CONTEMPORARY ADVANCES IN PHYSICS 639 



proton, this is so nearly the rule as to suggest very forcibly that the 

 major part of every nucleus consists of protons. All this strengthens 

 the belief that in witnessing these flashes of "long-range" particles one 

 is witnessing the signs of transmutation. 



The next step, then, consists in finding which of the elements may 

 be transmutable. I repeat that for the present, a strict assessment of 

 the evidence permits us to proclaim a transmutation only when there 

 are corpuscles of greater range than either the primary alpha-particles, 

 or hydrogen nuclei which suffer elastic impacts. The condition, how- 

 ever, is not quite so harsh as I have intimated. If hydrogen atoms be 



Fig. 4— Curve showing evidence that the particles emitted from aluminium bom- 

 barded by a-rays comprise protons and deflected alpha-particles (G. Stetter). 



Struck by alpha-particles, those and only those which are projected 

 straight ahead have the full computed range; those which bounce off 

 at an angle go less far; those which start off at 90° have no range at all 

 which is to say, no elastic impact can send a nucleus off in the plane 

 through the bombarded substance at right angles to the alpha-ray 

 stream. Thus, if one stations the fluorescent screen somewhere in this 

 plane, one may confidently count all of the scintillations as signs of 

 transmutation, excepting such as may be due to primary alpha-parti- 

 cles deflected through 90° by kernels which they approach without 

 disrupting them, or to hydrogen nuclei which after sufi'ering elastic 

 impacts got deflected. (The reader will have noticed in Fig. 3 that 

 the angle between the paths of the a-particles to the metal foil and 

 the paths of the protons from the foil is large, always more than 80°.) 



