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THE BECQUEREL RAYS
AND THE PROPERTIES OF RAP "M
THE BECQUEREL RAYS
AND THE PROPERTIES OF
HON. E. J. STRUTT
FELLOW OP TRINITY COLLEGE
41 AND 43 MADDOX STREET, BOND STREET, W,
[All rights reserved]
Edinburgh : T. and A. CONSTABLE, Printers to His Majesty
IN writing this book, my object has been to give
as clear and simple an account of the phenomena
of radio-activity as the subject admits of, without
sacrificing accuracy. The extraordinary properties
of radium have excited general interest outside the
scientific world, and there are probably many who
would be glad to learn something of the subject,
if they could find it explained without the use
of technicalities. However essential mathematical
methods may be in developing the subject, they
are seldom really necessary in presenting the results.
Moreover, some idea of the train of reasoning can
generally be given in ordinary language.
I have not found it possible to avoid assuming
some elementary scientific knowledge on the part
of the reader, but this has been reduced to the
smallest limits, and probably a great part of the
book will be intelligible without it.
I have not attempted to describe all the pheno-
mena which have been recorded, but have confined
vi THE BECQUEREL RAYS
myself to those which seemed most significant and
In revising the book, I have had the advantage of
consulting Professor Rutherford's excellent treatise
on ' Radio-activity,' which has recently appeared, and
have adopted one or two valuable and suggestive
remarks from it.
The names of authorities have not in all cases
been given, and it has been thought unnecessary,
in view of the general character of the work, to
give references to original papers.
TERLING PLACE, WITH AM,
I. ELECTRIC DISCHARGE IN HIGH VACUA T . . 1
II. DISCOVERY OF RADIO-ACTIVITY 25
III. THE PROPERTIES AND NATURE OF THE RADIATIONS . 50
IV. ABSORPTION AND IONISATION 87
V. THE CHANGES OCCURRING IN RADIO-ACTIVE BODIES . 112
VI. RADIO-ACTIVITY IN THE EARTH AND IN THE ATMO-
SPHERE ... 143
VII. THE ULTIMATE PRODUCTS OF RADIO-ACTIVE CHANGE . 153
VIII. THE ELECTRICAL THEORY OF THE NATURE OF MATTER 179
APPENDIX A. EXPERIMENTAL NOTES . . . . .189
APPENDIX B. THEORY OF THE MAGNETIC AND ELECTRO-
STATIC DEFLECTION OF THE CATHODE RAYS OR THE
/3-RAYS OF RADIO-ACTIVE SUBSTANCES .... 200
APPENDIX C. THE TREATMENT OF PITCHBLENDE RESIDUES
ON A LARGE SCALE 205
INDEX . . 209
ELECTRIC DISCHARGE IN HIGH VACUA
THE clue which led to the discovery of radio-
activity was obtained from the study of electric dis-
charge in high vacua ; and knowledge gained in the
same way has been indispensable in interpreting its
phenomena. Before entering on the main subject of
this book, it will be desirable, and indeed essential,
to give some account of the phenomena which accom-
pany the passage of electricity through rarefied gases.
When an electric discharge passes through air at
atmospheric pressure, a narrow, well-defined spark is
observed to pass between the electrodes of the in-
duction coil or electrical machine used to produce the
When, however, these electrodes are placed in an
air-tight vessel, and the air withdrawn by means of
a mercurial air-pump, a profound change in the
character of the discharge takes place. The spark
becomes broad and ill-defined as the air pressure is
reduced. Thus, for instance, if a cylindrical tube is
FIG. 1. Electric discharge at a moderately low pressure. The negative electrode
a is a disc, and is separated by a small dark interval from the blue negative glow e.
The glow along the rest of the tube to the anode b is. in air, a diffuse band of reddish
used, a glow spreads out from the two electrodes and
fills the tube (fig. 1).
2 THE BECQUEREL KAYS
In this case the luminosity is spread over a large
area. But by making the tube with a constriction in
the middle (fig. 2), the glow is concentrated in the
FIG. 2. Plticker's tube, for examining the spectra of rarefied gases under electric
discharge. The discharge passes between the platinum wires, a, b. These are sealed
airtight into the elongated glass bulbs, c, d. The discharge is constricted, and increased
in intensity by passing through the narrow tube e, which connects these bulbs. The
gas is introduced at /, which can then be sealed off if desired.
narrow portion, and its brightness greatly increased.
Such tubes are very useful for examining the spec-
trum of any desired gas under the electric discharge.
They are known by the name of Pliicker, who was
the first to use them.
In the majority of gases the glow is most con-
^/spicuous at a pressure of about ^^ of an atmosphere.
Let us take the case of atmospheric air. In this
case, at ^$ of the atmospheric pressure, the negative
electrode is seen to be surrounded by a blue glow,
different in colour from the reddish colour along the
length of the spark. The same thing can be seen in
the spark at atmospheric pressure, though in this
case the glow is almost confined to the surface of the
negative electrode. At the low pressure it can be
seen that the blue negative glow is separated by a
small dark interval from the electrode itself (see
Let the exhaustion now be carried further. It will
be found that the blue negative glow spreads out
and that the dark space becomes broader. A green
luminosity begins to be visible on the wall of the
tube opposite the negative electrode. At still higher
exhaustions, such as can only be obtained by pro-
longed use of the mercurial pump, it is found that a
sharp patch of green fluorescent light is visible on
ELECTRIC DISCHARGE IN HIGH VACUA 3
the wall opposite to the negative electrode ; this
patch corresponds in shape to the electrode itself, if
the electrode is flat. Thus a round disc electrode
will produce a round patch only a little larger than
itself. A faint streak of blue luminosity appears in
the gas, stretching from the cathode to the phos-
phorescent spot. It is evident from these phenomena
that some kind of influence is propagated out at right
angles to the cathode surface and travels through the
tube till it reaches the wall. This influence goes by
Fio. 3. Experiment showing that the cathode rays are always emitted at right angles
to the surface of the cathode, and do not necessarily follow the line of the discharge.
The rays from the flat cathode, a, strike the wall of the tube at ft, causing phosphores-
cence. They penetrate the glow of the main discharge, which curves away to the anode,
c, in a lateral tube joined on to the main one.
the name of the cathode rays. The cathode rays
always proceed at right angles to the cathode,
whether that is the direction in which the anode lies
or not. The spark or glow discharge which, at these
low pressures, is quite inconspicuous, of course pro-
ceeds from anode to cathode, and will turn a corner
without difficulty. This the cathode rays will not do.
Their path is essentially rectilinear (fig. 3).
A thick, solid obstacle placed in the path of the
cathode rays casts a perfectly sharp and definite
shadow. The outline of the object is seen on the
4 THE BECQUEREL RAYS
wall of the vessel as a black shadow on the bright
green fluorescent background (fig. 4).
Glass is not the
the influence of
the cathode rays.
Many other ma-
terials will do the
same, and some of
them give more
FIG. 4. Arrangement for showing that the cathode rays , -, -,
are stopped by a solid obstacle, and cast a sharp shadow. L il a n P^laSS.
A pear-shaped vessel, a, is used, which is highly exhausted. 1
The plane or slightly convex cathode, b, sends out the rays, J_nUS lOT 1 n-
which cause brilliant green fluorescent luminosity on the
opposite wall, c. The metal cross, d, which serves as anode, oforipp rlr annv
stands in the path of the rays, and casts a sharp black kLctllLe, LcllC
glows with a
beautiful orange-red colour. Rubies give a deep red
colour, far more effective than anything that they
show in the ordinary way. No rule can be given as
to whether a substance may be expected to fluoresce.
That can only be ascertained by trial.
The most characteristic and interesting property
of the cathode rays is that they are deflected by a
magnet. The deflection is very conspicuous and easy
to observe. It is only necessary to bring a common
horse-shoe magnet near any one of the tubes, and the
luminosity of the glass will be seen to move, showing
that the rays are now falling on a different spot to
what they did before.
To study the effect in detail, it is desirable to have
a form of tube (fig. 5) specially designed for the
purpose. The cathode rays from a flat cathode have
to pass through two successive diaphragms. By this
means the rays are confined to an exceedingly
sharp and narrow beam. They then pass through
ELECTRIC DISCHARGE IN HIGH VACUA 5
a wide bulb, and produce a well-defined green fluor-
escent patch on the end wall of the bulb. The
position of this patch can be read by means of a
scale fastened on to the outside of the tube. Before
FIG. 5. Tube for observing and measuring the magnetic and electrostatic deflection
of the cathode rays. The beam of cathode rays from the flat disc, a, represented by the
dotted line, is denned by passage through the two slits in the metal discs, b b. These
discs are connected to the anode. The cathode rays form a well-defined fluorescent
patch on the wall at c. They can be deflected magnetically by means of flat coils of wire
carrying a current, which lie on either side of the tube, in planes parallel to the paper.
Electrostatic deflection is produced by the plates d d, connected to the poles of a
entering the bulb, the rays have to pass between two
parallel metal plates, the object of which will be
In order to experiment upon the magnetic deflection,
the bulb is placed between the poles of an electro-
magnet, or, what is better, between two coils of wire
of large radius, through which an electric current can
be passed. This latter arrangement allows of a very
uniform magnetic field being obtained throughout the
When the current is turned on, so as to produce a
magnetic force, the cathode rays are deflected. In
other words, their path is bent into a curved form, so
that they strike against the glass at a different spot.
The direction in which the deflection takes place is at
right angles to the magnetic force, and it can be made
to change from one side to the other by reversing the
current through the magnet coils, which, of course,
6 THE BECQUEEEL RAYS
has the effect of exchanging the positions of the north
and south magnetic poles.
To describe the direction of the deflection, we may
say that if a little human figure were swimming in
the cathode rays, and looking along the lines of
magnetic force, he would be carried to his right.
Now, what is the most natural interpretation of this
magnetic effect ? We know that an electric current
in a wire, if free to move, is deflected, just like the
cathode rays are, by a magnetic force perpendicular to
it, in a direction perpendicular to the current itself,
and to the magnetic force. It is fair to conclude, at
least provisionally, that the cathode rays consist of a
series of electrically charged particles, fired off from
the cathode. This is the explanation of the nature
of the cathode rays which now obtains universal
The direction of magnetic deflection clearly shows
that the particles are negatively charged. Their
velocity is due to the intense electric force which
repels them from the cathode.
We can prove very directly that the cathode
rays do carry a charge of negative electricity. For
FIG. 6. Pen-ill's experiment for showing that the cathode rays carry a charge of negative
electricity. The rays from the flat cathode, a, pass through a hole in the front of the metal
canister, b, which is earthed, and serves to protect the inner canister, c, from electrical
disturbances. When the rays penetrate into c, it is found to acquire a negative charge.
this purpose the rays are made to fall into a little
metal cylinder, in which their charge can accumulate.
The cylinder is connected to an electroscope, and as
soon as the rays are turned on, the electroscope leaves
rapidly diverge with a charge of negative electricity.
ELECTRIC DISCHARGE IN HIGH VACUA 7
If the rays are deflected by a magnet, so as to pre-
vent their entering the cylinder, no charge is
obtained. This experiment is due to M. Perrin
Since the rays are charged with negative electricity,
they will be repelled by a negatively charged body,
and attracted by a positively charged one. The tube
(fig. 5) which has been described for observing the
magnetic deflection is also arranged for this experi-
ment. The two metal plates between which the rays
pass, are connected, one to the positive and the other
to the negative pole of a battery of, say, 20 or 30
cells. The fluorescent patch is deflected by the
electrostatic force, just as it was by the magnetic
force. The rays are found to move away from the
negative plate towards the positive one.
We have now to consider an important, though
difficult, part of the subject. It is very desirable to
get some idea of it, in order to understand what
follows, when we come to discuss the properties of
radio-active substances. It cannot be treated satis-
factorily without making use of mathematical symbols,
which would be foreign to the plan of this work.
Those who can follow that kind of reasoning will
find the matter set out in the simplest way that
it admits of in Appendix B. For others, a verbal
explanation, necessarily imperfect, must be attempted. 1
Let us consider, then, a particle charged with
electricity, and moving through a field of magnetic
force. As we have seen, the moving particle, since
it is equivalent to an electric current, must experience
a deflecting force, which will bend it to one side.
Now on what will the amount of the resulting de-
1 If the reader finds what follows too difficult, he must be content to take
it for granted that the cathode rays are negatively electrified particles of
about TT nro part of the mass of a hydrogen atom, and moving with an
enormous velocity, and pass on to p. 17.
8 THE BECQUEREL KAYS
flection depend ? Plainly on the distribution and
strength of the magnetic force, for one thing. That
can be measured experimentally, and there is nothing
doubtful about it. But the deflection depends also
on other things. The velocity with which the particle
is moving, its mass, and the quantity of electricity
it carries ; of these as yet we know nothing ; our
object is to obtain information about them.
We have spoken as if the mass of the particle
and its charge were each separately involved. But
that is not exactly true. The force tending to
deflect the particle depends entirely on its charge.
It is, in fact, proportional to the charge. But the
effect of the charge in producing sideways displace-
ment depends on the mass to which it is attached,
and which it has to drag with it. The heavier the
mass with which it is clogged, the more slowly will
it get up a sideways velocity, and the less it will
ultimately be deflected. Increase of mass acts in
the opposite way that increase of charge does. To
increase this mass acts in just the same way as
to decrease the charge. It is the ratio between the
mass and the charge, not the actual value of either,
that is essential in determining the amount of de-
This is so important that it will not be superfluous
to give an illustration of it. Suppose instead of one
single particle in the stream, we think of two of
them, close together, and imagine them joined so
as to make one large particle, twice as massive, and
carrying twice as much electricity as the original
small ones. It is quite apparent -that the doubled
particle will not move any differently from the single
ones, although yet it carries more electricity and
more matter. The absolute quantities of matter or
of electricity carried by a particle are not involved.
ELECTRIC DISCHARGE IN HIGH VACUA 9
It is the ratio between them that determines the
In considering the magnetic deflection, then, there
are two unknown quantities connected with each
particle that are concerned. These are its velocity
and the ratio of its electric charge to its mass.
The mechanical force on a charged particle moving
at right angles to a magnetic field, necessarily acts
in the same direction as the force which would act
on a wire carrying a current under the same circum-
stances ; that is, at right angles to its own direction and
to the magnetic force. Since there is a pull on the
particle at right angles to the direction it is moving,
it is constrained in just the same way as is a stone
tied to the end of a string, and whirled round in a
circle. For the string pulls on the stone in a direc-
tion at right angles to its path, just as the force
on the particle pulls it in a direction at right angles
to its path. The particle will accordingly move in
a circle, just as the stone does. The pull on the
particle counteracts its centrifugal force, just as the
pull on the stone counteracts its centrifugal force.
We know, or at least we can easily calculate from
observation, the radius of the circle into which the
rays are bent ; and we can compare their mechanical
behaviour quantitatively with that of a stone whirled
round in a circle of the same radius. We can cal-
culate the pull which the string exerts on the stone,
not of course absolutely, but in terms of the speed
of the stone and its mass ; and we can calculate in
just the same way the radial pull on the cathode
particle in terms of its speed and mass. The radial
pull can also be expressed, however, in terms of the
charge which the particle carries, its speed, and
the strength of the magnetic field through which
it moves. Now these two ways of computing the
10 THE BECQUEREL RAYS
radial pull, as they are both correct, must lead to
the same result. We can find by expressing this a
necessary relation between the speed of the particle
and its mass and electric charge, which are not
known, and the strength of the magnetic field and
the radius of the circle in which it moves, which
are known by direct measurement. The mass and
the charge, however, only appear as a ratio, as
already explained. So that we have a relation
between the ratio of charge to mass on the one
side, and velocity on the other. The relation is a
very simple one ; it is that the velocity is found
by multiplying this ratio by the magnetic force and
the radius. This is of course not sufficient to deter-
mine either the velocity or the ratio of charge
to mass. Before that can be done, a second
relation is necessary. In algebraical language, two
equations are necessary to determine two unknown
The second relation can be got in more than one
way. But the simplest and best is to measure the
deflection of the rays by a known electro-static
The theory of this experiment is perhaps simpler
than of that which we have been considering. The
particle moves, let us say, horizontally, and the
sideways electric force, at right angles to the
direction of motion, pulls it down. The particle
takes a certain time to travel the known length
of the electro-static field. During this time it is
pulled down by the electro-static field, and the
distance it falls is readily deduced from the observed
deflection of the phosphorescent patch. We know
then how far it fell in the time. The case is
exactly parallel to the fall of a rifle bullet, which
makes it necessary to elevate the rifle above the
ELECTRIC DISCHARGE IN HIGH VACUA 11
direction in which it is desired that the shot shall
travel ; but in our case the particle is pulled down
by the electric force upon it, not by gravity. We
can measure the strength and length of the electric
field, and these, in combination with the deflection
of the rays, give us a second relation between the
velocity and the ratio of mass to charge.
We have now a pair of equations, and a pair of
unknown quantities. It is accordingly a simple piece
of algebra to determine what the two unknown
The results are very astonishing. It is found that
the cathode particles move with a speed not insigni-
ficant when compared with the speed of light. The
speed of the rays depends on circumstances. If the
pressure of gas in the vessel is not very low indeed,
then a comparatively moderate electro-motive force
suffices to produce discharge, and to set the cathode
rays going. In this case the rays are not shot out by
a very intense electric force, and consequently they
move comparatively slowly, perhaps with ^ of the
speed of light. If, however, the exhaustion of the
vessel is carried to an extreme point, the electro-
motive force near the cathode becomes great, and
the rays travel faster ; speeds amounting to one-third
that of light have been so obtained. The velocity
of light is nearly two hundred thousand miles per
second. So that the cathode particles sometimes
travel at sixty thousand miles per second.
It is difficult to fully realise the meaning of such
speeds as this. A shot from a modern gun leaves
the muzzle at perhaps 3000 feet per second, so the
cathode particle goes twenty miles while the cannon
ball is going one foot ! In one second the particle
could go more than twice round the globe !
It is not found that the velocity of the rays is always
12 THE BECQUEREL EAYS
quite uniform in a given tube at a given pressure.
For, when the fluorescent patch is deflected, it is at
the same time drawn out into a band, with bright
intervals separated by dark ones. This appearance
is known as the magnetic spectrum of the rays. Each
bright band in the spectrum corresponds to rays of
one special velocity. The band most deflected from
the original position is produced by the impact of
the slowest cathode rays, that least deflected by
the fastest. The magnetic force is thus able to sort
out the fast moving particles from the slow ones.
The extended and discontinuous character of the
cathode ray spectrum is due to a peculiarity of the
induction coil used to produce discharge ; an induction
coil does riot produce a steady electro-motive force.
If, however, a battery of hundreds of storage cells is
employed instead, the bands are not obtained. The
deflected patch is no broader than the original one ;
the cathode rays produced are therefore all of the
We have now to consider the other information
which was got by measuring the deflection of the rays
by known electric and magnetic forces the ratio of
the electric charge to the mass. Before doing so, it
will be well to consider the ratio of the charge of a
chemical atom, that of hydrogen, for instance, to its
When water is decomposed by an electric current,
hydrogen and oxygen are set free at the electrodes.
We can measure how much electricity has passed
through the water, and how much hydrogen has been
set free, and it is found that the amount set free is
always the same for the same quantity of electricity.
It does not at all matter whether there was much
water or little, or whether a large current went
through for a short time, or a small current for a
ELECTRIC DISCHARGE IN HIGH VACUA 13
long time. The amount of hydrogen set free at the
electrode by a given quantity of electricity is always
fixed and invariable, unless, indeed, it is absorbed at
the electrode before it can get away by some chemical
agency. Now the electricity is carried through the
liquid by the oxygen and hydrogen atoms. Each
atom concerned in the process conveys a charge of
electricity. The hydrogen carries a positive charge
in this case, the oxygen a negative one. The positive
electrode attracts the negatively charged oxygen
atom, while the negative electrode attracts the
positively charged hydrogen atom.
If we find experimentally how much electricity has
to pass through the liquid for one gram of hydrogen
to be set free, it is evident that we have the charge
carried by a gramme of hydrogen. A gramme of
hydrogen no doubt contains an almost inconceivable
number of atoms. But the ratio of charge to mass
is the same for many atoms as for one. Thus we
have learned the ratio of the charge to the mass
of a hydrogen atom. It is found that the charge,
measured in electro-magnetic units, is very nearly
ten thousand times the mass, the mass being, of
course, expressed as a fraction of a gramme.
If the cathode particles were atoms of matter, as
was long believed to be the case, we should expect
their mass to be at all events not smaller than this in
comparison with the electric charge. For no other
atom is known which has so small a mass in pro-
portion to its charge. But, when we come to
determine, in the way that has been described, the
ratio of charge to mass for the cathode particles, it is
found that the ratio, instead of being ten thousand
to one, is about ten million to one. It cannot be
doubted, therefore, that the cathode particles are
something very different from atoms.
14 THE BECQUEREL RAYS
We cannot, without reference to other considera-
tions, decide whether they are heavier or lighter than
atoms. For, although the ratio of charge to mass has
been determined, we do not, so far, know anything
about the absolute value of either. We know that
the charge is very large relative to the mass; but
whether this is due to the charge being larger than
the charge of an atom, or the mass being smaller, we
cannot, without experiment, decide. What is wanted
is a direct measurement of the mass, or, what will do
as well, of the electric charge. When one of these is
known, the value of the other is quite determinate.
No one has yet succeeded in inventing a way of
measuring the mass of the particles ; and it is not
possible to measure the electric charge of the cathode
particles in a vacuum tube either. But this is not
the only way in which cathode rays can be produced.
It is found that they are also given off by metals
exposed to the action of ultra violet light ; light,
that is, of shorter wave-length than the eye is capable
It would carry us too far to discuss this subject in
detail, but it has been found that these particles have
the same ratio of charge to mass as the cathode
particles, and are beyond doubt identical with them.
We owe to the ingenuity and skill of Professor
J. J. Thomson a measurement of the charge which
Without entering into detail, some idea of the
way in which the experiment is made should be
It depends fundamentally on the discovery made
by Mr. C. T. R. Wilson, that the charged particles
are able to act as centres of condensation for water
vapour. If we have air more than saturated with
water vapour, the water vapour is ready to condense
ELECTRIC DISCHARGE IN HIGH VACUA 15
to liquid water, in the form of a fog. It cannot,
however, do this unless some kind of nuclei are
present for the raindrops to condense on. If there
is dust in the air, the dust particles are available
to act as nuclei of condensation, and there is nothing
to hinder that process from freely taking place. If
there is no dust, no condensation will, in an ordinary
way, occur, unless indeed the air is so heavily over-
charged with water vapour that condensation can
occur on the molecules of air themselves.
We shall not consider such great degrees of super-
saturation any further, but only those moderate
degrees of super-saturation which give no condensa-
tion in the absence of dust. It is found that in
this case the charged cathode particles, from metals
illuminated with ultra-violet light, are able to act
as centres of condensation, and to produce a fog. 1
Each particle gathers a drop of water about it, and
thus there are as many drops as there are particles.
We have got now, in exchange for the infinitesimal
cathode particles, an equal number of drops, quite
visible to the naked eye, or at all events under a very
moderate magnifying power. The number of drops
produced under given conditions can be estimated in
various ways. The simplest way would be to let
them fall on a glass plate and count them under the
microscope. This was not, however, the plan adopted
by Professor Thomson. He estimated the number of
drops by knowing, from the conditions of the ex-
periment, what quantity of water vapour had been
condensed, and by finding the average size of each
drop. This, of course, enables the number of drops
to be found.
The way in which the size of the drops was found
1 The particles require more super-saturation than dust does, to enable
them to produce condensation.
16 THE BECQUEREL RAYS
is very ingenious. These drops, which, taken together,
constitute a cloud, fall slowly through the air. Thus
the level of the top of the cloud gradually sinks ; the
rate at which it sinks is observed. This is the same
as the rate at which the individual drops sink. Now
the rate at which a spherical drop falls through air
depends on its size. For a small drop the rate of fall
soon becomes uniform. The smaller the drop, the
more slowly it falls, 1 and, if the rate of fall is deter-
mined, the size can be calculated, supposing, of course,
the viscous resistance of the air to be known, as it is.
In this way, then, the size of the drops has been
found, and the number of them calculated. This is
the same as the number of cathode particles. This
knowledge is of no use by itself. It is essential to
know also the quantity of electricity that the same
number of particles carry. If we know how much
electricity is carried by, say, 10,000 particles, it
will not be difficult to find how much is conveyed
In order to find the quantity of electricity on the
particles, it is necessary to collect their charges by
driving them up to a metal plate, on which their
charge can accumulate and be measured. They are
driven by repulsion from the metal plate from which
they came in the first instance, which is negatively
electrified for the purpose. If we know, as we do,
the rate at which the particles move under the
electric force employed, and the length of the vessel,
it will be easy to find how long it will take to collect
all that were present at the original moment, and
none that have been added since. The quantity of
electricity acquired in that time by the plate is the
1 The case of a small drop must not, of course, be confused with the case
of a heavy object like a stone. In that case the rate of fall does not depend
much on the resistance of the air, and, as Galileo showed, a small stone falls
at appreciably the same rate as a large one, and with accelerating velocity.
ELECTRIC DISCHARGE IN HIGH VACUA 17
aggregate charge on the particles. As we know how
many particles there are, we can find the charge
on each. It is found, in this way, that each
particle carries a charge equal to that carried by the
hydrogen atom, as nearly as the somewhat rough
estimates of the latter that we have will allow us to
We have come, then, to the conclusion that the
charge of the particle is the same as the charge of
the hydrogen atom. The charge is actually the same.
Relatively to the mass, however, it is very much
larger. The mass of the particle is, therefore, very
much smaller than the mass of an atom of hydrogen
about a thousand times smaller.
This is a momentous conclusion, for it shows that
the doctrine of the indivisibility of a chemical atom,
prevalent throughout the nineteenth century, must
be reconsidered. The cathode particles, or corpuscles,
as they are more conveniently named, are derived
from the atoms of gas in the vacuum tube, or from
the electrodes, and as these corpuscles are smaller
than atoms, the inference is inevitable that the atom
has been split up, and that one or more corpuscles,
which formed part of its original structure, have been
detached from it. This consideration opens up one
of the most promising lines of investigation in the
entire range of physical science. We shall defer
further consideration of it to a later chapter.
An important property of the cathode rays is their
power of penetrating thin solid obstacles. The first
observations of this kind were due to Hertz, the
1 We have explained how the charge carried by say, 1 gramme of hydro-
gen atoms is deduced. In order to find the charge carried by each, it is
necessary to know how many atoms there are in a gramme of hydrogen, or,
what comes to the same thing, how many in a cubic centimetre. This can
be calculated in various ways ; but they are all rather indirect, and the
results are not as concordant as might be wished. Still, the number is
certain within a few times.
THE BECQUEREL RAYS
celebrated discoverer of electro-magnetic waves. He
found that if a film of gold leaf was placed in the
path of the cathode rays, they were able, notwith-
standing, to produce some phosphorescence on the
glass wall of the tube, though the intensity of the
phosphorescence was greatly reduced by passage
of the rays through the leaf.
Lenard went a step further. He arranged a small
window of thin aluminium foil at the end of a vacuum
tube. This window was only about ^ inch in
diameter ; the object of making it so small was to
support the thin leaf as far as possible against the
atmospheric pressure, which tends to burst it inwards.
With this arrangement (fig. 7) Lenard was able to
FIG. 7. Tube for showing penetration of the cathode rays into the open, a is the
disc-shaped cathode ; a metal tube, b, serves as anode. Opposite the cathode is the thin
aluminium window, c, carried by the brass cap, d. The rays issue from this window.
get the cathode rays through the window and out
into the open air. This experiment does not succeed
unless the vacuum tube is very highly exhausted, so
as to obtain rays of great velocity. The rays of small
velocity, obtained at higher pressures, are unable to
penetrate the window. Lenard's experiments, at the
time when they were first published, were considered
by many to constitute a fatal objection to the
corpuscular theory of the rays. It was argued that
corpuscles could not penetrate a solid metal window,
however thin. This is no longer felt to be a serious
ELECTRIC DISCHARGE IN HIGH VACUA 19
difficulty, as we shall see when we come to discuss
modern ideas of the constitution of matter.
The cathode rays are not able to go any considerable
distance through air at atmospheric pressure. They
are diffused and spread out in a fan shape from the
window. If the air pressure in the space outside the
tube is reduced, the rays are able to go further, but
at atmospheric pressure they behave like a beam of
light in a smoky atmosphere. This is due to the
collision of the corpuscles with the molecules of the
air. This, if it does not altogether stop them, sends
them glancing off in a sideways direction.
Lenard made a series of very valuable determina-
tions of the comparative absorption of the cathode
rays by different solids and gases. The result was
to show that the densest substances are the most
absorbent for the cathode rays ; the lightest sub-
stances are the most transparent. This rule seems to
be applicable to all substances, from the densest, such
as gold, to the lightest, such as hydrogen, at a high
degree of rarefaction. The absorption of the rays is
proportional to the density. Gold is about fifteen
thousand times as dense as air ; thus if the rays were
partially absorbed by a piece of gold leaf it would
require a layer of air fifteen thousand times as thick
to absorb them as much.
Probably the law of density is only approximately
The transparency of different substances for the
cathode rays, since it depends on density, has no
connection whatever with their transparency for light.
Thus, for instance, mica, which is not very different
in density from aluminium, transmits the cathode
rays about equally well, whereas mica is transparent
to light and aluminium perfectly opaque.
The cathode rays, which have penetrated outside
20 THE BECQUEREL RAYS
the window, produce a blue glow in the air near it.
This glow is of the same nature as the glow inside
the tube near the negative electrode, which is most
conspicuous when the pressure is not so low as in
Lenard's experiments. The negative glow inside the
tube is due to the passage of the cathode rays
through the residual gas in the tube. If the tube is
filled with air the glow shows the bands of nitrogen
when examined with the spectroscope.
The cathode rays outside the tube are able to
act on a photographic plate. They are also able to
make the air through which they pass a conductor
When a highly exhausted tube, in which the
cathode rays are well developed, is brought near a
screen of fluorescent material, such as barium platino-
cyanide, the screen is observed to light up, even if
the vacuum tube is enveloped in black paper, and
has no thin window through which the cathode
rays can penetrate. This remarkable fact was
observed by Rontgen in 1896, and was the origin
of the discovery which has made his name famous.
He soon concluded that the new rays which pro-
duced this effect, and which have been named after
him, came from the place where the cathode rays
impinge on a solid obstacle ; from the green fluorescent
spot on the glass. It was natural to connect the
Rontgen rays with this fluorescence ; but it was found
a little later that the Rontgen rays were increased
in quantity rather than diminished when a metal
surface was arranged to receive the impact of the
cathode rays. As the metal surface was not fluor-
escent, it is evident that fluorescence is not the
essential condition for the production of Rontgen
rays. The essential thing is the sudden stoppage
of the cathode rays.
ELECTRIC DISCHARGE IN HIGH VACUA 21
The form of tube which experience has shown to
be the best for producing Rontgen rays is that
known as the focus tube (fig. 8). A saucer-shaped
FIG. 8. Focus bulb, a, for production of Rontgen rays. The cathode, b, is cup-
shaped, and the cathode rays from it (represented by dotted lines), converge on the
plate-shaped anode, c, inclined at 45 degrees. The Rontgen rays issue from the front of
cathode is used ; since the cathode rays are shot
out at right angles to the surface of the cathode,
they all converge to the centre of curvature of the
sphere of which the cathode forms a part. At that
point a slanting platinum target is placed to receive
them. Intense Rontgen rays issue from the point
at which the cathode rays strike. These rays, since
they issue from a single spot, are able to cast very
The Rontgen rays are able to pass to some extent
through all solid materials, but the facility with
which they are transmitted varies very much with
the material in question. The heavy metals are
the most opaque, and their opacity is greater than
that of light substances, such as wood or water,
quite out of proportion to their extra weight. In
this respect the Rontgen rays differ from the cathode
rays, for, as we have seen, the absorption of the
latter is nearly in proportion to the density. This
difference of transparency for different substances
is the property which enables the bones of a living
22 THE BECQUEREL RAYS
person to be seen or photographed. The rays are
able to penetrate flesh, which consists largely of
water. Bones, consisting of phosphate of lime, are not
nearly so transparent. The result is that, if the
hand or other part of the body is placed between the
tube and the fluorescent screen, the bones are seen
as a deep shadow, while the flesh gives only a faint
one. The rays are able to act on a photographic
plate, 1 and if that is used instead of the fluorescent
screen, it will record the appearance of the bone
It is found that the Rontgen rays are not at all
affected by a magnetic force. In this respect they
differ fundamentally from the cathode rays which
produced them. They are, moreover, very much
more penetrating than the latter.
The view of the nature of the Rontgen rays
which has gained general acceptance is that which
was first put forward by Sir George Stokes. He
considered that the Rontgen rays were thin pulses
of electric and magnetic force, due to the sudden
stoppage of the cathode particles. Electrical theory
shows that the stoppage of a moving charge may
be expected to produce such a pulse ; the pulse
would be of the same nature as ordinary light,
which is now regarded as the propagation of electric
waves ; but with this important difference, that the
length of the waves which are believed to constitute
Rontgen rays would be far shorter than that of the
waves which constitute visible light. We shall not,
however, be much concerned with Rontgen rays in
this book ; and it is unnecessary to discuss further
the difficult question of their origin and nature.
We have seen that the cathode rays are negatively
charged particles repelled from the cathode. If the
1 The plate is wrapped in black paper to pre\ 7 ent ordinary light affecting it.
ELECTBIC DISCHARGE IN HIGH VACUA 23
right experimental conditions are attained, positive
or anode rays, as they may be called, are attracted
up to the cathode. If the cathode is perforated,
(fig. 9) the momentum of these positive rays is
FIG. 9. Tube for showing the canal rays, a is the (perforated) cathode, and b the
anode. The canal rays issue through the hole at the back of the cathode, and cause
phosphorescence on the glass at c.
sufficient to carry them through it. They strike on
the walls of the tube, and, like the cathode rays,
produce phosphorescence at the point of impact.
The positive rays are far less conspicuous than
the cathode rays. They were discovered by Gold-
stein, long after the cathode rays were well known.
He called them canal rays, in allusion to the fact
that they were obtained through a channel in the
It has been found that the canal rays, like the
cathode rays, are deflected by a magnet ; this
deflection is in the opposite direction to that of the
cathode rays, and is far smaller. A magnetic form
sufficient to curl up the cathode rays into a very
small circle would scarcely deflect the canal rays
to any measurable extent. Canal rays, too, are
deflected by an electro-static field, in the opposite
direction to the cathode rays.
Measurements of the deflection exactly similar to
those for the cathode rays have been made by Wien
for the canal rays. His experiments proved that
the canal rays do not move quite so fast as the
cathode rays, and that the ratio of charge to mass.
24 THE BECQUEEEL BAYS
is beyond measure less, being of the same value as
for atoms of hydrogen, or, sometimes even smaller,
as for heavier atoms. This is not the place to
pursue the subject, but it will appear in the sequel
that positive charges are never associated with
masses of less than atomic dimensions. Negative
corpuscles have alone been shown to exist, and this
is probably the cause of the essential want of
symmetry in the behaviour of positive and negative
electricity towards matter.
In this chapter but a very small part of the
subject of electric discharge has been touched on.
Only those phenomena have been discussed which
bear on the interpretation of the phenomena of
radio-activity. We are now prepared to begin the
consideration of that subject.
DISCOVERY OF RADIO-ACTIVITY
The Active Elements
THE branch of science of which this work attempts
to give some account is of very recent growth. It
has excited extraordinary interest to the scientific
world, and has attracted a large number of workers.
Thanks to their labours, the subject has advanced
by leaps and bounds, and, though at the present
time our knowledge is still very imperfect in many
directions, we have the means of forming some
idea of the meaning and cause of the mysterious
phenomena which investigation has brought to
The first clue to the discovery of radio-activity
was given by the discovery of the Rontgen rays
We have already seen that these rays were, in
the earliest experiments, observed to issue from the
place where the cathode rays produced a green
fluorescence or luminescence of the glass.
In the early days of the discovery it was natural
to connect this greenish luminescence with the pro-
duction of the rays; and the question presented
itself, if we could produce the luminescence in other
ways, would it give rise to Eontgen rays as in
this case ? ,
Now many substances are known which, under
26 THE BECQUEREL RAYS
the influence of blue or violet light (itself of such
a quality as to be scarcely visible), are able to give
out a brilliant green luminescence. Conspicuous
among these are the salts of the rare metal uranium.
The commonest examples of uranium salts are
uranium nitrate, and potassium-uranyl sulphate. It
occurred to Prof. Henri Becquerel, of Paris, to try
whether these salts, when luminescent under the
influence of light, would give out Rontgen rays.
He exposed a photographic plate, wrapped in black
paper, to the action of the luminescent salts, and
found, after an exposure of some days, that a distinct
impression had been produced on the plate, which
appeared on development. It was natural to con-
clude that Rontgen rays were given off, as had
been thought likely.
Extraordinary as it may seem in face of the
result, this conclusion, as well as the reasoning
which led to it, was quite mistaken. We now
know that the fluorescence of the glass has nothing
to do with the production of the Eontgen rays.
We know, further, that the fluorescence of uranium
salts is quite unconnected with the invisible rays
which they emit. And lastly, we know that these
latter are of quite a different nature from the
Rontgen rays ! It seems a truly extraordinary
coincidence that so wonderful a discovery should
result from the following up of a series of false
clues. And it may well be doubted whether the
history of science affords any parallel to it. For
we can obtain the Rontgen rays even better by
letting the cathode rays fall on a metal surface
which is not fluorescent instead of a glass one
which is. We can obtain invisible radiation, able
to penetrate opaque substances, from uranium in
the metallic form, which is not fluorescent. And
DISCOVERY OF KADIO-ACTIVITY 27
lastly, as we shall see in the sequel, these uranium
rays differ altogether in their nature from Rontgen
This last conclusion, however, followed much later.
For most of them more easily ascertained character-
istics, the uranium rays, or Becquerel rays, as they
are now generally termed, in honour of their dis-
coverer, show a striking resemblance to the Rontgen
rays, and, on the other hand, a striking difference
from the rays of ordinary light.
We have already noticed the action on a photo-
graphic plate, and the penetration of opaque objects.
Another striking property is the absence of refraction.
One of the most familiar experiments on optics is the
bending of a ray of light by a glass prism; the behaviour
of the Rontgen rays is very different. They are able
to go straight through the prism without being turned
in the smallest degree out of their original path. The
same is true of the rays from uranium, though, on
account of their feebleness, it is more difficult to make
the experiment. If we place a little of the uranium
salt at the bottom of a narrow cavity in a block of
lead, so as to confine the rays into a narrow beam,
we shall find that, placing a photographic plate in
front of the opening a short distance away, we obtain
an impression marking the point at which the rays
strike the plate. If now we interpose a small prism
of wood, glass, aluminium, or any other material, in
the course of the beam, we shall find that the rays
strike on the plate in exactly the same position as at
first. The rays are not bent at all out of their
original direction. This experiment is difficult with
uranium, but it can now be easily repeated by
making use of radium, which, as we shall see later,
gives effects of the same kind as uranium but of
incomparably greater power.
THE BECQUEREL RAYS
One of the most remarkable properties of the
Rontgen rays is their power to make the air through
which they pass a conductor of electricity. It must
be understood that this conducting power is very
much inferior to that of the metals, or even to that
of acidulated water. It would not be possible, for
instance, to pass an electric current through the air
when made conducting by the Rontgen rays, which
would be sufficiently strong to ring an electric bell,
or to light an electric lamp.
The effect can readily be shown by means of a gold-
leaf electroscope (fig. 10). This instrument, it will
<c^ <=>e -~^> be remembered, consists of two
leaves of thin gold-leaf hung side
by side from an insulated support.
When a charge of electricity is
imparted to them, they repel one
another, being similarly electrified,
and, in consequence, they stand
apart. The more highly charged
they may be, the larger will be the
divergence. If we charge such an
instrument by touching it with a
piece of sealing-wax which has been
(^5^====^::^^ electrified by rubbing it on the coat-
sleeve, the leaves will stand apart
FIG. 10. Gold leaf electro- ? i i i . *r ii
scope of the ordinary form, tor a considerable time, it the air
A bell-jar, a, is fitted with an , ...
Through around the instrument is in its ordi-
nary non-conducting state. But,
,% p a a re stripf S if Rontgen rays are allowed to fall
tinfoil communicating with , ,1 ~i
the base of the instrument, Upon it, tlie air bCCOmCS a COn-
and so with the earth. When -, -, , < i * *
a charge of electricity is im- ductor, the charge of electricity
parted to t. t the leaves stand ** u J
apart by mutual repulsion, leaks away through it, and the gold
leaves fall together. The action is
very apparent indeed, and cannot
escape the most careless observation. I have shown
ebonite stopper, b.
this passes a brass rod,
DISCOVERY OF RADIO-ACTIVITY 29
it at the Royal Institution with the bulb producing
the rays up in the gallery of the theatre, while the
electroscope, projected on a screen by means of the
optical lantern, was near the table.
In the early days of the discovery of the uranium rays
Becquerel obtained similar effects from them. With
uranium, the collapse of the leaves is not nearly so
quick as with Rontgen rays, if these are of the
intensity commonly used for obtaining photographs
of the bones.
Still there is no difficulty in verifying Becquerel's
result. Some crystals of uranium metal may be
spread on the flat disc communicating with the gold
leaves with which most electroscopes are provided.
It will be found that the leaves do not go down
appreciably in the course of a few minutes 1 if the
uranium is withdrawn to a distance. But when it
is placed on the disc, the leaves lose their charge
altogether in that time.
The discovery of the activity of uranium raised the
question of whether any other of the seventy or more
known elements possessed similar properties. It was
certain that none of the materials, such as brass,
copper, tin, glass, or iron, which are used in the con-
struction of electrical instruments, could possess the
power to any extent. For if they did, the leakage
of electricity through the air due to their presence
could not have failed to make itself apparent. It
remained to make a systematic search among the
less-known elements. The result was to show that
one, and one only, of the elements up to that time
known possessed the same power as uranium. That
element rwas thorium. The element is rare indeed,
but not so scarce as to be difficult to obtain. It is
1 This, of course, supposes that the electroscope is in good working
30 THE BECQUEREL RAYS
remarkable as having the heaviest atom of any known
The activity of thorium is about the same as that
of uranium. The discovery of its activity is due to
Schmidt. Thorium is contained in the Welsbach
incandescent gas mantles. Plate I. is the photo-
graphic impression produced by one of these mantles
on a plate, due to its radio-activity.
Any one who has followed so far and who is
familiar with the doctrine of the conservation of energy,
will not fail to ask, Where does the energy come from
which has enabled the uranium to affect a photographic
plate and to throw the air into a conducting state ?
For,if there is any scientific doctrine which isthoroughly
well established by experience, it is that the amount
of energy in the universe remains strictly the same
at all times. If the uranium gives out energy, there,
are only two possible sources for this. Either the
uranium contains the energy stored up in itself, and
is slowly exhausting its stock, or, on the other hand,
it is able to draw its supplies from without and to
transmute them into the form of Becquerel radiation,
as the invisible radiation is called.
Becquerel was at first naturally inclined to the
former supposition. The simplest explanation of the
power which uranium possessed of giving out energy,
was to suppose that it had stored up this energy
while previously exposed to light. Such a storing
up of energy is by no means outside experience. A
conspicuous instance of it is in the phosphorescent
substances such as calcium sulphide. This substance
is familiar to every one in the form of Balmain's
luminous paint, used for making match-boxes luminous
so as to be easily found in a dark room.
Calcium sulphide glows in the dark after exposure
to sunlight. The glow gradually becomes fainter
DISCOVERY OF RADIO- ACTIVITY 31
and fainter until it has altogether disappeared. But
the substance must be left for a considerable time in
the dark before this happens. Exposure to light
revives the luminosity again. It was thought, then,
that the behaviour of uranium might be analogous to
this. But experiment did not confirm the idea. For
it was found that uranium salts which had just been
exposed to the sunshine were exactly equal in
photographic power to those which, had been long
kept in the dark. Indeed, it was found that if the
uranium nitrate was actually exposed to sunshine
while it was acting on the plate (the latter of course
wrapped in black paper), there was no increase in
effect. An even more crucial test might be made
with some mineral containing uranium. If a piece
of such a mineral were broken in half in the dark, and
the activity of the freshly exposed surfaces was
tested, it would be found that it was absolutely
normal. It is evident that the material constituting
these surfaces has never seen the light since the
formation of the mineral, which, there can be no
doubt, occurred countless ages before the appearance
of man on the globe. So that the experiment would
show conclusively that previous exposure to light had
nothing to do with the matter. I am not aware that
the experiment has been made in this form, but there
can be no doubt of what the result would be, in the
light of our present knowledge.
Another suggestion which was made as to the
source from which uranium got its energy, was that
it was able to draw energy from the surrounding
air. According to the kinetic theory of gases, air
consists of a number of minute particles called mole-
cules, which produce pressure on the walls of the
containing vessel by their constant impacts with it.
These molecules are believed to be flying about with
32 THE BECQUEREL RAYS
varying velocities, some below the general average,
others above. But the mean velocity 1 about which the
actual velocities fluctuate, depends on the tempera-
ture, and is fixed so long as the temperature remains
steady. At the freezing point it is, in the case of air,
more than a thousand feet per second.
Now the molecules moving with a velocity above
the mean, would, if they could be separated from
the rest, have a greater mean velocity than these.
Consequently these fast molecules would constitute
a hotter portion of air, and the others a colder
one, than the original air. Can such a separation
be practically effected ? All previous experience
goes to show that it cannot without calling in the
aid of other external sources of energy. Is it possible
that in uranium we have at last been able to find an
agent for utilising the energy which exists in the
heat of the surroundings, without making use of any
pre-existing difference of temperature ? It certainly
cannot be said to be a priori an impossibility, for we can
give no proof that energy cannot be made available
in this way. All we can say is that human experience
has failed up till now to furnish any case of such a
thing being done. 2
Attractive though this suggestion may seem, there
are insuperable objections to it. When we come to
deal with radium, an experiment will be described
which conclusively shows that radio-active bodies
do not acquire their energy in any such way. In
1 In strictness it is the velocity of mean square that is concerned. To
compute it, we take the square of the velocities, average them, and take the
square root of the result.
2 An article appeared in a popular magazine a few years ago, describing a
process of obtaining liquid air, and using it to obtain more liquid air, with a
surplus of power remaining over. It excited a good deal of enthusiasm among
the unscientific public. If this were really feasible it would be a case in
point. But those who had any knowledge of the theory of heat ventured to
doubt whether it had been done as asserted ; and the result has justified
DISCOVERY OF RADIO-ACTIVITY 33
the meantime we may note that if the air was the
source of energy, it might be expected that the
radio-activity would be reduced by placing the sub-
stance in an exhausted vessel. Such, however, is not
found to be the case.
We are, then, reduced to assume, either that the
uranium is acquiring its energy from some external
source by a process of which we can form no con-
ception, or that it is giving off potential energy of its
own which it had possessed all along, undergoing, of
course, some change in the process.
It may be asked, if this latter assumption is to be
considered, How is it that the change of other qualities
which must accompany this loss of energy does not
become visible ? How, in fact, can uranium remain
uranium ? The answer must be that it does not
that if we could only watch it for a sufficient time,
the change in its qualities, when it had given off its
energy, would become apparent. The rate of emission
of energy is so small when compared with the total
stock that the uranium possesses, that, during the
hundred years or so that the metal has been known,
the changes proceeding in it have not visibly altered
its properties. This is the view now generally held,
and we shall see in later chapters that there is very
strong evidence in support of it.
It has been explained how the wonderful radiating
properties of uranium were discovered. We now
come to the more sensational developments which
have attracted so much attention of late.
It is found that all the compounds of uranium emit
Becquerel rays. They are all able to affect a photo-
graphic plate which has been wrapped in black paper,
and they can all discharge a gold-leaf electroscope.
It was a natural and obvious experiment to compare
the various compounds of uranium amongst them-
THE BECQUEREL KAYS
selves, in order to see which was the most active.
The experiment is not difficult. It is merely neces-
sary to place equal quantities of the various com-
pounds successively on the plate of the gold-leaf
electroscope, and to compare the times \vhich the
leaves take to collapse in each case.
For this kind of work a special form of electroscope
has been devised, which is very convenient. The leaf
of the electroscope can be observed by means of a
microscope with a scale in the eye- piece, which is
focussed on it. Thus the rate at which the leaf goes
FIG. 11. Electroscope designed by M. Curie for measurements
in radio-activity. The instrument is enclosed in a brass case,
a b c. The front wall is of clear glass, the back one of obscured
glass, to provide a diffused light. A metal strip, d, hangs by an
insulating support, /, from the roof. To this strip the gold leaf,
e, is attached. A metal plate, h, is connected to d by a brass
rod, which passes, without touching, through a hole in the case.
A second metal plate, g, is in metallic communication with the
case. On this the radio-active substance is spread, and causes a
leak of electricity from the electrified plate, h. The leaf thus
gradually collapses. The rate of motion is read by means of a
microscope with scale in the eyepiece, focused on the leaf. For
charging the instrument, a knob (not shown) is provided, which
projects outside the case, carried by the rod, k. This knob,
also the plates h, g, are provided with metal covers to screen
them from external influence.
down can be read. A metal plate is provided to
carry the radio-active substance (fig. 11).
Comparisons of the uranium salts were carried out
by Madame Curie. She found that the various com-
pounds did not differ much among themselves in
activity. The compound of uranium with oxygen,
THE DISCOVERY OF RADIUM 35
for instance, known as uranium oxide, discharged the
electricity about as fast as the compound with nitric
acid, uranium nitrate. There were slight differences,
attributable to the fact that some of the compounds
contain a larger percentage of the metal than others.
But these differences were comparatively unimportant.
The essential thing was, apparently, the presence of
uranium. What other element it might be combined
with did not matter much. Indeed, an ingot of the
metal itself gave a result not very different from the
THE DISCOVERY OF RADIUM
Uranium, like most other metals, is obtained
from an ore which is quarried, or mined, out of the
earth. This ore is called pitchblende. It is not a
very common mineral. When it does occur, it is
usually found among igneous rocks, like granite or
felspar (Plate II). It is very valuable, as it consists
mainly of uranium oxide, which is in demand for
making canary glass.
Madame Curie turned her attention to this sub-
stance, to see whether it behaved like the artificial
compounds of uranium. To her surprise she found
that it was very much more powerful than any of
them. One specimen of pitchblende, for instance,
which had come from Joachimsthal in Bohemia, was
three or four times more active than uranium.
What could be the explanation of this ? Why
should the native compound of uranium be so much
more energetic than the artificial purified product ?
Madame Curie's answer was that pitchblende con-
tained, besides uranium, some new substance which
was far more radio-active.
It has been mentioned that pitchblende consists
mainly of uranium oxide. But in addition to this it
36 THE BECQUEKEL BAYS
contains small quantities of many metals, such as
iron, copper, lead, bismuth, and others. Indeed,
there are probably few metals which would not be
found in it, if the search were carried out with suffi-
cient care. The problem of separating pitchblende
into all its constituent parts is, accordingly, very
complex. The methods of chemical analysis, however,
make it possible to effect the separation.
Monsieur and Madame Curie proceeded to do this,
and they tested at every stage the radio-activity
of the products, using the gold-leaf electroscope as
It will not be necessary to enter into the details of
this process of separation, but some general idea of it
may be given. In the first place, the mineral is
heated with a suitable flux, and then dissolved in
hydrochloric acid. The solution is treated with the
gas known as sulphuretted hydrogen, a compound
of hydrogen and sulphur, which possesses the smell
characteristic of rotten eggs. A black mud, or pre-
cipitate, is immediately thrown down from the clear
solution. This is separated from the solution by
filtering. It is then dried, and tested with the
electroscope. It is found to be radio-active, and, on
further separating it into its constituents, which
include, amongst others, copper, bismuth, and lead,
the radio-activity is found to accompany the metal
The clear solution from which the black precipitate
was separated is now free from the metals which the
precipitate contained. Ammonia is added to it, and
a new precipitate is formed. This precipitate con-
tains iron, uranium, and other elements, and is, of
course, also radio-active, owing to the presence of
Separating the clear liquid by filtration, as before,
THE DISCOVERY OF RADIUM 37
we add ammonium sulphide, a reagent prepared by
passing a stream of sulphuretted hydrogen into
ammonia. This separates a further precipitate, which
is not appreciably active.
The clear liquid, or filtrate, as it is called,
which is left after this operation is treated
with carbonate of ammonia, often known as sal
volatile. This time a precipitate is obtained which
is far more active than the original pitchblende. The
principal active constituent has at last been run to
earth. The precipitate obtained by adding carbonate
of ammonia contains those metals which are called
metals of the alkaline earths. Calcium, which is the
metallic constituent of chalk and lime, is the most
abundantly distributed of these. Lime is the com-
pound of calcium and oxygen, just as rust is the
compound of iron and oxygen. It is, in fact, the rust
Another alkaline earth metal is known as barium.
This is much less common than calcium, but still
fairly abundant, and it is this metal which is chiefly
present in the precipitate which the solution of pitch-
blende gave when treated with carbonate of ammonia.
Is it, then, to be concluded that barium is radio-
active ? The answer must be in the negative, for it
is found that barium extracted from other minerals,
such as barytes or heavy spar, is not at all so. It is
evident that the barium from pitchblende contains
some other substance, which is so like it in chemical
properties that all the chemical operations which the
material has undergone have failed to separate them.
There is a method which hardly ever fails to sepa-
rate two metals, however closely they may resemble
one another, and that is the method of fractional
crystallisation. To explain this method a slight
digression will be necessary. Suppose that we have
38 THE BECQUEREL EAYS
a salt of barium, barium chloride for instance, and
dissolve it in water. It will be found that the
amount that will dissolve at the ordinary temperature
is limited and definite. But, if the water is heated
to the boiling point, we can dissolve much more. If
we dissolve as much as we can in boiling water, and
then let the clear solution cool down, what will
happen ? The solution cannot, when cold, hold so
much salt as it did when hot, and so some of the salt
will separate out in crystals on cooling. Suppose,
now, that we have a mixture containing both barium
chloride and calcium chloride. We dissolve this in
water, adding enough water to dissolve the whole of
it. We then heat the water to boiling, and boil it
away until salt just begins to separate out from the
boiling solution. Then we let it cool. Part of the
salt separates out ; but, since barium chloride is less
soluble than calcium chloride, it is more ready to
separate from the solution. So the separated salt
contains more, and the solution contains less, of the
barium chloride than the original salt did. One step
has been taken towards separation. Next we collect
the separated salt, dissolve it in water, and treat the
solution exactly as before. This time the salt which
separates is still richer in barium chloride, and, if we
go on long enough, we shall, in the end, be able bo
obtain the barium chloride quite free from calcium
chloride. The process is slow, but sure.
This example illustrates the process which M. and
Mme. Curie applied to the radio-active barium which
they had obtained from pitchblende. They converted
it into the chloride, dissolved it in water, and boiled
the water until crystallisation commenced. Then they
allowed it to cool, and collected the separated crystals.
These crystals when dried were found to be more
active than the material from which the solution had
THE DISCOVERY OF RADIUM 39
been made. The material still in solution when re-
covered was, of course, less active. It was evident,
therefore, that associated with the barium chloride
there was a radio-active metal, allied to barium, which
formed a chloride less soluble than barium chloride.
It had been partially separated from the barium
chloride, just as, in the example we gave, barium
chloride was separated from calcium chloride. This
new element was named Radium, in allusion to its
By working on large quantities of material, and
by repeated fractional crystallisations, M. and Mme.
Curie obtained products more and more active. At
every crystallisation the activity became greater, and
at first it seemed as if there was no limit. It will
be understood that, starting with a small quantity
of the mixture, the amount becomes less at every
operation, until, after a certain number of crystallisa-
tions, the amount remaining becomes too small for
further treatment. The only escape from this diffi-
culty is to take very large quantities of the mineral
to begin with. Pitchblende is a valuable mineral, as
we have seen, because of the uranium contained in
it. But the residues left after the uranium had been
extracted were worth little or nothing, and could be
had for the asking, though that is no longer the 'case.
These residues contained all, or nearly all, the barium
and radium, and several tons of this material were
worked up by M. and Mme. Curie. Now that the
properties of the material sought for were known, it
was no longer necessary to separate every constituent
in the mineral. It was merely necessary to obtain the
barium with its accompanying radium. For this purpose
a process which was simpler and more direct than that
which we have followed was devised (see Appendix C).
It will be unnecessary to follow this in detail ; suffice
40 THE BECQUEREL BAYS
it to say that many pounds of the barium chloride
containing radium were obtained. The mixture was
systematically treated by fractional crystallisation.
At every step stronger and stronger products were
obtained. After several operations a product no less
than thirty times more active than uranium was ob-
tained. Surely at last the product, if not pure, must
at least contain a large proportion of the new element ?
A piece of platinum wire was dipped in the salt, and
then into the blue non-luminous flame of a bunsen
gas burner. The characteristic green coloration pro-
duced by salts of barium was visible, and when the
light was analysed by means of a prism, the only rays
which were at all conspicuous in the spectrum were
those produced by barium salts. But, on closer
examination, faint traces of new lines were visible.
These were attributed to the new element, but it was
evident that, even in this intensely active product,
the proportion of the new element was small. It was
necessary to persist with the fractional crystallisation.
The products became more and more powerful, until
at last, when the salt was examined in a bunsen
flame, the green colour due to barium began to become
less conspicuous, and the red rays characteristic of
the new substance began to be predominant. Finally,
when the quantity of salt had been reduced to the
merest pinch, by the successive reductions at each
separation, the barium rays could no longer be seen.
Pure radium chloride had at last been isolated, at
the expense of almost incredible labour and persever-
ance. The activity of the product so obtained was
truly amazing. It discharged the electroscope a
million times faster than uranium. It would almost
instantly fog a photographic plate brought near
it. Sir William Huggins has mentioned a striking
example of its power in this respect. Five milli-
THE DISCOVERY OF RADIUM 41
grammes of the salt, which would, perhaps, be about
the size of a grain of corn, were put in the top drawer
of an ordinary writing-table. A packet of photo-
graphic plates happened to be in the bottom drawer
of the same table. These were required for use a few
days later ; but it was found that they had been com-
pletely fogged by the radium, which was, probably,
eighteen inches off, and separated from them by the
contents of the intervening drawers !
Different samples of pitchblende vary very much
in the quantity of radium that they contain. English
pitchblende, from Cornwall, contains much less than
the Joachims thai ore used by M. and Mme. Curie.
Pitchblende is the only source from which radium
has yet been extracted in the pure state. It is, how-
ever, by no means the only mineral which contains
the element. Most of those minerals which contain
uranium, thorium, and the rare earth metals, cerium,
yttrium, etc., contain traces of radium also (see
Appendix D). Examples of these are samarskite,
a black, heavy, lustrous mineral, which contains the
oxides of many metals, including niobium, tantalum,
and yttrium ; fergusonite, another black mineral from
felspar rocks in Sweden ; and cleveite, a substance
allied in its nature to pitchblende, and interesting
on account of the fact that it was the first discovered
source of terrestrial helium. All these substances
are of very rare occurrence, and the proportion of
radium contained in them is even smaller than in
Radium occurs also in certain mineral waters,
notably in those of Bath and Buxton. But the pro-
portion is very small indeed. Nor indeed could it
be otherwise ; for these waters all contain sulphates
of lime and other metals in solution. These sulphates
would precipitate almost all the radium that might
42 THE BECQUEREL EAYS
be in solution in the form of radium sulphate. No
saline substance that we know of is absolutely
insoluble. But the sulphate of barium only dissolves
very sparingly. And in all probability the sulphate
of radium, the next in the series of alkaline earth
metals, is very much less soluble still. It is inter-
esting to note in passing that the proportion of
barium sulphate in a saturated solution is about fifty
times as great as the proportion of radium in pitch-
blende. And it is usual to regard barium sulphate,
for the ordinary purposes of chemistry, as absolutely
I have found that the iron deposit left by the Bath
water contains much more radium than the salt
obtained by evaporating the water. It contains as
much, in fact, as some of the less active minerals.
The same is true of the Buxton deposit. But the
annual yield of deposit by the springs is very small
in both cases.
It would be very interesting to know whether the
medicinal value of these mineral waters is connected
with the presence of radium in them. Some of the
waters which have great repute are so nearly pure
that their virtue has always been regarded as very
mysterious. There does not seem to be any sufficient
peculiarity revealed by the analysis of the water to
explain why it should be better than any ordinary
spring water. If this could be traced to the presence
of radium, the mystery would be in great measure
explained. For the presence of even a very small
proportion of a substance so energetic might be
expected to produce remarkable results.
Comparatively few waters have been examined
as yet. But the subject is ' in the air/ and, no
doubt, large additions to our knowledge will soon
THE DISCOVERY OF EADIUM 43
Many kinds of soil and rock also contain traces of
radium. But these traces are very slight indeed,
and can only be detected by special methods. The
element, rare as it is, is none the less distributed
in recognisable traces almost everywhere.
The question is often asked, Is there any pro-
bability that radium will ever be found in large
It would be rash to make any confident assertion
on the subject. It must be admitted that several
elements, which at the time of their discovery were
very rare, have turned out to be in reality relatively
abundant. Thorium and vanadium are examples of
this. Thorium, for instance, was originally found in
the rare Norwegian mineral thorite. Berzelius, who
was the first to investigate the properties of the
element, was only able to obtain enough material to
furnish him with a few grammes of it. But now it
has been found in large quantities in the mineral
monazite in Brazil, and is worked up commercially
by the ton for use in preparing the mantles of incan-
descent gas lamps.
We cannot feel sure, therefore, that because the
supplies of a new element are scanty at first, they
necessarily remain permanently so. But in the case
of radium the prospects of obtaining large quantities
do not seem to be very encouraging. The presence
of radium in a mineral, even one part in one hundred
million, can be detected in one or two minutes by the
electrical test. So that it is far more easily searched
for than other elements, which require the applica-
tion of tedious chemical processes to determine their
presence or absence. Accordingly the search for
radium has been carried out very thoroughly. And
the result has been to make it certain that it is
indeed very much rarer than any other known
44 THE BECQUEREL RAYS
element, with the possible exception of the rare
gases, helium, neon, krypton, and xenon.
It is not beyond the bounds of possibility that a
mineral may be found which contains radium as a
principal constituent. But it does not seem likely
that such a mineral is to be found in any of the very
extensive collections which exist. Such collections
have been examined, and the radio-activity of promis-
ing minerals has been tested with the electroscope.
But nothing more active than pitchblende has been
found. Indeed, if any mineral containing a consider-
able percentage of radium existed in such a collection
it would not have escaped notice. For the glass of
the show-case containing it would have been coloured
a deep violet by the Becquerel rays.
There are no doubt large portions of the earth's
surface of which the mineral constituents have not
been minutely examined. It is by no means improb-
able that a richer variety of pitchblende than any
yet known may be found in such imperfectly explored
regions. It is even possible that a mineral may be
found which contains radium as a principal con-
stituent. These are, so far as can be seen at present,
the only ways in which the element can become
appreciably easier to procure than it is now. 1
It was mentioned, in describing the analysis of
pitchblende for radio-active substances, that the
precipitate formed by sulphuretted hydrogen in acid
solutions is radio-active. This activity is found on
further analysis of the precipitate to be mainly
associated with the bismuth contained in it.
1 In the light of the most recent investigation, it seems probable that
the radium in a mineral bears a fixed and very small ratio to the uranium.
If that is so, all hope of obtaining radium in quantity must be abandoned.
Bismuth is present to a considerable extent in the
pitchblende. Ordinary bismuth is quite without
radio-activity, so that it is clear that the radio-
activity is not due essentially to the bismuth itself,
but to some radio-active substance contained in it,
which is very similar in its chemical behaviour. This
new substance has been named by Mme. Curie,
polonium, in honour of her native country.
It is very doubtful whether polonium has been
isolated in a pure state. The separation from the
accompanying bismuth was found by Mme. Curie
very much more difficult than the separation of
radium from barium.
The methods employed by her were very similar in
principle to those used for the latter separation.
They depended, however, on fractional precipitation
instead of fractional crystallisation. Polonium-
bismuth was dissolved in nitric acid, and water
added to the solution. This process partially pre-
cipitated the material in the form of a basic salt,
which was found to be much richer in the active
constituent than the original substance had been.
The process was repeated many times, and eventually
a very concentrated product obtained, which possessed
intense radio-activity. It was not found, however, to
exhibit any characteristic spectrum distinct from that
The fractionation was very difficult to carry to
great lengths, for insoluble compounds were formed
which could only be got into solution by the tedious
process of first reducing them to the metallic form by
fusion with potassium cyanide. Attempts to frac-
tionate the radio-active bismuth by other methods
met with no better success. The most active pro-
ducts which could be got still evidently consisted for
the most part of pure bismuth.
46 THE BECQUEKEL RAYS
More recently Professor Marckwald has discovered
a far more simple and effective method of separation.
He does not make use of sulphuretted hydrogen in
separating the bismuth from the original pitchblende,
but precipitates it with water from a hydrochloric
acid solution of the mineral. The bismuth is dis-
solved up afresh in hydrochloric acid, and a rod or
plate of metallic bismuth immersed in it. The result
is that the radio-active material is wholly deposited
on the immersed metal in the course of a few days.
None at all remains in the solution.
This process of separation is quite analogous to
many other familiar ones. For instance, if an iron
knife is placed in a dilute solution of copper sulphate,
the copper will all be deposited upon it, while some of
the iron will go into solution and will replace the
copper in its combination with sulphuric acid.
It is found that copper as well as bismuth may be
used for the deposition of polonium.
The amount of deposit obtained is exceedingly
minute, but of very high activity, sufficient to pro-
duce conspicuous phosphorescent effects, such as will
be described in the next chapter. But even so, it
does not consist wholly, or even mainly, of polonium ;
the principal constituent is tellurium, a rare element
which occurs in pitchblende in minute traces. The
tellurium can be precipitated from a solution of the
deposit by means of hydrazin-hydrate, and polonium,
presumably in a pure state, remains in solution.
Only four milligrammes were obtainable from two tons
of pitchblende, so that the proportion present is only
one part in five hundred million.
The activity of the product is so great that an
electric current can be sent through the air in its
neighbourhood sufficiently strong to ring an electric
It is right to state that Professor Marckwald does
not regard the product obtained by him as identical
with Mme. Curie's polonium. But his reasons for
considering it different are not, so far as the author is
aware, generally regarded by current scientific opinion
as having much weight.
The nature of polonium is a somewhat obscure
subject. It is certain that it diminishes in activity
as time goes on, and some have for that reason re-
garded it as being a form of bismuth made active by
the neighbourhood of radium, rather than a true
radio-active element. But in the light of recent
investigation this objection has lost its cogency, as
we shall see in a later chapter.
Polonium appears to lose half its activity in the
course of about nine months. But this subject
requires further and more minute investigation than
it has yet received. Mme. Curie has recorded that
in some specimens of polonium the activity disappears
more quickly than in others. If this should be con-
firmed, the most probable explanation would seem to
be that two substances are present whose activity
decays at different rates. If these are present in
varying proportions, the aggregate effect might decay
at any intermediate rate.
The quantities of polonium hitherto prepared have
been too small to allow of a determination of its
atomic weight. Its general chemical behaviour is
very closely analogous to that of bismuth. Whether
other minerals besides pitchblende contain it has not
yet been ascertained.
Subsequently to the original investigation of M.
and Mme. Curie on pitchblende, which lead to the
discovery of radium and polonium, M. Debierne was
48 THE BECQUEEEL BAYS
able to establish the presence of a third new radio-
active element, to which he gave the name actinium.
This substance is nearly allied to thorium in its
chemical behaviour. It is precipitated, along with
uranium and many other metals, in the ammonia
group, in the process of analytical separation of the
constituents of pitchblende. But for preparing it in
practice, the waste residue, free from uranium, is
used. 1 M. Debierne has mentioned several reactions
which may be used for separating a product rich in
actinium from the mixture of other metals in the
ammonia precipitate. One of these is to dissolve
the precipitate in weak hydrofluoric acid ; it is found
the actinium remains in the least soluble portion.
An alternative method is to add some soluble barium
salt to a solution containing actinium, and then to
precipitate the barium again by means of sulphuric
acid. The sulphate of barium carries down the
actinium with it. This procedure is recommended as
being specially effective.
Much less is known of actinium than of radium and
polonium ; doubtless because the material is very
scarce indeed, and because the chemical separations
necessary for preparing it are difficult to carry out,
actinium has not yet become obtainable commercially.
Pure actinium has not been obtained. Even the
strongest products containing it consist mainly of
thorium. Thus we have no information as to its
atomic weight. Nor has it been observed to yield a
In addition to the radio-active elements which
have been discussed, and whose existence is well
established, a radio-active substance associated with
the lead of pitchblende has been described. Sufficient
evidence has scarcely yet been accumulated to deter-
1 See Appendix C.
mine whether this is a genuine radio-active element,
or whether it is only made active by traces of radium,
or by having been in the same solution with radio-
To sum up, the well-established radio-active ele-
ments are :
Polonium is less certainly a genuine element,
though the balance of evidence now seems to be in
favour of the view that it is one. Radio-active lead
is very doubtful.
The first three only have been obtained in weigh-
able quantities in the pure state.
THE PROPERTIES AND NATURE OF THE RADIATIONS
IN the foregoing chapters we have, in the main,
followed the historical order of development of our
subject. But, in pursuing it further, it will be more
convenient to abandon this plan. The number of
workers in the field of radio-activity has been very
large, and will probably increase still more, as the
supreme importance of the subject, and the chance
which it seems to give of far-reaching discoveries,
comes to be recognised. The work of different in-
vestigators is so interwoven, and so many have
assisted in the development of particular points, that
it would lead to hopeless confusion if an attempt
were made to present the subject in the order in
which it has been actually unfolded.
A question which cannot have failed to present
itself to every reader of the foregoing pages is this :
What is the nature of the mysterious influence
which is emitted from radio-active bodies, which can
penetrate, which can affect photographic plates, and
can discharge electricity, even after passing through
solid metal screens ?
Before coming to this, we shall describe some
farther experiments with the rays. We shall begin
with some which show that, whatever the Becquerel
rays may be, they are not all of the same kind.
Suppose we take a very small quantity of radium
salt and place it on the lower plate of the electro-
PROPERTIES OF THE RADIATIONS 51
scope (fig. 11),, the rate of discharge will be extremely
rapid. If the leaf is given a charge of electricity, it
will lose it almost immediately, owing to the discharg-
ing power of the radium. But if we place one sheet
of common tinfoil over the radium, and try again, we
shall find that the charge is lost, quickly, indeed, but
still very much less so than before. Perhaps the rate
of discharge will now be only one-tenth as great as
it was at first. Let us next put on a second sheet.
What is the rate of discharge to be now ? Will the
second sheet reduce it tenfold again, like the first ?
By no means. We find that the second sheet pro-
duces very little effect. It is evident that the rays
which are able to get through a sheet of tinfoil are
of quite a different kind from the bulk of those given
out by a bare radium salt. The sheet of foil has
acted like a filter. It has filtered out and stopped
an easily absorbed kind of rays, and it has allowed
another kind of rays, which are more penetrating
than the first, to get through. These penetrating
rays are not appreciably absorbed by a sheet of this
thickness, and consequently the insertion of the second
sheet does not affect them materially.
It may be asked, Are the rays which survive the
first and second sheet of tinfoil all of the same kind,
or do they include a third kind, even more penetrat-
ing ? Experiment shows that they do. If we place
a block of lead a quarter of an inch thick on the
radium, we shall find a great reduction of discharg-
ing power. It may now take some minutes for the
motion of the gold leaves past the divisions of the
scale to become apparent. If we add another such
block, the motion of the leaves 'will, indeed, be slower
still, but not very markedly so. The second kind of
rays have been filtered out, and a third kind, still
more penetrating, has been left.
52 THE BECQUEREL RAYS
The first kind of rays, those which are stopped by
very thin screens, have become known as the a-rays.
The second kind as the /3-rays, and the third kind,
the most penetrating of all, the y-rays.
There are certain properties common to all three
kinds of rays. They can all produce electric dis-
charge, as we have seen ; and they can all act on the
photographic plate. These are the methods which
are generally most convenient for the detection of
FLUORESCENCE PRODUCED BY THE RAYS
Another striking property which the rays possess
is that of causing fluorescence ; that is, they can
cause certain substances to glow with a luminosity
easily visible in the dark, but unaccompanied by
appreciable rise of temperature. The diamond is one
of the most conspicuously fluorescent substances.
The experiment is very easy. It is merely necessary
to go into a dark room provided with a small quantity
of radium bromide, and to bring it near a diamond
ornament. The stones at once glow with a bluish
light. If the ornament includes other stones, such
as rubies, these will appear quite black. A ring con-
sisting of rubies and diamonds alternately shows this
very effectively. The diamonds shine out brilliantly,
while the spaces between them, filled by the rubies,
This fluorescence of diamonds forms a very con-
venient test of their genuineness, quite within the
reach of any one who can obtain the use of a little
radium. For imitation diamonds do not fluoresce, or
at least so slightly compared with the real ones, that
there is no possibility of mistaking them. In this
simple way real diamonds can be distinguished from
false ones, without any expert knowledge whatever.
Other substances which are brilliantly fluorescent
are zinc sulphide, which glows most brilliantly under
the a-rays, and barium platino-cyanide, which is best
for the ft- and y-rays.
Screens are made of these substances spread on
cardboard, which are very convenient for experi-
mental purposes. The platino-cyanide screens sold
for showing the bones of the hand with the Rontgen
rays, are also suitable for observations with radium.
If a small quantity of radium be brought up to such
a screen, it will be seen to be faintly luminous with
the rays falling on it. On bringing the radium
nearer, the luminosity becomes stronger, but is of
course spread over a smaller area. The luminosity
is nearly as bright if the radium is brought up on
the blank side of the screen, since the rays can easily
penetrate the cardboard, and excite the luminosity
of the prepared surface. The luminosity will still
be seen if a piece of metal is inserted in the path of
the rays, before they fall on the screen. If the hand
be placed against the back of the screen, and the
radium held some inches off, the shadow of the fingers
will be seen on the screen, and it may even be possible
to make out something of the bones. The difference
between flesh and bone is not nearly so clear, how-
ever, as with Rontgen rays. We shall return to this
An interesting example of fluorescence under
Becquerel rays is afforded by the tissues of the
eye itself. The effect is easily observed. Close the
eye, and cover it with black paper, so as to exclude
all light from without. Now bring up the radium
outside. Distinct luminosity will be perceived, owing
to the fluorescence of the tissues under the Becquerel
rays, which penetrated the paper and the closed
54 THE BECQUEREL KAYS
The fluorescence is naturally most intense when the
fluorescent substance is very close to the radium.
The most brilliant effects of all are obtained by
mixing them. If some zinc sulphide is mixed with
radium bromide, and any design painted on a card-
board screen, it will glow perpetually in the dark.
It is natural to inquire whether there are any means
of foreseeing whether a substance will be fluorescent
under the Becquerel rays, or not. There are no means
by which we can be sure, for the reason why some
substances fluoresce, and others do not, is quite
obscure. There seems to be a connection between
fluorescence under Becquerel rays and fluorescence
under the action of light. Many of the substances
which fluoresce under one agent, also fluoresce under
the other. Barium platino- cyanide is a conspicuous
example of such coincidence. But the rule is far from
universal. Some of the aniline dyes are very fluor-
escent under the action of blue or violet light,
lighting up with a brilliant green colour. But they
remain quite dark in the neighbourhood of radium.
Again, the majority of diamonds do not fluoresce
much under the action of light, while they all appear
to do so to some extent under the Becquerel rays.
In some cases, luminosity produced by the
Becquerel rays persists after the rays have ceased to
act. It is said that some varieties of fluor-spar will
remain feebly luminous for days after they have been
exposed to radium.
CHEMICAL EFFECTS OF THE EAYS
One case of chemical action induced by the
Becquerel rays has already been encountered. That
is their action on photographic plate. The bromide
of silver contained in the gelatine film is reduced to a
CHEMICAL EFFECTS OF THE BAYS 55
lower state of oxidation, or rather bromination, and
this renders it susceptible to further reduction to
metallic silver by the developer. The action is, so far
as we know, identical with that produced by ordinary
light on the plate.
Many chemical changes are set going by the
neighbourhood of radium.
For example, if a radium salt is dissolved in water,
the water is continuously, though very slowly, decom-
posed into its elementary constituents, oxygen and
hydrogen. The mixture of gases is slowly evolved in
The Becquerel rays are able to change the yellow
inflammable kind of phosphorous into the red inert
The fluorescence of barium platino-cyanide under
the rays has been noticed. This substance is ordin-
arily lemon-yellow in colour, and very fluorescent.
But if it is kept near the radium for many hours, it
will become changed into an orange-red compound,
which is not fluorescent at all.
Another example of the same kind is the coloration
of glass by the rays. If a radium salt is kept for a
few days in a glass bottle, the glass will be seen to
have become distinctly violet. With prolonged ex-
posure to the rays the coloration becomes very deep.
The exact nature of the change here occurring is not
certain, but it is generally believed that the violet
colour is due to the separation of alkali metal (sodium
or potassium) in a finely divided state. The glass of a
Rontgen tube becomes coloured in the same way after
long use, though not nearly so strongly.
Most salts of the alkali and alkalium earth metals
acquire colour under the influence of the rays.
Common rock-salt becomes blue. Barium salts con-
taining radium, such as are obtained in the early
56 THE BECQUEREL RAYS
stages of preparation of the latter, become red under
the influence of their own rays.
In all cases the colour disappears when the sub-
stance is dissolved in water ; and by subsequent
drying, a white salt is again obtained.
The violet colour of the glass is destroyed by heat.
One other example of a chemical action set up by
the rays may be described not because it is in itself
specially interestipg theoretically, but because it has
been found very convenient as a practical means of
detecting the presence of the rays.
It has been found by Mr. Hardy and Miss Wilcox
that a solution of iodoform in chloroform turns violet
under the action of light. This violet colour is due to
the liberation of iodine. A little oxygen is necessary
for the reaction to take place ; but the atmosphere
If no light is acting, the solution remains colourless.
But in sunlight the violet colour soon becomes con-
spicuous. The exact nature of the change is not
known. The interesting point is that the Becquerel
rays are able to set it going, as well as light.
This reaction can be observed, even if the radium is
covered by thick lead, though, of course, under such
conditions it is very slow. The /3- and y-rays are con-
cerned in producing it. The a-rays do not appear to
have much effect, if any.
In most of the cases which have been described, the
question of which kind of rays is chiefly operative in
producing chemical action has not received much
attention. But probably the /3-rays are usually most
concerned. It is certainly so in the case of the action
on a photographic plate ; for in this instance the a-
rays have comparatively very little effect.
The examples which have been given by no means
exhaust the list of chemical actions which are promoted
PHYSIOLOGICAL PBOPEBTIES 57
by the Becquerel rays. Indeed, it is perhaps not too
much to say that they will be found to exert an
influence on the majority of chemical actions. The
subject offers a very wide field for experiment, and
one which has as yet hardly been touched upon. It
is very possible that the action of the rays may be
found in some cases to result in the formation of new
compounds, which cannot be produced in any other
The effects of fluorescence and chemical action
which we have been describing cannot, for the most
part, be detected with the feeble radiation of uranium.
It is necessary to make use of the much more power-
ful rays of radium in order to observe them.
PHYSIOLOGICAL PROPERTIES OF THE RAYS
For the sake of completeness, it is necessary to say
a few words about the physiological effects of radium ;
these may ultimately prove to be of the utmost
importance from the standpoint of practical medicine.
The effects were first brought to light in a some-
what dramatic way. M. Becquerel had for some time
carried in his waistcoat-pocket a small sealed tube
containing a radium preparation, in order to have it
ready to show to his friends. After a short time, the
skin underneath the pocket became red and inflamed.
Eventually it developed into a painful sore, which
healed only with great difficulty.
M. Curie has obtained a distinct reddening of the
skin after an exposure to the rays of only eight
minutes. This reddening appeared two months after
the exposure, and did not produce any serious result.
If the exposure is allowed to proceed for any con-
siderable length of time, it leaves a permanent scar.
It seems probable that the use of radium may
58 THE BECQUEREL RAYS
supersede ultra-violet light, and the Rontgen rays, in
the treatment of certain skin diseases. Cases have
been reported where its action has resulted in the
reduction of cancer growths. The author is not
competent to judge how much confidence can be
placed in these results ; but the subject seems to be
full of promise.
The rays seem also to have some effect in retarding
the growth of bacteria in certain cases. They are able
also to paralyse small animals, when the brain is
exposed to their action. Death ensues soon after.
The leaves of plants are destroyed by the Becquerel
rays. They turn yellow, and become friable. Lord
Blythswood has remarked a similar effect on linen
exposed to the rays. This also soon becomes quite
We shall now describe the experiment which first
gave a clue to the nature of the /3-rays. This
experiment shows that they are deflected by magnetic
force. It was made at about the same time, though
in somewhat different forms, by three independent
sets of investigators MM. Meyer and Sweidler,
M. Geisel, and M. Becquerel.
There are many ways of making the magnetic
deflection apparent. The simplest and crudest is
to bring the radium salt near a photographic plate,
wrapped in black paper. If no magnetic force is
applied, the result will be a blurred patch, symmetric-
ally disposed with regard to the position of the radium.
If now a magnet is brought up so that one pole is on
each side of the path of the rays, it will be found
that the image obtained on development is no longer
symmetrical, but is drawn out almost entirely on to
one side. The direction of the deviation is at right
angles to the path of the rays, and to the line between
the poles of the magnet. By turning the magnet
over, so as to make its poles change places, the
deflection takes place in the opposite direction to
what it did before.
The same experiment can be made with a fluor-
escent screen instead of a photographic plate. If an
electro-magnet is used, the luminosity is seen to
be drawn out to one side when the current is sent
one way, and to the opposite when it is reversed.
The electrical method may also be used for detect-
ing this deflection. M. Curie was the first to use
it in this way. He placed the radium salt at
the bottom of a
vessel (fig. 12).
This vessel was
the poles of an
When the mag-
_j. A , 7 o Q nr\f ov FlG - 12. Arrangement for observing magnetic deviation of
HOO Wcto 11UU t/Jv.- /j-rays. A metal tube closed at one end contains the radium
*f r\ T- a ^ ^ ie bottom. It is placed between the poles of an electro-
ClueQ, "tne rayS magnet. When the magnet current is off, the rays can get out
T and fall on the electroscope, producing the usual discharging
ISSUin*}* trOm tlie effect. When the magnet is on, they are thrown against the
side of the tube, and cannot get out so as to affect the electro-
mouth of the sc P e -
vessel fell on an electroscope, and this was accordingly
discharged. But when the magnet was excited, the
rays were curved and thrown against the side of the
vessel, so that they could not get out, and their
discharging effect was stopped. In this experiment
it is necessary to have some inches' distance between
the radio-active substance and the electroscope, so as to
have a sufficient stratum of air to absorb the a-rays,
and thus to experiment with the /3-rays only. 1
The magnetic deflection of the rays, as has been
1 The 7-rays do not produce conspicuous effect in this experiment.
THE BECQUEREL RAYS
mentioned, was the first clue to their true nature.
For it at once suggested that they were of the same
nature of the cathode rays, and quite distinct from
the Rontgen rays with which they were at first com-
pared. The deflection of the /3-rays is in the same
direction as that of the cathode rays. We may here
anticipate the experimental evidence, and state that
there is overwhelming reason to conclude that the
/3-rays are indeed cathode rays ; that they consist of
negatively electrified projectiles fired out from the
radio-active substance. It was a striking discovery
that the phenomenon of the cathode rays, before
only obtained by the use of complicated artificial
appliances batteries, induction coils, and mercurial
air-pumps, was spontaneously shown by a chemical
preparation, and even by a piece of stone (pitch-
blende), just as it came from the earth. Scientific
investigation often proceeds in the direction of com-
plicating its ap-
instances of the
dency are not
of detecting the
tion which we
FIG. 13. Another arrangement for observing the magnetic
deviation of the /3-rays. The radium is contained in a small
capsule, a. Its rays pass through the small hole or slit, b, and
fall on a photographic plate or fluorescent screen, c, after pass-
ing between the poles, d, e, of a magnet. When the current is
on, the impression is displaced, and drawn out into an elon- DUrDOSe,
gated form, in a direction perpendicular to the paper
very crude ; for
they do not admit of more than qualitative observa-
tion. A more refined method is the following
(fig. 13). The radio-active salt is placed in a very
THE /3-RAYS 61
narrow groove cut in a piece of metal. Over the
groove, and parallel to it at a little distance, is
placed a narrow slit, cut in a metal plate. The rays,
issuing from the radio-active salt, pass through the
slit, and are thereby confined to a narrow beam.
This beam may be made to fall on a photographic
plate, or on a fluorescent screen, perpendicular to
it, and the impression will be a narrow line. If a
magnetic force is applied across the path of the rays,
and in the direction of the slit, the beam will be
deflected. It is found, however, that the impression
produced by the deflected rays is not a mere displace-
ment of the impression produced by the original
undeflected beam. It is many times broader.
We saw in the first chapter that the cathode rays,
if produced by a steady and uniform discharge, were
not at all spread out by magnetic deflection. They
are all of the same velocity. With the cathode rays
of radium it is otherwise. Their velocities vary over
a wide range, some being much more deflected by
magnetic force than others ; this explains the broad-
ening out of the impression formed by the rays.
If we consider the deflected rays which reach some
given spot on the photographic plate, it is easy to
measure the circle into which they are bent ; for we
know that their path must pass through three points
the narrow radio-active source, the slit, and the
assigned spot on the plate (fig. 14). These three
positions being known, it is easy to calculate the
radius of the circle which passes through them.
This is the desired result. It is found that the
deflection of the /3-rays is less than that of the
cathode rays which can practically be produced by
electric discharge, even in the best vacua in which
the discharge can be got to pass at all. It is to
be concluded that the rays emitted by radium move
62 THE BECQUEREL RAYS
more rapidly. We shall have more to say on the
subject later on in this chapter.
FIG. 14. Magnetic spectram of 0-rays. Section of the path of
the rays. At a the radium is placed in a narrow groove in a piece
of metal. The rays pass through the slit, I, which confines them
to a narrow beam. In the absence of magnetic force, they would
fall on the plate at c, in a narrow band. When a magnetic force
is applied perpendicular to the paper, the rays are bent to varying
degrees, and reach the plate at various distances, dj, d%, d$, from
the undetected position. Thus a broad band or spectrum is
impressed on the plate.
Lenard found, as we have already seen, that the
cathode rays produced a blue luminosity in the gas
through which they pass. This luminosity is visible
both inside the vacuum tube and outside it, though
it is much more conspicuous inside in the neighbour-
hood of the cathode, especially where the pressure is
not too low. This luminosity is called the negative
glow, and possesses a spectrum characteristic of the
gas in question.
Sir William and Lady Huggins have recently
obtained evidence that the /3-rays of radium also
produce the same characteristic glow in gases. It
was noticed in the early days of the discovery of
radium that radium preparations were feebly self-
luminous in the dark. It was generally thought
that this was due to a slight fluorescence of the salt
under the influence of its own rays at least that was
THE /3-BAYS 63
the view which the author took of it, and probably
others thought the same. Sir William and Lady
Huggins, however, making use of their unrivalled
experience in the photography of feeble spectra gained
in working on the nebulae, were able to photograph
the spectrum of the radium glow, and they found it
to give the characteristic bands of nitrogen. Thus
it seems likely that the glow is due to the air, and
is produced by the action of the y8-rays upon it,
just as the cathode rays produce luminosity in
There is one outstanding difficulty about this
explanation which ought to be noticed. The /3-rays
are most concentrated in the immediate neighbour-
hood of the radium, and no doubt it is to be expected
that the glow should be strongest then. But it
ought, one would think, to extend to some visible
distance beyond the salt. This does not seem to be
the case, for the glow appears to stop at the boundary
of the solid. Professor Dewar has found that the
nitrogen bands cannot be observed in the glow
when the radium salt has been heated in a high
vacuum so as to remove all nitrogen from its
The /3-rays resemble the cathode rays in another
way. They carry a negative electric charge with
them. The radio-active substance loses this nega-
tive charge, and if it is insulated, it will become
positively charged in consequence. On the other
hand, anything which absorbs the rays necessarily
receives the negative charge which they carry, and
becomes negatively electrified. In short, a current
1 Some very weak preparations of radium in the author's possession are
quite as luminous, if not more so, than some pure radium bromide. This is
difficult to understand. Moreover, a trace of moisture suffices to destroy
the luminosity. Possibly a part of it really is in some cases due to
fluorescence xinder the rays.
THE BECQUEEEL RAYS
of negative electricity is constantly flowing spontane-
ously away from the radium.
It might seem to be the simplest matter possible
to detect these effects. But such is not the case.
The difficulties are two. In the first place, the
current is exceedingly small, so that the amount
of electricity hurled off by the radium in any
moderate time is difficult to detect, unless we have
a relatively considerable quantity of radium to work
with. In the second place, it will be remembered
that the rays are making the air all round them
conduct electricity. The result is that the charge
imparted by the rays to anything they fall upon
is conducted away by the air as fast as it comes ;
and no charge can accumulate so as to be observed.
M. and Mme. Curie were the first to succeed in de-
tecting the charge. The plan used by them was very
ingenious. They embedded the metal plate on which
the rays were to play, in paraffin wax, which is an
excellent insulator of electricity. 1
This prevented the electricity from leaking away,
although it allowed
the /3-rays to get
through to a suffi-
cient extent for the
purpose (fig. 15).
The result was that
the plate became
charged with nega-
tive electricity, and
its charge could be
made apparent by
means of an electrometer. M. Curie also tried the
1 It is known that paraffin loses its insulation to some extent while under
the influence of Becquerel rays ; but apparently, to judge by the result, the
failure of insulation is not serious enough to affect the use of paraffin in this
FIG. 15. Section of Curie's apparatus for observing
the charge conveyed by radium to surrounding conduc-
tors, a is the radium, contained in a cavity in a lead
block, b is the block of lead which receives the rays,
and becomes charged. It is entirely surrounded by
paraffin wax, d. This prevents the charge acquired from
leaking away, b is connected by the wire, c, to the
electrometer, which indicates the charge acquired. The
whole of the receiving arrangement is surrounded by a
metal case, e, connected to earth, but this metal is very
thin on the face where the rays enter.
complementary experiment, to show that the negative
electricity carried by the rays was lost by the radium.
In this case it was neces-
sary to surround the
radium with a thin layer
of paraffin. It then soon
became charged with posi-
tive electricity, showing
that negative electricity
had been shot off from it.
A simple way of ex-
hibiting this result was
published a short time ago
by the author. In this
case, instead of using
paraffin to prevent the
escape of electrification,
the alternative plan of re-
moving the air is adopted.
The radium is sealed up
in a thin-walled glass tube,
so that the rays can get
out through the sides of
the tube. The tube is
coated with a conducting
and a pair
-, i*ii* serves as an insulating support, d is covered
leaves, Constituting with a film of phosphoric acid, which makes
5 it a conductor. At the other end is a brass
electrOSCODe, are at- cap /.which carries the gold leaves r. The
whole is exhausted through &, which subse-
|-/-j T-J- T^Vip wVinlp quently serves as a means of supporting it.
J1CJ d loses negative electricity, thus acquiring a
ic nnnnr nr\ ir or P ositive charge, and the leaves, g, diverge.
ib llUllg Up 111 dll The divergence increases until the leaves
1 -i touch the tinfoil strips, h Ti, which are con-
VeSSeJ Dy an nected to the earth through a wire, k, sealed
, . . // \ into tne glass- This touching discharges the
insulating Support (rig!'. 16). leaves, but they begin to diverge again
\ o / immediately, if the radium preparation is
The p-rays carry negative ^definitely 18 oscillation of the leaves goes on
electricity away from the
system. It becomes charged with positive electricity
in consequence. The gold leaves begin to diverge,
Fia 16 ._ Arrangement for exhibiting the
r From this the thin glass tube, d, containing
Ol radium is hung by the glass rod, e, which
66 THE BECQUEEEL RAYS
and the divergence steadily increases until they
touch the walls of the containing vessel, and are
discharged. Then they begin to charge up again,
and so on indefinitely. The time that the leaves
take to diverge and collapse again once depends,
of course, on how much radium is used. But with
several milligrammes of a pure radium salt, it is easy
to make an apparatus which will go through its
cycle in the course of a minute. This apparatus
is probably the nearest approach to perpetual motion
that has ever been attained. The divergence and
collapse of the leaves is maintained without ceasing,
and can only stop when the radium loses its activity.
How long this may take, we do not very exactly
know ; but hundreds of years at least must elapse
before the forces which cause the leaves to move
have ceased to act.
Enough electricity has been accumulated by the
self- electrification of radium, even to give an electric
shock. It happened thus. Herr Dorn had sealed
up some radium salt hermetically in a tube of glass
which possessed good insulating power ; the negative
charge was shot out in the form of y8-rays, while
the positive charge remained in the tube, in which
it was retained by the good insulation of the glass.
This process went on for six months, after which
time, the radium being wanted for some purpose,
the tube was scratched with a file preparatory to
breaking it. The scratched and weakened glass was
no longer able to sustain the electric strain to
which it was subjected, and it was perforated by
an electric spark. At the same time a smart shock
was felt, as the accumulated electric charge passed
to earth through the experimenter's body. It may
be asked, Is there any limit to the extent to which
this self- charging of radium might proceed, if
THE -RAYS 67
enough time were allowed ? There is, of course, a
practical limit in that the insulation with which the
radium is surrounded would necessarily give way
in time, and be ruptured by a spark, as in the
case described. But there is another limit besides,
even if that one could be removed, for the positive
electricity left on the radium must hinder the escape
of more negative electricity. It drags on the escap-
ing negative particles ; it attracts them back to the
radium. The particles are shot out so fast that
they can escape without being appreciably hindered
by this in any practical case. But in the end, if
enough positive electricity had accumulated, it could
not fail to hold the negative particles back, and
altogether prevent their escape.
The /3-rays, like the cathode rays, are deflected
when an electric force acts at right angles to their
path. This was first proved by M. Becquerel, but,
instead of describing his original experiments, we
shall speak of some later ones made by Professor
Kauffmann, which show it more clearly, and enable
the electrostatic deflection to be compared with the
The principle of these experiments is very in-
genious. It is briefly this. A narrow and sharply
defined beam of rays from radium falls perpendi-
cularly on a photographic plate. A magnetic force
is applied at right angles to the direction of the
rays, and sorts them out according to their velocities ;
the fastest particles are deflected from their path
the least, the slowest particles are deflected the
most. Before the magnetic force was applied, the
rays all fell on one spot in the centre of the plate.
Now that the magnetic force acts, they are spread
out so as to fall on a straight line.
Now we apply in addition an electric force, so as
68 THE BECQUEREL EAYS
to deflect the rays, just as the magnetic did, but
in the perpendicular direction. This too, by itself,
would spread out the rays and cause them to inter-
sect the plate along a straight line at right angles
to the former one. But we must remember that
the magnetic force is acting simultaneously, though
independently. What will happen ? Every particle
which we might think of as moving vertically will
be deflected to a certain extent, say northwards, by
the magnetic force ; to
a certain extent in the
_, p perpendicular direc-
tion eastwards, by the
w 1 i E electrostatic force.
The result must be
that it strikes the
plate somewhere in
the region between
the north and east lines somewhere in the right-
hand top region of this diagram, say at the point P.
Since particles are present which have continuously
varying velocity, the impression on the plate will
be a curve. Each point on this curve will corre-
spond to some particular velocity. The perpendi-
cular distance of the point from the NS line, the
distance eastwards, that is, measures the electro-
static deflection. The perpendicular distance from
the EW line measures the magnetic deflection in
the same way.
This is the principle of the experiment. It is
now time to say something of the practical details.
The source of the rays was a small speck of radium
bromide, and the rays were confined into a definite
beam by passing through a round hole in a metal
plate. The rays, in passing from the radium to the
hole, had to pass between two parallel metal plates,
which were kept at a great potential difference, by
means of a special arrangement, into the details of
which we need not enter. After passing through
the hole, the rays had to traverse a further interval,
and then fell on the photographic plate. The whole
arrangement was enclosed in glass, and the air
completely pumped out. This was necessary, partly
to avoid the diffusion and absorption of the rays
by air, partly to prevent an electric discharge between
the plates used to produce electrostatic deflection.
The whole arrangement was placed between the
poles of a large magnet, so as to subject the rays
to a uniform magnetic force along their whole length.
The exposure of the plates had to last many hours
in order to get a visible impression.
It was possible, by measuring the position of
various points on the photographic curve, to find the
magnetic and electrostatic deflection corresponding
to each point. Each point represents the impact
of particles of some particular velocity, so that a
series of values was obtained for particles of various
velocities. We saw, in the first chapter, how a know-
ledge of the magnetic and electrostatic deflection
of the cathode rays could be used to calculate the
speed and electro-chemical equivalent of the parti-
cles. Professor Kauffmann used his measurements
to obtain this information. The results are very
interesting and important. They are as follows :
charge to mass.
1-31 XlO 7
70 THE BECQUEREL RAYS
The velocity of light is 3 x 10 10 centimetres per
second, so that the /3-particles are projected with
a speed nearly equal to this.
It is difficult fully to grasp the significance of this
amazing result. The apparently quiescent little speck
of white salt is hurling off projectiles with a speed
something like half a million times the speed of a
We see that the /3-particles of radium are some ten
times more rapid than the cathode rays. They move
much faster than any other natural body that is
known in the universe.
It will be noticed that the mass of the /3-particles
does not seem to remain constant when compared to
the charge, for the ratio between them varies in Pro-
fessor Kauffmann's experiments over a twofold range.
This is at first sight difficult to understand ; for
we have seen that the charge of electricity carried by
the particles of the cathode rays is in all probability
the same as the charge of a hydrogen atom ; and this
charge is believed to be fixed and indivisible. We
might have a charged particle carrying exactly twice
as much electricity as the hydrogen atom, or exactly
three times as much. But, unless the clearest proof
compels it, the existence of intermediate charges
cannot be admitted. There is an alternative possi-
bility, and that is that the particles have not all the
same mass that some have more inertia than others.
If this could be admitted, it would explain why the
ratio of charge to mass should alter. The particles
with the highest speed are those which (if the charge
is always the same) have also the largest mass. If
some of the particles are more massive than others, it
is not apparent, at first sight, why those shot out the
fastest should be the most massive. Exactly the
contrary might perhaps be expected.
THE /3-RAYS 71
Electro-magnetic theory beautifully accounts for
this. Some attempt must be made to follow the
explanation, though in truth the subject is not well
suited to an elementary book. The explanation will
probably only be intelligible to those who have some
idea of the phenomena of electro-magnetic induction.
Consider the motion of a charged particle. This
constitutes an electric current, and sets up a mag-
netic force in the surrounding space. Now to set up
a magnetic force requires the expenditure of energy.
Where did this energy come from ? Evidently from
the source which set the particle in motion. So it
is more difficult to set a particle in motion, if the
particle is electrified, than if it is not. In other
words, an electrified particle behaves as if it had a
greater mass than one which is not electrified.
Now the ordinary mechanical mass of the particle
does not depend at all on whether it is moving or
not, or on whether the motion is fast or slow. The
mass of a 10 Ib. cannon-ball remains 10 Ibs., whether
the ball is in the magazine, or whether it is flying
from the mouth of a gun. We must inquire closely
whether the same is true of the spurious mass which
a particle gets when it is electrified.
The magnetic force produced by the motion is pro-
portional to the speed. The energy which it takes
to produce the magnetic force is proportional to the
square of the magnetic force, and consequently to
the square of the speed. But the energy of a simple,
constant mass is also proportional to the square of
the speed. In other words, the electrical mass
behaves just like the ordinary mechanical one, and
does not depend on the speed at all.
All this is on the assumption that there is nothing
to take into account except the magnetic force pro-
duced by the motion of the charged particle.
72 THE BECQUEREL RAYS
But there is something else which must be taken
into account. There is an electric force induced by
the motion of the lines of magnetic force which have
been called into being by the motion of the charge.
Now the establishment of this electric force requires
energy too, just as the original magnetic force does.
This electric force depends on the strength of the
magnetic force, and on the rate of motion. But the
magnetic force itself depends on the rate of motion ; so
that the strength of the electric force depends on the
rate of motion twice over, or, in other words, on the
square of the speed of the particle.
Now the production of this electric field adds to
the apparent mass of the particle just as the produc-
tion of the magnetic field did. But there is this
important distinction. The added energy of motion,
which is proportional to the square of the electric
force, must be proportional to the fourth power of the
speed, instead of being proportional to the square of
it ; so that the addition of mass due to the induced
electric force depends on the speed, and becomes
greater the greater the speed is.
To calculate exactly the mass at any given speed
is a difficult problem, and there is a want of unani-
mity among the highest authorities on the subject.
But all are agreed that the increase of mass is unim-
portant until the speed of light is approached, but
that the apparent mass then rapidly increases. It is
believed that when the speed of light is reached the
mass becomes infinite in fact, that unlimited energy
would be required to make an electric charge move so
fast as that, and that consequently it never could
move so fast.
The particles of the cathode stream produced by
electric discharge do not move more than one-third
of the velocity of light, even when the conditions are
THE /3-RAYS 73
as favourable to the attainment of a high speed as it
is possible in practice to make them. More ordinarily
their velocity does not much exceed one-tenth of the
velocity of light. But even if one-third of the velo-
city of light is attained, the increased inertia would
scarcely be sufficient to be of practical importance.
It is otherwise with the cathode rays of high
velocity produced by radium, for the velocity of light
is nearly approached by them.
The trend of what has been said will now be
apparent. It is reasonable to assume that the
apparently increased mass of the faster moving par-
ticles is due to their electrical inertia, which might
be expected to increase in quite a similar way.
It is very important to know whether the whole
mass of the particles is electrical, or whether part of
it is mechanical and independent of speed. Kauff-
mann's experiments give the means of testing this to
a certain extent, for we can compare the masses at
different speeds, and see whether the increase of
mass with increasing speed is as fast as it ought to be
if the mass was wholly electrical. Professor Kauff-
mann is of opinion that his experiments are most
consistent with the view that the mass is altogether
of electrical origin that the /3- particles are, in fact,
disembodied charges of negative electricity, without
any material substratum. It is too soon to accept
this conclusion without reserve, for the assumptions
involved in the calculations are dubious, and the
accuracy attainable in the experiments is not sufficient
to put the theory to a very severe test.
But, when all this has been admitted, the sim-
plicity of the conclusion is very attractive, and it has
gained a great deal of support during the last two
years from current scientific opinion.
There is one misconception which must be carefully
74 THE BECQUEREL RAYS
guarded against. In speaking of the mass of a
charged particle, the effects of inertia have alone been
referred to. It is an undeniable inference from known
facts that the inertia of a body must be greater when
it is electrified. It necessarily becomes more difficult
to set in motion than before, though, in any case
within ordinary experience, their increase would be
too small to detect. We could not, for instance, give
any electric charge to a rifle bullet which would make
its inertia measurably greater. But, in the light of
electrical theory, there is no doubt at all that the
inertia has really been increased.
When we come to consider the weight of the body,
the force, that is, with which the earth attracts it,
we have no means of judging whether or not the
electrification produces any effect. 1 In all ordinary
cases inertia and weight are inseparably associated.
But whether the inertia of electrical origin is accom-
panied by corresponding weight, we do not know
at all. No experiment has been devised by which
the question can be tested. Indeed, as we do not
know why gravitation accompanies ordinary inertia,
it is not surprising that we are unable to decide
theoretically whether it should accompany electrical
inertia or not.
We have seen that the /3-rays are in all respects
similar to the cathode rays set in motion by an electric
discharge in a vacuum tube. The /3-rays, however,
though in some respects the most conspicuous, are by
no means the most important feature of radio-activity.
By far the greatest part of the discharging power is
due to the a-rays, and it is now time to consider these.
1 The electrical attraction -on surrounding conductors has, of course,
nothing to do with this.
THE a-RAYS 75
The /3-rays were easily shown to be deflected by a
magnetic force. For a long time it was thought that
the a-rays were not at all affected by magnetic force.
We now know that they too are bent, but to a much
less extent, and in the opposite direction to the
This result is due to the skill and perseverance of
Professor Eutherford, and it is of far-reaching im-
portance. We shall now describe the method used
by him for detecting the deflection. The observation
is difficult because of the extreme smallness of the
curvature which can be produced even by a very
strong magnetic force.
The experiment is in principle exactly the same as
M. Curie's experiment to show the deflection of the
/3-rays (fig. 12). But refinements are necessary
in this case which were not called for in that.
The method, it will be remembered, is to place the
radium at the bottom of a narrow vessel, and to place
the vessel between the poles of an electro -magnet.
When the magnet is not excited, the rays follow a
straight course, and are able to issue from the mouth
of the vessel, whence they penetrate into an electro-
scope, and cause the gold leaf to collapse. But when
the magnet is in action, the rays are bent into a
curve, and fall on the sides of the vessel, which stop
them. Thus they cannot get out of the vessel, and
the electroscope is not discharged.
In the case of the a-rays, it is only possible to
produce a very slight curvature of the rays. Thus
the vessel at the bottom of which the radium is
placed must be made very narrow. It must con-
sist of a mere slit between two parallel metal
plates ; for if the plates were not very close,
the rays could get out in spite of their slight
THE BECQUEREL RAYS
It is evident that one such slit cannot give out
very much radiation, on account of its narrowness.
To get enough rays to work with conveniently, it
is necessary to have a number of parallel slits, so that
the radium is placed in a layer at the bottom of a
box containing a number of parallel plates, with
narrow spaces between. The rays come through
these narrow in-
enter the elec-
troscope (fig. 17).
It is necessary
that the narrow
slits should be
in order that the
may act on the
rays along a con-
Air, however, ab-
sorbs the a-rays
very freely, and
if air filled the
cally no rays
the entire appa-
ratus, for the presence of some gas in the electro-
scope is necessary to enable the rays to discharge ;
and we cannot exhaust 1 the slits without exhaustiog
the . electroscope, for it would be impracticable to
make an air-tight partition thin enough to let the
a-rays get through. The difficulty was got over by
FIG. 17. Rutherford's arrangement for observing magnetic
deviation of a-rays. The rays pass up between the metal
slits, and enter the electroscope, causing discharge. When
a magnetic field is applied perpendicular to the paper, the
rays are curved, and can no longer get through the slits.
Thus discharge is stopped.
filling the apparatus with hydrogen, which is much
less opaque to the rays than air.
The vessel with the series of slits was placed
between the poles of a large and powerful electro-
magnet, so that the lines of magnetic force lay in the
same direction as the series of parallel plates, but
crossed the path of the rays at right angles. Under
these circumstances it was found that the discharge
of the electroscope was almost stopped when the
magnet was strongly excited. The a-rays had been
The experiment in this form does not show in
which direction the deflection takes
place whether it is in the same direc-
tion as for the /3-rays, or in the opposite.
It was necessary to modify it in order
to test this important point. For this
purpose a little ledge was made to pro-
ject over one side of each plate (fig. 18).
It will readily be understood that with
such a ledge the rays could get out
more when they were deflected to the
side away from the ledge than when
.1 -, . i , , -i FIG. 18. Arrange-
tney were bent so as to be intercepted ment of ledges for
termining direction in
which the rays are
bent. The rays can get
through when bent so
as to be concave to a.
When bent so as to
be concave to fe they are
stopped by the ledges.
by it. The rate of leak was accord-
ingly greater with magnetic force one
way than when it was reversed. By
this means the direction of deflection
could be inferred, and it was found to be opposite
to that of the cathode rays.
The argument which showed that the y8-rays were
negatively charged particles is equally applicable to
prove that the a-rays are positively charged ones.
It is found that the a-rays are deflected by an
electrostatic force, just as the /3-rays are. This
experiment is even more difficult than the corre-
78 THE BECQUEEEL KAYS
spending magnetic one which we have just described.
It was carried out by Professor Rutherford in quite a
similar way. A series of parallel brass plates were
used, but in this case it was necessary to insulate
them from one another by holding them at the sides
with ebonite. The alternate plates were connected
together, just like the plates of a storage cell, and an
electric force was made to act between each plate and
its neighbour by connecting the two sets of plates
to the poles of a battery consisting of many cells.
It was a great difficulty in the experiment that
only a moderate electric force could be applied
between the plates ; for if more was attempted, a
luminous electric discharge was produced between
the plates. This limitation made it very difficult to
deflect the rays completely, but still unmistakable
evidence of the deflection was obtained.
No one has yet succeeded in demonstrating directly
that the a-rays carry a positive charge, though there
can be no reasonable doubt of the fact that they do so.
The difficulty is the same as that which was overcome
in the case of the /3-rays by exhausting the vessel or
by the use of a solid insulator. With the a-rays we
cannot use a solid insulator, for they are scarcely able
to penetrate it. And all attempts to use the method
of exhaustion have failed. The reason is that the
conduction produced by the a-rays is very large, and
even at the best vacua there is still enough conduction
to enable the charge to leak away faster than it can
accumulate. 1 Another point which must not be
overlooked is that in the case of radium the negative
charge carried with the /3-rays will partially or
1 The author has recently satisfied himself that a charged polonium rod
loses its charge even in complete absence of air. This seems to indicate
that ions are torn off from the solid by the issuing a-particle, and carry the
current. There seems, therefore, to be no chance of detecting the charge
of the a-rays,
THE a-EAYS 79
perhaps wholly neutralise the positive one due to the
a-rays. Radio-active substances exist which only
give out a-rays. Polonium is one of these, and it will
no doubt be best to employ it in future attempts to
detect the charge carried by these rays.
By measuring the magnetic and electrostatic forces
necessary to completely deviate the rays, Professor
Rutherford was able to estimate the curvature of the
rays produced by a given electric or magnetic force,
though it is not to be expected that a measurement
of this kind should give results of great precision.
From these data we can reason exactly as for the
cathode rays ; and the result is to show that, for the
a-rays, the ratio of charge to mass, or electro-chemical
equivalent, is about the same as for an atom of
hydrogen. The velocity comes out about T ^ of the
velocity of light.
Thus the a-rays are very similar to the canal rays
of a vacuum discharge (see chap. i. p. 23).
M. Becquerel has been able to confirm Professor
Rutherford's observations on the magnetic deviations
of the a-rays. He used the photographic method.
The radio-active substance was placed in a narrow
groove in a piece of lead. A little distance above
it was a slit parallel to the groove. The rays were
confined by this means to a flat, narrow beam, like the
blade of a chisel. A photographic plate was placed
in a slanting position so that the beam grazed along
its surface. The image on the plate, after develop-
ment, gave a picture of the path which the rays
had pursued. The whole arrangement was placed
between the poles of a powerful electro-magnet. It
was necessary to give a long exposure. After a
sufficient exposure, the current was sent the opposite
way through the magnet and another equal exposure
given. On developing the plate it was found that the
THE BECQUEREL RAYS
rays had been bent by the magnetic force to a curved
path. The two trajectories of the rays curved away
in opposite directions
from the central line
(fig. 19) ; each cor-
responded to a differ-
ent direction of the
magnetic force. By
measuring the dis-
tance apart of the
two trajectories, the
deflection of the rays
could be quantita-
The result was in
good agreement with
The result to which
we have alluded,
that the electro-
of the a-particles is nearly the same as for a hydrogen
atom, is of very great importance. The electro-
chemical equivalent is, as we have seen, the quantity
of electricity transported by unit mass of the sub-
stance, or, what amounts to the same thing, the ratio
of the electric charge of each particle to its mass.
We cannot assume that the masses of the a-particles
are the same as the masses of the hydrogen atoms
until we know that the charge of electricity carried is
It cannot be said that we do know this with any
certainty. But it will appear in the sequel that there
are strong reasons for suspecting it. If we assume
it to be true, it would follow that the mass of the
FIG. 19. Figure representing Becquerel's photo-
graphs of the path of the a-rays under a magnetic
force. The rays are confined into a narrow beam,
and graze the plate. They are curved by the
magnetic force, which is supposed to work at right
angles to the plane of the paper. One branch gives
the path of the rays when the magnetic force acts
downwards, the other when it acts upwards.
THE a-EAYS 81
particle is (nearly at all events) the same as the mass
of an atom of hydrogen, and if that is so, the infer-
ence is almost irresistible that the two are identical.
Hydrogen has the lowest atomic weight of all the
elements its atom is the lightest. The next is
helium, with an atom four times as heavy. The
helium atom may be supposed to be capable of
carrying the same charge of electricity as the hydro-
gen atom, though this again is a matter of which we
have no direct knowledge. In that case, its electro-
chemical equivalent would be four times greater.
The actual measurements on the a-rays point to a
number about half as large again as for hydrogen,
but they are scarcely sufficiently precise to distin-
guish between a hydrogen atom and an atom of
It will be remembered that the ^-particles, or
corpuscles, of radium were not all ejected with equal
velocity. Some of them moved much more rapidly
than others, and the result was that a pencil of the
rays was splayed out into a fan-shape by magnetic
force. This is not found to be the case with the
a-rays ; M. Becquerel's photographs show that the
a-rays are not dispersed by magnetic deflection, or,
at all events, very little. It is to be concluded that
(with a given radio-active substance) they are all
shot out at about the same speed.
Sir William Crookes has made a very interesting
observation on the phosphorescence produced by the
a-rays. He brought a tiny speck of radium a short
distance off a zinc sulphide screen, and examined the
resulting fluorescent light with a magnifying lens.
He found that the fluorescence did not proceed uni-
formly from all parts of the screen, but that it came
from isolated points dotted about on the surface.
These points are constantly shifting about, and their
82 THE BECQUEEEL RAYS
appearance is very suggestive of the splashing of rain-
drops on a pond. This appearance is only produced
by the a-rays. The more penetrating kinds of rays
produce a uniform fluorescence, without scintillations,
and so do the Rontgen rays from an exhausted tube.
Sir William Crookes's experiment is easily re-
peated. A zinc sulphide screen is much better for
the purpose than a platino-cyanide one. A small piece
of apparatus, consisting of a zinc sulphide screen, a
small speck of radium in front of it, and a suitable
magnifying lens, all mounted in a brass tube, has
been designed by him for showing the effect, and
named a spinthariscope. The simple and direct in-
terpretation of the luminous specks is that each speck
corresponds to the impact of an a particle on the
phosphorescent screen. This was the view adopted
by Sir William Crookes, and generally accepted.
Though the explanation seems at first sight simple
and satisfactory, M. Becquerel has urged strong
arguments in favour of an alternative theory.
He is of opinion that, though the phenomena are
no doubt due to the impact of the a-particles, each
flash corresponds not to the impact of one particle,
but to the fracture of a crystal of the phosphorescent
substance by the battering which it gets from the
a-particles. Such a fracture will probably not occur
every time a particle strikes, but rather when a
number of particles happen to assault a weak spot
on the crystal simultaneously. The emission of light
by crystals in breaking is quite a familiar fact. Any
one can observe it for himself by grinding two lumps
of sugar together in the dark. M. Becquerel has
been able to imitate the spinthariscope by the crush-
ing of a crystal of zinc sulphide mechanically. When
the light emitted was examined by a lens, the charac-
teristic scintillating spots were observed. He has
THE y-RAYS OF EADIUM 83
found, moreover, that the scintillations produced by
radium are more numerous when the zinc sulphide
screen consists of small crystals than when it consists
of large ones. This proves conclusively that the
impact of an a-particle does not necessarily produce
a scintillation ; and it is favourable to the fracture
theory, for naturally small crystals would be more
easily broken by impact than large ones. Although
this theory is less attractive than the original one,
the balance of evidence seems to be in its favour.
Whichever be adopted, the spinthariscope affords an
interesting illustration of the mechanical nature of
THE y-RAYS OF RADIUM
It remains to deal with the most penetrating kind
of rays those known as the y-rays. Their most
striking characteristic is the great penetrating power,
for they are able to make their way through massive
blocks of iron or lead. I have succeeded in observing
the y-radiation from 10 milligrammes of radium
bromide, even through a thickness of 8 cm. (nearly
three inches) of solid lead, and there would be no
difficulty in observing their effect through 6 inches of
iron. In such observations the electrical method is
of course used, for it is much the most sensitive.
We have less information about the y-rays of
radium than about either of the other three varieties.
The feature which distinguishes them from the more
penetrating kind of /3-rays is the absence of the
characteristic magnetic deflection. No experiment
has yet succeeded in deflecting them by magnetic
force. Some observations have been made which
seem at first sight to suggest that they carry an
electric charge. But there is reason to doubt whether
they are conclusive.
84 THE BECQUEREL RAYS
Uranium and thorium as well as radium emit y-
rays. Polonium, however, does not do so. In fact,
the substances which emit /3-rays all emit y-rays ;
and y-rays are never, so far as we know, unaccom-
panied by /3-rays. We shall see in a later chapter
that radio-active products have been obtained from
thorium, radium, and uranium, whose activity dim-
inishes with time. In such cases the y-radiation
only appears in conjunction with the ft radiation, and
always bears a fixed proportion to it. The nature of
the y-rays is one of the most obscure questions in con-
nection with radio-activity. It will be interesting to
consider the merits of two alternative theories. The
view which is most popular is that which considers
the y-rays to be Rontgen rays, generated by the
/3-rays in striking the radium itself, or the absorbent
screen, if such be used. There is much to recom-
mend such a view. For we have seen that the fi-
rays are identical with cathode rays, and cathode
rays, when they strike a solid obstacle, generate
Rontgen rays, which are far more able to penetrate
matter than the cathode rays themselves. Thus the
relation between the /3-rays and y-rays seems to be
closely analogous to the relation between the cathode
rays in a vacuum tube, and the Rontgen rays which
they produce, when they strike a solid obstacle.
The invariable appearance of the two kinds of rays
in company is very well accounted for by this theory.
Moreover, the y-rays resemble Rontgen rays in their
complete indifference to magnetic force.
Simple and satisfactory as this view of the y-rays
now appears, there have been formidable objections
to it. We shall be in a position to understand these
when the absorption of the rays by solids and the
ionisation which they produce in gases comes to be
THE y-EAYS OF RADIUM 85
The other alternative is to consider that the y-rays
are corpuscular, like the /3-rays. This is easy to
reconcile with most of the facts. The mutual com-
panionship of the fi- and y-rays is thus intelligible,
and the phenomena of absorption and ionisation, to
which we have just alluded, are fully in accordance
with it. The great objection is the absence of
magnetic deflection. It must, indeed, be remem-
bered that the amount of magnetic deflection depends
on the velocity of the particles, and that if this
velocity were sufficiently high, the magnetic effect
might be too small for experimental detection. It
would be very important to know more definitely
whether or not there is a continuous transition
between the /3- and y-rays whether /3-rays exist
which just, but only just, show visible deflection in
a strong magnetic field. If this were found to be
the case, it would be in favour of the theory that the
y-rays were only an extreme variety of the /3-rays.
In any case it would seem that there must be
Eontgen rays, produced by the cathodic /3-rays. But
these might not be sufficiently conspicuous to produce
Professor Paschen has found that radium encased
in thick lead, so as to let out only the y-rays, still
gained a positive charge when insulated. This
experiment was made by means of an electroscope
apparatus similar to that described above (p. 34).
He regards this as proving that these rays carry a
negative charge. It must be remembered, however,
that when the Eontgen rays fall on a metal surface,
they set up secondary rays, which carry a negative
electric charge which are, in fact, cathode rays.
Paschen's experiment is open to the objection that
the charge observed by him may be due to such rays
set up at the surface of the lead by the y-rays, and
THE BECQUEREL RAYS
is not necessarily a proof that the y-rays themselves
are of a corpuscular nature. It is not inconsistent
with the alternative view that they are Rontgen
Let us now sum up our conclusions with regard
to the three kinds of rays emitted by radium.
Fig. 20, due to Mine. Curie, exhibits diagrammatically
the behaviour of
the three kinds
of rays in a
The ct-rays, giv-
ing rise to by
far the greater
part of the
are slightly de-
flected by a
3 netic force, in
Fto. 20. Diagram illustrative of the behaviour of the three that direction
varieties of Becquerel rays in a field of magnetic force. The a-
rays are bent very slightly in one direction. The /3-rays are bent "VVLLlC-h COrre
very much in the opposite direction ; while the y-rays are not
sponds to an
emission of positively charged masses. Measure-
ments of the ratio of charge to mass suggest that
they are comparable in mass to hydrogen or helium
The /?-rays, which are principally instrumental in
producing the photographic effect, are deflected in
the opposite direction to the a-rays, and consist of
negatively charged corpuscles of much less than
The y-rays, not deflected at all by magnetic force,
are probably Rontgen rays, produced by the cathodic
ABSORPTION AND IONISATION
WE have already seen that the three varieties of
rays emitted by radium can be separated by means
of solid 1 screens. Very thin screens allow all three
kinds of rays to pass. Screens of moderate thickness
suppress the a-rays, leaving the fi- and y-rays. Very
thick screens allow the y-rays only to pass. It is
now time to consider the absorption of the rays by
solids somewhat more in detail.
For studying the a-rays, it is convenient to make
use of polonium. For polonium emits these rays
only, and the effects are not complicated by the
presence of the other kinds.
In the first place, the a-rays are absorbed with
extraordinary facility by the air. For they can only
penetrate for a very small distance in the air not
more than two inches, if so much.
It was found by Mme. Curie that the percentage
of the polonium rays, transmitted through a leaf of
aluminium T J^ millimetre thick, was greater when
the rays had to traverse a considerable thickness of
air than when they had only a short distance to
traverse. For instance, it was found that when the
rays had to traverse 1*9 cm. of air, 5 per cent, only
survived passage through an aluminium leaf. But
when only *5 cm. of air lay between the polonium
and the apparatus for measuring the electrical effect,
.then 25 per cent, of the radiation was able to pass
88 THE BECQUEREL RAYS
the leaf. It would appear, therefore, that the pene-
trating power of the rays is diminished by passing
through the air.
It seems that the most probable explanation is as
We know that the rays consist of particles com-
parable in mass with the atoms of hydrogen. In
passing through the air, these must make many
collisions with the molecules of the latter. Now,
we have reason to think that the a-particles are
all shot out with about the same velocity : for, as
we have seen, they are all deflected to something
like the same extent by magnetic force. Thus,
every particle has, in the first instance, the same
chance of penetrating the absorbent screen. Now
some particles will be stopped altogether by the air.
Others will be partially hindered and their speed
reduced by grazing collisions with the air molecules.
The further the particles have to go through the air,
the larger will be the proportion of the survivors
which have undergone such hindrance ; so that the
average velocity will be smaller the further the
particles have gone ; and their power of penetrating
the aluminium leaf will be proportionately reduced.
This seems a plausible interpretation of the ob-
served facts. But, in the present state of our know-
ledge, it must be admitted to be speculative, and
must be received with due caution.
Experiments made by Professor Rutherford and
Miss Brooks have shown that the absorption of the
a-rays by different substances is nearly proportional
to the densities of those substances. For instance,
aluminium is some two thousand times heavier than
air, and it is found that a layer of aluminium will
reduce the rays to about the same extent as a layer
of air two thousand times thicker. The weight of
ABSORPTION AND IONISATION 89
the air interposed is the same as the weight of the
aluminium leaf, provided, of course, that we take
a stratum of the same cross-section in each case.
The law applies to most substances which have as
yet been tried. It is not accurate in every case.
Tin, for instance, is considerably less absorbent than
the law would require.
It has been found that the a-rays from various
radio-active bodies differ somewhat in penetrating
power. These differences are not, however, .very
great. They are probably due to a different velocity
of the particles, though it cannot yet be considered
quite certain that the a-particles are the same in every
case that uranium, for instance, emits exactly the
same kind of a-particles as radium.
The absorption of the /3-rays may next be
We have already seen that a pencil of the /3-rays
of radium is spread out into a variety of different
kinds of magnetic force, and that these varieties
correspond to different velocities of the corpuscles.
The rays least bent by the magnet are constituted
by the faster moving corpuscles ; the more de viable
rays by the slower ones. Since the /3-rays of radium
are so complex, we cannot expect that the absorp-
tion phenomena will be simple. And experiment
certainly shows that they are not ; for it is found
that after each successive absorption the rays which
are left are more penetrating (on the whole) than
For instance, in an experiment of Mine. Curie, the
/3-rays were absorbed by successive thicknesses of
lead, each equal to '155 mm. It was found that the
first sheet suppressed 60 per cent, of the radiation
falling on it. The succeeding ones each suppressed
a smaller percentage of what remained, until the
90 THE BECQUEREL RAYS
ninth sheet suppressed only 3 per cent, of the radia-
tion which had penetrated the first eight.
A probable explanation seems to be this :
The /3-rays are of various velocities. The fast-
moving corpuscles are able to penetrate further
through matter than the others before they are
stopped. Thus, after penetrating a sheet of metal,
the percentage of fast-moving corpuscles is increased,
and the power of penetrating a new sheet of metal
is greater than at first.
An experiment by M. Becquerel shows in a very
direct and satisfactory manner that the faster-moving
corpuscles are indeed those which have the greatest
penetrating power. It was explained in a former
chapter how the rays could be sorted out into a
spectrum by magnetic deflection, the fastest-moving
corpuscles being the least deflected. M. Becquerel
received the magnetic spectrum on a photographic
plate, arranging strips of various absorbent material
on the plate, so that the spectrum fell perpendicularly
upon them. On development, it was found that the
intensity of the spectrum was always more diminished
by absorption at that end which corresponded to large
deflection, or small corpuscular velocity. Practically,
it was found that rays of less than a certain critical
velocity were almost completely absorbed by each
screen. The limiting velocity in question was of
course lower for thin screens than for thick ones, and
also lower for screens of dense material than for
screens of light material.
Since there is a fairly well-defined minimum
velocity for penetration of a screen, the penetrating
power of a corpuscle must increase very rapidly with
increasing velocity. Exact measurements of pene-
trating power in different regions of the magnetic
spectrum would be of great value.
ABSORPTION AND IONISATION 91
By covering the radium salt with a sufficient thick-
ness of metal, we can suppress the /3-rays altogether.
The y-rays then alone remain ; but it is found that a
very considerable thickness of lead is required to
effect this. Some of the /3-corpuscles are able to
penetrate a sheet of lead one-tenth inch thick.
This fact is not a little remarkable. We saw that
the cathode rays were able to penetrate very thin
metal foil. The fact that they could do so was at one
time regarded as a fatal objection to the view that
they were of a corpuscular nature. But conclusive
proof has since been forthcoming to show that they
are so, and the same applies to the /3-rays of radium.
There are considerations which may help to explain
We must remember that Kauffmann's experi-
ments showed that some of the /3-particles are moving
with a velocity very little inferior to that of light.
Now as we have seen, a charged corpuscle moving
with such a speed will have a momentum out of all
proportion to what it would have according to the
laws of ordinary mechanics, if the same laws held
good for speeds like this that hold good for any speed
of which we have experience in an ordinary way.
The additional inertia, there is reason to believe,
would become infinite if the speed of light was
actually attained. Nothing could stop the corpuscle
under such conditions. It will be understood, there-
fore, that great penetrating power is quite consistent
with the known speed of the corpuscles, slightly
inferior to that of light.
Further consideration of how to explain the pene-
tration of solids by the rays must be deferred to the
Since the /3-rays of radium are so varied in their
penetrating power, it is difficult to compare their
92 THE BECQUEREL BAYS
absorption by various substances. The /3-rays of
uranium have been found to be very much more
homogeneous, and thus it is easier to obtain definite
results with them. It has been found that as in the
case of a-rays, the law of density is an approximation
to the truth. The absorption is nearly in proportion
to the density of the absorbing material (Plate III.).
There are exceptions to the law. For instance, the
metals tin and lead give excessive absorptions when
compared with most other substances. They are even
more absorbent than their high density would require.
It is noteworthy that tin departs from the law of
density in exactly the opposite direction when the
absorption of the a-rays is in question.
With regard to the y-rays, the available data are
not very extensive. Professor Rutherford has, how-
ever, examined the comparative absorption of these
rays by various substances. He found that in their
case too the law of density was approximately obeyed.
The exceptions are not always the same as those
for the /3-rays. Thus, for instance, tin, which is very
anomalous for the /3-rays, is not much so for the
y-rays. Lead, on the other hand, is anomalous for the
y-rays, and for the /3-rays also.
Some experiments by the author seem to make it
probable that the excessive absorption of the y-rays
by certain metals is connected with the emission of
secondary rays by those metals. If this view is
correct, it must be assumed that two distinct causes
are at work in producing absorption. One of these
kinds of absorption which is ordinarily much the
more important, depends only on the density of the
substance. The other kind, which is only impor-
tant in certain substances, depends on the power
which these substances possess of transforming the y-
rays which fall upon them into a variety of rays
ABSORPTION AND IONISATION 93
which have much less penetrating power. This action
is analogous to fluorescence in optics the property
which some substances, such as acidulated quinine,
possess of sending out light waves longer than those
which fall upon them. The process involves absorp-
tion of the incident waves. The absorption which
accompanies fluorescence too is of an exceptional
nature ; for ordinary absorption is not accompanied
In what has been said of the absorption of y-rays it
has been tacitly assumed that they are all of one
uniform kind. This is approximately true ; it is
found, however, that the rays which have passed
through great thicknesses of lead several centi-
metres are somewhat more penetrating than those
which have only traversed one centimetre. 1
We have often alluded to the property which the
Becquerel rays possess of causing the discharge of
electrified bodies. This property was one of the first
to be discovered, and it gives us the most generally
useful method of detecting the presence of the rays,
and measuring their comparative strength. It is now
time to consider this important subject in detail.
In experiments on the electrical conduction under
various conditions, the simple gold-leaf electroscope
which we have hitherto supposed to be used, is
scarcely available, and it is necessary to make use of
a quadrant electrometer. It is desirable that the
reader should make himself acquainted with the
principle of that instrument, for the greater part of
the experiments on radio-activity have involved its
use. A description of it will be found in elementary
works on electricity. It must suffice here to state
that it enables us to measure small quantities of
1 Professor M'Clelland has recently found that these most penetrating
rays of all obey the law of density very closely.
94 THE BECQUEREL RAYS
electricity. When the instrument is set up in work-
ing order, and a charge of electricity imparted to it,
the needle is deflected, to an extent proportional to
the potential or electric pressure which results. A
potential of one volt, which is about what can be got
from some forms of battery eel], will often suffice to
give a deflection of one hundred scale divisions on the
electrometer, though improved electrometers have
recently been devised which will give much more.
The quantity of electricity which is required to charge
the needle to one volt is very small. Accordingly a very
small current of electricity flowing to the electrometer
will cause its deflection to increase ; the rate at which
the electrometer needle is moving serves to measure
the current which is flowing to it. It is found in
practice that very much smaller currents can be
detected and measured in this way than by the
galvanometer ; though, when the current is large
enough, the latter instrument is more convenient.
A convenient piece of apparatus for experimenting
on the electrical effect is illustrated in the figure
(fig. 21). Two parallel metal plates are used. The
radio-active substance is placed on the bottom plate,
spread into a uniform layer. This bottom plate can
be charged to any desired potential or electric
pressure by means of a battery. The plate above is
connected to the electrometer, and is initially un-
charged. Means are provided for moving it to or
from the bottom plate. The whole arrangement is
enclosed air-tight in a glass jar.
The first experiment with the apparatus would be
this. Raise the lower plate to a high potential, say
100 volts. Having discharged the electrometer, by
connecting it to the earth by a wire, let the con-
nection be removed. The electrometer will now
receive a charge, and its deflection will increase
ABSORPTION AND IONISATION 95
rapidly ; for electricity is leaking, under the influence
of the radio-active substance, from the highly charged
lower plate to the upper one, which is connected to
the electrometer, and which was initially uncharged.
The upper plate thus receives a charge, and the
electrometer indicates it. It makes no difference
pruci of Battery.
FIG. 21. Parallel plate arrangement for investigating electrical conduc-
tion under Becquerel rays. The bottom plate, a, on which the radio-
active substance is spread, is kept at any desired high potential. The
upper plate, b, is movable, and parallel to the first. The distance can be
altered and measured by the micrometer screw. A stuffing box provides
for air-tightness. The top cover of the glass jar is of ebonite to provide
whether the bottom plate was charged positively
or negatively. A positive charge can leak across the
space between the plates just as well as a negative
one. Now let the air be exhausted from the bell jar
by means of a good air-pump. We shall find that
the electrical leakage has disappeared. No charge
now reaches the upper plate. 1
It is evident, therefore, that the true interpreta-
tion of the electrical leakage effect is that the
Becquerel rays are able to make the air through
1 It will be remembered that the radio-active substance is constantly firing
off negatively charged corpuscles ; these are no doubt conveying negative
electricity to the upper plate. But under ordinary conditions their effect
would be too small to be noticeable with this apparatus.
96 THE BECQUEREL RAYS
which they pass a conductor of electricity ; in the
absence of air no leakage takes place.
Let us now suppose the air readmitted to the
vessel, in order to investigate the leakage effect in
another way. The question to be tested is this.
What is the relation between the current of elec-
tricity flowing between the plates and the electric
pressure which causes it to flow ? In technical
language, What is the relation between the current
and the electro-motive force ?
In the most familiar case of electrical conduction
the conduction of electricity along a metal wire the
relation between the current and the electro -motive
force is the simplest that can be imagined. For the
current along the wire is simply in proportion to the
electric pressure which is applied to its ends. If the
electric pressure is doubled, the current is doubled also.
If the electric pressure is increased a thousand times,
the same is true of the current ; that is, provided
that the wire is not allowed to get hot a condition
which prevents our testing the law beyond a certain
point. But so far as the wire can be kept at a con-
stant temperature, the law, which is known as Ohm's
law, is rigorously and exactly true.
The question then is, Does the air made conducting
by Becquerel rays behave like the metal wire ? Both
of them are conductive of electricity. May not both
behave alike ?
It is a question for experiment, and the answer
which experiment gives is that they do not.
We can vary the potential or electric pressure of
the lower plate by connecting it to the high potential
end of an electric battery. The number of cells in-
cluded in the battery can be varied at pleasure. Each
successive cell increases the potential by a definite
amount, according to the construction of the cell. If
ABSORPTION AND IONISATION 97
we try one cell, and measure the leakage current, we
shall get a certain value for it. If we use two cells,
we shall get nearly twice as much, and, perhaps, with
three, not far from three times as much as with one.
But this state of things will not continue much
further. It will soon be found that the addition of
battery cells does not increase the current so much as
at first ; and when a certain number of cells has
been added, the current will have reached a limit.
A further addition of cells will not increase it at all,
or, at least, not to any measurable extent. The
greatest current which can be got through the air
is called the saturation current.
The number of cells required to produce saturation
of the current depends on the conditions of the
experiment the distance between the plates, and
still more the strength of the Becquerel rays. But
to give some idea, we may say that, with uranium
nitrate, and with the plates an inch apart, 100
volts difference of electric pressure between the
plates would be amply sufficient. This is a very
ordinary value for the electric pressure used in
domestic electric lighting.
A theory has been developed which gives a very
satisfactory explanation of the existence of the
saturation current. Before examining the experi-
mental facts further, it will be well to give an
account of this theory. For we shall then be able
to see their true bearing, instead of having to content
ourselves with recording them empirically.
It has now for many years been recognised that
when a current of electricity passes through a liquid
such as dilute sulphuric acid or salt and water, the
process consists in the motion of positively charged
particles up to the negative electrode, and negatively
charged ones up to the positive, under the influence
98 THE BECQUEREL RAYS
of electric attraction. The particles are called ions ;
and there is every reason to believe that such a solu-
tion partially decomposes of its own accord into ions,
so that they are ready, when the electric force is
applied, to move up to the electrodes, thus conveying
a current through the liquid.
With air in its ordinary state it is otherwise, for
under such circumstances it contains no ions, or
practically none, and cannot convey an electric
current. But the Becquerel rays are able to decom-
pose it into ions. Each atom of the a-rays, or each
corpuscle of the /3-rays, sooner or later strikes one of
the molecules of the air. The molecule is shattered
by the blow, and it breaks up into two ions, one
possessing a charge of positive electricity, the other
an equal charge of negative. When these ions were
joined to form a molecule, their charges served to
hold them together ; and the charges neutralised one
another so far as any external matter was concerned.
For the molecule contained as much positive elec-
tricity as negative, and so, taken altogether, it was
free from electrification, and would not move under
the influence of electric force. But now that the
ions constituting the molecule have been separated,
they are attracted by the electrodes, and move up to
them. It must be noticed, however, that the positive
ions are also attracted by the negative ones, and if
there are plenty of both in the gas, pairs of them are
sure to come into contact, and unite again to form
neutral molecules. If no external electric force is
acting, all the ions will do this sooner or later, and
the state of things will then be the same as at first,
if the radio-active substance is taken away. Of
course, if it remains, more ions are constantly brought
into being, and those again recombine. The forma-
tion of ions in a gas is called the ionisation of the
ABSORPTION AND IONISATION 99
gas. Thus in air exposed to the Becquerel rays there
is a constant ionisation in progress. As the number
of ions increases, the chance of a positive ion meeting
a negative one, and pairing with it, of course becomes
greater ; so that the rate of recombination becomes
greater. At last the rate at which the ions recom-
bine becomes equal to the rate at which they are
produced, so the supply reaches a stationary value.
This is practically attained in a fraction of a minute
after the rays begin acting.
Let us now consider what happens when an electric
force acts on the ions. They move across to the
electrodes, and are thus removed from the sphere of
action. If they move very slowly, most of them will
meet with partners on the way, and will be no longer
effective as carriers of electricity. A few of the ions,
however, are able to get across without recombining.
These few are the carriers of the current, which is
accordingly small. Now let double the electric force
be applied. The ions will move twice as fast as
Accordingly they will have only half the chance of
meeting partners before they get across, and conse-
quently nearly twice as many will get across uncom-
bined. But this will continue true only for small
electric forces, because when large forces are applied
and the ions move fast, a large proportion of the total
number will get across uncombined ; and then it is
evident that doubling the electric force cannot double
the number of ions that get across uncombined, for
there are not enough ions altogether to allow of that.
The most that can be done is to apply an electric
force large enough to snatch practically all the ions
away as soon as they are formed, giving them no time
1 It will be understood that the velocity is steady, and does not increase,
owing to the resistance of the air.
100 THE BECQUEEEL BAYS
to recombine. In that case, all the ions are utilised
in conveying the current, and the current is as large
as it can be. Nothing is gained by snatching the
ions away faster still by a larger electric force. So
the current has reached a limit independent of the
electric force. All this, it will be observed, is in
exact agreement with what was found experimentally
If a saturating electro-motive force be used, it is
found that the current between the two plates of our
apparatus increases instead of diminishing when the
plates are put further apart. This seems a very
strange result when we compare it with what happens
when electricity is flowing through a metal ; for the
less distance the current has to flow through a metal
the larger the current will be.
But the theory of ionisation affords a ready ex-
planation. For the further separated the plates are,
the more ions are formed in the air between them,
and, as the saturation current uses up all the ions,
this current is naturally increased.
The a-rays are readily absorbed by the air, as we
know ; so that if the distance between the plates is
considerable, the rays which reach the air near the
top plate will be much weakened by absorption, and
the air there will not be nearly so strongly ionised as
the air near the radio-active substance.
Now in most cases the ionisation due to a-rays
greatly exceeds that due to the other varieties, so that
if the plates are separated so far that the a-rays do
not extend to the top plate appreciably, we shall not
get many more ions produced between the plates by
separating them still further. This anticipation, again,
is confirmed by experiment ; for it is found that the
saturation current does not increase beyond a certain
point when the plates are widely separated.
ABSORPTION AND IONISATION 101
It is interesting to inquire what happens when the
air pressure is altered. Let us suppose the plates well
separated say two inches. When there is no air,
no ions can be produced, and there will be no current.
If now the air be admitted to a very feeble pressure,
sa y y^yth part of the atmosphere, the a-particles
will have very few chances of meeting molecules of
the gas, so as to ionise them. But some collisions do
occur, and accordingly a few ions are produced, and a
feeble current will pass. The number of collisions is
not, however, sufficient to impede the a-particles to
any serious extent.
If the air pressure be made twice as great, nearly
twice as many ionising collisions will occur, but not
quite twice as many, for the particles, after passing
through the first half of the gas, are now moving a
little more slowly, and are to some extent like a spent
bullet. Thus they will not be so well able to ionise
a molecule, if they do not hit it fairly, or, perhaps, a
few of them will have been stopped altogether, and
will therefore no longer be able to produce ions at all.
So that, with the doubled air pressure, we cannot
expect the current to be quite double ; but it will not
be much less. This point, like a very similar one we
had to deal with in considering the recombination
of ions, may be a little difficult to follow at first ;
but with consideration it will become quite clear.
The ionisation, and consequently the current, must
then be proportional to the pressure when the
pressure is small. Consider now the case when the
pressure is so large that the rays are altogether
absorbed in the space between the plates. It will be
evident that in this case all the a-particles are used
up in making ions; and no more ions can, under any
circumstances, be produced by the rays. So the
current has reached a maximum, and to increase
102 THE BECQUEREL RAYS
the pressure still more cannot increase it. All this
is entirely confirmed by experiment.
Everything that we have said will apply equally to
the a- and /3-rays, and probably to the y-rays also.
In the case of the more penetrating kinds of rays,
however, the pressure at which the saturation current
would reach a maximum would be very great.
It will be convenient to sum up the conclusions of
this somewhat difficult argument.
The leakage of electricity under the Becquerel rays
can be accounted for by the theory that the impact of
the projected particles on the molecules of the air
break them up into charged ions. For every ion
carrying a positive charge, another is formed carry-
ing a negative one. The convection of the electric
charge under the influence of the electro-motive force
constitutes the observed electric current. The ions
when left to themselves recombine to form neutral
This theory accounts for the following facts. The
current is proportional to the electro -motive force
when the electro-motive force is small. When the
electro-motive force is large, the current reaches a
maximum, or is said to be saturated. The saturation
current increases with the distance between the
electrodes. When the distance between the elec-
trodes is fixed, the saturation current is, for small
air pressures, proportional to the pressure. For large
air pressures, it reaches a maximum, independent of
further increase in the pressure.
Many experiments have been made on the pro-
perties of the ions. We cannot enter into the
details of these, but the main conclusions must be
The charge on the ions has been determined by
the same method that was found effective for the
ABSORPTION AND IONISATION 103
corpuscles emitted by metals under the influence of
ultra-violet light. 1 The result is to show that they
too carry the same charge as the hydrogen atom in
the electrolysis of liquids. It might perhaps be at
first concluded that the ions were identical with the
cathodic corpuscles. This view, however, cannot be
unreservedly accepted. For the velocity of the ions
moving through air under an electro-motive force is
very small. They do not move more than about
one centimetre per second, if the potential gradient
is one volt per centimetre. This has been ascer-
tained by making them move against an air blast,
and adjusting the blast so that they just could not
make headway against it, and convey an electric
current in the opposite direction, or by equivalent
methods. Now this velocity is much smaller than
the velocity which an atom would acquire if pulled
through air under an equal force. We must con-
clude that the ion, so far from being smaller than
an atom, is much larger. The most probable view is
that the negative ion consists of a corpuscle which
has attracted to itself a number of other uncharged
atoms ; these impede its motion. The positive ion
consists of a positively charged atom, also with
attendant uncharged atoms.
The rate of recombination of the ions has been
studied by blowing the ionised air away from the
radio-active substance at a known velocity, and de-
termining the amount of conductivity at different
distances along the tube. Recombination is in most
ordinary cases practically complete after two seconds.
For it is found that two seconds after the air has left
the radio-active substance, its conducting power has
Hitherto we have alone considered the ionisation
1 Chap. i. p. 14.
104 THE BEGQUEREL RAYS
of air by the rays. Some attention must now be
devoted to other gases.
The general phenomena are in all respects similar
to those already described for air. The point which
requires special consideration is the comparison be-
tween the amount of ionisation in air and in other
gases. We wish to compare the gases when they are
under the influence of rays of the same strength
throughout. To secure this, it is necessary that the
absorption of the rays by the layer of gas used shall
be small ; for otherwise the first layers of the gas
will appreciably screen the further layers from the
radiation to an unknown extent. This prevents
accurate comparisons being made.
To secure practical uniformity of the rays through-
out the gas, it is necessary to select a pressure within
that range for which the ionisation is proportional to
the pressure. Otherwise the conditions of the experi-
ment become very complicated, and the results difficult
For the a-rays it is necessary to select some quite
low pressure, say ^j^ n f the atmospheric pressure,
for the experiment. The apparatus with parallel
plates already described is very suitable for the
purpose. It should be set with the plates, say,
f-inch apart. Polonium may be conveniently em-
ployed as the radio-active substance, since it gives
a-rays only. The various gases, hydrogen, carbonic
acid, etc., are introduced into the glass jar, from which
the air has been previously removed by an air pump.
Each gas is in turn admitted to the apparatus, to a
pressure of -^th. f an atmosphere, and the saturation
current measured. Atmospheric air serves as a con-
venient standard with which to compare the other
gases. The radio-active substance must not be dis-
turbed throughout the series of experiments, or they
ABSORPTION AND IONISATION 105
will no longer be strictly comparable with each
In experimenting with the /3- and y-rays, it is not
necessary to work at reduced pressure ; for these rays
are not much absorbed by a reasonable thickness of
gas. In these cases it is more convenient to modify
the apparatus. The bottom plate should be made
thin, and the radio-active substance placed on a flat
dish, which is placed underneath the apparatus,
outside it. The rays then penetrate the bottom
plate and act on the gas between the plates. In
examining the y-rays, a thick slab of lead must be
interposed between the radium and the bottom
Experiments made on the lines described have
shown that all three kinds of rays give nearly the
same results ; for it is found that the ionisation of a
gas is in each case nearly proportional to its density.
Sulphurous acid, for instance, which is rather more
than twice as heavy as air, is also rather more than
twice as much ionised, other things being equal.
The law is only roughly true ; the most notable
exception is hydrogen. This gas is more than twice
as much ionised by all the varieties of rays as it
ought to be, if the relation between ionisation and
density were exact. There are other exceptions, but
these are comparatively unimportant. The /3- and
y-rays seem to give exactly the same results, as
nearly as the experiments can show. The a-rays
give slightly, but still distinctly, different values.
The general relation between ionisation and density
holds good for all the varieties of rays ; but the ex-
ceptions to it are not always the same.
The resemblance between the laws connecting
absorption and density, and on the other hand ionisa-
tion and density, cannot fail to attract notice. If both
106 THE BECQUEREL RAYS
ionisation and absorption are proportional to density,
they must also be proportional to one another.
Ionisation by the a- and /3-rays is regarded as dne
to the impact of projected particles on the molecules
of the gas. This leads very simply to an explanation
of the connection between ionisation and absorption.
Every particle stopped by impact with molecules
produces a certain number of ions. Thus the number
of ions produced is proportional to the number of
particles stopped. In other words, ionisation is pro-
portional to absorption.
Since ionisation is proportional to absorption, the
same amount of ionisation should always attend com-
plete absorption, whatever the gas in which the
absorption takes place. This affords a very direct
experimental test of the truth of the relation. It is
not difficult to completely absorb the a-rays in a
moderate thickness of gas, and to measure the result-
ing ionisation. Professor Rutherford made experi-
ments of this kind early in the history of the
development of the subject ; he found that the
ionisation was, at any rate nearly, the same whatever
gas was used to completely absorb the rays. This
is a very satisfactory confirmation of the theory that
ionisation is proportional to absorption.
Hitherto the ionisation of gases has alone been
considered ; but we have seen that the law of absorp-
tion and density is not at all confined to gases. It
holds approximately true for all classes of substances,
solids and liquids as well as gases. Is it natural to
inquire whether the law for ionisation does the same ?
It is probable that this is the case ; but we do not
know for certain, for there are formidable difficulties
in the way of measuring the ionisation of liquids and
We can show that some liquids, at all events, are
ABSOEPTION AND IONISATION 107
ionised by the Becquerel rays. It would be of no
use to try to observe the ionisation of water. For it
is impossible in practice to get water absolutely free
from saline impurities ; and the faintest trace of such
are sufficient to give it an enormous conductivity,
compared with anything that the Becquerel rays
could do. The effect of these would be lost in the
large conductivity already existent. To try to detect
the extra conduction due to them would be like
trying to see the rise of level of the water in a pond
when an extra thimbleful was emptied into it.
The only kind of liquids which it is of any use
to try are those which conduct electricity but very
little, if at all. Examples of such liquids are liquid
air, bisulphide of carbon, benzene, and petroleum
ether, such as is used in the propulsion of motor cars.
M. Curie has experimented on the ionisation of such
liquids by the rays. He was able to detect conduc-
tivity. The way he carried out the experiments was
practically equivalent to what has been described for
gases, but the space between the electrodes was filled
with a liquid instead of a gas.
The relation between current and electro-motive
force was investigated, just as for gases. It was not
found practicable to apply a large enough electro-
motive force to produce saturation, though some
signs were observed that saturation was being
There is no reason to be surprised that saturation
is difficult to attain in a liquid ; for it is natural that
the ions should encounter great resistance in pene-
trating so dense a medium. They will therefore
move very slowly under the electro- motive force, and
a very large electro-motive force will be necessary to
draw them away so quickly that none of them have
time to recombine. In other words, a very large
108 THE BECQUEREL BAYS
electro -motive force will be necessary to produce
As the saturation current has not been measured,
we have no precise information as to the ionisation
of liquids by the rays, and whether it is proportional
to the absorption.
With regard to the ionisation of solids, our in-
formation is still more scanty. M. Becquerel found
that paraffin wax conducted to some slight extent
under the influence of the rays. This conduction
persisted for a long time after the radio-active sub-
stance had been removed. The paraffin only slowly
recovered its usual insulating properties.
Probably some kinds of glass also conduct to a
certain extent. Radium may often be kept for
months sealed up hermetically in a glass tube. Now
negative corpuscles are constantly being shot through
the glass. If the positive charge which remains on
the radium were not able to escape by conduction,
it scarcely seems possible that the glass could ever
stand the electric stress for more than a few hours
without being pierced. Puncture does sometimes
It is not difficult to understand that in a solid
substance the ions produced by the rays may not
be free to move except with great difficulty under
electro-motive force. If this is the case, the current
can be conveyed, and we cannot measure the amount
of ionisation produced.
Conduction of electricity by metals is now gener-
ally regarded as due to a process analogous to the
motion of ions in an electrolyte, or in an ionised gas.
If this is really so, some kind of ions must exist in
a metal which are free to move through the metal.
It is hardly possible that these ions should be of the
same kind as the ions produced by Becquerel rays
ABSORPTION AND IONISATION 109
in gases; for the latter are, as we shall see, large
compared with molecules, and cannot be supposed to
move through a solid. Still, it is conceivable that
the rays produce some extra conduction in metals.
There would, however, be no chance of detecting
it, in presence of the enormous conduction already
It is now time to return to a matter which was
left over. This is the behaviour of the y-rays with
regard to ionisation and absorption.
These rays, it will be remembered, are of doubt-
ful nature. It is uncertain whether they consist of
corpuscles or of Rontgen rays generated by the stop-
page of corpuscles. Now the Rontgen rays emitted
by a vacuum tube are able to ionise gases, just as
the Becquerel rays do. How or why they do this is
not known ; for the simple explanation available in
the case of the a- and /3-rays of radium is not avail-
able. The Rontgen rays are believed to be of the
nature of electro-magnetic waves or pulses, and it is
not apparent why such a wave should be able to
separate the ions in a molecule. 1 But whatever the
reason, it is a fact that the Rontgen rays do produce
ionisation of the same order of intensity as the ionisa-
tion produced by radio-active substances ; moreover,
there is every reason to conclude that the ions are
of the same nature in each case. When we come to
compare the ionisation of different gases, however, an
extraordinary difference between the two ionising
agents makes its appearance. For it is found that
different gases are not ionised by the Rontgen rays
1 It is true that there is an electric field in the wave front which might
be conceived capable of tearing the ions apart. We do not know the
strength of this field, but it is difficult to believe that it can be strong
enough to produce such a result. Moreover, it is not clear why so few of
the molecules encountered by the wave are ionised, or what there is to dis-
tinguish the molecules which do get ionised from those which do not.
110 THE BECQUEREL KAYS
in anything like the same ratio as by the Becquerel
rays. The law of densities is not at all obeyed.
It is with gases or vapours containing elements of
high atomic weight that the difference is most con-
spicuous. Let us give an example. Iodine has a
high atomic weight one hundred and twenty-seven
times that of hydrogen. Methyl iodide, a compound
containing it, gives off a vapour about five times as
dense as the air. It is accordingly about five times
as much ionised by the Becquerel rays. But when
we come to try the effect of the Eontgen rays, the
ionisation of methyl iodide is no less than seventy
times the ionisation of air.
A corresponding difference appears in the absorp-
tion of the two kinds of rays by solids. Substances
of high atomic weight absorb much more in propor-
tion. Platinum is about twenty-one times as dense
as water, and a sheet of platinum would absorb the
Becquerel rays to about the same extent as a stratum
of water twenty-one times as thick. But with Eont-
gen rays, the absorption by such a sheet of water
would be quite inconspicuous in comparison with the
A simple illustration of the same thing has already
been encountered in the second chapter. It was
mentioned that when the shadow of the hand was
observed on a fluorescent screen by means of the
Rontgen rays, the bones were clearly visible, for they
gave a much darker shadow than the flesh ; but with
radium to illuminate the fluorescent screen, the differ-
ence between flesh and bone was much less con-
spicuous. The reason is that bone, which consists
mainly of calcium phosphate, is made up of elements
of much higher atomic weight than flesh, of which
the elementary constituents are oxygen, hydrogen,
nitrogen, and carbon ; accordingly, though the bone
ABSORPTION AND IONISATION 111
and flesh are about equally opaque to Becquerel rays,
the bone is far more opaque to Rontgen rays than
the flesh. The result is that they are seen in sharp
contrast, and the bones are clearly visible.
It has been explained that in regard to relative
ionisation and absorption, the y-rays of radium be-
have just like the a- and /3-rays, and thus absolutely
unlike ordinary Rontgen rays. This fact seems to be
a strong argument against the theory that identifies
them with Rontgen rays, and to be favourable to the
view which considers them to be an extreme kind of
Rontgen rays are of different kinds, for the rays
obtained from a highly exhausted vessel are much
more penetrating than those from one less perfectly
exhausted. There is reason to think that the relative
ioriisations of gases under Rontgen rays vary with
the quality of the rays. It is difficult, if indeed at
all practicable, to obtain Rontgen rays as penetrating
as the y-rays of radium, and the relative ionisations
under such rays have not yet been measured. But
Mr. Eve has recently made measurements which
indicate that the most penetrating Rontgen rays he
was able to obtain approach nearly in their properties
to the y-rays. This clears away the principal out-
standing difficulty in admitting that the y-rays are
THE CHANGES OCCURRING IN RADIO-ACTIVE BODIES
IF a specimen of radium bromide is placed in a glass
tube, and gently heated, it will be found that a small
quantity of gas can be extracted from it, which has
most remarkable properties. The gas has, in fact, all
the peculiarities of radium itself; for, if collected in
a glass tube, it will emit the Becquerel rays. A
fluorescent screen brought near it will light up, and,
indeed, the tube itself is sufficiently fluorescent under
the influence of the rays to be easily visible in the
dark. The air outside the tube is rendered con-
ducting by the rays emitted. Photographic plates
are acted upon. The glass of the containing tube is
turned perceptibly violet even in twenty-four hours.
In short, all the characteristic phenomena are obtained.
This radio-active gas has become known as the radium
The volume of emanation emitted by any such
quantity of radium as is at present procurable, is
absolutely infinitesimal. From 50 milligrammes of
radium bromide (of which the market price would be
about 2 5 -30) the volume of emanation procurable
at any one time probably would not exceed that of
a large pin's head, if, indeed, it amounted to so much.
But the gas is so active that, if it was mixed with
a million million times its own volume of air, the
electrical leakage through a sample of the mixture,
due to the action of the radio-active emanation on
the air containing it, would be quite conspicuous.
CHANGES IN EADIO- ACTIVE BODIES 113
It is only with great difficulty, and by taking
very special precautions, that the emanation can be
obtained in the pure state. But, for examining many
of its properties, that is by no means necessary. The
admixture of another gas, such as ordinary air, does
not at all interfere with many of the experiments.
In some cases, in fact, it facilitates them.
We have spoken of heating the solid radium
bromide. But the same result can be obtained by
dissolving it in water. The solution then gives off
the emanation. A mixture of oxygen and hydrogen
in their combining proportions is also given off, as
we have already seen. This mixture serves to wash
out the emanation, and the mixture may be extracted
and collected by means of a Sprengel air-pump.
A mixture of the emanation with air may readily
be obtained by bubbling air through the solution. If
this air be led into a tube containing some zinc sul-
phide, the fluorescence of the sulphide will be very
The electrical effects due to the emanation may
easily be shown. Air which has been bubbled
through radium bromide solution will rapidly dis-
charge the electroscope when blown on to it, or better,
admitted into the case of the instrument so as to
surround the gold leaves.
Air which contains the emanation behaves in all
respects like air which is exposed to radium ; but
it is able to keep itself constantly ionised by the
radio-active material which it contains ; whereas the
ionisation of ordinary air only continues as long as
the radium acts upon it. If the radium is with-
drawn, the conducting power disappears in the course
of one or two minutes, owing to recombination of the
ions already present, and the failure of the supply of
new ones. If the electro-motive force is acting all
THE BECQUEKEL RAYS
the time, the ions are used up immediately, and the
phenomena cease the moment the radium is with-
The emanation behaves like other gases in one very
important property. It can be condensed to the
liquid or solid form
by cooling. Differ-
ent gases differ
very greatly in this
respect. Steam or
water vapour, for
instance, can be
condensed out of a
space which con-
tains it by means
of ice, though to
traces of it would
require a lower
To condense the
Fio. 22. Arrangement for condensation of the emana-
tion by liquid air. a is a solution of radium bromide,
which yields the emanation. Air is bubbled through it,
rlpi~plv if 1
wc yes e emanaon. r s ue roug , P^WMJ, lb I
and carries the emanation through to the U tube, 6, im- OQY . TT f ^ maL-n nco r-T
mersed in liquid air, where it is condensed. The liquid air bcil J LU U**M3 UO Ul
is contained in a double walled vessel, c, with a vacuum . i i ,
between the walls. This serves to prevent the access of tile ULUCn greater
heat. While 6 is surrounded by liquid air, no emanation -, -. i i
can escape through d, and it has lost all discharging COlu. WniCIl W6 Can
power. When the liquid air is removed, the accumulated
emanation evaporates, and the issuing air is strongly command bv the
use of liquid air.
The condensation can be shown in the following
way. Air bubbled through the solution of radium
bromide is passed through a U-shaped glass tube,
immersed in liquid air (Fig. 22). This cools it to the
temperature of the liquid. At that temperature the
emanation no longer remains a gas, but separates out
CHANGES IN RADIO-ACTIVE BODIES 115
in the solid form, and is deposited on the walls of the
U tube. The whole process may be watched in the
dark. The air charged with emanation causes visible
fluorescence in the glass tubes. But the fluorescence
does not extend beyond the cooled portion of the
U tube when it condenses. The accumulated emana-
tion condensed in the tube shows a brilliant fluor-
escence, proving, incidentally, that even at this low
temperature it is still able to emit Becquerel rays.
of the emanation can
be shown even more
effectively by an-
The emanation, with
as little admixture
of any other gas as
possible, is intro-
duced into a glass
globe (Fig. 23) con-
taining zinc sul-
phide, which fluor-
FIG. 23. Arrangement for snowing the condensation
f the emanation. An inverted U-shaped tube carries
at each end a glasg bulb> The ]arger bulb) a> contaills
zinc sulphide or other fluorescent materials. The
arrangement is exhausted of air, and the emanation
introduced through the stopcock, c, which is then
closed. The zinc sulphide glows brilliantly. On
immersing 5 in liquid air, the emanation all condenses,
and a loses its luminosity. When 6 is allowed to heat
. up again, the luminosity is restored.
tube is another small
bulb which can be immersed in liquid air. As soon
as this is done, all the emanation condenses in the
bulb, so that there is none left in the large globe, and
the zinc sulphide is no longer luminous. If the bulb
is removed from the liquid air, the emanation evapo-
rates again, and the luminosity is restored.
The emanation seems to be absolutely unaffected
by any chemical treatment that we can submit it to.
Not even the most violent treatment, such as passing
ill l i
g 10 DO bV a SllOrt
116 THE BECQUEREL RAYS
it over red-hot magnesium, alters its radio-activity in
the slightest. The only other known gases which
can withstand such treatment are the inert con-
stituents of the atmosphere, which have been dis-
covered of recent years, helium, neon, argon, krypton,
and xenon. Any other gas, such as oxygen, carbonic
acid, or even nitrogen (which is generally very inert),
enters into chemical combination with red-hot mag-
nesium. In its chemical properties, or rather want
of properties, then, the emanation resembles the inert
We have not yet any very satisfactory means
of determining with accuracy the density of the
emanation. As we have already remarked, the
volume of pure emanation obtainable does not exceed
that of a pin's head, and it will readily be admitted
that to attempt to weigh such a quantity of gas
would not be a very hopeful task. Sir William
Ramsay and Dr. Travers succeeded in determining
the density of xenon with only 7 cc. of gas, equivalent
in English measure to about half a cubic inch. This
was justly regarded as a triumph of experimental
skill, But even that degree of refinement is of
course utterly insufficient for dealing with the emana-
tion. There is, however, a means by which we can
obtain some approximation to the desired result. It
depends on the diffusion of the emanation through a
Let us take a vessel made of porous (unglazed)
earthenware, such as is used for a Daniell's battery,
and fill it with carbonic acid gas, closing the mouth
air tight. After a little time we shall find that there
is less carbonic acid in the vessel than there was at
first. Some of it has diffused out through the porous
walls. How fast it has diffused out will, of course,
depend on how thick the porous vessel which we have
CHANGES IN RADIO-ACTIVE BODIES 117
chosen for the experiment may happen to be. But
we shall find that, however much or little carbonic
acid there may have been to begin with, the time
which elapses before half of that amount has escaped
will always be about the same. Suppose, for instance,
that there was one gramme of carbonic acid by weight
to begin with. We can find by experiment how long
a time elapses before half that quantity has escaped.
Suppose we found that it took an hour. Then after
another hour we should find that the quantity had
been halved again. There would then be only a
quarter gramme ; and so on. Whether there was air
mixed with the carbonic acid to begin with is quite
unimportant. The only thing we need consider is
the amount of carbonic acid inside. It is essential
that the carbonic acid should not be allowed to
accumulate outside, or some of it would diffuse back
to the inside again. But if the porous vessel is
exposed to the air of the room, the carbonic acid
which gets out is sure to be blown away effectually
by chance draughts.
Now, let us try the same thing with another gas,
hydrogen. We shall find that the hydrogen, which
is much lighter than carbonic acid, gets away much
faster from the same vessel. In fact, it is found
that the rate at which gases diffuse depends only on
their density. A dense gas always diffuses more
slowly than a light one. 1
Now it will be seen that this method is admirably
adapted to the case with which we are concerned.
For, as we have said, the admixture with air does
not affect the rate of diffusion. All that we have
to do is to find the rate at which the emanation
escapes through a porous vessel, whose behaviour
1 The quantitative law is that the rates of diffusion are inversely propor-
tional to the square roots of the densities.
118 THE BECQUEREL RAYS
with gases of known density has been found. We
can arrange to measure the quantity of emanation
in the porous vessel at any moment by observing
the rate of leak of electricity through it. The
absolute quantity present, which is so difficult to
measure, does not at all concern us. All we want
is a uniform scale of measurement which is consistent
with itself. We measure the rate of leak due to
the emanation from time to time, and find how long
a time elapses before it falls to half its initial value.
We are then in a position to compare its density with
those of the other gases.
Experiments made in this way are not susceptible
of any high degree of accuracy, but they point to
the conclusion that the emanation is about ninety
times as dense as hydrogen, or more than six times
as dense as air. This number must be accepted with
some reserve. But there is no doubt, at all events,
that the emanation is, for a gas, exceedingly heavy.
The density of a gas is a very important datum.
For every gas, observed under similar conditions, is
equally coarse grained. Each gas contains the same
number of ultimate particles or molecules in the same
volume. So that the weights of the molecules are in
the same ratio as the weights of equal volumes, or,
in other words, as the densities. By comparing the
densities, we can compare the weights of the mole-
cules. The molecule of the emanation is, then,
apparently about ninety times as heavy as the mole-
cule of hydrogen.
Unlike the radium salt which furnished it, the
emanation becomes perceptibly less active every
day, until the activity has become so small that it
is no longer perceptible. The activity is found to
decay every day by a fixed percentage of the amount
which it possessed at the beginning of that day. It
CHANGES IN RADIO-ACTIVE BODIES 119
goes down by about one-fifth, of its value every day.
This is nearly analogous to the way in which a sum
of money invested at compound interest increases.
But, to make the analogy exact, we must suppose the
interest to be subtracted from the capital, instead
of being added to it. In that ca,se the capital would
diminish in exactly the same kind of way that the
emanation diminishes in activity when left to itself.
We have already met with the same law in considering
the diffusion of gas out of a porous vessel. The rate
of decay of the emanation is conveniently indicated
by saying how long it would take to diminish to one-
half its initial activity. Careful experiments have
shown that the time required is 3*7 days.
It is a very remarkable fact that the rate of decay
of the emanation is absolutely independent of the
circumstances in which it may be. Hot or cold,
solid or gaseous, concentrated, or diluted with air,
it makes no difference whatever. We may condense
the emanation with liquid air, and keep it condensed
for some days. But it will be found that when we
let it evaporate again, it has diminished in activity
just so much and no more as it would have if it had
been left in the gaseous condition all the time. If
we start with a strong emanation, there will be no
difficulty in still recognising its presence after the
lapse of two months.
Sir William Ramsay and Mr. Soddy have recently
obtained the pure emanation, and have measured its
volume, by the use of a tube of very narrow bore,
and by working at a pressure much less than that
of the atmosphere. They have observed that
the volume contracts till no longer measurable,
and that this diminution follows the same law as
the radio-activity ; the volume reaches half its initial
value in 37 days. It is clear from this experiment
120 THE BECQUEREL RAYS
that the gaseous emanation is slowly turning into
solid products, which occupy a volume very small
compared with its own. The experimental difficulties
of this observation are very great, and the results,
as might be expected, not free from apparent ano-
malies ; but there can be little doubt of the general
We must now return to the radium salt, and con-
sider its behaviour after the emanation has been
taken from it. As the emanation is so very radio-
active, it will be readily understood that, while still
with the radium, it contributed a good deal to the
activity of the latter. So that the radium, deprived
of its emanation, is much less active than it was
before. But this enfeeblement does not last. If
the radium is tested from day to day, it is found
to be slowly becoming more active again. In fact,
it is brewing fresh emanation. It will be remem-
bered, however, that the emanation does not last.
So that there is a limit reached, when the emanation
dies away as fast as the radium can make more of it.
After that the activity of the radium will not increase
any further. This state of things is practically reached
after about a month.
The radium salt is not able to retain the whole of
its emanation, even at ordinary temperatures. A
little of it is always diffusing away.
The same behaviour can be traced when the radium
salt, instead of being heated, is dissolved in water.
This equally separates the emanation, which comes
away when the water is dried off. The salt recovered
from the solution is enfeebled like that which has
been heated, and the activity comes back in exactly
the same way.
We have seen that radium constantly generates
the emanation. It may be asked, Does the emanation
CHANGES IN RADIO-ACTIVE BODIES 121
in its turn generate anything else? The answer is
that it does.
The emanation, as we have seen, is only obtainable
in sufficient quantity to occupy a just visible volume,
and that notwithstanding that it is gaseous, and
consequently occupies a far larger volume weight
for weight than a liquid or solid would do. When
we state, therefore, that the substance which the
emanation generates is a solid, it will not be sur-
prising that no quantity large enough to be visible
at all, even under the microscope, has yet been
accumulated. If it is not visible, how can we be
sure that it exists at all ? The answer is, By its
radio-activity, which is exceedingly strong.
The solid substance, generated by the emanation,
has been called, not perhaps very happily, the induced
or deposited activity. This name was given before
the material nature of the deposit had been recog-
nised. Professor Rutherford has proposed the name
emanation X the unknown substance from the emana-
tion. This name, too, seems to leave something to be
desired, from a literary point of view. In the present
work the substance will be called the active deposit.
To demonstrate its formation, introduce a rod or
plate of any material into a vessel containing the
emanation (mixed, of course, with air), and leave it
there for a few hours. If the rod is withdrawn after
that time it will be found to be quite radio-active
when tested by the electroscope. A layer of the
active deposit, so thin as to be invisible and un-
weighable, has encrusted it. We can scrape off this
layer of deposited activity by means of emery
paper. The activity will then remain in the dust
left by the scraping process, or we can induce it to
evaporate or rather to sublime, if we make the
surface on which it is deposited intensely hot.
122 THE BECQUEREL RAYS
A curious peculiarity of the deposited activity
is that it deposits much more easily on a negatively
charged surface than on one that is positively
charged. It is evident, therefore, that the particles
of this substance, as they are formed throughout
the volume of the gaseous emanation, are charged
with positive electricity. This positive charge in-
duces them to move up to the negatively charged
wire, since opposite kinds of electricity attract one
another. When the emanation is allowed to remain
in the radium salt, it generates the deposited activity
in the salt, and this deposited activity, as well as the
activity due to the emanation, contributes to the
activity of the radium. The activity of a radium
salt which has not recently been deprived of its
emanation, accordingly consists of three parts. One
of these parts is contributed by the radium itself,
which emits Becquerel rays, while changing into the
emanation. A second part is due to the emanation,
while changing into the active deposit, and a third
part is due to the active deposit itself.
It will be remembered that radium yields three
kinds of rays. The a-rays, consisting of particles of
atomic dimensions positively charged. The y8-rays, con-
sisting of negative electrons, and the y-rays, of great
penetrating power, which are probably Rontgen
rays generated by the /3-rays. Does each of the
three constituents of ordinary radium radium itself,
its occluded emanation, and the active deposit give
off all three kinds of rays, or is each of them respon-
sible for one kind ? This question has been put
to the test of experiment. It is found that neither
of these alternatives represents the truth. For the
radium itself, when freed from the other products,
gives only the a-rays. Its radiation is completely
stopped by the thinnest screen of solid material.
CHANGES IN RADIO-ACTIVE BODIES 123
The same is true of the emanation. The active
deposit is responsible for the whole of the /3- and y-
rays. It contributes a part of the a-rays also.
In the present state of our subject, we can give
no further interpretation to these facts. But there
can be no doubt that, with the progress of knowledge,
their true bearing will become clear.
When describing the emanation, we stated that its
activity was apparent outside the glass tube con-
taining it. That is the superficial appearance of the
phenomenon. But we have now seen that the pene-
trating rays, which alone can produce effects outside
the tube, are exclusively due to the active deposit.
It is really this latter, deposited on the inside walls
of the tube, which enables the activity to be observed
outside. By measuring the activity of the rays
outside by the electrical method, we can follow the
development of the active deposit. When the
emanation has just been put in, no outside effect is
obtained. The outside effect gradually increases in
strength, but more and more slowly until a limit is
reached. This limit is attained in a few hours.
It represents the stage at which the active deposit
loses as much by its own decay, which we shall
describe immediately, as it gains by the development
of fresh supplies from the emanation.
The active deposit, like the emanation, is an
unstable substance. It loses its power much more
quickly than the emanation does. For in a few hours
after its formation it is no longer radio-active.
The changes which occur in the active deposit are
very complex, in contrast to the change in the
emanation. For while the emanation dies away
uniformly at a rate which is proportional to the
amount present, the active deposit behaves in quite
an eccentric manner. It begins to decay quite fast.
124 THE BECQUEREL RAYS
After some ten minutes this decay stops, and the
activity remains stationary for a time. Then it
starts decaying again, and goes on doing so till no
longer perceptible. It is evident that the change
takes place in several stages, each occurring at a
To analyse the experimental results, so as to find
out the number of changes which occur, and the
rate at which each proceeds, is not a very easy
matter. For it is not as if each change was quite
over before the next one began. In that case the
problem would be one of comparative simplicity.
But unfortunately the changes overlap : when a
portion of the deposit has undergone its first change,
this portion will proceed to the second change, while
the remainder goes on with the first one ; and so on
with the other changes. Professor Rutherford, to
whom, with Mr. Soddy, we owe most of our know-
ledge concerning the processes which occur in radio-
active bodies, has lately succeeded in specifying a
series of changes which will explain very satisfac-
torily the observed phenomena. It is not proposed
here to enter on the consideratioDS which led him
to this result, for they are only of technical interest.
The result itself, however, is of great importance. The
changes are three in number.
(1) A change which destroys half of the original
deposit in three minutes.
(2) A second change, in which half the material
changes in thirty-six minutes. During this change,
no rays are given out ; the material is, however, pre-
paring to get into the third stage.
(3) A third stage in which half the matter changes
in twenty-eight minutes.
After the third stage there is still a slight activity
left. This is exceedingly small, and only amounts to
CHANGES IN KADIO-ACTIVE BODIES 125
about 20000 of the initial amount. But, on the
other hand, it seems to be quite permanent, so far as
the experiments hitherto made have shown. We do
not yet know very much of the properties of this
last kind of activity, but it gives all three kinds of
We have considered the successive changes which
occur in radium, the production of the emanation,
its change into the active deposit, and the changes
of the activity of the latter.
We now turn to the changes in the other radio-
active elements uranium, thorium, actinium, and
polonium. The case of uranium is peculiarly inter-
esting, on account of its simplicity. For it only
undergoes one change before the activity vanishes
or becomes too small for detection. The one active
product is a solid one. In the case of radium, there
was no difficulty in separating the successive pro-
ducts, for the emanation, being a gas, could be
readily separated from the parent radium by heat ;
while the active deposit, being a solid, was readily
separated in its turn from the parent emanation.
Indeed, it may be said that the separation took
place of itself. In the case of uranium, the separa-
tion of solid from solid is not quite so simple.
Recourse must be had to the methods of chemical
separation, of which we gained some knowledge in
following the preparation of radium from pitchblende.
We may employ the method of fractional crystallisa-
tion, which, though tedious, seldom fails of ultimate
success. We may crystallise the uranium salts
repeatedly, and determine whether the more soluble
portions differed in their radio-active properties from
those which were less soluble. This is the method
which is used, as we saw, in separating radium from
barium ; and it was by this means that the active
126 THE BECQUEREL BAYS
product continuously produced by uranium was first
separated. But there is another process which
effects the desired separation much more easily. The
uranium salt is completely precipitated in an in-
soluble form if we add to the solution of it a solution
of ammonium carbonate. If we add more of this
reagent, the precipitated uranium salt is redissolved.
But a minute residue remains, which, though pre-
cipitated by the ammonium carbonate, in the first
instance, is not, like uranium, redissolved by an
excess of it. This residue is found to be intensely
active photographically, but not markedly so when
tested electrically. On the other hand, the uranium
itself is about as active electrically as before, while
its photographic activity has almost completely dis-
appeared. The /3-rays, it will be remembered, are
responsible for practically all the photographic action,
while the a-rays produce most of the electric leakage.
So that the substance which has been separated
emits only the /3-rays, the uranium itself only
a-rays. The new substance has been given the name
of uranium X, or the unknown substance from
uranium. Its /3-radiation gradually decays, but this
decay is much slower than any case of the kind
that we have yet dealt with. For twenty-two
days elapse before it sinks to one-half of its initial
In the meantime, the uranium salt, which had
been freed from all uranium X, gradually acquires
a fresh stock. The /3-radiation begins to assert
itself; and if the ammonium carbonate separation
is carried out again, a fresh stock of the uranium X
can be obtained.
The minute precipitate containing the uranium X
by no means consists entirely of that substance. Its
main constituent is iron, present as an impurity in
CHANGES IN RADIO-ACTIVE BODIES 127
the uranium salt. The quantity of uranium X is so
infinitesimal, that unless some impurity were present
for it to collect upon, it would be impracticable to
separate it. If no iron is present, it is necessary
to add a little, in order to provide a nucleus for
the trace of uranium X to gather on.
The case of thorium is complex. The first stage
of transformation is into a solid, which by analogy
with uranium X is called thorium X. In order to
effect the separation, thorium nitrate solution is
mixed with ammonia. The result is to precipitate
the thorium in an insoluble form, as thorium hy-
droxide. The nitric acid, formerly in combination
with the thorium, is now combined with ammonia
as ammonium nitrate, and this, being a soluble salt,
remains in the solution. If we filter off the solution
from the precipitated thorium hydroxide, and eva-
porate it, the residue will consist chiefly of ammon-
ium nitrate. This, however, like other ammonium
salts, can be got rid of by heat, which decomposes
it into volatile constituents. These are driven off,
and a minute residue remains, which is found to be
intensely radio-active, far more so than the original
thorium salt. In some cases, the activity, weight
for weight, will be found to be several hundred times
as large. This residue consists mainly of thorium,
which escaped precipitation by the ammonia. But
its high activity is due to the new substance thorium
X, which does not form an insoluble hydroxide, and
is consequently not precipitated by ammonia.
Thorium X decays at about the same rate as the
radium emanation, falling to one-half its quantity
value in four days.
The method we have described is the only one
by which thorium X has been satisfactorily separated
from the parent thorium. It is not, of course,
128 THE BECQUEKEL RAYS
asserted that it is the only possible method. The
study of the chemical reactions of thorium X, as
well as uranium X, is very important, and has not
perhaps yet received all the attention it deserves.
It is found that the thorium X is responsible for
about half the normal radio-activity of thorium. It-
is found, moreover, that the thorium which has been
freed from it gives only a-rays.
The next change in this series is from a solid to
a gas. Thorium X, in fact, gives off an emanation.
This differs very remarkably from the emanation
of radium, for it only lasts for a few minutes,
while the radium emanation is still perceptible after
a month. The time which elapses before the thorium
emanation falls to one-half its initial value is rather
under one minute.
In many respects the thorium emanation resembles
that of radium. But the very short duration makes
it more difficult to experiment with. The emanation
can, however, be condensed by liquid air, and
evaporates at a temperature only slightly different
from the radium emanation. The thorium emana-
tion, too, is chemically inert.
To obtain the thorium emanation, it is not, of
course, necessary to obtain the thorium X first in
a separated form. An ordinary thorium salt contains
a stock of thorium X, which it has manufactured,
and which it retains mixed with it. The salt will
therefore give out emanation. Indeed the emanation
from thorium salts was recognised long before the
intermediate product, thorium X, had been realised.
Many thorium salts give out the greater part of
their emanation in the cold. The emanation cannot
be extracted at a much greater rate by heating.
In this respect thorium differs from radium. Some
samples of thorium compounds are able to retain
CHANGES IN RADIO-ACTIVE BODIES 129
all or nearly all their emanation in an occluded
form. Others freely give up almost the whole of
it. Thorium nitrate gives off very little of its
emanation. Thorium oxide, on the other hand, gives
up nearly all the amount generated. If, however,
thorium oxide is heated to a white heat, it becomes
able to retain its emanation after cooling. On dis-
solving up in acid and reprecipitating, the original
properties are restored.
The thorium emanation, like that of radium, gives
an active deposit which concentrates itself on a
negatively electrified body. This active deposit,
instead of being less durable than the emanation, is
far more so. It appears to undergo two changes.
The first modification gives out no rays, and decays
to half the original amount in fifty-five minutes.
The second does give off rays, and the corresponding
time is eleven hours. The active deposit of thorium
can be dissolved off the surface on which it has
been formed, by means of acids. The acid, when
evaporated in a dish, leaves the activity on the
surface of the latter. Sulphuric or hydrochloric acid
dissolves the deposit more freely than nitric acid.
Lastly, we have the case of actinium. This has
been much less elaborately studied than the others,
because the material is so very scarce, and has
remained exclusively in the hands of very few.
Actinium gives off an emanation even less durable
than that of thorium : for it only takes four seconds
to fall to half its initial value. The emanation, as
in the other cases, leaves an active deposit, falling
to half value in forty minutes.
We may summarise the successive stages of degra-
dation of the various radio-active elements so far
as these have been traced in the following way.
Radium. Solid gas solid solid solid solid.
130 THE BECQUEREL RAYS
Uranium. Solid solid.
Thorium. Solid solid gas solid solid.
A ctinium. Solid gas solid.
There seems to be no rule of sequence which holds
good in every case. Nor are the various products
produced in the order of durability. For example,
the radium emanation gives a deposit less permanent
than itself. The thorium and actinium emanations
give one which is far more permanent.
With regard to polonium, it appears that the
activity decays, falling to half value in about a
year. But no experiments have been published, so
far as the author is aware, which give information
as to the products of its decay.
All chemical changes are accompanied by thermal
effects. When oxygen combines with hydrogen, heat
is given out, or, to take a still more familiar case,
when carbon combines with oxygen, as in the com-
bustion of coal. There are instances in which
the product is more complex than the original
constituents. But heat is often evolved in the de-
composition of a complex substance. The most
conspicuous examples are the explosive compounds
which have taken the place of gunpowder in modern
blasting operations. Nitro-glycerine, which is the
active principle of dynamite, gives off a large
amount of heat in its decomposition. It has been
found that the changes taking place in radium also
give rise to thermal effects. One of the original
experiments on this subject by MM. Curie and
Laborde, to whom we owe the discovery, was made
by means of Bunsen's ice calorimeter (fig. 24) an
instrument admirably adapted to the measurement
of small quantities of heat.
The working of the instrument depends on the fact
that water expands on freezing. This effect is
CHANGES IN RADIO-ACTIVE BODIES 131
familiar to all in the bursting of frozen water-pipes.
It is an obvious consequence that ice must contract on
melting. If, then, we can measure the contraction, it
will be evident that the amount of ice which has been
melted is quite determinate. The quantity of ice
Fia. 24. Bunsen's ice calorimeter, used in observing the heating effect of
radium, a is a vessel like a test tube, into which the sealed tube, I), containing the
radium, can be dropped, a is plugged at c with cotton wool, to guard against
external heating. Round a, is the outer chamber, d, which is quite full of freezing
water, with a lump of ice, e e e e, round a. The bottom contains mercury, as does
also the side tube, g g, and the horizontal graduated tube, h, of small diameter.
The whole instrument is packed in ice up to the level of the dotted line, to prevent
heat getting in from outside. While the radium is inside, the end, k, of the mercury
thread is observed to be constantly receding, owing to the melting and contraction
of the ice by the heat it liberates. As soon as the radium is withdrawn the thread
becomes again stationary.
melted serves to measure the amount of heat which
has been employed in melting it ; for standard
experiments have been made on the amount of heat
required to melt, say, a pound of ice.
To carry this method into practice, a double- walled
vessel is used, as shown in the figure. The space
132 THE BECQUEREL RAYS
between the walls is entirely filled with water, a part
of which is frozen. Below the water is mercury.
This mercury extends to the narrow graduated tube,
in which it forms a thread, as in a thermometer.
The position of the end of the thread can be read on
the scale, just as in reading a thermometer.
To prevent the ice between the walls of the vessel
getting melted by the heat of the room, the whole
instrument is packed in ice. The narrow graduated
tube, and the mouth of the vessel, alone project.
If now a little radium is introduced inside the
vessel, it is found that the end of the mercury thread
begins to recede. The contents of the vessel are
contracting, or, in other words, the ice between the
walls is melting ; and this goes on indefinitely. As
soon as the radium is withdrawn, the end of the
mercury thread remains steady. When the radium
is again introduced, the mercury thread begins to
recede steadily, and continues to do so, so long as the
radium is left inside.
We saw above (see p. 32) that it had been sug-
gested that radio-active bodies drew their supplies of
energy from the surrounding air. If this were so,
the molecules of the surrounding air would lose
energy, and its temperature would fall. The experi-
ment we have just described conclusively proves that
this is not the case. For, if the heat emitted by the
radium were drawn from the surrounding air, there
should be no heating effect on the whole from the air
and the radium taken together. But the air was
included with the radium inside the calorimeter, and
the heating effect is observed none the less. It is
evident, therefore, that radium does not draw its
energy from this source.
Let us return now to consider more quantitatively
the experiment with Bunsen's calorimeter.
CHANGES IN RADIO-ACTIVE BODIES 133
If we know the size of the graduated tube, and if
we found the rate at which the mercury moves, it is
easy to find the change of volume in an hour, and to
calculate the amount of ice which has been melted in
that time. The result is to show that radium can
melt something like its own weight of ice in an hour.
The amount of heat required to do this is not very
different from what is required to raise the same
quantity of water to the boiling point, starting at the
ordinary temperature. So that we may say, if we
prefer, that radium gives off enough heat in an hour
to boil its own weight of water.
A pound of radium, if we could obtain so much,
would be capable of boiling a pint of water in an hour.
It would give off as much heat as a small spirit lamp.
If we reflect that radium probably retains its
powers quite undiminished for hundreds if not
thousands of years, it will be admitted that this is one
of the most astonishing results in the whole range of
The quantities of radium available to experiment
with are, as we have seen so often, very small ; so
that they expose, in proportion to the amount of heat
liberated, a very large surface. This enables the heat
to get away easily. No part of a small mass of
radium is very far from the surface. And thus it
happens that the temperature does not rise very high.
In spite of this, M. Curie has been able to show the
heating effect of a fraction of a gramme of radium, by
means of an ordinary mercury thermometer, which
was heated several degrees hotter than the sur-
If a large mass of radium could be obtained, say
a few pounds, the inside would probably be vividly
incandescent, with the heat generated. For the out-
side layers would serve to hinder the escape of the
134 THE BECQUEEEL RAYS
heat generated by the internal portions. Thus the
temperature would be able to rise very high.
A most mistaken notion has got abroad that the
heating property of radium may have a direct
practical application. A little consideration will
show the utter futility of such an idea. Radium
can give off enough heat to melt its own weight of
ice in an hour. Coal can give off enough heat in the
process of complete combustion to melt about 100
times its own weight of ice. Perhaps this process of
complete combustion may take some five hours, in the
case of a domestic fire. Thus the burning coal gives
off heat at twenty times the rate that the same
amount of radium will do. The radium-filled grate
would require to be twenty times larger than the coal
one to produce the same effect. True, the radium
would practically not require replenishing. But if we
reflect that an ounce of radium costs 30,000, it will
be admitted that a good deal more heat might be
obtained by investing the money in consols, and
devoting the annual interest to the purchase of
The heating effect of radium even when so sparsely
distributed as it is in pitchblende, would probably be
perceptible inside a large block. A rough calculation,
on certain reasonable assumptions as to the rate at
which heat was conducted through the substance,
showed that, in the middle of a large slab a yard
thick, the temperature would be something like one-
fifth of a degree. It is not impossible that inside a
deep mine, where pitchblende was very abundant,
the temperature might be perceptibly higher from this
For a long time the source of the sun's heat was
regarded as very mysterious. We know, with fair
accuracy, the size of the sun, and the density of the
CHANGES IN KADIO-ACTIVE BODIES 135
material of which it is constituted. On any reason-
able view of the amount of heat which such a body
could give out in cooling, it seemed very difficult to
understand what could be the source of the enormous
quantities of heat which the sun pours out in all
directions. For we know with tolerable accuracy the
amount of heat which reaches a square yard of the
earth's surface, from the sun, every hour. And
assuming, as we safely may, that an equal amount of
heat is given out by the sun in every other direction,
the amount of heat given out altogether may be
When this is done, it is found that if the sun had
no resource but to draw on its own primeval heat, it
could not go on very long without becoming per-
As this does not happen, we must suppose that the
sun has some other source than its primeval supply to
The most natural source to look to is perhaps com-
bustion. But here again, if the sun was made of the
most combustible materials known, this source of
supply would not last nearly long enough.
The view which, until lately, has exclusively held
the field is that propounded by Yon Helmholtz. His
theory assumes that the sun is contracting ; that its
diameter becomes less every year. In short, that the
outer parts are falling down on the inner ones.
Whenever one body falls on another, heat is
developed. For instance, iron may be made visibly
red hot by skilful and vigorous hammering. The
method was employed by blacksmiths as a means of
kindling a fire before the invention of matches.
It is just in this way that the heat of the sun is
assumed to be maintained by contraction, and conse-
quent falling in of the outer parts on the inner ones.
136 THE BECQUEREL RAYS
It might be thought that this cause would be
inadequate to explain the origin of sun's heat.
But calculation shows that it is adequate. For a rate
of contraction altogether imperceptible by the most
refined measurements of the sun which we can make,
would suffice to maintain the output of energy.
The discovery of the wonderful supplies of energy
which radium possesses, and can give out, suggests
another possible explanation of the origin of the sun's
heat. It has been estimated that if the sun contained
two and a half parts of radium in a million, by
weight, the present output of solar energy would be
accounted for. If, in fact, the sun were only a little
richer in radium than the best pitchblende, we should
not have to look any further for the source of its
A circumstance strongly confirmatory of the idea
that a part at least of the sun's energy is derived
from radium is the abundance of helium in the solar
atmosphere. For we shall see in a later chapter that
helium is a product of the changes occurring in
radium. If the sun is really rich in radium it should
be radio-active. No radio-active effect due to the
sun has, however, yet been traced. An electroscope
will retain its charge just as long when exposed to the
sunlight l as when taken into a tunnel with many feet
of rock overhead. This has been regarded as a for-
midable objection to this theory of the sun's heat, but
it is not really so.
We must remember, in the first place, that the
radium is supposed to be disseminated uniformly
throughout the sun's volume. Now the Becquerel
rays cannot penetrate any great distance through
solid materials ; so that the rays from the internal
1 It is necessary to take precautions to exclude what are called photo-
electric effects, which have nothing to do with radio-activity.
CHANGES IN RADIO-ACTIVE BODIES 137
portions cannot escape. The radium contained in a
very thin outer shell could alone produce any ex-
ternal electrical effect.
If we reflect further, that the earth's atmosphere
produces an absorbent effect equivalent to a column
of water 32 feet high, it will not be difficult to
understand that the Becquerel rays from the sun
cannot easily get through. As a matter of fact,
calculation shows that the amount of effect to be
expected, assuming that the sun's heat is wholly
due to radium uniformly distributed throughout it,
would be utterly beyond the range of experimental
Professor Rutherford and Mr. Barnes have carried
the investigation of the heating effect in radium a
stage further than did the original discoverers of the
effect ; for they have investigated the amount of heat
liberated in the formation of each of the successive
They measured the heating effect from a specimen
of radium bromide. They then heated the salt and
removed the emanation, which was collected in a
small glass tube and sealed up.
It was found that the heating effect of the radium
salt was diminished by the loss of its emanation in
exactly the same ratio as the radio-activity was
diminished, that is, if the a-rays alone are taken into
account. On the other hand, the heating power
which had been lost by the solid was found to survive
unaltered in the gaseous emanation. The heating
power of the two added together was exactly the
same as if no separation had been effected.
It was found that the tube containing the emana-
tion lost its heating effect in just the same way as
the emanation loses its activity. For the heating
effect fell to half its initial strength in 3*7 days,
138 THE BECQUEREL BAYS
just as the activity of the emanation falls to half
value in that time.
The radium deprived of emanation recovers its
heating power just as it recovers its activity.
In fact, it was found that the heating effect of a
radio-active product was at all times proportional to
the amount of a-radiation it was emitting. This
applies equally to radium itself, to the emanation, and
to the active deposit.
The discovery of the enormous quantities of energy
liberated by radium is a proof that there lies latent
in the atom a quantity of energy absolutely gigantic
in comparison with anything which it was formerly
believed to contain.
Energy is often liberated, as we have seen, when
chemical combination takes place. The most con-
spicuous case is the combination of oxygen and
hydrogen to form water, for more heat is given out in
this case than in any other that has been investigated.
The energy liberated in the formation of a pound of
water would be supplied by a pound of radium in
forty hours. We do not yet know with certainty
how long radium will last, how long, in fact, it will
continue to evolve heat at this rate, but there can be
no doubt that the period is to be measured by
hundreds or thousands of years. So that the amount
of energy which the pound of radium possesses must
be millions of times greater than energy which is set
free in any known chemical change in an equal quan-
tity of material.
It was formerly thought that the energy liberated
in chemical changes was a considerable fraction of
what was present altogether, although, of course, it
was recognised that there was no proof of this. But
this view must now be altogether abandoned, for the
case of radium can hardly be regarded as exceptional.
CHANGES IN RADIO-ACTIVE BODIES 139
It is true that this is the only case where measurable
quantities of energy are being given off. 1 But it is
the giving off of the energy which we must alone
regard as exceptional. It must be assumed to be
present, in a latent form, in other chemical atoms also.
It is not inconceivable that some process might be
discovered which would enable us to liberate such
stores of energy on a large scale in the case of
ordinary materials to make large quantities of them
radio-active. If this could be achieved, the pro-
blem of the failure of the coal supply need never cause
any further anxiety.
We may conclude this chapter by saying something
about the effect of temperature on radio-active
M. Becquerel has experimented on metallic uranium
to see whether heating it or cooling it would alter the
radio-activity. As he has remarked, metallic uranium
has been subjected to a tremendous heat in the
electric furnace during the process of manufacture.
It is evident, therefore, that intense heating cannot
permanently destroy radio-activity. It remained to
investigate the question of whether a temporary effect
was produced while the uranium was still being
heated or cooled.
Experiments of this kind are difficult because heat-
ing or cooling the air may, and in fact certainly does,
affect the discharging power, apart from any change
in the activity of the radiating substance. For this
reason it is easiest to work with the ft- or y-rays,
which can be detected at a distance from the uranium.
The temperature of the uranium can then be varied
without altering the temperature of the air in which
the electric leakage is to be measured.
1 Uranium and thorium no doubt produce similar though much smaller
effects. But no attempt has been made, so far as I know, to detect these.
140 THE BEGQUEREL KAYS
When this arrangement was adopted it was found
that the activity of the uranium was quite unaffected
by changing its temperature. The discharging power
was just the same, whether the uranium was heated
to the temperature of boiling water, or cooled to the
temperature of liquid air.
With radium it is otherwise. Heating diminishes
the apparent activity. This, however, is not due to
any real change of activity, but to loss of the emana-
tion, which contributes largely to the activity of
radium as ordinarily observed. If the specimen is
sealed up in a glass tube, so that the emanation
cannot escape, no change of activity is observed.
Not only is the Becquerel radiation unaffected by
temperature ; the rate at which heat is liberated is
unaffected also. Just as much heat is given off by
radium maintained at a low temperature as at the
ordinary temperature. This has been shown by some
experiments of M. Curie, carried out in conjunction
with Professor Dewar. The method adopted differed
from those by which the earlier experiments were
made. It depended on the evaporation of a liquefied
To take a specific case, let us consider the evapora-
tion of liquid hydrogen. This liquid when boiling is
only some twenty degrees above the absolute zero of
temperature. Any heat which obtains access to this
liquid at its boiling point instantly causes actual
ebullition. Now, no matter what precautions may be
taken in the way of screening off external heat by
vacuum jackets or wool wrappings, some is certain to
find its way in and cause the liquid hydrogen to boil.
This boiling would interfere with the experiments ;
in order to avoid it the liquid hydrogen vessel was
immersed in a larger vessel, also containing liquid
hydrogen (fig. 25). Slight ebullition in the outer
CHANGES IN KADIO- ACTIVE BODIES 141
vessel was unimportant ; since, however, the tempera-
ture of the outer vessel could not rise above the
boiling point of liquid hydrogen, the access of heat to
the inner vessel was prevented, and the liquid in it
was perfectly quiescent.
As soon as a sealed tube containing radium was
introduced, the hydrogen in the inner vessel began to
boil off. That would occur with any substance, since
FIG. 25. Measurement of the heat emission of radium at the
temperature of liquid air or liquid hydrogen. A double-walled
vessel, a (vacuum jacketed) contains the liquid hydrogen. The
radium, b, in a sealed tube, is immersed in this, and causes
ebullition. The evolved gas is collected over water in d, in a
graduated tube, e. To prevent the access of external heat, a is
immersed in an outer vessel, c, also containing liquid hydrogen.
some ebullition may occur in the process of cooling
it down to the low temperature. After that no
further boiling would occur in ordinary cases. With
radium, however, the boiling went on quite steadily
and uniformly, as long as there remained any liquid
hydrogen in the vessel. This is due to the heat
142 THE BECQUEREL KAYS
steadily given off by radium, which we have dis-
The volume of gaseous hydrogen liberated by the
boiling evidently gives a measure of the quantity of
heat liberated. The evolved hydrogen was measured
by collecting it in the ordinary way over a pneumatic
trough, in a graduated tube. It is, of course, neces-
sary to know how much heat is liberated in boiling
off, say, one litre of hydrogen. This, however, has
been determined by standard experiments. The
first determinations seemed to show that even more
heat was liberated at the temperature of liquid
hydrogen than at the ordinary temperature. More
accurate experiments, made later, led to the con-
clusion that the rate of evolution of heat was quite
unaffected by cooling in liquid hydrogen.
The contrast, in this respect, between the changes
going on in radium, and ordinary chemical change,
is very striking. There is only one chemical reaction
the combination of hydrogen with fluorine which
is not arrested by cooling to this extent. The
enormous majority of chemical changes are altogether
stopped by cooling to the temperature of liquid air,
which is, on the absolute scale, some four times hotter
than liquid hydrogen; almost all of them are pro-
foundly affected by changes of temperature. The
changes occurring in radium are not influenced in
the least by heating or cooling.
RADIO-ACTIVITY IN THE EARTH AND IN THE
IT was mentioned in Chapter n. that some soils
contained radium. We shall now describe some of
the experiments by which this has been proved to
be the case. In the first place, it is found that the
air in cellars or caves possesses a marked ionisation. 1
This has been traced to the presence of radium in the
soil ; the emanation of this radium diffuses into the
air of the cave or cellar from the surrounding soil, and
ionises it. The same effect can be obtained still more
conspicuously by sinking a pipe into the earth, say
a yard down, and sucking up a sample of air from
the soil. Such air possesses marked conductivity ;
the conducting power falls to half its initial value in
a little less than four days. This shows that it is
due to the radium emanation.
No very complete investigation of the amount of
radium in various soils has yet been made. Clay
soils are usually the most active, but Professor J. J.
Thomson has found the sand from the beach at
Whitby to be very active too.
Much more active than any ordinary soil is the
deposit left by the water of certain thermal springs.
The author has found strong emanating power in the
deposits of Bath and Buxton. The water of these
1 This ionisation is much larger than that observable in closed vessels,
which will be discussed later.
144 THE BECQUEREL RAYS
springs rises from great depths below the earth, and
reaches the surface very hot. At Bath the tempera-
ture of the water at the spring is higher than can
easily be borne. As the water cools it deposits a fine
red mud in the tanks and pipes. This mud is found
to give off radium emanation in abundance, though
not, of course, nearly so strongly as pitchblende. The
mud deposited near the spring is stronger in emanating
power than what is deposited further away.
As the mud separates from the water, there is
necessarily some radium in solution in the water.
The quantity, however, is exceedingly small ; nor is
this surprising, for the water contains abundance of
sulphates in solution. These would precipitate any
radium that might be present in the form of insoluble
radium sulphates. Radium sulphate is one of the
most insoluble salts known, so that the quantity of
radium remaining unprecipitated is necessarily very
slight indeed. Whether the water is saturated with
radium sulphate has not yet been decided.
The author has estimated the quantity of radium
annually delivered by the spring as about ^ of a
gramme. This, if isolated, would be a quantity not
to be despised ; but to separate it economically from
the vast mass of water and saline material with which
it is associated would be quite impracticable.
It is interesting to note that the Bath spring yields
helium as well as radium. If the surface of the water
over the well of the King's Bath be watched, bubbles
of gas will be seen to constantly rise to the surface
and break there. The gas has been examined from
time to time by various chemists. The earlier in-
vestigators reported that it was mainly nitrogen.
After the discovery of argon it was realised that
the purely negative tests, before considered sufficient
to identify a gas as nitrogen, were insufficient; for
RADIO-ACTIVITY IN THE EARTH 145
they did not distinguish it from the other inert gases,
helium, neon, argon, krypton, and xenon.
In the light of this discovery Lord Rayleigh
re-examined the Bath gas, with the idea that it
might prove to be mainly argon. This hope was
disappointed ; for the gas was found to mainly con-
sist, as had previously been supposed, of nitrogen.
The percentage of argon in it was not very different
from the percentage of argon in the atmosphere ; but
the interesting observation was made that the gas
contained about one part of helium in a thousand, by
The presence of helium, it cannot be doubted, is
connected with the presence of radium. 2 In all pro-
bability the Bath spring draws its supplies of both
materials from the disintegration of radio-active
Many samples of ordinary well waters which have
percolated through the soil are found to contain
radium emanation in solution. The emanation is
fairly soluble in water, considerably more so than
oxygen or nitrogen, though less than carbonic acid.
Thus the dissolved gases boiled out from the water
are often quite markedly radio-active. The emana-
tion has been no doubt dissolved by the water in
percolating through strata which contain traces of
The first case of this was noted by Professor J. J.
'Thomson, who found that the Cambridge tap-water
would yield, on boiling, a gas containing the emana-
tion. When once the emanation has been boiled out,
no more, or at all events very little, can be got on
1 Professor Dewar has subsequently utilised the helium of the Bath spring
in his attempts to liquefy that gas. He has found that this helium contains
some six per cent, of neon. This points to a far larger proportion of neon in
the Bath gas than in the atmosphere.
2 See below, chap. vn.
146 THE BECQUEREL RAYS
a subsequent occasion. Thus the radium salt from
which the emanation was derived is not present in
In the case of the Bath springs, the water contains
far more emanation in solution than corresponds to
the dissolved radium. The gas bubbling up to the
surface also contains emanation. This suggests that
the water has had access to large quantities of radio-
active minerals, and that it has only disintegrated a
small portion of them.
Elster and Geitel were the first to discover that
the atmosphere contains radio-active matter. Their
experiment was as follows. A long negatively elec-
trified wire was hung up in the open air, and after
exposing it for some hours, it was coiled up to manage-
able dimensions and tested for radio-activity with the
electroscope. The result was to show that the wire
had become active. No effect of this kind is observed
when the wire is positively charged.
Now it will be remembered that a negatively
charged wire when immersed in the emanation of
thorium or radium, is able to collect the active
deposit formed by the degeneration of the emanation.
Thus it is fair to conclude that the active deposit
collected by a negatively charged wire in the open
is due to the presence of a radio-active emanation in
The rate of decay of the active deposit from the
air has been carefully investigated. The activity
does not fall off according to the simple law of com-
pound-interest, but behaves irregularly, like the
active deposit of radium. It is, of course, more
difficult to compare two irregular decay curves than
two regular ones, since the shape of the curve depends
on the time of exposure ; but, upon the whole, it
appears fairly certain that the active deposit obtained
RADIO-ACTIVITY IN THE EARTH 147
in the open may be identified with that of radium ;
and it would follow that the atmosphere contains
There is another and simpler way, in which the
active deposit of the atmosphere may be collected.
Mr. C. T. R. Wilson observed that freshly fallen rain,
quickly evaporated to dryness, left a radio-active
deposit on the basin in which it had been boiled
down. The same effect is obtained from snow or
hail. The active deposit thus collected dies away
in the same way as that collected by a negatively
charged wire, and is almost certainly identical
The amount of activity to be collected from the
atmosphere varies very much according to locality
and circumstances. Elster and Geitel carried out a
systematic series of measurements lasting over a whole
year. The largest activity they obtained was no less
than sixteen times the smallest. These experiments
were made at Wolfenbuttel, in the middle of Ger-
many. Mr. Simpson has recently made experiments
in the extreme north of Norway, with the interesting-
result that the activity there is seldom less than the
very highest ever found at Wolfenbuttel, and on one
occasion seven times as large. During these experi-
ments the ground was covered with deep snow.
It has been found that wind is favourable to large
radio-activity. This is not surprising, since larger
quantities of air come within the range of action of
the charged wire when a wind is blowing than when
the air is still.
The most probable source of atmospheric radio-
activity is radium emanation, which has diffused out
from the soil, and thus found its way into the air.
If this is the true explanation, we should expect less
activity on the sea-coast than inland ; this anticipa-
148 THE BECQUEREL RAYS
tion has been found to agree with experiment. But
it would be rash to infer too much from this until
more extensive measurements have been made,
especially in mid-ocean. On this view, the activity
of the air will depend on the quantity of radium in
the soil ; the high values obtained by Mr. Simpson
would be due to the comparative richness of the
surrounding strata in radium.
When the barometer falls, and the pressure of the
air becomes lower, the air imprisoned in the pores
of the soil will be to some extent forced out into
the atmosphere above. Thus high radio-activity in
the atmosphere should accompany a low barometer.
Elster and Geitel have found that this usually is the
case. Sometimes, however, the radio-activity quickly
increases when the barometer is rising. The explana-
tion of this is obscure. But it must be remembered
that wind and other causes probably intervene. The
whole subject affords an interesting field for in-
RADIO-ACTIVITY OF ORDINARY SUBSTANCES
A gold-leaf electroscope, well insulated, and placed
in an exhausted vessel, will retain its charge for very
long periods. Sir William Crookes, many years ago,
made an experiment on this subject. He hung up a
pair of gold leaves in an exhausted glass bulb, and
found that they remained divergent for months. It
is found, however, that if air be admitted to the
vessel, the leaves do not retain their charge for more
than a moderate number of hours perhaps twenty-
four or thirty -six hours.
For a long time this was regarded as due to a
failure of the insulation, owing to the admission of
dust or moisture with the air. But the subject did
RADIO- ACTIVITY IN THE EARTH 149
not attract much attention, and its importance does
not seem to have been realised.
Some of those who knew the facts and they were
f ew seem to have considered that the escape of
electricity was due to a brush or spark discharge
through the air, by which the electricity escaped
from the leaves. But there is a fatal objection to
such a view ; for a brush discharge cannot occur in
air unless the electric stress amounts to something
like thirty thousand volts per centimetre. In the
experiments we have mentioned it is more like one
hundred volts per centimetre ; so that this explana-
tion is untenable.
Light was first thrown on the subject in some
experiments by Mr. C. T. R. Wilson. He found that
the current which leaked away from the leaves became
saturated that is, that it was independent of the
electro-motive force if the latter was large enough.
Further, he found that the relative ionisation in
different gases was the same as the author had found
for the same gases when strongly ionised by the
Becquerel rays. Taking these facts together, it
seemed probable that the discharge through ordinary
air was intimately connected in some way with radio-
The next step was made at about the same time by
several experimenters. It was shown that the rate
at which the charge was lost depended on the nature
of the walls of the vessel. For instance, in some
experiments by the author it was found that when
the walls were lined with platinum, the rate of dis-
charge was three times as great as with zinc. It was
found, moreover, that the rate of discharge, though
quite definite with a given sample of platinum, was
different when another sample was employed. The
most natural interpretation of this result would be
150 THE BECQUEREL RAYS
that the activity was due, in part at least, to the
presence of a trace of some intensely active substance
such as radium in the platinum. This would explain
quite satisfactorily the want of constancy ; for some
samples would be sure to contain more of the im-
purity than others. In any case, it appears certain
that the loss of charge in ordinary air, which had
been known for so many years before the recognition
of radio-activity, was, in reality, an example of it.
It is rarely, indeed, that a discovery is so new that
nothing bearing upon it is to be found in scientific
The activity of these ordinary materials is exceed-
ingly minute. Thus the author estimated the activity
of a sample of platinum more active than most sub-
stances at TO Vo f the activity of uranium. So that
radium would be no less than three thousand million
times more active than this piece of platinum. A
very interesting discovery was made at about the
same time by Professor Maclennan and Mr. Burton
at Toronto, and Mr. H. L. Cooke at Montreal. It
was found that the rate of discharge of an enclosed
electroscope containing air could be diminished by
building a thick screen of lead round it, or by
immersing it in a tank of water. The diminution
which could thus be produced was about one-third of
the whole conductivity.
It seems, then, that there must exist everywhere
extremely penetrating rays, which are able to get
through ordinary thicknesses of metal, and which can
produce ionisation in gases, but which can be stopped
by great thicknesses of metal or water. These rays
are, so far at least, of the same nature as the y-rays
of radium, and their penetrating power seems to be
about the same.
Mr. Cooke made some experiments to find out
RADIO- ACTIVITY IN THE EARTH 151
what direction these rays came from, whether from
the earth, from the sky, or from a horizontal direc-
tion. He placed a thick metal screen, first over the
top of the electroscope, then at the side, then under-
neath, and came to the surprising conclusion that
the rays proceeded equally from all these directions.
This was equally the case in the open air as in
It has been mentioned in an earlier chapter that
some kinds of soil contain traces of radium. It
will be understood, therefore, that y-rays may be
expected to pass upwards from the earth, especi-
ally in localities where the soil is active. We
know, too, that the atmosphere usually contains
radium emanation ; there is, therefore, nothing
surprising in y-rays coming from above, and side-
ways. But it is very difficult to see why the amount
of radiation from above should be the same as the
amount from below. The strength of the radiation
must depend on the percentage of radium present,
and on the absorption by the material of the air;
and it would seem to be an extraordinary coincidence
that these should be so proportional as to give the
same radiation from above as from below. More-
over, the amount of emanation in the air varies from
day to day. So that even if the radiations were
equal on one occasion, it would seem inexplicable
that they should always be so.
For these and other reasons, Professor J. J.
Thomson is of opinion that the y-rays of radium
are not the rays principally concerned in these
experiments, but that some new kind of penetrating
radiation is given out by all substances in propor-
tion to their densities. It seems, however, in some
ways undesirable to postulate an entirely new cause
for the effect, until it has been more thoroughly
152 THE BECQUEREL RAYS
proved that the old one cannot be made to account
Mr. Campbell has recently given a preliminary
account of some experiments which have led him
to conclude that, in the case of some metals, the
apparent radio-activity of the walls of the vessel is
due in part to a kind of secondary radiation set up
by the penetrating radiation from outside. This is
analogous to the behaviour of heavy metals when
transmitting the Rontgen rays ; they are known to
give off a secondary radiation, much less penetrating
than the primary.
It will be seen from this brief sketch that the subject
of the apparent radio-activity of ordinary materials is
a subject of great complexity. No doubt in a short
time it will be possible to give a far more satisfactory
account of it than at present.
THE ULTIMATE PRODUCTS OF RADIO-ACTIVE
WE have followed in chapter v. the changes occurring
in radium, and the other radio-active elements, so far
as these are known. It must be observed, however,
that in all probability many more changes occur than
are yet recognised. In the case of radium we have
the apparently permanent residue (amounting to
20000 f the whole) from the active deposit. It
is true that this activity is slight. But then it
is very durable. Professor Rutherford considers it
probable that the same total amount of radiation is
given out at every stage of decay. Thus, the emana-
tion gives out as much radiation, weight for weight,
as the parent radium, and so on for the other changes.
This cannot hold universally, because some of the
products do not give out rays at all, and one at least,
uranium X, gives only /3-rays. But it often seems
to hold good. Thus, the slightly active permanent
residue may be just as important a step as the others.
We have spoken of it as permanent, but in all pro-
bability prolonged observation will show that it does
decay, though very slowly.
The activity of uranium X, and of the thorium
deposit, very possibly do not vanish, but have a
small residue, not yet detected.
In course of time, however, the activity must dis-
appear ; for the quantity of energy originally present
154 THE BECQUEKEL KAYS
was limited, and cannot keep up Becquerel radiation,
with attendant liberation of heat, for ever. We are
face to face, then, with the question, What becomes
of these products when they have lost all radio-
activity when, in fact, they have ceased changing,
and have settled down to a permanent condition ?
We have seen how exceedingly minute are the
quantities involved. While the radio-activity lasts,
that scarceness does not altogether bar the investiga-
tion of their properties. For it is possible to ascertain
the presence of the various products, and to measure
their relative amount, by observations on their radio-
activity. In fact, such observations answer, in large
measure, the same purpose as would be attained by
seeing and weighing the substances.
When, however, the radio-activity has gone, we
can no longer employ this method. And the difficulty
of pursuing the investigation further becomes very
great. In the first place, it will be remembered that
radium was found in certain minerals. Now, if radium
is degenerating by successive stages into some other
substance, is it not reasonable to assume that, in the
course of countless ages, the radium in such minerals
will have generated a fair amount of such products ?
If this be granted, we must consider what other sub-
stance is present in all the minerals which contain
Now these minerals are of extraordinary com-
plexity. There is hardly any element that is not
found in them in greater or less quantity. So that
the clue is a very poor one.
There is one substance in particular, however,
which is present very persistently in the radio-active
minerals, and which in some ways seemed to be a
likely product of radio-active change. That is the
PRODUCTS OF RADIO-ACTIVE CHANGE 155
It will be desirable to digress somewhat from our
subject, to give an account of this gas, and the history
of its discovery.
Lord Rayleigh had for many years been determin-
ing the density of gases, with a view to attaining the
highest possible precision. All previous experimenters
who had determined the density of nitrogen, the inert
constituent of the atmosphere, had contented them-
selves with removing the other known constituents
of the air, and weighing the inert residue as pure
nitrogen. Lord Rayleigh, however, considered it
desirable, as a measure of precaution, to determine
the density of nitrogen from other sources, such as
ammonia, or nitric acid, in addition. He found, to
his surprise, that these chemically prepared samples
of nitrogen were all consistently lighter by about
one part in three hundred than the nitrogen of the
air. This was a very surprising result. For no one
doubted, at that time, that the nitrogen of the air,
and the nitrogen of saltpetre, or, what is the same
thing, of nitric acid, were identical. The general
tendency was to attribute the discrepancy (which
after all was not large) to some oversight. But
further experimenting entirely confirmed its reality.
And, in the end, it was found (by Lord Rayleigh and
Sir William Ramsay, working in collaboration) to
be due to the presence of a hitherto unsuspected
constituent of the atmosphere, of about one and a
half times the density of nitrogen, present to the
extent of about one per cent. In order to obtain
it, it was necessary to absorb the nitrogen in which
Nitrogen is very inert. It was, and in some degree
still is, a matter of great difficulty to induce it to
enter into chemical combination. One of the methods
adopted for absorbing it, was to submit the gas to
156 THE BECQUEREL RAYS
the action of red-hot magnesium. The absorption
is very slow. But in the end all the nitrogen taken
was induced to combine with magnesium, and a
residue was found to remain over, which refused
to combine. This inert residue was termed argon,
and it was found to be absolutely indifferent to every
chemical re-agent, however active. It appeared, in
short, to be incapable of entering into combination
with any other substance. We have seen that in this
respect it resembles the emanations of radium and
After the discovery of argon, it became clear that
caution was necessary before any inert gas which
might be encountered in nature could be accepted
as nitrogen. It had been usual, in analysing gases,
to test for the oxygen and for hydrogen carbonic
oxide and combustible gases generally. The inert
residue was entered, without further examination,
as nitrogen. It was for this reason that argon had
remained so long undetected. But, at last, it had
been realised that the inert residue must be more
minutely examined before its nature could be stated.
Now it was known that certain minerals, notably
cleveite, a variety of pitchblende, gave off an inert
gas on heating. This gas, as usual, had been regarded
as nitrogen. In the light of the discovery of argon,
Sir William Ramsay thought it worth while to look
into the nature of the cleveite gas more closely. He
found that it was not nitrogen or argon, but another
new gas, distinct from either.
Many years before, Sir Norman Lockyer had ob-
served in the spectrum of the sun's chromosphere a
strong line in the yellow near the lines due to sodium.
He had not been able to identify this line as due to
any known terrestrial substance, and had concluded
that it was not due to any such substance, but to an
PEODUCTS OF RADIO-ACTIVE CHANGE 157
element present only in the sun. He accordingly
named it helium. The characteristic helium line was
afterwards observed, not only in the sun, but also in
many of the stars.
When the light emitted by the cleveite gas under
the influence of the electric discharge was examined,
it was found to give the characteristic yellow line
of helium. Terrestrial helium had at last been
Helium is quite inert chemically, like argon. It
cannot be made to enter into combination with any
known substance, nor can it be induced to recombine
with the original mineral when it has once been
driven off by heat.
Helium is a very light gas ; its atom is only four
times as heavy as that of hydrogen. It is even more
volatile than hydrogen, and enjoys the distinction of
being the only gas which has yet resisted liquefaction
by cold and pressure. If its liquefaction should ever
be achieved, we shall be able, by using the liquid, to
produce a lower temperature than in any other way.
The anticipation is that a temperature removed by
only five degrees (centigrade) from the absolute zero
would be attainable.
The suggestion that helium might be found to be
an ultimate product of radio-active change was first
made by Professor Rutherford and Mr. Soddy. The
honour of showing that radium did as an undoubted
fact produce helium is due to Sir William Ramsay
and Mr. Soddy. The observation is one of great
difficulty, and nothing but the highest experimental
skill could have brought it to a successful issue ; for
the quantity of radium available to experiment with
is necessarily small, and the yield of helium almost
In order to recognise the presence of helium it is
158 THE BECQUEREL RAYS
necessary, as we have said, to pass an electric dis-
charge through the gas, and to examine the luminosity
which results by means of the spectroscope. The
luminosity of the gas under the electric discharge
is greatest when at a low pressure, perhaps one-
hundredth part of the atmospheric pressure. This
circumstance is favourable when, as in the present
case, the quantity available for experiment is almost
In examining the spectrum of a gas at a low pres-
sure under the influence of the electric discharge, the
form of tube devised by Pliicker is commonly em-
ployed. 1 Sir William Ramsay and Mr. Soddy con-
structed a tube of this kind of the smallest possible
volume, so as to economise gas to the uttermost. They
exhausted this with the mercury pump, so as to get
rid of all traces of air. They then introduced the
pure accumulated emanation of fifty milligrammes of
It will be understood that to extract and manipu-
late quantities of gas of the size of a pin's head is no
easy matter. Sir William Ramsay, however, has had
unrivalled experience in this kind of work ; for it is
to him and to Dr. Towers that we owe our know-
ledge of the rare gases neon, krypton, and xenon pre-
sent in minute traces in the argon of the atmosphere.
In dealing with the very small quantities of these
gases which are procurable, methods of gas manipula-
tion were devised which enabled such small quantities
to be dealt with quite satisfactorily. It was by a
further refinement of these methods, which are of
too technical a character to be described here, that
the pure emanation was successfully transferred to
a small Pliicker tube.
The spectrum of this gas was observed immediately.
1 Chapter T., page '2.
PRODUCTS OF RADIO-ACTIVE CHANGE 159
It appeared to be different from any known spectrum. 1
After a few days, however, this unknown spectrum
was found to have faded away, and the helium spec-
trum had asserted itself in its place. The yellow
line known as D 3 , first observed by Sir Norman
Lockyer in the sun, and subsequently observed in
terrestrial helium, could be distinctly observed. In
short, the emanation had turned, partially at least,
It would scarcely be too much to say that this is
one of the most important experimental results ever
attained in chemical science ; for it is the first re-
corded instance of a transmutation of the elements.
There are some seventy elements known. Of these
a few, principally the common metals, such as iron,
copper, tin, lead, gold, were known to the ancients.
The large majority, however, have been left to modern
science to discover. Up till recently all attempts to
produce any one of these substances from another
had failed. The alchemist's dream of transmuting
silver into gold is familiar to all, though why silver
should be a specially more promising material to
produce gold from than any other is not, from the
chemist's point of view, very apparent. 2 It may have
had its origin in some vague notion that as silver
and gold were both ' noble' metals, there might not
be much difference in their nature. In spite of the
entire failure of any chemical process of solution,
distillation, or precipitation to effect the smallest
transmutation of the elements, it was felt that there
1 Sir William Ramsay and Professor Collie have recently made a detailed
study of the spectrum of the emanation. It can only be observed with
difficulty ; when obtained it resembles those of the inert gases, argon and its
2 This was a sufficient reason for feeling great scepticism about the
alleged success of an American attempt to produce gold, which achieved
some notoriety a few years ago. The process was to hammer silver at a
very low temperature.
160 THE BECQUEREL RAYS
must be some solution to the riddle of their separate
existence, and that that solution was not wholly
beyond what science might hope to reach ; for there
were hints of a connection between the elements-
hints, in fact, that they all formed part of one
It wa's n'oticed that when the weight of the other
atom's was "expressed in terms of the weight of the
hydrogen atom, the lightest of all, they approached,
in many instances, very closely to whole numbers.
The' other atoms were very nearly an exact number
of times as heavy as the hydrogen atom. It was not
at first unnatural to assume that an exact law of
nature was in question, and that the departure of
the atomic weights from whole numbers was due to
the unavoidable errors of experiment.
Such a theory was very attractive, for it would
almost irresistibly lead to the conclusion that the
other atoms were built up of a number of hydrogen
atoms. At the time the theory was propounded, no
great straining of the observed facts seemed to be
necessary in order to bring them into harmony with
it ; for the experiments undoubtedly in many cases
pointed very nearly indeed to the exact whole
numbers, arid it is beyond doubt that the tendency
of experimenters is to be far too sanguine as to the
accuracy which their measurements have achieved.
It is easy, after making a number of concordant
measurements, to feel confident that the error of the
result cannot much exceed the differences between
the several measurements ; but such conclusions must
be accepted with very great caution, for constant
errors may creep in which are often hard to detect,
and which affect every measurement equally. As a
simple example, we may carry out a weighing as
often and as carefully as we please ; but however
PRODUCTS OF RADIO-ACTIVE CHANGE 161
accurate the standard weights may be, and however
carefully they may be adjusted to balance the object
which is being weighed, the result will be erroneous
if there is any inequality in the length of the arms
of the balance.
This happens to be a case where the error can be
guarded against. It is merely necessary to exchange
the weights and the object which is being weighed,
and to place each in the pan formerly occupied by
the other. The weights should still be equal, and
if they are not, the balance is known to be in fault.
But there are many cases in which such errors may
occur, and where it is almost impossible to foresee
The process of taking a large number of measure-
ments affords no security against this. And even
when different methods are used, there may be
constant errors affecting them both to something
like the same extent ; this produces an apparent con-
cordance, which gives false confidence in the accuracy
of the result.
It became, then, necessary to make a most searching
investigation of the exact values of the atomic weights,
in order to arrive at certainty. This task was under-
taken by Stas, and, as the result of the labours of a
lifetime, he came to the conclusion that the law of
whole numbers was not in accordance with fact.
The most decisive case was that of chlorine, which
came out almost half-way between two whole
numbers thirty-five and thirty-six.
But when this has been admitted, it remains a
fact that the majority of the atomic weights are
very near whole numbers. It has been often thought
that this must be the result of chance. But there
is reason to feel practically certain that such is
not the case. We can calculate, by the theory
162 THE BECQUEREL RAYS
of probability, the chance that the numbers, if
assigned purely at random, should, on the average,
deviate so little from whole numbers as they do.
A calculation of this kind has been made by the
author. The methods of calculation are not suited
to a popular book. But the result was to show a
chance of something like a thousand to one against
the deviations from whole numbers being so small
as they are, if they were governed by nothing but
This, then, gave a very good reason for concluding
that there was some intimate connection between the
atoms of different elements, though it was, and, of
course, in large measure still is, impossible to say
what the nature of that connection might be.
This was not the only reason for suspecting that
different elements might, in reality, have a common
origin in spite of the failure of all experimental
attempts to break down the barrier between them.
The most important generalisation, known as the
Periodic Law, pointed strongly in the same direc-
This law is not of sufficient precision to easily
admit of simple and definite statement. But, broadly,
it is this. Let us make a list of the elements in the
order of their atomic weights. Let us also tabulate
the value of some other measurable quality of the
element such as the melting point, the boiling point,
or the refraction equivalent.
We shall then find that the properties of the
element do not increase regularly with the atomic
weight, but they show a regular fluctuation. Every
eighth shows a return to the properties of the first.
The best and clearest example of this is found,
not in the properties already mentioned, but in the
atomic volume of the element. The late Professor
PRODUCTS OF RADIO-ACTIVE CHANGE 163
Lothar Meyer of Tubingen was the first to draw
attention to the striking periodicity of this property.
The atomic volume of an element is defined as the
quotient of the atomic weight by the density. If we
take quantities of the different elements in the solid
state, proportional to their atomic weights, then it
is evident that these quantities will each contain an
equal number of atoms. Thus the volumes of these
4O 60 80 100 120
FIG. 26. Meyer's curve, showing dependence, of atomic volumes on atomic weights.
Horizontal distances represent atomic weights ; vertical ones atomic volumes.
Each element is denoted by its chemical symbol. The atomic volume alternately
increases and diminishes as the atomic weight increases. The elements of very high
atomic weight are not included, for there are so few of them that the further course
of the curve is not clearly indicated.
quantities are in the same ratio as the volumes
occupied by their respective atoms. The volume is
found by dividing the mass by the density.
The quotients of the atomic weights by the re-
spective densities are in the same ratio as the
volumes of their atoms, though, of course, not equal to
them. We cannot determine the volume of an atom
164 THE BECQUEREL RAYS
absolutely by considerations of this kind. But we
can compare the volume of one atom with that of
It is found that where the atomic volumes are
exhibited on a curve, as dependent on the atomic
weight, 1 that the curve goes through a regular
series of periodic fluctuations. A curve of this kind
is given in fig. 26.
The position for each element is indicated by its
chemical symbol. The maxima of atomic volume
always occur at the alkali metals, lithium, sodium,
potassium, rubidium, and caesum. This curve is
sufficient to show at a glance that the elements
are not entirely independent and isolated from one
another, but that they all form part of one common
The periodic law, though eminently suggestive,
has proved in some respects disappointing. It might
be thought that the discovery of this relation would
lead to the foundation of exact numerical laws con-
necting the atomic weights of the elements, which
would be able to survive the most searching experi-
mental test laws which would inevitably suggest
the true structure of chemical atoms and their
mutual relations. This has not proved to be the
case. It has been found that there are exceptions
and irregularities which make it impossible to accept
the periodic law as more than a rough approximation
to the truth.
Thus, for example, it is necessary, in order to put
the element tellurium into its proper relation with
the kindred elements, oxygen, sulphur, and selenium,
to assign it a place before iodine in the list of the
elements. If this were not done, tellurium, and iodine
1 On just the same principle as the height of the barometer is exhibited as
dependent on the time by the trace drawn by a self-recording barometer.
PRODUCTS OF EADIO-ACTIVE CHANGE 165
too, would be thrown into a position in the periodic
table altogether inconsistent with their known chemical
behaviour. Most careful experiments, however, have
made it certain that the atomic weight of tellurium is
slightly higher than that of iodine. It seems most pro-
bable that some disturbing cause exists which modifies
the atomic weights, not indeed to such an extent as to
altogether obscure their mutual relations, but enough
to deprive these relations of numerical exactness.
The problem of the evolution of the elements is
one of the most interesting and fundamental that
science presents. Although little definite progress
has yet been made, the door seems to be open ; and
the author, for one, feels little doubt that the pro-
cesses of radio-active change which have been observed
to take place in a few instances, are in reality repre-
sentative of the evolution of all the elements.
The information which we possess as yet about the
ultimate products of the degeneration of radium is very
imperfect. It will be remembered that the a-rays
consist of heavy particles whose mass was found by
the rough measurements which were alone feasible,
and by certain plausible assumptions, to be inter-
mediate between hydrogen and helium. It is doubt-
ful whether the measurements are good enough to
make it certain that the a-particles do not consist, on
the one hand, of hydrogen, or, on the other, of helium.
The point must remain open for the present. But
considering the fact that helium has been observed to
be formed in radio-active processes, the balance of
evidence seems to be greatly in its favour.
These a-rays are thrown off by every radio-active
body, and the probability is that the particles are in
each case the same. It is true that there is some differ-
ence of penetrating power in the various cases ; but
these are reasonably attributable to varying velocities.
166 THE BECQUEREL RAYS
If this view is correct, all the radio-active bodies must
be giving off the material constituting the a-particle,
whether it be hydrogen, helium, or something different
from either ; there is no means of telling whether
non-radio-active products are generated as by-pro-
ducts at each stage of the radio-active change. We
do not know, for instance, what happens when the
radium atom gives off one or more a-particles. Does this
suffice to convert it wholly into atoms of the emana-
tion, or is some other inactive substance produced
simultaneously ? The quantities of material are too
small to enable the question to be answered.
It will, perhaps, be objected that if helium has been
looked for and found, it is only necessary to apply
the same methods in searching for anything else, and
that a decisive answer could in that way be at once
Helium, however, is a substance which can be
detected spectroscopically in extremely small quan-
tities. It is doubtful whether any other substance is
known, with the possible exceptions of sodium and
hydrogen, which lends itself so well to spectroscopic
The reasons for this may easily be explained. In
the first place, helium is a gas, and moreover, a very
light gas. So that to charge a small Pliicker tube
with it at a very low pressure, requires a very small
quantity indeed, by weight. In the next place, the
gas is brilliantly luminous under the electric discharge ;
and the luminosity is concentrated very largely in
one line, the yellow line D 3 , whose position in the
spectrum is easily identified by reference to the soda
lines. This brings us to another point. There are
other substances, such as sodium and hydrogen, whose
spectra are conspicuous with very small quantities,
but these substances are to be met with everywhere.
PRODUCTS OF RADIO-ACTIVE CHANGE 167
It is, for instance, very difficult, if not impossible, to
prepare a Pllicker tube so free from moisture that the
characteristic red line of hydrogen cannot be seen in
it when the electric discharge is passed ; so that it
would be very difficult to infer the production of
hydrogen from radio-active substances from the
presence of this line. But with helium it is far other-
wise. For helium is very sparsely diffused over the
earth's surface, and in the atmosphere. And it is
very unlikely, indeed practically impossible, that any
helium which is observed should have accidentally
gained admission to the tube.
The majority of the elements do not lend themselves
very easily to spectroscopic detection, when only
traces are present. Oxygen, for instance, gives very
little luminosity, and its spectrum is not at all con-
spicuous or easily identified.
The metals of the alkalies and alkaline earths are
in some cases easily detected. If any of these occur
in the products of radio-activity, it may be possible to
detect them. But here again, sodium, which gives
the most easily visible spectrum, is so universally
present in everything, that it would be difficult to
draw any conclusion from its appearance.
There is another direction in which we may hope to
gain information as to the connection between the
We have seen that radium is gradually evolving the
emanation. Now, if there is any law of nature which
experiment has placed on a firm basis, it is the law of
the conservation of mass. Nothing we can do to any
portion of matter will in the smallest degree affect its
weight, so long as nothing is allowed to escape from
it. When any apparent exception to this law has
turned up, it has always been traced to a misinterpre-
tation of the facts. Take for instance the case of a
168 THE BECQUEREL RAYS
burning candle. What has become of the weight of
the candle when it has all burned away ? Experi-
ment shows that the constituents of the candle have
combined with the oxygen of the air, and have
remained in a gaseous form in the air. If we burned
the candle in a closed globe, the globe would be found
to weigh just as much after the candle had burned
away as it did before. And the same thing is found
in every case.
It is true that a transmutation of the elements is
a matter beyond the range of ordinary experience,
and that it is perhaps rash to feel absolutely confident
that the law of conservation of mass which has been
deduced from ordinary experience applies to such a
case. The law, however, has become so firmly estab-
lished in the minds of scientific men, that the burden
of proof must lie with those who doubt it.
Assuming, then, that the law holds good, radium
which has generated emanation must weigh less than
at first. In other words, the radium is gradually wast-
ing away. Recently an experimental determination
has been made of the rate at which the emanation is
generated. Sir William Ramsay and Mr. Soddy have
measured the volume of emanation yielded in a given
time by a known weight of radium salt. Now, as we
have seen (p. 118), the density of the emanation is
approximately known. Knowing the volume and the
density, the weight follows ; so that we know what
weight of emanation is yielded by say a gramme of
radium in a day. It is evident that we can find, by
calculation, how long it takes for half the radium to
turn into emanation, if we assume that that is the
only immediate product. The result is to show that
this process occupies something like a thousand years.
We cannot, of course, say how long it would take for
the whole of the radium to change to emanation, for
PRODUCTS OF RADIO-ACTIVE CHANGE 169
the process gets slower and slower, and does not come
to a definite end. But, practically, we may say that
radium cannot survive more than a few thousand
years. If another product is produced simultaneously
with the emanation, the life would be still less.
Other arguments, of a less direct character, 1 into
which we shall not here enter, point to about the
How then can it be that any radium has survived ?
For it is very certain that the earth was in existence
in much the same condition as at present, many
millions of years ago. Any radium which formed
part of its original constitution must have long since
degenerated. It would not have survived till now.
There is no alternative but to conclude that radium
is being in some way called into existence.
As before remarked, we cannot postulate the
creation of matter unless we are driven to it by the
clearest necessity ; so we must suppose that some
other elementary substance is being constantly trans-
formed into radium.
The only question is, What is the parent substance ?
We have not far to seek for a plausible answer.
Uranium is contained in all, or nearly all, the radium -
bearing minerals. And uranium is radio-active, and
therefore necessarily itself undergoing change. Is it
not reasonable to assume that uranium is changing into
radium? This view is due to Professor Rutherford
and Mr. Soddy, and has much to recommend it. We
learned (see p. 126) that uranium was constantly chang-
ing into a substance which has been called uranium
X. We saw also that the latter gradually lost its
radio-activity. In saying this, all that is meant is,
of course, that the activity becomes very much less
than at first so much less that experiment fails to
1 For an account of these, see Professor Rutherford's Radio-activity.
170 THE BECQUEREL EAYS
detect any activity with the quantity which has been
used in the tests.
But it must be remembered how very small that
quantity is. The amount of uranium X obtained is
too small to be detected at all by any test but its
intense radio-activity. It is doubtful whether it
would be visible under the microscope, if unmixed
with foreign matter. The activity of uranium X,
therefore, while it lasts, must be very great, even when
compared with that of an equal quantity of radium.
So that the radium produced by the decay of uranium
X might easily be too little for detection; the apparent
inactivity of decayed uranium X is accordingly no
serious objection to the view that uranium may be
slowly degenerating into radium, through the inter-
mediate stage of uranium X, and perhaps other inter-
Many considerations render this explanation of the
origin of radium plausible.
The weight of the uranium atom is greater than
that of the atom of radium. This is consistent with
what happens in other cases of radio-active change.
For we know that the atom of the emanation is
lighter than that of radium, and that the atom of
helium is lighter than that of the emanation. The
tendency is always towards a lighter atom. Nor
could it well be otherwise, when the process of change
is accompanied by the expulsion of a-particles from
the atoms, and consequent diminution of their weight.
There are many minerals which contain radium ;
Mr. Boltwood, and also the author, have recently
investigated these with a view to determining
whether uranium and radium always occur together,
and also more particularly, whether they are always
present in the same proportion. The result has been
to show that this is approximately so. This fact is
PRODUCTS OF RADIO-ACTIVE CHANGE 171
strongly confirmatory of the view that uranium is the
parent of radium. Since the life of radium is a few
thousand years, and since the mineral is much older
than that, it would be expected that the quantity of
radium present would have reached an equilibrium
value, when the rate of production is equal to the
rate of decay, just as the emanation from a given
quantity of radium reaches an equilibrium value.
This equilibrium quantity of radium would of course
be in proportion to the amount of uranium producing
it. That is exactly the state of things which analysis
of the various minerals seems to reveal. Mr. Soddy
has also made direct experiments to see if he could
actually observe the growth of radium from uranium.
He has not succeeded in doing so, and the fact that
he has not is somewhat disturbing ; for according to
the anticipated rate of production of radium, its
formation should have been easily detected. The
most probable explanation of this failure is that the
uranium X has to go through some intermediate stage,
before turning into radium. It is probable that
thorium, polonium, and actinium are also members of
a connected series of radio-active products. Thorium
occurs in some cases in the entire absence of uranium,
as in the minerals aeschynite and monazite, so that
there is no reason at present to connect it with the
series of products of which uranium is the parent.
Polonium and actinium, on the other hand, do occur
with uranium in pitchblende. Whether they ever
occur in the absence of uranium is a question which
has not yet been investigated.
The case of polonium, in particular, would seem
easy to attack. We know that polonium only retains
its activity for a few years ; or, at all events, that
after a year it is much less active than at first. This
makes it certain that polonium must be quite rapidly
172 THE BECQUEREL RAYS
formed in pitchblende ; for there is relatively a good
deal of it present, and a considerable supply must be
yearly formed, to make up the yearly loss. It would
seem a matter of no great difficulty to ascertain
experimentally which of the constituents of pitch-
blende is responsible for the formation of polonium.
It has been suggested by Professor Rutherford and
Mr. Soddy that the residual activity from the active
deposit of radium, which remains after the bulk of the
activity has decayed, is due to polonium. An objec-
tion to this view is that the material gives off /3-rays,
while polonium only gives a-rays. But that may be
due to the presence of some other radio-active con-
stituent as well as polonium. It will be necessary to
wait for data as to whether this activity decays at the
same rate as polonium or not.
If the views explained in this chapter are adopted,
it would follow that we must regard the intermediate
products, such as the emanations, and the active
deposits, as belonging to the same class of substances
as the ' permanent ' radio-active elements. The differ-
ence is only in degree. The durable radio-active
elements are feebly active considering the large
quantities involved. The intermediate products are
intensely active, but their activity is of short duration.
The total amount of radiation which the product
emits during its whole life is perhaps in many
instances not very different in the two cases. But it
is difficult to formulate any precise view, when we
consider the complication introduced by the existence
of the /3- and y-radiation, as well as the a variety. It
will now be realised that we have good hopes of being
able eventually to thoroughly understand the mutual
relations of the elements. The veil which has hitherto
shrouded the mystery of their separate existence has
to a very slight extent been torn aside.
PRODUCTS OF RADIO-ACTIVE CHANGE 173
The number of elements which have hitherto been
recognised as taking part in the scheme of radio-
active change is very small. This may partly be
due to the difficulty, so often insisted on, of recog-
nising the presence of minute traces ; but there are
Helium is one of the products of the series of
changes connected with the existence of radium.
Now heHum is only one of a series of five gases,
all having a graduated series of properties. These
gases have several times been mentioned in the
course of the present work. But it will be desirable
here to give a somewhat more detailed account of
them. The complete series is
helium, neon, argon, krypton, xenon.
We have already devoted some consideration to the
history of the discovery of argon, and of the discovery
of helium which resulted from it. As soon as these
gases had been obtained, the question arose of
whether they were simple homogeneous substances,
or mixtures of two or more constituents.
Suppose we possessed a liquid of unknown com-
position. How should we ascertain whether it was
homogeneous or complex ? Whether it consisted of
one chemical substance or of several ? The method
which generally succeeds is to distil it. Unless the
various constituents happen to boil at exactly the
same temperature (and this would be most unlikely)
the liquid first collected from the distillation would
be rich in the more volatile constituent, and by
redistilling this product several times we could
eventually obtain the volatile constituent in a freer
state. The process is quite a familiar one. For it
is employed for separating pure (or at least strong)
alcohol from fermented liquor, as well as for number-
less other technical purposes.
174 THE BECQUEREL RAYS
This is exactly the method which has to be
employed for separating the constituents of crude
argon. Argon being a gas, it has to be cooled to
a very low temperature to make it liquefy. This
can be done by a suitable use of the cooling agency
of liquid air. The liquid argon has then to be
distilled. This is done by allowing the temperature
to rise slightly. The liquid then boils off again
into the gaseous form. It was found by Sir William
and Dr. Travers that the first portions of argon
which distilled off contained traces of a lighter gas,
to which they gave the name neon. The dregs of
the liquid argon, which boiled off last, were found
to contain two new heavy gases, which were called
krypton and xenon. The three new gases were
finally, after many distillations, obtained in a pure
state, though in very small quantity, for the pro-
portion in which they exist in argon is very small.
By far the greater part of the crude argon of the
air consists of true argon. The neon, krypton, and
xenon are mere impurities. A trace of helium is
Helium has never been liquefied, and so it has
not been feasible to ascertain by distillation whether
it contains traces of anything else. But the pro-
bability is that it does not. The new gases, neon,
krypton, and xenon, have all been found to resemble
argon and helium in their inertness, and in other
properties which it would be beyond our limits to
describe. The five gases form a series of increasing
density and decreasing volatility.
We have seen that helium is produced in the
degeneration of radium. Is it not probable that the
other gases have had a similar origin ? That they
too have had their origin in radio-active substances ?
It is natural to inquire whether any of the gases
PRODUCTS OF RADIO-ACTIVE CHANGE 175
besides helium are found in minerals. So far as
the author is aware, there is only one such case.
A rare Norwegian mineral called malacone, which
is a variety of zircon, has been found to contain
traces of argon, as well as helium. This substance
has been tested, and found to possess marked
radio-activity. This was natural, for the mineral
contained helium also, and, no doubt, radium. The
mineral was heated, and the emanation extracted
from it. But this emanation was found to decay
at the same rate as the emanation of radium. So
that, if there is a radio-active substance present in
the mineral, which has produced the contained argon,
it does not seem likely that it produces a character-
istic emanation. It may be that the radio-active
change which produced the argon is over. That
the activity has exhausted itself, or it may be that
it still exists in the mineral, but gives no emanation.
The question deserves further investigation, but the
mineral is difficult to procure, and the proportion
of argon contained very small.
Upon the whole, the probability is that the sub-
stances which have produced the other inactive gases
no longer survive. Those which produce helium,
an extreme member of the series, are probably alone
in existence at the present time.
The activity of radium stands in remarkable con-
trast to the inactivity of the chemically allied metals,
barium, strontium, and calcium. It must be remem-
bered, however, that the number of molecules which
are taking part in the change, at any given instant,
is but a very small fraction of the whole. The
radium, we believe, takes thousands of years to
decompose altogether ; and the decomposition of an
atom, or at least the expulsion of an a-particle,
must be quite a sudden catastrophe. So that the
176 THE BECQUEREL RAYS
proportion of atoms which are in the act of decom-
posing at any given moment is very small. The
decomposition must be the result of a conjunction
of circumstances within the atom which is of
extremely unlikely, and consequently rare, occur-
rence. What those circumstances are, we do not
yet know. From this point of view, it will not be
difficult to understand that what is a very rare
event in the case of a radium atom, may be much
rarer still in the case of an atom of the allied metals,
so rare, in fact, that it may never occur at all within
human experience. So far, after all, the absence of
radio-activity does not necessarily imply any great
structural difference between the two kinds of
With regard to the elements not recognised to
be radio-active, there are several remarks to be
made. In the first place, as we have seen, every
obtainable material shows some traces of radio-
activity. But there is difficulty in feeling sure that
this is not due to traces of radium contained in it
as an impurity. For different samples of the same
substance seem to differ very greatly in activity.
One piece of tinfoil, for instance, may give double
the radio-active effect of another.
Still, it is possible that some of the activity is
really due to a specific property of the element in
We have seen that in some cases, inactive products
are formed by radio-active change, which themselves
yield radio-active products by further change. This
occurs in the course of degradation of the active
deposits of radium and thorium. Professor Ruther-
ford has very aptly remarked that the ordinary
inactive chemical elements may represent such in-
active phases, and that they may, none the less, be
PRODUCTS OF EADIO- ACTIVE CHANGE 177
undergoing changes of the same nature as those in
progress in the active elements, in spite of their
want of radio-activity. It is the absence of the
latter that prevents the detection of the changes.
Our best chance of obtaining further knowledge
on this question is to collect careful statistics of
the occurrence of different elements in minerals,
and, in particular, to ascertain whether there are
cases when elements are invariably associated, and
never found separate.
Even if such cases cannot be found, it will not
tell much against the theory. For, if the changes
are very slow (and we know nothing to limit their
possible duration), time enough may have elapsed for
the element produced to have been separated from
its parent by the course of geological changes.
The question may be raised, Is it possible to
stimulate a substance ordinarily inert, so as to make
it radio-active ? It is difficult to give a satisfactory
answer, but some experimental results have been
published which seem to suggest that it is possible.
M. Villard found that bismuth, exposed for some
time to the cathode rays, became temporarily
radio-active. The experiments, however, require
Quite recently, Sir William Ramsay has obtained
evidence that the radium rays are able to develop
activity in a material separated from the radium
by an air-tight partition. It was found that the
walls of a glass beaker, in which a closed bulb con-
taining radium solution had been kept for a year,
had become active, and gave off an emanation of
very quick decay, resembling that of actinium.
Further developments of this important observation
will be awaited with keen interest. There seems
to be a rich field for investigation, more particularly
178 THE BECQUEREL RAYS
with reference to the effect of the rays on different
Thus, it is possible that the actinium extracted
from pitchblende is a substance rendered active by
the neighbourhood of radium, or by exposure to
its radiation. We do not yet definitely know.
It may have been made active by the cathodic
bombardment at close quarters by the ($ radiation
of the active deposit.
On the other hand, the absolute constancy of
the rate of decay of the emanation, and other tem-
porarily active products, in spite of any change of
conditions to which we can expose them, seems to
point to the conclusion that the course of radio-active
change is beyond the range of human control.
It is unsatisfactory to be unable to make more
definite statements. But the present state of our
knowledge does not warrant them. The question
of whether it will be possible to induce radio-activity
artificially may be in the future one of no slight
practical importance. For, if this could be accom-
plished on a sufficient scale, it might be practicable
to set radio-active changes going which would result
in the formation of the precious metals. This is
but a dream at present. But it is not too much
to say that modern science has brought us a stage
nearer to the possibility of its fulfilment.
THE ELECTRICAL THEORY OF THE NATURE
IN this concluding chapter we shall consider, in the
light of radio-active phenomena, the nature of the
chemical atom, and the circumstances under which it
may be expected to break up.
The old-fashioned view of chemical atoms was
that they consisted of elastic spheres, of uniform
structure, or rather want of structure, within. In
the light of present knowledge, so simple a theory
is altogether untenable. We have seen that the
atoms of radio-active elements are able to emit
small particles or corpuscles of about x^o f the
mass of the hydrogen atom, the lightest atom known.
They are also able to emit positively charged particles,
comparable in mass with the hydrogen atoms. It
is also nearly certain that each atom of radium, for
instance, emits in the successive processes of degenera-
tion into the emanation and active deposit several of
these positive masses. It cannot, therefore, be doubted
that the atom of a radio-active substance is a struc-
ture of great complexity, very different to the simple
Since electrically charged particles are sometimes
emitted from the atom, it is inferred that the atom
is built up of such particles, which, in ordinary cases,
are in equilibrium under the influence of their mutual
attractions and repulsions, and of centrifugal forces.
180 THE BECQUEREL RAYS
There is every reason to think that the negative
electrification resides in all cases on corpuscles, of
the same kind as those which are expelled, and which
constitute the )8-rays.
These negative charges are assumed to be held in
equilibrium under the influence of positive electrifica-
tion ; how this positive electrification is distributed
it is very difficult to decide. No case is known in
which positive electricity is associated with a corpus-
cular mass. No positive corpuscles, complementary
to the negative ones, have ever made their presence
apparent. Whenever we obtain a positively charged
particle, it is always attached to a comparatively large
mass. So that there are considerable objections to
the assumption that positively charged corpuscles
exist in the atom. It may be that they are there,
but that the processes which succeed in detaching
an isolated negative corpuscle are not able to detach
the positive one.
Recently it has been preferred to suppose that the
positive electrification is not localised in particular
spots in the atom, but that it is diffused uniformly
over a considerable volume. The negative corpuscles
are assumed to be immersed in this positive fluid, as
it may be called.
It would be idle to pretend that this theory con-
stitutes an ultimate explanation of the nature of the
atom. In some ways it leaves this question more
mysterious than ever. The assumed positive volume
electrification which does not reside on matter presents
at least as formidable a problem as matter itself
appeared to do in the first instance. That, however,
is no reason for rejecting the theory, if it is found
to give a satisfactory explanation of the facts.
We have already seen, in discussing Kauffmann's
observations on the corpuscles, travelling with speeds
ELECTRICAL THEORY OF MATTER 181
approaching that of light, which radium emits, that
there is good ground for believing that the mass of
corpuscles is due entirely to the charge of electricity
which they carry. Now, since it must be admitted
that mass can originate in this way, it is well to
consider whether it is necessary to postulate any
other kind of mass. For evidently it is undesirable
to assume that some unknown kind of mass exists,
until it is quite clear that the electrical kind, whose
nature and origin we do to some extent understand,
is inadequate to fulfil all necessary requirements.
It is unphilosophical to assume two kinds of mass,
if one kind can be made to do.
The most important condition which any adequate
theory of mass must satisfy is that the mass is alto-
gether independent of the state of aggregation or
chemical combination of the material. For it is found
by the most careful experiments that the mass of a
chemical compound is exactly the same as the sum
of the masses of its constituents. Thus, for instance,
the mass of water is exactly the same as the com-
bined weight of its constituent gases, oxygen and
The electrical theory, properly interpreted, gives
a satisfactory account of the conservation of mass.
The question to be faced is this. Assuming the atom
to consist of corpuscles embedded in a volume of
positive electrification, can the electro-magnetic inertia
of each corpuscle be independent of the neighbourhood
of other corpuscles ?
Now the electro-magnetic inertia of the corpuscle is
due to the generation of a magnetic field by the lines of
1 Some recent experiments by Heidweiler should be mentioned, which
have led him to conclude that a slight change of weight does occur on
chemical combination. But these have not yet been confirmed, and it is
impossible to accept so revolutionary a conclusion until other experimenters
have been over the same ground with concordant results.
182 THE BECQUEKEL RAYS
electric force which radiate out from it in all directions.
By far the greater part of the energy which this mag-
netic field represents is localised in the space very near
the corpuscle, where the lines of electric force are very
crowded. The effect of bringing other corpuscles into
the neighbourhood of the one we are considering will
be to disturb its electrostatic lines. This disturbance,
however, will only affect the lines at some distance
from the corpuscle from which they spring, unless,
indeed, the other corpuscle comes to very close
quarters. The mutual repulsion of similar charges
will prevent that happening. So that the lines of
electric force in the immediate neighbourhood of a
corpuscle are secure from disturbance, and the
magnetic field in these regions, which represents
almost all the energy of motion, is not liable to be
affected. Thus, the mass of a corpuscle, if of elec-
trical origin, will not be affected by the presence of
other corpuscles. The mass of the atom, however,
is to be regarded as the sum of the masses of the
constituent corpuscles. That is accordingly nearly
independent of the presence of other atoms. It would
be very interesting if the slight changes of weight
which some experimenters have thought that they
could detect in chemical combination, could be traced
to the change of distribution of the electric field of
the corpuscles of one atom by the presence of
The mass associated with an electric, charge
depends on the extent to which the charge is con-
centrated. If the charge resides on a sphere, for
instance, the mass is greater if the sphere is small
than if it is large. Since the positive electrification
is assumed to be much less concentrated than the
negative, it does not contribute appreciably to the
mass of the atom.
ELECTEICAL THEORY OF MATTER 183
We know the charge on a detached corpuscle, and
its mass. If the mass is wholly electrical, these data
are sufficient to determine how large the sphere on
which the electricity resides must be. The result is
that the diameter is about 10~ 13 cm., or one five-
million-millionth of an inch.
The size of a chemical atom is commonly estimated
at something like 10 ~ 8 cm., or one fifty-millionth of an
inch, so that the diameter of a corpuscle is no more
than one hundred-thousandth of that of an atom.
There are in a hydrogen atom something between
500 and 1000 corpuscles. Thus the corpuscles are
very sparsely distributed, being separated by dis-
tances which, however small compared with the
distances which are within the range of microscopic
vision, are enormous compared with the dimensions of
the corpuscles themselves. The corpuscles in an atom
are of something like the same degree of scarcity as
the planets in the solar system. That is, the dis-
tances between them bear the same kind of relation
to their own size. This being so, it is not at all sur-
prising that the corpuscles emitted by radio-active
bodies the /3-rays, that is are able to penetrate
enormous numbers of atoms ; for they can readily go
straight through an atom without serious danger of
collision with another corpuscle. We assume that the
positive matrix of the atom is freely penetrable by
the corpuscles of the atom itself, or by external ones.
On this theory an a-particle consists of a part of
the positive nucleus of the atom torn off it, with
nearly the proportionate number of corpuscles.
Since, however, the a-particle is positively charged,
we must assume that it has one corpuscle short of the
full number. Thus there would be a surplus of
positive electrification, as experiment shows that
184 THE BECQUEKEL RAYS
The penetration of solid materials by the a-particles
presents a more formidable problem than the penetra-
tion by /3-particles ; it is not, however, by any means
impossible that our entire system of corpuscles, repre-
senting an a-particle, might pass through another,
representing an atom of ordinary matter, without
either of them being permanently affected. If we
remember that the a-particles move with a velocity
which is not beyond measure less than that of light, it
will be evident that the corpuscles in the a-particle
have not much time to perturb those in the atom
through which it passes, or to disturb their equili-
brium. Collisions, however, are naturally more
frequent when a complicated system of corpuscles
passes through an atom than when a single one only
does so. Thus the a-rays are naturally more easily
absorbed than the /3-rays, and are not able to get so
far through matter.
Little has yet been said about the forces which hold
the corpuscles in position in the molecule. The
positive matrix in which the corpuscles are immersed
will attract them to its centre. On the other hand,
they will be repelled electrostatically from one
another. There might, indeed, be static equilibrium
under forces of this kind. But on such a view it
is difficult to understand how there should be any
element of instability in the atom, such as might
account for the phenomena of radio-activity. More-
over, stationary clusters of corpuscles could not emit
vibrations with characteristic periods, as the atom
does in yielding a spectrum. It is believed, therefore,
that the corpuscles in an atom are in a state of
rapid rotation about the centre. This view would
strengthen the analogy between the constitution of
an atom and that of the planetary system, to which
attention has already been drawn.
ELECTRICAL THEORY OF MATTER 185
To deal by direct calculation with systems of 1000
corpuscles, which might be distributed anyhow in
three dimensioned space within the matrix, would
probably be impracticable. By dealing with a
moderate number of corpuscles, however, whose
movements are assumed to be confined to one plane,
Professor J. J. Thomson has obtained results which
illustrate in a general way the manner in which the
more complicated systems of real atoms are likely
to behave. In particular, he has found that certain
configurations recur periodically as the number of
corpuscles is continuously increased. This seems very
consistent with the observed properties of atoms, as
expressed by the periodic law.
It is probable that the rotation of corpuscles in the
atom is the cause which gives rise to light vibrations.
We know that light waves travel with the same
velocity as electrical ones, and no one now disputes
the identity of these two kinds of waves. The only
difference between light waves, such as are radiated
by atoms, and the electric waves used in wireless
telegraphy, is in their length from crest to crest. The
length of the light waves is something like a forty-
thousandth of an inch ; the length of the electrical
ones many yards.
Now the rotation of a corpuscle in a circular orbit
round a central positive charge involves a rapid
reversal of the electric field along the radius joining
them. For when the corpuscle has gone round half
the circle, the electric force, which was, say, north-
wards to begin with, is now southwards. After a
whole turn it will, of course, be northwards again.
This rapid alternate reversal of electric force is just
the condition for sending off electric waves. The
waves, for instance, which are used in wireless tele-
graphy, are produced by the surging of a charge of
186 THE BECQUEKEL RAYS
electricity to and fro in an oscillatory spark between
two knobs. The direction of the electric force is thus
reversed with great rapidity.
This case of a single corpuscle going round in a
circle is the simplest that can be taken as an example.
But the same general principle applies to the actual
case of many such corpuscles, with complex move-
ments and mutual perturbations. It will be under-
stood that the corpuscular atom is quite a sufficiently
complicated structure to account for the complexity
of the vibrations which most atoms give out, when
these vibrations are analysed and sorted out according
to their wave lengths by means of the spectroscope.
It may, perhaps, be thought that we have got very
far from the simple facts of radio-activity in applying
the theory of corpuscles to the spectrum of an atom.
There is, however, a very direct proof that they are
concerned in this phenomenon. It was discovered by
Zeeman in 1897 that the spectrum of sodium was
modified by placing the radiating material under the
influence of intense magnetic force. The lines were
broadened, and as closer observation had shown, split
up into three or more components. Though this
effect is very inconspicuous and difficult to observe, it
is an observation of far-reaching consequences. We
know that the motion of a corpuscle is modified in a
magnetic field, and its period of vibration altered
accordingly. Thus the Zeeman effect gives quite a
direct proof that corpuscles are concerned in the
mechanism of radiation. The amount of effect, indeed,
enables an estimate to be made of the ratio of the
charge of a corpuscle to its mass ; the result is quite in
accordance with the measurements made on the
corpuscles of radium, or of the cathode rays.
A few words must be said on the difficult subject
of what causes the rupture of an atom and the
ELECTRICAL THEORY OF MATTER 187
expulsion of a corpuscle or a-particle with great
Sir Oliver Lodge has pointed out that if the normal
condition of the corpuscles is one of rotation, this
must be accompanied by the emission of energy in the
form of electric waves. We have already given the
reason for this. Now it is evident that an atom
cannot go on indefinitely giving out energy ; sooner
or later the supply which it contains must fail, and
when that happens some catastrophe in the atom may
be anticipated. Sir Oliver Lodge considered the case
of a single electron revolving outside a positive
charge distributed over a considerable volume. As
the energy of motion diminishes owing to loss by
radiation, the corpuscle will get nearer to the centre.
In this position its velocity is necessarily greater than
before. The speed may increase until the velocity of
light is approached, when, as we have seen (see p. 72),
the mass will begin to increase also. When this
happens, the whole balance of forces may not im-
probably upset, and the corpuscle may escape with
the velocity it has acquired. The fact that the
corpuscular velocities observed with radio-active sub-
stances are of this order is favourable to such a theory.
It is difficult to believe that such a velocity could be
suddenly acquired in the process of expulsion. It
seems at least clear that no atom in which the
corpuscles are rotating can be permanent.
An obvious difficulty is that substances distinctly
radio-active are so exceptional ; for this theory seems
equally applicable to the case of any atom. It may be
remarked that we have a millionfold descent in radio-
activity when passing from radium to uranium ; while
experiments which have been already considered
(see p. 149) render it, to say the least, possible that
ordinary substances may possess an activity of their
188 THE BECQUEREL KAYS
owii, apart from radio-active impurities, which is much
more than a millionth that of uranium. What the
difference of atomic structure may be that causes so
profound a difference in radio-activity must be left to
the future to say.
A FEW practical hints will be given for the benefit of those
who may wish to observe for themselves the phenomena of
radio-activity. The experiments considered will only be those
which can be conducted without much difficulty or expense,
and with simple apparatus.
It cannot be too strongly recommended that some of the
experiments should be attempted ; for they will give an insight
into the subject and an interest in it which mere reading can
For most of the experiments, some radium will be required.
This may now be purchased from the majority of instrument
makers; advertisements of radium and other requisites for
experiments on radio-activity (such as fluorescent bodies) are
to be seen in the scientific journals, notably in Nature. Five
milligrammes of pure radium bromide is a useful quantity.
Such quantities are sold for about 5, though the price
fluctuates very much at present. The salt is usually sold in
a small capsule, which has a removable lid ; for most of the
experiments the lid is best left on.
If it is not desired to spend so much on the salt, the experi-
ments can be performed with an inferior product, containing
only a moderate or small percentage of pure radium. This
is much cheaper, but of course the effects are less brilliant.
It will be difficult to perform the experiment on magnetic
deflection with very inferior material. The photographic
experiments generally will, of course, require longer exposures
if this is used.
Photographic action of Radium
To illustrate the photographic effects of radium, and to show
190 THE BECQUEKEL RAYS
that metals are more absorbent for the radium rays than other
substances, a ' radiograph ' may be taken of metal objects in
a wooden box, or of coins in a purse. This is on the same
principle as those taken by the Kontgen rays, though, as has
been already explained, the results are not so clear.
Put a photographic plate in a black envelope. Lay the
object, say a purse with coins in it, on the film side, and put
the radium some inches above so as to radiate on to it. Leave
this arrangement for some time twenty-four hours will prob-
ably not be too long, but this must be found by trial. The
further away from the plate the radium is placed, the clearer
will be the results ; but a longer exposure must, of course, be
given at an increased distance.
Photographic action of the Salts of Thorium
The radio-activity of thorium salts can be tried by a simple
experiment devised by Mr. Richard Kerr. He uses one of the
mantles for the Welsbach incandescent gas-burners, which can
now be procured anywhere, and which contain thorium. Take
one of these mantles and cut it open with a pair of scissors, so
that it can be spread out flat. A new mantle must be used,
which has not been fired, for after that it becomes too brittle
to be touched. Spread out the mantle on the film side of a
photographic plate, and lay another glass plate on the top, so
as to keep it flat. Leave the whole in a light tight box in a
drawer or cupboard for at least a week. After that time has
elapsed, take the plate out and develop it.
A very clear picture of the mantle will be obtained, showing
the network black on a light ground. A print from this will,
of course, more clearly represent the appearance of the mantle
itself, for the network will then be white on a dark ground.
Photographic action of Pitchblende
Obtain from a mineral dealer a piece of pitchblende, select-
ing for preference a piece which contains other minerals, such
as quartz, felspar, and pyrites, embedded in it. Have one
surface of the piece cut flat and polished. This can be done
by an amateur, but practically it will save time and money to
PHOTOGRAPH OF A WELSBACH INCANDESCENT GAS MANTLE,
TAKEN BY MR. RlCHARD KERR.
THE MANTLE WAS OPENED OUT AND LAID FLAT ON A PLATE. THE EXPOSURE
LASTED FOR EIGHT DAYS. THE THORIUM CONTAINED IN THE MANTLE MAKES
IT RADIO-ACTIVE, AND ENABLES IT TO ACT ON THE PLATE.
RADIOGRAPH OF A PURSE CONTAINING COINS, TAKEN BY RADIUM.
THE METAL FRAME OK THE PUKSE AND THE ENCLOSED COINS STO1' THE RAYS MUCH
MOKE THAN THE LEATHER, SINCE THEY ARE DENSER. THUS IN THE POSITIVE
PICTURE THEY SHOW BLACK ON A LIGHT GROUND.
APPENDIX A 191
take it to a lapidary, or to have it done by the dealer from
whom the specimen is purchased. Lay the flat side on the
film side of a photographic plate, or of a piece of bromide
paper, placing the whole in a light tight box, which for further
security should be placed in a dark cupboard. Leave it for a
fortnight ; after that time, develop. A picture of the mineral
surface will be obtained (Plate II.), the pitchblende showing
out as a deep black on the plate, owing to the action of the
radio-active materials it contains on the latter. The parts of
the film opposite to the associated minerals, felspar, quartz,
etc., will be unaffected.
Since pitchblende is black, the direct ' negative ' impression
will give a better representation of the appearance of the
mineral surface than a print from it would do. Thus it is
better to use a piece of bromide paper in the first instance
instead of a plate.
Comparative absorptions of different materials
An experiment to illustrate the law that the absorption of
different materials is proportional to their density may be made
in the following way. Take a piece of sheet iron tinned iron
such as biscuit tins, etc., are made of, will do cut out with
shears, or strong scissors, a piece one inch square exactly.
Weigh it, and prepare a piece of wood also one inch square, of
the same weight as the iron. This need not be all in one piece,
successive layers of thin wood or card may be added- to make
up the exact weight. Now lay the iron and wood side by side
on a photographic plate, which should be wrapped in black
paper, or in one of the black envelopes obtainable from
photographic dealers. Bring the radium over the whole, at a
distance of two or three inches, taking care that its position is
symmetrical with regard to the iron and the wood. Expose
for a few minutes, or hours, if minutes are found insufficient
to give a good impression. This will, of course, depend on the
quantity and quality of the radium preparation employed. On
development, it will be found that there is intense blackening
of the plate where the wood and iron screens do not cover it.
Where they do, there is less darkening owing to the partial
absorption of the rays. It will be found that the shadows of
192 THE BECQUEREL RAYS
the wood and iron screens are about equally dense ; thus
showing that equal masses produce equal absorptions, inde-
pendently of the material used for the absorbent screen.
Fluorescence produced "by the Rays
This is easily observed with a few milligrammes of radium
bromide. A fluorescent screen, such as is usual for observing
the bones with Kontgen rays, will light up conspicuously when
the radium is brought up, either from the front or from behind.
In the latter case the rays penetrate the cardboard on which
the fluorescent material is spread. Interpose a piece of tin-
foil, or other thin metal, and the fluorescence will still be seen,
though not so brightly as before. With care, and by remaining
in the dark for some time so as to get the eye into its most
sensitive condition, the fluorescence may be observed even
when fairly thick materials, such as the palm of the hand or
a book, are interposed. In this case the 7-rays are chiefly
If the fluorescent screen is not available, the same pheno-
mena may be observed with a diamond. Imitation diamonds
may readily be distinguished by their far slighter fluorescence,
though they, as well as common glass, are quite visibly
luminous. To see the fluorescence of the tissues of the eye,
stay in the dark for some time, and when your eyes have
become sensitive to feeble lights, shut the eyelids, and bring
the radium near to one of them. Most people will easily
get an impression of luminosity, though some do not appear
to see it very readily. A piece of black paper should be
placed over the radium, to avoid any possibility of compli-
cation owing to its own feeble luminosity, which, by the way,
should be noticed.
Coloration of Glass ly the Rays
Lay a piece of ordinary sheet glass on the cell or tube con-
taining a few milligrammes of radium bromide. In a few days
the purple coloration will be quite visible. When feeble it
will best be observed by laying the glass on a sheet of white
APPENDIX A 193
paper. After the lapse of a few weeks the coloration will
be very strong, and of a deep violet colour.
Other substances may be tried also. Fluorspar, of the
colourless varieties, can readily be coloured. So can rock-salt,
or, if preferred, ordinary table salt. Dissolve the coloured
salt in a little water, and you will find that the solution is
colourless; boil it down, in a small porcelain basin over a
spirit lamp, and the salt, when dry, will be colourless as at
Electric Discharge by Radium
This is as easy an effect to observe as any. Any electro-
scope will serve, if well enough insulated, for ordinary use.
Charge the leaves in the usual way, by means of rubbed
sealing-wax, and bring the uncovered radium near them. It
will be found that the charge leaks away immediately, and
the leaves collapse. The electroscope should be charged, first
positively, then negatively ; it will be found that the discharge
occurs about as rapidly in each case.
Next, test the discharging action when the radium is
covered by various thin screens, tinfoil, paper, etc. It will be
found to be enormously reduced when one sheet of such
material is put over, but further reduced comparatively little
by the subsequent ones. The reason for this has already been
explained. The a-rays are completely absorbed by the first
screen ; the residual /3-rays comparatively little.
If you do not possess any radium, the same experiment
may be made, though, of course, not in so striking a way, by
means of pitchblende. In this case it will be best to have
some means of reading the electroscope with a scale, for the
rate of discharge will be too small to be very easily detected
without. If the electroscope insulates well, however, so that
the time which the leaves take to go down in the absence
of radio-active substances is long, it will not be difficult to
satisfy oneself that they go down considerably faster when
the pitchblende is held near. If, as is sometimes the case,
the electroscope has a flat top, the radio-active substance may
be spread upon this before charging. The leaves will then
go down faster than they would do in the absence of the
194 THE BECQUEREL KAYS
To make the electroscope satisfactory for this purpose, it
is desirable to see that the insulation is in good order. If this
is of ebonite, it can be improved by scraping with a knife
or emery paper. Other insulators are not, perhaps, so 'easy
to renovate. It is, however, almost always possible to get
good results while the instrument is warmed, so that no
moisture can condense on the insulating supports.
Observation of the Scintillations produced ly the a-Rays
As mentioned in the text, a piece of self-contained apparatus
has been devised for observing this effect. It has been put
on the market, and may be obtained from the instrument
makers for a few shillings. If preferred, the observer may
arrange the experiment for himself. A small zinc sulphide
screen is necessary. This may be purchased ready-made, or
made by spreading some of the bought sulphide 1 on paper
with gum. A very little radium is brought into a spot on
the screen. It is impossible to take too little. The best
plan is to touch the stock of radium with a wire, which may
be breathed on if necessary, and to transfer any speck of
radium which may adhere by rubbing the screen with the end
of the wire. A speck of light will be observed, when the
radium causes fluorescence. Examine this with either a
Coddington lens or a microscope, with an objective of very
low power. The scintillations will then be well seen.
Magnetic deflection of the Rays
This may be observed, either by the photographic plate or
fluorescent screen. A permanent magnet may be used, though
in that case a somewhat large one is desirable. A moderate-
sized electro-magnet is on the whole more convenient.
Whichever is used, two movable pole pieces of iron, say
f-inch square and l|-inch long, should be provided. These
can soon be cut from bar iron. by a blacksmith. Place the
pole pieces, one on each pole of the magnet, so as to leave
a narrow space between them. This space may be J-inch to
|- -inch wide.
1 This material has to be prepared in a special manner, and is best bought.
APPENDIX A 195
For this experiment it is desirable to have the rays confined
to some extent into a pencil or beam, before they enter the
magnetic field. For this purpose a piece of sheet lead, with
a slit in it, should be placed a little distance in front of the
radium, which, we assume, is itself confined to a narrow space.
The slit in the lead can easily be made by means of a chisel.
It may be T Vinch broad, and half an inch in front of the
The relative position of the various parts is indicated in
fig. 13. The beam from the radium crosses the space between
the poles, and falls on the plate, which is in a black envelope.
If the magnet is not excited, the impression on development
will be found to be a round patch. When, however, the
magnet is excited, this patch will be found to be drawn out
into a streak to one side of the plate, at right angles to the
direction of the magnetic force, and to the original direction
of the rays. On reversing the current, or, in the case of a
permanent magnet, turning it over so as to exchange the
position of the poles, the direction in which the impression
is drawn out will be found to be reversed.
The exposure must be found by trial, but, with five or ten
milligrammes of radium bromide, four to six hours will probably
This experiment refers to the /3-rays only, for the a-rays are
stopped by the black envelope in which the plate is wrapped.
The deflection of the a-rays is too difficult an observation for
the class of experimenters for whom these notes are intended.
If preferred, a fluorescent screen may be employed instead
of a plate for observing the deflection of the /3-rays. There
is no difference otherwise in carrying out the experiment.
Observation of the Emanation of Radium
. For this experiment it is necessary to have a test tube fitted
up for bubbling air through the solution of radium. Fig 27
shows the arrangement. Any book on elementary chemistry
will explain how the glass tubes are bent at right angles, and
how holes are bored in the corks, or a local druggist, from
whom the materials are purchased, will show how to do it.
Arrange a glass tube with two more corks, as indicated in the
THE BECQUEREL RAYS
figure, and put in it any fluorescent substance that you may
possess, confining it in the tube between plugs of cotton wool.
The best substance to use is the special zinc sulphide, already
mentioned. The minerals willemite and kunzite are also
FIG. 27. Apparatus for observing the connection, a is a test tube, containing
radium solution. This tube is closed by a cork with two holes. One of these, b,
passes to the bottom of the tute. The other, c, only just enters it. c is con-
nected by means of another cork to the wider tube./, /contains the fluorescent
substance, confined between plies of cotton wool, d, d. A small exit tube, e, is
fitted to /by a cork.
very brilliant, or a small diamond ornament may be placed
in the tube. Even broken glass will do.
Put the radium salt dry in the test tube, and insert the
cork air tight. Attach a piece of india-rubber tube to the
end of glass tube e ; then bring up a wine-glass containing water
so as to dip b into it, and suck gently through the india-rubber.
This will draw water into the test tube ; about a thimbleful
will be enough to suck in. This water dissolves the radium
salt, thereby releasing the gaseous emanation. It will now
be understood why the radium was not to be dissolved up
first, and the solution poured into the tube ; for in that case
the stored-up emanation would have been lost. Now attach
the india-rubber tube to b instead of e. Darken the room,
or, much better, perform the experiment at night. Wait until
your eyes are thoroughly accustomed to the darkness; then
blow very gently, so as to pass a few bubbles of air through
the solution and over the phosphorescent substance. These
bubbles carry the emanation with them, and this will cause
APPENDIX A 197
the fluorescent substance to light up. As soon as this occurs,
stop blowing, and close up the end of the tube e with a small
cork or a plug of wax. The luminosity of the zinc sulphide
will increase for two or three hours. This is due to the
formation of the active deposit upon it.
After a few hours, detach the tube containing the fluores-
cent material, and blow through it, so as to blow the emana-
tion altogether out. You will now observe fluorescence due
to the active deposit only. Watch this, from time to time,
and you will find that it rapidly decays, becoming imper-
ceptible after a few hours.
The growth of luminosity is best observed with the mineral
kunzite, 1 for this substance fluoresces only under the /3-rays.
The emanation itself does not yield these rays, though the
active deposit does; thus the effect of the active deposit is
not masked by that due to the emanation. The tube may
now be reattached, and some more emanation introduced into
it, by bubbling as before. It may then be closed at both ends.
The luminosity will then be found to persist for a long time,
falling to half its original brightness in the course of four
days. This loss of brightness, if the tube is properly closed
so that the emanation cannot escape, is due to the decay of
After these experiments it will be necessary to recover the
radium from the solution. Obtain a concave watch-glass from
a watchmaker, or from a chemical dealer ; lay it on a metal
tray containing sand; the lid of a round tin will serve.
Arrange the tray over a spirit lamp, so as to be able to heat
the watch-glass. The sand serves to diffuse the heat. Pour
the solution of radium into the glass, and gently evaporate
it ; do not heat it to the boiling point, for that might cause
spitting, and loss of radium salt. When dry, carefully scrape
up the salt and replace it in its capsule. You will find that
it has lost much of its activity, but this will gradually come
back, as a fresh stock of emanation is generated.
Preparation of Uranium X
Obtain an ounce of uranium nitrate, and dissolve it in two
1 Obtainable from Messrs. Griffin, 20 Sardinia Street, Lincoln's Inn Fields.
198 THE BECQUEREL EAYS
ounces of water, in a glass-beaker, or tumbler. Add a drop
of a strong solution of perchloride of iron. Now add a strong
solution of ammonium carbonate ('sal volatile'), little by
little. This will cause a yellow precipitate at first, but, by
adding more, and stirring, the yellow precipitate will be re-
dissolved, and the liquid will become comparatively clear,
.but not absolutely so, for the iron which was added is pre-
cipitated and not subsequently redissolved. With it is the
uranium X. There is so little of this latter that the iron
is necessary to act as a nucleus to collect it on. Otherwise
it could not be dealt with. Filter the liquid from the pre-
cipitate by means of bibulous paper contained in a glass
funnel. Any druggist will show you how to perform this
operation, or you will find it described in any elementary
book on chemistry. When the liquid has run through, pour
in some water to wash the precipitate which remains on the
funnel, then collect this precipitate. This will best be done
by making a hole in the bottom of the filter paper, and
washing the precipitate down through it with a fine stream
of water from a wash bottle, into a basin or watch-glass. Dry
the precipitate in this basin by gentle heat, with a spirit lamp
or on hot- water pipes.
Boil the solution of uranium in ammonium carbonate in
a beaker or flask, and continue this process until you cannot
smell any more ammonia in the issuing vapour. The uranium
will all be reprecipitated by this process, and should be
collected in a filter paper and dried, as in the former case.
Now take a piece of thin wood, or thick cardboard, and
make two holes in it, say half an inch in diameter, and one
clear inch apart. Paste a piece of paper on one side of it,
and let this dry. You will obtain in this way two cells, with
bottoms of equal thickness. Scrape up the two precipitates,
and put one of them into each cell. There will not, of course,
be nearly room enough for the whole of the uranium pre-
cipitate, but put in as much as the cell can contain. Now
lay this arrangement on a photographic plate, and leave it
in the dark for at least a week. On development you will
find a strong impression under the separated uranium X, and
scarcely any under the uranium. This is because the whole
of the /8-rays, which produce almost all the photographic
activity, are given out by the uranium X\ and since this has
APPENDIX A 199
been separated, the parent uranium is no longer able to
produce an impression.
The electrical effect of each should be tested with the
electroscope as already described. In this case, almost all
activity is still with the uranium. This, as already explained,
indicates that the uranium itself gives out all the a-rays.
Put the specimens on one side for several months, and
then take another photograph. You will now find that the
uranium X has lost its activity, while the uranium has re-
covered the activity it originally possessed. The old uranium X
has decayed, while the parent uranium has produced a fresh
THEORY OF THE MAGNETIC AND ELECTROSTATIC DE-
FLECTION OF THE CATHODE RAYS OR THE /3-RAYS
OF RADIO-ACTIVE SUBSTANCES
IN this note the mathematical theory of the subject, which
was omitted from the text, will be given in an elementary
In the first place we shall require an expression for the
mechanical force on a particle whose charge (e) is known when
it moves through a magnetic field, strength H. As this is not
given in any elementary book I have seen, it will be well to
show how it can be calculated.
In this, as in all calculations relating to the connection
between moving charges and magnetic phenomena, we depend
fundamentally on Eowland's experiment, by which he showed
that the convection of an electric charge produced the same
magnetic effect, not only qualitatively, but quantitatively, as
the corresponding electric current. In this experiment, an
electrically charged disc, divided into sectors insulated from
one another, was caused to rotate rapidly. The effect of this
rotation is, obviously, to carry a stream of electrified matter
round in circles concentric with the disc. If we know the
rate at which the disc rotates, and the density of electrification
at every point on it, it is evident that we can calculate at what
rate electricity is being carried past an externally fixed point,
in each of the circles concentric with the disc, which are
situated at different distances from the centre. Now, an
electric current in a wire bent into a circle, also involves
the transference of electricity at a definite rate past any
Let us imagine a large series of such circular wires, all placed
concentric on the surface of a disc similar to the rotating one,
APPENDIX B 201
and let us further suppose that the strength of the current in
each is so adjusted that the same quantity of electricity flows
through the wire past any fixed point in a second, as is carried
in the same time past a fixed point in the corresponding circle
of the rotating disc.
The question which Eowland set himself to decide was this :
Does the rotating disc produce the same effect on a magnetic
needle as the corresponding system of wires carrying currents ?
or, what amounts to the same thing, does the convection of
electricity on moving matter produce the same magnetic force
as the conduction at the same rate through a metal wire ?
The result of experiment was to show that it does, and that
the two actions are, in their magnetic behaviour, quite
Now, consider an electrified ring, consisting of a large
number (n) of electrified particles arranged at equal distances
round the circumference of a circle of unit radius. Let this
ring rotate with a uniform circumferential velocity v. Let e be
the charge of each particle.
Then the current going round the ring will be - , since
this is the quantity of electricity which passes any fixed point
in one second. The axial force acting on a magnetic pole of
strength H, due to this circular current, will be, at the centre,
nevH. This, accordingly, is the action of the ring of electrified
particles on the pole. Action and reaction, however, are in all
ordinary mechanical cases equal and opposite. If this law is
admitted in the present case, then there must be a reaction of
the magnetic pole on the ring of particles, also equal to nevH,
and in the opposite direction to the action of the electrified
ring on the pole. Thus the action on any particle of the ring
is at right angles to its own direction of movement, and to
the magnetic force due to the pole. Since the reaction on
n particles is nHev, the reaction on each must be equal to
ffev; If being the strength of the magnetic pole at unit
distance, represents also the strength of the magnetic field
at the particle.
Thus in this case the force on an electrified particle moving
at right angles to magnetic field of strength His equal to Hev t
at right angles to the magnetic force and to the direction of
202 THE BECQUEKEL RAYS
The force on a given electrified particle, however, can only
depend on the magnetic force at the particle itself, and on the
velocity and direction of motion. Thus we are entitled to con-
clude that the relation found for this particular case is quite
general, and that the force on an electrified particle moving at
right angles to a magnetic field is always to be found by the
rule given above.
There is one point which requires justification in the above
argument. That is the use that was made of the law that
action and reaction are equal and opposite. This law was
formulated by Newton without reference to electrical and
magnetic phenomena, and it is hardly justifiable to extend it
to them without further discussion. We have to consider
whether some part of the reaction of the magnetic pole may
not be exerted on the ether, the intangible fluid filling space
which is postulated to explain electrical and magnetic
phenomena. If this were the case, Newton's law could not
be said to be obeyed, for he did not contemplate a reaction
on anything but matter. If any of the reaction was exerted on
the surrounding ether, then the system of rotatory disc and
magnetic pole, held apart at a fixed distance, if floated on-
water so as to be free, would move off along the axial direction.
The experiment would not be practically feasible, for the forces
involved are much too small to produce perceptible motion.
But there is no reason to think that any such motion would
ensue, and all are agreed to assume that it would not. This
assumption, like many others which have been made in
developing electro-magnetic theory, is not absolutely satis-
factory and conclusive. But we have' to be content with it.
Now, consider a beam of cathode rays whose path is at right
angles to a field of magnetic force H. Let v be the velocity of
the particles, e their electric charge. Then, since there is a
deflecting force acting always at right angles to the path of
the particle, its path must be a circle. To find its radius, this
normal force is equal to Hev. It must, however, balance the
centrifugal force of the particle, equal to - , when r is the
radius of the circular path, and m the mass of the particle.
Hev = ,
APPENDIX B 203
TT m / 1 \
or Hr = v . . . . (1).
Let us now consider the deflection of the rays by a uniform
electrostatic force, at right angles to their initial direction.
The motion is exactly the same as that of a horizontal pro-
jectile, acted on by gravity, if the resistance of the air be
neglected ; for the electrostatic force produces a uniform
acceleration of the particle at right angles to the initial
direction of motion, and the path is in consequence a parabola,
just as in the case of gravity.
Let e be the length of the electrostatic field. Then the time
taken by the particle to travel along it will be . The force
acting on the particle is equal to Fe, if F is the intensity of
the field. The acceleration is -- . Thus if d is the sideways
displacement of the beam
-. - -n UV V , _ N
<*=** -r . (2).
Multiplying equations (1) and (2) together, we have
Now all the quantities in terms of which v is expressed can
be determined. Thus the strength of the magnetic field (H)
and the radius of curvature it produces (r) can be measured.
The strength (F) and length (I) of the electrostatic field can
be measured too, as can also the sideways displacement d of
the beam which it produces. Thus v, the velocity of the
cathode rays, is quite determinate.
Knowing v, we can readily find - - ; for, by equation (1)
This is not the only way in which these quantities (v and
) may be measured. Instead of measuring the electrostatic
deflection, we may obtain a second relation between them by
measuring the potential difference between the anode and
204 THE BECQUEREL KAYS
cathode of the tube, which sets the cathode particles going.
We do not, however, enter into this, for it has no application
to the a- or /3-rays of radio-active substances. It is doubtful
whether an electric field has anything to do with the expulsion
of these. If it has, this field must be situated entirely inside
the atom, and we cannot, of course, determine its strength
THE TREATMENT OF PITCHBLENDE RESIDUES ON A
IN the text we gave in outline the methods which first
indicated the existence of radium, and which served for the
extraction of the first samples of it. Some readers, however,
may desire full details of the methods which are actually
employed at the present time for the extraction of radium
on a large scale. For the operations are necessarily on a
large scale, in spite of the small bulk of the ultimate product.
We give, therefore, in this note, a free translation of Madame
Curie's account of the process.
' Since pitchblende is an expensive mineral we gave up
the idea of treating large quantities of it. In Europe, the
mineral is worked in the mines of Joachimsthal, in Bohemia.
The powdered mineral is roasted with carbonate of soda, and
the product of this operation is extracted, first with hot
water, then with dilute sulphuric acid. It is the solution,
which contains uranium, which makes pitchblende valuable.
The insoluble residue is thrown away. This residue contains
radio-active substances. Its activity is four and a half times
that of metallic uranium. The residue consists principally of
sulphates of lead calcium, of silica, of alumina, and of oxide
of iron. In addition, one finds, in greater or less quantity,
almost every metal copper, bismuth, zinc, cobalt, manganese,
nickel, vanadium, antimony, thallium, rare earths, niobium,
tantalum, arsenic, and barium. Radium exists in this mixture
in the form of sulphate, and its sulphate is the least soluble
of all. In order to get it into solution, it is necessary to get
rid of the sulphuric acid as completely as possible. For this
purpose the residue is first treated with a strong boiling
solution of caustic soda. The sulphuric acid which is in
206 THE BECQUEREL RAYS
combination with lead, aluminium, and calcium, passes in great
part into solution in the form of sulphate of soda, which can
be removed by washing with water. Some of the lead, silica,
and alumina is also removed by the treatment with alkali.
The insoluble portion, after washing with water, is treated
with hydrochloric acid. This process completely disintegrates
the material, and dissolves a great deal of it. Polonium
and actinium can be obtained from the solution. The former
is precipitated by sulphuretted hydrogen. The latter is con-
tained in the hydrates which are precipitated by ammonia,
after the solution has been separated from sulphides and per-
oxidised. As to the radium, that remains in the portion
insoluble in hydrochloric acid. This portion is washed
with water, then treated with a strong boiling solution of
carbonate of soda. If there was but a small quantity of
sulphates which had escaped decomposition, this process
results in the complete transformation of the sulphates of
barium and radium into carbonates. The material is then
very thoroughly washed with water, and dissolved in dilute
hydrochloric acid, which must be free from sulphuric acid.
The solution contains radium, as well as polonium and actinium.
It is filtered and precipitated with sulphuric acid. In this way
crude sulphates of radium and barium are obtained. But
they contain in addition a little calcium, lead, and iron, and
also a little actinium mechanically carried down with them.
The solution still contains a little actinium and polonium,
which can be recovered, as from the first solution in hydro-
chloric acid. One can obtain from a tonne (1000 kilos) of the
residue, 10 to 20 kilos of the crude sulphates, which are from
thirty to sixty times more active than metallic uranium. The
next step is the purification of them. For this purpose they
are boiled with carbonate of soda, and then converted into
chlorides. The solution is treated with sulphuretted hydrogen,
which separates a small quantity of active sulphides con-
taining polonium. The solution is filtered, oxidised with
chlorine, and precipitated with pure ammonia. The pre-
cipitated oxides and hydrates are very active, and the activity
is due to actinium. The filtered solution is precipitated by
carbonate of soda. The precipitated carbonates of the alkaline
earths are washed and converted into chlorides. The chlorides
are evaporated to dryness and washed with strong pure
APPENDIX C 207
hydrochloric acid. The chloride of calcium dissolves almost
entirely, while the chloride of barium, containing radium,
remains' insoluble. In this way, about 8 kilos of chloride
of barium, containing radium, can be obtained from 1000
kilos of the residue. Its activity is about sixty times as
crreat as that of metallic uranium. This chloride is ready for
CI-PARTICLES, nature of, 80, 81, 165,
a-Rays, the, 51, 52.
absorption of, 87, 88.
electric charge of, 80.
electrostatic deflection of, 77-80.
magnetic deflection of, 75-77.
of uranium, 126.
penetration of, 184.
ratio of charge to mass, 79.
scintillations produced by, 194.
velocity of, 79-81.
Absorption of a-rays, 87, 88.
of /3-rays, 89-92.
of cathode rays, 23.
connected with ionisation, 106.
experiment on, 191.
active deposit of, 129.
emanation of, 129.
extraction of, 206.
possible origin of, 177.
Active deposit from actinium, 129.
from the atmosphere, 146,
from radium, 121.
positive charge of, 122.
residue of, 153, 154.
sublimation of, 121.
changes in, 123-125.
from thorium, 129.
Air, conduction of electricity through,
under Becquerel rays, 93-105.
Alchemist, the, 159. f
Anode rays, 23.
Argon, 155, 156.
contained in malacone, 175.
distillation of, 173, 174.
possible connection of, with radio-
Atmosphere, radio-activity of the, 146-
rare gases of the, 173-175.
Atom, divisibility of the, 17.
energy of the, 138, 139.
structure of the, 179 et seq.
Atomic volumes, periodicity of, 162-
weights, approximation of, to whole
weights connected with atomic
/3-PARTiCLES, ratio of charge to mass,
mass of, 70-74.
/3-Rays, 51, 52.
absorption of, 89-92.
activity induced by, 177.
electric charge carried by, 63-66.
electrostatic deviation of, 67, 203.
identical with cathode rays, 60.
magnetic deviation of, 58, 194, 195,
penetration of, 183.
of uranium, 126.
velocity of, 69, 70.
Barnes, H. T., and Rutherford, E.,
heating effect of radium and its emana-
Bath, radium from thermal springs of,
helium from thermal springs of,
Becquerel, H., absorption and magnetic
deviation of /3-rays, 90.
discovery of uranium rays, 26-29.
effect of temperature on activity of
uranium, 139, 140.
electrostatic deflection of /3-rays,
ionisation of paraffin, 108.
magnetic deflection of a-rays, 79,
scars produced by radium, 57.
scintillations of zinc sulphate, 82.
rays, the, 30.
cause of emission of, 187.
chemical effects of, 55.
conduction produced by, 93-
THE BECQUEEEL EAYS
Becquerel rays, magnetic behaviour of,
penetration of matter by, 184.
photographic action of, 54, 55.
physiological action of, 57.
selective absorption of, 51.
See also a-rays, /3-rays, -y-rays.
Bismuth made active by cathode rays,
Boltwood, B., ratio of radium to uranium
in minerals, 170, 171.
Brooks, Miss, and Rutherford, E., ab-
sorption of a-rays, 88, 89.
Bunsen's ice calorimeter, 130-132.
Burton, E. F., and M'Leunan, J. C.,
radio-activity of ordinary materials,
Buxton, radium from thermal springs of,
CALC SPAR, fluorescence of, under cathode
Cambridge water, emanation in, 145, 146.
Campbell, N. E. , radio-activity of ordin-
ary materials, 152.
Canal rays, 23.
Cathode particles, cloud produced by,
method of counting, 14-17.
See also cathode rays.
bismuth made active by, 177.
electric charge carried by, 6, 7,
14, 15, 16, 17.
electric charge of, compared
with mass, 12.
electrostatic deviation of, 7, 10,
fluorescence produced by, 4.
magnetic deviation of, 4-9, 200-
magnetic spectrum of, 12.
penetration of solids by, 23.
shadow cast by, 3, 4.
Caves, emanation in the air of, 143.
Cellars, emanation in the air of, 143.
Chemical charge, energy liberated in,
effects of radium, 54.
Cleveite as a source of helium, 156.
Cold, effect of, on thermal emission of
Collie, Professor, and Ramsay, Sir W.,
spectrum of emanation, 159 note.
Colorations produced by radium, 55, 56,
Cooke, H. L., Penetrating radiation in
ordinary surroundings, 150-152.
Corpuscles, emission of light waves by
rotating, 185, 186.
in the atom, 179 et seq.
ratio of charge of, to mass, from
Zeeman effect, 186.
size of, 183.
See also cathode rays and /3-rays.
Crookes, Sir W. , insulation in high vacua,
scintillations produced by a-rays,
Crystallisation, fractional, 37, 38.
Curie, M. andMme., absorption of a-rays,
electric charge of /3-rays, 64-
electroscope for testing radio-
ionisation of liquids, 107.
magnetic deviation of /3-rays,
polonium, 44, 45.
M., and Dewar, Sir J., radium at
low temperature, 140-142.
and Laborde, A., heating effect of
DEBIEENE, H., on actinium, 47-49.
Deflection, electrostatic, of a-rays, 77-
of /3-rays, 67, 203.
of cathode rays, 7, 10, 11, 200-
magnetic, of a-rays, 75-77.
of /3-rays, 194, 195.
of cathode rays, 4-9, 200-204.
Deposit, active. See active deposit.
Dewar, Sir James, luminosity of radium,
and Curie, M., effect of low
temperature on thermal emission of
Diamonds, fluorescence of, 52.
Dorn, E., electric charge of radium, 66.
ELECTRIC discharge, canal rays pro-
duced by, 23.
cathode rays produced by, 3-
glow, 2, 3.
spark, 1, 2.
Electric discharge by radio-active sub
stances, 93-111, 193, 194. See also
by Kontgen rays, 28.
by uranium, discovery of, 29.
Electrometer, quadrant, 93, 94.
Electroscope, gold leaf, 28, 34.
Electrostatic deflection of a-rays, 77-80.
of /3-rays, 67, 203.
of cathode rays, 7, 10, 11, 200,
Elements, connection between the, 160-
possible origin of the common, 176,
the radio-active, 49.
transmutation of the, 159, 160.
Elster, J., and Geitel, H., radio-activity
of the atmosphere, 146-148.
Emanation of actinium, 129.
of radium, 112, 113.
chemical inertness of, 116.
condensation of, 114, 115.
contraction of, 119, 120.
density of, 116-118.
decay of, 118-120.
experiment on, 195-197.
generation of, 120.
heating effect of, 137.
in caves, 143, 144.
in cellars, 143, 144.
in thermal waters, 143, 144.
in common waters, 145.
radiation from, 112.
spectrum of, 158, 159.
of thorium, 129.
Energy of atoms generally, 138, 139.
of the radium atom, 138.
of uranium, source of, 30.
Eve, A. S., penetrating Kontgen rays,
Eye, fluorescence of tissues of, 54, 192.
FLUORESCENCE produced by a-rays, 81-
by Becquerel rays generally,
by cathode rays, 4.
by the emanation, experiment
GASES, the rare, 173-175.
various, ionisation of, 104, 105.
Geisel, F. , magnetic deflection of /3-rays,
Geitel, H., and Elster, J., radio-activity
of the atmosphere, 146-148.
Glass, coloration of, by radium, 55,
Gold leaf, penetration of, by cathode
rays, 17, 18.
Goldstein, E., canal rays, 23.
7-Rays, 51, 52, 83-86.
absorption of, 92, 93.
and ionisation by, 109-111.
HARDY, W. B. , and Wilcox, Miss, action
of rays on iodoform, 56.
Heat, effect of, on radio-activity, 139,
emission of, by the emanation, 137.
by pitchblende, 134.
by radium, 131-134.
by radium at low temperature,
Helium, detection of, 166.
in mineral waters, 145.
properties of, 157.
in radio-active minerals, 154-157.
from the emanation of radium, 157-
in the sun, 136.
Heidweiler, A., change of weight on
chemical combination, 181 note.
Hertz, H., penetration of cathode rays,
Huggins, Sir W., and Lady, spontaneous
luminosity of radium, 63.
Hydrogen atom, charge of, 17.
ionisation of, 105.
number of molecules in a cc. of,
ICE calorimeter, Bunsen's, 130-132.
Iodoform in chloroform, action of radium
lonisation of air, 98-102.
effect of pressure on, 101.
and absorption, 106.
of various gases, 104, 105.
of liquids, 106, 107.
of solids, 108.
Ions, electrolytic, 130, 132.
in air, under Becquerel rays, 98-
charge of the, 102, 103.
recombination of, 103, 198.
velocity of, under an E.M.F., 103.
See also lonisation.
THE BECQUEREL BAYS
KAUFFMANN, W., electrostatic and mag-
netic deviation of jS-rays, 67-74.
Krypton, 173, 174.
LABORDE, A., and Curie, P., heating
effect of radium, 130.
Lenard, P., penetration of cathode ray.s,
Light waves, emitted by rotating cor-
puscles, 185, 186.
Liquid air, condensation of emanation by,
hydrogen, chemical action at the
temperature of heat emitted by radium
cooled in, 140-142.
Lockyer, Sir N., helium in the sun, 156,
Lodge, Sir 0., cause of the rupture of an
Luminosity of radium, 62.
MAGNETIC deviation of a-rays, 75-77.
- of -rays, 58-61, 194, 195, 200-
connection of, with absorption,
of cathode rays, 4-9, 200-204.
force, action of,, on spectral lines,
spectrum of /3-rays, 61.
of cathode rays, 12.
Mass, conservation of, 167, 168, 181, 182.
electrical theory of, 180, 181.
Marckwald, Prof., separation of polonium,
M'Clelland, J. A., absorption of 7-rays,
M'Lennan, J. C., and Burton, E. F.,
penetrating rays from surroundings,
Metals, conduction through, 108, 109.
Meyer, Lothar, atomic volume curve,
and Von Sweidler, magnetic de-
viation of /3-rays, 58.
Minerals, radio-active, 35, 154, 170, 171.
Mineral waters, helium in, 145.
radium in, 143-145.
Nitrogen, bands of, in luminosity of
ORDINARY substances, activity of, 148,
PARAFFIN, ionisation of, 108.
Paschen, F., on -y-rays, 85, 86.
Penetrating radiation. >SV -y-ruvs.
from ordinary surroundings,
power of rays. See absorption.
Penetration of matter by Becquerel rays,
Periodic law, the, 162-165, 185.
Phosphorus, modification of, by radium,
Photographic action of pitchblende, 190.
of radium, 189.
of radio-active bodies, 54, 55.
of thorium, 190.
of uranium, 26, 27.
Physiological action of radium, 57.
Pitchblende, discovery of radio-active
bodies in, 35-37.
heating effect of, 134.
photographic action of, 190.
treatment of residues of, for extrac-
tion of radio-active bodies, 206.
Platino-cyanide of barium, fluorescence
Pliicker's tube, 2.
extraction of, 206.
possible origin of, 171, 172.
Products, ultimate, of radio-active change,
154 et sen.
QUADRANT electrometer, 93, 94.
RADIATION, a-, of radio-active substances.
/3-, of radio-active substances. See
cathodic. See cathode rays.
electric, 185, 186.
y-. See -y-rays.
luminous, emitted by moving cor-
puscles, 185, 186.
penetrating, in ordinary surround-
Rontgen. See Rontgen rays.
Radium, absorption of /3-rays from, 91.
- active deposit of. See active
as a source of solar heat, 135,
Radium, /9-rays of, 91, 122.
changes in, 112-125.
chemical effects of, nfj.
colorations produced by, 55, 56.
comparison of with chemically allied
elements, 175, 176.
discovery of in pitchblende, 35-
effect of heat on activity of,
emanation of. See emanation.
energy of, 138.
extraction of, 205-207.
fluorescence produced by, 52,
7-rays of, 122.
heating effects of, 131-134.
in the soil, 143.
in thermal waters, 41, 42, 143,
minerals containing, 41, 170, 171.
origin of, 169-171.
photographic activity of, 189.
physiological properties of, 57.
possible sources of, 43, 44.
production of helium by, 157,
probable life of, 168, 169.
ratio of, to uranium, in minerals,
spontaneous luminosity of, 62.
Rain, activity from, 147.
Ramsay, Sir W., activity induced by
and Collie, J. N., spectrum of the
emanation, 158, 159.
and Rayleigh, Lord, argon, 155.
and Boddy, F., contraction of the
emanation, 119, 120.
helium from radium, 155,
probable life of radium, 167-
and Travers, M. W., rare gases of
the atmosphere, 173-175.
Rayleigh, Lord, argon, 155, 156.
bath gas, 145.
Rontgen rays, properties of, 20-22, 25,
absorption of, 110, 111.
identity of, with ^-rays of
ionisation by, 109-111.
Rubies, fluorescence of, tinder cathode
Rutherford, B., absorption of 7-rays,
Rutherford, B., electrostatic deflection of
ionisation and absorption, 106.
magnetic deflection of a-rays, 76-
stages of radio-active change, 163-
and Barnes, H. T., heating effect of
radium and the emanation, 137.
and Miss Brooks, absorption of
and Soddy, F., radio-active change,
chap. V. passim.
changes in active deposit, 124.
probable origin of radium,
probable origin of polonium,
SALTS, coloration of, by radium, 65.
Saturation current, in gases, 97, 98.
in liquids, 107.
Scintillation produced by a-rays, 81-83,
Screens, fluorescent, 53.
Simpson, G. Y., atmospheric radio-
activity, 147, 148.
Skin, action of radium on the, 57.
Soddy, F. , attempt to observe formation
of radium, 171/ See also Ramsay,
Sir W., and Rutherford, E.
Solids, ionisation of, 108.
Spectrum, magnetic, of /3-rays, 61.
of cathode rays, 12.
lines, effect of magnetic force on,
of the emanation of radium, 158,
of gases, tube for observing, 2.
of helium from the emanation, 157-
of helium in the sun, 136.
of nitrogen, in the luminosity of
of radium, 40.
Sun, possible connection of the heat of,
with radium, 135, 136.
THERMAL effects of chemical change,
of radio-activity, 130-138.
springs, radium from, 143,
emanation from, 143, 144.
THE BECQUEREL RAYS
Thomson, Prof. J. J., charge of a cathode
emanation from spring water, 145.
radio-activity of ordinary materials,
structure of atoms, 185.
Thorium, activity of, 29, 30.
active deposit of, 129.
emanation of, 128, 129.
in minerals, 171.
photographic action of, 190.
Transmutation of elements, 159, 160.
Travers, M. W. See Ramsay, Sir W.
ULTIMATE products, the, of radio-active
change, 154 et seq.
Uranium, absorption of /3-rays from, 91.
activity of various components of,
as a possible parent of radium, 169-
changes occurring in, 125.
discharging power of, 29.
effect of temperature on activity of,
oxide in pitchblende, 36.
photographic action of, 26, 27.
ratio of, to radium in minerals, 170,
rays, discovery of, 26, 27.
salts, luminescence of, 26.
Uranium X, separation of, 125-127, 198,
VILLARD, P., bismuth made active by
cathode rays, 177.
Volumes, atomic, periodicity of, 162-
WATER, decomposition of, by radium,
spring, emanation dissolved in,
thermal, radio-activity associated
Waves, electric, emitted by rotating
Welsbach mantles, radio-activity of,
Wien, W., canal rays, 23.
Wilcox, Miss E. C., and Hardy, W. B.,
action of radium on iodoform, 56.
Wilson, C. T. E., activity from rain,
cloud formed by cathode particles,
leakage of electricity through gases,
XENON, 173, 174.
ZEEMAN effect, 186.
Zinc sulphide, fluorescence of, 53.
scintillations of, 82, 83.
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