i3o THE POPULAR SCIENCE MONTHLY 



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 elec- 

 trons 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 ex- 

 ternal 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 collid- 

 ing 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 3 X 10 12 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 Mars- 

 den with the scintillation method when a pencil of alpha rays passed 

 through hydrogen, and they were found to travel, as the theory pre- 

 dicted, about four times further than the alpha particle itself. Since 

 the energy gained by the hydrogen nucleus depends on the closeness 



