496 Professor Sir Ernest Rutherford [June 4, 



by measuring the deflection of a pencil of a rays in strong magnetic 

 and electric fields. With the aid of intense sources of radiation, it 

 was found that the value of the E/M — the ratio of the charge 

 carried by the a particle carried to its mass — was 4820 units, a value 

 to be expected if helium has an atomic weight 4 and carries two 

 unit charges. This experiment also shows that the mass of the 

 flying positive particle is uot affected appreciably by its swift motion. 

 From known data the initial velocity of the expalsion of the a 

 particles from all other radio-active substances can be deduced with 

 accuracy. 



If the expulsion of an a particle from an atom is the result of an 

 internal explosion, we should anticipate, from the analogy of a shot 

 from a gun, that the residual atom would recoil in a direction 

 opposite to the escaping p particle. The existence of these " recoil " 

 atoms can be shown in a variety of ways, for the velocity of recoil 

 is suificient to cause the atoms to leave the surface on which they 

 are deposited and to pass through a considerable distance in air at 

 a pressure of one millimetre before they are stopped. It is to be 

 anticipated that the momentum of a recoiling atom should be equal 

 and opposite to that of the escaping a particle. Since the deflection 

 of a charged particle in motion in a magnetic field is inversely 

 proportional to its momentum, the deflection of a stream of recoiling 

 atoms should be the same as for the a particles if the atoms carry 

 the same charge. Dr. Makower has examined the deflection of a 

 pencil of recoil atoms in a magnetic field, and found it to be exactly 

 half of that due to the a particle, proving definitely that the recoihng 

 atom carries only one unit of positive charge in place of two for the 

 a particle. 



We thus see that the simple application of momentum enables 

 us to deduce the mass and energy of the recoiling atoms. Since 

 the mass of the radio-active atoms is about fifty times that of the 

 a particle, the velocity, and also the energy, of recoil is only about 

 one-fiftieth of that of the escaping a particle. In a similar way, it 

 can be shown that the ejection of a swift (^ particle should cause a 

 vigorous recoil of the atom, though not so marked as in the case of 

 the more massive a particle. 



P Rays. 



During the last few years notable advances have been made in 

 our knowledge of the mode of emission of /8 particles from radio- 

 active atoms. The work of Baeyer, Hahn, and Meitner, and of 

 Danysz, has shown that the /? rays from a radio-active substance 

 like radium B or radium C contain a number of definite groups of 

 rays which are expelled with definite velocities. This is best shown 

 photographically by examining the deflection of a pencil of yS rays 

 in a magnetic field. In a uniform field, each of the groups of rays 



