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univalent atom in electrolytic liquids。 The number of unknown quantities was thus reduced to twothe mass and the velocity。 The measurement of the magnetic and electric deflections gave two independent relations between the unknowns； which could therefore be determined。 The velocities of the cathode ray particles were found to vary round a value about one…tenth that of light； but the mass was found always to be the same within the limits of error； whatever the nature of the terminals； of the residual gas in the vessel； and of the conditions of the experiment。 The mass of a cathode ray particle； or corpuscle； as Thomson； adopting Newton's name； called it； is about the eight…hundredth part of the mass of a hydrogen atom。
These corpuscles； found in so many different kinds of substance； are inevitably regarded as a common constituent of matter。 They are associated each with a unit of negative electricity。 Now electricity in motion possesses electromagnetic energy； and produces effects like those of mechanical inertia。 In other words； an electric charge possesses mass； and there is evidence to show that the effective mass of a corpuscle increases as its velocity approaches that of light in the way it would do if all its mass were electromagnetic。 We are led therefore to regard the corpuscle from one aspect as a disembodied charge of electricity； and to identify it with the electron of Lorentz and Larmor。
Thus； on this theory； matter and electricity are identified； and a great simplification of our conception of the physical structure of Nature is reached。 Moreover； from our present point of view； a common basis for matter suggests or implies a common origin； and a process of development possibly intelligible to our minds。 The idea of the evolution of matter becomes much more probable。
The question of the nature and physical meaning of a corpuscle or electron remains for consideration。 On the hypothesis of a universal luminiferous aether； Larmor has suggested a centre of aethereal strain 〃a place where the continuity of the medium has been broken and cemented together again （to use a crude but effective image） without accurately fitting the parts； so that there is a residual strain all round the place。〃 （Larmor； loc。 cit。） Thus he explains in quasi…mechanical terms the properties of an electron。 But whether we remain content for the time with our identification of matter and electricity； or attempt to express both of them in terms of hypothetical aether； we have made a great step in advance on the view that matter is made up of chemical atoms fundamentally distinct and eternally isolated。
Such was the position when the phenomena of radio…activity threw a new light on the problem； and； for the first time in the history of science； gave definite experimental evidence of the transmutation of matter from one chemical element to another。
In 1896 H。 Becquerel discovered that compounds of the metal uranium continually emitted rays capable of penetrating opaque screens and affecting photographic plates。 Like cathode and Rontgen rays； the rays from uranium make the air through which they pass a conductor of electricity； and this property gives the most convenient method of detecting the rays and of measuring their intensity。 An electroscope may be made of a strip of gold…leaf attached to an insulated brass plate and confined in a brass vessel with glass windows。 When the gold…leaf is electrified； it is repelled from the similarly electrified brass plate； and the angle at which it stands out measures the electrification。 Such a system； if well insulated； holds its charge for hours； the leakage of electricity through the air being very slow。 But； if radio…active radiation reach the air within； the gold…leaf falls； and the rate of its fall； as watched through a microscope with a scale in the eye…piece； measures the intensity of the radiation。 With some form of this simple instrument； or with the more complicated quadrant electrometer； most radio… active measurements have been made。
It was soon discovered that the activity of uranium compounds was proportional to the amount of uranium present in them。 Thus radio…activity is an atomic property dependent on the amount of an element and independent of its state of chemical combination。
In a search for radio…activity in different minerals； M。 and Mme Curie found a greater effect in pitch…blende than its contents of uranium warranted； and； led by the radio…active property alone； they succeeded； by a long series of chemical separations； in isolating compounds of a new and intensely radio…active substance which they named radium。
Radium resembles barium in its chemical properties； and is precipitated with barium in the ordinary course of chemical analysis。 It is separated by a prolonged course of successive crystallisation； the chloride of radium being less soluble than that of barium； and therefore sooner separated from an evaporating solution。 When isolated； radium chloride has a composition； which； on the assumption that one atom of metal combines with two of chlorine as in barium chloride； indicates that the relative weight of the atom of radium is about 225。 As thus prepared； radium is a well…marked chemical element； forming a series of compounds analogous to those of barium and showing a characteristic line spectrum。 But； unlike most other chemical elements； it is intensely radio…active； and produces effects some two million times greater than those of uranium。
In 1899 E。 Rutherford； then of Montreal； discovered that the radiation from uranium； thorium and radium was complex。 （Rutherford； 〃Radio…activity〃 （2nd edition）； Cambridge； 1905。） Three types of rays were soon distinguished。 The first； named by Rutherford alpha…rays； are absorbed by thin metal foil or a few centimetres of air。 When examined by measurements of the deflections caused by magnetic and electric fields； the alpha…rays are found to behave as would positively electrified particles of the magnitude of helium atoms possessing a double ionic charge and travelling with a velocity about one…tenth that of light。 The second or beta type of radiation is much more penetrating。 It will pass through a considerable thickness of metallic foil； or many centimetres of air； and still affect photographic plates or discharge electroscopes。 Magnetic and electric forces deflect beta…rays much more than alpha…rays； indicating that； although the speed is greater； approaching in some cases within five per cent。 that of light； the mass is very much less。 The beta…rays must be streams of particles； identical with those of cathode rays； possessing the minute mass of J。J。 Thomson's corpuscle； some eight…hundredth part of that of a hydrogen atom。 A third or gamma type of radiation was also detected。 More penetrating even than beta…rays； the gamma…rays have never been deflected by any magnetic or electric force yet applied。 Like Rontgen rays； it is probable that gamma…rays are wave…pulses in the luminiferous aether； though the possibility of explaining them as flights of non… electrified particles is before the minds of some physicists。
Still another kind of radiation has been discovered more recently by Thomson； who has found that in high vacua； rays become apparent which are absorbed at once by air at any ordinary pressure。
The emission of all these different types of radiation involves a continual drain of energy from the radio…active body。 When M。 and Mme Curie had prepared as much as a gramme of radium chloride； the energy of the radiation became apparent as an evolution of heat。 The radium salt itself； and the case containing it； absorbed the major part of the radiation； and were thus maintained at a temperature measurably higher than that of the surroundings。 The rate of thermal evolution was such that it appeared that one gramme of pure radium must emit about 100 gramme…calories of heat in an hour。 This observation； naturally as it follows from the phenomena previously discovered； first called attention to the question of the source of the energy which maintains indefinitely and without apparent diminution the wonderful stream of radiation proceeding from a radio…active substance。 In the solution of this problem lies the point of the present essay。
In order to appreciate the evidence which bears on the question we must now describe two other series of phenomena。
It is a remarkable fact that the intensity of the radiation from a radio… active body is independent of the external conditions of temperature； pressure； etc。 which modify so profoundly almost all other physical and chemical processes。 Exposure to the extreme cold of liquid air； or to the great heat of a furnace； leaves the radio…activity of a substance unchanged； apparent exceptions to this statement having been traced to secondary causes。
Then； it is found that radio…activity is always accompanied by some chemical change； a new substance always appears as the parent substance emits these radiations。 Thus by chemical reactions it is possible to separate from uranium and thorium minute quantities of radio…active materials to which the names of uranium…X and thorium…X have been given。 These bodies behave differently from t