RADIOACTIVITY— ARTIFICIAL AND NATURAL 



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pierce the wall; but many of these particles are much more energetic 

 than the impinging deuterons themselves, for they draw upon a 

 reserve of energy stored up in the nuclei.^ They shoot through the wall 

 into the air of the cloud-chamber itself, and if they are charged, they 

 make long trails of ions along their paths. The expansion is then 

 produced and the water-vapor, condensing upon these ions, makes 

 trails of droplets which are the paths made visible. 



Fig. 7 — Tracks of a proton and a H^ nucleus resulting from one of the deuteron- 

 deuteron reactions. (P. I. Dee, Cavendish Laboratory; Proceedings of the Royal 

 Society.) 



Figure 7 exhibits two of these visible paths of "tracks," made by 

 particles which sprang from the scene of the transmutation (inside the 

 knob) in practically opposite directions but with very different pene- 

 trative powers, since one of the tracks is seen to be much longer than 

 the other. The long one is the track of a proton, the short one is 

 that of a H^ nucleus which is a deuteron augmented by a captured 

 neuteron ; this picture shows a single example of the upper reaction of 

 Fig. 5. How can physicists be sure that these tracks are due to the 

 nuclei which I have named? This question is far too deep to be an- 

 swered in this place, and I can only assure the listener that while such 

 pictures by themselves cannot suffice for the proof, an unassailable 



" In the language employed in chemistry, these are "exothermic" reactions. 



