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A. T. CAMERON, M.A., B.Sc. 









IN this small volume an attempt has been made to 
deal with the most important facts of the science of 
Radioactivity in a manner simple enough to be under- 
stood by readers with a very elementary knowledge 
of Physics and Chemistry. The science is so 
intimately connected with these that no intelligent 
explanation can be given without assumption of a 
slight knowledge of general elementary science, and 
any attempt to impart this knowledge along with the 
treatment of the specific subject would not only result 
in a book far beyond the modest size of this volume, 
but is completely unnecessary in view of the many 
excellent treatises which already provide the desired 

I have to acknowledge my extreme indebtedness 
to Madame Curie's Traite de Radioactivite in the 
compilation of this book. 

Professor Swale Vincent and Dr. R. K. McClung 
have very kindly read through the manuscript of this 
volume, and I am indebted to them for numerous 

Jn 7 y, 1912. 

A. T. C. 
















ELEMENT . . . . . ' . l8l 



THE ATMOSPHERE . . . . .185 


Page 26, 1. 7, for ionisating read ionisation 

P- 3 2 
p. 46, 

P- Si, 
P- 65, 

p. 81, 
p. 86, 

p. 87, 

. 2 from foot, for lead, sulphide read lead sulphide 

. 5 from foot, for rarity read its rarity 

. 5 from foot, for /if read Z 

. 20, for dissolves read decomposes 

p. 68, ead p at top of diagram 
p. 74, note, for p. 159 read p. 105 

. 4 from foot, for radiations read radiation 
. 7 from foot, for was read were 
. 3, for amperes read ampere 

I7 for p. 94*M*p. 107. 
p. 99, delete C from middle of diagram and insert C at foot 

p. 114, 
P- 135. 

P. 143, 

PP- I5i 
P- I5i 
p. 160, 

p. 167, 
p. 170, 

. 4 from foot, for p. 57 read^. 44 

, i, insert (?) in middle of dotted line on the left 

, 12, for (Bismuth) read (? Bismuth) 

1 6, delete measured 

, 1 7, for calories per hour, read calories per gram per hour, 
153, headlines, for METHODS read CHANGES 
2 5> f or P- IO S reod P- 106 

18, for p. 52 read p. 53 

2 3> f or P- 5 read P. 5 1 

24, for not traceable read not wholly traceable 

i, for matter readt\\z uranium 

p. 184, col. 2, 1. 13, for 160 read 36 

,, col. 5, 1. ii to be one line lower, ranging with 3*86 (col. 4) 
,, col. I, 1. 15, for Mesothorium read Mesothorium * 
,, col. i, 1. 23, delete Thorium 




THE statement that radium is a bright shining metal > 
similar in appearance to other bright shining metals 
such as silver or platinum, except that it tarnishes 
instantly on exposure to air, while the salts of radium 
are white crystalline substances, scarcely to be dis- 
tinguished at a casual glance from common salt, 
seems to detract from the reputation of this wonderful 
element. The statement is nevertheless correct, and 
we must in consequence regard radium and its com- 
pounds more closely in order to find out with what 
properties they are endowed which have led to the 
foundation of a new branch of science radioactivity 
and have kept it in the public eye for a period of 
over fifteen years and for no mere nine days. 

The distinguishing property of radium and of a few 
other similar metals is the continual production of 
what are spoken of as radiations or rays. Sir Isaac 



Newton defined and used the word ray both in the 
sense of undulations or waves, and of corpuscles 
travelling in straight lines with considerable velocity. 
The rays which we 'have to consider are apparently 
of both kinds. There are three types of these rays. 
The first consists of particles of matter carrying an 
electric charge, the second of electric corpuscles, while 
the third type of radiation seems to consist of ethereal 
waves, strongly resembling those which after their 
discoverer are now universally known as Rontgen Rays. 

All three kinds of radiation are liberated with a 
velocity which appears to us enormous, and in some 
cases approximates to that of light itself. Yet the 
immensity of this latter figure, 186,000 miles a second, 
is largely a matter of the unit of comparison, since 
three years pass, nearly one hundred million seconds, 
before a wave of light travels to us from the nearest 
fixed star, 

The properties peculiar to radium are traceable to 
the actions of the radiations. Such properties are : 
action on photographic plates, power of producing 
beautiful scintillations and coloured phosphorescence 
with zinc blende and certain other minerals, power to 
bring about many chemical actions, continual and 
relatively considerable evolution of heat, and various 
beneficial therapeutic actions which have resulted 
from its application in medicine. 

We, must, therefore, study these radiations with 
'exactness, and must consider their mode of produc- 
tion. The substances producing them are known, 


from the results that they produce, as radioactive 
substances, that is, substances active in producing 
radiations (of the type described ; a lamp sends out 
rays, but it is not radioactive), while the science which 
treats of these is called radioactivity. The study of 
the production of the radiations is completely inter- 
locked with the study of the changes which take place 
in the atoms of the radioactive substances while the 
radiations are being emitted. And since the know- 
ledge we have acquired of these atoms has modified 
considerably the theory or theories (for there are 
several) of what an atom is, it is desirable before 
proceeding further to define exactly our notion of an 
atom, and also of an element. The historical develop- 
ment of the conceptions of atoms and elements is 
interesting. While the idea of an element reached its 
modern form only at the close of the seventeenth 
century, the atom wjiich Lucretius propounded, two 
thousand years ago, differed in little from the atom 
conceived by the majority of physicists before the 
discovery of radium at the close of the nineteenth 
century forced a newer conception upon them. 

Every old-world philosophy taught of some element 
or common principle of which Nature is formed. 
Certain sages held that this element was air, others 
that it was water, others earth. Empedokles enun- 
ciated four elements earth, air, fire and water 
incapable of being changed one into another, and 
forming all kinds of matter by intermixture in various 
proportions. Aristotle amplified the idea. He taught 


that there were four qualities hot and dry, cold and 
wet. Of these each element possessed two, and could 
by .adding or subtracting one of these be converted 
into another element. Thus, if fire, which is hot and 
dry, and in which heat predominates, is mixed with 
water, cold and wet, and in which wetness predomi- 
nates, there results air, hot and wet, in which wetness 
predominates. The Arab Geber (Abu-Moussah- 
Dschabir-Al-Sufi, 702-765) considered that the metals 
(of which several had already been prepared in a more 
or less pure condition) were compounds of sulphur and 
mercury, two more elements representing respectively 
the qualities of combustibility and volatility. Later, a 
third element, salt, was added, representing the quality 
of fixity in fire. In these philosophies matter was 
merely a formless void, upon which the qualities 
designated elements could be impressed. 

The modern definition of an element, as a substance 
which hitherto has resisted all efforts to resolve it into 
two or more dissimilar substances, is due to Robert 
Boyle (1626-1691), and he also distinguished between 
elements, and compounds compounded of more than 
one element. 

Through, all the ages matter was conceived as made 

o o 

up of minute particles or atoms in ceaseless motion. 
The Greeks learned this from Anaxagoras, Leukippos, 
Demokritos ; Lucretius taught it to the Romans. 
Then, as now, it was believed that atoms varied in 
size and weight, the smaller atoms being lighter ; they 
were impenetrable, so that no two could occupy the 


same space. Anaxagoras taught that the living body 
is built up of atoms derived from its food, that plants 
were living bodies, also composed of atoms, while 
every atom itself was a world in miniature. These 
wonderfully shrewd inductive guesses were rewarded 
by an accusation of sacrilege, and his life was 
preserved only by a hurried flight. 

The idea of minute concrete corpuscles or atoms 
was almost lost sight of in the Middle Ages, but was 
revived by Boyle and developed by Dalton. By the 
end of the eighteenth century observational science 
was becoming exact, and experimental science was 
commencing. Numerous elements were known and 
their properties recognised. Black had shown that 
a gas was not necessarily air, or even a species of 
air ; air was recognised to consist of nitrogen and 
oxygen, while Cavendish had even obtained, after 
removing the nitrogen and oxygen, a residue of one 
per cent., shown, a hundred years later (by Rayleigh 
and Ramsay), to be a rare element, argon. The same 
acute observer had proved that water was a combina- 
tion of hydrogen and oxygen. Thanks to Lavoisier, the 
principle of combustion was understood ; combustion 
was combination with oxygen. 

John Dalton (1766-1844), in a study of certain 
gaseous compounds, especially those of hydrogen 
with carbon, and of nitrogen with oxygen, found that 
these elements combined according to certain definite 
laws, that when the same two elements combined 
to form more than one compound, i. e. combined in 


more than one proportion, for a fixed amount 
of one element there is always a simple ratio 
between the amounts of the other element. He 
published a suggestion in 1807 that the regularities 
could be satisfactorily explained by the hypothesis 
that matter is composed of atoms, having sizes 
and weights differing with each element, but of 
identical size and weight for any particular element. 
From this idea of definite weight sprang that of the 
atomic weight of the element. Dalton's theory ulti- 
mately prevailed, and was universally accepted by 
the beginning of the second half of the nineteenth 
century. It was amplified, however, by the conception 
of the molecule, the smallest concrete particle of a 
substance which can separately exist, consisting of 
one or more like or unlike atoms. Thus a molecule 
of sodium chloride (table salt) consists of an atom of 
sodium united to an atom of chlorine ; a molecule of 
hydrogen, of two atoms of hydrogen joined together. 

The properties of the elements may be considered 
as the sum of the properties of the atoms in bulk, and 
the properties of the atoms individually. The former 
are the ordinary so-called physical properties of the 
elements, appearance, density, melting-point, and soon. 
The latter are more important for our present subject. 

The first property to be considered is that of weight. 
We cannot at present, and indeed can never, hope to 
weigh single atoms, but we can by various contriv- 
ances compare the weights of equal large numbers of 
atoms of different elements, and from these deter- 


minations we obtain certain definite ratios. At first 
the atom of hydrogen was taken as unit and its 
weight taken as i. That of oxygen was found to be 
15-88; of nitrogen 13*95, and so on. Since many 
more elements combine with oxygen than with 
hydrogen, and since the combining weights are usually 
determined in order to find the atomic weights, it has 
been found convenient to choose oxygen as the unit 
and to fix its atomic weight arbitrarily as i6'OO. It 
has been found that all the known elements have 
different atomic weights, and when a new element 
has been discovered, one of the most important 
proofs of its individuality is the determination of its 
atomic weight, which should be peculiar to itself. 

The second atomic property which we must consider 
is that of valency. It has been found that an atom of 
one element, when it is in combination, is combined 
with a definite number of atoms of a second element, 
and this number determines the valency or combining 
capacity of the element. Hydrogen is taken as 
standard, and has a valency of one ; it is univalent. 
A molecule of water is found to consist of two atoms 
of hydrogen (symbol H) united to one atom of oxygen 
(symbol 0). Oxygen has, therefore, a valency of two ; 
is divalent. The molecule of ammonia (formula NH 3 ) 
contains one atom of nitrogen (symbol N) united to 
three atoms of hydrogen. Nitrogen is, therefore, 
trivalent. Hydrochloric acid (formula HC1) has in 
its molecule an atom of chlorine (symbol Cl) united 
to one of hydrogen. Chlorine is univalent. Common 


salt, sodium chloride, has the formula NaCl, so that 
an atom of sodium replaces one of hydrogen. Sodium 
is also univalent. Lime, calcium oxide, CaO, has one 
atom of calcium combined with one of oxygen, a 
divalent element ; calcium is also divalent. Similarly 
by determining the composition of its compounds, 
the valency of any element can be determined. 

When the elements are arranged in the order of 
their atomic weights, it is found that at regular 
intervals similar elements recur. Each interval com- 
prises eight elements ; the first of one interval closely 
resembles the first of the next, separated from it by 
seven elements, and even more closely the first of the 
succeeding interval, separated by fifteen elements. 
In this way a periodic classification is obtained with 
groups of elements showing very similar physical and 
chemical properties. This arrangement is due to 
Mendeleeff. Thus sodium, potassium, rubidium and 
caesium form one such group ; copper, silver and gold 
a second ; calcium, strontium and barium a third. It 
is found that radium belongs to this third group, 
which is commonly known as the group of the 
alkaline earth metals, since their oxides arc the 
alkaline earths, lime, strontia and baryta. 

The last atomic property which must here be con- 
sidered is that which causes every element to have a 
different spectrum. Ordinary white light is produced 
by incandescence. When any solid or fluid is heated 
to a sufficiently high temperature, it vibrates strongly, 
and the vibrations are communicated to the surround- 


ing ether the elastic medium filling all space in 
the form of waves moving with tremendous velocity 
When these waves strike the retina of the eye they 
produce the impression called light. These waves 
are not all of the same length. The longer waves 
correspond to lower temperatures, and produce red 
light. Higher temperatures produce shorter waves. 
Certain of these shorter waves produce the impression 
of violet light. The light produced by waves shorter 
than these is invisible, and is known as ultraviolet 
light. Between red and violet the colours follow in 
the order exhibited by the rainbow. A mixture of 
all these colours gives the impression of white light. 
If a beam of white light is passed through a triangular 
piece of glass or quartz, it undergoes two successive 
processes of refraction, and emerges from the glass 
broken up into its components, and as a much wider 
beam a spectrum of colours. The rainbow effect is 
due to a similar action of small spheres of water (the 
raindrops) on sunlight. 

Sunlight gives an almost continuous spectrum. All 
the known elements are incandescent in the sun, and 
are sending out vibrations. When a single element 
is raised to incandescence, and the waves of light 
which it emits are examined, they are found to give, 
not a continuous spectrum, but either certain sharp 
lines, or bands, or patches of colour. Thus, sodium 
chloride, or any other salt of sodium, thrown on 
the flame, imparts to it a brilliant yellow colour. 
Seen through the spectroscope (the instrument for 


examining spectra) this is resofved into one sharp 
yellow line, having a definite position in the spectrum. 
The salts of potassium, on the other hand, give a 
spectrum consisting of a strong red and a strong 
violet line. Calcium salts impart a red colour to the 
non-luminous flame of a Bunsen burner. The spectrum 
of calcium consists of certain strong red and green 
lines, that of barium almost wholly of green lines. 
Each of these lines corresponds to a certain particular 
length of wave sent out by a particular kind of vibra- 
tion in the atoms of the element under examination. 
The atom of any one element is different from that of 
any other. Hence each element excites waves of differ- 
ent lengths, and has a spectrum peculiar to itself. None 
of the spectral lines from different elements coincide. 
The spectroscope allows us to pick outlines of known 
wave-length, and the position of other lines with 
reference to these standard lines permits the wave- 
lengths of the former to be calculated ; if these lines 
are measured in this way, we can determine what 
element has produced them ; and when we find lines 
not previously known, we are examining, in all 
probability, an element not previously discovered. 
One of the most important tests to be applied to a 
supposed new element is the inquiry whether it gives 
a distinct and new spectrum. 



THE initial observations of radioactivity were made 
by the use of photographic methods. The ordinary 
photographic plate consists of a layer of silver salt 
dissolved in gelatine and spread equally over a glass 
surface. Certain silver salts are remarkably sensitive 
to the action of light, and after exposure and sub- 
sequent treatment by suitable chemicals are rendered 
insoluble, with the result that a photographic image 
is developed. A layer of black paper serves to guard 
unexposed plates from the action of light ; light rays, 
therefore, do not penetrate this layer. 

It has been found that ordinary plates are affected 
to the greatest extent by light of short wave-length 
(violet and ultraviolet light), while light of longer wave- 
length has less action, so that the process of develop- 
ment can be carried out in red light. One of the 
properties of the Rontgen or X-rays (which are also 
waves of very short length) which excited greatest 
interest was that by which they were able to penetrate 
matter to a very marked degree. A photographic 
plate covered with black paper and exposed to X-rays 
B 17 


was fogged just as if it had been exposed to light with- 
out its protecting cover. Where the rays encountered 
metallic or other dense material, they were stopped 
in greater or less degree, with the result that a shadow 
image of this material was produced on the plate. 

In the earlier X-ray tubes the X-rays were pro- 
duced by the action of certain rays from the cathode 
in the tube impinging on the glass, and at the same 
time the glass was observed to phosphoresce. The 
phenomenon was not at first understood, and the 
production of rays was thought to be connected with 
the phosphorescence. It was known that various 
substances phosphoresced under the influence of 
light, especially under that of blue or violet light. 
These substances were examined to see if when 
phosphorescing they would also send out rays 
capable of penetrating black paper and acting on a 
photographic plate. Different observers stated that 
zinc sulphide and calcium sulphide, when phos- 
phorescent, possessed this property. This has not 
been confirmed by later experiments. Certain 
uranium salts had long been known to phosphoresce 
under the influence of light. These were tested by 
Henri Becquerel. He found that potassium uranium 
sulphate emitted rays that affected a photographic 
plate. This he attributed to its phosphorescence. 
But further experiments showed that the photo- 
graphic and phosphorescent properties were not con- 
nected with one another. He was able to show that 
all uranium salts, whether phosphorescent or not, and 


metallic uranium, which does not phosphoresce^ emit 
rays which fog a photographic plate. These experi- 
ments were carried out in a very simple manner. An 
unexposed plate was covered with black paper and 
the uranium salt laid on it. It was then allowed to 
remain in a dark chamber for one or more days, and 
the plate was then developed in the ordinary manner. 
An image of the uranium salt was in all cases found. 
From the time necessary to give an image of 
approximately the same depth, Becquerel was able 
to put forward the theory that the activity of the 
material was proportional to tJie amount of uranium it 
contained, and that, therefore, the phenomenon was an 
atomic property of uranium. 

Minerals containing uranium are also able to pro- 
duce the same photographic effect. One of the most 
important of these minerals is pitchblende, of which 
about seventy-five per cent, consists of an oxide of 
uranium having the formula U 3 O 8 . The first figure 
shows the photographic effect of a mineral containing 
a vein of pitchblende. The mineral surrounding the 
vein contained no uranium and hence had no effect. 1 

The photographic method showed us that uranium 
salts were able to give out a continuous supply of 
rays capable of a photographic effect, but it was not 
precise enough to give any accurate quantitative data. 
To obtain these, various electrical methods have been 
employed, which depend on the fact that Rontgen or 

1 Fig. i is reproduced from Madame Curie's Traite de 
Radioactivite, by kind permission. 
B 2 



X-rays and Becqucrel rays (as the* rays from uranium 
salts arc called) make air a conductor of electricity 
an infinitely weaker conductor, it is true, than the 
metal wires which carry our supply of electricity for 

FIG. i. 

lighting or heating, but a very much better conductor 
than air in its normal condition, when it is one of the 
best non-conductors of electricity known. The rays 
when traversing air or any other gas produce in it 
particles which are called ions and which are elec- 


trically charged. A detailed account of the pro- 
duction of these ions is given in Professor Pellat's 
The New State of Matter, one of the volumes of 
this series. 

A short account only can here be given. A 
salt such as sodium chloride when in the solid 
condition must be considered as made up of aggre- 
gates of molecules, each molecule having the compo- 
sition NaCl. When it is dissolved in water these 
molecules are separated from each other by molecules 
of water (water in the liquid condition is a mixture of 
H 2 O, H 4 O 2 , H 6 (X), and move backwards and forwards 
amidst these in a manner similar to, but much slower 
than that in which the molecules of oxygen move to 
and fro amidst those of nitrogen in ordinary air. By 
far the greater part of the sodium chloride, however, 
does not exist in solution in water as molecules, since, 
as soon as solution takes place, there is a condition 
of strain produced and the molecules are dissociated 
into atoms not ordinary atoms, but atoms carrying a 
charge of electricity. Each chlorine atom has attached 
to it a negative charge of electricity or an electron. 
The electrons are most simply regarded as atoms of 
electricity (although their precise nature is not yet 
determined with certainty), which have a mass about 
one seventeen-hundredth of that of the hydrogen 
atom (an atomic weight, therefore, of about crooo6). 
The electron attached to the chlorine atom has been 
obtained from the corresponding sodium atom which 
has, therefore, gained a positive charge of electricity. 


Such charged atoms have the property of attracting 
round themselves neutral molecules of water from 
the surrounding medium, and with these clusters form 
the ions. Therefore actually, of a solution of sodium 
chloride, the greater portion consists of unelectrified 
molecules of water, a smaller portion of equal numbers 
of atoms of sodium and chlorine, carrying unlike 
electric charges and each surrounded by a cluster 
of twenty or thirty molecules of water, and a much 
smaller number of undissociated and unelectrified 
molecules of sodium chloride. 

Ordinary gases or mixtures of gases, such as air, 
contain scarcely any ions. Under the influence of 
X-rays, or other rays of short wave-length, the number 
is increased very considerably. The same effect is 
produced when the gas is in contact with a uranium 
salt. The rays, whatever their nature, affect the 
molecules of gas in such a way that they lose 
electrons, and so become positively charged. These 
positively charged particles attract round themselves 
clusters of neutral molecules in exactly similar fashion 
to the electrically charged atoms in solution, and so 
form positive ions. The electrons which have been 
set free also attract to themselves a cluster of neutral 
molecules, so forming negative ions. The ions move 
freely in the gas space, behaving like very large gas 
molecules, until by the collision of clusters carrying 
unlike electric charges, these charges neutralise each 
other, or, in other words, the positive particle regains 
its electron, and the clysters are again resolved into 


simple gaseous molecules. This process is accelerated 
by the attraction which unlike charges of electricity 
exert on each other (on the other hand charges of 
electricity of like sign are found to repel each other). 

A disc of metal, charged with electricity, and 
insulated from its surroundings by glass or some 
other good non-conductor, will retain its electric 
charge for a considerable time, provided that the air 
surrounding it is perfectly dry. If the air, however, 
has been ionised from any source, the charge of 
electricity is quickly lost. The explanation is simple. 
If, for example, the disc is charged negatively, the 
metallic surface will attract all the positive ions in 
its neighbourhood, and these will neutralise its own 
electricity until the disc is completely discharged. 
This phenomenon affords a means of ascertaining 
whether ions are present, and further, by a measure- 
ment of the rate of discharge, the degree of ionisation 
can be found. Two instruments are used for accurate 
measurements of this kind : the electroscope, and the 

The gold-leaf electroscope has been used for elec- 
trical measurements for very many years. A simple 
form is shown in Fig. 2. It consists essentially of 
two strips of gold leaf, Z, Z 1 , hung parallel to one 
another from a brass bar, B, which connects them to 
a brass disc, D. The whole instrument is usually 
insulated from all sources of electricity, and especially 
from the earth, by supporting it in the neck of a glass 
bottle. When a glass rod is rubbed with catskin it 


acquires a positive charge. If it is then held near 
the brass disc the gold leaves are observed to move 
apart ; the degree of repulsion is proportional to the 
strength of the charge on the glass rod. The action 

FIG. 2. 

is shown diagrammatically in Fig. 3. The positive 
charge on the glass rod attracts a negative charge to 
the disc, and the positive charge which is at the same 
time liberated is repelled to the leaves. Each leaf 
acquires a charge of like kind ; they, therefore, repulse 


each other, and the repulsion is sufficient to cause the 
leaves to move apart. If now the disc is connected 
to earth (i. e. to a very large electrical capacity) while 
the glass rod is still near, the charge on the leaves 
is neutralised by an equal negative charge from the 
earth's surface (causing them to fall together), but 
the charge on the disc is held by the attraction of 
that on the rod. On removing the earth-connection 
and then the glass rod, the negative charge is left on 
the insulated system, and distributes itself over it, so 
that the leaves again receive a charge of like sign, 
and again diverge. If the air surrounding the instru- 
ment is dry, the leaves fall together very slowly. 
If, however, an ionising source is near, the leaves fall 
together rapidly. If a uranium preparation is placed 
inside the bo'.tle or close to the disc it will be found 
difficult to charge the electroscope, and it will be 
discharged very rapidly. 

The electroscope has been modified for radioactive 
purposes, although the principle remains the same. 
-Various forms have been constructed. That used by 
Pierre Curie in his radioactive measurements is shown 
in Fig. 4. A metal plate, A, has adhering to its 
upper end an easily movable metal leaf, B, also 
hanging vertically. The plate is held in position by 
a metal rod, C, which penetrates a cork suspended 
from the upper surface of the metallic containing 
vessel. The rod is terminated outside the vessel by 
a disc which serves to charge the system, and which 
is protected by a metal cap. A second rod, joined to 


the first, and at right angles to it, carries a metal 
plate, D. The whole system is completely insulated. 
Opposite D is a similar plate, E, supported by the 
containing vessel, and together with it, connected to 
earth so that it is always at zero potential. The part 
of the vessel containing D and E is known as the 
ionisating chamber, and E serves to support the radio- 
active substance. When the electroscope is charged 
at F, an electric field is established between D and 
E ; an electric stress is set up in the atmospheric 
medium between the two plates. If a radioactive 
substance is present the electroscope is steadily dis- 
charged, and the movable leaf steadily approaches 
the fixed plate. The movement of the leaf is observed 
through a microscope of small magnification, furnished 
with a scale, and the rapidity with which the leaf is 
seen to move over the scale allows a relative measure- 
ment of the degree of activity to which it is, roughly 
speaking, inversely proportional. 

By far the larger number of radioactive measure- 
ments are made with some form of quadrant electro- 
meter. This instrument was invented by Lord Kelvin, 
and has been considerably modified and improved 
by Curie, Dolezalek, and others. It consists essentially 
of a light metallic needle, A, Fig. 5, suspended 
by a very fine thread so that it moves very easily, and 
rotating inside four hollow quadrants,/?; these are 
arranged round it concentrically, and it is carefully 
insulated from them. The needle is connected to a 
powerful battery, from which it receives a strong 




charge of electricity ; it supports a small mirror, 
which serves to reflect a scale, by which means 



its movements can be observed by a telescope. 
Opposite quadrants are metallically connected. One 
pair is connected to earth, so that its electric potential 


is always zero. The second is connected to a metallic 
plate, C, and this part of the surface can be insulated 
at will. Opposite C is a similar plate, D, which 
carries the radioactive material, and is connected to a 
second battery, which keeps it strongly charged. If 
it is thus given a strong positive charge, all the ions 
of like charge, all the positive ions are immediately 
repelled towards the plate C. On removing the earth 
connection from the quadrants joined to C, this 
system commences to be charged electrically by these 
positive ions repelled from D. An electric stress is 
set up between the quadrants, and this affects the 
charge of the needle, causing the latter to move. The 
rate of movement can be measured by observing 
through a telescope the rate at which an image of a 
scale moves in the mirror. This rate is proportional 
to the rate at which the quadrants are being charged, 
and this to the magnitude of the electric current 
between C and D. This is proportional to the degree 
of activity of the substance under examination, so 
that the relative activities of two substances can be 

The relative activity of the salts of uranium was 
measured by Madame Curie by the electrometric 
method, and her results are shown in the follow- 
ing table. In each case the area of surface and 
thickness of the material used was approximately 


Metallic uranium (containing 

carbon) .... (?) 2'3 x io~ H 

Black uranium oxide . . 86 2*6 

Green uranium oxide 85 rS 

Hydrated uranic acid . . o'6 

Sodium uranate 75 1*2 

Potassium uranate ... 71 1*2 

Ammonium uranate 76 i'3 

Uranous sulphate . . . 41 07 

Potassium uranic sulphate . 41 07 

Uranic nitrate ... 47 07 

Copper uranium phosphate . 48 o'9 

Uranium oxysulphide 79 i'2 

It will be seen that the activity, which is pro- 
portional to the current produced, increases with the 
amount of uranium present. By these and similar 
experiments it has, therefore, been confirmed that 
uranium continuously emits radiations which appear 
to have many of the properties that are possessed by 
the Rontgen or X-rays, and which, to distinguish 
them from X-rays, were called Becquerel rays after 
their discoverer. 



IN the previous chapter a description has been 
given of the manner in which Becquerel discovered 
that the element uranium, and also its compounds 
with other elements, were active in sending out certain 
kinds of rays, rays which would affect a photographic 
plate, and which ionised gases. The results of Madame 
Curie have been quoted, and these pointed to the con- 
clusion that the activity was an atomic property of 
uranium itself. Madame Curie proceeded to examine 
all the other known elements, and as many of their 
compounds as were available, in order to find whether 
any of these possessed similar properties. The com- 
pounds of thorium were found to be active, but not 
those of any other element ; the activity of thorium 
was discovered independently by Schmidt and by 
Madame Curie in 1898. The rays emitted seemed to 
have all the properties of those from uranium ; they 
also were Becquerel rays. 

A number of minerals were also examined with the 
electrometer in the manner described already. All 
those found to be active were minerals containing 
either uranium or thorium, but in several cases 

3 1 


their activity was several times greater than that of 
the uranium or thorium contained. This anomaly is 
illustrated in the table following, containing the 

o * o 

results for comparable quantities of a number of 
uranium minerals- 
Current in amperes. 

Uranium 2'3 x io- n 

Pitchblende (from Johanngeorgenstadt) 8'3 

,, ( Joachimsthal) . . 7'o 

( ,, Pzibran) . . .6*5 

( Cornwall) . . . I'6 

Cleveite . . . . . .1*4 

Chalcolite . . . . . 5' 2 

Autunitc . . . . . . . 27 

The excess of activity is seen to be very con- 
siderable with the first three pitchblendes and the 
chalcolite. The latter is a naturally occurring copper 
uranium phosphate ; it can be made in the laboratory 
without difficulty. An artificial chalcolite was made 
and tested. Its activity was O'9 X io- n , a normal 
figure. These results suggested the presence of a 
strongly active element in the pitchblendes, and the 
natural chalcolite, an element hitherto undiscovered. 
An attempt was made to isolate this element. Pitch- 
blende was selected, as probably containing it in 
largest amount, and the mineral was subjected to a 
systematic chemical analysis, the property of activity 
affording a new test. 

Pitchblende consists of about seventy-five per cent, 
of uranium oxide, U 3 O 8 , small quantities of lead, 
sulphide, of calcium, iron, and magnesium, present 


chiefly as silicates, and traces of a large number of 
other elements, and even of the rare gases argon and 
helium. The pitchblende was treated chemically and 
obtained in solution and its various constituents 
separated. The activity was largely concentrated in 
two fractions, that containing barium and calcium, 
which had been precipitated as sulphates, and that 
containing bismuth. The ordinary salts of barium 
(or calcium) and of bismuth are inactive. Thus the 
results seemed to show the presence of two new 
elements, with chemical properties similar respectively 
to barium and to bismuth. The two fractions were 
subjected to a rigid examination, in order if possible 
to separate the active from the inactive constituents. 
It was found that by sublimation of bismuth sulphide 
in a vacuum, the first part which distils over, that is, 
the more volatile part, is the more active. When a 
solution of the bismuth fraction is made in nitric acid, 
and considerable excess of water added, part of the 
bismuth is precipitated as insoluble bismuth hydroxide, 
and the precipitate contains most of the activity. A 
strongly acid solution of the fraction in hydrochloric 
acid, when treated with sulphuretted hydrogen, is 
partly precipitated as sulphide, and the precipitate 
contains most of the active constituent. Thus by 
alternating chemical treatment with measurement of 
the degree of activity, clues were obtained to the kind 
of treatment which would effect the required separa- 
tion, and by repetition of one or more of these chemi- 
cal processes Madame Curie succeeded in partially 


concentrating the new element, an element which 
strongly resembled bismuth in all respects chemically, 
but had the additional property of being radioactive. 
The discovery of this new element Polonium (so 
named after her native land) was announced in July 
1898. Attempts to purify it further were not made 
until a much later date, and even at the present time 
neither the element nor its salts have been isolated in 
a perfectly pure condition. 

The method adopted with the barium fraction was 
to recrystallise fractionally the barium chloride salt. 
The part which crystallised first, the less soluble 
portion, was found to be the more active portion (and, 
therefore, to discharge an electroscope more quickly). 
By repeated recrystallisations a fraction was obtained, 
which, weight for weight, was sixty times more active 
than uranium oxide, which is usually taken as 
standard. This fraction was examined in the spectro- 
scope, and a new spectral line was discovered. Re- 
crystallisation was continued, and finally a fraction 
nine hundred times more active was obtained. The 
spectral line already noted appeared much more 
strongly, and in addition three rrew lines were seen- 
There was, therefore, strong evidence that a new 
element was present, and the discovery of radium, 
as this new element was named, was announced in 
December 1898. 

It was evident that radium and polonium were 
present in pitchblende only in the minutest quantities, 
and that it would be necessary to work with large 


masses of pitchblende in order to isolate the pure 
elements. Several tons of residues of pitchblende 
from which the uranium had been removed were 
passed through processes precisely similar to those 
described, the work occupying many months, and 
finally a fraction of a gram of pure radium chloride 
was obtained. 1 The activity of this fraction was 
nearly two million times the activity of the standard. 
The ordinary methods of comparing activity failed 
before this stage was reached, and other methods had 
to be employed finally in order to test the purity of 
the material. It was noticed, for example, that a 
strongly active preparation was colourless when 
freshly prepared, but on standing a few hours became 
yellow, then orange, and finally even rose-pink. The 
purer the substance (i. e. the less barium was present) 
the longer time was required to bring about this 
colour change and the pure radium salt remained 
uncoloured. The final tests of purity employed were 
examination of the spectrum and determination of the 
atomic weight. As the activity became greater the 
amount of barium present decreased, the spectral lines 
of barium slowly became weaker, until with the final 
fraction the strongest barium lines were barely visible. 
The atomic weight of barium is 137*4. Atomic 
weights of the various fractions were determined. 
The following table, from Madame Curie's Traite de 
Radioactivite, shows how the apparent atomic weight 

1 A description of recent modified methods of obtaining pure 
radium salts is given in Appendix A. 
C 2 


of the fraction increased as the spectrum of barium 
became fainter 

Activity. " Atomic'Weight. 

The spectrum of radium is very feeble. 

7500 H5'8 The spectrum of radium is strong, but is 

dominated by that of barium. 
'173-8 The two spectra have an almost equal 


223 Only the three strongest rays of barium 

Qr.i are visible, but their intensity is strong. 

, ie j 225-3 The same three rays are barely visible (an 

01 6 er "] experiment made with about cri gram 

of chloride). 

! 226-45 The strongest ray of barium is very feeble 
(an experiment made with about 0*4 
^ gram of chloride). 

The figure 225*3 was first put forward by Madame 
Curie as the atomic weight of radium. Later, when 
a larger amount of material was available, and conse- 
quently a more accurate measurement could be made, 
the figure 226^45 was obtained. The^method used to 
obtain this result was to weigh accurately a quantity 
of radium chloride, to dissolve it in pure water, and 
to add to the solution a solution of silver nitrate. 
The interaction produced silver chloride, a substance 
insoluble in water; this was accordingly precipi- 
tated. The precipitate was dried and weighed, and 
from the relative weights of silver chloride and radium 
chloride and the known atomic weights of silver and 
chlorine, the atomic weight of the radium was 
calculated. Successive determinations with 0*4 gram 
of pure material gave the figures 226-62, 22631, 


226*42, the average figure being that already stated, 
226-45. Sir Edward Thorpe has carried out a similar 
series of determinations with a quantity of radium 
chloride less than O'l gram, which had been purified 
in the same way. Three determinations gave the 
results 226'8, 2257, 2277, the mean of which is 2267, 
a figure in very close agreement with that of Madame 
Curie, so that it can be stated with considerable 
certainty that the atomic weight of radium] (symbol 
Ha) is 226'S. 1 

The chloride, bromide, nitrate, carbonate, and 
sulphate of radium have been prepared. They are 
all white salts. The first three are soluble in water, 
the two latter insoluble. When^the smallest trace of 
any of these salts is placed in the flame of a Bunsen 
burner, it imparts to the flame a beautiful crimson 
colour, very similar to that produced by strontium 
salts (the latter are used in pyrotechnics to produce 
such coloured flames). In their chemical properties 

Two recent series of determinations of the atomic weight 
of radium have been published. Honigschmid, using a gram 
of radium chloride, obtained the figure 225'95, and further 
recrystallisations of the salt did not alter this figure. 

Ramsay and Gray have used a method different from those 
previously employed. Radium chloride was transformed into 
radium bromide by the passage over it of hydrobromic acid, 
and the bromide retransformed into the chloride by similar treat- 
ment with gaseous hydrochloric acid, and the two weight-ratios 
compared. Actual determinations were made with only 2 or 3 
milligrams of salt, but the Steele type of micro-balance employed 
(see p. 60) admits great accuracy, and the figure they obtained, 
226'36, strongly supports Mme. Curie's figure, and is at variance 
with that of Honigschmid. 


radium salts closely resemble the corresponding salts 
of barium. Thus, for example, although the chloride 
has a different solubility from barium chloride is 
slightly less soluble the difference of solubility is so 
slight that some hundreds, even thousands, of crystal- 
lisations are necessary to effect a complete separation. 
Barium sulphate is one of the most insoluble sub- 
stances known ; radium sulphate is even more in- 
soluble. The formulae of these salts are, in the order 
named : RaCl,, RaBr 2 , Ra(NO 3 ),, RaCO 3 , RaSO 4 . 

These radium salts at present are sold at extremely 
high prices ; one milligram of radium chloride costs 
20. This immense value is obviously due, not only 
to the minute quantities in existence, but also to the 
prolonged work necessary in order to isolate the pure 
material. Owing to this fact, once the salt has been 
purified, further chemical manipulation is reduced to 
a minimum, and processes involving risk of even 
slight loss, such as the separation of the pure metal, 
have been avoided as far as possible. The pure 
metal has at length, however, been prepared, and it 
was fitting that this final step should have taken place 
in Madame Curie's laboratory (by Madame Curie and 
M. Debierne). A solution containing O'io6 gram of 
perfectly pure radium chloride dissolved in water was 
electrolysed between a cathode of mercury and an 
anode of platinum-iridium alloy. After electrolysis 
the solution contained only 0^0085 gram of radium 
salt, the difference, 0*0975 g ram > having been electro- 
lysed into radium and chlorine, and the radium 


having united with the mercury to form an amalgam. 
This amalgam was quite liquid, while the correspond- 
ing amalgam with the same quantity of barium is 
partly crystalline. The dried amalgam was trans- 
ferred to an iron boat ; this was placed inside a quartz 
tube ; the air contained in the tube was replaced 
by a current of hydrogen, and then the whole was 
cautiously heated. Most of the mercury distilled 
away at 270 Centigrade. At 400 C. the amalgam be- 
came solid. The temperature rose slowly to 700 C., at 
which temperature mercury had wholly distilled away 
and the radium itself commenced to volatilise and to 
attack the quartz containing-vessel. The boat now 
contained practically pure metallic radium, a brilliant 
white metal with a melting-point slightly above 
700 C. It blackened immediately on exposure to 
air, probably uniting with nitrogen to form radium 
nitride. A particle of the metal falling on white 
paper produced a mark on it analogous to a burn ; 
the metal reacted energetically with water, liberating 
hydrogen, and dissolving, so that the radium oxide 
formed by the interaction was evidently soluble in 
water. The element showed, as was to be expected, 
all the radioactive properties of its compounds. 
Metallic barium is also a brilliant white substance 
which dissolves in water with liberation of hydrogen, 
so that the parallelism already pointed out in regard 
to the salts holds for the elements themselves. 

The compounds of radium show, on the other hand, 
numerous properties which are not possessed by the 


corresponding salts of barium. They emit continuously 
and spontaneously a considerable quantity of heat, a 
quantity sufficient to keep themselves, and the vessel 
containing them, several degrees hotter than the 
surrounding atmosphere. They are spontaneously and 
continuously luminous^ whether kept in the dark or in 
sunlight. They bring about a number of chemical 
actions. Thus when in contact with air or with pure 
oxygen, they produce ozone continuously ; in contact 
with ammonia, this gas is decomposed into nitrogen 
and hydrogen. Sealed tubes of thin glass containing 
small quantities of radium salt decompose water in 
which they are placed into hydrogen peroxide, 
hydrogen, and oxygen ; they liberate iodine from iodo- 
form and from iodic acid, cause hydrogen and chlorine 
to combine slowly, decompose nitric acid, cause yellow 
phosphorus to change into the red modification. Pre- 
cious stones such as uncoloured rubies ; when kept near 
such tubes become coloured, in some cases brown, in 
others pink. Sir William Crookes exposed a yellow 
diamond to these conditions for seventy-eight days, 
at the end of which time the colour had become 
bluish green. The glass vessel containing the salts 
invariably becomes coloured ; the colour depends on 
the nature of the glass. Ordinary soda glass is 
rapidly coloured a dark purple, potash glass brown, 
while gold glass is coloured ruby. These colours are 
permanent under ordinary conditions, but disappear 
when the substances are heated to above 300 C. 
The chemical effects are produced in most cases 


when the radium salt is sealed up within a glass 
tube, so that they must be attributable to something 
which passes through the glass. Photographic and 
ionising effects are also produced by such sealed 
tubes of radium, so that the active rays can evidently 
pass through glass ; this leads to the supposition that 
the chemical effects are also due to the radiations, and 
that in all probability all the other abnormal effects 
of radium salts are traceable to the same source. 

One of the most marked phenomena is the pro- 
duction of phosphorescence. Zinc sulphide, a white 
substance, phosphoresces continuously .when placed 
near a small quantity of radium salt. When exam- 
ined under a microscope it appears to emit continuous 
flashes of light. The property has been taken 
advantage of in the instrument known as Crookes's 
Spinthariscope. The platinocyanides give beautiful 
colour effects when subjected to the influence of the 
radiations. Lithium platinocyanide shows a pink 
phosphorescence, the barium and calcium salts a deep 
green, the sodium salt a yellow colour. The same 
phosphorescent effect is produced whether the radium 
salt is uncovered, or is protected by a thin screen of 
glass or mica. A thick glass covering prevents the 
phosphorescence. This again suggests that the effect 
is due to the radiations, to something given off from 
the radium. 

If a current of air is passed over a uranium salt, or 
bubbled through a solution of the salt, the air current 
is ionised, and is capable of discharging an electro- 


scope with which it comes in contact, even at a con- 
siderable distance from the uranium salt, the source of 
ionisation. But it the air current after being ionised 
in this way is caused to pass through a filter of cotton- 
wool, the filter abstracts all the ions, and ordinary 
un-ionised air passes through, which is as usual with- 
out effect on an electroscope. If the salt of uranium 
be replaced by a solution of radium salt, the filter no 
longer prevents the discharge of the electroscope ; as 
all the ions are removed by the filter something must 
have passed through capable of producing fresh ions, 
a gas, mixed with the air. This gas is not by chance 
occluded in the salt solution, since the effect is con- 
tinuous ; a continual stream of air, de-ionised by the 
filter, will produce fresh ions for an indefinite period. 
Therefore not only is a gas given off by the radium salt 
solution, but this gas is also radioactive. This gas, still 
known generally as the emanation from radium, or 
radium emanation, from its method of production, has 
recently been shown to be a true element with a 
definite spectrum and a specific atomic weight, and 
has been named niton. 

A large number of the properties ot niton were 
determined before the gas had been obtained in a 
state of purity, indeed while it was present mixed 
with many thousand times its volume of air. The 
amount obtainable from a gram of radium per day is 
less than a cubic millimetre, /. e. less than the volume 
of a large pin's head. All our early information 
about the gas was determined from its radioactive 


properties, and largely from the study of its rate of 
decay (an expression which will be explained im- 
mediately). Measurements of the activity and rate 
of decay were made, and it was shown that these 
were the same before and after the various tests 
which were applied. It was proved in this way that 
niton was an inert gas which not only would not enter 
into combination with any other element, but was un- 
acted upon by the extremes of heat and cold to which 
it could be subjected, was unaltered by the strongest 
chemical reagents, and could not be broken up into 
any different elements, known or unknown, by any 
chemical treatment whatsoever. These facts strongly 
supported the theory that the emanation from radium 
was itself an element, and further a member of the 
inert series of gases of which argon and helium are 
the best known and the earliest discovered. 

When comparable measurements are made over a 
number of days of the total activity produced by a 
definite quantity of emanation this can be done by 
enclosing it, mixed with air, in a thin-walled glass 
vessel, and introducing this into an electroscope from 
time to time it is found that while equal quantities 
of emanation, freshly drawn from a radium solution, 
have equal activities within the limits of error 01 
measurement, the same .quantity rapidly changes in 
activity, becomes in fact less active, so that in a period 
of about four days it is only half as active as at first, 
in a period of eight days it is only a quarter as active, 
in a period of twelve days one-eighth as active, and 


so on. At the end of a month the activity has almost 
totally disappeared, and can only be detected by the 
most careful measurement. As one of the distin- 
guishing features of the gas is its activity, and since 
it is owing to this that it can be recognised when 
diluted with other gases, it must be concluded that 
the emanation has disappeared, decomposed, changed 
into something else. A definite quantity of niton has 
apparently a total existence, when separated from 
radium, of about one month. It is decaying all the 
time, and its rate of decay is such that one half of it 
has disappeared in about four days the exact figure 
has been proved by numerous measurements to be 
3-85 days. In a second period of 3-85 days, one-half 
of the remainder disappears, leaving only one-fourth 
of the total amount. At the end of a third such 
period one-eighth of the total amount remains, and 
this, as we have seen, gives rise to one-eighth of the 
total activity. Always in 3*85 days the amount 
remaining at the end of the period is exactly one- 
half of that at the beginning. This gives rise to the 
idea of a half-life, and the 3*85 clays is spoken of as 
the Jialf-life period. If the last statement holds true 
a given quantity of emanation will never completely 
disappear. At the end of N x 3*85 days (where N 
is a very large number) (1/2)^ of the original quantity 
of emanation will remain, its life is infinite, an infinite 
period of time must elapse before it is completely 
destroyed. But at the end of (8 x 3-85 =)3O'8 days 
the original quantity is reduced to (i/2) 8 or 1/256, a 



quantity within the limits of error of measurement, 
so that, as already stated, in a month the emanation 
has practically disappeared. 

100 1 



<J 40 



5 10 15 20 


FIG. 6. 

If points are plotted on squared paper, showing the 
amount of emanation at any given time, a regular 
curve is obtained. Such a curve is shown in Fig. 6. 
The total amount of emanation at zero time (at the 
commencement of an experiment) is taken as 100. 


Amounts of emanation arc plotted along the ordinates, 
the times (in days) along the abscissae. Such a curve 
is known as an exponential or logarithmic curve, and 
if the logarithms of the figures representing the 
amounts of emanation are taken, and plotted against 
the corresponding times, a straight line is obtained. 
This is shown in the figure. (A further discussion of 
these exponential curves is given in Appendix B.) 

As soon as the emanation was known to be a gas, 
and larger quantities of radium became available for 
experiment, attempts were made to isolate the gas 
produced, in order to study it in the pure condition. 
This work was largely carried out by Sir William 
Ramsay, and it is to the application of his delicate 
and exact methods of handling and analysing small 
quantities of gases that the absolute success of the 
attempts is due. 

Niton is liberated from a radium salt by heating it, 
when in the solid condition, or, better still, by pumping 
the gas away from it when dissolved in water. The 
latter method is that usually adopted. A quantity 
of radium salt containing 40 or 50 milligrams of the 
element is necessary in order to obtain an appreciable 
quantity of niton. It is better to work with ten times 
this quantity of radium, but unfortunately, on account 
of rarity this can only be done in two or three 
laboratories at the present time. The radium chloride 
or bromide is dissolved in pure water and placed in 
a small glass bulb such as A (Fig. 7). This bulb 
is sealed to. a mercury Topler pump; a trap, B, is 



FIG. 7. 


inserted between the bulb and the pump, so that any 
mercury driven over from the pump by accident is 
caught before it reaches the bulb. The pump shown 
in the diagram is one of the simplest forms in use, 
and is a modification due to von Antropoff. It 
consists essentially of a sloping barrel, D, connected 
by a fall tube at one end to a mercury reservoir, E, 
attached to the fall tube by thick rubber tubing. The 
other end of the barrel is closed by a thin capillary 
tube placed vertically and dipping beneath a cup 
containing mercury. A vacuum is first created in the 
apparatus. This is produced by repeatedly raising 
and lowering the reservoir E. Each time that E is 
lifted, the mercury rises in the barrel and forces all 
the air or other gas present in the barrel through the 
capillary tube F. Mercury is prevented from flowing 
over into the bulb by the valve C, a hollow glass tube 
the top of which is ground to fit the glass above it. 
As the mercury rises in the side tube the valve rises 
also, floating on the surface, and, jammed tightly 
against the glass surface above it, prevents all passage 
of liquid. On lowering the reservoir E the mercury 
sinks in the barrel until gas from the side tube forces 
its way out. F is sufficiently long to support a 
mercury column of over thirty inches, so that although 
as the pressure in the barrel falls, the air pressure on 
the external surface of the mercury in G (which is 
equal to that of a column of mercury thirty inches 
high) forces it up into the capillary, yet even when 
there is a perfect vacuum in D the mercury can only 


rise to the same height as it would in an ordinary 
barometer, i. e. about thirty inches, and will rapidly 
reach a position of equilibrium, will not flow back 
into the barrel. For the same reason the fall tube 
connecting with the reservoir is about the same 
length. The gaseous contents of the pump and the 
bulb joining it are in a closed space, are separated 
from the external atmosphere by glass and mercury, 
and the total quantity of gas is therefore limited. 
Each time the reservoir is raised a considerable 
quantity of this gas is forced out of the system 
through the tube F. If at the first stroke one-half 
of the gas is removed, then at the second one-half 
of the remainder will be removed, so that only one- 
quarter of the original will remain. For a reason 
similar to that in the case of the decay of the 
emanation, when twenty or thirty strokes of the pump 
have been made, there is only a trace of gas left in 
the pump and bulb, and by repeating the operation 
for a sufficient length of time, an almost perfect 
vacuum can be created. A partial vacuum equivalent 
to a pressure of one-tenth of a millimetre of mercury 
the normal atmospheric pressure is equivalent to that 
of a column of mercury 760 millimetres in height is 
sufficiently good for most purposes. When such a 
partial vacuum has been created in the apparatus, the 
mercury is allowed to rise slowly past the valve C 
and just through the tap above it. This is closed, 
and the small column of mercury above the tap 
prevents any trace of gas leakage through the latter. 


The process of creating a vacuum has of course 
removed any niton which was present in the solution, 
since under such conditions any gas dissolved in a 
liquid escapes from it. 

The apparatus is left sealed for four or five days, 
during which time a fresh amount of niton accu- 
mulates. This amount is so small, even when large 
quantities of radium are available, that it does not 
form a bubble of gas large enough to be passed 
through a mercury pump of the type described. But 
at the same time that the emanation is being pro- 
duced, the water in which the radium salt is dissolved 
is being decomposed into its constituents, hydrogen 
and oxygen, and in the period mentioned from ten 
to fifty cubic centimetres of the mixed gases are 
produced, 1 according to the amount of radium in the 
solution. Such an amount can be easily passed 
through a mercury pump without loss, and it carries 
the emanation along with it. 

The procedure is exactly as before. The tap 
connecting the pump to the bulb which contains the 
radium is opened, and the pressure of the gases in the 
bulb forces down the mercury seal. By repeatedly 
raising and lowering the reservoir the whole amount 
of gas is forced, little by little, through F, bubbles 
through the mercury in G, and is collected in an inverted 
thick-walled glass test-tube, H, which at the beginning 

1 With a strong solution of radium, this chemical action can 
be seen easily with the naked eye ; bubbles of gas are continually 
forming and rising to the surface of the solution. 


of the operation was completely filled with mercury. 
After no more gas can be pumped away from the 
bulb, it is sealed as before so that a fresh supply of 
emanation can accumulate. 

The test-tube contains hydrogen, oxygen, niton, 
and small traces of carbon dioxide and water 
vapour. In most cases before it is filled with mercury, 
a small amount of potash is fused to its upper interior 
surface. This rapidly absorbs moist carbon dioxide, 
so that by allowing the mixed gases to stand in 
contact with the deposit for some hours one of the 
impurities- is removed. 

The test-tube is then carefully transferred to a 
second apparatus sketched in Fig. 8. Initially, the 
tube P, the bulbs R and S, and the part of the 
bulb W above the mercury surface are completely 
evacuated of all traces of gas, by a pump similar 
to that already described, joined to the apparatus 
through the tap T. The mercury in W is kept at 
a constant level by tightening the clip M, and so 
closing the rubber tube connecting W to the small 
reservoir V. After a perfect vacuum has been pro- 
duced the tap T is closed, the three-way tap X is 
reversed, M opened, and by raising the reservoir V, 
mercury forced through the inverted syphon Y (of 
capillary tubing) into the mercury trough H. Air 
has now been removed completely from the whole 
apparatus. The test-tube H is placed over the end 
of the inverted syphon, and pressed downwards. The 
external pressure forces all the gas in H through the 

D 2 



inverted syphon into the burette W. Mercury is 
allowed to follow until all the gas has been displaced 
from the test-tube and the capillary, when the tap 
X is closed. The burette contains two electrodes of 
platinum wire, U, sealed through the top of the bulb 
and almost in contact. These are connected with an 
induction coil, and a small electric spark is passed 
across the gap between the electrodes. This causes 
union of the hydrogen and oxygen, forming water. 
It is found that there always remains a small quantity 
of hydrogen, amounting to from one to five per cent, 
of the volume present before the explosion. Had all 
the water decomposed produced hydrogen and oxygen 
only, these should have been in the correct propor- 
tions to form water again. It seems probable that 
the presence of the excess of hydrogen is accounted 
for by a second reaction, the decomposition of the 
water to form hydrogen peroxide and hydrogen. 
This goes on side by side with, the electrolytic 
decomposition but to a much smaller extent. The 
excess of hydrogen acts excellently by diluting the 
emanation and giving a volume of gas large enough 
to make manipulation easy. The tap X is turned 
so that the burette is in connection with the bulbs 
R and 5, and V is slowly raised. When the mercury 
has risen just above the tap X, this is closed. The 
bulb 5 contains phosphorus pentoxide, an excellent 
drying agent, which quickly removes all traces of the 
moisture formed by the explosion. After a few 
minutes the bulb R is surrounded with a paper cup, Q, 


moistened with water, and tied tightly round the 
narrow part of the tube. Into this is poured liquid 
air ; this immediately freezes the water moistening 
the paper, so that a cup of ice is produced. Since 
ice conducts heat very badly, this ice-cup forms a very 
efficient holder for the liquid air, which only slowly 
boils away within it. The cup is filled up from time 
to time, and the whole of the liquid air is not allowed 
to evaporate away until the end of the operations 
now to be described. Niton condenses to a solid at 
a temperature much above that at which liquid air 
boils (185 Centigrade). A ring of solid niton forms 
at once within the glass bulb, which of course also 
acquires a temperature approximating to that of the 
liquid air. The hydrogen and any trace of nitrogen 
present remain gaseous. After waiting five or six 
minutes to allow full condensation of all the niton 
present, the tap in the side tube is opened and the 
last traces of uacondensible gas are pumped away. 
During this time, if the operations are- carried out 
in darkness (or even in subdued daylight if much 
speaking comparably emanation is present) the bulb 
in contact with the liquid air is seen to glow with a 
magnificent green phosphorescence, showing that the 
emanation possesses properties very similar to those of 
radium itself. 

As soon as the last traces of gas have been com- 
pletely pumped away, the tap T is closed, the liquid 
air in the ice-cup is allowed to evaporate, and the 
apparatus allowed to warm up. When ordinary room 


temperature is reached the tap X is reopened, and 
mercury allowed to rise slowly, compressing the ema- 
nation into the small tube P t where the properties of 
the gas can be studied. By making P of very narrow 
capillary tube, and calibrating its volume, the volume 
of the emanation can be accurately measured ; by 
making it in the form of a small spectrum tube, into 
which platinum or aluminium-platinum electrodes are 
fixed, the spectrum of niton can be observed. Such a 
small spectrum tube is shown in Fig. 9. The elec- 
trodes serve simply to conduct an electric current 
from an induction coil to the extremities of the tube. 

Whenever a current of high voltage is connected to 
the terminals of such a tube and the gas within it is at 
a fairly low pressure say between one and seven milli- 
metres of mercury, z. e. between one-eight-hundredth 
and one-hundredth of atmospheric pressure it is 
found that the gas conducts the electric current 
easily, becomes luminescent, and if the light emitted 
is examined by a spectroscope, a distinct spectrum 
is observed specific for the particular gas under 

The emanation has all the physical properties of a 
gas. Itself it is colourless, although any tubs con- 
taining it begins to phosphoresce brilliantly within 
two or three minutes after it is introduced into the 


tube. The volume of the gas varies with the pressure 
to which it is subjected, just as with ordinary gases 
such as hydrogen or oxygen. .When heated the gas 
expands, and when cooled it contracts. When the 
emanation is allowed to remain in a tube for a number 
of days its volume is observed to decrease, and the 
rate of decrease of volume corresponds to tJie rate of 
loss of activity, so that the theory that the latter 
is actually due to the decay of the emanation is 
evidently correct. 

The volume of niton obtainable from a definite 
quantity of radium in a definite time has been mea- 
sured frequently. The datum is of very considerable 
importance. Since niton is produced by the destruc- 
tion of radium, if the rate at which the niton is 
produced is known, the rate of destruction of radium 
can be measured. Initial experiments with small 
quantities of radium were made by Sir William 
Ramsay and Mr. Soddy in 1903. Later, in 1908-9, 
exact and concordant measurements were made by 
Professor Rutherford, M. Debierne, and Sir William 
Ramsay and Dr. Gray. They found respectively that 
O'6i, 0*58, and O'6o cubic millimetre of niton was the 
maximum volume obtainable from one gram of radium. 
Since niton is constantly decaying at a fairly rapid 
rate, it is evident that if it is accumulating in contact 
with radium, a point will be reached at which as 
much niton is destroyed in any short interval of time 
as is produced from the radium in the same interval. 
The niton is then said to be in equilibrium with the 



radium producing it and the amount present. The 
equilibrium amount is evidently the maximum obtain- 
able. From the figures obtained it is calculated that 
the half-life period of radium is about 1800 years: 
in 1 800 years any given quantity of radium will have 
decayed to the extent of one-half. In making this cal- 
culation it is assumed that radium, like the emanation, 
decays at an exponential rate, so that in 3600 years 
only one-quarter of the original quantity of radium 
would remain, and so on. If the exponential law did 
not hold, and if the radium were continually pro- 
ducing niton at the rate observed, the life of any 
particular amount of radium would not be infinite 
but definite, and the whole of the radium would have 
disappeared in a comparatively short interval. But 
we have so many instances, as will be seen later, of 
radioactive substances decaying according to an ex- 
ponential law, that we can apply the law in this case 
also with confidence. Attempts to observe an actual 
decrease in the activity of a radium preparation have 
so far been unsuccessful. 

The proof has been given that niton is a gas be- 
having like any other gas, and from its chemical 
behaviour evidently an element strongly resembling 
the argon series of elements, all inert gases. As we 
have seen, the strongest proof of the individuality of 
an element is that it has a definite atomic weight and 
a definite spectrum. These proofs have been obtained 
in the case of niton. It was shown to possess a 
definite spectrum by Sir William Ramsay and Professor 


Collie in 1903. Later the spectrum has been mapped 
with exactness by Rutherford and Royds, and by 
Watson. Numerous attempts have been made from 
time to time to measure the atomic weight. The 
difficulties attending the direct measurement of the 
weight of a quantity of gas considerably less than 
one cubic millimetre are obviously so great that all 
the early experiments were indirect, using the method 
of diffusion. 

When a gas is allowed to stream from one vessel 
into another through a very small aperture, it is found 
that the rate of diffusion varies with the density of 
the gas is, in fact, inversely proportional to the 
square root of the density. Since the density of any 
gas is one-half the molecular weight, by comparing 
the rates of diffusion of two gases it is evident that if 
the molecular weight of one is known, that of the 
other can be calculated, at any rate approximately. 
The law holds even when the second gas is present 
diluted with a very large quantity of a third gas : it 
behaves as if the third gas were not present. Niton, 
mixed with many million times its volume of air, still 
obeys the law, and since the degree of its activity 
affords an accurate measurement of the amount of 
niton present in any given volume of air, it is possible 
to compare its rate of diffusion with that of any other 
gas. Early experiments gave very discrepant results, 
but they all showed that niton diffused very slowly, 
and must in consequence be a very heavy gas. The 
argon series of gases have the property that the mole- 


cule consists of but one atom, whereas the molecules 
of ordinary gases have at least two atoms. Mercury 
vapour is another instance of a monatomic gas. If 
the emanation belongs to the argon series it will also 
be monatomic, and its atomic and molecular weights 
will be identical. In order to make the conditions as 
parallel as possible Perkins compared directly the 
rates of diffusion of mercury vapour and niton. He 
found that niton diffused even more slowly than mer- 
cury, and that in consequence the atomic weight must 
be greater than that of mercury (200). His experi- 
mental data led to an atomic weight in the neigh- 
bourhood of 23-0, i.e. of the same order as that of 
radium (226*5). ^ n iQ 10 Debierne published the 
results of rigorously exact comparisons with sulphur 
dioxide, carbon dioxide, oxygen and argon. The 
results give an atomic weight for niton of approxi- 
mately 220 ; variations in different experiments were 
not greater than two to three per cent. 

In the same year Sir William Ramsay and Dr 
Gray succeeded in directly weighing the gas. Five 
experiments were made with amounts of niton of 
the order of one-tenth of a cubic millimetre, a volume 
roughly equal to that enclosed in the eye of a small 
needle. In each case the gas was compressed into 
a minute capillary tube, and this was sealed up and 
weighed. The tube was broken, the gas allowed to 
escape, and the broken pieces of tube again weighed. 
The difference was the weight of the gas. The 
weighings could not be carried out in an ordinary 


chemical balance, since the best of these are only 
sensitive to one fifty-thousandth of a gram, i. e. to 
one-fiftieth of a milligram ; various micro-balances 
which have been devised have a sensitiveness of 
about one-thousandth of a milligram. The actual 
weights of gas measured were from 572 to 729 mil- 
liontlis of a milligram. These measurements were 
carried out with a type of balance devised by 
Professor Steele, in which, instead of counter-balancing 
the unknown weight by known weights, as usual, 
the change of buoyancy of a bulb (to which known 
or unknown weights could be attached) was measured 
when the pressure in the air surrounding the bulb 
was changed. (The balance actually used was sensi- 
tive to about five-millionths of a milligram, i. e. it 
would discriminate between two, weights differing by 
this amount, about one hundred-thousand-millionth of 
a pound.) Numerous corrections were applied to the 
results, for the loss of niton through decay during the 
time of experiment, for the weight of the substances 
produced by this decay, for temperature, pressure, and 
so on. The figures found for the atomic weight in the 
respective experiments were 227, 226, 225, 220, 218. 
The mean figure was 223, and the extremes differed 
from this by only two per cent. a truly remark- 
able result, when we consider the extraordinary 
experimental difficulties, and the minuteness of the 
amounts weighed. 

Niton the name was suggested by Ramsay, after 
he had determined the atomic weight, and means 


" the shining one" is a gas with a definite spectrum, 
a definite atomic weight, and definite properties 
which show conclusively that it belongs to the rare 
or inert series of gases. It is by far the heaviest 
gas known. 1 Its atomic weight, as found by actual 
experiment, differs from that of radium by 3*5, and 
since we know from observation that radium is 
continually producing niton, it seems legitimate to 
conclude that this production is brought about by 
disintegration of atoms, each atom of radium giving 
rise to one atom of niton. 

Niton has been liquefied, and also obtained in 
the solid condition. This has been achieved by 
Rutherford, and by Ramsay and Gray. The method 
adopted, and found successful, was that which it is 
customary to use in liquefying any other gas appli- 
cation of pressure to the gas when cooled to a low 
temperature. The niton was prepared in a pure 
condition in the manner already described, and was 
forced into a narrow capillary tube under high 
pressure. The capillary tube was cooled to a very 
low temperature by means of liquid air. The niton 
condensed to a colourless liquid, and when the 
temperature was sufficiently low, to an opaque solid. 
Liquid niton is transparent like water. It produces 
phosphorescence to an extraordinarily marked degree ; 
the rays causing the effect are concentrated on a very 

1 This statement requires slight modification. There are 
two other radioactive emanations, and there is every reason to 
believe that these are also gaseous elements with atomic weights 
of the same order as niton. 



small space. The green colour observed under 
ordinary pressures with soda glass becomes bluish 
pink under the action of liquid niton. With silica 
the colour is blue. The solid glows intensely, and 
has the appearance of a small steel-blue arc light, 
which changes successively with lowered temperature 
to yellow, and at liquid air temperature to a brilliant 
orange red. On warming, the colour change is 
inverted. Solid niton melts at 71 C. ; the liquid 
boils at 62 C. The density of the liquid at its 
boiling-point is 57, that is to say it has a density 
almost half that of mercury (13*6), and is much 
heavier, bulk for bulk, than most minerals. 

The properties of the rare gases form a good 
illustration of the parallelism of properties exhibited 
by a group of elements. Some of these properties 
are given in the following table 

w. C ^ 




1) '"i- 4-1 ' 55 O* *H 




o "o O 

U ^ ^ 

Helium . 



268-5 c 

0*0138 O'I5 

Neon . 


abo u t 2 50 ! a bout 240 



39-9 188 186 

0-0379 I -212 

Krypton . 

82-9 169 ! 5 l0 

0-07 2-155 


I30-2 140 109 

0-II09 3-52 

Niton . . 

222 -c; 7I 62 
j i 

0-245 57 

Niton possesses, like radium, the property of con- 
tinually producing heat, and to an extent easily 


measurable. Sir William Ramsay has devised a 
very interesting experiment to show this heat pro- 
duction. A special thermometer was constructed in 
which the bulb contain- 
ing mercury was - re- 
placed by a double bulb. 
The arrangement is 
shown in Fig. 10. The 
space between the inner 
and outer bulbs was 
filled with mercury, as 
usual, and this part, 
forming the thermome- 
ter, was calibrated over 
the ordinary range of 
temperatures by com- 
parison with an exact 
thermometer. The inner 
bulb was joined to a 
capillary, and after cali- 
bration the capillary 
was sealed to the ap- 
paratus for purifying 
the emanation, and the 
excess hydrogen (see 
p. 53), and emanation 
from several days' accumulation of the gas from a 
solution of 50 milligrams of radium salt was forced 
into the inner bulb of the thermometer ; the capillary 
was then sealed off. The amount of emanation so 

FIG. 10. 


obtained was of the order of one hundredth of a 
cubic millimetre. The special thermometer was 
placed side by side with an ordinary exact ther- 
mometer, the two bulbs were wrapped in cotton wool 
and placed in a Dewar vacuum vessel (having all 
the properties of a Thermos flask, which is indeed 
a special form of Dewar vessel) so that they should 
be subject to exactly the same variations of tem- 
perature. The thermometers were read daily for 
three weeks. On the first day a difference of O'52 C. 
was observed. This increased to 073 on the second 
day, after which it decreased gradually, with the 
gradual decay of the emanation. No difference in 
the temperatures of the two thermometers could be 
observed at the end of a fortnight. 

The first attempts to see if niton possessed a 
definite spectrum were made by Ramsay and Soddy 
in 1902. They were unsuccessful. The spectrum 
was masked by the presence of other gases, chiefly 
carbon dioxide. After two or three days, however, 
the spectrum of helium was visible. It grew in 
intensity. Repetition of the experiment by the same 
and by other observers gave the same result. As the 
emanation decays its place is taken by helium. Helium 
is a definite element, the lightest member of the 
series of rare gases, only discovered by Ramsay 
himself in 1895 (by heating certain minerals; it was 
shown later to exist in the atmosphere to the extent 
of one part in 200,000 : see Appendix D). At the 
time that this discovery was made the emanation 


was not recognised as a definite element, but it 
appeared certain that radium had changed into helium^ 
so that this was the first definite evidence put forward 
for the transformation of one element into another. 1 
The experiment has been repeated so often, and by 
so many different observers, that not the slightest 
doubt rests on the accuracy of the result. 

The atomic weight of helium is 3-99. We have 
seen that the atomic weights of radium and niton 
differ by 3*5. It is then at least possible that an 
atom of radium breaks up, forming an atom of niton 
and one of helium. 

Niton can produce numerous chemical actions very 
similar in their nature to those produced by radium. 
Many of these actions have been closely studied. 
When niton and oxygen are left in contact with 
mercury a red crust of mercuric oxide is formed on 
the surface of the mercury ; the oxygen has been 
converted into ozone, and the ozone has attacked the 
mercury. Niton in contact with water dissolves it, 
producing hydrogen and oxygen and a trace of hydro- 
gen peroxide. We have seen that the same reactions 
are caused by radium. When niton is mixed with 
the gas carbon dioxide, a black deposit of carbon 
forms on the walls of the containing vessel, while 
oxygen and carbon monoxide are also produced. 
Carbon monoxide is similarly decomposed. Gaseous 
ammonia is decomposed into its constituents, nitrogen 

1 Some of the other radioactive changes had been investigated 
and appeared proven, but the proof was far less demonstrable. 



FIG. ii. 

and hydrogen. Hydrochloric 
acid is decomposed into hydro- 
gen and chlorine. These re- 
actions are produced whether 
the niton is directly mixed 
with the gas on which it acts 
or is allowed to act through 
a screen of thin glass. In the 
latter case the amounts of 
decomposition brought about 
are smaller. It would seem, 
therefore, that the chemical 
actions are produced, not by 
the niton itself, but by the radi- 
ations which it emits, which 
are capable of penetrating a 
thin layer of glass. 

A form of apparatus per- 
mitting accurate study of the 
chemical actions on various 
gases is shown in Fig. 11. It 
consists essentially of a small 
bulb, A, connected by a glass 
tube over thirty inches long, 
and by thick rubber pressure 
tubing to a mercury reservoir, 
C. A side tube connects 
through the tap E with an 
apparatus similar to that 
shown in Fig. 8, and used 


for purifying the emanation. By lowering or raising 
the reservoir the level of the mercury within the 
tube is brought to a point D, just below the side 
tube, and is fixed there by tightly screwing the 
clamp F, and so closing the rubber tube. The 
tap E is opened, and the bulb and tube completely 
evacuated by means of the pump connected to the 
other part of the apparatus. The gas which is to be 
experimented with is then allowed to enter through 
the side tube, and the tap E closed. Emanation is 
purified as usual, and is then forced through the side 
tube by filling the purification apparatus with mer- 
cury. As soon as the side tube is filled with mercury, 
the tap E is again closed, the clamp F opened, and 
mercury allowed to rise and partially fill the bulb. 
To the inner wall of the bulb is sealed a thin strip of 
blue glass, B, bent downwards, and ending in a very 
sharp point. When the mercury surface just touches 
the point, an absolutely definite volume is enclosed 
between glass and mercury. This volume is deter- 
mined, before sealing the bulb to the apparatus, by 
weighing the bulb completely filled to the tap G with 
mercury, and weighing a second time when only the 
part from the tap G to the blue point is filled with 
mercury. The difference in the two weights is the 
weight of mercury which occupies the definite volume 
under consideration. The weight and volume rela- 
tions of mercury are accurately known, and the 
volume can therefore be calculated. 

When the gas and emanation mixed together have 

E 2 






been enclosed in the bulb, the pressure is adjusted 
by raising or lowering the mercury reservoir, so that 
the blue point and mercury surface 
are just in contact, and the gas 
occupies the definite known volume. 
The amount of gas employed is usually 
such that the pressure it exerts in 
this space is less than atmospheric 
pressure, so that the surface of the 
mercury in the outer limb is below 
that in the bulb. The diagrammatic 
arrangement in Fig. 12 shows what is 
actually happening in such a case. 
The atmospheric pressure, which we 
can call P t exerted on the surface 
I of the reservoir, is balancing the in- 

^ ternal pressure of the* gas p plus the 

pressure exerted by the column of 
mercury MN, the difference in height 
of the two levels. The external 
pressure P is measured with a baro- 
meter, i.e. it is found in terms of a 
column of mercury. The difference 
between this figure and MN is the 
pressure exerted by the enclosed gas, 
also expressed in terms of a mercury 
FIG. 12. column. According to Boyle's law 

the pressure exerted by any definite 
amount of gas multiplied by the volume which it 
occupies at that pressure is a constant. We know 


the volume of the gas, and the pressure which it 
exerts, and we can, therefore, calculate the volume 
which it would occupy if it exerted a pressure of 
760 millimetres of mercury, the normal atmospheric 
pressure, and if it, therefore, occupied what we may 
call its normal volume. 

The pressure is measured immediately, and the 
corresponding volume calculated ; the tap G is then 
closed, and so the volume is kept constant through- 
out the experiment. Readings are taken daily, or 
even at shorter intervals if necessary, and these 
show whether the volume of the gas is changing, 
and therefore if the emanation is producing any 
effect on it. If the latter is the case, the rate of 
change of volume of the gas shows the rate at which 
the emanation is acting. It has been found in 
experiments with a mixture of hydrogen and oxygen, 
with carbon monoxide, with ammonia, and also with 
water in the presence of hydrogen and oxygen, that 
the chemical actions produced take place at a certain 
definite rate, and that this rate is the same as that 
at which the emanation is decaying. For example, in 
an experiment with electrolytic gas (hydrogen and 
oxygen in the proportions to form water) combina- 
tion takes place with the formation of water, and the 
gas volume becomes less. The amount of combina- 
tion becomes less from day to day, and at the end of 
about three weeks changes in pressure are no longer 
perceptible. The difference between the initial and 
final volumes is the total amount combined. If we 


draw a curve representing the percentage of this 
amount which is uncombined at any time, plotted 
against the percentage of the emanation which is 


w -F> CD CO C 
_ C 






























)0 80 60 40 20 

FIG. 13. 

undecayed at the same time, we obtain a straight 
line. Such a curve is shown in Fig. 13. It would 
appear, then, that the chemical change brought about 
at any given time is proportional to the amount of 


emanation decaying in the same time, and it can, 
therefore, be deduced that each portion, each atom of 
emanation in decaying can produce a definite quantitive 
chemical effect. 

We have seen that radium constantly produces the 
gas niton, and as matter is not only indestructible, 
but cannot be produced from nothing (ex nihilo nihil 
fit\ we can only conclude that the niton is produced by 
the destruction of the radium itself. We have seen 
also that niton decays very rapidly, so that if any 
definite volume of it be isolated, in four days one-half 
of that volume has disappeared, while after the space 
of a month the whole volume of gas has disappeared, 
or at any rate a scarcely measurable trace remains. 
What has become of it? Into what has this volume 
of gas changed ? 

It is found that if any vessel contains a trace of 
niton for a short time a few minutes suffices that 
vessel remains radioactive even after a current of air 
has been passed through it in quantity sufficient to 
remove completely the smallest traces of any gas 
which might adhere to the walls of the vessel. Some 
active substance has, therefore, been deposited on the 
vessel walls. The phenomenon is spoken of as the 
production of induced activity. It is studied most 
easily by allowing a wire charged with electricity to a 
high negative potential to remain in contact with 
air containing niton for a period of one or two hours. 
The wire is then removed, and when examined with 


an electroscope or electrometer is found to be strongly 
radioactive, so that a deposit of some strongly active 
substance has been formed on it. This substance is 
found to decay much more rapidly than niton ; in 
twenty-eight minutes the activity has fallen to half 
value, while in two or three hours it is no longer 

The decay curve from this active deposit is not a 
simple exponential curve. Mathematical analysis of 
the curve, and a detailed study of the radiations 
emitted by the deposit have shown that it does not 
consist of a single element, but of a number of 
elements produced successively. The mathematical 
analysis has been assisted by physical and chemical 
treatment ; by, for example, heating the wire to high 
temperatures, and observing whether the curve of 
decay has been affected ; by treating the wire with 
strong acids with subsequent similar testing. Such 
lines of experiment have established the following 

Niton, as it decays, produces a solid substance, 
soluble in strong acids, and which does not volatilise 
at temperatures below 800 Centigrade. This sub- 
stance, which is known as radium A, has a half-life 
period of only three minutes. It decays, producing 
radium B, also a solid substance, volatile below 700, 
and having a half-life period of 267 minutes. Radium 
B in its turn produces radium C, with a half-life 
period of 19*5 minutes. The decomposition product D 
of radium C is apparently not radioactive, for it does 


not emit radiations. 1 Nevertheless it slowly decays 
changing into a new active substance, radium E, and 
from the rate at which radium E is formed it is 
calculated that radium D has a half-life period of 
about seventeen years. Radium D has chemical 
properties strongly resembling those of lead, and is 
on that account also called radio-lead. The lead 
which is separated from pitchblende residues, contains 
appreciable quantities of radio-lead, and also becomes 
active on standing, owing to the slow production of 
radium E. 

Radium E having a half-life period of six or seven 
days, in its turn forms radium F. Radium F has a 
half-life period of 136 days, and its rate of decay and 
also the nature of the radiations which it emits 
identify it with the first of the radioactive substances 
discovered by Madame Curie, the element polonium, 
which also decays to one-half in 136 days. It has 
already been stated that polonium is found along 
with bismuth when the pitchblende residues are 
analysed, and that it has very similar chemical 
properties, so that its separation from bismuth is a 
matter of considerable difficulty. 

Recently, Madame Curie and M. Debierne have 
worked up the residues from several tons of uranium 
minerals, and after a tedious process of separation, 
obtained two milligrams of a substance which from its 
activity they estimated contained one milligram of 

1 Some recent observations seem to show that radium D is 
very feebly active. 


polonium. This substance examined in the spectro- 
scope showed the presence of traces of numerous 
elements, and in addition three unknown spectral 
lines, which were accordingly attributed to polonium. 
When the substance was dissolved in water, an 
evolution of gas was noticed, and this decomposition of 
water into hydrogen and oxygen has been confirmed 
by Bergwitz. The dry substance left in contact 
with air caused the continual formation of ozone. 

The solution of the substance in water produced 
not only hydrogen and oxygen, but also helium, the 
same rare gas that is produced during the decay of 
niton. The total amount of gas formed in a period 
of over one hundred days showed on analysis that 
1*3 cubic millimetres of helium had been produced. 

No further active substance is produced by the 
decomposition of polonium. The activity decreases 
according to the exponential law and at such a rate 
that in 136 days it has fallen to one-half. In the 
same time, as we have seen, an appreciable quantity 
of helium is formed. Is this the only product of the 
disintegration or are there other elements produced ? 
There is considerable evidence that lead is formed. 
The reasons for this will be mentioned later. 1 
Absolute experimental proof may be given at an 
early date. The small quantity of impure polonium 
recently obtained by Madame Curie is at present under 
careful observation. Within a reasonably short time 
the greater part of the polonium will have disappeared, 
1 See p. 159. 


and if the three new spectral lines were correctly 
attributed to it they should either have disappeared 
entirely, or should have become much weaker. In 
the original preparation the spectral lines of lead 
were very faint ; if lead really is formed by the 
disintegration of polonium the amount of lead present 
should continually increase, and its spectrum should 
become much stronger ; comparable photographs 
should show the lines due to lead much more strongly 

In this chapter a description has been given of two 
new elements, radium and niton, and of the methods 
by which they were discovered. The evidence that 
polonium is also a distinct and new element has been 
outlined. There is strong ground for believing that 
the intermediate substances are also true elements, 
although most of them decay so rapidly that it is 
improbable that any of the ordinary tests for the 
individuality of an element can ever be applied to 
them. Each of this series of elements resembles 
some one or other of the ordinary elements in 
chemical and physical properties ; each of them 
possesses in addition the property of emitting radia- 
tions ; and further, unlike ordinary elements which are 
usually assumed to have an infinite existence, each 
of these elements is continually undergoing destruction 
and this takes place at a perfectly definite rate, which 
is different for each element. 

The explanation of this phenomenon, which is now 


universally accepted, was put forward in 1903 by 
Professor Rutherford and Mr. Soddy. They sug- 
gested that there was a successive disintegration and 
transformation from one element into another, and 
that this change was atomic ; that an atom of radium 
changed into an atom of niton, that this then sooner 
or later changed into an atom of radium A, and so on, 
so that if we represent the atoms by the symbols of 
the elements we have the following scheme 

Ra> Nt > RaA > RaB > RaC > RaD > RaE > RaF 
or Polonium ( -> lead ?). 

We must suppose that the atoms of these elements 
are complex, and that sooner or later through some so 
far unexplained cause, some internal stress, they break 
up ; emitting radiations (we shall see in the next 
chapter that atoms of helium actually form part of these 
radiations] and giving an atom of an element, which 
is almost as large as the original atom, judging by the 
weight relations of radium and niton. Many experi- 
ments have been carried out to try and influence the 
rate of disintegration by external causes ; to take a 
single example, radium C has been subjected to the 
extremes of temperature and pressure available in the 
laboratory, but in no case has the slightest change 
been observed in its rate of decay. If we had a 
large number of complex atoms of the same kind, 
each subjected to some internal stress, so that at 
intervals they one by one disintegrated, and if this 
disintegration was regulated by the law of chance, 


then the rate of decay would also follow an exponen- 
tial law. 

The presence of polonium and radio-lead in appreci- 
ably large quantities in pitchblende and other minerals 
containing radium is now easily understood. In the 
long interval of time which has elapsed since the 
formation of these minerals radium has disintegrated 
continuously, and accordingly radio-lead and polonium 
have also been formed continuously. Since they 
disintegrate at a fairly rapid rate a time must be 
reached at which as much of either element is formed 
in a certain period as is destroyed in that period. 
The equilibrium amount will then remain present in 
the mineral. We have seen, however, that radium is 
forming niton at an appreciably rapid rate, and that 
from this fact it is deduced that radium has a half- 
life period of about 1800 years (see p. 57). This is 
only a minute fraction of the life of any of these 
minerals, so that were radium not continuously 
formed it would long since have completely ceased to 
exist in the minerals. From what element is it 
formed ? Has it any connection with uranium, which 
is also radioactive ? These are questions which must 
be answered in subsequent chapters: 



IT has been seen that the intensity of the radiations 
from equal amounts of different active substances 
varies over wide limits, and that it was in fact owing 
to this difference that the discoveries of radium and 
polonium were made. The difference is so marked 
that, for example, while the smallest trace of radium 
salt placed in the neighbourhood of a photographic 
plate produces an immediate effect, an equal amount 
of uranium, salt produces a similar effect only after 
acting over a period of twenty-four hours or even 
longer. A fluorescent screen is immediately lit up by 
a trace of radium or polonium, but is scarcely affected 
by uranium or thorium. 

Not only does this difference exist in the radiations 
from different sources, but those from the same source 
are also frequently not homogeneous. We know that 
if a radium salt is placed inside a charged electro- 
scope, the latter is immediately discharged. If this 
radium salt has been recently prepared evaporated 
from solution, for example, within a period of minutes 
then on covering the salt with a thin layer of tinfoil 
and recharging the electroscope, the latter is only very 



slowly discharged. Adding further layers of the foil 
or even sheets of metal produces no further marked 
effect. The foil has cut off practically all the radia- 
tions. If the radium salt, however, has been prepared 
some days or weeks previously, an entirely different 
series of phenomena is observed. The first layer of 
foil which is placed upon it produces a marked diminu- 
tion of the rate at which the leaves fall together, the 
rate of discharge of the electroscope. A second layer 
produces no marked effect. A layer of lead a quarter 
.of an inch in thickness produces again a further con- 
siderable diminution, but an equal layer placed on top 
of the first has no similar power, and indeed, a lead 
sheet an inch or more in thickness produces little 
further effect, although the leaves of the electroscope 
still fall together much faster than in the absence of 
the radium salt. Such an experiment shows that 
there are three distinct types of radiations ; the first, 
which ionise very powerfully, are entirely absorbed by 
a very thin sheet of tinfoil; the second, which also have 
a powerful action, are absorbed by a quarter of an inch 
of lead, while the third type can penetrate more than 
an inch of lead and still produce their normal ionising 
effect. Only one other kind of ray is known to have 
the same penetrating power as this last type the 
Rontgen rays or X-rays. The three types of rays 
were named by Professor Rutherford the alpha (a), 
beta (/?), and gamma (v) rays. The same experiment 
shows that there is a difference between radium freshly 
prepared and after standing for some days. The 


difference must be due to the gradual production of 
emanation and of the successive products radium A, 
B, C, etc., and the ft- and y- rays must come from one 
or other of these. 

The three types of radiations have been studied in 
great detail, in the course of which their behaviour 
has been tested under the action of magnetic and 
electric fields. It has been found that the first two 
types carry an electric charge, and the ratio of this 
charge to the mass of the particles forming the ray 
is so different in the two cases as completely to 
differentiate them. The velocity of projection of 
the radiations has also been measured, and here 
again there are marked differences. 

The radiations travel unde'r normal conditions in 
straight lines. This is shown in the following simple 
experiment due to Becquerel. A plate containing 
a small rectangular opening is placed above and 
parallel to a photographic plate, while the source of 
the radiations (uranium, radium or polonium) is placed 
vertically above the opening (see Fig. 14). The 
photographic impression shows a central region of 
greatest intensity, and two lateral regions of decreas- 
ing intensity, the phenomenon being exactly similar 
to that produced by light rays from a source of like 

When the radiations are examined in a strong 
magnetic field it 'is found that the ^-rays, the most 
penetrating, are unaffected, and continue to travel in 
straight lines. The /3-rays, on the other hand, are very 


markedly deflected, and their path becomes circular. 
Their deviation bears such a relation, to the direction of 
the magnetic field as to show that they carry a nega- 
tive charge of electricity. The a-rays show a deviation 
only when subjected to the strongest possible fields ; 
this deviation is opposite in direction to that of the 
/?-rays, showing that the electric charge is different, 
is positive instead of negative. The effect is shown 

FIG. 14. 

diagrammatically in the following hypothetical 
experiment suggested by Madame Curie. 

A capsule of lead has a deep hole bored centrally 
in it ; at the bottom of the hole a small quantity of 
radium salt is placed. The capsule is then placed on 
a photographic plate, P, and is subjected to a strong 
magnetic field. Radium, unless it is very recently 
prepared, emits all three types of radiations. These 
can only escape through the hole in the capsule, and 
therefore before the application of the magnetic field 
they are directed at right angles to the plate and pro- 



duce no effect on it. The field is supposed to act 
vertically to the plane of the paper. Under these 
conditions, as soon as the field is applied the /?-rays 
are caused to assume a circular path, as shown in the 
diagram, and are finally arrested by the photographic 


FIG. 15. 

plate, on which they produce an impression as usual. 
The ^-rays are unaffected. If the field is strong 
enough, the a-rays show a deviation in the direction 
opposite that taken by the /?-rays. The amount of 
this deviation is very small in the diagram it is 
purposely exaggerated. 


The ease with which /?-rays are deflected by a mag- 
netic field, and the direction in which they are deflected, 
suggest at once a close resemblance to the cathode rays, 
which have been shown to be electrons, or negative 
particles of electricity, are equally easily deflectible, 
and in the same direction. There are further resem- 
blances. The /?-rays also carry negative charges 
of electricity, and their penetrating power is only 
slightly greater. It is difficult to show by direct ex- 
periment that they carry negative charges on account 
of the ionisation which they produce, but the fact can 
be demonstrated very simply indirectly. A glass 
capsule containing a small quantity of radium salt 
sealed within it retains all the a-particles, since these 
can only penetrate the thinnest films of glass. On the 
other hand, most of the /5-particles can escape if the 
glass is only of moderate thickness, and if these are 
negatively charged, negative electricity is being re- 
moved continually from the system, which should in 
consequence become positively charged. It is found 
actually that such a capsule, when insulated in a 
vacuum and left undisturbed for some time, acquires 
a strong charge, so that on touching the capsule a 
distinct spark is produced. 

From the study of the action of a uniform magnetic 
field on a bundle of /?-rays, the radius of curvature of 
the rays can be measured, and from this figure the 
value of the quotient of the mass multiplied by the 
velocity and divided by the electric charge of a single 

ray can be calculated (the value of , where m is 


the mass, v the velocity, and e the electric charge). 
If the magnetic field is replaced by an electric field 
curvature is produced as in the first instance, but the 
degree of curvature is different, and from it is calcu- 
lated the value of the quotient of the mass multiplied 
by the square of the velocity, and divided by the 


electric charge (the value of- ). From the two 

expressions the values of the velocity, and of the ratio 
of the mass to charge, are easily calculated. Exactly 
similar experiments have been carried out for the 
cathode rays were, indeed, first carried out with them. 

Becquerel found that the value of -- for the #-ravs 

111 * r j 

was io 7 electromagnetic units, a value so closely 
approximating to that for the cathode rays, 177 x io 7 , 
that there could no longer be any doubt as to their 
identity. The /5-rays are, therefore, electrons or minute 
particles carrying negative electricity, having a mass 
which, according to the most recent estimations, is 
about one seventeen-hundredth of that of the hydrogen 
atom. They show some points of difference from the 
cathode rays. For example, the /?-rays measured by 
Becquerel had a velocity of 1*6 X io 10 centimetres per 
second, while the cathode rays from a Crookes tube 
have a velocity between io 9 and io 10 centimetres per 
second, i. e. between 6000 and 60,000 miles per second. 
Becquerel's figure is approximately 100,000 miles a 
second, a distinctly greater velocity, almost comparable 


with the velocity of light, which is 186,000 miles per 

The /?-rays themselves are not homogeneous. For 
example, they exhibit distinct differences when sub- 
jected to the action of a magnetic field. A fraction 
of the rays is deviated to a greater extent than the 
remainder ; it would be more correct to say that the 
degree of deviation varies for each ray. It can be 
shown easily, as by allowing the rays to act on a 
photographic plate covered by different thicknesses 
of metal, that the rays which suffer the greatest 
deviation, and in consequence strike the photographic 
plate at a point nearest their source, are the least pene- 
trating rays. Again, the rays have different velocities. 
Kaufman has recently made exact measurements 

of the ratio , and his results are of the utmost 

importance, since they show that the ratio is not con- 
stant, but decreases as the velocity of the particle 
increases. His figures are shown in the following 

Velocity of Ray. e_ j n Electromagnetic Units . 

Velocity of Light. tn 

0*94 0*63 x io 7 

0-9I 077 

0-88 0-975 

0-83 1-17 

079 1-31 

072 1-49 

0-59 1-68 

(The corresponding figures for the cathode ray are, 
according to Classen, 0*05 and 177.) It is seen that 


a decrease of velocity of about one-third is accom- 
panied by an increase in the ratio of over 150 per 
cent. Further experiments seem to show that in 
reality there exists no discontinuity in the velocities 
of the fi and cathode rays, that /3-rays as slow 
or slower than the fastest cathode rays exist. The 
importance of the results lies in the deduction that 
since the electric charge is a constant quantity, there- 
fore the mass of the particle is not a constant quantify^ 
but is at least partially, if not ivholly, a function of the 
speed of the particle. The experimental results are in 
accord with the electromagnetic theory of matter 
recently developed, according to which atoms are 
built up of large numbers of these small electrons, and 
from which Abraham has developed a theory that the 
mass of the corpuscle will be constant only so long as 
its velocity is small in comparison with that of light. 

Various attempts have been made to measure the 
actual number of /?-rays which are emitted in a given 
time from a definite quantity of radium. One such 
attempt, by Wien, was based on the property already 
mentioned, that by continual loss of electrons the 
radium acquires an electric charge. About four mil- 
ligrams of radium bromide was placed in a closed 
platinum capsule, and suspended by an insulating 
thread inside a glass vessel in which a good vacuum 
was created. An electrode was sealed through the 
wall of the glass vessel in such a way that it could, 
at will, be placed in contact with the platinum capsule. 
The electrode was connected with an electrometer. 


It was found that the capsule attained a potential of 
IOO volts, and gave rise to a current of 2^9 X io_ ia 
amperes. If each /?- particle carried the elementary 
charge of an electron (e = io_ 20 electromagnetic units) 
then the actual charge acquired corresponded to the 
emission of io 10 particles per gram of radium per 
second. This figure is evidently only a minimum one, 
since some proportion of the /^-radiations are absorbed 
by the platinum and by the radium salt itself. 
Makower has recently carried out more accurate 
measurements, and according to these the number 
of /^-particles emitted per second per gram of radium 
is io 11 , one hundred thousand million. 

It is found that /?-rays are absorbed by matter in 
proportion to its density. The same law holds also 
for the other types of rays, and some comparative 
figures will be given later (see p. 94). 

When /?- or y-rays traverse matter, then the bundle 
of primary rays passing in emerges scattered, spread 
out ; it has been dispersed. Further, the matter 
struck by the electrons or the y-rays also emits 
electrons. Similar effects are also produced by 
cathode and Rontgen rays. The secondary rays, as 
the electrons so produced are named, have usually 
a somewhat smaller velocity than the primary rays 
which produce them. 

Gamma rays seem to bear the same relation to 
/?-rays that X-rays do to cathode rays. They occur 
invariably when /?-rays are emitted from a radio- 
active preparation; they do not necessarily occur 


when a-rays are emitted. Thf most outstanding 
property of y-rays is their penetrability. They can 
penetrate from four to six inches of metal. They 
can penetrate the human body in the same way as 
X-rays, and if they arc then allowed to fall on a 
barium platinocyanicle screen, a similar shadowgraph 
is obtained. This high degree of penetrability is 
now often used to measure quantities of radioactive 
matter. The ionising effect of the /-rays is propor- 
tional to the degree of activity of the source producing 
them. Radioactive matter enclosed in an apparatus 
of glass or other material of such thickness as com- 
pletely to absorb any a- and /?-rays will still, provided 
it emits /-rays, produce an effect on an electroscope 
brought near the apparatus. The effect will vary 
quantitatively according to the degree of the activity, 
and this will be proportional as usual to the amount 
of active matter present which produces /-rays. In 
this way, changes in the amount can be observed 
although the radioactive matter is continuously sealed 
up, and in this way, for example, it has been shown by 
subjecting radium emanation to the greatest possible 
differences of pressure by compression in a steel 
bomb, that no effect was produced on its rate of 

The relationship, just pointed out, between /-and 
/9-rays similar to that between X- and cathode rays, 
and the fact that neither /- nor X-rays are deflected 
by a magnetic field, points to the great similarity 
between the two. The velocity of /-rays has not 


yet been measured. According to two independent 
observers, the velocity of X-rays is approximately 
that of light. The theory of X-rays usually accepted 
is that, like light-rays, they are undulations of the 
ether, waves travelling in straight lines, and not 
particles possessing mass. A similar explanation of 
y-rays is usually put forward, and is currently ac- 
cepted. It is easily conceivable that such waves 
should be produced whenever an electron, travelling 
at great speed, collides with matter. On the other 
hand, it is also very possible that a certain initial 
velocity is necessary before such a pulsation can be 
produced, and since, apparently, a -rays are not ac- 
companied by y-rays, we should expect, as is actually 
the case, that the velocity of the a-particles is much 

A second theory of the nature of y-rays must be 
mentioned ; it is due to Professor Bragg. He thinks 
that each y-ray consists of twin particles, held together 
by attraction due to the possession of unlike electric 
charges. Owing to these unlike charges the twin 
particles behave as though neutral, and are, in conse- 
quence, not affected by an electric or a magnetic 
field. This theory has not received much support, 
but at present it cannot be considered as definitely 
disproved, nor can the other be held to be definitely 

The most important of the three types of ray are 
undoubtedly the a-rays. Unlike y-rays they are not 
necessarily emitted along with /?-rays. Thus it was 


found, at a comparatively early period in the study 
of the subject, that polonium produces no /?-rays but 
emits a-rays alone. In that way a pure source of 
a-rays was obtained, and this enabled them to be 
studied easily and exactly. It was found also that 
where a- and /?-rays were simultaneously emitted, 
they did not necessarily arise from the same element. 
We have already discussed an experiment (p. 78) which 
demonstrated that freshly purified radium emitted 
a-rays alone, and it has been found, in a similar 
manner, that freshly purified niton also emits only 
a-particles. The /?- and /-particles which are emitted 
by old radium, come, as a matter of fact, only from 
radium B and C. Again, similar experiment has 
demonstrated that while a uranium salt emits both 
a- and ^-particles, yet freshly prepared uranium only 
emits a-particles. The substance emitting the /?-rays 
can be separated from an old uranium compound by 
various simple chemical processes, such as by adding 
excess of ammonium carbonate to a solution of a 
uranium salt. Uranium hydroxide is at first pre- 
cipitated, and then redissolves in the excess of the 
reagent ; but a light, cloudy precipitate remains, and 
this contains the element which produces /?-rays. 
This element is known as uranium X (ex uranid). 

It has been already mentioned that in very strong 
magnetic fields a-rays are very slightly deflected. 
This was first established by Professor Rutherford. 
It can be demonstrated by a simple experiment due 
to Professor Becquerel. The arrangement is shown 


diagrammatically in Fig. 16. The source of the 
a-rays, a small quantity of polonium conveniently 
is placed on a copper plate, C. Above, parallel to 
the plate, and at a distance of about a centimetre 
from it, is placed a metallic screen, S, pierced by 
a small aperture, which is situated directly above 
the active material. At an equal distance above 

3 c 

FIG. 16. 

this is placed a photographic plate, P. In the 
absence of a magnetic field the rays travel in straight 
lines and produce an effect on the plate vertically 
above the source. If we suppose a strong field, 
applied at right angles to the plane of the paper, 
deviation takes place, and on reversing the field the 
rays are deviated in the opposite direction, so that 
the total deviation is doubled and can be measured 
more accurately. Under the conditions of the ex- 


periment, if any /?-rays were produced, as would be 
the case if a radium salt were used, these were de- 
viated to such an extent when the field was applied, 
that they ceased to pass through the aperture. 

Professor Rutherford has also succeeded in showing 
that the oc-rays are deviated by an electric field, and 
from the fact that the deviations in each case are in 
a direction opposite to that taken by electrons, it 
has been established that these particles must carry 
a different electric charge are, in fact, charged 

The absorption of the rays by matter of different 
kinds has been studied very exactly. This can be 
done easily in the case of polonium. The total 
amount of ionisation produced by any definite quan- 
tity is measured, and then the polonium is covered 
by successive layers of aluminium leaf of a thickness 
of one-hundredth of a millimetre, and the decrease 
of ionisation is measured after each addition. By 
such experiments it has been demonstrated that the 
rays are completely absorbed by a thickness of 0*06 
millimetre of aluminium, and also that the absorption 
is not directly proportional to the thickness of the 
covering layer, but follows an exponential law, so 
that the second layer of the same thickness absorbs 
approximately the same percentage of the rays which 
reach it as does the first, and so on. It has also 
been found that the a-rays are only able to penetrate 
a small length of air, or any other gas, at the normal 
atmospheric pressure. The length traversed varies 


according to the density of the gas, and also accord- 
ing to the source of the rays. As an example of 
the second variation the a-rays from radium are 
completely absorbed after passing through 3*5 centi- 
metres of air ; they then cease to produce any ionising 
effect. The a-rays from radium C, on the other 
hand, are only completely absorbed by 7*08 centi- 
metres of air ; that is to say, they have twice the 
penetrating power. The penetrating power of the 
a-rays is different for almost every element producing 
them, and affords, in consequence, a means of testing 
for a particular element, and of ascertaining whether 
more than one a-emitting element is present. 

We have seen that the a-rays are deviated by both 
a magnetic and an electric field, and hence it is 
possible to calculate the ratio of their charge to their 
mass in exactly the same way as has been indicated 
in the case of the cathode and the /?-rays. This has 
been done by Rutherford. His first results for the 
a-rays from radium gave figures for efm of 6 x io 3 
electromagnetic units, and for the velocity 2*5 X io 9 
centimetres per second. His later more exact figures 
are 5-07 X io 3 for the ratio, and for the initial velocity 
3 06 x io 9 centimetres per second. 

The rays from different sources show a constant 
value for the ratio, although their velocity shows 
distinct variations. This velocity and their penetrating 
power are directly proportional, and this is shown 
clearly in the following table. The figures in the 
last column are the thicknesses of air at ordinary 



atmospheric pressure necessary to completely absorb 
the radiations, and are therefore proportional to their 
penetrability. (The elements actinium B and thorium 
C will be dealt with in the next chapter.) 



Initial Velocity 
(cms. sec.). 

Cms. Air. 

Radium C . 

5-07 x io 3 

2 '06 X IO 9 


Radium A . 

5 '6 x io :! 

177 x io 9 


Polonium . 

5-4 x io 3 

173 x io 9 


Actinium B 

47 x io 3 


Thorium C . 

5-6 x io 3 

2*27 x io 9 


Considering the difficulty of the experiment owing 
to the slight deviations which are actually measured 
the constancy of the ratio of the electric charge 
to the mass can be held as established. It follows 
that the a-particle is of the same nature, whatever its 
source. The velocity is in all cases distinctly of a 
lower order than that of light. (Thus in the table 
above the velocities vary between 10,000 and 14,000 
miles per second, while that of light is 186,000.) It 
is also distinctly of a lower order than that of the 
/?-rays. The value of the ratio is nearly 3500 times 
less than that of the electron (17 X io 7 ). If the a- 
particle also carries a single electric charge, it follows 
that its mass will be about 3500 times greater than 
that of the electron. The mass of the electron has 
been calculated to be about one seventeen-hundredth 
or one eighteen-hundredth of that of an atom of 


hydrogen (see p. 21). The a-particle would, there- 
fore, have a mass about twice that of the hydrogen 
atom, and would have dimensions comparable with 
the atoms of the chemical elements. No element is 
known with an atomic weight of 2, that is to say twice 
that of a hydrogen atom. But the helium atom has 
a weight four times that of the hydrogen atom (an 
atomic weight of 4). We have seen that helium is 
formed by the disintegration of niton, radium emana- 
tion, and also by the disintegration of polonkim. 
The suggestion that the a-particle is in some way 
connected with the helium atom is obvious. Ruther- 
ford and Soddy suggested either that it was a helium 
atom carrying two electric charges or that it consisted 
of a half-atom of helium. Later work by Rutherford 
has clearly substantiated the first supposition. 

The following experiment shows the transformation 
and identity in a direct manner. Radium emanation 
from three-tenths of a gram of radium was purified in 
the usual manner, and was compressed into a narrow 
capillary tube made of very thin glass, A, less than 
one-hundredth of a millimetre in thickness. Most 
of the a-particles could penetrate this thickness 
of glass. The tube was surrounded by a cylindrical 
tube of glass, B, above which was connected a 
capillary tube, C, of ordinary thickness ; into this 
were sealed platinum electrodes. At the commence- 
ment of the experiment as perfect a vacuum as 
possible was established in the outer tube. Accord- 
ing to the known properties of the a-particles a large 


proportion of them should succeed in penetrating the 
thin capillary tube, and they, or whatever substance 
they gave rise to, would be collected in the space 
between the two tubes. From time to time by rais- 
ing the mercury in the reservoir attached to the 
outer tube, any gas present was compressed into the 
capillary, and by passing an electric current and 
observing the spectrum the nature of the gas was 
ascertained. After twenty-four hours no trace of 
helium was found, but the strongest line in its 
spectrum was distinctly, although very feebly, visible 
after two days, and after six days all the lines 
of the helium spectrum were visible. 

The impermeability of the thin capillary to helium 
itself was proved conclusively by filling it with helium 
under the same conditions. Observations over a 
similar period of time showed that no helium had 
escaped from the inner tube. The only conclusion 
is that the helium produced is formed from the a- 
particles, and we must therefore consider that the 
a-particles are atoms of helium carrying two positive 
charges of electricity. 

Delicate experiments carried out with polonium in 
a vacuum as nearly perfect as possible, in order to 
avoid ionisation, have shown that the a-particles im- 
pinging on a metal surface connected to an electro- 
meter, communicate to it a positive electric charge, 
and that, therefore, the a-particles are themselves 
positively charged. Assuming that each particle 
carries only a single charge, from the magnitude of 



/ \ 

FIG. 17. 


the charge produced, it has been calculated that the 
number of a-particles emitted per second by a gram 
of radium in equilibrium with its emanation is about 
1 6 X io 10 . 

The actual number of a-particles has been 
measured by Professor Rutherford in two ways, 
quite distinct from that just mentioned. As we have 
seen, the range of an a-particle in air is small, only a 
few centimetres, when the air is at ordinary pressure. 
If, however, the pressure is reduced, the range of the 
particles is increased proportionately, and at very 
small pressures the range may even extend over 
several yards. Again, it has been found that an ion 
produced in a gas under reduced pressure, and under 
the influence of a very strong electric field, can itself 
by collision produce other ions, which in their turn 
by collision, produce more, so that the initial effect is 
continuously magnified. In this way the effect pro- 
duced by an a-particle can be registered at a very 
considerable distance from the source of the particle. 
Rutherford's first method, an electric method, took 
advantage of these facts. 

Radium C was used as the source of a-particles. 
Radium C emits all three types of rays. It is 
deposited along with its predecessors radium A and 
B on any negatively charged surface in contact with 
decaying emanation. A strongly active deposit was 
prepared. Radium A emits a-particles as well as 
radium C, but by waiting until, as shown by measure- 
ments in an electrometer, the rapid initial drop in 



activity was completed, correspond- 
ing to the rapid initial decay of the 
radium A, a deposit was obtained, the 
a-particles from which were emitted 
solely by radium C. The amount of 
radium C was measured by compar- 
ing its penetrating radiation with that 
of a known quantity of radium salt in 
equilibrium with its emanation, and, 
therefore, also with radium C, and 
the same method allowed the rate 
of decay of the radium C to be con- 
trolled after it had been introduced 
into the apparatus, and during the 
course of the experiment. The ap- 
paratus is shown in Fig. 18. It con- 
sisted essentially of two long cylin- 
drical glass tubes, separated by a tap 
of large bore. The first tube, A, was 
about five yards in length, the second, 
B, only twenty-five centimetres (just 
under a foot). The metal plate con- 
taining the active deposit was placed 
within the hollow tap, C, ground 
accurately to fit the apparatus, and 
inside an iron tube which could be 
brought to any position in the outer 
tube by a magnet. The tube A was 
connected to a mercury pump through 
the side tube shown in the diagram, 
G 2 


and was evacuated as far as possible. The tube B 
was also evacuated in a similar manner, but not to 
the same extent. On turning the tap between the 
tubes they remained separated by a mica window, D t 
set within a metal frame closing the tube connecting 
A and B. B contained a carefully insulated wire 
joining the apparatus to an electrometer. So long 
as the tap remained open, a certain number of a- 
particles penetrated the mica window and produced 
ions in B, which by the system of magnification 
previously explained, were duly registered by the 
electrometer. The distance of the radioactive matter 
was made so great that only a few ions penetrated 
the window per minute, and the electrometer was so 
arranged that each of these produced a deflection 
which could be observed separately. By counting 
the number of deflections (by the usual mirror 
arrangement on the electrometer needle) the number 
of a-particles was also counted. But a-particles were 
being emitted in all directions in the tube A. Only 
a certain small percentage was directed towards the 
window and penetrated it. This percentage was 
calculable from the law of chance, knowing the size 
of the window and its distance from the radiant 
source. From this percentage and the actual number 
of particles measured, the total number emitted from 
the deposit was at once found, and since the amount 
of radium C was known, the whole of the facts 
required had been ascertained. Rutherford found 
that the number of particles emitted by a quantity 


of radium C in equilibrium with a gram of radium 
was 3*4 X io 10 per second. 

The second method adopted was an actual enumera- 
tion of the scintillations produced by the particles on 
a zinc sulphide screen. The production of such a 
phosphorescence has already been mentioned, and its 
utilisation in the spinthariscope, invented by Sir 
William Crookes (see p, 41). When one looks 
through the microscope of this instrument, a picture* 
like that of a starry sky is seen, but a sky in which 
the stars have only an instantaneous appearance and 
are constantly replaced by new ones. On the assump- 
tion that each scintillation is produced by a single 
a-particle, and by removing the source of the par- 
ticles to such a distance that only a few scintillations 
take place per minute, it becomes possible to count 
these, and by means of a calculation similar to that 
employed in the electric method, to estimate the 
total number of particles emitted from a definite 
quantity of material. The figures actually found 
agreed within four per cent., an agreement truly 
remarkable when the great difficulties of the experi- 
ments are taken into consideration, and when it is 
remembered that in these experiments one is actually 
counting atoms. This agreement also shows that the 
hypothesis that each a-particle produces one scintilla- 
tion is a correct one, so that, when looking through a 
Crookes spinthariscope one actually sees effects which 
are produced by single atoms. 

From Rutherford's exact measurements of the 


total charge carried by the a-particles from a definite 
quantity of active material, and of the total number 
of particles emitted, he has calculated that the value 
of the charge e, carried by a single particle, is 
9'3 X 10 ~ 10 electrostatic unit. This is far greater 
than the elementary charge of an atom calculated 
by other methods, and, assuming that the a-particle 
carries two such charges, Rutherford concludes that 
*the elementary charge of electricity, the unit, is 
4-65 x 10 - 10 electrostatic unit. 

An a-particle is, as we have seen, shot out from the 
active element at a very high velocity which varies 
within slight limits, according to the nature of the 
element producing it. This velocity continually 
decreases, and after travelling through a relatively 
short thickness of air or other material, the a-particle 
disappears as such that is to say it loses its power 
to ionise, it loses its electric charge and its power 
to produce secondary rays, and there remains an 
ordinary atom of helium, uncharged. The velocity 
just sufficient to produce ionisation, and below which 
the a-particle loses its identity, is known as the 
critical velocity, and is the same whatever the source 
of the particle. It is about O'8 X io 9 centimetres per 
second, /.. about two-fifths of the original velocity 
of the particle emitted by radium C. The idea 
which most readily occurs to account for this decrease 
of velocity is that, in accordance with the kinetic 
theory of gases the a-particle suffers a continual 
series of collisions. But if this were really the case, 


as Sir Joseph Thomson has pointed out, the course 
of the particle would be much shorter than it actually 
is, and would quickly cease to be rectilinear, indeed 
the path would in all probability alter after the first 
collision with a gas molecule. It has been calculated 
that if perfect elasticity existed, such as is presupposed 
by the kinetic theory, four such collisions, if direct, 
would suffice to reduce the velocity of the particle to 
one-half, while a slightly larger number would be 
required if the collisions were not end on, but were 
in different directions. The number of collisions 
which one molecule would make in traversing a 
length of one centimetre of air at ordinary pressure 
has been calculated to be somewhat over 100,000 ; 
and so it is apparent that, according to the kinetic 
theory, the path of the particle would be only 
a minute fraction of that actually observed. The 
theory of collisions is also the one which most 
naturally accounts for the ionising properties of the 
particles, but it is evident that the collisions, if they 
take place, are not comparable with those which take 
place in a gas under normal conditions. Thomson 
thinks that the difference is to be attributed to the fact 
that the particles are electrically charged. Madame 
Curie is of the opinion that the determining factor is 
the velocity, that as soon as the velocity has fallen 
below the critical figure, the particle is discharged in 
the usual way by collision with some ion of opposite 
electric charge, and that a similar ionising phenomenon 
would be exhibited by an uncharged particle of 


helium, if a similar velocity could be imparted 
to it. 

We have seen that radium, by its disintegration, 
produces niton, and that their measured atomic 
weights are so nearly the same that it almost 
necessarily follows that one atom of niton is produced 
from one atom of radium. We have seen that radium 
emits a-particles, and that helium is also produced by 
its d^ay, and, further, that the a-particle is merely an 
atom of helium electrically charged and travelling 
with an enormous velocity. As far as we know, there 
are no other products from the decay of radium. If 
we write the change in the form of an equation, we 

Radium = Niton + Helium 
226'5 223 4 

and the numerical data lead us to conclude, finally, 
that from one atom of radium only one atom of niton 
and one atom of helium, i.e. one a-particle, are 
liberated. Since we know the atomic weights of 
radium and of helium more accurately than that of 
niton, we are now justified in deducing that by sub- 
traction it is, therefore, 222^5. 

Of the products of radium, four emit a-particles : 
niton, radium A and C, and polonium. (Each of 
radium B, C, and E emit ft- and y-rays.) We can 
apply similar reasoning to each of these four. The 
loss of a few electrons will not affect the atomic 
weights by a measurable amount, and, therefore, 


provided it be true in each case that but one a-particle 
be emitted per atom disintegrating we obtain the 
following results 

Niton (222-5) = Radium A (218-5) + Helium 


Radium A (218*5) = Radium B (214-5) + Helium 


Radium B (214-5) = Radium C (214-5) + one r 
more electrons. 

Rndium C (214-5) = Radium D (210*5) + Helium 
(4) + one or more electrons. 

Radium D (210*5) Radium E (210*5). 

Radium E (210-5) = Polonium (210-5) + one or 
more electrons. 

Polonium (210-5) = Unknown element (206*5) + 
Helium (4). 

The element whose atomic weight is nearest to 
206-5 is lead ; the latest determination for this element 
is 207*1. The figures approximate sufficiently to 
support the idea that the final product of this 
gradual atomic decay is an atom of lead. If this 
were the case, then the presence of lead should be 
expected in all minerals containing radium, and the 
older the mineral the larger the amount of lead which 
should be found. Some figures bearing on this 
question will be discussed in a later chapter. 1 

1 In Appendix C a full list is given of all the known radio- 
active elements and their most important properties. 


The rate of decay of radium D is so slow one-half 
is transformed in seventeen years that, when radium 
is allowed to remain until it is in equilibrium with its 
immediate products, the amount of radium E and 
polonium present is absolutely negligible ; there 
are produced in the course of disintegration of a single 
radium atom, until the radium D stage is reached, 
four ce-particles (from radium, niton, radium A, and 
radium C) and also ^-particles from radium B and 
radium C. The number of the latter, while not yet 
definitely fixed, is evidently of the same order, since, 
while Makower's figure for these is ic 11 (per second 
per gram of radium in equilibrium with its products), 
the number of a-particles will be, according to the 
hypothesis that one is emitted per atom for all four 
elements, four times 3*4 X io 10 , i.e. 13*6 X io 10 . This 
hypothesis receives support in the actual measure- 
ment of the helium produced from a definite quantity 
of radium over a definite period of time. According 
to the hypothesis, four atoms of helium are produced 
per atom of radium disintegrating. The rate of this 
disintegration is known the half-life period, as has 
been shown on p. 57, is 1760 years and, therefore, 
the number of radium atoms disintegrating in the 
time of measurement can be calculated, so that finally 
we can estimate exactly how much helium should 
be produced. The agreement between the calcu- 
lated and the measured amount is very good. The 
calculated figure per gram of radium is 0*43 cubic 
millimetre per day, or 158 cubic millimetres per year 



Sir James Dewar, by actual measurement, using 
70 milligrams of radium chloride, found that the 
figure per gram of radium was 0*463 cubic millimetre 
per day ; while Boltwood and Rutherford have 
obtained the figure 0*428 cubic millimetre ; so that 
there is every reason to believe that the hypothesis 
on which the calculation is based is a correct one. 
The extremely good agreement, considering the very 
great experimental difficulties, which these figures 
show, allows an extension of the method to determine 
the age of radioactive minerals. Such minerals in- 
variably contain helium; and by measuring the amount 
of helium present, on the assumption that it was 
originally produced in the form of o.-particles, the time 
necessary to allow such an accumulation can be readily 

The following table shows the relative thicknesses 
of air and of aluminium which will cut ofif half the 
respective radiations (it has already been pointed out 
that the absorption follows an exponential law) 

Type of Ray. 

Length of Air 
(in cms.). 

(in cms.). 

Cathode rays 



Alpha rays (uranium) . 



Beta rays (uranium) 



Hard Rontgen rays 



Gamma rays (radium) . 



Roughly the relative absorptions of the three rays 
are of the order I : 100: 10,000. 



The ionisation produced in any gas by any of the 
radiations is proportional to the density of the gas, 
other conditions temperature and pressure being 
the same. This is shown by the next table. In 
each case air has been chosen as the standard of 


Relative ionisation produced by 



! a-rays. 


7- rays. 


Air ... 
Oxygen . 
Carbon dioxide . 
Sulphur dioxide 



I j 


i '54 


I -21 





o 114 

I -60 
I-0 5 

The total ionisation had only been measured de- 
finitely in the case of the a-particles, but, roughly 
speaking, the ionising power is inversely proportional 
to the absorption in the three cases. 

It has been mentioned already that the extremes 
of pressure have no effect on the rate at which 
disintegration takes place ; we can compare with this 
the experiments of Sir James Dewar, and of Monsieur 
P. Curie on the effect of temperature. These savants 
have found that there is no change in the rate of decay 
as shown by the rate at which the radiations were 
emitted between the temperatures of 255 and + 
800 Centigrade ( 427 and + 1472 Fahrenheit). 
At the temperature of liquid air ( 185 C.) radium 
induces the double sulphate of uranium and potassium 


to phosphoresce as readily as it does at ordinary 
temperatures, showing that the emission of radiations 
is taking place normally. All experimental evidence 
tends to show that the disintegration of these active 
elements with all the specific phenomena to which it 
gives rise is completely independent of all external 
causes, and is only governed by some change 
taking place within that atom which is actually 

The production of phosphorescence was mentioned 
when dealing with radium and niton. It is not 
specific for these two elements, but is produced by all 
which emit radiations, the degree being proportional 
to the degree of activity. Moreover, all strongly 
active substances phosphoresce strongly themselves. 
Perhaps this statement requires a little modification. 
Pure niton does not phosphoresce itself, but every- 
thing in which it is contained, everything which is 
under the bombardment of its emitted rays phos- 
phoresces very brilliantly, so that the gas has the 
appearance also. The solid active substances, as long 
as their activity is considerable, even when they con- 
tain a considerable percentage of impurity, of inactive 
matter, certainly phosphoresce strongly themselves. 
They are strongly luminescent ; they give out light. 
By comparison we may conclude that particles of 
these substances are in a state of incandescence 
through the bombardment of radiations emitted by 
other particles. The effect of such luminescence is 
well shown by a comparison of the two parts of 



Fig. 19. The first is a photograph of a pure salt of 
radium taken by ordinary light, and gives a very 
good idea of the appearance of the freshly prepared 
pure salt. The second is a photograph of the salt 
taken by the light which the salt itself emits. 


FIG. 19. 

The similarity in the photographic effects produced 
by the rays from different elements is shown by the 
following two figures (20 and 2I), 1 and the slight 
differences are just what would be expected from the 
great differences of the activities ; the more strongly 

1 Figs. 19, 20 and 21 are reproduced from Madame Curie's 
Trdite dc Radioactivity by kind permission. 


active preparation, that from radium, gives a clearer 

One final effect of radium rays and all other radia- 
tions may be mentioned ; it is also produced by 
X-rays and cathode rays. Water in the state of gas 

FlG. 20. Radiograph of a medal, obtained by the action of 
uranium rays. 

is perfectly invisible. Steam, so called, consists ol 
very minute particles of liquid water. Air under 
normal conditions contains always a certain per- 
centage of water vapour. The amount which air 
can contain without condensation of liquid water 



depends on the temperature, increases . as that is 
raised, decreases as it is lowered, and by decreasing 
the temperature sufficiently condensation of water 
will always take place. The phenomenon produced 

FIG. 21. Radiograph obtained by the action of radium rays 

by bringing a cool surface into contact with hot moist 
air (when drops of moisture immediately condense on 
the surface) is known to everybody. Air which is 
saturated with water vapour can be cooled consider- 
ably, and no condensation will take place provided the 


air is absolutely free from dust particles. Introduc- 
tion of dust particles will at once cause condensation 
of water in the form of a cloud of moisture, and the 
same effect is produced by all radium rays and by the 
X-rays. Here ion particles take the place of dust 
particles. It was by the utilisation of this method 
that Sir Joseph Thomson, using X-rays to produce 
the effect, succeeded in measuring the actual electric 
charge carried by a single cathode ray, by a single 
electron. 1 

1 For the details of his experiments and the conclusions which 
he derived from them the reader must again be referred to 
Professor Pellat's volume, The New State of Matter, in this 



IN previous chapters three definite elements have 
been dealt with which show all the distinctive 
properties that usually define an element, and in 
addition are radioactive. These three are radium, 
niton, and polonium. It is possible that by further 
work enough radium D may be accumulated to show 
that it also has the qualities of a distinct element 
especially the emission of a definite spectrum, and 
the possession of a distinct atomic weight. But in the 
case of the elements radium A, B, C, and E, it is 
almost impossible that any quantity of these other 
than infinitesimal can be collected at any one time, 
no matter what quantity of radium becomes available, 
on account of their very short life-periods. Never- 
theless there is now strong ground for adding to the 
characterisation of an element the possession of 
definite radioactive properties such as (i) the posses- 
sion of a life-period (or, more correctly, a half-life 
period, cp. p. 57) different from all others since, up 
to the present time, no two elements have been 
discovered which have exactly the same half-life 
period ; (2) the emission of a definite radiation, a or/5, 



having a definite penetrating power, and so on. And 
since these elements, radium A, B, C, and E, possess 
such definite properties, we must consider them also 
as elements, until some new evidence is put forward 
against this theory. It should be added that such 
physical and chemical properties as they can be 
shown to exhibit by ascertaining with what elements 
they are precipitated from solution, and determining 
the temperatures at which they volatilise, also support 
the view of their individuality. The greater number 
of the substances which will be discussed in this 
chapter are considered elements on similar evidence. 

These elements are all new, and in all probability 
would never have been discovered without the aid of 
radioactive methods. But two elements have been 
mentioned, both of which are radioactive and both of 
which were discovered long before the science of 
radioactivity was dreamt of. These are uranium, the 
first element shown to be active, and thorium, whose 
activity was discovered independently by Madame 
Curie, and Schmidt. 

It has been pointed out that pitchblende, the source 
of most of the radium at present in existence (that is, 
in a condition of approximate purity), consists largely 
of uranium oxide, and it was in this mineral that 
uranium was actually discovered in 1789, 107 years 
before the discovery of its activity, by the German 
chemist, Klaproth. It is by no means a common 
element, but the minerals which contain it are found 
widely scattered. The richest ores are at Joachims- 

H 2 


thai, in Bohemia, and near St. Ives in Cornwall. The 
composition of the different uranium minerals varies 
largely ; pitchblende contains over seventy per cent, 
of uranium oxide (see p. 32), but many of the 
others are complex silicates, phosphates, or arsenates. 
Metallic uranium was isolated by Peligot in 1842 by 
the action of metallic potassium on the fused oxide. 
When obtained in this form it is a black metallic 
powder, but in the ordinary metallic form in which 
it can be easily obtained by fusing the powder in the 
absence of air it closely resembles such metals as 
iron and nickel. Heated to redness in air it burns 
brilliantly, forming the oxide, while it volatilises at 
the temperature of the electric arc. It dissolves in 
dilute acids with the formation of uranium salts and 
the evolution of the gas hydrogen. Its atomic weight 
is 238-5 ; this is greater than that of any other element 
yet discovered. Uranium salts are in many cases 
coloured yellow, and some of them have been observed 
to phosphoresce spontaneously. Figures have been 
quoted in Chapter II (see p. 30) which prove that 
the activity of uranium salts is an atomic property 
of uranium itself; the degree of phosphorescence 
exhibited is of the order to be expected from the 

Thorium was discovered by the Swedish chemist, 
Berzelius, in 1828. It does not occur in large deposits, 
although traces of thorium minerals, such as thorite, 
orangite, monazite sand, are found widely scattered 


throughout the earth's surface. Thorium is usually 
prepared in pure form as the oxalate. The metal has 
been obtained as a grey powder. Like uranium it 
volatilises in the electric arc, burns brilliantly when 
heated in air, and is easily dissolved in warm 
dilute acids. The oxide, ThO 2 , can be obtained by 
calcining either the oxalate or the sulphate. The 
salts of thorium do not show any spontaneous phos- 
phorescence such as has been observed in the case 
of uranium salts ; thorium salts are colourless. The 
activity of uranium and thorium will be dealt with 
later. All the evidence shows that each of these 
elements is decaying at an extremely slow rate, so 
that their half-life periods are at least hundreds of 
millions of years. There is so far little definite evidence 
to show which of the two is decaying at a more rapid 
rate. The atomic weight of thorium is 232-4 ; it is 
the second heaviest element known. Since all the radio- 
active transformations exhibit a decrease in atomic 
weight we may conclude with some degree of confid- 
ence that thorium does not give rise to uranium, 
although the converse statement is not so certain. 

We have seen that when radium and its products 
are allowed to remain without separation a condition 
of equilibrium results in which the ratio of the 
amount of radium to the amount of, say, niton 
becomes a constant. This is due simply to the com- 
paratively much slower rate at which radium dis- 
It will be remembered that the half-life 


periods, which are inversely proportional, roughly 
speaking, to the rates of decay, are respectively 1760 
years and 3'86 days. A condition is quickly reached 
at which as much niton is formed per second as is 
destroyed per second ; the actual amount present at 
any definite time has then attained a constant value 
and the condition of equilibrium has been reached. 
Again, since the half-life period of radium A is only 
a few minutes, is less than a thousandth part of that 
of niton, a condition of equilibrium will be quickly 
attained between niton and radium A, and if the 
niton is also in equilibrium with radium there will be 
equilibrium between the three, and the ratio of the 
amounts present will be constant. Considering th? 
slow rate at which radium changes it will even be 
correct to say that the actual amounts present will 
be constant (over a long period of time). 

The same reasoning applies in all cases, provided 
that the element which is formed ha; a shorter life- 
period than the element from which it is formed. If 
this is not true there are two possibilities. If any 
precursor of these elements is present which has a 
longer life-period than either, a condition of equi- 
librium will be obtained, since the amount of the 
intermediate element will be maintained at a constant 
figure by the decay of its precursor. As an example 
of this, radium, niton, and radium D may be selected. 
The last-named element has a half-life period of 
seventeen years. But if all three are present, the 


amount of niton is kept constant, and in spite of its 
shorter life and thos^ of the other intervening ele- 
ments, a condition of equilibrium will ultimately be 
reached at which just as much radium D is formed in 
any given time as decays in that time. The reason- 
ing may be extended even further to include polonium, 
which has a half-life period of 136 days. In the 
presence of radium, which has a much greater half- 
life period than any of the elements in its series, a 
condition will ultimately be attained in which all the 
series are present in amounts which for any one of 
them bears a constant ratio to the amount of radium 
present. Should only niton and its successors be 
present together the second possibility is exemplified. 
Radium D has a much longer period than niton ; its 
rate of decay is so much the slower. As we have 
seen, in a fairly short time at the en j of a month 
practically the whole of the niton has disintegrated, 
while after a few hours any definite amount of radium 
A, B, or C, separated from niton, has vanished. The 
radium D produced in these changes, however, is 
decaying very much more slowly, and its amount is 
scarcely affected by a month's loss. Practically the 
whole of the niton will have changed into radium D, 
while but the merest fraction of the latter has itseh 
changed, and a condition of equilibrium between 
these two elements can never be attained in the 
absence of radium. 

Supposing for one moment that uranium were 


decaying to give rise to thorium, then, if the half-life 
period of thorium is the smaller, a condition of 
equilibrium should be attained between the two, and 
in very old minerals the amounts of the two present 
should bear a constant ratio. Such a constant ratio 
has not been observed. If the life of thorium is 
longer than that of uranium, then equilibrium will 
never be attained, while if both elements have lives 
of many millions of years, as seems probable, then, 
even if the life of thorium is the shorter, the .minerals, 
the earth itself, may not yet have reached an age 
great enough for equilibrium. It is doubtful whether 
any uranium minerals exist which are absolutely free 
from thorium. There would seem to be no reason 
for this fact if there were no relationship between the 

An easier problem is furnished in the case of 
radium. Ever since its discovery, it seemed very 
improbable that its constant appearance in uranium 
minerals, and only in uranium minerals, was to be 
attributed solely to chance. Since the determination 
of its atomic weight, and the discovery of the part 
which helium plays in the atomic changes, any such 
fortuitous appearance seems quite impossible. The 
atomic weight of radium is 226*5. That of' uranium 
is 238'5. The difference, 12, is the weight of three 
atoms of helium. The period of radium is 1760 years. 
That of uranium is of the order of a thousand million 
ye.irs. If radium is produced from uranium by loss 


of helium, and if between the two there is not inter- 
posed an element of life longer than uranium, then 
in all uranium minerals a condition of equilibrium 
between uranium and radium will probably have been 
attained, and in that case there should be a constant 
ratio between the amounts of the two present. 
Careful analysis of the minerals should afford a test 
of the hypothesis. Such analyses have been carried 
out by a number of observers. The earlier experi- 
ments seemed to show that a constant ratio actually 
existed in different minerals. Boltwood's results can 
be taken as examples. His method of determining 
the amount of radium present in any given sample of 
mineral consisted in taking about a gram of mineral, 
finely powdering it, and allowing it to remain for the 
space of about a month ; in that time, as we have 
seen, a condition of equilibrium is always attained 
between the radium and niton. At the end of the 
time as much of the niton as possible was removed 
by passing a current of air over the powdered mineral. 
This amount was carefully measured by radioactive 
methods. The mineral was then dissolved in acid, 
the solution was boiled to expel all gas, and the niton 
in the expelled gas was also measured. From the 
total amount of niton found, the total amount of 
radium present was calculated. The method supposes 
that all the niton had been completely removed from 
the mineral. In the following table of Boltwood's 
results the amounts of emanation (to which the 



amounts of radium are proportional) are given in 
arbitrary units 

Amount of 

Amount of 




Niton in one 

(in grams] 

Ra Ur. 

gram mineral. 

in one gram 


N. Carolina 












N. Carolina 







2 2O 









Brazil . 



22 3 



Thorite . 

The constancy of the ratio in these results (which 
form only a small part of those actually obtained) 
gives strong support to the view that radium is 
derived from uranium. Some recent work by Mile. 
Gleditsch seems to show, however, that the ratio is 
perhaps not quite so constant as here indicated, and 
that the error in Boltwood's work is probably from 
loss of niton in the dissolution of the mineral. Her 
method consisted in taking about fifty grams of 
mineral in each case, dissolving it by means of acids, 
precipitating the radium by successive additions of 
barium chloride and sulphuric acid (when barium 
sulphate is precipitated, and with it radium sulphate) 
and estimating it by a determination of its activity, 
and, finally, determining the uranium in the solution 



from which the radium has been removed. Some of 
her results are shown in the following table 



Per cen 


Per cent. 

Ra Ur. 

Chalcolite . 


0714 X 1 

o- 5 


I 82 X I 

Or 7 

Carnotite . 







I -2O 




I '22 


2 59 


Joach imsth 


I- 4 8 





I 7 8 


3 23 

Bioggerite . 










Chalcolite . 



39 '03 

3 '33 

Gummite . 


o - 58 

17 37 


Samarskite . 


o 295 


3 '35 

Thorianite . 










The ratio is fairly constant for most of the minerals 
examined. The smallest figures are obtained for a 
chalcolite and the autunites, the largest for the 
thorianite and a sample of pitchblende. If the theory 
is correct, the discrepancies are far too great to be 
considered due to experimental error. The explana- 
tion put forward to account for them is that since 
from geological dati it seems likely that the chalcolite 
and autunites giving low results are the most recently 
formed of these minerals, it is probable that in them a 
condition of equilibrium between uranium and radium 
has not yet been reached. That the uranium-radium 
change actually occurs was determined by direct 
experiment before the publication of these figures, 


From the rate at which radium decays, and from 
the amount actually found in equilibrium with a 
definite quantity of uranium in the majority of the 
experiments quoted, it can be calculated, on the sup- 
position that uranium directly forms radium, that in 
one year about 1*3 X io~ 7 gram of radium, should be 
formed from one kilogram of uranium. Mr. Soddy 
has measured the amount formed. After the expiry 
of one year no trace of radium was observed, but a 
definite production was observed in four years, the 
amount being 5-2 X io~ n gram, 1 a quantity less than 
one two-thousandth of that expected. This experi- 
ment showed beyond question that radium is derived 
from uranium, but it also shows that there must be 
some intermediate product. This intermediate pro- 
duct must have a fairly long half-life period in 
comparison with radium, to account for the magnitude 
of the discrepancy, but not in comparison with 
uranium, in view of the approximate constancy of the 
uranium-radium ratio. 

One product of uranium has been mentioned 
already (p. 90), and a method described by which 
it can be separated from that clement. But a study 
of this product, uranium X, has shown that its half- 
life period is only 22 days, so that the existence of 
this link in the chain of changes would not materially 

1 At the end of four years it was possible to drive off suc- 
cessive quantities of radium emanation, which could be identified 
beyond doubt from its rate of decay; from the amount of emana- 
tion in any given time the amount of radium was calculated. 


affect the calculation. It is evident that it is not to 
this intermediate product that the discrepancy is due. 
From the magnitude of the discrepancy Soddy cal- 
culated that the half-life period of the unknown war. 
at least 35,000 years. 

A systematic search for this element was made and 
it was discovered by Boltwood, who named it ionium. 
It is an element which strongly resembles thorium in 
all its chemical properties. It emits a-particles, and 
these have the extremely short range of only 2*8 
centimetres in air; the figure is the smallest for any 
of the known elements which emit a-particles, and in 
itself serves to differentiate ionium completely from 
any other active element. 

It has recently been shown that ionium gives rise to 
helium ; in an actual experiment I'S grams of impure 
ionium, 3000 times more active than uranium, pro- 
duced 0*031 cubic millimetres of helium in 125 days. 

The complete series of changes between uranium 
and radium is not yet made out with certainty. As 
we have seen, the difference between their atomic 
weight suggests the emission of three a-particles in 
the changes before an atom of uranium is transformed 
into an atom of radium. Uranium itself emits an a- 
particle, so does ionium ; one remains unaccounted 
for. No other intermediate element has been dis- 
covered. There is some evidence that the atom of 
uranium may have the peculiar property of emitting 
two a-particles when it disintegrates. 1 The matter 

1 It has been found by actual measurement, using Ruther- 


cannot be regarded as settled. However, it is 
definitely proved that radium is formed from uranium 
whatever the intermediate steps, and we may regard 
the uranium series as commencing with uranium and 
ending with polonium (or perhaps lead). 

No relation can be shown to exist between thorium 
and radium ; on the other hand, thorium produces 
a series of elements the thorium series, including 
members almost as important as radium itself, and it 
appears likely that one of these, mesothoriuw^ will be 
employed largely as a substitute for radium. 

Thorium emits a-particles. The magnitude of its 
activity shows no change with lapse of time. From 
theoretical conclusions its life-period is calculated to 
be io 10 years. From the disintegration of thorium 
mesothorium is formed. This element was discovered 
by Hahn in 1907. It can be obtained from thorium 
salts by precipitating the thorium from solution by 
means of ammonia, and evaporating the ammoniacal 
filtrate. Mesothorium obtained in this way is not a 
simple substance. By further chemical treatment 
(addition of ammonia in presence of zirconium 
chloride) the mixture is resolved into a rayless 
substance, mesothorium i, and a smaller amount of 

ford's methods, that in old uranium minerals, in which radio- 
active equilibrium has been established, all the a-ray products 
emit an equal number of a-pai tides per second except uranium 
itself, which emits twice as many. Hence either uranium emits 
two a-particles per atom, or it is in equilibrium with an a-ray 
product which has not yet been separated from it. 


a substance emitting /?-ra) s, mesothorium 2. The 
first substance grows continually in activity, /?-rays 
being emitted, due to the production of the second, so 
that after a time a fresh quantity of mesothorium 2 
can be removed by repetition of the chemical treat- 
ment. Mesothorium i has a half-life period of 5-5 
years. In its chemical properties it resembles radium 
very closely, so much so that up to the present ^ime 
no means have been found of separating these two 
elements. They frequently occur together; they 
must do so, of course, in all minerals containing both 
thorium and uranium, and many such are known. 
Mesothorium 2 has a half-life period of only 6'2 hours. 
From it is formed the strongly radioactive substance 
radiothorium, which has a period of 713 days, and 
emits a-particles. Radiothorium by its decay pro- 
duces thorium X with a period of 371 days, a figure 
very nearly the same as that of niton. 

Radiothorium bears a very strong resemblance to 
thorium itself, and cannot be separated from it. 
This close resemblance is shared with ionium, which 
as we have seen is one of the intermediate products 
between radium and uranium. Thorium X resembles 
mesothorium and radium just as closely, so that we 
have here two triplets of elements between which all 
the resources of chemical analysis have so far failed 
to distinguish. Radioactive analysis at once dis- 
tinguishes them, both by the radiations they emit 
and by the products they give rise to ; it, however, 
gives no clue to a means of separation. 


From thorium X a gas is produced, a so-called 
radioactive emanation, emitting a-particles, and in all 
respects save two strongly resembling niton : it has 
a very much shorter life-period (one half of it disap- 
pears in fifty-three seconds), and it gives rise to a 
different series of products. All the chemical and 
physical properties of thorium emanation show that 
it is? like niton, one of the rare or inert gases. The 
most violent chemical reagents have no effect on it. 
No treatment will cause it to enter into chemical 
combination. No treatment affects its active pro- 
perties in the least degree. Its atomic weight seems 
also to approximate very closely to that of niton. 
We have seen that in the case of the latter we are 
justified in regarding the loss of an a-particle as 
accompanied by a decrease of four units in the atomic 
weight. If we make the same assumption here (this 
being, not that there is a loss of atomic weight of 
four that is certain, since the identity of helium and 
the a-particle is established, but that there is a loss of 
only four, that the atomic change is accompanied by 
the loss of only one helium atom), then, since the 
atomic weight of thorium is 232-5, and since between 
thorium and its emanation three a-particles are 
emitted (by thorium, radiothorinm, and thorium X), 
the atomic weight of the emanation should be 
232-5 12, i. c. 220-5. The atomic weight of niton 
has been found to be 222-5. 

On account of the extremely short life of the 
emanation of thorium its atomic weight can never be 


determined directly. Numerous attempts have been 
made to determine it by the method of diffusion 
already described (see p. 58), and these for the 
most part showed that the weights of the two emana- 
tions were of the same order of magnitude. The 
most recent determinations by this method point to a 
weight of approximately 200, so that since the experi- 
mental errors for such a short-lived product must be 
much greater than usual, and since the diffusion 
method itself is not capable of great accuracy, we 
shall probably be justified in accepting the figure 
220*5, theoretically calculated, as the correct one. 

Thorium emanation gives rise to four elements of 
very short life thorium A, B, C, and D (see Appendix 
C). Of these thorium B and C emit a-particles, and 
as the emanation has the same property, three a- 
particles are lost before the final inactive product is 
reached. This, therefore, should have an atomic 
weight of 220*5 12, i. e. 208-5. The atomic weight 
of the element bismuth is 2o8'O, and it has been sug- 
gested that bismuth is the final product of the thorium 

The emission of the a-particles results in the pro- 
duction of a relatively large amount of helium. 
Thorium itself is changing so slowly that the produc- 
tion of helium is much more difficult to ascertain 
than in the case of radium. It has been observed by 
Ramsay, while the presence of helium in minerals 
containing thorium, and its dependence on the pres- 
ence of this element has been demonstrated by Strutt. 


There is, so far, not much direct evidence to show 
that the radiations from the thorium series can bring 
about chemical action, though some experiments of 
Ramsay seem to show that a small quantity of 
hydrogen and oxygen is produced from a concen- 
trated solution of a thorium salt, and is produced 
presumably by the action of the radiations. 

We have seen that the chemical actions produced, 
the production of heat, all the abnormalities of radio- 
active substances are due, not to the elements, but to 
the radiations. Radium a few days old is in equili- 
brium with its emanation. Then as much niton is 
produced per second as decays per second. Therefore 
as many a-particles are emitted from the niton as 
from the radium. And extending the same argument, 
as many a-particles are in the same time emitted 
from radium A and radium C. If we remove the 
niton, its activity with that of its products is more 
than three times as great as that of the radium from 
which it is removed (since the /5-emitting elements 
are also removed), while the weights of the two are 
many thousand times different. Speaking roughly, 
the activities of equal quantities of two elements are 
inversely as their rates of decay. Radiothorium has 
a period of approximately two years. In the pure 
condition it would be many hundred times more 
strongly active than radium with a period of 1760 
years. And it gives rise to thorium X and a series 
of short-lived products emitting both a- and /?-rays, 
so that the combined activity will be several times 


greater than that of radiothorium itself. It cannot 
be easily separated from thorium, but mesothorium 
can ; and since mesothorium continually produces 
radiothorium, any method ivkick will place in our 
hands a moderately pure mesothorium product will give 
a source of radioactivity equal to or greater than 
radium itself. Such a method has been discovered 
by Hahn. He prepared the mesothorium (a mixture 
of mesothoriums I and 2, radiothorium, thorium X, and 
the subsequent products of short life) from monazite 
sand, which contains 0*3 per cent, of uranium, and four 
to five per cent, of thorium. The activity of the pre- 
pared product is due twenty-five per cent, to radium 
and, its series, and seventy-five per cent, to " meso- 
thorium." The maximum activity of the product is 
reached after a little more than three years, but after 
ten years it is still slightly greater than when pre- 
pared, and after twenty years is about half as great. 
When purified from inactive material as far as 
possible, the material is four times more active than 
radium, weight for weight. Since such a source of 
activity is available for a fairly long period, if it can 
be placed on the market at a reasonably low figure, 
it seems fairly certain that it will largely replace 
radium for all medicinal and other practical purposes. 
The price of radium is extremely high. The bromide 
costs at present about 20 a milligram. The sale 
of one-thirtieth of an ounce, a gram, were there a 
gram of material to be sold, would be only effected 
by the transfer of 20,000, and since there is no large 

I 2 


demand for uranium, there seems to be no reason to 
expect that the cost of radium production will ever 
be materially decreased. On the other hand, there is 
a fairly large market for thorium, so that thorium 
residues should become available for the preparation 
of mesothorium at a fairly cheap rate. We may also 
hope that further experiment will show a method for 
obtaining the series in a purer form. 

There remains a third series of radioactive elements 
to be described, the " actinium series." Actinium 
itself was discovered by Debierne in 1899, and inde- 
pendently by Giesel in 1902 ; the latter named it 
emanium. It is always found present in uranium 
minerals. It is precipitated with the so-called group 
of the rare earth metals, cerium, etc., and seems to 
resemble most strongly the element lanthanum. It 
has not yet been prepared in a pure state, though 
Debierne has succeeded in obtaining very strongly 
active preparations. It is apparently rayless, and 
neither its period nor its atomic weight is known, so 
that its position with regard to the other elements 
can only be conjectured. It gives rise to a series of 
strongly active products of short life. 

The first product is radioactinium, and was dis- 
covered by Hahn in 1906. It gives rise to actinium 
X, and from this the third emanation is produced, 
actinium emanation. This has all the inert properties 
exhibited by niton and thorium emanation ; it also 
must be considered as a member of the inert series of 


gases. It is particularised by having the shortest 
half-life period of the three emanations, 3^9 seconds. 
Since it disappears so rapidly, it is probable that it 
would have remained undiscovered had it not been a 
gas and so easily separable from the other members 
of the series. It gives rise to short-lived products 
much resembling those from the other emanations 
(see Appendix C). 

The most active preparations of actinium have 
been obtained by Debierne, and he has found that 
their general properties resemble those of radium 
preparations very closely. When dry, they ozonise 
the air with which they are in contact, and when dis- 
solved in water they decompose it, with formation of 
hydrogen and oxygen. Examination of the gases so 
produced shows that they contain an amount of 
helium of the same order as that obtained from radium 
solutions of equal activity. 

Neither the origin of actinium nor the final product 
of its disintegration is known. 

Attempts to measure the atomic weight of actinium 
emanation by diffusion measurements indicate that 
it is high, though the actual figures are usually dis- 
tinctly less than those for the other emanations. It 
is very doubtful, since this element decays so rapidly, 
whether any even approximately correct result can 
be obtained. 

Actinium emanation itself and two of its products 
emit a-particles, and if it be assumed that its atomic 
weight is in the neighbourhood of 222, then the 


atomic weight of the final product should be in the 
neighbourhood of 210. Since radioactinium and 
actinium X emit a- particles, on similar reasoning the 
atomic weight of actinium should be in the neigh- 
bourhood of 228, somewhat greater than radium. 
It invariably occurs with uranium, and it is usually 
regarded as being formed from that element, not in 
the line of descent of the radium series, but in a side 
line. Judging from the relative amounts of actinium 
and radium present in uranium minerals, if this theory 
is correct, Soddy has calculated that for every seven 
atoms of uranium which disintegrate, giving rise to 
radium products, one gives rise to actinium products. 

Below is shown what may be termed the radio- 
active family tree. The hypothetical relationships 
are shown by dotted lines. The nature of the radi- 
ations are shown in parentheses. The resemblance 
between the three emanations and their products is 
seen to be very great. 

Besides the elements described above, only two 
have been found which possess in the slightest degree 
the property of radioactivity. These are the common 
metal potassium and the element rubidium^ which is 
closely allied to it ; their radioactivity presents a 
hitherto insoluble enigma. The activity of these 
elements was discovered in 1906 by Campbell and 
Wood, and has been confirmed by numerous observers. 
The property is not shared by the closely allied 
elements sodium and caesium. 

Potassium and rubidium emit a continuous stream 

Uranium (a) . 


Thorium (a) 




Mesothorium I (a) 

(?) Radiouranium 

(?) Uranium Y (a) 




Mesothorium 2 (0) 

Uranium X (0) 

Actinium (a) 

Radiothorium (a) 

Ionium (a) 

Radioaciinium (a0) 




Thorium X (a) 

Radium (o) 

Actinium X (a) 




Thorium emanation (a) 

Niton (a) 

Actinium emanation (a) 




Thorium A (a) 

Radium A (a) 

Actinium A (a) 

Thorium B (0) 

Radium B (0) 

Actinium B (0) 




Thorium C (a) 

Radium C (a0) 

Actinium C (o) 




Thorium D (B} 

Radium D (?0) 

Actinium D (0) 





Radium E (0) 


Polonium (o) 



of ^-particles, very homogeneous and of small velocity 
in comparison with most of the other active elements. 
What causes this emission, and whether it is accom- 
panied by disintegration of atoms of the elements, as 
we should expect by analogy, and what are the pro- 
ducts of any such changes if they do take place, are 
questions absolutely unanswered up to the present 
time. It has been proved beyond doubt that the 
activity is not due to slight contamination with any of 
the elements more usually regarded as radioactive, 
and it has been shown that the degree of activity 


varies with the salt of potassium (or rubidium) and 
is always proportional to the amount of potassium (or 
rubidium} present, so that the activity is in these cases 
also an atomic property of the element, just as with 
the other active elements. 

In all some thirty-three or thirty-four elements 
have been found to be radioactive ; of these four 
were known previous to the discovery of radioactivity. 
It is extremely improbable that any of the others 
would ever have been discovered had they not 
possessed this remarkable property. Whether other 
elements are also undergoing slow changes or not it 
is impossible to state with certainty. We have seen 
several cases where changes are unaccompanied by 
emission of rays (actinium, radium D) ; had the pro- 
ducts of the change been inactive it could not have 
been discovered. Again, the emission of atoms of 
helium possessing an initial velocity of less than the 
critical figure (0*8 x io 9 centimetres per second, see 
p. 102) would not result in the production of ionisa- 
tion and consequently would not be detected. 
Furthermore, the unexpected cases of potassium and 
rubidium show that what are commonly regarded as 
ordinary elements can also possess active properties. 
// would seem, therefore, at least not impossible that 
many, if not all, of the elements are undergoing a slow 
process of change, of degradation, although this may be 
so slow tliat in millions of years the result would be 
barely perceptible. 



RADIOACTIVE changes are accompanied by a cpn- 
tinuous evolution of heat ; it has been remarked 
already that this heat evolution is sufficient to keep 
a salt of radium several degrees hotter than its 
surroundings, and an experiment has been described 
which demonstrated the effect for radium emanation 
in a very simple manner. The heat emission was 
first noticed by MM. Curie and Laborde in working 
with radium itself. They measured the effect in 
the following way. A small glass tube containing a 
gram of radium-barium chloride 1 of which about 
seventeen per cent, was radium chloride was placed 
inside a larger glass vessel, and in order to prevent radi- 
ation of heat this was enclosed inside a lead box, and 
that again inside a wooden box. An exactly similar 
arrangement contained, instead of the radium- 
barium chloride, a gram of pure barium chloride. 
Into each of the glass tubes was fixed one end of a 
thermo-couple, connected to a galvanometer, and the 
temperatures of the two salts were thus accurately 

1 A mixture of radium and barium chloride. 


measured. The difference of temperature actually 
found was 1-5 C. The rate at which heat was being 
emitted was measured by placing in the barium 
chloride a coil of wire of known resistance, which 
developed, therefore, a definite amount of heat on 
passage of an electric current ; the electric current 
was measured which was required in order to raise 
the temperature to that found for the radium prepara- 
tion. From this and similar experiments, Curie and 
Laborde deduced that one gram of pure radium emits 
98 gram-calories l per hour, i. e. practically the amount 
required to raise a gram of water from its freezing- 
point to its boiling-point. 

The number has been re-measured very accurately 
by different observers. The most accurate measure- 
ment, in all probability, is that carried out by von 
Schweidler and Hess, since they used a much larger 
quantity of radium than was previously available, a 
gram of the pure chloride belonging to the Viennese 
Academy. Their method resembled in general prin- 
ciples that already described, and they found an 
actual elevation of temperature of 5'5 C. They 
calculated that one gram of radium emits 118 gram- 
calories of heat per hour. 

Many interesting theoretical calculations have been 
made, based on this unceasing evolution of energy in 
the form of heat. Thus, a gram of radium would boil 

1 A gram-calorie is defined as the amount of heat which will 
raise one gram of water i C. in temperature (according to the 
exact definition, from I4*5-I5'5 C.)- 


away one-twelfth of a gram of water every hour, or, 
dealing with more customary quantities, twelve pounds 
of radium, could it be collected together, would 
evaporate every hour one pound of water. Sir 
William Ramsay is responsible for the statement that 
a ton of radium would not only boil away two hundred 
pounds of water every hour, but it would serve as 
efficient fuel to warm a house, to do the cooking and 
afford plenty of hot baths for a large family, and this 
not only during their own lives, but for a continuous 
period of about twenty generations before any marked 
decrease became noticeable. A quantity of the same 
order would replace all the fuel on a large Atlantic 
liner. Other simultaneous actions, however, might 
nullify these beneficent effects. Many chemical 
actions would ensue in the material surrounding the 
radium, and possibly with disastrous results. More- 
over, the physiological effects of the radiations are, as 
we shall see in the last chapter, very considerable, and 
it is quite possible that it may be found extremely 
dangerous to work with even such small quantities as 
a few grams. However, there seems no likelihood at 
the present time of any accumulation of pure radium 
salts of more than a few grams in amount, and this in 
spite of the statement that there is more than a million 
tons of radium scattered over the bed of the ocean. 

It is supposed that this, development of heat is 
attributable to the radiations, and it is easiest to follow 
their action by considering what takes place in a solid 
radium salt. Radiations are continually emitted, but 


of these nearly all the a-particles and a considerable 
proportion of the ^-particles never reach the surface 
of the material, but are absorbed by it. We have 
seen that both types are emitted at very high velocity, 
and the absorption must be brought about by sudden 
stoppage, by a series of violent collisions. Such 
sudden collisions invariably result in the development 
of heat. If this theory is correct, then the a-particles 
which possess by far the greater momentum since 
while their velocities are only between one-tenth and 
one-hundredth of the average velocity of the /?- 
particles, their mass is seven thousand times as great- 
should produce by far the greater heating effect. The 
heating effect of an active substance should be strictly 
proportional to its activity and practically propor- 
tional to the number of a-particles which it emits. 
The following considerations show that this is actually 
the case. 

When radium is in equilibrium with its emanation 
four a-particles are emitted for each atom of radium 
disintegrating, one each for radium, niton, radium A, 
and radium C. If the heating effect is due to the a- 
particles, then twenty-five per cent, of the total heat 
produced is due to radium, the remaining seventy-five 
per cent, to the three a-producing products. Pro- 
fessors Rutherford and Barnes have actually found by 
direct comparison that the relative heating effects are 
as 25 to 75. They have endeavoured further, by 
ascertaining the extent to which the amount of heat 
emitted varied as the emanation produced radium A 


and radium C, to calculate the heat effects of these 
products, and their results show moderately good 
agreement with theory. 

Radium. . . .25 percent. (Theory 25 per cent.) 
Niton plus radium A . 44 ,, 50 ,, 

Radium C . . . 31 ,,25 

Some idea has already been given of the magnitude 
of the heat production of these active elements. This 
is emphasised by the further calculation that the total 
amount of heat evolved by a cubic centimetre of 
niton during its complete disintegration is about seven 
million gram-calories. When the same volume of 
electrolytic gas, hydrogen and oxygen in the propor- 
tions to form water, is exploded, only three gram- 
calories are produced, and this reaction gives rise to 
more heat than any other known chemical change. 
Hence, during its disintegration the emanation gives 
rise to over two million times more heat than an equal 
volume of gases producing the most energetic chemical 
reaction known. 

According to the theory of heat development given, 
all active substances should give rise to heat, and this 
should be observable with relatively small amounts of 
substance. A heat evolution has even been observed 
in the case of pitchblende. Poole has found by care- 
ful measurement that a gram of pitchblende gives 
rise to 6 x io~ 5 gram-calorie per hour, while the 
theoretical figure calculated from its measured activity 
is 4-8 X io~ 5 . A number of measurements have been 



made by Duane, with a specially devised apparatus, 
and since this is the most delicate instrument of the 
type so far constructed, a detailed description may 
be given. He claims that by this instrument it is 
possible to measure one-thousandth of a gram-calorie 
per hour in an experiment lasting only a few minutes. 

FIG. 22. 

The method is based on the rapid increase of the 
pressure of the vapour of a volatile liquid when the 
liquid is subjected to an increase of temperature. 
Two glass vessels, A and A' (Fig. 22), are joined by 
a capillary tube, B. The vessels are half filled with 
ether, which volatilises very readily, since it boils at 
40 Centigrade. The remaining halves of the vessels 


are nearly evacuated and sealed off so that the ether 
is in the presence of its own vapour with a small 
amount of air. A small bubble of air is left in the 
capillary tube joining the two vessels. The two 
tubes, 7 and C' t act as recipients of the heat-producing 
substances. The effect of introducing a substance 
evolving heat into C, say, is to increase the vapour 
tension in B, so that the air-bubble is driven towards 
C'. The calorimeter is protected against outside 
variations of temperature by placing it inside a lead 
case. The actual amount of heat evolved was 
measured by a compensation effect, under such con- 
ditions that the air-bubble in the capillary remained 
in the same position throughout the experiment. The 
delicacy of the method is shown by an experiment 
with O'S milligram of radium chloride. The measured 
heat evolution was 120 gram-calories per hour, a 
result within two per cent, of that of von Schweidler 
and Hess, carried out with more than one thousand 
times the amount of radium. A small amount of 
radiothorium was tested and a heat evolution of 0*025 
calorie per hour was measured. The amount evolved 
by a radium salt of equal activity was calculated as 
0*039 calorie. The heat liberated by 0*2 gram of a 
polonium preparation was found to be 0*0117 calorie 
per hour, radium bromide of 'the same activity liberat- 
ing O'Oii calorie in the same period. These results 
lend support to the theory already enunciated that the 
heat effect is produced by the radiations, and is there- 
fore directly proportional to the degree of activity. 


Pegram and Webb have measured the heat pro- 
duction of thorium oxide, and have found that 
2'i X io~ 5 gram-calorie is emitted per hour per gram 
of thorium, so that thorium has only about one 
three-millionth of the heating effect of an equal 
weight of radium. 

We have seen that the energy liberated by disin- 
tegration may give rise to numerous other effects. 
Thus light is produced by the bombardment of zinc 
sulphide by a-particles ; numerous chemical reactions 
are produced by both a- and /2-particles. From exact 
measurements of the rate of decomposition of am- 
monia by the a- and ^-radiations from niton it has 
been calculated that, at a maximum, one per cent, of 
the total energy emitted is utilised in producing cJiemical 
action, while in experiments where only ^-particles 
acted the amount of energy utilised chemically was 
found to be of the order of O'Oi per cent. This smaller 
result was to be expected since the relative activities 
of the two types of radiations are, measured by their 
relative powers of ionisation, as 100 to i. 

We may conclude from the results so far obtained 
that all radioactive matter produces heat, and in 
amount proportional to its activity. This heat pro- 
duction has become of very considerable importance 
in estimations bearing on the age of the earth, and 
reconciles the conflicting figures obtained by Lord 
Kelvin from considerations dealing with the rate at 
which the earth is cooling, and by others from geo- 


logical considerations ; according to Professor Joly it 
leads to even more striking conclusions. 

In order that the importance of this application 
may be realised it is necessary to emphasise the fact 
that although the minerals containing large amounts 
of uranium and thorium are by no means common, 
yet these and the other active elements are present 
in small amounts over the whole globe, and in spite 
of the smallness of these amounts the total quantity 
over so wide an area is enormous, and so is conse- 
quently the heat to which it gives rise. This wide 
distribution is illustrated by the occurrence in infinit- 
esimally small quantities of the emanations of thorium 
and radium in ordinary air (see Appendix D). The 
ionisation produced by these occasional atoms of gas 
is responsible for the so-called "natural leak" of an 
electroscope, is responsible for the fact that, no 
matter how perfect the insulation, after some time 
the leaves of the instrument always fall together. 

If we regard the surface of the earth as made up 
of igneous and sedimentary material, then experiment 
has shown that both contain traces ,of radium in 
varying amount. The effect in some cases is great 
enough to render the waters percolating through such 
material distinctly radioactive, and to such a cause is 
to be attributed the radioactivity of the Bath waters, 
and those at various other renowned spas. The 
activity is measured by determinations of the amount 
of emanation which can be obtained from the material. 
Thorium emanation is often also present, and the 



relative amounts of the two can be ascertained by 
the study of the decay curves of the activity. Having 
determined these, the amounts of thorium and radium 
can at once be calculated. 

The largest number of experiments have been 
carried out for radium content. Professor Strutt has 
found from a large number of analyses of material 
from different sources that 

the mean radium content per gram igneous rock is 
17 X io~ 12 gram, 

the mean radium content per gram sedimentary rock 
is n X io~ 12 gram. 

The figures obtained by Professor Joly are of the same 
order of magnitude, but distinctly greater. From an 
examination of 126 specimens of igneous rocks from 
all over the earth's surface, many of the specimens 
having been obtained at considerable depths, he 
found that 

the mean radium content per gram igneous rock is 
7'O X io~ 12 gram, 

while from sixty-three rocks of sedimentary origin he 
obtained the result 

the mean radium content per gram sedimentary rock 
is 47 X io~ 12 gram. 

A cubic mile contains about 4 X io 15 cubic centi- 
metres. The density of an average igneous rock can 
perhaps be taken as about 2-5, while that of a sedi- 


mentary rock is as a mean figure about 2. A cubic 
mile of sedimentary material at the earth's surface 
will therefore weigh about 8 X io 15 grams (since a 
cubic centimetre of water of density one weighs one 
gram), while a cubic mile of average igneous rock 
will contain about io X io 15 grams. If Strutt's 
figures are accepted as correct, it can be easily cal- 
culated that a cubic mile of sedimentary material 
contains about 9 kilograms of radium, while the same 
quantity of igneous rock will contain about 17 kilo- 
grams (37 pounds). 1 If Joly's figures are correct these 
values will require to be quadrupled. 

Joly examined 24 specimens of sea water from the 
various oceans, and his average determination was 
0*017 X io~ 12 gram of radium per cubic centimetre. 
From this figure he calculated that in the combined 
ocean volumes there exists not less than 20,000 tons of 
radium. It has been found that river water is poorer 
in radium content, and further that material obtained 
by dredging the sea-floor is richer than dry land, so 
that Joly is of the opinion that the radium in the sea 
is derived from the sea-bed, and estimates that the 
latter must contain well over a million tons of 

The mean result of 82 determinations of the 
thorium content of igneous rocks is 1*3 X io~ 5 gram 
of thorium per gram of rock. 

1 So that a ton of radium would exist in a tract of land of 
igneous material, sixty square miles in area, and one mile in 
K 2 


According to Boltwood the heat given out by 
radium in complete radioactive equilibrium (from 
uranium to polonium) is 0*06 calorie per second per 
gram, and therefore each gram of the igneous material 
of the earth's crust emits by its radium content, if we 
take the mean of Joly's and Strutt's figures, 4-3 X 10 ~ 12 
X o - o6 = 2'6 x io~ 1:5 calorie per second, while the 
corresponding figure for sedimentary material is 
2'g x io~ 12 x O'o6 = 17 x io~ 13 calorie per second. 

The heat emitted by thorium in radioactive 
equilibrium is 5 x io~ calorie per second per gram 
thorium, so that each gram of igneous material emits 
by means of its thorium content 5 x io~ 9 x i'3X io~ f) 
= 6-5 x io~ 14 calorie per second, and the total heat 
emission per gram igneous material is 3 x io~ 13 
calorie per second. Since the sedimentary material 
gives results of the same order, we can probably state 
that the heat emission from radioactive sources per 
gram of material at the earth's surface is not less than 
2 x io~ 13 calorie per second. 

Various estimates of the age of the earth have 
been put forward. The first important calculation 
was made by Kelvin and was based on the rate at 
which the earth is losing heat. Geological estimates 
have been based on the time required for the sea to 
reach its present composition, and on the thickness 
of the various sedimentary strata. The most recent 
estimates are based on radioactive considerations. 
The results obtained by these different methods will 
be considered in order. 


Lord Kelvin assumed that originally the earth was 
in a molten condition, and that it cooled down at a 
uniform rate until the crust just solidified. The 
earth's interior was then at a definite temperature, 
which can be roughly estimated from the melting- 
point of the rocks forming the crust, while the surface 
rapidly attained a temperature comparable with that 
which we at present know. Under such conditions 
heat would slowly escape from the hot centre to the 
exterior, and the rate at which this took place 
depended on the rate at which the material of the 
crust could conduct it. According to this theory a 
regular temperature gradient should be set up between 
the exterior and interior, i. e. successive depths should 
exhibit successive increases of temperatures. This is 
actually found to be the case, and it should therefore 
conversely be possible to calculate the total time 
required to bring about the temperature conditions 
which actually exist. A number of measurements 
of the temperature gradient have been carried out. 
The figures vary between 24 and 39 metres as the 
depth at which successive increases of i Centigrade 
take place. Kelvin's figure was 24*5 metres, and his 
calculation, based on this figure, is that a period of 
forty million years has elapsed since the earth was 
in a molten condition. 

The age of the ocean has been calculated by several 
observers from the known sodium chloride content 
of sea water, by determinations of the average content 
of sodium chloride in river water. Knowing from the 


latter the average annual amount of sodium which is 
added to the ocean it is easy to calculate how long a 
period must have elapsed to bring about the present 
accumulation of sodium in the sea. The results vary 
between the limits of eiglity million and one hundred 
and fifty million years, the lower estimate being 
regarded as nearer the correct figure. 

The second geological method is based on deter- 
minations of the thicknesses of the various geological 
strata ; the most recent figures based on these 
determinations are between one hundred and one 
hundred and fifty million years. 

These values are minima, especially since we may 
suppose that the earth solidified at a period previous 
to the formation of the ocean, and that the sedi- 
mentary strata also represent a later period. But 
they are much greater than the figure put forward 
by Kelvin. The latter, however, did not take into 
account the then undiscovered fact of the large quan- 
tity of heat produced by radioactive matter, and this 
additional source of heat, by lessening the heat loss, 
would materially increase the period calculated by 
Kelvin and probably bring it into conformity with 
those calculated from geological data. 

Joly goes further. He estimates the total heat loss 
from the earth's surface per year as of the order of 
2'O x icr calories, which corresponds to 125 x io~ 8 
calorie per second per square centimetre of surface. 
We have seen that one gram of material at the earth's 
surface liberates on the average 2 x io~ 13 calorie, so 


that the earth's loss of heat should be neutralised 
according to Joly's method of calculation, by the 
heat evolved by a thickness of material of about forty 
miles, provided that the radioactive content throughout 
this thickness was the same as the average amount 
actually found at the earth's surface. If this is the case, 
then the earth can no longer be growing colder, and if 
radioactive material is emitting heat below this depth, 
the earth must be growing hotter instead of cooler. 

The radioactive methods of estimating the age of 
the earth are based in the first place on the amount 
of helium in radioactive rocks, and in the second 
place on the amount of lead present in such rocks. 
As we have seen, helium is liberated in many of the 
radioactive transformations ; in all of them in which 
a-particles are produced. So long as the radioactive 
material is in a molten condition or is in solution 
the helium will escape, but so soon as the material 
becomes solid the gas will become occluded to a 
greater or lesser extent. Knowing the amount of 
radium or thorium, and knowing the rate at which 
these produce helium (in the case of radium one gram 
in radioactive equilibrium with uranium and its pro- 
ducts emits 316 cubic millimetres of helium per year; 
cp. p. 105) from the measured quantity of helium 
present in any rock the age can be calculated, since 
it is simply the time taken for the helium to accumu- 
late, the time since the rock was molten (or in 
solution). Such calculations in order to be correct 


must include a correction for the amount of helium 
lost by diffusion ; this has been shown in many 
cases to be considerable. If this correction is in- 
adequate the result will be too small, and most of 
these results are hence to be regarded as minima. These 
calculations are, of course, based on the assumption 
that the rate at which the radioactive changes have 
taken place has never varied. There seems to be good 
ground for this assumption, since no experimental 
treatment of any kind has caused any measurable 
variation (compare in this connection pp. 88 and 108). 
Very many determinations of the helium contents 
of different rocks have been carried out by Professor 
Strutt In only one or two cases has the appearance 
of helium been unaccompanied by the presence of 
active material, notably in the case of certain beryls. 
These exceptions are so far unaccounted for. Strutt's 
results are not always concordant, but they invariably 
indicate a greater age for the earth than that based on 
geological data. For example, a sphene from Renfrew 
co., Ontario (Archaean period) was found to be 710 mil- 
lions of years old, another from Norway 449 millions of 
years. The results can alsobe applied in determining the 
relative ages of the different geological periods. Thus 
Strutt quotes the following of his results as typical 

Spha:rosiderite from Rhine provinces. 

Oligocene ..... 8*4 millions of years. 
Haematite, County Antrim. Eocene 31 ., 

,, Forest of Dean. Carboni- 

ferous Limestone . .150 ,, 

Sphene, County Renfrew, Ontario .710 ,, 


Not only does helium accompany radium in rocks, 
but it is found in the gases contained by the waters 
of all radioactive springs. The amount is often very 
considerable. One spring at Bourbon-Lancy pro- 
duces over ten thousand litres (two thousand gallons) 
of helium every year. In all probability the whole of 
the atmospheric helium (air contains one part of helium 
in 200,000, see Appendix D) is produced by radioactive 
disintegration . 

The calculations of the age of the earth .based on 
the amount of lead in uranium minerals of course 
assume the truth of the hypothesis that lead is the 
final product of the uranium series of transformations 
(see p. 105); although there is increasing experi- 
mental evidence for this hypothesis, yet it cannot 
be considered as definitely established. Nevertheless 
most uranium minerals contain an appreciable quan- 
tity of lead ; the exceptions which contain but traces, 
are found to be the youngest, geologically, and this 
would be expected, were the presence of lead 
dependent on radioactive change. However, less 
stress can be put on the results since there is always 
the possibility of the previous existence of lead in 
radioactive minerals, so that the figures based on the 
presence of lead must be regarded as maxima. It is 
interesting to observe, however, that while distinctly 
greater than the corresponding figures based on the 
helium content they are yet of the same order of 


Minerals from Carbonaceous period . 340 millions of years. 

,, Devonian period . . 370 ,, 

,, Pre- Carboniferous 

period . . . 410 

Silurian period . . 430 

Pre-Cambrian period: 

United States 


1 1435 

Ceylon . . 1649 

The discrepancies between these figures and those 
for helium can be explained on the assumption 'that 
the corrections for helium diffusion are insufficient ; 
they may also be due, as indicated above, to the 
presence, in the minerals examined, of lead which 
was not derived from uranium. In any case, unless 
strong reasons can be advanced in disproof of the 
theory that the radioactive changes are independent 
of external causes and take place at unvarying rate 
the helium figures must be regarded as minimal. 
The differences between the age of the earth as based 
on these and as derived from geological considerations 
are at present inexplicable. 



THE belief in the possibility of transmuting one 
element into another arose at a very early date. 
Indeed, the name Chemistry is probably derived from 
the practice of the art of transmutation by the ancient 
Egyptians. The old name for Egypt, Chemi, is sup- 
posed to be the origin of the term %fl/ua, by which 
the art was originally known. All early efforts to 
produce transmutation were grounded on the idea 
that base metals could be transformed into the much 
more valuable commodities, gold and silver. Such 
a belief persisted until the beginning of the sixteenth 
century, and was held by the credulous at a very 
much later period. The chemists of the Middle Ages 
were alchemists, and their most important chemical 
discoveries were often made accidentally in the course 
of vain attempts to manufacture gold. 

The idea of transmutation followed easily from the 
notions of elements which have been mentioned in 
the, first chapter. The elements were qualities, and by 
adding or subtracting one or other of these qualities 
to matter, the matter could be changed completely 
in appearance. Besides, accidental observations 



appeared to indicate that changes of the type which 
the alchemists sought did actually occur. It was 
noted that iron utensils when left in copper mines 
became coated with red metal from contact with the 
accumulated waters. That chemical change could 
take place between the iron, and copper salts dis- 
solved in the water was of course quite unknown ; 
it was evident that a transmutation of iron into 
copper had taken place. In a similar way it was 
noticed that copper could be changed into white or 
yellow substances by admixture with certain earths ; 
copper alloys were produced, but it seemed certain 
to those early observers that silver or gold had been 
formed. As chemical knowledge grew, such apparent 
transformations were gradually explained, and the 
processes of alchemistry, so far as they were con- 
cerned with transmutation, came to be identical with 

The ideas of elements promulgated by Boyle, and 
of hard indivisible atoms propounded by Dalton, 
scarcely admitted of transmutation, and while these 
ideas were accepted belief in it died. But we have 
seen that radioactivity has altered our conceptions. 
We have definite proof of the change of one element 
into another. Any theory that these radioactive 
substances are not real elements but are compounds 
of, say, lead with so much helium cannot be upheld, 
since we have seen that in a number of cases the 
substances can be prepared in a pure condition, and 
conform in every respect to the actual definitions by 


which elements are distinguished from other sub- 
stances. We can no longer conceive of a hard 
indivisible atom,. but must think of some complex 
system of electrons and particles, some infinitesimally 
small planetary system, perhaps a system of con- 
centric spheres arranged round some positive unknown 
atqm and consisting of numerous electrons, and in 
the case of the large radioactive atoms a large 
number of helium atoms, perhaps a whorl similar 
to that postulated in the spiral hypothesis of planet 
formation, unstable and continually shooting offhelium 
atoms and electrons until a condition of stability is 

Looking upon the atom in this wise it no longer 
appears impossible to change one kind of atom into 
another. Many of the larger atoms bear distinct 
points of similarity ; they all contain more or less 
helium, nor does it seem likely that the helium atom 
can be markedly different before and after its emission 
from a radioactive atom. In all probability other of 
the building stones of the atoms are identical ; indeed 
various theories have been put forward to the effect that 
all atoms are built up of varying proportions of such 
simple atoms as those of helium and hydrogen. If 
any such type of hypothesis were true and some of 
them must approximate to the truth then it ought 
to be possible, if sufficient force could be applied, to 
disrupt an atom, to resolve it into its constituents, 
and in that way to bring about a transmutation. The 
relatively greatest force at our disposal is, as we have 


seen, that provided by the disruption of radioactive 
substances. The changes which produce this force 
are not exactly comparable with those just discussed, 
since the atomic disruptions in these are apparently 
due to internal forces, whilst those sought in attempted 
transmutations must be brought about by external 
forces. Should the energy provided by the radiations 
be insufficient for the task, then the latter must be 
considered as impossible. This energy, however, is 
presumably of the order required, since it is itself 
produced by atomic changes, 

A number of experiments have been carried out by 
Sir William Ramsay and various co-workers, in which 
the effect of this concentrated form of energy in pro- 
ducing atomic change has been tested. His results 
appeared to be positive, to show that transmutation 
by this means was possible. They have been repeated 
by other observers who have obtained negative results, 
and until further work has been carried out no definite 
conclusions can be drawn one way or the other, and 
it cannot be claimed that actual transmutation has 
yet been accomplished. The work is of such 
importance, however, that a fairly full description of 
these experiments is given. 

Ramsay originally intended to test the effect of 
niton on solutions of copper salts. It has been shown 
already (p. 65) that niton decomposes water with 
evolution of hydrogen and oxygen, an action very 
similar to that brought about by the electric current 
in electrolysis. He thought that by a similar electro- 


lytic action copper might be produced from the 
copper solutions. That this hypothesis was justi- 
fiable is shown by the fact that silver salt solutions 
are decomposed with the liberation of metallic silver. 
However, in the case of copper no metallic copper 
was produced. A slight change of colour of the 
copper solution from blue to green took place, indi- 
cating slight chemical action, and a certain amount 
of gas was produced consisting chiefly of hydrogen 
and oxygen. On analysing the copper solution very 
carefully it was found to contain a minute trace of 
the somewhat rare element lithium. Numerous 
experiments were carried out to find whether 
this trace of lithium was present as an accidental 
impurity or had been formed in the solution. 
The latter hypothesis seemed plausible since 
lithium belongs to the copper group of elements, is 
the lightest of them in fact the atomic weight of 
copper is 63*6, while that of lithium is 6'Q. The 
most careful precautions were taken to ensure the 
purity of the copper salt and especially the absence 
of lithium. All preliminary tests seemed satisfactory. 
Nevertheless lithium was always present in the solu- 
tion after treatment with radium emanation. The 
final series of experiments are typical of the rest. 

Very pure copper nitrate was prepared from pure 
copper sulphate and the purest nitric acid obtainable. 
This was dissolved in pure water, and transferred to 
two bulbs made from the same piece of glass-tubing. 
One of these bulbs was sealed up and kept until 


the experiment with the other was completed, when 
the two solutions were simultaneously put through the 
same course of chemical analysis with the same 
utensils and the same reagents. In the first solution 
no trace of lithium was found, from which it appears 
definitely certain that the lithium did not come from 
the copper nitrate, nor from the reagents employed 
in analysis, nor apparently from the dissolution of the 
glass of the bulb; this might presumably contain 
lithium, since an ordinary soda-glass was employed, 
and in such glasses traces of lithium are sometimes 
found. (Actual tests of such glass in Ramsay's labor- 
atory, however, showed that lithium was absent.) 

The second bulb was sealed on to an apparatus 
similar to that shown in Fig. 23, forming the part A 
in the figure. The apparatus consisted first of a 
capillary intake tube, />, similar to that already 
described (see p. 52), connected to an explosion bulb, 
C y and through a side tube, D, to a mercury gas pump. 
The apparatus was first of all completely exhausted 
of air, and then pure niton and electrolytic gas were 
introduced through B, in the manner previously ex- 
plained (see p. 50), into the explosion burette C. 
After passing a spark there remained in the burette 
only niton and the small excess of hydrogen. By 
raising the mercury reservoir E this mixture was 
forced over into the bulb A. A was then surrounded 
by a Dewar vacuum vessel, half-filled with liquid air, 
and it rapidly attained the same low temperature 
( 185 Centigrade) at which the 'whole of the 



niton condenses to solid form (compare p. 54). 
After waiting some minutes to allow complete con- 
densation, the tap leading to the pump was opened 

FIG. 23. 

and the whole of the hydrogen was pumped away, 
since at -- 185 this element still remains gaseous. 
Finally the tap F was closed and the emanation was 
left to decay in contact with the copper solution. 


Since niton is fairly soluble in water at ordinary tem- 
peratures (about one part in three), a large part of the 
a- and /5-particles was liberated in close juxtaposition 
with copper atoms, many collisions between them 
must have occurred, and the conditions were almost 
an optimum for disruption of the copper atom, if 
disruption can be obtained by this means. 

After the niton had almost completely changed 
into radium D that is to say, at the expiration of 
about a month the tap was again opened, and the 
gases which had been produced in the interval were 
pumped away and analysed. The liquid was removed 
from the bulb and carefully analysed, the analyses 
being carried out in silica and platinum vessels ; as 
the same vessels were used in the simultaneous 
analysis of the sample of liquid not subjected to the 
radiations, and as this was proved to contain no 
lithium, there was no source of contamination, as has 
already been pointed out, in cither the vessels or the 
reagents employed. Yet lithium was observed in 
the solution after treatment with niton. The amount 
was almost infinitely small, it is true, but it was 
definitely present. As we have seen, the spectrum 
affords a very delicate and certain test for the pres- 
ence of an element, and lithium possesses a red 
spectrum line which cannot be mistaken. The 
alkaline residues from the copper solution (consisting 
chiefly of a trace of sodium derived probably from 
the glass vessel) when examined in the spectroscope 
showed the presence of this line unmistakably. It 


seemed that the lithium had been produced from 
the copper solution, and the most likely hypothesis 
appeared to be its derivation from the copper itself. 
It was still possible, however, that the lithium had 
been liberated from the glass by the action of the 
radiations. What mere solution, as in the blank 
experiment, could not effect, continuous bombard- 
ment with particles travelling with great momentum 
might bring about. In order to test this possibility, 
a third experiment was carried out in precisely the 
same manner except that pure water was substituted 
for the copper solution. No lithium was produced and 
the proof of the transformation seemed strengthened. 
Other interesting results were obtained. The gases 
from the copper solution did not appear to contain 
the helium which we have learnt to regard as the 
invariable result of radioactive transformation, but 
argon was present. However, the presence of argon 
in itself was explicable by air leakage through the 
taps of the apparatus. Over the relatively long 
period of time of the experiment it is almost im- 
possible to maintain a perfect vacuum with glass 
taps, even with the best lubricators which have so 
far been invented ; the amount of argon present was 
not greater than was to be expected from that of 
atmospheric nitrogen. The absence of helium rather 
emphasised the presence of this gas, however. Again, 
in the experiment with pure water both helium and 
neon were present in about equal amount. Neon was 
then considered as one of the rarest of the rare gases, 



and no accidental air leakage could on that assump- 
tion account for its presence. Ramsay suggested that 
niton in the presence of water disintegrated into 
neon, instead of into helium, and thought that in the 
presence of copper argon might be the disintegration 
product produced. If this were the case, of course 
the ordinary theory of disintegration, in which the 
production of helium plays an integral part, would 
require considerable modification. 

The experiments connected with the presumed 
production of lithium from copper were repeated by 
Madame Curie. Her methods were in general similar 
to those of Ramsay. From preliminary work, how- 
ever, she found that glass and silica-ware all contain 
appreciable amounts of lithium, amounts which 
could without doubt be detected by such spectro- 
scopic -methods as had actually been used. There- 
fore the whole of the experiments were carried out 
in platinum vessels. The amounts of niton employed 
were comparable with those in Ramsay's experiments. 
No lithium was detected. No satisfactory explanation 
can at present be advanced for the difference in these 
experiments. While at first sight it would appear 
that the lithium found in the first series came from 
the vessels employed, the negative results in the 
blank experiment and that with water and niton 
seem to preclude this conclusion. Further experi- 
ments of a similar nature and with larger quantities 
of niton are desirable to settle the important ques- 
tions involved. 


Perman has performed some experiments in which 
films of radium and copper salts were placed in close 
juxtaposition so that the radiations could act directly 
on copper in the solid condition. His experiments 
lead also to negative conclusions, but were carried 
out with such small amounts of radium that less 
stress can be laid on them. 

Rutherford has traced the presence of neon in such 
experiments to air leakage. Most elaborate pre- 
cautions were taken to exclude air, and in a 
series of five experiments carried out in a manner 
similar to that employed by Ramsay, neon was only 
observed in one instance, and this was directly traced 
to air leakage. Strutt had found already that it is 
possible to detect spectroscopically the neon from 
one-tenth of a cubic centimetre of air, and Rutherford 
confirmed this statement; he showed, indeed, that it 
is possible by observing the most elaborate pre- 
cautions to observe the spectrum of the neon obtained 
from one-fifteenth of a cubic centimetre of air. As 
neon exists in the atmosphere in one part in one 
hundred thousand, it is, therefore, possible to detect 
one fifteen-hundredth of a cubic millimetre of neon 
spectroscopically ; this exemplifies the extraordinary 
delicacy of a spectroscopic test. These results afford 
one more proof of the absolute unalterability of the 
radioactive changes under all conditions. 

More recently Ramsay has carried out an interest- 
ing series of experiments on salts of thorium and 
the other elements of its group, zirconium, titanium, 


and silicon. The element of the group -which has 
the lowest atomic weight is carbon (12). Thorium 
has the highest weight of the group (232*4). Ramsay 
considered it possible that the elements of this series 
under the action of radioactive bombardment might 
disintegrate, with the production of the lowest 
member of them, and in the presence of oxygen 
(from the nitrate) this would probably appear as 
carbon dioxide. Such a disintegration of the thorium 
atom would of course be quite distinct from the 
series of disintegration which normally occur ; if it 
can be proved it will illustrate the difference of an 
atomic change brought about by internal forces, and 
one produced by external forces. 

The first experiments were made with pure 
thorium nitrate, at once the source of the radia- 
tions and the material on which they were to act. 
Two hundred and seventy grams were dissolved 
in about 300 grams of pure water and stored in 
a round-bottomed flask of about half a litre capacity. 
This flask was joined to a capillary tube, closed by 
a tap, which in turn connected to a mercury pump. 
The apparatus was evacuated and allowed to stand 
for periods of from twenty to forty weeks. The 
gases which had accumulated were then pumped off 
and analysed. In every case carbon dioxide was 
present, together with a much smaller quantity of 
electrolytic gas (hydrogen and oxygen) and a some- 
what larger quantity of nitrogen. Thus in three 
experiments occupying respectively 250, 173, and 


228 days, the quantities of carbon dioxide were 
0*588, ro8, and 1*209 cubic centimetres, those of the 
electrolytic gases 0*017, 0*02, and 0*016, and 
those of nitrogen 5*145, 1*64, and 1*321 Were 
the presence of these gases due to traces absorbed 
by the glass surface, and only slowly liberated in a 
vacuum, or to the last traces of dissolved air, then 
the amounts should decrease with time. This is seen 
to be the case as regards the nitrogen, which can 
reasonably be traced to one or other of these sources. 
The electrolytic gas does not vary much in amount ; 
it is in all probability due to the decomposition of 
water by the radiations (compare the similar actions 
on p. 65). The carbon dioxide increased in amount 
with successive experiments, and some other source 
than those mentioned may, therefore, be indicated. 

In a similar experiment carried out with approxi- 
mately the same amount of a mercuric salt, where 
no such radioactive action could take place, only 
the merest trace of carbon dioxide was found in 
the gases analysed (0-015 although the amount 
of nitrogen was of the same order as for thorium 
(3*628 This result lends further support to the 
hypothesis that the carbon dioxide was not traceable 
to adsorption or to solution. 

Had a disintegration from thorium to carbon been 
brought about by the action of the radiations, then 
such action might be materially hastened by increas- 
ing their relative amount. The most convenient 
method of doing this is, as we have seen, the 


application of doses of niton. A second series of 
experiments were made, in the type of apparatus 
already described and pictured in Fig. 23. Solu- 
tions containing one or two grams of thorium, zir- 
conium, titanium, and silicon salts were successively 
subjected to the action of large doses of niton (the 
adjective is of course purely relative, the maximum 
dose used being O'l In all cases after the 
emanation had decayed, the gases which had been 
produced contained carbon dioxide in amounts varying 
from 0*054 to Q'55 1, amounts large enough to 
prevent any possibility of mistake in the analysis, and 
which appeared to be larger the greater the atomic 
weight of the element upon which the action was 
supposed to be produced. 

Ramsay accordingly suggested that under such 
conditions as these, atoms of thorium, zirconium, etc., 
can actually be broken up and that among the 
products are atoms of the element carbon. A series 
of similar experiments have been carried out by 
Herschfinkel. He also allowed niton to act on 
specially purified thorium emanation. He found 
that carbon dioxide was produced, while in experi- 
ments in which the solution was left untreated with 
emanation, or in which emanation was allowed to 
act on pure water, only minute traces of carbon 
dioxide were obtained. He found, however, that 
when the emanation was replaced by small quantities 
of potassium permanganate still more carbon dioxide 
was produced. Since potassium permanganate and 


oxalates react together to produce carbon dioxide, 
and since thorium is originally usually purified as 
oxalate, Herschfinkel thinks that the presence of the 
carbon dioxide is traceable to small quantities of 
thorium oxalate present in the nitrate employed for 
the experiments, and that these were decomposed 
by the emanation and the permanganate respectively, 
so that the change from the thorium to the carbon 
atom is as yet unproved. 

Ramsay claims that elaborate precautions were 
taken to free the thorium nitrate used from all 
traces of carbon (by first fusion to redness, which 
should burn away any trace of carbon present, and 
then by redissolution in nitric acid). After treatment 
with the emanation carbon dioxide was always found 
present. Other parallel experiments with bismuth, 
mercury and silver never gave any trace of this 

Here also further results must be awaited to settle 
finally this very interesting and important problem. 

We have seen that three parallel series of trans- 
formations are continually taking place, from uranium, 
thorium, and actinium, and that there is a possibility 
that uranium itself is the original source of all. We 
have seen that there is also a possibility of other 
transformations, in which much greater steps down 
the scale are taken. All of these changes represent 
degradations, changes accompanied by loss of weight. 
Can other changes occur, in which matter is built up ? 


How did matter originate ? These are questions 
which inevitably arise in view of what we have 
already discovered, and although they appear at 
present insoluble, yet many ingenious theories have 
been put forward, and some one of these may be 
found to approximate to the truth. More and more 
stress is being laid on the regularities in the atomic 
weights of the elements. In very many cases the 
difference between the weights is exactly four, the 
weight of an atom of helium, and the role played by 
the helium atom in the building up of others seems 
to be of major importance. 

It is interesting that the spectra of the simplest 
nebulae reveal the presence of four lines only two 
the strongest in the spectra of hydrogen and helium 
respectively, and the other two attributed to two 
other unknown elements of similarly low atomic 
weight. As the nebulae become more complex their 
spectrum grows in complexity, and it would seem 
that in these new worlds or new planetary systems 
an actual process of atom-building is taking place, 
but by what mechanism this is brought about one 
can at present scarcely conjecture. 



RADIUM has only one "practical" application 
its use in medicine. This, however, seems to be of 
the utmost importance. 

The physiological effect of radium rays was first 
observed in 1900 by Walkhoff, and his observation 
was soon corroborated. Thus, Becquerel in 1901 
accidentally carried in his waistcoat pocket for a few 
hours a small tube containing a radium salt. Fifteen 
days later the skin near where the tube had rested 
became inflamed, and the inflammation took some 
time to disappear. It resembled an ordinary burn 
in appearance, and the effect is spoken of as the 
production of a " radium burn." 

If a thin celluloid or rubber capsule containing a 
strongly active radium preparation is placed on the 
skin, and allowed to remain for some hours, then 
after -a time, which varies according to the time of 
action and the strength of the radium, the skin be- 
comes red, as after a burn, while in some cases a 
blister may be formed. With prolonged treatment 
an ulceration may be produced. These lesions heal 
very slowly. 



The effect of radium contained in metal capsules is 
much feebler. It can be concluded from these results 
that physiological effects are due to the radiations 
and not to the radium itself; this is in conformity 
with all the other actions of radium which have been 
dealt with. Moreover it is evident that the effect in 
these cases must be attributable to the penetrating 
radiations, and since metallic capsules produce much 
less effect, the ^-radiations, of which very few are 
kept back by such capsules, evidently have an action 
much weaker than that of the /3-rays. 1 

Many other similar actions have been observed. 
The action on the eye is marked. If a radium pre- 
paration is brought near the eye or the temple of .a 
person in complete darkness, a sensation of light is 
experienced. A fluorescent effect is produced on the 
retina. The blind whose retina is intact can expe- 
rience this sensation ; those in whom the retina is 
affected cannot do so. 

The leaves of plants submitted to the action of the 
rays become yellow and shrivel. Colonies of bacteria 
are killed. 

As soon as the physiological effects were discovered, 
medical applications were attempted, especially in the 
diseases where beneficial effects had been obtained by 
treatment with ultraviolet light or with X-rays ; these 

f Radium itself has in all probability a pharmacological 
actionvery similar to that of barium. The latter has a strongly 
poisonous effect. Niton, an inert gas, probably has no 
pharmacological effect per se. 


include several skin diseases. Already many impor- 
tant results have been obtained by these applications; 
it is probable that further study will reveal further 
scope for the utilisation of radium in medicine. One 
of the most marked and certain effects seems to be 
the permanent alleviation of pain. 

All three types of rays appear to produce action. 
The action of the a-rays must, however, be extremely 
localised on account of their very slight penetrability. 
The /?-rays are considered as most efficacious, since 
they not only produce distinct action, but are also 
capable of penetrating an appreciable layer of tissue. 

Many methods of application have been tested. 
Amongst these are the following 

Radium emanation is inhaled, mixed with air. 
The emanation is condensed at liquid air temperature 
on (a) such substances as lanoline, vaseline, glycerine, 
etc., which are then applied externally to the part 
affected ; or (&) such substances as quinine, bismuth, 
subnitrate, arsenic, etc., which are then taken inter- 
nally in order that a joint effect may be produced. 
Very dilute solutions of radium salts are made, and 
these are taken internally; any effects which they 
produce are comparable with those obtained with 
naturally occurring radioactive waters. Occasionally 
insoluble radium salts have been suspended in liquids 
and injected. In this way radium salts can be 
localised in certain tissues, but the treatment is 
extremely costly, since the radium is irremediably 


The rays are most usually applied externally. 
Copper plates of various sizes, or thin copper cylin- 
ders, with roughened surfaces, are covered with a very 
thin layer of varnish in which the radium salt has 
been dissolved. In this way a thin uniform layer of 
radium is obtained, and this will give the maximum 
surface e fleet. 

The actual application is carried out very simply. 
The surface of the skin is protected from the rays by 
a covering of lead foil, in which an aperture is cut 
exactly corresponding to the diseased tissue. This is 
covered with very thin muslin to protect the radium 
preparation from contact with the skin, and on the 
muslin the radium plaque rests. The time of applica- 
tion varies from a few minutes to an hour. In most 
cases several applications, each of an hour's duration, 
are made over a period of several days. 

Most marked effects have been produced with lupus 
and ulcerous growths. The most severe cases of lupus 
give way to the treatment, and after treatment only a 
slight difference of colour shows what had existed. 
Many ulcerous growths completely disappear, leaving 
no trace of diseased tissue. Birthmarks and scars 
are also removable by radium treatment. It is still 
doubtful whether deep-seated cancer has been cured, 
and in such cases the difficulty of application is great. 
It is claimed that many cases of superficial cancer 
and tubercular growths have yielded to treatment, 
but it is still perhaps uncertain whether sufficient 


time has elapsed to make certain ot their non- 

The beneficial effects of many spas are attributed 
to the radioactivity of the waters. That many of 
these springs are distinctly active has already been 
mentioned. It is possible that the internal consump- 
tion of these waters may produce a benefit which 
is in part due to their activity. It is certain that 
the degree of activity is insufficient to cause any 
deleterious action. 

Experiments with radium are limited in number 
on account of its great cost. There seems little like- 
lihood that the price of 20 per milligram (this is at 
the rate of 600,000 an ounce) will not be main- 
tained, at any rate for a number of years. Further, 
not only is the price prohibitive, but the amount 
available is extremely small. So far there are only 
two sources : the mines at Joachimsthal, worked by 
the Austrian Government, and those near St. Ives, 
under the control of the Radium Company. The 
chief output is hitherto from the former. It is doubt- 
ful whether they have yet produced more than twenty 
grams of approximately pure radium, and in all 
probability the total amount at present in existence 
in a more or less purified form is somewhere in the 
neighbourhood of this figure. In order to carry 
out chemical or physiological experiments with any 
degree of exactness quantities of the order of thirty 
to sixty milligrams are essential : it seems likely 


that much of the medical work \$ill be carried out by 
the system of loaning or hiring radium preparations, 
which is at present in application in Paris and London. 

As indicated in Chapter V, a second valuable source 
of the radiations seems to be mesothorium^ or more 
strictly speaking mesothorium together with its 
immediate products. As amongst these are elements 
producing ^-radiations, and as they nearly all produce 
a-radiations, there seems to be no reason why meso- 
thorium cannot be substituted for radium in medical 
applications. In all probability it will be placed on 
the market at a much lower cost, and in spite of the 
fact that most of its activity (and, therefore, its useful- 
,ness) disappears in a relatively short time a matter 
of ten or twelve years still, provided the cost is more 
reasonable, a fresh supply will be easily attainable. 
Experiments have already been carried out in which 
mesothorium and thorium emanation have been 
utilised instead of radium preparations, and in which 
the treatment has apparently been equally beneficial. 

Radium has other uses, which, although not 
" practical," are of the utmost importance theoretic- 
ally. These have been detailed elsewhere. Perhaps 
the most striking results from the study of this and 
the elements allied to it by the possession of the 
common property of activity are the discovery that 
in Nature a whole series of atomic transformations 
are continually taking place with the evolution of 


quantities of helium large enough to account for the 
whole of that gas found in the earth's atmosphere ; the 
discovery of the important role played by helium in 
the building up of the larger atoms, and the application 
of the facts of radioactivity in calculations of the age 
of our earth. 




RADIUM is prepared from pitchblende, a mineral 
containing fifty to eighty per cent, of uranium in the 
form of oxide, thorium in quantities varying from 
traces to ten per cent., and traces of silica, lead, the 
rare earths, and the rare gases. 

Haitinger and Ulrich in 1908 published a method by 
which in two years they succeeded in obtaining three 
grams of radium from 30,000 kilograms of pitchblende 
containing 53-4 per cent of uranium oxide. The 
radium was, therefore, present to the extent of one part 
in ten million. The work was carried out for the 
Austrian Government. 

After removal of the uranium by fusion with sodium 
sulphate, and then lixiviation with water and dilute 
sulphuric acid, whereby all the uranium was obtained 
in solution, the radioactive residue amounted to 
10,000 kilograms, and was largely present as sulphate. 
It was heated, in small portions at a time, for a period 
of ten or twelve hours with a twenty-five per cent, 
solution of sodium hydroxide. The residue was washed 
with water to remove the soluble sulphates as far as 
possible, was digested on the water bath with one and a 



half times its weight of fifty per cent, hydrochloric acid, 
then filtered and washed with water. The filtrate from 
this treatment contained practically no radium, but 
the whole of the bismuth-polonium, and the actinium. 
The residue was then heated with half its weight of a 
solution of sodium carbonate (twenty-five per cent.), 
and in this way most of the radium sulphate was 
converted into carbonate. After removing soluble 
sulphates by washing with water, treatment with pure 
hydrochloric acid converted the insoluble radium 
carbonate into soluble chloride. This treatment with 
sodium carbonate and hydrochloric acid was repeated 
three times and the final residue was practically free 
from radium. The whole of the radium was contained 
in the acid extracts, and these were treated with sul- 
phuric acid ; a precipitate was obtained consisting 
of lead, of metals of the alkaline earths (including 
radium) and of metals of the rare earths. This was 
repeatedly heated with an excess of concentrated 
sodium carbonate. The conversion of sulphates into 
carbonates by this process is never complete ; after 
extracting the mass with hydrochloric acid, the residue 
was boiled with water to remove lead chloride (which 
is soluble in hot water ; the solution was used for the 
preparation of radioactive lead), and was then added 
to the next batch of crude sulphates (see above ; only 
relatively very small amounts were worked up at one 
time). The hydrochloric acid solution containing 
the radium was treated with sulphuretted hydrogen, 
which removed all the lead ; it was then evaporated 
M 2 


to dryness, and the greater part of the calcium chloride 
removed by treating it with concentrated hydrochloric 
acid (in which the calcium salt was soluble, the salts 
of the other alkaline earth metals much less so). In 
this way an amount of crude insoluble chloride was 
obtained, weighing about twenty kilograms, and con- 
sisting of radium and barium chlorides with small 
amounts of strontium and calcium salts and other 

The chlorides were fractionally crystallised a 
number of times, the least soluble fraction always 
containing the bulk of the radium ; and in this way 
two large fractions were obtained, one, two kilograms 
in weight, containing almost all the radium ; the other, 
eleven kilograms in weight, containing scarcely any. 
The smaller fraction was re-fractionated until the 
radium portion was reduced to nine grams. This 
was dissolved, treated with sulphuretted hydrogen to 
remove any trace of impurity, and fractionated in 
silica vessels into five parts. The most pure of these 
fractions gave an atomic weight of 225, so that 
it consisted of almost pure radium chloride. The 
total amount of radium in the four fractions was cal- 
culated from the activities to be $'236 grams (as 
anhydrous radium chloride). 



As we have seen in Chapter III, any definite 
volume of radium emanation decreases to half in, 
roughly, four days. A further period of four days 
sees a further decrease to one-half of the half, and 
so on. It is evident that if this rate of decrease holds 
strictly, then no matter how long an interval of time 
elapses, some emanation will remain. The continual 
decay at once suggests that the element has a " life." 
Since, however, it never entirely disappears, this life 
is infinite at any rate, for some part of it. The 
same reasoning applies to any other element which 
decays ; the lives of all are infinite. Still, they can 
be differentiated by the fact that no two have the 
same period of half decay, the so-called "half-life," 
so that this half-life period affords one of the best 
means to discriminate between them. 

The curves, which are obtained by plotting amount 
of substance (or degree of activity) against time, are 
all very similar, provided the change is simple and 
refers to one transformation only. The curves all 
possess the property that if, instead of the amounts 
of substances, the logarithms of the figures represent- 



ing these amounts are employed, the curves become 
straight lines. Such curves are spoken of as ex- 
ponential or logarithmic curves, and are represented 
by a general formula which may be written in the 

I == V- A ' 

where I represents the activity at any time /, I that 
at zero time (the time at which observations were 
first made), e is the natural base of logarithms, and 
k is a constant. The half-life period can be deter- 
mined from this formula, since I is then equal to 
1 /2 ; and the formula becomes 

1/2 =*-" 

and if 1 is known t can be calculated, or vice versa. 

I represents the proportion of atoms which change 
per unit of time. The inverse of this quantity is, 
therefore, the average life of an atom of the sub- 
stance which is undergoing change. This is somewhat 
greater than the half-life ; it is, in fact, the half-life 
period multiplied by (log. 2 x log. e\ approximately 



AN hypothesis has been recently put forward by 
Hahn and is generally accepted, according to which 
the same element cannot emit both a- and /?-rays. 
If this is the case, then such substances as radium C, 
which emit both types of ray, are complex. Accept- 
ance of the theory requires their analysis into at least 
two elements : one emitting a-, the other /?-rays. 
Certain experiments support the theory, and the 
results of these analyses will be observed in the 
following tables 


1 84 


Substance. Half-life. 


P^ c 

Nature of Substance. 


rt 1/5 



rt C 



Uranium . . 6xio 9 years 



Metal: atomic weight, 238*5 

Radiouranium . 1 ? 

(? Ov 


Uranium X . . 24*6 days 

ft ' 

Ionium . . . (30,000 yrs. ?) a 


Radium. . . 1760 years | a 


Metal : atomic weight, 226*5 

Niton . . 3 "85 da> s a 

4 '23 

Gas : atomic weight, 222*5 

Radium A .3-0 minutes a 


Solid : volatile between 800 

and 900 C. 

Radium B . . 267 ,, ft 

Solid : volatile between 600 

and 700 C. 

r> j- /' fCi . 19*6 ,, a 
Radium Ll r L * ..o 

1^2 I 3 8 i P 



Radium D . . 15 years ! ft 

Solid: volatile below 1000 C. 

Radium E . 4*8 days ft 

Metal : volatile at 1000 C. 

Polonium . . 160 ,, a 3*86 

(Lead) . . . (infinite) 

(Metal : atomic weight, 207.) 

Thorium . . 3 X io 10 years , 

Metal : atomic weight, 232*4 

Mesothorium. . j 5 -5 years 

Mesothorium 2 . 6*2 hours ft 

Radiothorium . 2*0 years 3 '9 

Thorium X . 

3-6 days a 57 

Thorium emanation 

53 seconds 

a 5 '5 Gas : at. wt. of the order of 200 

Thorium A . 

0*14 ,, 


Thorium B . 

10*6 hours 


Thorium C f Ci 

55 minutes 

a $'0 

Thorium \C 2 



a , o o 

Thorium D . 

3*1 minutes 

Q [ 



Metal : group of rare earth 



19-5 days 

a(j8) 4'8 

Actinium X . 

10*5 ,, 

a 6'5 

Actinium emanation 

3 '9 seconds 

a 5 '8 Gas. 

Actinium A . 

0*002 second 

a 6*5 

Actinium B . 

36 minutes 


Actinium C . 


a 5'4 

Actinium D . 






Metal : atomic weight, 39*1 



Substance. Parts per Million of Air. 

Oxygen . . . . . . 209,500 

Nitrogen 780,500 

Carbon dioxide . . variable, about 300 

Water vapour . . . ,, 100 

Methane ...... 3*3 

Hydrogen ..... I 

Nitrogen compounds . variable, about 3 

Sulphur ,,..,, i 
Dust, sand, soot, organic 

material, etc. . . ,, ,, 100 

Ozone ...... i 

Hydrogen peroxide .... 0-003 

Argon ...... 9,37o 

Neon . . ... . 12 

Helium ...... 4*8 

Krypton . / .... 0^05 

Xenon ...... o - oo6 

Radium emanation (Niton) . . o '00000000000006 

Thorium ,, . . . . o'ciocc)ooooooooooooo2 

1 From an address given by Dr. Brill before the Gesell- 
schaft Deutscher Naturforscher und Arzte in 1909. 



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