SOME CONTEMPORARY ADVANCES IN PHYSICS 



103 



Fig. 4 shows curious collisions of alpha-particles passing through 

 helium gas, photographed by D. Bose and S. Ghosh of Calcutta. In 

 each of the two left-hand trails the alpha-particle has apparently 



Fig. 3 



knocked the nucleus and the two electrons of the atom in three differ- 

 ent directions. 2 The alpha-particle of the right-hand trail (iiib is a 

 magnification of iiia) seems to have produced quite an explosion; 

 this may be the disruption of a nucleus belonging to a stray molecule 



Fig. 4 



of nitrogen, but one would not expect the original particle to go on 

 as if unaffected. 



2 In hydrogen they found no cases of two electrons being driven off by the same 

 impact. This agrees with Millikan's conclusion that double ionization is much 

 more frequent with helium atoms than with molecules of any other kind, if indeed, 

 it is not a characteristic of helium alone. 



