The electrical nature of matter and radioactivity

Front matter

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Preface

The content of this book has already been published as a series of articles in the Electrical Review, The two correlated subjects under consideration are of such general interest that it has seemed desirable that the discussion of them should be made accessible in compact form.

The several chapters as they originally appeared have, therefore, been carefully revised and brought together in one volume. The author would extend his sincere thanks to his assistant, Dr. H. S. Uhler, for a number of valuable suggestions in connection with the revision of the work.

The aim of the writer has been to present the more important facts and conclusions in connection with the work on the "Electrical Nature of Matter and Radioactivity," as far as possible in non-mathematical language. This has been done with the belief that there are a large number of those who have a truly scientific interest in these most recent and important developments in Physics and Physical Chemistry, but to whoni a more technical and rigidly mathematical treatment might not appeal. To all who desire such a treatment, the admirable books by Thomson, on the *' Conductivity of Electricity through Gases," and by Rutherford, on "Radioactivity," are heartily recommended.

While this work is written in a semi-popular style, the attempt has been made to treat the subject with scientific accuracy. The facts presented have nearly always been taken directly from the original sources. Since, however,

IV PREFACE

this is a comparatively elementary discussion, references to the original papers are given chiefly in the cases of the more important contributions. All of those who desire to go more fully into these subjects are urged to read as many as possible of the original articles.

If this little book should contribute even in a small measure towards supplying the general demand for knowledge in the field which it covers, it will more than repay for the time and labor that have been spent in its preparation.

Harry C. Jones.

Preface To The Second Edition

The aim in preparing a second edition of this work is to bring it up to date as far as matters of fundamental importance are concerned. Most of the epoch-making discoveries in connection with radioactivity were made in the earlier stages of the work, but many important contributions to our knowledge in this field have been published in the last few years. This material, as far as is consistent with the scope and size of this book, has been incorporated in this edition.

The author gladly accepts this opportunity to express his thanks to his assistant, Dr. W. W. Strong, for valuable aid in revising this book. H. C. J.

Chapter I Page

The Electrical Conductivity of Gases i

Conditions which increase the conductivity of gases. How a conducting gas differs from a non-conducting. The ratio of the charge to the mass of the ion in a gas. The cathode ray. The

e e

value of — for the cathode particle. The ratio — constant for m m

e different gases. The ratio — varies for the different ions of in

e electrolytes. The value of — for gaseous ions produced by different means.

Chapter II

The Determination of the Mass of the Negative Ion in Gases, io Work of J. J. Thomson. Comparison of the charge on a gaseous ion with that on a univalent ion of an electrolyte. The ratio of the charge to the mass for the positive ion.

Chapter III

Nature of the Corpuscle. The Electrical Theory of Matter 19 Work of Thomson and Kaufmann. The electron the ultimate unit of matter. Earlier attempts to unify matter. Other relations between the elements.

CHAPTER IV The Nature of the Atom in Terms of the Electron Theory 29

Thomson's conception of the atom. The electron theory and the Periodic System. The atom in terms of the electron theory. Cations and anions in terms of the electron theory. The mass of an ion not exactly the same as that of the atom from which it is formed. The electron theory and radioactivity.

Chapter VIII

The ratio — for the alpha particle. The mass of the alpha partn tide. Critical velocity of the alpha particles. Alpha particles produce delta particles. Alpha particles are probably helium atoms. Action of the alpha particles on a photographic and on a fluorescent plate. Stopping the spinthariscope power of matter for the alpha particles.

Chapter IX

The beta rays. Nature of the charge carried by the beta particles. The determination of — for the beta particle. The mass m

of the beta-particle. Relation to the cathode particle. Cathode rays. Beta rays from radium. The gamma rays. Secondary radiations produced by beta rays. Summary of the properties of the alpha, beta, and ganuna rays. Total nxmiber of particles shot off by radium.

Chapter X

The self-limiinosity of radium compounds. Phosphorescence produced by radium salts. Radium increases the conductivity of dielectrics. Chemical effects produced by radioactive substances. Physiological action of the radiations from radium.

CHAPTER XI Production of Heat by Radium Salts 106

Measurement of the heat liberated by salts of radium. Method of the Bunsen ice calorimeter. Results of heat measurements. Source of the heat. Effect on solar heat. Does radium exist in the sun? Terrestrial heat produced by radium. Bearing on the calculated age of the earth. Theories as to the source of the heat produced by radium. Calculation of the amount of heat liberated by radium on the above theory that the heat is produced by the d particles. Three remarkable properties of radium.

CONTENTS Vii

Chapter XII Pagb

Emanation from Radioactive Substances ii8

Discovery of the thorium emanation by Rutherford. Method of obtaining the emanation. Amount of the emanation. Nature of the emanation. Diffusion of the emanation. Approximate determination of its molecular weight.

Chapter XIII

Recovery of emanating power. Decay of emanation. Heat evolved by the emanation. Helium produced from the emanation. This is not a tramsmutation of the elements. Further experiments on the production of helium from radium. Relation between the emanation and helium. Some remarkable results obtained by the action of the radium emanation.

Chapter XIV

Induced radioactivity produced by the emanation. Induced radioactivity undergoes decay. Induced radioactivity due to the deposit of radioactive matter. Properties of the radioactive matter deposited by the emanation from radioactive substances. Emanation X. Facts that must be taken into account in dealing with the decay of induced or excited radioactivity. Radium probably identical with polonium. Summary of the decomposition products of radium. Decomposition products of other radioactive substances. Interpretation of these facts. Radiothorium — a new radioactive element. Decomposition products of activium.

Chapter XV

Continuous formation of radioactive matter in uranium. Recovery of activity by uranium, and decay of activity in uranium X. Radiation from uranium X. Continuous formation of radioactive matter from thorium. Properties of thorium X. Decay of its radioactivity. Thorium X produces the thorium emanation. Recovery of radioactivity by thorium. Rate at which thorium recovers radioactivity independent of conditions. Radium does not give rise to substances corresponding to uranium X and thorium X.

Chapter XVI

Importance of a theory or generalization. The more important facts in connection with uranium. The more important facts in connection with thorium. The more important facts in connection with radium. Theory of Rutherford and Soddy to account for radioactive phenomena. The transformations of the radioactive elements differ fundamentally from ordinary chemical reactions. The electron theory of J. J. Thomson as applied to radioactivity. Is matter in general undergoing transformation?

Vm CONTENTS

Chapter XVII Page

Wide Distribution of Radioactive Matter and the Origin of

Radium 183

Radioactive matter in the earth and sea. Radioactive matter in the air. Is matter in general radioactive? The origin of radium. Ionium. The complete series of transformations in which radium is involved. Emanium. Conclusion.

ABBREVIATIONS OF THE TITLES OF JOURNALS

Amer. Chem. Joura. = American Chemical Journal.

Amer. Journ. Sci. = American Journal of Science.

Ann. Chim. Phys. = Annales de Chimie et de Physique.

Ber. d. deutsch. chem. Gesell. = Berichte der deutschen chemischen

Gesellschaft.

Cam. Phil. See. Proc. = Proceeding of the Cambridge Philosophical

Society.

Chem. News = Chemical News.

Joura. Chem. Soc. = Joumal of the Chemical Society of London. Joum. de Chim. Phys. = Journal de Chimie Physique. Nat. = Nature.

Phil. Mag. == Philosophical Magazine.

Phil. Trans. = Philosophical Transactions of the Royal Society. Phys. Rev. = Physical Review.

Phys. Zeit. « Physikalische Zeitschrift. Roy. Soc. Proc. = Proceedings of the Royal Society. Wied. Ann. = Wiedemann's Annalen.

The Electrical Nature of Matter and Radioactivity

Chapter I

The power of gases, under normal pressure and at ordinary temperatures, to conduct electricity is so small that it has been doubted whether pure, dust-free gases can conduct at all. Recent refined experiments, however, show that while pure, dust-free gases have only a small conductivity, they have a definite power to conduct electricity, which is measurable.

CONDITIONS WHICH INCREASE THE CONDUCTIVITY OF GASES

While gases under normal conditions have only slight conductivity, and are fairly good insulators, it is not a difiicult matter to increase greatly the conductivity of gases. This can be done in a number of ways. When gases are heated to high temperatures their electrical conductivity is greatly increased. According to Becquerel, when air is heated to a white heat, electricity will pass through it when the difference in potential is small. It is also known that gases in contact with incandescent solids have their conductivity increased. Some interesting and important facts, which it would lead us too far at present to discuss, have been brought to light through the study of these

phenomena.

I

Gases taken from flames have been found to show considerable conductivity, which is retained for some time after the gas has been removed from the flame and cooled down.

Other agents which increase the conductivity of gases are Rontgen rays, the presence of radioactive substances, and cathode rays. As these will be taken up later in some detail, they will not be discussed further in the present connection.

HOW A CONDUCTING GAS DIFFERS FROM A NON-CONDUCTING

We have seen that a gas in the normal condition has very small power to conduct electricity.

We have also seen that the conducting power of a gas can be greatly increased by a number of widely different agents. The question that would naturally arise in this connection is, how does a conducting gas differ from a non-conducting or normal gas? (We may term a normal gas non-conducting, since its conductivity is so slight.)

To answer this question we must study the properties of a conducting gas, and compare them with the properties of a non-conducting gas.

If the conducting gas is made to pass through a plug of glass-wool, or is drawn through water, it loses its conducting power. The conducting power of a gas is also removed by passing the gas through a metal tube of very fine bore; the finer the bore the more rapidly the conductivity is lost.

The removal of the conducting power by filtering through glass-wool shows that the conductivity of the gas is due to some constituent which is filtered out mechanically by the glass-wool. The experiments with the metal tube show that this constituent which can be filtered out by glass-wool is charged with electricity. These charged particles in a conducting gas are known as ions. Some of these particles are charged positively and others negatively. Since a conducting gas shows neither an excess of positive nor of negative electricity it is, as we say, electrically neutral.

THE RATIO OF THE CHARGE TO THE MASS OF THE ION IN A

GAS

When an acid, base, or salt is dissolved in water, we know that it breaks down into charged parts called ions. Every molecule of an electrolyte yields an equivalent number of posivitely charged parts or cations, and negatively charged parts or anions. The ratio of the charge carried by these ions to their mass has been determined. In the case of the hydrogen ion, which is the characteristic ion of all acids, it has been found to be of the order of magnitude of lo*.

It was recognized to be of importance to determine the ratio of the charge to the mass of the ion in gases. If we represent the charge carried by the gaseous ion by e, and

the mass of the ion by m, the ratio in question is — .

tn

We shall take up first the determination of the ratio —

m

for the cathode particle.

THE CATHODE RAY

When an electric discharge is passed through a highvacuum tube, rays are sent out from the cathode which generally produce a greenish yellow phosphorescence whera they fall upon the glass walls of the tube. These are known as the cathode rays. The nature of these rays was for some time in doubt. It was thought by some investigators that they were waves in the ether. It remained for Sir William

Crookes to give us the accepted explanation of the nature of the cathode rays. According to Crookes the cathode rays are charged particles, sent off from the cathode with very high velocity. They move towards the anode in a direction at right angles to the surface of the cathode. The properties of the cathode rays, in general, are in accord with this theory. The cathode rays can be deflected by a magnet.

A soUd body placed in their path casts a well-defined shadow.

Cathode rays can probably produce certain chemical changes, especially of a reducing nature.

Mechanical effects can readily be produced by the cathode rays, as was shown by Sir William Crookes. A glass paddlewheel is easily made to move along level glass tracks within the tube, by allowing the cathode rays to impinge upon the vanes.

Thermal effects are readily produced by the cathode rays. By suitably concentrating them upon platinum, the metal is rendered incandescent. All of these facts accord with the theory as to the nature of the cathode rays, advanced by Sir William Crookes.

The discovery that cathode rays can pass through thin films of metal seemed at first to argue against the Crookes theory. When we become familiar later with the exact nature of the cathode particle itself, we shall see that this argument is without foundation.

We shall, then, at present accept the Crookes theory, and regard the cathode rays as consisting of negatively ' electrified particles, moving with high velocities, in straight lines at right angles to the surface of the cathode.

In the light of the above theory and the facts upon which it is based, we shall now take up the work of J. J. Thomson, by which he determined the value of — for the cathode

m

particle.

e

THE VALUE OF — FOR THE CATHODE PARTICLE

m

The value of for the cathode particle was determined tn

by J. J. Thomson/ as follows: The cathode is placed near one end of an exhausted tube, and the anode removed only a short distance from the cathode. Beyond the anode on the side removed from the cathode is placed a metal plug connected with the earth. A small hole is bored through the centre of the anode and the metal plug. Cathode rays pass through these holes and fall on the wall of the vacuum tube at the end of the tube farthest removed from the cathode. Since the holes in the metal plates are small, we have a narrow beam of cathode rays striking the inner wall of the glass vessel, forming a small, phosphorescent spot on the glass. We have seen that the cathode rays are deviated by a magnetic field. If the whole tube is now properly placed in a magnetic field, the path of the cathode particles will be changed, and they will impinge upon the glass wall at some point different from that which they originally bombarded when no magnetic field was present. Measuring the magnitude of this deflection we can calculate the value

of — , in which v is the velocity of the ion. vtn

We have thus determined the ratio of e to vm.

We must now determine the value of z; in ordei to obtain

the ratio — . m

Into the above-mentioned vacuum tube are inserted two parallel plates of aluminium, which are so arranged that the beam of cathode particles passes between them. The plates are also parallel to the original, imdeflected beam. These metal plates are attached to some electrical source, and maintained at a known difference in potential. We thus have between the plates an electric field. The electrostatic intensity s, due to this field, deflects the ion with a force se, e being the charge upon the ion. The force due to the magnetic field already considered is fev, f being the strength of the field, e the charge carried by the ion, and V the velocity of the ion.

By suitably charging the metaJ plates, the electrical and magnetic forces can be made to act counter to one another. These two counter forces can readily be made equal to each other. This can easily be determined. We note the original position of the phosphorescent spot on the glass before placing the apparatus in the magnetic field. When the apparatus is placed in the magnetic field the beam of cathode particles is deflected, and the bright spot on the glass changes its position. The electrostatic force, acting counter to the magnetic, causes the beam to occupy more nearly its original position. When these two opposing forces are equal the phosphorescent spot occupies its original position. Thus we have an easy and efficient means of determining when these two opposite forces are equal. When they are, fev = se.

Klnowing now the value of v, and having previously determined, as we have seen, the ratio of e to vm^ we have

the value of — which is the quantity desired.

THE RATIO — CONSTANT FOR DIFFERENT GASES

m. w

ed Using a somewhat different method, J. J. Thomson found

. at first that the ratio — was a constant^ whether the gas in

[I tn

le the tube was air, carbon dioxide, methyl iodide, or hydrogen.

n This is a most important fact, as we shall see.

e Thomson and his coworkers then changed the nature

g of the metal of which the cathode was made, using plati-

num, aluminium, silver, copper, tin, zinc, lead, and iron, to see whether the nature of the metal from which the

I cathode discharge takes place has any effect on the value of

e e

the ratio — . They found the same value for — , regardless

m m

of the nature of the metal of which the cathode was made.

Thomson found that the value of — was equal to about

m

iXio7

e

THE RATIO — VARIES FOR THE DIFFERENT IONS OF

m

ELECTROLYTES

It will be seen that the value of — for the ions of elec-

m

trolytes varies with every kind of ion. This is necessarily the case, since the charge carried is the same for all univalent ions (and this quantity multiplied by the valency for all polyvalent ions, as is seen from Faraday's law), and the mass varies with every cation and every anion. Taking the

ion characteristic of acids, hydrogen, the value of — for

m

the hydrogen ion is i X lo^.

It is therefore obvious that the value of — for the cathode

m particle is one thousand times as great as the corresponding valine for the hydrogen ion produced when any acid is dissolved in a dissociating solvent.

Knowing the values of — in the two cases does not tell ftfi

us anything about the relative masses of the hydrogen ion

in solution, and the particle in the cathode discharge; since

the charges carried in the two cases might be the same or

might be very different. Before answering this question

we must know the relative charges carried by the ion in

electrolysis, and by the cathode particle.

e

THE VALUE OF — FOR GASEOUS IONS PRODUCED BY

m

DIFFERENT MEANS

Before taking up the beautiful method for determining the value of the charge carried by the cathode particle, we

shall ask and answer the question whether the value of —

for gaseous ions is the same, regardless of the means by which the gaseous ions are produced, or whether it varies with the means employed to produce the ions in the gas.

The answer to this question is unmistakably given by the results that have been obtained. The Lenard rays are nothing but cathode rays that have left the so-called vacuimi tube by passing through a thin sheet of aluminium.

The value of — for the particles in these rays has been ftfi

found to be about 4X10*.

The value of — for the gaseous ions produced in contact with incandescent metals is about 8.5X10*.

The value of — for the negative ion given off from radiopt

active substances is about iXio^.

It is obvious that the above values all refer to the negative, gaseous ion. We see from the results that the value

of — for this ion is practically constant, regardless of the m

means by which it is produced, and regardless of the nature

of the gas from which it is produced.

As to the value of — for the positive ion of gases, we fft

shall have something to say in the next chapter, and shall

also discuss the nature of this ion.

CHAPTER II The Determination of the Mass of the Negative Ion in Gases

work of j. j. thomson

The determination of the charge carried by the negative ion is of the very greatest importance. We have already

considered the method for determining the ratio — for the

m

negative ion. If we can now determine e, the charge carried

by this ion, we would know w, the mass of the negative ion.

One of the most ingenious experiments in modem physics has been devised by J. J. Thomson for solving this problem. The experiment is based on an observation made by C. T. R. Wilson,^ that gaseous ions, both positive and negative, can act as nuclei for the condensation of water- vapor, even if there are no dust particles present in the gas. If a given volume of a gas containing ions is allowed to expand, it cools itself, and a part of the water-vapor will condense around the ions, producing a fog or cloud in the apparatus containing the gas.

That the water-vapor actually condenses around the ions was proved conclusively by J. J. Thomson, by the following very simple experiment. Two parallel metal plates were placed a few centimetres apart in the vessel containing the gas which had been freed fiom dust. These

DETERMINATION CF THE MASS OF THE NEGATIVE ION II

plates were connected with the terminals of a battery, by which they could be charged to a relatively high difference in potential. Ions were produced in the gas by passing Rontgen rays through it.

If the gas was expanded before the plates were connected with the battery, condensation of the vapor took place; just as we should expect if the ions acted as nuclei around which the water-vapor would condense. If the plates are now connected with the battery and charged, the strong electrical field would remove the ions from the gas, and if the gas were then subjected to expansion we would not expect any appreciable condensation to take place, and such is the fact. Thomson says that under these conditions the condensation is scarcely greater than in unionized air.

This experiment shows conclusively that it is the ions that serve as the centres of condensation of the water-vapor — a drop of water condensing aroimd every ion if the ions are not too numerous. If we knew the number of droplets of water in a given voliune of the gas, we would know the number of ions in that volume. It is, however, obviously impossible to determine the number of water particles in a volimie of gas by any direct method. Thomson ^ solved this part of the problem by using an equation deduced by Stokes, connecting the rate at which the particles fall with their size. If we represent by v the velocity with which the particles fall, by g the acceleration of gravity, by c the viscosity coefficient of the gas, and by r the radius of the drop,

9 c By observing the rate at which the cloud settles we arrive at the value of v. Knowing v we determine at once the value of r, the radius of the drop. Knowing the radius of the drop we know its volume.

If we represent the mass of the water deposited by each cubic centimetre of the gas, by Af , the niunber of drops in a cubic centimetre n is given by the following equation:

SM n= — I,

47rr

The mass of water deposited from each cubic centimetre of the gas, M, must be determined indirectly. Thomson made use of the heat that is Uberated when the water-vapor condenses around the gaseous ions. Knowing M and r, we have all the data necessary for calculating n, the number of ions in a cubic centimetre of the gas, which is equal to the number of droplets in the same volume.

We now know the number of ions in a given volume of the gas. It still remains to determine the charge carried by a single ion.

If we knew the total quantity of electricity carried by the known munber of ions, we would know the amoimt carried by one ion. Let v be the mean velocities of both positive and negative ions when subjected to unit electrical force. We must measure the current carried by these ions across unit area, under an electric force F, in order to determine the charge carried by a single ion. If we represent the charge carried by a single ion as formerly by e, we have: Fvne = current through imit area perpendicular to the current. Measuring the current that passes through the gas, we know all of the above quantities except e, which is calculated at once.

In performing the condensation experiment it is neces-

DETERMINATION OF THE MASS OF THE NEGATIVE ION I3

sary, as Thomson points out, to work with gases that contain only a comparatively small number of ions. When the conductmg gas contains a large number of ions some of these are not carried down by the condensed water-vapor, as is shown by the fact that under these conditions a second expansion of the gas, which is no longer subjected to the ionizing agent, will produce still further condensation, demonstrating that it still contains ions that were not carried down by the first expansion.

The condition that the gas shall contain only a few ions is easily secured, especially when the gas is ionized by Rontgen rays. Either a weak stream of the rays is allowed to pass directly through the gas, or the intensity of the rays is diminished by inserting thin sheets of certain metals, such as aluminium, in their path.

The earlier experiments showed that the values of e for air ionized by Rontgen rays, and for hydrogen gas ionized by Rontgen rays, are equal to within the limit of experimental error, which proves that the gaseous ion carries the same charge whatever the gas from which it was produced. It is of the order of magnitude 4X10"*®.

COMPARISON OF THE CHARGE ON A GASEOUS ION WITH THAT ON A UNIVALENT ION OF AN ELECTROLYTE

Having determined the magnitude of the charge on a gaseous ion, we shall next determine the magnitude of the charge carried by a univalent ion of an electrolyte — say the hydrogen ion.

We know that the number of molecules in a cubic centimetre of a gas, at a pressure of 760 millimetres of mercury and at zero degrees, is between 2X10" and iXio^^ We know the amount of electricity required to liberate this amount of hydrogen gas. From these data we calculate that the charge carried by the hydrogen ion in solution is somewhere between iXio~^^ and 6Xio~^^

We thus see that the charge carried by the gaseous ion is the same as that carried by the hydrogen ion in electrolysis.

This conclusion is based upon a large amount of work with the ions produced from various gases and by various

ionizing agents. We have already seen that the value of —

in

for all of these gaseous ions is the same, no matter what the

nature of the gas from which they were produced, and no

matter what the nature of the ionizing agent. It has further

been shown that all of these gaseous ions carry the same

charge, and that this is the same charge as that carried by

the hydrogen ion in aqueous solution.

We have now all the data necessary for calculating the

relative masses of the gaseous ion, and the hydrogen ion in

solution.

The value of — for the hydrogen ion in solution is lo*. m

The value of — for the gaseous ion is lo^. The values of e m

in the two cases are the same. Therefore, the value of m