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Full text of "Radio-activity"

CAMBRIDGE PHYSICAL SERIES. 

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



EADIO-ACTIVITY 



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



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




lassgofo: 50, WELLINGTON STREET. 

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[All Rights reserved \ 



RADIO-ACTIVITY 



BY 



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

MACDONALD PROFESSOR OF PHYSICS, M C GILL UNIVERSITY, MONTREAL. 




CAMBRIDGE 

AT THE UNIVERSITY PRESS 
1904 



CambriUge : 

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



SENERAL 



J. J. THOMSON 

A TKIBUTE OF MY KESPECT AND ADMIRATION 



90ft (I 

<-W .* \J S* V/ 



PREFACE. 



TX this work, I have endeavoured to give a complete and 
*- connected account, from a physical standpoint, of the properties 
possessed by the naturally radio-active bodies. Although the 
subject is comparatively a new one, our knowledge of the pro- 
perties of the radio-active substances has advanced with great 
rapidity, and there is now a very large amount of information on 
the subject scattered throughout the various scientific journals. 

The phenomena exhibited by the -radio-active bodies are 
extremely complicated, and some form of theory is essential in 
order to connect in an intelligible manner the mass of experi- 
mental facts that have now been accumulated. I have found the 
theory that the atoms of the radio-active bodies are undergoing 
spontaneous disintegration, extremely serviceable not only in 
correlating the known phenomena, but also in suggesting new 
lines of research. 

The interpretation of the results has, to a large extent, been 
based on the disintegration theory, and the logical deductions to 
be drawn from the application of the theory to radio-active 
phenomena have also been considered. 

The rapid advance of our knowledge of radio-activity has 
been dependent on the information already gained by research 
into the electric properties of gases. The action possessed by the 
radiations from radio-active bodies of producing charged carriers 
or ions in the gas, has formed the basis of an accurate quantitative 
method of examination of the properties of the radiations and of 



vili PREFACE 

radio-active processes, and also allows us to determine with con- 
siderable certainty the order of magnitude of the different 
quantities involved. 

For these reasons, it has been thought advisable to give a brief 
account of the electric properties of gases, to the extent that is 
necessary for the interpretation of the results of measurements 
in radio-activity by the electric method. The chapter on the 
ionization theory of gases was written before the publication 
of J. J. Thomson's recent book on " Conduction of Electricity 
through Gases," in which the w_hole subject is treated in a 
complete and connected manner. 

A short chapter has been added, in which an account is given 
of the methods of measurement which, in the experience of the 
writer and others, are most suitable for accurate work in radio- 
activity. It is hoped that such an account may be of some service 
to those who may wish to obtain a practical acquaintance with the 
methods employed in radio-active measurements. 

My thanks are due to Mr W. C. Dampier Whetham, F.R.S., 
one of the editors of the Cambridge Physical Series, for many 
valuable suggestions, and for the great care and trouble he has 
taken in revising the proof sheets. I am also much indebted to 
my wife and Miss H. Brooks for their kind assistance in correcting 
the proofs, and to Mr R. K. M c Clung for revising the index. 



MACDONALD PHYSICS BUILDINGS, 
MONTREAL, 

February, 1904. 



TABLE OF CONTENTS. 



CHAP. 




PAGE 


I. 


Radio- active Substances 


1 


II. 


lonization Theory of Gases ....... 


28 


III. 


Methods of Measurement 


67 


IV. 


Nature of the Radiations ....... 


90 


V. 


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


149 


VI. 


Properties of the Radiations 


166 


VII. 


Continuous Production of Radio-active Matter . 


178 


VIII. 


Radio-active Emanations 


197 


IX. 


Excited Radio-activity 


250 


X. 


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


293 


XI. 


Radio-activity of the Atmosphere and of Ordinary Materials 


351 




Index ........ 


383 









Plate (Fig. 33) to face p. 169 



ABBREVIATIONS OF REFERENCES TO SOME OF 
THE JOURNALS. 

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

C. R. Comptes Rendus des Seances de F Academic des Sciences. Paris. 
Chem. News. Chemical News. London. 
Drude's Annal. Annalen der Physik. Leipzig. 
^9 Phil. Mag. Philosophical Magazine and Journal of Science. London. 

Phil. Trans. Philosophical Transactions of the Royal Society of London. 
Phys. Rev. Physical Review. New York. 
Phys. Z&it. Physikalische Zeitschrift. 

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

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

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

Wied. Annal. Annalen der Physik. Leipzig. 

ERRATA. 

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

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

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







CHAPTEK I. 

RADIO-ACTIVE SUBSTANCES. 

1. Introduction. The close of the old and the beginning v 
of the new century have been marked by a very rapid increase of ) 
our knowledge of that most important but comparatively little 
known subject the connection between electricity and matter. 
No study has been more fruitful in surprises to the investigator, 
both from the remarkable nature of the phenomena exhibited and 
from the laws controlling them. The more the subject has been 
examined, the more complex does the constitution of matter appear 
which can give rise to the remarkable effects observed. While 
the experimental results have led to the view that the constitution 
of the atom itself is very complex, at the same time they have 
strongly confirmed the old theory of the discontinuous or atomic 
structure of matter. The study of the radio-active substances and 
of the discharge of electricity through gases has supplied very 
strong experimental evidence in support of the fundamental ideas 
of the existing atomic theory. It has also indicated that the 
atom itself is not the smallest unit of matter, but is a complicated 
structure made up of a number of smaller bodies. 

A great impetus to the study t of this subject was initially 
given by the experiments of Lenard on the cathode rays, and 
by Rontgen's discovery of the X rays. An examination of the 
conductivity imparted to a gas by the X rays led to a clear view 
of the mechanism of the transport of electricity through gases 
by means of charged ions. This ionization theory of gases has 
been shown to afford a satisfactory explanation not only of the 
passage of electricity through flames and vapours, but also of the 
R. R.-A. 1 



2 KADIO-ACTIVE SUBSTANCES [CH. 

complicated phenomena observed when a discharge of electricity 
passes through a vacuum tube. At the same time, a further 
study of the cathode rays showed that they consisted of a stream 
of material particles, projected with great velocity, and possessing 
an apparent mass small compared with that of the hydrogen atom. 
The connection between the cathode and Rontgen rays and the 
nature of the latter were also elucidated. Much of this admirable 
experimental work on the nature of the electric discharge has 
been done by Professor J. J. Thomson and his students in the 
Cavendish Laboratory, Cambridge. 

An examination of natural substances, in order to see if they 
gave out dark radiations similar to X rays, led to the discovery of 
the radio-active bodies which possess the property of spontaneously 
emitting radiations, invisible to the eye, but readily detected by 
their action on photographic plates and their power of discharging 
electrified bodies. A detailed study of the radio-active bodies has 
led to the discovery of many new and surprising phenomena which 
have thrown much light, not only on the nature of the radiations 
themselves, but also on the processes occurring in those substances. 
Notwithstanding the complex nature of the phenomena, the know- 
ledge of the subject has advanced with great rapidity, and a large 
amount of experimental data has now been accumulated. 

In order to explain the phenomena of radio-activity, a theory 
has been put forward which regards the atoms of the radio-active 
elements as suffering spontaneous disintegration, and giving rise 
to a series of radio-active substances which differ in chemical 
properties from the parent elements. The radiations accompany 
the breaking-up of the atoms, and afford a comparative measure of 
the rate at which the disintegration takes place. This theory is 
found to account in a satisfactory way for all the known facts of 
radio-activity, and welds a mass of disconnected facts into one 
homogeneous whole. On this view, the continuous emission of 
energy from the active bodies is derived from the internal energy 
inherent in the atom, and does not in any way contradict the law 
of the conservation of energy. At the same time, however, it 
indicates that an enormous store of latent energy is resident in the 
radio-atoms themselves. This store of energy has previously not 
been observed, on account of the impossibility of breaking up into 



l] RADIO-ACTIVE SUBSTANCES 3 

simpler forms the atoms of the elements by the action of the 
chemical or physical forces at our command. 

On this theory we are witnessing in the radio-active bodies a 
veritable transformation of matter. This process of disintegration 
was investigated, not by direct chemical methods, but by means 
of the property possessed by the radio-active bodies of giving out 
specific types of radiation. Except in the case of a very active 
element like radium, the process of disintegration takes place so 
slowly, that hundreds if not thousands of years would be required 
before the amount transformed would come within the range of 
detection of the balance or the spectroscope. In radium, however, 
the process of disintegration takes place at such a rate that it 
should be possible within a limited space of time to obtain definite 
chemical evidence on this question. The recent discovery that 
helium can be obtained from radium adds strong confirmation to 
the theory ; for helium was indicated as a probable disintegration 
product of the radio-active elements before this experimental 
evidence was forthcoming. If the production of helium by radium 
is completely substantiated, the further study of radio-active bodies 
promises to open up new and important fields of chemical enquiry. 

In this book the experimental facts of radio-activity and the 
connection between them are interpreted on the disintegration 
theory. Many of the phenomena observed can be investigated in 
a quantitative manner, and prominence has been given to work of 
this character, for the agreement of any theory with the facts, 
which it attempts to explain, must ultimately depend upon the 
results of accurate measurement. 

The value of any working theory depends upon the number of 
experimental facts it serves to correlate, and upon its power of 
suggesting new lines of work. In these respects the disintegration 
theory, whether or not it may ultimately be proved to be correct, 
has already been justified by its results. 

2. Radio-active Substances. The term " radio-active " is 
now generally applied to a class of substances, such as uranium, 
thorium, radium, and their compounds, which possess the property 
of spontaneously emitting radiations capable of passing through 
plates of metal and other substances opaque to ordinary light. 

12 



4 RADIO-ACTIVE SUBSTANCES [CH. 

The characteristic property of these radiations, besides their 
penetrating power, is their action on a photographic plate and 
their power of discharging electrified bodies. In addition, a 
strongly radio-active body like radium is able to cause marked 
phosphorescence and fluorescence on some substances placed near 
it. In the above respects the radiations possess properties 
analogous to Rontgen rays, but it will be shown that, for the 
major part of the radiations emitted, the resemblance is only 
superficial. 

The most remarkable property of the radio-active bodies is 
their power of spontaneously and continuously radiating energy at 
a constant rate, without, as far as is known, the action upon them 
of any external exciting cause. The phenomena at first sight 
appear to be in direct contradiction to the law of conservation of 
energy,, since no obvious change with time occurs in the radiating 
material. The phenomena appear still more remarkable when it 
is considered that the radio-active bodies must have been steadily 
radiating energy since the time of their formation in the earth's 
crust. 

Immediately after Rontgen's discovery of the production of 
X rays, several scientists were led to examine if any natural 
bodies possessed the property of giving out radiations which could 
penetrate metals and other substances opaque to light. As the 
production of X rays seemed to be in some way connected with 
cathode rays, which cause strong fluorescent and phosphorescent 
effects on various bodies, the substances first examined were those 
that were phosphorescent when exposed to light. The first obser- 
vation in this direction was made by Niewenglowski 1 , who found 
that sulphide of calcium exposed to the sun's rays gave out some 
rays which were able to pass through black paper. A little later 
a similar result was recorded by H. Becquerel 2 for a special 
calcium sulphide preparation, and by Troost 3 for a specimen of 
hexagonal blend. These results were confirmed and extended in 
a later paper by Arnold 4 . No satisfactory explanations of these 
somewhat doubtful results have yet been given, except on the 
view that the black paper was transparent to some of the light 

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

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



I] RADIO-ACTIVE SUBSTANCES 5 

waves. At the same time Le Bon 1 showed that, by the action of 
sunlight on certain bodies, a radiation was given out, invisible to 
the eye, but active with regard to a photographic plate. These 
results have been the subject of much discussion; but there seems 
to be little doubt that the effects are due to short ultra-violet light 
waves, capable of passing through certain substances opaque to 
ordinary light. These effects, while interesting in themselves, are 
of quite a distinct character from those shown by the radio- 
active bodies which will now be considered. 

3. Uranium. The first important discovery in the subject of 
radio-activity was made in February, 1896, by M. Henri Becquerel 2 , 
who found that a maiiiiLm-~salt, the double sulphate of uranium 
and potassium, emitted some rays which gave an impression on a 
photographic plate enveloped in black paper. These rays were 
also able to pass through thin plates of metals and other substances 
opaque to light. The impressions on the plate could not have 
been due to vapours given off by the substances, since the same 
effect was produced whether the salt was placed directly on the 
black paper or on a thin plate of glass lying upon it. 

Becquerel found later that all the compounds of uranium as 
well as the metal itself possessed the same property, and, although 
the amount of action varied slightly for the different compounds, 
the effects in all cases were comparable. It was at first natural to 
suppose that the emission of these rays was in some way connected 
with the power of phosphorescence, but later observations showed 
that there was no connection whatever between them. The uranic 
salts are phosphorescent, while the uranous salts are not. The uranic 
salts, when exposed to ultra-violet light in the phosphoroscope, 
give a phosphorescent light lasting about "01 seconds. When the 
salts are dissolved in water, the duration is still less. The amount 
of action on the photographic plate does not depend on the par- 
ticular compound of uranium employed, but only on the amount of 
uranium present in the compound. The non-phosphorescent are 
equally active with the phosphorescent compounds. The amount 
of radiation given out is unaltered if the active body is kept 

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

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



6 KADIO-ACTIVE SUBSTANCES [CH. 

continuously in darkness. The rays are given out by solutions, 
and by crystals which have been deposited from solutions in the 
dark and never exposed to light. This shows that the radiation 
cannot be due in any way to the gradual emission of energy stored 
up in the crystal in consequence of exposure to a source of light. 

4. The power of giving out penetrating rays thus seems to be 
a specific property of the element uranium, since it is exhibited by 
the metal as well as by all its compounds. These radiations from 
uranium are persistent, and, as far as observations have yet gone, 
are unchanged, either in intensity or character, with lapse of time. 
Observations to test the constancy of the radiations for long 
periods of time have been made by Becquerel. Samples of uranic 
and uranous salts have been kept in a double box of thick lead, 
and the whole has been preserved from exposure to light. By a 
simple arrangement, a photographic plate can be introduced in a 
definite position above the uranium salts, which are covered with a 
layer of black paper. The plate is exposed at intervals for 48 hours, 
and the impression on the plate compared. No perceptible 
weakening of the radiation has been observed over a period of 
four years. Mme Curie 1 has made determinations of the activity of 
uranium over a space of five years by an electric method described 
later, but found no appreciable variation during that period. 

Since the uranium is thus continuously radiating energy from 
itself, without any known source of excitation, the question arises 
whether any known agent is able to affect the rate of its emission. 
No alteration was observed when the body was exposed to ultra- 
violet light or to ultra-red light or to X rays. Becquerel states 
that the double sulphate of uranium and potassium showed a 
slight increase of action when exposed to the arc light and to 
sparks, but he considers that the feeble effect observed was 
another action superimposed on the constant radiation from 
uranium. The intensity of the uranium radiation is not affected by 
a variation of temperature between 200 C. and the temperature of 
liquid air. This question is discussed in more detail later. 

5. In addition to these actions on a photographic plate, 
Becquerel showed that uranium rays, like Rontgen rays, possess the 

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



I] RADIO-ACTIVE SUBSTANCES 7 

important property of discharging both positively and negatively 
electrified bodies. These results were confirmed and extended by 
Lord Kelvin, Smolan and Beattie 1 . The writer made a detailed 
comparison 2 of the nature of the discharge produced by uranium 
with that produced by Rontgen rays, and showed that the dis- 
charging property of uranium is due to the production of charged 
ions by the radiation throughout the volume of the gas. The 
property has been made the basis of a qualitative and quantitative 
examination of the radiations from all radio-active bodies, and is 
discussed in detail in chapter n. 

The radiations from uranium are thus analogous, as regards 
their photographic and electrical actions, to Rontgen rays, but, 
compared with the rays from an ordinary X ray tube, these 
actions are extremely feeble. While with Rontgen rays a strong 
impression is produced on a photographic plate in a few minutes 
or even seconds, several days' exposure to the uranium rays is 
required to produce a well-marked action, even though the uranium 
compound, enveloped in black paper, is placed close to the plate. 
The discharging action, while very easily measurable by suitable 
methods, is also small compared with that produced by X rays 
from an ordinary tube. 

6. The rays from uranium show no evidence of direct re- 
flection, refraction, or polarization 3 . While there is no direct 
reflection of the rays, there is apparently a diffuse reflection set 
up where the rays strike a solid obstacle. This is in reality due 
to a secondary radiation set up when the primary rays impinge 
upon matter. The presence of this secondary radiation at first 
gave rise to the erroneous view that the rays could be reflected 
and refracted like ordinary light. The absence of reflection, re- 
fraction, or polarization in the penetrating rays from uranium 
necessarily follows in the light of our present knowledge of the 
rays. It is now known that the uranium rays, mainly responsible 
for the photographic action, are deviable by a magnetic field, and 
are similar in all respects to cathode rays, i.e. the rays are composed 

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

2 Phil. Mag. Jan. 1899. 

3 Rutherford, Phil. Mag. Jan. 1899. 



8 KADIO-ACTIVE SUBSTANCES [CH. 

of small particles projected at great velocities. The absence of the 
ordinary properties of transverse light waves is thus to be expected. 

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

8. The question naturally arose whether the property of 
spontaneously giving out penetrating radiations was confined to 
uranium and its compounds, or whether it was exhibited to any 
appreciable extent by other substances. 

By the electrical method, with an electrometer of ordinary 
sensitiveness, any body which possesses an activity of the order of 
1/100 of that of uranium can be detected. With an electroscope of 
special construction, such as has been designed by C. T. R. Wilson 
for his experiments on the natural ionization of air, a substance 
of activity 1/10000 and probably 1/100000 of that of uranium can 
be detected. 

If an active body like uranium be mixed with an inactive body, 
the resulting activity in the mixture is generally considerably less 
than that due to the active substance alone. This is due to the 
absorption of the radiation by the inactive matter present. The 
amount of decrease largely depends on the thickness of the layer 
from which the activity is determined. 

Mme Curie made a detailed examination by the electrical 
method of the great majority of known substances, including the 
very rare elements, to see if they possessed any activity. In cases 
when it was possible, several compounds of the elements were 
examined. With the exception of thorium and phosphorus, none 
of the other substances possessed an activity even of the order of 
1/100 of uranium. 



I] RADIO-ACTIVE SUBSTANCES 9 

The ionization of the gas by phosphorus does not, however, 
seem to be due to a penetrating radiation like that found in the 
case of uranium, but rather to a chemical action taking place at 
its surface. The compounds of phosphorus do not show any 
activity, and in this respect differ from uranium and the other 
active bodies. 

Le Bon 1 has also observed that quinine sulphate, if heated and 
then allowed to cool, possesses for a short time the property of 
discharging both positively and negatively electrified bodies. It 
is necessary, however, to draw a sharp line of distinction between 
phenomena of this kind and those exhibited by the naturally radio- 
active bodies. While both, under special conditions, possess the 
property of ionizing the gas, the laws controlling the phenomena 
are quite distinct in the two cases. For example, only one com- 
pound of quinine shows the property, and that compound only 
when it has been subjected to a preliminary heating. The action 
of phosphorus depends on the nature of the gas, and varies with 
temperature. On the other hand, the activity of the naturally 
radio-active bodies is spontaneous and permanent. It is exhibited 
by all compounds, and is not, as far as is yet known, altered by 
change in the chemical or physical conditions. 

9. The discharging and photographic action alone cannot be 
taken as -a criterion as to whether a substance is radio-active or 
not. It is necessary in addition to examine the radiations, and to 
test whether the actions take place through appreciable thicknesses 
of all kinds of matter opaque to ordinary light. For example, a 
body giving out short waves of ultra-violet light can be made to 
behave in many respects like a radio-active body. As Lenard 2 has 
shown, short waves of ultra-violet light will ionize the gas in their 
path, and will be rapidly absorbed in the gas. They will produce 
strong photographic action, and may pass through some substances 
opaque to ordinary light. The similarity to a radio-active body is 
thus fairly complete as regards these properties. On the other 
hand, the emission of these light waves, unlike that of the radiations 
from an active body, will depend largely on the molecular state 

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

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



10 KADIO- ACTIVE SUBSTANCES [CH. 

of the compound, or on temperature and other physical conditions. 
But the great point of distinction lies in the nature of the radia- 
tions from the bodies in question. In one case the radiations behave 
as transverse waves, obeying the usual laws of light waves, while in 
the case of a naturally active body, they consist for the most part 
of a continuous flight of material particles projected from the 
substance with great velocity. Before any substance can be called 
" radio-active " in the sense in which the term is used to describe 
the properties of the natural radio-active elements, it is thus 
necessary to make a close examination of its radiations ; for it is 
unadvisable to extend the use of the term " radio-active " to 
substances which do not possess the characteristic radiating 
properties of the radio-active elements which we have described, 
and the active products which can be obtained from them. Some 
of the pseudo-active bodies will however be considered later in 

10. Thorium. In the course of an examination of a large 
number of substances, Schmidt 1 found that thorium, its compounds, 
and the minerals containing thorium, possessed properties similar 
to those of uranium. The same discovery was made independently 
by Mme Curie 2 . The rays from thorium compounds, like those 
from uranium, possess the properties of discharging electrified 
bodies and acting on a photographic plate. Under the same 
conditions the discharging action of the rays is about equal in 
amount to that of uranium, but the photographic effect is 
distinctly weaker. 

The radiations from thorium are more complicated than those 
from uranium. It was early observed by several experimenters 
that the radiation from thorium compounds, especially the oxide, 
when tested by the electrified method, was very variable and 
uncertain. A detailed investigation of the radiations from thorium 
under various conditions was made by Owens 3 . He showed that 
thorium oxide, especially in thick layers, was able to produce 
conductivity in the gas when covered with a large thickness of 
paper, and that the amount of this conductivity ceuld be greatly 

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

3 Phil. Mag. Oct. 1899. 



I] RADIO- ACTIVE SUBSTANCES 11 

varied by blowing a current of air over the gas. In the course of 
an examination 1 of this action of the air current, the writer 
showed that thorium compounds gave out a material emanation 
made up of very small particles themselves radio-active. The 
emanation behaves like a radio-active gas; it diffuses rapidly 
through porous substances like paper, and is carried away by 
a current of air. The evidence of the existence of the emanation, 
and its properties, is considered in detail later in chapter vm. In 
addition to giving out an emanation, thorium behaves like uranium 
in emitting three types of radiation, each of which is similar in 
properties to the corresponding radiation from uranium. 

11. Radio-active minerals. Mme Curie has examined 
the radio-activity of a large number of minerals containing 
uranium and thorium. The electrical method was used, and the 
current measured between two parallel plates 8 cms. in diameter 
and 3 cms. apart, when one plate was covered with a uniform 
layer of the active matter. The following numbers give the order 
of the saturation current obtained in amperes. 

i 

Pitchblende from Johanngeorgenstadt 8-3 x 10 ~ n 

Joachimstahl ... 7'0 

Pzibran 6'5 

Cornwall ... ... 1*6 

Clevite 1'4 

Chalcolite 5-2 

Autunite 2'7 

Thorite from 0'3 to 1-4 

Orangite 2-0 

Monazite ... ... ... ... ... 0*5 

Xenotine 0*03 

Aeschynite O7 

Fergusonite 0*4 

Samarskite ... ... ... ... 1*1 

Niobite 0'3 

Carnotite 6-2 

Some activity is to be expected in these minerals, since they all 
contain either thorium or uranium or a mixture of both. An 
examination of the action of the uranium compounds with the 

i Phil. Mag. Jan. 1900. 



12 RADIO-ACTIVE SUBSTANCES [CH. 

same apparatus and under the same conditions led to the following 

results : 

i 

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

Black oxide of uranium ... ... 2'6 

Green 1-8 

Acid uranic hydrate 0'6 

Uranate of sodium ... 1-2 

Uranate of potassium 1-2 ,, 

Uranate of ammonia 1*3 

Uranous sulphate ... ... ... 0*7 

Sulphate of uranium and potassium 0*7 

Acetate 0'7 

Phosphate of copper and uranium O9 

Oxysulphide of uranium 1'2 

The interesting point in connection with these results is that 
some specimens of pitchblende have four times the activity of the 
metal uranium; chalcolite, the crystallized phosphate of copper 
and uranium, is twice as active as uranium ; and autunite, a 
phosphate of calcium and uranium, is as active as uranium. From 
the previous considerations, none of the substances should have 
shown as much activity as uranium or thorium. In order to be 
sure that the large activity was not due to the particular chemical 
combination, Mine Curie prepared chalcolite artificially, starting 
with pure products. This artificial chalcolite had the activity to 
be expected from its composition, viz. about 0'4 of the activity of 
the uranium. The natural mineral chalcolite is thus five times as 
active as the artificial mineral. 

It thus seemed probable that the large activity of some of 
these minerals, compared with uranium and thorium, was due to 
the presence of small quantities of some very active substance, 
which was different from the known bodies thorium and uranium. 

This supposition was completely verified by the work of M. and 
Mme Curie, who were able to separate from pitchblende by purely 
chemical methods two active bodies, one of which in the pure state 
is over a million times more active than the metal uranium. 

This important discovery was due entirely to the property 
of radio-activity possessed by the new bodies. The only guide 
in their separation was the activity of the products obtained. In 



I] KADIO-ACTIVE SUBSTANCES 13 

this respect the discovery of these bodies is quite analogous to the 
discovery of rare elements by the methods of spectrum analysis. 
The method employed in the separation consisted in examining 
the relative activity of the products after chemical treatment. In 
this way it was seen whether the radio-activity was confined to one 
or another of the products, or divided between both, and in what 
ratio such division occurred. 

The activity of the specimens thus served as a basis of rough 
qualitative and quantitative analysis, analogous in some respects 
to the indication of the spectroscope. To obtain comparative 
data it was necessary to test all the . products in the dry state. 
The chief difficulty lay in the fact that pitchblende is a very 
complex mineral, and contains in varying quantities nearly all the 
known metals. 

12. Radium. The analysis of pitchblende by chemical 
methods, using the procedure sketched above, led to the discovery 
of two very active bodies, polonium and radium. The nam^ejxjp- 
nium was given to the first substance discovered by Mme Curie 
in honour of theTcountry of her birth. The name radium was 
a very happy inspiration of the discoverers, for this substance in 
the pure state possesses the property of radio-activity to an 
astonishing degree. 

Radium is extracted from pitchblende by the same process 
necessary to separate barium, to which it is very closely allied in 
chemical properties 1 . After the removal of other substances, the 
radium remains behind mixed with barium. It can, however, be 
partially separated from the latter by the difference in solubility of 
the chlorides in water, alcohol, or hydrochloric acid. The chloride 
of radium is less soluble than that of barium, and can be separated 
from it by the method of fractional crystallization. After a large 
number of precipitations the radium can be almost completely freed 
from the barium. 

Both polonium and radium exist in infinitesimal quantities in 
pitchblende. In order to obtain a few decigrammes of very active 
radium, it is necessary to use several tons of pitchblende, or the 
residues obtained from the treatment of uranium minerals. It is 

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



14 RADIO-ACTIVE SUBSTANCES [CH. 

thus obvious that the expense and labour involved in preparation 
of a minute quantity of radium are very great. 

M. and Mme Curie were indebted for their first working 
material to the Austrian government, who generously presented 
them with a ton of the treated residue of uranium materials from 
the State manufactory of Joachimstahl in Bohemia. With the 
assistance of the Academy of Sciences and other societies in France, 
funds were given to carry out the laborious work of separation. 
Later the Curies were presented with a ton of residues from the 
treatment of pitchblende by the Societe Centrale de Produits 
Chimiques of Paris. The generous assistance afforded in thi& 
important work is a welcome sign of the active interest taken in 
these countries in the furthering of purely scientific research. 

The rough concentration and separation of the residues was 
performed in the chemical works, and there followed a large amount 
of labour in purification and concentration. In this manner, 
the Curies were able to obtain a small quantity of radium which 
was enormously active compared with uranium. No definite results 
have yet been given on the activity of pure radium but the Curies 
estimate that it is about one million times the activity of uranium, 
and may possibly be still higher. The difficulty of making a 
numerical estimate for such an intensely active body is very great. 
In the electric method, the activities are compared by noting the 
relative strength of the maximum or saturation current between 
two parallel plates, on one of which the active substance is spread. 
On account of the intense ionization of the gas between the plates, 
it is not possible to reach the saturation- current unless very high 
voltages are applied. Approximate comparisons can be made by 
the use of metal screens to cut down the intensity of the radiations, 
if the proportion of the radiation transmitted by such a screen has 
been determined by direct experiment on impure material of easily 
measurable activity. The value of the activity of radium compared 
with that of uranium will however vary to some extent according to 
which of the three types of rays is taken as a basis of comparison. 

It is thus difficult to control the final stages of the purification 
of radium by measurements of its activity alone. Moreover the 
activity of radium immediately after its preparation is only al^out 
one-fourth of its final value; it gradually rises to a maximum after 



l] RADIO-ACTIVE SUBSTANCES 15 

the radium salt has been kept in the dry state for about a month. 
For control experiments in purification, it is advisable to measure 
the initial rather than the final activity. 

Mme Curie has utilized the coloration of_the,_crystals of radi- 
ferous barium as a means of controlling the final process of puri- 
ficatiori. The crystals of salts of radium and barium deposited from 
acid solutions are indistinguishable. The crystals of radiferous 
barium are at first colourless, but, in the course of a few hours, 
become yellow, passing to orange and sometimes to a beautiful rose 
colour. The rapidity of this coloration depends on the amount of 
'barium present. Pure radium crystals do not colour, or at any rate 
not as rapidly as those containing barium. The coloration is a 
maximum for a definite proportion of radium, and this fact can be 
utilized as a means of testing the amount of barium present. When 
the crystals are dissolved in water the coloration disappears. 

Giesel 1 has observed that pure radium bromide gives a beautiful 
carmine colour to the Bunsen flame. If barium is present in any 
quantity, only the green colour due to barium is observed and a 
spectroscopic examination shows only the barium lines. This 
carmine coloration of the Bunsen flame is a good indication of the 
purity of the radium. 

Since the preliminary announcement of the discovery of 
radium, Giesel 2 has devoted a great deal of attention to the 
separation of radium, polonium and other active bodies from pitch- 
blende. He was indebted for his working material to the firm 
of P. de Haen of Hanover, who presented him with a ton of pitch- 
blende residues. Using the method of fractional crystallization of 
the bromide instead of the chloride, he has been able to prepare 
considerable quantities of pure radium. By this means the labour 
of final purification of radium has been much reduced. He states 
that six or eight crystallizations with the bromide are sufficient to 
almost completely free the radium from the barium. 

13. Spectrum of radium. It was of great importance to 
settle as soon as possible whether radium was in reality modified 
barium or a new element with a definite spectrum. For this 
purpose the Curies prepared some specimens of radium chloride, 

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

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



16 RADIO-ACTIVE SUBSTANCES [CH. 

and submitted them for examination of their spectrum to 
Demar9ay, an authority on that subject. The first specimen of 
radium chloride examined by Demar^ay 1 was not very active, but 
showed, besides the lines due to barium, a very strong new line in 
the ultra-violet. In another sample of greater activity, the line 
was still stronger and others also appeared, while the intensity of 
the new lines was comparable with those present due to barium. 
With a still more active specimen which was probably nearly pure, 
only three strong lines of barium appeared, while the new spectrum 
was very bright. The following table shows the wave-length of 
the new lines observed for radium. The wave lengths are expressed 
in Angstrom units and the intensity of each ray is denoted by a 
number, the ray of maximum intensity being 16. 

Wave length Intensity Wave length Intensity 

4826-3 10 4600-3 3 

4726-9 5 4533-5 9 

4099-6 3 4436-1 6 

4692-1 7 4340-6 12 

4683-0 14 38147 16 

4641-9 4 3649-6 12 

The lines are all sharply defined, and three or four of them 
have an intensity comparable with any known lines of other 
substances. There are also present in the spectrum two strong 
nebulous bands. In the visible part of the spectrum, which has 
not been photographed, the only noticeable ray has a wave 
length 5665, which is, however, very feeble compared with that of 
wave length 48 2 6 '3. The general aspect of the spectrum is similar 
to that of the alkaline earths ; it is known that these metals have 
strong lines accompanied by nebulous bands. 

' The principal line due to radium can be distinguished in 
impure radium of activity 50 times that of uranium. By the 
electrical method it is easy to distinguish the presence of radium 
in a body which has an activity only 1/100 of uranium. With a 
more sensitive electrometer 1/10000 of the activity of uranium 
could be observed. For the detection of radium, the examination 
of the radio-activity is thus a process nearly a million times more 
sensitive than spectrum analysis. 

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



l] RADIO-ACTIVE SUBSTANCES 17 

Later observations on the spectrum of radium have been made by 
Runge 1 , Exner and Haschek 2 , with specimens of radium prepared by 
Giesel. It has already been mentioned that the bromide of radium 
gives a characteristic pure carmine-red coloration to the Bunsen 
flame. The flame spectrum shows two broad bright bands in the 
orange-red, not observed in Demarcay's spectrum. In addition 
there is a line in the blue-green and two feeble lines in the violet. 

14. Atomic weight of radium. Mme Curie has made 
successive determinations of the atomic weight of the new element 
with specimens of steadily increasing purity. In the first obser- 
vation the radium was largely mixed with barium, and the atomic 
weight obtained was the same as that of barium, 137 '5. In 
successive observations with specimens of increasing purity the 
atomic weights of the mixture were 146 and 175. The final value 
obtained recently was 225, w r hich may be taken as the atomic 
weight of radium on the assumption that it is divalent. 

In these experiments about 0*1 gram of pure radium chloride has 
been obtained by successive fractionations. The difficulty involved 
in preparing a quantity of pure radium chloride large enough to 
test the atomic weight may be gauged from the fact that only a 
few centigrams of fairly pure radium, or a few decigrams of less 
concentrated material, are obtained from treatment of about 2 tons 
of the mineral from which it is derived. 

Runge and Precht 8 have examined the spectrum of radium in 
a magnetic field, and have shown the existence of series analogous 
to those observed for calcium, barium, and strontium. These series 
are connected with the atomic weights of the elements in question, 
and Runge and Precht have calculated by these means that the 
atomic weight of radium should be 258 a number considerably 
greater than the number 225 obtained by Mme Curie by means of 
chemical analysis. Marshall Watts 4 , on the other hand, using another 
relation between the lines of the spectrum 5 , deduced the value 
obtained by Mme Curie. Considering that the number found 

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

4 Phil Mag. July, 1903. 

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

R. R.-A. 2 



18 RADIO-ACTIVE SUBSTANCES [CH. 

by Mme Curie agrees with that required by the periodic system, 
it is advisable in the present state of our knowledge to accept the 
experimental number rather than the one deduced by Runge and 
Precht from spectroscopic evidence. 

There is no doubt that radium is a new element possessing 
remarkable physical properties. The detection and separation of 
this substance, existing in such minute proportions in pitchblende, 
has been due entirely to the characteristic property we are con- 
sidering, and is the first notable triumph of the study of radio- 
activity. As we shall see later in chapter vn, the property of radio- 
activity can be used, not only as a means of chemical research, but 
also as an extraordinarily delicate method of detecting chemical 
changes of a very special kind. 

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

As in the case of uranium or thorium, the photographic action 
is mainly due to the penetrating or cathodic [rays. The radio- 
graphs obtained with radium are very similar to those obtained 
with X rays, but lack the sharpness and detail of the latter. The 
rays are unequally absorbed by different kinds of matter, the 



l] RADIO-ACTIVE SUBSTANCES 19 

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

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

16. Compounds of radium. When first prepared in the 
solid state, all the salts of radium the chloride, bromide, acetate, 
sulphate, and carbonate are very similar in appearance to the 
corresponding salts of barium, but in time they gradually become 
coloured. In chemical properties the salts of radium are prac- 
tically the same as those of barium, with the exception that 
the chloride and bromide of radium are less soluble than the 
corresponding salts of barium. All the salts of radium are natu- 
rally phosphorescent. The phosphorescence of impure radium 
preparations is in some cases very marked. 

All the radium salts possess the property of causing rapid 
colorations of the glass vessel which contains them. For feebly 
active material the colour is usually violet, for more active material 
a yellowish-brown, and finally black. 

17. Polonium. Polonium was the first of the active sub- 
stances obtained from pitchblende. It has been investigated in 
detail by its discoverer Mme Curie 1 . The pitchblende was dissolved 
in acid and sulphuretted hydrogen added. The precipitated 
sulphides contained an active substance, which, after separation 
of impurities, was found associated with bismuth. This active 
substance, which has been named polonium, is so closely allied in 
chemical properties to bismuth that it has so far been found 
impossible to effect a complete separation. Partial^segaration of 
polonium can be made by successive fractionations based on one 
of the following modes of procedure : 

(1) Sublimation in a vacuum. The active sulphide is more 
volatile than that of bismuth. It is deposited as a black substance 
at portions of the tube, where the temperature is between 250 
1 C. R. 127, p. 175, 1898. 

2 2 



20 RADIO-ACTIVE SUBSTANCES [CH. 

and 300 C. In this way polonium of activity 700 times that of 
uranium was obtained. 

(2) Precipitation of nitric acid solutions by water. The 
precipitated sub-nitrate is much more active than the part that 
remains in solution. 

(3) Precipitation by sulphuretted hydrogen in a very acid 
hydrochloric acid solution. The precipitated sulphides are much 
more active than the salt which remains in solution. 

For concentration of the active substance Mme Curie 1 has made 
use of method (2). The process is, however, very slow and tedious, 
and is made still more complicated by the tendency to form 
precipitates insoluble either in strong or weak acids. After a 
large number of fractionations, a small quantity of matter was 
obtained, enormously active compared with uranium. On exami- 
nation of the substance spectroscopically, only the bismuth lines 
were observed. A spectroscopic examination of the active bismuth 
by Demarcay and by Runge and Exner has led to the discovery 
of no new lines. On the other hand Sir William Crookes 2 states that 
he found one new line in the ultra-violet, while Berndt 3 , working 
with polonium of activity 300, observed a large number of new 
lines in the ultra-violet. These results await further confirmation. 

The polonium prepared by Mme Curie differs from the other 
radio-active bodies in several particulars. In the first place the 
radiations include only very easily absorbable rays. The two 
penetrating types of radiation given out by uranium, thorium, 
and radium are absent. In the second place the activity does 
not remain constant, but diminishes continuously with the time. 
Mme Curie found that the polonium lost half its original activity 
in the course of eleven months. 

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

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

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

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



I] RADIO-ACTIVE SUBSTANCES 21 

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

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

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

19. The discussion of the nature of polonium was renewed by 
the discovery of Marckwald 1 that a substance similar to polonium, 
of which the activity did not decay with time, could be separated 
from pitchblende. The method of separation from the bismuth 
chloride solution obtained from uranium residues was very simple. 
A rod of bismuth, dipped in the active solution, rapidly became 
coated with a black deposit, which was intensely active. This 
deposit was continued until the whole of the activity was removed 
from the solution. From 850 grammes of bismuth solution, 

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



22 RADIO-ACTIVE SUBSTANCES [CH. 

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

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

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

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

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

Actinium. Debierne 1 has obtained from pitchblende a very 
active substance which he named actinium. This active substance 

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



I] RADIO-ACTIVE SUBSTANCES 23 

is precipitated with the iron group, and appears to be very closely 
allied in chemical properties to thorium, though it is many thousand 
times more active. It is very difficult to separate from thorium 
and the rare earths. Debierne has made use of the following 
methods for partial separation: 

(1) Precipitation in hot solutions, slightly acidulated with 
hydrochloric acid, by excess of hyposulphite of soda. The active 
matter is present almost entirely in the precipitate. 

(2) Action of hydrofluoric acid upon the hydrates freshly 
precipitated, and held in suspension in water. The portion 
dissolved is only slightly active. By this method titanium may 
be separated. 

(3) Precipitation of neutral nitrate solutions by oxygenated 
water. The precipitate carries down the active body. 

(4) Precipitation of insoluble sulphates. If barium sulphate, 
for example, is precipitated in the solution containing the active 
body, the barium carries down the active matter. The thorium 
and actinium are freed from the barium by conversion of the 
sulphate into the chloride and precipitation by ammonia. 

In this way Debierne has obtained a substance comparable 
in activity with radium. The separation, which is difficult and 
laborious, has so far not been carried far enough to bring out 
any new lines in the spectrum. Actinium gives out easily ab- 
sorbed and penetrating deviable rays like those of radium, 
and also a radio-active emanation 1 , which is more allied to the 
emanation of thorium than to that of radium. The emanation 
has a distinctive rate of decay ; it loses its activity in the course 
of a few seconds, while the thorium emanation loses half its activity 
in one minute. The distinctive character of the radiations and 
emanations, together with the permanence of the activity, make 
it very probable that actinium will prove to be a new element 
of very great activity. 

21. Giesel 2 also has obtained from pitchblende a radio-active 
substance which in many respects is similar to the actinium of 
Debierne. The active substance belongs to the group of cerium 

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

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



24 RADIO-ACTIVE SUBSTANCES [CH. 

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

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

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

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

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

22. The similarity of the chemical properties of actinium and 
thorium has led to the suggestion at different times that the 
activity of thorium is not due to thorium itself but to the presence 
of a slight trace of actinium. In view of the difference in the rate 
of decay of activity of the emanations of thorium and actinium, 
this position is not tenable. If the activity of thorium were due 



l] RADIO-ACTlVE SUBSTANCES 25 

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

Baskerville 1 , working with thorium minerals, was able to obtain 
thorium less active to the photographic plate than ordinary 
thorium. He put forward the view that thorium was a mixture 
of two elements, one of which was active and the other inactive. 
These results were probably due to the separation of the active 
product Th. X from the thorium (see section 119). This process 
would temporarily greatly reduce the activity as tested by the 
photographic method. 

Until thorium is obtained permanently free from activity, the 
question whether the radio-activity is due to a small trace of very 
active matter, or to the thorium itself, must remain in doubt 2 . The 
fact that ordinary commercial thorium and the purest chemical 
preparation show equal activity supports the view that the effect 
is not due to a radio-active impurity, but to the element itself. 
If the activity of thorium is due to a small trace of active matter, 
this active substance is certainly not radium or actinium or any 
other known material. 

Hofmann and Zerban 3 obtained a substance from pitchblende 
similar in radio-active properties to thorium. The activity of this 
product did not dimmish much in four months' interval. The 
substance was probably the same as Debierne's actinium. They 
also examined the thorium minerals broggerite and clevite, but 
obtained only some active residues the activity of which decreased 
rapidly with the time. 

23. Radio-active lead. Elster and Geitel 4 found that lead 
sulphate obtained from pitchblende was very active. They con- 
sidered that the activity was due to admixture with radium, and 
by suitable treatment the lead sulphate was obtained in an inactive 
state. 

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

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

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

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



26 KADIO-ACTIVE SUBSTANCES [CH. 

Hofmann and Strauss 1 found that lead sulphate obtained from 
pitchblende was active. This was not due to admixture with either 
uranium or radium or polonium. They gave the name of radio- 
active lead to the substance. This radio-active lead, in most of 
its reactions, resembled ordinary lead, but showed differences in 
the behaviour of the sulphide and the sulphate. The sulphate was 
very strongly phosphorescent. This sulphate apparently lost its 
activity with time, but recovered it in a few minutes after exposure 
to cathode rays in a vacuum tube. 

Giesel 2 also was able to obtain radio-active lead, but found that 
the activity diminished with time, while Hofmann states that his 
preparations preserve their activity. It thus appears probable 
that radio-active lead is either one of the numerous examples of 
substances made active for the time by solution with radio- 
elements, or lead with a slight admixture of a radio-element. The 
peculiar action of the cathode rays in causing an increase of the 
photographic and electric action of radio-lead sulphate has ap- 
parently nothing whatever to do with the activity proper of the 
substance, but seems to be an additional effect due to the strong 
phosphorescence set up. The sulphide does not show any such 
action. The phosphorescent light probably includes some short 
ultra-violet light waves which are capable of ionizing the gas. 

24. If elements heavier than uranium exist, it is probable that 
they will be radio-active. The extreme delicacy of radio-activity 
as a means of chemical analysis would enable such elements to 
be recognized even if present in infinitesimal quantities. It is 
probable that considerably more than the three or four radio- 
elements at present recognized exist in minute quantity, and that 
the number at present known will be augmented in the future. 
In the first stage of the search, a purely chemical examination is 
of little value, for it is not probable that the new element should 
exist in sufficient quantity to be detected by chemical or spectro- 
scopic analysis. The main criteria of importance are the existence 
or absence of distinctive radiations or emanations, and the perman- 
ence of the radio-activity. The presence of a radio-active emanation 

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

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



I] RADIO-ACTIVE SURSTAXCES 27 

with a rate of decay different from those already known would 
afford strong evidence that a new radio-active body was present. 
The presence of either thorium or radium in matter can very 
readily be detected by observing the rate of decay of the emana- 
tions given out by them. When once the presence of a new 
radio-element has been inferred by an examination of its radio- 
active properties, chemical methods of separation can be devised, 
the radiating or emanating property being used as a guide in 
qualitative and quantitative analysis. 



CHAPTER II. 

IONIZATION THEORY OF GASES. 

25. lonization of gases by radiation. The mostimportant 
property possessed by the radiations from radio-active bodies is 
their power of discharging bodies whether positively or negatively 
electrified. As this property has been made the basis of a method 
for an accurate quantitative analysis and comparison of the 
radiations, the variation of the rate of discharge under different 
conditions and the processes underlying it will be considered in 
some detail. 

In order to explain the similar discharging power of Rontgen 
rays, the theory 1 has been put ^r Earth 

forward that the rays pro- f 

duce positively and negatively * 

charged carriers throughout + 

the volume of the gas sur- ^ 

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

that the rate of production is 

proportional to the intensity 

of the radiation. These carriers, Fl 8- l - 

or ions 2 as they have been termed, move with a uniform velocity 

through the gas under a constant electric field, and their velocity 

varies directly as the strength of the field. 

Suppose we have a gas between two metal plates A and B 
(Fig. 1) exposed to the radiation, and that the" plates are kept 
at a constant difference of potential. A definite number of ions 
will be produced per second by the radiation, and the number 

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

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



CH. Il] IOXIZATION TflEORY OF GASES 29 

produced will in general depend upon the nature and pressure of 
the gas. In the electric field the positive ions travel towards the 
negative plate, and the negative ions towards the other plate, and 
consequently a current will pass through the gas. Some of the 
ions will also recombine, the rate of recombination being propor- 
tional to the square of the number present. For a given intensity 
of radiation, the current passing through the gas will increase at 
first with the potential difference between the plates, but it will 
finally reach a maximum when all the ions are removed by the 
electric field before any recombination occurs. 

This theory accounts also for all the characteristic properties of 
gases made conducting by the rays from active substances, though 
there are certain differences observed between the conductivity 
phenomena produced by active substances and by X rays. These 
differences are for the most part the result of unequal absorption 
of the two types of rays. Unlike Rontgen rays a large proportion 
of the radiation from active bodies consists of rays which are 
absorbed in their passage through a few centimetres of air. The 
ionization of the gas is thus not uniform, but falls off rapidly with 
increase of distance from the active substance. 

26. Variation of the current with voltage. Suppose that 
a layer of radio-active matter is spread uniformly on the lower of 
two horizontal plates A and B (Fig. 1). The lower plate A is 
connected with one pole of a battery of cells the other pole of which 
is connected with earth. The plate B is connected with one pair of 
quadrants of an electrometer, the other pair being connected with 
earth. 

The current 1 between the plates, determined by the rate of 
movement of the electrometer needle, is observed at first to in- 
crease rapidly with the voltage, then more slowly, finally reaching 
a value which increases very slightly with a large increase in the 
voltage. This, as we have indicated, is simply explained on the 
ionization theory. 

The radiation produces ions at a constant rate, and, before the 
electric field is applied, the number pe^ unit volume increases 

1 A minute current is observed between the plates even if no radio-active matter 
is present. This has been found to be due mainly to a slight natural radio-activity 
of the matter composing them. (See sections 218220.) 



30 IONIZATION THEORY OF GASES [CH. 

until the rate of production of fresh ions is exactly balanced by the 
recombination of the ions already produced. On application of a 
small electric field, the positive ions travel to the negative electrode 
and the negative to the positive. 

Since the velocity of the ions between the plates is directly 
proportional to the strength of the electric field, in a weak field 
the ions take so long to travel between the electrodes that most of 
them recombine on the way. 

The current observed is consequently small. With increase of 
the voltage there is an increase of speed of the ions and a smaller 
number recombine. The current consequently increases, and will 
reach a maximum value when the electric field is sufficiently 
strong to remove all the ions before appreciable recombination has 
occurred. The value of the current will then remain constant even 
though the voltage is largely increased. 

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

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




Saturation Curve 



Folk 
Fig. 2. 

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



n] 



IONIZATION THEORY OF GASES 



31 



Although the variation of the current with voltage depends 
only on the velocity of the ions and their rate of recombination, 
the full mathematical analysis is intricate, and the equations, 
expressing the relation between current and voltage, are only 
integrable for the case of uniform ionization. The question is com- 
plicated by the inequality in the velocity of the ions and by the 
disturbance of the potential gradient between the plates by the 
movement of the ions. J. J. Thomson 1 has worked out the case 
for uniform production of ions between two parallel plates, and has 
found that the relation between the current i and the potential 
difference V applied is expressed by 



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

In certain cases of unsymmetrical ionization, which arise in the 
study of the radiations from active bodies, the relation between 
current and voltage is very different from that expressed by 



00 

M 
80 

70 








.-. 


_ 








^x 














X 












/ 












50 
40 
30 




/ 












/ 














/ 






Sat 


uration Cu 
m, activity 
es 4-5 cms. 


rve 




/ 






plat 


apart 




JU 
10 


/ 














/ 












100 200 300 400 500 600 70C 



Fig. 3. 

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



32 IONIZAT1ON THEOKY OF GASES [CH. 

the above equation. Some of these cases will be considered in 
section 47. 

27. The general shape of the current-voltage curves for gases 
exposed to the radiations from active bodies is shown in Fig. 3. 

This curve was obtained for '45 grams of impure radium 
chloride, of activity 1000 times that of uranium, spread over an 
area of 33 sq. cms. on the lower of two large parallel plates, 
4'5 cms. apart. The maximum value of the current observed, 
which is taken as 100, was 1*2 x 10~ 8 amperes, the current for low 
voltages was nearly proportional to the voltage, and about 600 
volts between the plates was required to ensure approximate 
saturation. 

In dealing with slightly active bodies like uranium or thorium, 
approximate saturation is obtained for much lower voltages. 
Tables I. and II. show the results for the current between two 
parallel plates distant 0'5 cms. and 2*5 cms. apart respectively, when 
one plate was covered with a thin uniform layer of uranium oxide. 

TABLE I. TABLE II. 

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

Volts Current Volts Current 

125 18 -5 7'3 

25 36 1 14 

5 55 2 27 

1 67 4 47 

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

16 88 112 90 

100 94 375 97 

335 100 800 100 

The results are shown graphically in Fig. 4. 

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



IONIZATION THEORY OF GASES 



33 



Taking into consideration the early part of the curves, the 
current does not reach a practical maximum as soon as would be 
expected on the simple ionization theory. It seems probable that 



100 



Saturation Curves 

for 
Uranium rays 




40 60 

Volts 



10 



Fig. 4. 

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

28. The equation expressing the relation between the current 
and the voltage is very complicated even in the case of a uniform 
rate of production of ions between the plates. An approximate 
R. R.-A. - 3 



34 IONIZATION THEORY OF GASES [CH. 

theory, which is of utility in interpreting the experimental results, 
can however be simply deduced if the disturbance of the potential 
gradient is disregarded, and the ionization assumed uniform be- 
tween the plates. 

Suppose that the ions are produced at a constant rate q per 
cubic centimetre per second in the gas between parallel plates 
distant I cms. from each other. When no electric field is applied, 
the number N present per c.c., when there is equilibrium between 
the rates of production and recombination, is given by q == aN 2 , 
where a is a constant. 

If a small potential difference V is applied, which gives only a 
small fraction of the maximum current, and consequently has not 
much effect on the value of N, the current % per sq. cm. of the 
plate, is given by 

.NeuV 



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

uV 

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

I 

y 

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

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

Thus 7 = q .1 . e, 

i NuV uV 

and = = 

7 ql 2 I 2 \fqa ' 

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

Let j = 0, 

then F = ' 

u 



II] IONIZATION THEOKY OF GASES 35 

Now the greater the value of V required to obtain a given 
value of p (supposed small compared with unity), the greater the 
potential required to produce saturation. 

It thus follows from the equation that : 

(1) For a given intensity of radiation, the saturation P.D. 
increases with the distance between the plates. In the equation, 
for small values of p, V varies as I 2 . This is found to be the case 
for uniform ionization, but it only holds approximately for non- 
uniform ionization. 

(2) For a given distance between the plates, the saturation 
p. D. is greater, the greater the intensity of ionization between the 
plates. This is found to be the case for the ionization produced 
by radio-active substances. With a very active substance like 
radium, the ionization produced is so intense that very large 
voltages are required to produce approximate saturation. On the 
other hand, only a fraction of a volt per cm. is necessary to produce 
saturation in a gas where the ionization is very slight, for example, 
in the case of the natural ionization observed in a closed vessel, 
where no radio-active substances are present. 

For a given intensity of radiation, the saturation P. D. decreases 
rapidly with the lowering of the pressure of the gas. This is due 
to two causes operating in the same direction, viz. a decrease in 
the intensity of the ionization and an increase in the velocity of 
the ions. The ionization varies directly as the pressure, while the 
velocity varies inversely as the pressure. This will obviously have 
the effect of causing more rapid saturation, since the rate of 
recombination is slower and the time taken for the ions to travel 
between the electrodes is less. 

The saturation curves observed for the gases hydrogen and 
carbon dioxide 1 are very similar in shape to those obtained for air. 
For a given intensity of radiation, saturation is more readily 
obtained in hydrogen than in air, since the ionization is less than 
in air while the velocity of the ions is greater. Carbon dioxide on 
the other hand requires a greater p. D. to produce saturation than 
does air, since the ionization is more intense and the velocity of 
the ions less than in air. 

1 Rutherford, Phil. Mag, Jan. 1899. 

32 



36 



IONIZATION THEORY OF GASES 



[CH. 



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




Volts 
Fig. 5. 

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

1 Phil. Mag. Feb. 1901. 



II] IONIZATION THEORY OF GASES 37 

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




Uranium'] 



Fig. 6. 

A dry current of air or any other gas is passed at a constant 
rate through a long metal tube TL. The current of air after 
passing through a quantity of cotton-wool to remove dust particles, 
passes over a vessel T containing a radio-active body such as 
uranium, which does not give off a radio-active emanation. By 
means of insulated electrodes A and B, charged to a suitable 
potential, the current through the gas can be tested at various 
points along the tube. 

A gauze screen, placed over the cross-section of the tube at D, 
serves to prevent any direct action of the electric field in abstracting 
ions from the neighbourhood of T. 

If the electric field is sufficiently strong, all the ions travel 
in to the electrodes at A, and no current is observed at the elec- 
trode B. If the current is observed successively at different distances 
along the tube, all the electrodes except the one under consideration 
being connected to earth, it is found that the current diminishes 
with the distance from the active body. If the tube is of fairly 
wide bore, the loss of the ions due to diffusion is small, and the 
decrease in conductivity of the gas is due to recombination of the 
ions alone. 

On the ionization theory, the number dn of ions per unit volume 
which recombine in the time dt is proportional to the square of 
the number present. Thus 

dn 

where a is a constant. 



38 IONIZATION THEORY OF GASES [CH. 

Integrating this equation, 



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

The experimental results obtained 1 have been shown to agree 
very well with this equation. 

In an experiment similar to that illustrated in Fig. 6, using 
uranium oxide as a source of ionization, it was found that half the 
number of ions present in the gas recombined in 2*4 seconds, and 
that at the end of 8 seconds one-fourth of the ions were still 
uncombined. 

Since the rate of recombination is proportional to the square of 
the number present, the time taken for half of the ions present in 
the gas to recombine decreases very rapidly with the intensity of 
the ionization. If radium is used, the ionization is so intense that 
the rate of recombination is extremely rapid. It is on account of 
this rapidity of recombination that large voltages are necessary to 
produce saturation in the gases exposed to very active preparations 
of radium. 

The value of a, which may be termed the coejftdent_of_recom- 
l&LLation, has been determined in absolute measure by Townsend 2 , 
M c Clung 3 and Langevin 4 by different experimental methods but 
with very concordant results. Suppose, for example, with the 
apparatus of Fig. 6, the time T, taken for half the ions to recombine 
after passing by the electrode A, has been determined experi- 

mentally. Then - = aT, where N is the number of ions per c.c. 

present at A. If the saturation current i is determined at the 
electrode A, i = NVe where e is the charge on an ion and V is the 
volume of uniformly ionized gas carried by the electrode 'A per 

Ve 
second. Then a = . m . 

11. 

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

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

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



II] IONIZATION THEORY OF GASES 39 

Value of a. 



Gas 

Air 

Carbon Dioxide 
Hydrogen 


Town send 

3420 x e 
3500 x e 
3020 x e 


M c Clung 

3384 x e 
3492 x e 


Langevin 
3200 xe 
3400 xe 



The latest determination of the value of e (see section 36) is 
3-4 x 10- 10 E. s. units ; thus a = Tl x lO" 6 . 

Using this value, it can readily be shown from the equation of 
recombination that, if 10 6 ions are present per c.c., half of them 
recombine in about 0'9 sec. and 99% in 90 sees. 

M e Clung (loc. tit.) showed that the value of a was approximately 
independent of the pressure between '125 and three atmospheres. 
In later observations, Langevin has found that the value of a 
decreases rapidly when the pressure is lowered below the limits 
used by M c Clung. 

31. In experiments on recombination it is essential that the 
gas should be free from dust or other suspended particles. In 
dusty air, the rate of recombination is much more rapid than in 
dust-free air, as the ions diffuse rapidly to the comparatively large 
dust particles distributed throughout the gas. The effect of the 
suspension of small particles in a conducting gas is very .well 
illustrated by an experiment of Owens 1 . If tobacco smoke is 
blown between two parallel plates as in Fig. 1, the current at once 
diminishes to a small fraction of its former value, although a P.D. 
is applied sufficient to produce saturation under ordinary con- 
ditions. A much larger voltage is then necessary to produce 
saturation. If the smoke particles are removed by a stream of air, 
the current at once returns to its original value. 

32. Mobility of the ions. Determinations of the mobility 
of the ions, i.e. the velocity of the ions under a potential gradient 
of 1 volt per cm., have been made by Rutherford 2 , Zeleny 3 , and 
Langevin 4 for gases exposed to Rontgen rays. Although widely 
different methods have been employed, the results have been very 
concordant and fully support the view that the ions move with a 

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

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



40 IONIZATION THEORY OF GASES [CH. 

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

Zeleny 1 first drew attention to the fact that the positive and 
negative ions had different velocities. The velocity of the negative 
ion is always greater than that of the positive, and varies with the 
amount of water vapour present in the gas. 

The results, previously discussed, of the variation of the current 
with voltage and of the rate of recombination of the ions do not of 
themselves imply that the ions produced in gases by the radiations 
from active bodies are of the same size as those produced by 
Rontgen rays under similar conditions. They merely show that 
the conductivity under various conditions can be satisfactorily 
explained by the view that charged ions are produced throughout 
the volume of the gas. The same general relations would be 
observed if the ions differed considerably in size and velocity from 
those produced by Kontgen rays. The most satisfactory method 
of determining whether the ions are identical in the two cases is 
to determine the velocity of the ions under similar conditions. 

In order to compare the velocity of the ions 2 , the writer has 
used an apparatus similar to that shown in Fig. 6 on p. 37. 

The ions were carried with a rapid constant stream of air 
past the charged electrode A , and the conductivity of the gas tested 
immediately afterwards at an electrode B, which was placed close 
to A. The insulated electrodes A and B were fixed centrally in 
the metal tube L, which was connected with earth. 

For convenience of calculation, it is assumed that the electric 
field between the cylinders is the same as if the cylinders were 
infinitely long. 

Let a and b be the radii of the electrode A, and of the tube L 
respectively, and let V = potential of A. 

The electromotive intensity X (without regard to sign) at a 
distance r from the centre of the tube is given by 

v 



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



H] IONIZATION THEORY OF GASES 41 

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

dr = Xu^dt, 

b 
or log e - r dr 

dt = - ^ - . 

VUt 

Let r. 2 be the greatest distance measured from the axis of the 
tube from which the negative ion can just reach the electrode A 
in the time t taken for the air to pass along the electrode. 

'- 



If p 2 be the ratio of the number of the negative ions that reach 
the electrode A to the total number passing by, then 



Therefore 

/> 2 (6 2 -a 2 )log e - 



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



"'= 



2 



In the above equations it is assumed that the current of air is 
uniform over the cross-section of the tube, and that the ions are 
uniformly distributed over the cross-section ; also, that the move- 
ment of the ions does not appreciably disturb the electric field. 
Since the value of t can be calculated from the velocity of the 
current of air and the length of the electrode, the values of the 
velocities of the ions under unit potential gradient can at once be 
determined. 

The equation (1) shows that p 2 is proportional to F, i.e. that 



42 



IONIZATION THEORY OF GASES 



[CH. 



fche rate of discharge of the electrode A varies directly as the 
potential of A, provided that the value of V is not large enough to 
remove all the ions from the gas as it passes by the electrode. 
This was found experimentally to be the case. 

In the comparison of the velocities, the potential V was adjusted 
to such a value that p z was about one half, when uranium oxide 
was placed in the tube at L. The active substance was then 
removed, and an aluminium cylinder substituted for the brass 
tube. X rays were allowed to fall on the centre of this aluminium 
cylinder, and the strength of the rays adjusted to give about the 
same conductivity to the gas as the uranium had done. Under 
these conditions the value of p. 2 was found to be the same as for, 
the first experiment. 

This experiment shows conclusively that the ions produced 
by Rontgen rays and by uranium move with the same velocity 
and are probably identical in all respects. The method described 
above is not very suitable for an accurate determination of the 
velocities, but gave values for the positive ions of about 1*4 cms. 
per second per volt per centimetre, and slightly greater values for 
the negative ions. 

33. The most accurate determinations of the mobility of the 
ions produced by Rontgen rays have been made by Zeleny 1 and 
Langevin 2 . Zeleny used a method similar in principle to that 
explained above. His results are shown in the following table, 
where K^ is the mobility of the positive ion and K z that of the 
negative ion. 



Gas 


K, 


K 2 


K 2 
KI 


Temperature 


Air. dry 


1-36 


1-87 


1-375 


13-5C. 


moist 


1-37 


1-51 


1-10 


14 


Oxygen, dry 


1-36 


1-80 


1-32 


17 


moist 


1-29 


1-52 


1-18 


16 


Carbon dioxide, dry 


0-76 


0-81 


1-07 


17'5 


moist 


0-81 


0-75 


0-915 


17 


Hydrogen, dry 


6-70 


7-95 


1-15 


20 


,, moist ... 


5-30 


5-60 


1-05 


20 



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

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



II] IONIZATION THEORY OF GASES 43 

Langevin determined the velocity of the ions by a direct method 
in which the time taken for the ion to travel over a known distance 
was observed. 

The following table shows the comparative values obtained for 
air and carbon dioxide. 

Air CO 2 

K, K, ^ Kl K 9 | 

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

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

These results show that for all gases except CO 2 , there is a 
marked increase in the velocity of the negative ion with the dry- 
ness of the gas, and that, even in moist gases, the velocity of the 
negative ions is always greater than that of the positive ions. The 
velocity of the positive ion is not much affected by the presence 
of moisture in the gas. 

The velocity of the ions varies inversely as the pressure of the 
gas. This has been shown by Rutherford 1 for the negative ions 
produced by ultra-violet light falling on a negatively charged sur- 
face, and later by Langevin 2 for both the positive and negative ions 
produced by Rontgen rays. Langevin has shown that the velocity 
of the positive ion increases more slowly with the diminution of 
pressure than that of the negative ion. It appears as if the nega- 
tive ion, especially at pressures of about 10 mm. of mercury, 
begins to diminish in size. 

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

It has long been known that if air saturated with water- vapour 
is suddenly expanded, a cloud of small globules of water is formed. 
These drops are formed round the dust particles present in the gas, 

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



44 IONIZATION THEORY OF GASES [CH. 

which act as nuclei for the condensation of water around them. 
The experiments of R. von Helmholtz and Bicharz 1 had shown that 
chemical reactions, for example the combustion of flames, taking 
place in the neighbourhood, affected the condensation of a steam- 
jet. Lenard showed that a similar action was produced when ultra- 
violet light fell on a negatively charged zinc surface placed near 
the steam-jet. These results suggested that the presence of electric 
charges in the gas facilitated condensation. 

A very complete study of the conditions of condensation of 
water on nuclei has been made by C. T. R. Wilson 2 . An apparatus 
was constructed which allowed a very sudden expansion of the air 
over a wide range of pressure. The amount of condensation was 
observed in a small glass vessel. A beam of light was passed 
into the apparatus which allowed the drops formed to be readily 
observed by the eye. 

Preliminary small expansions caused a condensation of the 
water round the dust nuclei present in the air. These dust nuclei 
were removed by allowing the drops to settle. After a number of 
successive small expansions, the air was completely freed from 
dust, so that no condensation was produced. 

Let Vi = initial volume of the gas in the vessel, 
v 2 = volume after expansion. 

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

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

roughly constant until = 1'38, when the number suddenly in- 
creases and a very dense cloud of fine drops is produced. 

If the radiation from an X ray tube or a radio-active substance 
is now passed into the condensation vessel, a new series of phenomena 

is observed. As before, if <1'25 no drops are formed, but if 

^i 

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

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

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

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



Il] IONIZATION THEORY OF GASES 45 

greater the intensity of the rays. This point at which condensa- 
tion begins is very marked, and a slight variation of the amount of 
expansion causes either a dense cloud or no cloud at all. 

It now remains to be shown that the formation of a cloud by 
the action of the rays is due to the productions of ions in the 
gas. If the expansion vessel is provided with two parallel plates 
between which an electric field can be applied, it is seen that the 
number of drops, formed by the expansion with the rays acting, 
decreases with increase of the electric field. The stronger the 
field the smaller the number of drops formed. This result is to be 
expected if the ions are the centres of condensation ; for in a strong 
electric field the ions are at once carried to the electrodes, and thus 
disappear from the gas. If no electric field is acting, a cloud can 
be produced some time after the rays have been cut off; but if a 
strong electric field is applied, under the same conditions, no cloud 
is formed. This is in agreement with experiments showing the 
time required for the ions to disappear by recombination. In 
addition it can be shown that each one of the fine drops carries an 
electric charge and can be made to move in a strong uniform 
electric field. 

The small number of drops produced without the action of the 

rays when > 1*25 is due to a very slight natural ionization of 

the gas. That this ionization exists has been clearly shown by 
electrical methods (section 218). 

The evidence is thus complete that the ions themselves serve 
as centres for the condensation of water around them. These ex- 
periments show conclusively that the passage of electricity through 
a gas is due to the production of charged ions distributed through- 
out the volume of the gas, and verify in a remarkable way the 
hypothesis of the discontinuous structure of the electric charges 
carried by matter. 

This property of the ions of acting as nuclei of condensation 
gives a very delicate method of detecting the presence of ions in 
the gas. If only an ion or two is present per c.c., their presence 
after expansion is at once observed by the drops formed. In this 
way the ionization due to a small quantity of uranium held a yard 
away from the condensation vessel is at once made manifest. 



46 



IONIZATION THEORY OF GASES 



[CH. 



35. Difference between the positive and negative ions. 

In the course of experiments to determine the charge carried by 
an ion, J. J. Thomson 1 observed that the cloud formed under the 
influence of X rays increased in density when the expansion was 
about 1*31 and suggested in explanation that the positive and 
negative ions had different condensation points. 

This difference in behaviour of the positive and negative ions 
was investigated in detail by C. T. R. Wilson 2 in the following way. 
X rays were made to pass in a narrow beam on either side of a 
plate AB (Fig. 7) dividing the condensation vessel into two equal 



Earth 




Fig. 7. 

parts. The opposite poles of a battery of cells were connected 
with two parallel plates C and D, placed symmetrically with regard 
to A. The middle point of the battery and the plate A were con- 
nected with earth. If the plate C is positively charged, the ions in 
the space CA at a short distance from A are all negative in sign. 
Those to the right are all positive. It was found that condensation 

occurred only for the negative ions in AC when = T25 but did 

M 

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

i 

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

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



Il] IONIZATION THEORY OF GASES 47 

The negative ion thus more readily acts as a centre of conden- 
sation than the positive ion. The greater effect of the negative 
ion in causing condensation has been suggested as an explanation 
of the positive charge always observed in the upper atmosphere. 
The negative ions under certain conditions become centres for the 
formation of small drops of water and are removed to the earth by 
the action of gravity, while the positive ions remain suspended. 

With the apparatus described above, it has been shown that 
the positive and negative ions are equal in number. If the ex- 
pansion is large enough, to ensure condensation on both ions, the 
numbers of drops formed on the right and left of the vessel in 
Fig. 7 are equal in number and fall at the same rate, i.e. are equal 
in size. 

Since the ions are produced in equal numbers from a gas 
electrically neutral, this experiment shows that the charge on 
positive and negative ions is equal in value but opposite in sign. 

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



~ O ~ ~I" 
9 fM 

where g is the acceleration due to gravity. The radius of the drop 
and consequently the weight of water in each drop can thus be 
determined. Since the total weight of water precipitated is known, 
the number of drops present is at once obtained. 

This method has been used by J. J. Thomson 1 to determine the 
charge carried by an ion. If the expansion exceeds the value T31, 
both positive and negative ions become centres of condensation. 
From the rate of fall it can be shown that the drops are approxi- 
mately all of the same size. 

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



48 IONIZATION THEORY OF GASES [CH. 

The condensation vessel was similar to that employed by 
C. T. R. Wilson. Two parallel horizontal plates were fitted in the 
vessel and the radiation from an X ray tube or radio-active substance 
ionized the gas between them. A difference of potential V, small 
compared with that required to saturate the gas, was applied 
between the parallel plates distant I cms. from each other. The 
small current i through the gas is given (section 28) by 

._NuVe 
I ' 

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

Since the value of N is the same as the number of drops and the 
velocity u is known, the value of e can be determined. 
In his last determination J. J. Thomson found that 

e = 3*4 x 10~ 10 electrostatic units. 

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

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

A direct determination of the coefficient of diffusion of the ions 

1 Phil. Mag. April, 1903. 



n] 



IONIZATION THEORY OF GASES 



49 



produced in gases by Rb'ntgen rays or by the rays from active 
substances has been made by Townsend 1 . The general method 
employed was to pass a stream of ionized gas through a diffusion 
vessel made up of a number of fine metal tubes arranged in parallel. 
Some of the ions in their passage through the tubes diffuse to the 
sides, the proportion being greater the slower the motion of the 
gas and the narrower the tube. Observations were made of the 
conductivity of the gas before and after passage through the tubes. 
In this way, correcting if necessary for the recombination during 
the time taken to pass through the tubes, the proportion R of 
either positive or negative ions which are abstracted can be 
deduced. The value of R can be mathematically expressed by 
the following equation in terms of K, the coefficient of diffusion 
of the ions into the gas with which they are mixed 2 , 

_3WKZ _^K Z 

R = 4< (-195<?. 2F + -0243e~~ ~*r + &c.), 
where a = radius of the tube, 

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

Only the first two terms of the series need be taken into 
account when narrow tubes are used. 

In this equation R, V, and a are determined experimentally, 
and K can thus be deduced. 

The following table shows the results obtained by Townsend 
when X rays were used. Almost identical results were obtained 
later, when the radiations from active substances replaced the 
X rays. 

Coefficients of diffusion of ions into gases. 



Gas 


-K"for + ions 


K for -ions 


Mean value 
of K 


Eatio of 
values of K 


Air, dry 


028 


043 


0347 


1-54 


moist 


032 


035 


0335 


1-09 


Oxygen, dry 


025 


0396 -0323 


1-58 


moist 
Carbonic acid, dry . . . 


0288 
023 


0358 -0323 
026 -0245 


1-24 
1-13 


moist 


0245 


0255 


025 


1-04 


Hydrogen, dry 


123 


190 -156 


1-54 


moist ... 


128 


142 


135 


I'll 



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



2 Townsend, loc. cit. p. 139. 



50 IONIZATION THEORY OF GASES [CH. 

The moist gases were saturated with water vapour at a tem- 
perature of 15 C. 

It is seen that the negative ion in all cases diffuses faster than 
the positive. Theory shows that the coefficients of diffusion should 
be directly proportional to the velocities of the ions, so that this 
result is in agreement with the observations on the greater velocity 
of the negative ion. 

This difference in the rate of diffusion of the ions at once 
explains an interesting experimental result. If ionized gases are 
blown through a metal tube, the tube gains a negative charge 
while the gas itself retains a positive charge. The number of 
positive and negative ions present in the gas is originally the same, 
but, in consequence of the more rapid diffusion of the negative ions, 
more of the negative ions than of the positive give up their charges 
to the tube. The tube consequently gains a negative charge and 
the gas a positive charge. 

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



where e is the charge on an ion, 

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

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

dp nXeK 

-f- and u = . 
dx p 

Let N be the number of molecules in a cubic centimetre of 
gas at the pressure P and at the temperature 15C., for which 

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

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



Il] IONIZATIOX THEORY OF GASES 51 

3X 10 8 . M! 

Ne -- electrostatic units, 
J\. 

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

It is known that one absolute electro-magnetic unit of 
electricity in passing through water liberates 1/23 c.c. of hydrogen 
at a temperature of 15 C. and standard pressure. The number of 
atoms in this volume is 2'46 N, and, if e is the charge on the 
hydrogen atore in the electrolysis of water, 

ion 

2-46 Ne = 3 x 10 10 E. s. units, 
Ne' = 1-22 x 10 10 E. s. units. 

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

6 -TL 

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

e 2-46 1-37 



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

39. Number of the ions. We have seen that, from experi- 
mental data, Townsend has found that N, the number of molecules 
present in 1 c.c. of gas at 15 C. and standard pressure, is given by 

A r e= 1-22x10. 

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

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



42 



52 IONIZATION THEORY OF GASES [CH. 

The saturation current through air was found to be 1*2 x 10~ 8 
amperes, i.e. 36 E.s. units, for parallel plates, 4'5 cms. apart, when '45 
gramme of radium of activity 1000 times that of uranium was spread 
over an area of 33 sq. cms. of the lower plate. This corresponds to a 
production of about 10 11 ions per second. Assuming, for the purpose 
of illustration, that the ionization was uniform between the plates, 
the volume of air acted on by the rays was about 148 c.c., and the 
number of ions produced per c.c. per second about 7 x 10 8 . Since 
JV" 3-g x 10 19 , it is thus seen that, if one molecule produces two 
ions, the proportion of the gas ionized per second is about 10~ u of the 
whole. For uranium the fraction is about 10~ 14 , and for pure radium, 
of activity one million times that of uranium, about 10~ 8 . Thus 
even in the case of pure radium, only about one molecule of gas is 
acted on per second in every 100 millions. 

The electrical methods are so delicate that the production of 
one ion per cubic centimetre per second can readily be detected. 
This corresponds to the ionization of about one molecule in every 
10 19 present in the gas. 

40. Size and nature of the ions. A}i approximate estimate 
of the mass of an ion, compared with the m)ass of the molecule of 
the gas in which it is produced, can be made from the known data 
of the coefficient K of inter-diffusion of the ions into gases. The 
value of K for the positive ions in moist carbon dioxide has been 
shown to be '0245, while the value of K for the inter-diffusion of 
carbon dioxide with air is '14. The value of K for different gases 
has been found to be approximately inversely proportional to the 
square root of the products of the masses of the molecules of the 
two inter-diffusing gases ; thus, the positive ion in carbon dioxide 
behaves as if its mass were large compared with that of the 
molecule. Similar results hold for the negative as well as for the 
positive ion, and for other gases besides carbon dioxide. 

This has led to the view that the ion consists of a charged 
centre surrounded by a cluster of molecules travelling with it, 
which are kept in position round the charged nucleus by electrical 
forces. A rough estimate shows that this cluster consists of about 
30 molecules of the gas. This idea is supported by the variation 
in velocity, i.e. the variation of the size of the negative ion, in the 



Il] IONIZATION THEORY OF GASES 53 

presence of water vapour ; for the negative ion undoubtedly has a 
greater mass in moist than in dry gases. At the same time it is 
possible that the apparently large size of the ion, as determined 
by diffusion methods, may be in part a result of the charge carried 
by the ion. The presence of a charge on a moving body would 
increase the frequency of collision with the molecules of the gas, 
and consequently diminish the rate of diffusion. The ion on this 
view may not actually be of greater size than the molecule from 
which it is produced. 

The negative and positive ions certainly differ in size, and this 
difference becomes very pronounced for low pressures of the gas. 
At atmospheric pressure, the negative ion, produced by the action 
of ultra-violet light on a negatively charged body, is of the 
same size as the ion produced by X rays, but at low pressures 
J. J. Thomson has shown that it is identical with the corpuscle or 
electron, which has an apparent mass of about 1/1000 of the mass 
of the hydrogen atom. A similar result has been shown by 
Townsend to hold for the negative ion produced by X rays at a 
low pressure. It appears that the negative ion at low pressure 
sheds its attendant cluster. The result of Langevin, that the 
velocity of the negative ion increases more rapidly with the 
diminution of pressure than that of the positive ion, shows that 
this process of removal of the cluster is quite appreciable at a 
pressure of 10 mms. of mercury. 

It must thus be supposed that the process of ionization in 
gases consists in a removal of a negative corpuscle or electron from 
the molecule of the gas. At atmospheric pressure this corpuscle 
immediately becomes the centre" of an aggregation of molecules 
which moves with it and is the negative ion. After removal of 
the negative ion the molecule retains a positive charge, and probably 
also becomes the centre of a cluster of new molecules. 

The terms electron and ion as used in this work may therefore 
be denned as follows : 

The electron or corpuscle is the body of smallest mass yet 
known to science. It carries a negative charge of value 3*4 x 10~ 10 
electrostatic units. Its presence has only been detected when in 
rapid motion, when it has, for speeds up to about 10 10 cms. a second, 
an apparent mass m given by e/m = T86 x 10 7 electromagnetic 



54 ION1ZATION THEORY OF GASES [CH. 

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

The ions which are produced in gases at ordinary pressure have 
an apparent size, as determined from their rates of diffusion, large 
compared with the molecule of the gas in which they are produced. 
The negative ion consists of an electron with a cluster of molecules 
attached to and moving with it. The positive ion consists of a 
molecule from which an electron has been expelled, with a cluster 
of molecules attached; at low pressures under the action of an 
electric field the electron does not form a cluster. The positive ion 
is always atomic in size, even at low pressure of the gas. Each of 
the ions carries a charge of value 3'4 x 10~ 10 electrostatic units. 

41. Ions produced by collision. The greater part of the 
radiation from the radio-active bodies consists of a stream of charged 
particles travelling with great velocity. Of this radiation, the a 
particles, which cause most of the ionization observed in the gas, 
consist of positively charged bodies projected with a velocity about 
one-tenth the velocity of light. The ft rays consist of negatively 
charged particles, which are identical with the cathode rays pro- 
duced in a vacuum tube and travel with a speed about one-half 
the velocity of light (chapter iv.). Each of these projected 
particles possesses such great kinetic energy that it is able to 
produce a large number of ions by collision with the gas molecules 
in its path. No definite experimental evidence has yet been 
obtained of the number of ions produced by a single particle, or 
of the way the ionization varies with the speed, but there is no 
doubt that each projected body produces many thousand ions in 
its path before its energy of motion is destroyed. 

It has already been mentioned (section 29) that at low pressures 
ions moving under the action of an electric field are able to pro- 
duce fresh ions by collision with the molecules of the gas. At low 
pressures the negative ion is identical with the electron produced 
in a vacuum tube, or emitted by a radio-active substance. 

The mean free path of the ion is inversely proportional to the 
pressure of the gas. Consequently, if an ion moves in an electric 
field, the velocity acquired between collisions increases with diminu- 
tion of the pressure. Townsend has shown that fresh ions are 



Il] IONIZATION THEORY OF GASES 55 

occasionally produced by collision when the negative ion moves 
freely between two points differing in potential by 10 volts. If 
the difference be about V = 20 volts, fresh ions are produced at 
each collision 1 . 

Now the energy W, acquired by an ion of charge e moving 
freely between two points at a difference of potential V, is given by 

W=Ve. 

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

ion is given by 

TF=2-3 xlO- u ergs. 

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

mu* = Ve, 



and u = 

v m 

p 

Now - = 1'86 x 10 7 electromagnetic units for the electron at 
slow speeds (section 76). 
Taking F= 20 volts, 

m = 2 '7 x 10 8 cms. per sec. 

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

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

An estimate of the energy required to produce an ion by X rays 
has been made by Rutherford and M c Clung. The energy of the 
rays was measured by their heating effect, and the total number of 
ions produced determined. On the assumption that all the energy 
of the rays is used up in producing ions, it was found that F= 175 

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



56 IONIZATION THEORY OF GASES [CH. 

volts a value considerably greater than that observed by Town- 
send from data of ionization by collision. The ionization in the two 
cases, however, is produced under very different conditions, and: it 
is impossible to estimate how much of the energy of the rays is 
dissipated in the form of heat. 

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

43. Variation of the current with distance between the 
plates. It has been found experimentally 1 that the intensity of 
the ionization, due to a large plane surface of active matter, falls 
off approximately in an exponential law with the distance from the 
plate. On the assumption that the rate of production of ions at 
any point is a measure of the intensity / of the radiation, the 

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

-*o 

constant, x the distance from the plate, and 7 the intensity of the 
radiation at the surface of the plate. This result can be deduced 
theoretically on the assumption that the ionization at any point is 
proportional to the intensity of the radiation, and that the energy 
of the rays is used up in producing ions. 

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

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

Let I = Kq, where K is a constant. 

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



Il] IONIZATION THEORY OF GASES 57 

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

dl=qa). dx 



ay 

Integrating, iog e 1 = -~ . x + A , 

where A is a constant. 

Since 7 = 7 when x = 0, A = log e / , and 



/o 



-A* 



where X = = a constant. 

K. 

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

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

plate, - = er**, 

<?o 

Consider two parallel plates placed as in Fig. 1, one of which is 
covered with a uniform layer of radio-active matter. If the distance 
d between the plates is small compared with the dimensions of the 
plates, the ionization near the centre of the plates will be sensibly 
uniform over any plane parallel to the plates and lying between 
them. The saturation current i per unit area is given by 

I'd 

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



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

JO A, 



when \d is small, i.e. when the ionization between the plates is 
nearly constant, 

i = q e'd. 

The current is thus proportional to the distance between the 
plates. When \d is large, the saturation current i Q is equal to Q , 

A/ 

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



58 IONIZATION THEORY OF GASES [CH. 

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

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

Distance Saturation Current 

2-5 mms. 32 

5 55 

V5 72 

10 85 

12-5 96 

15 100 

Thus the increase of current for equal increments of distance 
between the plates decreases rapidly with the distance traversed by 
the radiation. 

The distance of 15 mms. was not sufficient to completely absorb 
all the radiation, so that the current had not reached its limiting 
value. 

When more than one type of radiation is present, the saturation 
current between parallel plates is given by 

A 1 - e~^ d + &c. 



where A, A l are constants and X, \ the absorption constants of 
the radiations in the gas. 

Since the radiations are unequally absorbed in different gases, 
the variation of current with distance depends on the nature of the 
gas between the plates. 

44. Variation of the current with pressure. The rate 

I of production of ions by the radiations from active substances is 

\ directly proportional to the pressure of the gas. The absorption of 

the radiation in the gas also varies directly as the pressure. The 

latter result necessarily follows if the energy required to produce 

an ion is independent of the pressure. 

In cases where the ionization is uniform between two parallel 
plates, the current will vary directly as the pressure; when however 



II] IONIZATION THEORY OF GASES 59 

the ionization is not uniform, on account of the absorption of the 
radiation in the gas, the current does not decrease directly as the 
pressure until the pressure is reduced so far that the ionization 
is sensibly uniform. Consider the variation with pressure of the 
saturation current i between two large parallel plates, one of which 
is covered with a uniform layer of active matter. 

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

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

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

*o 

[ d pldx = I d ple-^*. dx = ^(l- PM). 
Jo -' o *i 

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



The ratio is thus dependent on the distance d between the 
plates and the absorption of the radiation by the gas. 

The difference in the shape of the pressure-current curves 1 is 
well illustrated in Fig. 8, where curves are given for hydrogen, air, 
and carbonic acid for plates 3*5 cms. apart. 

For the purpose of comparison, the current at atmospheric 
pressure and temperature in each case is taken as unity. The 
actual value of the current was greatest in carbonic acid and 
least in hydrogen. In hydrogen, where the absorption is small, 
the current over the whole range is nearly proportional to the 
pressure. In carbonic acid, where the absorption is large, the 
current diminishes at first slowly with the pressure, but is nearly 
proportional to it below the pressure of 235 mms. of mercury. 
The curve for air occupies an intermediate position. 

In cases where the distance between the plates is large, the 
saturation current will remain constant with diminution of pres- 
sure until the absorption is so reduced that the radiation reaches 
the other plate. 

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



60 



IONIZATION THEOKY OF GASES 



[CH. 



An interesting result follows from the rapid absorption of 
radiation by the gas. If the current is observed between two 
fixed parallel plates, distant d^ and d 2 respectively from a large 
plane surface of active matter, the current at first increases with 
diminution of pressure, passes through a maximum value, and 
then diminishes. In such an experimental case the lower plate 
through which the radiations pass is made either of open gauze or 
of thin metal foil to allow the radiation to pass through readily. 







I'rrxxun- in inms. 



150 



300 450 

Fig. 8. 



600 



750 



The saturation current i is obviously proportional to 

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

1,-x.r, ^ 1 ,-\ f J J \ 



II] IONIZATION THEORY OF GASES 61 

For example, if the active matter is uranium, p\ l= T6 for the 
a rays at atmospheric pressure. If c? 2 = 3, and d^ =1, the saturation 
current reaches a maximum when the pressure is reduced to about 
1/3 of an atmosphere. This result has been verified experimentally. 

45. Conductivity of different gases when acted on by 
the rays. For a given intensity of radiation, the rate of pro- 
duction of ions in a gas varies for different gases and increases 
with the density of the gas. Strutt 1 has made a very complete 
examination of the relative conductivity of gases exposed to the 
different types of rays emitted by active substances. To avoid 
correction for any difference of absorption of the radiation in the 
various gases, the pressure of the gas was always reduced until 
the ionization was directly proportional to the pressure, when, as 
we have seen above, the ionization must everywhere be uniform 
throughout the gas. For each type of rays, the ionization of 
air is taken as unity. The currents through the gases were 
determined at different pressures, and were reduced to a common 
pressure by assuming that the ionization was proportional to the 
pressure. 

With unscreened active material, the ionization is almost 
entirely due to a rays. When the active substance is covered with 
a layer of aluminium '01 cms. in thickness, the ionization is mainly 
due to the ft or cathodic rays, and when covered with 1 cm. of lead 
the ionization is solely due to the 7 or very penetrating rays. 
Experiments on the 7 rays of radium were made by observing the 
rate of discharge of a special gold-leaf electroscope filled with the 
gas under examination and exposed to the action of the rays. 
The following table gives the relative conductivities of gases 
exposed to various kinds of ionizing radiations. 

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

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



62 



IONIZATION THEORY OF GASES 



[CH. 



This difference of conductivity in gases is due to unequal 
absorptions of the radiations. The writer has shown 1 that the 







RELATIVE CONDUCTIVITY 


rijio 


Eelative 






Density 


a rays 


[3 rays 


7 rays 


Rontgen 
rays 


Hydrogen 
Air ... ... ... 


0-0693 
1-00 


0-226 
1-00 


0-157 
1-00 


0-169 
1-00 


0-114 
1-00 


Oxygen 


1-11 


1-16 


1-21 


1-17 1-39 


Carbon dioxide 


1-53 


1-54 


1-57 


1-53 


1-60 


Cyanogen 


1-86 


1-94 


1-86 


1-71 


1-05 


Sulphur dioxide ... 


2-19 


2-04 


2-31 


2-13 7-97 


Chloroform 


4-32 


4-44 


4-89 


4-88 31-9 


Methyl iodide 


5-05 


3'51 


5-18 


4-80 


72-0 


Carbon tetrachloride 


5-31 


5-34 


5-83 


5-67 


45-3 



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

Gas 



Air 

Hydrogen 

Oxygen 

Carbonic acid 
Hydrochloric acid gas 
Ammonia 



Total 
lonization 

100 

95 
106 

96 
102 
101 



The numbers, though only approximate in character, seem to 
show that the energy required to produce an ion is probably not 
very different for the various gases. Assuming that the energy 
required to produce an ion in different gases is about the same, it 
follows that the relative conductivities are proportional to the 
relative absorption of the radiations. 

A similar result has been found by M c Lennan for cathode rays. 
He proved that the ionization was directly proportional to the 
absorption of the rays in the gas, thus showing that the same 
energy is required to produce an ion in all the gases examined. 

46. Potential Gradient. The normal potential gradient 
between two charged electrodes is always disturbed when the gas 

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



II 



IONIZATION THEORY OF GASES 



63 



is ionized in the space between them. If the gas is uniformly 
ionized between two parallel plates, Child and Zeleny have shown 
that there is a sudden drop of potential near the surface of both 
plates, and that the electric field is sensibly uniform for the inter- 
mediate space between them. The disturbance of the potential 
gradient depends upon the difference of potential applied, and is 
different at the surface of the two plates. 

In most measurements of radio-activity the material is spread 
over one plate only. In such a case the ionization is to a large 
extent confined to the volume of the air close to the active plate. 
The potential gradient in such a case is shown in Fig. 9. The 
dotted line shows the variation of potential at any point between 
the plates when no ionization is produced between the plates; 




Distance 




-V 



Fig. 9. 



curve A for weak ionization, such as is produced by uranium, 
curve B for the intense ionization produced by a very active 
substance. In both cases the potential gradient is least near the 
active plate, and greatest near the opposite plate. For very 



64 IONIZATION THEOKY OF GASES [CH. 

intense ionization it is very small near the active surface. The 
potential gradient varies slightly according as the active plate is 
charged positively or negatively. 

47. Variation of current with voltage for surface ion- 
ization. Some very interesting results, giving the variation of the 
current with voltage, are observed when the ionization is intense, 
and confined to the space near the surface of one of two parallel 
plates between which the current is measured. 

The theory of this subject has been worked out independently 
by Child 1 and Rutherford' 2 . Let V be the potential difference 
between two parallel plates at a distance d apart. Suppose that 
the ionization is confined to a thin layer near the surface of the 
plate A (see Fig. 1) which is charged positively. When the electric 
field is acting, there is a distribution of positive ions between the 
plates A and B. 

Let n t = number of positive ions per unit volume at a distance 
x from the plate A, 

K l = mobility of the positive ions, 
e charge on an ion. 

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



By Poisson's equation 

d*V 



K, dV S?V 

Then t l = - -=- . -j- . 

4-7T dx dx z 

fd 
Integrating 



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

1 Phys. Rev. Vol. 12, 1901. 

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



II] IONIZATIOX THEORY OF GASES 65 

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

dx 
can be made extremely small. 

Putting A = 0, 

dV ' 



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

Integrating between the limits and d, 



9V 2 



If i 2 is the value of the current when the electric field is 
reversed, and K* the velocity of the negative ion, 

9V' 2 



?- 

* 2 A 2 

The current in the two directions is thus directly proportional 
to the velocities of the positive and negative ions. The current 
should vary directly as the square of the potential difference 
applied, and inversely as the cube of the distance between the 
plates. 

The theoretical condition of surface ionization cannot be fulfilled 
by the ionization due to active substances, as the ionization extends 
some centimetres from the active plate. If, however, the distance 
between the plates is large compared with the distance over which 
the ionization extends, the results will be in rough agreement with 
the theory. Using an active preparation of radium, the writer has 
made 1 some experiments on the variation of current with voltage 
between parallel plates distant about 10 cms. from each other. 

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



66 IONIZATION THEORY OF GASES [CH. II 

The results showed 

(1) That the current through the gas for small voltages 
increased more rapidly than the potential difference applied, but 
not as rapidly as the square of that potential difference. 

(2) The current through the gas depended on the direction of 
the electric field ; the current was always smaller when the active 
plate was charged positively on account of the smaller mobility of 
the positive ion. The difference between ^ and i 2 was greatest 
when the gas was dry, which is the condition for the greatest 
difference between the velocities of the ions. 

An interesting result follows from the above theory. For given 
values of V and d, the current cannot exceed a certain definite 
value, however much the ionization may be increased. In a 
similar way, when an active preparation of radium is used as a 
source of surface ionization, it is found that, for a given voltage 
and distance between the plates, the current does not increase 
beyond a certain value however much the activity of the material 
is increased. 

In this chapter an account of the ionization theory of gases has 
been given to the extent that is necessary for the interpretation of 
the measurements of radio-activity by the electric method. It 
would be out of place here discuss the development of that 
theory in detail, to explain the passage of electricity through 
flames and vapours, the discharge of electricity from hot bodies, 
and the very complicated phenomena observed in the passage of 
electricity through a vacuum tube. This chapter was written 
before the publication of J. J. Thomson's recent book Conduction 
of Electricity through Gases (Cambridge University Press, 1903), 
to which the reader is referred for further information on this 
important subject. 



CHAPTER III. 

METHODS OF MEASUREMENT. 

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

(1) The action of the rays on a photographic plate. 
^ (2) The ionizing action of the rays on the surrounding gas. 

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

The third method is very restricted in its application, and can 
only be employed for intensely active substances like radium or 
polonium. 

The photographic method has been very widely used, especially 
in the earlier development of the subject, but has gradually been 
displaced by the electrical method as a quantitative determination 
of the radiations became more and more necessary. In certain 
directions, however, it possesses distinct advantages over the elec- 
trical method. For example, it has proved a very valuable means 
of investigating the curvature of the path of the rays, when 
deflected by a magnetic or electric field, and has allowed us to 
determine the constants of these rays with considerable accuracy. 

On the other hand, the photographic method as a general 
method of study of the radiations is open to many objections. A 
day's exposure is generally required to produce an appreciable 
darkening of the sensitive film when exposed to a weak source of 
radiation like uranium and thorium. It cannot, in consequence, be 
employed to investigate the radiations of those active products 

52 



68 METHODS OF MEASUREMENT [CH. 

which rapidly lose their activity. Moreover, W. J. Russell has 
shown that the darkening of a photographic plate can be produced 
by many agents which do not give out rays like those of the radio- 
active bodies. This darkening of the plate is produced under very 
many conditions, and very special precautions are necessary when 
long exposures to a weak source of radiation are required. 

The main objection to the photographic method, however, lies 
in the fact that the radiations which produce the strongest electrical 
effect are very weak photographically. For example, Soddy 1 has 
shown that the photographic action of uranium is due almost 
entirely to the more penetrating rays, and that the easily absorbed 
rays produce in comparison very little effect. Speaking generally 
the penetrating rays are the most active photographically, and the 
action on the plate under ordinary conditions is almost entirely due 
to them. 

Most of the energy radiated from active bodies is in the form 
of easily absorbed rays which are comparatively inactive photo- 
graphically. These rays are difficult to study by the photographic 
method, as the layer of black paper which, in many cases, is 
necessary to absorb the phosphorescent light from active substances, 
cuts off at the same time most of the rays under examination. 
These rays will be shown to play a far more important part in the 
processes occurring in radio-active bodies than the rays which are 
more active photographically. 

The electrical method, on the other hand, offers a rapid and 
accurate method of quantitatively examining the radiations. It can 
be used as a means of measurement of all the types of radiation 
emitted, excluding light waves, and is capable of accurate measure- 
ment over an extremely wide range. With proper precautions 
it can be used to measure effects produced by radiations of 
extremely small intensity. 

49. Electrical Methods. The electrical methods employed 
in studying radio-activity are all based on the property of the 
radiation in question of ionizing the gas, i.e. of producing positively 
and negatively charged carriers throughout the volume of the gas. 
The discussion of the application of the ionization theory of gases to 

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



m ] METHODS OF MEASUREMENT 



69 



measurements of radio-activity has been given in the last chapter. 
It has there been shown that the essential condition to be fulfilled 
for comparative measurements of the intensity of the radiations 
is that the electrical field should in all cases be strong enough to 
obtain the maximum or saturation current through the gas. 

The electric field required to produce practical saturation 
varies with the intensity of the ionization and consequently with 
the activity of the preparations to be examined. For preparations 
which have an activity not more than 500 times that of uranium, 
under ordinary conditions, a field of 100 volts per cm. is sufficient to 
produce a practical saturation current. For very active samples of 
radium, it is often impossible to obtain conveniently a high enough 
electromotive force to give even approximate saturation. Under 
such conditions comparative measurement could be made by 
measuring the current under diminished pressure of the gas, 
when saturation is more readily obtained. 

The method to be employed in the measurement of this ioniza- 
tion current depends largely on the intensity of the current to be 
measured. If some very active radium is spread on the lower of 
two insulated plates as in Fig. 1, and a saturating electric field 
applied, the current may be readily measured by a sensitive gal- 
vanometer of high resistance. For example, a weight of '45 gr. 
of radium chloride of activity 1000 times that of uranium oxide, 
spread over a plate of area 33 sq. cms. gave a maximum current of 
1*1 x 10~ 8 amperes when the plates were 4'5 cms. apart. In this 
case the difference of potential to be applied to produce practical 
saturation was about 600 volts. Since most of the ionization is 
due to rays which are absorbed in passing through a few centi- 
metres of air, the current is not much increased by widening the 
distance between the two plates. In cases where the current is 
not quite large enough for direct deflection, the current may be 
determined by connecting the upper insulated plate with a well 
insulated condenser. After charging for a definite time, say 1 or 
more minutes, the condenser is discharged through the galvano- 
meter, and the current can be readily deduced. 

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



70 



METHODS OF MEASUEEMENT 



[CH. 



material, it is necessary io_emploj methods for measuring currents 
much smaller than can be conveniently detected by an ordinary 
galvanometer. The most convenient apparatus to employ for this 
purpose is one of the numerous types of quadrant electrometer or 
an electroscope of special design. For many observations, especially 
where the activity of the two substances is to be compared under 
constant conditions, an electroscope offers a very certain and simple 
method of measurement. As an example of a simple apparatus 
of this kind, a brief description will be given of the electroscope 
used by M. and Mme Curie in many of their earlier observations. 



I 




''Earth 



Fig. 10. 



The connections are clearly seen from Figure 10. The active 
material is placed on a plate laid on top of the fixed circular plate 
P, connected with the case of the instrument and to earth. The 
upper insulated plate P r is connected to the insulated gold-leaf 
system LL'. S is an insulating support and L the gold-leaf. 

The system is first charged to a suitable potential by means of 
the rod C. The rate of movement of the gold-leaf is observed by 
means of a microscope. In comparisons of the activity of two 
specimens, the time taken to pass over a certain number of 
divisions of the micrometer scale in the eyepiece is observed. 
Since the capacity of the charged system is constant, the average 
rate of movement of the gold-leaf is directly proportional to the 
ionization current between P and P', i.e. to the intensity of the 



Ill] 



METHODS OF MEASUREMENT 



71 



radiation emitted by the active substance. Unless very active 
material is being examined, the difference of potential between P 
and P' can easily be made sufficient to produce saturation. 

When necessary, a correction can readily be made for the rate 
of leak when no active material is present. In order to avoid 
external disturbances, the plates PP and the rod 6 f , are surrounded 
by metal cylinders, E and F, connected with earth. 

til. A modified form of the gold-leaf electroscope can be used 
to determine extraordinarily minute cur- 
rents with accuracy, and can be employed 
in cases where a sensitive electrometer is 
unable to detect the current. A special 
type of electroscope has been used by 
Elster and Geitel, in their experiments on 
the natural ionization of the atmosphere. 
A very convenient type of electroscope to 
measure the current due to minute ioniza- 
tion of the gas is shown in Fig. 11. 

This type of electroscope was first used 
by C. T. R. Wilson 1 in his experiments of 
the natural ionization of air in closed 
vessels. A brass cylindrical vessel is taken 
of about 1 litre capacity. The gold-leaf 
system consisting of a narrow strip of gold-leaf L attached to a flat 
rod R is insulated inside the vessel by the small sulphur bead S, 
supported from the rod P. In a dry atmosphere a clean sulphur 
bead is almost a perfect insulator. The system is charged by a 
light bent rod CO ' passing through the ebonite cork D. The rod 
C is connected to one terminal of a battery of small accumulators 
of 200 to 300 volts. If these are absent the system can be charged 
by means of a rod of sealing-wax. The charging rod CO' is then 
removed from contact with the gold-leaf system. The rods P and 
C and the cylinder are then connected with earth. 

The rate of movemejit_ofthe ^gold-leaf is_pbserved by a reading 
^g ***& rlindfiT, covered with thin 




Fig. 11. 



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



72 METHODS OF MEASUREMENT [CH. 

air in the cylinder is to be accurately measured, it is advisable to 
enclose the supporting and charging rod and sulphur bead inside a 
small metal cylinder M connected to earth, so that only the charged 
gold-leaf system is exposed in the main volume of the air. 

In an apparatus of this kind the small leakage over the sulphur 
bead can be almost completely eliminated by keeping the rod P 
charged to the average potential of the gold-leaf system during 
the observation. This method has been used with great success by 
C. T. R. Wilson (loc. cit). Such refinements, however, are generally 
unnecessary, except in investigations of the natural ionization of 
gases at low pressures, when the conduction leak over the sulphur 
bead is comparable with the discharge due to the ionized gas. 

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

GV 
l = ' 

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

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



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

The number of ions produced in the gas can be calculated if 
the charge on an ion is known. J. J. Thomson has shown that the 
charge e on an ion is equal to 3'4 x 10~ 10 electrostatic units or 
1*13 x 10~ 19 coulombs. 

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

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

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



Ill] METHODS OF MEASUREMENT 73 

Now for an electroscope with a volume of 1000 c.c., i was equal 
to about 1'9 x 10~ 15 amperes. Substituting the values given above 
q = 17 ions per cubic centimetre per second. 

With suitable precautions an electroscope can thus readily 
measure an ionization current corresponding to the production of 
1 ion per cubic centimetre per second. 

The great advantage of an apparatus of this kind lies in the 
fact that the current measured is due to the ionization inside the 
vessel and is not influenced by the ionization of the external air or 
by electrostatic disturbances. Such an apparatus is very convenient 
for investigating the very penetrating radiations from the radio- 
elements, since these rays pass readily through the walls of the 
electroscope. When the electroscope is placed on a lead plate 3 or 
4 mms. thick, the ionization in the electroscope, due to a radio- 
active body placed under the lead, is due entirely to the very 
penetrating rays, since the other two types of rays are completely 
absorbed in the lead plate. 

53. A modified form of electroscope, which promises to be of 
great utility for measuring currents even more minute than those 
to be observed with the type of instrument already described, has 
recently been devised by C. T. R. Wilson 1 . The construction of the 
apparatus is shown in Fig. 12. 




I 



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



74 METHODS OF MEASUREMENT [CH. 

The case consists of a rectangular brass box 4 cms. x 4 cms. 
x 3 cms. A narrow gold-leaf L is attached to a rod R passing 
through a clean sulphur cork. Opposite the gold-leaf is fixed an 
insulated brass plate P, placed about 1 mm. from the wall of the 
box. The movement of the gold-leaf is observed through two 
small windows by means of a microscope provided with a micrometer 
scale. The plate P is maintained at a constant potential (generally 
about 200 volts). The electrometer case is placed in an inclined 
position as shown in the figure, the angle of inclination and the 
potential of the plate being adjusted to give the desired sensitive- 
ness. The gold-leaf is initially connected to the case, and the 
microscope adjusted so that the gold-leaf is seen in the centre of 
the scale. For a given potential of the plate, the sensitiveness 
depends on the angle of tilt of the case. There is a certain critical 
inclination below which the gold-leaf is unstable. The most 
sensitive position lies just above the critical angle. In a particular 
experiment Wilson found that with an angle of tilt of 30 and with 
the plate at a constant potential of 207 volts, the gold-leaf, when 
raised to a potential of one volt above the case, moved over 200 
scale divisions of the eyepiece, 54 divisions corresponding to one 
millimetre. 

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

54. Electrometers. Although the electroscope can be used 
with advantage in special cases, it is limited in its application. 
The most generally convenient apparatus for measurement of 
ionization currents through gases is one of the numerous types of 
quadrant electrometers. With the use of auxiliary capacities, the 
electrometer can be used to measure currents with accuracy over 
a wide range, and can be employed for practically every kind of 
measurement required in radio-activity. 

The elementary theory of the symmetrical quadrant electrometer 



Ill] METHODS OF MEASUREMENT 75 

as given in the text-books is very imperfect. It is deduced that 
the sensibility of the electrometer measured by the deflection of 
the needle for 1 volt P.D. between the quadrants varies directly 
as the potential of the charged needle, provided that this potential 
is high compared with the P.D. between the quadrants. In most 
electrometers however, the sensibility rises to a maximum, and then 
decreases with increase of potential of the needle. For electrometers 
in which the needle lies close to the quadrants, this maximum 
sensibility is obtained for a comparatively low potential of the 
needle. A theory of the quadrant electrometer, accounting for this 
action, has been recently given by G. W. Walker 1 . The effect 
appears to be due to the presence of the air space that necessarily 
exists between adjoining quadrants. 

Suppose that it is required to measure with an electrometer 
the ionization current between two 
horizontal metal plates A and B 
(Fig. 13) on the lower of which some 
active material has been spread. If 
the saturation current is required, 
the insulated plate A is connected 
with one pole of a battery of sufficient 
E.M.F. to produce saturation, the 
other pole being connected to earth. 
The insulated plate B is connected 

with one pair of quadrants of the T ll~~ *^ Earth 

electrometer, the other pair being 
earthed. By means of a suitable key 

K, the plate B and the pair of quadrants connected with it may be 
either insulated or connected with earth. When a measurement 
is to be taken the earth connection is broken. If the positive pole 
of the battery is connected with A, the plate B and the electro- 
meter connections immediately begin to be charged positively, and 
the potential, if allowed, will steadily rise until it is very nearly 
equal to the potential of A. As soon as the potential of the 
electrometer system begins to rise, the electrometer needle com- 
mences to move at a uniform rate. Observations of the angular 
movement of the needle are made either by the telescope and scale 
1 Phil Mag. Aug. 1903. 




76 METHODS OF MEASUREMENT [CH. 

or by the movement of the spot of light on a scale in the usual 
way. If the needle is damped so as to give a uniform motion 
over the scale, the rate of movement of the needle, i.e. the number 
of divisions of the scale passed over per second, may be taken as 
a measure of the current through the gas. The rate of movement 
is most simply obtained by observing with a stop-watch the time 
taken for the spot of light, after the motion has become steady, to 
pass over 100 divisions of the scale. As soon as the observation is 
made, the plate B is again connected with earth, and the electro- 
meter needle returns to its original position. 

In most experiments on radio-activity only comparative measures 
of saturation currents are required. If these comparative measures 
are to extend over weeks or months, as is sometimes the case, it is 
necessary to adopt some method of standardizing the electrometer 
from day to day, so as to correct for variation in its sensibility. 
This is most simply done by comparing the current to be measured 
with that due to a standard sample of uranium oxide, which is 
placed in a definite position in a small testing vessel, always kept 
in connection with the electrometer. Uranium oxide is a very 
constant 'source of radiation, and the saturation current due to it 
is the same from day to day. By this method of comparison 
accurate observations may be made on the variation of activity of 
a substance over long intervals of time, although the sensibility 
of the electrometer may vary widely between successive measure- 
ments. 

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



Ill] METHODS OF MEASUREMENT 77 

is to a large extent avoided in the White pattern of the Kelvin 
electrometer, which is provided with a replenisher and attracted 
disc for keeping the potential of the needle at a definite value. If 
sufficient trouble is taken in insulating and setting up this type 
of electrometer, it proves a very useful instrument of moderate 
sensibility, and will continue in good working order for a year or 
more without much attention. 

Simpler types of electrometer of greater sensibility can however 
be readily constructed to give accurate results. The old type of 
quadrant electrometer, to be found in every laboratory, can readily 
be modified to prove a useful and trustworthy instrument. A light 
needle can be simply made of thin aluminium, of silvered paper or 
of a thin plate of mica, covered with gold-leaf to make it conducting. 
The aluminium wire and mirror attached should be made as light 
as possible. The needle should be suspended either by a fine 
quartz fibre or a long bifilar suspension of silk. A very fine 
phosphor bronze wire of some length is also very satisfactory. 
A magnetic control is not very suitable, as.it is disturbed by coils 
or dynamos working in the neighbourhood. In addition, the zero 
point of the needle is not as steady as with the quartz or bifilar 
suspension. 

When an electrometer is used to measure a current by noting 
the rate of movement of the needle, it is essential that the needle 
should be damped sufficiently to give a uniform motion of the spot 
of light over the scale. The damping requires fairly accurate 
adjustment. If it is too little, the needle has an oscillatory move- 
ment superimposed on the steady motion; if it is too great, it 
moves too sluggishly from rest and takes some time to attain 
a state of uniform motion. With a light needle, very little, if any, 
extra damping is required. A light platinum wire with a single 
loop dipping in sulphuric acid is generally sufficient for the purpose. 
* With light needle systems and delicate suspensions, it is only 
necessary to charge the needle to a potential of a few hundred volts 
to give a sensibility of several thousand divisions for a volt. With 
such low potentials, the difficulty of insulation of the condenser, 
with which the needle is in electrical connection, is much reduced. 
It is convenient to use a condenser such that the potential of the 
needle does not fall more than a few per cent, per day. The 



78 



METHODS OF MEASUREMENT 



[CH. 



ordinary short glass jar partly filled with sulphuric acid is, in most 
cases, not easy to insulate to this extent. It is better to replace 
it by an ebonite (or sulphur) condenser 1 such as is shown in 
Fig. 14. 

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

circular recess a brass plate B fits / JL N 

loosely. The ebonite plate rests 
on another brass plate C connect- 
ed with earth. The condenser 
thus formed has a considerable 
capacity and retains a charge for 
a long time. In order to make 
connection with the needle, a 
small glass vessel D, partly filled 
with sulphuric acid, is placed on 
the plate B and put in connec- 
tion with the needle by means C 
of a fine platinum wire. The 
platinum wire from the needle 

dips into the acid, and serves to damp the needle. In a dry atmo- 
sphere, a condenser of this kind will not lose more than 20 per cent, 
of its charge in a week. If the insulation deteriorates, it can 
readily be made good by rubbing the edge of the ebonite A with 
sand-paper, or removing its surface in a lathe. 

If a sufficient and steady E.M.F. is available, it is much better to 
keep it constantly connected with the needle and to avoid the use 
of the condenser altogether. If a battery of small accumulators is 
used, their potential can always be kept at a constant value, and 
the electrometer always has a constant sensibility. 

56. A very useful electrometer of great sensibility has recently 
been devised by Dolezalek 2 . It is of the ordinary quadrant type 
with a very light needle of silvered paper, spindle shaped, which 
lies fairly close to the quadrants. A very fine quartz suspension is 

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

2 Instrumentenkunde, p. 345, Dec. 1901. 



Ill] METHODS OF MEASUREMENT 79 

employed. In consequence of the lightness of the needle and the 
nearness to the quadrants it acts as its own damper. This is 
a great advantage, for difficulties always arise with the wire dipping 
into sulphuric acid, on account of the thin film which collects after 
some time on the surface of the acid. This film obstructs the 
motion of the platinum wire dipping into the acid, and has to be 
removed at regular intervals. These instruments can be readily 
made to give a sensibility of several thousand divisions for a volt 
when the needle is charged to about one hundred volts. The 
sensibility of the electrometer passes through a maximum as the 
potential of the needle is increased. It is always advisable to 
charge the needle to about the value of this critical potential. The 
capacity of the electrometer is in general high (about 50 electro- 
static units) but the increased sensibility more than compensates 
for this. The needle may either be charged by lightly touching 
it with one terminal of a battery, or it may be kept charged to 
a constant potential through the quartz suspension. The quartz 
fibre can be made sufficiently conducting for this purpose by 
dipping it into a dilute solution of calcium chloride. In addition 
to its great sensibility, the advantages of this instrument lie in the 
steadiness of the zero and in the self-damping. 

57. Adjustment and screening. In adjusting an electro- 
meter, it is important to arrange that the needle lies symmetrically 
with regard to the quadrants. This is best tested by observing 
whether the needle is deflected on charging, the quadrants all 
being earthed. In most electrometers there is an adjustable 
quadrant, the position of which may be altered until the needle is 
not displaced on charging. When this condition is fulfilled, the 
zero reading of the electrometer remains unaltered as the needle 
loses its charge, and the deflection on both sides of the zero should 
be the same for equal and opposite quantities of electricity. 

The supports of the quadrants require to be well insulated. 
Ebonite rods are as a rule more satisfactory for this purpose than 
glass. In testing for the insulation of the quadrants and the 
connections attached, the system is charged to give a deflection 
of about 200 scale divisions. If the needle does not move more 
than one or two divisions after standing for one minute, the 



80 METHODS OF MEASUREMENT [CH. 

insulation may be considered quite satisfactory. When a suitable 
desiccator is placed inside the tight-fitting electrometer case, the 
insulation of the quadrants should remain good for months. If the 
insulation of the ebonite deteriorates, it can easily be made good 
by removing the surface of the ebonite in a lathe. 

In working with a sensitive instrument like the Dolezalek 
electrometer, it is essential that the electrometer and the testing 
apparatus should be completely enclosed in a screen of wire-gauze 
connected with earth, in order to avoid electrostatic disturbances. 
If an apparatus is to be tested at some distance from the electro- 
meter, the wires leading to it should be insulated in metal cylinders 
connected to earth. The size of the insulators used at various 
points should be made as small as possible in order to avoid 
disturbances due to their electrification. In damp climates, paraffin 
or sulphur insulates better than ebonite. The objection to paraffin 
as an insulator for sensitive electrometers lies in the difficulty of 
getting entirely rid of any electrification on its surface. When 
once paraffin has been charged, the residual charge, after dis- 
electrifying it with a flame, continues to leak out for a long interval. 
All insulators should be diselectrified by means of a spirit-lamp or 
still better by leaving some uranium near them. Care should be 
taken not to touch the insulation when once diselectrified. 

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




Ill] METHODS OF MEASUREMENT 81 

In course of time the walls of the room become radio-active owing 
to the dissemination of dust and the action of the radio-active 
emanations. 

58. Electrometer key. For work with electrometers of 
high sensibility, a special key is 

necessary to make and break from , 

a distance the connection of the 

quadrants to earth in order to 

avoid electrostatic disturbances at 

the moment the current is to be 

measured. The simple key shown 

in Fig. 15 has been found very 

satisfactory for this purpose. A 

small brass rod BM, to which a 

string is attached, can be moved 

Vertically Up and down in a braSS Etetrmneter Tenting Vessel 

tube A, which is rigidly attached Fi g- 15 - 

to a bent metal support connected 

to earth. When the string is released this rod makes contact with, 
the mercury M, which fills a hole in the small block of ebonite P. 
The electrometer and testing vessel are connected with the mercury. 
When the string is pulled the rod BM is removed from the 
mercury and the earth connection of the electrometer system is 
broken. On release of the string, the rod BM falls and the electro- 
meter is again earthed. By means of this key, which may be 
operated at any distance from the electrometer, the earth con- 
nection may be made and broken at definite intervals without 
any appreciable disturbance of the needle. 

59. Testing apparatus. The arrangement shown in Fig. 16 
is very convenient for many measurements in radio-activity. Two 
parallel insulated metal plates A and B are placed inside a metal 
vessel V, provided with a side door. The plate A is connected with 
one terminal of a battery of small storage cells, the other pole of 
which is earthed ; the plate B with the electrometer, and the vessel 
V with earth. The shaded areas in the figure indicate the position 
of ebonite insulators. The active material to be tested is spread 
uniformly in a shallow groove (about 5 cms. square and 2 mms. 

R. R.-A. 6 



82 



METHODS OF MEASUREMENT 



[CH. 



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







XO JOilCCl 

V. 


omctcr 
3 








j 






( 




1 


B 

Active 'Material 
\ / A 






N 


I 


=? 



"To Battery 



Fig. 16. 

from the battery is permanently connected to the metal block N 
resting on the ebonite support. In this arrangement there is no 
possibility of a conduction leak from the plate A to B, since the 
earth-connected vessel V intervenes. 

An apparatus of this kind is very convenient for testing the 
absorption of the radiations by solid screens, as well as for making- 
comparative studies of the activity of different bodies. Unless 
very active preparations of radium are employed, a battery of 
300 volts is sufficient to ensure saturation when the plates are not 
more than 5 centimetres apart. If substances are being tested which 
give off a radio-active emanation, the effect of the emanation can 
be eliminated by passing a steady current of air from a gas bag 
between the plates. This removes the emanation as fast as it is 
produced. 

If a clean plate is put in the place of A, a small movement of 
the electrometer needle is always observed. If there is no radio- 
active substance in the neighbourhood, this effect is due to the 
small natural ionization of the air. We can always correct for this 
natural leak when necessary. 

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



Ill] METHODS OF MEASUREMENT 83 

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




Earth 



Earth 

Fig. 17. 

the battery in the usual way, and the central conductor A with the 
electrometer. This central rod is insulated from the external 
cylinder by an ebonite cork, which is divided into two parts by a 
metal ring CC' connected to earth. This ring acts the part of a 
guard-ring, and prevents any conduction leak between B and A. 
The ebonite is thus only required to insulate satisfactorily for the 
small rise of potential produced on A during the experiment. In all 
accurate measurements of current in radio-activity the guard-ring 
principle should always be used to ensure good insulation. This 
is easily secured when the ebonite is only required to insulate 
for a fraction of a volt, instead of for several hundred volts, as is 
the case when the guard-ring is absent. 

61. For measurements of radio-activity with an electrometer, 
a steady source of E.M.F. of at least 300 volts is necessary. This 
is best obtained by a battery of small cells simply made by 
immersing strips of lead in dilute sulphuric acid, or by a battery 
of small accumulators of the usual construction. Small accumu- 
lators of capacity about one-half ampere hour can now be obtained 
at a moderate price, and are more constant and require less 
attention than simple lead cells. 

In order to measure currents over a wide range, a graduated 
series of capacities is required. The capacity of an electrometer and 
testing apparatus is usually about 50 electrostatic units or "000056 
microfarads. Subdivided condensers of mica are constructed in 
which capacities varying from '001 to *2 microfarads are provided. 
With such a condenser, another extra capacity is required to 

62 



84 METHODS OF MEASUREMENT [CH. 

bridge over the gap between the capacity of the electrometer 
and the lowest capacity of the condenser. This capacity of value 
about 200 electrostatic units can readily be made of parallel plates 
or still better of concentric cylinders. With this series of capacities, 
currents may be measured between 3 x 10~ 14 and 3 x 10~ 8 amperes 
a range of over one million times. Still larger currents can be 
measured if the sensibility of the electrometer is reduced, or if 
larger capacities are available. 

In a room devoted to electrometer measurements of radio- 
activity, it is desirable to have no radio-active matter present 
except that to be tested. The room should also be as free from 
dust as possible. The presence of a large quantity of dust in the 
air (see section 31) is a very disturbing factor in all radio-active 
measurements. A larger E.M.F. is required to produce saturation 
on account of the diffusion of the ions to the dust particles. The 
presence of dust in the air also leads to uncertainty in the dis- 
tribution of excited activity in an electric field (see section 171). 

62. Measurement of Current. In order to determine 
the current in the electrometer circuit by measuring the rate of 
movement of the needle, it is necessary to know both the capacity 
of the circuit and the sensibility of the electrometer. 

Let C = capacity of electrometer and its connections in E.s. units. 
d = number of divisions of the scale passed over per second. 

D = sensibility of the electrometer measured in scale divi- 
sions for 1 volt P.D. between the quadrants. 

The current i is given by the product of the capacity of the 
system and the rate of rise of potential. 

ThUS ; = .s. units, 

Cd 



Suppose, for example, 

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



Ill] METHODS OF MEASUREMENT 85 

Since the electrometer can readily measure a current corre- 
sponding to a movement of half a scale division per second, 
it is easily seen that an electrometer can measure a current of 
3 x 10~ 14 amperes, which is considerably below the range of the 
most sensitive galvanometer. 

The capacity of the electrometer itself must not be considered 
as only that of the pair of quadrants and the needle when in a 
position of rest. The actual capacity is very much larger than this, 
on account of the motion of the charged needle. Suppose, for 
example, the needle is charged to a high negative potential, and 
kept at the zero position by an external constraint. If a quantity Q 
of positive electricity is given to the electrometer and its connections, 
the whole system is raised to a potential V, such that Q = CV, 
where C is the capacity of the system. When however the needle 
is allowed to move, it is attracted into the charged pair of quad- 
rants. This corresponds to the introduction of a negatively charged 
body between the quadrants, and in consequence the potential of 
the system is lowered to V. The actual capacity C' of the system 
when the needle moves is thus greater than (7, and is given by 



The capacity of the electrometer is thus not a constant, but 
depends on the potential of the needle, i.e. on the sensibility of the 
electrometer. 

An interesting result of practical importance follows from the 
variation of the capacity of the electrometer with the potential of 
the needle. If the external capacity attached to the electrometer 
is small compared with that of the electrometer itself, the rate of 
movement of the needle for a constant current is, in some cases, 
independent of the sensibility. An electrometer may be used for 
several days or even weeks to give nearly equal deflections for 
a constant current, without recharging the needle, although its 
potential has been steadily falling during the interval. In such 
a case the decrease in sensibility is nearly proportional to the 
decrease in capacity of the electrometer, so that the deflection for 
a given current is not much altered. The theory of this action has 
been given by J. J. Thomson 1 . 

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



86 



METHODS OF MEASUREMENT 



[CH. 



63. The capacity of the electrometer and its connections 
cannot be measured by any of the commutator methods used for 
the determination of small capacities, for in such cases the needle 
does not move, and the capacity measured is not that of the 
electrometer system when in actual use. The value of the capacity 
may, however, be determined by the method of mixtures. 

Let G = capacity of electrometer and connections. 
C l = capacity of a standard condenser. 

The electrometer and its connections are charged to a potential 
V l by a battery, and the deflection d l of the needle is noted. By 
means of an insulated key, the capacity of the standard condenser 
is added in parallel with the electrometer system. Let V 2 be the 
potential of the system, and d 2 the new deflection. 



Then 



C+C, V, 



V 9 d, 



and 



c= a 



d l - 



A simple standard capacity for this purpose can be constructed 
of two concentric brass tubes the diameters of which can be 
accurately measured. The external cylinder D (Fig. 18) is mounted 



Earth 




Battery 



Fig. 18. 



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



Ill] METHODS OF MEASUREMENT 87 

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




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

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

Then C+C != |, 

and C 



d l c? 2 

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

64. Quartz piezo-electrique. In measurements of the 
strength of currents by electrometers, it is always necessary to 
determine the sensibility of the instrument and the capacity of the 
electrometer and the apparatus attached thereto. By means of the 
quartz piezo-electrique devised by the brothers MM. J. and P. Curie 1 , 
measurements of the current can be made with rapidity and 
accuracy over a wide range. These measurements are quite inde- 
pendent of the capacity of the electrometer and external circuit. 

The essential part of this instrument consists of a plate of 
quartz which is cut in a special manner. When this plate is 
placed under tension, there is a liberation of electricity equal in 
amount but opposite in sign on the two sides of the plate. The 
plate of quartz AB (Fig. 19) is hung vertically and weights are 
added to the lower end. The plate is .cut so that the optic axis of 

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



88 METHODS OF MEASUREMENT [CH. 

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

The two faces A and B are normal to one of the binary axes 
(or electrical axes) of the crystal. The tension must be applied in 
a direction normal to the optic and electric axes. The two faces 
A and B are silvered, but the main portion of the plate is electrically 



To Support 




Earth 



Earth 

To Weight 

Fig. 19. 

insulated by removing a narrow strip of the silvering near the upper 
and lower ends of the plate. One side of the plate is connected to 
the electrometer and to the conductor, the rate of leak of which is 
to be measured. The quantity of electricity set free on one face of 
the plate is accurately given by 

Q = 0-063 j . F, 

where L is the length of the insulated portion of the plate, b the 
thickness AB, and F the weight attached in kilogrammes. Q is 
then given in electrostatic units. 

Suppose, for example, that it is required to measure the current 
between the plates CD (Fig. 19), due to some radio-active material 
on the plate G, for a given difference of potential between C and D. 



Ill] METHODS OF MEASUREMENT 89 

At a given instant the connection of the quadrants of the electro- 
meter with the earth is broken. The weight is attached to the 
quartz plate, and is held in the hand so as to gradually apply the 
tension. This causes a release of electricity opposite in sign to 
that given to the plate D. The electrometer needle is kept at the 
position of rest as nearly as possible by adjusting the tension by 
hand. The tension being fully applied, the moment the needle 
commences to move steadily from zero is noted. The current 

between the plates CD is then given by -^ where t is the time of 

t 

the observation. The value of Q is known from the weight attached. 
In this method the electrometer is only used as a detector to 
show that the system is kept at zero potential. No knowledge of 
the capacity of the insulated system is required. With practice, 
measurements of the current can be made in this way with rapidity 
and certainty. 



CHAPTER IV. 

NATURE OF THE RADIATIONS. 

PART I. 

COMPARISON OF THE RADIATIONS. 

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

Two general methods can be used to distinguish the types of 
the radiations given out by the same body, and also to compare 
the radiations from the different active substances. These methods 
are as follows : 

:(1) By observing whether the rays are .appreciably deflected 
in a magnetic field. 

(2) By comparing the relative absorption of the rays by solids 
and gases. 



CH. IV] NATURE OF THE RADIATIONS 91 

Examined in these ways, it has been found that there are three 
different types of radiation emitted from radio-active bodies, which 
for brevity and convenience have been termed the a, ft, and 7 rays. 

(i) The a rays are very readily absorbed by thin metal foil 
and by a few centimetres of air. They have been shown to consist 
of positively charged bodies projected with a velocity of about 
1/10 the velocity of light. They are deflected by intense mag- 
netic and electric fields, but the amount of deviation is minute 
in comparison with the deviation, under the same conditions, of 
the cathode rays produced in a vacuum tube. 

(ii) The ft rays are far more penetrating in character than the 
a rays, and consist of negatively charged bodies projected with 
velocities of the same order as the velocity of light. They are far 
more readily deflected than the a rays and are in fact identical 
with the cathode rays produced in a vacuum tube. 

(iii) The 7 rays are extremely penetrating, and non-deviable 
by a magnetic field. Their true nature is not yet known, but they 
are analogous in some respects to very penetrating Rontgen rays. 

The three best known radio-active substances, uranium, thorium, 
and radium, all give out these three types of rays, each in an amount 
approximately proportional to its relative activity. Polonium 
stands alone in giving only the a. or easily absorbed rays 1 . 

66. Deflection of the rays. The rays emitted from the 
active bodies thus present a very close analogy with the rays which 
are produced in a highly exhausted vacuum tube when an electric 

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



92 



NATURE OF THE RADIATIONS 



[CH. 



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

The effect of a magnetic field on a pencil of rays from a 
radio-active substance giving out the three kinds of rays is very well 
illustrated in Fig. 20 1 . 

Some radium is placed in the bottom of a narrow cylindrical 
lead vessel R. A narrow pencil 
of rays consisting of a, ft, and 
7 rays escapes from the open- 
ing. If a strong uniform 
magnetic field is applied at 
right angles to the plane of 
the paper, and directed towards 
the paper, the three types of 
rays are separated from one 
another. The 7 rays continue 
in a straight line without any . A 
deviation. The ft rays are Fig. 20. 

deflected to the right, describ- 
ing circular orbits the radius of which varies within wide limits. 
If the photographic plate AC is placed under the radium vessel, 
ike r j3 rays produce a diffuse photographic impression on the right 
of the vessel R. The a rays are bent in the direction opposite to 
that of the ft rays and describe a portion of the arc of a circle of 
large radius, but they are rapidly absorbed after traversing a 
distance of a few centimetres from the vessel R. The amount 




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



IV] 



NATURE OF THE RADIATIONS 



93 



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

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

rays, the a rayjy)roducej^ 



in the gas and the 7 rays ^Ke'IeaSE With a thin layer of un- 
screened active material spread on the lower of two parallel plates 
5 cms. apart, the amount of ionization due to the a, ft, and 7 ravs 
is of the relative order 10,000, 100, andjl. These numbers are only 
rough approximations, and the differences become less marked 
as the thickness of the radio-active layer increases. 

The average penetrating power of the rays is shown below. In 
the first column is given the thickness of the aluminium, which 
cuts each radiation down to half its value, and in the second the 
relative power of penetration of the rays. 





Thickness of 




Radiation 


Aluminium in cms. 
which cuts off half 


Relative power 
of penetration 




the radiation 




a rays 


0-0005 cms. 


1 


18 


0'05 cms. 


100 


7 


8 cms. 


10000 



rejative_powerof jDenetration is thus approximately inversely 
proportional to the relative ionization. These numbers, however, 
only indicate the order of relative penetrating power. This power 
varies considerably for the different active bodies. 

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

68. Difficulties of comparative measurements. It is 

difficult to make quantitative or even qualitative measurements of 
the relative intensity of the three types of rays from active sub- 
stances. The three general methods employed depend upon the 
action of the rays in ionizing the gas, in acting on a photographic 



94 NATURE OF THE RADIATIONS [CH. 

plate, and in causing phosphorescent or fluorescent effects in certain 
substances. In each of these methods the fraction of the rays which 
is absorbed and transformed into another form of energy is different 
for each type of ray. Even when one specific kind of ray is under 
observation, comparative measurements are rendered difficult by 
the complexity of that type of rays. For example, the /3 rays from 
radium consist of negatively charged particles projected with a 
wide range of velocity, and, in consequence, they are absorbed 
in different amounts in passing through a definite thickness of 
matter. In each case, only a fraction of the energy absorbed 
is transformed into the particular type of energy, whether ionic, 
chemical, or luminous, which serves as a means of measurement. 

The rayswhich are the most active electrically are the least 
r active photographically. Under ordinary conditions most of the 
photographic action of uranium, thorium, and radium, is due to the 
/3 or cathodic rays. The a. rays from uranium and thorium, on 
account of their weak action, have not yet been detected photo- 
graphically. With active substances like radium and polonium, 
the a rays readily produce a photographic impression. So far the 
7 rays have been detected photographically from radium only. 
That no photographic action of these rays has yet been established 
for uranium and thorium is probably due merely to the fact that 
the effect sought for is very small, and during exposures for long 
intervals it is very difficult to avoid fogging of the plates owing to 
other causes. Considering the similarity of the radiations in other 
respects, there can be little doubt that the 7 rays do produce some 
photographic action, though it is too small to observe with certainty. 

These differences in the photographic and ionizing properties 
of the radiations must always be taken into account in comparing 
results obtained by the two methods. The apparent contradiction 
of results obtained by different observers using these two methods 
is found to be due to their differences in relative photographic 
and ionizing action. For example, with the unscreened active 
material, the ionization observed by the electrical method is due 
almost entirely to a rays, while the photographic action under the 
same condition is due almost entirely to the (3 rays. 

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



IV] NATURE OF THE RADIATIONS 95 

aluminium or mica of '01 cms. or a sheet of ordinary writing-paper 
is sufficient to completely absorb all the a rays. With such a 
screen over the active material, the effects are due only to the 
ft and 7 rays, which pass through with a very slight absorption. 
Most of the /3 rays are absorbed in 5 mms. of aluminium or 2 mms. 
of lead. The radiation passing through such screens consists very 
largely of the 7 rays. As a rough working rule it may be taken 
that a thickness of matter required to absorb any type of rays is 
inversely proportional to the density of the substance, i.e. the 
absorption is proportional to the density. This rule holds ap- 
proximately for light substances, but, in heavy substances like 
mercury and lead, the radiations are about twice as readily absorbed 
as the density rule would lead us to expect. 



PART II. 
THE ft OR CATHODIC RAYS. 

69. Discovery of the /3 rays. A discovery which gave 
a great impetus to the study of the radiations from active bodies 
was made in 1899, almost simultaneously in Germany, France, and 
Austria, when it was observed that preparations of radium gave 
out some rays deviable by a magnetic field, and very similar in 
character to the cathode rays produced in a vacuum tube. The 
observation of Elster and Geitel that a magnetic field altered 
the conductivity produced in air by radium rays, led Giesel 1 to 
examine the effect of a magnetic field on the radiations. In his 
experiments, the radio-active preparation was placed in a small 
vessel between the poles of an electromagnet. The vessel was 
arranged to give a pencil of rays which was approximately per- 
pendicular to the field. The rays caused a small fluorescent patch 
on the screen. On exciting the electromagnet, the fluorescent 
zone was observed to broaden out on one side. On reversing the 
field, the extension of the zone was in the opposite direction. The 
deviation of the rays thus indicated was in the same direction and 
of the same order of magnitude as that for cathode rays. 

S. Meyer and Schweidler 2 also obtained similar results. They 
1 Wied. Annal. 69, p. 831, 1899. 2 Phys. Zeit. 1, pp. 90, 113, 1899. 



96 NATURE OF THE RADIATIONS [CH. 

showed, in addition, the deviation of the rays when a change 
occurred in the conductivity ; of the air under the influence of 
a magnetic field. Becquerel 1 , a little later, showed the magnetic 
deflection of the radium rays ? by using the photographic method. 
P. Curie 2 , by the electrical method, showed furthermore that the 
rays from radium consisted of two kinds, one apparently non- 
deviable and easily absorbed (now known as the a rays), and the 
other penetrating and deviable by a magnetic field (now known 
as the ft rays). The ionization effect due to the (3 rays was 
only a small fraction of that due to the a rays. At a later date 
Becquerel, by the photographic method, showed that uranium gave 
out some deflectable rays. It had been shown previously 3 that the 
rays from uranium consisted of a and ft rays. The deflected rays 
in Becquerel's experiment consisted entirely of /3 rays, as the 
a rays from uranium produce no appreciable photographic action. 
Rutherford and Grier 4 , using the electric method, showed that 
compounds of thorium, like those of uranium, gave out beside 
a rays some penetrating ft rays, deviable in a magnetic field. As 
in the case of radium, the ionization due to the a. rays of uranium 
and thorium is large compared with that due to the ft rays. 

70. Examination of the magnetic deviation by the 
photographic method. . Becquerel has made a very complete 
study, by the photographic method, of the ft rays from radium, 
and has shown that they behave in all respects like cathode rays, 
which are known to be negatively charged particles moving with 
a high velocity. J. J. Thomson (Recent Researches, p. 136) has 
obtained the equation for the path of a charged particle moving 
in a uniform magnetic field. If a particle of mass m and charge 
e is projected with a velocity V, at an angle a with the direction of 
a field of strength H, it ^vill describe a curved path, whose radius 
R of curvature is given by 

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

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

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

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



IV] NATURE OF THE RADIATIONS 97 



7T 



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



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

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



*x ^ -7T1 P~ 

V;:::;>7 

Fig. 21. 

A photographic plate P, with the film downwards, is enveloped 
in black paper and placed horizontally in the uniform horizontal 
magnetic field of an electromagnet. The magnetic field is sup- 
posed to be uniform and, in the figure, is at right angles to the 
plane of the paper. The plate was covered with a sheet of lead, 
and on the edge of the plate, in the centre of the magnetic field 
is placed a small lead vessel R containing the radio-active matter. 

On exciting the magnet, so that the rays are bent to the left 
of the figure, it is observed that a photographic impression is pro- 
duced directly below the source of the rays, which have been bent 
round by the magnetic field. The active matter sends out rays 
equally in all directions. The rays perpendicular to the field 
describe circles, which strike the plate immediately under the 
source. A few of these rays, A l} A 2 , A 3 , are shown in the figure. 
The rays, normal to the plate, strike the plate almost normally, 
R. R.-A. 7 



98 NATURE OF THE RADIATIONS [CH. 

while the rays nearly parallel to the plate strike the plate at 
nearly grazing incidence. The rays, inclined to the direction of 
the field, describe spirals and produce effects on an axis parallel 
to the field passing through the source. In consequence of this, 
any opaque screen placed in the path of the rays has its shadow 
thrown near the edge of the photographic plate. 

71. Complexity of the rays. The deviable rays from 
radium are complex, i.e. they are composed of a flight of particles 
projected with a wide range of velocity. In a magnetic field every 
ray describes a path, of which the radius of curvature is directly 
proportional to the velocity of projection. The complexity of 
the radiation has been very clearly shown by Becquerel 1 in the 
following way. 

An uncovered photographic plate, with the film upwards, was 
placed horizontally in the horizontal uniform magnetic field of 
an electro-magnet. A small, open, lead box, containing the 
radio-active matter, was placed in the centre of the field, on 
the photographic plate. The light, due to the phosphorescence 
of the radio-active matter, therefore, could not reach the plate. 
The whole apparatus was placed in a dark room. The impression 
on the plate takes the form of a large, diffuse, but continuous 
band, elliptic in shape, produced on one side of the plate. 

Such an impression is to be expected if the rays are sent out 
in all directions, even if their velocities of projection are the same, 
for it can readily be shown theoretically, that the path of the rays 
is confined within an ellipse whose minor axis, which is at right 
angles to the field, is equal to 2R, and whose major axis is equal 
to TrR. If, however, the active matter is placed in the bottom of a 
deep lead cylinder of small diameter, the rays have practically all 
the same direction of projection, and in that case each part of the 
plate is acted on by rays of a definite curvature. 

In this case also, a diffuse impression is observed on the plate, 
giving, so to speak, a continuous spectrum of the rays and showing 
that the radiation is composed of rays of widely different curvatures. 
Fig. 22 shows a photograph of this kind, obtained by Becquerel, 
with strips of paper, aluminium, and platinum placed on the plate. 

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



IV] 



NATURE OF THE RADIATIONS 



99 



If screens of various thickness are placed on the plate, it is 
observed that the plate is not appreciably affected within a certain 




distance from the active matter, and that this distance increases 
with the thickness of the screen. This distance is obviously equal 
to twice the radius of curvature of the path of the rays, which are 
just able to produce an impression through the screen. 

These experiments show very clearly that the most deviable 
rays are the most readily absorbed by matter. By observations of 
this kind Becquerel has determined approximately the inferior 
limit of the value of HR for rays which are transmitted through 
different thicknesses of matter. 

The results are given in the table below : 



Substance 


Thickness 
in mms. 


Inferior limit 
of HR for 
transmitted rays 


Black paper ... 


0-065 


650 


Aluminium . . . 


o-oio 


350 





o-ioo 


1000 




0-200 


1480 


Mica ... 


0-025 


520 


Glass 


0-155 


1130 


Platinum 


0-030 


1310 


SST... ::: 


0-085 
0-130 


1740 
2610 




I 





If is a constant for all the rays, the value of HR is propor- 
tional to the velocity of the rays, and it follows from the table that 
the velocity of the rays which just produce an effect on the plate 
through '13 mms. of lead is about 7 times that of the rays which 

72 



100 



NATURE OF THE RADIATIONS 



[CH. 



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

/> 

will be shown, however, in section 76, that is not a constant for 

m 

all speeds, but decreases with increase of velocity of the rays. The 
difference in velocity between the rays is in consequence not as 
great as this calculation would indicate. On examination of the 
rays from uranium, Becquerel found that the radiation is not as 
complex as that from radium, but consists wholly of rays for 
which the value of HR is about 2000. 



Sattery 



"Electrometer 



72. Examination of the fi rays by the electric method. 

The presence of easily deviable rays given off from an active 
substance can most readily be shown by the photographic method, 
but it is necessary, in addition, to show that the penetrating rays 
which produce the ionization in the gas are the same as those 
which cause the photographic action. This can be conveniently 
tested in an arrangement similar to that shown in Fig. 23. 

The radio-active matter A is placed on a lead block E" between 
the two parallel lead plates BB '. The 
rays pass between the parallel plates and 
ionize the gas between the^plates PP' of 
the testing vessel. The magnetic field is 
applied at right angles to the plane of 
the paper. The dotted rectangle EEEE 
represents the position of the pole piece. 
If a compound of radium or thorium is 
under investigation, a stream of air is 
required to prevent the diffusion of the 
radio-active emanations into the testing 
vessel. When a layer of uranium, thorium 
or radium compound is placed at A, the 
ionization in the testing vessel is due 

mainly to the action of the a and /3 rays. The a rays are cut 
off by adding a layer of aluminium '01 cm. thick over the active 
material. When the layer of active matter is not more than a few 
millimetres thick, the ionization due to the 7 rays is small com- 
pared with that produced by the /9 rays, and may be neglected. 
On the application of a magnetic field at right angles to the mean 




Fig, 23. 



IV] NATURE OF THE RADIATIONS 101 

direction of the rays, the ionization in the testing vessel due to 
the rays steadily decreases as the strength of the field increases, 
and in a strong field it is reduced to a very small fraction of its 
original value. In this case the rays are bent so that none of 
them enter the testing vessel. 

Examined in this way it has been found that the rays of 
uranium, thorium, and radium consist entirely of rays readily 
deflected by a magnetic field. The rays from polonium consist 
entirely of a rays, the deviation of which can be detected only in 
very intense magnetic fields. 

When the screen covering the active material is removed, in 
a strong magnetic field, the ionization in the vessel is mainly due 
to the a. rays. On account of the slight deviation of the a rays 
under ordinary experimental conditions, a still greater increase of 
the magnetic field does not appreciably alter the current due to 
them in the testing vessel. 

The action of a magnetic field on a very active substance like 
radium is easily shown by the electrical method, as the ionization 
current due to the deviable rays is large. With substances of 
small activity like uranium and thorium, the ionization current 
due to the deviable rays is very small, and a sensitive electrometer 
or an electroscope is required to determine the variation, in a 
magnetic field, of the very small current involved. This is 
especially the case for thorium oxide, which gives out only about 
1/5 of the amount of deviable rays that the same weight of uranium 
oxide gives. 

73. Experiments with a fluorescent screen. The ft 

rays from a few milligrams of pure radium bromide produce 
intense fluorescence in barium platino-cyanide and other substances 
which can be made luminous under the influence of the cathode 
rays. Using a centigram of radium bromide, the luminosity on 
a screen, placed upon it, is bright enough to be observed in 
daylight. With the aid of such a screen in a dark room many 
of the properties of the rays may be simply illustrated and their 
complex nature clearly shown. A small quantity of radium is 
placed in the bottom of a short, narrow, lead tube open at one end. 
This is placed between the pole pieces of an electro-magnet, and 



102 NATURE OF THE RADIATIONS [CH. 

the screen placed below it. With no magnetic field, a faint 
luminosity of the screen is observed due to the very penetrating 
7 rays which readily pass through the lead. When the magnetic 
field is put on, the screen is brightly lighted up on one side over 
an area elliptical in shape (section 71). The direction of deviation 
is reversed by reversal of the field. The broad extent of the 
illumination shows the complex nature of the /3 rays. On placing 
a metallic object /at various points above the screen, the trajectory 
of the rays can readily be traced by noticing the position of the 
shadow cast upon the screen. By observing the density of the 
shadow, it can readily be seen that the rays most easily deviated 
are the least penetrating. 



Comparison of the /3 rays with cathode rays. 

74. Means of comparison. In order to prove the identity 
of the /3 rays from active bodies with the cathode rays produced 
in a vacuum tube, it is necessary to show 

(1) That the rays carry with them a negative charge ; 

(2) That they are deviated by an electric as well as by a 
magnetic field ; 

(3) That the ratio e/m is the same as for the cathode rays. 

Electric charge carried by the /3 rays. The experiments 
of. Perrin and J. J. Thomson have shown that the cathode rays 
carry with them a negative charge. In addition, Lenard has shown 
that the rays still carry with them a charge after traversing thin 
layers of matter. When the rays are absorbed, they give up their 
charge to the body which absorbs them. The total amount of 
charge carried by the (3 rays from even a very active preparation 
of radium is, in general, small compared with that carried by the 
whole of the cathode rays in a vacuum tube, and can be detected 
only by very delicate methods. 

Suppose that a layer of very active radium is spread on a metal 
plate connected to earth, and that the rays are absorbed by a 
parallel plate connected with an electrometer. If the rays are 
negatively charged, the top plate should receive a negative charge 
increasing with the time. On account, however, of the great 



IV] NATURE OF THE RADIATIONS 103 

ionization produced by the rays between the plates, any charge 
given to one of them is almost instantly dissipated. In many 
cases the plate does become charged to a definite positive or 
negative potential depending on the metal, but this is due to the 
contact difference of potential between the plates, and would be 
produced whether the rays were charged or not. The ionization of 
the gas is greatly diminished by placing over the active material a 
metal screen which absorbs the a rays, but allows the ft rays to 
pass through with little absorption. 

The rapid loss of any charge communicated to the top plate 
can be very much reduced, either by diminishing the pressure 
of the gas surrounding it or by enclosing the plate with suitable 
insulators. In their experiments to determine the amount of 
charge carried by the radium rays, M. and Mme Curie 1 used 
the second method/ 

A metal disc MM (Fig. 24) is connected with an electrometer 
by the wire T. The disc and wire are completely surrounded by 
insulating matter ^^. The whole is surrounded by a metal envelope 
EEEE connected with earth. On the lower side of the disc, the 
insulator and the metallic covering are ,very thin. This side is 
exposed to the rays of the radium R placed in a depression in 
a lead plate A A. 




Fig. 24. 

The rays of the radium pass through the metal cover and 
insulator with little absorption, but they are completely absorbed 
by the disc MM. It was observed that the disc received a negative 
charge which increased uniformly with the time, showing that the 
rays carry with them a negative charge. The current observed 
was very small. With an active preparation of radium 2 , forming a 

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

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






104 NATURE OF THE RADIATIONS [CH. 

layer 2*5 sq. cms. in area and 2 mms. thick, a current of the order 
of 10~ n amperes was observed after the rays had traversed a layer 
of aluminium "01 mm. thick and a layer of ebonite '3 mm. thick. 
The current was the same with discs of lead, copper, and zinc, and 
also when the ebonite was replaced by paraffin. 

Curie also observed in another experiment of a similar character 
that the radium itself acquired a positive charge. This necessarily 
follows if the rays carry with them a negative charge. If the 
ft rays alone carried with them a charge, a pellet of radium, if* 
perfectly insulated, and surrounded by a non-conducting medium, 
would in the course of time be raised to a high positive potential. 
Since, however, the a rays carry with them a charge opposite in 
sign to the ft rays, the ratio of the charge carried off by the two 
types of rays must be determined, before it can be settled whether 
the radium would acquire a positive or a negative charge. If, 
however, the radium is placed in an insulated metal vessel of a 
thickness sufficient to absorb all the a rays, but not too thick to 
allow most of the ft rays to escape, the vessel will acquire a 
positive charge in a vacuum. 

An interesting experimental result bearing upon this point 
has been described by Dorn 1 . A small quantity of radium was 
placed in a sealed glass tube and left for several months. On 
opening the tube with a file, a bright electric spark was observed 
at the moment of fracture, showing that there was a large differ- 
ence of potential between the inside of the tube and the earth. 

In this case the a rays were absorbed in the walls of the tube, 
but a large proportion of the ft rays escaped. The inside of the 
tube thus became charged, in the course of time, to a high positive 
potential ; a steady state would be reached when the rate of escape 
of negative electricity was balanced by the leakage of positive elec- 
tricity through the walls of the tube. The external surface of the 
glass would be always practically at zero potential, on account of 
the ionization of the air around it. 

Strutt 2 has recently described a simple experiment to illus- 
trate still more clearly that a radium preparation acquires a 
positive charge, if it is enclosed in an envelope thick enough to 

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

2 Phil. Mag. Nov. 1903. 



IV] NATURE OF THE RADIATIONS 105 

absorb all the a particles, but thin enough to allow most of the 
ft particles to escape. A sealed tube, containing the radium, was 
attached at one end to a pair of thin gold leaves in metallic 
connection with the radium, and was insulated inside a larger 
tube by means of a quartz rod. The air in the outer tube was 
exhausted as completely as possible by means of a mercury pump, 
in order to reduce as much as possible the ionization in the gas, 
and consequently the loss of any charge gained by the gold leaves. 
After an interval of 20 hours, the gold leaves were observed to 
diverge to their full extent, indicating that they had acquired 
a large positive charge. In this experiment Strutt used gram 
of radiferous barium of activity only 100 times that of uranium. 
It can readily be calculated that 10 milligrams of pure radium 
bromide would have caused an equal divergence of the leaves 
in a few minutes. 

A determination of the amount of the charge carried off by the 
rays of radium has been made recently by Wien 1 . A small quantity 
of radium, placed in a sealed platinum vessel, was hung by an 
insulating thread inside a glass cylinder which was exhausted to 
a low pressure. A connection between the platinum vessel and an 
electrode sealed on the external glass cylinder could be made, when 
required, by tilting the tube. Wien found that in a good vacuum 
the platinum vessel became charged to about 100 volts. The rate 
of escape of negative electricity from the platinum vessel containing 
4 milligrams of radium bromide corresponded to 2'91 x 10~ 12 am- 
peres. If the charge on each particle is taken as 1*1 x 10" 20 electro- 
magnetic units, this corresponds to an escape of 2*66 x 10 7 particles 
per second. From 1 gram of radium bromide the corresponding 
number would be 6'6 x 10 9 per second. Since some of the ft rays 
are absorbed in their passage through the walls of the containing 
vessel, the actual number projected per second from 1 gram of 
radium bromide must be greater than the above value. 

75. Determination of e/m. J. J. Thomson has shown that 
in their passage between the plates of a condenser the cathode 
rays are deflected towards the positive plate. Shortly after the 
discovery of the magnetic deviation of the ft rays from radium, 

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



106 NATURE OF THE RADIATIONS [CH. 

Dorn 1 and Becquerel 2 showed that they also were deflected by an 
electrostatic field. 

By observing the amount of the electrostatic and magnetic 
deviation, Becquerel was able to determine the ratio of e/m and 
the velocity of the projected particles. Two rectangular copper 
plates, 3'45 cms. high and 1 cm. apart, were placed in a vertical 
plane and insulated on paraffin blocks. One plate was charged to 
a high potential by means of an influence machine, and the other 
was connected to earth. The active matter was placed in a narrow 
groove cut in a lead plate parallel to the copper plates and placed 
midway between them. The photographic plate, enveloped in 
black paper, was placed horizontally above the plate containing 
the active substance. The large and diffuse pencil of rays thus 
obtained was deflected by the electric field, but the deviation 
amounted to only a few millimetres and was difficult to measure. 
The method finally adopted was to place vertically above the 
active matter a thin screen of mica, which cut the field into two 
equal parts. Thus, in the absence of an electric field, a narrow 
rectangular shadow was produced on the plate. 

When the electric field was applied, the rays were deflected and 
a part of the pencil of rays was stopped by the mica screen. A 

shadow was thus cast on the plate which showed the direction of 

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

an impression through the black paper. 

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

Xe 

a . 
ra 

Since the particle moves with a constant acceleration parallel to 
the field, the path of the particle is the same as that of a body 
projected horizontally from a height with a constant velocity and 
acted on by gravity. The path of the particle is thus a parabola, 
whose axis is parallel to the field and whose apex is at the point 
where the particle enters the electric field. The linear deviation 

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



IV] NATURE OF THE RADIATIONS 107 

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



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



a 
tan0= --. 

mv 2 

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

d 2 = h tan 6 + d l 



In the experimental arrangement the values were 
c/ 2 = -4 cms. ; 
Z = T02 x 10 12 ; 
I = 3-45 cms. ; 
h = 1*2 cms. 

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

e_ V_ 
m HR' 

Combining these two equations we get 



H.R.d a ' 

A difficulty arose in identifying the radiations for which the 
electric and magnetic deviations were determined. Becquerel 
estimated that the value of HR for the rays deflected by the 
electric field was about 1600 c.G.s. units. Thus 

v = 1*6 x 10 10 cms. per second, 

and = 10 7 . 

m 



108 NATURE OF THE RADIATIONS [CH. 

Thus these rays had a velocity more than half the velocity of light, 
and an apparent mass about the same as the cathode ray particles, 
i.e. about 1/1000 of the mass of the hydrogen atom./ The /3 ray is 
therefore analogous in all respects to the cathode ray, except that 
it differs in velocity/ In a vacuum tube the cathode rays generally 
have a velocity of about 2 x 10 9 cms. per sec. In special tubes 
with strong fields the velocity may be increased to about 10 10 cms. 
per sec. These charged particles behave like isolated units of 
negative electricity. The conception of such disembodied charges, 
known as electrons, has been examined mathematically among 
others by Larmor, who sees in this conception the ultimate basis 
of a theory of matter. The /3 rays from radium may also be 
considered as electrons, but when obtained from this source have 
velocities varying from about 1/3 V to at least *96F, where Fis the 
velocity of light, and thus have an average velocity considerably 
greater than that of the electrons produced in a vacuum tube. 
These moving electrons are, it seems, able to pass through much 
greater thicknesses of matter before they are absorbed than the 
slower electrons produced in a vacuum tube, but the difference 
is one merely of degree and not of kind. Electrons are thus con- 
tinuously and spontaneously expelled from radium with enormous 
velocities. It is difficult to avoid the conclusion, that this velocity 
has not been suddenly impressed on the electron. Such a sudden 
gain of velocity would mean an immense and sudden concentration 
of energy on a small particle, and it is more probable that the 
electron has been in rapid orbital or oscillatory motion in the atom, 
and, by some means or other, suddenly escapes from its orbit. 
According to this view, the energy of the electron is not suddenly 
created, but is only made obvious by its escape from the system to 
which it belongs. 

76. Variation of with the velocity of the electron. 

ra 

The fact that radium throws off electrons with rates of speed 
varying from 1/3 to 9/10 the velocity of light has been utilised by 
Kaufmann 1 to examine whether the ratio e/m of the electrons 
varies with the speed. It has been shown by J. J. Thomson 2 , 

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



IV] NATURE OF THE RADIATIONS 109 

Heaviside 1 , and Searle 2 that, according to the electromagnetic 
theory, a charge of electricity in motion behaves as if it had 
apparent mass. For small speeds this additional electrical mass 

202 

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

rapidly as the speed of light is approached. It is very im- 
portant to settle whether the mass of the electron is due partly 
to mechanical and partly to electrical mass, or whether it can be 
explained by virtue of electricity in motion independent of the 
usual conception of mass. 

Slightly different formulae expressing the variation of mass 
with speed have been developed by J. J. Thomson, Heaviside, 
and Searle. To interpret his results Kauftnann used a formula 
developed by M. Abraham 3 . 

Let m = mass of electron for slow speeds ; 

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

Let /8=. 

Then it can be shown that 

Mi 

- 8/4* 08) (1), 



i ri + B- i + & 

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

The experimental method employed to determine e/m and u is 
similar to the method of crossed spectra. Some strongly active 
radium was placed at the bottom of a brass box. The rays from 
this passed between two brass plates insulated and about 1*2 mm. 
apart. These rays fell on a platinum diaphragm, in which was 
a small tube about 0'2 mm. in diameter, which allowed a narrow 
bundle of rays to pass. The rays fell on a photographic plate 
enveloped in a thin layer of aluminium. 

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

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

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



110 



NATURE OF THE RADIATIONS 



[CH. 



When the whole apparatus was placed in a vacuum, a P.D. of 
from 2000 to 5000 volts could be applied between the plates 
without a spark. The rays were deflected in their passage through 
the electric field and produced what may be termed an electric 
spectrum on the plate. 

If a magnetic field is superimposed parallel to the electric field 
by means of an electromagnet, a magnetic spectrum is obtained 
perpendicular to the electric spectrum. The combination of tha 
two spectra gives rise to a jcurved line on the plate. Disregarding 
some small corrections, it can readily be shown that if y and z are 
the electric and magnetic deviations respectively, 



and 



e 

in 



(3), 
.(4). 



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

...(5), 




where K, /c I} /c. 2 are constants. 

Equation (5) gives the curve that should be obtained on the 
plate according to the electromagnetic theory. This is compared 
by trial with the actual curve obtained on the plate. 

In this way Kaufmann 1 found that the value of e/m decreased 
with the speed, showing that, assuming the charge constant, the 
mass of the electron increased with the speed. 

The following numbers give some of the preliminary results 
obtained by this method. 



Velocity of electron 




m 


2-36 x 10 10 cms. per sec. 
2-48 
2-59 
2-72 
2-85 


1-SlxlO 7 
1-17 xlO 7 
0-97 x 10 7 
0-77 x 10 7 
0-63 x 10 7 



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



IV] 



NATURE OF THE RADIATIONS 



111 



For the cathode rays S. Simon 1 obtained a value of e/m of 
T86 x 10 7 for an average speed of about 7 x 10 9 cms. per second. 

In a later paper 2 with some very active radium, more satis- 
factory photographs were obtained which allowed of accurate 
measurement. The given equation of the curve was found to 
agree satisfactorily with experiment. 

The following table, deduced from the results given by 
Kaufmann, shows the agreement between the theoretical and 
experimental values, u being the velocity of the electron and V 
that of light. 



Value of 
u 
V 


Observed value of 

ni 
m 


Percentage difference 
from theoretical 
values 


Small 


1 




:732 


1-34 


-1-5% 


752 


1-37 


-0-9,, 


'111 


1-42 


-0-6,, 


801 


1-47 


+0-5 


830 


1-545 


' +0-5,, 


860 


1-65 





883 


1-73 


+2-8 


933 


2-05 


-7-8,,? 


949 


2-145 


-1-2,, 


963 


2-42 


+ 0-4 



The average percentage error between the observed and calcu- 
lated value is thus not much more than one per cent. It is 
remarkable how nearly the velocity of the electron has to approach 

the velocity of light before the value of - - becomes large. This 

WQ 

is shown in the following table which gives the calculated values 

?72/ 

of - - for different velocities of the electron. 
ra 



Value of ^. small *1 



9 -99 '999 -9999 -999999 



Calculated m 
value m 



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



10-1 



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

1 Wied. AnnaL p. 589, 1899. 

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



112 NATURE OF THE RADIATIONS [CH. 

the mass of the electron is practically constant. The increase of 
mass becomes appreciable at about half the velocity of light, and 
increases steadily as the velocity of light is approached. Theo- 
retically the mass becomes infinite at the velocity of light, but 
even when the velocity of the electron only differs from that of 
light by one part in a million, its mass is only 10 times the value 
for slow speeds. 

The above results are therefore in agreement with the view 
that the mass of the electron is altogether electrical in origin and 
can be explained purely by electricity in motion. The value of 
e/m , for slow speeds, deduced from the results was T84 x 10 7 , 
which is in very close agreement with the value obtained by 
Simon for the cathode rays, viz. 1'86 x 10 7 . 

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

2 e 2 
compared with the velocity of light, the apparent mass ra = ^ - . 

o L 

2 e 

Therefore a = ~ . e. 

3m 

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

77. Absorption of the $ rays by matter. The absorption 
of the /3 rays by matter can readily be investigated by noting the 
variation of the ionization current in a testing vessel when the 
active matter is covered by screens differing in material and thick- 
ness. When the active matter is covered with aluminium foil of 
thickness '1 mm., the current in a testing vessel such as is shown 
in Fig. 16, is due almost entirely to the @ rays. If a uranium 
compound is used, it is found that the saturation current decreases 
with the thickness of matter traversed very nearly according to an 
exponential law. Taking the saturation current as a measure of 
the intensity of the rays, the intensity / after passing through a 
thickness d of matter is given by 

/ 

TQ~ e ' 

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



IV] NATURE OF THE RADIATIONS 113 

of matter, and / is the initial intensity. For uranium rays the 
current is reduced to half its value after passing through about 
- 5 mm. of aluminium. 

If a compound of thorium or radium is examined in the same 
way, it is found that the current does not decrease regularly 
according to the above equation. Results of this kind for radium 
rays have been given by Meyer and Schweidler 1 . The amount of 
absorption of the rays by a certain thickness of matter decreases 
with the thickness traversed. This is exactly opposite to what is 
observed for the a rays. This variation in the absorption is due to 
the fact that the y9 rays are made up of rays which vary greatly in 
penetrating power. The rays from uranium are fairly homogeneous 
in character, i.e. they consist of rays projected with about the same 
velocity. The rays from radium and thorium are complex, i.e. they 
consist of rays projected with a wide range of velocity and con- 
sequently with a wide range of penetrating power. The electrical 
examination of the deviable rays thus leads to the same results as. 
their examination by the photographic method. 

Results on the absorption of cathode rays have been given by 
Lenard 2 , who has shown that the absorption of cathode rays is- 
nearly proportional to the density of the absorbing matter, and is 
independent of its chemical state. If the deviable rays from active 
bodies are similar to cathode rays, a similar law of absorption is to- 
be expected. Strutt 3 , working with radium rays, has determined 
the law of absorption and has found it roughly proportional to the 
density of matter over a range of densities varying from 0'041 for 
sulphur dioxide to 21 '5 for platinum. In the case of mica and 
cardboard, the values of X divided by the density were 3*94 and 
3'84 respectively, while the value for platinum was 7 '34. In order 
to deduce the absorption coefficient, he assumed that the radiation 
fell off according to an exponential law with the distance traversed. 
As the rays from radium are complex, we have seen that this is 
only approximately the case. 

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

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

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

3 Nature, p. 539, 1900. 

R. R.-A. 8 



114 



NATURE OF THE RADIATIONS 



[CH. 



suitable for such a determination than the complex rays of radium. 
I have in consequence made some experiments with uranium rays 
to determine the dependence of absorption on the density. The 
results obtained are given in the following table : where X, is the 
^coefficient of absorption. 



Substance 


X 


Density 


X 
Density 


Glass ... 


14-0 


2-45 


5-7 


Mica ... 


14-2 


2-78 


5-1 


Ebonite 


6-5 


1-14 


5-7 


Wood ... 


2-16 


40 


5-4 


Cardboard 


37 


70 


5-3 


Iron ... 


44 


7'8 


5-6 


Aluminium 


14-0 


2-60 


5-4 


Copper 


60 


8-6 


7-0 


Silver ... 


75 


10-5 


7-1 


Lead ... 


122 


11-5 


10-8 


Tin ... 


96 


7-3 


13-2 



It will be observed that the value of the absorption constant 
divided by the density is very nearly the same for such different 
substances as glass, mica, ebonite, wood, iron and aluminium. The 
divergences from the law are great, however, for the other metals 
examined, viz. copper, silver, lead and tin. In tin the value of X 
divided by the density is 2*5 times its value for iron and aluminium. 
These differences show that a law for the absorption of the (3 rays 
depending only on the density does not hold for all substances. 
With an exception in the case of tin, the value of X divided by the 
density for the metals increases in the same order as their atomic 
weights. 

The absorption of the /3 rays by matter decreases very rapidly 
with increase of speed. For example, the absorption of cathode 
rays in Lenard's experiment (loc. cit.) is about 500 times as great 
as for the uranium ft rays. The velocity of the ft rays of uranium 
was found by Becquerel to be about 1*6 x 10 10 cms. per sec. The 
velocity of the cathode rays used in Lenard's experiment was 
certainly not less than 1/10 of this, so that, for a decrease of 
speed of less than 10 times, the absorption has increased over 
500 times. 



IV] NATURE OF THE RADIATIONS 115 

79. Variation of the amount of radiation with the 
thickness of the layer of radiating material. The radiations 
are sent out equally from all portions of the active mass, but the 
ionization of the gas which is measured is due only to the radiations 
which escape into the air. The depth from which the radiations 
can reach the surface depends on the absorption of the radiation 
by the active matter itself. 

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



where 7 is the intensity at the surface due to a very thick layer. 

This equation has been confirmed experimentally by observing 
the current due to the ft rays for different thicknesses of uranium 
oxide. In this case / = / for a thickness of oxide corresponding 
to 11 gr. per sq. cm. This gives a value of \ divided by density of 
6 '3. This is a value slightly greater than that observed for the 
absorption of the same rays in aluminium. Such a result shows 
clearly that the substance which gives rise to the ft rays does not 
absorb them to a much greater extent than does ordinary matter 
of the same density. 

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



PART III. 

THE a RAYS. 

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

82 



116 NATURE OF THE RADIATIONS [CH. 

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

81. The nature of the a rays. The nature of the a rays 
was difficult to determine, for a magnetic field sufficient to cause 
considerable deviation of the ft rays produced no appreciable effect 
on the a rays. It was suggested by several observers that they 
were, in reality, secondary rays set up by the (3 or cathode rays in 
the active matter from which they were produced. Such a view, 
however, failed to explain the radio-activity of polonium, which 
gave out a rays only. Later work also showed that the matter, 
which gave rise to the ft rays from uranium, could be chemically 
separated from the uranium, while the intensity of the a rays was 
unaffected. These and other results show that the a and ft rays 
are produced quite independently of one another. The view that 
they are an easily absorbed type of Rontgen rays fails to explain 
a characteristic property of the a rays, viz. that the absorption of 
the rays in a given thickness of matter, determined by the elec- 
trical method, increases with the thickness of matter previously 
traversed. It does not seem probable, that such an effect could 
be produced by a radiation like X rays, but the result is to be 
expected if the rays consist of projected bodies, which fail to 
ionize the gas when their velocity is reduced to below a certain 
value. From observations of the relative ionization produced in 
gases by the a and ft rays, Strutt 1 suggested in 1901 that the 
rays might consist of positively charged bodies projected with 
great velocity. Sir William Crookes 2 , in 1902, advanced the same 
hypothesis. From a study of the a rays of polonium Mme Curie 3 
in 1900 suggested the probability that these rays consisted of 
bodies, projected with great velocity, which lost their energy by 
passing through matter. 

1 Phil. Trans, p. 507, 1901. 

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

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



IV] NATURE OF THE RADIATIONS 117 

The writer was led independently to the same view by a mass 
of indirect evidence which received an explanation only on the 
hypo thesis that the rays consisted of matter projected with great 
velocity. Preliminary experiments with radium of activity 1000 
showed that it was very difficult to determine the magnetic devia- 
tion of the a rays. When the rays were passed through slits 
sufficiently narrow to enable a minute deviation of the rays to be 
detected, the ionizing effect of the issuing rays was too small to 
measure with certainty. It was not until radium of activity 19,000 
was obtained that it was possible to detect the deviation of these 
rays in an intense magnetic field. How small the magnetic devia- 
tion is may be judged from the fact that the a rays, projected at 
right angles to a magnetic field of 10,000 C.G.S. units, describe the 
arc of a circle of about 39 cms. radius, while under the same con- 
ditions the cathode rays produced in a vacuum tube would describe 
a circle of about '01 cm. radius. It is therefore not surprising 
that the a rays were for some time thought to be non-deviable in 
a magnetic field. 

82. Magnetic deviation of the a rays. The general 
method employed 1 to detect the magnetic deviation of the a rays 
was to allow the rays to pass through narrow slits and to observe 
whether the rate of discharge of an electroscope, due to the issuing 
rays, was altered by the application of a strong magnetic field. 
Fig. 25 shows the general arrangement of the experiment. The 
rays from a thin layer of radium of activity 19,000 passed upwards 
through a number of narrow slits G, in parallel, and then through 
a thin layer of aluminium foil, '00034 cm. thick, into the testing 
vessel V. The ionization produced by the rays in the testing 
vessel was measured by the rate of movement of the leaves of a 
gold-leaf electroscope B. The gold-leaf system was insulated inside 
the vessel by a sulphur bead C, and could be charged by means of 
a movable wire I), which was afterwards earthed. The rate of 
movement of the gold-leaf was observed through small mica 
windows in the testing vessel by means of a microscope provided 
with a micrometer eye-piece. 

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

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



118 



NATURE OF THE RADIATIONS 



[CH. 



rays passed through 20 to 25 slits of equal width, placed side by 
side. This was arranged by cutting grooves at regular intervals in 
side-plates into which brass plates were slipped. The width of the 
slit varied in different experiments between '042 cm. and '1 cm. 




mfTttflow of Hydrogen 



Earth 



Outflow of Hydrogen 
Fig. 25. 

The magnetic field was applied perpendicular to the plane of the 
paper, and parallel to the plane of the slits. The rays are thus 
deflected in a direction perpendicular to the plane of the slits and 
a very srnall amount of deviation is sufficient to cause the rays to 
impinge on the sides of the plate where they are absorbed. 

The testing vessel and system of plates were waxed to a lead 
plate P so that the rays entered the vessel V only through the 
aluminium foil. It is necessary in these experiments to have a 
steady stream of gas passing downwards between the plates in 
order to prevent the diffusion of the emanation from the radium 
upwards into the testing vessel. The presence in the testing 
vessel of a small amount of this emanation, which is always given 
out by radium, would produce great ionization and completely 
mask the effect to be observed. For this purpose, a steady 
current of dry electrolytic hydrogen of about 2 c.c. per second was 
passed into the testing vessel, streamed through the porous alu- 
minium foil, and passed between the plates carrying the emanation 
with it away from the apparatus. The use of a stream of hydrogen 



IV] NATURE OF THE RADIATIONS 119 

instead of air greatly simplifies the experiment, for it increases the 
ionization current due to the a rays in the testing vessel, and at 
the same time greatly diminishes that due to the ft and 7 rays. 
This is caused by the fa^t^hatjLhe_gLrays. are much more readily 
absorbed in air than in hydrogen, while the rate of production of 
ions due to the ft and 7 rays is much less in hydrogen than in air. 
The intensity of the a rays after passing between the plates is 
consequently greater when hydrogen is used; and since the rays 
pass through a sufficient distance of hydrogen in the testing vessel 
to be largely absorbed, the total amount of ionization produced by 
them is greater with hydrogen than with air. 

The following is an example of an observation on the magnetic 
deviation : 

Pole-pieces T90 x 2'50cms. 

Strength of field between pole-pieces 8370 units. 

Apparatus of 25 parallel plates of length 370 cms., width 

*70 cm., with an average air-space between plates of 

042 cm. 
Distance of radium below plates 1'4 cm. 

Rate of discharge 
of electroscope in 
volts per minute 
'(1) Without magnetic field 8-33 

(2) With magnetic field 172 

(3) Radium covered with thin layer of mica to 

absorb all a rays 0'93 

(4) Radium covered with mica and magnetic field 

applied 0'92 

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

The amount of a rays not deviated by the field is thus about 
11% of the total. The small difference between (3) and (4) 
measures the small ionization due to the ft rays, for they would 



120 NATURE OF THE RADIATIONS [CH. 

be completely deviated by the magnetic field. (4) comprises the 
effect of the <y rays together with the natural leak of the electro- 
scope in hydrogen. 

In this experiment there was a good deal of stray magnetic 
field acting on the rays before they reached the pole-pieces. The 
diminution of the rate of discharge due to the a rays was found to 
be proportional to the strength of field between the pole-pieces. 
With a more powerful magnetic field, the whole of the a rays were 
deviated, showing that they consisted entirely of projected charged 
particles. 

In order to determine the direction of deviation of the rays, 
the rays were passed through slits one mm. in width, each of which 
was half covered with a brass strip. The diminution of the rate of 
discharge in the testing vessel for a given magnetic field in such a 
case depends upon the direction of the field. In this way it was 
found that the rays were deviated in the opposite sense to the 
cathode rays. Since the latter consist of negatively charged 
particles, the a rays must consist of positively charged particles. 

These results were soon after confirmed by Becquerel 1 , by the 
photographic method, which is very well adapted to determine the 
character of the path of the rays acted on by a magnetic field. 
The radium was placed in a linear groove cut in a small block of 
lead. Above this source, at a distance of about 1 centimetre, was 
placed a metallic screen, formed of two plates, leaving between them 
a narrow opening paralled to the groove.- Above this was placed 
the photographic plate. The whole apparatus was placed in a 
strong magnetic field parallel to the groove. The strength of the 
magnetic field was sufficient to reflect the ft rays completely away 
from the plate. When the plate was parallel to the opening, 
there was produced on it an impression, due to the a rays alone, 
which became more and more diffuse as the distance from the 
opening increased. This distance should not exceed 1 or 2 centi- 
metres on account of the absorption of the rays in air. If, during 
the exposure, the magnetic field is reversed for equal lengths of 
time, on developing the plate two images of the a rays are 
observed which are deflected in opposite directions. This devia- 
tion, even in a strong field, is small though quite appreciable and 
1 C. R. 136, p. 199, 1903. 



IV] NATURE OF THE RADIATIONS 121 

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

M. Becquerel 1 , by the same method, found that the a rays from 
polonium were deviated in the same direction as the a rays from 
radium ; and thus that they also consist of projected positive bodies. 
In both cases, the photographic impressions were sharply marked 
and did not show the same diffusion which always appears in 
photographs of the rays. 

83. Electrostatic deviation of the a rays. If the rays 
are charged bodies, they should be deflected in passing through a 
strong electric field. This was found by the writer to be the case, 
but the electric deviation is still more difficult to detect than the 
magnetic deviation, as the intensity of the electric field must of 
necessity be less than that required to produce a spark in the 
presence of radium. The apparatus was similar to that employed 
for the magnetic deviation (Fig. 25) with this exception, that the 
brass sides which held the plates in position, were replaced by 
ebonite. Alternate plates were connected together and charged 
to a high potential by means of a battery of small accumulators. 
The discharge in the electroscope, due to the a rays, was found to 
be diminished by application of the electric field. With plates 
055 cm. apart and 4*5 cms. high, the diminution was only 7 / 
with a P. D. of 600 volts between the slits. With a special arrange- 
ment of plates, with slits only '01 cm. apart, the discharge was 
diminished about 45 % with an electric field corresponding to 
10,000 volts per cm. 

84. Determination of the constants of the rays. If the 

deviation of the rays in both an electric and magnetic field is 
known, the values of the velocity of the rays, and the ratio e/m of 
the charge of the particle to its mass can be determined by the 
method first used by J. J. Thomson for the cathode rays. From 
the equations of a moving charged body, the radius p of curva- 
ture of the path of the rays in a magnetic field of strength H 
perpendicular to the path of the rays is given by 



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



122 NATUEE OF THE RADIATIONS [CH. 

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

2pd l = Zj 2 

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

If the rays pass through a uniform electric field of strength X and 
length 1 2 with a deviation d, 



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

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

From equations (1) and (2) 

L <ft&3T 

~~d,tfH' 

e 24 V 

and - = - ~ -==. . 

m ff H 

The values of V and e/m are thus completely determined from the 
combined results of the electric and magnetic deviation. It was 
found that 

V = 2'5 x 10 9 cms. per sec. 

- = 6 x 10 3 . 
m 

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

The results on the magnetic and electric deviation of the 
a rays of radium have been confirmed by Des Coudres 1 , by the 
photographic method. Some pure radium bromide was used as a 
source of radiation. The whole apparatus was enclosed in a vessel 
which was exhausted to a low vacuum. In this way, not only 
was he able to determine the photographic action of the rays at 
a much greater distance from the source, but he was also able 
1 Phys. Zeit. 4, No. 17, p. 483, 1903. 



IV] NATURE OF THE RADIATIONS 123 

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

V= 1*65 x 10 9 cms. per sec. 



These values are in very good agreement with the numbers found 
by the electric method. The 2 rays from radium are complex, and 
probably consist of a stream of positively charged bodies projected 
at velocities lying between certain limits. The amount of devia- 
tion of the particles in a magnetic field will thus differ according 
to the velocity of the particle. The photographic results of 
Becquerel seem to indicate that the velocity of the rays of radium 
can vary only within fairly narrow limits, since the trajectory of 
the rays in a magnetic field is sharply marked and not nearly as 
diffuse as in similar experiments with the ft rays. 

85. Becquerel 1 has examined the amount of magnetic devia- 
tion of the a. rays at different distances from the source of the rays 
in a very simple way. A narrow vertical pencil of the rays, after 
its passage through a narrow slit, fell on a photographic plate, 
which was inclined at a small angle to the vertical and had its 
lower edge perpendicular to the slit. The trajectory of the rays 
is shown by a fine line traced on the plate. If a strong magnetic 
field is applied parallel to the slit, the trajectory of the rays is 
displaced to the right or left according to the direction of the 
field. If equal times of exposure are given for the magnetic field 
in the two directions, on developing the plate two fine diverging 
lines are found traced on the plate. The distance between these 
lines at any point is a measure of twice the average deviation 
at that point, corresponding to the value of the magnetic field. 
By measuring the distance between the trajectories at various 
points, Becquerel found that the radius of curvature of -the path of 
the rays increased with the distance from the slit. The product 
Hp of the strength of the field and the radius of curvature of the 
path of the rays is shown in the following table. 

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



124 NATURE OF THE RADIATIONS [CH. 



Distance in mms. 




from the slit 


Hp 


1 


2-91 x 10 5 


3 


2-99 


5 


3-06 


7 


3-15 


8 


3-27 


9 


3-41 



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

fty\ 

thus in good agreement. Since Hp V these results show 

p 

that the values either of V or of for the projected particles vary 

at different distances from the source. Becquerel considered that 
the rays were homogeneous, and, in order to explain the results, 
has suggested that the charge on the projected particles may 
gradually decrease with the distance traversed, so that the radius 
of curvature of the path steadily increases with the distance from 
the source. It, however, seems more probable that the rays con- 
sist of particles projected with different velocities, and that the 
slower particles are more quickly absorbed in the gas. In conse- 
quence of this, only the swifter particles are present some distance 
from the source. Before any definite conclusion can be reached, it 
will be necessary to determine the actual values of e/m and V for 
different points of the trajectory. 

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

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

e 

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

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

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



IV] NATURE OF THE RADIATIONS 125 



charge and are difficult to H^fWt hy a. Trmgnptip field. The experi- 
ments of Wien have shown that the velocity of projection of the 
Canal rays varies with the gas in the tube and the intensity of the 
electric field applied, but it is generally about 1/10 of the velocity 
of the a. particle from radium. The value of e/m is also variable, 
depending upon the gas in the tube. 

It has been shown that for the a. rays of radium 

V= 2-5 x 10 9 and e/m =6 x 10 3 . 

Now the value of e/m for the hydrogen atom, liberated in the 
electrolysis of water, is 10 4 . Assuming the charge carried by the 
a particle to be the same as that carried by the hydrogen atom, the 
mass of the a particle ^s about twice_that of the hydrogen atom. If 
the a particle consists of any known kind of matter, this result 
indicates that it consists either of projected helium or hydrogen. 
Further evidence on this important question is given in section 202. 

The a. rays from all the radio-active substances and their 
products, such as the radio-active emanations and the matter 
causing excited activity, possess the same general properties and 
do not vary very much in penetrating power. It is thus probable 
that in all cases the a rays from the different radio-active sub- 
stances consist of positively charged bodies projected with great 
velocity. Since the rays from radium are made up in part of a 
rays from the emanation stored in the radium, and from the 
excited activity which it produces, the a rays from each of these 
products must consist of positively charged bodies ; for it has been 
shown that all the a rays from radium are deviated in a strong 
magnetic field. 

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

= i m V* = i V*e = 5'9 x 10~ 6 ergs. 

Taking the velocity of a rifle bullet as 10 5 cms. per second, it is 
seen that, mass for mass, the energy of motion of the a rays is 
6 x 10 8 times as great as that of the rifle bullet. In this projection 
of bodies atomic in size with great velocity probably lies the 
principal cause of the heating effects produced by radium (section 
106). 



126 NATURE OF THE RADIATIONS [CH. 

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

It will be shown later that only a minute fraction of the atoms 
of the radio-element need break up per second in order to account 
for the radiations even of an enormously active element like 
radium. The question of the possible causes which lead to this 
atomic disintegration and the consequences which follow from it 
will be discussed later in chapter X. 

88. Experiments with a zinc sulphide screen. A screen 



IV] NATURE OF THE RADIATIONS 127 

of Sidot's hexagonal bleiid_(phQspboi^aceiiL-aii^-^ulphidp) lights 
up brightly under the action of the a rajs of radium and polonium. 
If the surface of the screerLJs examined with ajnagnifying glass, 
the light from the screen is found not to be uniformly distributed 
but to consist of a number of scintillating points of light. No two 
flashes succeed one another at the same point, but they are scattered 
over the surface, coming and going rapidly without any movement 
of translation. This remarkable action of the radium and polonium 
rays on a zinc sulphide screen was discovered by Sir William 
Crookes 1 , and independently by Elster and Geitel 2 , who observed 
it with the rays given out from a wire which has been charged 
negatively either in the open air or in a vessel containing the 
emanation of thorium. 

In order to show the scintillations of radium on the screen, 
Sir William Crookes has devised a simple apparatus which he has 
called the " Spinthariscope." A small piece_o__mtal, which has 
been dipped in a radium solution, is fixed several millimetres #way 
from a small zinc sulphide screen. This screen is fixed at one 
end of a short brass tube and is looked at through a lens fixed at 
the other end of the tube. Viewed in this way, the surface of the 
screen is seen as a dark background, dotted with brilliant points 
of light which come and go with great rapidity. The number of 
points of light per unit area to be seen at one time falls off rapidly 
as the distance from the radium increases, and, at several centi- 
metres distance, only an occasional one is seen. The experiment 
is extremely beautiful, and brings vividly before the observer the 
fact that the radium is shooting out a stream of projectiles, the 
impact of each of which on the screen is marked by a flash of light. 

The scintillatingjgpints of light on the screen are due to the 
impact of the a particles on its surface. If the radium is covered 
with a layer of foil of sufficient thickness to absorb all the a rays 
the scintillations cease. There is still a phosphorescence to be 
observed on the screen due to the /3 and 7 rays, but this luminosity 
is not marked by scintillations to any appreciable extent. Sir 
William Crookes showed that the number of scintillations was 
about the same in vacuo as in air at atmospheric pressure. If the 

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

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



128 NATUKE OF THE RADIATIONS [CH. 

screen was kept at a constant temperature, but the radium cooled 
down to the temperature of liquid air, no appreciable difference in 
the number of scintillations was observed. If, however, the screen 
was gradually cooled to the temperature of liquid air, the scintilla- 
tions diminished in number and finally ceased altogether. This is 
due to the fact that the screen loses to a large extent its power of 
phosphorescence at such a low temperature. 

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

In the scintillations of zinc sulphide, we are actually witnessing 
the effect produced by the impact on the screen of single atoms of 
matter projected with enormous velocity. Each of the particles 
carries an amount of energy corresponding to 5'9 x 10~ 6 ergs. On 
account of the ease with which these particles are stopped, most of 
this energy is given up at the surface of the screen, and a portion 
of the energy is transformed into light. Zinc sulphide is very 
sensitive to mechanical shocks. Luminosity is observed if a pen- 
knife is drawn across the screen, or if a current of air is directed on 
to the screen. The disturbance effected by the impact of the a 
particle extends over a distance very large compared with the size 
of the impinging particle, so that the spots of light produced have 
an appreciable area. Becquerel 1 recently has made an examination 
of the scintillations produced by different substances and has 
concluded that the scintillations are due to irregular cleavages 
in the crystals composing the screen, produced by the action 
of the a rays. Scintillations can be mechanically produced by 
crushing a crystal. Tomrnasina 2 found that a zinc sulphide screen 
1 C. R. 137, Oct. 27, 1903. 2 C. E. 137, Nov. 9, 1903. 



IV] NATURE OF THE RADIATIONS 129 

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

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

89. Absorption of the a rays by matter. The a rays from 
the different radio-active substances can be- distinguished from 
each other by the relative amounts of their absorption by gases 
or by thin screens of solid substances. When examined under 
the same conditions, the a rays from the active substances can be 
arranged in a definite order with reference to the amount of 
absorption in a given thickness of matter. 

In order to test the amount of absorption of the a rays for 
different thicknesses of matter, an apparatus similar to that shown 
in Fig. 16, p. 82 was employed 1 . A thin layer of the active 
material was spread uniformly over an area of about 30 sq. cms., 
and the saturation current observed between two plates 3'5 cms. 
apart. With a thin layer 2 of active material, the ionization between 
the plates is almost entirely due to the a. rays. The ionization 
due to the and 7 rays is generally less than 1/ of the total. 

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

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

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

R. R.-A. 9 



130 



NATURE OF THE RADIATIONS 



[CH. 



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

Polonium. Radium. 



Layers of 
aluminium 


Current 


Ratio of 
decrease for 
each layer 





100 








41 


1 


41 








31 


2 


12-6 








17 


3 


2-1 








067 


4 


14 




5 








Layers of 
aluminium 


Current 


Ratio of 
decrease for 
each layer 





100 








48 


1 


48 








48 


2 


23 








60 


3 


13-6 








47 


4 


6-4 








39 


5 


2-5 








36 


6 


9 




7 








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

** - o-M 



or 



where i t is the current for a thickness t, and i the initial current. 
In the case of polonium, the decrease is far more rapid than would 
be indicated by the exponential law. By the first layer, the 
current is reduced to the ratio '41. The addition of the third 
layer cuts the current down to a ratio of '17. For most of the 
active bodies, the current diminishes slightly faster than the 
exponential law would lead one to expect, especially when the 
radiation is nearly all absorbed. 



IV] 



XATURE OF THE RADIATIONS 



131 



Battery 




Electrometer 



Fig. 26. 



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

The saturation current was 
measured between two parallel 
plates PP' 3 cms. apart. The 
polonium A was placed in the 
metal box CO, and the rays from it, after passing through an 
opening in the lower plate P', covered with a layer of thin foil T, 
ionized the gas between the plates. For a certain distance AT, of 
4 cms. or more, no appreciable current was observed between P 
and P. As the distance A T was diminished, the current increased 
in a very sudden manner, so that for a small variation of the distance 
A T there was a large increase of current. With still further decrease 
of distance the current increases in a more regular manner. The 
results are shown in the following table, where the screen T con- 
sisted of one and two layers of aluminium foil respectively. The 
current due to the rays, without the aluminium screen, is in each 
case taken as 100. 



Distance AT in cms. 


3-5 


2-5 


1-9 


1-45 


0-5 


For 100 rays transmitted by one layer 








5 


10 


25 


For 100 rays transmitted by two layers 














07 












! 



The metallic screen thus cuts off a greater proportion of 
the rays the greater the distance of air which the radiations 
traverse. The effects are still more marked if the plates PP are 
close together. Results similar but not so marked are found if 
radium is substituted for the polonium. 

It follows from these experiments that the ionization per unit 
volume, due to a large plate uniformly covered with the radio- 
active matter, falls off rapidly with the distance from the plate. 
At a distance of 7 or 8 cms. the a rays from uranium, thorium, or 

92 



132 



NATURE OF THE RADIATIONS 



[CH. 



radium have been completely absorbed in the gas, and the small 
ionization then observed in the gas is due to the more penetrating 
/3 and 7 rays. The relative amount of the ionization observed at 
a distance from the source will increase with the thickness of the 
layer of active matter, but will reach a maximum for a layer of a 
certain thickness. The greater proportion of the ionization, due 
to unscreened active matter, is thus entirely confined to a shell of 
air surrounding it not more than 7 cms. in depth. 

91. The a rays from different compounds of the same active 
element, although differing in amount, have about the same average 
penetrating power. Experiments on this point have been made by 
the writer 1 and by Owens 2 . For the purpose of comparison of the 
relative power of penetration of the a rays from the different radio- 
elements, it is thus only necessary to determine the penetrating 
power for one compound of each of the radio-elements. Rutherford 



100 



Absorption of Radiation 
by Aluminium Poil 




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

Fig. 27. 



and Miss Brooks 3 have determined the amount of absorption of 
the a rays from the different active substances in their passage 
through successive layers of aluminium foil '00034 cm. thick. The 

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



IV] NATURE OF THE RADIATIONS 133 

curves of absorption are given in Fig. 27. For the purpose of 
comparison in each case, the initial current with the bare active 
compound is taken as 100. A very thin layer of the active 
substance was used, and, in the case of thorium and radium, the 
emanations given off were removed by a slow current of air through 
the testing vessel. A potential difference of 300 volts was applied 
between the plates, which was sufficient to give the maximum 
current in each case. 

Curves for the minerals organite and thorite were very nearly 
the same as for thoria. 

For the purpose of comparison, the absorption curves of the 
excited radiations of thorium and radium are given, as well as the 
curve for the radio-elements uranium, thorium, radium, and polo- 
nium. The a radiations may be arranged in the following order, 
as regards their power of penetration, beginning with the most 
penetrating. 

Thorium ) 

-r, ,. } excited radiartion. 

Kadium 1 

Thorium. 
Radium. 
Polonium. 
Uranium. 

The same order is observed for all the absorbing substances 
examined, viz., aluminium, Dutch metal, tinfoil, paper, and air and 
other gases. The differences in the absorption of the a rays from 
the active bodies are thus considerable, and must be ascribed either 
to a difference of mass or of velocity of the a particles or to a 
variation in both these quantities. 

Since the a rays differ either in mass or velocity, it follows 
that they cannot be ascribed to any single radio-active impurity 
common to all radio-active bodies. 

92. Absorption of the a rays by gases. The a rays from 
the different radio-active substances are quickly absorbed in their 
passage through a few centimetres of air at atmospheric pressure 
and temperature. In consequence of this, the ionization of the air, 
due to the a rays, is greatest near the surface of the radiating body 
and falls off very rapidly with the distance (see section 90). 



134 



NATURE OF THE RADIATIONS 



[CH. 



TT Electrometer 



^Battery 



A simple method of determining the absorption in gases is 
shown in Fig. 28. The maximum 
current is measured between two 
parallel plates A and B kept at a 
fixed distance of 2 cms. apart, and 
then moved by means of a screw to 
different distances from the radio- 
active surface. The radiation from 
this active surface passed through a 
circular opening in the plate A, 
covered with thin aluminium foil, 
and was stopped by the upper plate. 
For observations on other gases be- 
sides air, and for examining the Fig. 28. 
effect at different pressures, the apparatus is enclosed in an air- 
tight cylinder. 

If the radius of the active surface is large compared with the 
distance of the plate A from it, the intensity of the radiation is 
approximately uniform over the opening in the plate A, and falls 
off with the distance x traversed according to an exponential law. 
Thus 




where A, is the " absorption constant " of the radiation for the gas 
under consideration 1 . Let 

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

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

to 

g \x _ e -\ d+x) _ g \x / 1 e~* 1 } 

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

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



IV] 



NATURE OF THE RADIATIONS 



135 



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




\ 




5 Distance in mm*. 10 15 

Fig. 29. 

The variation of the current between A and B with the distance 
from a thin layer of uranium oxide is shown in Fig. 29 for different 
gases. The initial measurements were taken at a distance of about 
3'5 mms. from the active surface. The actual values of this initial 
current were different for the different gases, but, for the purposes 
of comparison, the value is in each case taken as unity. 

It will be seen that the current falls off with the distance 
approximately in a geometrical progression, a result which is in 
agreement with the simple theory given above. The distance 
through which the rays pass before they are absorbed is given 
below for different gases. 



Gas 

Carbonic acid 

Air 

Coal-gas 
Hydrogen . . . 



Distance in mms. to 
absorb half of radiation 

3 
4-3 

7-5 
16 



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



136 



NATURE OF THE RADIATIONS 



[CH. 



The absorption is least in hydrogen and greatest in carbonic 
acid, and follows the same order as the densities of the gases. 
In the case of air and carbonic acid, the absorption is proportional 
to the density, but this rule is widely departed from in the case 
of hydrogen. Results for the relative absorption by air of the a. rays 
from the different active bodies are shown in Fig. 30. 



100 



90 



60 



\ 



Absorption of Radiation by Air 





10 12 14 16 
Distance in mms. 



18 



20 22 



24 



Fig. 30. 



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

Distance in mms. 
Uranium ... ... ... ... ... ... 4-3 

Radium 7-5 

Thorium 10 

Excited radiation from Thorium and Radium ... 16-5 

The order of absorption by air of the radiations from the active 
substances is the same as the order of absorption by the metals 
and solid substances examined. 



IV] NATURE OF THE RADIATIONS 137 

93. Connection between absorption and density. Since 
in all cases the radiations first diminish approximately according 
to an exponential law with the distance traversed, the intensity / 
after passing through a thickness x is given by / = I e~^ where \ 
is the absorption constant and I the initial intensity. 

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

Radiation X for aluminium X for air 

Excited radiation ... 830 -42 

Thorium 1250 -69 

Radium 1600 -90 

Uranium 2750 1'6 

Taking the density of air at 20 C. and 760 mms. as 0'00120 
compared with water as unity, the following table shows the value 
of X divided by density for the different radiations. 

Radiation Aluminium Air 

Excited radiation ... 320 350 

Thorium 480 " 550 

Radium 620 740 

Uranium 1060 1300 

Comparing aluminium and air, the absorption is thus roughly 
proportional to the density for all the radiations. The divergence, 
however, between the absorption-density numbers is large when 
two metals like tin and aluminium are compared. The value of X 
for tin is not much greater than for aluminium, although the 
density is nearly three times as great. 

If the absorption is proportional to the density, the absorption 
in a gas should vary directly as the pressure, and this is found to 
be the case. Some results on this subject have been given by the 
writer (loc. cit.) for uranium rays between pressures of 1/4 and 1 
atmosphere. Owens (loc. cit.) examined the absorption of the a 
radiation in air from thoria between the pressures of 0*5 to 3 
atmospheres and found that the absorption varied directly as the 
pressure. 

The variation of absorption with density for the projected 
positive particles is thus very similar to the law for the projected 
negative particles and for cathode rays. The absorption, in both 
cases, depends mainly on the density, but is not in all cases directly 



138 NATURE OF THE RADIATIONS [CH. 

proportional to it. Since the absorption of the a rays in gases is 
probably mainly due to the exhaustion of the energy of the rays 
by the production of ions in the gas, it seems probable that the 
absorption in metals is due to a similar cause. 

94. Relation between ionization and absorption in 
gases. It has been shown (section 45) that if the a rays are 
completely absorbed in a gas, the total ionization produced is about 
the same for all the gases examined. Since the rays are unequally 
absorbed in different gases, there should be a direct connection 
between the relative ionization and the relative absorption. This 
is seen to be the case if the results of Strutt (section 45) are com- 
pared with the relative absorption constants (section 92). 

,< Relative Relative 

absorption ionization 

Air 1 1 

Hydrogen ... -27 '226 

Carbon dioxide 1-43 1'53 

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

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

As we have seen, experiment shows that the ionization of the gas 
due to the a rays from a large plane surface of radio-active matter 
falls off approximately according to an exponential law until most 
of the rays are absorbed, whereupon the ionization decreases at a 
much faster rate. The ionization of the gas is due_ta the collision 
of the positively^^r^edjmrticles with the molecules in their path. 
Each projected particle carries with it sufficient energy to produce, 
on an average, several thousand ions in its path before its velocity 
is reduced to a value below which it fails to ionize the gas. This 
minimum velocity for the a and /3 particles is probably about 10 8 cms. 
per second. More experimental data are required on the variation of 
the amount of ionization of the gas with the speed of the projected 



IV] NATURE OF THE RADIATIONS 139 

particles. The experiments of Townsend 1 and Durack 2 point to the 
conclusion that the amount of ionization per unit distance passes 
through a maximum and then decreases as the velocity of the 
particle increases. For example, Townsend found that the number 
of ions produced by an electron moving in an electric field was 
small at first for weak fields, but increased with the strength of 
the electric field to a maximum corresponding to the production 
of 21 ions per cm. in air at a pressure of 1 mm. of mercury ; 
while for a much higher velocity of about 5 x 10 9 cms. per second 
Durack found that the electrons only produce '4 ions per cm. at 
1 mm. pressure. In a later paper, Durack 3 showed that for the 
electrons from radium, which are projected with a velocity of 
about half the velocity of light, the corresponding number of ions 
per cm. of path is '19 or only about 1/100 of the maximum number 
observed by Townsend. 

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

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

Then mvdv = 

where k is a constant and 



- = -- - 

A, A, A. 

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

3 Phil. Ma . May, 1903. 



140 NATURE OF THE RADIATIONS [CH. 

for A = 0, since q = when v = 0. q should thus be proportional 
to v 2 . This conclusion is contrary to the experimental results, for 
it has been shown, at any rate for the {3 particles, that the ioniza- 
tion per unit distance decreases with increase of velocity. 

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

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

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

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

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



IV] NATURE OF THE RADIATIONS 141 

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



PART IV. 

THE 7 OR VERY PENETRATING RAYS. 

96. In addition to the a and /3 rays, the three active sub- 
stances, uranium, thorium, and radium, all give out a radiation of 
an extraordinarily penetrating character. These 7 rays are con- 
siderably more penetrating than the X rays produced in a " hard " 
vacuum tube. Their presence can readily be observed for an active 
substance like radium, but is difficult to detect for uranium and 
thorium unless a large quantity of active material is used. 

Villard 1 , using the photographic method, first drew attention 
to the fact that radium gave out these very penetrating rays, and 
found that they were non-de viable by a magnetic field. This result 
was confirmed by Becquerel*. 

Using a few milligrams of radium bromide, the 7 rays, can 
readily be detected in a dark room by_the luminosity they excite 
in the mineral willemite or a screen of ^platinocyanide of barium. 
The a and {3 rays are completely absorbed by placing a thickness 
of 1 centimetre of lead over the radium, and the rays which then 
pass through the lead consist entirely of 7 rays. The very great 
penetrating power of these rays is readily observed by noting the 
slight diminution of the luminosity of the screen when plates of 
metal several centimetres thick are placed between the radium and 
the screen. These rays also produce ionization in gases and are 
best investigated by the electrical method. The presence of the 
7 rays from 30 mgs. of radium bromide can be observed in an 
electroscope after passing through 30 cms. of iron. 

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



142 NATURE OF THE RADIATIONS [CH. 

97. In an examination of the active substances by the elec- 
trical method the writer 1 found that both uranium and thorium 
gave out 7 rays in amount roughly proportional to their activity. 
An electroscope of the type shown in Fig. 11 was employed. This 
was placed on a large lead plate '65 cm. thick, the active substance 
being placed in a closed vessel beneath. 

The discharge due to the natural ionization of the air in the 
electroscope was first observed. The additional ionization due to 
the active substance must be that produced by rays which have 
passed through the lead plate and the walls of the electroscope. 
The following table shows that the discharge due to these rays 
decreases according to an exponential law with the thickness of 
lead traversed. 



Thickness of lead 


Kate of discharge 


62 cms. 


100 


. , -f- o4 cms. 


67 


+2-86 


23 


+5-08 


8 



Using 100 gr. of uranium and thorium, the discharge due to 
the rays through 1 cm. of lead was quite appreciable and readily 
measured. The results showed that the amount of y rays was 
about the same for equal weights of thorium and uranium oxides. 
The penetrating power was also about the same as for the radium 
rays. 

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

M , i Thickness of metal to 

absorb half of the rays 

Mercury ... -75 cms. 

Lead ... -9 

Tin ... 1-8 

Copper ... 2-2 

Zinc ... 2'5 

Iron ... 2-5 

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



IV 



NATURE OF THE RADIATIO1S T S 



143 



98. Connection between absorption and density. The 

absorption constant X of the rays was determined from the 

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

__ *0 

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

The results are included in the following table : 





y rays 


/3 rays from 
uranium 


Substance 


x 


X 


\ 


\ 






density 




density 


Water 


033 


033 






Glass 


086 


035 


14-0 


5-7 


Iron ... 


28 


036 


44 


5'6 


Zinc... 


28 


039 








Copper 
Tin 


31 
38 


035 
052 


60 
96 


7-7 
13-2 


Lead 


77 


068 


1-22 


10-8 


Mercury 


92 


068 











1 



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

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

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

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



144 NATURE OF THE RADIATIONS [CH. 

cathode than to Rb'ntgen rays. It is well known that Rontgen 
rays produce much greater ionization in gases such as sulphuretted 
hydrogen and hydrochloric acid gas than in air, although the 
differences in density are not large. For example, under the 
influence of X rays sulphuretted hydrogen has six times the con- 
ductivity of air, but under the cathode rays the conductivity is 
only slightly greater than that of air. In an experiment made 
by the writer, in which the testing vessel was filled with sulphu- 
retted hydrogen, it was found that the current for the 7 rays 
from radium was only slightly greater than it was when the vessel 
contained air. 

Strutt 1 has recently made a detailed investigation of the rela- 
tive conductivity of gases exposed to the 7 rays of radium. The 
results have already been given in the table in section 45. He found 
that the relative conductivities of different gases compared with 
air were about the same as for the /3 rays of radium, but were very 
different from the conductivities for Rontgen rays. 

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

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

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

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

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



IV] NATURE OF THE RADIATIONS 145 

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

(1) Great penetrating power. 

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

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

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

Three possible hypotheses may thus be considered : 

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

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

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

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

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

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

R. R.-A. 10 



146 NATUKE OF' THE RADIATIONS [CH. 

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



PART V. 
SECONDARY RAYS. 

100. Production of secondary rays. It has long been 
known that Rontgenjrays, when they impinge on solid^^bstacles, 
produce secondary rays of much less penetrating power than the 
incident rays. This was first shown by Perrin and has been 
investigated in detail by Sagnac, Langevin, Townsend and others. 
Thus it is not surprising that similar phenomena should be 
observed for the radiation from radio-active substances. By 
means of the photographic method, Becquerel 1 has made a close 

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



IV] NATURE OF THE RADIATIONS 147 

study of the secondary radiations produced by radio-active sub- 
stances. In his earliest observations he noticedJthat^adiographs 
of metallic objects were always surrounded- by a diffuse border. 
This effect is due to the secondary rays set up by the incident 
rays at the surface of the screen. 

The secondary rays produced by the a rays are very feeble. 
They are best shown by polonium, which gives out only a rays, 
when, in consequence, the results are not complicated by the 
action of the ft rays. Strong secondary rays are set up at the 
point of impact -of the ft or cathodic rays. Becquerel found that 
the magnitude of this action depended greatly on the velocity 
of the rays. The rays of lowest velocity gave the most intense 
secondaiy action, while the penetrating rays gave, in comparison, 
scarcely any secondary effect. In consequence of the presence of 
this secondary radiation, the photographic impression of a screen 
pierced with holes is not clear and distinct. In each case there is 
a double photographic impression, due to the primary rays and the 
secondary rays set up by them. 

These secondary rays are de viable by a magnetic field, and in 
turn produce tertiary rays and so on. The secondary rays are in all 
cases more readily deviated and absorbed than the primary rays, 
from which they_arise. The very penetrating 7 rays give rise to 
secondary rays, which cause intense action on the photographic 
plate. When some radium was placed in a cavity inside a deep 
lead block, rectangular in shape, besides the impression due to the 
direct rays through the lead, Becquerel observed that there was 
also a strong impression due to the secondary rays emitted from 
the surface of the lead. The action of these secondary rays on 
the plate is so strong that the effect on the plate is, in many cases, 
increased by adding a metal screen between the active material 
and the plate. 

The comparative photographic action of the primary and 
secondary rays cannot be taken as a relative measure of the 
intensity of their radiations. For example, only a small portion 
of the energy of the ft rays is in general absorbed in the sensitive 
film. Since the secondary rays are far more easily absorbed than 
the primary rays, a far greater proportion of their energy is ex- 
pended in producing photographic action than in the case of the 

102 



148 



NATURE OF THE RADIATIONS 



[CH. IV 



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

On account of these secondary rays, radiographs produced by 
the ft rays of radium in general show a diffuse border round the 
shadow of the object. For this reason radiographs of this kind 
lack the sharpness of outline of X ray photographs. 

101. Mme Curie 1 has shown by the electric method that the 
a rays of polonium produce secondary rays. The method adopted 
was to compare the ionization current between two parallel plates, 
when two screens of different material, placed over the polonium, 
were interchanged. 



Screens employed 


Thickness 
in mms. 


Current 
observed 


Aluminium 
Cardboard 


o-oi 

0-005 


17-9 


Cardboard 
Aluminium 


0-005 

o-oi 


6-7 


Aluminium 
Tin 


o-oi 

0-005 


150 


Tin 

Aluminium 


0-005 

o-oi 


126 


Tin 

Cardboard 


0-005 
0-005 


13-9 


Cardboard 
Tin 


0-005 
0-005 


4-4 



These results show that the a rays of polonium are modified in 
passing through matter, and that the amount of secondary rays set 
up varies with screens of different material. Mme Curie, using the 
same method, was unable to observe any such effect for the ft rays 
of radium. The production of secondary rays by the ft rays of 
radium is, however, readily shown by the photographic method. 



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



CHAPTER V. 

RATE OF EMISSION OF ENERGY. 

102. Comparison of the ionization produced by the a 
and /3 rays. With unscreened active material the ionization 
produced between two parallel plates, placed as in Fig. 16, is mainly 
due to the a rays. On account of the slight penetrating power of 
the a rays, the current due to them practically reaches a maximum 
with a small thickness of radio-active material. The following 
saturation currents were observed 1 for "different thicknesses of 
uranium oxide between parallel plates sufficiently far apart for all 
the a rays to be absorbed in the gas between them. 

Surface of uranium oxide 38 sq. cms. 







Weight of uranium oxide 


Saturation current 


in grammes per sq. cm. 


in amperes per sq. cm. 


of surface 


of surface 


0036 


1-7 x 10~ 13 


0096 


3-2xlO~ 13 


0189 


4-OxlO- 13 


0350 


4'4xlO- 13 


0955 


4-7xlO~ 13 



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



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



150 



KATE OF EMISSION OF ENERGY 



[CH. 



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

On account of the difference in the penetrating power of the a 
and ft rays, the ratio of the ionization currents produced by them 
depends on the thickness of the radio-active layer under examina- 
tion. The following comparative values of the current due to the 
a and ft rays were obtained for very thin layers of active matter 1 . A 
weight of 1/10 gramme of fine powder, consisting of uranium oxide, 
thorium oxide, or radium chloride of activity 2000, was spread as 
uniformly as possible over an area of 80 sq. cms. The saturation 
current was observed between parallel plates 5'7 cms. apart. This 
distance was sufficient to absorb most of the a rays from the active 
substances. A layer of aluminium '009 cm. thick absorbed all 
the a rays. 





Current due 
to a rays 


Current due 
to ]8 rays 


,q 

Katio currents - 
a 


Uranium . . . 


1 


1 


0074 


Thorium 


1 


27 


0020 


Eadium 


2000 


1350 


0033 



In the above table the saturation current due to the a and 
ft rays of uranium is, in each case, taken as unity. The third 
column gives the ratio of the currents observed for equal weights 
of substance. The results are only approximate in character, for 
the ionization due to a given weight of substance depends on its 
fineness of division. In all cases, the current due to the ft rays is 
small compared with that due to the a rays, being greatest for 
uranium and least for thorium. As the thickness of layer increases, 

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

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

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



V] RATE OF EMISSION OF ENERGY 151 

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

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

Jo A, 

Now X is difficult to measure experimentally for air, but an 
approximate estimate can be made of its value from the known 
fact that the absorption of ft rays is approximately proportional to 
the density of any given substance. 

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

X for air = '0065. 

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

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

Thus we have approximately 

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

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

This calculation takes into account only the energy which is 
radiated out into the surrounding gas ; but on account of the ease 
with which the a rays are absorbed, even with a thin layer, the 



152 RATE OF EMISSION OF ENERGY [CH. 

greater proportion of the radiation is absorbed by the radio-active 
substance itself. This is seen to be the case when it is recalled 
that the a radiation of thorium or radium is reduced to half 
value after passing through a thickness of about 0*0005 cm. of 
aluminium. Taking into consideration the great density of the 
radio-active substances, it is probable that most of the radiation 
which escapes into the air is due to a thin skin of the powder not 
much more than '0001 cm. in thickness. 

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

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



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



f 

/ 



if \d is large. 

In a similar way it may be shown that the energy TF 2 of the 

ET 

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



V] RATE OF EMISSION OF EXERGY 153 

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



Xj and X 2 are difficult to determine directly for the radio-active 
substance itself, but it is probable that the ratio Xj/X*, is not very 
different from the ratio for the absorption coefficients for another 
substance like aluminium. This follows from the general result 
that the absorption of both a and ft rays is proportional to the 
density of the substance; for it has already been shown in the 
case of the ft rays from uranium that the absorption of the rays in 
the radio-active material is about the same as for non-radio-active 
matter of the same density. 

With a thick layer of uranium oxide spread over an area of 
22 sq. cms., it was found that the saturation current between 
parallel plates 6*1 cms. apart, due to the a rays, was 12'7 times 
as great as the current due to the ft rays. Since the a rays were 
entirely absorbed between the plates and the total ionization 
produced by the ft rays is 154 times the value at the surface of the 
plates, 

W 1 total number of ions due to a rays 
W 2 total number of ions due to ft rays 

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

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

=100 approximately. 



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

This estimate is confirmed by calculations based on indepen- 
dent data. Let m lf ra 2 be the masses of the a and ft particles 
respectively. Let v l} v 2 be their velocities. 

* 
Energy of one a particle _ 



Energy of one ft particle m 2 v<? in< t 

e 



154 RATE OF EMISSION OF ENERGY [CH. 

Now it has been shown that for the a rays of radium 
v 1 = 2-5 x 10 9 , 

= 6 x 10 s . 
m^ 

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

w s =ro x 10 10 , 

= 1-8 x 10 7 , 
ra 2 

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

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

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

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



v> 

V] RATE OF EMISSION OF ENERGY 155 

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

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

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

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

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



156 RATE OF EMISSION OF ENERGY [CH. 

hours afterwards the activity, measured by the a rays, reached a 
minimum corresponding to 25/ of its maximum value when in a 
state of radio-active equilibrium (see section 191). The saturation 
current between parallel plates, sufficiently far apart to absorb all 
the a rays in the gas between them, was measured by a galvano- 
meter and found to be 2 '6 x 10~ 8 amperes. In this case the film 
of radium bromide was so thin that the absorption of the a rays by 
the radium itself was very small. Taking into account that half of 
the a radiation from the radium was absorbed in the plate, it can 
readily be deduced that the total current corresponding to 1 gram 
of radium when in a state of radio-active equilibrium is equal to 
1*2 x 10~ 4 electromagnetic units. Taking the charge on each ion 
as 1*13 x 10" 20 electromagnetic units, this corresponds to the pro- 
duction of 10 16 ions per second per gram 1 . 

Langevin 2 has deduced from the results of Townsend on ioniza- 
tion by collision, that the energy required to produce fresh ions at 
every collision is equal to the energy acquired by an ion moving 
freely between two points, which differ in potential by about 60 
volts. This corresponds to an amount of energy of 6'8 x 10~ u ergs. 
The total rate of emission of energy on the production of 10 16 ions 
per second is thus 7 x 10 5 ergs per second or about 60 gram- 
calories per hour. 

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

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

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

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



V] RATE OF EMISSION OF ENERGY 157 

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

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

The total number of ions produced for 1 gram of radium is 
10 16 . This corresponds to an emission of 1*4 x 10 11 a particles from 
1 gram of radium per second and an emission of energy of 70 gram- 
calories per hour. 

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

(1) about 10 11 a particles are projected per second ; 

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

probably equal to about 50 gram-calories per hour. 

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

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

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



158 



RATE OF EMISSION OF ENERGY 



[CH. 





Number of 
a particles 
per sec. 


Emission of energy 
in form of a rays 
per hour 


Emission of energy 
per year 


Uranium 
Thorium ... 
Radium 


70000 
70000 
10 11 


3'5 x 10~ 6 gram-cal. 
3-5 x 10~ 5 
50 


3 gram-cal. 
3 
4 '4 x 10 gram-cal. 



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

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



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



RATE OF EMISSION OF ENERGY 



159 



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

In some later experiments using 0'7 gram of pure radium 
bromide, P. Curie 2 found that the temperature of the radium 
indicated by a mercury thermometer was* 3 C. above that of the 
surrounding air. This result was confirmed by Giesel who obtained 
a difference of temperature of 5 C. with 1 gram of radium bromide. 
The actual rise of temperature observed will obviously depend upon 
the size and nature of the vessel con- 
taining the radium. 

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

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




Fig. 31. 



2 Societe de Physique, 1903. 



160 RATE OF EMISSION OF ENERGY [CH. 

The small closed Dewar flask A contains the radium in a glass 
tube R, immersed in the liquid to be employed. The flask A is 
surrounded by another Dewar bulb B, containing the same liquid, 
so that no heat is communicated to A from the outside. The gas, 
liberated in the tube A, is collected in the usual way over water or 
mercury and its volume determined. By this method, measurements 
were made with liquid carbon dioxide, oxygen, and liquid hydrogen. 
Especial interest attaches to the results with liquid hydrogen. 
The rate of heat emission of the radium was found to be about 
the same in boiling carbon dioxide and oxygen, but Dewar 1 states 
that it was distinctly greater in liquid hydrogen. This result, if 
confirmed, is of great interest, for it shows that while the rate of 
heat emission is practically unchanged between the range of tem- 
perature of liquid oxygen and carbon dioxide, the great relative 
drop in absolute temperature between liquid oxygen and hydrogen 
causes an increase in the heat emission. It will be shown in the 
next section that the heat emission of radium is directly connected 
with the radio-activity of that element. A change in the rate of 
heat emission must then involve a change in the radio-activity of 
radium. The conclusion that the heat emission of radium is greater 
in liquid hydrogen than at ordinary temperatures thus requires 
confirmation by direct measurements of the radio-activity. 

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

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

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

1 Dewar, British Association, 1903. 



V] RATE OF EMISSION OF ENERGY 161 

depended upon the age of the radium preparation pointed to the 
conclusion that the phenomenon of heat emission of radium was 
connected with the radio-activity of that element. It had long 
been known that radium compounds increased in activity for about 
a month after their preparation, when they reached a steady state. 
This increase of activity is due to the continuous production by the 
radium of the radio-active emanation or gas, which is occluded in 
the radium compound and adds its radiation to that of the radium 
proper. It thus seemed probable that the heating effect was in 
some way connected with the presence of the emanation. Some 
experiments upon this point have been made recently by Ruther- 
ford and Barnes 1 . In order to measure the small amounts of heat 
emitted, a form of differential air calorimeter was employed. Two 
equal glass flasks of about 500 c.c. were filled with dry air at 
atmospheric pressure. These flasks were connected through a 
glass U-tube filled with xylene, which served as a manometer 
to determine any variation of pressure of the air in the flasks. 
A small glass tube, closed at the lower end, was introduced into 
the middle of each of the flasks. When a continuous source of 
heat was introduced into the glass tube, the air surrounding it was 
heated and the pressure was increased. The difference of pressure, 
when a steady state was reached, was observed on the manometer 
by means of a microscope with a micrometer scale in the eye- 
piece. On placing the source of heat in the similar tube in the 
other flask, the difference in pressure was reversed. In order to 
keep the apparatus at a constant temperature, the two flasks were 
immersed in a water bath, which was kept well stirred. 

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

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



162 



RATE OF EMISSION OF ENERGY 



[CH. 



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

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



1-6 




80 



160 240 

Ho urn 



320 



Fig. 32. 



V] RATE OF EMISSION OF ENERGY 163 

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

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

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

112 



164 RATE OF EMISSION OF ENERGY [CH. 

store of latent energy, which can only be manifested when the 
atom breaks up. For some reason, the atomic system becomes 
unstable, and an a particle, of mass about twice that of the hydro- 
gen atom, escapes, carrying with it its energy of motion. Since 
the a. particles would be practically absorbed in a thickness of 
radium of less than "001 cm., the greater proportion of the a 
particles, expelled from a mass of radium, would be stoppe'd in the 
radium itself and their energy of motion would be manifested in 
the form of heat. The radium would thus be heated by its own 
bombardment above the temperature of the surrounding air. The 
suggestion that the heat emission of radium was connected with 
the expulsion of the a. rays was first given by Sir Oliver Lodge 1 . 
The energy of the expelled a particles does not account for the 
whole emission of heat by radium. It is evident that the violent 
expulsion of a part of the atom must result in intense electrical 
disturbances in the atom. At the same time, the residual parts of 
the disintegrated atom rearrange themselves to form a permanently 
or temporarily stable system. During this process also, energy is 
emitted, which is manifested in the form of heat in the radium 
itself. 

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

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

1 Nature, April 2, 1903. 



V] RATE OF EMISSION OF ENERGY 165 

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

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



r 



CHAPTER VI. 

PROPERTIES OF THE RADIATIONS. 

108. BESIDES their power of acting on a photographic plate, 
and of ionizing gases, the radiations from active bodies are able 
to produce marked chemical and physical actions in various sub- 
stances. Most of these effects are due either to the. a or /3 rays. 
The y rays produce little effect in comparison. Since the ft rays 
are similar in all respects to high velocity cathode rays, it is to be 
expected that they will produce effects similar in character to 
those produced by the cathode rays in a vacuum tube. 

/ 

Phosphorescent action. 

Becquerel 1 has studied the action of radium rays in producing 
phosphorescence in various bodies. The substance to be tested 
was placed above the radium in the form of powder on a very thin 
mica plate. Examination was made of the sulphides of calcium 
and strontium, ruby, diamond, varieties of spar, phosphorus and 
hexagonal blende. Substances like the ruby and spar, which phos- 
phoresce under luminous rays, did not phosphoresce under the 
radium rays. On the other hand, those which were made luminous 
by ultra-violet light were also luminous under the action of radium 
rays. The radium rays show distinct differences from X rays. For 
example, a diamond which was very luminous with radium rays 
was unaffected by X rays. It has been mentioned previously that 
the a rays from Marckwald's preparation of polonium produce 
marked phosphorescence in the diamond. The double sulphate of 

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



CH. VI] PROPERTIES OF THE RADIATIONS 167 

uranium and potassium is more luminous than hexagonal blende 
under X rays, but the reverse is true for radium rays ; under the 
influence of these rays, sulphide of calcium gave a blue luminosity 
but was hardly affected by X rays. 

The following table shows the relative phosphorescence excited 
in various bodies. 



, 

Without screen. 
Substance Intensity 


Across screen 
of black 
paper 


Hexagonal blende . . ... . . . : 


13-36 
1-99 
1-14 
1-00 
30 


04 
05 

01 

31 

02 


Platino-cyanide of barium 


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


! 



In the last column the intensity without the screen is in each 
case taken as unity. The great diminution of intensity after the 
rays have passed through black paper shows that most of the phos- 
phorescence developed without the screen is, in the majority of 
cases, due to the a rays. 

Bary 1 has made a very complete examination of the class of 
substances which become luminous under radium rays. He found 
that the great majority of substances belong to the alkali metals 
and alkaline earths. All these substances were also phosphorescent 
under the action of X rays. 

Zinc sulphide (Sidot's blende) phosphoresces very brightly 
under the influence of the rays from radium and other very active 
substances. This was observed by Curie and Debierne in their 
study of the radium emanation and the excited activity produced 
by it. It has also been largely used by Giesel as an optical means 
of detecting the presence of emanations from very active sub- 
stances. It is an especially sensitive means of detecting the 
presence of a rays, when it exhibits the " scintillating " property 
already discussed in section 88. In order to show the luminosity 
due to the a rays, the screen should be held close to the active 
substance, as the rays are absorbed in their passage through a few 

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



168 PROPERTIES OF THE RADIATIONS [CH. 

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

Platino-cyanide of barium fluoresces under the action of all 
three kinds of rays, but is especially suitable for a study of the 
and 7 rays. With a decigram of radium, the luminosity on the 
screen can be seen at a distance of a metre from the radium. The 
rays produce quite an appreciable luminosity on the screen after 
their passage through the human body. The mineral willemite (zinc 
silicate) was recently found by Kunz to be an even more sensitive 
means of detecting the presence of the radiations than platino- 
cyanide of barium. It fluoresces a beautiful greenish colour, and 
a piece of the mineral appears quite translucent under the action 
of the rays. Baskerville 1 has recently shown that kunzite, a new 
variety of mineral spodumene discovered by Kunz 2 , becomes 
luminous when exposed to the action of radium rays and retains 
its luminosity for some time. 

Both zinc sulphide and platino-cyanide of barium diminish in 
luminosity after exposure for some time to the action of the rays. 
To regenerate a screen of the latter, exposure to solar light is 
necessary. A similar phenomenon has been observed by Villard 
for a screen exposed to Rontgen rays. Giesel made a screen of 
platino-cyanide of radio-active barium. The screen, very luminous 
at first, gradually turned brown in colour, and at the same time 
the crystals became dichroic. In this condition the luminosity 
was much less, although the active substance had increased in 
activity after preparation. Many of the substances which are 
luminous under the rays from active substances lose this property 
to a large extent at low temperatures. 

109. Luminosity of radium compounds. All radium 
compounds are spontaneously luminous. This luminosity is es- 
pecially brilliant in the dry haloid salts, and persists for long 
intervals of time. In damp air the salts lose a large amount of 
their luminosity, but they recover it on drying. With very active 
radium chloride, the Curies have observed that the light changes 

1 Science, Sept. 4, 1903. 

2 Science, Aug. 28, 1903. 



f 

I 

4 




0091 U 



Vl] PROPERTIES OF THE RADIATIONS 169 

in colour and intensity with time. The original luminosity is 
recovered if the salt is dissolved and dried. Many inactive pre- 
parations of radiferous barium are strongly luminous. The writer 
has seen a preparation of impure radium bromide which gave out 
a light sufficient to read by in a dark room. The luminosity of 
radium persists over a wide range of temperature and is as bright 
at the temperature of liquid air as at ordinary temperatures. A 
slight luminosity is observed in a solution of radium, and if crystals 
are being formed in the solution, they can be clearly distinguished 
in the liquid by their greater luminosity. 

110. Spectrum of the phosphorescent light of radium. 

Compounds of radium, with a large admixture of barium, are 
usually strongly self-luminous. This luminosity decreases with 
increasing purity, and pure radium bromide is only very feebly 
self-luminous. A spectroscopic examination of the slight phos- 
phorescent light of pure radium bromide has been made by Sir 
William and Lady Huggins 1 . On viewing 'the light with a direct 
vision spectroscope, there were faint indications of a variation of 
luminosity at different points along the spectrum. In order to get 
a photograph of the spectrum within a reasonable time, they made 
use of a quartz spectroscope of special design which had been 
previously employed in a spectroscopic examination of faint celestial 
objects. After three days' exposure with a slit of 1/450 of an inch 
in width, a negative was obtained which showed a number of 
bright lines. The magnified spectrum is shown in Fig. 33. The 
lines of this spectrum were found to agree not only in position but 
also in relative intensity with the band spectrum of nitrogen. The 
band spectrum of nitrogen and also the spark spectrum 2 of radium 
are shown in the same figure. 

Some time afterwards Sir William Crookes and Prof. Dewar 3 
showed that this spectrum of nitrogen was not obtained if the 
radium was contained in a highly exhausted tube. Thus it 

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

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

3 British Assoc. 1903. 



170 PROPERTIES OF THE RADIATIONS [CH. 

appears that the spectrum is due to the action of the radium rays 
either on occluded nitrogen or the nitrogen in the atmosphere 
surrounding the radium. 

It is very remarkable that a phosphorescent light, like that of 
radium bromide, should show a bright line spectrum of nitrogen. 
It shows that radium at ordinary temperatures is able to set up 
radiations which are produced only by the electric discharge under 
special conditions. 

Sir William and Lady Huggins were led to examine the 
spectrum of the natural phosphorescent light of radium with the 
hope that some indications might thereby be obtained of the 
processes occurring in the radium atom. Since the main radiation 
from radium consists of positively charged atoms projected with 
great velocity, radiations must be set up both in the expelled body 
and in the system from which it escapes. Further experiments in 
this direction are much to be desired at the present time. 

111. Thermo-luminescence. E. Wiedemann and Schmidt 1 
have shown that certain bodies after exposure to .the cathode rays 
or the electric spark become luminous when they are heated to 
a temperature much below that required to cause incandescence. 
This property of thermo-luminescence is most strikingly exhibited 
in certain cases where two salts, one of which is much in excess of 
the other, are precipitated together. It is to be expected that 
such bodies would also acquire the property when exposed to the 
/3 or cathodic rays of radium. This has been found to be the case 
by Wiedemann 2 . Becquerel showed that fluor-spar, exposed to the 
radium rays, was luminous when heated. The glass tubes in which 
radium is kept are rapidly blackened. On heating the tube, a 
strong luminosity is observed, and the coloration to a large extent 
disappears. The peculiarity of many of these bodies lies in the 
fact that the property of becoming luminous when heated is retained 
for a long interval of time after the body is removed from the 
influence of the exciting cause. It appears probable that the rays 
cause chemical changes in these bodies, which are permanent until 
heat is applied. A portion of the chemical energy is then released 
in the form of visible light. 

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



VI] PROPERTIES OF THE RADIATIONS 171 



Physical actions. 

112. Some electric effects. Radium rays have the same 
effect as ultra-violet light and Rontgen rays in. increasing the 
facility with which a spark passes between electrodes. Elster and 
Geitel 1 showed thatTf two electrodes were separated by a distance 
such that the spark just refused to pass, on bringing near a specimen 
of radium the spark at once passes. This effect is best shown with 
short sparks from a small induction coil. The Curies have ob- 
served that radium completely enveloped by a lead screen 1 cm. 
thick produces a similar action. The effect in that case is due to 
the 7 rays alone. This action of the rays can be very simply 
illustrated by connecting two spark-gaps with the induction coil in 
parallel. The spark-gap of one circuit is adjusted so that the 
discharge just refuses to pass across it, but passes by the other. 
When some radium is brought near the silent spark-gap, the spark 
at once passes and ceases in the other. 

Hemptinne 2 found that the electrodeless discharge in a vacuum 
tube began at a higher pressure when a strong preparation of 
radium was brought near the tube. In one experiment the dis- 
charge without the rays began at 51 mms. but with the radium 
rays at 68 mms. The colour of the discharge was also altered. 

Himstedt 3 found that the resistance of selenium was diminished 
by the action of radium rays in the same way as by ordinary light. 

F. Henning 4 examined the electrical resistance of a barium 
chloride solution containing radium of activity 1000, but could 
observe no appreciable difference between it and a similar pure 
solution of barium chloride. This experiment shows that the 
action of the rays from the radium does not produce any appreciable 
change in the conductivity of the barium solution. The amount 
of radium present was too small to obtain the relative conductivity 
of the radium and barium solution. 

Specimens of strongly active material have been employed to 
obtain the potential at any point of the atmosphere. The ionization 
due to the active substance is so intense that the body to which it 

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

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



172 



PROPERTIES OF THE RADIATIONS 



[CH. 



is attached rapidly takes up the potential of the air surrounding 
the active substance. In this respect it is more convenient and 
rapid in its action than the ordinary taper or water dropper, but on 
account of the disturbance of the electric field by the strong 
ionization produced, it is probably not so accurate a method as 
the water dropper. 

113. Effect on liquid and solid dielectrics. P. Curie 1 
made the very important observation that liquid dielectrics became 
partial conductors under the influence of radium rays. In these 
experiments the radium, contained in a glass tube, was placed in 
an inner thin cylinder of copper. This was surrounded by a con- 
centric copper cylinder, and the liquid to be examined filled the 
space between. A strong electric field was applied, and the current 
through the liquid measured by means of an electrometer. 

The following numbers illustrate the results obtained : 



Substance 


Conductivity iu 
megohms per 1 cm. 3 


Carbon bisulphide 






20xlO~ 14 


Petroleum ether 






15 




Amyline 






14 




Carbon chloride 






8 




Benzene 






4 




Liquid air 






1-3 




Vaseline oil ... 







1-6 





Liquid air, vaseline oil, petroleum ether, amyline, are normally 
nearly perfect insulators. The conductivity of amyline and petro- 
leum ether due to the rays at -17C. was only 1/10 of its 
value at C. There is thus a marked action of temperature on 
the conductivity. For very active material the current was pro- 
portional to the voltage. With material of only 1/500 of the 
activity, it was found that Ohm's law was not obeyed. 

The following numbers were obtained : 



Volts 

50 
100 
200 
400 



Current 
109 
185 
255 
335 



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



VI] PROPERTIES OF THE RADIATIONS 173 

For an increase of voltage of 8 times, the current only increases 
about 3 times. The current in the liquid thus tends to become 
" saturated " as does the ordinary ionization current through a gas. 
These results have an important bearing on the ionization theory, 
and show that the radiation probably produces ions in the liquid as 
well as in the gas. It was also found that X rays increased the 
conductivity to about the same extent as the radium rays. 

Becquerel 1 has recently shown that solid paraffin exposed to 
the and 7 rays of radium acquires the property of conducting 
electricity to a slight extent. After removal of the radium the 
conductivity diminishes with time according to the same law as for 
an ionized gas. These results show that a solid as well as a liquid 
and gaseous dielectric is ionized under the influence of radium 
rays. 

114. Effect of temperature on the radiations. Becquerel 2 , 
by the electric method, determined the activity of uranium at the 
temperature of liquid air, and found that it did not differ more 
than 1 per cent, from the activity at ordinary temperatures. In 
his experiments, the a rays from the uranium were absorbed before 
reaching the testing vessel, and the electric current measured was 
due to the ft rays alone. P. Curie 3 found that the luminosity of 
radium and its power of exciting fluorescence in bodies were 
retained at the temperature of liquid air. Observations by the 
electric method showed that the activity of radium was unaltered 
at the temperature of liquid air. If a radium compound is heated 
in an open vessel, it is found that the activity, measured by the 
a rays, falls to about 25 per cent, of its original value. This is 
however not due to a change in the radio-activity, but to the 
release of the radio-active emanation, which is stored in the 
radium. No alteration is observed if the radium is heated in 
a closed vessel where none of the radio-active products are able 
to escape. 

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

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

3 Societe de Physique, March 2, 1900. 



174 PROPERTIES OF THE RADIATIONS [CH. 

Chemical actions. 

115. Rays from active radium preparations change oxygen 
into ozone 1 ' 2 . Its presence can be detected by the smell or by the 
action on iodide of potassium paper. This effect is. due to the 
a and ft rays from the radium, and not to the luminous rays from 
it. Since energy is required to produce ozone from oxygen, this 
must be derived from the energy of the radiations. 

The Curies found that radium compounds rapidly produced 
coloration in glass. For moderately active material the colour 
is violet, for more active material it is yellow. Long continued 
action blackens the glass, although the glass may have no lead in 
its composition. This coloration gradually extends through the 
glass, and is dependent to some extent on the kind of glass used. 

Giesel 2 found that he could obtain as much coloration in rock- 
salt and fluor-spar by radium rays, as by exposure to the action of 
cathode rays in a vacuum tube. The coloration, however, extended 
much deeper than that produced by the cathode rays. This is to 
be expected, since the radium rays have a higher velocity, and 
consequently greater penetrating power, than the cathode rays 
produced in an ordinary vacuum tube. Goldstein observed that 
the coloration is far more intense and rapid when the salts are 
melted or heated to a red heat. Melted potassium sulphate, 
under the action of a very active preparation of radium, was 
rapidly coloured a strong greenish blue which gradually changed 
into a dark green. 

The cause of these colorations by cathode and radium rays 
has been the subject of much discussion. Elster and Geitel 3 
observed that a specimen of potassium sulphate, coloured green by 
radium rays, showed a strong photo-electric action, i.e. it rapidly 
lost a negative charge of electricity when exposed to the action of 
ultra-violet light. All substances coloured by cathode rays show 
a strong photo-electric action, and, since the metals sodium and 
potassium themselves show photo-electric action to a very remark- 
able degree, Elster and Geitel have suggested that the colorations 
are caused by a solid solution of the metal in the salt. 

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

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

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



VI] PROPERTIES OF THE RADIATIONS 175 

Although the coloration due to radium rays extends deeper 
than that due to the cathode rays, when exposed to light the 
colour fades away at about the same rate in the two cases. 

BecquereJ 1 found that white phosphorus is changed into the 
red variety by the action of radium rays. This action was shown 
to be due mainly to the /8 rays. The secondary radiation set up 
by the primary rays also produced a marked effect. Radium rays, 
like ordinary light rays, also caused a precipitate of calomel in the 
presence of oxalic acid. 

Hardy and Miss Willcock 2 found that a solution of iodoform in 
chloroform turned purple after exposure for 5 minutes to the rays 
from 5 milligrams of radium bromide. This action is due to the 
liberation of iodine. By testing the effect of screens of different 
thicknesses, over the radium, this action was found to be mainly 
due to the ft rays from the radium. Rontgen rays produce a 
similar coloration. 

Hardy 3 also observed an action of the radium rays on the 
coagulation of globulin. Two solutions of globulin from ox serum 
were used, one made electro-positive by adding acetic acid, and the 
other electro-negative by adding ammonia. When the globulin 
was exposed close to the radium in naked drops, the opalescence of 
the electro-positive solution rapidly diminished, showing that the 
solution became more complete. The electro-negative solution was 
rapidly turned to a jelly and became opaque. These actions were 
found to be due to the a rays of radium alone. 

This is further evidence in favour of the view that the a rays 
consist of projected positively charged bodies of atomic dimensions, 
for a similar coagulation effect is produced by the metallic ions of 
liquid electrolytes, and has been shown by W. C. D. Whetham 4 to 
be due to the electric charges carried by the ions. 

116. Gases evolved from radium. Curie and Debierne 5 
observed that radium preparations placed in a vacuum tube con- 
tinually lowered the vacuum. The gas evolved was always accom- 

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

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

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

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

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



176 PROPERTIES OF THE RADIATIONS [CH. 

panied by the emanation, but no new lines were observed in its 
spectrum. Giesel 1 has observed a similar evolution of gas from 
solutions of radium bromide. Giesel forwarded some active material 
to Runge and Bodlander, in order that they might test the gas 
spectroscopically. From 1 gram of a 5 per cent, radium prepara- 
tion they obtained 3'5 c.c. of gas in 16 days. This gas was found, 
however, to be mainly hydrogen, with 12 per cent, of oxygen. In 
later experiments Ramsay and Soddy 2 found that 50 milligrams of 
radium bromide evolved gases at the rate of about 0'5 c.c. per day. 
This is a rate of evolution about twice that observed by Runge and 
Bodlander. On analysing the gases about 28*9 per cent, was found 
to consist of oxygen, and the rest was hydrogen. The slight excess 
of hydrogen over that attained in the decomposition of water, they 
consider to be due to the action of oxygen on the grease of the 
stop-cocks. The radio-active emanation from radium has a strong 
oxidizing action and rapidly produces carbon dioxide, if carbonaceous 
matter is present. The production of gas is probably due to the 
action of the radiations in decomposing water. The amount of 
energy required to produce the rate of decomposition observed by 
Ramsay and Soddy about 10 c.c. per day for 1 gram of radium 
bromide corresponds to about 30 gram -calories per day. This 
amount of energy is about two per cent, of the total energy emitted 
in the form of heat. 

Ramsay and Soddy (loc. cit.) have also observed the presence of 
helium in the gases evolved by solution of radium bromide. This 
important result is considered in detail in section 201. 

Physiological actions. 

117. Walkhoff first observed that radium rays produce burns 
of much the same character as those caused by Rontgen rays. 
Experiments in this direction have been made by Giesel, Curie and 
Becquerel, and others, with very similar results. There is at first 
a painful irritation, then inflammation sets in, which lasts from 10 
to 20 days. This effect is produced by all preparations of radium, 
and appears to be due mainly to the a and /3 rays. 

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



VI] PROPERTIES OF THE RADIATIONS 177 

Care has to be taken in handling radium on account of the 
painful inflammation set up by the rays. If a finger is held for 
some minutes at the base of a capsule containing a radium prepara- 
tion, the skin becomes inflamed for about 15 days and then peels 
off. The painful feeling does not disappear for two months. 

Danysz 1 found that this action is mainly confined to the skin, 
and does not extend to the underlying tissue. Caterpillars sub- 
jected to the action of the rays lost their power of motion in 
several days and finally died. 

Radium rays have been found beneficial in certain cases of 
cancer. The effect is apparently similar to that produced by 
Rontgen rays, but the use of radium possesses the great advantage 
that the radiating source can be enclosed in a fine tube and intro- 
duced at the particular point at which the action of the rays is 
required. The rays have also been found to hinder or stop the 
development of microbes 8 . 

Another interesting action of the radium rays has "been ob- 
served by Giesel. On bringing up a radium preparation to the 
closed eye, in a dark room, a sensation of diffuse light is observed. 
This effect has been examined by Himstedt and Nagel 3 who have 
shown that it is due to a fluorescence produced by the rays in the 
eye_Jtself. The blind are able to perceive this luminosity if the 
retina is intact, but do not do so if the retina is diseased. Hardy 
and Anderson 4 have recently examined this effect in some detail. 
The sensation of light is produced both by the $ and 7 rays. The 
eyelid practically absorbs all the ft rays, so that the luminosity 
observed with a closed eye is due to the 7 rays alone. The lens 
and retina of the eye are strongly phosphorescent under the action 
of the ft and 7 rays. Hardy and Anderson consider that the 
luminosity observed in a dark room with the open eye (the phos- 
phorescent light of the radium itself being stopped by black paper) 
is to a large extent due to the phosphorescence set up in the 
eyeball. The 7 rays, for the most part, produce the sensation of 
light when they strike the retina. 

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

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

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

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

R. R.-A. 12 



CHAPTER VII. ^ 

CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER. 

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

In 1900, however, Sir William Crookes 1 showed that, by a single 
chemical operation, uranium could be obtained photographically 
inactive while the whole of the activity could be concentrated 
in a small residue free from uranium. This residue, to which 
he gave the name UrX, was many hundred times more active 
photographically, weight for weight, than the uranium from which 
it had been separated. The method employed for this separation 
was to precipitate a solution of the uranium with ammonium car- 
bonate. On dissolving the precipitate in an excess of the reagent, a 
light precipitate remained behind. This was filtered, and constituted 
the Ur X. The active substance Ur X was probably present in 
very small quantity, mixed with impurities derived from the 
uranium. No new lines were observed in its spectrum. A par- 
tial separation of the activity of uranium was also effected by 
another method. Crystallized uranium nitrate was dissolved in 
ether, when it was found that the uranium divided itself between 
the ether and water present in two unequal fractions. The small 
part dissolved in the water layer was found to contain practically 
1 Proc. Roy. Soc. 66, p. 409, 1900. 



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

all the activity when examined by the photographic method, while 
the other fraction was almost inactive. These results, taken by 
themselves, pointed very strongly to the conclusion that the 
activity of uranium was not due to the element itself, but to 
some other substance, associated with it, which had distinct 
chemical properties. 

Results of a similar character were observed by Becquerel 1 . 
It was found that barium could be made photographically very 
active by adding barium chloride to the uranium solution and 
precipitating the barium as sulphate. By a succession of precipi- 
tations the uranium was rendered photographically almost inactive,, 
while the barium was strongly active. 

The inactive uranium and the active barium were laid aside; 
but, on examining them a year later, it was found that the uranium 
had completely regained its activity, while that of the barium had 
completely disappeared. The loss of activity of uranium was thus 
only temporary in character. 

In the above experiments, the activity of uranium was examined 
by the photographic method. The photographic action produced 
by uranium is due almost entirely to the ft rays. The a rays, in 
comparison, have little if any effect. Now the radiation from Ur X 
consists entirely of ft rays, and is consequently photographically 
very active. If the activity of uranium had been measured 
electrically without any screen over it, the current observed would 
have been due very largely to the a rays, and little change would 
have been observed after the removal of Ur X, since only the con- 
stituent responsible for the ft rays was removed. This important 
point is discussed in more detail in section 189. 

119. Thorium X. Rutherford and Soddy 2 , working with 
thorium compounds, found that an intensely active constituent 
could be separated from thorium by a single chemical operation. 
If ammonia is added to a thorium solution, the thorium is precipi- 
tated, but a large amount of the activity is left behind in the 
nitrate, which is chemically free from thorium. This filtrate was 
evaporated to dry ness, and the ammonium salts driven off by 

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

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

122 



180 CONTINUOUS PRODUCTION OF KADIO- ACTIVE M 



ATTER [ 



CH. 



ignition. A small residue was obtained which, weight for weight, was 
in some cases several thousand times more active than the thorium 
from which it was obtained, while the activity of the precipitated 
thorium was reduced to less than one half of its original value. 
This active constituent was named Th X from analogy to Crookes' 
UrX. 



120 



100 




8 12 

Time in Dayt- 

Fig. 34. 



The active residue was found to consist mainly of impurities 
from the thorium ; the Th X could not be examined chemically, 
and probably was present only in minute quantity. It was also 
found that an active constituent could be partly separated from 
thorium oxide by shaking it with water for some time. On 



VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 181 



filtering the water, and evaporating down, a very active residue 
was obtained which was analogous in all respects to Th X. 

On examining the products a month later, it was found that 
the Th X was no longer active, ivhile the thorium had completely 
regained its activity. A long series of measurements was then 
undertaken to examine the time-rate of these processes of decay 
and recovery of activity. 

The results are shown graphically in Fig. 34, where the final 
activity of the thorium and the initial activity of the Th X are in 
each case taken as 100. The ordinates represent the activities 



iooc 



60 



60 



\ 







40 



\ 



20 




20 



24- 



04 8 12 16 

Time in Days 

Fig. 35. 

determined by means of the ionization current, and the abscissae 
represent the time in days. It will be observed that both curves 
are irregular for the first two days. The activity of the Th X 
increased at first, while the activity of the thorium diminished. 
Disregarding these initial irregularities of the curves, which will be 
explained in detail in section 190, it will be seen that, after the 
first two days, the time taken for the thorium to recover half its 
lost activity is about equal to the time taken by the Th X to lose 



182 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH. 

half its activity. This time in each case is about four days. The 
percentage proportion of the activity regained by the thorium, over 
any given interval, is approximately equal to the precentage pro- 
portion of the activity lost by the Th X during the same interval. 

If the recovery curve is produced backwards in the normal 
direction to meet the vertical axis, it does so at a minimum of 
25 per cent., and the above conclusions hold more accurately, if the 
recovery is assumed to start from this minimum. This is clearly 
shown by Fig. 35, where the percentages of activity recovered, 
reckoned from the 25 per cent, minimum, are plotted as ordinates. 
In the same figure the decay curve, after the second day, is shown 
on the same scale. The activity of the Th X decays with the time 
according to an exponential law, falling to half value in about 
four days. If 7 is the initial activity and I t is the activity after 
a time t, then 

*-" -V : ft 

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

r ( = i--, 

*o 

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

120. Uranium X. Similar results were obtained when 
uranium was examined. The UrX was separated by Becquerel's 
method of successive precipitations with barium. The decay of 
the separated activity and the recovery of the lost activity are 
shown graphically in Fig. 36. A more detailed discussion of this 
experiment is given in section 189. 

The curves of decay and recovery exhibit the same peculiarities 
and can be expressed by the same equations as in the case of 
thorium. The time-rate of decay and recovery is, however, much 
slower than for thorium, the activity of the Ur X falling to half its 
value in about ,2 days. 



VIl] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 183 

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



too 




20 



60 



80 100 120 

Time in Days 



Fig. 36. 

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

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

(1) That there is a constant rate of production of fresh 

radio-active matter by the radio-active body. 

(2) That the activity of the matter decreases according to 

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

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



184 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH. 

to the particles produced in the time dt, is, at the moment of their 
formation, equal to Kq dt where K is a constant. 

It is required to find the activity due to the whole matter 
produced after the process has continued for a time T. 

The activity dl, due to the matter produced during the time dt 
at the time t, decays according to an exponential law during the 
time T t that elapses before its activity is estimated, and in 
consequence is given by 



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



The activity reaches a maximum value J when T is very great, 
and is then given by 



Thus 



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

A state of equilibrium is reached when the rate of loss of 
activity of the matter already produced is balanced by the activity 
supplied by the production of new active matter. According to 
this view the radio-active bodies are undergoing change, but the 
activity remains constant owing to the action of two opposing 
processes. Now if this active matter can at any time be sepa- 
rated from the substance in which it is produced, the decay of 
its activity, as a whole, should follow an exponential law with 
the time, since each portion of the matter decreases in activity 
according to an exponential law with the time, whatever its age 



VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 185 

may be. If / is the initial activity of the separated product, the 
activity I t after an interval t is given by 



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

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

Thorium and uranium behave quite differently with regard to 



186 CONTINUOUS PKODUCTION OF KADIO- ACTIVE MATTER [CH. 

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

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

1 Phil. Mag. Sept. 1902. 



VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 187 

cessive filtrates. In three successive precipitations the activities of 
the residues were proportional to 100, 8, T6 respectively. Thus two 
precipitations are nearly sufficient to free the thorium from Th X. 
The thorium freed from Th X was then allowed to stand for 
a definite time, and the amount of Th X formed during that time 
found by precipitating it, and measuring its radio-activity. Ac- 
cording to the theory, the activity I t of the thorium formed in the 
time t is given by 



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

If \t is small, 



Since the activity of Th X falls to half value in 4 days, the 
value of \ expressed in hours = '0072. After standing a period 
of 1 hour about 1/140, after 1 day 1/6, after 4 days 1/2 of the 
maximum should be obtained. The experimental results obtained 
showed an agreement as good as could be expected, with the equa- 
tion expressing the result that the Th X was being produced at 
a constant rate. 

The thorium-nitrate which had been freed from Th X was 
allowed to stand for one month, and then it was again subjected 
to the same process. The activity of the Th X was found to be 
the same as that obtained from an equal amount of the original 
thorium-nitrate. In one month, therefore, the Th X had been 
regenerated, and had reached a maximum value. By leaving the 
thorium time to fully recover its activity, this process can be re- 
peated indefinitely, and equal amounts of Th X are obtained at 
each precipitation. Ordinary commercial thorium-nitrate and the 
purest nitrate obtainable showed exactly the same action, and 
equal amounts of Th X could be obtained from equal weights. 
These processes thus appear to be independent of the chemical 
purity of the substance 1 . 

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



188 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH. 

The process of the production of Th X is continuous, and no 
alteration has been observed in the amount produced in the given 
time after repeated separations. After 23 precipitations extending 
over 9 days, the amount produced in a given interval was about 
the same as at the beginning of the process. 

These results are all in agreement with the view that the 
Th X is being continuously produced from the thorium compound 
at a constant rate. The amount of active matter produced from 
1 gram of thorium is probably extremely minute, but the elec- 
trical effects due to its activity are so large that the process of 
production can be followed after extremely short intervals. With 
a sensitive electrometer the amount of Th X produced per minute 
in 10 grams of thorium-nitrate gives a rapid movement to the 
electrometer needle. For larger intervals it is necessary to add 
additional capacity to the system to bring the effects within range 
of the instrument. 

124. Rate of decay of activity. It has been shown that 
the activity of Ur X and Th X decays according to an exponential 
law with the time. This, we shall see later, is the general law of 
decay of activity in any type of active matter, obtained by itself, 
and freed from any secondary active products which it may, itself, 
produce. In any case, when this law is not fulfilled, it can be 
shown that the activity is due to the superposition of two or 
more effects, each of which decays in an exponential law with 
the time. The physical interpretation of this law still remains 
to be discussed. 

It has been shown that in uranium and thorium compounds 
there is a continuous production of active matter which keeps the 
compound in radio-active equilibrium. The changes by which 
the active matter is produced must be chemical in nature, since 
the products of the action are different in chemical properties 
from the matter in which the changes take place. The activity 
of the products has afforded the means of following the changes 
occurring in them. It now remains to consider the connection 
between the activity at any time, and the amount of chemical 
change taking place at that time. 

In the first place, it is found experimentally that the saturation 



VI I] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 189 

ionization current i t , after the active product has been allowed to 
decay for a time t, is given by 



where i is the initial saturation current and \ the constant of 
decay. 

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



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



N t =( 

J t 



. 

A, 

If N is the number of atoms at the beginning, 



Thus = e ~ 



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



J90 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH. 

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

This is the same as the law of mono-molecular change in 
chemistry, and expresses the fact that there is only one changing 
system. If the change depended on the mutual action of two 
systems, the law of decay would be different, since the rate of 
decay in that case would depend on the relative concentration 
of the two reacting substances. This is not so, for there is not 
a single case yet observed in which the law of decay was affected 
by the amount of active matter present. 

From the above equation (1) 



or the number of systems changing in unit time is proportional to 
the number unchanged at that time. 

In the case of recovery of activity, after an active product has 
been removed, the number of systems changing in unit time, when 
radio-active equilibrium is produced, is equal to \N . This must 
be equal to the number q of new systems supplied in unit time, or 



and x-; 

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

125. Influence of conditions on the rate of decay. 

Since the activity of any product, at any time, may be taken as 
a measure of the rate at which chemical change takes place, it 
may be used as a means of determining the effect of conditions 
on the changes occurring in radio-active matter. If the rate of 
change should be accelerated or retarded, it is to be expected 
that the value of the radio-active constant X would be increased or 



VII] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 191 

decreased, i.e. that the decay curve would be different under 
different conditions. 

No such effect, however, has yet been observed in any case of 
radio-active change, where none of the active products produced 
are allowed to escape from the system. The rate of decay is 
unaltered by any chemical or physical agency, and in this respect 
the changes in radio-active matter are sharply distinguished from 
ordinary chemical change. For example, the rate of decay of 
activity from any product takes place at the same rate when the 
substance is exposed to light as when it is kept in the dark, at 
the same rate in a vacuum as in air or any other gas at atmo- 
spheric pressure. Its rate of decay is unaltered by surrounding 
the active matter by a thick layer of lead under conditions where 
no ordinary radiation from outside can affect it. The activity of 
the matter is unaffected by ignition or chemical treatment. The 
material giving rise to the activity can be dissolved in acid and 
re-obtained by evaporation of the solution without altering the 
activity. The rate of decay is the same whether the active 
matter is retained in the solid state or kept in solution. When 
a product has lost its activity, resolution or heat does not re- 
generate it, and as we shall see later, the rate of decay of the 
active products, so far examined, is the same at a red heat as at 
the temperature of liquid air. In fact, no variation of physical or 
chemical conditions has led to any observable difference in the 
decay of activity of any of the numerous types of active matter 
which have been examined. 

126. Effect of conditions on the rate of recovery of 
activity. The recover} 7 of the activity of a radio-element with 
time, when an active product is separated from it, is governed by 
the rate of production of fresh active matter and by the decay of 
activity of that already produced. Since the rate of decay of the 
activity of the separated product is independent of conditions, the 
rate of recovery of activity can be modified only by a change of 
the rate of production of fresh active matter. As far as experi- 
ments have gone, the rate of production, like the rate of decay, is 
independent of chemical or physical conditions. There are indeed 
certain cases which are apparent exceptions to this rule. For 



192 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH. 

example, the escape of the radio-active emanations from thorium 
and radium is readily affected by heat, moisture and solution. 
A more thorough investigation, however, shows that the excep- 
tion is only apparent and not real. These cases will be discussed 
more in detail in chapter vm, but it may be stated here that 
the differences observed are due to differences in the rate of escape 
of the emanations into the surrounding gas, and not to differences 
in the rate of production. For this reason it is difficult to test the 
question at issue in the case of the thorium compounds, which 
in most cases readily allow the emanation produced by them to 
escape into the air. 

In order to show that the rate of production is independent 
of molecular state, temperature, etc., it is necessary in such a 
case to undertake a long series of measurements extending 
over the whole time of recovery. It is impossible to make accu- 
rate relative comparisons to see if the activity is altered by the 
conversion of one compound into another. The relative activity 
in such a case, when measured by spreading a definite weight of 
material uniformly on a metal plate, varies greatly with the physical 
conditions of the precipitate, although the total activity of two 
compounds may be the same. 

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




VII] CONTINUOUS PRODUCTION OF RADIO-A 



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

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

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

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

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



194 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER [CH. 

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

The view that the atoms of the radio-elements are undergoing 
-spontaneous disintegration was put forward by Mr Soddy and the 
writer as a result of evidence of this character. The discovery of 
the material nature of the a rays added strong confirmation to 
the hypothesis ; for it has been pointed out (section 87) that the 
expulsion of a particles must be the result of a disintegration 
of the atoms of the radio-element. Taking the case of thorium 
as an example, the processes occurring in the atom may be 
pictured in the following way. It must be supposed that the 
thorium atoms are not permanently stable systems, but, on an 
average, a constant small proportion of them about one atom in 
every 10 16 will suffice breaks up per second. The disintegration 
consists in the expulsion from the atom of one or more a particles 
with great velocity. For simplicity, it will be supposed that each 
atom expels one a particle. It has been shown that the a particle 
of radium has a mass about twice that of the hydrogen atom. 
From the similarity of the a rays from thorium and radium, it is 
probable that the a particle of thorium does not differ much in 
mass from that of radium, and may be equal to it. After the 
escape of an a particle, the part of the atom left behind, which 
has a mass slightly less than that of the thorium atom, tends to 
rearrange its components to form a temporarily stable system. It 
is to be expected that it will differ in chemical properties from 
the thorium atom from which it was derived. The atom of the 
substance Th X is, on this view, the thorium atom minus one a 
particle. The atoms of Th X are far more unstable than the atoms 
of thorium, and one after the other they break up, each atom ex- 
pelling one a particle as before. These projected a particles give rise 
to the radiation from the Th X. Since the activity of Th X falls to 
half its original value in about four days, on an average half of the 



VIl] CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER 195 

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

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

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

128. Magnitude of the changes. It can be calculated 
by several independent methods that, in order to account for the 
changes occurring in thorium, probably not more than 10 s and 
not less than 10 4 atoms in each gram of thorium suffer disintegra- 
tion per second. It is well known (section 39) that 1 cubic centi- 
metre of hydrogen at atmospheric pressure and temperature contains 
about 2 x 10 19 molecules. From this it follows that one gram of 
thorium contains about 10 21 atoms. The fraction which breaks 
up per second thus lies between 10~ 17 and 10~ 16 . This is an 
extremely small ratio, and it is evident that the process could 
continue for long intervals of time, before the amount of matter 
changed would be capable of detection by the spectroscope or 

132 



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

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



CHAPTER VIII. 

KADIO-ACTIVE EMANATIONS. 

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

Thorium Emanation. 

130. Discovery of the emanation. In the course of 
examination of the radiations of thorium, several observers had 
noted that some of the thorium compounds, and especially the 
oxide, were very inconstant sources of radiation, when examined in 
open vessels by the electrical method. Owens 1 found that this 
inconstancy was due to the presence of air currents. When a 
closed vessel was used, the current, immediately after the intro- 
duction of the active matter, increased with the time, and finally 

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



198 RADIO-ACTIVE EMANATIONS [CH. 

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

In an investigation of these peculiar properties of thorium 
compounds, the writer 1 found that the effects were due to an 
emission of radio-active particles of some kind from the thorium 
compounds. This " emanation," as it was termed for convenience, 
possesses the properties of ionizing the gas and acting on a photo- 
graphic plate, and is able to diffuse rapidly through porous 
substances like paper and thin metal foil. 

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

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

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



VIII] 



RADIO-ACTIVE EMANATIONS 



199 



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

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

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

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



To Electrometer 




Fig. 37. 

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



200 RADIO-ACTIVE EMANATIONS [CH. 

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

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

132. Duration of the activity of the emanation. The 

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

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

Time in seconds Current 

100 

28 69 

62 51 

118 25 

155 14 

210 67 

272 4'1 

360 1-8 

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



VIII] RADIO-ACTIVE EMANATIONS 201 

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




4 5 

Time in Minutes 

Fig. 38. 

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

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

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



202 RADIO-ACTIVE EMANATIONS [CH. 

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

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

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

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

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



VIII] 



RADIO-ACTIVE EMANATIONS 



203 



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

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



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



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



No. of layers 
of paper 


Current 




i 


1 


1 -37 


2 


16 


3 


08 



No. of layers 
of paper 


Current 





1 


1 


74 


2 


74 


5 


72 


10 


67 


20 


55 



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



204 RADIO-ACTIVE EMANATIONS [CH. 

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

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

Time in seconds Current 

9 

23 25 

53 49 

96 67 

125 76 

194 88 

244 98 

304 99 

484 100 

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

With the previous notation, the decay curve is given by 




and the recovery curve by 



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

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



VIII] RADIO-ACTIVE EMANATIONS 205 

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

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

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

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



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

X = l/87, 

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

Radium Emanation. 

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



206 RADIO-ACTIVE EMANATIONS [CH. 

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

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

Experiments to determine accurately the rate of decay of 
activity of the emanation have been made by P. Curie 1 , and 
Rutherford and Soddy 2 . In the experiments of the latter, the 
emanation mixed with air was stored over mercury in an ordinary 
gas holder. From time to time, equal quantities of air mixed with 
the emanation were measured off by a gas pipette and delivered 
into a testing vessel. The latter consisted of an air-tight brass 
cylinder carrying a central insulated electrode. A saturation voltage 
was applied to the cylinder, and the inner electrode was connected 
to the electrometer with a suitable capacity in parallel. The 
saturation current was observed immediately after the introduction 
of the active gas into the testing vessel, and was taken as a measure 
of the activity of the emanation present. The current increased 
rapidly with the time owing to the production of excited activity 
on the walls of the containing vessel. This effect is described in 
detail in chapter IX. 

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

Time in hours Kelative Activity 

100 

20-8 85-7 

187-6 24-0 

354-9 6-9 

521-9 1-5 

786-9 0-19 

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



VIII] 



RADIO-ACTIVE EMANATIONS 



207 



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



-^-Battery 

E 



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

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

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



A 

A 



Electrometer 



vessel were thus not due to the 

a rays from the emanation at lg ' 

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

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

was actually measured was thus the decay of the excited activity 

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

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

the amount of excited activity is nearly proportional at any time 

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



208 RADIO-ACTIVE EMANATIONS [CH. 

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

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

The experiments were performed under the most varied con- 
ditions but the rate of decay was found to remain unaltered. The 
rate of decay did not depend on the material of the vessel contain- 
ing the emanation or on the nature or pressure of the gas with 
which the emanation was mixed. It was unaffected by the amount 
of emanation present, or by the time of exposure to the radium, 
provided sufficient time had elapsed to allow the excited activity 
to reach a maximum value before the observations were begun. 
P. Curie 1 found that the rate of decay of activity was unaffected 
by exposing the vessel containing the emanation to different 
temperatures ranging from + 450 to 180 C. 

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

Emanations from Actinium. 

137. Debierne 2 found that actinium gives out an emanation 
similar to the emanations of thorium and radium. The loss 
of activity of the emanation is even more rapid than for the 
thorium emanation, for its activity falls to half value in a few 
seconds. In consequence of the rapid decay of activity, the 
emanation is able to diffuse through the air only a short distance 
from the active matter before it loses the greater proportion of its 

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



VIII] RADIO-ACTIVE EMANATIONS 209 

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

Experiments with large amounts of Radium Emanation. 



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

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

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

Giesel 1 has recorded some interesting observations of the effect of 
the radium emanation on a screen of phosphorescent zinc sulphide. 

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



210 RADIO-ACTIVE EMANATIONS [CH. 

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

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

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

139. Curie and Debierne 1 have made an examination of the 
emanation from radium, and the excited activity produced by it. 
They have examined the emanation given off from radium under 
very low pressures. The tube containing the emanation was ex- 
hausted to a good vacuum by a mercury pump. It was observed 
that a gas was given off from the radium which produced excited 
activity on the glass walls. This gas was extremely active, and 
rapidly affected a photographic plate through the glass. It caused 
fluorescence on the surface of the glass and rapidly blackened it, 
and was still active after standing ten days. When spectroscopi- 
cally examined, this gas did not show any new lines, but gene- 
rally those of the spectra of carbonic acid, hydrogen, and mercury. 

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



VIIl] RADIO-ACTIVE EMANATIONS 211 

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

In further experiments, Curie and Debierne 1 found that many 
substances were phosphorescent under the action of the emanation 
and the excited activity produced by it. In their experiments, two 





Active Material 

Fig. 40. 

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

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

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

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

142 



212 RADIO-ACTIVE EMANATIONS [CH. 

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

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

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



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

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



VIII] RADIO-ACTIVE EMANATIONS 213 

Measurements of Emanating Power. 

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

In this way comparisons of the emanating power of thorium 
compounds have been made by Rutherford and Soddy 1 , using an 
apparatus similar to that shown in Fig. 37 on page 199. 

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

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

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

be tested. 



, emanating power of specimen _ ^ &> 

emanating power of standard i\ o> 



i 

2 

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



214 RADIO-ACTIVE EMANATIONS [CH. 

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

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

141. Effect of condition^ on emanating power. The 

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

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

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

Temperature has a very marked effect on the emanating power. 



VIII] RADIO-ACTIVE EMANATIONS 215 

The writer 1 showed that the emanating power of ordinary thoria 
was increased three to four times by heating the substance to a dull 
red heat in a platinum tube. If the temperature was kept con- 
stant, the emanation continued to escape at the increased rate, 
but returned to its original value on cooling. If, however, the 
compound was heated to a white heat, the emanating power was 
greatly reduced, and it returned on cooling to about 10/ of the 
original value. Such a compound is said to be "de-emanated" 
The emanating power of radium compounds varies in a still more 
striking manner with rise of temperature. The rate of escape 
of the emanation is momentarily increased even 10,000 times by 
heating to a dull red heat. This effect does not continue, for the 
large escape of the emanation by heating is in reality due to the 
release of the emanation stored up in the radium compound. Like 
thoria, when the compound has once been heated to a very high 
temperature, it loses its emanating power and does not regain it. 

A further examination of the effect of temperature was made 
by Rutherford and Soddy 2 . The emanating power of thoria decreases 
very rapidly with lowering of temperature, and at the temperature 
of solid carbonic acid it is only about 10/ c of its ordinary value. 
It rapidly returns to its original value when the cooling agent is 
removed. 

Increase of temperature from 80 C. to a dull red heat of plati- 
num thus increases the emanating power about 40 times, and the 
effects can be repeated again and again, with the same compound, 
provided the temperature is not raised to the temperature at which 
de-emanation begins. De-emanation sets in above a red heat, and 
the emanating power is then permanently diminished, but even 
long continued heating at a white heat never entirely destroys the 
emanating power. 

142. Regeneration of emanating power. An interesting 
question arises whether the de-emanation of thorium and radium is 
due to a removal or alteration of the substance which produces the 
emanation, or whether intense ignition merely changes the rate 
of escape of the emanation from the solid into the surrounding 
atmosphere. 

1 Phys. Zeit. 2, p. 429, 1901. 2 Phil. Mag. Nov. 1902. 



216 KADIO- ACTIVE EMANATIONS [CH. 

It is evident that the physical properties of the thoria are 
much altered by intense ignition. The compound changes in 
colour from white to pink ; it becomes denser and also far less readily 
soluble in acids. In order to see if the emanating power could be 
regenerated by a cyclic chemical process, the de-emanated thoria 
was dissolved, precipitated as hydroxide and again converted into 
oxide. At the same time a specimen of the ordinary oxide was 
subjected to an exactly parallel process. The emanating power of 
both these compounds was the same and was from two to three 
times greater than that of ordinary thoria. 

Thus de-emanation does not permanently destroy the power 
of thorium of giving out an emanation, but merely produces an 
alteration of the amount of the emanation which escapes from the 
compound. 

143. Rate of production of the emanation. The eman- 
ating power of thorium compounds, then, is a very variable quantity, 
much affected by moisture, heat, and solution. Speaking generally, 
increased temperatures and solution greatly increase the emanating 
power of both thorium and radium. 

The wide differences between the emanating powers of these 
substances in the solid state and in solution pointed to the conclu- 
sion that the differences were probably due to the rate of escape of 
the emanation into the surrounding gas, and not to a variation of 
the rate of reaction which gave rise to the emanation. It is 
obvious that a very slight retardation in the rate of escape of the 
thorium emanation from the compound into the gas, will, on account 
of the rapid decay of activity of the emanation, produce great 
changes in emanating power. The regeneration of the emanating 
power of de-emanated thoria and radium by solution and chemical 
treatment made it evident that the original power of thorium and 
radium of producing the emanation still persisted in an unaltered 
degree. 

The question whether the emanation was produced at the same 
rate in emanating as in non-emanating compounds can be put to a 
sharp quantitative test. If the rate of production of emanation 
goes on at the same rate in the solid compound, where very 
little escapes, as in the solution, where probably all escapes, the 



VIII] RADIO-ACTIVE EMANATIONS 217 

emanation must be occluded in the compound, and there must in 
consequence be a sudden release of this emanation on solution of 
the compound. On account of the very slow decay of the activity 
of the emanation of radium, the effects should be far more marked 
in that compound than in thorium. 

From the point of view developed in section 124, the expo- 
nential law of decay of the emanation expresses the result that N t 
the number of particles remaining unchanged at a time t is given 
by 

'*-<- 

where N is the initial number of particles present. When a 
steady state is reached, the rate of production q of fresh emanation 
particles is exactly balanced by the rate of change of the particles 
N already present, i.e. 

q = \N , 

N Q in this case represents the amount of emanation " occluded " in 
the compound. Substituting the value of X found for the radium 
emanation in section 136, 

^= * = 463,000. 

<?o X 

The amount of emanation stored in a non-emanating radium 
compound should therefore be nearly 500,000 times the amount 
produced per second by the compound. This result was tested in 
the following way 1 : 

A weight of '03 gr. of radium chloride of activity 1000 times that 
of uranium was placed in a Drechsel bottle and a sufficient amount of 
water drawn in to dissolve it. The released emanation was swept 
out by a current of air into a small gas holder and then into a testing 
cylinder. The initial saturation current was proportional to N . A 
rapid current of air was then passed through the radium solution 
for some time in order to remove any slight amount of emanation 
which had not been removed initially. The Drechsel bottle was 
closed air-tight, and allowed to stand undisturbed for a definite 
time t. The accumulated emanation was then swept out as before 
into the testing vessel. The new ionization current represents 
1 Rutherford and Soddy, Phil. Mag. April, 1903. 



218 RADIO-ACTIVE EMANATIONS [CH. 

the value of Nt the amount of emanation formed in the compound 
during the interval t. 

In the experiment t 105 minutes, 
and observed value 

J = -0131. 

^0 

Assuming that there is no decay during the interval, 
#,= 105xt>0xgo. 

Thus ^ = 480,000. 

ft 

Making flhe small correction for the decay of activity during 
the interval 



We have previously shown that from the theory 

= i = 463,000. 

q \ 

The agreement between theory and experiment is thus as close 
as could be expected from the nature of the experiments. This 
experiment proves conclusively that the rate of production of 
emanation in the solid compound is the same as in the solution. 
In the former case it is occluded, in the latter it escapes as fast as 
it is produced. 

It is remarkable how little emanation, compared with the 
amount stored up in the compound, escapes from solid radium 
chloride in a dry atmosphere. One experiment showed that the 
emanating power in the dry solid state was less than ^ / of the 
emanating power of the solution. Since nearly 500,000 times as 
much emanation is stored up as is produced per second, this result 
showed that the amount of emanation which escaped per second was 
less than 10~ 8 of that occluded in the compound. 

If a solid radium chloride compound is kept in a moist atmo- 
sphere, the emanating power becomes comparable with the amount 
produced per second in the solution. In such a case, since the rate 



VIII] RADIO-ACTIVE EMANATIONS 219 

of escape is continuous, the amount occluded will be much less than 
the amount for the non-emanating material. 

The phenomenon of occlusion of the radium emanation is prob- 
ably not connected in any way with its radio-activity, although this 
property has in this case served to measure it. The occlusion of 
helium by minerals presents almost a complete analogy to the 
occlusion of the radium emanation. The helium is given off by 
fergusonite, for example, in part when it is heated and completely 
on dissolving the mineral. 

144. Similar results hold for thorium, but, on account of the 
rapid loss of activity of the emanation, the amount of emanation 
occluded in a non-emanating compound is very small compared 
with that observed for radium. If the production of the thorium 
emanation proceeds at the same rate under all conditions, the 
solution of a solid non-emanating compound should be accompanied 
by a rush of emanation greater than that subsequently produced. 
With the same notation as before we have for the thorium emana- 
tion, 

X 1 



This result was tested as follows : a quantity of finely powdered 
thorium nitrate, of emanating power 1/200 of ordinary thoria, 
was dropped into a Drechsel bottle containing hot water and the 
emanation rapidly swept out into the testing vessel by a current of 
air. The ionization current rose quickly to a maximum, but soon 
fell again to a steady value ; showing that the amount of emanation 
released when the nitrate dissolves, is greater than the subsequent 
amount produced from the solution. 

The rapid loss of the activity of the thorium emanation makes 
a quantitative comparison like that made for radium very difficult. 
By slightly altering the conditions of the experiment, however, a 
definite proof was obtained that the rate of production of emana- 
tion is the same in the solid compound as in the solution. After 
dropping in the nitrate, a rapid air stream was blown through the 
solution for 25 seconds into the testing vessel. The air stream was 
stopped and the ionization current immediately measured. The 
solution was then allowed to stand undisturbed for 10 minutes. 



220 RADIO-ACTIVE EMANATIONS [CH. 

In that* time the accumulation of the emanation again attained a 
practical maximum and again represented a steady state. The 
stream of air was blown through, as before, for 25 seconds, stopped 
and the current again measured. In both cases, the electrometer 
recorded a movement of 14'6 divisions per second. By blowing 
the same stream of air continuously through the solution the final 
current corresponded to 7 - 9 divisions per second or about one-half 
of that observed after the first rush. 

Thus the rate of production of emanation is the same in the 
solid nitrate as in the solution, although the emanating power, i.e. 
the rate of escape of the emanation, is over 600 times greater in 
the solution than in the solid. 

It seems probable that the rate of production of emanation 
by thorium, like the rate of production of Ur X and Th X, is inde- 
pendent of conditions. The changes of emanating power of the 
various compounds by moisture, heat, and solution must therefore 
be ascribed solely to an alteration in the rate of escape of the 
emanation into the surrounding gas and not to an alteration in 
the rate of its production in the compound. 

On this view, it is easy to see that slight changes in the mode 
of preparation of a thorium compound may produce large changes 
in emanating power. Such effects have been often observed, and 
must be ascribed to slight physical changes in the precipitate. 
The fact that the rate of production of the emanation is indepen- 
dent of the physical or chemical conditions of the thorium, in which 
it is produced, is thus in harmony with what had previously been 
observed for the radio-active products Ur X and Th X. 



Source of the Thorium Emanation. 

145. Some experiments of Rutherford and Soddy 1 will now 
be considered, which show that the thorium emanation is pro- 
duced, not directly by the thorium itself, but by the active 
product ThX. 

When the Th X, by precipitation with ammonia, is removed from 
a quantity of thorium nitrate, the precipitated thorium hydroxide 

1 Phil. Mag. Nov. 1902. 



VIII] RADIO-ACTIVE EMANATIONS 221 

does not at first possess appreciable emanating power. This loss 
of emanating power is not due, as in the case of the de-emanated 
oxide, to a retardation in the rate of escape of the emanation 
produced ; for the hydroxide, when dissolved in acid, still gives 
off no emanation. On the other hand, the solution, containing 
the Th X, possesses emanating power to a marked degree. 
On leaving the precipitated hydroxide and the Th X for some 
time, it is found that the Th X decreases in emanating power, 
while the hydroxide gradually regains its emanating power. After 
about a month's interval, the emanating power of the hydroxide 
has nearly reached a maximum, while the emanating power of 
the Th X has almost disappeared. 

The curves of decay and recovery of emanating power with 
time are found to be exactly the same as the curves of decay 
and recovery of activity of Th X and the precipitated hydroxide 
respectively, shown in Fig. 35. The emanating power of Th X, 
as well as its activity, falls to half value in four days, while the 
hydroxide regains half its final emanating power as well as half its 
lost activity in the same interval. 

It follows from these results that the emanating power of Th X 
is directly proportional to its activity, i.e. that the rate of produc- 
tion of emanating particles is always proportional to the number 
of a particles, projected from the Th X per second. The radiation 
from Th X thus accompanies the change of the Th X into the 
emanation. Since the emanation has chemical properties distinct 
from those of the Th X, and also a distinctive rate of decay, it 
cannot be regarded as a vapour of Th X, but it is a distinct 
chemical substance, produced by the changes occurring in Th X. 
On the view advanced in section 127, the atom of the emanation 
consists of the part of the atom of Th X left behind after the 
expulsion of one or more a particles. The atoms of the emana- 
tion are unstable, and in turn expel a particles. This projection 
of a particles constitutes the radiation from the emanation, which 
serves as a measure of the amount of emanation present. Since 
the activity of the emanation falls to half value in one minute 
while that of Th X falls to half value in four days, the emanation 
consists of atoms, which disintegrate at intervals nearly 6000 times 
shorter than do the atoms of Th X. 



222 RADIO-ACTIVE EMANATIONS [CH. 

Source of the Radium Emanation. 

146. No intermediate stage Radium X between radium 
and its emanation, corresponding to the Th X for thorium, has 
so far been observed. The emanation from radium is probably 
produced directly from that element. In this respect, the radium 
emanation holds the same position in regard to radium as Th X 
does to thorium, and its production from radium can be explained 
on exactly similar lines. 

Radiations from the Emanations. 

147. Special methods are necessary to examine the nature of 
the radiation from the emanations, for the radiations arise from 
the volume of the gas in which the emanations are distributed. 
Some experiments to examine the radiations from the thorium 
emanation were made by the writer in the following way. 

A highly emanating thorium compound wrapped in paper was 
placed inside a lead box B about 1 cm. deep, shown in Fig. 41. 
There was an opening cut in ^ TO Electrometer 

the top of the box, over which ' 

a very thin sheet of mica was 

waxed. The emanation rapidly ^j/-/ (V , 

diffused through the paper into ||g Emanation igl V 
the vessel, and after ten minutes ^"~ 

reached a state of radio-active Flg - 4L 

equilibrium. The penetrating power of the radiation from the 
emanation which passed through the thin mica window was 
examined by the electrical method in the usual way by adding 
screens of thin aluminium foil. The results are expressed in the 
following table : 

Thickness of mica window -0015 cm. 
Thickness of aluminium foil -00034 cm. 
Layers of foil Current 

100 

1 59 

2 30 

3 10 

4 3-2 



VIII] RADIO-ACTIVE EMANATIONS 223 

The greater proportion of the conductivity is thus due to 
a rays, as in the case of the radio-active elements. The amount 
of absorption of these a rays by aluminium foil is about the same 
as that of the rays from the active bodies. No direct comparison 
can be made, for the a. rays from the emanation show the charac- 
teristic property of increased rate of absorption with thickness 
of matter traversed. Before testing, the rays have been largely 
absorbed by the mica window, and the penetrating power has 
consequently decreased. 

No alteration in the radiation from the emanation was ob- 
served on placing an insulated wire inside the emanation vessel, 
and charging it to a high positive or negative potential. When 
a stream of air through the vessel carried away the emanation as 
fast as it was produced, the intensity of the radiation fell to a small 
fraction of its former value. 

N<> evidence of any rays in the radiations was found in 
these experiments, although a very small effect would have been 
detected. After standing some hours, however, & rays began to 
appear. These were due to the excited activity deposited on the 
walls of the vessel from the emanation, and not directly to the 
emanation itself. 

The radium emanation, like that of thorium, only gives rise to 
a. rays. This was tested in the following way 1 : 

A large amount of emanation was introduced into a cylinder 
made of sheet copper '005 cm. thick, which absorbed all the 
a rav^s but allowed the /3 and 7 rays, if present, to pass through 
with but little loss. The external radiation from the cylinder 
was determined at intervals, commencing about two minutes after 
the introduction of the emanation. The amount observed at first 
was extremely small, but increased rapidly and practically reached 
a maximum in three or four hours. Thus the radium emanation 
only gives a rays, the ft rays appearing as the excited activity is 
produced on the walls of the vessel. On sweeping out the emana- 
tion by a current of air, there was no immediately appreciable 
decrease of the radiation. This is another proof that the emanation 
does not give out any /9 rays. In a similar way it can be shown 

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



224 RADIO-ACTIVE EMANATIONS [CH. 

that the emanation does not give rise to 7 rays ; these rays always 
make their appearance at the same time as the /3 rays. 

The method of examination of the radiations from the 
emanations has been given in some detail, as the results are of 
considerable importance in the discussion, which will be given 
later in chapter x, of the connection between the changes oc- 
curring in radio-active products and the radiations they emit. 
There is no doubt that the emanations, apart from the excited 
activity to which they give rise, only give out a rays, consisting 
most probably of positively charged bodies projected with great 
velocity. 



Effect of Pressure on the rate of production of the Emanation. 

148. It has already been mentioned that the conductivity 
due to the thorium emanation is proportional to the pressure of 
the gas, pointing to the conclusion that the rate of production 
of the emanation is independent of the pressure, as well as of the 
nature of the surrounding gas. This result was directly confirmed 
with the apparatus of Fig. 41. When the pressure of the gas 
under the vessel was slowly reduced, the radiation, tested outside 
the window, increased to a limit, and then remained constant 
over a wide range of pressure. This increase, which was far more 
marked in air than in hydrogen, is due to the fact that the a rays 
from the emanation were partially absorbed in the gas inside the 
vessel when at atmospheric pressure. At pressures of the order 
of 1 millimetre of mercury the external radiation decreased, but 
experiment showed that this must be ascribed to a removal of the 
emanation by the pump, and not to a change in the rate of pro- 
duction. The thorium compounds very readily absorb water- vapour, 
which is slowly given off at low pressures, and in consequence 
some of the emanation is carried out of the vessel with the water- 
vapour. 

Curie and Debierne 1 found that both the amount of excited 
activity produced in a closed vessel containing active samples of 
radium, and also the time taken to reach a maximum value, were 

1 C. R. 133, p. 931, 1901. 



VIIl] RADIO-ACTIVE EMANATIONS 225 

independent of the pressure and nature of the gas. This was true 
in the case of a solution down to the pressure of the saturated 
vapour, and in the case of solid salts to very low pressures. When 
the pump was kept going at pressures of the order of '001 mm. of 
mercury, the amount of excited activity was much diminished. 
This was probably not due to any alteration of the rate of escape 
of the emanation, but to the removal of the emanation by the 
action of the pump as fast as it was formed. 

Since the amount of excited activity, when in a state of 
radio-active equilibrium, is a measure of the amount of emana- 
tion producing it, these results show that the amount of emanation 
present when the rate of production balances the rate of decay is 
independent of the pressure and nature of the gas. It was also 
found that the time taken to reach the point of radio-active equi- 
librium was independent of the size of the vessel or the amount 
of active matter present. These results show that the state of 
equilibrium cannot in any way be ascribed to the possession by the 
emanation of any appreciable vapour pressure ; for if such were the 
case, the time taken to reach the equilibrium value should depend 
on the size of the vessel and the amount of active matter present. 
The results are, however, in agreement with the view that the 
emanation is present in minute quantity in the tube, and that the 
equilibrium is governed purely by the radio-active constant X, the 
constant of decay of activity of the emanation. This has been seen 
to be the same under all conditions of concentration, pressure and 
temperature, and, provided the rate of supply of the emanation 
from the active compound is not changed, the time-rate of increase 
of activity to the equilibrium value will always be the same, 
whatever the size of the vessel or the nature and pressure of the 
surrounding gas. 

Chemical Nature of the Emanations. 

149. Earlier experiments. We shall now consider some 
experiments on the physical and chemical properties of the emana- 
tions themselves, without reference to the material producing them, 
in order to see if they possess any properties which identify them 
with any known kind of matter. 

R. R.-A. 15 



226 RADIO-ACTIVE EMANATIONS [CH. 

It was soon observed that the thorium emanation passed 
unchanged through acid solutions, and later the same result was 
shown to hold true in the case of both emanations for every 
reagent that was tried. Preliminary observations 1 showed that the 
thorium emanation, obtained in the usual way by passing air over 
thoria, passed unchanged in amount through a platinum tube 
heated electrically to the highest temperature obtainable. The 
tube was then filled with platinum-black, and the emanation passed 
through it in the cold, and with gradually increasing temperatures, 
until the limit was reached. In another experiment, the emana- 
tion was passed through a layer of red-hot lead-chromate in a 
glass tube. The current of air was replaced by a current of 
hydrogen, and the emanation was sent through red-hot magnesium- 
powder and red-hot palladium-black, and, by using a current of 
carbon dioxide, through red-hot zinc-dust. In every case the 
emanation passed through without sensible change in the amount. 
If anything, a slight increase occurred, owing to the time taken for 
the gas-current to pass through the tubes when hot being slightly 
less than when cold, the decay en route being consequently less. 
The only known gases capable of passing in unchanged amount 
through all the reagents employed are the recently discovered 
members of the argon family. 

But another possible interpretation might be put upon the 
results. If the emanation were the manifestation of a type of 
excited radio-activity on the surrounding atmosphere, then, since 
from the nature of the experiments it was necessary to employ in 
each case as the atmosphere, a gas not acted on by the reagent 
employed, the result obtained might be expected. Red-hot mag- 
nesium would not retain an emanation consisting of radio-active 
hydrogen, or red-hot zinc-dust, an emanation consisting of radio- 
active carbon dioxide. The incorrectness of this explanation was 
shown in the following way. Carbon dioxide was passed over 
thoria, then through a T-tube, where a current of air met and 
mixed with it, both passing on to the testing-cylinder. But 
between this and the T-tube a large soda-lime tube was intro- 
duced, and the current of gas was thus freed from its admixed 
carbon dioxide, before being tested in the cylinder for the emana- 

1 Rutherford and Soddy, Phil. Mag. Nov. 1902. 



VI II] RADIO-ACTIVE EMANATIONS 227 

tion. The amount of emanation found was quite unchanged, 
whether carbon dioxide was sent over thoria in the manner de- 
scribed, or whether, keeping the other arrangements as before, 
an equally rapid current of air was substituted for it. The theory 
that the emanation is an effect of the excited activity on the 
surrounding medium is thus excluded. 

Experiments of a similar kind on the radium emanation were 
made later. A steady stream of gas was passed through a radium 
chloride solution and then through the reagent to be employed, 
into a testing-vessel of small volume, so that any change in the 
amount of emanation passing through could readily be detected. 
The radium emanation, like that of thorium, passed unchanged in 
amount through every reagent used. 

Later experiments. In later experiments by Sir William 
Ramsay and Mr Soddy 1 , the emanation from radium was exposed to 
still more drastic treatment. The emanation in a glass tube was 
sparked for several hours with oxygen over alkali. The oxygen 
was then removed by ignited phosphorus and no visible residue was 
left. When, however, another gas was introduced, mixed with 
the minute amount of emanation in the tube and withdrawn, 
the activity of emanation was found to be unaltered. In another 
experiment, the emanation was introduced into a magnesium lime 
tube, which was heated for three hours at a red heat. The 
emanation was then removed and tested, but no diminution in its 
discharging power was observed. 

The emanations of thorium and radium thus withstand chemical 
treatment in a manner hitherto unobserved except in gases of the 
argon family. 

150. Ramsay and Soddy (loc. cit.) record an interesting 
experiment to illustrate the gaseous nature of the emanation. 
A large amount of the radium emanation was collected in a 
small glass tube. This tube phosphoresced brightly under the 
influence of the rays from the emanation. The passage of the 
emanation from point to point was observed in a darkened 
room by the luminosity excited in the glass. On opening the 
stop-cock connecting with the Topler pump, the slow flow through 

1 Proc. Roy. Soc. 72, p. 204, 1903. 

152 



228 RADIO-ACTIVE EMANATIONS [CH. 

the capillary tube was noticed, the rapid passage along the wider 
tubes, the delay in passing through a plug of phosphorous pent- 
oxide, and the rapid expansion into the reservoir of the pump. 
When compressed, the luminosity of the emanation increased, and 
became very bright as the small bubble containing the emanation 
was expelled through the fine capillary tube. 



Diffusion of the Emanations. 

151. It has been shown that the emanations of thorium and 
radium behave like radio-active gases, distributed in minute amount 
in the air or other gas in which they are tested. With the small 
quantities of active material so far investigated, the emanations 
have not yet been collected in sufficient amount to allow the 
examination of their spectrum or to detect them by the balance. 
Although the molecular weight of the emanations cannot yet be 
obtained by direct chemical methods, an indirect estimate of it 
can be made by determining the rate of their inter-diffusion into 
air or other gases. The coefficients of inter-diffusion of various 
gases have long been known, and the results show that the 
coefficient of diffusion of one gas into another is, for the simpler 
gases, approximately inversely proportional to the square root of 
the product of their molecular weights. If, therefore, the coefficient 
of diffusion of the emanation into air is found to have a value, 
lying between that of two known gases A and B, it is probable 
that the molecular weight of the emanation lies between that of 
A and B. 

Although the amount of emanation given off from radium is 
too small to be detected by volume 1 , the electrical conductivity 
produced by the emanation in the gas, with which it is mixed, 
is often very large, and offers a ready means of measuring the 
emanation present. 

Some experiments have been made by Miss Brooks and the 
writer 2 to determine the rate of the diffusion of the radium emana- 
tion into air, by a method similar to that employed by Loschmidt 3 

1 See, however, p. 313 (Feb. 1904). 

2 Rutherford and Miss Brooks, Trans. Roy. Soc. Canada 1901, Chem. News 1902. 

3 Sitzungsber. d. Wiener Akad. 61, n. p. 367, 1871. 



VIII] RADIO-ACTIVE EMANATIONS 229 

in 1871, in his investigations of the coefficient of inter-diffusion 
of gases. 

Fig. 42 shows the general arrangement. A long brass cylinder 
AB, of length 73 cms., and diameter 6 cms., was divided into two 




From 

Gasometer - 

Radium 



Fig. 42. 



equal parts by a moveable metal slide S. The ends of the cylinder 
were closed with ebonite stoppers. Two insulated brass rods, a 
and b, each half the length of the tube, passed through the ebonite 
stoppers and were supported centrally in the tube. The cylinder 
was insulated and connected with one pole of a battery of 300 
volts, the other pole of which was earthed. The central rods could 
be connected with a sensitive quadrant electrometer. The cylinder 
was covered with a thick layer of felt, and placed inside a metal 
box filled with cotton-wool in order to keep temperature con- 
ditions as steady as possible. 

In order to convey a sufficient quantity of emanation into 
the half-cylinder A, it was necessary to heat the radium slightly. 
The slide S was closed and the side tubes opened. A slow 
current of dry air from a gasometer was passed through a platinum 
tube, in which a small quantity of a radium compound was placed. 
The emanation was carried with the air into the cylinder A. When 
a sufficient quantity had been introduced, the stream of air was 
stopped. The side tubes were closed by fine capillary tubes. 
These prevented any appreciable loss of gas due to the diffusion, 
but served to keep the pressure of the gas inside A at the pressure 
of the outside air. The three entrance tubes into the cylinder, 
shown in the figure, were for the purpose of initially mixing the 
emanation and gas as uniformly as possible. 

After standing several hours to make temperature conditions 
steady, the slide was opened, and the emanation began to diffuse 



230 RADIO-ACTIVE EMANATIONS [CH. 

into the tube B. The current through the tubes A and B was 
measured at regular intervals by an electrometer, with a suitable 
capacity in parallel. Initially there is no current in B, but after 
the opening of the slide, the amount in A decreased and the 
amount in B steadily increased. After several hours the amount 
in each half is nearly the same, showing that the emanation is 
nearly uniformly diffused throughout the cylinder. 
It can readily be shown 1 that if 

K coefficient of diffusion of the emanation into air, 
t = duration of diffusion experiments in sees., 
a = total length of cylinder, 

Si = partial pressure of emanation in tube A at end of diffusion, 
$ 2 = partial pressure of emanation in tube B at end of diffusion, 
then 



*&*."* 



Now the values of S 1 and $ 2 are proportional to the saturation 
ionization currents due to the emanations in the two halves of the 
cylinder. From this equation K can be determined, if the relative 
values of Si and $ 2 are observed after diffusion has been in progress 
for a definite interval t. 

The determination of Si and $ 2 is complicated by the excited 
activity produced on the walls of the vessel. The ionization due 
to this must be subtracted from the total ionization observed in 
each half of the cylinder, for the excited activity is produced from 
the material composing the emanation, and is removed to the 
electrodes in an electric field. The ratio of the current due to 
excited activity to the current due to the emanation depends on 
the time of exposure to the emanation, and is only proportional to 
it for exposures of several hours. 

The method generally adopted in the experiments was to open 
the slide for a definite interval, ranging in the experiments from 
15 to 120 minutes. The slide was then closed and the currents 
in each half determined at once. The central rods, which had 

1 See Stefan, Sitzungsber. d. Wien. Akad. 63, n. p. 82, 1871. 



VIII] RADIO-ACTIVE EMANATIONS 231 

been kept negatively charged during the experiments, had most 
of the excited activity concentrated on their surfaces. These 
were removed, new rods substituted and the current immediately 
determined. The ratio of the currents in the half cylinders under 
these conditions was proportional to S^ and $ 2 , the amounts of 
emanation present in the two halves of the cylinder. 

The values of K, deduced from different values of t, were found 
to be in good agreement. In the earlier experiments the values 
of K were found to vary between '08 and '12. In some later 
experiments, where great care was taken to ensure that tempera- 
ture conditions were very constant, the values of K were found to 
vary between '07 and '09. The lower value '07 is most likely 
nearer the true value, as temperature disturbances tend to give 
too large a value of K. No certain differences were observed in 
the value of K whether the air was dry or damp, or whether an 
electric field was acting or not. 

152. Some experiments on the rate of diffusion of the radium 
emanation into air were made at a later date by P. Curie and Danne 1 . 
If the emanation is contained in a closed reservoir, it has been shown 
that its activity, which is a measure of the amount of emanation 
present, decreases according to an exponential law with the time. 
If the reservoir is put in communication with the outside air 
through a capillary tube, the emanation slowly diffuses out, and 
the amount of emanation in the reservoir is found to decrease 
according to the same law as before, but at a faster rate. Using 
tubes of different lengths and diameters, the rate of diffusion was 
found to obey the same laws as a gas. The value of K was found 
to be O'lOO. This is a slightly greater value of K than the lowest 
value 0'07 found by Rutherford and Miss Brooks. No mention 
is made by Curie and Danne of having taken any special precau- 
tions against temperature disturbances, and this may account for 
the higher value of K obtained by them. 

They also found that the emanation, like a gas, always divided 
itself between two reservoirs, put in connection with one another, 
in the proportion of their volumes. In one experiment one reser- 
voir was kept at a temperature of 10 C. and the other at 350 C. 

1 C. E. 136, p. 1314, 1903. 



232 



KADIO-ACTIVE EMANATIONS 



[CH. 



The emanation divided itself between the. two reservoirs in the 
same proportion as a gas under the same conditions. 

153. For the purpose of comparison, a few of the coefficients 
of interdiffusion of gases, compiled from Landolt and Bernstein's 
tables, are given below. 



Gas or vapour 


Coefficient of 
diffusion into air 


Molecular weight 


Water vapour 


0-198 


18 


Carbonic acid gas 


0-142 


44 


Alcohol vapour 


0-101 


46 


Ether vapour 


0-077 


74 


Radium emanation ... 


0-07 


? 



The tables, although not very satisfactory for the purpose of 
comparison, show that the coefficient of interdiffusion follows the 
inverse order of the molecular weights. The value of K for the 
radium emanation is slightly less than for ether vapour, of which 
the molecular weight is 74. We may thus conclude that the 
emanation is of greater molecular weight than 74. It seems 
likely that the emanation has a molecular weight somewhere in 
the neighbourhood of 100, and is probably greater than this, 
for the vapours of ether and alcohol have higher diffusion 
coefficients compared with carbonic acid than the theory would 
lead us to anticipate. Comparing the diffusion coefficients of the 
emanation and carbonic acid into air, the value of the molecular 
weight of the emanation should be about 176 if the result 
observed for the simple gases, viz. that the coefficient of diffusion 
is inversely proportional to the square root of the molecular 
weights, holds true in the present case. On the disintegration 
theory developed in chapter x, it is to be expected that the 
atomic weight of the emanation should be slightly less than 225, 
the atomic weight of radium. 

It is of interest to compare the value of K = '07 with the value 
of K determined by Townsend (section 37) for the gaseous ions 
produced in air at ordinary pressure and temperature, by Rontgen 
rays or by the radiations from active substances. Townsend found 
that the value of K in dry air was *028 for the positive ions 



VIII] 



RADIO-ACTIVE EMANATIONS 



233 



and '043 for the negative ions. The radium emanation thus 
diffuses more rapidly than the ions produced by its radiation in 
the gas, and behaves as if its mass were smaller than that of 
the ions produced in air, but considerably greater than that of 
the air molecules with which it is mixed. 

It is not possible to regard the emanation as a temporarily 
modified condition of the gas originally in contact with the active 
body. Under such conditions a much larger value of K would be 
expected. The evidence derived from the experiments on diffusion 
strongly supports the view that the emanation is a gas of heavy 
molecular weight. 

Diffusion of the Thorium Emanation. 

154. On account of the rapid decay of the activity of the 
thorium emanation, it is not possible to determine the value of K 
its coefficient of diffusion into air by the methods employed for the 
radium emanation. The value of K has been determined by the 
writer in the following way. A plate C, 
Fig. 43, covered with thorium hydroxide, was 
placed horizontally near the base of a long 
vertical brass cylinder P. The emanation 
released from the thorium compound diffuses 
upwards in the cylinder. 

Let p be the partial pressure of the emana- 
tion at a distance x from the source C. This 
will be approximately uniform over the cross 
section of the cylinder. From the general 
principles of diffusion we get the equation 

d?p dp 

da: 2 dt 

Fig. 43. 
The emanation is continuously breaking 

up and expelling a particles. The emanation-residue gains a posi- 
tive charge, and, in an electric field, is at once removed from the 
gas to the negative electrode. 

Since the activity of the emanation at any time is always 
proportional to the number of particles which have not broken up, 
and since the activity decays with the time according to an 




234 RADIO-ACTIVE EMANATIONS [CH. 

exponential law, p pjer M where p l is the value of p when t = 
and X is the radio-active constant of the emanation. 



, 

and 8=-^-; ; ; ; ;; . i ':;; i ;r Klli , 

Thus p=Ae~ Y/ ^' x +B. 

Since p = when x = oo , = 0. 
If p = p when x = 0, ^L = p . 

\/I 
Thus p=p e ' K - x . 

It was not found convenient in the experiments to determine 
the activity of the emanation along the cylinder, but an equivalent 
method was used which depends upon measuring the distribution 
of "excited activity," produced along a central rod AB, which is 
charged negatively. 

It will be shown later (section 167) that the amount of excited 
activity at any point is always proportional to the amount of 
emanation at that point. The distribution of " excited activity " 
along the central rod from the plate C upwards thus gives the 
variation of p for the emanation along the tube. 

In the experiments, the cylinder was filled with dry air at 
atmospheric pressure and was kept at a constant temperature. 
The central rod was charged negatively and exposed from one to 
two days in the presence of the emanation. The rod was then 
removed, and the distribution of the excited activity along it 
determined by the electric method. It was found that the amount 
of excited activity fell off with the distance x according to an 
exponential law, falling to half value in about 1*9 cms. This is in 
agreement with the above theory. 

Since the activity of the emanation falls to half value in 
1 minute, X = '0115. The value ^ = '09 was deduced from the 
average of a number of experiments. This is a slightly greater 
value than K = '07, obtained for the radium emanation, but the 
results show that the two emanations do not differ much from 
one another in molecular weight. 



VIII] RADIO-ACTIVE EMANATIONS 235 

Diffusion of the Emanation into Liquids. 

155. Experiments have been made by Wallstabe 1 on the 
coefficient of diffusion of the radium emanation into various liquids. 
The radium emanation was allowed to diffuse into a closed reservoir, 
containing a cylinder of the liquid under observation. The cylinder 
was provided with a tube and a stop-cock extending beyond the 
closed vessel, so that different layers of the liquid could be removed. 
The liquid was then placed in a closed testing vessel, where the 
ionization current due to the escape of the emanation from the 
liquid was observed to rise to a maximum after several hours, and 
then to decay. This maximum value of the current was taken as 
a measure of the amount of emanation absorbed in the liquid. 

The coefficient of diffusion K of the emanation into the liquid 
can be obtained from the same equation used to determine the 
diffusion of the thorium emanation into air, 

/T 
p=p e V,JT* 

where X is the constant of decay of activity of the radium emana- 
tion and x the depth of the layer of water from the surface. 

Putting a = A/ g , it was found that 

for water a = 1/6, 
for toluol a = *75. 

The value of X expressed in terms of a day as the unit of time 
is about 17. 

Thus the value of K for the diffusion of the radium emanation 

cm " 
into water = '066 , 

day 

The value of K found by Stefan 2 for the diffusion of carbon 

dioxide into water was T36 --, These results are thus in har- 

day 

mony with the conclusion drawn from the diffusion of the radium 
emanation into air, and show that the radium emanation behaves 
as a gas of high molecular weight. 

1 Phys. Zeit. 4, p. 721, 1903. 

2 Wien. Sitzungsber. 2, p. 371, 1878. 



236 RADIO-ACTIVE EMANATIONS [CH. 

Condensation of the Emanations. 

156. Condensation of the emanations. During an in- 
vestigation of the effect of physical and chemical agencies on 
the thorium emanation, Rutherford and Soddy 1 found that the 
emanation passed unchanged in amount through a white-hot 
platinum tube and through a tube cooled to the temperature 
of solid carbon dioxide. In later experiments, the effects of still 
lower temperatures were examined, and it was then found that at 
the temperature of liquid air both emanations were condensed 2 . 

If either emanation is conveyed by a slow stream of hydrogen, 
oxygen or air through a metal spiral immersed in liquid air, and 
placed in connection with a testing vessel as in Fig. 37, no trace 
of emanation escapes in the issuing gas. When the liquid air is 
removed and the spiral plunged into cotton-wool, several minutes 
elapse before any deflection of the electrometer needle is observed, 
and then the condensed emanation volatilizes rapidly, and the 
movement of the electrometer needle is very sudden, especially 
in the case of radium. With a fairly large amount of radium 
emanation, under the conditions mentioned, a very few seconds 
elapse after the first sign of movement before the electrometer 
needle indicates a deflection of several hundred divisions per 
second. It is not necessary in either case that the emanating 
compound should be retained in the gas stream. After the 
emanation is condensed in the spiral, the thorium or radium 
compound may be removed and the gas stream sent directly 
into the spiral. But in the case of thorium under these condi- 
tions, the effects observed are naturally small owing to the rapid 
loss of the activity of the emanation with time, which proceeds at 
the same rate at the temperature of liquid air as at ordinary 
temperatures. ' 

If a large amount of radium emanation is condensed in a glass 
U tube, the progress of the condensation can be followed by the 
eye, by means of the phosphorescence which the radiations excite 
in the glass. If the ends of the tube are sealed and the tempera- 
ture allowed to rise, the glow diffuses uniformly throughout the 

1 Phil. Mag. Nov. 1902. 

2 Phil. Mag. May 1903. 



VIII] 



RADIO-ACTIVE EMANATIONS 



237 



tube, and can be concentrated at any point to some extent by 
local cooling of the tube with liquid air. 

157. Experimental arrangements. A simple experimental 
arrangement to illustrate the condensation and volatilization of the 
emanation and some of its charac- 
teristic properties is shown in Fig. 
44. The emanation obtained by 
solution or heating, from a few milli- 
grams of radium bromide, is con- 
densed in the glass U tube T im- 
mersed in liquid air. This U tube 
is then put into connection with a 
larger glass tube V, in the upper 
part of which is placed a piece of 
zinc sulphide screen Z, and in the 
lower part of the tube a piece of the 
mineral willemite. The stop-cock 
A is closed and the U tube and 
the vessel V are partially exhausted 



w 



Fig. 44. 



by a pump through the stop-cock B. This lowering of the pressure 
causes a more rapid diffusion of the emanation when released. The 
emanation does not escape if the tube T is kept immersed in liquid 
air. The stop-cock B is then closed, and the liquid air removed. 
No luminosity of the screen or the willemite in the tube V is 
observed for several minutes, until the temperature of T rises 
above the point of volatilization of the emanation. The emana- 
tion is then rapidly carried into the vessel F, partly by expansion 
of the gas in the tube T with rising temperature, and partly by 
the process of diffusion. The screen Z and the willemite are 
caused to phosphoresce brilliantly under the influence of the rays 
from the emanation surrounding them. 

If the end of the vessel V is then plunged into liquid air, the 
emanation is again condensed in the lower end of the tube, and the 
willemite phosphoresces much more brightly than before. This is 
not due to an increase of the phosphorescence of willemite at the 
temperature of the liquid air, but to the effect of the rays from 
the emanation condensed around it. At the same time the lumin- 



238 RADIO-ACTIVE EMANATIONS [CH. 

osity of the zinc sulphide gradually diminishes, and practically 
disappears after several hours if the end of the tube is kept in 
the liquid air. If the tube is removed from the liquid air, 
the emanation again volatilizes and lights up the screen Z. The 
luminosity of the willemite returns to its original value after the 
lapse of several hours. This slow change of the luminosity of 
the zinc sulphide screen and of the willemite is due to the gradual 
decay of the "excited activity" produced by the emanation on 
the surface of all bodies exposed to its action (chapter vm). 
The luminosity of the screen is thus due partly to the radiation 
from the emanation and partly to the excited radiation caused 
by it. As soon as the emanation is removed from the upper 
to the lower part of the tube, the " excited " radiation gradually 
diminishes in the upper and increases in the lower part of the 
tube. 

The luminosity of the screen gradually diminishes with the 
time as the enclosed emanation loses its activity, but is still 
appreciable after several weeks interval. 

An apparatus of a similar character to illustrate the condensa- 
tion of the radium emanation has been described by P. Curie 1 . 

158. Determination of the temperature of condensa- 
tion. A detailed investigation was made by Rutherford and 
Soddy (loc. cit.) of the temperatures at which condensation and 
volatilization commenced for the two emanations. The experi- 
mental arrangement of the first method is shown clearly in Fig. 45. 
A slow constant stream of gas, entering at A, was passed through 
a copper spiral S, over 3 metres in length, immersed in a bath 
of liquid ethylene. The copper spiral was made to act as its 
own thermometer by determining its electrical resistance. The 
resistance temperature curve was obtained by observation of the 
resistances at 0, the boiling point of liquid ethylene 103'5, 
the solidification point of ethylene 169 and in liquid air. The 
temperature of the liquid air was deduced from the tables given 
by Baly for the boiling point of liquid air for different percentages 
of oxygen. The resistance temperature curve, for the particular 
spiral employed, was found to be nearly a straight line between 

1 Societe de Physique, 1903. 



VIIl] 



RADIO-ACTIVE EMANATIONS 



239 



and 192 C., cutting the temperature axis if produced nearly 
at the absolute zero. The resistance of the spiral, deduced from 



Ammeter 




To Earth 



Fig. 45. 



readings on an accurately calibrated Weston millivoltmeter, with 
a constant current through the spiral, was thus very approximately 
proportional to the absolute temperature. The liquid ethylene was 
kept vigorously stirred by an electric motor, and was cooled to any 
desirable temperature by surrounding the vessel with liquid air. 

The general method employed for the radium emanation was 
to pass a suitable amount of emanation, mixed with the gas to be 
employed, from the gas holder B into the spiral, cooled below the 
temperature of condensation. After the emanation was condensed 
in the spiral, a current of electrolytic hydrogen or oxygen was 
passed through the spiral. The temperature was allowed to 
rise gradually, and was noted at the instant when a deflection of 
the electrometer, due to the presence of emanation in the testing 
vessel T, was observed. The resistance, subject to a slight correc- 
tion due to the time taken for the emanation to be carried into 
the testing vessel, gave the temperature at which some of the 
emanation commenced to volatilize. The ionization current in 
the testing vessel rose rapidly to a maximum value, showing that, 
for a small increase of temperature, the whole of the radium 
emanation was volatilized. The following table gives an illustration 



240 



RADIO-ACTIVE EMANATIONS 



[CH. 



of the results obtained for a current of hydrogen of T38 cubic 
centimetres per second. 



Temperature 


Divisions per second 
of the electrometer 


-160 





-156 





- 154 '3 


1 


- 153-8 


21 


-152 -5 


24 



The following table shows the results obtained for different 
currents of hydrogen and oxygen. 





Current of Gas 


Z\ 


T* 


Hydrogen . . . 


25 c.c. per sec. 


-151-3 


-150 


> 


32 


- 1537 


-151 


,, 


92 


-152 


-151 


,, 


1-38 


-154 


-153 


)5 ... 


2-3 


- 162-5 


-162 


Oxygen 


34 


-152-5 


-151-5 


5? 


58 


-155 


-153 



The temperature T l in the above table gives the temperature 
of initial volatilization, jT 2 the temperature for which half of the 
condensed emanation had been released. For slow currents of 
hydrogen and oxygen, the values of 7\ and T^ are in good agree- 
ment. For a stream of gas as rapid as 2*3 cubic centimetres per 
second the value of T l is much lower. Such a result is to be 
expected, for, in too rapid a stream, the gas is not cooled to the 
temperature of the spiral, and, in consequence, the inside surface 
of the spiral is above the mean temperature, and some of the 
emanation escapes at a temperature apparently much lower. In 
the case of oxygen, this effect appears for a gas stream of 0'58 cubic 
centimetres per second. 

In the experiments on the thorium emanation, a slightly dif- 
ferent method was necessary, on account of the rapid loss of its 
activity. The steady stream of gas was passed over the thorium 



VIII] 



RADIO-ACTIVE EMANATIONS 



241 



compound, and the temperature was observed at the instant an 
appreciable movement of the electrometer was observed. This 
gave the temperature at which a small fraction of the thorium 
emanation escaped condensation, and not the value T^ observed 
for the radium emanation, which gave the temperature for which 
a small fraction of the previously condensed emanation was 
volatilized. 

The following table illustrates the results obtained. 





Current of Gas 


Temperature 


Hydrogen 

n 

Oxygen 


71 c.c. per sec. 
1-38 
58 


-155C. 
-159 C. 
-155C. 



On comparing these results with the values obtained for the 
radium emanation, it will be observed that with equal gas streams 
the temperatures are nearly the same. 

A closer examination of the thorium emanation showed, how- 
ever, that this apparent agreement was only accidental, and that 
there was, in reality, a very marked difference in the effect of tem- 
perature in the two emanations. It was found experimentally that 
the radium emanation was condensed very near the temperature 
at which volatilization commenced, and that the points of conden- 
sation and volatilization were fairly sharply defined. 

On the other hand, the thorium emanation required a range 
of over 30 C. after condensation had started in order to ensure 
complete condensation. Fig. 46 is an example of the results 
obtained with a steady gas stream of 1*38 c.c. per sec. of oxygen. 
The ordinates represent the percentage proportion of the emana- 
tion uncondensed at different temperatures. It will be observed 
that condensation commences about 120, and very little of the 
emanation escapes condensation at 155 C. 

To investigate this difference of behaviour in the two emana- 
tions, a static method was employed, which allowed determinations 
of the two emanations to be made under comparable conditions. 
The emanation, mixed with a small amount of the gas to be used, 
was introduced into the cool spiral, which had previously been 

16 



R. R.-A. 



242 



RADIO-ACTIVE EMANATIONS 



[CH. 



exhausted by means of a mercury pump. The amount of emana- 
tion remaining uncondensed after definite intervals was rapidly 
removed by means of the pump, and was carried with a constant 
auxiliary stream of gas into the testing vessel. 



! 

t 



90 




^N 


\ 












\ 








70 

J 60 

1 

| 50 

* 

: 40 
30 
20 
10 







\ 












\ 


\ 












\ 












\ 














\ 






Condensation Curve 
Thorium Emanation 




\ 








\ 


V 












\ 



-100 



-160 



-110 -120 -130 -140 -150 

Temperature Centigrade 

Fig. 46. 

Tested in this way, it was found that the volatilization point 
of the radium emanation was very nearly the same as that ob- 
tained by the blowing method, viz. 150 C. With thorium, on 
the other hand, the condensation started at about 120 C., and, 
as in the blowing method, continued over a range of about 30 C. 
The proportion of the emanation condensed at any temperature 
was found to depend on a variety of conditions, although the point 
at which condensation commenced, viz. 120 C., was about the 
same in each case. It depended on the pressure and nature of the 
gas, on the concentration of the emanation, and on the time for 
which it was left in the spiral. For a given temperature a greater 
proportion of the emanation was condensed, the lower the pressure 
and the longer the time it was left in the spiral. Under the 
same conditions, the emanation was more rapidly condensed in 
hydrogen than in oxygen. 



VIII] RADIO-ACTIVE EMANATIONS 243 

159. Thus there is no doubt that the thorium emanation 
begins to condense at a temperature higher than that at which 
the radium emanation condenses. The explanation of the pecu- 
liar behaviour of the thorium emanation is clear when the small 
number of emanation particles present in the gas are taken into 
consideration. It has been shown that both emanations give 
out only a rays. It is probable that the a particles from the 
two emanations are similar in character and produce about the 
same number of ions in their passage through the gas. The 
number of ions produced by each a particle before its energy 
is dissipated is probably about 70,000. (See section 104.) 

Now in the experiment the electrometer readily measured 
a current of 10~ 3 electrostatic units. Taking the charge on an ion 
as 3*4 x 10~ 10 electrostatic units, this corresponds to a production in 
the testing vessel of about 3 x 10 6 ions per sec., which would be 
produced by about 40 expelled a. particles per second. Each 
radiating particle cannot expel less than one a particle and may 
expel more, but it is likely that the number expelled by an atom 
of the thorium emanation is not greatly different from the number 
by an atom of the radium emanation. 

In section 124 it has been shown that, according to the law of 
decay, \N particles change per second when N are present. Thus 
to produce 40 a particles, \N cannot be greater than 40. Since for 
the thorium emanation X is 1/87, it follows that N cannot be greater 
than 3500. The electrometer thus detected the presence of 3500 
particles of the thorium emanation, and since in the static method 
the volume of the condensing spiral was about 15 c.c., this corre- 
sponds to a concentration of about 230 particles per c.c. An 
ordinary gas at atmospheric pressure and temperature probably 
contains about 3'6 x 10 19 molecules per c.c. Thus the emanation 
would have been detected on the spiral if it possessed a partial 
pressure of less than 10~ 17 of an atmosphere. 

It is thus not surprising that the condensation point of the 
thorium emanation is not sharply defined. It is rather a matter 
of remark that condensation should occur so readily with so sparse 
a distribution of emanation particles in the gas ; for, in . order 
that condensation may take place, it is probable that the particles 
must approach within one another's sphere of influence. 

162 



244 RADIO-ACTIVE EMANATIONS [CH. 

Now in the case of the radium emanation, the rate of decay 
is about 5000 times slower than that of the thorium emanation, 
and consequently the actual number of particles that must be 
present to produce the same number of rays per second in the two 
cases must be about 5000 times greater in the case of radium 
than in the case of thorium. This conclusion involves only the 
assumption that the same number of rays is produced by a 
particle of emanation in each case, and that the expelled particles 
produce in their passage through the gas the same number of 
ions. The number of particles present, in order to be detected 
by the electrometer, in this experiment, must therefore have 
been about 5000 x 3500, i.e. about 2 x 10 7 . The difference of 
behaviour in the two cases is well explained by the view 
that, for equal electrical effects, the number of radium emana- 
tion particles must be far larger than the number of thorium 
emanation particles. It is to be expected that the probability 
of the particles coming into each other's sphere of influence will 
increase very rapidly as the concentration of the particles in- 
creases, and that, in the case of the radium emanation, once the 
temperature of condensation is attained, all but a small proportion 
of the total number of particles present will condense in a very 
short time. In the case of the thorium emanation, however, the 
temperature might be far below that of condensation, and yet 
a considerable portion remain uncondensed for comparatively long 
intervals. On this view the experimental results obtained are 
^exactly what is to be expected. A greater proportion "condenses, 
Ibhe longer the time allowed for condensation under the same con- 
ditions. The condensation occurs more rapidly in hydrogen than 
in oxygen, as the diffusion is greater in the former gas. For the 
same reason the condensation occurs faster the lower the pressure 
of the gas present. Finally, when the emanation is carried by 
a steady gas stream, a smaller proportion condenses than in the 
other cases, because the concentration of emanation particles per 
unit volume of gas is less in these conditions. 

It is possible that the condensation of the emanations may not 
occur in the gas itself but at the surface of the containing vessel. 
Accurate observations of the temperature of condensation have so 
far only been made in a copper spiral, but condensation certainly 



VIII] EADIO-ACTIVE EMANATIONS 245 

occurs in tubes of lead or glass at about the same temperature as 
in tubes of copper. 

160. In experiments that were made by the static method 
with a very large quantity of radium emanation, a slight amount 
of escape of the condensed emanation was observed several degrees 
below the temperature at which most of the emanation was released. 
This is to be expected, since under such conditions the electrometer 
is able to detect a very minute proportion of the whole quantity of 
the emanation condensed. 

Special experiments, with a large quantity of emanation, that 
were made with the spiral immersed in a bath of rapidly boiling 
nitric oxide, showed this effect very clearly. For example, the con- 
densed emanation began to volatilize at -155C. In 4 minutes 
the temperature had risen to 153'5, and the amount volatilized 
was four times as great as at 155. In the next 5 minutes the 
temperature had increased to 152'3 and practically the whole 
quantity, which was at least fifty times the amount at the 
temperature of 153*5, had volatilized. 

It thus seems probable that, if the temperature were kept 
steady at the point at which volatilization was first observed, 
and the released emanation removed at intervals, the whole of 
the emanation would in course of time be liberated at that tem- 
perature. These results also point to the probability that the 
condensed emanation possesses a true vapour pressure, but great 
refinements in experimental methods would be necessary before 
such a conclusion could be definitely established. 

The true temperature of condensation of the thorium emana- 
tion is probably about 120 C., and that of radium about 
150 C. Thus there is no doubt that the two emanations are 
quite distinct from each other in this respect, and also with regard 
to their radio-activity, although they both possess the property 
of chemical inertness. These results on the temperatures of 
condensation do not allow us to make any comparison of the 
condensation points of the emanations with those of known gases, 
since the lowering of the condensation point of gases with diminu- 
tion of pressure has not been studied at such extremely minute 
pressures. 



246 RADIO-ACTIVE EMANATIONS [CH. 

161. It was found 1 that the activity of the thorium emanation 
decayed at the same rate, when condensed in the spiral at the 
temperature of liquid air, as at ordinary temperatures. This is in 
accord with results of a similar kind obtained by P. Curie for the 
radium emanation (section 136), and shows that the value of the 
radio-active constant is unaffected by wide variations of tempera- 
ture. 

Amount of Emanation from Radium and Thorium. 

162. It has been shown in section 104, that 1 gram of radium 
emits about 10 11 a particles per second. Since the activity due to 
the emanation stored up in radium, when in a state of radio-active 
equilibrium, is about one quarter of the whole, the number of a 
particles projected per second by the emanation from 1 gram of 
radium is about 2'5 x 10 10 . It has been shown in section 143, 
that 463,000 times the amount of emanation produced per second 
is stored up in the radium. But in a state of radio-active equi- 
librium, the number of emanation particles breaking up per second 
is equal to the number produced per second. Assuming that each 
emanation particle in breaking up expels one a particle, it follows 
that the number of emanation particles, present in 1 gram of 
radium in radio-active equilibrium, is 463,000 x 2'5 x 10 10 , i.e. 
1*2 x 10 16 . Taking the number of hydrogen molecules in 1 c.c. of gas 
at atmospheric pressure and temperature as 3'6 x 10 19 (section 39), 
the volume of the emanation from 1 gram of radium is 3'3 x 10~ 4 
cubic centimetre at atmospheric pressure and temperature. Quite 
independently of any method of calculation, it is evident that the 
volume of the emanation is very small, for attempts made to 
detect its presence by its volume have so far failed. It is probable, 
however, from the above calculation, that, when larger quantities 
of radium are available for experiment, the emanation will be 
collected in volume sufficiently large to measure. 

In the case of thorium, the maximum quantity of emanation to 

be obtained from 1 gram of the solid is very minute, both on account 

of the small activity of thorium and of the rapid break up of the 

emanation after its production. Since the amount of emanation, 

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



VIIl] RADIO-ACTIVE EMANATIONS 247 

stored in a non-emanating thorium compound, is only 87 times 
the rate of production, while in radium it is 463,000 times, and the 
rate of production of the emanation by radium is about 1 million 
times faster than by thorium, it follows that the amount of emana- 
tion to be obtained from 1 gram of thorium is not greater than 
10~ 10 of the amount from an equal weight of radium, i.e. its volume 
is not greater than 5 x 10~ 14 c.c. at the ordinary pressure and 
temperature. Even with large quantities of thorium, the amount 
of emanation is too small ever to be detected by its volume. 

Heal Emission of the Radium Emanation. 

163. It has been shown in section 106, that the radium 
emanation emitsjieatjat a rapid rate and is responsible for about 
70 : , of the heating effect of radium. The emanation from 1 gram 
of radium, together wiflT the heat effect due to the excited activity 
on the walls of the containing vessel, thus gives rise to an emission 
of heat of about 70 gram-calories per hour. This rate of heat 
emission decays according to an exponential law with the time, 
decreasing to half value in about four days. The total quantity of 

heat given out during the life of the emanation is , where q is 

X 

the initial rate of heat production and X is the radio-active 
constant of the emanation. Since the value of X expressed in 
hours (section 136) is 1/128 and q is 70, the total quantity of 
heat emitted from the emanation from 1 gram of radium is about 
10,000 gram-calories. But the volume of this emanation is about 
3'3 x 10~ 4 c.c. Thus the total heat emitted from one cubic centi- 
metre of the emanation at standard pressure and temperature 
would be about 3 x 10 7 gram-calories. The initial rate of emission 
of heat is 2 x 10 5 gram-calories per hour or 60 gram-calories per 
second. This rapid emission of heat would be sufficient to heat 
to redness if not to melt down the tube which contains the 
emanation. 

If the atomic weight of the emanation is taken to be about 200, 
it can be calculated that 1 pound weight of the emanation would 
initially radiate heat at the rate of about 8000 horse-power, and in 
the whole course of its heat emission would radiate an amount of 



248 RADIO-ACTIVE EMANATIONS [CH. 

energy corresponding to 40,000 horse-power days. In order to 
obtain such an amount of emanation about 70 tons of radium 
would be required. 



Summary of Results. 

164. The investigations into the nature of the radio-active 
emanations have thus led to the folio wing conclusions: The radio- 
elements thorium and radium continuously produce from themselves 
radio-active emanations at a rate which is constant under all con- 
ditions. In some cases, the emanations continuously diffuse from 
the radio-active compounds into the surrounding gas; in other 
cases, the emanations are unable to escape from the material in 
which they are produced but are occluded, and can only be released 
by the action of solution or heat. 

The emanations possess all the properties of radio-active gases. 
They diffuse through gases, liquids, and porous substances, and can 
be occluded in some solids. Under varying conditions of pressure, 
volume, and temperature, the emanations distribute themselves in 
the- same way and according to the same laws as does a gas. 

The emanations possess the important property of condensation 
under the influence of extreme cold, and by that means can be 
separated from the gases with which they are mixed. The radia- 
tion from the emanation is material in nature, and consists of a 
stream of positively charged particles projected with great velocity. 

Taking all these properties into consideration, it is difficult to 
avoid the conclusion that the emanations are material and exist 
in the gaseous state. The emanations possess the property of 
chemical inertness, and in this respect resemble the gases of the 
argon family. The emanations are produced in minute amount; 
sufficient quantity has not yet been obtained to examine by 
ordinary chemical methods. With regard to their rates of dif- 
fusion, the emanations of both thorium and radium behave like 
gases of high molecular weight. 

These emanations have been detected and their properties 
investigated by the property they possess of emitting radiations of 
a special character. These radiations consist entirely of a rays, 
i.e. particles, projected with great velocity, which carry a positive 



VIIl] RADIO-ACTIVE EMANATIONS 249 

charge and have a mass about twice that of the hydrogen atom. 
The emanations do not possess the property of permanently radiat- 
ing, but the intensity of the radiations diminishes according to an 
exponential law with the time, falling to half value, in the case of 
thorium in one minute, and in case of radium in about four days. 
The law of decay of activity does not seem to be influenced by 
any physical or chemical agency. 

The emanation particles gradually break up, each particle as it 
breaks up expelling a charged body. The emanation after it has 
radiated ceases to exist as such, but is transformed into a new 
kind of matter, which is deposited on the surface of bodies and 
gives rise to the phenomena of excited activity. This last property, 
and the connection of the emanation with it, is discussed in detail 
in the succeeding chapter. 



CHAPTER IX. 

EXCITED RADIO-ACTIVITY. 

165. Excited radio-activity. One of the most interesting 
and remarkable properties of thorium and radium is their power of 
" exciting " or " inducing " temporary activity on all bodies in their 
neighbourhood. A substance which has been exposed for some 
time in the presence of radium or thorium, behaves as if its surface 
were covered with an invisible deposit of intensely radio-active 
material. The " excited" body emits radiations capable of affecting 
a photographic plate and of ionizing a gas. Unlike the radio- 
elements themselves however, the activity of the body does not 
remain constant after it has been removed from the influence of 
the exciting active material, but decays with the time. The 
activity lasts for several hours when due to radium and several 
days when due to thorium. 

This property was first observed by M. and Mme Curie 1 for 
radium, and independently by the writer 2 for thorium 3 . 

1 C. R. 129, p. 714, 1899. 2 Phil, Mag. Jan. and Feb. 1900. 

3 As regards date of publication, the priority of the discovery of "excited 
activity " belongs to M. and Mme Curie. A short paper on this subject, entitled 
" Sur la radioactivite provoquee. par les rayons de Becquerel," was communicated 
by them to the Comptes Rendus, Nov. 6, 1899. A shor*t note was added to the 
paper by Becquerel in which the phenomena of excited activity were ascribed to a 
type of phosphorescence. On my part, I had simultaneously discovered the 
emission of an emanation from thorium compounds and the excited activity 
produced by it, in July, 1899. I, however, delayed publication in order to work 
out in some detail the properties of the emanation and of the excited activity and 
the connection between them. The results were published in two papers in the 
Philosophical Magazine (Jan. and Feb. 1900) entitled " A radio-active substance 
emitted from thorium compounds" and " Kadio-activity produced in substances by 
the action of thorium compounds." 



CH. IX] 



EXCITED RADIO-ACTIVITY 



251 



If any solid body is placed inside a closed vessel containing an 
emanating compound of thorium or radium, its surface becomes 
radio-active. For thorium compounds the amount of excited ac- 
tivity on a body is in general greater the nearer it is to the active 
material. In the case of radium, however, provided the body has 
been exposed for several hours, the amount of excited activity is to 
a large extent independent of the position of the body in the vessel 
containing the active material. Bodies are made active whether 
exposed directly to the action of the radio-active substance or 
screened from the action of the direct rays. This has been clearly 
shown in some experiments of P. Curie. A small open vessel a 
(Fig. 47) containing a solution of radium is placed inside a larger 
closed vessel V. 




Fig. 47. 

Plates A, B, C, D, E are placed in various positions in the 
enclosure. After exposure for a day, the plates after removal are 
found to be radio-active even in positions completely shielded from 
the action of the direct rays. For example, the plate D shielded 
from the direct radiation by the lead plate P is as active as the 
plate E, exposed to the direct radiation. The amount of activity 
produced in a given time on a plate of given area in a definite 
position is independent of the material of the plate. Plates of 
mica, copper, cardboard, ebonite, all show equal amounts of activity. 
The amount of activity depends on the area of the plate and on 



252 



EXCITED RADIO-ACTIVITY 



[CH. 



the amount of free space in its neighbourhood. Excited radio- 
activity is also produced in water if exposed to the action of an 
emanating compound. 

166. Concentration of excited radio-activity on the 
negative electrode. When thorium or radium is placed in a 
closed vessel, the whole interior surface becomes strongly active. 
In a strong electric field, on the other hand, the writer found that 
the activity was confined entirely to the negative electrode. By 
suitable arrangements, the whole of the excited activity, which 
was previously distributed over the surface of the vessel, can be 
concentrated on a small negative electrode placed inside the vessel. 
An experimental arrangement for this purpose is shown in Fig. 48. 




The metal vessel V containing a large amount of thoria is con- 
nected to the positive pole of a battery of about 300 volts. The 
wire AB to be made active is fastened to a stouter rod BC, passing 
through an ebonite cork inside a short cylinder D, fixed in the side 
of the vessel. This rod is connected with the negative pole of the 
battery. In this way the wire AB is the only conductor exposed 
in the field with a negative charge, and it is found that the whole 
of the excited activity is concentrated upon it. 

In this way it is possible to make a short thin metal wire over 
10,000 times as active per unit surface as the thoria from which 
the excited activity is derived. In the same way, the excited 
activity due to radium can be concentrated mainly on the negative 



IX] EXCITED RADIO-ACTIVITY 253 

electrode. In the case of thorium, if the central wire is charged 
positively, it shows no appreciable activity. With radium, however, 
a positively charged body becomes slightly active. In most cases, 
the amount of activity produced on the positive electrode is not 
more than 5% f ^ ne corresponding amount when the body is 
negatively charged. For both thorium and radium, the amount of 
excited activity on electrodes of the same size is independent of 
their material. 

All metals are made active to equal extents for equal times of 
exposure. When no electric field is acting, the same amount 
of activity is produced on insulators like mica and glass as on 
conductors of equal dimensions. 

167. Connection between the emanations and excited 
activity. An examination of the conditions under which excited 
activity is produced shows that there is a very close connection 
between the emanation and the excited activity. If a thorium 
compound is covered with several sheets of paper, which cut off the 
a rays but allow the emanation to pass through, excited activity is 
still produced in the space above it. If a thin sheet of mica is 
waxed down over the active material, thus preventing the escape of 
the emanation, no excited activity is produced outside it. Uranium 
and polonium which do not give off an emanation are not able to 
produce excited activity on bodies. Not only is the presence of 
the emanation necessary to cause excited activity, but the amount 
of excited activity is always proportional to the amount of emana- 
tion present. For example, de-emanated thoria produces very 
little excited activity compared with ordinary thoria. In all cases 
the amount of excited activity produced is proportional to the 
emanating power. The emanation when passing through an 
electric field loses its property of exciting activity at the same 
rate as the radiating power diminishes. This was shown by the 
following experiment. 

A slow constant current of air from a gasometer, freed from 
dust by its passage through cotton-wool, passed through a rectangu- 
lar wooden tube 70 cms. long. Four equal insulated metal plates 
A, B, C, D, were placed at regular intervals along the tube. The 
positive pole of a battery of 300 volts was connected to a metal 



254 



EXCITED RADIO-ACTIVITY 



[CH. 



plate placed in the bottom of the tube, while the negative pole 
was connected with the four plates. A mass of thoria was placed 
in the bottom of the tube under the plate A , and the current due 
to the emanation determined at each of the four plates. After 
passing a current of air of 0'2 cm. per second, for 7 hours along the 
tube, the plates were removed and the amount of excited activity 
produced on them was tested by the electric method. The follow- 
ing results were obtained. 





Eelative current 


Eelative excited 




due to emanation 


activity 


Plate A ... 


1 


1 


B ... 


55 


43 


,, o ... 


18 


16 


D ... 


072 


061 



Within the errors of measurement, the amount of excited 
activity is thus proportional to the radiation from the emanation, 
i.e. to the amount of emanation present. The same considerations 
hold for the radium emanation. The emanation in this case, on 
account of the slow loss of its activity, can be stored mixed with 
air for long periods in a gasometer, and its effects tested quite 
independently of the active matter from which it is produced. 
The ionization current due to the excited activity produced by the 
emanation is always proportional to the current due to the emana- 
tion for the period of one month or more that its activity is large 
enough to be conveniently measured by an electrometer. 

If at any time during the interval, some of the emanation is 
removed and introduced into a new testing vessel, the ionization 
current will immediately commence to increase, rising in the course 
of four or five hours to about twice its original value. This increase 
of the current is due to the excited activity produced on the walls 
of the containing vessel. On blowing out the emanation, the 
excited activity is left behind, and at once begins to decay. 
Whatever its age, the emanation still possesses the property of 
causing excited activity, and in amount always proportional to its 
activity, i.e. to the amount of emanation present. 

These results show that the power of exciting activity on 



IX] EXCITED RADIO-ACTIVITY 255 

inactive substances is a property of the radio-active emanations, 
and is proportional to the amount of emanation present. 

The phenomenon of excited activity cannot be ascribed to a 
type of phosphorescence produced by the rays from the emanation 
on bodies ; for it has been shown that the activity can be concen- 
trated on the negative electrode in a strong electric field, even if 
the electrode is shielded from the direct radiation from the active 
substance which gives off the emanation. The amount of excited 
activity does not seem in any way connected with the ionization 
produced by the emanation in the gas with which it is mixed. 
For example, if a closed vessel is constructed with two large 
parallel insulated metal plates on the lower of which a layer of 
thoria is spread, the amount of the excited activity on the upper 
plate when charged negatively, is independent of the distance 
between the plates when that distance is varied from 1 millimetre 
to 2 centimetres. This experiment shows that the amount of 
excited activity depends only on the amount of emanation, emitted 
from the thoria; for the ionization produced with a distance of 
2 centimetres between the plates is about ten times as great as 
with a distance of 1 millimetre. 

168. If a platinum wire is made active by exposure to the 
emanation of thoria, its activity 1 can be removed by treating the 
wire with certain acids. For example, the activity is not much 
altered by immersing the wire in hot or cold water or nitric acid, 
but more than 80% of it is removed by dilute or concentrated 
solutions of sulphuric or hydrochloric acid. The activity has not 
been destroyed by this treatment but is manifested in the solution. 
If the solution is evaporated, the activity remains behind on the 
dish. 

These results show that the excited activity is due to a deposit 
on the surface of bodies of radio-active matter which has definite 
properties as regards solution in acids. This active matter is 
dissolved in some acids, but, when the solvent is evaporated, the 
active matter is left behind. This active matter is deposited on 
the surface of bodies, for it can be partly removed by rubbing the 
body with a cloth, and almost completely by scouring the plate 

1 Rutherford, Phil. Mag. Feb. 1900. 



256 EXCITED RADIO-ACTIVITY [CH. 

with sand or emery paper. The amount of active matter deposited 
is extremely small, for no difference of weight has been detected 
in a platinum wire when made extremely active. On examining 
the wire under a microscope, no trace of foreign matter is observed. 
It follows from these results that the matter which causes excited 
activity is many thousand times more active, weight for weight, 
than radium itself. 

It is convenient to have a definite name for this radio-active 
matter, for the term " excited activity " only refers to the radiation 
from the active matter and not to the matter itself. Since the 
matter which produces the phenomena of excited radio-activity is 
derived from the emanation of thorium and of radium, the name 
emanation X will be given to it. This is chosen from analogy to the 
active products Ur X and Th X which are continuously produced 
from uranium and thorium respectively. The emanation X from 
thorium is different in chemical and other properties from the 
emanation X from radium. For example, each type of matter has 
a distinctive rate of decay of activity, as well as some differences 
in solubility by acids. 

On the view developed in section 127, the emanation X is the 
residue left behind from each atom of the emanation of thorium 
or of radium after one or more a particles have been expelled. The 
emanation X is an unstable substance, and its atoms again break 
up, giving rise to "excited activity," i.e. to the radiation from 
" emanation X." 

The emanation X is quite distinct in chemical and physical 
properties from the emanation which produces it. For example, 
emanation X behaves as a solid, which is deposited on the surface 
of bodies, while the emanation exists in the gaseous state. The 
emanation is insoluble in hydrochloric or sulphuric acids, while 
emanation X is readily soluble in both. 

169. Decay of the excited activity produced by thorium. 

The excited activity produced in a body after a long exposure to 
the emanations of thorium, decays in an exponential law with the 
time, falling to half value in about 11 hours. The following table 
shows the rate of decay of the excited activity produced on a brass 
rod. 



IX] 



EXCITED RADIO-ACTIVITY 



257 



Time in hours 


7-9 
11-8 
23-4 
29-2 
32-6 
49-2 
62-1 
71-4 



Current 

100 
64 
47-4 
19-6 
13-8 
10-3 
3-7 
1-86 
0-86 



The results are shown graphically in Fig. 49, Curve A. 



100 




40 60 8O 100 
Time in Hours 
Fig. 49. 

The intensity of the radiation / after any time t is given by 
= e~ u where \ is the radio-active constant. 

The rate of decay of excited activity, like that of the activity of 
other radio-active products, is not appreciably affected by change of 
conditions. The rate of decay is independent of the concentration of 
the excited activity, and of the material of the body in which it is 
produced. It is independent also of the nature and pressure of the 



R. R.-A. 



17 



258 EXCITED KADIO-ACTIVITY [CH 

gas in which it decays. The rate of decay is unchanged whether 
the excited activity is produced on the body with or without an 
electric field. 

The amount of excited activity produced on a body increases 
at first with the time, but reaches a maximum after an exposure 
of several days. An example of the results is given in the following 
table. In this experiment, a rod was made the cathode in a closed 
vessel containing thoria. It was removed at intervals for the short 
time necessary to test its activity and then replaced. 

Time in hours Current 

1-58 6'3 

3'25 10-5 

5-83 29 

9-83 40 

14-00 59 

23-41 77 

29-83 83 

47-00 90 

72-50 95 

96-00 100 

These results are shown graphically in Curve B, Fig. 49. It is 
seen that the decay and recovery curves may be represented 
approximately by the following equations. 

For the decay curve A, -=- = e~ xt . 

^o 

For the recovery curve B, ^ = 1 e~^. 

J-Q 

The two curves are thus complementary to one another ; they 
are connected in the same way as the decay and recovery curves of 
Ur X, and are susceptible of a similar explanation. 

The amount of excited radio-activity reaches a maximum value 
when the rate of supply of fresh radio-active particles balances the 
rate of change of those already deposited. 

170. Excited radio-activity produced by a short ex- 
posure. The initial portion of the recovery curve B, Fig. 49, is 
not accurately represented by the above equation. The activity 
for the first few hours increases more slowly than would be 



IX] EXCITED RADIO-ACTIVITY 259 

expected from the equation. This result, however, is completely 
explained in the light of later results. The writer 1 found that, for 
a short exposure of a body to the thorium emanation, the excited 
activity upon it after removal, instead of at once decaying at the 
normal rate, increased for several hours. In some cases the activity 
of the body increased three to four times its original value in the 
course of a few hours and then decayed with the time. 

Some of the results obtained are shown in the following tables. 
Table I. is for a platinum wire exposed as cathode for 15 minutes; 
Table II. for aluminium foil with 41 minutes' exposure to the 
emanation. About 5 minutes elapsed between removal and the 
first observation. 

TABLE I. TABLE II. 

Time Current Time Current 

01 01 

7*5 minutes 1-5 21 minutes T6 

24 2-1 31 1-8 

43 2-4 57 2-0 

58 2-7 70 2-2 

78 3-1 91 2-5 

99 3-4 120 2-9 

160 2-9 
180 2-9 
22 hours 1*0 
49 -21 

The initial current is, in each case, taken as unity. In Table II. 
the activity after increasing nearly to three times its original value 
decreases again at about the normal rate, falling to half value in 
about 11 hours. 

With a longer time of exposure to the emanation, the ratio of 
the increase after removal is much less marked. For a long 
interval of exposure, the activity after removal begins at once to 
diminish. In this case, the increase of activity of the matter 
deposited in the last few hours does not compensate for the 
decrease of activity of the active matter as a whole, and conse- 
quently the activity at once commences to decay. This increase of 
activity with time explains the initial irregularity in the recovery 
1 Phys. Zeit. 3, No. 12, p. 254, 1902. Phil. Mag. Jan. 1903. 

172 



260 



EXCITED RADIO-ACTIVITY 



[CH. 



curve, for the active matter deposited during the first few hours 
takes some time to reach its maximum activity, and the initial 
activity is, in consequence, smaller than would be expected from 
the equation. 

The increase of activity on a rod exposed for a short interval in 
the presence of the thorium emanation has been further investigated 
by Miss Brooks. The curve C in Fig. 50 shows the variation with 
time of the activity of a brass rod exposed for 10 minutes in the 
emanation vessel rilled with dust-free air. The excited activity 
after removal increased in the course of 3'7 hours to five times its 
initial value, and afterwards decayed at the normal rate. 



IOC 



80 



7 



20 



E 40 80 120 160 200 240 280 320 

Time in Minutes 
Fig. 50. 

171. Effect of dust on the distribution of excited activity. 

Miss Brooks, working in the Cavendish Laboratory, observed that 
the excited activity due to the thorium emanation appeared in 
some cases on the anode in an electric field, and that the distribu- 
tion of excited activity varied in an apparently capricious manner. 
This effect was finally traced to the presence of dust in the air of 
the emanation vessel. For example, with an exposure of 5 minutes 



IX] EXCITED RADIO-ACTIVITY 261 

the amount of excited activity to be observed on a rod depended 
on the time that the air had been allowed to remain undisturbed 
in the emanation vessel beforehand. The effect increased with the 
time of standing, and was a maximum after about 18 hours. The 
amount of excited activity obtained on the rod was then about 
20 times as great as the amount observed for air freshly introduced. 
The activity of this rod did not increase after removal, but with 
fresh air, the excited activity, for a 5 minutes' exposure, increased 
to five or six times its initial value. 

This anomalous behaviour was found to be due to the presence 
of dust particles in the air of the vessel, in which the bodies were 
made radio-active. These particles of dust, when shut up in the 
presence of the emanation, become radio-active. When a nega- 
tively charged rod is introduced into the vessel, a part of the 
radio-active dust is concentrated on the rod and its activity is 
added to the normal activity produced on the wire. After the air 
in the vessel has been left undisturbed for an interval sufficiently 
long to allow each of the particles of dust to reach a state of radio- 
active equilibrium, on the application of an electric field, all the 
positively charged dust particles will at once be carried to the 
negative electrode. The activity of the electrode at once com- 
mences to decay, since the decay of the activity of the dust particles 
on the wire quite masks the initial rise of the normal activity 
produced on the wire. 

Part of the radio-active dust is also carried to the anode, and 
the proportion increases with the length of time during which the 
air has been undisturbed. The greatest amount obtained on the 
anode was about 60/ of that on the cathode. 

These anomalous effects were found to disappear if the air was 
made dust-free by passing through a plug of glass wool, or by 
application for some time of a strong electric field. 

172. Decay of excited activity from radium. The excited 
activity produced on bodies by exposure to the radium emanation 
decays much more rapidly than the thorium excited activity. For 
short times of exposure 1 to the emanation the decay curve is very 
irregular. This is shown in Fig. 51. 

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



262 



EXCITED RADIO-ACTIVITY 



[CH. 



It was found that the intensity of the radiation decreased 
rapidly for the first 10 minutes after removal, but about 15 minutes 
after removal reached a value which is maintained nearly constant 
for an interval of about 20 minutes. It then decays, following an 




40 



120 



140 



60 80 100 
Time in Minutes 

Fig. 51. 

exponential law to zero, the intensity falling to half value in about 
30 minutes. With longer times of exposure, the irregularities 
in the curve are not so marked. 

Later, P. Curie and Danne 1 made a detailed investigation of the 
decay of excited activity for times of exposure to the emanation 
from 10 seconds to 6 days. The results are shown graphically 
in Fig. 52, where the ordinates represent the logarithm of the 
intensity of the radiation, and the abscissae time in hours. Curve 
A represents the decay for a long time of exposure. This decay 

1 C. R. 136, p. 364, 1903. 



IX] 



EXCITED RADIO-ACTIVITY 



263 



curve is unaltered for all times of exposure to the emanation greater 
than 24 hours. 

After an interval of 2*5 hours, the logarithmic decay curve for 
long times of exposure is a straight line, i.e. the activity falls off 
in an exponential law with the time, falling to half value in 
28 minutes. P. Curie and Danne found that for any time t 




Time in Hourg 
Fig. 52. 



after removal the intensity I t was given by the difference of two 
exponentials, viz. 



where Xj = ^^ and X 2 = y^. with the second as the unit of time. 
The numerical constant a = 4*20. The explanation of this law of 
decay is given in section 177. 

The decay curve varies greatly with the time of exposure. For 
example, in an exposure of 5 minutes, the activity at first decreases 



264 EXCITED KADIO-ACTIVITY [CH. 

very rapidly, then passes through a minimum after 8 minutes, and 
increases to a maximum after 40 minutes, and after 2'5 hours 
decays in an exponential law to zero 1 . As in the case of the excited 
activity from thorium, the rate of decay of the excited activity 
from radium is for the most part independent of the nature of the 
body made active. Curie and Danne (loc. cit.) observed that the 
active bodies gave off an emanation itself capable of exciting 
activity in neighbouring bodies. This property rapidly disappeared 
and was inappreciable 2 hours after removal. In certain substances 
like celluloid and caoutchouc, the decay of activity is very much 
slower than for the metals. This effect becomes more marked 
with increase of time of exposure to the emanation. A similar 
effect is exhibited by lead, but to a less marked degree. During 
the time the activity lasts, these substances continue to give off 
an emanation. 

It is probable that these divergencies from the general law are 
not due to an actual change in the rate of decay of the true excited 
activity but to an occlusion of the emanation by these substances 
during the interval of exposure. After exposure the emanation 
gradually diffuses out, and thus the activity due to this occluded 
emanation and the excited activity produced by it decays very 
slowly with the time. 

173. Excited radio-activity of very slow decay. M. and 

Mme Curie 2 have observed that bodies which have been exposed 
for a long interval in the presence of the radium emanation do not 
lose all their activity. The excited activity at first decays rapidly 
at the normal rate, falling to half value in about 30 minutes, but a 
residual activity always remains of the order of 1/20,000 of the 
initial activity. This residual activity either does not diminish 
at all, or so slowly that the decrease is not appreciable after 
an interval of six months. 

1 The writer has not observed the rise to a maximum found by Curie and 
Danne for the decay curves of the excited activity due to radium (see Fig. 52), but 
has always obtained curves of decay, for short exposures, similar to that shown in 
Fig. 51. This has been the case whether the excited activity has been produced on 
a body by the action of an electric field or not. In the experiments, a slow current 
of air was always passed through the testing apparatus to remove any emanation 
from the body made active. 

2 Thesis, Paris, 1903, p. 116. 



IX] EXCITED RADIO-ACTIVITY 265 

Giesel 1 has also observed that a platinum wire which has been 
exposed in the presence of the radium emanation possesses residual 
activity, and he has shown that the radiation consists entirely of 
a rays. A further discussion of this residual activity and its 
possible connection with polonium is given later in section 188. 

174. Connection between decay curves for different 
times of exposure. The decay of excited activity, in cases 
where there is no occlusion of the emanation by the substance 
made active, is a function only of the time of exposure. The 
decay curves are all intimately connected with each other, and can 
be theoretically deduced provided the decay curve for a very short 
exposure is accurately known. 

It is supposed that the excited activity produced on a body is 
due to a deposit of radio-active particles. On an average a certain 
number of these particles will break up per second, giving rise 
to rays which ionize the gas. If a large number of particles is 
deposited, the rate of production of ions iff the gas by the rays will 
be practically a continuous function of the time. The rate of pro- 
duction of ions, at any time, divided by the total number of radio- 
active particles deposited, will be called the average number of 
ions produced by each particle at that time. 

Suppose the radio-active particles which cause excited activity 
are deposited at a uniform rate of q per second. The number 
deposited in a short time dt = qdt. 

Let n = the average number of ions produced in the gas per 
second by each particle, at the instant of removal. 

n = the average number of ions per particle per second 
after an interval t. 

Suppose n = w /(0 where f(t) is a function of t such that 
f(t) = 1 when t = 0, 
f(t) = when = oo , 

f(t) may in some cases pass through a maximum value greater 
than unity. The variation of the rate of production of ions with 

1 Ber. deutsch. Chem. Ges. p. 2368, 1903. Chem. News, Aug. 7, 1903. 



266 EXCITED RADIO-ACTIVITY [CH. 

time is supposed to include the effects of different kinds of 
radiation emitted during the succession of changes which may 
occur. 

The number of ions produced per second after a time t by the 
active particles deposited for the first short interval of exposure is 
given by qn f(t)dt. 

The number N t of ions produced per second at the time t by 
the radio-active matter deposited during the interval t is given by 

y(0 fc 

o 

A steady state is reached when the rate of supply of fresh ions 
per second by the addition of the radio-active material is balanced 
by the rate of diminution of the production of ions by the excited 
radiation as a whole. This steady state is reached after a long 
interval of exposure, and the maximum rate of production of ions 
N is given by 



f(t)dt, 

o 

and 

' '" V Nt />* 
~ 



r 7 

J 



If the curve of decay of the excited activity for a very short 
exposure is plotted with the ionization current as ordinate and 
time as abscissa, as in Fig. 51, the values of these integrals are at 
once determined from the experimental curve by measuring the 
area included between the curve and the ordinates erected at the 
points corresponding to the time limits of the integrals. 

The curve of rise of excited activity can thus be deduced from 
the decay curve and vice versd. 

N l} the rate of production of ions due to the excited radiation, 
after removal from the emanation for a time t l} is given by 



f(t)dt, 

.*, 

if t is the time of exposure. 



IX] EXCITED RADIO-ACTIVITY 267 

If N is the number of ions produced immediately after removal, 



. 

('f(t)dt 

.'0 



The decay curve for any time of exposure can thus also be 
deduced from the curve of decay for a short exposure. For a very 
long interval of exposure the value J^ at a time t after removal is 
given by 



AT 




r x 
f(f)dt 

JO 



Now the curve of rise -^ is given by 

Nt j'f(t)dt 

N /"/(*)*' 

Jo 

Thus * ~ F = F ' 

Thus the decay and rise curves are very simply connected, 
whatever the law of decay of the radiations. This relation may 
be expressed as follows : For a long exposure, the percentage activity Y 
lost after removal for a time t is equal to the percentage of the final 
activity gained by a body exposed during the same interval. 

This result, which has already been shown to apply to the 
decay and recovery curves of Ur X, Th X, and other radio-active 
products, is of general application to all cases of radio-active 
change when the rate of supply is a constant. The connection 
between the decay curves of radium and thorium excited activity, 
for different times of exposure, can also be shown to hold equally 
for all types of active products. 

The relation that holds between the decay and recovery curves 
can easily be deduced from a priori considerations. 



268 EXCITED RADIO-ACTIVITY [CH. 

Let us suppose, for example, that a body has been exposed for 
a long interval in a vessel containing a constant quantity of the 
radium emanation. The excited activity in the body will have 
reached a maximum value when the rate of supply is balanced by 
the rate of change. Suppose this body is removed and an exactly 
similar body immediately substituted. The sum of the excited 
activity on these two bodies will at any time be the same as on 
the single body before removal. If this were not the case, there 
would be a change in energy of the radiations from the radio- 
active system, as a whole, purely by removal of one body and 
substitution of another. This is contrary to the general experi- 
mental fact that the processes occurring in radio-activity are 
independent of control, and that the radiation from a system in 
radio-active equilibrium remains constant. 

Thus if It = intensity of radiation from the excited body at any 
time t after removal. 

It = intensity of radiation from the new body exposed 
under the same conditions for a time t. 

Then I t + // = / where / is the initial activity on the removed 
body. 

Thus 1 j- ~ ~f > which is the same relation that has been 

*0 ^0 

developed from other considerations. 

These results are particular cases of what may be termed the 
" conservation of radio-activity," which is discussed in detail in 
section 196. 

175. Theory of successive changes. It has been pointed 
out that the excited activity produced in a body exposed for a very 
short interval in the presence of the thorium or radium emana- 
tions does not decay according to a simple exponential law. In 
the case of a^ody^^dJgdJcgLJ^ activity 

increases for a few hours, passes through a maximum where the 
activity is five to six times the initial value, and then slowly decays 
in an exponential law with the time, falling to half value after 
a further interval of 11 hours. After the maximum is reached, 



IX] EXCITED RADIO-ACTIVITY 269 

the activity decays at the normal rate observed for bodies exposed 
for a long interval in the presence of the thorium emanation. 

The increase of activity with time cannot be ascribed to a 
possible occlusion of the radio-active matter in the pores of the 
substance and a gradual passage to the surface after removal; 
for it has been found that a very thin sheet of aluminium foil, 
which absorbs very little of the radiation, exhibits the same effect 
as a solid plate. The effect is, however, similar in some respects 
to the increase of activity with time observed in a closed vessel in 
which the radium emanation has been introduced. This is known 
to be due to the production from the emanation of radio-active 
matter, which is deposited on the walls of the vessel and adds its 
radiation to that of the emanation proper. In a similar way the 
activity of Th X increases for the first day after separation from 
the thorium, and this is ascribed (see section 190) to the produc- 
tion of excited activity in the mass of the Th X. 

The most probable explanation of the initial increase of activity 
with time, observed for the excited activity produced by the thorium 
emanation, is that there are two successive changes occurring in 
the emanation X of thorium after the deposit of the active matter 
on the surface of the body. 

The theory of these secondary changes will now be considered. 
Let n be the number of radio-active particles deposited on the 
body during the exposure to the emanation. The exposure is 
supposed to be so short that only a very small proportion of the 
particles have undergone change during the time of exposure. 
These particles are supposed to undergo change in an exponen- 
tial law with the time, and the product of the first change to 
break up again according to the same law, but at a different rate. 
Let X,, A 2 be the constants of the first and second changes respec- 
tively. After removal for a time t, the number n of particles 
remaining unchanged is given by 

the number which change in the time dt at the time t is given by 



Some of this number at once begin to go through the second 



270 EXCITED RADIO-ACTIVITY [CH. 

change, but the number which has undergone the first but not the 
second change at the time T after removal is given by 

~V dt. 



The number q of particles which have undergone the first of 
the two changes at a time T after removal is thus given by 



r 

1^0 e~' 
Jo 



^-V 

Now the number of these particles breaking up in unit time is 
proportional to A^, and is a measure of the radiation accompany- 
ing the change (section 124). 

If K is the ratio of the ionization produced in the second 
change to that produced in the first change, the saturation current 
It resulting from the two successive changes is given by 

I t _ AjJlo* 

T,- 



where / is the initial value of the saturation current. 

This equation will be applied later with satisfactory results 
in section 190, to explain the rise of activity of Th X after its 
separation. 

176. On examination of the curve shown in Fig. 50, which 
shows the rise of activity of a rod exposed for ten minutes in the 
presence of the thorium emanation, it is seen that the curve C, 
showing a rise to a maximum, is roughly similar to the curves of 
recovery of uranium and thorium when the Ur X and Th X 
respectively have been removed. If the curve is produced back- 
wards, it is seen to pass very nearly through the origin. The 
abscissae measure the time from the moment the rod was intro- 
duced into the emanation vessel. 

If the increase of activity with time is due to a secondary 
change of the type already considered, it follows at once that the 



IX] EXCITED RADIO-ACTIVITY 271 

total number of ions produced during the first change is not much 
more than one per cent, of that produced in the second change. 
If, for example, the initial activity be taken as due to the radia- 
tion from the first change, the activity due to the first change 
alone should fall off in an exponential law with the time, 
following the dotted curve D shown in Fig. 50. The area 
EABE serves as a comparative measure of the total number 
of ions produced by the first change, and this area is seen to 
be small compared to the corresponding area included by the 
main curve C. 

There is, however, no reason to suppose that the first change is 
accompanied by any ionizing radiation at all. The initial activity 
observed is due to the fact that some of the deposited matter has 
undergone change before the rod is tested ; for it will be shown 
that the experimental curve obtained can be completely deduced 
if the first change is supposed to take place without any emission 
of ionizing rays, but that ionizing rays are emitted in the second 
change. 

It has been shown that after removal of the body for a time T 
the number of particles q which have undergone the first change 
but not the second change is given by 



where X x is the constant of decay in the first change and Xj for the 
second change. 

Since it is supposed that only the second change gives rise to 
a radiation, the activity at any time T after removal is propor- 
tional to q. The value of q passes through a maximum when 



i.e. when = e~^~^ T . 

A,! 

Now it is known, from experiments for a long interval of ex- 
posure, that in the second change the activity falls to half value 
in 11 hours, i.e. X^'063, when the time is expressed in hours. 
Since the maximum activity is reached when T = 220 minutes 
approximately, the value of AJ = '75. Substituting the values of 



272 



EXCITED RADIO-ACTIVITY 



[CH. 



Xj, Xa in the equation for q, the theoretical value of the activity for 
any time T after exposure is shown in the following table. The 
observed experimental values are also shown. The maximum 
activity is taken as unity. 



Time in 
minutes 


Theoretical value 
of activity 


Observed value 
of activity 


15 


22 


23 


30 


38 


37 


60 


64 


63 


120 


90 


91 


220 


1-00 


1-00 


305 


97 


96 



The curve drawn from zero is thus almost in exact agreement 
with the equation, taking \ '75. 

It may thus be concluded that the matter emanation X under- 
goes at least two changes : 

(1) A change which is not accompanied by ionizing radia- 
tions, but in which the amount of matter undergoes change 
according to an exponential law with the time, falling to half 
value in 55 minutes. 

(2) A second change, accompanied by the emission of rays, in 
which half the matter undergoes change in 11 hours. 

The existence of such a well-marked change in the matter 
emanation X of thorium, not accompanied by the emission of 
ionizing rays, is very interesting. It will be shown later that 
there is strong evidence of a change of a similar character in the 
emanation X of radium. It may be supposed that the change 
consists in a rearrangement of the components of the atom 
which is not of such a violent character as to cause a portion of 
the atom to be expelled. Since there is only one changing system 
involved, it is to be expected that the law of change would be the 
same as for a monomolecular change in chemistry. 

177. Secondary changes in emanation X of radium. 
The decay curves of the activity produced on a rod by a short 
exposure to the radium emanation are of a very different character 



IX] EXCITED RADIO-ACTIVITY 273 

from those observed from thorium. In the first place, there is 
a rapid decay of the activity to less than 1/8 of the initial value, 
then a very slow variation for about 20 minutes, and then a gradual 
decay according to an exponential law. It is not possible to explain 
this variation of the activity on the assumption of two changes. It 
is necessary to suppose that there are three, the second of which is 
a change not accompanied by ionizing rays. 

Some evidence will first be considered of the decay of activity 
of a body exposed for several days in the presence of the radium 
emanation. P. Curie and Danne (loc. cit.) state that the law of 
decay of the activity of such a body is expressed accurately by 
the equation 



where ^i = x ^ = Ts> an(i a = 4'20. 



Curie and Danne do not state definitely whether the law of 
decay holds for the first ten minutes after 'removal. The shape of 
the decay curve for short exposure suggests from theoretical con- 
siderations that there should be in addition a small but rapid 
initial drop of activity during the first ten minutes after removal. 
Such a rapid initial drop of activity has been experimentally 
observed by the writer. 

It seems probable that the equation of P. Curie and Danne 
applies for the decay of excited activity starting from a time about 
ten minutes after removal. During that short interval the un- 
changed deposited matter rapidly passes through the first change, 
for half the matter is changed in about three minutes. At the 
time at which the measurements of Curie and Danne began, 
probably nearly all of the deposited matter had gone through the 
first change. 

Since the decay of activity after that time can be expressed by 
two exponentials, it is probable that there are two farther changes 
occurring. The view that the. first of these changes is a change 
unaccompanied by ionizing rays, followed by another change with 
the emission of rays, will be found to be in very close agreement 
with the results of P. Curie and Danne. It has been shown 
that, after a short exposure for a time dt to the emanation, during 
R. R.-A. 18 



274 EXCITED KA DIG- ACTIVITY [CH. 

which n particles were deposited on the body, the number q of 
particles which have passed through the first change but not the 
second is at any time T given by 



=./(). 

(See section 174.) 

But, since only the second change is accompanied by rays, the 
intensity of the radiation is always proportional to q the number 
unchanged, i.e. to n f(t). 

It has been shown in section 174 that, for a very long exposure, 
the activity 7 f , after removal for a time t, is given by 

dt 



1 n f(t)dt 
o 



where 7 is the initial intensity after removal. Substituting the 
value of f(t) and integrating 



This is of the same form as the equation of the decay curve 
found by Curie and Danne. Substituting the values Xj = 1/2420, 

\2= 1/1860, which were found by them, the value of -- - is 4'3 

Xj X, 2 

and of -- is 3'3. 

A! \2 

The experimental value found by Curie and Danne for these 
constants was 4'2 and 3'2 respectively. The agreement between 
the theory and the experiment is as close as could be expected. 

There are thus three distinct changes in the emanation X of 
radium, viz. 

(1) A very rapid initial change. Half of the matter changes 
in about three minutes and is accompanied by ionizing rays. 

(2) A slower change, which is not accompanied by ionizing 
rays. Half of the matter undergoes change in 1* minutes. 



IX] EXCITED RADIO-ACTIVITY 275 

(3) A third change, which is faster than the second, and 
is accompanied by ionizing rays. Half the matter changes in 
about 28 minutes. 

178. Physical and chemical properties of the active 
matter. On account of the slow decay of the activity of emana- 
tion X of thorium, its physical and chemical properties have been 
more closely examined than the emanation X of radium. It has 
already been mentioned that the emanation X of thorium is 
soluble in some acids. The writer 1 found that the active matter 
was dissolved off the wire by strong or dilute solutions of sul- 
phuric, hydrochloric and hydrofluoric acids, but was only slightly 
soluble in water or nitric acid. The active matter was left behind 
when the solvent was evaporated. The rate of decay of activity 
was unaltered by dissolving the active matter in sulphuric acid, 
and allowing it to decay in the solution. In the experiment, the 
active matter was dissolved off an active platinum wire and then 
equal portions of the solutions were taken at definite intervals, 
evaporated down in a platinum dish, and the activity of the residue 
tested by the electric method. The rate of decay was found to be 
exactly the same as if the active matter had been left on the wire. 
In another experiment, an active platinum wire was made the 
cathode in a copper sulphate solution, and a thin film of copper 
deposited on it. The rate of decay of the activity was unchanged 
by the process. 

A detailed examination of the physical and chemical properties 
of the emanation X of thorium has been recently made by F. von 
Lerch 2 , and some important and interesting results have been 
obtained. A solution of emanation X was prepared by dissolving 
the metal which had been exposed for some time in the presence 
of the thorium emanation. In most cases the active matter was 
precipitated with the metal. For example, an active copper wire 
was dissolved in nitric acid and then precipitated by caustic potash. 
The precipitate was strongly active. An active magnesium wire, 
dissolved in hydrochloric acid and then (precipitated as phosphate, 
also gave an active precipitate. The activity of the precipitates 



1 Phys. Zeit. 3, No. 12, p. 254, 1902. 

2 Dnide's Annal. Nov. 1903. 



182 



276 EXCITED RADIO- ACTIVITY [CH. 

decayed at the normal rate, i.e. the activity fell to half value in 
about 11 hours. 

Experiments were also made on the solubility of emanation X 
in different substances. A platinum plate was made active and 
then placed in different solutions, and the decrease of the activity 
observed. In addition to the acids already mentioned, a large 
number of substances were found to dissolve the emanation X to 
some extent. The active matter was however not dissolved to an 
appreciable extent in ether or alcohol. Many substances became 
active if added to the active solution of emanation X and then 
precipitated. For example, an active solution of hydrochloric acid 
was obtained by dissolving the emanation X from an active 
platinum wire. Barium chloride was then added and precipitated 
as sulphate. The precipitate was strongly active, thus suggesting 
that the emanation X was carried down by the barium. 

179. Electrolysis of solutions. Dorn showed that, if solu- 
tions of radiferous barium chloride were electrolysed, both electrodes 
became temporarily active, but the anode to a greater degree than 
the cathode. F. von Lerch (loc. cit.) has made a detailed examina- 
tion of the action of electrolysis on an active solution of emanation 
X of thorium. The active matter was dissolved off an active 
platinum plate by hydrochloric acid and then electrolysed between 
platinum electrodes. The cathode was very active, but there was 
no trace of activity on the anode. The cathode lost its activity at 
a rate much faster than the normal. With an amalgamated zinc 
cathode on the other hand, the rate of decay was normal. When 
an active solution of hydrochloric acid was electrolysed with an 
electromotive force smaller than that required to decompose water, 
the platinum became active and the activity decayed to half value 
in 4*75 hours while the normal fall is to half value in 11 hours. 
These results point to the conclusion that the matter emanation X 
is complex and consists of two parts which have different rates of 
decay of activity, and can be separated by electrolysis. 

Under special conditions it was found possible to make the 
anode active. This was the case if the anion attached itself to 
the anode. For example, if an active hydrochloric solution was 
electrolysed with a silver anode, the chloride of silver formed was 



IX] EXCITED RADIO-ACTIVITY 277 

strongly active and its activity decayed at a normal rate. Von 
Lerch found that the amount of activity obtained by placing 
different metals in active solutions for equal times varied greatly 
with the metal. For example, he found that if a zinc plate and 
an amalgamated zinc plate, which show equal potential differences 
with regard to hydrochloric acid, were dipped for equal times in 
two solutions of equal activity, the zinc plate was seven times as 
active as the other. The activity was almost removed from the 
solution in a few minutes by dipping a zinc plate into it. Some 
metals became active when dipped into an active solution while 
others did not. Platinum, palladium, and silver remained inactive, 
while copper, tin, lead, nickel, iron, zinc, cadmium, magnesium, 
and aluminium became active. These results strongly confirm the 
view that excited activity is due to a deposit of active matter 
which has distinctive chemical behaviour. 

G. B. Pegram 1 has made a detailed study of the active deposits 
obtained by electrolysis of pure and commercial thorium salts. 
The commercial thorium nitrate obtained from P. de Haen gave, 
when electrolysed, a deposit of lead peroxide on the anode. This 
deposit was radio-active, and its activity decayed at the normal 
rate of the excited activity due to thorium. From solutions of 
pure thorium nitrate, no visible deposit was obtained on the anode, 
but it was, however, found to be radio-active. The activity 
decayed rapidly, falling to half value in about one hour. Some 
experiments were also made on the effect of adding metallic salts 
to thorium solutions and then electrolysing them. Anode and 
cathode deposits of the oxides or metals obtained in this way were 
found to be radio-active, but the activity fell to half value in a few 
minutes. The gases produced by electrolysis were radio-active, 
but this was due to the presence of the thorium emanation. The 
results of Pegram and von Lerch would seem to indicate that, 
besides those already known, other radio-active products with 
a distinctive rate of decay are produced during the changes 
occurring in thorium. 

180. Effect of temperature. The activity of a platinum 
wire which has been exposed in the presence of the thorium 

1 Phys. Review, p. 424, Dec. 1903. 



278 



EXCITED RADIO-ACTIVITY 



[CH. 



emanation is almost completely lost by heating the wire to a white 
heat. Miss F. Gates 1 found that the activity was not destroyed 
by the intense heat, but manifested itself on neighbouring bodies. 
When the active wire was heated electrically in a closed cylinder, 
the activity was transferred from the wire to the interior surface 
of the cylinder in unaltered amount. The rate of decay of the 
activity was not altered by the process. By blowing a current of 
air through the cylinder during the heating, a part of the active 
matter was removed from the cylinder. Similar results were found 
for the excited activity due to radium. 

F. von Lerch (loc. cit.) determined the amount of activity 
removed at different temperatures. The results are shown in the 
following table for a platinum wire excited by the thorium ema- 
nation. 





Temperature 


Percentage of 
activity removed 


Heated 2 minutes 
then i minute more 
j> "% > 

55 55 U ?) 55 


800 C. 
1020 C. 
1260 C. 
1460 C. 



16 
52 
99 



It is not possible to settle definitely from these experiments 
whether the active matter is actually volatilized at a high tempe- 
rature or is removed by disintegration of the surface of the wire. 
All the metals so far tried apparently lose their activity at about 
the same temperature. 

181. Emission of heat. It has been shown in sections 105, 
106, and 163, that the radium emanation, together with the 
secondary products which arise from it, is responsible for about 
75 per cent, of the total heat emission observed for radium. 
The gradual decay to a minimum of the heat emission of the 
radium for the first few hours after the emanation is removed 
is due to the gradual decay of the excited activity produced by 
the occluded emanation in the radium itself. In a similar way, the 
gradual increase of the heating effect of the separated emanation 

1 Phys. Review, p. 300, 1903. 



IX] EXCITED RADIO-ACTIVITY 279 

for the first few hours after removal, is due to the excited activity 
produced by the emanation on the walls of the containing vessel. 

Some experiments were recently made by H. T. Barnes and 
the writer 1 on the division of the heating effect of radium between 
the successive products of radium. For measurement of the 
heating effects, a pair of differential platinum thermometers, 
wound spirally in the inside of a glass tube, were used. The 
radium or its emanation, enclosed in a fine glass tube, was 
placed inside the platinum spiral and the rise of temperature 
observed. 

The heating effect of 30 milligrams of radium bromide was 
first determined. The emanation was then removed from it by 
heating, and condensed in a small glass tube. The heating effect 
of the de-emanated radium was determined ten minutes after the 
removal of the emanation, and was found to have diminished to 
59 per cent, of its original value. It then diminished more slowly 
with time to a minimum corresponding to 25 per cent, of its 
original value (see section 106). 

The curve of diminution with time of the heating effect of 
radium to the minimum of 25 per cent, should be identical with 
the corresponding curve of diminution with time of the heating 
effect of the emanation tube to zero after removal of the emanation. 
This was found to be the case. The emanation was allowed to 
remain for several hours in a small glass tube in order that the 
excited activity should reach a maximum value. The emanation 
was then rapidly withdrawn from the tube, and the heating effect 
of the tube determined at regular intervals. There was a similar 
initial drop within the first 10 minutes, then a slower variation, 
and finally a decrease to zero according to an exponential law with 
the time, falling to half value in about 30 minutes. 

The curve of increase of the heating effect of the emanation 
tube to a maximum after the introduction of the emanation was 
found to be complementary to the curve of decrease of the heating 
effect to zero after withdrawal of the emanation. It was not found 
possible to separate the heating effect of the emanation itself 
from the first rapid change in emanation X, since temperature 

1 Phil. Mag. Feb. 1904. 



280 



EXCITED KADIO-ACTIVITY 



[CH. 



conditions did not become steady until an interval of 10 minutes 
after introducing or withdrawing the emanation, and in that time, 
the first change in emanation X was nearly completed. 

The division of the heating effect amongst the radio-active 
products of radium is given in the following table. The activity 
of each product measured by the a rays is also given for com- 
parison : 



Active products 


Nature of 
rays 


Percentage 
proportion of 
total activity 
measured by 


Percentage 
proportion of 
total heating 
effect 






the rays 


i 


Radium 


a rays 


25 


25 


(freed from active products) 








Emanation 


a rays 


18 } 




t 




\ 33 


41 


Emanation X (first change) 


a rays 


15 j 




t 








(second change) 


No a rays 


\ 




1 




I 42 


34 


(third change) 


a, /3, and y rays 


42 ) 





The heating effect of the active products is approximately 
proportional to their activity measured by the a. rays. There can 
be very little doubt that the emanation supplies an amount of the 
heating effect proportional to its activity. 

The decay curve of the activity of radium to a minimum of 
25 per cent, after removal of the emanation is approximately the 
same as the corresponding decay curve of the activity of radium 
measured by the a. rays. 

There is no doubt that the heating effect of radium is a result 
of the succession of radio-active changes occurring in it. The 
heating effect accompanies the expulsion of a particles, and is 
approximately proportional to the number expelled. The time- 
variation of the heating effect of the radio-active products is 
the same as the time-variation of their activity measured by 
the a rays. 

182. Effect of variation of E. M. F. on amount of 
excited activity from thorium. It has been shown that the 



IX] EXCITED RADIO-ACTIVITY 281 

excited activity is confined to the cathode in a strong electric field. 
In weaker fields the activity is divided between the cathode and 
the walls of the vessel. This was tested in an apparatus 1 shown in 
Fig. 53. 



Electrometer -^ Battery 

f a ^ 

C R8 B 



Fig. 53. 

A is a cylindrical vessel of 5*5 cms. diameter, B the negative 
electrode passing through insulating ends C, D. For a potential 
difference of 50 volts, most of the excited activity was deposited 
on the electrode B. For about 3 volts, half of the total excited 
activity was produced on the rod B, and half on the walls of the 
vessel. Whatever the voltage applied, the sum of the activities 
on the central rod and the walls of the* cylinder was found to 
be a constant when a steady state was reached. 

When no voltage was applied, diffusion alone was operative, 
and in that case about 13 per cent, of the total activity was on the 
rod B. The application of an electric field has thus no influence 
on the sum total of excited activity, but merely controls the pro- 
portion concentrated on the negative electrode. 

A more detailed examination of the variation with strength of 
field of the amount on the negative electrode was made in a similar 
manner by F. Henning 2 . He found that in a strong electric field 
the amount of excited activity was practically independent of the 
diameter of the rod B, although the diameter varied between 
'59 mm. and 6'0 mms. With a small voltage, the amount on the 
negative electrode varied with its diameter. The curves showing 
the relation between the amount of excited activity and voltage 
are very similar in character to those obtained for the variation of 
the current through an ionized gas with the voltage applied. 

The amount of excited activity reaches a maximum when all 
the emanation X is removed from the gas as rapidly as it is 

1 Rutherford, Phil. Mag. Feb. 1900. 
- Drude's Annal. p. 562, 1902. 



282 EXCITED RADIO-ACTIVITY [CH. 

formed. With weaker fields, a portion diffuses to the sides of the 
vessel, and produces excited activity on the positive electrode. 

183. Effect of pressure on distribution of excited 
activity. In a strong electric field, the amount of excited activity 
produced on the cathode is independent of the pressure down to a 
pressure of about 10 mms. of mercury. In some experiments made 
by the writer 1 , the emanating thorium compound was placed 
inside a closed cylinder about 4 cms. in diameter, through which 
passed an insulated central rod. The central rod was connected to 
the negative pole of a battery of 50 volts. When the pressure was 
reduced below 10 mms. of mercury, the amount of excited activity 
produced on the negative electrode diminished, and was a very 
small fraction of its original value at a pressure of -^ mm. Some 
excited activity was in this case found to be distributed over the 
interior surface of the cylinder. It may thus be concluded that at 
low pressures the excited activity appears on both anode and 
cathode, even in a strong electric field. 

Curie and Debierne 2 observed that, if a vessel containing an 
emanating radium compound was kept pumped down to a low 
pressure, the amount of excited activity produced on the vessel 
was much reduced. In this case the emanation given off by the 
radium was removed by the pump with the other gases con- 
tinuously evolved from the radium compound. On account of the 
very slow decay of activity of the emanation, the amount of excited 
activity produced on the walls of the vessel, in the passage of the 
emanation through it, was only a minute fraction of the amount 
produced when all the emanation given off was not allowed to 
escape. 

184. Transmission of excited activity. The characteristic 
property of excited radio-activity is that it can be confined to the 
cathode in a strong electric field. Since the activity is due to a 
deposit of radio-active matter on the electrified surface, the matter 
must be transported by positively charged carriers. The experi- 
ments of Fehrle 3 showed that the carriers of excited activity travel 

1 Phil. Mag. Feb. 190Q. 2 Cm R 132> p . 768? 190 i. 

:J Phys. Zeit. 3, No. 7, p. 130, 1902. 



IX] EXCITED RADIO-ACTIVITY 283 

along the lines of force in an electric field. For example, if a small 
negatively charged metal plate was placed in the centre of a metal 
vessel containing an emanating thorium compound, more excited 
activity was produced on the sides and corners of the plate than at 
the central part. 

A difficulty however arises in connection with the positive 
charge of the carrier. According to the view developed in sec- 
tion 127 and later in section 200, the matter emanation X, which 
is deposited on bodies and gives rise to excited activity, is itself 
derived from the emanation. The emanations of thorium and 
radium emit only a rays, i.e. positively charged particles. After 
the expulsion of an a particle, the residue, which is supposed to 
constitute the emanation X, should retain a negative charge, and 
be carried to the anode in an electric field. The exact opposite is 
however observed to be the case. The experimental evidence does 
not support the view that the positively charged a particles, 
expelled from the emanation, are directly responsible for the 
phenomena of excited activity ; for no excited activity is produced 
in a body exposed to the a rays of the emanation, provided the 
emanation itself does not come in contact with it. It may be 
supposed that in gases the matter emanation X, immediately after 
its production, attaches itself to the positive ions, produced in 
the gas by the radiation, on the same sort of principle that water 
vapour condenses round the negative ion. The active matter is 
then transported by these positive carriers to the cathode. In the 
case of radium, there is evidence that some of the carriers of 
excited activity do not acquire a positive charge until they have 
been present in the gas for some time. 

Whatever view is taken of the process by which these carriers 
obtain a positive charge, there can be little doubt that the expul- 
sion of an a particle with great velocity from the atom of the 
emanation must set the residue in motion. On account of the 
comparatively large mass of this residue, which constitutes the 
emanation X, the velocity acquired will be small compared with 
that of the expelled a particle, and the moving mass will be 
rapidly brought to rest at atmospheric pressure by collision with 
the gas molecules in its path. At low pressures, however, the 
collisions will be so few that it will not be brought to rest until it 



284 



EXCITED RADIO-ACTIVITY 



[CH. 



strikes the boundaries of the vessel. A strong electric field would 
have very little effect in controlling the motion of such a heavy 
mass, unless it has been initially brought to rest by collision with 
the gas molecules. This would explain why the active matter is 
not deposited on the cathode at low pressures in an electric field. 
Some direct evidence of a process of this character, obtained by 
Debierne on examination of the excited activity produced by 
actinium, is discussed in section 186. 

185. The following method has been employed by the writer to 
determine the velocity of the positive carriers of excited activity of 
radium and thorium in an electric field. Suppose A and B (Fig. 54) 



B ' 



I E,_ _ 



Emanation 
A 




Fig. 54. 

are two parallel plates exposed to the influence of the emanation, 
which is uniformly distributed between them. If an alternating 
E.M.F. E is applied between the plates, the same amount of 
excited activity is produced on each electrode. If in series with 
the source of the alternating E.M.F. a battery is placed of E.M.F. 
E l less than E , the positive carrier moves in a stronger electric 
field in one half alternation than in the other. A carrier con- 
sequently moves over unequal distances during the two half 
alternations, since the velocity of the carrier is proportional to the 
strength of the electric field in which it moves. The excited 
activity will in consequence be unequally distributed over the two 
electrodes. If the frequency of alternation is sufficiently great, 
only the positive carriers within a certain small distance of one 



IX] EXCITED RADIO-ACTIVITY 285 

plate can be conveyed to it, and the rest, in the course of several 
succeeding alternations, are carried to the other plate. 

When the plate B is negatively charged, the E.M.F. between 
the plates is E E l , when B is positive the E.M.F. is E + E+. 
Let d = distance between the plates, 
T time of a half alternation, 
p = ratio of the excited radio-activity on the plate B to the 

sum of the radio-activities on the plates A and B, 
K = velocity of the positive carriers for a potential-gradient 
of 1 volt per centimetre. 

On the assumption that the electric field between the plates is 
uniform, and that the velocity of the carrier is proportional to the 
electric field, the velocity of the positive carrier towards B is 

EO- El TT 

~~~ 



and in the course of the next half alternation 



towards the plate A. 

If x l is less than d, the greatest distances x l , x z passed over by 
the positive carrier during two succeeding half alternations is thus 
given by 

EQ EI vrp , E + E! ^ T 

#! = -- = &-JL, and # 2 = -- j JK.J.. 
a a 

Suppose that the positive carriers are produced at a uniform 
rate of q per second for unit distance between the plates. The 
number of positive carriers which reach B during a half alter- 
nation consists of two parts : 

(1) One half of those carriers which are produced within the 
distance x of the plate B. This number is equal to 



(2) All the carriers which are left within the distance x^ from 
B at the end of the previous half alternation. The number of 
these can readily be shown to be 



286 



EXCITED RADIO-ACTIVITY 



[CH. 



The remainder of the carriers, produced between A and B 
during a complete alternation, will reach the other plate A in the 
course of succeeding alternations, provided no appreciable recombi- 
nation takes place. This must obviously be the case, since the 
positive carriers travel further in a half alternation towards A than 
they return towards B during the next half alternation. The 
carriers thus move backwards and forwards in the changing electric 
field, but on the whole move towards the plate A. 

The total number of positive carriers produced between the 
plates during a complete alternation is 2dqT. The ratio p of the 
number which reach B to the total number produced is thus 
given by 



Substituting the values of x-^ and # 2 



#2 

obtain 



In the experiments the values of E , E l} d, and T were varied, 
and the results obtained were in general agreement with the above 
equation. 

The following results were obtained for thorium : 
Plates T30 cms. apart. 







Alternations 






EO+EI 


%0 E I 


per second 


P 


K 


152 


101 


57 


27 


1-25 


225 


150 


57 


38 


1-17 


300 


200 


57 


44 


1-24 



Plates 2 cms. apart. 







Alternations 






Eo + El 


E E l 


per second 


P 


K 


273 


207 


44 


37 


1-47 


300 


200 


53 


286 


1-45 



JX] EXCITED RADIO-ACTIVITY 287 

The average mobility K deduced from a large number of 
experiments was 1*3 cms. per sec. per volt per cm. for atmospheric 
pressure and temperature. This velocity is about the same as 
the velocity of the positive ion produced by Rontgen rays in air, 
viz. 1 '37 cms. per sec. The results obtained with the radium 
emanation were more uncertain than those for thorium on account 
of the distribution of some excited activity on the positive elec- 
trode. The values of the velocities of the carriers were however 
found to be roughly the same for radium as for thorium. 

These results show that the carriers of the emanation X travel 
in the gas with about the same velocity as the positive or negative 
ions produced by the radiations in the gas. This shows either that 
the emanation X becomes attached to positive ions, or that the 
emanation X itself, acquiring in some way a positive charge, forms 
a cluster of neutral molecules which travel with it. 

186. Excited activity from actinium and " emanation 
substance." Giesel 1 observed that the " emanating substance " 
gave off a large quantity of emanation, and that this emanation 
gave rise to a type of radiation which he termed the E rays. 
A narrow metal cylinder containing the active substance was 
placed with the open end downwards, about 5 cms. above the 
surface of a zinc sulphide screen. The screen was charged nega- 
tively to a high potential by an electric machine, and the cylinder 
connected with earth. A luminous spot of light was observed on 
the screen, which was brighter at the edge than at the centre. A 
conductor, connected with earth, brought near the luminous spot 
apparently repelled it. An insulator did not show such a marked 
effect. On removal of the active substance, the luminosity of the 
screen persisted for some time. This was probably due to the 
excited activity produced on the screen. 

The results obtained by Giesel support the view that the 
carriers of excited activity of the " emanation substance " have a 
positive charge. In a strong electric field the carriers travel along 
the lines of force to the cathode, and there cause excited activity 
on the screen. The movement of the luminous zone on the ap- 
proach of a conductor is due to the disturbance of the electric field. 

1 Ber. deutscli. Chem. GeselL 36, p. 342, 1903. 



288 



EXCITED RADIO-ACTIVITY 



[CH. 



Debierne 1 found that actinium also gave off a large amount of 
emanation, the activity of which decayed very rapidly with the 
time. At some distance from the source, the activity of the emana- 
tion fell to half value in one-and-a-half minutes. This is not very- 
different from the rate of decay of the activity of the thorium 
emanation, which falls to half value in about one minute. 

This emanation produces excited activity on surrounding objects, 
and at diminished pressure the emanation produces a uniform 
distribution of excited activity in the enclosure containing the 
emanation. No data have yet been published of the rate of decay 
of the excited activity produced by the emanation of actinium. 

Debierne observed that the distribution of excited activity was 
altered by a strong magnetic field. The experi- 
mental arrangement is shown in Fig. 55. The 
active matter was placed at M, and two plates 
A and B were placed symmetrically with regard 
to the source. On the application of a strong 
magnetic field normal to the plane of the paper, 
the excited activity was unequally distributed 
between the plates A and B. The results showed 
that the carriers of excited activity were deviated 
by a magnetic field in the opposite sense to the 
cathode rays, i.e. the carriers were positively 
charged. In some cases, however, the opposite 
effect was obtained. Debierne considers that the excited activity 
of actinium is due to " ions activants," the motion of which is 
altered by a magnetic field. Other experiments showed that the 
magnetic field acted on the "ions activants" and not on the 
emanation. 

The results of Debierne thus lead to the conclusion that the 
carriers of excited activity are derived from the emanation and are 
projected with considerable velocity. This result supports the 
view advanced in section 184 that the expulsion of a particles 
from the emanation must set the part of the system left behind in 
rapid motion. A close examination of the mode of transference of 
the excited activity by actinium and the emanation substance is 




Fig. 55. 



1 C. R. 136, pp. 446 and 671, 1903. 



IX] EXCITED RADIO-ACTIVITY 289 

likely to throw further light on the processes which give rise to 
the deposit of active matter on electrodes. 

187. Radio-active induction. In carrying out experiments 
on the separation of radium from pitchblende, M. and Mme Curie 1 
observed that the separation of the active substance is fairly com- 
plete, if the stage of purification is not far advanced. Copper, 
antimony, and arsenic can be separated practically inactive, but 
other bodies, like lead and iron, always show slight activity. When 
the stage of purification is more advanced, every body separated 
from the active solution exhibits activity. 

Debierne 2 showed that barium was made active by solution 
with actinium. The active barium removed from the actinium 
still preserved its activity after chemical treatment. In this way 
Debierne obtained barium chloride 6000 times as active as uranium. 
Although the activity of the barium chloride could be concentrated 
in the same way as the activity of radiferous barium chloride, it 
did not show any of the spectroscopic lines of radium. The activity 
however of the barium was not permanent, but decayed to about 
one-third of its value in three months. 

Giesel showed in 1900 that bismuth could be made active by 
placing it in a radium solution, and suggested that polonium was 
in reality bismuth made active by its mixture with the radium in 
pitchblende. Mme Curie also found that bismuth was made active 
by solution with a radium compound, and succeeded in fractionat- 
ing the above bismuth in the same way as polonium. In this way 
bismuth was obtained 2000 times as active as uranium, but the 
activity decreased with time. These experiments are rendered 
very uncertain by the difficulty of completely separating the radium 
from the bismuth. 

Giesel 3 in 1903 showed that a bismuth plate dipped in a radium 
solution remained active after every care had been taken to remove 
all traces of radium. This active bismuth gave out only a rays, 
and in this respect was analogous to polonium or Marckwald's 
radio-active tellurium. The absence of the a rays in the bismuth 
indicates that no radium adhered to the bismuth. The activity of 

1 Thesis, Paris, 1903, p. 117. 2 C. R. 131, p. 137, 1900. 

3 Ber. deutsch. Chem. Ges. p. 2368, 1903. 

R. R.-A. 19 



290 EXCITED RADIO-ACTIVITY [CH. 

the bismuth did not decay over the period of examination, but 
observations were not made over sufficient lengths of time to make 
certain of this. There are two points of view that have been taken 
in regard to radio-active induction. Some have supposed that the 
inactive molecules of the substance themselves temporarily acquire 
the property of radio-activity, after admixture with a very active 
substance like radium or actinium. On this view the radio-active 
bismuth is in reality bismuth, some of the matter of which has 
temporarily acquired radio-active property. 

On the other point of view, production of activity in inactive 
bodies is either due to a slight admixture of the active element, 
or to a removal with the substance of a radio-active product 
of the element. In the former case, the activity of the body is 
permanent ; in the latter, it decays with the time, according to the 
same law as the decay of activity of the separated product. For 
example, if barium is precipitated in an uranium solution, the 
barium is active, and its activity decays at the same rate as the 
separated Ur X. In fact, the barium precipitate carried down with 
it the matter Ur X. 

So far, however, no case has yet been observed when any body 
has acquired the property of radio-activity by exposure to the 
radiations alone of the radio-elements. The evidence at present 
supports the view that the activity produced in inactive bodies is 
due to a separation with it of an active product. The experiments 
of F. von Lerch, described in sections 178, 179, show that many 
metals are able to become active when placed in a solution of 
the emanation X of thorium. This activity is due to a deposit 
of the emanation X on the metal. The activity is removable by 
precipitation and also in some cases by electrolysis. In the case of 
solutions obtained from pitchblende, it is thus not surprising that 
a similar action occurs, and that many substances possess some 
temporary activity at the time of their separation. One or more 
of the numerous active products in pitchblende is precipitated 
with the substance, and the activity then decays with the time. 

188. Possible origin of polonium. Mme Curie has not 
yet been able to purify polonium sufficiently to obtain any spectro- 
scopic evidence of a new element. Giesel has consistently taken 



IX] EXCITED RADIO-ACTIVITY 291 

the view, that polonium is in reality " induced " bismuth. At the 
same time, it has not yet been definitely settled whether Marck- 
wald's radio-active tellurium contains the same active constituent 
as polonium or not. Taking the view that every case of induction 
is due to a removal with the inactive element of an active product 
of the radio-elements, some evidence will now be considered which 
points to the probability that polonium is a disintegration product 
of the element radium. 

It has been pointed out that Mme Curie was able to fractionate 
bismuth, made active in a radium solution, in the same way as 
polonium, i.e. that the active matter in the bismuth had chemical 
properties similar to polonium. Giesel, in addition, showed that a 
platinum or bismuth plate placed in a radium solution acquired 
strong activity, and, like polonium, gave rise to a rays only. If 
the active bismuth or platinum decays at the same rate as 
polonium, it would be very strong evidence that polonium was a 
product of radium. Further experiments are, however, required 
on this point. 

It has been mentioned that bodies exposed for a long interval 
in the presence of the radium emanation, always retain about 
1/20,000 of their original activity. Giesel found that the residual 
activity of a platinum wire exposed in the presence of the radium 
emanation, gave out only a. rays, and in that respect resembled 
polonium. 

The writer has recently found that active matter can be dis- 
solved by sulphuric acid from the inside of a glass tube, which 
has at one time contained the radium emanation. On evapora- 
ting the acid, an active deposit was left behind which gave out 
a and /3 rays. The activity of this deposit, as far as observations 
have yet gone, has not decayed with the time. This active sub- 
stance gives out a far greater proportion of /9 rays than either 
radium or thorium. The a rays showed about the same amount of 
absorption in aluminium foil as the a rays from polonium, and 
possessed also the characteristic property exhibited by the 
polonium rays (section 90) of rapidly increasing absorption with 
thickness of matter traversed. It is thus possible that this 
active matter may contain polonium with another product giving 
rise to ft rays. If it be assumed that the a rays, which are given 

192 



292 EXCITED RADIO-ACTIVITY [CH. IX 

out by the active residue, accompany another very slow change 
occurring in the matter emanation X, the time taken for the 
activity of this product to decay to half value can be deduced 
from general considerations. It will be shown later, in section 
195, that each of the successive changes in radium or thorium, 
which is accompanied by a rays, gives rise to about the same 
total amount of ionization. This is merely an expression of the 
fact that the same number of systems must undergo change in 
each successive product, and that each system probably expels 
the same number of a particles with about the same velocity. 
Now it was found experimentally that the ionization current due 
to the active residue was about 1/20,000 of the initial ionization 
due to the emanation, which in its further changes had given 
rise to the slowly decaying active matter. Since the ionization 
current due to the emanation was 20,000 times that due to the 
active matter, its rate of change was 20,000 times faster. But the 
activity of the emanation decays to half value in four days, so that 
the activity of this other active matter would decay to half value 
in about 80,000 days or about 200 years. 

The existence of such a slow change in the emanation X of 
radium probably accounts in part for the radio-activity which is 
produced on the walls of the laboratory in which radium prepara- 
tions have been kept in open vessels. The emanation diffuses into 
the air and produces emanation X, which is deposited on the walls 
of the room, and there gives rise to a deposit of very slowly decay- 
ing matter. This activity persists in a room even though no 
radio-active matter has been kept in it for some time. 



CHAPTER X. 

RADIO- ACTIVE PROCESSES. 

189. Radio-activity of uranium. It has already been 
shown in section 118 that a radio-active constituent Ur X can 
be separated from uranium by several different processes. The 
activity of the separated Ur X decays with the time, falling to half 
value in about 22 days. At the same time the uranium, from 
which the Ur X has been separated, gradually regains its lost 
activity. The law of decay of Ur X and the recovery of the lost 
activity of the uranium are expressed by the equations 

^ = e-** and ^=l- e -^, 

-*0 -*0 

where X is the radio-active constant of Ur X. The constant radio- 
activity of uranium thus represents a state of equilibrium, where 
the rate of production of new active matter is balanced by the rate 
of change of the Ur X already produced. 

The radio-active processes occurring in uranium present several 
points of difference from the processes occurring in thorium and 
radium. In the first place, uranium does not give off an emanation, 
and in consequence does not produce any excited activity on bodies. 
So far only one active product Ur X has been observed in uranium. 
This active product UrX differs from ThX and the emanations, 
inasmuch as the radiation from it consists almost entirely of /8 rays. 
This peculiarity of the radiations from Ur X initially led to some 
confusion in the interpretation of observations on Ur X and the 
uranium from which it had been separated. When examined by 
the photographic method, the uranium freed from Ur X showed no 
photographic action, while the Ur X possessed it to an intense 



294 RADIO-ACTIVE PROCESSES [CH. 

degree. With the electric method, on the other hand, the results 
obtained were exactly the reverse. The uranium freed from Ur X 
showed very little loss of activity while the activity of the Ur X 
was very small. The explanation of these results was given by 
Soddy 1 and by Rutherford and Grier 2 . The a rays of uranium are 
photographically almost inactive but produce most of the ioniza- 
tion in the gas. The ft rays, on the other hand, produce a strong 
photographic action, but very little ionization compared with the a 
rays. When the Ur X is separated from the uranium, the uranium 
does not at first give out any {3 rays. In the course of time, fresh 
Ur X is produced from the uranium, and ft rays begin to appear, 
gradually increasing in intensity until they reach the original value 
shown before the separation of the Ur X. 

In order to determine the recovery curves of uranium after the 
separation of Ur X, it was thus necessary to measure the rate of 
increase of the ft rays. This was done by covering the uranium 
with a layer of aluminium of sufficient thickness to absorb all the 
a rays, and then measuring the ionization due to the rays in an 
apparatus similar to Fig. 16. 

Uranium has not yet been obtained inactive when tested by 
the electric method. Becquerel 3 has stated that he was able to 
obtain inactive uranium, but in his experiments the uranium was 
covered with a layer of black paper, which would entirely absorb 
the a rays. There is no evidence that the a radiation of uranium 
has been altered either in character or amount by any chemical 
treatment. The a rays appear to be inseparable from the uranium, 
and it will be shown later that the other radio-active elements as 
well as uranium also possess a non-separable activity consisting 
entirely of a rays. The changes occurring in uranium must then 
be considered to be of two kinds, (1) the change which gives rise 
to the a rays and the product Ur X, (2) the change which gives 
rise to the ft rays from Ur X. 

The possibility of separating the Ur X, which gives rise to the 
ft rays of uranium, shows that the a and ft rays are produced quite 
independently of one another, and by matter of different chemical 
properties. 

1 Tram. Chem. Soc. 81, p. 460, 1902. 2 Phil. Mag. Sept. 1902. 

3 C. R. 131, p. 137, 1900. 



X] RADIO-ACTIVE PROCESSES 295 

190. Radio-activity of thorium. The radio-active pro- 
cesses occurring in thorium are far more complicated than those 
in uranium. It has been shown that a radio-active product 
Th X is continuously produced from the thorium. This Th X 
breaks up, giving rise to the radio-active emanation. This radio- 
active emanation, in turn, produces from itself the active matter, 
emanation X, which is responsible for the phenomenon of excited 
radio-activity. It has also been shown (section 176) that there is 
very strong evidence that the emanation X of thorium goes through 
two further changes, before the radio-active processes are at an 
end. 

The peculiarities of the initial portions of the decay and recovery 
curves of Th X and thorium respectively (Curves A and B, Fig. 34, 
p. 180), will now be considered. It was shown that when the 
Th X was removed from the thorium by precipitation with ammonia, 
the radiation increased about 15 per cent, during the first day, passed 
through a maximum, and then fell off according to an exponential 
law, decreasing to half value in four days. > At the same time the 
activity of the separated hydroxide decreased for the first day, 
passed through a minimum, and then slowly increased again, rising 
to its original value after the lapse of about one month. 

When a thorium compound is in a state of radio-active equi- 
librium, the series of changes in which Th X, the emanation, and 
emanation X are produced go on simultaneously. Since a state of 
equilibrium has been reached for each of these products, the 
amount of each product changing in unit time is equal to the 
amount of that product supplied from the preceding change in 
unit time. Now the matter Th X is soluble in ammonia, while 
the matter emanation X is not. The Th X is thus removed from 
the thorium by precipitation with ammonia, but the emanation X 
is left behind with the thorium. Since the emanation X is pro- 
duced from the emanation, which in turn arises from Th X, on the 
removal of the exciting cause Th X, the radiation due to this 
emanation X will decay, since the rate of production of fresh 
emanation X no longer balances its own rate of change. Disregard- 
ing the initial irregularity in the decay curve of emanation X 
(section 170), the activity of the emanation X will have decayed 
to half value in about 11 hours and to one quarter value at the 



296 KADIO-ACTIVE PROCESSES [CH. 

end of 22 hours. As soon, however, as the Th X has been separated, 
new Th X is produced in the thorium compound. The activity of 
this new Th X is not, however, sufficient to compensate at first for 
the loss of activity due to the change in emanation X, so that, as 
a whole, the activity will at first decrease, then pass through a 
minimum, then increase again. 

The correctness of this point of view has been tested by Ruther- 
ford and Soddy 1 as follows : If the precipitated thorium hydroxide 
after the removal of Th X is put through a series of precipitations 
with ammonia at short intervals, the Th X is removed almost as 
fast as it is formed, and, at the same time, the activity of the 
emanation X decays. 

The following table indicates the results obtained. A portion 
of the precipitated hydroxide was removed after each series of 
precipitations and its activity tested in the usual way. 

Activity of 
hydroxide per cent. 

After 1 precipitation 46 

After 3 precipitations at intervals of 24 hours ... 39 

After 3 more precipitations at intervals of 24 hours and 

3 at intervals of 8 hours 22 

After 3 more each of 8 hours 24 

After 6 more each of 4 hours ... ... ... ... 25 

The differences in the last three numbers are not significant, 
for it is difficult to make accurate comparisons of the activity of 
thorium compounds, which have been precipitated under slightly 
different conditions. It is thus seen that as a result of successive 
precipitations, the activity is reduced to a minimum of about 25 per 
cent. The recovery curve of the activity of this 23 times precipitated 
hydroxide is shown in Fig. 56. The initial drop in the curve is 
quite absent, and the curve, starting from the minimum, is practi- 
cally identical with the curve shown in Fig. 35, which gives the 
recovery curve of thorium hydroxide after the first two days. This 
residual activity about 25 per cent, of the maximum is non- 
separable from the thorium by any chemical process that has been 
tried. 

The initial rise of activity of Th X, after it has been separated, 
will now be considered. In all cases, it was found that the activity 

1 Tram. Chem. Soc. 81, p. 837, 1902. Phil. Mag. Nov. 1902. 



KADIO-ACTIVE PROCESSES 



297 



of the separated Th X had increased about 15 per cent, at the 
end of 24 hours, and then steadily decayed, falling to half value in 
about four days. 

This peculiarity of the Th X curve follows, of necessity, from the 
considerations already advanced to explain the drop in the recovery 
curve. As soon as the Th X is separated, it at once produces from 
itself the emanation, and this in turn produces the emanation X. The 
activity due to the emanation X at first more than compensates 
for the decay of activity of the Th X itself. The total activity 



100 



80 




40 



20 



12 



16 



20 



Time in 



Fig. 56. 

thus increases to a maximum, and then slowly decays to zero 
according to an exponential law with the time. The curve ex- 
pressing the variation of the activity of the separated Tb X with 
time can be deduced from the theory of successive changes already 
considered in section 175. In the present case there are three 
successive changes occurring at the same time, viz. the change of 
Th X into the emanation, of the emanation into emanation X, and 
the final changes giving rise to the activity of emanation X. Since, 
however, the change of the emanation into emanation X (about 
half changed in one minute) is far more rapid than the changes 
occurring in Th X or emanation X, for the purposes of calculation 
it may be assumed without serious error that the Th X changes at 



298 



RADIO-ACTIVE PROCESSES 



[CH. 



once into emanation X. The initial change of emanation X 
about half changed in 55 minutes will also .be disregarded for 
the same reason. 

Let \i and X 2 be the constants of decay of activity of Th X 
and emanation X respectively. Since the activity of Th X and 
of emanation X falls to half value in 4 days and 11 hours respec- 
tively, the value of \ = '0072 and of X 2 = '063 where 1 hour is 
taken as the unit of time. 

It has already been shown (section 175) that after a time t 
the activity I t , of a product in which there is a secondary change, 
is given by 



where 7 is the intensity of the radiation immediately after separa- 
tion and K the ratio of the ionization produced in the secondary 
change to that produced in the primary change. By comparison 
of this equation with the curve of variation of the activity of Th X 
with time shown in Fig. 57, Curve A, it is found that the value 
of K is about '44. 

The calculated values of -=* for different values of t are shown 



in the second column of the following table, and the observed values 
in the third column. 



; 

Time 


Theoretical 
value 


Observed 
value 





1-00 


1-00 


25 days 


1-09 





5 


1-16 





1 


'1-15 


1-17 


1-5 


1-11 





2 


1-04 





3 


875 


88 


* 


75 


72 


6 


53 


53 


9 


315 


295 


13 


157 


152 


1 







The theoretical and observed values thus agree within the 
limit of error in the measurements. The theoretical curve is 



RADIO-ACTIVE PROCESSES 



299 



shown in Curve A, Fig. 57 (with the observed points marked, for 
comparison). The curve B shows the theoretical curve of the decay 
of the activity of Th X and the emanation, supposing there is no 
secondary change into emanation X. Curve C shows the differ- 
ence curve between the curves A and B, i.e. the proportion of the 
activity at different times due to the emanation X. The activity 
due to emanation X thus rises to a maximum about two days after 
removal of the Th X, and then decays with the time at the same 




Time in Days. 
Fig. 57. 

rate as the Th X itself, i.e. the activity falls to half amount every 
four days. When the value of t exceeds four days, the value of 
0-^-W m the theoretical equation is very small. 
The equation of decay is thus expressed by 



i.e. the curve decays in an exponential law with the time. 



300 RADIO-ACTIVE PROCESSES [CH. 

191. Radio-activity of radium. Notwithstanding the 
enormous difference in their relative activities, the radio-activity 
of radium presents many close analogies to that of thorium. Both 
substances give rise to emanations which in turn produce " excited 
activity" on bodies in their neighbourhood. Radium, however, 
does not give rise to any intermediate product between the 
element itself and the emanation it produces, or in other words 
there is no product in radium corresponding to Th X in thorium. 

Giesel first drew attention to the fact that a radium compound 
gradually increased in activity after preparation, and only reached 
a constant value after a month's interval. If a radium compound 
is dissolved in water and boiled for some time, or a current of air 
drawn through the solution, on evaporation it is found that the 
activity has been diminished. The same result is observed if 
a solid radium compound is heated in the open air. This loss 
of activity is due to the removal of the emanation by the process 
of solution or heating. Consider the case of a radium compound 
which has been kept for some time in solution in a shallow vessel, 
exposed to the open air, and then evaporated to dryness. The 
emanation which, in the state of solution, was removed as fast as 
it was formed, is now occluded, and, together with the emana- 
tion X which it produces, adds its radiations to that of the original 
radium. The activity will increase to a maximum value where the 
rate of production of fresh emanation balances the rate of change 
of that already produced. 

If now the compound is dissolved or heated, the emanation 
escapes. Since the emanation X is not volatile and is insoluble 
in water, it is not removed by the process of solution or heating. 
Since, however, the exciting cause is removed, its activity will 
immediately begin to decay, and in the course of a few hours 
will have almost disappeared. The activity of the radium mea- 
sured by the rays is then found to be about 25 per cent, of its 
original value. This residual activity of radium, consisting entirely 
of a rays, is non-separable, and has not been further diminished by 
chemical or physical means. Rutherford and Soddy 1 examined the 
effect of aspiration for long intervals through a radium chloride 
solution. After the first few hours the activity was found to be 

1 Phil. Mag. April, 1903. 



x] 



RADIO-ACTIVE PROCESSES 



301 



reduced to 25 per cent., and further aspiration for three weeks 
did not produce any further diminution. The radium was then 
evaporated to dryness, and the rise of its activity with time 
determined. The results are shown in the following table. The 
final activity in the second column is taken as one hundred. In 
column 3 is given the percentage proportion of the activity re- 
covered. 



Time in days 


Activity 


Percentage 
Activity recovered 





250 





0-70 


33-7 


11-7 


1-77 


42-7 


23-7 


4-75 


68-5 


58-0 


7-83 


83-5 


78-0 


16-0 


96-0 


95-0 


21-0 


100-0 


100-0 



The results are shown graphically in Fig. 58. 



100 




Fig. 58. 

The decay curve of the radium emanation is shown in the 
same figure. The curve of recovery of the lost activity of radium 



302 RADIO-ACTIVE PROCESSES [CH. 

is thus analogous to the curves of recovery of uranium and thorium 
which have been freed from the active products Ur X and Th X 
respectively. The intensity I t of the recovered activity at any 

time is given by -j- = 1 e~ Kt , where / is the final value, and \ is 

M 

the radio-active constant of the emanation. The decay and recovery 
curves are complementary to one another. 

Knowing the rate of decay of activity of the radium emanation, 
the recovery curve of the activity of radium can thus at once be 
deduced, provided all of the emanation formed is occluded in the 
radium compound. 

When the emanation is removed from a radium compound by 
solution or heating, the activity measured by the (3 rays falls 
almost to zero, but increases in the course of a month to its 
original value. The curve showing the rise of rays with time 
is practically identical with the curve, Fig. 58, showing the re- 
covery of the lost activity of radium measured by the a. rays. The 
explanation of this result lies in the fact that the ft rays from 
radium only arise from emanation X, and that the non-separable 
activity of radium gives out only a rays. On removal of the 
emanation, the activity of the emanation X decays nearly to 
zero, and in consequence the ft rays almost disappear. When 
the radium is allowed to stand, the emanation begins to ac- 
cumulate, and produces in turn emanation X, which gives rise to 
ft rays. The amount of ft rays (allowing for a period of retarda- 
tion of a few hours) will then increase at the same rate as the 
activity of the emanation, which is continuously produced from 
the radium. 

192. If the radium allows some of the emanation produced to 
escape into the air, the curves of recovery will be different from 
that shown in Fig. 58. For example, suppose that the radium 
compound allows a constant fraction a of the amount of emana- 
tion, present in the compound at any time, to escape per second. 
If n is the number of emanation particles present in the com- 
pound at the time t, the number of emanation particles changing 
in the time dt is \ndt, where X is the constant of decay of activity 
of the emanation. If q is the rate of production of emanation 



X] RADIO-ACTIVE PROCESSES 303 

particles per second, the increase of the number dn in the time dt 
is given by 

dn qdt \ndt andt, 

or -jj = q - (X + a) n. 

The same equation is obtained when no emanation escapes, 
with the difference that the constant X + a is replaced by \. 

fj (Yl 

When a steady state is reached, -j- is zero, and the maximum value 
of n is equal to i * . 



If no escape takes place, the maximum value of n is equal to ^ . 

A 

The escape of emanation will thus lower the amount of activity 
recovered in the proportion - -- . If r? is the final number of 

A T* tt 

emanation particles stored up in the compound, the integration of 
the above equation gives = 1 e~ (K+a)t . 

The curve of recovery of activity is thus of the same general 
form as the curve when no emanation escapes, but the constant 
\ is replaced by X -I- a. 

For example, if a = X = 1/463000, the equation of rise of activity 

is given by = 1 e~ 2M , and, in consequence, the increase of 

activity to the maximum will be far more rapid than in the 
case of no escape of emanation. 

A very slight escape of emanation will thus produce large altera- 
tions both in the final maximum and in the curve of recovery of 
activity. 

A large number of experiments have been described by Mme Curie 
in her These pt'esentee d la Faculte des Sciences de Paris on the 
effect of solution and of heat in diminishing the activity of radium. 
The results obtained are in general agreement with the above view, 
that 75 per cent, of the activity of radium is due to the emana- 
tion and the excited activity it produces. If the emanation is 
wholly or partly removed by solution or heating, the activity of 



304 RADIO-ACTIVE PROCESSES [CH. 

the radium is correspondingly diminished, but the activity of 
the radium compound is spontaneously recovered owing to the 
production of fresh emanation. A state of radio-active equi- 
librium is reached, when the rate of production of fresh emanation 
balances the rate of change in the emanation stored up in the 
compound. The differences observed in the rate of recovery of 
radium under different conditions were probably due to variations 
in the rate of escape of the emanation. 

193. Non-separable activity. It has been shown that, for 
all three radio-elements, uranium, thorium, and radium, there is 
a non-separable activity consisting entirely of a. rays. In the 
case of uranium the activity is the same as the activity of the 
uranium, measured by the a rays, before the product Ur X, which 
gives rise only to ft rays, is removed. In the case of thorium 
and radium, where the active products produced give out a. rays, 
the non-separable activity is about 25 per cent, of the maximum 
activity measured by the a rays. 

The existence of a non-separable activity follows from the 
point of view of regarding radio-active processes which has been 
advanced in sections 87 and 127. The three radio-elements 
are supposed to be undergoing atomic disintegration, which is 
accompanied by the expulsion of a rays. If the number of atoms 
which break up per second is almost infinitesimal compared with 
the total number present, the same number, on an average, will 
break up per second. The number of a particles expelled per 
second will thus be a constant for each radio-element. There 
will thus always be a non-separable activity of the radio-elements, 
which is an inherent property of the elements and cannot be 
removed from them by any chemical or physical process. 

194. Radiations from the active products. Most of the 

changes occurring in the different radio-active products are accom- 
panied by the emission of a rays alone. The and 7 rays appear 
only in the final stages of the radio-active changes. 

It has been shown that the non-separable activity of the three 
radio-elements consists entirely of a rays. The two emanations give 
rise only to a rays (section 147). It also seems probable that the 
product Th X, if the emanation X which it produces is completely 



X] RADIO-ACTIVE PROCESSES 305 

separated from it, would give rise only to a rays. On the other 
hand, the activity of Ur X consists only of ft rays. The changes 
in the matter emanation X of thorium and radium give rise to 
both a and ft rays. It has been shown (section 176) that the 
emanation X of thorium goes through two changes, the first of 
which does not give rise to rays at all, and the second of which 
gives rise to a. and ft rays. The first change occurring in emana- 
tion X of radium gives rise to a. rays but not to ft rays; the 
second change probably does not give rise to rays at all, while 
the third change again includes all three kinds of rays. The 
absence of ft rays in the first change, taking place in the emana- 
tion X of radium, can readily be shown by exposing a negatively 
charged wire in the presence of the radium emanation for a few 
minutes. The activity on removal, measured by the a rays, falls 
rapidly, as is shown in Fig. 51, p. 262 ; but the activity measured 
by the ft rays alone is at first small, and increases for some time 
instead of diminishing. If a and ft rays had been both given out 
in the first change, it is to be expected that the amount of the ft 
radiation would initially decay at the same rate|.as the a radiation, 
but no such effect is observed. 

The ft and probably also the 7 rays of the three radio-elements 
thus only appear in the last of the series of radio-active changes. 
It is remarkable that the last change, which is readily detected 
by the radio-active property, should in each of the three radio- 
elements be accompanied by the expulsion of a single electron 
with great velocity, and that all the other changes, with the ex- 
ception of two that probably give rise to no rays at all, should 
be accompanied by the expulsion of a rays, i.e. of material particles 
atomic in size. 

The polonium of Mme Curie and the radio-tellurium of 
Marckwald emit only a rays. Becquerel 1 states that he has 
detected some rays of a penetrating character from polonium 
by the photographic method. The writer has examined by the 
electric method the radiations from the active preparation of 
radio- tellurium, but was unable to detect any trace of 7 rays. 
The evidence so far obtained points to the conclusion that the 

1 C. R. pp. 136, 977. 1903. 
R. R.-A. 20 



306 KADIO-ACTIVE PROCESSES [CH. 

7 rays appear at the same time as the ft rays, and in an amount 
proportional to them. 

The character of the radiations from each of the radio-active 
products must always be taken into consideration in the interpre- 
tation of results obtained by different methods of measurement. 
For example, a radium compound, which has been heated in an 
open glass tube, after a few hours practically loses its power of 
exciting fluorescence on a screen of platino-cyanide of barium 
placed near it. This is due to the fact already mentioned, that 
the ft and 7 rays practically disappear from radium for several 
hours after the emanation is removed. For the same reason the 
radium, with a screen placed over it of sufficient thickness to 
absorb all the a rays, would appear, when measured by the electric 
method, to be almost inactive. 

Since the a rays are photographically very inactive compared 
with the /3 rays, the non-separable activity of uranium, thorium, 
and radium, although producing marked ionization, would be 
almost inappreciable if tested by the photographic method. It has 
been stated by some observers that uranium and thorium have been 
obtained which showed no trace of activity. On examination of the 
results, however, it is found that the methods employed were not 
suitable to definitely settle the question. It is true that, by certain 
chemical processes, uranium and thorium can be obtained tempo- 
rarily inactive, when tested by the photographic method, or by the 
electric method if the compound is covered by a screen of sufficient 
thickness to absorb all the a. rays. If however the activity is tested 
electrically with unscreened active matter, there is always found to be 
a residual activity. In the course of time, the uranium and thorium 
compounds spontaneously regain the whole of their lost activity. 

195. Division of the activity amongst the products. 

It has been shown in section 190 that the activity of thorium 
hydroxide, after the removal of Th X, falls to 46 per cent, of its 
original value. When the Th X is removed from the thorium at 
short intervals, in order to allow the activity of the emanation X 
left behind to decay, there is a residual activity of 25 per cent, of 
the maximum. About 21 per cent, of the total activity is thus 
due to the emanation X. 



X] RADIO-ACTIVE PROCESSES 307 

This result is confirmed by observations on the increase of 
activity of Th X after removal. It has been shown in section 
190 that the activity due to the emanation X, produced from 
the Th X, is equal to '44 when the activity of the Th X, together 
with the emanation it produces, is taken as unity. Now the 
activity of the separated Th X and the emanation is equal to '54 
of the total. The proportion of the total activity due to emana- 
tion X is thus 24 per cent., a result which is not very different 
from the value of 21 per cent, obtained previously. It is difficult 
to make an accurate estimate of the activity of the emanation, 
compared with that due to the emanation X. An approximate 
estimate was however made in the following way. 

Some thorium hydroxide enclosed in a paper envelope was 
placed inside a closed cylinder with an insulated central electrode 
kept charged negatively. After an interval of several days, a state 
of radio-active equilibrium was reached, and the ionization was 
measured : 

(1) with the thorium inside the cylinder, and 

(2) with the thorium removed. 

(1) gave the ionization current due to the emanation and the 
emanation X on the central electrode, and (2) gave the current due 
to the emanation X. Taking into consideration that half of the 
radiation from the emanation X was absorbed in the central elec- 
trode, it was deduced that the amount of ionization produced by the 
emanation in the gas was not very different from that produced by 
the emanation X. This result points to the conclusion that the 
emanation and emanation X in a thorium compound supply about 
an equal proportion of the total activity. The relative activities 
of the different products are shown in the following table. The 
numbers must only be considered as approximate. 

Residual activity of thorium 25% 

Activity of ThX alone 21 / 

emanation alone ... ... ... ... 24 / 

due to first change emanation X ... ... / 

second 24 % 

Leaving out of account the first change in emanation X, which 
is of a character quite distinct from the others, it is seen that the 
activity is approximately equally divided amongst the products. 

202 



308 RADIO-ACTIVE PROCESSES [CH. 

Similar results hold in the case of radium. The emanation 
and the residual activity of radium supply about 18 and 25 per cent, 
respectively of the total activity, and the rest is supplied by the 
changes in emanation X. These results are thus also in rough 
agreement with those obtained for thorium, and indicate that each 
change which gives rise to a rays supplies about an equal fraction 
of the total activity. This is an important result, for it indicates 
that about the same number of a particles is expelled at each 
change, which gives rise to a rays. This deduction is based on the 
observed fact that the penetrating power and consequently the 
ionization produced by such a particles is not very different. It 
therefore seems probable that, when a compound of thorium or of 
radium is in radio-active equilibrium, the same number of systems 
change per second in each of the products, and that the change in 
all cases but one is accompanied by the expulsion of about the 
same number of a particles. 

196. Conservation of radio-activity. The early observa- 
tions on uranium and thorium had shown that their radio-activity 
remained constant over the period of several years during which 
they were examined. The possibility of separating from uranium 
and thorium the active products Ur X and Th X respectively, the 
activity of which decayed with the time, seemed at first sight to be 
contradictory to this point of view. Further observation, however, 
showed that the total radio-activity of these bodies was not altered 
by the chemical processes, for it was found that the uranium 
and thorium from which the active products were removed, spon- 
taneously regained their radio-activity. At any time after removal 
of the active product, the sum total of the radio-activity of the 
separated product together with that of the substance from which 
it has been separated is always equal to that of the original com- 
pound before separation. In cases where the active products, like 
Ur X and the radium emanation, decay with time according to an 
exponential law, this follows at once from the experimental results. 
If i t is the activity of the product at any time t after separation, 

and / the initial value, we know that j =e~^. At the same 

*o 

time the activity I t recovered during the interval t is given by 



X] RADIO-ACTIVE PROCESSES 309 

y = 1 e~ M , where \ is the same constant as before. It thus 

*o 

follows that i t + I t = / , which is an expression of the above result. 
The same is also true whatever the law of decay of activity of the 
separated product (see section 174). For example, the activity of 
Th X after separation from thorium at first increases with the 
time. At the' same time, the activity of the residual thorium 
compound at first decreases, and at such a rate, that the sum of 
the activities of the thorium and its separated product is always 
equal to that of the original thorium. 

This principle of " conseivatior^p^^dioj-actiyity 1 ,'' as it may be 
termed, follows from the general result that the radio-active pro- 
cesses cannot in any way be changed by the action of known 
forces. It may be recalled that the rate of decay of the activity of 
a radio-active product cannot be altered by any known agency. 
The rate of decay is independent of the concentration of the active 
matter, of the pressure and of the nature of the gas in which it is 
placed, and is not affected by wide ranges of temperature. In the 
same way, it has not been found possible to alter the rate of 
production of active matter from the radio- elements. In addition, 
there is not a single case yet observed where radio-activity has 
been altered or destroyed in any active body or created in an 
inactive element. 

Certain cases have been observed, which at first sight seem to 
indicate a destruction of radio-activity. For example, the excited 
radio-activity is removed from a platinum wire when heated above 
a red heat. It has been shown, however, by Miss Gates (sec- 
tion 180) that the radio^agtiviiby^ is jiot destroyed, but is deposited 
in unaltered amount on the colder bodies surrounding it. Thorium 
oxide has beeTT shown to lose its power in emanating to a large 
extent by ignition to a white heat. But a close examination shows 
that the emanation is still being produced at the samejrate, but is 
occluded in the compound. 

The total radio-activity of a given mass of a radio-element, 
measured by the peculiar radiations emitted, is a quantity which 
can neither be increased nor diminished, although it may be mani- 
fested in a series of products which are capable of separation from 
1 Rutherford and Soddy, Phil. Mag. May, 1903. 



310 KADIO-ACTIVE PROCESSES [CH. 

the radio-element. The term "conservation of radio-activity" is 
thus a convenient expression of the facts known at the present 
time. It is quite possible, however, that further experiments at 
very high or very low temperatures may show that the radio- 
activity does vary. For example, Dewar states that the heat 
emission of radium seems to be rather greater in liquid hydrogen 
than in liquid oxygen. An increase of heat emission would 
probably entail an increase of the radio-activity of the radium 
immersed in liquid hydrogen. Accurate experiments have not, 
however, yet been made on the radio-activity of radium at such 
low temperatures. 

Although no difference has been observed in the radio-activity 
of uranium over an interval of five years, it will be shown (sec- 
tion 203) that on theoretical grounds the radio-activity of a given 
quantity of a radio-element should decrease with the time. The 
change will, however, be so slow in uranium and thorium, that 
probably thousands if not millions of years must elapse before a 
measurable change would have taken place. In radium, however, 
the change takes place about one million times faster, so that a 
measurable alteration should be detected in the course of a few 
years 1 . The total radio-activity of a given quantity of matter left 
to itself should thus decrease, but it should be constant for a 
constant mass of the radio-element. It is only in this restricted 
sense that the principle can be employed. 

The conservation of radio-activity applies not only to the 
radiations taken as a whole, but also to each specific type of 
radiation. If the emanation is removed from a radium compound, 
the amount of (3 radiation of the radium at once commences to 
decrease, but this is compensated by the appearance of /3 rays 
in the radiations from the vessel in which the separated emanation 
is stored. At any time the sum total of the ft radiations from the 
radium and the emanation vessel is always the same as that from 
the radium compound before the emanation was removed. 

1 It seems probable however that the radio-activity of radium, measured by the 
a rays, will increase rather than diminish for several hundred years after its 
separation. This is due to the fact that the increase of the activity due to the last 
slow change of radium (about half changed in 200 years) will probably more than 
compensate for the change in the radium itself. Ultimately, however, the radio- 
activity of the radium must decrease with time. 



X] RADIO-ACTIVE PROCESSES 311 

Similar results have also been found to hold for the 7 rays. 
This was tested by the writer in the following way. The emana- 
tion from some solid radium bromide was released by heat, 
and condensed in a small glass tube which was then sealed off. 
The radium so treated, and the emanation tube, were placed 
together under an electroscope, with a screen of lead 1 cm. thick 
interposed in order to let through only the 7 rays. The experi- 
ments were continued over three weeks, but the sum total of the 
7 rays from the radium and the emanation tube was, over the 
whole interval, equal to that of the original radium. During this 
period the amount of 7 rays from the radium at first decreased to 
only a few per cent, of the original value, and then slowly increased 
again, until at the end of the three weeks it had nearly regained 
its original value, before the emanation was removed. At the same 
time the amount of 7 rays from the emanation tube rose from zero 
to a maximum and then slowly decreased again at the same rate 
as the decay of the activity of the emanation in the tube. This 
result shows that the amount of 7 rays from radium was a constant 
quantity over the interval of observation, although the amount of 
7 rays from the radium and emanation tube had passed through a 
cycle of changes. 

197. Resume of results. Before discussing the general 
theory advanced to account for the processes occurring in the radio- 
element, a brief resume will be given of the more important results 
already described in detail in previous chapters. 

The radio-activity of uranium, thorium, and radium has been 
shown to be maintained by the production at a constant rate of new 
kinds of matter, which possess temporary activity. The constant 
activity of the radio-elements is due to a state of equilibrium where 
the rate of production of new active matter compensates for the 
change in that already produced. In some cases, the active products 
possess well-defined chemical properties different from those of the 
parent elements and can be separated from them by chemical 
means. The separation of Ur X and Th X from uranium and 
thorium are good examples of this process. In other cases, the 
new products, as in the case of the thorium and radium emana- 
tions, are gaseous in character and are released from the radio- 



312 RADIO-ACTIVE PROCESSES [CH. 

elements by the process of diffusion. These emanations have been 
shown to possess the properties of gases. The radium emanation 
diffuses and distributes itself between two reservoirs kept at 
different temperatures according to the laws of gases. Both the 
emanations can be condensed by cold, and by that means can be 
removed from the other gases with which they are mixed. The 
emanations possess the property of being occluded in some bodies, 
including the radio-elements themselves, and can be liberated by 
heating or solution. They diffuse through porous partitions, and 
in general behave like chemically inert gases of high molecular 
weight. Other radio-active products, for example the emanations 
X of radium and of thorium, are not gaseous, but attach themselves 
to the surface of bodies and can be removed from them by solution 
or heating. The emanation X of thorium, for example, possesses 
some chemical properties which distinguish it not only from the 
emanation from which it is derived, but also from the other active 
product Th X. It is soluble in some acids and not in others. If 
the emanation X of thorium is removed from an active wire by 
solution in hydrochloric acid, the active matter attaches itself to 
some metals dipped in the acid but not to others, and in general 
possesses the properties of matter of definite chemical behaviour. 
The differences in the chemical and physical properties of the 
different products of a radio-element can be well illustrated in the 
case of thorium. Thorium X is soluble in ammonia, while thorium 
is not. Thorium X produces the emanation which is a gas, in- 
soluble in acids but condensed by cold. This, in turn, produces 
the matter emanation X, which is insoluble in ammonia but soluble 
in hydrochloric and sulphuric acids. There can be little doubt 
that these active products are material in nature. They differ 
from ordinary matter in their power of emitting rays of a special 
character, and by the fact that they exist in the radio-elements in 
minute quantities which are, in most cases, too small to be detected 
by the balance or the spectroscope. Approximate estimates (sec- 
tion 162) can be made of the amount of these active products that 
are present in a radio-element when in radio-active equilibrium, 
and it has been shown that, except in the case of a very active 
body like radium, the amount is too small to be detected by 
ordinary chemical means. 



OF TH: 
UNIVERSITY 

c OF 
RADIO-ACTIVE PROCESSES 

The case of the radium emanation however is different. It 
can be shown (section 162) that probably the emanation exists in 
greater proportion in radium than radium does in pitchblende. 
Yet radium was detected in pitchblende by the property of radiat- 
ing which it possesses, and has been isolated from it and found to 
be an element of well-marked chemical and physical properties. 
It has been estimated that 1 gram of radium in a state of radio- 
active equilibrium probably contains a volume of about 0'3 cubic 
millimetre of the emanation, measured at standard pressure and 
temperature. From a kilogram of radium 0'3 cubic centimetre 
would be produced. When larger quantities of radium are available 
for experiments, there can thus be little doubt that a sufficient 
amount of the radium emanation will be obtained to examine 
chemically. In fact, as will be shown in section 201, even with 
the small quantities of radium now available, some very important 
spectroscopic evidence has already been obtained, in regard to the 
processes occurring in the emanation. 

[Volume of the Emanation 1 . 

Sir William Ramsay and Mr Soddy have kindly placed at my 
disposal some preliminary results of a recent investigation by them 
on the volume of the emanation released from radium. In these 
experiments, 60 milligrams of pure radium bromide were used, in 
which the emanation had been allowed to collect for 8 days. This 
emanation, freed as far as possible from all other gases, was forced 
into a capillary tube in which its volume was measured. The 
following table shows the results obtained. 

Initial volume 0*124 cubic millimetre. 
Volume after 1 day 0*027 
3 days O'Oll 
6 0-0063 
9 0-0041 

12 0-0011 

Final volume 0-0004 

The volume of the gas obtained shrank rapidly during the first 
day, then more slowly, and after the third day decreased approxi- 
mately according to an exponential law with the time, decreasing 
1 Added Feb. 1, 1904. 



314 RADIO-ACTIVE PROCESSES [CH. 

to half value in about four days. According to the views already 
advanced, it is to be expected that the volume of the emanation 
itself should decrease according to an exponential law with the 
time, falling to half value in about 3*7 days. This is a result of the 
theory that half of the emanation at the end of 3'7 days has been 
transformed into the substance emanation X, which behaves as a 
solid and is deposited on the surface of the containing vessel. It 
seems not improbable that the rapid decrease, observed during the 
first day, may have been due to the presence, with the emanation, 
of another gas which was quickly absorbed either by the walls of 
the capillary tube or by the mercury. 

It can readily be deduced that the volume of the emanation at 
the end of the first day is equivalent to the amount derived from 
0'022 gram of pure radium in radio-active equilibrium. Taking 
the volume of the gas, 0'027 cubic millimetre, at the end of the 
first day as the true volume of emanation, it follows that the 
volume of the emanation to be obtained from 1 gram of radium 
in radio-active equilibrium is 1*2 cubic millimetres. Taking the 
volume observed on the third day, the corresponding value is 
0*9 cubic millimetre. The calculated value of the volume of the 
emanation to be derived from 1 gram of radium is 0*3 cubic 
millimetre. The calculated value is thus of the right order of 
magnitude. This is an indication of the general correctness of 
the different methods of calculation (see sections 104 and 162) on 
which the theoretical determination of the volume of the emanation 
has been based. 

It will be shown later, in section 201, that the emanation of 
radium produces helium from itself. The shrinkage of the volume 
to a very small fraction of its original value indicates that the 
helium produced was buried in the walls of the tube. This is to 
be expected if the helium consists in reality of the a particles 
expelled from the emanation and its products. The a. particle is 
projected with sufficient velocity to penetrate a distance of about 
02 millimetre into the walls of the capillary tube. It is to be 
expected that a portion, at least, of the buried helium should be 
released when the tube is strongly heated.] 

Of the three types of rays from the active bodies, the a and ft 



X] RADIO-ACTIVE PROCESSES 315 

rays are material in nature. The mass and velocity of the a 
particles, projected from radium, have been measured. They have 
been shown to be positively charged particles projected with a 
velocity of about 1/10 the velocity of light, and a mass about 
twice that of the hydrogen atom. The particles carry a negative 
charge, and have an apparent mass about 1/1000 the mass of the 
hydrogen atom. They are identical in character with the cathode 
ray particle produced in a vacuum tube. The nature of the 7 rays 
has not yet been determined. 

The a rays play by far the most important part in radio-active 
processes. Most of the energy radiated in the form of ionizing rays 
is due to them. In addition, most of the active products emit only 
a rays. The and 7 rays in most cases only appear in the last 
stage of the radio-active processes. 

The activity of most of the products decays according to an 
exponential law with the time. In cases where this does not hold, 
the activity can be shown to be due to several successive changes, 
the rate of each of which decays according to an exponential law 
but with a different radio-active constant. The rate of decay of 
activity has not yet been found to be in any way influenced by 
wide variation in chemical and physical conditions. 

The activity of any product at any time (section 124), is 
proportional to the rate of change of the product, and is also pro- 
portional to the amount of matter left unchanged. In cases where 
one active product gives rise to another, the activity of the first 
product is, at any time, a measure of the rate of production of the 
second product. In other words, the radiations accompany the 
change of one product into another, and serve as a measure of the 
rate of change. This point of view at once follows if the expulsion 
of rays is taken to be the cause of the change from one product 
into another. The rate of emission of a. particles is a measure of 
the rate of change of the first product, i.e. it is proportional to the 
rate at which the second product is produced. For example, the 
amount of emanation X of thorium produced in a given time by 
the thorium emanation is proportional to the activity of the ema- 
nation. In cases where the rate of change of the second product is 
rapid compared with that of the first, after sufficient interval has 
elapsed in order to reach a state of approximate radio-active equi- 



316 RADIO-ACTIVE PROCESSES [CH. 

librium, the activity of the second product is always found to vary 
at the same rate as that of the parent product. For example, the 
amount of the emanation produced by Th X is always proportional 
to the activity of the Th X, and decays at the same rate, i.e. it 
falls to half value in about four days. In the same way, the activity 
of the emanation X, produced by the radium emanation, after some 
hours have elapsed for conditions to become steady, is always found 
to be proportional to the activity of the emanation. In other 
words, the activity of the emanation X decays according to the 
same law, and at the same rate, as the radium emanation which 
produces it, i.e. to half value in a little less than four days. 

The rapid rate of heat emission of radium is connected with 
the radio-activity of that element. More than two-thirds of the 
heat emission of radium is due to the radium emanation and the 
secondary products to which the emanation gives rise. The heat 
emission seems to be for the most part connected with the emission 
of a. rays. 

The total energy which would be given out by a given quantity 
of radium is of quite a different order of magnitude to that ob- 
served in ordinary chemical reactions. 

198. Theories of radio-activity. A brief review will now 
be given of the working hypotheses which have served as a guide 
to the investigators in the field of radio-activity. These working 
theories have in many cases been modified or extended with the 
growth of experimental knowledge. 

The early experiments of Mme Curie had indicated that radio- 
activity was an atomic and not a molecular phenomenon. This 
was still further substantiated by later work, and the detection and 
isolation of radium from pitchblende was a brilliant verification of 
the truth of this hypothesis. 

The discovery that the /3 rays of the radio-elements were 
similar to the cathode rays produced in a vacuum tube was an 
important advance, and has formed a basis of several subsequent 
theories. J. Perrin 1 , in 1901, following the views of J. J. Thomson 
and others, suggested that the atoms of bodies consisted of parts 
and might be likened to a miniature planetary system. In the 

1 Revue Scientifique, April 13, 1901. 



X] RADIO-ACTIVE PROCESSES 317 

atoms of the radio-elements, the parts composing the atoms more 
distant from the centre might be able to escape from the central 
attraction and thus give rise to the radiation of energy observed. 
In December 1901, Becquerel 1 put forward the following hypo- 
thesis, which he stated had served him as a guide in his investi- 
gations. According to the view of J. J. Thomson, radio-active 
matter consists of negatively and positively charged particles. The 
former have a mass about 1/1000 of the mass of the hydrogen 
atom, while the latter have a mass about one thousand times 
greater than the negative particle. The negatively charged par- 
ticles (the /9 rays) would be projected with great velocity, but 
the larger positive particles would have much lower velocity and 
would form as a sort of gas (the emanation) which would deposit 
itself on the surface of bodies. This in turn would subdivide 
giving rise to rays (excited activity). 

In a paper communicated to the Royal Society in June 1900, 
Rutherford and McClung 2 showed that the energy, radiated in 
the form of ionizing rays into the gas, was- 3000 gram-calories per 
year for radium of activity 100,000 times that of uranium. Taking 
the latest estimate 1,500,000 of the activity of a pure radium com- 
pound, this would correspond to an emission of energy into the 
gas in the form of a rays of about 45,000 gram-calories per gram 
per year. The suggestion was put forward that this energy might 
be derived from a re-grouping of the constituents of the atom of 
the radio-elements, and it was pointed out that the possible energy 
to be derived from a greater concentration of the components of 
the atom was large compared with that given out in molecular 
reactions. 

In the original papers 3 giving an account of the discovery of the 
emanation of thorium and the excited radio-activity produced by 
it, the writer took the view that both of these manifestations were 
due to radio-active material. The emanation behaved like a gas, 
while the matter which caused excited activity attached itself to 
solids and could be dissolved in some acids but not in others. In 
conjunction with Miss Brooks, it was shown that the radium 
emanation diffused through air like a gas of heavy molecular 

1 C. R. 133, p. 979, 1901. 2 Phil. Trans. A, p. 25, 1901. 

3 Phil. Mag. Jan. and Feb. 1900. 



318 RADIO-ACTIVE PROCESSES [CH. 

weight. At a later date Mr Soddy and the writer showed that the 
radium and thorium emanations behaved like chemically inert 
gases, since they were unaffected by the most drastic physical 
and chemical treatment. 

On the other hand, P. Curie, who, in conjunction with Debierne, 
had made a series of researches on the radium emanation, expressed 
dissent from this view. P. Curie 1 did not consider that there was 
sufficient evidence that the emanation was material in nature, and 
pointed out that no spectroscopic evidence of its presence had yet 
been obtained, and also that the emanation disappeared when 
contained in a sealed vessel. It was pointed out by the writer 2 
that the failure to detect spectroscopic lines was probably a conse- 
quence of the minute quantity of the emanation present, under 
ordinary conditions, although the electrical and phosphorescent 
actions produced by this small quantity are very marked. This 
contention is borne out by calculations of the probable amount of 
the emanation released from 1 gram of radium given in section 162. 
P. Curie took the view that the emanation was not material, 
but consisted of centres of condensation of energy attached to the 
gas molecules and moving with them. 

M. and Mme Curie have throughout taken a very general view 
of the phenomena of radio-activity, and have not put forward any 
definite theory. In Jan. 1902, they gave an account of the general 
working theory 3 which had guided them in their researches. 
Radio-activity is an atomic property, and the recognition of this 
fact had created their methods of research. Each atom acts as a 
constant source of emission of energy. This energy may either 
be derived from the potential energy of the atom itself, or each 
atom may act as a mechanism which instantly regains the energy 
which is lost. They suggested that this energy may be borrowed 
from the surrounding air in some way not contemplated by the 
principle of Carnot. 

In the course of a detailed study of the radio-activity of thorium, 
Mr Soddy and the writer 4 found that it was necessary to suppose 
that thorium was continuously producing from itself new kinds of 

1 C. R. 136, p. 223, 1903. 2 PhiL Mag ^ April> 19Q3 

3 C. R. 134, p. 85, 1902. 

4 Trans. Chem. Soc. 81, pp. 321, 837, 1902. Phil. Mag. Sept. and Nov. 1902. 



X] RADIO-ACTIVE PROCESSES 319 

active matter, which possess temporary activity and differ in 
chemical properties from the thorium. The constant radio-activity 
of thorium was shown to be the result of equilibrium between the 
processes of production of active matter and the change of that 
already produced. At the same time, the theory was advanced 
that the production of active matter was a consequence of the dis- 
integration of the atom. The work of the following year was 
devoted to an examination of the radio-activity of uranium and 
radium on similar lines, and it was found that the conclusions 
already advanced for thorium held equally for uranium and radium 1 . 
The discovery of a condensation of the radio-active emanations 2 
gave additional support to the view that the emanations were 
gaseous in character. In the meantime, the writer 3 had found that 
the rays consisted of positively charged bodies atomic in size, 
projected with great velocity. The discovery of the material 
nature of these rays served to strengthen the theory of atomic 
disintegration, and at the same time to offer an explanation of 
the connection between the a rays and the changes occurring in 
the radio-elements. In a paper entitled "Radio-active Change," 
Mr Soddy and the writer 4 put forward in some detail the theory 
of atomic disintegration as an explanation of the phenomena of 
radio-activity, and at the same time some of the more important 
consequences which follow from the theory were discussed. 

In a paper announcing the discovery of the heat emission of 
radium, P. Curie and Laborde 8 state that the heat energy may be 
equally well supposed to be derived from a breaking up of the 
radium atom or from energy absorbed by the radium from some 
external source. 

J. J. Thomson in an article on "Radium," communicated to 
Nature*, put forward the view that the emission of energy from 
radium is probably due to some change within of the atom, and 
pointed out that a large store of energy would be released by a 
contraction of the atom. 

Sir William Crookes 7 , in 1899, proposed the theory that the 

1 Phil. Mag. April, 1903. 2 Phil. Mag. May, 1903. 

Phys. Zeit. 4, p. 235, 1902. Phil. Mag. Feb. 1903. 
4 Phil. Mag. May, 1903. 5 C. R. 136, p. 673, 1903. 

6 Nature, p. 601, 1903. 7 C. R. 128, p. 176, 1899. 



320 RADIO-ACTIVE PROCESSES [CH. 

radio-active elements possess the property of abstracting energy 
from the gas. If the moving molecules, impinging more swiftly 
on the substance, were released from the active substance at a 
much lower velocity, the energy released from the radio-elements 
might be derived from the atmosphere. This theory was advanced 
again later to account for the large heat emission of radium, 
discovered by P. Curie and Laborde. 

Fillipo Re 1 recently advanced a very general theory of matter 
with a special application to radio-active bodies. He supposes 
that the parts of the atom were originally free, constituting a 
nebula of extreme tenuity. These parts have gradually become 
united round centres of condensation, and have thus formed the 
atoms of the elements. On this view an atom may be likened 
to an extinct sun. The radio-active atoms occupy a transition 
stage between the original nebula and the more stable chemical 
atoms, and in the course of their contraction give rise to the 
heat emission observed. 

Lord Kelvin in a paper to the British Association, 1903, has 
suggested that radium may obtain its energy from external sources. 
If a piece of white paper is put into one vessel and a piece of black 
paper into an exactly similar vessel, on exposure of both vessels to 
the light the vessel containing the black paper is found to be at a 
higher temperature. He suggests that radium in a similar manner 
may keep its temperature above the surrounding air by its power 
of absorption of unknown radiations. 

199. Discussion of Theories. From the survey of the 
general hypotheses advanced as possible explanations of radio- 
activity, it is seen that they may be broadly divided into two 
classes^one of which assumes that the energy emitted from the 
radio-elements is derived at the expense of the internal energy of 
the atom, and the other that the energy is derived from external 
sources, but that the radio-elements act as mechanisms capaBleT of 
transforming this borrowed energy into the special forms manifested 
in the phenomena of radio-activity. Of these two sets of hypo- 
theses the first appears to be the most probable, and to be best 
supported by the experimental evidence. Up to the present not 
1 C. R. p. 136, p. 1393, 1903. 



X] RADIO-ACTIVE PROCESSES 321 

the slightest experimental evidence has been adduced to show 
that the energy of radium is derived from external sources. 

J. J. Thomson (loc. cit.) has discussed the question in the 
following way : 

" It has been suggested that the radium derives its energy from 
the air surrounding it, that the atoms of radium possess the faculty 
of abstracting the kinetic energy from the more rapidly moving air 
molecules while they are able to retain their own energy when in 
collision with the slowly moving molecules of air. I cannot see, 
however, that even the possession of this property would explain 
the behaviour of radium ; for imagine a portion of radium placed 
in a cavity in a block of ice ; the ice around the radium gets 
melted ; where does the energy for this come from ? By the hypo- 
theses there is no change in the air-radium system in the cavity, 
for the energy gained by the radium is lost by the air, while heat 
cannot flow into the cavity from the outside, for the melted ice 
round the cavity is hotter than the ice surrounding it." 

The writer has recently found that the activity of radium is 
not altered by surrounding it with a large mass of lead. A cylinder 
of lead was cast 10 cms. in diameter and 10 cms. high. A hole 
was bored in one end of the cylinder to the centre, and the radium, 
enclosed in a small glass tube, was placed in the cavity. The 
opening was then hermetically closed. The activity was measured 
by the rate of discharge of an electroscope by the 7 rays trans- 
mitted through the lead, but no appreciable change was observed 
during a period of one month. 

M. and Mme Curie early made the suggestion that the radiation 
of energy from the radio-active bodies might be accounted for by 
supposing that space is traversed by a type of Rontgen rays, and 
that the radio-elements possess the property of absorbing them. 
Recent experiments (section 215) have shown that there is present 
at the surface of the earth a very penetrating type of rays, similar 
to the 7 rays of radium. Even if it were supposed that the radio- 
elements possessed the power of absorbing this radiation, the 
energy of the rays is far too minute to account even for the energy 
radiated from an element of small activity like uranium. In 
addition, all the evidence so far obtained points to the conclusion 
that the radio-active bodies do not absorb the type of rays they 
R. R.-A. 21 



322 RADIO-ACTIVE PROCESSES [CH. 

emit to any greater extent than would be expected from their 
density. It has been shown (section 79) that this is true in the 
case of uranium. Even if it were supposed that the radio-elements 
possess the property of absorbing the energy of some unknown 
type of radiation, which is able to pass through ordinary matter 
with little absorption, there still remains the fundamental difficulty 
of accounting for the peculiar radiations from the radio-elements, 
and the series of changes that occur in them. It is not sufficient 
to account for the heat emission only, for it has been shown (section 
106) that the emission of heat is directly connected with the radio- 
activity. 

In addition, the distribution of the heat emission of radium 
amongst the radio-active products which arise from it is extremely 
difficult to explain on the hypothesis that the energy emitted 
is borrowed from external sources. It has been shown that more 
than two-thirds of the heat emitted by radium is due to the 
emanation together with the matter emanation X, which is pro- 
duced by the emanation. When the emanation is separated from 
the radium, its power of emitting heat, after reaching a maximum, 
decreases with the time according to an exponential law. It 
would thus be necessary on the absorption hypothesis to postulate 
that most of the heat emission of radium, observed under ordinary 
conditions, is not due to the radium itself but to something pro- 
duced by the radium, the power of which of absorbing energy from 
external sources diminishes with time. 

The strongest evidence against the hypothesis of absorption of 
external energy is that such a theory ignores the fact, that, when- 
ever radio-activity is observed, it is always accompanied by some 
change which can be- detected by the appearance of new products 
having chemical properties distinct from those of the original 
substances. This leads to some form of " chemical " theory, and 
other results show that the change is atomic and not molecular. 

200. Theory of radio-active change. The processes occur- 
ring in the radio-elements are of a character quite distinct from any 
previously observed in chemistry. Although it has been shown 
that the radio-activity is due to the spontaneous and continuous 
production of new types of active matter, the laws which control 



X] KADIO-ACTIVE PROCESSES 323 

this production are different from the laws of ordinary chemical 
reactions. It has not been found possible in any way to alter 
either the_j-ate at which the matter is produced or its rate of 
change when produced. Temperature, which is such an important 
factor in altering the rate of chemical reactions, is, in these cases, 
entirely without influence. In addition, no ordinary chemical 
change is known which is accompanied by the expulsion of charged 
atoms with great velocity. It has been suggested by Armstrong 
and Lowry 1 that radio-activity may be an exaggerated form of 
fluorescence or phosphorescence with a very slow rate of decay. 
But no form of phosphorescence has yet been shown to be accom- 
panied by radiations of the character of those emitted by the 
radio-elements. Whatever hypothesis is put forward to explain 
radio-activity must account not only for the production of a series 
of active products, which differ in chemical and physical properties 
from each other and from the parent element, but also for the 
emission of rays of a special character. Besides this, it is necessary 
to account for the large amount of energy continuously radiated 
from the radio-elements. 

The radio-elements, besides their high atomic weights, do not 
possess in common any special chemical characteristics which differ- 
entiate them from the elements, which do not possess the property 
of radio-activity to an appreciable degree. Of all the known ele- 
ments, uranium, thorium, and radium possess the heaviest atomic 
weights, viz.: radium 225, thorium 232*5, and uranium 240. 

If a high atomic weight is taken as evidence of a complicated 
structure of the atom, it might be expected that disintegration 
might occur more readily in heavy than in light atoms. At the 
same time, there is no reason to suppose that the elements of the 
highest atomic weight should be the most radio-active; in fact, 
radium is far more active than uranium, although its atomic weight 
is less. This is seen to be the case also in the radio-active pro- 
ducts; for example, the radium emanation is enormously more 
active weight for weight than the radium itself, and there is 
every reason to believe that the emanation has an atom lighter 
than that of radium. 

1 Proc. Roy. Soc. 1903. 

212 



\324 
Un 



EADIO-ACTIVE PROCESSES [CH. 



order to explain the phenomena of radio-activity, Rutherford 
aVi Soddy have advanced the theory that jJn-ftk)ms of the radio- 
el\inents suffer spontaneous disintegration, and that each disinte- 
grated atom passes through a succession of well-marked changes, 
accompanied in most cases by the emission of a rays. 

A preliminary account of this hypothesis to explain special 
phenomena has already been given in sections 87 and 127. It 
will now be applied generally to the radio-elements, and the con- 
sequences, which follow from it, will be considered. It is supposed 
that, on an average, a definite small proportion of the atoms of each 
radio- element becomes unstable at a given time. As a result of 
this instability, an a. particle is expelled with great velocity. The 
continuous expulsion of these a particles gives rise to the non- 
separable activity of the radio-elements, which has been shown to 
consist entirely of a rays (section 193). The expulsion of an 2 par- 
ticle, of mass about twice that of the hydrogen atom, leaves behind it 
a new system lighter than the original one, and possessing chemical 
and physical properties quite different from those of the original 
element. This new system again becomes unstable, and expels 
another a particle. The process of disintegration, once started, 
proceeds from stage to stage at a definite measurable rate in each 
case. At each stage, with the exception of one change in thorium 
and one in radium which are not accompanied by rays at all, one 
or more a particles are projected, until the last stages are reached, 
when the ft and 7 rays appear. The term metabolon has been 
suggested as a convenient expression for each of these changing 
atoms, derived from the successive disintegration of the atoms of 
the radio-elements. Each metabolon, on an average, exists only 
for a limited time. In a collection of metabolons of the same kind 
the number N, which are unchanged at a time t after production, 
is given by N = N e~ M where N Q is the original number. Now 

, = \N, or the fraction of the metabolons present, which change 

in unit time, is equal to X. The value l/\ may thus be taken as 
the average life of each metabolon. 

The various metabolons from the radio-elements are distin- 
guished from ordinary matter by their great instability and conse- 
quently rapid rate of change. Since a body which is radio-active 



X] RADIO-ACTIVE PROCESSES 325 

must ipso facto be undergoing change, it follows that none of the 
active products, for example, the emanations and Th X, can consist 
of any known kind of matter; for there is no evidence to show that 
inactive matter can be made radio-active, or that two forms of the 
same element can exist, one radio-active and the other not. For 
example, half of the matter constituting the radium emanation 
has undergone change after an interval of four days. After the 
lapse of about one month the emanation as such has nearly 
disappeared, having been transformed through several stages into 
other and more stable types of matter, which are in consequence 
difficult to detect by their radio-activity. 

The following table gives the list of the active products or 
metabolons known to result from the disintegration of the three 
radio-elements. In the second column is given the value of the 
radio-active constant X for each active product, i.e. the proportion 
of the active matter undergoing change per second ; in the third 
column, the time r required for the activity to fall to one-half, i.e. 
the time taken for half the active product to undergo change ; in 
the third column, the nature of the rays from each active product, 
not including the rays from the products which result from it ; in 
the fourth column, a few of the more marked physical and chemical 
properties of each metabolon. 

There are two well-marked changes in uranium, five in 
thorium, and six in radium. It is quite probable that a closer 
examination of the active products may lead to the discovery of 
still further changes. For example, the evidence obtained by 
von Lerch (section 179) from the electrolysis of a solution of 
emanation X of thorium points to the conclusion that there is an 
additional change occurring in emanation X, for which the value 
of T is 3 or 4 hours. The experiments of Pegram (section 179) 
also suggest that another radio-active product, of which the value 
of r is about 3 minutes, is present in thorium. The change of 
thorium X into the emanation would have been difficult to detect 
if the product of the change had not been gaseous in character. 
Besides the changes mentioned above, it is thus quite possible 
that other and more rapid changes may be taking place which 
have not yet been detected. 

It has been pointed out in section 188 that the fourth change 



326 



RADIO-ACTIVE PROCESSES 



[CH. 



Radio-active -v 




Nature of Chemical and Physical 


Products 


A 


T 


the Rays Properties 


URANIUM 




a Soluble in excess of ammo- 


1 




nium carbonate 


Uranium X 3*6 x W~ 7 


22 days 


j8 (and 7?) Insoluble in excess of am- 


1 




monium carbonate 


Final product 






THORIUM ,. f u.- 




a 


Insoluble in ammonia 


Thorium X 2-OxlO- 


4 days 


a (and /3?) 


Soluble in ammonia 


Thorium Emanation 1-15 x 10~ 2 


1 minute 


a 


Behaves like a chemically 








inert gas of heavy mole- 


i 






cular weight. Condenses 








at - 120 C. 


Emanation X 






\ Attaches itself to the sur- 


(first change) 2-2 x 10~ 4 


55 minutes 


no rays 


face of bodies concen- 










trated on the cathode 


i 








in an electric field 


t 
Second change 1'74 x 10~ 5 


11 hours 


a, /3, 7 


[Soluble in some acids and 
) not in others. Possesses 










well-marked chemical 


1 








properties in solution 


Final product 


... 








RADIUM 






a 




I 










Radium Emanation 

i 


2-14xlO~ 6 to 
2-00 x 10~ 6 


3-7 to 4 days a 


Behaves like a chemically 
inert gas of heavy mole- 


1 






cular weight 


Emanation X 




Condenses at - 150 C. 


(first change) about 4 x 10~ 3 


about 3 a 


Attaches itself to the sur- 






minutes 


face of bodies ; mainly 


t 




41 




concentrated on the 


Second change 3-18 x 10~ 4 


S& minutes 


no rays 


cathode in an electric 


i 




T field 


Third change 4-1 x 10~ 4 


28 minutes 


a, j8, 7 ; 1 Soluble in some acids and 


L 1 ' 




1 not in others; volati- 


t 


. 


' lized at a white heat 


Fourth change 


200 years (?) 


a, jS Soluble in sulphuric acid 


1 






Final product 




... 





in emanation X of radium may possibly be the radio-active con- 
stituent present in the polonium of Mme Curie or the radio-tellu- 
rium of Marckwald. After the disintegrated radio-atoms have 
undergone the succession of changes mentioned above, a final 
stage will be reached where the atoms are either permanently 
stable or change so slowly that it is difficult to detect their 



X] RADIO-ACTIVE PROCESSES 327 

presence by the property of radio-activity. Consequently, there 
will always be a residual inactive product or products of the 
changes occurring in each of the radio-elements. In addition, 
since the a particles, projected from the radio-elements, are 
material in nature and are not radio-active, they must also be 
considered as a residual product. 

The value of r, which may be taken as a comparative measure 
of the stability of the different metabolons, varies over a very wide 
range. The thorium emanation is the most unstable of the meta- 
bolons. and (leaving out of account the fourth change in emanation 
X of radium) uranium X the least unstable. The metabolons 
constituting uranium X are about 30,000 times as stable as the 
metabolons constituting the thorium emanation. 

The only two metabolons of about the same stability are the 
metabolons constituting thorium X and the radium emanation. 
In each case the activity falls to half value in about four days. I 
consider that the approximate agreement of the numbers is a mere 
coincidence, and that the two types of matter are quite distinct 
from one another ; for, if the metabolons were identical, it would 
be expected that the changes which follow would take place in the 
same way and at the same rate, but such is not the case. More- 
over Th X and the radium emanation have chemical and physical 
properties quite distinct from one another. 

201. Production of helium from radium and the radium 
emanation. Since the final products, resulting from a disinte- 
gration of the radio-elements, are not radio-active, they should in 
the course of geologic ages collect in some quantity, and should 
always be found associated with the radio-elements. Now the 
inactive products resulting from the radio-active changes are the a 
particles expelled at each stage, and the final inactive product or 
products which remain, when the process of disintegration can no 
longer be traced by the property of radio-activity. 

Pitchblende, in which the radio-elements are mostly found, 
contains in small quantity a large proportion of all the known 
elements. In searching for a possible disintegration product 
common to all the radio-elements, the presence of helium in the 
radio-active minerals is noteworthy; for helium is only found in 



328 RADIO-ACTIVE PROCESSES [CH. 

the radio-active minerals and is an invariable companion of the 
radio-elements. In addition the presence of a light, inert gas like 
helium in minerals had always been a matter of surprise. The 
production by radium and thorium of the radio-active emanations, 
which behaved like chemically inert gases of the helium-argon 
family, suggested the possibility that one of the final inactive 
products of the disintegration of the radio-elements might prove 
to be a chemically inert gas. The discovery later of the material 
nature of the a rays added weight to the suggestion; for the 
measurement of the ratio e/m of the a particle indicated that if 
the a particle consisted of any known kind of matter, it must either 
be hydrogen or helium. For these reasons, it was suggested in 
1902 by Rutherford and Soddy 1 that helium might be a product 
of the disintegration of the radio-elements. 

Sir William Ramsay and Mr Soddy in 1903 undertook an in- 
vestigation of the radium emanation, with the purpose of seeing if 
it were possible to obtain any spectroscopic evidence of the presence 
of a new substance. First of all, they exposed the emanation to 
very drastic treatment (section 149), and confirmed and extended 
the results previously noted by Rutherford and Soddy that the 
emanation behaved like a chemically inert gas, and in this respect 
possessed properties analogous to the gases of the helium-argon 
group. 

On obtaining 30 milligrams of pure radium bromide (pre- 
pared about three months previously) Ramsay and Soddy 2 ex- 
amined the gases, liberated by solution of the radium bromide in 
water, for the presence of helium. A considerable quantity of 
hydrogen and oxygen was released by the solution (see section 
116). The hydrogen and oxygen were removed by passing the 
liberated gases over a red-hot spiral of partially oxidized copper- 
wire and the resulting water vapour by a phosphorous pentoxide 
tube. 

The gas was then passed into a small vacuum tube which was 
in connection with a small U tube. By placing the U tube in 
liquid air, most of the emanation present was condensed, and also 
most of the CO 2 present in the gas. On examining the spectrum 

1 Phil Mag. p. 582, 1902 ; pp. 453 and 579, 1903. 

2 Nature, July 16, p. 246, 1903. Proc. Roy. Soc. 72, p. 204, 1903. 



X] RADIO-ACTIVE PROCESSES 329 

of the gas in the vacuum tube the characteristic line D 3 of helium 
was observed. 

This experiment was repeated with 30 milligrams of radium 
bromide about four months old, lent for the purpose by the writer. 
The emanation and CO 2 were removed by passing them through a 
U tube immersed in liquid air. A practically complete spectrum 
of helium was observed, including the lines of wave-length 6677, 
5876, 5016, 4972, 4713 and 4472. There were also present three 
other lines of wave-length about 6180, 5695, 5455 which have not 
yet been identified. 

In later experiments the emanation from 50 milligrams of the 
radium bromide was conveyed with oxygen into a small U tube, 
cooled in liquid air, in which the emanation was condensed. Fresh 
oxygen was added and the U tube again pumped out. The small 
vacuum tube, connected with the U tube, showed at first no 
helium lines when the liquid air was removed. The spectrum 
obtained was a new one, and Ramsay and Soddy considered it 
to be probably that of the emanation itself. After allowing the 
emanation tube to stand for four days, the helium spectrum appeared 
with all the characteristic lines, and in addition, three new lines 
present in the helium obtained by solution of the radium. These 
results have since been confirmed. The experiments, which have 
led to such striking and important results, were by no means easy 
of performance, for the quantity of helium and of emanation released 
from 50 mgrs. of radium bromide is extremely small. It was 
necessary, in all cases, to remove almost completely the other gases, 
which were present in sufficient quantity to mask the spectrum of 
the substance under examination. The success of the experiments 
has been largely due to the application to the investigation of the 
refined methods of gas analysis, which had been previously employed 
by Sir William Ramsay with so much success in the separation of 
the rare gases xenon and krypton, which exist in minute pro- 
portions in the atmosphere. The fact that the helium spectrum 
was not present at first, but appeared after tHe emanation had 
remainecfm theliuBerfbr some days, shows that the helium must 
have been produced from the emanation. The emanation cannot 
be helium itself, for in the first place, helium is not radio-active, 
and in the second place, the helium spectrum was not present at 



330 RADIO-ACTIVE PROCESSES [CH. 

first, when the quantity of emanation in the tube was at its 
maximum. In addition, the diffusion experiments, already dis- 
cussed, point to the conclusion that the emanation is of high 
molecular weight. There can thus be no doubt that the helium is 
derived from the emanation of radium in consequence of changes 
or* some kind occurring in it. 

In order to explain the presence of helium in radium on ordi- 
nary chemical lines, it has been suggested that radium is not 
a true element, but a molecular compound of helium with some 
substance known or unknown. The helium compound gradually 
breaks down, giving rise to the helium observed. It is at once 
obvious that this postulated helium compound is of an entirely 
different character to any other compound previously observed 
in chemistry. Weight for weight, it emits during its change an 
amount of energy at least one million times greater than any 
molecular compound known (see section 205). In addition, it must 
be supposed that the rate of breaking up of the helium compound 
is independent of great ranges of temperature a result never 
before observed in any molecular change. The helium compound 
in its breaking up must give rise to the peculiar radiations and 
also pass through the successive radio-active changes observed in 
radium. 

In order to explain the production of helium and radio-activity 
on this view, a unique kind of molecule must thus be postulated 
a molecule, in fact, which is endowed with every single property 
which on the disintegration theory is ascribed to the atom of the 
radio-elements. On the other hand, radium as far as it has been 
examined, has fulfilled every test required for an element. It has 
a well-marked and characteristic spectrum, and there is no reason 
to suppose that it is not an element in the ordinarily accepted 
sense of the term. 

On the theory that the radio-elements are undergoing atomic 
disintegration, the helium must either be considered to exist 
within the radium atom, or else to be formed from its constituent 
corpuscles during the process of disintegration. The theory that 
the heavy atoms are all built up of some simple fundamental 
unit of matter or protyle has been advanced at various times by 
many prominent chemists and physicists. Prout's hypothesis, 



X] RADIO-ACTIVE PROCESSES 331 

that all elements are built up out of hydrogen, is an example of 
this point of view of regarding the subject. 

On the disintegration theory, the changes occurring in the 
radio-atoms involve an actual transformation of the atoms through 
successive changes. This change is so slow in uranium and thorium 
that at least a million years would be required before the amourft 
of change is measurable by the balance. In radium it is a million 
times faster, but even in that case it is doubtful whether any 
appreciable change would be observed by ordinary chemical methods 
for many years, if the possibility of such a change had not been 
suggested from other lines of research. 

202. Amount of helium from radium. The appearance of 
helium in a tube containing the radium emanation may indicate 
either that the helium is one of the final products, which appear 
at the end of .the series of radio-active changes, or that^the Jielium 
is in realjty tho expelled a particle The evidence at present 
points to the latter as being the more probable explanation. In 
the first place, the emanation diffuses like a gas of heavy molecular \ 
weight, and it appears probable that, after the expulsion of a few \ 
fji particles, the atomic weight of the final product is comparable 
with that of the emanation. On the other hand, the value of e/m I 
determined for the projected OL particle points to the conclusion / 
that, if it consists of any known kind of matter, it is either * 
hydrogen or helium. 

If the a. particles, when released, can exist in the gaseous state, 
an estimate can readily be made of the volume of the total number 
of particles liberated per year. It has been calculated that one 
gram of radium expels about 10 11 a. particles per second. Since 
there are 3'6 x 10 19 molecules in one cubic centimetre of any gas 
at standard pressure and temperature, the volume of the a particles 
released per second from 1 gram of radium is 2'8 x 10" 9 c.c. and 
per year 90 cubic millimetres. 

It has already been shown that the emanation released from 
1 gram of radium in a state of radio-active equilibrium is probably 
about 3 x 10~ 4 c.c. Since the emanation passes through at least 
three stages, each of which gives rise to a rays, the volume of the 
a. particles from the emanation, released from 1 gram of radium, is 
about one cubic millimetre. 



332 RADIO-ACTIVE PROCESSES [CH. 

Ramsay and Soddy state that the amount of helium, present 
in the gases from radium, was very minute. From the above 
estimates, it can readily be shown that the amount of helium 
liberated in the experiments described in section 201 was about 
0'5 cubic millimetres. If the a. particles are helium, it is to be 
expected that the greater portion of the helium, which is produced 
in a tube containing the radium emanation, would be buried in 
the walls of the glass tube ; for the a particles are projected with 
sufficient velocity to penetrate some distance into the glass. 

203. Rate of change of the radio-elements. Since the 
atoms of the radio-elements themselves are continuously breaking 
up, they must also be considered to be metabolons, the only dif- 
ference between them and the metabolons such as the emanations, 
Th X and others, being their comparatively great stability and con- 
sequent very slow rate of change. There is no evidence that the 
process of disintegration, traced above, is reversible, and, in the 
course of time, a quantity of radium, uranium or thorium, left 
to itself must gradually be transformed into inactive matter of 
different kinds. 

An approximate estimate of the rate of change of radio-elements 
can be deduced from the number of atoms breaking up per second. 
It has been calculated from several lines of evidence (section 104) 
that from 1 gram of radium about 10 11 a particles are expelled 
per second. The number for uranium and thorium is about 
7 x 10 4 . 

Now it has been shown that there are at least four rapid changes 
in radium, each of which gives rise to a. rays. In the absence of 
evidence of the number of a particles expelled at each change, the 
assumption, which seems most probable, will be made, viz., that 
each metabolon expels only one a. particle. Since there are four 
changes in radium, the number of atoms in one gram of radium 
breaking up per second is 2'5 x 10 10 . Now it has been shown, from 
data based on experimental evidence, that one cubic centimetre of 
hydrogen, at standard pressure and temperature, contains about 
3'6 x 10 19 molecules. Taking the atomic weight of radium as 225, 
there will be T8 x 10 21 atoms in 1 gram of radium. The fraction 
\ of one gram of radium which changes is thus 1*4 x 10~ n per 
second and 4'4x 10~ 4 per year. It thus follows that, in each gram 



X] RADIO-ACTIVE PROCESSES 333 

of radium, about half a milligram disintegrates per year. Since 
the amount of radium which is unchanged will diminish according 
to an exponential law with the time, half of a given weight of 
radium will be transformed in about 1500 years. Only one per 
cent, will remain unchanged after a lapse of about 10,000 years. 
In a gram of uranium or thorium, where the change takes place 
at about one-millionth the rate, about a million years would be 
required before half a milligram would be changed. All but one 
per cent, of the uranium and thorium would be transformed in 
about 10 10 years. 

This is the minimum estimate of the life of radio-elements on 
the assumption that one a particle is expelled at each change. A 
maximum limit to the life of the radio-elements can be deduced 
by supposing that the radium is completely disintegrated into a 
particles. Since the mass of the a particle is about twice that of 
the hydrogen atom there cannot be many more than 100 a particles 
produced from each atom of the radio-elements. The 'maximum 
estimate of the life of radium is thus about 5'0 times greater 
than the minimum estimate. The minimum estimate is however 
probably nearer the truth : for there is no evidence to show that 
more than one a particle is expelled at each change. The agree- 
ment between the calculated and experimental values of the 
volume of the emanation (see section 197) is strong evidence in 
support of the minimum estimate ; for in the calculation only one 
a particle was supposed to be expelled at each change. 

The changes in radium are thus fairly rapid, and a mass of 
radium if left to itself should in the course of a few thousand years 
have lost a large proportion of its radio-activity. Taking the 
minimum estimate of the life of radium, the value of X is 4'4 x 10~ 4 , 
with a year as the unit of time. A mass of radium left to itself 
should thus be half transformed in 1500 years and only one- 
millionth part would remain after 30,000 years. Thus supposing, 
for illustration, that the earth was originally composed of pure 
radium, its activity per gram 30,000 years later would not be 
greater than the activity observed to-day in a good specimen of 
pitchblende. Even taking the maximum estimate of the life of 
radium, the time required for the radium to practically disappear 
is short compared with the probable age of the earth. We are 



334 RADIO-ACTIVE PROCESSES [CH. 

thus forced to the conclusion that radium is being continuously 
produced in the earth, unless the very improbable assumption is 
made, that radium was in some way suddenly formed at a date 
recent in comparison with the age of the earth. It has been 
suggested that radium may be a disintegration product of one 
of the radio-elements found in pitchblende. Both uranium and 
thorium fulfil the conditions required in a possible source of 
production of radium. Both are present in pitchblende, have 
atomic weights greater than that of radium, and have rates of 
change which are slow compared with that of radium. In some 
respects, uranium filfils the conditions required better than thorium ; 
for it has not been observed that minerals rich in thorium contain 
much radium, while on the other hand, the pitchblendes contain- 
ing the most radium contain a large proportion of uranium. 

If radium is not produced from uranium, it is certainly a 
remarkable coincidence that the greatest activity of pitchblende 
yet observed is about five or six times that of uranium. Since 
radium has a life short compared with that of uranium, the 
amount of radium produced should reach a maximum value after 
a few thousand years, when the rate of production of fresh radium 
which is also a measure of the rate of change of uranium 
balances the rate of change of that product. In this respect the 
process would be exactly analogous to the production of the 
emanation by radium, with the difference that the radium changes 
much more slowly than the emanation. But since radium itself 
in its disintegration gives rise to at least four changes with the 
corresponding production of a rays, the activity due to the radium 
(measured by the a rays), when in a state of radio-active equili- 
brium with uranium, should be about four times that of the 
uranium that produces it ; for it has been shown that only one 
change has so far been observed in uranium in which a rays are 
expelled. Taking into account the presence of polonium and 
actinium in pitchblende, the activity in the best pitchblende is 
about the same as would be expected if the radium were a dis- 
integration product of uranium. If this hypothesis is correct, the 
amount of radium in any pitchblende should be proportional to 
the amount of uranium present, provided the radium is not 
removed from the mineral by percolating water. On the other 



X] RADIO-ACTIVE PROCESSES 335 

hand it should be noticed that while the greatest amount of 
radium has been observed in a pitchblende rich in uranium, some 
pitchblendes rich in uranium contain very little radium. 

The general evidence, which has been advanced to show that 
radium must continually be produced from some other substance, 
applies also to actinium, which has an activity of the same order 
of magnitude as that of radium. It is very remarkable that the 
three radio-active substances, radium, thorium and actinium, should 
exhibit such a close similarity in the succession of changes which 
occur in them. Each of them at one stage of its disintegration 
emits a radio-active gas, and in each case this gas is transformed 
into a solid which is deposited upon the surface of bodies. It 
would appear that, after disintegration of an atom of any of these 
has once begun, there is a similar succession of changes, in which 
the resulting systems have allied chemical and physical properties. 
Such a connection is of interest as indicating a possible origin 
of the recurrence of properties in the atoms of the elements, as 
exemplified by the periodic law. 

204. Loss of weight of the radio-element. Since the 
radio-elements are continuously throwing off a particles atomic 
in size, an active substance, enclosed in a vessel sufficiently thin to 
allow the a particles to escape, must gradually lose in weight. 
This loss of weight w r ill be small under ordinary conditions, since 
the greater proportion of the a rays produced are absorbed in the 
mass of the substance. If a very thin layer of a radium compound 
were spread on a very thin sheet of substance, which did not 
appreciably absorb the a particles, a loss of weight due to the 
expulsion of a particles might be detectable. Since e/m = 6 x 10 3 
for the a. particle, and e=l'l x 10" 20 electro-magnetic units, and 
10 n a particles are expelled per second per gram of radium, the 
fraction of the mass expelled is 1*8 x 10~ 13 per second and 
6 x 10~ 6 per year. There is one condition, however, under which 
the radium should lose in weight fairly rapidly. If a current of 
air is slowly passed over a radium solution, the emanation produced 
would be removed as fast as it was formed. Since the atom of 
the emanation has a mass probably not much smaller than the 
radium atom, the fraction of the mass removed per year should 



336 RADIO-ACTIVE PROCESSES [CH. 

be nearly equal to the fraction of the radium which changes per 
year, i.e. one gram of radium should diminish in weight half a 
milligram (section 203) per year on a maximum estimate and 
1/100 of a milligram on a minimum estimate. 

If it is supposed that the /3 particles have weight, the loss of 
weight due to their expulsion is very small compared with that 
due to the emission of a particles. Taking the estimate deduced 
from the observation of Wien (section 104), that 6 '6 x 10 9 ft particles 
are projected per second from 1 gram of radium bromide, the loss 
of weight would only be about 1'2 x 10~ 10 gram per year. 

Except under very special experimental conditions, it would 
thus be very difficult to detect the loss of weight of radium due to 
the expulsion of {3 particles from its mass. There is, however, a 
possibility that radium might change in weight even though none 
of the radio-active products were allowed to escape. For example, 
if the view is taken that gravitation is the result of forces having 
their origin in the atom, it is possible that, if the atom were 
disintegrated, the weight of the parts might not be equal to that 
of the original atom. 

A large number of experiments have been made to see if 
radium preparations, kept in a sealed tube, alter in weight. With 
the small quantities of radium available to the experimenter, no 
difference of weight of radium preparations with time has yet 
been established with certainty. Heydweiller stated that he had 
observed a loss of weight of radium, and Dorn also obtained a 
slight indication of change in weight. These results have not, 
however, been confirmed. Forch, later, was unable to observe any 
appreciable change. 

205. Total emission of energy from the radio-element. 

It has been shown that 1 gram of radium emits energy at the 
rate of 100 gram-calories per hour or 876,000 gram-calories per 
year. If 1 gram of radium were set apart, its radio-activity and 
consequent heat emission at a time t is given by qe~ M , where \ is 
the constant of decay of activity of radium. Thus the total heat 

/OC 
qe~ M = - . 
) ^ 

Now on the minimum estimate of the life of radium, the value 



X] RADIO-ACTIVE PROCESSES 337 

of X is 4'4 x 10~ 4 , and on the maximum estimate 1*76 x 10~ 5 when 
1 year is taken as the unit of time. The total heat emission from 
1 gram of radium during its life thus lies between 2 x 10 9 and 
5 x 10 10 gram-calories. The minimum estimate is probably nearer 
the truth than the maximum. The heat emitted in the union of 
hydrogen and oxygen to form 1 gram of water is about 4 x 10 3 
gram-calories, and in this reaction more heat is given out for 
equal weights than in any other chemical reaction known. It is 
thus seen that the total energy emitted from 1 gram of radium 
during its changes is about one million times greater than that in- 
volved in any known molecular change. That matter is able, under 
special conditions, to emit an enormous amount of energy, is well 
exemplified by the case of the radium emanation. The total heat 
emission from the emanation released from 1 gram of radium, 
and from the secondary products, corresponds to about 10 4 
gram-calories, and this amount of heat is given out as a conse- 
quence of changes in a minute volume of gas. Taking the 
estimate that the volume of the emanation is 3 x 10~ 4 cubic 
centimetres at standard pressure and temperature, and its atomic 
weight about 200, it can be calculated that 1 gram of emanation 
gives out during its life about 10 9 gram-calories. Quite inde- 
pendently of any theory, a result of the same order of magnitude 
can be deduced from the experiments. 

Since the other radio-elements only differ from radium in the 
slowness of their change, the total heat emission from uranium 
and thorium must be of a similar high order of magnitude. There 
is thus reason to believe that an enormous store of latent energy 
is resident in the atoms of the radio-elements. This store of 
energy could not have been recognized if the atoms were not 
undergoing a slow process of disintegration. The energy emitted 
in radio-active changes may thus be supposed to be derived from 
the internal energy of the atoms. The emission of this energy 
does not disobey the law of the conservation of energy, for it is 
only necessary to suppose that, when the radio-active changes 
have ceased, the energy stored up in the atoms of the final 
products is less than that in the original atoms of the radio- 
elements. The difference between the energy originally possessed 
by the matter, which has undergone the change, and the final 

R. R.-A. 22 



338 RADIO-ACTIVE PROCESSES [CH. 

inactive products which arise, is a measure of the total amount of 
energy released. 

There seems to be no reason to suppose that the atomic energy 
of all the elements is not of a similar high order of magnitude. 
With the exception of their high atomic weights, the radio- 
elements do not possess any special chemical characteristics which 
differentiate them from the inactive elements. The existence of 
a latent store of energy in the atoms is a necessary consequence 
of the modern view developed by J. J. Thomson, Larmor, and 
Lorentz, of regarding the atom as a complicated structure consisting 
of charged parts in rapid oscillatory or orbital motion in regard to 
one another. The energy may be partly kinetic and partly potential, 
but the mere arrangement of the charged particles, which probably 
constitute the atom, in itself implies a large store of internal 
atomic energy. 

It is not to be expected that the existence of this store of 
latent energy would have ordinarily manifested itself, since the 
atoms cannot be broken up into simpler forms by the physical or 
chemical agencies at our disposal. Its existence at once explains 
the failure of chemistry to transform the atoms, and also accounts 
for the independence of the rate of change of the radio-active 
processes of all external agencies. It has not so far been found 
possible to alter in any way the rate of emission of energy from 
the radio-elements. If it were ever found possible to control at 
will the rate of disintegration of the radio-elements, an enormous 
amount of energy could be obtained from a small quantity of 
matter. 

206. Possible causes of disintegration. In order to ex- 
plain the phenomena of radio-activity, it has been supposed that a 
certain small fraction of the radio-atoms undergoes disintegration 
per second, but no assumptions have been made as to the cause 
which produces the instability and consequent disintegration. 
The instability of the atoms may be supposed to be brought about 
either by the action of external forces or of forces inherent in the 
atoms themselves. It is conceivable, for example, that the appli- 
cation of some slight external force might cause instability and 
consequent disintegration, accompanied by the liberation of a large 



X] RADIO-ACTIVE PROCESSES 339 

amount of energy, on the same principle that a detonator is 
necessary to start some explosives. It has been shown that the 
number of atoms of any radio-active product which break up per 
second is always proportional to the number present. This law 
of change does not throw any light on the question, for it would 
be expected equally on either hypothesis. It has not been found 
possible to alter the rate of change of any product by the appli- 
cation of any known physical or chemical forces, unless possibly it 
is assumed that the force of gravitation which is not under our 
control may influence in some way the stability of the radio-atoms. 

It has been suggested by J. J. Thomson 1 that the rate of dis- 
integration of radium may be influenced by its own radiations. 
This, at first sight, appears very probable, for a small mass of pure 
radium compound is subjected to an intense bombardment by the 
radiations arising from it, and the radiations are of such a character 
that they might be expected to produce a breaking up of the 
atoms of matter which they traverse. If this is the case the 
radio-activity of a given quantity of radium should be a function 
of its concentration, and should be greater in the solid state than 
when disseminated through a large mass of matter. 

I have recently tried an experiment to see if this were the 
case. Two glass tubes were taken, in one of which was placed a 
few milligrams of pure radium bromide in a state of radio-active 
equilibrium, and in the other a solution of barium chloride. The 
two tubes were connected near the top by a short cross tube and 
the open ends sealed off. The activity of the radium in the solid 
state was tested immediately after its introduction by placing it 
in a definite position near an electroscope made of thin metal of 
the type shown in Fig. 11. The increased rate of discharge of the 
electroscope was observed. This rate of discharge was due to the 
ft and 7 rays from the radium. By placing a lead plate 6 mms. 
in thickness between the radium and the electroscope, the rate of 
discharge observed was then due to the 7 rays alone. By slightly 
tilting the apparatus, the barium solution flowed into the radium 
tube and dissolved the radium. The tube was well shaken so as 
to distribute the radium uniformly throughout the solution. No 
appreciable change of the activity measured by the 7 rays was 

1 Nature, April 30, p. 601, 1903. 

222 



340 RADIO-ACTIVE PROCESSES [CH. 

observed over the period of one month. The activity measured 
by the ft and 7 rays was somewhat reduced, but this was not due 
to a decrease of the radio-activity, but to an increased absorption 
of the /3 rays in their passage through the solution. The volume 
of the solution was at least 1000 times greater than that of the 
solid radium bromide, and, in consequence, the radium was sub- 
jected to the action of a much weaker radiation. I think we may 
conclude from this experiment that the radiations emitted by 
radium have little if any influence in causing the disintegration 
of the radium atoms. 

This result is in general agreement with other observations; 
for it has not been observed that the decay of activity of any 
product is influenced by the degree of concentration of that 
product. 

It thus seems likely that the cause of the disruption of the 
toms of the radio-elements and their products is resident in the 
toms themselves. "According to the modern views of the consti- 
ution of the atom, it is not so much a matter of surprise that 
3me atoms disintegrate as that the atoms of the elements are so 
ermanent as they appear to be. In accordance with the hypothesis 
of J. J. Thomson, it may be supposed that the atoms consist of a 
number of small positively and negatively charged particles in 
rapid internal movement, and held in equilibrium by their mutual 
forces. In a complex atom, where the possible variations in the 
relative motion of the parts are very great, the atom may arrive 
at such a phase that one part acquires sufficient kinetic energy 
to escape from the system, or that the constraining forces are 
momentarily neutralised, so that the part escapes from the system 
with the velocity possessed by it at the instant of its release. 

Sir Oliver Lodge 1 has advanced the view that the instability of 
the atom may be a result of radiation of energy by the atom. Larmor 
has shown that an electron, subject to acceleration, radiates energy 
at a rate proportional to the square of its acceleration. An electron 
moving uniformly in a straight line does not radiate energy, but 
an electron, constrained to move in a circular orbit with constant 
velocity, is a powerful radiator, for in such a case the electron is 
continuously accelerated towards the centre. Lodge considered 

1 Lodge, Nature, June 11, p. 129, 1903. 



X] RADIO-ACTIVE PROCESSES 341 

the simple case of a negatively charged electron revolving round 
an atom of mass relatively large but having an equal positive 
charge and held in equilibrium by electrical forces. This system 
will radiate energy, and since the radiation of energy is equivalent 
to motion in a resisting medium, the particle tends to move 
towards the centre, and its speed consequently increases. The 
rate of radiation of energy will increase rapidly with the speed 
of the electron. When the speed of the electron becomes very 
nearly equal to the velocity of light, according to Lodge, another 
effect supervenes. It has been shown (section 76) that the 
apparent mass of an electron increases very rapidly as the speed 
of light is approached, and is theoretically infinite at the speed 
of light. There will be at this stage a sudden increase of the 
mass of the revolving atom and, on the supposition that this stage 
can be reached, a consequent disturbance of the balance of forces 
holding the system together. Lodge considers it probable that, 
under these conditions, the parts of the system will break asunder 
and escape from the sphere of one another's influence. 

It seems probable that the primary cause of the disintegration 
of the atom must be looked for in the loss of energy of the atomic 
system due to electro-magnetic radiation. Larmor 1 has shown 
that the condition to be fulfilled in order that a system of rapidly 
moving electrons may persist without loss of energy is that the 
vector sum of the accelerations towards the centre should be 
permanently null. While a single electron moving in a circular 
orbit is a powerful radiator of energy, it is remarkable how rapidly 
the radiation of energy diminishes if several electrons are revolv- 
ing in a ring. This has recently been shown by J. J. Thomson 2 , 
who examined mathematically the case of a system of negatively 
electrified corpuscles, situated at equal intervals round the circum- 
ference of a circle, and rotating in one plane with uniform velocity 
round its centre. For example, he found that the radiation from 
a group of six particles moving with a velocity of ^ of the velocity 
of light is less than one-millionth part of the radiation from a 
single particle describing the same orbit with the same velocity. 
When the velocity is T J^ of that of light the amount of radiation 

1 Larmor, Aether and Matter, p. 233. 

2 J. J. Thomson, Phil. May. p. 681, Dec. 1903. 



342 RADIO-ACTIVE PROCESSES [CH. 

is only 10~ 16 of that of the single particle moving with the same 
velocity in the same orbit. 

Results of this kind indicate that an atom consisting of a large 
number of revolving electrons may radiate energy extremely slowly, 
and yet, finally, this minute but continuous drain of energy from 
the atom must result either in a rearrangement of its component 
parts into a new system, or of an expulsion of electrons or groups 
of electrons from the atom. 

A suggestion, due to J. J. Thomson 1 , of a possible mechanism 
to account for the expulsion from the radio-atoms of an a particle, 
i.e. of a connected group of electrons, has recently been explained 
by Whetham 2 as follows : " The sub-atomic corpuscles, when their 
velocity is changing, must radiate ethereal waves. Their energy 
is thus gradually diminished ; and systems of revolving corpuscles, 
permanent while moving fast, may become unstable. As a simple 
example, six bodies at the corners of a plane hexagon under the 
influence of mutual forces may continue, while their velocity 
exceeds a certain limit, to revolve about a central point while 
keeping their relative positions. When there is no motion, how- 
ever, this arrangement is impossible, and the six bodies must place 
themselves, five at the corners of a pentagon and one at the centre. 
Thus, as the velocity falls to a certain value, a sudden and ex- 
plosive rearrangement occurs, during which, in the complex 
system constituting an atom, the ejection of parts of the system 
becomes possible." 

207. Radio-activity and the heat of the sun and earth. 

It was pointed out by Mr Soddy and the writer 3 that the 
maintenance of the sun's heat for long intervals of time did not 
present any fundamental difficulty if a process of disintegration, 
such as occurs in the radio-elements, were supposed to be taking 
place in the sun. In a letter to Nature (July 9, 1903) W. E. 
Wilson showed that the presence of 3'6 grams of radium in each 
cubic metre of the sun's mass was sufficient to account for the 
present rate of emission of energy by the sun. This calculation 
was based on the estimate of Curie and Laborde, that one gram 

1 Prof. Thomson's paper has just appeared. Phil. May. March, 1904. 

2 Quarterly Review, p. 123, Jan. 1904. 3 Phil. Mag. May, 1903. 



X] RADIO- ACTIVE PROCESSES 343 

of radium emits 100 gram-calories per hour, and on the observa- 
tion of Langley that each square centimetre of the sun's surface 
emits 8-28 x 10 6 gram-calories per hour. Since the average density 
of the sun is 1'44, the presence of radium in the sun, to the 
extent of 2'5 parts by weight in a million, would account for its 
present rate of emission of energy. 

An examination of the spectrum of the sun has not so far 
revealed any of the radium lines. It is known, however, from 
spectroscopic evidence that helium is present, and this indirectly 
suggests the existence of radio-active matter also. It can readily 
be shown 1 that the absence of penetrating rays from the sun at 
the surface of the earth does not imply that the radio-elements 
are not present in the sun. Even if the sun were composed of 
pure radium, it would hardly be expected that the 7 rays emitted 
would be appreciable at the surface of the earth, since the rays 
would be almost completely absorbed in passing through the 
atmosphere, which corresponds to a thickness of 76 centimetres of 
mercury. 

In the Appendix E of Thomson and Tait's Natural Philosophy, 
Lord Kelvin has calculated the energy lost in the concentration of 
the sun from a condition of infinite dispersion, and concludes that 
it seems " on the whole probable that the sun has not illuminated 
the earth for 100,000,000 years and almost certain that he has not 
done so for 500,000,000 years. As for the future we may say, with 
equal certainty, that inhabitants of the earth cannot continue to 
enjoy the light and heat essential to their life for many million 
years longer, unless sources now unknown to us are prepared in 
the great storehouses of creation." 

The discovery that a small mass of a substance like radium 
can emit spontaneously an enormous quantity of heat renders 
it possible that this estimate of the age of the sun's heat 
may be much increased. In a letter to Nature (Sept. 24, 1903) 
G. H. Darwin drew attention to this probability, and stated that, 
" The lost energy of the sun, supposed to be a homogeneous sphere 
of mass M and radius a, is f/i3/ 2 /a where p is the constant of 
gravitation. On introducing numerical values for the symbols in 
this formula, I find the lost energy to be 2*7 x 10 7 M calories where 

1 See Strutt and Joly, Xature, Oct. 15, 1903. 



344 RADIO-ACTIVE PROCESSES [CH. 

M is expressed in grams. If we adopt Langley's value of the solar 
constant, this heat suffices to give a supply for 12 million years. 
Lord Kelvin used Pouillet's value for that constant, but if he had 
been able to use Langley's, his 100 million would have been 
reduced to 60 million. The discrepancy between my results of 
12 million and his of 60 million is explained by a conjectural 
augmentation of the lost energy to allow for the concentration 
of the solar mass towards its central parts." Now it has been 
shown (section 205) that one gram of radium emits during its 
life an amount of heat which probably lies between 2 x 10 9 and 
5 x 10 10 gram-calories. It has also been pointed out that there is 
every reason to suppose that a similar amount of energy is resident 
in the chemical atoms of the inactive elements. It is not impro- 
bable that, at the enormous temperature of the sun, the breaking 
up of the elements into simpler forms may be taking place at 
a more rapid rate than on the earth. If the energy resident 
in the atoms of the elements is thus available, the time during 
which the sun may continue to emit heat at the present rate may 
be from 50 to 500 times longer than was computed by Lord Kelvin 
from dynamical data. 

Similar considerations apply to the question of the probable 
age of the earth. A full discussion of the probable age of the 
earth, computed from its secular cooling from a molten mass, is 
given by Lord Kelvin in Appendix D of Thomson and Tait's Natural 
Philosophy. He has there shown that about 100 million years 
after the earth was a molten mass, the gradual cooling due to 
radiation from its surface would account for the average tempera- 
ture gradient of 1/50 F. per foot, observed to-day near the earth's 
surface. 

Some considerations will now be discussed which point to the 
probability that the present temperature gradient observed in the 
earth cannot be used as a guide to estimate the length of time 
that has elapsed since the earth has been at a temperature capable 
of supporting animal and vegetable life ; for it will be shown that 
probably there is sufficient radio-active matter on the earth to 
supply as much heat to the earth as is lost by radiation from its 
surface. Taking the average conductivity K of the materials of 
the earth as '004 (C.G.S. units) and the temperature gradient T near 



X] RADIO-ACTIVE PROCESSES 345 

the surface as "00037 C. per cm., the heat Q in gram-calories 
conducted to the surface of the earth per second is given by 



where R is the radius of the earth. 

Let X be the average amount of heat liberated per second per 
cubic centimetre of the earth's volume owing to the presence of 
radio-active matter. If the heat Q radiated from the earth is 
equal to the heat supplied by the radio-active matter on the 
earth, 



3KT 

Z = -7T- 

Substituting the values of these constants, 

X = 7 x 10~ 15 gram-calorie per second 
= 2'2 x 10~ 7 gram-calorie per year. 

Since 1 gram of radium emits 864,000 gram-calories per year, 
the presence of 2'6 x 10~ 13 gram of radium per unit volume or 
4'6 x 10~ 14 gram per unit mass, would compensate for the heat lost 
from the earth by conduction. 

Now it will be shown in the following chapter that radio-active 
matter seems to be distributed fairly uniformly through the earth 
and atmosphere. In addition it has been found that all substances 
are radio-active to a feeble degree, although it is not yet settled 
whether this radio-activity may not be due mainly to the presence 
of a radio-element as an impurity. For example, Strutt 1 observed 
that a platinum plate was about 1/3000 as active as a crystal of 
uranium nitrate, or about 2 x 10~ 10 as active as radium. This cor- 
responds to a far greater activity than is necessary to compensate 
for the loss of heat of the earth. A more accurate deduction, 
however, can be made from data of the radio-activity exhibited by 
matter dug out from the earth. Elster and Geitel 2 filled a dish of 
volume 3'3 x 10 3 c.c. with clay dug up from the garden, and placed 
it in a vessel of 30 litres capacity in which was placed an electro- 

1 Strutt, Phil. Mag. June, 1903. 

- Elster and Geitel, Phys. Zeit. 4, No. 19, p. 522, 1903. Chem. Xeicg, July 17, 
p. 30, 1903. 



346 RADIO-ACTIVE PROCESSES [CEL 

scope to determine the conductivity of the enclosed gas. After 
standing for several days, he found that the conductivity of the air 
reached a constant maximum value, corresponding to three times 
the normal. It will be shown later (section 218) that the normal 
conductivity observed in sealed vessels corresponds to the produc- 
tion of about 30 ions per c.c. per second. The number of ions 
produced per second in the vessel by the radio-active earth was 
thus about 2 x 10 6 . This would give a saturation current through 
the gas of 2'2 x 10~ 14 electro-magnetic units. Now the emanation 
from 1 gram of radium stored in a metal cylinder gives a satura- 
tion current of about 3'2 x 10~ 3 electro-magnetic units. Elster and 
Geitel considered that most of the conductivity observed in the 
gas was due to a radio-active emanation, which gradually diffused 
from the clay into the air in the vessel. The increased conduc- 
tivity in the gas observed by Elster and Geitel would thus be 
produced by the emanation from 7 x 10~ 10 gram of radium. 
Taking the density of clay as 2, this corresponds to about 10~ 13 
gram of radium per gram of clay. But it has been shown that if 
4*6 x 10~ u gram of radium was present in each gram of earth, the 
heat emitted would compensate for the loss of heat of the earth by 
conduction and radiation. The amount of activity observed in the 
earth is thus about the right order of magnitude to account for the 
heat emission required. In the above estimate, the presence of 
uranium and thorium minerals in the earth has not been con- 
sidered. In addition, it is probable that the total amount of radio- 
activity in clay was considerably greater than that calculated, for it 
is likely that other radio-active matter was present which did not 
give off an emanation. 

I think we may conclude that the present rate of loss of heat 
of the earth might have continued unchanged for long periods of 
time in consequence of the supply of heat from radio-active matter 
in the earth. It thus seems probable that the earth may have 
remained for very long intervals of time at a temperature not very 
different from that observed to-day, and that in consequence the 
time during which the earth has been at a temperature capable of 
supporting the presence of animal and vegetable life may be very 
much longer than the estimate made by Lord Kelvin from other 
data. 



X] RADIO-ACTIVE PROCESSES 347 

208. Evolution of matter. Although the hypothesis that 
all matter is composed of some elementary unit of matter or pro- 
tyle has been advanced as a speculation at various times by many 
prominent physicists and chemists, the first definite experimental 
evidence showing that the chemical atom was not the smallest 
unit of matter was obtained in 1897 by J. J. Thomson in his classic 
research on the nature of the cathode rays produced by an electric 
discharge in a vacuum tube. Sir William Crookes, who was the first 
to demonstrate the remarkable properties of these rays, had sug- 
gested that they consisted of streams of projected charged matter 
and represented as he termed it a new or "fourth state of matter." 

J. J. Thomson showed by two distinct methods that the cathode 
rays consisted of a stream of negatively charged particles projected 
with great velocity. The particles behaved as if their mass was 
only about 1/1000 of the mass of the atom of hydrogen, which is 
the lightest atom known. These corpuscles, as they were termed by 
Thomson, were found at a later date to be produced from a glowing 
carbon filament and from a zinc plate exposed to the action of 
ultra-violet light. They acted as isolated units of negative elec- 
tricity, and, as we have seen, may be identified with the electrons 
studied mathematically by Larmor and Lorentz. Not only were 
these electrons produced by the action of light, heat, and the 
electric discharge, but they were also found to be spontaneously 
emitted from the radio-elements with a velocity far greater than 
that observed for the electrons in a vacuum tube. 

The electrons produced in these different ways were all found to 
carry a negative charge and to be apparently identical ; for the 
ratio e/m of the charge of the electron to its mass was in all cases 
the same within the limit of experimental errors. Since elec- 
trons, produced from different kinds of matter and under different 
conditions, were in all cases identical, it seemed probable that they 
were a constituent part of all matter. J. J. Thomson suggested 
that the atom is built up of a number of these negatively charged 
electrons combined in some way with corresponding positively 
charged bodies. 

On this view the atoms of the chemical elements differ from 
one another only in the number and arrangement of the component 
electrons. 



348 RADIO-ACTIVE PROCESSES [CH. 

The removal of an electron from the atom does not appear to 
permanently affect the stability of the system, for no evidence has 
so far been obtained to show that the passage of an intense electric 
discharge through a gas results in a permanent alteration of the 
structure of the atom. On the other hand, in the case of the 
radio-active bodies, a positively charged particle of mass about 
twice that of the hydrogen atom escapes from the heavy radio- 
atom. This appears to result at once in a permanent alteration of 
the atom, and causes a marked change in its physical and chemical 
properties. In addition there is no evidence that the process is 
reversible. 

The only direct experimental evidence of the transformation 
of matter has been derived from a study of the radio-active 
bodies. If the disintegration theory, advanced to account for the 
phenomena of radio-activity, is correct in the main essentials, then 
the radio-elements are undergoing a spontaneous and continu- 
ous process of transformation into other and different kinds of 
matter. The rate of transformation is slow in uranium and thorium, 
but is fairly rapid in radium. It has been shown that the fraction 
of a mass of radium which is transformed per year lies between 
1/2000 and 1/10000 of the total amount present. In the case of 
uranium and thorium probably a million years would be required 
to produce a similar amount of change. The process of trans- 
formation in uranium and thorium is thus far too slow to be 
detected within a reasonable time by the use of the balance or 
spectroscope, but the radiations which accompany the transforma- 
tion can readily be detected. Although the process of change is 
slow it is continuous, and in the course of ages the uranium and 
thorium present in the earth must be transformed into other and 
simpler types of matter. 

Those who have considered the possibility of atoms undergoing 
a process of transformation, have generally thought that the 
matter as a whole would undergo a progressive change, with a 
gradual alteration of physical and chemical properties of the whole 
mass of substance. On the theory of disintegration this is not the 
case. Only a minute fraction of the matter present breaks up in 
unit time, and in each of the succession of stages through which 
the disintegrated atoms pass, there is in most cases a marked 



X] RADIO-ACTIVE PROCESSES 349 

alteration in the chemical and physical properties of the matter. 
The transformation of the radio-elements is thus a transformation 
of a part per saltum, and not a progressive change of the whole. 
At any time after the process of transformation has been in 
progress there will thus remain a part of the matter which is 
unchanged, and, mixed with it, the products which have resulted 
from the transformation of the remainder. 

The question naturally arises whether the process of degrada- 
tion of matter is confined to the radio-elements or is a universal 
property of matter. It will be shown in chapter xi. that all 
matter, so far examined, exhibits the property of radio-activity to 
a slight degree. It still remains to be shown, however, that the 
observed radio-activity is not due to the presence in the matter of 
a slight trace of a radio-element. If ordinary matter is radio- 
active, it is certain that its activity is not greater than that of 
uranium, and consequently that its rate of transformation must 
be excessively slow. There is, however, another possibility to be 
considered. The changes occurring in the radio-elements would 
probably never have been detected if the change had not been 
accompanied by the expulsion of charged particles with great 
velocity. It does not seem unlikely that an atom may undergo 
disintegration without projecting a part of its system with great 
velocity. In fact, we have seen that, even in the radio-elements, 
one of the series of changes in both thorium and radium is unac- 
companied by ionizing rays. It may thus be possible that all 
matter is undergoing a slow process of transformation, which has 
so far only been detected in the radio-elements on account of the 
expulsion of charged particles during the change. This process of 
degradation of matter continuing for ages must reduce the con- 
stituents of the earth to the simpler and more stable forms of 
matter. 

The idea that helium is a transformation product of radium, 
suggests the probability that helium is one of the more elementary 
substances of which the heavier atoms are composed. Sir Norman 
Lockyer, in his interesting book on "Inorganic Evolution," has 
pointed out that the spectrum of helium and of hydrogen pre- 
dominates in the hottest stars. In the cooler stars the more 
complex types of matter appear. Sir Norman Lockyer has based 



350 RADIO-ACTIVE PROCESSES [CH. X 

his theory of evolution of matter on evidence of a spectroscopic 
examination of the stars, and considers that temperature is the 
main factor in breaking up matter into its simpler forms. The 
transformation of matter occurring in the radio-elements is on the 
other hand spontaneous, and independent of temperature over the 
range examined. 



CHAPTER XL 

RADIO-ACTIVITY OF THE ATMOSPHERE AND OF 
ORDINARY MATERIALS. 

209. Radio -activity of the atmosphere. The experiments 
of Geitel 1 and C. T. R Wilson 2 in 1900 had shown that a positively 
or negatively charged conductor placed inside a closed vessel gradu- 
ally lost its charge. This loss of charge was shown to be due to a 
small ionization of the air inside the vessel. In addition, Elster 
and Geitel had found that a charged body exposed in the open 
air lost its charge rapidly, and that the rate of discharge was 
dependent on the locality and on atmospheric conditions. A more 
detailed description and discussion of these results will be given 
later in section 218. 

In the course of these experiments Geitel had observed that 
the rate of discharge increased slightly for some time after the 
vessel had been closed. He considered that this might possibly 
be due to the existence of some radio-active substances in the air, 
which produced excited activity in the walls of the vessel and so 
increased the rate of dissipation of the charge. In 1901 Elster 
and Geitel 3 tried the bold experiment of seeing if it were possible 
to extract a radio-active substance from the air. The experiments 
of the writer had shown that the excited radio-activity from the 
thorium emanation could be concentrated on the negative electrode 
in a strong electric field. This result indicated that the carriers 
of the radio-activity had a positive charge of electricity. Elster 
and Geitel therefore tried an experiment to see if positively charged 

1 Phys. Zeit. 2, p. 116, 1900. 

2 Proc. Camb. Phil. Soc. 11, p. 32, 1900. Proc. Roy. Soc. 68, p. 151, 1901. 

3 Phys. Zeit. 2, p. 590, 1901. 



352 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 

carriers, possessing a similar property, were present in the atmo- 
sphere. For this purpose a cylinder of wire-netting, charged nega- 
tively to 600 volts, was exposed for several hours in the open air. 
This was then removed and quickly placed in a large bell-jar, inside 
which was placed an electroscope to detect the rate of discharge. 
They found that the rate of discharge was increased to a slight 
extent. In order to multiply the effect, a wire of about 20 metres 
long was exposed at some height from the ground, and was kept 
charged to a high potential by connecting it to the negative 
terminal of an influence machine. After exposure for some hours, 
this wire was removed and placed inside the dissipation vessel. 
The rate of discharge was found to be increased many times by the 
presence of the wire. No increase was observed if the wire had 
been charged positively instead of negatively. The results also 
showed that the radio-active matter could be removed from the 
wire in the same way as from a wire made active by exposure in the 
presence of the thorium emanation. A piece of leather moistened 
with ammonia was rubbed over the active wire. On testing the 
leather it was found to be strongly radio-active. If a long wire 
were used, the amount of activity obtained on the leather was 
comparable to that possessed by a gram of uranium oxide. 

The activity produced on the wire was not permanent, but 
disappeared to a large extent in the course of a few hours. The 
amount of activity produced on a wire of given size, exposed under 
similar conditions, was independent of the material of the wire. 
Lead, iron and copper wires gave about equal effects. 

The amount of activity obtained was greatly increased by ex- 
posing a negatively charged wire in a mass of air which had been 
undisturbed for a long time. Experiments were made in the great 
cave of Wolfenbiittel, and a very large amount of activity was 
observed. By transferring the activity to a piece of leather it 
was found 1 that the rays could appreciably light up a screen of 
barium platinocyanide in the dark. The rays also darkened a 
photographic plate through a piece of aluminium O'l mm. in 
thickness. 

These remarkable experiments show that the excited radio- 
activity obtained from the atmosphere is very similar in character 
1 Phys. Zeit. 3, p. 76, 1901. 



XI] AXD OF ORDINARY MATERIALS 353 

to the excited activity produced by the emanations of radium and 
thorium. No investigators have contributed more to our know- 
ledge of the radio-activity and ionization of the atmosphere than 
Elster and Geitel. The experiments here described have been the 
starting-point of a series of researches by Elster and Geitel and 
others on the radio-active properties of the atmosphere which have 
led to a great extension of our knowledge of that important subject. 

Eutherford and Allan 1 determined the rate of decay of the 
excited activity produced on a negatively charged wire exposed in 
the open air. A wire about 15 metres long was exposed in the 
open air, and kept charged by an influence machine to a potential 
of about -10000 volts. An hour's exposure was sufficient to obtain 
a large amount of excited activity on the wire. The wire was 
then rapidly removed and wound on a framework which formed 
the central electrode in a large cylindrical metal vessel. The 
ionization current for a saturation voltage was measured by 
means of a sensitive Dolezalek electrometer. The current, which 
is a measure of the activity of the wire, was found to diminish 
according to an exponential law with the time, falling to half value 
in about 45 minutes. The rate of decay was independent of the 
material of the wire, of the time of exposure, and of the potential 
of the wire. 

An examination was also made of the nature of the rays emitted 
by the radio-active wire. For this purpose a lead wire was made 
radio-active in the manner described, and then rapidly wound into 
the form of a flat spiral. The penetrating power of the rays was 
tested in a vessel similar to that shown in Fig. 16. Most of the 
ionization was found to be due to some very easily absorbed rays, 
which were of a slightly more penetrating character than the a 
rays emitted from a wire made active by the radium or thorium 
emanations. The intensity of the rays was cut down to half value 
by about O'OOl cm. of aluminium. The photographic action ob- 
served by Elster and Geitel through O'l mm. of aluminium showed 
that some penetrating rays were also present. This was afterwards 
confirmed by Allan, using the electric method. These penetrating 
rays are probably similar in character to the (3 rays from the 
radio-elements. 

1 Phil Mag. Dec. 1902. 
R. R.-A. 23 



354 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 

210. The excited activity produced on the negatively charged 
wire cannot be due to an action of the strong electric field on the 
surface of the wire ; for very little excited activity is produced if 
the wire is charged to the same potential inside a closed cylinder. 

We have seen that the excited activity produced on the wire 
can be partially removed by rubbing, and by solution in acids, and, 
in this respect, it is similar to the excited activity produced in 
bodies by the emanations of radium and thorium. The very close 
similarity of the excited activity obtained from the atmosphere 
to that obtained from the radium and thorium emanations sug- 
gests the probability that a radio-active emanation exists in the 
atmosphere. This view is confirmed by a large amount of indirect 
evidence discussed in sections 212 and 213. 

Assuming the presence of a radio-active emanation in the 
atmosphere, the radio-active effects observed receive a simple 
explanation. The emanation in the air gradually breaks up, 
giving rise in some way to positively charged radio-active carriers. 
These are driven to the negative electrode in the electric field, 
and there undergo a further change, giving rise to the radiations 
observed at the surface of the wire. The matter which causes 
excited activity will thus be analogous to the emanation X of 
radium and thorium. 

Since the earth is negatively electrified with regard to the 
upper atmosphere, these positive radio-active carriers produced in 
the air are continuously deposited on the surface of the earth. 
Everything on the surface of the earth, including the external 
surface of buildings, the grass, and leaves of trees, must be covered 
with an invisible deposit of radio-active material. A hill or 
mountain peak, or any high mass of rock or land, concentrates the 
earth's electric field at that point and consequently will receive 
more excited radio-activity per unit area than the plain. Elster 
and Geitel have pointed out that the greater ionization of the air 
observed in the neighbourhood of projecting peaks receives a 
satisfactory explanation on this view. 

If the radio-active carriers are produced at a uniform rate in 
the atmosphere, the amount of excited activity I t , produced on 
a wire exposed under given conditions, will, after exposure for a 
time t, be given by I t = I (l -e~ u ) where / is the maximum 



XI] AND OF ORDINARY MATERIALS 355 

activity on the wire and X is the constant of decay of the excited 
activity. Since the activity of a wire after removal falls to half 
value in about 45 minutes, the value of X is 0'92 with one hour as 
the unit of time. Some experiments made by Allan are in rough 
agreement with the above equation. Accurate comparative results 
are difficult to obtain on account of the inconstancy of the radio- 
activity of the open air. After an exposure of a wire for several 
hours, the activity reached a practical maximum, and was not 
much increased by continued exposure. 

A wire charged to a high potential in the open air abstracts 
the positive radio-active carriers from a large volume of air. Very 
little excited activity, in comparison, is produced in a closed vessel 
or by drawing a rapid current of the outside air through a cylinder 
in the centre of which a negatively charged rod is placed. In one 
experiment a current of air of 500 cms. per second was drawn 
through a cylinder of 141 litres capacity. The amount of activity 
produced on the negative electrode was only a few per cent, of the 
amount observed on the same electrode charged to the same 
potential in the open air. 

The amount of excited activity produced on a wire, supported 
some distance from the surface of the earth, should increase steadily 
with the voltage, for the greater the potential, the greater the 
volume of air from which the radio-active carriers are abstracted. 

The presence of radio-active matter in the atmosphere will 
account for a portion of the ionization of the air observed near 
the earth. It seems unlikely, however, that the whole of the 
ionization observed in the air is due to this cause alone. 

211. Radio-activity of freshly fallen rain and snow. 

C. T. R. Wilson 1 tried experiments to see if any of the radio- 
active material from the air was carried down by rain. For this 
purpose a quantity of freshly fallen rain was collected, rapidly 
evaporated to dryness in a platinum vessel, and the activity of the 
residue tested by placing the vessel in an electroscope. In all 
cases, the rate of discharge of the electroscope was considerably 
increased. From about 50 c.c. of rain water, an amount of activity 
was obtained sufficient to increase the rate of discharge of the 

1 Proc. Camb. Phil. Soc. 11, Pt. vi. p. 428, 1902. 

232 



356 KADIO-ACTIVITY OF THE ATMOSPHERE [CH. 

electroscope four or five times, after the rays had traversed a thin 
layer of aluminium or gold leaf. The activity disappeared in the 
course of a few hours, falling to half value in about 30 minutes. 
Bain water, which had stood for some hours, showed no trace of 
activity. Tap water, when evaporated, left no active residue. 

The amounts of activity obtained from a given quantity of rain 
water were all of the same order of magnitude, whether the rain 
was precipitated in fine or in large drops, by night or by day, or 
whether the rain was tested at the beginning or at the end of a 
heavy rainfall lasting several hours. 

The activity obtained from rain is not destroyed by heating 
the platinum vessel to a red heat. In this and other respects it 
resembles the excited activity obtained on negatively charged 
wires exposed in the open air. 

C. T. R. Wilson 1 obtained a radio-active precipitate from rain 
water by adding a little barium chloride and precipitating the 
barium with sulphuric acid. An active precipitate was also 
obtained if alum was added to the water, and the aluminium 
precipitated by ammonia. The precipitates obtained in this way 
showed a large activity. The filtrate when boiled down was quite 
inactive, showing that the active matter had been completely 
removed by precipitation. The production of active precipitates 
from rain water is quite analogous to the production of active 
precipitates from a solution containing the emanation X of thorium 
(see section 178). 

The radio-activity of freshly fallen snow was independently ob- 
served by C. T. R. Wilson 2 in England, and Allan 3 and McLennan 4 
in Canada. In order to obtain a large amount of activity, the 
surface layer of snow was removed, and evaporated to dry ness 
in a metal vessel. An active residue was obtained with radio- 
active properties similar to those observed for freshly fallen rain. 
Both Wilson and Allan found that the activity of rain and snow 
decayed at about the same rate, the activity falling to half value 
in about 30 minutes. McLennan states that he found a smaller 
amount of radio-activity in the air after a prolonged fall of snow. 

1 Proc. Roy. Soc. Vol. 12, 1902. 

2 Proc. Camb. Phil. Soc. 12, p. 85, 1903. 

3 Phys. Rev. 16, p. 106, 1903. * Phys. Rev. 16, p. 184, 1903. 



XI] AXD OF ORDINARY MATERIALS 357 

Schmauss 1 has observed that drops of water falling through air 
ionized by Rb'ntgen rays acquire a negative charge. This effect is 
ascribed to the fact that the negative ions in air diffuse faster 
than the positive. On this view the drops of rain and flakes of 
snow would acquire a negative charge in falling through the air. 
They would in consequence act as collectors of the positive radio- 
active carriers from the air. On evaporation of the water the 
radio-active matter would be left behind. 

212. Radio-active emanations from the earth. Elster 
and Geitel observed that the air in caves and cellars was, in most 
cases, abnormally radio-active, and showed very strong ionization. 
This action might possibly be due to an effect of stagnant air, by 
which it produced a radio-active emanation from itself, or to a 
diffusion of a radio-active emanation from the soil. In order to 
test if this emanation was produced by the air itself, Elster and 
Geitel shut up the air for several weeks in a large boiler, but no 
appreciable increase of the activity or ionization was observed. In 
order to test if the air imprisoned in the capillaries of the soil was 
radio-active Elster and Geitel 2 put a pipe into the earth and sucked 
up the air into a testing vessel by means of a water pump. 

The apparatus employed to test the ionization of the air is 
shown in Fig. 59. C is an electroscope connected with a wire net, 
Z. The active air was introduced into the large bell-jar of 27 litres 
capacity, the inside of which was covered with wire netting, MM'. 
The bell-jar rested on an iron plate AB. The electroscope could 
be charged by the rod S. The rate of discharge of the electro- 
scope, before the active air was introduced, was noted. On allowing 
the active air to enter, the rate of discharge increased rapidly, 
rising in the course of a few hours in one experiment to 30 times 
the original value. They found that the emanation produced 
excited activity on the walls of the containing vessel. The air 
sucked up from the earth was even more active than that observed 
in caves and cellars. There can thus be little doubt that the 
abnormal activity observed in caves and cellars is due to a radio- 
active emanation, present in the earth, which gradually diffuses to 
the surface and collects in places where the air is not disturbed. 

1 Drude's Annal. 9, p. 224, 1902. 2 Phys. Zeit. 3 ; p. 574, 1902. 



358 



RADIO-ACTIVITY OF THE ATMOSPHERE 



[CH. 



The results obtained by Elster and Geitel for the air removed 
from the earth at Wolfenbiittel were also obtained later by Ebert 
and Ewers 1 at Munich. They found a strongly active emanation 
in the soil, and, in addition, examined the variation with time of 
the activity due to the emanation in a sealed vessel. After the 
introduction of the active air into the testing vessel, the activity 
was observed to increase for several hours, and then to decay, 
according to an exponential law, with the time, falling to half 
value in about 3 '2 days. This rate of decay is more rapid than 
that observed for the radium emanation, which decays to half 



H c= 




M 




M 1 



V 



Fig. 59. 

value in a little less than four days. The increase of activity with 
time is probably due to the production of excited activity on the 
walls of the vessel by the emanation. In this respect it is analogous 
to the increase of activity observed when the radium emanation 
is introduced into a closed vessel. No definite experiments were 
made by Ebert and Ewers on the rate of decay of this excited 
activity. In one experiment the active emanation, after standing 
in the vessel for 140 hours, was removed by sucking ordinary air 

1 Phys. Zeit. 4, p. 162, 1902. 



Xl] AND OF ORDINARY MATERIALS 359 

of small activity through the apparatus. The activity rapidly fell 
to about half value, and this was followed by a very slow decrease 
of the activity with time. This result indicates that about half 
the rate of discharge observed was due to the radiation from the 
emanation and the other half to the excited activity produced by it. 

The apparatus employed by Ebert and Ewers in these experi- 
ments was very similar to that employed by Elster and Geitel, 
shown in Fig. 59. Ebert and Ewers observed that when the wire 
net attached to the electroscope was charged negatively the rate 
of discharge observed was always greater than when it was charged 
positively. The differences observed between the two rates of 
discharge varied between 10 and 20 per cent. This difference in 
the rates of discharge for positive and negative electricity is 
probably connected with the presence of particles of dust or small 
water globules suspended in the gas. The experiments of Miss 
Brooks (section 171) have shown that the particles of dust present 
in the air containing the thorium emanation become radio-active. 
A large proportion of these dust particles acquire a positive charge 
and are carried to the negative electrode in an electric field. This 
effect would increase the rate of discharge of the electroscope when 
charged negatively. In later experiments, Ebert and Ewers 
observed that, in some cases, if the air had been kept in the vessel 
for several days, the effect was reversed, and the electroscope 
showed a great rate of discharge when charged positively. 

J. J. Thomson 1 has observed that the magnitude of the ioniza- 
tion current depends on the direction of the electric field, if fine 
water globules are suspended in the ionized gas. 

In later experiments, Ebert 2 found that the radio-active emana- 
tion could be removed from the air by condensation in liquid air. 
This property of the emanation was independently discovered by 
Ebert before he was aware of the results of Rutherford and Soddy 
on the condensation of the emanations of radium and thorium. In 
order to increase the amount of radio-active emanation in a given 
volume of air, a quantity of the active air, obtained by sucking the 
air from the soil, was condensed by a liquid air machine. The air 
was then allowed to partially evaporate, but the process was stopped 

1 Phil. Mag. Sept. 1902. 

2 Site. Akad. d. Wiss. Munich, 33, p. 133, 1903. 



360 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 

before the point of volatilization of the emanation was reached. 
This process was repeated with another quantity of air and the 
residues added together. Proceeding in this way, he was able to 
concentrate the emanation in a small volume of air. On allowing 
the air to evaporate, the ionization of the air in the testing vessel 
increased rapidly for a time and then slowly diminished. Ebert 
states that the maximum was reached earlier for the emanation 
which had been liquefied for some time than for fresh air. The rate 
of decay of activity of the emanation was not altered by keeping 
it at the temperature of liquid air for some time. In this respect 
it behaves like the emanations of radium and thorium. 

J. J. Thomson 1 found that air bubbled through Cambridge tap 
water showed much greater conductivity than ordinary air. The 
air was drawn through the water by means of a water pump into a 
large gasometer, when the ionization current was tested with a 
sensitive electrometer. When a rod charged negatively was intro- 
duced into this conducting air it became active. After an exposure 
for a period of 15 to 30 minutes in the conducting gas, the rod, 
when introduced into a second testing vessel, increased the saturation 
current in the vessel to about five times the normal amount. Very 
little effect was produced if the rod was uncharged or charged 
positively for the same time. The activity of the rod decayed 
with the time, falling to half value in about 40 minutes. The 
amount of activity produced in a wire under constant conditions 
was independent of the material of the wire. The rays from the 
rod were readily absorbed in a few centimetres of air. 

These effects were, at first, thought to be due to the action of 
the small water drops suspended in the gas, for it was well known 
that air rapidly drawn through water causes a temporary increase 
in its conductivity. Later results, however, showed that there 
was a radio-active emanation present in Cambridge tap water. 
This led to an examination of the waters from deep wells in 
various parts of England, and J. J. Thomson found that, in some 
cases, a large amount of emanation could be obtained from the 
well water. The emanation was released either by bubbling air 
through the water or by boiling the water. The gases obtained by 
boiling the water were found to be strongly active. A sample of 

1 Phil. Hag. Sept. 1902. 



XI] AND OF ORDINARY MATERIALS 361 

air mixed with the radio-active emanation was condensed. The 
liquefied gas was allowed to evaporate, and the earlier and last 
portions of the gas were collected in separate vessels. The final 
portion was found to be about 30 times as active as the first portion. 

An examination of the radio-active properties of the active 
gases so obtained has been made by Adams 1 . He found that the 
activity of the emanation decayed in an exponential law with the 
time, falling to half value in about 3'4 days. This is not very 
different from the rate of decay of the activity of the radium 
emanation, which falls to half value in a little less than four days. 
The excited activity produced by the emanation decayed to half 
value in about 35 minutes. The decay of the excited activity 
from radium is at first irregular, but after some time falls off in an 
exponential law, diminishing to half value in 28 minutes. Taking 
into account the uncertainty attaching to measurements of the 
very small ionization observed in these experiments, the results 
indicate that the emanation obtained from well water in England 
is similar to, if not identical with, the radium emanation. Adams 
observed that the emanation was slightly soluble in water. After 
well water had been boiled for some time and then put aside, it 
was found to recover its power of giving off an emanation with 
time. The amount obtained after standing for some time was 
never more than 10 per cent, of the amount first obtained. Thus 
it is probable that the well water, in addition to the emanations 
mixed with it, has also a slight amount of a permanent radio-active 
substance dissolved in it. Ordinary rain water or distilled water 
does not give off an emanation. 

Bumstead and Wheeler 2 have recently made a very careful 
examination of the radio-activity of the emanation obtained from 
the surface water and soil at New Haven, Connecticut. The 
emanation, obtained from the water by boiling, was passed into 
a large testing cylinder, and measurements of the current were 
made by means of a sensitive electrometer. The current gradually 
rose to a maximum after the introduction of the emanation, in 
exactly the same way as the current increases in a vessel after the 
introduction of the radium emanation. The decay of activity of 

1 Phil. Mag. Nov. 1903. 

- Amer. Journ. Science, 17, p. 97, Feb. 1904. 



362 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 

the emanations obtained from the water and soil was carefully 
measured, and within the limit of experimental error agreed with 
the decay of activity observed with the radium emanation. The 
identity of the emanations from the water and soil with the 
radium emanation was still further established by experiments 
on the rate of diffusion of the emanation through a porous plate. 
By comparative tests it was found that the coefficient of diffusion 
of the emanations from the water and soil was the same as for 
the radium emanation. In addition, by comparison of the rate of 
diffusion of carbonic acid, it was found that the density of the 
emanation was about four times that of carbonic acid, a result in 
good accord with that found for the radium emanation (section 
153). 

213. Radio-activity of constituents of the earth. Elster 
and Geitel 1 observed that, although in many cases the conductivity 
of the air was abnormally high in underground enclosures, the 
conductivity varied greatly for different places. In the Baumann 
Cave, for example, the conductivity of the air was nine times the 
normal, but in the Iberg Cave only three times the normal. In a 
cellar at Clausthal the conductivity was only slightly greater than 
the normal, but the excited radio-activity obtained on a negatively 
charged wire exposed in it was only 1/11 of the excited radio- 
activity obtained when the wire was exposed in the free air. It 
was concluded from these experiments that the amount of radio- 
activity in the different places probably varied with the nature 
of the soil. Observations were then made on the conductivity of 
the air sucked up from the earth at different parts of the country. 
The clayey and limestone soils at Wolfenbiittel were found to be 
strongly active, the conductivity varying from four to sixteen times 
the normal amount. A sample of air from the shell limestone of 
Wiirzburg and from the basalt of Wilhelmshohe showed very little 
activity. 

Experiments were made to see if any radio-active substance 
could be detected in the soil itself. For this purpose some earth 
was placed on a dish and introduced under a bell-jar, similar to that 
shown in Fig. 59. The conductivity of the air in the bell-jar 

1 Phys. Zeit. 4, p. 522, 1903. 



XI] AND OF ORDINARY MATERIALS 363 

increased with the time, rising to three times the normal value 
after several days. Little difference was observed whether the 
earth was dry or moist. The activity of the soil seemed to be 
permanent, for no change in the activity was observed after the 
earth had been laid aside for eight months. 

Attempts were then made to separate the radio-active con- 
stituent from the soil by chemical treatment. For this purpose 
a sample of clay was tested. By extraction with hydrochloric 
acid all the calcium carbonate was removed. On drying the 
clay, the activity was found to be reduced, but it spontaneously 
regained its original activity in the course of a few days. It thus 
seems probable that an active product had been separated from 
the soil by the acid. Elster and Geitel consider that an active 
substance was present in the clay, which formed a product more 
readily soluble in hydrochloric acid than the active material itself. 
There seemed to be a process analogous to the separation of Th X 
from thorium by precipitation with ammonia. 

Experiments were also made to see if substances placed in the 
earth acquired any radio-activity. For this purpose samples of 
potter's clay, whitening, and heavy spar, wrapped in linen, were 
placed in the earth 50 cms. below the surface. After an interval 
of a month, these were dug up and their activity examined. The 
clay was the only substance which showed any activity. The 
activity of the clay diminished with the time, showing that activity 
had been excited in it by the emanations present in the soil. 

Elster and Geitel 1 have recently found that a large quantity of 
the radio-active emanation can be obtained by sucking air through 
clay. In some cases, the conductivity of the air in the testing 
vessel was increased over 100 times. They have also found that 
" fango " a fine mud obtained from hot springs in Battaglia, 
Northern Italy gives off three or four times as much emanation 
as clay. By treating the fango with acid, the active substance 
present was dissolved. On adding some barium chloride to the 
solution, and precipitating the barium as sulphate, the active 
substance was removed, and in this way a precipitate was obtained 
over 100 times as active, weight for weight, as the original fango. 
Comparisons were made of the rate of decay of the excited activity, 

1 Phys. Zeit. 5, No. 1, p. 11, 1903. 



364 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 

due to the emanation from fango, with that due to the radium 
emanation, and within the limit of error, the decay curves obtained 
were found to be identical. There can thus be little doubt that 
the activity observed in fango is due to the presence of a small 
quantity of radium. Elster and Geitel calculate that .the amount 
of radium, contained in it, is only about one thousandth of the 
amount to be obtained from an equal weight of pitchblende from 
Joachimstahl. 

The natural carbonic acid arising from great depths of old 
volcanic soil was also tested. The carbon dioxide was obtained 
from the works in the liquid state. The gas was found to show 
distinct activity, and was able to produce excited activity on the 
surface of the vessel. After an interval of 16 days the gas was 
again tested and found to be inactive. 

These results are similar to those of J. J. Thomson, who found 
an active emanation in the water obtained from deep wells. 

f 

214. Effect of meteorological conditions upon* the 
radio-activity of the atmosphere. The original experiments 
of Elster and Geitel on the excited radio-activity derived from 
the atmosphere were repeated by Rutherford and Allan 1 in 
Canada. It was found that a large amount of excited radio- 
activity could be derived from the air, and that the effects were 
similar to those observed by Elster and Geitel in Germany. This 
was the case even on the coldest day in winter, when the ground 
was covered deeply with snow and the wind was blowing from the 
north over snow-covered lands. The results showed that the 
radio-activity present in the air was not much affected by the 
presence of moisture, for the air during a Canadian winter is 
extremely dry. The greatest amount of excited activity on a 
negatively charged wire was obtained in a strong wind. In some 
cases the amount produced for a given time of exposure was ten 
to twenty times the normal amount. A cold bright day of winter 
usually gave more effect than a warm dull day in summer. 

Elster and Geitel 2 have made a detailed examination of the 
effect of meteorological conditions on the amount of excited radio- 
activity to be derived from the atmosphere. For this purpose a 

1 Phil. Mag. Dec. 1902. -> Phys. Zeit. 4, pp. 137, 138. 1902. 



XI] AND OF ORDINARY MATERIALS 365 

simple portable apparatus 1 was devised by them and used for the 
whole series of experiments. A large number of observations were 
taken, extending over a period of twelve months. They found 
that the amount of excited activity obtained was subject to great 
variations. The extreme values obtained varied in the ratio of 
16 to 1. No direct connection could be traced between the amount 
of ionization in the atmosphere and the amount of excited activity 
produced. They found that the greatest amount of excited activity 
was obtained during a fog, while the amount of ionization in the 
air is then small. This result, however, is not necessarily contra- 
dictory to the view that the ionization and activity of the air 
are to a certain extent connected. From the experiments of 
Miss Brooks on the effect of dust in acting as carriers of excited 
activity, it is to be expected that more excited activity would be 
obtained during a fog than in clear air. The particles of water 
become centres for the deposit of radio-active matter. The 
positive carriers are thus anchored and are not removed from 
the air by the earth's field. In a strong electric field, these 
small drops will be carried to the negative electrode and manifest 
their activity on the surface of the wire. On the other hand, the 
distribution of water globules throughout the air causes the ions 
in the air to disappear rapidly in consequence of their diffusion to 
the surface of the drops (see section 31). For this reason the 
denser the fog, the smaller will be the conductivity observed in 
the air. 

Lowering the temperature of the air had a decided influence. 
The average activity observed below 0C. was T44 times the 
activity observed above C. The height of the barometer was 
found to exert a marked influence on the amount of excited activity 
to be derived from the air. The lower the barometer the greater 
was the amount of excited activity in the air. The effect of 
variation of the height of the barometer is intelligible, when it is 
considered that probably a large proportion of the radio-activity 
observed in the air is due to the radio-active emanations- which 
are continuously diffusing from the earth into the atmosphere. 
Elster and Geitel have suggested that a lowering of the pressure 
of the air would cause the air from the ground to be drawn up 

1 Phys. Zeit. 4, p. 522, 1903. 



366 KADIO- ACTIVITY OF THE ATMOSPHERE [CH. 

from the capillaries of the earth into the atmosphere. This, how- 
ever, need not necessarily be the case if the conditions of the escape 
of the emanation into the atmosphere are altered by the variation 
of the position of underground water or by a heavy fall of rain. 

The amount of excited activity to be derived from the air on 
the Baltic Coast was only one-third of that observed inland at 
Wolfenblittel. Experiments on the radio-activity of the air in 
mid-ocean would be of great importance in order to settle whether 
the radio-activity observed in the air is due to the emanations 
from the soil alone. It is to be expected that the radio-activity 
of the air at different points of the earth would vary widely, and 
would largely depend on the nature of the soil. 

Some interesting experiments have been made by McLennan 1 
on the amount of excited radio-activity to be derived from the air 
when filled with fine spray. The experiments were made at the 
foot of the American Fall at Niagara. An insulated wire was 
suspended near the foot of the Fall, and the amount of excited 
activity on the wire compared with the amount to be obtained on 
the same wire for the same exposure in Toronto. The amount of 
activity obtained from the air at Toronto was generally five or six 
times that obtained from the air at the Falls. In these experi- 
ments it was not necessary to use an electric machine to charge 
the wire negatively, for the falling spray kept the insulated wire 
permanently charged to a potential of about 7500 volts. These 
results indicate that the falling spray had a negative charge and 
electrified the wire. The small amount of the excited radio- 
activity at the Falls was probably due to the fact that the 
negatively charged drops abstracted the positively charged radio- 
active carriers from the atmosphere, and in falling carried them 
to the river below. On collecting the spray and evaporating it, 
no active residue was obtained. Such a result is, however, to be 
expected on account of the minute proportion of the spray tested 
compared with that present in the air. 

215. A very penetrating radiation firom the earth's 
surface. McLennan 2 , and Rutherford and Cooke 3 independently 

1 Phys. Rev. 16, p. 184, 1903, and Phil. Mag. 5, p. 419, 1903. 

2 Phys. Rev. No. 4, 1903. 3 Americ. Phys. Soc. Dec. 1902. 



XI] AND OF ORDINARY MATERIALS 367 

observed the presence of a very penetrating radiation inside build- 
ings. McLennan measured the natural conductivity of the air in 
a large closed metal cylinder by means of a sensitive electrometer. 
The cylinder was then placed inside another and the space between 
filled with water. For a thickness of water between the cylinders 
of 25 cms. the conductivity of the air in the inner cylinder fell to 
about 63 per cent, of its initial value. This result shows that part 
of the ionization in the inner cylinder was due to a penetrating 
radiation from an external source, which radiation was partially or 
wholly absorbed in water. 

Rutherford and Cooke observed that the rate of discharge of a 
sealed brass electroscope was diminished by placing a lead screen 
around the electroscope. A detailed investigation of the decrease 
of the rate of discharge in the electroscope, when surrounded by 
metal screens, was made later by Cooke 1 . A thickness of 5 cms. of 
lead round the electroscope decreased the rate of discharge about 
30 per cent. Further increase of the thickness of the screen had 
no effect. When the apparatus was surrounded by 5 tons of pig- 
lead the rate of discharge was about the same as when surrounded 
by a plate about 3 cms. thick. An iron screen also diminished the 
rate of discharge to about the same extent as the lead. By suitably 
arranging lead screens it was found that the radiation came equally 
from all directions. It was of the same intensity by night as by 
day. In order to be sure that this penetrating radiation did not 
arise from the presence of radio-active substances in the laboratory, 
the experiments were repeated in buildings in which radio-active 
substances had never been introduced, and also on the open ground 
far removed from any building. In all cases a diminution of the rate 
of discharge of the electroscope, when surrounded by lead screens, 
was observed. These results show that a penetrating radiation is 
present at the surface of the earth, arising partly from the earth 
itself and partly from the atmosphere. 

This result is not unexpected, when the radio-activity of the 
earth and atmosphere is taken into account. The writer has 
found that bodies made active by exposure to the emanations from 
thorium and radium give out 7 rays. It is then to be expected 
that the very similar excited radio-activity which is present in 
1 Phil. Mag. Oct. 1903. 



368 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 

the earth and atmosphere should also give rise to 7 rays of 
a similar character. 

216. Comparison of the radio-activity of the atmo- 
sphere with that produced by the radio-elements. The 

radio-active phenomena observed in the earth and atmosphere are 
very similar in character to those produced by thorium and radium. 
Radio-active emanations are present in the air of caves and cellars, 
in natural carbonic acid, and in deep well water, and these emana- 
tions produce excited radio-activity on all bodies in contact with 
them. The question now arises whether these effects are due to 
known radio-elements present in the earth or to unknown kinds 
of radio-active matter ? The simplest method of testing this point 
is to compare the rates of decay of the radio-active products in 
the atmosphere with those of the known radio-active products of 
thorium and radium. A cursory examination of the facts at once 
shows that the radio-activity of the atmosphere is much more 
closely allied to effects produced by radium than to those due to 
thorium. The activity of the emanation released from well water, 
and also that sucked up from the earth, decays to half value in 
about 3'3 days, while the activity of the radium emanation decays 
to half value in an interval of 3'7 to 4 days. Considering the 
difficulty of making accurate determinations of these quantities, 
the rates of decay of the activity of the emanations from the earth 
and from radium agree within the limits of experimental error. 
Bumstead and Wheeler have shown that the emanation from the 
soil and surface water of New Haven is identical with the radium 
emanation. If the emanation from the earth is the same as that 
from radium, the excited activity produced should have the same 
rate of decay as that from radium. The emanation from well 
water in England approximately fulfils this condition (section 212), 
but an observation recorded by Ebert and Ewers (section 212) 
seems to show that the excited activity due to the emanation 
sucked up from the earth decays at a very slow rate compared 
with that due to radium. 

On comparing the rates of decay of the excited activity derived 
from the atmosphere and of that produced by radium, the evidence 
is to some extent conflicting. The activity of a negatively charged 



XI] AND OF ORDINARY MATERIALS 369 

wire, exposed in the open air, decays according to an exponential 
law with the time, falling to half value in 45 minutes. On the 
other hand, the activity of freshly fallen rain and snow falls to 
half value in about 30 minutes. Now the activity of a wire, made 
active by exposure to the radium emanation, is at first irregular, 
but about an hour after removal it decays according to an expo- 
nential law with the time, falling to half value in 28 minutes. 
The agreement between the rates of decay of the activity of the 
emanation in the air and the excited activity produced on rain and 
snow, with the similar effects produced by radium, strongly sup- 
ports the view that the radium emanation is present in the soil 
and atmosphere. Allan 1 has also obtained evidence to show that 
the rate of decay of the excited activity produced on a negatively 
charged wire is the resultant of the rates of decay of several types 
of matter which have different rates of decay. For example, the 
activity transferred from the active wire to a piece of leather 
moistened with ammonia, fell to half value in 38 minutes, while on 
a piece of absorbent felt treated similarly the activity fell to half 
value in 60 minutes. Thus it seems probable that different types 
of active matter are collected by the negatively charged wire, 
which are soluble in ammonia in different degrees. An accurate 
determination of the rate of decay of the excited activity from 
actinium would be of interest, in order to see if the activity derived 
from the air may be due in part to the presence of the actinium 
emanation. 

Considering the results as a whole, there is evidence that other 
radio-active substances besides radium and thorium are present in 
the earth. There can be little doubt, however, that part of the 
radio-activity of the atmosphere is due to the radium emanation,, 
which is continually diffusing into the atmosphere from the pores 
of the earth. Since radio-activity has been observed in the 
atmosphere at all points at which observations have, so far, been 
made, there can be little doubt that radio-active matter is dis- 
tributed in minute quantities throughout the soil of the earth. 
The volatile emanations escape into the atmosphere by diffusion, 
or are carried to the surface in spring water or by the escape of 
underground gases, and cause the radio-active phenomena observed 

1 Phys. Rev. 16, p. 306, 1903. 
R. B.-A. 24 



370 KADIO- ACTIVITY OF THE ATMOSPHERE [CH. 

in the atmosphere. The observation of Elster and Geitel that the 
radio-activity of the air is much less near the sea than inland is 
at once explained, if the radio-activity of the atmosphere is due 
mainly to the diffusion of emanations from the soil into the air 
above it. 

The rare gases helium and xenon which exist in the atmosphere 
have been tested and found to be non-radio-active. The radio- 
activity of the air cannot be ascribed to a slight radio-activity 
possessed by either of these gases. 

In order to account for the effect observed, it is only necessary 
to suppose that the radio-active substance is present in minute 
quantity mixed with the soil. Suppose, for the purpose of illustra- 
tion, that the radio-activity of the atmosphere is due to the radium 
emanation escaping from the earth's surface. The air sucked from 
the soil in many cases shows 20 times the conductivity of ordinary 
air. Now it will be shown (section 218) that the natural 
conductivity of air observed in closed vessels corresponds to a 
production of about 30 ions per c.c. per second. The active air of 
20 times the normal conductivity thus gives rise to about 600 ions 
per c.c. per second. In 100 litres of this active air the number of 
ions produced per second is therefore 6 x 10 7 . Now it has been 
found that the saturation current in a sealed vessel, due to the 
emanation from one gram of radium chloride, corresponds to a 
current of 2'5 x 10~ 8 electro-magnetic units. Taking the charge of 
an ion as I'l x 10~ 20 electro-magnetic units, this corresponds to a 
production of 2*3 x 10 15 ions in the gas per second. The emanation 
present in 100 litres of air, of activity 20 times the normal activity, 
would thus correspond to the amount released by solution of 
3 x 10~ 8 of a gram of radium chloride. A very minute amount of 
radium per cubic foot of soil would account for the radio-active 
effects observed. 

217. Radio-activity of ordinary materials. It has been 
shown that radio-active matter seems to be distributed fairly 
uniformly over the surface of the earth and in the atmosphere. 
The very important question arises whether the small radio-activity 
observed is due to known or unknown radio-elements present in 
the earth and atmosphere, or to a feeble radio-activity of matter 



XI] AND OF ORDINARY MATERIALS 371 

in general, which is only readily detectable when large quantities 
of matter are present. The experimental evidence is not yet 
sufficient to answer this question, but undoubted proof has been 
obtained that many of the metals show a very feeble radio-activity. 
Whether this radio-activity is due to the presence of a slight trace 
of the radio-elements or is an actual property of the metals them- 
selves still remains in doubt. This point will be discussed in 
more detail later in section 220. 

Schuster 1 has pointed out that every physical property hitherto 
discovered for one element has been found to be shared by all 
the others in varying degrees. For example, the property of 
magnetism is most strongly marked in iron, nickel, and cobalt, but 
all other substances are found to be either feebly magnetic or 
diamagnetic. It might thus be expected on general principles 
that all matter should exhibit the property of radio-activity in 
varying degrees. On the view developed in chapter x. the 
presence of this property is an indication that the matter is 
undergoing change accompanied by the expulsion of charged 
particles. It does not, however, by any means follow that because 
the atom of one element in the course of time becomes unstable 
and breaks up, that, therefore, the atoms of all the other elements 
pass through similar phases of instability. 

It has already been mentioned (section 8), that Mme Curie 
made a very extensive examination of most of the elements and 
their compounds for radio-activity. The electric method was 
used, and any substance possessing an activity of 1/100 of that of 
uranium would certainly have been detected. With the exception 
of the known radio-elements and the minerals containing uranium 
and thorium, no other substances were found to be radio-active 
even to that degree. 

Certain substances like phosphorus 2 possess the property of 
ionizing a gas under special conditions. The air which is drawn 
over the phosphorus is conducting, but it has not yet been settled 
whether this conductivity is due merely to ions formed at the 
surface of the phosphorus or to ions produced by the phosphorus 
nuclei or emanations, as they have been termed, which are carried 

1 British Assoc. 1903. 

2 J. J. Thomson, Conduction of Electricity through Gases, p. 324, 1903. 

242 



372 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 

along with the current of air. It does not however appear that 
the ionization of the gas is in any way due to the presence of a 
penetrating type of radiation such as is emitted by the radio- 
active bodies. Le Bon (section 8) observed that quinine sulphate, 
after being heated to a temperature below the melting point and 
then allowed to cool, showed for a time strong phosphorescence 
and was able rapidly to discharge an electroscope. The discharging 
action of quinine sulphate under varying conditions has been very 
carefully examined by Miss Gates 1 . The ionization could not be 
observed through thin aluminium foil or gold-leaf but appeared 
to be confined to the surface. The current observed by an electro- 
meter was found to vary with the direction of the electric field, 
indicating that the positive and negative ions had very different 
mobilities. The discharging action appears to be due either to an 
ionization of the gas very close to the surface by some short ultra- 
violet light waves, accompanying the phosphorescence, or to a 
chemical action taking place at the surface. 

Thus, neither phosphorus nor quinine sulphate can be con- 
sidered to be radio-active, even under the special conditions when 
they are able to discharge an electrified body. No evidence in 
either case has been found that the ionization is due to the 
emission of a penetrating radiation. 

No certain evidence has yet been obtained that any body can 
be made radio-active by exposure to Rontgen or cathode rays. 
A metal exposed to the action of Rontgen rays gives rise to a 
secondary radiation which is very readily absorbed in a few 
centimetres of air. It is possible that this secondary radiation 
may prove to be analogous in some respects to the a rays from 
the radio-elements. The secondary radiation, however, ceases 
immediately the Rontgen rays are cut off. Villard 2 observed that 
a piece of bismuth produced a feeble photographic action after it 
had been exposed for some time to the action of the cathode 
rays in a vacuum. It has not however been shown that the 
bismuth gives out rays of a character similar to those of the 
radio-active bodies. 

The existence of a very feeble radio-activity of ordinary matter 

1 Amer. Phys. Soc. Oct. 1903. 

2 Societe de Physique, July, 1900. 



Xl] AXD OF ORDINARY MATERIALS 373 

has been deduced from the study of the conductivity of gases in 
closed vessels. The conductivity is extremely minute, and special 
methods are required to determine it with accuracy. A brief 
account will now be given of the gradual growth of our knowledge 
on this important question. 

218. Conductivity of air in closed vessels. Since the 
time of Coulomb onwards several investigators have believed that 
a charged conductor placed inside a closed vessel lost its charge 
more rapidly than could be explained by the conduction leak 
across the insulating support. Matte ucci, as early as 1850, observed 
that the rate of loss of charge w r as independent of the potential. 
Boys, by using quartz insulators of different lengths and diameters, 
arrived at the conclusion that the leakage must in part take place 
through the air. This loss of charge in a closed vessel was believed 
to be due in some way to the presence of dust particles in the air. 

On the discovery that gases became temporary conductors of 
electricity under the influence of Rontgen rays and the rays from 
radio-active substances, attention was again drawn to this question. 
Geitel 1 and C. T. R. Wilson- independently attacked the problem 
and both came to the conclusion that the loss of charge was due 
to a constant ionization of the air in the closed vessel. Geitel 
employed in his experiments an apparatus similar to that shown 
in Fig. 59. The loss of charge of an Exner electroscope, with the 
cylinder of wire netting Z attached, was observed in a closed vessel 
containing about 30 litres of air. The electroscope system was 
found to diminish in potential at the rate of about 40 volts per 
hour, and this leakage was shown not to be due to a want of 
insulation of the supports. 

Wilson, on the other hand, used a vessel of very small volume, 
in order to work with air which could be completely freed from 
dust. In the first experiments a silvered glass vessel with a 
volume of only 163 c.c. was employed. The experimental arrange- 
ment is shown in Fig. 60. 

The conductor, of which the loss of charge was to be measured, 
was placed near the centre of the vessel A. It consisted of a 

1 Phys. Zeit. 2, p. 116, 1900. 

2 Proc. Camb. Phil. Soc. 11, p. 52, 1900. Proc. Boy. Soc. 68, p. 152, 1901. 



374 



KADIO-ACTIVITY OF THE ATMOSPHERE 



[CH. 



narrow strip of metal with a gold-leaf attached. The strip of 
metal was fixed to the upper rod by means of a small sulphur bead. 
The upper rod was connected to a sulphur condenser with an 
Exner electroscope B attached to indicate its potential. The 
gold-leaf system was initially charged to the same potential as 
the upper rod and condenser by means of a fine steel wire which 
was caused to touch the gold-leaf system by the attraction of a 
magnet brought near it. The rate of movement of the gold-leaf 




Earth 



Earth 



Fig. 60. 



was measured by means of a microscope provided with a micro- 
meter eye-piece. By keeping the upper rod at a slightly higher 
potential than the gold-leaf system, it was ensured that the loss 
of charge of the gold-leaf system was not in any way due to a 
conduction leakage across the sulphur bead. 

The method employed by Wilson in these experiments is 
very certain and convenient when an extremely small rate of 
discharge is to be observed. In this respect the electroscope is 
able to measure with certainty a rate of loss of charge much 
smaller than can be measured by a sensitive electrometer. 



XI] AND OF ORDINARY MATERIALS 375 

Both Geitel and Wilson found that the leakage of the insulated 
system in dust-free air was the same for a positive as for a negative 
charge, and was independent of the potential over a considerable 
range. The leakage was the same in the dark as in diffuse 
daylight. The independence of leakage of the potential is strong 
evidence that the loss of charge is due to a constant ionization of 
the air. When the electric field acting on the gas exceeds a 
certain value all the ions are carried to the electrodes before re- 
combination occurs. A saturation current is reached, and it will 
be independent of further increase of the electric field, provided, 
of course, a potential sufficiently high to cause a spark to pass is 
not applied. 

C. T. R. Wilson has recently devised a striking experiment to 
show the presence of ions in dust-free air which is not exposed to 
any external ionizing agency. Two large metal plates are placed 
in a glass vessel connected to an expansion apparatus similar to 
that described in section 34. On expanding the air the presence 
of the ions is shown by the appearance of a slight cloud between 
the plates. These condensation nuclei carry an electric charge 
and are apparently similar in all respects to the ions produced 
in gases by X rays or by the rays from active substances. 

Wilson found that the loss of charge of the insulated system 
was independent of the locality. The rate of discharge was un- 
altered when the apparatus was placed in a deep tunnel, so that 
it did not appear that the loss of charge was due to an external 
radiation. From experiments already described, however (section 
215), it is probable that about 30 per cent, of the rate of discharge 
observed was due to a very penetrating radiation. This experiment 
of Wilson's indicates that the intensity of the penetrating radiation 
was the same in the tunnel as at the earth's surface. Wilson 
found that the ionization of the air was about the same in a brass 
vessel as in one of glass, and came to the conclusion that the 
air was spontaneously ionized. 

Using a brass vessel of volume about 471 c.c., Wilson de- 
termined the number of ions that must be produced in air 
per unit volume per second, in order to account for the loss of 
charge of the insulated system. The leakage system was found 
to have a capacity of about I'l electrostatic units, and lost its 



376 



RADIO-ACTIVITY OF THE ATMOSPHERE 



[CH. 



charge at the rate of 4*1 volts per hour for a potential of 210 volts, 
and 4'0 volts per hour for a potential of 120 volts. Taking the 
charge on an ion as 3*4 x 10~ 10 electrostatic units, this corresponds 
to a production of 26 ions per second. 

Rutherford and Allan 1 repeated the results of Geitel and 
Wilson, using an electrometer method. The saturation current 
was observed between two concentric zinc cylinders of diameter 
25'5 and 7*5 cms. respectively and length 154 cms. It was found 
that the saturation current could practically be obtained with a 
potential of a few volts. Saturation was however obtained with 
a lower voltage after the air had remained undisturbed in the 
cylinders for several days. This was probably due to the gradual 
settling of the dust originally present in the air. 

Later observations of the number of ions produced in air in 
sealed vessels have been made by Patterson 2 , by Harms 3 , and by 
Cooke 4 . The results obtained by different observers are shown 
in the following table. The value of the charge on an ion is taken 
as 3*4 x 10~ 10 electrostatic units : 





Number of ions 




Material of vessel 


produced per c.c. 


Observer 




per second 




Silvered glass . . 


36 


C. T. R Wilson 


Brass 


26 




Zinc 


27 


Rutherford and Allan 


Glass 


53 to 63 


Harms 


Iron 


61 


Patterson 


Cleaned brass . . 


10 


Cooke 



It will be shown later that the differences in these results are 
probably due to differences in the radio-activity of the containing 
vessel. 

219. Effect of pressure and nature of gas. C. T. R. Wilson 
(loc. cit.) found that the rate of leakage of a charged conductor 
varied approximately as the pressure of the air between the pres- 
sures examined, viz. 43 mms. and 743 mms. of mercury. These 
results point to the conclusion that, in a good vacuum, a charged 

1 Phil. Mag. Dec. 1902. . 2 PhiL Mag ^ August, 1903. 

3 Phys. Zeit. 4, No. 1, p. 11, 1902 4 PhiL Magm Oct 1903 



XI] 



AND OF ORDINARY MATERIALS 



377 



body would lose its charge extremely slowly. This is in agreement 
with an observation of Crookes, who found that a pair of gold- 
leaves retained their charge for several months in a high vacuum. 
Wilson 1 at a later date investigated the leakage for different 
gases. The results are included in the following table, where the 
ionization produced in air is taken as unity: 



(TOO 


Relative 


Relative ionization 




ionization 


density 


Air 


1-00 


1-00 


Hydrogen . . 


0-184 


2-7 


Carbon dioxide . . 


1-69 


MO 


Sulphur dioxide. . 


2-64 


1-21 


Chloroform . . 


4-7 


1-09 



With the exception of hydrogen, the ionization produced in 
different gases is approximately proportional to the density. The 
relative ionization is very similar to that observed by Strutt 
(section 45) for gases exposed to the influence of the a and @ rays 
from radio-active substances, and points to the conclusion that the 
ionization observed may be due either to a radiation from the 
walls of the vessel or from external sources. 

Patterson 2 examined the variation of the ionization of air 
with pressure in a large iron vessel of diameter 30 cms. and length 
20 cms. The current between a central electrode and the cylinder 
was measured by means of a sensitive Dolezalek electrometer. 
He found that the saturation current was practically independent 
of the pressure for pressures greater than 300 mms. of mercury. 
Below a pressure of 80 mms. the current varied directly as the 
pressure. For air at atmospheric pressure, the current was inde- 
pendent of the temperature up to 450 C. With further increase 
of temperature, the current began to increase, and the increase 
was more rapid when the central electrode was charged negatively 
than when it was charged positively. This difference was ascribed 
to the production of positive ions at the surface of the iron vessel. 
The results obtained by Patterson render it very improbable that 
the ionization observed in air is due to a spontaneous ionization of 
the enclosed air: for it would be expected that the amount of 
this ionization would depend on the temperature of the gas. On 
1 Proc. Roy. Soc. 69, p. 277, 1901. 2 Phil. Hag. Aug. 1903. 



378 



RADIO-ACTIVITY OF THE ATMOSPHERE 



[CH. 



the other hand, the results are to be expected if the ionization 
of the enclosed air is mainly due to an easily absorbed radiation 
from the walls of the vessel. If this radiation had a penetrating 
power about equal to that observed for the a rays of the radio- 
elements, the radiation would be absorbed in a few centimetres of 
air. With diminution of pressure, the radiations would traverse 
a greater distance of air before complete absorption, but the total 
ionization produced by the rays would still remain about the same, 
until the pressure was reduced sufficiently to allow the radiation 
to traverse the air space in the vessel without complete absorption. 
With still further diminution of pressure, the total ionization 
produced by the radiation, and in consequence the current observed, 
will vary directly as the pressure. 

220. Examination of ordinary matter for radio-activity. 

Strutt 1 , McLennan and Burton 2 , and Cooke 3 , independently ob- 
served about the same time that ordinary matter is radio-active 
to a slight degree. Strutt, by means of an electroscope, observed 
that the ionization produced in a closed vessel varied with the 
material of the vessel. A glass vessel with a removeable base 
was employed and the vessel was lined with the material to be 
examined. The following table shows the relative results obtained. 
The amount of leakage observed is expressed in terms of the 
number of scale divisions of the eye-piece passed over per hour 
by the gold-leaf: 



Material of lining of vessel 


Leakage in scale 
divisions per hour 


Tinfoil 


3-3 


another sample 
Glass coated with phosphoric acid 
Silver chemically deposited on glass 
Zinc ... 


2-3 
1-3 

1-6 
1*2 


Lead 


2 - 2 


Copper (clean) 
(oxidized) 
Platinum (various samples) 
Aluminium ... 


2-3 
1-7 
2-0, 2-9, 3-9 
1*4 







1 Phil. Mag. June, 1903. Nature, Feb. 19, 1903. 

2 Phys. Rev. No. 4, 1903. J. J. Thomson, Nature, Feb. 26, 1903. 

3 Phil. Mag. Aug. 6, 1903. Eutherford, Nature, April 2, 1903. 



XI] AND OF ORDINARY MATERIALS 379 

There are thus marked differences in the leakage observed for 
different materials and also considerable differences in different 
samples of the same metal. For example, one specimen of platinum 
caused nearly twice the leakage of another sample from a different 
stock. 

McLennan and Burton, on the other hand, measured by means 
of a sensitive electrometer the ionization current produced in the 
air in a closed iron cylinder 25 cms. in diameter and 130 cms. in 
length, in which an insulated central electrode was placed. The 
open cylinder was first exposed for some time at the open window 
of the laboratory. It was then removed, the top and bottom 
closed, and the saturation current through the gas determined as 
soon as possible. In all cases it was observed that the current 
diminished for two or three hours to a minimum and then very 
slowly increased again. In one experiment, for example, the initial 
current observed corresponded to 30 on an arbitrary scale. In the 
course of four hours the current fell to a minimum of 6'6, and 
44 hours later had risen to a practical maximum of 24. The 
initial decrease observed is probably due to a radio-activity of 
the enclosed air or walls of the vessel, which decayed rapidly 
with the time. The decay of the excited activity produced on 
the interior surface of the cylinder when exposed to the air was 
probably responsible for a part of the decrease observed. McLennan 
ascribes the increase of current with time to a radio-active ema- 
nation which is given off from the cylinder, and ionizes the enclosed 
air. On placing linings of lead, tin, and zinc in the iron cylinder, 
considerable differences were observed both of the minimum current 
and also of the final maximum. Lead gave about twice the cur- 
rent due to zinc, while tin gave an intermediate value. These 
results are similar in character to those obtained by Strutt. 

McLennan and Burton also investigated the effect of dimi- 
nution of pressure on the current. The cylinder was filled with 
air to a pressure of 7 atmospheres, and allowed to stand until 
the current reached a constant value. The air was then allowed 
to escape and the pressure reduced to 44 mms. of mercury. The 
current was found to vary approximately as the pressure over the 
whole range. These results are not in agreement with the results 
of Patterson already described, nor with some later experiments 



380 KADIO-ACTIVITY OF THE ATMOSPHERE [CH. 

of Strutt. McLennan's results however point to the conclusion 
that the ionization was mainly due to an emanation emitted from 
the metal. Since the air was rapidly removed, a proportionate 
amount of the emanation would be removed also, and it might 
thus be expected that the current would vary directly as the 
pressure. If this is the case the current through the gas at low 
pressures should increase again to a maximum if time is allowed 
for a fresh emanation to form. 

H. L. Cooke, using an electroscopic method, obtained results 
very similar to those given by Strutt. Cooke observed that a pene- 
trating radiation was given out from brick. When -a brass vessel 
containing the gold-leaf system was surrounded by brick, the 
discharge of the electroscope was increased by 40 to 50 per cent. 
This radiation was of about the same penetrating power as the 
rays from radio-active substances. The rays were completely 
absorbed by surrounding the electroscope by a sheet of lead 2 mms. 
in thickness. This result is in agreement with the observation 
of Elster and Geitel, already mentioned, that radio-active matter 
was present in clay freshly dug up from the earth. 

Cooke also observed that the ionization of the air in a brass 
electroscope could be reduced to about one-third of its usual 
value if the interior surface of the brass was carefully cleaned. By 
removing the interior surface of the brass he was able to reduce 
the ionization of the enclosed air from 30 to 10 ions per c.c. per 
second. This is an important observation, and indicates that a 
large proportion of the radio-activity observed in ordinary matter 
is (fiie to a deposit of radio-active matter on its surface. It has 
already been shown (sections 173 and 188) that bodies which 
have been exposed in the presence of the radium emanation 
retain a residual activity which decays extremely slowly. There 
can be no doubt that the radium emanation is present in the 
atmosphere, and the exposed surface of matter, in consequence, 
will become coated with an invisible film of radio-active matter, 
deposited from the atmosphere. On account of the slow decay of 
this activity it is probable that the activity of matter exposed in 
the open air would steadily increase for a long interval. Metals, 
even if they are originally inactive, would thus acquire a fairly 
permanent activity, but it should be possible to get rid of this 



Xl] AND OF ORDINARY MATERIALS 381 

by removing the surface of the metal or by chemical treat- 
ment. 

It must be borne in mind that the activity observed in ordinary 
matter is excessively minute. The lowest rate of production of 
ions yet observed is 10 per cubic centimetre per second in a 
brass vessel. Suppose a spherical brass vessel is taken of capacity 
1 litre. The area of the interior surface would be about 480 sq. 
cms. and the total number of ions produced per second would be 
about 10 4 . Now it has been shown in section 104 that an a 
particle projected from radium probably gives rise to 7 x 10 4 ions 
before it is absorbed in the gas. An expulsion of one a particle 
every 7 seconds from the whole vessel, or of one a particle from 
each square centimetre of surface per hour would thus account for 
the minute conductivity observed. Even if it were supposed that 
this activity is the result of a breaking up of the matter com- 
posing the vessel, the disintegration of one atom per second per 
gram, provided it was accompanied by the expulsion of an a 
particle, would fully account for the conductivity observed. 

Strutt 1 has recently observed that a radio-active emanation can 
be obtained by bubbling air through mercury. The emanation 
appears to be very similar in character to that emitted by radium 
emanation, for its activity decays to half value in 318 days and 
the excited activity to half value in 20 minutes. An emanation 
was also obtained by drawing air over red-hot copper. 

Bumstead and Wheeler 2 have repeated the experiments of 
Strutt of bubbling air through mercury, but were unable to 
detect any increase of the conductivity of air, which had been 
circulated through hot mercury for fourteen hours, although an 
increase of 10 per cent, of the natural conductivity could have 
been detected. These results indicate that the emanation from 
mercury obtained in the experiments of Strutt was probably due 
to the presence of a minute amount of radium as an impurity. 

There is not yet sufficient evidence to decide with certainty 
whether ordinary matter possesses the property of radio-activity. 
There is no doubt that, if matter possesses the property at all, 
it does so to a minute extent. The extreme minuteness of the 

1 Phil Mag. July, 1903. 

2 Amer. Jour. Science, 17, p. 110, Feb. 1904. 



382 RADIO-ACTIVITY OF THE ATMOSPHERE, ETC. [CH. XI 

radio-activity observed, and the distribution of radio-active matter 
throughout the constituents of the earth, render it difficult to be 
certain that any substance, however carefully prepared, is freed 
from possible radio-active impurities. A careful comparison of 
the rates of decay of the activity of the emanations obtained from 
ordinary matter, and of the excited activity, with the corresponding 
rates of decay of the activity of the products of the known radio- 
active substances, may throw some light on the question. 



or THE 
UNIVERSITY 

OF 




INDEX. 



The numbers refer to the pages. 



a rays 

discovery of, 115 et seq. 

nature of, 115 et seq. 

magnetic deviation of, 117 et seq. 

electrostatic deviation of, 121 

velocity of, 122 et seq. 

value of e/m for, 122 

mass and energy of, 125 et seq. 

origin of, in atomic disintegration, 126 

scintillations produced by, 127 

absorption of, by matter, 129 et seq. 

increase of absorption with thickness 
of matter traversed, 129 et seq. 

relative absorption of a rays from 
radio -elements, 132 

absorption of, by gases, 133 

connection between absorption and 
density, 137 

relation between ionization and ab- 
sorption, 138 

theory of absorption 'of, 138 

effect of thickness of layer of radiating 
matter on emission of, 149 

relative ionization produced by a and 
/3 rays, 149 

relative energy emitted in form of a 
and ft rays, 150 et seq. 

number and energy of a particles 
from radio-elements, 154 

emission of energy from radio-ele- 
ments in form of a rays, 154 

connection of heat emission of radium 
with a rays, 160 et seq. 

connection of, with radio-active 
changes, 193, 322 

from the emanations, 222 

absence of, in change in emanation 
X of thorium, 270 et seq. 

absence of, in change in emanation 
X of radium, 273 

non-separable activity of radio-ele- 
ments consists of, 304 

emission from all active products 
except last change, 304 

a particles consist of helium, 331 

loss of weight due to expulsion of, 335 



Abraham 

apparent mass of moving charged 

body, 109 
Absorption 

law of, .in gases, 56 et seq. 

relative absorption of a, ft and 7 rays 

by matter, 93 

of ft rays by solids, 112 et seq. 
of a rays by solids, 129 et seq. 
of 7 rays by solids, 142 
connection between absorption and 

density for ft rays, 113 et seq. 
connection between absorption and 

density for a rays, 137 
connection between absorption and 

density for 7 rays, 143 
of ft rays in radio-active matter, 115 
of a rays in gases, 133 et seq. 
connection between absorption and 

ionization, 138 
theory of, 138 
of a rays by radium, 164 
of rays from the emanations, 222 
of penetrating rays from the earth, 367 
Actinium 

methods of separation of, 22 et seq. 

properties of, 23 

similarity to "emanating substance" 

of Giesel, 24 
possible connection with radio-activity 

of thorium, 25 
emanation from, 208 
excited activity produced by, 288 
effect of magnetic field on excited 

activity from, 288 
Adams 

decay of activity of emanation from 

well water, 361 
decay of excited activity from the 

emanation, 361 
Age 

of radium, 333 
of sun and earth, 343 
Allan 

increase with time of excited activity 

from atmosphere, 355 



384 



INDEX 



Allan (cont.) 

radio-activity of snow, 356 
effect of conditions on decay of ac- 
tivity from air, 369 
Allan and Eutherford 

decay of excited activity from atmo- 
sphere, 353 

ionization of air in closed vessels, 376 
Anderson and Hardy 

action of radium rays on eye, 177 
Armstrong and Lowry 

radio-activity and phosphorescence, 

323 
Arnold 

rays from phosphorescent substances, 

4 
Aschkinass and Caspar! 

action of radium rays on eye, 177 
Atmosphere 

excited radio-activity from, 351 et seq. 
radio-activity of, due to emanations, 

354 
diffusion of emanations into, from 

earth, 357 
effect of temperature, pressure, &c. on 

radio-activity of, 364 
presence of very penetrating radiation 

in, 366 
comparison of radio-activity of, with 

radio-elements, 368 
Atom 

number of, per c.c., 51 

complex nature of, 126 

disintegration of, 126 

number of, transformed per second, 

332 

changing atoms, 322 et seq. 
possible causes of disintegration of, 

338 et seq. 

evolution of, 347 et seq. 
Atomic weight 

of radium by chemical methods, 17 
from spectroscopic evidence, 18 
of emanations, 232 
of radio-elements and connection with 
radio-activity, 323 et seq. 

B rays 

discovery of, 95 

magnetic deflection of, 95 et seq. 
complexity of, 98 et seq. 
examination by the electrical method, 

100 

effect of, on a fluorescent screen, 101 
charge carried by the, 102 et seq. 
electrostatic deviation of, 106 
velocity of, and value of elm for, 106 

et seq. 
variation of ejm with velocity of, 108 

et seq. 






j8 rays (cont.) 

absorption of, 112 

variation of absorption with density, 
113 et seq. 

variation of intensity of, with thick- 
ness of layer, 115 

relative ionizatiou produced by a and 
/3 rays, 149 

relative energy emitted in form of 
a and /3 rays, 150 et seq. 

phosphorescent action of, 166 

physical action produced by, 171 et seq. 

chemical action of, 174 

physiological action of, 176 

from UrX, 293 

from emanation X of radium, 302 

appearance of, only in last of radio- 
active changes, 304 

change of weight due to expulsion of, 

336 
Barium platinocyanide 

phosphorescence of, under radium 
rays, 167 

change of colour due to radium rays, 

168 
Barnes and Eutherford 

heating effect of radium emanation, 
161 

connection of heating effect with radio- 
activity, 160 et seq. 

heating effect of emanation, 247 

heating effect of excited activity, 279 

division of heating effect among active 

products, 279 
Baskerville 

activity of thorium, 25 

phosphorescence of kunzite under ra- 
dium rays, 168 

phosphorescence produced by radium 

rays, 168 
Beattie, Smolan and Kelvin 

discharging power of uranium rays, 7 
Becquerel 

rays from calcium sulphide, 4 

rays from uranium, 5 et seq. 

permanence of uranium rays, 6 

discharging power of uranium rays, 6 

magnetic deflection of radium rays by 
photographic method, 96 

curvature of radium rays in a mag- 
netic field, 96 et seq. 

complexity of radium rays, 98 et seq. 

electrostatic deflection of /3 rays of 
radium, 106 

value of elm for 8 rays of radium, 
107 

magnetic deviation of a rays of radium 
and polonium, 120 

trajectory of rays of radium in mag- 
netic field, 123 et seq. 



INDEX 



385 



Becquerel (con*.) 

scintillations due to cleavage of crys- 
tals, 128 

7 rays from radium, 141 
secondary rays produced by active 

substances, 146 
phosphorescence produced by radium 

rays, 166 
conductivity of paraffin under radium 

radiation, 173 
effect of temperature' on uranium rays, 

173 

chemical action of radium rays, 175 
removal of activity from uranium by 

precipitation with barium, 179 
recovery of activity of uranium after 

precipitation with barium, 179 
penetrating rays from polonium, 305 
theory of radio-activity, 317 
Bemont et M. et Mme Curie 

discovery of radium, 13 
Benoist 

variation of absorption of Rontgen 

rays in matter, 144 
Berndt 

spectrum of polonium, 20 
Bodlander and Runge 

evolution of gases from radium, 176 
Boys 

rate of dissipation of charge, 373 
Brooks, Miss 

variation of excited activity from 

thorium for short exposures, 260 
effect of dust on distribution of ex- 
cited activity, 260 
Brooks and Rutherford 

absorption of a rays by matter, 129 
comparison of absorption of a rays 

from radio-elements. 132 et seq. 
diffusion of radium emanation, 228 
decay of excited activity from radium, 

261 
Bumstead and Wheeler 

emanation from surface water and the 

soil, 361, 368 
identity of emanation from soil with 

radium emanation, 361 
absence of emanation in mercury, 381 
Burton and McLennan 

penetrating radiation from the earth, 

366 
radio-activity of ordinary materials, 

378 
emanation from ordinary matter, 379 

Canal rays 

similarity of to a rays, 92 
Capacity 

of electroscopes, 72 

of electrometers, 79, 85 

R. E.-A. 



Capacity (cont.) 

standards of, 86 
Carbonic acid 

radio-activity of natural, 364 
Caspari and Aschkinass 

action of radium rays on eye, 177 
Cathode rays 

comparison of, with p rays, 102 et seq. 
absorption of, by matter, 113 
see also rays, 95 et seq. ' 
Caves 

radio-active matter present in air of, 

357 

radio-activity of air of, due to emana- 
tion from the soil, 357 
Charge 

carried by the ions, 47 et seq. 
negative charge carried by /S rays, 102 

et seq. 
measurement of charge carried by /3 

rays, 104 et seq. 

positive charge carried by a rays, 120 
Chemical nature 
of emanation, 225 
of emanation X, 275 
Chemical actions of radium rays 
production of ozone, 174 
coloration of glass and rock salt, 174 
on phosphorus, 175 
on iodoform, 175 
on globulin, 175 
evolution of hydrogen and oxygen, 

176* 
Child 

potential gradient between electrodes, 

63 
variation of current with voltage for 

surface ionization, 64 
Clouds 
formation by condensation of water 

round ions, 43 et seq. 
difference between positive and nega- 
tive ions in formation of, 46 
Collision 

ionization by, 54 

number of ions produced by rays 

per cm. of path, 139 
total number of ions produced by 

collisions of a particles, 156 
Coloration 

of crystals of radiferous barium, 15 
of bunsen flame by radium, 17 
of glass by radium rays, 174 
of rock salt, fluor spar' and potassium 

sulphate by radium rays, 174 
Concentration 

of excited activity on negative elec- 
trode, 252 

activity of radium independent of, 
339 

25 



386 



INDEX 



Condensation 

of water round the ions, 43 et seq. 
of emanations, 236 et seq. 
experimental illustration of, 237 
temperature of, 238 
difference between point of, for eman- 
ations of thorium and radium, 243 
from air sucked up from the earth, 

357 
Conductivity 

of gases exposed to radiations, 28 

et seq. 

variation of, with pressure, 58 et seq. 
variation of, with nature of gas, 61 
comparison of, for gases exposed to 

a, /3 and 7 rays, 61 et seq. 
of insulators, 172 
of air in closed vessels, 351, 373 
increase of, with time, in a closed 

vessel, 351 

of air in caves and cellars, 357 
variation of in closed vessels with 

pressure and nature of gas, 376 
variation of with temperature for air 

in closed vessels, 377 
Conservation of radio-activity 

examples of, 308 
Cooke, H. L. 

penetrating rays from the earth, 366 
number of ions per c.c. in closed 

vessels, 376 
radio-activity from ordinary matter, 

378 
Corpuscle 

(see Electron) 
Crookes (Sir W.) 
spectrum of polonium, 20 
separation of Ur X, 178 
nature of a rays, 116 
scintillations produced by radium, 127 
spinthariscope, 127 
number of scintillations independent 

of pressure and temperature, 128 
theory of radio-activity, 319 
cathode rays, 347 
Crookes and Dewar 

absence of nitrogen spectrum in phos- 
phorescent light of radium at low 
pressures, 169 
Curie, Mme 

permanence of uranium rays, 6 
discovery of radio-activity of thorium, 

10 
radio-activity of uranium and thorium 

minerals, 11 

relative activity of compounds of ura- 
nium, 12 

coloration of radium crystals, 15 
spectrum of radium, 15 
nature of a rays, 116 



Curie, Mme (cont.) 

absorption of a rays from polonium, 

131 

secondary radiation tested by electric 
method, 148 

slowly decaying excited activity from 
radium, 264 

bismuth made active by solution of 
barium, 289 

recovery of activity of radium, 303 
Curie, P. 

magnetic deviation of radium rays by 
electric method, 96 

heat emission of radium at low tem- 
perature and variation of heat emis- 
sion with age of radium, 159, 160 

conductivity of dielectrics under 
radium rays, 172 

radio-activity of radium unaffected by 
temperature, 173 

decay of activity of radium emana- 
tion, 206 et seq. 

discovery of excited radio-activity 
from radium, 250 

nature of the emanation, 318 
Curie, M. et Mme 

discovery of radium, 13 

charge carried by radium rays, 102 

luminosity of radium compounds, 168 

production of ozona by radium rays, 
174 

coloration of glass by radium rays, 174 

theory of radio-activity, 318 

possible absorption by radio-elements 

of unknown radiations, 321 
Curie, J. et P. 

quartz piezo-electrique, 87 et seq. 
Curie, P. et Danne 

diffusion of radio-active emanation, 
231 

decay of excited activity from radium, 
262 

occlusion of radium emanation in 

solids, 264 
Curie, P. and Debierne 

active gases evolved from radium, 210 

phosphorescence produced by radium 
emanation, 211 

distribution of luminosity, 211 

rate of production of emanation inde- 
pendent of pressure, 224 

effect of pressure on amount of ex- 
cited activity, 282 
Curie, P. and Laborde 

heat emission of radium, 158 

origin of heat from radium, 319 
Current 

through gases, 28 et seq. 

variation of with distance between 
the plates, 56 et seq. 



INDEX 



387 



Current (cont.) 

variation of with pressure of gas, 

58 et seq. 

variation of, with nature of gas, 61 
measurement of, by galvanometer, 69 
measurement of, by electroscope, 70 

et seq. 

measurement of, by electrometer, 84 
measurement of, by quartz piezo- 

electrique, 87 
magnitude of, for one gram of radium, 

156 

Danne and Curie, P. 

diffusion of radium emanation, 231 
decay of excited activity from radium, 

262 
occlusion of radium emanation in 

solids, 264 
Danysz 

action of radium rays on skin, 177 
Darwin, G. H. 

age of sun, 343 
Debierne 

actinium, 22 

emanation from actinium, 208 

effect of magnetic field on activity 

excited from actinium, 288 
barium made active by actinium, 289 
Debierne and Curie 

evolution of gas from radium, 175 
active gas evolved from radium, 210 
phosphorescence produced by radium 

emanation, 211 

distribution of luminosity, 211 
rate of production of emanation inde- 
pendent of pressure, 224 
effect of pressure on excited activity, 

282 
Decay 

of heating effect of emanation, 162 
of activity of Th X, 180 
of activity of UrX, 182 
significance of law of, 189 
effect of conditions on the rate of, 190 
of activity of thorium emanation, 200 
of activity of radium emanation, 206 
excited activity due to thorium for 

long exposure, 256 
excited activity due to thorium for 

short exposure, 258 
excited activity due to radium, 261 
excited activity of slow decay due to 

radium, 264, 291 
excited activity from atmosphere, 351 

et seq. 

of activity of rain and snow, 356 
of emanation from earth, 358 
differences in, of excited activity from 

atmosphere, 368 



Deflection 

of rays in a magnetic field, 92 - 
of /3 rays in a magnetic field, 95 et seq. 
of /3 rays in an electrostatic field, 106 
of a rays in a magnetic field, 117 et seq. 
of a rays in an electrostatic field, 121 
of "ions activants" in a magnetic 

field, 288 
Demarcay 

spectrum of radium, 16 
Des Coudres 

magnetic and electric deviation of a 

rays of radium, 122 
determination of elm for a rays, 122 
velocity of cathode rays diminished in 

passage through matter, 139> 
Dewar 

emission of heat from radium in liquid 

hydrogen, 160 
Dewar and Crookes 

absence of nitrogen spectrum in phos- 
phorescent light of radium at low 
pressures, 169 
Dielectrics 

conduction of, under radium rays, 172 
Diffusion 
of ions, 49 
of radium emanation into gases, 228 

et seq. 

of thorium emanation into gases, 233 
of radium emanation into liquids, 235 
Discharge 

action of rays on spark and electrode- 
less, 171 
Disintegration 

account of theory of, 126, 193, 323 et 

seq. 

list of products of, 326 
helium a product of, 327 et seq. 
rate of, in radio-elements, 332 et seq. 
emission of energy in consequence of, 

336 et seq. 

possible causes of, 338 et seq. 
of matter in general, 347 et seq. 
Dissipation of charge 
in closed vessels, 351, 373 
in caves and cellars, 357 
effect of pressure and nature of gas 

on, 376 et seq. 

effect of material of vessel on, 378 et seq . 
Dolezalek 

electrometer, construction of, 78 et seq. 
Dorn 

charge carried by radium rays, 104 
electrostatic deflection of ft rays from 

radium, 106 

discovery of radium emanation, 205 
effect of moisture on emanating power 

of thorium, 214 
electrolysis of radium solution, 276 

252 



388 



INDEX 



Dorn (cont.) 

loss of weight of radium, 336 
Durack 

ionization by collision of electrons of 

great velocity, 139 
Dust 

effect of, in recombination of ions, 39 
effect of, on distribution of excited 
activity, 260 

Earth 

amount of radium in, 344 

age of, 344 

excited activity deposited on, 354 

activity concentrated on peaks of, 354 

emanation from, 363 

very penetrating radiation from, 366 
Ebert 

condensation of emanation from the 

earth, 359 
Ebert and Ewers 

emanation from the earth, 358 
Electric field 

deflection of j8 rays by, 106 

deflection of a rays by, 121 

movement in, of carriers of excited 
activity, 282 et seq. 

action on, of carriers of excited ac- 
tivity from "emanating substance," 
287 
Electrolysis 

separation of radio-tellurium by, 21 

of solutions of emanation X, 276 

of thorium solutions, 277 

of radium solutions, 276 
Electrometer 

description of, 74 et seq. 

use of, in measurements, 74 

construction of, 76 et seq. 

Dolezalek, 78 

adjustment and screening of, 79 

special key for, 81 

application of, to measurements of 
radio-activity, 81 et seq. 

measurement of current by, 84 

capacity of, 85 

use with quartz piezo-electrique, 87 
Electron 

definition of, 53 

identity of /3 rays with electrons, 102 
et seq., 107 

variation of apparent mass of electron 
with velocity, 108 et seq. 

evidence that mass of electron is 
electro-magnetic, 112 

diameter of, 112 

production of, under different condi- 
tions, 347 
Electroscope 

description of, used by Curie, 70 



Electroscope (cont.) 

construction of, for accurate measure- 
ments, 71 
use of, in measurements of minute 

currents, 71 
of C. T. E. Wilson, 73 
use of, in measuring conductivity of 

air in closed vessels, 373 et seq. 
use of, for determining radio-activity 

of ordinary matter, 378, 380 
Elster and Geitel 
radio-active lead, 25 
effect of magnetic field on conductivity 

produced in air by radium rays, 95 
scintillations produced by active sub- 
stances, 127 

action of radium rays on spark, 171 
photo-electric action of body, coloured 

by radium rays, 174 
radio-active matter in earth, 345 
discovery of excited activity in atmo- 
sphere, 351 

emanations from the earth, 357 
radio-activity of air in caves, 362 
radio-activity of the earth, 362 
radio-activity of natural carbonic acid, 

364 

variation of radio-activity in atmo- 
sphere with meteorological condi- 
tions, 364 et seq. 
effect of temperature and pressure on 

atmospheric radio-activity, 365 
Emanation 

* rate of heat emission by, 158 
A variation of heat emission with time, 

160 
of thorium, discovery and properties, 

197 

methods of measurement of, 199 
decay of activity of, 200 
effect of thickness of layer, on 

amount of, 202 

increase of, with time, to a maxi- 
mum, 204 
of radium, 205 

decay of activity of, 206 
of actinium, properties of, 208 
of radium, phosphorescence produced 

by, 209 et seq. 
rate of emission of, 213 
effect of conditions, on rate of emis- 
sion of, 214 

regeneration of emanating power, 215 
continuous rate of production of, 216 

et seq. 

source of thorium emanation, 220 
source of radium emanation, 222 
radiations from, 222 
effect of pressure on production of, 224 
chemical nature of, 225 



INDEX 



389 



Emanation (cont.) 

experiments to illustrate gaseous na- 
ture of, 227 

rate of diffusion of radium emanation, 
228 et seq. 

rate of diffusion of thorium emana- 
tion, 233 

diffusion of, into liquids, 235 

condensation of, 236 et seq. 

temperature of condensation of, 238 
et seq. 

volume of, from 1 gram of radium, 
and thorium, 246 

heat emission of, 247 

connection between emanations and 
excited activity, 253 et seq. 

effect of removal of, on activity of 
radium, 300 

effect of rate of escape of, on activity 
of radium, 302 

fraction of activity of radium, due to, 
304 

experimental separation, and volume 
of, 311 et seq. 

decrease of volume of, with time, 313 

radio-activity of atmosphere, due to 
emanations, 354 

sucked up from the earth, 357 

rate of decay of activity of, from the 
earth, 358 

condensation of, from atmosphere, 359 

in caves, 357 

in well-water, and springs, 360 

in natural carbonic acid gas, 364 

from " fango," 363 

effect of meteorological conditions on 
amount of, in atmosphere, 364 

from metals, 378 
Emanation X (see Excited radio-activity) 

definition of, 256 

chemical and physical properties of, 
275 

electrolysis of, 276 

effect of temperature on, 277 

emission of heat by, 278 

transmission of, to negative elec- 
trode, 282 et seq. 

irregularities in decay of activity of 
ThX due to, 295 

removal of, by successive precipita- 
tions, 296 

theory of effect of production of, on 

activity of ThX, 295 et seq. 
Emanating power 

measurement of, 213 

effect of conditions on, 214 

regeneration of, 215 
" Emanating substance " of Giesel 

separation and properties of, 23 et seq. 

similarity of, to actinium, 24 



"Emanating substance" of Giesel (cont.) 

emanation from, 209 

excited activity produced by, 287 

action of an electric field on, 287 
Energy 

of a particle, 125 

of /9 particle, 151 

comparison of, for a and /3 particles, 
150 

emitted from radio-elements, in form 
of a rays, 154 

emitted from radium in form of heat, 
158 et seq. 

emission of, from the emanation, 161 
et seq., 247 

emission of, from radio-active pro- 
ducts of radium, 278 

total emission of, from 1 gram of 
radio-elements, 336 

latent store of, in matter, 337 
Eve 

conductivity of gases for very pene- 
trating Rontgen rays, 145 
Evolution of matter 

evidence of, 348 
Ewers and Ebert 

emanation from the earth, 358 
Excited radio-activity 

heat emission due to, 161 et seq. 

discovery and properties of, 250 

concentration of, on negative elec- 
trode, 252 

connection of, with the emanations, 
253 

removal of, by acids, 255 

decay of, due to thorium, 256 

decay of, for short exposurgTo thorium, 
259 

effect of dust on distribution of, 260 

decay of, from radium, 261 

of radium, of very slow decay, 264 

connection between decay curves of, 
and times of exposure, 265 

theory of successive changes to give 
rise to, 268 

changes in emanation X of thorium, 
270 et seq. 

changes in emanation X of radium, 
272 

physical and chemical properties of 
emanation X, 275 

electrolysis of active solutions, 276 

effect of temperature on, 277 

emission of heat, due to, 278 

variation with electric field, of amount 
of, 280 

effect of pressure on distribution of, 282 

transmission of, 282 et seq. 

from actinium, and " emanating sub- 
stance," 287 



390 



INDEX 



Excited radio-activity (cont.) 

possible connection of polonium and 
radio-tellurium with, 290 et seq. 

from the atmosphere, 351 et seq. 

concentration of, on negative elec- 
trode, 351 

decay of, 353 

due to emanation in atmosphere, 
354 

distribution of, on surface of earth, 
354 

concentration of, on prominences of 
the earth, 354 

of rain and snow, 356 

decay of, on rain and snow, 356 

due to emanation from earth, 357 

produced by emanation from tap 
water, 360 

effect of meteorological conditions on 
amount of, 364 

amount of, at Niagara Falls, 366 

rate of decay of, dependent on con- 
ditions, 368 
Exner and Haschek 

spectrum of radium, 17 
Eye 

action of radium rays on, 177 

Fehrle 

distribution of excited activity on a 

plate, in electric field, 282 
Fluorescence 

produced in substances by radium 

rays, 18 
produced in substances by radium 

and polonium rays, 166 
Fog 

large amount of excited activity, 

during, 365 
Forch 
loss of weight of radium, 336 

7 rays 

discovery of, 141 

absorption of, by matter, 142 

connection between absorption of, and 
density, 143 

discussion of nature of rays, 143 et seq. 

conservation of radio-activity mea- 
sured by, 311 

measurement of radio-activity by 

means of, 321, 339 
Gases 

evolved by radium, 175 

presence of helium in gases from 

radium, 176, 327 et seq. 
Gates, Miss F. 

effect of temperature on excited ac- 
tivity, 278 

discharge of quinine sulphate, 372 



Geitel 

natural conductivity of air in closed 

vessels, 351, 373 
Geitel and Elster 

radio-active lead, 25 

effect of magnetic field on conducti- 
vity produced by radium rays, 95 

scintillations produced by active sub- 
stances, 127 

action of radium rays on spark, 171 

photo-electric action of bodies co- 
loured by radium rays, 174 

radio-active matter in earth, 345 

discovery of radio-active matter in 
atmosphere, 351 

emanations from the earth, 357 

radio-activity of air in caves, 362 

radio-activity of the soil, 362 

radio-activity of natural carbonic acid, 
364 

variation of radio-activity of air, with 
meteorological conditions, 364 et seq. 

effect of temperature and pressure on 

radio-activity in atmosphere, 365 
Giesel 

coloration of bunsen flame by radium, 
15 

separation of radium by crystallization 
of bromide, 15 

emanating substance, 23 

radio-active lead, 26 

magnetic deviation of ft rays, 95 

temperature of radium bromide above 
air, 159 

decrease with time of luminosity of 
radio-active screen, 168 

coloration of bodies by radium rays, 
174 

evolution of gases from radium, 176 

action of radium rays on eye, 177 

emanation from the emanating sub- 
stance, 209 

luminosity produced by radium ema- 
nation, 209 

excited activity from emanating 
substance, 287 

bismuth made active by radio-active 
solution, 289 

activity of radium dependent on age, 

300 
Glass 

coloration produced in, by radium 
rays, 174 

phosphorescence produced by emana- 
tion, 210 
Globulin 

action of radium rays on, 175 
Goldstein 

canal strahlen, 92 

coloration of bodies by radium rays, 174 



INDEX 



391 



Grier and Kutherford 

magnetic deviation of /3 rays of 

thorium, 96 

relative current due to a and /3 rays, 150 
nature of rays from Ur X, 294 

Hardy 

coagulation of globulin by radium 

rays, 175 
Hardy and Miss Willcock 

coloration of iodoform solutions by 

radium rays, 175 
Hardy and Anderson 

action of radium rays on the eye, 177 
Harms 
number of ions per c.c., in closed 

vessel, 376 
Haschek and Exner 

spectrum of radium, 17 
Heat 

rate of emission of, from radium, 158 
emission of, from radium at low 

temperatures, 159 
rate of emission of, after removal of 

the emanation, 162 
rate of emission of, by the emanation, 

162 

variation with time of heat emission 

of radium, and of its emanation, 162 

connection of heat emission with the 

radio-activity, 161 
source of heat energy, 163 
heating effect of the emanation, 247 
heating effect of emanation X, 278 

et seq. 
proportion of heating effect, due to 

radio-active products, 280 
total heat emission during life of 

radio -elements, 336 
heating of earth by radio-active 

matter, 344 
Heaviside 

apparent mass of moving charged 

body, 109 
Helium 

produced by radium and its emana- 
tion, 327 

amount of, from radium, 331 
origin of, 331 
Helmholtz and Kicharz 

action of ions on steam jet, 44 
Hemptinne 

action of rays on spark, and electrode- 
less discharge, 171 
Henning 

resistance of radium solutions, 171 
effect of voltage on amount of ex- 
cited activity, 281 
Heydweiler 

loss of weight of radium, 336 



Himstedt 

action of radium rays on selenium, 171 
Himstedt and Nagel 

action of radium rays on eye, 177 
Hofmann and Strauss 

radio-active lead, 26 
Hofmann and Zerban 

active substance from pitchblende, 25 
Huggius, Sir W. and Lady 

spectrum of phosphorescent light of 

radium bromide, 169 
Hydrogen 

production of, by radium rays, 176 

Induced radio-activity (see Excited 

radio-activity) 
Induction 

radio-active, 21 

meaning, and examples of, 289 
Insulators 

conduction of, under radium rays, 172 
Iodoform 

coloration produced in, by radium 

rays, 175 
lonization 

theory of, to explain conductivity of 
gases, 28 et seq. 

by collision, 36, 54 

variation of, with pressure of gas, 58 
et seq. 

variation of, with nature of gas, 62 

comparison of, produced by rays, 93, 
149 

total, produced by 1 gram radium, 154 

production of, in insulators, 172 

natural ionization of gases, 373 et seq. 
Ions 

in explanation of conductivity of gases, 
28 et seq. 

rate of recombination of, 37 et seq. 

mobility of, 39 et seq. 

difference between mobility of positive 
and negative, 42, 43 

condensation of water around, 43 et 
seq. 

difference between positive and nega- 
tive, 46 

charges carried by, 47 

diffusion of, 48 et seq. 

charge on ion same as on hydrogen 
atom, 51 

number of, produced per c.c., 52 

size and nature of, 52 et seq. 

definition of, 52 et seq. 

production of, by collision, 36, 54 

velocity acquired by, between colli- 
sions, 55 

energy required to produce, 55 

comparative number of, produced in 
gases, 62 



392 



INDEX 



Ions (cont.) 

disturbance of potential gradient by 

movement of, 63 

number of, produced by a particle, 155 
production of, in insulators, 172 
number produced per c.c., in closed 

vessels, 375 

Joly 

absorption of radium rays by at- 
mosphere, 343 (see foot-note) 

Kauffmann 

variation of ejm with velocity of 

electron, 108 et seq. 
Kelvin 

theory of radio-activity, 320 
age of sun and earth, 343, 344 
Kelvin, Smolan and Beattie 

discharging power of uranium rays, 7 
Kunz 

phosphorescence of willemite, and 

kunzite, 168 
Kunzite 

phosphorescence of, under radium 
rays, 168 

Laborde and Curie 

heat emission of radium, 19, 158 
origin of heat from radium, 319 
Langevin 

coefficient of recombination of ions, 38 
velocity of ions, 39 et seq. 
energy required to produce an ion, 55 
secondary radiation produced by X- 

rays, 146 
Larmor 

electrons and matter, 108 
structure of the atom, 126 
radiation from accelerated electrons, 

340 

Lead, radio-active 
preparation of, 26 
radiations from, 26 
Le Bon 

rays from bodies exposed to sunlight, 

5 
discharging power of quinine sulphate, 

9, 372 
Lenard 

ionization of gases by ultra-violet 

light, 9 

action of ions on a steam jet, 44 
negative charge carried by Lenard 

rays, 102 

absorption of cathode rays propor- 
tional to the density, 113 
Lerch, von 

chemical properties of emanation X, 
275 



Lerch, von (cont.) 

electrolysis of solution of emanation 

X, 276 

effect of temperature on excited act- 
ivity, 278 
Lockyer 

inorganic evolution, 349 
Lodge 

connection of heat emission with a 

rays, 164 

instability of atoms, 340 
Lorentz 

structure of atoms, 126 
Lowry and Armstrong 

radio-activity and phosphorescence, 

323 
Luminosity 

of radium compounds, 168 

change of, in radium compounds with 

time, 168 
spectrum of phosphorescent light from 

radium bromide, 169 
of radium compounds unaffected by 
temperature, 173 

Magnetic field 

effect of on rays, 92 
deflection of /3 rays by, 95 et seq. 
,, ,, a rays by, 117 et seq. 
,, ,, "ionsactivants"by, 288 
Marckwald 

preparation of radio-tellurium, 21 



apparent mass of electron, 107 et seq. 
variation of mass of electron with 

speed, 108 et seq. 
of a particle, 122, 125 
Materials 

radio-activity of ordinary, 370, 378 
Matteucci 

rate of dissipation of charge in closed 

vessels, 373 
McClung 

coefficient of recombination of ions, 

38 
McClung and Eutherford 

energy required to produce an ion, 55 
variation of current with thickness of 

layer of uranium, 149 
estimate of energy radiated from radio- 
elements, 154 

radiation of energy from radium, 317 
McLennan 

absorption of cathode rays, 62 

radio-activity of snow, 356 

excited radio-activity at Niagara Falls, 

366 
McLennan and Burton 

penetrating radiation from the earth, 
366 



INDEX 



393 



McLennan and Burton (cont.) 

radio-activitv of ordinary materials, 
378 

emanation from ordinary matter, 379 
Mercury 

emanation from, 381 
Metabolon 

definition of, 324 

table of metabolons, 326 

radio-elements as metabolons, 332 
Meteorological conditions 

effect of, on radio-activity of atmo- 
sphere, 364 
Methods of measurement 

in radio-activity, 67 et seq. 

comparison of photographic and elec- 
trical, 67 et seq. 

description of electrical, 68 et seq. 
Meyer and Schweidler 

magnetic deviation of rays by elec- 
trical method, 95 

absorption of ft rays of radium by 

matter, 113 
Mobility 

of ions, 39 et seq. 
Moisture 

effect of, on velocity of ions, 40, 42 

effect of, on emanating power, 214 
Molecule 

number of, in 1 c.c. of hydrogen, 51 

molecular weight of radium emana- 
tion, 232 

molecular weight of thorium emana- 
tion, 234 
Molecular weight 

of radium emanation, 232 

of thorium emanation, 234 

Nagel and Himstedt 

action of radium rays on eye, 177 
Niewenglowski 

rays from sulphide of calcium, 4 
Number 

of molecules per c.c. of hydrogen, 51 
of ions produced in gas by active 

substances, 52 
of a particles emitted per gram of 

radium, 155 

of ions, produced per c.c. in closed 
vessels, 375 

Occlusion 

of emanation in thorium and radium, 

217 

of radium emanation by solids, 264 
Owens 

saturation current affected by dust, 

39 

effect of air currents on conductivity 
produced by thorium, 197 



Owens (cont.) 

penetrating power of rays independent 

of compound, 132 
absorption of a rays varies directly as 

pressure of gas, 137 
Oxygen 

change into ozone, by radium rays, 174 
production of from radium solutions, 

176 
Ozone 

production of, by radium rays, 174 

Paraffin 

objection to as an insulator, 80 

conductivity of, under radium rays, 

173 
Patterson 

number of ions per c.c. in closed 
vessel, 376 

natural conductivity of air due to an 
easily absorbed radiation, 377 

effect of temperature on natural con- 
ductivity of air, 377 
Pegram 

electrolysis of thorium solutions, 277 
Penetrating power 

comparison of for a, ft and, y rays, 93 

variation in, of ft rays, 98 et seq. 

comparison of, for a rays from radio- 
elements, 136 

variation of, with density for ft rays, 
112 et seq. 

variation of, with density for a rays, 
137 

variation of, with density for y rays, 

143 
Penetrating radiation 

from the earth and atmosphere, 366 
Perrin 

negative charge of cathode rays, 102 

theory of radio-activity, 316 
Phosphorescence 

production of, by radium, 18 

production of, by radium and polon- 
ium rays, 166 

comparison of, produced by a and ft 
rays, 168 

of zinc sulphide, 167 

of barium platino-cyanide, 168 

of willemite and kunzite, 168 

diminution of, with time, 168 

of radium compounds, 168 

spectrum of phosphorescent light of 
radium bromide, 169 

production of by heat (thermo-lumin- 
escence), 170 

produced by radium emanation in 
substances, 210, 227 

use of, to illustrate condensation of 
emanations, 237 



394 



INDEX 



Phosphorus 

action of radium rays on, 175 

ionization produced by, 371 
Photo-electric action 

produced by radium rays in certain 

substances, 174 
Photographic 

method, advantages and disadvan- 
tages of, 67 

relative photographic action of rays, 

68 
Physical action of radium rays 

on sparks, 171 

on electrodeless discharge, 171 

on selenium, 171 

on conductivity of insulators, 172 
Physical properties 

of emanation X, 275 
Physiological action of radium rays 

production of burns, 176 

effect on bacteria, 177 

effect on cancer, 177 

effect on eye, 177 
Piezo-electrique of quartz 

description of, 87 
Pitchblendes 

comparison of radio-activity of, 11 

radio-elements separated from, 13 et 
seq. 

radium continually produced from, 

334 
Polarization of uranium rays 

absence of, 7 
Polonium 

methods of separation of, 19 

rays from, 20 

decay of activity of, 20 

discussion of nature of, 21 

similarity to radio-tellurium, 22 

magnetic deviation of a rays from, 
121 

increase of absorption with thickness 
of matter traversed, 131 

phosphorescent action of rays from, 
166 

possible origin of polonium and con- 
nection with radium, 290 et seq. 

penetrating rays from, 305 
Potential 

required to produce saturation, 30 
et seq. 

fall of potential, to produce ions each 
collision, 55 

gradient, due to movement of ions, 

63 
Precht and Eunge 

atomic weight of radium, 17 

heating effect of radium, 164 
Pressure 

effect of, on velocity of ions, 43 



Pressure (cont.) 
effect of, on current through gases, 

58 et seq. 
production of emanation, independent 

of, 224 
effect of, on distribution of excited 

activity, 282 
effect of, on natural conductivity of 

air in closed vessels, 376 
Products, radio-active ' 
radiations from, 304 
division of activity amongst, 306 
list of from radio-elements, 326 

Quartz piezo-electrique 

use of, in measurement of current, 37 
Quinine sulphate 

discharging power of, 9, 372 

phosphorescence of, 372 

Kadiations 

emitted by uranium, 8 

emitted by thorium, 10 

emitted by radium, 18 

emitted by polonium, 20 

emitted by actinium, 23 

methods of measurement of, 67 et seq. 

methods of comparison of, 90 

three kinds of, 91 

analogy to rays from a Crookes tube, 
92 * 

relative ionizing and penetrating 
power of, 93 

difficulties of comparative measure- 
ment of, 93 et seq. 

/3 rays, 95 et seq. 

a rays, 115 et seq. 

7 rays, 141 et seq. 

secondary rays, 146 

comparison of ionization of a and /5 
rays, 149 et seq. 

connection of, with heat emission, 
160 

phosphorescent effect of, 166 

physical actions of, 171 et seq. 

chemical actions of, 174 et seq. 

physiological actions of, 176 

from the emanation, 222 

from Ur X, 293 

non-separable activity of radio-ele- 
ments consists of a rays, 304 

from different active products, 304 

conservation of energy of each specific 

type of, 308 
Kadium 

discovery of, 13 

separation of, 13 

spectrum of, 15 

atomic weight of, 17 

radiations from, 18 



INDEX 



395 



Radium (cont.) 
compounds of, 19 

nature of radiations from, 90 et seq. 
/3 rays from, 95 et seq. 
a rays from, 115 et seq. 
y rays from, 141 et seq. 
secondary rays from, 147 
heat emission of, 158 et seq. 
production of phosphorescence by, 

166 et seq. 
spectrum of phosphorescent light of, 

169 

physical actions of, 171 et seq. 
chemical actions of, 174 
physiological actions of, 176 
emanation from, 205 et seq. 
properties of emanation from, 205 et 

seq. 
chemical nature of emanation from, 

225 et seq. 

diffusion of emanation from, 228 et seq. 
condensation of emanation from, 236 

et seq. 

amount of emanation from, 246, 312 
heat emission of emanation from, 247, 

278 

excited radio-activity from, 251 et seq. 
decay of excited activity from, 261 

et seq. 
successive changes in emanation X of, 

272 

properties of emanation X of, 275 
heating effect due to products of, 278 
radio-active induction due to, 289 
connection of, with polonium, 291 
alteration of activity of, by removal 

of emanation, 300 
recovery of activity of, after removal 

of emanation, 301 

effect of escape of emanation on 
. recovery of activity of, 302 
non-separable acthity of. 302, 304 
radiations from active products of, 304 
division of activity amongst active 

products of, 306 

conservation of radio-activity of, 308 
determination of volume of emanation 

of and diminution with time, 313 

et seq. 

theories of radio-activity of, 316 
discussions of theories of radio-activity 

of, 320 
energy of radiations, not derived from 

external source, 321 
theory of radio-active change, 322 
list of active products of, 326 
polonium possible product of, 326 
production of helium from, 327 
helium disintegration product of, 327 
amount of helium from, 331 



Radium (cont.) 

rate of change of, 332 

life of, 333 

origin of, 333 

possible production of, by uranium, 
334 

loss of weight of, 335 

experiments to determine loss of 
weight of, 336 

total emission of energy from 1 gram 
of, 336 

possible causes of disintegration of, 338 

amount of, to account for heat of sun, 
342 

possible connection of with heat of 
sun, 342 

probable amount of, in earth, 345 

possible connection with heat of earth, 

344 
Rain 

radio-activity of, 355 

decay of activity of, 356 
Ramsay and Soddy 

evolution of gas from radium, 176 

chemical nature of the emanation, 
227 

gaseous nature of the emanation, 227 

volume of emanation, and change 
witfi time, 313 

helium from radium emanation, 328 
Re, Filippo 

theory of radio-activity, 320 
Recombination 

of ions, 37 et seq. 

constant of, 39 
Recovery 

of heating effect of radium, 162 

of activity of thorium after removal 
of Th X, 181 

of activity of uranium after removal 
of Ur X, 182 

significance of law of, 185 

effect of conditions on rate of, 191 

of activity of radium, after removal 

of emanation, 301 
Reflection 

no evidence of direct reflection for 
uranium rays, 7 

diffuse reflection of rays, 7 
Refraction 

no evidence of, for uranium rays, 7 
Regeneration 

of emanating power, 215 
Richarz and von Helmholtz 

action of ions on steam jet, 44 
Runge 

spectrum of radium, 17 
Runge and Precht 

atomic weight of radium, 17 

heating effect of radium, 164 



396 



INDEX 



Eunge and Bodlander 

evolution of gas from radium, 176 
Kussell 

photographic action of substances, 68 

Saturation current 

meaning of, 30 et seq. 

application of, to measurements of 
radio-activity, 69 

measurement of, 82 et seq. 
Schmidt 

discovery of radio-activity of thorium, 

10 
Schmidt and Wiedemann 

thermo-luminescence, 170 
Schuster 

radio-activity of matter, 371 
Schweidler and Meyer 

magnetic deviation of /3 rays by electric 
method, 95 

absorption of /3 rays of radium, 113 
Scintillations 

discovery of in zinc sulphide screen, 
127 

connection of, with a rays, 127 

illustration of by spinthariscope, 127 

cause of, 128 

production of, by action of electric 

field, 128 
Searle 

apparent mass of moving charged 

body, 109 
Secondary rays 

examination of, by photographic 
method, 146 

examination of, by electrical method, 

148 
Selenium 

action of radium rays on, 171 
Simon 

value of e/m for cathode rays, 111 
Smolan, Beattie and Kelvin 

discharging power of uranium rays, 7 
Snow 

radio-activity of, 356 

decay of activity of, 356 
Soddy 

comparison of photographic and elec- 
trical action of uranium rays, 68 

nature of rays from Ur X, 294 
Soddy and Kamsay 

evolution of gas from radium, 176 

chemical nature of the emanation, 
227 

gaseous nature of the emanation, 227 

volume of the emanation, and change 
with time, 313 

helium from radium emanation, 320 
Soddy and Rutherford 

separation of ThX, 179 



Soddy and Rutherford (cont.) 
decay of activity of Th X, 181 
recovery of activity of thorium, freed 

from Th X, 181 

decay of activity of UrX, 182 
recovery of activity of uranium, freed 

from Ur X, 182 
explanation of decay and recovery 

curves, 183 

rate of production of ThX, 186 
theory of decay of activity, 188 
influence of conditions on rate of 
decay and recovery of activity, 190 
et seq. 

disintegration hypothesis, 194, 324 
decay of activity of radium emanation, 

206 
measurements of emanating power, 

213 

effect of temperature, moisture, and 
solution, on emanating power, 214 
regeneration of emanating power, 215 
constant rate of production of emana- 
tion of radium and thorium, 216 
et seq. 

source of thorium emanation, 220 
radiations from the emanation, 222 
chemical nature of emanation, 226 
condensation of emanations of radium 

and thorium, 236 et seq. 
temperature of condensation of emana- 
tion, 238 et seq. 
effect of successive precipitations on 

activity of thorium, 2J6 
recovery of activity of radium, 300 
conservation of radio-activity, 309 
theory of radio-activity, 318 
theory of radio-active change, 324 
Soil 

radio-activity of, 362 
difference in activity of, 362 
Solution 

coloration of, by radium, 15 
of emanation X in acids, 275 
electrolysis of active, 276 
Source 

of the thorium emanation, 220 
of radium emanation, 222 
Spark 

action of radium rays on, 171 
Spectrum 

spark spectrum of radium, 15, 16 
flame spectrum of radium, 17 
effect of a magnetic field on spectrum 

of radium, 17 
of polonium, 20 
of phosphorescent light of radium 

bromide, 169 

of helium in radium gases and emana- 
tion, 329 






INDEX 



397 



Spectrum (cont.) 

of emanation, 329 
Spinthariscope 

description of, 127 
Springs 

emanation from water of, 360 
Stark 

energy to produce an ion, 55 
Strauss and Hofmann 

radio-active lead, 25 
Strutt 

conductivity of gases for radiation, 
61, 62 

negative charge carried by radium 
rays, 104 

absorption of /3 rays proportional to 
density, 113 

nature of a rays, 116 

conductivity of gases produced by 
7 rays, 62, 144 

absorption of radium rays from sun 
by atmosphere, 343 

radio-activity of ordinary matter, 378 

emanation from mercury, 381 
Sun 

effect of radium in, 342 

age of, 343 

Temperature 

of radium above surrounding space, 

158 
effect of, on intensity of radiations 

from uranium and radium, 173 
effect of, on luminosity, 173 
rate of decay of radium emanation 

unaffected by, 208 
of condensation of emanations, 238 

et seq. 
rate of decay of thorium emanation 

unaffected by, 246 
effect of, on excited activity, 277 
effect of, on amount of excited activity 

in atmosphere, 364 et seq. 
effect of, on natural ionizationofair,377 
Theories 

of radio-activity, review of, 316 
discussion of, 320 
disintegration theory, 324 
Thermo-luminescence, 170 
Thomson, J. J. 

relation between current and voltage 

for ionized gases, 31 
difference between ions as condensa- 
tion nuclei, 46 
charge on ion, 47 
theory of electrometers, 85 
path of charged particle in uniform 

magnetic field, 96 
apparent mass of moving charged 

body, 108 



Thomson, J. J. (cont.) 

structure of atom, 126, 347 
theory of radio-activity, 319 
cause of heat emission from radium, 

321 
possible causes of disintegration of 

radium, 342 
nature of electrons, 347 
emanation from tap-water and deep 

wells, 360 
Thomson, J. J., and Kutherford 

ionization theory of gases, 28 et seq. 
Thorium 

discovery of radio-activity of, 10 
emanation from, 11 
preparation of non-radio-active tho- 
rium, 25 

nature of radiations from, 90 et seq. 
)3 rays from, 95 et seq. 
a rays from, 115 et seq. 
7 rays from, 141 et seq. 
rate of emission of energy by, 154 
separation of Th X from, 179 
recovery of activity of, 181 
disintegration of thorium, 193 
emanation from, 197 
properties of emanation from, 198 et 

seq. f 

diffusion of emanation from, 233 
condensation of emanation from, 236 

et seq. 

excited radio-activity from, 250 et seq. 
successive changes in emanation X of, 

272 
explanation of initial portion of decay 

curve, 295 et seq. 

explanation of initial portion of re- 
covery curve, 295 et seq. 
effect of successive precipitations on, 

296 
recovery curve after large number of 

precipitations, 297 
theory of decay curve of Th X, 298 
non- separable activity of, 296, 304 
radiations from active products of, 

304 et seq. 
division of activity amongst active 

products of, 306 et seq. 
conservation of radio-activity of, 308 

et seq. 

resume of results, 311 et seq. 
theories of radio-activity of, 316 et seq. 
discussion of theories of radio-activity, 

320 et seq. 
source of energy of radiations, 320 et 

seq. 
theory of radio-active change, 322 et 

seq. 

table of radio-active products of, 326 
rate of change of, 332 et seq. % 



398 



INDEX 



Thorium (cont.) 
life of, 333 
total emission of energy from 1 gram 

of, 337 
possible causes of disintegration of, 

338 et seq. 
Thorium X 

methods of separation of, 179 
law of decay of activity of, 182 
law of recovery of activity of, 182 
theory to explain production of, 183 
material nature of, 185 
continuous production of, 186 
explanation of decay of activity of, 

188 et seq. 
effect of conditions on the rate of 

change of, 190 et seq. 
disintegration hypothesis to explain 

production of, 193 et seq. 
minute amount of, produced, 195 
effect of successive separations of, on 

activity of thorium, 296 
theory of decay curve of, 296 et seq. 
Tommasina 

scintillations produced by electrifica- 
tion, 128 
Townsend 

ions by collision, 36, 54 
coefficient of recombination, 38 
diffusion of ions, 49 et seq. 
comparison of charge on ion with that 

on hydrogen atom in electrolysis, 51 
number of molecules per c.c. of gas, 51 
ionization by collision for different 

speeds, 139 
Transmission 

of excited radio-activity of radium 

and thorium, 282 et seq. 
of excited radio-activity of actinium, 

287 
Troost 

rays from hexagonal blende, 4 

Uranium 

discovery of radio-activity of, 5 
persistence of radiations of, 6 
discharging power of rays, 7 
absence of reflection, refraction and 

polarization, 7 
examination of uranium minerals, 11 

et seq. 
relative activity of compounds of 

uranium, 12 

nature of radiation from, 90 et seq. 
/3 rays from, 95 et seq. 
a rays from, 115 et seq. 
y rays from, 142 et seq. 
emission of energy by, 154 
separation of Ur X from, 179 
recovery of activity of, 182 



Uranium (cont.) 

non-separable activity of, 294, 304 

radiations from Ur X, 293 

method of measurement of activity 

of Ur X, 294 
changes in, 294 
conservation of radio-activity of, 308 

et seq. 

resume of results, 311 et seq. 
theories of radio-activity, 316 et seq. 
discussion of theories of radio- 
activity, 320 et seq. 
source of energy of radiation, 320 et 

seq. 
theory of radio-active change, 322 et 

seq. 

table of active products, 326 
rate of change of, 332 et seq. 
life of, 333 

radium possible product of, 334 
total emission of energy from 1 gram 

of, 337 
possible causes of disintegration of, 

338 et seq. 
Uranium X 

separation of, by Crookes, 178 
separation of, by Becquerel, 179 
decay of activity of, 182 
recovery of activity of, 182 
theory to explain production of, 183 
material nature of, 185 
explanation of decay of activity of, 

188 et seq. 

radiations from, 293 
method of measurement of radiations 

from, 294 
changes in, 294 

Velocity 

of ions in electric field, 39 et seq. 
difference between, of positive and 

negative ions, 42 et seq. 
of [3 particle or electron, 107 et seq., 

110 et seq. 
variation of mass of electron with, 

108 et seq. 

of a particle, 122 et seq. 
of transmission of carriers of excited 

activity, 284 
Villard 

discovery of 7 rays from radium, 141 
alteration of X ray screen with time, 

168 

activity produced by cathode rays, 372 
Volume 

of radium emanation, calculation of, 

246 

of emanation, determination of, 313 
decrease of, of radium emanation, 

313 



INDEX 



399 



Walker, G. W. 

theory of electrometer, 75 
Walkhoff 

action of radium rays on skin, 176 
Wallstabe 

diffusion of radium emanation into 

liquids, 235 
Water 

emanation from, 360 

decay of activity of emanation from, 

361 
Water-falls 

amount of excited activity produced 

at Niagara, 366 

electrification produced near, 366 
Watts, Marshall 

atomic weight of radium, 17 
Weight 

loss of by radio-elements, 335 
attempts to measure loss of, in 

radium, 336 
Wheeler and Bumstead 

emanation from surface water and 

the soil, 361, 368 
identity of emanation from soil with 

radium emanation, 361 
absence of emanation in mercury, 

381 
Whetham 

effect of valency of ion on colloidal 

solutions, 175 
possible cause of disintegration of 

atom, 342 
Wiedemann and Schmidt 

thermo-luminescence, 170 
Wiedemann 

thermo-luminescence produced by 

radium rays, 170 
Wien 

amount of charge carried by radium 

rays, 105 
positive charge of canal rays, 125 



Willcock, Miss, and Hardy 

coloration of iodoform solution by 

radium rays, 175 
Willemite 

phosphorescence of under radium 

rays, 168 

use to show condensation of emana- 
tion, 237 
Wilson, W. E. 

radium in sun, 342 
Wilson, H. A. 

charge on ion, 48 
Wilson, C. T. E. 

ions as nuclei of condensation, 44 

et seq. 

difference between positive and nega- 
tive ions as condensation nuclei, 46 
equality of charges carried by positive 

and negative ions, 47 
construction of electroscope, 73 
natural ionization of air in vessels, 351 
radio-activity of rain and snow,355,356 
loss of charge in closed vessels, 373 
presence of ions in free air shown by 

condensation, 375 

number of ions produced per c.c., 375 

effect of pressure and nature of gas 

on. ionization in sealed vessels, 376 

Zeleny 

velocity of ions, 39 et seq. 
difference of velocity of ions, 40, 42 
potential gradient between electrodes, 

63 
Zerban and Hofmann 

active substances from pitchblende, 25 
Zinc Sulphide 

scintillations produced in by a rays, 

127 

cause of luminosity of, 127 et seq. 
scintillations due to cleavage of 
crystals, 128 




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