CHEMISTRY 95 



which the energy could be obtained is suggested by the view 

 that in the atom we have electrified systems of very different 

 types, one small, the other, large; the radius of one type is 

 comparable with 1O 13 cm. ; that of the other is about 100,000 

 times greater. The electrostatic potential energy in the 

 smaller bodies is enormously greater than in the larger ones ; 

 if one of these small bodies were to explode and expand to 

 the size of the larger ones, we should have a liberation of 

 energy large enough to endow an a particle with the energy 

 it possesses. Is it possible that the positive units of electricity 

 were, to begin with, quite as small as the negative, but while 

 in the course of ages most of these have passed from the 

 smaller stage to the larger, there are some small ones still 

 lingering in radio-active substances, and it is the explosion of 

 these which liberates the energy during radio-active disinte- 

 grations ? 



The a particles have been detected and counted separately 

 by Rutherford, using a special electric method, the passage of 

 a single a particle into a suitable detecting vessel. In his own 

 words : 



"The entrance of an a particle through a small open- 

 ing was marked by a sudden movement of the needle of the 

 electrometer which was used as a measuring instrument. In 

 this way by counting the number of separate impulses com- 

 municated to the electrometer needle, it was possible to deter- 

 mine by direct counting the number of a particles expelled per 

 second from one gram of radium. But we can go further 

 and confirm the result of counting the number of a particles 

 by an entirely distinct method. Sir William Crookes has 

 shown that when the a rays are allowed to fall upon a screen 

 of phosphorescent zinc sulphide, a number of bright scintilla- 

 tions are observed. It appears as if the impact of each a 

 particle produced a visible flash of light where it struck the 

 screen. Using suitable screens the number of scintillations 



