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 scattered 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
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 distribution 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 rôle 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
