concentrated 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 when it passes through the intense electric field close to the nucleus of the colliding atom. The nearer it passes to the nucleus, the greater the deflection of the particle. Assuming that the forces between the alpha particle and the nucleus of the colliding atom are mainly electrical and vary according to an inverse square law, the alpha particle describes a hyperbolic orbit round the nucleus, and the relative number of alpha particles deflected through different angles can be simply calculated.
It was thus possible to test this theory of atomic structure by actual experiment. This was undertaken by Geiger and Marsden in a very important but difficult investigation. They examined the relative number of alpha particles scattered through various angles by their passage through thin films of matter, e. g., aluminium, silver and gold, by actually counting the alpha particles by means of the scintillations on a zinc sulphide screen. The experimental results were found to be in very good accord with the theory, while Darwin, in addition, showed that any other law of force except the inverse square was incompatible with the observations.
From these results, it is a simple matter to show that the radium of the nucleus of the gold atom can not be greater than cm.—an exceedingly small distance and only about one ten-thousandth part of the diameter of the atom. While the results thus indicated that the nucleus of a heavy atom was of minute dimensions, it was of interest to see whether a still lower limit could be obtained for lighter atoms. On the theory, the helium atom has a nucleus of two unit positive charges, and the lighter atom, hydrogen, should have a nucleus of only one unit. When an alpha particle passes through hydrogen gas, there should be occasional very close encounters between the particle and nucleus of the hydrogen atom. Since the mass of the hydrogen atom is only one quarter of that of helium, it is to be anticipated that the former should be set in very swift motion by a close collision with an alpha particle, and in special cases should be given a velocity 1.6 times greater than that of the colliding alpha particle, and should travel four times as far. Such swiftly moving hydrogen nuclei were actually observed by Marsden with the scintillation method when a pencil of alpha rays passed through hydrogen, and they were found to travel, as the theory predicted, about four times further than the alpha particle itself. Since the energy gained by the hydrogen nucleus depends on the closeness
