally distributed throughout the vessel.

The luminosity due to it should thus be equal at each end of
the tube. Even with a capillary tube connecting the two bulbs, the
gas continuously given off by the radium will always carry the
emanation with it and cause a practically uniform distribution.

The gradual increase-of-4he-amount of emanation throughout
the tube will be given by___the-£quation

where Nt is the number of emanation particles present at the
time t, NQ the number present when radio-active equilibrium is
reached, and X is the radio-active constant of the emanation. The
phosphorescent action, which is due partly to the radiations from
the emanation and partly to the excited activity on the walls,
should thus reach half the maximum value in four days and should
practically reach its limit after three weeks interval.

The variation of luminosity with different distances between
the screens is to be expected. The amount of excited activity
deposited on the boundaries is proportional to the amount of
emanation present. Since the emanation is equally distributed,
the amount of excited activity deposited on the screens, due to the
emanation between them, varies directly as the distance, provided
the distance between the screens is small compared with their
dimensions. Such a result would also follow if the phosphorescence
were due to the radiation from the emanation itself, provided that
the pressure of the gas was low enough to prevent absorption of
the radiation from the emanation in the gas itself between the
screens.

VIII] RADIO-ACTIVE EMANATIONS 213

Measurements of Emanating Power.

140. Emanating power. The compounds of thorium in the
solid state vary very widely in the amount of emanation they emit
under ordinary conditions. It is convenient to use the term
emanating power to express the amount of emanation given off per
second by one gram of the compound. Since, however, we have
no means of determining absolutely the amount of emanation
present, all measurements of emanating power are of necessity
comparative. In most cases, it is convenient to take a given weight
of a thorium compound kept under conditions as nearly as possible
constant, and to compare the amount of emanation of the compound
to be examined with this standard.

In this way comparisons of the emanating power of thorium
compounds have been made by Rutherford and Soddy1, using an
apparatus similar to that shown in Fig. 37 on page 199.

A known weight of the substance to be tested was spread on a
shallow dish, placed in the glass tube C. A stream of dry dust-free
air, kept constant during all the experiments, was passed over the
compound and carried the emanation into the testing vessel. After
ten minutes interval, the current due to the emanation in the
testing vessel reached a constant value. The compound was then
removed, and the standard comparison sample of equal weight
substituted; the saturation current was observed when a steady
state was again reached. The ratio of these two currents gives
the ratio of the emanating power of the two samples.

It was found experimentally, that, for the velocities of air
current employed, the saturation current in the testing vessel was
directly proportional to the weight of thorium, for weights up to
20 grams. This is explained by the supposition that the emanation
is removed by the current of air from the mass of the compound,
as fast as it is formed.

Let i\ = saturation current due to a weight &>! of the standard,
i2 = „ „ „ „ o>2 of the sample to

be tested.

,™ emanating power of specimen _ ^ &>

emanating power of standard i\ o>

1 Trans. Chem. Soc., p. 321, 1902. Phil. Mag. Sept. 1902.

214 RADIO-ACTIVE EMANATIONS [CH.

By means of this relation the emanating power of compounds
which are not of equal weight can be compared.

It was found that thorium compounds varied enormously in
emanating power, although the percentage proportion of thorium
present in the compound was not very different. For example,
the emanating power of thorium hydroxide was generally 3 to 4
times greater than that of ordinary thoria, obtained from the manu-
facturer. Thorium nitrate, in the solid state, had only 1/200 of the
emanating power of ordinary thoria, while preparations of the
carbonate were found to vary widely among themselves in emanat-
ing power, which depended upon slight variations in the method
of preparation.

141. Effect of condition^ on emanating power. The

emanating power of different compounds of thorium ahd radrfmi is
much affected by the alteration of chemical and physical conditions.
In this respect the emanating power, which is a measure of the
rate of escape of the emanation into the surrounding gas, must not
be confused with the rate of decay of the activity of the emanations
themselves, which has already been shown to be unaffected by
external conditions.

Dorn (loc. cit.) first observed that the emanating power of
thorium and radium compounds was much affected by moisture.
In a fuller investigation of this point by Rutherford and Soddy, it
was found that the emanating power of thoria is from two to three
times greater in a moist than in a dry gas. Continued desiccation
of the thoria in a glass tube, containing phosphorus pentoxide, did
not reduce the emanating power much below that observed in
ordinary dry air. In the same way radium chloride in the solid
state gives off very little emanation when in a dry gas, but the
amount is much increased in a moist gas.

The rate of escape of emanation is much increased by solution
of the compound. For example, thorium nitrate, which has an
emanating power of only 1/200 that of thoria in the solid state,
has in solution an emanating power of 3 to 4 times that of thoria.
P. Curie and Debierne observed that the emanating power of
radium was also much increased by solution.

Temperature has a very marked effect on the emanating power.

VIII] RADIO-ACTIVE EMANATIONS 215

The writer1 showed that the emanating power of ordinary thoria
was increased three to four times by heating the substance to a dull
red heat in a platinum tube. If the temperature was kept con-
stant, the emanation continued to escape at the increased rate,
but returned to its original value on cooling. If, however, the
compound was heated to a white heat, the emanating power was
greatly reduced, and it returned on cooling to about 10°/0 of the
original value. Such a compound is said to be "de-emanated"
The emanating power of radium compounds varies in a still more
striking manner with rise of temperature. The rate of escape
of the emanation is momentarily increased even 10,000 times by
heating to a dull red heat. This effect does not continue, for the
large escape of the emanation by heating is in reality due to the
release of the emanation stored up in the radium compound. Like
thoria, when the compound has once been heated to a very high
temperature, it loses its emanating power and does not regain it.

A further examination of the effect of temperature was made
by Rutherford and Soddy2. The emanating power of thoria decreases
very rapidly with lowering of temperature, and at the temperature
of solid carbonic acid it is only about 10°/c of its ordinary value.
It rapidly returns to its original value when the cooling agent is
removed.

Increase of temperature from 80° C. to a dull red heat of plati-
num thus increases the emanating power about 40 times, and the
effects can be repeated again and again, with the same compound,
provided the temperature is not raised to the temperature at which
de-emanation begins. De-emanation sets in above a red heat, and
the emanating power is then permanently diminished, but even
long continued heating at a white heat never entirely destroys the
emanating power.

142. Regeneration of emanating power. An interesting
question arises whether the de-emanation of thorium and radium is
due to a removal or alteration of the substance which produces the
emanation, or whether intense ignition merely changes the rate
of escape of the emanation from the solid into the surrounding
atmosphere.

1 Phys. Zeit. 2, p. 429, 1901. 2 Phil. Mag. Nov. 1902.

216 KADIO- ACTIVE EMANATIONS [CH.

It is evident that the physical properties of the thoria are
much altered by intense ignition. The compound changes in
colour from white to pink ; it becomes denser and also far less readily
soluble in acids. In order to see if the emanating power could be
regenerated by a cyclic chemical process, the de-emanated thoria
was dissolved, precipitated as hydroxide and again converted into
oxide. At the same time a specimen of the ordinary oxide was
subjected to an exactly parallel process. The emanating power of
both these compounds was the same and was from two to three
times greater than that of ordinary thoria.

Thus de-emanation does not permanently destroy the power
of thorium of giving out an emanation, but merely produces an
alteration of the amount of the emanation which escapes from the
compound.

143. Rate of production of the emanation. The eman-
ating power of thorium compounds, then, is a very variable quantity,
much affected by moisture, heat, and solution. Speaking generally,
increased temperatures and solution greatly increase the emanating
power of both thorium and radium.

The wide differences between the emanating powers of these
substances in the solid state and in solution pointed to the conclu-
sion that the differences were probably due to the rate of escape of
the emanation into the surrounding gas, and not to a variation of
the rate of reaction which gave rise to the emanation. It is
obvious that a very slight retardation in the rate of escape of the
thorium emanation from the compound into the gas, will, on account
of the rapid decay of activity of the emanation, produce great
changes in emanating power. The regeneration of the emanating
power of de-emanated thoria and radium by solution and chemical
treatment made it evident that the original power of thorium and
radium of producing the emanation still persisted in an unaltered
degree.

The question whether the emanation was produced at the same
rate in emanating as in non-emanating compounds can be put to a
sharp quantitative test. If the rate of production of emanation
goes on at the same rate in the solid compound, where very
little escapes, as in the solution, where probably all escapes, the

VIII] RADIO-ACTIVE EMANATIONS 217

emanation must be occluded in the compound, and there must in
consequence be a sudden release of this emanation on solution of
the compound. On account of the very slow decay of the activity
of the emanation of radium, the effects should be far more marked
in that compound than in thorium.

From the point of view developed in section 124, the expo-
nential law of decay of the emanation expresses the result that Nt
the number of particles remaining unchanged at a time t is given
by

'*-<-•

where N0 is the initial number of particles present. When a
steady state is reached, the rate of production q0 of fresh emanation
particles is exactly balanced by the rate of change of the particles
N0 already present, i.e.

q0 = \N0,

NQ in this case represents the amount of emanation " occluded " in
the compound. Substituting the value of X found for the radium
emanation in section 136,

^°= * = 463,000.

<?o X

The amount of emanation stored in a non-emanating radium
compound should therefore be nearly 500,000 times the amount
produced per second by the compound. This result was tested in
the following way1 :

A weight of '03 gr. of radium chloride of activity 1000 times that
of uranium was placed in a Drechsel bottle and a sufficient amount of
water drawn in to dissolve it. The released emanation was swept
out by a current of air into a small gas holder and then into a testing
cylinder. The initial saturation current was proportional to N0. A
rapid current of air was then passed through the radium solution
for some time in order to remove any slight amount of emanation
which had not been removed initially. The Drechsel bottle was
closed air-tight, and allowed to stand undisturbed for a definite
time t. The accumulated emanation was then swept out as before
into the testing vessel. The new ionization current represents
1 Rutherford and Soddy, Phil. Mag. April, 1903.

218 RADIO-ACTIVE EMANATIONS [CH.

the value of Nt the amount of emanation formed in the compound
during the interval t.

In the experiment t — 105 minutes,
and observed value

J = -0131.

Assuming that there is no decay during the interval,
#,= 105xt>0xgo.

Thus ^ = 480,000.

Making flhe small correction for the decay of activity during
the interval

We have previously shown that from the theory

—0 = i = 463,000.

q0 \

The agreement between theory and experiment is thus as close
as could be expected from the nature of the experiments. This
experiment proves conclusively that the rate of production of
emanation in the solid compound is the same as in the solution.
In the former case it is occluded, in the latter it escapes as fast as
it is produced.

It is remarkable how little emanation, compared with the
amount stored up in the compound, escapes from solid radium
chloride in a dry atmosphere. One experiment showed that the
emanating power in the dry solid state was less than ^ °/0 of the
emanating power of the solution. Since nearly 500,000 times as
much emanation is stored up as is produced per second, this result
showed that the amount of emanation which escaped per second was
less than 10~8 of that occluded in the compound.

If a solid radium chloride compound is kept in a moist atmo-
sphere, the emanating power becomes comparable with the amount
produced per second in the solution. In such a case, since the rate

VIII] RADIO-ACTIVE EMANATIONS 219

of escape is continuous, the amount occluded will be much less than
the amount for the non-emanating material.

The phenomenon of occlusion of the radium emanation is prob-
ably not connected in any way with its radio-activity, although this
property has in this case served to measure it. The occlusion of
helium by minerals presents almost a complete analogy to the
occlusion of the radium emanation. The helium is given off by
fergusonite, for example, in part when it is heated and completely
on dissolving the mineral.

144. Similar results hold for thorium, but, on account of the
rapid loss of activity of the emanation, the amount of emanation
occluded in a non-emanating compound is very small compared
with that observed for radium. If the production of the thorium
emanation proceeds at the same rate under all conditions, the
solution of a solid non-emanating compound should be accompanied
by a rush of emanation greater than that subsequently produced.
With the same notation as before we have for the thorium emana-
tion,

X 1

This result was tested as follows : a quantity of finely powdered
thorium nitrate, of emanating power 1/200 of ordinary thoria,
was dropped into a Drechsel bottle containing hot water and the
emanation rapidly swept out into the testing vessel by a current of
air. The ionization current rose quickly to a maximum, but soon
fell again to a steady value ; showing that the amount of emanation
released when the nitrate dissolves, is greater than the subsequent
amount produced from the solution.

The rapid loss of the activity of the thorium emanation makes
a quantitative comparison like that made for radium very difficult.
By slightly altering the conditions of the experiment, however, a
definite proof was obtained that the rate of production of emana-
tion is the same in the solid compound as in the solution. After
dropping in the nitrate, a rapid air stream was blown through the
solution for 25 seconds into the testing vessel. The air stream was
stopped and the ionization current immediately measured. The
solution was then allowed to stand undisturbed for 10 minutes.

220 RADIO-ACTIVE EMANATIONS [CH.

In that* time the accumulation of the emanation again attained a
practical maximum and again represented a steady state. The
stream of air was blown through, as before, for 25 seconds, stopped
and the current again measured. In both cases, the electrometer
recorded a movement of 14'6 divisions per second. By blowing
the same stream of air continuously through the solution the final
current corresponded to 7-9 divisions per second or about one-half
of that observed after the first rush.

Thus the rate of production of emanation is the same in the
solid nitrate as in the solution, although the emanating power, i.e.
the rate of escape of the emanation, is over 600 times greater in
the solution than in the solid.

It seems probable that the rate of production of emanation
by thorium, like the rate of production of Ur X and Th X, is inde-
pendent of conditions. The changes of emanating power of the
various compounds by moisture, heat, and solution must therefore
be ascribed solely to an alteration in the rate of escape of the
emanation into the surrounding gas and not to an alteration in
the rate of its production in the compound.

On this view, it is easy to see that slight changes in the mode
of preparation of a thorium compound may produce large changes
in emanating power. Such effects have been often observed, and
must be ascribed to slight physical changes in the precipitate.
The fact that the rate of production of the emanation is indepen-
dent of the physical or chemical conditions of the thorium, in which
it is produced, is thus in harmony with what had previously been
observed for the radio-active products Ur X and Th X.

Source of the Thorium Emanation.

145. Some experiments of Rutherford and Soddy1 will now
be considered, which show that the thorium emanation is pro-
duced, not directly by the thorium itself, but by the active
product ThX.

When the Th X, by precipitation with ammonia, is removed from
a quantity of thorium nitrate, the precipitated thorium hydroxide

1 Phil. Mag. Nov. 1902.

VIII] RADIO-ACTIVE EMANATIONS 221

does not at first possess appreciable emanating power. This loss
of emanating power is not due, as in the case of the de-emanated
oxide, to a retardation in the rate of escape of the emanation
produced ; for the hydroxide, when dissolved in acid, still gives
off no emanation. On the other hand, the solution, containing
the Th X, possesses emanating power to a marked degree.
On leaving the precipitated hydroxide and the Th X for some
time, it is found that the Th X decreases in emanating power,
while the hydroxide gradually regains its emanating power. After
about a month's interval, the emanating power of the hydroxide
has nearly reached a maximum, while the emanating power of
the Th X has almost disappeared.

The curves of decay and recovery of emanating power with
time are found to be exactly the same as the curves of decay
and recovery of activity of Th X and the precipitated hydroxide
respectively, shown in Fig. 35. The emanating power of Th X,
as well as its activity, falls to half value in four days, while the
hydroxide regains half its final emanating power as well as half its
lost activity in the same interval.

It follows from these results that the emanating power of Th X
is directly proportional to its activity, i.e. that the rate of produc-
tion of emanating particles is always proportional to the number
of a particles, projected from the Th X per second. The radiation
from Th X thus accompanies the change of the Th X into the
emanation. Since the emanation has chemical properties distinct
from those of the Th X, and also a distinctive rate of decay, it
cannot be regarded as a vapour of Th X, but it is a distinct
chemical substance, produced by the changes occurring in Th X.
On the view advanced in section 127, the atom of the emanation
consists of the part of the atom of Th X left behind after the
expulsion of one or more a particles. The atoms of the emana-
tion are unstable, and in turn expel a particles. This projection
of a particles constitutes the radiation from the emanation, which
serves as a measure of the amount of emanation present. Since
the activity of the emanation falls to half value in one minute
while that of Th X falls to half value in four days, the emanation
consists of atoms, which disintegrate at intervals nearly 6000 times
shorter than do the atoms of Th X.

222 RADIO-ACTIVE EMANATIONS [CH.

Source of the Radium Emanation.

146. No intermediate stage — Radium X — between radium
and its emanation, corresponding to the Th X for thorium, has
so far been observed. The emanation from radium is probably
produced directly from that element. In this respect, the radium
emanation holds the same position in regard to radium as Th X
does to thorium, and its production from radium can be explained
on exactly similar lines.

Radiations from the Emanations.

147. Special methods are necessary to examine the nature of
the radiation from the emanations, for the radiations arise from
the volume of the gas in which the emanations are distributed.
Some experiments to examine the radiations from the thorium
emanation were made by the writer in the following way.

A highly emanating thorium compound wrapped in paper was
placed inside a lead box B about 1 cm. deep, shown in Fig. 41.
There was an opening cut in ^ TO Electrometer

the top of the box, over which '

a very thin sheet of mica was

waxed. The emanation rapidly ^j/-/(V,

diffused through the paper into ||g Emanation igl V
the vessel, and after ten minutes ^"~

reached a state of radio-active Flg- 4L

equilibrium. The penetrating power of the radiation from the
emanation which passed through the thin mica window was
examined by the electrical method in the usual way by adding
screens of thin aluminium foil. The results are expressed in the
following table :

Thickness of mica window -0015 cm.
Thickness of aluminium foil -00034 cm.
Layers of foil Current

0 100

1 59

2 30

3 10

4 3-2

VIII] RADIO-ACTIVE EMANATIONS 223

The greater proportion of the conductivity is thus due to
a rays, as in the case of the radio-active elements. The amount
of absorption of these a rays by aluminium foil is about the same
as that of the rays from the active bodies. No direct comparison
can be made, for the a. rays from the emanation show the charac-
teristic property of increased rate of absorption with thickness
of matter traversed. Before testing, the rays have been largely
absorbed by the mica window, and the penetrating power has
consequently decreased.

No alteration in the radiation from the emanation was ob-
served on placing an insulated wire inside the emanation vessel,
and charging it to a high positive or negative potential. When
a stream of air through the vessel carried away the emanation as
fast as it was produced, the intensity of the radiation fell to a small
fraction of its former value.

N<> evidence of any 0 rays in the radiations was found in
these experiments, although a very small effect would have been
detected. After standing some hours, however, & rays began to
appear. These were due to the excited activity deposited on the
walls of the vessel from the emanation, and not directly to the
emanation itself.

The radium emanation, like that of thorium, only gives rise to
a. rays. This was tested in the following way1:

A large amount of emanation was introduced into a cylinder
made of sheet copper '005 cm. thick, which absorbed all the
a rav^s but allowed the /3 and 7 rays, if present, to pass through
with but little loss. The external radiation from the cylinder
was determined at intervals, commencing about two minutes after
the introduction of the emanation. The amount observed at first
was extremely small, but increased rapidly and practically reached
a maximum in three or four hours. Thus the radium emanation
only gives a rays, the ft rays appearing as the excited activity is
produced on the walls of the vessel. On sweeping out the emana-
tion by a current of air, there was no immediately appreciable
decrease of the radiation. This is another proof that the emanation
does not give out any /9 rays. In a similar way it can be shown

1 Rutherford and Soddy, Phil. Mag. April, 1903.

224 RADIO-ACTIVE EMANATIONS [CH.

that the emanation does not give rise to 7 rays ; these rays always
make their appearance at the same time as the /3 rays.

The method of examination of the radiations from the
emanations has been given in some detail, as the results are of
considerable importance in the discussion, which will be given
later in chapter x, of the connection between the changes oc-
curring in radio-active products and the radiations they emit.
There is no doubt that the emanations, apart from the excited
activity to which they give rise, only give out a rays, consisting
most probably of positively charged bodies projected with great
velocity.

Effect of Pressure on the rate of production of the Emanation.

148. It has already been mentioned that the conductivity
due to the thorium emanation is proportional to the pressure of
the gas, pointing to the conclusion that the rate of production
of the emanation is independent of the pressure, as well as of the
nature of the surrounding gas. This result was directly confirmed
with the apparatus of Fig. 41. When the pressure of the gas
under the vessel was slowly reduced, the radiation, tested outside
the window, increased to a limit, and then remained constant
over a wide range of pressure. This increase, which was far more
marked in air than in hydrogen, is due to the fact that the a rays
from the emanation were partially absorbed in the gas inside the
vessel when at atmospheric pressure. At pressures of the order
of 1 millimetre of mercury the external radiation decreased, but
experiment showed that this must be ascribed to a removal of the
emanation by the pump, and not to a change in the rate of pro-
duction. The thorium compounds very readily absorb water- vapour,
which is slowly given off at low pressures, and in consequence
some of the emanation is carried out of the vessel with the water-
vapour.

Curie and Debierne1 found that both the amount of excited
activity produced in a closed vessel containing active samples of
radium, and also the time taken to reach a maximum value, were

1 C. R. 133, p. 931, 1901.

VIIl] RADIO-ACTIVE EMANATIONS 225

independent of the pressure and nature of the gas. This was true
in the case of a solution down to the pressure of the saturated
vapour, and in the case of solid salts to very low pressures. When
the pump was kept going at pressures of the order of '001 mm. of
mercury, the amount of excited activity was much diminished.
This was probably not due to any alteration of the rate of escape
of the emanation, but to the removal of the emanation by the
action of the pump as fast as it was formed.

Since the amount of excited activity, when in a state of
radio-active equilibrium, is a measure of the amount of emana-
tion producing it, these results show that the amount of emanation
present when the rate of production balances the rate of decay is
independent of the pressure and nature of the gas. It was also
found that the time taken to reach the point of radio-active equi-
librium was independent of the size of the vessel or the amount
of active matter present. These results show that the state of
equilibrium cannot in any way be ascribed to the possession by the
emanation of any appreciable vapour pressure ; for if such were the
case, the time taken to reach the equilibrium value should depend
on the size of the vessel and the amount of active matter present.
The results are, however, in agreement with the view that the
emanation is present in minute quantity in the tube, and that the
equilibrium is governed purely by the radio-active constant X, the
constant of decay of activity of the emanation. This has been seen
to be the same under all conditions of concentration, pressure and
temperature, and, provided the rate of supply of the emanation
from the active compound is not changed, the time-rate of increase
of activity to the equilibrium value will always be the same,
whatever the size of the vessel or the nature and pressure of the
surrounding gas.

Chemical Nature of the Emanations.

149. Earlier experiments. We shall now consider some
experiments on the physical and chemical properties of the emana-
tions themselves, without reference to the material producing them,
in order to see if they possess any properties which identify them
with any known kind of matter.

R. R.-A. 15

226 RADIO-ACTIVE EMANATIONS [CH.

It was soon observed that the thorium emanation passed
unchanged through acid solutions, and later the same result was
shown to hold true in the case of both emanations for every
reagent that was tried. Preliminary observations1 showed that the
thorium emanation, obtained in the usual way by passing air over
thoria, passed unchanged in amount through a platinum tube
heated electrically to the highest temperature obtainable. The
tube was then filled with platinum-black, and the emanation passed
through it in the cold, and with gradually increasing temperatures,
until the limit was reached. In another experiment, the emana-
tion was passed through a layer of red-hot lead-chromate in a
glass tube. The current of air was replaced by a current of
hydrogen, and the emanation was sent through red-hot magnesium-
powder and red-hot palladium-black, and, by using a current of
carbon dioxide, through red-hot zinc-dust. In every case the
emanation passed through without sensible change in the amount.
If anything, a slight increase occurred, owing to the time taken for
the gas-current to pass through the tubes when hot being slightly
less than when cold, the decay en route being consequently less.
The only known gases capable of passing in unchanged amount
through all the reagents employed are the recently discovered
members of the argon family.

But another possible interpretation might be put upon the
results. If the emanation were the manifestation of a type of
excited radio-activity on the surrounding atmosphere, then, since
from the nature of the experiments it was necessary to employ in
each case as the atmosphere, a gas not acted on by the reagent
employed, the result obtained might be expected. Red-hot mag-
nesium would not retain an emanation consisting of radio-active
hydrogen, or red-hot zinc-dust, an emanation consisting of radio-
active carbon dioxide. The incorrectness of this explanation was
shown in the following way. Carbon dioxide was passed over
thoria, then through a T-tube, where a current of air met and
mixed with it, both passing on to the testing-cylinder. But
between this and the T-tube a large soda-lime tube was intro-
duced, and the current of gas was thus freed from its admixed
carbon dioxide, before being tested in the cylinder for the emana-

1 Rutherford and Soddy, Phil. Mag. Nov. 1902.

VI II] RADIO-ACTIVE EMANATIONS 227

tion. The amount of emanation found was quite unchanged,
whether carbon dioxide was sent over thoria in the manner de-
scribed, or whether, keeping the other arrangements as before,
an equally rapid current of air was substituted for it. The theory
that the emanation is an effect of the excited activity on the
surrounding medium is thus excluded.

Experiments of a similar kind on the radium emanation were
made later. A steady stream of gas was passed through a radium
chloride solution and then through the reagent to be employed,
into a testing-vessel of small volume, so that any change in the
amount of emanation passing through could readily be detected.
The radium emanation, like that of thorium, passed unchanged in
amount through every reagent used.

Later experiments. In later experiments by Sir William
Ramsay and Mr Soddy1, the emanation from radium was exposed to
still more drastic treatment. The emanation in a glass tube was
sparked for several hours with oxygen over alkali. The oxygen
was then removed by ignited phosphorus and no visible residue was
left. When, however, another gas was introduced, mixed with
the minute amount of emanation in the tube and withdrawn,
the activity of emanation was found to be unaltered. In another
experiment, the emanation was introduced into a magnesium lime
tube, which was heated for three hours at a red heat. The
emanation was then removed and tested, but no diminution in its
discharging power was observed.

The emanations of thorium and radium thus withstand chemical
treatment in a manner hitherto unobserved except in gases of the
argon family.

150. Ramsay and Soddy (loc. cit.) record an interesting
experiment to illustrate the gaseous nature of the emanation.
A large amount of the radium emanation was collected in a
small glass tube. This tube phosphoresced brightly under the
influence of the rays from the emanation. The passage of the
emanation from point to point was observed in a darkened
room by the luminosity excited in the glass. On opening the
stop-cock connecting with the Topler pump, the slow flow through

1 Proc. Roy. Soc. 72, p. 204, 1903.

15—2

228 RADIO-ACTIVE EMANATIONS [CH.

the capillary tube was noticed, the rapid passage along the wider
tubes, the delay in passing through a plug of phosphorous pent-
oxide, and the rapid expansion into the reservoir of the pump.
When compressed, the luminosity of the emanation increased, and
became very bright as the small bubble containing the emanation
was expelled through the fine capillary tube.

Diffusion of the Emanations.

151. It has been shown that the emanations of thorium and
radium behave like radio-active gases, distributed in minute amount
in the air or other gas in which they are tested. With the small
quantities of active material so far investigated, the emanations
have not yet been collected in sufficient amount to allow the
examination of their spectrum or to detect them by the balance.
Although the molecular weight of the emanations cannot yet be
obtained by direct chemical methods, an indirect estimate of it
can be made by determining the rate of their inter-diffusion into
air or other gases. The coefficients of inter-diffusion of various
gases have long been known, and the results show that the
coefficient of diffusion of one gas into another is, for the simpler
gases, approximately inversely proportional to the square root of
the product of their molecular weights. If, therefore, the coefficient
of diffusion of the emanation into air is found to have a value,
lying between that of two known gases A and B, it is probable
that the molecular weight of the emanation lies between that of
A and B.

Although the amount of emanation given off from radium is
too small to be detected by volume1, the electrical conductivity
produced by the emanation in the gas, with which it is mixed,
is often very large, and offers a ready means of measuring the
emanation present.

Some experiments have been made by Miss Brooks and the
writer2 to determine the rate of the diffusion of the radium emana-
tion into air, by a method similar to that employed by Loschmidt3

1 See, however, p. 313 (Feb. 1904).

2 Rutherford and Miss Brooks, Trans. Roy. Soc. Canada 1901, Chem. News 1902.

3 Sitzungsber. d. Wiener Akad. 61, n. p. 367, 1871.

VIII] RADIO-ACTIVE EMANATIONS 229

in 1871, in his investigations of the coefficient of inter-diffusion
of gases.

Fig. 42 shows the general arrangement. A long brass cylinder
AB, of length 73 cms., and diameter 6 cms., was divided into two

From

Gasometer -

Radium

Fig. 42.

equal parts by a moveable metal slide S. The ends of the cylinder
were closed with ebonite stoppers. Two insulated brass rods, a
and b, each half the length of the tube, passed through the ebonite
stoppers and were supported centrally in the tube. The cylinder
was insulated and connected with one pole of a battery of 300
volts, the other pole of which was earthed. The central rods could
be connected with a sensitive quadrant electrometer. The cylinder
was covered with a thick layer of felt, and placed inside a metal
box filled with cotton-wool in order to keep temperature con-
ditions as steady as possible.

In order to convey a sufficient quantity of emanation into
the half-cylinder A, it was necessary to heat the radium slightly.
The slide S was closed and the side tubes opened. A slow
current of dry air from a gasometer was passed through a platinum
tube, in which a small quantity of a radium compound was placed.
The emanation was carried with the air into the cylinder A. When
a sufficient quantity had been introduced, the stream of air was
stopped. The side tubes were closed by fine capillary tubes.
These prevented any appreciable loss of gas due to the diffusion,
but served to keep the pressure of the gas inside A at the pressure
of the outside air. The three entrance tubes into the cylinder,
shown in the figure, were for the purpose of initially mixing the
emanation and gas as uniformly as possible.

After standing several hours to make temperature conditions
steady, the slide was opened, and the emanation began to diffuse

230 RADIO-ACTIVE EMANATIONS [CH.

into the tube B. The current through the tubes A and B was
measured at regular intervals by an electrometer, with a suitable
capacity in parallel. Initially there is no current in B, but after
the opening of the slide, the amount in A decreased and the
amount in B steadily increased. After several hours the amount
in each half is nearly the same, showing that the emanation is
nearly uniformly diffused throughout the cylinder.
It can readily be shown1 that if

K — coefficient of diffusion of the emanation into air,
t = duration of diffusion experiments in sees.,
a = total length of cylinder,

Si = partial pressure of emanation in tube A at end of diffusion,
$2 = partial pressure of emanation in tube B at end of diffusion,
then

*&*."*

Now the values of S1 and $2 are proportional to the saturation
ionization currents due to the emanations in the two halves of the
cylinder. From this equation K can be determined, if the relative
values of Si and $2 are observed after diffusion has been in progress
for a definite interval t.

The determination of Si and $2 is complicated by the excited
activity produced on the walls of the vessel. The ionization due
to this must be subtracted from the total ionization observed in
each half of the cylinder, for the excited activity is produced from
the material composing the emanation, and is removed to the
electrodes in an electric field. The ratio of the current due to
excited activity to the current due to the emanation depends on
the time of exposure to the emanation, and is only proportional to
it for exposures of several hours.

The method generally adopted in the experiments was to open
the slide for a definite interval, ranging in the experiments from
15 to 120 minutes. The slide was then closed and the currents
in each half determined at once. The central rods, which had

1 See Stefan, Sitzungsber. d. Wien. Akad. 63, n. p. 82, 1871.

VIII] RADIO-ACTIVE EMANATIONS 231

been kept negatively charged during the experiments, had most
of the excited activity concentrated on their surfaces. These
were removed, new rods substituted and the current immediately
determined. The ratio of the currents in the half cylinders under
these conditions was proportional to S^ and $2, the amounts of
emanation present in the two halves of the cylinder.

The values of K, deduced from different values of t, were found
to be in good agreement. In the earlier experiments the values
of K were found to vary between '08 and '12. In some later
experiments, where great care was taken to ensure that tempera-
ture conditions were very constant, the values of K were found to
vary between '07 and '09. The lower value '07 is most likely
nearer the true value, as temperature disturbances tend to give
too large a value of K. No certain differences were observed in
the value of K whether the air was dry or damp, or whether an
electric field was acting or not.

152. Some experiments on the rate of diffusion of the radium
emanation into air were made at a later date by P. Curie and Danne1.
If the emanation is contained in a closed reservoir, it has been shown
that its activity, which is a measure of the amount of emanation
present, decreases according to an exponential law with the time.
If the reservoir is put in communication with the outside air
through a capillary tube, the emanation slowly diffuses out, and
the amount of emanation in the reservoir is found to decrease
according to the same law as before, but at a faster rate. Using
tubes of different lengths and diameters, the rate of diffusion was
found to obey the same laws as a gas. The value of K was found
to be O'lOO. This is a slightly greater value of K than the lowest
value 0'07 found by Rutherford and Miss Brooks. No mention
is made by Curie and Danne of having taken any special precau-
tions against temperature disturbances, and this may account for
the higher value of K obtained by them.

They also found that the emanation, like a gas, always divided
itself between two reservoirs, put in connection with one another,
in the proportion of their volumes. In one experiment one reser-
voir was kept at a temperature of 10° C. and the other at 350° C.

1 C. E. 136, p. 1314, 1903.

232

KADIO-ACTIVE EMANATIONS

[CH.

The emanation divided itself between the. two reservoirs in the
same proportion as a gas under the same conditions.

153. For the purpose of comparison, a few of the coefficients
of interdiffusion of gases, compiled from Landolt and Bernstein's
tables, are given below.

Gas or vapour

Coefficient of
diffusion into air

Molecular weight

Water vapour

0-198

Carbonic acid gas

0-142

Alcohol vapour

0-101

Ether vapour

0-077

Radium emanation ...

0-07

The tables, although not very satisfactory for the purpose of
comparison, show that the coefficient of interdiffusion follows the
inverse order of the molecular weights. The value of K for the
radium emanation is slightly less than for ether vapour, of which
the molecular weight is 74. We may thus conclude that the
emanation is of greater molecular weight than 74. It seems
likely that the emanation has a molecular weight somewhere in
the neighbourhood of 100, and is probably greater than this,
for the vapours of ether and alcohol have higher diffusion
coefficients compared with carbonic acid than the theory would
lead us to anticipate. Comparing the diffusion coefficients of the
emanation and carbonic acid into air, the value of the molecular
weight of the emanation should be about 176 if the result
observed for the simple gases, viz. that the coefficient of diffusion
is inversely proportional to the square root of the molecular
weights, holds true in the present case. On the disintegration
theory developed in chapter x, it is to be expected that the
atomic weight of the emanation should be slightly less than 225,
the atomic weight of radium.

It is of interest to compare the value of K = '07 with the value
of K determined by Townsend (section 37) for the gaseous ions
produced in air at ordinary pressure and temperature, by Rontgen
rays or by the radiations from active substances. Townsend found
that the value of K in dry air was *028 for the positive ions

VIII]

RADIO-ACTIVE EMANATIONS

233

and '043 for the negative ions. The radium emanation thus
diffuses more rapidly than the ions produced by its radiation in
the gas, and behaves as if its mass were smaller than that of
the ions produced in air, but considerably greater than that of
the air molecules with which it is mixed.

It is not possible to regard the emanation as a temporarily
modified condition of the gas originally in contact with the active
body. Under such conditions a much larger value of K would be
expected. The evidence derived from the experiments on diffusion
strongly supports the view that the emanation is a gas of heavy
molecular weight.

Diffusion of the Thorium Emanation.

154. On account of the rapid decay of the activity of the
thorium emanation, it is not possible to determine the value of K
its coefficient of diffusion into air by the methods employed for the
radium emanation. The value of K has been determined by the
writer in the following way. A plate C,
Fig. 43, covered with thorium hydroxide, was
placed horizontally near the base of a long
vertical brass cylinder P. The emanation
released from the thorium compound diffuses
upwards in the cylinder.

Let p be the partial pressure of the emana-
tion at a distance x from the source C. This
will be approximately uniform over the cross
section of the cylinder. From the general
principles of diffusion we get the equation

„ d?p dp

da:2 dt

Fig. 43.
The emanation is continuously breaking

up and expelling a particles. The emanation-residue gains a posi-
tive charge, and, in an electric field, is at once removed from the
gas to the negative electrode.

Since the activity of the emanation at any time is always
proportional to the number of particles which have not broken up,
and since the activity decays with the time according to an

234 RADIO-ACTIVE EMANATIONS [CH.

exponential law, p — pjerM where pl is the value of p when t = 0
and X is the radio-active constant of the emanation.

and 8=-^-;; ; ; ;; .i':;;i;rKlli,

Thus p=Ae~Y/^'x+B.

Since p = 0 when x = oo , £ = 0.
If p = p0 when x = 0, ^L = p0 .

\/I
Thus p=p0e ' K -x.

It was not found convenient in the experiments to determine
the activity of the emanation along the cylinder, but an equivalent
method was used which depends upon measuring the distribution
of "excited activity," produced along a central rod AB, which is
charged negatively.

It will be shown later (section 167) that the amount of excited
activity at any point is always proportional to the amount of
emanation at that point. The distribution of " excited activity "
along the central rod from the plate C upwards thus gives the
variation of p for the emanation along the tube.

In the experiments, the cylinder was filled with dry air at
atmospheric pressure and was kept at a constant temperature.
The central rod was charged negatively and exposed from one to
two days in the presence of the emanation. The rod was then
removed, and the distribution of the excited activity along it
determined by the electric method. It was found that the amount
of excited activity fell off with the distance x according to an
exponential law, falling to half value in about 1*9 cms. This is in
agreement with the above theory.

Since the activity of the emanation falls to half value in
1 minute, X = '0115. The value ^ = '09 was deduced from the
average of a number of experiments. This is a slightly greater
value than K = '07, obtained for the radium emanation, but the
results show that the two emanations do not differ much from
one another in molecular weight.

VIII] RADIO-ACTIVE EMANATIONS 235

Diffusion of the Emanation into Liquids.

155. Experiments have been made by Wallstabe1 on the
coefficient of diffusion of the radium emanation into various liquids.
The radium emanation was allowed to diffuse into a closed reservoir,
containing a cylinder of the liquid under observation. The cylinder
was provided with a tube and a stop-cock extending beyond the
closed vessel, so that different layers of the liquid could be removed.
The liquid was then placed in a closed testing vessel, where the
ionization current due to the escape of the emanation from the
liquid was observed to rise to a maximum after several hours, and
then to decay. This maximum value of the current was taken as
a measure of the amount of emanation absorbed in the liquid.

The coefficient of diffusion K of the emanation into the liquid
can be obtained from the same equation used to determine the
diffusion of the thorium emanation into air,

/T
p=p0e V,JT*

where X is the constant of decay of activity of the radium emana-
tion and x the depth of the layer of water from the surface.

Putting a = A/ g , it was found that

for water a = 1/6,
for toluol a = *75.

The value of X expressed in terms of a day as the unit of time
is about 17.

Thus the value of K for the diffusion of the radium emanation

cm "
into water = '066 ,

day

The value of K found by Stefan2 for the diffusion of carbon

dioxide into water was T36 --, These results are thus in har-

day

mony with the conclusion drawn from the diffusion of the radium
emanation into air, and show that the radium emanation behaves
as a gas of high molecular weight.

1 Phys. Zeit. 4, p. 721, 1903.

2 Wien. Sitzungsber. 2, p. 371, 1878.

236 RADIO-ACTIVE EMANATIONS [CH.

Condensation of the Emanations.

156. Condensation of the emanations. During an in-
vestigation of the effect of physical and chemical agencies on
the thorium emanation, Rutherford and Soddy1 found that the
emanation passed unchanged in amount through a white-hot
platinum tube and through a tube cooled to the temperature
of solid carbon dioxide. In later experiments, the effects of still
lower temperatures were examined, and it was then found that at
the temperature of liquid air both emanations were condensed2.

If either emanation is conveyed by a slow stream of hydrogen,
oxygen or air through a metal spiral immersed in liquid air, and
placed in connection with a testing vessel as in Fig. 37, no trace
of emanation escapes in the issuing gas. When the liquid air is
removed and the spiral plunged into cotton-wool, several minutes
elapse before any deflection of the electrometer needle is observed,
and then the condensed emanation volatilizes rapidly, and the
movement of the electrometer needle is very sudden, especially
in the case of radium. With a fairly large amount of radium
emanation, under the conditions mentioned, a very few seconds
elapse after the first sign of movement before the electrometer
needle indicates a deflection of several hundred divisions per
second. It is not necessary in either case that the emanating
compound should be retained in the gas stream. After the
emanation is condensed in the spiral, the thorium or radium
compound may be removed and the gas stream sent directly
into the spiral. But in the case of thorium under these condi-
tions, the effects observed are naturally small owing to the rapid
loss of the activity of the emanation with time, which proceeds at
the same rate at the temperature of liquid air as at ordinary
temperatures. • '

If a large amount of radium emanation is condensed in a glass
U tube, the progress of the condensation can be followed by the
eye, by means of the phosphorescence which the radiations excite
in the glass. If the ends of the tube are sealed and the tempera-
ture allowed to rise, the glow diffuses uniformly throughout the

1 Phil. Mag. Nov. 1902.

2 Phil. Mag. May 1903.

VIII]

RADIO-ACTIVE EMANATIONS

237

tube, and can be concentrated at any point to some extent by
local cooling of the tube with liquid air.

157. Experimental arrangements. A simple experimental
arrangement to illustrate the condensation and volatilization of the
emanation and some of its charac-
teristic properties is shown in Fig.
44. The emanation obtained by
solution or heating, from a few milli-
grams of radium bromide, is con-
densed in the glass U tube T im-
mersed in liquid air. This U tube
is then put into connection with a
larger glass tube V, in the upper
part of which is placed a piece of
zinc sulphide screen Z, and in the
lower part of the tube a piece of the
mineral willemite. The stop-cock
A is closed and the U tube and
the vessel V are partially exhausted

Fig. 44.

by a pump through the stop-cock B. This lowering of the pressure
causes a more rapid diffusion of the emanation when released. The
emanation does not escape if the tube T is kept immersed in liquid
air. The stop-cock B is then closed, and the liquid air removed.
No luminosity of the screen or the willemite in the tube V is
observed for several minutes, until the temperature of T rises
above the point of volatilization of the emanation. The emana-
tion is then rapidly carried into the vessel F, partly by expansion
of the gas in the tube T with rising temperature, and partly by
the process of diffusion. The screen Z and the willemite are
caused to phosphoresce brilliantly under the influence of the rays
from the emanation surrounding them.

If the end of the vessel V is then plunged into liquid air, the
emanation is again condensed in the lower end of the tube, and the
willemite phosphoresces much more brightly than before. This is
not due to an increase of the phosphorescence of willemite at the
temperature of the liquid air, but to the effect of the rays from
the emanation condensed around it. At the same time the lumin-

238 RADIO-ACTIVE EMANATIONS [CH.

osity of the zinc sulphide gradually diminishes, and practically
disappears after several hours if the end of the tube is kept in
the liquid air. If the tube is removed from the liquid air,
the emanation again volatilizes and lights up the screen Z. The
luminosity of the willemite returns to its original value after the
lapse of several hours. This slow change of the luminosity of
the zinc sulphide screen and of the willemite is due to the gradual
decay of the "excited activity" produced by the emanation on
the surface of all bodies exposed to its action (chapter vm).
The luminosity of the screen is thus due partly to the radiation
from the emanation and partly to the excited radiation caused
by it. As soon as the emanation is removed from the upper
to the lower part of the tube, the " excited " radiation gradually
diminishes in the upper and increases in the lower part of the
tube.

The luminosity of the screen gradually diminishes with the
time as the enclosed emanation loses its activity, but is still
appreciable after several weeks interval.

An apparatus of a similar character to illustrate the condensa-
tion of the radium emanation has been described by P. Curie1.

158. Determination of the temperature of condensa-
tion. A detailed investigation was made by Rutherford and
Soddy (loc. cit.) of the temperatures at which condensation and
volatilization commenced for the two emanations. The experi-
mental arrangement of the first method is shown clearly in Fig. 45.
A slow constant stream of gas, entering at A, was passed through
a copper spiral S, over 3 metres in length, immersed in a bath
of liquid ethylene. The copper spiral was made to act as its
own thermometer by determining its electrical resistance. The
resistance temperature curve was obtained by observation of the
resistances at 0°, the boiling point of liquid ethylene — 103'5°,
the solidification point of ethylene — 169° and in liquid air. The
temperature of the liquid air was deduced from the tables given
by Baly for the boiling point of liquid air for different percentages
of oxygen. The resistance temperature curve, for the particular
spiral employed, was found to be nearly a straight line between

1 Societe de Physique, 1903.

VIIl]

RADIO-ACTIVE EMANATIONS

239

0° and — 192° C., cutting the temperature axis if produced nearly
at the absolute zero. The resistance of the spiral, deduced from

Ammeter

To Earth

Fig. 45.

readings on an accurately calibrated Weston millivoltmeter, with
a constant current through the spiral, was thus very approximately
proportional to the absolute temperature. The liquid ethylene was
kept vigorously stirred by an electric motor, and was cooled to any
desirable temperature by surrounding the vessel with liquid air.

The general method employed for the radium emanation was
to pass a suitable amount of emanation, mixed with the gas to be
employed, from the gas holder B into the spiral, cooled below the
temperature of condensation. After the emanation was condensed
in the spiral, a current of electrolytic hydrogen or oxygen was
passed through the spiral. The temperature was allowed to
rise gradually, and was noted at the instant when a deflection of
the electrometer, due to the presence of emanation in the testing
vessel T, was observed. The resistance, subject to a slight correc-
tion due to the time taken for the emanation to be carried into
the testing vessel, gave the temperature at which some of the
emanation commenced to volatilize. The ionization current in
the testing vessel rose rapidly to a maximum value, showing that,
for a small increase of temperature, the whole of the radium
emanation was volatilized. The following table gives an illustration

240

RADIO-ACTIVE EMANATIONS

[CH.

of the results obtained for a current of hydrogen of T38 cubic
centimetres per second.

Temperature

Divisions per second
of the electrometer

-160°

-156°

- 154° '3

- 153°-8

-152° -5

The following table shows the results obtained for different
currents of hydrogen and oxygen.

Current of Gas

Hydrogen . . .

•25 c.c. per sec.

-151-3

-150

>»

•32

- 1537

-151

•92

-152

-151

1-38

-154

-153

)5 ...

2-3

- 162-5

-162

Oxygen

•34

-152-5

-151-5

•58

-155

-153

The temperature Tl in the above table gives the temperature
of initial volatilization, jT2 the temperature for which half of the
condensed emanation had been released. For slow currents of
hydrogen and oxygen, the values of 7\ and T^ are in good agree-
ment. For a stream of gas as rapid as 2*3 cubic centimetres per
second the value of Tl is much lower. Such a result is to be
expected, for, in too rapid a stream, the gas is not cooled to the
temperature of the spiral, and, in consequence, the inside surface
of the spiral is above the mean temperature, and some of the
emanation escapes at a temperature apparently much lower. In
the case of oxygen, this effect appears for a gas stream of 0'58 cubic
centimetres per second.

In the experiments on the thorium emanation, a slightly dif-
ferent method was necessary, on account of the rapid loss of its
activity. The steady stream of gas was passed over the thorium

VIII]

RADIO-ACTIVE EMANATIONS

241

compound, and the temperature was observed at the instant an
appreciable movement of the electrometer was observed. This
gave the temperature at which a small fraction of the thorium
emanation escaped condensation, and not the value T^ observed
for the radium emanation, which gave the temperature for which
a small fraction of the previously condensed emanation was
volatilized.

The following table illustrates the results obtained.

Current of Gas

Temperature

Hydrogen

Oxygen

•71 c.c. per sec.
1-38 „ „
•58 „ „

-155°C.
-159° C.
-155°C.

On comparing these results with the values obtained for the
radium emanation, it will be observed that with equal gas streams
the temperatures are nearly the same.

A closer examination of the thorium emanation showed, how-
ever, that this apparent agreement was only accidental, and that
there was, in reality, a very marked difference in the effect of tem-
perature in the two emanations. It was found experimentally that
the radium emanation was condensed very near the temperature
at which volatilization commenced, and that the points of conden-
sation and volatilization were fairly sharply defined.

On the other hand, the thorium emanation required a range
of over 30° C. after condensation had started in order to ensure
complete condensation. Fig. 46 is an example of the results
obtained with a steady gas stream of 1*38 c.c. per sec. of oxygen.
The ordinates represent the percentage proportion of the emana-
tion uncondensed at different temperatures. It will be observed
that condensation commences about —120°, and very little of the
emanation escapes condensation at — 155° C.

To investigate this difference of behaviour in the two emana-
tions, a static method was employed, which allowed determinations
of the two emanations to be made under comparable conditions.
The emanation, mixed with a small amount of the gas to be used,
was introduced into the cool spiral, which had previously been

R. R.-A.

242

RADIO-ACTIVE EMANATIONS

[CH.

exhausted by means of a mercury pump. The amount of emana-
tion remaining uncondensed after definite intervals was rapidly
removed by means of the pump, and was carried with a constant
auxiliary stream of gas into the testing vessel.

J 60

| 50

: 40
30
20
10
0

Condensation Curve
Thorium Emanation

-100

-160

-110 -120 -130 -140 -150

Temperature Centigrade

Fig. 46.

Tested in this way, it was found that the volatilization point
of the radium emanation was very nearly the same as that ob-
tained by the blowing method, viz. — 150° C. With thorium, on
the other hand, the condensation started at about —120° C., and,
as in the blowing method, continued over a range of about 30° C.
The proportion of the emanation condensed at any temperature
was found to depend on a variety of conditions, although the point
at which condensation commenced, viz. — 120° C., was about the
same in each case. It depended on the pressure and nature of the
gas, on the concentration of the emanation, and on the time for
which it was left in the spiral. For a given temperature a greater
proportion of the emanation was condensed, the lower the pressure
and the longer the time it was left in the spiral. Under the
same conditions, the emanation was more rapidly condensed in
hydrogen than in oxygen.

VIII] RADIO-ACTIVE EMANATIONS 243

159. Thus there is no doubt that the thorium emanation
begins to condense at a temperature higher than that at which
the radium emanation condenses. The explanation of the pecu-
liar behaviour of the thorium emanation is clear when the small
number of emanation particles present in the gas are taken into
consideration. It has been shown that both emanations give
out only a rays. It is probable that the a particles from the
two emanations are similar in character and produce about the
same number of ions in their passage through the gas. The
number of ions produced by each a particle before its energy
is dissipated is probably about 70,000. (See section 104.)

Now in the experiment the electrometer readily measured
a current of 10~3 electrostatic units. Taking the charge on an ion
as 3*4 x 10~10 electrostatic units, this corresponds to a production in
the testing vessel of about 3 x 106 ions per sec., which would be
produced by about 40 expelled a. particles per second. Each
radiating particle cannot expel less than one a particle and may
expel more, but it is likely that the number expelled by an atom
of the thorium emanation is not greatly different from the number
by an atom of the radium emanation.

In section 124 it has been shown that, according to the law of
decay, \N particles change per second when N are present. Thus
to produce 40 a particles, \N cannot be greater than 40. Since for
the thorium emanation X is 1/87, it follows that N cannot be greater
than 3500. The electrometer thus detected the presence of 3500
particles of the thorium emanation, and since in the static method
the volume of the condensing spiral was about 15 c.c., this corre-
sponds to a concentration of about 230 particles per c.c. An
ordinary gas at atmospheric pressure and temperature probably
contains about 3'6 x 1019 molecules per c.c. Thus the emanation
would have been detected on the spiral if it possessed a partial
pressure of less than 10~17 of an atmosphere.

It is thus not surprising that the condensation point of the
thorium emanation is not sharply defined. It is rather a matter
of remark that condensation should occur so readily with so sparse
a distribution of emanation particles in the gas ; for, in . order
that condensation may take place, it is probable that the particles
must approach within one another's sphere of influence.

16—2

244 RADIO-ACTIVE EMANATIONS [CH.

Now in the case of the radium emanation, the rate of decay
is about 5000 times slower than that of the thorium emanation,
and consequently the actual number of particles that must be
present to produce the same number of rays per second in the two
cases must be about 5000 times greater in the case of radium
than in the case of thorium. This conclusion involves only the
assumption that the same number of rays is produced by a
particle of emanation in each case, and that the expelled particles
produce in their passage through the gas the same number of
ions. The number of particles present, in order to be detected
by the electrometer, in this experiment, must therefore have
been about 5000 x 3500, i.e. about 2 x 107. The difference of
behaviour in the two cases is well explained by the view
that, for equal electrical effects, the number of radium emana-
tion particles must be far larger than the number of thorium
emanation particles. It is to be expected that the probability
of the particles coming into each other's sphere of influence will
increase very rapidly as the concentration of the particles in-
creases, and that, in the case of the radium emanation, once the
temperature of condensation is attained, all but a small proportion
of the total number of particles present will condense in a very
short time. In the case of the thorium emanation, however, the
temperature might be far below that of condensation, and yet
a considerable portion remain uncondensed for comparatively long
intervals. On this view the experimental results obtained are
^exactly what is to be expected. A greater proportion "condenses,
Ibhe longer the time allowed for condensation under the same con-
ditions. The condensation occurs more rapidly in hydrogen than
in oxygen, as the diffusion is greater in the former gas. For the
same reason the condensation occurs faster the lower the pressure
of the gas present. Finally, when the emanation is carried by
a steady gas stream, a smaller proportion condenses than in the
other cases, because the concentration of emanation particles per
unit volume of gas is less in these conditions.

It is possible that the condensation of the emanations may not
occur in the gas itself but at the surface of the containing vessel.
Accurate observations of the temperature of condensation have so
far only been made in a copper spiral, but condensation certainly

VIII] EADIO-ACTIVE EMANATIONS 245

occurs in tubes of lead or glass at about the same temperature as
in tubes of copper.

160. In experiments that were made by the static method
with a very large quantity of radium emanation, a slight amount
of escape of the condensed emanation was observed several degrees
below the temperature at which most of the emanation was released.
This is to be expected, since under such conditions the electrometer
is able to detect a very minute proportion of the whole quantity of
the emanation condensed.

Special experiments, with a large quantity of emanation, that
were made with the spiral immersed in a bath of rapidly boiling
nitric oxide, showed this effect very clearly. For example, the con-
densed emanation began to volatilize at -155°C. In 4 minutes
the temperature had risen to — 153'5°, and the amount volatilized
was four times as great as at — 155°. In the next 5£ minutes the
temperature had increased to 152'3° and practically the whole
quantity, which was at least fifty times the amount at the
temperature of — 153*5°, had volatilized.

It thus seems probable that, if the temperature were kept
steady at the point at which volatilization was first observed,
and the released emanation removed at intervals, the whole of
the emanation would in course of time be liberated at that tem-
perature. These results also point to the probability that the
condensed emanation possesses a true vapour pressure, but great
refinements in experimental methods would be necessary before
such a conclusion could be definitely established.

The true temperature of condensation of the thorium emana-
tion is probably about — 120° C., and that of radium about
— 150° C. Thus there is no doubt that the two emanations are
quite distinct from each other in this respect, and also with regard
to their radio-activity, although they both possess the property
of chemical inertness. These results on the temperatures of
condensation do not allow us to make any comparison of the
condensation points of the emanations with those of known gases,
since the lowering of the condensation point of gases with diminu-
tion of pressure has not been studied at such extremely minute
pressures.

246 RADIO-ACTIVE EMANATIONS [CH.

161. It was found1 that the activity of the thorium emanation
decayed at the same rate, when condensed in the spiral at the
temperature of liquid air, as at ordinary temperatures. This is in
accord with results of a similar kind obtained by P. Curie for the
radium emanation (section 136), and shows that the value of the
radio-active constant is unaffected by wide variations of tempera-
ture.

Amount of Emanation from Radium and Thorium.

162. It has been shown in section 104, that 1 gram of radium
emits about 1011 a particles per second. Since the activity due to
the emanation stored up in radium, when in a state of radio-active
equilibrium, is about one quarter of the whole, the number of a
particles projected per second by the emanation from 1 gram of
radium is about 2'5 x 1010. It has been shown in section 143,
that 463,000 times the amount of emanation produced per second
is stored up in the radium. But in a state of radio-active equi-
librium, the number of emanation particles breaking up per second
is equal to the number produced per second. Assuming that each
emanation particle in breaking up expels one a particle, it follows
that the number of emanation particles, present in 1 gram of
radium in radio-active equilibrium, is 463,000 x 2'5 x 1010, i.e.
1*2 x 1016. Taking the number of hydrogen molecules in 1 c.c. of gas
at atmospheric pressure and temperature as 3'6 x 1019 (section 39),
the volume of the emanation from 1 gram of radium is 3'3 x 10~4
cubic centimetre at atmospheric pressure and temperature. Quite
independently of any method of calculation, it is evident that the
volume of the emanation is very small, for attempts made to
detect its presence by its volume have so far failed. It is probable,
however, from the above calculation, that, when larger quantities
of radium are available for experiment, the emanation will be
collected in volume sufficiently large to measure.

In the case of thorium, the maximum quantity of emanation to

be obtained from 1 gram of the solid is very minute, both on account

of the small activity of thorium and of the rapid break up of the

emanation after its production. Since the amount of emanation,

1 Rutherford and Soddy, Phil. Mag. May, 1903.

VIIl] RADIO-ACTIVE EMANATIONS 247

stored in a non-emanating thorium compound, is only 87 times
the rate of production, while in radium it is 463,000 times, and the
rate of production of the emanation by radium is about 1 million
times faster than by thorium, it follows that the amount of emana-
tion to be obtained from 1 gram of thorium is not greater than
10~10 of the amount from an equal weight of radium, i.e. its volume
is not greater than 5 x 10~14 c.c. at the ordinary pressure and
temperature. Even with large quantities of thorium, the amount
of emanation is too small ever to be detected by its volume.

Heal Emission of the Radium Emanation.

163. It has been shown in section 106, that the radium
emanation emitsjieatjat a rapid rate and is responsible for about
70 : , of the heating effect of radium. The emanation from 1 gram
of radium, together wiflT the heat effect due to the excited activity
on the walls of the containing vessel, thus gives rise to an emission
of heat of about 70 gram-calories per hour. This rate of heat
emission decays according to an exponential law with the time,
decreasing to half value in about four days. The total quantity of

heat given out during the life of the emanation is — , where q is

the initial rate of heat production and X is the radio-active
constant of the emanation. Since the value of X expressed in
hours (section 136) is 1/128 and q is 70, the total quantity of
heat emitted from the emanation from 1 gram of radium is about
10,000 gram-calories. But the volume of this emanation is about
3'3 x 10~4 c.c. Thus the total heat emitted from one cubic centi-
metre of the emanation at standard pressure and temperature
would be about 3 x 107 gram-calories. The initial rate of emission
of heat is 2 x 105 gram-calories per hour or 60 gram-calories per
second. This rapid emission of heat would be sufficient to heat
to redness if not to melt down the tube which contains the
emanation.

If the atomic weight of the emanation is taken to be about 200,
it can be calculated that 1 pound weight of the emanation would
initially radiate heat at the rate of about 8000 horse-power, and in
the whole course of its heat emission would radiate an amount of

248 RADIO-ACTIVE EMANATIONS [CH.

energy corresponding to 40,000 horse-power days. In order to
obtain such an amount of emanation about 70 tons of radium
would be required.

Summary of Results.

164. The investigations into the nature of the radio-active
emanations have thus led to the folio wing conclusions: — The radio-
elements thorium and radium continuously produce from themselves
radio-active emanations at a rate which is constant under all con-
ditions. In some cases, the emanations continuously diffuse from
the radio-active compounds into the surrounding gas; in other
cases, the emanations are unable to escape from the material in
which they are produced but are occluded, and can only be released
by the action of solution or heat.

The emanations possess all the properties of radio-active gases.
They diffuse through gases, liquids, and porous substances, and can
be occluded in some solids. Under varying conditions of pressure,
volume, and temperature, the emanations distribute themselves in
the- same way and according to the same laws as does a gas.

The emanations possess the important property of condensation
under the influence of extreme cold, and by that means can be
separated from the gases with which they are mixed. The radia-
tion from the emanation is material in nature, and consists of a
stream of positively charged particles projected with great velocity.

Taking all these properties into consideration, it is difficult to
avoid the conclusion that the emanations are material and exist
in the gaseous state. The emanations possess the property of
chemical inertness, and in this respect resemble the gases of the
argon family. The emanations are produced in minute amount;
sufficient quantity has not yet been obtained to examine by
ordinary chemical methods. With regard to their rates of dif-
fusion, the emanations of both thorium and radium behave like
gases of high molecular weight.

These emanations have been detected and their properties
investigated by the property they possess of emitting radiations of
a special character. These radiations consist entirely of a rays,
i.e. particles, projected with great velocity, which carry a positive

VIIl] RADIO-ACTIVE EMANATIONS 249

charge and have a mass about twice that of the hydrogen atom.
The emanations do not possess the property of permanently radiat-
ing, but the intensity of the radiations diminishes according to an
exponential law with the time, falling to half value, in the case of
thorium in one minute, and in case of radium in about four days.
The law of decay of activity does not seem to be influenced by
any physical or chemical agency.

The emanation particles gradually break up, each particle as it
breaks up expelling a charged body. The emanation after it has
radiated ceases to exist as such, but is transformed into a new
kind of matter, which is deposited on the surface of bodies and
gives rise to the phenomena of excited activity. This last property,
and the connection of the emanation with it, is discussed in detail
in the succeeding chapter.

CHAPTER IX.

EXCITED RADIO-ACTIVITY.

165. Excited radio-activity. One of the most interesting
and remarkable properties of thorium and radium is their power of
" exciting " or " inducing " temporary activity on all bodies in their
neighbourhood. A substance which has been exposed for some
time in the presence of radium or thorium, behaves as if its surface
were covered with an invisible deposit of intensely radio-active
material. The " excited" body emits radiations capable of affecting
a photographic plate and of ionizing a gas. Unlike the radio-
elements themselves however, the activity of the body does not
remain constant after it has been removed from the influence of
the exciting active material, but decays with the time. The
activity lasts for several hours when due to radium and several
days when due to thorium.

This property was first observed by M. and Mme Curie1 for
radium, and independently by the writer2 for thorium3.

1 C. R. 129, p. 714, 1899. 2 Phil, Mag. Jan. and Feb. 1900.

3 As regards date of publication, the priority of the discovery of "excited
activity " belongs to M. and Mme Curie. A short paper on this subject, entitled
" Sur la radioactivite provoquee. par les rayons de Becquerel," was communicated
by them to the Comptes Rendus, Nov. 6, 1899. A shor*t note was added to the
paper by Becquerel in which the phenomena of excited activity were ascribed to a
type of phosphorescence. On my part, I had simultaneously discovered the
emission of an emanation from thorium compounds and the excited activity
produced by it, in July, 1899. I, however, delayed publication in order to work
out in some detail the properties of the emanation and of the excited activity and
the connection between them. The results were published in two papers in the
Philosophical Magazine (Jan. and Feb. 1900) entitled " A radio-active substance
emitted from thorium compounds" and " Kadio-activity produced in substances by
the action of thorium compounds."

CH. IX]

EXCITED RADIO-ACTIVITY

251

If any solid body is placed inside a closed vessel containing an
emanating compound of thorium or radium, its surface becomes
radio-active. For thorium compounds the amount of excited ac-
tivity on a body is in general greater the nearer it is to the active
material. In the case of radium, however, provided the body has
been exposed for several hours, the amount of excited activity is to
a large extent independent of the position of the body in the vessel
containing the active material. Bodies are made active whether
exposed directly to the action of the radio-active substance or
screened from the action of the direct rays. This has been clearly
shown in some experiments of P. Curie. A small open vessel a
(Fig. 47) containing a solution of radium is placed inside a larger
closed vessel V.

Fig. 47.

Plates A, B, C, D, E are placed in various positions in the
enclosure. After exposure for a day, the plates after removal are
found to be radio-active even in positions completely shielded from
the action of the direct rays. For example, the plate D shielded
from the direct radiation by the lead plate P is as active as the
plate E, exposed to the direct radiation. The amount of activity
produced in a given time on a plate of given area in a definite
position is independent of the material of the plate. Plates of
mica, copper, cardboard, ebonite, all show equal amounts of activity.
The amount of activity depends on the area of the plate and on

252

EXCITED RADIO-ACTIVITY

[CH.

the amount of free space in its neighbourhood. Excited radio-
activity is also produced in water if exposed to the action of an
emanating compound.

166. Concentration of excited radio-activity on the
negative electrode. When thorium or radium is placed in a
closed vessel, the whole interior surface becomes strongly active.
In a strong electric field, on the other hand, the writer found that
the activity was confined entirely to the negative electrode. By
suitable arrangements, the whole of the excited activity, which
was previously distributed over the surface of the vessel, can be
concentrated on a small negative electrode placed inside the vessel.
An experimental arrangement for this purpose is shown in Fig. 48.

The metal vessel V containing a large amount of thoria is con-
nected to the positive pole of a battery of about 300 volts. The
wire AB to be made active is fastened to a stouter rod BC, passing
through an ebonite cork inside a short cylinder D, fixed in the side
of the vessel. This rod is connected with the negative pole of the
battery. In this way the wire AB is the only conductor exposed
in the field with a negative charge, and it is found that the whole
of the excited activity is concentrated upon it.

In this way it is possible to make a short thin metal wire over
10,000 times as active per unit surface as the thoria from which
the excited activity is derived. In the same way, the excited
activity due to radium can be concentrated mainly on the negative

IX] EXCITED RADIO-ACTIVITY 253

electrode. In the case of thorium, if the central wire is charged
positively, it shows no appreciable activity. With radium, however,
a positively charged body becomes slightly active. In most cases,
the amount of activity produced on the positive electrode is not
more than 5% °f ^ne corresponding amount when the body is
negatively charged. For both thorium and radium, the amount of
excited activity on electrodes of the same size is independent of
their material.

All metals are made active to equal extents for equal times of
exposure. When no electric field is acting, the same amount
of activity is produced on insulators like mica and glass as on
conductors of equal dimensions.

167. Connection between the emanations and excited
activity. An examination of the conditions under which excited
activity is produced shows that there is a very close connection
between the emanation and the excited activity. If a thorium
compound is covered with several sheets of paper, which cut off the
a rays but allow the emanation to pass through, excited activity is
still produced in the space above it. If a thin sheet of mica is
waxed down over the active material, thus preventing the escape of
the emanation, no excited activity is produced outside it. Uranium
and polonium which do not give off an emanation are not able to
produce excited activity on bodies. Not only is the presence of
the emanation necessary to cause excited activity, but the amount
of excited activity is always proportional to the amount of emana-
tion present. For example, de-emanated thoria produces very
little excited activity compared with ordinary thoria. In all cases
the amount of excited activity produced is proportional to the
emanating power. The emanation when passing through an
electric field loses its property of exciting activity at the same
rate as the radiating power diminishes. This was shown by the
following experiment.

A slow constant current of air from a gasometer, freed from
dust by its passage through cotton-wool, passed through a rectangu-
lar wooden tube 70 cms. long. Four equal insulated metal plates
A, B, C, D, were placed at regular intervals along the tube. The
positive pole of a battery of 300 volts was connected to a metal

254

EXCITED RADIO-ACTIVITY

[CH.

plate placed in the bottom of the tube, while the negative pole
was connected with the four plates. A mass of thoria was placed
in the bottom of the tube under the plate A , and the current due
to the emanation determined at each of the four plates. After
passing a current of air of 0'2 cm. per second, for 7 hours along the
tube, the plates were removed and the amount of excited activity
produced on them was tested by the electric method. The follow-
ing results were obtained.

Eelative current

Eelative excited

due to emanation

activity

Plate A ...

„ B ...

•55

•43

,, o ...

•18

•16

„ D ...

•072

•061

Within the errors of measurement, the amount of excited
activity is thus proportional to the radiation from the emanation,
i.e. to the amount of emanation present. The same considerations
hold for the radium emanation. The emanation in this case, on
account of the slow loss of its activity, can be stored mixed with
air for long periods in a gasometer, and its effects tested quite
independently of the active matter from which it is produced.
The ionization current due to the excited activity produced by the
emanation is always proportional to the current due to the emana-
tion for the period of one month or more that its activity is large
enough to be conveniently measured by an electrometer.

If at any time during the interval, some of the emanation is
removed and introduced into a new testing vessel, the ionization
current will immediately commence to increase, rising in the course
of four or five hours to about twice its original value. This increase
of the current is due to the excited activity produced on the walls
of the containing vessel. On blowing out the emanation, the
excited activity is left behind, and at once begins to decay.
Whatever its age, the emanation still possesses the property of
causing excited activity, and in amount always proportional to its
activity, i.e. to the amount of emanation present.

These results show that the power of exciting activity on

IX] EXCITED RADIO-ACTIVITY 255

inactive substances is a property of the radio-active emanations,
and is proportional to the amount of emanation present.

The phenomenon of excited activity cannot be ascribed to a
type of phosphorescence produced by the rays from the emanation
on bodies ; for it has been shown that the activity can be concen-
trated on the negative electrode in a strong electric field, even if
the electrode is shielded from the direct radiation from the active
substance which gives off the emanation. The amount of excited
activity does not seem in any way connected with the ionization
produced by the emanation in the gas with which it is mixed.
For example, if a closed vessel is constructed with two large
parallel insulated metal plates on the lower of which a layer of
thoria is spread, the amount of the excited activity on the upper
plate when charged negatively, is independent of the distance
between the plates when that distance is varied from 1 millimetre
to 2 centimetres. This experiment shows that the amount of
excited activity depends only on the amount of emanation, emitted
from the thoria; for the ionization produced with a distance of
2 centimetres between the plates is about ten times as great as
with a distance of 1 millimetre.

168. If a platinum wire is made active by exposure to the
emanation of thoria, its activity1 can be removed by treating the
wire with certain acids. For example, the activity is not much
altered by immersing the wire in hot or cold water or nitric acid,
but more than 80% of it is removed by dilute or concentrated
solutions of sulphuric or hydrochloric acid. The activity has not
been destroyed by this treatment but is manifested in the solution.
If the solution is evaporated, the activity remains behind on the
dish.

These results show that the excited activity is due to a deposit
on the surface of bodies of radio-active matter which has definite
properties as regards solution in acids. This active matter is
dissolved in some acids, but, when the solvent is evaporated, the
active matter is left behind. This active matter is deposited on
the surface of bodies, for it can be partly removed by rubbing the
body with a cloth, and almost completely by scouring the plate

1 Rutherford, Phil. Mag. Feb. 1900.

256 EXCITED RADIO-ACTIVITY [CH.

with sand or emery paper. The amount of active matter deposited
is extremely small, for no difference of weight has been detected
in a platinum wire when made extremely active. On examining
the wire under a microscope, no trace of foreign matter is observed.
It follows from these results that the matter which causes excited
activity is many thousand times more active, weight for weight,
than radium itself.

It is convenient to have a definite name for this radio-active
matter, for the term " excited activity " only refers to the radiation
from the active matter and not to the matter itself. Since the
matter which produces the phenomena of excited radio-activity is
derived from the emanation of thorium and of radium, the name
emanation X will be given to it. This is chosen from analogy to the
active products Ur X and Th X which are continuously produced
from uranium and thorium respectively. The emanation X from
thorium is different in chemical and other properties from the
emanation X from radium. For example, each type of matter has
a distinctive rate of decay of activity, as well as some differences
in solubility by acids.

On the view developed in section 127, the emanation X is the
residue left behind from each atom of the emanation of thorium
or of radium after one or more a particles have been expelled. The
emanation X is an unstable substance, and its atoms again break
up, giving rise to "excited activity," i.e. to the radiation from
" emanation X."

The emanation X is quite distinct in chemical and physical
properties from the emanation which produces it. For example,
emanation X behaves as a solid, which is deposited on the surface
of bodies, while the emanation exists in the gaseous state. The
emanation is insoluble in hydrochloric or sulphuric acids, while
emanation X is readily soluble in both.

169. Decay of the excited activity produced by thorium.

The excited activity produced in a body after a long exposure to
the emanations of thorium, decays in an exponential law with the
time, falling to half value in about 11 hours. The following table
shows the rate of decay of the excited activity produced on a brass
rod.

IX]

EXCITED RADIO-ACTIVITY

257

Time in hours
0

7-9
11-8
23-4
29-2
32-6
49-2
62-1
71-4

Current

100
64
47-4
19-6
13-8
10-3
3-7
1-86
0-86

The results are shown graphically in Fig. 49, Curve A.

100

40 60 8O 100
Time in Hours
Fig. 49.

The intensity of the radiation / after any time t is given by
— = e~u where \ is the radio-active constant.

The rate of decay of excited activity, like that of the activity of
other radio-active products, is not appreciably affected by change of
conditions. The rate of decay is independent of the concentration of
the excited activity, and of the material of the body in which it is
produced. It is independent also of the nature and pressure of the

R. R.-A.

258 EXCITED KADIO-ACTIVITY [CH

gas in which it decays. The rate of decay is unchanged whether
the excited activity is produced on the body with or without an
electric field.

The amount of excited activity produced on a body increases
at first with the time, but reaches a maximum after an exposure
of several days. An example of the results is given in the following
table. In this experiment, a rod was made the cathode in a closed
vessel containing thoria. It was removed at intervals for the short
time necessary to test its activity and then replaced.

Time in hours Current

1-58 6'3

3'25 10-5

5-83 29

9-83 40

14-00 59

23-41 77

29-83 83

47-00 90

72-50 95

96-00 100

These results are shown graphically in Curve B, Fig. 49. It is
seen that the decay and recovery curves may be represented
approximately by the following equations.

For the decay curve A, -=- = e~xt.

For the recovery curve B, ^ = 1 — e~^.

J-Q

The two curves are thus complementary to one another ; they
are connected in the same way as the decay and recovery curves of
Ur X, and are susceptible of a similar explanation.

The amount of excited radio-activity reaches a maximum value
when the rate of supply of fresh radio-active particles balances the
rate of change of those already deposited.

170. Excited radio-activity produced by a short ex-
posure. The initial portion of the recovery curve B, Fig. 49, is
not accurately represented by the above equation. The activity
for the first few hours increases more slowly than would be

IX] EXCITED RADIO-ACTIVITY 259

expected from the equation. This result, however, is completely
explained in the light of later results. The writer1 found that, for
a short exposure of a body to the thorium emanation, the excited
activity upon it after removal, instead of at once decaying at the
normal rate, increased for several hours. In some cases the activity
of the body increased three to four times its original value in the
course of a few hours and then decayed with the time.

Some of the results obtained are shown in the following tables.
Table I. is for a platinum wire exposed as cathode for 15 minutes;
Table II. for aluminium foil with 41 minutes' exposure to the
emanation. About 5 minutes elapsed between removal and the
first observation.

TABLE I. TABLE II.

Time Current Time Current

01 01

7*5 minutes 1-5 21 minutes T6

24 „ 2-1 31 „ 1-8

43 „ 2-4 57 „ 2-0

58 „ 2-7 70 „ 2-2

78 „ 3-1 91 „ 2-5

99 „ 3-4 120 „ 2-9

160 „ 2-9
180 „ 2-9
22 hours 1*0
49 „ -21

The initial current is, in each case, taken as unity. In Table II.
the activity after increasing nearly to three times its original value
decreases again at about the normal rate, falling to half value in
about 11 hours.

With a longer time of exposure to the emanation, the ratio of
the increase after removal is much less marked. For a long
interval of exposure, the activity after removal begins at once to
diminish. In this case, the increase of activity of the matter
deposited in the last few hours does not compensate for the
decrease of activity of the active matter as a whole, and conse-
quently the activity at once commences to decay. This increase of
activity with time explains the initial irregularity in the recovery
1 Phys. Zeit. 3, No. 12, p. 254, 1902. Phil. Mag. Jan. 1903.

17—2

260

EXCITED RADIO-ACTIVITY

[CH.

curve, for the active matter deposited during the first few hours
takes some time to reach its maximum activity, and the initial
activity is, in consequence, smaller than would be expected from
the equation.

The increase of activity on a rod exposed for a short interval in
the presence of the thorium emanation has been further investigated
by Miss Brooks. The curve C in Fig. 50 shows the variation with
time of the activity of a brass rod exposed for 10 minutes in the
emanation vessel rilled with dust-free air. The excited activity
after removal increased in the course of 3'7 hours to five times its
initial value, and afterwards decayed at the normal rate.

IOC

0 E 40 80 120 160 200 240 280 320

Time in Minutes
Fig. 50.

171. Effect of dust on the distribution of excited activity.

Miss Brooks, working in the Cavendish Laboratory, observed that
the excited activity due to the thorium emanation appeared in
some cases on the anode in an electric field, and that the distribu-
tion of excited activity varied in an apparently capricious manner.
This effect was finally traced to the presence of dust in the air of
the emanation vessel. For example, with an exposure of 5 minutes

IX] EXCITED RADIO-ACTIVITY 261

the amount of excited activity to be observed on a rod depended
on the time that the air had been allowed to remain undisturbed
in the emanation vessel beforehand. The effect increased with the
time of standing, and was a maximum after about 18 hours. The
amount of excited activity obtained on the rod was then about
20 times as great as the amount observed for air freshly introduced.
The activity of this rod did not increase after removal, but with
fresh air, the excited activity, for a 5 minutes' exposure, increased
to five or six times its initial value.

This anomalous behaviour was found to be due to the presence
of dust particles in the air of the vessel, in which the bodies were
made radio-active. These particles of dust, when shut up in the
presence of the emanation, become radio-active. When a nega-
tively charged rod is introduced into the vessel, a part of the
radio-active dust is concentrated on the rod and its activity is
added to the normal activity produced on the wire. After the air
in the vessel has been left undisturbed for an interval sufficiently
long to allow each of the particles of dust to reach a state of radio-
active equilibrium, on the application of an electric field, all the
positively charged dust particles will at once be carried to the
negative electrode. The activity of the electrode at once com-
mences to decay, since the decay of the activity of the dust particles
on the wire quite masks the initial rise of the normal activity
produced on the wire.

Part of the radio-active dust is also carried to the anode, and
the proportion increases with the length of time during which the
air has been undisturbed. The greatest amount obtained on the
anode was about 60°/0 of that on the cathode.

These anomalous effects were found to disappear if the air was
made dust-free by passing through a plug of glass wool, or by
application for some time of a strong electric field.

172. Decay of excited activity from radium. The excited
activity produced on bodies by exposure to the radium emanation
decays much more rapidly than the thorium excited activity. For
short times of exposure1 to the emanation the decay curve is very
irregular. This is shown in Fig. 51.

1 Rutherford and Miss Brooks, Phil. Mag. July, 1902.

262

EXCITED RADIO-ACTIVITY

[CH.

It was found that the intensity of the radiation decreased
rapidly for the first 10 minutes after removal, but about 15 minutes
after removal reached a value which is maintained nearly constant
for an interval of about 20 minutes. It then decays, following an

120

140

60 80 100
Time in Minutes

Fig. 51.

exponential law to zero, the intensity falling to half value in about
30 minutes. With longer times of exposure, the irregularities
in the curve are not so marked.

Later, P. Curie and Danne1 made a detailed investigation of the
decay of excited activity for times of exposure to the emanation
from 10 seconds to 6 days. The results are shown graphically
in Fig. 52, where the ordinates represent the logarithm of the
intensity of the radiation, and the abscissae time in hours. Curve
A represents the decay for a long time of exposure. This decay

1 C. R. 136, p. 364, 1903.

IX]

EXCITED RADIO-ACTIVITY

263

curve is unaltered for all times of exposure to the emanation greater
than 24 hours.

After an interval of 2*5 hours, the logarithmic decay curve for
long times of exposure is a straight line, i.e. the activity falls off
in an exponential law with the time, falling to half value in
28 minutes. P. Curie and Danne found that for any time t

Time in Hourg
Fig. 52.

after removal the intensity It was given by the difference of two
exponentials, viz.

where Xj = ^^ and X2 = y^. with the second as the unit of time.
The numerical constant a = 4*20. The explanation of this law of
decay is given in section 177.

The decay curve varies greatly with the time of exposure. For
example, in an exposure of 5 minutes, the activity at first decreases

264 EXCITED KADIO-ACTIVITY [CH.

very rapidly, then passes through a minimum after 8 minutes, and
increases to a maximum after 40 minutes, and after 2'5 hours
decays in an exponential law to zero1. As in the case of the excited
activity from thorium, the rate of decay of the excited activity
from radium is for the most part independent of the nature of the
body made active. Curie and Danne (loc. cit.) observed that the
active bodies gave off an emanation itself capable of exciting
activity in neighbouring bodies. This property rapidly disappeared
and was inappreciable 2 hours after removal. In certain substances
like celluloid and caoutchouc, the decay of activity is very much
slower than for the metals. This effect becomes more marked
with increase of time of exposure to the emanation. A similar
effect is exhibited by lead, but to a less marked degree. During
the time the activity lasts, these substances continue to give off
an emanation.

It is probable that these divergencies from the general law are
not due to an actual change in the rate of decay of the true excited
activity but to an occlusion of the emanation by these substances
during the interval of exposure. After exposure the emanation
gradually diffuses out, and thus the activity due to this occluded
emanation and the excited activity produced by it decays very
slowly with the time.

173. Excited radio-activity of very slow decay. M. and

Mme Curie2 have observed that bodies which have been exposed
for a long interval in the presence of the radium emanation do not
lose all their activity. The excited activity at first decays rapidly
at the normal rate, falling to half value in about 30 minutes, but a
residual activity always remains of the order of 1/20,000 of the
initial activity. This residual activity either does not diminish
at all, or so slowly that the decrease is not appreciable after
an interval of six months.

1 The writer has not observed the rise to a maximum found by Curie and
Danne for the decay curves of the excited activity due to radium (see Fig. 52), but
has always obtained curves of decay, for short exposures, similar to that shown in
Fig. 51. This has been the case whether the excited activity has been produced on
a body by the action of an electric field or not. In the experiments, a slow current
of air was always passed through the testing apparatus to remove any emanation
from the body made active.

2 Thesis, Paris, 1903, p. 116.

IX] EXCITED RADIO-ACTIVITY 265

Giesel1 has also observed that a platinum wire which has been
exposed in the presence of the radium emanation possesses residual
activity, and he has shown that the radiation consists entirely of
a rays. A further discussion of this residual activity and its
possible connection with polonium is given later in section 188.

174. Connection between decay curves for different
times of exposure. The decay of excited activity, in cases
where there is no occlusion of the emanation by the substance
made active, is a function only of the time of exposure. The
decay curves are all intimately connected with each other, and can
be theoretically deduced provided the decay curve for a very short
exposure is accurately known.

It is supposed that the excited activity produced on a body is
due to a deposit of radio-active particles. On an average a certain
number of these particles will break up per second, giving rise
to rays which ionize the gas. If a large number of particles is
deposited, the rate of production of ions iff the gas by the rays will
be practically a continuous function of the time. The rate of pro-
duction of ions, at any time, divided by the total number of radio-
active particles deposited, will be called the average number of
ions produced by each particle at that time.

Suppose the radio-active particles which cause excited activity
are deposited at a uniform rate of q per second. The number
deposited in a short time dt = qdt.

Let n0 = the average number of ions produced in the gas per
second by each particle, at the instant of removal.

n = the average number of ions per particle per second
after an interval t.

Suppose n = w0/(0 where f(t) is a function of t such that
f(t) = 1 when t = 0,
f(t) = 0 when £ = oo ,

f(t) may in some cases pass through a maximum value greater
than unity. The variation of the rate of production of ions with

1 Ber. deutsch. Chem. Ges. p. 2368, 1903. Chem. News, Aug. 7, 1903.

266 EXCITED RADIO-ACTIVITY [CH.

time is supposed to include the effects of different kinds of
radiation emitted during the succession of changes which may
occur.

The number of ions produced per second after a time t by the
active particles deposited for the first short interval of exposure is
given by qn0f(t)dt.

The number Nt of ions produced per second at the time t by
the radio-active matter deposited during the interval t is given by

y(0 «fc

A steady state is reached when the rate of supply of fresh ions
per second by the addition of the radio-active material is balanced
by the rate of diminution of the production of ions by the excited
radiation as a whole. This steady state is reached after a long
interval of exposure, and the maximum rate of production of ions
N0 is given by

f(t)dt,

and

' '" V Nt />*
~

r°7

J 0

If the curve of decay of the excited activity for a very short
exposure is plotted with the ionization current as ordinate and
time as abscissa, as in Fig. 51, the values of these integrals are at
once determined from the experimental curve by measuring the
area included between the curve and the ordinates erected at the
points corresponding to the time limits of the integrals.

The curve of rise of excited activity can thus be deduced from
the decay curve and vice versd.

Nl} the rate of production of ions due to the excited radiation,
after removal from the emanation for a time tl} is given by

f(t)dt,

.*,

if t is the time of exposure.

IX] EXCITED RADIO-ACTIVITY 267

If N is the number of ions produced immediately after removal,

('f(t)dt

.'0

The decay curve for any time of exposure can thus also be
deduced from the curve of decay for a short exposure. For a very
long interval of exposure the value J^ at a time t after removal is
given by

AT
°

rx
f(f)dt

Now the curve of rise -^ is given by

Nt j'f(t)dt

N° /"/(*)*'

Thus * ~ F = F '

Thus the decay and rise curves are very simply connected,
whatever the law of decay of the radiations. This relation may
be expressed as follows : For a long exposure, the percentage activity Y
lost after removal for a time t is equal to the percentage of the final
activity gained by a body exposed during the same interval.

This result, which has already been shown to apply to the
decay and recovery curves of Ur X, Th X, and other radio-active
products, is of general application to all cases of radio-active
change when the rate of supply is a constant. The connection
between the decay curves of radium and thorium excited activity,
for different times of exposure, can also be shown to hold equally
for all types of active products.

The relation that holds between the decay and recovery curves
can easily be deduced from a priori considerations.

268 EXCITED RADIO-ACTIVITY [CH.

Let us suppose, for example, that a body has been exposed for
a long interval in a vessel containing a constant quantity of the
radium emanation. The excited activity in the body will have
reached a maximum value when the rate of supply is balanced by
the rate of change. Suppose this body is removed and an exactly
similar body immediately substituted. The sum of the excited
activity on these two bodies will at any time be the same as on
the single body before removal. If this were not the case, there
would be a change in energy of the radiations from the radio-
active system, as a whole, purely by removal of one body and
substitution of another. This is contrary to the general experi-
mental fact that the processes occurring in radio-activity are
independent of control, and that the radiation from a system in
radio-active equilibrium remains constant.

Thus if It = intensity of radiation from the excited body at any
time t after removal.

It = intensity of radiation from the new body exposed
under the same conditions for a time t.

Then It + // = /0 where /0 is the initial activity on the removed
body.

Thus 1 — j- ~ ~f > which is the same relation that has been

*0 ^0

developed from other considerations.

These results are particular cases of what may be termed the
" conservation of radio-activity," which is discussed in detail in
section 196.

175. Theory of successive changes. It has been pointed
out that the excited activity produced in a body exposed for a very
short interval in the presence of the thorium or radium emana-
tions does not decay according to a simple exponential law. In
the case of a^ody^^dJgdJcgLJ^ activity

increases for a few hours, passes through a maximum where the
activity is five to six times the initial value, and then slowly decays
in an exponential law with the time, falling to half value after
a further interval of 11 hours. After the maximum is reached,

IX] EXCITED RADIO-ACTIVITY 269

the activity decays at the normal rate observed for bodies exposed
for a long interval in the presence of the thorium emanation.

The increase of activity with time cannot be ascribed to a
possible occlusion of the radio-active matter in the pores of the
substance and a gradual passage to the surface after removal;
for it has been found that a very thin sheet of aluminium foil,
which absorbs very little of the radiation, exhibits the same effect
as a solid plate. The effect is, however, similar in some respects
to the increase of activity with time observed in a closed vessel in
which the radium emanation has been introduced. This is known
to be due to the production from the emanation of radio-active
matter, which is deposited on the walls of the vessel and adds its
radiation to that of the emanation proper. In a similar way the
activity of Th X increases for the first day after separation from
the thorium, and this is ascribed (see section 190) to the produc-
tion of excited activity in the mass of the Th X.

The most probable explanation of the initial increase of activity
with time, observed for the excited activity produced by the thorium
emanation, is that there are two successive changes occurring in
the emanation X of thorium after the deposit of the active matter
on the surface of the body.

The theory of these secondary changes will now be considered.
Let n0 be the number of radio-active particles deposited on the
body during the exposure to the emanation. The exposure is
supposed to be so short that only a very small proportion of the
particles have undergone change during the time of exposure.
These particles are supposed to undergo change in an exponen-
tial law with the time, and the product of the first change to
break up again according to the same law, but at a different rate.
Let X,, A 2 be the constants of the first and second changes respec-
tively. After removal for a time t, the number n of particles
remaining unchanged is given by

the number which change in the time dt at the time t is given by

Some of this number at once begin to go through the second

270 EXCITED RADIO-ACTIVITY [CH.

change, but the number which has undergone the first but not the
second change at the time T after removal is given by

~V dt.

The number q of particles which have undergone the first of
the two changes at a time T after removal is thus given by

1^0 e~'
Jo

^-V

Now the number of these particles breaking up in unit time is
proportional to A^, and is a measure of the radiation accompany-
ing the change (section 124).

If K is the ratio of the ionization produced in the second
change to that produced in the first change, the saturation current
It resulting from the two successive changes is given by

It _ AjJlo*

T,-

where /0 is the initial value of the saturation current.

This equation will be applied later with satisfactory results
in section 190, to explain the rise of activity of Th X after its
separation.

176. On examination of the curve shown in Fig. 50, which
shows the rise of activity of a rod exposed for ten minutes in the
presence of the thorium emanation, it is seen that the curve C,
showing a rise to a maximum, is roughly similar to the curves of
recovery of uranium and thorium when the Ur X and Th X
respectively have been removed. If the curve is produced back-
wards, it is seen to pass very nearly through the origin. The
abscissae measure the time from the moment the rod was intro-
duced into the emanation vessel.

If the increase of activity with time is due to a secondary
change of the type already considered, it follows at once that the

IX] EXCITED RADIO-ACTIVITY 271

total number of ions produced during the first change is not much
more than one per cent, of that produced in the second change.
If, for example, the initial activity be taken as due to the radia-
tion from the first change, the activity due to the first change
alone should fall off in an exponential law with the time,
following the dotted curve D shown in Fig. 50. The area
EABE serves as a comparative measure of the total number
of ions produced by the first change, and this area is seen to
be small compared to the corresponding area included by the
main curve C.

There is, however, no reason to suppose that the first change is
accompanied by any ionizing radiation at all. The initial activity
observed is due to the fact that some of the deposited matter has
undergone change before the rod is tested ; for it will be shown
that the experimental curve obtained can be completely deduced
if the first change is supposed to take place without any emission
of ionizing rays, but that ionizing rays are emitted in the second
change.

It has been shown that after removal of the body for a time T
the number of particles q which have undergone the first change
but not the second change is given by

where Xx is the constant of decay in the first change and Xj for the
second change.

Since it is supposed that only the second change gives rise to
a radiation, the activity at any time T after removal is propor-
tional to q. The value of q passes through a maximum when

i.e. when — = e~^~^T.

A,!

Now it is known, from experiments for a long interval of ex-
posure, that in the second change the activity falls to half value
in 11 hours, i.e. X^'063, when the time is expressed in hours.
Since the maximum activity is reached when T = 220 minutes
approximately, the value of AJ = '75. Substituting the values of

272

EXCITED RADIO-ACTIVITY

[CH.

Xj, Xa in the equation for q, the theoretical value of the activity for
any time T after exposure is shown in the following table. The
observed experimental values are also shown. The maximum
activity is taken as unity.

Time in
minutes

Theoretical value
of activity

Observed value
of activity

•22

•23

•38

•37

•64

•63

120

•90

•91

220

1-00

305

•97

•96

The curve drawn from zero is thus almost in exact agreement
with the equation, taking \ — '75.

It may thus be concluded that the matter emanation X under-
goes at least two changes : —

(1) A change which is not accompanied by ionizing radia-
tions, but in which the amount of matter undergoes change
according to an exponential law with the time, falling to half
value in 55 minutes.

(2) A second change, accompanied by the emission of rays, in
which half the matter undergoes change in 11 hours.

The existence of such a well-marked change in the matter
emanation X of thorium, not accompanied by the emission of
ionizing rays, is very interesting. It will be shown later that
there is strong evidence of a change of a similar character in the
emanation X of radium. It may be supposed that the change
consists in a rearrangement of the components of the atom
which is not of such a violent character as to cause a portion of
the atom to be expelled. Since there is only one changing system
involved, it is to be expected that the law of change would be the
same as for a monomolecular change in chemistry.

177. Secondary changes in emanation X of radium.
The decay curves of the activity produced on a rod by a short
exposure to the radium emanation are of a very different character

IX] EXCITED RADIO-ACTIVITY 273

from those observed from thorium. In the first place, there is
a rapid decay of the activity to less than 1/8 of the initial value,
then a very slow variation for about 20 minutes, and then a gradual
decay according to an exponential law. It is not possible to explain
this variation of the activity on the assumption of two changes. It
is necessary to suppose that there are three, the second of which is
a change not accompanied by ionizing rays.

Some evidence will first be considered of the decay of activity
of a body exposed for several days in the presence of the radium
emanation. P. Curie and Danne (loc. cit.) state that the law of
decay of the activity of such a body is expressed accurately by
the equation

where ^i = x^ = Ts> an(i a = 4'20.

Curie and Danne do not state definitely whether the law of
decay holds for the first ten minutes after 'removal. The shape of
the decay curve for short exposure suggests from theoretical con-
siderations that there should be in addition a small but rapid
initial drop of activity during the first ten minutes after removal.
Such a rapid initial drop of activity has been experimentally
observed by the writer.

It seems probable that the equation of P. Curie and Danne
applies for the decay of excited activity starting from a time about
ten minutes after removal. During that short interval the un-
changed deposited matter rapidly passes through the first change,
for half the matter is changed in about three minutes. At the
time at which the measurements of Curie and Danne began,
probably nearly all of the deposited matter had gone through the
first change.

Since the decay of activity after that time can be expressed by
two exponentials, it is probable that there are two farther changes
occurring. The view that the. first of these changes is a change
unaccompanied by ionizing rays, followed by another change with
the emission of rays, will be found to be in very close agreement
with the results of P. Curie and Danne. It has been shown
that, after a short exposure for a time dt to the emanation, during
R. R.-A. 18

274 EXCITED KA DIG- ACTIVITY [CH.

which n0 particles were deposited on the body, the number q of
particles which have passed through the first change but not the
second is at any time T given by

=«./(«).

(See section 174.)

But, since only the second change is accompanied by rays, the
intensity of the radiation is always proportional to q the number
unchanged, i.e. to n0f(t).

It has been shown in section 174 that, for a very long exposure,
the activity 7f, after removal for a time t, is given by

1 n0f(t)dt
o

where 70 is the initial intensity after removal. Substituting the
value of f(t) and integrating

This is of the same form as the equation of the decay curve
found by Curie and Danne. Substituting the values Xj = 1/2420,

\2= 1/1860, which were found by them, the value of -- - — is 4'3

Xj — X,2

and of -- — is 3'3.

A! — \2

The experimental value found by Curie and Danne for these
constants was 4'2 and 3'2 respectively. The agreement between
the theory and the experiment is as close as could be expected.

There are thus three distinct changes in the emanation X of
radium, viz.

(1) A very rapid initial change. Half of the matter changes
in about three minutes and is accompanied by ionizing rays.

(2) A slower change, which is not accompanied by ionizing
rays. Half of the matter undergoes change in 1*£ minutes.

IX] EXCITED RADIO-ACTIVITY 275

(3) A third change, which is faster than the second, and
is accompanied by ionizing rays. Half the matter changes in
about 28 minutes.

178. Physical and chemical properties of the active
matter. On account of the slow decay of the activity of emana-
tion X of thorium, its physical and chemical properties have been
more closely examined than the emanation X of radium. It has
already been mentioned that the emanation X of thorium is
soluble in some acids. The writer1 found that the active matter
was dissolved off the wire by strong or dilute solutions of sul-
phuric, hydrochloric and hydrofluoric acids, but was only slightly
soluble in water or nitric acid. The active matter was left behind
when the solvent was evaporated. The rate of decay of activity
was unaltered by dissolving the active matter in sulphuric acid,
and allowing it to decay in the solution. In the experiment, the
active matter was dissolved off an active platinum wire and then
equal portions of the solutions were taken at definite intervals,
evaporated down in a platinum dish, and the activity of the residue
tested by the electric method. The rate of decay was found to be
exactly the same as if the active matter had been left on the wire.
In another experiment, an active platinum wire was made the
cathode in a copper sulphate solution, and a thin film of copper
deposited on it. The rate of decay of the activity was unchanged
by the process.

A detailed examination of the physical and chemical properties
of the emanation X of thorium has been recently made by F. von
Lerch2, and some important and interesting results have been
obtained. A solution of emanation X was prepared by dissolving
the metal which had been exposed for some time in the presence
of the thorium emanation. In most cases the active matter was
precipitated with the metal. For example, an active copper wire
was dissolved in nitric acid and then precipitated by caustic potash.
The precipitate was strongly active. An active magnesium wire,
dissolved in hydrochloric acid and then (precipitated as phosphate,
also gave an active precipitate. The activity of the precipitates

1 Phys. Zeit. 3, No. 12, p. 254, 1902.

2 Dnide's Annal. Nov. 1903.

18—2

276 EXCITED RADIO- ACTIVITY [CH.

decayed at the normal rate, i.e. the activity fell to half value in
about 11 hours.

Experiments were also made on the solubility of emanation X
in different substances. A platinum plate was made active and
then placed in different solutions, and the decrease of the activity
observed. In addition to the acids already mentioned, a large
number of substances were found to dissolve the emanation X to
some extent. The active matter was however not dissolved to an
appreciable extent in ether or alcohol. Many substances became
active if added to the active solution of emanation X and then
precipitated. For example, an active solution of hydrochloric acid
was obtained by dissolving the emanation X from an active
platinum wire. Barium chloride was then added and precipitated
as sulphate. The precipitate was strongly active, thus suggesting
that the emanation X was carried down by the barium.

179. Electrolysis of solutions. Dorn showed that, if solu-
tions of radiferous barium chloride were electrolysed, both electrodes
became temporarily active, but the anode to a greater degree than
the cathode. F. von Lerch (loc. cit.) has made a detailed examina-
tion of the action of electrolysis on an active solution of emanation
X of thorium. The active matter was dissolved off an active
platinum plate by hydrochloric acid and then electrolysed between
platinum electrodes. The cathode was very active, but there was
no trace of activity on the anode. The cathode lost its activity at
a rate much faster than the normal. With an amalgamated zinc
cathode on the other hand, the rate of decay was normal. When
an active solution of hydrochloric acid was electrolysed with an
electromotive force smaller than that required to decompose water,
the platinum became active and the activity decayed to half value
in 4*75 hours while the normal fall is to half value in 11 hours.
These results point to the conclusion that the matter emanation X
is complex and consists of two parts which have different rates of
decay of activity, and can be separated by electrolysis.

Under special conditions it was found possible to make the
anode active. This was the case if the anion attached itself to
the anode. For example, if an active hydrochloric solution was
electrolysed with a silver anode, the chloride of silver formed was

IX] EXCITED RADIO-ACTIVITY 277

strongly active and its activity decayed at a normal rate. Von
Lerch found that the amount of activity obtained by placing
different metals in active solutions for equal times varied greatly
with the metal. For example, he found that if a zinc plate and
an amalgamated zinc plate, which show equal potential differences
with regard to hydrochloric acid, were dipped for equal times in
two solutions of equal activity, the zinc plate was seven times as
active as the other. The activity was almost removed from the
solution in a few minutes by dipping a zinc plate into it. Some
metals became active when dipped into an active solution while
others did not. Platinum, palladium, and silver remained inactive,
while copper, tin, lead, nickel, iron, zinc, cadmium, magnesium,
and aluminium became active. These results strongly confirm the
view that excited activity is due to a deposit of active matter
which has distinctive chemical behaviour.

G. B. Pegram1 has made a detailed study of the active deposits
obtained by electrolysis of pure and commercial thorium salts.
The commercial thorium nitrate obtained from P. de Haen gave,
when electrolysed, a deposit of lead peroxide on the anode. This
deposit was radio-active, and its activity decayed at the normal
rate of the excited activity due to thorium. From solutions of
pure thorium nitrate, no visible deposit was obtained on the anode,
but it was, however, found to be radio-active. The activity
decayed rapidly, falling to half value in about one hour. Some
experiments were also made on the effect of adding metallic salts
to thorium solutions and then electrolysing them. Anode and
cathode deposits of the oxides or metals obtained in this way were
found to be radio-active, but the activity fell to half value in a few
minutes. The gases produced by electrolysis were radio-active,
but this was due to the presence of the thorium emanation. The
results of Pegram and von Lerch would seem to indicate that,
besides those already known, other radio-active products with
a distinctive rate of decay are produced during the changes
occurring in thorium.

180. Effect of temperature. The activity of a platinum
wire which has been exposed in the presence of the thorium

1 Phys. Review, p. 424, Dec. 1903.

278

EXCITED RADIO-ACTIVITY

[CH.

emanation is almost completely lost by heating the wire to a white
heat. Miss F. Gates1 found that the activity was not destroyed
by the intense heat, but manifested itself on neighbouring bodies.
When the active wire was heated electrically in a closed cylinder,
the activity was transferred from the wire to the interior surface
of the cylinder in unaltered amount. The rate of decay of the
activity was not altered by the process. By blowing a current of
air through the cylinder during the heating, a part of the active
matter was removed from the cylinder. Similar results were found
for the excited activity due to radium.

F. von Lerch (loc. cit.) determined the amount of activity
removed at different temperatures. The results are shown in the
following table for a platinum wire excited by the thorium ema-
nation.

Temperature

Percentage of
activity removed

Heated 2 minutes
then „ i minute more
» j> "% >» »» •••

55 55 U ?) 55

800° C.
1020° C.
1260° C.
1460° C.

0
16
52
99

It is not possible to settle definitely from these experiments
whether the active matter is actually volatilized at a high tempe-
rature or is removed by disintegration of the surface of the wire.
All the metals so far tried apparently lose their activity at about
the same temperature.

181. Emission of heat. It has been shown in sections 105,
106, and 163, that the radium emanation, together with the
secondary products which arise from it, is responsible for about
75 per cent, of the total heat emission observed for radium.
The gradual decay to a minimum of the heat emission of the
radium for the first few hours after the emanation is removed
is due to the gradual decay of the excited activity produced by
the occluded emanation in the radium itself. In a similar way, the
gradual increase of the heating effect of the separated emanation

1 Phys. Review, p. 300, 1903.

IX] EXCITED RADIO-ACTIVITY 279

for the first few hours after removal, is due to the excited activity
produced by the emanation on the walls of the containing vessel.

Some experiments were recently made by H. T. Barnes and
the writer1 on the division of the heating effect of radium between
the successive products of radium. For measurement of the
heating effects, a pair of differential platinum thermometers,
wound spirally in the inside of a glass tube, were used. The
radium or its emanation, enclosed in a fine glass tube, was
placed inside the platinum spiral and the rise of temperature
observed.

The heating effect of 30 milligrams of radium bromide was
first determined. The emanation was then removed from it by
heating, and condensed in a small glass tube. The heating effect
of the de-emanated radium was determined ten minutes after the
removal of the emanation, and was found to have diminished to
59 per cent, of its original value. It then diminished more slowly
with time to a minimum corresponding to 25 per cent, of its
original value (see section 106).

The curve of diminution with time of the heating effect of
radium to the minimum of 25 per cent, should be identical with
the corresponding curve of diminution with time of the heating
effect of the emanation tube to zero after removal of the emanation.
This was found to be the case. The emanation was allowed to
remain for several hours in a small glass tube in order that the
excited activity should reach a maximum value. The emanation
was then rapidly withdrawn from the tube, and the heating effect
of the tube determined at regular intervals. There was a similar
initial drop within the first 10 minutes, then a slower variation,
and finally a decrease to zero according to an exponential law with
the time, falling to half value in about 30 minutes.

The curve of increase of the heating effect of the emanation
tube to a maximum after the introduction of the emanation was
found to be complementary to the curve of decrease of the heating
effect to zero after withdrawal of the emanation. It was not found
possible to separate the heating effect of the emanation itself
from the first rapid change in emanation X, since temperature

1 Phil. Mag. Feb. 1904.

280

EXCITED KADIO-ACTIVITY

[CH.

conditions did not become steady until an interval of 10 minutes
after introducing or withdrawing the emanation, and in that time,
the first change in emanation X was nearly completed.

The division of the heating effect amongst the radio-active
products of radium is given in the following table. The activity
of each product measured by the a rays is also given for com-
parison :

Active products

Nature of
rays

Percentage
proportion of
total activity
measured by

Percentage
proportion of
total heating
effect

the rays

Radium

a rays

(freed from active products)

Emanation

a rays

18 }

\ 33

Emanation X (first change)

a rays

15 j

„ (second change)

No a rays

0 \

I 42

„ (third change)

a, /3, and y rays

42 )

The heating effect of the active products is approximately
proportional to their activity measured by the a. rays. There can
be very little doubt that the emanation supplies an amount of the
heating effect proportional to its activity.

The decay curve of the activity of radium to a minimum of
25 per cent, after removal of the emanation is approximately the
same as the corresponding decay curve of the activity of radium
measured by the a. rays.

There is no doubt that the heating effect of radium is a result
of the succession of radio-active changes occurring in it. The
heating effect accompanies the expulsion of a particles, and is
approximately proportional to the number expelled. The time-
variation of the heating effect of the radio-active products is
the same as the time-variation of their activity measured by
the a rays.

182. Effect of variation of E. M. F. on amount of
excited activity from thorium. It has been shown that the

IX] EXCITED RADIO-ACTIVITY 281

excited activity is confined to the cathode in a strong electric field.
In weaker fields the activity is divided between the cathode and
the walls of the vessel. This was tested in an apparatus1 shown in
Fig. 53.

Electrometer -^ Battery

f a ^—

C R8° B

Fig. 53.

A is a cylindrical vessel of 5*5 cms. diameter, B the negative
electrode passing through insulating ends C, D. For a potential
difference of 50 volts, most of the excited activity was deposited
on the electrode B. For about 3 volts, half of the total excited
activity was produced on the rod B, and half on the walls of the
vessel. Whatever the voltage applied, the sum of the activities
on the central rod and the walls of the* cylinder was found to
be a constant when a steady state was reached.

When no voltage was applied, diffusion alone was operative,
and in that case about 13 per cent, of the total activity was on the
rod B. The application of an electric field has thus no influence
on the sum total of excited activity, but merely controls the pro-
portion concentrated on the negative electrode.

A more detailed examination of the variation with strength of
field of the amount on the negative electrode was made in a similar
manner by F. Henning2. He found that in a strong electric field
the amount of excited activity was practically independent of the
diameter of the rod B, although the diameter varied between
'59 mm. and 6'0 mms. With a small voltage, the amount on the
negative electrode varied with its diameter. The curves showing
the relation between the amount of excited activity and voltage
are very similar in character to those obtained for the variation of
the current through an ionized gas with the voltage applied.

The amount of excited activity reaches a maximum when all
the emanation X is removed from the gas as rapidly as it is

1 Rutherford, Phil. Mag. Feb. 1900.
- Drude's Annal. p. 562, 1902.

282 EXCITED RADIO-ACTIVITY [CH.

formed. With weaker fields, a portion diffuses to the sides of the
vessel, and produces excited activity on the positive electrode.

183. Effect of pressure on distribution of excited
activity. In a strong electric field, the amount of excited activity
produced on the cathode is independent of the pressure down to a
pressure of about 10 mms. of mercury. In some experiments made
by the writer1, the emanating thorium compound was placed
inside a closed cylinder about 4 cms. in diameter, through which
passed an insulated central rod. The central rod was connected to
the negative pole of a battery of 50 volts. When the pressure was
reduced below 10 mms. of mercury, the amount of excited activity
produced on the negative electrode diminished, and was a very
small fraction of its original value at a pressure of -^ mm. Some
excited activity was in this case found to be distributed over the
interior surface of the cylinder. It may thus be concluded that at
low pressures the excited activity appears on both anode and
cathode, even in a strong electric field.

Curie and Debierne2 observed that, if a vessel containing an
emanating radium compound was kept pumped down to a low
pressure, the amount of excited activity produced on the vessel
was much reduced. In this case the emanation given off by the
radium was removed by the pump with the other gases con-
tinuously evolved from the radium compound. On account of the
very slow decay of activity of the emanation, the amount of excited
activity produced on the walls of the vessel, in the passage of the
emanation through it, was only a minute fraction of the amount
produced when all the emanation given off was not allowed to
escape.

184. Transmission of excited activity. The characteristic
property of excited radio-activity is that it can be confined to the
cathode in a strong electric field. Since the activity is due to a
deposit of radio-active matter on the electrified surface, the matter
must be transported by positively charged carriers. The experi-
ments of Fehrle3 showed that the carriers of excited activity travel

1 Phil. Mag. Feb. 190Q. 2 Cm R 132> p. 768? 190i.

:J Phys. Zeit. 3, No. 7, p. 130, 1902.

IX] EXCITED RADIO-ACTIVITY 283

along the lines of force in an electric field. For example, if a small
negatively charged metal plate was placed in the centre of a metal
vessel containing an emanating thorium compound, more excited
activity was produced on the sides and corners of the plate than at
the central part.

A difficulty however arises in connection with the positive
charge of the carrier. According to the view developed in sec-
tion 127 and later in section 200, the matter emanation X, which
is deposited on bodies and gives rise to excited activity, is itself
derived from the emanation. The emanations of thorium and
radium emit only a rays, i.e. positively charged particles. After
the expulsion of an a particle, the residue, which is supposed to
constitute the emanation X, should retain a negative charge, and
be carried to the anode in an electric field. The exact opposite is
however observed to be the case. The experimental evidence does
not support the view that the positively charged a particles,
expelled from the emanation, are directly responsible for the
phenomena of excited activity ; for no excited activity is produced
in a body exposed to the a rays of the emanation, provided the
emanation itself does not come in contact with it. It may be
supposed that in gases the matter emanation X, immediately after
its production, attaches itself to the positive ions, produced in
the gas by the radiation, on the same sort of principle that water
vapour condenses round the negative ion. The active matter is
then transported by these positive carriers to the cathode. In the
case of radium, there is evidence that some of the carriers of
excited activity do not acquire a positive charge until they have
been present in the gas for some time.

Whatever view is taken of the process by which these carriers
obtain a positive charge, there can be little doubt that the expul-
sion of an a particle with great velocity from the atom of the
emanation must set the residue in motion. On account of the
comparatively large mass of this residue, which constitutes the
emanation X, the velocity acquired will be small compared with
that of the expelled a particle, and the moving mass will be
rapidly brought to rest at atmospheric pressure by collision with
the gas molecules in its path. At low pressures, however, the
collisions will be so few that it will not be brought to rest until it

284

EXCITED RADIO-ACTIVITY

[CH.

strikes the boundaries of the vessel. A strong electric field would
have very little effect in controlling the motion of such a heavy
mass, unless it has been initially brought to rest by collision with
the gas molecules. This would explain why the active matter is
not deposited on the cathode at low pressures in an electric field.
Some direct evidence of a process of this character, obtained by
Debierne on examination of the excited activity produced by
actinium, is discussed in section 186.

185. The following method has been employed by the writer to
determine the velocity of the positive carriers of excited activity of
radium and thorium in an electric field. Suppose A and B (Fig. 54)

B '

I E,_ _

Emanation
A

Fig. 54.

are two parallel plates exposed to the influence of the emanation,
which is uniformly distributed between them. If an alternating
E.M.F. E0 is applied between the plates, the same amount of
excited activity is produced on each electrode. If in series with
the source of the alternating E.M.F. a battery is placed of E.M.F.
El less than E0, the positive carrier moves in a stronger electric
field in one half alternation than in the other. A carrier con-
sequently moves over unequal distances during the two half
alternations, since the velocity of the carrier is proportional to the
strength of the electric field in which it moves. The excited
activity will in consequence be unequally distributed over the two
electrodes. If the frequency of alternation is sufficiently great,
only the positive carriers within a certain small distance of one

IX] EXCITED RADIO-ACTIVITY 285

plate can be conveyed to it, and the rest, in the course of several
succeeding alternations, are carried to the other plate.

When the plate B is negatively charged, the E.M.F. between
the plates is E0—El, when B is positive the E.M.F. is E0 + E+.
Let d = distance between the plates,
T — time of a half alternation,
p = ratio of the excited radio-activity on the plate B to the

sum of the radio-activities on the plates A and B,
K = velocity of the positive carriers for a potential-gradient
of 1 volt per centimetre.

On the assumption that the electric field between the plates is
uniform, and that the velocity of the carrier is proportional to the
electric field, the velocity of the positive carrier towards B is

EO- El TT

~~~

and in the course of the next half alternation

towards the plate A.

If xl is less than d, the greatest distances xl, xz passed over by
the positive carrier during two succeeding half alternations is thus
given by

EQ — EI vrp , E0 + E! ^T

#! = -- = — &-JL, and #2 = -- j — JK.J..
a a

Suppose that the positive carriers are produced at a uniform
rate of q per second for unit distance between the plates. The
number of positive carriers which reach B during a half alter-
nation consists of two parts :

(1) One half of those carriers which are produced within the
distance x± of the plate B. This number is equal to

(2) All the carriers which are left within the distance x^ from
B at the end of the previous half alternation. The number of
these can readily be shown to be

286

EXCITED RADIO-ACTIVITY

[CH.

The remainder of the carriers, produced between A and B
during a complete alternation, will reach the other plate A in the
course of succeeding alternations, provided no appreciable recombi-
nation takes place. This must obviously be the case, since the
positive carriers travel further in a half alternation towards A than
they return towards B during the next half alternation. The
carriers thus move backwards and forwards in the changing electric
field, but on the whole move towards the plate A.

The total number of positive carriers produced between the
plates during a complete alternation is 2dqT. The ratio p of the
number which reach B to the total number produced is thus
given by

Substituting the values of x-^ and #2

obtain

In the experiments the values of E0, El} d, and T were varied,
and the results obtained were in general agreement with the above
equation.

The following results were obtained for thorium :
Plates T30 cms. apart.

Alternations

EO+EI

%0 EI

per second

152

101

•27

1-25

225

150

•38

1-17

300

200

•44

1-24

Plates 2 cms. apart.

Alternations

Eo + El

E0 El

per second

273

207

•37

1-47

300

200

•286

1-45

JX] EXCITED RADIO-ACTIVITY 287

The average mobility K deduced from a large number of
experiments was 1*3 cms. per sec. per volt per cm. for atmospheric
pressure and temperature. This velocity is about the same as
the velocity of the positive ion produced by Rontgen rays in air,
viz. 1 '37 cms. per sec. The results obtained with the radium
emanation were more uncertain than those for thorium on account
of the distribution of some excited activity on the positive elec-
trode. The values of the velocities of the carriers were however
found to be roughly the same for radium as for thorium.

These results show that the carriers of the emanation X travel
in the gas with about the same velocity as the positive or negative
ions produced by the radiations in the gas. This shows either that
the emanation X becomes attached to positive ions, or that the
emanation X itself, acquiring in some way a positive charge, forms
a cluster of neutral molecules which travel with it.

186. Excited activity from actinium and " emanation
substance." Giesel1 observed that the " emanating substance "
gave off a large quantity of emanation, and that this emanation
gave rise to a type of radiation which he termed the E rays.
A narrow metal cylinder containing the active substance was
placed with the open end downwards, about 5 cms. above the
surface of a zinc sulphide screen. The screen was charged nega-
tively to a high potential by an electric machine, and the cylinder
connected with earth. A luminous spot of light was observed on
the screen, which was brighter at the edge than at the centre. A
conductor, connected with earth, brought near the luminous spot
apparently repelled it. An insulator did not show such a marked
effect. On removal of the active substance, the luminosity of the
screen persisted for some time. This was probably due to the
excited activity produced on the screen.

The results obtained by Giesel support the view that the
carriers of excited activity of the " emanation substance " have a
positive charge. In a strong electric field the carriers travel along
the lines of force to the cathode, and there cause excited activity
on the screen. The movement of the luminous zone on the ap-
proach of a conductor is due to the disturbance of the electric field.

1 Ber. deutscli. Chem. GeselL 36, p. 342, 1903.

288

EXCITED RADIO-ACTIVITY

[CH.

Debierne1 found that actinium also gave off a large amount of
emanation, the activity of which decayed very rapidly with the
time. At some distance from the source, the activity of the emana-
tion fell to half value in one-and-a-half minutes. This is not very-
different from the rate of decay of the activity of the thorium
emanation, which falls to half value in about one minute.

This emanation produces excited activity on surrounding objects,
and at diminished pressure the emanation produces a uniform
distribution of excited activity in the enclosure containing the
emanation. No data have yet been published of the rate of decay
of the excited activity produced by the emanation of actinium.

Debierne observed that the distribution of excited activity was
altered by a strong magnetic field. The experi-
mental arrangement is shown in Fig. 55. The
active matter was placed at M, and two plates
A and B were placed symmetrically with regard
to the source. On the application of a strong
magnetic field normal to the plane of the paper,
the excited activity was unequally distributed
between the plates A and B. The results showed
that the carriers of excited activity were deviated
by a magnetic field in the opposite sense to the
cathode rays, i.e. the carriers were positively
charged. In some cases, however, the opposite
effect was obtained. Debierne considers that the excited activity
of actinium is due to " ions activants," the motion of which is
altered by a magnetic field. Other experiments showed that the
magnetic field acted on the "ions activants" and not on the
emanation.

The results of Debierne thus lead to the conclusion that the
carriers of excited activity are derived from the emanation and are
projected with considerable velocity. This result supports the
view advanced in section 184 that the expulsion of a particles
from the emanation must set the part of the system left behind in
rapid motion. A close examination of the mode of transference of
the excited activity by actinium and the emanation substance is

Fig. 55.

1 C. R. 136, pp. 446 and 671, 1903.

IX] EXCITED RADIO-ACTIVITY 289

likely to throw further light on the processes which give rise to
the deposit of active matter on electrodes.

187. Radio-active induction. In carrying out experiments
on the separation of radium from pitchblende, M. and Mme Curie1
observed that the separation of the active substance is fairly com-
plete, if the stage of purification is not far advanced. Copper,
antimony, and arsenic can be separated practically inactive, but
other bodies, like lead and iron, always show slight activity. When
the stage of purification is more advanced, every body separated
from the active solution exhibits activity.

Debierne2 showed that barium was made active by solution
with actinium. The active barium removed from the actinium
still preserved its activity after chemical treatment. In this way
Debierne obtained barium chloride 6000 times as active as uranium.
Although the activity of the barium chloride could be concentrated
in the same way as the activity of radiferous barium chloride, it
did not show any of the spectroscopic lines of radium. The activity
however of the barium was not permanent, but decayed to about
one-third of its value in three months.

Giesel showed in 1900 that bismuth could be made active by
placing it in a radium solution, and suggested that polonium was
in reality bismuth made active by its mixture with the radium in
pitchblende. Mme Curie also found that bismuth was made active
by solution with a radium compound, and succeeded in fractionat-
ing the above bismuth in the same way as polonium. In this way
bismuth was obtained 2000 times as active as uranium, but the
activity decreased with time. These experiments are rendered
very uncertain by the difficulty of completely separating the radium
from the bismuth.

Giesel3 in 1903 showed that a bismuth plate dipped in a radium
solution remained active after every care had been taken to remove
all traces of radium. This active bismuth gave out only a rays,
and in this respect was analogous to polonium or Marckwald's
radio-active tellurium. The absence of the a rays in the bismuth
indicates that no radium adhered to the bismuth. The activity of

1 Thesis, Paris, 1903, p. 117. 2 C. R. 131, p. 137, 1900.

3 Ber. deutsch. Chem. Ges. p. 2368, 1903.

R. R.-A. 19

290 EXCITED RADIO-ACTIVITY [CH.

the bismuth did not decay over the period of examination, but
observations were not made over sufficient lengths of time to make
certain of this. There are two points of view that have been taken
in regard to radio-active induction. Some have supposed that the
inactive molecules of the substance themselves temporarily acquire
the property of radio-activity, after admixture with a very active
substance like radium or actinium. On this view the radio-active
bismuth is in reality bismuth, some of the matter of which has
temporarily acquired radio-active property.

On the other point of view, production of activity in inactive
bodies is either due to a slight admixture of the active element,
or to a removal with the substance of a radio-active product
of the element. In the former case, the activity of the body is
permanent ; in the latter, it decays with the time, according to the
same law as the decay of activity of the separated product. For
example, if barium is precipitated in an uranium solution, the
barium is active, and its activity decays at the same rate as the
separated Ur X. In fact, the barium precipitate carried down with
it the matter Ur X.

So far, however, no case has yet been observed when any body
has acquired the property of radio-activity by exposure to the
radiations alone of the radio-elements. The evidence at present
supports the view that the activity produced in inactive bodies is
due to a separation with it of an active product. The experiments
of F. von Lerch, described in sections 178, 179, show that many
metals are able to become active when placed in a solution of
the emanation X of thorium. This activity is due to a deposit
of the emanation X on the metal. The activity is removable by
precipitation and also in some cases by electrolysis. In the case of
solutions obtained from pitchblende, it is thus not surprising that
a similar action occurs, and that many substances possess some
temporary activity at the time of their separation. One or more
of the numerous active products in pitchblende is precipitated
with the substance, and the activity then decays with the time.

188. Possible origin of polonium. Mme Curie has not
yet been able to purify polonium sufficiently to obtain any spectro-
scopic evidence of a new element. Giesel has consistently taken

IX] EXCITED RADIO-ACTIVITY 291

the view, that polonium is in reality " induced " bismuth. At the
same time, it has not yet been definitely settled whether Marck-
wald's radio-active tellurium contains the same active constituent
as polonium or not. Taking the view that every case of induction
is due to a removal with the inactive element of an active product
of the radio-elements, some evidence will now be considered which
points to the probability that polonium is a disintegration product
of the element radium.

It has been pointed out that Mme Curie was able to fractionate
bismuth, made active in a radium solution, in the same way as
polonium, i.e. that the active matter in the bismuth had chemical
properties similar to polonium. Giesel, in addition, showed that a
platinum or bismuth plate placed in a radium solution acquired
strong activity, and, like polonium, gave rise to a rays only. If
the active bismuth or platinum decays at the same rate as
polonium, it would be very strong evidence that polonium was a
product of radium. Further experiments are, however, required
on this point.

It has been mentioned that bodies exposed for a long interval
in the presence of the radium emanation, always retain about
1/20,000 of their original activity. Giesel found that the residual
activity of a platinum wire exposed in the presence of the radium
emanation, gave out only a. rays, and in that respect resembled
polonium.

The writer has recently found that active matter can be dis-
solved by sulphuric acid from the inside of a glass tube, which
has at one time contained the radium emanation. On evapora-
ting the acid, an active deposit was left behind which gave out
a and /3 rays. The activity of this deposit, as far as observations
have yet gone, has not decayed with the time. This active sub-
stance gives out a far greater proportion of /9 rays than either
radium or thorium. The a rays showed about the same amount of
absorption in aluminium foil as the a rays from polonium, and
possessed also the characteristic property exhibited by the
polonium rays (section 90) of rapidly increasing absorption with
thickness of matter traversed. It is thus possible that this
active matter may contain polonium with another product giving
rise to ft rays. If it be assumed that the a rays, which are given

19—2

292 EXCITED RADIO-ACTIVITY [CH. IX

out by the active residue, accompany another very slow change
occurring in the matter emanation X, the time taken for the
activity of this product to decay to half value can be deduced
from general considerations. It will be shown later, in section
195, that each of the successive changes in radium or thorium,
which is accompanied by a rays, gives rise to about the same
total amount of ionization. This is merely an expression of the
fact that the same number of systems must undergo change in
each successive product, and that each system probably expels
the same number of a particles with about the same velocity.
Now it was found experimentally that the ionization current due
to the active residue was about 1/20,000 of the initial ionization
due to the emanation, which in its further changes had given
rise to the slowly decaying active matter. Since the ionization
current due to the emanation was 20,000 times that due to the
active matter, its rate of change was 20,000 times faster. But the
activity of the emanation decays to half value in four days, so that
the activity of this other active matter would decay to half value
in about 80,000 days or about 200 years.

The existence of such a slow change in the emanation X of
radium probably accounts in part for the radio-activity which is
produced on the walls of the laboratory in which radium prepara-
tions have been kept in open vessels. The emanation diffuses into
the air and produces emanation X, which is deposited on the walls
of the room, and there gives rise to a deposit of very slowly decay-
ing matter. This activity persists in a room even though no
radio-active matter has been kept in it for some time.

CHAPTER X.

RADIO- ACTIVE PROCESSES.

189. Radio-activity of uranium. It has already been
shown in section 118 that a radio-active constituent Ur X can
be separated from uranium by several different processes. The
activity of the separated Ur X decays with the time, falling to half
value in about 22 days. At the same time the uranium, from
which the Ur X has been separated, gradually regains its lost
activity. The law of decay of Ur X and the recovery of the lost
activity of the uranium are expressed by the equations

^ = e-** and ^=l-e-^,

-*0 -*0

where X is the radio-active constant of Ur X. The constant radio-
activity of uranium thus represents a state of equilibrium, where
the rate of production of new active matter is balanced by the rate
of change of the Ur X already produced.

The radio-active processes occurring in uranium present several
points of difference from the processes occurring in thorium and
radium. In the first place, uranium does not give off an emanation,
and in consequence does not produce any excited activity on bodies.
So far only one active product Ur X has been observed in uranium.
This active product UrX differs from ThX and the emanations,
inasmuch as the radiation from it consists almost entirely of /8 rays.
This peculiarity of the radiations from Ur X initially led to some
confusion in the interpretation of observations on Ur X and the
uranium from which it had been separated. When examined by
the photographic method, the uranium freed from Ur X showed no
photographic action, while the Ur X possessed it to an intense

294 RADIO-ACTIVE PROCESSES [CH.

degree. With the electric method, on the other hand, the results
obtained were exactly the reverse. The uranium freed from Ur X
showed very little loss of activity while the activity of the Ur X
was very small. The explanation of these results was given by
Soddy1 and by Rutherford and Grier2. The a rays of uranium are
photographically almost inactive but produce most of the ioniza-
tion in the gas. The ft rays, on the other hand, produce a strong
photographic action, but very little ionization compared with the a
rays. When the Ur X is separated from the uranium, the uranium
does not at first give out any {3 rays. In the course of time, fresh
Ur X is produced from the uranium, and ft rays begin to appear,
gradually increasing in intensity until they reach the original value
shown before the separation of the Ur X.

In order to determine the recovery curves of uranium after the
separation of Ur X, it was thus necessary to measure the rate of
increase of the ft rays. This was done by covering the uranium
with a layer of aluminium of sufficient thickness to absorb all the
a rays, and then measuring the ionization due to the rays in an
apparatus similar to Fig. 16.

Uranium has not yet been obtained inactive when tested by
the electric method. Becquerel3 has stated that he was able to
obtain inactive uranium, but in his experiments the uranium was
covered with a layer of black paper, which would entirely absorb
the a rays. There is no evidence that the a radiation of uranium
has been altered either in character or amount by any chemical
treatment. The a rays appear to be inseparable from the uranium,
and it will be shown later that the other radio-active elements as
well as uranium also possess a non-separable activity consisting
entirely of a rays. The changes occurring in uranium must then
be considered to be of two kinds, (1) the change which gives rise
to the a rays and the product Ur X, (2) the change which gives
rise to the ft rays from Ur X.

The possibility of separating the Ur X, which gives rise to the
ft rays of uranium, shows that the a and ft rays are produced quite
independently of one another, and by matter of different chemical
properties.

1 Tram. Chem. Soc. 81, p. 460, 1902. 2 Phil. Mag. Sept. 1902.

3 C. R. 131, p. 137, 1900.

X] RADIO-ACTIVE PROCESSES 295

190. Radio-activity of thorium. The radio-active pro-
cesses occurring in thorium are far more complicated than those
in uranium. It has been shown that a radio-active product
Th X is continuously produced from the thorium. This Th X
breaks up, giving rise to the radio-active emanation. This radio-
active emanation, in turn, produces from itself the active matter,
emanation X, which is responsible for the phenomenon of excited
radio-activity. It has also been shown (section 176) that there is
very strong evidence that the emanation X of thorium goes through
two further changes, before the radio-active processes are at an
end.

The peculiarities of the initial portions of the decay and recovery
curves of Th X and thorium respectively (Curves A and B, Fig. 34,
p. 180), will now be considered. It was shown that when the
Th X was removed from the thorium by precipitation with ammonia,
the radiation increased about 15 per cent, during the first day, passed
through a maximum, and then fell off according to an exponential
law, decreasing to half value in four days. > At the same time the
activity of the separated hydroxide decreased for the first day,
passed through a minimum, and then slowly increased again, rising
to its original value after the lapse of about one month.

When a thorium compound is in a state of radio-active equi-
librium, the series of changes in which Th X, the emanation, and
emanation X are produced go on simultaneously. Since a state of
equilibrium has been reached for each of these products, the
amount of each product changing in unit time is equal to the
amount of that product supplied from the preceding change in
unit time. Now the matter Th X is soluble in ammonia, while
the matter emanation X is not. The Th X is thus removed from
the thorium by precipitation with ammonia, but the emanation X
is left behind with the thorium. Since the emanation X is pro-
duced from the emanation, which in turn arises from Th X, on the
removal of the exciting cause Th X, the radiation due to this
emanation X will decay, since the rate of production of fresh
emanation X no longer balances its own rate of change. Disregard-
ing the initial irregularity in the decay curve of emanation X
(section 170), the activity of the emanation X will have decayed
to half value in about 11 hours and to one quarter value at the

296 KADIO-ACTIVE PROCESSES [CH.

end of 22 hours. As soon, however, as the Th X has been separated,
new Th X is produced in the thorium compound. The activity of
this new Th X is not, however, sufficient to compensate at first for
the loss of activity due to the change in emanation X, so that, as
a whole, the activity will at first decrease, then pass through a
minimum, then increase again.

The correctness of this point of view has been tested by Ruther-
ford and Soddy1 as follows : If the precipitated thorium hydroxide
after the removal of Th X is put through a series of precipitations
with ammonia at short intervals, the Th X is removed almost as
fast as it is formed, and, at the same time, the activity of the
emanation X decays.

The following table indicates the results obtained. A portion
of the precipitated hydroxide was removed after each series of
precipitations and its activity tested in the usual way.

Activity of
hydroxide per cent.

After 1 precipitation 46

After 3 precipitations at intervals of 24 hours ... 39

After 3 more precipitations at intervals of 24 hours and

3 at intervals of 8 hours 22

After 3 more each of 8 hours 24

After 6 more each of 4 hours ... ... ... ... 25

The differences in the last three numbers are not significant,
for it is difficult to make accurate comparisons of the activity of
thorium compounds, which have been precipitated under slightly
different conditions. It is thus seen that as a result of successive
precipitations, the activity is reduced to a minimum of about 25 per
cent. The recovery curve of the activity of this 23 times precipitated
hydroxide is shown in Fig. 56. The initial drop in the curve is
quite absent, and the curve, starting from the minimum, is practi-
cally identical with the curve shown in Fig. 35, which gives the
recovery curve of thorium hydroxide after the first two days. This
residual activity — about 25 per cent, of the maximum — is non-
separable from the thorium by any chemical process that has been
tried.

The initial rise of activity of Th X, after it has been separated,
will now be considered. In all cases, it was found that the activity

1 Tram. Chem. Soc. 81, p. 837, 1902. Phil. Mag. Nov. 1902.

KADIO-ACTIVE PROCESSES

297

of the separated Th X had increased about 15 per cent, at the
end of 24 hours, and then steadily decayed, falling to half value in
about four days.

This peculiarity of the Th X curve follows, of necessity, from the
considerations already advanced to explain the drop in the recovery
curve. As soon as the Th X is separated, it at once produces from
itself the emanation, and this in turn produces the emanation X. The
activity due to the emanation X at first more than compensates
for the decay of activity of the Th X itself. The total activity

100

Time in

Fig. 56.

thus increases to a maximum, and then slowly decays to zero
according to an exponential law with the time. The curve ex-
pressing the variation of the activity of the separated Tb X with
time can be deduced from the theory of successive changes already
considered in section 175. In the present case there are three
successive changes occurring at the same time, viz. the change of
Th X into the emanation, of the emanation into emanation X, and
the final changes giving rise to the activity of emanation X. Since,
however, the change of the emanation into emanation X (about
half changed in one minute) is far more rapid than the changes
occurring in Th X or emanation X, for the purposes of calculation
it may be assumed without serious error that the Th X changes at

298

RADIO-ACTIVE PROCESSES

[CH.

once into emanation X. The initial change of emanation X —
about half changed in 55 minutes — will also .be disregarded for
the same reason.

Let \i and X2 be the constants of decay of activity of Th X
and emanation X respectively. Since the activity of Th X and
of emanation X falls to half value in 4 days and 11 hours respec-
tively, the value of \ = '0072 and of X2 = '063 where 1 hour is
taken as the unit of time.

It has already been shown (section 175) that after a time t
the activity It, of a product in which there is a secondary change,
is given by

where 70 is the intensity of the radiation immediately after separa-
tion and K the ratio of the ionization produced in the secondary
change to that produced in the primary change. By comparison
of this equation with the curve of variation of the activity of Th X
with time shown in Fig. 57, Curve A, it is found that the value
of K is about '44.

The calculated values of -=* for different values of t are shown

•••

in the second column of the following table, and the observed values
in the third column.

;

Time

Theoretical
value

Observed
value

1-00

•25 days

1-09

—

•5 „

1-16

—

'1-15

1-17

1-5 „

1-11

—

1-04

—

•875

•88

•*

•75

•72

6 „

•53

•315

•295

•157

•152

The theoretical and observed values thus agree within the
limit of error in the measurements. The theoretical curve is

RADIO-ACTIVE PROCESSES

299

shown in Curve A, Fig. 57 (with the observed points marked, for
comparison). The curve B shows the theoretical curve of the decay
of the activity of Th X and the emanation, supposing there is no
secondary change into emanation X. Curve C shows the differ-
ence curve between the curves A and B, i.e. the proportion of the
activity at different times due to the emanation X. The activity
due to emanation X thus rises to a maximum about two days after
removal of the Th X, and then decays with the time at the same

Time in Days.
Fig. 57.

rate as the Th X itself, i.e. the activity falls to half amount every
four days. When the value of t exceeds four days, the value of
0-^-W m the theoretical equation is very small.
The equation of decay is thus expressed by

i.e. the curve decays in an exponential law with the time.

300 RADIO-ACTIVE PROCESSES [CH.

191. Radio-activity of radium. Notwithstanding the
enormous difference in their relative activities, the radio-activity
of radium presents many close analogies to that of thorium. Both
substances give rise to emanations which in turn produce " excited
activity" on bodies in their neighbourhood. Radium, however,
does not give rise to any intermediate product between the
element itself and the emanation it produces, or in other words
there is no product in radium corresponding to Th X in thorium.

Giesel first drew attention to the fact that a radium compound
gradually increased in activity after preparation, and only reached
a constant value after a month's interval. If a radium compound
is dissolved in water and boiled for some time, or a current of air
drawn through the solution, on evaporation it is found that the
activity has been diminished. The same result is observed if
a solid radium compound is heated in the open air. This loss
of activity is due to the removal of the emanation by the process
of solution or heating. Consider the case of a radium compound
which has been kept for some time in solution in a shallow vessel,
exposed to the open air, and then evaporated to dryness. The
emanation which, in the state of solution, was removed as fast as
it was formed, is now occluded, and, together with the emana-
tion X which it produces, adds its radiations to that of the original
radium. The activity will increase to a maximum value where the
rate of production of fresh emanation balances the rate of change
of that already produced.

If now the compound is dissolved or heated, the emanation
escapes. Since the emanation X is not volatile and is insoluble
in water, it is not removed by the process of solution or heating.
Since, however, the exciting cause is removed, its activity will
immediately begin to decay, and in the course of a few hours
will have almost disappeared. The activity of the radium mea-
sured by the rays is then found to be about 25 per cent, of its
original value. This residual activity of radium, consisting entirely
of a rays, is non-separable, and has not been further diminished by
chemical or physical means. Rutherford and Soddy1 examined the
effect of aspiration for long intervals through a radium chloride
solution. After the first few hours the activity was found to be

1 Phil. Mag. April, 1903.

RADIO-ACTIVE PROCESSES

301

reduced to 25 per cent., and further aspiration for three weeks
did not produce any further diminution. The radium was then
evaporated to dryness, and the rise of its activity with time
determined. The results are shown in the following table. The
final activity in the second column is taken as one hundred. In
column 3 is given the percentage proportion of the activity re-
covered.

Time in days

Activity

Percentage
Activity recovered

250

0-70

33-7

11-7

1-77

42-7

23-7

4-75

68-5

58-0

7-83

83-5

78-0

16-0

96-0

95-0

21-0

100-0

The results are shown graphically in Fig. 58.

100

Fig. 58.

The decay curve of the radium emanation is shown in the
same figure. The curve of recovery of the lost activity of radium

302 RADIO-ACTIVE PROCESSES [CH.

is thus analogous to the curves of recovery of uranium and thorium
which have been freed from the active products Ur X and Th X
respectively. The intensity It of the recovered activity at any

time is given by -j- = 1 — e~Kt, where /0 is the final value, and \ is

•M

the radio-active constant of the emanation. The decay and recovery
curves are complementary to one another.

Knowing the rate of decay of activity of the radium emanation,
the recovery curve of the activity of radium can thus at once be
deduced, provided all of the emanation formed is occluded in the
radium compound.

When the emanation is removed from a radium compound by
solution or heating, the activity measured by the (3 rays falls
almost to zero, but increases in the course of a month to its
original value. The curve showing the rise of 0 rays with time
is practically identical with the curve, Fig. 58, showing the re-
covery of the lost activity of radium measured by the a. rays. The
explanation of this result lies in the fact that the ft rays from
radium only arise from emanation X, and that the non-separable
activity of radium gives out only a rays. On removal of the
emanation, the activity of the emanation X decays nearly to
zero, and in consequence the ft rays almost disappear. When
the radium is allowed to stand, the emanation begins to ac-
cumulate, and produces in turn emanation X, which gives rise to
ft rays. The amount of ft rays (allowing for a period of retarda-
tion of a few hours) will then increase at the same rate as the
activity of the emanation, which is continuously produced from
the radium.

192. If the radium allows some of the emanation produced to
escape into the air, the curves of recovery will be different from
that shown in Fig. 58. For example, suppose that the radium
compound allows a constant fraction a of the amount of emana-
tion, present in the compound at any time, to escape per second.
If n is the number of emanation particles present in the com-
pound at the time t, the number of emanation particles changing
in the time dt is \ndt, where X is the constant of decay of activity
of the emanation. If q is the rate of production of emanation

X] RADIO-ACTIVE PROCESSES 303

particles per second, the increase of the number dn in the time dt
is given by

dn — qdt — \ndt — andt,

or -jj = q - (X + a) n.

The same equation is obtained when no emanation escapes,
with the difference that the constant X + a is replaced by \.

fj (Yl

When a steady state is reached, -j- is zero, and the maximum value
of n is equal to i— * — .

If no escape takes place, the maximum value of n is equal to ^ .

The escape of emanation will thus lower the amount of activity
recovered in the proportion - -- . If r?0 is the final number of

A» T* tt

emanation particles stored up in the compound, the integration of
the above equation gives — = 1 — e~(K+a)t .

The curve of recovery of activity is thus of the same general
form as the curve when no emanation escapes, but the constant
\ is replaced by X -I- a.

For example, if a = X = 1/463000, the equation of rise of activity

is given by — = 1 — e~2M, and, in consequence, the increase of

activity to the maximum will be far more rapid than in the
case of no escape of emanation.

A very slight escape of emanation will thus produce large altera-
tions both in the final maximum and in the curve of recovery of
activity.

A large number of experiments have been described by Mme Curie
in her These pt'esentee d la Faculte des Sciences de Paris on the
effect of solution and of heat in diminishing the activity of radium.
The results obtained are in general agreement with the above view,
that 75 per cent, of the activity of radium is due to the emana-
tion and the excited activity it produces. If the emanation is
wholly or partly removed by solution or heating, the activity of

304 RADIO-ACTIVE PROCESSES [CH.

the radium is correspondingly diminished, but the activity of
the radium compound is spontaneously recovered owing to the
production of fresh emanation. A state of radio-active equi-
librium is reached, when the rate of production of fresh emanation
balances the rate of change in the emanation stored up in the
compound. The differences observed in the rate of recovery of
radium under different conditions were probably due to variations
in the rate of escape of the emanation.

193. Non-separable activity. It has been shown that, for
all three radio-elements, uranium, thorium, and radium, there is
a non-separable activity consisting entirely of a. rays. In the
case of uranium the activity is the same as the activity of the
uranium, measured by the a rays, before the product Ur X, which
gives rise only to ft rays, is removed. In the case of thorium
and radium, where the active products produced give out a. rays,
the non-separable activity is about 25 per cent, of the maximum
activity measured by the a rays.

The existence of a non-separable activity follows from the
point of view of regarding radio-active processes which has been
advanced in sections 87 and 127. The three radio-elements
are supposed to be undergoing atomic disintegration, which is
accompanied by the expulsion of a rays. If the number of atoms
which break up per second is almost infinitesimal compared with
the total number present, the same number, on an average, will
break up per second. The number of a particles expelled per
second will thus be a constant for each radio-element. There
will thus always be a non-separable activity of the radio-elements,
which is an inherent property of the elements and cannot be
removed from them by any chemical or physical process.

194. Radiations from the active products. Most of the

changes occurring in the different radio-active products are accom-
panied by the emission of a rays alone. The 0 and 7 rays appear
only in the final stages of the radio-active changes.

It has been shown that the non-separable activity of the three
radio-elements consists entirely of a rays. The two emanations give
rise only to a rays (section 147). It also seems probable that the
product Th X, if the emanation X which it produces is completely

X] RADIO-ACTIVE PROCESSES 305

separated from it, would give rise only to a rays. On the other
hand, the activity of Ur X consists only of ft rays. The changes
in the matter emanation X of thorium and radium give rise to
both a and ft rays. It has been shown (section 176) that the
emanation X of thorium goes through two changes, the first of
which does not give rise to rays at all, and the second of which
gives rise to a. and ft rays. The first change occurring in emana-
tion X of radium gives rise to a. rays but not to ft rays; the
second change probably does not give rise to rays at all, while
the third change again includes all three kinds of rays. The
absence of ft rays in the first change, taking place in the emana-
tion X of radium, can readily be shown by exposing a negatively
charged wire in the presence of the radium emanation for a few
minutes. The activity on removal, measured by the a rays, falls
rapidly, as is shown in Fig. 51, p. 262 ; but the activity measured
by the ft rays alone is at first small, and increases for some time
instead of diminishing. If a and ft rays had been both given out
in the first change, it is to be expected that the amount of the ft
radiation would initially decay at the same rate|.as the a radiation,
but no such effect is observed.

The ft and probably also the 7 rays of the three radio-elements
thus only appear in the last of the series of radio-active changes.
It is remarkable that the last change, which is readily detected
by the radio-active property, should in each of the three radio-
elements be accompanied by the expulsion of a single electron
with great velocity, and that all the other changes, with the ex-
ception of two that probably give rise to no rays at all, should
be accompanied by the expulsion of a rays, i.e. of material particles
atomic in size.

The polonium of Mme Curie and the radio-tellurium of
Marckwald emit only a rays. Becquerel1 states that he has
detected some rays of a penetrating character from polonium
by the photographic method. The writer has examined by the
electric method the radiations from the active preparation of
radio- tellurium, but was unable to detect any trace of 7 rays.
The evidence so far obtained points to the conclusion that the

1 C. R. pp. 136, 977. 1903.
R. R.-A. 20

306 KADIO-ACTIVE PROCESSES [CH.

7 rays appear at the same time as the ft rays, and in an amount
proportional to them.

The character of the radiations from each of the radio-active
products must always be taken into consideration in the interpre-
tation of results obtained by different methods of measurement.
For example, a radium compound, which has been heated in an
open glass tube, after a few hours practically loses its power of
exciting fluorescence on a screen of platino-cyanide of barium
placed near it. This is due to the fact already mentioned, that
the ft and 7 rays practically disappear from radium for several
hours after the emanation is removed. For the same reason the
radium, with a screen placed over it of sufficient thickness to
absorb all the a rays, would appear, when measured by the electric
method, to be almost inactive.

Since the a rays are photographically very inactive compared
with the /3 rays, the non-separable activity of uranium, thorium,
and radium, although producing marked ionization, would be
almost inappreciable if tested by the photographic method. It has
been stated by some observers that uranium and thorium have been
obtained which showed no trace of activity. On examination of the
results, however, it is found that the methods employed were not
suitable to definitely settle the question. It is true that, by certain
chemical processes, uranium and thorium can be obtained tempo-
rarily inactive, when tested by the photographic method, or by the
electric method if the compound is covered by a screen of sufficient
thickness to absorb all the a. rays. If however the activity is tested
electrically with unscreened active matter, there is always found to be
a residual activity. In the course of time, the uranium and thorium
compounds spontaneously regain the whole of their lost activity.

195. Division of the activity amongst the products.

It has been shown in section 190 that the activity of thorium
hydroxide, after the removal of Th X, falls to 46 per cent, of its
original value. When the Th X is removed from the thorium at
short intervals, in order to allow the activity of the emanation X
left behind to decay, there is a residual activity of 25 per cent, of
the maximum. About 21 per cent, of the total activity is thus
due to the emanation X.

X] RADIO-ACTIVE PROCESSES 307

This result is confirmed by observations on the increase of
activity of Th X after removal. It has been shown in section
190 that the activity due to the emanation X, produced from
the Th X, is equal to '44 when the activity of the Th X, together
with the emanation it produces, is taken as unity. Now the
activity of the separated Th X and the emanation is equal to '54
of the total. The proportion of the total activity due to emana-
tion X is thus 24 per cent., a result which is not very different
from the value of 21 per cent, obtained previously. It is difficult
to make an accurate estimate of the activity of the emanation,
compared with that due to the emanation X. An approximate
estimate was however made in the following way.

Some thorium hydroxide enclosed in a paper envelope was
placed inside a closed cylinder with an insulated central electrode
kept charged negatively. After an interval of several days, a state
of radio-active equilibrium was reached, and the ionization was
measured :

(1) with the thorium inside the cylinder, and

(2) with the thorium removed.

(1) gave the ionization current due to the emanation and the
emanation X on the central electrode, and (2) gave the current due
to the emanation X. Taking into consideration that half of the
radiation from the emanation X was absorbed in the central elec-
trode, it was deduced that the amount of ionization produced by the
emanation in the gas was not very different from that produced by
the emanation X. This result points to the conclusion that the
emanation and emanation X in a thorium compound supply about
an equal proportion of the total activity. The relative activities
of the different products are shown in the following table. The
numbers must only be considered as approximate.

Residual activity of thorium 25%

Activity of ThX alone 21 °/0

„ „ emanation alone ... ... ... ... 24 °/0

„ due to first change emanation X ... ... 0 °/0

„ „ second „ „ „ 24 %

Leaving out of account the first change in emanation X, which
is of a character quite distinct from the others, it is seen that the
activity is approximately equally divided amongst the products.

20—2

308 RADIO-ACTIVE PROCESSES [CH.

Similar results hold in the case of radium. The emanation
and the residual activity of radium supply about 18 and 25 per cent,
respectively of the total activity, and the rest is supplied by the
changes in emanation X. These results are thus also in rough
agreement with those obtained for thorium, and indicate that each
change which gives rise to a rays supplies about an equal fraction
of the total activity. This is an important result, for it indicates
that about the same number of a particles is expelled at each
change, which gives rise to a rays. This deduction is based on the
observed fact that the penetrating power and consequently the
ionization produced by such a particles is not very different. It
therefore seems probable that, when a compound of thorium or of
radium is in radio-active equilibrium, the same number of systems
change per second in each of the products, and that the change in
all cases but one is accompanied by the expulsion of about the
same number of a particles.

196. Conservation of radio-activity. The early observa-
tions on uranium and thorium had shown that their radio-activity
remained constant over the period of several years during which
they were examined. The possibility of separating from uranium
and thorium the active products Ur X and Th X respectively, the
activity of which decayed with the time, seemed at first sight to be
contradictory to this point of view. Further observation, however,
showed that the total radio-activity of these bodies was not altered
by the chemical processes, for it was found that the uranium
and thorium from which the active products were removed, spon-
taneously regained their radio-activity. At any time after removal
of the active product, the sum total of the radio-activity of the
separated product together with that of the substance from which
it has been separated is always equal to that of the original com-
pound before separation. In cases where the active products, like
Ur X and the radium emanation, decay with time according to an
exponential law, this follows at once from the experimental results.
If it is the activity of the product at any time t after separation,

and /0 the initial value, we know that j =e~^. At the same

•*o

time the activity It recovered during the interval t is given by

X] RADIO-ACTIVE PROCESSES 309

y = 1 — e~M, where \ is the same constant as before. It thus

•*o

follows that it + It = /0, which is an expression of the above result.
The same is also true whatever the law of decay of activity of the
separated product (see section 174). For example, the activity of
Th X after separation from thorium at first increases with the
time. At the' same time, the activity of the residual thorium
compound at first decreases, and at such a rate, that the sum of
the activities of the thorium and its separated product is always
equal to that of the original thorium.

This principle of " conseivatior^p^^dioj-actiyity1,'' as it may be
termed, follows from the general result that the radio-active pro-
cesses cannot in any way be changed by the action of known
forces. It may be recalled that the rate of decay of the activity of
a radio-active product cannot be altered by any known agency.
The rate of decay is independent of the concentration of the active
matter, of the pressure and of the nature of the gas in which it is
placed, and is not affected by wide ranges of temperature. In the
same way, it has not been found possible to alter the rate of
production of active matter from the radio- elements. In addition,
there is not a single case yet observed where radio-activity has
been altered or destroyed in any active body or created in an
inactive element.

Certain cases have been observed, which at first sight seem to
indicate a destruction of radio-activity. For example, the excited
radio-activity is removed from a platinum wire when heated above
a red heat. It has been shown, however, by Miss Gates (sec-
tion 180) that the radio^agtiviiby^ is jiot destroyed, but is deposited
in unaltered amount on the colder bodies surrounding it. Thorium
oxide has beeTT shown to lose its power in emanating to a large
extent by ignition to a white heat. But a close examination shows
that the emanation is still being produced at the samejrate, but is
occluded in the compound.

The total radio-activity of a given mass of a radio-element,
measured by the peculiar radiations emitted, is a quantity which
can neither be increased nor diminished, although it may be mani-
fested in a series of products which are capable of separation from
1 Rutherford and Soddy, Phil. Mag. May, 1903.

310 KADIO-ACTIVE PROCESSES [CH.

the radio-element. The term "conservation of radio-activity" is
thus a convenient expression of the facts known at the present
time. It is quite possible, however, that further experiments at
very high or very low temperatures may show that the radio-
activity does vary. For example, Dewar states that the heat
emission of radium seems to be rather greater in liquid hydrogen
than in liquid oxygen. An increase of heat emission would
probably entail an increase of the radio-activity of the radium
immersed in liquid hydrogen. Accurate experiments have not,
however, yet been made on the radio-activity of radium at such
low temperatures.

Although no difference has been observed in the radio-activity
of uranium over an interval of five years, it will be shown (sec-
tion 203) that on theoretical grounds the radio-activity of a given
quantity of a radio-element should decrease with the time. The
change will, however, be so slow in uranium and thorium, that
probably thousands if not millions of years must elapse before a
measurable change would have taken place. In radium, however,
the change takes place about one million times faster, so that a
measurable alteration should be detected in the course of a few
years1. The total radio-activity of a given quantity of matter left
to itself should thus decrease, but it should be constant for a
constant mass of the radio-element. It is only in this restricted
sense that the principle can be employed.

The conservation of radio-activity applies not only to the
radiations taken as a whole, but also to each specific type of
radiation. If the emanation is removed from a radium compound,
the amount of (3 radiation of the radium at once commences to
decrease, but this is compensated by the appearance of /3 rays
in the radiations from the vessel in which the separated emanation
is stored. At any time the sum total of the ft radiations from the
radium and the emanation vessel is always the same as that from
the radium compound before the emanation was removed.

1 It seems probable however that the radio-activity of radium, measured by the
a rays, will increase rather than diminish for several hundred years after its
separation. This is due to the fact that the increase of the activity due to the last
slow change of radium (about half changed in 200 years) will probably more than
compensate for the change in the radium itself. Ultimately, however, the radio-
activity of the radium must decrease with time.

X] RADIO-ACTIVE PROCESSES 311

Similar results have also been found to hold for the 7 rays.
This was tested by the writer in the following way. The emana-
tion from some solid radium bromide was released by heat,
and condensed in a small glass tube which was then sealed off.
The radium so treated, and the emanation tube, were placed
together under an electroscope, with a screen of lead 1 cm. thick
interposed in order to let through only the 7 rays. The experi-
ments were continued over three weeks, but the sum total of the
7 rays from the radium and the emanation tube was, over the
whole interval, equal to that of the original radium. During this
period the amount of 7 rays from the radium at first decreased to
only a few per cent, of the original value, and then slowly increased
again, until at the end of the three weeks it had nearly regained
its original value, before the emanation was removed. At the same
time the amount of 7 rays from the emanation tube rose from zero
to a maximum and then slowly decreased again at the same rate
as the decay of the activity of the emanation in the tube. This
result shows that the amount of 7 rays from radium was a constant
quantity over the interval of observation, although the amount of
7 rays from the radium and emanation tube had passed through a
cycle of changes.

197. Resume of results. Before discussing the general
theory advanced to account for the processes occurring in the radio-
element, a brief resume will be given of the more important results
already described in detail in previous chapters.

The radio-activity of uranium, thorium, and radium has been
shown to be maintained by the production at a constant rate of new
kinds of matter, which possess temporary activity. The constant
activity of the radio-elements is due to a state of equilibrium where
the rate of production of new active matter compensates for the
change in that already produced. In some cases, the active products
possess well-defined chemical properties different from those of the
parent elements and can be separated from them by chemical
means. The separation of Ur X and Th X from uranium and
thorium are good examples of this process. In other cases, the
new products, as in the case of the thorium and radium emana-
tions, are gaseous in character and are released from the radio-

312 RADIO-ACTIVE PROCESSES [CH.

elements by the process of diffusion. These emanations have been
shown to possess the properties of gases. The radium emanation
diffuses and distributes itself between two reservoirs kept at
different temperatures according to the laws of gases. Both the
emanations can be condensed by cold, and by that means can be
removed from the other gases with which they are mixed. The
emanations possess the property of being occluded in some bodies,
including the radio-elements themselves, and can be liberated by
heating or solution. They diffuse through porous partitions, and
in general behave like chemically inert gases of high molecular
weight. Other radio-active products, for example the emanations
X of radium and of thorium, are not gaseous, but attach themselves
to the surface of bodies and can be removed from them by solution
or heating. The emanation X of thorium, for example, possesses
some chemical properties which distinguish it not only from the
emanation from which it is derived, but also from the other active
product Th X. It is soluble in some acids and not in others. If
the emanation X of thorium is removed from an active wire by
solution in hydrochloric acid, the active matter attaches itself to
some metals dipped in the acid but not to others, and in general
possesses the properties of matter of definite chemical behaviour.
The differences in the chemical and physical properties of the
different products of a radio-element can be well illustrated in the
case of thorium. Thorium X is soluble in ammonia, while thorium
is not. Thorium X produces the emanation which is a gas, in-
soluble in acids but condensed by cold. This, in turn, produces
the matter emanation X, which is insoluble in ammonia but soluble
in hydrochloric and sulphuric acids. There can be little doubt
that these active products are material in nature. They differ
from ordinary matter in their power of emitting rays of a special
character, and by the fact that they exist in the radio-elements in
minute quantities which are, in most cases, too small to be detected
by the balance or the spectroscope. Approximate estimates (sec-
tion 162) can be made of the amount of these active products that
are present in a radio-element when in radio-active equilibrium,
and it has been shown that, except in the case of a very active
body like radium, the amount is too small to be detected by
ordinary chemical means.

OF TH:
UNIVERSITY

c OF
RADIO-ACTIVE PROCESSES

The case of the radium emanation however is different. It
can be shown (section 162) that probably the emanation exists in
greater proportion in radium than radium does in pitchblende.
Yet radium was detected in pitchblende by the property of radiat-
ing which it possesses, and has been isolated from it and found to
be an element of well-marked chemical and physical properties.
It has been estimated that 1 gram of radium in a state of radio-
active equilibrium probably contains a volume of about 0'3 cubic
millimetre of the emanation, measured at standard pressure and
temperature. From a kilogram of radium 0'3 cubic centimetre
would be produced. When larger quantities of radium are available
for experiments, there can thus be little doubt that a sufficient
amount of the radium emanation will be obtained to examine
chemically. In fact, as will be shown in section 201, even with
the small quantities of radium now available, some very important
spectroscopic evidence has already been obtained, in regard to the
processes occurring in the emanation.

[Volume of the Emanation1.

Sir William Ramsay and Mr Soddy have kindly placed at my
disposal some preliminary results of a recent investigation by them
on the volume of the emanation released from radium. In these
experiments, 60 milligrams of pure radium bromide were used, in
which the emanation had been allowed to collect for 8 days. This
emanation, freed as far as possible from all other gases, was forced
into a capillary tube in which its volume was measured. The
following table shows the results obtained.

Initial volume 0*124 cubic millimetre.
Volume after 1 day 0*027
„ „ 3 days O'Oll
„ „ 6 „ 0-0063
„ 9 „ 0-0041

„ 12 „ 0-0011

Final volume 0-0004

The volume of the gas obtained shrank rapidly during the first
day, then more slowly, and after the third day decreased approxi-
mately according to an exponential law with the time, decreasing
1 Added Feb. 1, 1904.

314 RADIO-ACTIVE PROCESSES [CH.

to half value in about four days. According to the views already
advanced, it is to be expected that the volume of the emanation
itself should decrease according to an exponential law with the
time, falling to half value in about 3*7 days. This is a result of the
theory that half of the emanation at the end of 3'7 days has been
transformed into the substance emanation X, which behaves as a
solid and is deposited on the surface of the containing vessel. It
seems not improbable that the rapid decrease, observed during the
first day, may have been due to the presence, with the emanation,
of another gas which was quickly absorbed either by the walls of
the capillary tube or by the mercury.

It can readily be deduced that the volume of the emanation at
the end of the first day is equivalent to the amount derived from
0'022 gram of pure radium in radio-active equilibrium. Taking
the volume of the gas, 0'027 cubic millimetre, at the end of the
first day as the true volume of emanation, it follows that the
volume of the emanation to be obtained from 1 gram of radium
in radio-active equilibrium is 1*2 cubic millimetres. Taking the
volume observed on the third day, the corresponding value is
0*9 cubic millimetre. The calculated value of the volume of the
emanation to be derived from 1 gram of radium is 0*3 cubic
millimetre. The calculated value is thus of the right order of
magnitude. This is an indication of the general correctness of
the different methods of calculation (see sections 104 and 162) on
which the theoretical determination of the volume of the emanation
has been based.

It will be shown later, in section 201, that the emanation of
radium produces helium from itself. The shrinkage of the volume
to a very small fraction of its original value indicates that the
helium produced was buried in the walls of the tube. This is to
be expected if the helium consists in reality of the a particles
expelled from the emanation and its products. The a. particle is
projected with sufficient velocity to penetrate a distance of about
•02 millimetre into the walls of the capillary tube. It is to be
expected that a portion, at least, of the buried helium should be
released when the tube is strongly heated.]

Of the three types of rays from the active bodies, the a and ft

X] RADIO-ACTIVE PROCESSES 315

rays are material in nature. The mass and velocity of the a
particles, projected from radium, have been measured. They have
been shown to be positively charged particles projected with a
velocity of about 1/10 the velocity of light, and a mass about
twice that of the hydrogen atom. The 0 particles carry a negative
charge, and have an apparent mass about 1/1000 the mass of the
hydrogen atom. They are identical in character with the cathode
ray particle produced in a vacuum tube. The nature of the 7 rays
has not yet been determined.

The a rays play by far the most important part in radio-active
processes. Most of the energy radiated in the form of ionizing rays
is due to them. In addition, most of the active products emit only
a rays. The 0 and 7 rays in most cases only appear in the last
stage of the radio-active processes.

The activity of most of the products decays according to an
exponential law with the time. In cases where this does not hold,
the activity can be shown to be due to several successive changes,
the rate of each of which decays according to an exponential law
but with a different radio-active constant. The rate of decay of
activity has not yet been found to be in any way influenced by
wide variation in chemical and physical conditions.

The activity of any product at any time (section 124), is
proportional to the rate of change of the product, and is also pro-
portional to the amount of matter left unchanged. In cases where
one active product gives rise to another, the activity of the first
product is, at any time, a measure of the rate of production of the
second product. In other words, the radiations accompany the
change of one product into another, and serve as a measure of the
rate of change. This point of view at once follows if the expulsion
of rays is taken to be the cause of the change from one product
into another. The rate of emission of a. particles is a measure of
the rate of change of the first product, i.e. it is proportional to the
rate at which the second product is produced. For example, the
amount of emanation X of thorium produced in a given time by
the thorium emanation is proportional to the activity of the ema-
nation. In cases where the rate of change of the second product is
rapid compared with that of the first, after sufficient interval has
elapsed in order to reach a state of approximate radio-active equi-

316 RADIO-ACTIVE PROCESSES [CH.

librium, the activity of the second product is always found to vary
at the same rate as that of the parent product. For example, the
amount of the emanation produced by Th X is always proportional
to the activity of the Th X, and decays at the same rate, i.e. it
falls to half value in about four days. In the same way, the activity
of the emanation X, produced by the radium emanation, after some
hours have elapsed for conditions to become steady, is always found
to be proportional to the activity of the emanation. In other
words, the activity of the emanation X decays according to the
same law, and at the same rate, as the radium emanation which
produces it, i.e. to half value in a little less than four days.

The rapid rate of heat emission of radium is connected with
the radio-activity of that element. More than two-thirds of the
heat emission of radium is due to the radium emanation and the
secondary products to which the emanation gives rise. The heat
emission seems to be for the most part connected with the emission
of a. rays.

The total energy which would be given out by a given quantity
of radium is of quite a different order of magnitude to that ob-
served in ordinary chemical reactions.

198. Theories of radio-activity. A brief review will now
be given of the working hypotheses which have served as a guide
to the investigators in the field of radio-activity. These working
theories have in many cases been modified or extended with the
growth of experimental knowledge.

The early experiments of Mme Curie had indicated that radio-
activity was an atomic and not a molecular phenomenon. This
was still further substantiated by later work, and the detection and
isolation of radium from pitchblende was a brilliant verification of
the truth of this hypothesis.

The discovery that the /3 rays of the radio-elements were
similar to the cathode rays produced in a vacuum tube was an
important advance, and has formed a basis of several subsequent
theories. J. Perrin1, in 1901, following the views of J. J. Thomson
and others, suggested that the atoms of bodies consisted of parts
and might be likened to a miniature planetary system. In the

1 Revue Scientifique, April 13, 1901.

X] RADIO-ACTIVE PROCESSES 317

atoms of the radio-elements, the parts composing the atoms more
distant from the centre might be able to escape from the central
attraction and thus give rise to the radiation of energy observed.
In December 1901, Becquerel1 put forward the following hypo-
thesis, which he stated had served him as a guide in his investi-
gations. According to the view of J. J. Thomson, radio-active
matter consists of negatively and positively charged particles. The
former have a mass about 1/1000 of the mass of the hydrogen
atom, while the latter have a mass about one thousand times
greater than the negative particle. The negatively charged par-
ticles (the /9 rays) would be projected with great velocity, but
the larger positive particles would have much lower velocity and
would form as a sort of gas (the emanation) which would deposit
itself on the surface of bodies. This in turn would subdivide
giving rise to rays (excited activity).

In a paper communicated to the Royal Society in June 1900,
Rutherford and McClung2 showed that the energy, radiated in
the form of ionizing rays into the gas, was- 3000 gram-calories per
year for radium of activity 100,000 times that of uranium. Taking
the latest estimate 1,500,000 of the activity of a pure radium com-
pound, this would correspond to an emission of energy into the
gas in the form of a rays of about 45,000 gram-calories per gram
per year. The suggestion was put forward that this energy might
be derived from a re-grouping of the constituents of the atom of
the radio-elements, and it was pointed out that the possible energy
to be derived from a greater concentration of the components of
the atom was large compared with that given out in molecular
reactions.

In the original papers3 giving an account of the discovery of the
emanation of thorium and the excited radio-activity produced by
it, the writer took the view that both of these manifestations were
due to radio-active material. The emanation behaved like a gas,
while the matter which caused excited activity attached itself to
solids and could be dissolved in some acids but not in others. In
conjunction with Miss Brooks, it was shown that the radium
emanation diffused through air like a gas of heavy molecular

1 C. R. 133, p. 979, 1901. 2 Phil. Trans. A, p. 25, 1901.

3 Phil. Mag. Jan. and Feb. 1900.

318 RADIO-ACTIVE PROCESSES [CH.

weight. At a later date Mr Soddy and the writer showed that the
radium and thorium emanations behaved like chemically inert
gases, since they were unaffected by the most drastic physical
and chemical treatment.

On the other hand, P. Curie, who, in conjunction with Debierne,
had made a series of researches on the radium emanation, expressed
dissent from this view. P. Curie1 did not consider that there was
sufficient evidence that the emanation was material in nature, and
pointed out that no spectroscopic evidence of its presence had yet
been obtained, and also that the emanation disappeared when
contained in a sealed vessel. It was pointed out by the writer2
that the failure to detect spectroscopic lines was probably a conse-
quence of the minute quantity of the emanation present, under
ordinary conditions, although the electrical and phosphorescent
actions produced by this small quantity are very marked. This
contention is borne out by calculations of the probable amount of
the emanation released from 1 gram of radium given in section 162.
P. Curie took the view that the emanation was not material,
but consisted of centres of condensation of energy attached to the
gas molecules and moving with them.

M. and Mme Curie have throughout taken a very general view
of the phenomena of radio-activity, and have not put forward any
definite theory. In Jan. 1902, they gave an account of the general
working theory3 which had guided them in their researches.
Radio-activity is an atomic property, and the recognition of this
fact had created their methods of research. Each atom acts as a
constant source of emission of energy. This energy may either
be derived from the potential energy of the atom itself, or each
atom may act as a mechanism which instantly regains the energy
which is lost. They suggested that this energy may be borrowed
from the surrounding air in some way not contemplated by the
principle of Carnot.

In the course of a detailed study of the radio-activity of thorium,
Mr Soddy and the writer4 found that it was necessary to suppose
that thorium was continuously producing from itself new kinds of

1 C. R. 136, p. 223, 1903. 2 PhiL Mag^ April> 19Q3

3 C. R. 134, p. 85, 1902.

4 Trans. Chem. Soc. 81, pp. 321, 837, 1902. Phil. Mag. Sept. and Nov. 1902.

X] RADIO-ACTIVE PROCESSES 319

active matter, which possess temporary activity and differ in
chemical properties from the thorium. The constant radio-activity
of thorium was shown to be the result of equilibrium between the
processes of production of active matter and the change of that
already produced. At the same time, the theory was advanced
that the production of active matter was a consequence of the dis-
integration of the atom. The work of the following year was
devoted to an examination of the radio-activity of uranium and
radium on similar lines, and it was found that the conclusions
already advanced for thorium held equally for uranium and radium1.
The discovery of a condensation of the radio-active emanations2
gave additional support to the view that the emanations were
gaseous in character. In the meantime, the writer3 had found that
the rays consisted of positively charged bodies atomic in size,
projected with great velocity. The discovery of the material
nature of these rays served to strengthen the theory of atomic
disintegration, and at the same time to offer an explanation of
the connection between the a rays and the changes occurring in
the radio-elements. In a paper entitled "Radio-active Change,"
Mr Soddy and the writer4 put forward in some detail the theory
of atomic disintegration as an explanation of the phenomena of
radio-activity, and at the same time some of the more important
consequences which follow from the theory were discussed.

In a paper announcing the discovery of the heat emission of
radium, P. Curie and Laborde8 state that the heat energy may be
equally well supposed to be derived from a breaking up of the
radium atom or from energy absorbed by the radium from some
external source.

J. J. Thomson in an article on "Radium," communicated to
Nature*, put forward the view that the emission of energy from
radium is probably due to some change within of the atom, and
pointed out that a large store of energy would be released by a
contraction of the atom.

Sir William Crookes7, in 1899, proposed the theory that the

1 Phil. Mag. April, 1903. 2 Phil. Mag. May, 1903.

» Phys. Zeit. 4, p. 235, 1902. Phil. Mag. Feb. 1903.
4 Phil. Mag. May, 1903. 5 C. R. 136, p. 673, 1903.

6 Nature, p. 601, 1903. 7 C. R. 128, p. 176, 1899.

320 RADIO-ACTIVE PROCESSES [CH.

radio-active elements possess the property of abstracting energy
from the gas. If the moving molecules, impinging more swiftly
on the substance, were released from the active substance at a
much lower velocity, the energy released from the radio-elements
might be derived from the atmosphere. This theory was advanced
again later to account for the large heat emission of radium,
discovered by P. Curie and Laborde.

Fillipo Re1 recently advanced a very general theory of matter
with a special application to radio-active bodies. He supposes
that the parts of the atom were originally free, constituting a
nebula of extreme tenuity. These parts have gradually become
united round centres of condensation, and have thus formed the
atoms of the elements. On this view an atom may be likened
to an extinct sun. The radio-active atoms occupy a transition
stage between the original nebula and the more stable chemical
atoms, and in the course of their contraction give rise to the
heat emission observed.

Lord Kelvin in a paper to the British Association, 1903, has
suggested that radium may obtain its energy from external sources.
If a piece of white paper is put into one vessel and a piece of black
paper into an exactly similar vessel, on exposure of both vessels to
the light the vessel containing the black paper is found to be at a
higher temperature. He suggests that radium in a similar manner
may keep its temperature above the surrounding air by its power
of absorption of unknown radiations.

199. Discussion of Theories. From the survey of the
general hypotheses advanced as possible explanations of radio-
activity, it is seen that they may be broadly divided into two
classes^one of which assumes that the energy emitted from the
radio-elements is derived at the expense of the internal energy of
the atom, and the other that the energy is derived from external
sources, but that the radio-elements act as mechanisms capaBleT of
transforming this borrowed energy into the special forms manifested
in the phenomena of radio-activity. Of these two sets of hypo-
theses the first appears to be the most probable, and to be best
supported by the experimental evidence. Up to the present not
1 C. R. p. 136, p. 1393, 1903.

X] RADIO-ACTIVE PROCESSES 321

the slightest experimental evidence has been adduced to show
that the energy of radium is derived from external sources.

J. J. Thomson (loc. cit.) has discussed the question in the
following way : —

" It has been suggested that the radium derives its energy from
the air surrounding it, that the atoms of radium possess the faculty
of abstracting the kinetic energy from the more rapidly moving air
molecules while they are able to retain their own energy when in
collision with the slowly moving molecules of air. I cannot see,
however, that even the possession of this property would explain
the behaviour of radium ; for imagine a portion of radium placed
in a cavity in a block of ice ; the ice around the radium gets
melted ; where does the energy for this come from ? By the hypo-
theses there is no change in the air-radium system in the cavity,
for the energy gained by the radium is lost by the air, while heat
cannot flow into the cavity from the outside, for the melted ice
round the cavity is hotter than the ice surrounding it."

The writer has recently found that the activity of radium is
not altered by surrounding it with a large mass of lead. A cylinder
of lead was cast 10 cms. in diameter and 10 cms. high. A hole
was bored in one end of the cylinder to the centre, and the radium,
enclosed in a small glass tube, was placed in the cavity. The
opening was then hermetically closed. The activity was measured
by the rate of discharge of an electroscope by the 7 rays trans-
mitted through the lead, but no appreciable change was observed
during a period of one month.

M. and Mme Curie early made the suggestion that the radiation
of energy from the radio-active bodies might be accounted for by
supposing that space is traversed by a type of Rontgen rays, and
that the radio-elements possess the property of absorbing them.
Recent experiments (section 215) have shown that there is present
at the surface of the earth a very penetrating type of rays, similar
to the 7 rays of radium. Even if it were supposed that the radio-
elements possessed the power of absorbing this radiation, the
energy of the rays is far too minute to account even for the energy
radiated from an element of small activity like uranium. In
addition, all the evidence so far obtained points to the conclusion
that the radio-active bodies do not absorb the type of rays they
R. R.-A. 21

322 RADIO-ACTIVE PROCESSES [CH.

emit to any greater extent than would be expected from their
density. It has been shown (section 79) that this is true in the
case of uranium. Even if it were supposed that the radio-elements
possess the property of absorbing the energy of some unknown
type of radiation, which is able to pass through ordinary matter
with little absorption, there still remains the fundamental difficulty
of accounting for the peculiar radiations from the radio-elements,
and the series of changes that occur in them. It is not sufficient
to account for the heat emission only, for it has been shown (section
106) that the emission of heat is directly connected with the radio-
activity.

In addition, the distribution of the heat emission of radium
amongst the radio-active products which arise from it is extremely
difficult to explain on the hypothesis that the energy emitted
is borrowed from external sources. It has been shown that more
than two-thirds of the heat emitted by radium is due to the
emanation together with the matter emanation X, which is pro-
duced by the emanation. When the emanation is separated from
the radium, its power of emitting heat, after reaching a maximum,
decreases with the time according to an exponential law. It
would thus be necessary on the absorption hypothesis to postulate
that most of the heat emission of radium, observed under ordinary
conditions, is not due to the radium itself but to something pro-
duced by the radium, the power of which of absorbing energy from
external sources diminishes with time.

The strongest evidence against the hypothesis of absorption of
external energy is that such a theory ignores the fact, that, when-
ever radio-activity is observed, it is always accompanied by some
change which can be- detected by the appearance of new products
having chemical properties distinct from those of the original
substances. This leads to some form of " chemical " theory, and
other results show that the change is atomic and not molecular.

200. Theory of radio-active change. The processes occur-
ring in the radio-elements are of a character quite distinct from any
previously observed in chemistry. Although it has been shown
that the radio-activity is due to the spontaneous and continuous
production of new types of active matter, the laws which control

X] KADIO-ACTIVE PROCESSES 323

this production are different from the laws of ordinary chemical
reactions. It has not been found possible in any way to alter
either the_j-ate at which the matter is produced or its rate of
change when produced. Temperature, which is such an important
factor in altering the rate of chemical reactions, is, in these cases,
entirely without influence. In addition, no ordinary chemical
change is known which is accompanied by the expulsion of charged
atoms with great velocity. It has been suggested by Armstrong
and Lowry1 that radio-activity may be an exaggerated form of
fluorescence or phosphorescence with a very slow rate of decay.
But no form of phosphorescence has yet been shown to be accom-
panied by radiations of the character of those emitted by the
radio-elements. Whatever hypothesis is put forward to explain
radio-activity must account not only for the production of a series
of active products, which differ in chemical and physical properties
from each other and from the parent element, but also for the
emission of rays of a special character. Besides this, it is necessary
to account for the large amount of energy continuously radiated
from the radio-elements.

The radio-elements, besides their high atomic weights, do not
possess in common any special chemical characteristics which differ-
entiate them from the elements, which do not possess the property
of radio-activity to an appreciable degree. Of all the known ele-
ments, uranium, thorium, and radium possess the heaviest atomic
weights, viz.: radium 225, thorium 232*5, and uranium 240.

If a high atomic weight is taken as evidence of a complicated
structure of the atom, it might be expected that disintegration
might occur more readily in heavy than in light atoms. At the
same time, there is no reason to suppose that the elements of the
highest atomic weight should be the most radio-active; in fact,
radium is far more active than uranium, although its atomic weight
is less. This is seen to be the case also in the radio-active pro-
ducts; for example, the radium emanation is enormously more
active weight for weight than the radium itself, and there is
every reason to believe that the emanation has an atom lighter
than that of radium.

1 Proc. Roy. Soc. 1903.

21—2

\324
Un

EADIO-ACTIVE PROCESSES [CH.

order to explain the phenomena of radio-activity, Rutherford
aVi Soddy have advanced the theory that jJn»-ftk)ms of the radio-
el\inents suffer spontaneous disintegration, and that each disinte-
grated atom passes through a succession of well-marked changes,
accompanied in most cases by the emission of a rays.

A preliminary account of this hypothesis to explain special
phenomena has already been given in sections 87 and 127. It
will now be applied generally to the radio-elements, and the con-
sequences, which follow from it, will be considered. It is supposed
that, on an average, a definite small proportion of the atoms of each
radio- element becomes unstable at a given time. As a result of
this instability, an a. particle is expelled with great velocity. The
continuous expulsion of these a particles gives rise to the non-
separable activity of the radio-elements, which has been shown to
consist entirely of a rays (section 193). The expulsion of an 2 par-
ticle, of mass about twice that of the hydrogen atom, leaves behind it
a new system lighter than the original one, and possessing chemical
and physical properties quite different from those of the original
element. This new system again becomes unstable, and expels
another a particle. The process of disintegration, once started,
proceeds from stage to stage at a definite measurable rate in each
case. At each stage, with the exception of one change in thorium
and one in radium which are not accompanied by rays at all, one
or more a particles are projected, until the last stages are reached,
when the ft and 7 rays appear. The term metabolon has been
suggested as a convenient expression for each of these changing
atoms, derived from the successive disintegration of the atoms of
the radio-elements. Each metabolon, on an average, exists only
for a limited time. In a collection of metabolons of the same kind
the number N, which are unchanged at a time t after production,
is given by N = N0e~M where NQ is the original number. Now

, = — \N, or the fraction of the metabolons present, which change

in unit time, is equal to X. The value l/\ may thus be taken as
the average life of each metabolon.

The various metabolons from the radio-elements are distin-
guished from ordinary matter by their great instability and conse-
quently rapid rate of change. Since a body which is radio-active

X] RADIO-ACTIVE PROCESSES 325

must ipso facto be undergoing change, it follows that none of the
active products, for example, the emanations and Th X, can consist
of any known kind of matter; for there is no evidence to show that
inactive matter can be made radio-active, or that two forms of the
same element can exist, one radio-active and the other not. For
example, half of the matter constituting the radium emanation
has undergone change after an interval of four days. After the
lapse of about one month the emanation as such has nearly
disappeared, having been transformed through several stages into
other and more stable types of matter, which are in consequence
difficult to detect by their radio-activity.

The following table gives the list of the active products or
metabolons known to result from the disintegration of the three
radio-elements. In the second column is given the value of the
radio-active constant X for each active product, i.e. the proportion
of the active matter undergoing change per second ; in the third
column, the time r required for the activity to fall to one-half, i.e.
the time taken for half the active product to undergo change ; in
the third column, the nature of the rays from each active product,
not including the rays from the products which result from it ; in
the fourth column, a few of the more marked physical and chemical
properties of each metabolon.

There are two well-marked changes in uranium, five in
thorium, and six in radium. It is quite probable that a closer
examination of the active products may lead to the discovery of
still further changes. For example, the evidence obtained by
von Lerch (section 179) from the electrolysis of a solution of
emanation X of thorium points to the conclusion that there is an
additional change occurring in emanation X, for which the value
of T is 3 or 4 hours. The experiments of Pegram (section 179)
also suggest that another radio-active product, of which the value
of r is about 3 minutes, is present in thorium. The change of
thorium X into the emanation would have been difficult to detect
if the product of the change had not been gaseous in character.
Besides the changes mentioned above, it is thus quite possible
that other and more rapid changes may be taking place which
have not yet been detected.

It has been pointed out in section 188 that the fourth change

326

RADIO-ACTIVE PROCESSES

[CH.

Radio-active -v

Nature of Chemical and Physical

Products

the Rays Properties

URANIUM

a Soluble in excess of ammo-

nium carbonate

Uranium X 3*6 x W~7

22 days

j8 (and 7?) Insoluble in excess of am-

monium carbonate

Final product

THORIUM ,. fu.-

Insoluble in ammonia

Thorium X 2-OxlO-«

4 days

a (and /3?)

Soluble in ammonia

Thorium Emanation 1-15 x 10~2

1 minute

Behaves like a chemically

inert gas of heavy mole-

cular weight. Condenses

at - 120° C.

Emanation X

\ Attaches itself to the sur-

(first change) 2-2 x 10~4

55 minutes

no rays

face of bodies concen-

trated on the cathode

in an electric field

t
Second change 1'74 x 10~5

11 hours

a, /3, 7

[Soluble in some acids and
) not in others. Possesses

well-marked chemical

properties in solution

Final product

...

RADIUM

Radium Emanation

2-14xlO~6to
2-00 x 10~6

3-7 to 4 days a

Behaves like a chemically
inert gas of heavy mole-

cular weight

Emanation X

Condenses at - 150° C.

(first change) about 4 x 10~3

about 3 a

Attaches itself to the sur-

minutes

face of bodies ; mainly

concentrated on the

Second change 3-18 x 10~4

•S& minutes

no rays

cathode in an electric

T field

Third change 4-1 x 10~4

28 minutes

a, j8, 7 ; 1 Soluble in some acids and

L 1 '

1 not in others; volati-

' lized at a white heat

Fourth change

200 years (?)

a, jS Soluble in sulphuric acid

Final product

...

in emanation X of radium may possibly be the radio-active con-
stituent present in the polonium of Mme Curie or the radio-tellu-
rium of Marckwald. After the disintegrated radio-atoms have
undergone the succession of changes mentioned above, a final
stage will be reached where the atoms are either permanently
stable or change so slowly that it is difficult to detect their

X] RADIO-ACTIVE PROCESSES 327

presence by the property of radio-activity. Consequently, there
will always be a residual inactive product or products of the
changes occurring in each of the radio-elements. In addition,
since the a particles, projected from the radio-elements, are
material in nature and are not radio-active, they must also be
considered as a residual product.

The value of r, which may be taken as a comparative measure
of the stability of the different metabolons, varies over a very wide
range. The thorium emanation is the most unstable of the meta-
bolons. and (leaving out of account the fourth change in emanation
X of radium) uranium X the least unstable. The metabolons
constituting uranium X are about 30,000 times as stable as the
metabolons constituting the thorium emanation.

The only two metabolons of about the same stability are the
metabolons constituting thorium X and the radium emanation.
In each case the activity falls to half value in about four days. I
consider that the approximate agreement of the numbers is a mere
coincidence, and that the two types of matter are quite distinct
from one another ; for, if the metabolons were identical, it would
be expected that the changes which follow would take place in the
same way and at the same rate, but such is not the case. More-
over Th X and the radium emanation have chemical and physical
properties quite distinct from one another.

201. Production of helium from radium and the radium
emanation. Since the final products, resulting from a disinte-
gration of the radio-elements, are not radio-active, they should in
the course of geologic ages collect in some quantity, and should
always be found associated with the radio-elements. Now the
inactive products resulting from the radio-active changes are the a
particles expelled at each stage, and the final inactive product or
products which remain, when the process of disintegration can no
longer be traced by the property of radio-activity.

Pitchblende, in which the radio-elements are mostly found,
contains in small quantity a large proportion of all the known
elements. In searching for a possible disintegration product
common to all the radio-elements, the presence of helium in the
radio-active minerals is noteworthy; for helium is only found in

328 RADIO-ACTIVE PROCESSES [CH.

the radio-active minerals and is an invariable companion of the
radio-elements. In addition the presence of a light, inert gas like
helium in minerals had always been a matter of surprise. The
production by radium and thorium of the radio-active emanations,
which behaved like chemically inert gases of the helium-argon
family, suggested the possibility that one of the final inactive
products of the disintegration of the radio-elements might prove
to be a chemically inert gas. The discovery later of the material
nature of the a rays added weight to the suggestion; for the
measurement of the ratio e/m of the a particle indicated that if
the a particle consisted of any known kind of matter, it must either
be hydrogen or helium. For these reasons, it was suggested in
1902 by Rutherford and Soddy1 that helium might be a product
of the disintegration of the radio-elements.

Sir William Ramsay and Mr Soddy in 1903 undertook an in-
vestigation of the radium emanation, with the purpose of seeing if
it were possible to obtain any spectroscopic evidence of the presence
of a new substance. First of all, they exposed the emanation to
very drastic treatment (section 149), and confirmed and extended
the results previously noted by Rutherford and Soddy that the
emanation behaved like a chemically inert gas, and in this respect
possessed properties analogous to the gases of the helium-argon
group.

On obtaining 30 milligrams of pure radium bromide (pre-
pared about three months previously) Ramsay and Soddy2 ex-
amined the gases, liberated by solution of the radium bromide in
water, for the presence of helium. A considerable quantity of
hydrogen and oxygen was released by the solution (see section
116). The hydrogen and oxygen were removed by passing the
liberated gases over a red-hot spiral of partially oxidized copper-
wire and the resulting water vapour by a phosphorous pentoxide
tube.

The gas was then passed into a small vacuum tube which was
in connection with a small U tube. By placing the U tube in
liquid air, most of the emanation present was condensed, and also
most of the CO2 present in the gas. On examining the spectrum

1 Phil Mag. p. 582, 1902 ; pp. 453 and 579, 1903.

2 Nature, July 16, p. 246, 1903. Proc. Roy. Soc. 72, p. 204, 1903.

X] RADIO-ACTIVE PROCESSES 329

of the gas in the vacuum tube the characteristic line D3 of helium
was observed.

This experiment was repeated with 30 milligrams of radium
bromide about four months old, lent for the purpose by the writer.
The emanation and CO2 were removed by passing them through a
U tube immersed in liquid air. A practically complete spectrum
of helium was observed, including the lines of wave-length 6677,
5876, 5016, 4972, 4713 and 4472. There were also present three
other lines of wave-length about 6180, 5695, 5455 which have not
yet been identified.

In later experiments the emanation from 50 milligrams of the
radium bromide was conveyed with oxygen into a small U tube,
cooled in liquid air, in which the emanation was condensed. Fresh
oxygen was added and the U tube again pumped out. The small
vacuum tube, connected with the U tube, showed at first no
helium lines when the liquid air was removed. The spectrum
obtained was a new one, and Ramsay and Soddy considered it
to be probably that of the emanation itself. After allowing the
emanation tube to stand for four days, the helium spectrum appeared
with all the characteristic lines, and in addition, three new lines
present in the helium obtained by solution of the radium. These
results have since been confirmed. The experiments, which have
led to such striking and important results, were by no means easy
of performance, for the quantity of helium and of emanation released
from 50 mgrs. of radium bromide is extremely small. It was
necessary, in all cases, to remove almost completely the other gases,
which were present in sufficient quantity to mask the spectrum of
the substance under examination. The success of the experiments
has been largely due to the application to the investigation of the
refined methods of gas analysis, which had been previously employed
by Sir William Ramsay with so much success in the separation of
the rare gases xenon and krypton, which exist in minute pro-
portions in the atmosphere. The fact that the helium spectrum
was not present at first, but appeared after tHe emanation had
remainecfm theliuBerfbr some days, shows that the helium must
have been produced from the emanation. The emanation cannot
be helium itself, for in the first place, helium is not radio-active,
and in the second place, the helium spectrum was not present at

330 RADIO-ACTIVE PROCESSES [CH.

first, when the quantity of emanation in the tube was at its
maximum. In addition, the diffusion experiments, already dis-
cussed, point to the conclusion that the emanation is of high
molecular weight. There can thus be no doubt that the helium is
derived from the emanation of radium in consequence of changes
or* some kind occurring in it.

In order to explain the presence of helium in radium on ordi-
nary chemical lines, it has been suggested that radium is not
a true element, but a molecular compound of helium with some
substance known or unknown. The helium compound gradually
breaks down, giving rise to the helium observed. It is at once
obvious that this postulated helium compound is of an entirely
different character to any other compound previously observed
in chemistry. Weight for weight, it emits during its change an
amount of energy at least one million times greater than any
molecular compound known (see section 205). In addition, it must
be supposed that the rate of breaking up of the helium compound
is independent of great ranges of temperature — a result never
before observed in any molecular change. The helium compound
in its breaking up must give rise to the peculiar radiations and
also pass through the successive radio-active changes observed in
radium.

In order to explain the production of helium and radio-activity
on this view, a unique kind of molecule must thus be postulated—
a molecule, in fact, which is endowed with every single property
which on the disintegration theory is ascribed to the atom of the
radio-elements. On the other hand, radium as far as it has been
examined, has fulfilled every test required for an element. It has
a well-marked and characteristic spectrum, and there is no reason
to suppose that it is not an element in the ordinarily accepted
sense of the term.

On the theory that the radio-elements are undergoing atomic
disintegration, the helium must either be considered to exist
within the radium atom, or else to be formed from its constituent
corpuscles during the process of disintegration. The theory that
the heavy atoms are all built up of some simple fundamental
unit of matter or protyle has been advanced at various times by
many prominent chemists and physicists. Prout's hypothesis,

X] RADIO-ACTIVE PROCESSES 331

that all elements are built up out of hydrogen, is an example of
this point of view of regarding the subject.

On the disintegration theory, the changes occurring in the
radio-atoms involve an actual transformation of the atoms through
successive changes. This change is so slow in uranium and thorium
that at least a million years would be required before the amourft
of change is measurable by the balance. In radium it is a million
times faster, but even in that case it is doubtful whether any
appreciable change would be observed by ordinary chemical methods
for many years, if the possibility of such a change had not been
suggested from other lines of research.

202. Amount of helium from radium. The appearance of
helium in a tube containing the radium emanation may indicate
either that the helium is one of the final products, which appear
at the end of .the series of radio-active changes, or that^the Jielium
is in realjty tho expelled a particle The evidence at present
points to the latter as being the more probable explanation. In
the first place, the emanation diffuses like a gas of heavy molecular \
weight, and it appears probable that, after the expulsion of a few \
fji particles, the atomic weight of the final product is comparable
with that of the emanation. On the other hand, the value of e/m I
determined for the projected OL particle points to the conclusion /
that, if it consists of any known kind of matter, it is either *
hydrogen or helium.

If the a. particles, when released, can exist in the gaseous state,
an estimate can readily be made of the volume of the total number
of particles liberated per year. It has been calculated that one
gram of radium expels about 1011 a. particles per second. Since
there are 3'6 x 1019 molecules in one cubic centimetre of any gas
at standard pressure and temperature, the volume of the a particles
released per second from 1 gram of radium is 2'8 x 10"9 c.c. and
per year 90 cubic millimetres.

It has already been shown that the emanation released from
1 gram of radium in a state of radio-active equilibrium is probably
about 3 x 10~4 c.c. Since the emanation passes through at least
three stages, each of which gives rise to a rays, the volume of the
a. particles from the emanation, released from 1 gram of radium, is
about one cubic millimetre.

332 RADIO-ACTIVE PROCESSES [CH.

Ramsay and Soddy state that the amount of helium, present
in the gases from radium, was very minute. From the above
estimates, it can readily be shown that the amount of helium
liberated in the experiments described in section 201 was about
0'5 cubic millimetres. If the a. particles are helium, it is to be
expected that the greater portion of the helium, which is produced
in a tube containing the radium emanation, would be buried in
the walls of the glass tube ; for the a particles are projected with
sufficient velocity to penetrate some distance into the glass.

203. Rate of change of the radio-elements. Since the
atoms of the radio-elements themselves are continuously breaking
up, they must also be considered to be metabolons, the only dif-
ference between them and the metabolons such as the emanations,
Th X and others, being their comparatively great stability and con-
sequent very slow rate of change. There is no evidence that the
process of disintegration, traced above, is reversible, and, in the
course of time, a quantity of radium, uranium or thorium, left
to itself must gradually be transformed into inactive matter of
different kinds.

An approximate estimate of the rate of change of radio-elements
can be deduced from the number of atoms breaking up per second.
It has been calculated from several lines of evidence (section 104)
that from 1 gram of radium about 1011 a particles are expelled
per second. The number for uranium and thorium is about
7 x 104.

Now it has been shown that there are at least four rapid changes
in radium, each of which gives rise to a. rays. In the absence of
evidence of the number of a particles expelled at each change, the
assumption, which seems most probable, will be made, viz., that
each metabolon expels only one a. particle. Since there are four
changes in radium, the number of atoms in one gram of radium
breaking up per second is 2'5 x 1010. Now it has been shown, from
data based on experimental evidence, that one cubic centimetre of
hydrogen, at standard pressure and temperature, contains about
3'6 x 1019 molecules. Taking the atomic weight of radium as 225,
there will be T8 x 1021 atoms in 1 gram of radium. The fraction
\ of one gram of radium which changes is thus 1*4 x 10~n per
second and 4'4x 10~4 per year. It thus follows that, in each gram

X] RADIO-ACTIVE PROCESSES 333

of radium, about half a milligram disintegrates per year. Since
the amount of radium which is unchanged will diminish according
to an exponential law with the time, half of a given weight of
radium will be transformed in about 1500 years. Only one per
cent, will remain unchanged after a lapse of about 10,000 years.
In a gram of uranium or thorium, where the change takes place
at about one-millionth the rate, about a million years would be
required before half a milligram would be changed. All but one
per cent, of the uranium and thorium would be transformed in
about 1010 years.

This is the minimum estimate of the life of radio-elements on
the assumption that one a particle is expelled at each change. A
maximum limit to the life of the radio-elements can be deduced
by supposing that the radium is completely disintegrated into a
particles. Since the mass of the a particle is about twice that of
the hydrogen atom there cannot be many more than 100 a particles
produced from each atom of the radio-elements. The 'maximum
estimate of the life of radium is thus about 5'0 times greater
than the minimum estimate. The minimum estimate is however
probably nearer the truth : for there is no evidence to show that
more than one a particle is expelled at each change. The agree-
ment between the calculated and experimental values of the
volume of the emanation (see section 197) is strong evidence in
support of the minimum estimate ; for in the calculation only one
a particle was supposed to be expelled at each change.

The changes in radium are thus fairly rapid, and a mass of
radium if left to itself should in the course of a few thousand years
have lost a large proportion of its radio-activity. Taking the
minimum estimate of the life of radium, the value of X is 4'4 x 10~4,
with a year as the unit of time. A mass of radium left to itself
should thus be half transformed in 1500 years and only one-
millionth part would remain after 30,000 years. Thus supposing,
for illustration, that the earth was originally composed of pure
radium, its activity per gram 30,000 years later would not be
greater than the activity observed to-day in a good specimen of
pitchblende. Even taking the maximum estimate of the life of
radium, the time required for the radium to practically disappear
is short compared with the probable age of the earth. We are

334 RADIO-ACTIVE PROCESSES [CH.

thus forced to the conclusion that radium is being continuously
produced in the earth, unless the very improbable assumption is
made, that radium was in some way suddenly formed at a date
recent in comparison with the age of the earth. It has been
suggested that radium may be a disintegration product of one
of the radio-elements found in pitchblende. Both uranium and
thorium fulfil the conditions required in a possible source of
production of radium. Both are present in pitchblende, have
atomic weights greater than that of radium, and have rates of
change which are slow compared with that of radium. In some
respects, uranium filfils the conditions required better than thorium ;
for it has not been observed that minerals rich in thorium contain
much radium, while on the other hand, the pitchblendes contain-
ing the most radium contain a large proportion of uranium.

If radium is not produced from uranium, it is certainly a
remarkable coincidence that the greatest activity of pitchblende
yet observed is about five or six times that of uranium. Since
radium has a life short compared with that of uranium, the
amount of radium produced should reach a maximum value after
a few thousand years, when the rate of production of fresh radium
— which is also a measure of the rate of change of uranium —
balances the rate of change of that product. In this respect the
process would be exactly analogous to the production of the
emanation by radium, with the difference that the radium changes
much more slowly than the emanation. But since radium itself
in its disintegration gives rise to at least four changes with the
corresponding production of a rays, the activity due to the radium
(measured by the a rays), when in a state of radio-active equili-
brium with uranium, should be about four times that of the
uranium that produces it ; for it has been shown that only one
change has so far been observed in uranium in which a rays are
expelled. Taking into account the presence of polonium and
actinium in pitchblende, the activity in the best pitchblende is
about the same as would be expected if the radium were a dis-
integration product of uranium. If this hypothesis is correct, the
amount of radium in any pitchblende should be proportional to
the amount of uranium present, provided the radium is not
removed from the mineral by percolating water. On the other

X] RADIO-ACTIVE PROCESSES 335

hand it should be noticed that while the greatest amount of
radium has been observed in a pitchblende rich in uranium, some
pitchblendes rich in uranium contain very little radium.

The general evidence, which has been advanced to show that
radium must continually be produced from some other substance,
applies also to actinium, which has an activity of the same order
of magnitude as that of radium. It is very remarkable that the
three radio-active substances, radium, thorium and actinium, should
exhibit such a close similarity in the succession of changes which
occur in them. Each of them at one stage of its disintegration
emits a radio-active gas, and in each case this gas is transformed
into a solid which is deposited upon the surface of bodies. It
would appear that, after disintegration of an atom of any of these
has once begun, there is a similar succession of changes, in which
the resulting systems have allied chemical and physical properties.
Such a connection is of interest as indicating a possible origin
of the recurrence of properties in the atoms of the elements, as
exemplified by the periodic law.

204. Loss of weight of the radio-element. Since the
radio-elements are continuously throwing off a particles atomic
in size, an active substance, enclosed in a vessel sufficiently thin to
allow the a particles to escape, must gradually lose in weight.
This loss of weight wrill be small under ordinary conditions, since
the greater proportion of the a rays produced are absorbed in the
mass of the substance. If a very thin layer of a radium compound
were spread on a very thin sheet of substance, which did not
appreciably absorb the a particles, a loss of weight due to the
expulsion of a particles might be detectable. Since e/m = 6 x 103
for the a. particle, and e=l'l x 10"20 electro-magnetic units, and
10n a particles are expelled per second per gram of radium, the
fraction of the mass expelled is 1*8 x 10~13 per second and
6 x 10~6 per year. There is one condition, however, under which
the radium should lose in weight fairly rapidly. If a current of
air is slowly passed over a radium solution, the emanation produced
would be removed as fast as it was formed. Since the atom of
the emanation has a mass probably not much smaller than the
radium atom, the fraction of the mass removed per year should

336 RADIO-ACTIVE PROCESSES [CH.

be nearly equal to the fraction of the radium which changes per
year, i.e. one gram of radium should diminish in weight half a
milligram (section 203) per year on a maximum estimate and
1/100 of a milligram on a minimum estimate.

If it is supposed that the /3 particles have weight, the loss of
weight due to their expulsion is very small compared with that
due to the emission of a particles. Taking the estimate deduced
from the observation of Wien (section 104), that 6 '6 x 109 ft particles
are projected per second from 1 gram of radium bromide, the loss
of weight would only be about 1'2 x 10~10 gram per year.

Except under very special experimental conditions, it would
thus be very difficult to detect the loss of weight of radium due to
the expulsion of {3 particles from its mass. There is, however, a
possibility that radium might change in weight even though none
of the radio-active products were allowed to escape. For example,
if the view is taken that gravitation is the result of forces having
their origin in the atom, it is possible that, if the atom were
disintegrated, the weight of the parts might not be equal to that
of the original atom.

A large number of experiments have been made to see if
radium preparations, kept in a sealed tube, alter in weight. With
the small quantities of radium available to the experimenter, no
difference of weight of radium preparations with time has yet
been established with certainty. Heydweiller stated that he had
observed a loss of weight of radium, and Dorn also obtained a
slight indication of change in weight. These results have not,
however, been confirmed. Forch, later, was unable to observe any
appreciable change.

205. Total emission of energy from the radio-element.

It has been shown that 1 gram of radium emits energy at the
rate of 100 gram-calories per hour or 876,000 gram-calories per
year. If 1 gram of radium were set apart, its radio-activity and
consequent heat emission at a time t is given by qe~M, where \ is
the constant of decay of activity of radium. Thus the total heat

/OC
qe~M = - .
) ^

Now on the minimum estimate of the life of radium, the value

X] RADIO-ACTIVE PROCESSES 337

of X is 4'4 x 10~4, and on the maximum estimate 1*76 x 10~5 when
1 year is taken as the unit of time. The total heat emission from
1 gram of radium during its life thus lies between 2 x 109 and
5 x 1010 gram-calories. The minimum estimate is probably nearer
the truth than the maximum. The heat emitted in the union of
hydrogen and oxygen to form 1 gram of water is about 4 x 103
gram-calories, and in this reaction more heat is given out for
equal weights than in any other chemical reaction known. It is
thus seen that the total energy emitted from 1 gram of radium
during its changes is about one million times greater than that in-
volved in any known molecular change. That matter is able, under
special conditions, to emit an enormous amount of energy, is well
exemplified by the case of the radium emanation. The total heat
emission from the emanation released from 1 gram of radium,
and from the secondary products, corresponds to about 104
gram-calories, and this amount of heat is given out as a conse-
quence of changes in a minute volume of gas. Taking the
estimate that the volume of the emanation is 3 x 10~4 cubic
centimetres at standard pressure and temperature, and its atomic
weight about 200, it can be calculated that 1 gram of emanation
gives out during its life about 109 gram-calories. Quite inde-
pendently of any theory, a result of the same order of magnitude
can be deduced from the experiments.

Since the other radio-elements only differ from radium in the
slowness of their change, the total heat emission from uranium
and thorium must be of a similar high order of magnitude. There
is thus reason to believe that an enormous store of latent energy
is resident in the atoms of the radio-elements. This store of
energy could not have been recognized if the atoms were not
undergoing a slow process of disintegration. The energy emitted
in radio-active changes may thus be supposed to be derived from
the internal energy of the atoms. The emission of this energy
does not disobey the law of the conservation of energy, for it is
only necessary to suppose that, when the radio-active changes
have ceased, the energy stored up in the atoms of the final
products is less than that in the original atoms of the radio-
elements. The difference between the energy originally possessed
by the matter, which has undergone the change, and the final

R. R.-A. 22

338 RADIO-ACTIVE PROCESSES [CH.

inactive products which arise, is a measure of the total amount of
energy released.

There seems to be no reason to suppose that the atomic energy
of all the elements is not of a similar high order of magnitude.
With the exception of their high atomic weights, the radio-
elements do not possess any special chemical characteristics which
differentiate them from the inactive elements. The existence of
a latent store of energy in the atoms is a necessary consequence
of the modern view developed by J. J. Thomson, Larmor, and
Lorentz, of regarding the atom as a complicated structure consisting
of charged parts in rapid oscillatory or orbital motion in regard to
one another. The energy may be partly kinetic and partly potential,
but the mere arrangement of the charged particles, which probably
constitute the atom, in itself implies a large store of internal
atomic energy.

It is not to be expected that the existence of this store of
latent energy would have ordinarily manifested itself, since the
atoms cannot be broken up into simpler forms by the physical or
chemical agencies at our disposal. Its existence at once explains
the failure of chemistry to transform the atoms, and also accounts
for the independence of the rate of change of the radio-active
processes of all external agencies. It has not so far been found
possible to alter in any way the rate of emission of energy from
the radio-elements. If it were ever found possible to control at
will the rate of disintegration of the radio-elements, an enormous
amount of energy could be obtained from a small quantity of
matter.

206. Possible causes of disintegration. In order to ex-
plain the phenomena of radio-activity, it has been supposed that a
certain small fraction of the radio-atoms undergoes disintegration
per second, but no assumptions have been made as to the cause
which produces the instability and consequent disintegration.
The instability of the atoms may be supposed to be brought about
either by the action of external forces or of forces inherent in the
atoms themselves. It is conceivable, for example, that the appli-
cation of some slight external force might cause instability and
consequent disintegration, accompanied by the liberation of a large

X] RADIO-ACTIVE PROCESSES 339

amount of energy, on the same principle that a detonator is
necessary to start some explosives. It has been shown that the
number of atoms of any radio-active product which break up per
second is always proportional to the number present. This law
of change does not throw any light on the question, for it would
be expected equally on either hypothesis. It has not been found
possible to alter the rate of change of any product by the appli-
cation of any known physical or chemical forces, unless possibly it
is assumed that the force of gravitation which is not under our
control may influence in some way the stability of the radio-atoms.

It has been suggested by J. J. Thomson1 that the rate of dis-
integration of radium may be influenced by its own radiations.
This, at first sight, appears very probable, for a small mass of pure
radium compound is subjected to an intense bombardment by the
radiations arising from it, and the radiations are of such a character
that they might be expected to produce a breaking up of the
atoms of matter which they traverse. If this is the case the
radio-activity of a given quantity of radium should be a function
of its concentration, and should be greater in the solid state than
when disseminated through a large mass of matter.

I have recently tried an experiment to see if this were the
case. Two glass tubes were taken, in one of which was placed a
few milligrams of pure radium bromide in a state of radio-active
equilibrium, and in the other a solution of barium chloride. The
two tubes were connected near the top by a short cross tube and
the open ends sealed off. The activity of the radium in the solid
state was tested immediately after its introduction by placing it
in a definite position near an electroscope made of thin metal of
the type shown in Fig. 11. The increased rate of discharge of the
electroscope was observed. This rate of discharge was due to the
ft and 7 rays from the radium. By placing a lead plate 6 mms.
in thickness between the radium and the electroscope, the rate of
discharge observed was then due to the 7 rays alone. By slightly
tilting the apparatus, the barium solution flowed into the radium
tube and dissolved the radium. The tube was well shaken so as
to distribute the radium uniformly throughout the solution. No
appreciable change of the activity measured by the 7 rays was

1 Nature, April 30, p. 601, 1903.

22—2

340 RADIO-ACTIVE PROCESSES [CH.

observed over the period of one month. The activity measured
by the ft and 7 rays was somewhat reduced, but this was not due
to a decrease of the radio-activity, but to an increased absorption
of the /3 rays in their passage through the solution. The volume
of the solution was at least 1000 times greater than that of the
solid radium bromide, and, in consequence, the radium was sub-
jected to the action of a much weaker radiation. I think we may
conclude from this experiment that the radiations emitted by
radium have little if any influence in causing the disintegration
of the radium atoms.

This result is in general agreement with other observations;
for it has not been observed that the decay of activity of any
product is influenced by the degree of concentration of that
product.

It thus seems likely that the cause of the disruption of the
toms of the radio-elements and their products is resident in the
toms themselves. "According to the modern views of the consti-
ution of the atom, it is not so much a matter of surprise that
3me atoms disintegrate as that the atoms of the elements are so
ermanent as they appear to be. In accordance with the hypothesis
of J. J. Thomson, it may be supposed that the atoms consist of a
number of small positively and negatively charged particles in
rapid internal movement, and held in equilibrium by their mutual
forces. In a complex atom, where the possible variations in the
relative motion of the parts are very great, the atom may arrive
at such a phase that one part acquires sufficient kinetic energy
to escape from the system, or that the constraining forces are
momentarily neutralised, so that the part escapes from the system
with the velocity possessed by it at the instant of its release.

Sir Oliver Lodge1 has advanced the view that the instability of
the atom may be a result of radiation of energy by the atom. Larmor
has shown that an electron, subject to acceleration, radiates energy
at a rate proportional to the square of its acceleration. An electron
moving uniformly in a straight line does not radiate energy, but
an electron, constrained to move in a circular orbit with constant
velocity, is a powerful radiator, for in such a case the electron is
continuously accelerated towards the centre. Lodge considered

1 Lodge, Nature, June 11, p. 129, 1903.

X] RADIO-ACTIVE PROCESSES 341

the simple case of a negatively charged electron revolving round
an atom of mass relatively large but having an equal positive
charge and held in equilibrium by electrical forces. This system
will radiate energy, and since the radiation of energy is equivalent
to motion in a resisting medium, the particle tends to move
towards the centre, and its speed consequently increases. The
rate of radiation of energy will increase rapidly with the speed
of the electron. When the speed of the electron becomes very
nearly equal to the velocity of light, according to Lodge, another
effect supervenes. It has been shown (section 76) that the
apparent mass of an electron increases very rapidly as the speed
of light is approached, and is theoretically infinite at the speed
of light. There will be at this stage a sudden increase of the
mass of the revolving atom and, on the supposition that this stage
can be reached, a consequent disturbance of the balance of forces
holding the system together. Lodge considers it probable that,
under these conditions, the parts of the system will break asunder
and escape from the sphere of one another's influence.

It seems probable that the primary cause of the disintegration
of the atom must be looked for in the loss of energy of the atomic
system due to electro-magnetic radiation. Larmor1 has shown
that the condition to be fulfilled in order that a system of rapidly
moving electrons may persist without loss of energy is that the
vector sum of the accelerations towards the centre should be
permanently null. While a single electron moving in a circular
orbit is a powerful radiator of energy, it is remarkable how rapidly
the radiation of energy diminishes if several electrons are revolv-
ing in a ring. This has recently been shown by J. J. Thomson2,
who examined mathematically the case of a system of negatively
electrified corpuscles, situated at equal intervals round the circum-
ference of a circle, and rotating in one plane with uniform velocity
round its centre. For example, he found that the radiation from
a group of six particles moving with a velocity of ^ of the velocity
of light is less than one-millionth part of the radiation from a
single particle describing the same orbit with the same velocity.
When the velocity is TJ^ of that of light the amount of radiation

1 Larmor, Aether and Matter, p. 233.

2 J. J. Thomson, Phil. May. p. 681, Dec. 1903.

342 RADIO-ACTIVE PROCESSES [CH.

is only 10~16 of that of the single particle moving with the same
velocity in the same orbit.

Results of this kind indicate that an atom consisting of a large
number of revolving electrons may radiate energy extremely slowly,
and yet, finally, this minute but continuous drain of energy from
the atom must result either in a rearrangement of its component
parts into a new system, or of an expulsion of electrons or groups
of electrons from the atom.

A suggestion, due to J. J. Thomson1, of a possible mechanism
to account for the expulsion from the radio-atoms of an a particle,
i.e. of a connected group of electrons, has recently been explained
by Whetham2 as follows : — " The sub-atomic corpuscles, when their
velocity is changing, must radiate ethereal waves. Their energy
is thus gradually diminished ; and systems of revolving corpuscles,
permanent while moving fast, may become unstable. As a simple
example, six bodies at the corners of a plane hexagon under the
influence of mutual forces may continue, while their velocity
exceeds a certain limit, to revolve about a central point while
keeping their relative positions. When there is no motion, how-
ever, this arrangement is impossible, and the six bodies must place
themselves, five at the corners of a pentagon and one at the centre.
Thus, as the velocity falls to a certain value, a sudden and ex-
plosive rearrangement occurs, during which, in the complex
system constituting an atom, the ejection of parts of the system
becomes possible."

207. Radio-activity and the heat of the sun and earth.

It was pointed out by Mr Soddy and the writer3 that the
maintenance of the sun's heat for long intervals of time did not
present any fundamental difficulty if a process of disintegration,
such as occurs in the radio-elements, were supposed to be taking
place in the sun. In a letter to Nature (July 9, 1903) W. E.
Wilson showed that the presence of 3'6 grams of radium in each
cubic metre of the sun's mass was sufficient to account for the
present rate of emission of energy by the sun. This calculation
was based on the estimate of Curie and Laborde, that one gram

1 Prof. Thomson's paper has just appeared. Phil. May. March, 1904.

2 Quarterly Review, p. 123, Jan. 1904. 3 Phil. Mag. May, 1903.

X] RADIO- ACTIVE PROCESSES 343

of radium emits 100 gram-calories per hour, and on the observa-
tion of Langley that each square centimetre of the sun's surface
emits 8-28 x 106 gram-calories per hour. Since the average density
of the sun is 1'44, the presence of radium in the sun, to the
extent of 2'5 parts by weight in a million, would account for its
present rate of emission of energy.

An examination of the spectrum of the sun has not so far
revealed any of the radium lines. It is known, however, from
spectroscopic evidence that helium is present, and this indirectly
suggests the existence of radio-active matter also. It can readily
be shown1 that the absence of penetrating rays from the sun at
the surface of the earth does not imply that the radio-elements
are not present in the sun. Even if the sun were composed of
pure radium, it would hardly be expected that the 7 rays emitted
would be appreciable at the surface of the earth, since the rays
would be almost completely absorbed in passing through the
atmosphere, which corresponds to a thickness of 76 centimetres of
mercury.

In the Appendix E of Thomson and Tait's Natural Philosophy,
Lord Kelvin has calculated the energy lost in the concentration of
the sun from a condition of infinite dispersion, and concludes that
it seems " on the whole probable that the sun has not illuminated
the earth for 100,000,000 years and almost certain that he has not
done so for 500,000,000 years. As for the future we may say, with
equal certainty, that inhabitants of the earth cannot continue to
enjoy the light and heat essential to their life for many million
years longer, unless sources now unknown to us are prepared in
the great storehouses of creation."

The discovery that a small mass of a substance like radium
can emit spontaneously an enormous quantity of heat renders
it possible that this estimate of the age of the sun's heat
may be much increased. In a letter to Nature (Sept. 24, 1903)
G. H. Darwin drew attention to this probability, and stated that,
" The lost energy of the sun, supposed to be a homogeneous sphere
of mass M and radius a, is f/i3/2/a where p is the constant of
gravitation. On introducing numerical values for the symbols in
this formula, I find the lost energy to be 2*7 x 107 M calories where

1 See Strutt and Joly, Xature, Oct. 15, 1903.

344 RADIO-ACTIVE PROCESSES [CH.

M is expressed in grams. If we adopt Langley's value of the solar
constant, this heat suffices to give a supply for 12 million years.
Lord Kelvin used Pouillet's value for that constant, but if he had
been able to use Langley's, his 100 million would have been
reduced to 60 million. The discrepancy between my results of
12 million and his of 60 million is explained by a conjectural
augmentation of the lost energy to allow for the concentration
of the solar mass towards its central parts." Now it has been
shown (section 205) that one gram of radium emits during its
life an amount of heat which probably lies between 2 x 109 and
5 x 1010 gram-calories. It has also been pointed out that there is
every reason to suppose that a similar amount of energy is resident
in the chemical atoms of the inactive elements. It is not impro-
bable that, at the enormous temperature of the sun, the breaking
up of the elements into simpler forms may be taking place at
a more rapid rate than on the earth. If the energy resident
in the atoms of the elements is thus available, the time during
which the sun may continue to emit heat at the present rate may
be from 50 to 500 times longer than was computed by Lord Kelvin
from dynamical data.

Similar considerations apply to the question of the probable
age of the earth. A full discussion of the probable age of the
earth, computed from its secular cooling from a molten mass, is
given by Lord Kelvin in Appendix D of Thomson and Tait's Natural
Philosophy. He has there shown that about 100 million years
after the earth was a molten mass, the gradual cooling due to
radiation from its surface would account for the average tempera-
ture gradient of 1/50° F. per foot, observed to-day near the earth's
surface.

Some considerations will now be discussed which point to the
probability that the present temperature gradient observed in the
earth cannot be used as a guide to estimate the length of time
that has elapsed since the earth has been at a temperature capable
of supporting animal and vegetable life ; for it will be shown that
probably there is sufficient radio-active matter on the earth to
supply as much heat to the earth as is lost by radiation from its
surface. Taking the average conductivity K of the materials of
the earth as '004 (C.G.S. units) and the temperature gradient T near

X] RADIO-ACTIVE PROCESSES 345

the surface as "00037° C. per cm., the heat Q in gram-calories
conducted to the surface of the earth per second is given by

where R is the radius of the earth.

Let X be the average amount of heat liberated per second per
cubic centimetre of the earth's volume owing to the presence of
radio-active matter. If the heat Q radiated from the earth is
equal to the heat supplied by the radio-active matter on the
earth,

3KT

Z = -7T-

Substituting the values of these constants,

X = 7 x 10~15 gram-calorie per second
= 2'2 x 10~7 gram-calorie per year.

Since 1 gram of radium emits 864,000 gram-calories per year,
the presence of 2'6 x 10~13 gram of radium per unit volume or
4'6 x 10~14 gram per unit mass, would compensate for the heat lost
from the earth by conduction.

Now it will be shown in the following chapter that radio-active
matter seems to be distributed fairly uniformly through the earth
and atmosphere. In addition it has been found that all substances
are radio-active to a feeble degree, although it is not yet settled
whether this radio-activity may not be due mainly to the presence
of a radio-element as an impurity. For example, Strutt1 observed
that a platinum plate was about 1/3000 as active as a crystal of
uranium nitrate, or about 2 x 10~10 as active as radium. This cor-
responds to a far greater activity than is necessary to compensate
for the loss of heat of the earth. A more accurate deduction,
however, can be made from data of the radio-activity exhibited by
matter dug out from the earth. Elster and Geitel2 filled a dish of
volume 3'3 x 103 c.c. with clay dug up from the garden, and placed
it in a vessel of 30 litres capacity in which was placed an electro-

1 Strutt, Phil. Mag. June, 1903.

- Elster and Geitel, Phys. Zeit. 4, No. 19, p. 522, 1903. Chem. Xeicg, July 17,
p. 30, 1903.

346 RADIO-ACTIVE PROCESSES [CEL

scope to determine the conductivity of the enclosed gas. After
standing for several days, he found that the conductivity of the air
reached a constant maximum value, corresponding to three times
the normal. It will be shown later (section 218) that the normal
conductivity observed in sealed vessels corresponds to the produc-
tion of about 30 ions per c.c. per second. The number of ions
produced per second in the vessel by the radio-active earth was
thus about 2 x 106. This would give a saturation current through
the gas of 2'2 x 10~14 electro-magnetic units. Now the emanation
from 1 gram of radium stored in a metal cylinder gives a satura-
tion current of about 3'2 x 10~3 electro-magnetic units. Elster and
Geitel considered that most of the conductivity observed in the
gas was due to a radio-active emanation, which gradually diffused
from the clay into the air in the vessel. The increased conduc-
tivity in the gas observed by Elster and Geitel would thus be
produced by the emanation from 7 x 10~10 gram of radium.
Taking the density of clay as 2, this corresponds to about 10~13
gram of radium per gram of clay. But it has been shown that if
4*6 x 10~u gram of radium was present in each gram of earth, the
heat emitted would compensate for the loss of heat of the earth by
conduction and radiation. The amount of activity observed in the
earth is thus about the right order of magnitude to account for the
heat emission required. In the above estimate, the presence of
uranium and thorium minerals in the earth has not been con-
sidered. In addition, it is probable that the total amount of radio-
activity in clay was considerably greater than that calculated, for it
is likely that other radio-active matter was present which did not
give off an emanation.

I think we may conclude that the present rate of loss of heat
of the earth might have continued unchanged for long periods of
time in consequence of the supply of heat from radio-active matter
in the earth. It thus seems probable that the earth may have
remained for very long intervals of time at a temperature not very
different from that observed to-day, and that in consequence the
time during which the earth has been at a temperature capable of
supporting the presence of animal and vegetable life may be very
much longer than the estimate made by Lord Kelvin from other
data.

X] RADIO-ACTIVE PROCESSES 347

208. Evolution of matter. Although the hypothesis that
all matter is composed of some elementary unit of matter or pro-
tyle has been advanced as a speculation at various times by many
prominent physicists and chemists, the first definite experimental
evidence showing that the chemical atom was not the smallest
unit of matter was obtained in 1897 by J. J. Thomson in his classic
research on the nature of the cathode rays produced by an electric
discharge in a vacuum tube. Sir William Crookes, who was the first
to demonstrate the remarkable properties of these rays, had sug-
gested that they consisted of streams of projected charged matter
and represented — as he termed it — a new or "fourth state of matter."

J. J. Thomson showed by two distinct methods that the cathode
rays consisted of a stream of negatively charged particles projected
with great velocity. The particles behaved as if their mass was
only about 1/1000 of the mass of the atom of hydrogen, which is
the lightest atom known. These corpuscles, as they were termed by
Thomson, were found at a later date to be produced from a glowing
carbon filament and from a zinc plate exposed to the action of
ultra-violet light. They acted as isolated units of negative elec-
tricity, and, as we have seen, may be identified with the electrons
studied mathematically by Larmor and Lorentz. Not only were
these electrons produced by the action of light, heat, and the
electric discharge, but they were also found to be spontaneously
emitted from the radio-elements with a velocity far greater than
that observed for the electrons in a vacuum tube.

The electrons produced in these different ways were all found to
carry a negative charge and to be apparently identical ; for the
ratio e/m of the charge of the electron to its mass was in all cases
the same within the limit of experimental errors. Since elec-
trons, produced from different kinds of matter and under different
conditions, were in all cases identical, it seemed probable that they
were a constituent part of all matter. J. J. Thomson suggested
that the atom is built up of a number of these negatively charged
electrons combined in some way with corresponding positively
charged bodies.

On this view the atoms of the chemical elements differ from
one another only in the number and arrangement of the component
electrons.

348 RADIO-ACTIVE PROCESSES [CH.

The removal of an electron from the atom does not appear to
permanently affect the stability of the system, for no evidence has
so far been obtained to show that the passage of an intense electric
discharge through a gas results in a permanent alteration of the
structure of the atom. On the other hand, in the case of the
radio-active bodies, a positively charged particle of mass about
twice that of the hydrogen atom escapes from the heavy radio-
atom. This appears to result at once in a permanent alteration of
the atom, and causes a marked change in its physical and chemical
properties. In addition there is no evidence that the process is
reversible.

The only direct experimental evidence of the transformation
of matter has been derived from a study of the radio-active
bodies. If the disintegration theory, advanced to account for the
phenomena of radio-activity, is correct in the main essentials, then
the radio-elements are undergoing a spontaneous and continu-
ous process of transformation into other and different kinds of
matter. The rate of transformation is slow in uranium and thorium,
but is fairly rapid in radium. It has been shown that the fraction
of a mass of radium which is transformed per year lies between
1/2000 and 1/10000 of the total amount present. In the case of
uranium and thorium probably a million years would be required
to produce a similar amount of change. The process of trans-
formation in uranium and thorium is thus far too slow to be
detected within a reasonable time by the use of the balance or
spectroscope, but the radiations which accompany the transforma-
tion can readily be detected. Although the process of change is
slow it is continuous, and in the course of ages the uranium and
thorium present in the earth must be transformed into other and
simpler types of matter.

Those who have considered the possibility of atoms undergoing
a process of transformation, have generally thought that the
matter as a whole would undergo a progressive change, with a
gradual alteration of physical and chemical properties of the whole
mass of substance. On the theory of disintegration this is not the
case. Only a minute fraction of the matter present breaks up in
unit time, and in each of the succession of stages through which
the disintegrated atoms pass, there is in most cases a marked

X] RADIO-ACTIVE PROCESSES 349

alteration in the chemical and physical properties of the matter.
The transformation of the radio-elements is thus a transformation
of a part per saltum, and not a progressive change of the whole.
At any time after the process of transformation has been in
progress there will thus remain a part of the matter which is
unchanged, and, mixed with it, the products which have resulted
from the transformation of the remainder.

The question naturally arises whether the process of degrada-
tion of matter is confined to the radio-elements or is a universal
property of matter. It will be shown in chapter xi. that all
matter, so far examined, exhibits the property of radio-activity to
a slight degree. It still remains to be shown, however, that the
observed radio-activity is not due to the presence in the matter of
a slight trace of a radio-element. If ordinary matter is radio-
active, it is certain that its activity is not greater than that of
uranium, and consequently that its rate of transformation must
be excessively slow. There is, however, another possibility to be
considered. The changes occurring in the radio-elements would
probably never have been detected if the change had not been
accompanied by the expulsion of charged particles with great
velocity. It does not seem unlikely that an atom may undergo
disintegration without projecting a part of its system with great
velocity. In fact, we have seen that, even in the radio-elements,
one of the series of changes in both thorium and radium is unac-
companied by ionizing rays. It may thus be possible that all
matter is undergoing a slow process of transformation, which has
so far only been detected in the radio-elements on account of the
expulsion of charged particles during the change. This process of
degradation of matter continuing for ages must reduce the con-
stituents of the earth to the simpler and more stable forms of
matter.

The idea that helium is a transformation product of radium,
suggests the probability that helium is one of the more elementary
substances of which the heavier atoms are composed. Sir Norman
Lockyer, in his interesting book on "Inorganic Evolution," has
pointed out that the spectrum of helium and of hydrogen pre-
dominates in the hottest stars. In the cooler stars the more
complex types of matter appear. Sir Norman Lockyer has based

350 RADIO-ACTIVE PROCESSES [CH. X

his theory of evolution of matter on evidence of a spectroscopic
examination of the stars, and considers that temperature is the
main factor in breaking up matter into its simpler forms. The
transformation of matter occurring in the radio-elements is on the
other hand spontaneous, and independent of temperature over the
range examined.

CHAPTER XL

RADIO-ACTIVITY OF THE ATMOSPHERE AND OF
ORDINARY MATERIALS.

209. Radio -activity of the atmosphere. The experiments
of Geitel1 and C. T. R Wilson2 in 1900 had shown that a positively
or negatively charged conductor placed inside a closed vessel gradu-
ally lost its charge. This loss of charge was shown to be due to a
small ionization of the air inside the vessel. In addition, Elster
and Geitel had found that a charged body exposed in the open
air lost its charge rapidly, and that the rate of discharge was
dependent on the locality and on atmospheric conditions. A more
detailed description and discussion of these results will be given
later in section 218.

In the course of these experiments Geitel had observed that
the rate of discharge increased slightly for some time after the
vessel had been closed. He considered that this might possibly
be due to the existence of some radio-active substances in the air,
which produced excited activity in the walls of the vessel and so
increased the rate of dissipation of the charge. In 1901 Elster
and Geitel3 tried the bold experiment of seeing if it were possible
to extract a radio-active substance from the air. The experiments
of the writer had shown that the excited radio-activity from the
thorium emanation could be concentrated on the negative electrode
in a strong electric field. This result indicated that the carriers
of the radio-activity had a positive charge of electricity. Elster
and Geitel therefore tried an experiment to see if positively charged

1 Phys. Zeit. 2, p. 116, 1900.

2 Proc. Camb. Phil. Soc. 11, p. 32, 1900. Proc. Roy. Soc. 68, p. 151, 1901.

3 Phys. Zeit. 2, p. 590, 1901.

352 RADIO-ACTIVITY OF THE ATMOSPHERE [CH.

carriers, possessing a similar property, were present in the atmo-
sphere. For this purpose a cylinder of wire-netting, charged nega-
tively to 600 volts, was exposed for several hours in the open air.
This was then removed and quickly placed in a large bell-jar, inside
which was placed an electroscope to detect the rate of discharge.
They found that the rate of discharge was increased to a slight
extent. In order to multiply the effect, a wire of about 20 metres
long was exposed at some height from the ground, and was kept
charged to a high potential by connecting it to the negative
terminal of an influence machine. After exposure for some hours,
this wire was removed and placed inside the dissipation vessel.
The rate of discharge was found to be increased many times by the
presence of the wire. No increase was observed if the wire had
been charged positively instead of negatively. The results also
showed that the radio-active matter could be removed from the
wire in the same way as from a wire made active by exposure in the
presence of the thorium emanation. A piece of leather moistened
with ammonia was rubbed over the active wire. On testing the
leather it was found to be strongly radio-active. If a long wire
were used, the amount of activity obtained on the leather was
comparable to that possessed by a gram of uranium oxide.

The activity produced on the wire was not permanent, but
disappeared to a large extent in the course of a few hours. The
amount of activity produced on a wire of given size, exposed under
similar conditions, was independent of the material of the wire.
Lead, iron and copper wires gave about equal effects.

The amount of activity obtained was greatly increased by ex-
posing a negatively charged wire in a mass of air which had been
undisturbed for a long time. Experiments were made in the great
cave of Wolfenbiittel, and a very large amount of activity was
observed. By transferring the activity to a piece of leather it
was found1 that the rays could appreciably light up a screen of
barium platinocyanide in the dark. The rays also darkened a
photographic plate through a piece of aluminium O'l mm. in
thickness.

These remarkable experiments show that the excited radio-
activity obtained from the atmosphere is very similar in character
1 Phys. Zeit. 3, p. 76, 1901.

XI] AXD OF ORDINARY MATERIALS 353

to the excited activity produced by the emanations of radium and
thorium. No investigators have contributed more to our know-
ledge of the radio-activity and ionization of the atmosphere than
Elster and Geitel. The experiments here described have been the
starting-point of a series of researches by Elster and Geitel and
others on the radio-active properties of the atmosphere which have
led to a great extension of our knowledge of that important subject.

Eutherford and Allan1 determined the rate of decay of the
excited activity produced on a negatively charged wire exposed in
the open air. A wire about 15 metres long was exposed in the
open air, and kept charged by an influence machine to a potential
of about -10000 volts. An hour's exposure was sufficient to obtain
a large amount of excited activity on the wire. The wire was
then rapidly removed and wound on a framework which formed
the central electrode in a large cylindrical metal vessel. The
ionization current for a saturation voltage was measured by
means of a sensitive Dolezalek electrometer. The current, which
is a measure of the activity of the wire, was found to diminish
according to an exponential law with the time, falling to half value
in about 45 minutes. The rate of decay was independent of the
material of the wire, of the time of exposure, and of the potential
of the wire.

An examination was also made of the nature of the rays emitted
by the radio-active wire. For this purpose a lead wire was made
radio-active in the manner described, and then rapidly wound into
the form of a flat spiral. The penetrating power of the rays was
tested in a vessel similar to that shown in Fig. 16. Most of the
ionization was found to be due to some very easily absorbed rays,
which were of a slightly more penetrating character than the a
rays emitted from a wire made active by the radium or thorium
emanations. The intensity of the rays was cut down to half value
by about O'OOl cm. of aluminium. The photographic action ob-
served by Elster and Geitel through O'l mm. of aluminium showed
that some penetrating rays were also present. This was afterwards
confirmed by Allan, using the electric method. These penetrating
rays are probably similar in character to the (3 rays from the
radio-elements.

1 Phil Mag. Dec. 1902.
R. R.-A. 23

354 RADIO-ACTIVITY OF THE ATMOSPHERE [CH.

210. The excited activity produced on the negatively charged
wire cannot be due to an action of the strong electric field on the
surface of the wire ; for very little excited activity is produced if
the wire is charged to the same potential inside a closed cylinder.

We have seen that the excited activity produced on the wire
can be partially removed by rubbing, and by solution in acids, and,
in this respect, it is similar to the excited activity produced in
bodies by the emanations of radium and thorium. The very close
similarity of the excited activity obtained from the atmosphere
to that obtained from the radium and thorium emanations sug-
gests the probability that a radio-active emanation exists in the
atmosphere. This view is confirmed by a large amount of indirect
evidence discussed in sections 212 and 213.

Assuming the presence of a radio-active emanation in the
atmosphere, the radio-active effects observed receive a simple
explanation. The emanation in the air gradually breaks up,
giving rise in some way to positively charged radio-active carriers.
These are driven to the negative electrode in the electric field,
and there undergo a further change, giving rise to the radiations
observed at the surface of the wire. The matter which causes
excited activity will thus be analogous to the emanation X of
radium and thorium.

Since the earth is negatively electrified with regard to the
upper atmosphere, these positive radio-active carriers produced in
the air are continuously deposited on the surface of the earth.
Everything on the surface of the earth, including the external
surface of buildings, the grass, and leaves of trees, must be covered
with an invisible deposit of radio-active material. A hill or
mountain peak, or any high mass of rock or land, concentrates the
earth's electric field at that point and consequently will receive
more excited radio-activity per unit area than the plain. Elster
and Geitel have pointed out that the greater ionization of the air
observed in the neighbourhood of projecting peaks receives a
satisfactory explanation on this view.

If the radio-active carriers are produced at a uniform rate in
the atmosphere, the amount of excited activity It, produced on
a wire exposed under given conditions, will, after exposure for a
time t, be given by It = I0(l -e~u) where /0 is the maximum

XI] AND OF ORDINARY MATERIALS 355

activity on the wire and X is the constant of decay of the excited
activity. Since the activity of a wire after removal falls to half
value in about 45 minutes, the value of X is 0'92 with one hour as
the unit of time. Some experiments made by Allan are in rough
agreement with the above equation. Accurate comparative results
are difficult to obtain on account of the inconstancy of the radio-
activity of the open air. After an exposure of a wire for several
hours, the activity reached a practical maximum, and was not
much increased by continued exposure.

A wire charged to a high potential in the open air abstracts
the positive radio-active carriers from a large volume of air. Very
little excited activity, in comparison, is produced in a closed vessel
or by drawing a rapid current of the outside air through a cylinder
in the centre of which a negatively charged rod is placed. In one
experiment a current of air of 500 cms. per second was drawn
through a cylinder of 141 litres capacity. The amount of activity
produced on the negative electrode was only a few per cent, of the
amount observed on the same electrode charged to the same
potential in the open air.

The amount of excited activity produced on a wire, supported
some distance from the surface of the earth, should increase steadily
with the voltage, for the greater the potential, the greater the
volume of air from which the radio-active carriers are abstracted.

The presence of radio-active matter in the atmosphere will
account for a portion of the ionization of the air observed near
the earth. It seems unlikely, however, that the whole of the
ionization observed in the air is due to this cause alone.

211. Radio-activity of freshly fallen rain and snow.

C. T. R. Wilson1 tried experiments to see if any of the radio-
active material from the air was carried down by rain. For this
purpose a quantity of freshly fallen rain was collected, rapidly
evaporated to dryness in a platinum vessel, and the activity of the
residue tested by placing the vessel in an electroscope. In all
cases, the rate of discharge of the electroscope was considerably
increased. From about 50 c.c. of rain water, an amount of activity
was obtained sufficient to increase the rate of discharge of the

1 Proc. Camb. Phil. Soc. 11, Pt. vi. p. 428, 1902.

23—2

356 KADIO-ACTIVITY OF THE ATMOSPHERE [CH.

electroscope four or five times, after the rays had traversed a thin
layer of aluminium or gold leaf. The activity disappeared in the
course of a few hours, falling to half value in about 30 minutes.
Bain water, which had stood for some hours, showed no trace of
activity. Tap water, when evaporated, left no active residue.

The amounts of activity obtained from a given quantity of rain
water were all of the same order of magnitude, whether the rain
was precipitated in fine or in large drops, by night or by day, or
whether the rain was tested at the beginning or at the end of a
heavy rainfall lasting several hours.

The activity obtained from rain is not destroyed by heating
the platinum vessel to a red heat. In this and other respects it
resembles the excited activity obtained on negatively charged
wires exposed in the open air.

C. T. R. Wilson1 obtained a radio-active precipitate from rain
water by adding a little barium chloride and precipitating the
barium with sulphuric acid. An active precipitate was also
obtained if alum was added to the water, and the aluminium
precipitated by ammonia. The precipitates obtained in this way
showed a large activity. The filtrate when boiled down was quite
inactive, showing that the active matter had been completely
removed by precipitation. The production of active precipitates
from rain water is quite analogous to the production of active
precipitates from a solution containing the emanation X of thorium
(see section 178).

The radio-activity of freshly fallen snow was independently ob-
served by C. T. R. Wilson2 in England, and Allan3 and McLennan4
in Canada. In order to obtain a large amount of activity, the
surface layer of snow was removed, and evaporated to dry ness
in a metal vessel. An active residue was obtained with radio-
active properties similar to those observed for freshly fallen rain.
Both Wilson and Allan found that the activity of rain and snow
decayed at about the same rate, the activity falling to half value
in about 30 minutes. McLennan states that he found a smaller
amount of radio-activity in the air after a prolonged fall of snow.

1 Proc. Roy. Soc. Vol. 12, 1902.

2 Proc. Camb. Phil. Soc. 12, p. 85, 1903.

3 Phys. Rev. 16, p. 106, 1903. * Phys. Rev. 16, p. 184, 1903.

XI] AXD OF ORDINARY MATERIALS 357

Schmauss1 has observed that drops of water falling through air
ionized by Rb'ntgen rays acquire a negative charge. This effect is
ascribed to the fact that the negative ions in air diffuse faster
than the positive. On this view the drops of rain and flakes of
snow would acquire a negative charge in falling through the air.
They would in consequence act as collectors of the positive radio-
active carriers from the air. On evaporation of the water the
radio-active matter would be left behind.

212. Radio-active emanations from the earth. Elster
and Geitel observed that the air in caves and cellars was, in most
cases, abnormally radio-active, and showed very strong ionization.
This action might possibly be due to an effect of stagnant air, by
which it produced a radio-active emanation from itself, or to a
diffusion of a radio-active emanation from the soil. In order to
test if this emanation was produced by the air itself, Elster and
Geitel shut up the air for several weeks in a large boiler, but no
appreciable increase of the activity or ionization was observed. In
order to test if the air imprisoned in the capillaries of the soil was
radio-active Elster and Geitel2 put a pipe into the earth and sucked
up the air into a testing vessel by means of a water pump.

The apparatus employed to test the ionization of the air is
shown in Fig. 59. C is an electroscope connected with a wire net,
Z. The active air was introduced into the large bell-jar of 27 litres
capacity, the inside of which was covered with wire netting, MM'.
The bell-jar rested on an iron plate AB. The electroscope could
be charged by the rod S. The rate of discharge of the electro-
scope, before the active air was introduced, was noted. On allowing
the active air to enter, the rate of discharge increased rapidly,
rising in the course of a few hours in one experiment to 30 times
the original value. They found that the emanation produced
excited activity on the walls of the containing vessel. The air
sucked up from the earth was even more active than that observed
in caves and cellars. There can thus be little doubt that the
abnormal activity observed in caves and cellars is due to a radio-
active emanation, present in the earth, which gradually diffuses to
the surface and collects in places where the air is not disturbed.

1 Drude's Annal. 9, p. 224, 1902. 2 Phys. Zeit. 3; p. 574, 1902.

358

RADIO-ACTIVITY OF THE ATMOSPHERE

[CH.

The results obtained by Elster and Geitel for the air removed
from the earth at Wolfenbiittel were also obtained later by Ebert
and Ewers1 at Munich. They found a strongly active emanation
in the soil, and, in addition, examined the variation with time of
the activity due to the emanation in a sealed vessel. After the
introduction of the active air into the testing vessel, the activity
was observed to increase for several hours, and then to decay,
according to an exponential law, with the time, falling to half
value in about 3 '2 days. This rate of decay is more rapid than
that observed for the radium emanation, which decays to half

H c=

Fig. 59.

value in a little less than four days. The increase of activity with
time is probably due to the production of excited activity on the
walls of the vessel by the emanation. In this respect it is analogous
to the increase of activity observed when the radium emanation
is introduced into a closed vessel. No definite experiments were
made by Ebert and Ewers on the rate of decay of this excited
activity. In one experiment the active emanation, after standing
in the vessel for 140 hours, was removed by sucking ordinary air

1 Phys. Zeit. 4, p. 162, 1902.

Xl] AND OF ORDINARY MATERIALS 359

of small activity through the apparatus. The activity rapidly fell
to about half value, and this was followed by a very slow decrease
of the activity with time. This result indicates that about half
the rate of discharge observed was due to the radiation from the
emanation and the other half to the excited activity produced by it.

The apparatus employed by Ebert and Ewers in these experi-
ments was very similar to that employed by Elster and Geitel,
shown in Fig. 59. Ebert and Ewers observed that when the wire
net attached to the electroscope was charged negatively the rate
of discharge observed was always greater than when it was charged
positively. The differences observed between the two rates of
discharge varied between 10 and 20 per cent. This difference in
the rates of discharge for positive and negative electricity is
probably connected with the presence of particles of dust or small
water globules suspended in the gas. The experiments of Miss
Brooks (section 171) have shown that the particles of dust present
in the air containing the thorium emanation become radio-active.
A large proportion of these dust particles acquire a positive charge
and are carried to the negative electrode in an electric field. This
effect would increase the rate of discharge of the electroscope when
charged negatively. In later experiments, Ebert and Ewers
observed that, in some cases, if the air had been kept in the vessel
for several days, the effect was reversed, and the electroscope
showed a great rate of discharge when charged positively.

J. J. Thomson1 has observed that the magnitude of the ioniza-
tion current depends on the direction of the electric field, if fine
water globules are suspended in the ionized gas.

In later experiments, Ebert2 found that the radio-active emana-
tion could be removed from the air by condensation in liquid air.
This property of the emanation was independently discovered by
Ebert before he was aware of the results of Rutherford and Soddy
on the condensation of the emanations of radium and thorium. In
order to increase the amount of radio-active emanation in a given
volume of air, a quantity of the active air, obtained by sucking the
air from the soil, was condensed by a liquid air machine. The air
was then allowed to partially evaporate, but the process was stopped

1 Phil. Mag. Sept. 1902.

2 Site. Akad. d. Wiss. Munich, 33, p. 133, 1903.

360 RADIO-ACTIVITY OF THE ATMOSPHERE [CH.

before the point of volatilization of the emanation was reached.
This process was repeated with another quantity of air and the
residues added together. Proceeding in this way, he was able to
concentrate the emanation in a small volume of air. On allowing
the air to evaporate, the ionization of the air in the testing vessel
increased rapidly for a time and then slowly diminished. Ebert
states that the maximum was reached earlier for the emanation
which had been liquefied for some time than for fresh air. The rate
of decay of activity of the emanation was not altered by keeping
it at the temperature of liquid air for some time. In this respect
it behaves like the emanations of radium and thorium.

J. J. Thomson1 found that air bubbled through Cambridge tap
water showed much greater conductivity than ordinary air. The
air was drawn through the water by means of a water pump into a
large gasometer, when the ionization current was tested with a
sensitive electrometer. When a rod charged negatively was intro-
duced into this conducting air it became active. After an exposure
for a period of 15 to 30 minutes in the conducting gas, the rod,
when introduced into a second testing vessel, increased the saturation
current in the vessel to about five times the normal amount. Very
little effect was produced if the rod was uncharged or charged
positively for the same time. The activity of the rod decayed
with the time, falling to half value in about 40 minutes. The
amount of activity produced in a wire under constant conditions
was independent of the material of the wire. The rays from the
rod were readily absorbed in a few centimetres of air.

These effects were, at first, thought to be due to the action of
the small water drops suspended in the gas, for it was well known
that air rapidly drawn through water causes a temporary increase
in its conductivity. Later results, however, showed that there
was a radio-active emanation present in Cambridge tap water.
This led to an examination of the waters from deep wells in
various parts of England, and J. J. Thomson found that, in some
cases, a large amount of emanation could be obtained from the
well water. The emanation was released either by bubbling air
through the water or by boiling the water. The gases obtained by
boiling the water were found to be strongly active. A sample of

1 Phil. Hag. Sept. 1902.

XI] AND OF ORDINARY MATERIALS 361

air mixed with the radio-active emanation was condensed. The
liquefied gas was allowed to evaporate, and the earlier and last
portions of the gas were collected in separate vessels. The final
portion was found to be about 30 times as active as the first portion.

An examination of the radio-active properties of the active
gases so obtained has been made by Adams1. He found that the
activity of the emanation decayed in an exponential law with the
time, falling to half value in about 3'4 days. This is not very
different from the rate of decay of the activity of the radium
emanation, which falls to half value in a little less than four days.
The excited activity produced by the emanation decayed to half
value in about 35 minutes. The decay of the excited activity
from radium is at first irregular, but after some time falls off in an
exponential law, diminishing to half value in 28 minutes. Taking
into account the uncertainty attaching to measurements of the
very small ionization observed in these experiments, the results
indicate that the emanation obtained from well water in England
is similar to, if not identical with, the radium emanation. Adams
observed that the emanation was slightly soluble in water. After
well water had been boiled for some time and then put aside, it
was found to recover its power of giving off an emanation with
time. The amount obtained after standing for some time was
never more than 10 per cent, of the amount first obtained. Thus
it is probable that the well water, in addition to the emanations
mixed with it, has also a slight amount of a permanent radio-active
substance dissolved in it. Ordinary rain water or distilled water
does not give off an emanation.

Bumstead and Wheeler2 have recently made a very careful
examination of the radio-activity of the emanation obtained from
the surface water and soil at New Haven, Connecticut. The
emanation, obtained from the water by boiling, was passed into
a large testing cylinder, and measurements of the current were
made by means of a sensitive electrometer. The current gradually
rose to a maximum after the introduction of the emanation, in
exactly the same way as the current increases in a vessel after the
introduction of the radium emanation. The decay of activity of

1 Phil. Mag. Nov. 1903.

- Amer. Journ. Science, 17, p. 97, Feb. 1904.

362 RADIO-ACTIVITY OF THE ATMOSPHERE [CH.

the emanations obtained from the water and soil was carefully
measured, and within the limit of experimental error agreed with
the decay of activity observed with the radium emanation. The
identity of the emanations from the water and soil with the
radium emanation was still further established by experiments
on the rate of diffusion of the emanation through a porous plate.
By comparative tests it was found that the coefficient of diffusion
of the emanations from the water and soil was the same as for
the radium emanation. In addition, by comparison of the rate of
diffusion of carbonic acid, it was found that the density of the
emanation was about four times that of carbonic acid, a result in
good accord with that found for the radium emanation (section
153).

213. Radio-activity of constituents of the earth. Elster
and Geitel1 observed that, although in many cases the conductivity
of the air was abnormally high in underground enclosures, the
conductivity varied greatly for different places. In the Baumann
Cave, for example, the conductivity of the air was nine times the
normal, but in the Iberg Cave only three times the normal. In a
cellar at Clausthal the conductivity was only slightly greater than
the normal, but the excited radio-activity obtained on a negatively
charged wire exposed in it was only 1/11 of the excited radio-
activity obtained when the wire was exposed in the free air. It
was concluded from these experiments that the amount of radio-
activity in the different places probably varied with the nature
of the soil. Observations were then made on the conductivity of
the air sucked up from the earth at different parts of the country.
The clayey and limestone soils at Wolfenbiittel were found to be
strongly active, the conductivity varying from four to sixteen times
the normal amount. A sample of air from the shell limestone of
Wiirzburg and from the basalt of Wilhelmshohe showed very little
activity.

Experiments were made to see if any radio-active substance
could be detected in the soil itself. For this purpose some earth
was placed on a dish and introduced under a bell-jar, similar to that
shown in Fig. 59. The conductivity of the air in the bell-jar

1 Phys. Zeit. 4, p. 522, 1903.

XI] AND OF ORDINARY MATERIALS 363

increased with the time, rising to three times the normal value
after several days. Little difference was observed whether the
earth was dry or moist. The activity of the soil seemed to be
permanent, for no change in the activity was observed after the
earth had been laid aside for eight months.

Attempts were then made to separate the radio-active con-
stituent from the soil by chemical treatment. For this purpose
a sample of clay was tested. By extraction with hydrochloric
acid all the calcium carbonate was removed. On drying the
clay, the activity was found to be reduced, but it spontaneously
regained its original activity in the course of a few days. It thus
seems probable that an active product had been separated from
the soil by the acid. Elster and Geitel consider that an active
substance was present in the clay, which formed a product more
readily soluble in hydrochloric acid than the active material itself.
There seemed to be a process analogous to the separation of Th X
from thorium by precipitation with ammonia.

Experiments were also made to see if substances placed in the
earth acquired any radio-activity. For this purpose samples of
potter's clay, whitening, and heavy spar, wrapped in linen, were
placed in the earth 50 cms. below the surface. After an interval
of a month, these were dug up and their activity examined. The
clay was the only substance which showed any activity. The
activity of the clay diminished with the time, showing that activity
had been excited in it by the emanations present in the soil.

Elster and Geitel1 have recently found that a large quantity of
the radio-active emanation can be obtained by sucking air through
clay. In some cases, the conductivity of the air in the testing
vessel was increased over 100 times. They have also found that
" fango " — a fine mud obtained from hot springs in Battaglia,
Northern Italy — gives off three or four times as much emanation
as clay. By treating the fango with acid, the active substance
present was dissolved. On adding some barium chloride to the
solution, and precipitating the barium as sulphate, the active
substance was removed, and in this way a precipitate was obtained
over 100 times as active, weight for weight, as the original fango.
Comparisons were made of the rate of decay of the excited activity,

1 Phys. Zeit. 5, No. 1, p. 11, 1903.

364 RADIO-ACTIVITY OF THE ATMOSPHERE [CH.

due to the emanation from fango, with that due to the radium
emanation, and within the limit of error, the decay curves obtained
were found to be identical. There can thus be little doubt that
the activity observed in fango is due to the presence of a small
quantity of radium. Elster and Geitel calculate that .the amount
of radium, contained in it, is only about one thousandth of the
amount to be obtained from an equal weight of pitchblende from
Joachimstahl.

The natural carbonic acid arising from great depths of old
volcanic soil was also tested. The carbon dioxide was obtained
from the works in the liquid state. The gas was found to show
distinct activity, and was able to produce excited activity on the
surface of the vessel. After an interval of 16 days the gas was
again tested and found to be inactive.

These results are similar to those of J. J. Thomson, who found
an active emanation in the water obtained from deep wells.

214. Effect of meteorological conditions upon* the
radio-activity of the atmosphere. The original experiments
of Elster and Geitel on the excited radio-activity derived from
the atmosphere were repeated by Rutherford and Allan1 in
Canada. It was found that a large amount of excited radio-
activity could be derived from the air, and that the effects were
similar to those observed by Elster and Geitel in Germany. This
was the case even on the coldest day in winter, when the ground
was covered deeply with snow and the wind was blowing from the
north over snow-covered lands. The results showed that the
radio-activity present in the air was not much affected by the
presence of moisture, for the air during a Canadian winter is
extremely dry. The greatest amount of excited activity on a
negatively charged wire was obtained in a strong wind. In some
cases the amount produced for a given time of exposure was ten
to twenty times the normal amount. A cold bright day of winter
usually gave more effect than a warm dull day in summer.

Elster and Geitel2 have made a detailed examination of the
effect of meteorological conditions on the amount of excited radio-
activity to be derived from the atmosphere. For this purpose a

1 Phil. Mag. Dec. 1902. -> Phys. Zeit. 4, pp. 137, 138. 1902.

XI] AND OF ORDINARY MATERIALS 365

simple portable apparatus1 was devised by them and used for the
whole series of experiments. A large number of observations were
taken, extending over a period of twelve months. They found
that the amount of excited activity obtained was subject to great
variations. The extreme values obtained varied in the ratio of
16 to 1. No direct connection could be traced between the amount
of ionization in the atmosphere and the amount of excited activity
produced. They found that the greatest amount of excited activity
was obtained during a fog, while the amount of ionization in the
air is then small. This result, however, is not necessarily contra-
dictory to the view that the ionization and activity of the air
are to a certain extent connected. From the experiments of
Miss Brooks on the effect of dust in acting as carriers of excited
activity, it is to be expected that more excited activity would be
obtained during a fog than in clear air. The particles of water
become centres for the deposit of radio-active matter. The
positive carriers are thus anchored and are not removed from
the air by the earth's field. In a strong electric field, these
small drops will be carried to the negative electrode and manifest
their activity on the surface of the wire. On the other hand, the
distribution of water globules throughout the air causes the ions
in the air to disappear rapidly in consequence of their diffusion to
the surface of the drops (see section 31). For this reason the
denser the fog, the smaller will be the conductivity observed in
the air.

Lowering the temperature of the air had a decided influence.
The average activity observed below 0°C. was T44 times the
activity observed above 0° C. The height of the barometer was
found to exert a marked influence on the amount of excited activity
to be derived from the air. The lower the barometer the greater
was the amount of excited activity in the air. The effect of
variation of the height of the barometer is intelligible, when it is
considered that probably a large proportion of the radio-activity
observed in the air is due to the radio-active emanations- which
are continuously diffusing from the earth into the atmosphere.
Elster and Geitel have suggested that a lowering of the pressure
of the air would cause the air from the ground to be drawn up

1 Phys. Zeit. 4, p. 522, 1903.

366 KADIO- ACTIVITY OF THE ATMOSPHERE [CH.

from the capillaries of the earth into the atmosphere. This, how-
ever, need not necessarily be the case if the conditions of the escape
of the emanation into the atmosphere are altered by the variation
of the position of underground water or by a heavy fall of rain.

The amount of excited activity to be derived from the air on
the Baltic Coast was only one-third of that observed inland at
Wolfenblittel. Experiments on the radio-activity of the air in
mid-ocean would be of great importance in order to settle whether
the radio-activity observed in the air is due to the emanations
from the soil alone. It is to be expected that the radio-activity
of the air at different points of the earth would vary widely, and
would largely depend on the nature of the soil.

Some interesting experiments have been made by McLennan1
on the amount of excited radio-activity to be derived from the air
when filled with fine spray. The experiments were made at the
foot of the American Fall at Niagara. An insulated wire was
suspended near the foot of the Fall, and the amount of excited
activity on the wire compared with the amount to be obtained on
the same wire for the same exposure in Toronto. The amount of
activity obtained from the air at Toronto was generally five or six
times that obtained from the air at the Falls. In these experi-
ments it was not necessary to use an electric machine to charge
the wire negatively, for the falling spray kept the insulated wire
permanently charged to a potential of about — 7500 volts. These
results indicate that the falling spray had a negative charge and
electrified the wire. The small amount of the excited radio-
activity at the Falls was probably due to the fact that the
negatively charged drops abstracted the positively charged radio-
active carriers from the atmosphere, and in falling carried them
to the river below. On collecting the spray and evaporating it,
no active residue was obtained. Such a result is, however, to be
expected on account of the minute proportion of the spray tested
compared with that present in the air.

215. A very penetrating radiation firom the earth's
surface. McLennan2, and Rutherford and Cooke3 independently

1 Phys. Rev. 16, p. 184, 1903, and Phil. Mag. 5, p. 419, 1903.

2 Phys. Rev. No. 4, 1903. 3 Americ. Phys. Soc. Dec. 1902.

XI] AND OF ORDINARY MATERIALS 367

observed the presence of a very penetrating radiation inside build-
ings. McLennan measured the natural conductivity of the air in
a large closed metal cylinder by means of a sensitive electrometer.
The cylinder was then placed inside another and the space between
filled with water. For a thickness of water between the cylinders
of 25 cms. the conductivity of the air in the inner cylinder fell to
about 63 per cent, of its initial value. This result shows that part
of the ionization in the inner cylinder was due to a penetrating
radiation from an external source, which radiation was partially or
wholly absorbed in water.

Rutherford and Cooke observed that the rate of discharge of a
sealed brass electroscope was diminished by placing a lead screen
around the electroscope. A detailed investigation of the decrease
of the rate of discharge in the electroscope, when surrounded by
metal screens, was made later by Cooke1. A thickness of 5 cms. of
lead round the electroscope decreased the rate of discharge about
30 per cent. Further increase of the thickness of the screen had
no effect. When the apparatus was surrounded by 5 tons of pig-
lead the rate of discharge was about the same as when surrounded
by a plate about 3 cms. thick. An iron screen also diminished the
rate of discharge to about the same extent as the lead. By suitably
arranging lead screens it was found that the radiation came equally
from all directions. It was of the same intensity by night as by
day. In order to be sure that this penetrating radiation did not
arise from the presence of radio-active substances in the laboratory,
the experiments were repeated in buildings in which radio-active
substances had never been introduced, and also on the open ground
far removed from any building. In all cases a diminution of the rate
of discharge of the electroscope, when surrounded by lead screens,
was observed. These results show that a penetrating radiation is
present at the surface of the earth, arising partly from the earth
itself and partly from the atmosphere.

This result is not unexpected, when the radio-activity of the
earth and atmosphere is taken into account. The writer has
found that bodies made active by exposure to the emanations from
thorium and radium give out 7 rays. It is then to be expected
that the very similar excited radio-activity which is present in
1 Phil. Mag. Oct. 1903.

368 RADIO-ACTIVITY OF THE ATMOSPHERE [CH.

the earth and atmosphere should also give rise to 7 rays of
a similar character.

216. Comparison of the radio-activity of the atmo-
sphere with that produced by the radio-elements. The

radio-active phenomena observed in the earth and atmosphere are
very similar in character to those produced by thorium and radium.
Radio-active emanations are present in the air of caves and cellars,
in natural carbonic acid, and in deep well water, and these emana-
tions produce excited radio-activity on all bodies in contact with
them. The question now arises whether these effects are due to
known radio-elements present in the earth or to unknown kinds
of radio-active matter ? The simplest method of testing this point
is to compare the rates of decay of the radio-active products in
the atmosphere with those of the known radio-active products of
thorium and radium. A cursory examination of the facts at once
shows that the radio-activity of the atmosphere is much more
closely allied to effects produced by radium than to those due to
thorium. The activity of the emanation released from well water,
and also that sucked up from the earth, decays to half value in
about 3'3 days, while the activity of the radium emanation decays
to half value in an interval of 3'7 to 4 days. Considering the
difficulty of making accurate determinations of these quantities,
the rates of decay of the activity of the emanations from the earth
and from radium agree within the limits of experimental error.
Bumstead and Wheeler have shown that the emanation from the
soil and surface water of New Haven is identical with the radium
emanation. If the emanation from the earth is the same as that
from radium, the excited activity produced should have the same
rate of decay as that from radium. The emanation from well
water in England approximately fulfils this condition (section 212),
but an observation recorded by Ebert and Ewers (section 212)
seems to show that the excited activity due to the emanation
sucked up from the earth decays at a very slow rate compared
with that due to radium.

On comparing the rates of decay of the excited activity derived
from the atmosphere and of that produced by radium, the evidence
is to some extent conflicting. The activity of a negatively charged

XI] AND OF ORDINARY MATERIALS 369

wire, exposed in the open air, decays according to an exponential
law with the time, falling to half value in 45 minutes. On the
other hand, the activity of freshly fallen rain and snow falls to
half value in about 30 minutes. Now the activity of a wire, made
active by exposure to the radium emanation, is at first irregular,
but about an hour after removal it decays according to an expo-
nential law with the time, falling to half value in 28 minutes.
The agreement between the rates of decay of the activity of the
emanation in the air and the excited activity produced on rain and
snow, with the similar effects produced by radium, strongly sup-
ports the view that the radium emanation is present in the soil
and atmosphere. Allan1 has also obtained evidence to show that
the rate of decay of the excited activity produced on a negatively
charged wire is the resultant of the rates of decay of several types
of matter which have different rates of decay. For example, the
activity transferred from the active wire to a piece of leather
moistened with ammonia, fell to half value in 38 minutes, while on
a piece of absorbent felt treated similarly the activity fell to half
value in 60 minutes. Thus it seems probable that different types
of active matter are collected by the negatively charged wire,
which are soluble in ammonia in different degrees. An accurate
determination of the rate of decay of the excited activity from
actinium would be of interest, in order to see if the activity derived
from the air may be due in part to the presence of the actinium
emanation.

Considering the results as a whole, there is evidence that other
radio-active substances besides radium and thorium are present in
the earth. There can be little doubt, however, that part of the
radio-activity of the atmosphere is due to the radium emanation,,
which is continually diffusing into the atmosphere from the pores
of the earth. Since radio-activity has been observed in the
atmosphere at all points at which observations have, so far, been
made, there can be little doubt that radio-active matter is dis-
tributed in minute quantities throughout the soil of the earth.
The volatile emanations escape into the atmosphere by diffusion,
or are carried to the surface in spring water or by the escape of
underground gases, and cause the radio-active phenomena observed

1 Phys. Rev. 16, p. 306, 1903.
R. B.-A. 24

370 KADIO- ACTIVITY OF THE ATMOSPHERE [CH.

in the atmosphere. The observation of Elster and Geitel that the
radio-activity of the air is much less near the sea than inland is
at once explained, if the radio-activity of the atmosphere is due
mainly to the diffusion of emanations from the soil into the air
above it.

The rare gases helium and xenon which exist in the atmosphere
have been tested and found to be non-radio-active. The radio-
activity of the air cannot be ascribed to a slight radio-activity
possessed by either of these gases.

In order to account for the effect observed, it is only necessary
to suppose that the radio-active substance is present in minute
quantity mixed with the soil. Suppose, for the purpose of illustra-
tion, that the radio-activity of the atmosphere is due to the radium
emanation escaping from the earth's surface. The air sucked from
the soil in many cases shows 20 times the conductivity of ordinary
air. Now it will be shown (section 218) that the natural
conductivity of air observed in closed vessels corresponds to a
production of about 30 ions per c.c. per second. The active air of
20 times the normal conductivity thus gives rise to about 600 ions
per c.c. per second. In 100 litres of this active air the number of
ions produced per second is therefore 6 x 107. Now it has been
found that the saturation current in a sealed vessel, due to the
emanation from one gram of radium chloride, corresponds to a
current of 2'5 x 10~8 electro-magnetic units. Taking the charge of
an ion as I'l x 10~20 electro-magnetic units, this corresponds to a
production of 2*3 x 1015 ions in the gas per second. The emanation
present in 100 litres of air, of activity 20 times the normal activity,
would thus correspond to the amount released by solution of
3 x 10~8 of a gram of radium chloride. A very minute amount of
radium per cubic foot of soil would account for the radio-active
effects observed.

217. Radio-activity of ordinary materials. It has been
shown that radio-active matter seems to be distributed fairly
uniformly over the surface of the earth and in the atmosphere.
The very important question arises whether the small radio-activity
observed is due to known or unknown radio-elements present in
the earth and atmosphere, or to a feeble radio-activity of matter

XI] AND OF ORDINARY MATERIALS 371

in general, which is only readily detectable when large quantities
of matter are present. The experimental evidence is not yet
sufficient to answer this question, but undoubted proof has been
obtained that many of the metals show a very feeble radio-activity.
Whether this radio-activity is due to the presence of a slight trace
of the radio-elements or is an actual property of the metals them-
selves still remains in doubt. This point will be discussed in
more detail later in section 220.

Schuster1 has pointed out that every physical property hitherto
discovered for one element has been found to be shared by all
the others in varying degrees. For example, the property of
magnetism is most strongly marked in iron, nickel, and cobalt, but
all other substances are found to be either feebly magnetic or
diamagnetic. It might thus be expected on general principles
that all matter should exhibit the property of radio-activity in
varying degrees. On the view developed in chapter x. the
presence of this property is an indication that the matter is
undergoing change accompanied by the expulsion of charged
particles. It does not, however, by any means follow that because
the atom of one element in the course of time becomes unstable
and breaks up, that, therefore, the atoms of all the other elements
pass through similar phases of instability.

It has already been mentioned (section 8), that Mme Curie
made a very extensive examination of most of the elements and
their compounds for radio-activity. The electric method was
used, and any substance possessing an activity of 1/100 of that of
uranium would certainly have been detected. With the exception
of the known radio-elements and the minerals containing uranium
and thorium, no other substances were found to be radio-active
even to that degree.

Certain substances like phosphorus2 possess the property of
ionizing a gas under special conditions. The air which is drawn
over the phosphorus is conducting, but it has not yet been settled
whether this conductivity is due merely to ions formed at the
surface of the phosphorus or to ions produced by the phosphorus
nuclei or emanations, as they have been termed, which are carried

1 British Assoc. 1903.

2 J. J. Thomson, Conduction of Electricity through Gases, p. 324, 1903.

24—2

372 RADIO-ACTIVITY OF THE ATMOSPHERE [CH.

along with the current of air. It does not however appear that
the ionization of the gas is in any way due to the presence of a
penetrating type of radiation such as is emitted by the radio-
active bodies. Le Bon (section 8) observed that quinine sulphate,
after being heated to a temperature below the melting point and
then allowed to cool, showed for a time strong phosphorescence
and was able rapidly to discharge an electroscope. The discharging
action of quinine sulphate under varying conditions has been very
carefully examined by Miss Gates1. The ionization could not be
observed through thin aluminium foil or gold-leaf but appeared
to be confined to the surface. The current observed by an electro-
meter was found to vary with the direction of the electric field,
indicating that the positive and negative ions had very different
mobilities. The discharging action appears to be due either to an
ionization of the gas very close to the surface by some short ultra-
violet light waves, accompanying the phosphorescence, or to a
chemical action taking place at the surface.

Thus, neither phosphorus nor quinine sulphate can be con-
sidered to be radio-active, even under the special conditions when
they are able to discharge an electrified body. No evidence in
either case has been found that the ionization is due to the
emission of a penetrating radiation.

No certain evidence has yet been obtained that any body can
be made radio-active by exposure to Rontgen or cathode rays.
A metal exposed to the action of Rontgen rays gives rise to a
secondary radiation which is very readily absorbed in a few
centimetres of air. It is possible that this secondary radiation
may prove to be analogous in some respects to the a rays from
the radio-elements. The secondary radiation, however, ceases
immediately the Rontgen rays are cut off. Villard2 observed that
a piece of bismuth produced a feeble photographic action after it
had been exposed for some time to the action of the cathode
rays in a vacuum. It has not however been shown that the
bismuth gives out rays of a character similar to those of the
radio-active bodies.

The existence of a very feeble radio-activity of ordinary matter

1 Amer. Phys. Soc. Oct. 1903.

2 Societe de Physique, July, 1900.

Xl] AXD OF ORDINARY MATERIALS 373

has been deduced from the study of the conductivity of gases in
closed vessels. The conductivity is extremely minute, and special
methods are required to determine it with accuracy. A brief
account will now be given of the gradual growth of our knowledge
on this important question.

218. Conductivity of air in closed vessels. Since the
time of Coulomb onwards several investigators have believed that
a charged conductor placed inside a closed vessel lost its charge
more rapidly than could be explained by the conduction leak
across the insulating support. Matte ucci, as early as 1850, observed
that the rate of loss of charge wras independent of the potential.
Boys, by using quartz insulators of different lengths and diameters,
arrived at the conclusion that the leakage must in part take place
through the air. This loss of charge in a closed vessel was believed
to be due in some way to the presence of dust particles in the air.

On the discovery that gases became temporary conductors of
electricity under the influence of Rontgen rays and the rays from
radio-active substances, attention was again drawn to this question.
Geitel1 and C. T. R. Wilson- independently attacked the problem
and both came to the conclusion that the loss of charge was due
to a constant ionization of the air in the closed vessel. Geitel
employed in his experiments an apparatus similar to that shown
in Fig. 59. The loss of charge of an Exner electroscope, with the
cylinder of wire netting Z attached, was observed in a closed vessel
containing about 30 litres of air. The electroscope system was
found to diminish in potential at the rate of about 40 volts per
hour, and this leakage was shown not to be due to a want of
insulation of the supports.

Wilson, on the other hand, used a vessel of very small volume,
in order to work with air which could be completely freed from
dust. In the first experiments a silvered glass vessel with a
volume of only 163 c.c. was employed. The experimental arrange-
ment is shown in Fig. 60.

The conductor, of which the loss of charge was to be measured,
was placed near the centre of the vessel A. It consisted of a

1 Phys. Zeit. 2, p. 116, 1900.

2 Proc. Camb. Phil. Soc. 11, p. 52, 1900. Proc. Boy. Soc. 68, p. 152, 1901.

374

KADIO-ACTIVITY OF THE ATMOSPHERE

[CH.

narrow strip of metal with a gold-leaf attached. The strip of
metal was fixed to the upper rod by means of a small sulphur bead.
The upper rod was connected to a sulphur condenser with an
Exner electroscope B attached to indicate its potential. The
gold-leaf system was initially charged to the same potential as
the upper rod and condenser by means of a fine steel wire which
was caused to touch the gold-leaf system by the attraction of a
magnet brought near it. The rate of movement of the gold-leaf

Earth

Fig. 60.

was measured by means of a microscope provided with a micro-
meter eye-piece. By keeping the upper rod at a slightly higher
potential than the gold-leaf system, it was ensured that the loss
of charge of the gold-leaf system was not in any way due to a
conduction leakage across the sulphur bead.

The method employed by Wilson in these experiments is
very certain and convenient when an extremely small rate of
discharge is to be observed. In this respect the electroscope is
able to measure with certainty a rate of loss of charge much
smaller than can be measured by a sensitive electrometer.

XI] AND OF ORDINARY MATERIALS 375

Both Geitel and Wilson found that the leakage of the insulated
system in dust-free air was the same for a positive as for a negative
charge, and was independent of the potential over a considerable
range. The leakage was the same in the dark as in diffuse
daylight. The independence of leakage of the potential is strong
evidence that the loss of charge is due to a constant ionization of
the air. When the electric field acting on the gas exceeds a
certain value all the ions are carried to the electrodes before re-
combination occurs. A saturation current is reached, and it will
be independent of further increase of the electric field, provided,
of course, a potential sufficiently high to cause a spark to pass is
not applied.

C. T. R. Wilson has recently devised a striking experiment to
show the presence of ions in dust-free air which is not exposed to
any external ionizing agency. Two large metal plates are placed
in a glass vessel connected to an expansion apparatus similar to
that described in section 34. On expanding the air the presence
of the ions is shown by the appearance of a slight cloud between
the plates. These condensation nuclei carry an electric charge
and are apparently similar in all respects to the ions produced
in gases by X rays or by the rays from active substances.

Wilson found that the loss of charge of the insulated system
was independent of the locality. The rate of discharge was un-
altered when the apparatus was placed in a deep tunnel, so that
it did not appear that the loss of charge was due to an external
radiation. From experiments already described, however (section
215), it is probable that about 30 per cent, of the rate of discharge
observed was due to a very penetrating radiation. This experiment
of Wilson's indicates that the intensity of the penetrating radiation
was the same in the tunnel as at the earth's surface. Wilson
found that the ionization of the air was about the same in a brass
vessel as in one of glass, and came to the conclusion that the
air was spontaneously ionized.

Using a brass vessel of volume about 471 c.c., Wilson de-
termined the number of ions that must be produced in air
per unit volume per second, in order to account for the loss of
charge of the insulated system. The leakage system was found
to have a capacity of about I'l electrostatic units, and lost its

376

RADIO-ACTIVITY OF THE ATMOSPHERE

[CH.

charge at the rate of 4*1 volts per hour for a potential of 210 volts,
and 4'0 volts per hour for a potential of 120 volts. Taking the
charge on an ion as 3*4 x 10~10 electrostatic units, this corresponds
to a production of 26 ions per second.

Rutherford and Allan1 repeated the results of Geitel and
Wilson, using an electrometer method. The saturation current
was observed between two concentric zinc cylinders of diameter
25'5 and 7*5 cms. respectively and length 154 cms. It was found
that the saturation current could practically be obtained with a
potential of a few volts. Saturation was however obtained with
a lower voltage after the air had remained undisturbed in the
cylinders for several days. This was probably due to the gradual
settling of the dust originally present in the air.

Later observations of the number of ions produced in air in
sealed vessels have been made by Patterson2, by Harms3, and by
Cooke4. The results obtained by different observers are shown
in the following table. The value of the charge on an ion is taken
as 3*4 x 10~10 electrostatic units :

Number of ions

Material of vessel

produced per c.c.

Observer

per second

Silvered glass . .

C. T. R Wilson

Brass

Zinc

Rutherford and Allan

Glass

53 to 63

Harms

Iron

Patterson

Cleaned brass . .

Cooke

It will be shown later that the differences in these results are
probably due to differences in the radio-activity of the containing
vessel.

219. Effect of pressure and nature of gas. C. T. R. Wilson
(loc. cit.) found that the rate of leakage of a charged conductor
varied approximately as the pressure of the air between the pres-
sures examined, viz. 43 mms. and 743 mms. of mercury. These
results point to the conclusion that, in a good vacuum, a charged

1 Phil. Mag. Dec. 1902. . 2 PhiL Mag^ August, 1903.

3 Phys. Zeit. 4, No. 1, p. 11, 1902 4 PhiL Magm Oct 1903

XI]

AND OF ORDINARY MATERIALS

377

body would lose its charge extremely slowly. This is in agreement
with an observation of Crookes, who found that a pair of gold-
leaves retained their charge for several months in a high vacuum.
Wilson1 at a later date investigated the leakage for different
gases. The results are included in the following table, where the
ionization produced in air is taken as unity:

(TOO

Relative

Relative ionization

ionization

density

Air

1-00

Hydrogen . .

0-184

2-7

Carbon dioxide . .

1-69

Sulphur dioxide. .

2-64

1-21

Chloroform . .

4-7

1-09

With the exception of hydrogen, the ionization produced in
different gases is approximately proportional to the density. The
relative ionization is very similar to that observed by Strutt
(section 45) for gases exposed to the influence of the a and @ rays
from radio-active substances, and points to the conclusion that the
ionization observed may be due either to a radiation from the
walls of the vessel or from external sources.

Patterson2 examined the variation of the ionization of air
with pressure in a large iron vessel of diameter 30 cms. and length
20 cms. The current between a central electrode and the cylinder
was measured by means of a sensitive Dolezalek electrometer.
He found that the saturation current was practically independent
of the pressure for pressures greater than 300 mms. of mercury.
Below a pressure of 80 mms. the current varied directly as the
pressure. For air at atmospheric pressure, the current was inde-
pendent of the temperature up to 450° C. With further increase
of temperature, the current began to increase, and the increase
was more rapid when the central electrode was charged negatively
than when it was charged positively. This difference was ascribed
to the production of positive ions at the surface of the iron vessel.
The results obtained by Patterson render it very improbable that
the ionization observed in air is due to a spontaneous ionization of
the enclosed air: for it would be expected that the amount of
this ionization would depend on the temperature of the gas. On
1 Proc. Roy. Soc. 69, p. 277, 1901. 2 Phil. Hag. Aug. 1903.

378

RADIO-ACTIVITY OF THE ATMOSPHERE

[CH.

the other hand, the results are to be expected if the ionization
of the enclosed air is mainly due to an easily absorbed radiation
from the walls of the vessel. If this radiation had a penetrating
power about equal to that observed for the a rays of the radio-
elements, the radiation would be absorbed in a few centimetres of
air. With diminution of pressure, the radiations would traverse
a greater distance of air before complete absorption, but the total
ionization produced by the rays would still remain about the same,
until the pressure was reduced sufficiently to allow the radiation
to traverse the air space in the vessel without complete absorption.
With still further diminution of pressure, the total ionization
produced by the radiation, and in consequence the current observed,
will vary directly as the pressure.

220. Examination of ordinary matter for radio-activity.

Strutt1, McLennan and Burton2, and Cooke3, independently ob-
served about the same time that ordinary matter is radio-active
to a slight degree. Strutt, by means of an electroscope, observed
that the ionization produced in a closed vessel varied with the
material of the vessel. A glass vessel with a removeable base
was employed and the vessel was lined with the material to be
examined. The following table shows the relative results obtained.
The amount of leakage observed is expressed in terms of the
number of scale divisions of the eye-piece passed over per hour
by the gold-leaf:

Material of lining of vessel

Leakage in scale
divisions per hour

Tinfoil

3-3

„ another sample
Glass coated with phosphoric acid
Silver chemically deposited on glass
Zinc ...

2-3
1-3

1-6
1*2

Lead

2 -2

Copper (clean)
„ (oxidized)
Platinum (various samples)
Aluminium ...

2-3
1-7
2-0, 2-9, 3-9
1*4

1 Phil. Mag. June, 1903. Nature, Feb. 19, 1903.

2 Phys. Rev. No. 4, 1903. J. J. Thomson, Nature, Feb. 26, 1903.

3 Phil. Mag. Aug. 6, 1903. Eutherford, Nature, April 2, 1903.

XI] AND OF ORDINARY MATERIALS 379

There are thus marked differences in the leakage observed for
different materials and also considerable differences in different
samples of the same metal. For example, one specimen of platinum
caused nearly twice the leakage of another sample from a different
stock.

McLennan and Burton, on the other hand, measured by means
of a sensitive electrometer the ionization current produced in the
air in a closed iron cylinder 25 cms. in diameter and 130 cms. in
length, in which an insulated central electrode was placed. The
open cylinder was first exposed for some time at the open window
of the laboratory. It was then removed, the top and bottom
closed, and the saturation current through the gas determined as
soon as possible. In all cases it was observed that the current
diminished for two or three hours to a minimum and then very
slowly increased again. In one experiment, for example, the initial
current observed corresponded to 30 on an arbitrary scale. In the
course of four hours the current fell to a minimum of 6'6, and
44 hours later had risen to a practical maximum of 24. The
initial decrease observed is probably due to a radio-activity of
the enclosed air or walls of the vessel, which decayed rapidly
with the time. The decay of the excited activity produced on
the interior surface of the cylinder when exposed to the air was
probably responsible for a part of the decrease observed. McLennan
ascribes the increase of current with time to a radio-active ema-
nation which is given off from the cylinder, and ionizes the enclosed
air. On placing linings of lead, tin, and zinc in the iron cylinder,
considerable differences were observed both of the minimum current
and also of the final maximum. Lead gave about twice the cur-
rent due to zinc, while tin gave an intermediate value. These
results are similar in character to those obtained by Strutt.

McLennan and Burton also investigated the effect of dimi-
nution of pressure on the current. The cylinder was filled with
air to a pressure of 7 atmospheres, and allowed to stand until
the current reached a constant value. The air was then allowed
to escape and the pressure reduced to 44 mms. of mercury. The
current was found to vary approximately as the pressure over the
whole range. These results are not in agreement with the results
of Patterson already described, nor with some later experiments

380 KADIO-ACTIVITY OF THE ATMOSPHERE [CH.

of Strutt. McLennan's results however point to the conclusion
that the ionization was mainly due to an emanation emitted from
the metal. Since the air was rapidly removed, a proportionate
amount of the emanation would be removed also, and it might
thus be expected that the current would vary directly as the
pressure. If this is the case the current through the gas at low
pressures should increase again to a maximum if time is allowed
for a fresh emanation to form.

H. L. Cooke, using an electroscopic method, obtained results
very similar to those given by Strutt. Cooke observed that a pene-
trating radiation was given out from brick. When «-a brass vessel
containing the gold-leaf system was surrounded by brick, the
discharge of the electroscope was increased by 40 to 50 per cent.
This radiation was of about the same penetrating power as the
rays from radio-active substances. The rays were completely
absorbed by surrounding the electroscope by a sheet of lead 2 mms.
in thickness. This result is in agreement with the observation
of Elster and Geitel, already mentioned, that radio-active matter
was present in clay freshly dug up from the earth.

Cooke also observed that the ionization of the air in a brass
electroscope could be reduced to about one-third of its usual
value if the interior surface of the brass was carefully cleaned. By
removing the interior surface of the brass he was able to reduce
the ionization of the enclosed air from 30 to 10 ions per c.c. per
second. This is an important observation, and indicates that a
large proportion of the radio-activity observed in ordinary matter
is (fiie to a deposit of radio-active matter on its surface. It has
already been shown (sections 173 and 188) that bodies which
have been exposed in the presence of the radium emanation
retain a residual activity which decays extremely slowly. There
can be no doubt that the radium emanation is present in the
atmosphere, and the exposed surface of matter, in consequence,
will become coated with an invisible film of radio-active matter,
deposited from the atmosphere. On account of the slow decay of
this activity it is probable that the activity of matter exposed in
the open air would steadily increase for a long interval. Metals,
even if they are originally inactive, would thus acquire a fairly
permanent activity, but it should be possible to get rid of this

Xl] AND OF ORDINARY MATERIALS 381

by removing the surface of the metal or by chemical treat-
ment.

It must be borne in mind that the activity observed in ordinary
matter is excessively minute. The lowest rate of production of
ions yet observed is 10 per cubic centimetre per second in a
brass vessel. Suppose a spherical brass vessel is taken of capacity
1 litre. The area of the interior surface would be about 480 sq.
cms. and the total number of ions produced per second would be
about 104. Now it has been shown in section 104 that an a
particle projected from radium probably gives rise to 7 x 104 ions
before it is absorbed in the gas. An expulsion of one a particle
every 7 seconds from the whole vessel, or of one a particle from
each square centimetre of surface per hour would thus account for
the minute conductivity observed. Even if it were supposed that
this activity is the result of a breaking up of the matter com-
posing the vessel, the disintegration of one atom per second per
gram, provided it was accompanied by the expulsion of an a
particle, would fully account for the conductivity observed.

Strutt1 has recently observed that a radio-active emanation can
be obtained by bubbling air through mercury. The emanation
appears to be very similar in character to that emitted by radium
emanation, for its activity decays to half value in 318 days and
the excited activity to half value in 20 minutes. An emanation
was also obtained by drawing air over red-hot copper.

Bumstead and Wheeler2 have repeated the experiments of
Strutt of bubbling air through mercury, but were unable to
detect any increase of the conductivity of air, which had been
circulated through hot mercury for fourteen hours, although an
increase of 10 per cent, of the natural conductivity could have
been detected. These results indicate that the emanation from
mercury obtained in the experiments of Strutt was probably due
to the presence of a minute amount of radium as an impurity.

There is not yet sufficient evidence to decide with certainty
whether ordinary matter possesses the property of radio-activity.
There is no doubt that, if matter possesses the property at all,
it does so to a minute extent. The extreme minuteness of the

1 Phil Mag. July, 1903.

2 Amer. Jour. Science, 17, p. 110, Feb. 1904.

382 RADIO-ACTIVITY OF THE ATMOSPHERE, ETC. [CH. XI

radio-activity observed, and the distribution of radio-active matter
throughout the constituents of the earth, render it difficult to be
certain that any substance, however carefully prepared, is freed
from possible radio-active impurities. A careful comparison of
the rates of decay of the activity of the emanations obtained from
ordinary matter, and of the excited activity, with the corresponding
rates of decay of the activity of the products of the known radio-
active substances, may throw some light on the question.

or THE
UNIVERSITY

INDEX.

The numbers refer to the pages.

a rays

discovery of, 115 et seq.

nature of, 115 et seq.

magnetic deviation of, 117 et seq.

electrostatic deviation of, 121

velocity of, 122 et seq.

value of e/m for, 122

mass and energy of, 125 et seq.

origin of, in atomic disintegration, 126

scintillations produced by, 127

absorption of, by matter, 129 et seq.

increase of absorption with thickness
of matter traversed, 129 et seq.

relative absorption of a rays from
radio -elements, 132

absorption of, by gases, 133

connection between absorption and
density, 137

relation between ionization and ab-
sorption, 138

theory of absorption 'of, 138

effect of thickness of layer of radiating
matter on emission of, 149

relative ionization produced by a and
/3 rays, 149

relative energy emitted in form of a
and ft rays, 150 et seq.

number and energy of a particles
from radio-elements, 154

emission of energy from radio-ele-
ments in form of a rays, 154

connection of heat emission of radium
with a rays, 160 et seq.

connection of, with radio-active
changes, 193, 322

from the emanations, 222

absence of, in change in emanation
X of thorium, 270 et seq.

absence of, in change in emanation
X of radium, 273

non-separable activity of radio-ele-
ments consists of, 304

emission from all active products
except last change, 304

a particles consist of helium, 331

loss of weight due to expulsion of, 335

Abraham

apparent mass of moving charged

body, 109
Absorption

law of, .in gases, 56 et seq.

relative absorption of a, ft and 7 rays

by matter, 93

of ft rays by solids, 112 et seq.
of a rays by solids, 129 et seq.
of 7 rays by solids, 142
connection between absorption and

density for ft rays, 113 et seq.
connection between absorption and

density for a rays, 137
connection between absorption and

density for 7 rays, 143
of ft rays in radio-active matter, 115
of a rays in gases, 133 et seq.
connection between absorption and

ionization, 138
theory of, 138
of a rays by radium, 164
of rays from the emanations, 222
of penetrating rays from the earth, 367
Actinium

methods of separation of, 22 et seq.

properties of, 23

similarity to "emanating substance"

of Giesel, 24
possible connection with radio-activity

of thorium, 25
emanation from, 208
excited activity produced by, 288
effect of magnetic field on excited

activity from, 288
Adams

decay of activity of emanation from

well water, 361
decay of excited activity from the

emanation, 361
Age

of radium, 333
of sun and earth, 343
Allan

increase with time of excited activity

from atmosphere, 355

384

INDEX

Allan (cont.)

radio-activity of snow, 356
effect of conditions on decay of ac-
tivity from air, 369
Allan and Eutherford

decay of excited activity from atmo-
sphere, 353

ionization of air in closed vessels, 376
Anderson and Hardy

action of radium rays on eye, 177
Armstrong and Lowry

radio-activity and phosphorescence,

323
Arnold

rays from phosphorescent substances,

4
Aschkinass and Caspar!

action of radium rays on eye, 177
Atmosphere

excited radio-activity from, 351 et seq.
radio-activity of, due to emanations,

354
diffusion of emanations into, from

earth, 357
effect of temperature, pressure, &c. on

radio-activity of, 364
presence of very penetrating radiation

in, 366
comparison of radio-activity of, with

radio-elements, 368
Atom

number of, per c.c., 51

complex nature of, 126

disintegration of, 126

number of, transformed per second,

332

changing atoms, 322 et seq.
possible causes of disintegration of,

338 et seq.

evolution of, 347 et seq.
Atomic weight

of radium by chemical methods, 17
from spectroscopic evidence, 18
of emanations, 232
of radio-elements and connection with
radio-activity, 323 et seq.

B rays

discovery of, 95

magnetic deflection of, 95 et seq.
complexity of, 98 et seq.
examination by the electrical method,

100

effect of, on a fluorescent screen, 101
charge carried by the, 102 et seq.
electrostatic deviation of, 106
velocity of, and value of elm for, 106

et seq.
variation of ejm with velocity of, 108

et seq.

j8 rays (cont.)

absorption of, 112

variation of absorption with density,
113 et seq.

variation of intensity of, with thick-
ness of layer, 115

relative ionizatiou produced by a and
/3 rays, 149

relative energy emitted in form of
a and /3 rays, 150 et seq.

phosphorescent action of, 166

physical action produced by, 171 et seq.

chemical action of, 174

physiological action of, 176

from UrX, 293

from emanation X of radium, 302

appearance of, only in last of radio-
active changes, 304

change of weight due to expulsion of,

336
Barium platinocyanide

phosphorescence of, under radium
rays, 167

change of colour due to radium rays,

168
Barnes and Eutherford

heating effect of radium emanation,
161

connection of heating effect with radio-
activity, 160 et seq.

heating effect of emanation, 247

heating effect of excited activity, 279

division of heating effect among active

products, 279
Baskerville

activity of thorium, 25

phosphorescence of kunzite under ra-
dium rays, 168

phosphorescence produced by radium

rays, 168
Beattie, Smolan and Kelvin

discharging power of uranium rays, 7
Becquerel

rays from calcium sulphide, 4

rays from uranium, 5 et seq.

permanence of uranium rays, 6

discharging power of uranium rays, 6

magnetic deflection of radium rays by
photographic method, 96

curvature of radium rays in a mag-
netic field, 96 et seq.

complexity of radium rays, 98 et seq.

electrostatic deflection of /3 rays of
radium, 106

value of elm for 8 rays of radium,
107

magnetic deviation of a rays of radium
and polonium, 120

trajectory of rays of radium in mag-
netic field, 123 et seq.

INDEX

385

Becquerel (con*.)

scintillations due to cleavage of crys-
tals, 128

7 rays from radium, 141
secondary rays produced by active

substances, 146
phosphorescence produced by radium

rays, 166
conductivity of paraffin under radium

radiation, 173
effect of temperature' on uranium rays,

173

chemical action of radium rays, 175
removal of activity from uranium by

precipitation with barium, 179
recovery of activity of uranium after

precipitation with barium, 179
penetrating rays from polonium, 305
theory of radio-activity, 317
Bemont et M. et Mme Curie

discovery of radium, 13
Benoist

variation of absorption of Rontgen

rays in matter, 144
Berndt

spectrum of polonium, 20
Bodlander and Runge

evolution of gases from radium, 176
Boys

rate of dissipation of charge, 373
Brooks, Miss

variation of excited activity from

thorium for short exposures, 260
effect of dust on distribution of ex-
cited activity, 260
Brooks and Rutherford

absorption of a rays by matter, 129
comparison of absorption of a rays

from radio-elements. 132 et seq.
diffusion of radium emanation, 228
decay of excited activity from radium,

261
Bumstead and Wheeler

emanation from surface water and the

soil, 361, 368
identity of emanation from soil with

radium emanation, 361
absence of emanation in mercury, 381
Burton and McLennan

penetrating radiation from the earth,

366
radio-activity of ordinary materials,

378
emanation from ordinary matter, 379

Canal rays

similarity of to a rays, 92
Capacity

of electroscopes, 72

of electrometers, 79, 85

R. E.-A.

Capacity (cont.)

standards of, 86
Carbonic acid

radio-activity of natural, 364
Caspari and Aschkinass

action of radium rays on eye, 177
Cathode rays

comparison of, with p rays, 102 et seq.
absorption of, by matter, 113
see also £ rays, 95 et seq. '
Caves

radio-active matter present in air of,

357

radio-activity of air of, due to emana-
tion from the soil, 357
Charge

carried by the ions, 47 et seq.
negative charge carried by /S rays, 102

et seq.
measurement of charge carried by /3

rays, 104 et seq.

positive charge carried by a rays, 120
Chemical nature
of emanation, 225
of emanation X, 275
Chemical actions of radium rays
production of ozone, 174
coloration of glass and rock salt, 174
on phosphorus, 175
on iodoform, 175
on globulin, 175
evolution of hydrogen and oxygen,

176*
Child

potential gradient between electrodes,

63
variation of current with voltage for

surface ionization, 64
Clouds
formation by condensation of water

round ions, 43 et seq.
difference between positive and nega-
tive ions in formation of, 46
Collision

ionization by, 54

number of ions produced by £ rays

per cm. of path, 139
total number of ions produced by

collisions of a particles, 156
Coloration

of crystals of radiferous barium, 15
of bunsen flame by radium, 17
of glass by radium rays, 174
of rock salt, fluor spar' and potassium

sulphate by radium rays, 174
Concentration

of excited activity on negative elec-
trode, 252

activity of radium independent of,
339

386

INDEX

Condensation

of water round the ions, 43 et seq.
of emanations, 236 et seq.
experimental illustration of, 237
temperature of, 238
difference between point of, for eman-
ations of thorium and radium, 243
from air sucked up from the earth,

357
Conductivity

of gases exposed to radiations, 28

et seq.

variation of, with pressure, 58 et seq.
variation of, with nature of gas, 61
comparison of, for gases exposed to

a, /3 and 7 rays, 61 et seq.
of insulators, 172
of air in closed vessels, 351, 373
increase of, with time, in a closed

vessel, 351

of air in caves and cellars, 357
variation of in closed vessels with

pressure and nature of gas, 376
variation of with temperature for air

in closed vessels, 377
Conservation of radio-activity

examples of, 308
Cooke, H. L.

penetrating rays from the earth, 366
number of ions per c.c. in closed

vessels, 376
radio-activity from ordinary matter,

378
Corpuscle

(see Electron)
Crookes (Sir W.)
spectrum of polonium, 20
separation of Ur X, 178
nature of a rays, 116
scintillations produced by radium, 127
spinthariscope, 127
number of scintillations independent

of pressure and temperature, 128
theory of radio-activity, 319
cathode rays, 347
Crookes and Dewar

absence of nitrogen spectrum in phos-
phorescent light of radium at low
pressures, 169
Curie, Mme

permanence of uranium rays, 6
discovery of radio-activity of thorium,

10
radio-activity of uranium and thorium

minerals, 11

relative activity of compounds of ura-
nium, 12

coloration of radium crystals, 15
spectrum of radium, 15
nature of a rays, 116

Curie, Mme (cont.)

absorption of a rays from polonium,

131

secondary radiation tested by electric
method, 148

slowly decaying excited activity from
radium, 264

bismuth made active by solution of
barium, 289

recovery of activity of radium, 303
Curie, P.

magnetic deviation of radium rays by
electric method, 96

heat emission of radium at low tem-
perature and variation of heat emis-
sion with age of radium, 159, 160

conductivity of dielectrics under
radium rays, 172

radio-activity of radium unaffected by
temperature, 173

decay of activity of radium emana-
tion, 206 et seq.

discovery of excited radio-activity
from radium, 250

nature of the emanation, 318
Curie, M. et Mme

discovery of radium, 13

charge carried by radium rays, 102

luminosity of radium compounds, 168

production of ozona by radium rays,
174

coloration of glass by radium rays, 174

theory of radio-activity, 318

possible absorption by radio-elements

of unknown radiations, 321
Curie, J. et P.

quartz piezo-electrique, 87 et seq.
Curie, P. et Danne

diffusion of radio-active emanation,
231

decay of excited activity from radium,
262

occlusion of radium emanation in

solids, 264
Curie, P. and Debierne

active gases evolved from radium, 210

phosphorescence produced by radium
emanation, 211

distribution of luminosity, 211

rate of production of emanation inde-
pendent of pressure, 224

effect of pressure on amount of ex-
cited activity, 282
Curie, P. and Laborde

heat emission of radium, 158

origin of heat from radium, 319
Current

through gases, 28 et seq.

variation of with distance between
the plates, 56 et seq.

INDEX

387

Current (cont.)

variation of with pressure of gas,

58 et seq.

variation of, with nature of gas, 61
measurement of, by galvanometer, 69
measurement of, by electroscope, 70

et seq.

measurement of, by electrometer, 84
measurement of, by quartz piezo-

electrique, 87
magnitude of, for one gram of radium,

156

Danne and Curie, P.

diffusion of radium emanation, 231
decay of excited activity from radium,

262
occlusion of radium emanation in

solids, 264
Danysz

action of radium rays on skin, 177
Darwin, G. H.

age of sun, 343
Debierne

actinium, 22

emanation from actinium, 208

effect of magnetic field on activity

excited from actinium, 288
barium made active by actinium, 289
Debierne and Curie

evolution of gas from radium, 175
active gas evolved from radium, 210
phosphorescence produced by radium

emanation, 211

distribution of luminosity, 211
rate of production of emanation inde-
pendent of pressure, 224
effect of pressure on excited activity,

282
Decay

of heating effect of emanation, 162
of activity of Th X, 180
of activity of UrX, 182
significance of law of, 189
effect of conditions on the rate of, 190
of activity of thorium emanation, 200
of activity of radium emanation, 206
excited activity due to thorium for

long exposure, 256
excited activity due to thorium for

short exposure, 258
excited activity due to radium, 261
excited activity of slow decay due to

radium, 264, 291
excited activity from atmosphere, 351

et seq.

of activity of rain and snow, 356
of emanation from earth, 358
differences in, of excited activity from

atmosphere, 368

Deflection

of rays in a magnetic field, 92 -
of /3 rays in a magnetic field, 95 et seq.
of /3 rays in an electrostatic field, 106
of a rays in a magnetic field, 117 et seq.
of a rays in an electrostatic field, 121
of "ions activants" in a magnetic

field, 288
Demarcay

spectrum of radium, 16
Des Coudres

magnetic and electric deviation of a

rays of radium, 122
determination of elm for a rays, 122
velocity of cathode rays diminished in

passage through matter, 139>
Dewar

emission of heat from radium in liquid

hydrogen, 160
Dewar and Crookes

absence of nitrogen spectrum in phos-
phorescent light of radium at low
pressures, 169
Dielectrics

conduction of, under radium rays, 172
Diffusion
of ions, 49
of radium emanation into gases, 228

et seq.

of thorium emanation into gases, 233
of radium emanation into liquids, 235
Discharge

action of rays on spark and electrode-
less, 171
Disintegration

account of theory of, 126, 193, 323 et

seq.

list of products of, 326
helium a product of, 327 et seq.
rate of, in radio-elements, 332 et seq.
emission of energy in consequence of,

336 et seq.

possible causes of, 338 et seq.
of matter in general, 347 et seq.
Dissipation of charge
in closed vessels, 351, 373
in caves and cellars, 357
effect of pressure and nature of gas

on, 376 et seq.

effect of material of vessel on, 378 et seq .
Dolezalek

electrometer, construction of, 78 et seq.
Dorn

charge carried by radium rays, 104
electrostatic deflection of ft rays from

radium, 106

discovery of radium emanation, 205
effect of moisture on emanating power

of thorium, 214
electrolysis of radium solution, 276

25—2

388

INDEX

Dorn (cont.)

loss of weight of radium, 336
Durack

ionization by collision of electrons of

great velocity, 139
Dust

effect of, in recombination of ions, 39
effect of, on distribution of excited
activity, 260

Earth

amount of radium in, 344

age of, 344

excited activity deposited on, 354

activity concentrated on peaks of, 354

emanation from, 363

very penetrating radiation from, 366
Ebert

condensation of emanation from the

earth, 359
Ebert and Ewers

emanation from the earth, 358
Electric field

deflection of j8 rays by, 106

deflection of a rays by, 121

movement in, of carriers of excited
activity, 282 et seq.

action on, of carriers of excited ac-
tivity from "emanating substance,"
287
Electrolysis

separation of radio-tellurium by, 21

of solutions of emanation X, 276

of thorium solutions, 277

of radium solutions, 276
Electrometer

description of, 74 et seq.

use of, in measurements, 74

construction of, 76 et seq.

Dolezalek, 78

adjustment and screening of, 79

special key for, 81

application of, to measurements of
radio-activity, 81 et seq.

measurement of current by, 84

capacity of, 85

use with quartz piezo-electrique, 87
Electron

definition of, 53

identity of /3 rays with electrons, 102
et seq., 107

variation of apparent mass of electron
with velocity, 108 et seq.

evidence that mass of electron is
electro-magnetic, 112

diameter of, 112

production of, under different condi-
tions, 347
Electroscope

description of, used by Curie, 70

Electroscope (cont.)

construction of, for accurate measure-
ments, 71
use of, in measurements of minute

currents, 71
of C. T. E. Wilson, 73
use of, in measuring conductivity of

air in closed vessels, 373 et seq.
use of, for determining radio-activity

of ordinary matter, 378, 380
Elster and Geitel
radio-active lead, 25
effect of magnetic field on conductivity

produced in air by radium rays, 95
scintillations produced by active sub-
stances, 127

action of radium rays on spark, 171
photo-electric action of body, coloured

by radium rays, 174
radio-active matter in earth, 345
discovery of excited activity in atmo-
sphere, 351

emanations from the earth, 357
radio-activity of air in caves, 362
radio-activity of the earth, 362
radio-activity of natural carbonic acid,

364

variation of radio-activity in atmo-
sphere with meteorological condi-
tions, 364 et seq.
effect of temperature and pressure on

atmospheric radio-activity, 365
Emanation

«* rate of heat emission by, 158
A variation of heat emission with time,

160
of thorium, discovery and properties,

197

methods of measurement of, 199
decay of activity of, 200
effect of thickness of layer, on

amount of, 202

increase of, with time, to a maxi-
mum, 204
of radium, 205

decay of activity of, 206
of actinium, properties of, 208
of radium, phosphorescence produced

by, 209 et seq.
rate of emission of, 213
effect of conditions, on rate of emis-
sion of, 214

regeneration of emanating power, 215
continuous rate of production of, 216

et seq.

source of thorium emanation, 220
source of radium emanation, 222
radiations from, 222
effect of pressure on production of, 224
chemical nature of, 225

INDEX

389

Emanation (cont.)

experiments to illustrate gaseous na-
ture of, 227

rate of diffusion of radium emanation,
228 et seq.

rate of diffusion of thorium emana-
tion, 233

diffusion of, into liquids, 235

condensation of, 236 et seq.

temperature of condensation of, 238
et seq.

volume of, from 1 gram of radium,
and thorium, 246

heat emission of, 247

connection between emanations and
excited activity, 253 et seq.

effect of removal of, on activity of
radium, 300

effect of rate of escape of, on activity
of radium, 302

fraction of activity of radium, due to,
304

experimental separation, and volume
of, 311 et seq.

decrease of volume of, with time, 313

radio-activity of atmosphere, due to
emanations, 354

sucked up from the earth, 357

rate of decay of activity of, from the
earth, 358

condensation of, from atmosphere, 359

in caves, 357

in well-water, and springs, 360

in natural carbonic acid gas, 364

from " fango," 363

effect of meteorological conditions on
amount of, in atmosphere, 364

from metals, 378
Emanation X (see Excited radio-activity)

definition of, 256

chemical and physical properties of,
275

electrolysis of, 276

effect of temperature on, 277

emission of heat by, 278

transmission of, to negative elec-
trode, 282 et seq.

irregularities in decay of activity of
ThX due to, 295

removal of, by successive precipita-
tions, 296

theory of effect of production of, on

activity of ThX, 295 et seq.
Emanating power

measurement of, 213

effect of conditions on, 214

regeneration of, 215
" Emanating substance " of Giesel

separation and properties of, 23 et seq.

similarity of, to actinium, 24

"Emanating substance" of Giesel (cont.)

emanation from, 209

excited activity produced by, 287

action of an electric field on, 287
Energy

of a particle, 125

of /9 particle, 151

comparison of, for a and /3 particles,
150

emitted from radio-elements, in form
of a rays, 154

emitted from radium in form of heat,
158 et seq.

emission of, from the emanation, 161
et seq., 247

emission of, from radio-active pro-
ducts of radium, 278

total emission of, from 1 gram of
radio-elements, 336

latent store of, in matter, 337
Eve

conductivity of gases for very pene-
trating Rontgen rays, 145
Evolution of matter

evidence of, 348
Ewers and Ebert

emanation from the earth, 358
Excited radio-activity

heat emission due to, 161 et seq.

discovery and properties of, 250

concentration of, on negative elec-
trode, 252

connection of, with the emanations,
253

removal of, by acids, 255

decay of, due to thorium, 256

decay of, for short exposurgTo thorium,
259

effect of dust on distribution of, 260

decay of, from radium, 261

of radium, of very slow decay, 264

connection between decay curves of,
and times of exposure, 265

theory of successive changes to give
rise to, 268

changes in emanation X of thorium,
270 et seq.

changes in emanation X of radium,
272

physical and chemical properties of
emanation X, 275

electrolysis of active solutions, 276

effect of temperature on, 277

emission of heat, due to, 278

variation with electric field, of amount
of, 280

effect of pressure on distribution of, 282

transmission of, 282 et seq.

from actinium, and " emanating sub-
stance," 287

390

INDEX

Excited radio-activity (cont.)

possible connection of polonium and
radio-tellurium with, 290 et seq.

from the atmosphere, 351 et seq.

concentration of, on negative elec-
trode, 351

decay of, 353

due to emanation in atmosphere,
354

distribution of, on surface of earth,
354

concentration of, on prominences of
the earth, 354

of rain and snow, 356

decay of, on rain and snow, 356

due to emanation from earth, 357

produced by emanation from tap
water, 360

effect of meteorological conditions on
amount of, 364

amount of, at Niagara Falls, 366

rate of decay of, dependent on con-
ditions, 368
Exner and Haschek

spectrum of radium, 17
Eye

action of radium rays on, 177

Fehrle

distribution of excited activity on a

plate, in electric field, 282
Fluorescence

produced in substances by radium

rays, 18
produced in substances by radium

and polonium rays, 166
Fog

large amount of excited activity,

during, 365
Forch
loss of weight of radium, 336

7 rays

discovery of, 141

absorption of, by matter, 142

connection between absorption of, and
density, 143

discussion of nature of rays, 143 et seq.

conservation of radio-activity mea-
sured by, 311

measurement of radio-activity by

means of, 321, 339
Gases

evolved by radium, 175

presence of helium in gases from

radium, 176, 327 et seq.
Gates, Miss F.

effect of temperature on excited ac-
tivity, 278

discharge of quinine sulphate, 372

Geitel

natural conductivity of air in closed

vessels, 351, 373
Geitel and Elster

radio-active lead, 25

effect of magnetic field on conducti-
vity produced by radium rays, 95

scintillations produced by active sub-
stances, 127

action of radium rays on spark, 171

photo-electric action of bodies co-
loured by radium rays, 174

radio-active matter in earth, 345

discovery of radio-active matter in
atmosphere, 351

emanations from the earth, 357

radio-activity of air in caves, 362

radio-activity of the soil, 362

radio-activity of natural carbonic acid,
364

variation of radio-activity of air, with
meteorological conditions, 364 et seq.

effect of temperature and pressure on

radio-activity in atmosphere, 365
Giesel

coloration of bunsen flame by radium,
15

separation of radium by crystallization
of bromide, 15

emanating substance, 23

radio-active lead, 26

magnetic deviation of ft rays, 95

temperature of radium bromide above
air, 159

decrease with time of luminosity of
radio-active screen, 168

coloration of bodies by radium rays,
174

evolution of gases from radium, 176

action of radium rays on eye, 177

emanation from the emanating sub-
stance, 209

luminosity produced by radium ema-
nation, 209

excited activity from emanating
substance, 287

bismuth made active by radio-active
solution, 289

activity of radium dependent on age,

300
Glass

coloration produced in, by radium
rays, 174

phosphorescence produced by emana-
tion, 210
Globulin

action of radium rays on, 175
Goldstein

canal strahlen, 92

coloration of bodies by radium rays, 174

INDEX

391

Grier and Kutherford

magnetic deviation of /3 rays of

thorium, 96

relative current due to a and /3 rays, 150
nature of rays from Ur X, 294

Hardy

coagulation of globulin by radium

rays, 175
Hardy and Miss Willcock

coloration of iodoform solutions by

radium rays, 175
Hardy and Anderson

action of radium rays on the eye, 177
Harms
number of ions per c.c., in closed

vessel, 376
Haschek and Exner

spectrum of radium, 17
Heat

rate of emission of, from radium, 158
emission of, from radium at low

temperatures, 159
rate of emission of, after removal of

the emanation, 162
rate of emission of, by the emanation,

162

variation with time of heat emission

of radium, and of its emanation, 162 •

connection of heat emission with the

radio-activity, 161
source of heat energy, 163
heating effect of the emanation, 247
heating effect of emanation X, 278

et seq.
proportion of heating effect, due to

radio-active products, 280
total heat emission during life of

radio -elements, 336
heating of earth by radio-active

matter, 344
Heaviside

apparent mass of moving charged

body, 109
Helium

produced by radium and its emana-
tion, 327

amount of, from radium, 331
origin of, 331
Helmholtz and Kicharz

action of ions on steam jet, 44
Hemptinne

action of rays on spark, and electrode-
less discharge, 171
Henning

resistance of radium solutions, 171
effect of voltage on amount of ex-
cited activity, 281
Heydweiler

loss of weight of radium, 336

Himstedt

action of radium rays on selenium, 171
Himstedt and Nagel

action of radium rays on eye, 177
Hofmann and Strauss

radio-active lead, 26
Hofmann and Zerban

active substance from pitchblende, 25
Huggius, Sir W. and Lady

spectrum of phosphorescent light of

radium bromide, 169
Hydrogen

production of, by radium rays, 176

Induced radio-activity (see Excited

radio-activity)
Induction

radio-active, 21

meaning, and examples of, 289
Insulators

conduction of, under radium rays, 172
Iodoform

coloration produced in, by radium

rays, 175
lonization

theory of, to explain conductivity of
gases, 28 et seq.

by collision, 36, 54

variation of, with pressure of gas, 58
et seq.

variation of, with nature of gas, 62

comparison of, produced by rays, 93,
149

total, produced by 1 gram radium, 154

production of, in insulators, 172

natural ionization of gases, 373 et seq.
Ions

in explanation of conductivity of gases,
28 et seq.

rate of recombination of, 37 et seq.

mobility of, 39 et seq.

difference between mobility of positive
and negative, 42, 43

condensation of water around, 43 et
seq.

difference between positive and nega-
tive, 46

charges carried by, 47

diffusion of, 48 et seq.

charge on ion same as on hydrogen
atom, 51

number of, produced per c.c., 52

size and nature of, 52 et seq.

definition of, 52 et seq.

production of, by collision, 36, 54

velocity acquired by, between colli-
sions, 55

energy required to produce, 55

comparative number of, produced in
gases, 62

392

INDEX

Ions (cont.)

disturbance of potential gradient by

movement of, 63

number of, produced by a particle, 155
production of, in insulators, 172
number produced per c.c., in closed

vessels, 375

Joly

absorption of radium rays by at-
mosphere, 343 (see foot-note)

Kauffmann

variation of ejm with velocity of

electron, 108 et seq.
Kelvin

theory of radio-activity, 320
age of sun and earth, 343, 344
Kelvin, Smolan and Beattie

discharging power of uranium rays, 7
Kunz

phosphorescence of willemite, and

kunzite, 168
Kunzite

phosphorescence of, under radium
rays, 168

Laborde and Curie

heat emission of radium, 19, 158
origin of heat from radium, 319
Langevin

coefficient of recombination of ions, 38
velocity of ions, 39 et seq.
energy required to produce an ion, 55
secondary radiation produced by X-

rays, 146
Larmor

electrons and matter, 108
structure of the atom, 126
radiation from accelerated electrons,

340

Lead, radio-active
preparation of, 26
radiations from, 26
Le Bon

rays from bodies exposed to sunlight,

5
discharging power of quinine sulphate,

9, 372
Lenard

ionization of gases by ultra-violet

light, 9

action of ions on a steam jet, 44
negative charge carried by Lenard

rays, 102

absorption of cathode rays propor-
tional to the density, 113
Lerch, von

chemical properties of emanation X,
275

Lerch, von (cont.)

electrolysis of solution of emanation

X, 276

effect of temperature on excited act-
ivity, 278
Lockyer

inorganic evolution, 349
Lodge

connection of heat emission with a

rays, 164

instability of atoms, 340
Lorentz

structure of atoms, 126
Lowry and Armstrong

radio-activity and phosphorescence,

323
Luminosity

of radium compounds, 168

change of, in radium compounds with

time, 168
spectrum of phosphorescent light from

radium bromide, 169
of radium compounds unaffected by
temperature, 173

Magnetic field

effect of on rays, 92
deflection of /3 rays by, 95 et seq.
,, ,, a rays by, 117 et seq.
,, ,, "ionsactivants"by, 288
Marckwald

preparation of radio-tellurium, 21

apparent mass of electron, 107 et seq.
variation of mass of electron with

speed, 108 et seq.
of a particle, 122, 125
Materials

radio-activity of ordinary, 370, 378
Matteucci

rate of dissipation of charge in closed

vessels, 373
McClung

coefficient of recombination of ions,

38
McClung and Eutherford

energy required to produce an ion, 55
variation of current with thickness of

layer of uranium, 149
estimate of energy radiated from radio-
elements, 154

radiation of energy from radium, 317
McLennan

absorption of cathode rays, 62

radio-activity of snow, 356

excited radio-activity at Niagara Falls,

366
McLennan and Burton

penetrating radiation from the earth,
366

INDEX

393

McLennan and Burton (cont.)

radio-activitv of ordinary materials,
378

emanation from ordinary matter, 379
Mercury

emanation from, 381
Metabolon

definition of, 324

table of metabolons, 326

radio-elements as metabolons, 332
Meteorological conditions

effect of, on radio-activity of atmo-
sphere, 364
Methods of measurement

in radio-activity, 67 et seq.

comparison of photographic and elec-
trical, 67 et seq.

description of electrical, 68 et seq.
Meyer and Schweidler

magnetic deviation of £ rays by elec-
trical method, 95

absorption of ft rays of radium by

matter, 113
Mobility

of ions, 39 et seq.
Moisture

effect of, on velocity of ions, 40, 42

effect of, on emanating power, 214
Molecule

number of, in 1 c.c. of hydrogen, 51

molecular weight of radium emana-
tion, 232

molecular weight of thorium emana-
tion, 234
Molecular weight

of radium emanation, 232

of thorium emanation, 234

Nagel and Himstedt

action of radium rays on eye, 177
Niewenglowski

rays from sulphide of calcium, 4
Number

of molecules per c.c. of hydrogen, 51
of ions produced in gas by active

substances, 52
of a particles emitted per gram of

radium, 155

of ions, produced per c.c. in closed
vessels, 375

Occlusion

of emanation in thorium and radium,

217

of radium emanation by solids, 264
Owens

saturation current affected by dust,

effect of air currents on conductivity
produced by thorium, 197

Owens (cont.)

penetrating power of rays independent

of compound, 132
absorption of a rays varies directly as

pressure of gas, 137
Oxygen

change into ozone, by radium rays, 174
production of from radium solutions,

176
Ozone

production of, by radium rays, 174

Paraffin

objection to as an insulator, 80

conductivity of, under radium rays,

173
Patterson

number of ions per c.c. in closed
vessel, 376

natural conductivity of air due to an
easily absorbed radiation, 377

effect of temperature on natural con-
ductivity of air, 377
Pegram

electrolysis of thorium solutions, 277
Penetrating power

comparison of for a, ft and, y rays, 93

variation in, of ft rays, 98 et seq.

comparison of, for a rays from radio-
elements, 136

variation of, with density for ft rays,
112 et seq.

variation of, with density for a rays,
137

variation of, with density for y rays,

143
Penetrating radiation

from the earth and atmosphere, 366
Perrin

negative charge of cathode rays, 102

theory of radio-activity, 316
Phosphorescence

production of, by radium, 18

production of, by radium and polon-
ium rays, 166

comparison of, produced by a and ft
rays, 168

of zinc sulphide, 167

of barium platino-cyanide, 168

of willemite and kunzite, 168

diminution of, with time, 168

of radium compounds, 168

spectrum of phosphorescent light of
radium bromide, 169

production of by heat (thermo-lumin-
escence), 170

produced by radium emanation in
substances, 210, 227

use of, to illustrate condensation of
emanations, 237

394

INDEX

Phosphorus

action of radium rays on, 175

ionization produced by, 371
Photo-electric action

produced by radium rays in certain

substances, 174
Photographic

method, advantages and disadvan-
tages of, 67

relative photographic action of rays,

68
Physical action of radium rays

on sparks, 171

on electrodeless discharge, 171

on selenium, 171

on conductivity of insulators, 172
Physical properties

of emanation X, 275
Physiological action of radium rays

production of burns, 176

effect on bacteria, 177

effect on cancer, 177

effect on eye, 177
Piezo-electrique of quartz

description of, 87
Pitchblendes

comparison of radio-activity of, 11

radio-elements separated from, 13 et
seq.

radium continually produced from,

334
Polarization of uranium rays

absence of, 7
Polonium

methods of separation of, 19

rays from, 20

decay of activity of, 20

discussion of nature of, 21

similarity to radio-tellurium, 22

magnetic deviation of a rays from,
121

increase of absorption with thickness
of matter traversed, 131

phosphorescent action of rays from,
166

possible origin of polonium and con-
nection with radium, 290 et seq.

penetrating rays from, 305
Potential

required to produce saturation, 30
et seq.

fall of potential, to produce ions each
collision, 55

gradient, due to movement of ions,

63
Precht and Eunge

atomic weight of radium, 17

heating effect of radium, 164
Pressure

effect of, on velocity of ions, 43

Pressure (cont.)
effect of, on current through gases,

58 et seq.
production of emanation, independent

of, 224
effect of, on distribution of excited

activity, 282
effect of, on natural conductivity of

air in closed vessels, 376
Products, radio-active '
radiations from, 304
division of activity amongst, 306
list of from radio-elements, 326

Quartz piezo-electrique

use of, in measurement of current, 37
Quinine sulphate

discharging power of, 9, 372

phosphorescence of, 372

Kadiations

emitted by uranium, 8

emitted by thorium, 10

emitted by radium, 18

emitted by polonium, 20

emitted by actinium, 23

methods of measurement of, 67 et seq.

methods of comparison of, 90

three kinds of, 91

analogy to rays from a Crookes tube,
92 *

relative ionizing and penetrating
power of, 93

difficulties of comparative measure-
ment of, 93 et seq.

/3 rays, 95 et seq.

a rays, 115 et seq.

7 rays, 141 et seq.

secondary rays, 146

comparison of ionization of a and /5
rays, 149 et seq.

connection of, with heat emission,
160

phosphorescent effect of, 166

physical actions of, 171 et seq.

chemical actions of, 174 et seq.

physiological actions of, 176

from the emanation, 222

from Ur X, 293

non-separable activity of radio-ele-
ments consists of a rays, 304

from different active products, 304

conservation of energy of each specific

type of, 308
Kadium

discovery of, 13

separation of, 13

spectrum of, 15

atomic weight of, 17

radiations from, 18

INDEX

395

Radium (cont.)
compounds of, 19

nature of radiations from, 90 et seq.
/3 rays from, 95 et seq.
a rays from, 115 et seq.
y rays from, 141 et seq.
secondary rays from, 147
heat emission of, 158 et seq.
production of phosphorescence by,

166 et seq.
spectrum of phosphorescent light of,

169

physical actions of, 171 et seq.
chemical actions of, 174
physiological actions of, 176
emanation from, 205 et seq.
properties of emanation from, 205 et

seq.
chemical nature of emanation from,

225 et seq.

diffusion of emanation from, 228 et seq.
condensation of emanation from, 236

et seq.

amount of emanation from, 246, 312
heat emission of emanation from, 247,

278

excited radio-activity from, 251 et seq.
decay of excited activity from, 261

et seq.
successive changes in emanation X of,

272

properties of emanation X of, 275
heating effect due to products of, 278
radio-active induction due to, 289
connection of, with polonium, 291
alteration of activity of, by removal

of emanation, 300
recovery of activity of, after removal

of emanation, 301

effect of escape of emanation on
. recovery of activity of, 302
non-separable acthity of. 302, 304
radiations from active products of, 304
division of activity amongst active

products of, 306

conservation of radio-activity of, 308
determination of volume of emanation

of and diminution with time, 313

et seq.

theories of radio-activity of, 316
discussions of theories of radio-activity

of, 320
energy of radiations, not derived from

external source, 321
theory of radio-active change, 322
list of active products of, 326
polonium possible product of, 326
production of helium from, 327
helium disintegration product of, 327
amount of helium from, 331

Radium (cont.)

rate of change of, 332

life of, 333

origin of, 333

possible production of, by uranium,
334

loss of weight of, 335

experiments to determine loss of
weight of, 336

total emission of energy from 1 gram
of, 336

possible causes of disintegration of, 338

amount of, to account for heat of sun,
342

possible connection of with heat of
sun, 342

probable amount of, in earth, 345

possible connection with heat of earth,

344
Rain

radio-activity of, 355

decay of activity of, 356
Ramsay and Soddy

evolution of gas from radium, 176

chemical nature of the emanation,
227

gaseous nature of the emanation, 227

volume of emanation, and change
witfi time, 313

helium from radium emanation, 328
Re, Filippo

theory of radio-activity, 320
Recombination

of ions, 37 et seq.

constant of, 39
Recovery

of heating effect of radium, 162

of activity of thorium after removal
of Th X, 181

of activity of uranium after removal
of Ur X, 182

significance of law of, 185

effect of conditions on rate of, 191

of activity of radium, after removal

of emanation, 301
Reflection

no evidence of direct reflection for
uranium rays, 7

diffuse reflection of rays, 7
Refraction

no evidence of, for uranium rays, 7
Regeneration

of emanating power, 215
Richarz and von Helmholtz

action of ions on steam jet, 44
Runge

spectrum of radium, 17
Runge and Precht

atomic weight of radium, 17

heating effect of radium, 164

396

INDEX

Eunge and Bodlander

evolution of gas from radium, 176
Kussell

photographic action of substances, 68

Saturation current

meaning of, 30 et seq.

application of, to measurements of
radio-activity, 69

measurement of, 82 et seq.
Schmidt

discovery of radio-activity of thorium,

10
Schmidt and Wiedemann

thermo-luminescence, 170
Schuster

radio-activity of matter, 371
Schweidler and Meyer

magnetic deviation of /3 rays by electric
method, 95

absorption of /3 rays of radium, 113
Scintillations

discovery of in zinc sulphide screen,
127

connection of, with a rays, 127

illustration of by spinthariscope, 127

cause of, 128

production of, by action of electric

field, 128
Searle

apparent mass of moving charged

body, 109
Secondary rays

examination of, by photographic
method, 146

examination of, by electrical method,

148
Selenium

action of radium rays on, 171
Simon

value of e/m for cathode rays, 111
Smolan, Beattie and Kelvin

discharging power of uranium rays, 7
Snow

radio-activity of, 356

decay of activity of, 356
Soddy

comparison of photographic and elec-
trical action of uranium rays, 68

nature of rays from Ur X, 294
Soddy and Kamsay

evolution of gas from radium, 176

chemical nature of the emanation,
227

gaseous nature of the emanation, 227

volume of the emanation, and change
with time, 313

helium from radium emanation, 320
Soddy and Rutherford

separation of ThX, 179

Soddy and Rutherford (cont.)
decay of activity of Th X, 181
recovery of activity of thorium, freed

from Th X, 181

decay of activity of UrX, 182
recovery of activity of uranium, freed

from Ur X, 182
explanation of decay and recovery

curves, 183

rate of production of ThX, 186
theory of decay of activity, 188
influence of conditions on rate of
decay and recovery of activity, 190
et seq.

disintegration hypothesis, 194, 324
decay of activity of radium emanation,

206
measurements of emanating power,

213

effect of temperature, moisture, and
solution, on emanating power, 214
regeneration of emanating power, 215
constant rate of production of emana-
tion of radium and thorium, 216
et seq.

source of thorium emanation, 220
radiations from the emanation, 222
chemical nature of emanation, 226
condensation of emanations of radium

and thorium, 236 et seq.
temperature of condensation of emana-
tion, 238 et seq.
effect of successive precipitations on

activity of thorium, 2J6
recovery of activity of radium, 300
conservation of radio-activity, 309
theory of radio-activity, 318
theory of radio-active change, 324
Soil

radio-activity of, 362
difference in activity of, 362
Solution

coloration of, by radium, 15
of emanation X in acids, 275
electrolysis of active, 276
Source

of the thorium emanation, 220
of radium emanation, 222
Spark

action of radium rays on, 171
Spectrum

spark spectrum of radium, 15, 16
flame spectrum of radium, 17
effect of a magnetic field on spectrum

of radium, 17
of polonium, 20
of phosphorescent light of radium

bromide, 169

of helium in radium gases and emana-
tion, 329

INDEX

397

Spectrum (cont.)

of emanation, 329
Spinthariscope

description of, 127
Springs

emanation from water of, 360
Stark

energy to produce an ion, 55
Strauss and Hofmann

radio-active lead, 25
Strutt

conductivity of gases for radiation,
61, 62

negative charge carried by radium
rays, 104

absorption of /3 rays proportional to
density, 113

nature of a rays, 116

conductivity of gases produced by
7 rays, 62, 144

absorption of radium rays from sun
by atmosphere, 343

radio-activity of ordinary matter, 378

emanation from mercury, 381
Sun

effect of radium in, 342

age of, 343

Temperature

of radium above surrounding space,

158
effect of, on intensity of radiations

from uranium and radium, 173
effect of, on luminosity, 173
rate of decay of radium emanation

unaffected by, 208
of condensation of emanations, 238

et seq.
rate of decay of thorium emanation

unaffected by, 246
effect of, on excited activity, 277
effect of, on amount of excited activity

in atmosphere, 364 et seq.
effect of, on natural ionizationofair,377
Theories

of radio-activity, review of, 316
discussion of, 320
disintegration theory, 324
Thermo-luminescence, 170
Thomson, J. J.

relation between current and voltage

for ionized gases, 31
difference between ions as condensa-
tion nuclei, 46
charge on ion, 47
theory of electrometers, 85
path of charged particle in uniform

magnetic field, 96
apparent mass of moving charged

body, 108

Thomson, J. J. (cont.)

structure of atom, 126, 347
theory of radio-activity, 319
cause of heat emission from radium,

321
possible causes of disintegration of

radium, 342
nature of electrons, 347
emanation from tap-water and deep

wells, 360
Thomson, J. J., and Kutherford

ionization theory of gases, 28 et seq.
Thorium

discovery of radio-activity of, 10
emanation from, 11
preparation of non-radio-active tho-
rium, 25

nature of radiations from, 90 et seq.
)3 rays from, 95 et seq.
a rays from, 115 et seq.
7 rays from, 141 et seq.
rate of emission of energy by, 154
separation of Th X from, 179
recovery of activity of, 181
disintegration of thorium, 193
emanation from, 197
properties of emanation from, 198 et

seq.f

diffusion of emanation from, 233
condensation of emanation from, 236

et seq.

excited radio-activity from, 250 et seq.
successive changes in emanation X of,

272
explanation of initial portion of decay

curve, 295 et seq.

explanation of initial portion of re-
covery curve, 295 et seq.
effect of successive precipitations on,

296
recovery curve after large number of

precipitations, 297
theory of decay curve of Th X, 298
non- separable activity of, 296, 304
radiations from active products of,

304 et seq.
division of activity amongst active

products of, 306 et seq.
conservation of radio-activity of, 308

et seq.

resume of results, 311 et seq.
theories of radio-activity of, 316 et seq.
discussion of theories of radio-activity,

320 et seq.
source of energy of radiations, 320 et

seq.
theory of radio-active change, 322 et

seq.

table of radio-active products of, 326
rate of change of, 332 et seq. %

398

INDEX

Thorium (cont.)
life of, 333
total emission of energy from 1 gram

of, 337
possible causes of disintegration of,

338 et seq.
Thorium X

methods of separation of, 179
law of decay of activity of, 182
law of recovery of activity of, 182
theory to explain production of, 183
material nature of, 185
continuous production of, 186
explanation of decay of activity of,

188 et seq.
effect of conditions on the rate of

change of, 190 et seq.
disintegration hypothesis to explain

production of, 193 et seq.
minute amount of, produced, 195
effect of successive separations of, on

activity of thorium, 296
theory of decay curve of, 296 et seq.
Tommasina

scintillations produced by electrifica-
tion, 128
Townsend

ions by collision, 36, 54
coefficient of recombination, 38
diffusion of ions, 49 et seq.
comparison of charge on ion with that

on hydrogen atom in electrolysis, 51
number of molecules per c.c. of gas, 51
ionization by collision for different

speeds, 139
Transmission

of excited radio-activity of radium

and thorium, 282 et seq.
of excited radio-activity of actinium,

287
Troost

rays from hexagonal blende, 4

Uranium

discovery of radio-activity of, 5
persistence of radiations of, 6
discharging power of rays, 7
absence of reflection, refraction and

polarization, 7
examination of uranium minerals, 11

et seq.
relative activity of compounds of

uranium, 12

nature of radiation from, 90 et seq.
/3 rays from, 95 et seq.
a rays from, 115 et seq.
y rays from, 142 et seq.
emission of energy by, 154
separation of Ur X from, 179
recovery of activity of, 182

Uranium (cont.)

non-separable activity of, 294, 304

radiations from Ur X, 293

method of measurement of activity

of Ur X, 294
changes in, 294
conservation of radio-activity of, 308

et seq.

resume of results, 311 et seq.
theories of radio-activity, 316 et seq.
discussion of theories of radio-
activity, 320 et seq.
source of energy of radiation, 320 et

seq.
theory of radio-active change, 322 et

seq.

table of active products, 326
rate of change of, 332 et seq.
life of, 333

radium possible product of, 334
total emission of energy from 1 gram

of, 337
possible causes of disintegration of,

338 et seq.
Uranium X

separation of, by Crookes, 178
separation of, by Becquerel, 179
decay of activity of, 182
recovery of activity of, 182
theory to explain production of, 183
material nature of, 185
explanation of decay of activity of,

188 et seq.

radiations from, 293
method of measurement of radiations

from, 294
changes in, 294

Velocity

of ions in electric field, 39 et seq.
difference between, of positive and

negative ions, 42 et seq.
of [3 particle or electron, 107 et seq.,

110 et seq.
variation of mass of electron with,

108 et seq.

of a particle, 122 et seq.
of transmission of carriers of excited

activity, 284
Villard

discovery of 7 rays from radium, 141
alteration of X ray screen with time,

168

activity produced by cathode rays, 372
Volume

of radium emanation, calculation of,

246

of emanation, determination of, 313
decrease of, of radium emanation,

313

INDEX

399

Walker, G. W.

theory of electrometer, 75
Walkhoff

action of radium rays on skin, 176
Wallstabe

diffusion of radium emanation into

liquids, 235
Water

emanation from, 360

decay of activity of emanation from,

361
Water-falls

amount of excited activity produced

at Niagara, 366

electrification produced near, 366
Watts, Marshall

atomic weight of radium, 17
Weight

loss of by radio-elements, 335
attempts to measure loss of, in

radium, 336
Wheeler and Bumstead

emanation from surface water and

the soil, 361, 368
identity of emanation from soil with

radium emanation, 361
absence of emanation in mercury,

381
Whetham

effect of valency of ion on colloidal

solutions, 175
possible cause of disintegration of

atom, 342
Wiedemann and Schmidt

thermo-luminescence, 170
Wiedemann

thermo-luminescence produced by

radium rays, 170
Wien

amount of charge carried by radium

rays, 105
positive charge of canal rays, 125

Willcock, Miss, and Hardy

coloration of iodoform solution by

radium rays, 175
Willemite

phosphorescence of under radium

rays, 168

use to show condensation of emana-
tion, 237
Wilson, W. E.

radium in sun, 342
Wilson, H. A.

charge on ion, 48
Wilson, C. T. E.

ions as nuclei of condensation, 44

et seq.

difference between positive and nega-
tive ions as condensation nuclei, 46
equality of charges carried by positive

and negative ions, 47
construction of electroscope, 73
natural ionization of air in vessels, 351
radio-activity of rain and snow,355,356
loss of charge in closed vessels, 373
presence of ions in free air shown by

condensation, 375

number of ions produced per c.c., 375

effect of pressure and nature of gas

on. ionization in sealed vessels, 376

Zeleny

velocity of ions, 39 et seq.
difference of velocity of ions, 40, 42
potential gradient between electrodes,

63
Zerban and Hofmann

active substances from pitchblende, 25
Zinc Sulphide

scintillations produced in by a rays,

127

cause of luminosity of, 127 et seq.
scintillations due to cleavage of
crystals, 128

CAMBRIDGE: PRINTED BY j. AND c. F. CLAY, AT THE UNIVERSITY PRESS.

14 DAY USE

RETURN TO DESK FROM WHICH

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Kenewed books are subject to immediate recall

LD21-35m-8 '72
(Q4189slO)476 — A-32

General Library

University of California

Berkeley

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