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SCIENCE PROGRESS 



it over millions of molecules of the plate material. It could 

 not have pushed them all to one side unless their combined 

 mass was much less than that of the particle itself, which is 

 certainly not the case. What other conclusion can we come 

 to than that it must have gone through them all, since it cannot 

 have gone round them? 



*- Then we naturally ask, what then is this a particle and how 

 does it come to possess this extraordinary power ? 



It is of course abundantly proved that the a particles are 

 helium atoms. The continual presence of helium in materials 

 containing radioactive matter would naturally point to this 



lorMftohcN 



Fig. ii. 



conclusion and various workers have actually grown helium 

 from radium. But the most simple and direct proof is due 

 to Rutherford. Placing some emanation in a glass tube with 

 very thin walls, on the outside of which a vacuum at first 

 existed, he found that helium gradually appeared in this outside 

 space. Without the emanation nothing of the kind took place, 

 so the radium emanation shot helium through the glass wall. 

 When the emanation was withdrawn from the inner tube and 

 helium was substituted for it, nothing of the sort took place. 

 The effect is not due to a leak. 



Our experiment on the passage of the a particles through 

 the screen shows therefore that the helium atom must pass 

 through all the millions of atoms it meets in the screen. Here 



