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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 



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. 

August 1904. 








SPHERE ... 143 






INDEX . . 209 



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air-tight vessel, and the air withdrawn by means of 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 
light, d. 

used, a glow spreads out from the two electrodes and 
fills the tube (fig. 1). 


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 

%. i). 

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 


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 


wall of the vessel as a black shadow on the bright 

green fluorescent background (fig. 4). 

Glass is not the 
only substance 
which becomes 
fluorescent under 
the influence of 
the cathode rays. 
Many other ma- 
terials will do the 
same, and some of 
them give more 
brilliant effects 

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 


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 
explained later. 

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 
whole bulb. 

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, 


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. 


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 
(fig. 6). 

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. 


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. 


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 


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 


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 
quantities are. 

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 


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 
same velocity. 

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 


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. 


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 
of detecting. 

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 
they carry. 

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 


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. 


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 
by each. 

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. 


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 
judge. 1 

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. 




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 

b ci, 

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 


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 


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 
of electricity. 

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. 


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 
this anode. 

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 
sharp shadows. 

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 


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 
shadow permanently. 

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. 


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. 


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. 



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 
in 1896. 

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 



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 


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. 



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, 


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 


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 


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 


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 
their scepticism. 




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- 




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, 


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 


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 


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, 


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 
of calcium. 

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 


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 


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 
radio-active properties. 

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 


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- 


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 
good pitchblende. 

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 


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 
insoluble ! 

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 
be made. 


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 
quantities ? 

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 


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 
of bismuth. 

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. 


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 


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 
characteristic spectrum. 

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- 
active elements. 

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. 




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- 



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. 


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 
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, 
are dark. 

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 
point later. 

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 


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. 


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 


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 
minute bubbles. 

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 


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 
supplies this. 

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 


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. 


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 


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 
narrow leaden 
vessel (fig. 12). 
This vessel was 
placed between 
the poles of an 
electro- magnet. 
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. 



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- 
pliances. But 
instances of the 
opposite ten- 
dency are not 

The methods 
of detecting the 
tion which we 
have described, 

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 


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 
neighbourhood. 1 

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. 



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 


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 


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 


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 : 

Velocity, centimetres 
per second. 

Ratio of 
charge to mass. 




77xl0 7 


97xl0 7 


M7X10 7 


1-31 XlO 7 


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. 


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 


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 




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- 
terspaces and 
enter the elec- 
troscope (fig. 17). 
It is necessary 
that the narrow 
slits should be 
reasonably long 
in order that the 
magnetic force 
may act on the 
rays along a con- 
siderable length. 
Air, however, ab- 
sorbs the a-rays 
very freely, and 
if air filled the 
spaces, practi- 
cally no rays 
would get 
through. We 
cannot exhaust 
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 

-Outflow of 

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- 


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 



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- 
tively estimated. 
The result was in 
good agreement with 
Professor Ruther- 
ford's estimate. 

The result to which 
we have alluded, 
that the electro- 
chemical equivalent 
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 
the same. 

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 


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 


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 a-radiation. 


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. 


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 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 
visible effects. 

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 



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 
magnetic field. 
The ct-rays, giv- 
ing rise to by 
far the greater 
part of the 
electrical effect, 
are slightly de- 
flected by a 
transverse mag- 
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 
atomic dimensions. 

The y-rays, not deflected at all by magnetic force, 
are probably Rontgen rays, produced by the cathodic 



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 



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 
follows : 

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 


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 


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. 


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 
the difficulty. 

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 
last chapter. 

Since the /3-rays of radium are so varied in their 
penetrating power, it is difficult to compare their 


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 


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 
by fluorescence. 

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. 


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 


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 

To Tiiglv 

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 
for insulation. 

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. 


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 


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 



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 


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 
before. 1 

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. 


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 
to occur. 

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. 


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 


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 


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 
practically vanished. 

Hitherto we have alone considered the ionisation 

1 Chap. i. p. 14. 


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 
to interpret. 

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 


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 


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 


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 


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 


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. 


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 
platinum sheet. 

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 


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 
Rontgen rays. 



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. 



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 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 
almost completely 
condensed out of a 
space which con- 
tains it by means 
of ice, though to 
remove absolutely 
all perceptible 
traces of it would 
require a lower 
temperature still. 
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 

radio-active. * 

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 


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. 

The condensation 
of the emanation can 
be shown even more 
effectively by an- 
other experiment. 
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 

nnrl^r ifcj 

ill l i 

g 10 DO bV a SllOrt 


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 
porous plate. 

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 


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. 


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 


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 


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 


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. 


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. 


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. 


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 
characteristic rate. 

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 


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 


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 


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, 


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 


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. 


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 


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 


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. 


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 


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 
coal ! 

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 


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- 
ceptibly colder, 

As this does not happen, we must suppose that the 
sun has some other source than its primeval supply to 
draw upon. 

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. 


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. 


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 
radio-active products. 

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, 


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. 


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. 


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 


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 


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. 



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. 



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 


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 
volume. 1 

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. 



a subsequent occasion. Thus the radium salt from 
which the emanation was derived is not present in 
the water. 

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 atmosphere. 

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 


in the open may be identified with that of radium ; 
and it would follow that the atmosphere contains 
radium emanation. 

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 
with it. 

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- 


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- 


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 


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 


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 


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 


proved that the old one cannot be made to account 
for it. 

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. 



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 



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 
element helium. 


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 
it existed. 

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 


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 


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 
inconceivably minute. 

In order to recognise the presence of helium it is 


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 
radium bromide. 

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. 


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, 
into helium. 

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. 


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 
scheme.- -., 

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 


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 



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 


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 

Atcnvio Weights 



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 


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. 


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. 


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. 


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 


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 


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 
same figure. 

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. 


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- 
mediate stages. 

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 


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 


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. 


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 
other possibilities. 

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. 


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 
also present. 

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 


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 


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 


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 



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. 



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 
elastic sphere. 

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. 



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 


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 
hydrogen. 1 

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. 


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. 


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 
there is. 


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. 


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 


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 


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 


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 
never give. 

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 



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 













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 


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 


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 



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. 


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 



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 


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 
the latter. 

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. 


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 


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 



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 
iixed point. 

Let us imagine a large series of such circular wires, all placed 
concentric on the surface of a disc similar to the rotating one, 


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 


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. 

TT mi? 
Hev = , 



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 


or v 

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) 


m Hr' 
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 


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 



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 


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 


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. 

Actinium, 47. 

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- 
activity, 175. 

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 

numbers, 160-162. 
weights connected with atomic 

volumes, 162-164. 

/3-PARTiCLES, ratio of charge to mass, 
69, 203. 

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- 
tion, 137. 

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- 





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, 

143, 144. 

CALC SPAR, fluorescence of, under cathode 

rays, 4. 

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. 

rays, 3. 

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, 

11, 200-204. 

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 

radium, 140. 
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, 
87, 88. 

/3-rays, 89. 

electric charge of /3-rays, 64- 


electroscope for testing radio- 
activity, 34. 

ionisation of liquids, 107. 

magnetic deviation of /3-rays, 


radium, 35. 

polonium, 44, 45. 

M., and Dewar, Sir J., radium at 

low temperature, 140-142. 

and Laborde, A., heating effect of 

radium, 130-133. 

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 

radium, 140-142. 
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- 

83, 194. 
by Becquerel rays generally, 

52, 192. 

by cathode rays, 4. 

by the emanation, experiment 

on, 196. 

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, 

17, 18. 
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, 

17 note. 

ICE calorimeter, Bunsen's, 130-132. 
Iodoform in chloroform, action of radium 

on, 56. 
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. 



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, 

114, 115. 
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 

atom, 187. 
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, 

46, 47. 
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. 

NEON, 173,174. 

Nitrogen, bands of, in luminosity of 
radium, 63. 

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, 

183, 184. 

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 

of, 53-58. 
Pliicker's tube, 2. 
Polonium, 44-47. 

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. 
See a-rays. 

/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- 
ings, 150-152. 

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, 
170, 171. 

spontaneous luminosity of, 62. 
Rain, activity from, 147. 

Ramsay, Sir W., activity induced by 
0-rays, 177. 

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 
.^radium, 85. 

ionisation by, 109-111. 
Rubies, fluorescence of, tinder cathode 

rays, 4. 

Rutherford, B., absorption of 7-rays, 

Rutherford, B., electrostatic deflection of 
a-rays, 78-79. 

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 
a-rays, 88. 

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 
radium, 62. 

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. 



Thomson, Prof. J. J., charge of a cathode 

particle, 14-17. 

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. 

X, 127-129. 

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 

with, 143-145. 
Waves, electric, emitted by rotating 

corpuscles, 185-187. 

Welsbach mantles, radio-activity of, 
' 190. 

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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teaching of one of Darwin's greatest successors, who has been for many years a 
leader in biological progress. As Professor Weismann has from time to time 
during the last quarter of a century frankly altered some of his positions, this 
deliberate summing up of his mature conclusions is very valuable. In the second 
place, as the volumes discuss all the chief problems of organic evolution, they 
form a reliable guide to the whole subject, and may be regarded as furnishing 
what is much needed a Text-book of Evolution Theory. 


Principal of University College Bristol, author of ' Animal Life and Intelligence,' 
etc. With numerous Illustrations. Large crown 8vo., los. 6d. 


Photogravure Frontispiece. viii + 352 pages. Demy 8vo., cloth, i6s. 


Lond., Lecturer on Biology at the London School of Medicine for Women, 
and the Polytechnic Institute, Regent Street. With about 200 original Illustra- 
tions. Crown 8vo., cloth, 7s. 6d. 


M.A., Assistant Master at Tonbridge School. With numerous Illustrations and 
Diagrams. Crown 8vo., cloth, 45. 6d. 


Lond., F.Z.S , and A. J. MASLEN, F.L.S., Lecturer on Botany at the Wool- 
wich Polytechnic. With over 200 Illustrations. Crown 8vo., 75. 6d. 

ACTIVE SUBSTANCES. By the Hon. R. J. STRUTT, Fellow of Trinity 
College, Cambridge. With Diagrams. Demy 8vo. 8s. 6d. net. 

Fully Illustrated. Crown 8vo., cloth, js. 6d. net. 

WOOD. A Manual of the Natural History and Industrial Applications 
of the Timbers of Commerce. By G. S. BOULGER, F.L.S., F.G.S. Fully 
Illustrated. Crown 8vo., 7s. 6d. net. 


F.R.S. xii + 25i pages. Crown 8vo., 33. 6d. 


viii + 312 pages, with 52 Illustrations (many of them full-page). Crown 8vo., 
cloth, 35. 6d. 





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