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A BRIEF ACCOUNT OF
RADIO-ACTIVITY
BY
FRANCIS P. VENABLE, PH.D., D.SC., LL.D.
PROFESSOR OF CHEMISTRY, UNIVERSITY OF NORTH CAROLINA
AUTHOR OF
"A SHORT HISTORY OF CHEMISTRY,"
"PERIODIC LAW," ETC.
D. C. HEATH & CO., PUBLISHERS
BOSTON NEW YORK CHICAGO
COPYRIGHT, 1917,
BY D. C. HEATH & CO.
IA7
PREFACE
I have gathered the material for this little book because I have found
it a necessary filling out of the course for my class in general
chemistry. Such a course dealing with the composition and structure of
matter is left unfinished and in the air, as it were, unless the
marvellous facts and deductions from the study of radio-activity are
presented and discussed. The usual page or two given in the present
text-books are too condensed in their treatment to afford any
intelligent grasp of the subject, so I have put in book form the
lectures which I have hitherto felt forced to give.
Perhaps the book may prove useful also to busy men in other branches
of science who wish to know something of radio-activity and have scant
leisure in which to read the larger treatises.
It is needless to say that there is nothing original in the book
unless it be in part the grouping of facts and order of their
treatment. I have made free use of the writings of Rutherford, Soddy,
and J. J. Thomson, and would here express my debt to them--just a part
of that indebtedness which we all feel to these masters. I wish also
to acknowledge my obligations to Professor Bertram B. Boltwood for his
helpful suggestions in connection with this work.
CONTENTS
CHAPTER I
DISCOVERY OF RADIO-ACTIVITY PAGE
The beginning--Radio-active bodies--An atomic
property--Discovery of new radio-active bodies--Discovery
of Polonium--Discovery of Radium--Other radio-active
bodies found 1
CHAPTER II
PROPERTIES OF THE RADIATIONS
Ionization of Gases--Experimental confirmation--Application
of electric field--Size and nature of ions--Photographing
the track of the ray--Action of radiations on photographic
plates--Discharge of electrified bodies--Scintillations
on phosphorescent bodies--Penetrating power--Magnetic
deflection--Three types of rays--Alpha rays--Beta rays--Gamma
rays--Measurement of radiations--Identifications of the rays 7
CHAPTER III
CHANGES IN RADIO-ACTIVE BODIES
Radio-activity a permanent property--Induced
activity--Discovery of Uranium X--Conclusions drawn--Search
for new radio-active bodies--Methods of investigation--Nature
of the radiations--Life-periods--Equilibrium series 17
CHAPTER IV
NATURE OF THE ALPHA PARTICLE
Disintegrating of the elements--Identification of the
rays--The alpha rays--Alpha rays consist of solid
particles--Electrical charge--Helium formed from alpha
particles--Discovery of Helium--Characteristics of
Helium--Table of constants 25
CHAPTER V
THE STRUCTURE OF THE ATOM
Properties of Radium--Energy evolved by radium--Necessity for
a disintegration theory--Disintegration theory--Constitution
of the atom--Rutherford's atom--Scattering of alpha
particles--Stopping power of substances 32
CHAPTER VI
RADIO-ACTIVITY AND CHEMICAL THEORY
Influence upon chemical theory--The periodic system--Basis
of the periodic system--Influence of positive
nucleus--Determination of the atomic number--Use of X-ray
spectra--Changes caused by ray-emission--Atomic weight
losses--Lead the end product--Changes of position
in the periodic system--Changes from loss of beta
particles--Isotopes--Radio-activity in nature--Radio-active
products in the earth's crust--Presence in air and soil
waters--Cosmical radio-activity 41
INDEX 53
A BRIEF ACCOUNT OF RADIO-ACTIVITY
CHAPTER I
DISCOVERY OF RADIO-ACTIVITY
The object of this brief treatise is to give a simple account of the
development of our knowledge of radio-activity and its bearing on
chemical and physical science. Mathematical processes will be omitted,
as it is sufficient to give the assured results from calculations
which are likely to be beyond the training of the reader. Experimental
evidence will be given in detail wherever it is fundamental and
necessary to a confident grasp of some of the marvelous deductions in
this new branch of science. Theories cannot be avoided, but the facts
remain while theories grow old and are discarded for others more in
accord with the facts.
The Beginning
As so often happens in the history of science, the opening up of this
new field with its fascinating disclosures was due to an investigation
undertaken for another purpose but painstakingly carried out with a
mind open to the truth wherever it might lead.
In 1895, Roentgen modestly announced his discovery of the _X_ rays.
This attracted immediate and intense interest. Among those who
undertook to follow up these phenomena was Becquerel, who, because of
the apparent connection with phosphorescence, tried the action of a
number of phosphorescent substances upon the photographic plate, the
most striking characteristic of the _X_ rays being their effect upon
such sensitive plates. In these experiments he obtained no results
until he tried salts of uranium, recalling previous observations of
his as to their phosphorescence. Distinct action was noted.
Furthermore, he proved that this had no connection with the phenomenon
of phosphorescence, as both uranic and uranous salts were active and
the latter show no phosphorescence. Becquerel announced his
discoveries in 1896 and this was the beginning of the new science of
radio-activity.
Radio-active bodies
The rays given off by uranium and its salts were found to differ from
the _X_ rays. They showed no appreciable variation in intensity, no
previous exposure of the substance to light was necessary, and neither
changes of temperature nor any other physical or chemical agency
affected them.
At first uranium and its compounds were the only known source of these
new radiations, but many other substances were examined and two years
later thorium and its compounds were added to the list. In general the
discharging action seemed about the same. Other elements and ordinary
substances show a minute activity. Only potassium and rubidium have a
greater activity than this, and theirs is only about one-thousandth
that of uranium.
An Atomic Property
In the examination of uranium and thorium compounds it was found that
the activity was determined by the uranium and thorium present; it was
proportto the amount ofional these elements present and independent of
the nature of the other elements composing the compound. The
conclusion was, therefore, that the activity was an inherent property
of the atoms of uranium and thorium, that is, an atomic property. This
was a long step forward and introduced into science the conception of
a new property of matter, or at least of certain forms of matter.
Discovery of New Radio-active Bodies
In examining a large number of minerals containing uranium and
thorium, Mme. Curie made the important observation that many of these
were more active than the elements themselves. In measuring the
activity she made use of the electrical method which will be described
later. In the following table giving her results for uranium minerals
the numbers under _i_ give the maximum current in amperes. They serve
simply for comparison.
_i_
Pitchblende from Joachimsthal 7.0 x 10^{-11}
Clevite 1.4 x 10^{-11}
Chalcolite 5.2 x 10^{-11}
Autunite 2.7 x 10^{-11}
Carnotite 6.2 x 10^{-11}
Uranium 2.3 x 10^{-11}
Uranium and potassium sulphate 0.7 x 10^{-11}
Uranium and copper phosphate 0.9 x 10^{-11}
The last three are pure uranium and compounds of that element given
for comparison with the first five, which are naturally occurring
minerals. The last compound has the same composition as chalcolite and
is simply the artificially prepared mineral. It has the activity which
would be calculated from the proportion of uranium present, the copper
and phosphoric acid contributing no activity.
Since the activity is not dependent upon the composition but upon the
amount of uranium present, the activity in all of the minerals should
be less than that of uranium. On the contrary, it is several times
greater. Natural and artificial chalcolite also show a marked
difference in favor of the former. The supposition was a natural one,
therefore, that these minerals contained small quantities of an
element, or elements, undetected by ordinary analysis and having a
much greater activity than uranium. Similar results were obtained in
the examination of thorium minerals and thorium salts.
Discovery of Polonium
Following up this supposition, M. and Mme. Curie set themselves the
task of separating this unknown substance. Starting with pitchblende,
a systematic chemical examination was made. This is an exceedingly
complex mineral, containing many elements. The processes were
laborious and demanded much time and minute care. They need not be
described here. It is sufficient to say that along with bismuth a very
active substance was separated, to which Mme. Curie gave the name of
polonium for Poland, her native land. Its complete isolation is very
difficult and sufficient quantities of the pure substance have not
been obtained to determine its atomic weight and other properties, but
some of the lines of its spectrum have been determined. Chemically it
is very closely analogous to bismuth.
Discovery of Radium
In a similar manner a barium precipitate was obtained from pitchblende
which contained a highly active substance. The pure chloride of this
body and barium can be prepared together and then separated by
fractional crystallization. To the new body thus found the name of
radium was given. It is similar in chemical properties to barium. Its
atomic weight has been determined by several careful investigators and
is accepted as 226. Its spectrum has been mapped and its general
properties are known. It is a silvery white, oxidizable metal. In one
ton of pitchblende about 0.2 gram of radium is present; this is about
5000 times greater than the amount of polonium present. The activity
of the products was depended upon as the guide in these separations.
The radium found is relatively enormously more active than the
pitchblende or uranium.
Other Radio-active Bodies Found
In the above separations use was made of relationships to bismuth and
barium. Similarly, by taking advantage of chemical relationship to the
iron group of elements, another body was partially separated by
Debierne, to which he gave the name actinium. Boltwood discovered in
uranium minerals the presence of a body which he named ionium, and
which is so similar to thorium that it cannot be separated from it.
It, however, far exceeds thorium in activity.
The lead which is present in uranium and thorium minerals--apparently
in fairly definite ratio to the amount of uranium and thorium--is
found, on separation and purification, to possess radio-active
properties. This activity is due to the presence of a very small
proportion of an active constituent called radio-lead, which has
chemical properties identical with those of ordinary lead. The bulk of
the lead obtained from radio-active minerals differs in atomic weight
from ordinary lead and appears also to be different according to
whether its source is a thorium or a uranium mineral.
A large number of other radio-active substances have been separated
and some of their properties determined, but these were found by
different means and will be noted in their proper place. They number
in all more than thirty. The sources or parents of these are the
original uranium or thorium, and the products form regular series with
distinctive properties for each member.