ELECTRONS 87 



the kernel of the helium atom. He used a glass tube 

 with a deep dimple in one side. First he tested the 

 atmosphere within the tube for the presence of helium 

 and found none. Then in the dimple he placed a radio- 

 active substance. Again he tested; and helium was 

 then present. The enormous velocities of the alpha 

 particles had carried them through the glass wall into 

 the enclosed space, where they showed in the spectro- 

 scope the lines which are characteristic of helium. 



More recently Professor Rutherford has let these 

 alpha particles shoot through nitrogen gas. Imagine, 

 if you care to, some enormously solid comet plunging 

 into our solar system so rapidly as not to perturb our 

 planet until the fatal moment when it collides with the 

 sun. The violent impacts of alpha particles and nuclei 

 of nitrogen were relatively infrequent since both are 

 very tiny; but when they occurred the nitrogen atom 

 was disrupted and some of the protons knocked out of 

 its nucleus. 



There is dove-tailing evidence enough on all these 

 matters to require several graduate courses in physics 

 for its exposition, so the reader whose time is limited 

 must accept it on faith. He might justly ask, however, 

 how these protons were recognized? By the distance 

 they were knocked ! And the reason is this : Alpha par- 

 ticles rushing from their former nuclear homes will 

 penetrate a fairly definite distance in air before they are 

 so slowed down that they will not produce little scin- 

 tillations when they strike a properly prepared screen. 

 If instead of air they rush through an atmosphere of 

 hydrogen gas the effect is the same except that occa- 



