NOBEL LECTURE
December 11, 1908.
The study of the properties of the α-rays has played a notable part inthe development of radioactivity and has been instrumental in bringingto light a number of facts and relationships of the first importance.With increase of experimental knowledge there has been a growingrecognition that a large part of radioactive phenomena is intimatelyconnected with the expulsion of the α-particles. In this lecture anattempt will be made to give a brief historical account of thedevelopment of our knowledge of the α-rays and to trace the long andarduous path trodden by the experimenter in the attempts to solve thedifficult question of the chemical nature of the α-particles. α-rayswere first observed in 1899 as a special type of radiation and duringthe last six years there has been a persistent attack on this greatproblem, which has finally yielded to the assault when the resources ofthe attack seemed almost exhausted.
Shortly after his discovery of the radiating power of uranium by thephotographic method, Becquerel showed that the radiation from uraniumlike the Röntgen-rays possessed the property of discharging anelectrified body. In a detailed investigation of this property, Iexamined the effect on the rate of discharge by placing successivelayers of thin aluminium foil over the surface of a layer of uraniumoxide and was led to the conclusion that two types of radiation of verydifferent penetrating power were present. The conclusions at that periodwere summed up as follows:
"These experiments show that the uranium, radiation is complex andthat there are present at least two distinct types of radiation—onethat is very readily absorbed, which will be termed for convenience theα-radiation, and the other of a more penetrative character, which willbe termed the β-radiation."[1] When other radioactive substances werediscovered, it was seen that the types of radiation present wereanalogous to the β– and α-rays of uranium and when a still morepenetrating type of radiation from radium was discovered by Villard, theterm γ-rays was applied to them. The names thus given soon came intogeneral use as a convenient nomenclature for the three distinct types ofradiation emitted from uranium, radium, thorium, and actinium. Onaccount of their insignificant penetrating power, the α-rays were atfirst considered of little importance and attention was mainly directedto the more penetrating β-rays. With the advent of active preparationsof radium, Giesel in 1899 showed that the β-rays from this substancewere easily deflected by a magnetic field in the same direction as astream of cathode rays and consequently appeared to be a stream ofprojected particles carrying a negative charge. The proof of theidentity of the β-particles with the electrons constituting the cathoderays was completed in 1900 by Becquerel, who showed that the β-particlesfrom radium had about the same small mass as the electrons and wereprojected at a speed comparable with the velocity of light. Time doesnot allow me to enter into the later work of Kaufmann and others on thissubject, which has greatly extended our knowledge of the constitutionand mass of electrons.
In the meantime, further investigation had disclosed that theα-particles produced most of the ionization observed in theneighbourhood of an unscreened radioactive substance, and that most ofthe energy radiated was in the form of α-rays. It was calculated byRutherford and McClung in