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+The Project Gutenberg EBook of A Brief Account of Radio-activity, by
+Francis Preston Venable
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever. You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Brief Account of Radio-activity
+
+Author: Francis Preston Venable
+
+Release Date: May 9, 2010 [EBook #32307]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK A BRIEF ACCOUNT OF RADIO-ACTIVITY ***
+
+
+
+
+Produced by The Online Distributed Proofreading Team at
+http://www.pgdp.net (This file was produced from images
+generously made available by The Internet Archive/American
+Libraries.)
+
+
+
+
+
+
+
+
+
+ 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, Röntgen 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 × 10^{-11}
+ Clevite 1.4 × 10^{-11}
+ Chalcolite 5.2 × 10^{-11}
+ Autunite 2.7 × 10^{-11}
+ Carnotite 6.2 × 10^{-11}
+ Uranium 2.3 × 10^{-11}
+ Uranium and potassium sulphate 0.7 × 10^{-11}
+ Uranium and copper phosphate 0.9 × 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.
+
+
+
+
+CHAPTER II
+
+PROPERTIES OF THE RADIATIONS
+
+
+The activity of these radio-active bodies consists in the emission of
+certain radiations which may be separated into rays and studied
+through the phenomena which they cause.
+
+
+Ionization of Gases
+
+One of these phenomena is the power of forming ions or carriers of
+electricity by the passage of the rays through a gas, thus ionizing
+the gas. The details of an experiment will serve to make the meaning
+of this ionization clear.
+
+ [Illustration: FIG. 1.--IONIZATION OF GASES.]
+
+When this apparatus is set up a minute current will be observed
+without the introduction of any radio-active matter. This, as
+Rutherford says, has been found due mainly to a slight natural
+radio-activity of the matter composing the plates. If radio-active
+matter is spread on plate _A_, which is connected with one pole of a
+grounded battery, and if plate _B_ is connected with an electrometer
+which is also connected with the earth, a current is caused which
+increases rapidly with the difference of potential between the plates,
+then more slowly until a value is reached that changes only slightly
+with a larger increase in the voltage.
+
+According to the theory of ionization, the radiation produces ions at
+a constant rate. The ions carrying a positive charge are attracted to
+plate _B_, while those negatively charged are attracted to plate _A_,
+thus causing a current. These ions will recombine and neutralize their
+charges if the opportunity is given. The number, therefore, increases
+to a point at which the ions produced balance the number recombining.
+
+When an electric field is produced between the plates, the velocity of
+the ions between the plates is increased in proportion to the strength
+of the electric field. In a weak field the ions travel so slowly that
+most of them recombine on the way and consequently the observed
+current is very small. On increasing the voltage the speed of the ions
+is increased, fewer recombine, the current increases, and, when the
+condition for recombination is practically removed, it will have a
+maximum value. This maximum current is called the saturation current
+and the value of the potential difference required to give this
+maximum current is called the saturation P.D. or saturation voltage.
+
+The picture, then, is this. The radiations separate the components of
+the gas into ions, or carriers of electricity, half of which are
+charged negatively and half positively. In the electric field those
+negatively charged seek the positive plate and those positively
+charged seek the negative plate. If time is given, these ions meet and
+recombine, their charges are neutralized, and there is no current.
+
+
+Experimental Confirmation
+
+This theory of the ionization of gases has been most interestingly
+confirmed by direct experiment. For instance, the ions may form nuclei
+for the condensation of water, and in this way the existence of the
+separate ions in the gas may be shown and the number present actually
+counted.
+
+When air saturated with water vapor is allowed to expand suddenly, the
+water present forms a mist of small globules. There are always small
+dust particles in air and around these as nuclei the drops are formed.
+These drops will settle and thus by repeated small expansions all dust
+nuclei may be removed and no mist or cloud will be formed by further
+expansions.
+
+If now the radiation from a radio-active body be introduced into the
+condensation vessel, a new cloud is produced in which the water drops
+are finer and more numerous according to the intensity of the rays. On
+passing a strong beam of light through the condensation chamber, the
+drops can readily be seen. These drops form on the ions produced by
+the radiation.
+
+
+Application of Electric Field
+
+If the condensation chamber has two parallel plates for the
+application of an electric field like that already described, the
+ions will be carried at once to the electrodes and disappear. The
+rapidity of this action depends upon the strength of the electric
+field and experiment shows that the stronger the field the smaller the
+number of condensation drops formed. If there is no electric field, a
+cloud can be produced some time after the shutting off of the source
+of radiation, showing that time is required for the recombination of
+the ions.
+
+
+Size and Nature of Ions
+
+If the drops are counted (there being special methods for this) and
+the total current carried accurately measured, then the charge carried
+by each ion may be calculated. This has been determined. The mass of
+an ion compared with the mass of the molecules of gas in which it was
+produced can also be approximately estimated. In the study of these
+ions the view has been held that the charged ion attracted to itself a
+cluster of molecules which surrounded the charged nucleus and traveled
+with it. It is roughly estimated that about thirty molecules of the
+gas cluster around each charged ion.
+
+
+Photographing the Track of the Ray
+
+Utilizing the fact that these ions with their clusters of molecules
+form nuclei for the condensation of water vapor, C. T. R. Wilson has
+by instantaneous photography been able to photograph the track of an
+ionizing ray through air. The number of the ions produced, and hence
+the number of drops, is so great that the trail is shown as a
+continuous line. In the copy of this photograph it will be seen that
+at some distance from its source the straight trail is slightly but
+abruptly bent. Near the end of its course there is another abrupt and
+much sharper bend. These bends show where the ionizing ray, in this
+case an alpha particle, has been deflected by more or less direct
+collision with an atom. These collisions and the final disappearance
+of the ray will be discussed later.
+
+ [Illustration: FIG. 2.--PHOTOGRAPH OF THE TRACK OF AN
+ IONIZING RAY.]
+
+
+Action of Radiations on Photographic Plates
+
+Taking up now other means of examining these radiations, it is well to
+consider their action upon a photographic or sensitive plate. It will
+be recalled that this was the method by which their existence was
+originally detected. To illustrate the method, the following account
+of how one such photograph was taken may be given.
+
+The plate was wrapped in two thicknesses of black paper. The objects
+were placed upon this and the radio-active ore, separated by a board
+one inch thick, was placed above. The exposure lasted five days. The
+action is much less rapid and the result not so clearly defined as in
+the case of photographs taken by _X_ rays. Of course, the removal of
+the board and the use of more concentrated preparations of radium
+would give quicker and better results. The method, however, on
+account of time consumed and lack of definition is ill adapted to
+accurate work.
+
+ [Illustration: FIG. 3--PHOTOGRAPH OF VARIOUS OBJECTS TAKEN
+ BY MEANS OF PITCHBLENDE]
+
+
+Discharge of Electrified Bodies
+
+The radiations from radio-active bodies can discharge both positively
+and negatively electrified bodies by making the air surrounding them a
+conductor of electricity. To demonstrate this, use is made of an
+electroscope. If the hinged leaf of such an instrument be electrically
+charged and a radio-active body be brought into its neighborhood, the
+electricity will be discharged and the leaf return to its original
+position. The rapidity of this discharge is used to measure the degree
+of activity of the body giving off the radiation.
+
+ [Illustration: FIG. 4.--GOLD-LEAF ELECTROSCOPE.
+
+ The gold-leaf _L_ is attached to a flat rod _R_ and is
+ insulated inside the vessel by a piece of amber _S_ supported
+ from the rod _P_. The system is charged by a bent rod _CC'_
+ passing through an ebonite stopper. After charging, it is
+ removed from contact with the gold-leaf system. The rods _P_
+ and _C_ and the cylinder are then connected with the earth.]
+
+
+Scintillations on Phosphorescent Bodies
+
+It was found by Crookes that a screen covered with phosphorescent zinc
+sulphide was brightly lighted up when exposed to the radiations. This
+is due to the bombardment of the zinc sulphide by a type of ray called
+the alpha ray. Under a magnifying glass this light is seen to be made
+up of a number of scintillating points of light and is not continuous,
+each scintillation being of very short duration. By proper subdivision
+of the field under the lens, the number of scintillations can be
+counted with close accuracy.
+
+A simple form of apparatus called the spinthariscope has been devised
+to show these scintillations. A zinc sulphide screen is fixed in one
+end of a small tube and a plate carrying a trace of radium is placed
+very close to it. The scintillations can be observed through an
+adjustable lens at the other end of the tube. Outer light should be
+cut off, as in a dark room. The screen then appears to be covered with
+brilliant flashes of light. Other phosphorescent substances, such as
+barium platino-cyanide, may be substituted for the zinc sulphide, but
+they do not answer so well.
+
+
+Penetrating Power
+
+By penetrating power is meant the power exhibited by the rays of
+passing through solids of different thicknesses and gases of various
+depths. This power varies with different radiations and with the
+nature of the solid or gas. For instance, a sheet of metallic foil may
+be used and the effect of aluminum will differ from that of gold and
+the different rays vary in penetrating power. In the case of gases
+air will differ from hydrogen, and it is noticed that certain rays
+disappear after penetrating a short distance, while others can
+penetrate further before being lost.
+
+
+Magnetic Deflection
+
+If the radiations are subjected to the action of a strong magnetic
+field, it is found that part of them are much deflected in the
+magnetic field and describe circular orbits, part are only slightly
+deflected and in the opposite direction from the first, and the
+remaining rays are entirely unaffected.
+
+ [Illustration: FIG. 5.--SHOWING MAGNETIC DEFLECTION OF
+ [alpha], [beta], AND [gamma] RAYS.]
+
+
+Three Types of Rays
+
+By the use of these methods of investigation it is learned that the
+radiations consist of three types of rays. These have been named the
+alpha, beta, and gamma rays, respectively. Some radio-active bodies
+emit all three types, some two, and some only one. The distinguishing
+characteristic of these types of rays may be summed up as follows:
+
+
+Alpha Rays
+
+The alpha rays have a positive electrical charge and a comparatively
+low penetrating power. They are slightly deflected in strong magnetic
+and electric fields. They have a great ionizing power and a velocity
+about one-fifteenth that of light.
+
+
+Beta Rays
+
+The beta rays are negatively charged and have a greater penetrating
+power than the alpha rays. They show a strong deflection in magnetic
+and electric fields, have less ionizing power than the alpha rays, and
+a velocity of the same order as light.
+
+
+Gamma Rays
+
+The gamma rays are very penetrating and are not deflected in the
+magnetic or electric fields. They have the least ionizing power and a
+very great velocity.
+
+The penetrating power of each type is complex and varies with the
+source, so the statements given are but generalizations. The alpha
+rays are projected particles which lose energy in penetrating matter.
+As to the power of ionizing gases, if that for the [alpha] rays is
+taken as 10,000, then the [beta] rays would be approximately 100 and
+the [gamma] rays 1.
+
+
+Measurement of Radiations
+
+The rays are examined and measured in several ways:
+
+1. By their action on the sensitive photographic plates. The use
+of this method is laborious, consumes time, and for comparative
+measurements of intensity is uncertain as to effect.
+
+2. By electrical methods, using electroscopes, quadrant
+electrometers, etc. These are the methods most used.
+
+3. By exposure to magnetic and electric fields, noting extent and
+direction of deflection.
+
+4. By their relative absorption by solids and gases.
+
+5. By the scintillations on a zinc sulphide screen.
+
+
+Identification of the Rays
+
+The alpha rays have been identified as similar to the so-called canal
+rays. These were first observed in the study of the _X_ rays. When an
+electrical discharge is passed through a vacuum tube with a cathode
+having holes in it, luminous streams pass through the holes toward the
+side away from the anode and the general direction of the stream. They
+travel in straight lines and render certain substances phosphorescent.
+These rays are slightly deflected by a magnetic field and in an
+opposite direction from that taken by the cathode rays in their
+deflection. The rays seem to be positive ions with masses never less
+than that of the hydrogen atom. Their source is uncertain, but they
+may be derived from the electrodes.
+
+The beta rays are identical in type with the cathode rays and are
+negative electrons.
+
+The gamma rays are analogous to the _X_ rays and are of the order of
+light. They are in general considerably more penetrating than _X_
+rays. For example, the gamma rays sent out by 30 milligrams of radium
+can be detected by an electroscope after passing through 30
+centimeters of iron, a much greater thickness than can be penetrated
+by the ordinary _X_ rays.
+
+
+
+
+CHAPTER III
+
+CHANGES IN RADIO-ACTIVE BODIES
+
+
+Is Radio-activity a Permanent Property?
+
+Is this power of emitting radiations a permanent property or is it
+lost with the passage of time? The first investigations of the
+activity of uranium and thorium showed no loss of intensity at the end
+of several years, and radium also seemed to show no decrease in its
+enormous activity. Polonium, however, was found to lose most of its
+activity in a year, and later it appeared that some radio-active
+substances lost most of their activity in the course of a few minutes
+or hours.
+
+
+Induced Activity
+
+A phenomenon called induced or secondary radio-activity was also
+observed. Thus a metal plate or wire exposed to the action of thorium
+oxide for some hours became itself active. This induced activity was
+not permanent but decreased to half its value in about eleven hours
+and practically disappeared within a week. Similar phenomena were
+observed when radium was substituted for thorium.
+
+
+Discovery of Uranium X
+
+In 1900 Crookes precipitated a solution of an active uranium salt with
+ammonium carbonate. The precipitate was dissolved so far as possible
+in an excess of the reagent, leaving an insoluble residue. This
+residue was many hundred times more active, weight for weight, than
+the original salt, and the solution containing the salt was
+practically inactive. At the end of a year the uranium salt had
+regained its activity while the residue had become inactive.
+
+Another method of obtaining the same result is to dissolve
+crystallized uranium nitrate in ether. Two layers of solution are
+formed, one ether and the other water coming from the water of
+crystallization. The aqueous layer is active, while the water layer is
+inactive. Similarly, by adding barium chloride solution to a solution
+of a salt of uranium and then precipitating the barium as sulphate,
+the activity is transferred to this precipitate. These experiments
+give proof of the formation and separation of a radio-active body by
+ordinary chemical operations.
+
+So, too, in the case of thorium salts a substance can be obtained by
+means of ammonium hydroxide which is several thousand times more
+active than an equal weight of the original salt. After standing a
+month, the separated material has lost its activity and the thorium
+salt has regained it. Here, again, there is the formation, separation,
+and loss of a radio-active body.
+
+
+Conclusions Drawn
+
+Now, these are ordinary chemical processes for the separation of
+distinct chemical individuals. The results, therefore, lead naturally
+to the conclusions: (1) it would seem that uranium and thorium are
+themselves inactive and the activity is due to some other substance
+formed by these elements; (2) this active substance is produced by
+some transformation in those elements, for on standing the activity
+is regained. This latter conclusion is startling, for it indicates a
+change in the atom which, up to the time of this discovery, was deemed
+unchangeable under the influence of such physical and chemical changes
+as were known to us.
+
+
+Search for New Radio-active Bodies
+
+The search for new radio-active bodies and the study of their
+characteristics has been systematically and successfully carried on.
+The bodies obtained in the above experiments were named uranium _X_
+and thorium _X_, respectively. Further, it became clear from the
+investigation of uranium minerals that radium, polonium, actinium, and
+ionium originated from uranium. From thorium minerals a body was
+separated called mesothorium, which was analogous to radium. Both
+thorium and radium were found to give off a radio-active gas. The
+first lost half of its activity in less than one minute. The second
+was more stable and lost half of its activity in about four days. The
+name radium emanation was given to the latter and it was found
+chemically and physically to belong to the class of monatomic or noble
+gases, such as helium, argon, neon, etc., which had been discovered by
+Ramsay. In some cases the chemical action was determined and these new
+bodies were found analogous to well-known elements, as radium to
+barium, polonium to bismuth. The physical properties were investigated
+and, where possible, spectra were mapped and atomic weights
+determined.
+
+It is clear, therefore, that these bodies are elemental in character
+and as such are made up of distinct, similar atoms, just as the
+commonly recognized elements are believed to be. In this way more than
+thirty new elements have been added to the list. These new elements
+are called radio-active elements, but it is an open question whether
+all atoms do not possess this property in greater or less degree.
+Certainly, it is possessed in varying degree by four of the old
+elements widely separated in the Periodic System, namely, uranium,
+thorium, rubidium, and potassium. The last two, while feebly active
+themselves, do not form any secondary radio-active substance so far as
+is known. Only two of the elements, then, can definitely be said to go
+through these transformations. It is just possible that radio-activity
+may be found to be a common property of all atoms and of all matter.
+
+
+Methods of Investigation
+
+It is important to know how these new bodies were discovered and
+distinguished from one another. Two properties are relied upon. One is
+the nature of the rays emitted and the other is the duration of the
+activity. Of course, knowledge of the physical and chemical properties
+is also of great importance whenever obtainable.
+
+
+Nature of the Radiations
+
+The nature of the radiation is a distinguishing characteristic, though
+similarity here does not prove identity of substances. Some emit
+[alpha] rays only, some emit [beta] rays, some emit two of the
+possible rays, as for instance, [beta] and [gamma], and some emit all
+three. The rays may also differ in the velocity with which they are
+emitted by different radio-active substances. Thus, in the case of one
+substance the [alpha] rays may have a slightly greater or less
+penetrating power than those emitted by some other substance, and this
+may be true also of the other rays.
+
+
+Life Periods
+
+The duration of the activity is called the life period. This is
+absolutely fixed for each body and furnishes the most important mode
+of differentiating among them. It measures the relative stability and
+is the time which must elapse before their activity is lost and they,
+changing into something else, entirely disappear. The measure usually
+adopted is the half-value period. Two hypotheses are made use of:
+
+1. That there is a constant production of fresh radio-active matter by
+the radio-active body.
+
+2. That the activity of the matter so formed decreases according to an
+exponential law with the time from the moment of its formation.
+
+These hypotheses agree with the experimental results. The decrease and
+rise of activity, for example, of uranium and uranium _X_, and also of
+thorium and thorium _X_, have been measured, plotted, and the
+equations worked out.
+
+Manifestly, a state of equilibrium will be reached when the rate of
+loss of activity of the matter already produced is balanced by the
+activity of the new matter produced. This equilibrium and the
+knowledge of the rate of decrease in general will have little value if
+this rate, like chemical changes, is subject to the influence of
+chemical and physical conditions. The rate of decrease has been found
+to be unaltered by any known chemical or physical agency. For
+instance, neither the highest temperatures applicable nor the cold of
+liquid air have any appreciable effect.
+
+
+Equilibrium Series
+
+In order to measure the disintegration of a radio-active body in units
+of time so that the rate may be comparable with that of other
+radio-active bodies, the relation between the amounts under
+consideration must be a definite one. For this purpose equal weights
+of the bodies are not taken, but use is made of the amounts which are
+in equilibrium with a fixed amount of the parent substance.
+
+One gram of radium has been settled upon as the standard for that
+series and a unit known as the "curie" has been adopted to express the
+equilibrium quantity of radium emanation. Thus, a curie of radium
+emanation (or niton) is the weight (or, as this is a gas, the volume
+at standard pressure and temperature) of the emanation in equilibrium
+with one gram of radium. This, by calculation and experiment, is found
+to be 0.63 cubic millimeter. When this amount has been produced by one
+gram of radium, the formation and decay will exactly balance one
+another. This is, therefore, one curie of emanation.
+
+The measurement of the rate of decay is difficult but can be carried
+out with great accuracy, even down to seconds, in the case of certain
+short-lived bodies. Errors crept in at first from the failure to
+completely separate the substances produced in the series, and
+sometimes because of the simultaneous production of two substances.
+
+As stated, the decay follows an exponential law. The time required for
+the decay of activity to half-value does not mean, therefore, that
+there will be total decay in twice that time. Thus the half-value
+period for uranium _X_ is about 22 days. The period for complete decay
+is about 160 days. This half-value period corresponds to the
+half-value recovery period of uranium, which is also 22 days.
+
+These were the earlier figures obtained for uranium _X_ and they
+illustrate some of the difficulties surrounding such determinations.
+It was found later that the body examined as uranium _X_ was really a
+constant mixture and of course the decay and recovery periods were
+also composite. It required later and very skilful work to separate
+them into the bodies indicated in the disintegration series.
+
+The half-value period for thorium _X_ is much shorter, namely, a
+little over four days, and this is also the recovery period for
+thorium _X_. The plotted decay and recovery curves will intersect at
+this point.
+
+The consecutive disintegration series, with the half-value periods,
+for the uranium and thorium series as given by Soddy are seen in the
+following tables. They are probably subject to some changes on further
+and more accurate determination. The nature of the rays emitted is
+also given.
+
+ [Illustration:
+
+ Uranium (8 × 10^9 years) 238.5 -> [alpha]
+ -> [alpha]
+ \/
+ Uranium X (35.5 days) (230.5) -> [beta]&[gamma]
+ -> ([beta])
+ \/
+
+ \/
+
+ \/
+
+ Ionium (5 × 10^4 to 10^6 years) (230.5) -> [alpha]
+ \/
+ Radium (2,500 years) 226.4 -> [alpha]
+ \/
+ Emanation (5.57 days) (222.4) -> [alpha]
+ \/
+ Radium A (4.3 minutes) (218.4) -> [alpha]
+ \/
+ Radium B (38.5 minutes) (214.4) -> ([beta])
+ \/
+ Radium C_{1} (28.1 minutes) { (214.4) -> [alpha]
+ { -> [beta]&[gamma]
+ { \/
+ Radium C_{2} (1.9 minutes) { (210.4) -> [beta]&[gamma]
+ \/
+ Radium D (24 years?) (210.4) -> ([beta])
+ \/
+ Radium E (7.25 days) (210.4) -> [beta]&[gamma]
+ \/
+ Radium F (Polonium 202 days) (210.4) -> [alpha]
+ \/
+ Radium G (probably lead) (206.8)
+
+
+ Actinium (?)
+ \/
+ Radio-Actinium (28.1 days) -> [alpha]
+ -> ([beta])
+ \/
+ Actinium X (15 days) -> [alpha]
+ \/
+ Emanation (5.6 seconds) -> [alpha]
+ \/
+ Actinium A (0.0029 second) -> [alpha]
+ \/
+ Actinium B (52.1 minutes) -> ([beta])
+ \/
+ Actinium C_{1} (3.10 mins.) { -> [alpha]
+ { \/
+ Actinium C_{2} (?) { -> [alpha]
+ \/
+ Actinium D (7.4 minutes) -> [beta]&[gamma]
+ \/
+ Actinium E (unknown)
+
+
+ Thorium (4 × 10^{10} years?) 232.4 -> [alpha](?)
+ \/
+ Mesothorium_{1} (7.9 years)
+ \/
+ Mesothorium_{2} (8.9 hours) -> [beta]&[gamma]
+ \/
+ Radiothorium (2.91 years?) -> [alpha]
+ \/
+ Thorium X (5.35 days) -> [alpha]
+ \/
+ Emanation (76 seconds) -> [alpha]
+ \/
+ Thorium A (0.203 second) -> [alpha]
+ \/
+ Thorium B (15.3 hours) -> ([beta])
+ \/
+ Thorium C_{1} (79 minutes) { -> [alpha]
+ { \/
+ Thorium C_{2} (?) { -> [alpha]
+ \/
+ Thorium D (4.5 minutes) -> [beta]&[gamma]
+ \/
+ Thorium E (unknown)
+
+ FIG. 6.--DISINTEGRATION SERIES FOR URANIUM, ACTINIUM, AND
+ THORIUM, AS GIVEN BY SODDY.]
+
+
+
+
+CHAPTER IV
+
+NATURE OF THE ALPHA PARTICLE
+
+
+Disintegration of the Elements
+
+The remarkable disintegrations related in the last chapter, in which
+the heaviest known elementary atom--that of uranium (at. wt. 238)--is
+by successive stages changed into others of lower atomic weight,
+afford a clue to the nature of the atom and to that goal of the
+chemist, the final constitution of matter. The composite nature of the
+atom and some sort of interrelation of the elements had previously
+been made apparent from a study of the Periodic System and data
+gathered still earlier, but all attempts at working out a so-called
+genesis of the elements had proved vague and unsatisfactory.
+
+
+Identification of the Rays
+
+To get an understanding of the disintegration occurring in
+radio-active substances, the nature of the rays produced must be
+known. These rays are the cause of the activity and their emission
+accompanies the changes or disintegration. They have for the sake of
+convenience been called the alpha, beta, and gamma rays. The gamma
+rays have been identified with the _X_ rays discovered by Röntgen and
+are a form of energy analogous to light. The beta rays are particles
+of negative electricity or electrons. With these, then, we have some
+degree of familiarity. But what are the alpha rays? An answer to this
+question should make clearer the character of the changes taking
+place, and should give some insight into the composition and mechanism
+of the atom.
+
+
+The Alpha Rays
+
+It has already been stated that these alpha rays are similar or
+analogous to the canal rays, but this advances the matter very little,
+as the nature of these canal rays has not been fully determined. The
+full identity with them, if proved, should have an important
+theoretical bearing.
+
+
+Alpha Rays Consist of Solid Particles
+
+In the first place, these alpha rays have been found to be made up of
+solid particles, that is, of what we are accustomed to call matter.
+Since it has become more and more difficult to draw a clear
+distinction between matter and energy, it would perhaps be better to
+say that these particles appear to have some of the properties
+hitherto attributed solely to matter. The best evidence that these
+particles are of atomic mass is furnished by their deflection in
+electric and magnetic fields.
+
+
+Electrical Charge
+
+It is not of first importance to discuss this or other proofs of the
+material nature of these particles. That they carry a charge of
+positive electricity is, however, a fact of very great import. The
+value of this charge has been carefully determined by a number of
+investigators working with different sources of the alpha particles
+and has been found to be 9.3 × 10^{-10} electrostatic units
+(.000,000,000,93 e.s.). From the consideration of the charge upon an
+electron previously obtained by J. J. Thomson and others, it was
+concluded that the alpha particle carried two unit positive charges;
+the fundamental unit charge, therefore, is half this value, or
+4.65 × 10^{-10} e.s.
+
+
+Helium Formed from Alpha Particles
+
+To determine the nature of the alpha particle a crucial experiment was
+carried out by Rutherford and Royds, which was described as follows:
+
+ [Illustration: FIG. 7.--APPARATUS USED IN EXPERIMENT BY
+ RUTHERFORD AND ROYDS.]
+
+A large quantity of radium emanation was compressed into a fine glass
+tube _A_, about 1.5 cm. long. This tube, which was sealed to a larger
+capillary tube _B_, was sufficiently thin to allow the alpha particles
+from the emanation and its products to pass through, but sufficiently
+thick to withstand atmospheric pressure. The thickness of the glass
+wall was in most cases less than .01 mm. On introducing the emanation
+into the tube, the escape of the alpha particles from the emanation
+was clearly seen by the scintillations produced at some distance on a
+zinc sulphide screen. After this test the glass tube _A_ was
+surrounded by a glass tube _T_ and a small spectrum tube _V_ attached
+to it. The tube _T_ was exhausted to a charcoal vacuum. By means of
+the mercury column _H_, the gases in the tube _T_ could at any time be
+compressed into the spectrum tube _V_ and the nature of the gases
+which had been produced determined spectroscopically. It was found
+that two days after the introduction of the emanation into _A_ the
+spectrum showed the yellow line of helium, and after six days the
+whole helium spectrum was observed. In order to be certain that the
+helium, coming possibly from some other source, had not diffused
+through the thin walls of the tube _A_, the emanation was pumped out
+and helium substituted. No trace of helium could be observed in the
+vacuum tube after several days, showing that the helium observed in
+the first experiment must have originated from the alpha particles
+which had been propelled through the thin glass tube into the outer
+tube.
+
+Most of the alpha particles are propelled with such force that they
+penetrate some distance into the walls of the outer tube and some of
+these gradually diffuse out into the exhausted space. The presence of
+helium in the spectrum tube can be detected after a shorter interval
+if a thin cylinder of lead is placed over the emanation tube, since
+the particles fired into the lead diffuse out more rapidly than from
+glass.
+
+A still more definite proof of the identity of the alpha particle with
+the helium atom was obtained by removing the outer glass tube _T_ and
+placing a cylinder of lead over the emanation tube in the open air.
+Helium was always detected in the lead after it had remained several
+hours over the thin tube containing a large quantity of the emanation.
+In order to test for the presence of helium in the lead, the gases
+present were released by melting the lead in a closed vessel. There
+can thus be no doubt that the alpha particle becomes a helium atom
+when its positive charge is neutralized.
+
+Thus the chemist was afforded the experience of the building up of at
+least one element under his observation, and both the analysis and
+synthesis of matter have been revealed through the discoveries of
+radio-activity.
+
+
+Discovery of Helium
+
+It is of interest at this point to learn something of the history of
+helium and its occurrence. In 1868 there was discovered by Janssen and
+Lockyer a bright yellow line in the spectrum of the sun's
+chromosphere. Because of its origin the name helium was given to the
+supposed new element causing it. Later it was found in the spectra of
+many of the stars, and because of its predominance in some of these
+they were called helium stars. Its existence on our planet was not
+detected for nearly thirty years.
+
+In 1895, in connection with the discovery of argon in the atmosphere,
+a search was made to see if the latter element could be obtained from
+mineral sources. In analyzing certain uranium minerals Hillebrand had
+found considerable quantities of a gas which he took to be a peculiar
+form of nitrogen. Ramsay made a further examination of the gas coming
+from these minerals and the spectroscope revealed the yellow line of
+helium, thus at last proving the presence of this element on the
+earth. It is known now to be present in thorium minerals, in the
+waters of radio-active wells, and in minute amounts in the atmosphere.
+Its occurrence in every case, in the light of the experiment described
+above, would seem to be due to the presence of radio-active changes.
+
+
+Characteristics of Helium
+
+Helium, on account of its chemical inactivity and physical properties,
+is classed along with argon, neon, krypton, and xenon in the zero
+group of the Periodic System, and forms with them the monatomic, inert
+gases. In this class are now placed also the three radio-active gases,
+emanating respectively from radium, thorium, and actinium. These are
+generally known as radium emanation, thorium emanation, and actinium
+emanation. The first mentioned was once called niton. Emanium was the
+name originally proposed by Giesel for the body now known as actinium.
+
+The calculated rate of production of helium in the series in
+equilibrium with one gram of radium is 158 cubic millimeters per year.
+This corresponds quite well with the experimental results.
+
+
+Table of Constants
+
+Some of the more important atomic and radio-active constants are given
+in the following table. They are recorded here to show how helpful the
+study of radio-activity has been in working out the composition of
+matter, and to give some idea of the magnitude of the numbers and the
+minuteness of the quantities dealt with.
+
+ Electric charge carried by each H atom in
+ electrolysis 4.65 × 10^{-10} e.s.[1]
+ Electric charge carried by each [alpha]
+ particle 9.3 × 10^{-10} e.s.
+ Number of atoms in 1 gram of H 6.2 × 10^{23}
+ Mass of 1 atom of H 1.6 × 10^{-24} gram
+ Number of molecules per cc. of any gas at
+ standard pressure and temperature 2.72 × 10^{19}
+ Number of [alpha] particles expelled per
+ second per gram of radium itself 3.6 × 10^{10}
+ Number of [alpha] particles expelled per
+ second per gram of radium in
+ equilibrium with its products 14.3 × 10^{10}
+
+ [1] The expression 10^{-10} means multiplying by .000,000,000,1;
+ 10^{10} means multiplying by 10,000,000,000.
+
+
+
+
+CHAPTER V
+
+THE STRUCTURE OF THE ATOM
+
+
+Properties of Radium
+
+A study of the properties of radium will aid in throwing light upon
+the question as to the building up of the atom. First to be considered
+are the usual properties which distinguish an elementary body.
+Metallic radium has been prepared by a method similar to that used in
+the preparation of barium. It is a pure white metal, melting at 700°,
+and far more volatile than barium. It rapidly alters on exposure to
+the air, probably forming a nitride. It energetically decomposes water
+and the product dissolves in the water. Its atomic weight is 226.
+
+Radium forms a series of salts analogous in appearance and chemical
+action to those of barium. In the course of time they become colored,
+especially if mixed barium salts. The radiations from radium produce
+marked chemical effects in a number of substances. Carbon dioxide is
+changed into carbon, oxygen, and carbon monoxide, and the latter is
+changed into carbon and oxygen. Ammonia is dissociated into nitrogen
+and hydrogen; hydrochloric acid into chlorine and hydrogen. Oxygen is
+condensed into ozone. In general, the action upon gases appears to be
+similar to that of the silent electric discharge. Water is decomposed
+into hydrogen and oxygen. If moist radium chloride or a salt of radium
+containing water of crystallization is sealed in a glass tube, the
+gradual accumulation of hydrogen and oxygen will burst the tube.
+
+The radiations rapidly decompose organic matter with the evolution of
+gases. Thus grease from stopcocks of apparatus used with radium or
+paraffin will give off carbon dioxide. Under an intense alpha
+radiation paraffin or vaseline become hard and infusible. White
+phosphorus is changed into red.
+
+The action upon living tissue is most noteworthy, as its possible use
+as a remedial agent is dependent upon this. A small amount of a radium
+salt enclosed in a glass tube will cause a serious burn on flesh
+exposed to it. It therefore has to be handled with care and undue
+exposure to the radiations must be avoided. Cancer sacs shrivel up and
+practically disappear under its action. Whether the destruction of
+whatever causes the cancer is complete is at least open to serious
+doubt.
+
+The coagulating effect upon globulin is interesting. When two
+solutions of globulin from ox serum are taken and acetic acid added to
+one while ammonia is added to the other, the opalescence in drops of
+the former is rapidly diminished on exposure to radium, showing a more
+complete solution, whereas the latter solution rapidly turns to a
+jelly and becomes opaque, indicating a greatly decreased solubility.
+
+
+Energy Evolved by Radium
+
+The greater part of the tremendous energy evolved by radium is due to
+the emission of the alpha particles, and in comparison the beta and
+gamma rays together supply only a small fraction. This energy may be
+measured as heat. It was first observed that a radium compound
+maintained a temperature several degrees higher than that of the air
+around it. The rate of heat production was later measured by means of
+an ice calorimeter and also by noting the strength of the current
+required to raise a comparison tube of barium salt to the same
+temperature. Both methods showed that the heat produced was at the
+rate of about 135 gram calories per hour. As the emission is
+continuous, one gram of radium would therefore emit about 1,180,000
+gram calories in the course of a year. At the end of 2000 years it
+would still emit 590,000 gram calories per year. Such a production of
+energy so far surpasses all experience that it becomes almost
+inconceivable. It is futile to speak of it in terms of the heat
+evolved by the combustion of hydrogen, which is the greatest that can
+be produced by chemical means.
+
+This effect is unaltered at low temperatures, as has been tested by
+immersing a tube containing radium in liquid air. It should be stated
+that these measurements were made after the radium had reached an
+equilibrium with its products; that is, after waiting at least a month
+after its preparation. The evolution of heat from radium and the
+radio-active substances is, in a sense, a secondary effect, as it
+measures the radiant energy transformed into heat energy by the
+active matter itself and whatever surrounds it. Let us repeat,
+therefore, that the total amount of energy pent up in a single atom of
+radium almost passes our powers of conception.
+
+
+Necessity for a Disintegration Theory
+
+The facts gathered so far justify and necessitate a theory which shall
+satisfactorily explain them, and since these phenomena are not caused
+by nor subject to the influence of external agencies, they must refer
+to changes taking place within the atom--in other words, a theory of
+disintegration. In the main, these facts may be summed up as the
+emission of certain radiations from known elemental matter: the
+material alpha particles with positive charge, the beta particles or
+negative electrons, and the gamma rays analogous to _X_ rays. The
+emission of these rays results in the production of great heat. Then
+there is the law of transformations by which whole series of new
+elements are generated from the original element and maintain a
+constant equilibrium of growth and decay in the series. Lastly, we
+have the production of helium from the alpha particles.
+
+
+Disintegration Theory
+
+In explanation of these phenomena, Rutherford offered the hypothesis
+that the atoms of certain elements were unstable and subject to
+disintegration. The only elements definitely known to come under this
+description are the two having atoms of the greatest known mass,
+thorium (232) and uranium (238).
+
+The atoms of uranium, for instance, are supposed to be not permanent
+but unstable systems. According to the hypothesis, about 1 atom in
+every 10^{18} becomes unstable each second and breaks up with a
+violent explosion for so small a mass of matter. One, or possibly two
+alpha particles are expelled with great velocity. This alpha particle
+corresponds to an atom of helium with an atomic weight of 4, and its
+loss reduces the original atomic weight to 234 with the formation of a
+new element, having changed properties corresponding to the new atomic
+weight. This new element is uranium X_{1}.
+
+These new atoms are far more unstable than those of uranium, and the
+decomposition proceeds at a new rate of 1 in 10^{7} per second. So at
+a definite, measurable rate this stepwise disintegration proceeds. The
+explosions are not in all cases equally violent in going from element
+to element, nor are the results the same. Sometimes alpha particles
+alone are expelled, sometimes beta, or two of them together, as alpha
+and beta.
+
+The new product may remain with the unchanged part of the original
+matter. Thus there would be an accumulation of it until its own decay
+balances its production, resulting eventually in a state of
+equilibrium.
+
+
+Constitution of the Atom
+
+In order to explain the electrical and optical properties of matter,
+the hypothesis was made that the atom consisted of positively and
+negatively electrified particles. Later it was shown that negative
+electrons exist in all kinds of matter. Various attempts were made to
+work out a model of such an atom in which these particles were held in
+equilibrium by electrical forces. The atom of Lord Kelvin consisted of
+a uniform sphere of positive electrification throughout which a number
+of negative electrons were distributed, and J. J. Thomson has
+determined the properties of this type as to the number of particles,
+their arrangement and stability.
+
+
+Rutherford's Atom
+
+According to Rutherford, the atom of uranium may be looked upon as
+consisting of a central charge of positive electricity surrounded by a
+number of concentric rings of negative electrons in rapid motion. The
+positively charged centre is made up of a complicated system in
+movement, consisting in part of charged helium and hydrogen atoms, and
+practically the whole charge and mass of the atom is concentrated at
+the centre. The central system of the atom is from some unknown cause
+unstable, and one of the helium atoms escapes from the central mass as
+an alpha particle.
+
+There are, confessedly, difficulties connected with this conception of
+the atom which need not, however, be discussed here. Much remains to
+be learned as to the mechanics of the atom, and the hypothesis
+outlined above will probably have to be materially altered as
+knowledge grows. Perhaps it may have to be entirely abandoned in favor
+of some more satisfactory solution. Until such time it at least
+suffices as a mental picture around which the known facts group
+themselves. In this picture energy and matter lose their old-time
+distinctness of definition. Discrete subdivisions of energy are
+recognized which may be called charged particles without losing their
+significance. Some of these subdivisions charged in a certain way or
+with neutralized charge exhibit the properties of so-called matter.
+
+
+Scattering of Alpha Particles
+
+This conception of the atom would doubtless fail of much support were
+it not for certain experimental facts which lend great weight to it.
+Certain suppositions can be based on this theory mathematically
+reasoned out and tested by experiment. Predictions thus based on
+mathematical reasoning and afterward confirmed by experiment give a
+very convincing impression that truth lies at the bottom.
+
+The first of these experimental proofs comes under the head of what is
+known as the scattering of the alpha particles, a phenomenon which,
+when first observed, proved hard to explain. If an alpha particle in
+its escape from the parent atom should come within the influence of
+the supposed outer electrical field of some other atom, it should be
+deflected from its course and, the intensity of the two charges being
+known, the angle of deflection could be calculated. For instance, if
+it came to what might be called a head-on collision with the positive
+central nucleus of another atom, it would recoil if it were itself of
+lesser mass, or would propel the other forward if that were the
+lighter.
+
+The experiment is carried out by placing a thin metal foil over a
+radio-active body, as radium _C_, which expels alpha particles with a
+high velocity, and counting the number of alpha particles which are
+scattered through an angle greater than 90° and so recoil toward their
+source. This has been done by a number of investigators and it has
+been found that the angle of scattering and the number of recoil
+particles depend upon the atomic weight of the metal used as foil. For
+example, if gold is used, the number of recoil atoms is one in
+something less than 8,000.
+
+Taking the atomic weight of gold into consideration, Rutherford
+calculated mathematically that this was about the number which should
+be driven backward. But he went further and calculated also the number
+which should be returned by aluminum, which has an atomic weight of
+only about one-seventh that of gold. Two investigators determined
+experimentally the number for aluminum and their results agreed with
+Rutherford's calculations.
+
+The metals from aluminum to gold have been examined in this way. The
+number of recoil particles increases with the atomic weight of the
+metal. Comparing experiment with theory, the central charge in an atom
+corresponds to about one-half the atomic weight multiplied by the
+charge on an electron, or, as it is expressed, 1/2 Ae.
+
+There is only one lighter atom than helium, namely, hydrogen, which
+has a mass only one-fourth as great. When alpha particles are
+discharged into hydrogen, a few of the latter atoms are found to be
+propelled to a distance four times as great as that reached by the
+alpha particles.
+
+
+Stopping Power of Substances
+
+Parallel with the experiments mentioned, there is what is called
+the stopping power of substances. This means the depth or thickness
+of a substance necessary to put a stop to the course of the alpha
+particles. This gives the range of the alpha particles in such
+substances and is connected in a simple way with the atomic weight,
+that is, it is again fixed by the mass of the opposing atom. This
+stopping power of an atom for an alpha particle is approximately
+proportional to the square root of its atomic weight.
+
+Considering gases, for instance, if the range in hydrogen be 1,
+then the range in oxygen, the atomic weight of which is 16, is only
+(1/16)^{1/2} or 1/4. Generally in the case of metals the weight of
+matter per unit area required to stop the alpha particle is found to
+vary according to the square root of the atomic weight of the metal
+taken.
+
+
+
+
+CHAPTER VI
+
+RADIO-ACTIVITY AND CHEMICAL THEORY
+
+
+Influence upon Chemical Theory
+
+It can easily be seen that the revelations of radio-activity must have
+a far-reaching effect upon chemical theory, throwing light upon, and
+so bringing nearer, the solution of some of the problems which have
+been long discussed without arriving at any satisfactory solution. The
+so-called electro-chemical nature of the elements will certainly be
+made much clearer. The changes in valence should become intelligible
+and valence itself should be explained. A fuller understanding of the
+ionization of electrolytes also becomes possible. As these matters are
+debatable and the details are still unsettled, it is scarcely
+appropriate to give here the hypotheses in detail or to enter into any
+discussion of them. But the promise of solution in accord with the
+facts is encouraging.
+
+
+The Periodic System
+
+Such progress has been made, however, in regard to a better
+understanding of the Periodic System that the new facts and their
+interpretation may well be given. No reliable clue to the meaning of
+this system and the true relationship between the elements had been
+found up to the time when new light was thrown upon it by the
+discoveries of radio-activity. The underlying principle was unknown
+and even the statement of what was sometimes erroneously called the
+Periodic Law was manifestly incorrect and its terms were ignored.
+
+
+Basis of the Periodic System
+
+The ordinary statement of the fundamental principle of the Periodic
+System has been that the properties of the elements were periodic
+functions of the atomic weights, and that when the elements were
+arranged in the order of their atomic weights they fell into a natural
+series, taking their places in the proper related groups.
+
+In accepting this, the interpretation of function was both
+unmathematical and vague, and the order of the atomic weights was not
+strictly adhered to but unhesitatingly abandoned to force the group
+relationship. Wherever consideration of the atomic weight would have
+placed an element out of the grouping with other elements to which it
+was clearly related in physical and chemical properties, the guidance
+of these properties was accepted and that of the atomic weights
+disregarded. Such shiftings are noted in the cases of tellurium and
+iodine; cobalt and nickel; argon and potassium. It was most helpful
+that, following the order of atomic weights, the majority of the
+elements fell naturally into their places. Otherwise the
+generalization known as the Periodic System might have remained for a
+long time undiscovered and the progress of chemistry would have been
+greatly retarded.
+
+
+Influence of Positive Nucleus
+
+It is evident that the order of the elements is determined by
+something else than their atomic weights. From the known facts of
+radio-activity it would seem that this determining factor is the
+positive nucleus. And this nucleus also determines the mass or weight
+of the atom. Taking the elements in their order in the Periodic Series
+and numbering the positions held by them in this series as 1, 2, 3,
+etc., we get the position number or what is called the atomic number.
+This designates the order or position of the element in the series.
+We must learn that this number marks a position rather than a single
+element, a statement which will be explained later.
+
+
+Determination of the Atomic Number
+
+Since the atomic weight is unreliable as a means of settling the
+position of an element in the series and so fixing its atomic number,
+how is this number to be determined? Of course, one answer to this
+question is that we may rely upon a consideration of the general
+properties, as has been done in the past. Fortunately, other methods
+have been found by which this may be confirmed. For instance, the
+stopping and scattering power of the element for alpha particles has
+been suggested and successfully used.
+
+
+Use of X-Ray Spectra
+
+A most interesting method is due to Moseley's observations upon the
+_X_-ray spectra of the various elements. It has been found that
+crystals, such as those of quartz, have the power of reflecting and
+defining the _X_ rays. The spectra given by these rays can be
+photographed and the wave lengths measured. These _X_ rays are emitted
+by various substances under bombardment by the cathode rays (negative
+electrons) and have great intensity and very minute wave lengths.
+Moseley made use of various metals as anti-cathodes for the production
+of these rays. These metals ranged from calcium to zinc in the
+Periodic System. In each case he observed that two characteristic
+types of _X_ rays of definite intensity and different wave lengths
+were emitted. From the frequency of these waves there is deduced a
+simple relation connected with a fundamental quantity which increases
+in units from one element to the next. This is due to the charge of
+the positive central nucleus. The number found in this way is one less
+than the atomic number. Thus the number for calcium is 19 instead of
+20 and that for zinc is 29 instead of 30. So, by adding 1 to the
+number found the atomic number is obtained.
+
+The atomic weight can usually be followed in fixing the atomic number,
+but where doubt exists the method just given can be resorted to. Thus
+doubt arises in the case of iron and nickel and cobalt. This would be
+the order according to the atomic weights. The _X_-ray method gives
+the order as iron, cobalt, and nickel, and this is the accepted order
+in the Periodic System.
+
+
+Changes Caused by Ray Emission
+
+On studying the properties of the elements in a transformation series
+in connection with the ray emission which produced them, it was seen
+that these properties were determined in each case by the nature of
+the ray emitted from the preceding transformation product or parent
+element.
+
+
+Atomic Weight Losses
+
+Each alpha particle emitted means a loss of 4 in the atomic weight.
+This is the mass of a helium atom. Thus from uranium with an atomic
+weight of 238 to radium there is a loss of three alpha particles.
+Therefore, 12 must be subtracted from 238, leaving 226, which agrees
+closely with the atomic weight of radium as actually determined by the
+ordinary methods. Uranium X_{1}, then, would have an atomic weight
+of 234 and that of ionium would be 230. The other intermediate
+elements, whose formation is due to the loss of beta particles only,
+show no decrease in atomic weight.
+
+
+Lead the End Product
+
+From uranium to lead there is a loss of 8 alpha particles, or 32 units
+in atomic weight. This would give for the final product an atomic
+weight of 206. The atomic weight of lead is 207.17. It is not at all
+certain that the final product of this series is ordinary lead. The
+facts are such that they would lead one to think that it is not. It is
+known only that the end product would probably be some element closely
+resembling lead chemically and hence difficult or impossible to
+separate from it. Several accurate determinations of lead coming from
+uranium minerals, which always carry this element and in an
+approximately definite ratio to the amount of uranium present, show
+atomic weights of 206.40; 206.36; and 206.54. Even the most rigid
+methods of purification fail to change these results. The lead in
+these minerals might therefore be considered as coming in the main
+from the disintegration of the uranium atom and, though chemically
+resembling lead, as being in reality a different element with
+different atomic weight.
+
+Furthermore, in the thorium series 6 alpha particles are lost before
+reaching the end product, which again is perhaps the chemical analogue
+of lead. The atomic weight here should be 232 less 24, or 208.
+Determinations of the atomic weight of lead from thorite, a thorium
+mineral nearly free from uranium, gave 208.4.
+
+The end product of the actinium series is also an element resembling
+lead, but both the beginning and ending of this series are still in
+obscurity.
+
+
+Changes of Position in the Periodic System
+
+The loss of 4 units in the atomic weight of an element on the
+expulsion of an alpha particle is accompanied by a change of chemical
+properties which removes the new element two groups toward the
+positive side in the Periodic System.
+
+Thus ionium is so closely related to thorium and so resembles it
+chemically that it is properly classed along with thorium as a
+quadrivalent element in the fourth group. Ionium expels an alpha
+particle and becomes radium, which is a bivalent element resembling
+barium belonging to the second group. Radium then expels an alpha
+particle and becomes the gas, radium emanation, which is an analogue
+of argon and belongs to the zero group. Other instances might be cited
+which go to show that in all cases the loss of an alpha particle makes
+a change of two places toward the left or positive side of the System.
+
+
+Changes from Loss of Beta Particles
+
+The loss of a beta particle causes no change in the atomic weight but
+does cause a shift for each beta particle of one group toward the
+right or negative side of the System. Two such losses, then, will
+counterbalance the loss of an alpha particle and bring the new element
+back to the group originally occupied by its progenitor. Thus uranium
+in the sixth group loses an alpha particle and the product UX_{1}
+falls in the fourth group. One beta particle is then lost and UX_{2}
+belonging to the fifth group is formed. With the loss of one more beta
+particle the new element returns to the sixth group from which the
+transformation began.
+
+The table on page 48, as adapted from Soddy, affords a general view of
+these changes.
+
+
+Isotopes
+
+An examination of the table will show a number of different elements
+falling in the same position in a group of the Periodic System
+irrespective of their atomic weights. These are chemically inseparable
+so far as the present limitations of chemical analysis are concerned.
+Even the spectra of these elements seem to be identical so far as
+known. This identity extends to most of the physical properties, but
+this demands much further investigation. For this new phenomenon Soddy
+has suggested the word isotope for the element and isotopic for the
+property, and these names have come into general use.
+
+ [Illustration: RADIO-ACTIVE ELEMENTS FROM URANIUM AND THORIUM
+ PLACED IN THE PERIODIC SYSTEMS Adapted from Soddy]
+
+Manifestly, we have come across a phenomenon here which quite
+eliminates the atomic weight as a determining factor as to position in
+the Periodic or Natural System or of the elemental properties in
+general. All of the properties of the bodies which we call elements,
+and consequently of their compounds and hence of matter in general,
+seem to depend upon the balance maintained between the charges of
+negative and positive electricity which, according to Rutherford's
+theory, go to make up the atom.
+
+It is evident that any study of chemical phenomena and chemical theory
+is quite incomplete without a study of radio-activity and the
+transformations which it produces.
+
+
+Radio-activity in Nature
+
+In concluding this outline of the main facts of radio-activity, it is
+of interest to discuss briefly the presence of radio-active material
+on this planet and in the stars. Facts enough have been gathered to
+show the probable universality of this phenomenon of radio-activity.
+Whether this means solely the disintegration of the uranium and
+thorium atoms, or whether other elements are also transformed under
+the intensity of the agencies at work in the universe, is of course a
+question as yet unsolved.
+
+
+Radio-active Products in the Earth's Crust
+
+The presence of uranium and thorium widely distributed throughout
+the crust of the earth would lead to the conclusion that their
+disintegration products would be found there also. Various rocks of
+igneous origin have been examined revealing from 4.78 × 10^{-12}
+to 0.31 × 10^{-12} grams of radium per gram of the rock. Aqueous
+rocks have shown a lesser amount, ranging from 2.92 × 10^{-12} to
+0.86 × 10^{-12} grams. As the soil is formed by the decomposition
+of these rocks, radium is present in varying amounts in all kinds of
+soil.
+
+
+Presence in Air and Soil Waters
+
+As radium is transformed into the gaseous emanation, this will escape
+wherever the soil is not enclosed. For instance, a larger amount of
+radio-activity is found in the soil of caves and cellars than in open
+soils. If an iron pipe is sunk into a soil and the air of the soil
+sucked up into a large electroscope, the latter instrument will show
+the effect of the rays emitted and will measure the degree of
+activity. Also the interior of the pipe will receive a deposit of the
+radio-active material and will show appreciable radio-activity after
+being removed from the soil.
+
+This radium emanation is dissolved in the soil waters, wells, springs,
+and rivers, rendering them more or less radio-active, and sometimes
+the muddy deposit at the bottom of a spring shows decided
+radio-activity.
+
+The emanation also escapes into the air so that many observations made
+in various places show that the radium emanation is everywhere present
+in the atmosphere. Neither summer nor winter seems to affect this
+emanation, and it extends certainly to a height of two or three miles.
+Rain, falling through the air, dissolves some of the emanation, so
+that it may be found in freshly-fallen rain water and also in
+freshly-fallen snow. Radio-active deposits are found upon electrically
+charged wires exposed near the earth's surface.
+
+As helium is the resulting product of the alpha particles emitted by
+the emanation and other radio-active bodies, it is found in the soil
+air, soil waters, and atmosphere.
+
+Average measurements of the radio-activity of the atmosphere have led
+to the calculation that about one gram of radium per square kilometer
+of the earth's surface is requisite to keep up the supply of the
+emanation.
+
+A number of estimates have been given as to the heat produced by the
+radio-active transformations going on in the material of this planet.
+Actual data are scarce and mere assumptions unsatisfactory, so little
+that is worth while can be deduced. It is possible that this source of
+heat may have an appreciable effect upon or serve to balance the
+earth's rate of cooling.
+
+
+Cosmical Radio-activity
+
+Meteorites of iron coming from other celestial bodies have not shown
+the presence of radium. Aerolites or stone meteorites have been found
+to contain as much as similar terrestrial rock. Since the sun
+contains helium and some stars show its presence as predominating,
+this suggests the presence of radio-active matter in these bodies. In
+addition, the spectral lines of uranium, radium, and the radium
+emanation have been reported as being found in the sun's spectrum and
+also in the new star, _Nova Geminorum 2_. These observations await
+further investigation and confirmation. So far as the sun's
+chromosphere is concerned, the possible amount of radium present would
+seem to be very small. If this is true, radio-active processes could
+have little to do with the sun's heat. The statement is made by
+Rutherford that indirect evidence obtained from the study of the
+aurora suggests that the sun emits rays similar in type to the alpha
+and beta rays. Such rays would be absorbed, and the gamma rays
+likewise, in passing through the earth's atmosphere and so escape
+ordinary observation. All of this is but further evidence of the unity
+of matter and of forces in the universe.
+
+
+
+
+INDEX
+
+
+ Actinium, discovery of, 6
+
+ Activity, induced, 17
+
+ Alpha particles, effect of loss on Atomic Weight, 45
+ electrical charge of, 26
+ form helium, 27
+ nature of, 25
+ penetrating power of, 39
+ position of element changed by its loss, 46
+ recoil, 39
+ scattering of, 38
+ solid, 26
+
+ Atom, constitution of, 36
+ Kelvin's, 37
+ models of, 37
+ Rutherford's, 37
+
+ Atomic number, determination of, 43
+
+
+ Becquerel's experiments, 2
+
+ Beta particles, change in position of element by loss of, 47
+
+
+ Chalcolite, natural and artificial, 4
+
+ Constants, table of, 31
+
+ Curie unit, 22
+
+
+ Disintegration of the element, 25
+
+ Disintegration series, 24
+
+ Disintegration theory, 35
+
+
+ Electroscope, 12
+
+ Equilibrium series, 22
+
+
+ Helium, characteristics of, 30
+ discovery of, 29
+
+
+ Ionium, discovery of, 6
+
+ Ionization, application of electric field to, 10
+ experimental confirmation, 9
+
+ Ionization of gases, 7
+ theory of, 8
+
+ Ions, size and nature of, 10
+
+ Isotopes, 47
+
+
+ Lead, atomic weight varies with source, 45
+ radio-active, 6
+ the end product, 45
+
+ Life-periods of radio-active bodies, 21
+
+
+ Periodic system, 41
+ basis of, 42
+
+ Polonium, discovery of, 4
+
+ Positive nucleus, influence of, 43
+
+ Potassium, radio-activity of, 3
+
+
+ Radiations, action on phosphorescent bodies, 13
+ action on photographic plates, 11
+ discharge electrified bodies, 12
+ magnetic deflection of, 14
+ measurements of, 15
+ penetrating power of, 13, 15
+
+ Radio-active bodies, elemental nature of, 20
+ examination of, 20
+ life periods of, 21
+
+ Radio-activity, an atomic property, 3
+ cosmical, 51
+ influence on chemical theory, 41
+ products in atmosphere, 51
+ products in earth's crust, 50
+ products in soil waters, 50
+
+ Radium, action on organic matter, etc., 33
+ amount in pitchblende, 5
+ discovery of, 5
+ emanation, 22
+ energy evolved by, 34
+ properties of, 5, 32
+
+ Rays, alpha, 15, 16, 26
+ beta, 15, 16
+ gamma, 15, 16
+ identification of, 16, 25
+ magnetic deflection of, 14
+ photographing track of, 10
+ types of, 14
+
+ Rubidium, radio-activity of, 3
+
+
+ Spinthariscope, 13
+
+ Stopping power of substances, 39
+
+
+ Thorium X, discovery of, 18, 21
+
+
+ Uranium atom, disintegration of, 36
+
+ Uranium minerals, radio-activity of, 3
+
+ Uranium X, discovery of, 17, 21, 23
+
+
+ X-ray spectra, 44
+
+
+ Zinc sulphide screen, 13
+
+
+
+
+TRANSCRIBER'S NOTES
+
+
+1. Passages in italics are surrounded by _underscores_.
+
+2. Images have been moved from the middle of a paragraph to the
+closest paragraph break.
+
+3. The original text includes certain Greek alphabets. For this text
+version [alpha], [beta], [gamma] indicate first three letters of Greek
+alphabet respectively.
+
+4. In this version, the number following carat character ^ is to be
+interpreted as follows. The expression 10^{-2} means multiplying by
+0.01; 10^{10} means multiplying by 10,000,000,000.
+
+5. In this version, the subscripted text has been replaced by an
+underline character _ followed by the same with curly braces { and }.
+For example, X_{1} indicates X with subscript 1.
+
+6. The fractions are indicated with the help of forward character /.
+For example, 1/4 indicates one-fourth.
+
+7. Other than the changes listed above, the original text has been
+reproduced as such.
+
+
+
+
+
+End of the Project Gutenberg EBook of A Brief Account of Radio-activity, by
+Francis Preston Venable
+
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+
+The Project Gutenberg EBook of A Brief Account of Radio-activity, by
+Francis Preston Venable
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever. You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Brief Account of Radio-activity
+
+Author: Francis Preston Venable
+
+Release Date: May 9, 2010 [EBook #32307]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK A BRIEF ACCOUNT OF RADIO-ACTIVITY ***
+
+
+
+
+Produced by The Online Distributed Proofreading Team at
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+
+
+
+
+<h1>A BRIEF ACCOUNT OF<br />
+RADIO-ACTIVITY</h1>
+
+<h4>BY</h4>
+
+<h2><span class="smcap">FRANCIS P. VENABLE, Ph.D., D.Sc., LL.D.</span></h2>
+
+<h5>PROFESSOR OF CHEMISTRY, UNIVERSITY OF NORTH CAROLINA<br />
+AUTHOR OF<br />
+"A SHORT HISTORY OF CHEMISTRY,"<br />
+"PERIODIC LAW," ETC.</h5>
+
+<h3>D. C. HEATH &amp; CO., PUBLISHERS<br />
+<small>BOSTON &nbsp; &nbsp; NEW YORK &nbsp; &nbsp; CHICAGO</small></h3>
+
+
+
+<h4><span class="smcap">Copyright, 1917,<br />
+By D. C. Heath &amp; Co.</span><br />
+<br />
+IA7</h4>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'>[Pg iii]</span></p>
+<h2>PREFACE</h2>
+
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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&mdash;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.
+<span class='pagenum'>[Pg iv]</span></p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'>[Pg v]</span></p>
+<h2>CONTENTS</h2>
+
+
+<table border="0" cellpadding="5" cellspacing="0" summary="Table of Contents" width="80%">
+<tr>
+ <td colspan="2" align="center"><a href="#CHAPTER_I"><b>CHAPTER I</b></a></td>
+</tr>
+<tr>
+ <td align="center"><b>DISCOVERY OF RADIO-ACTIVITY</b></td>
+ <td align="right"><small>PAGE</small></td>
+</tr>
+<tr>
+ <td>The beginning&mdash;Radio-active bodies&mdash;An atomic property&mdash;Discovery
+of new radio-active bodies&mdash;Discovery of Polonium&mdash;Discovery
+of Radium&mdash;Other radio-active bodies found</td>
+ <td align="right"><a href="#Page_1">1</a></td>
+</tr>
+<tr>
+ <td colspan="2" align="center"><a href="#CHAPTER_II"><b>CHAPTER II</b></a></td>
+</tr>
+<tr>
+ <td colspan="2" align="center"><b>PROPERTIES OF THE RADIATIONS</b></td>
+</tr>
+<tr>
+ <td>Ionization of Gases&mdash;Experimental confirmation&mdash;Application of
+electric field&mdash;Size and nature of ions&mdash;Photographing the
+track of the ray&mdash;Action of radiations on photographic plates&mdash;Discharge
+of electrified bodies&mdash;Scintillations on phosphorescent
+bodies&mdash;Penetrating power&mdash;Magnetic deflection&mdash;Three
+types of rays&mdash;Alpha rays&mdash;Beta rays&mdash;Gamma rays&mdash;Measurement
+of radiations&mdash;Identifications of the rays</td>
+ <td align="right"><a href="#Page_7">7</a></td>
+</tr>
+<tr>
+ <td colspan="2" align="center"><a href="#CHAPTER_III"><b>CHAPTER III</b></a></td>
+</tr>
+<tr>
+ <td colspan="2" align="center"><b>CHANGES IN RADIO-ACTIVE BODIES</b></td>
+</tr>
+<tr>
+ <td>Radio-activity a permanent property&mdash;Induced activity&mdash;Discovery
+of Uranium X&mdash;Conclusions drawn&mdash;Search for new radio-active
+bodies&mdash;Methods of investigation&mdash;Nature of the
+radiations&mdash;Life-periods&mdash;Equilibrium series</td>
+ <td align="right"><a href="#Page_17">17</a></td>
+</tr>
+<tr>
+ <td colspan="2" align="center"><a href="#CHAPTER_IV"><b>CHAPTER IV</b></a></td>
+</tr>
+<tr>
+ <td colspan="2" align="center"><b>NATURE OF THE ALPHA PARTICLE</b></td>
+</tr>
+<tr>
+ <td>Disintegrating of the elements&mdash;Identification of the rays&mdash;The
+alpha rays&mdash;Alpha rays consist of solid particles&mdash;Electrical
+charge&mdash;Helium formed from alpha particles&mdash;Discovery of
+Helium&mdash;Characteristics of Helium&mdash;Table of constants</td>
+ <td align="right"><a href="#Page_25">25</a></td>
+</tr>
+<tr>
+ <td colspan="2" align="center"><a href="#CHAPTER_V"><b>CHAPTER V</b></a><span class='pagenum'>[Pg vi]</span></td>
+</tr>
+<tr>
+ <td colspan="2" align="center"><b>THE STRUCTURE OF THE ATOM</b></td>
+</tr>
+<tr>
+ <td>Properties of Radium&mdash;Energy evolved by radium&mdash;Necessity for a
+disintegration theory&mdash;Disintegration theory&mdash;Constitution
+of the atom&mdash;Rutherford's atom&mdash;Scattering of alpha particles&mdash;Stopping
+power of substances</td>
+ <td align="right"><a href="#Page_32">32</a></td>
+</tr>
+<tr>
+ <td colspan="2" align="center"><a href="#CHAPTER_VI"><b>CHAPTER VI</b></a></td>
+</tr>
+<tr>
+ <td colspan="2" align="center"><b>RADIO-ACTIVITY AND CHEMICAL THEORY</b></td>
+</tr>
+<tr>
+ <td>Influence upon chemical theory&mdash;The periodic system&mdash;Basis of the
+periodic system&mdash;Influence of positive nucleus&mdash;Determination
+of the atomic number&mdash;Use of X-ray spectra&mdash;Changes caused
+by ray-emission&mdash;Atomic weight losses&mdash;Lead the end product&mdash;Changes
+of position in the periodic system&mdash;Changes from
+loss of beta particles&mdash;Isotopes&mdash;Radio-activity in nature&mdash;Radio-active
+products in the earth's crust&mdash;Presence in air and
+soil waters&mdash;Cosmical radio-activity</td>
+ <td align="right"><a href="#Page_41">41</a></td>
+</tr>
+<tr>
+ <td><a href="#INDEX"><span class="smcap">Index</span></a></td>
+ <td align="right"><a href="#Page_53">53</a></td>
+</tr>
+</table>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'>[Pg vii]</span></p>
+<h2>A BRIEF ACCOUNT OF<br />
+RADIO-ACTIVITY</h2>
+
+<p><span class='pagenum'>[Pg viii]</span></p>
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_1" id="Page_1">[Pg 1]</a></span></p>
+<h1>A BRIEF ACCOUNT OF<br />
+RADIO-ACTIVITY</h1>
+
+<h2><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I</h2>
+
+<h3>DISCOVERY OF RADIO-ACTIVITY</h3>
+
+
+<p>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.</p>
+
+<div class="sidenote">The Beginning</div>
+
+<p>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.</p>
+
+<p><span class='pagenum'><a name="Page_2" id="Page_2">[Pg 2]</a></span>
+In 1895, R&ouml;ntgen modestly announced his discovery
+of the <i>X</i> 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 <i>X</i> 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.</p>
+
+<div class="sidenote">Radio-active bodies</div>
+
+<p>The rays given off by uranium and its salts were
+found to differ from the <i>X</i> 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.</p>
+
+<p>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
+<span class='pagenum'><a name="Page_3" id="Page_3">[Pg 3]</a></span>
+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.</p>
+
+<div class="sidenote">An Atomic Property</div>
+
+<p>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.</p>
+
+<div class="sidenote">Discovery of New Radio-active Bodies</div>
+
+<p>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>i</i> give the
+maximum current in amperes. They serve simply for
+comparison.</p>
+
+
+<table border="0" cellpadding="2" cellspacing="0" summary="Uranium Minerals" width="80%">
+<tr>
+ <td align="center">&nbsp;</td>
+ <td align="center"><i>i</i></td>
+</tr>
+<tr>
+ <td>Pitchblende from Joachimsthal</td>
+ <td align="center">7.0 &times; 10<sup>-11</sup></td>
+</tr>
+<tr>
+ <td>Clevite</td>
+ <td align="center">1.4 &times; 10<sup>-11</sup></td>
+</tr>
+<tr>
+ <td>Chalcolite</td>
+ <td align="center">5.2 &times; 10<sup>-11</sup></td>
+</tr>
+<tr>
+ <td>Autunite</td>
+ <td align="center">2.7 &times; 10<sup>-11</sup></td>
+</tr>
+<tr>
+ <td>Carnotite</td>
+ <td align="center">6.2 &times; 10<sup>-11</sup></td>
+</tr>
+<tr>
+ <td>Uranium</td>
+ <td align="center">2.3 &times; 10<sup>-11</sup></td>
+</tr>
+<tr>
+ <td>Uranium and potassium sulphate</td>
+ <td align="center">0.7 &times; 10<sup>-11</sup></td>
+</tr>
+<tr>
+ <td>Uranium and copper phosphate</td>
+ <td align="center">0.9 &times; 10<sup>-11</sup></td>
+</tr>
+</table>
+
+<p><span class='pagenum'><a name="Page_4" id="Page_4">[Pg 4]</a></span>
+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.</p>
+
+<p>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.</p>
+
+<div class="sidenote">Discovery of Polonium</div>
+
+<p>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
+<span class='pagenum'><a name="Page_5" id="Page_5">[Pg 5]</a></span>
+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.</p>
+
+<div class="sidenote">Discovery of Radium</div>
+
+<p>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.</p>
+
+<div class="sidenote">Other Radio-active Bodies Found</div>
+
+<p>In the above separations use was made of relationships
+<span class='pagenum'><a name="Page_6" id="Page_6">[Pg 6]</a></span>
+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.</p>
+
+<p>The lead which is present in uranium and thorium
+minerals&mdash;apparently in fairly definite ratio to the amount of
+uranium and thorium&mdash;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.</p>
+
+<p>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.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_7" id="Page_7">[Pg 7]</a></span></p>
+<h2><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II</h2>
+
+<h3>PROPERTIES OF THE RADIATIONS</h3>
+
+
+<p>The activity of these radio-active bodies consists in the emission of
+certain radiations which may be separated into rays and studied
+through the phenomena which they cause.</p>
+
+<div class="sidenote">Ionization of Gases</div>
+
+<p>One of these phenomena is the power of forming ions or carriers of
+electricity by the passage of the rays through a gas, thus ionizing
+the gas. The details of an experiment will serve to make the meaning
+of this ionization clear.</p>
+
+<div class="figcenter" style="width: 60%;">
+<img src="images/i015.jpg" width="100%" alt="Fig. 1." title="Fig. 1." />
+<span class="caption"><span class="smcap">Fig. 1.&mdash;Ionization of Gases.</span></span>
+</div>
+
+<p>When this apparatus is set up a minute current will be observed
+without the introduction of any radio-active matter. This, as
+Rutherford says, has been found due mainly to a slight natural
+radio-activity of the matter composing the plates. If radio-active matter
+<span class='pagenum'><a name="Page_8" id="Page_8">[Pg 8]</a></span>
+is spread on plate <i>A</i>, which is connected with one pole of a grounded
+battery, and if plate <i>B</i> is connected with an electrometer which is
+also connected with the earth, a current is caused which increases
+rapidly with the difference of potential between the plates, then more
+slowly until a value is reached that changes only slightly with a
+larger increase in the voltage.</p>
+
+<p>According to the theory of ionization, the radiation produces ions at
+a constant rate. The ions carrying a positive charge are attracted to
+plate <i>B</i>, while those negatively charged are attracted to plate <i>A</i>,
+thus causing a current. These ions will recombine and neutralize their
+charges if the opportunity is given. The number, therefore, increases
+to a point at which the ions produced balance the number recombining.</p>
+
+<p>When an electric field is produced between the plates, the velocity of
+the ions between the plates is increased in proportion to the strength
+of the electric field. In a weak field the ions travel so slowly that
+most of them recombine on the way and consequently the observed
+current is very small. On increasing the voltage the speed of the ions
+is increased, fewer recombine, the current increases, and, when the
+condition for recombination is practically removed, it will have a
+maximum value. This maximum current is called the saturation current
+and the value of the potential difference required to give this
+maximum current is called the saturation P.D. or saturation voltage.</p>
+
+<p>The picture, then, is this. The radiations separate
+<span class='pagenum'><a name="Page_9" id="Page_9">[Pg 9]</a></span>
+the components of the gas into ions, or carriers of electricity, half
+of which are charged negatively and half positively. In the electric
+field those negatively charged seek the positive plate and those
+positively charged seek the negative plate. If time is given, these
+ions meet and recombine, their charges are neutralized, and there is
+no current.</p>
+
+<div class="sidenote">Experimental Confirmation</div>
+
+<p>This theory of the ionization of gases has been most interestingly
+confirmed by direct experiment. For instance, the ions may form nuclei
+for the condensation of water, and in this way the existence of the
+separate ions in the gas may be shown and the number present actually
+counted.</p>
+
+<p>When air saturated with water vapor is allowed to expand suddenly, the
+water present forms a mist of small globules. There are always small
+dust particles in air and around these as nuclei the drops are formed.
+These drops will settle and thus by repeated small expansions all dust
+nuclei may be removed and no mist or cloud will be formed by further
+expansions.</p>
+
+<p>If now the radiation from a radio-active body be introduced into the
+condensation vessel, a new cloud is produced in which the water drops
+are finer and more numerous according to the intensity of the rays. On
+passing a strong beam of light through the condensation chamber, the
+drops can readily be seen. These drops form on the ions produced by
+the radiation.</p>
+
+<div class="sidenote">Application of Electric Field</div>
+
+<p>If the condensation chamber has two parallel plates
+for the application of an electric field like that already
+<span class='pagenum'><a name="Page_10" id="Page_10">[Pg 10]</a></span>
+described, the ions will be carried at once to the electrodes and
+disappear. The rapidity of this action depends upon the strength of
+the electric field and experiment shows that the stronger the field
+the smaller the number of condensation drops formed. If there is no
+electric field, a cloud can be produced some time after the shutting
+off of the source of radiation, showing that time is required for the
+recombination of the ions.</p>
+
+<div class="sidenote">Size and Nature of Ions</div>
+
+<p>If the drops are counted (there being special methods for this) and
+the total current carried accurately measured, then the charge carried
+by each ion may be calculated. This has been determined. The mass of
+an ion compared with the mass of the molecules of gas in which it was
+produced can also be approximately estimated. In the study of these
+ions the view has been held that the charged ion attracted to itself a
+cluster of molecules which surrounded the charged nucleus and traveled
+with it. It is roughly estimated that about thirty molecules of the
+gas cluster around each charged ion.</p>
+
+<div class="figright" style="width: 20%;">
+<img src="images/i019.jpg" width="100%" alt="Fig. 2." title="Fig. 2." />
+<span class="caption"><span class="smcap">Fig. 2.&mdash;Photograph
+of the Track of an Ionizing Ray.</span></span>
+</div>
+
+<div class="sidenote">Photographing the Track of the Ray</div>
+
+<p>Utilizing the fact that these ions with their clusters of molecules
+form nuclei for the condensation of water vapor, C. T. R. Wilson has
+by instantaneous photography been able to photograph the track of an
+ionizing ray through air. The number of the ions produced, and hence
+the number of drops, is so great that the trail is shown as a
+continuous line. In the copy of this photograph it will be seen
+<span class='pagenum'><a name="Page_11" id="Page_11">[Pg 11]</a></span>
+that at some distance from its source the straight trail is slightly
+but abruptly bent. Near the end of its course there is another abrupt
+and much sharper bend. These bends show where the ionizing ray, in
+this case an alpha particle, has been deflected by more or less direct
+collision with an atom. These collisions and the final disappearance
+of the ray will be discussed later.</p>
+
+<div class="sidenote">Action of Radiations on Photographic Plates</div>
+
+<p>Taking up now other means of examining these radiations, it is well to
+consider their action upon a photographic or sensitive plate. It will
+be recalled that this was the method by which their existence was
+originally detected. To illustrate the method, the following account
+of how one such photograph was taken may be given.</p>
+
+<p>The plate was wrapped in two thicknesses of black paper. The objects
+were placed upon this and the radio-active ore, separated by a board
+one inch thick, was placed above. The exposure lasted five days. The
+action is much less rapid and the result not so clearly defined as in
+the case of photographs taken by <i>X</i> rays. Of course, the removal of
+the board and the use of more concentrated preparations of radium
+would give quicker and better results. The method, however, on
+<span class='pagenum'><a name="Page_12" id="Page_12">[Pg 12]</a></span>
+account of time consumed and lack of definition is ill adapted to
+accurate work.</p>
+
+<div class="sidenote">Discharge of Electrified Bodies</div>
+
+<p>The radiations from radio-active bodies can discharge
+both positively and negatively electrified bodies by
+making the air surrounding them a conductor
+of electricity. To demonstrate this,
+use is made of an electroscope. If the
+hinged leaf of such an instrument be electrically charged
+and a radio-active body be brought into its neighborhood,
+the electricity will be discharged and the leaf
+return to its original position. The rapidity of this
+discharge is used to measure the degree of activity of
+the body giving off the radiation.</p>
+
+<div class="figcenter" style="width: 70%;">
+<img src="images/i021.jpg" width="100%" alt="Fig. 3." title="Fig. 3." />
+<span class="caption"><span class="smcap">Fig. 3&mdash;Photograph of Various Objects
+taken by means of Pitchblende</span></span>
+</div>
+
+
+<div class="figcenter" style="width: 50%;">
+<img src="images/i020.jpg" width="100%" alt="Fig. 4." title="Fig. 4." />
+<span class="caption"><span class="smcap">Fig. 4.&mdash;Gold-leaf Electroscope.</span></span>
+</div>
+
+<div class="blockquot"><p>The gold-leaf <i>L</i> is attached to a flat rod <i>R</i>
+and is insulated inside the vessel by a piece of amber <i>S</i> supported
+from the rod <i>P</i>. The system is charged by a bent rod <i>CC'</i> passing
+through an ebonite stopper. After charging, it is removed from contact
+with the gold-leaf system. The rods <i>P</i> and <i>C</i> and the cylinder are
+then connected with the earth.</p></div>
+
+
+<p><span class='pagenum'><a name="Page_13" id="Page_13">[Pg 13]</a></span></p>
+
+<div class="sidenote">Scintillations on Phosphorescent Bodies</div>
+
+<p>It was found by Crookes that a screen covered with phosphorescent zinc
+sulphide was brightly lighted up when exposed to the radiations. This
+is due to the bombardment of the zinc sulphide by a type of ray called
+the alpha ray. Under a magnifying glass this light is seen to be made
+up of a number of scintillating points of light and is not continuous,
+each scintillation being of very short duration. By proper subdivision
+of the field under the lens, the number of scintillations can be
+counted with close accuracy.</p>
+
+<p>A simple form of apparatus called the spinthariscope has been devised
+to show these scintillations. A zinc sulphide screen is fixed in one
+end of a small tube and a plate carrying a trace of radium is placed
+very close to it. The scintillations can be observed through an
+adjustable lens at the other end of the tube. Outer light should be
+cut off, as in a dark room. The screen then appears to be covered with
+brilliant flashes of light. Other phosphorescent substances, such as
+barium platino-cyanide, may be substituted for the zinc sulphide, but
+they do not answer so well.</p>
+
+<div class="sidenote">Penetrating Power</div>
+
+<p>By penetrating power is meant the power exhibited by the rays of
+passing through solids of different thicknesses and gases of various
+depths. This power varies with different radiations and with the nature
+of the solid or gas. For instance, a sheet of metallic foil may be used
+and the effect of aluminum will differ from that of gold and the different
+<span class='pagenum'><a name="Page_14" id="Page_14">[Pg 14]</a></span>
+rays vary in penetrating power. In the case of gases air will differ
+from hydrogen, and it is noticed that certain rays disappear after
+penetrating a short distance, while others can penetrate further
+before being lost.</p>
+
+<div class="sidenote">Magnetic Deflection</div>
+
+<p>If the radiations are subjected to the action of a strong magnetic
+field, it is found that part of them are much deflected in the
+magnetic field and describe circular orbits, part are only slightly
+deflected and in the opposite direction from the first, and the
+remaining rays are entirely unaffected.</p>
+
+<div class="figcenter" style="width: 60%;">
+<img src="images/i024.jpg" width="100%" alt="Fig. 5." title="Fig. 5." />
+<span class="caption"><span class="smcap">Fig. 5.&mdash;Showing Magnetic Deflection
+of</span> &#945;, &#946;, <span class="smcap">and</span> &#947; <span class="smcap">Rays</span>.</span>
+</div>
+
+
+<div class="sidenote">Three Types of Rays</div>
+
+<p>By the use of these methods of investigation it is learned that the
+radiations consist of three types of rays. These have been named the
+alpha, beta, and gamma rays, respectively. Some radio-active bodies
+emit all three types, some two, and some only one. The distinguishing
+characteristic of these types of rays may be summed up as follows:
+<span class='pagenum'><a name="Page_15" id="Page_15">[Pg 15]</a></span></p>
+
+<div class="sidenote">Alpha Rays</div>
+
+<p>The alpha rays have a positive electrical charge and a comparatively
+low penetrating power. They are slightly deflected in strong magnetic
+and electric fields. They have a great ionizing power and a velocity
+about one-fifteenth that of light.</p>
+
+<div class="sidenote">Beta Rays</div>
+
+<p>The beta rays are negatively charged and have a greater penetrating
+power than the alpha rays. They show a strong deflection in magnetic
+and electric fields, have less ionizing power than the alpha rays, and
+a velocity of the same order as light.</p>
+
+<div class="sidenote">Gamma Rays</div>
+
+<p>The gamma rays are very penetrating and are not deflected in the
+magnetic or electric fields. They have the least ionizing power and a
+very great velocity.</p>
+
+<p>The penetrating power of each type is complex and varies with the
+source, so the statements given are but generalizations. The alpha
+rays are projected particles which lose energy in penetrating matter.
+As to the power of ionizing gases, if that for the &#945; rays is
+taken as 10,000, then the &#946; rays would be approximately 100 and
+the &#947; rays 1.</p>
+
+<div class="sidenote">Measurement of Radiations</div>
+
+<p>The rays are examined and measured in several ways: 1. By their action
+on the sensitive photographic plates. The use of this method is
+laborious, consumes time, and for comparative measurements of
+intensity is uncertain as to effect.</p>
+
+<p>2. By electrical methods, using electroscopes, quadrant
+<span class='pagenum'><a name="Page_16" id="Page_16">[Pg 16]</a></span>
+electrometers, etc. These are the methods most used.</p>
+
+<p>3. By exposure to magnetic and electric fields, noting extent and
+direction of deflection.</p>
+
+<p>4. By their relative absorption by solids and gases.</p>
+
+<p>5. By the scintillations on a zinc sulphide screen.</p>
+
+<div class="sidenote">Identification of the Rays</div>
+
+<p>The alpha rays have been identified as similar to the so-called canal
+rays. These were first observed in the study of the <i>X</i> rays. When an
+electrical discharge is passed through a vacuum tube with a cathode
+having holes in it, luminous streams pass through the holes toward the
+side away from the anode and the general direction of the stream. They
+travel in straight lines and render certain substances phosphorescent.
+These rays are slightly deflected by a magnetic field and in an
+opposite direction from that taken by the cathode rays in their
+deflection. The rays seem to be positive ions with masses never less
+than that of the hydrogen atom. Their source is uncertain, but they
+may be derived from the electrodes.</p>
+
+<p>The beta rays are identical in type with the cathode rays and are
+negative electrons.</p>
+
+<p>The gamma rays are analogous to the <i>X</i> rays and are of the order of
+light. They are in general considerably more penetrating than <i>X</i>
+rays. For example, the gamma rays sent out by 30 milligrams of radium
+can be detected by an electroscope after passing through 30
+centimeters of iron, a much greater thickness than
+can be penetrated by the ordinary <i>X</i> rays.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_17" id="Page_17">[Pg 17]</a></span></p>
+<h2><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III</h2>
+
+<h3>CHANGES IN RADIO-ACTIVE BODIES</h3>
+
+
+<div class="sidenote">Is Radio-activity a Permanent Property?</div>
+
+<p>Is this power of emitting radiations a permanent property or is it
+lost with the passage of time? The first investigations of the
+activity of uranium and thorium showed no loss of intensity at the end
+of several years, and radium also seemed to show no decrease in its
+enormous activity. Polonium, however, was found to lose most of its
+activity in a year, and later it appeared that some radio-active
+substances lost most of their activity in the course of a few minutes
+or hours.</p>
+
+<div class="sidenote">Induced Activity</div>
+
+<p>A phenomenon called induced or secondary radio-activity was also
+observed. Thus a metal plate or wire exposed to the action of thorium
+oxide for some hours became itself active. This induced activity was
+not permanent but decreased to half its value in about eleven hours
+and practically disappeared within a week. Similar phenomena were
+observed when radium was substituted for thorium.</p>
+
+<div class="sidenote">Discovery of Uranium X</div>
+
+<p>In 1900 Crookes precipitated a solution of an active uranium
+salt with ammonium carbonate. The precipitate was dissolved so
+far as possible in an excess of the reagent, leaving an insoluble
+residue. This residue was many hundred times more
+<span class='pagenum'><a name="Page_18" id="Page_18">[Pg 18]</a></span>
+active, weight for weight, than the original salt, and the solution
+containing the salt was practically inactive. At the end of a year the
+uranium salt had regained its activity while the residue had become
+inactive.</p>
+
+<p>Another method of obtaining the same result is to dissolve
+crystallized uranium nitrate in ether. Two layers of solution are
+formed, one ether and the other water coming from the water of
+crystallization. The aqueous layer is active, while the water layer is
+inactive. Similarly, by adding barium chloride solution to a solution
+of a salt of uranium and then precipitating the barium as sulphate,
+the activity is transferred to this precipitate. These experiments
+give proof of the formation and separation of a radio-active body by
+ordinary chemical operations.</p>
+
+<p>So, too, in the case of thorium salts a substance can be obtained by
+means of ammonium hydroxide which is several thousand times more
+active than an equal weight of the original salt. After standing a
+month, the separated material has lost its activity and the thorium
+salt has regained it. Here, again, there is the formation, separation,
+and loss of a radio-active body.</p>
+
+<div class="sidenote">Conclusions Drawn</div>
+
+<p>Now, these are ordinary chemical processes for the separation of
+distinct chemical individuals. The results, therefore, lead naturally
+to the conclusions: (1) it would seem that uranium and thorium are
+themselves inactive and the activity is due to some other substance
+formed by these elements; (2) this active substance is produced by some
+<span class='pagenum'><a name="Page_19" id="Page_19">[Pg 19]</a></span>
+transformation in those elements, for on standing the activity is
+regained. This latter conclusion is startling, for it indicates a
+change in the atom which, up to the time of this discovery, was deemed
+unchangeable under the influence of such physical and chemical changes
+as were known to us.</p>
+
+<div class="sidenote">Search for New Radio-active Bodies</div>
+
+<p>The search for new radio-active bodies and the study of their
+characteristics has been systematically and successfully carried on.
+The bodies obtained in the above experiments were named uranium <i>X</i>
+and thorium <i>X</i>, respectively. Further, it became clear from the
+investigation of uranium minerals that radium, polonium, actinium, and
+ionium originated from uranium. From thorium minerals a body was
+separated called mesothorium, which was analogous to radium. Both
+thorium and radium were found to give off a radio-active gas. The
+first lost half of its activity in less than one minute. The second
+was more stable and lost half of its activity in about four days. The
+name radium emanation was given to the latter and it was found
+chemically and physically to belong to the class of monatomic or noble
+gases, such as helium, argon, neon, etc., which had been discovered by
+Ramsay. In some cases the chemical action was determined and these new
+bodies were found analogous to well-known elements, as radium to
+barium, polonium to bismuth. The physical properties were investigated
+and, where possible, spectra were mapped and atomic weights
+determined.</p>
+
+<p><span class='pagenum'><a name="Page_20" id="Page_20">[Pg 20]</a></span>
+It is clear, therefore, that these bodies are elemental in character
+and as such are made up of distinct, similar atoms, just as the
+commonly recognized elements are believed to be. In this way more than
+thirty new elements have been added to the list. These new elements
+are called radio-active elements, but it is an open question whether
+all atoms do not possess this property in greater or less degree.
+Certainly, it is possessed in varying degree by four of the old
+elements widely separated in the Periodic System, namely, uranium,
+thorium, rubidium, and potassium. The last two, while feebly active
+themselves, do not form any secondary radio-active substance so far as
+is known. Only two of the elements, then, can definitely be said to go
+through these transformations. It is just possible that radio-activity
+may be found to be a common property of all atoms and of all matter.</p>
+
+<div class="sidenote">Methods of Investigation</div>
+
+<p>It is important to know how these new bodies were discovered and
+distinguished from one another. Two properties are relied upon. One is
+the nature of the rays emitted and the other is the duration of the
+activity. Of course, knowledge of the physical and chemical properties
+is also of great importance whenever obtainable.</p>
+
+<div class="sidenote">Nature of the Radiations</div>
+
+<p>The nature of the radiation is a distinguishing characteristic,
+though similarity here does not prove identity of substances.
+Some emit &#945; rays only, some emit &#946; rays, some emit
+two of the possible rays, as for instance, &#946; and &#947;, and
+<span class='pagenum'><a name="Page_21" id="Page_21">[Pg 21]</a></span>
+some emit all three. The rays may also differ in the velocity with
+which they are emitted by different radio-active substances. Thus, in
+the case of one substance the &#945; rays may have a slightly greater
+or less penetrating power than those emitted by some other substance,
+and this may be true also of the other rays.</p>
+
+<div class="sidenote">Life Periods</div>
+
+<p>The duration of the activity is called the life period. This is
+absolutely fixed for each body and furnishes the most important mode
+of differentiating among them. It measures the relative stability and
+is the time which must elapse before their activity is lost and they,
+changing into something else, entirely disappear. The measure usually
+adopted is the half-value period. Two hypotheses are made use of:</p>
+
+<p>1. That there is a constant production of fresh radio-active matter by
+the radio-active body.</p>
+
+<p>2. That the activity of the matter so formed decreases according to an
+exponential law with the time from the moment of its formation.</p>
+
+<p>These hypotheses agree with the experimental results. The decrease and
+rise of activity, for example, of uranium and uranium <i>X</i>, and also of
+thorium and thorium <i>X</i>, have been measured, plotted, and the
+equations worked out.</p>
+
+<p>Manifestly, a state of equilibrium will be reached
+when the rate of loss of activity of the matter already
+produced is balanced by the activity of the new matter
+<span class='pagenum'><a name="Page_22" id="Page_22">[Pg 22]</a></span>
+produced. This equilibrium and the knowledge of the rate of decrease
+in general will have little value if this rate, like chemical changes,
+is subject to the influence of chemical and physical conditions. The
+rate of decrease has been found to be unaltered by any known chemical
+or physical agency. For instance, neither the highest temperatures
+applicable nor the cold of liquid air have any appreciable effect.</p>
+
+<div class="sidenote">Equilibrium Series</div>
+
+<p>In order to measure the disintegration of a radio-active body in units
+of time so that the rate may be comparable with that of other
+radio-active bodies, the relation between the amounts under
+consideration must be a definite one. For this purpose equal weights
+of the bodies are not taken, but use is made of the amounts which are
+in equilibrium with a fixed amount of the parent substance.</p>
+
+<p>One gram of radium has been settled upon as the standard for that
+series and a unit known as the "curie" has been adopted to express the
+equilibrium quantity of radium emanation. Thus, a curie of radium
+emanation (or niton) is the weight (or, as this is a gas, the volume
+at standard pressure and temperature) of the emanation in equilibrium
+with one gram of radium. This, by calculation and experiment, is found
+to be 0.63 cubic millimeter. When this amount has been produced by one
+gram of radium, the formation and decay will exactly balance one
+another. This is, therefore, one curie of emanation.</p>
+
+<p><span class='pagenum'><a name="Page_23" id="Page_23">[Pg 23]</a></span>
+The measurement of the rate of decay is difficult but can be carried
+out with great accuracy, even down to seconds, in the case of certain
+short-lived bodies. Errors crept in at first from the failure to
+completely separate the substances produced in the series, and
+sometimes because of the simultaneous production of two substances.</p>
+
+<p>As stated, the decay follows an exponential law. The time required for
+the decay of activity to half-value does not mean, therefore, that
+there will be total decay in twice that time. Thus the half-value
+period for uranium <i>X</i> is about 22 days. The period for complete decay
+is about 160 days. This half-value period corresponds to the
+half-value recovery period of uranium, which is also 22 days.</p>
+
+<p>These were the earlier figures obtained for uranium <i>X</i> and they
+illustrate some of the difficulties surrounding such determinations.
+It was found later that the body examined as uranium <i>X</i> was really a
+constant mixture and of course the decay and recovery periods were
+also composite. It required later and very skilful work to separate
+them into the bodies indicated in the disintegration series.</p>
+
+<p>The half-value period for thorium <i>X</i> is much shorter, namely, a
+little over four days, and this is also the recovery period for
+thorium <i>X</i>. The plotted decay and recovery curves will intersect at
+this point.</p>
+
+<p>The consecutive disintegration series, with the
+half-value periods, for the uranium and thorium series as
+<span class='pagenum'><a name="Page_24" id="Page_24">[Pg 24]</a></span>
+given by Soddy are seen in the following tables. They are probably
+subject to some changes on further and more accurate determination.
+The nature of the rays emitted is also given.</p>
+
+<div class="figcenter" style="width: 80%;">
+<img src="images/i034.jpg" width="100%" alt="Fig. 6." title="Fig. 6." />
+<span class="caption"><span class="smcap">Fig. 6.&mdash;Disintegration Series for Uranium, Actinium,
+and Thorium, as Given by Soddy.</span></span>
+</div>
+
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_25" id="Page_25">[Pg 25]</a></span></p>
+<h2><a name="CHAPTER_IV" id="CHAPTER_IV"></a>CHAPTER IV</h2>
+
+<h3>NATURE OF THE ALPHA PARTICLE</h3>
+
+
+<div class="sidenote">Disintegration of the Elements</div>
+
+<p>The remarkable disintegrations related in the last chapter, in which
+the heaviest known elementary atom&mdash;that of uranium (at. wt.
+238)&mdash;is by successive stages changed into others of lower atomic
+weight, afford a clue to the nature of the atom and to that goal of
+the chemist, the final constitution of matter. The composite nature of
+the atom and some sort of interrelation of the elements had previously
+been made apparent from a study of the Periodic System and data
+gathered still earlier, but all attempts at working out a so-called
+genesis of the elements had proved vague and unsatisfactory.</p>
+
+<div class="sidenote">Identification of the Rays</div>
+
+<p>To get an understanding of the disintegration occurring in
+radio-active substances, the nature of the rays produced must be
+known. These rays are the cause of the activity and their emission
+accompanies the changes or disintegration. They have for the sake of
+convenience been called the alpha, beta, and gamma rays. The gamma
+rays have been identified with the <i>X</i> rays discovered by R&ouml;ntgen
+and are a form of energy analogous to light. The beta rays are
+particles of negative electricity or electrons. With
+<span class='pagenum'><a name="Page_26" id="Page_26">[Pg 26]</a></span>
+these, then, we have some degree of familiarity. But what are the
+alpha rays? An answer to this question should make clearer the
+character of the changes taking place, and should give some insight
+into the composition and mechanism of the atom.</p>
+
+<div class="sidenote">The Alpha Rays</div>
+
+<p>It has already been stated that these alpha rays are similar or
+analogous to the canal rays, but this advances the matter very little,
+as the nature of these canal rays has not been fully determined. The
+full identity with them, if proved, should have an important
+theoretical bearing.</p>
+
+<div class="sidenote">Alpha Rays Consist of Solid Particles</div>
+
+<p>In the first place, these alpha rays have been found to be made up of
+solid particles, that is, of what we are accustomed to call matter.
+Since it has become more and more difficult to draw a clear
+distinction between matter and energy, it would perhaps be better to
+say that these particles appear to have some of the properties
+hitherto attributed solely to matter. The best evidence that these
+particles are of atomic mass is furnished by their deflection in
+electric and magnetic fields.</p>
+
+<div class="sidenote">Electrical Charge</div>
+
+<p>It is not of first importance to discuss this or other proofs
+of the material nature of these particles. That they carry a
+charge of positive electricity is, however, a fact of very
+great import. The value of this charge has been carefully
+determined by a number of investigators working with different
+sources of the alpha particles and has been found to
+<span class='pagenum'><a name="Page_27" id="Page_27">[Pg 27]</a></span>
+be 9.3 &times; 10<sup>-10</sup> electrostatic units (.000,000,000,93 e.s.).
+From the consideration of the charge upon an electron previously obtained
+by J. J. Thomson and others, it was concluded that the alpha particle
+carried two unit positive charges; the fundamental unit charge,
+therefore, is half this value, or 4.65 &times; 10<sup>-10</sup> e.s.</p>
+
+<div class="sidenote">Helium Formed from Alpha Particles</div>
+
+<p>To determine the nature of the alpha particle a crucial experiment was
+carried out by Rutherford and Royds, which was described as follows:</p>
+
+<div class="figright" style="width: 30%;">
+<img src="images/i037.jpg" width="100%" alt="Fig. 7." title="Fig. 7." />
+<span class="caption"><span class="smcap">Fig. 7.&mdash;Apparatus
+Used in Experiment by Rutherford and Royds.</span></span>
+</div>
+
+<p>A large quantity of radium emanation was compressed into a fine glass
+tube <i>A</i>, about 1.5 cm. long. This tube, which was sealed to a larger
+capillary tube <i>B</i>, was sufficiently thin to allow the alpha particles
+from the emanation and its products to pass through, but sufficiently
+thick to withstand atmospheric pressure. The thickness of the glass
+wall was in most cases less than .01 mm. On introducing the emanation
+into the tube, the escape of the alpha particles from the emanation
+was clearly seen by the scintillations produced at some distance on a
+zinc sulphide screen. After this test the glass tube <i>A</i> was
+surrounded by a glass tube <i>T</i> and a small spectrum tube
+<i>V</i> attached to it. The tube
+<span class='pagenum'><a name="Page_28" id="Page_28">[Pg 28]</a></span>
+<i>T</i> was exhausted to a charcoal vacuum. By means of the mercury column
+<i>H</i>, the gases in the tube <i>T</i> could at any time be compressed into
+the spectrum tube <i>V</i> and the nature of the gases which had been
+produced determined spectroscopically. It was found that two days
+after the introduction of the emanation into <i>A</i> the spectrum showed
+the yellow line of helium, and after six days the whole helium
+spectrum was observed. In order to be certain that the helium, coming
+possibly from some other source, had not diffused through the thin
+walls of the tube <i>A</i>, the emanation was pumped out and helium
+substituted. No trace of helium could be observed in the vacuum tube
+after several days, showing that the helium observed in the first
+experiment must have originated from the alpha particles which had
+been propelled through the thin glass tube into the outer tube.</p>
+
+<p>Most of the alpha particles are propelled with such force that they
+penetrate some distance into the walls of the outer tube and some of
+these gradually diffuse out into the exhausted space. The presence of
+helium in the spectrum tube can be detected after a shorter interval
+if a thin cylinder of lead is placed over the emanation tube, since
+the particles fired into the lead diffuse out more rapidly than from
+glass.</p>
+
+<p>A still more definite proof of the identity of the alpha particle with
+the helium atom was obtained by removing the outer glass tube <i>T</i> and
+placing a cylinder of lead over the emanation tube in the open air. Helium was
+<span class='pagenum'><a name="Page_29" id="Page_29">[Pg 29]</a></span>
+always detected in the lead after it had remained several hours over
+the thin tube containing a large quantity of the emanation. In order
+to test for the presence of helium in the lead, the gases present were
+released by melting the lead in a closed vessel. There can thus be no
+doubt that the alpha particle becomes a helium atom when its positive
+charge is neutralized.</p>
+
+<p>Thus the chemist was afforded the experience of the building up of at
+least one element under his observation, and both the analysis and
+synthesis of matter have been revealed through the discoveries of
+radio-activity.</p>
+
+<div class="sidenote">Discovery of Helium</div>
+
+<p>It is of interest at this point to learn something of the history of
+helium and its occurrence. In 1868 there was discovered by Janssen and
+Lockyer a bright yellow line in the spectrum of the sun's
+chromosphere. Because of its origin the name helium was given to the
+supposed new element causing it. Later it was found in the spectra of
+many of the stars, and because of its predominance in some of these
+they were called helium stars. Its existence on our planet was not
+detected for nearly thirty years.</p>
+
+<p>In 1895, in connection with the discovery of argon in the atmosphere, a
+search was made to see if the latter element could be obtained from mineral
+sources. In analyzing certain uranium minerals Hillebrand had found considerable
+quantities of a gas which he took to be a peculiar form of nitrogen.
+Ramsay made a further examination of the gas coming from these minerals and
+<span class='pagenum'><a name="Page_30" id="Page_30">[Pg 30]</a></span>
+the spectroscope revealed the yellow line of helium, thus at last
+proving the presence of this element on the earth. It is known now to
+be present in thorium minerals, in the waters of radio-active wells,
+and in minute amounts in the atmosphere. Its occurrence in every case,
+in the light of the experiment described above, would seem to be due
+to the presence of radio-active changes.</p>
+
+<div class="sidenote">Characteristics of Helium</div>
+
+<p>Helium, on account of its chemical inactivity and physical properties,
+is classed along with argon, neon, krypton, and xenon in the zero
+group of the Periodic System, and forms with them the monatomic, inert
+gases. In this class are now placed also the three radio-active gases,
+emanating respectively from radium, thorium, and actinium. These are
+generally known as radium emanation, thorium emanation, and actinium
+emanation. The first mentioned was once called niton. Emanium was the
+name originally proposed by Giesel for the body now known as actinium.</p>
+
+<p>The calculated rate of production of helium in the series in
+equilibrium with one gram of radium is 158 cubic millimeters per year.
+This corresponds quite well with the experimental results.</p>
+
+<div class="sidenote">Table of Constants</div>
+
+<p>Some of the more important atomic and radio-active constants
+are given in the following table. They are recorded here to show
+how helpful the study of radio-activity has been in working
+out the composition of matter, and to give some idea of
+<span class='pagenum'><a name="Page_31" id="Page_31">[Pg 31]</a></span>
+the magnitude of the numbers and the minuteness of the quantities
+dealt with.</p>
+
+<table border="0" cellpadding="2" cellspacing="0" summary="Constants">
+<colgroup><col width="60%" /><col width="35%" /><col width="5%" /></colgroup>
+<tr>
+ <td></td>
+ <td align="right"></td>
+ <td></td>
+</tr>
+<tr>
+ <td></td>
+ <td align="right"></td>
+ <td></td>
+</tr>
+<tr>
+ <td>Electric charge carried by each H atom inelectrolysis</td>
+ <td align="right">4.65 &times; 10<sup>-10</sup></td>
+ <td>e.s.<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a></td>
+</tr>
+<tr>
+ <td>Electric charge carried by each &#945; particle</td>
+ <td align="right">9.3 &times; 10<sup>-10</sup></td>
+ <td>e.s.</td>
+</tr>
+<tr>
+ <td>Number of atoms in 1 gram of H</td>
+ <td align="right">6.2 &times; 10<sup>23</sup></td>
+ <td></td>
+</tr>
+<tr>
+ <td>Mass of 1 atom of H</td>
+ <td align="right">1.6 &times; 10<sup>-24</sup></td>
+ <td>gram</td>
+</tr>
+<tr>
+ <td>Number of molecules per cc. of any gas at standard pressure and temperature</td>
+ <td align="right">2.72 &times; 10<sup>19</sup></td>
+ <td>&nbsp;</td>
+</tr>
+<tr>
+ <td>Number of &#945; particles expelled per second per gram of radium itself</td>
+ <td align="right">3.6&nbsp; &times; 10<sup>10</sup></td>
+ <td>&nbsp;</td>
+</tr>
+<tr>
+ <td>Number of &#945; particles expelled per second per gram of radium in equilibrium with its products</td>
+ <td align="right">14.3&nbsp; &times; 10<sup>10</sup></td>
+ <td>&nbsp;</td>
+</tr>
+</table>
+
+<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> The expression 10<sup>-10</sup> means multiplying by .000,000,000,1; 10<sup>10</sup> means
+multiplying by 10,000,000,000.</p></div>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_32" id="Page_32">[Pg 32]</a></span></p>
+<h2><a name="CHAPTER_V" id="CHAPTER_V"></a>CHAPTER V</h2>
+
+<h3>THE STRUCTURE OF THE ATOM</h3>
+
+
+<div class="sidenote">Properties of Radium</div>
+
+<p>A study of the properties of radium will aid in throwing light upon
+the question as to the building up of the atom. First to be considered
+are the usual properties which distinguish an elementary body.
+Metallic radium has been prepared by a method similar to that used in
+the preparation of barium. It is a pure white metal, melting at
+700&deg;, and far more volatile than barium. It rapidly alters on
+exposure to the air, probably forming a nitride. It energetically
+decomposes water and the product dissolves in the water. Its atomic
+weight is 226.</p>
+
+<p>Radium forms a series of salts analogous in appearance and chemical
+action to those of barium. In the course of time they become colored,
+especially if mixed barium salts. The radiations from radium produce
+marked chemical effects in a number of substances. Carbon dioxide is
+changed into carbon, oxygen, and carbon monoxide, and the latter is
+changed into carbon and oxygen. Ammonia is dissociated into nitrogen and
+hydrogen; hydrochloric acid into chlorine and hydrogen. Oxygen is condensed
+into ozone. In general, the action upon gases appears to be similar to that
+<span class='pagenum'><a name="Page_33" id="Page_33">[Pg 33]</a></span>
+of the silent electric discharge. Water is decomposed into hydrogen
+and oxygen. If moist radium chloride or a salt of radium containing
+water of crystallization is sealed in a glass tube, the gradual
+accumulation of hydrogen and oxygen will burst the tube.</p>
+
+<p>The radiations rapidly decompose organic matter with the evolution of
+gases. Thus grease from stopcocks of apparatus used with radium or
+paraffin will give off carbon dioxide. Under an intense alpha
+radiation paraffin or vaseline become hard and infusible. White
+phosphorus is changed into red.</p>
+
+<p>The action upon living tissue is most noteworthy, as its possible use
+as a remedial agent is dependent upon this. A small amount of a radium
+salt enclosed in a glass tube will cause a serious burn on flesh
+exposed to it. It therefore has to be handled with care and undue
+exposure to the radiations must be avoided. Cancer sacs shrivel up and
+practically disappear under its action. Whether the destruction of
+whatever causes the cancer is complete is at least open to serious
+doubt.</p>
+
+<p>The coagulating effect upon globulin is interesting. When two
+solutions of globulin from ox serum are taken and acetic acid added to
+one while ammonia is added to the other, the opalescence in drops of
+the former is rapidly diminished on exposure to radium, showing a more
+complete solution, whereas the latter solution rapidly turns to a
+jelly and becomes opaque, indicating a greatly decreased solubility.</p>
+
+<div class="sidenote">Energy Evolved by Radium</div>
+
+<p>The greater part of the tremendous energy evolved
+<span class='pagenum'><a name="Page_34" id="Page_34">[Pg 34]</a></span>
+by radium is due to the emission of the alpha particles, and in
+comparison the beta and gamma rays together supply only a small
+fraction. This energy may be measured as heat. It was first observed
+that a radium compound maintained a temperature several degrees higher
+than that of the air around it. The rate of heat production was later
+measured by means of an ice calorimeter and also by noting the
+strength of the current required to raise a comparison tube of barium
+salt to the same temperature. Both methods showed that the heat
+produced was at the rate of about 135 gram calories per hour. As the
+emission is continuous, one gram of radium would therefore emit about
+1,180,000 gram calories in the course of a year. At the end of 2000
+years it would still emit 590,000 gram calories per year. Such a
+production of energy so far surpasses all experience that it becomes
+almost inconceivable. It is futile to speak of it in terms of the heat
+evolved by the combustion of hydrogen, which is the greatest that can
+be produced by chemical means.</p>
+
+<p>This effect is unaltered at low temperatures, as has been tested by
+immersing a tube containing radium in liquid air. It should be stated that
+these measurements were made after the radium had reached an equilibrium
+with its products; that is, after waiting at least a month after its
+preparation. The evolution of heat from radium and the radio-active
+substances is, in a sense, a secondary effect, as it measures the radiant
+<span class='pagenum'><a name="Page_35" id="Page_35">[Pg 35]</a></span>
+energy transformed into heat energy by the active matter itself and
+whatever surrounds it. Let us repeat, therefore, that the total amount
+of energy pent up in a single atom of radium almost passes our powers
+of conception.</p>
+
+<div class="sidenote">Necessity for a Disintegration Theory</div>
+
+<p>The facts gathered so far justify and necessitate a theory which shall
+satisfactorily explain them, and since these phenomena are not caused
+by nor subject to the influence of external agencies, they must refer
+to changes taking place within the atom&mdash;in other words, a theory
+of disintegration. In the main, these facts may be summed up as the
+emission of certain radiations from known elemental matter: the
+material alpha particles with positive charge, the beta particles or
+negative electrons, and the gamma rays analogous to <i>X</i> rays. The
+emission of these rays results in the production of great heat. Then
+there is the law of transformations by which whole series of new
+elements are generated from the original element and maintain a
+constant equilibrium of growth and decay in the series. Lastly, we
+have the production of helium from the alpha particles.</p>
+
+<div class="sidenote">Disintegration Theory</div>
+
+<p>In explanation of these phenomena, Rutherford offered the hypothesis
+that the atoms of certain elements were unstable and subject to
+disintegration. The only elements definitely known to come under this
+description are the two having atoms of the greatest known mass,
+thorium (232) and uranium (238).</p>
+
+<p><span class='pagenum'><a name="Page_36" id="Page_36">[Pg 36]</a></span>
+The atoms of uranium, for instance, are supposed to be not permanent
+but unstable systems. According to the hypothesis, about 1 atom in
+every 10<sup>18</sup> becomes unstable each second and breaks up with a violent
+explosion for so small a mass of matter. One, or possibly two alpha
+particles are expelled with great velocity. This alpha particle
+corresponds to an atom of helium with an atomic weight of 4, and its
+loss reduces the original atomic weight to 234 with the formation of a
+new element, having changed properties corresponding to the new atomic
+weight. This new element is uranium <i>X</i><sub>1</sub>.</p>
+
+<p>These new atoms are far more unstable than those of uranium, and the
+decomposition proceeds at a new rate of 1 in 10<sup>7</sup> per second. So at a
+definite, measurable rate this stepwise disintegration proceeds. The
+explosions are not in all cases equally violent in going from element
+to element, nor are the results the same. Sometimes alpha particles
+alone are expelled, sometimes beta, or two of them together, as alpha
+and beta.</p>
+
+<p>The new product may remain with the unchanged part of the original
+matter. Thus there would be an accumulation of it until its own decay
+balances its production, resulting eventually in a state of
+equilibrium.</p>
+
+<div class="sidenote">Constitution of the Atom</div>
+
+<p>In order to explain the electrical and optical properties
+of matter, the hypothesis was made that the atom consisted of
+positively and negatively electrified particles. Later it was
+shown that negative electrons exist in all kinds of matter.
+<span class='pagenum'><a name="Page_37" id="Page_37">[Pg 37]</a></span>
+Various attempts were made to work out a model of such an atom in
+which these particles were held in equilibrium by electrical forces.
+The atom of Lord Kelvin consisted of a uniform sphere of positive
+electrification throughout which a number of negative electrons were
+distributed, and J. J. Thomson has determined the properties of this
+type as to the number of particles, their arrangement and stability.</p>
+
+<div class="sidenote">Rutherford's Atom</div>
+
+<p>According to Rutherford, the atom of uranium may be looked upon as
+consisting of a central charge of positive electricity surrounded by a
+number of concentric rings of negative electrons in rapid motion. The
+positively charged centre is made up of a complicated system in
+movement, consisting in part of charged helium and hydrogen atoms, and
+practically the whole charge and mass of the atom is concentrated at
+the centre. The central system of the atom is from some unknown cause
+unstable, and one of the helium atoms escapes from the central mass as
+an alpha particle.</p>
+
+<p>There are, confessedly, difficulties connected with this conception of
+the atom which need not, however, be discussed here. Much remains to
+be learned as to the mechanics of the atom, and the hypothesis
+outlined above will probably have to be materially altered as
+knowledge grows. Perhaps it may have to be entirely abandoned in favor
+of some more satisfactory solution. Until such time it at least suffices
+as a mental picture around which the known facts group themselves. In
+<span class='pagenum'><a name="Page_38" id="Page_38">[Pg 38]</a></span>
+this picture energy and matter lose their old-time distinctness of
+definition. Discrete subdivisions of energy are recognized which may
+be called charged particles without losing their significance. Some of
+these subdivisions charged in a certain way or with neutralized charge
+exhibit the properties of so-called matter.</p>
+
+<div class="sidenote">Scattering of Alpha Particles</div>
+
+<p>This conception of the atom would doubtless fail of much support were
+it not for certain experimental facts which lend great weight to it.
+Certain suppositions can be based on this theory mathematically
+reasoned out and tested by experiment. Predictions thus based on
+mathematical reasoning and afterward confirmed by experiment give a
+very convincing impression that truth lies at the bottom.</p>
+
+<p>The first of these experimental proofs comes under the head of what is
+known as the scattering of the alpha particles, a phenomenon which,
+when first observed, proved hard to explain. If an alpha particle in
+its escape from the parent atom should come within the influence of
+the supposed outer electrical field of some other atom, it should be
+deflected from its course and, the intensity of the two charges being
+known, the angle of deflection could be calculated. For instance, if
+it came to what might be called a head-on collision with the positive
+central nucleus of another atom, it would recoil if it were itself of
+lesser mass, or would propel the other forward if that were the lighter.</p>
+
+<p><span class='pagenum'><a name="Page_39" id="Page_39">[Pg 39]</a></span>
+The experiment is carried out by placing a thin metal foil over a
+radio-active body, as radium <i>C</i>, which expels alpha particles with a
+high velocity, and counting the number of alpha particles which are
+scattered through an angle greater than 90&deg; and so recoil toward
+their source. This has been done by a number of investigators and it
+has been found that the angle of scattering and the number of recoil
+particles depend upon the atomic weight of the metal used as foil. For
+example, if gold is used, the number of recoil atoms is one in
+something less than 8,000.</p>
+
+<p>Taking the atomic weight of gold into consideration, Rutherford
+calculated mathematically that this was about the number which should
+be driven backward. But he went further and calculated also the number
+which should be returned by aluminum, which has an atomic weight of
+only about one-seventh that of gold. Two investigators determined
+experimentally the number for aluminum and their results agreed with
+Rutherford's calculations.</p>
+
+<p>The metals from aluminum to gold have been examined in this way. The
+number of recoil particles increases with the atomic weight of the
+metal. Comparing experiment with theory, the central charge in an atom
+corresponds to about one-half the atomic weight multiplied by the
+charge on an electron, or, as it is expressed, &frac12; Ae.</p>
+
+<p>There is only one lighter atom than helium, namely,
+hydrogen, which has a mass only one-fourth as great.
+<span class='pagenum'><a name="Page_40" id="Page_40">[Pg 40]</a></span>
+When alpha particles are discharged into hydrogen, a few of the latter
+atoms are found to be propelled to a distance four times as great as
+that reached by the alpha particles.</p>
+
+<div class="sidenote">Stopping Power of Substances</div>
+
+<p>Parallel with the experiments mentioned, there is what is called the
+stopping power of substances. This means the depth or thickness of a
+substance necessary to put a stop to the course of the alpha
+particles. This gives the range of the alpha particles in such
+substances and is connected in a simple way with the atomic weight,
+that is, it is again fixed by the mass of the opposing atom. This
+stopping power of an atom for an alpha particle is approximately
+proportional to the square root of its atomic weight.</p>
+
+<p>Considering gases, for instance, if the range in hydrogen be 1, then
+the range in oxygen, the atomic weight of which is 16, is only
+&#8730;(1/16) or &frac14;. Generally in the case of metals the weight of
+matter per unit area required to stop the alpha particle is found to vary
+according to the square root of the atomic weight of the
+metal taken.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_41" id="Page_41">[Pg 41]</a></span></p>
+<h2><a name="CHAPTER_VI" id="CHAPTER_VI"></a>CHAPTER VI</h2>
+
+<h3>RADIO-ACTIVITY AND CHEMICAL THEORY</h3>
+
+
+<div class="sidenote">Influence upon Chemical Theory</div>
+
+<p>It can easily be seen that the revelations of radio-activity must have
+a far-reaching effect upon chemical theory, throwing light upon, and
+so bringing nearer, the solution of some of the problems which have
+been long discussed without arriving at any satisfactory solution. The
+so-called electro-chemical nature of the elements will certainly be
+made much clearer. The changes in valence should become intelligible
+and valence itself should be explained. A fuller understanding of the
+ionization of electrolytes also becomes possible. As these matters are
+debatable and the details are still unsettled, it is scarcely
+appropriate to give here the hypotheses in detail or to enter into any
+discussion of them. But the promise of solution in accord with the
+facts is encouraging.</p>
+
+<div class="sidenote">The Periodic System</div>
+
+<p>Such progress has been made, however, in regard to a better
+understanding of the Periodic System that the new facts and
+their interpretation may well be given. No reliable clue to the
+meaning of this system and the true relationship between the
+<span class='pagenum'><a name="Page_42" id="Page_42">[Pg 42]</a></span>
+elements had been found up to the time when new light was thrown upon
+it by the discoveries of radio-activity. The underlying principle was
+unknown and even the statement of what was sometimes erroneously
+called the Periodic Law was manifestly incorrect and its terms were
+ignored.</p>
+
+<div class="sidenote">Basis of the Periodic System</div>
+
+<p>The ordinary statement of the fundamental principle of the Periodic
+System has been that the properties of the elements were periodic
+functions of the atomic weights, and that when the elements were
+arranged in the order of their atomic weights they fell into a natural
+series, taking their places in the proper related groups.</p>
+
+<p>In accepting this, the interpretation of function was both
+unmathematical and vague, and the order of the atomic weights was not
+strictly adhered to but unhesitatingly abandoned to force the group
+relationship. Wherever consideration of the atomic weight would have
+placed an element out of the grouping with other elements to which it
+was clearly related in physical and chemical properties, the guidance
+of these properties was accepted and that of the atomic weights
+disregarded. Such shiftings are noted in the cases of tellurium and
+iodine; cobalt and nickel; argon and potassium. It was most helpful
+that, following the order of atomic weights, the majority of the
+elements fell naturally into their places. Otherwise
+the generalization known as the Periodic System might
+<span class='pagenum'><a name="Page_43" id="Page_43">[Pg 43]</a></span>
+have remained for a long time undiscovered and the progress of
+chemistry would have been greatly retarded.</p>
+
+<div class="sidenote">Influence of Positive Nucleus</div>
+
+<p>It is evident that the order of the elements is determined by
+something else than their atomic weights. From the known facts of
+radio-activity it would seem that this determining factor is the
+positive nucleus. And this nucleus also determines the mass or weight
+of the atom. Taking the elements in their order in the Periodic Series
+and numbering the positions held by them in this series as 1, 2, 3,
+etc., we get the position number or what is called the atomic number.
+This designates the order or position of the element in the series. We
+must learn that this number marks a position rather than a single
+element, a statement which will be explained later.</p>
+
+<div class="sidenote">Determination of the Atomic Number</div>
+
+<p>Since the atomic weight is unreliable as a means of settling the
+position of an element in the series and so fixing its atomic number,
+how is this number to be determined? Of course, one answer to this
+question is that we may rely upon a consideration of the general
+properties, as has been done in the past. Fortunately, other methods
+have been found by which this may be confirmed. For instance, the
+stopping and scattering power of the element for alpha particles has
+been suggested and successfully used.</p>
+
+<div class="sidenote">Use of X-Ray Spectra</div>
+
+<p>A most interesting method is due to Moseley's
+<span class='pagenum'><a name="Page_44" id="Page_44">[Pg 44]</a></span>
+observations upon the <i>X</i>-ray spectra of the various elements. It has
+been found that crystals, such as those of quartz, have the power of
+reflecting and defining the <i>X</i> rays. The spectra given by these rays
+can be photographed and the wave lengths measured. These <i>X</i> rays are
+emitted by various substances under bombardment by the cathode rays
+(negative electrons) and have great intensity and very minute wave
+lengths. Moseley made use of various metals as anti-cathodes for the
+production of these rays. These metals ranged from calcium to zinc in
+the Periodic System. In each case he observed that two characteristic
+types of <i>X</i> rays of definite intensity and different wave lengths
+were emitted. From the frequency of these waves there is deduced a
+simple relation connected with a fundamental quantity which increases
+in units from one element to the next. This is due to the charge of
+the positive central nucleus. The number found in this way is one less
+than the atomic number. Thus the number for calcium is 19 instead of
+20 and that for zinc is 29 instead of 30. So, by adding 1 to the
+number found the atomic number is obtained.</p>
+
+<p>The atomic weight can usually be followed in fixing the atomic
+number, but where doubt exists the method just given can be resorted
+to. Thus doubt arises in the case of iron and nickel and cobalt.
+This would be the order according to the atomic weights. The
+<i>X</i>-ray method gives the order as iron, cobalt, and
+<span class='pagenum'><a name="Page_45" id="Page_45">[Pg 45]</a></span>
+nickel, and this is the accepted order in the Periodic System.</p>
+
+<div class="sidenote">Changes Caused by Ray Emission</div>
+
+<p>On studying the properties of the elements in a transformation series
+in connection with the ray emission which produced them, it was seen
+that these properties were determined in each case by the nature of
+the ray emitted from the preceding transformation product or parent
+element.</p>
+
+<div class="sidenote">Atomic Weight Losses</div>
+
+<p>Each alpha particle emitted means a loss of 4 in the atomic weight.
+This is the mass of a helium atom. Thus from uranium with an atomic
+weight of 238 to radium there is a loss of three alpha particles.
+Therefore, 12 must be subtracted from 238, leaving 226, which agrees
+closely with the atomic weight of radium as actually determined by the
+ordinary methods. Uranium <i>X</i><sub>1</sub>, then, would have an atomic weight of
+234 and that of ionium would be 230. The other intermediate elements,
+whose formation is due to the loss of beta particles only, show no
+decrease in atomic weight.</p>
+
+<div class="sidenote">Lead the End Product</div>
+
+<p>From uranium to lead there is a loss of 8 alpha particles, or 32 units
+in atomic weight. This would give for the final product an atomic
+weight of 206. The atomic weight of lead is 207.17. It is not at all
+certain that the final product of this series is ordinary lead. The
+facts are such that they would lead one to think that it is not.
+It is known only that the end product would probably be some
+element closely resembling lead chemically and hence
+<span class='pagenum'><a name="Page_46" id="Page_46">[Pg 46]</a></span>
+difficult or impossible to separate from it. Several accurate
+determinations of lead coming from uranium minerals, which always
+carry this element and in an approximately definite ratio to the
+amount of uranium present, show atomic weights of 206.40; 206.36; and
+206.54. Even the most rigid methods of purification fail to change
+these results. The lead in these minerals might therefore be
+considered as coming in the main from the disintegration of the
+uranium atom and, though chemically resembling lead, as being in
+reality a different element with different atomic weight.</p>
+
+<p>Furthermore, in the thorium series 6 alpha particles are lost before
+reaching the end product, which again is perhaps the chemical analogue
+of lead. The atomic weight here should be 232 less 24, or 208.
+Determinations of the atomic weight of lead from thorite, a thorium
+mineral nearly free from uranium, gave 208.4.</p>
+
+<p>The end product of the actinium series is also an element resembling
+lead, but both the beginning and ending of this series are still in
+obscurity.</p>
+
+<div class="sidenote">Changes of Position in the Periodic System</div>
+
+<p>The loss of 4 units in the atomic weight of an element on the
+expulsion of an alpha particle is accompanied by a change of chemical
+properties which removes the new element two groups toward the
+positive side in the Periodic System.</p>
+
+<p>Thus ionium is so closely related to thorium and so
+resembles it chemically that it is properly classed along
+with thorium as a quadrivalent element in the fourth
+<span class='pagenum'><a name="Page_47" id="Page_47">[Pg 47]</a></span>
+group. Ionium expels an alpha particle and becomes radium, which is a
+bivalent element resembling barium belonging to the second group.
+Radium then expels an alpha particle and becomes the gas, radium
+emanation, which is an analogue of argon and belongs to the zero
+group. Other instances might be cited which go to show that in all
+cases the loss of an alpha particle makes a change of two places
+toward the left or positive side of the System.</p>
+
+<div class="sidenote">Changes from Loss of Beta Particles</div>
+
+<p>The loss of a beta particle causes no change in the atomic weight but
+does cause a shift for each beta particle of one group toward the
+right or negative side of the System. Two such losses, then, will
+counterbalance the loss of an alpha particle and bring the new element
+back to the group originally occupied by its progenitor. Thus uranium
+in the sixth group loses an alpha particle and the product UX<sub>1</sub> falls
+in the fourth group. One beta particle is then lost and UX<sub>2</sub> belonging
+to the fifth group is formed. With the loss of one more beta particle
+the new element returns to the sixth group from which the
+transformation began.</p>
+
+<p>The table on page 48, as adapted from Soddy, affords a general view of
+these changes.<span class='pagenum'><a name="Page_48" id="Page_48">[Pg 48]</a></span></p>
+
+<div class="figcenter" style="width: 100%;">
+<img src="images/i058.jpg" width="100%" alt="Fig. 6." title="Fig. 6." />
+<span class="caption"><span class="smcap">Radio-active Elements from Uranium and Thorium Placed in the Periodic Systems</span><br />
+<small>Adapted from Soddy</small></span>
+</div>
+
+<div class="sidenote">Isotopes</div>
+
+<p>An examination of the table will show a number of different
+elements falling in the same position in a group of the Periodic
+System irrespective of their atomic weights. These are
+chemically inseparable so far as the present limitations of
+<span class='pagenum'><a name="Page_49" id="Page_49">[Pg 49]</a></span>
+chemical analysis are concerned. Even the spectra of these elements
+seem to be identical so far as known. This identity extends to most of
+the physical properties, but this demands much further investigation.
+For this new phenomenon Soddy has suggested the word isotope for the
+element and isotopic for the property, and these names have come into
+general use.</p>
+
+<p>Manifestly, we have come across a phenomenon here which quite
+eliminates the atomic weight as a determining factor as to position in
+the Periodic or Natural System or of the elemental properties in
+general. All of the properties of the bodies which we call elements,
+and consequently of their compounds and hence of matter in general,
+seem to depend upon the balance maintained between the charges of
+negative and positive electricity which, according to Rutherford's
+theory, go to make up the atom.</p>
+
+<p>It is evident that any study of chemical phenomena and chemical theory
+is quite incomplete without a study of radio-activity and the
+transformations which it produces.</p>
+
+<div class="sidenote">Radio-activity in Nature</div>
+
+<p>In concluding this outline of the main facts of radio-activity,
+it is of interest to discuss briefly the presence
+of radio-active material on this planet and
+in the stars. Facts enough have been
+gathered to show the probable universality of this
+phenomenon of radio-activity. Whether this means
+solely the disintegration of the uranium and thorium
+<span class='pagenum'><a name="Page_50" id="Page_50">[Pg 50]</a></span>
+atoms, or whether other elements are also transformed under the
+intensity of the agencies at work in the universe, is of course a
+question as yet unsolved.</p>
+
+<div class="sidenote">Radio-active Products in the Earth's Crust</div>
+
+<p>The presence of uranium and thorium widely distributed throughout the
+crust of the earth would lead to the conclusion that their
+disintegration products would be found there also. Various rocks of
+igneous origin have been examined revealing from 4.78 &times; 10<sup>-12</sup> to
+0.31 &times; 10<sup>-12</sup> grams of radium per gram of the rock. Aqueous rocks
+have shown a lesser amount, ranging from 2.92 &times; 10<sup>-12</sup> to 0.86
+&times; 10<sup>-12</sup> grams. As the soil is formed by the decomposition of
+these rocks, radium is present in varying amounts in all kinds of
+soil.</p>
+
+<div class="sidenote">Presence in Air and Soil Waters</div>
+
+<p>As radium is transformed into the gaseous emanation, this will escape
+wherever the soil is not enclosed. For instance, a larger amount of
+radio-activity is found in the soil of caves and cellars than in open
+soils. If an iron pipe is sunk into a soil and the air of the soil
+sucked up into a large electroscope, the latter instrument will show
+the effect of the rays emitted and will measure the degree of
+activity. Also the interior of the pipe will receive a deposit of the
+radio-active material and will show appreciable radio-activity after
+being removed from the soil.</p>
+
+<p>This radium emanation is dissolved in the soil waters, wells, springs,
+and rivers, rendering them more or less radio-active, and sometimes
+the muddy deposit at the bottom of a spring shows decided radio-activity.</p>
+
+<p><span class='pagenum'><a name="Page_51" id="Page_51">[Pg 51]</a></span>
+The emanation also escapes into the air so that many observations made
+in various places show that the radium emanation is everywhere present
+in the atmosphere. Neither summer nor winter seems to affect this
+emanation, and it extends certainly to a height of two or three miles.
+Rain, falling through the air, dissolves some of the emanation, so
+that it may be found in freshly-fallen rain water and also in
+freshly-fallen snow. Radio-active deposits are found upon electrically
+charged wires exposed near the earth's surface.</p>
+
+<p>As helium is the resulting product of the alpha particles emitted by
+the emanation and other radio-active bodies, it is found in the soil
+air, soil waters, and atmosphere.</p>
+
+<p>Average measurements of the radio-activity of the atmosphere have led
+to the calculation that about one gram of radium per square kilometer
+of the earth's surface is requisite to keep up the supply of the
+emanation.</p>
+
+<p>A number of estimates have been given as to the heat produced by the
+radio-active transformations going on in the material of this planet.
+Actual data are scarce and mere assumptions unsatisfactory, so little
+that is worth while can be deduced. It is possible that this source of
+heat may have an appreciable effect upon or serve to balance the
+earth's rate of cooling.</p>
+
+<div class="sidenote">Cosmical Radio-activity</div>
+
+<p>Meteorites of iron coming from other celestial bodies have not
+shown the presence of radium. Aerolites or stone meteorites
+have been found to contain as much as similar terrestrial rock.
+<span class='pagenum'><a name="Page_52" id="Page_52">[Pg 52]</a></span>
+Since the sun contains helium and some stars show its presence as
+predominating, this suggests the presence of radio-active matter in
+these bodies. In addition, the spectral lines of uranium, radium, and
+the radium emanation have been reported as being found in the sun's
+spectrum and also in the new star, <i>Nova Geminorum 2</i>. These
+observations await further investigation and confirmation. So far as
+the sun's chromosphere is concerned, the possible amount of radium
+present would seem to be very small. If this is true, radio-active
+processes could have little to do with the sun's heat. The statement
+is made by Rutherford that indirect evidence obtained from the study
+of the aurora suggests that the sun emits rays similar in type to the
+alpha and beta rays. Such rays would be absorbed, and the gamma rays
+likewise, in passing through the earth's atmosphere and so escape
+ordinary observation. All of this is but further evidence of the unity
+of matter and of forces in the universe.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_53" id="Page_53">[Pg 53]</a></span></p>
+<h2><a name="INDEX" id="INDEX"></a>INDEX</h2>
+
+<p>
+Actinium, discovery of, <a href="#Page_6">6</a><br />
+<br />
+Activity, induced, <a href="#Page_17">17</a><br />
+<br />
+Alpha particles, effect of loss on Atomic Weight, <a href="#Page_45">45</a><br />
+<span style="margin-left: 1em;">electrical charge of, <a href="#Page_26">26</a></span><br />
+<span style="margin-left: 1em;">form helium, <a href="#Page_27">27</a></span><br />
+<span style="margin-left: 1em;">nature of, <a href="#Page_25">25</a></span><br />
+<span style="margin-left: 1em;">penetrating power of, <a href="#Page_39">39</a></span><br />
+<span style="margin-left: 1em;">position of element changed by its loss, <a href="#Page_46">46</a></span><br />
+<span style="margin-left: 1em;">recoil, <a href="#Page_39">39</a></span><br />
+<span style="margin-left: 1em;">scattering of, <a href="#Page_38">38</a></span><br />
+<span style="margin-left: 1em;">solid, <a href="#Page_26">26</a></span><br />
+<br />
+Atom, constitution of, <a href="#Page_36">36</a><br />
+<span style="margin-left: 1em;">Kelvin's, <a href="#Page_37">37</a></span><br />
+<span style="margin-left: 1em;">models of, <a href="#Page_37">37</a></span><br />
+<span style="margin-left: 1em;">Rutherford's, <a href="#Page_37">37</a></span><br />
+<br />
+Atomic number, determination of, <a href="#Page_43">43</a><br />
+<br />
+<br />
+Becquerel's experiments, <a href="#Page_2">2</a><br />
+<br />
+Beta particles, change in position of element by loss of, <a href="#Page_47">47</a><br />
+<br />
+<br />
+Chalcolite, natural and artificial, <a href="#Page_4">4</a><br />
+<br />
+Constants, table of, <a href="#Page_31">31</a><br />
+<br />
+Curie unit, <a href="#Page_22">22</a><br />
+<br />
+<br />
+Disintegration of the element, <a href="#Page_25">25</a><br />
+<br />
+Disintegration series, <a href="#Page_24">24</a><br />
+<br />
+Disintegration theory, <a href="#Page_35">35</a><br />
+<br />
+<br />
+Electroscope, <a href="#Page_12">12</a><br />
+<br />
+Equilibrium series, <a href="#Page_22">22</a><br />
+<br />
+<br />
+Helium, characteristics of, <a href="#Page_30">30</a><br />
+<span style="margin-left: 1em;">discovery of, <a href="#Page_29">29</a></span><br />
+<br />
+<br />
+Ionium, discovery of, <a href="#Page_6">6</a><br />
+<br />
+Ionization, application of electric field to, <a href="#Page_10">10</a><br />
+<span style="margin-left: 1em;">experimental confirmation, <a href="#Page_9">9</a></span><br />
+<br />
+Ionization of gases, <a href="#Page_7">7</a><br />
+<span style="margin-left: 1em;">theory of, <a href="#Page_8">8</a></span><br />
+<br />
+Ions, size and nature of, <a href="#Page_10">10</a><br />
+<br />
+Isotopes, <a href="#Page_47">47</a><br />
+<br />
+<br />
+Lead, atomic weight varies with source, <a href="#Page_45">45</a><br />
+<span style="margin-left: 1em;">radio-active, <a href="#Page_6">6</a></span><br />
+<span style="margin-left: 1em;">the end product, <a href="#Page_45">45</a></span><br />
+<br />
+Life-periods of radio-active bodies, <a href="#Page_21">21</a><br />
+<br />
+<br />
+Periodic system, <a href="#Page_41">41</a><br />
+<span style="margin-left: 1em;">basis of, <a href="#Page_42">42</a></span><br />
+<br />
+Polonium, discovery of, <a href="#Page_4">4</a><br />
+<br />
+Positive nucleus, influence of, <a href="#Page_43">43</a><br />
+<br />
+Potassium, radio-activity of, <a href="#Page_3">3</a><br />
+<br />
+<br />
+Radiations, action on phosphorescent bodies, <a href="#Page_13">13</a><br />
+<span style="margin-left: 1em;">action on photographic plates, <a href="#Page_11">11</a></span><br />
+<span style="margin-left: 1em;">discharge electrified bodies, <a href="#Page_12">12</a></span><br />
+<span style="margin-left: 1em;">magnetic deflection of, <a href="#Page_14">14</a></span><br />
+<span style="margin-left: 1em;">measurements of, <a href="#Page_15">15</a></span><br />
+<span style="margin-left: 1em;">penetrating power of, <a href="#Page_13">13</a>, <a href="#Page_15">15</a></span><br />
+<span class='pagenum'><a name="Page_54" id="Page_54">[Pg 54]</a></span><br />
+Radio-active bodies, elemental nature of, <a href="#Page_20">20</a><br />
+<span style="margin-left: 1em;">examination of, <a href="#Page_20">20</a></span><br />
+<span style="margin-left: 1em;">life periods of, <a href="#Page_21">21</a></span><br />
+<br />
+Radio-activity, an atomic property, <a href="#Page_3">3</a><br />
+<span style="margin-left: 1em;">cosmical, <a href="#Page_51">51</a></span><br />
+<span style="margin-left: 1em;">influence on chemical theory, <a href="#Page_41">41</a></span><br />
+<span style="margin-left: 1em;">products in atmosphere, <a href="#Page_51">51</a></span><br />
+<span style="margin-left: 1em;">products in earth's crust, <a href="#Page_50">50</a></span><br />
+<span style="margin-left: 1em;">products in soil waters, <a href="#Page_50">50</a></span><br />
+<br />
+Radium, action on organic matter, etc., <a href="#Page_33">33</a><br />
+<span style="margin-left: 1em;">amount in pitchblende, <a href="#Page_5">5</a></span><br />
+<span style="margin-left: 1em;">discovery of, <a href="#Page_5">5</a></span><br />
+<span style="margin-left: 1em;">emanation, <a href="#Page_22">22</a></span><br />
+<span style="margin-left: 1em;">energy evolved by, <a href="#Page_34">34</a></span><br />
+<span style="margin-left: 1em;">properties of, <a href="#Page_5">5</a>, <a href="#Page_32">32</a></span><br />
+<br />
+Rays, alpha, <a href="#Page_15">15</a>, <a href="#Page_16">16</a>, <a href="#Page_26">26</a><br />
+<span style="margin-left: 1em;">beta, <a href="#Page_15">15</a>, <a href="#Page_16">16</a></span><br />
+<span style="margin-left: 1em;">gamma, <a href="#Page_15">15</a>, <a href="#Page_16">16</a></span><br />
+<span style="margin-left: 1em;">identification of, <a href="#Page_16">16</a>, <a href="#Page_25">25</a></span><br />
+<span style="margin-left: 1em;">magnetic deflection of, <a href="#Page_14">14</a></span><br />
+<span style="margin-left: 1em;">photographing track of, <a href="#Page_10">10</a></span><br />
+<span style="margin-left: 1em;">types of, <a href="#Page_14">14</a></span><br />
+<br />
+Rubidium, radio-activity of, <a href="#Page_3">3</a><br />
+<br />
+<br />
+Spinthariscope, <a href="#Page_13">13</a><br />
+<br />
+Stopping power of substances, <a href="#Page_39">39</a><br />
+<br />
+<br />
+Thorium X, discovery of, <a href="#Page_18">18</a>, <a href="#Page_21">21</a><br />
+<br />
+<br />
+Uranium atom, disintegration of, <a href="#Page_36">36</a><br />
+<br />
+Uranium minerals, radio-activity of, <a href="#Page_3">3</a><br />
+<br />
+Uranium X, discovery of, <a href="#Page_17">17</a>, <a href="#Page_21">21</a>, <a href="#Page_23">23</a><br />
+<br />
+<br />
+X-ray spectra, <a href="#Page_44">44</a><br />
+<br />
+<br />
+Zinc sulphide screen, <a href="#Page_13">13</a><br />
+</p>
+
+
+
+<hr style="width: 65%;" />
+<h2>TRANSCRIBER'S NOTES</h2>
+
+
+<p>Images have been moved from the middle of a paragraph to the
+closest paragraph break. Other than that, the original text has been
+reproduced as such.</p>
+
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of A Brief Account of Radio-activity, by
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+</body>
+</html>
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+The Project Gutenberg EBook of A Brief Account of Radio-activity, by
+Francis Preston Venable
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever. You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Brief Account of Radio-activity
+
+Author: Francis Preston Venable
+
+Release Date: May 9, 2010 [EBook #32307]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK A BRIEF ACCOUNT OF RADIO-ACTIVITY ***
+
+
+
+
+Produced by The Online Distributed Proofreading Team at
+http://www.pgdp.net (This file was produced from images
+generously made available by The Internet Archive/American
+Libraries.)
+
+
+
+
+
+
+
+
+
+ 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.
+
+
+
+
+CHAPTER II
+
+PROPERTIES OF THE RADIATIONS
+
+
+The activity of these radio-active bodies consists in the emission of
+certain radiations which may be separated into rays and studied
+through the phenomena which they cause.
+
+
+Ionization of Gases
+
+One of these phenomena is the power of forming ions or carriers of
+electricity by the passage of the rays through a gas, thus ionizing
+the gas. The details of an experiment will serve to make the meaning
+of this ionization clear.
+
+ [Illustration: FIG. 1.--IONIZATION OF GASES.]
+
+When this apparatus is set up a minute current will be observed
+without the introduction of any radio-active matter. This, as
+Rutherford says, has been found due mainly to a slight natural
+radio-activity of the matter composing the plates. If radio-active
+matter is spread on plate _A_, which is connected with one pole of a
+grounded battery, and if plate _B_ is connected with an electrometer
+which is also connected with the earth, a current is caused which
+increases rapidly with the difference of potential between the plates,
+then more slowly until a value is reached that changes only slightly
+with a larger increase in the voltage.
+
+According to the theory of ionization, the radiation produces ions at
+a constant rate. The ions carrying a positive charge are attracted to
+plate _B_, while those negatively charged are attracted to plate _A_,
+thus causing a current. These ions will recombine and neutralize their
+charges if the opportunity is given. The number, therefore, increases
+to a point at which the ions produced balance the number recombining.
+
+When an electric field is produced between the plates, the velocity of
+the ions between the plates is increased in proportion to the strength
+of the electric field. In a weak field the ions travel so slowly that
+most of them recombine on the way and consequently the observed
+current is very small. On increasing the voltage the speed of the ions
+is increased, fewer recombine, the current increases, and, when the
+condition for recombination is practically removed, it will have a
+maximum value. This maximum current is called the saturation current
+and the value of the potential difference required to give this
+maximum current is called the saturation P.D. or saturation voltage.
+
+The picture, then, is this. The radiations separate the components of
+the gas into ions, or carriers of electricity, half of which are
+charged negatively and half positively. In the electric field those
+negatively charged seek the positive plate and those positively
+charged seek the negative plate. If time is given, these ions meet and
+recombine, their charges are neutralized, and there is no current.
+
+
+Experimental Confirmation
+
+This theory of the ionization of gases has been most interestingly
+confirmed by direct experiment. For instance, the ions may form nuclei
+for the condensation of water, and in this way the existence of the
+separate ions in the gas may be shown and the number present actually
+counted.
+
+When air saturated with water vapor is allowed to expand suddenly, the
+water present forms a mist of small globules. There are always small
+dust particles in air and around these as nuclei the drops are formed.
+These drops will settle and thus by repeated small expansions all dust
+nuclei may be removed and no mist or cloud will be formed by further
+expansions.
+
+If now the radiation from a radio-active body be introduced into the
+condensation vessel, a new cloud is produced in which the water drops
+are finer and more numerous according to the intensity of the rays. On
+passing a strong beam of light through the condensation chamber, the
+drops can readily be seen. These drops form on the ions produced by
+the radiation.
+
+
+Application of Electric Field
+
+If the condensation chamber has two parallel plates for the
+application of an electric field like that already described, the
+ions will be carried at once to the electrodes and disappear. The
+rapidity of this action depends upon the strength of the electric
+field and experiment shows that the stronger the field the smaller the
+number of condensation drops formed. If there is no electric field, a
+cloud can be produced some time after the shutting off of the source
+of radiation, showing that time is required for the recombination of
+the ions.
+
+
+Size and Nature of Ions
+
+If the drops are counted (there being special methods for this) and
+the total current carried accurately measured, then the charge carried
+by each ion may be calculated. This has been determined. The mass of
+an ion compared with the mass of the molecules of gas in which it was
+produced can also be approximately estimated. In the study of these
+ions the view has been held that the charged ion attracted to itself a
+cluster of molecules which surrounded the charged nucleus and traveled
+with it. It is roughly estimated that about thirty molecules of the
+gas cluster around each charged ion.
+
+
+Photographing the Track of the Ray
+
+Utilizing the fact that these ions with their clusters of molecules
+form nuclei for the condensation of water vapor, C. T. R. Wilson has
+by instantaneous photography been able to photograph the track of an
+ionizing ray through air. The number of the ions produced, and hence
+the number of drops, is so great that the trail is shown as a
+continuous line. In the copy of this photograph it will be seen that
+at some distance from its source the straight trail is slightly but
+abruptly bent. Near the end of its course there is another abrupt and
+much sharper bend. These bends show where the ionizing ray, in this
+case an alpha particle, has been deflected by more or less direct
+collision with an atom. These collisions and the final disappearance
+of the ray will be discussed later.
+
+ [Illustration: FIG. 2.--PHOTOGRAPH OF THE TRACK OF AN
+ IONIZING RAY.]
+
+
+Action of Radiations on Photographic Plates
+
+Taking up now other means of examining these radiations, it is well to
+consider their action upon a photographic or sensitive plate. It will
+be recalled that this was the method by which their existence was
+originally detected. To illustrate the method, the following account
+of how one such photograph was taken may be given.
+
+The plate was wrapped in two thicknesses of black paper. The objects
+were placed upon this and the radio-active ore, separated by a board
+one inch thick, was placed above. The exposure lasted five days. The
+action is much less rapid and the result not so clearly defined as in
+the case of photographs taken by _X_ rays. Of course, the removal of
+the board and the use of more concentrated preparations of radium
+would give quicker and better results. The method, however, on
+account of time consumed and lack of definition is ill adapted to
+accurate work.
+
+ [Illustration: FIG. 3--PHOTOGRAPH OF VARIOUS OBJECTS TAKEN
+ BY MEANS OF PITCHBLENDE]
+
+
+Discharge of Electrified Bodies
+
+The radiations from radio-active bodies can discharge both positively
+and negatively electrified bodies by making the air surrounding them a
+conductor of electricity. To demonstrate this, use is made of an
+electroscope. If the hinged leaf of such an instrument be electrically
+charged and a radio-active body be brought into its neighborhood, the
+electricity will be discharged and the leaf return to its original
+position. The rapidity of this discharge is used to measure the degree
+of activity of the body giving off the radiation.
+
+ [Illustration: FIG. 4.--GOLD-LEAF ELECTROSCOPE.
+
+ The gold-leaf _L_ is attached to a flat rod _R_ and is
+ insulated inside the vessel by a piece of amber _S_ supported
+ from the rod _P_. The system is charged by a bent rod _CC'_
+ passing through an ebonite stopper. After charging, it is
+ removed from contact with the gold-leaf system. The rods _P_
+ and _C_ and the cylinder are then connected with the earth.]
+
+
+Scintillations on Phosphorescent Bodies
+
+It was found by Crookes that a screen covered with phosphorescent zinc
+sulphide was brightly lighted up when exposed to the radiations. This
+is due to the bombardment of the zinc sulphide by a type of ray called
+the alpha ray. Under a magnifying glass this light is seen to be made
+up of a number of scintillating points of light and is not continuous,
+each scintillation being of very short duration. By proper subdivision
+of the field under the lens, the number of scintillations can be
+counted with close accuracy.
+
+A simple form of apparatus called the spinthariscope has been devised
+to show these scintillations. A zinc sulphide screen is fixed in one
+end of a small tube and a plate carrying a trace of radium is placed
+very close to it. The scintillations can be observed through an
+adjustable lens at the other end of the tube. Outer light should be
+cut off, as in a dark room. The screen then appears to be covered with
+brilliant flashes of light. Other phosphorescent substances, such as
+barium platino-cyanide, may be substituted for the zinc sulphide, but
+they do not answer so well.
+
+
+Penetrating Power
+
+By penetrating power is meant the power exhibited by the rays of
+passing through solids of different thicknesses and gases of various
+depths. This power varies with different radiations and with the
+nature of the solid or gas. For instance, a sheet of metallic foil may
+be used and the effect of aluminum will differ from that of gold and
+the different rays vary in penetrating power. In the case of gases
+air will differ from hydrogen, and it is noticed that certain rays
+disappear after penetrating a short distance, while others can
+penetrate further before being lost.
+
+
+Magnetic Deflection
+
+If the radiations are subjected to the action of a strong magnetic
+field, it is found that part of them are much deflected in the
+magnetic field and describe circular orbits, part are only slightly
+deflected and in the opposite direction from the first, and the
+remaining rays are entirely unaffected.
+
+ [Illustration: FIG. 5.--SHOWING MAGNETIC DEFLECTION OF
+ [alpha], [beta], AND [gamma] RAYS.]
+
+
+Three Types of Rays
+
+By the use of these methods of investigation it is learned that the
+radiations consist of three types of rays. These have been named the
+alpha, beta, and gamma rays, respectively. Some radio-active bodies
+emit all three types, some two, and some only one. The distinguishing
+characteristic of these types of rays may be summed up as follows:
+
+
+Alpha Rays
+
+The alpha rays have a positive electrical charge and a comparatively
+low penetrating power. They are slightly deflected in strong magnetic
+and electric fields. They have a great ionizing power and a velocity
+about one-fifteenth that of light.
+
+
+Beta Rays
+
+The beta rays are negatively charged and have a greater penetrating
+power than the alpha rays. They show a strong deflection in magnetic
+and electric fields, have less ionizing power than the alpha rays, and
+a velocity of the same order as light.
+
+
+Gamma Rays
+
+The gamma rays are very penetrating and are not deflected in the
+magnetic or electric fields. They have the least ionizing power and a
+very great velocity.
+
+The penetrating power of each type is complex and varies with the
+source, so the statements given are but generalizations. The alpha
+rays are projected particles which lose energy in penetrating matter.
+As to the power of ionizing gases, if that for the [alpha] rays is
+taken as 10,000, then the [beta] rays would be approximately 100 and
+the [gamma] rays 1.
+
+
+Measurement of Radiations
+
+The rays are examined and measured in several ways:
+
+1. By their action on the sensitive photographic plates. The use
+of this method is laborious, consumes time, and for comparative
+measurements of intensity is uncertain as to effect.
+
+2. By electrical methods, using electroscopes, quadrant
+electrometers, etc. These are the methods most used.
+
+3. By exposure to magnetic and electric fields, noting extent and
+direction of deflection.
+
+4. By their relative absorption by solids and gases.
+
+5. By the scintillations on a zinc sulphide screen.
+
+
+Identification of the Rays
+
+The alpha rays have been identified as similar to the so-called canal
+rays. These were first observed in the study of the _X_ rays. When an
+electrical discharge is passed through a vacuum tube with a cathode
+having holes in it, luminous streams pass through the holes toward the
+side away from the anode and the general direction of the stream. They
+travel in straight lines and render certain substances phosphorescent.
+These rays are slightly deflected by a magnetic field and in an
+opposite direction from that taken by the cathode rays in their
+deflection. The rays seem to be positive ions with masses never less
+than that of the hydrogen atom. Their source is uncertain, but they
+may be derived from the electrodes.
+
+The beta rays are identical in type with the cathode rays and are
+negative electrons.
+
+The gamma rays are analogous to the _X_ rays and are of the order of
+light. They are in general considerably more penetrating than _X_
+rays. For example, the gamma rays sent out by 30 milligrams of radium
+can be detected by an electroscope after passing through 30
+centimeters of iron, a much greater thickness than can be penetrated
+by the ordinary _X_ rays.
+
+
+
+
+CHAPTER III
+
+CHANGES IN RADIO-ACTIVE BODIES
+
+
+Is Radio-activity a Permanent Property?
+
+Is this power of emitting radiations a permanent property or is it
+lost with the passage of time? The first investigations of the
+activity of uranium and thorium showed no loss of intensity at the end
+of several years, and radium also seemed to show no decrease in its
+enormous activity. Polonium, however, was found to lose most of its
+activity in a year, and later it appeared that some radio-active
+substances lost most of their activity in the course of a few minutes
+or hours.
+
+
+Induced Activity
+
+A phenomenon called induced or secondary radio-activity was also
+observed. Thus a metal plate or wire exposed to the action of thorium
+oxide for some hours became itself active. This induced activity was
+not permanent but decreased to half its value in about eleven hours
+and practically disappeared within a week. Similar phenomena were
+observed when radium was substituted for thorium.
+
+
+Discovery of Uranium X
+
+In 1900 Crookes precipitated a solution of an active uranium salt with
+ammonium carbonate. The precipitate was dissolved so far as possible
+in an excess of the reagent, leaving an insoluble residue. This
+residue was many hundred times more active, weight for weight, than
+the original salt, and the solution containing the salt was
+practically inactive. At the end of a year the uranium salt had
+regained its activity while the residue had become inactive.
+
+Another method of obtaining the same result is to dissolve
+crystallized uranium nitrate in ether. Two layers of solution are
+formed, one ether and the other water coming from the water of
+crystallization. The aqueous layer is active, while the water layer is
+inactive. Similarly, by adding barium chloride solution to a solution
+of a salt of uranium and then precipitating the barium as sulphate,
+the activity is transferred to this precipitate. These experiments
+give proof of the formation and separation of a radio-active body by
+ordinary chemical operations.
+
+So, too, in the case of thorium salts a substance can be obtained by
+means of ammonium hydroxide which is several thousand times more
+active than an equal weight of the original salt. After standing a
+month, the separated material has lost its activity and the thorium
+salt has regained it. Here, again, there is the formation, separation,
+and loss of a radio-active body.
+
+
+Conclusions Drawn
+
+Now, these are ordinary chemical processes for the separation of
+distinct chemical individuals. The results, therefore, lead naturally
+to the conclusions: (1) it would seem that uranium and thorium are
+themselves inactive and the activity is due to some other substance
+formed by these elements; (2) this active substance is produced by
+some transformation in those elements, for on standing the activity
+is regained. This latter conclusion is startling, for it indicates a
+change in the atom which, up to the time of this discovery, was deemed
+unchangeable under the influence of such physical and chemical changes
+as were known to us.
+
+
+Search for New Radio-active Bodies
+
+The search for new radio-active bodies and the study of their
+characteristics has been systematically and successfully carried on.
+The bodies obtained in the above experiments were named uranium _X_
+and thorium _X_, respectively. Further, it became clear from the
+investigation of uranium minerals that radium, polonium, actinium, and
+ionium originated from uranium. From thorium minerals a body was
+separated called mesothorium, which was analogous to radium. Both
+thorium and radium were found to give off a radio-active gas. The
+first lost half of its activity in less than one minute. The second
+was more stable and lost half of its activity in about four days. The
+name radium emanation was given to the latter and it was found
+chemically and physically to belong to the class of monatomic or noble
+gases, such as helium, argon, neon, etc., which had been discovered by
+Ramsay. In some cases the chemical action was determined and these new
+bodies were found analogous to well-known elements, as radium to
+barium, polonium to bismuth. The physical properties were investigated
+and, where possible, spectra were mapped and atomic weights
+determined.
+
+It is clear, therefore, that these bodies are elemental in character
+and as such are made up of distinct, similar atoms, just as the
+commonly recognized elements are believed to be. In this way more than
+thirty new elements have been added to the list. These new elements
+are called radio-active elements, but it is an open question whether
+all atoms do not possess this property in greater or less degree.
+Certainly, it is possessed in varying degree by four of the old
+elements widely separated in the Periodic System, namely, uranium,
+thorium, rubidium, and potassium. The last two, while feebly active
+themselves, do not form any secondary radio-active substance so far as
+is known. Only two of the elements, then, can definitely be said to go
+through these transformations. It is just possible that radio-activity
+may be found to be a common property of all atoms and of all matter.
+
+
+Methods of Investigation
+
+It is important to know how these new bodies were discovered and
+distinguished from one another. Two properties are relied upon. One is
+the nature of the rays emitted and the other is the duration of the
+activity. Of course, knowledge of the physical and chemical properties
+is also of great importance whenever obtainable.
+
+
+Nature of the Radiations
+
+The nature of the radiation is a distinguishing characteristic, though
+similarity here does not prove identity of substances. Some emit
+[alpha] rays only, some emit [beta] rays, some emit two of the
+possible rays, as for instance, [beta] and [gamma], and some emit all
+three. The rays may also differ in the velocity with which they are
+emitted by different radio-active substances. Thus, in the case of one
+substance the [alpha] rays may have a slightly greater or less
+penetrating power than those emitted by some other substance, and this
+may be true also of the other rays.
+
+
+Life Periods
+
+The duration of the activity is called the life period. This is
+absolutely fixed for each body and furnishes the most important mode
+of differentiating among them. It measures the relative stability and
+is the time which must elapse before their activity is lost and they,
+changing into something else, entirely disappear. The measure usually
+adopted is the half-value period. Two hypotheses are made use of:
+
+1. That there is a constant production of fresh radio-active matter by
+the radio-active body.
+
+2. That the activity of the matter so formed decreases according to an
+exponential law with the time from the moment of its formation.
+
+These hypotheses agree with the experimental results. The decrease and
+rise of activity, for example, of uranium and uranium _X_, and also of
+thorium and thorium _X_, have been measured, plotted, and the
+equations worked out.
+
+Manifestly, a state of equilibrium will be reached when the rate of
+loss of activity of the matter already produced is balanced by the
+activity of the new matter produced. This equilibrium and the
+knowledge of the rate of decrease in general will have little value if
+this rate, like chemical changes, is subject to the influence of
+chemical and physical conditions. The rate of decrease has been found
+to be unaltered by any known chemical or physical agency. For
+instance, neither the highest temperatures applicable nor the cold of
+liquid air have any appreciable effect.
+
+
+Equilibrium Series
+
+In order to measure the disintegration of a radio-active body in units
+of time so that the rate may be comparable with that of other
+radio-active bodies, the relation between the amounts under
+consideration must be a definite one. For this purpose equal weights
+of the bodies are not taken, but use is made of the amounts which are
+in equilibrium with a fixed amount of the parent substance.
+
+One gram of radium has been settled upon as the standard for that
+series and a unit known as the "curie" has been adopted to express the
+equilibrium quantity of radium emanation. Thus, a curie of radium
+emanation (or niton) is the weight (or, as this is a gas, the volume
+at standard pressure and temperature) of the emanation in equilibrium
+with one gram of radium. This, by calculation and experiment, is found
+to be 0.63 cubic millimeter. When this amount has been produced by one
+gram of radium, the formation and decay will exactly balance one
+another. This is, therefore, one curie of emanation.
+
+The measurement of the rate of decay is difficult but can be carried
+out with great accuracy, even down to seconds, in the case of certain
+short-lived bodies. Errors crept in at first from the failure to
+completely separate the substances produced in the series, and
+sometimes because of the simultaneous production of two substances.
+
+As stated, the decay follows an exponential law. The time required for
+the decay of activity to half-value does not mean, therefore, that
+there will be total decay in twice that time. Thus the half-value
+period for uranium _X_ is about 22 days. The period for complete decay
+is about 160 days. This half-value period corresponds to the
+half-value recovery period of uranium, which is also 22 days.
+
+These were the earlier figures obtained for uranium _X_ and they
+illustrate some of the difficulties surrounding such determinations.
+It was found later that the body examined as uranium _X_ was really a
+constant mixture and of course the decay and recovery periods were
+also composite. It required later and very skilful work to separate
+them into the bodies indicated in the disintegration series.
+
+The half-value period for thorium _X_ is much shorter, namely, a
+little over four days, and this is also the recovery period for
+thorium _X_. The plotted decay and recovery curves will intersect at
+this point.
+
+The consecutive disintegration series, with the half-value periods,
+for the uranium and thorium series as given by Soddy are seen in the
+following tables. They are probably subject to some changes on further
+and more accurate determination. The nature of the rays emitted is
+also given.
+
+ [Illustration:
+
+ Uranium (8 x 10^9 years) 238.5 -> [alpha]
+ -> [alpha]
+ \/
+ Uranium X (35.5 days) (230.5) -> [beta]&[gamma]
+ -> ([beta])
+ \/
+
+ \/
+
+ \/
+
+ Ionium (5 x 10^4 to 10^6 years) (230.5) -> [alpha]
+ \/
+ Radium (2,500 years) 226.4 -> [alpha]
+ \/
+ Emanation (5.57 days) (222.4) -> [alpha]
+ \/
+ Radium A (4.3 minutes) (218.4) -> [alpha]
+ \/
+ Radium B (38.5 minutes) (214.4) -> ([beta])
+ \/
+ Radium C_{1} (28.1 minutes) { (214.4) -> [alpha]
+ { -> [beta]&[gamma]
+ { \/
+ Radium C_{2} (1.9 minutes) { (210.4) -> [beta]&[gamma]
+ \/
+ Radium D (24 years?) (210.4) -> ([beta])
+ \/
+ Radium E (7.25 days) (210.4) -> [beta]&[gamma]
+ \/
+ Radium F (Polonium 202 days) (210.4) -> [alpha]
+ \/
+ Radium G (probably lead) (206.8)
+
+
+ Actinium (?)
+ \/
+ Radio-Actinium (28.1 days) -> [alpha]
+ -> ([beta])
+ \/
+ Actinium X (15 days) -> [alpha]
+ \/
+ Emanation (5.6 seconds) -> [alpha]
+ \/
+ Actinium A (0.0029 second) -> [alpha]
+ \/
+ Actinium B (52.1 minutes) -> ([beta])
+ \/
+ Actinium C_{1} (3.10 mins.) { -> [alpha]
+ { \/
+ Actinium C_{2} (?) { -> [alpha]
+ \/
+ Actinium D (7.4 minutes) -> [beta]&[gamma]
+ \/
+ Actinium E (unknown)
+
+
+ Thorium (4 x 10^{10} years?) 232.4 -> [alpha](?)
+ \/
+ Mesothorium_{1} (7.9 years)
+ \/
+ Mesothorium_{2} (8.9 hours) -> [beta]&[gamma]
+ \/
+ Radiothorium (2.91 years?) -> [alpha]
+ \/
+ Thorium X (5.35 days) -> [alpha]
+ \/
+ Emanation (76 seconds) -> [alpha]
+ \/
+ Thorium A (0.203 second) -> [alpha]
+ \/
+ Thorium B (15.3 hours) -> ([beta])
+ \/
+ Thorium C_{1} (79 minutes) { -> [alpha]
+ { \/
+ Thorium C_{2} (?) { -> [alpha]
+ \/
+ Thorium D (4.5 minutes) -> [beta]&[gamma]
+ \/
+ Thorium E (unknown)
+
+ FIG. 6.--DISINTEGRATION SERIES FOR URANIUM, ACTINIUM, AND
+ THORIUM, AS GIVEN BY SODDY.]
+
+
+
+
+CHAPTER IV
+
+NATURE OF THE ALPHA PARTICLE
+
+
+Disintegration of the Elements
+
+The remarkable disintegrations related in the last chapter, in which
+the heaviest known elementary atom--that of uranium (at. wt. 238)--is
+by successive stages changed into others of lower atomic weight,
+afford a clue to the nature of the atom and to that goal of the
+chemist, the final constitution of matter. The composite nature of the
+atom and some sort of interrelation of the elements had previously
+been made apparent from a study of the Periodic System and data
+gathered still earlier, but all attempts at working out a so-called
+genesis of the elements had proved vague and unsatisfactory.
+
+
+Identification of the Rays
+
+To get an understanding of the disintegration occurring in
+radio-active substances, the nature of the rays produced must be
+known. These rays are the cause of the activity and their emission
+accompanies the changes or disintegration. They have for the sake of
+convenience been called the alpha, beta, and gamma rays. The gamma
+rays have been identified with the _X_ rays discovered by Roentgen and
+are a form of energy analogous to light. The beta rays are particles
+of negative electricity or electrons. With these, then, we have some
+degree of familiarity. But what are the alpha rays? An answer to this
+question should make clearer the character of the changes taking
+place, and should give some insight into the composition and mechanism
+of the atom.
+
+
+The Alpha Rays
+
+It has already been stated that these alpha rays are similar or
+analogous to the canal rays, but this advances the matter very little,
+as the nature of these canal rays has not been fully determined. The
+full identity with them, if proved, should have an important
+theoretical bearing.
+
+
+Alpha Rays Consist of Solid Particles
+
+In the first place, these alpha rays have been found to be made up of
+solid particles, that is, of what we are accustomed to call matter.
+Since it has become more and more difficult to draw a clear
+distinction between matter and energy, it would perhaps be better to
+say that these particles appear to have some of the properties
+hitherto attributed solely to matter. The best evidence that these
+particles are of atomic mass is furnished by their deflection in
+electric and magnetic fields.
+
+
+Electrical Charge
+
+It is not of first importance to discuss this or other proofs of the
+material nature of these particles. That they carry a charge of
+positive electricity is, however, a fact of very great import. The
+value of this charge has been carefully determined by a number of
+investigators working with different sources of the alpha particles
+and has been found to be 9.3 x 10^{-10} electrostatic units
+(.000,000,000,93 e.s.). From the consideration of the charge upon an
+electron previously obtained by J. J. Thomson and others, it was
+concluded that the alpha particle carried two unit positive charges;
+the fundamental unit charge, therefore, is half this value, or
+4.65 x 10^{-10} e.s.
+
+
+Helium Formed from Alpha Particles
+
+To determine the nature of the alpha particle a crucial experiment was
+carried out by Rutherford and Royds, which was described as follows:
+
+ [Illustration: FIG. 7.--APPARATUS USED IN EXPERIMENT BY
+ RUTHERFORD AND ROYDS.]
+
+A large quantity of radium emanation was compressed into a fine glass
+tube _A_, about 1.5 cm. long. This tube, which was sealed to a larger
+capillary tube _B_, was sufficiently thin to allow the alpha particles
+from the emanation and its products to pass through, but sufficiently
+thick to withstand atmospheric pressure. The thickness of the glass
+wall was in most cases less than .01 mm. On introducing the emanation
+into the tube, the escape of the alpha particles from the emanation
+was clearly seen by the scintillations produced at some distance on a
+zinc sulphide screen. After this test the glass tube _A_ was
+surrounded by a glass tube _T_ and a small spectrum tube _V_ attached
+to it. The tube _T_ was exhausted to a charcoal vacuum. By means of
+the mercury column _H_, the gases in the tube _T_ could at any time be
+compressed into the spectrum tube _V_ and the nature of the gases
+which had been produced determined spectroscopically. It was found
+that two days after the introduction of the emanation into _A_ the
+spectrum showed the yellow line of helium, and after six days the
+whole helium spectrum was observed. In order to be certain that the
+helium, coming possibly from some other source, had not diffused
+through the thin walls of the tube _A_, the emanation was pumped out
+and helium substituted. No trace of helium could be observed in the
+vacuum tube after several days, showing that the helium observed in
+the first experiment must have originated from the alpha particles
+which had been propelled through the thin glass tube into the outer
+tube.
+
+Most of the alpha particles are propelled with such force that they
+penetrate some distance into the walls of the outer tube and some of
+these gradually diffuse out into the exhausted space. The presence of
+helium in the spectrum tube can be detected after a shorter interval
+if a thin cylinder of lead is placed over the emanation tube, since
+the particles fired into the lead diffuse out more rapidly than from
+glass.
+
+A still more definite proof of the identity of the alpha particle with
+the helium atom was obtained by removing the outer glass tube _T_ and
+placing a cylinder of lead over the emanation tube in the open air.
+Helium was always detected in the lead after it had remained several
+hours over the thin tube containing a large quantity of the emanation.
+In order to test for the presence of helium in the lead, the gases
+present were released by melting the lead in a closed vessel. There
+can thus be no doubt that the alpha particle becomes a helium atom
+when its positive charge is neutralized.
+
+Thus the chemist was afforded the experience of the building up of at
+least one element under his observation, and both the analysis and
+synthesis of matter have been revealed through the discoveries of
+radio-activity.
+
+
+Discovery of Helium
+
+It is of interest at this point to learn something of the history of
+helium and its occurrence. In 1868 there was discovered by Janssen and
+Lockyer a bright yellow line in the spectrum of the sun's
+chromosphere. Because of its origin the name helium was given to the
+supposed new element causing it. Later it was found in the spectra of
+many of the stars, and because of its predominance in some of these
+they were called helium stars. Its existence on our planet was not
+detected for nearly thirty years.
+
+In 1895, in connection with the discovery of argon in the atmosphere,
+a search was made to see if the latter element could be obtained from
+mineral sources. In analyzing certain uranium minerals Hillebrand had
+found considerable quantities of a gas which he took to be a peculiar
+form of nitrogen. Ramsay made a further examination of the gas coming
+from these minerals and the spectroscope revealed the yellow line of
+helium, thus at last proving the presence of this element on the
+earth. It is known now to be present in thorium minerals, in the
+waters of radio-active wells, and in minute amounts in the atmosphere.
+Its occurrence in every case, in the light of the experiment described
+above, would seem to be due to the presence of radio-active changes.
+
+
+Characteristics of Helium
+
+Helium, on account of its chemical inactivity and physical properties,
+is classed along with argon, neon, krypton, and xenon in the zero
+group of the Periodic System, and forms with them the monatomic, inert
+gases. In this class are now placed also the three radio-active gases,
+emanating respectively from radium, thorium, and actinium. These are
+generally known as radium emanation, thorium emanation, and actinium
+emanation. The first mentioned was once called niton. Emanium was the
+name originally proposed by Giesel for the body now known as actinium.
+
+The calculated rate of production of helium in the series in
+equilibrium with one gram of radium is 158 cubic millimeters per year.
+This corresponds quite well with the experimental results.
+
+
+Table of Constants
+
+Some of the more important atomic and radio-active constants are given
+in the following table. They are recorded here to show how helpful the
+study of radio-activity has been in working out the composition of
+matter, and to give some idea of the magnitude of the numbers and the
+minuteness of the quantities dealt with.
+
+ Electric charge carried by each H atom in
+ electrolysis 4.65 x 10^{-10} e.s.[1]
+ Electric charge carried by each [alpha]
+ particle 9.3 x 10^{-10} e.s.
+ Number of atoms in 1 gram of H 6.2 x 10^{23}
+ Mass of 1 atom of H 1.6 x 10^{-24} gram
+ Number of molecules per cc. of any gas at
+ standard pressure and temperature 2.72 x 10^{19}
+ Number of [alpha] particles expelled per
+ second per gram of radium itself 3.6 x 10^{10}
+ Number of [alpha] particles expelled per
+ second per gram of radium in
+ equilibrium with its products 14.3 x 10^{10}
+
+ [1] The expression 10^{-10} means multiplying by .000,000,000,1;
+ 10^{10} means multiplying by 10,000,000,000.
+
+
+
+
+CHAPTER V
+
+THE STRUCTURE OF THE ATOM
+
+
+Properties of Radium
+
+A study of the properties of radium will aid in throwing light upon
+the question as to the building up of the atom. First to be considered
+are the usual properties which distinguish an elementary body.
+Metallic radium has been prepared by a method similar to that used in
+the preparation of barium. It is a pure white metal, melting at 700 deg.,
+and far more volatile than barium. It rapidly alters on exposure to
+the air, probably forming a nitride. It energetically decomposes water
+and the product dissolves in the water. Its atomic weight is 226.
+
+Radium forms a series of salts analogous in appearance and chemical
+action to those of barium. In the course of time they become colored,
+especially if mixed barium salts. The radiations from radium produce
+marked chemical effects in a number of substances. Carbon dioxide is
+changed into carbon, oxygen, and carbon monoxide, and the latter is
+changed into carbon and oxygen. Ammonia is dissociated into nitrogen
+and hydrogen; hydrochloric acid into chlorine and hydrogen. Oxygen is
+condensed into ozone. In general, the action upon gases appears to be
+similar to that of the silent electric discharge. Water is decomposed
+into hydrogen and oxygen. If moist radium chloride or a salt of radium
+containing water of crystallization is sealed in a glass tube, the
+gradual accumulation of hydrogen and oxygen will burst the tube.
+
+The radiations rapidly decompose organic matter with the evolution of
+gases. Thus grease from stopcocks of apparatus used with radium or
+paraffin will give off carbon dioxide. Under an intense alpha
+radiation paraffin or vaseline become hard and infusible. White
+phosphorus is changed into red.
+
+The action upon living tissue is most noteworthy, as its possible use
+as a remedial agent is dependent upon this. A small amount of a radium
+salt enclosed in a glass tube will cause a serious burn on flesh
+exposed to it. It therefore has to be handled with care and undue
+exposure to the radiations must be avoided. Cancer sacs shrivel up and
+practically disappear under its action. Whether the destruction of
+whatever causes the cancer is complete is at least open to serious
+doubt.
+
+The coagulating effect upon globulin is interesting. When two
+solutions of globulin from ox serum are taken and acetic acid added to
+one while ammonia is added to the other, the opalescence in drops of
+the former is rapidly diminished on exposure to radium, showing a more
+complete solution, whereas the latter solution rapidly turns to a
+jelly and becomes opaque, indicating a greatly decreased solubility.
+
+
+Energy Evolved by Radium
+
+The greater part of the tremendous energy evolved by radium is due to
+the emission of the alpha particles, and in comparison the beta and
+gamma rays together supply only a small fraction. This energy may be
+measured as heat. It was first observed that a radium compound
+maintained a temperature several degrees higher than that of the air
+around it. The rate of heat production was later measured by means of
+an ice calorimeter and also by noting the strength of the current
+required to raise a comparison tube of barium salt to the same
+temperature. Both methods showed that the heat produced was at the
+rate of about 135 gram calories per hour. As the emission is
+continuous, one gram of radium would therefore emit about 1,180,000
+gram calories in the course of a year. At the end of 2000 years it
+would still emit 590,000 gram calories per year. Such a production of
+energy so far surpasses all experience that it becomes almost
+inconceivable. It is futile to speak of it in terms of the heat
+evolved by the combustion of hydrogen, which is the greatest that can
+be produced by chemical means.
+
+This effect is unaltered at low temperatures, as has been tested by
+immersing a tube containing radium in liquid air. It should be stated
+that these measurements were made after the radium had reached an
+equilibrium with its products; that is, after waiting at least a month
+after its preparation. The evolution of heat from radium and the
+radio-active substances is, in a sense, a secondary effect, as it
+measures the radiant energy transformed into heat energy by the
+active matter itself and whatever surrounds it. Let us repeat,
+therefore, that the total amount of energy pent up in a single atom of
+radium almost passes our powers of conception.
+
+
+Necessity for a Disintegration Theory
+
+The facts gathered so far justify and necessitate a theory which shall
+satisfactorily explain them, and since these phenomena are not caused
+by nor subject to the influence of external agencies, they must refer
+to changes taking place within the atom--in other words, a theory of
+disintegration. In the main, these facts may be summed up as the
+emission of certain radiations from known elemental matter: the
+material alpha particles with positive charge, the beta particles or
+negative electrons, and the gamma rays analogous to _X_ rays. The
+emission of these rays results in the production of great heat. Then
+there is the law of transformations by which whole series of new
+elements are generated from the original element and maintain a
+constant equilibrium of growth and decay in the series. Lastly, we
+have the production of helium from the alpha particles.
+
+
+Disintegration Theory
+
+In explanation of these phenomena, Rutherford offered the hypothesis
+that the atoms of certain elements were unstable and subject to
+disintegration. The only elements definitely known to come under this
+description are the two having atoms of the greatest known mass,
+thorium (232) and uranium (238).
+
+The atoms of uranium, for instance, are supposed to be not permanent
+but unstable systems. According to the hypothesis, about 1 atom in
+every 10^{18} becomes unstable each second and breaks up with a
+violent explosion for so small a mass of matter. One, or possibly two
+alpha particles are expelled with great velocity. This alpha particle
+corresponds to an atom of helium with an atomic weight of 4, and its
+loss reduces the original atomic weight to 234 with the formation of a
+new element, having changed properties corresponding to the new atomic
+weight. This new element is uranium X_{1}.
+
+These new atoms are far more unstable than those of uranium, and the
+decomposition proceeds at a new rate of 1 in 10^{7} per second. So at
+a definite, measurable rate this stepwise disintegration proceeds. The
+explosions are not in all cases equally violent in going from element
+to element, nor are the results the same. Sometimes alpha particles
+alone are expelled, sometimes beta, or two of them together, as alpha
+and beta.
+
+The new product may remain with the unchanged part of the original
+matter. Thus there would be an accumulation of it until its own decay
+balances its production, resulting eventually in a state of
+equilibrium.
+
+
+Constitution of the Atom
+
+In order to explain the electrical and optical properties of matter,
+the hypothesis was made that the atom consisted of positively and
+negatively electrified particles. Later it was shown that negative
+electrons exist in all kinds of matter. Various attempts were made to
+work out a model of such an atom in which these particles were held in
+equilibrium by electrical forces. The atom of Lord Kelvin consisted of
+a uniform sphere of positive electrification throughout which a number
+of negative electrons were distributed, and J. J. Thomson has
+determined the properties of this type as to the number of particles,
+their arrangement and stability.
+
+
+Rutherford's Atom
+
+According to Rutherford, the atom of uranium may be looked upon as
+consisting of a central charge of positive electricity surrounded by a
+number of concentric rings of negative electrons in rapid motion. The
+positively charged centre is made up of a complicated system in
+movement, consisting in part of charged helium and hydrogen atoms, and
+practically the whole charge and mass of the atom is concentrated at
+the centre. The central system of the atom is from some unknown cause
+unstable, and one of the helium atoms escapes from the central mass as
+an alpha particle.
+
+There are, confessedly, difficulties connected with this conception of
+the atom which need not, however, be discussed here. Much remains to
+be learned as to the mechanics of the atom, and the hypothesis
+outlined above will probably have to be materially altered as
+knowledge grows. Perhaps it may have to be entirely abandoned in favor
+of some more satisfactory solution. Until such time it at least
+suffices as a mental picture around which the known facts group
+themselves. In this picture energy and matter lose their old-time
+distinctness of definition. Discrete subdivisions of energy are
+recognized which may be called charged particles without losing their
+significance. Some of these subdivisions charged in a certain way or
+with neutralized charge exhibit the properties of so-called matter.
+
+
+Scattering of Alpha Particles
+
+This conception of the atom would doubtless fail of much support were
+it not for certain experimental facts which lend great weight to it.
+Certain suppositions can be based on this theory mathematically
+reasoned out and tested by experiment. Predictions thus based on
+mathematical reasoning and afterward confirmed by experiment give a
+very convincing impression that truth lies at the bottom.
+
+The first of these experimental proofs comes under the head of what is
+known as the scattering of the alpha particles, a phenomenon which,
+when first observed, proved hard to explain. If an alpha particle in
+its escape from the parent atom should come within the influence of
+the supposed outer electrical field of some other atom, it should be
+deflected from its course and, the intensity of the two charges being
+known, the angle of deflection could be calculated. For instance, if
+it came to what might be called a head-on collision with the positive
+central nucleus of another atom, it would recoil if it were itself of
+lesser mass, or would propel the other forward if that were the
+lighter.
+
+The experiment is carried out by placing a thin metal foil over a
+radio-active body, as radium _C_, which expels alpha particles with a
+high velocity, and counting the number of alpha particles which are
+scattered through an angle greater than 90 deg. and so recoil toward their
+source. This has been done by a number of investigators and it has
+been found that the angle of scattering and the number of recoil
+particles depend upon the atomic weight of the metal used as foil. For
+example, if gold is used, the number of recoil atoms is one in
+something less than 8,000.
+
+Taking the atomic weight of gold into consideration, Rutherford
+calculated mathematically that this was about the number which should
+be driven backward. But he went further and calculated also the number
+which should be returned by aluminum, which has an atomic weight of
+only about one-seventh that of gold. Two investigators determined
+experimentally the number for aluminum and their results agreed with
+Rutherford's calculations.
+
+The metals from aluminum to gold have been examined in this way. The
+number of recoil particles increases with the atomic weight of the
+metal. Comparing experiment with theory, the central charge in an atom
+corresponds to about one-half the atomic weight multiplied by the
+charge on an electron, or, as it is expressed, 1/2 Ae.
+
+There is only one lighter atom than helium, namely, hydrogen, which
+has a mass only one-fourth as great. When alpha particles are
+discharged into hydrogen, a few of the latter atoms are found to be
+propelled to a distance four times as great as that reached by the
+alpha particles.
+
+
+Stopping Power of Substances
+
+Parallel with the experiments mentioned, there is what is called
+the stopping power of substances. This means the depth or thickness
+of a substance necessary to put a stop to the course of the alpha
+particles. This gives the range of the alpha particles in such
+substances and is connected in a simple way with the atomic weight,
+that is, it is again fixed by the mass of the opposing atom. This
+stopping power of an atom for an alpha particle is approximately
+proportional to the square root of its atomic weight.
+
+Considering gases, for instance, if the range in hydrogen be 1,
+then the range in oxygen, the atomic weight of which is 16, is only
+(1/16)^{1/2} or 1/4. Generally in the case of metals the weight of
+matter per unit area required to stop the alpha particle is found to
+vary according to the square root of the atomic weight of the metal
+taken.
+
+
+
+
+CHAPTER VI
+
+RADIO-ACTIVITY AND CHEMICAL THEORY
+
+
+Influence upon Chemical Theory
+
+It can easily be seen that the revelations of radio-activity must have
+a far-reaching effect upon chemical theory, throwing light upon, and
+so bringing nearer, the solution of some of the problems which have
+been long discussed without arriving at any satisfactory solution. The
+so-called electro-chemical nature of the elements will certainly be
+made much clearer. The changes in valence should become intelligible
+and valence itself should be explained. A fuller understanding of the
+ionization of electrolytes also becomes possible. As these matters are
+debatable and the details are still unsettled, it is scarcely
+appropriate to give here the hypotheses in detail or to enter into any
+discussion of them. But the promise of solution in accord with the
+facts is encouraging.
+
+
+The Periodic System
+
+Such progress has been made, however, in regard to a better
+understanding of the Periodic System that the new facts and their
+interpretation may well be given. No reliable clue to the meaning of
+this system and the true relationship between the elements had been
+found up to the time when new light was thrown upon it by the
+discoveries of radio-activity. The underlying principle was unknown
+and even the statement of what was sometimes erroneously called the
+Periodic Law was manifestly incorrect and its terms were ignored.
+
+
+Basis of the Periodic System
+
+The ordinary statement of the fundamental principle of the Periodic
+System has been that the properties of the elements were periodic
+functions of the atomic weights, and that when the elements were
+arranged in the order of their atomic weights they fell into a natural
+series, taking their places in the proper related groups.
+
+In accepting this, the interpretation of function was both
+unmathematical and vague, and the order of the atomic weights was not
+strictly adhered to but unhesitatingly abandoned to force the group
+relationship. Wherever consideration of the atomic weight would have
+placed an element out of the grouping with other elements to which it
+was clearly related in physical and chemical properties, the guidance
+of these properties was accepted and that of the atomic weights
+disregarded. Such shiftings are noted in the cases of tellurium and
+iodine; cobalt and nickel; argon and potassium. It was most helpful
+that, following the order of atomic weights, the majority of the
+elements fell naturally into their places. Otherwise the
+generalization known as the Periodic System might have remained for a
+long time undiscovered and the progress of chemistry would have been
+greatly retarded.
+
+
+Influence of Positive Nucleus
+
+It is evident that the order of the elements is determined by
+something else than their atomic weights. From the known facts of
+radio-activity it would seem that this determining factor is the
+positive nucleus. And this nucleus also determines the mass or weight
+of the atom. Taking the elements in their order in the Periodic Series
+and numbering the positions held by them in this series as 1, 2, 3,
+etc., we get the position number or what is called the atomic number.
+This designates the order or position of the element in the series.
+We must learn that this number marks a position rather than a single
+element, a statement which will be explained later.
+
+
+Determination of the Atomic Number
+
+Since the atomic weight is unreliable as a means of settling the
+position of an element in the series and so fixing its atomic number,
+how is this number to be determined? Of course, one answer to this
+question is that we may rely upon a consideration of the general
+properties, as has been done in the past. Fortunately, other methods
+have been found by which this may be confirmed. For instance, the
+stopping and scattering power of the element for alpha particles has
+been suggested and successfully used.
+
+
+Use of X-Ray Spectra
+
+A most interesting method is due to Moseley's observations upon the
+_X_-ray spectra of the various elements. It has been found that
+crystals, such as those of quartz, have the power of reflecting and
+defining the _X_ rays. The spectra given by these rays can be
+photographed and the wave lengths measured. These _X_ rays are emitted
+by various substances under bombardment by the cathode rays (negative
+electrons) and have great intensity and very minute wave lengths.
+Moseley made use of various metals as anti-cathodes for the production
+of these rays. These metals ranged from calcium to zinc in the
+Periodic System. In each case he observed that two characteristic
+types of _X_ rays of definite intensity and different wave lengths
+were emitted. From the frequency of these waves there is deduced a
+simple relation connected with a fundamental quantity which increases
+in units from one element to the next. This is due to the charge of
+the positive central nucleus. The number found in this way is one less
+than the atomic number. Thus the number for calcium is 19 instead of
+20 and that for zinc is 29 instead of 30. So, by adding 1 to the
+number found the atomic number is obtained.
+
+The atomic weight can usually be followed in fixing the atomic number,
+but where doubt exists the method just given can be resorted to. Thus
+doubt arises in the case of iron and nickel and cobalt. This would be
+the order according to the atomic weights. The _X_-ray method gives
+the order as iron, cobalt, and nickel, and this is the accepted order
+in the Periodic System.
+
+
+Changes Caused by Ray Emission
+
+On studying the properties of the elements in a transformation series
+in connection with the ray emission which produced them, it was seen
+that these properties were determined in each case by the nature of
+the ray emitted from the preceding transformation product or parent
+element.
+
+
+Atomic Weight Losses
+
+Each alpha particle emitted means a loss of 4 in the atomic weight.
+This is the mass of a helium atom. Thus from uranium with an atomic
+weight of 238 to radium there is a loss of three alpha particles.
+Therefore, 12 must be subtracted from 238, leaving 226, which agrees
+closely with the atomic weight of radium as actually determined by the
+ordinary methods. Uranium X_{1}, then, would have an atomic weight
+of 234 and that of ionium would be 230. The other intermediate
+elements, whose formation is due to the loss of beta particles only,
+show no decrease in atomic weight.
+
+
+Lead the End Product
+
+From uranium to lead there is a loss of 8 alpha particles, or 32 units
+in atomic weight. This would give for the final product an atomic
+weight of 206. The atomic weight of lead is 207.17. It is not at all
+certain that the final product of this series is ordinary lead. The
+facts are such that they would lead one to think that it is not. It is
+known only that the end product would probably be some element closely
+resembling lead chemically and hence difficult or impossible to
+separate from it. Several accurate determinations of lead coming from
+uranium minerals, which always carry this element and in an
+approximately definite ratio to the amount of uranium present, show
+atomic weights of 206.40; 206.36; and 206.54. Even the most rigid
+methods of purification fail to change these results. The lead in
+these minerals might therefore be considered as coming in the main
+from the disintegration of the uranium atom and, though chemically
+resembling lead, as being in reality a different element with
+different atomic weight.
+
+Furthermore, in the thorium series 6 alpha particles are lost before
+reaching the end product, which again is perhaps the chemical analogue
+of lead. The atomic weight here should be 232 less 24, or 208.
+Determinations of the atomic weight of lead from thorite, a thorium
+mineral nearly free from uranium, gave 208.4.
+
+The end product of the actinium series is also an element resembling
+lead, but both the beginning and ending of this series are still in
+obscurity.
+
+
+Changes of Position in the Periodic System
+
+The loss of 4 units in the atomic weight of an element on the
+expulsion of an alpha particle is accompanied by a change of chemical
+properties which removes the new element two groups toward the
+positive side in the Periodic System.
+
+Thus ionium is so closely related to thorium and so resembles it
+chemically that it is properly classed along with thorium as a
+quadrivalent element in the fourth group. Ionium expels an alpha
+particle and becomes radium, which is a bivalent element resembling
+barium belonging to the second group. Radium then expels an alpha
+particle and becomes the gas, radium emanation, which is an analogue
+of argon and belongs to the zero group. Other instances might be cited
+which go to show that in all cases the loss of an alpha particle makes
+a change of two places toward the left or positive side of the System.
+
+
+Changes from Loss of Beta Particles
+
+The loss of a beta particle causes no change in the atomic weight but
+does cause a shift for each beta particle of one group toward the
+right or negative side of the System. Two such losses, then, will
+counterbalance the loss of an alpha particle and bring the new element
+back to the group originally occupied by its progenitor. Thus uranium
+in the sixth group loses an alpha particle and the product UX_{1}
+falls in the fourth group. One beta particle is then lost and UX_{2}
+belonging to the fifth group is formed. With the loss of one more beta
+particle the new element returns to the sixth group from which the
+transformation began.
+
+The table on page 48, as adapted from Soddy, affords a general view of
+these changes.
+
+
+Isotopes
+
+An examination of the table will show a number of different elements
+falling in the same position in a group of the Periodic System
+irrespective of their atomic weights. These are chemically inseparable
+so far as the present limitations of chemical analysis are concerned.
+Even the spectra of these elements seem to be identical so far as
+known. This identity extends to most of the physical properties, but
+this demands much further investigation. For this new phenomenon Soddy
+has suggested the word isotope for the element and isotopic for the
+property, and these names have come into general use.
+
+ [Illustration: RADIO-ACTIVE ELEMENTS FROM URANIUM AND THORIUM
+ PLACED IN THE PERIODIC SYSTEMS Adapted from Soddy]
+
+Manifestly, we have come across a phenomenon here which quite
+eliminates the atomic weight as a determining factor as to position in
+the Periodic or Natural System or of the elemental properties in
+general. All of the properties of the bodies which we call elements,
+and consequently of their compounds and hence of matter in general,
+seem to depend upon the balance maintained between the charges of
+negative and positive electricity which, according to Rutherford's
+theory, go to make up the atom.
+
+It is evident that any study of chemical phenomena and chemical theory
+is quite incomplete without a study of radio-activity and the
+transformations which it produces.
+
+
+Radio-activity in Nature
+
+In concluding this outline of the main facts of radio-activity, it is
+of interest to discuss briefly the presence of radio-active material
+on this planet and in the stars. Facts enough have been gathered to
+show the probable universality of this phenomenon of radio-activity.
+Whether this means solely the disintegration of the uranium and
+thorium atoms, or whether other elements are also transformed under
+the intensity of the agencies at work in the universe, is of course a
+question as yet unsolved.
+
+
+Radio-active Products in the Earth's Crust
+
+The presence of uranium and thorium widely distributed throughout
+the crust of the earth would lead to the conclusion that their
+disintegration products would be found there also. Various rocks of
+igneous origin have been examined revealing from 4.78 x 10^{-12}
+to 0.31 x 10^{-12} grams of radium per gram of the rock. Aqueous
+rocks have shown a lesser amount, ranging from 2.92 x 10^{-12} to
+0.86 x 10^{-12} grams. As the soil is formed by the decomposition
+of these rocks, radium is present in varying amounts in all kinds of
+soil.
+
+
+Presence in Air and Soil Waters
+
+As radium is transformed into the gaseous emanation, this will escape
+wherever the soil is not enclosed. For instance, a larger amount of
+radio-activity is found in the soil of caves and cellars than in open
+soils. If an iron pipe is sunk into a soil and the air of the soil
+sucked up into a large electroscope, the latter instrument will show
+the effect of the rays emitted and will measure the degree of
+activity. Also the interior of the pipe will receive a deposit of the
+radio-active material and will show appreciable radio-activity after
+being removed from the soil.
+
+This radium emanation is dissolved in the soil waters, wells, springs,
+and rivers, rendering them more or less radio-active, and sometimes
+the muddy deposit at the bottom of a spring shows decided
+radio-activity.
+
+The emanation also escapes into the air so that many observations made
+in various places show that the radium emanation is everywhere present
+in the atmosphere. Neither summer nor winter seems to affect this
+emanation, and it extends certainly to a height of two or three miles.
+Rain, falling through the air, dissolves some of the emanation, so
+that it may be found in freshly-fallen rain water and also in
+freshly-fallen snow. Radio-active deposits are found upon electrically
+charged wires exposed near the earth's surface.
+
+As helium is the resulting product of the alpha particles emitted by
+the emanation and other radio-active bodies, it is found in the soil
+air, soil waters, and atmosphere.
+
+Average measurements of the radio-activity of the atmosphere have led
+to the calculation that about one gram of radium per square kilometer
+of the earth's surface is requisite to keep up the supply of the
+emanation.
+
+A number of estimates have been given as to the heat produced by the
+radio-active transformations going on in the material of this planet.
+Actual data are scarce and mere assumptions unsatisfactory, so little
+that is worth while can be deduced. It is possible that this source of
+heat may have an appreciable effect upon or serve to balance the
+earth's rate of cooling.
+
+
+Cosmical Radio-activity
+
+Meteorites of iron coming from other celestial bodies have not shown
+the presence of radium. Aerolites or stone meteorites have been found
+to contain as much as similar terrestrial rock. Since the sun
+contains helium and some stars show its presence as predominating,
+this suggests the presence of radio-active matter in these bodies. In
+addition, the spectral lines of uranium, radium, and the radium
+emanation have been reported as being found in the sun's spectrum and
+also in the new star, _Nova Geminorum 2_. These observations await
+further investigation and confirmation. So far as the sun's
+chromosphere is concerned, the possible amount of radium present would
+seem to be very small. If this is true, radio-active processes could
+have little to do with the sun's heat. The statement is made by
+Rutherford that indirect evidence obtained from the study of the
+aurora suggests that the sun emits rays similar in type to the alpha
+and beta rays. Such rays would be absorbed, and the gamma rays
+likewise, in passing through the earth's atmosphere and so escape
+ordinary observation. All of this is but further evidence of the unity
+of matter and of forces in the universe.
+
+
+
+
+INDEX
+
+
+ Actinium, discovery of, 6
+
+ Activity, induced, 17
+
+ Alpha particles, effect of loss on Atomic Weight, 45
+ electrical charge of, 26
+ form helium, 27
+ nature of, 25
+ penetrating power of, 39
+ position of element changed by its loss, 46
+ recoil, 39
+ scattering of, 38
+ solid, 26
+
+ Atom, constitution of, 36
+ Kelvin's, 37
+ models of, 37
+ Rutherford's, 37
+
+ Atomic number, determination of, 43
+
+
+ Becquerel's experiments, 2
+
+ Beta particles, change in position of element by loss of, 47
+
+
+ Chalcolite, natural and artificial, 4
+
+ Constants, table of, 31
+
+ Curie unit, 22
+
+
+ Disintegration of the element, 25
+
+ Disintegration series, 24
+
+ Disintegration theory, 35
+
+
+ Electroscope, 12
+
+ Equilibrium series, 22
+
+
+ Helium, characteristics of, 30
+ discovery of, 29
+
+
+ Ionium, discovery of, 6
+
+ Ionization, application of electric field to, 10
+ experimental confirmation, 9
+
+ Ionization of gases, 7
+ theory of, 8
+
+ Ions, size and nature of, 10
+
+ Isotopes, 47
+
+
+ Lead, atomic weight varies with source, 45
+ radio-active, 6
+ the end product, 45
+
+ Life-periods of radio-active bodies, 21
+
+
+ Periodic system, 41
+ basis of, 42
+
+ Polonium, discovery of, 4
+
+ Positive nucleus, influence of, 43
+
+ Potassium, radio-activity of, 3
+
+
+ Radiations, action on phosphorescent bodies, 13
+ action on photographic plates, 11
+ discharge electrified bodies, 12
+ magnetic deflection of, 14
+ measurements of, 15
+ penetrating power of, 13, 15
+
+ Radio-active bodies, elemental nature of, 20
+ examination of, 20
+ life periods of, 21
+
+ Radio-activity, an atomic property, 3
+ cosmical, 51
+ influence on chemical theory, 41
+ products in atmosphere, 51
+ products in earth's crust, 50
+ products in soil waters, 50
+
+ Radium, action on organic matter, etc., 33
+ amount in pitchblende, 5
+ discovery of, 5
+ emanation, 22
+ energy evolved by, 34
+ properties of, 5, 32
+
+ Rays, alpha, 15, 16, 26
+ beta, 15, 16
+ gamma, 15, 16
+ identification of, 16, 25
+ magnetic deflection of, 14
+ photographing track of, 10
+ types of, 14
+
+ Rubidium, radio-activity of, 3
+
+
+ Spinthariscope, 13
+
+ Stopping power of substances, 39
+
+
+ Thorium X, discovery of, 18, 21
+
+
+ Uranium atom, disintegration of, 36
+
+ Uranium minerals, radio-activity of, 3
+
+ Uranium X, discovery of, 17, 21, 23
+
+
+ X-ray spectra, 44
+
+
+ Zinc sulphide screen, 13
+
+
+
+
+TRANSCRIBER'S NOTES
+
+
+1. Passages in italics are surrounded by _underscores_.
+
+2. Images have been moved from the middle of a paragraph to the
+closest paragraph break.
+
+3. The original text includes certain Greek alphabets. For this text
+version [alpha], [beta], [gamma] indicate first three letters of Greek
+alphabet respectively.
+
+4. In this version, the number following carat character ^ is to be
+interpreted as follows. The expression 10^{-2} means multiplying by
+0.01; 10^{10} means multiplying by 10,000,000,000.
+
+5. In this version, the subscripted text has been replaced by an
+underline character _ followed by the same with curly braces { and }.
+For example, X_{1} indicates X with subscript 1.
+
+6. The fractions are indicated with the help of forward character /.
+For example, 1/4 indicates one-fourth.
+
+7. Other than the changes listed above, the original text has been
+reproduced as such.
+
+
+
+
+
+End of the Project Gutenberg EBook of A Brief Account of Radio-activity, by
+Francis Preston Venable
+
+*** END OF THIS PROJECT GUTENBERG EBOOK A BRIEF ACCOUNT OF RADIO-ACTIVITY ***
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