IXAUGUHAL ADDRESS. li> 



from a somewhat different point of view in order that I may be able 

 to suggest an alternative hypothesis as to their natxire. Let me take 

 you back to the history of scientific research in the last century. The 

 wave theory of light had completely triumphed over the coi-puscular 

 theory which Newton had advocated; and the progress of physical 

 science for scores of years consisted in a long series of successful 

 explanations of new discoveries in terms of the undulatory theory. 

 When, therefore, new radiations were discovered, it was natural that 

 determined efforts should be made to explain them in terms of the 

 winning hypothesis. For a long time many physicists claimed that 

 the cathode rays were aether pulses, and Sir William Crookes had to 

 fight hai'd for his idea of a fourth state of matter. The later investi- 

 gations of J. J. Thomson established firmly the material view, and 

 laid the foundation of the electronic theory. When Becquerel first 

 experimented with the rays of uranium, some of his earliest tests were 

 made in order to discover whether the new radiation possessed the 

 characteristic properties of aether waves. Pioneering woi'k is always 

 difficult: Becquerel's experiments led him to false conclusions. He 

 announced that he had found reflection, refraction, and polarisation ; 

 it was. tlierefore, concluded that the rays were of the nature of light, 

 i^nd this view was accepted by m.ost physicists for some years. Finally, 

 Rutherford, in 1899, repeated the expei'iments, and reversed Bec- 

 quereFs conclusions. Thanks mainly to his labours, we now know 

 that two at least of the three forms of radiation which radium and 

 uranium emit are material. The alpha rays are positively charged 

 atoms of helium, the beta rays are negative electrons. There remain 

 tlie gamma rays, and with these must be classed the Rontgen rays, 

 which resemble them so closely. Many years ago Sir George Stokes, 

 as I have already said, applied the pulse hypothesis to explain the 

 properties of the newly-discovered X rays; and later it was naturally 

 appliet-i to the gamma rays also. But it is a very striking fact, I 

 think, that the more closely these various radiations are examined the 

 clearer does it become that there is a strong family likeness between 

 the properties of them all. If for no other reason, we are forced to 

 inquire whether they are not more nearly allied in Nature than is 

 compatible with the classification into coi-puscular and pulse radia- 

 tions. The differences between the various rays — alpha, beta, gamma, 

 and Rontgen rays — are rather differences in the degree to which 

 various properties are exhibited; there is no very radical difference 

 between the properties themselves. The most obvious difference at all 

 stages of the inquiry has been, perhaps, the fact that the alpha and 

 beta rays can be deflected by electric and magnetic fields, whilst the 



