UCSB LIBRAKf
 
 MODERN THEORY 
 
 OF 
 
 PHYSICAL PHENOMENA
 
 MODERN THEORY 
 
 OF 
 
 PHYSICAL PHENOMENA 
 
 RADIO-ACTIVITY, IONS, ELECTRONS 
 
 BY 
 AUGUSTO RIGHI 
 
 PROFESSOR OF PHYSICS IN THE UNIVERSITY OF BOLOGNA 
 
 AUTHORIZED TRANSLATION 
 
 BY 
 AUGUSTUS TROWBRIDGE 
 
 PROFESSOR OF MATHEMATICAL PHYSICS IN THE 
 UNIVERSITY OF WISCONSIN 
 
 KTefo ff 0tfe 
 THE MACMILLAN COMPANY 
 
 LONDON : MACMILLAN & CO., LTD. 
 1909 
 
 All rigfiti reterveJ
 
 COPYRIGHT, 1904, 
 BY THE MACMILLAN COMPANY. 
 
 Set up and electrotyped. Published December, 1904. Reprinted 
 January, 1909. 
 
 Notfoaoto 
 
 J. B. Cashing Co. Berwick <k Smith Co. 
 Norwood, Mass., U.S.A.
 
 PREFACE 
 
 IT is with a certain trepidation that I 
 have entertained the proposal of an English 
 translation of an entirely unpretentious book, 
 which is, in fact, only an extension of a 
 chapter added to " Telegrafia senza Filo " 
 (Telegraphy without Wires). 
 
 The fact that this book has gone through 
 two Italian editions in a very short time 
 certainly shows that my work has not been 
 useless in my own country. But to issue an 
 edition in the language of those illustrious 
 men to whom, above all, we owe the ad- 
 mirable theory which forms the subject of 
 this little book is quite a different thing. 
 
 That which has weighed with me in my 
 decision was the thought that this English 
 translation might at least serve to show with 
 what great favour the development of the 
 ideas concerning the first cause of physical
 
 vi PREFACE 
 
 phenomena has been received in Italy, and 
 also to show how fully the works of those 
 philosophers to whom this development is 
 due are appreciated in this country. 
 
 AUGUSTO RIGHI. 
 
 BOLOGNA, ITALY, 
 August, 1904.
 
 TRANSLATOR'S PREFACE 
 
 FOR more than twenty-five years Profes- 
 sor Righi has been an indefatigable investi- 
 gator and constant contributor to electrical 
 science, and has attained a foremost rank 
 among Italian scientists. While he is not 
 one of the relatively small group of investi- 
 gators to whom we owe the extremely im- 
 portant theory outlined in this book, he is, 
 nevertheless, admirably qualified to discuss 
 and explain the theory, both by reason of 
 his deep insight into electrical phenomena 
 and because of his ability to explain intri- 
 cate physical processes without the aid of 
 mathematical formulae. 
 
 As Professor Righi states in his preface 
 to the first edition, the Italian original was 
 written more with the object of interesting 
 the greatest possible number of readers in 
 this new and important branch of physics
 
 viii TRANSLATOR'S PREFACE 
 
 than as a book of reference for physicists. 
 With this end in view the subject-matter 
 was presented in an elementary form, and 
 the book met with a very marked success. 
 
 I have made this translation believing that 
 there are at least as many English as Italian 
 readers to whom an elementary treatment of 
 the electron theory as it stands at present 
 will be acceptable. 
 
 Professor Righi has read the proofs of the 
 translation, thus insuring its accuracy, and 
 has kindly provided me with a special preface 
 in English. 
 
 AUGUSTUS TROWBRIDGE. 
 
 MADISON, WISCONSIN, 
 September, 1904.
 
 CONTENTS 
 
 PACK 
 
 INTRODUCTION . . . . . xi 
 
 I. ELECTROLYTIC IONS AND ELECTRONS . . i 
 II. THE ELECTRONS AND THE PHENOMENA OF 
 
 LIGHT n 
 
 III. NATURE OF THE CATHODE RAYS ... 28 
 
 IV. THE IONS IN GASES AND IN SOLIDS . . 40 
 V. RADIO-ACTIVITY 54 
 
 VI. MASS, VELOCITY, AND ELECTRIC CHARGE OF 
 
 THE IONS AND OF THE ELECTRONS . . Io8 
 
 VII. THE ELECTRONS AND THE CONSTITUTION OF 
 
 MATTER 141 
 
 BIBLIOGRAPHY . 153
 
 INTRODUCTION 
 
 A NEW and interesting branch of science 
 has been formed, partly as a result of the 
 numerous recent experimental researches on 
 the electric discharge, and partly as a conse- 
 quence of the discovery of radio-activity and 
 new phenomena in magneto optics. At the 
 same time these researches have caused a 
 theory to spring up which harmonizes all 
 the facts, and which has profoundly modified 
 the dominant ideas concerning the imme- 
 diate cause of electrical phenomena, and of 
 physical phenomena in general. 
 
 When the old hypothesis of the electric 
 fluid was abandoned, chiefly because of the 
 disinclination to admit "action at a dis- 
 tance/' it seemed for a time as though Fara- 
 day's ideas, formulated later by Maxwell, 
 must lead to a new concept regarding the 
 cause of electrical phenomena, since, accord- 
 ing to Faraday, the ether, and not the so-
 
 Xll INTRODUCTION 
 
 called electrified bodies, is the seat of the 
 phenomena. But the impossibility of find- 
 ing a satisfactory mechanical representation 
 of the supposed elastic deformations of the 
 ether, to which were attributed in Maxwell's 
 theory the apparent forces at a distance, and 
 the necessity of admitting at any rate the 
 existence of an entity distinct from the ether 
 and from matter, made it soon apparent that, 
 even under the new order of ideas, the 
 conception of the nature of electricity still 
 remained obscure. 
 
 At present a new evolution is being ac- 
 complished, since, without knowing anything 
 more about the first cause, an atomic struc- 
 ture is attributed to electricity. This new 
 conception, suggested by the studies men- 
 tioned farther on, already shows promise of 
 becoming as fruitful as the analogous one 
 which has long been admitted regarding the 
 constitution of matter, inasmuch as it per- 
 mits us to place in reciprocal relation, often 
 even quantitative, phenomena which seem 
 utterly different and independent of each 
 other.
 
 INTRODUCTION xm 
 
 What the electrons or electric atoms really 
 are remains a mystery; but in spite of this 
 the new theory may perhaps acquire not a 
 little importance in the future even from 
 the philosophic point of view, since it points 
 out a new mode of considering the structure 
 of ponderable matter and tends to bring back 
 to a single origin all the phenomena of the 
 physical world. 
 
 It is indeed true that, with the modern 
 positivist and utilitarian tendencies, many 
 people do not appreciate this advantage, and 
 prefer to consider a theory principally as a 
 convenient means to arrange and coordinate 
 facts, or as a guide for the investigation of 
 new phenomena. But if hitherto men have 
 confided too much in the power of human 
 ingenuity and have too easily believed them- 
 selves to be on the point of discovering the 
 ultimate cause of things, we perhaps fall into 
 the contrary excess. 
 
 In this book I shall set forth the principal 
 facts which have led to the electron theory, 
 and I shall endeavour to make this theory 
 clear at least along its general lines.
 
 THE MODERN THEORY OF 
 PHYSICAL PHENOMENA 
 
 CHAPTER I 
 
 ELECTROLYTIC IONS AND ELECTRONS 1 
 
 THE hypothesis of electrolytic dissociation 
 is generally admitted in order to explain 
 electrolysis in accord with the well-known 
 laws of Faraday which this phenomenon 
 obeys. Each molecule of an electrolyte may 
 break up into two ions; that is, into two 
 atoms or atomic groups having equal charges 
 of opposite sign. Thus when a solid, such 
 as chloride of sodium, or common salt, is 
 dissolved in water, some of its molecules 
 undergo dissociation; that is, these mole- 
 cules cease to exist as such, and their ions 
 become separated and free. Owing to the 
 
 1 The numbers in parentheses inserted in the text refer to 
 the bibliography at the end of the book. 
 
 B I
 
 2 ELECTROLYTIC IONS AND ELECTRONS 
 
 invisible molecular and atomic motions, 
 whose energy constitutes the heat contained 
 in a body, these ions wander through the 
 liquid, without, however, any given direction 
 of motion predominating. In the resulting 
 mutual collisions it happens at one instant 
 that a molecule breaks up into ions, at 
 another instant that individual ions recom- 
 bine into molecules. There are, so to speak, 
 incessant unions and separations, in spite of 
 which the number of dissociated molecules 
 remains sensibly independent of the time. 
 
 When two electrodes connected with the 
 poles of a battery are immersed in the solu- 
 tion, the ions of the two kinds for ex- 
 ample, the positive ions of sodium and the 
 negative ions of chlorine no longer wander 
 at haphazard in any direction ; but, obeying 
 the electric force, the first approach the nega- 
 tive electrode, or cathode, the second the 
 positive electrode, or anode. On arriving 
 at the electrodes, the ions give up their 
 charges and become neutral atoms, which 
 remain free, at least if no special chemical 
 action takes place between these atoms and
 
 ELECTROLYTIC IONS AND ELECTRONS 3 
 
 the surrounding bodies (this, by the way, is 
 precisely what would happen in the case of 
 the sodium). The electric current in the 
 liquid consists in this transport of electricity 
 brought about by the ions. 
 
 Electrolysis obeys the two laws which 
 were enunciated by Faraday. The first of 
 these laws, which asserts the proportionality 
 existing between the quantity of electricity 
 traversing the liquid and the quantity of 
 matter deposited on the electrodes, is em- 
 bodied in the statement that all the ions 
 existing in the liquid possess charges equal 
 in absolute value. Thus, in the case of 
 chloride of sodium, the ions of the metal 
 have positive charges all equal among them- 
 selves, and equal, but of opposite sign, to the 
 charge of any one of the ions of chlorine. 
 
 In order to satisfy Faraday's second law, 
 according to which a proportionality exists 
 between the relative chemical equivalents of 
 different electrolytes and the amount of each 
 decomposed when the same quantity of elec- 
 tricity is transmitted through them all, as 
 would be the case when all were placed in
 
 4 ELECTROLYTIC IONS AND ELECTRONS 
 
 series in a single circuit, it is necessary to 
 admit that all univalent atoms possess a 
 charge equal in absolute value to that of the 
 sodium or of the chlorine ion : that all atoms 
 which possess twice this charge behave as 
 bivalent, etc. The following example will 
 make this point clear. If a current is passed 
 through a solution of the chloride of copper 
 the molecules of which contain two atoms of 
 copper (univalent) and two of chlorine, and 
 also through a solution of the other chloride, 
 the molecules of which contain one atom of 
 copper (bivalent) and two of chlorine, there 
 will collect on the cathode of the first solu- 
 tion a quantity of copper double that on the 
 second cathode, although, naturally, the quan- 
 tity of positive electricity transported through 
 the two liquids will have been the same. 
 
 In 1 88 1 the illustrious Helmholtz pointed 
 out that the laws of electrolysis suggest the 
 idea that the electric charge pertaining to 
 any valency of an ion may be a fixed quan- 
 tity having a separate existence ; and, since 
 a material atom is a fixed and determinate 
 portion of a certain kind of matter and is
 
 ELECTROLYTIC IONS AND ELECTRONS 5 
 
 considered indivisible, it is thus natural to 
 consider this electric charge as fixed and in- 
 divisible, all the more because a quantity of 
 electricity smaller than this is never encoun- 
 tered. The charge of the ion (univalent) 
 may therefore be called an atom of electric- 
 ity, or better, as Mr. Stoney has proposed, 
 an electron (electric ion). 
 
 In fact, as early as 1871, considerably in 
 advance of Helmholtz, Weber conceived the 
 idea of the atomic structure of electricity. 
 This famous physicist and mathematician 
 proposed the well-known theory, according 
 to which electric phenomena are due to par- 
 ticles or atoms of positive and negative elec- 
 tricity acting on each other at a distance, 
 with forces depending, not only upon the 
 distance itself, but also on the velocity of the 
 particles and on their accelerations ; that is, 
 on the manner in which these velocities vary. 
 Naturally this theory in which action at a 
 distance was still admitted has nothing in 
 common with that now in favour except the 
 fundamental concept of the electric atom ; 
 and although Weber, searching in his theory
 
 6 ELECTROLYTIC IONS AND ELECTRONS 
 
 for the cause of the forces which govern the 
 atomic structure of bodies, advanced the 
 hypothesis " that to every ponderable atom 
 there is united an electric atom" (i), the re- 
 lation now admitted between ions and elec- 
 trons seems to have been better perceived 
 by Helmholtz. 
 
 We do not believe that the atomic hy- 
 pothesis concerning the nature of electricity 
 imposes on us the necessity of considering it 
 as matter, since we are still free to suppose 
 than an electron may be simply a special 
 localized condition of the universal ether. 
 We may, on the contrary, from now on add 
 that instead of considering electricity as 
 matter, we are led to the exactly opposite 
 hypothesis that the atoms of various bodies 
 are systems of electrons. 
 
 When the ions arrive on the electrodes 
 and become neutral atoms, the electrons en- 
 ter into the circuit to constitute the electric 
 current. Now it seems natural to suppose 
 that these electrons, instead of merging, so 
 to speak, into a homogeneous whole (the 
 old electric fluid), preserve their individual-
 
 ELECTROLYTIC IONS AND ELECTRONS 7 
 
 ity; this is all the more natural because, if 
 they are to pass from one atom to another, 
 it is most probable that they must exist mo- 
 mentarily isolated ; thus the electric current 
 in conductors would be nothing else than a 
 motion of free electrons across interatomic 
 space. It remains undetermined whether 
 the current consists in the motion of posi- 
 tive electrons in one direction and negative 
 in the opposite direction, or in the motion in 
 a given direction of one of the two kinds of 
 electrons, say the negative ; but preference 
 is given to the latter opinion, because, while 
 there is reason to hold that the negative 
 electrons may exist in a free state, this is not 
 true of the positive electrons. Only the 
 former, as it appears, suffer displacement, 
 separate themselves from ponderable matter 
 or unite with one another, and vibrate in 
 light sources, as we shall soon see. There- 
 fore, while a negative ion in being deposited 
 on the anode gives up its electron, a positive 
 ion arriving at the cathode does not give up 
 the positive electron, but takes away a nega- 
 tive electron from the cathode itself.
 
 8 ELECTROLYTIC IONS AND ELECTRONS 
 
 Here, then, is the old theory of the electric 
 fluid in a certain sense called back to life, 
 but profoundly modified. It is no longer 
 a question of a continuous fluid, but of 
 special atoms (the electrons), which, however, 
 as has already been observed, are not neces- 
 sarily to be considered as material in the 
 ordinary sense of the word. 
 
 Besides, and this is of more importance, 
 we do not attribute to the atoms of electricity 
 that mysterious faculty of acting at a dis- 
 tance, with which the old fluid was supposed 
 to be endowed, but instead we suppose that 
 the reciprocal forces between the electrons 
 have their origin in the special elastic 
 deformations of the ether, identical with 
 those called for in Maxwell's theory to take 
 account of the electric forces between 
 conductors. 
 
 To explain the phenomena of electrolysis 
 it is sufficient to admit, as is always done, 
 the hypothesis of electrolytic dissociation ; 
 but this hypothesis is not well adapted to 
 explain the propagation of electricity in 
 gases and certain other phenomena. How-
 
 ELECTROLYTIC IONS AND ELECTRONS 9 
 
 ever, with the admission of electric dissocia- 
 tion, that is, the separation of the negative 
 electrons from the neutral atoms, we assign 
 a reason both for electrolysis and for these 
 other phenomena as well. 
 
 In order that a negative electron may 
 separate itself from a neutral atom, energy 
 must be expended to overcome the attrac- 
 tion by which the electron is held to the 
 positive ion, which is what remains of the 
 atom when the negative electron is taken 
 from it, precisely as it is necessary to fur- 
 nish heat energy to separate the molecules 
 of a liquid from one another in evaporation, 
 or as it is necessary to do mechanical work 
 in lifting a weight from the earth. 
 
 The energy necessary to ionize or dissoci- 
 ate an atom naturally varies according to its 
 chemical nature. Experiment indicates that 
 this energy is a minimum for the so-called 
 electropositive bodies, such as the metals, 
 and gradually becomes greater as we pro- 
 ceed toward the more electronegative bodies, 
 which, moreover, may even take on new 
 negative electrons. This energy depends
 
 10 ELECTROLYTIC IONS AND ELECTRONS 
 
 also upon the nature and on the condition 
 of the atoms surrounding the one which 
 is about to break up into an electron and 
 a positive ion ; it is extremely small for 
 bodies in aqueous solution. 
 
 This being so, electrolytic dissociation, or 
 the separation of a molecule into two ions, 
 for example, sodium chloride into a positive 
 ion of sodium and a negative ion of chlorine, 
 should be considered to be a consequence 
 of the dissociation of the metallic atom. 
 This atom breaks up into a positive ion of 
 sodium and into a negative electron, which 
 is seized by the chlorine atom, transforming 
 the latter into a negative ion. Once we 
 adopt this mode of considering electrolytic 
 dissociation, it, with all its very important 
 consequences, enters into the more general 
 electron theory.
 
 CHAPTER II 
 
 THE ELECTRONS AND THE PHENOMENA 
 OF LIGHT 
 
 WHILE the hypothesis of the electrons 
 springs in such a natural manner from elec- 
 trolytic phenomena, it is in an entirely differ- 
 ent field of physics, namely, that of optics, 
 that it finds an unexpected and brilliant con- 
 firmation. 
 
 It is a fact, now recognized by all, that 
 light is a vibratory phenomenon, and may no 
 longer be considered to be due to the emis- 
 sion of discrete corpuscles by luminous 
 bodies, as Newton supposed. In support of 
 this many beautiful and classic experiments 
 exist, with which the names of Young, of 
 Fresnel, and of Foucault are connected. 
 And when we speak of light, we necessarily 
 include radiant heat, because since the cele- 
 brated researches of Melloni there can exist 
 no doubt concerning the identity of the
 
 12 THE PHENOMENA OF LIGHT 
 
 nature of these phenomena, which appear to 
 be so different. 
 
 But the undulatory theory demands a 
 medium capable of propagating the waves; 
 from this arises the necessity of admitting 
 the existence of the ether ; that is, of a sub- 
 stance distributed everywhere throughout 
 interplanetary and interstellar space and 
 throughout interatomic space as well. The 
 hypothesis of the ether forces itself on us in 
 an irresistible manner, and almost seems to 
 acquire the character of reality and certitude, 
 when we consider the perfection with which 
 the undulatory hypothesis takes account of 
 all optical phenomena even quantitatively 
 and in the minutest detail. 
 
 Following the example of Fresnel, light 
 vibrations were considered for a long while 
 to be true mechanical vibrations of the 
 ethereal and material particles, but later it 
 was recognized, especially in consequence of 
 the work of Maxwell, that light waves could 
 be considered as electromagnetic waves; 
 thus two distinct classes of physical phe- 
 nomena were united. The electromagnetic
 
 THE PHENOMENA OF LIGHT 13 
 
 theory of light has received in recent years 
 a material support from the well-known ex- 
 periments of Hertz, and later from those of 
 other physicists ; and at present there is per- 
 haps no one who will refuse to admit that 
 the phenomena of light are in reality electro- 
 magnetic phenomena, and that light waves 
 differ from those which Hertz has shown 
 how to produce, only in so far as numerical 
 values are concerned. 
 
 However, the electromagnetic theory of 
 light, derived as it is from the properties of 
 the electromagnetic field, does not serve to 
 explain those phenomena to take account of 
 which it was necessary, under the old me- 
 chanical theory, to resort to an action of pon- 
 derable matter on the ether. To complete 
 the theory accepted at present, it was there- 
 fore necessary to take account, in some 
 manner, of the material atoms; and the 
 Dutch physicist, Lorentz, had the fortunate 
 idea of considering the electric charges of 
 the atoms together with the atoms them- 
 selves. If merely the negative or merely 
 the positive charges take part in the light
 
 14 THE PHENOMENA OF LIGHT 
 
 vibrations, and if both the electric and mag- 
 netic forces generated by their motion be 
 taken into account, one arrives at an electro- 
 magnetic theory of light capable of explain- 
 ing even those phenomena which eluded the 
 theory based simply on the formulae of 
 Maxwell or of Hertz. 
 
 Let us here consider a most interesting 
 phenomenon discovered by Zeeman, a former 
 pupil of Lorentz, because it is one of those 
 by which the relative independence of the 
 negative electrons and their characteristic 
 freedom of motion is most clearly demon- 
 strated. 
 
 It is a well-known fact that a luminous 
 gas emits radiations of definite periods of 
 vibration, and not those corresponding to a 
 continuous series of intermediate periods. 
 As a result the spectrum of the light emitted 
 by the gas reduces to a limited number of 
 narrow lines, which are the images of the 
 slit through which the light is passed in 
 order to analyze it with the prism. For ex- 
 ample, the spectrum of the light emitted by 
 sodium in the gaseous state consists of two
 
 THE PHENOMENA OF LIGHT 15 
 
 yellow lines very near each other, which with 
 low-power spectroscopes appear blended into 
 a single line. Now Zeeman showed that if 
 a gas is placed in an intense magnetic field, 
 say between the poles of a powerful electro- 
 magnet, each simple line of its spectrum is 
 in general broken up into a group of new 
 lines. 
 
 There are two important cases to be con- 
 sidered : First, that in which the luminous 
 ray which we are considering is parallel to 
 the lines of magnetic force ; second, that in 
 which the ray is perpendicular to the lines of 
 force. The general case is naturally a trifle 
 complicated, and for its treatment I refer the 
 reader to original articles on this subject (2). 
 
 Let us suppose that we have a luminous 
 gas between the two opposite magnetic poles ; 
 for example, vapour of cadmium obtained by 
 passing electric sparks between two wires of 
 that metal. If we examine the light which 
 is propagated in the direction of the lines of 
 force (Case I), that is, from one pole toward 
 the other, we easily ascertain that, while the 
 green line of the spectrum of cadmium ap-
 
 16 
 
 THE PHENOMENA OF LIGHT 
 
 pears sharp and simple as in A (Fig. i) be- 
 fore the magnetic field is set up, the instant 
 the field is created the line A vanishes, and 
 instead of it there appear two new lines, B 
 and C, one on either side of the position A\ 
 
 A 
 
 B K C 
 
 FIG. i. 
 
 which the single line at first occupied, and at 
 equal distances from it. 
 
 If we examine with the spectroscope a ray 
 of light in the equatorial direction (Case II), 
 that is, perpendicular to the direction of the 
 magnetic field, the single line, A (Fig. 2), is 
 replaced by three lines, C, A', B, of which 
 the one in the middle, equidistant from
 
 THE PHENOMENA OF LIGHT 
 
 the other two, occupies the position of the 
 original line. 
 
 In the case of other spectrum lines, either 
 the phenomena are identical with those 
 which are exhibited by the green cadmium 
 
 .A 
 
 FIG. 2. 
 
 line, or slightly more complicated effects 
 are obtained. Thus, for example, of the 
 two sodium lines, the one usually called /?, 
 is transformed in the second case into four 
 lines, A, B, C, D (Fig. 3), while the line Z? 2 
 changes into a group of six lines (Fig. 4), 
 A, B, C, D, E, F. 
 
 Complete explanation of these phenomena,
 
 IS 
 
 THE PHENOMENA OF LIGHT 
 
 at any rate in the least complicated cases, is 
 furnished by Lorentz's theory; but for our 
 purpose it will be sufficient to treat the 
 single case, to which Figure i refers, of light 
 emitted by the vapour of cadmium in the 
 direction of the lines of force. 
 
 D, 
 
 A B 
 
 C D 
 
 FIG. 3. 
 
 Let us consider an electrified particle 
 which, attracted toward a position of equi- 
 librium, O (Fig. 5), vibrates about this point 
 with circular motion, describing a circumfer- 
 ence of radius OA. The vibrating electri- 
 fied particle generates light waves. Suppose 
 we study the light which is propagated in
 
 THE PHENOMENA OF LIGHT 
 
 the direction perpendicular to the plane of 
 the circumference. If a magnetic force acts 
 
 
 
 
 
 A B C . D E F 
 
 
 
 
 
 
 
 
 FIG. 4. 
 
 in this direction, there exists at every instant 
 an electromagnetic force analogous to that 
 which would act on a short portion of an 
 electric current coin- 
 ciding in direction with 
 the velocity of the 
 particle. This force, 
 therefore, will be di- 
 rected along the radius 
 OA, passing through 
 the moving particle, FIG. 5.
 
 20 THE PHENOMENA OF LIGHT 
 
 and will act, as the case may be, either 
 from A toward O, or in the opposite direc- 
 tion. The effect of this new force, which 
 augments or diminishes the force which 
 maintains the particle in its orbit, is to vary 
 its vibratory period, that is, the time re- 
 quired for the particle to describe the cir- 
 cumference, just as a change in the 
 intensity of the force of gravity has the 
 effect of varying the period of oscillation 
 of a pendulum. 
 
 From the effect produced by a magnetic 
 field on a circular vibration we may easily 
 pass to that produced on any vibration 
 whatsoever by a consideration like the fol- 
 lowing : 
 
 Light vibrations are perfectly well under- 
 stood. They follow the same laws as do the 
 small oscillations of a pendulum, and are, in 
 general, elliptical ; in special cases they may 
 be rectilinear or circular; they are always 
 transverse, that is, they lie in the plane 
 perpendicular to the light ray. Now it may 
 be shown that every elliptical vibration is 
 kinematically equivalent to the resultant of
 
 THE PHENOMENA OF LIGHT 
 
 21 
 
 two circular vibrations of opposite directions 
 of rotation, the one right-handed (motion in 
 the same sense as that 
 of the hands of a 
 watch), the other left- 
 handed ; and in ad- 
 dition the circular 
 vibration which has the 
 same direction of rota- 
 tion as the ellipse EF 
 (Fig. 6), namely AB, L ' 
 has a diameter equal 
 to half the sum of the axes of the ellipse, 
 while the other circular vibration, CD, has 
 a diameter equal to half the difference of 
 the same axes. 1 If we do not wish to have 
 
 1 Employing the usual symbols, the elliptical vibration, 
 referred to its axes, may be represented by means of its rec- 
 tangular components 
 
 FIG. 6. 
 
 This is evidently equivalent to the resultant of two circular vi- 
 brations, one of which is right-handed like the given ellipse, 
 and has the components 
 
 a -^- o f\ (i -4- o /\ 
 
 x * sin \j v * cos v 
 
 2 2 
 
 while the other is left-handed and has the components
 
 22 
 
 THE PHENOMENA OF LIGHT 
 
 recourse to a mathematical demonstration, 
 we may convince ourselves of the proof of 
 this statement by employing a special piece 
 of apparatus which serves, among other pur- 
 
 FlG. 7. 
 
 poses, that of effecting the composition of two 
 pendular vibrations of circular character (3). 
 
 Two pendulums (Fig. 7) are suspended 
 from two fixed points, which for the sake of 
 simplicity are not represented in the figure,
 
 THE PHENOMENA OF LIGHT 23 
 
 placed one above the other on the same 
 vertical line. One of the pendulums con- 
 sists simply of a wire carrying at its lower 
 end a heavy ring and a funnel, A, filled with 
 sand ; the lower part of the other consists of 
 a platform BC, situated below A, which car- 
 ries on one of its sides a second funnel, D, 
 also filled with sand. The length of the 
 first pendulum may be varied at will, but 
 for the experiment with which we have to 
 deal this length should be such that the 
 two pendulums have the same period of 
 vibration. A simple electric device controls 
 the openings of the funnels and thus pre- 
 vents or allows the flow of sand. 
 
 At first let us suppose the pendulum BC 
 to be fixed while we impart a circular motion 
 to A ; it is easy to ascertain whether or not 
 this motion be circular from the trace left by 
 the sand on the platform BC. Let us then 
 impart a circular motion to the pendulum 
 BC as well, but in the direction opposite to 
 that of the first pendulum ; we may easily 
 ascertain if we succeed in doing this by 
 observing the trace left by the sand from the
 
 24 THE PHENOMENA OF LIGHT 
 
 funnel D on the plane of support. If, now 
 that the two pendulums are vibrating, we 
 allow the sand to run out of the funnel A, 
 it will form an elliptical trace on the plat- 
 form BC. This ellipse becomes a straight 
 line if the two component circular vibrations 
 have equal diameters. We can thus con- 
 vince ourselves of the truth of the statement 
 made, and in addition we learn that, when 
 the two component circular vibrations of op- 
 posite rotation have equal amplitudes, the 
 resulting vibration is rectilinear. 
 
 Returning now to the case of the particle 
 vibrating in a magnetic field, it in general 
 executes an elliptical vibration, for which we 
 may conceive the two equivalent circular 
 vibrations to be substituted. But these last 
 are of opposite sign of rotation; if one of 
 them is accelerated by the magnetic field, 
 the other must be retarded. As soon as 
 their periods cease to be equal, they can no 
 longer cause a single spectrum line, but 
 cause instead two new lines situated on 
 either side of the single primitive line. This 
 explanation, furnished by Lorentz's theory
 
 THE PHENOMENA OF LIGHT 25 
 
 to explain the experiment of Zeeman, was 
 proved correct by this able experimenter in 
 a series of new experiments, which showed 
 that the two new lines were in fact due to 
 circular vibrations, one right-handed and the 
 other left-handed. 
 
 Suitable qualitative and quantitative ex- 
 periments made it possible to deduce two 
 very interesting results from the Zeeman 
 phenomenon. On investigating which of 
 the two new lines was due, for a given 
 direction of the magnetic field, to the right- 
 handed vibrations and which to the left- 
 handed, the sign of the charge of the 
 vibrating particles could be determined, and 
 it was recognized that, in order to make the 
 observed facts accord with their explanation, 
 it was necessary to admit that these particles 
 possessed a negative rather than a positive 
 charge. In the second place, it was possible 
 to obtain an approximate evaluation of the 
 ratio of the electric charge of the vibrating 
 particle to its mass. The result to which 
 this led was, that this ratio is more than a 
 thousand times greater than that which re-
 
 26 THE PHENOMENA OF LIGHT 
 
 lates to the atom of hydrogen in electrolysis, 
 and hence still greater than that pertaining 
 to the atoms of other substances. 
 
 This result may be interpreted in several 
 ways, the more important of which are the 
 following: either the vibrating particles are 
 ions, and the charge of each is more than 
 one thousand times as great as that which 
 pertains to each valency in electrolysis; or 
 the vibrating particles have a charge equal 
 to that of the electrolytic ions, and their 
 mass is less than one-thousandth as great as 
 that of an ion of hydrogen. The second 
 interpretation is naturally the one accepted, 
 and the vibrating particles are considered to 
 be free electrons. These therefore possess, 
 or at least there is united with them, a small 
 material mass; but we shall see that this 
 same mass probably has an electromagnetic 
 cause. At any rate, this result is corrobo- 
 rated by those which are reached in other 
 ways, as will be shown later. 
 
 Lorentz's theory receives, therefore, a splen- 
 did confirmation through the experiments of 
 Zeeman ; and hence it may be retained, that
 
 THE PHENOMENA OF LIGHT 27 
 
 the structure of the material atoms is such 
 as to permit the negative electrons, which 
 form a part of them, to vibrate freely, while 
 the positive part remains relatively fixed. 
 Thus we conceive a neutral atom to consist 
 of one portion which in the aggregate has a 
 positive charge, and of one or more negative 
 electrons, which move about it like the sat- 
 ellites about a planet, held in their orbits by 
 the electric force. 
 
 The so-called oscillators, or the apparatus 
 used to generate electromagnetic waves, have 
 recently come into considerable prominence. 
 One of the possible but impractical forms 
 consists of an electrified body executing vi- 
 bratory motion ; for example, one driven by 
 a sonorous body in vibration. Now if one 
 imagines the electrified particle to be replaced 
 by a simple electron, and if one supposes the 
 period of vibration to be so small that it 
 would be expressed by a fraction whose 
 numerator is unity and whose denominator 
 is a number of fifteen places, then this 
 electron would generate ordinary light waves 
 instead of Hertzian electromagnetic waves.
 
 CHAPTER III 
 
 NATURE OF THE CATHODE RAYS (4) 
 
 THE phenomena on which we shall now 
 touch show the negative electrons in the act 
 of undergoing very rapid motions of transla- 
 tion instead of the vibratory motions which 
 we have considered in the preceding chapter. 
 Hence they present themselves under condi- 
 tions favourable for a closer study, and thus, 
 by modifying their motion in various ways, 
 new and interesting effects may be produced. 
 But for the sake of clearness it will be well 
 first to state the principal characteristics of 
 electric discharges in rarefied gases. 
 
 Let us consider a glass tube, AC (Fig. 8), 
 through the end walls of which are fused two 
 platinum wires terminating in aluminium elec- 
 trodes, A, C. If the air pressure in the tube 
 is somewhat less than that of the atmosphere, 
 for example, eight or ten millimeters of 
 mercury, and if an electric discharge is 
 
 28
 
 NATURE OF THE CATHODE RAYS 29 
 
 . 
 
 mm:::':. 
 
 FIG. 8. 
 
 caused to pass from one electrode to the other, 
 instead of the well-known loud and brilliant 
 spark which is formed in the open air, a char-
 
 30 NATURE OF THE CATHODE RAYS 
 
 acteristic luminous phenomenon is obtained, 
 in which two regions are distinguishable : the 
 positive luminous column, a sort of ill-defined, 
 rose-coloured spark having a blurred contour 
 which reaches from the anode up to within a 
 short distance of the cathode ; and the nega- 
 tive column, or negative glow, violet in colour 
 and contiguous with the cathode. Between 
 these two luminous regions there is an inter- 
 val called the Faraday dark space. 
 
 If now the pressure of the air is diminished, 
 the luminous character changes. We will con- 
 cern ourselves with the negative light with- 
 out considering further the positive column 
 which, with increasing rarefaction, gradually 
 diminishes both in size and in luminous in- 
 tensity, often subdividing into distinct regions 
 separated by relatively dark intervals (striated 
 discharge). At first the negative light ex- 
 tends over the entire cathode, as at C\ in 
 case initially it only covered the extremity ; 
 but later, with still further rarefaction, it ex- 
 tends all around to greater and greater dis- 
 tances, detaching itself at the same time from 
 the electrode, as at C". In the meantime a
 
 NATURE OF THE CATHODE RAYS 31 
 
 new luminous stratum forms in contact with 
 the electrode, and thus the negative light is 
 divided into two parts ; namely, the first 
 negative column, adhering to the cathode, 
 and the second negative column, or nega- 
 tive glow, separated from each other by a 
 relatively dark region which, to distinguish 
 it from the Faraday dark space, is called the 
 cathode dark space. Continuing the rare- 
 faction still further, the two luminous nega- 
 tive columns extend farther and farther out, 
 becoming continually less bright and sharp 
 in outline (C'", Fig. 8). The interval which 
 separates them also becomes greater; and, 
 when the highest rarefaction is obtained, that 
 is to say when the air pressure is reduced to 
 less than one-thousandth of a millimeter of 
 mercury, almost every trace of luminosity in 
 the gas disappears. 
 
 But before this stage is reached a new 
 phenomenon appears. At first the portion 
 of the walls of the tube about the cathode, 
 and later that in front of it, becomes lumi- 
 nous, diffusing a brilliant light, usually 
 green, due to a species of phosphorescence,
 
 32 NATURE OF THE CATHODE RAYS 
 
 or perhaps better of fluorescence, since this is 
 the name given to the emission of light by 
 fluorspar and certain other substances, which 
 does not last appreciably after the cause 
 which produced it has ceased. The cause 
 of this phenomenon is to be sought in the 
 cathode, because, if an obstacle is placed 
 between it and the wall, a very sharp shadow 
 is thrown,' as if the fluorescence were excited 
 by invisible radiations sent out from the 
 cathode. We will now turn our attention 
 to these radiations, which are called Cathode 
 Rays. 
 
 They are propagated in straight lines and 
 leave the cathode in a direction at right 
 angles to its surface ; hence if this has the 
 form of a concave mirror, the cathode rays 
 converge practically to the centre of curva- 
 ture. When concentrated in this manner 
 their singular properties become more evi- 
 dent, as Sir William Crookes has shown in 
 a very brilliant and suggestive manner with 
 the aid of cleverly devised apparatus. 
 
 The principal properties of the cathode 
 rays are the following : they excite phospho-
 
 NATURE OF THE CATHODE RAYS 33 
 
 rescence not only in glass, as we have seen, 
 but in a large number of other bodies, includ- 
 ing those which phosphoresce under the ac- 
 tion of light. The cathode rays heat bodies 
 which they strike and tend to move them as 
 if the impact were mechanical. It is pos- 
 sible, however, that this mechanical action 
 may be, at least in a large part, a simple con- 
 sequence of the preceding effect. Finally, 
 bodies struck by cathode rays become sources 
 of new radiations ; namely, the famous X-rays 
 discovered by Professor Rbntgen. In order 
 to explain all these phenomena, Crookes 
 brought forward his hypothesis of radiant 
 matter. 
 
 As early as 1816 the celebrated Faraday (5) 
 pointed out the possibility of a fourth state 
 of matter, as a consequence of a hypothetical 
 transformation which transcends evaporation 
 by as much as evaporation transcends the 
 fluid state ; or, he expressed his thought still 
 better by saying that he looked forward with 
 the greatest impatience to the discovery of 
 a new state of the chemical elements. He 
 suggested further, and this has an especial
 
 34 NATURE OF THE CATHODE RAYS 
 
 interest with reference to the theory with 
 which we are now occupied, that the decom- 
 position of the metals, their recomposition 
 and the realization of the formerly absurd 
 idea of transmutation, were problems which 
 chemistry one day must solve. 
 
 According to Crookes, when the electric 
 discharge takes place in a highly rarefied 
 gas, very minute negatively electrified mate- 
 rial particles are projected from the cathode 
 and, forming a fourth state of matter tran- 
 scending the gaseous state, produce the ob- 
 served effects as a result of their collisions ; 
 moreover, the trajectories of these particles 
 constitute the cathode rays. It was later 
 thought that the particles were the actual 
 atoms of the gas residue which on account 
 of its extreme rarefaction presented such 
 new properties as were brought to light by 
 the rotation of the vanes of Crookes' famous 
 radiometer. 
 
 But some people, among whom was the 
 illustrious Hertz, preferred to consider the 
 cathode rays as an undulatory phenomenon 
 similar to light, having its origin at the sur-
 
 NATURE OF THE CATHODE RAYS 35 
 
 face of the cathode and its seat in the ether. 
 However, this opinion had very soon to be 
 given up on account of subsequent experi- 
 ments. Thus, while many physicists, notably 
 J. J. Thomson (6) in England, to whom we 
 owe so much of the present electron theory, 
 and Q. Maiorana (7) in Italy, were finding 
 out that the velocity of the cathode rays is 
 noticeably less than that of light, J. Perrin (8) 
 was making it evident that the cathode 
 rays produce a transport of negative electric- 
 ity. This last effect may be obtained 
 even when the rays have passed through a 
 thin metallic plate, as was later shown by 
 Lenard (9). 
 
 Perrin's experiment may be made with a dis- 
 charge tube similar to that shown in Figure 9. 
 The cathode consists of an aluminium disk, 
 and the anode ABDE is a cylindrical box 
 with circular openings at the centre of 
 the bases. This box is in connection with 
 the earth, and contains the conductor /% 
 which is connected to an electroscope. The 
 conductor F usually has the form of a hol- 
 low cylinder with an opening turned toward
 
 36 NATURE OF THE CATHODE RAYS 
 
 that in the base, DE, of the anode. A nega- 
 tive charge collects on the conductor /''when 
 the discharge enters the tube. Evidently 
 this can only be due to a transport of charge 
 effected by the cathode rays. Moreover, at 
 the approach of a magnet, the action of 
 which, as we shall see, is to make the cathode 
 rays assume a curved path, the rays cease to 
 
 FIG. 9. 
 
 enter the cylinder AD and cause a luminous 
 spot to appear on the base DE, which for 
 this purpose is usually coated with a phos- 
 phorescent substance. Now just as soon as 
 the conductor F ceases to receive the rays, 
 it, in turn, ceases to receive electricity. 
 
 Naturally the discovery of these facts fur- 
 nished the strongest support for Crookes' 
 theory. But countless recent experiments 
 due to many physicists have led to a slight
 
 NATURE OF THE CATHODE RAYS 37 
 
 modification, and a better statement of the 
 original hypothesis and to the admission that 
 the particles, which in their rapid motion 
 constitute the cathode rays, can be nothing 
 but the negative electrons themselves. This 
 opinion, at present held by all, rests princi- 
 pally on the following facts which have been 
 accurately verified, and with which we shall 
 concern ourselves in detail a little later. In 
 the first place, the cathode rays always have 
 identical properties whatever may be the 
 rarefied gas in which they are formed, and 
 whatever may be the nature of the cathode ; 
 in the second place, the moving negative 
 particles all possess that same small mass 
 less than one-thousandth that of a hydrogen 
 atom which is encountered, as we have seen, 
 in the study of the Zeeman effect, and which 
 is deduced from the results of various other 
 phenomena as well. 
 
 Cathode rays may also be produced with- 
 out having recourse to the electric discharge. 
 Thus, for example, a body exposed to the 
 action of light, or better of ultraviolet rays, 
 emits electrons. Unless the surrounding
 
 38 NATURE OF THE CATHODE RAYS 
 
 gas is extremely rarefied they unite with 
 neutral atoms and form negative ions; but 
 if the gas is almost entirely removed, the 
 electrons remain free, and on leaving the 
 body form true cathode rays (10), which in 
 general possess a velocity less than that 
 which they have in discharge tubes. This 
 velocity decreases as the negative potential 
 of the illuminated body is diminished. 
 
 Also in the case of the cathode rays the 
 mass of the electron was not separately de- 
 termined, but rather the ratio between the 
 electric charge of any electron and its mass. 
 Such a determination as this is based on the 
 effects produced on the cathode rays by elec- 
 tric or magnetic forces, and these effects are 
 in good accord with the accepted hypothesis. 
 In fact, it is clear that when an electric force 
 acts on negative particles in motion, they 
 should deviate from their ordinary rectilinear 
 path ; and since an electrified particle in 
 motion should behave in a manner analo- 
 gous to a current, or more accurately to an 
 element of current, it follows that the par- 
 ticle itself should deviate from its ordinary
 
 NATURE OF THE CATHODE RAYS 39 
 
 path when it is exposed to the action of a 
 magnetic field. But we shall consider such 
 phenomena and the measurements relating 
 to them a little farther on.
 
 CHAPTER IV 
 
 THE IONS IN GASES AND IN SOLIDS 
 
 IN electrolytes the electrons are joined to 
 the neutral atoms to form free ions, and the 
 motion of these ions is what constitutes the 
 electric current. At present the opinion is 
 held that the same thing happens in gases ; 
 namely, that when a gas possesses electric 
 conductivity, it owes it to the presence of 
 ions, and to their motion under the action 
 of electric forces. The hypothesis of the 
 ionization of gases, which, for a long time, 
 was held by very few, is now generally 
 admitted in consequence of the numerous 
 experiments made in recent years. 
 
 We are, then, of the opinion that a gas 
 contains free ions. These are ordinarily 
 present in such a small number that the 
 resulting conductivity is very small. But 
 there are circumstances in which, by the 
 
 40
 
 THE IONS IN GASES AND IN SOLIDS 41 
 
 action of appropriate external energy, the 
 gas is ionized ; that is to say, many of its 
 atoms are broken up into positive ions and 
 negative electrons. If the gas is not suf- 
 ficiently rarefied, the electrons unite with 
 the neutral atoms and form negative ions. 
 Moreover, certain facts seem to indicate that 
 atoms or neutral molecules can unite with 
 ions to form groups which, while having the 
 usual charge of the ions, possess masses 
 much greater than those which can pertain 
 to a simple ion. 
 
 The most natural explanation of the 
 known facts, and in particular of those 
 about to be mentioned, is that the electrical 
 conductivity of gases is due to the presence 
 of electrified particles which are free to move 
 between its molecules. 
 
 An ionized gas loses its conductivity 
 when passed through minute interstices, say 
 through a mass of glass wool, or through 
 long and fine metallic tubes, or is made to 
 bubble through a conducting liquid (n), 
 which, however, should not contain any 
 radio-active substance. The same result is
 
 42 THE IONS IN GASES AND IN SOLIDS 
 
 obtained if the gas is made to pass between 
 two oppositely electrified conductors in such 
 a way that it may serve as a conductor for 
 the current. In the first case, the phenome- 
 non is explained by the attraction exerted on 
 the ions by the bodies near which they pass ; 
 in the second case, the two conductors 
 attract and hold the ions which carry a 
 charge opposite in sign to their own and so 
 remove them from the gas. 
 
 The manner in which an ionized gas be- 
 haves when it is carrying an electric cur- 
 rent is also in perfect accord with the 
 accepted hypothesis. Let us suppose that 
 we have, for example, two parallel metallic 
 disks, one of which communicates with the 
 insulated pole of a battery, and the other 
 with an electrometer. If we ionize the air 
 between the disks by passing Rontgen rays 
 through it, and if we vary the value of the 
 potential furnished by the battery, we find 
 that the gas fails to follow the well-known 
 law of Ohm, which holds for constant elec- 
 tric currents, and according to which the 
 intensity of the current in a conductor in-
 
 THE IONS IN GASES AND IN SOLIDS 43 
 
 creases in proportion to the difference of 
 potential between its ends. In fact, the 
 intensity of the current measured by the 
 charge, which is acquired in a given time by 
 the disk in communication with the elec- 
 trometer, increases considerably less rapidly 
 than the potential. The intensity even 
 finally assumes a limiting value which does 
 not increase as the potential of the battery is 
 raised. When the current has attained this 
 value, called the saturation value, all the 
 ions generated in a given time by the Rbnt- 
 gen rays (or in general produced by what- 
 ever source of ionization is employed) are 
 utilized in transmitting the current in this 
 same time. An increase in potential is of 
 no effect, as there are not a greater number 
 of ions to be disposed of. 
 
 Moreover, a curious phenomenon met 
 with by the writer (12), and which was 
 confirmed and rightly interpreted by J. J. 
 Thomson and E. Rutherford, is obviously 
 explained by the accepted theory. The phe- 
 nomenon is the following: if the distance 
 between the two metallic disks considered
 
 44 THE IONS IN GASES AND IN SOLIDS 
 
 above is varied, the intensity of the current 
 which traverses the ionized air between them 
 varies as well, but in a manner contrary to 
 that which one would suppose. In fact, the 
 intensity of the current increases, within 
 certain limits, with an increase of the dis- 
 tance. This is easily explained when we 
 reflect that with an increase in the distance 
 between the plates there is an increase in 
 the amount of air in which the phenomenon 
 takes place, and in consequence also in the 
 number of ions, which, by their motion, con- 
 stitute the saturation current. 
 
 The ions in gases move around between 
 the molecules, frequently colliding with 
 them. New ions may form by the breaking 
 up of neutral molecules, and ions of opposite 
 sign may recombine into molecules. This 
 last action, namely, the disappearance of 
 ions, is continually taking place, and it is 
 because of this that the number of ions does 
 not increase beyond a certain limit under 
 the action of an ionizing cause. 
 
 If the ions are generated in a single region 
 of the gas, they diffuse into the remaining
 
 THE IONS IN GASES AND IN SOLIDS 45 
 
 portion. In gases under ordinary pressure 
 the velocity of diffusion is usually extremely 
 small because of frequent collisions; but if 
 an electric field acts, the velocity of diffusion 
 becomes large ; the first time that a measure- 
 ment of this kind was made (13) the velocity 
 was found to be several decimeters per 
 second. 
 
 Ultraviolet rays, cathode rays, Rbntgen 
 rays, rays emitted by radio-active substances, 
 heating to a relatively high temperature, are 
 all causes of ionization. This is greater or 
 less according to circumstances, and is limited, 
 as has already been mentioned, by a continual 
 recomposition of atoms and neutral mole- 
 cules. 
 
 But there exists still another cause of 
 ionization, to which in reality some of the 
 above causes reduce ; this is the collision of 
 the ions (and, in fact, of the electrons as well, 
 since probably some of these exist, at least 
 momentarily, in the free state in gas under 
 ordinary pressure) with the atoms and mole- 
 cules. When an ion possesses a sufficiently 
 high velocity, it can furnish the energy neces-
 
 46 THE IONS IN GASES AND IN SOLIDS 
 
 sary to transform an atom into a positive 
 ion and a negative electron, and hence also 
 to transform a molecule into two ions of 
 opposite sign. Let us briefly take up these 
 various means of ionizing a gas. 
 
 Light radiations, and especially the ultra- 
 violet, may ionize a gas in two different 
 ways. If they strike a solid or liquid body, 
 they produce an emission of negative elec- 
 trons which results in the rapid discharge of 
 the body, if it was negatively electrified, and 
 even the formation on it of a positive charge, 
 as has been demonstrated by the writer (14). 
 Ordinarily the experiment is performed with 
 metals, because the effect is rather weak 
 with liquids, and solid insulators are not so 
 well adapted to quantitative determinations. 
 As a source of active radiations, the invisible 
 ultraviolet rays emitted by an arc light or 
 by an electric spark are employed, although 
 certain bodies, such as the alkaline metals 
 and amalgamated zinc, give a marked effect 
 even with visible radiations. Now if the 
 electric field determined by the negative 
 charge of the body is sufficiently intense,
 
 THE IONS IN GASES AND IN SOLIDS 47 
 
 the negative electrons which are emitted may 
 acquire a velocity great enough to ionize the 
 neutral atoms by impact. 
 
 But even directly the more refrangible 
 ultraviolet radiations given off by the electric 
 spark cause ionization of the gas through 
 which they pass, as was demonstrated by 
 Lenard (15), who allowed the radiations from 
 a spark formed between aluminium wires 
 to fall on electrified bodies. These became 
 discharged with about the same rapidity 
 whether they were charged positively or 
 negatively and whatever was the nature and 
 condition of their surfaces. All this could 
 not be attributed to a surface action, but 
 rather to an action on the mass of the air 
 traversed by the radiations; that is, to the 
 ionization which they produced. An experi- 
 ment which may be repeated also with other 
 sources of ionization confirmed this explana- 
 tion. It consists in blowing the air from the 
 place where it is ionized to another place, 
 where, as a result of the conductivity which 
 it retains for a certain time, it brings about 
 the discharge of electrified bodies. In order
 
 48 THE IONS IN GASES AND IN SOLIDS 
 
 to cause the effect to cease it is sufficient 
 to intercept the radiations. 
 
 It appears that only the more rapid ultra- 
 violet vibrations produce direct ionization 
 of gases in any marked degree. Indeed, the 
 experiments described above do not succeed 
 except when the path traversed in the air 
 by the radiations is reduced to but a few 
 centimeters, and it is a well-known fact that 
 the most refrangible ultraviolet radiations 
 are very rapidly absorbed by air at ordinary 
 pressure. 
 
 The cathode rays which are, as we have 
 seen, nothing but negative electrons in 
 motion, ionize a gas, as will be explained 
 before long in some detail. 
 
 With regard to the Rontgen rays, which 
 apparently are the manifestation of ether 
 waves generated by the sudden variations 
 in velocity of the electrons, the ionization 
 of gases produced by them seems to be due 
 to a sudden electric impulse produced in the 
 electrons of the gaseous atoms. 
 
 Finally, a rise in temperature, which is 
 equivalent to an increase in atomic velocity
 
 THE IONS IN GASES AND IN SOLIDS 49 
 
 and apparently to an increase in the velocity 
 of the negative electrons as well, naturally 
 tends to free the latter from their bond with 
 the positive part of the atom. A red-hot 
 metallic wire ionizes the gas in contact with 
 it, and gases in flames always appear to be 
 strongly ionized. 
 
 In order that the molecules of the gas may 
 become ionized by the impact of those ions 
 which already exist in it, it is generally neces- 
 sary to expose the gas to the action of suffi- 
 ciently intense electrical forces. In too 
 weak a field, the ions, while they follow the 
 electric force, do not acquire a sufficient 
 velocity between one collision and the next, 
 and the effect of the collisions is to keep this 
 velocity constantly at a low value, since natu- 
 rally a part of the energy of motion of the 
 ions is given up to the molecules which have 
 been struck. Under such circumstances the 
 paths described by the ions cannot differ 
 much from the lines of electric force, or, in 
 other words, the ions must continually move 
 very nearly in the direction of the force which 
 urges them. The so-called phenomena of
 
 50 THE IONS IN GASES AND IN SOLIDS 
 
 the electric shadow and similar ones of which 
 the reader will find a partial treatment in 
 another place (16), are the immediate conse- 
 quence of this. 
 
 But if a sufficiently strong electric field 
 acts on the gas, ionization by impact takes 
 place, and it is on this phenomenon that a 
 satisfactory explanation of the complex and 
 varied phenomena of the electric discharge is 
 at present founded. In order to take this 
 matter up, we would be obliged to go beyond 
 the limits assigned to this chapter; but in 
 view of that which follows, it will be useful 
 to call attention by way of an example, to 
 the explanation which is given of the forma- 
 tion of the two columns of negative light and 
 the dark space included between them in the 
 case of the electric discharge in highly rare- 
 fied gases. 
 
 The phenomenon is started by the few 
 electrons existing in the gas, or perhaps also 
 by the negative electrons expelled from the 
 cathode. These electrons move with accel- 
 erated motion and rapidly acquire a velocity 
 sufficient to make them capable of ionizing
 
 THE IONS IN GASES AND IN SOLIDS 51 
 
 by impact the gas molecules at some distance 
 from the cathode. This gives rise to the 
 second negative column, or negative glow, 
 which is thus a region of the gas where ion- 
 ization takes place. The electric force drives 
 the positive ions created in this manner 
 toward the cathode, close to which they 
 possess the velocity required to ionize the 
 gas molecules. This causes the formation 
 of the first column of negative light. 
 
 The electrons produced in this region 
 move away from the cathode, and in this way 
 the two regions of ionization furnish each 
 other the necessary ions or electrons. The 
 cathode dark space is thus simply the region 
 traversed by the electrons constituting the 
 cathode rays, and especially by the positive 
 ions which move toward the cathode, before 
 they have acquired the velocity necessary to 
 produce ionization. 
 
 We will not concern ourselves further 
 with the negative electrons after they have 
 arrived at the second negative column ; how- 
 ever, it is of interest to us to know what be- 
 comes of the positive ions aftet they arrive
 
 52 THE IONS IN GASES AND IN SOLIDS 
 
 at the cathode. Some of them naturally 
 become neutralized by the negative elec- 
 trons ; but others, on account of their velocity, 
 which varies in direction as a result of colli- 
 sions, may bend around the cathode, or even 
 pass through it, if it is provided with open- 
 ings or canals, or if it is made of wire gauze. 
 Beyond the cathode the positive ions then 
 constitute the positive or anode rays, analo- 
 gous to the cathode rays, and which are often 
 called canal rays, the name given them by 
 Goldstein. 
 
 An electric or a magnetic field causes a 
 deviation of the positive rays, but in the 
 direction opposite to that which would be 
 observed with cathode rays; it is precisely 
 on account of this that the conclusion is 
 reached, that these rays consist of positively 
 electrified particles in motion. However, the 
 deviation is noticeably smaller for the posi- 
 tive rays than for the cathode rays under 
 similar conditions. From measurements of 
 the deviation it may be shown that the par- 
 ticles in motion do not possess a minute mass 
 as in the case of the cathode rays, but a mass
 
 THE IONS IN GASES AND IN SOLIDS 53 
 
 comparable with that of atoms or of electro- 
 lytic ions. In this case, therefore, we have 
 to deal not with positive electrons, but with 
 ions and probably with groups of greater 
 mass as well. 
 
 If we admit electric conductivity to be a 
 phenomenon of convection in gases as well 
 as in liquids, the analogous hypothesis for 
 solid conductors, already stated in Chapter I, 
 becomes all the more natural. And, inas- 
 much as it appears that only the negative 
 electrons, and not the positive, can exist iso- 
 lated, it is held that the electric current in a 
 conductor consists, at least principally, in the 
 motion of negative electrons. The experi- 
 mental evidence that metals do not offer an 
 insurmountable obstacle to the motion of the 
 electrons is furnished by their perviousness 
 to the cathode rays. Without entering into 
 details we may add that this mode of consid- 
 ering the current permits us to explain fairly 
 well various observed facts, as, for example, 
 the proportionality between the thermal and 
 electric conductivity of various bodies, and to 
 explain such phenomena as those relative to
 
 54 THE IONS IN GASES AND IN SOLIDS 
 
 the optical properties of metals. Thus the 
 electron theory not only finds no contradic- 
 tion in phenomena of this sort, but also 
 shows itself able to furnish a simple explana- 
 tion of them.
 
 CHAPTER V 
 
 RADIO-ACTIVITY 
 
 THE discovery of the so-called X-rays by 
 Professor Rontgen early in 1896 gave rise to 
 numerous experiments relating to the possi- 
 ble existence of other radiations capable of 
 affecting photographic films and of passing 
 through opaque bodies. Effects of this sort 
 were described by Le Bon; but we will not 
 consider these, as it was recognized that the 
 effects attributed by this experimenter to a 
 new radiation which he called " black light " 
 proceeded, at least in nearly every case, from 
 causes which have no intimate relation with 
 the subject which we are now about to treat. 
 
 Certain experiments which have, on the 
 other hand, a relationship with radio-activity 
 were made by Henry (17) with phosphores- 
 cent sulphide of zinc ; by Niewenglowski (18) 
 with calcium sulphide ; and by Becquerel (19) 
 
 55
 
 56 RADIO-ACTIVITY 
 
 with double sulphate of uranium and potas- 
 sium. The result of these experiments is 
 that these substances emit rays capable of 
 passing through opaque bodies and acting 
 on a photographic film, when phosphores- 
 cence is excited in them by exposure to 
 light or to X-rays. 
 
 When Becquerel began his experimenta- 
 tion, he had a special object in view. It was 
 already known that the X-rays had their 
 origin at that part of the wall which is ren- 
 dered luminescent where the cathode rays 
 strike ; it was consequently natural to sup- 
 pose that phosphorescence and the emission 
 of X-rays were related phenomena, though 
 later it was recognized that such is not the 
 case. H. Becquerel therefore wished to de- 
 termine whether bodies rendered phospho- 
 rescent by the action, not of cathode rays, 
 but of light, would emit X-rays. And since 
 these rays can affect a photographic film 
 even when it is surrounded by opaque ob- 
 jects, the French physicist placed various 
 bodies above a film, protected in this man- 
 ner from the light, and exposed the whole
 
 RADIO-ACTIVITY 57 
 
 to sunlight. After several unsuccessful at- 
 tempts, he obtained a marked effect with 
 crystalline laminae of the double sulphate of 
 uranium and potassium, since on developing 
 the film he saw an image of the laminae and 
 the shadow of a silver coin which had been 
 placed below one of them. It seemed, there- 
 fore, that the phenomenon he had predicted 
 had really taken place. But Becquerel hap- 
 pened also to obtain the same effect with 
 poor illumination on a cloudy day, from 
 which he suspected that the effect itself did 
 not depend on the action of the light. And, 
 in fact, he very soon proved (20) that the 
 uranium salt continually and spontaneously 
 emitted rays able to go through opaque 
 bodies and to act on photographic films 
 without the necessity of its being exposed 
 to the light. 
 
 Further research showed (21) that the rays 
 from the uranium salt share with the X-rays, 
 not only the property of traversing opaque 
 bodies, of acting on photographic films, of 
 rendering phosphorescent bodies luminous, 
 and, as was later shown, the negative prop-
 
 58 RADIO-ACTIVITY 
 
 erty of not being susceptible of reflection 
 and refraction, and hence of polarization, but 
 also another property which was recognized 
 shortly after the X-rays were discovered (22); 
 namely, that of ionizing gases through 
 which they pass. A method of studying 
 the Becquerel rays, which is more rapid 
 than the photographic method, is based on 
 this property; it consists in the measure- 
 ment of the velocity with which an electri- 
 fied body becomes discharged when the 
 surrounding gas is exposed to the action of 
 the rays. 
 
 For this purpose we may make use of any 
 electrometer connected to a metallic disk 
 placed at a short distance from a second disk ; 
 and we may experiment in two ways. Either 
 the second disk is put in connection with the 
 earth and then one notes the rate of diminu- 
 tion of an electric charge communicated to 
 the electrometer when the air between the 
 two disks is ionized by the radio-active body ; 
 or the second disk is charged and the rapidity 
 with which the electrometer deviates is ob- 
 served. With very active bodies a galvanom-
 
 RADIO-ACTIVITY 59 
 
 eter may be employed, but if bodies of weak 
 radio-activity are to be investigated, it is 
 better to use a gold-leaf electrometer, or bet- 
 ter still, an electroscope, with but a single 
 leaf consisting simply of a vertical metallic 
 rod, AB (Fig. 10), at the upper end of which 
 a very light gold or aluminium leaf, 
 CD, is fastened. In order to secure 
 good insulation the rod is supported 
 by a small piece of sulphur, S. The 
 electrical capacity of the conduct- 
 ing system ABCD is extremely 
 small, and hence it results that the 
 diminution in the divergence of the 
 leaf CD is not too slow. The elec- 
 troscope becomes an electrometer, ' IG ' Ia 
 if the position of CD is observed by means 
 of a microscope and ocular scale. The 
 potential corresponding to each division of 
 the scale may be determined in advance by 
 the use of a battery of small accumulators. 
 The writer prefers a form of electrometer 
 slightly different from that which has become 
 classic for the study of feebly radio-active 
 bodies. The sulphur or fused quartz insu-
 
 6o 
 
 RADIO-ACTIVITY 
 
 lator 6 1 (Fig. n) is quite slender and held 
 with mastic or gutta-percha to the bottom of 
 a small bell-shaped piece of metal connected 
 with the rod AB. This form of support 
 obviates or reduces the creeping of the 
 charge from the rod to the surface of the 
 insulator. Moreover, the writer in 
 again adopting an arrangement 
 which he devised many years ago, 
 uses an ordinary millimeter scale 
 placed at several meters' distance 
 from the electrometer, instead of 
 one annexed to the ocular. A con- 
 verging acromatic lens forms a real 
 FIG. ii. image of the scale in the plane in 
 which the metallic leaf moves, and thus both 
 the leaf and the scale are seen simultaneously 
 in the field of the microscope. 
 
 In one of the electrometers lately con- 
 structed by the writer the rod and the gold 
 leaf have scarcely one-fourth the dimensions 
 shown in Figure n. It is particularly well 
 adapted to the demonstration of radio-activity, 
 since it is so sensitive that, when one of the 
 uranium salts is approached in such a way
 
 RADIO-ACTIVITY 6 1 
 
 that the rays emitted by it pass through the 
 thin aluminium walls of the electrometer 
 case, the gold leaf, when previously electri- 
 fied, may even with the unaided eye be seen 
 to droop. 
 
 As more and more experiments were 
 made, it was established that all the com- 
 pounds of uranium are radio-active, that is, 
 they emit Becquerel rays, and their activity 
 is in proportion to the quantity of uranium 
 which they contain; this shows that radio- 
 activity is a property of the uranium atom 
 and one which remains unaltered when the 
 atom itself enters into combination with the 
 atoms of other chemical elements. 
 
 Thorium, as well as uranium, is radio- 
 active, to a slightly different degree, as was 
 independently established by Schmidt (23) 
 and by Mme. Curie (24). 
 
 If it had not been for the discovery of cer- 
 tain bodies whose radio-activity is hundreds 
 and even thousands of times greater than 
 that of uranium, it would have been perhaps 
 very difficult to have studied radio-activity in 
 a thorough manner and to have determined
 
 62 RADIO-ACTIVITY 
 
 the peculiarities and the probable original 
 cause of this interesting phenomenon, or, at 
 least, all this would have required much 
 time and most accurate research. 
 
 M. and Mme. Curie chanced to find cer- 
 tain specimens of chalcolite and pitchblende 
 (in particular that which is mined at Joachims- 
 thai), which were somewhat more active than 
 pure uranium. Recalling that radio-activity 
 is an atomic property, the phenomenon could 
 not be attributed to uranium contained in 
 these minerals, and it was necessary to sup- 
 pose that an unknown substance, more active 
 than uranium itself, was present. Having 
 recourse to physical and chemical methods 
 of separation, certain compounds of bismuth 
 were extracted from these minerals having a 
 radio-activity as much as four hundred times 
 that of uranium (25). The name polonium 
 was given to the unknown substance con- 
 tained in these compounds, the radio-activity 
 of which diminishes slowly as time goes on. 
 Later the Curies and M. Bemont (26) extracted 
 from pitchblende a small quantity of a very 
 active body, chemically analogous to barium
 
 RADIO-ACTIVITY 63 
 
 and associated with it in almost every re- 
 action, to which they gave the name radium. 
 Another radio-active body associated with 
 thorium and possessing similar chemical prop- 
 erties was discovered by M. Debierne (27), 
 who called it actinium. 
 
 Various other investigators have suc- 
 ceeded in deriving still other noticeably 
 radio-active substances from many different 
 minerals, but especially from pitchblende; 
 the nature of these substances is not yet well 
 understood. In particular Elster and Gei- 
 tel (28) obtained a rather radio-active sulphate 
 of lead which seems to contain simply radif- 
 erous barium. Nor is the radio-active lead 
 obtained by Giesel (29) any better character- 
 ized, while that found by Hofmann and 
 Strauss (30) would, in some respects, seem 
 to resemble polonium. The active lead, or 
 radio-lead of these authors, exhibited one 
 property worthy of mention. Under certain 
 conditions the active sulphate of lead loses a 
 large part of its radio-activity, which it later 
 slowly recovers ; but if it is exposed to the 
 bombardment of the cathode rays, its radio-
 
 64 RADIO-ACTIVITY 
 
 active properties are fully restored in a few 
 minutes. 
 
 Recently Hofmann and Wblfl (31) have 
 partially separated the active substance from 
 the inactive lead, and have determined that 
 it differs from the polonium of Mme. Curie 
 in the constancy of its radio-activity. 
 
 The substance called radio- tellurium by 
 Marckwald (32) also resembles polonium in 
 its properties, the only difference being that 
 its radio-activity does not seem to diminish 
 with time. It would seem that radio-tellu- 
 rium is the body to which the activity of 
 the bismuth extracted from the Joachimsthal 
 pitchblende is due, and it is considered by 
 this author to be a new body belonging to 
 the sulphur and tellurium series, because it is 
 deposited on sticks of bismuth or antimony 
 introduced into an acid solution of active 
 chloride of bismuth. Marckwald obtained 
 in this manner about six decigrams of quite 
 active material from 850 grams of the salt. 
 An active bismuth quite similar to that of 
 the other investigators was discovered by 
 Giesel (33), who later separated from it a prod-
 
 RADIO-ACTIVITY 65 
 
 uct identical with that obtained by Marck- 
 wald. Finally a supposed new radio-active 
 element was separated from thorium by 
 Baskerville, who called it carolinium (34). 
 
 As we see, a great uncertainty still exists 
 regarding the nature and even the separate 
 existence of the greater part of the radio- 
 active substances; so much so, in fact, that 
 there are some who consider the effects at- 
 tributed to Mme. Curie's polonium as due to 
 an induced radio-activity^ that is, to a transi- 
 tory phenomenon, of which we shall speak 
 later; there are others who believe radio- 
 active thorium and actinium to be identical. 
 The chemistry of the radio-active bodies is 
 only in its initial stages, and at present 
 radium, whose characteristic spectrum is now 
 known, is the only substance the existence 
 of which, as an element distinct from the 
 others, seems to be sure. 
 
 Radium has never been prepared in the 
 free state, but, nevertheless, several of its salts 
 are known. For example, the small quan- 
 tity of radio-active substance, which the 
 Curies were first able patiently to obtain from
 
 66 RADIO-ACTIVITY 
 
 several tons of pitchblende residues, remain- 
 ing after the uranium had been extracted, 
 was the chloride of radium. These residues 
 contained compounds of almost all the met- 
 als, among which were barium, bismuth, and 
 the metals of the rare earths. Chemical 
 processes, which it would take too long to 
 describe here, permit the extraction of the 
 barium with the radium, the bismuth with 
 the polonium, and the rare earths with the 
 actinium ; after this there remains the sepa- 
 ration of each of the radio-active bodies from 
 the body which accompanied it, in spite of 
 previous chemical reactions. This separa- 
 tion, to which Mme. Curie has devoted many 
 years of labour, has not yet been completely 
 successful, except in the case of radium, 
 where the following process was employed. 
 The radiferous chloride of barium, of 
 which about eight kilograms is extracted 
 from a ton of pitchblende residues, is dis- 
 solved in hot water, so as to form a saturated 
 solution. On cooling, crystals, which we 
 will call A, are thrown down, while on evap- 
 orating the remaining solution, another
 
 RADIO-ACTIVITY 67 
 
 chloride, which we will call B, is obtained. 
 Since the chloride of radium is slightly less 
 soluble than the barium chloride, it results 
 that the chloride A is richer, and the chloride 
 B is poorer, in radium than the chloride used 
 in forming the solution. This may be ascer- 
 tained by determining the relative radio-activ- 
 ity. A second operation exactly similar to 
 the above is carried out with each of the 
 products A and B ; now since the least active 
 part obtained from A and the most active 
 part obtained from B are about equally 
 radio-active, they are reunited, and thus the 
 four portions which result from the second 
 operation reduce to three of different rich- 
 ness in radium. The process is continued 
 in the same manner ; however, to prevent an 
 inordinate increase in the number of por- 
 tions, no use is made of those whose radio- 
 activity is very small, and the operation is 
 discontinued on those portions whose radio- 
 activity is sufficiently great. It is also found 
 advantageous to make use of the mother 
 liquor obtained from one operation to dis- 
 solve the crystals obtained in the succeeding
 
 68 RADIO-ACTIVITY 
 
 operation. When the greater part of the 
 inactive substances is eliminated by this pro- 
 cess, it is carried on still further, but now the 
 least active parts are more freely discarded. 
 In this way a small amount of radium is lost, 
 but the purification proceeds more rapidly, 
 to accomplish which it is advisable to acidu- 
 late the solvent more and more with pure 
 hydrochloric acid. Finally, practically pure 
 chloride of radium is separated to the amount 
 of two or three decigrams per ton of the 
 residues employed. At present bromide of 
 radium is also prepared in the pure state. 
 
 Besides the measurement of the degree of 
 increasing radio-activity a spectroscopic ex- 
 amination of the product may be employed 
 to estimate its purity, since radium possesses 
 a spectrum which perfectly characterizes it 
 and which was studied by Dema^ay. If an 
 impure product is examined, the spectrum 
 of barium appears with the spectrum of 
 radium; but as the process of purification 
 goes on, the barium lines become weaker 
 and finally almost completely disappear. 
 
 After the principal effects due to radio-
 
 RADIO-ACTIVITY 69 
 
 active bodies were discovered, the attention 
 of physicists was turned to the direct study 
 of the rays emitted. 
 
 All known radiations are, or at least we 
 conceive that they are, of two sorts; those 
 due to waves propagated in the ether, and 
 those due to the motion of electrified mate- 
 rial particles. Not only the luminous rays 
 properly so called, and the invisible heat and 
 ultraviolet rays, but also, as is thought, the 
 Rontgen rays belong to the first sort. To 
 the second sort belong the cathode rays, 
 which are, in fact, considered to be due to 
 the motion of negative electrons. 
 
 It is not difficult to decide on the nature 
 of new radiations which belong to one or the 
 other of these two categories ; in fact, while 
 an electric or magnetic field cannot in the 
 least degree modify the form of luminous or 
 of Rontgen rays, etc., they should cause a 
 marked curvature of the path described by 
 electrified particles, unless the velocity is 
 exceedingly great. However, there exists, as 
 it appears, certain new radiations to which 
 their discoverer, Blondlot, has given the name
 
 70 RADIO-ACTIVITY 
 
 of n-rays (35), which seem to possess some 
 of the properties of heat rays, but which 
 appear to possess other very strange proper- 
 ties. We shall not attempt to describe them 
 here, since their nature is still enigmatic. 
 
 In order to acquire a clear conception of 
 the nature of the rays emitted by radio-active 
 bodies, it was necessary to subject them to 
 magnetic or electric forces, and therefore to 
 place the radio-active bodies either between 
 the poles of a powerful magnet or between 
 two metal plates oppositely electrified. More- 
 over, in order that the possible deformation 
 of the rays might be recognized, they had to 
 be passed through a small opening or dia- 
 phragm, on the far side of which either a 
 phosphorescent body or a photographic plate 
 protected from the light by a black paper 
 covering was placed. With the phosphores- 
 cent body the displacement of the luminous 
 spot is at once seen when an electric or a 
 magnetic field deforms the rays, and the 
 same result is detected when the plate is 
 developed. It is generally preferable to use 
 the photographic plate, because the length
 
 RADIO-ACTIVITY 71 
 
 of exposure, which may be as great as is 
 desired, eventually compensates for the low 
 intensity of the radiations which are studied. 
 
 Applying these methods, it was recognized 
 from the very first that in general a radio- 
 active body emits rays which are deviated 
 by the magnetic and the electric field, and 
 at the same time other rays which are not 
 deviated. It was further determined that 
 the former behave in all respects like cathode 
 rays of high velocity, that is, as though they 
 consisted of negative electrons projected in 
 straight lines with enormous velocity. We 
 shall see further on that this velocity may be 
 measured, and that the ratio between electric 
 charge and mass has practically the same 
 value as that found in the case of the cathode 
 rays. 
 
 Subsequent researches have proved that 
 radium and other strongly radio-active bodies 
 also produce rays deviated somewhat less 
 than the cathode rays by the electric and 
 magnetic forces, but in the opposite direc- 
 tion. Hence it may be said that radio-active 
 bodies emit three varieties of rays. It is
 
 72 RADIO-ACTIVITY 
 
 probable that this is true of all bodies, since, 
 even if some of these varieties of rays have 
 not yet been found in the complex emission 
 of certain radio-active bodies, this is in all 
 probability due to their being less intense 
 and hence less easy to detect. An example 
 of this is found in Mme. Curie's polonium, 
 which practically only emits rays whose 
 deviation is opposite to that of the cathode 
 rays. 
 
 The rays which are subject to deviation 
 can only be considered as consisting in the 
 emission of electrified particles. The direc- 
 tion of the deviation shows the sign of the 
 charges of the particles, while the amount of 
 the deviation permits an evaluation of the 
 velocities with which the particles move, and 
 in addition, of the ratio which exists between 
 the charge and the mass of each; conse- 
 quently the mass itself may be determined 
 if we assume that its electric charge has that 
 constant value which pertains to the electro- 
 lytic ion of hydrogen. We will now sum up 
 the results of the research relating to radio- 
 activity, adopting Rutherford's designation,
 
 RADIO-ACTIVITY 73 
 
 a, /?, y, for the three kinds of rays emitted 
 by radium, and perhaps in general by every 
 radio-active body. 
 
 The a-rays behave in a manner which 
 confirms the hypothesis made by Strutt 
 (36), which is that they consist of positive 
 ions projected in all directions from the 
 radio-active body. In fact, Rutherford (37) 
 proved that they transport positive charges, 
 while Becquerel (38) determined that their 
 deviation in a magnetic field is in the oppo- 
 site direction to that of the cathode rays. 
 
 If, for example, the rays emanate from a 
 small quantity of one of the salts of radium 
 contained in a little lead vessel, P (Fig. 12), 
 they are propagated in the direction of the 
 straight line PC] but if a magnetic field is 
 set up perpendicular to the plane of the fig- 
 ure, the a-rays separate themselves from the 
 others and curve in the direction of the arc 
 PA. 
 
 It was found, on measuring the velocity 
 and the ratio between the charge and the 
 mass of the particles constituting the a-rays, 
 that the velocity may attain a value about a
 
 74 
 
 RADIO-ACTIVITY 
 
 tenth of the velocity of light, and that the 
 ratio mentioned above would indicate that 
 the particles possess a mass of atomic order. 
 We may say, therefore, that the a-rays are 
 identical with canal rays of high velocity. 
 
 FIG. 12. 
 
 The ionization of a gas seems to be espe- 
 cially due to the a-rays, evidently as a result 
 of collisions. The phenomenon is rather 
 restricted if the gas which surrounds the 
 radio-active body is at ordinary pressure. 
 In fact, the a-rays from radium are only 
 slightly penetrating, as either a layer of
 
 RADIO-ACTIVITY 75 
 
 ordinary air ten centimeters thick, or a thin 
 plate of aluminium less than a tenth of a 
 millimeter thick, is sufficient to absorb the 
 greater part of the rays. 
 
 The /3-rays behave in all respects like very 
 penetrating cathode rays. That is, they are 
 negative electrons projected in all directions ; 
 their velocity is enormous, since it may attain 
 a value but slightly inferior to the velocity 
 of light. This result is deduced from the 
 effect produced on the rays by the magnetic 
 field, which was determined almost at the 
 same time by Becquerel (39), Giesel (40), 
 Meyer and Von Schweidler (41), and Dorn 
 (42). Instead of curving like the a-rays, 
 they bend considerably more in the opposite 
 direction, as is shown in Figure 12. The cur- 
 vature of some of the /2-rays may be so great 
 that they end by striking a photographic 
 plate, ZZ, on which the lead vessel P rests 
 in the region B^ B%. 
 
 The production of rays curved to a greater 
 or less degree by the magnetic field is due 
 to the fact that the velocities of the various 
 negative electrons, of which the /3-rays are
 
 76 RADIO-ACTIVITY 
 
 only the trajectory, have different values. 
 The magnetic field causes those which move 
 less rapidly to describe semicircles of small 
 radius and those with the high velocity to 
 describe the arcs of large radius. It is thus 
 that the elongated image obtained on the 
 photographic plate is explained. By means 
 of this image it is easy to prove that the less 
 deflected rays, consisting of electrons with 
 great velocity, are also the more penetrating. 
 In fact, a photographic plate placed in the 
 path of the /8-rays absorbs most strongly 
 those rays which are the most deviated (B^ 
 Fig. 12). Moreover, the variety of /3-rays is 
 quite great; thus, while some are stopped 
 by aluminium foil one-hundredth of a milli- 
 meter thick, others are able to traverse sev- 
 eral millimeters of lead. It was proved 
 directly by the Curies (43) that the /8-rays 
 really transport negative charges. In their 
 experiments the a-rays were arrested by a 
 plate of aluminium so that only the effect of 
 the /2-rays was shown by the electrometer. 
 
 While the a and (3 rays are deviated in an 
 electric or a magnetic field, the y-rays are
 
 RADIO-ACTIVITY 77 
 
 not affected at all. Thus those emitted by 
 the radio-active body contained in P (Fig. 
 12) preserve their rectilinear motion, PC, 
 even when the magnetic field is set up. 
 
 The y-rays, like the y6-rays, are not homo- 
 geneous, there being a more penetrating and 
 a less penetrating variety. This property 
 renders them very similar to the Rontgen 
 rays, and as such they are now generally 
 considered. It is true it has been observed 
 that the conductivity excited by the y-rays 
 in various gases is not proportional to that 
 produced by the X-rays, and this seemed to 
 establish a difference in the nature of the 
 two sorts of rays. But recent experiment 
 has shown that this dissimilarity in behav- 
 iour depends solely on the fact that the y-rays 
 in the aggregate are only comparable with 
 the most penetrating X-rays. In fact, the 
 ratio between the conductivity produced in 
 various gases by y and X rays tends toward 
 unity, when a comparison is made between 
 the former and Rontgen rays which are 
 furnished by " hard " tubes and are made to 
 pass through a lead plate before arriving at 
 the gas to be ionized.
 
 78 RADIO-ACTIVITY 
 
 When the three sorts of rays strike or 
 traverse various bodies, they produce effects 
 which differ according to the nature of the 
 bodies themselves. This is particularly evi- 
 dent when one of the salts of radium is em- 
 ployed and certain of these effects have only 
 thus far been observed with this substance. 
 It is not possible to completely isolate the 
 rays of one species from the others and to 
 study separately the phenomena produced by 
 them ; but it is possible, by means of the in- 
 terposition of absorbing plates to keep back 
 the least penetrating rays, such as the a, or 
 the a together with a part of the ft rays, or 
 finally, to allow only the y-rays to pass 
 through ; and this is sufficient in many cases 
 to allow the effects produced by the one or 
 the other to be recognized. 
 
 The effects produced by radio-active bodies, 
 and in particular by radium, may be classified 
 as luminous, chemical, electrical, mechanical, 
 thermal, and physiological. Besides what has 
 been previously stated, the following may be 
 said with reference to these effects. 
 
 Phosphorescence and fluorescence seem to
 
 RADIO-ACTIVITY 79 
 
 result especially from the action of the a and 
 the ft rays ; but certain bodies become more 
 brilliantly luminescent when they are struck 
 by the a-rays, others when they are struck by 
 the /3-rays. For example, hexagonal blende 
 is particularly luminescent under the action 
 of the a-rays from radium. 
 
 Crookes has constructed a small instru- 
 ment called the spinthariscope (44), by means 
 of which the effect produced by radium on a 
 screen coated with phosphorescent sulphide 
 of zinc is observed. A small particle of the 
 radium salt is placed at a distance of about 
 a half millimeter from the screen, which is 
 viewed through a lens or microscope. Brill- 
 iant points appear here and there and imme- 
 diately become extinguished and thus give 
 an effect of scintillation. According to 
 Crookes each luminous point is caused by 
 the impact of a positive ion. According to 
 Becquerel, the scintillation is due to the rup- 
 ture or cleavage of the little crystals forming 
 the phosphorescent films, since he obtained 
 similar effects by crushing the body between 
 two glass plates (45). In any case the singu-
 
 80 RADIO-ACTIVITY 
 
 lar luminous phenomenon of the spinthari- 
 scope is only indirectly due to the supposed 
 bombardment of the phosphorescent body by 
 the a-rays. 
 
 The luminosity of the salts of radium has 
 been attributed to a phosphorescence excited 
 by the rays which they themselves generate. 
 But it may be that the light is produced by 
 the collision of the a ions or of the /8 elec- 
 trons, not, in fact, with the molecules of the 
 radio-active body, but with those of the sur- 
 rounding gas or of the gases which are slowly 
 released from the radium salt; and it might 
 also be supposed that the cause of the lumi- 
 nous phenomenon is the transformation of 
 the radium into an emanation, of which we 
 shall speak later. However, it has been 
 found by Sir William and Lady Huggins (46) 
 that the spectrum of the light emitted by 
 radium consists of lines coincident with 
 some of those of nitrogen, and almost iden- 
 tical with that of the negative glow produced 
 in air at ordinary pressure. 
 
 Recently Becquerel (47) has observed that 
 the salts of uranium are also spontaneously
 
 RADIO-ACTIVITY 8 1 
 
 luminous. In order to convince oneself of 
 this, it is only necessary to look, at night, 
 when the eye is well rested, at a small glass 
 vessel containing uranium nitrate. The 
 writer has, under such conditions, observed 
 a very vivid scintillation when the vessel con- 
 taining the salt was shaken ; for this reason 
 it is advisable not to shake the vessel too 
 strongly if one wishes to verify the sponta- 
 neous emission of light and not confound it 
 with that which is produced by concussions, 
 and which is probably electrical in origin. 
 
 Phosphorescent substances are altered by 
 the action of the rays emitted by radium and 
 often change colour ; glass becomes violet or 
 blackish, and thus modified, it is thermo- 
 luminescent, since it becomes luminous when 
 heated to about 500 C. These modifications 
 are probably chemical in nature, like those 
 to which images on photographic plates are 
 due. 
 
 Inasmuch as the rays from radium are 
 unequally absorbed by bodies of different 
 density, it is possible by their use to obtain 
 radiographs similar to those, now so well
 
 82 RADIO-ACTIVITY 
 
 known, which are obtained with X-rays. 
 However, the latter are always very much 
 the more perfect. 
 
 Radio-active bodies, and especially radium, 
 produce still other chemical actions ; thus 
 according to Becquerel, a solution of bichlo- 
 ride of mercury and oxalic acid (6J grams 
 of the bichloride, 12^ of oxalic acid, and 
 100 of water) deposits calomel under the 
 action of the rays from radium ; their action 
 is therefore in this case similar to that of 
 light rays. Under the action of the rays 
 from radium ordinary phosphorus is trans- 
 formed into red phosphorus, and oxygen into 
 ozone. And it seems as though the changes 
 in colour which are shown by glass, barium- 
 platino-cyanide, etc., should be attributed to 
 chemical modifications. The last of these 
 substances becomes at the same time less 
 phosphorescent, but later reacquires this 
 property when exposed to sunlight. It is 
 a rather singular fact that the rays from 
 radium do not sensibly modify the iodide 
 of silver of a daguerreotype plate. 
 
 Since the a and /3 rays consist in the
 
 RADIO-ACTIVITY 83 
 
 emission of positive ions and negative elec- 
 trons respectively, it is natural that they 
 should produce an electric charge whenever 
 the one variety and not the other is arrested. 
 In this manner radio-active substances be- 
 come a continuous source of electricity. 
 Radio-active bodies should become electri- 
 fied even when free motion is allowed to all 
 the rays, if the quantity of positive electricity 
 transported by the a-rays differs from the 
 quantity of negative electricity carried away 
 in the same time by the /3-rays. Appropriate 
 experiments have shown that this does not 
 happen (48); and hence there must be a 
 compensation between the charges of the two 
 signs. If, however, the a-rays are arrested 
 by a sheathing of not too thin glass, this 
 becomes positively electrified. This experi- 
 ment was made by Wien with a small 
 amount of bromide of radium placed in a 
 glass tube wrapped in aluminium foil and 
 hung by a wire in a vacuum tube. The 
 gas must be removed from the region sur- 
 rounding the small tube in order that the 
 conductivity of the ionized gas may not cause
 
 RADIO-ACTIVITY 
 
 the charge which collects to be dissipated. 
 Under some conditions an electric spark 
 may be obtained, as, for 
 example, when a small tube 
 containing radium (49) is 
 opened. 
 
 An ingenious little piece 
 of apparatus (Fig. 13) which 
 was devised by Strutt (50) 
 demonstrates the continual 
 production of electricity by 
 radium. In a hermetically 
 sealed glass vessel, in which 
 a good vacuum has been 
 formed, a small closed tube, 
 a, is placed which contains 
 a radio-active body. This 
 tube, which is supported by 
 an insulator of fused quartz, 
 6, has its surface covered 
 with a conducting film (for 
 example, painted with phos- 
 phoric acid), and terminates 
 in two gold leaves, cc, which form an electro- 
 scope. These leaves are always in motion. 
 
 FIG. 13.
 
 RADIO-ACTIVITY 85 
 
 They spread apart little by little until they 
 touch the strips of tin foil, ee, pasted on the 
 walls of the vessel and communicating with 
 the earth. Then they instantly fall together, 
 only to slowly diverge again, and this goes 
 on continuously. The effect is due to the 
 positive charge left on the little tube by the 
 a-rays which are absorbed while the negative 
 charge is carried away by the /3-rays. 
 
 Another effect of an electrical nature, 
 which is due to the rays from radium, is 
 obtained, according to Curie (51), in the 
 following manner: conductors are placed 
 in such a way that the discharge of an 
 electric machine or an induction coil may 
 follow two separate paths, in each of which 
 an air gap is provided for the formation of 
 the sparks. If these two air gaps are equiv- 
 alent, so that the discharge takes place equally 
 well in either of the two paths open to it, it 
 is sufficient to bring the radium near one of 
 the two air gaps in order that the spark may 
 form at this one and not at the other. There- 
 fore the rays from radium produce an action 
 similar to that which, in certain circum-
 
 86 RADIO-ACTIVITY 
 
 stances, is caused by the Rontgen rays, in 
 that they tend to assist the formation of 
 sparks. 
 
 Finally, another electrical effect of radium 
 is the one which has been recently an- 
 nounced, namely, that a diminution in the 
 electrical resistance of bismuth is produced 
 when the radio-active substance is brought 
 to within a very small distance of a thin 
 plate of this metal. 
 
 Since the a-rays consist of positive ions, it 
 follows that a body which emits them must 
 necessarily send out matter as well ; from a 
 certain standpoint the same thing may be 
 said of the ^8-rays, as we shall see later. It is 
 to be expected, therefore, that there should 
 be a continual diminution in the weight of 
 radio-active substances. Heydweiller (52) 
 thought that he had detected this ; but Dorn 
 (53) could not confirm this result in spite of 
 the fact that he employed a body of greater 
 radio-activity. Thus it would seem that we 
 are dealing with a decrease in mass so small 
 as to elude ordinary determinations. 
 
 In the case of radium, the emission of the
 
 RADIO-ACTIVITY 87 
 
 various sorts of rays is accompanied by a 
 continual production of heat. In fact, the 
 Curies and Laborde (54) found that a small 
 quantity of a radium salt continually main- 
 tained itself at a higher temperature than 
 that of the surrounding air. The experiment, 
 which is quite simple, is thus described by 
 Mme. Curie. A closed glass flask contain- 
 ing seven decigrams of pure bromide of 
 radium and a mercury thermometer are 
 placed inside a double-walled glass vessel 
 closed with a cotton stopper. A vacuum is 
 produced between the two walls of the vessel, 
 which is, in fact, one of those which are used 
 to preserve liquid air because of the good 
 heat insulation which may be obtained in this 
 manner. A precisely similar thermometer, 
 together with a similar glass flask, which, 
 however, contains an inactive body such as 
 chloride of barium, instead of the radio-active 
 substance, are placed in a second glass vessel 
 similar to the first. As soon as thermal equi- 
 librium is established, it may be observed that 
 the first thermometer registers a temperature 
 about three degrees higher than the second.
 
 88 RADIO-ACTIVITY 
 
 In order to measure the quantity of heat 
 continuously evolved by the radium salt, a 
 small quantity of it was introduced into a 
 Bunsen calorimeter. From the quantity of 
 ice which melted in a given time, it was 
 deduced that the heat evolved by the radio- 
 active body in one hour would be sufficient 
 to melt a weight of ice equal to its own, or 
 to heat an equal weight of water through 
 about 80 C. Thus we see that the quan- 
 tity of heat furnished by radium is relatively 
 great. 
 
 A thermal effect of another sort is that 
 demonstrated by Georgiewski (55). Accord- 
 ing to this author, while the velocity with 
 which a hot body cools does not vary when 
 the gas which surrounds it is ionized by the 
 rays from radium, the cooling itself is accel- 
 erated if the hot body is electrified, and espe- 
 cially if it is negatively electrified. This 
 effect was observed not only with air, but 
 also with other gases, and the result was the 
 same whether all the rays from radium were 
 allowed to act, or whether the a-rays were 
 prevented from acting.
 
 RADIO-ACTIVITY 89 
 
 When the rays from radium, and from 
 radio-active bodies in general, strike an inac- 
 tive body, they cause it to emit new rays 
 which are called secondary rays. This emis- 
 sion is contemporaneous with the action of 
 the incident rays, which distinguishes this 
 phenomenon from that of induced radio- 
 activity, of which we shall speak farther on. 
 The property of exciting secondary rays per- 
 tains also to other radiations, such as X-rays 
 and cathode rays, and in all cases the second- 
 ary rays differ from those which cause them. 
 Thus it may be said that the X-rays are sec- 
 ondary rays produced by cathode rays, which 
 may themselves be generated by X-rays (56). 
 Moreover, these latter produce secondary 
 rays of a nature similar to their own but 
 which are less penetrating. Luminous and 
 ultraviolet rays may produce cathode rays 
 as secondary rays, etc. The rays from radio- 
 active bodies produce effects of this sort, 
 which, however, have not yet been studied 
 in a thorough manner. 
 
 Besides the effects already described, the 
 rays emitted by radium have a most pro-
 
 90 RADIO-ACTIVITY 
 
 nounced action on living tissues and may 
 even cause severe lesions hard to be cured. 
 Becquerel and the Curies proved this to 
 their cost (57). According to Danysz (58) 
 a tube containing a strongly radio-active 
 body placed on the skin of an animal caused 
 a complete destruction of the epidermis and 
 the dermis, though the action on the lower 
 tissues was relatively quite weak. The ner- 
 vous system is, on the contrary, quite sensi- 
 tive to the action of radium, since lesions are 
 soon formed which may result in paralysis 
 and death. 
 
 The effect produced on bacteria is worthy 
 of note; for, according to Aschkinass and 
 Caspari (59), the development of certain 
 species becomes arrested when they are sub- 
 jected to these rays. The larvae of some 
 insects die, and the prolonged action of the 
 rays from radium on the seeds of cer- 
 tain plants deprives them of the ability to 
 germinate. It seems, moreover, that, in 
 compensation for the deleterious actions 
 which they exert, the rays emitted by radio- 
 active substances may become useful in the
 
 RADIO-ACTIVITY 91 
 
 cure of certain diseases, such as cancer and 
 lupus. 
 
 The sensation of light which suffuses the 
 field of vision when one of the radium prepa- 
 rations is brought close to the eye is not a 
 direct physiological effect, but one which is 
 explained by the phosphorescence of the 
 parts which constitute the organ of sight. 
 The sensation is produced even in the eye 
 of a blind person, provided the retina and 
 the optic nerve are in normal condition. 
 
 Finally, according to Bloch (61), the action 
 of the rays from radium on crystalline sele- 
 nium is similar to that of light, since they 
 increase its electrical conductivity. 
 
 In addition to the emission of a, ft, and y 
 rays radio-active bodies, or at least radium 
 and thorium, continually lose a part of their 
 substance in another form. Certain experi- 
 ments relative to the ionization of the air 
 produced by thorium compounds led Ruther- 
 ford to the discovery of the radio-active ema- 
 nations. Before this Owens (62) had noticed 
 a strange irregularity in the ionizing action 
 of the rays from thorium oxide, but it was
 
 92 RADIO-ACTIVITY 
 
 Rutherford who furnished the explanation 
 of this (63) by demonstrating that thorium 
 continually emits radio-active particles, whose 
 ionizing action reenforces that due to the 
 a, y8, and y rays. Radium itself produces its 
 own emanation, and the particles which form 
 this emanation are distinct from those which 
 constitute the a and ft rays, since the former 
 diffuse slowly like the molecules of a gas 
 while the latter are projected with enormous 
 velocity. 
 
 In fact, the emanation mixes with the sur- 
 rounding gas and may be carried with it from 
 one place to another. Recent experiments 
 show that the emanation behaves like a true 
 gas, since, according to Traubenberg (64), not 
 only the emanation contained in the city 
 water of Freiburg (and, in general, in water 
 from springs containing any radio-active sub- 
 stances whatsoever), but also the emanation 
 from radium obeys the laws of Dalton and 
 Henry, which hold for gases. 
 
 The emanation easily passes through nar- 
 row apertures or fissures through which an 
 ordinary gas could only circulate with ex-
 
 RADIO-ACTIVITY 93 
 
 treme slowness. It is only temporarily radio- 
 active ; that is, its radio-activity decays in a 
 continuous manner to such an extent, that at 
 the end of a minute the activity of the tho- 
 rium emanation is reduced to one-half of the 
 initial value, while that of radium loses its 
 activity considerably less rapidly, being re- 
 duced to one-half only after about four days. 
 
 Rutherford and Soddy (65) have shown 
 that the radium emanation condenses at a 
 temperature of about 1 50 below zero centi- 
 grade. In fact, a current of gas which has 
 passed over a compound of radium and into 
 a tube surrounded by liquid air, issues from 
 the tube entirely robbed of the emanation, 
 and the emanation itself returns to the gas- 
 eous state when the temperature is raised to 
 the value mentioned above. It is easy to 
 follow the motion of the emanation, because 
 it is luminous and renders the glass tube 
 which contains it luminous. The emanation 
 from thorium condenses at a slightly higher 
 temperature. 
 
 The emanation has the property of render- 
 ing bodies with which it comes in contact
 
 94 RADIO-ACTIVITY 
 
 temporarily active. The radio-activity ac- 
 quired in this manner by bodies which do 
 not possess it on their own account was, it 
 seems, first observed by the Curies, who 
 called it induced radio-activity. According 
 to Rutherford it is due to an invisible solid 
 substance which the emanation deposits on 
 bodies in unappreciable quantities, and which 
 is soluble in certain acids but not in others. 
 Upon evaporating the solution formed in 
 this way, a radio-active residue is obtained. 
 Moreover, it seems that the emanation ad- 
 heres to the bodies which it has rendered 
 active, and also, in a certain sense, is ab- 
 sorbed by them, since these bodies give off 
 emanations of their own. Celluloid, india 
 rubber, and paraffin absorb and then give up 
 the emanation in appreciable quantities. In- 
 dependently of this it has been found that 
 a body which has become radio-active by 
 induction, in a large measure loses this prop- 
 erty when it is heated; this lost activity is 
 acquired by surrounding cold bodies. 
 
 The presence of a negative charge on a 
 body facilitates the production of induced
 
 RADIO-ACTIVITY 95 
 
 radio-activity. It is even possible to cause 
 a conductor which is strongly charged nega- 
 tively to become radio-active by merely plac- 
 ing it for several hours in the open air, as 
 has been shown by Elster and Geitel (67). 
 According to Sella (68) the same result may 
 be obtained in certain cases when the con- 
 ductor is charged positively. These two 
 experiments point to the existence in the 
 atmosphere of an emanation analogous to 
 those which are given off by radium and 
 thorium. 
 
 Radio-activity may also be produced in 
 substances which in themselves are inactive, 
 by dissolving the inactive body in a liquid 
 together with an active body and then sepa- 
 rating the two by chemical methods. In 
 this manner active bismuth is obtained after 
 having been dissolved in a solution contain- 
 ing radium. Generally the body which has 
 become radio-active in this way slowly loses 
 its activity ; and it is because of this that 
 polonium has been considered by some to be 
 bismuth which has been rendered active by 
 solution. A salt of barium becomes radio-
 
 96 RADIO-ACTIVITY 
 
 active, in a similar way, when it is derived 
 from a solution of uranium, while this solu- 
 tion loses a part of its radio-activity, which 
 it later slowly regains. 
 
 A phenomenon analogous to this, and 
 one to which we shall return later, is that 
 described by Rutherford, who was able to 
 chemically separate thorium into two por- 
 tions ; the one which he called thorium-X is 
 quite active and loses its radio-activity little 
 by little as time goes on, while the other in 
 time reacquires this lost property. 
 
 The continued evolution of heat and the 
 emission of a-rays as well do not pertain 
 exclusively to radium, but are also properties 
 of the emanation which it evolves. If a 
 radium salt is heated in a tube until so much 
 of its emanation as it is possible to liberate 
 is driven off and collected separately in a 
 tube cooled by liquid air, then, when the tube 
 containing the radium and that containing 
 the emanation come into thermal equilibrium 
 with the surrounding atmosphere, it is found 
 that the quantity of heat produced in unit 
 time by the radium diminishes, while that
 
 RADIO-ACTIVITY 97 
 
 evolved by the emanation increases, in such 
 a way that the sum of the two remains prac- 
 tically constant (69). If the emission of a- 
 rays on the part of the radium and on that of 
 the emanation are compared by means of the 
 ionization which they produce, a behaviour 
 exactly similar to that in the case of the 
 emission of heat is noted. This leads one to 
 suppose (70) that the heat evolved by the 
 radium is due to the collision of the positive 
 ions (a-rays) with the air molecules or with 
 those of the radio-active substance itself. 
 The velocity of the a-rays which has been 
 found to attain a value of one-tenth of the 
 velocity of light, is such as to satisfy this 
 hypothesis ; but at any rate the origin of the 
 energy, of which radium seems to be an inex- 
 haustible source, still remains unexplained. 
 
 It is probable that radio-activity is a prop- 
 erty possessed to a greater or less degree by 
 all bodies. In fact, recent experiments have 
 shown that the speed with which a charged 
 electroscope, for example that shown in Fig- 
 ure 10, becomes discharged, depends on the 
 nature of the walls of the vessel in which it
 
 98 RADIO-ACTIVITY 
 
 is inclosed (71). This is attributed to the 
 radio-activity of the walls themselves. It 
 seems, moreover, that very penetrating radia- 
 tions, which proceed perhaps from the atmos- 
 phere or from surrounding bodies, continually 
 exist all about us and produce a slight ioniza- 
 tion of gases, since the rapidity of discharge 
 mentioned above becomes less, when the ap- 
 paratus is surrounded by a thick shield of 
 lead (72). Thus one might even suspect 
 that the slight radio-activity of ordinary sub- 
 stances may be, at least in part, an emission 
 of secondary rays excited by these radia- 
 tions, which are supposed to be continually 
 present. 
 
 The fact that air which has been drawn up 
 from beneath the soil has an electrical con- 
 ductivity greater than that of ordinary air is 
 especially important. Moreover, it has been 
 found that the water from many springs and 
 wells, in particular the water supplied to the 
 city of Cambridge (73), contains a radio-active 
 gas which impregnates air made to bubble 
 through the water and causes it to become 
 feebly conducting. These two facts lead one
 
 RADIO-ACTIVITY 99 
 
 to suppose with Elster and Geitel (74) that a 
 small quantity of some radio-active substance, 
 which may possibly be radium, exists in the 
 ground, and that the emanation from this sub- 
 stance is carried by the ground air and is 
 dissolved to a slight extent by spring water. 
 Mr. Allen (75), who discovered that the 
 water of the spring called " King's Bath " is 
 radio-active, advances the idea, worthy of be- 
 ing taken into consideration in this connec- 
 tion, that the therapeutic action of mineral 
 waters may be due at least in part to an em- 
 anation or to a radio-active gas contained in 
 them ; thus it would be easy to account for 
 the fact, which is so often asserted, even if it 
 has never been explicitly demonstrated, that 
 mineral waters become less efficacious when 
 exported. 
 
 The presence of radio-active emanations 
 in springs which contain carbonic acid gas, 
 and in certain thermal waters, as well as the 
 relatively great radio-activity of the mud 
 deposited from these waters, leads one to 
 suppose that the proportion of radium, or of 
 some other very radio-active substance, con-
 
 100 RADIO-ACTIVITY 
 
 tained in the earth's crust, must increase with 
 the depth below the surface (76). Hence it 
 would seem very well worth while to insti- 
 tute an investigation of volcanic products. 
 
 Freshly fallen snow is radio-active. The 
 writer has been able to ascertain that the 
 conductivity of the air on a snowy day was 
 more than double that on ordinary days. 
 
 In this chapter we have presented a long 
 series of facts which have been, for the most 
 part, ascertained with absolute certainty, but 
 which at any time may assume a different 
 aspect in the light of some future discovery. 
 It would, therefore, be premature to formu- 
 late a theory of radio-activity at the present 
 time, were it not that the nature of this 
 phenomenon has been rendered so manifest 
 by the properties which the radiations and 
 emanations have been found to possess. In- 
 deed, it is impossible to doubt that a radio- 
 active body continually emits part of its own 
 substance and that hence it should have a 
 limited existence : the electron theory, in the 
 form in which it will be presented in the last 
 chapter, and according to which atoms are
 
 RADIO-ACTIVITY IOI 
 
 systems of electrons, seems designed ex- 
 pressly to confirm this belief. But a theory 
 of radio-activity should also explain the 
 origin of the energy which becomes avail- 
 able during the disintegration of radio-active 
 bodies. 
 
 It is thought by some that this energy 
 comes from an unknown radiation which is 
 passing continually through all space. This 
 radiation might impart enormous velocities 
 to the constituents of the atoms of certain 
 bodies velocities great enough to separate 
 these constituents from each other. Prob- 
 ably if such a radiation really did exist it 
 would be absorbed by radio-active bodies. 
 Thus it would seem that the radio-activity of 
 a body might be reduced by surrounding it 
 with other radio-active bodies. Probably no 
 one has ever thought of experimenting in 
 just this way, but Elster and Geitel (77) have 
 found that the radio-activity of a body does 
 not diminish when it is placed underground, 
 for example in a mine, so that it is covered 
 by a layer of the earth's crust at least eight 
 hundred meters thick ; and it is hardly prob-
 
 102 RADIO-ACTIVITY 
 
 able that this should not absorb, at least to 
 an appreciable degree, the radiations which 
 are supposed to pass through space. 
 
 Hence it seems preferable to adopt the 
 hypothesis that the origin of the energy of 
 radio-active bodies is analogous to that of the 
 thermal energy furnished by chemical com- 
 binations, with the difference that, while in 
 the latter case we are dealing with the atoms 
 which taken in the free state or issuing from 
 other combinations unite together to form 
 new molecules, in the case of the radio-active 
 bodies we are dealing instead with electrons 
 derived from unstable atoms, and which unite 
 to form new and more enduring atoms. 
 Certain experiments, and especially those of 
 Rutherford and of his collaborators, render 
 these atomic modifications and transforma- 
 tions highly probable and lead to the fol- 
 lowing consideration of the phenomena of 
 radio-activity (78). 
 
 The atoms of the radio-active bodies are 
 unstable systems of electrons. Every now 
 and then some of these atoms subdivide into 
 free negative electrons and groups of elec-
 
 RADIO-ACTIVITY 103 
 
 trons with positive properties, or positive 
 ions. The former constitute the ^8-rays ; the 
 latter the a-rays. The emanation probably 
 consists of these same positive ions or of 
 modifications of them. If one portion only 
 of the disintegrated atoms is radiated out 
 into the surrounding space, the remaining 
 portion will constitute a new body, which 
 may be itself radio-active; and in this case 
 its atoms will be subject to further division. 
 The same thing may be said of the new 
 atoms which constitute the emanation, as 
 well as of the substance which this emana- 
 tion deposits on inert bodies, thereby ren- 
 dering them temporarily active (induced 
 activity). The atomic transformations cease 
 only when the electrons constitute stable 
 atoms; that is, a non-radio-active substance. 
 For example, let us consider uranium. 
 Crookes (79) and Becquerel (80) separated 
 from this substance an active and a non- 
 active part. The former, which Crookes and 
 Rutherford call uranium-X, in time loses its 
 activity, while the latter slowly acquires it. 
 This inactive uranium little by little becomes
 
 104 RADIO-ACTIVITY 
 
 transformed into uranium-X, and the trans- 
 formation is accompanied by radiation. At 
 the same time the uranium-X becomes trans- 
 formed into an unknown substance. A sim- 
 ilar thing happens in the case of thorium, 
 with this difference, however, that in addi- 
 tion to a transformation of inactive thorium 
 into thorium-X accompanied by the emission 
 of the usual rays, there is the production of 
 an emanation as a result of a final disintegra- 
 tion of the atoms which constitute the tho- 
 rium-X. The emanation itself is radio-active ; 
 that is, able to transform itself into other 
 substances, among which is the substance 
 which adheres to neighbouring bodies and 
 causes induced radio-activity. Also in the 
 case of thorium we are ignorant of what is 
 the final and stable atomic condition. 
 
 The successive transformations of the 
 radium atom are analogous to those of tho- 
 rium, save that there apparently exists no 
 radium-X. Thus there is a splitting up of 
 the radium atoms and the formation of an 
 emanation, or perhaps better of emanations, 
 inasmuch as there seem to exist between the
 
 RADIO-ACTIVITY 10$ 
 
 radium and the condensible emanation inter- 
 mediate stages of transformation,- which are 
 the transformations taking place during the 
 emission of the a, /3, and y rays. The ema- 
 nation itself is radio-active, and in under- 
 going transformation gives rise to the 
 substance producing induced radio-activity; 
 that is, to a substance temporarily radio- 
 active and thus in the stage of further trans- 
 formation. Neither in the case of radium 
 nor of the other radio-active bodies are we 
 able to say what other successive aggrega- 
 tions are possible before a stable condition is 
 attained; but the final non-radio-active sub- 
 stance has at least been found in the case of 
 radium. It has in fact been recognized (81) 
 that in time the emanation of radium en- 
 closed in a tube becomes modified ; its spec- 
 trum slowly changes in aspect and ends by 
 exhibiting the characteristic lines of helium. 
 This gas, recently discovered in our atmos- 
 phere and in certain minerals, is one whose 
 lines are found in the solar spectrum, and 
 hence the name helium. The minerals in 
 which it is found are, in general, those
 
 106 RADIO-ACTIVITY 
 
 from which radium is obtained ; a fact which 
 is naturally explained by what has just been 
 mentioned. 1 
 
 A very recent experiment of the Curies 
 and Dewar (82) seems to establish beyond 
 all doubt the transformation of the radium 
 emanation into helium. About four deci- 
 grams of pure and dry bromide of radium 
 stood for three months in a small flask com- 
 municating with a Geissler tube. This was 
 provided with two electrodes fused through 
 the glass to allow the passage of an electric 
 discharge, by means of which the gas con- 
 tained in the tube could be rendered lumi- 
 nous. At first a good vacuum was produced 
 in both flask and tube, and it was noted 
 that the radio-active substance continuously 
 evolved a gas which amounted to a cubic 
 centimeter per month (under ordinary press- 
 ure). On passing a discharge through the 
 
 1 E. Rutherford stated, in a paper read before the Interna- 
 tional Congress of Arts and Sciences in St. Louis (September 
 19-25, 1904), that the evidence at present indicates that the 
 a-particle is helium. See the important work by Rutherford 
 entitled "Radio-activity," recently published in the Cam- 
 bridge Physical Series. -TRANSLATOR.
 
 RADIO-ACTIVITY IO/ 
 
 tube and on examining the spectrum of the 
 light produced in this manner, the hydrogen 
 and mercury lines were seen, the latter being 
 due to the fact that a mercury pump was 
 employed in obtaining the vacuum. Thus 
 far no trace of helium had appeared. 
 
 The same bromide of radium was then 
 placed in a quartz tube and heated until the 
 salt fused. While the heating lasted, a pump 
 was kept working in order to draw the gas, 
 which was being given off, into a tube cooled 
 by liquid air. 
 
 In this manner about 2.6 cubic centimeters 
 of gas were collected, which was luminous 
 on account of the emanation it contained. 
 When this gas was brought into a Geissler 
 tube, it showed only the spectrum of nitro- 
 gen ; this was still the case after an attempt 
 had been made to eliminate the nitrogen by 
 means of condensation produced by cooling 
 with liquid hydrogen. At this point the 
 quartz tube containing the radium salt was 
 hermetically sealed while it was being pumped 
 out. Twenty days later, when the gas con- 
 tained in the tube was rendered luminous by
 
 108 RADIO-ACTIVITY 
 
 means of a discharge between external elec- 
 trodes of tin-foil, the spectroscope clearly 
 showed the entire spectrum of helium. 
 
 After this it seems difficult to doubt that 
 helium is formed as a result of the radio- 
 active transformations of radium. Never- 
 theless, many other proofs of this character 
 will be necessary before the transformation 
 of the chemical atoms may be considered as 
 a demonstrated fact ; that is to say, before 
 the idea of the absolute invariability of the 
 atoms, which with time has become so deeply 
 rooted in our minds, is abandoned.
 
 CHAPTER VI 
 
 MASS, VELOCITY, AND ELECTRIC CHARGE OF 
 THE IONS AND OF THE ELECTRONS 
 
 THE time has now come to give some 
 idea of the methods by which it has been 
 possible to measure not only the ratio be- 
 tween the electric charge and the mass of 
 the electrons or of the ions and the separate 
 values of these two quantities, but also the 
 velocity with which they move under various 
 conditions. Such methods for the most part 
 are based upon the effects which electric and 
 magnetic fields, either acting separately or 
 simultaneously, produce on the moving elec- 
 trified particles; or they are based on the 
 heat which is developed by the particles 
 themselves when they strike an obstacle ; or, 
 finally, on the property which the particles 
 possess of acting as nuclei for the condensa- 
 tion of vapours. Without entering into the 
 details of the experiments and of the calcula- 
 
 109
 
 1 10 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 tions which relate to them, let us examine 
 these different phenomena. 
 
 First, let us consider the effect which the 
 magnetic field produces on cathode rays, 
 which move so as to render the wall of the 
 discharge tube luminous at a point opposite 
 the cathode, and whose direction is perpen- 
 dicular to that of the field. For example, 
 the tube in which the cathode rays are gen- 
 erated may be placed between the poles of 
 a magnet. The electrons in motion, which 
 constitute the cathode rays, deviate from 
 their rectilinear path, and the rays them- 
 selves assume the form of circular arcs. In 
 fact, the magnetic field produces an electro- 
 magnetic force which affects the moving 
 electron. This force is in the direction of 
 that which the field would produce on an 
 electric current coinciding with the trajec- 
 tory, and hence it is perpendicular both to 
 the trajectory and to the magnetic force ; it 
 has no effect on the speed of the electron, 
 but only on the direction of its motion, which 
 is circular and uniform. The electromag- 
 netic force must then be equal and opposite
 
 MASS, VELOCITY, AND ELECTRIC CHARGE ill 
 
 to the centrifugal force of this circular mo- 
 tion. Now the centrifugal force depends in 
 a simple and known manner on the mass and 
 the velocity of the particle and on the radius 
 of its trajectory ; and, on the other hand, the 
 electromagnetic force is proportional to the 
 charge of the electron and to its velocity, 
 since the product of these two factors repre- 
 sents the intensity of the equivalent electric 
 current. We thus arrive at a simple relation 
 between the following quantities: first, charge 
 and mass of the electron, or, more exactly, the 
 ratio between the two ; second, the velocity 
 of the electron; third, the intensity of the 
 magnetic field ; fourth, the radius of the cir- 
 cular arc which is described. 1 The last two 
 quantities may be measured, and it is only 
 necessary to know one of the others in order 
 to calculate the remaining one. 
 
 If one assumes, as, in fact, at first it was 
 assumed, that the velocity of the electron in 
 
 1 Let e represent the charge of each particle, m its mass, 
 V its velocity, H the intensity of the magnetic field, p the 
 radius of the trajectory. The relation which exists between 
 
 a 
 
 these quantities is V Hp .
 
 112 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 the cathode rays is of the order of the mag- 
 nitude of the molecular velocities in gases, 
 one finds, for the ratio between charge and 
 mass, a value of an order similar to that 
 which holds for ions in electrolysis. Thus 
 one would be led to suppose that the mass 
 of the electron is of the order of magnitude 
 of that of the material atom. Fortunately, it 
 was recognized very soon that this assump- 
 tion was incorrect, and, in addition to the 
 magnetic deviation of the cathode rays, 
 other effects were sought and other theo- 
 retical considerations were undertaken which 
 might lead to a simultaneous determination 
 of the velocity and of the ratio mentioned 
 above. For example, it may be assumed 
 that the velocity of the electron is such that 
 its kinetic energy must equal the electric 
 work done in the passage of the charge of 
 the electron from the potential of the cathode 
 to that of the anode. Such a method as this 
 was adopted not long ago by Kaufmann (83) 
 and Simon (84). 
 
 J. J. Thomson (85) determined the velocity 
 of the electrons by measuring the negative
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 113 
 
 charge which they gave up on entering a 
 hollow conductor connected with an elec- 
 trometer, after having suffered a deviation 
 in a magnetic field, and by measuring at 
 the same time, by means of a thermo-elec- 
 tric couple, the energy transported by them. 
 Thus two more relations may be added to 
 the relation which exists between the quanti- 
 ties mentioned above. 1 One of these states 
 that the electric charge transported to the 
 conductor (this is found by direct measure- 
 ment) is equal to the number of electrons 
 multiplied by the constant charge of each of 
 them ; the other states that the energy trans- 
 ported and transformed into heat by impact 
 (and measured by means of the thermo-elec- 
 tric couple) is equal to the number of elec- 
 trons multiplied by the kinetic energy of 
 each, which is the half-product of the mass 
 and the square of the velocity. 
 
 To the two unknown quantities which we 
 had at first, we have thus added a new one, 
 
 tn 
 1 The two relations are Q = We, N= W, where Q is 
 
 the total quantity of electricity transported, N" the number of 
 electrons, and IV their kinetic energy.
 
 114 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 which is the number of electrons set in 
 motion during the experiment ; but now we 
 have three relations instead of the single 
 one we had at first; hence by eliminating 
 the number of electrons it becomes possible 
 to calculate both the ratio between the charge 
 and mass of each electron, and the velocity 
 as well. 1 The first results of any importance 
 were obtained by this method. Values of 
 the ratio between charge and mass of the 
 electrons were obtained, which differed but 
 slightly from each other when the rarefied 
 gas in which the cathode rays were produced 
 was changed (air, hydrogen, carbonic acid 
 gas). In every case the value found clearly 
 indicated that, if every electron represents an 
 electric charge equal to that of an electrolytic 
 ion, its mass must be, on the contrary, very 
 much smaller than that of an ion of hydro- 
 gen. The values found for the velocity of 
 the electrons showed, that it is very much 
 
 1 The elimination of N from the three equations leads to 
 the two following : 
 
 v _ ?.W e _ 2 W 
 
 QHp m 
 
 which serve to calculate V and 
 
 m
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 11$ 
 
 greater than the molecular velocities of 
 gases, and that, in fact, it was in this in- 
 stance about one-tenth of the velocity of 
 light. 
 
 Another method for the determination of 
 the velocity of the electrons of the cathode 
 rays, which is based on the deviation pro- 
 duced by an electric field, is also due to 
 Thomson (86). 
 
 FIG. 14. 
 
 The cathode rays leave the cathode C 
 (Fig. 14) and are reduced to a narrow bun- 
 dle by two thick metallic diaphragms A and 
 B, communicating with the earth and pro- 
 vided with narrow horizontal apertures, and 
 then pass between two metal plates D, E. 
 If these are oppositely charged, the rays 
 should deviate from their rectilinear path, 
 since the negative electrons, which consti-
 
 Il6 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 tute the rays, will be attracted by the posi- 
 tive plate and repelled by the other. Hertz 
 had been unable to obtain the expected devia- 
 tion of cathode rays in an electric field, and 
 neither did Thomson succeed in obtaining it 
 at first, because the conductivity which a 
 rarefied gas acquires when cathode rays pass 
 through it makes it impossible to maintain 
 the two plates at a sufficient difference of 
 potential. But when the highest attainable 
 rarefaction of the gas is reached, it is possible 
 to obtain the effect and to see the displace- 
 ment of the luminous spot at the end of the 
 tube, due to the phosphorescence excited by 
 the cathode rays. If, for example, the plate 
 E is positive and D negative, the luminous 
 spot descends, which shows that the path of 
 the electrons has been curved downward. 
 
 In fact, the electric force, which may be 
 assumed to be constant if the two plates are 
 sufficiently large and near together, will cause 
 each electron to describe a parabola, and thus 
 the cathode rays will curve, precisely as a jet 
 of water issuing from a horizontal tube curves 
 under the influence of gravity.
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 117 
 
 In the case of gravity the force is propor- 
 tional to the mass of the moving body, and 
 the acceleration is independent of the mass. 
 Here, on the contrary, the force acting on an 
 electron, and hence also the acceleration 
 which it produces, is proportional to the elec- 
 tric charge; and since, for a given charge, 
 and hence for a given electric force, the 
 acceleration is in inverse ratio to the mass, 
 it may be said that the acceleration itself, 
 on which the form of the parabola depends 
 in a known manner, is proportional to the 
 ratio between the charge and the mass of 
 the electron. Just as in the case of the mag- 
 netic deviation, so, also, in this case of the 
 electric deviation there exists a relation 
 which contains the above ratio and the ini- 
 tial velocity of the electron. Hence, if simul- 
 taneously with the electric field a magnetic 
 field is produced, the lines of force of which 
 are perpendicular both to the lines of elec- 
 tric force and the direction of the rays, it 
 will be possible to calculate not only the 
 ratio between the charge and mass of each 
 electron, but also its velocity. Moreover, it
 
 Il8 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 is possible to arrange matters in such a way 
 that the effects due to the two fields shall 
 neutralize each other. Then the two quan- 
 tities to be determined may be deduced from 
 the intensity of each of the fields, and from 
 the deviation produced when only one of 
 them acts. 1 In this manner Thomson ob- 
 tained a value for the velocity about one- 
 tenth that of light, and a value for the ratio 
 between the charge and mass of an electron 
 in agreement with that found by the other 
 method. 
 
 Similar measurements were made by Mr. 
 H. A. Wilson (87), who used a series of 
 cathodes made of different metals and proved 
 that the results obtained did not depend on 
 the nature of the cathode. 
 
 Almost at the same time Thomson's 
 method was used by Lenard (88), who ex- 
 
 1 Denoting by the deviation produced separately either 
 by the electric or by the magnetic field, by / the length of the 
 path along which the electron is exposed to the deflecting 
 forces, by F the intensity of the electric field, and by H the 
 magnetic field strength, we have 
 
 V= F -- L 
 
 H ' m
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 119 
 
 perimented with the rays which bear his 
 name; that is to say, with cathode rays which 
 are made to issue through thin aluminium 
 foil from the tube in which they are pro- 
 duced. 
 
 Lenard also (89) applied a new method, in 
 which the electrons were subjected to the 
 action of an electric field parallel to the 
 
 FIG. 15. 
 
 direction of their motion. For this purpose 
 the Lenard rays generated by the cathode 
 C (Fig. 15) issue from a hole A, which is 
 closed with aluminium foil, and thus pene- 
 trate into the apparatus V, containing a 
 highly rarefied gas. In this apparatus a con- 
 denser is placed consisting of two parallel 
 metallic disks, a, b, and having a small open- 
 ing at the centre, through which the rays
 
 120 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 coming from a and directed toward the 
 phosphorescent screen S, must pass. Two 
 metallic tubes, m, n, protect the rays from 
 the electric action of the disk a which may 
 be insulated and charged, while b is kept 
 always in communication with the earth. 
 Hence it is only in the path between a and 
 b that the electrons are exposed to an elec- 
 tric force which increases or diminishes their 
 velocity according as the disk a is positively 
 or negatively charged. These same rays 
 are, however, subjected farther on to a de- 
 flecting force, as in the preceding method, 
 produced by a magnetic field or by a trans- 
 verse electric field caused by the parallel 
 disks d, e, and the deviation thus produced 
 is measured. 
 
 As one might expect, for a given electric 
 field between d and e the deviation depends 
 upon the sign of the charge on a, since the 
 velocity with which the electrons arrive in 
 the field, producing the deviation, depends 
 on the sign of this charge. By measuring 
 this deviation and the intensity of the two 
 fields the ratio so often referred to may be
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 121 
 
 calculated ; the value which Lenard obtained 
 by this method will be given farther on. 
 
 Finally, I will cite a most ingenious, 
 but rather complicated, method which was 
 adopted by Wiechert (90) in making a di- 
 rect and accurate determination of the veloc- 
 ity of the cathode rays. By combining his 
 results with those obtained by the method 
 of magnetic deviation, he was able to calcu- 
 late also the ratio between charge and mass 
 of the electron. 
 
 But it is not only on the electrons of the 
 cathode rays that measurements designed to 
 determine their characteristic constants have 
 been performed ; other measurements have 
 been made both on the negative electrons 
 emitted by metals subjected to ultraviolet 
 rays and on those emitted by incandescent 
 or by radio-active bodies. Thus, J. J. 
 Thomson (91) resorted to an experimental 
 method in which a magnetic field was 
 caused to act upon the electrons emitted 
 by an illuminated metal. By means of an 
 almost identical experimental arrangement, 
 the writer (92) had previously determined
 
 122 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 that a magnetic field reduces the transport 
 of negative electricity from the body which 
 receives the ultraviolet rays to neighbour- 
 ing bodies ; Thomson 
 interpreted this fact 
 in terms of the new the- 
 ory, and availed himself 
 of it in the following 
 manner. 
 
 A small, negatively 
 charged zinc disk AB 
 (Fig. 1 6), supported by 
 a metallic rod L t is 
 placed in a vessel con- 
 taining highly rarefied 
 air in such a way that 
 it may be brought more 
 or less close to a metal 
 grating CZ?, which is 
 parallel with the disk 
 and connected with an 
 electrometer. The vessel is closed by a 
 quartz plate EF, in order that the more 
 refrangible ultraviolet radiations caused by a 
 shower of sparks passing between zinc wires 
 
 FIG. 16.
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 123 
 
 may not be absorbed before they arrive at 
 the charged disk. When the apparatus has 
 been tested to make sure that the electrom- 
 eter becomes discharged by the radiations, 
 even when the disk AB is relatively far from 
 the grating, a magnetic field is produced the 
 lines of force of which are parallel to the 
 disk and to the grating. It is then found 
 that if the distance between the disk and the 
 
 FIG. 17. 
 
 grating is increased to more than a certain 
 amount, the transport of the negative elec- 
 tricity from AB to CD ceases almost entirely. 
 The mechanism to which the phenomenon 
 is due is the following : before the magnetic 
 field is set up the negative electrons which 
 the radiations expel from the zinc disk AB 
 (Fig. 17) pass directly from AB toward 
 CD ; but after a magnetic field perpendicular 
 to the plane of the figure is produced, each 
 electron describes a curve which may be 
 shown to be a cycloid, and as a result the
 
 124 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 electron after having gone a certain distance 
 from the disk returns to it without having 
 been able to reach the grating if the latter 
 is too far distant. 1 A few of the paths de- 
 scribed by the electrons are shown as broken 
 lines in the figure. 
 
 The maximum distance attained by an 
 electron depends in a known manner upon 
 the intensity of the electric field between 
 the disk and the grating, upon the intensity 
 of the magnetic field, and on the ratio be- 
 
 1 Choosing the J^-axis in the direction perpendicular to 
 the disk and the grating, the K-axis perpendicular to the X- 
 axis and to the magnetic field, we have as the equations of 
 motion : 
 
 m d^ =Fe _ He dj L 
 dt* dt" 
 
 dP dt 
 
 Supposing the initial values of x, y, , , to be zero, the 
 solution of these equations is : 
 
 x= a (i cos /), y = a (bt sin bt), in which 
 
 The radius of the circle which generates the cycloid is a, and 
 hence the maximum distance from the zinc disk which an 
 electron may attain is : 
 
 2 ^L 
 H**
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 125 
 
 tween the charge and mass of the electron. 
 Hence it is possible to determine this ratio 
 by measuring the first two quantities and the 
 maximum distance between the disk and the 
 grating beyond which the latter no longer 
 receives a sensible charge. The accuracy of 
 the determination is limited by the fact that 
 the electrons do not all leave the disk with 
 equal velocities, and hence the maximum 
 distance from it which they attain is not the 
 same for all. 
 
 Thomson (93) applied a similar method 
 to the electrons emitted by an incandescent 
 metal. Lenard (94) measured the ratio by 
 resorting to the action of ultraviolet rays on 
 metals in a vacuum; and finally Becquerel 
 (95) and others determined the constants for 
 electrons emitted by radio-active bodies. 
 Without at present entering into the details 
 of these last determinations and of others 
 which have given analogous results, it will 
 be well to tabulate the principal values of the 
 ratio between the charge and mass of an elec- 
 tron in order that these values may be com- 
 pared with each other.
 
 126 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 SOURCE OF 
 THE ELEC- 
 TRONS 
 
 EXPERIMENTER 
 
 DATE 
 
 METHOD USED 
 
 RATIO 
 
 BETWEEN 
 
 CHARGE AND 
 MASS 
 
 Cathode 
 
 J.J.Thomson 
 
 1897 
 
 Electric Deviation and 
 
 231 io 15 * 
 
 Rays 
 
 
 
 Magnetic Deviation. 
 
 
 Cathode 
 
 J.J.Thomson 
 
 1897 
 
 Magnetic Deviation, 
 
 351 io 
 
 Rays 
 
 
 
 charge transported, 
 
 
 
 
 
 and heat evolved, j 
 
 
 Cathode 
 
 Kaufmann 
 
 1897- 
 
 Magnetic Deviation, 
 
 558-101* 
 
 Rays 
 
 
 1898 
 
 and 
 
 
 
 
 
 Potential Difference. 
 
 
 Lenard 
 
 Lenard 
 
 1898 
 
 Electric Deviation and 
 
 191.7. lo 16 
 
 Rays 
 
 
 
 Magnetic Deviation. 
 
 
 Lenard 
 
 Lenard 
 
 1898 
 
 Deviation and Electric 
 
 204 io 18 
 
 Rays 
 
 
 
 Field. 
 
 
 Cathode 
 
 Simon 
 
 1899 
 
 Magnetic Deviation 
 
 559-5 Io15 
 
 Rays 
 
 
 
 and 
 
 
 
 
 
 Potential Difference. 
 
 
 Cathode 
 
 Weichert 
 
 1899 
 
 Magnetic Deviation 
 
 (303 - io 
 
 Rays 
 
 
 
 and Velocity. 
 
 \46s io 
 
 Ultra- 
 
 J.J.Thomson 
 
 1899 
 
 Diminution of the 
 
 228 - io 16 
 
 violet 
 
 
 
 discharge by the action 
 
 
 Rays 
 
 
 
 of a magnetic field. 
 
 
 Metal 
 
 J.J.Thomson 
 
 1899 
 
 Diminution of the 
 
 261 io* 
 
 heated to 
 
 
 
 discharge by the action 
 
 
 Redness 
 
 
 
 of a magnetic field. 
 
 
 Ultra- 
 
 Lenard 
 
 1900 
 
 Magnetic Deviation 
 
 345.10^ 
 
 violet 
 
 
 
 and 
 
 
 Rays 
 
 
 
 Electric Field. 
 
 
 /3-Rays 
 
 Becquerel 
 
 1900 
 
 Electric Deviation 
 
 about 
 
 from 
 
 
 
 and 
 
 300- io 16 
 
 Radium 
 
 
 
 Magnetic Deviation. 
 
 
 * 231- io 18 signifies the number obtained by writing 13 zeros to the right of the 
 231; that is to say, the number 231,000,000,000,000,000. The unit of charge 
 adopted here is the electrostatic, which is defined as the quantity of electricity 
 which repels a like charge placed at a distance of one centimeter with unit force; 
 that is, with the force of one dyne.
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 127 
 
 If account be taken of the great variety 
 of phenomena in which the negative elec- 
 trons make themselves manifest, and of the 
 diversity of the methods adopted in the 
 measurement of the ratio between the charge 
 and the mass of each electron, the accord 
 between the results is most remarkable. 
 There can then remain no doubt as to the 
 order of magnitude of this ratio since it turns 
 out to be between 663 and 1937 times greater 
 than the analogous ratio for the hydrogen ion 
 in electrolysis, which is equal to 0.289- io 15 , 
 and still greater than that which relates 
 to the ions of other bodies. Therefore, the 
 particles constituting the cathode rays and 
 the /8-rays from radio-active bodies cannot 
 be atoms, but must be particles of very much 
 smaller mass. Thus it is that the existence 
 of masses which are much smaller than that 
 of the smallest of the atoms of known sub- 
 stances, has been demonstrated in the surest 
 possible manner and by purely physical 
 methods. 
 
 The difference between the values found 
 by various investigators for the ratio between
 
 128 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 the charge and mass of the electrons is not 
 solely due to errors in measurement, since 
 the very accurate experiments made by Kauf- 
 mann (96) prove that this ratio varies with 
 the velocity of the electrons, becoming rapidly 
 smaller as the velocity approaches that of 
 light. Kaufmann caused a magnetic and an 
 electric field to act on the /3-rays, emitted by 
 one of the salts of radium, in such a way 
 that the fields had a common direction per- 
 pendicular to that of the rays. The action 
 of the magnetic force is to cause the rays to 
 deviate in a certain direction, while that of 
 the electric force is to cause the rays to devi- 
 ate in a direction at right angles to this. 
 Thus the experimental arrangement resem- 
 bles the well-known one of the crossed 
 prisms, in which two prisms cause light to 
 undergo two successive deviations at right 
 angles to each other. Just as in this latter 
 experiment each coloured ray, separated from 
 white light by the action of the first prism, 
 suffers a new deviation due to the second 
 prism, and these deviations may be sepa- 
 rately measured, so also in Kaufmann's ex-
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 129 
 
 periment the deviation of each of the /3-rays, 
 which differ as regards the velocity of the 
 electrons which constitute them, may be 
 measured. 
 
 By such a method as this Kaufmann found 
 for the ratio between the charge and mass of 
 the electrons values approximately equal to 
 those obtained by other experimenters so long 
 as he was dealing with electrons of relatively 
 low velocity, or electrons constituting feebly 
 penetrating /2-rays, but he found much 
 smaller values for the more penetrating rays ; 
 in fact, the ratio fell off to about one-half of 
 the usual value for electrons whose velocity 
 was about nine-tenths that of light. 
 
 Inasmuch as everything seems to point to 
 the fact that the electric charge is always the 
 same for all electrons, it must be held that 
 their mass is not constant, but increases 
 rapidly as their velocity approaches that of 
 light. This result is of great importance, 
 since it is in conformity with the suppo- 
 sition that the electrons do not possess a 
 material mass in the ordinary sense of the 
 word, but only an apparent mass due to the
 
 130 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 fact that they are electric charges in motion. 
 But we shall return to this subject in the last 
 chapter. 
 
 Wien (97) and Thomson (98) carried out 
 measurements on the positive ions which 
 constitute the canal rays, similar to those on 
 the negative electrons. The first of these 
 physicists obtained as a result a velocity of 
 3600 kilometers per second, and the value 
 0.009. i o t5 for the ratio; Thomson found the 
 ratio to be o.oi2-io 15 . Thus it is certain that 
 the positive particles in the canal rays are 
 not electrons, but are, instead, atoms or per- 
 haps heavy electrified atomic groups. 
 
 The preceding conclusion relative to the 
 small mass of the electrons is based on the 
 hypothesis that the electric charge of the ions 
 in gases is equal to that which is associated 
 with every ion in electrolysis (or better, with 
 every valency of the ion). From now on 
 this may be considered an established fact 
 rather than a hypothesis. In fact, the study 
 of the diffusion of ions in gases (99) has led 
 to the important conclusion that the electric 
 charge of each ion is sensibly equal to that
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 131 
 
 which the ions have in electrolysis. But 
 even before this result had been obtained 
 Thomson (100) measured directly the charge 
 of each ion in a gas, employing an extremely 
 clever method of which we will now attempt 
 to give an idea. 
 
 When air saturated with water vapour sud- 
 denly expands, a part of the vapour condenses 
 in a cloud as a result of the cooling which 
 accompanies the expansion, and each little 
 globule generally has as a nucleus a particle 
 of the microscopic dust which is ordinarily 
 contained in the atmosphere. It seems, in 
 fact, as though the presence of very minute 
 bodies was required for the condensation of 
 vapour, and that the condensation begins on 
 the surface of these bodies whose radius of 
 curvature must certainly be very small. At 
 any rate, the presence of these corpuscles is 
 favourable to the liquefaction of the vapour ; 
 for if the dust is carefully removed, it is 
 necessary to expand the moist air considera- 
 bly more in order that the cloud may form. 
 
 Mr. C. T. R. Wilson (101) showed that a 
 gas containing ions behaves like air charged
 
 132 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 with dust, since the ions themselves act as 
 nuclei or centres of condensation for the 
 vapour. And, in fact, an expansion of the 
 air which may be too small to cause a cloud 
 to form, may, on the contrary, be sufficient 
 to precipitate the vapour when the air itself 
 is ionized ; for example, by passing Rontgen 
 rays through it. That the change in be- 
 haviour of the gas is really due to the pres- 
 ence of ions was confirmed by the fact that 
 the formation of the cloud is again retarded, 
 or what amounts to the same thing, a greater 
 expansion is necessary, when the ions are 
 removed by the passage of an electric cur- 
 rent through the gas. 
 
 The apparatus used by Thomson is some- 
 what complicated, but the essential part is a 
 vessel which contains the moist air, and in 
 which two horizontal conductors are placed 
 one above the other. The upper, which may 
 serve to close the vessel, generally consists 
 of a disk of thin aluminium which is kept 
 in electrical connection with the earth ; the 
 lower, which may consist of the mass of water 
 which keeps the air above it saturated, is con-
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 133 
 
 nected to an electrometer. At the proper 
 moment the air is ionized by allowing X-rays 
 or the rays emitted by a radio-active body to 
 enter the vessel by passing through the alu- 
 minium plate. Ions may also be introduced 
 into the vessel by charging the lower con- 
 ductor negatively and by letting ultraviolet 
 rays fall on its surface. In this case the alu- 
 minium disk is replaced by a metallic grat- 
 ing, the vessel is closed by a quartz plate, 
 and a zinc or other metallic disk is chosen 
 as the lower conductor. 
 
 In the first part of the experiment a meas- 
 urement is made of the quantity of electricity 
 which passes from one conductor to the other 
 in unit time ; that is to say, the intensity of 
 the current which traverses the ionized air is 
 measured. This is found by multiplying the 
 diminution of potential in unit time, which 
 is calculated from the decrease in the devia- 
 tion of the electrometer, by the capacity of 
 the system formed by the lower conductor 
 and the electrometer. But, on the other 
 hand, this current is due to the motion of 
 the ions under the action of the electric field
 
 134 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 which exists between the two disks ; and the 
 intensity of the current may be calculated 
 in terms of the following quantities : the 
 strength of the electric field, the number of 
 ions which exist in unit volume, the charge 
 of each ion, and the velocity with which they 
 move from one disk to the other. By equat- 
 ing the two values for the current strength a 
 relation is obtained which contains the num- 
 ber of ions, their charge and their velocities, 
 in addition to the numerical data resulting 
 directly from the experiment. 1 But the ve- 
 locity of the ions in an electric field of known 
 
 1 Calling the intensity of the field between the two disks 
 E, the number of ions per cubic centimeter JV, the velocity 
 of the ions in a field of unit intensity F, the charge of each 
 e, the area of the disks A, the intensity of the current from 
 one disk to the other is obviously 
 
 NVEeA; 
 
 and if C is the capacity of the insulated disk, P the diminu- 
 tion of its potential in unit time, the equation which is men- 
 tioned in the text is 
 
 NVEeA = CP. 
 
 By measuring E, A, C, F, and by assuming for V the value 
 given by special experiments which Rutherford undertook to 
 determine this velocity (480 centimeters per second in the 
 case of ions produced in air by X-rays), only A 7 " and e remain 
 unknown. N was measured in the manner described farther 
 on in the text.
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 135 
 
 intensity has been measured and is known ; 
 hence, in order to arrive at a determination 
 of the charge of the ions, only the number 
 which exist in unit volume remains to be 
 measured. 
 
 The second part of the experiment serves 
 to accomplish this. If a cloud is produced 
 by a suitable expansion of the air, it is suffi- 
 cient to count in some way the globules of 
 water of which it is formed in order that the 
 object of the experiment may be attained. 
 In fact, assuming that each of the ions is a 
 nucleus of one of the drops, the number of 
 the former would equal the number of the 
 latter. Now, to estimate the number of 
 drops, it is sufficient to divide the total mass 
 by the mass of each drop. The calculation of 
 the total mass is based on the measurement of 
 the lowest temperature which the air reaches 
 in the act of expansion and on the measure- 
 ment of the temperature of the air itself after 
 the formation of the cloud ; 1 the mass of 
 each drop is inferred from its diameter, and 
 
 1 Calling the two temperatures /, /', the specific heat of the 
 air at constant volume C, the latent heat of vaporization of
 
 136 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 this in turn from the velocity with which it 
 falls, or from the rate of descent of the upper 
 surface of the cloud, which is always suffi- 
 ciently sharp in outline. Naturally a rela- 
 tion exists between the dimensions of a 
 sphere and the velocity with which it falls. 
 While it is true that all bodies fall with equal 
 velocity in a vacuum, this is not the case in 
 air, the resistance of which more or less slows 
 down their motion. In the case of a sphere 
 this slowing down is the greater, the smaller 
 the sphere, because, while the weight is pro- 
 portional to the volume and hence to the 
 cube of the diameter of the sphere, the re- 
 sistance offered by the air is proportional to 
 the surface, and hence to the square of the 
 diameter. Therefore, as this is diminished, 
 the weight falls off more rapidly than the 
 resistance of the air. This is the reason why 
 bodies reduced to fragments, such as powders 
 and small drops, fall so slowly that they often 
 seem to stand still. 
 
 the water Z, the mass of the air in unit volume M, the mass 
 of vapour condensed in a cubic centimeter of air g, we have 
 
 Lq=CM(t' -/).
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 137 
 
 To determine the diameter of the drops of 
 water, a formula 1 is employed which contains 
 this diameter in terms of the velocity of 
 descent and the viscosity of the medium in 
 which the particles move. The number of 
 ions and therefore the charge of each may 
 be calculated in the manner described above 
 when the diameter is known. 
 
 The final result which Thomson obtained 
 from a series of observations made on air 
 ionized by X-rays was that the charge of 
 each ion is (in electrostatic units) 6.5 . io~ 10 , 
 or o.ooo ooo ooo 65. He deduced from his 
 other experiments on the action of ultra- 
 violet rays on zinc the value of 6.8 io~ 10 , 
 which differs only very slightly from the 
 above result. 
 
 However, Thomson (102) himself later 
 found that a correction should have been 
 applied to these values. It has been proved 
 that the condensation of vapour on the nega- 
 
 1 If V is the velocity with which a drop falls, r its radius, 
 ft. the coefficient of viscosity of the air, and g the acceleration 
 of gravity, this formula is : 
 
 9 n y= 2 gr z . 
 For air ju, = 0.00018.
 
 138 MASS, VELOCITY, AND ELECTRIC CHARGE 
 
 tive ions takes place with an expansion less 
 than the minimum necessary in the case of 
 condensation on positive ions, and that prob- 
 ably, in the experiment described, a few of the 
 positive ions acted as nuclei. In fact, it is 
 now known that, when the volume of the 
 moist air is suddenly increased in the ratio 
 of i to 1.25, the negative and not the positive 
 ions act as nuclei in the formation of the 
 drops; it is only when the expansion is in 
 the ratio of i to 1.31 that the positive ions 
 begin to take part in the phenomenon. 
 
 A series of new experiments, in which 
 account was taken of this, led to the more 
 accurate result of 3.4 io~ 10 electrostatic units 
 as the charge of each single ion. 
 
 Shortly after this, new research on the 
 same subject was undertaken by Mr. H. A. 
 Wilson (103), who adopted a slightly different 
 method, which has the advantage of not re- 
 quiring the evaluation of the number of ions 
 existing in the air subjected to the Rbntgen 
 rays. 
 
 In this case also, by a sudden expansion of 
 the air, the water vapour is condensed into
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 139 
 
 drops having an ion as a nucleus, but the 
 cloud is made to form between two parallel 
 metallic disks. When these are oppositely 
 electrified, the velocity with which the drops 
 fall is different from that which they pos- 
 sessed before the electric field was set up 
 between the disks. Whether the change is 
 an increase or decrease depends on the direc- 
 tion of the electric force. By measuring this 
 velocity, sufficient data are available for the 
 calculation of the charge of each drop. 1 
 
 Without entering into further details rela- 
 tive to these very ingenious experiments, I 
 will point out that they confirm the fact, 
 
 1 Calling the velocity with which the drops fall when there 
 is no electric field z/, that when the field accelerates the fall 
 v", the intensity of the field X, the charge of the ion e, the 
 mass of a drop m, the acceleration of gravity g, it is evident 
 
 that we have 
 
 mgv" = (mg + Xe) T/ ; 
 
 but from the equation 
 
 of the preceding footnote we may deduce 
 m = 3.14 lo~ 9 'is'*. 
 Hence, on eliminating m, we have 
 
 which permits a calculation of e based on the measurement of 
 X, v', and v".
 
 previously noted by Thomson, that when the 
 intensity of the X-rays employed to ionize the 
 air is very great, a number of ions may unite 
 and form the nucleus of a single drop. In 
 this case it follows that the charge of the 
 drop is either twice, or three times, etc., that 
 of a drop which has but a single ion as a 
 nucleus. Wilson's experiments show that 
 this really may happen, since the cloud sepa- 
 rates into several horizontal layers which fall 
 with different velocities. Evidently one of 
 the layers is formed by drops having but one 
 ion as a nucleus, another layer by those hav- 
 ing two ions as a nucleus, etc. It is clear 
 that, by taking this into account, a measure- 
 ment of the velocity with which any one of 
 the various layers falls may be used in cal- 
 culating the quantity sought. The result 
 which Wilson obtained (3.1 io~ 10 ) differs but 
 slightly from Thomson's result. This value 
 is almost identical with that found by calcu- 
 lating the charge of the hydrogen ion in elec- 
 trolysis, assuming the mass of the ion to have 
 the value calculated from the kinetic theory 
 of gases.
 
 MASS, VELOCITY, AND ELECTRIC CHARGE 141 
 
 Thus we see that the numerical agreement 
 which the theory would lead one to expect 
 is substantiated within the limits of precision, 
 which in this sort of research may at present 
 be reasonably demanded.
 
 CHAPTER VII 
 
 THE ELECTRONS AND THE CONSTITUTION OF 
 MATTER 
 
 ON account of the facility with which the 
 electron theory lends itself to the formation 
 of a model of the mechanism of physical 
 phenomena, it possesses a decided utility, 
 even in the minds of those who see in it 
 only an instrument for research. In reality, 
 the theory itself is hardly in its initial stages, 
 and it would be premature to consider it 
 just yet as a solid basis for a new system 
 of natural philosophy. Nevertheless, since 
 even from this point of view it is constantly 
 acquiring importance, it will be useful to 
 dedicate this last chapter to a concise expo- 
 sition of the hypothesis, according to which 
 matter is now coming to be considered as 
 built up of electrons. 
 
 A role of fundamental importance is 
 assigned to the electrons in this new mode 
 
 142
 
 THE CONSTITUTION OF MATTER 143 
 
 of conceiving the constitution of bodies ; but 
 in order that it may be possible to explain 
 the known phenomena in terms of the elec- 
 trons, it is necessary to suppose them to be 
 endowed with certain essential properties. 
 Thus, for example, it is held that there exist 
 electrons of two sorts, which are in a certain 
 sense mutually antagonistic, namely, negative 
 electrons and positive electrons ; it is held 
 that the first and not the second may exist 
 in the free state; it is admitted that the 
 separation of a negative electron takes place 
 more easily, or with less expenditure of 
 energy, from certain atoms, like those of 
 the metals, than from others. However, 
 the fundamental property attributed to the 
 electrons is that, in their essence, they con- 
 sist of electric charges acting on each other 
 in the manner expressed by the formulae of 
 Hertz or Maxwell. It follows from this that 
 the new theory does not pretend to give a 
 reason for the cause of electric phenomena. 
 This still remains a mystery. While for- 
 merly, starting with the existence of cosmic 
 ether and that of ponderable matter, char-
 
 144 THE CONSTITUTION OF MATTER 
 
 acterized by its principal attribute, inertia, 
 the attempt was made to give a mechanical 
 explanation for all phenomena ; now, on the 
 contrary, starting with the ether and the 
 electrons, the attempt is made to construct, 
 so to speak, ponderable matter out of these 
 and to take account of the phenomena 
 which it presents. Hence it may be said 
 that the electron theory is much more a 
 theory of matter than a theory of electricity ; 
 or, rather, in the new system electricity is 
 set up in the place of matter, the existence 
 of which was, on the whole, not much better 
 understood than is the essence of the elec- 
 trons at the present time. 
 
 In order to better understand the impor- 
 tance of the hypothesis and the fundamental 
 attributes of the electrons, it will be well for 
 us to consider synthetically the phenomena 
 due to electrified bodies, either at rest or in 
 motion. Following a line of reasoning due 
 to Lodge (104), let us suppose that we bring 
 into contact two bodies of different material 
 and then separate them. They soon present 
 that group of properties which constitute the
 
 THE CONSTITUTION OF MATTER 145 
 
 two opposite electric states and, in particular, 
 they attract each other and create the electric 
 field which surrounds them. If one of the 
 two bodies, say that which has the positive 
 electric properties, is removed to an infinite 
 distance, only the negative body need be 
 taken into consideration. If we also suppose 
 it to be very small, the electric field will be 
 represented by rectilinear lines of force arriv- 
 ing at the body from all directions. The 
 surrounding ether is now deformed, giving 
 to this word the very broadest meaning ; that 
 is to say, it exists in a strained condition, 
 which is shown not only by the tensions 
 along the lines of force, which cause the 
 apparent forces at a distance, but also by 
 the transverse pressures. What the cause of 
 this special state of the ether is, how it can be 
 susceptible of a dual aspect, that is, how it 
 can correspond to a positive and to a negative 
 charge, are questions which we are absolutely 
 unable to answer, just as we are unable to 
 answer the question, what is the real struc- 
 ture and real nature of the entity which exists 
 everywhere and which is called the ether.
 
 146 THE CONSTITUTION OF MATTER 
 
 Let us now suppose that the small nega- 
 tively electrified body moves with uniform 
 speed; that is, we will suppose the special 
 state of deformation just defined changes its 
 position in the ether. It may be deduced 
 from Maxwell's theory and from direct ex- 
 periment as well, that this propagation of 
 the ethereal strain from place to place pro- 
 duces the magnetic field. This may then be 
 considered to be due to a deformation differ- 
 ent in nature from the electric deformation 
 but analogous to it, since tensions along the 
 lines of force and pressures in the transverse 
 direction also exist in the magnetic field. 
 And if the motion is rectilinear, the lines of 
 force are circles with their centres on the 
 trajectory and lying in the planes perpen- 
 dicular to this. A series of electrified bodies 
 following each other with uniform motion has 
 the properties of an electric current. Thus a 
 constant current may be considered as a flow 
 of equidistant electrons in uniform motion, 
 and a variable current as a flow of electrons 
 in variable motion, or of electrons which do 
 not follow one another at equal intervals.
 
 THE CONSTITUTION OF MATTER 147 
 
 If the small electrified body moves with 
 non-uniform motion, the magnetic field which 
 it creates is variable, and the phenomenon 
 of induction takes place. If the motion is 
 periodic, the phenomena of light occur. 
 Every variation in the velocity of electrified 
 bodies causes a variation in the magnetic 
 field, this produces a variation in the electric 
 field, and these two variations are propagated 
 together with the velocity of light. 
 
 Let us now suppose that at a given mo- 
 ment we wish to increase the velocity of the 
 electrified body, which we will suppose to 
 move up to this moment with uniform speed. 
 On account of the relations existing between 
 the electric and magnetic force in an electro- 
 magnetic field, it is not possible to accelerate 
 the motion of the electrified body without 
 the expenditure of energy. In fact, an in- 
 crease in the velocity results in a variation 
 of the magnetic field, which in turn produces 
 an electric force tending to oppose the accel- 
 eration of the motion. In the same way a 
 decrease in the velocity is opposed by the 
 generation of an electric force which tends
 
 148 THE CONSTITUTION OF MATTER 
 
 to conserve the velocity of the electrified 
 particle. In both cases the electromagnetic 
 phenomenon is such as to simulate inertia, 
 and the body by the mere fact of its being 
 electrified behaves as though its mass were 
 larger than it really is. 
 
 What has been said of the small electrified 
 body holds for an electron, and its mass, 
 which we have stated to be less than one- 
 thousandth that of the hydrogen atom, is at 
 least in part not real, but apparent. 
 
 This species of apparent inertia, which an 
 electrified body or an electron presents, is a 
 manifestation of the phenomenon called self- 
 induction in the case of electric currents. In 
 fact, if there are, instead of a single moving 
 electron, a great number of such electrons 
 following one another at small and equal 
 intervals along the same path, these repre- 
 sent an electric current. An increase or 
 diminution in the velocity of the electrons 
 gives rise to an increase or diminution of the 
 number which pass a given point on the path 
 in unit time, and hence correspond to an 
 increase or diminution in the intensity of the
 
 THE CONSTITUTION OF MATTER 149 
 
 current. Now what has been said about the 
 effect due to the variation in the velocity of 
 a single moving electrified body or of a 
 single electron, is substantially true of any 
 number of electrons, and thus each variation 
 in their velocity generates a force which tends 
 to hinder the variation itself. Each change 
 in the intensity of the current generates an 
 electromotive force, which tends to oppose 
 the change or to produce a new current in 
 such a direction as to diminish the change. 
 As is seen, this current is the extra current, 
 and the electromotive force is the electromo- 
 tive force of self-induction. 
 
 In summing up, it may be said that the 
 electrons determine the production of the 
 so-called electrostatic phenomena, when they 
 are stationary; of the phenomena of magneto- 
 statics and of constant currents when they 
 are in uniform motion ; of electromagnetic 
 phenomena when they move non-uniformly, 
 and of optical phenomena when they move 
 with periodic motion. A sudden variation 
 in the velocity of an electron which may be 
 due, for example, to a collision, generates an
 
 150 THE CONSTITUTION OF MATTER 
 
 electromagnetic wave in the ether analogous 
 to the waves caused by an explosion in air. 
 The X-rays are the manifestation of these 
 waves. 
 
 We are now in a position to comprehend 
 in what consists the modern hypothesis, 
 according to which matter is built up of elec- 
 trons. First of all, we may admit that the 
 electrons are not matter, in the ordinary sense 
 of the word ; that is, they do not possess mass 
 other than that which they seem to have by 
 reason of their motion and electric charge. 
 Kaufmann's experiment, referred to in the 
 preceding chapter, renders this hypothesis 
 very probable. He found, in fact, that the 
 ratio between the charge and the mass of the 
 moving electrons increases rapidly as the ve- 
 locity approaches that of light. And since 
 the hypothesis of a varying charge would 
 be too improbable, it only remains to be sup- 
 posed that the mass rapidly increases. Now 
 such a result as this is in accord with the 
 hypothesis according to which the mass of 
 the electrons is entirely electromagnetic in 
 its origin.
 
 THE CONSTITUTION OF MATTER 151 
 
 Nothing prevents us from supposing that 
 matter, and hence all known bodies, consist 
 of aggregations or systems of electrons since 
 the electrons, which may be considered as 
 simple electric charges devoid of matter or 
 as consisting in a modification of the ether 
 symmetrically distributed about a point, per- 
 fectly simulate inertia by reason of the laws 
 of the electromagnetic field and thus show 
 the fundamental property of matter. 
 
 Therefore it may be admitted that a mate- 
 rial atom is nothing but a system consisting 
 of a certain number of positive and an equal 
 number of negative electrons, and that the 
 latter, or at least some of them, move about 
 the remaining portion like satellites. Mo- 
 lecular and atomic forces would then be 
 nothing but the manifestations of the elec- 
 tromagnetic forces of the electrons, and grav- 
 itation itself might be explained with these 
 concepts as a basis. In fact, this has already 
 been attempted. 
 
 If we suppose one or more negative elec- 
 trons to be taken away from an atom, it be- 
 comes a positive ion, while the addition of
 
 152 THE CONSTITUTION OF MATTER 
 
 one or more negative electrons to a neutral 
 atom produces a negative ion. The manner 
 in which various bodies behave when sub- 
 jected to free electrons in motion, as in the 
 case of the cathode rays, is such as to indorse 
 the hypothesis. It has indeed been found 
 that a body prevents the passage of the elec- 
 trons, or absorbs the cathode rays, in propor- 
 tion to its density; that is, in proportion to 
 the total number of electrons, which consti- 
 tute it, independently of the manner in which 
 they are grouped to form chemical atoms of 
 various kinds. 
 
 The electrons would seem to be, therefore, 
 the elements of construction in the architec- 
 ture of the atoms. When such a hypothesis 
 as this is once adopted, the dogma of the 
 invariability of the chemical atom or of the 
 impossibility of transmutation of chemical 
 substances is forever banished from science, 
 since according to this hypothesis everything 
 is built up of electrons. We have already 
 seen how the phenomena of radio-activity 
 seem to show transformations of this kind. 
 
 If, in addition, it is held that all bodies are
 
 THE CONSTITUTION OF MATTER 153 
 
 at least slightly radio-active and hence emit 
 ions and electrons, these new views relative 
 to the structure of matter become quite simi- 
 lar to those which were advanced as the basis 
 of a general explanation of physical phenom- 
 ena more than a half-century ago by a clever 
 and original Italian physicist, Ambrogio Fu- 
 sinieri (105). In spite of the fact that the 
 concepts of this physicist have hitherto been 
 open to objections and now have lost a part 
 of their value in the light of subsequently 
 discovered facts, one thinks at once of what 
 he called attenuated matter emitted by all 
 bodies, when one speaks of emanations sent 
 out by radio-active bodies, or of electrons 
 which, like a species of slow and invisible 
 evaporation, are probably given off in a con- 
 tinuous manner by every material substance, 
 and diffuse into surrounding space.
 
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 INDEX 
 
 [THE NUMBERS REFER TO THE PAGES.] 
 
 o-rays, 73. 
 
 Absorption of, 74. 
 
 Magnetic deviation of, 73. 
 
 Nature of, 73. 
 
 Velocity of, 74, 97. 
 Actinium, 63. 
 Anode rays, 52. 
 Aschkinass and Caspari : 
 
 Effect of radium on bacteria, 90. 
 
 /J-rays, 75. 
 
 Absorption of, 76. 
 
 Magnetic deviation of, 75. 
 
 Negative charge transported by, 
 
 76. 
 Baskerville : 
 
 Carolinium, 65. 
 Becquerel : 
 
 Magnetic deviation of a-rays, 73. 
 
 Magnetic deviation of (3-rays, 75. 
 
 Rays, 58, 61. 
 
 Theory of spinthariscope, 79. 
 Bemont : 
 
 Preparation of radium, 62. 
 
 Canal rays, 52. 
 Carolinium, 65. 
 Cathode : 
 
 Dark space, 31. 
 Cathode rays, 32. 
 
 lonization by, 48. 
 
 Transport of charge by, 35. 
 
 Velocity of, 35. 
 Charge of electron : 
 
 Ratio to mass, 118, 121, 125. 
 
 Variation of ratio, 128. 
 
 Charge of ion, 131 etseq. 
 Curie, M. : 
 
 Discharging action of radium 
 
 rays, 85. 
 Curie, Mme. : 
 
 Preparation of radium, 66. 
 
 Radio-activity of thorium, 6x. 
 Curie, M. and Mme. : 
 
 Charge transported by |3-rays, 76. 
 
 Preparation of polonium, 62. 
 Curie, M. and Mme. and Laborde : 
 
 Evolution of heat by radium, 87. 
 Crookes, Sir W. : 
 
 Radiant matter, 34. 
 
 Spinthariscope, 79. 
 
 Danysz : 
 
 Physiological effects of radium, 
 
 90. 
 Debierne : 
 
 Preparation of actinium, 63. 
 Dewar and Curie : 
 
 Production of helium, 106 et seq. 
 Dissociation : 
 
 Electric, 9. 
 
 Electrolytic, 10. 
 Dorn: 
 
 Loss of weight of radium, 86. 
 
 Magnetic deviation of 0-rays, 75. 
 
 , 118, 121, 125, 130. 
 
 M 
 
 Variation of, 128, 130. 
 Electric field : 
 
 Deviation of rays by, 115 et seq, 
 Electric shadow, 49. 
 
 163
 
 1 64 
 
 INDEX 
 
 Electrolysis, 3. 
 Electron : 
 
 Charge of, 26, 129. 
 
 Mass of, 37, 129, 150. 
 Elster and Geitel : 
 
 Excited activity in atmosphere,95- 
 
 Faraday : 
 
 Dark space, 30. 
 
 Fourth state of matter, 33. 
 Fluorescence, 32. 
 
 y-rays, 77. 
 Georgiewski : 
 
 Thermal effect, 88. 
 Giesel : 
 
 Magnetic deviation of 0-rays, 75. 
 
 Radio-lead, 63. 
 
 Helium : 
 
 Produced from radium emana- 
 tion, 106 et seq. 
 Heydweiller : 
 
 Loss of weight of radium, 86. 
 Hofmann and Strauss : 
 
 Radio-lead, 63. 
 Hofmann and Wolfl : 
 
 Radio-lead, 64. 
 Huggins, Sir W. and Lady : 
 
 Spectrum of radium, 80. 
 
 Ions: 
 
 As nuclei of condensation, 132. 
 Charge, mass, and velocity of, 
 
 13, 137 ft *eq- 
 
 lonization, 40. 
 By impact, 45, 50. 
 Causes of, 45. 
 Due to Lenard rays, 47. 
 
 Kaufmann : 
 
 Variation of 128, 150. 
 m 
 
 Lenard : 
 , 118 et seq. 
 
 Transport of charge by cathode 
 
 rays, 35. 
 Lenard rays : 
 
 lonization by, 47. 
 Lorenz : 
 
 Electromagnetic theory of light, 
 
 Magnetic field : 
 
 Effect on a-rays, 73. 
 
 Effect on P-rays, 75. 
 
 Effect on cathode rays, no. 
 
 Effect on circular vibration, 20 
 
 et seq. 
 Maiorana : 
 
 Velocity of cathode rays, 35. 
 Marckwald : 
 
 Radio-tellurium, 64. 
 Meyer and Von Schweidler : 
 
 Magnetic deviation of P-rays, 
 75- 
 
 Perrin: 
 
 Transport of charge by cathode 
 
 rays, 35. 
 Pitchblende, 62. 
 Polonium, 62. 
 
 Rays emitted by, 72. 
 
 Radiant matter, 34. 
 Radiation : 
 
 From radio-active bodies, 69. 
 Radio-active bodies : 
 
 Chemical actions due to, 82. 
 
 Effects produced by, 78. 
 
 Evolution of heat by, 96. 
 
 Loss of weight by, 86. 
 Radio-active emanations, 91. 
 
 Gaseous nature of, 92. 
 
 Presence in the atmosphere, 95. 
 Radio-activity : 
 
 Induced, 65, 94. 
 
 Of snow, 100. 
 
 Of spring water, 98. 
 Radio-lead, 63. 
 Radio-tellurium, 64.
 
 INDEX 
 
 I6 5 
 
 Radium, 63. 
 
 Evolution of heat by, 87. 
 
 Physiological effect of rays from, 
 90. 
 
 Preparation of, 65 et seq. 
 
 Spectrum of, 68, 80. 
 Rutherford : 
 
 Induced radio-activity, 94. 
 
 Radio-active emanations, 91 et 
 seg. 
 
 Thorium-X, 96. 
 
 Transport of charge by a-rays, 73. 
 Rutherford and Soddy : 
 
 Theory of atomic disintegration, 
 102 et seg. 
 
 Saturation current, 43. 
 Schmidt : 
 
 Radio-activity of thorium, 61. 
 Secondary rays, 85. 
 Selenium : 
 
 Effect of rays from radium on, 91. 
 Sella : 
 
 Excited activity in the atmos- 
 phere, 95. 
 Spinthariscope, 79. 
 Strutt: 
 
 Nature of -rays, 73. 
 
 Production of electricity by 
 radium, 84. 
 
 Theory of atomic disintegration, 
 
 102 et seg. 
 Thomson, J. J. 
 Charge of the ion, 131. 
 Velocity of cathode rays, 35. 
 Velocity of the electrons, 112 ei 
 seq. 
 
 Velocity and of the positive 
 
 
 
 ions, 130. 
 Thorium, 61. 
 Thorium-X, 96, 104. 
 
 Uranium : 
 Radio-active compounds of, 57, 
 
 61. 
 Uranium-X, 103. 
 
 Weber : 
 
 Atomic theory of electricity, 5. 
 Wien: 
 
 Velocity and of positive ions, 
 m 
 
 130- . 
 
 Wilson, C. T. R. 
 Ions as nuclei of condensation, 
 
 132. 
 Wilson, H. A. 
 
 Charge of the ion, 138. 
 
 Zeeman effect, 14 et seq.
 
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