190 ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1915. 



a A^ery 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., aluminum, 

 silver, and gold — by actually counting the aljDha particles by means 

 of the scintillations on a zinc sulphide screen. The experimental 

 fesults 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 w^as incompatible with the observations. 



From these results it is a simple matter to show that the radius 

 of the nucleus of the gold atom can not be greater than 3X10'^" cm. — 

 an exceedingly small distance and only about one ten-thousandth 

 part of the diameter of the atom. Wliile 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 this 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 be- 

 tween 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 mo- 

 tion 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 farther than the alpha particle itself. Since the 

 energy gained by the hydrogen nucleus depends on the closeness of 

 its approach to tlie alpha particle, it can be simply calculated that 

 the centers of the nuclei must have passed within 10"^^ cm. of each 

 other. This is an extraordinarily small distance, even smaller 

 than the diameter of the electron itself. It is thus clear that the 

 nuclei of hydrogen and of helium must be exceedingly minute. It 

 should be borne in mind that such observations only give a maximum 

 limit to the size of the nucleus, and there is no experimental evidence 

 against the view that the nucleus of the hydrogen atom may not 

 actually prove to be minute in volume compared even with the nega- 

 tive electron. If this be the case, it appears probable that the hydro- 

 gen nucleus is the positive electron and that its great mass, compared 

 with the negative electron, is due to the greater concentration of its 

 charge. According to modern theory the electrical mass of a charged 

 particle varies inversely as its radius. The greater mass of the posi- 

 tive than of the negative electron would thus be explained if its 



