568 Professor Sir Ernest Rutherford [June 6, 



is an arresting fact, showing that the a-particle must encounter very 

 intense forces in penetrating the structure of the atom. In order to 

 explain such results, the idea of the nucleus atom was developed 

 in which the main mass of the atom is concentrated in a positively 

 charged nucleus of very small dimensions compared with the space 

 occupied by the electrons which surround it. The scattering of 

 a-particles through large angles was shown to he the result of a 

 single collision where the a-particle passed close to this charged 

 nucleus. From a study of the distribution of the particles scattered 

 at different angles, results of first importance emerged. It was found 

 that the results could be explained only if the electric forces between 

 the a-particle and charged nucleus followed the law of inverse 

 squares for distances apart of the order of 10"^^ cm. Darwin pointed 

 out that the variation of scattering with velocity was explicable only 

 on the same law. This is an important step, for it affords an experi- 

 mental proof that, at any rate to a first approximation, the ordinary 

 law of force holds for electrified bodies at such exceedingly minute 

 distances. It was also found that a resultant charge on the nucleus 

 measured in fundamental units was about equal to the atomic 

 number of the element. In the case of gold this number is believed 

 from the work of Moseley to be 79. 



Knowing the mass of the impinging a-particle and of the atom 

 with which it collides, we can determine from direct mechanical 

 principles the distribution of velocities after the collision, assuming 

 that there is no loss of energy due to radiation or other causes. It 

 is important to notice that in such a calculation we need make no 

 assumption as to the nature of the atoms or of the forces involved 

 in the approach and separation of the atoms. For example, if an 

 a-particle collides with another helium atom, we should expect the 

 a-particle to give its energy to the helium atom, which could thus travel 

 on with the speed of the a-particle. If an a-particle collides directly 

 with a heavy atom — e.g. of gold of atomic weight 107 — the a-particle 

 should retrace its path with only slightly diminished velocity, while 

 the gold atom moves onward in the original direction of the a-particle, 

 but with about one-fiftieth of its velocity. Next, consider the 

 important case where the a-particle of mass 4 makes a direct collision 

 with a hydrogen atom of mass 1. From the laws of impact, the 

 hydrogen atom is shot forward with a velocity 1 • 6 times that of the 

 impinging a-particle, while the a-particle moves forward in the same 

 direction, but with only * 6 of its initial speed. Marsden showed 

 that swift hydrogen atoms set in motion by impact with a-particles 

 can be detected like a-particles by the scintillations produced in a 

 zinc sulphide crystal. Recently I have been able to measure the 

 speed of such H atoms and found it to be in good accord with the 

 calculated value, so that we may conclude that the ordinary laws of 

 impact may be applied with confidence in such cases. The relative 

 velocities of the a-particles and recoil atom after collision can thus 



