PROBLEMS OF MODERN SCIENCE 



their characteristics are investigated from a quan- 

 titative standpoint. They may, for example, be 

 able to penetrate for a greater or less distance in 

 air, they may be deviated to a greater or to a 

 less degree by an electric or by a magnetic field. 

 These are the tests which serve to differentiate 

 them, the presence of either class of projectile at 

 any point being ascertainable by the scintillations 

 which they cause on a zinc sulphide screen. By 

 tests of this kind Rutherford has been able to prove 

 that hydrogen can be knocked out of the nucleus 

 of the nitrogen atom but not out of oxygen or 

 carbon. He has also adduced considerable evidence 

 in favour of the view that a body of mass 3 and 

 charge 2 can be ejected from the nuclei both of 

 nitrogen and of oxygen. Such a body would be 

 an isotope of helium, which has a nuclear charge 

 of 2 units and mass 4. 



The consideration of the energy which may 

 be stored in the nuclei of atoms furnishes problems 

 of the greatest interest and importance. We have 

 seen that the heavy radioactive atoms are con- 

 tinuously undergoing spontaneous disintegration, 

 and that this process consists primarily in a decom- 

 position of the nucleus. This decomposition is 

 accompanied by a continuous evolution of heat 

 energy, as is evidenced by the fact that radioactive 

 bodies are always at a somewhat higher tempera- 

 ture than their surroundings. Precise measure- 

 go 



