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happening to disintegrate just as the alpha-particle passed by, are 

 admissible but highly improbable. Another such collision in argon 

 is shown in Fig. 2; this too was the only encounter with four diverging 



Fig. 1 



tracks observed in many thousand photographs with the same gas. 1 

 A collision in air, in which the struck nucleus was not broken, but 

 knocked to one side while the alpha-particle rebounded in the manner 

 demanded by the principle of conservation of momentum, is shown 



Fig. 2 



in Fig. 3. These results show how small the atom-nuclei must be, 

 compared to the extension of their electron-systems; for the 80,000 

 alpha-particles observed in air had traversed the electron-systems of 

 about ten billion molecules altogether. 



1 As Rutherford's experiments indicate that argon atoms are especially stable 

 against disintegration, this may be a case of two consecutive collisions with adjacent 

 atoms. 



