688 Scattering of a and j3 Particles by Matter. 



experimentally whether such a simple relation holds also for 

 the lighter atoms. In cases where the mass of the deflecting 

 atom (for example, hydrogen, helium, lithium) is not very 

 different from that of the a particle, the general theory of 

 single scattering will require modification, for it is necessary 

 to take into account the movements of the atom itself 

 (see § 4).^ 



It is of interest to note that Nagaoka * has mathematically 

 considered the properties of a " Saturnian " atom which he 

 supposed to consist of a central attracting mass surrounded 

 by rings of rotating electrons. He showed that such a 

 system was stable if the attractive force was large. From 

 the point of view considered in this paper, the chance of 

 large deflexion would practically be unaltered, whether the 

 atom is considered to be a disk or a sphere. It may be 

 remarked that the approximate value found for the central 

 charge of the atom of gold (100^) is about that to be 

 expected if the atom of gold consisted of 49 atoms of helium, 

 each carrying a charge 2 e. This may be only a coincidence, 

 but it is certainly suggestive in view of the expulsion of 

 helium atoms carrying two unit charges from radioactive 

 matter. 



The deductions from the theory so far considered are 

 independent of the sign of the central charge, and it has not 

 so far been found possible to obtain definite evidence to 

 determine whether it be positive or negative. It may be 

 possible to settle the question of sign by consideration of the 

 difference of the laws of absorption of the /3 particle to be 

 expected on the two hypotheses, for the effect of radiation in 

 reducing the velocity of the /3 particle should be far more 

 marked with a positive than with a negative centre. If the 

 central charge be positive, it is easily seen that a positively 

 charged mass if released from the centre of a heavy atom, 

 would acquire a great velocity in moving through the electric 

 field. It may be possible in this way to account for the high 

 velocity of expulsion of a particles without supposing that 

 they are initially in rapid motion within the atom. 



Further consideration of the application of this theory to 

 these and other questions will be reserved for a later paper, 

 when the main deductions of the theory have been tested 

 experimentally. Experiments in this direction are already 

 in progress by Geiger and Marsden. 



University of Manchester, 

 April 1911. 



* Nagaoka, Phil. Mag-, vii. p. 445 (1904). 



