CONSTITUTION OF MATTER RUTHERFOKD. 189 



rays opens out into a diffuse or scattered beam. The alpha and beta 

 particles move so swiftly that they are actually able to pass through 

 the structure of the atom and are deflected by the intense forces with- 

 in the atom. Geiger first drew attention to a very unexpected effect 

 with alpha particles. When a pencil of alpha rays falls on a thin 

 film of gold, for example, the great majority of the particles pass 

 through with little absorption. A few, however, are found to be so 

 scattered that they are turned back through an angle of more than a 

 right angle. Taking into consideration the great energy of motion 

 of the alpha particle, such a result is as surprising as it would be to 

 a gunner if an occasional shot at a light target was deflected back 

 toward the gun. It was found that these large deflections must 

 result from an encounter with a single atom. The occasional sudden 

 deflection of an alpha particle is well illustrated in one of the later 

 photographs of the trail of an alpha particle obtained by Mr. C. T. K, 

 Wilson, and shown in plate 5, figure 1. It is seen that the rectilinear 

 path of the particle suffers two sharp bends, no doubt resulting in 

 each case from a single close encounter with an atom. In the sharp 

 bend near the end a slight spur is seen, indicating that the atom was set 

 in such swift motion by the encounter with the alpha particle that it 

 was able to ionize gas at a short distance. If the forces causing the 

 deflection Avere electrical, it was at once evident that the electric field 

 within the atom must be exceedingly intense. The distribution of 

 positive electricity assumed in the Thomson atom was much too 

 diff'use to produce the intense fields required. To overcome this diffi- 

 culty the Avriter inverted the role of positive electricity. Instead of 

 being distributed through a sphere comparable in size with the sphere 

 of action of the atom, the positive electricity is supposed to be con- 

 centrated in a very minute volume or nucleus, and the greater part 

 of the mass of the atom is supposed to be resident in this nucleus. 

 The latter is supposed to be surrounded by a distribution of negative 

 electrons extending over a distance comparable with the diameter of 

 the atom as ordinarily understood. On this point of view the alpha 

 particle is the minute nucleus of the helium atom, which has lost its 

 two external electrons. In this type of atom the large deviations of 

 the alpha particle take place w^hen it passes through the intense elec- 

 tric field close to the nucleus of the colliding atom. The nearer it 

 passes to the nucleus, the greater the deflection of the particle. As- 

 suming that the forces between the alpha particle and the nucleus of 

 the colliding atom are mainly electrical and vary according to an in- 

 verse square law, the alpha particle describes a hyperbolic orbit 

 around the nucleus, and the relative number of alpha particles de- 

 flected through different angles can be simply calculated. 



It was thus possible to test this theory of atomic structure by ac- 

 tual experiment. This was undertaken by Geiger and Marsden in 



