THE CONSTITUTION OF MATTER 129 



with new and powerful methods of attack on this problem, and has 

 allowed us to distinguish to some extent between various theories of 

 atomic structure. One of these methods depends on the study of the 

 deflection of swiftly moving bodies like alpha and beta particles in their 

 passage through matter. It is found that these rays are always scat- 

 tered in their passage through matter, i. e., a narrow pencil of 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 within 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 towards 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. R. Wilson, and shown in Fig. 13. It is 



Fig. 13. Track of Alpha Particles showing Shabp Deviations (Wilson). 



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 were electrical, it was at once evident that 

 the electric field within the atom must be exceedingly intense. The dis- 

 tribution of positive electricity assumed in the Thomson atom was much 

 too diffuse to produce the intense fields required. To overcome this 

 difficulty, the writer 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 



