822 Sir E. Rutherford and Dr. J. Chad wick on the 



ultimate distribution of kinetic energy between the three 

 bodies involved in a collision — viz., the a particle,, the H satel- 

 lite, and the central nucleus. It is also necessary to account 

 for the surprising fact that the H atoms from aluminium 

 are shot out in all directions. The latter fact suggests at 

 once the possibility of an atomic explosion in which the 

 energy of the a particle plays the part of a detonator, and 

 the escaping H particle gains most of its energy from the 

 .nucleus. Such a general conclusion, however, is not sup- 

 ported hy the observed result that the energy of escape 

 appears to be nearly proportional to the energy of the 

 incident a particle. On account of the difficulty of deter- 

 ming the ranges in these experiments, too much stress must 

 not be laid on this point. 



The escape of the H atoms in all directions may be 

 explained in a simple way, which is illustrated in fig. 6. 



Fig. 6. 



The H atom is supposed to be moving in an orbit round 

 the central nucleus N. If the collision occurs, as in A, the 

 H atom is driven in the forward direction of the ol particle 

 and away from the nucleus ; if, as in B, the H atom is driven 

 towards the nucleus, it describes an orbit close to the nucleus 

 and escapes in the backward direction. The difference of 

 velocity of the H atoms in the forward and backward 

 directions is probably due to the fact that the nucleus has 

 been set in motion in the direction of the a, particle before 

 the close collision with the H satellite occurs. On this view, 

 the relative velocity of the H atom and residual nucleus is 

 the same whether the H atom escapes in the backward or 

 forward direction, but the actual velocity in the backward 

 direction is less. 



It is clear that in the case of aluminium, the law of con- 

 servation of energy does not hold, unless we take into 



